Curcumol Suppresses Breast Cancer Cell Metastasis by Inhibiting MMP-9 Via JNK1/2 and Akt-Dependent NF-κB Signaling Pathways

Abstract

Curcumolhas been reported to possess antitumor activity. However, its effect and mechanisms against tumor metastasis are still unclear. This study is to investigate the inhibitory effect of curcumol on breast cancer cell metastasis and elucidate the underlying molecular mechanisms. Our results showed that noncytotoxicity was caused by curcumol within 10 to 40 µg/mL in MDA-MB-231 and 4T1 cells for 24 hours, whereas sustained treatment with curcumol for 14 days significantly suppressed the clonogenic activity of cells. Importantly, curcumol at noncytotoxic concentrations suppressed the migration ability of both MDA-MB-231 and 4T1 cells. Moreover, curcumol suppressed the migration and invasion of MDA-MB-231 cells in the Boyden chamber migration and invasion assay and inhibited the adhesion of MDA-MB-231 cells onto the matrigel. Further investigations revealed that curcumol decreased the enzyme activity and protein expression of matrix metalloproteinase (MMP-9) in MDA-MB-231 cells. Moreover, curcumol inhibited the activation of c-Jun N-terminal kinase (JNK) 1/2 and Akt (Ser473). Meanwhile, it also inhibited the nuclear translocation and transcriptional activity of nuclear factor κB (NF-κB). Furthermore, JNK inhibitor SP600125 and Akt (Ser473) inhibitor LY294002 enhanced the inhibition of curcumol on NF-κB p65 nuclear translocation. Finally, supplementation with SP600125, LY294002, or NF-κB inhibitor Ammonium pyrrolidinedithiocarbamate (PDTC) significantly enhanced the inhibitory effect of curcumol on MMP-9 expression and cell migration, invasion, and adhesion in MDA-MB-231 cells. Our findings provide evidence for the suppression of breast cancer cell metastasis by curcumol and suggest that the inhibition of MMP-9 via JNK1/2 and Akt (Ser473)-dependent NF-κB signaling pathways may be the underlying mechanisms.

Introduction

Breast cancer is one of the most common causes of cancer-related death for women in the world.¹ Breast cancer metastasis, which refers to the spread of cancer cells from the primary sites to distant sites and colonies, is the main cause of poor clinical outcome or death in the majority of breast cancer patients.² It is reported that localized breast cancer has a 5-year survival rate of 98%; conversely, women diagnosed with distant metastasis have a survival rate of only 27%.³ Targeting the process of metastasis, including detachment from the primary site, motility, and invasion into the second site as well as adhesion profile to other tissues could be considered as the potential targets for antimetastatic therapy.⁴

Tumor metastasis is a complex cascade of events. A number of proteolytic enzymes, such as matrix metalloproteinases (MMPs) play a central role in the breakdown of basement membrane.³ For example, MMP-9 is always constitutively expressed in highly metastatic tumors and necessary for metastasizing to other tissue during breast cancer progression. Transcription factors, such as nuclear factor κB (NF-κB), have been reported to regulate the expression of MMPs after its translocation to the nucleus.⁵ In addition, protein kinases, including mitogen-activated protein kinases (MAPKs) and Akt, are known to be upstream regulators of NF-κB and also play important roles in controlling MMPs.^6-8

Curcumol (Figure 1) is one of the major active components in the essential oil of Rhizoma Curcumae, which is traditionally used for the treatment of gynecological tumors in China. It is reported that curcumol inhibited the proliferation and attenuated the total RNA synthesis of MCF-7, MDA-MB-231, HeLa, and OV-UL-2 gynecological cancer cells with negligible effect on normal cells.⁹ In addition, curcumol also inhibited the proliferation and induced the apoptosis of nasopharyngeal carcinoma CNE-2 cells.¹⁰ These studies demonstrated that curcumol exhibited significant antitumor effects. However, its effect and mechanisms against tumor metastasis are still largely unknown. In this study, the inhibitory effects of curcumol on the migration, invasion, and adhesion of highly invasive breast cancer cells and the underlying molecular mechanisms were investigated.

Figure 1.

Chemical structure of curcumol.

