HUMAN-MELANOMA CELL-LINES DIFFER IN THEIR CAPACITY TO RELEASE ADP AND AGGREGATE PLATELETS

In this study we have investigated, using three different human melanoma cell lines (M(1)Do., M(3)Da., M(4)Be.), the varying capacity of melanoma cells to induce platelet aggregation in the presence or absence of inhibitors of ADP or thrombin. The expression levels of different integrins (alpha(v), beta(3), alpha(v) beta(3), alpha(IIb), alpha(v) beta(3)) were evaluated by immunoprecipitation, binding and now cytometry studies. The level of ADP in supernatants of melanoma cells were quantified by ADP bioassay and HPLC. Platelets were irreversibly aggregated by M(3)Da. as shown by electron microscopy, in contrast to M(1)Do. which induced a slow reversible aggregation. M(4)Be. did not induce platelet aggregation. In both cases, with M(3)Da. or M(1)Do., apyrase but not PPACK inhibited platelet induced aggregation. An anti-alpha(v) beta(3) monoclonal antibody (LYP18) or polyclonal antibody inhibited platelet aggregation. A similar number of LYP18 molecules bound to the surface of M(1)Do., M(3)Da. and M(4)Be. cell lines. Biological HPLC assays of ADP present in the supernatant of tumour cell lines showed the highest concentration of ADP to be secreted by M(3)Da., followed by M(1)Do., and none detected for M(4)Be. These results show that differences in in vitro aggregating potential of the three human melanoma cell lines are not related to low integrin expression levels but to their ability to generate ADP. Generation of ADP by human melanoma cells may act as important modulator of melanoma-platelet interactions.