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1 Academic Biomedical Centre, Faculty of Veterinary Medicine, Utrecht University, Utrecht, The Netherlands
2 Institute of Biochemistry and Molecular Biology-II, University Medical Center Hamburg-Eppendorf, Hamburg, Germany
3 Academic Biomedical Centre, Faculty of Biology, Utrecht University, Utrecht, The Netherlands
* To whom correspondence should be addressed. E-mail: H.P.Haagsman{at}vet.uu.nl.
Myofiber atrophy is the final outcome of muscle wasting induced by catabolic factors such as glucocorticoids and thyroid hormones. We set up an in vitro system to define the catabolic reaction based on myotube atrophy. Both mouse C2C12 and rat L6 cells were used. C2C12 myotube formation was improved by replacing horse-serum with the serum substitute Ultroser G. A new method was developed to quantify size changes of large (0.5-1mm) myotubes only, excluding remaining myoblasts and small myotubes. Dexamethasone reduced myotube size by 30% in L6 but not in C2C12 myotubes. Expression of the glucocorticoid receptor (GR) was twofold higher in L6 myotubes than in C2C12 myotubes. In both cell lines L-triiodothyronine (T3) did not induce a significant size reduction. Expression of the major T3 receptor (T3R
1) was higher in L6 myotubes. It was investigated whether the changes in myotube size are related to changes in atrogin-1 expression, as this enzyme is thought to be a key factor in the initiation of muscle atrophy. Dexamethasone induced a twofold increase of atrogin-1 mRNA; again only L6 myotubes were susceptible. Interestingly, atrogin-1 expression in Ultroser G fused C2C12 myotubes was lower than that in horse serum fused myotubes. Furthermore, dexamethasone treatment increased atrogin-1 expression only in horse serum fused myotubes, but not in Ultroser G fused myotubes. Ultroser G induced fusion may result in atrophy-resistant C2C12 myotubes. Therefore, C2C12 myotubes offer an ideal system to study skeletal muscle atrophy as, depending on differentiation conditions, C2C12 cells produce atrophy-inducible and atrophy-resistant myotubes.
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