A potential role for Akt/FOXO signalling in both protein loss and the impairment of muscle carbohydrate oxidation during sepsis in rodent skeletal muscle

Abstract

Sepsis causes muscle atrophy and insulin resistance, but the underlying mechanisms are unclear. Therefore, the present study examined the effects of lipopolysaccharide (LPS)-induced endotoxaemia on the expression of Akt, Forkhead Box O (FOXO) and its downstream targets, to identify any associations between changes in FOXO-dependent processes influencing muscle atrophy and insulin resistance during sepsis. Chronically instrumented male Sprague-Dawley rats received a continuous intravenous infusion of LPS (15 μg kg −1 h −1 ) or saline for 24 h at 0.4 ml h −1 . Animals were terminally anaesthetized and the extensor digitorum longus muscles from both hindlimbs were removed and snap-frozen. Measurements were made of mRNA and protein expression of selected signalling molecules associated with pathways regulating protein synthesis and degradation and carbohydrate metabolism. LPS infusion induced increases in muscle tumour necrosis factor-α (8.9-fold, P < 0.001) and interleukin-6 (8.4-fold, P < 0.01), paralleled by reduced insulin receptor substrate-1 mRNA expression (−0.7-fold, P < 0.01), and decreased Akt1 protein and cytosolic FOXO1 and FOXO3 phosphorylation. These changes were accompanied by significant increases in muscle atrophy F-box mRNA (5.5-fold, P < 0.001) and protein (2-fold, P < 0.05) expression, and pyruvate dehydrogenase kinase 4 mRNA (15-fold, P < 0.001) and protein (1.6-fold, P < 0.05) expression. There was a 29% reduction in the muscle protein : DNA ratio, a 56% reduction in pyruvate dehydrogenase complex (PDC) activity (P < 0.05), and increased glycogen degradation and lactate accumulation. The findings of this study suggest a potential role for Akt/FOXO in the simultaneous impairment of carbohydrate oxidation, at the level of PDC, and up-regulation of muscle protein degradation, in LPS-induced endotoxaemia.

Peroxisome proliferator-activated receptor γ agonism attenuates endotoxaemia-induced muscle protein loss and lactate accumulation in rats

“…PDK4 (15-fold, P<0.001) relative to Con/Sal-treated animals. These findings are consistent with previous observations during endotoxaemia in EDL muscle [15,16]. Rosi treatment markedly supressed the LPS-induced increases in mRNA expression for all genes analysed, with the exception of MAFbx, where the difference did not reach statistical significance (P=0.07 versus Con/LPS; Figure 2E).…”

“…Down-regulation of AKT signalling in inflammatory catabolic conditions is associated with dephosphorylation (activation) of FOXO transcription factors and increased mRNA expression of FOXO downstream gene targets [12,19], which we have confirmed in skeletal muscle in the same rat model of LPS-induced endotoxaemia as used in the present study [15]. Furthermore, we have demonstrated in this same rodent model of clinical endotoxaemia that suppression of LPS-induced muscle cytokine elevation achieved by concurrent low dose Dex infusion preserves phosphorylation of AKT (activation) and FOXO1 (inhibition), reduces abundance and activation of their downstream targets and maintains muscle protein content [16].…”

“…It also provides mechanistic insight of the molecular events underpinning this protection. We hypothesized that dietary administration of the PPARγ agonist Rosi to rodents for 2 weeks before and during 24 h of LPS-induced endotoxaemia would reduce skeletal muscle cytokine mRNA abundance that we have previously observed in skeletal muscle during endotoxaemia [15]. We hypothesized further that as a direct consequence of this, cytokine provoked up-regulation of several molecular events known to increase muscle protein breakdown and lactate accumulation, and reduce carbohydrate oxidation, would be suppressed.…”

“…), after which the extensor digitorum longus (EDL) muscle from both hindlimbs was removed and immediately snap-frozen in liquid nitrogen. The EDL muscle was selected due to its high susceptibility to atrophy during endotoxaemia [15,24].…”

Cachexia – The Interplay Between the Immune System, Brain Control and Metabolism

“…Impaired insulin sensitivity is another symptom frequently present during cachexia in humans and animal models (Crossland et al, 2008;Smiechowska et al, 2010;Asp et al, 2010;Doehner et al, 2010). This metabolic disorder also develops due to the excessive activation of inflammatory pathways.…”