Therapeutic targeting of TWEAK/Fnl4 in cancer: exploiting the intrinsic tumor cell killing capacity of the pathway

Lower Expression of TWEAK is Associated with Poor Survival and Dysregulate TIICs in Lung Adenocarcinoma

Background. Lung cancer remains the leading cause of cancer death worldwide, and the most subtype is lung adenocarcinoma (LUAD). Tumor-infiltrating immune cells (TIICs) greatly impact the prognosis of LUAD. Tumor necrosis factor–like weak inducer of apoptosis (TWEAK), signal via its receptor fibroblast growth factor-inducible 14 (Fn14), dysregulates immune cell recruitment within tumor environment, thus promoting the progression of autoimmune diseases and cancer. We aimed to explore its role in LUAD. Methods. The expression level of TWEAK was explored in Tumor Immune Estimation Resource 2.0 (TIMER2.0) and Oncomine databases. The Tumor Immune Dysfunction and Exclusion (TIDE) and Lung Cancer Explorer (LCE) databases were applied to evaluate the survival in correlation to TWEAK expression. TIICs were assessed with TIMER2.0 and TIDE datasets. The expression of TWEAK protein was detected in LUAD cell lines and also in tissue samples from LUAD patients via western blotting or combination with immunochemistry. Results. Our results showed that TWEAK was downregulated in LUAD tumors compared to normal tissues in TIMER2.0, Oncomine, cell lines, and clinical specimens. Poor survival was uncovered in lower TWEAK expression of LUAD patients in LCE ( $\text{meta}-\text{HR}=0.84$ [95% CI, 0.76-0.92]) and TCGA ( $\text{Continuous}\text{Z}=-1.97$ , $p=0.0486$ ) and GSE13213@PRECOG ( $\text{Continuous}\text{Z}=-4.25$ , $p=2.12\text{e}-5$ ) in TIDE. Multiple tumor-infiltrating immune cells (TIICs) were found closely correlated with TWEAK expression in LUAD, especially hematopoietic stem cell ( $\text{Rho}=0.505$ , $p=2.78\text{e}-33$ ), common lymphoid progenitor ( $\text{Rho}=-0.504$ , $p=3.79\text{e}-33$ ), and myeloid-derived suppressor cells (MDSCs) ( $\text{Rho}=-0.615$ , $p=1.36\text{e}-52$ ). Conclusion. Lower level of TWEAK was linked with poor survival and aberrant recruitment and phenotype of TIICs in LUAD, which might motivate immune escape and weaken the effects of immunotherapy.

TWEAK/Fn14 Is Overexpressed in Crohn's Disease and Mediates Experimental Ileitis by Regulating Critical Innate and Adaptive Immune Pathways

BACKGROUND & AIMS

Crohn's disease (CD) is a debilitating inflammatory disorder that affects more than 1.6 million people in North America alone. Members of the tumor necrosis factor superfamily are key regulators of intestinal inflammation; specifically, tumor necrosis factor-like weak inducer of apoptosis (TWEAK) and its receptor, fibroblast growth factor-inducible 14 (Fn14), are involved in normal and pathologic tissue remodeling. Our aim was to determine the role of TWEAK/Fn14 in CD and a murine model of CD-like ileitis (ie, SAMP1/YitFc [SAMP] strain).

METHODS

SAMP mice deficient in Fn14 (SAMP × Fn14-/-) were developed and a detailed time-course study was performed evaluating ileal tissues by histology and stereomicroscopy, as well as quantitative polymerase chain reaction and NanoString technology (Seattle, WA). Reciprocal bone marrow chimeras were generated to assess the relevance of Fn14 in hematopoietic vs nonhematopoietic compartments. Surgically resected intestinal tissues and mucosal biopsy specimens from patients with CD, ulcerative colitis, and healthy controls were analyzed for the expression of TWEAK/Fn14 by quantitative polymerase chain reaction, Western blot, immunohistochemistry, and immunofluorescence.

RESULTS

SAMP × Fn14-/- showed a marked decrease in ileitis severity at 20 weeks of age compared with SAMP WT controls. Bone marrow chimeras showed that Fn14 was required in both hematopoietic and nonhematopoietic compartments for ileitis to develop. Transcriptome data showed multiple cellular pathways regulated by Fn14 signaling. Finally, increased expression of TWEAK and Fn14 was observed in tissue lesions from CD patients compared with ulcerative colitis and healthy controls.

