The objectives were to investigate the roles of different calpains and protein kinase C (PKC) isoforms in muscle differentiation. Concentrations of - and m-calpain increased significantly whereas PKC and declined significantly during L8 myoblast differentiation. Both -calpain and m-calpain antisense oligonucleotides inhibited myotube formation and creatine kinase activity during L8 myoblast differentiation. These results implied that both - and m-calpain were involved in L8 myoblast differentiation. To investigate the involvement of calpain in regulation of PKC concentrations, -calpain antisense oligonucleotides were added to L8 myoblasts. PKC remained unchanged and PKC declined. By adding m-calpain antisense oligonucleotides instead, PKC level remained unchanged and PKC concentrations increased significantly during differentiation. These results suggest that PKC , but not PKC , is the substrate for -calpain and PKC and are the substrates for the m-calpain. In addition, more phosphorylated myogenin was found in day 2 antisense oligonucleotides treated L8 cells. It is concluded that the decline of PKC mediated by m- and -calpain is essential for L8 myoblast differentiation. The decline of PKC during myoblast differentiation may cause
hypo-phosphorylation of myogenin, which in turn activates muscle-specific genes during myogenesis.
The International Journal of Biochemistry & Cell Biology 38(4):662-70