EVIDENCE FOR THE PARTICIPATION OF THE PROTEASOME AND CALPAIN IN EARLYPHASES OF MUSCLE-CELL DIFFERENTIATION

Objectives were to investigate the role of the proteasome and nz-calpa in to muscle cell differentiation. Accordingly, we investigated the ef fects of lactacystin, a proteasome inhibitor, and calpain inhibitor-II (CI-II) on L8 muscle cell differentiation and assessed concentrations of proteasomal and calpain subunit mRNAs during differentiation. L8 m yoblasts were induced to differentiate by culturing in mitogen-deplete d medium. To assess the importance of the proteasome and calpain to di fferentiation, we examined effects of lactacystin and CI-II on creatin e kinase (CK) activity. In the absence of inhibitor, CK activity was d etectable within 48 h of mitogen depletion and myotubes were formed. A ddition of lactacystin or CI-II to cultures drastically reduced CK act ivity and prevented formation of myotubes. Hence, proteasome and calpa in are both necessary for differentiation. In order to identify which proteasomal subunits were regulated during differentiation, we examine d the concentrations of two 20S core subunits (C8 and C9) and three 22 S ATPases (MSS1, S4 and TBP1) during differentiation. Concentrations o f m-calpain and beta-tubulin mRNAs were also assessed. Differentiation was associated with slight increases (ca. 30%) in concentrations of m RNAs encoding the proteasomal 20S core subunits (C8 and C9) and with l arge increases (approximately 2-fold) in mRNAs encoding the regulatory subunit ATPases, nz-calpain mRNA concentration also increased two-fol d following mitogen depletion. beta-tubulin mRNA concentration remaine d unchanged early in the differentiation process and thereafter declin ed. Of interest, changes in proteasomal and m-calpain mRNAs occurred w ithin 6-24 h of mitogen depletion (i.e., at least 24-36 h prior to det ectable changes in creatine kinase activity). These results indicate t hat changes in expression of proteasome and calpains subunits occur ea rly in the differentiation process. These changes may be required for the normal course of differentiation to proceed. Differentiation is as sociated with larger changes in proteasomal ATPase mRNAs than in 20S c ore particle mRNAs indicating that either turnover rates of the 22S AT Pase subunits are more rapid in differentiating cells than of the 20S core particles or that functions of the regulatory subunits become mor e important during muscle cell differentiation. (C) 1998 Elsevier Scie nce Ltd. All rights reserved.