Differential effects of phorbol ester on tumor cells induced by avian sarcoma virus

Avian sarcoma virus-induced tumors usually grow progressively for several weeks and then regress. We have injected phorbol myristate acetate (PMA) directly into tumors in an effort to stimulate neoplastic growth. The results show instead that PMA exerted an inhibitory effect in this regard and, in fact, caused an acceleration of tumor regression. At the same time, treatment of cultured avian sarcoma cells with PMA resulted in greatly diminished levels of the kinase activity associated with the src gene product, pp60src. PMA-treated tumor cells from regressing sarcomas were, however, stimulated to express viral antigens at their surface and produced more progeny virus than did untreated tumor cells.