Materials and Methods

Materials

Curcumol with a purity >98% was purchased from Chengdu Must Bio-Technology Co, Ltd (Chengdu, China) and dissolved in dimethyl sulfoxide (DMSO) at a stock solution of 40 mg/mL and stored at −20°C. The antibodies against phospho-c-Jun N-terminal kinase (p-JNK) 1/2, phospho-extracellular signal-regulated kinases (p-ERK) 1/2, p-p38, p-Akt (Ser473), and NF-κB p65 (8242) were purchased from Cell Signaling Technology (Boston, MA). The antibody against lamin B was purchased from Sigma-Aldrich (St Louis, MO). The antibodies against MMP-9 and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were ordered from BIOSS (Beijing, China) and Beyotime (Jiangsu, China), respectively. The NF-κB-Luc firefly luciferase construct was a kind gift from Dr Weiling Jin of Shanghai Jiaotong University, China. Other chemicals were obtained from Sigma-Aldrich, unless indicated otherwise indicated.

Cell Culture

Human breast cancer cells MDA-MB-231 and mouse breast tumor cells 4T1 were obtained from the American Type Cell Culture Collection (Manassas, VA). MDA-MB-231 cells were maintained in Dulbecco’s modified Eagle’s medium supplemented with 10% fetal bovine serum (FBS; Invitrogen, Carlsbad, CA) and 1% penicillin/streptomycin (Invitrogen). 4T1 cells were cultured in RPMI1640 medium supplemented with 10% FBS and 1% penicillin/streptomycin. The 2 cell lines were cultured at 37°C in a 5% CO₂ and 95% air atmosphere.

Measurement of Cell Viability

As previously described,¹¹ cell viability was evaluated by a sulforhodamine B (SRB) assay. Cells (0.8 × 10⁴ cells/well) were treated with various concentrations of curcumol (10, 20, 30, and 40 µg/mL). After 24 hours of incubation, cells were fixed with 10% trichloroacetic acid and stained with 0.4% SRB for 30 minutes. The protein-bound dye is dissolved in 10 mM Tris base solution for optical density (OD) determination at a wavelength of 490 nm using a multiwell spectrophotometer microplate reader (Biotek, Winooski, VT). Cell viability was expressed as a percentage of that of the control (untreated) cells.

Colony Formation Assay

MDA-MB-231 and 4T1 cells were seeded and cultured overnight in 6-well plates. Cells were treated with curcumol for 14 days. Medium with curcumol or vehicle was replaced every 3 days. At the end of treatment, cells were fixed in 100% methanol and stained with 0.005% crystal violet. Finally, images were captured by a CCD camera, and the colonies were counted.

Wound Healing Assay

Cells were seeded on each side of an ibidi culture insert (Munich, Germany) at a density of 4 × 10⁵ cells. After appropriate cell attachment, the insert was gently removed. Cells of each well were treated with various concentrations of curcumol (10, 20, and 40 µg/mL). Then, they were photographed using a BDS200 inverted biological microscope (Optec, Chongqing, China) at insert removal (0 hours) and following 6, 8, 12, and 24 hours of incubation.

Analysis of MMP-9 Activity by Zymography

Conditioned media from cells cultured in the absence of serum for 24 hours were collected. Samples were mixed with loading buffer and separated on 8% SDS (sodium dodecyl sulfate)-polyacrylamide gel containing 0.1% gelatin, without boiling. After electrophoresis, the gels were washed twice in zymography washing buffer (2.5% Triton X-100, 50 mmol/L Tris-HCl 5 mmol/L CaCl₂, and 1 µmol/L ZnCl₂ in double-distilled H₂O) at room temperature to remove SDS, followed by incubation at 37°C for 48 hours in zymography reaction buffer (50 mM Tris-HCl [pH 8.0], 5 mM CaCl₂, 1 µmol/L ZnCl₂, and 0.02% NaN₃). Finally, gels were stained with Coomassie blue R-250 (0.05% Comassie blue R-250, 30% methanol, and 10% acetic acid) for 3 hours and destained with destaining solution (20% methanol, 10% acetic acid, and 70% double-distilled H₂O). Nonstaining bands representing the levels of the latent forms of MMP-9 were captured by a CCD camera.

