Hepatic insulin resistance in mice with hepatic overexpression of diacylglycerol acyltransferase 2

Abstract

Mice overexpressing acylCoA:diacylglycerol (DAG) acyltransferase 2 in the liver (Liv-DGAT2) have been shown to have normal hepatic insulin responsiveness despite severe hepatic steatosis and increased hepatic triglyceride, diacylglycerol, and ceramide content, demonstrating a dissociation between hepatic steatosis and hepatic insulin resistance. This led us to reevaluate the role of DAG in causing hepatic insulin resistance in this mouse model of severe hepatic steatosis. Using hyperinsulinemic-euglycemic clamps, we studied insulin action in Liv-DGAT2 mice and their wild-type (WT) littermate controls. Here, we show that Liv-DGAT2 mice manifest severe hepatic insulin resistance as reflected by decreased suppression of endogenous glucose production (0.8 ± 41.8 vs. 87.7 ± 34.3% in WT mice, P < 0.01) during the clamps. Hepatic insulin resistance could be attributed to an almost 12-fold increase in hepatic DAG content (P < 0.01) resulting in a 3.6-fold increase in protein kinase Cε (PKCε) activation (P < 0.01) and a subsequent 52% decrease in insulin-stimulated insulin receptor substrate 2 (IRS-2) tyrosine phosphorylation (P < 0.05), as well as a 64% decrease in fold increase pAkt/Akt ratio from basal conditions (P < 0.01). In contrast, hepatic insulin resistance in these mice was not associated with increased endoplasmic reticulum (ER) stress or inflammation. Importantly, hepatic insulin resistance in Liv-DGAT2 mice was independent of differences in body composition, energy expenditure, or food intake. In conclusion, these findings strengthen the link between hepatic steatosis and hepatic insulin resistance and support the hypothesis that DAG-induced PKCε activation plays a major role in nonalcoholic fatty liver disease (NAFLD)-associated hepatic insulin resistance.protein kinase Cε | ceramides, insulin receptor kinase | type 2 diabetes H epatic insulin resistance associated with nonalcoholic fatty liver disease (NAFLD) is a major risk factor for the development of type 2 diabetes (1-6). However, whether there is a causal link between NAFLD and hepatic insulin resistance remains controversial (7,8). Although intracellular triglyceride is unlikely to be the direct mediator of insulin resistance (3, 4), other related intracellular lipid intermediates such as diacylglycerol (3, 4, 9-12) and ceramide (13) have been postulated to block insulin signaling at the level of the insulin receptor kinase and Akt2, respectively, leading to insulin resistance. Indeed, diacylglycerol (DAG) has been shown to activate protein kinase Cε (PKCε) in the liver, which in turn binds to the insulin receptor and inhibits its tyrosine kinase activity (14).However, a recent paper reported that mice overexpressing acylCoA:diacylglycerol acyltransferase 2 (Liv-DGAT2) in the liver were not insulin resistant despite severe hepatic steatosis associated with increased liver DAG content (15). This study was of critical importance because it dissociated hepatic DAG content from hepatic insulin resistance and implied that hepatic DAG was not a causal fa...

Metabolic syndrome and nonalcoholic fatty liver disease: Is insulin resistance the link?

“…Indeed, numerous studies in animal models of NAFLD reveal a clear link between NAFLD and hepatic insulin resistance (Birkenfeld et al, 2011b, Camporez et al, 2013a, 2013bCantley et al, 2013;Jornayvaz et al, 2010aJornayvaz et al, , 2011Lee et al, 2011). Notably, evidence suggests that certain lipid intermediates, such as diacylglycerols, activate novel protein kinase Cε (PKCε) (Fig.…”

XBP1s Is an Anti-lipogenic Protein

“…2C). Cellular localization of PKC⑀, which can be analyzed by the ratio of cytosolic to membrane PKC⑀, is an important determinant for PKC⑀ activity (32,33). Thus, we next analyzed PKC⑀ levels in cytosolic and membrane compartments of livers of adXBP1s-and adLacZ-injected mice.…”

Identification and Validation of a Selective Small Molecule Inhibitor Targeting the Diacylglycerol Acyltransferase 2 Activity

“…90% TG reduction) and have impaired skin barrier function, leading to early death, 7) while DGAT1 knockout mice are still viable with a moderate reduction in TG (ca. 50% TG reduction).8) In addition, hepatic suppression of DGAT2 with antisense oligonucleotides reduced hepatic TG content, increased fatty acid oxidation, and thereby reversed diet-induced hepatic steatosis and insulin resistance in rodents.9,10) Conversely, upregulation of DGAT2 expression induced increase of cytoplasmic TG content and lipid droplets (LDs) in rat hepatocytes 11) and adipocytes, 12) and similarly liver-directed DGAT2 overexpression caused hepatic TG accumulation 11) and promoted hepatic insulin resistance 13) in mice. Therefore, inhibition of DGAT2 enzyme, particularly by small molecule, is expected to be a feasible therapeutic strategy for hepatic steatosis and its complications, such as insulin resistance.…”