Intestinal Monoacylglycerol Acyltransferase Research Articles

Humanized Monoacylglycerol Acyltransferase 2 Mice Develop Non‐alcoholic Steatohepatitis and Fibrosis that Responds to Treatment with Elafibranor

The number of individuals with non-alcoholic fatty liver disease (NAFLD) is at epidemic levels worldwide. Although thought of as benign, NAFLD can progress to more severe forms of the disease that include non-alcoholic steatohepatitis (NASH), fibrosis, cirrhosis, and in some cases hepatocellular carcinoma. Presently no therapies exist that can treat NAFLD and/or NASH even though this area of drug discovery has been highly active. The accumulation of triglycerides and fatty acids in the liver are linked to the initiation and progression of NAFLD. Intestinal monoacylglycerol acyltransferase 2 (MGAT2) catalyzes the resynthesis of triglycerides from dietary triglyceride-derived monoacylglycerol and fatty acids. Mice lacking mMGAT2 are resistant to obesity-dependent hepatic steatosis, a phenotype that is partially restored by expressing hMGAT2 in the intestine, suggesting that inhibiting human MGAT2 activity may be a viable option for treating NAFLD. Here, we generated transgenic hMgat2+/+ mice and characterized their responses to being fed different metabolic diets in order to test whether this cohort could be used as a model to test drug efficacy. hMgat2+/+ mice fed a steatotic diet acquired NAFLD and had elevated levels of several inflammatory cytokines in the liver, indicating the presence of ongoing NASH. Hydroxyproline and galectin-3 levels were highly elevated, and correlated with the degree of fibrosis seen by histology. hMgat2+/+ mice fed a high fat diet became obese, were glucose intolerant, and acquired insulin resistance, suggesting the onset of diabetes. The majority of phenotypes observed with both diets were attenuated by treatment with the PPARa/d agonist, elafibranor. Thus, hMgat2+/+ mice may be an excellent “humanized” model that can be used for testing drug efficacy targeting systemic hMGAT2 activity, and undoubtedly be invaluable in furthering our understanding of how hMGAT2 activity affects overall triglyceride metabolism.

Discovery of Human Intestinal MGAT Inhibitors Using High-Throughput Mass Spectrometry

Monoacylglycerol acyltransferase (MGAT) activity catalyzes the synthesis of diacylglycerol (DAG) from fatty acyl-CoA and monoacylglycerol as substrates. It is important for the resynthesis of triacylglycerol (TAG) in the intestine. In the present study, we developed a MGAT enzymatic assay of human intestinal microsomes using a high-throughput mass spectrometry (MS)–based detection system. After screening with small-molecular-weight libraries for compounds exhibiting inhibitions against DAG and the consequent TAG syntheses, we identified multiple compounds that specifically inhibit intestinal MGAT activity. The inhibitory activities of these compounds were correlated to those determined using a recombinant human MGAT2 enzyme. An aryl-sulfonamide compound T1 showed potent inhibitory activity toward human intestinal MGAT and recombinant human MGAT2, with selectivity over MGAT3. This high-throughput MS-based assay provides a novel platform for the discovery of DAG or TAG synthesis inhibitors. The identified aryl-sulfonamide compound T1 is a promising starting compound for optimization studies of inhibitors with selectivity toward MGAT2.

Pharmacological characterization of a series of aryl-sulfonamide derivatives that potently and selectively inhibit monoacylglycerol acyltransferase 2

Monoacylglycerol acyltransferase (MGAT) 2 is an endoplasmic reticulum membrane enzyme that catalyzes the synthesis of diacylglycerol (DAG) from fatty acyl-CoA and monoacylglycerol as substrates. It is important for the resynthesis of triacylglycerol in the intestine. We have identified a series of aryl-sulfonamide MGAT2 inhibitors and demonstrated pharmacological inhibition of MGAT2 improved hyperlipidemia, obesity, and diabetes in animal models. However, its mechanism of action has not been elucidated in molecular and cellular levels. In the present study, we have characterized a series of aryl-sulfonamide derivatives that potently and selectively inhibit human MGAT2 and determined their pharmacological profiles. Analyses on the molecular mechanism of a representative aryl-sulfonamide MGAT2 inhibitor revealed a reversible inhibitory activity and a binding activity to MGAT2. The aryl-sulfonamide derivatives exhibited potent inhibitory activities against both human and mouse intestinal MGAT activities, which were correlated to those determined using recombinant human and mouse MGAT enzymes. We have developed a cellular assay using Liquid Chromatography-Mass Spectrometry and confirmed that the aryl-sulfonamide derivatives suppressed DAG synthesis in the cellular context. We have thus elucidated their pharmacological profiles and provided the fundamental clues for understanding the molecular and cellular actions of the aryl-sulfonamide MGAT2 inhibitors.

