Effect of orthovanadate on commitment of avian myoblasts transformed with Rous sarcoma virus to myogenic...

Myogenic differentiation of quail myoblasts transformed with a temperature-sensitive mutant of Rous sarcoma virus (QM-RSV cells) depends on the temperature: at 35.5 degrees C, the permissive temperature for the virus, the transformed myoblasts proliferate, without fusion, but at 41 degrees C, the nonpermissive temperature, they become committed to myogenic differentiation until about 10 h and then myoblast fusion occurs within 24 h. This temperature dependency of the differentiation reaction is derived from protein kinase activity of src gene product. Thus, at 41 degrees C, the differentiation proceeds with dephosphorylation of the proteins. For further clarification of relationship between the protein phosphorylation and the control of differentiation, the events during differentiation were examined using inhibitors of tyrosine kinase and phosphatase, respectively. To examine the role of phosphotyrosyl protein phosphatases in skeletal muscle differentiation, these cells were treated with sodium orthovanadate, a potent inhibitor of the enzyme. The treatment with the drug inhibited myoblast fusion and creatine kinase activity of the cells at 41 degrees C, with inhibition of tyrosine dephosphorylation. Moreover, the treatment of the cells with vanadate for 12 h at 41 degrees C, followed by the removal of the drug resulted in myoblast fusion after the lag time about 12 h. On the other hand, herbimycin A was needed to acquire the fusion commitment at 35.5 degrees C. These results are suggestive evidence to reflect that tyrosine dephosphorylation is a key step in commitment of QM-RSV cells to myogenic differentiation. Examination of the tyrosine phosphorylated proteins indicated that vanadate mainly inhibited the dephosphorylations of 70-, 58-, and 38-kDa proteins, suggesting that these proteins may be closely associated with the steps involved in commitment to myogenic differentiation.