| dc.description.abstract | During the process of postnatal myogenesis, quiescent muscle satellite cell will be activated by myogenic signals, and basic fibroblast growth factor (bFGF, also known as FGF2) is one of the key myogenic signals. Myogenesis is critically regulated by the myogenic regulatory factors (MRF) family during the determination and differentiation of myogenic cells. FGF2 has been found to repress the expression of MyoD but promote proliferation of satellite cells at the same time, however, the mechanisms mediating FGF2 effects on both events remain to be understood. In this study, we found FGF2 increased C2C12 proliferation dose-dependently from 1.25 to 10 ng/ml and saturated effect was seen at higher doses. Similar cell proliferation effect was observed in MyoD overexpressed C2C12 cells (C2-tTA-MyoD), suggesting MyoD repression is not necessary for the cell proliferation effect of FGF2. Using C2-tTA-MyoD, we further found MyoD mRNA was not targeted by FGF2, implying the observed repression of MyoD mRNA should be regulated at transcriptional level. In our lab, a previous study has found that FGF2 affects MyoD promoter activity through several upstream genomic fragments, but the exact transcription factors mediating this effect have not been identified. These putative regulative regions have been further narrowed down for identifying key TFs mediating this effect and site-directed mutagenesis will be employed to delete putative cis-elements for confirming their involvement. The involvement of one transcription factor, AP1, was examined by rescuing the repressed MyoD expression with its inhibitor SP100030 but no positive rescue was found, suggesting involvement of other TFs. Therefore, further study is needed to identify TFs mediating the repressive effect of FGF2. Since MyoD repression was also seen in the muscle of cancer patients with cachexia, it prompted us to examine whether MyoD over-expression could rescue muscle cachexia. It was surprising to find that the myogenic differentiation of C2-tTA-MyoD treated with cachexia medium from C26 colon cancer cells was not rescued by MyoD over-expression. On the contrary, their myotube atrophy induced by cachexia medium was largely rescued by MyoD over-expression. Taken together, these observations suggest that different mechanisms are employed by cachexia factors to repress myogenesis and to induce myotube atrophy. Furthermore, these evidence also imply that maintaining a functional level of MyoD in myoblasts and myofibers might significantly attenuate the progression of cancer cachexia. | en_US |