Cell cycle progression and transmitotic apoptosis resistance promote escape from extrinsic apoptosis

Extrinsic apoptosis relies on TNF-family receptor activation by immune cells or receptor-activating drugs. Here, we monitored cell cycle progression at a resolution of minutes to relate apoptosis kinetics and cell-to-cell heterogeneities in death decisions to cell cycle phases. Interestingly, we found that cells in S phase delay TRAIL receptor-induced death in favour of mitosis, thereby passing on an apoptosis-primed state to their offspring. This translates into two distinct fates, apoptosis execution post mitosis or cell survival from inefficient apoptosis. Transmitotic resistance is linked to Mcl-1 upregulation and its increased accumulation at mitochondria from mid-S phase onwards, which allows cells to pass through mitosis with activated caspase-8, and with cells escaping apoptosis after mitosis sustaining sublethal DNA damage. Antagonizing Mcl-1 suppresses cell cycle-dependent delays in apoptosis, prevents apoptosis-resistant progression through mitosis and averts unwanted survival after apoptosis induction. Cell cycle progression therefore modulates signal transduction during extrinsic apoptosis, with Mcl-1 governing decision making between death, proliferation and survival. Cell cycle progression thus is a crucial process from which cell-to-cell heterogeneities in fates and treatment outcomes emerge in isogenic cell populations during extrinsic apoptosis. This article has an associated First Person interview with the first author of the paper.

Stress-induced TRAILR2 expression overcomes TRAIL resistance in cancer cell spheroids

The influence of 3D microenvironments on apoptosis susceptibility remains poorly understood. Here, we studied the susceptibility of cancer cell spheroids, grown to the size of micrometastases, to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Interestingly, pronounced, spatially coordinated response heterogeneities manifest within spheroidal microenvironments: In spheroids grown from genetically identical cells, TRAIL-resistant subpopulations enclose, and protect TRAIL-hypersensitive cells, thereby increasing overall treatment resistance. TRAIL-resistant layers form at the interface of proliferating and quiescent cells and lack both TRAILR1 and TRAILR2 protein expression. In contrast, oxygen, and nutrient deprivation promote high amounts of TRAILR2 expression in TRAIL-hypersensitive cells in inner spheroid layers. COX-II inhibitor celecoxib further enhanced TRAILR2 expression in spheroids, likely resulting from increased ER stress, and thereby re-sensitized TRAIL-resistant cell layers to treatment. Our analyses explain how TRAIL response heterogeneities manifest within well-defined multicellular environments, and how spatial barriers of TRAIL resistance can be minimized and eliminated.

Monovalent TNF receptor 1-selective antibody with improved affinity and neutralizing activity

Selective inhibition of tumor necrosis factor (TNF) signaling through the proinflammatory axis of TNF-receptor 1 (TNFR1) while leaving pro-survival and regeneration-promoting signals via TNFR2 unaffected is a promising strategy to circumvent limitations of complete inhibition of TNF action by the approved anti-TNF drugs. A previously developed humanized antagonistic TNFR1-specific antibody, ATROSAB, showed potent inhibition of TNFR1-mediated cellular responses. Because the parental mouse antibody H398 possesses even stronger inhibitory potential, we scrutinized the specific binding parameters of the two molecules and revealed a faster dissociation of ATROSAB compared to H398. Applying affinity maturation and re-engineering of humanized variable domains, we generated a monovalent Fab derivative (13.7) of ATROSAB that exhibited increased binding to TNFR1 and superior inhibition of TNF-mediated TNFR1 activation, while lacking any agonistic activity even in the presence of cross-linking antibodies. In order to improve its pharmacokinetic properties, several Fab13.7-derived molecules were generated, including a PEGylated Fab, a mouse serum albumin fusion protein, a half-IgG with a dimerization-deficient Fc, and a newly designed Fv-Fc format, employing the knobs-into-holes technology. Among these derivatives, the Fv13.7-Fc displayed the best combination of improved pharmacokinetic properties and antagonistic activity, thus representing a promising candidate for further clinical development.

Bcl-2-mediated control of TRAIL-induced apoptotic response in the non-small lung cancer cell line NCI-H460 is effective at late caspase processing steps

Dysregulation of the mitochondrial signaling pathway of apoptosis induction represents a major hurdle in tumor therapy. The objective of the presented work was to investigate the role of the intrinsic (mitochondrial) apoptotic pathway in the non-small lung cancer cell line NCI-H460 upon induction of apoptosis using the highly bioactive TRAIL derivative Db-scTRAIL. NCI-H460 cells were TRAIL sensitive but an only about 3 fold overexpression of Bcl-2 was sufficient to induce a highly TRAIL resistant phenotype, confirming that the mitochondrial pathway is crucial for TRAIL-induced apoptosis induction. TRAIL resistance was paralleled by a strong inhibition of caspase-8, -9 and -3 activities and blocked their full processing. Notably, especially the final cleavage steps of the initiator caspase-8 and the executioner caspase-3 were effectively blocked by Bcl-2 overexpression. Caspase-9 knockdown failed to protect NCI-H460 cells from TRAIL-induced cell death, suggesting a minor role of this initiator caspase in this apoptotic pathway. Rather, knockdown of the XIAP antagonist Smac resulted in enhanced caspase-3 degradation after stimulation of cells with TRAIL. Of note, downregulation of XIAP had only limited effects on TRAIL sensitivity of wild-type NCI-H460 cells, but resensitized Bcl-2 overexpressing cells for TRAIL-induced apoptosis. In particular, XIAP knockdown in combination with TRAIL allowed the final cleavage step of caspase-3 to generate the catalytically active p17 fragment, whose production was otherwise blocked in Bcl-2 overexpressing cells. Together, our data strongly suggest that XIAP-mediated inhibition of final caspase-3 processing is the last and major hurdle in TRAIL-induced apoptosis in NCI-H460 cells, which can be overcome by Smac in a Bcl-2 level dependent manner. Quantitative investigation of the XIAP/Smac interplay using a mathematical model approach corroborates our experimental data strengthening the suggested roles of XIAP and Smac as critical determinants for TRAIL sensitivity.

