GENERATION OF ADP BY HUMAN AND MURINE TUMOR CELLS IS SPECIFIC BUT IS UNRELATED TO METASTATIC POTENTIAL
The ability of tumor cells to activate platelets may facilitate the metastatic process. It has been generally assumed that the production of ADP by tumor cells is due to non-specific damage during harvesting in vitro or, in vivo, by frictional interactions with the capillary wall. The present work shows that tumor cell ADP arises not from cell damage but by a specific process under metabolic control. The human 253J urinary carcinoma cell line activated heparinized human platelets by an ADP-dependent mechanism based on inhibition by CP/CPK and the identification of aggregating concentrations (1 uM) of ADP in the cell-free supernatant by HPLC. Tumor cell damage during harvesting was shown not to be a factor since (i) the amount of ADP secreted was unrelated to the appearance of LDH, (ii) was similar when measured in confluent monolayers, in tumor cells after detachment and resuspension or following crossover studied in HBSS and MEM, and (iii) was constant at varying tumor cell concentrations. Metabolic control of ADP generation or transport was indicated by the fact that it was reduced 50 in tumor cells treated with p-chloromercuribenzene sulfonate and was completely abolished in those treated with iodoacetic acid. In order to determine whether this metabolically controlled generation of ADP was related to metastatic potential, we carried out identical experiments with the FI (low) and F10 (high) metastatic variants of the Bl6 murine melanoma line. The amounts of ADP produced by the B16 cells were about twice as great as with the human 253J cells but there was no significant difference between the amounts of ADP generated by FI and F10 variants. These studies demonstrate that ADP production by tumor cells is a discrete process under metabolic control but is not directly related to the metastatic potential of individual tumor cell lines.