Docosahexaenoic acid prevents palmitate-induced activation of proteolytic systems in C2C12 myotubes

Abstract

Saturated fatty acids like palmitate contribute to muscle atrophy in a number of conditions (e.g., Type II diabetes) by altering insulin signaling. Akt is a key modulator of protein balance that inhibits the FoxO transcription factors (e.g., FoxO3) which selectively induce the expression of atrophy-inducing genes (atrogenes) in the ubiquitin-proteasome and autophagy-lysosome systems. Conversely, omega-3 polyunsaturated fatty acids have beneficial effects on insulin signaling and may preserve muscle mass. In an earlier report, the omega-3 fatty acid docosahexaenoic acid (DHA) protected myotubes from palmitate-induced atrophy; the mechanisms underlying the alterations in protein metabolism were not identified. This study investigated whether DHA prevents a palmitate-induced increase in proteolysis by restoring Akt/FoxO signaling. Palmitate increased the rate of protein degradation, while co-treatment with DHA prevented the response. Palmitate reduced the activation state of Akt and increased nuclear FoxO3 protein while decreasing its cytosolic level. Palmitate also increased the mRNAs of two FoxO3 atrogene targets, the E3 ubiquitin ligase atrogin-1/MAFbx and the autophagy mediator Bnip3. DHA attenuated the effects of palmitate on Akt activation, FoxO3 localization, and atrogene mRNAs. DHA, alone or in combination with palmitate, decreased the ratio of LC3B-II:LC3B-I protein as well as the rate of autophagosome formation, as indicated by reduced LC3B-II protein in the presence of 10 mmol/L methylamine, suggesting an independent effect of DHA on the macroautophagy pathway. These data indicate that palmitate induces myotube atrophy, at least in part, by activating multiple proteolytic systems and that DHA counters the catabolic effects of palmitate by restoring Akt/FoxO signaling.

Docosahexaenoic acid‐supplementation prior to fasting prevents muscle atrophy in mice

“…Overall, this suggests that the effect of DHA on Akt and FoxO3A phosphorylations resulted in a lesser activation of autophagy and ubiquitination. This is in agreement with a recent study showing that DHA counteracted the palmitate‐induced myotube atrophy by restoring the Akt/FoxO3 signalling and limiting activation of proteolysis . n‐3 LC‐PUFAs were previously reported to modulate the activity of the ECS, which can regulate Akt‐dependent signalling in muscle .…”

“…This may modulate the endocannabinoid system (ECS), which can regulate the Akt‐dependent signalling pathway in muscle . In addition, DHA countered the catabolic effects of palmitate in C2C12 cells by restoring Akt signalling and attenuated the activation of multiple proteolytic systems …”

The role of E3 ubiquitin-ligases MuRF-1 and MAFbx in loss of skeletal muscle mass

“…C2C12 myotubes treated with palmitate demonstrated increased rates of protein degradation via inactivation of Akt, activation of FOXO3, and up-regulation of both MuRF-1 and MAFbx [92]. On the other hand, the omega-3 fatty acid docosahexaenoic acid (DHA) protected myotubes from palmitate-induced atrophy and restored Akt/FOXO signaling [92]. In support with the protective role of omega 3 fatty acids on skeletal muscle, dietary supplementation of fish oil was shown to attenuate lipopolysaccharide-induced muscle atrophy in weanling piglets.…”

“…Saturated fatty acids such as palmitate were shown to contribute to muscle atrophy. C2C12 myotubes treated with palmitate demonstrated increased rates of protein degradation via inactivation of Akt, activation of FOXO3, and up-regulation of both MuRF-1 and MAFbx [92]. On the other hand, the omega-3 fatty acid docosahexaenoic acid (DHA) protected myotubes from palmitate-induced atrophy and restored Akt/FOXO signaling [92].…”

Lipid modulation of skeletal muscle mass and function

“…For example, exposure of C2C12 myotubes to palmitate (C16:0), the most abundant circulating saturated fatty acid, has been shown to decrease myotube diameter and suppress insulin signalling . In accord with this, palmitate provision in muscle cells has been reported to induce the expression of pro‐atrophic genes such as atrogin‐1/MAFbx, concomitant with increased nuclear localization of its transcriptional regulator FoxO3 . In contrast, application of docosahexaenoic acid (DHA), an omega‐3 polyunsaturated fatty acid (PUFA), did not alter myotube morphology when applied alone and was shown to counter‐modulate palmitate‐induced atrophy in C2C12 myotubes .…”