[Use of purine rescue pathways by L1210 cells exposed to methotrexate in vitro]

Methotrexate (MTX) blocks the de novo synthesis of purines and pyrimidines due to a diminution of reduced folates, and produces an accumulation of intracellular 5-phosphoribosyl-1-pyrophosphate (PRPP). Phosphoribosyltransferases, in the presence of PRPP, synthesize nucleotides starting on purine bases, that are activated to triphosphates ("rescue pathway"). The authors study the activity of the rescue pathways in L1210 leukemia cells in vitro to restore the levels of ATP and GTP that have been lowered by MTX. Hypoxanthine (Hpx) was used as a preformed source of purines, high efficiency liquid chromatography was employed to estimate the intracellular concentrations of ATP and GTP. Two hours after 0.1 mM Hpx addition to in vitro L1210 cells in the presence of 0.1 microM MTX, the amount of ATP and GTP increased 5 to 7 times. In former experiments the author has shown that the uptake of PRPP increases when Hpx is added to L1210 cells exposed to MTX. The results of this work show that L1210 leukemia cells in vitro can use the rescue pathways of purine synthesis when Hpx is present in the growth medium, so preventing the anti-purine effect of MTX.