Regulation of metabolic transcriptional co-activators and transcription factors with acute exercise

Endurance exercise improves insulin sensitivity and increases fat oxidation, which are partly facilitated by the induction of metabolic transcription factors. Next to exercise, increased levels of FFA's also increase the gene expression of transcription factors, hence making it difficult to discern the effects from contractile signals produced during exercise, from those produced by increased circulatory FFA's. We aimed to investigate, in human skeletal muscle, whether acute exercise affects gene expression of metabolic transcriptional co-activators and transcription factors, including PGC-1α, PRC, PPARα, β/δ, and γ and RXR, SREBP-1c and FKHR, and to discern the effect of exercise per se from those of elevated levels of FFA. Two hours of endurance exercise was performed either in the fasted state, or following carbohydrate ingestion prior to and during exercise, thereby blunting the fasting-induced increase in FA availability and oxidation. Of the genes measured, PGC-1α and PRC mRNA increased immediately after, while PPARβ/δ and FKHR mRNA increased 1–4 h after exercise, irrespective of the increases in FFA's. Our results suggest that the induction in vivo of metabolic transcription factors implicated in mitochondrial biogenesis are under the control of inherent signals, (PGC-1α, PRC), while those implicated in substrate selection are under the control of associated signals (PPARβ/δ, FKHR) stimulated from the contracting skeletal muscle that are independent of circulating FFA levels.

Human skeletal muscle atrophy in amyotrophic lateral sclerosis reveals a reduction in Akt and an increase in atrogin-1

The molecular mechanisms influencing muscle atrophy in humans are poorly understood. Atrogin-1 and MuRF1, two ubiquitin E3-ligases, mediate rodent and cell muscle atrophy and are suggested to be regulated by an Akt/Forkhead (FKHR) signaling pathway. Here we investigated the expression of atrogin-1, MuRF1, and the activity of Akt and its catabolic (FKHR and FKHRL1) and anabolic (p70s6k and GSK-3β) targets in human skeletal muscle atrophy. The muscle atrophy model used was amyotrophic lateral sclerosis (ALS). All measurements were performed in biopsies from 22 ALS patients and 16 healthy controls as well as in G93A ALS mice. ALS patients had a significant increase in atrogin-1 mRNA and protein content, which was associated with a decrease in Akt activity. There was no difference in the mRNA and protein content of FKHR, FKHRL1, p70s6k, and GSK-3β. Similar observations were made in the G93A ALS mice. Human skeletal muscle atrophy, as seen in the ALS model, is associated with an increase in atrogin-1 and a decrease in Akt. The transcriptional regulation of human atrogin-1 may be controlled by an Akt-mediated transcription factor other than FKHR or via another signaling pathway.

Pyruvate dehydrogenase activation and kinase expression in human skeletal muscle during fasting

Fasting forces adaptive changes in whole body and skeletal muscle metabolism that increase fat oxidation and decrease the oxidation of carbohydrate. We tested the hypothesis that 40 h of fasting would decrease pyruvate dehydrogenase (PDH) activity and increase PDH kinase (PDK) isoform mRNA expression in human skeletal muscle. The putative transcriptional activators of PDK isozymes, peroxisome proliferator-activated receptor-α (PPAR-α) protein, and forkhead homolog in rhabdomyosarcoma (FKHR) mRNA were also measured. Eleven healthy adults fasted after a standard meal (25% fat, 60% carbohydrate, 15% protein) with blood and skeletal muscle samples taken at 3, 15, and 40 h postprandial. Fasting increased plasma free fatty acid, glycerol, and β-hydroxybutyrate concentrations and decreased glucose and insulin concentrations. PDH activity decreased from 0.88 ± 0.11 mmol acetyl-CoA · min^-1 · kg wet muscle wt^-1 at 3 h to 0.62 ± 0.10 (P = not significant) and 0.39 ± 0.06 (P < 0.05) mmol · min^-1 · kg wet mass^-1 after 15 and 40 h of fasting. Although all four PDK isoforms were expressed in human skeletal muscle, PDK^-2 and -4 mRNA were the most abundant. PDK^-1 and ^-3 mRNA abundance was ~1 and 15% of the PDK^-2 and 4^- levels, respectively. The 40-h fast had no effect on PDK^-1, ^-2, and ^-3 mRNA expression. PDK-4 mRNA was significantly increased ~3-fold after 15 h and ~14-fold after 40 h of fasting. Skeletal muscle PPAR-α protein and FKHR mRNA abundance were unaffected by the fast. The results suggest that decreased PDH activation after 40 h of fasting may have been a function of the large increase in PDK-4 mRNA expression and possible subsequent increase in PDK protein and activity. The changes in PDK-4 expression and PDH activity did not coincide with increases in the transcriptional activators PPAR-α and FKHR.

Human sarcopenia reveals an increase in SOCS-3 and myostatin and a reduced efficiency of Akt phosphorylation

Age-related skeletal muscle sarcopenia is linked with increases in falls, fractures, and death and therefore has important socioeconomic consequences. The molecular mechanisms controlling age-related muscle loss in humans are not well understood, but are likely to involve multiple signaling pathways. This study investigated the regulation of several genes and proteins involved in the activation of key signaling pathways promoting muscle hypertrophy, including GH/STAT5, IGF-1/Akt/GSK-3β/4E-BP1, and muscle atrophy, including TNFα/SOCS-3 and Akt/FKHR/atrogene, in muscle biopsies from 13 young (20 ± 0.2 years) and 16 older (70 ± 0.3 years) males. In the older males compared to the young subjects, muscle fiber cross-sectional area was reduced by 40–45% in the type II muscle fibers. TNFα and SOCS-3 were increased by 2.8 and 1.5 fold, respectively. Growth hormone receptor protein (GHR) and IGF-1 mRNA were decreased by 45%. Total Akt, but not phosphorylated Akt, was increased by 2.5 fold, which corresponded to a 30% reduction in the efficiency of Akt phosphorylation in the older subjects. Phosphorylated and total GSK-3β were increased by 1.5 and 1.8 fold, respectively, while 4E-BP1 levels were not changed. Nuclear FKHR and FKHRL1 were decreased by 73 and 50%, respectively, with no changes in their atrophy target genes, atrogin-1 and MuRF1. Myostatin mRNA and protein levels were significantly elevated by 2 and 1.4 fold. Human sarcopenia may be linked to a reduction in the activity or sensitivity of anabolic signaling proteins such as GHR, IGF-1, and Akt. TNFα, SOCS-3, and myostatin are potential candidates influencing this anabolic perturbation.