ACTIVATION OF ERBB2 AND C-SRC IN PHORBOL ESTER-TREATED MOUSE EPIDERMIS - POSSIBLE ROLE IN MOUSE SKIN TUMOR PROMOTION

In recent work eve showed that the EGF receptor (EGFr) was activated i n tumor promoter treated mouse epidermis (Cell Growth & Differentiatio n, 6: 1447-1455, 1995). In the present study, we have investigated the possible role of other erbB family members in the process of tumor pr omotion. Both erbB2 and erbB3, but not erbB4, were expressed in cultur ed mouse keratinocytes and in mouse epidermis in vivo. In cultured mou se keratinocytes, EGF stimulated rapid tyrosine phosphorylation of erb B2 followed by a time-dependent degradation of erbB2 protein. Furtherm ore, an increase in erbB2:EGFr heterodimer formation was also induced by EGF. In contrast to the results with erbB2, EGF did not induce tyro sine phosphorylation, the degradation of erbB3, or erbB3:EGFr heterodi mer formation in cultured keratinocytes. Further analyses revealed tha t c-src kinase activity was dramatically elevated in cultured mouse ke ratinocytes exposed to EGF. In mouse epidermis following multiple trea tments with 12-O-tetradecanoylphorbol-13-acetate (TPA), the phosphotyr osine content of erbB2 was significantly elevated in a dose-dependent manner. Concomittantly, erbB2:EGFr heterodimer formation and c-src kin ase activity were also elevated in TPA-treated epidermis. Structure-ac tivity relationships with several phorbol ester analogs showed that th e elevated phosphorylation of erbB2 in mouse epidermis followed closel y with tumor promoting ability. Activation of erbB2 and c-src kinase w ere also observed in the epidermis of TGF alpha transgenic mice where expression of human TGF alpha was targeted to basal keratinocytes with the human K14 promoter. Collectively, the current data suggest that t he activation of erbB2 in phorbol ester treated skin can be explained solely by a mechanism involving elevation of EGFr ligands and activati on of the EGFr. In addition, activation of c-spe may be an important d ownstream effector in mouse keratinocytes both ill vivo and in vitro, following activation of the EGFr, erbB2, or both.