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. Author manuscript; available in PMC: 2020 Jul 1.
Published in final edited form as: Mol Cancer Res. 2019 Oct 22;18(1):79–90. doi: 10.1158/1541-7786.MCR-19-0691

A TAZ-ANGPTL4-NOX2 axis regulates ferroptotic cell death and chemoresistance in epithelial ovarian cancer

Wen-Hsuan Yang 1,2,3, Zhiqing Huang 4,5, Jianli Wu 1,2, Chien-Kuang C Ding 1,2, Susan K Murphy 4,5, Jen-Tsan Chi 1,2,§
PMCID: PMC6942206  NIHMSID: NIHMS1541480  PMID: 31641008

Abstract

Ovarian cancer (OVCA) is the deadliest gynecologic cancer. Despite recent advances, clinical outcomes remain poor, necessitating novel therapeutic approaches. To investigate metabolic susceptibility, we performed nutrigenetic screens on a panel of clear-cell and serous OVCA cells and identified cystine addiction and vulnerability to ferroptosis, a novel form of regulated cell death. Our results may have therapeutic potential, but little is known about the determinants of ferroptosis susceptibility in OVCA. We found that vulnerability to ferroptosis in OVCA cells is enhanced by lower cell confluency. Since the Hippo pathway effectors YAP/TAZ are recognized as sensors of cell density, and TAZ is the predominant effector in the tested OVCA cell lines, we investigated the role of TAZ in ferroptosis of OVCA. TAZ removal confers ferroptosis resistance, while TAZS89A overexpression sensitizes cells to ferroptosis. In addition, we found that lower TAZ level in chemo-resistant recurrent OVCA is responsible for reduced ferroptosis susceptibility. The integrative genomic analysis identified ANGPTL4 as a direct TAZ-regulated target gene that sensitizes ferroptosis by activating NOX2. Collectively, cell density-regulated ferroptosis in OVCA is mediated by TAZ through the regulation of the ANGPTL4-NOX2 axis, suggesting therapeutic potentials for OVCAs and other TAZ-activated tumors.

Keywords: Ferroptosis, Ovarian Cancer, Erastin, Hippo pathway, WW Domain Containing Transcription Regulator 1 (TAZ), Angiopoietin-Like 4 (ANGPTL4), NADPH Oxidase 2 (NOX2)

Introduction

The impact of OVCA and the critical need for novel therapeutics

Epithelial ovarian cancer is the deadliest gynecologic cancer and claims the lives of approximately 150,000 women every year worldwide (1). The symptoms of ovarian cancer are vague and often attributed to other more common ailments. As a result, the correct diagnosis usually only occurs after cancer has spread beyond the ovaries. Standard therapy involves surgical debulking followed by chemotherapy with a platinum-taxane doublet (2). While many patients initially respond favorably to this combined treatment, most patients relapse with a recurrent disease that is often resistant to platinum and taxane drugs. Other chemotherapeutic options are used mainly in an effort to prolong survival. Recently, platinum-PARP inhibitor combinations have been proven to be beneficial for OVCA regardless of the BRCA1/2 mutation status (3,4). However, the outcomes for most women with OVCA are still unsatisfactory, therefore, novel therapeutic options are still urgently needed.

Ferroptosis as a novel cell death involving lipid peroxidation

One possible therapeutic approach is the induction of ferroptosis, a novel and distinct form of iron-dependent programmed cell death (5,6). Ferroptosis sensitivity is found to be affected by various biological processes, such as loss of p53 (7), DNA damage pathway (8), metabolisms (9–11), or epithelial-mesenchymal transition (EMT) (12,13), which are often dysregulated in OVCA. Ferroptosis can be induced by the small molecule, erastin (14), that reduces cystine import and result in a redox imbalance by reducing intracellular glutathione levels. Glutathione is a cofactor for glutathione peroxidase (GPX4), an enzyme that resolves the accumulation of lipid-based reactive oxygen species (ROS). Therefore, ferroptosis and lipid peroxidation can also be induced by chemical or genetic inhibition of GPX4(15). A previous study has indicated that the levels of GPX4, regulated by the EMT-activator ZEB1, may dictate ferroptosis sensitivity of drug-resistant cancer cells, implicating GPX4 as a major determinant of ferroptosis (12,13). On the other hand, accumulation of lipid-based ROS and ferroptosis can also be induced by the generation of superoxide and hydrogen peroxide upon upregulation of NADPH oxidases (NOXs) (5).

