Regulation of TAK1 Activity
TAK1 was originally found to be activated by transforming growth factor β (TGF-β) and bone morphogenetic protein (BMP) (Yamaguchi et al. 1995). Many protein kinases are activated by a conformational change upon phosphorylation on the activation loop. Autophosphorylation of Thr-187 and Ser-192 residues in the activation loop of TAK1 are essential for the activation (Sakurai et al. 2000; Kishimoto et al. 2000). TAK1 forms a constitutive complex with TAK1 binding protein 1 (TAB1) and TAB2 or the structurally related TAB3. TAB1 and either TAB2 or TAB3 bind to the N-terminal kinase domain and the C-terminal region of TAK1, respectively. TAB2 and TAB3 are close homologs and share similar functionality, whereas TAB1 is structurally distinct from TAB2/3. These binding proteins regulate TAK1 activity in different signaling pathways. For instance, TAB1 is essential for osmotic stress-induced TAK1 activation through enhancing oligomerization of the kinase (Inagaki et al. 2008). In contrast, TAB2 and TAB3 play an important role in the activation of TAK1 in cytokine signaling. TAB2 and TAB3 each have a zinc-finger domain, which is necessary for recruitment of TAK1 to the polyubiquitin chain where TAB2 and TAB3 facilitate autophosphorylation and activation of TAK1 (Takaesu et al. 2000; Kanayama et al. 2004). Accordingly, inhibition of TAB2 reduces TAK1 activity in several tissues (Ishitani et al. 2003). However, in some exceptional cases, loss of TAB2 rather prolongs and increases the activation of TAK1 (Morioka et al. 2014). This is attributed to the additional function of TAB2 to recruit protein phosphatases to terminate TAK1 signaling cascade in order to avoid unnecessary sustained activation of TAK1 (Kajino et al. 2006).
Role of TAK1 in Cytokine and Environmental Stressor Signalings
Transforming growth factor-β (TGF-β) triggers several intracellular signal transduction pathways to induce cell proliferation, differentiation, migration, and survival. TGF-β ligand binds to and activates Ser/Thr kinase receptors, which lead to the phosphorylation and the activation of receptor-regulated Smad family (which is called R-Smad), Smad2 and Smad3. Phosphorylated R-Smads interact with the comediator Smad (which is called Co-Smad), Smad4. This (R-smad/Co-smad) complex translocates into the nucleus and regulates expression of the TGF-β-responsive genes. As described above, TAK1 is catalytically activated by TGF-β stimulation. TGF-β activates the TAK1-MKK6-p38 kinase cascade leading to the phosphorylation of ATF-2, which induces interaction with Smad4 where the complex modulates transcription of TGF-β-responsive genes (Hanafusa et al. 1999). In addition to activation of TAK1-MAPKK-MAPK pathway by TGF-β, it has been shown that TAK1 also regulates SnoN oncoprotein. SnoN is found to be an inhibitor of TGF-β signaling where SnoN recruits a transcriptional repressor complex to block Smad-dependent transcriptional activation of TGF-β-responsive genes. TGF-β stimulation causes SnoN protein degradation, allowing the induction of TGF-β target genes. In this signaling cascade, TAK1 interacts with and phosphorylates SnoN, and this phosphorylation destabilizes SnoN. Indeed, inactivation of TAK1 has been shown to prevent TGF-β-induced SnoN degradation and impairs subsequent induction of the TGF-β-responsive genes (Kajino et al. 2007).
TAK1 Control of Programmed Cell Death
During the past decade, it has been become increasingly evident that TAK1 is a central regulator of programmed cell death. TAK1 inhibition leads to cell death in response to a variety of stressors. Among them, regulation of TNFα-induced cell death by TAK1 has been well studied and characterized. Following TNFα receptor complex formation (Complex I, described above), under some circumstances such as lack of survival pathway activation, the complex dissociates from TNFR1, leading to the formation of another complex including TRADD, FAS-associated protein with a death domain (FADD), RIPK1, and, more importantly, caspase 8 (known as Complex IIa) (Fig. 2). Within the complex, caspase 8 is dimerized, self-cleaved, and activated to further activate downstream executioner caspases such as caspase 3, which leads to enzymatic digestion of intracellular substrates to execute apoptotic cell death. Furthermore, if caspase 8 is perturbed by gene deletion or inhibited by a viral protein CrmA or by pharmacological inhibitors, TNFα induces of third shift of protein complexes toward Complex IIb that include FADD, RIPK1, and RIPK3 (Fig. 2). RIPK3 and RIPK1 phosphorylate and activate each other, resulting in the recruitment of another cell death mediator, mixed lineage kinase domain-like (MLKL). MLKL has been proposed to form a pore on the plasma membrane to govern cell death, which is often called necroptosis, in order to ensure cell death when apoptosis fails to be activated (Wallach et al. 2016). In this signaling cascade, inhibition of TAK1 causes caspase 8 and caspase 3 activation in response to TNFα, which suggests that TAK1 inhibits caspase activation to block apoptotic cell death (Morioka et al. 2014). Thus, TAK1 is likely to blunt the formation or activation of Complex IIa. Recent studies revealed two major mechanisms by which TAK1 inhibits caspase activation (Fig. 2). One pathway is TAK1-NF-κB pathway, which is important for production of caspase inhibitory molecules such as cIAPs and cFLIP (Mihaly et al. 2014b). The other pathway is TAK1-ROS pathway. It has been well documented that loss of TAK1 accumulates reactive oxygen species (ROS) following TNFα stimulation, which triggers caspase cascade activation (Morioka et al. 2009). The source of ROS and the mechanism by which TAK1 regulates ROS level have not been elucidated and warrant further study.
