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

Abstract

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.

Keywords: dietary fat; neutral lipid metabolism; triacylglycerol.

Fig. 1.

Generation and characterization of mice expressing human MGAT2 in the intestine of Mogat2^−/− mice. (A) Human MOGAT2 transgene driven by villin promoter (Vil-hMOGAT2 transgene fragment). P1, P2, and P3 are PCR primers used for genotyping. (B) Mouse endogenous Mogat2 and human transgenic MOGAT2 detected by a 625 bp amplicon and a 415 bp amplicon, respectively. Ntg, nontransgenic; Tg, transgenic. (C) MGAT2 mRNA levels in the proximal and distal small intestine of wild-type, Mogat2^−/−, M2^Int-L, and M2^Int-H mice measured by quantitative PCR using cyclophilin as an internal control. To assess the overall expression levels of both endogenous mouse Mogat2 and transgenic human MOGAT2, primers were designed annealing to identical sequences shared by mouse and human gene encoding MGAT2. ND, not detected. (D) MGAT activity in the proximal and distal intestine of wild-type, Mogat2^−/−, M2^Int-L, and M2^Int-H mice. n = 4−8 per group; Bars represent mean ± SEM. Differences between bars without the same letter are statistically significant.

Fig. 2.

Expression of human MGAT2 in the intestine of Mogat2^−/− mice promotes the uptake and esterification of MAG. Micelle preparation containing fatty acids and 2-monooleoyl-rac-glycerol [¹⁴C (U)] tracers were injected into ligated intestinal pouches created in the proximal and the distal intestine. Bars represent accumulation of tracers incorporated into MAG, DAG, TAG, and PL in each intestine segment of wild-type, Mogat2^−/−, M2^Int-L, and M2^Int-H mice. n = 4 per group. Differences between bars without the same letter are statistically significant.

Fig. 3.

Restoration of MGAT2 in the intestine of Mogat2^−/− mice corrects the absorption of dietary triacylglycerol. (A) Plasma radioactivity level in mice after injection of lipase inhibitor P407 and gavage with olive oil containing trioleoylglycerol [carboxyl-¹⁴C]. n = 9 for wild-type, 9 for Mogat2^−/−, 9 for M2^Int-L, and 5 for M2^Int-H mice. *P < 0.05 versus wild-type. (B) Radiolabeled lipids extracted from plasma samples from (A) and resolved by TLC. Each lane represents a sample collected from one mouse.

Fig. 4.

Restoration of MGAT2 in the intestine partially attenuates the increased oxygen consumption but does not correct the altered respiratory exchange ratio of Mogat2^−/− mice. Mice (12–14 weeks old) were fed sequentially semipurified (defined) diets containing 10, 45, or 60% calories from fat for three days per diet. (A) Oxygen consumption rates adjusted for baseline body weights of each mouse at the start of each diet treatment. (B) RERs were calculated by dividing carbon dioxide production with oxygen consumption (VCO₂/VO₂). (C) Food intake per hour. Data from each mouse were pooled from the same time of the day of the same diet treatment. Graphs represent average days. n = 12−19 mice per group. Gray areas mark dark phase of the light cycle (6 PM to 6 AM). Error bars for M2^Int-L and M2^Int-H mice were omitted to enhance clarity.

Fig. 5.

Restoration of MGAT2 in the intestine partially restores metabolic efficiency in Mogat2^−/− mice in response to high-fat feeding. Body weights of 12-week-old wild-type, Mogat2^−/−, M2^Int-L, and M2^Int-H mice were measured weekly after switching from chow to 60% diet for 10 weeks. n = 13−26 per group.

Fig. 6.

Effects of restoring intestinal MGAT2 on diet-induced hepatic steatosis and glucose intolerance in Mogat2^−/− mice. (A) Liver mass and (B) hepatic triacylglycerol of 6-month-old male mice fed the 60% fat diet for three months. (C) Fasting plasma insulin and (D) blood glucose concentration following an intraperitoneal glucose challenge in 5- to 6-month-old mice fed the 60% fat diet for two months. n = 8−12 per group; bars represent mean ± SEM. Differences between bars without the same letter are statistically significant.