CONCLUSIONS

TWEAK/Fn14 are up-regulated in CD, and also mediate experimental CD-like ileitis, by regulation of multiple innate and adaptive cellular pathways. Therefore, TWEAK/Fn14 may represent a novel therapeutic target for the treatment of small intestinal inflammation in CD.

Bioinformatic prediction and experimental validation of a PE38-based recombinant immunotoxin targeting the Fn14 receptor in cancer cells

AIM

AFn14R can serve as an ideal target for cancer immunotherapy. Here, a combined bioinformatic and experimental approach was applied to characterize an immunotoxin consisting of single-chain variable fragment antibody that targets Fn14 and a toxin fragment (PE38).

METHODS & RESULTS

Flow cytometry results showed that the rate of PE38-P4A8 binding to Fn14 was approximately 60 and 40% in HT-29 and A549 cells, respectively. Moreover, 1 ng/µl of immunotoxin was able to lyse approximately 53 and 41% of HT-29 and A549, respectively. PE38-P4A8 showed stability in mouse serum (∼90%) after 3-h incubation. Most importantly, using bioinformatics for determining the structure and function of fusion proteins can be very helpful in designing of experiments.

CONCLUSION

Coupled with bioinformatics, experimental approaches revealed that PE38-P4A8 could be used as a promising therapeutic agent for cancer cells expressing Fn14.

Connective Tissue Growth Factor Promotes Efficient Generation of Human Induced Pluripotent Stem Cell-Derived Choroidal Endothelium

Age‐related macular degeneration (AMD) is a leading cause of irreversible blindness in the Western world. Although, the majority of stem cell research to date has focused on production of retinal pigment epithelial (RPE) and photoreceptor cells for the purpose of evaluating disease pathophysiology and cell replacement, there is strong evidence that the choroidal endothelial cells (CECs) that form the choriocapillaris vessels are the first to be lost in this disease. As such, to accurately evaluate disease pathophysiology and develop an effective treatment, production of patient‐specific, stem cell‐derived CECs will be required. In this study, we report for the first time a stepwise differentiation protocol suitable for generating human iPSC‐derived CEC‐like cells. RNA‐seq analysis of the monkey CEC line, RF/6A, combined with two statistical screens allowed us to develop media comprised of various protein combinations. In both screens, connective tissue growth factor (CTGF) was identified as the key component required for driving CEC development. A second factor tumor necrosis factor (TNF)‐related weak inducer of apoptosis receptor was also found to promote iPSC to CEC differentiation by inducing endogenous CTGF secretion. CTGF‐driven iPSC‐derived CEC‐like cells formed capillary tube‐like vascular networks, and expressed the EC‐specific markers CD31, ICAM1, PLVAP, vWF, and the CEC‐restricted marker CA4. In combination with RPE and photoreceptor cells, patient‐specific iPSC derived CEC‐like cells will enable scientists to accurately evaluate AMD pathophysiology and develop effective cell replacement therapies. Stem Cells Translational Medicine2017;6:1533–1546

Scavenger Receptors: Emerging Roles in Cancer Biology and Immunology

Scavenger receptors constitute a large family of evolutionally conserved protein molecules that are structurally and functionally diverse. Although scavenger receptors were originally identified based on their capacity to scavenge modified lipoproteins, these molecules have been shown to recognize and bind to a broad spectrum of ligands, including modified and unmodified host-derived molecules or microbial components. As a major subset of innate pattern recognition receptors, scavenger receptors are mainly expressed on myeloid cells and function in a wide range of biological processes, such as endocytosis, adhesion, lipid transport, antigen presentation, and pathogen clearance. In addition to playing a crucial role in maintenance of host homeostasis, scavenger receptors have been implicated in the pathogenesis of a number of diseases, e.g., atherosclerosis, neurodegeneration, or metabolic disorders. Emerging evidence has begun to reveal these receptor molecules as important regulators of tumor behavior and host immune responses to cancer. This review summarizes our current understanding on the newly identified, distinct functions of scavenger receptors in cancer biology and immunology. The potential of scavenger receptors as diagnostic biomarkers and novel targets for therapeutic interventions to treat malignancies is also highlighted.

