Effects of some inhibitors on myogenic differentiation of avian myoblasts transformed with Rous sarcoma virus

The differentiation of quail myoblasts transformed with a temperature-sensitive mutant of Rous sarcoma virus (QM-RSV cells) depends on culture temperature. At 35.5 degrees C, the permissive temperature for RSV, QM-RSV cells repeatedly proliferate without differentiation. However, myogenic differentiation proceeds at 41 degrees C, the nonpermissive temperature for RSV. To obtain useful inhibitors for the analysis of myogenic differentiation in QM-RSV cells, various drugs of myoblast fusion reported previously were examined in the QM-RSV system. Thirteen of twenty-seven drugs tested blocked myoblast fusion of QM-RSV cells. Among inhibitors that blocked the fusion of QM-RSV cells, four of them, acetylsalicylic acid (aspirin), 12-o-tetradecanoyl phorbol-13-acetate (TPA), doxorubicin, and N,N'-hexamethylenebisacetamide (HMBA), were chosen for further analysis. Two parameters of myogenic differentiation, myotube formation by myoblast fusion and creatine kinase activity, were examined. Aspirin, doxorubicin, and HMBA, inhibited myoblast fusion by acting on both steps before and after commitment for myoblast fusion. TPA affected the step before commitment for fusion. The effects of these inhibitors on creatine kinase activity were not always in parallel with myoblast fusion, suggesting that the process of myoblast fusion and the expression of creatine kinase activity are separate phenomena. Aspirin and doxorubicin did not affect creatine kinase activity. On the contrary, doxorubicin accelerated creatine kinase activity about twofold when the cells were treated with this drug after commitment in spite of strong inhibition of myoblast fusion. The expression of Mb-N3 and H145 antigens, which are closely related to the differentiation of QM-RSV cells, was affected variously by these inhibitors and sodium orthovanadate, an inhibitor of protein-phosphotyrosine phosphatase, suggesting that these inhibitors act on different steps during differentiation.