Role of FoxO1 in MDR1 gene expression: Novel target to overcome chemoresistance in Adriamycin-resistant breast cancer cells

Abstract

Increased expression of MDR1 is believed to be one of the major causes for the chemoresistance acquisition of various cancer cells, including adriamycin-resistant breast cancer cells (MCF-7/ADR). Forkhead bOX-containing protein, O subfamily (FoxO) transcription factors are involved in the diverse cellular responses, such as differentiation, proliferation and metabolism, but possible roles of FoxO in the expression of drug efflux pumps have not been studied. In this study, we found that the expression and activity of MDR1 are enhanced in MCF-7/ADR cells and these are essential for the doxorubicin resistance. A putative binding site to FoxO was identified at the proximal promoter region of human MDR1 gene and this site was partially overlapped with C/EBP?? binding region. Gel shift and immunoblot analysis of subcellular fractions revealed that nuclear levels of FoxO1 and its DNA binding activity were selectively enhanced in MCF-7/ADR cells, which was reversed by FoxO1 antibody. Reporter gene assays showed that transcription of MDR1 gene is stimulated by FoxO1 overexpression. Moreover, the expression and transactivation of MDR1 gene in MCF-7/ADR cells were completely inhibited by FoxO1 siRNA. The MDR1 expression in MCF-7/ADR cells was also inhibited by a functional FoxO1 inactivator, insulin. In conclusion, FoxO1 functions as a novel transcriptional activator of MDR1 gene and is crutial for MDR1 induction in MCF-7/ADR cells, which might be a therapeutic target to overcome doxorubicin resistance in MCF-7/ADR cells.