Partnering role of FOXM1 and EPS8 in maintaining chemoresistance in human ovarian cancer cells

Abstract

Ovarian cancer is the most lethal gynecologic malignancies with low 5-year survival rate. Usually diagnosed late, ovarian cancer shows resistance to chemotherapy and tends to recur after treatment. Different studies have shown that the Forkhead box transcription factor M1 (FOXM1) is overexpressed in advanced ovarian cancer and its expression levels correlate with chemosensitivity. Independently, Epidermal Growth Factor Receptor Substrate 8 (EPS8) was found to be involved in the acquisition of chemoresistance in several cancer types with unclear mechanism. The recent findings that EPS8 contributed to upregulation of FOXM1 expression in HN4 cells and that there was direct physical interaction between FOXM1 and EPS8 in yeast two hybrid assays led us to hypothesize that FOXM1 and EPS8 would functionally interact to regulate chemoresistance. This study aims to investigate the role of EPS8 in FOXM1-mediated chemoresistance in ovarian cancer cells. First, the interaction between FOXM1 and EPS8 was investigated. The interaction was confirmed using co-immunoprecipitation assay. Overexpression and depletion of FOXM1 in ovarian cancer cells followed by qPCR analysis suggested that FOXM1 might be a potential regulator of EPS8. Interestingly, FOXM1c showed a stronger stimulating effect than FOXM1b onEPS8expression. However, further study of theEPS8 promoter is required to confirm direct transcriptional regulation by FOXM1. Second, the role of EPS8 in maintaining cisplatin resistance in ovarian cancer cells was examined. EPS8 levels were depleted using shRNAs in ES2 cells. Cell viability assay was performed with increasing concentrations of cisplatin and the IC50 values determined for ES2 cells before and after EPS8 depletion. Decreased cell viability after EPS8 depletion was suggestive of its role in the regulation of cisplatin chemoresistance. Third, to study the functional interaction between FOXM1 and EPS8in the regulation of cisplatin chemoresistance,FOXM1 function was suppressed using the specific inhibitor FDI-6 together withEPS8 depletion using shRNAs. FDI-6 was confirmed to perturb FOXM1 function based on its suppression of expression of FOXM1 known targets, CCNB1and CDC25B. Decreased cell viability in FOXM1-suppressed and EPS8-depleted ES2 cells suggested that both FOXM1 and EPS8 are required to maintain cisplatin chemoresistance inES2 cells. Four, the involvement of EPS8 in a known FOXM1-mediated cisplatin resistance mechanism was investigated by studying the regulation of the DNA repair genes EXO1and RAD51by EPS8.EPS8 knockdown led to EXO1downregulation, indicating that EPS8 may be a potential regulator of EXO1. Study of the binding/recruitment ofEPS8 to the EXO1promoter is required to further understand the mode of regulation. Five, similar to FOXM1, EPS8 was also upregulated upon cisplatin treatment. The underlying mechanism requires further analysis. To conclude, the present study reported for the first time the requirement of EPS8 in maintaining chemosensitivity towards cisplatin in ovarian cancer cells. FOXM1c showed a higher upregulating effect on EPS8 expression than FOXM1b. Ovarian cancer was sensitized to cisplatin by EPS8 depletion, and further sensitized after FOXM1 inhibition. Moreover, EXO1downregulation after EPS8 knockdown indicated that EPS8 may regulate chemoresistance via modulating expression ofEXO1, a known FOXM1target, in ovarian cancer.