Relationship between chemically induced ha‐ras mutation and transformation of BALB/c 3T3 cells: Evidence for chemical‐specific activation and cell type—specific recruitment of oncogene in transformation

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Abstract
BALB/c 3T3 cells were exposed to 7, 12‐dimethylbenz[a]anthracene (DMBA) and resultant transformed foci were analyzed for the presence of A182 → T mutation at codon 61 of Ha‐ras (a mutation found in many DMBA‐induced animal tumors). None of the 30 independently cloned transformed cell lines contained such a mutation. In order to see whether DMBA is able to induce this mutation in BALB/c 3T3 cells, we developed a method sensitive enough to detect this specific mutation at the frequency of 10–6. Employing this assay, we found time‐and dose‐dependent induction by DMBA of Ha‐ras A182 → T mutation in BALB/c 3Tc cells; for example, 2 wk after exposure to 100 μg/mL DMBA, 1.4 in 1 × 104 cells contained this specific mutation. On the other hand, other agents that also induce BALB/c 3T3 cell transformation, such as 3‐methylcholanthrene (MCA), 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA), N‐methyl‐N'‐nitro‐N‐nitrosoguanidine (MNNG), or ultraviolet light, did not induce the mutation at detectable frequency (< 10–6). These results suggest that DMBA efficiently induces Ha‐ras mutation in BALB/c 3T3 cells but that this mutation is not recruited in the process of cell transformation. A hypothesis of carcinogen‐specific mutation of Ha‐ras gene and its tissue (cell type)‐specific recruitment in carcinogenesis is proposed.