Intestine-specific expression of MOGAT2 partially restores metabolic efficiency in Mogat2-deficient mice

Acyl CoA:monoacylglycerol acyltransferase (MGAT) catalyzes the resynthesis of triacylglycerol, a crucial step in the absorption of dietary fat. Mice lacking the gene Mogat2, which codes for an MGAT highly expressed in the small intestine, are resistant to obesity and other metabolic disorders induced by high-fat feeding. Interestingly, these Mogat2⁻/⁻ mice absorb normal amounts of dietary fat but exhibit a reduced rate of fat absorption, increased energy expenditure, decreased respiratory exchange ratio, and impaired metabolic efficiency. MGAT2 is expressed in tissues besides intestine. To test the hypothesis that intestinal MGAT2 enhances metabolic efficiency and promotes the storage of metabolic fuels, we introduced the human MOGAT2 gene driven by the intestine-specific villin promoter into Mogat2⁻/⁻ mice. We found that the expression of MOGAT2 in the intestine increased intestinal MGAT activity, restored fat absorption rate, partially corrected energy expenditure, and promoted weight gain upon high-fat feeding. However, the changes in respiratory exchange ratio were not reverted, and the recoveries in metabolic efficiency and weight gain were incomplete. These data indicate that MGAT2 in the intestine plays an indispensable role in enhancing metabolic efficiency but also raise the possibility that MGAT2 in other tissues may contribute to the regulation of energy metabolism.

Properties of the Mouse Intestinal Acyl-CoA:Monoacylglycerol Acyltransferase, MGAT2

Acyl-CoA:monoacylglycerol acyltransferase (MGAT) plays an important role in dietary fat absorption by catalyzing a rate-limiting step in the re-synthesis of diacylglycerols in enterocytes. The present study reports further characterization of MGAT2, a newly identified intestinal MGAT (Cao, J., Lockwood, J., Burn, P., and Shi, Y. (2003) J. Biol. Chem. 278, 13860-13866) for its substrate specificity, requirement for lipid cofactors, optimum pH and Mg2+, and other intrinsic properties. MGAT2 enzyme expressed in COS-7 cells displayed a broad range of substrate specificity toward fatty acyl-CoA derivatives and monoacylglycerols, among which the highest activities were observed with oleoyl-CoA and rac-1-monolauroylglycerol, respectively. MGAT2 appeared to acylate monoacylglycerols containing unsaturated fatty acyls in preference to saturated ones. Lipid cofactors that play roles in signal transduction were shown to modulate MGAT2 activities. In contrast to oleic acid and sphingosine that exhibited inhibitory effects, phosphatidylcholine, phosphatidylserine, and phosphatidic acid stimulated MGAT2 activities. Using recombinant murine MGAT2 expressed in Escherichia coli, we demonstrated conclusively that MGAT2 also possessed an intrinsic acyl-CoA:diacylglycerol acyltransferase (DGAT) activity, which could provide an alternative pathway for triacylglycerol synthesis in the absence of DGAT. In contrast to the inhibitory effect on MGAT2 activities, nonionic and zwitterionic detergents led to a striking activation of DGAT activity of the human DGAT1 expressed in mammalian cells, which further distinguished the behaviors of the two enzymes. The elucidation of properties of MGAT2 will facilitate future development of compounds that inhibit dietary fat absorption as a means to treat obesity.

