TPL2 mediates autoimmune inflammation through activation of the TAK1 axis of IL-17 signaling

An autonomous activation of interleukin-17 receptor signaling sustains inflammation and promotes disease progression

Anti-interleukin-17 (IL-17) therapy has been used in various autoimmune diseases. However, the efficacy is unexpectedly limited in several IL-17-associated diseases, and the mechanism of limited efficacy remains unclear. Here, we show that a molecular complex containing the adaptor molecule Act1 and tyrosine phosphatase SHP2 mediated autonomous IL-17R signaling that accelerated and sustained inflammation. SHP2, aberrantly augmented in various autoimmune diseases, was induced by IL-17A itself in astrocytes and keratinocytes, sustaining chemokine production even upon anti-IL-17 therapies. Mechanistically, SHP2 directly interacted with and dephosphorylated Act1, which replaced Act1-TRAF5 complexes and induced IL-17-independent activation of IL-17R signaling. Genetic or pharmacologic inactivation of SHP2, or blocking Act1-SHP2 interaction, paralyzed both IL-17-induced and IL-17-independent signaling and attenuated primary or relapsing experimental autoimmune encephalomyelitis. Therefore, Act1-SHP2 complexes mediate an alternative pathway for autonomous activation of IL-17R signaling, targeting which could be a therapeutic option for IL-17-related diseases in addition to current antibody therapies.

Unleashing nature's potential and limitations: Exploring molecular targeted pathways and safe alternatives for the treatment of multiple sclerosis (Review)

Driven by the limitations and obstacles of the available approaches and medications for multiple sclerosis (MS) that still cannot treat the disease, but only aid in accelerating the recovery from its attacks, the use of naturally occurring molecules as a potentially safe and effective treatment for MS is being explored in model organisms. MS is a devastating disease involving the brain and spinal cord, and its symptoms vary widely. Multiple molecular pathways are involved in the pathogenesis of the disease. The present review showcases the recent advancements in harnessing nature's resources to combat MS. By deciphering the molecular pathways involved in the pathogenesis of the disease, a wealth of potential therapeutic agents is uncovered that may revolutionize the treatment of MS. Thus, a new hope can be envisioned in the future, aiming at paving the way toward identifying novel safe alternatives to improve the lives of patients with MS.

TPL2 kinase activity regulates microglial inflammatory responses and promotes neurodegeneration in tauopathy mice

Tumor progression locus 2 (TPL2) (MAP3K8) is a central signaling node in the inflammatory response of peripheral immune cells. We find that TPL2 kinase activity modulates microglial cytokine release and is required for microglia-mediated neuron death in vitro. In acute in vivo neuroinflammation settings, TPL2 kinase activity regulates microglia activation states and brain cytokine levels. In a tauopathy model of chronic neurodegeneration, loss of TPL2 kinase activity reduces neuroinflammation and rescues synapse loss, brain volume loss, and behavioral deficits. Single-cell RNA sequencing analysis indicates that protection in the tauopathy model was associated with reductions in activated microglia subpopulations as well as infiltrating peripheral immune cells. Overall, using various models, we find that TPL2 kinase activity can promote multiple harmful consequences of microglial activation in the brain including cytokine release, iNOS (inducible nitric oxide synthase) induction, astrocyte activation, and immune cell infiltration. Consequently, inhibiting TPL2 kinase activity could represent a potential therapeutic strategy in neurodegenerative conditions.

The Role of IL-17 in the Pathogenesis of Oral Squamous Cell Carcinoma

Elucidating the inflammatory mechanisms underlying formation and progression of oral squamous cell carcinoma (OSCC) is crucial for discovering new targeted therapeutics. The proinflammatory cytokine IL-17 has proven roles in tumor formation, growth, and metastasis. The presence of IL-17 is demonstrated in both in vitro and in vivo models, and in OSCC patients, is mostly accompanied by enhanced proliferation and invasiveness of cancer cells. Here we review the known facts regarding the role of IL-17 in OSCC pathogenesis, namely the IL-17 mediated production of proinflammatory mediators that mobilize and activate myeloid cells with suppressive and proangiogenic activities and proliferative signals that directly induce proliferation of cancer cells and stem cells. The possibility of a potential IL-17 blockade in OSCC therapy is also discussed.

YEATS2 regulates the activation of TAK1/NF-κB pathway and is critical for pancreatic ductal adenocarcinoma cell survival

The prognosis of pancreatic ductal adenocarcinoma (PDAC) is poor despite diagnostic progress and new chemotherapeutic regimens. Constitutive activation of NF-κB is frequently observed in PDAC. In this study, we found that YEATS2, a scaffolding protein of ATAC complex, was highly expressed in human PDAC. Depletion of YEATS2 reduced the growth, survival, and tumorigenesis of PDAC cells. The binding of YEATS2 is crucial for maintaining TAK1 activation and NF-κB transcriptional activity. Of importance, our results reveal that YEATS2 promotes NF-κB transcriptional activity through modulating TAK1 abundance and directly interacting with NF-κB as a co-transcriptional factor.

Therapeutic potential of targeting IL-17 and its receptor signaling in neuroinflammation

T helper 17 cells are thought to significantly contribute to the neuroinflammation process during neurogenerative diseases via their signature cytokine, interleukin (IL)-17. Recently, an emerging key role of IL-17 and its receptors has been documented in inflammatory and autoimmune diseases. The clinical studies conducted on patients with neurodegenerative disease have also shown an increase in IL-17 levels in serum as well as cerebrospinal fluid samples. Therapeutic targeting of either IL-17 receptors or direct IL-17 neutralizing antibodies has shown a promising preclinical and clinical proof of concept for treating chronic autoimmune neurodegenerative diseases such as multiple sclerosis. Thus, IL-17 and its receptors have a central role in regulation of neuroinflammation and can be considered as one of the major therapeutic targets in chronic neuroinflammatory diseases.

ERK1/2 in immune signalling

Extracellular signal-related kinases 1 and 2 (ERK1/2) are the final components of the mitogen-activated protein kinase (MAPK) phosphorylation cascade, an integral module in a diverse array of signalling pathways for shaping cell behaviour and fate. More recently, studies have shown that ERK1/2 plays an essential role downstream of immune receptors to elicit inflammatory gene expression in response to infection and cell or tissue damage. Much of this work has studied ERK1/2 activation in Toll-like receptor (TLR) pathways, providing mechanistic insights into its recruitment, compartmentalisation and activation in cells of the innate immune system. In this review, we summarise the typical activation of ERK1/2 in growth factor receptor pathways before discussing its known roles in immune cell signalling with a focus downstream of TLRs. We examine emerging research uncovering evidence of dysfunctional ERK1/2 signalling in inflammatory diseases and discuss the potential therapeutic benefit of targeting ERK1/2 pathways in inflammation.

E3 ubiquitin ligase NEDD4L negatively regulates inflammation by promoting ubiquitination of MEKK2

Aberrant activation of inflammation signaling triggered by tumor necrosis factor α (TNF‐α), interleukin‐1 (IL‐1), and interleukin‐17 (IL‐17) is associated with immunopathology. Here, we identify neural precursor cells expressed developmentally down‐regulated gene 4‐like (NEDD4L), a HECT type E3 ligase, as a common negative regulator of signaling induced by TNF‐α, IL‐1, and IL‐17. NEDD4L modulates the degradation of mitogen‐activated protein kinase kinase kinase 2 (MEKK2) via constitutively and directly binding to MEKK2 and promotes its poly‐ubiquitination. In interleukin‐17 receptor (IL‐17R) signaling, Nedd4l knockdown or deficiency enhances IL‐17‐induced p38 and NF‐κB activation and the production of proinflammatory cytokines and chemokines in a MEKK2‐dependent manner. We further show that IL‐17‐induced MEKK2 Ser520 phosphorylation is required not only for downstream p38 and NF‐κB activation but also for NEDD4L‐mediated MEKK2 degradation and the subsequent shutdown of IL‐17R signaling. Importantly, Nedd4l‐deficient mice show increased susceptibility to IL‐17‐induced inflammation and aggravated symptoms of experimental autoimmune encephalomyelitis (EAE) in IL‐17R signaling‐dependent manner. These data suggest that NEDD4L acts as an inhibitor of IL‐17R signaling, which ameliorates the pathogenesis of IL‐17‐mediated autoimmune diseases.

