Biochemical properties of the 75-kDa tumor necrosis factor receptor. Characterization of ligand binding, internalization, and receptor phosphorylation

Implications of ADAM17 activation for hyperglycaemia, obesity and type 2 diabetes

In this review, we focus specifically on the role that the metalloproteinase, A Disintegrin and Metalloproteinase 17 [ADAM17] plays in the development and progression of the metabolic syndrome. There is a well-recognised link between the ADAM17 substrate tumour necrosis factor α (TNF-α) and obesity, inflammation and diabetes. In addition, knocking out ADAM17 in mice leads to an extremely lean phenotype. Importantly, ADAM17-deficient mice exhibit one of the most pronounced examples of hypermetabolism in rodents to date. It is vital to further understand the mechanistic role that ADAM17 plays in the metabolic syndrome. Such studies will demonstrate that ADAM17 is a valuable therapeutic target to treat obesity and diabetes.

Lack of Evidence for a Direct Interaction of Progranulin and Tumor Necrosis Factor Receptor-1 and Tumor Necrosis Factor Receptor-2 From Cellular Binding Studies

Progranulin (PGRN) is a secreted anti-inflammatory protein which can be processed by neutrophil proteases to various granulins. It has been reported that at least a significant portion of the anti-inflammatory effects of PGRN is due to direct high affinity binding to tumor necrosis factor receptor-1 (TNFR1) and TNFR2 and inhibition of tumor necrosis factor (TNF)-induced TNFR1/2 signaling. Two studies failed to reproduce the interaction of TNFR1 and TNFR2 with PGRN, but follow up reports speculated that this was due to varying experimental circumstances and/or the use of PGRN from different sources. However, even under consideration of these speculations, there is still a striking discrepancy in the literature between the concentrations of PGRN needed to inhibit TNF signaling and the concentrations required to block TNF binding to TNFR1 and TNFR2. While signaling events induced by 0.2–2 nM of TNF have been efficiently inhibited by low, near to equimolar concentrations (0.5–2.5 nM) of PGRN in various studies, the reported inhibitory effects of PGRN on TNF-binding to TNFR1/2 required a huge excess of PGRN (100–1,000-fold). Therefore, we investigated the effect of PGRN on TNF binding to TNFR1 and TNFR2 in highly sensitive cellular binding studies. Unlabeled TNF inhibited >95% of the specific binding of a Gaussia princeps luciferase (GpL) fusion protein of TNF to TNFR1 and TNFR2 and blocked binding of soluble GpL fusion proteins of TNFR1 and TNFR2 to membrane TNF expressing cells to >95%, too. Purified PGRN, however, showed in both assays no effect on TNF–TNFR1/2 interaction even when applied in huge excess. To rule out that tags and purification- or storage-related effects compromise the potential ability of PGRN to bind TNF receptors, we directly co-expressed PGRN, and as control TNF, in TNFR1- and TNFR2-expressing cells and looked for binding of GpL-TNF. While expression of TNF strongly inhibited binding of GpL-TNF to TNFR1/2, co-expression of PGRN had not effect on the ability of the TNFR1/2-expressing cells to bind TNF.

Expression of TNFα membrane-bound receptors in the peripheral blood mononuclear cells (PMBC) in rheumatoid arthritis patients

OBJECTIVE

To investigate the expression of TNFα membrane-bound receptors: the percentage of cells expressing these receptors and the number of molecules expressed on different immune cell subsets, and to evaluate serum concentrations of soluble TNFα and its receptors (sTNFRI and sTNFRII) in patients with rheumatoid arthritis in acute stage and after response to treatment compared to healthy donors.

METHODS

The objects of the study are peripheral blood mononuclear cells (PBMC) of healthy donors (n=150) and RA patients (n=40) subjected to hospital treatment with either biological agents (Rituximab) or glucocorticosteroids (methylprednisolone). To determine PBMC phenotype antibodies anti-hCD3-APC, anti-hCD19 PECy7, anti-hCD14 FITC (eBioscience), as well as anti-hTNFRI-PE and anti-hTNFRII-PE (R&D Systems) were used. To determine receptor number on the cells Quantibrite PE Beads (BD) were used.

RESULTS

Cells obtained from patients who responded to therapy and achieved disease remission exhibited either an increase in the percentage of TNFRI+ cells or elevated expression density of this receptor type.

CONCLUSION

Subsets of immunocompetent cells from RA patients show variation in the percentage of membrane-bound receptor positive cells and receptor expression density, which influences the development and progression of the pathological processes in RA. Response to therapy and achievement of disease remission are associated with an increase of TNFRI expression.

Levels of TNF, TNF autoantibodies and soluble TNF receptors in patients with bronchial asthma

OBJECTIVES

The aim of the study was to evaluate the potential contribution made by tumor necrosis factor (TNF) autoantibodies to the pathogenesis of bronchial asthma (BA).

METHODS

We used affinity chromatography methods and a magnetic separation procedure to purify human autoantibodies specific to TNF. The autoantibodies were used as a calibration material to determine the absolute content of autoantibodies to TNF using enzyme-linked immunosorbent assay (ELISA). TNF content and levels of soluble receptors to TNF were determined using the ELISA commercial test kits.

RESULTS

We demonstrated significant increases in the levels of TNF and soluble TNF receptors in the sera of patients with uncontrolled and controlled BA, as compared with healthy donors. Levels of autoantibodies of the IgG2 and IgG4 subclasses were significantly higher in sera from patients with uncontrolled BA than in healthy donors. Levels of IgG2 autoantibodies were significantly higher in sera from patients with uncontrolled BA than in patients with controlled BA.

CONCLUSIONS

BA is associated with changes in the levels of not only TNF and soluble receptors for TNF, but also autoantibodies to TNF. Given the magnitude of the changes in the levels of different subclasses of autoantibodies to TNF, we propose that these autoantibodies might contribute to the pathogenesis of BA.