TNF Signaling in Peritoneal Mesothelial Cells: Pivotal Role of cFLIPL

♦ BACKGROUND: Peritoneal dialysis (PD) coincides with high concentrations of proinflammatory cytokines, such as tumor necrosis factor (TNF), in the peritoneal cavity. During treatment, chronic inflammatory processes lead to damage of the peritoneal membrane and a subsequent ultrafiltration failure. Human peritoneal mesothelial cells (HPMCs) play a central role as mediators and targets of PD-related inflammatory changes. Although TNF Receptor 1 (TNFR1) is expressed in high numbers on the cells, TNF-induced apoptosis is inhibited. Here, the underlying molecular mechanisms of TNFR1 signaling in HPMCs are investigated. ♦ METHODS: Human peritoneal mesothelial cells were isolated from the omentum of healthy donors and the dialysis solution of PD patients. Flow cytometry was applied to determine cell surface expression of TNFR1 on HPMCS from healthy donors in absence or presence of TNF or PD fluid (PDF) and were compared to TNFR1 expression on cells from PD patients. To investigate TNFR1-mediated signaling, HPMCs were treated with PDF or TNF, and expression patterns of proteins involved in the TNFR1 signaling pathway were assessed by western blot. ♦ RESULTS: Incubation with PDF led to a significant up-regulation of TNFR1 on the cell surface correlating with elevated TNFR1 numbers on HPMCs from PD patients. Investigations of underlying molecular mechanisms of TNFR1 signaling showed that PDF affects TNFR1 signaling at the proapoptotic signaling pathway by upregulation of IκBα and downregulation of cFLIP_L. In contrast, TNF exclusively induces the activation of NFκB by an increase of phosphorylated IκBα. ♦ CONCLUSIONS: Novel and relevant insights into the mechanisms of TNFR1-mediated signaling in HPMCs with an impact on our understanding of PD-associated damage of the peritoneal membrane are shown.

Selective Blocking of TNF Receptor 1 Attenuates Peritoneal Dialysis Fluid Induced Inflammation of the Peritoneum in Mice

Chronic inflammatory conditions during peritoneal dialysis (PD)-treatment lead to the impairment of peritoneal tissue integrity. The resulting structural and functional reorganization of the peritoneal membrane diminishes ultrafiltration rate and thereby enhances mortality by limiting dialysis effectiveness over time. Tumour necrosis factor (TNF) and its receptors TNFR1 and TNFR2 are key players during inflammatory processes. To date, the role of TNFR1 in peritoneal tissue damage during PD-treatment is completely undefined. In this study, we used an acute PD-mouse model to investigate the role of TNFR1 on structural and morphological changes of the peritoneal membrane. TNFR1-mediated TNF signalling in transgenic mice expressing human TNFR1 was specifically blocked by applying a monoclonal antibody (H398) highly selective for human TNFR1 prior to PD-treatment. Cancer antigen-125 (CA125) plasma concentrations were measured by enzyme-linked immunosorbent assay (ELISA). Western blot analyses were applied to determine TNFR2 protein concentrations. Histological staining of peritoneal tissue sections was performed to assess granulocytes within the peritoneal membrane as well as the content of hyaluronic acid and collagen. We show for the first time that the number of granulocytes within the peritoneal membrane is significantly reduced in mice pre-treated with H398. Moreover, we demonstrate that blocking of TNFR1 not only influences CA125 values but also hyaluronic acid and collagen contents of the peritoneal tissue in these mice. These results strongly suggest that TNFR1 inhibition attenuates peritoneal damage caused by peritoneal dialysis fluid (PDF) and therefore may represent a new therapeutic approach in the treatment of PD-related side effects.

Dominant negative effects of tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) receptor 4 on TRAIL receptor 1 signaling by formation of heteromeric complexes

The cytokine TNF-related apoptosis-inducing ligand (TRAIL) and its cell membrane receptors constitute an elaborate signaling system fulfilling important functions in immune regulation and tumor surveillance. Activation of the death receptors TRAILR1 and TRAILR2 can lead to apoptosis, whereas TRAILR3 and TRAILR4 are generally referred to as decoy receptors, which have been shown to inhibit TRAIL-induced apoptosis. The underlying molecular mechanisms, however, remain unclear. Alike other members of the TNF receptor superfamily, TRAIL receptors contain a pre-ligand binding assembly domain (PLAD) mediating receptor oligomerization. Still, the stoichiometry of TRAIL receptor oligomers as well as the issue of whether the PLAD mediates only homotypic or also heterotypic interactions remained inconclusive until now. Performing acceptor-photobleaching FRET studies with receptors 1, 2, and 4, we demonstrate interactions in all possible combinations. Formation of dimers was shown by chemical cross-linking experiments for interactions of TRAILR2 and heterophilic interactions between the two death receptors or between either of the death receptors and TRAILR4. Implications of the demonstrated receptor-receptor interactions on signaling were investigated in suitable cellular models. Both apoptosis induction and activation of the transcription factor NFκB were significantly reduced in the presence of TRAILR4. Our experimental data combined with mathematical modeling show that the inhibitory capacity of TRAILR4 is attributable to signaling-independent mechanisms, strongly suggesting a reduction of signaling competent death receptors through formation heteromeric receptor complexes. In summary, we propose a model of TRAIL receptor interference driven by PLAD-mediated formation of receptor heterodimers on the cell membrane.

An anti-TNFR1 scFv-HSA fusion protein as selective antagonist of TNF action

IZI-06.1 is a humanized anti-TNFR1 single-chain fragment variable (scFv) that selectively inhibits binding of tumor necrosis factor (TNF) and lymphotoxin alpha to tumor necrosis factor receptor 1 (TNFR1) but not TNFR2. Recently, IZI-06.1 was converted into a fully human IgG1 antibody (ATROSAB) for the treatment of inflammatory diseases. Here, we compare the bivalent ATROSAB with a monovalent scFv-human serum albumin (HSA) fusion protein lacking any antibody-associated effector functions and possessing approximately only half the molecular mass of an IgG, which should facilitate accumulation in inflamed tissues. Furthermore, the half-life of the scFv should be strongly extended while maintaining monovalent binding, avoiding a possible signal transduction by receptor cross-linking in the absence of TNF. The scFv-HSA fusion protein was produced by stably transfected Chinese hamster ovary cells and purified by affinity chromatography. The fusion protein bound specifically to TNFR1 in enzyme-linked immunosorbent assay and TNFR1-transfected mouse embryonic fibroblasts. Affinity determined by quartz crystal microbalance was reduced compared with ATROSAB, which resulted also in a reduced inhibitory activity. Compared with the scFv fragment, the half-life of the fusion protein was significantly increased, although not reaching the long half-life of ATROSAB. In summary, the scFv-HSA may provide an alternative to the full-length IgG1 with the ability to selectively inhibit TNFR1 and exploiting the pharmacokinetic properties of albumin.

