Suppression of Diacylglycerol Acyltransferase-2 (DGAT2), but Not DGAT1, with Antisense Oligonucleotides Reverses Diet-induced Hepatic Steatosis and Insulin Resistance

Abstract

Nonalcoholic fatty liver disease (NAFLD) is a major contributing factor to hepatic insulin resistance in type 2 diabetes. Diacylglycerol acyltransferase (Dgat), of which there are two isoforms (Dgat1 and Dgat2), catalyzes the final step in triglyceride synthesis. We evaluated the metabolic impact of pharmacological reduction of DGAT1 and -2 expression in liver and fat using antisense oligonucleotides (ASOs) in rats with diet-induced NAFLD. Dgat1 and Dgat2 ASO treatment selectively reduced DGAT1 and DGAT2 mRNA levels in liver and fat, but only Dgat2 ASO treatment significantly reduced hepatic lipids (diacylglycerol and triglyceride but not long chain acyl CoAs) and improved hepatic insulin sensitivity. Because Dgat catalyzes triglyceride synthesis from diacylglycerol, and because we have hypothesized that diacylglycerol accumulation triggers fat-induced hepatic insulin resistance through protein kinase C⑀ activation, we next sought to understand the paradoxical reduction in diacylglycerol in Dgat2 ASO-treated rats. Within 3 days of starting Dgat2 ASO therapy in high fat-fed rats, plasma fatty acids increased, whereas hepatic lysophosphatidic acid and diacylglycerol levels were similar to those of control rats. These changes were associated with reduced expression of lipogenic genes (SREBP1c, ACC1, SCD1, and mtGPAT) and increased expression of oxidative/thermogenic genes (CPT1 and UCP2). Taken together, these data suggest that knocking down Dgat2 protects against fat-induced hepatic insulin resistance by paradoxically lowering hepatic diacylglycerol content and protein kinase C⑀ activation through decreased SREBP1c-mediated lipogenesis and increased hepatic fatty acid oxidation. Nonalcoholic fatty liver disease (NAFLD)5 is the most frequent cause of abnormal liver function tests in the United States (estimated prevalence of 14 -20%) (1, 2). It is caused by triglyceride (TG) accumulation within the liver and is strongly associated with insulin resistance, type 2 diabetes mellitus (T2DM), and the metabolic syndrome (3, 4). Accumulating evidence suggests that hepatic lipid accumulation causes hepatic insulin resistance. For example, increasing hepatic lipid stores in mice by overexpressing lipoprotein lipase in the liver (5) and in rats by short term high fat feeding (6) results in liver-specific fat accumulation and hepatic insulin resistance. Several strategies have been employed to reduce hepatic steatosis in rodents; these include treatment with a mitochondrial uncoupling agent (2,4-dinitrophenol) (6), antisense oligonucleotide inhibition of acetyl-coenzyme A carboxylase I and II (7), adenoviral overexpression of malonyl-CoA decarboxylase (8), and transgenic overexpression of uncoupling protein 1 (9), all of which successfully ameliorated hepatic insulin resistance. We have also shown that moderate weight loss in patients with T2DM lowers liver triglycerides and specifically improves hepatic insulin sensitivity (10). Although the above data strongly suggest that hepatic lipid accumulation causes hepatic insulin r...

SirT1 knockdown in liver decreases basal hepatic glucose production and increases hepatic insulin responsiveness in diabetic rats

“…1F). ASOs have been previously shown not to distribute to muscle and not to cause reduction in gene expression in this tissue (6,17,18). Additionally, we tested the transcription of SirT1 in the hypothalamus as a possible explanation for the decreased food intake and found no differences in mRNA (control ASO 1.00 Ϯ 0.14 vs. SirT1 ASO 1.08 Ϯ 0.13) on the normal chow diet.…”

Distinct patterns of tissue-specific lipid accumulation during the induction of insulin resistance in mice by high-fat feeding

“…While TGs are now considered to be a metabolically inert storage pool [11,17], it remains unclear which bioactive lipid species may be most important in the induction of insulin resistance. Many studies have reported increased levels of DGs or ceramides in insulin-resistant states and have demonstrated that reductions in these lipid species can improve insulin action [5,[9][10][11][12][14][15][16]. One mechanism through which these lipid species have been proposed to drive insulin resistance is through antagonism of the insulin-signalling cascade [4,12].…”

“…Elevated DGs in muscle and liver are associated with insulin resistance in humans [5,6] and animals [7,8]. Genetic manipulations in rodents that alter DG levels also support a role for this lipid intermediate as an important mediator of insulin action [9][10][11].…”

Hepatitis C Virus Core Protein Decreases Lipid Droplet Turnover

“…DGAT1 and DGAT2 account for nearly all TG synthesis in mammalian cells (29), and both enzymes reside in the ER, although they are found in distinct ER microdomains (30). Both DGATs have been implicated in the development of diet-induced hepatic steatosis (31)(32)(33). However, although DGAT2 deficiency in mice is lethal early after birth, DGAT1 deficiency protects against hepatic steatosis in both diet-induced obesity and fasting paradigms, but it has no effect on steatosis in situations where de novo lipogenesis is increased (32).…”