Abstract 597: Disruptions in Hepatic Insulin Signaling Reveal an Enterohepatic Signaling Pathway that Regulates the ABCG5 ABCG8 Sterol Transporter and Biliary Cholesterol Secretion

Insulin resistance is associated with increased risk for cholesterol gallstones as well as the development of diabetic dyslipidemia. HDL is the primary cholesterol carrier in the reverse cholesterol transport (RCT) pathway, the process by which cholesterol is delivered from peripheral organs to the liver for elimination in bile. Therefore, we hypothesized that insulin signaling regulates hepatobiliary cholesterol transport in the RCT pathway. To test the role of insulin signaling, we utilized mice harboring insulin receptor flanked by loxP sites (IR fl/fl ) in combination with adenoassociated viral vectors containing no transgene (empty) or Cre recombinase to generate control and liver insulin receptor knock out (LIRKO) mice, respectively. As with previous LIRKO models, our mice showed markedly reduced insulin receptor mRNA and protein in liver, but not skeletal muscle or adipose tissue, and impaired glucose tolerance. LIRKO mice had increased biliary cholesterol secretion as well as increased expression of the ABCG5 ABCG8 sterol transporter, the primary mediator of biliary cholesterol secretion. Levels of SR-BI, the primary HDL receptor, were unchanged as were rates of HDL clearance from plasma and selective delivery of HDL cholesterol to the liver. We also observed increased ileal fibroblast growth factor (FGF)15 mRNA. Wild-type mice treated with FGF19 exhibited increased ABCG5 ABCG8 protein expression in the liver. Immunofluorescence microscopy was used to examine the subcellular localization of ABCG5 in the liver following FGF19 administration. Under control conditions, G5 appeared in puncta, diffusely distributed within hepatocytes. Following treatment with FGF19, G5 signal intensity was substantially increased and juxtaposed to zonula occcludin-1 (ZO-1), a tight junction protein the delineates the canalicular channels within the liver. In conclusion, depletion of hepatic insulin receptors increases G5G8 abundance and promotes its localization to the canalicular surface. This effect is associated with an increase in ileal FGF15 expression which promotes its localization to the apical surface and drives biliary cholesterol secretion.