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. 2024 Mar 25;19(3):e0300051. doi: 10.1371/journal.pone.0300051

Combination of ethyl acetate fraction from Calotropis gigantea stem bark and sorafenib induces apoptosis in HepG2 cells

Pattaraporn Chaisupasakul 1,2, Dumrongsak Pekthong 2,3,4, Apirath Wangteeraprasert 5, Worasak Kaewkong 6, Julintorn Somran 7, Naphat Kaewpaeng 2,8, Supawadee Parhira 2,4,9,*,#, Piyarat Srisawang 1,2,10,*,#
Editor: Nafees Ahemad11
PMCID: PMC10962855  PMID: 38527038

Abstract

The cytotoxicity of the ethyl acetate fraction of the Calotropis gigantea (L.) Dryand. (C. gigantea) stem bark extract (CGEtOAc) has been demonstrated in many types of cancers. This study examined the improved cancer therapeutic activity of sorafenib when combined with CGEtOAc in HepG2 cells. The cell viability and cell migration assays were applied in HepG2 cells treated with varying concentrations of CGEtOAc, sorafenib, and their combination. Flow cytometry was used to determine apoptosis, which corresponded with a decline in mitochondrial membrane potential and activation of DNA fragmentation. Reactive oxygen species (ROS) levels were assessed in combination with the expression of the phosphatidylinositol-3-kinase (PI3K)/ protein kinase B (Akt)/ mammalian target of rapamycin (mTOR) pathway, which was suggested for association with ROS-induced apoptosis. Combining CGEtOAc at 400 μg/mL with sorafenib at 4 μM, which were their respective half-IC50 concentrations, significantly inhibited HepG2 viability upon 24 h of exposure in comparison with the vehicle and each single treatment. Consequently, CGEtOAc when combined with sorafenib significantly diminished HepG2 migration and induced apoptosis through a mitochondrial-correlation mechanism. ROS production was speculated to be the primary mechanism of stimulating apoptosis in HepG2 cells after exposure to a combination of CGEtOAc and sorafenib, in association with PI3K/Akt/mTOR pathway suppression. Our results present valuable knowledge to support the development of anticancer regimens derived from the CGEtOAc with the chemotherapeutic agent sorafenib, both of which were administered at half-IC50, which may minimize the toxic implications of cancer treatments while improving the therapeutic effectiveness toward future medical applications.

Introduction

The World Health Organization estimates that by 2040, hepatocellular carcinoma (HCC), which contributes 75–85% of the overall incidence of primary carcinoma of the liver, will cause the deaths of more than 1,300,000 individuals [1]. Sorafenib is commonly administered and considered to be an effective treatment for HCC due to its efficiency in generating a survival advantage in patients with advanced HCC, which cannot be treated by liver surgery [2]. Sorafenib inhibits the activity of RAF proto-oncogene serine/threonine-protein kinase, B-Raf proto-oncogene, serine/threonine kinase, and kinases in the Rat sarcoma virus/Rapidly accelerated fibrosarcoma (Raf)/ Mitogen-activated protein kinase kinase (MEK)/Extracellular signal-regulated kinase (ERK) signaling pathway, resulting in decreased tumor cell proliferation. Sorafenib can also interfere with angiogenesis by suppressing the hepatocyte factor receptor, Fms-like tyrosine kinase, vascular endothelial growth factor receptor-2/-3, platelet-derived growth factor receptor and other tyrosine kinases [3]. However, numerous side effects of sorafenib have been noted including, hypertension, hand-foot skin reaction, diarrhea, fatigue, weight loss, alopecia, anorexia, and vocal abnormalities [1, 4].

The anticancer-promoting effect of sorafenib against Bel7402 HCC cells is accompanied by a rise in reactive oxygen species (ROS) levels. This increases the protein expression levels of Bax, caspase-3, and caspase-9 while decreasing those of Bcl-2 [5]. Sorafenib triggers apoptosis in human liver cancer cells by suppressing the signaling pathways, phosphatidylinositol-3-kinase (PI3K), protein kinase B (Akt), mammalian target of rapamycin (mTOR), and ERK [6], resulting in the activation of Bax and suppression of Bcl-2, which induces caspase-3 and Poly (ADP-ribose) polymerase expression in LM3 HCC cells [7].

The anticancer efficacy of sorafenib is enhanced when combined with other anticancer drugs compared to single therapy. Apoptosis in LM3 HCC cells was enhanced when sorafenib was combined with capsaicin [7]. In combination, sorafenib and bufalin promoted apoptosis in the cells of non-small cell lung cancer NCI-H292 by enhancing ROS production and suppressing mitochondrial membrane potential (MMP) [8]. Sorafenib combined with celastrol upregulated apoptosis in HepG2 and Hep1-6 HCC cells [9]. Similarly, silibinin combined with sorafenib increased apoptosis in HCC Bel-7402 cells via the activation of Akt, ERK, signal transducer and activator of transcription 3 (STAT3), and mitogen-activated protein kinase p38, leading to the activation of cleaved-caspase3 and cleaved-poly (ADP-ribose) polymerase [10]. Thus, combining sorafenib with plant extracts could be an effective form of anticancer therapy.

Calotropis gigantea (L.) Dryand (Apocynaceae), widely referred to as a crown flower or gigantic milkweed, appears natively in Africa, Eastern Asia, and Southeast Asia. As an abundant source of cardenolides, it has applications in traditional Chinese and Ayurvedic medical treatments for a wide range of diseases, including cancer [11, 12]. The leaf and root ethanolic extract of C. gigantea exhibited cytotoxicity in T47D breast cancer cells [13]. An ethanolic extract of leaves from C. gigantea was found to activate cytotoxicity in WiDr human colon cancer cells [14]. The ethanolic extract of the entire C. gigantea plant upregulated the expression of the death receptor, Fas, Fas ligand, and caspase-8 in non-small cell lung cancer cells [15]. The leaves and stem methanolic extract of C. gigantea triggered apoptosis by elevating the expression of pro-apoptotic proteins in human breast cancer cells [16]. Furthermore, C. gigantea leaf dichloromethane and ethyl acetate fractions exhibited cytotoxicity in WiDr human colon cancer cells [14]. Several phytochemicals in C. gigantea extracts, including cardenolides, antifeedant nonprotein amino acids, naphthalene, and terpene derivatives, flavonol glycosides, pregnanes, ursanetype triterpenoids, and steroids, have an essential function in the biological properties of the extracts [17]. In addition, the cardiac glycosides, flavonoids, alkaloids, phenolics, and triterpenoids in C. gigantea extract also showed anti-cancer activities in cancer cells [1824].

According to our previous study, the C. gigantea stem bark extracts were abundant with several secondary metabolites including calactin, cardiac glycosides, flavonoids, triterpenoids, phenolics, and alkaloids [25]. The anti-cancer properties of the ethanolic extract of the C. gigantea stem bark have been demonstrated, and HCT116 had the lowest IC50 value among HT-29 and HepG2 cells [25, 26]. Calotropin, a particular of cardenolide derived from C. gigantea stem bark extracts, reportedly exhibits cytotoxicity to enforce malignancies [27].

The ethyl acetate fraction of C. gigantea stem bark extracts (CGEtOAc) contains high levels of cardiac glycosides and phenolics but lower levels of triterpenoids, flavonoids, and calotropins than other fractions [25, 26]. Although a monotreatment with CGEtOAc exhibited a less potent anticancer effect in HepG2 cells than in HCT116 cells, studying the enhanced anticancer efficacy of a standard chemotherapeutic treatment when applied in combination with CGEtOAc is important for developing future alternative treatments. To investigate the potential anticancer properties of the C. gigantea stem bark extract and the reduction of chemotherapeutic side effects on HepG2 cells, this study examined the combination treatment with a minimized dose of CGEtOAc and sorafenib that demonstrated apoptotic activity mechanisms.

Materials and methods

Materials

Hepatocellular carcinoma HepG2 cells (JCRB1054) and normal lung fibroblast IMR-90 cells (JCRB9054) were obtained from the Japanese Collection of Research Bioresources Cell Bank (JCRB Cell Bank), Japan. Dulbecco’s Modified Eagle Medium (DMEM) (10-014-CM) was obtained from Corning, USA. Fetal bovine serum (FBS) (16000–044) and penicillin and streptomycin (15140–122) were purchased from Gibco, USA. 3-(4, 5-Dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) (D0801) was purchased from Tokyo Chemical Industry CO., LTD., Japan. Anti-PI3 Kinase Antibody, p110β (PI3K) (09482), and Luminata Forte Western HRP Substrate (WBLUF0100) were purchased from Merck, Germany. Annexin V conjugated Alexa Fluor 488 (A13201), Propidium iodide (PI) (P3566), Hoechst 33342 (H1399), 5,5,6,6’-tetrachloro-1,1’,3,3’ tetraethylbenzimidazoylcarbocyanine iodide (JC-1) (T3168), Horseradish peroxidase-conjugated goat anti-rabbit (65–6120). goat anti-mouse secondary antibody (A28177), 2’, 7’-dichlorodihydrofluorescein diacetate (H2DCFDA) (D399), and AKT Pan Monoclonal Antibody (E.32.10) (MA5-14999) were purchased from Invitrogen, USA. H2DCFDA-Cellular ROS Assay Kit (ab113851) and Anti-Mammalian target of rapamycin (mTOR) (ab32028) were purchased from Abcam, USA. β-Actin (8H10D10) Mouse mAb (3700) were purchased from Cell Signaling Technology, USA. E.Z.N.A.® Total RNA Kit I (R6834-01) was purchased from Omega Bio-tek, Inc., USA. TetroTM cDNA Synthesis Kit (BIO-65043) was purchased from Meridian Bioscience, USA. 5× HOT FIREPol® EvaGreen® qPCR Mix Plus (no ROX) (08-25-00001) was purchased from Solis Bio-Dyne, Germany. A proteinase inhibitor cocktail (ML051) was purchased from HIMEDIA, India. Mammalian Protein Extraction Reagent (M-PERTM) (78501) was purchased from Thermo Fisher Scientific, USA.

Preparation of standardized CGEtOAc

The CGEtOAc was prepared and standardized for calotropin content with high-performance liquid chromatography, as detailed in our previous study [26]. The C. gigantea stem bark was harvested, dried at a temperature of 35 ± 7°C, and ground into powder. The ultrasonic-assisted extraction with 95% ethanol was used to prepare the crude extract before fractionation using the liquid-liquid chromatography of water and dichloromethane, followed by the rest of the water layer and ethyl acetate. The ethyl acetate fraction was evaporated at 50°C to obtain the dark brown sticky extract. The content of calotropin in CGEtOAc (2.7 ± 0.06 mg calotropin/10 g CGEtOAc) was determined and used as a bioactive marker to standardize the CGEtOAc. The possible m/z of other cardenolides or other compounds found in CGEtOAc are illustrated in S1 Fig. The plant collection followed the national guideline of Thailand for using plants for research (Approval number 0278 under the Plant Varieties Protect Act B.E. 2542 (1999) section 53 from the Department of Agriculture, Ministry of Agricultural and Cooperatives, Thailand). The voucher specimen of dry plant number 005194 was identified in our previous report [26] and preserved for reference in a recognized herbarium in Thailand at PNU herbarium, Faculty of Science, Naresuan University, Phitsanulok, Thailand, 65000.

Cell culture

Hepatocellular carcinoma HepG2 (JCRB1054) and fibroblast IMR-90 cells (JCRB9054) were cultured in Dulbecco’s Modified Eagle Medium containing 10% fetal bovine serum and 1% penicillin and streptomycin under a humidified incubator at 5% CO2 at 37°C. Cells were cultured in a T-25 culture flask using complete media and subcultured when they reached 80–90% confluency every 4 days during the incubation cycle. Cell count was monitored during each subculture to measure the consistency of growth rates before proceeding with the cell treatment procedure. The human cell culture research was approved by the Naresuan University Institutional Review Board (NU-IRB) in Panel 1: Health Sciences, with the approval number P1-0166/2565.

MTT assay evaluation of cell viability

After incubating HepG2 cells at a density of 1.5×104 cells/well/150 μL in a 96-well plate for 24 h and exposing them to the extracts for an additional 24 h, 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was applied at a concentration of 2 mg/mL to assess viable cells based on mitochondrial reductase activity. We then measured absorbance at 595 nm using dimethyl sulfoxide (DMSO) to dissolve the formazan crystals produced by MTT, employing a microplate reader (SpectraMax ABS, Molecular Devices, USA). The IC50 values were calculated using GraphPad Prism version 9.

Analysis of cell migration activity using wound healing technique

After incubating HepG2 cells at a density of 4×105 cells/well/1 mL in a 12-well plate and allowing them to reach 80–90% confluency within 48 h, we induced a cell monolayer to form physical wounds by scratching the attached cells in a continuous straight line at the center of each well, using a 200 μL sterile tip. The scratch wounds were consistent in size in each well to avoid variations between conditions [2831]. We removed debris and non-attached cells before subsequently exposing the cells to the extracts, sorafenib, and their combination for 0–72 h.

The gap width of the wound was measured and recorded in three regions of each image between the closest points on both sides of the wound using an inverted microscope (IX71, Olympus Corporation, Japan). The results were analyzed as the percentage of the scratch gap using cellSens Standard Ver.2.3. Triplicate measurements of each well were made to obtain the mean value. The scratch widths of the control and treatment wells were measured and normalized according to their respective values at 0 h. The migrated distance was defined as the following formula:

migrated rate (%) = (width of initial wound—width of remaining wound) / width of initial wound × 100 [32, 33].

Determination of apoptosis using flow cytometry

HepG2 cells were incubated at a density of 2 × 105 cells/1 mL in a 12-well plate for 24 h and exposed them to the extracts for an additional 24 h. The degree of apoptosis was then evaluated after harvesting by double staining with Annexin V conjugated Alexa Fluor 488 at 5 μL (25 μg/mL) in 100 μL of 1x annexin-binding buffer and propidium iodide (PI) at 1 μL of 100 μg/mL, according to the manufacturer’s instruction. After a 15-min incubation in the dark, 400 μL of cold annexin-binding buffer was added to the cell suspension and cell apoptosis was measured using flow cytometry.

Annexin V-binding phosphatidylserine results in translocation from the inner plasma membrane to the outside membrane, signaling an early apoptotic stage. PI can enter the cell and bind DNA when the cell membrane becomes permeable. The double labeling of Annexin V and PI reveals late-stage apoptosis. CytoFLEX flow cytometry (CytoFLEX S V2-B2-Y3-R2 Flow Cytometer, Beckman Coulter, USA) and CytExpert Version 2.4.0.28 were used to detect the apoptotic cells.

Determination of DNA fragmentation by staining with Hoechst 33342

After incubating HepG2 cells at a density of 5×105 cells/2 mL on a glass coverslip in a 35 mm culture dish for 24 h and exposing them to the extracts for an additional 24 h, HepG2 cells were treated with a 4% formaldehyde fixative solution for 15 min. Subsequently, they were incubated with Hoechst 33342 at a concentration of 10 μg/mL for 10 min to identify DNA fragmentation following the manufacturer’s instructions. DNA fragmentation was indicated by fluorescence intensity, observed under a fluorescent microscope (BX53F2, Olympus Corporation, Japan).

Fluorescence labeling with JC-1 to evaluate mitochondrial membrane potential (MMP)

After incubating HepG2 cells at a density of 5×105 cells/2 mL on a glass coverslip in a 35 mm culture dish for 24 h and exposing them to the extracts for an additional 24 h, HepG2 cells were treated with 4% formaldehyde for 15 min before being labeled with 10 μg/mL 5,5,6,6’-tetrachloro-1,1’,3,3’ tetraethylbenzimidazoylcarbocyanine iodide (JC-1) according to the manufacturer’s instructions. Healthy mitochondria show that the cationic dye JC-1 can enter to shift to a dimeric form and display red fluorescence. In the cytoplasm of damaged cells, JC-1 exists as a monomeric structure emitting green fluorescence. Using a fluorescent microscope, the fluorescence intensity of the monomeric and dimeric forms of JC-1 was analyzed (BX53F2, Olympus Corporation, Japan).

ROS production was assessed using an assay kit and fluorescence imaging

After incubating HepG2 cells at a density of 5×105 cells/2 mL on a glass coverslip in a 35 mm culture dish for 24 h and exposing them to the extracts for an additional 24 h, the cellular ROS production levels were detected in HepG2 cells by applying the H2DCFDA-Cellular ROS Assay Kit according to the manufacturer’s instruction. The fluorescence degree of cellular ROS levels was detected with a microplate reader at Ex/Em = 485/535 nm (SpectraMax iD3, Molecular Devices, USA).

Staining with 2’,7’-dichlorodihydrofluorescein diacetate (H2DCFDA) was also applied to detect cellular ROS production. HepG2 cells were exposed to H2DCFDA at 20 μM, which is deacetylated by esterase to form 2′,7′-H2DCF, which is then oxidized by ROS to form fluorescent 2’,7’-dichlorofluorescein and ROS levels was monitored using a fluorescent microscope (BX53F2, Olympus Corporation, Japan) at 40× magnification.

Real-time quantitative reverse transcription polymerase chain reaction for PI3K/Akt/mTOR gene expression

After incubating HepG2 cells at a density of 2×106 cells/4 mL in a 60 mm culture dish for 24 h and exposing them to the extracts for an additional 24 h, total RNA was extracted using E.Z.N.A.® Total RNA Kit I following the manufacturer’s instructions. The RNA contents were analyzed at absorbances of 260 and 280 nm using a spectrophotometer (NanoDropTM One/Onec Microvolume, Thermo Scientific, USA). TetroTM cDNA Synthesis Kit was used for the complementary DNA (cDNA) synthesis from messenger RNA templates at 5 μg/μL for quantitative reverse transcription-polymerase chain reaction for PI3K, Akt, and mTOR following the manufacturer’s instructions. Then, cDNA at 1 ng/μL was used for real-time PCR analysis, which was carried out using 5× HOT FIREPol® EvaGreen® qPCR Mix Plus (no ROX) in a CFX Connect Real-Time PCR System (Bio-rad CFX manager version 3.1) at the following conditions: 40 cycles 95°C for 30 s; 60°C for 30 s; 72°C for 30 s. The relative expression level of genes was quantified and normalized to that of GAPDH as the housekeeping gene using the 2-ΔΔCt method [22]. The sequences of the primers (10 pg/μL) are listed in Table 1.