Potential neurotoxic activity of diverse molecules released by astrocytes

Astrocytes are the main support cells of the central nervous system. They also participate in neuroimmune reactions. In response to pathological and immune stimuli, astrocytes transform to reactive states characterized by increased release of inflammatory mediators. Some of these molecules are neuroprotective and inflammation resolving while others, including reactive oxygen species (ROS), nitric oxide (NO), matrix metalloproteinase (MMP)- 9, L-glutamate, and tumor necrosis factor α (TNF), are well-established toxins known to cause damage to surrounding cells and tissues. We hypothesized that similar to microglia, the brain immune cells, reactive astrocytes can release a broader set of diverse molecules that are potentially neurotoxic. A literature search was conducted to identify such molecules using the following two criteria: 1) evidence of their expression and secretion by astrocytes and 2) direct neurotoxic action. This review describes 14 structurally diverse molecules as less-established astrocyte neurotoxins, including C-X-C motif chemokine ligand (CXCL)10, CXCL12/CXCL12(5-67), FS-7-associated surface antigen ligand (FasL), macrophage inflammatory protein (MIP)- 2α, TNF-related apoptosis inducing ligand (TRAIL), pro-nerve growth factor (proNGF), pro-brain-derived neurotrophic factor (proBDNF), chondroitin sulfate proteoglycans (CSPGs), cathepsin (Cat)B, group IIA secretory phospholipase A₂ (sPLA₂-IIA), amyloid beta peptides (Aβ), high mobility group box (HMGB)1, ceramides, and lipocalin (LCN)2. For some of these molecules, further studies are required to establish either their direct neurotoxic effects or the full spectrum of stimuli that induce their release by astrocytes. Only limited studies with human-derived astrocytes and neurons are available for most of these potential neurotoxins, which is a knowledge gap that should be addressed in the future. We also summarize available evidence of the role these molecules play in select neuropathologies where reactive astrocytes are a key feature. A comprehensive understanding of the full spectrum of neurotoxins released by reactive astrocytes is key to understanding neuroinflammatory diseases characterized by the adverse activation of these cells and may guide the development of novel treatment strategies.

Cellular mechanism underlying the facilitation of contractile response induced by IL-25 in mouse tracheal smooth muscle

Asthma is a common heterogeneous respiratory disease characterized by airway inflammation and airway hyperresponsiveness (AHR) which is associated with abnormality in smooth muscle contractility. The epithelial cell-derived cytokine IL-25 is implicated in type 2 immune pathology including asthma, whereas the underlying mechanisms have not been fully elucidated. This study aims to investigate the effects of IL-25 on mouse tracheal smooth muscle contractility and elucidate the cellular mechanisms. Incubation with IL-25 augmented the contraction of mouse tracheal smooth muscles, which could be suppressed by the L-type voltage-dependent Ca²⁺ channel (L-VDCC) blocker nifedipine. Furthermore, IL-25 enhanced the cytosolic Ca²⁺ signals and triggered up-regulation of α1C L-VDCC (Ca_V1.2) in primary cultured mouse tracheal smooth muscle cells. Knocking down IL-17RA/IL-17RB receptors or inhibiting the transforming growth factor-β-activated kinase 1 (TAK1)-tumor progression locus 2 (TPL2)-MAPK kinase 1/2 (MEK1/2)-ERK1/2-activating protein-1 (AP-1) signaling pathways suppressed the IL-25-elicited up-regulation of Ca_V1.2 and hyperreactivity in tracheal smooth muscles. Moreover, inhibition of TPL2, ERK1/2 or L-VDCC alleviated the AHR symptom induced by IL-25 in a murine model. This study revealed that IL-25 potentiated the contraction of tracheal smooth muscle and evoked AHR via activation of TPL2-ERK1/2-Ca_V1.2 signaling, providing novel targets for the treatment of asthma with a high-IL-25 phenotype.

Viral Mimicry of Interleukin-17A by SARS-CoV-2 ORF8

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection triggers cytokine-mediated inflammation, leading to a myriad of clinical presentations in COVID-19. The SARS-CoV-2 open reading frame 8 (ORF8) is a secreted and rapidly evolving glycoprotein. Patients infected with SARS-CoV-2 variants with ORF8 deleted are associated with mild disease outcomes, but the molecular mechanism behind this is unknown. Here, we report that SARS-CoV-2 ORF8 is a viral cytokine that is similar to but distinct from interleukin 17A (IL-17A) as it induces stronger and broader human IL-17 receptor (hIL-17R) signaling than IL-17A. ORF8 primarily targeted blood monocytes and induced the heterodimerization of hIL-17RA and hIL-17RC, triggering a robust inflammatory response. Transcriptome analysis revealed that besides its activation of the hIL-17R pathway, ORF8 upregulated gene expression for fibrosis signaling and coagulation dysregulation. A naturally occurring ORF8 L84S variant that was highly associated with mild COVID-19 showed reduced hIL-17RA binding and attenuated inflammatory responses. This study reveals how SARS-CoV-2 ORF8 by a viral mimicry of the IL-17 cytokine contributes to COVID-19 severe inflammation. IMPORTANCE Patients infected with SARS-CoV-2 variants lacking open reading frame 8 (ORF8) have been associated with milder infection and disease outcome, but the molecular mechanism behind how this viral accessory protein mediates disease pathogenesis is not yet known. In our study, we revealed that secreted ORF8 protein mimics host IL-17 to activate IL-17 receptors A and C (IL-17RA/C) and induces a significantly stronger inflammatory response than host IL-17A, providing molecular insights into the role of ORF8 in COVID-19 pathogenesis and serving as a potential therapeutic target.

TH Cells and Cytokines in Encephalitogenic Disorders

The invasion of immune cells into the central nervous system (CNS) is a hallmark of the process we call neuroinflammation. Diseases such as encephalitides or multiple sclerosis (MS) are characterised by the dramatic influx of T lymphocytes and monocytes. The communication between inflammatory infiltrates and CNS resident cells is primarily mediated through cytokines. Over the years, numerous cytokine networks have been assessed to better understand the development of immunopathology in neuroinflammation. In MS for instance, many studies have shown that CD4+ T cells infiltrate the CNS and subsequently lead to immunopathology. Inflammatory CD4+ T cells, such as TH1, TH17, GM-CSF-producing helper T cells are big players in chronic neuroinflammation. Conversely, encephalitogenic or meningeal regulatory T cells (TREGs) and TH2 cells have been shown to drive a decrease in inflammatory functions in microglial cells and thus promote a neuroprotective microenvironment. Recent studies report overlapping as well as differential roles of these cells in tissue inflammation. Taken together, this suggests a more complex relationship between effector T cell subsets in neuroinflammation than has hitherto been established. In this overview, we review the interplay between helper T cell subsets infiltrating the CNS and how they actively contribute to neuroinflammation and degeneration. Importantly, in this context, we will especially focus on the current knowledge regarding the contribution of various helper cell subsets to neuroinflammation by referring to their helper T cell profile in the context of their target cell.

Single-nucleus chromatin accessibility profiling highlights distinct astrocyte signatures in progressive supranuclear palsy and corticobasal degeneration

Tauopathies such as progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) exhibit characteristic neuronal and glial inclusions of hyperphosphorylated Tau (pTau). Although the astrocytic pTau phenotype upon neuropathological examination is the most guiding feature in distinguishing both diseases, regulatory mechanisms controlling their transitions into disease-specific states are poorly understood to date. Here, we provide accessible chromatin data of more than 45,000 single nuclei isolated from the frontal cortex of PSP, CBD, and control individuals. We found a strong association of disease-relevant molecular changes with astrocytes and demonstrate that tauopathy-relevant genetic risk variants are tightly linked to astrocytic chromatin accessibility profiles in the brains of PSP and CBD patients. Unlike the established pathogenesis in the secondary tauopathy Alzheimer disease, microglial alterations were relatively sparse. Transcription factor (TF) motif enrichments in pseudotime as well as modeling of the astrocytic TF interplay suggested a common pTau signature for CBD and PSP that is reminiscent of an inflammatory immediate-early response. Nonetheless, machine learning models also predicted discriminatory features, and we observed marked differences in molecular entities related to protein homeostasis between both diseases. Predicted TF involvement was supported by immunofluorescence analyses in postmortem brain tissue for their highly correlated target genes. Collectively, our data expand the current knowledge on risk gene involvement (e.g., MAPT, MAPK8, and NFE2L2) and molecular pathways leading to the phenotypic changes associated with CBD and PSP.

