Altered lipid metabolism in apolipoprotein E-deficient mice does not affect cholesterol balance across the liver

Adaptation of cholesterol and bile acid synthesis and of biliary cholesterol secretion represent key metabolic responses to maintain cholesterol homeostasis and have been suggested to be influenced by apolipoprotein E (apoE) phenotype in humans. We have investigated hepatic metabolism and secretion of cholesterol into bile in homozygous apoE-deficient (apoE -/-) mice fed normal lab chow. Plasma cholesterol levels were 10 times higher in apoE (-/-) mice than in controls (+/+); triacylglycerol levels were only minimally affected. Hepatic cholesterol (+56%) and triacylglycerol (+232%) contents were significantly increased in apoE (-/-) mice, whereas those of cholesteryl ester and of phospholipids were similar in both groups. Lipid accumulated predominantly in periportal areas of apoE (-/-) livers. Hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG CoA reductase) messenger RNA (mRNA) level and activity were reduced by 45% and 50%, respectively, in apoE (-/-) mice. In contrast, plasma lathosterol/cholesterol ratios, indicative for whole-body cholesterol synthesis, were fourfold increased in these mice. Acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity was similar in livers of both groups. Despite the marked changes in hepatic cholesterol metabolism, neither hepatic bile acid synthesis, bile acid pool size and composition, nor hepatic cholesterol 7alpha-hydroxylase and sterol 27-hydroxylase mRNA levels differed between apoE (-/-) and (+/+) mice. In addition, biliary cholesterol secretion was unaffected in the knock-out mice. Our results show that lack of apoE leads to marked changes in hepatic cholesterol metabolism without altering cholesterol balance across the liver. The data are compatible with increased peripheral cholesterol biosynthesis in apoE-deficient mice.