Diacylglycerol activation of protein kinase Cε and hepatic insulin resistance

Abstract

Nonalcoholic fatty liver disease (NAFLD) is now the most frequent chronic liver disease in Western societies, affecting one in four adults in the USA, and is strongly associated with hepatic insulin resistance, a major risk factor in the pathogenesis of type 2 diabetes. Although the cellular mechanisms underlying this relationship are unknown, hepatic accumulation of diacylglycerol (DAG) in both animals and humans has been linked to hepatic insulin resistance. In this Perspective, we discuss the role of DAG activation of protein kinase Cε as the mechanism responsible for NAFLD-associated hepatic insulin resistance seen in obesity, type 2 diabetes, and lipodystrophy.

Figure 1. Molecular Mechanism of Diacylglycerol-PKCε Mediated Hepatic Insulin Resistance

The accumulation of diacylglycerol (DAG) in the liver leads to the activation of protein kinase Cε (PKCε), which subsequently inhibits the insulin receptor kinase. This then leads to decreased insulin-stimulated tyrosine phosphorylation (pY) of insulin receptor substrate 1 and 2 (IRS1, IRS2), resulting in reduced insulin activation of 1-phosphoinositol 3-kinase (PI 3-kinase) and Akt2. Reduced Akt2 activation results in decreased glycogen synthase (GS)-mediated glycogen synthesis and decreased suppression of gluconeogenesis, which in turn leads to glucose release through glucose transporter 2 (GLUT2). FATP5, fatty acid transport protein 5; FOXO, forkhead box protein O; G6Pase, glucose-6-phosphatase; GSK3, glycogen synthase kinase-3; LCoAs, long chain fatty acids; PDK, pyruvate dehydrogenase kinase; PEPCK, phosphoenolpyruvate carboxykinase; PIP2, phosphatidylinositol bisphosphate; PIP3, phosphatidylinositol trisphosphate; PH, pleckstrin homology domain; PTB, phosphotyrosine binding domain; SH2, src homology domain.

Figure 2. Mechanisms of Hepatocellular Diacylglycerol Accumulation

Increases in hepatic diacylglycerol (DAG) content results from an imbalance in rates of fatty acid delivery/uptake relative to rates of mitochondrial fatty acid oxidation and conversion of DAGs to triglycerides (TAGs). Increased energy intake exceeding rates of energy expenditure, is the most common cause of NAFLD and DAG-PKCε induced hepatic insulin resistance, which is most often seen in exogenous obesity. Predisposing genetic factors such as ApoC3 gene variants resulting in increased plasma ApoC3 concentrations results in suppression of lipoprotein lipase activity, increased postprandial chylomicron remnants and increased hepatic fat uptake, increased hepatic DAG content, PKCε-mediated hepatic insulin resistance. Defects in adipocyte fat storage as seen in lipodystrophy or due to genetic alterations (e.g. mutations in PPARγ or perilipin (Agostini et al., 2006; Gandotra et al., 2011) also can result in increased fat delivery to the liver, NAFLD and hepatic insulin resistance. Genetic or acquired defects in mitochondrial fatty acid oxidation may also predispose to NAFLD and DAG-PKCε mediated hepatic insulin resistance. Finally, fatty acids released from adipocytes, can enter the liver through the liver specific fatty acid transport protein 5 (FATP5), resulting in increased long chain fatty acids (LCoAs) (Schaffer and Lodish, 1994), which can then be converted to DAG.

Figure 3. Metabolic Pathways Leading to Hepatic Diacylglycerol Accumulation

The glycerol 3-phosphate (or phosphatidic acid) pathway represents the de novo lipogenesis route in the synthesis of triglycerides (TAG) and phospholipids. Acyl-CoA:glycerol-sn-3-phosphate acyltransferase (GPAT) catalyzes the acylation of sn-glycerol-3-phosphate with acyl-coenzyme A (acyl-CoA) to generate lysophosphatidic acid (LPA). LPA is thought to be the rate-controlling step in TAG synthesis. Subsequently, the enzymes acyl-CoA:1-acylglycerol-sn-3-phosphate acyltransferase (AGPAT), phosphatidic acid phosphatase (PAP) and diacylglycerol:acyl-CoA acyltransferase (DGAT) generate phosphatidic acid (PA), diacylglycerol (DAG) and TAG. In the liver, TAG is either deposited in intracellular vacuoles or exported in very low-density lipoproteins (VLDL) particles. LPA and PA require translocation through the cytosol for TAG synthesis at the endoplasmic reticulum if they are not synthesized in the endoplasmic reticulum. DAG can be hydrolyzed to monoacylglycerol (MAG) by hormone-sensitive lipase (HSL) and subsequently to glycerol by monoglyceride lipase (MGL). These reactions release fatty acids. Glycerol can be used as a substrate for gluconeogenesis. The conversion from TAG to DAG is mediated by adipose triglyceride lipase (ATGL). Comparative Gene Identification-58 (CGI-58) is an activator of ATGL. DAG activate PKCε membrane translocation to inhibit the insulin receptor kinase. Phospholipase C can release DAG from membrane lipids. Whether DAG derived from the phospholipase C pathway and from lipid droplets can lead to PKCε activation and hepatic insulin resistance remains to be determined.