The tumor necrosis factor-inducible zinc finger protein A20 interacts with TRAF1/TRAF2 and inhibits NF-kappaB activation

A20 inhibits NF-κB activation by dual ubiquitin-editing functions

Deregulation of the transcription factor NF-kappaB can mediate several inflammatory diseases in addition to cancer. Therefore, several proteins, including the zinc finger protein A20, tightly control its activation. Recently, the underlying mechanism by which A20 downregulates NF-kappaB activation in response to the pro-inflammatory cytokine tumor necrosis factor (TNF) has been described. A20 was shown to exert two opposing activities: sequential de-ubiquitination and ubiquitination of the TNF receptor-interacting protein (RIP), thereby targeting RIP to proteasomal degradation.

Antiapoptotic effect both in vivo and in vitro of A20 gene when transfected into rat hippocampal neurons

AIM

To evaluate the antiapoptotic effect of the A20 gene in primary hippocampal neurons both in vivo and in vitro.

METHODS

Primary hippocampal neurons in embryonic day 18 (E18) rats were transfected with the A20 gene by using the new Nucleofector electroporation transfection method. We then examined, whether A20-neurons possessed anti-apoptotic abilities after TNF-alpha stimulation in vitro. A20-neurons and pcDNA3-neurons were transplanted into the penumbra of the brains of rats that had been subjected to 90-min of ischemia induced by left middle cerebral artery occlusion (MCAO).

RESULTS

A20-neurons resisted TNF-alpha induced apoptosis in vitro. The apoptosis rate of neurons overexpressing A20 (28.46%+/-3.87%) was lower than that in neurons transfected with pcDNA3 (53.06%+/-5.36%). More A20-neurons survived in the penumbra both 3-d and 7-d after transplantation than did sham pcDNA3 neurons.

CONCLUSION

The novel function of A20 may make it a potential targets for the gene therapy for neurological diseases.

A novel caspase-2 complex containing TRAF2 and RIP1

The enzymatic activity of caspases is implicated in the execution of apoptosis and inflammation. Here we demonstrate a novel nonenzymatic function for caspase-2 other than its reported proteolytic role in apoptosis. Caspase-2, unlike caspase-3, -6, -7, -9, -11, -12, and -14, is a potent inducer of NF-kappaB and p38 MAPK activation in a TRAF2-mediated way. Caspase-2 interacts with TRAF1, TRAF2, and RIP1. Furthermore, we demonstrate that endogenous caspase-2 is recruited into a large and inducible protein complex, together with TRAF2 and RIP1. Structure-function analysis shows that NF-kappaB activation occurs independent of enzymatic activity of the protease and that the caspase recruitment domain of caspase-2 is sufficient for the activation of NF-kappaB and p38 MAPK. These results demonstrate the inducible assembly of a novel protein complex consisting of caspase-2, TRAF2, and RIP1 that activates NF-kappaB and p38 MAPK through the caspase recruitment domain of caspase-2 independently of its proteolytic activity.

Endogenous Inhibitors of Nuclear Factor-κB, An Opportunity for Cancer Control: Fig. 1

Excessive and prolonged activation of nuclear factor-kappaB (NF-kappaB) has been linked to numerous human diseases, especially cancer, because of the elevated expression of genes encoding antiapoptotic proteins, cytokines, chemokines, cell adhesion molecules, and so on. Eukaryotic cells have developed multiple mechanisms to keep this ubiquitous transcription factor in check. In addition to the inhibitor of kappaB family proteins, a number of endogenous molecules that negatively regulate the activation or activity of NF-kappaB have been identified. These molecules include A20, CYLD, cyPG15-deoxy-Delta(12,14)-prostaglandin J(2), Foxj1, Twist proteins, and beta-arrestins. The extended list of these endogenous inhibitors of NF-kappaB may provide new opportunities for the development of novel strategies for the intervention of malignant transformation. The question to be asked is how NF-kappaB is sustained activated in a number of cancers in which so many antagonists are surrounded.

Luteolin sensitizes tumor necrosis factor-alpha-induced apoptosis in human tumor cells

Tumor necrosis factor-alpha (TNFalpha) activates both cell death and cell survival pathways, which render most cancer cells resistant to its cytotoxicity. In this study, we found that pretreatment with luteolin, a plant flavonoid, greatly sensitized TNFalpha-induced apoptotic cell death in a number of human cancer cell lines; including colorectal cancer COLO205, HCT116 cells and cervical cancer HeLa cells. In the search of the molecular mechanisms responsible for the sensitization effect of luteolin, we discovered that luteolin inhibited TNFalpha-induced activation of nuclear transcription factor-kappa B (NF-kappaB), the main survival factor in TNFalpha signaling. As a result, luteolin suppressed the expression of NF-kappaB-targeted antiapoptotic genes, including A20 and cellular inhibitor of apoptosis protein-1 (c-IAP1). The role of A20 and c-IAP1 was further confirmed by ectopic expression of these two genes, which significantly protected cell death induced by luteolin followed by TNFalpha. In addition, inhibition of NF-kappaB by luteolin led to augmentation and prolongation of c-Jun N-terminal kinase (JNK) activation induced by TNFalpha. Suppression of JNK activation, either by a synthetic JNK inhibitor (SP600125) or by overexpression of the dominant negative forms of JNK kinase 1 (JNKK1) and JNK kinase 2 (JNKK2), conferred significant protection against apoptotic cell death induced by luteolin and TNFalpha, suggesting that NF-kappaB and JNK are closely associated with the sensitization effect of luteolin. Data from this study reveal a novel function of luteolin and enhance the value of luteolin as an anticancer agent.

Signaling to NF-kappaB

The transcription factor NF-kappaB has been the focus of intense investigation for nearly two decades. Over this period, considerable progress has been made in determining the function and regulation of NF-kappaB, although there are nuances in this important signaling pathway that still remain to be understood. The challenge now is to reconcile the regulatory complexity in this pathway with the complexity of responses in which NF-kappaB family members play important roles. In this review, we provide an overview of established NF-kappaB signaling pathways with focus on the current state of research into the mechanisms that regulate IKK activation and NF-kappaB transcriptional activity.

Antioxidant effect of zinc in humans

Oxidative stress is known to be an important contributing factor in many chronic diseases. We tested the hypothesis that in healthy normal volunteers zinc acts as an effective anti-inflammatory and antioxidant agent. Ten normal volunteers were administered daily oral zinc supplementation (45 mg zinc as gluconate) and 10 volunteers received placebo for 8 weeks. Plasma zinc, MDA, HAE, and 8-OHdG levels; LPS-induced TNF-alpha and IL-1beta mRNA; and ex vivo TNF-alpha-induced NF-kappaB activity in mononuclear cells (MNC) were determined before and after supplementation. In subjects receiving zinc, plasma levels of lipid peroxidation products and DNA adducts were decreased, whereas no change was observed in the placebo group. LPS-stimulated MNC isolated from zinc-supplemented subjects showed reduced mRNA for TNF-alpha and IL-1beta compared to placebo. Ex vivo, zinc protected MNC from TNF-alpha-induced NF-kappaB activation. In parallel studies using HL-60, a promyelocytic cell line, we observed that zinc enhances the upregulation of mRNA and DNA-specific binding for A20, a transactivating factor which inhibits the activation of NF-kappaB. Our results suggest that zinc supplementation may lead to downregulation of the inflammatory cytokines through upregulation of the negative feedback loop A20 to inhibit induced NF-kappaB activation. Zinc administration to human subjects with conditions associated with increased oxidative stress should be explored.

