Increased diacylglycerol acyltransferase activity is associated with triglyceride accumulation in tissues of diet-induced insulin-resistant hyperlipidemic hamsters

Abstract

Over-accumulation of triglyceride (TG) in insulin-sensitive tissues is associated with the development of insulin resistance. We investigated whether enhanced de novo lipogenesis via diacylglycerol acyltransferase (DGAT) may contribute to the over-accumulation of TG in various tissues (liver, adipose, muscle, and intestine) using 2 well-characterized hyperlipidemic, insulin-resistant hamster models. In general, a marked increase in TG accumulation was noted in most tissues. Interestingly, the increase in TG accumulation corresponded to an increase in microsomal DGAT activity which ranged from 114% to 575% in all of the examined tissues (n = 7 per group). To delineate the mechanism for the increase in DGAT activity, we measured the expression of DGAT-1 and DGAT-2 messenger RNA by relative reverse transcriptase polymerase chain reaction (RT-PCR). In general, DGAT gene expression changed with DGAT-1 changing the most in the liver and adipose tissue, whereas DGAT-2 showed responses mainly in muscle and intestine. The increases in messenger RNA expression were not remarkable (averaging 35%; n = 4 per group) indicating that posttranscriptional mechanism(s) may play a larger role in regulating DGAT activity. In summary, the data suggest that elevated DGAT activity/expression and the subsequent increase in de novo lipogenesis could in part induce the insulin-resistant state.

Introduction

The accumulation of triglyceride (TG) in insulin-sensitive tissues has been associated with insulin resistance and type 2 diabetes [1], [2]. The term used to describe this phenomenon has been referred to as lipotoxicity. It is thought that lipotoxicity or the accumulation of TG in insulin-sensitive tissues impairs insulin action in their respective organs causing insulin resistance [1], [2]. De novo lipogenesis is known to be one possible mechanism resulting in the accumulation of TG in insulin-sensitive tissues [2]. Activation of the lipogenic transcriptional factor, sterol regulatory element-binding protein-1 that regulates their target enzymes, fatty acid synthase (FAS), and acetyl CoA carboxylase (ACC), appears to be a major contributory factor underlying the increase in lipogenesis [3]. Surprisingly, few studies have examined diacylglycerol acyltransferase (DGAT), the enzyme that catalyzes the final step in TG synthesis, as another possible mechanism for increased lipogenesis and impaired insulin action [4], [5]. Part of the reason may be that progress in understanding DGAT has only emerged since the cloning of DGAT-1 and DGAT-2 in recent years. Although some studies have recently supported the idea that over-expression of DGAT may play a role in insulin resistance [6], [7], these studies have not examined the relationship of TG accumulation with DGAT activity and gene expression in various insulin-sensitive tissues. Therefore, the aim of our study was to characterize intracellular TG mass, DGAT activity, and the expression of DGAT-1 and DGAT-2 genes in various insulin-sensitive tissues (liver, adipose, muscle, and intestine) using 2 well-characterized diet-induced hyperlipidemic, insulin-resistant hamster models—the high fructose–fed hamster and the high fat-fed hamster [8], [9]. Such study could provide a greater insight as to the role of DGAT in the development of insulin resistance and to which DGAT genes may be of greater importance in the development of insulin resistance.