Transforming growth factor-beta1 (TGF-beta)-induced apoptosis of prostate cancer cells involves Smad7-dependent activation of p38 by TGF-beta-activated kinase 1 and mitogen-activated protein kinase kinase 3

The inhibitory Smad7, a direct target gene for transforming growth factor-beta (TGF-beta), mediates TGF-beta1-induced apoptosis in several cell types. Herein, we report that apoptosis of human prostate cancer PC-3U cells induced by TGF-beta1 or Smad7 overexpression is caused by a specific activation of the p38 mitogen-activated protein kinase pathway in a TGF-beta-activated kinase 1 (TAK1)- and mitogen-activated protein kinase kinase 3 (MKK3)-dependent manner. Expression of dominant negative p38, dominant negative MKK3, or incubation with the p38 selective inhibitor [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole], prevented TGF-beta1-induced apoptosis. The expression of Smad7 was required for TGF-beta-induced activation of MKK3 and p38 kinases, and endogenous Smad7 was found to interact with phosphorylated p38 in a ligand-dependent manner. Ectopic expression of wild-type TAK1 promoted TGF-beta1-induced phosphorylation of p38 and apoptosis, whereas dominant negative TAK1 reduced TGF-beta1-induced phosphorylation of p38 and apoptosis. Endogenous Smad7 was found to interact with TAK1, and TAK1, MKK3, and p38 were coimmunoprecipitated with Smad7 in transiently transfected COS1 cells. Moreover, ectopically expressed Smad7 enhanced the coimmunoprecipitation of HA-MKK3 and Flag-p38, supporting the notion that Smad7 may act as a scaffolding protein and facilitate TAK1- and MKK3-mediated activation of p38.     

The type I TGF-beta receptor engages TRAF6 to activate TAK1 in a receptor kinase-independent manner

Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine that regulates embryonic development and tissue homeostasis; however, aberrations of its activity occur in cancer. TGF-beta signals through its Type II and Type I receptors (TbetaRII and TbetaRI) causing phosphorylation of Smad proteins. TGF-beta-associated kinase 1 (TAK1), a member of the mitogen-activated protein kinase kinase kinase (MAPKKK) family, was originally identified as an effector of TGF-beta-induced p38 activation. However, the molecular mechanisms for its activation are unknown. Here we report that the ubiquitin ligase (E3) TRAF6 interacts with a consensus motif present in TbetaRI. The TbetaRI-TRAF6 interaction is required for TGF-beta-induced autoubiquitylation of TRAF6 and subsequent activation of the TAK1-p38/JNK pathway, which leads to apoptosis. TbetaRI kinase activity is required for activation of the canonical Smad pathway, whereas E3 activity of TRAF6 regulates the activation of TAK1 in a receptor kinase-independent manner. Intriguingly, TGF-beta-induced TRAF6-mediated Lys 63-linked polyubiquitylation of TAK1 Lys 34 correlates with TAK1 activation. Our data show that TGF-beta specifically activates TAK1 through interaction of TbetaRI with TRAF6, whereas activation of Smad2 is not dependent on TRAF6.     

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

Bone morphogenetic protein 2 (BMP2), 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 BMP2 induces apoptosis has not been fully elucidated. Here we propose a BMP2 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. BMP2 has no inhibitory effect on the IL-6-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 BMP2 induces activation of TGF-beta-activated kinase (TAK1) and subsequent phosphorylation of p38 stress-activated protein kinase. In addition, forced expression of kinase-negative TAK1 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 BMP2. Interestingly, this ectopic expression of Smad6 blocks BMP2-induced TAK1 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.     

(-)-Epigallocatechin gallate reduces transforming growth factor beta-stimulated HSP27 induction through the suppression of stress-activated protein kinase/c-Jun N-terminal kinase in osteoblasts

We previously reported that transforming growth factor-beta (TGF-beta) stimulates heat shock protein 27 (HSP27) induction through p38 mitogen-activated protein (MAP) kinase and extracellular signal-regulated kinase 1/2 (ERK1/2) in osteoblast-like MC3T3-E1 cells. In the present study, we investigated whether (-)-epigallocatechin gallate (EGCG), the major polyphenol found in green tea, affects the TGF-beta-stimulated induction of HSP27 in these cells, and its underlying mechanism. EGCG significantly suppressed the HSP27 induction stimulated by TGF-beta in a dose-dependent manner between 10 and 30 microM without affecting the HSP70 levels. TGF-beta with or without EGCG did not affect the advanced oxidation protein products. The TGF-beta-induced phosphorylation of p38 MAP kinase and ERK1/2 was not affected by EGCG. SP600125, a specific inhibitor of stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK), markedly reduced the HSP27 expression induced by TGF-beta. EGCG significantly suppressed the TGF-beta-induced phosphorylation of SAPK/JNK without affecting the phosphorylation of Smad2. EGCG attenuated the phosphorylation of both MKK4 and TAK1 induced by TGF-beta. These results strongly suggest that EGCG suppresses the TGF-beta-stimulated induction of HSP27 via the attenuation of the SAPK/JNK pathway in osteoblasts, and that this effect is exerted at a point upstream from TAK1.