Table 1. The primer sequences used in RT-qPCR.

Gene Forward primer (5’-3) Reverse primer (5’-3)
GAPDH AACGGGAAGCTTGTCATCAATGGAAA GCATCAGCAGAGGGGGCAGAG
PI3K CCACGETOACCCATCATCAGGTGAA CCTCACGGAGGCATTCTAAAGT
Akt CCTCCACGETOACCATCGCACTG TCACAAAGAGCCCTCCATTATCA
mTOR ATGCAGCTGTCCTGGTTCTC AATCAGACAGGCACGETOACAGGG

Immunoblotting

After incubating HepG2 cells at a density of 2×106 cells/4 mL in a 60 mm culture dish for 24 h and exposing them to the extracts for an additional 24 h, the total cellular protein was extracted with the Mammalian Protein Extraction Reagent, adding 1% proteinase inhibitor cocktail and quantifying with a spectrophotometer (The NanoDrop® ND-1000, Thermo Scientific, USA). Protein samples at 60 μg/μL were separated with 8–12% sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis with 1X running buffer (125 mM Tris-base, 959 mM glycine, 0.1% SDS), transferred to a polyvinylidene fluoride (PVDF) membrane using transfer buffer (25 mM Tris-base, 192 mM glycine, 20% methanol, 0.1% SDS), and nonspecific proteins were blocked with Immobilon® Block-CH (Chemiluminescent Blocker). Membranes were incubated overnight with anti-PI3K (1:1,000), anti-Akt (1:1,000), anti-mTOR (1:1,000) primary antibodies. Then, membranes were incubated for 2–4 h with 1:5,000 concentrations of horseradish peroxidase-conjugated goat anti-rabbit or goat anti-mouse secondary antibody at 4°C. The expression of β-actin (at 1:1,500 concentration) behaves as a loading control. The 1X phosphate-buffered saline with Tween 20 was used for washing each step of primary and secondary incubation. The image of protein expressions was measured by applying the Luminata Forte Western HRP Substrate and Chemiluminescence western blot detection (Image Quant LAS 4000; GE Healthcare Life Sciences, USA). Protein expression levels relative to β-actin were calculated using ImageJ version 1.46.

Statistical analysis

The significant differences in data presented as the mean ± SD from at least three different experiments were analyzed with a one-way analysis of variance (ANOVA) using Tukey’s honestly significant difference (HSD) test at p<0.05 using GraphPad Prism version 9.

Results

Cytotoxicity of CGEtOAc and sorafenib in HepG2 cells

The cytotoxic properties in HepG2 cells assessed using the MTT technique revealed that CGEtOAc, sorafenib, and their combination diminished cell viability in a concentration-responsive manner after 24 h of exposure (Fig 1A–1D). The IC50 values for CGEtOAc (0–2,000 μg/mL) and sorafenib (0–40 μM) were 707.87 ± 49.05 μg/mL and 8.65 ± 0.33 μM, respectively. The IC50 graphs are presented in S2 Fig. Subsequently, the combination of sub-IC50 and around-IC50 concentrations of CGEtOAc at 200, 400, 600, and 800 μg/mL and sorafenib at 2, 4, and 8 μM was examined. Fig 1C demonstrates that CGEtOAc at 400 μg/mL in combination with sorafenib at 4 μM significantly decreased cell viability compared to the vehicle and single treatments with CGEtOAc and sorafenib at the same concentration. Furthermore, the cytotoxic effect of this combination was comparable to CGEtOAc at 600 and 800 μg/mL in combination with sorafenib at 4 μM. Notably, the combination of CGEtOAc and sorafenib at an IC50 dose of 8 μM was extremely cytotoxic to HepG2 cells.

Fig 1. Cytotoxicity of the ethyl acetate fraction of C. gigantea stem bark extracts (CGEtOAc) and sorafenib against HepG2 cells.

Fig 1

The 3-(4, 5-Dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) technique was used to assess the cell viability after being treated for 24 h with (A) CGEtOAc, (B) sorafenib, and (C) their combination demonstrated in the bar graph and (D) heat map analysis. (E) CGEtOAc and (F) the combination of CGEtOAc and sorafenib treatment on IMR-90 cells. Cells treated with 0.8% dimethyl sulfoxide (DMSO) represented the vehicle control. The significant differences in data, presented as the mean ± standard deviation (SD) from at least three different experiments, were investigated with a one-way analysis of variance (ANOVA) using Tukey’s honestly significant difference (HSD) test: *; p < 0.05 vs the vehicle control, a; p < 0.05 vs a single CGEtOAc treatment at their own dose, b; p < 0.05 vs a single sorafenib treatment at their own dose.

The combination efficacy of CGEtOAc with sorafenib after 24 h of incubation was analyzed using the combination index (CI) based on the Chou-Talalay method [3436] using the CompuSyn version 1.0.1 Software (ComboSyn Inc., NJ, USA), as shown in S3 Fig. We found that CGEtOAc at 400 μg/mL in combination with sorafenib at 4 μM had a CI value of 0.5, and the inhibition rate was approximately 60%, indicating the synergistic effect. However, a combination of CGEtOAc at 200 μg/mL and sorafenib at 4 mM also yielded a CI of 0.5, but the inhibition rate was 40%. Although CI values less than 1 were found when combining CGEtOAc at 200, 400, 600, and 800 μg/mL with sorafenib at a high dose of 8 μM, it is important to note that sorafenib was used at a high concentration. Therefore, we concluded that CGEtOAc at 400 μg/mL and sorafenib at 4 μM were suitable for the following experiments.

In addition, the toxicity to IMR-90 cells of CGEtOAc at 200, 400, and 800 μg/mL was significant in comparison to the vehicle, but to a lesser degree than that observed in HepG2 cells, while the toxicity of CGEtOAc at 1,000, 1,500, and 2,000 μg/mL was not significantly different from that of the 800 μg/mL solution (Fig 1E). The IC50 of CGEtOAc to IMR-90 cells was approximately greater than 5,000 μg/mL. Compared to the vehicle control, the combination of 400 μg/mL CGEtOAc and 4 μM sorafenib had a significant cytotoxic effect on IMR-90 cells, but to a lesser extent than HepG2 cells, where no combination effect was observed (Fig 1F).

CGEtOAc and sorafenib, singly and in combination inhibited the migration of HepG2 cells

At a concentration of 400 μg/mL for CGEtOAc, 4 μM for sorafenib, and their combination, cell migration was significantly inhibited. This was demonstrated by a higher percentage of gap distance in a wound healing assay, compared to each hour of incubation in the vehicle group, when 0 h set at 100% for each group (Fig 2A and 2B). The percentage of gap distance for the combination of CGEtOAc and sorafenib in each incubation period did not differ from the single treatments. However, the migration data for the combination of CGEtOAc and sorafenib at 72 h disappeared due to the complete loss of cell viability. Cells detached from the plate, rendering it impossible to measure the gap between cell scratches, attributed to the toxic effects of the combination treatment.

Fig 2. The migratory capacity of HepG2 cells was assessed through a 0–72 h incubation with CGEtOAc at 400 μg/mL and sorafenib at 4 μM, both singly and in combination.

Fig 2

(A) Wound healing images were captured with a magnification bar of 500 μm. (B) The percentage of the gap was depicted as a bar graph. Cells treated with 0.8% DMSO represented the vehicle control. The significant differences in data, presented as the mean ± SD from at least three different experiments, were investigated with a one-way ANOVA using Tukey’s HSD test: *; p < 0.05 vs the 0 h of the vehicle control, a; p < 0.05 compared to 24 h of incubation in the vehicle group, b; p < 0.05 compared to 48 h of incubation in the vehicle group, and c; p < 0.05 compared to 72 h of incubation in the vehicle group, with 0 hours set at 100% for each group.

The migration rate, as shown in S4 Fig, confirmed a significant inhibition of HepG2 cell migration rate by CGEtOAc at 400 μg/mL, sorafenib at 4 μM, and their combination compared to each hour of incubation in the vehicle group. The migration rate for the combination of CGEtOAc and sorafenib in each incubation period did not differ from the single treatments.

Our findings indicate that CGEtOAc, sorafenib, and their combination upregulated the anti-proliferative and anti-migratory activities in HepG2 cells.

Combination of CGEtOAc and sorafenib enhanced apoptosis in HepG2 cells

A combination of CGEtOAc and sorafenib was applied to examine apoptotic HepG2 cells. Annexin V/PI analysis revealed that 400 μg/mL CGEtOAc combined with 4 μM sorafenib significantly increased the proportion of apoptotic cells compared to the vehicle control and single treatments (Fig 3A and 3B), as shown in the gating strategy of flow cytometry demonstrated in S1 Raw images. In addition, JC-1 staining for detecting the dissipation of mitochondrial membrane potential (MMP) was examined. Depolarization of MMP following treatment with 400 μg/mL CGEtOAc combined with 4 μM sorafenib is demonstrated in Fig 4. These results indicate that the promotion of apoptosis in HepG2 cells by the combination of CGEtOAc and sorafenib correlates with MMP damage.

Fig 3. The apoptotic effect of CGEtOAc, sorafenib, and their combination against HepG2 cells after exposure for 24 h.

Fig 3

(A) Apoptotic levels in HepG2 cells were examined by flow cytometry after double labeling using annexin V and PI. (B) The levels of apoptotic cells were displayed as a bar graph. 0.8% DMSO represented the vehicle control. The significant differences in data, presented as the mean ± SD from at least three different experiments, were investigated with a one-way ANOVA using Tukey’s HSD test: *; p < 0.05 compared to the vehicle control. a; p < 0.05 compared to a single CGEtOAc treatment. b; p < 0.05 compared to a single sorafenib treatment.

Fig 4. Determination of mitochondrial membrane potential (MMP) in HepG2 cells after exposure to CGEtOAc, sorafenib, and their combination for 24 h by labeling with 5,5,6,6′-tetrachloro-1,1′,3,3′ tetraethylbenzimidazoylcarbocyanine iodide (JC-1) and Hoechst 33342.

Fig 4

The images were detected using a fluorescent microscope with a magnification bar of 50 μM. Cells treated with 0.8% DMSO represented the vehicle control.

The combination of CGEtOAc and sorafenib triggered HepG2 apoptosis associated with increasing cellular ROS levels

The underlying mechanism of apoptotic cells in HepG2 was identified by measuring ROS production using H2DCFDA staining. As shown in Fig 5A, 400 μg/mL CGEtOAc combined with 4 μM sorafenib upregulated ROS production with the green fluorescence intensity of dichlorofluorescein compared to the vehicle control and their respective treatment groups. In addition, Fig 5B demonstrated that the formation of cellular ROS significantly increased following exposure to CGEtOAc, sorafenib, and their combination. ROS levels were approximately 175% and 178% in CGEtOAc and sorafenib treatments, respectively. The treatment with a combination of CGEtOAc and sorafenib significantly increased ROS levels to approximately 193% compared to single treatments. Two hours of pretreatment with N-acetylcysteine (NAC) to inhibit ROS formation confirmed that CGEtOAc, sorafenib, and their combination upregulated ROS generation in HepG2 cells.

Fig 5. The intracellular production of reactive oxygen species (ROS) in HepG2 cells subjected to CGEtOAc, sorafenib, and their combination for 24 h.

Fig 5

(A) Cellular ROS levels were analyzed using H2DCFDA fluorescence labeling and visualized under a fluorescence microscope with a magnification bar of 50 μM. (B) Bar graph displaying the percentage of ROS generation. Cells treated with 0.8% DMSO represented the vehicle control. The significant differences in data, presented as the mean ± SD from at least three different experiments, were investigated with a one-way ANOVA using Tukey’s HSD test: *; p < 0.05 compared to the vehicle control. a; p < 0.05 compared to a single CGEtOAc treatment. b; p < 0.05 compared to a single sorafenib treatment. c; p < 0.05 compared to a combination treatment of CGEtOAc and sorafenib.

Additionally, when ROS generation was inhibited, the apoptotic effect of CGEtOAc, sorafenib, and their combination was considerably reduced but not to baseline levels, in comparison to the vehicle and their single treatment groups (Fig 6A and 6B). However, the downregulation in apoptosis in NAC pretreated at 10 mM for 2 h followed by combined treatment was still significantly greater than in the vehicle and single treatment groups, indicating that ROS is the major regulatory mediator but not the only mechanism mediating the apoptotic effect in HepG2 cells treated with CGEtOAc combined with sorafenib.

Fig 6. The apoptosis-inducing effect was associated with ROS generation in HepG2 cells after being exposed to the combination of CGEtOAc and sorafenib for 24 h.

Fig 6

Following N-acetylcysteine (NAC) pre-incubation, cells were exposed to CGEtOAc, sorafenib, and their combination for 24 h. (A) Apoptotic levels were analyzed using annexin V and PI double labeling, followed by flow cytometry. (B) Bar graph depicting the percentage of the total apoptotic rate. Cells treated with 0.8% DMSO represented the vehicle control. The significant differences in data, presented as the mean ± SD from at least three different experiments, were investigated with a one-way ANOVA using Tukey’s HSD test: *; p < 0.05 compared to the vehicle control. a; p < 0.05 compared to a single CGEtOAc treatment. b; p < 0.05 compared to a single sorafenib treatment. c; p < 0.05 compared to a combination treatment of CGEtOAc and sorafenib.

The apoptotic response following treatment with a combination of CGEtOAc and sorafenib may involve the inhibition of the PI3K/Akt/mTOR pathway in HepG2 cells

Apoptosis in cancer cells has been discovered to be mediated by the signaling pathway proteins PI3K/AKT/mTOR [24, 37]. Compared to vehicle control and their respective single treatment groups, 400 μg/mL CGEtOAc combined with 4 μM sorafenib significantly downregulated the PI3K, Akt, and mTOR protein expression. Combined treatment also significantly downregulated the expressions of PI3K, Akt, and mTOR genes (Fig 7). The original uncropped and unadjusted images underlying all blot results are demonstrated in S2 Raw images. Thus, it is hypothesized that the apoptotic mechanism of CGEtOAc combined with sorafenib is associated with the downregulation of the upstream PI3K/AKT/mTOR signaling cascade pathway.

Fig 7. The combination of CGEtOAc and sorafenib downregulated the expression of the phosphatidylinositol-3-kinase (PI3K)/ protein kinase B (Akt)/ mammalian target of rapamycin (mTOR) signaling pathway after the incubation of HepG2 cells for 24 h.

Fig 7

(A-B) Representative western blot images, bar graph, and quantitative reverse transcription-polymerase chain reaction analysis of the relative expression levels of (A-C) PI3K, (D-F) Akt, and (G-I) mTOR. Cells treated with 0.8% DMSO represented the vehicle control. The significant differences in data, presented as the mean ± SD from at least three different experiments, were investigated with a one-way ANOVA using Tukey’s HSD test: *; p < 0.05 compared to the vehicle control. a; p < 0.05 compared to a single CGEtOAc treatment. b; p < 0.05 compared to a single sorafenib treatment.

Discussion

Since 2010, there has been considerable focus on studying the anticancer effects of C. gigantea extracts in many cancer models [25, 26, 3841]. However, biological activities involving C. gigantea stem bark extracts have rarely been conducted. The C. gigantea stem bark extracts generated cellular toxicity in HCT116, HT-29, and HepG2 cancer cells, of which the dichloromethane fraction exhibited the most notable apoptotic activity compared to other fractions. HepG2 cell apoptotic responses were much substantially sensitive to each fraction of the extract, and the response to the ethyl acetate fraction treatment was weaker than in HCT116 and HT-29 cells. The ethyl acetate fraction contains the highest levels of cardiac glycosides and phenolics, however, triterpenoids, flavonoids, and calotropins are present in lower levels than dichloromethane and other extract fractions. Nevertheless, this suggests that the cytotoxicity of each fraction from the C. gigantea stem bark is not entirely influenced by a single component present in each fraction [25, 26]. Although the overall phytochemical contents and efficacy against HepG2 cells of the ethyl acetate fraction are lower than those of the dichloromethane fraction, including the calotropin content, which was selected as a bioactive marker for standardization [26], the highest amount of total cardiac glycosides in the ethyl acetate fraction is intriguing and merits further investigation when combined with a conventional chemotherapeutic agent, sorafenib at low dose usage, to enhance anticancer efficacy and reduce adverse effects.

Several pieces of evidence suggest that cardiac glycosides derived from plant extracts have a crucial part in the anticancer properties against several cancer cells [42, 43]. Apoptosis in MCF-7 and HepG2 cells was observed by the cardiac glycoside lanatoside C from Digitalis ferruginea [22]. Cardenolides, a class of cardiac glycosides also exhibited cytotoxicity to many cancer cells [44, 45]. Consequently, based on the findings in HPLC-EIS-MS, it may be postulated that the cardiac glycoside composition in the ethyl acetate fraction of the C. gigantea stem bark extract is one of the key components that confer anticancer properties to the ethyl acetate fraction when combined with sorafenib in HepG2 cells.