A recent study has revealed that TAK1 is not only involved in caspase-dependent cell death but also necroptosis. As described above, phosphorylation of RIPK1 and RIPK3 are important for stable RIPK1-RIPK3 complex formation and subsequent execution of necroptotic cell death. A surprising finding regarding the regulation of RIPK1 and RIPK3 phosphorylation came from the analysis of TAB2-deficient fibroblasts which exhibit sustained and hyper-activation of TAK1 following TNFα stimulation. Despite the presence of TAK1 activation, TNFα is able to induce cell death in TAB2-deficient fibroblasts. This cell death can be completely blocked by RIPK3 deletion suggesting necroptotic cell death. Furthermore, it has been shown that activation of TAK1 through overexpression induces RIPK3 phosphorylation and necroptosis. Moreover, additional deletion of TAK1 in TAB2-deficient fibroblasts solely induces apoptosis following TNFα stimulation. These results collectively reveal an unexpected role of TAK1 in cell death pathways as an inducer of necroptosis. In summary, despite the fact that loss of TAK1 induces apoptosis, aberrant activation of TAK1 triggers another type of programmed cell death (necroptosis). Although it is clear that TAK1 promotes RIPK3 activity, the importance of TAK1 in necroptosis needs to be carefully considered. Our results suggest that TAK1 deletion induces caspase 8 activation but, even when caspase 8 is blocked, cells still undergo cell death. In line with this, combined inhibition of caspase and RIPK3 partially rescues TNFα-induced cell death in TAK1-deficient cells (Morioka et al. 2014). These results suggest that, although the default pathway of cell death in TAK1-deficient cells is apoptosis, further blocking caspase 8 activity might engage an alternative RIPK1-RIPK3 necroptosis pathway. In addition, several studies suggest that TAK1 deletion induces both apoptosis and necroptosis (Lamothe et al. 2013). Therefore, molecular composition of the complex leading to necroptosis might be different dependent on the presence or absence of TAK1.
Physiological Importance of TAK1 Signal Transduction Pathway
The physiologic importance of TAK1 and TAK1-binding proteins has been revealed using a number of knockout mouse models. TAK1 is ubiquitously expressed in the human body and basally active to some extent (Omori et al. 2012). TAK1 signaling deficit has been shown to cause severe damage to tissues and even mouse mortality emphasizing the importance of TAK1 in the maintenance of tissue homeostasis. For instance, epithelial cell-specific deletion of TAK1 in the epidermis or intestinal epithelium causes caspase activation and cell death leading to severe inflammation, which resembles psoriasis in the case of epidermal TAK1 deletion, and inflammatory bowel disease (IBD) in intestinal epithelium TAK1 deletion (Omori et al. 2008; Kajino-Sakamoto et al. 2008). Double deletion of TAB1 and TAB2 in these tissues severely abolishes TAK1 activity presumably due to failed response to a variety of upstream stimuli in vivo and induces a phenotype similar to the TAK1 KO phenotype (Omori et al. 2012). TAK1 in endothelial tissues is important for prevention of blood vessel death and regression (Morioka et al. 2012). Hematopoietic-specific TAK1 deletion depletes hematopoietic progenitor cells, T cells, and macrophages (Mihaly et al. 2014b). Deletion of TNFR1 largely rescues these injuries and animal mortalities, demonstrating that TNFα is the major killer of TAK1-deficient cells in vivo. In line with in vitro studies, these tissue injuries are associated with caspase activation and are not rescued by RIPK3 deletion, indicating that the damaging effects are mediated primarily by apoptosis. Hepatocyte-specific deletion of TAK1 also causes apoptosis and liver injury. However, unlike epithelial and endothelial tissues, TAK1 deficiency in hepatocytes does not cause profound tissue damage and is not associated with immediate animal mortality. Instead, TAK1-deficient hepatocyte death induces inflammation and promotes compensatory proliferation, which eventually leads to hepatocellular carcinoma within 6 weeks (Morioka et al. 2016). TAB2-deficient hepatocytes reduce lipopolysaccharides (LPS)-induced TAK1 activation in hepatocyte and exacerbates LPS-dependent liver damage (Ikeda et al. 2014). TAK1-deficient satellite cells exhibit increased oxidative stress and undergo spontaneous cell death, which perturbs muscle regeneration in adult mice (Ogura et al. 2015). Besides the regulation of apoptosis by TAK1, it was also recently shown that TAB2-deletion exaggerates wound-induced cell death and delays wound healing in vivo. Given that TAB2 deletion potentially results in hyperactivation of TAK1 in dermal fibroblasts as described above and the delay in wound healing is restored by blocking necroptosis, TAK1 is likely to be actively involved in induction of necroptosis at least in the wound response (Morioka et al. 2014).