Identification of key processes that control tumor necrosis factor availability in a tuberculosis granuloma

Tuberculosis (TB) granulomas are organized collections of immune cells comprised of macrophages, lymphocytes and other cells that form in the lung as a result of immune response to Mycobacterium tuberculosis (Mtb) infection. Formation and maintenance of granulomas are essential for control of Mtb infection and are regulated in part by a pro-inflammatory cytokine, tumor necrosis factor-α (TNF). To characterize mechanisms that control TNF availability within a TB granuloma, we developed a multi-scale two compartment partial differential equation model that describes a granuloma as a collection of immune cells forming concentric layers and includes TNF/TNF receptor binding and trafficking processes. We used the results of sensitivity analysis as a tool to identify experiments to measure critical model parameters in an artificial experimental model of a TB granuloma induced in the lungs of mice following injection of mycobacterial antigen-coated beads. Using our model, we then demonstrated that the organization of immune cells within a TB granuloma as well as TNF/TNF receptor binding and intracellular trafficking are two important factors that control TNF availability and may spatially coordinate TNF-induced immunological functions within a granuloma. Further, we showed that the neutralization power of TNF-neutralizing drugs depends on their TNF binding characteristics, including TNF binding kinetics, ability to bind to membrane-bound TNF and TNF binding stoichiometry. To further elucidate the role of TNF in the process of granuloma development, our modeling and experimental findings on TNF-associated molecular scale aspects of the granuloma can be incorporated into larger scale models describing the immune response to TB infection. Ultimately, these modeling and experimental results can help identify new strategies for TB disease control/therapy.

Tuberculosis is a common and deadly infectious disease caused by a highly successful bacterium, Mycobacterium tuberculosis (Mtb). Multiple host immune factors control the formation of a self-organizing aggregate of immune cells termed a granuloma in the lungs after inhalation of Mtb. One such factor, tumor necrosis factor-α (TNF), is a protein that regulates inflammatory immune responses. Availability of TNF within a TB granuloma has been proposed to have a critical role in the protective immunity against TB. However, direct measurement of the level of TNF in a granuloma is not experimentally feasible. Therefore, we develop a mathematical model based on an experimental model of granuloma developed in mice to predict TNF availability in a granuloma. We measure values of critical model parameters and explore mechanisms that influence TNF availability in the granuloma. We find that cellular organization in a granuloma and intracellular trafficking of TNF control TNF availability in a granuloma. Further, our model analysis also highlights anti-TNF drug properties that determine their TNF neutralization power. Our findings complement and extend those of recent studies on the role of TNF in the immune response against TB.

Tumor necrosis factor and stroke: role of the blood-brain barrier

The progression and outcome of stroke is affected by the intricate relationship between the blood-brain barrier (BBB) and tumor necrosis factor alpha (TNFalpha). TNFalpha crosses the intact BBB by a receptor-mediated transport system that is upregulated by CNS trauma and inflammation. In this review, we discuss intracellular trafficking and transcytosis of TNFalpha, regulation of TNFalpha transport after stroke, and the effects of TNFalpha on stroke preconditioning. TNFalpha can activate cytoprotective pathways by pretreatment or persistent exposure to low doses. This explains the paradoxical observation that transport of this proinflammatory cytokine improves the survival and function of hypoxic cells and of mice with stroke. The dual effects of TNFalpha may be related to differential regulation of TNFalpha trafficking downstream to TNFR1 and TNFR2 receptors. As we better understand how peripheral TNFalpha affects its own transport and modulates neuroregeneration, we may be in a better position to pharmacologically manipulate its regulatory transport system to treat stroke.

ATP-depleting carbohydrates prevent tumor necrosis factor receptor 1-dependent apoptotic and necrotic liver injury in mice

We demonstrated previously that depletion of hepatic ATP by endogenous metabolic shunting of phosphate after fructose treatment renders hepatocytes resistant to tumor necrosis factor (TNF)-induced apoptosis. We here address the question whether this principle extends to TNF receptor 1-mediated caspase-independent apoptotic and to necrotic liver injury. As in the apoptotic model of galactosamine/lipopolysaccharide (LPS)-induced liver damage, the necrotic hepatotoxicity initiated by sole high-dose LPS treatment was abrogated after depletion of hepatic ATP. Although systemic TNF and interferon-gamma levels were suppressed, animals still were protected when ATP depletion was initiated after the peak of proinflammatory cytokines upon LPS injection, showing that fructose-induced ATP depletion affects both cytokine release and action. In T cell-dependent necrotic hepatotoxicity elicited by concanavalin A or galactosamine + staphylococcal enterotoxin B, ATP depletion prevented liver injury as well, but here without modulating cytokine release. By attenuating caspase-8 activation, ATP depletion of hepatocytes in vitro impaired TNF receptor signaling by the death-inducing signaling complex, whereas receptor internalization and nuclear factor-kappaB activation upon TNF stimulation were unaffected. These findings demonstrate that sufficient target cell ATP levels are required for the execution of both apoptotic and necrotic TNF-receptor 1-mediated liver cell death.

Intracellular signals and events activated by cytokines of the tumor necrosis factor superfamily: From simple paradigms to complex mechanisms

Tumor necrosis factor (TNF) and several related cytokines can induce opposite effects such as cell activation and proliferation or cell death. How the cell maintains the balance between these seemingly mutually exclusive pathways has long remained a mystery. TNF receptor I (TNFRI) initially emerged as a potent activator of NFkappaB and AP-1 transcription factors, while the related CD95 (Fas, Apo-1) was recognized as a prototype death receptor. Advances in research have uncovered critical molecular players in these intracellular processes. They have also revealed a much more complex picture than originally thought. Several new signaling pathways, including the alternative NFkappaB activation cascade, have been uncovered, and previously unknown modes of cross-talk between intracellular signaling molecules were revealed. It also turned out that signaling mechanisms mediated by the TNF receptor superfamily members can operate not only in the immune system but also in organ development.

