Identification of acyl coenzyme A:monoacylglycerol acyltransferase 3, an intestinal specific enzyme implicated in dietary fat absorption.

Acyl coenzyme A:monoacylglycerol acyltransferase (MGAT) catalyzes the synthesis of diacylglycerol using 2-monoacylglycerol and fatty acyl coenzyme A. This enzymatic reaction is believed to be an essential and rate-limiting step for the absorption of fat in the small intestine. Although the first MGAT-encoding cDNA, designated MGAT1, has been recently isolated, it is not expressed in the small intestine and hence cannot account for the high intestinal MGAT enzyme activity that is important for the physiology of fat absorption. In the current study, we report the identification of a novel MGAT, designated MGAT3, and present evidence that it fulfills the criteria to be the elusive intestinal MGAT. MGAT3 encodes a approximately 36-kDa transmembrane protein that is highly homologous to MGAT1 and -2. In humans, expression of MGAT3 is restricted to gastrointestinal tract with the highest level found in the ileum. At the cellular level, recombinant MGAT3 is localized to the endoplasmic reticulum. Recombinant MGAT3 enzyme activity produced in insect Sf9 cells selectively acylates 2-monoacylglycerol with higher efficiency than other stereoisomers. The molecular identification of MGAT3 will facilitate the evaluation of using intestinal MGAT as a potential point of intervention for antiobesity therapies.

Concerted elevation of acyl-coenzyme A:diacylglycerol acyltransferase (DGAT) activity through independent stimulation of mRNA expression of DGAT1 and DGAT2 by carbohydrate and insulin.

Glucose and insulin are anabolic signals which upregulate the transcriptions of a series of lipogenic enzymes to convert excess carbohydrate into triglycerides for efficient energy storage. These enzymes include ATP-citrate lyase (ACL), acetyl-coenzyme A carboxylase (ACC), fatty acid synthase (FAS), and glycerol-3-phosphate acyltransferase (G3PA). Acyl-coenzyme A:diacylglycerol acyltransferase (DGAT) is important to synthesize fatty acids into triglycerides. Two DGATs from different gene families have recently been identified. In the current study, we report that glucose preferentially enhances DGAT1 mRNA expression, whereas insulin specifically increases the level of DGAT2 mRNA. Treatment of adipocytes with glucose and insulin together results in higher DGAT activity in the membrane than cells treated with either of the agents alone, indicating that glucose and insulin have additive effect on DGAT activation. In mice treated with fast/refeeding protocol, DGAT2 mRNA decreased upon fasting and was replenished upon refeeding in adipose tissue and liver. This pattern of change was not observed for DGAT1. Inasmuch as DGAT1 mRNA is less abundant in liver, we suggest that DGAT1 is more involved in fat absorption in the intestine and in basal level triglyceride synthesis in adipose tissue where it is more highly expressed. In contrast, DGAT2 is more likely to play important roles in assembly of de novo synthesized fatty acids into VLDL particles in the liver.