Cell Line-Dependent Changes in Sensitivity to Mitomycin C by 1-Methyl-3-isobutylxanthine Is Due to an Altered Intracellular Dose of the Anticancer Drug and/or to Changes in DT-diaphorase Activity

Sensitization by 1-methyl-3-isobutylxanthine (MIX), a potent inhibitor of cAMP phosphodiesterase, of cellular sensitivity to mitomycin C (MC) was tested using various cell lines. Sensitization was seen with CHO cells and Balb/c 3T3 cells, but a decrease in sensitivity was seen with HeLa, K-balb and other cell lines. To measure the extent of crosslinks produced by MC, we used an alkaline elution assay. The extent of crosslinks was increased by MIX in CHO cells and decreased in K-balb cells. These changes in extent are well reflected by changes in sensitivity to MC in both cell lines. In CHO cells, an intracellular dose of MC was increased by MIX with no change in the rate of uptake or efflux of MC. There was a slight decrease in the dose in HeLa or K-balb cells. Among the enzymes which engage in the reductive activation of MC, we tested DT-diaphorase and NADPH-cytochrome P450 reductase using CHO, HeLa and K-balb cells. MIX had almost no effect on the activity of NADPH-cytochrome P450 in each cell line, whereas it suppressed the activity of DT-diaphorase, significantly in HeLa and K-balb cells, and slightly in CHO cells. All these facts indicate that MIX caused an increase in the extent of crosslinks via an increase in the intracellular dose of MC in CHO cells, and that these increases may lead to the sensitization. On the other hand, the decrease in the sensitivity shown in HeLa and K-balb cells may be due to the suppression of DT-diaphorase activity and/or to a decrease in MC dose. The effect of forskolin, another cAMP raising agent, on the sensitivity to MC was examined in CHO cells to test whether the sensitization by MIX involves the intracellular cAMP level. The treatment with forskolin had little effect, indicating that there was no such involvement.