In vitro neoplastic progression of rat tracheal epithelial cells transformed by diverse carcinogens

Preneoplastic transformants were isolated from primary rat tracheal epithelial cells after treatment with (a) mutagenic concentrations of the alkylating agent N-methyl-N-nitro-N'-nitrosoguanidine, (b) nonmutagenic concentrations of the DNA hypomethylating agent 5-azacytidine, or (c) after arising spontaneously. We have addressed the question of whether preneoplastic transformants induced by different carcinogens differ in their ability to progress to the immortal stage and to become neoplastic. Spontaneous transformants occurred with a very low frequency, and 5-azacytidine induced preneoplastic transformants half as efficiently as N-methyl-N-nitro-N'-nitrosoguanidine. However, no phenotypic differences could be detected between the 70 preneoplastic colonies isolated from the 3 groups; colony size, cell density, and clonogenicity were not statistically different. Clones from all 3 groups became immortal and further progressed to become neoplastic with similar frequencies. The level of expression of the oncogenes H-ras, K-ras, and raf was also similar in all 3 groups. These experiments indicated that there was no difference in the ability of spontaneous transformants or those induced by N-methyl-N-nitro-N'-nitrosoguanidine or 5-azacytidine to progress to become immortal or neoplastic. This suggests that whereas the nature of the carcinogen influenced the frequency of the initial transforming event, progression to the neoplastic stage was independent of the nature of the transforming insult.