Mechanisms of transcriptional activation of bcl-2 gene expression by 17β-estradiol in breast cancer cells

Abstract

bcl-2 gene expression is induced by 17β-estradiol (E2) in T47D and MCF- 7 human breast cancer cells, and the mechanism of E2 responsiveness was further investigated by analysis of the bcl-2 gene promoter. The -1602 to - 1534 distal region (bcl-2j) of the promoter was E2-responsive; however, in gel mobility shift assays, the estrogen receptor α (ER(α)) did not bind [32p]bcl-2j, whereas Sp1 protein formed a retarded band complex. Further analysis demonstrated that the upstream region (-1603 to -1579) of the bcl-2 gene promoter contained two GC/GA-rich sites at -1601 (5'-GGGCTGG-3') and - 1588 (3'-GGAGGG-5') that bound Sp1 protein. Subsequent studies confirmed that transactivation by E2 was dependent on ER(α)/Sp1 interactions with both GC- rich sites, and this was confirmed by in vitro footprinting. In contrast, a 21-base pair E2-responsive downstream region (-1578 to -1534) did not bind Sp1 or ER(α) protein; however, analysis of a complex binding pattern with nuclear extracts showed that ATF-1 and CREB-1 bound to this motif. These data coupled with results of transient transfection studies demonstrated that transcriptional activation by E2 of the -1578 to -1534 region of the bcl-2 gene promoter was dependent on induction of cAMP and subsequent activation through a cAMP response element. Thus, hormone regulation of bcl-2 gene expression in breast cancer cells involves multiple enhancer elements and E2- mediated transactivation does not require direct binding of the estrogen receptor with promoter DNA.