Catabolism of low density lipoproteins by perfused rabbit livers: Cholestyramine promotes receptor-dependent hepatic catabolism of low density lipoproteins (hepatic low density lipoprotein receptor/low density lipoprotein metabolism/hypercholesterolemia/casein diet/high density lipoprotein

Rabbits fed a wheat starch/casein diet develop a marked hypercholesterolemia accompanied by a decrease in the number of EDTA-sensitive binding sites on plasma membrane fractions of the liver for low density lipoproteins (LDL) and 13migrating very low density lipoproteins [Chao, Y.-S., Yamin, T.T. & Alberts, A. W. (1982)J. BioL Chem., in press]. Inclusion of 1% cholestyramine resin in this diet prevents the increase in plasma cholesterol, increases the removal of LDL from plasma, and increases the number of hepatic plasma membrane LDLbinding sites. To determine the functional role of hepatic LDLbinding sites in the catabolism ofLDL, we studied the catabolism of "MI-labeled LDL (125I-LDL) by in situ perfused rabbit livers in a recirculating system. The rate of catabolism was measured from the increment of nonprotein-bound radioiodine in the perfusate. The receptor-dependent catabolism of LDL by the liver was calculated from the difference of hepatic catabolism of 125I-LDL and catabolism of LI-labeled cyclohexanedione-modified LDL, which does not bind to LDL receptors. The data show that about 74% of LDL catabolized by perfused livers from chow-fed rabbits is through the receptor-dependent pathway and 26% is through the receptor-independent pathway. In rabbits fed a cholesterol diet, the hepatic catabolism of 125I-LDL is reduced, and the receptordependent catabolism of 125I-LDL is abolished. In rabbits fed the wheat starch/casein diet, the receptor-dependent catabolism of 125I-LDL is reduced by 40% when compared with hepatic catabolism in chow-fed rabbits. Perfused livers from rabbits fed the wheat starch/casein diet supplemented with 1% cholestyramine show a 5.4-fold increase of receptor-dependent catabolism of 125ILDLwhen compared with that of livers from rabbits fed the wheat starch/casein diet alone. Thus, these studies demonstrate that the change in the number of rabbit hepatic membrane LDL receptors induced by dietary manipulation and drugs is correlated to the functional rate of removal of LDL by the liver. In cultured extrahepatic cells, low density lipoproteins (LDL) are taken up by the cells after binding to the high-affinity receptor on the cellular surface. The activity of the LDL receptor is regulated by the concentration ofLDL apolipoprotein in the culture medium (1). Several lines of evidence suggest that the LDL receptor is involved in the catabolism of LDL in vivo. Patients with homozygous familial hypercholesterolemia, who have elevated plasma LDL concentration, lack the high-affinity LDL receptor in their cultured skin fibroblasts (2). Shepherd et al. (3) showed that in heterozygous familial hypercholesterolemia, the removal of lmI-labeled LDL('2I-LDL) from plasma by the receptor-dependent pathway is reduced. Thompson et al. (4) recently reported that in homozygous familial hypercholesterolemia, the removal of 125I-LDL by the receptordependent pathway is essentially abolished. Cholesterol, the major component of LDL, is metabolized mainly by the liver. There is ample evidence that suggests that a receptor similar to that found in extrahepatic tissue also exists on the plasma membranes of the liver. Administration of pharmacological doses of 17a-ethinyl estradiol to rats increases the number of high-affinity binding sites for LDL on the liver membranes (5), and the increased number ofbinding sites is accompanied by increased hepatic catabolism of 125I-LDL in isolated perfused rat livers (6). A high-affinity binding site for LDL is also present in cultured pig (7) and rabbit hepatocytes (8) and in liver membranes prepared from dogs (9, 10) and rabbits (11). The rabbit is a suitable animal model for the study of lipoprotein metabolism. Recently, the Watanabe heritable hyperlipidemic (WHHL) rabbit line was discovered and found to be a useful model for the study of lipoprotein metabolism in familial hypercholesterolemia (12). Homozygous WHHL rabbits have greatly reduced numbers of LDL receptor in both cultured fibroblasts (13) and hepatic membranes (11). Normal rabbits also can become hypercholesterolemic ifthey are fed either a cholesterol diet (14) or a cholesterol-free wheat starch/casein diet (15). The hepatic LDL receptor in normal rabbits can be regulated by the addition of dietary constituents to the animal diet. Feeding rabbits a cholesterol diet caused reduced activity of hepatic LDL receptor (16). Slater et al. (17) have demonstrated that cholestyramine resin, a bile acid sequestrant, increases the hepatic uptake of LDL by promoting the activity of hepatic LDL receptor. We have shown that the hepatic EDTA-sensitive binding sites for 125I-LDL is down-regulated in rabbits fed a wheat starch/casein diet and that this binding site can be induced when cholestyramine is included in the diet (18). In the current study, we investigated the catabolism of 1"ILDL by perfused rabbit livers and the regulation of the activity of hepatic LDL receptor in rabbits fed different diets.