title: T3 rapidly increases SLC2A4 gene expression and GLUT4 trafficking to the plasma membrane in skeletal muscle of rat and improves glucose homeostasis.
abstract: BACKGROUND
  Glucose transporter 4 (GLUT4) is highly expressed in muscle and fat tissue, where triiodothyronine (T(3)) induces solute carrier family 2 facilitated glucose transporter member 4 (SLC2A4) gene transcription. T(3) was also shown to rapidly increase glucose uptake in myocytes exposed to cycloheximide, indicating that it might act nongenomically to regulate GLUT4 availability. We tested this hypothesis by evaluating, in thyroidectomized rats (Tx rats), the acute and/or chronic T(3) effects on GLUT4 mRNA expression and polyadenylation, protein content, and trafficking to the plasma membrane (PM) in skeletal muscle, as well as on blood glucose disappearance rate (kITT) after insulin administration.
  
  
  METHODS
  Rats were surgically thyroidectomized and treated with T(3) (0.3 to 100 μg/100 g body weight) from 10 minutes to 5 days, and killed thereafter. Sham-operated (SO) rats were used as controls. Total RNA was extracted from the skeletal muscles (soleus [SOL] and extensorum digitalis longus [EDL]) and subjected to Northern blotting analysis using rat GLUT4 cDNA probe. Total protein was extracted and subjected to specific centrifugations for subcellular fractionation, and PM as well as microsomal (M) fractions were subjected to Western blotting analysis, using anti-GLUT4 antiserum as a probe. GLUT4 mRNA polyadenylation was examined by a rapid amplification of cDNA ends-poly(A) test (RACE-PAT).
  
  
  RESULTS
  Thyroidectomy reduced skeletal muscle GLUT4 mRNA, mRNA poly(A) tail length, protein content, and trafficking to the PM, as well as the kITT. The acute T(3) treatment rapidly (30 minutes) increased all these parameters compared with Tx rats. The 5-day T(3) treatment increased GLUT4 mRNA and protein expression, and restored GLUT4 trafficking to the PM and kITT to SO values.
  
  
  CONCLUSIONS
  The results presented here show for the first time that, in parallel to its transcriptional action on the SLC2A4 gene, T(3) exerts a rapid post-transcriptional effect on GLUT4 mRNA polyadenylation, which might increase transcript stability and translation efficiency, leading to the increased GLUT4 content and availability to skeletal muscle, as well as on GLUT4 translocation to the PM, improving the insulin sensitivity, as shown by the kITT.
authors.affiliation: Department of Physiology and Biophysics, Institute of Biomedical Sciences, University of São Paulo, São Paulo, Brazil.
authors.forename: Erika Lia
authors.initials: EL
authors.lastname: Brunetto
authors.forename: Silvania da Silva
authors.initials: Sda S
authors.lastname: Teixeira
authors.forename: Gisele
authors.initials: G
authors.lastname: Giannocco
authors.forename: Ubiratan Fabres
authors.initials: UF
authors.lastname: Machado
authors.forename: Maria Tereza
authors.initials: MT
authors.lastname: Nunes
mesh_terms: D000818:Animals; D015152:Blotting, Northern; D002462:Cell Membrane / Q000378:metabolism; D015870:Gene Expression / Q000502:physiology; D005947:Glucose / Q000378:metabolism; D051275:Glucose Transporter Type 4 / Q000235:genetics / Q000378:metabolism*; D007328:Insulin / Q000502:physiology; D007333:Insulin Resistance / Q000235:genetics; D018482:Muscle, Skeletal / Q000502:physiology; D026723:Polyadenylation / Q000502:physiology; D021381:Protein Transport / Q000235:genetics; D012333:RNA, Messenger / Q000032:analysis / Q000378:metabolism*; D051381:Rats; D013963:Thyroid Hormones / Q000494:pharmacology*; D013965:Thyroidectomy; D014284:Triiodothyronine / Q000502:physiology*
chemical_list: D051275:Glucose Transporter Type 4; D007328:Insulin; D012333:RNA, Messenger; C495694:Slc2a4 protein, rat; D013963:Thyroid Hormones; D014284:Triiodothyronine; D005947:Glucose