Linking JNK signaling to NF-kappaB: a key to survival

In addition to marshalling immune and inflammatory responses, transcription factors of the NF-kappaB family control cell survival. This control is crucial to a wide range of biological processes, including B and T lymphopoiesis, adaptive immunity, oncogenesis and cancer chemoresistance. During an inflammatory response, NF-kappaB activation antagonizes apoptosis induced by tumor necrosis factor (TNF)-alpha, a protective activity that involves suppression of the Jun N-terminal kinase (JNK) cascade. This suppression can involve upregulation of the Gadd45-family member Gadd45beta/Myd118, which associates with the JNK kinase MKK7/JNKK2 and blocks its catalytic activity. Upregulation of XIAP, A20 and blockers of reactive oxygen species (ROS) appear to be important additional means by which NF-kappaB blunts JNK signaling. These recent findings might open up entirely new avenues for therapeutic intervention in chronic inflammatory diseases and certain cancers; indeed, the Gadd45beta-MKK7 interaction might be a key target for such intervention.

The multifaceted roles of TRAFs in the regulation of B-cell function

Tumour-necrosis factor receptor (TNFR)-associated factors (TRAFs) are cytoplasmic adaptor proteins that are important in lymphocyte activation and apoptosis. Many studies of TRAFs have used models of exogenous overexpression by non-lymphoid cells. However, the actions of TRAFs present at normal levels in lymphoid cells often differ considerably from those that have been established in non-lymphocyte overexpression models. As I discuss here, information obtained from studying these molecules in physiological settings in B cells reveals that they have several roles, which are both unique and overlapping. These include activation of kinases and transcription factors, and interactions with other signalling proteins, culminating in the induction or inhibition of biological functions.

Angiotensin II early regulated genes in H295R human adrenocortical cells

Evidence for the dysregulation of aldosterone synthesis in cardiovascular pathophysiology has renewed interest in the control of its production. Cellular mechanisms by which angiotensin II (ANG II) stimulates aldosterone synthesis in the adrenal zona glomerulosa are incompletely understood. To elucidate the mechanism of intracellular signaling by ANG II stimulation in the adrenal, we have studied immediate-early regulated genes in human adrenal H295R cells using cDNA microarrays. H295R cells were stimulated with ANG II for 3 h. Gene expression was analyzed by microarray technology and validated by real-time RT-PCR. Eleven genes were found to be upregulated by ANG II. These encode the proteins for ferredoxin, Nor1, Nurr1, c6orf37, CAT-1, A20, MBLL, M-Ras, RhoB, GADD45α, and a novel protein designated FLJ45273 . Maximum expression levels for all genes occurred 3–6 h after ANG II stimulation. This increase was dose dependent and preceded maximal aldosterone production. Other aldosterone secretagogues, K+and endothelin-1 (ET-1), also induced the expression of these genes with variable efficiency depending on the gene and with lower potency than ANG II. ACTH had negligible effect on gene expression except for the CAT-1 and Nurr1 genes. These ANG II-stimulated genes are involved in several cellular functions and are good candidate effectors and regulators of ANG II-mediated effects in adrenal zona glomerulosa.

A20 is a potent inhibitor of TLR3- and Sendai virus-induced activation of NF-κB and ISRE and IFN-β promoter

Toll-like receptor 3 (TLR3) recognizes dsRNA generated during viral infection and activation of TLR3 results in induction of type I interferons (IFNs) and cellular anti-viral response. TLR3 is associated with a TIR domain-containing adapter protein TRIF, which activates distinct downstream pathways leading to activation of NF-kappaB and ISRE sites in the promoters of type I IFNs. We show here that A20, a NF-kappaB-inducible zinc finger protein that has been demonstrated to be an inhibitor of TNF-induced NF-kappaB activation and a physiological suppressor of inflammatory response, potently inhibited TLR3- and Sendai virus-mediated activation of ISRE and NF-kappaB and IFN-beta promoter in reporter gene assays. A20 also inhibited TRIF-, but not its downstream signaling components TBK1-, IKKbeta-, and IKKepsilon-mediated activation of ISRE and NF-kappaB and IFN-beta promoter. Moreover, A20 interacted with TRIF in co-immunoprecipitation experiments. Finally, expression of A20 could be induced at protein level by Sendai virus infection. These data suggest that A20 targets TRIF to inhibit TLR3-mediated induction of IFN-beta transcription and functions as a feedback negative regulator for TLR3 signaling and cellular anti-viral response.

The ubiquitin-modifying enzyme A20 is required for termination of Toll-like receptor responses

A20 is a cytoplasmic protein required for the termination of tumor necrosis factor (TNF)-induced signals. We show here that mice doubly deficient in either A20 and TNF or A20 and TNF receptor 1 developed spontaneous inflammation, indicating that A20 is also critical for the regulation of TNF-independent signals in vivo. A20 was required for the termination of Toll-like receptor-induced activity of the transcription factor NF-kappaB and proinflammatory gene expression in macrophages, and this function protected mice from endotoxic shock. A20 accomplished this biochemically by directly removing ubiquitin moieties from the signaling molecule TRAF6. The critical function of this deubiquitinating enzyme in the restriction of TLR signals emphasizes the importance of the regulation of ubiquitin conjugation in innate immune cells.

A20 protects endothelial cells from TNF-, Fas-, and NK-mediated cell death by inhibiting caspase 8 activation

A20 is a stress response gene in endothelial cells (ECs). A20 serves a dual cytoprotective function, protecting from tumor necrosis factor (TNF)-mediated apoptosis and inhibiting inflammation via blockade of the transcription factor nuclear factor-kappaB (NF-kappaB). In this study, we evaluated the molecular basis of the cytoprotective function of A20 in EC cultures and questioned whether its protective effect extends beyond TNF to other apoptotic and necrotic stimuli. Our data demonstrate that A20 targets the TNF apoptotic pathway by inhibiting proteolytic cleavage of apical caspases 8 and 2, executioner caspases 3 and 6, Bid cleavage, and release of cytochrome c, thus preserving mitochondrion integrity. A20 also protects from Fas/CD95 and significantly blunts natural killer cell-mediated EC apoptosis by inhibiting caspase 8 activation. In addition to protecting ECs from apoptotic stimuli, A20 safeguards ECs from complement-mediated necrosis. These data demonstrate, for the first time, that the cytoprotective effect of A20 in ECs is not limited to TNF-triggered apoptosis. Rather, A20 affords broad EC protective functions by effectively shutting down cell death pathways initiated by inflammatory and immune offenders.

