SREBP-2 positively regulates transcription of the cholesterol efflux gene, ABCA1, by generating oxysterol ligands for LXR

Abstract

Cholesterol accumulation and removal are regulated by two different transcription factors. SREBP-2 (sterol-regulatory-elementbinding protein-2) is best known to up-regulate genes involved in cholesterol biosynthesis and uptake, whereas LXR (liver X receptor) is best known for up-regulating cholesterol efflux genes. An important cholesterol efflux gene that is regulated by LXR is the ATP-binding cassette transporter, ABCA1 (ATPbinding cassette transporter-A1). We have previously shown that statin treatment down-regulated ABCA1 expression in human macrophages, probably by inhibiting synthesis of the LXR ligand 24(S),25-epoxycholesterol. However, it was subsequently reported that ABCA1 expression is down-regulated by SREBP-2 through binding of SREBP-2 to an E-box element in ABCA1's proximal promoter. As statin treatment induces SREBP-2 activation, this may provide an alternative explanation for the statinmediated down-regulation of ABCA1. In the present study, we employed a set of CHO (Chinese-hamster ovary) mutant cell lines to investigate the role of SREBP-2 in the regulation of ABCA1. We observed increased ABCA1 mRNA levels in SREBP-2-overexpressing cells and decreased levels in cells lacking a functional SREBP-2 pathway, which were restored when the SREBP-2 pathway was reinstated. Moreover, ABCA1 gene expression was positively associated with synthesis of 24(S),25-epoxycholesterol in these cell lines. In studies using a human ABCA1 promoter reporter assay, mutation of the E-box motif had a similar response as the wild-type construct to either statin treatment or addition of 24(S),25-epoxycholesterol. By contrast, these responses were completely ablated when the DR4 element to which LXR binds was mutated. These results support the idea that 24(S),25-epoxycholesterol and statin treatment influence ABCA1 transcription via supply of an LXR ligand and not through an SREBP-2/E-boxrelated mechanism. In addition, our results indicate a critical role of SREBP-2 as a positive regulator of ABCA1 gene expression by enabling the generation of oxysterol ligands for LXR.

Sulfated oxysterol, 25HC3S, is a potent regulator of lipid metabolism in human hepatocytes

“…1). SREBPs play important roles not only in cholesterol synthesis and fatty acid metabolism but also many important biological events (33)(34)(35)(36)(37)(38)(39). It will be interesting to investigate whether these oxysterols are also involved in these events.…”

Dysregulation of Plasmalogen Homeostasis Impairs Cholesterol Biosynthesis

“…Real-time PCR-Extraction of total RNA from cells, synthesis of first-strand cDNA, and quantitative real-time PCR were performed using the housekeeping gene porphobilinogen deaminase (PBGD) (23) or ribosomal protein 3 (RPL3) (24) as internal control (17). Primers for Chinese hamster SQLE (12) and LSS (25), and human LSS and HMGCR (24) were listed in supplemental Table S1.…”

Amyloid-β Pathology and APOE Genotype Modulate Retinoid X Receptor Agonist Activity in Vivo

“…The mechanism through which A␤ down-regulates the expression of NRs may involve decreased production of oxysterol ligands, positive modulators of NR expression. Overall, activation of the sterol regulatory element-binding protein 2 (SREBP2) results in the production of oxysterol ligands from cholesterol, activating NRs, including LXR and RXR, to increase ABCA1 expression (74). Recent data demonstrate that A␤ down-regulates SREBP-2 cleavage (75), supporting disruptions in lipid metabolism as a potential pathogenic process in AD (76).…”