c-erbB was introduced into normal human fibroblasts, MRC-5, which expressed normal levels of EGF receptor and in a SV40-transformed cell line, MRC-5V1, derived from them, which expressed markedly reduced levels of EGF receptor mRNA. MRC-5 overexpressing c-erbB, responded mitogenically to EGF. However, addition of high EGF concentrations markedly reduced DNA synthesis and resulted in the inhibition of cellular growth. In contrast, MRC-5V1 exhibited an increase in DNA synthesis in an EGF-dependent manner which was enhanced by overexpression of c-erbB. These cells, unlike MRC-5, also produced TGF alpha, an EGF receptor ligand which is often associated with cellular transformation. Ligand-activation of EGF receptor did not alter the lifespan, induce focus formation or anchorage-independence of MRC-5 and all the cell types remained non-tumourigenic in nude mice. However, c-erbB induced the expression of tPA, c-jun and junB in both MRC-5 and MRC-5V1. The data suggest that overexpression and activation of c-erbB is unlikely to play a role in immortalisation of human diploid fibroblasts but it may contribute to cellular transformation.

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