In addition to cardiac glycoside as the major component of the ethyl acetate fraction, other low-level components identified in C. gigantea, and other plant extracts have been demonstrated to exhibit anticancer efficacy. Coroglaucigenin from C. gigantea stem and leaf extracts exhibited cancer cytotoxicity concomitant with an increase in intracellular ROS in A549 human lung cancer cells [46]. Calotroposid A, isolated from the ethyl acetate fraction of C. gigantea roots, triggered the extrinsic apoptosis in WiDr colon cancer cells [47]. A cardenolide calotropin from C. gigantea downregulated the proliferation of human colorectal cancer cells [27]. Bacaba, a phenolic from the extract of Oenocarpus bacaba, promoted apoptosis in breast cancer cells [48]. Pectolinarigenin, a flavonoid isolated from Cirsium japonioum, Eupatorium odoratum, and Trollius chinensis, caused ROS generation-mediated apoptosis in A375 non-pigmented human melanoma cells [19]. Raddeanin A, a triterpenoid, enhanced ROS production and promoted apoptotic cells in non-small cell lung cancer [21].

According to the findings of this study, HepG2 cell exposed to a combination of 400 μg/mL CGEtOAc and 4 μM sorafenib (half of their respective IC50) for 24 h demonstrated increased mitochondrial-association with apoptosis. It was discovered that the ROS generation was predominantly responsible for the apoptosis-inducing effects of combined therapy, however, ROS is not the only mechanism participating in the apoptotic action; other mechanisms may work in conjunction to generate this treatment efficiency. Inhibition of PI3K/Akt/mTOR expression also supports the hypothesis that the apoptotic mechanism of CGEtOAc, when combined with sorafenib, may be associated with the downregulation of the upstream PI3K/AKT/mTOR signaling cascade pathway. However, the experiments presented did not provide sufficient evidence of this inhibition in direct association with the apoptotic effects following the treatment. Furthermore, consistent with our findings, there have been reports on the effect of C. procera extracts and cardiac glycosides in the inhibition of the expression of the PI3K/AKT/mTOR pathway in correlation to induce apoptosis cancer cells [22, 4953].

Several studies have established that the apoptotic effect of C. gigantea and other plant extracts is primarily related to an upregulation in ROS generation and oxidative damage to cancer cells, for example, a whole plant ethanolic extract treatment in non-small cell lung cancer [15] and the dichloromethane extract fraction of the C. gigantea stem bark in HCT116 cells [26]. The apoptotic effect, which primarily involved ROS formation, was also demonstrated in many other extract treatments, for example, in pectolinarigenin (flavonoid) from Cirsium japonioum, Eupatorium odoratum, and Trollius chinesis used against A375 non-pigmented human melanoma cells [19]; a natural flavonoid, apigenin from Clerodendrum viscosum leaves used against human breast adenocarcinoma cells [54]; isoorientin from several plants, such as, Phyllostachys pubescens Patrinia, and Drosophyllum lusitanicum used against HepG2 cells [55]; piperlongumine, an alkaloid from Piper longum L. used against MG63 human osteosarcoma cells [24]; berberis, an alkaloid from Berberis hispanican root bark used against human laryngeal epidermoid carcinoma Hep-2 [56]; ginsenoside Rk1 used against MCF-7 human breast cancer cell [37]; delicaflavone, a biflavonoid from Selaginella doederleinii used against HCT116 and HT29 colorectal cancer cells [57]; 3-deoxysappanchalcone from Caesalpinia sappan L. (Leguminosae) used against human esophageal cancer cells KYSE30 and KYSE410 [20]; and raddeanin a, a triterpenoid from Anemone raddeana Regel used against non-small cell lung cancer A549 and H1299 cells [21].

Enhanced ROS generation promoted apoptosis by inducing MMP loss in nonpigmented human melanoma cells as a consequence of pectolinarigenin treatment [19]. Similarly, ROS production by 3-deoxysappanchalcone treatment caused MMP loss that resulted in apoptosis in human esophageal cancer cells [20]. In addition, apoptosis mediated by ROS production was found to be involved in activating p53 expression in human breast cancer cells treated with apigenin from Clerodendrum viscosum leaves [54] and Berberis hispanica alkaloid extracts [56]. Furthermore, ROS levels enhancing apoptosis in non-small cell lung cancer A549 and H1299 cells following treatment with raddeanin A were also reported to increase the phosphorylation of the STAT3 pathway [21]. Triterpenoid pristimerin from Celastraceae and Hippocrateaceae [58], as well as Musca domestica pupae [59], upregulated apoptosis via ROS-induced endoplasmic reticulum (ER) stress in CAL-27 and SCC-15 oral squamous carcinoma and human liver cancer HepG2cells, respectively. Additionally, triterpenoids from Euphorbia macrostegia impaired mitochondrial respiration, leading to increased ROS accumulation, ultimately contributing to ER stress and apoptosis in cancer cells [60].

Increased ROS production has been reported as a regulator of PI3K/Akt/mTOR expression, which results in cancer cell apoptosis, examples include kaemperol, a flavonoid found in Chinese herbs belonging to Zingiberaceae [23], piperlongumine isolated from Piper longum L. [24], isoorientin treatment [55], and ginsenoside Rk1 [37]. The PI3K/Akt/mTOR (including mTOR complex 1, mTORC1, and mTORC2) signaling pathway regulates apoptosis in many human diseases, including malignancies and proliferative disorders. Phosphatidylinositol 3,4,5-triphosphate is a significant regulator of Akt following the activation of PI3K-activated receptor tyrosine kinases in the treatment of growth stimuli [61]. Activated Akt results in the regulation of downstream substrates to control the cell cycle, growth, proliferation, and energy metabolism, whereas mTOR targets protein kinases that enhance ribosome production and protein synthesis [62]. As a result, inhibiting the PI3K/Akt/mTOR pathway is considered a potentially attractive treatment for cancer cells [61] as reported when the following compounds were isolated: tanshinone I (from Salvia miltiorrhiza) [63], lanatoside C [22], isoorientin [55], mahanine [18], piperlongumine [24], and apigetrin [64].

Notably, several studies have reported the apoptotic effect of cardiac glycosides, with this compound identified as one of the major components in C. gigantea extracts that exhibit anticancer activity. This effect is achieved through the mechanism of inhibiting Na+/K+ ATPase, leading to an increase in intracellular Ca2+ via the Na+/Ca2+ exchanger [65, 66]. Cardenolides from C. gigantea exhibited potent anticancer effects against breast cancers by inhibiting Na+/K+ ATPase, resulting in an increase in intracellular Ca2+ via the Na+/Ca2+ exchanger-dependent manner, ultimately leading to apoptosis [67]. Cardenolide UNBS1450, having lower affinity for the α-2 Na+/K+ ATPase than other cardenolides, induced downregulation of myeloid cell leukemia-1, a member of the anti-apoptotic protein family downstream of Na+/K+ ATPase activity, contributing to apoptosis in cancer cells [68]. Various signaling pathways downstream of Na+/K+ ATPase inhibition has been demonstrated to contribute to the inhibition of cancer cell proliferation. Oleandrin, a cardiac glycoside extracted from the leaves of Neriaum oleander, induced accumulation of Ca2+, leading to ER stress-mediated cytotoxic effects, enhancing immunogenic cell death in both in vitro and in vivo cancer models [42]. Ascleposide, a natural cardenolide isolated from Reevesia formosana, downregulated the oncoprotein c‐Myc protein and inhibited the phosphorylation of tumor suppressor protein Rb, resulting in the inhibition of cancer cell cycle progression and blocking cell proliferation. This effect was attributed to an increase of α‐tubulin acetylation, which contributed to the inhibition of Na+/K+ ATPase activity by enhancing the endocytosis of this protein in cancer cells through activation of the mitogen‐activated protein kinases (MAPKs) pathway [69].

Additionally, toxicarioside G, a cardenolide isolated from C. gigantea, exhibited another mechanism of anticancer activity by inhibiting cancer cell viability and proliferation. It enhanced Yes1 associated transcriptional regulator (YAP) dephosphorylation and nuclear localization, along with downstream target gene expression [70]. Calotropin, a cardiac glycoside derived from C. gigantea, demonstrates an inhibitory effect on the regulation of metabolic reprogramming in cancer, specifically targeting aerobic glycolysis (the Warburg effect). This leads to activation of cell cycle arrest and inhibition metastasis in cancer cells, ultimately contributing to activation of apoptosis [71]. Another cardiac glycoside, 3′-epi-12β-hydroxyfroside, isolated from the roots of C. gigantea, induced autophagy, leading to enhanced apoptosis. This effect was mediated by downregulation of the heat shock protein 90 (Hsp90)-regulated Akt/mTOR pathway in lung cancer cells [72]. Lanatoside C, a cardiac glycoside, has been shown to induce apoptosis in cancer cells by suppressing the Janus kinase (JAK)/signal transducer and activator of transcription (STAT)/cytokine signaling (SOCS) (JAK2/STAT6/SOCS2) pathway [73].

Several cellular pathways have been demonstrated in the anticancer-apoptotic activity of sorafenib. Sorafenib triggers apoptosis by increasing intracellular ROS levels in Bel7402 hepatocellular carcinoma [5] and NCI-H292 human non-small lung cancer cells [8]. Sorafenib suppresses cell invasion and cell proliferation in hepatocellular carcinoma HepG2 and Huh-7 cells by up-regulating p53 and Forkhead box M1 transcription factor expressions, resulting in the inhibition of matrix metalloproteinase 2 and Ki-67 expression [74]. Sorafenib inhibits cyclin D1 expression in NB4 acute promyelocytic leukemia cells [75], SW982, and HS-SY-II synovial sarcoma cells [76]. Sorafenib activates apoptotic cells in human cancer by reducing the levels of the p-Akt, p-mTOR, and p-ERK pathways [57, 76, 77]. Furthermore, sorafenib treatment upregulates the expression of phosphorylation-Jun N-terminal kinase and -JUN in human hepatocellular carcinoma Huh-7 cells [78], downregulates the expression of phosphorylation-STAT3 in human oral cancer MC-3 cells [79], and downregulates the expression of the ERK and MEK pathways in HepG2 and PLC/PRF5 cells [80]. Additionally, sorafenib reduces intracellular ATP levels, which may influence the downregulation of the expression of Na+/K+ ATPase, as has been reported following ouabain administration in cancer cells [81].

Combining sorafenib with anticancer agents promotes cancer cell apoptosis. Artesunate combined with sorafenib had a synergistic effect on HCC cell proliferation by activating ERK and STAT3 signaling pathways [82]. Sorafenib combined with the triterpenoid cucurbitacin from Cucurbitaceae enhanced apoptosis in HepG2 and Huh7 human hepatocellular carcinoma cells by inhibiting STAT3 activity [83]. Sorafenib combined with betulinic acid caused endoplasmic reticulum stress-associated apoptosis in non-small cell lung cancer A549, H358, and A427 cells [84]. Apigenin (4’,5,7-trihydroxyflavone) in combination with sorafenib caused extrinsic apoptosis in hepatocellular carcinoma cells [85]. Berberine combined with sorafenib promoted apoptosis in human hepatocellular carcinoma cells [86]. The combination of bufalin and sorafenib enhanced ROS production and MMP-dependent apoptotic response in non-small cell lung cancer NCI-H292 cells [8]. Furthermore, osthole (a coumarin), sorafenib, and the combination of both compounds suppressed PI3K and Raf kinases, resulting in cytotoxic effects in cancer cells [87]. Additionally, sorafenib in combination with capsaicin downregulated the expression levels of Akt, mTOR, and p70S6K in LM3 human hepatocellular carcinoma cells, thereby inducing apoptosis [7].

In addition to the possible mechanism proposed in this study for CGEtOAc, combined with sorafenib-induced apoptosis in cancer cells, which involves increased ROS accumulation and inhibition of PI3K/Akt/mTOR expression, the apoptotic effects resulting from the combination treatment of compounds found in C. gigantea extracts and anticancer agents may be attributed to several mechanisms. For instance, the synergistic effect of sorafenib in combination with the Na+/K+ ATPase inhibitor berbamine, an alkaloid isolated from Berberis amurensis, and cardiac glycosides like ouabain, is proposed to be contributed by the potentiation of epidermal growth factor receptor (EGFR)-mediated ERK1/2 and p38MAPK activation by berbamine [88]. Similarly, the combination of digitoxin and sorafenib has been reported to suppress p-ERK, hypoxia inducible factor (HIF)-1α, HIF-2α, and VEGF expression, contributing to apoptosis in cancer cells [89]. Furthermore, the combination of cardenolide derivative, AMANTADIG (3β-[2-(1-amantadine)-1-on-ethylamine]-digitoxigenin) and docetaxel induced apoptosis through the inhibition of surviving protein expression in human androgen-insensitive prostate cancer cells [90].

Thus, the anticancer-apoptotic activity of CGEtOAc and sorafenib was enhanced when administered in combination with HepG2 cells. A reduced dose of both the extract and sorafenib may be beneficial for future alternative cancer treatments, resulting in fewer adverse effects and a positive outcome. However, the limitation of this study is that it did not evaluate the direct association of the apoptosis-mitochondrial dependent pathway or the regulation of the PI3K/Akt/mTOR pathway in apoptosis following CGEtOAc and sorafenib treatment in cancer cells. Additional research is required to verify the selective mechanism of the cancer therapeutic efficacy of this combination therapy.

Conclusions

In this study, a combination of CGEtOAc and sorafenib at a dosage half their respective IC50s activated MMP-dependent apoptosis in association with the production of ROS as the major regulator in HepG2 cells. This ROS production was suggested to be related to PI3K/Akt/mTOR pathway suppression. Thus, our findings provide valuable fundamental data for future research on the development of anticancer regimens derived from the C. gigantea stem bark extract, the ethyl acetate fraction, and the reduction of the chemotherapy dosage used to treat cancer patients with potentially less severe side effects.

Supporting information

S1 Fig. The high-pressure liquid chromatography-electrospray ionisation-mass spectroscopic (HPLC-ESI-MS) chromatogram of C. gigantea stem bark extracts (CGEtOAc) in negative mode.

(PDF)

pone.0300051.s001.pdf (208.2KB, pdf)
S2 Fig. The IC50 curves for (A) CGEtOAc and (B) sorafenib in HepG2 cells, and (C) CGEtOAc in IMR-90 cells for 24 h of incubation.

(PDF)

pone.0300051.s002.pdf (276.2KB, pdf)
S3 Fig. The combination index (CI) vs. fraction affected (Fa) graph for CGEtOAc in combination with sorafenib after 24 h of incubation.

(PDF)

pone.0300051.s003.pdf (99.8KB, pdf)
S4 Fig. The migration rate of HepG2 cells treated with CGEtOAc at 400 μg/mL and sorafenib at 4 μM, both singly and in combination, was evaluated using a wound healing assay after 0–72 h of incubation and compared to the vehicle group.

Cells treated with 0.8% DMSO represented the vehicle control. The significant differences in data, presented as the mean ± SD from at least three different experiments, were investigated with a one-way ANOVA using Tukey’s HSD test: a; p < 0.05 compared to 24 h of incubation in the vehicle group, b; p < 0.05 compared to 48 h of incubation in the vehicle group, and c; p < 0.05 compared to 72 h of incubation in the vehicle group.

(PDF)

pone.0300051.s004.pdf (282.5KB, pdf)
S1 Raw images. Raw images displaying the gating strategies used in flow cytometry of annexin V and propidium iodide (PI) staining in HepG2 cells after 24 h of incubation, with a combination of 400 μg/mL CGEtOAc and 4 μM sorafenib.

(PDF)

pone.0300051.s005.pdf (2.1MB, pdf)
S2 Raw images. Raw images of the original uncropped and unadjusted western blot images for HepG2 cells treated with a combination of 400 μg/mL CGEtOAc and 4 μM sorafenib for a 24-h incubation period.

(PDF)

pone.0300051.s006.pdf (212.4KB, pdf)

Data Availability

We have used the public repository "Zenodo" to deposit the Supporting information, S1-S6, (DOI: 10.5281/zenodo.10799946).

Funding Statement

SP and PS received grant supported from National Science Research and Innovation Fund (NSRF) of Thailand [Grant NO. R2564B007]. AW received grant supported from National Science Research and Innovation Fund (NSRF) of Thailand [Grant NO. R2564B033]. SP received grant supported from Agricultural Research Development Agency (Public Organization) [Grant NO. CRP6505030030]. PC received grant supported from Center of Excellence for Innovation in Chemistry (PERCH-CIC) [Grant NO. NUPMEM02/63] and Faculty of Medical Science, Naresuan University, Phitsanulok, Thailand [Grant NO. 63064382]. SP, PS and DP received partial support from the Global and Frontier Research University Fund, Naresuan University [Grant NO. R2567C003]. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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Decision Letter 0

Nafees Ahemad

12 Sep 2023

PONE-D-23-18216

Enhanced apoptosis of sorafenib in HepG2 cells by combining with the ethyl acetate fraction from the Calotropis gigantea stem bark extract

PLOS ONE

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Reviewer #1: Yes

Reviewer #2: Partly

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Reviewer #1: I Don't Know

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #2: No

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Reviewer #1: In this study, authors examined the effect of combination treatment of Sorafenib and CGEtOAc on HepG2 cells, aiming that combination treatment maximizes the effect of killing cancer cells and minimizes side-effects.

Here are several points need to be clarified:

[1] In Fig.1D, * (asterisk) applies to all bars starting 200 to 2,000µg/ml; therefore it appears to be statistically significant. However, in the text, 1,000~2,000 are not significant. Which one is accurate?

[2] Fig.1E: Does CGE treatment more toxic in normal fibroblasts than Sorafenib?

[3] The intensity of Hoechst 33342 staining seems to be low in vehicle control and NAC-treated samples (Fig.3A). Why?

[4] Discussion is very long (7 pages). It would be nice to state briefly.

