NUCLEOTIDE METABOLIZING ECTOENZYMES ARE UP-REGULATED IN A431 CELLS PERIODICALLY TREATED WITH CYTOSTATIC ATP LEADING TO PARTIAL RESISTANCE WITHOUT PREVENTING APOPTOSIS

Extracellular ATP, when added as a single dose at concentrations highe r than 0.1 mM to the culture medium, was growth inhibitory or even cyt otoxic for human epidermoid carcinoma cells (A431). Adenosine at the s ame concentrations was much less potent, The molecular mechanism under lying the inhibitory effect of extracellular ATP has been investigated . The cytostatic as well as the cytotoxic effects of ATP could be prev ented by supplying uridine as a pyrimidine source and, alternatively, by simultaneous addition of dipyridamole, which inhibits the uptake of adenosine. The data suggest that the long-term production and continu ous uptake of adenosine, which is enzymatically generated from the ATP in the medium, led to an intracellular nucleotide imbalance with pyri midine starvation. This triggered suicidal processes ending up in apop tosis of the cells. The tumor cells have been adapted to extracellular ATP with the aim to obtain cells which are more resistant to ATP. The refore, growing cells were periodically treated with extracellular ATP . These cells were characterized by an enlargement of cell size, a dec reased proliferation rate, and a reduced but not abolished sensitivity to cytostatic and cytotoxic ATP doses. The calcium response of adapte d cells was shortened. The nucleotide hydrolyzing ectoenzyme activitie s (ecto-ATPase, ecto-ADPase, ecto-AMPase, ecto-Ap(4)Aase) were simulta neously upregulated. All phenotypic alterations of the adapted cells d isappeared after cultivation for several generations in the absence of extracellular ATP. Considering ATP as a potential chemotherapeutic ag ent the adaptive phenomena of treated cells might be important. 0167-4 889/98/$-see front matter (C) 1998 Published by Elsevier Science B.V. All rights reserved.