Liver‐directed overexpression of mitochondrial glycerol‐3‐phosphate acyltransferase results in hepatic steatosis, increased triacylglycerol secretion and reduced fatty acid oxidation

Abstract

Glycerol-3-phosphate acyltransferase (GPAT) catalyzes the first committed step in triacylglycerol (TAG) and phospholipid biosynthesis. GPAT activity has been identified in both ER and mitochondrial subcellular fractions. The ER activity dominates in most tissues except in liver, where the mitochondrial isoform (mtGPAT) can constitute up to 50% of the total activity. To study the in vivo effects of hepatic mtGPAT overexpression, mice were transduced with adenoviruses expressing either murine mtGPAT or a catalytically inactive variant of the enzyme. Overexpressing mtGPAT resulted in massive 12- and 7-fold accumulation of liver TAG and diacylglycerol, respectively but had no effect on phospholipid or cholesterol ester content. Histological analysis showed extensive lipid accumulation in hepatocytes. Furthermore, mtGPAT transduction markedly increased adipocyte differentiation-related protein and stearoyl-CoA desaturase-1 (SCD-1) in the liver. In line with increased SCD-1 expression, 18:1 and 16:1 in the hepatic TAG fraction increased. In addition, mtGPAT overexpression decreased ex vivo fatty acid oxidation, increased liver TAG secretion rate 2-fold, and increased plasma TAG and cholesterol levels. These results support the hypothesis that increased hepatic mtGPAT activity associated with obesity and insulin resistance contributes to increased TAG biosynthesis and inhibition of fatty acid oxidation, responses that would promote hepatic steatosis and dyslipidemia.

Thematic Review Series: Glycerolipids. Mammalian glycerol-3-phosphate acyltransferases: new genes for an old activity

“…In vitro experiments show that overexpression of GPAT1 in hepatocytes increases incorporation of fatty acids into triglycerides and decreases fatty acid oxidation (40,66). Similarly, in vivo, GPAT1 overexpression leads to decreased hepatic fatty acid oxidation and increased triglyceride biosynthesis (64). In GPAT1 knock-out mice, hepatic fatty acid oxidation rates and serum ketone bodies are increased (60,61), consistent with a preferential utilization of fatty acids through b-oxidation.…”

“…Knock-down of GPAT1 in ob/ob mice, a model of type 2 diabetes, showed no significant improvement in glucose homeostasis (65). Overexpression of GPAT1 in mice (64) or rats (63) showed no effect (64) or an impairment in insulin sensitivity (63), respectively. It is likely that multiple alterations in hepatic lipid metabolism occur upon GPAT1 modulation that are integrated together, leading to either improved or impaired glucose metabolism, depending on dietary conditions.…”

Regulation of Triglyceride Metabolism II. Function of mitochondrial GPAT1 in the regulation of triacylglycerol biosynthesis and insulin action

“…14; Nagle CA, An J, Shiota M, Torres TP, Cline GW, Liu ZX, Wang S, Catlin RL, Shulman GI, Newgard CB, Coleman RA, unpublished observations). In mice treated with adenovirus-GPAT1, hepatic TAG increases 12-fold, and adipocyte differentiation-related protein and stearoyl-CoA desaturase-1 expression are induced (14). Although body and fat pad weights are similar in rats that overexpress GPAT1 and control rats that express green fluorescent protein, a hyperinsulinemic-euglycemic clamp study shows that hepatic insulin resistance develops, with a glucose output that is 2.5-fold higher than present in control mice (Nagle CA, An J, Shiota M, Torres TP, Cline GW, Liu ZX, Wang S, Catlin RL, Shulman GI, Newgard CB, Coleman RA, unpublished observations).…”

AGPAT6 Is a Novel Microsomal Glycerol-3-phosphate Acyltransferase

“…GPAT1 appears to prefer saturated fatty acyl-CoAs as substrates, and its enzymatic activity is resistant to sulfhydryl-modifying agents, such as N-ethylmaleimide (NEM) (4). Overexpression of GPAT1 in primary hepatocytes or in the liver leads to increased TG synthesis and reduced fatty acid oxidation (8,9). Conversely, mice deficient in GPAT1 exhibit reduced fat mass, lower body weight, reduced hepatic TG content, and improved insulin sensitivity (10,11).…”