Cell death induced by uridine 5'-triphosphate (UTP) in contrast to adenosine 5'-triphosphate (ATP) in human epidermoid carcinoma cells (A-431).

OBJECTIVE Extracellular ATP has been reported as an important signaling molecule mediating quite divergent specific biological effects. Recent clinical trials suggest a potential role of ATP in cancer treatment. The aim of our study was to analyze the mechanisms of UTP in comparison to ATP-mediated cell death in an established cell line model (A-431). METHODS Cell culture and proliferation assays, separation of nucleotides by thin-layer technique, measurement of cytosolic free Ca(2+), flow cytometry analysis (annexin V), ultra structure, semi-quantitative RT-PCR, standard statistics. RESULTS ATP, when added as a single dose between 100 and 500 microM to A-431 cell cultures, showed increasing cytotoxicity, mainly as apoptosis, with a paradoxically decreasing effect at higher concentrations up to 1500 microM. Doses exceeding 1500 microM again led to increasing cytotoxicity. UTP at doses between 500 and 3000 microM resulted in increasing cell death following a normal sigmoid dose-response model. ATP and UTP were degraded by membrane-bound ectoenzymes. ATP degradation products, e.g. adenosine, also induced cell death. Dipyridamole, an adenosine uptake inhibitor, was able to abolish ATP toxicity, which was also counteracted by the addition of uridine. In addition, we found functional and transcriptional evidence for P2 receptors on A-431 cells. CONCLUSION Extracellular ATP seems to act via degradation to adenosine and consecutive induction of apoptosis. In contrast, we were not able to demonstrate analogous mechanisms for cell death mediated by extracellular UTP, but were able to provide some evidence of classical ligand-receptor action of uncleaved UTP on A-431 cells.