The transmembrane domains of TNF-related apoptosis-inducing ligand (TRAIL) receptors 1 and 2 co-regulate apoptotic signaling capacity

TNF-related apoptosis-inducing ligand (TRAIL) is a member of the tumor necrosis factor (TNF) ligand family that exerts its apoptotic activity in human cells by binding to two transmembrane receptors, TRAILR1 and TRAILR2. In cells co-expressing both receptors the particular contribution of either protein to the overall cellular response is not well defined. Here we have investigated whether differences in the signaling capacities of TRAILR1 and TRAILR2 can be attributed to certain functional molecular subdomains. We generated and characterized various chimeric receptors comprising TRAIL receptor domains fused with parts from other members of the TNF death receptor family. This allowed us to compare the contribution of particular domains of the two TRAIL receptors to the overall apoptotic response and to identify elements that regulate apoptotic signaling. Our results show that the TRAIL receptor death domains are weak apoptosis inducers compared to those of CD95/Fas, because TRAILR-derived constructs containing the CD95/Fas death domain possessed strongly enhanced apoptotic capabilities. Importantly, major differences in the signaling strengths of the two TRAIL receptors were linked to their transmembrane domains in combination with the adjacent extracellular stalk regions. This was evident from receptor chimeras comprising the extracellular part of TNFR1 and the intracellular signaling part of CD95/Fas. Both receptor chimeras showed comparable ligand binding affinities and internalization kinetics. However, the respective TRAILR2-derived molecule more efficiently induced apoptosis. It also activated caspase-8 and caspase-3 more strongly and more quickly, albeit being expressed at lower levels. These results suggest that the transmembrane domains together with their adjacent stalk regions can play a major role in control of death receptor activation thereby contributing to cell type specific differences in TRAILR1 and TRAILR2 signaling.

A minimal mathematical model for the initial molecular interactions of death receptor signalling

Tumor necrosis factor (TNF) is the name giving member of a large cytokine family mirrored by a respective cell membrane receptor super family. TNF itself is a strong proinflammatory regulator of the innate immune system, but has been also recognized as a major factor in progression of autoimmune diseases. A subgroup of the TNF ligand family, including TNF, signals via so-called death receptors, capable to induce a major form of programmed cell death, called apoptosis. Typical for most members of the whole family, death ligands form homotrimeric proteins, capable to bind up to three of their respective receptor molecules. But also unligated receptors occur on the cell surface as homomultimers due to a homophilic interaction domain. Based on these two interaction motivs (ligand/receptor and receptor/receptor) formation of large ligand/receptor clusters can be postulated which have been also observed experimentally. We use here a mass action kinetics approach to establish an ordinary differential equations model describing the dynamics of primary ligand/receptor complex formation as a basis for further clustering on the cell membrane. Based on available experimental data we develop our model in a way that not only ligand/receptor, but also homophilic receptor interaction is encompassed. The model allows formation of two distict primary ligand/receptor complexes in a ligand concentration dependent manner. At extremely high ligand concentrations the system is dominated by ligated receptor homodimers.

A visual analytics approach for models of heterogeneous cell populations

In recent years, cell population models have become increasingly common. In contrast to classic single cell models, population models allow for the study of cell-to-cell variability, a crucial phenomenon in most populations of primary cells, cancer cells, and stem cells. Unfortunately, tools for in-depth analysis of population models are still missing. This problem originates from the complexity of population models. Particularly important are methods to determine the source of heterogeneity (e.g., genetics or epigenetic differences) and to select potential (bio-)markers. We propose an analysis based on visual analytics to tackle this problem. Our approach combines parallel-coordinates plots, used for a visual assessment of the high-dimensional dependencies, and nonlinear support vector machines, for the quantification of effects. The method can be employed to study qualitative and quantitative differences among cells. To illustrate the different components, we perform a case study using the proapoptotic signal transduction pathway involved in cellular apoptosis.

Heterogeneity reduces sensitivity of cell death for TNF-stimuli

Background

Apoptosis is a form of programmed cell death essential for the maintenance of homeostasis and the removal of potentially damaged cells in multicellular organisms. By binding its cognate membrane receptor, TNF receptor type 1 (TNF-R1), the proinflammatory cytokine Tumor Necrosis Factor (TNF) activates pro-apoptotic signaling via caspase activation, but at the same time also stimulates nuclear factor κB (NF-κB)-mediated survival pathways. Differential dose-response relationships of these two major TNF signaling pathways have been described experimentally and using mathematical modeling. However, the quantitative analysis of the complex interplay between pro- and anti-apoptotic signaling pathways is an open question as it is challenging for several reasons: the overall signaling network is complex, various time scales are present, and cells respond quantitatively and qualitatively in a heterogeneous manner.

Results

This study analyzes the complex interplay of the crosstalk of TNF-R1 induced pro- and anti-apoptotic signaling pathways based on an experimentally validated mathematical model. The mathematical model describes the temporal responses on both the single cell level as well as the level of a heterogeneous cell population, as observed in the respective quantitative experiments using TNF-R1 stimuli of different strengths and durations. Global sensitivity of the heterogeneous population was quantified by measuring the average gradient of time of death versus each population parameter. This global sensitivity analysis uncovers the concentrations of Caspase-8 and Caspase-3, and their respective inhibitors BAR and XIAP, as key elements for deciding the cell's fate. A simulated knockout of the NF-κB-mediated anti-apoptotic signaling reveals the importance of this pathway for delaying the time of death, reducing the death rate in the case of pulse stimulation and significantly increasing cell-to-cell variability.

Conclusions

Cell ensemble modeling of a heterogeneous cell population including a global sensitivity analysis presented here allowed us to illuminate the role of the different elements and parameters on apoptotic signaling. The receptors serve to transmit the external stimulus; procaspases and their inhibitors control the switching from life to death, while NF-κB enhances the heterogeneity of the cell population. The global sensitivity analysis of the cell population model further revealed an unexpected impact of heterogeneity, i.e. the reduction of parametric sensitivity.

Nanotube action between human mesothelial cells reveals novel aspects of inflammatory responses

A well-known role of human peritoneal mesothelial cells (HPMCs), the resident cells of the peritoneal cavity, is the generation of an immune response during peritonitis by activation of T-cells via antigen presentation. Recent findings have shown that intercellular nanotubes (NTs) mediate functional connectivity between various cell types including immune cells - such as T-cells, natural killer (NK) cells or macrophages - by facilitating a spectrum of long range cell-cell interactions. Although of medical interest, the relevance of NT-related findings for human medical conditions and treatment, e.g. in relation to inflammatory processes, remains elusive, particularly due to a lack of appropriate in vivo data. Here, we show for the first time that primary cultures of patient derived HPMCs are functionally connected via membranous nanotubes. NT formation appears to be actin cytoskeleton dependent, mediated by the action of filopodia. Importantly, significant variances in NT numbers between different donors as a consequence of pathophysiological alterations were observable. Furthermore, we show that TNF-α induces nanotube formation and demonstrate a strong correlation of NT connectivity in accordance with the cellular cholesterol level and distribution, pointing to a complex involvement of NTs in inflammatory processes with potential impact for clinical treatment.

