L-asparaginase-induced modulation of methotrexate polyglutamylation in murine leukemia L5178Y.

The modulation of methotrexate polyglutamylation by L-asparaginase has been examined in mice bearing sublines of leukemia L5178Y that have different sensitivities to asparaginase. A single i.p. injection of 200 IU/kg of asparaginase completely inhibited ascites tumor cell growth in the parental L5178Y/S+ tumor for 120 h compared to 72 and 30 h in the L5178Y/S and L5178Y/S+/- sublines, respectively. Similarly, DNA and protein synthesis were completely inhibited by asparaginase for 96 h in L5178Y/S+ cells, but only for 72 and 24 h in L5178Y/S and L5178Y/S+/- cells. In each tumor the temporal patterns of depletion and recovery of S-phase cells were similar to the patterns of suppression and recovery of DNA and protein synthesis observed in that tumor. When methotrexate was administered at either 96 or 24 h after asparaginase during the asparaginase-induced S-phase nadirs of L5178Y/S+ and L5178Y/S+/- cells, respectively, subsequent methotrexate polyglutamylation was inhibited 83 and 92% compared to tumor cells exposed to methotrexate only. Recovery of methotrexate polyglutamylation in both tumors following L-asparaginase pretreatment coincided in time with the return in the fraction of S-phase cells towards the pretreatment values. The inhibition of methotrexate polyglutamate accumulation by asparaginase was associated with decreased retention of methotrexate in tumor cells. In contrast, asparaginase had no significant effect on methotrexate polyglutamate accumulation and methotrexate retention when administered after methotrexate. These data indicated that the asparaginase-induced modulation of methotrexate polyglutamylation in mice was directly related to the time course of inhibition and recovery of tumor cell proliferation by asparaginase, and thus varied with the intrinsic sensitivity of the individual tumor to the enzyme.