Boyden Chamber Invasion and Migration Assay

A polycarbonate filter with 8 µm pore size (Corning, NY) was coated with matrigel (BD, Franklin Lakes, NJ). MDA-MB-231 cells were pretreated with curcumol (10, 20, and 40 µg/mL) for 24 hours. The upper chambers were seeded with 1 × 10⁵ cells in 200 µL serum-free medium, and the lower chambers were filled with medium containing 10% FBS. After 24 hours, the filter was fixed with 100% methanol and stained with 0.05% crystal violet. Randomly chosen fields were captured using fluorescence microscopy (OLYMPUS IX71) at 100× magnification, and invading cells were quantified by manual counting. The inhibition percentage of invading cells was quantified with untreated cells representing 100%. To measure the migration ability, MDA-MB-231 cells were seeded into a Boyden chamber that was not coated with matrigel. The subsequent Boyden chamber migration assay was measured as described in the invasion assay.

Adhesion Assay

Cells were preincubated with curcumol (10, 20, and 40 µg/mL) for 24 hours and seeded onto a 96-well plate coated with matrigel for 4 hours. Attached cells were fixed in 100% methanol and stained with 0.05% crystal violet solution. Randomly chosen fields were obtained using fluorescence microscopy (OLYMPUS IX71) at 100× magnification. Then, attached cells were quantified by manual counting. Inhibition percentage of adhesion cells was quantified, with untreated (control) cells representing 100%.

Western Blotting Analysis

The total cellular proteins were extracted from cells in ice-cold RIPA buffer (Cell Signaling Technologies, USA) supplemented with 1% (v/v) protein inhibitor cocktail and 1 mM phenylmethylsulfonyl fluoride. The nuclear proteins were extracted with nuclear extraction kit (Millipore, USA) following the manufacturer’s instructions. Briefly, the cell pellet was incubated with ice cold 1× cytoplasmic lysis buffer on ice for 15 minutes. Then, the cell suspension was centrifuged at 250g for 5 minutes at 4°C, and the cell pellet was incubated again with ice cold 1× cytoplasmic lysis buffer. A syringe with a needle (27 gauge) was used to disrupt the cell membrane (repeatedly drawing and ejecting approximately 5 times). The suspension was then centrifuged at 8000g for 20 minutes at 4°C, and the supernatant was used as the cytosolic fraction. The remaining pellet was suspended in ice cold nuclear extraction buffer, and the nuclei were disrupted with a fresh syringe with a 27-gauge needle (repeatedly drawing and ejecting approximately 5 times). The nuclear suspension was gently agitated at 4°C for 60 minutes on a rotator and then centrifuged at 16 000g for 5 minutes at 4°C. Finally, the supernatant was transferred to a fresh tube as the nuclear fraction.

Proteins were then analyzed for protein expression as previously described.¹² Briefly, 30 µg of the cellular proteins were resolved by electrophoresis in 10% SDS-polyacrylamide gel and subsequently transferred to a polyvinylidene difluoride membrane. The blots were incubated with primary antibody, washed, and incubated with horseradish peroxidase–conjugated secondary antibody. The activity of peroxidase on the blot was visualized by enhanced chemiluminescence (ECL) detection reagents (GE Healthcare, Sweden). The blots were acquired and quantified by a gel imaging system (Tanon, China). The concentration of the loaded cellular proteins was normalized against the internal control GAPDH or lamin B, and then the value was expressed as normalized data relative to control.

Luciferase Assay

The luciferase assay was performed as previously described.¹³ The reporter construct (NF-κB-Luc) used in this study contains a firefly luciferase gene under the control of a consensus NF-κB site and premixed with constitutively expressing Renilla luciferase vector, which serves as an internal control for transfection efficiency. MDA-MB-231 cells were seeded in 6-well plates, at a confluency of 70%; cells were transfected with the reporter plasmid by lipofectamine 2000 (Invitrogen) following the manufacturer’s instructions. After 24 hours of growth, cells were treated with curcumol (10, 20, and 40 µg/mL) for 24 hours. Luciferase assays were performed using the Dual-Luciferase Reporter Assay System (Beyotime, China) according to the manufacturer’s instructions. The firefly luciferase activity value was normalized to the Renilla activity value. Promoter activity was presented as a percentage of change compared with the vehicle-treated control.

