Insights into the structural basis of the GADD45beta-mediated inactivation of the JNK kinase, MKK7/JNKK2

NF-kappaB/Rel factors control programmed cell death (PCD), and this control is crucial to oncogenesis, cancer chemoresistance, and antagonism of tumor necrosis factor (TNF) alpha-induced killing. With TNFalpha, NF-kappaB-mediated protection involves suppression of the c-Jun-N-terminal kinase (JNK) cascade, and we have identified Gadd45beta, a member of the Gadd45 family, as a pivotal effector of this activity of NF-kappaB. Inhibition of TNFalpha-induced JNK signaling by Gadd45beta depends on direct targeting of the JNK kinase, MKK7/JNKK2. The mechanism by which Gadd45beta blunts MKK7, however, is unknown. Here we show that Gadd45beta is a structured protein with a predicted four-stranded beta-sheet core, five alpha-helices, and two acidic loops. Association of Gadd45beta with MKK7 involves a network of interactions mediated by its putative helices alpha3 and alpha4 and loops 1 and 2. Whereas alpha3 appears to primarily mediate docking to MKK7, loop 1 and alpha4-loop 2 seemingly afford kinase inactivation by engaging the ATP-binding site and causing conformational changes that impede catalytic function. These data provide a basis for Gadd45beta-mediated blockade of MKK7, and ultimately, TNFalpha-induced PCD. They also have important implications for treatment of widespread diseases.     

An antiapoptotic protein, c-FLIPL, directly binds to MKK7 and inhibits the JNK pathway

Inhibition of NF-kappaB activation increases susceptibility to tumor necrosis factor (TNF)alpha-induced cell death, concurrent with caspases and prolonged c-Jun N-terminal kinase (JNK) activation, and reactive oxygen species (ROS) accumulation. However, the detailed mechanisms are unclear. Here we show that cellular FLICE-inhibitory protein (c-FLIP) is rapidly lost in NF-kappaB activation-deficient, but not wild-type fibroblasts upon TNFalpha stimulation, indicating that NF-kappaB normally maintains the cellular levels of c-FLIP. The ectopic expression of the long form of c-FLIP (c-FLIPL) inhibits TNFalpha-induced prolonged JNK activation and ROS accumulation in NF-kappaB activation-deficient fibroblasts. Conversely, TNFalpha induces prolonged JNK activation and ROS accumulation in c-Flip-/- fibroblasts. Moreover, c-FLIPL directly interacts with a JNK activator, MAP kinase kinase (MKK)7, in a TNFalpha-dependent manner and inhibits the interactions of MKK7 with MAP/ERK kinase kinase 1, apoptosis-signal-regulating kinase 1, and TGFbeta-activated kinase 1. This stimuli-dependent interaction of c-FLIPL with MKK7 might selectively suppress the prolonged phase of JNK activation. Taken that ROS promote JNK activation and activation of the JNK pathway may promote ROS accumulation, c-FLIPL might block this positive feedback loop, thereby suppressing ROS accumulation.     

Reactive oxygen species mediate crosstalk between NF-kappaB and JNK

The activation of NF-kappaB inhibits apoptosis via a mechanism involving upregulation of various antiapoptotic genes, such as cellular FLICE-inhibitory protein (c-FLIP), Bcl-xL, A1/Bfl-1, and X chromosome-liked inhibitor of apoptosis (XIAP). In contrast, the activation of c-Jun N-terminal kinase (JNK) promotes apoptosis in a manner that is dependent on the cell type and the context of the stimulus. Recent studies have indicated that one of the antiapoptotic functions of NF-kappaB is to downregulate JNK activation. Further studies have also revealed that NF-kappaB inhibits JNK activation by suppressing accumulation of reactive oxygen species (ROS). In this review, we will focus on the signaling crosstalk between the NF-kappaB and JNK cascades via ROS.     

Co-induction of cell death and survival pathways by phosphoinositide 3-kinase

A single stimulus can induce both the cell death and survival pathway, suggesting that these pathways share common upstream signaling components. In order to define these components, human U937 cells grown in 10% serum were exposed to serum-free media. This treatment resulted in apoptosis, which was found to be mediated by SAPK/JNK. It was previously reported that the serum withdrawal (SW)-induced SAPK activation is mediated by a positive mutual interaction between the reactive oxygen species (ROS) and phosphoinositide 3-kinase (PI3K). This study shows that the ROS/PI3K interaction also induces a NF-kappaB-dependent survival pathway. Despite the role of PI3K, Akt was found to be irrelevant to the activation of SAPK and NF-kappaB. Comparative analyses of SAPK and NF-kappaB for their responses to exogenous H(2)O(2) revealed that SAPK activation requires much higher H(2)O(2) concentrations than those required for NF-kappaB activation. Moreover, high lethal concentrations of H(2)O(2) were found to activate NF-kappaB and SAPK in a PI3K-independent manner. These results suggest that ROS induce both the SAPK-dependent apoptotic and NF-kappaB-mediated survival pathways, and these inducer signals are amplified by PI3K in the SW-triggered pathway. Cell death appears to be favored as this amplification proceeds.     

