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

Response of human pulmonary epithelial cells to lipopolysaccharide involves Toll-like receptor 4 (TLR4)-dependent signaling pathways: evidence for an intracellular compartmentalization of TLR4

Pulmonary epithelial cells are continuously exposed to microbial challenges as a result of breathing. It is recognized that immune myeloid cells express Toll-like receptors (TLRs), which play a major role in detecting microbes and initiating innate immune responses. In contrast, little is known concerning the expression of TLR in pulmonary epithelial cells per se, their distribution within the cell, their function, and the signaling pathways involved. In this work, we demonstrated by reverse transcription-PCR and/or immunoblot that TLR4 and the accessory molecule MD-2 are constitutively expressed in distinct human alveolar and bronchial epithelial cells. We further characterized by flow cytometry, biotinylation/precipitation, and confocal microscopy the intracellular localization of TLR4 in these cells. Despite this intracellular compartmentalization of TLR4, pulmonary epithelial cells were responsive to the TLR4 activator lipopolysaccharide (LPS), a potent Gram-negative bacteria-associated molecular pattern. Using respiratory epithelial cells isolated from TLR4 knock-out and wild type mice, we demonstrated that TLR4 is the actual activating receptor for LPS in these cells. Furthermore we showed that this cell response to LPS involves a signaling complex including the kinases interleukin-1 receptor-associated kinase (IRAK), p38, Jnk, and ERK1/2. Moreover, using vectors expressing dominant-negative forms of MyD88 and TRAF6, we established that LPS-induced activation of respiratory epithelial cells is largely dependent on TLR4 signaling intermediates. Altogether these data demonstrate that TLR4 is a key element in the response of pulmonary epithelial cells to molecules derived from Gram-negative bacteria. The intracellular localization of TLR4 in lung epithelia is expected to play an important role in the prevention of the development of chronic inflammatory disease.

TRAF6 is a critical factor for dendritic cell maturation and development

IL-1 receptor (IL-1R)/Toll-like receptor (TLR) family and TNF receptor (TNFR) superfamily members are critical for regulating multiple aspects of dendritic cell (DC) biology. Several signaling pathways associated with each family utilize the adapter molecule, TRAF6, but its role in DCs is unclear. By examining TRAF6-deficient mice and bone marrow (BM) chimeras reconstituted with TRAF6-deficient fetal liver cells, we show that proper DC maturation requires TRAF6. In response to either microbial components or CD40L, TRAF6-deficient DCs fail to upregulate surface expression of MHCII and B7.2, or produce inflammatory cytokines. Moreover, LPS-treated TRAF6-deficient DCs do not exhibit an enhanced capacity to stimulate naive T cells. Interestingly, a major population of splenic DCs, the CD4(+)CD8alpha(-) subset, is nearly absent in both TRAF6-deficient mice and BM chimeras. Together these results indicate that TRAF6 regulates the critical processes required for maturation, activation, and development of DCs, the primary cellular bridge between innate and adaptive immunity.

Structurally distinct recognition motifs in lymphotoxin-beta receptor and CD40 for tumor necrosis factor receptor-associated factor (TRAF)-mediated signaling

Lymphotoxin-beta receptor (LTbetaR) and CD40 are members of the tumor necrosis factor family of signaling receptors that regulate cell survival or death through activation of NF-kappaB. These receptors transmit signals through downstream adaptor proteins called tumor necrosis factor receptor-associated factors (TRAFs). In this study, the crystal structure of a region of the cytoplasmic domain of LTbetaR bound to TRAF3 has revealed an unexpected new recognition motif, 388IPEEGD393, for TRAF3 binding. Although this motif is distinct in sequence and structure from the PVQET motif in CD40 and PIQCT in the regulator TRAF-associated NF-kappaB activator (TANK), recognition is mediated in the same binding crevice on the surface of TRAF3. The results reveal structurally adaptive "hot spots" in the TRAF3-binding crevice that promote molecular interactions driving specific signaling after contact with LTbetaR, CD40, or the downstream regulator TANK.

Identification and characterization of the cytoplasmic protein TRAF4 as a p53-regulated proapoptotic gene

The role of p53 in tumor suppression partly relies on its ability to transcriptionally regulate target genes involved in the initiation of cell cycle arrest or the activation of programmed cell death. In recent years many genes have been identified as p53-regulated genes; however, no single target gene has been shown to be required for the full apoptotic effect. We have identified TRAF4 as a p53-regulated gene in a microarray screen using a Murine 11K Affymetrix GeneChip hybridized with cRNA from the p53 temperature-sensitive cell line, Vm10. TRAF4 is a member the TRAF family of adaptor proteins that mediate cellular signaling by binding to various members of the tumor necrosis family receptor superfamily and interleukin-1/Toll-like receptor super-family. In contrast to its other family members, TRAF4 has not been shown to bind to a member of the tumor necrosis factor receptor superfamily in vivo, nor has it been shown to regulate signaling pathways common to its other family members. Therefore the role of TRAF4 in a signaling pathway has not yet been established and requires further study. TRAF4 is specifically regulated by p53 in response to temperature sensitive p53, overexpression of p53 by use of an adenovirus, and stabilization of p53 in response to DNA damage. The murine TRAF4 promoter contains a functional p53 DNA-binding site approximately 1 kilobase upstream of the initiating methionine. TRAF4 localizes to the cytoplasm and appears to remain in the cytoplasm following DNA damage. Interestingly, the overexpression of TRAF4 induces apoptosis and suppresses colony formation. These data suggest a correlation that the orphan adaptor protein TRAF4 may play a role in p53-mediated proapoptotic signaling in the response to cellular stress.

