Elevated expression or activity of the transcription factor forkhead box M1 (FOXM1) is associated with the development and progression of many malignancies, including breast cancer. In this study, we show that the thiazole antibiotic thiostrepton selectively induces cell cycle arrest and cell death in breast cancer cells through down-regulating FOXM1 expression. Crucially, our data show that thiostrepton treatment reduced FOXM1 expression in a time-and dose-dependent manner, independent of de novo protein synthesis and predominantly at transcriptional and gene promoter levels. Our results indicate that thiostrepton can induce cell death through caspase-dependent intrinsic and extrinsic apoptotic pathways as well as through caspase-independent death mechanisms, as observed in MCF-7 cells, which are deficient of caspase-3 and caspase-7. Cell cycle analysis showed that thiostrepton induced cell cycle arrest at G 1 and S phases and cell death, concomitant with FOXM1 repression in breast cancer cells. Furthermore, thiostrepton also shows efficacy in repressing breast cancer cell migration, metastasis, and transformation, which are all downstream functional attributes of FOXM1. We also show that overexpression of a constitutively active FOXM1 mutant, #N-FOXM1, can abrogate the antiproliferative effects of thiostrepton. Interestingly, thiostrepton has no affect on FOXM1 expression and proliferation of the untransformed MCF-10A breast epithelial cells. Collectively, our data show that FOXM1 is one of the primary cellular targets of thiostrepton in breast cancer cells and that thiostrepton may represent a novel lead compound for targeted therapy of breast cancer with minimal toxicity against noncancer cells.

The transcription factor Forkhead box M1 (FOXM1) is a key regulator of cell proliferation and is overexpressed in many forms of primary cancers, leading to uncontrolled cell division and genomic instability. To address the role of FOXM1 in chemoresistance, we generated a cisplatin-resistant breast cancer cell line (MCF-7-CIS R ), which had an elevated level of FOXM1 protein and mRNA expression relative to the parental MCF-7 cells. A close correlation was observed between FOXM1 and the expression of its proposed downstream targets that are involved in DNA repair; breast cancer-associated gene 2 (BRCA2) and X-ray cross-complementing group 1 (XRCC1) were expressed at higher levels in the resistant cell lines compared with the sensitive MCF-7 cells. Moreover, cisplatin treatment induced DNA damage repair in MCF-7-CIS R and not in MCF-7 cells. Furthermore, the expression of a constitutively active FOXM1 (ΔN-FOXM1) in MCF-7 cells alone was sufficient to confer cisplatin resistance. Crucially, the impairment of DNA damage repair pathways through the small interfering RNA knockdown inhibition of either FOXM1 or BRCA2/XRCC1 showed that only the silencing of FOXM1 could significantly reduce the rate of proliferation in response to cisplatin treatment in the resistant cells. This suggests that the targeting of FOXM1 is a viable strategy in circumventing acquired cisplatin resistance. Consistently, the FOXM1 inhibitor thiostrepton also showed efficacy in causing cell death and proliferative arrest in the cisplatin-resistant cells through the downregulation of FOXM1 expression. Taken together, we have identified a novel mechanism of acquired cisplatin resistance in breast cancer cells through the induction of FOXM1. Mol Cancer Res; 8(1); 24-34. ©2010 AACR.