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VILAS BOAS, V.F Effect of the Triiodothyronine (T3) and the Thyroid Receptor Beta selective Agonist GC-24 upon rat skeletal muscle trophism: expression of proteasome-dependent genes. 2008. 107 folhas. Dissertacao (Mestrado em Biologia Celular e do Desenvolvimento) – Instituto de Ciencias Biomedicas, Universidade de Sao Paulo, Sao Paulo. Triiodothyronine (T3) is known to play a key role in the function of several tissues/organs via the thyroid hormone receptor isoforms alpha (TRα) and beta (TRβ). Abnormalities in skeletal muscle function have been associated with increased levels of T3, which is a major sarcopenia (loss of sarcomeres). Although the phenomenon of sarcopenia induced by T3 has been widely reported, little is known about the molecular mechanisms involved in proteolysis induced by T3. In this study we have investigated the effects of T3 and GC-24, a novel synthetic TRβselective compound, on the ubiquitin proteasome pathway. We analyzed the effect of T3 and GC-24 on the radial trophism, ubiquitination levels and gene expression of the ubiquitin-proteasome pathway, which are important regulators of muscle proteolysis in the skeletal muscle. We have addressed the ubiquitin ligases (Atrogin1, MuRF-1 and E3α) and the deubiquitinating enzymes (UBP45, UBP69 and USP28). Wistar male rats (170-200g) were divided in 4 groups (Control, 12, 1 and 7 days). Rats received T3 (30μg/100g) and GC-24 (16 μg/100g). After decapitation, EDL and soleus muscles were removed for histological analysis, protein expression and gene expression. Cross sectional area was determined in histological sections through the software "Image-Pro Plus. The ubiquitination levels was determined by Western Blot and gene expression determined by Real Time PCR analysis. T3 and GC-24 reduced the diameter of the muscle fibers vs control group. Both T3 and GC-24 incresed the ubiquitination levels, in the soleus and EDL. Regarding gene expression analysis, T3 and GC-24 modulate the gene expression in a differential manner. In the soleus, T3 increased Atrogin-1 and E3 alpha gene expression, while did not alter Murf-1 gene expression. On the other hand, in EDL Atrogin-1 gene expression is not altered, while E3 alpha and Murf-1 are elevated by T3. In the soleus and EDL deubiquitinating gene expression is mostly not altered, exception made for UBP 45, which is reduced by T3 in soleus muscle. GC-24, increased gene expression of E3α and MuRF-1 in the soleus, while did not alter Atrogin-1 gene expression. However, in EDL muscle, GC-24 increased Atrogin-1 and E3α mRNA, while did not alter MuRF-1. Finally, GC-24 decreased UBP 45 gene expression in EDL muscle and USP 28 gene expression was robustly elevated by GC-24 in both muscles analyzed. This data shows that GC-24 is able to strongly modulate genes that are less responsive or even unresponsive to T3, pointing that the GC-24-TRb complex might trans-activate differently target genes. However, both T3 and GC-24 are able to modulate the muscle proteolysis.

Efeito da triiodotironina (T3) e do agonista TRb seletivo GC-24 sobre o trofismo muscular esquelético de ratos: aspectos envolvendo a proteólise dependente de proteassoma.