Recovery from exposure to DNA-damaging chemicals in radiation-resistant insect cells

Radioresistant TN-368 lepidopteran insect cells were examined with respect to their sensitivity to the chemical agents methyl methanesulfonate (MMS), N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), propane sultone (PS), mitomycin C (MMC), and 4-nitroquinoline 1-oxide (4NQO). Based on survival ability, the TN-368 cells were more resistant than most mammalian cells to each of these agents. Concentrations of these agents which reduce survival to about 10% were used to assess recovery ability assayed by colony formation in liquid-holding and split-dose experiments. Liquid-holding experiments were performed by exposing cells in the plateau phase of growth for 1 h to 8 mM MMS, 50 microM MNNG, 9 mM PS, 110 microM MMC, or 175 microM 4NQO, removing the drug and incubating cells in spent medium for 6 h, and plating for colony formation. Split-dose experiments were performed by exposing exponentially growing cells to the above drug concentrations for 1 h, incubating in fresh medium for 6 h, exposing the cells to the agent for an additional hour, and plating. The TN-368 cells were able to significantly recover from MMS, MNNG and PS in both types of experiment. Recovery from 4NQO was observed in liquid-holding experiments and not assessed in split-dose experiments. In all cases where recovery was observed, survival enhancement was approximately 2-fold. Recovery from MMC (a cross-linking agent) exposure was not observed in either type of experiment. In addition, recovery from 8-methoxypsoralen plus UVA light (PUVA), another cross-linking treatment, was not observed. These studies indicate that DNA-DNA and/or DNA-protein crosslinking may be important molecular lesions causing death in the lepidopteran cells and that these cells may have some difficulty in repairing such damage.