Reactive oxygen species promote TNFalpha-induced death and sustained JNK activation by inhibiting MAP kinase phosphatases

TNFalpha is a pleiotropic cytokine that induces either cell proliferation or cell death. Inhibition of NF-kappaB activation increases susceptibility to TNFalpha-induced death, concurrent with sustained JNK activation, an important contributor to the death response. Sustained JNK activation in NF-kappaB-deficient cells was suggested to depend on reactive oxygen species (ROS), but how ROS affect JNK activation was unclear. We now show that TNFalpha-induced ROS, whose accumulation is suppressed by mitochondrial superoxide dismutase, cause oxidation and inhibition of JNK-inactivating phosphatases by converting their catalytic cysteine to sulfenic acid. This results in sustained JNK activation, which is required for cytochrome c release and caspase 3 cleavage, as well as necrotic cell death. Treatment of cells or experimental animals with an antioxidant prevents H(2)O(2) accumulation, JNK phosphatase oxidation, sustained JNK activity, and both forms of cell death. Antioxidant treatment also prevents TNFalpha-mediated fulminant liver failure without affecting liver regeneration.     

Different involvement of the mitochondrial, plastidial and cytosolic ascorbate–glutathione redox enzymes in heat shock responses

Plant survival under heat stress requires the activation of proper defence mechanisms to avoid the impairment of metabolic functions. Heat stress leads to the overproduction of reactive oxygen species (ROS) in the cell. In plants, the ascorbate (ASC)-GSH cycle plays a pivotal role in controlling ROS levels and cellular redox homeostasis. Ascorbate peroxidase (APX) is the enzyme of this cycle mainly involved in ROS detoxification. In this study, the ASC-GSH cycle enzymes were analysed in the cytosol, mitochondria and plastids of tobacco Bright Yellow-2 cultured cells. The cells were also subjected to two different heat shocks (HSs; 35 or 55°C for 10 min) and the cell compartments were isolated in both conditions. The results reported here indicate that moderate HS (35°C) does not affect cell viability, whereas cell exposure to 55°C HS induces programmed cell death (PCD). In relation to ASC-GSH cycle, the three analysed compartments have specific enzymatic profiles that are diversely altered by the HS treatments. The cytosol contains the highest activity of all ASC-GSH cycle enzymes and the data reported here suggest that it acts as a redox buffer for the whole cells. In particular, the cytosolic APX seems to be the most versatile enzyme, being its activity enhanced after moderate HS and reduced during PCD induction, whereas the other APX isoenzymes are only affected in the cells undergoing PCD. The relevance of the changes in the different ASC-GSH cycle isoenzymes in allowing cell survival or promoting PCD is discussed.     

NF-kappaB activation prevents apoptotic oxidative stress via an increase of both thioredoxin and MnSOD levels in TNFalpha-treated Ewing sarcoma cells

Repression of activation of c-Jun N-terminal kinase (JNK) participates in the anti-apoptotic effect of nuclear factor-kappaB (NF-kappaB) in TNFalpha-treated Ewing sarcoma cells. As oxidative stress is one of the most prominent activators of JNK, we investigated the relationship between TNFalpha-induced NF-kappaB activation and the control of oxidative stress. Inhibition of NF-kappaB activation resulted in an increase in TNFalpha-induced ROS production, lipid peroxidation and protein oxidation. Those ROS and lipid peroxides were both involved in TNFalpha-induced apoptosis, whereas only ROS elevation triggered sustained JNK activation. TNFalpha increased the level of two antioxidant enzymes, thioredoxin and manganese superoxide dismutase by an NF-kappaB-dependent mechanism. Inhibition of expression or activity of these enzymes sensitized cells to TNFalpha-induced apoptosis, indicating their functional role in protection from cell death. Thus, agents that inhibit activities of these enzymes may prove helpful in the treatment of Ewing tumors.     

Stress is an agonist for the induction of programmed cell death: A review

The prevailing models of stress induced Programmed Cell Death (PCD) posit that excess extracellular chemicals interact with or enter cells and disrupts cellular homeostasis. This activates signalling cascades involving the mitochondria, an increase in the steady state levels of Reactive Oxygen Species (ROS) as well as the activation of Bax and caspases. Further, the increased ROS also causes cellular damage that triggers or enhances PCD responses. The models have been modified in a number of ways, for example to include the existence of caspase and Bax independent forms of PCD. More recently, the ubiquity of ROS has also been challenged in part based on the failure of anti-oxidants to protect from diseases with increased intensity of oxidative stress. Here we focus on a number of other, often overlooked, observations regarding stress mediated responses that may further increase our mechanistic understanding of PCD. These include the concept of the “milieu intérieur” which suggests that cells actively protect themselves (adaptive homeostasis) in part by limiting entry to most extracellular chemicals. Of similar importance, stress also increases the levels of other stress inducible second messengers including ceramide, iron and calcium. This review focuses on the concept that stress is an agonist that conveys information that is transduced into the cell to activate the appropriate genetically encoded cell death and survival responses.     

