Binding and degradation of thrombospondin-1 mediated through heparan sulphate proteoglycans and low-density-lipoprotein receptor-related protein: localization of the functional activity to the trimeric N-terminal heparin-binding region of thrombospondin-1.

Thrombospondin-1 (TSP-1) is a multimodular trimeric protein involved in cell adhesion, motility and growth. TSP-1 binds to cells and is internalized and degraded in a process that requires the presence of heparan sulphate proteoglycan; the process is inhibited by heparin or receptor-associated protein (RAP), an antagonist of the low-density-lipoprotein receptor (LDLR) family. We characterized the attributes of TSP-1 that mediate the process. TSP277, which is truncated at Gln-277 of TSP-1 and contains the heparin-binding domain and the heptad repeat region that mediates trimerization, bound to and was degraded by a variety of cells with kinetics similar to those of the binding and degradation of intact TSP-1. Degradation of TSP277 was inhibited by heparin or RAP with dose responses similar to those for inhibition of degradation of TSP-1. Binding and degradation of TSP277 were decreased in Chinese hamster ovary cells lacking heparan sulphate. These results indicate that the N-terminal heparin-binding domain in a trivalent configuration is sufficient to mediate binding and degradation of TSP-1 via the proteoglycan-LDLR family pathway.