Statistical Analysis

All data were presented as mean ± SD for 3 independent experiments. Statistical analysis was performed using the 2-tailed Student’s t-test. A P value of <.05 was considered to be statistically significant.

Results

Cytotoxicity of Curcumol to MDA-MB-231 and 4T1 Cells

In this study, we first examined the cytotoxicity of curcumol on breast cancer cell lines MDA-MB-231 and 4T1. Both cells were treated with various concentrations of curcumol (10-40 µg/mL) for 24 hours followed by the SRB assay. We found that curcumol showed no toxicity in MDA-MB-231 and 4T1 cells (Figures 2A and 2B). However, sustained treatment, over 14 days, with curcumol significantly suppressed the clonogenic activity and resulted in substantial cell death at 20 and 40 µg/mL in MDA-MB-231 cells and 40 µg/mL in 4T1 cells (Figures 2C and 2D). These results demonstrated that curcumol showed no cytotoxicity with a short duration of treatment, whereas it exhibited potent cytotoxicity after a relatively long duration of treatment.

Figure 2.

Inhibitory effects of curcumol on breast cancer cells. (A and B) Cytotoxicity of curcumol to MDA-MB-231 and 4T1 cells. Cells were treated with curcumol as indicated for 24 hours, and cell viability was determined by sulforhodamine B assay and expressed as a percentage of that of the control (CTRL; untreated) cells. (C and D) Colony formation assay in MDA-MB-231 and 4T1 cells. Cells (2.5 × 10² cells per well) were treated with curcumol as indicated for 14 days. Images were captured by a CCD camera. Values represent mean ± SD. ***P < .001 compared with control.

Inhibition of Migration, Invasion, and Adhesion by Curcumol in Breast Cancer Cells

To determine the effect of curcumol on the migration capability of MDA-MB-231 and 4T1 cells, a wound healing assay was first carried out. Both cells were exposed to various concentrations of curcumol (10-40 µg/mL) for 24 hours to exclude the influence of cytotoxicity caused by curcumol. Wound closure was monitored and photographed at 0, 4, 8, 12, and 24 hours. As representative fields shown in Figure 3A, after treatment for 8 hours, curcumol with 20 and 40 µg/mL markedly inhibited the flattening and spread of both cell lines along the edges of the wound compared with controls. Furthermore, a Boyden chamber transwell migration assay also showed that curcumol exhibited an inhibitory effect on the motility of MDA-MB-231 cells in a concentration-dependent manner after 24 hours of treatment (Figure 3B). The invasion assay was performed in highly invasive MDA-MB-231 cells using matrigel-coated 24-well Boyden chambers. As shown in Figure 3C, curcumol (10-40 µg/mL) significantly inhibited the invasion of MDA-MB-231 cells. Finally, we evaluated the effect of curcumol on cell adhesion. Curcumol also exhibited a concentration-dependent inhibition of adhesion in MDA-MB-231 cells onto the matrigel (Figure 3D). These results indicate that curcumol exhibited a significant inhibitory effect on the migration, invasion, and adhesion of breast cancer cells at noncytotoxic concentrations.

Figure 3.

Inhibition of the migration, invasion, and adhesion by curcumol in breast cancer cells. A. Ibidi wound healing assay: MDA-MB-231 and 4T1 cells were seeded on each side of an Ibidi culture insert overnight as described in the Materials and Methods section and then treated with curcumol for 24 hours, respectively. Cells were photographed following 6, 8, 12, and 24 hours of incubation. The representative images were captured at 8 hours after treatment. B. Boyden chamber transwell migration assay: MDA-MB-231 cells were pretreated with curcumol for 24 hours and then seeded in the chamber of a 24-transwell plate for another 24-hour migration assay as described in the Materials and Methods section. C. Boyden chamber transwell invasion assay: MDA-MB-231 cells were pretreated with curcumol for 24 hours and then seeded in the chamber with coated matrigel of a 24-transwell plate for another 24-hour invasion assay as described in the Materials and Methods section. D. Adhesion assay: MDA-MB-231 cells were preincubated with curcumol for 24 hours and seeded onto a 96-well plate coated with matrigel for 4 hours. Then, attached cells were stained and counted as described in the Materials and Methods section. Randomly chosen fields were obtained using an optical microscope (100× magnification). Values represent mean ± SD. **P < .01 and ***P < .001 compared with control (CTRL).