Endothelial Tpl2 regulates vascular barrier function via JNK-mediated degradation of claudin-5 promoting neuroinflammation or tumor metastasis

Increased vascular permeability and leakage are hallmarks of several pathologies and determine disease progression and severity by facilitating inflammatory/metastatic cell infiltration. Using tissue-specific genetic ablation in endothelial cells, we have investigated in vivo the role of Tumor progression locus 2 (Tpl2), a mitogen-activated protein kinase kinase kinase (MAP3K) member with pleiotropic effects in inflammation and cancer. In response to proinflammatory stimuli, endothelial Tpl2 deletion alters tight junction claudin-5 protein expression through inhibition of JNK signaling and lysosomal degradation activation, resulting in reduced vascular permeability and immune cell infiltration. This results in significantly attenuated disease scores in experimental autoimmune encephalomyelitis and fewer tumor nodules in a hematogenic lung cancer metastasis model. Accordingly, pharmacologic inhibition of Tpl2 or small interfering RNA (siRNA)-mediated Tpl2 knockdown recapitulates our findings and reduces lung metastatic tumor invasions. These results establish an endothelial-specific role for Tpl2 and highlight the therapeutic potential of blocking the endothelial-specific Tpl2 pathway in chronic inflammatory and metastatic diseases.

TAK1-TABs Complex: A Central Signalosome in Inflammatory Responses

Transforming growth factor-β (TGF-β)-activated kinase 1 (TAK1) is a member of the MAPK kinase kinase (MAPKKK) family and has been implicated in the regulation of a wide range of physiological and pathological processes. TAK1 functions through assembling with its binding partners TAK1-binding proteins (TAB1, TAB2, and TAB3) and can be activated by a variety of stimuli such as tumor necrosis factor α (TNFα), interleukin-1β (IL-1β), and toll-like receptor ligands, and they play essential roles in the activation of NF-κB and MAPKs. Numerous studies have demonstrated that post-translational modifications play important roles in properly controlling the activity, stability, and assembly of TAK1-TABs complex according to the indicated cellular environment. This review focuses on the recent advances in TAK1-TABs-mediated signaling and the regulations of TAK1-TABs complex by post-translational modifications.

Suppression of monosodium urate crystal-induced inflammation by inhibiting TGF-β-activated kinase 1-dependent signaling: role of the ubiquitin proteasome system

Monosodium urate (MSU) crystals activate inflammatory pathways that overlap with interleukin-1β (IL-1β) signaling. However, the post-translational mechanisms involved and the role of signaling proteins in this activation are unknown. In the present study, we investigated the intracellular signaling mechanisms involved in MSU-induced activation of THP-1 macrophages and human nondiseased synovial fibroblasts (NLSFs) and the in vivo efficacy of an inhibitor of tumor growth factor-β (TGF-β)-activated kinase 1 (TAK1), 5Z-7-oxozeaenol, in MSU-induced paw inflammation in C57BL/6 mice. THP-1 macrophage activation with MSU crystals (25-200 µg/ml) resulted in the rapid and sustained phosphorylation of interleukin-1 receptor-activated kinase 1 (IRAK1 Thr209) and TAK1 (Thr184/187) and their association with the E3 ubiquitin ligase TRAF6. At the cellular level, MSU inhibited the deubiquitinases A20 and UCHL2 and increased 20s proteasomal activity, leading to a global decrease in K63-linked ubiquitination and increase in K48-linked ubiquitination in THP-1 macrophages. While MSU did not stimulate cytokine production in NLSFs, it significantly amplified IL-1β-induced IL-6, IL-8, and ENA-78/CXCL5 production. Docking studies and MD simulations followed by TAK1 in vitro kinase assays revealed that uric acid molecules are capable of arresting TAK1 in an active-state conformation, resulting in sustained TAK1 kinase activation. Importantly, MSU-induced proinflammatory cytokine production was completely inhibited by 5Z-7-oxozeaenol but not IRAK1/4 or TRAF6 inhibitors. Administration of 5Z-7-oxozeaenol (5 or 15 mg/kg; orally) significantly inhibited MSU-induced paw inflammation in C57BL/6 mice. Our study identifies a novel post-translational mechanism of TAK1 activation by MSU and suggests the therapeutic potential of TAK1 in regulating MSU-induced inflammation.

MAP3K8 Regulates Cox-2-Mediated Prostaglandin E2 Production in the Lung and Suppresses Pulmonary Inflammation and Fibrosis

Idiopathic pulmonary fibrosis (IPF) is characterized by exuberant deposition of extracellular matrix components, leading to the deterioration of lung architecture and respiratory functions. Profibrotic mechanisms are controlled by multiple regulatory molecules, including MAPKs, in turn regulated by multiple phosphorylation cascades. MAP3K8 is an MAPK kinase kinase suggested to pleiotropically regulate multiple pathogenic pathways in the context of inflammation and cancer; however, a possible role in the pathogenesis of IPF has not been investigated. In this report, MAP3K8 mRNA levels were found decreased in the lungs of IPF patients and of mice upon bleomycin-induced pulmonary fibrosis. Ubiquitous genetic deletion of Map3k8 in mice exacerbated the modeled disease, whereas bone marrow transfer experiments indicated that although MAP3K8 regulatory functions are active in both hematopoietic and nonhematopoietic cells, Map3k8 in hematopoietic cells has a more dominant role. Macrophage-specific deletion of Map3k8 was further found to be sufficient for disease exacerbation thus confirming a major role for macrophages in pulmonary fibrotic responses and suggesting a main role for Map3k8 in the homeostasis of their effector functions in the lung. Map3k8 deficiency was further shown to be associated with decreased Cox-2 expression, followed by a decrease in PGE₂ production in the lung; accordingly, exogenous administration of PGE₂ reduced inflammation and reversed the exacerbated fibrotic profile of Map3k8 ^-/- mice. Therefore, MAP3K8 has a central role in the regulation of inflammatory responses and Cox-2-mediated PGE₂ production in the lung, and the attenuation of its expression is integral to pulmonary fibrosis development.

Activation of c-Jun N-Terminal Kinase, a Potential Therapeutic Target in Autoimmune Arthritis

The c-Jun-N-terminal kinase (JNK) is a critical mediator involved in various physiological processes, such as immune responses, and the pathogenesis of various diseases, including autoimmune disorders. JNK is one of the crucial downstream signaling molecules of various immune triggers, mainly proinflammatory cytokines, in autoimmune arthritic conditions, mainly including rheumatoid arthritis, ankylosing spondylitis, and psoriatic arthritis. The activation of JNK is regulated in a complex manner by upstream kinases and phosphatases. Noticeably, different subtypes of JNKs behave differentially in immune responses. Furthermore, aside from biologics targeting proinflammatory cytokines, small-molecule inhibitors targeting signaling molecules such as Janus kinases can act as very powerful therapeutics in autoimmune arthritis patients unresponsiveness to conventional synthetic antirheumatic drugs. Nevertheless, despite these encouraging therapies, a population of patients with an inadequate therapeutic response to all currently available medications still remains. These findings identify the critical signaling molecule JNK as an attractive target for investigation of the immunopathogenesis of autoimmune disorders and for consideration as a potential therapeutic target for patients with autoimmune arthritis to achieve better disease control. This review provides a useful overview of the roles of JNK, how JNK is regulated in immunopathogenic responses, and the potential of therapeutically targeting JNK in patients with autoimmune arthritis.

