BMP2-induced apoptosis is mediated by activation of the TAK1-p38 kinase pathway that is negatively regulated by Smad6

Bone morphogenetic protein 2 (BMPS), a member of the transforming growth factor-beta (TGF-beta) superfamily, regulates a variety of cell fates and functions. At present, the molecular mechanism by which BMPS induces apoptosis has not been fully elucidated. Here we propose a BMPS signaling pathway that mediates apoptosis in mouse hybridoma MH60 cells whose growth is interleukin-6 (IL-6)-dependent. BMP2 dose-dependently induces apoptosis in MH60 cells even in the presence of IL-6. BMPS has no inhibitory effect on the IL-g-induced tyrosine phosphorylation of STAT3, and the bcl-2 gene expression which is known to be regulated by STAT3, suggesting that BMP2-induced apoptosis is not attributed to alteration of the IL-6-mediated bcl-2 pathway. We demonstrate that BMPS induces activation of TGF-beta-activated kinase (TAK1) and subsequent phosphorylation of p38 stress-activated protein kinase. In addition, forced expression of kinase-negative TAXI in MH60 cells blocks BMP2-induced apoptosis. These results indicate that BMP2-induced apoptosis is mediated through the TAK1-p38 pathway in MH60 cells. We also show that MH60-derived transfectants expressing Smad6 are resistant to the apoptotic signal of BMPB. Interestingly, this ectopic expression of Smad6 blocks BMP2-induced TAXI activation and p38 phosphorylation. Moreover, Smad6 can directly bind to TAK1. These findings suggest that Smad6 is likely to function as a negative regulator of the TAK1 pathway in the BMP2 signaling, in addition to the previously reported Smad pathway.