Chi-Liang Eric Yen

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 acyl-CoA: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.

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.

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.

Deficiency of the intestinal enzyme acyl CoA:monoacylglycerol acyltransferase-2 protects mice from metabolic disorders induced by high-fat feeding

Animals are remarkably efficient in absorbing dietary fat and assimilating this energy-dense nutrient into the white adipose tissue (WAT) for storage. Although this metabolic efficiency may confer an advantage in times of calorie deprivation, it contributes to obesity and associated metabolic disorders when dietary fat is abundant. Here we show that the intestinal lipid synthesis enzyme acyl CoA:monoacylglycerol acyltransferase-2 (MGAT2) has a crucial role in the assimilation of dietary fat and the accretion of body fat in mice. Mice lacking MGAT2 have a normal phenotype on a low-fat diet. However, on a high-fat diet, MGAT2-deficient mice are protected against developing obesity, glucose intolerance, hypercholesterolemia and fatty livers. Caloric intake is normal in MGAT2-deficient mice, and dietary fat is absorbed fully. However, entry of dietary fat into the circulation occurs at a reduced rate. This altered kinetics of fat absorption apparently results in more partitioning of dietary fat toward energy dissipation rather than toward storage in the WAT. Thus, our studies identify MGAT2 as a key determinant of energy metabolism in response to dietary fat and suggest that the inhibition of this enzyme may prove to be a useful strategy for treating obesity and other metabolic diseases associated with excessive fat intake.

Intestine-specific Deletion of Acyl-CoA:Monoacylglycerol Acyltransferase (MGAT) 2 Protects Mice from Diet-induced Obesity and Glucose Intolerance

Background: Global MGAT2 knock-out mice are protected from obesity. Results: Intestine-specific MGAT2 knock-out mice showed increased energy expenditure and were protected against excess weight gain and metabolic disorders induced by high fat feeding. Conclusion: Intestinal triacylglycerol metabolism is crucial in regulating systemic energy balance. Significance: Intestinal MGAT2 may be a feasible intervention target for diseases associated with excess caloric intake. The absorption of dietary fat involves the re-esterification of digested triacylglycerol in the enterocytes, a process catalyzed by acyl-CoA:monoacylglycerol acyltransferase (MGAT) 2. Mice without a functional gene encoding MGAT2 (Mogat2−/−) are protected from diet-induced obesity. Surprisingly, these mice absorb normal amounts of dietary fat but increase their energy expenditure. MGAT2 is expressed in tissues besides intestine, including adipose tissue in both mice and humans. To test the hypothesis that intestinal MGAT2 regulates systemic energy balance, we generated and characterized mice deficient in MGAT2 specifically in the small intestine (Mogat2IKO). We found that, like Mogat2−/− mice, Mogat2IKO mice also showed a delay in fat absorption, a decrease in food intake, and a propensity to use fatty acids as fuel when first exposed to a high fat diet. Mogat2IKO mice increased energy expenditure although to a lesser degree than Mogat2−/− mice and were protected against diet-induced weight gain and associated comorbidities, including hepatic steatosis, hypercholesterolemia, and glucose intolerance. These findings illustrate that intestinal lipid metabolism plays a crucial role in the regulation of systemic energy balance and may be a feasible intervention target. In addition, they suggest that MGAT activity in extraintestinal tissues may also modulate energy metabolism.

Intestinal triacylglycerol synthesis in fat absorption and systemic energy metabolism

The intestine plays a prominent role in the biosynthesis of triacylglycerol (triglyceride; TAG). Digested dietary TAG is repackaged in the intestine to form the hydrophobic core of chylomicrons, which deliver metabolic fuels, essential fatty acids, and other lipid-soluble nutrients to the peripheral tissues. By controlling the flux of dietary fat into the circulation, intestinal TAG synthesis can greatly impact systemic metabolism. Genes encoding many of the enzymes involved in TAG synthesis have been identified. Among TAG synthesis enzymes, acyl-CoA:monoacylglycerol acyltransferase 2 and acyl-CoA:diacylglycerol acyltransferase (DGAT)1 are highly expressed in the intestine. Their physiological functions have been examined in the context of whole organisms using genetically engineered mice and, in the case of DGAT1, specific inhibitors. An emerging theme from recent findings is that limiting the rate of TAG synthesis in the intestine can modulate gut hormone secretion, lipid metabolism, and systemic energy balance. The underlying mechanisms and their implications for humans are yet to be explored. Pharmacological inhibition of TAG hydrolysis in the intestinal lumen has been employed to combat obesity and associated disorders with modest efficacy and unwanted side effects. The therapeutic potential of inhibiting specific enzymes involved in intestinal TAG synthesis warrants further investigation.