14-3-3ζ-TRAF5 axis governs interleukin-17A signaling

IL-17A is a therapeutic target in many autoimmune diseases. Most nonhematopoietic cells express IL-17A receptors and respond to extracellular IL-17A by inducing proinflammatory cytokines. The IL-17A signal transduction triggers two broad, TRAF6- and TRAF5-dependent, intracellular signaling pathways to produce representative cytokines (IL-6) and chemokines (CXCL-1), respectively. Our limited understanding of the cross-talk between these two branches has generated a crucial gap of knowledge, leading to therapeutics indiscriminately blocking IL-17A and global inhibition of its target genes. In previous work, we discovered an elevated expression of 14-3-3 proteins in inflammatory aortic disease, a rare human autoimmune disorder with increased levels of IL-17A. Here we report that 14-3-3ζ is essential for IL-17 signaling by differentially regulating the signal-induced IL-6 and CXCL-1. Using genetically manipulated human and mouse cells, and ex vivo and in vivo rat models, we uncovered a function of 14-3-3ζ. As a part of the molecular mechanism, we show that 14-3-3ζ interacts with several TRAF proteins; in particular, its interaction with TRAF5 and TRAF6 is increased in the presence of IL-17A. In contrast to TRAF6, we found TRAF5 to be an endogenous suppressor of IL-17A-induced IL-6 production, an effect countered by 14-3-3ζ. Furthermore, we observed that 14-3-3ζ interaction with TRAF proteins is required for the IL-17A-induced IL-6 levels. Together, our results show that 14-3-3ζ is an essential component of IL-17A signaling and IL-6 production, an effect that is suppressed by TRAF5. To the best of our knowledge, this report of the 14-3-3ζ-TRAF5 axis, which differentially regulates IL-17A-induced IL-6 and CXCL-1 production, is unique.

Regeneration of Damaged Tendon-Bone Junctions (Entheses)-TAK1 as a Potential Node Factor

Musculoskeletal dysfunctions are highly prevalent due to increasing life expectancy. Consequently, novel solutions to optimize treatment of patients are required. The current major research focus is to develop innovative concepts for single tissues. However, interest is also emerging to generate applications for tissue transitions where highly divergent properties need to work together, as in bone-cartilage or bone-tendon transitions. Finding medical solutions for dysfunctions of such tissue transitions presents an added challenge, both in research and in clinics. This review aims to provide an overview of the anatomical structure of healthy adult entheses and their development during embryogenesis. Subsequently, important scientific progress in restoration of damaged entheses is presented. With respect to enthesis dysfunction, the review further focuses on inflammation. Although molecular, cellular and tissue mechanisms during inflammation are well understood, tissue regeneration in context of inflammation still presents an unmet clinical need and goes along with unresolved biological questions. Furthermore, this review gives particular attention to the potential role of a signaling mediator protein, transforming growth factor beta-activated kinase-1 (TAK1), which is at the node of regenerative and inflammatory signaling and is one example for a less regarded aspect and potential important link between tissue regeneration and inflammation.

Interleukin-17 in Chronic Inflammatory Neurological Diseases

A critical role for IL-17, a cytokine produced by T helper 17 (Th17) cells, has been indicated in the pathogenesis of chronic inflammatory and autoimmune diseases. A positive effect of blockade of IL-17 secreted by autoreactive T cells has been shown in various inflammatory diseases. Several cytokines, whose production is affected by environmental factors, control Th17 differentiation and its maintenance in tissues during chronic inflammation. The roles of IL-17 in the pathogenesis of chronic neuroinflammatory conditions, multiple sclerosis (MS), experimental autoimmune encephalomyelitis (EAE), Alzheimer's disease, and ischemic brain injury are reviewed here. The role of environmental stimuli in Th17 differentiation is also summarized, highlighting the role of viral infection in the regulation of pathogenic T helper cells in EAE.

microRNA-381-3p Confers Protection Against Ischemic Stroke Through Promoting Angiogenesis and Inhibiting Inflammation by Suppressing Cebpb and Map3k8

Ischemic stroke is a serious disease with limited prevention methods, and various genes and microRNAs (miRNAs) have been found to be dysregulated in the pathogenesis of this disease. This study aims to explore the potential role of miR-381-3p in ischemic stroke, along with its underlying mechanism. A mouse model of ischemic stroke was developed using middle cerebral artery occlusion. Next, the expression of mitogen-activated protein kinase kinase kinase 8 (Map3k8) and CCAAT enhancer binding protein beta (Cebpb) was determined by RT-qPCR. Gain- and loss-of-function approaches were applied to analyze the effects of miR-381-3p, Cebpb and Map3k8 on the biological functions of endothelial progenitor cells (EPCs) with the involvement of the tumor necrosis factor-α (TNF-α) signaling pathway. In addition, dual luciferase reporter gene assay was performed for the analysis of the relationship among miR-381-3p, Map3k8 and Cebpb. Further, rescue experiment was performed with the help of JNK/p38 specific agonist, Anisomycin. Map3k8 and Cebpb were highly expressed in ischemic stroke. Loss-of-function of Map3k8 or Cebpb in EPCs contributed to accelerated proliferation, migration and angiogenesis of EPCs. Next, miR-381-3p downregulated the expression of its two target genes, Map3k8 and Cebpb. miR-381-3p overexpression promoted angiogenesis of EPCs, and inhibited inflammation, which could be reversed by restoration of Map3k8 or Cebpb. Additionally, silencing Map3k8 or Cebpb inhibited the activation of TNF-α signaling pathway. Furthermore, Anisomycin treatment could enhance inflammation and inhibit angiogenesis. Taken together, miR-381-3p downregulates Map3k8 and Cebpb to protect against ischemic stroke, broadening our understanding of the pathogenesis of ischemic stroke.

TAK1 inhibition in mouse astrocyte cultures ameliorates cytokine-induced chemokine production and neutrophil migration

Systemic inflammation can exacerbate symptoms of many neurological diseases. This effect may be facilitated by glial cells of the central nervous system (CNS) that alter their transcriptional responses and up-regulate cytokine and chemokine expression which can, in turn trigger immune surveillance. In this study, we sought to determine the effects of pro-inflammatory cytokine stimulation (TNF, IL-1α, IL-1β) on astrocyte and microglia chemokine secretion. Primary cultures of astrocytes or microglia were stimulated with the recombinant cytokines and the levels of secreted chemokines were semi-quantitatively determined using a chemokine-specific proteome profiler array and densitometry. Pharmacological inhibitors were used to determine the effects of p38 MAPK, JNK, ERK1/2, NFkB, and transforming growth factor beta-associated kinase 1 (TAK1) in controlling chemokine production. Finally, neutrophil migration assays were performed to demonstrate functionality. Our data show that stimulated astrocytes secrete at least eight chemokines as a response to cytokine stimulation. These include those involved in neutrophil chemo-attraction and proved capable of promoting neutrophil migration in vitro. In contrast, microglia up-regulated few chemokines in response to cytokine stimulation and did not promote neutrophil migration. However, microglia readily secreted chemokines following stimulation with the toll-like receptor agonists. Finally, we show that both the production of chemokines and neutrophil migration resulting from cytokine stimulation of astrocytes was dependent on TAK1 signaling. Collectively, this study adds to the understanding of how astrocytes and microglia respond to stimuli and their role in promoting neutrophil migration to the CNS during inflammatory conditions.