In our current study, we perform a nutrigenetic screen and show that most OVCA cell lines are addicted to cystine and sensitive to ferroptosis. Furthermore, we found that ferroptosis susceptibility of OVCA cells is affected by cell density. Low density, but not high density OVCA cells, were highly susceptible to erastin-induced ferroptosis. The density-dependent phenotypes of cancer cells are sensed and regulated by the evolutionarily conserved Hippo pathway (16) converging into two transcriptional co-activators, YAP (Yes-associated protein 1) and TAZ (transcriptional coactivator with PDZ-binding motif). YAP/TAZ activities are regulated by their phosphorylation and intracellular localization. When grown at high cell density, YAP/TAZ are phosphorylated, retained in the cytosol, and subjected to proteasomal degradation. Upon shifting to low cell density, YAP/TAZ become dephosphorylated and translocate into the nucleus to associate with TEAD transcriptional factors to drive gene expression regulating cell proliferation, differentiation, and migration (17). Recent studies have also identified the novel role of YAP and TAZ in regulating ferroptosis (18,19). However, the relevance of these findings for OVCA remains unknown. Here, we have established the role of cell density and TAZ in regulating ferroptosis of OVCA. In addition, we found that TAZ regulates erastin-induced ferroptosis through the induction of ANGPTL4, which in turn activates NOX2, resulting in ferroptosis. Thus, these data support the role of TAZ in regulating ferroptosis through ANGPTL4-NOX2 and that inducing ferroptosis may be a novel therapeutic strategy for OVCA and other TAZ-activated tumors.

Materials and Methods

Materials and reagents

Erastin was obtained from the Duke University Small Molecule Synthesis Facility. The following antibodies, their catalog numbers, sources and diltuionswere indicated below: YAP/TAZ (#8418, Cell Signaling Technology, 1:1000), βa-tubulin (#86298, Cell Signaling Technology, 1:2000), vinculin (sc-73614, Santa Cruz, 1:2000), V5 tag (#13202, Cell Signaling Technology, 1:2000), H3 (#4499, Cell Signaling Technology, 1:2000), GAPDH (sc-25778, Santa Cruz, 1:2000), ANGPTL4 (#40–9800, ThermoFisher Scientific, 1:1000), NOX2 (sc-130543, Santa Cruz, 1:1000), anti-rabbit IgG, horseradish peroxidase (HRP)-linked antibody (#7074, Cell Signaling Technology, 1:2000–1:4000) and anti-mouse IgG, HRP-linked Antibody (#7072, Cell Signaling Technology, 1:2000–1:4000). Plasmids were obtained from Addgene (TAZS89A #52084; ANGPTL4-V5 #102446). The NOX2 inhibitor, gp91 ds-tat, was purchased from Eurogentec (cat #: AS-63818) and recombinant human ANGPTL4 protein was purchased from Novus Biologicals (4487-AN). VAS2870 (Calbiochem-492000), GKT136901 (Calbiochem-534032), Z-VAD-FMK (SML0583), ferrostatin-1 (SML0583) and liproxstatin-1 (SML1414) were purchased from Sigma.

Cell culture and transfection

Cell lines are maintained as part of the cell line repository within the Division of Reproductive Sciences at Duke University. For the details, please refer to (18). STR profiling was performed at the Duke University DNA Analysis Facility each time when frozen stocks are prepared; they were last genotyped on 6/5/18. Mycoplasma testing were conducted at the Cell Culture Facility, Duke Universityon 6/5/18. For nutrigenetic screens, all OVCA cells were cultured in a humidified incubator at 37°C and 5% CO2 using custom-made Dulbecco’s Modified Eagle Medium (11995-DMEM, ThermoFisher Scientific) with 10% heat-inactivated and dialyzed Fetal Bovine Serum (HyCloneTM FBS, GE Healthcare Life Sciences #SH30070.03HI) with the indicated amino acid removed as previously described (20,21). Transfections were performed according to the manufacturer’s instructions with TransIT-LT1 transfection reagent (Mirus Bio) or RNAiMax transfection reagent (ThermoFisher Scientific).

siRNA-mediated gene knockdown

All human siRNAs were purchased from Dharmacon or Qiagen: Non-targeting control, siNT (Qiagen SI03650318); siTAZ (Dharmacon M-016083); siANGPTL4 (Dharmacon M-007807); siNOX2 (Dharmacon M-011021); siTAZ#1 (target sequence: AGA CAT GAG ATC CAT CAC TAA); siTAZ#2 (target sequence: ACA GTA GTA CCA AAT GCT TTA); siANGPTL4 #1 (target sequence: CTG CGA ATT CAG CAT CTG CAA); siANGPTL4 #2 (target sequence: CAC CAT GTT GAT CCA GCC CAT); siNOX2 #1 (target sequence: GAA GAC AAC TGG ACA GGA A); siNOX2 #2 (target sequence: GAA ACT ACC TAA GAT AGC G). Knockdown efficacy was validated by RT-qPCR and/or western blots.

RNA isolation and quantitative real-time PCR

Total RNAs from cultured cells were extracted using the RNeasy Mini Kit (Qiagen #74104) with DNase I treatment (Qiagen #79254). cDNAs were synthesized from 1μg of total RNA using SuperScript™ II Reverse Transcriptase (ThermoFisher Scientific #18064) with random hexamers following protocols from the manufacturer. The levels of gene expression were measured by quantitative PCR (qPCR) with Power SYBR Green PCR Mix (Applied Biosystems, ThermoFisher Scientific #4367659). Primers used included (listed 5’ to 3’): β-actin-F’: GGG GTG TTG AAG GTC TCA AA; β-actin-R’: GGC ATC CTC ACC CTG AAG TA; TAZ-F’: TGC TAC AGT GTC CCC ACA AC; TAZ-R’: GAA ACG GGT CTG TTG GGG AT; ANGPTL4-F’: GGC TCA GTG GAC TTC AAC CG; ANGPTL4-R’: CCG TGA TGC TAT GCA CCT TCT; NOX2-F’: TGG AGT TGT CAT CAC GCT GTG; NOX2-R’: CTG CCC ACG TAC AAT TCG TTC; 18S-F’: CTG GAT ACC GCA GCT AGG AA; 18S-R’: CCC TCT TAA TCA TGG CCT CA.

