Selective role of sterol regulatory element binding protein isoforms in aggregated LDL-induced vascular low density...

Low density lipoprotein receptor-related protein (LRP1) is upregulated in vascular smooth muscle cells by intravascular aggregated LDL (agLDL) - LDL trapped in the arterial intima and systemic LDL. LRP1 upregulation in hypercholesterolemic aortas is concomitant with SREBP downregulation. However, the specific role of SREBP isoforms in LRP1 transcription and LDL-induced LRP1 upregulation in human vascular smooth muscle cells (VSMC) is unknown. In the present study we report that specific silencing of either SREBP-1 or SREBP-2 enhanced LRP1 whereas overexpression of the active SREBP isoforms decreased LRP1 expression. Gel mobility shift and ChIP assays demonstrated that SREBP-1a, SREBP-1c and SREBP-2 were able to bind to three putative SRE sequences; SRE-A (-1042 to -1028), SRE-B (-115 to -101) and SRE-C (+226 to +234). ChIP assays demonstrated that agLDL (100μg/mL, 24h) significantly and specifically decreased SREBP-2 binding to the LRP1 promoter. Luciferase assays demonstrated that agLDL increased the transcriptional activity of A/B or A/C double mutants but failed to increase that of the double B/C mutant. Our results show that both SREBP-1 and SREBP-2 negatively modulated LRP1 transcription. Furthermore, agLDL exerted an upregulatory effect on LRP1 expression by decreasing SREBP-2 binding to LRP1 promoter. Two SRE-like sequences control the response of LRP1 to agLDL.