Hes-6, an inhibitor of Hes-1, is regulated by 17β-estradiol and promotes breast cancer cell proliferation

INTRODUCTION: Hes-6 is a member of the basic helix-loop-helix (bHLH) family of transcription factors, and its overexpression has been reported in metastatic cancers of different origins. Hes-6 has been described as an inhibitor of Hes-1 during neuronal development, although its function in cancer is not known. In this study, we investigated the function of Hes-6 in breast cancer and tested the hypothesis that Hes-6 enhances breast cancer cell proliferation and is regulated by estrogen. METHODS: To investigate the function of Hes-6, T47D cells stably expressing Hes-6 were generated by lentiviral transduction, and conversely, siRNA also was used to knock down Hes-6 expression in breast cancer cells. The Hes-6-expressing T47D cells were transplanted into immunodeficient mice to study effects on tumor growth. RESULTS: We found that Hes-6 expression was significantly higher in the high-grade, estrogen receptor (ER)α-negative SKBR3 and MDA-MB-231 cells compared with the ERα-positive, non-metastasizing T47D and MCF-7 breast carcinoma cells. Moreover, the level of Hes-6 mRNA was 28 times higher in breast cancer samples compared with normal breast samples. In Hes-6-expressing T47D cells, Hes-6 ectopic expression was shown to stimulate cell proliferation in vitro as well as breast tumor growth in xenografts. Moreover, expression of Hes-6 resulted in induction of E2F-1, a crucial target gene for the transcriptional repressor Hes-1. Consistently, silencing of Hes-6 by siRNA resulted in downregulation of E2F-1 expression, whereas estrogen treatment caused induction of Hes-6 and downstream targets hASH-1 and E2F-1 in MCF-7 cells. CONCLUSIONS: Together, the data suggest that Hes-6 is a potential oncogene overexpressed in breast cancer, with a tumor-promoting and proliferative function. Furthermore, Hes-6 is a novel estrogen-regulated gene in breast cancer cells. An understanding of the role and regulation of Hes-6 could provide insights into estrogen signaling and endocrine resistance in breast cancer and, hence, could be important for the development of novel anticancer drugs.