Activation of Discoidin Domain Receptor 1 Facilitates the Maturation of Human Monocyte-Derived Dendritic Cells Through the TNF Receptor Associated Factor 6/TGF-β-Activated Protein Kinase 1 Binding Protein 1β/p38α Mitogen-Activated Protein Kinase Signaling Cascade

Maturation of dendritic cells (DCs) is critical for their ability to stimulate resting naive T cells in primary immune responses. Previous studies demonstrated that collagen, such as type I collagen, could facilitate DC maturation; however, the basis of collagen-mediated DC maturation remains unclear. Discoidin domain receptor 1 (DDR1) is a nonintegrin collagen receptor constitutively expressed in a variety of epithelial cells, including tumor cells, and is inducible in leukocytes. In this study, we evaluated the role of DDR1 in DC maturation using human monocyte-derived DCs. Two DDR1 isoforms, DDR1a and DDR1b, were expressed in both immature and mature DCs. Activation of DDR1 on immature DCs resulted in their partial maturation; however, DDR1 activation markedly amplified TNF-alpha- and LPS-induced phenotypic and functional maturation of DCs through activation of p38 mitogen-activated protein kinase (MAPK), suggesting the involvement of DDR1b in this process. Activation of DDR1b on differentiated DDR1b-overexpressing THP-1 cells or DDR1 on mature DCs induced the formation of TNFR associated factor 6 (TRAF6)/TGF-beta-activated kinase 1 binding protein 1beta/p38alpha MAPK complex and p38alpha autophosphorylation. Transfection of differentiated DDR1b-overexpressing THP-1 cells with dominant negative TRAF6 completely abrogated DDR1b-mediated p38 MAPK phosphorylation, indicating a critical role of TRAF6 in DDR1b-mediated p38 MAPK activation. Taken together, our data suggest that DDR1b-collagen interaction augments the maturation of DCs in a tissue microenvironment through a unique TRAF6/TGF-beta-activated kinase 1 binding protein 1beta/p38alpha MAPK signaling cascade and contributes to the development of adaptive immune responses.

A flow cytometric method to detect protein-protein interaction in living cells by directly visualizing donor fluorophore quenching during CFP?YFP fluorescence resonance energy transfer (FRET)

BACKGROUND

Protein interactions at the molecular level can be measured by fluorescence resonance energy transfer (FRET) using a pair of fluorescent proteins, such as CFP and YFP, in which the emission spectrum of CFP significantly overlaps the excitation spectrum of YFP. The resulting energy given off from the donor CFP protein can directly excite the acceptor YFP protein when the proteins are closely approximated. During FRET, there is quenching of the emission of the donor CFP protein that is directly related to the efficiency of energy transfer and inversely proportional to the sixth power of the distance between the donor and acceptor proteins. In this study we describe a new approach to visualize donor CFP quenching during CFP-->YFP FRET and demonstrate how this parameter can be used to calculate FRET efficiency.

METHODS

A novel flow cytometric method to detect protein-protein interactions in living cells was developed that utilized assessment of CFP donor quenching during CFP-->YFP FRET by comparing CFP intensity between FRET-positive and -negative populations. To accomplish this, we compared the CFP intensity in FRET-positive and FRET-negative cells within the same population transfected with a CFP/YFP fusion protein, in which the molar ratio of CFP:YFP was one. By using separate lasers to excite CFP and YFP, the detection of FRET was separated from that of YFP. Therefore, after direct excitation, the YFP emission spectrum remained constant in all transfected cells, whereas the emission spectrum of CFP varied with the extent of FRET in individual cells. Specific CFP/YFP fusion constructs were prepared to evaluate this approach. The first one consisted of CFP and YFP separated by two caspase cleavage sites (CFP-LEVD-YFP). A second construct consisted of CFP and YFP separated by a structurally restricted 232-amino acid (aa) spacer. No FRET was observed by transfectants expressing this construct.

RESULTS

Transfection of CFP-LEVD-YFP into Hela cells resulted in a FRET-positive population and a FRET-negative one. The appearance of the FRET-negative population was inhibited by the caspase inhibitor z-VAD. Moreover, substituting D for A in the caspase cleavage sites of this probe abolished the FRET-negative population, demonstrating the probe's specificity for caspase activity. Comparison of the CFP emission in the FRET-positive and FRET-negative population was used to document the relationship of FRET to donor quenching and permit the calculation of FRET efficiency and relative molecular distance between CFP and YFP. Similar results were noted when cells transfected with the caspase-sensitive probe (in the presence of z-VAD) were mixed with cells expressing the CFP-YFP construct with the 232-aa spacer and therefore were FRET negative. This demonstrated the validity of calculating CFP donor quenching and FRET efficiency by comparing emission spectra of an unknown construct with that of a known positive control, both expressed by the same population of cells. Using this approach, we confirmed that members of the TNF receptor-associated factor (TRAF) family engaged in both homotypic and heterotypic interactions.

