IDENTIFICATION OF THE LOW-DENSITY-LIPOPROTEIN RECEPTOR-RELATED PROTEIN (LRP) AS AN ENDOCYTIC RECEPTOR FOR THROMBOSPONDIN-1

Thrombospondin-1 (TSP1) has potent biological effects on vasculature s mooth muscle cells (SMCs) and endothelial cells. The regulation of ext racellular accumulation of TSP1 is mediated by a previously obscure pr ocess of endocytosis which leads to its lysosomal degradation, Since m embers of the low density lipoprotein receptor (LDLR) family have been found to mediate endocytosis which leads to degradation of a diverse array of ligands, we evaluated their possible role in the uptake and d egradation of TSP1 by vascular SMCs, endothelial cells and fibroblasts , I-125-TSP1 was found to be internalized and degraded lysosomally by all these cell types, Both the internalization and degradation of I-12 5-TSP1 could be inhibited by a specific antagonist of the LDLR family, the 39-kD receptor-associated protein (RAP), Antibodies to the LDLR-r elated protein (LRP) completely blocked the uptake and degradation of I-125-TSP1 in SMCs and fibroblasts but not endothelial cells. Solid-ph ase binding assays confirmed that LRP bound to TSP1 and that the inter action was of high affinity (K-d = 5 nM). Neither RAP nor LRP antibodi es inhibited the binding of I-125-TSP1 to surfaces of SMCs, However, c ell surface binding, as well as, endocytosis and degradation could be blocked by heparin or by pre-treatment of the cells with either hepari tinase, chondroitinase or beta-D-xyloside. The data indicates that cel l surface proteoglycans are involved in the LRP-mediated clearance of TSP1, A model for the clearance of TSP1 by these cells is that TSP1 bo und to proteoglycans is presented to LRP for endocytosis, In endotheli al cells, however, the internalization of TSP1 was not mediated by LRP but since RAP inhibited TSP1 uptake and degradation, we postulate tha t another member of the LDLR family is likely to be involved.