Roles of fibroblast growth factor-inducible 14 in hepatocellular carcinoma

The prognostic value of the fibroblast growth factor-inducible 14 (Fn14) expression in hepatocellular carcinoma (HCC) is unknown. Real-time PCR (RT-PCR), western blot assays and immunohistochemistry analysis were here performed in order to compare Fn14 expressions in paired liver samples of HCC and normal liver tissue. Most of the tumor tissues expressed significantly higher levels of Fn14 compared to adjacent non-tumor tissues, with Fn14High accounting for 54.6% (142/260) of all patients. The Pearson χ(2) test indicated that Fn14 expression was closely associated with serum alpha fetal protein (AFP) (P=0.002) and tumor number (p=0.019). Univariate and multivariate analyses revealed that along with tumor diameter and portal vein tumor thrombosis (PVTT ) type, Fn14 was an independent prognostic factor for both overall survival (OS) (HR=1.398, p=0.008) and recurrence (HR=1.541, p=0.001) rates. Fn14 overexpression HCC correlated with poor surgical outcome, and this molecule may be a candidate biomarker for prognosis as well as a target for therapy.

Nuclear Factor κB is Required for Tumor Growth Inhibition Mediated by Enavatuzumab (PDL192), a Humanized Monoclonal Antibody to TweakR

TweakR is a TNF receptor family member, whose natural ligand is the multifunctional cytokine TWEAK. The growth inhibitory activity observed following TweakR stimulation in certain cancer cell lines and the overexpression of TweakR in many solid tumor types led to the development of enavatuzumab (PDL192), a humanized IgG1 monoclonal antibody to TweakR. The purpose of this study was to determine the mechanism of action of enavatuzumab’s tumor growth inhibition and to provide insight into the biology behind TweakR as a cancer therapeutic target. A panel of 105 cancer lines was treated with enavatuzumab in vitro; and 29 cell lines of varying solid tumor backgrounds had >25% growth inhibition in response to the antibody. Treatment of sensitive cell lines with enavatuzumab resulted in the in vitro and in vivo (xenograft) activation of both classical (p50, p65) and non-classical (p52, RelB) NFκB pathways. Using NFκB DNA binding functional ELISAs and microarray analysis, we observed increased activation of NFκB subunits and NFκB-regulated genes in sensitive cells over that observed in resistant cell lines. Inhibiting NFκB subunits (p50, p65, RelB, p52) and upstream kinases (IKK1, IKK2) with siRNA and chemical inhibitors consistently blocked enavatuzumab’s activity. Furthermore, enavatuzumab treatment resulted in NFκB-dependent reduction in cell division as seen by the activation of the cell cycle inhibitor p21 both in vitro and in vivo. The finding that NFκB drives the growth inhibitory activity of enavatuzumab suggests that targeting TweakR with enavatuzumab may represent a novel cancer treatment strategy.

TWEAK/Fn14 Axis-Targeted Therapeutics: Moving Basic Science Discoveries to the Clinic

The TNF superfamily member TWEAK (TNFSF12) is a multifunctional cytokine implicated in physiological tissue regeneration and wound repair. TWEAK is initially synthesized as a membrane-anchored protein, but furin cleavage within the stalk region can generate a secreted TWEAK isoform. Both TWEAK isoforms bind to a small cell surface receptor named Fn14 (TNFRSF12A) and this interaction stimulates various cellular responses, including proliferation and migration. Fn14, like other members of the TNF receptor superfamily, is not a ligand-activated protein kinase. Instead, TWEAK:Fn14 engagement promotes Fn14 association with members of the TNFR associated factor family of adapter proteins, which triggers activation of various signaling pathways, including the classical and alternative NF-κB pathways. Numerous studies have revealed that Fn14 gene expression is significantly elevated in injured tissues and in most solid tumor types. Also, sustained Fn14 signaling has been implicated in the pathogenesis of cerebral ischemia, chronic inflammatory diseases, and cancer. Accordingly, several groups are developing TWEAK- or Fn14-targeted agents for possible therapeutic use in patients. These agents include monoclonal antibodies, fusion proteins, and immunotoxins. In this article, we provide an overview of some of the TWEAK/Fn14 axis-targeted agents currently in pre-clinical animal studies or in human clinical trials and discuss two other potential approaches to target this intriguing signaling node.