CONCLUSIONS

We have established a novel flow cytometric approach to assess donor CFP quenching during CFP-->YFP FRET, which can be used for the calculation of FRET efficiency and relative biological molecular distance between CFP and YFP moieties. This method can be used not only to analyze cells that express a CFP and YFP fusion protein, but also independent CFP-coupled and YFP-coupled interacting proteins.

Mice lacking TNFalpha receptors 1 and 2 are resistant to death and fulminant liver injury induced by agonistic anti-Fas antibody

The liver is particularly susceptible to Fas-mediated cytotoxicity. Mice given an adequate parenteral dose of agonistic anti-Fas antibody (aFas) or of FasL are known to develop a devastating liver injury and to die in a few hours. The present work shows that mice lacking TNFR1 and TNFR2 (R(-)) both survive a single dose of aFas, otherwise rapidly lethal, and develop a mild form of hepatic damage, compared to the much more severe liver injury that in a few hours strikes wild-type mice (R(+)), eventually involving increased activity of proteases of different families (caspase 3-, 8-, and 9-like, calpains, cathepsin B). Neither the overall tissue levels of Fas and FasL nor Fas expression at the hepatocyte surface are altered in the liver of R(-) animals. The DNA-binding activity of the NF-kappaB transcription factor is enhanced after aFas treatment, but much more markedly in R(-) than in R(+) mice. Bcl2, while unchanged in untreated animals, is markedly upregulated in R(-) but not in R(+) mice challenged with aFas. The requirement of a normal TNFR1/TNFR2 phenotype for full deployment of the general and liver-specific aFas toxicity in mice most likely implies that treatment with aFas in some way results in activation of the TNFalpha-TNFRs system and that this activation synergizes with Fas-mediated signals in causing the fulminant liver injury and the animal death. The precise cellular and molecular details underlying this interplay between Fas- and TNFRs-mediated signaling systems in the general and liver-specific aFas toxicity largely remain to be clarified.

Ubiquitination of the Epstein-Barr virus-encoded latent membrane protein 1 depends on the integrity of the TRAF binding site

The latent membrane protein 1 (LMP1) encoded by the Epstein-Barr virus functions as a constitutively activated receptor of the tumor necrosis factor receptor family. LMP1 is a short-lived protein that is ubiquitinated and degraded by the proteasome. We have previously shown that LMP1 recruits the adapter protein tumor necrosis factor receptor-associated factor 3 (TRAF3) to lipid rafts. To test if TRAFs are involved in LMP1's ubiquitination, we have mutated the LMP1 CTAR1 site that has been identified as a TRAF binding site. We show that the CTAR1 mutant (CTAR1(-)) is expressed after transfection at a similar level to wild-type LMP1, and behaves as wild-type LMP1 with respect to membrane localization. However, CTAR1(-) does not bind TRAF3. We demonstrate that ubiquitination of CTAR1(-) is significantly reduced when compared to wild-type LMP1. In addition, the expression of wild-type LMP1 induces the ubiquitination, an effect that is significantly reduced when the CTAR1(-) is expressed. Taken together, our results suggest that TRAF proteins are involved in the ubiquitination of LMP1, and that their binding to LMP1 may facilitate their own ubiquitination.

TWEAK mediates signal transduction and differentiation of RAW264.7 cells in the absence of Fn14/TweakR. Evidence for a second TWEAK receptor

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) is a member of the tumor necrosis factor family that is implicated in apoptosis, proliferation, migration, and inflammation. We describe our findings showing that TWEAK mediated the differentiation of RAW264.7 (RAW) monocyte/macrophage cells into multinuclear, functional osteoclasts. The effect of TWEAK was direct and not mediated by the receptor activator of nuclear factor-kappa B (NF-kappa B) ligand (RANKL) as shown by the use of TWEAK- or RANKL-neutralizing antibodies and by osteoprotegerin, a decoy receptor for RANKL. Recently, fibroblast growth factor-inducible 14 (Fn14) was suggested to be a receptor for TWEAK. We show that the Fn14/TWEAK receptor (TweakR) was not responsible for the osteoclastic effect of TWEAK on RAW cells. Flow cytometry analysis did not reveal the expression of Fn14/TweakR on RAW cells. Moreover, Fn14/TweakR-neutralizing antibodies did not block TWEAK-induced RAW cell differentiation into osteoclasts. This indicated that a second TweakR, TweakR2, exists on RAW cells and is responsible for mediating TWEAK-induced differentiation. We next compared the signaling pathways that are activated by the two receptors. TWEAK binding to TweakR2 activated the NF-kappa B, mitogen-activated protein kinase and c-Jun N-terminal kinase signaling cascades in RAW cells. In contrast, TWEAK binding to Fn14/TweakR activated the NF-kappa B and c-Jun N-terminal kinase pathways but induced only a weak activation of MAPK in HT-29 human colon adenocarcinoma cells expressing endogenous Fn14/TweakR. We propose that the biological effects of TWEAK are mediated by binding to one of at least two distinct receptors that induce differential activation of downstream signaling pathways.

