DIFFERENTIAL-EFFECTS OF ESTRADIOL AND ITS ANALOGS ON CYCLIN D1 AND CDK4 EXPRESSION IN ESTROGEN-RECEPTOR POSITIVE MCF-7 AND ESTROGEN RECEPTOR-TRANSFECTED MCF-10AE(WT5) CELLS

Estradiol stimulates the growth of a majority of human breast tumors c ontaining the estrogen receptors (ERs), proteins that mediates estroge n function. Opposing effects of estradiol have been found in cells exp ressing endogenous ER and those containing a transfected ER. To unders tand the role of estradiol structure in diverse estrogenic responses r elated to cell cycle regulation, we evaluated the effects of estradiol and its analogs on cell cycle progression, and the expression of cycl in D1 and the cyclin dependent kinase 4 (CDK4) in ER-positive MCF-7 an d ER-transfected MCF-10AE(wt5) cells. Four analogs of estradiol, with a re-positioned or a deleted hydroxyl group, were used. Our results sh ow that estradiol and all of the analogs facilitated cell cycle progre ssion of MCF-7 cells. In contrast, only estradiol inhibited the cell c ycle progression of MCF-10AE(wt5) cells significantly. Western blot an alysis revealed that cyclin D1 protein increased to the maximal level by 6 h after the initiation of cell cycle from G(1) phase of MCF-7 cel ls. The least effective analog in inducing cyclin D1 was 3-hydroxyestr atriene. However, this analog was most effective at inducing CDK4, con tributing to its efficacy in facilitating MCF-7 cell cycle. In contras t to MCF-7 cells, the level of cyclin D1 protein was not influenced si gnificantly by estradiol or its analogs in MCF-10AE(wt5) cells. Sucros e gradient sedimentation analysis of ER from MCF-7 cells showed that t he major peak of [H-3]-estradiol bound to ER could be displaced by a 1 0-fold excess of unlabelled estradiol or any of the analogs. In contra st, several analogs were less effective than unlabelled estradiol in c ompetitive displacement of [(3H)]- estradiol bound to ER from MCF-10AE (wt5) cells. These data indicate that the induction of cyclin D1 is an important part of the growth stimulatory effects of estrogens in MCF- 7 cells, but it may not be involved in growth inhibition of MCF-10AE(w t5) cells. Our results also show that estrogenic compounds interact wi th ER from MCF-10AE(wt5) cells with altered ligand binding affinity, p ossibly due to the absence or dysfunction of certain transcription fac tors or ER-associated proteins that co-regulate ER function.