Tpl2 Protects Against Fulminant Hepatitis Through Mobilization of Myeloid-Derived Suppressor Cells

Myeloid derived suppressor cells (MDSC) in the liver microenvironment protects against the inflammation-induced liver injury in fulminant hepatitis (FH). However, the molecular mechanism through which MDSC is recruited into the inflamed liver remain elusive. Here we identified a protein kinase Tpl2 as a critical mediator of MDSC recruitment into liver during the pathogenesis of Propionibacterium acnes/LPS-induced FH. Loss of Tpl2 dramatically suppressed MDSC mobilization into liver, leading to exaggerated local inflammation and increased FH-induced mortality. Mechanistically, although the protective effect of Tpl2 for FH-induced mortality was dependent on the presence of MDSC, Tpl2 neither directly targeted myeloid cells nor T cells to regulate FH pathogenesis, but functioned in hepatocytes to mediate the induction of MDSC-attracting chemokine CXCL1 and CXCL2 through modulating IL-25 (also known as IL-17E) signaling. As a consequence, increased MDSC in the inflamed liver specifically restrained the local proliferation of infiltrated pathogenic CD4⁺ T cells, and thus protected against the inflammation-induced acute liver failure. Together, our findings established Tpl2 as a critical mediator of MDSC recruitment and highlighted the therapeutic potential of Tpl2 for the treatment of FH.

ABIN-2, of the TPL-2 Signaling Complex, Modulates Mammalian Inflammation

Mammalian TPL-2 kinase (MAP3K8) mediates Toll-like receptor activation of ERK1/2 and p38α MAP kinases and is critical for regulating immune responses to pathogens. TPL-2 also has an important adaptor function, maintaining stability of associated ABIN-2 ubiquitin-binding protein. Consequently, phenotypes detected in Map3k8^-/- mice can be caused by lack of TPL-2, ABIN-2, or both proteins. Recent studies show that increased inflammation of Map3k8^-/- mice in allergic airway inflammation and colitis results from reduced ABIN-2 signaling, rather than blocked TPL-2 signaling. However, Map3k8^-/- mice have been employed extensively to evaluate the potential of TPL-2 as an anti-inflammatory drug target. We posit that Map3k8^D270A/D270A mice, expressing catalytically inactive TPL-2 and physiologic ABIN-2, should be used to evaluate the potential effects of TPL-2 inhibitors in disease.

Inflammatory Responses Reprogram TREGS Through Impairment of Neuropilin-1

Chronic inflammatory insults compromise immune cell responses and ultimately contribute to pathologic outcomes. Clinically, it has been suggested that bone debris and implant particles, such as polymethylmethacrylate (PMMA), which are persistently released following implant surgery evoke heightened immune, inflammatory, and osteolytic responses that contribute to implant failure. However, the precise mechanism underlying this pathologic response remains vague. T_REGS, the chief immune-suppressive cells, express the transcription factor Foxp3 and are potent inhibitors of osteoclasts. Using an intra-tibial injection model, we show that PMMA particles abrogate the osteoclast suppressive function of T_REGS. Mechanistically, PMMA particles induce T_REG instability evident by reduced expression of Foxp3. Importantly, intra-tibial injection of PMMA initiates an acute innate immune and inflammatory response, yet the negative impact on T_REGS by PMMA remains persistent. We further show that PMMA enhance T_H17 response at the expense of other T effector cells (T_EFF), particularly T_H1. At the molecular level, gene expression analysis showed that PMMA particles negatively regulate Nrp-1/Foxo3a axis to induce T_REG instability, to dampen T_REG activity and to promote phenotypic switch of T_REGS to T_H17 cells. Taken together, inflammatory cues and danger signals, such as bone and implant particles exacerbate inflammatory osteolysis in part through reprogramming T_REGS.

Peli1 controls the survival of dopaminergic neurons through modulating microglia-mediated neuroinflammation

Chronic neuroinflammation is known to contributes to the toxicity of neurodegeneration of Parkinson’s disease (PD). However, the molecular and cellular mechanisms controlling inflammatory responses in the central nervous system remain poorly understood. Here we found that a E3 ubiquitin ligase Peli1 is dramatically induced only in the substantia nigra (SN) of the human and mouse PD brains. The ablation of Peli1 significantly suppressed LPS-induced production of neurotoxic mediators and proinflammatory cytokines in SN and in primary microglia, whereas Peli1 is dispensable for the inflammatory responses in astrocyte. Accordingly, Peli1 deficiency markedly inhibited neuron death induced by the conditioned medium from LPS-stimulated microglia. Mechanistical study suggested that Peli1 acts as a positive regulator of inflammatory response in microglia through activation of NF-κB and MAP kinase. Our results established Peli1 as a critical mediator in the regulation of microglial activation and neuroinflammation-induced death of dopaminergic neurons during PD pathogenesis, suggesting that targeting Peli1 may have therapeutic effect in neuroinflammation.

The aryl hydrocarbon receptor is a novel negative regulator of interleukin-17-mediated signaling and inflammation in vitro

Interleukin (IL)-17 plays a critical role in the pathogenesis of inflammation and autoimmune diseases. The aryl hydrocarbon receptor (AHR) is a transcription factor responsible for the elimination of xenobiotic chemicals. However, it remains unknown whether AHR is involved in IL-17 signaling. Here, we demonstrate that knockdown of AHR significantly enhances, while overexpression or activation of AHR inhibits IL-17-induced inflammation in Hela cells. AHR specifically suppresses IL-17-induced p38 activation, and inhibition of p38 activity markedly reverses the effect of AHR knockdown on IL-17-induced inflammation. Mechanistically, AHR physically interacts with TAK1 and mitogen-activated protein kinase kinase 3/6 (MKK3/6) and disrupts TAK1-MKK3/6 interaction, leading to impaired IL-17 signaling. Thus, our study indicates that AHR negatively regulates IL-17-mediated signaling and inflammation at least partially through interfering with the interaction between TAK1 and MKK3/6.

Tumor Progression Locus 2 in Hepatocytes Potentiates Both Liver and Systemic Metabolic Disorders in Mice

Tumor progression locus 2 (TPL2), a serine/threonine kinase, has been regarded as a potentially interesting target for the treatment of various diseases with an inflammatory component. However, the function of TPL2 in regulating hepatocyte metabolism and liver inflammation during the progression of nonalcoholic fatty liver disease (NAFLD) is poorly understood. Here, we report that TPL2 protein expression was significantly increased in fatty liver from diverse species, including humans, monkeys, and mice. Further investigations revealed that compared to wild-type (WT) littermates, hepatocyte-specific TPL2 knockout (HKO) mice exhibited improved lipid and glucose imbalance, reserved insulin sensitivity, and alleviated inflammation in response to high-fat diet (HFD) feeding. Overexpression of TPL2 in hepatocytes led to the opposite phenotype. Regarding the mechanism, we found that mitogen-activated protein kinase kinase 7 (MKK7) was the specific substrate of TPL2 for c-Jun N-terminal kinase (JNK) activation. TPL2-MKK7-JNK signaling in hepatocytes represents a promising drugable target for treating NAFLD and associated metabolic disorders. Conclusion: In hepatocytes, TPL2 acts as a key mediator that promotes both liver and systemic metabolic disturbances by specifically increasing MKK7-JNK activation.

Hypoxia potentiates monocyte-derived dendritic cells for release of tumor necrosis factor α via MAP3K8

Dendritic cells (DCs) constantly sample peripheral tissues for antigens, which are subsequently ingested to derive peptides for presentation to T cells in lymph nodes. To do so, DCs have to traverse many different tissues with varying oxygen tensions. Additionally, DCs are often exposed to low oxygen tensions in tumors, where vascularization is lacking, as well as in inflammatory foci, where oxygen is rapidly consumed by inflammatory cells during the respiratory burst. DCs respond to oxygen levels to tailor immune responses to such low-oxygen environments. In the present study, we identified a mechanism of hypoxia-mediated potentiation of release of tumor necrosis factor α (TNF-α), a pro-inflammatory cytokine with important roles in both anti-cancer immunity and autoimmune disease. We show in human monocyte-derived DCs (moDCs) that this potentiation is controlled exclusively via the p38/mitogen-activated protein kinase (MAPK) pathway. We identified MAPK kinase kinase 8 (MAP3K8) as a target gene of hypoxia-induced factor (HIF), a transcription factor controlled by oxygen tension, upstream of the p38/MAPK pathway. Hypoxia increased expression of MAP3K8 concomitant with the potentiation of TNF-α secretion. This potentiation was no longer observed upon siRNA silencing of MAP3K8 or with a small molecule inhibitor of this kinase, and this also decreased p38/MAPK phosphorylation. However, expression of DC maturation markers CD83, CD86, and HLA-DR were not changed by hypoxia. Since DCs play an important role in controlling T-cell activation and differentiation, our results provide novel insight in understanding T-cell responses in inflammation, cancer, autoimmune disease and other diseases where hypoxia is involved.