Loss of the cylindromatosis tumour suppressor inhibits apoptosis by activating NF-kappaB

Protein modification by the conjugation of ubiquitin moieties--ubiquitination--plays a major part in many biological processes, including cell cycle and apoptosis. The enzymes that mediate ubiquitin-conjugation have been well-studied, but much less is known about the ubiquitin-specific proteases that mediate de-ubiquitination of cellular substrates. To study this gene family, we designed a collection of RNA interference vectors to suppress 50 human de-ubiquitinating enzymes, and used these vectors to identify de-ubiquitinating enzymes in cancer-relevant pathways. We report here that inhibition of one of these enzymes, the familial cylindromatosis tumour suppressor gene (CYLD), having no known function, enhances activation of the transcription factor NF-kappaB. We show that CYLD binds to the NEMO (also known as IKKgamma) component of the IkappaB kinase (IKK) complex, and appears to regulate its activity through de-ubiquitination of TRAF2, as TRAF2 ubiquitination can be modulated by CYLD. Inhibition of CYLD increases resistance to apoptosis, suggesting a mechanism through which loss of CYLD contributes to oncogenesis. We show that this effect can be relieved by aspirin derivatives that inhibit NF-kappaB activity, which suggests a therapeutic intervention strategy to restore growth control in patients suffering from familial cylindromatosis.

Caspase-mediated cleavage converts the tumor necrosis factor (TNF) receptor-associated factor (TRAF)-1 from a selective modulator of TNF receptor signaling to a general inhibitor of NF-kappaB activation

The role of tumor necrosis factor (TNF) receptor-associated factor (TRAF)-1 in NF-kappaB activation by various members of the TNF receptor family is not well understood, and conflicting data have been published. Here, we show that TRAF1 differentially affects TRAF2 recruitment and activation of NF-kappaB by members of the TNF receptor family. Interestingly, a naturally occurring caspase-derived cleavage product of TRAF1 solely comprising its TRAF domain (TRAF1-(164-416)) acted as a general inhibitor of NF-kappaB activation. In contrast, a corresponding fragment generated by cleavage of TRAF3 showed no effect in this regard. In accordance with these functional data, TRAF1, but not TRAF3, interacted with the IKK complex via its N-TRAF domain. Endogenous TRAF1 and the overexpressed TRAF domain of TRAF1 were found to be constitutively associated with the IKK complex, whereas endogenous receptor interacting protein was only transiently associated with the IKK complex upon TNF stimulation. Importantly, the caspase-generated TRAF1-fragment, but not TRAF1 itself inhibited IKK activation. Our results suggest that TRAF1 and TRAF1-(164-416) exert their regulatory effects on receptor-induced NF-kappaB activation not only by modulation of TRAF2 receptor interaction but especially TRAF1-(164-416) also by directly targeting the IKK complex.

TNF-receptor-associated factors as targets for drug development

TNF-receptor-associated factors (TRAFs) are the bottleneck of the TNF-receptor (TNF-R) family signal transduction. They integrate the signalling from many members of the TNF-R family and initiate intracellular signalling cascades aimed at the activation of NF-kappaB and c-jun, the reprogramming of gene expression and the control of cell death. Deregulation of these pathways is the cause of several autoimmune and inflammatory diseases. The specificity and interaction of the members of the TRAF family with the TNF-R entails the recognition of just a 4 - 6 amino acid motif in the cytosolic region of the receptor, suitable as an attractive target for drug discovery. This review summarises the current knowledge on TRAFs and discusses the pros and cons of their application as targets for drug discovery.

Comparative analysis of apoptosis and inflammation genes of mice and humans

Apoptosis (programmed cell death) plays important roles in many facets of normal mammalian physiology. Host-pathogen interactions have provided evolutionary pressure for apoptosis as a defense mechanism against viruses and microbes, sometimes linking apoptosis mechanisms with inflammatory responses through NFkappaB induction. Proteins involved in apoptosis and NFkappaB induction commonly contain evolutionarily conserved domains that can serve as signatures for identification by bioinformatics methods. Using a combination of public (NCBI) and private (RIKEN) databases, we compared the repertoire of apoptosis and NFkappaB-inducing genes in humans and mice from cDNA/EST/genomic data, focusing on the following domain families: (1) Caspase proteases; (2) Caspase recruitment domains (CARD); (3) Death Domains (DD); (4) Death Effector Domains (DED); (5) BIR domains of Inhibitor of Apoptosis Proteins (IAPs); (6) Bcl-2 homology (BH) domains of Bcl-2 family proteins; (7) Tumor Necrosis Factor (TNF)-family ligands; (8) TNF receptors (TNFR); (9) TIR domains; (10) PAAD (PYRIN; PYD, DAPIN); (11) nucleotide-binding NACHT domains; (12) TRAFs; (13) Hsp70-binding BAG domains; (14) endonuclease-associated CIDE domains; and (15) miscellaneous additional proteins. After excluding redundancy due to alternative splice forms, sequencing errors, and other considerations, we identified cDNAs derived from a total of 227 human genes among these domain families. Orthologous murine genes were found for 219 (96%); in addition, several unique murine genes were found, which appear not to have human orthologs. This mismatch may be due to the still fragmentary information about the mouse genome or genuine differences between mouse and human repertoires of apoptotic genes. With this caveat, we discuss similarities and differences in human and murine genes from these domain families.

