Role of dt-diaphorase as a determinant of sensitivity to mitomycin analogs in nonsmall cell lung-cancer cell-lines.

DT-diaphorase (DT-D) is regarded as a two-electron reductase that plays an important role in the biotransformation of mitomycin C (MMC) to antitumor metabolites, which is enhanced under hypoxic conditions. To evaluate the role of DT-D as a bioactivator of MMC and its analogue, KW-2149, in non-small cell lung cancer (NSCLC) cell lines under an aerobic or hypoxic condition, we examined the inhibitory effect of dicumarol which was regarded as an inhibitor of DT-D on the sensitivity to these anticancer agents in vitro. In this study, we used an MMC-resistant NSCLC cell line (PC-9/MC4) which was established in our laboratory from a PC-9 cell line as a parent cell line by continuous exposure to MMC. The subline PC-9/MC4 was 6.7-fold more resistant to MMC than PC-9, the parent cell line, under aerobic conditions, and 5.2-fold more resistant even under hypoxic conditions. PC-9/MC4 cell lines did not show collateral resistance to KW-2149, a newly developed MMC analogue. The IC50 value of PC-9 against MMC significantly decreased by co-incubation with dicumarol under aerobic, but not under hypoxic conditions. KW-2149 was cytotoxic to PC-9/MC4 as well as PC-9 cells, and the sensitivity to KW-2149 was not altered by coincubation with dicumarol or exposure to hypoxia. There were no significant differences in intracellular uptake of MMC and the activities of cytosolic detoxification enzymes, GST and GSH, between PC-9 and PC-9/MC4 cell lines under aerobic conditions. These findings suggest a partial role of DT-D in bioactivation of MMC, but not of KW-2149, under aerobic conditions. However, the detailed mechanisms of drug resistance to MMC under hypoxic conditions are still not clear.