The E3 Ligase MuRF1 Degrades Myosin Heavy Chain Protein in Dexamethasone-Treated Skeletal Muscle

Abstract

Skeletal muscle atrophy occurs as a side effect of treatment with synthetic glucocorticoids such as dexamethasone (DEX) and is a hallmark of cachectic syndromes associated with increased cortisol levels. The E3 ubiquitin ligase MuRF1 (muscle RING finger protein 1) is transcriptionally upregulated by DEX treatment. Differentiated myotubes treated with DEX undergo depletion of myosin heavy chain protein (MYH), which physically associates with MuRF1. This loss of MYH can be blocked by inhibition of MuRF1 expression. When wild-type and MuRF1(-/-) mice are treated with DEX, the MuRF1(-/-) animals exhibit a relative sparing of MYH. In vitro, MuRF1 is shown to function as an E3 ubiquitin ligase for MYH. These data identify the mechanism by which MYH is depleted under atrophy conditions and demonstrate that inhibition of a single E3 ligase, MuRF1, is sufficient to maintain this important sarcomeric protein.

Smad3 signaling is required for satellite cell function and myogenic differentiation of myoblasts

“…In addition, both mRNA expression and protein level of MuRF1 were up-regulated, while no change in the expression of Atrogin1 was observed, in the muscle tissue from Smad3-null mice when compared to controls ( Figure 4A and 4B). MuRF1 has been previously demonstrated to interact with and promote the degradation of MyHC protein in myotubes treated with dexamethasone (DEX) [25]. Consistent with this, we find decreased expression of MyHC at the protein level in Smad3-null mice, with no appreciable change in mRNA expression of MyHC detected between WT and Smad3-null muscle tissue ( Figure 4A and 4B).…”

“…Alternatively, this could be due to an increase in the activity of NF-κB transcription factors, which can be activated by inflammatory cytokines [14,35]. Consistent with increased MuRF1 levels, MyHC, a known target of MuRF1 [25], was down-regulated ( Figure 4A and 4B). Thus, these results clearly suggest that the muscular atrophy seen in Smad3-null mice is much akin to cachecticlike muscle wasting.…”

Small‐molecule inhibition of MuRF1 attenuates skeletal muscle atrophy and dysfunction in cardiac cachexia

“…Importantly, additional experiments also provided evidence that the compound was able to prevent the reduction in both the content and ubiquitination of the key sarcomeric protein actin and a corresponding significant reduction in the activity of the ubiquitin proteasome pathway. Interestingly, while myosin heavy chain is generally suggested to be the key target of MuRF1‐mediated degradation,6, 15 other studies have also found that actin is similarly susceptible40: In vitro and in vivo data have demonstrated lower protein content in line with a greater ubiquitination of actin in catabolic conditions, with MuRF1 found to interact with and polyubiquitinate actin. As such, our data provide strong support that the developed compound was able to perturb the myofibril‐directed MuRF1 atrophy axis, which, in turn, inhibited actin ubiquitination and degradation and thus overall muscle proteolysis.…”

“…Previous studies have reported that small‐molecule knockdowns of MuRF1 can prevent atrophy ex vivo in myotubes 13, 14, 15. However, to our knowledge, this concept had yet to be translated to an in vivo setting, such as animal models that mimic clinically relevant disease conditions.…”

“…glucocorticoids, inflammatory cytokines, reactive oxygen species7, 8, 9, 10), while its gene inactivation confers partial resistance to muscle wasting conditions 7, 8, 11, 12. Therefore, several approaches have attempted to down‐regulate MuRF1 activity therapeutically, either directly in vitro via targeted inhibition of MuRF1 by a muscle‐specific small molecule13 and adenoviral knockdowns14, 15 or indirectly in vivo via exercise training16 or antioxidant administration17, which resulted in a significant reduction in muscle wasting.…”