Lymphotoxin beta receptor triggering induces activation of the nuclear factor kappaB transcription factor in some cell types

Stimulation of Non-canonical NF-κB Through Lymphotoxin-β-Receptor Impairs Myogenic Differentiation and Regeneration of Skeletal Muscle

Myogenic differentiation, muscle stem cell functionality, and regeneration of skeletal muscle are cellular processes under tight control of various signaling pathways. Here, we investigated the role of non-canonical NF-κB signaling in myogenic differentiation, muscle stem cell functionality, and regeneration of skeletal muscle. We stimulated non-canonical NF-κB signaling with an agonistically acting antibody of the lymphotoxin beta receptor (LTβR). Interestingly, we found that stimulation of non-canonical NF-κB signaling through the LTβR agonist impairs myogenic differentiation, muscle stem cell function, and regeneration of skeletal muscle. Furthermore, we show that stimulation of non-canonical NF-κB signaling by the LTβR agonist coincides with activation of canonical NF-κB signaling. We suggest a direct crosstalk between canonical and non-canonical NF-κB signaling during myogenic differentiation which is required for proper myogenic differentiation and thereby regeneration of skeletal muscle.

Fragmentation of Small-Cell Lung Cancer Regulatory States in Heterotypic Microenvironments

Small-cell lung cancers derive from pulmonary neuroendocrine cells, which have stem-like properties to reprogram into other cell types upon lung injury. It is difficult to uncouple transcriptional plasticity of these transformed cells from genetic changes that evolve in primary tumors or secondary metastases. Profiling of single cells is also problematic if the required sample dissociation activates injury-like signaling and reprogramming. Here we defined cell-state heterogeneities in situ through laser capture microdissection-based 10-cell transcriptomics coupled with stochastic-profiling fluctuation analysis. In labeled cells from a small-cell lung cancer mouse model initiated by neuroendocrine deletion of Rb1-Trp53, variations in transcript abundance revealed cell-to-cell differences in regulatory state in vitro and in vivo. Fluctuating transcripts in spheroid culture were partly shared among Rb1-Trp53-null models, and heterogeneities increased considerably when cells were delivered intravenously to colonize the liver. Colonization of immunocompromised animals drove a fractional appearance of alveolar type II-like markers and poised cells for paracrine stimulation from immune cells and hepatocytes. Immunocompetency further exaggerated the fragmentation of tumor states in the liver, yielding mixed stromal signatures evident in bulk sequencing from autochthonous tumors and metastases. Dozens of transcript heterogeneities recurred irrespective of biological context; their mapped orthologs brought together observations of murine and human small-cell lung cancer. Candidate heterogeneities recurrent in the liver also stratified primary human tumors into discrete groups not readily explained by molecular subtype but with prognostic relevance. These data suggest that heterotypic interactions in the liver and lung are an accelerant for intratumor heterogeneity in small-cell lung cancer. SIGNIFICANCE: These findings demonstrate that the single-cell regulatory heterogeneity of small-cell lung cancer becomes increasingly elaborate in the liver, a common metastatic site for the disease.See related articles by Singh and colleagues, p. 1840 and Sutcliffe and colleagues, p. 1868.

Tissue Transglutaminase-Mediated AT1 Receptor Sensitization Underlies Pro-inflammatory Cytokine LIGHT-Induced Hypertension

BACKGROUND

Although numerous recent studies have shown a strong link between inflammation and hypertension, the underlying mechanisms by which inflammatory cytokines induce hypertension remain to be fully elucidated. Hypertensive disorders are also associated with elevated pressor sensitivity. Tissue transglutaminase (TG2), a potent cross-linking enzyme, is known to be transcriptionally activated by inflammatory cytokines and stabilize angiotensin II (Ang II) receptor AT₁ (AT₁R) via ubiquitination-preventing posttranslational modification. Here we sought to investigate the TG2-mediated AT₁R stabilization in inflammation-induced hypertension and its functional consequences with a focus on receptor abundance and Ang II responsiveness.

METHODS AND RESULTS

Using an experimental model of inflammation-induced hypertension established by introducing the pro-inflammatory tumor necrosis factor cytokine LIGHT, we provide pharmacologic and genetic evidence that TG2 is required for LIGHT-induced hypertension (systolic pressure on day 6: LIGHT = 152.3 ± 7.4 vs. LIGHT+ERW1041E [TG2 inhibitor] = 105.8 ± 13.1 or LIGHT+TG2^−/− = 114.3 ± 4.3 mm Hg, P < 0.05, n = 4–5) and renal compromise (urine albumin/creatinine: LIGHT = 0.17 ± 0.05 vs. LIGHT+ERW1041E = 0.03 ± 0.01 or LIGHT+TG2^−/− = 0.06 ± 0.01 mg/mg; plasma creatinine: LIGHT = 1.11 ± 0.04 vs. LIGHT+ERW1041E = 0.94 ± 0.04 or LIGHT+TG2^−/− = 0.88 ± 0.09 mg/dl; urine volume: LIGHT = 0.23 ± 0.1 vs. LIGHT+ERW1041E = 0.84 ± 0.13 or LIGHT+TG2^−/− = 1.02 ± 0.09 ml/24 hour on day 14, P < 0.05, n = 4–5). Our mechanistic studies showed that the TG2-mediated AT₁R modification and accumulation (relative renal AT₁R level: phosphate-buffered saline [PBS] = 1.23 ± 0.22, LIGHT = 3.49 ± 0.37, and LIGHT+ERW1041E = 1.77 ± 0.46, P < 0.05, n = 3; LIGHT+TG2^+/+ = 85.28 ± 36.11 vs. LIGHT+TG2^−/− = 7.01 ± 5.68, P < 0.05, n = 3) induced by LIGHT is associated with abrogated β-arrestin binding (AT₁R/associated β-arrestin ratio: PBS = 2.62 ± 1.07, LIGHT = 38.60 ± 13.91, and LIGHT+ERW1041E = 6.97 ± 2.91, P < 0.05, n = 3; LIGHT+TG2^+/+ = 66.43 ± 44.81 vs. LIGHT+TG2^−/− = 2.45 ± 1.78, P < 0.01, n = 3) and could be found in renal medulla tubules of kidneys (relative tubular AT₁R level: PBS = 5.91 ± 2.93, LIGHT = 92.82 ± 19.54, LIGHT+ERW1041E = 28.49 ± 11.65, and LIGHT+TG2^−/− = 0.14 ± 0.10, P < 0.01, n = 5) and the blood vasculature (relative vascular AT₁R level: PBS = 0.70 ± 0.30, LIGHT = 13.75 ± 2.49, and LIGHT+ERW1041E = 3.28 ± 0.87, P < 0.01, n = 3), 2 of the tissues highly related to the genesis of hypertension. Our in vitro cellular assays showed that LIGHT stimulation triggered a rapid TG2-dependent increase in the abundance of AT₁Rs (relative AT₁R level after 2-hour LIGHT treatment: AT₁R (WT)+TG2 = 2.21 ± 0.23, AT₁R (Q315A)+TG2 = 0.18 ± 0.23, P < 0.05 vs. starting point = 1, n = 2) and downstream calcium signaling (fold increase in NFAT-driven luciferase activity: Saline = 0.02 ± 0.03, Ang II = 0.17 ± 0.08, LIGHT = 0.05 ± 0.04, LIGHT+Ang II = 0.90 ± 0.04 (P < 0.01 vs. Ang II), and LIGHT+Ang II+ERW1041E = 0.15 ± 0.15 (P < 0.01 vs. LIGHT+Ang II), n = 3).

CONCLUSIONS

Our data indicate an essential and systemic role for TG2 in bridging inflammation to hypertension via its posttranslational modifications stabilizing AT₁ receptor and sensitizing Ang II. Our findings also suggest that TG2 inhibitors could be used as a novel group of cardiovascular agents.