Neuroendocrine Modulation Induced by Selective Blockade of TNF- in Rheumatoid Arthritis

Tumor necrosis factor-alpha (TNFalpha) is a main actor in the pathogenesis of rheumatoid arthritis (RA), interacting with other molecules in complex mechanisms. The neuroendocrine system is known to be involved and Chromogranin A (CHGA) serum levels are elevated in patients with RA. We evaluated the effect of the selective blockade of TNF-alpha, induced by treatment with anti-TNF-alpha monoclonal antibodies (mAbs), on the serum levels of CHGA and on its correlation with TNF-alpha and TNF-alpha receptors (TNFRs) serum levels. Seven patients with RA have been treated with the anti-TNF-alpha mAb, infliximab. We measured the serum levels of TNF-alpha, its receptors (tumor necrosis factor receptor-I [TNFR-I] and tumor necrosis factor receptor-II [TNFR-II]), and CHGA before and during the treatment. We also measured, as a control, the serum levels of CHGA, TNF-alpha, and soluble TNFRs in 14 patients who were being treated with infliximab, adalimumab, or etanercept and in 20 matching negative controls. The serum levels of TNFR-I and TNFR-II, which are a sensitive marker for the TNF-alpha pathway, correlated with those of CHGA before treatment (Pearson's coefficient, respectively, 0.59 and 0.53). Treatment with anti-TNF-alpha mAb provided a significant clinical response in all patients and the correlation between CHGA and TNFR-I and TNFR-II was no more evident during treatment (respectively, -0.09 and -0.07). TNF-alpha blockade allows a clinical effect in patients with RA and modifies the correlation between CHGA and TNFRs, suggesting that TNF-alpha and CHGA reciprocally interfere in the pathogenesis of RA, through intermediate adaptors, whose identification warrants further studies.

Colocalization of endogenous TNF with a functional intracellular splice form of human TNF receptor type 2

Background

Tumor necrosis factor (TNF) is a pleiotropic cytokine involved in a broad spectrum of inflammatory and immune responses including proliferation, differentiation, and cell death. The biological effects of TNF are mediated via two cell surface TNF receptors: p55TNFR (TNFR1; CD120a) and p75TNFR (TNFR2; CD120b). Soluble forms of these two receptors consisting of the extracellular domains are proteolytically cleaved from the membrane and act as inhibitors. A novel p75TNFR isoform generated by the use of an additional transcriptional start site has been described and was termed hicp75TNFR. We focused on the characterization of this new isoform as this protein may be involved in chronic inflammatory processes.

Methods

Cell lines were retroviraly transduced with hp75TNFR isoforms. Subcellular localization and colocalization studies with TNF were performed using fluorescence microscopy including exhaustive photon reassignment software, flow cytometry, and receptosome isolation by magnetic means. Biochemical properties of the hicp75TNFR were determined by affinity chromatography, ELISA, and western blot techniques.

Results

We describe the localization and activation of a differentially spliced and mainly intracellularly expressed isoform of human p75TNFR, termed hicp75TNFR. Expression studies with hicp75TNFR cDNA in different cell types showed the resulting protein mostly retained in the trans-Golgi network and in endosomes and colocalizes with endogenous TNF. Surface expressed hicp75TNFR behaves like hp75TNFR demonstrating susceptibility for TACE-induced shedding and NFκB activation after TNF binding.

Conclusion

Our data demonstrate that intracellular hicp75TNFR is not accessible for exogenously provided TNF but colocalizes with endogenously produced TNF. These findings suggest a possible intracellular activation mechanism of hicp75TNFR by endogenous TNF. Subsequent NFκB activation might induce anti-apoptotic mechanisms to protect TNF-producing cells from cytotoxic effects of TNF. In addition, the intracellular and not TACE-accessible splice form of the hp75TNFR could serve as a pool of preformed, functional hp75TNFR.

Coupling Tumor Necrosis Factor-α with αVIntegrin Ligands Improves Its Antineoplastic Activity

Despite the impressive results obtained in animal models, the clinical use of tumor necrosis factor-alpha (TNF) as an anticancer drug is limited by severe toxicity. We have shown previously that targeted delivery of TNF to aminopeptidase N (CD13), a marker of angiogenic vessels, improved the therapeutic index of this cytokine in tumor-bearing mice. To assess whether the vascular-targeting approach could be extended to other markers of tumor blood vessels, in this work, we have fused TNF with the ACDCRGDCFCG peptide, a ligand of alpha(V) integrins by recombinant DNA technology. We have found that subnanogram doses of this conjugate are sufficient to induce antitumor effects in tumor-bearing mice when combined with melphalan, a chemotherapeutic drug. Cell adhesion assays and competitive binding experiments with anti-integrin antibodies showed that the Arg-Gly-Asp moiety interacts with cell adhesion receptors, including alpha(V)beta(3) integrin, as originally postulated. In addition, ACGDRGDCFCG-mouse TNF conjugate induced cytotoxic effects in standard cytolytic assays, implying that ACGDRGDCFCG-mouse TNF conjugate can also bind TNF receptors and trigger death signals. These results indicate that coupling TNF with alpha(V) integrin ligands improves its antineoplastic activity and supports the concept that vascular targeting is a strategy potentially applicable to different endothelial markers, not limited to CD13.

Signaling to gene activation and cell death by tumor necrosis factor receptors and Fas

Tumor necrosis factor (TNF) receptors and Fas elicit a wide range of biological responses, including cell death, cell proliferation, inflammation, and differentiation. The pleiotropic character of these receptors is reflected at the level of signal transduction. The cytotoxic effects of TNF and Fas result from the activation of an apoptotic/necrotic program. On the other hand, TNF receptors, and under certain conditions also Fas, exert a proinflammatory function that results from the induction of several genes. In this context, the transcription factor nuclear factor-kappa B (NF-kappaB) plays an important role. NF-kappaB is also important for the induction of several antiapoptotic genes, which explains at least partially why several cell types can only be killed by TNF in the presence of transcription or translation inhibitors. It is the balance between proapoptotic and antiapoptotic pathways that determines whether a cell will finally die or proliferate. A third signal transduction pathway that is activated in response to TNF is the mitogen-activated protein kinase cascade, which plays an important role in the modulation of transcriptional gene activation.

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.

