Adult-onset deficiency of acyl CoA:monoacylglycerol acyltransferase 2 protects mice from diet-induced obesity and glucose intolerance

Intestinal Acyl-CoA synthetase 5 (ACSL5) deficiency potentiates postprandial GLP-1 & PYY secretion, reduces food intake, and protects against diet-induced obesity

OBJECTIVE

In the small intestine, the products of digestion of dietary triacylglycerol (TAG), fatty acids (FA) and monoacylglycerol, are taken up by absorptive cells, enterocytes, for systemic energy delivery. These digestion products can also bind receptors on endocrine cells to stimulate the release of hormones capable of influencing systemic energy metabolism. The initial phase of intestinal FA absorption involves the acylation of FAs to acyl-CoA by the acyl-CoA long chain synthetase (ACSL) enzymes. ACSL5 is abundantly expressed in the small intestinal epithelium where it is the major ACSL isoform, contributing approximately 80% of total ACSL activity. In mice with whole body deficiency of ACSL5, the rate of dietary fat absorption is reduced and energy expenditure is increased. However, the mechanisms by which intestinal ACSL5 contributes to intestinal FA metabolism, enteroendocrine signaling, and regulation of energy expenditure remain undefined. Here, we test the hypothesis that intestinal ACSL5 regulates energy metabolism by influencing dietary fat absorption and enteroendocrine signaling.

METHODS

To explore the role of intestinal ACSL5 in energy balance and intestinal dietary fat absorption, a novel mouse model of intestine specific ACSL5 deficiency (ACSL5IKO) was generated by breeding ACSL5 floxed (ACSL5loxP/loxP) to mice harboring the tamoxifen inducible, villin-Cre recombinase. ACSL5IKO and control, ACSL5loxP/loxP mice were fed chow (low in fat) or a 60% high fat diet (HFD), and metabolic phenotyping was performed including, body weight, body composition, insulin and glucose tolerance tests, energy expenditure, physical activity, and food intake studies. Pair-feeding studies were performed to determine the role of food intake in regulating development of obesity. Studies of dietary fat absorption, fecal lipid excretion, intestinal mucosal FA content, and circulating levels of glucagon like peptide 1 (GLP-1) and peptide YY (PYY) in response to a TAG challenge were performed. Treatment with a GLP-1 receptor antagonist was performed to determine the contribution of GLP-1 to acute regulation of food intake.

RESULTS

We found that ACSL5IKO mice experienced rapid and sustained protection from body weight and fat mass accumulation during HFD feeding. While intestine specific deficiency of ACSL5 delayed gastric emptying and reduced dietary fat secretion, it did not result in increased excretion of dietary lipid in feces. Energy expenditure and physical activity were not increased in ACSL5IKO mice. Mice deficient in intestinal ACSL5 display significantly reduced energy intake during HFD, but not chow feeding. When HFD intake of control mice was matched to ACSL5IKO during pair-feeding studies, no differences in body weight or fat mass gain were observed between groups. Postprandial GLP-1 and PYY were significantly elevated in ACSL5IKO mice secondary to increased FA content in the distal small intestine. Blockade of GLP-1 signaling by administration of a long-acting GLP-1 receptor antagonist partially restored HFD intake of ACSL5IKO.

CONCLUSIONS

These data indicate that intestinal ACSL5 serves as a critical regulator of energy balance, protecting mice from diet-induced obesity exclusively by increasing satiety and reducing food intake during HFD feeding. The reduction in food intake observed in ACSL5IKO mice is driven, in part, by increased postprandial GLP-1 and PYY secretion. These effects are only observed during HFD feeding, suggesting that altered processing of dietary fat following intestinal ACSL5 ablation contributes to GLP-1 and PYY mediated increases in satiety.

Monoacylglycerol O-acyltransferase 1 lowers adipocyte differentiation capacity in vitro but does not affect adiposity in mice

OBJECTIVE

Monoacylglycerol O-acyltransferase 1 (Mogat1), a lipogenic enzyme that converts monoacylglycerol to diacylglycerol, is highly expressed in adipocytes and may regulate lipolysis by re-esterifying fatty acids released during times when lipolytic rates are low. However, the role of Mogat1 in regulating adipocyte fat storage during differentiation and diet-induced obesity is relatively understudied.