Study of Arabidopsis thaliana resistome in response to cucumber mosaic virus infection using whole genome microarray

The plant innate immune response is mediated by resistance (R) genes and involves hypersensitive response (HR) cell death. During resistance responses, the host undergoes net changes in the transcriptome. To understand these changes, we generated a whole genome transcript profile for RCY1-mediated resistance to cucumber mosaic virus strain Y (CMV-Y) in Arabidopsis. Using a very stringent selection criterion, we identified 444 putative factors belonging to nine different functional classes that show significant transcript regulation during Arabidopsis-CMV-Y interaction. Genes with unknown function formed the largest class. Other functional classes represented in the resistome include kinases and phosphatases, protein degradation machinery/proteases, transcriptional regulators, and others. Interestingly, several of the unknown function genes possess well characterized domains and secondly many genes encode small peptides with less than 100 amino acids. Analysis of 1.1 kb promoter regions of the 444 genes revealed that 9 out of the 12 known cis-binding elements are significantly associated with pathogen responsive cluster. Location and distribution of five prominent binding elements for select group of disease resistance related and unknown function genes is presented. The analysis also revealed 80 defense-responsive genes that might participate in R gene-mediated defense against both viral and bacterial pathogens. In addition, chromosome distribution of genes that respond to bacterial and viral pathogens suggests that they are located in small gene clusters and may be transcriptionally co-regulated. Exploring the precise function of the new genes identified in this analysis will offer new insights into plant defense.

Zinc-finger protein A20, a regulator of inflammation and cell survival, has de-ubiquitinating activity

Ubiquitination regulates the stability and/or activity of numerous cellular proteins. The corollary is that de-ubiquitinating enzymes, which 'trim' polyubiquitin chains from specific substrate proteins, play key roles in controlling fundamental cellular activities. Ubiquitin is essential at several stages during the activation of NF-kappaB (nuclear factor kappaB), a central co-ordinator of inflammation and other immune processes. Ubiquitination is known to cause degradation of the inhibitory molecule IkappaBalpha (inhibitor of kappaB). In addition, activation of TRAF (tumour-necrosis-factor-receptor-associated factor) and IKKgamma (IkappaB kinase gamma)/NEMO (NF-kappaB essential modifier) signal adaptors relies on their modification with 'nonclassical' forms of polyubiquitin chains. Ubiquitin also plays a key role in determining cell fate by modulating the stability of numerous pro-apoptotic or anti-apoptotic proteins. The zinc-finger protein A20 has dual functions in inhibiting NF-kappaB activation and suppressing apoptosis. The molecular mechanisms of these anti-inflammatory and cytoprotective effects are unknown. Here we demonstrate that A20 is a de-ubiquitinating enzyme. It contains an N-terminal catalytic domain that belongs to the ovarian-tumour superfamily of cysteine proteases. A20 cleaved ubiquitin monomers from branched polyubiquitin chains linked through Lys48 or Lys63 and bound covalently to a thiol-group-reactive, ubiquitin-derived probe. Mutation of a conserved cysteine residue in the catalytic site (Cys103) abolished these activities. A20 did not have a global effect on ubiquitinated cellular proteins, which indicates that its activity is target-specific. The biological significance of the catalytic domain is unknown.

Caspase-1 Activates Nuclear Factor of the κ-Enhancer in B Cells Independently of Its Enzymatic Activity

The proteolytic activity of caspases is involved in apoptosis and inflammation. In this regard, caspase-1 is required for pro-interleukin (IL)-1beta and pro-IL-18 maturation. We report here on a novel function of caspase-1 as an activator of nuclear factor of the kappa-enhancer in B-cells (NF-kappaB) and p38 mitogen-activated protein kinase (MAPK). This function is not shared by the murine caspase-1 homologues caspase-11 and -12. In contrast to pro-IL-1beta maturation, caspase-1-induced NF-kappaB activation is not inhibited by the virus-derived caspase-1 inhibitor cytokine response modifier A and is equally induced by the enzymatically inactive caspase-1 C285A mutant. Although the general NF-kappaB-inhibiting protein A20 inhibits caspase-1-derived activation of NF-kappaB, dominant-negative forms of TRAF2 and RIP1 have no effect. We demonstrate that caspase-1 interacts with RIP2 and that dominant-negative forms of RIP2 and IkappaB kinase complex-beta inhibit caspase-1-mediated NF-kappaB activation. Structure-function analysis shows that the caspase recruitment domain of caspase-1 mediates the activation of NF-kappaB and p38 MAPK. These data demonstrate that caspase-1 contributes to inflammation by two distinct pathways: proteolysis of pro-IL-1beta, and RIP2-dependent activation of NF-kappaB and p38 MAPK mediated by the caspase recruitment domain.

Differential regulation of gene expression following CD40 activation of leukemic compared to healthy B cells

It is possible to differentiate malignant from healthy cells and to classify diseases based on identification of specific gene expression profiles. We hypothesized that gene expression profiling could also be used to identify differential activation of healthy and malignant cells, and as a model for this, we examined the molecular sequelae of CD40 activation of healthy and B-cell chronic lymphocytic leukemia (CLL) cells. Hierarchical clustering analysis of gene expression signatures grouped samples by CD40 activation status and further subclassified CD40-activated CLL cells from healthy B cells. Supervised analyses in healthy B cells compared to CLL cells identified differential regulation of genes governing cell cycle progression and apoptosis. CD40 signaling of CLL cells increases their susceptibility to immune recognition, but promotes survival and cell cycle arrest, making these cells potentially more resistant to chemotherapy. These results illustrate the utility of gene expression profiling to elucidate the molecular sequelae of signaling in healthy cells and altered signaling pathways in malignant cells. This type of approach should be useful to identify targets of therapy of malignant diseases.

Mathematical model of NF-kappaB regulatory module

The two-feedback-loop regulatory module of nuclear factor kappaB (NF-kappaB) signaling pathway is modeled by means of ordinary differential equations. The constructed model involves two-compartment kinetics of the activators IkappaB (IKK) and NF-kappaB, the inhibitors A20 and IkappaBalpha, and their complexes. In resting cells, the unphosphorylated IkappaBalpha binds to NF-kappaB and sequesters it in an inactive form in the cytoplasm. In response to extracellular signals such as tumor necrosis factor or interleukin-1, IKK is transformed from its neutral form (IKKn) into its active form (IKKa), a form capable of phosphorylating IkappaBalpha, leading to IkappaBalpha degradation. Degradation of IkappaBalpha releases the main activator NF-kappaB, which then enters the nucleus and triggers transcription of the inhibitors and numerous other genes. The newly synthesized IkappaBalpha leads NF-kappaB out of the nucleus and sequesters it in the cytoplasm, while A20 inhibits IKK converting IKKa into the inactive form (IKKi), a form different from IKKn, no longer capable of phosphorylating IkappaBalpha. After parameter fitting, the proposed model is able to properly reproduce time behavior of all variables for which the data are available: NF-kappaB, cytoplasmic IkappaBalpha, A20 and IkappaBalpha mRNA transcripts, IKK and IKK catalytic activity in both wild-type and A20-deficient cells. The model allows detailed analysis of kinetics of the involved proteins and their complexes and gives the predictions of the possible responses of whole kinetics to the change in the level of a given activator or inhibitor.

