Mode of action of methylating carcinogens: comparative studies of murine and human cells

Murine and human cells (mainly lymphocytes) were ethylated in vitro with either N-[14C]methyl-N-nitrosourea (MNU) or di[14C]methyl sulphate (DMS) and the extents of methylation of DNA at O-6 and N-7 of guanine and N-3 of adenine were determined. The cytotoxic action of MNU was also compared with that of DMS, as assessed by their effects on cell division following stimulation of these lymphocytes in culture by concanavalin A (Con A). The overall extent of methylation of the DNA of human cells was about 70% of that of murine cells after exposure to MNU and DMS. Mouse cells, responding to Con A in culture, were found to be much more sensitive to both agents than could be accounted for by these differences in overall extent of methylation of the DNA. Significant differences were found between cells in their ability to rapidly remove O6-methylguanine from DNA. Normal human lymphocytes were always proficient, but some human lymphoid lines were deficient in this respect, while all the murine cells tested were deficient, as found in vivo for mouse lymphoid tissues. No correlation has yet been found between the susceptibility of various mouse strains to the carcinogenic action of MNU and their ability to remove methylated bases from DNA. The cytotoxicity studies, showing that normal human lymphocytes were relatively more resistant than murine cells to methylation by both MNU and DMS, suggested that the ability to remove O6-methylguanine (which is produced in a very low proportion by DMS) was not of prime importance in conferring resistance in this type of assay.