METHODS

Here, adipocyte-specific Mogat1 knockout mice were generated and subjected to a high-fat diet to determine the effects of Mogat1 deficiency on diet-induced obesity. Mogat1 floxed mice were also used to develop preadipocyte cell lines wherein Mogat1 could be conditionally knocked out to study adipocyte differentiation in vitro.

RESULTS

In preadipocytes, it was found that Mogat1 knockout at the onset of preadipocyte differentiation prevented the accumulation of glycerolipids and reduced the differentiation capacity of preadipocytes. However, the loss of adipocyte Mogat1 did not affect weight gain or fat mass induced by a high-fat diet in mice. Furthermore, loss of Mogat1 in adipocytes did not affect plasma lipid or glucose concentrations or insulin tolerance.

CONCLUSIONS

These data suggest Mogat1 may play a role in adipocyte differentiation in vitro but not adipose tissue expansion in response to nutrient overload in mice.

Global deficiency of stearoyl-CoA desaturase-2 protects against diet-induced adiposity

In mouse, there are four stearoyl-CoA desaturase isoforms (SCD1-4) that catalyze the synthesis of monounsaturated fatty acids. Previously, we have shown that mice harboring a whole body deletion of the SCD1 isoform (SCD1KO) are protected from diet and genetically induced adiposity. Here, we report that global deletion of the SCD2 isoform (SCD2KO) provides a similar protective effect against the onset of both high-fat diet (HFD) and high-carbohydrate diet (HCD) induced adiposity. After 10 weeks of HFD feeding or 6 weeks of HCD feeding, SCD2KO mice failed to gain weight and had decreased fat mass. On HFD, SCD2KO mice remained glucose and insulin tolerant. Lastly, the markers for energy expenditure, UCP1 and PGC-1α, were increased in the brown adipose tissue of HFD fed SCD2KO mice.

Mice lacking have reduced signs of metabolic syndrome and non-alcoholic fatty liver disease

Metabolic syndrome (MetS) is a term used to characterize individuals having at least three of the following diseases: obesity, dyslipidemia, hyperglycemia, insulin resistance, hypertension, and nonalcoholic fatty liver disease (NAFLD). It is widespread, and the number of individuals with MetS is increasing. However, the events leading to the manifestation of MetS are not well-understood. Here, we show that loss of murine ARV1 (mARV1) results in resistance to acquiring diseases associated with MetS. Arv1^-/- animals fed a high-fat diet were resistant to diet-induced obesity, had lower blood cholesterol and triglyceride levels, and retained glucose tolerance and insulin sensitivity. Livers showed no gross morphological changes, contained lower levels of cholesterol, triglycerides, and fatty acids, and showed fewer signs of NAFLD. Knockout animals had elevated levels of liver farnesol X receptor (FXR) protein and its target, small heterodimer protein (SHP). They also had decreased levels of CYP7α1, CYP8β1, and mature SREBP1 protein, evidence suggesting that liver FXR signaling was activated. Strengthening this hypothesis was the fact that peroxisome proliferator-activating receptor α (PPARα) protein was elevated, along with its target, fibroblast growth factor 21 (FGF21). Arv1^-/- animals excreted more fecal cholesterol, free fatty acids, and bile acids. Their small intestines had 1) changes in bile acid composition, 2) an increase in the level of the intestinal FXR antagonist, tauromuricholic acid, and 3) showed signs of attenuated FXR signaling. Overall, we believe that ARV1 function is deleterious when consuming a high-fat diet. We further hypothesize that ARV1 is critical for initiating events required for the progression of diseases associated with MetS and NAFLD.

AIDA Selectively Mediates Downregulation of Fat Synthesis Enzymes by ERAD to Retard Intestinal Fat Absorption and Prevent Obesity

The efficiency of intestinal absorption of dietary fat constitutes a primary determinant accounting for individual vulnerability to obesity. However, how fat absorption is controlled and contributes to obesity remains unclear. Here, we show that inhibition of endoplasmic-reticulum-associated degradation (ERAD) increases the abundance of triacylglycerol synthesis enzymes and fat absorption in small intestine. The C2-domain protein AIDA acts as an essential factor for the E3-ligase HRD1 of ERAD to downregulate rate-limiting acyltransferases GPAT3, MOGAT2, and DGAT2. Aida^-/- mice, when grown in a thermal-neutral condition or fed high-fat diet, display increased intestinal fatty acid re-esterification, circulating and tissue triacylglycerol, accompanied with severely increased adiposity without enhancement of adipogenesis. Intestine-specific knockout of Aida largely phenocopies its whole-body knockout, strongly indicating that increased intestinal TAG synthesis is a primary impetus to obesity. The AIDA-mediated ERAD system may thus represent an anti-thrifty mechanism impinging on the enzymes for intestinal fat absorption and systemic fat storage.

