Differential effects on cell killing in metallothionein overexpressing CHO mutant cell lines

An increased level of metallothionein (MT) within the cytosol has been associated with the development of resistance to the cytotoxic effects of alkylating agents, but the mechanism of this effect is unclear. To verify the possible role of MT as a direct scavenger of alkylating agents we transfected two alkylating agent sensitive CHO mutant cell lines, methyl methanesulphate-1 and -2 (MMS-1 and MMS-2), with the human metallothionein II-A (hMTII-A) gene. From each mutant two transfected cell clones were isolated which were found to bear episomally 50 to 70 copies of the plasmid pMTII-BPV coding for the hMTII-A gene. The transfected cell lines contained between 1.4 and 4.9 micrograms of MT per 10(7) cells. This could be increased 2- to 9-fold by heavy metal induction. Transfectants derived from mutant MMS-1 were equally sensitive as the parental cells to a set of alkylating agents. However, transfectants derived from mutant cell line MMS-2 gained significant resistance to both N-methyl-N'-nitro-N-nitrosoguanidine and N-nitroso-N-methylurea, whilst retaining sensitivity to MMS. The degree of resistance was not increased by MT induction after heavy metal ion exposure. These results indicate that although resistance to alkylating agents may be mediated by a process which involves MT expression, this is not due to simple scavenging of the cytotoxic agent by the MT protein.