A TNF receptor 2 selective agonist rescues human neurons from oxidative stress-induced cell death

Tumor necrosis factor (TNF) plays a dual role in neurodegenerative diseases. Whereas TNF receptor (TNFR) 1 is predominantly associated with neurodegeneration, TNFR2 is involved in tissue regeneration and neuroprotection. Accordingly, the availability of TNFR2-selective agonists could allow the development of new therapeutic treatments of neurodegenerative diseases. We constructed a soluble, human TNFR2 agonist (TNC-scTNF(R2)) by genetic fusion of the trimerization domain of tenascin C to a TNFR2-selective single-chain TNF molecule, which is comprised of three TNF domains connected by short peptide linkers. TNC-scTNF(R2) specifically activated TNFR2 and possessed membrane-TNF mimetic activity, resulting in TNFR2 signaling complex formation and activation of downstream signaling pathways. Protection from neurodegeneration was assessed using the human dopaminergic neuronal cell line LUHMES. First we show that TNC-scTNF(R2) interfered with cell death pathways subsequent to H(2)O(2) exposure. Protection from cell death was dependent on TNFR2 activation of the PI3K-PKB/Akt pathway, evident from restoration of H(2)O(2) sensitivity in the presence of PI3K inhibitor LY294002. Second, in an in vitro model of Parkinson disease, TNC-scTNF(R2) rescues neurons after induction of cell death by 6-OHDA. Since TNFR2 is not only promoting anti-apoptotic responses but also plays an important role in tissue regeneration, activation of TNFR2 signaling by TNC-scTNF(R2) appears a promising strategy to ameliorate neurodegenerative processes.

TNFR1-induced activation of the classical NF-κB pathway

The molecular mechanisms underlying activation of the IκB kinase (IKK) complex are presumably best understood in the context of tumor necrosis factor (TNF) receptor-1 (TNFR1) signaling. In fact, it seems that most, if not all, proteins relevant for this process have been identified and extensive biochemical and genetic data are available for the role of these factors in TNF-induced IKK activation. There is evidence that protein modification-independent assembly of a core TNFR1 signaling complex containing TNFR1-associated death domain, receptor interacting kinase 1, TNF receptor-associated factor 2 and cellular inhibitor of apoptosis protein 1 and 2 starts a chain of nondegrading ubiquitination events that culminate in the recruitment and activation of IKK complex-stimulating kinases and the IKK complex itself. Here, we sum up the known details of TNFR1-induced IKK activation, address arising contradictions and discuss possible explanations resolving the apparent discrepancies.

ATROSAB, a humanized antagonistic anti-tumor necrosis factor receptor one-specific antibody

Tumor necrosis factor (TNF) signals through two membrane receptors, TNFR1 and TNFR2, and TNFR1 is known to be the major pathogenic mediator of chronic and acute inflammatory diseases. Present clinical intervention is based on neutralization of the ligand TNF. Selective inhibition of TNF receptor 1 (TNFR1) provides an alternative opportunity to neutralize the pro-inflammatory activity of TNF while maintaining the advantageous immunological responses mediated by TNFR2, including immune regulation, tissue homeostasis and neuroprotection. We recently humanized a mouse anti-human TNFR1 monoclonal antibody exhibiting TNFR1-neutralizing activity. This humanized antibody has been converted into an IgG1 molecule (ATROSAB) containing a modified Fc region previously demonstrated to have greatly reduced effector functions. Purified ATROSAB, produced in CHO cells, showed strong binding to human and rhesus TNFR1-Fc fusion protein and mouse embryonic fibroblasts transfected with a recombinant TNFR1 fusion protein with an affinity identical to the parental mouse antibody H398. Using chimeric human/mouse TNFR1 molecules, the epitope of ATROSAB was mapped to the N-terminal region (amino acid residues 1-70) comprising the first cysteine-rich domain (CRD1) and the A1 sub-domain of CRD2. In vitro, ATROSAB inhibited typical TNF-mediated responses like apoptosis induction and activation of NFκB-dependent gene expression such as IL-6 and IL-8 production. These findings open the way to further analyze the therapeutic activity of ATROSAB in relevant disease models in non-human primates.

Ligand-induced internalization of TNF receptor 2 mediated by a di-leucin motif is dispensable for activation of the NFκB pathway

Endocytosis is an important mechanism to regulate tumor necrosis factor (TNF) signaling. In contrast to TNF receptor 1 (TNFR1; CD120a), the relevance of receptor internalization for signaling as well as the fate and route of internalized TNF receptor 2 (TNFR2; CD120b) is poorly understood. To analyze the dynamics of TNFR2 signaling and turnover at the plasma membrane we established a human TNFR2 expressing mouse embryonic fibroblast cell line in a TNFR1(-/-)/TNFR2(-/-) background. TNF stimulation resulted in a decrease of constitutive TNFR2 ectodomain shedding. We hypothesized that reduced ectodomain release is a result of TNF/TNFR2 complex internalization. Indeed, we could demonstrate that TNFR2 was internalized together with its ligand and cytoplasmic binding partners. Upon endocytosis the TNFR2 signaling complex colocalized with late endosome/lysosome marker Rab7 and entered the lysosomal degradation pathway. Furthermore, we identified a di-leucin motif in the cytoplasmic part of TNFR2 suggesting clathrin-dependent internalization of TNFR2. Internalization defective TNFR2 mutants are capable to signal, i.e. activate NFκB, demonstrating that the di-leucin motif dependent internalization is dispensable for this response. We therefore propose that receptor internalization primarily serves as a negative feed-back to limit TNF responses via TNFR2.

ON/OFF and beyond--a boolean model of apoptosis

Apoptosis is regulated by several signaling pathways which are extensively linked by crosstalks. Boolean or logical modeling has become a promising approach to capture the qualitative behavior of such complex networks. Here we built a large-scale literature-based Boolean model of the central intrinsic and extrinsic apoptosis pathways as well as pathways connected with them. The model responds to several external stimuli such as Fas ligand, TNF-α, UV-B irradiation, interleukin-1β and insulin. Timescales and multi-value node logic were used and turned out to be indispensable to reproduce the behavior of the apoptotic network. The coherence of the model was experimentally validated. Thereby an UV-B dose-effect is shown for the first time in mouse hepatocytes. Analysis of the model revealed a tight regulation emerging from high connectivity and spanning crosstalks and a particular importance of feedback loops. An unexpected feedback from Smac release to RIP could further increase complex II formation. The introduced Boolean model provides a comprehensive and coherent description of the apoptosis network behavior. It gives new insights into the complex interplay of pro- and antiapoptotic factors and can be easily expanded to other signaling pathways.

