Protein kinase C activity and phorbol ester binding to rat myogenic cells during growth and differentiation

Phorbol esters have been reported to induce opposite responses in fetal myoblasts and in satellite cells isolated from adult skeletal muscles. We examined the possibility that different levels of protein kinase C (PKC) activity and different phorbol ester binding characteristics account for these responses. For this purpose, the subcellular distributions of PKC were compared in primary cultures of myogenic cells from fetal and adult rat muscles and in the L6 cell line. Cells were used at the proliferative stage or after differentiation into myotubes. Binding of phorbol dibutyrate (PDBu) was assayed. In all three cell types, the levels of PKC specific activity were comparable at the proliferating and the differentiated stages, and partial translocation of PKC activity from the membrane to the cytosolic compartment was observed after differentiation into myotubes. PDBu binding, which had a Kd of 6 to 13 nM in proliferative cells, rose to between 30 and 52 nM in myotubes. Simultaneously, a small increase was observed in the total number of PDBu binding sites. These results suggest that the role of PKC might change with the stage of differentiation. They also imply that the difference described by others between the sensitivity to phorbol esters of fetal myoblasts and satellite cells is not connected with the phorbol ester receptor (i.e., PKC), but might be caused by events subsequent to PKC activation.