17β-estradiol inhibits apoptosis in MCF-7 cells, inducing bcl-2 expression via two estrogen-responsive elements present in the coding sequence

We have found that 17 beta-estradiol induces bcl-2 transcription in human breast cancer MCF-7 cells. To identify cis-acting elements involved in this regulation, we have analyzed hormone responsiveness of transiently transfected reporter constructs containing the bcl-2 major promoter (P-1), Hormone inducibility was observed only when either of two sequences, located within the bcl-2 coding region and showing one and two mutations with respect to the consensus estrogen-responsive element, were inserted downstream from the P-1 promoter. Both sequences behaved as enhancers exclusively in cells expressing the estrogen receptor and were able to bind this receptor in in vitro assays. Transfections into MCF-7 cells of plasmids carrying a bcl-2 cDNA fragment which included these two elements revealed that their simultaneous presence resulted in an additive effect on reporter gene activity, whose size resembled the increase of endogenous bcl-2 mRNA level observed in untransfected cells after hormone treatment. Moreover. the identified elements were able to mediate up-regulation of bcl-2 expression by 17 beta-estradiol, since exogenous bcl-2 mRNA was induced by hormone challenge of MCF-7 cells transiently transfected with a vector containing the bcl-2 coding sequence cloned under the control of a non-estrogen-responsive promoter. Finally, we show that hormone prevention of apoptosis, Induced by incubating MCF-7 cells with hydrogen peroxide, was strictly related to bcl-2 up-regulation. Our results indicate that the bcl-2 major promoter does not contain cis-acting elements directly involved in transcriptional control by 17 beta-estradiol and that hormone treatment inhibits programmed cell death in MCF-7 cells, inducing bcl-2 expression via two estrogen-responsive elements located within its coding region.