Reviewer #2: This manuscript by Chaisupasakul et al. investigates the potential of the ethyl acetate fraction of the stem bark extract of Calotropis gigantea for use in combination therapy with sorafenib against HepG2 cells. They showed this by testing the cytotoxicity caused by each agent singly, and in combination. They have also shown that the combination of the extract and sorafenib induces early apoptosis. Possible related mechanisms have been explored such as ROS production and PI3K/Akt/mTOR pathway inhibition. This is a good starting point; however, the study presented still lacks some key experiments that would solidify the conclusions proposed. Also, further compound purification should be explored to support the combinatorial experiments. Major rewriting of the paper is also recommended so that important points can be clear to the reader as the statements in its current form have grammatical errors ad are confusing. Hence, the manuscript in its current form is not recommended for publication yet. Further elaboration of the review, as well as suggestions to improve the manuscript is written below.

1) Change short title to C. gigantea stem bark ethyl acetate fraction combined “with” sorafenib induces apoptosis in HepG2 cells.

2) Shorten keyword “The ethyl acetate fraction of Calotropis gigantea stem bark extract”.

3) The statement “The leaf and root ethanolic extract of C. gigantea enhanced cytotoxicity in T47D breast cancer cells” should be replaced with “The leaf and root ethanolic extract of C. gigantea exhibited cytotoxicity in T47D breast cancer cells”.

4) Change “curative properties on cancer” or “cancer curative” to “anti-cancer properties”.

5) Room temperature stated in the methods is not standard room temperature.

6) Define EA.

7) There are a lot of missing statements for reagent sources or suppliers and concentrations used for the experiments in the methodology section. Please take a closer look and fix.

8) “under a humidified 5% CO2 at 37°C incubator” should be “humidified incubator at 5% CO2, 37°C”.

9) Include seeding densities for all cell lines used for all of the experiments.

10) State what reservoir the cells were cultured for testing the test compounds and extracts.

11) Was there no solubilization step of the resulting formazan pellet once MTT is added?

12) Specify how the wounds were made for the cell migration assay.

13) Indicate model of flow cytometer used.

14) Indicate concentrations of the stains and antibodies used.

15) Complete statement header for qRT-PCR. What are you running PCR with?

16) Show the list of mRNA templates in Supplementary Information.

17) Indicate concentration of primers used.

18) Table 1 title should be a complete statement.

19) The note in Table 1 can be removed since it should be stated in the methods section.

20) Indicate incubation time, antibody concentration, and what buffer was used for Western blots.

21) There should at least be an MS profile, HPLC trace, or proof of further purification efforts in a Figure or Supplementary Information since the components of CGEtOAc may contain known compounds that have already been reported to synergize with sorafenib.

22) The curve and statistical model used to compute for the IC50 should be shown in a Figure, as well as methods.

23) The IC50 of the extracts seemed a bit high for exhibiting cytotoxic effects. This may still show potential but is mostly not considered as a candidate in the long run. This may be solved if the causative compound is isolated.

24) Figure 1 C would be better visualized if it were presented as a heat map to show a checkerboard analysis.

25) The fractional inhibitory concentration should be computed to determine if the combinatorial effect is either synergistic, potentiation, or just an additive effect.

26) According to Figure 1 E, CGEtOAc is still cytotoxic in normal cells tested. Hence, the IC50 of CGEtOAc should be computed and a therapeutic index determined.

27) Migration studies should be placed as a separate figure.

28) The results shown in Figure 1E and 1G did not show that the gap in the vehicle and CGEtOAc-treated cells were different and hence conclusion from this is not supported. The gap made from the wound healing assay for vehicle alone is still comparable with the baseline, hence, this assay should still be optimized before using any test compounds and extracts.

29) For all flow cytometry experiments, gating paradigms prior to the final image should be shown in the Supplementary Information.

30) Show how many events were collected for all flow cytometry experiments.

31) The statement “the combination of CGEtOAc and sorafenib promoted apoptosis in HepG2 cells depending on MMP damage” is not fully supported by the results presented. All the results have shown is that the combination of CGEtOAc and sorafenib causes early apoptosis and MMP disruption but it doesn’t necessarily mean that they are directly related to each other or the events are sequential. A temporal aspect to the experiment could be introduced to determine which events come first.

32) JC-1 and Hoechst 33342 imaging should be a separate Figure.

33) The header “The combination of CGEtOAc and sorafenib triggered HepG2 apoptosis through increasing cellular ROS levels” should be changed as this experiment only proves that early apoptosis caused by CGEtOAc and sorafenib combination is reduced when cellular ROS is blocked but it does not fully support that increased ROS happens before early apoptosis.

34) In Figure 3B, despite being statistically significant, the ROS levels are already high even if it is just in the single treatment of sorafenib and CGEtOAc, hence, the combination of the two triggering an increase of ROS levels may not be necessarily true and this is just an additive effect rather than synergy.

35) Figure legends stating that these are histograms should be changed as all data shown are in bar graphs.

36) The statement “Additionally, when ROS generation was inhibited, the apoptotic effect of CGEtOAc, sorafenib, and their combination was considerably reduced to baseline levels…” should be changed as they are reduced but not to baseline levels.

37) Indicate what $ mean in Figure 4B.

38) Indicate concentration of NAC used.

39) There is no direct evidence that the downregulation of PI3K/Akt/mTOR proteins and mRNA causes early apoptosis, hence, the statement that CGEtOAc and sorafenib combination causes early apoptosis thru PI3K/Akt/mTOR inhibition should be reconstructed and conclusions be changed.

40) Explain clearly why would the cytotoxicity of each fraction from C. gigantea ST is not entirely influenced by a single component present in each fraction.

41) If it is hypothesized that a cardiac glycoside would be a key component in the EA fraction of C. gigantea ST extract, this should be isolated and further tested in the battery of assays.

**********

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Reviewer #1: No

Reviewer #2: No

**********

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PLoS One. 2024 Mar 25;19(3):e0300051. doi: 10.1371/journal.pone.0300051.r002

Author response to Decision Letter 0


27 Oct 2023

Journal Requirements:

Please state what role the funders took in the study. If the funders had no role, please state: "The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript."

If this statement is not correct you must amend it as needed.

Please include this amended Role of Funder statement in your cover letter; we will change the online submission form on your behalf.

Response:

Thank you for these suggestions.

We stated, "Role of Funder statement:

“The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript."

This statement was also included in the cover letter.

3. We note that you have stated that you will provide repository information for your data at acceptance. Should your manuscript be accepted for publication, we will hold it until you provide the relevant accession numbers or DOIs necessary to access your data. If you wish to make changes to your Data Availability statement, please describe these changes in your cover letter and we will update your Data Availability statement to reflect the information you provide.

Response:

Thank you for these valuable suggestions.

We stated the Data Availability statement as follows:

“Data availability will be provided, including repository information for the data, when the paper is accepted for publication, along with the relevant accession numbers or DOIs necessary for access.”

This statement was also included in the cover letter.

4. PLOS ONE now requires that authors provide the original uncropped and unadjusted images underlying all blot or gel results reported in a submission’s figures or Supporting Information files. This policy and the journal’s other requirements for blot/gel reporting and figure preparation are described in detail at https://journals.plos.org/plosone/s/figures#loc-blot-and-gel-reporting-requirements and https://journals.plos.org/plosone/s/figures#loc-preparing-figures-from-image-files. When you submit your revised manuscript, please ensure that your figures adhere fully to these guidelines and provide the original underlying images for all blot or gel data reported in your submission. See the following link for instructions on providing the original image data: https://journals.plos.org/plosone/s/figures#loc-original-images-for-blots-and-gels.

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Response:

Thank you for these suggestions.

In the revised manuscript submission, we included the original, uncropped, and unadjusted images that underlie all protein blot and other raw image results reported in the Supporting Information files. The revised manuscript has 6 figure files in the Supporting Information, as addressed in the following. This ensures that these figures fully adhere to the journal's policy and requirements for blot reporting. We also mentioned in the cover letter that we included the original, uncropped, and unadjusted images for all blot and raw image results, including 6 file images in the Supporting Information files.

Supporting information

S1 Fig: The high-pressure liquid chromatography-electrospray ionisation-mass spectroscopic (HPLC-ESI-MS) chromatogram of CGEtOAc in negative mode.

S2 Fig: The IC50 curve of (A) CGEtOAc and (B) sorafenib in HepG2 cells, and (C) CGEtOAc in IMR-90 cells after 24 h of incubation.

S3 Fig: The combination index (CI) vs. fraction affected (Fa) graph of CGEtOAc in combination with sorafenib after 24 h of incubation.

S4 Fig: The migration rate of CGEtOAc at 400 µg/mL in combination with 4 µM sorafenib in HepG2 cells after 24 h of incubation.

S5 Raw images displaying the gating strategies used in flow cytometry of annexin V and propidium iodide (PI) staining in HepG2 cells after 24 h of incubation with a combination of 400 µg/mL CGEtOAc and 4 µM sorafenib.

S6 Raw images of the original uncropped and unadjusted western blot images for HepG2 cells treated with a combination of 400 µg/mL CGEtOAc and 4 µM sorafenib for a 24-h incubation period.

5. Please include your full ethics statement in the ‘Methods’ section of your manuscript file. In your statement, please include the full name of the IRB or ethics committee who approved or waived your study, as well as whether or not you obtained informed written or verbal consent. If consent was waived for your study, please include this information in your statement as well.

Response:

Thank you for these valuable suggestions.

In the Methods section of the revised manuscript, we stated and provided the full name of the ethics committee that approved the human cell culture research as follows:

“The human cell culture research was approved by the Naresuan University Institutional Review Board (NU-IRB) in Panel 1: Health Sciences, with the approval number P1-0166/2565."

Reviewers' comments:

Dear Reviewers,

Thank you very much for your valuable suggestions and comments on our manuscript. These comments have been immensely helpful in improving and revising our paper. We have carefully studied your comments and made corrections in line with your suggestions. The revised portions are marked in blue in the paper. The main corrections in the paper and our responses to the reviewers’ comments and remarks are as follows:

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

Response:

We appreciate the positive comments from the reviewers.

________________________________________

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: I Don't Know

Reviewer #2: Yes

Response:

We are grateful for the favorable remarks provided by the reviewers.

________________________________________

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

Response:

We are grateful for the favorable remarks provided by the reviewers.

________________________________________

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: No

Response:

We express our gratitude for the positive comments and valuable suggestions provided by the reviewers.

________________________________________

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1:

In this study, authors examined the effect of combination treatment of Sorafenib and CGEtOAc on HepG2 cells, aiming that combination treatment maximizes the effect of killing cancer cells and minimizes side-effects.

Here are several points need to be clarified:

Response:

We appreciate the reviewer's comprehensive and insightful analysis of our paper, as well as constructive and necessary suggestions.

This manuscript represents an ongoing project aimed at conducting purification experiments and investigating potential biomarker compounds in the promising extract from C. gigantea stem barks. We propose the potential of the EtOAc fraction of C. gigantea stem barks and publish the possibility of using natural products in combination with the chemotherapeutic agent sorafenib for further research into future anticancer agents. It requires additional experimental phases and substantial support to achieve our objective. We hope that our manuscript will meet the journal's standards and provide valuable knowledge worldwide. We sincerely regret any grammatical errors and typos; throughout the manuscript, they have been carefully corrected.

The revised manuscript has been thoroughly rechecked, the English has been meticulously reviewed, and it now meets the required standards for publication with the support of Editage's editorial services (www.editage.com). An editing certification is also attached.

[1] In Fig.1D, * (asterisk) applies to all bars starting 200 to 2,000µg/ml; therefore it appears to be statistically significant. However, in the text, 1,000~2,000 are not significant. Which one is accurate?

Response:

Thank you for your question, which provides valuable feedback and prompts us to provide a clearer explanation of this issue in the revised manuscript.

Figure 1D demonstrates that the toxicity of CGEtOAc to IMR-90 cells at concentrations ranging from 200 to 2,000 µg/mL is significant compared to the vehicle. However, these concentrations are less cytotoxic to IMR-90 cells than what was observed in HepG2 cells. Notably, CGEtOAc at 1,000, 1,500, and 2,000 µg/mL did not show a significant difference from the effect at 800 µg/mL. This finding suggests that from 200 µg/mL to the highest dose of CGEtOAc at 2,000 µg/mL, it exhibits lower cytotoxicity to normal IMR-90 cells while causing a dose-dependent increase in cytotoxicity to HepG2 cells.

[2] Fig.1E: Does CGE treatment more toxic in normal fibroblasts than Sorafenib?

Response:

Thank you for your question.

For a 24-hour incubation period, the IC50 of CGEtOAc on IMR-90 cells was greater than 5,000 µg/mL. We did not evaluate the IC50 of sorafenib on IMR-90 cells. In contrast to IMR-90 cells, the IC50 of CGEtOAc and sorafenib on HepG2 cells were 771.00 ± 69.74 µg/mL and 8.64 ± 0.32 µM, respectively, as shown in Figure 1. We then used half-IC50 doses of CGEtOAc at 400 µg/mL and sorafenib at 4 µM, as well as a combination of both to treat IMR-90 cells.

As observed, the toxicity of CGEtOAc at 200, 400, and 800 µg/mL to IMR-90 cells was significant compared to the vehicle, although it was less pronounced than that observed in HepG2 cells, as shown in Figure 1E of the revised manuscript. Additionally, the combination of CGEtOAc at 400 µg/mL and sorafenib at 4 µM had a significant cytotoxic effect on IMR-90 cells compared to the vehicle control, but it was less pronounced than the effect in HepG2 cells (as seen in Figure 1F of the revised manuscript). This combination also caused more cytotoxicity in HepG2 cells, resulting in approximately 60% inhibition of cell viability, as shown in Figure 1C of the revised manuscript.

In summary, when using the same doses of CGEtOAc and CGEtOAc in combination with sorafenib, it exhibited less cytotoxicity to IMR-90 cells, while causing more cytotoxicity to cancer cells.

[3] The intensity of Hoechst 33342 staining seems to be low in vehicle control and NAC-treated samples (Fig.3A). Why? CGEtOAc 400 µg/mL and sorafenib 4 µM

Response:

We are grateful for the favorable remarks provided by the reviewers.

Hoechst 33342 is used to assess cell death by apoptosis, characterized by DNA fragmentation or nuclear condensation. The vehicle or the vehicle with NAC treatment showed fewer apoptotic specific hallmarks, suggesting reduced ROS-induced apoptosis, as indicated in this manuscript. Consequently, the reduced staining of cells with Hoechst 33342 resulted in fewer apoptotic cells.

Example references with their DOIs:

https://doi.org/10.2147/DDDT.S189969

https://doi.org/10.1111/jmi.12133

https://doi.org/10.1002/jbt.22616

https://doi.org/10.1093/jpp/rgac037

[4] Discussion is very long (7 pages). It would be nice to state briefly.

Response:

We appreciate the reviewer's insightful comments and suggestions for improving the readability of the manuscript.

The revised manuscript has been edited, and the discussion section has been modified to be more concise.

We are genuinely appreciative of the Editor and Reviewers’ decision to allow us the opportunity to revise this manuscript to fully meet PLOS ONE's publication criteria. We are thankful for the time, effort, and assistance you have provided in enhancing the quality of this work. We sincerely hope that our revisions have elevated the manuscript's quality, and we are confident that the reviewers will find it suitable for publication.

Sincerely yours,

On behalf of all co-authors,

Corresponding authors

Reviewer #2:

This manuscript by Chaisupasakul et al. investigates the potential of the ethyl acetate fraction of the stem bark extract of Calotropis gigantea for use in combination therapy with sorafenib against HepG2 cells. They showed this by testing the cytotoxicity caused by each agent singly, and in combination. They have also shown that the combination of the extract and sorafenib induces early apoptosis. Possible related mechanisms have been explored such as ROS production and PI3K/Akt/mTOR pathway inhibition. This is a good starting point; however, the study presented still lacks some key experiments that would solidify the conclusions proposed. Also, further compound purification should be explored to support the combinatorial experiments. Major rewriting of the paper is also recommended so that important points can be clear to the reader as the statements in its current form have grammatical errors ad are confusing. Hence, the manuscript in its current form is not recommended for publication yet. Further elaboration of the review, as well as suggestions to improve the manuscript is written below.

Response:

We appreciate the reviewer's comprehensive and insightful analysis of our paper, as well as constructive and necessary suggestions.

This manuscript represents an ongoing project aimed at conducting purification experiments and investigating potential biomarker compounds in the promising extract from C. gigantea stem barks. We propose the potential of the EtOAc fraction of C. gigantea stem barks and publish the possibility of using natural products in combination with the chemotherapeutic agent sorafenib for further research into future anticancer agents. It requires additional experimental phases and substantial support to achieve our objective. We hope that our manuscript will meet the journal's standards and provide valuable knowledge worldwide. We sincerely regret any grammatical errors and typos; throughout the manuscript, they have been carefully corrected.

The revised manuscript has been thoroughly rechecked, the English has been meticulously reviewed, and it now meets the required standards for publication with the support of Editage's editorial services (www.editage.com). An editing certification is also attached.

1) Change short title to C. gigantea stem bark ethyl acetate fraction combined “with” sorafenib induces apoptosis in HepG2 cells.

Response:

We appreciate the reviewer's thorough and thoughtful analysis of our paper, as well as the constructive comments.

The title of the revised manuscript has been changed to "Combination of Ethyl acetate fraction from Calotropis gigantea stem bark and sorafenib induces apoptosis in HepG2 cells." We hope the reviewers will agree.

2) Shorten keyword “The ethyl acetate fraction of Calotropis gigantea stem bark extract”.

Response:

Thank you for your suggestions.

The keywords in the revised manuscript include “The ethyl acetate fraction, The Calotropis gigantea stem bark”.

