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β‐hydroxy‐β‐methyl butyrate (HMB) stimulates mammalian target of rapamycin complex 1 (mTORc1) signaling via a phosphatidylinositol‐3‐kinase (PI3K)‐dependent pathway
Author(s) -
Hill Derek Samuel,
Szewczyk Nathaniel,
Smith Kenneth,
Brookfield Rosalind,
Loughna Paul,
Rathmacher John,
Rennie Michael J,
Atherton Philip J
Publication year - 2011
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.25.1_supplement.1059.10
Subject(s) - mtorc1 , p70 s6 kinase 1 , pi3k/akt/mtor pathway , protein kinase b , ribosomal protein s6 , leucine , phosphorylation , signal transduction , chemistry , biochemistry , microbiology and biotechnology , biology , amino acid
HMB is formed in liver via oxidation of the leucine transamination product α‐ketoisocaproate. In C 2 C 12 muscle cells, HMB attenuates experimentally induced catabolism (e.g. lipopolysaccharide incubations) and stimulates protein synthesis in an mTORc1‐dependent manner, through upstream mechanisms which are unclear. We tested the hypothesis that HMB, like leucine, would stimulate mTORc1 independent of PI3K signaling. C 2 C 12 myotubes were incubated with 0.5 mM leucine alone or 0.5 mM HMB +/− inhibitors of mTORc1 (100 nM rapamycin) or PI3K (20 μm LY290042); n=6. Phosphorylation of protein kinase B (AKT) and mTORc1 substrates, ribosomal protein S6 kinase (S6K1) and 4E‐binding protein‐1 (4EBP1) were measured by immunoblotting. HMB stimulated the phosphorylation of AKT Ser473 (+129±19%; P <0.01), S6K1 Thr389 (+50±6% P <0.05) and 4EBP1 Thr65/70 (+51±21% P <0.05). HMB stimulated anabolic signaling with greater potency than leucine, e.g. S6K1 Thr389 +50% vs. +17%; respectively. As expected, incubation of HMB with rapamycin ablated increases in mTORc1 signaling, but not AKT phosphorylation (+188±59% P <0.01). In contrast, incubation with LY290042 abolished HMB‐induced increases in both AKT and mTORc1 signaling, suggesting HMB signals to mTORc1 in a PI3K‐dependent manner. We conclude that HMB, despite being a leucine metabolite, signals to mTORc1 through mechanisms distinct from those of leucine. Support: RCUK