Metabolic tracing of monoacylglycerol acyltransferase-2 activity in vitro and in vivo

Monoacylglycerol acyltransferase 2 (MGAT2) catalyzes the synthesis of diacylglycerol (DAG) from free fatty acids (FFA) and sn-monoacylglycerol (MG), the two major hydrolysis products of dietary fat. To demonstrate MGAT2-mediated cellular activity of triglyceride (TG) synthesis, we utilized 1-oleoyl-glycerol-d5 as a substrate to trace MGAT2-driven 1-oleoyl-glycerol-d5 incorporation into TG in HEK293 cells stably expressing human MGAT2. The oleoyl-glycerol-d5 incorporated major TG species were then quantified by liquid chromatography electrospray ionization tandem mass spectrometry (LC/ESI/MS/MS) in a 96-well format. Conventional MGAT2 target-engagement in vivo assays measure the elevation of total plasma TG by orally dosing a bolus of TG oil. We developed a novel LC/ESI/MS/MS-based fat absorption assay to assess the ability of MGAT2 inhibitors to inhibit fat absorption in CD1 mice by a meal tolerance test consisting of a mixture of liquid Boost plus^® and 0.59 g/kg ^U13C-TG oil. The newly resynthesized plasma heavy TGs containing three ¹³C in the glycerol backbone and two ^U13C-acyl-chains, which represented the digested, absorbed and resynthesized TGs, were then quantitated by LC/ESI/MS/MS. With this assay, we identified a potent MGAT2 inhibitor that blocked MGAT2-mediated activity in vitro and in vivo. The use of 1-oleoyl-glycerol-d5 and ^U13C-TG oil followed by LC/ESI/MS/MS detection of stable-isotopic labeled DAG, TG, or glycerol provides a wide range of applications to study pathophysiological regulation of the monoacylglycerol pathway and MGAT2 activity.

Redundant roles of the phosphatidate phosphatase family in triacylglycerol synthesis in human adipocytes

AIMS/HYPOTHESIS

In mammals, the evolutionary conserved family of Mg(2+)-dependent phosphatidate phosphatases (PAP1), involved in phospholipid and triacylglycerol synthesis, consists of lipin-1, lipin-2 and lipin-3. While mutations in the murine Lpin1 gene cause lipodystrophy and its knockdown in mouse 3T3-L1 cells impairs adipogenesis, deleterious mutations of human LPIN1 do not affect adipose tissue distribution. However, reduced LPIN1 and PAP1 activity has been described in participants with type 2 diabetes. We aimed to characterise the roles of all lipin family members in human adipose tissue and adipogenesis.

METHODS

The expression of the lipin family was analysed in adipose tissue in a cross-sectional study. Moreover, the effects of lipin small interfering RNA (siRNA)-mediated depletion on in vitro human adipogenesis were assessed.

RESULTS

Adipose tissue gene expression of the lipin family is altered in type 2 diabetes. Depletion of every lipin family member in a human Simpson-Golabi-Behmel syndrome (SGBS) pre-adipocyte cell line, alters expression levels of adipogenic transcription factors and lipid biosynthesis genes in early stages of differentiation. Lipin-1 knockdown alone causes a 95% depletion of PAP1 activity. Despite the reduced PAP1 activity and alterations in early adipogenesis, lipin-silenced cells differentiate and accumulate neutral lipids. Even combinatorial knockdown of lipins shows mild effects on triacylglycerol accumulation in mature adipocytes.

CONCLUSIONS/INTERPRETATION

Overall, our data support the hypothesis of alternative pathways for triacylglycerol synthesis in human adipocytes under conditions of repressed lipin expression. We propose that induction of alternative lipid phosphate phosphatases, along with the inhibition of lipid hydrolysis, contributes to the maintenance of triacylglycerol content to near normal levels.

