Steroid hormones induce bcl-X gene expression through direct activation of distal promoter P4.

ABSTRACT

Bcl-X exists in at least five different isoforms with complex effects on programmed cell death. Glucocorticoids and progestins control bcl-X expression and influence the ratio between bcl-X(L) (antiapoptotic isoform) and bcl-X(S) (proapoptotic isoform) in different tissues. The 5'-UTR region of the mouse bcl-X gene contains at least five different promoters, which exhibit a tissue-specific pattern of promoter usage. Several mRNAs with different 5'-leading exons can be generated upon promoter activation. Here we explore the potential of the various bcl-X gene promoters to be regulated by glucocorticoids or progestins. We found that the region located immediately upstream of promoter 4 (P4) contains two hormone response element (HRE)-like sequences at positions -3040 (HRE I) and -3001 (HRE II) relative to the translation initiation codon. These HRE-like sequences confer hormone responsiveness to a core promoter and bind glucocorticoid or progesterone receptors in vitro. Point mutations of both HREs that prevent steroid receptor binding also eliminate hormonal inducibility. In cells treated with glucocorticoids, the hormone receptor is recruited to the P4 region containing the HREs. Analysis of the products of the endogenous bcl-X in epithelial mammary cells showed that only transcripts originating from P4 increased upon hormone treatment. This observation correlates with the induction of the bcl-X(L) mRNA, suggesting that P4 is one of the bcl-X promoters responsible for the generation of this antiapoptotic isoform.