3) The statement “The leaf and root ethanolic extract of C. gigantea enhanced cytotoxicity in T47D breast cancer cells” should be replaced with “The leaf and root ethanolic extract of C. gigantea exhibited cytotoxicity in T47D breast cancer cells”.

Response:

Thank you for your valuable suggestions.

We have edited this statement as suggested.

4) Change “curative properties on cancer” or “cancer curative” to “anti-cancer properties”.

Response:

Thank you for your suggestions.

The revised manuscript has been edited as suggested.

5) Room temperature stated in the methods is not standard room temperature.

Response:

Thank you for your constructive comments.

The revised manuscript has been edited as suggested. The statement is as follows: (Page ...): In brief, the C. gigantea ST was harvested, dried at a temperature of 35 ± 7 ºC, and ground into a powder.

6) Define EA.

Response:

Thank you for your comments. Please accept our apologies for this misspelling.

The ethyl acetate fraction of the Calotropis gigantea stem bark extract has been abbreviated as “CGEtOAc” throughout the revised manuscript.

7) There are a lot of missing statements for reagent sources or suppliers and concentrations used for the experiments in the methodology section. Please take a closer look and fix.

Response:

Thank you for your comments. We apologize for this error.

The revised manuscript has been edited thoroughly.

8) “under a humidified 5% CO2 at 37°C incubator” should be “humidified incubator at 5% CO2, 37°C”.

Response:

Thank you for your constructive suggestions.

The revised manuscript has been thoroughly edited as suggested.

9) Include seeding densities for all cell lines used for all of the experiments.

Response:

Thank you for your constructive suggestions.

The revised manuscript has been thoroughly edited as suggested.

10) State what reservoir the cells were cultured for testing the test compounds and extracts.

Response:

Thank you for your valuable suggestions.

The revised manuscript has been thoroughly edited, and we have addressed this issue as suggested in the Cell Culture Method section.

"Cells were cultured in T-25 culture flasks in complete media and sub-cultured after reaching 80-90% confluency every 4 days of the incubation cycle. Cell numbers were monitored during each subculture to measure the consistency of cell growth rates before proceeding with the cell treatment plating procedure. The human cell culture research was approved by the Naresuan University Institutional Review Board (NU-IRB) in Panel 1: Health Sciences, with the approval number P1-0166/2565."

11) Was there no solubilization step of the resulting formazan pellet once MTT is added?

Response:

We very much appreciate the valuable suggestions from the reviewer.

The revised manuscript has been thoroughly edited, and we have addressed this issue as suggested in the MTT assay method.

“After incubating HepG2 cells at a density of 1.5×104 cells per well/150 µL in a 96-well plate for 24 hours and exposing them to the extracts for an additional 24 hours, we applied 3-(4,5-Dimethylthiazol-2-yl)-2,5-Diphenyltetrazolium Bromide (MTT) at a concentration of 2 mg/ml to assess viable cells based on mitochondrial reductase activity. We then measured absorbance at 595 nm using DMSO to dissolve the formazan crystals produced by MTT, with a microplate reader (SpectraMax ABS, Molecular Devices, USA). The IC50 values were calculated using Graph Prism Software version 9.”

12) Specify how the wounds were made for the cell migration assay.

Response:

Thank you for your constructive suggestions.

The revised manuscript has been thoroughly edited, and we have addressed this issue as suggested in the “Analysis of Cell Migration Activity by Wound Healing Method” section.

“After incubating HepG2 cells at a density of 4×10^5 cells per well/1 mL in a 12-well plate and allowing them to reach 80-90% confluency within 48 hours, we induced the formation of physical wounds in the cell monolayer by scratching the attached cells in a continuous straight line at the center of each well using a 200 µL sterile tip. The scratch wounds were consistently of the same size in each well to minimize variations between conditions (https://doi.org/10.3389/fcell.2019.00107 , https://doi.org/10.3791%2F51046 , https://doi.org/10.3389/fcell.2021.640972 , https://doi.org/10.1186/s40360-018-0284-4 ).

We removed debris and non-attached cells before subsequently exposing the cells to extracts, sorafenib, and their combination for 0-72 hours.

The gap width of the wound was measured and recorded in three regions, with triplicate measurements of each region performed to obtain the mean value. This was done using an inverted microscope (IX71, Olympus, Japan) to measure the distance between the closest points on both sides of the wound. The results were analyzed as the percentage of the scratch gap using cellSens Standard [Ver.2.3] software. The scratch widths of the control and treatment wells were measured and normalized according to their respective values at 0 h. The migrated distance was defined by the following formula:

Migrated rate (%) = (the width of the initial wound - the width of the remaining wound) / the width of the initial wound × 100 (https://doi.org/10.1016/j.phymed.2019.153112 , https://doi.org/10.1186/s12885-022-09684-0).” These results are presented in the Supporting Information (S4 Fig).

13) Indicate model of flow cytometer used.

Response:

Thank you for your suggestions.

The revised manuscript has been thoroughly edited, and we have incorporated this issue as suggested in the “Determination of Apoptosis by Flow Cytometric Analysis” method.

14) Indicate concentrations of the stains and antibodies used.

Response:

Thank you for your suggestions.

The revised manuscript has been thoroughly edited, and we have addressed this issue as suggested.

15) Complete statement header for qRT-PCR. What are you running PCR with?

Response:

Thank you for your suggestions.

The revised manuscript has been thoroughly edited, and we have updated the header to read: “Real-time quantitative reverse transcription polymerase chain reaction (RT-qPCR) for PI3K/Akt/mTOR gene expression.”

16) Show the list of mRNA templates in Supplementary Information.

Response:

Thank you for your suggestions.

We conducted a real-time RT-PCR assay, consisting of three essential steps:

(i) cellular RNA extraction for the conversion of RNA into cDNA templates by reverse transcriptase,

(ii) the amplification of the cDNA, and

(iii) the real-time detection and quantification of amplification products.

Specific gene primers for PI3K, Akt, and mTOR are detailed in the table provided in the methods section.

The groups of cells studied for gene expression were treated with CGEtOAc at 400 µg/mL, 4 µM sorafenib, and a combination of both. RNA was extracted from these groups to convert it into cDNA templates, which were then compared to the vehicle control group.

17) Indicate concentration of primers used.

Response:

Thank you for your suggestions.

The revised manuscript has been thoroughly edited to include the primer concentration (10 pg/µL) in the method section.

18) Table 1 title should be a complete statement.

Response:

Thank you for your suggestions.

The revised manuscript has been thoroughly edited, and the title of Table 1 has been changed to “The Primer Sequences Used in RT-qPCR”.

19) The note in Table 1 can be removed since it should be stated in the methods section.

Response:

Thank you for your feedback.

The revised manuscript has been meticulously edited in accordance with your suggestions.

20) Indicate incubation time, antibody concentration, and what buffer was used for Western blots.

Response:

Thank you for your comments.

The revised manuscript has been thoroughly edited and now includes the suggested issue.

21) There should at least be an MS profile, HPLC trace, or proof of further purification efforts in a Figure or Supplementary Information since the components of CGEtOAc may contain known compounds that have already been reported to synergize with sorafenib.

Response:

We sincerely appreciate your valuable suggestions.

The HPLC chromatogram of CGEtOAc, along with negative mode mass spectra and m/z values of major ions, has been included in the Supporting Information (S1 Fig). However, due to the lack of other standards to confirm the corresponding peaks and ions, we have reported m/z values and possible ions corresponding to compounds found in Calotropis species. These compounds include the formic acid adduct ions [HCOO]- of Calactinic acid methyl ester (a singly-linked cardenolide) at m/z 607.27 and calactin or calotropin (an isomer, a doubly-linked cardenolide) at m/z 577.26.

Synergistic effects of certain cardiac glycosides, such as digoxin and ouabain (singly-linked cardenolides), have been previously reported.

S1 Fig: The high-pressure liquid chromatography-electrospray ionisation-mass spectroscopic (HPLC-ESI-MS) chromatogram of CGEtOAc in negative mode.

The HPLC-ESI-MS chromatogram of CGEtOAc was obtained from an Agilent 6540 UHD LCMS instrument. CGEtOAc was dissolved in methanol (MS grade, 5 mg/mL, 10 μL) then injected into a stationary phase using a Phenomenex Luna® 3 μm C18 column (150 mm × 4.6 mm i). The temperatures of autosampler and column were 4 and 25 °C, respectively. A mobile phase was a gradient solution of 0.1% formic acid-water solution and 0.1% formic acid-acetonitrile solution (5–95% acetonitrile, 20 min, 0.8 mL/min flow rate). The electrospray ionization mass spectrometry in negative mode (m/z range 200–800) setup with a 30 psi nebulizer pressure (N2), a 10 L/min drying gas flow rate, and a 350 °C temperature was performed. The retention time in the base peak chromatogram (BPC) and mass spectra of possible m/z of CGEtOAc were illustrated.

This manuscript is part of an ongoing project focused on purifying compounds and exploring potential biomarkers in the promising extract from C. gigantea stem barks. Additional experimental phases and substantial support are needed to accomplish our objectives. Currently, we have isolated calactinic acid methyl ester and plan to conduct further experiments in the future.

References:

1. Xiao Y, Yan W, Guo L, Meng C, Li B, Neves H, Chen PC, Li L, Huang Y, Kwok HF, Lin Y. Digitoxin synergizes with sorafenib to inhibit hepatocelluar carcinoma cell growth without inhibiting cell migration. Mol Med Rep. 2017 Feb;15(2):941-947. doi: 10.3892/mmr.2016.6096. Epub 2016 Dec 30. PMID: 28035421.

2. Yang, S., Yang, S., Zhang, H., Hua, H., Kong, Q., Wang, J., & Jiang, Y. (2021). Targeting Na+/K+-ATPase by berbamine and ouabain synergizes with sorafenib to inhibit hepatocellular carcinoma. British Journal of Pharmacology, 178(21), 4389–4407. https://doi.org/10.1111/bph.15616]

22) The curve and statistical model used to compute for the IC50 should be shown in a Figure, as well as methods.

Response:

We appreciate your valuable suggestions.

The revised manuscript has been updated to include the Supporting Information, which features the IC50 graph of CGEtOAc and sorafenib in HepG2 cells, as well as the IC50 of CGEtOAc in IMR-90 cells (S2 Fig).

23) The IC50 of the extracts seemed a bit high for exhibiting cytotoxic effects. This may still show potential but is mostly not considered as a candidate in the long run. This may be solved if the causative compound is isolated.

Response:

We thank the reviewer for careful and thorough assessment of our manuscript and for providing detailed, valuable, and constructive comments that have enhanced the quality of our work.

We agree with the reviewer's comment. In the first experiment, we screened CGEtOAc for its potential anticancer activity. After discovering its cytotoxicity, we conducted several purification experiments and obtained some pure compounds. These compounds are currently being enriched and their chemical structures elucidated for further investigation.

The fractions of C. gigantea obtained from solvents with varying polarity properties resulted in each fraction containing different phytochemicals. These C. gigantea fractions exhibited a variety of phytochemicals, such as phenolics, triterpenoids, cardiac glycosides, and flavonoids, each with distinct biological activities due to their chemical structures, physicochemical properties, and affinities to target proteins. Consequently, the biological activities, including cytotoxicity, of each fraction were attributed to a combination of chemicals, as previously mentioned and discussed in our report (Winitchaikul T et al., 2021, Sawong S et al., 2022).

After conducting the study on the cytotoxic effects of the extracts and fractions on cancer cells, it was observed that the IC50 values were higher than those of the isolated compounds. Several factors influence the response in various cancer cell models, including the incubation time, the plant parts from which the extract fractions were obtained, and variations in cell models.

Mutiah R. et al. (2018) reported varying IC50 values for the fractionated ethanol extract of C. gigantea roots: ethyl acetate fraction (IC50 0.063 μg/ml), dichloromethane fraction (IC50 0.367 μg/ml), butanol fraction (IC50 12.18 g/ml), and water fraction (IC50 8,493 μg/ml) in cancer cells (https://doi.org/10.22034/APJCP.2018.19.6.1457 ). Our study is consistent with these findings, as a 24-hour incubation of the ethanol extract from the whole plant of C. gigantea demonstrated less than a 20% cytotoxic effect on cancer cells at 15 μg/ml, as reported by Lee J. et al. (2019) (https://doi.org/10.1186/s12906-019-2561-1 ). The cytotoxic activity of the C. gigantea latex showed IC50 values ranging from 400-150 µg after a 24-hour incubation against A549 cancer cells (https://doi.org/10.1016/j.biopha.2016.12.133 ).

Our previous reports demonstrated that in HepG2, HCT116, and HT-29 cells, the IC50 of the extract fractions from the stem bark of C. gigantea was the lowest in the DCM (dichloromethane) fraction, followed by the EtOAc (ethyl acetate), EtOH (ethanol), and water fractions (https://doi.org/10.1038/s41598-022-16321-0, https://doi.org/10.1371/journal.pone.0254392 ).

It has been reported that compounds isolated from the leaves of C. gigantea exhibit varying cytotoxic effects against cancer cells. These effects are influenced by factors such as the polarity of solvents, chemical structures, and the physicochemical properties of the isolated compounds. The presence of an aromatic methoxy group in pinoresinol results in decreased cytotoxicity, with an IC50 greater than 100 µM. Similarly, the presence of a (6-O-vanilloyl)-β-D-glucopyranosyl group decreases the cytotoxicity of pinoresinol against cancer cells (https://doi.org/10.1016/j.bmcl.2017.04.087 ).

We conducted bio-guided fractionation and purification of major cardiac glycosides or cardenolides to further substantiate our hypothesis. Our ongoing preliminary research and unpublished results suggest the potential of calotropin and calactin, which are major cardenolides found in Calotropis species, to have a combined effect with several commercial chemotherapeutic agents. However, these experiments were based on the key findings of this manuscript. Therefore, we would like to present our novel results to propose the potential use of plant-derived compounds in combination with chemotherapeutic agents for enhanced benefits in cancer therapy.

Because of the high IC50 values observed in cancer treatment when using the extract fraction, combining it with a chemotherapeutic drug presents a promising approach. This strategy aims to concurrently reduce the concentration of the extract and the dosage of the anticancer drug. Such combination has the potential to enhance treatment efficacy while minimizing the side effects associated with high-dosage usage. This advantage becomes particularly evident when compared to using the extract alone, especially in cases where the isolated compound is not employed.

24) Figure 1 C would be better visualized if it were presented as a heat map to show a checkerboard analysis.

Response:

Thank you for your valuable comments.

The revised manuscript now includes the representation of the combination effect in Figure 1C and in the heat map graph (Figure 1D in the revised manuscript) as well.

25) The fractional inhibitory concentration should be computed to determine if the combinatorial effect is either synergistic, potentiation, or just an additive effect.

Response:

Thank you for your comments.

In the revised manuscript, we have addressed the combination index (CI), which assesses the synergistic efficacy of a combination treatment using the Chou-Talalay method. A CI value less than 1 indicates a synergistic mechanism for the combination treatment. We found that the combination of CGEtOAc at 400 µg/mL and sorafenib at 4 mM resulted in a CI of 0.5, with an inhibition rate of approximately 60%, demonstrating a synergistic effect. However, when CGEtOAc was used at 200 µg/mL in combination with sorafenib at 4 mM, the CI was 0.5, but the inhibition rate was 40%.

Additionally, when CGEtOAc using concentrations of 200, 400, 600, and 800 µg/mL was combined with sorafenib at 8 mM (IC50), the CI was consistently less than 1. As a result, we determined that CGEtOAc at 400 µg/mL and sorafenib at 4 µM were the most suitable for the subsequent anticancer experiments. We have also included the Supporting Information in S3 Fig, which illustrates the combination index (CI) vs. fraction affected (Fa) graph of CGEtOAc in combination with sorafenib after 24 hours of incubation.

References related to the CI analysis:

https://doi.org/10.1016/j.synres.2018.04.001

https://doi.org/10.1016/j.biopha.2016.10.096

https://doi.org/10.31557/apjcp.2023.24.5.1495

26) According to Figure 1 E, CGEtOAc is still cytotoxic in normal cells tested. Hence, the IC50 of CGEtOAc should be computed and a therapeutic index determined.

Response:

Thank you for your comments.

In the revised manuscript, we have addressed the IC50 of CGEtOAc on IMR90 cells for a 24-hour period, which was approximately greater than 5,000 µg/mL. This value was notably higher than the effect observed on HepG2 cells, where the IC50 for CGEtOAc was approximately 771.00 ± 69.74 µg/mL, around 6.5 times lower.

The toxicity of CGEtOAc at concentrations of 200, 400, and 800 µg/mL was significantly higher compared to the vehicle control for both IMR-90 and HepG2 cells, though the effect was more pronounced in HepG2 cells. Furthermore, when considering the combination of CGEtOAc at 400 µg/mL and sorafenib at 4 µM, it resulted in a significant cytotoxic effect on IMR-90 cells compared to the vehicle control, although the effect was less pronounced than what was observed in HepG2 cells. Specifically, the combination of CGEtOAc at 400 µg/mL and sorafenib at 4 µM resulted in a 60% cytotoxic effect on HepG2 cells, while it induced a 20% cytotoxic effect on IMR-90 cells. It's important to note that the combination index (CI) of CGEtOAc with sorafenib in IMR-90 cells cannot be calculated

27) Migration studies should be placed as a separate figure.

Response:

Thank you for your suggestions.

The revised manuscript has been edited, and the order of the figures has been rearranged to include seven figures. The migration results were separated into Figure 2A and 2B with some modifications. This was done as a result of repeating the migration experiment to confirm the obvious migration inhibition of the combination of CGEtOAc at 400 µg/mL and sorafenib at 4 µM on HepG2 cells, in comparison to the vehicle. Additionally, we have provided the migration rate (%) in the Supporting Information (S4 Fig.).

