Excessive glucocorticoid-induced muscle MuRF1 overexpression is independent of Akt/FoXO1 pathway

Abstract

The ubiquitin-proteasome system (UPS)-dependent proteolysis plays a major role in the muscle catabolic action of glucocorticoids (GCs). Atrogin-1 and muscle-specific RING finger protein 1 (MuRF1), two E3 ubiquitin ligases, are uniquely expressed in muscle. It has been previously demonstrated that GC treatment induced MuRF1 and atrogin-1 overexpression. However, it is yet unclear whether the higher pharmacological dose of GCs induced muscle protein catabolism through MuRF1 and atrogin-1. In the present study, the role of atrogin-1 and MuRF1 in C2C12 cells protein metabolism during excessive dexamethasone (DEX) was studied. The involvement of Akt/forkhead box O1 (FoXO1) signaling pathway and the cross-talk between anabolic regulator mammalian target of rapamycin (mTOR) and catabolic regulator FoXO1 were investigated. High concentration of DEX increased MuRF1 protein level in a time-dependent fashion (P<0.05), while had no detectable effect on atrogin-1 protein (P>0.05). FoXO1/3a (Thr24/32) phosphorylation was enhanced (P<0.05), mTOR phosphorylation was suppressed (P<0.05), while Akt protein expression was not affected (P>0.05) by DEX. RU486 treatment inhibited the DEX-induced increase of FoXO1/3a phosphorylation (P<0.05) and MuRF1 protein; LY294002 (LY) did not restore the stimulative effect of DEX on the FoXO1/3a phosphorylation (P>0.05), but inhibited the activation of MuRF1 protein induced by DEX (P<0.05); rapamycin (RAPA) inhibited the stimulative effect of DEX on the FoXO1/3a phosphorylation and MuRF1 protein (P<0.05).

Detecting the Effects of the Glucocorticoid Dexamethasone on Primary Human Skeletal Muscle Cells—Differences to the Murine Cell Line

“…However MuRF-1 was also expressed and detected in the control groups at each incubation time. Similar effects were also determined in C2C12 myotubes [49].…”

“…A possible explanation could be that Akt and mTOR have not been phosphorylated, which led to their inactivation and to the activation of Foxo at higher dex concentrations [49]. Similar results were observed in the murine C2C12 myotubes, indicating that dex had a comparable effect on human and murine myotubes [49]. However, a concentration of 100 µM dex resulted in the highest Foxo expression in the human primary myotubes ( Figure 4C).…”

“…The transcription factor Foxo was significantly upregulated in myotubes after incubation with 10 and 100 µM dex after 48 and 72 h ( Figure 4C). A possible explanation could be that Akt and mTOR have not been phosphorylated, which led to their inactivation and to the activation of Foxo at higher dex concentrations [49]. Similar results were observed in the murine C2C12 myotubes, indicating that dex had a comparable effect on human and murine myotubes [49].…”

Polycystin-2 Is Required for Starvation- and Rapamycin-Induced Atrophy in Myotubes

“…Even if the status of phosphorylation of AKT was sensitive to starvation, it was unaffected by PC2 downregulation, indicating that AKT pathway does not participate in the regulation of atrophy by PC2. In this regard, other studies have also shown events of atrophy independent of AKT/FOXO1 pathway, especially in glucocorticoid-induced muscle atrophy (29).…”

“…Thus, given that PC2 was required for starvation- and rapamycin-induced atrophy and that autophagy was not implicated in the mechanism by which PC2 regulates atrophy, it is possible that calcium microdomains controlled by PC2 may be also required to induce atrophy in C2C12. In fact, agents such as angiotensin II, tumor necrosis factor-α (TNFα) and lipopolysaccharide not only induce muscle atrophy but also a rise in Ca 2+ which is necessary for both proteolysis and decreasing in protein synthesis in muscle cells (29, 32–34). This work did not evaluate the role of Ca 2+ .…”