Control of Treg cell homeostasis and immune equilibrium by Lkb1 in dendritic cells

To balance immunity and tolerance, the endogenous pool of Foxp3⁺ regulatory T (T_reg) cells is tightly controlled, but the underlying mechanisms of this control remain poorly understood. Here we show that the number of T_reg cells is negatively regulated by the kinase Lkb1 in dendritic cells (DCs). Conditional knockout of the Lkb1 gene in DCs leads to excessive T_reg cell expansion in multiple organs and dampens antigen-specific T cell immunity. Lkb1-deficient DCs are capable of enhancing, compared with wild-type DCs, T_reg cell proliferation via cell-cell contact involving the IKK/IKBα-independent activation of the NF-κB/OX40L pathway. Intriguingly, treating wild-type mice with lipopolysaccharide selectively depletes Lkb1 protein in DCs, resulting in T_reg cell expansion and suppressed inflammatory injury upon subsequent challenge. Loss of Lkb1 does not obviously upregulate proinflammatory molecules expression on DCs. We thus identify Lkb1 as a regulatory switch in DCs for controlling T_reg cell homeostasis, immune response and tolerance.

Regulatory T (Treg) cells are important for maintaining immune homeostasis by suppressing immune cell activation, but how the Treg cell pool is maintained is still unclear. Here the authors show that a kinase, Lkb1, operates in dendritic cells (DC) to inhibit Treg cell expansion and immunosuppression via mechanisms involving NF-kB/OX40L signalling.

IL-17A contributes to perioperative neurocognitive disorders through blood-brain barrier disruption in aged mice

BACKGROUND

Perioperative neurocognitive disorders (PND) occur frequently after surgery, especially in aged patients. Surgery-induced neuroinflammation and blood-brain barrier (BBB) dysfunction play a crucial role in the pathogenesis of PND. Interleukin-17A (IL-17A) increases after surgical stress and will be involved in BBB dysfunction. However, the effect of IL-17A on BBB function during PND remains poorly understood.

METHODS

Male wild-type C57BL/6J mice (15 months old) received tibial fracture surgery and fixation to establish the PND model. All the mice were injected intraperitoneally with an IL-17A-neutralizing antibody (Abs) or isotype-control Abs 30 min before tibial fracture surgery. Animal behaviour tests conducted 24 h after surgery included the contextual fear conditioning and Y maze tests. Serum and hippocampus IL-17A levels and hippocampus IL-6 and IL-1β levels were detected by ELISA. BBB function was detected by Evans blue (EB) test. Hippocampus matrix metalloproteinase-2 (MMP-2)- and MMP-9-positive cells were detected by immunohistochemistry. Hippocampus albumin, occludin, claudin-5 and IL-17A receptors were detected by Western blot. For the in vitro experiment, bEnd.3 cells were incubated with IL-17A. Cell IL-17A receptors were detected by immunofluorescence. Cellular MMP-2, MMP-9, occludin, and claudin-5 were detected by Western blot.

RESULTS

Tibial fracture surgery promoted memory impairment, increased levels of IL-17A and IL-17A receptors, inflammatory factor production and BBB dysfunction. IL-17A Abs inhibited this effect, including improving memory function, decreasing inflammatory factor production and alleviating BBB disruption, indicated by decreased tight junctions (TJs) and increased MMPs after surgery. The in vitro study suggested that recombinant IL-17A could upregulate the expression of IL-17A receptors, decrease TJs and increase the level of MMPs in bEnd.3 cells.

CONCLUSIONS

Our results suggested that IL-17A-promoted BBB disruption might play an important role in the pathogenesis of PND.

A20-binding inhibitor of NF-κB (ABIN) 2 negatively regulates allergic airway inflammation

Airway allergic responses are shown to be inhibited by binding of ABIN-2 to A20, a key negative regulator of inflammation. In contrast, the catalytic activity of the ABIN-2–associated kinase TPL-2 does not regulate airway allergic responses, an important consideration for the development of TPL-2 inhibitors to treat inflammatory diseases.

TPL-2 MAP 3-kinase promotes inflammation in numerous mouse disease models and is an attractive anti-inflammatory drug target. However, TPL-2–deficient (Map3k8^−/−) mice develop exacerbated allergic airway inflammation to house dust mite (HDM) compared with wild type controls. Here, we show that Map3k8^D270A/D270A mice expressing kinase dead TPL-2 had an unaltered response to HDM, indicating that the severe airway inflammation observed in Map3k8^−/− mice is not due to blockade of TPL-2 signaling and rather reflects a TPL-2 adaptor function. Severe allergic inflammation in TPL-2–deficient mice was likely due to reduced levels of ABIN-2 (TNIP2), whose stability depends on TPL-2 expression. Tnip2^E256K knock-in mutation, which reduced ABIN-2 binding to A20, augmented the HDM-induced airway inflammation, but did not affect TPL-2 expression or signaling. These results identify ABIN-2 as a novel negative regulator of allergic airway responses and importantly indicate that TPL-2 inhibitors would not have unwanted allergic comorbidities.

Protein Kinase Serine/Threonine Kinase 24 Positively Regulates Interleukin 17-Induced Inflammation by Promoting IKK Complex Activation

Interleukin 17 (IL-17) is a key inflammatory cytokine that plays a critical role in tissue inflammation and autoimmune diseases. However, its signaling remains poorly understood. In this study, we identified serine/threonine kinase 24 (Stk24) as a positive modulator of IL-17-mediated signaling and inflammation. Stk24 deficiency or knockdown markedly inhibited IL-17-induced phosphorylation of NF-κB and impaired IL-17-induced chemokines and cytokines expression. Stk24 overexpression greatly enhanced IL-17-induced NF-κB activation and expression of chemokines and cytokines in a kinase activity-independent manner. The IL-17-induced inflammatory response was significantly reduced in Stk24-deficient mice. In addition, the severity of experimental autoimmune encephalomyelitis was markedly reduced in mice with a deficiency of Stk24 in non-hematopoietic cells. We further demonstrated that Stk24 directly interacts with TAK1 and IKKβ and promotes the formation of TAK1/IKK complexes, leading to enhanced IKKβ/NF-κB activation and downstream cytokines and chemokines induction. Collectively, our findings suggest that Stk24 plays an important role in controlling IL-17-triggered inflammation and autoimmune diseases and provides new insight into the therapeutic targets of IL-17-mediated inflammatory disease.

RKIP mediates autoimmune inflammation by positively regulating IL-17R signaling

Th17 cells contribute to the development of autoimmune diseases by secreting interleukin-17 (IL-17), which activates its receptor (IL-17R) that is expressed on epithelial cells, macrophages, microglia, and resident neuroectodermal cells. However, the mechanisms through which IL-17R-mediated signaling contributes to the development of autoimmune disease have not been completely elucidated. Here, we demonstrate that Raf-1 kinase inhibitor protein (RKIP) deficiency in mice ameliorates the symptoms of experimental autoimmune encephalomyelitis (EAE). Adoptive T-cell-transfer experiments demonstrate that RKIP plays a predominant role in Th17-mediated, but not in Th1-mediated immune responses. RKIP deficiency has no effect on Th17-cell differentiation ex vivo, nor does it affect Th17-cell differentiation in EAE mice. However, RKIP significantly promotes IL-17R-induced proinflammatory cytokine and chemokine production. Mechanistically, RKIP directly interacts with IL-17RA and Act1 to promote the formation of an IL-17R-Act1 complex, resulting in enhanced MAPK- and P65-mediated NF-κB activation and downstream cytokine production. Together, these findings indicate that RKIP functions as an essential modulator of the IL-17R-Act1 axis in IL-17R signaling, which promotes IL-17-induced inflammation and autoimmune neuroinflammation.