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.

Biology of the TRANCE axis

As the TNF and TNFR superfamilies have grown to more than two dozen combined members over the past 30 years, their involvement in interactions between immune cells, with regard to the events governing cellular differentiation, activation, and survival have been well established. The recently identified TNF superfamily cytokine, TRANCE (RANKL/OPGL/ODF/TNFSF11), which interacts with two receptors-one functional, TRANCE-R (RANK/TNFRSF11A), and one decoy, OPG (TNFRSF11B)-is a survival factor for activated dendritic cells, and may also be important for the maintenance of immune tolerance. TRANCE is also the key cytokine involved in osteoclast differentiation and activation, making TRANCE signaling crucial for proper bone homeostasis, and a potential therapeutic target in diseases such as osteoporosis, osteolytic metastatic cancer, arthritis, and periodontitis. Importantly, the positive role that TRANCE has in activating the immune system, appears to significantly contribute to pathologic bone loss. These observations have spurred intense study of the various ways in which the immune system can influence bone. Furthermore, TRANCE has also been demonstrated to play essential roles in the developmental processes leading to both lymph node formation, and the expansion and function of mammary glands during pregnancy and lactation. Thus, TRANCE is quickly emerging as a cytokine of significant importance to further understanding unique aspects of mammalian biology.

Expression of TNF-receptor-associated factor genes in murine tooth development

Tumor necrosis factor receptor-associated factors (TRAFs) belong to a family of intracellular adaptor proteins that mediate signaling downstream of various cell surface receptors. We carried out comparative in situ hybridization analysis of five Traf genes Traf1, Traf2, Traf3, Traf4 and Traf6 during murine odontogenesis from the formation of the epithelial thickening to the early bell stage. Traf2, Traf3 and Traf6 showed weak expression in the thickened epithelium. Expression of Traf1, Traf2 and Traf6 were observed in the outer edges of the bud epithelium whereas Traf3 was strongly expressed at the tip of the bud epithelium. Expression of Traf1, Traf4 and Traf6 were detected in the dental papilla mesenchyme. Traf2 showed restricted expression in the internal enamel epithelium of the bell stage while expression of Traf1, Traf3, Traf4 and Traf6 were observed in both the internal and the external enamel epithelium. During early odontogenesis, all five genes show dynamic spatiotemporal expression patterns.

TWEAK is an endothelial cell growth and chemotactic factor that also potentiates FGF-2 and VEGF-A mitogenic activity

OBJECTIVE

TWEAK, a member of the tumor necrosis factor superfamily, binds to the Fn14 receptor and stimulates angiogenesis in vivo. In this study, we investigated Fn14 gene expression in human endothelial cells (ECs) and examined the effect of TWEAK, added either alone or in combination with fibroblast growth factor-2 (FGF-2) or vascular endothelial growth factor-A (VEGF-A), on EC proliferation, migration, and survival in vitro. We also determined whether a soluble Fn14-Fc fusion protein could inhibit TWEAK biologic activity on ECs and investigated TWEAK signal transduction in ECs.

METHODS AND RESULTS

We found that both FGF-2 and VEGF-A could induce Fn14 mRNA expression in ECs. TWEAK was a mitogen for ECs, and this proliferative activity could be inhibited by an Fn14-Fc decoy receptor. Furthermore, TWEAK treatment activated several intracellular signaling pathways in ECs and potentiated FGF-2--and VEGF-A--stimulated EC proliferation. TWEAK also had EC chemotactic activity, but it did not promote EC survival.

CONCLUSIONS

These results indicate that TWEAK is an EC growth and migration factor but not a survival factor. TWEAK can also enhance both FGF-2 and VEGF-A mitogenic activity on ECs. Thus, TWEAK may act alone as well as in combination with FGF-2 or VEGF-A to regulate pathological angiogenesis.

Profiling of differentially expressed apoptosis-related genes by cDNA arrays in human cord blood CD34+ cells treated with etoposide

OBJECTIVE

Understanding the molecular events that contribute to survival of and drug-induced apoptosis in hematopoietic stem and progenitor cells (HSC/P) can have impact on more rational approaches to blood cancer therapeutic design, as well as on strategies to minimize toxic side effects of chemotherapeutic drugs. Here we sought to systematically evaluate the basic molecular components and main pathways that govern and mediate cellular response initiated within human CD34(+) cells to etoposide-induced apoptosis.

MATERIALS AND METHODS

Human CD34(+) cells were isolated from umbilical cord blood (CB) and expanded in vitro. Expression of apoptosis-related genes in the control and etoposide treated cells was determined using cDNA array and quantitative real-time RT-PCR.

RESULTS

We identified a set of apoptosis-related genes expressed in highly purified normal human CB CD34(+) cells and determined how the expression of these genes changed in response to etoposide treatment. In addition, TRAIL does not induce apoptosis of normal human CD34(+) cells, and it has no cytotoxic effect on human CD34(+) cells that are undergoing apoptosis in response to growth factor withdrawal. This may be due to upregulation of cytotoxic receptors as well as the decoy receptor for TRAIL, and c-FLIP.

