| p38 mitogen-activated protein kinase (MAPK) has been well established as a critical positive regulator of myogenic differentiation. However, it is unclear which of the four p38 isoforms in the mouse genome participates in this process. Using C2C12 myogenic cells as a model, we showed here that p38alpha, beta, and gamma are expressed with distinct expression patterns during differentiation. Knockdown of any of them by siRNA inhibits myogenic differentiation, which suggests that the functions of the three p38 isoforms are not completely redundant. To further elucidate the unique role of each p38 isoform in myogenic differentiation, we individually knocked down one p38 isoform at a time in C2C12 cells and compared the whole-genome gene expression profiles by microarrays. We found that some genes are co-regulated by all three p38 isoforms, while others are uniquely regulated by one particular p38 isoform. Furthermore, several novel p38 target genes (i.e., E2F2, cyclin D3, and WISP1) are found to be required for myogenin expression, which provides a molecular basis to explain why different p38 isoforms are required for myogenic differentiation.;Upstream of p38, MKK3 and MKK6 are known to be involved in myogenic differentiation. However, the identity of the MAP3K and other upstream signaling molecules involved in myogenic differentiation remain to be elucidated. Using siRNA technique, we found that TAK1 is one of the MAP3Ks involved in myogenesis. In addition, TRAF6, an adaptor protein mediating the activation of TAK1 in cytokine signaling and innate immune responses, was also found to be required for myogenic differentiation. Knockdown of TAK1 and TRAF6 separately by siRNA specifically deceased the levels of active p38 as well as the expression of myogenin. Our data suggested that TAK1 and TRAF6 exert their myogenic function through the p38 MAPK pathway. |
