| Exendin-4,a long-acting agonist of the glucagon-like peptide 1(GLP-1),has been used as a therapeutic agent for diabetes and improves hepatic steatosis.ATP-binding cassette transporter A1(ABCA1),which is identified as a mutated molecular in Tangier Disease,is a pivotal regulator of cholesterol efflux in reverse cholesterol transport(RCT).Specific deletion of hepatic ABCA1 results in cholesterol accumulation and interrupts the RCT.In this study,we checked the effects of exendin-4 on ABCA1 expression in hepatocytes and investigated the detailed mechanism.Firstly,we demonstrated that exendin-4,binding to GLP-1 receptor,increased the expression and transcription of ABCA1 in hepatocytes via CaMKK/CaMKIV pathway.As a result,exendin-4 decreased cholesterol content in HepG2 cells.To understand the detailed mechanism,we employed chromatin immunoprecipitation(ChIP)assay to prove that prolactin regulatory element binding(PREB)mediates ABCA1 transcriptional activity and this effect was enhanced by exendin-4.Moreover,when PREB-binding site in ABCA1 promoter region was mutated,ABCA1 promoter activity induced by exendin-4 did not persist.Further,silence of PREB attenuated the effects of exendin-4 on ABCA1 expression while silence of CaMKIV or blocking of CaMKK by its specific inhibitor STO609 canceled the upregulation of both ABCA1 and PREB induced by exendin-4,pointing out that exendin-4 increased ABCA1 expression via CaMKK/CaMKIV/PREB.It was confirmed by in vivo experiments that treatment with exendin-4(1 nmol/kg/day,4 weeks)or overexpression of PREB induced the expression of hepatic ABCA1 and decreased the cholesterol accumulation in the liver of mice with high fat diet.In summary,our data demonstrated that exendin-4 facilitated cholesterol efflux in hepatocytes via elevation of ABCA1,which was mediated by CaMKK/CaMKIV/PREB signaling pathway,suggesting that exendin-4 used for diabetes has a potential function in suppressing cholesterol accumulation in the liver. |
