The cellular receptors involved in the catabolism of high-density lipoproteins have been the focus of research for the last several years. The liver and kidney are the two major sites of HDL metabolism. Previous studies have identified cubilin, a multiligand cell surface extrinsic membrane protein, as a receptor which binds apoA-I and apoA-II. Cubilin is present on the apical surface of the renal tubular cell and is proposed to interact with megalin as a co-receptor for the uptake and degradation of poorly delipidated HDL. Megalin was identified as a potential co receptor for cubilin and the degradation of HDL since antibodies against megalin decreased HDL degradation in cell culture systems. In all previous cell culture in vitro studies, both cubilin and megalin were present. In order to determine the potential independent roles of cubilin and megalin in the degradation of HDL we have constructed a 11kb cubilin cDNA. Transfection studies utilizing 125I apoA-I or 125HDL and the cubilin cDNA in CHO cells revealed increased apoA-I and HDL binding however there was no apoA-I or HDL degradation. Therefore cubilin alone is not able to initiate cellular uptake and catabolism of apoA-I or HDL. The independent role of megalin in the kidney degradation was evaluated using a cell line that contained only megalin receptors with no significant expression of cubilin, low density lipoprotein (LDL) receptor, or LDL receptor related protein as well as a megalin knockout cell line. In in vitro experiments with radiolabeled apoA-I, apoA-II, and HDL the megalin positive cell line rapidly degraded apolipoproteins A-I, A-II and HDL. The megalin knockout cell line did not degraded HDL or the radiolabeled apolipoproteins. Thus megalin alone without cubilin is able to bind and degrade apoA-I as well as HDL. The combined results from these studies have indicated that there are two potential pathways for the renal mediated catabolism of HDL. The first pathway involves the binding of poorly lipidated HDL to cubilin, which then interacts with megalin to facilitate the degradation of HDL. The second pathway involves the binding, uptake, and degradation of HDL by megalin alone. Based on this data we propose that megalin is the major renal receptor for the degradation of HDL.