Curcumol Inhibited MMP-9, Activation of JNK1/2 and Akt, and Nuclear Translocation of NF-κB p65 in MDA-MB-231 Cells

Cancer metastasis is associated with increased expression of MMPs.^14,15 To determine whether the antimetastasis potential of curcumol was associated with its suppression on MMPs, gelatin zymography assay was performed in our study. As shown in Figure 4A, curcumol markedly reduced the gelatinolytic activity of MMP-9 in MDA-MB-231 cells, suggesting that the inhibitory effect of curcumol on the metastasis of breast cancer cells may be regulated by the inhibition of MMP-9 activity. To further determine whether this inhibitory effect resulted from an alteration in the MMP-9 expression, we determined its protein level by Western blotting after curcumol treatment. The result showed that curcumol (10-40 µg/mL) significantly reduced MMP-9 protein expression in MDA-MB-231 cells (Figure 4B). These results indicate that curcumol mediated metastasis inhibition of breast cancer cells by downregulation of MMP-9 expression and thereafter inhibition of MMP-9 activity.

Figure 4.

Curcumol inhibited gelatinolytic activity and protein expression of MMP-9, activation of JNK1/2 and Akt, and nuclear translocation of NF-κB in MDA-MB-231 cells. Cells were treated with curcumol for 24 hours. A. The curcumol-treated serum-free medium was subsequently assayed by gelatin zymography. B. The MMP-9 protein expression was detected by Western blotting. GAPDH was used as a housekeeping gene. C. The expression levels of p-JNK1/2, p-ERK1/2, p-p38, and p-Akt (Ser473) in total proteins were analyzed by Western blotting. GAPDH was used as a housekeeping gene. D. The expression of NF-κB in nuclear protein was analyzed by Western blotting. Lamin B was used as a nuclear envelope marker. The blots were a representative of 3 independent experiments. E. MDA-MB-231 cells were transfected with the NF-κB luciferase plasmids construct for 6 hours and then treated with curcumol. Bioluminescence was measured using the luciferase assay system. Values represent mean ± SD. **P < .01 and ***P < .001 compared with control.

Abbreviations: MMP, matrix metalloproteinase; JNK, Jun N-terminal kinase; NF, nuclear factor; p-ERK, phospho-extracellular signal-regulated kinase; GAPDH, glyceraldehyde-3-phosphate dehydrogenase.

To elucidate the mechanism underlying MMP-9 regulation by curcumol, the phosphorylated status of MAPKs (JNK1/2, ERK1/2 and p38) and Akt were investigated by Western blotting analysis. As show in Figure 4C, curcumol significantly inhibited the phosphorylation of JNK1/2 and Akt, indicating that curcumol could suppress the activation of JNK1/2 and Akt. In contrast, curcumol did not affect the phosphorylation of ERK1/2 and p38.

It is reported that transcription factor NF-κB, which is known to be downstream of MAPKs and Akt, translocates to the nucleus and regulates the expression of multiple genes involved in MMP secretion. To further understand the inhibitory mechanisms of curcumol on MMP-9 regulation, the NF-κB pathway was investigated using the luciferase assay and Western blotting. As shown in Figure 4D, curcumol significantly inhibited the nuclear translocation of NF-κB p65 in MDA-MB-231 cells. Furthermore, curcumol also inhibited NF-κB transcriptional activity at 20 and 40 µg/mL (Figure 4E). These results indicated that curcumol-induced metastatic behavior inhibition of breast cancer cells may be involved in MMP-9, JNK1/2, Akt, and NF-κB.

Inhibition of MMP-9 by Curcumol Involving JNK1/2 and Akt-Dependent NF-κB Pathways in MDA-MB-231 Cells

To study the role that JNK1/2, Akt, and NF-κB play in the inhibitory effect of curcumol on MMP-9 protein expression, further experiments were conducted by treating MDA-MB-231 cells with curcumol in the presence and absence of specific inhibitor of JNK (SP600125), Akt (LY294002), or NF-κB (PDTC). In Figure 5A, MMP-9 protein expression was significantly inhibited by curcumol. When combining SP600125, LY294002, or PDTC with curcumol, the MMP-9 protein expression was further inhibited. To elucidate whether JNK1/2 and Akt were the upstream signaling molecules of NF-κB activation, NF-κB p65 nuclear translocation was examined in MDA-MB-231 cells treated with curcumol in the presence and absence of SP600125 or LY294002. The results (Figure 5B) show that combining curcumol with SP600125 or LY294002 further reduced the NF-κB p65 nuclear translocation, indicating that both JNK1/2 and Akt are involved in the NF-κB regulation of curcumol.

