Stimulatory role of transforming growth factors in multistage skin carcinogenesis: possible explanation for the tumor-inducing effect of wounding in initiated NMRI mouse skin.

Mechanical wounding provides a convertogenic ("stage I tumor-promoting") stimulus in initiated NMRI mouse skin, indicating that this stage of carcinogenesis can be entirely controlled by endogenous factors. A search for such factors led to the finding that both platelet-derived Epstein-Barr-virus-inducing factor (EIF), alias human TGF beta 1 and porcine TGF beta, exhibited--upon intracutaneous injection--convertogenic efficacy in initiated NMRI-mouse skin in vivo provided that their injection was combined with a single topical application of the non-convertogenic tumor promoter 12-O-retinolyphorbol-13-acetate (RPA). Since TGF beta inhibits epidermal cell proliferation, the RPA treatment is thought to provide a mitogenic stimulus required for conversion. The RPA treatment can be replaced by intracutaneous injection of transforming growth factor alpha (TGF alpha). These results indicate that the stage of conversion consists of two components, one of which is related to mitogenesis (RPA or TGF alpha), the other to a still unknown activity exhibited by TGF beta-like factors. Thus, endogenous factors with the quality of "wound hormones" may be involved in multistage skin carcinogenesis. This finding could explain the convertogenic effect of skin wounding.