Mechanisms of hepatic steatosis in mice fed a lipogenic methionine choline-deficient diet

Abstract

The methionine choline-deficient (MCD) diet results in liver injury similar to human nonalcoholic steatohepatitis (NASH). The aims of this study were to define mechanisms of MCD-induced steatosis in insulin-resistant db/db and insulin-sensitive db/m mice. MCD-fed db/db mice developed more hepatic steatosis and retained more insulin resistance than MCD-fed db/m mice. Both subcutaneous and gonadal fat were reduced by MCD feeding: gonadal fat decreased by 23% in db/db mice and by 90% in db/m mice. Weight loss was attenuated in the db/db mice, being only 13% compared with 35% in MCD-fed db/db and db/m mice, respectively. Both strains had upregulation of hepatic fatty acid transport proteins as well as increased hepatic uptake of [14C]oleic acid: 3-fold in db/m mice (P < 0.001) and 2-fold in db/db mice (P < 0.01) after 4 weeks of MCD feeding. In both murine strains, the MCD diet reduced triglyceride secretion and downregulated genes involved in triglyceride synthesis. Therefore, increased fatty acid uptake and decreased VLDL secretion represent two important mechanisms by which the MCD diet promotes intrahepatic lipid accumulation in this model. Feeding the MCD diet to diabetic rodents broadens the applicability of this model for the study of human NASH.

Fig. 1.

Impact of the methionine choline-deficient (MCD) diet on the gonadal and subcutaneous fat compartments. Subcutaneous and gonadal fat were excised from db/m and db/db mice fed either a control diet or an MCD diet for 4 weeks. In both strains, the MCD diet led to significant reductions in both subcutaneous and gonadal fat depots. However, in db/db mice, gonadal fat loss was much less marked compared with db/m mice on the MCD diet. Values represent means of five mice ± SEM. * P < 0.01, effect of diet; ^† P < 0.01, effect of strain.

Fig. 2.

Hepatic [¹⁴C]oleic acid uptake at 4 h after oral gavage in db/m and db/db mice fed a control or MCD diet for 4 weeks. The MCD diet increased the hepatic uptake of [¹⁴C]oleic acid by 3-fold in db/m mice and by nearly 2-fold in db/db mice. Values represent means of five mice ± SEM. * P < 0.01. Open bars, control diet; closed bars, MCD diet.

Fig. 3.

Serum triglyceride levels after Triton WR-1339 injection in db/m and db/db mice fed a control or MCD diet for 4 weeks. Serum triglyceride secretion after Triton WR-1339 administration is blunted in MCD diet-fed db/m (A) and db/db (B) mice. Open and closed bars represent the control diet and the MCD diet, respectively. Values represent means of four mice ± SEM.

Fig. 4.

Serum triglyceride secretion after Triton WR-1339 injection in db/m and db/db mice fed control and MCD diets for 4 weeks. Open and closed bars represent the control diet and the MCD diet, respectively. Values represent means of four mice ± SEM. * P < 0.05, significant difference between strains on a control diet, as determined by t-test.

Fig. 5.

Western blot of serum apolipoprotein B-100 (apoB-100) and apoB-48 in db/db and db/m mice fed either the MCD or control diet for 4 weeks. Time points represent hours after the administration of Triton WR-1339. In db/m mice, the MCD diet reduced apoB secretion. In db/db mice, baseline levels of apoB-100 and apoB-48 were increased in the MCD diet cohort. The rate of accumulation was more pronounced in apoB-48 than in apoB-100.

Fig. 6.

Summary of the proposed mechanisms by which the MCD diet affects hepatic lipid metabolism. The MCD diet significantly increases fatty acid uptake and reduces VLDL secretion. Key genes involved in triglyceride synthesis are downregulated by MCD feeding.