Exploring the formation of Alzheimer's disease senile plaques in silico

An experimental simulation environment suitable for exploring the neuroinflammatory hypothesis of Alzheimer's disease (AD) has been developed. Using scientific literature, we have calculated parameters and rates and constructed an interactive model system. The simulation can be manipulated to explore competing hypotheses about AD pathology, i.e. can be used as an experimental "in silico" system. In this paper, we outline the assumptions and aspects of the model, and illustrate qualitative and quantitative findings. The interactions of amyloid beta deposits, glial cell dynamics, inflammation and secreted cytokines, and the stress, recovery, and death of neuronal tissue are investigated. The model leads to qualitative insights about relative roles of the cells and chemicals in the disease pathology.

TNF receptor subtype signalling: differences and cellular consequences

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

Reactive oxygen species are downstream products of TRAF-mediated signal transduction

Members of the TNFR (tumor necrosis factor receptor) superfamily are involved in regulating activation and differentiation of cells as well as cell survival and programmed cell death/apoptosis. Multimerization of TNFRs can lead to recruitment of TRAFs (TNFR-associated factors) by the receptors resulting in activation of kinases and transcription factors, such as c-Jun N-terminal kinase and nuclear factor kappaB (NF-kappaB). Signal transduction triggered by TNF-alpha also induces an increase in intracellular reactive oxygen species (ROS). ROS have been suggested to play a role in NF-kappaB activation, which is thought to promote cell survival. However, oxidation of proteins and lipids by ROS can also result in apoptosis. The processes generating intracellular ROS and the mechanism(s) regulating the cellular redox status have not been fully elucidated. We investigated whether TRAFs play a role in controlling intracellular ROS levels. Our results indicate that recruitment of TRAFs to the plasma membrane of human embryonic kidney (HEK) 293 cells is crucial for activation of signaling pathways, which regulate ROS production in mitochondria. TRAF-mediated changes in ROS levels enhanced NF-kappaB activation but were not dependent on NF-kappaB-inducing kinase. Consistent with its anti-apoptotic function, Bcl-x(L) interfered with TRAF-mediated ROS generation but not NF-kappaB activation. Taken together, our results suggest a novel role of TRAFs in signal transduction pathways triggered by TNFR-related proteins, which balance cell survival and apoptosis by regulating the electron transport in mitochondria.

BLyS BINDS TO B CELLS WITH HIGH AFFINITY AND INDUCES ACTIVATION OF THE TRANSCRIPTION FACTORS NF-kappaB AND ELF-1

B lymphocyte stimulator (BLyS) is a novel member of the TNF family of proteins expressed by myeloid cells as membrane-bound and soluble forms. BLyS was shown to act specifically on B cells, inducing proliferation and immunoglobulin production both in vitro and in vivo. The present study was undertaken to characterize binding of radiolabeled BLyS to its cognate receptor on human B lymphocytes and examine intracellular events initiated by BLyS binding. Similar to other TNF family members, BLyS is present in solution as a homotrimer as determined by gel filtration chromatography and light scattering analysis. BLyS binding to B cells is specific as other TNF family members tested did not compete for(125)I-BLyS binding. Analysis of equilibrium binding of(125)I-labeled BLyS to purified human tonsillar B cells demonstrated saturable binding. Scatchard analysis of the binding data revealed a single class of high-affinity binding on human B cells with approximately 2600 binding sites per cell and an apparent dissociation constant (K(D)) of about 0.1 nM. In addition we report that BLyS binding to B cells results in the activation of NF-kappaB and the Ets family transcription factor, ELF-1, and in the induction of mRNA for Polo-like kinase (PLK).

Defining therapeutic targets by using adenovirus: blocking NF-kappaB inhibits both inflammatory and destructive mechanisms in rheumatoid synovium but spares anti-inflammatory mediators

The role of the transcription factor NF-kappaB in the pathogenesis of rheumatoid arthritis has long been a subject of controversy. We used an adenoviral technique of blocking NF-kappaB through overexpression of the inhibitory subunit IkappaBalpha, which has the advantage that it can be used in the diseased tissue itself, with >90% of the synovial macrophages, fibroblasts, and T cells infected. We found that the spontaneous production of tumor necrosis factor alpha and other pro-inflammatory cytokines is NF-kappaB-dependent in rheumatoid synovial tissue, in contrast to the main anti-inflammatory mediators, like IL-10 and -11, and the IL-1 receptor antagonist. Of even more interest, IkappaBalpha overexpression inhibited the production of matrix metalloproteinases 1 and 3 while not affecting their tissue inhibitor. Blocking NF-kappaB in the rheumatoid joint thus has a very beneficial profile, reducing both the inflammatory response and the tissue destruction. The adenoviral technique described here has widespread applicability, allowing rapid testing of the effects of blocking a potential therapeutic target in either cultures of normal cells or in the diseased tissue itself.

Soluble tumor necrosis factor receptor (p75) fusion protein (ENBREL) as a therapy for rheumatoid arthritis

Soluble tumor necrosis factor (TNF) receptor fusion protein (p75) (Enbrel) is a reversible inhibitor of the biologic effects of TNF. Enbrel has been shown in placebo-controlled trials to significantly improve the signs and symptoms of rheumatoid arthritis. Clinical trials are now in progress to assess the safety and efficacy of Enbrel in combination with methotrexate in refractory rheumatoid arthritis along with trials to compare Enbrel to methotrexate in patients with early rheumatoid arthritis.

Osteoprotegerin is a receptor for the cytotoxic ligand TRAIL

TRAIL is a tumor necrosis factor-related ligand that induces apoptosis upon binding to its death domain-containing receptors, DR4 and DR5. Two additional TRAIL receptors, TRID/DcR1 and DcR2, lack functional death domains and function as decoy receptors for TRAIL. We have identified a fifth TRAIL receptor, namely osteoprotegerin (OPG), a secreted tumor necrosis factor receptor homologue that inhibits osteoclastogenesis and increases bone density in vivo. OPG-Fc binds TRAIL with an affinity of 3.0 nM, which is slightly weaker than the interaction of TRID-Fc or DR5-Fc with TRAIL. OPG inhibits TRAIL-induced apoptosis of Jurkat cells. Conversely, TRAIL blocks the anti-osteoclastogenic activity of OPG. These data suggest potential cross-regulatory mechanisms by OPG and TRAIL.

