Abstract
Endocrine therapies targeting estrogen action are pivotal for the prevention and treatment of ER-positive breast cancers. Previous studies sought to recreate hormone responsiveness by the stable expression of ERalpha in the ER-negative MDA-MB-231 breast cancer cells. Paradoxically, estrogen inhibits breast cancer cell growth when an exogenous ERalpha is expressed. In this study, we have built on previous studies by developing a Tet-off adenoviral system to express ERalpha in the ER-negative SKBr3 breast cancer cells that over-express both EGFR and HER2. This system efficiently delivers ERalpha and the expression level of ERalpha is controlled by doxycycline in a concentration-dependent manner. The growth of SKBr3 was inhibited by ERalpha expression and further inhibited in the presence of 1 nM 17beta-estradiol. SKBr3 cells were arrested at G0/G1 cell cycle upon ERalpha expression, which corresponded to an increase of p21Cip1/Waf1, hypo-phosphorylation of pRb and decrease of E2F1. Estrogen also reduced EGFR and HER2 expression in SKBr3 cells after ERalpha was expressed. Given that estrogen-induced increase of p21Cip1/Waf1 and decrease of E2F1 was also observed in MDA-MB-231 cells stably transfected with ERalpha, our results suggest that a common pathway might be shared by different breast cancer cell lines whose growth is suppressed by ectopic ERalpha and estrogen.