Blocking VLDL secretion causes hepatic steatosis but does not affect peripheral lipid stores or insulin sensitivity in mice

Abstract

The liver secretes triglyceride-rich VLDLs, and the triglycerides in these particles are taken up by peripheral tissues, mainly heart, skeletal muscle, and adipose tissue. Blocking hepatic VLDL secretion interferes with the delivery of liver-derived triglycerides to peripheral tissues and results in an accumulation of triglycerides in the liver. However, it is unclear how interfering with hepatic triglyceride secretion affects adiposity, muscle triglyceride stores, and insulin sensitivity. To explore these issues, we examined mice that cannot secrete VLDL [due to the absence of microsomal triglyceride transfer protein (Mttp) in the liver]. These mice exhibit markedly reduced levels of apolipoprotein B-100 in the plasma, along with reduced levels of triglycerides in the plasma. Despite the low plasma triglyceride levels, triglyceride levels in skeletal muscle were unaffected. Adiposity and adipose tissue triglyceride synthesis rates were also normal, and body weight curves were unaffected. Even though the blockade of VLDL secretion caused hepatic steatosis accompanied by increased ceramides and diacylglycerols in the liver, the mice exhibited normal glucose tolerance and were sensitive to insulin at the whole-body level, as judged by hyperinsulinemic euglycemic clamp studies. Normal hepatic glucose production and insulin signaling were also maintained in the fatty liver induced by Mttp deletion. Thus, blocking VLDL secretion causes hepatic steatosis without insulin resistance, and there is little effect on muscle triglyceride stores or adiposity.

Fig. 1

Analysis of Mttp^Δ/Δ mice. A: Distribution of cholesterol and triglycerides in the plasma lipoproteins of Mttp^Δ/Δ and control mice that had been fasted overnight. Open circles, control mice; closed circles, Mttp^Δ/Δ. B: Western blot analysis of apolipoprotein B-100 (apoB-100) and apoB-48 in the plasma of Mttp^Δ/Δ and control mice that had been fasted overnight. C: Liver triglycerides in Mttp^Δ/Δ and control mice after overnight fasting (n = 5, * P < 0.05) (mean ± SEM). D: Oil Red O staining of liver sections from Mttp^Δ/Δ and control mice following overnight fasting (original magnification × 10).

Fig. 2

Body weight and adipose tissue studies in Mttp^Δ/Δ mice. A: Body fat measured by dual energy X-ray absorptiometry on a chow diet (n = 9–10) and after 4 months on a high-fat diet (n = 15–18). Open bar, control mice; closed bar, MttpD/D mice. (mean ± SEM). B: Weight gain on a high-fat diet (n = 16–18). Open circles, control mice; closed circles, Mttp^Δ/Δ mice. C: Triglyceride synthesis in inguinal fat as judged by mass isotopomer distribution analysis on chow (n = 8) and high-fat diets (n = 7–9) (mean ± SEM). Similar results were observed in epididymal fat. D: Fractional de novo lipogenesis in adipose tissue in Mttp^Δ/Δ and control mice on chow and high-fat diets, as measured by mass isotopomer distribution analysis (n = 6–10). E: Similar absolute levels of de novo lipogenesis in adipose tissue of Mttp^Δ/Δ and control mice on chow and high-fat diets. F: Expression of three genes in adipose tissue that are upregulated during lipogenesis [ATP citrate lyase (ATPCL), fatty acid synthase (FAS), and stearoyl-CoA desaturase-1 (SCD-1)]. Studies were performed on a chow diet and after 1 month on a high-carbohydrate diet (HCHO). Samples from Mttp^Δ/Δ and control mice were pooled and analyzed in triplicate. Results were normalized to cyclophilin mRNA expression. Fold change was calculated compared with chow-fed control mice.

Fig. 3

Skeletal muscle triglyceride levels in Mttp^Δ/Δ mice. A: Gastrocnemius triglyceride levels in Mttp^Δ/Δ and control mice that had been fasted overnight (n = 5) (mean ± SEM). B: Fractional triglyceride synthesis (%) in Mttp^Δ/Δ and control mice measured by mass isotopomer distribution analysis (n = 4) (mean ± SEM).

Fig. 4

Normal sensitivity to insulin in Mttp^Δ/Δ mice. A: Glucose levels in Mttp^Δ/Δ and control mice during a glucose tolerance test (n = 10) (mean ± SEM). Open circles, control mice; closed circles, Mttp^Δ/Δ mice. B: Glucose infusion rate during hyperinsulinemic euglycemic clamp study with a high insulin dose (n = 5–8). C: Suppression of hepatic glucose production (HGP) during a clamp with a low dose of insulin infusion (n = 9) (mean ± SEM). D: Akt phosphorylation before and after insulin infusion and hepatic phosphoenolpyruvate carboxykinase (PEPCK) protein levels in the liver of Mttp^Δ/Δ and control mice. E: Ceramides and diacylglycerols in the liver of Mttp^Δ/Δ and control mice after an overnight fast (n = 6) (mean ± SEM). * P < 0.05, significantly different from a control group.