Signaling pathways initiated by beta-hydroxy-beta-methylbutyrate to attenuate the depression of protein synthesis in skeletal muscle in response to cachectic stimuli

• Published in: American journal of physiology: endocrinology and metabolism Vol. 293; no. 4; pp. E923 - E931
• Main Authors: Eley, Helen L, Russell, Steven T, Baxter, Jeffrey H, Mukerji, Pradip, Tisdale, Michael J
• Format: Journal Article
• Language: English
• Published: United States 01.10.2007
• Subjects: Animals; Cachexia - etiology; Cachexia - metabolism; Cachexia - pathology; Carrier Proteins - metabolism; Cells, Cultured; Male; Mice; Mice, Inbred Strains; Muscle Fibers, Skeletal - drug effects; Muscle Neoplasms - complications; Muscle Neoplasms - metabolism; Muscle, Skeletal - drug effects; Muscle, Skeletal - metabolism; Neoplasm Transplantation; Phosphoproteins - metabolism; Phosphorylation - drug effects; Protein Biosynthesis - drug effects; Protein Kinases - metabolism; Proteoglycans - pharmacology; Ribosomal Protein S6 Kinases, 70-kDa - metabolism; Signal Transduction - drug effects; TOR Serine-Threonine Kinases; Valerates - pharmacology
• ISSN: 0193-1849; 1522-1555
• DOI: 10.1152/ajpendo.00314.2007

1 Nutritional Biomedicine, School of Life and Health Sciences, Aston University, Birmingham B4 7ET, United Kingdom; and 2 Ross Products Division, Abbott Laboratories, Columbus, Ohio Submitted 29 May 2007 ; accepted in final form 2 July 2007 To investigate the mechanism by which -hydroxy- -methylbutyrate (HMB) attenuates the depression of protein synthesis in the skeletal muscle of cachectic mice, a study has been carried out in murine myotubes in the presence of proteolysis-inducing factor (PIF). PIF inhibited protein synthesis by 50% within 4 h, and this was effectively attenuated by HMB (25–50 µM). HMB (50 µM) alone stimulated protein synthesis, and this was attenuated by rapamycin (27 nM), an inhibitor of mammalian target of rapamycin (mTOR). Further evidence for an involvement of this pathway was shown by an increased phosphorylation of mTOR, the 70-kDa ribosomal S6 kinase (p70 S6k ), and initiation factor 4E-binding protein (4E-BP1) and an increased association of eukaryotic initiation factor 2 (eIF4E) with eIF4G. PIF alone induced a transient (1–2 h) stimulation of phosphorylation of mTOR and p70 S6k . However, in the presence of HMB, phosphorylation of mTOR, p70 S6k , and 4E-BP1 was increased, and inactive 4E-BP1-eIF4E complex was reduced, whereas the active eIF4G·eIF4E complex was increased, suggesting continual stimulation of protein synthesis. HMB alone reduced phosphorylation of elongation factor 2, but this effect was not seen in the presence of PIF. PIF induced autophosphorylation of the double-strand RNA-dependent protein kinase (PKR), leading to phosphorylation of eIF2 on the -subunit, which would inhibit protein synthesis. However, in the presence of HMB, phosphorylation of PKR and eIF2 was attenuated, and this was also observed in skeletal muscle of cachectic mice administered HMB (0.25 g/kg). These results suggest that HMB attenuates the depression of protein synthesis by PIF in myotubes through multiple mechanisms. proteolysis-inducing factor; eukaryotic initiation factor 2 ; 4E-binding protein 1 Address for reprint requests and other correspondence: M. J. Tisdale, Nutritional Biomedicine, School of Life and Health Sciences, Aston Univ., Birmingham B4 7ET, UK (e-mail: m.j.tisdale{at}aston.ac.uk )