Sultan Karim R, Henkel Birgit, Terlou Maarten, Haagsman Henk P
Faculty of Veterinary Medicine, Academic Biomedical Centre, Utrecht University, Utrecht, The Netherlands.
Am J Physiol Cell Physiol. 2006 Feb;290(2):C650-9. doi: 10.1152/ajpcell.00163.2005. Epub 2005 Sep 21.
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 C(2)C(12) and rat L6 cells were used. C(2)C(12) 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 (0.5-1 mm) myotubes only, excluding remaining myoblasts and small myotubes. Dexamethasone reduced myotube size by 30% in L6 but not in C(2)C(12) myotubes. Expression of the glucocorticoid receptor was twofold higher in L6 myotubes than in C(2)C(12) myotubes. In both cell lines, 3,3',5-triiodo-l-thyronine (T(3)) did not induce a significant size reduction. Expression of the major T(3) receptor (T(3)Rbeta1) was higher in L6 myotubes. We 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 C(2)C(12) 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 C(2)C(12) myotubes. Therefore, C(2)C(12) myotubes offer an ideal system in which to study skeletal muscle atrophy because, depending on differentiation conditions, C(2)C(12) cells produce atrophy-inducible and atrophy-resistant myotubes.
肌纤维萎缩是由分解代谢因子（如糖皮质激素和甲状腺激素）诱导的肌肉消耗的最终结果。我们建立了一个体外系统，以基于肌管萎缩来定义分解代谢反应。使用了小鼠C2C12细胞和大鼠L6细胞。用血清替代品优思得G替代马血清可改善C2C12肌管的形成。开发了一种新方法，仅对大的（0.5 - 1毫米）肌管的大小变化进行量化，排除剩余的成肌细胞和小肌管。地塞米松使L6肌管的大小减小了30%，但对C2C12肌管没有影响。糖皮质激素受体在L6肌管中的表达比在C2C12肌管中高两倍。在两种细胞系中，3,3',5 - 三碘 - L - 甲状腺原氨酸（T3）均未诱导明显的大小减小。主要的T3受体（T3Rβ1）在L6肌管中的表达更高。我们研究了肌管大小的变化是否与萎缩基因1的表达变化有关，因为这种酶被认为是肌肉萎缩起始的关键因素。地塞米松使萎缩基因1的mRNA增加了两倍；同样，只有L6肌管对此敏感。有趣的是，在优思得G融合的C2C12肌管中，萎缩基因1的表达低于马血清融合的肌管。此外，地塞米松处理仅在马血清融合的肌管中增加了萎缩基因1的表达，而在优思得G融合的肌管中没有增加。优思得G诱导的融合可能导致抗萎缩的C2C12肌管。因此，C2C12肌管提供了一个理想的系统来研究骨骼肌萎缩，因为根据分化条件，C2C12细胞可产生可诱导萎缩和抗萎缩的肌管。