A20 inhibits toll-like receptor 2- and 4-mediated interleukin-8 synthesis in airway epithelial cells

The zinc finger protein A20 is encoded by an immediate early response gene and acts as an inhibitor of nuclear factor (NF)-kappaB-dependent gene expression induced by different stimuli, including tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta). Toll-like receptor 2 (TLR2) and TLR4 have been found to transduce, respectively, peptidoglycan (PGN) and lipopolysaccharide (LPS) signals for the activation of NF-kappaB and the production of inflammatory cytokines. Here, we have examined the role of A20 in TLR-mediated NF-kappaB-dependent gene expression in human airway epithelial cells (AECs). Stimulation with LPS and PGN resulted in a significant increase in the level of A20 mRNA in primary cultured AECs and in NCI-H292 AECs. LPS and PGN induced activation of the IL-8 promoter both in NCI-H292 AECs and in HEK293 cells expressing either TLR2 or TLR4 plus MD-2. Dominant-negative myeloid differentiation protein and a mutant form of IkappaBalpha attenuated this PGN- or LPS-induced activation of the IL-8 promoter. Furthermore, overexpression of A20 inhibited activation of both NF-kappaB and the IL-8 promoter by PGN or LPS in these cells. Taken together, our results suggest that A20 may function as a negative regulator of TLR-mediated inflammatory responses in the airway, thereby protecting the host against harmful overresponses to pathogens.

Overexpression of a zinc-finger protein gene from rice confers tolerance to cold, dehydration, and salt stress in transgenic tobacco

Stress perception and signal transduction leading to tolerance involve a complex interplay of different gene products. We describe here the isolation and characterization of an intronless gene (OSISAP1) from rice encoding a zinc-finger protein that is induced after different types of stresses, namely cold, desiccation, salt, submergence, and heavy metals as well as injury. The gene is also induced by stress hormone abscisic acid. Overexpression of the gene in transgenic tobacco conferred tolerance to cold, dehydration, and salt stress at the seed-germination/seedling stage as reflected by the percentage of germination/green seedlings, the fresh weight of seedlings, and their developmental pattern. Thus, OSISAP1 seems to be an important determinant of stress response in plants.

ZNF216 Is an A20-like and IκB Kinase γ-Interacting Inhibitor of NFκB Activation

The transcription factor NFkappaB plays important roles in immune regulation, inflammatory responses, and anti-apoptosis. Activation of NFkappaB requires the activity of IkappaB kinase, a kinase complex that contains two catalytic subunits, IKKalpha and IKKbeta, and a non-enzymatic regulatory subunit, IKKgamma. To understand how NFkappaB activation is regulated at the IKKgamma level, we searched for IKKgamma-interacting proteins by the yeast two-hybrid system. This search identified ZNF216, a zinc finger protein with unknown biological functions. ZNF216 contains an A20-like zinc finger domain (ZnF-A20) at its N terminus and an AN1-like domain (ZnF-AN1) at its C terminus. Similar to A20, ZNF216 interacted with IKKgamma, RIP, and TRAF6 in co-immunoprecipitation experiments. Domain mapping experiments indicated that the ZnF-A20 domain was responsible for interacting with IKKgamma and RIP, whereas the ZnF-AN1 domain interacted with TRAF6. ZNF216 inhibited NFkappaB activation triggered by overexpression of RIP and TRAF6 but not of p65. ZNF216 also inhibited tumor necrosis factor (TNF)-, interleukin-1-, and Toll-like receptor 4-induced NFkappaB activation in a dose-dependent manner. The ZnF-A20 domain was essential for ZNF216-mediated inhibition of NFkappaB activation. The ZnF-A20 and ZnF-AN1 domains of ZNF216 could interact with each other, whereas ZNF216 could form homo-oligomers or hetero-oligomers with A20. Unlike A20, which inhibits TNF-induced apoptosis, overexpression of ZNF216 sensitized cells to TNF-induced apoptosis. Our findings suggest that ZNF216 and A20 have redundant and distinct roles in regulating NFkappaB activation and apoptosis.

Identification of potential stroke targets by lentiviral vector mediated overexpression of HIF-1 alpha and HIF-2 alpha in a primary neuronal model of hypoxia

The identification of genes differentially regulated by ischemia will lead to an improved understanding of cell death pathways such as those involved in the neuronal loss observed following a stroke. Furthermore, the characterization of such pathways could facilitate the identification of novel targets for stroke therapy. We have used a novel approach to amplify differential gene expression patterns in a primary neuronal model of stroke by employing a lentiviral vector system to specifically bias the transcriptional activation of hypoxically regulated genes. Overexpression of the hypoxia-induced transcription factor subunits HIF-1 alpha and HIF-2 alpha elevated hypoxia-mediated transcription of many known HIF-regulated genes well above control levels. Furthermore, many potentially novel HIF-regulated genes were discovered that were not previously identified as hypoxically regulated. Most of the novel genes identified were activated by a combination of HIF-2 alpha overexpression and hypoxic insult. These included several genes with particular importance in cell survival pathways and of potential therapeutic value. Hypoxic induction of HIF-2 alpha may therefore be a critical factor in mediating protective responses against ischemic injury. Further investigation of the genes identified in this study may provide increased understanding of the neuronal response to hypoxia and may uncover novel therapeutic targets for the treatment of cerebral ischemia.

NF kappa B controls the balance between Fas and tumor necrosis factor cell death pathways during T cell receptor-induced apoptosis via the expression of its target gene A20

Activation-induced cell death (AICD), a term originally coined for the anti-CD3-induced apoptosis of T cell hybridomas and thymocytes, is predominantly driven by death receptors and has been involved in the control of autoreactive T cells in the periphery. In the Do-11.10 T cell hybridoma model of AICD, activation of the T cell receptor (TCR) results in Fas-dependent apoptosis. Here, we show that inhibition of the transcription factor nuclear factor kappa B (NF kappa B) in Do-11.10 cells resulted in increased sensitivity to TCR-mediated apoptosis, correlating with defective induction of the anti-apoptotic NF kappa B target gene A20. Stable expression of the zinc finger protein A20 in NF kappa B-negative Do-11.10 cells rescued the phenotype. TCR activation in NF kappa B-deficient Do-11.10 cells resulted predominantly in tumor necrosis factor (TNF) receptor 2 (TNFR2)-dependent bystander cell death rather than classical Fas-dependent AICD. Strikingly, A20 blocked TNF-mediated apoptosis and simultaneously restored TCR-induced Fas-dependent AICD. In addition, NF kappa B downstream of TNFR was required for up-regulation of Fas expression by endogenous TNF secreted in response to TCR stimulation. Together, these results suggest that NF kappa B can play both pro- and anti-apoptotic roles during AICD. We propose that NF kappa B controls the balance between Fas and TNF cell death pathways during AICD via the expression of the zinc finger protein A20.

