Tumor repressive functions of estrogen receptor β in SW480 colon cancer cells

Estrogen receptor β (ERβ) is the predominant ER in the colorectal epithelium. Compared with normal colon tissue, ERβ expression is reduced in colorectal cancer. Our hypothesis is that ERβ inhibits proliferation of colon cancer cells. Hence, the aim of this study has been to investigate the molecular function of ERβ in colon cancer cells, focusing on cell cycle regulation. SW480 colon cancer cells have been lentivirus transduced with ERβ expression construct with or without mutated DNA-binding domain or an empty control vector. Expression of ERβ resulted in inhibition of proliferation and G₁ 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 ERβ-expressing cells. Furthermore, the c-Myc target gene p21^(Waf1/Cip1) was induced and Cdc25A was reduced by ERβ at the transcriptional level. The second cdk2-inhibitor, p27^Kip1, was induced by ERβ, but this regulation occurred at the posttranscriptional level, probably through ERβ-mediated repression of the F-box protein p45^Skp2. Expression of the ERβ-variant with mutated DNA binding domain resulted in completely different cell cycle gene regulation. We performed in vivo studies with SW480 cells ± ERβ transplanted into severe combined immunodeficient/ beige mice; after three weeks of ERβ-expression, a 70% reduction of tumor volume was seen. Our results show that ERβ inhibits proliferation as well as colon cancer xenograft growth, probably as a consequence of ERβ-mediated inhibition of cell-cycle pathways. Furthermore, this ERβ-mediated cell cycle repression is dependent on functional ERE binding.