Myostatin reduces Akt/TORC1/p70S6K signaling, inhibiting myoblast differentiation and myotube size

Am J Physiol Cell Physiol. 2009 Jun;296(6):C1258-70. doi: 10.1152/ajpcell.00105.2009. Epub 2009 Apr 8.

Abstract

Myostatin is a negative regulator of skeletal muscle size, previously shown to inhibit muscle cell differentiation. Myostatin requires both Smad2 and Smad3 downstream of the activin receptor II (ActRII)/activin receptor-like kinase (ALK) receptor complex. Other transforming growth factor-beta (TGF-beta)-like molecules can also block differentiation, including TGF-beta(1), growth differentiation factor 11 (GDF-11), activins, bone morphogenetic protein 2 (BMP-2) and BMP-7. Myostatin inhibits activation of the Akt/mammalian target of rapamycin (mTOR)/p70S6 protein synthesis pathway, which mediates both differentiation in myoblasts and hypertrophy in myotubes. Blockade of the Akt/mTOR pathway, using small interfering RNA to regulatory-associated protein of mTOR (RAPTOR), a component of TOR signaling complex 1 (TORC1), increases myostatin-induced phosphorylation of Smad2, establishing a myostatin signaling-amplification role for blockade of Akt. Blockade of RAPTOR also facilitates myostatin's inhibition of muscle differentiation. Inhibition of TORC2, via rapamycin-insensitive companion of mTOR (RICTOR), is sufficient to inhibit differentiation on its own. Furthermore, myostatin decreases the diameter of postdifferentiated myotubes. However, rather than causing upregulation of the E3 ubiquitin ligases muscle RING-finger 1 (MuRF1) and muscle atrophy F-box (MAFbx), previously shown to mediate skeletal muscle atrophy, myostatin decreases expression of these atrophy markers in differentiated myotubes, as well as other genes normally upregulated during differentiation. These findings demonstrate that myostatin signaling acts by blocking genes induced during differentiation, even in a myotube, as opposed to activating the distinct "atrophy program." In vivo, inhibition of myostatin increases muscle creatine kinase activity, coincident with an increase in muscle size, demonstrating that this in vitro differentiation measure is also upregulated in vivo.

MeSH terms

  • Activin Receptors, Type I / antagonists & inhibitors
  • Activin Receptors, Type I / metabolism
  • Adaptor Proteins, Signal Transducing
  • Animals
  • Benzamides / pharmacology
  • Carrier Proteins / metabolism
  • Cell Differentiation* / drug effects
  • Cell Size* / drug effects
  • Cells, Cultured
  • Creatine Kinase / metabolism
  • Dioxoles / pharmacology
  • Follistatin / pharmacology
  • Humans
  • Insulin-Like Growth Factor I / metabolism
  • Mice
  • Mice, SCID
  • Muscle Fibers, Skeletal / drug effects
  • Muscle Fibers, Skeletal / enzymology*
  • Muscle Fibers, Skeletal / pathology
  • Muscle Proteins / metabolism
  • Myoblasts, Skeletal / drug effects
  • Myoblasts, Skeletal / enzymology*
  • Myoblasts, Skeletal / pathology
  • Myostatin / antagonists & inhibitors
  • Myostatin / metabolism*
  • Organ Size
  • Phosphorylation
  • Protein Kinase Inhibitors / pharmacology
  • Protein Kinases / genetics
  • Protein Kinases / metabolism*
  • Proteins / metabolism
  • Proto-Oncogene Proteins c-akt / metabolism*
  • RNA Interference
  • RNA, Small Interfering / metabolism
  • Rapamycin-Insensitive Companion of mTOR Protein
  • Regulatory-Associated Protein of mTOR
  • Ribosomal Protein S6 Kinases, 70-kDa / metabolism*
  • SKP Cullin F-Box Protein Ligases / metabolism
  • Signal Transduction
  • Smad2 Protein / genetics
  • Smad2 Protein / metabolism
  • Smad3 Protein / genetics
  • Smad3 Protein / metabolism
  • TOR Serine-Threonine Kinases
  • Transfection
  • Tripartite Motif Proteins
  • Ubiquitin-Protein Ligases / metabolism

Substances

  • 4-(5-benzo(1,3)dioxol-5-yl-4-pyridin-2-yl-1H-imidazol-2-yl)benzamide
  • Adaptor Proteins, Signal Transducing
  • Benzamides
  • Carrier Proteins
  • Dioxoles
  • Follistatin
  • Muscle Proteins
  • Myostatin
  • Protein Kinase Inhibitors
  • Proteins
  • RICTOR protein, human
  • RNA, Small Interfering
  • RPTOR protein, human
  • Rapamycin-Insensitive Companion of mTOR Protein
  • Regulatory-Associated Protein of mTOR
  • SMAD2 protein, human
  • SMAD3 protein, human
  • Smad2 Protein
  • Smad3 Protein
  • Tripartite Motif Proteins
  • Insulin-Like Growth Factor I
  • FBXO32 protein, human
  • SKP Cullin F-Box Protein Ligases
  • TRIM63 protein, human
  • Ubiquitin-Protein Ligases
  • Protein Kinases
  • MTOR protein, human
  • TOR Serine-Threonine Kinases
  • mTOR protein, mouse
  • Proto-Oncogene Proteins c-akt
  • Ribosomal Protein S6 Kinases, 70-kDa
  • Activin Receptors, Type I
  • Creatine Kinase