Figure 5.

Curcumol inhibited MMP-9 involving the JNK1/2 and Akt-dependent NF-κB pathway in MDA-MB-231 cells. (A) Cells were pretreated with specific inhibitor of JNK (SP600125), Akt (LY294002), or NF-κB (PDTC) for 1 hour and then treated with or without curcumol (20 µg/mL) for 24 hours. The expression of MMP-9 was analyzed by Western blotting. GAPDH was used as a housekeeping gene. (B) Cells were pretreated with specific inhibitor of JNK (SP600125) or Akt (LY294002) for 1 hour and then treated with or without curcumol (20 µg/mL) for 24 hours. The expression of NF-κB in nuclear protein was analyzed by Western blotting. Lamin B was used as nuclear envelope marker. The blots were a representative of 3 independent experiments.

Abbreviations: PDTC, Ammonium pyrrolidinedithiocarbamate; MMP, matrix metalloproteinase; JNK, Jun N-terminal kinase; NF, nuclear factor; GAPDH, glyceraldehyde-3-phosphate dehydrogenase.

Curcumol Suppressed Breast Cancer Cell Metastasis Involving JNK1/2, Akt (Ser473), and NF-κB

To determine the role of JNK1/2, Akt, and NF-κB in the antimetastasis action of curcumol, the effects of curcumol on the migration, invasion, and adhesion of MDA-MB-231 cells were examined again in the presence and absence of specific inhibitors of JNK1/2, Akt, or NF-κB. The results revealed that single treatment with curcumol or SP600125, LY294002, and PDTC decreased cell migration (Figures 6A and 6B), invasion (Figure 6C), and adhesion (Figure 6D), respectively. Importantly, the combination of curcumol with SP600125, LY294002, or PDTC could further reduce cell migration (Figures 6A and 6B), invasion (Figure 6C), and adhesion (Figure 6D).

Figure 6.

Curcumol suppressed the migration, invasion, and adhesion of MDA-MB-231 cells involving JNK1/2, Akt, and NF-κB. Cells were pretreated with specific inhibitor of JNK (SP600125), Akt (LY294002), or NF-κB (PDTC) for 1 hour and then incubated in the presence or absence of curcumol (20 µg/mL) for 24 hours. Then, (A) ibidi wound healing assay, (B) Boyden chamber transwell migration assay, (C) Boyden chamber transwell invasion assay, and (D) adhesion assay were carried out as described in the Materials and Methods section. Values represent mean ± SD. *P < .05, **P < .01, and ***P < .001 compared with control (CTRL).

Abbreviations: PDTC, Ammonium pyrrolidinedithiocarbamate; MMP, matrix metalloproteinase; JNK, Jun N-terminal kinase; NF, nuclear factor.

Discussion

In women, about 23% of all diagnosed cases of cancer are breast cancer, which is the highest rate among all the cancers, accounting for about 7.6 million deaths worldwide each year.¹ Breast cancer is also one of the tumors with the poorest prognosis. The failure of treatment for breast cancer is often caused by lymph node metastasis and invasion to the neighboring organs. Moreover, approximately 80% of patients with breast cancer also have bone metastases developing at some point during the course of their disease.¹⁶ In addition, about 10% to 16% of breast cancer patients present with the problem of brain metastasis.¹⁷

Rhizoma Curcumae is traditionally used for the treatment of gynecological tumors in China. Curcumol, one of its major active components, has been reported to effectively inhibit the proliferation of breast cancer cells, including MCF-7, MDA-MB-231, and so on. However, its effects on the metastasis of breast cancer cells and the associated molecular mechanisms are still unknown. In this study, we first discovered that curcumol obviously reduced the migration of MDA-MB-231 and 4T1 cells in wound healing assay at noncytotoxic concentrations. Furthermore, we found that curcumol exhibited significant inhibitory effect on the Boyden chamber transwell migration and invasion and the adhesion of highly metastatic breast cancer cells, suggesting that curcumol has the potential for inhibiting metastasis of breast cancer cells.

