Hepatic insulin resistance integrates T2D and NAFLD

Boston Children'S Hospital, Boston MA

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Abstract

This proposal investigates mechanisms that combine T2D (type 2 diabetes) with NAFLD (non-alcoholic fatty liver disease), which progress during chronic hepatic insulin resistance to life-threatening NASH (non-alcoholic steatohepatitis). Several studies argue that selective hepatic insulin resistance is required to integrate T2D with NAFLD/NASH. Regardless, we posit that complete hepatic insulin resistance exacerbates NAFLD/NASH and T2D in mice fed the western GAN diet—which models common sugar-sweetened food and beverages associated with T2D and NAFLD/NASH in patients. We model complete hepatic insulin resistance with ‘LDKO’ mice that lack hepatic Irs1 (insulin receptor substrate 1) and Irs2, which activates FoxO1 mediated transcription to induce hepatic Fst (follistatin) expression and secretion. Circulating hepatic Fst causes WAT (white adipose tissue) insulin resistance and uncontrolled lipolysis. Circulating FST in human NAFLD patients correlates with insulin resistance in leg fat to release fatty acids from this benign storage site. Our proposal investigates whether hepatic FoxO1 and Fst promote the progression of NAFLD to NASH during hepatic insulin resistance. We use mouse genetics to determine whether inactivation of FoxO1 or Fst in LDKO mice fed the GAN diet can attenuate both NAFLD/NASH trajectory and liver inflammatory gene expression to identify pathways promoting NAFLD and its progression to NASH during hepatic insulin resistance. We test this hypothesis by inactivating TNFα in LDKO mice fed the GAN diet. A simpler high-fructose diet (HFruD60%) is used to investigate whether FoxO1 and Fst promote NAFLD/NASH from hepatic fructose metabolism. Re- esterification of circulating fatty acids with hepatic Gro3P (glycerol-3-phosphate)—a fructose metabolite—is a major source of hepatic triacylglycerol in NAFLD patients. To understand whether fructose promotes NAFLD/NASH by re-esterification of circulating fatty acid during complete hepatic insulin resistance, we can investigate LDKO mice fed the HFruD60% without or after deletion of hepatic FoxO1 or Fst—or Khk (Ketohexokinase) that is essential for hepatic metabolism of fructose. Feeding mice [13C]fructose enables LC- MS to determine the incorporation of fructose metabolites into the glycerol or fatty acid moieties of liver or circulating triacylglycerol. In humans and mice, NAFLD might arise from ‘selective insulin resistance’—owing to uncontrolled hepatic glucose output in conjunction with some insulin-stimulated lipogenesis; however, we posit that selective hepatic insulin resistance might have the opposite effect and attenuate NAFLD/NASH owing to inhibition of FoxO1 and Fst. Since chronic nutrient excess can suppress hepatic Irs2, we investigate GAN diet- induced NAFLD/NASH and T2D in mice lacking hepatic Irs2 (LKO2 mice) or Irs1 (LKO1 mice), as well as our novel transgenic mice expressing nutrient-insensitive Irs2tg in hepatocytes. Completion of our proposal can impact human health by identifying systemic and hepatic metabolic regulatory mechanisms by which hepatic insulin resistance integrates T2D with NAFLD/NASH.