DGAT enzymes and triacylglycerol biosynthesis

Triacylglycerols (triglycerides) (TGs) are the major storage molecules of metabolic energy and FAs in most living organisms. Excessive accumulation of TGs, however, is associated with human diseases, such as obesity, diabetes mellitus, and steatohepatitis. The final and the only committed step in the biosynthesis of TGs is catalyzed by acylCoA:diacylglycerol acyltransferase (DGAT) enzymes. The genes encoding two DGAT enzymes, DGAT1 and DGAT2, were identified in the past decade, and the use of molecular tools, including mice deficient in either enzyme, has shed light on their functions. Although DGAT enzymes are involved in TG synthesis, they have distinct protein sequences and differ in their biochemical, cellular, and physiological functions. Both enzymes may be useful as therapeutic targets for diseases. Here we review the current knowledge of DGAT enzymes, focusing on new advances since the cloning of their genes, including possible roles in human health and diseases.—Yen, C-L. E., S. J. Stone, S. Koliwad, C. Harris, and R. V. Farese, Jr. DGAT enzymes and triacylglycerol biosynthesis. J. Lipid Res. 2008. 49: 2283–2301. Supplementary key words triacylglycerols • triglycerides • acyl-CoA: diacylglycerol acyltransferase • diacylglycerol • fatty acyl-CoA • lipoprotein • adipose • obesity • intestine • mammary gland Triacylglycerols (triglycerides) (TGs), a major type of neutral lipid, are a heterogeneous group of molecules with a glycerol backbone and three FAs attached by ester bonds. The physical and chemical properties of TG differ based on chain length and the degree to which their FAs are desaturated. TGs serve multiple important functions in living organisms. Chief among these, they are the major storage molecules of FA for energy utilization and the synthesis of membrane lipids. Because they are highly reduced and anhydrous, TGs store 6-fold more energy than the same amount of hydrated glycogen (1). In plants, TGs are a major component of seed oils, which are valuable resources for dietary consumption and industrial uses. TG from plants and microorganisms can also be used for biofuels. In animals, energy stores of TG are concentrated primarily in adipocytes, although TGs are also found prominently in myocytes, hepatocytes, enterocytes, and mammary epithelial cells. In addition to energy storage, TG synthesis in cells may protect them from the potentially toxic effects of excess FA. In the enterocytes and hepatocytes of most mammals, TGs are synthesized for the assembly and secretion of lipoproteins, which transport dietary and endogenously synthesized FA between tissues. Also, TGs in secreted lipids acts as a component of the skinʼs surface water barrier, and collections of TG in adipose tissue provide insulation for organisms. Although TGs are essential for normal physiology, the excessive accumulation of TG in human adipose tissue results in obesity and, in nonadipose tissues, is associated with organ dysfunction. For example, excessive TG deposition in skeletal muscle and the liver is associated with insulin resistance, in the liver with nonalcoholic steatohepatitis, and in the heart with cardiomyopathy (2, 3). Owing to worldwide increases in the prevalence of obesity and other diseases of excessive TG accumulation, an understanding of the basic processes that govern TG synthesis and storage is of considerable biomedical importance. This work was supported by American Heart Association Scientist Development Grants (C-L.E.Y., S.J.S.), an A.P. Giannini Foundation Award (S.K.), National Institutes of Health Grant DK-56084 (R.V.F.), the Sandler Family Supporting Foundation, and the J. David Gladstone Institutes. Manuscript received 8 August 2008 and in revised form 29 August 2008. Published, JLR Papers in Press, August 29, 2008. DOI 10.1194/jlr.R800018-JLR200 Abbreviations: ACAT, acyl-CoA:cholesterol acyltransferase; AMPK, AMP-activated kinase; apoB, apolipoprotein B; ARAT, acyl-CoA:retinol acyltransferase; ASO, anti-sense oligonucleotide; DG, diacylglycerol; DGAT, acyl-CoA:diacylglycerol acyltransferase; ER, endoplasmic reticulum; GPAT, glycerol-phosphate acyltranserase; MG, monoacylglycerol; MGAT, acyl-CoA:monoacylglycerol acyltransferase; SCD, stearoyl-CoA desaturase; SREBP, sterol-regulatory element binding protein; TG, triacylglycerol (triglyceride); WAT, white adipose tissue; XBP1, X-box binding protein 1. 1 To whom correspondence should be addressed. e-mail: [email protected] Copyright © 2008 by the American Society for Biochemistry and Molecular Biology, Inc. This article is available online at http://www.jlr.org Journal of Lipid Research Volume 49, 2008 2283 at P E N N S T A T E U N IV E R S IT Y , on F ebuary 1, 2013 w w w .j.org D ow nladed fom Two major pathways for TG biosynthesis, elucidated in the 1950s and 1960s, are known: the glycerol phosphate or Kennedy pathway (4) and the monoacylglycerol (MG) pathway (Fig. 1A) (for reviews of pathway biochemistry, see Refs. 5–8). Both pathways use fatty acyl-CoAs, the “activated” forms of FA synthesized by intracellular acyl-CoA synthases, as acyl donors (9). The glycerol phosphate pathway is present in most cells. In contrast, the MG pathway is found in specific cell types, such as enterocytes, hepatocytes, and adipocytes, where it may participate in the reesterification of hydrolyzed TG (10). The MG pathway is the dominant mode of TG synthesis in human small intestine, where TGs are synthesized from components of hydrolyzed dietary fats (11, 12). In the final reaction of both pathways, a fatty acyl-CoA and diacylglycerol (DG) molecule are covalently joined to form TG. This reaction (Fig. 1B) is catalyzed by acyl-CoA:diacylglycerol acyltransferase (DGAT, E.C. 2.3.1.20) enzymes. TG biosynthesis is believed to occur mainly at the endoplasmic reticulum (ER) (13). Newly synthesized TGs are thought to be released into the associated lipid bilayer, where they are channeled into cytosolic lipid droplets or, in cells that secrete TG, nascent lipoproteins (Fig. 2). The precise mechanism by which TGs are deposited into lipid droplets is unknown. Several models have been proposed (as reviewed in Ref. 14). Transfer of TG into lipoproteins involves the cotranslational addition of lipids to apolipoprotein B (apoB) in a process catalyzed by the microsomal triglyceride transfer protein (MTP) (as reviewed in Refs. 15–17). DGAT activity was first reported in 1956 (13, 18). Although there was much interest subsequently in the biochemistry of TG synthesis, the purification of a DGAT proved to be difficult. Only in the last decade have DGAT genes been cloned, and the molecular tools for studying TG synthesis become available. At least two DGAT enzymes exist in a wide variety of eukaryotes. Interestingly, these two DGAT enzymes are not similar at the level of DNA or protein sequences. In this review, we summarize progress over the past decade in understanding these two key enzymes of TG synthesis. DGAT1 AND DGAT2: DISTINCT GENE FAMILIES