Assaying kinase activity of the TPL-2/NF-κB1 p105/ABIN-2 complex using an optimal peptide substrate

The MKK1/2 kinase tumour progression locus 2 (TPL-2) is critical for the production of tumour necrosis factor alpha (TNFα) in innate immune responses and a potential anti-inflammatory drug target. Several earlier pharmaceutical company screens with the isolated TPL-2 kinase domain have identified small-molecule inhibitors that specifically block TPL-2 signalling in cells, but none of these have progressed to clinical development. We have previously shown that TPL-2 catalytic activity regulates TNF production by macrophages while associated with NF-κB1 p105 and ABIN-2, independently of MKK1/2 phosphorylation via an unknown downstream substrate. In the present study, we used a positional scanning peptide library to determine the optimal substrate specificity of a complex of TPL-2, NF-κB1 p105 and ABIN-2. Using an optimal peptide substrate based on this screen and a high-throughput mass spectrometry assay to monitor kinase activity, we found that the TPL-2 complex has significantly altered sensitivities versus existing ATP-competitive TPL-2 inhibitors than the isolated TPL-2 kinase domain. These results imply that screens with the more physiologically relevant TPL-2/NF-κB1 p105/ABIN-2 complex have the potential to deliver novel TPL-2 chemical series; both ATP-competitive and allosteric inhibitors could emerge with significantly improved prospects for development as anti-inflammatory drugs.

TPL2 kinase action and control of inflammation

Tumor progression locus 2 (TPL2, also known as COT or MAP3K8) is a mitogen-activated protein kinase kinase (MAP3K) activated downstream of TNFαR, IL1R, TLR, CD40, IL17R, and some GPCRs. TPL2 regulates the MEK1/2 and ERK1/2 pathways to regulate a cascade of inflammatory responses. In parallel to this, TPL2 also activates p38α and p38δ to drive the production of various inflammatory mediators in neutrophils. We discuss the implications of this finding in the context of various inflammatory diseases.

NF-κB and the Transcriptional Control of Inflammation

The NF-κB transcription factor was discovered 30 years ago and has since emerged as the master regulator of inflammation and immune homeostasis. It achieves this status by means of the large number of important pro- and antiinflammatory factors under its transcriptional control. NF-κB has a central role in inflammatory diseases such as rheumatoid arthritis, inflammatory bowel disease, and autoimmunity, as well as diseases comprising a significant inflammatory component such as cancer and atherosclerosis. Here, we provide an overview of the studies that form the basis of our understanding of the role of NF-κB subunits and their regulators in controlling inflammation. We also describe the emerging importance of posttranslational modifications of NF-κB in the regulation of inflammation, and highlight the future challenges faced by researchers who aim to target NF-κB transcriptional activity for therapeutic benefit in treating chronic inflammatory diseases.

Tpl2 Promotes Innate Cell Recruitment and Effector T Cell Differentiation To Limit Citrobacter rodentium Burden and Dissemination

Tumor progression locus 2 (Tpl2) is a serine-threonine kinase that regulates Th1 differentiation, secretion of the inflammatory cytokine gamma interferon (IFN-γ), and host defense against the intracellular pathogens Toxoplasma gondii, Listeria monocytogenes, and Mycobacterium tuberculosis However, relatively little is known about the contribution of Tpl2 to Th17 differentiation and immune cell function during infection with an extracellular pathogen. The goal of this study was to determine whether Tpl2 influences the immune response generated to the extracellular bacterium Citrobacter rodentium, which induces a mixed Th1 and Th17 response. During peak infection with C. rodentium, Tpl2-/- mice experienced greater bacterial burdens with evidence of dissemination to the liver and spleen but ultimately cleared the bacteria within 3 weeks postinfection, similar to the findings for wild-type mice. Tpl2-/- mice also recruited fewer neutrophils and monocytes to the colon during peak infection, which correlated with increased bacterial burdens. In mixed bone marrow chimeras, Tpl2 was shown to play a T cell-intrinsic role in promoting both IFN-γ and interleukin-17A production during infection with C. rodentium However, upon CD4 T cell transfer into Rag-/- mice, Tpl2-/- CD4 T cells were as protective as wild-type CD4 T cells against the dissemination of bacteria and mortality. These data indicate that the enhanced bacterial burdens in Tpl2-/- mice are not caused primarily by impairments in CD4 T cell function but result from defects in innate immune cell recruitment and function.

IL-17-triggered downregulation of miR-497 results in high HIF-1α expression and consequent IL-1β and IL-6 production by astrocytes in EAE mice

Interleukin 17 (IL-17) is increasingly recognized as a key factor that contributes to the pathogenesis of multiple sclerosis (MS) and its experimental mouse autoimmune encephalomyelitis (EAE) model. However, the roles and regulatory mechanisms of IL-17-induced pro-inflammatory cytokine production in EAE mice remain largely unclear. In this study, the expression of IL-17, hypoxia inducible factor-1α (HIF-1α), IL-1β, IL-6 and microRNA-497 (miR-497), as well as their intrinsic associations, was investigated using EAE model mice and cultured astrocytes exposed to IL-17 in vitro. We observed markedly increased production of IL-17, HIF-1α, IL-1β and IL-6 in the brain tissues of EAE mice, while the expression and secretion of HIF-1α, IL-1β and IL-6 were also significantly increased when cultured primary astrocytes from mice were stimulated with IL-17. Meanwhile, the expression of miR-497 was downregulated both in vivo and in vitro. Subsequent in vitro experiments revealed that IL-17 induced the production of IL-1β and IL-6 in astrocytes through the upregulation of HIF-1α as a transcriptional factor, indicating that IL-17-mediated downregulation of miR-497 enhanced HIF-1α expression. Furthermore, astrocyte-specific knockdown of IL-17RA and HIF-1α or astrocyte-specific overexpression of miR-497 by infection with different lentiviral vectors containing an astrocyte-specific promotor markedly decreased IL-1β and IL-6 production in brain tissues and alleviated the pathological changes and score of EAE mice. Collectively, these findings indicate that decreased miR-497 expression is responsible for IL-17-triggered high HIF-1α expression and consequent IL-1β and IL-6 production by astrocytes in EAE mice.Cellular & Molecular Immunology advance online publication, 1 May 2017; doi:10.1038/cmi.2017.12.

The kinase TPL2 activates ERK and p38 signaling to promote neutrophilic inflammation

Tumor progression locus 2 (TPL2; also known as MAP3K8) is a mitogen-activated protein kinase (MAPK) kinase kinase (MAP3K) that phosphorylates the MAPK kinases MEK1 and MEK2 (MEK1/2), which, in turn, activate the MAPKs extracellular signal-regulated kinase 1 (ERK1) and ERK2 (ERK1/2) in macrophages stimulated through the interleukin-1 receptor (IL-1R), Toll-like receptors (TLRs), or the tumor necrosis factor receptor (TNFR). We describe a conserved and critical role for TPL2 in mediating the effector functions of neutrophils through the activation of the p38 MAPK signaling pathway. Gene expression profiling and functional studies of neutrophils and monocytes revealed a MEK1/2-independent branch point downstream of TPL2 in neutrophils. Biochemical analyses identified the MAPK kinases MEK3 and MEK6 and the MAPKs p38α and p38δ as downstream effectors of TPL2 in these cells. Genetic ablation of the catalytic activity of TPL2 or therapeutic intervention with a TPL2-specific inhibitor reduced the production of inflammatory mediators by neutrophils in response to stimulation with the TLR4 agonist lipopolysaccharide (LPS) in vitro, as well as in rodent models of inflammatory disease. Together, these data suggest that TPL2 is a drug target that activates not only MEK1/2-dependent but also MEK3/6-dependent signaling to promote inflammatory responses.