Induction of the Epithelial-to-Mesenchymal Transition of Human Colorectal Cancer by Human TNF-β (Lymphotoxin) and its Reversal by Resveratrol

Objective: Tumor necrosis factor-beta (TNF-β), as an inflammatory mediator that has been shown to promote tumorigenesis, induces NF-κB. Natural multi-targeted agent resveratrol in turn shows anti-inflammatory and anti-cancer properties. Epithelial-to-mesenchymal transition (EMT) allows cancer cells to turn into a motile state with invasive capacities and is associated with metastasis and development of cancer stem cells (CSC). However, TNF-β-induced EMT and the anti-invasion mechanism of resveratrol on CRC are not yet completely understood. Methods: We investigated the underlying molecular mechanisms of resveratrol on TNF-β/TNF-βR-induced EMT and migration of CRC cells (HCT116, RKO, SW480) in monolayer or 3D alginate cultures. Results: TNF-β, similar to TNF-α, induced significant cell proliferation, morphological change, from an epithelial to a spindle-like mesenchymal shape with the formation of filopodia and lamellipodia associated with the expression of EMT parameters (elevated vimentin and slug, reduced E-cadherin), increased migration/invasion, and formation of CSC in all CRC cells. Interestingly, these effects were dramatically decreased in the presence of resveratrol or anti-TNF-βR with TNF-β co-treatment, inducing biochemical changes to the mesenchymal-epithelial transition (MET), with a planar cell surface and suppressed formation of CSC cells. This was associated with a significant increase in apoptosis. Furthermore, we found that resveratrol suppressed TNF-β-induced NF-κB and NF-κB-regulated gene biomarkers associated with growth, proliferation, and invasion. Finally, TNF-βR interacts directly with focal adhesion kinase (FAK) and NF-κB. Conclusion: These results suggest that resveratrol down-regulates TNF-β/TNF-βR-induced EMT, at least in part via specific suppression of NF-κΒ and FAK in CRC cells.

Association of LTA gene haploblock with periodontal disease in Italian adults

BACKGROUND

Periodontitis is a common oral disease caused by host inflammatory response towards bacteria biofilm. The chronic activation of immune response leads to destruction of teeth supporting tissue, bone loss and tooth detachment. Different factors could be involved in the development and severity of the disease; among them the host genetic background should be considered.

OBJECTIVES

In our study, we analysed haploblocks in a genomic region within major histocompatibility complex (MHC) locus aimed at disclosing a possible correlation with the risk of periodontal disease in 602 adult subjects from North-East Italy.

RESULTS

The CTTAC haploblock (formed by LTA-rs2857709, LTA-rs2844484, LTA- rs2229094, LTA-rs2229092 and LTA-rs1041981 polymorphisms) correlated with protection towards periodontitis condition, after regression analysis including age and smoking status as covariates (P-value = 0.015).

CONCLUSION

Our results suggest that a haplotype within LTA gene (encoding for lymphotoxin alpha) is involved in the susceptibility towards chronic periodontitis.

LST1: A multifunctional gene encoded in the MHC class III region

The LST1 gene is located in the MHC class III cluster between the MHC class I and II regions. While most genes in this cluster have been sufficiently characterised, a definitive function and expression pattern for LST1 still remains elusive. In the present review we describe its promotor, gene organisation, splice variants and expression in human tissues, cell lines and cancer. We focus on LST1 expression in inflammation and discuss known correlations with autoimmune diseases and cancer. Current data on LST1 polymorphisms and their known associations with pathologies are also discussed in detail. We summarize the potential functions that have been described for the full-length LST1 protein including its function as a transmembrane adaptor protein with inhibitory signal transduction and its role as a membrane scaffold facilitating the formation of tunnelling nanotubes. We also discuss further potential functions by compiling all known LST1-interacting proteins. Furthermore, we address knowledge gaps and conflictive issues regarding disease association, non-hematopoietic expression and the discrepancy between RNA and protein expression data.

Inflammation, Autoimmunity, and Hypertension: The Essential Role of Tissue Transglutaminase

Inflammatory cytokines cause hypertension when introduced into animals. Additional evidence indicates that cytokines induce the production of autoantibodies that activate the AT1 angiotensin receptor (AT1R). Extensive evidence shows that these autoantibodies, termed AT1-AA, contribute to hypertension. We review here recent studies showing that cytokine-induced hypertension and AT1-AA production require the ubiquitous enzyme, tissue transglutaminase (TG2). We consider 3 mechanisms by which TG2 may contribute to hypertension. (i) One involves the posttranslational modification (PTM) of AT1Rs at a glutamine residue that is present in the epitope sequence (AFHYESQ) recognized by AT1-AA. (ii) Another mechanism by which TG2 may contribute to hypertension is by PTM of AT1Rs at glutamine 315. Modification at this glutamine prevents ubiquitination-dependent proteasome degradation and allows AT1Rs to accumulate. Increased AT1R abundance is likely to account for increased sensitivity to Ang II activation and in this way contribute to hypertension. (iii) The increased TG2 produced as a result of elevated inflammatory cytokines is likely to contribute to vascular stiffness by modification of intracellular contractile proteins or by crosslinking vascular proteins in the extracellular matrix. This process, termed inward remodeling, results in reduced vascular lumen, vascular stiffness, and increased blood pressure. Based on the literature reviewed here, we hypothesize that TG2 is an essential participant in cytokine-induced hypertension. From this perspective, selective TG2 inhibitors have the potential to be pharmacologic weapons in the fight against hypertension.

Context-dependent roles for lymphotoxin-β receptor signaling in cancer development

The LTα1β2 and LIGHT TNF superfamily cytokines exert pleiotropic physiological functions through the activation of their cognate lymphotoxin-β receptor (LTβR). Interestingly, since the discovery of these proteins, accumulating evidence has pinpointed a role for LTβR signaling in carcinogenesis. Early studies have shown a potential anti-tumoral role in a subset of solid cancers either by triggering apoptosis in malignant cells or by eliciting an anti-tumor immune response. However, more recent studies provided robust evidence that LTβR signaling is also involved in diverse cell-intrinsic and microenvironment-dependent pro-oncogenic mechanisms, affecting several solid and hematological malignancies. Consequently, the usefulness of LTβR signaling axis blockade has been investigated as a potential therapeutic approach for cancer. Considering the seemingly opposite roles of LTβR signaling in diverse cancer types and their key implications for therapy, we here extensively review the different mechanisms by which LTβR activation affects carcinogenesis, focusing on the diverse contexts and different models assessed.

The Tumor Necrosis Factor Superfamily Molecule LIGHT Promotes Keratinocyte Activity and Skin Fibrosis

Several inflammatory diseases including scleroderma and atopic dermatitis display dermal thickening, epidermal hypertrophy, or excessive accumulation of collagen. Factors that might promote these features are of interest for clinical therapy. We previously reported that LIGHT, a TNF superfamily molecule, mediated collagen deposition in the lungs in response to allergen. We therefore tested whether LIGHT might similarly promote collagen accumulation and features of skin fibrosis. Strikingly, injection of recombinant soluble LIGHT into naive mice, either subcutaneously or systemically, promoted collagen deposition in the skin and dermal and epidermal thickening. This replicated the activity of bleomycin, an antibiotic that has been previously used in models of scleroderma in mice. Moreover skin fibrosis induced by bleomycin was dependent on endogenous LIGHT activity. The action of LIGHT in vivo was mediated via both of its receptors, HVEM and LTβR, and was dependent on the innate cytokine TSLP and TGF-β. Furthermore, we found that HVEM and LTβR were expressed on human epidermal keratinocytes and that LIGHT could directly promote TSLP expression in these cells. We reveal an unappreciated activity of LIGHT on keratinocytes and suggest that LIGHT may be an important mediator of skin inflammation and fibrosis in diseases such as scleroderma or atopic dermatitis.

Global transcriptomic analysis of model human cell lines exposed to surface-modified gold nanoparticles: the effect of surface chemistry

Gold nanoparticles (Au NPs) are attractive for biomedical applications not only for their remarkable physical properties, but also for the ease of which their surface chemistry can be manipulated. Many applications involve functionalization of the Au NP surface in order to improve biocompatibility, attach targeting ligands or carry drugs. However, changes in cells exposed to Au NPs of different surface chemistries have been observed, and little is known about how Au NPs and their surface coatings may impact cellular gene expression. The gene expression of two model human cell lines, human dermal fibroblasts (HDF) and prostate cancer cells (PC3) was interrogated by microarray analysis of over 14,000 human genes. The cell lines were exposed to four differently functionalized Au NPs: citrate, poly(allylamine hydrochloride) (PAH), and lipid coatings combined with alkanethiols or PAH. Gene functional annotation categories and weighted gene correlation network analysis were used in order to connect gene expression changes to common cellular functions and to elucidate expression patterns between Au NP samples. Coated Au NPs affect genes implicated in proliferation, angiogenesis, and metabolism in HDF cells, and inflammation, angiogenesis, proliferation apoptosis regulation, survival and invasion in PC3 cells. Subtle changes in surface chemistry, such as the initial net charge, lability of the ligand, and underlying layers greatly influence the degree of expression change and the type of cellular pathway affected.