Western blot analysis

For immunoblotting, please refer to previous publication for details (18). In short, cells were collected and quantified by BCA protein assay. After separating by SDS-PAGE, the proteins were transferred to PVDF membranes that were further blocked with 5% non-fat milk or BSA and probed with indicated antibodies following by HRP-conjugated secondary antibodies. The signals were developed and detected by Amersham ECL prime western blotting detection reagent and the Bio-Rad ChemiDoc™ Imaging System.

Cell viability and cytotoxicity assays

After seeded and transfected with indicated siRNAs for two days, cells (~60–70% confluence) were treated with erastin or cystine deprivation for another 24–48 hours. Cell viability was evaluated using crystal violet staining or the CelltiterGlo luminescent cell viability assay kit (Promega G7571) according to the manufacturer’s instructions. The CelltiterGlo luminescent cell viability assay is based on quantitation of the cellular ATP levels as an indicator of metabolically active cells and cellular viability. The cytotoxicity and cell death of treated cells were determined by CytoTox-Fluor™ Cytotoxicity Assay (Promega G9260) which measured the released DNA as indicator of cell death according to the manufacturer’s instructions.

Enzyme-linked immunosorbent assay (ELISA)

Quantification of secreted ANGPTL4 in the culture media from cells following a two-day incubation was performed by Human ANGPTL4 ELISA kit (RAB0017, Sigma Aldrich) according to the manufacturer’s instruction.

ChIP analysis

ChIP-qPCR experiment was carried out according to the Myers Lab ChIP-seq protocol (22). Please refer to the previous publication for the experimental details(18). Primers targeting chromosome 10 was used as a negative control for potential non-specific binding (23). Primer sequences are as follows (listed 5’ to 3’): CTGF-F’: GCC AAT GAG CTG AAT GGA GT; CTGF-R’: CAA TCC GGT GTG AGT TGA TG; ANGPTL4-F’: GTC TCC CAC GGT TCG TAG AG; ANGPTL4-R’: TAT AAG TTG GGT GCG GAG TGG; Ch10-F’: ACC AAC ACT CTT CCC TCA GC; Ch10-R’: TTA TTT TGG TTC AGG TGG TTG A.

Lipid ROS assay using flow cytometry

10 μM of C11-BODIPY dye (D3861, ThermoFisher Scientific) was used for lipid ROS staining according to the manufacturer’s instructions. Please refer to the previous publication for the experimental details (18). In short, cells were collected after siRNAs treatments for two days, 10 μM erastin treatment for 18 hours, and 10 μM C11-BODIPY-containing medium for 1 hour and the ROS levels were determined by flow cytometry analysis (FACSCanto™ II, BD Biosciences).

Statistical Analyses

The results were evaluated with two-tailed Student’s t-test or ANOVA (one- or two-way) using GraphPad Prism version 8.0.1 (GraphPad Software). Statistically significant differences were set to P < 0.05 between experimental groups (*< 0.05; **<0.01; ***<0.001). The statistical data were represented as mean ± SEM. The number of biological replicates is listed in each figure.

Data availability

RNA-seq for transcriptional profiles of CAOV2 primary and CAOV2 recurrent cells (GSE133663) and CAOV2 ovarian cancer cells with TAZ silencing (GSE133664) have been deposited in the NCBI Genome Expression Omnibus (GEO) under SuperSeries GSE 133673.

Results

Ovarian cancer cells are sensitive to cystine deprivation

Many tumor cells have dysregulated metabolisms that render them addicted to certain nutrients, such as amino acids (24). To reveal such nutrient addiction in OVCAs, we established a nutrigenetic screen by removing glucose or each of the 15 individual amino acids usually included in Dulbecco’s Modified Eagle Medium (25,26). The nutrigenetic screen was applied to a panel of eight OVCA cell lines including four cell lines of clear-cell subtype (TOV-21G, ES-2, RMG-2, RMG-V) and four cell lines of serous subtype (OVCA432, OVCA429, OVCA420, 41M). The screen revealed that the removal of cystine, the dimeric form of cysteine, dramatically reduced the viability of most clear-cell and serous subtypes of OVCA cells (Figure 1A). Glutamine or glucose deprivation also decreases cell viability, especially in the clear-cell subtype, reminiscent of the glutamine addiction of basal-type breast cancer cells (27). Knowing that cystine deprivation is reported to trigger ferroptosis (5), we determined whether ferroptosis inhibitors can rescue the cell death triggered by cystine deprivation. Both liproxstatin-1 and ferrostatin-1, two ferroptosis inhibitors, significantly rescued cystine deprivation-induced cell death of TOV-21G cells (Figure 1B–C), indicating that cystine deprivation indeed induces ferroptosis. We further confirm that the erastin-induced cell death can be rescued by ferrostatin-1, but not apoptosis inhibitor, Z-VAD-FMK (Figure 1D). Therefore, we used erastin, a canonical inducer of ferroptosis, to investigate the ferroptosis mechanisms of OVCA in our subsequent studies.

