The in vivo and in vitro effects of 3,5,3'-triiodothyronine (T3) on control and mdx muscle cell proliferation
Moor, Andrea Nichole
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The effects of excess thyroid hormone (T3) on in vivo and in vitro muscle regeneration in control and mdx muscles were examined. The mdx mouse is a genetic model of Duchenne muscular dystrophy (DMD), but unlike DMD, the course of the degenerative disease in this rodent stabilizes after most of its muscle tissue is successfully replaced with new muscle. The regenerative capabilities of control and mdx muscles were examined using in vivo histology and morphometric analyses, and in vitro cell cycle analyses. In vivo experiments showed that mdx muscle maintains a larger regenerative capacity than control muscle under hyperthyroid conditions. T3 differentially affected muscle repair and was deleterious only to controls. $\sp3$H-thymidine incorporation and propidium iodide (PI) studies (in vitro) revealed that T3 primarily affected the control and mdx cell cycles during the G$\sb0$/G$\sb1$ and S-phases. More mdx myoblasts were observed in G$\sb0$/G$\sb1$ with T3 treatment compared to untreated mdx myoblasts. Results suggest that the large regenerative capability of mdx myoblasts may be a result of differential regulation of cell cycling. Distinct responses were observed in control and mdx fibroblasts with T3 treatment, and it was evident that myoblast and fibroblast interactions did occur in vitro. Bromodeoxyuridine (BrdU) and PI double-labelling experiments and MyoD staining showed differences in cycling activity between control and mdx myoblast cultures as they developed over time. Control myoblasts cycled less uniformly than mdx myoblasts. The proportions of myoblasts in early and late S-phases were higher in mdx cultures compared to control cultures and mdx cells appeared to cycle faster. T3 treatment only altered the cycling of control myoblasts, suggesting that these myoblasts may be more sensitive to the effects of T3 before MyoD expression during myogenic progression. Overall results suggest that T3 affects control muscle regeneration by reducing the growth of new myotubes in vivo, and that reduction in growth is a direct result of the effects of T3 on cycling behavior of control myoblasts and fibroblasts as shown in vitro. Surprisingly, this negative effect of T3 was not observed in mdx muscle, and has led us to question further the molecular mechanisms of this successful muscle regeneration.