Abstract
Dexamethasone-induced muscle atrophy is due to an increase in protein breakdown and a decrease in protein synthesis, associated with an over-stimulation of the autophagy-lysosomal pathway. These effects are mediated by alterations in IGF-1 and PI3K/Akt signaling. In this study, we have investigated the effects of β-Hydroxy-β-methylbutyrate (HMB) on the regulation of autophagy and proteosomal systems. Rats were treated during 21 days with dexamethasone as a model of muscle atrophy. Co-administration of HMB attenuated the effects promoted by dexamethasone. HMB ameliorated the loss in body weight, lean mass and the reduction of the muscle fiber cross-sectional area (shrinkage) in gastrocnemius muscle. Consequently, HMB produced an improvement in muscle strength in the dexamethasone-treated rats. To elucidate the molecular mechanisms responsible for these effects, rat L6 myotubes were used. In these cells, HMB significantly attenuated lysosomal proteolysis induced by dexamethasone by normalizing the changes observed in autophagosome formation, LC3 II, p62 and Bnip3 expression after dexamethasone treatment. HMB effects were mediated by an increase in FoxO3a phosphorylation and concomitant decrease in FoxO transcriptional activity. The HMB effect was due to the restoration of Akt signaling diminished by dexamethasone treatment. Moreover, HMB was also involved in the regulation of the activity of ubiquitin and expression of MurF1 and Atrogin-1, components of the proteasome system that are activated or up-regulated by dexamethasone. In conclusion, in vivo and in vitro studies suggest that HMB exerts protective effects against dexamethasone-induced muscle atrophy by normalizing the Akt/FoxO axis that controls autophagy and ubiquitin proteolysis.
Highlights
Muscle atrophy occurs in various conditions such as fasting or disuse as well as in diseases including cancer, diabetes, AIDS, sepsis, denervation or glucocorticoid treatment
In a well-established in vitro model of muscle atrophy induced by DEX, we studied the effects of HMB on the autophagic-lysosomal and ubiquitin proteasome pathways
Since Forkhead box O (FoxO) has been established as the key element for DEX-induced autophagy, we studied the effects of HMB on the phosphorylative status of FoxO3a and FoxO-dependent transcriptional activity
Summary
Muscle atrophy occurs in various conditions such as fasting or disuse as well as in diseases including cancer, diabetes, AIDS, sepsis, denervation or glucocorticoid treatment. It happens when proteolysis exceeds protein synthesis, which leads to a reduction of muscle fiber crosssectional area and a decrease in muscle strength [1]. Autophagy is a physiological process utilized by skeletal muscle to sequester cytoplasmic proteins and organelles into vacuoles known as autophagosomes. These fuse with lysosomes, leading to the digestion of the contents by lysosomal hydrolases [3]. Upregulation of autophagy and lysosomal genes has been documented at the transcript and protein level (microtubule-associated protein I light chain 3 (LC3), p62 and Bnip3) in atrophying muscles [2]
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