THROMBOSPONDIN MODULATES MELANOMA-PLATELET INTERACTIONS AND MELANOMA TUMOR-CELL GROWTH IN-VIVO

In this study we have investigated the role of thrombospondin (TSP) as a possible ligand playing a key role in human M(3)Da. melanoma cell interaction with platelets and in tumour growth. TSP is secreted (80 +/- 6 ng TSP 10(-6) cells) and bound to the surface of M(3)Da. cells via receptors different from CD36, as shown by biosynthetic labelling and immunofluorescence studies. The levels of TSP binding to M(3)Da. cells evaluated by binding studies, using an anti-TSP monoclonal antibody (MAb) (LYP8), shows 367 000 +/- 58 000 (mean +/- s.d.) LYP8 binding sites per cell with a dissociation constant (K-d) of 67 nM. TSP binding to M(3)Da. cells shows 400 000 +/- 50 000 TSP binding sites per cell with a K-d Of 10 nM. The capacity of anti-TSP MAb (LYP8) to inhibit M(3)Da.-platelet interactions was followed on an aggregometer and evaluated by electron microscopy studies. Toe biological role of TSP binding to M(3)Da. cells was investigated by implanting subcutaneously the M(3)Da. cell line in nude mice and following the size and time of in vivo tumour growth. Reducing the availability or the functional level of TSP by using an anti-TSP MAb (LYP8) resulted in a significant decrease in platelet aggregates interacting with M(3)Da. melanoma cells. Using an enzyme-linked immunosorbent assay, purified alpha(v) beta(3) was shown to bind TSP. Moreover, LYP8-coated M(3)Da. cells showed a reduced capacity to form rumours in vivo. M(3)Da. cells were observed to attach and spread on human platelet TSP-coated plastic wells. This attachment by M(3)Da. cells was inhibited in a similar way by LYP8 and an anti-alpha(v) beta(3) MAb (LYP18). The results obtained in this study show that TSP secreted and bound to the surface of a human melanoma cell line (M(3)Da.) acts as a link between aggregated platelets and the M(3)Da. cell surface. Moreover, these results shows that TSP can modulate tumour growth in vivo. Reagents such as MAbs directed against TSP and peptides derived from TSP could not only be used as a new therapeutic approach in the control of tumour metastasis of melanoma, but may also contribute to elucidation of the role of TSP in cancer biology.