CHANGES OF PROTEASOMES AND CATHEPSINS ACTIVITIES AND THEIR EXPRESSIONDURING DIFFERENTIATION OF C2C12 MYOBLASTS

C2C12 myoblasts fuse to form multinucleated myotubes and express muscl e specific proteins during differentiation. To elucidate developmental regulation of intracellular proteolytic systems, enzymatic activities , protein and mRNA levels of proteasomes (20S and 26S) and lysosomal c athepsins (B, L, and H) were examined. Myoblasts were differentiated f ully to myotubes 6 days after starting differentiation. In this develo pmental process, the 26S proteasome activity decreased, while the 20S proteasome activity increased, Expression of proteasome subunits of 20 S (RC2, RC8) and regulatory components of 26S (S4, S7) was down-regula ted, though total protein levels of proteasomes showed no remarkable c hanges. On the other hand, enzymatic activities of cathepsins B and BL increased in association with an increase of their transcriptional a nd translational levels. Expression of their specific endogenous inhib itor, cystatin beta, also increased. Maturation of the lysosomal prote olytic system was tightly linked to the differentiation process, These results suggested that signals for differentiation of myoblasts media te a change of intracellular proteolytic systems, involving up-regulat ion of lysosomal cathepsins and down-regulation of proteasomes.