Glucagon-like peptide 1 (GLP-1) is a naturally occurring peptide secreted by the L cells of the small intestine. GLP-1 functions as an incretin and stimulates glucose-mediated insulin production by pancreatic beta cells. In this study, we demonstrate that exendin-4/GLP-1 has a cognate receptor on human hepatocytes and that exendin-4 has a direct effect on the reduction of hepatic steatosis in the absence of insulin. Both glucagon-like peptide 1 receptor (GLP/R) messenger RNA and protein were detected on primary human hepatocytes, and receptor was internalized in the presence of GLP-1. Exendin-4 increased the phosphorylation of 3-phosphoinositide-dependent kinase-1 (PDK-1), AKT, and protein kinase C zeta (PKC-zeta) in HepG2 and Huh7 cells. Small interfering RNA against GLP-1R abolished the effects on PDK-1 and PKC-zeta. Treatment with exendin-4 quantitatively reduced triglyceride stores compared with control-treated cells.
Conclusion: This is the first report that the G protein-coupled receptor GLP-1R is present on human hepatocytes. Furthermore, it appears that exendin-4 has the same beneficial effects in vitro as those seen in our previously published in vivo study in ob/ob mice, directly reducing hepatocyte steatosis. Future use for human nonalcoholic fatty liver disease, either in combination with dietary manipulation or other pharmacotherapy, may be a significant advance in treatment of this common form of liver disease.