Testosterone signals through mTOR and androgen receptor to induce muscle hypertrophy

Med Sci Sports Exerc. 2013 Sep;45(9):1712-20. doi: 10.1249/MSS.0b013e31828cf5f3.

Abstract

Purpose: The anabolic hormone testosterone induces muscle hypertrophy, but the intracellular mechanisms involved are poorly known. We addressed the question whether signal transduction pathways other than the androgen receptor (AR) are necessary to elicit hypertrophy in skeletal muscle myotubes.

Methods: Cultured rat skeletal muscle myotubes were preincubated with inhibitors for ERK1/2 (PD98059), PI3K/Akt (LY294002 and Akt inhibitor VIII) or mTOR/S6K1 (rapamycin), and then stimulated with 100 nM testosterone. The expression of α-actin and the phosphorylation levels of ERK1/2, Akt and S6K1 (a downstream target for mTOR) were measured by Western blot. mRNA levels were evaluated by real time RT-PCR. Myotube size and sarcomerization were determined by confocal microscopy. Inhibition of AR was assessed by bicalutamide.

Results: Testosterone-induced myotube hypertrophy was assessed as increased myotube cross-sectional area (CSA) and increased α-actin mRNA and α-actin protein levels, with no changes in mRNA expression of atrogenes (MAFbx and MuRF-1). Morphological development of myotube sarcomeres was evident in testosterone-stimulated myotubes. Known hypertrophy signaling pathways were studied at short times: ERK1/2 and Akt showed an increase in phosphorylation status after testosterone stimulus at 5 and 15 min, respectively. S6K1 was phosphorylated at 60 min. This response was abolished by PI3K/Akt and mTOR inhibition but not by ERK1/2 inhibition. Similarly, the CSA increase at 12 h was abolished by inhibitors of the PI3K/Akt pathway as well as by AR inhibition.

Conclusions: These results suggest a crosstalk between pathways involving fast intracellular signaling and the AR to explain testosterone-induced skeletal muscle hypertrophy.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Actins / genetics
  • Actins / metabolism
  • Androgens / metabolism*
  • Androgens / pharmacology
  • Animals
  • Cells, Cultured
  • Hypertrophy / chemically induced
  • Hypertrophy / metabolism
  • Hypertrophy / pathology
  • MAP Kinase Signaling System
  • Muscle Fibers, Skeletal / metabolism*
  • Muscle Fibers, Skeletal / pathology
  • Muscle Proteins / genetics
  • Muscular Diseases / chemically induced
  • Muscular Diseases / metabolism*
  • Muscular Diseases / pathology
  • Phosphatidylinositol 3-Kinases / metabolism
  • Phosphorylation / drug effects
  • Proto-Oncogene Proteins c-akt / metabolism
  • RNA, Messenger / metabolism
  • Rats
  • Receptors, Androgen / metabolism*
  • Ribosomal Protein S6 Kinases / metabolism
  • SKP Cullin F-Box Protein Ligases / genetics
  • Sarcomeres / drug effects
  • Signal Transduction*
  • TOR Serine-Threonine Kinases / metabolism*
  • Testosterone / metabolism*
  • Testosterone / pharmacology
  • Tripartite Motif Proteins
  • Ubiquitin-Protein Ligases / genetics

Substances

  • Actins
  • Androgens
  • Muscle Proteins
  • RNA, Messenger
  • Receptors, Androgen
  • Tripartite Motif Proteins
  • Testosterone
  • Fbxo32 protein, rat
  • SKP Cullin F-Box Protein Ligases
  • Trim63 protein, rat
  • Ubiquitin-Protein Ligases
  • TOR Serine-Threonine Kinases
  • mTOR protein, rat
  • Proto-Oncogene Proteins c-akt
  • Ribosomal Protein S6 Kinases
  • Rps6kb1 protein, rat