New insights into cellular cholesterol acquisition: promoter analysis of human HMGCR and SQLE, two key control enzymes in cholesterol synthesis.

BACKGROUND: The two control points of cholesterol synthesis, 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) and squalene monooxygenase (SQLE) are known targets of the transcription factor sterol-regulatory element binding protein-2 (SREBP-2). Yet the location of the sterol-regulatory elements (SREs) and cofactor binding sites, nuclear factor-Y (NF-Y) and specificity protein 1 (Sp1), have not been satisfactorily mapped in the human SQLE promoter, or at all in the human HMGCR promoter.
METHODS: We used luciferase reporter assays to screen the sterol-responsiveness of a library of predicted SRE, Sp1 and NF-Y site mutants and hence identify bone fide binding sites. We confirmed SREs via an electrophoretic mobility shift assay (EMSA) and ChIP-PCR.
RESULTS: We identified two SREs in close proximity in both the human HMGCR and SQLE promoters, as well as one NF-Y site in HMGCR and two in SQLE. In addition, we found that HMGCR expression is highly activated only when SREBP-2 levels are very high, in contrast to the low density lipoprotein receptor (LDLR), a result reflected in mouse models used in other studies.
CONCLUSIONS: Both HMGCR and SQLE promoters have two SREs that may act as a homing region to attract a single SREBP-2 homodimer, with HMGCR being activated only when there is absolute need for cholesterol synthesis. This ensures preferential uptake of exogenous cholesterol via LDLR, thereby conserving energy. GENERAL SIGNIFICANCE: We provide the first comprehensive investigation of SREs and NF-Ys in the human HMGCR and SQLE promoters, increasing our fundamental understanding of the transcriptional regulation of cholesterol synthesis.