CONCLUSION

p53, c-Myc, and BAFF pathways are main pathways utilized by CD34(+) cells to arrest cell-cycle progression at multiple checkpoints, to halt proliferation, and to induce apoptosis as part of their cellular response to etoposide. Multiple known pro-survival and pro-apoptotic pathways are simultaneously activated in etoposide-treated CD34(+) cells. Also, TRAIL, used alone or in concert with chemotherapeutic drugs, may be of use as a safe blood cancer therapeutic with no or low toxicity for HSC/P.

Essential roles for NF-kappa B and a Toll/IL-1 receptor domain-specific signal(s) in the induction of I kappa B-zeta

I kappa B-zeta, a new negative-regulator of nuclear factor-kappa B (NF-kappa B), is strongly induced by lipopolysaccharide or interleukin-1 beta stimulation, but not by tumor necrosis factor-alpha. Here, we analyzed the mechanisms for transcriptional induction of I kappa B-zeta. I kappa B-zeta mRNA was induced by overexpression of MyD88 or TRAF6, but not TRAF2. Stimulation of macrophages with peptidoglycan or CpG DNA, which activated Toll-like receptor 2 or 9, respectively, also resulted in I kappa B-zeta induction. Thus, activation of the MyD88-dependent signaling pathway, commonly found downstream of different Toll/interleukin-1 receptor (TIR) domains, is sufficient for I kappa B-zeta induction. The induction was inhibited by treatment with various inhibitors of NF-kappa B activation or by overexpressing I kappa B-alpha or beta, indicating essential roles for NF-kappa B in I kappa B-zeta induction. However, overexpression of the NF-kappa B subunits induced I kappa B-alpha, but not I kappa B-zeta. These results indicate the existence of another signal essential for I kappa B-zeta induction, which is specifically mediated by the TIR domain-mediated signaling pathway.

RANK-Fc: a therapeutic antagonist for RANK-L in myeloma

BACKGROUND

Severe bone destruction due to inappropriate osteoclastogenesis is a prominent feature of multiple myeloma (MM). MM increases bone loss by disrupting the checks that normally control signaling by receptor activator of nuclear factor kappaB ligand (RANK-L, also called TRANCE [tumor necrosis factor-related, activation-induced cytokine], osteoprotegerin ligand [OPG-L], osteoclast differentiation factor [ODF], and tumor necrosis factor superfamily member 11 [TNFSF11]), a TNF-family cytokine required for osteoclast differentiation and activation. RANK-L binds to its functional receptor RANK (TNF receptor superfamily member 11a [TNF RSF11a]) to stimulate osteoclastogenesis. Osteotropic cytokines regulate this process by controlling bone marrow stromal expression of RANK-L. Further control over osteoclastogenesis is maintained by regulated expression of osteoprotegerin (OPG, also called osteoclastogenesis inhibitory factor and TNFRSF11b), a soluble decoy receptor for RANK-L. In normal bone marrow, abundant stores of OPG in stroma, megakaryocytes, and myeloid cells provide a natural buffer against increased RANK-L. MM disrupts these controls by increasing expression of RANK-L and decreasing expression of OPG. Concurrent deregulation of RANK-L and OPG expression is found in bone marrow biopsies from patients with MM but not in specimens from patients with non-MM hematologic malignancies.

METHODS

RANK-Fc is a recombinant RANK-L antagonist that is formed by fusing the extracellular domain of RANK to the Fc portion of human immunoglobulin G(1) (hIgG(1)). In vitro, addition of RANK-Fc virtually eliminates the formation of osteoclasts in cocultures of MM with bone marrow and osteoblast/stromal cells. The severe combined immunodeficiency (SCID)/ARH77 mouse model and the SCID-hu-MM mouse model of human MM were used to assess the ability of RANK-Fc to block the development of MM-induced bone disease in vivo. Mice received either RANK-Fc or hIgG(1) 200 microg intravenously three times per week.

RESULTS

RANK-Fc limited bone destruction in both the SCID/ARH-77 model and the SCID-hu-MM model. Administration of RANK-Fc also caused a marked reduction in tumor burden and serum paraprotein in SCID-hu-MM mice that was associated with the restoration of OPG and a reduction in RANK-L expression in the xenograft.

CONCLUSIONS

MM-induced bone destruction requires increased RANK-L expression and is facilitated by a concurrent reduction in OPG, a natural decoy receptor for RANK-L. Administration of the RANK-L antagonist RANK-Fc limits MM-induced osteoclastogenesis, development of bone disease, and MM tumor progression.

