Role of FOXM1 in ovarian cancer tumorigenesis and chemoresistance

Abstract

Ovarian carcinoma is the most lethal gynecological malignancy. The high relapse and mortality rates are attributable to late diagnosis and development of drug resistance. Identifying novel prognostic and therapeutic targets for ovarian carcinoma is crucial for improving patients' long-term survival rate.

Forkhead box protein M1 (FOXM1), which is a widely studied member of the FOX superfamily of proteins, participates in cell proliferation and apoptosis affecting the developmental function of many organs. Recently, there has been emerging evidence supporting the biological significance of FOXM1 in carcinogenesis. Overexpression of FOXM1 has been reported in multiple human malignancies including primary breast cancer, lung cancer, prostate cancer, etc. However, whether FOXM1 participates in the development of ovarian cancer, with emphasis on the association with clinicopathological parameters and chemoresistance, remains unknown. This study aims at elucidating the functional role of FOXM1 in the tumorigenesis of ovarian cancer.

Immunohistochemical study showed higher nuclear FOXM1 expression was significantly associated with advanced stages of ovarian cancer (P=0.035). Though not reaching statistical significance, FOXM1 overexpression displayed association with serous histologic subtype, high grade cancers (poor differentiation) and chemoresistance. Patients with a low FOXM1 level had a significantly longer overall (P=0.019) and disease-free survival (P=0.014) than those with high FOXM1 expression. Multivariate progression analysis established high expression of FOXM1, advanced cancer stages and poor histological differentiation (high grade) as independent prognostic factors for short overall and disease-free survival. Consistently, in vitro Transwell assays demonstrated transient knockdown of FOXM1 was capable of reducing SKOV-3 migration and invasion. Furthermore, paclitaxel treatment down-regulated FOXM1 expression in the sensitive cell line but not the resistant one. Immunofluorescence and flow cytometric analyses demonstrated FOXM1 knockdown could enhance paclitaxel-mediated mitotic catastrophe in ovarian cancer cells.

Recent attention has been drawn to the oncogenic roles of kinesin-like protein KIF2C and p21-activated kinase 4 (PAK4) in human cancers. Interestingly, the expressions of KIF2C and PAK4 altered in a similar pattern to FOXM1 expression upon paclitaxel treatment by displaying down-regulation only in the paclitaxel sensitive cell line but not the resistant one. FOXM1 silencing, qPCR, luciferase reporter assay and chromatin immunoprecipitation confirmed KIF2C and PAK4 to be novel transcriptional targets of FOXM1. Clonogenic assay showed KIF2C knockdown could re-sensitize resistant cell line to paclitaxel treatment. Flow cytometry demonstrated KIF2C silencing was able to increase the number of cells blocked at G2/M cell cycle phase in sensitive cell line and raise the number of apoptotic cells in resistant cell line. Up-regulations of miR-590 and miR-370 were also observed in a panel of drug resistant ovarian and breast cancer cell lines. While ectopic expression of miR-590 reduced FOXM1 expression, FOXM1 also seemed to be able to regulate the expression of miR-590.

In summary, this study showed overexpression of FOXM1 in ovarian cancer correlated with poor survival of patients and paclitaxel resistance. KIF2C and PAK4 were identified as novel transcriptional targets of FOXM1 implicated in chemoresistance.