Figure 1. Ovarian cancer cells are sensitive to cystine deprivation.

Figure 1.

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(A) Normalized cell viability of the indicated ovarian cancer cells after deprivation of individual amino acids or glucose. Error bars represent mean ± SEM (n=3, biological replicates).

(B-C) Two ferroptosis inhibitors rescued cystine deprivation-induced cell death. TOV-21G cells were seeded in 10% (20 μM of cystine) or 20% (40 μM of cystine) of full DMEM media supplied with or without either 200 nM liproxstatin-1 (B) or 2 μM ferrostatin-1 (C). After 2 days, the cell viability was determined by CelltiterGlo and normalized to the signal of full DMEM media (200 μM of cystine) of each group. (n=3 per group; mean ± SEM; two-way ANOVA).

(D) Cell viability of CAOV2 was determined by CelltiterGlo after cells were treated with 16 μM erastin, 20μM Z-VAD-FMK (Z-VAD), or 1μM ferrostatin-1 (Fer-1).

Cell density affects the sensitivity of OVCAs to erastin-induced ferroptosis

While investigating ferroptosis in OVCA cells, we observed that cell density affects the ferroptosis sensitivity of CAOV2, a serous ovarian cancer cell line. When CAOV2 cells were plated at low density, they were highly sensitive to erastin based on CelltiterGlo assay (Figure 2A) and crystal violet staining (Figure 2B and S1A). On the other hand, when plated at higher densities, CAOV2 cells were much less sensitive to erastin-induced death using CelltiterGlo and crystal violet assays (Figure 2A–B and S1A). To exclude the possibility that these differences are caused by different levels of available erastin per cell, the same number of TOV-21G cells were seeded in larger or smaller areas to recreate lower or higher cell densities. We found that cells plated at lower cell density were consistently more sensitive to ferroptosis (Figure S1B). Therefore, cell density impacts ferroptosis sensitivity.

Figure 2. TAZ regulates the sensitivity of CAOV2 to erastin-induced ferroptosis.

Figure 2.

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(A) The relative cell viability of CAOV2 cells grown at low/medium/high (4000/8000/16000 cells per 96 well) cell densities after treated with indicated concentrations of erastin. Data are represented as mean ± SEM with three biological replicates per group; two-way ANOVA; ***p < 0.001.

(B) Crystal violet staining of CAOV2 cultured at low/medium/high (1.6×105/3.2×105/6.4×105 cells per 6 well) cell densities and treated with 16 μM erastin.

(C) Western blot analysis of YAP and TAZ proteins in the ovarian cancer cells (TOV-21G, CAOV2, and CAOV2R) and one breast cancer cell line (MDA-MB-231). Representative data of at least three independent experiments.

(D) Western blot measuring YAP and TAZ protein levels in the cytosolic (Cyto) /nucleic (Nuc) fractions of CAOV2 extracts when grown at low (L) or high (H) cell density. Histone H3 and GAPDH serve as nuclear and cytosolic marker, respectively.

(E) The relative cell viability of CAOV2 cells after silencing of control (siNT), TAZ (siTAZ), or two individual TAZ-targeting siRNAs (siTAZ #1 and siTAZ #2) for 1 day before they were treated with indicated dosages of erastin for 2 days. (n=2; mean ± SEM; two-way ANOVA; ***p < 0.001).

(F) The relative cell viability of TOV-21G cells after silencing of control (siNT) or TAZ (siTAZ) for 2 days before they were treated by indicated dosages of erastin treatment for 1 day. (n=3; mean ± SEM; two-way ANOVA; ***p < 0.001).

See also Figure S1.

Since the two closely related Hippo pathway paralogues, YAP/TAZ, sense and mediate cell density-dependent responses (16), we investigated the potential role of YAP/TAZ in regulating ferroptosis in OVCAs. Among the two paralogues (YAP and TAZ), western blot reveals that TAZ is the predominantly expressed protein in both TOV-21G (clear-cell subtype) and CAOV2 cells (serous subtype) (Figure 2C). In contrast, YAP is the predominant Hippo effector protein in the breast cancer cell, MDA-MB-231, which serves as a control for YAP expression and detection (Figure 2C). We further performed cytosolic and nuclear fractionations of CAOV2 cells under low or high cell densities. Consistently, TAZ is the main protein whose nuclear levels were elevated when grown in low cell density (Figure 2D). Therefore, we focused on the potential role of TAZ as the major effector of the Hippo pathway in regulating density-dependent ferroptosis in OVCAs.