28) The results shown in Figure 1E and 1G did not show that the gap in the vehicle and CGEtOAc-treated cells were different and hence conclusion from this is not supported. The gap made from the wound healing assay for vehicle alone is still comparable with the baseline, hence, this assay should still be optimized before using any test compounds and extracts.

Response:

Thank you for your comments.

The revised manuscript has been edited to address the details of the wound healing assay used to evaluate the anti-migration properties of CGEtOAc on HepG2 cancer cells. We repeated the experiments to validate the migration inhibition of the treatment, and the results showed some modifications. The gap or migration distance of the vehicle group was significantly reduced in a time-dependent manner, while the treatment with CGEtOAc, sorafenib, and the combination showed no change over the 0-72 hour period, suggesting an increase in the anti-proliferation and anti-migration activities in HepG2 cells. Additionally, we have provided the migration rate (%) in Supporting Information S4 Fig.

We would like to clarify to the reviewer that we optimized the gap distance at the time of initiating the scratching in all culture wells to minimize variations. To ensure consistency in gap measurements, we followed these steps:

1. HepG2 cells were seeded in a 12-well culture plate to allow cells reaching 80-90% confluency.

2. The scratch must be done gently at the center with the straight line of each well.

3. Experiments were carried out in triplicate wells and repeated at least three times for the collection of validity of mean value.

4. Gap distance was measured at the first time of the scratching to make sure no variation of the gap before treatment started.

5. The gap width of the wound was measured and recorded in three regions, with triplicate measurements of each region performed to obtain the mean value, to measure the distance between the closest points on both sides of the wound

6. The scratch widths of the control and treatment wells were measured and normalized according to their respective values at 0 hour.

7. The gap or migrated distance (%) was defined as well as the migration rate (%).

29) For all flow cytometry experiments, gating paradigms prior to the final image should be shown in the Supplementary Information.

Response:

Thank you for your suggestions.

The revised manuscript now includes the Supporting Information, S5 Raw images, which contains raw images demonstrating the gating strategy for flow cytometry of Annexin V/PI staining for CGEtOAc at 400 µg/mL, sorafenib at 4 µM, and the combination of CGEtOAc at 400 µg/mL and sorafenib at 4 µM. Additionally, we tested sorafenib at 4, 8, and 20 µM as a positive control for apoptosis. The raw images in S5 demonstrate the flow cytometry gating strategies for both scatter and singlets gating, and they also provide apoptotic bar graphs. It's worth noting that the apoptotic rates, obtained from both the scatter and singlets gating methods, showed similar values. Importantly, the apoptotic rates presented in Figure 3 and Figure 6 in the revised manuscript are based on the scatter gating method.

30) Show how many events were collected for all flow cytometry experiments.

Response:

Thank you for your comments.

We have included Supporting Information, S5 Raw images, which demonstrate the gating strategy for flow cytometry and the flow analysis settings.

31) The statement “the combination of CGEtOAc and sorafenib promoted apoptosis in HepG2 cells depending on MMP damage” is not fully supported by the results presented. All the results have shown is that the combination of CGEtOAc and sorafenib causes early apoptosis and MMP disruption but it doesn’t necessarily mean that they are directly related to each other or the events are sequential. A temporal aspect to the experiment could be introduced to determine which events come first.

Response:

Thank you for your comments.

Our manuscript suggests apoptosis activity of the extract and sorafenib treatment in HepG2 cells, which correlated with mitochondrial damage. Our study's limitation is that it does not assess the direct relationship or sequence of apoptotic events that follow mitochondrial damage. Therefore, we stated in the suggestion that “the combination of CGEtOAc and sorafenib promoted apoptosis in HepG2 cells is correlated with MMP damage” However, several studies have been reported that apoptosis pathway mediated by mitochondrion lead to activate of caspases cascade to trigger cell apoptosis. Mitochondria plays a vital role in regulation of apoptosis pathway by trigger the delivery of cytochrome c to activate the initiator caspase, eventually resulting in cell apoptosis.

Example of references:

https://doi.org/10.1016/j.phymed.2022.154528

https://doi.org/10.1016/j.ejphar.2017.12.027

http://dx.doi.org/10.2174/1871520620666200624145217

https://doi.org/10.1002/ptr.7054

http://www.ncbi.nlm.nih.gov/pmc/articles/pmc4502999/

32) JC-1 and Hoechst 33342 imaging should be a separate Figure.

Response:

Thank you for your comments.

The revised manuscript has been edited, and the order of figures has been adjusted to include 7 figures. The JC-1 and Hoechst 33342 imaging have been separated and are now presented in Figures 4 and 5.

33) The header “The combination of CGEtOAc and sorafenib triggered HepG2 apoptosis through increasing cellular ROS levels” should be changed as this experiment only proves that early apoptosis caused by CGEtOAc and sorafenib combination is reduced when cellular ROS is blocked but it does not fully support that increased ROS happens before early apoptosis.

Response:

Your comments are appreciated.

The revised manuscript has been edited, and the heading of the results section has been changed to read: "The combination of CGEtOAc and sorafenib triggered HepG2 apoptosis associated with an increase in cellular ROS levels.”

34) In Figure 3B, despite being statistically significant, the ROS levels are already high even if it is just in the single treatment of sorafenib and CGEtOAc, hence, the combination of the two triggering an increase of ROS levels may not be necessarily true and this is just an additive effect rather than synergy.

Response:

Thank you for your comments.

We acknowledge the reviewer's comment regarding the significant increase in ROS levels with single treatments of CGEtOAc or sorafenib, compared to the 100% vehicle control. Furthermore, the combination of these treatments significantly augmented ROS levels, reaching 193% in comparison to the single treatments (with sorafenib at 175% and CGEtOAc at 175%).

It has been reported that apoptotic cell death can also occur through the initiation of various types of stress-induced damage, with ROS production being a critical stressor. Additionally, cancer cells inherently produce higher levels of ROS than normal cells. Therefore, the environmental stress in cancer cells can dramatically activate ROS generation, posing a significant risk of oxidative-induced apoptosis. Increased intracellular ROS generation is indeed crucial for inducing apoptosis with chemotherapeutic agents in various cancer cell types. ROS levels were significantly increased approximately two-fold following the anticancer treatments. (References are provided in the section below.)

References:

http://dx.doi.org/10.1016/j.biopha.2016.10.096

https://doi.org/10.3892/ijmm.2018.3807

https://doi.org/10.1038/bjc.2013.334

https://doi.org/10.3390/ijms20184407

https://doi.org/10.31557/APJCP.2023.24.5.1495

https://doi.org/10.1016/j.semcdb.2017.05.023

https://doi.org/10.7150/thno.46728

https://doi.org/10.1007/s00253-016-7930-9

Several studies have reported that treatment with extracts or compounds isolated from C. gigantea results in apoptosis due to a significant increase in oxidative ROS generation in cancer cells. For instance, the ethanol extract of the whole plant of C. gigantea induced apoptosis in cancer cells, leading to an approximately threefold increase in ROS levels and a reduction in antioxidant enzyme levels compared to the control group (https://doi.org/10.1186/s12906-019-2561-1 ). ROS generation also significantly increased in A549 cells after treatment with coroglaucigenin, which was isolated from the stems and leaves of C. gigantea (https://doi.org/10.18632/oncotarget.16454 ).

As a result, ROS associated with apoptosis in combination therapy was identified as one of the major mediators of apoptosis. The levels of ROS demonstrated an additive effect rather than a synergistic effect following the combination treatment with CGEtOAc and sorafenib. This issue is discussed in detail in the discussion section.

35) Figure legends stating that these are histograms should be changed as all data shown are in bar graphs.

Response:

Thank you for your suggestions.

The revised manuscript has been edited, and all instances of the term "histogram" have been changed to "bar graphs," as recommended.

36) The statement “Additionally, when ROS generation was inhibited, the apoptotic effect of CGEtOAc, sorafenib, and their combination was considerably reduced to baseline levels…” should be changed as they are reduced but not to baseline levels.

Response:

Thank you for your suggestions.

The revised manuscript has been edited and changed as suggested. The sentence now reads: “When ROS generation was inhibited, the apoptotic effect of CGEtOAc, sorafenib, and their combination was significantly reduced but did not return to baseline levels, in comparison to the vehicle and their single treatment groups.”

37) Indicate what $ mean in Figure 4B.

Response:

Thank you for your suggestions.

The revised manuscript has been edited. Figure 6 in the revised manuscript displays a significant comparison of the combination treatment (in the presence of NAC) with both single CGEtOAc and sorafenib treatments, represented by the symbol $.

38) Indicate concentration of NAC used.

Response:

Thank you for your comments.

The revised manuscript has been edited to address the NAC concentration (10 mM).

39) There is no direct evidence that the downregulation of PI3K/Akt/mTOR proteins and mRNA causes early apoptosis, hence, the statement that CGEtOAc and sorafenib combination causes early apoptosis thru PI3K/Akt/mTOR inhibition should be reconstructed and conclusions be changed.

Response:

Thank you for the constructive comments.

We agree with the reviewer that a limitation of our work is the lack of direct evaluation of how the extract, combined with sorafenib, downregulates the PI3K/Akt/mTOR pathway in HepG2 cells. In the revised manuscript, we addressed this by stating, “Thus, we hypothesize that the apoptotic mechanism of CGEtOAc combined with sorafenib is associated with the downregulation of the upstream PI3K/AKT/mTOR signaling cascade pathway. Further experiments are needed to assess how CGEtOAc in combination with sorafenib downregulates the PI3K/Akt/mTOR pathway to regulate apoptosis in HepG2 cells.”

40) Explain clearly why would the cytotoxicity of each fraction from C. gigantea ST is not entirely influenced by a single component present in each fraction.

Response:

Thank you for your question and for providing feedback to us.

The fractions of C. gigantea obtained from different solvents, each with varying polarity properties, resulted in each fraction containing different phytochemicals. The presence of a variety of phytochemicals in each C. gigantea fraction, such as phenolics, triterpenoids, cardiac glycosides, and flavonoids, led to various biological activities with differing magnitudes. These variations are attributed to differences in their chemical structures, physicochemical properties, and affinities to target proteins. Therefore, the biological activities, including cytotoxicity of each fraction, were caused by a combination of chemicals within, as previously reported and discussed (Winitchaikul T et al., 2021, Sawong S et al., 2022).

41) If it is hypothesized that a cardiac glycoside would be a key component in the EA fraction of C. gigantea ST extract, this should be isolated and further tested in the battery of assays.

Response:

Thank you for your valuable suggestion.

We conducted bio-guided fractionation and purification to isolate major cardiac glycosides or cardenolides, as a means of further supporting our hypothesis. Our ongoing preliminary research and unpublished results have indicated the potential of calotropin and calactin, two major cardenolides found in Calotropis species, in combination with several commercially available chemotherapeutic agents. However, these experiments are an extension of the key results presented in this manuscript. Therefore, we are currently sharing our novel findings to propose the potential benefits of using plant-derived compounds in conjunction with chemotherapeutic agents for cancer therapy.

We are genuinely appreciative of the Editor and Reviewers' decision to allow us the opportunity to revise this manuscript to fully meet PLOS ONE's publication criteria. We are thankful for the time, effort, and assistance you have provided in enhancing the quality of this work. We sincerely hope that our revisions have elevated the manuscript's quality, and we are confident that the reviewers will find it suitable for publication.

Sincerely yours,

On behalf of all co-authors,

Corresponding authors

________________________________________

Attachment

Submitted filename: Response to Reviewers.docx

pone.0300051.s007.docx (147KB, docx)

Decision Letter 1

Nafees Ahemad

4 Jan 2024

PONE-D-23-18216R1Combination of Ethyl acetate fraction from Calotropis gigantea stem bark and sorafenib induces apoptosis in HepG2 cellsPLOS ONE

Dear Dr. Srisawang,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

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Reviewer #2: In this study conducted by Chaisupasakul and colleagues, the researchers explore the therapeutic potential of the ethyl acetate fraction derived from the stem bark extract of Calotropis gigantea. The focus is on its synergistic effects in combination with sorafenib against HepG2 cells. The research findings demonstrate that the combined application of the extract and sorafenib leads to early apoptosis. The study delves into potential underlying mechanisms, including the examination of factors such as reactive oxygen species (ROS) production and the inhibition of the PI3K/Akt/mTOR pathway. The manuscript has greatly improved since the first submission. However, problems with the determination of the IC50 is an issue because of the lack of data points towards the bottom of the asymptotic curve used to compute for the IC50. This would in turn, have a cascading effect on the downstream experiments presented since the assessment whether the effect is synergistic or additive would be hard to determine. Further suggestions are written below.

1) In the abstract, “The cytotoxicity of the ethyl acetate fraction of the Calotropis gigantea (L.) Dryand. (C.gigantea) stem bark extract (CGEtOAc) of has been demonstrated in many types of cancers.” should be changed to “The cytotoxicity of the ethyl acetate fraction of the Calotropis gigantea (L.) Dryand. (C.gigantea) stem bark extract (CGEtOAc) has been demonstrated in many types of cancers.”.

2) In the Materials section of the Methodology, there should be a comma after (DMEM)(Corning, USA) instead of a period.

3) Include catalog numbers of FBS, DMEM, MTT, and Lumina Forte Western HRP Substrate.

4) “The ultrasonic-assisted extraction with 95% ethanol was used to prepare the crude extract before fractionation using the liquid-liquid chromatography of water and dichloromethane, followed by the rest of the water layer and ethyl acetate.” should be “The ultrasonic-assisted extraction with 95% ethanol was used to prepare the crude extract before fractionation using liquid-liquid chromatography of water and dichloromethane, followed by the rest of the water layer and ethyl acetate.”

5) The 50 in IC50 should be a subscript.

6) Graph Prism version 9 should be changed to GraphPad Prism version 9.

7) “The scratch wounds were consistently in size in each well to avoid variations between conditions” should be “The scratch wounds were consistent in size in each well to avoid variations between conditions”

8) There should be no indentation before HepG2 cells in the Fluorescence labeling with JC-1 to evaluate mitochondrial membrane potential section of the Methodology.

9) 5-Diphenyltetrazolium Bromide should be 5-diphenyltetrazolium bromide.

10) In S2 Fig, adjust the graph such that the origin would meet at y=0 and x=-2.0.

11) Make sure that all supplementary figures are titled properly just like how you would normally title a main text figure.

12) There is a major concern on how the effectivity of CGEtOAc is and the how the CGEtOAc and sorafenib are set up. The asymptotic curve used to determine the IC50 of the extract and the compound is missing the flat slope to the right (observations with 0% inhibition), hence the accuracy of the IC50 evaluation is compromised since the formula from GraphPad would have no bottom response to be used. This could potentially cause a cascade effect since the next experiments were based off of the computed CIs with the IC50 of CGEtOAc and sorafeninb as a baseline.

13) The subtitle “CGEtOAc in combination with sorafenib inhibited the migration ability of HepG2 cells” should be CGEtOA and sorafenib, singly and in combination inhibited the migration of HepG2 cells”.

14) Comple this Figure tile in Figure 2. The migratory capacity of HepG2 cells was assessed using a 72-h.

15) Why is there no data point for 72h treatment for CGEtOAc and sorafenib combination?

16) There should be a T-test comparing the combination group with the single treatment to determine whether the combination is actually acting on the migration rate and not just the individual ability of CGEtOAc and sorafenib. An experiment with lower doses combination can be done to determine if it is truly the combination of the two that is inhibiting migration.

17) Fix S4 Fig title as it doesn’t only show 24 h of incubation and not just the combination treatment. This applies to all the Figure titles, as they should be complete and should completely describe the data presented.

18) Define MMP.

19) Figure 3 title should be The apoptotic effect of CGEtOAc, sorafenib, and their combination against HepG2 cells after exposure for 24h.

20) “As shown in Fig 5A, 400 CGEtOAc μg/mL combined with 4 μM sorafenib upregulated ROS production with the green fluorescence intensity of dichlorofluorescein compared to the vehicle control and their respective treatment groups.” should be “As shown in Fig 5A, 400 μg/mL CGEtOAc combined with 4 μM sorafenib upregulated ROS production with the green fluorescence intensity of dichlorofluorescein compared to the vehicle control and their respective treatment groups.”

21) The subtitle “The apoptotic response following treatment with a combination of CGEtOAc and sorafenib involved the inhibition of the PI3K/Akt/mTOR pathway in HepG2 cells” should be “The apoptotic response following treatment with a combination of CGEtOAc and sorafenib may involve the inhibition of the PI3K/Akt/mTOR pathway in HepG2 cells” since the blot experiments did not show whether inhibition of PI3K/Akt/mTor by CGEtoAc and sorafenib and their combination is happening dependently or independently of apoptosis.

22) “The ethyl acetate fraction contains the highest levels of cardiac glycosides and phenolics, however, triterpenoids, flavonoids, and calotropins are present in lover levels than dichloromethane and other extract fractions.” should be “The ethyl acetate fraction contains the highest levels of cardiac glycosides and phenolics, however, triterpenoids, flavonoids, and calotropins are present in lower levels than dichloromethane and other extract fractions.”

23) Make sure that all compounds that are written in the article are in lower case (i.e. lanatoside C, raddeanin A, kaemperol, etc.) since these chemical compounds are not proper nouns.

24) The statement “Inhibition of PI3K/Akt/mTOR expression was also demonstrated as a mediator of apoptosis following combination therapy.” should be rewritten as a hypothesis or a possibility as the experiments presented are not a direct evidence of its association with apoptosis.

Reviewer #3: The revised version of this manuscript from Chaisupasakul et al. has improved considerably from its previous version. However, there are some minor suggestions in the discussion section that need to be addressed before recommending it for publication.

