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We found recently that TNF-like weak inducer of apoptosis (TWEAK) and fibroblast growth factor–inducible-14 (Fn14) by virtue of their strong capability to reduce the freely available cytoplasmic pool of TNFR-associated factor (TRAF)2 and cellular inhibitors of apoptosis (cIAPs) antagonize the functions of these molecules in TNFR1 signaling, resulting in sensitization for apoptosis and inhibition of classical NF-κB signaling. In this study, we demonstrate that priming of cells with TWEAK also interferes with activation of the classical NF-κB pathway by CD40. Likewise, there was strong inhibition of CD40 ligand (CD40L)–induced activation of MAPKs in TWEAK-primed cells. FACS analysis and CD40L binding studies revealed unchanged CD40 expression and normal CD40L–CD40 interaction in TWEAK-primed cells. CD40L immunoprecipitates, however, showed severely reduced amounts of CD40 and CD40-associated proteins, indicating impaired formation or reduced stability of CD40L–CD40 signaling complexes. The previously described inhibitory effect of TWEAK on TNFR1 signaling has been traced back to reduced activity of the TNFR1-associated TRAF2–cIAP1/2 ubiquitinase complex and did not affect the stability of the immunoprecipitable TNFR1 receptor complex. Thus, the inhibitory effect of TWEAK on CD40 signaling must be based at least partly on other mechanisms. In line with this, signaling by the CD40-related TRAF2-interacting receptor TNFR2 was also attenuated but still immunoprecipitable in TWEAK-primed cells. Collectively, we show that Fn14 activation by soluble TWEAK impairs CD40L–CD40 signaling complex formation and inhibits CD40 signaling and thus identify the Fn14-TWEAK system as a potential novel regulator of CD40-related cellular functions.

Fn14 is a therapeutic target in various diseases.Anti-Fn14 antibodies activate the alternative NFκB pathway but not other Fn14-related activities induced by soluble or membrane-bound TWEAK. FcγR-bound anti-Fn14 antibodies, however, activate the full spectrum of Fn14-associated activities.Anti-Fn14 antibodies elicit agonistic activities differing from those of the natural Fn14 ligand TWEAK.These findings influence the rationale of designing Fn14-targeted therapies. The Fn14-specific monoclonal antibodies PDL192 and P4A8, which are under consideration in clinical trials, showed no agonistic activity with respect to IL8 production and cell death induction. However, oligomerization with protein G or binding to Fcγ receptors converted both anti-Fn14 antibodies into potent agonists. TNF-like weak inducer of apoptosis (TWEAK), the ligand of Fn14, occurs naturally in two forms with partly different signaling capabilities, as a membrane-bound ligand and as a soluble trimeric molecule. Although membrane TWEAK strongly triggers all Fn14-associated pathways, soluble TWEAK predominately triggers the alternative nuclear factor κB (NFκB) pathway and enhances TNF-induced cell death but has only a poor effect on the classical NFκB pathway and chemokine production. Thus, the oligomerized and FcγR-bound anti-Fn14 mAbs mimicked the activity of membrane TWEAK. Notably, both anti-Fn14 antibodies significantly triggered p100 processing, the hallmark of the alternative NFκB pathway, and therefore resembled soluble TWEAK. In contrast to the latter, however, the anti-Fn14s showed no effect on TNF receptor 1-induced cell death and P4A8 even blocked the corresponding TWEAK response. Thus, we showed that Fn14 antibodies display an alternative NFκB pathway-specific agonistic activity but fail to phenocopy other activities of soluble TWEAK, whereas oligomerized or FcγR-bound Fn14 antibodies fully mimic the activity of membrane TWEAK. In view of the trivalent nature of the TWEAK-Fn14 interaction, this suggests that the alternative NFκB pathway is uniquely responsive already to Fn14 dimerization enabling antibodies to elicit an unnatural response pattern distinct from that of the naturally occurring Fn14 ligands.

TNF-like weak inducer of apoptosis, TWEAK, is a typical member of the TNF ligand family. Thus, it is initially expressed as a type II transmembrane protein from which a soluble variant can be released by proteolytic processing. In this study, we show that membrane TWEAK is superior to soluble variant of TWEAK (sTWEAK) with respect to the activation of the classical NF-κB pathway, whereas both TWEAK variants are potent inducers of TNFR-associated factor-2 depletion, NF-κB–inducing kinase accumulation and p100 processing, hallmarks of activation of the noncanonical NF-κB pathway. Like other soluble TNF ligands with a poor capability to activate their corresponding receptor, sTWEAK acquires an activity resembling those of the transmembrane ligand by oligomerization or cell surface-immobilization. Blockade of the Fn14 receptor inhibited NF-κB signaling irrespective of the TWEAK form used for stimulation, indicating that the differential activities of the two TWEAK variants on classical and noncanonical NF-κB signaling is not related to the use of different receptors.

