Non alcoholic fatty liver disease (NAFLD) hepatic insulin resistance and type

Non alcoholic fatty liver disease (NAFLD) hepatic insulin resistance and type 2 diabetes are all strongly associated and are all reaching epidemic proportions. insulin resistance and type 2 diabetes. The DAG-PKCε hypothesis can explain the occurrence of hepatic insulin resistance observed in most LOR-253 cases of NAFLD associated with obesity lipodystrophy and type 2 diabetes. synthesis of TAG from carbohydrate and protein metabolism exceeds the rate of hepatic TAG catabolism due to fatty acid oxidation and export of TAG as very low density lipoproteins (VLDL). The liver derives most of its energy for LOR-253 metabolism from fatty acid oxidation during both fasting and feeding and the contributions of fatty LOR-253 acid oxidation to hepatic energy metabolism approaches 100% with hepatic steatosis (7) Circulating fatty acids are taken up into the liver through specific membrane proteins i.e. FATP2 and FATP5 FAT/CD36 and caveolins (3 8 Part of the intracellular pathways of lipid storage mobilization synthesis oxidation and export are portrayed in Figures 1 and ?and22. Figure 1 Molecular Regulation of Intrahepatic TAG and DAG Turnover Figure 2 Mechanism of Diacylglycerol-PKCε Mediated Hepatic Insulin Resistance Hepatic lipid metabolism DAGs and hepatic insulin resistance Numerous studies have demonstrated a strong relationship between intramyocellular lipids and muscle insulin resistance (3 9 10 Studies in normal weight nondiabetic adults found that intramyocellular triglyceride content is a far stronger predictor of muscle insulin resistance than circulating fatty acids (11) suggesting that intramyocellular lipids may be playing a causal role in muscle insulin resistance. In fact insulin sensitive and resistant obese subjects can be separated on the basis of muscle and liver lipid accumulation (12). In rodent models when plasma fatty acids were increased by infusing Liposyn along with heparin to activate lipoprotein lipase muscle insulin resistance developed at ~3 hours into the infusion when diacylglycerols (DAG) increased and PKCθ was activated (13). In contrast there were no changes in muscle triglyceride or ceramide content at this time thus LOR-253 disassociating these lipids as causal factors in the pathogenesis of lipid-induced muscle insulin resistance. DAGs are second messengers activating members of novel protein kinase C (nPKC) family. These findings of DAG-mediated muscle insulin resistance have subsequently been translated and confirmed in humans (14-16). Hepatic steatosis and hepatic insulin resistance can be induced in mice and rats with 3 days of high-fat diet (HFD) LOR-253 before the development of obesity (17). Livers PP2Bbeta of these 3-day HFD fed rats showed increases in hepatic DAG species originating mainly from dietary sources. Similar to the muscle studies there were no alterations in liver ceramide content thus disassociating hepatic ceramide content from hepatic insulin resistance in these studies. The connection between hepatic DAG accumulation and hepatic insulin resistance could be attributed to activation of PKCε which is highly expressed in liver (17). These changes were associated with reductions in insulin-stimulated insulin receptor substrate-2 (IRS-2) tyrosine phosphorylation by the insulin receptor kinase leading to reductions in insulin stimulation of hepatic glycogen synthesis and suppression of hepatic glucose production (Figure 2). The fact that hepatic insulin resistance occurred prior to any changes in systemic insulin resistance inflammation or adipose tissue mass argues strongly in support of a primary causal role of DAG-PKCε in mediating hepatic insulin resistance (18). The specific role of PKCε in causing hepatic insulin resistance was also directly examined. Knock-down of hepatic expression of PKCε using antisense oligonucleotides in rats as well as PKCε gene knockout mice were both found to protected from lipid-induced hepatic insulin resistance when fed a HFD despite the development of hepatic steatosis (18 19 Complementary lines of evidence confirm a pivotal role of DAGs in the development of hepatic insulin resistance. First mitochondrial acyl-CoA:glycerol-sn-3-phosphate acyltransferase (mtGPAT) catalyzes the formation of lysophosphatidic acid (LPA) from fatty LOR-253 acyl CoA and glycerol 3-phosphate (Figure 1). When mtGPAT-deficient (mtGPAT1?/?) mice are placed on a high-fat diet they accumulate.