Effects of phorbol 12-myristate 13-acetate on the differentiation program of embryonic chick skeletal myoblasts

The effects of phorbol 12-myristate 13-acetate (PMA) on three aspects of myogenesis have been analyzed: (a) fusion of mononucleated myogenic cells to form myotubes; (b) synthesis and accumulation of two muscle-specific proteins; and (c) DNA synthesis. Using autoradiography combined with immunofluorescent localization of muscle-specific light meromyosin and the muscle-specific intermediate filament protein desmin, we have found that embryonic chick myogenic cells cultured in the presence of PMA (50 nM) initiate the synthesis of both desmin and muscle-specific light meromyosin and, by these criteria, partially differentiate. These cells differ from normal definitive postmitotic myoblasts, however, since they (a) do not fuse; (b) do not assemble normal myofibrils; and (c) incorporate [3H]thymidine. PMA does not appear to induce DNA synthesis in postmitotic myoblasts, but it apparently permits cells to initiate expression of muscle-specific proteins while preventing complete withdrawal from the cell cycle. Inhibition of fusion by PMA has been reported, but continued incorporation of [3H]thymidine in nuclei of cells expressing muscle-specific proteins is a previously undescribed effect of PMA. This effect is not achieved by 4-alpha-phorbol-12, 13-didecanoate, a nonpromoting phorbol ester, and may be relevant to the action of PMA as a tumor promoter.