Lipid-induced insulin resistance: unravelling the mechanism

Abstract

Insulin resistance has long been associated with obesity. More than 40 years ago, Randle and colleagues postulated that lipids impaired insulin-stimulated glucose use by muscles through inhibition of glycolysis at key points. However, work over the past two decades has shown that lipid-induced insulin resistance in skeletal muscle stems from defects in insulin-stimulated glucose transport activity. The steatotic liver is also resistant to insulin in terms of inhibition of hepatic glucose production and stimulation of glycogen synthesis. In muscle and liver, the intracellular accumulation of lipids-namely, diacylglycerol-triggers activation of novel protein kinases C with subsequent impairments in insulin signalling. This unifying hypothesis accounts for the mechanism of insulin resistance in obesity, type 2 diabetes, lipodystrophy, and ageing; and the insulin-sensitising effects of thiazolidinediones.

Figure 1. Glucose-fatty-acid cycle proposed by Randle and colleagues

CoA=coenzyme A. PDH=pyruvate dehydrogenase. PFK=phosphofructokinase. G6P=glucose-6-phosphate. HK=hexokinase. Red circle with minus sign represents inhibition. Black line with arrowhead represents increase or accumulation of substrate. Blue dotted line with arrowhead indicates a pathway that is inhibited.

Figure 2. Mechanisms of insulin sensitivity and resistance in muscle and liver

(A) Insulin-sensitive muscle. (B) Insulin-resistant muscle. (C) Insulin-sensitive liver. (D) Insulin-resistant liver. IRS=insulin-receptor substrate. IR=insulin receptor. PI3K=1-phosphatidylinositol 3-kinase. GLUT4=glucose transporter 4. DAG=diacylglycerol. PKC=protein kinase C. Ser=serine. Thr=threonine. FOX01=forkhead box O1. FOXA2=forkhead box A2. G6P=glucose-6-phosphate. GS=glycogen synthase. GSK=glycogen synthase kinase. Green circle with plus sign represents activation. Red circle with minus sign represents inactivation. Solid line with arrowhead represents increase or accumulation of substrate. Dotted line indicates inhibition of pathway.