Tpl2 promotes neutrophil trafficking, oxidative burst, and bacterial killing

Tumor progression locus 2 (Tpl2) is a serine/threonine kinase that promotes inflammatory cytokine production by activating the MEK/ERK pathway. Tpl2 has been shown to be important for eliciting the inflammatory properties of macrophages; however, there is relatively little known about the contribution of Tpl2 to neutrophil effector functions. This is an important consideration, as neutrophils provide the first line of defense against infection in the innate immune system. We found that Tpl2 is expressed in both human and murine neutrophils, suggesting a potential function for Tpl2 in this lineage. Despite significantly higher proportions of bone marrow (BM) neutrophils in Tpl2-deficient (Tpl2^-/- ) mice compared with wild-type (WT) mice, Tpl2^-/- mice have significantly reduced proportions of circulating neutrophils. Tpl2^-/- neutrophils show impaired recruitment to thioglycollate, which was primarily a result of neutrophil-extrinsic factors in the host. In response to infection, neutrophils secrete inflammatory cytokines and produce reactive oxygen species (ROS), which promote bacterial killing. Tpl2 ablation impaired neutrophil TNF secretion in response to LPS stimulation, superoxide generation in response to the chemotactic peptide fMLP, and killing of the extracellular bacterium, Citrobacter rodentium, despite normal bacterial phagocytosis. These results implicate Tpl2 in the regulation of multiple neutrophil antimicrobial pathways, including inflammatory cytokine secretion and oxidative burst. Furthermore, they indicate that Tpl2 functions early during infection to bolster neutrophil-mediated innate immunity against extracellular bacteria.

IL-17 Signaling: The Yin and the Yang

Interleukin (IL)-17 is the founding member of a novel family of inflammatory cytokines. While the proinflammatory properties of IL-17 are key to its host-protective capacity, unrestrained IL-17 signaling is associated with immunopathology, autoimmune disease, and cancer progression. In this review we discuss both the activators and the inhibitors of IL-17 signal transduction, and also the physiological implications of these events. We highlight the surprisingly diverse means by which these regulators control expression of IL-17-dependent inflammatory genes, as well as the major target cells that respond to IL-17 signaling.

Post-Translational Modifications of the TAK1-TAB Complex

Transforming growth factor-β (TGF-β)-activated kinase 1 (TAK1) is a member of the mitogen-activated protein kinase kinase kinase (MAPKKK) family that is activated by growth factors and cytokines such as TGF-β, IL-1β, and TNF-α, and mediates a wide range of biological processes through activation of the nuclear factor-κB (NF-κB) and the mitogen-activated protein (MAP) kinase signaling pathways. It is well established that activation status of TAK1 is tightly regulated by forming a complex with its binding partners, TAK1-binding proteins (TAB1, TAB2, and TAB3). Interestingly, recent evidence indicates the importance of post-translational modifications (PTMs) of TAK1 and TABs in the regulation of TAK1 activation. To date, a number of PTMs of TAK1 and TABs have been revealed, and these PTMs appear to fine-tune and coordinate TAK1 activities depending on the cellular context. This review therefore focuses on recent advances in the understanding of the PTMs of the TAK1-TAB complex.

Map3k8 Modulates Monocyte State and Atherogenesis in ApoE-/- Mice

OBJECTIVE

Map3k8 (Cot/Tpl2) activates the MKK1/2-ERK1/2, MAPK pathway downstream from interleukin-1R, tumor necrosis factor-αR, NOD-2R (nucleotide-binding oligomerization domain-like 2R), adiponectinR, and Toll-like receptors. Map3k8 plays a key role in innate and adaptive immunity and influences inflammatory processes by modulating the functions of different cell types. However, its role in atherogenesis remains unknown. In this study, we analyzed the role of this kinase in this pathology.

APPROACH AND RESULTS

We show here that Map3k8 deficiency results in smaller numbers of Ly6C^highCD11c^low and Ly6C^lowCD11c^high monocytes in ApoE^-/- mice fed a high-fat diet (HFD). Map3k8^-/-ApoE^-/- monocytes displayed high rates of apoptosis and reduced amounts of Nr4a1, a transcription factor known to modulate apoptosis in Ly6C^lowCD11c^high monocytes. Map3k8^-/-ApoE^-/- splenocytes and macrophages showed irregular patterns of cytokine and chemokine expression. Map3k8 deficiency altered cell adhesion and migration in vivo and decreased CCR2 expression, a determinant chemokine receptor for monocyte mobilization, on circulating Ly6C^highCD11c^low monocytes. Map3k8^-/-ApoE^-/- mice fed an HFD showed decreased cellular infiltration in the atherosclerotic plaque, with low lipid content. Lesions had similar size after Map3k8^+/+ApoE^-/- bone marrow transplant into Map3k8^-/-ApoE^-/- and Map3k8^+/+ApoE^-/- mice fed an HFD, whereas smaller plaques were observed after the transplantation of bone marrow lacking both ApoE and Map3k8.

CONCLUSIONS

Map3k8 decreases apoptosis of monocytes and enhances CCR2 expression on Ly6C^highCD11c^low monocytes of ApoE^-/- mice fed an HFD. These findings explain the smaller aortic lesions in ApoE^-/- mice with Map3k8^-/-ApoE^-/- bone marrow cells fed an HFD, supporting further studies of Map3k8 as an antiatherosclerotic target.

A TPL2 (MAP3K8) disease-risk polymorphism increases TPL2 expression thereby leading to increased pattern recognition receptor-initiated caspase-1 and caspase-8 activation, signalling and cytokine secretion

OBJECTIVE

IBD is characterised by dysregulated intestinal immune homeostasis and cytokine secretion. In the intestine, properly regulating pattern recognition receptor (PRR)-mediated signalling and cytokines is crucial given the ongoing host-microbial interactions. TPL2 (MAP3K8, COT) contributes to PRR-initiated pathways, yet the mechanisms for TPL2 signalling contributions in primary human myeloid cells are incompletely understood and its role in intestinal myeloid cells is poorly defined. Furthermore, functional consequences for the IBD-risk locus rs1042058 in TPL2 are unknown.

METHODS

We analysed protein, cytokine and RNA expression, and signalling in human monocyte-derived macrophages (MDMs) through western blot, ELISA, real-time PCR and flow cytometry.

RESULTS

PRR-induced cytokine secretion was increased in MDMs from rs1042058 TPL2 GG risk individuals. TPL2 activation by the Crohn's disease-associated PRR nucleotide-oligomerisation domain (NOD)2 required PKC, and IKKβ, IKKα and IKKγ signalling. TPL2, in turn, significantly enhanced NOD2-induced ERK, JNK and NFκB signalling. We found that another major mechanism for the TPL2 contribution to NOD2 signalling was through ERK-dependent and JNK-dependent caspase-1 and caspase-8 activation, which in turn, led to early autocrine interleukin (IL)-1β and IL-18 secretion and amplification of long-term cytokines. Importantly, Salmonella typhimurium-induced cytokines from human intestinal myeloid-derived cells required TPL2 as well as autocrine IL-1β and IL-18. Finally, rs1042058 GG risk carrier MDMs from healthy individuals and patients with Crohn's disease had increased TPL2 expression and NOD2-initiated TPL2 phosphorylation, ERK, JNK and NFκB activation, and early autocrine IL-1β and IL-18 secretion.

CONCLUSIONS

Taken together, the rs1042058 GG IBD-risk polymorphism in TPL2 results in a gain-of-function by increasing TPL2 expression and signalling, thereby amplifying PRR-initiated outcomes.

NF-κB Pathways in the Pathogenesis of Multiple Sclerosis and the Therapeutic Implications

Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling pathways are involved in cell immune responses, apoptosis and infections. In multiple sclerosis (MS), NF-κB pathways are changed, leading to increased levels of NF-κB activation in cells. This may indicate a key role for NF-κB in MS pathogenesis. NF-κB signaling is complex, with many elements involved in its activation and regulation. Interestingly, current MS treatments are found to be directly or indirectly linked to NF-κB pathways and act to adjust the innate and adaptive immune system in patients. In this review, we will first focus on the intricacies of NF-κB signaling, including the activating pathways and regulatory elements. Next, we will theorize about the role of NF-κB in MS pathogenesis, based on current research findings, and discuss some of the associated therapeutic implications. Lastly, we will review four new MS treatments which interrupt NF-κB pathways—fingolimod, teriflunomide, dimethyl fumarate (DMF) and laquinimod (LAQ)—and explain their mechanisms, and the possible strategy for MS treatments in the future.