Stimulation of myofibrillar protein degradation and expression of mRNA encoding the ubiquitin‐proteasome system in C2C12 myotubes by dexamethasone: effect of the proteasome inhibitor MG‐132

Addition of the synthetic glucocorticoid, dexamethasone (Dex) to serum-deprived C2C12 myotubes elicited time- and concentration-dependent changes in Nτ-methylhistidine (3-MH), a marker of myofibrillar protein degradation. Within 24 h, 100 nM Dex significantly decreased the cell content of 3-MH and increased release into the medium. Both of these responses had increased in magnitude by 48 h and then declined toward basal values by 72 h. The increase in the release of 3-MH closely paralleled its loss from the cell protein. Furthermore, Dex also decreased the 3-MH:total cell protein ratio, suggesting that myofibrillar proteins were being preferentially degraded. Incubation of myotubes with the peptide aldehyde, MG-132, an inhibitor of proteolysis by the (ATP)-ubiquitin (Ub)-dependent proteasome, prevented both the basal release of 3-MH (>95%) and the increased release of 3-MH into the medium in response to Dex (>95%). Northern hybridization studies demonstrated that Dex also elicited similar time- and concentration-dependent increases in the expression of mRNA encoding two components (14 kDa E2 Ub-conjugating enzyme and Ub) of the ATP-Ub-dependent pathway. The data demonstrate that Dex stimulates preferential hydrolysis of myofibrillar proteins in C2C12 myotubes and suggests that the ATP-Ub-dependent pathway is involved in this response. J. Cell. Physiol. 181:455–461, 1999. © 1999 Wiley-Liss, Inc.