Low Density Lipoprotein Receptor-related Protein/α2-Macroglobulin Receptor Mediates the Cellular Internalization and Degradation of Thrombospondin: A PROCESS FACILITATED BY CELL-SURFACE PROTEOGLYCANS

Abstract

Thrombospondin (TSP) is a cell and matrix glycoprotein that interacts with a variety of molecules. Newly synthesized thrombospondin is either incorporated into the extracellular matrix, or binds to the cell surface where it is rapidly internalized and degraded (McKeown-Longo, P. J., Hanning, R., and Mosher, D. F. (1984) J. Cell Biol. 98, 22-28). In the current investigation we identify the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP) as a receptor responsible for mediating the internalization of TSP leading to its degradation. LRP is a large cell surface receptor consisting of a 515-kDa heavy chain and an 85-kDa light chain proteolytically derived from a 600-kDa precursor. A specific and high affinity interaction between purified LRP and TSP was demonstrated by homologous ligand competition experiments, where a KD of 3-20 nM was measured using different preparations of TSP. The binding of TSP to purified LRP was completely inhibited by the 39-kDa receptor-associated protein, a known antagonist of ligand binding by LRP. Cultured fibroblasts rapidly internalize and degrade 125I-labeled TSP via a receptor-mediated process. This process is inhibited by receptor-associated protein and by antibodies against LRP, indicating that LRP is mediating the cellular internalization of TSP. Our studies also confirm that the efficient catabolism of TSP requires the participation of cell surface proteoglycans, since digestion of cells with heparitinase markedly reduces the extent of LRP-mediated TSP degradation. The ability of LRP to directly bind and mediate the cellular internalization and degradation of TSP indicates that this receptor may play an important role in the catabolism of TSP in vivo.