Synergy between all-trans retinoic acid and tumor necrosis factor pathways in acute leukemia cells

The nuclear receptor ligand all-trans retinoic acid (ATRA) causes dramatic terminal differentiation of acute promyelocytic leukemia (APL) cells in vitro and in patients, but it is less active in other malignancies. However, downstream mediators of the effects of ATRA are not well understood. We used a cDNA microarray to search for ATRA-regulated genes in the APL cell line NB4 and found that ATRA regulated several members of the tumor necrosis factor (TNF) pathway. Here we show that TNF can synergize with ATRA to induce differentiation, showing monocytic characteristics more typical of differentiation mediated by TNF than by ATRA. ATRA and TNF can also induce differentiation of the non-APL cell line U937. Underlying this response was an increase in TNF-induced nuclear factor-kappaB (NF-kappaB) DNA binding within 2 hours in the presence of ATRA and activation of NF-kappaB DNA binding and transcriptional activity in response to ATRA alone within 48 hours of ATRA treatment. Furthermore, we found a synergistic induction of the NF-kappaB target genes BCL-3, Dif-2, and TNF receptor 2 (TNFR2) in response to the combination of TNF and ATRA. These genes have been previously shown to play a role in TNF signaling, and amplification of such genes may represent a mechanism whereby TNF and ATRA can act synergistically. We propose that ATRA can prime cancer cells for differentiation triggered by TNF and suggest that targeting the TNF pathway in combination with ATRA may represent a novel route to treat leukemias.

A novel type of deubiquitinating enzyme

A previous report from this laboratory described two novel proteins that have sequence similarity to A20, a negative regulator of NF-kappaB (Evans, P. C., Taylor, E. R., Coadwell, J., Heyninck, K., Beyaert, R., and Kilshaw, P. J. (2001) Biochem. J. 357, 617-623). One of these molecules, cellular zinc finger anti-NF-kappaB (Cezanne), a 100-kDa cytoplasmic protein, also suppressed NF-kappaB. Here we demonstrate that Cezanne is a novel deubiquitinating enzyme, distinct from the two known families of deubiquitinases, Types I and II. We show that Cezanne contains an N-terminal catalytic domain that belongs to the recently discovered ovarian tumor protein (OTU) superfamily, a group of proteins displaying structural similarity to cysteine proteases but having no previously described function. Recombinant Cezanne cleaved ubiquitin monomers from linear or branched synthetic ubiquitin chains and from ubiquitinated proteins. Mutation of a conserved cysteine residue in the catalytic site of the proteolytic domain caused Cezanne to co-precipitate polyubiquitinated cellular proteins. We also provide evidence for an additional ubiquitin binding site in the C-terminal part of the molecule. Our data provide the first demonstration of functional activity among OTU proteins.

Role of SODD in regulation of tumor necrosis factor responses

Signaling from tumor necrosis factor receptor type 1 (TNFR1) can elicit potent inflammatory and cytotoxic responses that need to be properly regulated. It was suggested that the silencer of death domains (SODD) protein constitutively associates intracellularly with TNFR1 and inhibits the recruitment of cytoplasmic signaling proteins to TNFR1 to prevent spontaneous aggregation of the cytoplasmic death domains of TNFR1 molecules that are juxtaposed in the absence of ligand stimulation. In this study, we demonstrate that mice lacking SODD produce larger amounts of cytokines in response to in vivo TNF challenge. SODD-deficient macrophages and embryonic fibroblasts also show altered responses to TNF. TNF-induced activation of NF-kappaB is accelerated in SODD-deficient cells, but TNF-induced c-Jun N-terminal kinase activity is slightly repressed. Interestingly, the apoptotic arm of TNF signaling is not hyperresponsive in the SODD-deficient cells. Together, these results suggest that SODD is critical for the regulation of TNF signaling.

The signaling adaptors and pathways activated by TNF superfamily

Members of the TNF receptor superfamily play pivotal roles in numerous biological events in metazoan organisms. Ligand-mediated trimerization by corresponding homo- or heterotrimeric ligands, the TNF family ligands, causes recruitment of several intracellular adaptors, which activate multiple signal transduction pathways. While recruitment of death domain (DD) containing adaptors such as Fas associated death domain (FADD) and TNFR associated DD (TRADD) can lead to the activation of a signal transduction pathway that induces apoptosis, recruitment of TRAF family proteins can lead to the activation of transcription factors such as, NF-kappaB and JNK thereby promoting cell survival and differentiation as well as immune and inflammatory responses. Individual TNF receptors are expressed in different cell types and have a range of affinities for various intracellular adaptors, which provide tremendous signaling and biological specificities. In addition, numerous signaling modulators are involved in regulating activities of signal transduction pathways downstream of receptors in this superfamily. Most of the TNF receptor superfamily members as well as many of their signaling mediators, have been uncovered in the last two decades. However, much remains unknown about how individual signal transduction pathways are regulated upon activation by any particular TNF receptor, under physiological conditions.

Otubains: a new family of cysteine proteases in the ubiquitin pathway

The modification of cellular proteins by ubiquitin (Ub) is an important event that underlies protein stability and function in eukaryotes. Protein ubiquitylation is a dynamic and reversible process; attached Ub can be removed by deubiquitylating enzymes (DUBs), a heterogeneous group of cysteine proteases that cleave proteins precisely at the Ub-protein bond. Two families of DUBs have been identified previously. Here, we describe new, highly specific Ub iso-peptidases, that have no sequence homology to known DUBs, but which belong to the OTU (ovarian tumour) superfamily of proteins. Two novel proteins were isolated from HeLa cells by affinity purification using the DUB-specific inhibitor, Ub aldehyde (Ubal). We have named these proteins otubain 1 and otubain 2, for OTU-domain Ubal-binding protein. Functional analysis of otubains shows that the OTU domain contains an active cysteine protease site.