Antitumor activity of a humanized, bivalent immunotoxin targeting fn14-positive solid tumors

The TNF-like weak inducer of apoptosis (TWEAK; TNFSF12) receptor Fn14 (TNFRSF12A) is expressed at low levels in normal tissues but frequently highly expressed in a wide range of tumor types such as lung, melanoma, and breast, and therefore it is a potentially unique therapeutic target for these diverse tumor types. We have generated a recombinant protein containing a humanized, dimeric single-chain anti-fibroblast growth factor-inducible 14-kDa protein (Fn14) antibody fused to recombinant gelonin toxin as a potential therapeutic agent (designated hSGZ). The hSGZ immunotoxin is a highly potent and selective agent that kills Fn14-positive (Fn14(+)) tumor cells in vitro. Treatment of cells expressing the MDR protein MDR1 (ABCB1B) showed no cross-resistance to hSGZ. Induced overexpression of Fn14 levels in MCF7 cells through HER2 (ERBB2) signaling translated to an improved therapeutic index of hSGZ treatment. In combination with trastuzumab, hSGZ showed an additive or synergistic cytotoxic effect on HER2(+)/Fn14(+) breast cancer cell lines. Also, hSGZ treatment inhibited Erb3/Akt signaling in HER2-overexpressing breast cancer cells. Pharmacokinetic studies in mice revealed that hSGZ exhibited a biexponential clearance from plasma with a rapid initial clearance (t1/2α = 1.26 hours) followed by a seven-fold longer plasma half-life (t1/2β = 7.29 hours). At 24, 48, and 72 hours after injection, uptake of the hSGZ into tumors was 5.1, 4.8, and 4.7%ID/g, with a tumor-to-muscle ratio of 5.6, 6.2, and 9.0, respectively. Therapeutic efficacy studies showed significant tumor inhibition effects using an MDA-MB-231/Luc breast cancer xenograft model. Our findings show that hSGZ is an effective anticancer agent and a potential candidate for clinical studies.

Fibroblast growth factor inducible (Fn14)-specific antibodies concomitantly display signaling pathway-specific agonistic and antagonistic activity

BACKGROUND

Fn14 is a therapeutic target in various diseases.

RESULTS

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.

CONCLUSION

Anti-Fn14 antibodies elicit agonistic activities differing from those of the natural Fn14 ligand TWEAK.

SIGNIFICANCE

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.

Structure of the extracellular domains of human and Xenopus Fn14: implications in the evolution of TWEAK and Fn14 interactions

TWEAK (TNF homologue with weak apoptosis-inducing activity) and Fn14 (fibroblast growth factor-inducible protein 14) are members of the tumor necrosis factor (TNF) ligand and receptor super-families. Having observed that Xenopus Fn14 cross-reacts with human TWEAK, despite its relatively low sequence homology to human Fn14, we examined the conservation in tertiary fold and binding interfaces between the two species. Our results, combining NMR solution structure determination, binding assays, extensive site-directed mutagenesis and molecular modeling, reveal that, in addition to the known and previously characterized β-hairpin motif, the helix-loop-helix motif makes an essential contribution to the receptor/ligand binding interface. We further discuss the insight provided by the structural analyses regarding how the cysteine-rich domains of the TNF receptor super-family may have evolved over time.

DATABASE

Structural data are available in the Protein Data Bank/BioMagResBank databases under the accession codes 2KMZ, 2KN0 and 2KN1 and 17237, 17247 and 17252.

STRUCTURED DIGITAL ABSTRACT

TWEAK binds to hFn14 by surface plasmon resonance (View interaction) xeFn14 binds to TWEAK by enzyme linked immunosorbent assay (View interaction) TWEAK binds to xeFn14 by surface plasmon resonance (View interaction) hFn14 binds to TWEAK by enzyme linked immunosorbent assay (View interaction).