Mutations which abolish phosphorylation of the TRAF-binding domain of TNF receptor 2 enhance receptor-mediated NF-kappa B activation

We demonstrate that a 41 amino acid region (amino acids 379 to 419) in the cytoplasmic domain of tumor necrosis factor receptor 2 (TNFR2) is phosphorylated by unidentified kinase(s) both in vitro and in vivo. This domain (denoted x1c) corresponds almost exactly to the previously identified TRAF-binding domain and is by itself sufficient as a substrate for phosphorylation. In addition, the x1c domain is also crucial for TNFR2-mediated NF-kappa B activation. The cytoplasmic domain of TNFR2 lacks tyrosines, and conversion of all 12 potential serine and threonine phosphorylation targets in x1c to alanines either had no effect on NF-kappa B activation or resulted in enhanced NF-kappa B activity, depending on the structural context of x1c. The results show that while the TRAF-binding domain of TNFR2 is a major target of kinases, its phosphorylation is not required for NF-kappa B activation. Our data moreover suggest that phosphorylation of x1c negatively regulates the activation of NF-kappa B.

The type 1 receptor (CD120a) is the high-affinity receptor for soluble tumor necrosis factor

Tumor necrosis factor (TNF) can induce a variety of cellular responses at low picomolar concentrations. This is in apparent conflict with the published dissociation constants for TNF binding to TNF receptors in the order of 100-500 pM. To elucidate the mechanisms underlying the outstanding cellular sensitivity to TNF, we determined the binding characteristics of TNF to both human TNF receptors at 37 degrees C. Calculation of the dissociation constant (Kd) from the association and dissociation rate constants determined at 37 degrees C revealed a remarkable high affinity for TNF binding to the 60-kDa TNF type 1 receptor (TNF-R1; Kd = 1.9 x 10(-11) M) and a significantly lower affinity for the 80-kDa TNF type 2 receptor (TNF-R2; Kd = 4.2 x 10(-10) M). The high affinity determined for TNF-R1 is mainly caused by the marked stability of ligand-receptor complexes in contrast to the transient interaction of soluble TNF with TNF-R2. These data can readily explain the predominant role of TNF-R1 in induction of cellular responses by soluble TNF and suggest the stability of the TNF-TNF receptor complexes as a rationale for their differential signaling capability. In accordance with this reasoning, the lower signaling capability of homotrimeric lymphotoxin, compared with TNF, correlates with a lower stability of the lymphotoxin-TNF-R1 complex at 37 degrees C.

Serine/threonine kinase activity associated with the cytoplasmic domain of the lymphotoxin-beta receptor in HepG2 cells

The lymphotoxin-beta receptor (LT-betaR) has been shown to be the receptor for the membrane-bound lymphotoxin heterotrimers LTalpha1/beta2 and LTalpha2/beta1. The extracellular domain of LT-betaR shows extensive similarity with members of the tumor necrosis factor receptor family, while its cytoplasmic domain is distinct and lacks any inherent enzymatic activity. This suggests that the interaction of LT-betaR with other molecules might be important for signal transduction. Here we demonstrate the association of a fusion protein, comprising glutathione S-transferase and the cytoplasmic domain of LT-betaR (GST-LT-betaR(CD)), with several proteins in the size range 29-80 kDa from HepG2 cell lysates. We present evidence that two of these proteins are serine/threonine kinases, which associate with amino acids 324-377 of the cytoplasmic domain of LT-betaR and phosphorylate this receptor. The characteristics of these novel kinases indicate that they are distinct from the previously described tumor necrosis factor receptor-associated kinases. This suggests the presence of novel signal transduction pathway(s) for LT-betaR.

The p80 TNF receptor-associated kinase (p80TRAK) associates with residues 354-397 of the p80 cytoplasmic domain: similarity to casein kinase

The cytoplasmic domain of the p80 TNF receptor associates with a protein kinase, termed p80TRAK, that phosphorylates both the p60 and p80 TNF receptors. To determine the region of the cytoplasmic domain that is necessary for binding of p80TRAK and the region that it phosphorylates, a series of deletions of the p80 cytoplasmic domain were constructed and expressed as glutathione-S-transferase fusion proteins. These fusions were then used to examine the binding of p80TRAK derived from cellular extracts. We found that out of 174 residues (266-439) in the cytoplasmic domain of p80 receptor, 44 residues (354-397) were sufficient for binding of p80TRAK. Interestingly, this was also the region that contained the phosphorylation site for p80TRAK. Phosphoamino acid analysis of this region revealed phosphorylation primarily on serine residues. Furthermore, we found that, like p80TRAK, purified casein kinase 1 (CK1) also binds to residues 354-397 of the p80 TNF receptor and causes its phosphorylation. Additionally, the activity of p80TRAK was inhibited by CK1-7, the CK1-specific inhibitor. Thus, our results indicate that p80TRAK associates with a short stretch of approximately 44 residues located in the cytoplasmic domain of the p80 TNF receptor and that this kinase is similar to CK1.

Cytokine priming reduces dependence on TNF-R2 for TNF-alpha-mediated induction of macrophage nitric oxide generation

In the presence of interferon-gamma (IFN-gamma), human tumor necrosis factor-alpha (Hu-TNF-alpha), which binds to murine TNF-alpha receptor type 1 (TNF-R1) but not to murine TNF-R2, was effective in inducing nitric oxide (NO) production in spleen-derived macrophages (M phi), albeit at concentrations 12.5-fold greater than those required by murine TNF-alpha (Mu-TNF-alpha), to achieve the same result. Addition of anti-TNF-R1 completely inhibited the Mu-TNF-alpha-mediated induction of NO, demonstrating that TNF-R1 is critical to the IFN-gamma-dependent TNF-alpha-mediated induction of M phi effector function. However, treatment with anti-TNF-R2 resulted in a partial inhibition of M phi activation. Spleen-derived M phi were more dependent on TNF-R2 than RAW 264.7 or peritoneal M phi based on their responsiveness to Hu-TNF-alpha. Priming of spleen-derived M phi with either IFN-gamma or granulocyte-macrophage colony-stimulating factor (GM-CSF) heightened the maximal responses to both TNF-alpha species and increased the overall effectiveness of Hu-TNF-alpha without increasing expression of either TNF-alpha receptor. The dependence of spleen-derived M phi on both TNF-alpha receptors for signaling the induction of effector function supports an active signaling role for TNF-R2 in its synergy with TNF-R1 rather than a passive ligand passing role.

