Tumor Repressive Functions of Estrogen Receptor β in SW480 Colon Cancer Cells

Estrogen receptor beta (ER beta) is the predominant ER in the colorectal epithelium. Compared with normal colon tissue, ER beta expression is reduced in colorectal cancer. Our hypothesis is that ER beta inhibits proliferation of colon cancer cells. Hence, the aim of this study has been to investigate the molecular function of ER beta in colon cancer cells, focusing on cell cycle regulation. SW480 colon cancer cells have been lentivirus transduced with ER beta expression construct with or without mutated DNA-binding domain or an empty control vector. Expression of ER beta resulted in inhibition of proliferation and G(1) phase cell cycle arrest and this effect was dependent on a functional DNA-binding region. c-Myc is overexpressed in an overwhelming majority of colorectal tumors. By Western blot and real-time PCR, we found c-Myc to be down-regulated in the ER beta-expressing cells. Furthermore, the c-Myc target gene p21((Wuf1/Cip1)) was induced and Cdc25A was reduced by ER beta at the transcriptional level. The second cdk2-inhibitor, p27(Kip1), was induced by ER beta, but this regulation occurred at the posttranscriptional level, probably through ER beta-mediated repression of the F-box protein p45(Skp2). Expression of the ER beta-variant with mutated DNA binding domain resulted in completely different cell cycle gene regulation. We performed in vivo studies with SW480 cells +/- ER beta transplanted into severe combined immunodeficient/beige mice; after three weeks of ER beta-expression, a 70% reduction of tumor volume was seen. Our results show that ER beta inhibits proliferation as well as colon cancer xenograft growth, probably as a consequence of ER beta-mediated inhibition of cell-cycle pathways. Furthermore, this ER beta-mediated cell cycle repression is dependent on functional ERE binding. [Cancer Res 2009;69(15):6100-6]