title: Testosterone signals through mTOR and androgen receptor to induce muscle hypertrophy.
abstract: OBJECTIVE
  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.
authors.affiliation: Institute of Biomedical Sciences, University of Chile, Santiago, Chile.
authors.forename: Carla
authors.initials: C
authors.lastname: Basualto-Alarcón
authors.forename: Gonzalo
authors.initials: G
authors.lastname: Jorquera
authors.forename: Francisco
authors.initials: F
authors.lastname: Altamirano
authors.forename: Enrique
authors.initials: E
authors.lastname: Jaimovich
authors.forename: Manuel
authors.initials: M
authors.lastname: Estrada
mesh_terms: D000199:Actins / Q000235:genetics / Q000378:metabolism; D000728:Androgens / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D002478:Cells, Cultured; D006984:Hypertrophy / Q000139:chemically induced / Q000378:metabolism / Q000473:pathology; D020935:MAP Kinase Signaling System; D018485:Muscle Fibers, Skeletal / Q000378:metabolism* / Q000473:pathology; D009124:Muscle Proteins / Q000235:genetics; D009135:Muscular Diseases / Q000139:chemically induced / Q000378:metabolism* / Q000473:pathology; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D011944:Receptors, Androgen / Q000378:metabolism*; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism; D044843:SKP Cullin F-Box Protein Ligases / Q000235:genetics; D012518:Sarcomeres / Q000187:drug effects; D015398:Signal Transduction; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism*; D013739:Testosterone / Q000378:metabolism* / Q000494:pharmacology; D000070584:Tripartite Motif Proteins; D044767:Ubiquitin-Protein Ligases / Q000235:genetics
chemical_list: D000199:Actins; D000728:Androgens; D009124:Muscle Proteins; D019869:Phosphatidylinositol 3-Kinases; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger; D011944:Receptors, Androgen; D019893:Ribosomal Protein S6 Kinases; D044843:SKP Cullin F-Box Protein Ligases; D058570:TOR Serine-Threonine Kinases; D013739:Testosterone; D000070584:Tripartite Motif Proteins; D044767:Ubiquitin-Protein Ligases; C486692:Fbxo32 protein, rat; C499086:Trim63 protein, rat; C546845:mTOR protein, rat; C526901:Rps6kb1 protein, rat