Modulation of Cell Cycle Progression by 5-Azacytidine Is Associated with Early Myogenesis Induction in Murine Myoblasts

Myogenesis is a multistep process, in which myoblasts withdraw from the cell cycle, cease to divide, elongate and fuse to form multinucleated myotubes. Cell cycle transition is controlled by a family of cyclin-dependent protein kinases (CDKs) regulated by association with cyclins, negative regulatory subunits and phosphorylation. Muscle differentiation is orchestrated by myogenic regulatory factors (MRFs), such as MyoD and Myf-5. DNA methylation is crucial in transcriptional control of genes involved in myogenesis. Previous work has indicated that treatment of fibroblasts with the DNA-demethylating agent 5-azacytidine (AZA) promotes MyoD expression. We studied the effects of AZA on cell cycle regulation and MRFs synthesis during myoblast proliferation and early myogenesis phases in C2C12 cells. During the proliferation phase, cells were incubated in growth medium with 5 mu M AZA (GMAZA) or without AZA (GM) for 24 hours. At 70% confluence, cells were kept in growth medium in order to spontaneously achieve differentiation or transferred to differentiation medium with 5 mu M AZA (DMAZA) or without AZA (DM) for 12 and 24 hours. Cells used as control were unstimulated. In the proliferation phase, AZA-treated cells seemed to lose their characteristic circular shape and become elongated. The presence of AZA resulted in significant increases in the protein contents of Cyclin-D (FC:1.23 GMAZA vs GM p <= 0.05), p21 (FC: 1.23 GMAZA vs GM p <= 0.05), Myf-5 (FC: 1.21 GMAZA vs GM p <= 0.05) and MyoD (FC: 1.20 GMAZA vs GM p <= 0.05). These results propose that AZA could inhibit cell proliferation. During 12 hours of differentiation, AZA decreased the downregulation of genes involved in cell cycle arrest and in restriction point (G1 and G1/S phase) and the expression of several cyclins, E2F Transcription Factors, cyclin-dependent kinase inhibitors, specific genes responsible of cell cycle negative regulation. During 24 hours of differentiation, AZA induced an increment in the protein expression of Myf-5 (FC: 1.57 GMAZA vs GM p <= 0.05), MyoD (FC: 1.14 DM vs GM p <= 0.05; FC: 1.47 DMAZA vs GM p <= 0.05), p21 (FC: 1.36 GMAZA vs GM p <= 0.01; FC: 1.49 DM vs GM p <= 0.05; FC: 1.82 DMAZA vs GM p <= 0.01) and MyHC (FC: 1.40 GMAZA vs GM p <= 0.01; FC: 2.39 DM vs GM p <= 0.05; FC: 3.51 DMAZA vs GM p <= 0.01). Our results suggest that AZA-induced DNA demethylation can modulate cell cycle progression and enhance myogenesis. The effects of AZA may open novel clinical uses in the field of muscle injury research and treatment.