Deficiency of MGAT2 increases energy expenditure without high-fat feeding and protects genetically obese mice from excessive weight gain

Acyl CoA:monoacylglycerol acyltransferase 2 (MGAT2) is thought to be crucial for dietary fat absorption. Indeed, mice lacking the enzyme (Mogat2−/−) are resistant to obesity and other metabolic disorders induced by high-fat feeding. However, these mice absorb normal quantities of fat. To explore whether a high level of dietary fat is an essential part of the underlying mechanism(s), we examined metabolic responses of Mogat2−/− mice to diets containing varying levels of fat. Mogat2−/− mice exhibited 10−15% increases in energy expenditure compared with wild-type littermates; although high levels of dietary fat exacerbated the effect, this phenotype was expressed even on a fat-free diet. When deprived of food, Mogat2−/− mice expended energy and lost weight like wild-type controls. To determine whether MGAT2 deficiency protects against obesity in the absence of high-fat feeding, we crossed Mogat2−/− mice with genetically obese Agouti mice. MGAT2 deficiency increased energy expenditure and prevented these mice from gaining excess weight. Our results suggest that MGAT2 modulates energy expenditure through multiple mechanisms, including one independent of dietary fat; these findings also raise the prospect of inhibiting MGAT2 as a strategy for combating obesity and related metabolic disorders resulting from excessive calorie intake.

Glycomacropeptide, a low-phenylalanine protein isolated from cheese whey, supports growth and attenuates metabolic stress in the murine model of phenylketonuria.

Phenylketonuria (PKU) is caused by a mutation in the phenylalanine (phe) hydroxylase gene and requires a low-phe diet plus amino acid (AA) formula to prevent cognitive impairment. Glycomacropeptide (GMP) contains minimal phe and provides a palatable alternative to AA formula. Our objective was to compare growth, body composition, and energy balance in Pah(enu2) (PKU) and wild-type mice fed low-phe GMP, low-phe AA, or high-phe casein diets from 3-23 wk of age. The 2 × 2 × 3 design included main effects of genotype, sex, and diet. Fat and lean mass were assessed by dual-energy X-ray absorptiometry, and acute energy balance was assessed by indirect calorimetry. PKU mice showed growth and lean mass similar to wild-type littermates fed the GMP or AA diets; however, they exhibited a 3-15% increase in energy expenditure, as reflected in oxygen consumption, and a 3-30% increase in food intake. The GMP diet significantly reduced energy expenditure, food intake, and plasma phe concentration in PKU mice compared with the casein diet. The high-phe casein diet or the low-phe AA diet induced metabolic stress in PKU mice, as reflected in increased energy expenditure and intake of food and water, increased renal and spleen mass, and elevated plasma cytokine concentrations consistent with systemic inflammation. The low-phe GMP diet significantly attenuated these adverse effects. Moreover, total fat mass, %body fat, and the respiratory exchange ratio (CO(2) produced/O(2) consumed) were significantly lower in PKU mice fed GMP compared with AA diets. In summary, GMP provides a physiological source of low-phe dietary protein that promotes growth and attenuates the metabolic stress induced by a high-phe casein or low-phe AA diet in PKU mice.

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.

Triacylglycerol synthesis and energy metabolism: a gut reaction?

Abstract ““...excess accumulation of TG, resulting from chronic surplus of energy substrates, is the hallmark of obesity and related metabolic diseases,including hepatic steatosis and Type 2 diabetes.””