Activated Ha-ras oncogenes were introduced into early passage normal human adult dermal fibroblasts, 149BR and established mouse Swiss 3T3 fibroblasts by retroviral-mediated gene transfer or by DNA transfection, respectively. The presence and expression of Ha-ras oncogenes was followed by Southern and Northern blotting and immunoprecipitation. Overexpression of the human mutant c-Ha-ras1 T24 oncogene in mouse cells induced morphological transformation, focus formation and growth in soft agar with low frequency. Tumourigenicity studies showed that the malignant transformation of these cells by p21Ha-ras T24 oncoprotein was concentration-dependent and it required additional events suggesting that in vitro establishment is not a sufficient prerequisite for tumourigenic conversion by activated Ha-ras oncogenes. In contrast, constitutive expression of the viral Ha-ras oncogene in human diploid fibroblasts failed to immortalise or transform them. All ras-expressing human cells remained flat, anchorage-dependent and non-tumourigenic suggesting that these cells are resistant to transformation by activated oncogenes. The role of Ha-ras oncogenes in the transformation of mammalian cells in discussed.

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