Human melanoma cell lines differ in their capacity to release ADP and aggregate platelets

Summary. In this study we have investigated, using three different human melanoma cell lines (M₁Do., M₃Da., M₄Be.), 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 (α_v, β₃, α_vβ₃, α_IIb, α_vβ₃) were evaluated by immunoprecipitation, binding and flow cytometry studies. The level of ADP in supernatants of melanoma cells were quantified by ADP bioassay and HPLC. Platelets were irreversibly aggregated by M₃Da. as shown by electron microscopy, in contrast to M₁Do, which induced a slow reversible aggregation. M₄Be. did not induce platelet aggregation. In both cases, with M₃Da. or M₁Do., apyrase but not PPACK inhibited platelet induced aggregation. An anti-α_vβ₃ monoclonal antibody (LYP18) or polyclonal antibody inhibited platelet aggregation. A similar number of LYP18 molecules bound to the surface of M₁Do., M₃Da. and M₄Be. 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 M₃Da., followed by M₁Do., and none detected for M₄Be. 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.