Mouse
leukemia L1210 cells were generated for resistance to
4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone (
MAIQ), a potent inhibitor of
ribonucleotide reductase that is directed in the nonheme
iron subunit (NHI) of the
enzyme. The resistant cells, MQ-580, showed an 8-fold increase in IC50 toward
MAIQ, a 4-fold increase in IC50 toward
hydroxyurea, and also showed resistance to other
ribonucleotide reductase inhibitors. In addition, the MQ-580 cell line was resistant to nonribonucleotide
reductase inhibitors such as
etoposide,
daunomycin and
vinblastine, but not to
cisplatin. The
mRNA for the NHI subunit was increased 7-fold in the MQ-580 cells with essentially no change in the
mRNA level for the effector-binding subunit. The
ribonucleotide reductase activity in the cell-free extracts prepared from the MQ-580 cells was only slightly elevated (30%). However, passage of the cell-free extract from the MQ-580 cells over
Sephadex G-25 resulted in a 4.8-fold increase in specific activity over that of the wild-type cells. While the
reductase activity in the cell-free extract from the MQ-580 cells did not show altered sensitivity to
MAIQ, the
reductase activity in the cell-free extract from the MQ-580 cells was much more sensitive to the effects of the
iron-chelating agents Desferal and
EDTA. The cell pellets from the MQ-580 cells were much darker in color than the pellets from the wild-type cells or
hydroxyurea-resistant cells. The supernatant fraction from the MQ-580 cells after-SDS-PAGE showed the appearance of a strong
Coomassie blue-staining band at 50 kDA that was not apparent in either the wild-type or
hydroxyurea-resistant cells. This new resistant cell line offers an opportunity to explore differences in resistance mechanisms of drugs (e.g.
MAIQ and
hydroxyurea) that are directed at the same subunit of
ribonucleotide reductase.