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.

Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine, whose numerous cell and tissue activities include cell-cycle control, the regulation of early development, differentiation, extracellular matrix formation, hematopoesis, angiogenesis, chemotaxis, immune functions, and the induction of apoptosis. TGF-beta-mediated growth inhibition and apoptosis can be correlated with its function as a tumor suppressor. The apoptosis-inducing capacity has been investigated in many cell types. Data from cell-culture experiments and in vivo studies argue for a pivotal role of TGF-beta-mediated apoptosis in the maintenance of B- and T-cell homeostasis. The importance of TGF-beta in the control of liver cell apoptosis and cell death of prostate epithelial cells has been confirmed in many studies. Inactivation of TGF-beta in animal models via a knockout approach or neutralizing antibodies suggests that TGF-beta-mediated apoptosis plays an important part during tissue formation and remodeling and during the phase of ontogenetic neuron death. The molecular mechanisms involved in these processes seem to involve the activation of SMAD proteins. Many studies have described an interaction of TGF-beta with other signaling cascades as exemplified by the requirement of AP1 transcription factor for the induction of apoptosis in liver cells. The aim of this review is (1) to summarize and classify data in the TGF-beta apoptosis literature with respect to the affected cell types, (2) to provide insights into the intracellular mechanisms involved in TGF-beta-mediated apoptosis, and (3) to set TGF-beta-mediated apoptosis in a physiological context.

The inhibitory Smad7, a direct target gene for transforming growth factor-β (TGF-β), mediates TGF-β1–induced apoptosis in several cell types. Herein, we report that apoptosis of human prostate cancer PC-3U cells induced by TGF-β1 or Smad7 overexpression is caused by a specific activation of the p38 mitogen-activated protein kinase pathway in a TGF-β–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-β1–induced apoptosis. The expression of Smad7 was required for TGF-β–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-β1–induced phosphorylation of p38 and apoptosis, whereas dominant negative TAK1 reduced TGF-β1–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.

We show that following immunoneutralization of endogenous transforming growth factors beta (TGF-beta) in the chick embryo, ontogenetic neuron death of ciliary, dorsal root and spinal motor neurons was largely prevented, and neuron losses following limb bud ablation were greatly reduced. Likewise, preventing TGF-beta signaling by treatment with a TbetaR-II fusion protein during the period of ontogenetic cell death in the ciliary ganglion rescued all neurons that normally die. TUNEL staining revealed decreased numbers of apoptotic cells following antibody treatment. Exogenous TGF-beta rescued the TGF-beta-deprived phenotype. We conclude that TGF-beta is critical in regulating ontogenetic neuron death as well as cell death following neuronal target deprivation.