Glucagon-like peptide-1 receptor agonists improve cholesterol metabolism by inhibiting SREBP-2 via SIRT6-AMPK pathway in HepG2 cells treated with palmitic acid

Background: Glucagon-like peptide-1 receptor agonists (GLP-1RAs) exhibit not only hypoglycemic effects but also protective effects against nonalcoholic fatty liver disease and cardiovascular diseases, conditions that are associated with dyslipidemia.Methods: To evaluate the beneficial effects of GLP-1RAs on hepatic cholesterol metabolism, HepG2 cells were exposed to palmitic acid (PA) and subsequently treated with or without the GLP-1RAs, exendin-4 and liraglutide. Cholesterol levels and the expression of cholesterol metabolism-related factors were measured.Results: Exendin-4 and liraglutide reduced cholesterol levels in both cell lysates and culture media of PA-treated HepG2 cells. They also decreased the expression of genes involved in cholesterol synthesis (ACAT1, SREBP-2, HMGCR, and SQLE), bile acid synthesis (LXRα and CYP7A1), and PCSK9, while increasing the expression of genes involved in the reverse cholesterol transport pathway (ABCA1 and SR-B1) and low-density lipoprotein cholesterol uptake (LDLR). SREBP-2 inhibition by small interfering RNA in GLP-1RAs treated cells amplified the reduction in the expression of HMGCR, SQLE, LXRα, CYP7A1, and PCSK9 genes and HMGCR protein, as well as the increase in expression of the LDLR gene. However, inhibition of SIRT6 and AMPK, which were increased by GLP-1RAs, reversed the suppression of SREBP-2 and its downstream factor genes and amplified the increase in expression of the LDLR gene in PA-treated HepG2 cells.Conclusions: These findings demonstrate that GLP-1RAs improve cholesterol metabolism through activation of the SIRT6-AMPK pathway, resulting in the inhibition of SREBP-2 in PA-treated HepG2 cells. Moreover, the upregulation of LDLR gene expression in cells treated with GLP-1RAs occurs through both SREBP-2-dependent and SREBP-2-independent pathways.