Testosterone regulation of Akt/mTORC1/FoxO3a signaling in skeletal muscle

Mol Cell Endocrinol. 2013 Jan 30;365(2):174-86. doi: 10.1016/j.mce.2012.10.019. Epub 2012 Oct 29.

Abstract

Low endogenous testosterone production, known as hypogonadism is commonly associated with conditions inducing muscle wasting. Akt signaling can control skeletal muscle mass through mTOR regulation of protein synthesis and FoxO regulation of protein degradation, and this pathway has been previously identified as a target of androgen signaling. However, the testosterone sensitivity of Akt/mTOR signaling requires further understanding in order to grasp the significance of varied testosterone levels seen with wasting disease on muscle protein turnover regulation. Therefore, the purpose of this study is to determine the effect of androgen availability on muscle Akt/mTORC1/FoxO3a regulation in skeletal muscle and cultured C(2)C(12) myotubes. C57BL/6 mice were either castrated for 42 days or castrated and treated with the nandrolone decanoate (ND) (6 mg/kg bw/wk). Testosterone loss (TL) significantly decreased volitional grip strength, body weight, and gastrocnemius (GAS) muscle mass, and ND reversed these changes. Related to muscle mass regulation, TL decreased muscle IGF-1 mRNA, the rate of myofibrillar protein synthesis, Akt phosphorylation, and the phosphorylation of Akt targets, GSK3β, PRAS40 and FoxO3a. TL induced expression of FoxO transcriptional targets, MuRF1, atrogin1 and REDD1. Muscle AMPK and raptor phosphorylation, mTOR inhibitors, were not altered by low testosterone. ND restored IGF-1 expression and Akt/mTORC1 signaling while repressing expression of FoxO transcriptional targets. Testosterone (T) sensitivity of Akt/mTORC1 signaling was examined in C(2)C(12) myotubes, and mTOR phosphorylation was induced independent of Akt activation at low T concentrations, while a higher T concentration was required to activate Akt signaling. Interestingly, low concentration T was sufficient to amplify myotube mTOR and Akt signaling after 24 h of T withdrawal, demonstrating the potential in cultured myotubes for a T initiated positive feedback mechanism to amplify Akt/mTOR signaling. In summary, androgen withdrawal decreases muscle myofibrillar protein synthesis through Akt/mTORC1 signaling, which is independent of AMPK activation, and readily reversible by anabolic steroid administration. Acute Akt activation in C(2)C(12) myotubes is sensitive to a high concentration of testosterone, and low concentrations of testosterone can activate mTOR signaling independent of Akt.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Adenylate Kinase / metabolism
  • Androgens / pharmacology
  • Animals
  • Cell Line
  • Enzyme Activation
  • Forkhead Box Protein O3
  • Forkhead Transcription Factors / metabolism*
  • Gene Expression
  • Insulin-Like Growth Factor I / genetics
  • Insulin-Like Growth Factor I / metabolism
  • Male
  • Mechanistic Target of Rapamycin Complex 1
  • Mice
  • Mice, Inbred C57BL
  • Multiprotein Complexes
  • Muscle Fibers, Skeletal / drug effects
  • Muscle Fibers, Skeletal / metabolism*
  • Muscle Fibers, Skeletal / physiology
  • Muscle Strength
  • Muscle, Skeletal / drug effects
  • Muscle, Skeletal / metabolism
  • Muscle, Skeletal / physiology
  • Nandrolone / analogs & derivatives
  • Nandrolone / pharmacology
  • Nandrolone Decanoate
  • Orchiectomy
  • Phosphorylation
  • Protein Processing, Post-Translational
  • Proteins / metabolism*
  • Proto-Oncogene Proteins c-akt / metabolism*
  • Receptors, Androgen / genetics
  • Receptors, Androgen / metabolism
  • Signal Transduction
  • TOR Serine-Threonine Kinases
  • Testosterone / physiology*
  • Transcriptional Activation

Substances

  • Androgens
  • Forkhead Box Protein O3
  • Forkhead Transcription Factors
  • FoxO3 protein, mouse
  • Multiprotein Complexes
  • Proteins
  • Receptors, Androgen
  • insulin-like growth factor-1, mouse
  • Testosterone
  • Insulin-Like Growth Factor I
  • Nandrolone
  • TOR Serine-Threonine Kinases
  • Mechanistic Target of Rapamycin Complex 1
  • Proto-Oncogene Proteins c-akt
  • Adenylate Kinase
  • Nandrolone Decanoate