Regulation of Toll-like receptor 4 signalling by A20 zinc finger protein

A20 is a zinc finger protein that renders cells resistant to apoptosis. However, the recent demonstration that A20-deficient mice develop severe inflammation and are hyper-responsive to LPS suggests that A20 may play a key role in regulating the inflammatory response. This study, for the first time, explores the likely mechanism by which A20 can regulate the pro-inflammatory effects of LPS. More specifically it characterises the ability of A20 to modulate TLR-4 signalling since TLR-4 acts as the signalling receptor system for LPS. Full length A20 inhibited the ability of TLR-4 to activate the transcription factors, NF-kappa B and AP-1, and induce the chemokine IL-8. The inhibitory capacity of A20 on NF-kappa B was localised to the C-terminal zinc finger domain of A20 whereas full length A20 was required to effect inhibition of AP-1 and IL-8. Furthermore full length and C-terminal A20 showed similar regulatory effects on MEKK-1 activation of NF-kappa B and AP-1 and induction of IL-8. The findings increase our mechanistic understanding of the anti-inflammatory effects of A20 and suggest that it modulates TLR-4 signalling at or downstream of MEKK-1.

Mutations in the v-Rel transactivation domain indicate altered phosphorylation and identify a subset of NF-kappaB-regulated cell death inhibitors important for v-Rel transforming activity

Consistent with the constitutive activation of Rel/NF-kappaB in human hematopoietic tumors, the v-Rel oncoprotein induces aggressive leukemia/lymphomas in animal models. v-Rel is thus a valuable tool to characterize the role of Rel/NF-kappaB in cancer and the mechanisms involved. Prior studies by our group identified a serine-rich domain in v-Rel that was required for biological activity. Here, we investigated the molecular basis for the transformation defect of specific serine mutants. We show that the transforming efficiency of these mutants in primary lymphoid cells is correlated with their ability to mediate kappaB site-dependent transactivation and with specific changes in phosphorylation profiles. Interestingly, coexpression of the death antagonists Bcl-xL and Bcl-2 significantly increased their oncogenicity, whereas other NF-kappaB-regulated death inhibitors showed little or no effect. The fact that a subset of apoptosis inhibitors could rescue v-Rel transactivation mutants suggests that their reduced transcriptional activity may critically affect expression of defined death antagonists essential for oncogenesis. Consistent with this hypothesis, we observed selection for high endogenous expression of Bcl-2-related death antagonists in cells transformed by weakly transforming v-Rel mutants. These results emphasize the need for Rel/NF-kappaB to efficiently activate expression of a subset of antiapoptotic genes from the Bcl-2 family to manifest its oncogenic phenotype.

Structure-function analysis of the A20-binding inhibitor of NF-kappa B activation, ABIN-1

Nuclear factor kappa B (NF-kappa B)-dependent gene expression plays an important role in numerous cellular processes including stress responses, inflammation and cell proliferation. Therefore, the activity of this transcription factor needs to be tightly regulated. Among others, the NF-kappa B-dependent zinc finger protein A20 is involved in the negative feedback regulation of NF-kappa B activation in response to tumor necrosis factor (TNF). We previously demonstrated that A20 can interact with A20-binding inhibitors of NF-kappa B activation (ABINs), which have the potential to inhibit TNF-induced activation of NF-kappa B upon overexpression. The ABIN proteins were therefore proposed to mediate the NF-kappa B inhibiting function of A20. Here we demonstrate the presence of a short homologous region in ABINs and I kappa B kinase gamma, the regulatory subunit of the I kappa B kinase complex. Site-specific mutagenesis of this region abolished the NF-kappa B inhibiting function of ABIN-1, without affecting the interaction with A20. Furthermore, coexpression of these ABIN-1 mutants interfered in a dominant negative manner with the NF-kappa B inhibiting function of ABIN-1, whereas the A20-mediated inhibition was unaffected. These results suggest that A20 and ABIN-1 probably act independently of their mutual interaction.

Tumor necrosis factor-regulated biphasic activation of NF-kappa B is required for cytokine-induced loss of skeletal muscle gene products

NF-kappaB activation is classically defined as a transient response initiated by the degradation of IkappaB inhibitor proteins leading to nuclear import of NF-kappaB and culminating with the resynthesis of IkappaBalpha and subsequent inactivation of the transcription factor. Although this type of regulation is considered the paradigm for NF-kappaB activation, other regulatory profiles are known to exist. By far the most common of these is chronic or persistent activation of NF-kappaB. In comparison, regulation of NF-kappaB in a biphasic manner represents a profile that is scarcely documented and whose biological significance remains poorly understood. Here we show using differentiated skeletal muscle cells, that tumor necrosis factor (TNF) induces NF-kappaB activation in a biphasic manner. Unlike the first transient phase, which is terminated within 1 h of cytokine addition, the second phase persists for an additional 24-36 h. Biphasic activation is mediated at both the levels of NF-kappaB DNA binding and transactivation function, and both phases are dependent on the IKK/26 S proteasome pathway. We find that regulation of the first transient phase is mediated by the degradation and subsequent resynthesis of IkappaBalpha, as well as by a TNF-induced expression of A20. Second phase activity correlates with persistent down-regulation of both IkappaBalpha and IkappaBbeta proteins, derived from a continuous TNF signal. Finally, we demonstrate that inhibition of NF-kappaB prior to initiation of the second phase of activity inhibits cytokine-mediated loss of muscle proteins. We propose that the biphasic activation of NF-kappaB in response to TNF may play a key regulatory role in skeletal muscle wasting associated with cachexia.

Tumor necrosis factor signaling

A single mouse click on the topic tumor necrosis factor (TNF) in PubMed reveals about 50,000 articles providing one or the other information about this pleiotropic cytokine or its relatives. This demonstrates the enormous scientific and clinical interest in elucidating the biology of a molecule (or rather a large family of molecules), which began now almost 30 years ago with the description of a cytokine able to exert antitumoral effects in mouse models. Although our understanding of the multiple functions of TNF in vivo and of the respective underlying mechanisms at a cellular and molecular level has made enormous progress since then, new aspects are steadily uncovered and it appears that still much needs to be learned before we can conclude that we have a full comprehension of TNF biology. This review shortly covers some general aspects of this fascinating molecule and then concentrates on the molecular mechanisms of TNF signal transduction. In particular, the multiple facets of crosstalk between the various signalling pathways engaged by TNF will be addressed.

NF-kappaB activator Act1 associates with IL-1/Toll pathway adaptor molecule TRAF6

NF-kappaB activator 1 (Act1), also called CIKS, is a recently identified protein with NF-kappaB and AP-1 activation activities through its association with the IkappaB kinase complex. We identified and confirmed that Act1 interacts with tumor necrosis factor receptor-associated factor 6 (TRAF6); notably, Act1 binds to TRAF6 only among TRAF family proteins. The amino-terminal half of Act1 is required for its interaction with the TRAF domain. Act1-mediated NF-kappaB activation was inhibited by a dominant-negative mutant of TRAF6 in a dose-dependent manner, and IL-1-induced NF-kappaB activation was inhibited by a high level of Act1 expression. Our results suggest that Act1 is involved in IL-1/Toll-mediated signaling through TRAF6.