Herpesvirus entry mediator (HVEM) attenuates signals mediated by the lymphotoxin β receptor (LTβR) in human cells stimulated by the shared ligand LIGHT

Signals mediated by members of the tumor necrosis factor receptor superfamily modulate a network of diverse processes including initiation of inflammatory responses and altering cell fate between pathways favoring survival and death. Although such pathways have been well-described for the TNF-α receptor, less is known about signaling induced by the TNF superfamily member LIGHT and how it is differentially altered by expression of its two receptors LTβR and HVEM in the same cell. We used cell lines with different relative expression of HVEM and LTβR to show that LIGHT-induced signals mediated by these receptors were associated with altered TRAF2 stability and RelA nuclear translocation. Production of the inflammatory chemokine CXCL10 was primarily mediated by LTβR. Higher expression of HVEM was associated with cell survival, while unopposed LTβR signaling favored pathways leading to apoptosis. Importantly, restoring HVEM expression in cells with low endogenous expression recapitulated the phenotype of cells with higher endogenous expression. Together, our data provide evidence that relative expression of HVEM and LTβR modulates canonical NF-κB and pro-apoptotic signals stimulated by LIGHT.

Critical role of CD4 T cells in maintaining lymphoid tissue structure for immune cell homeostasis and reconstitution

Loss of the fibroblastic reticular cell (FRC) network in lymphoid tissues during HIV-1 infection has been shown to impair the survival of naive T cells and limit immune reconstitution after antiretroviral therapy. What causes this FRC loss is unknown. Because FRC loss correlates with loss of both naive CD4 and CD8 T-cell subsets and decreased lymphotoxin-β, a key factor for maintenance of FRC network, we hypothesized that loss of naive T cells is responsible for loss of the FRC network. To test this hypothesis, we assessed the consequences of antibody-mediated depletion of CD4 and CD8 T cells in rhesus macaques and sooty mangabeys. We found that only CD4 T-cell depletion resulted in FRC loss in both species and that this loss was caused by decreased lymphotoxin-β mainly produced by the CD4 T cells. We further found the same dependence of the FRC network on CD4 T cells in HIV-1-infected patients before and after antiretroviral therapy and in other immunodeficiency conditions, such as CD4 depletion in cancer patients induced by chemotherapy and irradiation. CD4 T cells thus play a central role in the maintenance of lymphoid tissue structure necessary for their own homeostasis and reconstitution.

Adiponectin inhibits lymphotoxin-β receptor-mediated NF-κB signaling in human umbilical vein endothelial cells

Adiponectin exerts anti-diabetic and anti-atherogenesis properties through its 2 receptors (AdipoR1 and AdipoR2). However, the signaling pathways responsible for the anti-inflammatory effects of adiponectin are largely unknown. In this study, we identified the lymphotoxin (LT)-β receptor (LTBR) as an interacting partner of human AdipoR1 by using a yeast two-hybrid screening. The interaction between LTBR and AdipoR1 was confirmed by co-immunoprecipitation and co-localization analysis. Furthermore, adiponectin incubation inhibited lymphotoxin-induced NF-κB activation and the expression of adhesion molecules in human umbilical vein endothelial cells. These results indicated that AdipoR1 interacted with LTBR and mediated the inhibition of LTBR-activated NF-κB pathway.

Requiem protein links RelB/p52 and the Brm-type SWI/SNF complex in a noncanonical NF-kappaB pathway

The SWI/SNF chromatin remodeling complex plays pivotal roles in mammalian transcriptional regulation. In this study, we identify the human requiem protein (REQ/DPF2) as an adaptor molecule that links the NF-kappaB and SWI/SNF chromatin remodeling factor. Through in vitro binding experiments, REQ was found to bind to several SWI/SNF complex subunits and also to the p52 NF-kappaB subunit through its nuclear localization signal containing the N-terminal region. REQ, together with Brm, a catalytic subunit of the SWI/SNF complex, enhances the NF-kappaB-dependent transcriptional activation that principally involves the RelB/p52 dimer. Both REQ and Brm were further found to be required for the induction of the endogenous BLC (CXCL13) gene in response to lymphotoxin stimulation, an inducer of the noncanonical NF-kappaB pathway. Upon lymphotoxin treatment, REQ and Brm form a larger complex with RelB/p52 and are recruited to the BLC promoter in a ligand-dependent manner. Moreover, a REQ knockdown efficiently suppresses anchorage-independent growth in several cell lines in which the noncanonical NF-kappaB pathway was constitutively activated. From these results, we conclude that REQ functions as an efficient adaptor protein between the SWI/SNF complex and RelB/p52 and plays important roles in noncanonical NF-kappaB transcriptional activation and its associated oncogenic activity.

Secondary lymphoid organs: responding to genetic and environmental cues in ontogeny and the immune response

Secondary lymphoid organs (SLOs) include lymph nodes, spleen, Peyer's patches, and mucosal tissues such as the nasal-associated lymphoid tissue, adenoids, and tonsils. Less discretely anatomically defined cellular accumulations include the bronchus-associated lymphoid tissue, cryptopatches, and isolated lymphoid follicles. All SLOs serve to generate immune responses and tolerance. SLO development depends on the precisely regulated expression of cooperating lymphoid chemokines and cytokines such as LTalpha, LTbeta, RANKL, TNF, IL-7, and perhaps IL-17. The relative importance of these factors varies between the individual lymphoid organs. Participating in the process are lymphoid tissue initiator, lymphoid tissue inducer, and lymphoid tissue organizer cells. These cells and others that produce crucial cytokines maintain SLOs in the adult. Similar signals regulate the transition from inflammation to ectopic or tertiary lymphoid tissues.

Lymphotoxin-alpha 1 beta 2 and LIGHT induce classical and noncanonical NF-kappa B-dependent proinflammatory gene expression in vascular endothelial cells

Activation of the classical and noncanonical NF-kappaB pathways by ligation of the lymphotoxin (LT)-beta receptor (LTbetaR) plays a crucial role in lymphoid organogenesis and in the generation of ectopic lymphoid tissue at sites of chronic inflammation. Within these microenvironments, LTbetaR signaling regulates the phenotype of the specialized high endothelial cells. However, the direct effects of LTbetaR ligation on endothelial cells remain unclear. We therefore questioned whether LTbetaR ligation could directly activate endothelial cells and regulate classical and noncanonical NF-kappaB-dependent gene expression. We demonstrate that the LTbetaR ligands LIGHT and LTalpha1beta2 activate both NF-kappaB pathways in HUVECs and human dermal microvascular endothelial cells (HDMEC). Classical pathway activation was less robust than TNF-induced signaling; however, only LIGHT and LTalpha1beta2 and not TNF activated the noncanonical pathway. LIGHT and LTalpha1beta2 induced the expression of classical NF-kappaB-dependent genes in HUVEC, including those encoding the adhesion molecules E-selectin, ICAM-1, and VCAM-1. Consistent with this stimulation, LTbetaR ligation up-regulated T cell adhesion to HUVEC. Furthermore, the homeostatic chemokine CXCL12 was up-regulated by LIGHT and LTalpha1beta2 but not TNF in both HUVEC and HDMEC. Using HUVEC retrovirally transduced with dominant negative IkappaB kinase alpha, we demonstrate that CXCL12 expression is regulated by the noncanonical pathway in endothelial cells. Our findings therefore demonstrate that LTbetaR ligation regulates gene expression in endothelial cells via both NF-kappaB pathways and we identify CXCL12 as a bona fide noncanonical NF-kappaB-regulated gene in these cells.

High levels of Lymphotoxin-Beta (LT-Beta) gene expression in rheumatoid arthritis synovium: clinical and cytokine correlations

Lymphotoxin-Beta (LT-Beta) is implicated in lymphoid follicle development, production of pro-inflammatory cytokines, and can enhance the proliferation of fibroblasts and synoviocytes. The objective of this study was to investigate LT-Beta and LT-BetaReceptor (LT-BetaR) gene expression in RA patient synovium and blood samples compared with control individuals, and correlate with LT-Alpha and TNF-Alpha gene expression and disease parameters. RT-PCR was used to investigate the gene expression of LT-Beta, LT-BetaR, TNF-Alpha and LT-Alpha in the blood and synovium of RA patients and a control group of individuals. LT-Beta gene expression was significantly higher in RA patient synovium compared to control synovium (P = 0.005). There was a significant positive correlation between LT-Beta and LT-Alpha gene expression in both the synovium (P = 0.001) and blood (P = 0.002) of RA patients. LT-Beta gene expression was significantly higher in RA patient synovial samples that were inflamed to a moderately severe degree compared to those inflamed to a minimal degree (P = 0.02). Analysis of clinical variables revealed a significant positive correlation between LT-BetaR gene expression in RA patient synovium and Pain VAS Score (P = 0.01) and also HAQ Score (P = 0.01). Increased LT-Beta gene expression occurs in RA synovium and correlates with the degree of inflammation. LT-Beta may play a role in RA disease pathogenesis by contributing to a more intense inflammatory reaction in the synovium.

