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

Abstract

Forkhead box protein M1 (FOXM1) is a transcription factor ubiquitously expressed in proliferating cells. Its levels are overexpressed in various cancers, including ovarian cancer. The promoter activity of FOXM1 was recently found to be unregulated by Epidermal growth factor receptor kinase substrate 8 (EPS8), a substrate for tyrosine kinases. Interestingly, depletion of FOXM1 or EPS8 in cancer cells led to chemosensitivity against cisplatin. Mechanistically, FOXM1 is known to transcriptionally up-regulate copper transporter 1 (hCTR1) and other DNA repair genes, and to enhance β-catenin activation. Our recent finding that FOXM1 and EPS8 physically interacted in yeast two-hybrid and immunoprecipitation assays suggests that EPS8 might mediate its chemoresistant effect in cancer cells via modulation of FOXM1 function. To test whether FOXM1 and EPS8 regulate cancer chemosensitivity synergistically, we studied chemosensitivity in the ovarian cancer cell line ES2 with a series of cisplatin concentrations using XTT assay. First, treatment with FDI6, a small molecule inhibitor of FOXM1 DNA binding activity, was found to increase chemosensitivity and experiments are ongoing to determine the effect of shRNA-mediated EPS8 knockdown. Second, after either FOXM1 inhibition or EPS8 knockdown, more cells were shown to undergo apoptosis under cisplatin treatment as reflected by the increased expression of cleaved PARP-1 protein in immunoblotting analysis. Third, mRNA levels of the DNA repair genes EXO1 and RAD51 were found to be suppressed after FOXM1 inhibition and/or EPS8 knockdown by RT-qPCR analysis. Taken together, our findings support that FOXM1 and EPS8 interact to mediate regulation of chemosensitivity in ovarian cancer cells.