Rabbits fed low-fat, cholesterol-free diets containing casein as the sole protein source develop endogenous hypercholesterolemia (EH). To test the hypothesis that lipoprotein cholesteryl esters in EH rabbits are acyl coenzyme A: cholesterol acyltransferase (ACAT) derived, we treated EH rabbits with CI-976, a potent and selective ACAT inhibitor. In addition, since cholesterol and bile acid synthesis as well as low-density lipoprotein (LDL) receptor activity are reduced in EH rabbits, we determined whether changes in gene expression for 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, 7alpha-hydroxylase, and the LDL receptor might be associated with the efficacy due to ACAT inhibition. Compared with EH controls, CI-976-treated rabbits (50 mg/kg per day for 5 weeks) had decreased plasma total cholesterol (-43%), very-low-density lipoprotein (VLDL) cholesterol (-62%), LDL cholesterol (-43%), plasma apolipoprotein B (-23%), liver cholesteryl esters (-39%), LDL size, VLDL and LDL cholesteryl ester content (percent of total lipids), cholesteryl oleate/cholesteryl linoleate ratios in VLDL and LDL (25% to 30%), and ex vivo liver ACAT activity. The triglyceride/cholesteryl ester ratio increased twofold to fourfold in these apolipoprotein B-containing B-containing lipoproteins. Endogenous cholesterol absorption appeared to be unaffected by drug treatment. CI-976 failed to alter specific hepatic mRNAs involved in cholesterol metabolism, but comparisons among dietary control groups revealed a marked reduction in 7alpha-hydroxylase mRNA, no change in LDL receptor mRNA, and an increase in HMG-CoA reductase mRNA in EH rabbits compared with normal chow-fed controls. In cholesterol-fed rabbits (exogenous hypercholesterolemia) mRNA levels for all three proteins were lower than in chow-fed rabbits, especially that for HMG-CoA reductase. We conclude that (1) EH is due to the combined effects of hepatic hypersecretion of ACAT-derived lipoproteins and the relative absence of 7alpha-hydroxylase gene expression; (2) ACAT inhibition in this animal model results in a population of LDL particles that are fewer in number and smaller than those found in EH controls; and (3) marked changes in plasma LDL cholesterol can occur by diet (EH) or drug treatment (ACAT inhibitor) in rabbits in the absence of changes in LDL receptor gene expression.