Hijacking of host cell IKK signalosomes by the transforming parasite Theileria

Parasites have evolved a plethora of mechanisms to ensure their propagation and evade antagonistic host responses. The intracellular protozoan parasite Theileria is the only eukaryote known to induce uncontrolled host cell proliferation. Survival of Theileria-transformed leukocytes depends strictly on constitutive nuclear factor kappa B (NF-kappaB) activity. We found that this was mediated by recruitment of the multisubunit IkappaB kinase (IKK) into large, activated foci on the parasite surface. IKK signalosome assembly was specific for the transforming schizont stage of the parasite and was down-regulated upon differentiation into the nontransforming merozoite stage. Our findings provide insights into IKK activation and how pathogens subvert host-cell signaling pathways.

Transcription factors as targets for cancer therapy

A limited list of transcription factors are overactive in most human cancer cells, which makes them targets for the development of anticancer drugs. That they are the most direct and hopeful targets for treating cancer is proposed, and this is supported by the fact that there are many more human oncogenes in signalling pathways than there are oncogenic transcription factors. But how could specific transcription-factor activity be inhibited?

A20 inhibits tumor necrosis factor (TNF) alpha-induced apoptosis by disrupting recruitment of TRADD and RIP to the TNF receptor 1 complex in Jurkat T cells

Tumor necrosis factor receptor 1 (TNFR1) can trigger distinct signaling pathways leading to either the activation of NF-kappaB transcription factors or apoptosis. NF-kappaB activation results in the expression of antiapoptotic genes that inhibit the apoptosis pathway that is activated in parallel. However, the molecular mechanism of this inhibition remains poorly characterized. We have isolated a Jurkat T-cell mutant that exhibits enhanced sensitivity to TNF-induced apoptosis as a result of a deficiency in I-kappaB kinase gamma (IKKgamma)/NEMO, an essential component of the IKK complex and NF-kappaB pathway. We show here that the zinc finger protein A20 is an NF-kappaB-inducible gene that can protect the IKKgamma-deficient cells from TNF-induced apoptosis by disrupting the recruitment of the death domain signaling molecules TRADD and RIP to the receptor signaling complex. Our study, together with reports on the role of other antiapoptotic proteins such as c-FLIP and c-IAP, suggests that, in order to ensure an effective shutdown of the apoptotic pathway, TNF induces multiple NF-kappaB-dependent genes that inhibit successive steps in the TNFR1 death signaling pathway.

The promyelocytic leukemia protein represses A20-mediated transcription

The promyelocytic leukemia (PML) protein is a tumor suppressor that is disrupted by the chromosomal translocation t(15;17), a consistent cytogenetic feature of acute promyelocytic leukemia. A role of PML in multiple pathways of apoptosis was conclusively demonstrated using PML(-/-) animal and cell culture models. In a previous study, we found that PML sensitizes tumor necrosis factor-induced apoptosis in tumor necrosis factor (TNF)-resistant U2OS cells. This finding helped to explain the mechanism of PML-induced apoptosis. The zinc finger protein A20 is a target gene of NF kappa B inducible by TNF alpha, and it is a potent inhibitor of TNF-induced apoptosis. In the this study, we demonstrated that PML is a transcriptional repressor of the A20 promoter and that PML represses A20 expression induced by TNF alpha. We showed that PML inhibits A20 transactivation through the NF kappa B site by interfering with its binding to the promoter. We also showed that stable overexpression of A20 inhibits apoptosis and caspase activation induced by PML/TNF alpha. The results of this study suggest that A20 is a downstream target of PML-induced apoptosis and supports a role of A20 in modulating cell death induced by PML/TNF alpha in TNF-resistant cells.

Regulation of cytokine signaling and inflammation

Inflammation progresses by the action of pro-inflammatory cytokines, including interleukin-1 (IL-1), the tumor necrosis factor (TNF), gamma-interferon (IFNgamma), IL-12, IL-18, and the granulocyte-macrophage colony-stimulating factor, and is resolved by anti-inflammatory cytokines such as IL-4, IL-10, IL-13, IFNalpha, and the transforming growth factor (TGF)beta. The intracellular signal transduction pathways of these cytokines have been studied extensively, and these pathways ultimately activate transcription factors, such as NF-kappaB, Smad, and STATs. Recently, the negative-feedback regulation of these pathways has been identified. In this review, we provide examples of the relationship between cytokine signal transduction, negative-signal regulation, and inflammatory disease models. Furthermore, we illustrate several approaches for treating inflammatory diseases by modulating extracellular and intracellular signaling pathways.

Alpha 5 beta 1 integrin activates an NF-kappa B-dependent program of gene expression important for angiogenesis and inflammation

GeneCalling, a genome-wide method of mRNA profiling, reveals that endothelial cells adhering to fibronectin through the alpha 5 beta 1 integrin, but not to laminin through the alpha 2 beta 1 integrin, undergo a complex program of gene expression. Several of the genes identified are regulated by the NF-kappa B transcription factor, and many are implicated in the regulation of inflammation and angiogenesis. Adhesion of endothelial cells to fibronectin activates NF-kappa B through a signaling pathway requiring Ras, phosphatidylinositol 3-kinase, and Rho family proteins, whereas adhesion to laminin has a limited effect. Retroviral transfer of the superrepressor of NF-kappa B, I kappa B-2A, blocks basic fibroblast growth factor-induced angiogenesis in vivo. These results suggest that engagement of the alpha 5 beta 1 integrin promotes an NF-kappa B-dependent program of gene expression that coordinately regulates angiogenesis and inflammation.

Pleiotropic signal transduction mediated by human CD30: a member of the tumor necrosis factor receptor (TNFR) family

CD30, a member of the tumor necrosis factor receptor (TNFR) family, is a characteristic cell surface receptor for activated T-cells and the malignant cells of Hodgkin's disease (HD), anaplastic large cell lymphoma (ALCL) and a few other non-Hodgkin's lymphomas. As an independent predictor of disease progression and poor prognosis, high serum levels of soluble CD30 (sCD30) have prognostic significance for patients with CD30-positive lymphomas and viral infections. Activation of CD30 by ligand binding or cross-linking with immobilized antibody leads to trimerization of the receptor, recruitment of signaling proteins and transducing of numerous effects. Due to the lack of an intrinsic enzymatic domain, signal transduction is exclusively mediated by the members of the TNFR-associated factor (TRAF) family and the various TRAF-binding proteins. CD30 signaling can induce several pathways including the activation of NFkappaB and the MAP kinases. CD30 mediated signal transduction is capable of promoting cell proliferation and cell survival as well as antiproliferative effects and cell death depending on cell type and co-stimulatory effects. Some data indicate the opposite signaling of CD30 in HD or ALCL cells, while other information point to pleiotropic signaling pathways in both malignancies. The pro and contra of this controversy is discussed in this review.