Alterations in TNF-alpha signal transduction in resistant human papillary thyroid carcinoma cells

Tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) inhibit the growth of the human papillary thyroid carcinoma (PTC) cell line, NP. Exposure of NP cells to TNF-alpha resulted in the development of several PTC cell lines (R30, R45, and R60) with graded loss to the TNF-alpha-induced antiproliferation, termed resistance. In contrast, the NP cells and the resistant cells were equally sensitive to the antiproliferative action of interferon-gamma. Utilizing TNF-alpha receptor-specific agonist monoclonal antibodies, we demonstrated that the TNF-alpha receptor p55 mediated the antiproliferative action of TNF-alpha, while the p75 receptor did not affect cell proliferation in the NP cell line. The resistant PTC cell lines, however, showed a graded loss of p55 receptor-mediated antiproliferation and a concomitant activation of a p75 receptor-mediated growth stimulation. Shedding of TNF receptors is an important mechanism of TNF-alpha receptor metabolism. The p55 receptor mediated the TNF-alpha-induced up-regulation of the shedding of the p75 TNF-alpha receptor. The p75 receptor mediated the TNF-alpha-induced down-regulation of the shedding of the p55 receptor. However, the shedding of the p75 receptor was decreased and the shedding of the p55 receptor was increased in the resistant R60 cell line compared with the NP cell line, in the presence and absence of TNF-alpha. In contrast, IFN-gamma increased shedding of both p55 and p75 TNF-alpha receptors in NP and R60 cell lines with equal potency. Furthermore, the resistant PTC cell lines have increased basal manganous superoxide dismutase (MnSOD) expression and blunted induction of MnSOD mRNA upon short-term. TNF-alpha treatment (less than 2 h of treatment). The results indicate that a decrease in signal transduction via the p55 TNF-alpha receptor and concomitant increase in signal transduction via the p75 TNF-alpha receptor are involved in the development of PTC cell resistance.

Myxoma virus T2 protein, a tumor necrosis factor (TNF) receptor homolog, is secreted as a monomer and dimer that each bind rabbit TNFalpha, but the dimer is a more potent TNF inhibitor

The myxoma virus T2 (M-T2) gene expresses a secreted protein that contains significant sequence similarity to the ligand binding domains of the cellular tumor necrosis factor (TNF) receptors, specifically inhibits the cytolytic activity of rabbit TNFalpha and is an important virulence factor for myxoma virus infection in rabbits. M-T2 protein was overexpressed from vaccinia virus vectors, purified to apparent homogeneity, and found to specifically protect mouse and rabbit cells from lysis by rabbit TNFalpha at molar ratios comparable with the soluble versions of the host tumor necrosis factor receptors. M-T2 secreted from virus-infected cells is detected as both a monomer and a disulfide-linked dimer, both of which were shown by Scatchard analysis to bind rabbit TNFalpha (Kd values of 170 pM and 195 pM, respectively), values that are comparable with the affinities of mammalian TNFs with their receptors. In contrast to the rabbit ligand, M-T2 interacts with mouse TNFalpha with a much lower affinity, Kd of 1.7 nM, and was unable to inhibit the cytolytic activity of this ligand on mouse cells. Although both monomeric and dimeric forms bound rabbit TNFalpha with comparable affinity, the dimeric M-T2 protein was a far more potent inhibitor of rabbit TNFalpha, presumably because it can more effectively prevent dimerization of TNF receptors than can the M-T2 monomer.

Effects of chronic ethanol administration on the endocytosis of cytokines by rat hepatocytes

The effects of chronic ethanol administration on the endocytosis of three representative cytokines were investigated in isolated rat hepatocytes. When hepatocytes were isolated from rats that were fed an ethanol liquid diet for 12 to 13 weeks, these cells exhibited a decreased ability to internalize and degrade transforming growth factor-alpha, tumor necrosis factor-alpha and interleukin-6, compared with hepatocytes from the pair-fed controls. This impaired endocytosis of all three cytokines was accompanied by significant decreases in the amount of hepatocyte surface-bound cytokine. Changes in cytokine binding to surface receptors and reduced rates of receptor-cytokine complex internalization into the cells seem to be major contributors to defective endocytosis in hepatocytes from the ethanol-fed rats. Impaired hepatocyte endocytosis could lead to altered steady-state levels of cytokines in the liver and modified physiological responses to cytokines. These changes could affect homeostasis among the various cell types in the liver and could contribute to liver dysfunction and injury.

Tumor necrosis factor-alpha and tumor necrosis factor receptors in the failing human heart

BACKGROUND

Tumor necrosis factor-alpha (TNF-alpha) is a proinflammatory cytokine that produces negative inotropic effects in the heart. Recently, elevated levels of TNF-alpha have been reported in patients with advanced congestive heart failure. Although TNF-alpha is thought to exert its deleterious effects by binding to two cell surface receptors, TNFR1 and TNFR2, the level of expression and regulation of TNF receptors in the heart in cardiac disease states is not known.

METHODS AND RESULTS

We examined mRNA and protein levels for TNFR1, TNFR2, and TNF-alpha in explanted hearts from organ donors as well as in patients with end-stage dilated cardiomyopathy (DCM) and ischemic heart disease (IHD). Northern blot analysis revealed that mRNA for TNFR1 and TNFR2 was present in nonfailing, DCM, and IHD hearts. TNFR1 and TNFR2 receptor protein levels, as measured by ELISA, were decreased 60% in DCM and IHD patients compared with nonfailing hearts (P < .005). To determine a potential mechanism for the decrease in TNF receptor expression, we measured levels of circulating soluble TNF receptors (sTNFRs) in DCM and IHD patients. This analysis showed that there was a significant one-and-a-half to threefold increase in sTNFRs in DCM (P < .03) and IHD patients (P < .001). Another important finding was that TNF-alpha mRNA and TNF-alpha protein were present in the explanted hearts from DCM and IHD patients but not in nonfailing hearts.