信号分子介导藻类细胞程序性死亡的研究进展

藻类是水生态系统中的重要初级生产者,在物质转换和能量迁移过程中发挥重要作用。细胞程序性死亡(PCD)作为一种细胞自我调控的死亡模式,受到多种信号分子的控制。研究发现藻类细胞在遭受环境胁迫的情况下,在形态和生理上均表现出类PCD的特征,同时伴随着活性氧/一氧化氮/钙离子(ROS/NO/Ca²⁺)水平的变化。研究认为, ROS/NO/Ca²⁺作为信号分子介导藻细胞内的caspase-like酶活性变化,从而触发藻细胞的类程序性死亡。然而,对信号分子是如何在环境胁迫下的藻类细胞中引发类PCD仍知之甚少。文章综述了信号分子ROS/NO/Ca²⁺介导藻类类PCD的研究进展以及信号分子间的级联关系,并对今后类PCD在该领域待开展的研究进行了展望。     

Mitochondrial c-Jun N-terminal kinase (JNK) signaling initiates physiological changes resulting in amplification of reactive oxygen species generation

The JNK signaling cascade is critical for cellular responses to a variety of environmental and cellular stimuli. Although gene expression aspects of JNK signal transduction are well studied, there are minimal data on the physiological impact of JNK signaling. To bridge this gap, we investigated how JNK impacted physiology in HeLa cells. We observed that inhibition of JNK activity and JNK silencing with siRNA reduced the level of reactive oxygen species (ROS) generated during anisomycin-induced stress in HeLa cells. Silencing p38 had no significant impact on ROS generation under anisomycin stress. Moreover, JNK signaling mediated amplification of ROS production during stress. Mitochondrial superoxide production was shown to be the source of JNK-induced ROS amplification, as an NADPH oxidase inhibitor demonstrated little impact on JNK-mediated ROS generation. Using mitochondrial isolation from JNK null fibroblasts and targeting the mitochondrial scaffold of JNK, Sab, we demonstrated that mitochondrial JNK signaling was responsible for mitochondrial superoxide amplification. These results suggest that cellular stress altered mitochondria, causing JNK to translocate to the mitochondria and amplify up to 80% of the ROS generated largely by Complex I. This work demonstrates that a sequence of events exist for JNK mitochondrial signaling whereby ROS activates JNK, thereby affecting mitochondrial physiology, which can have effects on cell survival and death.     

Tumor necrosis factor-induced nonapoptotic cell death requires receptor-interacting protein-mediated cellular reactive oxygen species accumulation

The mechanism of tumor necrosis factor (TNF)-induced nonapoptotic cell death is largely unknown, although the mechanism of TNF-induced apoptosis has been studied extensively. In wild-type mouse embryonic fibroblast cells under a caspase-inhibited condition, TNF effectively induced cell death that morphologically resembled necrosis. In this study, we utilized gene knockout mouse embryonic fibroblasts cells and found that tumor necrosis factor receptor (TNFR) I mediates TNF-induced necrotic cell death, and that RIP, FADD, and TRAF2 are critical components of the signaling cascade of this TNF-induced necrotic cell death. Inhibitors of NF-kappaB facilitated TNF-induced necrotic cell death, suggesting that NF-kappaB suppresses the necrotic cell death pathway. JNK, p38, and ERK activation seem not to be required for this type of cell death because mitogen-activated protein kinase inhibitors did not significantly affect TNF-induced necrotic cell death. In agreement with the previous reports that the reactive oxygen species (ROS) may play an important role in this type of cell death, the ROS scavenger butylated hydroxyanisole efficiently blocked TNF-induced necrotic cell death. Interestingly, during TNF-induced necrotic cell death, the cellular ROS level was significantly elevated in wild type, but not in RIP(-/-), TRAF2(-/-), and FADD(-/-) cells. These results suggest that RIP, TRAF2, and FADD are crucial in mediating ROS accumulation in TNF-induced necrotic cell death.     

Redox regulation in plant programmed cell death

Programmed cell death (PCD) is a genetically controlled process described both in eukaryotic and prokaryotic organisms. Even if it is clear that PCD occurs in plants, in response to various developmental and environmental stimuli, the signalling pathways involved in the triggering of this cell suicide remain to be characterized. In this review, the main similarities and differences in the players involved in plant and animal PCD are outlined. Particular attention is paid to the role of reactive oxygen species (ROS) as key inducers of PCD in plants. The involvement of different kinds of ROS, different sites of ROS production, as well as their interaction with other molecules, is crucial in activating PCD in response to specific stimuli. Moreover, the importance is stressed on the balance between ROS production and scavenging, in various cell compartments, for the activation of specific steps in the signalling pathways triggering this cell suicide process. The review focuses on the complexity of the interplay between ROS and antioxidant molecules and enzymes in determining the most suitable redox environment required for the occurrence of different forms of PCD.