Catabolic Signaling Pathways, Atrogenes, and Ubiquitinated Proteins Are Regulated by the Nutritional Status in the Muscle of the Fine Flounder

Abstract

A description of the intracellular mechanisms that modulate skeletal muscle atrophy in early vertebrates is still lacking. In this context, we used the fine flounder, a unique and intriguing fish model, which exhibits remarkably slow growth due to low production of muscle-derived IGF-I, a key growth factor that has been widely acknowledged to prevent and revert muscle atrophy. Key components of the atrophy system were examined in this species using a detailed time-course of sampling points, including two contrasting nutritional periods. Under basal conditions high amounts of the atrogenes MuRF-1 and Atrogin-1 were observed. During fasting, the activation of the P38/MAPK and Akt/FoxO signaling pathways decreased; whereas, the activation of the IκBα/NFκB pathway increased. These changes in signal transduction activation were concomitant with a strong increase in MuRF-1, Atrogin-1, and protein ubiquitination. During short-term refeeding, the P38/MAPK and Akt/FoxO signaling pathways were strongly activated, whereas the activation of the IκBα/NFκB pathway decreased significantly. The expression of both atrogenes, as well as the ubiquitination of proteins, dropped significantly during the first hour of refeeding, indicating a strong anti-atrophic condition during the onset of refeeding. During long-term refeeding, Akt remained activated at higher than basal levels until the end of refeeding, and Atrogin-1 expression remained significantly lower during this period. This study shows that the components of the atrophy system in skeletal muscle appeared early in the evolution of vertebrates and some mechanisms have been conserved, whereas others have not. These results represent an important achievement for the area of fish muscle physiology, showing an integrative view of the atrophy system in a non-mammalian species and contributing to novel insights on the molecular basis of muscle growth regulation in earlier vertebrates.

Transcriptional dynamics of immune, growth and stress related genes in skeletal muscle of the fine flounder ( Paralichthys adpersus) during different nutritional statuses

“…These previous findings for the roles of IL-1b and TNFa in muscle atrophy were supported by the present study, which recorded the activation of signaling pathways mediated by these cytokines, such as P38/ MAPK and IkBa/NFkB. A previous study found that the activation of the IkBa/NFkB signaling pathway rapidly increases during fasting, with significant differences in the activation of IkBa from the first until the last week of fasting (Fuentes et al, 2012b). This is associated with the results obtained in this study, where both IL-1b and TNFa would be involved in activating this signaling pathway.…”

“…The expression of these pro-atrophic molecules is regulated by different mechanisms. For murf1, expression is due to the activation of the IkBa/NFkB pathway (Fuentes et al, 2012b), which might be triggered by external stimuli such as il-1b and tnfa, as described in mammals (Cai et al, 2004). Furthermore, the high expression of atrogin1 could be controlled via FOXO/Akt, due to the high RNA content of foxo1/3.…”

“…down-regulated and reached levels even lower than those of the control group, suggesting the down-regulation of the entire stress response system. Furthermore, activation of the FOXO/Akt pathway was previously analyzed, finding that FOXO1 and FOXO3 phosphorylation decreased after three weeks of fasting, increased during short-term refeeding, and returned to basal levels during longterm refeeding, thus suggesting the activation of these factors (Fuentes et al, 2012b). Moreover, no changes in FOXO1 protein levels during long-term fasting and refeeding were observed (Fuentes et al, 2012b).…”

“…Furthermore, activation of the FOXO/Akt pathway was previously analyzed, finding that FOXO1 and FOXO3 phosphorylation decreased after three weeks of fasting, increased during short-term refeeding, and returned to basal levels during longterm refeeding, thus suggesting the activation of these factors (Fuentes et al, 2012b). Moreover, no changes in FOXO1 protein levels during long-term fasting and refeeding were observed (Fuentes et al, 2012b). These prior results at the protein level are consistent with the results found in the current study at the mRNA level, where expression of foxo1 and foxo3 in the skeletal muscle of the fine flounder was stable during long-term fasting and refeeding but decreased during the first 24 h of refeeding.…”

“…Furthermore, a strong increase in activity occurs for the IkBa/NFkB pathway (Fuentes et al, 2012b). Signaling pathway mediated by IkBa/NFkB, which would be up-regulating the expression of murf1, suggesting an activation of this pathway by cytokines such as il-1b.…”