D. Lopez et Mp. Mclean, Sterol regulatory element-binding protein-1a binds to cis elements in the promoter of the rat high density lipoprotein receptor SR-BI gene, ENDOCRINOL, 140(12), 1999, pp. 5669-5681
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.