Role of intracellular proteases in differentiation of L6 myoblast cells.

The activities of cathepsins B, L, and H increased by 3.0, 2.3, and 1.9 fold in differentiation of L6 myoblasts. Separating from the intrinsic inhibitor, both mu- and m-calpain activities also increased by 4.9 and 1.7 fold during differentiation. Calpeptin (10(-5)M), cell penetrating calpain inhibitor, blocked fusion of myoblast by 55% and increment of creatine phosphokinase (CPK) activity by 20.3% with inhibiting the activities of cathepsin L and mu-calpain. E-64(10(-4)M) did not block fusion of myoblasts but increment of CPK activity by 25.0% with inhibiting the activities of cathepsin L and B. Calpain may be involved in the process of myoblast fusion.