Preneoplastic potential of morphologically distinct transformed foci induced by 3-methylcholanthrene.

Individual variability of scoring foci positive for transformation presents a difficult problem in assessing the transformation assay. In this study, an attempt was made to identify five morphologically distinct types of transformed foci based on size (2-3, 3-4, and > or = 4 mm in diameter), invasiveness (smooth vs. invading margins), and other properties (piling vs. spread) induced by 3-methylcholanthrene in Balb/c-3T3 cells. The transformed focal cells were used in in vitro studies including anchorage-independent analysis, focal reconstruction, gene transfection using NIH-3T3 host cells, and Southern blotting to assess amplification of five proto-oncogenes (K-ras, H-ras, c-fos, c-jun, c-myc) and a tumor suppressor (p53) gene. Results showed that 1) there was a significant increase in anchorage-independent growth of all five types of foci ranging from 7-12%; 2) all five morphological types of transformed foci showed 8-15% focal reconstruction; 3) DNA from all five types of transformed foci induced transformation in NIH-3T3 cells at a level significantly above the control DNA; 4) gene amplification studies indicated amplification in both K-ras and H-ras proto-oncogenes; however, c-fos, c-jun, and c-myc did not show DNA amplification. The tumor suppressor gene (p53) was activated and the increase was up to 3-fold over the normal Balb/c-3T3 DNA. These findings are consistent with our hypothesis that all five morphologically different foci have preneoplastic potential and that any foci of size > or = 2 mm regardless of invasiveness and piling should be scored as positive during the transformation assay.