Lipoprotein binding to canine hepatic membranes. Metabolically distinct apo-E and apo-B,E receptors

Abstract

Hepatic membranes from adult dog livers have receptors which bind to lipoproteins containing the E apoprotein (the apo-E HDLc) but lack specific receptors for the apo-B-containing low density lipoproteins (LDL). Scatchard analysis of direct binding data for 125I-apo-E-HDLc revealed nonlinearity of the binding which could be resolved into two components, suggesting the presence of two separate binding sites. The binding site for apo-E HDLc that possessed the highest affinity (Kd = 0.23 x 10(-9) M) was calcium-dependent and was sensitive to proteolytic digestion with pronase. The lower affinity (Kd = 20 x 10(-9) M) binding site for apo-E HDLc did not require calcium and was resistant to pronase digestion. Chemical modification of the arginyl or lysyl residues of the apo-E HDLc prevented the HDLc from binding to the higher affinity receptor but had no effect on their binding to the lower affinity site. Adult canine liver membranes also bound canine 125I-HDL. However, the binding of HDL was of lower affinity (Kd = 8.2 x 10(-8) M), did not require calcium, was not blocked by modification of the lysyl or arginyl residues, and may not be of physiologic significance. Although the liver membranes from normal chow-fed adult dogs did not bind canine LDL, it was possible to demonstrate specific high affinity binding of LDL under certain metabolic conditions in dogs. When adult dogs were treated with the hypocholesterolemic agent cholestyramine, the liver membranes from these animals readily bound canine LDL. The Kd for canine LDL binding to these liver membranes was 15 x 10(-9) M. Furthermore, it was possible to demonstrate high affinity binding of LDL to the liver membranes from young rapidly growing puppies (Kd = 11 x 10(-9) M). The binding of the 125I-LDL to the liver membranes from the adult cholestyramine-treated dogs or from the puppies appeared to be mediated by apo-B,E receptors which resembled the LDL receptor of human skin fibroblasts. The 125I-LDL binding of these liver membranes was competitively inhibited by the addition of unlabeled LDL or apo-E HDLc. On the other hand, 125I-apo-E HDLc, capable of binding to the apo-B,E or to the apo-E receptors, were only partially displaced by the addition of unlabeled LDL but were totally displaced by apo-E HDLc. In summary, the adult dog liver possessed only the apo-E receptor. An apo-B,E receptor capable of binding LDL and HDLc could be induced by treatment of adult dogs with cholestyramine. Similarly, the liver membranes of young growing puppies possessed the apo-E and po-B,E receptors and were capable of binding both apo-E HDLc and LDL. The mechanism responsible for the control of the expression of the hepatic apo-B,E and/or apo-E receptors remains to be determined. These data indicate that a unique receptor capable of interacting specifically with apo-E-containing lipoproteins, and not with apo-B-containing lipoproteins (LDL), exists in the adult canine liver.