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

Abstract

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 (p70(s6k) and GSK-3beta) 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, p70(s6k), and GSK-3beta. 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.

Human Sarcopenia Reveals an Increase in SOCS-3 and Myostatin and a Reduced Efficiency of Akt Phosphorylation

“…53,54 This suggests that other transcription factors may be involved in regulating human atrogin-1 and MuRF1. 35,36 A reduction in the efficiency to phosphorylate Akt, observed in the present study and by others in rodents, 45 may be a crucial factor in controlling sarcopenia. Therefore identifying the factor(s) that may inhibit Akt phosphorylation is of importance.…”

“…35 PCR primer and probe sequences (Stratagene, La Jolla, CA) for atrogin-1, MuRF1, and ribosomal phosphoprotein PO (RPLPO; 36B4) have been reported previously. 35,36 Primers for TNF␣, SOCS-3, IGF-1 and myostatin were forward, CG TCT CCT ACC AGA CCA AGG; TTC AGC ATC TCT GTC GGA AGA C; GTG GAG ACA GGG GCT TTT ATT TC, AGG TAT ACT GGA ATC CGA TCT CTG A and reverse, CC AAA GTA GAC CTG CCC AGA; CGG CAG CTG GGT GAC TTT; CTT GTT GTT TCC TGC ACT CCC TCT ACT; CAC TGT CTT CAC ATC AAT GCT CTG.…”

“…Electrophoresis was performed using 10-12% SDS PAGE gels in cold buffer (4°C) containing 25 mM Tris (pH 8.8), 192 mM glycine, and 10% methanol, as published previously. 36 Conditions of incubation for Akt, phospho-Akt Ser473 , phospho-GSK-3␤ Ser9 , phospho-4E-BP1 Thr37/46 , FKHR, FKHRL1, ␣-tubulin, and Lamin A have been published previously. 35,36 The antibodies raised against SOCS-3 and IGF-1 were from Santa Cruz Biotechnology (Santa Cruz, CA); myostatin from Biovendor (Modrice, Czech Republic); STAT5, STAT5 Tyr694 , GSK-3␤, and 4E-BP1 from Cell Signaling Technologies (Danvers, MA); and GHR (MAB263) antibody from AbD Serotec (Oxford, United Kingdom).…”

Myogenic differentiation during regrowth of atrophied skeletal muscle is associated with inactivation of GSK-3β

“…Subsequently, consequent to decreased Akt activity, reduced mRNA translation (2, 10) and increased protein degradation resulting from decreased suppression of atrogin expression (46) likely contributed to the loss of muscle mass. It has been shown that Akt plays a role in preservation of muscle mass not only in mice; a recent study in human subjects revealed a reduction in Akt activity in amyotrophic lateral sclerosis, a degenerating muscle disease (35).…”

Signaling Pathways that Mediate Skeletal Muscle Hypertrophy: Effects of Exercise Training

“…The signaling pathway IGF-1/PI3K/Akt (growth factor like Insulin-1, phosphatidylinositol 3-kinase and protein kinase B, respectively) is considered the main mediator of normal muscle development and one of the most studied signaling molecular systems involved in muscle hypertrophy. This pathway plays a key role in the hypertrophic process, since it coordinates the molecular basis related to protein degradation and synthesis [7,9,10,[15][16][17][18][19][20][21].…”