Author:
Eley Helen L.,Russell Steven T.,Baxter Jeffrey H.,Mukerji Pradip,Tisdale Michael J.
Abstract
To investigate the mechanism by which β-hydroxy-β-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 (p70S6k), 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 p70S6k. However, in the presence of HMB, phosphorylation of mTOR, p70S6k, 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 α-subunit, which would inhibit protein synthesis. However, in the presence of HMB, phosphorylation of PKR and eIF2α 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.
Publisher
American Physiological Society
Subject
Physiology (medical),Physiology,Endocrinology, Diabetes and Metabolism
Reference44 articles.
1. Orally Administered Leucine Stimulates Protein Synthesis in Skeletal Muscle of Postabsorptive Rats in Association with Increased eIF4F Formation
2. Leucine Stimulates Translation Initiation in Skeletal Muscle of Postabsorptive Rats via a Rapamycin-Sensitive Pathway
3. Ashford AJ, Pain VM. Effect of diabetes on the rates of synthesis and degradation of ribosomes in rat muscle and liver in vivo. J Biol Chem 261: 4059–4065, 1986.
4. Avruch J, Belham QP, Weng K, Hara K, Yonezawa K. The p70S6k kinase integrates nutrient and growth signals to control translational capacity. Prog Mol Subcell Biol 76: 115–154, 2001.
5. Bibby MC, Double JA, Ali SA, Fearon KCH, Brennan RA, Tisdale MJ. Characterisation of a transplantable adenocarcinoma of the mouse colon producing cachexia in recipient animals. J Natl Cancer Inst 78: 539–546, 1987.