TAZ regulates sensitivity to erastin-induced ferroptosis

To test the possibility that TAZ regulates susceptibility of OVCA cells to erastin, we first reduced TAZ expression (>85% knockdown) by siRNAs in CAOV2 cells (Figures S1C–D). Knockdown of TAZ by pool siRNA as well as two more independent siRNAs all reduce erastin-induced cell death based on CelltiterGlo assays (Figure 2E). To determine if TAZ also regulates ferroptosis in TOV-21G cells, we found that TAZ knockdown by siRNAs reduces sensitivity to erastin (Figures 2F and S1E). Collectively, these data strongly indicate that TAZ regulates the sensitivity of OVCA cells to erastin-induced ferroptosis.

Resistance to ferroptosis following treatment with carboplatin in vivo

We performed our experiments in CAOV2 because we have established paired CAOV2 xenograft cells from a mouse model of ovarian cancer that mimics the course of disease (28). In short, the CAOV2 cells were stably transduced with a construct containing GFP and luciferase (pGreenFire plasmid constructs, SBI) to generate CAOV2-GFP/LUC cells, allowing us to monitor tumor growth using in vivo bioluminescence imaging. CAOV2-GFP/LUC cells (3.5×105 per mouse) were injected intraperitoneally into female nude mice with tumor formation monitored weekly using the IVIS® in vivo imaging system. Once tumor formation was evident, we initiated treatment with carboplatin (60mg/kg intraperitoneally twice a week for two weeks) and monitored tumor volume based on the luciferase flux. In this model, tumor signal was reduced following carboplatin treatment, but then recurred approximately 2–4 weeks after carboplatin treatment had stopped. Therefore, we refer to the residual tumor cells that eventually emerged after carboplatin treatment as CAOV2R (recurrent). We then compared cystine dependency (Figure 3A–B) and erastin sensitivity (Figures 3C–D) between the CAOV2 and CAOV2R cells. These experiments revealed that CAOV2R was more resistant to ferroptosis based on crystal violet staining (Figure 3A, C), cytotoxicity assay (Figure 3B), and CelltiterGlo viability assay (Figure 3D). Since TAZ regulates ferroptosis sensitivity (Fig 2), we compared TAZ protein abundance in CAOV2 and CAOV2R cells and found that CAOV2R contained a lower level of TAZ protein (Figure 3E). We, therefore, overexpressed the constitutively active TAZS89A (29) in the CAOV2R cells (Figure 3F) and found that increased TAZ significantly sensitized the CAOV2R cells to erastin (Figure 3G). These findings suggest that comparisons of CAOV2 vs. CAOV2R can be used to elucidate the molecular mechanisms of TAZ-regulated ferroptosis.

Figure 3. Regrowth of chemo-residual tumor cells are more resistant to ferroptosis.

Figure 3.

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(A) Crystal violet staining of CAOV2 and CAOV2R cells when culture with full or cystine-deprived (Cys−) media. Representative data of at least three independent experiments.

(B) Cytotoxicity was determined by CytoTox-Fluor assay after treatment of 10% (20 μM of cystine) of full media (n=3; two-way ANOVA; ***p < 0.001; ns: not significant).

(C) Crystal violet staining of CAOV2 and CAOV2R cells when cultured with DMSO or 15μM erastin. Representative data of at least three independent experiments.

(D) The relative cell viability of CAOV2 and CAOV2R cells when they were treated with the indicated dosage of erastin (n=3; mean ± SEM; two-way ANOVA; ***p < 0.001).

(E) Western blot analysis of TAZ and Vinculin (control) protein levels in CAOV2 and CAOV2R cells. Representative data of at least three independent experiments.

(F) Validation of TAZS89A overexpression by Western blots in CAOV2R cells. Representative data of at least three independent experiments.

(G) The relative cell viability of CAOV2R cells after the transfection of control vector or constitutively active TAZ, TAZS89A, following by treating with indicated dosages of erastin. Data are represented as mean±SEM, n=3 biological replicates; two-way ANOVA; ***p < 0.001.