Targeting the lymphotoxin-beta receptor with agonist antibodies as a potential cancer therapy

The lymphotoxin-beta receptor (LT beta R) is a tumor necrosis factor receptor family member critical for the development and maintenance of various lymphoid microenvironments. Herein, we show that agonistic anti-LT beta R monoclonal antibody (mAb) CBE11 inhibited tumor growth in xenograft models and potentiated tumor responses to chemotherapeutic agents. In a syngeneic colon carcinoma tumor model, treatment of the tumor-bearing mice with an agonistic antibody against murine LT beta R caused increased lymphocyte infiltration and necrosis of the tumor. A pattern of differential gene expression predictive of cellular and xenograft response to LT beta R activation was identified in a panel of colon carcinoma cell lines and when applied to a panel of clinical colorectal tumor samples indicated 35% likelihood a tumor response to CBE11. Consistent with this estimate, CBE11 decreased tumor size and/or improved long-term animal survival with two of six independent orthotopic xenografts prepared from surgical colorectal carcinoma samples. Targeting of LT beta R with agonistic mAbs offers a novel approach to the treatment of colorectal and potentially other types of cancers.

LIGHT, a member of the TNF superfamily, activates Stat3 mediated by NIK pathway

Stat3, a member of the signal transducers and activators of transcription (STAT) family, is a key signal transduction protein activated by numerous cytokines, growth factors, and oncoproteins that controls cell proliferation, differentiation, development, survival, and inflammation. Constitutive activation of Stat3 has been found frequently in a wide variety of human tumors and induces cellular transformation and tumor formation. In this study, we demonstrated that LIGHT, a member of tumor necrosis factor superfamily, activates Stat3 in cancer cells. LIGHT induces dose-dependent activation of Stat3 by phosphorylation at both the tyrosine 705 and serine 727 residues. The activation of Stat3 by LIGHT appears to be mediated by NIK phosphorylation. Expression of a kinase-inactive NIK mutant abolished LIGHT induced Stat3 activation. Overexpression of an active NIK induces Stat3 activation by phosphorylation at the both tyrosine 705 and serine 727 residues. Activation of Stat3 by NIK requires NIK kinase activity as showed by kinase assays. In addition, LIGHT increases the expression of Stat3 target genes including cyclin D1, survivin, and Bcl-xL, and stimulates human LNCaP prostate cancer cell growth in vitro which can be blocked by expression of a dominant-negative Stat3 mutant. Taken together, these results indicate that in addition to activating NF-kappaB/p52, LIGHT also activates Stat3. Activation of Stat3 together with activating non-canonical NF-kappaB/p52 signaling by LIGHT may maximize its effects on cellular proliferation, survival, and inflammation.

Lymphotoxin a-dependent and -independent signals regulate stromal organizer cell homeostasis during lymph node organogenesis

Lymph nodes provide specialized stromal microenvironments that support the recruitment and organization of T cells and B cells, enabling them to effectively participate in immune responses. While CD4(+)3(-) lymphoid tissue inducer cells (LTic's) are known to play a key role in influencing lymph node (LN) development, the mechanisms that regulate the development of stromal organizer cells are unclear. Here, we define an ontogenetic program of lymph node stromal cell maturation in relation to the requirement for LTic's. We also describe a lymph node reaggregation assay to study cell-cell interactions and lymphocyte recruitment to these organs that reproduces the in vivo events during lymph node development. In addition, analysis of the lymph node anlagen in normal and lymphotoxin a (LTa)-deficient embryos shows that LTa-mediated signaling is required to sustain proliferation and survival of stromal cells in vivo. Our data identify LTa-independent and LTa-dependent stages of lymph node development, and provide direct evidence for the role of LTic's during LN organogenesis.

The TNF superfamily member LIGHT contributes to survival and activation of synovial fibroblasts in rheumatoid arthritis

OBJECTIVES

The TNF superfamily member LIGHT has a T-cell co-stimulatory role and has previously been associated with inflammation and autoimmunity. To investigate its role in rheumatoid arthritis (RA), a disease where activated T cells contribute in a prominent way, we have analysed the expression of LIGHT and its receptors in RA and analysed its effects on synovial fibroblasts in vitro.

METHODS

The expression of LIGHT was measured in synovial tissues and fluids and the receptors of LIGHT were detected on synovial fibroblasts derived from patients with RA and osteoarthritis (OA). The effects of recombinant LIGHT on the production of proinflammatory cytokines and proteases and on the apoptosis of synovial fibroblasts was assessed.

RESULTS

LIGHT mRNA was present in synovial tissues of patients with RA but not with OA. Correspondingly, soluble LIGHT protein could be detected in RA synovial fluid samples at much higher levels than in synovial fluid from patients with OA. Immunohistochemical detection of LIGHT and analysis of synovial fluid cells by flow cytometry revealed CD4 T cells as the major source of LIGHT in the rheumatoid joint. Synovial fibroblasts from RA patients were found to express the LIGHT receptors HVEM and LTbetaR. Recombinant LIGHT induced RA synovial fibroblasts to upregulate MMP-9 mRNA, CD54 and IL-6 in an NF-kappaB-dependent fashion. In vitro, exposure of cultured synovial fibroblasts to LIGHT reduced FAS-mediated apoptosis significantly, without affecting the rate of spontaneous apoptosis.

CONCLUSIONS

The results provide evidence for a novel T-cell-dependent activation of synovial fibroblasts by LIGHT in joints of patients with RA, contributing to an inflammatory and destructive phenotype.

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.

Inhibition of Lymphotoxin-β Receptor–Mediated Cell Death by Survivin-ΔEx3

TNFSF14/LIGHT is a member of the tumor necrosis factor superfamily that binds to lymphotoxin-beta receptor (LTbetaR) to induce cell death via caspase-dependent and caspase-independent pathways. It has been shown that cellular inhibitor of apoptosis protein-1 inhibits cell death by binding to LTbetaR-TRAF2/TRAF3 complexes and caspases. In this study, we found that both Kaposi's sarcoma-associated herpesvirus K7 (KSHV-K7), a viral inhibitor of apoptosis protein, and the structurally related protein survivin-DeltaEx3 could inhibit LTbetaR-mediated caspase-3 activation. However, only survivin-DeltaEx3 could protect cells from LTbetaR-mediated cell death. The differential protective effects of survivin-DeltaEx3 and KSHV-K7 can be attributed to the fact that survivin-DeltaEx3, but not KSHV-K7, is able to maintain mitochondrial membrane potential and inhibit second mitochondria-derived activator of caspase/DIABLO release. Moreover, survivin-DeltaEx3 is able to inhibit production of reactive oxygen species and can translocate from nucleus to cytosol to associate with apoptosis signal-regulating kinase 1 after activation of LTbetaR. Furthermore, survivin-DeltaEx3 protects LTbetaR-mediated cell death in caspase-3-deficient MCF-7 cells. Thus, survivin-DeltaEx3 is able to regulate both caspase-dependent and caspase-independent pathways, whereas inhibition of caspase-independent pathway is both sufficient and necessary for its protective effect on LTbetaR-mediated cell death.

TRAF2 plays a key, nonredundant role in LIGHT-lymphotoxin beta receptor signaling

LIGHT is a member of the tumor necrosis factor (TNF) superfamily, and its function is mediated by at least two receptors, including lymphotoxin beta receptor (LTbetaR) and herpes simplex virus entry mediator. However, the molecular mechanism of LIGHT signaling mediated by LTbetaR has not been clearly defined. In this report, we demonstrate that TRAF2 is critical for LIGHT- and LTbetaR-mediated activation of both the transcription factor NF-kappaB and the mitogen-activated protein kinase JNK. In HeLa cells, LIGHT induces NF-kappaB and JNK activation, which can be blocked by the dominant negative mutant of TRAF2. In these cells, LIGHT causes the recruitment of TRAF2, TRAF3, and IkappaB kinase into the LTbetaR complex. Importantly, while both NF-kappaB and JNK are activated by LIGHT in wild-type mouse embryonic fibroblasts, no activation of either of these two pathways is observed in TRAF2 null fibroblasts. However, LIGHT-induced NF-kappaB and JNK activation can be restored by ectopic expression of TRAF2 in TRAF2-/- cells. Interestingly, in contrast to TNF signaling, the activation of both NF-kappaB and JNK by LIGHT was normal in RIP-/- and TRAF5-/- cells. Taken together, our data demonstrate that TRAF2, an important effector molecule of TNF signaling, plays a critical, nonredundant role in LIGHT-LTbetaR signaling.