The TWEAK receptor Fn14 is a therapeutic target in melanoma: immunotoxins targeting Fn14 receptor for malignant melanoma treatment

Fn14, the cell surface receptor for TWEAK, is over-expressed in various human solid tumor types and can be a negative prognostic indicator. We detected Fn14 expression in ~60% of the melanoma cell lines we tested, including both B-Raf WT and B-Raf^V600E lines. Tumor tissue microarray analysis indicated that Fn14 expression was low in normal skin but elevated in 173/190 (92%) of primary melanoma specimens and in 86/150 (58%) of melanoma metastases tested. We generated both a chemical conjugate composed of the rGel toxin and the anti-Fn14 antibody ITEM-4 (designated ITEM4-rGel) and a humanized, dimeric single-chain antibody of ITEM-4 fused to rGel (designated hSGZ). Both ITEM4-rGel and hSGZ were highly cytotoxic to a panel of different melanoma cell lines. Mechanistic studies showed that both immunotoxins induced melanoma cell necrosis. Also, these immunotoxins could up-regulate the cellular expression of Fn14 and trigger cell signaling events similar to the Fn14 ligand TWEAK. Finally, treatment of mice bearing human melanoma MDA-MB-435 xenografts with either ITEM4-rGel or hSGZ showed significant tumor growth inhibition compared to controls. We conclude that Fn14 is a novel therapeutic target in melanoma and the hSGZ construct appears to warrant further development as a novel therapeutic agent against Fn14-positive melanoma.

TWEAK signals through JAK-STAT to induce tumor cell apoptosis

The TWEAK receptor Fn14 (TNFRSF12), a member of the TNF Receptor superfamily, can mediate many processes, including apoptosis. Fn14 agonists have therefore been the subject of interest as potential cancer therapeutics. In cell culture experiments, interferon gamma (IFNγ) is typically required for induction of apoptotic activity by either TWEAK or Fn14 agonistic antibodies in most cell lines. We have investigated the mechanism of IFNγ signaling and the role of JAK-STAT signaling in TWEAK/Fn14-mediated tumor cell killing. We found that IFNγ-mediated enhancement of tumor cell killing is JAK-STAT dependent, as JAK inhibitors block IFNγ-dependent TWEAK induced apoptosis. Exposure of tumor cells to IFNγ results in an increase in Fn14 expression on the cell surface, which may be a mechanism by which IFNγ induces sensitivity to TWEAK. In a reciprocal fashion, we observed that IFNγ receptor levels increase in response to TWEAK treatment in WiDr cells. Significantly, we found that TWEAK alone can induce STAT1 phosphorylation in WiDr tumor cells. Moreover, TWEAK induction of tumor cell apoptosis in WiDr cells in the absence of IFNγ is mediated by the JAK-STAT pathway. Correspondingly, we show that treatment of tumor bearing mice with mBIIB036, an Fn14 agonistic antibody, results in STAT1 phosphorylation in the tumors. Notably, the level of STAT1 phosphorylation appears to correlate with the degree of tumor growth inhibition by BIIB036 in vivo. Additionally, in WiDr cells, TWEAK induces a soluble factor, which we have identified as IFNβ, capable of independently inducing STAT1 phosphorylation when transferred to naïve cells. Finally, either IFNα or IFNβ can partially substitute for IFNγ in sensitizing tumor cells to Fn14 agonists. In summary, we show that TWEAK/Fn14 can signal through the JAK-STAT pathway to induce IFNβ, and that the ability of TWEAK to induce tumor cell apoptosis is mediated by JAK-STAT signaling. We also demonstrate that IFNγ enhancement of TWEAK/FN14-mediated tumor cell death is JAK-dependent and may occur by IFNγ-dependent upregulation of Fn14 on tumor cells. These findings may have implications for the appropriately targeted clinical development of Fn14 agonists as anti-cancer therapy.

Inflammatory cytokines and survival factors from serum modulate tweak-induced apoptosis in PC-3 prostate cancer cells