ANGPTL4 is a direct target gene of TAZ that regulates sensitivity to ferroptosis

TAZ is a transcriptional coactivator that affects cellular phenotypes through regulating the expression of target genes upon association with transcriptional factors such as TEAD. Assuming that knockdown of TAZ may repress the relevant target genes essential for ferroptosis, we determined the transcriptional response to knockdown of TAZ in CAOV2 by RNA-seq (Figure S2A; GSE133673). Next, we integrated these TAZ-affected genes with the genes that were downregulated in CAOV2R when compared to CAOV2 (Figure 4A) to identify 1179 candidate genes. To identify the TAZ-regulated genes that are essential for ferroptosis across different cells, we further narrowed the list of candidate genes by comparing to the gene lists identified from our renal cell studies through both RNAi screen and TAZ-affected genes (18) (Figure S2B). Among the three candidate genes from these analyses, ATP6V1B2 was not pursued because it was only one subunit of a multi-component protein complex. The knockdown of GPSM3 did not confer erastin resistance (Figure S2C–D). Therefore, ANGPTL4 (Angiopoietin-Like 4) emerged as the most promising candidate gene based on the following evidence. First, ANGPTL4 mRNA was down-regulated upon TAZ knockdown, which was confirmed by RT-qPCR (Figure 4B). Second, there is a significant correlation between the expression of ANGPTL4 and TAZ (encoded by WWTR1) in the TCGA ovarian tumor dataset. The expression of WWTR1 is also consistently correlated with CTGF (30) and CYR61 (31), two canonical YAP/TAZ target genes (Figures S2E–G). Third, the knockdown of ANGPTL4 in both CAOV2 and TOV-21G protects cells from erastin-induced death (Figures 4C–D and S2H–I). This protective effect was further validated by using two more independent ANGPTL4-targeting siRNAs in CAOV2 cells (Figure 4E and S2J). Consistently, overexpression of V5-tag ANGPTL4 sensitized TOV-21G cells to ferroptosis (Figures 4F and S2K). Collectively, these data indicate that ANGPTL4 is a prominent TAZ-regulated determinant of ferroptosis in OVCA.

Figure 4. ANGPTL4 is a direct target gene of TAZ that regulates ferroptosis sensitivity.

Figure 4.

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(A) Venn diagram showing genes that are both downregulated upon TAZ knockdown in CAOV2 cells (3370 +1179 genes) and downregulated genes (2540 +1179 genes) in CAOV2R cells compared to CAOV2 cells by RNA-seq.

(B) Validation of downregulated ANGPTL4 mRNA level by RT-qPCR upon TAZ knockdown (n=3; mean ± SEM; Student’s t-test; *p < 0.05).

(C-D) The relative cell viability of CAOV2 (C) and TOV-21G (D) cells after knockdown of ANGPTL4 following by treating with indicated dosages of erastin. Data are represented as mean±SEM, n=3 biological replicates, ***p < 0.001.

(E) The relative cell viability of CAOV2 cells after silencing of ANGPTL4 by two individual siRNAs for 2 days before treated with indicated dosages of erastin for 1 day. Data are represented as mean±SEM, n=3 biological replicates, ***p < 0.001.

(F) The relative cell viability of TOV-21G cells after overexpression of V5-tagged ANGPTL4, followed by treating indicated dosages of erastin. Data are represented as mean±SEM, n=3 biological replicates, ***p < 0.001.

(G) The relative levels of TAZ-associated genomic DNA in the indicated promoters associated with endogenous TAZ protein using ChIP-qPCR with TAZ antibody in CAOV2 protein lysate. CTGF promoter serves as a positive control for a TAZ target gene, while Ch10 serves as a negative control.

See also Figure S2.

Next, we analyzed previous ChIP-seq studies and found the regulatory regions of ANGPTL4 were physically associated with YAP/TAZ/TEAD complexes (23,32). To further validate that ANGPTL4 is a direct target gene of TAZ, we performed ChIP-qPCR using an antibody specific for endogenous TAZ protein. As shown in Figure 4G, we found that the promoter region of ANGPTL4, similar to the CTGF (positive control), was enriched in the TAZ pull-down, indicating that the ANGPTL4 promoter is associated with TAZ. Thus, ANGPTL4 is a direct downstream target gene of TAZ that may contribute to TAZ-regulated ferroptosis.

Differential ANGPTL4 expression contributes to the distinct ferroptosis sensitivities among CAOV2 and CAOV2R cells

We next determined whether the differential expression of ANGPTL4 could explain the different ferroptosis sensitivity between CAOV2 and CAOV2R cells. From the results of RT-qPCR and western blot, we found CAOV2R has a lower ANGPTL4 expression at both mRNA level and protein level (Figures 5A–B). Since ANGPTL4 encodes a secreted protein, we also examined the level of extracellular ANGPTL4 in the culture media by enzyme-linked immunosorbent assay (ELISA). Consistently, extracellular ANGPTL4 proteins are less abundant in the CAOV2R cells as compared to the CAOV2 cells (Figure 5C). To test the possibility that the lower expression of ANGPTL4 in CAOV2R may contribute to its relative ferroptosis resistance, we determined whether the addition of recombinant ANGPTL4 proteins may enhance the ferroptosis sensitivity of CAOV2R. As shown in Figure 5D, soluble ANGPTL4 sensitized CAOV2R, but not CAOV2, cells to ferroptosis.

Figure 5. Differential ANGPTL4 expression contributes to the different ferroptosis sensitivities among CAOV2 and CAOV2R cells.

Figure 5.

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(A) The relative levels of ANGPTL4 mRNA were measured by RT-qPCR in CAOV2 and CAOV2R cells (n=3; mean ± SEM; Student’s t-test; *p < 0.05).

(B) Western blot of ANGPTL4 and β-tubulin proteins in CAOV2 and CAOV2R cells. Representative data of at least three independent experiments.

(C) ELISA of extracellular ANGPTL4 proteins in the culture media of CAOV2 and CAOV2R cells incubated for 2 days (n=4, mean ± SEM; Student’s t-test; *p < 0.05).

