Initiation of C3H/10T1/2 cell transformation by N-methyl-N''-nitro-N-nitrosoguanidine and aflatoxin B1

The utility of C3H/10T1/2 mouse embryo fibroblasts for the detectionof carcinogenic substances has been limited by their apparentinsensitivity to the oncogenic effects of direct-acting alkylatingagents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) andprocarcinogens such as aflatoxin B₁ (AfB₁). Because the processof C3H/10T1/2 transformation can be observed to proceed throughdiscrete stages of initiation and promotion, we have consideredthe possibility that MNNG and AfB₁ may only initiate C3H/10T1/2transformation. Treatment of asynchronous C3H/10T1/2 cells withMNNG or AfB₁ alone generally produced few transformed foci.If MNNG or AfB₁ treatment was followed by the exposure of cellsto the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate(TPA), numerous transformed foci were produced. Phorbol didnot enhance transformation by either substance. MNNG and AfB₁thus appear to be initiating agents for transformation. TPAalso enhanced the transformation of C3H/10T1/2 cells by lowdoses of 3-methylcholanthrene (3-MCA), but transformation byhigh concentrations of 3-MCA was inhibited by the presence ofTPA. These studies suggest that the sensitivity of the C3H/10T1/2transformation system to potential carcinogens can be dramaticallyheightened if the bioassay is conducted in the presence andabsence of TPA.