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK, TNFSF12) is a member of the tumor necrosis factor superfamily. TWEAK activates the Fn14 receptor, and may regulate cell death, survival and proliferation in tumor cells. However, there is little information on the function and regulation of this system in prostate cancer. Fn14 expression and TWEAK actions were studied in two human prostate cancer cell lines, the androgen-independent PC-3 cell line and androgen-sensitive LNCaP cells. Additionally, the expression of Fn14 was analyzed in human biopsies of prostate cancer. Fn14 expression is increased in histological sections of human prostate adenocarcinoma. Both prostate cancer cell lines express constitutively Fn14, but, the androgen-independent cell line PC-3 showed higher levels of Fn14 that the LNCaP cells. Fn14 expression was up-regulated in PC-3 human prostate cancer cells in presence of inflammatory cytokines (TNFα/IFNγ) as well as in presence of bovine fetal serum. TWEAK induced apoptotic cell death in PC-3 cells, but not in LNCaP cells. Moreover, in PC-3 cells, co-stimulation with TNFα/IFNγ/TWEAK induced a higher rate of apoptosis. However, TWEAK or TWEAK/TNFα/IFNγ did not induce apoptosis in presence of bovine fetal serum. TWEAK induced cell death through activation of the Fn14 receptor. Apoptosis was associated with activation of caspase-3, release of mitochondrial cytochrome C and an increased Bax/BclxL ratio. TWEAK/Fn14 pathway activation promotes apoptosis in androgen-independent PC-3 cells under certain culture conditions. Further characterization of the therapeutic target potential of TWEAK/Fn14 for human prostate cancer is warranted.

Role of TWEAK in lupus nephritis: a bench-to-bedside review

There is significant unmet need in the treatment of lupus nephritis (LN) patients. In this review, we highlight the role of the TWEAK/Fn14 pathway in mediating key pathologic processes underlying LN involving both glomerular and tubular injury, and thus the potential for renal protection via blockade of this pathway. The specific pathological mechanisms of TWEAK - namely promoting inflammation, renal cell proliferation and apoptosis, vascular activation and fibrosis - are described, with supporting data from animal models and in vitro systems. Furthermore, we detail the translational relevance of these mechanisms to clinical readouts in human LN. We present the opportunity for an anti-TWEAK therapeutic as a renal protective agent to improve efficacy relative to current standard of care treatments hopefully without increased safety risk, and highlight a phase II trial with BIIB023, an anti-TWEAK neutralizing antibody, designed to assess efficacy in LN patients. Taken together, targeting the TWEAK/Fn14 axis represents a potential new therapeutic paradigm for achieving renal protection in LN patients.

A Bioinformatics Resource for TWEAK-Fn14 Signaling Pathway

TNF-related weak inducer of apoptosis (TWEAK) is a new member of the TNF superfamily. It signals through TNFRSF12A, commonly known as Fn14. The TWEAK-Fn14 interaction regulates cellular activities including proliferation, migration, differentiation, apoptosis, angiogenesis, tissue remodeling and inflammation. Although TWEAK has been reported to be associated with autoimmune diseases, cancers, stroke, and kidney-related disorders, the downstream molecular events of TWEAK-Fn14 signaling are yet not available in any signaling pathway repository. In this paper, we manually compiled from the literature, in particular those reported in human systems, the downstream reactions stimulated by TWEAK-Fn14 interactions. Our manual amassment of the TWEAK-Fn14 pathway has resulted in cataloging of 46 proteins involved in various biochemical reactions and TWEAK-Fn14 induced expression of 28 genes. We have enabled the availability of data in various standard exchange formats from NetPath, a repository for signaling pathways. We believe that this composite molecular interaction pathway will enable identification of new signaling components in TWEAK signaling pathway. This in turn may lead to the identification of potential therapeutic targets in TWEAK-associated disorders.

TWEAK/Fn14 pathway: an immunological switch for shaping tissue responses

Our immune system performs the vital function of recognizing and eliminating invading pathogens and malignancies. There is an increasing appreciation that the immune system also actively mediates tissue responses under both physiological and pathological conditions, significantly impacting the inflammatory, fibrogenic, and regenerative components. Likewise, there is a growing understanding of how epithelial, endothelial, and other non-hematopoietic tissue cell types actively contribute to the interplay that shapes tissue responses. While much of the molecular basis underlying the immune regulation of tissue responses remains to be delineated, the tumor necrosis factor (TNF) superfamily ligand/receptor pair of TNF-like weak inducer of apoptosis (TWEAK) and fibroblast growth factor-inducible molecule 14 (Fn14) has now emerged as a key piece of this puzzle. In this review, we first discuss how the usually 'dormant' TWEAK/Fn14 pathway becomes activated specifically in injury and disease contexts. We then summarize how TWEAK-mediated Fn14 signaling triggers a wide range of activities in tissue parenchymal and stromal cells as well as progenitor cells. Finally, we review recent experimental evidence that further supports the functional dichotomy of TWEAK/Fn14 activation in physiological versus pathological tissue responses and its potential therapeutic implications. Whereas transient TWEAK/Fn14 activation promotes productive tissue responses after injury, excessive or persistent TWEAK/Fn14 activation drives pathological tissue responses, leading to progressive damage and degeneration.

