Signaling pathways initiated by beta-hydroxy-beta-methylbutyrate to attenuate the depression of protein synthesis in skeletal muscle in response to cachectic stimuli.
To investigate the mechanism by which beta-hydroxy-beta-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 muM). HMB (50 muM) 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 alpha-subunit, which would inhibit protein synthesis. However, in the presence of HMB, phosphorylation of PKR and eIF2alpha 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.
Eley
Helen L
HL
Nutritional Biomedicine, School of Life and Health Sciences, Aston Univ., Birmingham B4 7ET, UK.
Russell
Steven T
ST
Baxter
Jeffrey H
JH
Mukerji
Pradip
P
Tisdale
Michael J
MJ
D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D002100:Cachexia / Q000209:etiology / Q000378:metabolism* / Q000473:pathology; D002352:Carrier Proteins / Q000378:metabolism; D018797:Cell Cycle Proteins; D002478:Cells, Cultured; D039642:Eukaryotic Initiation Factors; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D018485:Muscle Fibers, Skeletal / Q000187:drug effects; D019042:Muscle Neoplasms / Q000150:complications / Q000378:metabolism; D018482:Muscle, Skeletal / Q000187:drug effects* / Q000378:metabolism*; D009368:Neoplasm Transplantation; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D014176:Protein Biosynthesis / Q000187:drug effects*; D011494:Protein Kinases / Q000378:metabolism; D011509:Proteoglycans / Q000494:pharmacology; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects*; D058570:TOR Serine-Threonine Kinases; D014631:Valerates / Q000494:pharmacology*
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; C499527:Eif4ebp1 protein, mouse; D039642:Eukaryotic Initiation Factors; D010750:Phosphoproteins; D011509:Proteoglycans; D014631:Valerates; C042057:proteolysis-inducing peptide; C004961:beta-hydroxyisovaleric acid; D011494:Protein Kinases; C546843:mTOR protein, mouse; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases