LDL Receptor mRNA Abundance

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-hydroxy3-methylglutaryl coenzyme A (HMG-CoA) reductase, 7a-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 trigryceride/cholesteryi ester ratio increased twofold to fourfold in these apolipoprotein B-containing lipoproteins. Endogenous cholesterol absorption appeared to be unaffected by drug treatment Cl-976 failed to alter specific Aendogenous hypercholesterolemia (EH) can be produced experimentally in rabbits by feeding a cholesterol-free, purified diet containing casein as the sole protein source.' Unlike cholesterol-fed rabbits or Watanabe heritable hyperlipidemic rabbits, there is a selective increase in the cholesterol concentration of lowdensity lipoprotein (LDL) in the EH rabbit and no change in plasma trigh/cerides. Therefore, the plasma lipid and lipoprotein profile of the EH rabbit phenotypicalry resembles human type Ha hyperlipoproteinemia/ Although a reduction in receptor-mediated catabolism of LDL is thought to be responsible in part for the EH condition in rabbits, increased LDL production may also contribute. Perfused livers from EH rabbits secrete an Received May 5, 1993; revision accepted December 27, 1993. From the Department of Atherosclerosis Therapeutics, ParkeDavis Pharmaceutical Research Division of Warner-Lambert Co, Ann Arbor, MI. Presented in part at the 64th Scientific Sessions of the American Heart Association, November 11-14, 1991, and published in abstract form (Artcrioscler Thromb. 1991;ll:1398a and Circulation. 1991;84[suppl II]:II-138). Correspondence to Brian R. Krause, PhD, Department of Atherosclerosis Therapeutics, Parke-Davis Research Division of Warner Lambert Co, Ann Arbor, MI 48105. hepatic mRNAs involved in cholesterol metabolism, but comparisons among dietary control groups revealed a marked reduction in 7o-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 7a-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. (Arterioscler Thromb.