Differential lymphotoxin-beta and interferon gamma signaling during mouse liver regeneration induced by chronic and acute injury

The liver regenerates after acute injury via hepatocyte cell division; during chronic injury, when hepatocyte replication is impaired or blocked, liver progenitor oval cells mediate liver regeneration. If both regeneration options are blocked in animal models, then liver failure and death ensues. The mechanisms underlying oval cell induction, proliferation, and subsequent liver regeneration remain poorly characterized. In particular, cell-signaling pathways that distinguish the alternative pathways are unknown. This study shows that in a mouse model, hepatic expression of lymphotoxin-beta (LTbeta) and interferon gamma (IFNgamma) transcripts is increased in response to the choline-deficient, ethionine-supplemented (CDE) diet, which induces oval cell-mediated liver regeneration. Oval cells express LTbeta and IFNgamma transcripts, contributing to the increased expression in the liver of mice fed the CDE diet. An attenuated oval cell response to such a diet was observed in LTbeta receptor-, LTbeta-, and IFNgamma-gene targeted mice. Loss of LTbeta and LTbeta receptor signaling reduced the number of oval cells expressing A6 and muscle pyruvate kinase. The lack of IFNgamma signaling reduced muscle pyruvate kinase(+), but not A6(+), oval cells. In contrast, partial hepatectomy suppressed LTbeta and IFNgamma transcripts. We also show that IFNgamma induces STAT-3 phosphorylation in an oval cell line. In conclusion, LTbeta, LTbeta receptor, and IFNgamma are involved in oval cell-mediated, but not hepatocyte-mediated, liver regeneration, and the absence of these pathways impairs the oval cell-dependent regenerative response.

I kappa B kinase complex alpha kinase activity controls chemokine and high endothelial venule gene expression in lymph nodes and nasal-associated lymphoid tissue

The lymphotoxin (LT) beta receptor plays a critical role in secondary lymphoid organogenesis and the classical and alternative NF-kappaB pathways have been implicated in this process. IKKalpha is a key molecule for the activation of the alternative NF-kappaB pathway. However, its precise role and target genes in secondary lymphoid organogenesis remain unknown, particularly with regard to high endothelial venules (HEV). In this study, we show that IKKalpha(AA) mutant mice, who lack inducible kinase activity, have hypocellular lymph nodes (LN) and nasal-associated lymphoid (NALT) tissue characterized by marked defects in microarchitecture and HEV. In addition, IKKalpha(AA) LNs showed reduced lymphoid chemokine CCL19, CCL21, and CXCL13 expression. IKKalpha(AA) LN- and NALT-HEV were abnormal in appearance with reduced expression of peripheral node addressin (PNAd) explained by a severe reduction in the HEV-associated proteins, glycosylation-dependent cell adhesion molecule 1 (GlyCAM-1), and high endothelial cell sulfotransferase, a PNAd-generating enzyme that is a target of LTalphabeta. In this study, analysis of LTbeta(-/-) mice identifies GlyCAM-1 as another LTbeta-dependent gene. In contrast, TNFRI(-/-) mice, which lose classical NF-kappaB pathway activity but retain alternative NF-kappaB pathway activity, showed relatively normal GlyCAM-1 and HEC-6ST expression in LN-HEV. In addition, in this communication, it is demonstrated that LTbetaR is prominently expressed on LN- and NALT-HEV. Thus, these data reveal a critical role for IKKalpha in LN and NALT development, identify GlyCAM-1 and high endothelial cell sulfotransferase as new IKKalpha-dependent target genes, and suggest that LTbetaR signaling on HEV can regulate HEV-specific gene expression.

Prophylactic protection by N-acetylcysteine against the pulmonary injury induced by 2-chloroethyl ethyl sulfide, a mustard analogue

Mustard gas exposure causes adult respiratory distress syndrome associated with lung injury. The purpose of this study was to investigate whether an antioxidant, such as N-acetylcysteine (NAC), has any protective effect. Guinea pigs were given single exposure (0.5-6 mg/kg body weight) of 2-chloroethyl ethyl sulfide (CEES) as a mustard analogue intratracheally and maintained for various lengths of time (1 h to 21 days). Within 1 h of CEES infusion at 4 mg/kg, high levels of tumor necrosis factor alpha (TNF-alpha), ceramides, and nuclear factor kappaB accumulated in lung and alveolar macrophages. Both acid and neutral sphingomyelinases were activated within 4 h. These signal transduction events were associated with alteration in the oxygen defense system. Within 1 h of exposure to CEES (6 mg/kg body weight), there was 10-fold increase in the (125)I-BSA leakage into lung tissue, indicating severe lung injury. Although low level of CEES exposure (0.5 mg/kg body weight) produced symptoms of chemical burn in lung as early as 1 h after exposure, the severity of edema, congestion, hemorrhage, and inflammation increased progressively with time (1 h to 21 days). Feeding of single dose of NAC (0.5 g) by gavage just before the CEES infusion was ineffective to counteract these effects. However, consumption of the antioxidant in drinking water for 3 or 30 days prior to CEES exposure significantly inhibited the induction of TNF-alpha, activation of neutral and acid sphingomyelinases, production of ceramides, activation of caspases, leakage of (125)I-bovine serum albumin ((125)I-BSA) into lung tissue, and histological alterations in lung. Pretreatment with NAC for 3 and 30 days protected against 69-76% of the acute lung injury. Therefore, NAC may be an antidote for CEES-induced lung injury.

Signal transduction events in lung injury induced by 2-chloroethyl ethyl sulfide, a mustard analog

Sulfur mustard has been used as a vesicant chemical warfare agent. To understand the mechanism by which mustard gas exposure causes respiratory damage, we have used 2-chloroethyl ethyl sulfide (CEES) as a mustard analog. Our initial studies have shown that guinea pigs exposed to CEES intratracheally accumulate high levels of TNF-alpha. Accumulation of TNF-alpha leads to activation of both acid and neutral sphingomyelinases, resulting in high accumulation of ceramides, a second messenger involved in cell apoptosis. In addition, NF-kappa B was activated for a short period (1-2 h after exposure) as determined by mobility shift assay. Supershift assays indicated that both p50 and p65 of NF-kappa B were activated due to CEES exposure. However, NF-kappa B rapidly disappeared after 2 h. It is possible that the initial activation of NF-kappa B was an adaptive response to protect the cells from damage since NF-kappa B is known to inhibit TNF-alpha/ceramide-induced cell apoptosis. Since NF-kappa B disappeared after 2 h, the cells continued being damaged owing to accumulation of ceramides and activation of several caspases, leading to apoptosis.

Nuclear transcription factor-kappaB in Hodgkin's disease

Nuclear factor-kappaB (NF-kappaB) is a transcriptional factor that was originally discovered in the nucleus of B cells that bind to the kappa light chain of the immunoglobulins. Research during 15 years, however, has revealed that NF-kappaB is present in its inactive state in the cytoplasm of almost every cell type. When activated, NF-kappaB translocates to the nucleus, binds the DNA and regulates the expression of over 200 different genes. The product of these genes regulate the immune system, cell proliferation, tumor metastasis, inflammation and viral replication. Several tumor cell types express constitutively activated form of NF-kappaB and it is required for the proliferation of the tumor cells. Numerous studies have shown that Hodgkin's disease cells exhibit constitutive active NF-kappaB. The present review examines the mechanism how NF-kappaB is activated and its relevance to Hodgkin's disease.

Endogenous association of TRAF2, TRAF3, cIAP1, and Smac with lymphotoxin beta receptor reveals a novel mechanism of apoptosis

Lymphotoxin-beta receptor (LT beta R) is a member of tumor necrosis factor receptor family and plays essential roles in the embryonic development and organization of secondary lymphoid tissues. It binds two types of tumor necrosis factor family cytokines, heterotrimer LT alpha 1 beta 2 and homotrimer LIGHT, and activates multiple signaling pathways including transcriptional factor NF kappa B, c-Jun N-terminal kinase, and cell death. However, the molecular mechanism of the activation of these signaling pathways by LT beta R is not clear. Because there is no enzymatic activity associated with the receptor itself, the signal transduction of LT beta R is mediated by cytoplasmic proteins recruited to receptors. To identify these proteins, we took a proteomic approach. The endogenous LIGHT.LT beta R complex was affinity-purified from U937 cells, and proteins associated with the complex were identified by mass spectrometry. Four of five proteins identified, TRAF2, TRAF3, cIAP1, and Smac, are reported here. Their association with LT beta R was further confirmed by coimmunoprecipitation in U937 cells and HEK293 cells. The presence of cIAP1 and Smac in LIGHT.LT beta R complex revealed a novel mechanism of LIGHT.LT beta R-induced apoptosis.