Proteolysis of ECM is the necessary step for cancer metastasis. Increase of the expression and activity of proteolytic enzymes, such as MMPs, contributes to tumor cells invasion into the bloodstream or lymphatic system to spread to other organs.¹⁴ Therefore, the influence of curcumol on MMPs was investigated in our subsequent study. Our zymography assay demonstrated that curcumol reduced the zymography activity of MMP-9. Meanwhile, curcumol also decreased the protein expression of MMP-9. These results suggest that the antimetastasis activity of curcumol may be associated with the regulation of MMP-9.

MAPKs (JNK1/2, ERK1/2, and p38) play an important role in regulating the expression of MMPs.³ Inhibition of the MAPKs pathway exhibits the potential to prevent metastasis for a wide range of tumors.¹⁸ Our study revealed that curcumol inhibited the phosphorylation of JNK1/2. Additionally, the PI3K/Akt signaling pathway also plays a crucial role in MMPs regulation, cell survival, and tumor cell invasion.^8,19 Our study further demonstrated that curcumol also inhibited the phosphorylation of Akt. These findings indicated that suppression of the activation of JNK1/2 and Akt may be involved in the inhibition of metastasis and regulation of MMP-9 by curcumol.

The transcription of MMPs is regulated by upstream sequences, including motifs corresponding to NF-κB.²⁰ NF-κB is maintained in cytoplasm through the interaction with an inhibitor of NF-κB, but on dissociation, it moves into the nucleus and promotes cancer cell proliferation, angiogenesis, and metastasis.²¹ Previous studies also reported that NF-κB is the key downstream regulator of MAPKs and Akt in MMPs expression.^8,22 In our study, we found that curcumol exhibited a significantly inhibitory effect on NF-κB p65 nuclear translocation and transcriptional activity, suggesting that NF-κB may play an important role in the inhibition of metastasis and regulation of MMP-9 by curcumol. Similarly, NF-κB p65 protein was also observed to be diminished by curcumol in db/db mice.²³

Then, we further explored whether the inhibitory effect of curcumol on the MMP-9 and metastasis of breast cancer cells was correlated with inhibition of JNK1/2, Akt, and NF-κB. Our results demonstrated that specific inhibitor of JNK (SP600125), Akt (LY294002), or NF-κB (PDTC) significantly enhanced the inhibition of curcumol on MMP-9 and the migration, invasion, and adhesion of MDA-MB-231 cells. Meanwhile, specific inhibitor of JNK (SP600125) and Akt (LY294002) also enhanced the inhibition of curcumol on NF-κB nuclear translocation. These results suggest that the antimetastasis effects of curcumol in breast cancer cells may be related to MMP-9 inhibition via a JNK1/2 and Akt-dependent NF-κB pathway.

Although curcumol deactivated JNK1/2 and Akt in our study, leading to the suppression of the NF-κB signaling pathway, it remains unknown what is the upstream event by which curcumol suppresses the activity of JNK1/2 and Akt. Increasing data show that reactive oxygen species (ROS) may be involved in the cell death and metastasis of cancer cells. Moreover, ROS also play a part in signal transduction pathways as mediators. It is reported that ROS are tightly related to the modulation of MAPK and Akt pathways.^24,25 Meanwhile, curcumol also exhibited different effects on ROS in previous reports.^26,27 Although we do not provide direct evidence, the speculation is that curcumol’s suppression of JNK1/2 and Akt activity may be related to ROS, and more specific studies are needed to directly address this question.

Conclusion

In conclusion, curcumol suppressed the migration, invasion, and adhesion of breast cancer cells. A mechanistic study demonstrated that curcumol inhibited the zymography activity and expression of MMP-9. Moreover, curcumol suppressed the activation of JNK1/2 and Akt and the nuclear translocation and transcriptional activity of NF-κB in MDA-MB-231. Importantly, inhibition of MMP-9 via the JNK1/2 and Akt-dependent NF-κB pathways was further proved to play a critical role in the antimetastasis effect of curcumol.