The ongoing debate regarding pathway selective hepatic insulin resistance (IR) focuses on potential insulin signaling branchpoints beyond which regulation of glucose metabolism is attenuated and regulation of de novo lipogenesis (DNL) remains intact. These models suggest that impaired insulin receptor kinase activity, proximal to any potential branchpoint, only plays a minor role in clinical hepatic IR. We compared insulin signaling in liver biopsies from human subjects with nonalcoholic fatty liver disease (NAFLD) and hepatic IR vs. weight matched controls, using glucose ingestion to drive insulin secretion and fructose ingestion as a low insulin comparator. Insulin signaling through phosphorylation of insulin receptor β, Akt, and GSK3β was suppressed in NAFLD (all P < 0.05). Insulin regulation of mTORC1 via PRAS40 phosphorylation was absent in NAFLD subjects; no differences were seen between controls and NAFLD subjects in AMPK dependent RAPTOR phosphorylation. Pathway selective IR implies SREBP1c is primarily responsible for DNL in the insulin resistant liver; we probed the transcriptional regulation of DNL by assessing the in vivo occupancy of lipogenic target gene promoters by SREBP1c and ChREBP using ChIP experiments in mice with diet-induced IR. Fasted-refed mice were refed with high fat diet (HFD) and 1% dextrose drinking water. Mice with hepatic IR (9 day HFD fed) or hepatic and peripheral IR (4 week HFD fed) were compared with more insulin sensitive 2 day HFD fed mice. SREBP1c was not bound to the Fasn promoter in insulin resistant livers, while ChREBP was bound to the Fasn promoter equally in all three groups of mice.
Conclusions: There is a proximal block in insulin signaling in humans with hepatic IR and NAFLD, dissociating previously measured changes in DNL rates from insulin signaling. SREBP does not mediate DNL in insulin resistant mice while ChREBP action can explain the maintenance of the transcriptional program of lipogenesis.
D.F. Vatner: None. K.W. ter Horst: None. L. Goedeke: None. D. Zhang: None. V. Samuel: None. M.J. Serlie: Advisory Panel; Self; Fresenius Medical Care. G.I. Shulman: Advisory Panel; Self; AstraZeneca, Janssen Research & Development, Merck & Co., Inc. Advisory Panel; Spouse/Partner; Merck & Co., Inc. Board Member; Self; Novo Nordisk A/S. Consultant; Self; Aegerion Pharmaceuticals, IMetabolic BioPharma Corporation, Longitude Capital, Nimbus Discovery, Inc., Staten Biotechnology B.V.
National Institutes of Health