CONCLUSIONS

In summary, the results of this study constitute the initial demonstration that TNF receptor proteins are dynamically regulated in patients with advanced congestive heart failure. Moreover, the observation that failing hearts express elevated levels of TNF-alpha suggests that overexpression of this cytokine may be one of several different maladaptive mechanisms responsible for the progressive cardiac decompensation that occurs in advanced heart failure.

Identification of an interferon-gamma receptor alpha chain sequence required for JAK-1 binding

We have shown previously that a four-amino acid block residing at positions 266-269 (LPKS) in the intracellular domain of the human interferon-gamma (IFN-gamma) receptor alpha chain is critical for IFN-gamma-dependent tyrosine kinase activation and biologic response induction. Herein we show that this sequence is required for the constitutive attachment of the tyrosine kinase JAK-1. Using a vaccinia expression system, a receptor alpha chain-specific monoclonal antibody coprecipitated JAK-1 from cells coexpressing JAK-1 and either (a) wild type IFN-gamma receptor alpha chain, (b) a receptor alpha chain truncation mutant containing only the first 59 intracellular domain amino acids, or (c) a receptor mutant containing alanine substitutions for the functionally irrelevant residues 272-275. In contrast, JAK-1 was not coprecipitated when coexpressed with a receptor alpha chain mutant containing alanine substitutions for the functionally critical residues 266-269 (LPKS). Mutagenesis of the LPKS sequence revealed that Pro-267 is the only residue obligatorily required for receptor function. In addition, Pro-267 is required for JAK-1 binding. These results thus identify a site in the IFN-gamma receptor alpha chain required for constitutive JAK-1 association and establish that this association is critical for IFN-gamma signal transduction.

The role of tumor necrosis factor receptors in tumor necrosis factor-alpha-mediated cytolysis of ovarian cancer cell lines

OBJECTIVE

Our purpose was to define the expression of tumor necrosis factor receptors on ovarian cancer cells and determine what role these receptors play in tumor necrosis factor-alpha-mediated cytolysis.

STUDY DESIGN

Cell surface expression of tumor necrosis factor-alpha receptors was determined on ovarian cancer cell lines Caov-3, SK-OV-3, NIH:OVCAR-3, and A2780 by a tumor necrosis factor-alpha-binding assay that used iodine 125-labeled tumor necrosis factor-alpha. Monoclonal antibodies specific for the 55 to 60 kd (TR60) and 75 to 80 kd (TR80) tumor necrosis factor receptors were used to determine the relative density of each receptor type. To elucidate which receptor(s) was responsible for mediating the signal for cytolysis, 24-hour MTT cytolytic assays that used tumor necrosis factor-alpha and emetine were performed in the presence or absence of receptor-specific monoclonal antibodies.

RESULTS

The four ovarian cell lines expressed a similar number of surface receptors, 4500 to 7000 per cell, had similar dissociation constants, 0.3 to 0.6 nmol/L, and expressed predominately the TR60 receptor subtype. Receptor function studies showed that the presence of the monoclonal antibody to the TR60 receptor completely inhibited tumor necrosis factor-alpha-mediated cytolysis, whereas the monoclonal antibody to the TR80 receptor only partially blocked cytolysis.

CONCLUSIONS

Ovarian cancer cell lines express both tumor necrosis factor receptors, with the TR60 receptor being the dominant subtype. Tumor necrosis factor-alpha-mediated cytolysis appears to be dependent on the presence of a functional TR60 receptor. The TR80 receptor does not appear requisite for cytolysis; however, a complementary role cannot be excluded. Manipulation of tumor necrosis factor receptor subtypes on ovarian cancer cells may enhance the cytotoxic effects, thus improving the therapeutic efficacy of tumor necrosis factor-alpha.

The TNFR2-TRAF signaling complex contains two novel proteins related to baculoviral inhibitor of apoptosis proteins

The 75 kDa tumor necrosis factor receptor (TNFR2) transduces extracellular signals via receptor-associated cytoplasmic proteins. Two of these signal transducers, TRAF1 and TRAF2, were isolated and characterized previously. We report here the biochemical purification and subsequent molecular cloning of two novel TNFR2-associated proteins, designated c-IAP1 and c-IAP2, that are closely related mammalian members of the inhibitor of apoptosis protein (IAP) family originally identified in baculoviruses. The viral and cellular IAPs contain N-terminal baculovirus IAP repeat (BIR) motifs and a C-terminal RING finger. The c-IAPs do not directly contact TNFR2, but rather associate with TRAF1 and TRAF2 through their N-terminal BIR motif-comprising domain. The recruitment of c-IAP1 or c-IAP2 to the TNFR2 signaling complex requires a TRAF2-TRAF1 heterocomplex.

Casein kinase-1 phosphorylates the p75 tumor necrosis factor receptor and negatively regulates tumor necrosis factor signaling for apoptosis

Cellular responses initiated by tumor necrosis factor (TNF) are mediated by two different cell surface receptors with respective molecular masses of 55 kDa (p55) and 75 kDa (p75). p55 is functional in almost every cell type and can independently transmit most biological activities of TNF. In contrast, TNF signaling via p75 seems so far largely restricted to cells of lymphoid origin, where it can induce proliferation, cytokine production, and/or apoptosis. The mechanisms that regulate TNF receptor activity are largely unknown. Here we report that the p75 of unstimulated p75-responsive PC60 T cells is phosphorylated on serine by a kinase activity present in p75 immune complexes. Several lines of evidence indicate that the latter kinase is casein kinase-1 (CK-1). Previous results have shown that the p75 TNF receptor is constitutively phosphorylated in vivo. Our data show that the latter in vivo phosphorylation is also at least partially due to CK-1. Pretreatment of cells with TNF had no detectable effect on p75 phosphorylation in vitro or in vivo. However, a specific CK-1 inhibitor potentiated TNF-induced apoptosis mediated by p75, suggesting an inhibitory role for phosphorylation by CK-1. Although in vivo p75 phosphorylation could be seen in both p75-unresponsive and p75-responsive cell lines, in vitro p75 phosphorylation in p75 coimmunoprecipitates could not be observed in cell lines that were biologically unresponsive to p75 stimulation. The latter observation further indicates a regulatory role for p75 phosphorylation in p75-mediated signaling. Taken together, our data demonstrate that the p75 TNF receptor is phosphorylated and associated with CK-1, which negatively regulates p75-mediated TNF signaling.

