METABOLISM OF THROMBOSPONDIN-2 - BINDING AND DEGRADATION BY 3T3 CELLSAND GLYCOSAMINOGLYCAN-VARIANT CHINESE-HAMSTER OVARY CELLS

Thrombospondin 1 (TSP1) and thrombospondin 2 (TSP2) are members of the thrombospondin family that have a similar structural organization but somewhat different functional activities. Iodinated recombinant mouse TSP2 bound to NIH 3T3 cells and was internalized and degraded through a chloroquine-inhibitable pathway. TSP2 degradation was saturable, sp ecific, and similar to the kinetics of degradation of TSP1. Human plat elet TSP1, recombinant mouse TSP1, and recombinant mouse TSP2 cross-co mpeted with one another for degradation by 3T3 cells. Degradation of T SP2 was less sensitive to inhibition by heparin than degradation of TS P1. This is in agreement with differences in heparin-binding affinity of the two TSPs. Degradation of TSP2 was slower in cultures of Chinese hamster ovary (CHO) cells lacking heparan sulfate proteoglycans than in wild type CHO cells or in cultures of 3T3 cells treated with hepari tinase than in untreated 3T3 cells. Degradation of TSP2 was inhibited by antibodies against the low density lipoprotein receptor-related pro tein (LRP) or by the 39-kDa receptor-associated protein, a known antag onist of LRP. This study indicates that TSP2 and TSP1 are metabolized by a common internalization and degradation pathway involving heparan sulfate proteoglycan and LRP. Competition for this pathway is a possib le mechanism whereby cells can control the levels and ratio of TSP1 an d TSP2 in the extracellular milieu.