TNF receptor subtype signalling: differences and cellular consequences

Tumour necrosis factor-alpha (TNF alpha) is a multifunctional cytokine belonging to a family of ligands with an associated family of receptor proteins. The pleiotropic actions of TNF range from proliferative responses such as cell growth and differentiation, to inflammatory effects and the mediation of immune responses, to destructive cellular outcomes such as apoptotic and necrotic cell death mechanisms. Activated TNF receptors mediate the association of distinct adaptor proteins that regulate a variety of signalling processes including kinase or phosphatase activation, lipase stimulation, and protease induction. Moreover, the cytokine regulates the activities of transcription factors, heterotrimeric or monomeric G-proteins and calcium ion homeostasis in order to orchestrate its cellular functions. This review addresses the structural basis of TNF signalling, the pathways employed with their cellular consequences, and focuses on the specific role played by each of the two TNF receptor isotypes, TNFR1 and TNFR2.

A novel zinc finger protein interacts with receptor-interacting protein (RIP) and inhibits tumor necrosis factor (TNF)- and IL1-induced NF-kappa B activation

Receptor-interacting protein (RIP) is a serine/threonine protein kinase that is critically involved in tumor necrosis factor receptor-1 (TNF-R1)-induced NF-kappa B activation. In a yeast two-hybrid screening for potential RIP-interacting proteins, we identified ZIN (zinc finger protein inhibiting NF-kappa B), a novel protein that specifically interacts with RIP. ZIN contains four RING-like zinc finger domains at the middle and a proline-rich domain at the C terminus. Overexpression of ZIN inhibits RIP-, IKK beta-, TNF-, and IL1-induced NF-kappa B activation in a dose-dependent manner in 293 cells. Domain mapping experiments indicate that the RING-like zinc finger domains of ZIN are required for its interaction with RIP and inhibition of RIP-mediated NF-kappa B activation. Overexpression of ZIN also potentiates RIP- and TNF-induced apoptosis. Moreover, immunofluorescent staining indicates that ZIN is a cytoplasmic protein and that it colocalizes with RIP. Our findings suggest that ZIN is an inhibitor of TNF- and IL1-induced NF-kappa B activation pathways.

A novel zinc finger protein that inhibits osteoclastogenesis and the function of tumor necrosis factor receptor-associated factor 6

A variety of surface receptors eliciting diverse cellular responses have been shown to recruit tumor necrosis factor receptor-associated factor (TRAF) adaptor molecules. However, a few TRAF-interacting intracellular proteins that serve as downstream targets or regulators of TRAF function have been identified. In search of new intracellular molecules that bind TRAF6, we carried out a yeast two-hybrid cDNA library screening with an N-terminal segment of TRAF6 as the bait. A novel human C(2)H(2)-type zinc finger family protein was identified, which when coexpressed with TRAF6 led to a suppression of TRAF6-induced activation of NF-kappa B and c-Jun N-terminal kinase. This novel protein was designated TIZ (for TRAF6-inhibitory zinc finger protein). TIZ expression also inhibited the signaling of RANK (receptor activator of NF-kappa B), which together with TRAF6 has been shown to be essential for osteoclastogenesis. Furthermore, the expression level of TIZ appeared to be regulated during the differentiation of human peripheral blood monocytes into osteoclasts. More significantly, transfection of TIZ into the monocyte/macrophage cell line Raw264.7 reduced the RANK ligand-induced osteoclastogenesis of this cell line. Our findings suggest that the novel zinc finger protein TIZ may play a role during osteoclast differentiation by modulating TRAF6 signaling activity.

All TRAFs are not created equal: common and distinct molecular mechanisms of TRAF-mediated signal transduction

The tumor necrosis factor (TNF) receptor associated factors (TRAFs) have emerged as the major signal transducers for the TNF receptor superfamily and the interleukin-1 receptor/Toll-like receptor (IL-1R/TLR) superfamily. TRAFs collectively play important functions in both adaptive and innate immunity. Recent functional and structural studies have revealed the individuality of each of the mammalian TRAFs and advanced our understanding of the underlying molecular mechanisms. Here, we examine this functional divergence among TRAFs from a perspective of both upstream and downstream TRAF signal transduction pathways and of signaling-dependent regulation of TRAF trafficking. We raise additional questions and propose hypotheses regarding the molecular basis of TRAF signaling specificity.

Mechanism of cardioprotective action of TNF-alpha in the isolated rat heart

BACKGROUND

Tumour necrosis factor alphs (TNF-alpha), a proinflammatory cytokine, is synthesized in the heart under pathologic conditions; however, it is not clear whether this cytokine results in heart dysfunction or serves as a cardioprotective agent.

OBJECTIVE

To examine whether TNF-alpha in low concentrations exerts a cardioprotective effect on the heart and prevents the occurrence of intracellular calcium overload.

ANIMALS AND METHODS

The effect of TNF-alpha was studied in vivo on hemodynamic parameters in anesthetized rats. The cardioprotective action of TNF-alpha was tested against ischemia-reperfusion-induced changes in cardiac performance in the isolated perfused rat hearts. The effect of TNF-alpha on intracellular free calcium was evaluated in freshly isolated adult rat cardiomyocytes by Fura 2 technique.

RESULTS

An intravenous injection of TNF-alpha (200 mug/kg) in rats produced a transient but significant depressant effect on cardiac function and an increase in heart rate. TNF-alpha (25 mug/mL) did not affect cardiac function in the isolated heart; however, it attenuated the ischemia-reperfusion-induced changes in the left ventricular pressures (developed pressure, end diastolic pressure, +dP/dt and -dP/dt). In the isolated cardiomyocytes, TNF-alpha did not produce any change in the level of intracellular free calcium, but this agent (10 to 100 ng/mL) significantly decreased the potassium chloride (30 mM) -induced increase in free calcium.

CONCLUSIONS

The inhibitory effect of low concentrations of TNF-alpha on calcium influx may reduce the occurrence of intracellular calcium overload, and this may be responsible for improving left ventricular dysfunction due to ischemia-reperfusion injury in the heart.

A mammalian homolog of Drosophila schnurri, KRC, regulates TNF receptor-driven responses and interacts with TRAF2

The cytokine TNFalpha launches cascades of gene activation that control inflammation and apoptosis through NFkappaB and JNK/SAPK signal transduction pathways. Here we describe a function for the zinc finger transcription factor kappa recognition component (KRC) in regulating patterns of gene activation in response to proinflammatory stimuli. We demonstrate that KRC overexpression inhibits while antisense or dominant-negative KRC enhances NFkappaB-dependent transactivation and JNK phosphorylation and consequently, apoptosis and cytokine gene expression. The effect of KRC is mediated through its interaction with the adaptor protein TRAF2, which intersects both pathways. KRC is a hitherto unrecognized participant in the signal transduction pathway leading from the TNF receptor to gene activation and may play a critical role in inflammatory and apoptotic responses.