The NF-kappa B activation in lymphotoxin beta receptor signaling depends on the phosphorylation of p65 at serine 536

NF-kappaB-inducing kinase (NIK) has been shown to play an essential role in the NF-kappaB activation cascade elicited by lymphotoxin beta receptor (LTbetaR) signaling. However, the molecular mechanism of this pathway remains unclear. In this report we demonstrate that both NIK and IkappaB kinase alpha (IKKalpha) are involved in LTbetaR signaling and that the phosphorylation of the p65 subunit at serine 536 in its transactivation domain 1 (TA1) plays an essential role. We also found that NF-kappaB could be activated in the LTbetaR pathway without altering the level of the phosphorylation of IkappaB and nuclear localization of p65. By using a heterologous transactivation system in which Gal4-dependent reporter gene is activated by the Gal4 DNA-binding domain in fusion with various portions of p65, we found that TA1 serves as a direct target in the NIK-IKKalpha pathway. In addition, mutation studies have revealed the essential role of Ser-536 within TA1 of p65 in transcriptional control mediated by NIK-IKKalpha. Furthermore, we found that Ser-536 was phosphorylated following the stimulation of LTbetaR, and this phosphorylation was inhibited by the kinase-dead dominant-negative mutant of either NIK or IKKalpha. These observations provide evidence for a crucial role of the NIK-IKKalpha cascade for NF-kappaB activation in LTbetaR signaling.

RelB is required for Peyer's patch development: differential regulation of p52-RelB by lymphotoxin and TNF

Targeted disruption of the Rel/NF-kappaB family members NF-kappaB2, encoding p100/p52, and RelB in mice results in anatomical defects of secondary lymphoid tissues. Here, we report that development of Peyer's patch (PP)-organizing centers is impaired in both NF-kappaB2- and RelB-deficient animals. IL-7-induced expression of lymphotoxin (LT) in intestinal cells, a crucial step in PP development, is not impaired in RelB-deficient embryos. LTbeta receptor (LTbetaR)-deficient mice also lack PPs, and we demonstrate that LTbetaR signaling induces p52-RelB and classical p50-RelA heterodimers, while tumor necrosis factor (TNF) activates only RelA. LTbetaR-induced binding of p52-RelB requires the degradation of the inhibitory p52 precursor, p100, which is mediated by the NF-kappaB-inducing kinase (NIK) and the IkappaB kinase (IKK) complex subunit IKKalpha, but not IKKbeta or IKKgamma. Activation of RelA requires all three IKK subunits, but is independent of NIK. Finally, we show that TNF increases p100 levels, resulting in the specific inhibition of RelB DNA binding via the C-terminus of p100. Our data indicate an important role of p52-RelB heterodimers in lymphoid organ development downstream of LTbetaR, NIK and IKKalpha.

LIGHT, a member of the tumor necrosis factor ligand superfamily, prevents tumor necrosis factor-alpha-mediated human primary hepatocyte apoptosis, but not Fas-mediated apoptosis

LIGHT is a member of tumor necrosis factor (TNF) superfamily, and its receptors have been identified as lymphotoxin-beta receptor (LTbetaR) and the herpesvirus entry mediator (HVEM)/ATAR/TR2, both of which lack the cytoplasmic sequence termed the "death domain." The present study has demonstrated that LIGHT inhibits TNFalpha-mediated apoptosis of human primary hepatocytes sensitized by actinomycin D (ActD), but not Fas- or TRAIL-mediated apoptosis. Furthermore, LIGHT does not prevent some cell lines such as HepG2 or HeLa from undergoing ActD/TNFalpha-induced apoptosis. This protective effect requires LIGHT pretreatment at least 3 h prior to ActD sensitization. LIGHT stimulates nuclear factor-kappaB (NF-kappaB)-dependent transcriptional activity in human hepatocytes like TNFalpha. The time course of NF-kappaB activation after LIGHT administration is similar to that of the pretreatment required for the anti-apoptotic effect of LIGHT. LIGHT inhibits caspase-3 processing on the apoptotic protease cascade in TNFalpha-mediated apoptosis but not Fas-mediated apoptosis. In addition, increased caspase-3 and caspase-8 activities in ActD/TNFalpha-treated cells are effectively blocked by LIGHT pretreatment. However, LIGHT does not change the expression of TNFRp55, TNFRp75, and Fas. These results indicate that LIGHT may act as an anti-apoptotic agent against TNFalpha-mediated liver injury by blocking the activation of both caspase-3 and caspase-8.

Lymphotoxin-beta receptor mediates NEMO-independent NF-kappaB activation

Lymphotoxin-beta receptor (LTbetaR) is a member of the tumor necrosis factor receptor (TNFR) superfamily that activates nuclear factor-kappaB (NF-kappaB) through the IkappaB kinase (IKK) complex, the core of which is comprised of IKK1, IKK2 and NF-kappaB essential modulator (NEMO). We demonstrate here that the LTbetaR signaling to NF-kappaB activation does not necessarily require NEMO, which is essential for TNFR signaling. In the absence of NEMO, the p50 and RelB, but not RelA subunits of NF-kappaB are found in the nuclear DNA binding complexes induced by the LTbetaR signaling. Our results thus disclose NEMO-independent NF-kappaB activation by LTbetaR.

Appearance and phenotype of murine follicular dendritic cells expressing VCAM-1

The architecture of lymphoid follicles is determined by a series of interactions between lymphoid and follicular stromal cells. A cardinal population in the non-lymphoid compartment is the follicular dendritic cell (FDC), whose communication with resting and activated B cells involves various adhesive interactions. The FDC phenotype variably includes the display of vascular cell adhesion molecule (VCAM-1). In this report we investigated the appearance and follicular tissue distribution of VCAM-1 in murine peripheral lymphoid tissues, and compared VCAM-1 with other FDC markers using immunohistochemistry. Correlating the appearance of VCAM-1 with other murine FDC-associated markers (CR1.2 [complement receptor 1.2 or CD35/21] and FDC-M1) revealed that the display of VCAM-1 is restricted to a subset of CR1.2-positive FDCs. We found that the expression of VCAM-1 antigen in the spleen or peripheral lymph nodes on FDCs requires antigenic stimulus, and that it coincides with germinal center formation. The VCAM-1 expression is associated with the appearance of mucosal addressin cell adhesion molecule (MAdCAM-1), with some slight differences in occurrence. The appearance of VCAM-1 and MAdCAM-1 antigens on FDCs may serve as indicators of FDC activation.

Differential requirement for NF-kappaB-inducing kinase in the induction of NF-kappaB by IL-1beta, TNF-alpha, and Fas

In this study, we examined the role of the nuclear factor-kappaB (NF-kappaB)-inducing kinase (NIK) in distinct signaling pathways leading to NF-kappaB activation. We show that a dominant-negative form of NIK (dnNIK) delivered by adenoviral (Ad5dnNIK) vector inhibits Fas-induced IkappaBalpha phosphorylation and NF-kappaB-dependent gene expression in HT-29 and HeLa cells. Interleukin (IL)-1beta- and tumor necrosis factor-alpha (TNF-alpha)-induced NF-kappaB activation and kappaB-dependent gene expression are inhibited in HeLa cells but not in Ad5dnNIK-infected HT-29 cells. Moreover, Ad5dnNIK failed to sensitize HT-29 cells to TNF-alpha-induced apoptosis at an early time point. However, cytokine- and Fas-induced signals to NF-kappaB are finally integrated by the IkappaB kinase (IKK) complex, since IkappaBalpha phosphorylation, NF-kappaB DNA binding activity, and IL-8 gene expression were strongly inhibited in HT-29 and HeLa cells overexpressing dominant-negative IKKbeta (Ad5dnIKKbeta). Our findings support the concept that cytokine signaling to NF-kappaB is redundant at the level of NIK. In addition, this study demonstrates for the first time the critical role of NIK and IKKbeta in Fas-induced NF-kappaB signaling cascade.

Requirement for the NF-kappaB family member RelA in the development of secondary lymphoid organs

The transcription factor nuclear factor (NF)-kappaB has been suggested to be a key mediator of the development of lymph nodes and Peyer's patches. However, targeted deletion of NF-kappaB/ Rel family members has not yet corroborated such a function. Here we report that when mice lacking the RelA subunit of NF-kappaB are brought to term by breeding onto a tumor necrosis factor receptor (TNFR)1-deficient background, the mice that are born lack lymph nodes, Peyer's patches, and an organized splenic microarchitecture, and have a profound defect in T cell-dependent antigen responses. Analyses of TNFR1/RelA-deficient embryonic tissues and of radiation chimeras suggest that the dependence on RelA is manifest not in hematopoietic cells but rather in radioresistant stromal cells needed for the development of secondary lymphoid organs.

