Response of MTX-resistant V79 cells to some DNA-damaging agents.

Three methotrexate (MTX)-resistant clones selected at 200 nM MTX have been isolated from UV-irradiated V79 cells and were marked M1, M2 and M3. Two further clones were also isolated by gradually exposing M1 to higher concentrations of MTX and marked M4 and M5, isolated at 1200 nM and 2400 nM MTX respectively. The frequency of the induction of MTX-resistant clones by UV light was found to increase significantly on treatment of the irradiated cells with either 3-aminobenzamide or caffeine. The resistant clones had the same sensitivity as the parental cells to UV light, but were resistant to gamma-rays, MNNG and H2O2. In MTX-resistant cells low-dose H2O2-induced repair function was absent. UV-induced mutation at the HGPRT locus was the same in MTX-resistant as sensitive cells, whereas for MNNG-induced mutation it was higher in resistant cells. The rate of SCE induction by UV light was higher in MTX-resistant cells compared to the parental ones, but was not significantly different for induction by MNNG and H2O2.