The interaction of lipoproteins with lipoprotein receptors is a fundamental factor influencing the levels of plasma lipoproteins and delivery of lipids to tissues. Abnormalities affecting this interaction may involve either the receptor or the lipoprotein ligands and lead to accelerated arteriosclerosis. This is the case in two hereditary disorders, familial hypercholesterolemia, in which the apo-B,E(LDL) receptor is defective, and familial dysbetalipoproteinemia (type III hyperlipoproteinemia), in which the ligand apolipoprotein (apo-) E is defective. Using current techniques, it is not possible to quantitate defects in lipoprotein receptor-ligand interactions on a molar basis in vivo. The major goal of this proposal is to develop methods that allow estimation of lipoprotein receptor number and rate of association with lipoprotein ligands in vivo. The liver is the major site of receptor-mediated clearance of lipoproteins and will be studied in detail. Radiolabeled lipoproteins will be injected in experimental animals and their distribution in vivo monitored continuously using gamma camera imaging. A three-compartment model consisting of ligand in extrahepatic blood, hepatic blood, and ligand bound to receptors will be designed and validated. The proposed studies will focus primarily on apo-E-containing lipoproteins. Comparisons will be made between normal apo-E and mutants of apo-E that are defective in receptor binding. Kinetic modeling of receptor-mediated uptake in vivo is a novel approach to the investigation of lipoprotein metabolism that could introduce a new dimension to our understanding of the interaction between lipoprotein receptors and ligands. ultimately it could lead to diagnostic tests for familial hypercholesterolemia and other hyperlipidemic disorders associated with abnormal lipoprotein receptor function. We will delineate more precisely the in vitro determinants of receptor binding by normal and mutant forms of apo-E. The effect on binding of the number of apo-E molecules per particle, the lipid composition, and the presence of other apolipoproteins will be investigated on native and synthetic lipoproteins in vitro. We will correlate in vitro receptor binding data with the metabolic fate of lipoproteins containing apo-E in vivo. Human subjects with mutations in apo-E will be studied to gain insight into the normal role of this protein. Our goal determine the origin of beta-very low density lipoproteins, a potentially atherogenic lipoprotein that is the hallmark of defective function of apo-E in vivo.
