Inhibiting monoacylglycerol acyltransferase 1 ameliorates hepatic metabolic abnormalities but not inflammation and injury in mice

Abstract

Abnormalities in hepatic lipid metabolism and insulin action are believed to play a critical role in the etiology of nonalcoholic steatohepatitis. Monoacylglycerol acyltransferase (MGAT) enzymes convert monoacylglycerol to diacylglycerol, which is the penultimate step in one pathway for triacylglycerol synthesis. Hepatic expression of Mogat1, which encodes an MGAT enzyme, is increased in the livers of mice with hepatic steatosis, and knocking down Mogat1 improves glucose metabolism and hepatic insulin signaling, but whether increased MGAT activity plays a role in the etiology of nonalcoholic steatohepatitis is unclear. To examine this issue, mice were placed on a diet containing high levels of trans fatty acids, fructose, and cholesterol (HTF-C diet) or a low fat control diet for 4 weeks. Mice were injected with antisense oligonucleotides (ASOs) to knockdown Mogat1 or a scrambled ASO control for 12 weeks while remaining on diet. The HTF-C diet caused glucose intolerance, hepatic steatosis, and induced hepatic gene expression markers of inflammation, macrophage infiltration, and stellate cell activation. Mogat1 ASO treatment, which suppressed Mogat1 expression in liver and adipose tissue, attenuated weight gain, improved glucose tolerance, improved hepatic insulin signaling, and decreased hepatic triacylglycerol content compared with control ASO-treated mice on HTF-C chow. However, Mogat1 ASO treatment did not reduce hepatic diacylglycerol, cholesterol, or free fatty acid content; improve histologic measures of liver injury; or reduce expression of markers of stellate cell activation, liver inflammation, and injury. In conclusion, inhibition of hepatic Mogat1 in HTF-C diet-fed mice improves hepatic metabolic abnormalities without attenuating liver inflammation and injury.

Keywords: Diacylglycerol; Fatty Acid Metabolism; Inflammation; Insulin Resistance; Liver; Nonalcoholic Steatohepatitis.

FIGURE 1.

Hepatic gene expression in DIO mice after Mogat1 inhibition. A, pathway analyses of hepatic gene expression array studies of DIO mice treated with control or Mogat1 ASO. B, hepatic expression of the indicated genes in livers of lean and DIO mice treated with control or Mogat1 ASOs. *, p < 0.05 versus lean controls; **, p < 0.05 versus lean and DIO controls. Error bars, S.E. AU, arbitrary units.

FIGURE 2.

Mogat1 ASO treatment improved glucose tolerance. A, Mogat1 inhibition led to decreased weight gain. B, reduced body weight after Mogat1 inhibition is associated with diminished weight of subcutaneous (SQ) and epididymal (EPI) white adipose tissue (WAT) depots. C, results of GTT studies in mice fed an LF or HTF-C diet and treated with control or Mogat1 ASOs. The bar graph at the right represents area under the curve values among the groups. n = 10 mice for all groups. *, p < 0.05 versus LF control and HTF-C Mogat1; **, p < 0.05 versus LF and HTF-C control. D, insulin-stimulated phosphorylation of Akt in hepatocytes isolated from mice treated with LF or HTF-C diet and control or Mogat1 ASO. Error bars, S.E.

FIGURE 3.

Mogat1 inhibition reduces hepatic TAG but not DAG content. A, expression of Mogat1 and Mogat2 in the indicated tissues of mice fed LF or HTF-C diet and treated with control or Mogat1 ASOs. *, p < 0.05 versus LF controls. **, p < 0.05 versus LF and HTF-C diet controls. AU, arbitrary units. B, liver weight and liver weight/body weight (BW) ratio. *, p < 0.05 versus LF controls. C, liver TAG content. Averages ± S.E. (error bars) for each group are shown. *, p < 0.05 versus LF controls. **, p < 0.05 versus both HTF-C and LF. D, liver free cholesterol content. *, p < 0.05 versus LF controls.

FIGURE 4.

Mogat1 inhibition does not affect liver DAG or free fatty acid content. A, total liver DAG content. Averages ± S.E. (error bars) for each group are shown. Data shown are the ion intensity of the specified DAG molecular species normalized to the ion intensity of the internal standard 20:0–20:0 DAG/mg of tissue. *, p < 0.05 versus LF. B, liver DAG species by fatty acid chain length and saturation index. C, liver FFA content; D, FFA species. Averages ± S.E. for each group are shown. *, p < 0.05 versus LF.

FIGURE 5.

Mogat1 ASO increases fatty acid oxidation and suppresses lipogenesis. A, rates of hepatocyte palmitate oxidation. Averages ± S.E. (error bars) for each group are shown. *, p < 0.05 versus LF and HTF-C controls. B, rates of de novo lipogenesis. *, p < 0.05 versus LF controls. **, p < 0.05 versus LF and HTF-C controls. C, rates of triglyceride synthesis and turnover. *, p < 0.05 versus LF controls. **, p < 0.05 versus LF and HTF-C controls. D, expression of genes encoding fatty acid oxidation and lipogenic enzymes. *, p < 0.05 versus controls. AU, arbitrary units.

FIGURE 6.

Hepatic Mogat1 inhibition does not result in histological improvement of steatohepatitis. A, H&E- and trichrome-stained liver sections from mice fed an LF or HTF-C diet and treated with control or Mogat1 ASOs. B, graphs depict results of NAS by a histopathologist blinded to treatment group. Data points represent individual mice, and averages (open boxes) for each group are also shown. Steatosis grading was done on low to medium power evaluation of parenchymal involvement by steatosis (0 for <5%, 1 for 5–33%, 2 for 33–66%, and 3 for more than 66%). Inflammation was graded by overall assessment of all inflammatory foci on a ×200 field (0 for no foci, 1 for <2, 2 for 2–4 foci, and 3 for >4 foci). Ballooning score was 0 for no ballooned cells per field, 1 for few, and 2 for many. Fibrosis was graded 0 for no fibrosis, 1 for perisinusoidal or periportal/portal, 2 for perisinusoidal and periportal/portal, 3 for bridging fibrosis, and 4 for cirrhosis. NAS is the sum of steatosis, inflammation, and ballooning scores. C, plasma ALT and AST concentrations are shown. U/L, units/liter. *, p < 0.05 versus LF. Error bars, S.E.

FIGURE 7.

Gene expression markers for liver injury, inflammation, and NASH. Hepatic expression of inflammatory and cytokine markers (A) and fibrogenic and hepatic stellate cell activation markers (B) in livers of mice fed an LF or HTF-C diet and treated with control or Mogat1 ASOs. *, p < 0.05 versus LF; **, p < 0.05 versus both HTF-C and LF. Error bars, S.E. AU, arbitrary units.