(D) Relative cell viability of CAOV2 and CAOV2R cells when treated with recombinant ANGPTL4 protein in media followed by 8 μM or 16 μM erastin and then normalized to DMSO control group (n=3, mean ± SEM; two-way ANOVA; *p < 0.05; **p < 0.01).

ANGPTL4 regulates ferroptosis through NOX2

With regard to the mechanistic link between ANGPTL4 and ferroptosis, it is interesting to note that ANGPTL4 has been reported to activate the NADPH oxidase 1, NOX1, in the keratinocyte carcinoma cells (33). The NADPH oxidases are recognized to generate ROS (34), promoting lipid peroxidation and ferroptosis (5). Therefore, we investigated the expression levels of all seven members of the NOX protein family (NOX1–5 and DUOX1–2) in OVCA. Among different NOXs, we found that NOX2 is the most abundantly expressed member from the analysis of RNA-seq data from the TCGA dataset (Figure S3A). We also noticed NOX2 protein was lower in CAOV2R when compared to CAOV2 cells (Figure S3B). Therefore, we focused on the role of NOX2 protein. First, we found a pan NOX inhibitor, VAS2870, protected CAOV2 cells from ferroptosis (Figure 6A). Another NOX inhibitor, GKT136901, also protected cells from ferroptosis (Figure 6B). We further used a NOX2 specific inhibitor, gp91dstat peptide, in the CAOV2 cells and found it protected ferroptosis as well (Figure 6C). In addition, silencing of NOX2 by siRNA also conferred ferroptosis resistance in both CAOV2 cells (Figures 6D and S3C) and TOV-21G cells (Figures 6E and S3D). To examine the relevance of TAZ-ANGPTL4-NOX2 for the varying ferroptosis sensitivity between CAOV2 and CAOV2R, we found that gp91dstat protected CAOV2, but not CAOV2R, from ferroptosis (Figure 6F). Furthermore, NOX inhibitor GKT136901 also abolishes the ferroptosis-sensitizing effect when TAZS89A was overexpressed in CAOV2R cells (Figure 6G). Therefore, NOX2 activity is essential for ferroptosis regulation by TAZ. Finally, we measured the lipid-based reactive oxygen species (lipid ROS), the hallmark of ferroptosis, by C11-BODIPY staining and validated that knockdown of TAZ, ANGPTL4, or NOX2 decreases the erastin-induced lipid peroxidation (Figure 7A). Taken together, we propose a signaling mechanism (Figure 7B) by which TAZ is a cell-density-dependent determinant of ferroptosis sensitivity in OVCA through regulating levels of ANGPTL4, which in turn regulates NOX2 activity and ferroptotic death.

Figure 6. ANGPTL4 regulates ferroptosis through NOX2.

Figure 6.

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(A-E) The relative cell viability of erastin-treated CAOV2 cells (A-D) or TOV-21G cells (E) when NOXs was inhibited by (A) 20 μM pan-NOX inhibitor VAS2870; (B) 20 μM NOX inhibitor, GKT136901; (C) specific NOX2 inhibitor, gp91dstat (33 μg/ml); or (D-E) pooled siRNAs (siNOX2) and/or two independent NOX2-targeting siRNAs (siNOX2 #1 and #2). n=3, mean ± SEM; two-way ANOVA; ***p < 0.001.

(F) The relative cell viability of CAOV2 or CAOV2R cells was determined by CelltiterGlo after 24 hours of 8 μM erastin with or without NOX2 inhibitor, gp91dstat (33 μg/ml). Data are represented as mean±SEM, n=3 after normalized to the DMSO controls (two-way ANOVA; *p < 0.05; **p < 0.01; ***p < 0.001; ns: not significant).

(G) The relative cell viability of CAOV2R expressing control vector or TAZS89A was determined by CelltiterGlo after 24 hours of 10 μM erastin with or without 20 μM NOX inhibitor, GKT136901. Data are represented as mean±SEM, n=6 after normalized to the DMSO controls (two-way ANOVA; ***p < 0.001).

See also Figure S3.

Figure 7. The proposed molecular mechanism on how TAZ-regulated ferroptosis sensitivity through ANGPTL4-NOX2 axis in ovarian cancer cells.

Figure 7.

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(A) Inhibition of TAZ, ANGPTL4, or NOX2 abolishes the elevated lipid-ROS induced by erastin treatment. Lipid ROS in CAOV2 cells was assessed by flow cytometry using C11-BODIPY after transfected with siTAZ, siANGPTL4 or siNOX2. Representative data from one out of three independent experiments are shown.

(B) Schematic representing the model of TAZ-regulated ferroptosis of OVCA through ANGPTL4-NOX2.

Discussion

Here, we employed nutrigenetic screens and identified cystine addiction and ferroptosis susceptibility of OVCA cells, implying that ferroptosis-inducing agents may hold therapeutic potential for OVCA. However, little is known about ferroptosis in OVCA other than being briefly mentioned in two recent papers (35,36) without mechanistic investigation. By studying how cell density regulates ferroptosis sensitivity, as described for other cancer types, we have elucidated the role of TAZ in regulating ferroptosis through the ANGPTL4-NOX2 axis in OVCA. This study enhances our understanding of the role of TAZ and how it regulates ferroptosis in OVCA.

