Terminal Differentiation of Myogenic Cells Requires Stable Form of Tumor Suppressor Protein, P53

Although, the role of tumor suppressor protein, p53 is well accepted in differentiation, it is not clear how p53 enhances differentiation in some cell lineages and inhibits others. C2C12 myoblasts permanently withdraw from cell cycle and terminally differentiate into multinucleated myotubes. BC3H1 myogenic cells however differentiate but reversibly withdraw from cell cycle and do not fuse into myotubes. We examined the levels of p53 in both cytoplasm and nucleus of C2C12 and BC3H1 myogenic cells undergoing differentiation. We found quite a good amount of full-length p53 in the cytoplasm and extensive degradation in the nucleus of control and differentiated C2C12 myoblasts. In addition, in the nucleus we detected very low level of full-length p53 during early differentiation and quite a significant amount during late differentiation when C2C12 myoblasts were fusing into myotubes. On the other hand, we found quite a good amount of p53 in control and early differentiated BC3H1 myogenic cells both in the cytoplasm and nucleus. However, its level gradually decreased in both locations while these cells were differentiating. Although, p53 is extensively degraded in the nucleus in these differentiated cells, this degradation pattern is quite different from C2C12 myoblasts, p53 mutant-expressing C33A cells, and testicular somatic (Leydig and Sertoli) cells. Altogether, our data indicate that the lack of differentiation of BC3H1 cells in multinucleated myotubes is possibly due to the extensive degradation of p53 in the nucleus. Abbreviations used: p53, a tumor suppressor protein; 8-Br-cAMP, 8-bromo-cyclic adenosine monophosphate; h, hour; and kDa, kilo-Dalton