Apoptosis is one of the most investigated topics in the life sciences, especially as this kind of programmed cell death has been linked to several diseases. The strong desire to understand the function and regulation of apoptosis is unfortunately confronted with its complexity and its high degree of cross linking within the cell. Therefore we apply the so-called logical or Boolean mathematical modeling approach to comprehensively describe the numerous interactions in the apoptotic network. Classical Boolean modeling assumes that a certain cellular signal is either present (on) or absent (off). We use extensions of classical Boolean models, namely timescale constants and multi-value nodes, which allow the model to emulate typical apoptotic features. The mathematical model describes for the first time the numerous relevant interactions and signals that control apoptosis in a single and coherent framework. The logical model of apoptosis provides valuable information about the topology of the network including feedback loops and crosstalk effects. Proper investigation of the mutual interactions between species points towards hubs in the network with outstanding relevance. These species are of special interest concerning experimental intervention as well as drug target search. The model we present here is easy to use and freely available.

Nanoscale arrangement of apoptotic ligands reveals a demand for a minimal lateral distance for efficient death receptor activation

Cellular apoptosis, the prototype of programmed cell death, can be induced by activation of so-called death receptors. Interestingly, soluble and membrane-bound members of death receptor ligands can differentially activate their receptors. Using the death receptor ligand tumor necrosis factor (TNF) presented on a surface in a nanoscaled pattern with spacings between 58 and 290 nm, we investigated its requirements for spatial arrangement and motility to efficiently activate TNF receptor (TNFR)1 and TNFR2 as well as its chimeras TNFR1-Fas and TNFR2-Fas. We show that the mere mechanical fixation of TNF is insufficient to efficiently activate TNFR2 that is responsive to only the membrane bound form of TNF but not its soluble form. Rather, an additional stabilization of TNFR2(-Fas) by cluster formation seems to be mandatory for efficient activation. In contrast, TNFR1(-Fas) is strongly activated by TNF spaced within up to 200 nm distances, whereas larger spacings of 290 nm fails completely. Furthermore, unlike for TNFR2(-Fas) no dose-response relationship to increasing distances of nanostructured ligands could be observed for TNFR1-(Fas), suggesting that compartmentalization of the cell membrane in confinement zones of approximately 200 nm regulates TNFR1 activation.

Targeted lipid-coated nanoparticles: delivery of tumor necrosis factor-functionalized particles to tumor cells

Polymeric nanoparticles displaying tumor necrosis factor on their surface (TNF nanocytes) are useful carrier systems capable of mimicking the bioactivity of membrane-bound TNF. Thus, TNF nanocytes are potent activators of TNF receptor 1 and 2 leading to a striking enhancement of apoptosis. However, in vivo applications are hampered by potential systemic toxicity. Here, using TNF nanocytes as a model system, we developed a procedure to generate targeted lipid-coated particles (TLP) in which TNF activity is shielded. The TLPs generated here are composed of an inner single-chain TNF (scTNF)-functionalized, polymeric nanoparticle core surrounded by a lipid coat endowed with polyethylene glycol (PEG) for sterical stabilization and a single-chain Fv (scFv) fragment for targeting. Using a scFv directed against the tumor stroma marker fibroblast activation protein (FAP) we show that TLP and scTNF-TLP specifically bind to FAP-expressing, but not to FAP-negative cells. Lipid coating strongly reduced nonspecific binding of particles and scTNF-mediated cytotoxicity towards FAP-negative cells. In contrast, an increased cytotoxicity of TLP was observed for FAP-positive cells. Thus, through liposome encapsulation, nanoparticles carrying bioactive molecules, which are subject to nonselective uptake and activity towards various cells and tissues, can be converted into target cell-specific composite particles exhibiting a selective activity towards antigen-positive target cells. Besides safe and targeted delivery of death ligands such as TNF, TLP should be suitable for various diagnostic and therapeutic applications, which benefit from a targeted delivery of reagents embedded into the particle core or displayed on the core particle surface.

Single-chain TNF, a TNF derivative with enhanced stability and antitumoral activity

The inflammatory and proapoptotic cytokine TNF possesses a compelling potential as an antitumoral therapeutic agent. Possible target cells include the malignant cells themselves, the tumor vasculature, or the immune system. As the clinical use of TNF is limited by systemic toxicity, targeting strategies using TNF-based fusion proteins are currently used. A major obstacle, however, is that homotrimeric TNF ligands are prone to activity loss due to dissociation into their monomers. In this study, we report the construction of single-chain TNF molecule, a TNF mutant consisting of three TNF monomers fused by short peptide linkers. In comparison to wild-type TNF, single-chain TNF was found to possess increased stability in vitro and in vivo, displayed reduced systemic toxicity yet slightly enhanced antitumoral activity in mouse models. Creation of single-chain variants is a new approach for improvement of functional activity of therapeutics based on TNF family ligands.

Steady state and (bi-) stability evaluation of simple protease signalling networks

Signal transduction networks are complex, as are their mathematical models. Gaining a deeper understanding requires a system analysis. Important aspects are the number, location and stability of steady states. In particular, bistability has been recognised as an important feature to achieve molecular switching. This paper compares different model structures and analysis methods particularly useful for bistability analysis. The biological applications include proteolytic cascades as, for example, encountered in the apoptotic signalling pathway or in the blood clotting system. We compare three model structures containing zero-order, inhibitor and cooperative ultrasensitive reactions, all known to achieve bistability. The combination of phase plane and bifurcation analysis provides an illustrative and comprehensive understanding of how bistability can be achieved and indicates how robust this behaviour is. Experimentally, some so-called "inactive" components were shown to have a residual activity. This has been mostly ignored in mathematical models. Our analysis reveals that bistability is only mildly affected in the case of zero-order or inhibitor ultrasensitivity. However, the case where bistability is achieved by cooperative ultrasensitivity is severely affected by this perturbation.

Tumor Necrosis Factor (TNF)-Functionalized Nanostructured Particles for the Stimulation of Membrane TNF-Specific Cell Responses

Most members of the tumor necrosis factor (TNF) ligand family occur in both a membrane-bound and a soluble form, which can possess differential bioactivities. The aim of this work was the construction of a synthetic-biological hybrid system consisting of chemically nanostructured core-shell particles with a diameter of 100 nm, 1 microm, or 10 microm and the cytokine TNF to obtain a tool that mimics the bioactivity of naturally occurring membrane-bound TNF. Synthetic core-shell nanoparticles consisting of an inorganic silica core and an ultrathin organic shell bearing a maleimide group at the shell surface which allowed for a covalent and site-directed coupling of CysHisTNF mutants were prepared. The TNF mutants were modified at the N-terminus by PCR cloning by introducing a His-Tag for purification and a free cysteine group for reaction with the particle-attached maleimide group. The resulting nanostructured hybrid particles initiated strong TNF receptor type 2 specific responses, otherwise only seen for the membrane-bound form of TNF, but not the soluble cytokine, thus clearly demonstrating new and membrane TNF-like properties of the bioconjugated soluble TNF.