Soluble TNF-like weak inducer of apoptosis (TWEAK) trimers induce, in a variety of cell lines, translocation of cytosolic tumor necrosis factor (TNF) receptor-associated factor-2 (TRAF2) to a triton X-100-insoluble compartment without changes in the total cellular TRAF2 content. TWEAK-induced TRAF2 translocation is paralleled by a strong increase in nuclear factor kappaB 2 (NFkappaB2)/p100 processing to p52, indicating that TRAF2 redistribution is sufficient for activation of the alternative NFkappaB pathway. In accordance with the crucial role of TRAF2 in proinflammatory, anti-apoptotic TNF receptor-1 (TNFR1) signaling, we observed that TWEAK-primed cells have a reduced capacity to activate the classical NFkappaB pathway or JNK (cJun N-terminal kinase) in response to TNF. Furthermore, TWEAK-primed cells are sensitized for the TNFR1-mediated induction of apoptotic and necrotic cell death. Notably, the expression of the NFkappaB-regulated, TRAF2-interacting TRAF1 protein can attenuate TWEAK-induced depletion of the triton X-100-soluble TRAF2 fraction and improve TNFR1-induced NFkappaB signaling in TWEAK-primed cells. Taken together, we demonstrate that soluble TWEAK desensitizes cells for proinflammatory TNFR1 signaling and thus identify TWEAK as a modifier of TNF signaling.

It has been shown that tumor necrosis factor receptor-2 (TNFR2) stimulation leads to degradation of TNF receptor associated factor-2 (TRAF2) and inhibition of TNFR1-induced activation of NFkappaB and JNK. Here, we show that TRAF1 inhibits TNFR2-induced proteasomal degradation of TRAF2 and relieves TNFR1-induced activation of NFkappaB from the inhibitory effect of TNFR2. TRAF1 co-recruited with TRAF2 to both TNF receptors. Despite lacking an amino-terminal RING/zinc-finger domain, TRAF1 did not interfere with TNFR1-induced activation of JNK and NFkappaB. It is noted that physiological expression levels of TRAF1 enhanced NFkappaB activation and interleukin-8 (IL8) production induced by TNFR2. Thus, TRAF1 shifts the quality of integrated TNFR1-TNFR2 signaling from apoptosis induction to proinflammatory NFkappaB signaling.

TWEAK, like many other ligands of the TNF family, occurs naturally in two forms, as a type II transmembrane protein and as soluble ligand released from the latter by proteases of the furin family. Both TWEAK variants interact with high affinity with Fn14, an unusual small member of the TNF receptor family. TWEAK and Fn14 activate a variety of intracellular signaling pathways but regulation of TNF-induced cell death and stimulation of the classical and alternative NFκB pathway are certainly the best understood ones. Intriguingly, soluble and membrane TWEAK significantly differ in their ability to trigger these responses. While activation of the alternative NFκB pathway and enhancement of TNF-induced cell death are efficiently induced by both forms of TWEAK, membrane TWEAK has a much higher capacity than soluble TWEAK to stimulate the classical NFκB pathway. Importantly, soluble TWEAK gains a membrane TWEAK-like Fn14 stimulating activity upon oligomerization or artificial anchoring to the cell surface. On the example of NFκB signaling and enhancement of TNF-induced cell death, we summarize here protocols that allow the identification of signaling pathways/cellular responses that preferentially respond to membrane TWEAK. These protocols base either on the side-by-side analysis of soluble TWEAK and oligomerized or cell surface-anchorable TWEAK variants or on the use of transfectants expressing soluble and membrane TWEAK.

TWEAK, like many other ligands of the TNF family, occurs naturally in two forms, as a type II transmembrane protein and as soluble ligand released from the latter by proteases of the furin family. Both TWEAK variants interact with high affinity with Fn14, an unusual small member of the TNF receptor family. TWEAK and Fn14 activate a variety of intracellular signaling pathways but regulation of TNF-induced cell death and stimulation of the classical and alternative NFκB pathway are certainly the best understood ones. Intriguingly, soluble and membrane TWEAK significantly differ in their ability to trigger these responses. While activation of the alternative NFκB pathway and enhancement of TNF-induced cell death are efficiently induced by both forms of TWEAK, membrane TWEAK has a much higher capacity than soluble TWEAK to stimulate the classical NFκB pathway. Importantly, soluble TWEAK gains a membrane TWEAK-like Fn14 stimulating activity upon oligomerization or artificial anchoring to the cell surface. On the example of NFκB signaling and enhancement of TNF-induced cell death, we summarize here protocols that allow the identification of signaling pathways/cellular responses that preferentially respond to membrane TWEAK. These protocols base either on the side-by-side analysis of soluble TWEAK and oligomerized or cell surface-anchorable TWEAK variants or on the use of transfectants expressing soluble and membrane TWEAK.