YAP/TAZ and other components of the Hippo signaling pathway are important in oncogenesis and migration of OVCA cells (37). In our experiments, TAZ, rather than YAP, is the dominant effector in the tested OVCA cells. While the role of YAP was the first recognized, many studies have also supported the role of TAZ in OVCA. For example, increased expression of TAZ mRNA is correlated with poor prognosis and TAZ manipulation affects migration, proliferation, treatment response, and EMT of OVCA (38,39). Therefore, our findings suggest that inducing ferroptosis will be an effective strategy for eradicating TAZ-activated tumors which are particularly aggressive and resistant to current standard treatments. In the future, it will be interesting to test whether YAP or other regulators of the Hippo pathway also play a role in regulating ferroptosis.

ANGPTL4 is a member of the angiopoietin family and the members of which act as regulators of lipid and glucose metabolism (40,41). ANGPTL4 also plays a role in tumor biology. ANGPTL4 is upregulated in several human cancers and this is associated with metastasis and poor outcome (42,43). In addition, ANGPTL4 is induced by various oncogenic pathways to promote angiogenesis, invasion, and metastasis (44). Especially relevant is a study that ANGPTL4 stimulates oncogenic ROS and anoikis resistance through the activation of NADPH oxidase (33). Our current study reveals that ANGPTL4 is a direct transcriptional target of TAZ, consistent with the repression of ANGPTL4 by the TAZ/YAP inhibitor, verteporfin (45). Based on the previous studies, ANGPTL4 is expected to increase ROS and predispose cells to signal that induces ferroptosis. Therefore, our findings implicate high ANGPTL4 levels and anoikis resistance as novel and highly relevant oncogenic properties that can be targeted by inducing ferroptosis. Since anoikis resistance is essential for tumor metastasis (46), our results imply that ferroptosis may target tumor cells at different stages of tumor progression. Anoikis resistance is one of the phenotypic changes that occur during EMT (47,48). Our findings are also in agreement with other studies showing increased ferroptosis sensitivity during EMT that is in part due to the regulation of GPX4 by ZEB1 (12).

We have found that the carboplatin-treated CAOV2R cells are less sensitive to ferroptosis and have a lower level of TAZ. These results seemingly contradict previous reports on the ferroptosis sensitivity of persister cells (12) due to increased GPX4 expression. However, our results are consistent with the supplemental data in (14) that show cells re-growing at two-months, similar to the timeframe of carboplatin-treated CAOV2R cells in our studies, are much less responsive to GPX4 inhibitor RSL3-induced cell death. Of course, the discrepancy between results may also be due to the different cell lines used, different cancer drugs or different ferroptosis inducers in these studies.

Our results may have therapeutic implications. While inducing ferroptosis may have substantial anti-tumor potential, challenges remain. It is not clear which tumors would respond to ferroptosis-inducing agents. Our results indicate that TAZ-activated tumors may be particularly sensitive to various ferroptosis-inducing therapies. Thus, TAZ activation and induction of its canonical target genes may serve as predictive biomarkers. In addition, it is important to consider the compounds or therapeutic agents that would be most effective for inducing ferroptosis. While erastin and various GPX4 inhibitors can induce ferroptosis, their toxicity and stability may limit in vivo application and translation potential. One promising agent is recombinant human cyst(e)inase that can trigger ferroptosis by depleting plasma cystine (49). Since its anti-tumor efficacy and in vivo safety have been demonstrated in murine models for multiple tumors, it may have the potentials to trigger in vivo ferroptosis of OVCA and improve clinical outcomes.

Supplementary Material

1

Implications.

This study reveals that TAZ promotes ferroptosis in ovarian cancers by regulating ANGPTL4 and NOX, offering a novel therapeutic potential for ovarian tumors with TAZ activation

Acknowledgments

We thank David Corcoran for RNA-seq data analysis and thank Carole Grenier for technical assistance with experiments. This work was supported by Duke Cancer Institute Pilot Project, Department of Defense grants (W81XWH-17-1-0143, W81XWH-15-1-0486, W81XWH-19-1-0842) and NIH (1R01GM124062 and 1R01NS111588) to JTC.

This work was supported by DCI Pilot Project, Department of Defense grants (W81XWH-17-1-0143, W81XWH-15-1-0486, W81XWH-19-1-0842) and NIH (1R01GM124062 and 1R01NS111588) to JTC.

Footnotes

The authors declare no potential conflicts of interest

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Associated Data

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

Supplementary Materials

1
NIHMS1541480-supplement-1.docx (948.1KB, docx)

Data Availability Statement

RNA-seq for transcriptional profiles of CAOV2 primary and CAOV2 recurrent cells (GSE133663) and CAOV2 ovarian cancer cells with TAZ silencing (GSE133664) have been deposited in the NCBI Genome Expression Omnibus (GEO) under SuperSeries GSE 133673.

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