ATP and adenosine prevent via different pathways the activation of caspases in apoptotic AKR-2B fibroblasts

Confluent AKR-2B fibroblasts rapidly disintegrate after serum deprivation.(27) ATP or adenosine added immediately after serum removal afforded substantial protection against cell death even for a long period of 24 h, ED50 values were 14 and 110 mu M for ATP and adenosine, respectively. In the presence of 5 mu g/ml cycloheximide the protective effect of both substances was suppressed, indicating that protein synthesis is required. The protective effect of ATP was highly specific since among numerous tested derivatives only ATP-[gamma-S] exhibited a substantial protective effect. The ability of ATP and adenosine to modulate cell division was analyzed, Both substances did not exhibit any mitogenic effect. Adenosine completely blocked PDGF-BB induced cell division, whereas ATP had no effect. Unlike adenosine, ATP strongly stimulated Ca2+-release from intracellular stores. On the other hand, adenosine stimulated an increase in the intracellular concentration of cAMP from 0.4-1.5 mu M, whereas ATP decreased the content below 0.1 mu M. ATP stimulated the phosphorylation of MAP-kinase, RSK and p70(S6)-kinase; adenosine was inactive. After complexation of [Ca2+](i) the protective effect of AIP was greatly lost while adenosine was still active. Surprisingly neither ATP nor adenosine caused an activation of PKC-isoforms. After incubation with pertussis toxin, the protection by ATP was reduced indicating an involvement of Gi-proteins in the signal transduction induced by AIP. Our results indicate that ATP as well as adenosine are potent inhibitors of cell death caused by serum deprivation and that this protective effect apparently occurs via distinct pathways. However, both pathways must converge at the point of caspase activation, since the stimulation of DEVDase- and VEIDase-activities, respectively, are suppressed by either ATP or adenosine.