Expression of fibroblast growth factor (FGF)-inducible 14 (Fn14), a member of the tumor necrosis factor receptor superfamily, is typically low in healthy adult organisms, but strong Fn14 expression is induced in tissue injury and tissue remodeling. High Fn14 expression is also observed in solid tumors, which is why this receptor is under consideration as a therapeutic target in oncology. Here, we describe various novel mouse-human cross-reactive llama-derived recombinant Fn14-specific antibodies (5B6, 18D1, 4G5) harboring the human IgG1 Fc domain. In contrast to recombinant variants of the established Fn14-specific antibodies PDL192 and P4A8, all three llama-derived antibodies efficiently bound to the W42A and R56P mutants of human Fn14. 18D1 and 4G5, but not 5B6, efficiently blocked TNF-like weak inducer of apoptosis(TWEA K) binding at low concentrations (0.2–2 μg/ml). Oligomerization and Fcγ receptor (FcγR) binding converted all antibodies into strong Fn14 agonists. Variants of 18D1 with enhanced and reduced antibody-dependent cell-mediated cytotoxicity (ADCC) activity were further analyzed in vivo with respect to their effect on metastasis. In a xenogeneic model using human colon carcinoma cancer cells, both antibody variants were effective in reducing metastasis to the liver. In contrast, only the 18D1 variant with enhanced ADCC activity, but not its ADCC-defective counterpart, suppressed lung metastasis in the RE NCA model. In sum, this suggests that Fn14 targeting might primarily act by triggering of antibody effector functions, but also by blockade of TWEA K-Fn14 interaction in some cases

Introduction: TNF-related weak inducer of apoptosis (TWEAK) triggers multiple cellular responses involved in left ventricular remodelling after myocardial infarction (MI), including cytokine production, inflammation, and wound healing. We therefore hypothesized that TWEAK-Fn14 interactions improve wound healing after experimental MI. Methods/Results: We investigated the effects of exogenous TWEAK by applying (1) cross-linked TWEAK - activating the classical NF-kB-signalling pathway, (2) TWEAK and (3) human serum albumin-tagged (HSA)-TWEAK - both activating the alternative NFkB-signalling pathway, or (4) placebo after the induction of experimental MI. Treatment with TWEAK and HSA-TWEAK surprisingly resulted in significantly higher mortality after MI (survival, placebo vs. TWEAK vs. HSA-TWEAK, 66.7 (10 of 15) vs. 14.3 (4 of 26) vs. 7.1 % (1 of 19)) due to cardiac ruptures in the TWEAK- and HSA-TWEAK groups. Ruptures were not related to changes in cardiac architecture since left ventricular dimensions as measured by echocardiography were identical between HSA-TWEAK and the placebo group three days after MI, a time point where cardiac ruptures had not taken place yet. We rule out direct remodelling defects, since MMP9 mRNA expression and proteolytic activity (gelatine zymography), and collagen1α1 mRNA expression were not altered between the groups. HSA-TWEAK induced an exaggerated inflammatory response: Infiltration of neutrophils into the border zone was significantly increased as assessed by immunohistochemistry (placebo vs. HSA-TWEAK, 297.91 vs. 455.75 neutrophils/ mm2, p < 0.001). FACS-Analysis showed a significant up-regulation of CD45+-cells in the infarcted area (CD45+-cells, placebo vs. HSA-TWEAK, 4.52 vs. 9.38 %, p < 0.05). This was associated with the up-regulation of genes important for the recruitment (MIP2, MCP-1 and MCP-5) and differentiation (IL5, IL12 and IFN-γ) of leukocytes (pg/ml, placebo vs. HSA-TWEAK: p < 0.05: MIP2, 1.41 vs. 7.83; IL5, 4.13 vs. 18.75; p < 0.001: MCP-1, N.D. vs. 28.72; MCP-5, N.D. vs. 7.36; IL12, 2.07 vs. 44.51; IFN-γ, N.D. vs. 7.47). Conclusion: Treatment with TWEAK reduces survival after experimental MI due to left ventricular rupture. This may be mediated by an exaggerated inflammatory response.

In this study, we have analysed the apoptotic effects of the ubiquitous environmental toxin benzo[ a] pyrene (BP) in HaCaT cells and human keratinocytes. Although prolonged exposure to BP was not cytotoxic on its own, a strong enhancement of CD95 (Fas)-mediated apoptosis was observed with BP at concentrations activating the aryl hydrocarbon receptor (AhR). Importantly, the ultimately mutagenic BP-metabolite, that is, (+)-anti-BP-7,8-diol-9,10-epoxide (BPDE), failed to enhance CD95-mediated cell death, suggesting that the observed pro-apoptotic effect of BP is neither associated with DNA adducts nor DNA-damage related signalling. CD95-induced apoptosis was also enhanced by beta-naphtoflavone, a well-known agonist of the AhR that does not induce DNA damage, thus suggesting a crucial role for AhR activation. Consistently, BP failed to sensitise for CD95L-induced apoptosis in AhR knockdown HaCaT cells. Furthermore, inhibition of CYP1A1 and/or 1B1 expression did not affect the pro-apoptotic crosstalk. Exposure to BP did not increase expression of CD95, but led to augmented activation of caspase-8. Enhancement of apoptosis was also observed with the TRAIL death receptors that activate caspase-8 and apoptosis by similar mechanisms as CD95. Together, these observations indicate an interference of AhR signalling with the activity of receptor-associated signalling intermediates that are shared by CD95 and TRAIL receptors. Our data thus suggest that AhR agonists can enhance cytokine-mediated adversity upon dermal exposure.