Interestingly, the role of TAK1 in vivo is not limited to regulation cell death. For example, TAK1-deficient neutrophils hyperproliferate rather than dying spontaneously, leading to splenomegaly and myelomonocytic leukemia (Ajibade et al. 2012). Myeloid-specifies deletion of TAK1 leads to osteoporosis in mice (Swarnkar et al. 2014). In addition, accumulating evidence suggest that TAK1 is a central signaling molecule in metabolic regulation. Several studies have reported that TAK1 also activates AMPK and autophagy. TAK1-mediated autophagy and fatty acid oxidation prevent hepatosteatosis and tumorigenesis (Inokuchi-Shimizu et al. 2014). Furthermore, TAK1 has been shown to directly bind and inhibit the activity of sterol regulatory element-binding protein (SREBP). TAK1 deletion in the liver causes hyperactivation of SREBP and induction of lipid generation, leading to hyperlipidema and steatosis, which results in development of hepatocarcinoma (Morioka et al. 2016). Notably, deletion of TAK1 increased ER volume and facilitated ER stress tolerance in cultured cells, which was mediated by upregulation of sterol-regulatory element binding proteins (SREBPs)-dependent lipogenesis. In line with this, central nervous system (CNS)-specific Tak1 deletion upregulated SREBP target lipogenic genes and blocked ER stress in the hypothalamus. Furthermore, CNS-specific TAK1 deletion prevented ER stress-induced hypothalamic leptin resistance and hyperphagic obesity under high fat diet (HFD) (Sai et al. 2016).
TAK1, a member of the MAPKKK family, has emerged as a key regulator of signal transduction cascades. The activity of TAK1 is tightly regulated by its binding partners, TAB proteins, in distinct signaling pathways. As a MAPKKK, TAK1 activates MAPKK and MAPK leading to a variety of biological responses such as cell proliferation, migration, and differentiation. Aside from MAPK pathways, TAK1 has been shown to activate and deactivate a myriad of downstream signaling molecules such as NF-κB transcription factor, AMPK, RIPKs, and SREBPs. Upstream signals include cytokines such as TNFα and IL-1, TLR ligands, BCR, TCR and environmental changes such as osmotic stress, UV irradiation, ischemia, and nutrient starvation. The importance of TAK1 has been extensively studied in these signaling pathways. Among these studies, the most notable functions of TAK1 are regulation of inflammatory responses and cell death. TAK1 senses these stressors and activates signaling pathways leading to production of cytokines to ameliorate the level of insult or to maintain tissue homeostasis. In contrast, when TAK1 function, which is frequently perturbed by pathogens, is lost, cells are programmed to undergo cell death predominantly through apoptosis in order to remove such malfunctioning cells. Therefore, activation and inactivation of TAK1 trigger different signaling pathways, which eventually contribute to the maintenance of tissue homeostasis. In addition, it has been shown that TAK1 inhibition promotes apoptosis and kills cancer cells in vitro as well as in vivo. Therefore, TAK1 is now an attractive molecular target for the treatment of human cancers. Collectively, further study of TAK1 signal transduction cascades will provide us with better understanding of biological events occurring in the body as well as potential therapeutic method for several human diseases such as cancers.
- Inagaki M, Omori E, Kim JY, Komatsu Y, Scott G, Ray MK, Yamada G, Matsumoto K, Mishina Y, Ninomiya-Tsuji J. TAK1-binding protein 1, TAB1, mediates osmotic stress-induced TAK1 activation but is dispensable for TAK1-mediated cytokine signaling. J Biol Chem. 2008;283:33080–6.PubMedPubMedCentralCrossRefGoogle Scholar