Hepatic fatty acid synthesis is suppressed in mice with fatty livers due to targeted apolipoprotein B38.9 mutation

Humans and aenetically engineered mice with hypobetalipoproteinemia due to truncation-producing mutations of the apolipoprotein B (apoB) gene frequently have fatty livers, because the apoB defect impairs the capacity of livers to export triglycerides (TGs). We assessed the adaptation of hepatic lipid metabolism in our apoB-38.9 -bearing mice. Hepatic TG contents were 2- and 4-fold higher in heterozygous and homozygous mice, respectively, compared with wild-type mice. Respective in vivo hepatic fatty acid synthetic rates were reduced to 40% and 15% of the wild-type rate. Hepatic mRNAs for sterol regulatory element-binding protein (SREBP)-Ic, fatty acid synthase (FAS), and stearoyl coenzyme A desaturase-1 were coordinately decreased. FAS and SREBP-Ic mRNA levels were strongly and positively correlated with each other and inversely correlated with hepatic TGs, suggesting that impaired TG export is a potent inhibitor of fatty acid synthesis. In contrast, levels of plasma beta-hydroxybutyrate and of hepatic carnitine palmitoyl transferase and peroxisome proliferator-activated receptor-alpha mRNAs were not altered, implying that beta-oxidation was not affected. Fasting followed by refeeding increased hepatic fatty acid synthesis 56-fold over fasting in normal and heterozygous mice but only 24-fold in homozygous mice. Parallel changes occurred in FAS and SREBP-Ic mRNAs. Thus. impairment of very low density lipoprotein export downregulates hepatic fatty acid synthesis, but the adaptation is incomplete, resulting in fatty livers. The signals mediating suppression of FAS and SREBP-Ic levels remain to be identified.