Targeted disruption of the meprin beta gene in mice leads to underrepresentation of knockout mice and changes in renal gene expression profiles

Meprins are multidomain zinc metalloproteases that are highly expressed in mammalian kidney and intestinal brush border membranes and in leukocytes and certain cancer cells. Mature meprins are oligomers of evolutionarily related, separately encoded alpha and/or beta subunits. Homooligomers of meprin alpha are secreted; oligomers containing meprin beta are plasma membrane associated. Meprin substrates include bioactive peptides and extracellular matrix proteins. Meprins have been implicated in cancer and intestinal inflammation. Additionally, meprin beta is a candidate gene for diabetic nephropathy. To elucidate in vivo functions of these metalloproteases, meprin beta null mice were generated by targeted disruption of the meprin beta gene on mouse chromosome 18q12. Analyses of meprin beta knockout mice indicated that (i) 50% fewer null mice are born than the Mendelian distribution predicts, (ii) null mice that survive develop normally and are viable and fertile, (iii) meprin beta knockout mice lack membrane-associated meprin alpha in kidney and intestine, and (iv) null mice have changes in renal gene expression profiles compared to wild-type mice as assessed by microarray analyses. Thus, disruption of the meprin beta allele in mice affects embryonic viability, birth weight, renal gene expression profiles, and the distribution of meprin alpha in kidney and intestine.

All in the family: evolutionary and functional relationships among death receptors

Over the last decade, significant progress has been made towards identifying the signaling pathways within mammalian cells that lead to apoptosis mediated by death receptors. The simultaneous expression of more than one death receptor in many, if not all, cell types suggests that functional innovation has driven the divergence of these receptors and their cognate ligands. To better understand the physiological divergence of the death receptors, a phylogenetic analysis of vertebrate death receptors was conducted based upon amino-acid sequences encoding the death domain regions of currently known and newly identified members of the family. Evidence is presented to indicate an ancient radiation of death receptors that predates the emergence of vertebrates, as well as ongoing divergence of additional receptors both within several receptor lineages as well as modern taxonomic lineages. We speculate that divergence among death receptors has led to their functional specialization. For instance, some receptors appear to be primarily involved in mediating the immune response, while others play critical roles during development and tissue differentiation. The following represents an evolutionary approach towards an understanding of the complex relationship among death receptors and their proposed physiological functions in vertebrate species.

TRAF1 is a critical regulator of JNK signaling by the TRAF-binding domain of the Epstein-Barr virus-encoded latent infection membrane protein 1 but not CD40

The oncogenic Epstein-Barr virus (EBV)-encoded latent infection membrane protein 1 (LMP1) mimics a constitutive active tumor necrosis factor (TNF) family receptor in its ability to recruit TNF receptor-associated factors (TRAFs) and TNF receptor-associated death domain protein (TRADD) in a ligand-independent manner. As a result, LMP1 constitutively engages signaling pathways, such as the JNK and p38 mitogen-activated protein kinases (MAPK), the transcription factor NF-kappaB, and the JAK/STAT cascade, and these activities may explain many of its pleiotropic effects on cell phenotype, growth, and transformation. In this study we demonstrate the ability of the TRAF-binding domain of LMP1 to signal on the JNK/AP-1 axis in a cell type- dependent manner that critically involves TRAF1 and TRAF2. Thus, expression of this LMP1 domain in TRAF1-positive lymphoma cells promotes significant JNK activation, which is blocked by dominant-negative TRAF2 but not TRAF5. However, TRAF1 is absent in many established epithelial cell lines and primary nasopharyngeal carcinoma (NPC) biopsy specimens. In these cells, JNK activation by the TRAF-binding domain of LMP1 depends on the reconstitution of TRAF1 expression. The critical role of TRAF1 in the regulation of TRAF2-dependent JNK signaling is particular to the TRAF-binding domain of LMP1, since a homologous region in the cytoplasmic tail of CD40 or the TRADD-interacting domain of LMP1 signal on the JNK axis independently of TRAF1 status. These data further dissect the signaling components used by LMP1 and identify a novel role for TRAF1 as a modulator of oncogenic signals.

Traf6 is essential for murine tooth cusp morphogenesis

Ectodermal appendages such as skin, hair, teeth, and sweat glands are affected in patients with hypohidrotic (anhydrotic) ectodermal dysplasia (HED). It has been established that mutations in the tumor necrosis factor (TNF) superfamily of molecules, i.e., ectodysplasin (EDA), EDA receptor (EDAR), and EDAR-associated death domain (EDARADD; the intracellular adaptor for EDAR), are responsible for several forms of HED in humans and mice. We show here by in situ hybridisation that another TNF family (orphan) receptor, TROY (also known TAJ, TAJ-alpha, TRADE, and TNFRSF19), is strongly coexpressed with Edar in the epithelial enamel knot signalling centres that are believe to regulate cuspal morphogenesis during murine tooth development. Traf6 is known to function as an intracellular adaptor protein for Troy and examination of Traf6 mutant mice revealed abnormalities in molar teeth that are similar but more severe than those produced by mutations in Eda signalling molecules. This finding suggests that, in additional to ectodysplasin, another TNF pathway involving Troy/Traf6 is involved in molar tooth cusp formation and identifies an essential role for a Traf in tooth development. Developmental Dynamics 229:131-135, 2004.

CD40 and its viral mimic, LMP1: similar means to different ends

CD40 is an important regulator of diverse aspects of the immune response including the T-cell-dependent humoral immune response, the development of antigen-presenting cells (APCs) and inflammation. Latent membrane protein 1 (LMP1), a protein encoded by Epstein-Barr Virus (EBV), appears to mimic CD40 in multiple ways. CD40 and LMP1 bind similar sets of cellular signalling proteins and activate overlapping signalling pathways. Despite many similarities shared between CD40 and LMP1, they also differ substantively. In this review, we will compare and contrast the signalling mediated by CD40 and LMP1.