MGAT2, a Monoacylglycerol Acyltransferase Expressed in the Small Intestine

Acyl CoA:monoacylglycerol acyltransferase (MGAT) catalyzes the synthesis of diacylglycerol, a precursor of triacylglycerol. In the intestine, MGAT plays a major role in the absorption of dietary fat by catalyzing the resynthesis of triacylglycerol in enterocytes. This resynthesis is required for the assembly of lipoproteins that transport absorbed fat to other tissues. Despite intense efforts, a gene encoding an intestinal MGAT has not been found. Previously, we identified a gene encoding MGAT1, which in mice is expressed in the stomach, kidney, adipose tissue, and liver but not in the intestine. We now report the identification of homologous genes in humans and mice encoding MGAT2. Expression of the MGAT2 cDNA in either insect or mammalian cells markedly increased MGAT activity in cell membranes. MGAT activity was proportional to the level of MGAT2 protein expressed, and the amount of diacylglycerol produced depended on the concentration of MGAT substrates (fatty acyl CoA or monoacylglycerol). In humans, the MGAT2 gene is highly expressed in the small intestine, liver, stomach, kidney, colon, and white adipose tissue; in mice, it is expressed predominantly in the small intestine. The discovery of the MGAT2 gene will facilitate studies to determine the functional role of MGAT2 in fat absorption in the intestine and to determine whether blocking MGAT activity in enterocytes is a feasible approach to inhibit fat absorption and treat obesity.

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.

Identification of a gene encoding MGAT1, a monoacylglycerol acyltransferase.

Acyl-CoA:monoacylglycerol acyltransferase (MGAT) catalyzes the synthesis of diacylglycerol, the precursor of physiologically important lipids such as triacylglycerol and phospholipids. In the intestine, MGAT plays a major role in the absorption of dietary fat because resynthesis of triacylglycerol is required for the assembly of lipoproteins that transport absorbed fat to other tissues. MGAT activity has also been reported in mammalian liver and white adipose tissue. However, MGAT has never been purified to homogeneity from mammalian tissues, and its gene has not been cloned. We identified a gene that encodes an MGAT (MGAT1) in mice. This gene has sequence homology with members of a recently identified diacylglycerol acyltransferase gene family. Expression of the MGAT1 cDNA in insect cells markedly increased MGAT activity in cell membranes. In addition, MGAT activity was proportional to the level of MGAT1 protein expressed, and the amount of diacylglycerol produced depended on the concentration of either of its substrates, oleoyl-CoA or monooleoylglycerol. In mice, MGAT1 expression and MGAT activity were detected in the stomach, kidney, white and brown adipose tissue, and liver. However, MGAT1 was not expressed in the small intestine, implying the existence of a second MGAT gene. The identification of the MGAT1 gene should greatly facilitate research on the identification of the intestinal MGAT gene and on the function of MGAT enzymes in mammalian glycerolipid metabolism.

L-glutamine and transforming growth factor-alpha enhance recovery of monoacylglycerol acyltransferase and diacylglycerol acyltransferase activity in porcine postischemic ileum.

Recovery of the ability to digest and absorb lipids is essential to the maintenance of normal nutrition in infants with bowel damage. Two intrinsic microsomal enzymes, monoacylglycerol acyltransferase (MGAT) and diacylglycerol acyltransferase (DGAT), catalyze the major pathway for intestinal triacylglycerol biosynthesis. This study describes the effects of intestinal ischemia on epithelial DGAT and MGAT activities and their recovery in response to two luminal treatments: L-glutamine (Gln), the primary intestinal fuel, and transforming growth factor-alpha (TGF-alpha), a mitogenic hormone similar to epidermal growth factor present in breast milk. Ischemic damage and recovery were analyzed in mucosa from Thiry-Vella loops in the mid-ileum of 7-wk-old pigs. Loops were subjected to 2-h occlusion of local mesenteric arteries, followed by 6 or 72 h of recovery in the presence of luminal glucose (control), Gln, or TGF-alpha. Ischemic tissue followed by 6-h recovery exhibited an approximate 50% decrease in both MGAT and DGAT activities compared with nonischemic loop tissue. At 72 h, MGAT and DGAT recovery in Gln plus TGF-alpha-treated loops was significantly greater than their corresponding 6-h peak damage levels (p < 0.05). From 6 to 72 h, MGAT increased 4-fold and DGAT increased 3.6-fold after Gln plus TGF-alpha treatment. With other treatments, MGAT and DGAT activities increased <2.5-fold from 6 to 72 h. This study shows that intestinal MGAT and DGAT activities decrease after ischemic damage, yet recover rapidly in bowel exposed to Gln and/or TGF-alpha. By stimulating the rate of recovery of the villi and lipid synthesizing enzymes, these treatments could improve the efficacy of enteral feeding in infants recovering from bowel damage.