Sterol regulatory element-binding protein-1a binds to cis elements in the promoter of the rat high density lipoprotein receptor SR-BI gene

The high density lipoprotein (HDL) receptor, or scavenger receptor class B type I (SR-BI), is critical for cholesterol transport and a potential targe t for hypercholesterolemic drugs. Thus, elucidation of the mechanism underl ying regulation of the HDL receptor SR-BI gene is essential. It has been pr eviously shown that there is a correlation between depletion in ovarian cho lesteryl ester content and increased HDL receptor SR-BI expression in respo nse to hormonal stimulation. We wanted to determine whether the levels of m ature sterol response element-binding protein-1a (SREBP-1a), a key protein in the transcriptional regulation of several genes by sterols, are affected under these conditions. Thus, Western blot analysis was carried out. Consi stent with the possibility that SREBP-1a may be involved in the regulation of the HDL receptor SR-BI gene, we found that mature SREBP-1a levels increa sed up to 11-fold in the ovary after treatment with 50 U hCG. This increase in mature SREBP-1a protein levels correlated with a 30% decrease in ovaria n cholesterol levels. These changes in both SREBP-1a and cholesterol levels preceded a 2-fold induction of HDL receptor SR-BI protein levels. To deter mine whether SREBP-1a could directly regulate the expression of the rat HDL receptor SR-BI gene, approximately 2.2 kb of the receptor SR-BI promoter w ere cloned and sequenced, and deletion analysis and mobility shift assays w ere performed. The results of these studies demonstrate that the rat HDL re ceptor SR-BI promoter contains two sterol response elements (pSRE and dSRE) through which SREBP-1a can bind and activate transcription of this gene. T hese motifs are similar to known SRE motifs reported for sterol-sensitive g enes, and the pSRE is located between two Sp1 sites, similar to the SRE-1 m otif in the low density lipoprotein receptor. The cysteine protease inhibit or N-acetyl-leucyl-leucyl-norleucinal, which inhibits SREBP degradation, en hanced the effect of SREBP-1a on the regulation of the rat HDL receptor SR- BI gene. It has previously been shown that tropic hormones such as hCG can also influence gene expression by increasing cAMP levels. Consistent with t his fact, me have recently shown that steroidogenic factor-1 (SF-1) mediate s cAMP activation of the HDL receptor SR-BI gene. Thus, we decided to exami ne whether SREBP-1a could cooperate with SF-1 to enhance transcription this gene. The results confirm that indeed both SF-1 and SREBP-1a synergize to induce HDL receptor SR-BI gene expression.