Activin A is a member of the transforming growth factor (TGF)-β superfamily that regulates cell proliferation and differentiation. Using the p38 inhibitor SB203580, our previous studies demonstrated that p38 was involved in activin A-mediated hemoglobin (Hb) synthesis in K562 cells. SB203580 is an inhibitor of p38α and p38β isoforms. In this study, we show that p38α and p38β mRNA were expressed in K562 cells and that activin A activated the kinase activities of these isoforms. To investigate the roles of p38α and p38β isoforms in activin A-mediated erythroid differentiation, we generated stable clones that over-expressed the dominant negative p38 isoforms p38α(AF) and p38β(AF) in K562 cells. The expressions of either p38α(AF) or p38β(AF) reduced activin A-induced p38 activation, Hb synthesis, and ζ-globin promoter activity. Similarly, down-regulation of either p38α or p38β by isoform-specific siRNAs also reduced activin A-induced ζ-globin promoter activity. Co-expressions of p38α(AF) and p38β(AF), together, greatly inhibited the transcription activity of the ζ-globin promoter. Conversely, expression of mitogen-activated protein kinase kinase (MKK) 6b(E), a constitutive activator of p38, significantly activated ζ-globin promoter. Co-expressions of either p38α or p38β with MKK6b had a similar activation of ζ-globin promoter. Activin A induction of erythroid differentiation was inhibited by follistatin. Activin A-induced phosphorylation of MKK6 and p38 was also inhibited by follistatin. Moreover, over-expression of MKK6b(E) reverted follistatin inhibition of activin A-induced ζ-globin promoter activity. These results demonstrate that activin A induces erythroid differentiation of K562 cells through activation of MKK6-p38α/p38β pathway and follistatin inhibits those effects.
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