Two tumour necrosis factor receptors: structure and function

Tumour necrosis factor (TNF) exerts two main effects: a beneficial one as an anti-infection, anti-tumour cytokine, and a detrimental one in the systemic inflammatory response syndrome (SIRS). Two receptors (TNF-R) mediate these effects, but their precise role in different cell types is far from solved. TNF induces receptor oligomerization, an event that is believed to connect the receptors to downstream signalling pathways. Recent research suggests that several TNF-R-associated proteins, including kinases, may initiate cytoplasmic signal transduction.

TRAF2-mediated activation of NF-kappa B by TNF receptor 2 and CD40

TNF receptor-associated factor (TRAF) proteins are candidate signal transducers that associate with the cytoplasmic domains of members of the tumor necrosis factor (TNF) receptor superfamily. The role of TRAFs in the TNF-R2 and CD40 signal transduction pathways, which result in the activation of transcription factor NF-kappa B, was investigated. Overexpression of TRAF2, but not TRAF1 or TRAF3, was sufficient to induce NF-kappa B activation. A truncated derivative of TRAF2 lacking an amino-terminal RING finger domain was a dominant-negative inhibitor of NF-kappa B activation mediated by TNF-R2 and CD40. Thus, TRAF2 is a common mediator of TNF-R2 and CD40 signaling.

A novel family of putative signal transducers associated with the cytoplasmic domain of the 75 kDa tumor necrosis factor receptor

Mutational analysis identified a C-terminal region of 78 amino acids within the cytoplasmic domain of the human 75 kDa tumor necrosis factor receptor (TNF-R2) that is required for signal transduction. This region was subsequently shown to mediate the interaction of cytoplasmic factors with TNF-R2. Two of these factors were isolated and molecularly cloned using biochemical purification and the yeast two-hybrid system. TNF receptor-associated factor 1 (TRAF1) and TRAF2 are the first two members of a novel protein family containing a novel C-terminal homology region, the TRAF domain. In addition, TRAF2 contains an N-terminal RING finger motif. TRAF1 and TRAF2 can form homo- and heterotypic dimers. Our analysis indicates that TRAF1 and TRAF2 are associated with the cytoplasmic domain of TNF-R2 in a heterodimeric complex in which TRAF2 contacts the receptor directly. TRAF1 interacts with TNF-R2 indirectly through heterodimer formation with TRAF2.

Differential effects of interleukin 1-alpha (IL-1 alpha) or tumor necrosis factor-alpha (TNF-alpha) on motility of human melanoma cell lines on fibronectin

Interleukin-1 alpha (IL-1 alpha) and tumor necrosis factor-alpha (TNF-alpha) induce a motogenic response in a number of benign and malignant cells. We examined the chemokinetic effects of these cytokines on the cell migration of four melanoma cell lines on fibronectin using modified Boyden chambers and video-time lapse analysis. Flow cytometry analysis of IL-1 receptors, TNF receptors, and shifts in beta 1 integrin expression were correlated with the effects of these cytokines on cell migration on fibronectin. The four melanoma cell lines exhibited heterogeneous expression of types I and II IL-1 receptors as well as p60 TNF receptors. Scant p80 TNF receptor expression was detected on only one cell line. Three of four melanoma cell lines demonstrated type I IL-1 receptors by Western blotting. IL-1 alpha and TNF-alpha induced heterogeneous modulation of beta 1 integrin expression in the four melanoma cell lines tested; downward shift of the alpha 2, alpha 3, alpha 4, and beta 1 integrin subunits was detected among three of the melanoma cell lines as were upward shifts of the alpha 4, alpha 5, and alpha 6 integrin subunits among three of the melanoma cell lines. IL-1 alpha and TNF-alpha induced enhanced migration on fibronectin in one of the melanoma cell lines and were related to an upward shift in the alpha 4 and alpha 5 integrin subunit expression. Taken together, the findings indicate that expression of a particular receptor for IL-1 or TNF does not necessarily signal a motogenic response in melanoma cells, but induces heterogeneous shifts in beta 1 integrin expression. However, upregulation in alpha 4 and alpha 5 integrin subunits appears to relate to enhanced migration on fibronectin.

Biochemical characterization of the extracellular domain of the 75-kilodalton tumor necrosis factor receptor

An expression plasmid encoding the extracellular domain of the 75-kDa human tumor necrosis factor (TNF) type 2 receptor (TNF-R2) was constructed and used to generate a stable cell line secreting soluble TNF-R2 (sTNF-R2). Purified sTNF-R2 was resolved by SDS-PAGE into one band of approximate M(r) 43,000, consistent with a molecular weight of 36,000 +/- 4800 obtained by sedimentation equilibrium analysis. The apparent molecular weight observed by gel filtration chromatography was approximately 136,000. Glycosylation analysis revealed that Asn-149 is fully glycosylated, while Asn-171 is incompletely glycosylated (approximately 50%), and that a proline-, serine-, and threonine-rich region (residues 175-234) contains O-linked carbohydrate structures. Scatchard analysis of [125I]TNF-alpha and [125I]TNF-beta binding to sTNF-R2 gave dissociation constants (Kd) of 0.3 and 0.75 nM, respectively, comparable to those observed for intact cell-surface TNF-R2. The sTNF-R2 was found to block the cytotoxicity of both TNF-alpha and TNF-beta in a murine L-M cell assay. The sizes of the sTNF-R2.TNF-alpha and sTNF-R2.TNF-beta complexes determined by gel filtration chromatography were approximately 322 and 204 kDa, respectively. The stoichiometry of the sTNF-R2.TNF-alpha and sTNF-R2.TNF-beta complexes were examined by size-exclusion chromatography, sedimentation equilibrium, and cross-linking. The data from these studies suggest that at least two molecules of sTNF-R2 can bind to a single TNF-alpha or TNF-beta trimer.