Abstract
Blocking hepatic very low-density lipoprotein secretion through genetic or pharmacologic inhibition of microsomal triglyceride transfer protein (Mttp) causes hepatic steatosis, yet the risks for developing hepatic fibrosis are poorly understood. We report that liver-specific Mttp knockout mice (Mttp-LKO) exhibit both steatosis and fibrosis, which is exacerbated by a high-transfat/fructose diet. When crossed into germline liver fatty acid (FA) binding protein null mice (Mttp-LKO, i.e., double knockout mice) hepatic steatosis was greatly diminished and fibrosis prevented, on both low-fat and high-fat diets. The mechanisms underlying protection include reduced long chain FA uptake, shifts in FA distribution (lipidomic profiling), and metabolic turnover, specifically decreased hepatic 18:2 FA and triglyceride species and a shift in 18:2 FA use for oxidation versus incorporation into newly synthesized triglyceride. Double knockout mice were protected against fasting-induced hepatic steatosis (a model of enhanced exogenous FA delivery) yet developed steatosis upon induction of hepatic de novo lipogenesis with fructose feeding. Mttp-LKO mice, on either the liver FA binding protein null or Apobec-1 null background (i.e., apolipoprotein B100 only) exhibited only subtle increases in endoplasmic reticulum stress, suggesting that an altered unfolded protein response is unlikely to account for the attenuated phenotype in double knockout mice. Acute, antisense-mediated liver FA binding protein knockdown in Mttp-LKO mice also reduced FA uptake, increased oxidation versus incorporation of 18:2 species with complete reversal of hepatic steatosis, increased hepatic injury, and worsened fibrosis. CONCLUSION: Perturbing exogenous hepatic FA use modulates both hepatic steatosis and fibrosis in the setting of hepatic Mttp deletion, adding new insight into the pathophysiological mechanisms and consequences of defective very low-density lipoprotein secretion. (Hepatology 2017;65:836-852).