1. The authors have mentioned the possible role of cardiac glycoside in conferring anticancer properties, and so based on their findings in HPLC-ESI-MS, the line in discussion “the cardiac glycoside composition in the ethyl acetate fraction of the C. gigantea stem bark extract may be postulated to be one of the key components that confer anticancer properties to the ethyl acetate fraction when combined with sorafenib in HepG2 cells” should be modified accordingly.

2. The present study reports exposure to CGEtOAc and sorafenib reduces the expression of PI3K/Akt/mTOR. In contrast, previous studies from Dai et al. 2018 and Wang et al. 2020 have shown reduced activation of PI3K/Akt/mTOR signaling pathways when sorafenib was given with other combinations. Does CGEtOAc has any role in the transcription of PI3K/Akt/mTOR? Are there any available studies showing transcription regulation of PI3K/Akt/mTOR from any other plant extract?

3. In the discussion, the author also should shed lights on other possible mechanism participating in the apoptotic action of the current combination therapy

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Reviewer #2: No

Reviewer #3: No

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PLoS One. 2024 Mar 25;19(3):e0300051. doi: 10.1371/journal.pone.0300051.r004

Author response to Decision Letter 1


14 Feb 2024

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: (No Response)

Reviewer #3: All comments have been addressed

________________________________________

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Partly

Reviewer #3: Yes

________________________________________

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

Reviewer #3: Yes

________________________________________

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: Yes

Reviewer #3: Yes

________________________________________

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: Yes

Reviewer #3: Yes

________________________________________

6. Review Comments to the Author

Reviewer #2:

In this study conducted by Chaisupasakul and colleagues, the researchers explore the therapeutic potential of the ethyl acetate fraction derived from the stem bark extract of Calotropis gigantea. The focus is on its synergistic effects in combination with sorafenib against HepG2 cells. The research findings demonstrate that the combined application of the extract and sorafenib leads to early apoptosis. The study delves into potential underlying mechanisms, including the examination of factors such as reactive oxygen species (ROS) production and the inhibition of the PI3K/Akt/mTOR pathway. The manuscript has greatly improved since the first submission.

However, problems with the determination of the IC50 is an issue because of the lack of data points towards the bottom of the asymptotic curve used to compute for the IC50. This would in turn, have a cascading effect on the downstream experiments presented since the assessment whether the effect is synergistic or additive would be hard to determine. Further suggestions are written below.

1) In the abstract, “The cytotoxicity of the ethyl acetate fraction of the Calotropis gigantea (L.) Dryand. (C.gigantea) stem bark extract (CGEtOAc) of has been demonstrated in many types of cancers.” should be changed to “The cytotoxicity of the ethyl acetate fraction of the Calotropis gigantea (L.) Dryand. (C.gigantea) stem bark extract (CGEtOAc) has been demonstrated in many types of cancers.”.

Response:

We appreciate the positive comments from the reviewer and thank you for these suggestions.

The revised manuscript has been edited as suggested.

2) In the Materials section of the Methodology, there should be a comma after (DMEM)(Corning, USA) instead of a period.

3) Include catalog numbers of FBS, DMEM, MTT, and Lumina Forte Western HRP Substrate.

Response:

Thank you for these suggestions.

The revised manuscript has been edited as suggested.

4) “The ultrasonic-assisted extraction with 95% ethanol was used to prepare the crude extract before fractionation using the liquid-liquid chromatography of water and dichloromethane, followed by the rest of the water layer and ethyl acetate.” should be “The ultrasonic-assisted extraction with 95% ethanol was used to prepare the crude extract before fractionation using liquid-liquid chromatography of water and dichloromethane, followed by the rest of the water layer and ethyl acetate.”

Response:

Thank you for these suggestions.

The revised manuscript has been edited as suggested.

5) The 50 in IC50 should be a subscript.

Response:

Thank you for these suggestions.

The revised manuscript has been edited as suggested.

6) Graph Prism version 9 should be changed to GraphPad Prism version 9.

Response:

Thank you for these suggestions.

The revised manuscript has been edited as suggested.

7) “The scratch wounds were consistently in size in each well to avoid variations between conditions” should be “The scratch wounds were consistent in size in each well to avoid variations between conditions”

Response:

Thank you for these suggestions.

The revised manuscript has been edited as suggested.

8) There should be no indentation before HepG2 cells in the Fluorescence labeling with JC-1 to evaluate mitochondrial membrane potential section of the Methodology.

Response:

Thank you for these suggestions.

The revised manuscript has been edited as suggested.

9) 5-Diphenyltetrazolium Bromide should be 5-diphenyltetrazolium bromide.

Response:

Thank you for these suggestions.

The revised manuscript has been edited as suggested.

10) In S2 Fig, adjust the graph such that the origin would meet at y=0 and x=-2.0.

Response:

Thank you for these suggestions.

The S2 Fig. has been revised as suggested and is shown below.

S2 Fig.-revised

S2 Fig. The IC50 curves for (A) CGEtOAc and (B) sorafenib in HepG2 cells, and (C) CGEtOAc in IMR-90 cells for 24 h of incubation.

11) Make sure that all supplementary figures are titled properly just like how you would normally title a main text figure.

Response:

Thank you for these suggestions.

The revised manuscript and all figures have been edited as suggested.

12) There is a major concern on how the effectivity of CGEtOAc is and the how the CGEtOAc and sorafenib are set up. The asymptotic curve used to determine the IC50 of the extract and the compound is missing the flat slope to the right (observations with 0% inhibition), hence the accuracy of the IC50 evaluation is compromised since the formula from GraphPad would have no bottom response to be used. This could potentially cause a cascade effect since the next experiments were based off of the computed CIs with the IC50 of CGEtOAc and sorafenib as a baseline.

Response:

Thank you for these suggestions. We appreciate the reviewer’s insights.

We have revised cell viability test to enhance the accuracy and reliability of IC50 value (Figure 1-revised and S2 Fig-revised with correction of Y-axis name to “% cell viability”). The revised manuscript has been edited as follows.

CGEtOAc was tested at concentrations ranging from 0 to 2,000 µg/mL, while sorafenib concentrations ranged from 0 to 40 µM, as illustrated in Figure 1 and S2 Fig. The cytotoxic effect revealed that cell viability decreased to approximately 18% at CGEtOAc 2,000 µg/mL, with an IC50 value of 707.87±49.05 µg/mL. The IC50 of CGEtOAc from 0 to 1,000 µg/mL was 771.00 ± 69.74 µg/mL. Additionally, for sorafenib concentrations ranging from 0 to 40 µM, which also exhibited a decrease in cell viability to approximately 10%, the IC50 was 8.65±0.33 µM, and at 0 to 20 µM, it was 8.64 ± 0.32 µM, demonstrating a negligible difference.

However, concerning the reviewer’s concern about the missing the flat slope to the right of the asymptotic curve used to determine the IC50 of the extract and sorafenib, it’s important to note that the potentially cascade effect on the next experiments was not affected. This is because the cytotoxic effect of CGEtOAc and sorafenib revealed a decrease in cell viability to less than 10-20%. Additionally, in the consequence experiments aimed at evaluating the combination effect, we utilized sub-IC50 concentrations and those around IC50 for both CGEtOAc (at 200,400, 600, and 800 µg/mL) and sorafenib (at 2, 4, and 8 µM). The objective was to employ concentrations as low as possible to generate the combination effect of CGEtOAc and sorafenib.

Additionally, the absence of a flat slope reaching 0% cell viability was due to the limited solubility of CGEtOAc extracts at doses higher than 2,000 µg/mL, which we preliminarily explored. Therefore, the present study restricted the maximum dose of CGEtOAc to 2,000 µg/mL. Concerning the IC50 of sorafenib, we consistently obtained IC50 values (8.65±0.33 µM for concentrations from 0 to 40 µM) comparable to those reported in other publication (https://doi.org/10.21873/invivo.13190, https://doi.org/10.2147/DDDT.S344750 ). Consequently, the combination effect of CGEtOAc and sorafenib, as demonstrated in Figure 1C, where CGEtOAc was at 400 µg/mL and sorafenib at 4 µM, was deemed suitable for the subsequent experiments and potential future applications in treatment.

Figure 1-revised is shown below.

Fig 1. Cytotoxicity of the ethyl acetate fraction of C. gigantea stem bark extracts (CGEtOAc) and sorafenib against HepG2 cells. The 3-(4, 5-Dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) technique was used to assess the cell viability after being treated for 24 h with (A) CGEtOAc, (B) sorafenib, and (C) their combination, demonstrated in the bar graph and (D) heat map analysis. (E) CGEtOAc and (F) the combination of CGEtOAc and sorafenib treatment on IMR-90 cells. Cells treated with 0.8% dimethyl sulfoxide (DMSO) represented the vehicle control. The significant differences in data, presented as the mean ± standard deviation (SD) from at least three different experiments, were investigated with a one-way analysis of variance (ANOVA) using Tukey's honestly significant difference (HSD) test: *; p < 0.05 vs the vehicle control, a; p < 0.05 vs a single CGEtOAc treatment at their own dose, b; p < 0.05 vs a single sorafenib treatment at their own dose.

13) The subtitle “CGEtOAc in combination with sorafenib inhibited the migration ability of HepG2 cells” should be CGEtOA and sorafenib, singly and in combination inhibited the migration of HepG2 cells”.

Response:

Thank you for these suggestions.

The revised manuscript has been edited as suggested.

14) Comple this Figure tile in Figure 2. The migratory capacity of HepG2 cells was assessed using a 72-h.

Response:

Thank you for these suggestions.

The revised manuscript has been edited, and the figure title in Figure 2 has been completed as followed:

“Fig 2. The migratory capacity of HepG2 cells was assessed through a 0-72-h incubation with CGEtOAc at 400 µg/mL and sorafenib at 4 µM, both singly and in combination.”

15) Why is there no data point for 72h treatment for CGEtOAc and sorafenib combination?

Response:

We appreciate the reviewer’s suggestions.

In acknowledgment of your valuable question, we have incorporated this issue into the Results section as follows:

“The migration data for the combination of CGEtOAc and sorafenib at 72 h disappeared due to the complete loss of cell viability. Cells detached from the plate, rendering it impossible to measure the gap between cell scratches, attributed to the toxic effects of the combination treatment.”

16) There should be a T-test comparing the combination group with the single treatment to determine whether the combination is actually acting on the migration rate and not just the individual ability of CGEtOAc and sorafenib. An experiment with lower doses combination can be done to determine if it is truly the combination of the two that is inhibiting migration.

Response:

We appreciate the reviewer’s suggestions.

The revised Figure 2 and S4 Fig. have been updated to include comparisons between the combination group with the single treatment, as stated as below:

“At a concentration of 400 µg/mL for CGEtOAc, 4 µM for sorafenib, and their combination, cell migration was significantly inhibited. This was demonstrated by a higher percentage of gap distance in a wound healing assay compared to each hour of incubation in the vehicle group, when 0 h set at 100% for each group (Fig 2A and 2B). The percentage of gap distance for the combination of CGEtOAc and sorafenib in each incubation period did not differ from the single treatments. However, the migration data for the combination of CGEtOAc and sorafenib at 72 h disappeared due to the complete loss of cell viability. Cells detached from the plate, rendering it impossible to measure the gap between cell scratches, attributed to the toxic effects of the combination treatment.

The migration rate, as shown in S4 Fig, confirmed a significant inhibition of HepG2 cell migration by CGEtOAc at 400 µg/mL, sorafenib at 4 µM, and their combination compared to each hour of incubation in the vehicle group. The migration rate for the combination of CGEtOAc and sorafenib in each incubation period did not differ from the single treatments.

Our findings indicate that CGEtOAc, sorafenib, and their combination upregulated the anti-proliferative and anti-migratory activities in HepG2 cells.”

Notably, suppression of migration was observed within 24 hours after incubation with CGEtOAc at 400 µg/mL, sorafenib at 4 µM, and their combination, with no significant difference among the 24-, 48- and 72-hour incubation periods. This finding aligns with reports demonstrating that the inhibition of migration remains consistent across different incubation times at a particular concentration of the agent that significant exhibited cytotoxic effects on cancer cells (https://doi.org/10.21873/anticanres.15241 , https://doi.org/10.1093/gastro/goaa072 ).

In addition, we found that the suppression of migration by the combination of CGEtOAc and sorafenib did not differ from that observed with their respective single treatments. However, cell viability showed a greater reduction in the combination group compared to each single treatment. Our findings are consistent with report demonstrating that the combination of the chemotherapeutic agent docetaxel and plant extract had a similar inhibitory effect on migration compared to single treatments, as evaluated by measuring gap width in breast cancer cells at every incubation time (https://doi.org/10.1155/2021/5517944 ). These findings suggest that cell migration was inhibited prior to cell death. After the first 24 hours post-treatment with both lethal and non-lethal doses of radiation and chemotherapy drug paclitaxel, migration of glioblastomas was inhibited prior to cell death. This suggestion proposed that cell migration should be a therapeutic target in anti-metastasis/anti-invasion strategies for improve cancer therapeutic outcomes (https://doi.org/10.1016%2Fj.bbrep.2021.101071 ).

Thus, our manuscript suggests that each single treatment, CGEtOAc and sorafenib, possesses migration inhibition properties, which may contribute to enhancing the synergistic effect of the combination treatment on anticancer efficiency.

The study evaluated the effects of CGEtOAc at 400 µg/mL, sorafenib at 4 µM, and their combination on the inhibition of cell migration. The results from both MTT and combination index analysis indicated that the combination of CGEtOAc at 400 µg/mL with sorafenib at 4 µM exhibited a combination index (CI) value of 0.5, correlated with an inhibition rate of approximately 60%, suggestive of a synergistic effect. However, combinations with lower concentrations, such as CGEtOAc at 200 µg/mL and sorafenib at 4 mM also yielded a CI of 0.5, albeit with a lesser inhibition rate of 40%. Therefore, we concluded that CGEtOAc at 400 µg/mL and sorafenib at 4 µM were selected for the further experimentation.

Below are the revised versions of Figure 2 and S4 Fig.

Figure 2-revised

Fig 2. The migratory capacity of HepG2 cells was assessed through a 0-72-h incubation with CGEtOAc at 400 µg/mL and sorafenib at 4 µM, both singly and in combination. (A) Wound healing images were captured with a magnification bar of 500 µm. (B) The percentage of the gap was depicted as a bar graph. Cells treated with 0.8% DMSO represented the vehicle control. The significant differences in data, presented as the mean ± SD from at least three different experiments were investigated with a one-way ANOVA using Tukey's HSD test: *; p < 0.05 vs the 0 h of the vehicle control, a; p < 0.05 compared to 24 h of incubation in the vehicle group, b; p < 0.05 compared to 48 h of incubation in the vehicle group, and c; p < 0.05 compared to 72 h of incubation in the vehicle group, with 0 hours set at 100% for each group. 

S4 Fig-revised

S4 Fig. The migration rate of HepG2 cells treated with CGEtOAc at 400 µg/mL and sorafenib at 4 µM, both singly and in combination, was evaluated using a wound healing assay after 0-72 h of incubation and compared to the vehicle group. Cells treated with 0.8% DMSO represented the vehicle control. The significant differences in data, presented as the mean ± SD from at least three different experiments were investigated with a one-way ANOVA using Tukey's HSD test: a; p < 0.05 compared to 24 h of incubation in the vehicle group, b; p < 0.05 compared to 48 h of incubation in the vehicle group, and c; p < 0.05 compared to 72 h of incubation in the vehicle group.

17) Fix S4 Fig title as it doesn’t only show 24 h of incubation and not just the combination treatment. This applies to all the Figure titles, as they should be complete and should completely describe the data presented.

Response:

We appreciate the reviewer’s suggestions.

This title has been revised and changed to

“S4 Fig. The migration rate of HepG2 cells treated with CGEtOAc at 400 µg/mL and sorafenib at 4 µM, both singly and in combination, was evaluated using a wound healing assay after 0-72 h of incubation and compared to the vehicle group.”

18) Define MMP.

Response:

We appreciate the reviewer’s suggestions.

The revised manuscript has been edited to address the issue of “mitochondrial membrane potential (MMP)”, as suggested.

19) Figure 3 title should be the apoptotic effect of CGEtOAc, sorafenib, and their combination against HepG2 cells after exposure for 24h.

Response:

Thank you for these suggestions.

The revised manuscript has been edited as suggested.

20) “As shown in Fig 5A, 400 CGEtOAc μg/mL combined with 4 μM sorafenib upregulated ROS production with the green fluorescence intensity of dichlorofluorescein compared to the vehicle control and their respective treatment groups.” should be “As shown in Fig 5A, 400 μg/mL CGEtOAc combined with 4 μM sorafenib upregulated ROS production with the green fluorescence intensity of dichlorofluorescein compared to the vehicle control and their respective treatment groups.”

Response:

Thank you for these suggestions.

The revised manuscript has been edited as suggested.

21) The subtitle “The apoptotic response following treatment with a combination of CGEtOAc and sorafenib involved the inhibition of the PI3K/Akt/mTOR pathway in HepG2 cells” should be “The apoptotic response following treatment with a combination of CGEtOAc and sorafenib may involve the inhibition of the PI3K/Akt/mTOR pathway in HepG2 cells” since the blot experiments did not show whether inhibition of PI3K/Akt/mTor by CGEtoAc and sorafenib and their combination is happening dependently or independently of apoptosis.

Response:

Thank you for these suggestions.

The revised manuscript has been edited as suggested.

22) “The ethyl acetate fraction contains the highest levels of cardiac glycosides and phenolics, however, triterpenoids, flavonoids, and calotropins are present in lover levels than dichloromethane and other extract fractions.” should be “The ethyl acetate fraction contains the highest levels of cardiac glycosides and phenolics, however, triterpenoids, flavonoids, and calotropins are present in lower levels than dichloromethane and other extract fractions.”

