RATIO OF 2'-DEOXYADENOSINE-5'-TRIPHOSPHATE THYMIDINE-5'TRIPHOSPHATE INFLUENCES THE COMMITMENT OF HUMAN COLON-CARCINOMA CELLS TO THYMINELESSDEATH

In colon cancers induction of a thymineless state following inhibition of thymidylate synthase (TS) by 5-fluorouracil combined with leucovor in can initiate a cytotoxic response. Using a 5-fluorouracil-leucovori n-treated human colon carcinoma cell line (GC(3)/c1) and a clonally de rived TS- mutant, initiation events that dictate the onset of and comm itment to thymineless death have been examined. Initial events related to a temporally associated decrease in dTTP and elevation in the dATP pools; no depletion of dGTP or elevation in dCTP was detected. Nucleo somal degradation of DNA commenced at 24 h in TS- and 49 h in GC(3)/cl , and was associated with the more rapid development of an imbalance i n the dATP and dTTP pools and a higher dATP:dTTP ratio in TS- cells. T he contribution of elevated dATP or depleted dTTP pools to thymineless death was subsequently determined by treatment of GC(3)/cl or TS- cel ls with deoxyadenosine to elevate the dATP pool either under thymidine -replete or thymineless conditions. Thus, deoxyadenosine supplementati on under dATP-replete conditions elevated the dATP pool for 16 h and w as cytotoxic to cells, During dTTP depletion elevated dATP was maintai ned, and cytotoxicity was significantly and rapidly enhanced by deoxya denosine but could be reversed by thymidine, Data suggest that mainten ance of elevated dATP and the dATP:dTTP ratio are essential initiation events in the commitment of colon carcinoma cells to thymineless deat h.