Estrogen stimulates SREBP2 expression in hepatic cell lines via an estrogen response element in the SREBP2 promoter

Abstract

Objective: Hypoestrogenism in women is strongly associated with menopause and it can lead to lipid disorder, which predisposes people to premature cardiovascular disease. However, the mechanism of lipid disorder remains unclear. Sterol regulatory element-binding protein 2 (SREBP2) is the key transcription factor regulating cholesterol metabolism. We hypothesize that estrogen regulates SREBP2 transcription through an estrogen response element (ERE) in the SREBP2 promoter region. Methods: Human hepatoblastoma cells (HepG2) were treated with dose-dependent concentrations of estradiol (E2) for 24 h. Then, SREBP2 expression was determined via real-time PCR and immunofluorescence. The expressions of the SREBP2 downstream target genes HMGCR and LDLR were determined via real-time PCR. Lipid secretion in the culture media of HepG2 cells was measured using ELISA. Through bioinformatics analysis, we identified high-scoring ERE-like sequences in the SREBP2 gene promoter. Chromatin immunoprecipitation analysis was used to confirm the ERE. DNA fragments of the putative or mutated ERE-like sequence were synthesized and ligated into pGL3-basic plasmid to construct the SREBP2 promoter luciferase reporter systems. SREBP2-Luciferase (SREBP2-Luc), SREBP2-Mutation (SREBP2-Mut) and the blank control were transfected into hepatic cell lines. Luciferase activities were measured using the dual-luciferase reporter assay system. Chromatin immunoprecipitation analysis and the luciferase reporter assay were repeated in human hepatoma cells (HuH-7). Results: We found that E2 dose-dependently increased the expression of SREBP2 in HepG2 cells and that the increased levels were blocked when treated with an estrogen receptor-alpha antagonist. Additionally, E2 increased both HMGCR and LDLR expression and lipid secretion in HepG2 cells. Notably, we identified a functional ERE in the SREBP2 gene promoter, to which E2 could specifically bind and induce transcription. Conclusions: An ERE was identified in the SREBP2 gene promoter. It mediates the regulation of SREBP2 expression by estrogen in hepatocytes. This study provides a mechanism to link cardiovascular disease with estrogen.