Studies of binding of tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) to fibroblast growth factor inducible 14 (Fn14)

To perform highly sensitive cellular binding studies with TNF-like weak inducer of apoptosis (TWEAK), we developed a bioluminescent variant of soluble TWEAK (GpL-FLAG-TNC-TWEAK) by fusing it genetically to the C terminus of the luciferase of Gaussia princeps (GpL). Equilibrium binding studies on human (HT1080 and HT29) and murine (Renca and B16) cell lines at 37 °C revealed high affinities of human TWEAK from 53 to 112 pm. The dissociation rate constant of the TWEAK-Fn14 interaction was between 0.48×10(-3) s(-1) (HT29) and 0.58×10(-3) s(-1) (HT1080) for the human molecules, and the association rate constant obtained was 3.3×10(6) m(-1) s(-1) for both cell lines. It has been shown previously that oligomerization of soluble TWEAK trimers results in enhanced Fn14-mediated activation of the classical NFκB pathway. Binding studies with GpL-FLAG-TNC-TWEAK trimers oligomerized by help of a FLAG tag-specific antibody gave no evidence for a major increase in Fn14 occupancy by oligomerized ligand despite strongly enhanced induction of the NFκB target IL8. Thus, aggregated complexes of soluble TWEAK and Fn14 have a higher intrinsic activity to stimulate the classical NFκB pathway and qualitatively differ from isolated trimeric TWEAK-Fn14 complexes. Furthermore, determination of IL8 induction as a function of occupied activated receptors revealed that the intrinsic capability of TNFR1 to stimulate the classical NFκB pathway and IL8 production was ∼100-fold higher than Fn14. Thus, although ∼25 activated TNFR1 trimers were sufficient to trigger half-maximal IL8 production, more than 2500 cell-bound oligomerized TWEAK trimers were required to elicit a similar response.

Expression of TWEAK in normal human skin, dermatitis and epidermal neoplasms: association with proliferation and differentiation of keratinocytes

BACKGROUND

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) has been implicated in the pathogenesis of various inflammatory pathologies and cancer. We aimed to investigate its expression in normal human skin, inflammatory skin diseases and epidermal neoplasms.

METHODS

Immunohistochemistry for TWEAK was performed in samples of healthy skin, plaque psoriasis, lichen planus, prurigo nodularis, discoid lupus erythematosus, lichen sclerosus, seborrheic keratosis, common warts, actinic keratosis, Bowen's disease, keratoacanthoma and basal and squamous cell carcinoma. Double immunofluorescence was used to investigate co-localization of TWEAK with cytokeratin-10 and proliferating cell nuclear antigen (PCNA).

RESULTS

TWEAK was robustly expressed in the epidermis of healthy skin and decreased in inflammatory conditions, both in the context of epidermal hyperplasia and atrophy. Decreased TWEAK immunoreactivity was regularly observed in common warts, actinic keratosis and Bowen's disease, particularly in areas of marked proliferation as evidenced by PCNA-positive nuclei. In squamous cell carcinoma, expression of TWEAK ranged from strong to completely absent, and it mostly corresponded with the expression of cytokeratin-10. TWEAK was absent in keratoacanthoma and basal cell carcinoma.

CONCLUSIONS

TWEAK is a constitutively expressed epidermal protein whose downregulation might be an early indicator of disturbed differentiation or pathologic proliferation of keratinocytes that accompany inflammatory and neoplastic skin diseases.