Cationic cell-penetrating peptides interfere with TNF signalling by induction of TNF receptor internalization

Cationic cell-penetrating peptides (CPPs) have been used widely as delivery vectors for the import of molecules that otherwise do not cross the plasma membrane of eukaryotic cells. In this work, we demonstrate that the three cationic CPPs, Antennapedia homeodomain-derived peptide (Antp), nona-arginine and Tat-derived peptide, inhibit tumour necrosis factor (TNF)-mediated signal transduction. This inhibition is based on the downregulation of TNF receptors at the cell surface by induction of internalization. In contrast to TNF-dependent receptor internalization, no receptor activation occurs. The receptor downregulation is not restricted to the CPPs. Remarkably, the HIV-1 Tat protein itself also induces the internalization of TNF receptors. The dynamin dependence of the internalization, as well as the fact that epidermal growth factor receptors are also internalized, suggest a general induction of clathrin-dependent endocytosis as the mechanism of action. The significance of these findings for the use of cationic CPPs in the import of bioactive peptides is demonstrated here using a conjugate consisting of Antp and a Smac protein-derived cargo peptide. The cargo alone, when introduced into cells by electroporation, enhanced TNF-induced apoptosis by inhibiting the anti-apoptotic action of IAPs (inhibitor of apoptosis proteins). For the Antp-Smac conjugate at concentrations below 40 μM the inhibitory effect of the Antp peptide compensated for the pro-apoptotic activity of the cargo, and led to the protection of cells against TNF-mediated apoptosis. These data provide important new information for the use of cationic CPPs for the cellular delivery of bioactive molecules.

Ligands working as receptors: reverse signaling by members of the TNF superfamily enhance the plasticity of the immune system

The inflammatory cytokine tumor necrosis factor (TNF), as well as most other ligand members of the TNF superfamily, exist both as classical soluble cytokines, but also in the form of type II transmembrane proteins. Both forms possess bioactivity, although some effects are distinct. In addition, an increasing body of evidence suggests that the membrane integrated ligands can receive signals, i.e. act as receptors which can transmit positive and negative feedback signals into the ligand bearing cell. Thus, reverse signaling enables a two-way communication in cell-to-cell signaling, and it is conceivable that this bi-directional signal exchange contributes to the plasticity of the ligand-receptor systems. Reverse signaling mainly has been observed in the immune system and within the TNF superfamily. Its function is only beginning to emerge warranting additional investigation, especially when it comes to therapeutic strategies involving cytokine modulation. This review provides an update of the literature about reverse signaling of transmembrane TNF family members and discusses its potential biological and clinical impact.

The Met-196 -> Arg variation of human tumor necrosis factor receptor 2 (TNFR2) affects TNF-alpha-induced apoptosis by impaired NF-kappaB signaling and target gene expression

Tumor necrosis factor-alpha (TNF-alpha)-induced signaling is pivotally involved in the pathogenesis of chronic inflammatory diseases. A polymorphism in the TNF receptor 2 (TNFR2) gene resulting in a juxtamembrane inversion from methionine (TNFR2(196MET)) to arginine (TNFR2(196ARG)) has been genetically associated with an increased risk for systemic lupus erythematosus and familial rheumatoid arthritis. Albeit the mutation does not affect the TNF binding kinetics of TNFR2, the present study provides evidence that the mutation results in a significantly lower capability to induce TNFR2-mediated NF-kappaB activation. Pretriggering of TNFR2 with a receptor-specific mutein leads to an enhancement of TNFR1-induced apoptosis, which is further increased in cells carrying the TNFR2(196ARG) variant. A diminished induction of NF-kappaB-dependent target genes conveying either anti-apoptotic or pro-inflammatory functions, such as cIAP1, TRAF1, IL-6, or IL-8 is observed. The mutated form TNFR2(196ARG) shows a reduction of inducible TRAF2 recruitment upon TNF-alpha stimulation. The findings suggest a common molecular mechanism for the involvement of the TNFR2(196ARG) variant in the etiopathogenesis of different chronic inflammatory disorders.

NFkappaB activation by Fas is mediated through FADD, caspase-8, and RIP and is inhibited by FLIP

Fas (APO-1/CD95) is the prototypic death receptor, and the molecular mechanisms of Fas-induced apoptosis are comparably well understood. Here, we show that Fas activates NFκB via a pathway involving RIP, FADD, and caspase-8. Remarkably, the enzymatic activity of the latter was dispensable for Fas-induced NFκB signaling pointing to a scaffolding-related function of caspase-8 in nonapoptotic Fas signaling. NFκB was activated by overexpressed FLIP_L and FLIP_S in a cell type–specific manner. However, in the context of Fas signaling both isoforms blocked FasL-induced NFκB activation. Moreover, down-regulation of both endogenous FLIP isoforms or of endogenous FLIP_L alone was sufficient to enhance FasL-induced expression of the NFκB target gene IL8. As NFκB signaling is inhibited during apoptosis, FasL-induced NFκB activation was most prominent in cells that were protected by Bcl2 expression or caspase inhibitors and expressed no or minute amounts of FLIP. Thus, protection against Fas-induced apoptosis in a FLIP-independent manner converted a proapoptotic Fas signal into an inflammatory NFκB-related response.

Bistability analyses of a caspase activation model for receptor-induced apoptosis

Apoptosis is an important physiological process crucially involved in development and homeostasis of multicellular organisms. Although the major signaling pathways have been unraveled, a detailed mechanistic understanding of the complex underlying network remains elusive. We have translated here the current knowledge of the molecular mechanisms of the death-receptor-activated caspase cascade into a mathematical model. A reduction down to the apoptotic core machinery enables the application of analytical mathematical methods to evaluate the system behavior within a wide range of parameters. Using parameter values from the literature, the model reveals an unstable status of survival indicating the need for further control. Based on recent publications we tested one additional regulatory mechanism at the level of initiator caspase activation and demonstrated that the resulting system displays desired characteristics such as bistability. In addition, the results from our model studies allowed us to reconcile the fast kinetics of caspase 3 activation observed at the single cell level with the much slower kinetics found at the level of a cell population.

Analogies between Drosophila and mammalian TRAF pathways

A central event in innate immunity is the activation of the NF-kappaB signaling pathway and up-regulation of NF-kappaB-dependent defense genes. Attack of mammals as well as of insects by microorganisms leads, among other things, to the activation of receptors of the Toll-like receptor group. Various adaptor proteins involving members of the TNF receptor-associated factor (TRAF) family channel these receptor-generated signals to conserved intracellular kinase cascades that finally lead to the activation of NF-kappaB and JNK. In vertebrates, TRAF proteins link these pathways also to IL-1R-related molecules and members of the TNF receptor superfamily, which orchestrate a variety of immunoregulatory processes of the innate but also of the adaptive immune system. In this review, we will focus on the similarities but also the differences in TRAF-dependent signaling pathways of mammals and insects.

