Smads, TAK1, and their common target ATF-2 play a critical role in cardiomyocyte differentiation

We previously demonstrated that bone morphogenetic proteins (BMPs) induce c ardiomyocyte differentiation through the mitogen-activated protein kinase k inase kinase TAK1. Transcription factors Smads mediate transforming growth factor-beta signaling and the ATF/CREB family transcription factor ATF-2 ha s recently been shown to act as a common target of the Smad and the TAK1 pa thways. We here examined the role of Smads and ATF-2 in cardiomyocyte diffe rentiation of P19CL6, a clonal derivative of murine P19 cells. Although P19 CL6 efficiently differentiates into cardiomyocytes when treated with dimeth yl sulfoxide, P19CL6noggin, a P19CL6 cell line constitutively overexpressin g the BMP antagonist noggin, did not differentiate into cardiomyocytes. Coo verexpression of Smad1, a ligand-specific Smad, and Smad4, a common Smad, r estored the ability of P19CL6noggin to differentiate into cardiomyocytes, w hereas stable overexpression of Smad6, an inhibitory Smad, completely block ed differentiation of P19CL6, suggesting that the Smad pathway is necessary for cardiomyocyte differentiation. ATF-2 stimulated the beta MHC promoter activity by the synergistic manner with Smad1/4 and TAK1 and promoted termi nal cardiomyocyte differentiation of P19CL6noggin, whereas overexpression o f the dominant negative form of ATF-2 reduced the promoter activities of se veral cardiac-specific genes and inhibited differentiation of P19CL6. These results suggest that Smads, TAK1, and their common target ATF-2 cooperativ ely play a critical role in cardiomyocyte differentiation.