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

Abstract

Addition of the synthetic glucocorticoid, dexamethasone (Dex) to serum-deprived C(2)C(12) myotubes elicited time- and concentration-dependent changes in N(tau)-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 E(2) Ub-conjugating enzyme and Ub) of the ATP-Ub-dependent pathway. The data demonstrate that Dex stimulates preferential hydrolysis of myofibrillar proteins in C(2)C(12) myotubes and suggests that the ATP-Ub-dependent pathway is involved in this response.

AMPK Activation Stimulates Myofibrillar Protein Degradation and Expression of Atrophy-Related Ubiquitin Ligases by Increasing FOXO Transcription Factors in C2C12 Myotubes

“…In muscle cell cultures, the amount of N -methylhistidine released into the medium, as an index of myofibrillar protein degradation, was increased by serum deprivation 36) and glucocorticoid treatment. 37) In this study, AICAR induced the release of N -methylhistidine into the medium from C2C12 myotubes (Fig. 2B).…”

Glucocorticoids differentially regulate degradation of MyoD and Id1 by N-terminal ubiquitination to promote muscle protein catabolism

“…The catabolic effects of glucocorticoid on overall muscle protein metabolism are well known. For example, glucocorticoid increased overall protein degradation in rat L8 and L6 myotubes and enhanced the degradation of myofibrillar proteins in C2C12 myotubes (1)(2)(3). In response to fasting, muscles of adrenalectomized animals, unlike normals, are unable to increase protein breakdown, whereas complete restoration occurs after glucocorticoid administration.…”

“…In response to fasting, muscles of adrenalectomized animals, unlike normals, are unable to increase protein breakdown, whereas complete restoration occurs after glucocorticoid administration. mRNA levels for several components of the ubiquitin system are up-regulated by glucocorticoids (2,(4)(5)(6). For example, muscle-specific E3 ligases Atrogin-1/MAFbx and Murf1 were both up-regulated upon glucocorticoid administration to skeletal muscle (4).…”

Activation of AMPK inhibits cardiomyocyte hypertrophy by modulating of the FOXO1/MuRF1 signaling pathway in vitro

“…Previous studies have demonstrated that 3-MH cannot be reutilized for protein synthesis, and therefore, the output of 3-MH has been used as an index of myofibrillar protein degradation [21,27,28] . The concentration of 3-MH was measured as previously described [27,28] .…”