Tumor Necrosis Factor Receptor-associated Factor (TRAF) 1 Regulates CD40-induced TRAF2-mediated NF-κB Activation

To investigate CD40 signaling complex formation in living cells, we used green fluorescent protein (GFP)-tagged CD40 signaling intermediates and confocal life imaging. The majority of cytoplasmic TRAF2-GFP and, to a lesser extent, TRAF3-GFP, but not TRAF1-GFP or TRAF4-GFP, translocated into CD40 signaling complexes within a few minutes after CD40 triggering with the CD40 ligand. The inhibitor of apoptosis proteins cIAP1 and cIAP2 were also recruited by TRAF2 to sites of CD40 signaling. An excess of TRAF2 allowed recruitment of TRAF1-GFP to sites of CD40 signaling, whereas an excess of TRAF1 abrogated the interaction of TRAF2 and CD40. Overexpression of TRAF1, however, had no effect on the interaction of TRADD and TRAF2, known to be important for tumor necrosis factor receptor 1 (TNF-R1)-mediated NF-kappaB activation. Accordingly, TRAF1 inhibited CD40-dependent but not TNF-R1-dependent NF-kappaB activation. Moreover, down-regulation of TRAF1 with small interfering RNAs enhanced CD40/CD40 ligand-induced NF-kappaB activation but showed no effect on TNF signaling. Because of the trimeric organization of TRAF proteins, we propose that the stoichiometry of TRAF1-TRAF2 heteromeric complexes ((TRAF2)2-TRAF1 versus TRAF2-(TRAF1)2) determines their capability to mediate CD40 signaling but has no major effect on TNF signaling.

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.

Tumor necrosis factor-receptor 2 is up-regulated on lamina propria T cells in Crohn's disease and promotes experimental colitis in vivo

Tumor necrosis factor (TNF) plays a pivotal role in the pathogenesis of Crohn's disease (CD). However, little is known about the role of TNF receptors (TNF-R) in this disease. Here, we found that TNF-R2 (in contrast to TNF-R1) was significantly up-regulated on lamina propria and peripheral blood T cells in CD compared to control patients. To directly test the functional role of TNF-R2 in Th1-mediated experimental colitis in vivo, we took advantage of transgenic animals overexpressing TNF-R2 in T cells. Reconstitution of SCID mice with CD4+ CD62L+ T cells from TNF-R2 transgenic mice led to an earlier wasting syndrome, a more severe colitis and augmented Th1 cytokine production than reconstitution with cells from wild-type littermates. In addition, TUNEL staining revealed a significantly decreased apoptosis rate of lamina propria mononuclear cells in mice reconstituted with TNF-R2 transgenic T cells compared to mice reconstituted with wild-type T cells. In summary, our data suggest a critical regulatory role of TNF-R2 signaling for disease exacerbation in Th1-mediated chronic colitis. Taken together with the increased expression of TNF-R2 in CD, selective targeting of TNF-R2 signaling thus emerges as a potentially novel approach to the treatment of CD.

Non-apoptotic Fas signaling

Fas (Apo-1, CD95) and Fas-Ligand (FasL, CD95L) are typical members of the TNF receptor and TNF ligand family, respectively, with a pivotal role in the regulation of apoptotic processes, including activation-induced cell death, T-cell-induced cytotoxicity, immune privilege and tumor surveillance. Impairment of the FasL/Fas system has been implicated in liver failure, autoimmune diseases and immune deficiency. Thus, the FasL/Fas system was mainly appreciated with respect to its death-inducing capabilities. However, there is increasing evidence that activation of Fas can also result in non-apoptotic responses like cell proliferation or NF-kappaB activation. While the apoptotic features of the FasL/Fas system and the pathways involved are comparably well investigated, the pathways that are utilized by Fas to transduce proliferative and activating signals are poorly understood. This review is focused on the non-apoptotic functions of the FasL/Fas system. In particular, the similarities and differences of the molecular mechanisms of apoptotic and non-apoptotic Fas signaling are addressed.

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.

Control of receptor-induced signaling complex formation by the kinetics of ligand/receptor interaction

Tumor necrosis factor (TNF) exists both as a membrane-integrated type II precursor protein and a soluble cytokine that have different bioactivities on TNFR2 (CD120b) but not on TNFR1 (CD120a). To identify the molecular basis of this disparity, we have investigated receptor chimeras comprising the cytoplasmic part of Fas (CD95) and the extracellular domains of the two TNF receptors. The membrane form of TNF, but not its soluble form, was capable of inducing apoptosis as well as activation of c-Jun N-terminal kinase and NF-kappaB via the TNFR2-derived chimera. In contrast, the TNFR1-Fas chimera displayed strong responsiveness to both TNF forms. This pattern of responsiveness is identical to that of wild type TNF receptors, demonstrating that the underlying mechanisms are independent of the particular type of the intracellular signaling machinery and rather are controlled upstream of the intracellular domain. We further demonstrate that the signaling strength induced by a given ligand/receptor interaction is regulated at the level of adaptor protein recruitment, as shown for FADD, caspase-8, and TRAF2. Since both incidents, strong signaling and robust adapter protein recruitment, are paralleled by a high stability of individual ligand-receptor complexes, we propose that half-lives of individual ligand-receptor complexes control signaling at the level of adaptor protein recruitment.

Tumor necrosis factor-receptor 2 is up-regulated on lamina propria T cells in Crohn's disease and promotes experimental colitis in vivo

Tumor necrosis factor (TNF) plays a pivotal role in the pathogenesis of Crohn's disease (CD). However, little is known about the role of TNF receptors (TNF-R) in this disease. Here, we found that TNF-R2 (in contrast to TNF-R1) was significantly up-regulated on lamina propria and peripheral blood T cells in CD compared to control patients. To directly test the functional role of TNF-R2 in Th1-mediated experimental colitis in vivo, we took advantage of transgenic animals overexpressing TNF-R2 in T cells. Reconstitution of SCID mice with CD4+ CD62L+ T cells from TNF-R2 transgenic mice led to an earlier wasting syndrome, a more severe colitis and augmented Th1 cytokine production than reconstitution with cells from wild-type littermates. In addition, TUNEL staining revealed a significantly decreased apoptosis rate of lamina propria mononuclear cells in mice reconstituted with TNF-R2 transgenic T cells compared to mice reconstituted with wild-type T cells. In summary, our data suggest a critical regulatory role of TNF-R2 signaling for disease exacerbation in Th1-mediated chronic colitis. Taken together with the increased expression of TNF-R2 in CD, selective targeting of TNF-R2 signaling thus emerges as a potentially novel approach to the treatment of CD.

