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Method of identifying inhibitors of oncogenic transformation: selective inhibition of cell growth in serum-free medium.

We developed a new method for evaluating inhibitors of oncogenic signal transduction pathways based on different growth abilities between normal and transformed cells in a defined serum-free medium. The growth rates of src, abl or ras oncogene-transformed cells, activated raf proto-oncogene transformed cells, and normal NIH-3T3 cells were 60-90%, 20-30% and 10% in a serum-free medium, respectively, compared to the growth rates in a serum-containing medium. An addition of a growth factor (PDGF, FGF or TGF-beta) stimulated the growth of normal NIH3T3 cells by 40-80% in a serum-free medium. Herbimycin A, a specific cytoplasmic protein tyrosine kinase inhibitor, selectively inhibited the growth of src or abl transformed cells in the serum-free medium resulting in about 10-fold or fivefold lower IC50 than those in the serum-containing medium. The antibiotic did not show such an effect on ras transformed cells, and the treatment of src transformed cells with other protein kinase inhibitors or cytotoxic drugs showed little IC50 shifts between the two media. Thus, this method of comparing growth inhibition in the serum-free and the serum-containing media may be useful in evaluating specific inhibitors of signaling pathways mediated by growth factors and certain oncogene products.