Biphasic cytotoxic mechanism of extracellular ATP on U-937 human histiocytic leukemia cells: involvement of adenosine generation

Since extracellular ATP can exhibit cytotoxic activity in vivo and in vitro, its application has been proposed as an alternative anticancer therapy. In this study we investigated the mechanisms of ATP-induced cytotoxicity in a human leukemic cell line (U-937). ATP added as a single dose exceeding 50 CIM was cytostatic or even cytotoxic for U-937 cells. Interestingly. growth inhibition by ATP (50-3500 muM) showed a biphasic dose response. Up to 800 muM. ATP was cytotoxic in a dose-dependent manner (EC50 90 muM). In a range between 800 and 2000 muM, cell count was markedly higher despite the higher ATP concentrations. The cytotoxic effect of ATP could be antagonized by addition of uridine as a pyrimidine source and, alternatively, by addition of the nucleoside transmembrane inhibitor dipyridamole, The apoptosis-inducing adenosine A(3) receptor was not involved in measurable quantities, sines (1) adenosine did not lead to an elevation of intracellular calcium levels, and (2) tin unselective A(1-3) antagonist (ULS-II-80) could net abrogate the cytotoxic effect. Experiments monitoring extracellular nucleotide metabolism confirmed the assumption that the long-term production and continuous uptake of adenosine, which is extracellularly generated by degradation of ATP, led to an intracellular nucleotide imbalance with pyrimidine starvation. The biphasic dose response to higher ATP concentrations could be explained by the rapid degradation of lower ATP concentrations (300 muM) to adenosine by serum-derived enzymes, whereas higher concentrations (900 muM) only produced small amounts of adenosine due to forward inhibition of AMP hydrolysis by prolonged high ADP levels. FAGS analysis revealed that at lower adenosine concentrations (300 muM) a reversible G(1) phase attest of the cell cycle in as induced, whereas higher concentrations (1000 muM) triggered apoptosis. Considering ATP as a potential cytostatic drug, our data have important implications concerning metabolic interactions of administered nucleotides. (C) 2001 Elsevier Science B.V. Ah rights reserved.