Essential role of nuclear factor (NF)-kappaB-inducing kinase and inhibitor of kappaB (IkappaB) kinase alpha in NF-kappaB activation through lymphotoxin beta receptor, but not through tumor necrosis factor receptor I

Both nuclear factor (NF)-kappaB-inducing kinase (NIK) and inhibitor of kappaB (IkappaB) kinase (IKK) have been implicated as essential components for NF-kappaB activation in response to many external stimuli. However, the exact roles of NIK and IKKalpha in cytokine signaling still remain controversial. With the use of in vivo mouse models, rather than with enforced gene-expression systems, we have investigated the role of NIK and IKKalpha in signaling through the type I tumor necrosis factor (TNF) receptor (TNFR-I) and the lymphotoxin beta receptor (LTbetaR), a receptor essential for lymphoid organogenesis. TNF stimulation induced similar levels of phosphorylation and degradation of IkappaBalpha in embryonic fibroblasts from either wild-type or NIK-mutant mice. In contrast, LTbetaR stimulation induced NF-kappaB activation in wild-type mice, but the response was impaired in embryonic fibroblasts from NIK-mutant and IKKalpha-deficient mice. Consistent with the essential role of IKKalpha in LTbetaR signaling, we found that development of Peyer's patches was defective in IKKalpha-deficient mice. These results demonstrate that both NIK and IKKalpha are essential for the induction of NF-kappaB through LTbetaR, whereas the NIK-IKKalpha pathway is dispensable in TNFR-I signaling.

Discrete signaling regions in the lymphotoxin-beta receptor for tumor necrosis factor receptor-associated factor binding, subcellular localization, and activation of cell death and NF-kappaB pathways

Lymphotoxin-beta receptor (LTbetaR), a member of the tumor necrosis factor receptor superfamily, is essential for the development and organization of secondary lymphoid tissue. Wild type and mutant LTbetaR containing successive truncations of the cytoplasmic domain were investigated by retrovirus-mediated gene transfer into HT29.14s and in 293T cells by transfection. Wild type receptors accumulated in perinuclear compartments and enhanced responsiveness to ligand-induced cell death and ligand-independent activation of NFkappaB p50 dimers. Coimmunoprecipitation and confocal microscopy mapped the TRAF3 binding site to amino acids PEEGDPG at position 389. However, LTbetaR truncated at position Pro(379) acted as a dominant positive mutant that down-modulated surface expression and recruited TRAF3 to endogenous LTbetaR. This mutant exhibited ligand-independent cell death and activated NF-kappaB p50 dimers. By contrast, truncation at Gly(359) created a dominant-negative mutant that inhibited ligand-induced cell death and activation of NF-kappaB p50/p65 heterodimers. This mutant also blocked accumulation of wild type receptor into perinuclear compartments, suggesting subcellular localization may be crucial for signal transduction. A cryptic TRAF-independent NF-kappaB activating region was identified. These mutants define discrete subregions of a novel proline-rich domain that is required for subcellular localization and signal transduction by the LTbetaR.

Regulated commitment of TNF receptor signaling: a molecular switch for death or activation

Tumor necrosis factor receptor (TNFR) superfamily members can induce a context-dependent apoptosis or cell activation. However, the mechanisms by which these opposing programs are selected remain unclear. We show that in T cells, TNFR2 (TNFRSF1B) signaling is dramatically affected by the intracellular mediator RIP, a protein Ser/Thr kinase required for NF-kappaB activation by TNFR1 (TNFRSF1A). In the presence of RIP, TNFR2 triggers cell death, whereas in the absence of RIP, TNFR2 activates NF-kappaB. RIP is induced during IL2-driven T cell proliferation, and its inhibition reduces susceptibility to TNF-dependent apoptosis. Evidence that signaling outputs are shaped by intracellular constraints helps reconcile conflicting views of TNFR1 and TNFR2 as apoptotic mediators.

Tumor necrosis factor receptor and Fas signaling mechanisms

Four members of the tumor necrosis factor (TNF) ligand family, TNF-alpha, LT-alpha, LT-beta, and LIGHT, interact with four receptors of the TNF/nerve growth factor family, the p55 TNF receptor (CD120a), the p75 TNF receptor (CD120b), the lymphotoxin beta receptor (LT beta R), and herpes virus entry mediator (HVEM) to control a wide range of innate and adaptive immune response functions. Of these, the most thoroughly studied are cell death induction and regulation of the inflammatory process. Fas/Apo1 (CD95), a receptor of the TNF receptor family activated by a distinct ligand, induces death in cells through mechanisms shared with CD120a. The last four years have seen a proliferation in knowledge of the proteins participating in the signaling by the TNF system and CD95. The downstream signaling molecules identified so far--caspases, phospholipases, the three known mitogen activated protein (MAP) kinase pathways, and the NF-kappa B activation cascade--mediate the effects of other inducers as well. However, the molecules that initiate these signaling events, including the death domain- and TNF receptor associated factor (TRAF) domain-containing adapter proteins and the signaling enzymes associated with them, are largely unique to the TNF/nerve growth factor receptor family.

Signaling Through the Lymphotoxin-β Receptor Stimulates HIV-1 Replication Alone and in Cooperation with Soluble or Membrane-Bound TNF-α

The level of ongoing HIV-1 replication within an individual is critical to HIV-1 pathogenesis. Among host immune factors, the cytokine TNF-α has previously been shown to increase HIV-1 replication in various monocyte and T cell model systems. Here, we demonstrate that signaling through the TNF receptor family member, the lymphotoxin-β (LT-β) receptor (LT-βR), also regulates HIV-1 replication. Furthermore, HIV-1 replication is cooperatively stimulated when the distinct LT-βR and TNF receptor systems are simultaneously engaged by their specific ligands. Moreover, in a physiological coculture cellular assay system, we show that membrane-bound TNF-α and LT-α1β2 act virtually identically to their soluble forms in the regulation of HIV-1 replication. Thus, cosignaling via the LT-β and TNF-α receptors is probably involved in the modulation of HIV-1 replication and the subsequent determination of HIV-1 viral burden in monocytes. Intriguingly, surface expression of LT-α1β2 is up-regulated on a T cell line acutely infected with HIV-1, suggesting a positive feedback loop between HIV-1 infection, LT-α1β2 expression, and HIV-1 replication. Given the critical role that LT-α1β2 plays in lymphoid architecture, we speculate that LT-α1β2 may be involved in HIV-associated abnormalities of the lymphoid organs.

Dermatan sulfate activates nuclear factor-kappab and induces endothelial and circulating intercellular adhesion molecule-1

Proteoglycans (PGs) can influence cell behaviors through binding events mediated by their glycosaminoglycan (GAG) chains. This report demonstrates that chondroitin sulfate B, also known as dermatan sulfate (DS), a major GAG released during the inflammatory phase of wound repair, directly activates cells at the physiologic concentrations of DS found in wounds. Cultured human dermal microvascular endothelial cells exposed to DS responded with rapid nuclear translocation of nuclear factor-kappaB (NF-kappaB), increased expression of intercellular adhesion molecule-1 (ICAM-1) mRNA, and increased ICAM-1 cell surface protein. Heparan sulfate and chondroitin sulfates A and C had no effect on activation of NF-kappaB or induction of ICAM-1. Inhibition of NF-kappaB activation blocked the effect of DS. The increase in cell surface ICAM-1 did not involve TNF-alpha or IL-1 release by endothelial cells, but it was facilitated by autocrine factors whose release was initiated by DS. The ICAM-1-inductive activity of DS was confirmed in vivo. Injection of DS, but not heparin or other chondroitin sulfates, into mice greatly increased circulating levels of soluble ICAM. These data provide evidence that DS, but not other GAGs, initiates a previously unrecognized cell signaling event that can act during the response to injury.

