Evidence against a role of general protein kinase C downregulation in skintumor promotion

Using isoenzyme-specific antibodies, we have performed an immunoblot analys is of the PKC isoenzyme pattern during the course of TPA-induced tumor prom otion in the epidermis of NMRI mice. The TPA-sensitive PKC isoforms alpha, delta, epsilon, eta, mu (and TPA-insensitive PKC zeta), but not PKC beta an d gamma, were found to be expressed in both normal and neoplastic epidermis , The immune signals of all TPA-sensitive PKC isoforms were moderately and reversibly attenuated upon a single TPA treatment. Using different antibodi es against PKC eta and PKC mu, this apparent downregulation could mainly be attributed to epitope changes of these enzymes, whereas for the other PKC species no such conclusion could be drawn. Except for PKC epsilon, no subst antial long-term attenuation of the immune signals of the other PKC isoform s occurred upon chronic phorbol ester treatment (i.e., 14 applications of 5 nmol TPA each over 7 weeks), which led to tumor development in initiated m ouse skin. Specific PKC activity (related to tissue weight) was 40-50% lowe r in TPA-treated as compared with control epidermis whereby no clearcut dif ference was found between single and chronic TPA treatment. Benign and mali gnant skin tumors generated according to the initiation-promotion protocol did not exhibit consistent alterations in the immune pattern of the PKC iso enzymes with the exception of a decrease of PKC epsilon and an increase of PKC mu signal in carcinomas. Our data indicate that, in contrast with earli er assumptions, no general long-lasting PKC downregulation plays a critical role in skin tumor promotion. Int. J. Cancer 85:503-507, 2000, (C) 2000 Wi ley-Liss, Inc.