Development of an Fn14 agonistic antibody as an anti-tumor agent

TWEAK, a TNF family ligand with pleiotropic cellular functions, was originally described as capable of inducing tumor cell death in vitro. TWEAK functions by binding its receptor, Fn14, which is up-regulated on many human solid tumors. Herein, we show that intratumoral administration of TWEAK, delivered either by an adenoviral vector or in an immunoglobulin Fc-fusion form, results in significant inhibition of tumor growth in a breast xenograft model. To exploit the TWEAK-Fn14 pathway as a therapeutic target in oncology, we developed an anti-Fn14 agonistic antibody, BIIB036. Studies described herein show that BIIB036 binds specifically to Fn14 but not other members of the TNF receptor family, induces Fn14 signaling, and promotes tumor cell apoptosis in vitro. In vivo, BIIB036 effectively inhibits growth of tumors in multiple xenograft models, including colon (WiDr), breast (MDA-MB-231), and gastric (NCI-N87) tumors, regardless of tumor cell growth inhibition response observed to BIIB036 in vitro. The anti-tumor activity in these cell lines is not TNF-dependent. Increasing the antigen-binding valency of BIB036 significantly enhances its anti-tumor effect, suggesting the contribution of higher order cross-linking of the Fn14 receptor. Full Fc effector function is required for maximal activity of BIIB036 in vivo, likely due to the cross-linking effect and/or ADCC mediated tumor killing activity. Taken together, the anti-tumor properties of BIIB036 validate Fn14 as a promising target in oncology and demonstrate its potential therapeutic utility in multiple solid tumor indications.

TWEAK, a multifunctional cytokine in kidney injury

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) is a cytokine of the TNF superfamily that activates the Fn14 receptor. TWEAK may regulate cell proliferation, cell death, cell differentiation, and inflammation. TWEAK and Fn14 are constitutively present in the kidney. Sources of TWEAK and Fn14 include intrinsic renal cells and infiltrating leukocytes. Basal Fn14 expression is low, but Fn14 is greatly upregulated during kidney injury. TWEAK contributes to kidney inflammation promoting chemokine secretion by renal cells through canonical and non-canonical NFκB activation. TWEAK also promotes tubular cell proliferation. However, TWEAK induces mesangial and tubular cell apoptosis under proinflammatory conditions. These data indicate that TWEAK is a multifunctional cytokine in the kidney, the actions of which are modulated by the cell microenvironment. Confirmation of the role of TWEAK in kidney injury came from functional studies in experimental animal models. The TWEAK/Fn14 pathway contributed to cell death and interstitial inflammation during acute kidney injury, to glomerular injury in lupus nephritis, to hyperlipidemia-associated kidney injury, and to tubular cell hyperplasia following unilateral nephrectomy. Circulating soluble TWEAK (sTWEAK) levels are a potential biomarker of adverse outcomes in chronic kidney disease and urinary sTWEAK is a potential biomarker of lupus nephritis activity. The available evidence suggests that TWEAK may provide diagnostic information and be a therapeutic target in renal injury. Its role in human kidney disease should be further explored.

Development and characterization of a potent immunoconjugate targeting the Fn14 receptor on solid tumor cells

TNF-like weak inducer of apoptosis (TWEAK) and fibroblast growth factor (FGF)-inducible 14 (Fn14) are a TNF superfamily ligand-receptor pair involved in many cellular processes including proliferation, migration, differentiation, inflammation, and angiogenesis. The Fn14 receptor is expressed at relatively low levels in normal tissues, but it is known to be dramatically elevated in a number of tumor types, including brain and breast tumors. Thus, it seems to be an excellent candidate for therapeutic intervention. We first analyzed Fn14 expression in human tumor cell lines. Fn14 was expressed in a variety of lines including breast, brain, bladder, skin, lung, ovarian, pancreatic, colon, prostate, and cervical cancer cell lines. We then developed an immunoconjugate containing a high-affinity anti-Fn14 monoclonal antibody (ITEM-4) conjugated to recombinant gelonin (rGel), a highly cytotoxic ribosome-inactivating N-glycosidase. Both ITEM-4 and the conjugate were found to bind to cells to an equivalent extent. Confocal microscopic analysis showed that ITEM4-rGel specifically and rapidly (within 2 hours) internalized into Fn14-positive T-24 bladder cancer cells but not into Fn14-deficient mouse embryonic fibroblasts. Cytotoxicity studies against 22 different tumor cell lines showed that ITEM4-rGel was highly cytotoxic to Fn14-expressing cells and was 8- to 8 × 10(4)-fold more potent than free rGel. ITEM4-rGel was found to kill cells by inducing apoptosis with high-mobility group box 1 protein release. Finally, ITEM4-rGel immunoconjugate administration promoted long-term tumor growth suppression in nude mice bearing T-24 human bladder cancer cell xenografts. Our data support the use of an antibody-drug conjugate approach to selectively target and inhibit the growth of Fn14-expressing tumors.