Response:

Thank you for these suggestions.

The revised manuscript has been edited as suggested. 

23) Make sure that all compounds that are written in the article are in lower case (i.e. lanatoside C, raddeanin A, kaemperol, etc.) since these chemical compounds are not proper nouns.

Response:

We appreciate the reviewer's suggestions.

The revised manuscript has been carefully reviewed and edited as suggested.

24) The statement “Inhibition of PI3K/Akt/mTOR expression was also demonstrated as a mediator of apoptosis following combination therapy.” should be rewritten as a hypothesis or a possibility as the experiments presented are not a direct evidence of its association with apoptosis.

Response:

Thank you for these suggestions.

The revised manuscript has been edited as follows, incorporating your recommendations.

“Inhibition of PI3K/Akt/mTOR expression also supports the hypothesis that the apoptotic mechanism of CGEtOAc, when combined with sorafenib, may be associated with the downregulation of the upstream PI3K/AKT/mTOR signaling cascade pathway. However, the experiments presented did not provide sufficient evidence of this inhibition in a direct association with the apoptotic effects following the treatment.”

We would like to inform you that our manuscript's title has been edited using “ethyl acetate” to replace “Ethyl acetate”, thus the title is “Combination of ethyl acetate fraction from Calotropis gigantea stem bark and sorafenib induces apoptosis in HepG2 cells".

In conclusion, we sincerely thank the editor of the PLOS ONE and the reviewers for their insightful comments and suggestions. We appreciate the time and effort you have dedicated to assisting our work to be suitable for publication. We have carefully noted all your suggestions and comments and have addressed them, as they are beneficial for strengthening this study.

Sincerely yours,

Corresponding authors

Reviewer #3:

The revised version of this manuscript from Chaisupasakul et al. has improved considerably from its previous version. However, there are some minor suggestions in the discussion section that need to be addressed before recommending it for publication.

1. The authors have mentioned the possible role of cardiac glycoside in conferring anticancer properties, and so based on their findings in HPLC-ESI-MS, the line in discussion “the cardiac glycoside composition in the ethyl acetate fraction of the C. gigantea stem bark extract may be postulated to be one of the key components that confer anticancer properties to the ethyl acetate fraction when combined with sorafenib in HepG2 cells” should be modified accordingly.

Response:

We appreciate the reviewer's suggestions.

The revised manuscript has been carefully reviewed and edited incorporating the suggested changes as follows:

“Consequently, based on the findings in HPLC-EIS-MS, it may be postulated that the cardiac glycoside composition in the ethyl acetate fraction of the C. gigantea stem bark extract is one of the key components that confer anticancer properties to the ethyl acetate fraction when combined with sorafenib in HepG2 cells.”

2. The present study reports exposure to CGEtOAc and sorafenib reduces the expression of PI3K/Akt/mTOR. In contrast, previous studies from Dai et al. 2018 and Wang et al. 2020 have shown reduced activation of PI3K/Akt/mTOR signaling pathways when sorafenib was given with other combinations. Does CGEtOAc has any role in the transcription of PI3K/Akt/mTOR? Are there any available studies showing transcription regulation of PI3K/Akt/mTOR from any other plant extract?

Response:

We acknowledge the reviewer’s comments regarding the reduced activation of PI3K/Akt/mTOR signaling pathways instead of the transcription of protein expression following various treatments with plant extracts, sorafenib, both individually and in combination. However, it is worth noting that several reports have demonstrated the effects of plant extracts, sorafenib, and their combination on the transcription of protein signaling pathway expression in cancer cells. However, there have been no reports on the effect of C. gigantea extracts, either alone or in combination with sorafenib, on the inhibition of PI3K/Akt/mTOR expression in cancer cells. The following publications serve as examples of C. procera (the family Apocynaceae) extracts and cardiac glycosides in the inhibition of the expression of the PI3K/AKT/mTOR pathway in cancer cells.

The leaves of C. procera extracts exhibited inhibition on AKT/mTOR expression, inducing apoptosis in cancer cells (https://doi.org/10.1016/j.heliyon.2023.e16706 ). Cardiac glycosides, peruvoside, inhibited the expression of PI3K and mTOR in cancer cells (https://doi.org/10.1016/j.lfs.2019.117147 ). Cardiac glycoside ouabain exhibited inhibition of the expression of mTOR and p-AKT in cancer cells (https://doi.org/10.3892/mmr.2018.8587 ). Lanatoside C, the cardiac glycoside, downregulated gene expression of AKT, PI3K, and mTOR in cancer cells (https://doi.org/10.3390/biom9120792 ). Digoxin decreased PI3K and p-Akt protein expression levels in cancer cells (https://doi.org/10.1016/j.redox.2019.101131 ). Ouabain significantly decreased the levels of PI3K and p-AKT(Thr308) in cancer cells (https://doi.org/10.21873/anticanres.15241 ).

Thus, we have addressed the following points in the discussion:

“Inhibition of PI3K/Akt/mTOR expression also supports the hypothesis that the apoptotic mechanism of CGEtOAc, when combined with sorafenib, may be associated with the downregulation of the upstream PI3K/AKT/mTOR signaling cascade pathway. However, the experiments presented did not provide sufficient evidence of this inhibition in direct association with the apoptotic effects following the treatment. Furthermore, consistent with our findings, there have been reports on the effect of C. procera extracts and cardiac glycosides in the inhibition of the expression of the PI3K/AKT/mTOR pathway in correlation to induce apoptosis cancer cells (Ref: ).”

The following publications demonstrate examples of reports revealing the effects of plant extracts on the inhibition of the expression of the PI3K/AKT/mTOR pathway in cancer cells.

Matrine (MAT), a compound extracted from Sophora flavescens Aiton, exhibited anticancer against breast cancer cells in a 24-h incubation by inhibiting the expression of PI3K/AKT pathway (https://doi.org/10.1038%2Fs41598-023-39655-9 ).

The anticancer activity against renal cell carcinoma of gypenosides of Gynostemma pentaphylla (Thunb.) Makino was demonstrated through the downregulation of mRNA and protein expressions of the AKT/P-AKT-mTOR-P-mTOR pathway (https://doi.org/10.1016/j.jep.2021.113907 ).

Bupleurum, the dried root of Bupleurum chinensis DC. downregulates both the genes and protein expressions of PI3K, Akt, mTOR and phosphorylated proteins P-PI3K, P-Akt, P-MTOR (https://doi.org/10.1016/j.jep.2021.114742 ), along with other publications including Celastrus orbiculatus extracts ((http://dx.doi.org/10.2174/1871520619666190731162722), Scutellaria barbata D.Don (https://doi.org/10.3892/or.2017.5892 ).

Additionally, anticancer drugs have been reported to exhibit potential anticancer effects through the inhibition of protein expressions in PI3K/Akt/mTOR signaling pathways. Sorafenib has been shown to suppress PI3K expression (https://doi.org/10.3389/fonc.2022.852095 , https://doi.org/10.3892%2Fol.2018.8536, http://www.ncbi.nlm.nih.gov/pmc/articles/pmc6789287/, ), decrease the expression of the PI3K/Akt pathway (doi: 10.1186/s12964-023-01355-2, https://doi.org/10.18632/oncotarget.9168 ), suppress AKT expression downstream effectors including 4E-BP1 and eIF4E (DOI: 10.1097/CAD.0000000000001056), and downregulate the PI3K/AKT/mTOR pathway (PMID: 25778319).

We also addressed in the discussion that there have been reports indicating that sorafenib, in combination with other plant extracts, exhibited suppression of PI3K/Akt/mTOR expression in cancer cells.

“Furthermore, osthole (coumarins), sorafenib, and the combination of both compounds suppressed PI3K and Raf kinases, resulting in cytotoxic effects in cancer cells (https://doi.org/10.3390/molecules25215192 ). Additionally, sorafenib in combination with capsaicin downregulated the expression levels of Akt, mTOR and p70S6K in LM3 human hepatocellular carcinoma cells, thereby inducing apoptosis (https://doi.org/10.3892/or.2018.6754).”

3. In the discussion, the author also should shed lights on other possible mechanism participating in the apoptotic action of the current combination therapy

Response:

Thank you for these suggestions, which contribute to strengthening the manuscript's correlation between results and the potential mechanisms underlying the anticancer effect of the combination of CGEtOAc and sorafenib.

This manuscript has provided a discussion into the possible mechanism of HepG2 cells exposed to a combination of CGEtOAc and sorafenib, leading to activation of apoptosis. ROS generation was found to be predominantly responsible for the apoptosis-inducing effects of the combined therapy; however, ROS is not the sole mechanism participating in the apoptotic action. Inhibition of PI3K/Akt/mTOR expression also supports the hypothesis that the apoptotic mechanism of CGEtOAc, when combined with sorafenib, may be associated with the downregulation of the upstream PI3K/AKT/mTOR signaling cascade pathway. Other mechanisms may work in conjunction to generate this treatment efficiency.

We also discussed that ROS production has been reported as a regulator of PI3K/Akt/mTOR expression, which results in cancer cell apoptosis. Furthermore, we discussed sorafenib's mechanism in triggering apoptosis in cancer cells. Moreover, this manuscript addressed the possible mechanism of a combination of sorafenib with anticancer agents, which promotes cancer cell apoptosis.

Regarding the review’s comment that provides strength to the potential discussion of this study, the revised manuscript has addressed the possible mechanism of the anticancer effect of a combination of C. gigantea extracts and sorafenib as follows:

“Notably, several studies have reported the apoptotic effect of cardiac glycosides, with this compound identified as one of the major components in C. gigantea extracts that exhibit anticancer activity. This effect is achieved through the mechanism of inhibiting Na+/K+ ATPase, leading to an increase in intracellular Ca2+ via the Na+/Ca2+ exchanger https://doi.org/10.1007/s10549-005-9053-3 , https://doi.org/10.1371/journal.pone.0287769 . Cardenolides from C. gigantea exhibited potent anticancer effects against breast cancers by inhibiting Na+/K+ ATPase, resulting in an increase in intracellular Ca2+ via the Na+/Ca2+ exchanger-dependent manner, ultimately leading to apoptosis https://doi.org/10.1021/acs.jnatprod.0c00423 . Cardenolide UNBS1450, having lower affinity for the α-2 Na+/K+ ATPase than other cardenolides, induced downregulation of myeloid cell leukemia-1, a member of the anti-apoptotic protein family downstream of Na+/K+ ATPase activity, contributing to apoptosis in cancer cells https://doi.org/10.1038/cddis.2015.134 . Various signaling pathways downstream of Na+/K+ ATPase inhibition has been demonstrated to contribute to the inhibition of cancer cell proliferation. Oleandrin, a cardiac glycoside extracted from the leaves of Neriaum oleander, induced accumulation of Ca2+, leading to ER stress-mediated cytotoxic effects, enhancing immunogenic cell death in both in vitro and in vivo cancer models https://doi.org/10.1038/s41419-021-03605-y . Ascleposide, a natural cardenolide isolated from Reevesia formosana, downregulated the oncoprotein c‐Myc protein and inhibited the phosphorylation of tumor suppressor protein Rb, resulting in the inhibition of cancer cell cycle progression and blocking cell proliferation. This effect was attributed to an increase of α‐tubulin acetylation, which contributed to the inhibition of Na+/K+ ATPase activity by enhancing the endocytosis of this protein in cancer cells through activation of the mitogen‐activated protein kinases (MAPKs) pathway https://doi.org/10.1002/pros.23944 .

Additionally, toxicarioside G, a cardenolide isolated from C. gigantea, exhibited another mechanism of anticancer activity by inhibiting cancer cell viability and proliferation. It enhanced Yes1 associated transcriptional regulator (YAP) dephosphorylation and nuclear localization, along with downstream target gene expression https://doi.org/10.3892/or.2021.8175 . Calotropin, a cardiac glycoside derived from C. gigantea, demonstrates an inhibitory effect on the regulation of metabolic reprogramming in cancer, specifically targeting aerobic glycolysis (the Warburg effect). This leads to activation of cell cycle arrest and inhibition metastasis in cancer cells, ultimately contributing to activation of apoptosis https://doi.org/10.1016/j.jobcr.2023.09.002. Another cardiac glycoside, 3′-epi-12β-hydroxyfroside, isolated from the roots of C. gigantea, induced autophagy, leading to enhanced apoptosis. This effect was mediated by downregulation of the heat shock protein 90 (Hsp90)-regulated Akt/mTOR pathway in lung cancer cells https://doi.org/10.7150/thno.23304. Lanatoside C, a cardiac glycoside, has been shown to induce apoptosis in cancer cells by suppressing the Janus kinase (JAK)/signal transducer and activator of transcription (STAT)/cytokine signaling (SOCS) (JAK2/STAT6/SOCS2) pathway http://www.ncbi.nlm.nih.gov/pmc/articles/pmc8387865/ , DOI: 10.3892/ol.2021.13001.”

“In addition to the possible mechanism proposed in this study for CGEtOAc, combined with sorafenib-induced apoptosis in cancer cells, which involves increased ROS accumulation and inhibition of PI3K/Akt/mTOR expression, the apoptotic effects resulting from the combination treatment of compounds found in C. gigantea extracts and anticancer agents may be attributed to several mechanisms. For instance, the synergistic effect of sorafenib in combination with the Na+/K+ ATPase inhibitor berbamine, an alkaloid isolated from Berberis amurensis, and cardiac glycosides like ouabain, is proposed to be contributed by the potentiation of epidermal growth factor receptor (EGFR)-mediated ERK1/2 and p38MAPK activation by berbamine https://doi.org/10.1111/bph.15616 . Similarly, the combination of digitoxin and sorafenib has been reported to suppress p-ERK, hypoxia inducible factor (HIF)-1α, HIF-2α, and VEGF expression, contributing to apoptosis in cancer cells https://doi.org/10.3892/mmr.2016.6096 . Furthermore, the combination of cardenolide derivative, AMANTADIG (3β-[2-(1-amantadine)-1-on-ethylamine]-digitoxigenin) and docetaxel induced apoptosis through the inhibition of surviving protein expression in human androgen-insensitive prostate cancer cells. (https://doi.org/10.1016/j.biopha.2018.08.028 ).”

We would like to inform you that our manuscript's title has been edited using “ethyl acetate” to replace “Ethyl acetate”, thus the title is “Combination of ethyl acetate fraction from Calotropis gigantea stem bark and sorafenib induces apoptosis in HepG2 cells".

In conclusion, we sincerely thank the editor of PLOS ONE and the reviewers for their insightful comments and suggestions. We appreciate the time and effort you have dedicated to assisting our work to be suitable for publication. We have carefully noted all your suggestions and comments and have addressed them, as they are beneficial for strengthening this study.

Sincerely yours,

Corresponding authors

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Submitted filename: Response to Reviewers.docx

pone.0300051.s008.docx (2.1MB, docx)

Decision Letter 2

Nafees Ahemad

21 Feb 2024

Combination of ethyl acetate fraction from Calotropis gigantea stem bark and sorafenib induces apoptosis in HepG2 cells

PONE-D-23-18216R2

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Acceptance letter

Nafees Ahemad

14 Mar 2024

PONE-D-23-18216R2

PLOS ONE

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Associated Data

    This section collects any data citations, data availability statements, or supplementary materials included in this article.

    Supplementary Materials

    S1 Fig. The high-pressure liquid chromatography-electrospray ionisation-mass spectroscopic (HPLC-ESI-MS) chromatogram of C. gigantea stem bark extracts (CGEtOAc) in negative mode.

    (PDF)

    pone.0300051.s001.pdf (208.2KB, pdf)
    S2 Fig. The IC50 curves for (A) CGEtOAc and (B) sorafenib in HepG2 cells, and (C) CGEtOAc in IMR-90 cells for 24 h of incubation.

    (PDF)

    pone.0300051.s002.pdf (276.2KB, pdf)
    S3 Fig. The combination index (CI) vs. fraction affected (Fa) graph for CGEtOAc in combination with sorafenib after 24 h of incubation.

    (PDF)

    pone.0300051.s003.pdf (99.8KB, pdf)
    S4 Fig. The migration rate of HepG2 cells treated with CGEtOAc at 400 μg/mL and sorafenib at 4 μM, both singly and in combination, was evaluated using a wound healing assay after 0–72 h of incubation and compared to the vehicle group.

    Cells treated with 0.8% DMSO represented the vehicle control. The significant differences in data, presented as the mean ± SD from at least three different experiments, were investigated with a one-way ANOVA using Tukey’s HSD test: a; p < 0.05 compared to 24 h of incubation in the vehicle group, b; p < 0.05 compared to 48 h of incubation in the vehicle group, and c; p < 0.05 compared to 72 h of incubation in the vehicle group.

    (PDF)

    pone.0300051.s004.pdf (282.5KB, pdf)
    S1 Raw images. Raw images displaying the gating strategies used in flow cytometry of annexin V and propidium iodide (PI) staining in HepG2 cells after 24 h of incubation, with a combination of 400 μg/mL CGEtOAc and 4 μM sorafenib.

    (PDF)

    pone.0300051.s005.pdf (2.1MB, pdf)
    S2 Raw images. Raw images of the original uncropped and unadjusted western blot images for HepG2 cells treated with a combination of 400 μg/mL CGEtOAc and 4 μM sorafenib for a 24-h incubation period.

    (PDF)

    pone.0300051.s006.pdf (212.4KB, pdf)
    Attachment

    Submitted filename: Response to Reviewers.docx

    pone.0300051.s007.docx (147KB, docx)
    Attachment

    Submitted filename: Response to Reviewers.docx

    pone.0300051.s008.docx (2.1MB, docx)

    Data Availability Statement

    We have used the public repository "Zenodo" to deposit the Supporting information, S1-S6, (DOI: 10.5281/zenodo.10799946).


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