Polyoma middle T abrogates TPA requirement of murine melanocytes and induces malignant melanoma.

We have transfected the polyoma middle T oncogene into an immortal murine melanocyte cell line, Mel-ab. This highly pigmented line is phorbol ester (TPA) dependent for in vitro growth, suggesting activation and/or down regulation of Protein Kinase C (PKC) is essential for mitogenesis. Moreover, cells of this line do not form tumours when injected subcutaneously into immunocompetent or immunoincompetent mice. Here we show that PyMT alone is sufficient to produce TPA-independence and transformation to the tumourigenic state in transfected Mel-ab cells. Western blot analysis shows that middle T overcomes the TPA requirement by a mechanism independent of PKC down regulation though this does appear to occur when Mel-ab cells are grown continuously in TPA. These results suggest that PyMT is not exerting its transforming effect by PKC down regulation, but conceivably at some later stage of second messenger signalling, possibly through PyMT-c-src protein kinase activity.