Crystal structure of a UBP-family deubiquitinating enzyme in isolation and in complex with ubiquitin aldehyde

The ubiquitin-specific processing protease (UBP) family of deubiquitinating enzymes plays an essential role in numerous cellular processes. HAUSP, a representative UBP, specifically deubiquitinates and hence stabilizes the tumor suppressor protein p53. Here, we report the crystal structures of the 40 kDa catalytic core domain of HAUSP in isolation and in complex with ubiquitin aldehyde. These studies reveal that the UBP deubiquitinating enzymes exhibit a conserved three-domain architecture, comprising Fingers, Palm, and Thumb. The leaving ubiquitin moiety is specifically coordinated by the Fingers, with its C terminus placed in the active site between the Palm and the Thumb. Binding by ubiquitin aldehyde induces a drastic conformational change in the active site that realigns the catalytic triad residues for catalysis.

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.

The neurotrophin receptor p75(NTR): novel functions and implications for diseases of the nervous system

Neurotrophins have long been known to promote the survival and differentiation of vertebrate neurons. However, these growth factors can also induce cell death through the p75 neurotrophin receptor (p75(NTR)), a member of the tumor necrosis factor receptor superfamily. Consistent with a function in controlling the survival and process formation of neurons, p75(NTR) is mainly expressed during early neuronal development. In the adult, p75(NTR) is re-expressed in various pathological conditions, including epilepsy, axotomy and neurodegeneration. Potentially toxic peptides, including the amyloid beta- (Abeta-) peptide that accumulates in Alzheimer's disease, are ligands for p75(NTR). Recent work also implicates p75(NTR) in the regulation of both synaptic transmission and axonal elongation. It associates with the Nogo receptor, a binding protein for axonal growth inhibitors, and appears to be the transducing subunit of this receptor complex.

Structurally adaptive hot spots at a protein interaction interface on TRAF3

Tumor necrosis factor (TNF) signaling is controlled by receptors and intracellular signaling pathways that activate the NF-kappaB transcription factor. The resulting signals elicit immune responses and have important implications for disorders such as autoimmunity or allergic reactions. TNF-receptor-associated factors (TRAFs) bind to the cytoplasmic portion of TNFRs as well as downstream regulators and thus are co-inducers of the signal transduction. TRAF3 binds to diverse receptors and regulators by accomodating a conserved motif that is embedded in completely different structural frameworks. Thus, the protein-protein contact region on TRAF3 represents a binding interface that is structurally and functionally adaptive. In this report, three 'hot spots' at the TRAF3 protein-interaction interface are defined that provide the principal contact regions for different binding partners. The side-chains of residues at these 'hot spots' are flexible and undergo movements on binding the different partners. These side chain rearrangements provide a structural adaptability that promotes interaction with a variety of distinct proteins. It is proposed that similar adaptive 'hot spots' are also present on the binding surfaces of TRAF1, TRAF2 and TRAF5.

Immune and cell therapy of hematologic malignancies

Advances in molecular biology and immunology have identified means to activate the immune response against leukemia-associated antigens. Recent studies indicate that the stealth-like phenotype of leukemia cells can be reversed through transfer of genes encoding recombinant membrane-stabilized proteins of the tumor necrosis factor (TNF) imily, such as the one encoding CD154, the ligand for CD40. A phase I clinical trial using autologous CD154-transduced leukemia cells as a cellular vaccine has provided encouraging results. Treatment not only appears capable of inducing a cellular anti-leukemia immunity, but also may have a direct effect on leukemia cells by inducing latent sensitivity to Fas (CD95)-dependent leukemia-cell apoptosis. Phase II studies currently are underway using multiple injections of autologous leukemia cells made to express recombinant CD154 via gene transfer. Conceivably, we may be entering an era of effective gene therapy for hematologic malignancies.

Distinct molecular mechanism for initiating TRAF6 signalling

Tumour-necrosis factor (TNF) receptor-associated factor 6 (TRAF6) is the only TRAF family member that participates in signal transduction of both the TNF receptor (TNFR) superfamily and the interleukin-1 receptor (IL-1R)/Toll-like receptor (TLR) superfamily; it is important for adaptive immunity, innate immunity and bone homeostasis. Here we report crystal structures of TRAF6, alone and in complex with TRAF6-binding peptides from CD40 and TRANCE-R (also known as RANK), members of the TNFR superfamily, to gain insight into the mechanism by which TRAF6 mediates several signalling cascades. A 40 degrees difference in the directions of the bound peptides in TRAF6 and TRAF2 shows that there are marked structural differences between receptor recognition by TRAF6 and other TRAFs. The structural determinant of the petide TRAF6 interaction reveals a Pro-X-Glu-X-X-(aromatic/acidic residue) TRAF6-binding motif, which is present not only in CD40 and TRANCE-R but also in the three IRAK adapter kinases for IL-1R/TLR signalling. Cell-permeable peptides with the TRAF6-binding motif inhibit TRAF6 signalling, which indicates their potential as therapeutic modulators. Our studies identify a universal mechanism by which TRAF6 regulates several signalling cascades in adaptive immunity, innate immunity and bone homeostasis.