Differential influence of adjacent normal cells on the proliferation of mammalian cells transformed by the viral oncogenes myc, ras and src.

We investigated the role of adjacent normal cells in the modulation of focal outgrowth of mammalian fibroblasts transformed by different viral oncogenes (myc, src and ras). NIH3T3 cells transformed by these three oncogenes were derived by transfection or infection and showed comparable cloning efficiencies in semi-solid medium. However, upon replating in liquid medium a small number of transformed cells together with a vast excess of normal mouse embryo fibroblasts C3H10T1/2, ras- and src-transformed cells were able to overgrow the monolayer and formed distinct foci, whereas myc-transformed cells lacked this ability. Conditioned medium from normal cells did not affect the proliferation of myc-transformed cells at clonal density. Addition of phorbol ester tumour promoters, either at the time of plating or as late as after one week, efficiently rescued focus formation by myc-transformed cells. In contrast, when myc-transformed cells were cultivated alone, their clone size and cloning efficiency were slightly reduced by the addition of tumour promoters. These results indicate that cell-cell contacts between transformed cells and adjacent normal cells specifically inhibit the growth of myc- but not of ras- or src-transformed cells. The ability of tumour promoters and phospholipase-C to rescue the focus forming ability of myc-transformed cells is consistent with the possibility that activation of protein-kinase C is involved in the clonal expansion of 'suppressed' myc-bearing cells.