TNF-related apoptosis inducing ligand (TRAIL) and its receptors in tumor surveillance and cancer therapy

TNF-related apoptosis-inducing ligand (TRAIL/APO-2L) is a typical member of the TNF ligand family that induces apoptosis by activating the death receptors TRAIL-R1 and TRAIL-R2. TRAIL has attracted great attention in recent years as a promising anti cancer reagent because recombinant soluble TRAIL derivatives induce apoptosis in a broad range of tumor cells but not or only rarely in non-transformed cells. In this review we will address the putative role of TRAIL in cancer treatment in the light of the emerging importance of TRAIL in tumor surveillance and discuss the molecular basis of the cooperation of TRAIL and chemotherapeutic drugs. In particular, we debate controversial data in the literature concerning the cytotoxicity of different TRAIL derivatives on primary human cells.

Intracellular localization and transcriptional regulation of tumor necrosis factor (TNF) receptor-associated factor 4 (TRAF4)

To gain insight in the subcellular localization of tumor necrosis factor receptor-associated factor (TRAF4) we analyzed GFP chimeras of full-length TRAF4 and various deletion mutants derived thereof. While TRAF4-GFP (T4-GFP) was clearly localized in the cytoplasm, the N-terminal deletion mutant, T4(259-470), comprising the TRAF domain of the molecule, and a C-terminal deletion mutant consisting mainly of the RING and zinc finger domains of TRAF4 were both localized predominantly to the nucleus. Passive nuclear localization of T4(259-470) can be ruled out as the TRAF domain of TRAF4 was sufficient to form high molecular weight complexes. T4(259-470) recruited full-length TRAF4 into the nucleus whereas TRAF4 was unable to change the nuclear localization of T4(259-470). Thus, it seems that individual T4(259-470) mutant molecules are sufficient to direct the respective TRAF4-T4(259-470) heteromeric complexes into the nucleus. In cells forming cell-cell contacts, TRAF4 was recruited to the sites of contact via its C-TRAF domain. The expression of some TRAF proteins is regulated by the NF-kappaB pathway. Thus, we investigated whether this pathway is also involved in the regulation of the TRAF4 gene. Indeed, in primary T-cells and Jurkat cells stimulated with the NF-kappaB inducers TNF or phorbol 12-myristate 13-acetate (PMA), TRAF4-mRNA was rapidly up-regulated. In Jurkat T-cells deficient for I-kappaB kinase gamma (IKKgamma, also known as NEMO), an essential component of the NF-kappaB-inducing-IKK complex, induction of TRAF4 was completely inhibited. In cells deficient for RIP (receptor interactive protein), an essential signaling intermediate of TNF-dependent NF-kappaB activation, TNF-, but not PMA-induced up-regulation of TRAF4 was blocked. These data suggest that activation of the NF-kappaB pathway is involved in up-regulation of TRAF4 in T-cells.

Endogenous membrane tumor necrosis factor (TNF) is a potent amplifier of TNF receptor 1-mediated apoptosis

The heat shock protein 90 (Hsp-90) inhibitor, geldanamycin, and the proteasome inhibitor, MG-132, both inhibited tumor necrosis factor receptor 1 (TNF-R1)- but not TRAIL-induced apoptosis in Kym-1 cells, suggesting that TNF-R1-induced cell death is dependent on NF-kappaB activation in this model. Triggering of TNF-R1 by agonistic antibodies led to cell-type specific induction of endogenous TNF and apoptosis, the latter of which was abrogated by neutralizing TNF specific antibodies. TNF-R1-stimulated cells expressed TNF mainly in a cell-associated form, suggesting that the endogenously produced TNF act in its membrane-bound form. Geldanamycin failed to inhibit apoptosis induction by a combination of agonistic TNF-R1- and TNF-R2-specific antibodies, indicating that both TNF receptors co-operate in TNF-R1-triggered apoptosis in Kym-1 cells. Thus, TNF-R1 stimulation can elicit a strong and rapid apoptotic response via induction of membrane TNF and subsequent cooperation of TNF-R1 and TNF-R2. Moreover, we give evidence that this mechanism circumvents the need of the prolonged presence of exogenous soluble TNF for TNF-R1-mediated apoptosis induction.

Apoptotic crosstalk of TNF receptors: TNF-R2-induces depletion of TRAF2 and IAP proteins and accelerates TNF-R1-dependent activation of caspase-8

We have recently shown that stimulation of TNF-R2 selectively enhances apoptosis induction by the death receptor TNF-R1. Here, we demonstrate that stimulation of CD30 or CD40 also leads to selective enhancement of TNF-R1-induced cell death. Enhancement of apoptosis was correlated with the depletion of endogenous TRAF2 within 1 to 6 hours. Selective prestimulation of TNF-R2 for several hours inhibited TNF-R2-induced activation of the anti-apoptotic NF-κB pathway up to 90% and dramatically enhanced apoptosis induction by this receptor. When both TNF-receptors were stimulated simultaneously, TNF-R1-induced NF-κB activation remained unaffected but TNF-R1-induced apoptosis was still significantly enhanced. Compared with FasL-induced cell death TNF-R1-induced activation of caspase-8 was significantly weaker and delayed. Costimulation or prestimulation of TNF-R2 enhanced caspase-8 processing. Life cell imaging and confocal microscopy revealed that both TNF-R1 and TNF-R2 recruited the anti-apoptotic factor cIAP1 in a TRAF2-dependent manner. Thus, TNF-R2 may compete with TNF-R1 for the recruitment of newly synthesized TRAF2-bound anti-apoptotic factors, thereby promoting the formation of a caspase-8-activating TNF-R1 complex. Hence,TNF-R2 triggering can interfere with TNF-R1-induced apoptosis by inhibition of NF-κB-dependent production of anti-apoptotic factors and by blocking the action of anti-apoptotic factors at the post-transcriptional level.

Tumor necrosis factor (TNF) and phorbol ester induce TNF-related apoptosis-inducing ligand (TRAIL) under critical involvement of NF-kappa B essential modulator (NEMO)/IKKgamma

We show that tumor necrosis factor (TNF) and phorbol 12-myristate 13-acetate (PMA) induce TNF-related apoptosis-inducing ligand (TRAIL) in T cells. In cells deficient for NF-kappaB essential modulator (NEMO)/IKKgamma, an essential component of the NF-kappaB-inducing I-kappaB kinase (IKK) complex, induction of TRAIL expression was completely abrogated but was recovered in cells restored for IKKgamma expression. In cells deficient for receptor-interacting protein expression TNF, but not PMA-induced TRAIL expression was blocked. Inhibition of protein synthesis with cycloheximide blocked PMA, but not TNF-induced up-regulation of TRAIL. As both TNF and PMA rapidly induce NF-kappaB activation this suggests that NEMO/IKKgamma-dependent activation of the NF-kappaB pathway is necessary but not sufficient for up-regulation of TRAIL in T cells. The capability of the NF-kappaB pathway to induce the potent death ligand TRAIL may explain the reported proapoptotic features of this typically antiapoptotic pathway.