Characterization of monoacylglycerol acyltransferase 2 inhibitors by a novel probe in binding assays

Monoacylglycerol acyltransferase 2 (MGAT2) is a membrane-bound lipid acyltransferase that catalyzes the formation of diacylglycerol using monoacylglycerol and fatty acyl CoA as substrates. MGAT2 is important for intestinal lipid absorption and is an emerging target for the treatment of metabolic diseases. In the current study, we identified and characterized four classes of novel MGAT2 inhibitors. We established both steady state and kinetic binding assay protocols using a novel radioligand, [(3)H]compound A. Diverse chemotypes of MGAT2 inhibitors were found to compete binding of [(3)H]compound A to MGAT2, indicating the broad utility of [(3)H]compound A for testing various classes of MGAT2 inhibitors. In the dynamic binding assays, the kinetic values of MGAT2 inhibitors such as Kon, Koff, and T1/2 were systematically defined. Of particular value, the residence times of inhibitors on MGAT2 enzyme were derived. We believe that the identification of novel classes of MGAT2 inhibitors and the detailed kinetic characterization provide valuable information for the identification of superior candidates for in vivo animal and clinical studies. The current work using a chemical probe to define inhibitory kinetics can be broadly applied to other membrane-bound acyltransferases.

Pharmacological Inhibition of Monoacylglycerol O-Acyltransferase 2 Improves Hyperlipidemia, Obesity, and Diabetes by Change in Intestinal Fat Utilization

Monoacylglycerol O-acyltransferase 2 (MGAT2) catalyzes the synthesis of diacylglycerol (DG), a triacylglycerol precursor and potential peripheral target for novel anti-obesity therapeutics. High-throughput screening identified lead compounds with MGAT2 inhibitory activity. Through structural modification, a potent, selective, and orally bioavailable MGAT2 inhibitor, compound A (compA), was discovered. CompA dose-dependently inhibited postprandial increases in plasma triglyceride (TG) levels. Metabolic flux analysis revealed that compA inhibited triglyceride/diacylglycerol resynthesis in the small intestine and increased free fatty acid and acyl-carnitine with shorter acyl chains than originally labelled fatty acid. CompA decreased high-fat diet (HFD) intake in C57BL/6J mice. MGAT2-null mice showed a similar phenotype as compA-treated mice and compA did not suppress a food intake in MGAT2 KO mice, indicating that the anorectic effects were dependent on MGAT2 inhibition. Chronic administration of compA significantly prevented body weight gain and fat accumulation in mice fed HFD. MGAT2 inhibition by CompA under severe diabetes ameliorated hyperglycemia and fatty liver in HFD-streptozotocin (STZ)-treated mice. Homeostatic model assessments (HOMA-IR) revealed that compA treatment significantly improved insulin sensitivity. The proximal half of the small intestine displayed weight gain following compA treatment. A similar phenomenon has been observed in Roux-en-Y gastric bypass-treated animals and some studies have reported that this intestinal remodeling is essential to the anti-diabetic effects of bariatric surgery. These results clearly demonstrated that MGAT2 inhibition improved dyslipidemia, obesity, and diabetes, suggesting that compA is an effective therapeutic for obesity-related metabolic disorders.

MOGAT2: A New Therapeutic Target for Metabolic Syndrome

Metabolic syndrome is an ever-increasing health problem among the world’s population. It is a group of intertwined maladies that includes obesity, hypertriglyceridemia, hypertension, nonalcoholic fatty liver disease (NAFLD), and diabetes mellitus type II (T2D). There is a direct correlation between high triacylglycerol (triglyceride; TAG) level and severity of metabolic syndrome. Thus, controlling the synthesis of TAG will have a great impact on overall systemic lipid metabolism and thus metabolic syndrome progression. The Acyl-CoA: monoacylglycerolacyltransferase (MGAT) family has three members (MGAT1, -2, and -3) that catalyze the first step in TAG production, conversion of monoacylglycerol (MAG) to diacylglycerol (DAG). TAG is then directly synthesized from DAG by a Acyl-CoA: diacylglycerolacyltransferase (DGAT). The conversion of MAG → DAG → TAG is the major pathway for the production of TAG in the small intestine, and produces TAG to a lesser extent in the liver. Transgenic and pharmacological studies in mice have demonstrated the beneficial effects of MGAT inhibition as a therapy for treating several metabolic diseases, including obesity, insulin resistance, T2D, and NAFLD. In this review, the significance of several properties of MGAT physiology, including tissue expression pattern and its relationship to overall TAG metabolism, enzymatic biochemical properties and their effects on drug discovery, and finally what is the current knowledge about MGAT small molecule inhibitors and their efficacy will be discussed. Overall, this review highlights the therapeutic potential of inhibiting MGAT for lowering TAG synthesis and whether this avenue of drug discovery warrants further clinical investigation.