The cytoplasmic domain of the lymphotoxin-beta receptor mediates cell death in HeLa cells

Activation of lymphotoxin-beta receptor (LT-betaR) by conjugation with heterotrimeric lymphotoxin, LT-alpha1/beta2, or by cross-linking with anti-LT-betaR antibodies can trigger apoptosis. We have observed that overexpression of either LT-betaR or the cytoplasmic domain of LT-betaR (LT-betaR(CD)) also induces apoptosis, which may be attributed to the tendency of LT-betaR(CD) to self-associate. The self-association domain of LT-betaR(CD) was mapped to amino acids 324-377, a region of the protein that is also essential for LT-betaR-induced apoptosis. Furthermore, we have shown that LT-betaR(CD)-induced apoptosis could be inhibited by a TRAF3 dominant negative mutant and by the caspase inhibitors Z-VAD-FMK, DEVD-FMK, and CrmA. The ligand-independent apoptosis induced by LT-betaR(CD) will help us to further dissect LT-betaR signaling pathway.

Hepatitis C virus core protein enhances NF-kappaB signal pathway triggering by lymphotoxin-beta receptor ligand and tumor necrosis factor alpha

Our previous study indicated that the core protein of hepatitis C virus (HCV) can associate with tumor necrosis factor receptor (TNFR)-related lymphotoxin-beta receptor (LT-betaR) and that this protein-protein interaction plays a modulatory effect on the cytolytic activity of recombinant form LT-betaR ligand (LT-alpha1beta2) but not tumor necrosis factor alpha (TNF-alpha) in certain cell types. Since both TNF-alpha/TNFR and LT-alpha1beta2/LT-betaR are also engaged in transcriptional activator NF-kappaB activation or c-Jun N-terminal kinase (JNK) activation, the biological effects of the HCV core protein on these regards were elucidated in this study. As demonstrated by the electrophoretic mobility shift assay, the expression of HCV core protein prolonged or enhanced the TNF-alpha or LT-alpha1beta2-induced NF-kappaB DNA-binding activity in HuH-7 and HeLa cells. The presence of HCV core protein in HeLa or HuH-7 cells with or without cytokine treatment also enhanced the NF-kappaB-dependent reporter plasmid activity, and this effect was more strongly seen with HuH-7 cells than with HeLa cells. Western blot analysis suggested that this modulation of the NF-kappaB activity by the HCV core protein was in part due to elevated or prolonged nuclear retention of p50 or p65 species of NF-kappaB in core protein-producing cells with or without cytokine treatment. Furthermore, the HCV core protein enhanced or prolonged the IkappaB-beta degradation triggering by TNF-alpha or LT-alpha1beta2 both in HeLa and HuH-7 cells. In contrast to that of IkappaB-beta, the increased degradation of IkappaB-alpha occurred only in LT-alpha1beta2-treated core-producing HeLa cells and not in TNF-alpha-treated cells. Therefore, the HCV core protein plays a modulatory effect on NF-kappaB activation triggering by both cytokines, though the mechanism of NF-kappaB activation, in particular the regulation of IkappaB degradation, is rather cell line and cytokine specific. Studies also suggested that the HCV core protein had no effect on TNF-alpha-stimulated JNK activity in both HeLa and HuH-7 cells. These findings, together with our previous study, strongly suggest that among three signaling pathways triggered by the TNF-alpha-related cytokines, the HCV core protein potentiates NF-kappaB activation in most cell types, which in turn may contribute to the chronically activated, persistent state of HCV-infected cells.

Autocrine tumor necrosis factor (TNF) and lymphotoxin (LT) alpha differentially modulate cellular sensitivity to TNF/LT-alpha cytotoxicity in L929 cells

Tumor necrosis factor (TNF) and lymphotoxin (LT) alpha are structurally and functionally related cytokines. We expressed the TNF and LT-alpha genes in murine fibrosarcoma L929r2 cells, which can be sensitized to TNF/LT-alpha-dependent necrosis by inhibitors of transcription or translation. Autocrine production of murine TNF in L929r2 cells completely downmodulated the expression of the 55- and 75-kD TNF receptors, resulting in resistance to TNF/LT-alpha cytotoxicity. Partial downmodulation of the 55-kD receptor was observed in human TNF-producing L929r2 cells. In contrast, an unaltered TNF receptor expression was found on LT-alpha L929r2 transfectants. Hence, although similar cytotoxic effects are induced by extracellularly administered TNF and LT-alpha, endogenous expression of these cytokines fundamentally differs in the way they modulate TNF receptor expression. Unlike LT-alpha, secreted by the classical pathway, TNF is first formed as a membrane-bound protein, which is responsible for receptor downmodulation. To explore whether the different pathways for secretion of TNF and LT-alpha explain this difference, we examined the effect of membrane-bound LT-alpha expression. This was obtained by exchange of the classical signal sequence of LT-alpha for the membrane anchor of chicken hepatic lectin. Membrane retention of LT-alpha resulted indeed in receptor downmodulation and TNF/LT-alpha resistance. We conclude that membrane retention of newly synthesized TNF or LT-alpha is absolutely required for receptor downmodulation and TNF/LT-alpha resistance.

Imbalance between interstitial collagenase and tissue inhibitor of metalloproteinases 1 in synoviocytes and fibroblasts upon direct contact with stimulated T lymphocytes: involvement of membrane-associated cytokines

OBJECTIVE

To determine whether direct cell-cell contact with stimulated T lymphocytes (a) differentially modulates the production of interstitial collagenase (matrix metalloproteinase 1 [MMP-1]) and tissue inhibitor of metalloproteinases 1 (TIMP-1) on human synoviocytes and dermal fibroblasts, and (b) induces the production of prostaglandin E2 (PGE2); and to identify the membrane-associated factors on T cell surfaces involved in these mechanisms.

METHODS

Dermal fibroblasts and fibroblast-like synovial cells (synoviocytes) were cultured with fixed T cells, isolated plasma membranes from T cells, interleukin-1beta (IL-1beta; 250 pg/ml), or transforming growth factor beta (TGFbeta; 5 ng/ml). Culture supernatants were assayed for the production of MMP-1, TIMP-1, and PGE2. The expression of MMP-1 and TIMP-1 messenger RNA was analyzed by Northern blot of total fibroblast RNA.

RESULTS

Membranes of stimulated T cells, i.e., human peripheral blood T lymphocytes (PBTL) and the human T cell line HUT-78, induced the production of PGE2 and MMP-1 on both synoviocytes and dermal fibroblasts. TIMP-1 production was enhanced upon contact with PBTL stimulated for short periods of time (2-4 hours) but not for longer periods. Similar results were obtained with CD4+ and CD8+ synovial tissue T cell clones (TCCs), which induced the production of TIMP-1 by fibroblasts when stimulated for short (2-4 hours), but not long, periods of time. This time dependency was not observed with HUT-78 cells. The production of MMP-1 by fibroblasts and synoviocytes upon cellular contact with stimulated T cells was higher than that induced by an optimum concentration of IL-1beta, whereas the production of PGE2 was equivalent or slightly lower. Cell membrane-associated IL-1alpha and tumor necrosis factor a, but not CD69, CD40 ligand, or CD11b, were involved in the induction of MMP-1 and PGE2 production, as shown by blockade experiments using monoclonal antibodies and cytokine antagonists.

CONCLUSION

Synovial tissue TCCs and PBTL stimulated for long periods of time trigger the production of PGE2 and MMP-1, but not TIMP-1, in synoviocytes and dermal fibroblasts, thus inducing an imbalance between the metalloenzyme and its inhibitor. These results demonstrate that T cells may affect fibroblast and synoviocyte functions directly (i.e., by contact activation) and indirectly (i.e., by activation of cytokine production in monocyte/macrophages, which in turn, trigger stromal cell functions). Since the production of MMPs in monocyte/macrophages is also induced upon contact with stimulated T cells, our results strongly suggest that contact of synovial cells with chronically stimulated T lymphocytes favors matrix catabolism. By analogy, this mechanism may trigger tissue destruction in vivo and, thus, may potentiate tissue destruction in chronic inflammatory diseases such as RA.

A role for tumor necrosis factor receptor type 1 in gut-associated lymphoid tissue development: genetic evidence of synergism with lymphotoxin beta

Lymphotoxin alpha (LTalpha) signals via tumor necrosis factor receptors (TNFRs) as a homotrimer and via lymphotoxin beta receptor (LTbetaR) as a heterotrimeric LTalpha1beta2 complex. LTalpha-deficient mice lack all lymph nodes (LNs) and Peyer's patches (PPs), and yet LTbeta-deficient mice and TNFR-deficient mice have cervical and mesenteric LN. We now show that mice made deficient in both LTbeta and TNFR type 1 (TNFR1) lack all LNs, revealing redundancy or synergism between TNFR1 and LTbeta, acting presumably via LTbetaR. A complete lack of only PPs in mice heterozygous for both ltalpha and ltbeta, but not ltalpha or ltbeta alone, suggests a similar two-ligand phenomenon in PP development and may explain the incomplete lack of PPs seen in tnfr1-/- mice.