Expression of the MAPK kinases MKK-4 and MKK-7 in rheumatoid arthritis and their role as key regulators of JNK

Comprehensive insights into rheumatoid arthritis: Pathophysiology, current therapies and herbal alternatives for effective disease management

Rheumatoid arthritis is a chronic autoimmune inflammatory disease characterized by immune response overexpression, causing pain and swelling in the synovial joints. This condition is caused by auto-reactive antibodies that attack self-antigens due to their incapacity to distinguish between self and foreign molecules. Dysregulated activity within numerous signalling and immunological pathways supports the disease's development and progression, elevating its complexity. While current treatments provide some alleviation, their effectiveness is accompanied by a variety of adverse effects that are inherent in conventional medications. As a result, there is a deep-rooted necessity to investigate alternate therapeutic strategies capable of neutralizing these disadvantages. Medicinal herbs display a variety of potent bioactive phytochemicals that are effective in the complementary management of disease, thus generating an enormous potency for the researchers to delve deep into the development of novel phytomedicine against autoimmune diseases, although additional evidence and understanding are required in terms of their efficacy and pharmacodynamic mechanisms. This literature-based review highlights the dysregulation of immune tolerance in rheumatoid arthritis, analyses the pathophysiology, elucidates relevant signalling pathways involved, evaluates present and future therapy options and underscores the therapeutic attributes of a diverse array of medicinal herbs in addressing this severe disease.

Network Pharmacology and Experimental Validation Explore the Pharmacological Mechanisms of Herb Pair for Treating Rheumatoid Arthritis

OBJECTIVE

This study aimed to elucidate the multitarget mechanism of the Mori Ramulus - Taxilli Herba (MT) herb pair in treating rheumatoid arthritis (RA).

METHODS

The targets of the herb pair and RA were predicted from databases and screened through cross-analysis. The core targets were obtained using protein-protein interaction (PPI) network analysis. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis were performed. Finally, animal experiments were conducted to validate the anti-RA effect and mechanism of this herb pair.

RESULTS

This approach successfully identified 9 active compounds of MT that interacted with 6 core targets (AKT1, TNF, IL6, TP53, VEGFA, and IL1β). Pathway and functional enrichment analyses revealed that MT had significant effects on the TNF and IL-17 signaling pathways. The consistency of interactions between active components and targets in these pathways was confirmed through molecular docking. Moreover, the potential therapeutic effect of MT was verified in vivo, demonstrating its ability to effectively relieve inflammation by regulating these targeted genes and pathways.

CONCLUSION

The present work suggests that the therapeutic effect of MT herb pair on RA may be attributed to its ability to regulate the TNF signaling pathway and IL-17 signaling pathway.

Systems pharmacology approach to investigate the mechanism of Artemisia argyi in treating rheumatic diseases

Artemisia argyi (AA) has been proven to be effective in the adjuvant treatment of rheumatism (RA), but the mechanism of its action in RA is not clear. This study aims to clarify the molecular mechanism of AA as a potential therapy for RA by using network pharmacology. The TCM systems pharmacology (TCMSP) was used to screen the active components of AA, and identification of the potential target genes of active compounds and rheumatism was performed with PharmMapper and GeneCards, respectively. Construction of complex target networks and protein-protein interaction networks was based on the Cytoscape software. The biological functions and pathway analysis of targets and effective targets were analyzed using DAVID. Our study demonstrated that 105 target genes were associated with these active compounds and RA. ALB, AKT1, and MAPK1 were the first three hub genes, and the metabolic and signaling pathways related to these hub genes were remarkably abundant. Results showed that AA might play a role in RA by affecting multiple targets and multiple ways, reflecting that TCM was characterized by multicomponents and multitargets. AA has the potential to be a promising new candidate for the treatment of RA and has value for further research and development.

Regulation of transforming growth factor-β signalling by SUMOylation and its role in fibrosis

Fibrosis is an abnormal healing process that only repairs the structure of an organ after injury and does not address damaged functions. The pathogenesis of fibrosis is multifactorial and highly complex; numerous signalling pathways are involved in this process, with the transforming growth factor-β (TGF-β) signalling pathway playing a central role. TGF-β regulates the generation of myofibroblasts and the epithelial-mesenchymal transition by regulating transcription and translation of downstream genes and precisely regulating fibrogenesis. The TGF-β signalling pathway can be modulated by various post-translational modifications, of which SUMOylation has been shown to play a key role. In this review, we focus on the function of SUMOylation in canonical and non-canonical TGF-β signalling and its role in fibrosis, providing promising therapeutic strategies for fibrosis.

Screening and verification of hub genes involved in osteoarthritis using bioinformatics

Osteoarthritis (OA) is one of the most common causes of disability and its development is associated with numerous factors. A major challenge in the treatment of OA is the lack of early diagnosis. In the present study, a bioinformatics method was employed to filter key genes that may be responsible for the pathogenesis of OA. From the Gene Expression Omnibus database, the datasets GSE55457, GSE12021 and GSE55325 were downloaded, which comprised 59 samples. Of these, 30 samples were from patients diagnosed with osteoarthritis and 29 were normal. Differentially expressed genes (DEGs) were obtained by downloading and analyzing the original data using bioinformatics. The Gene Ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathways were analyzed using the Database for Annotation, Visualization and Integrated Discovery online database. Protein-protein interaction network analysis was performed using the Search Tool for the Retrieval of Interacting Genes/proteins online database. BSCL2 lipid droplet biogenesis associated, seipin, FOS-like 2, activator protein-1 transcription factor subunit (FOSL2), cyclin-dependent kinase inhibitor 1A (CDKN1A) and kinectin 1 (KTN1) genes were identified as key genes by using Cytoscape software. Functional enrichment revealed that the DEGs were mainly accumulated in the ErbB, MAPK and PI3K-Akt pathways. Reverse transcription-quantitative PCR analysis confirmed a significant reduction in the expression levels of FOSL2, CDKN1A and KTN1 in OA samples. These genes have the potential to become novel diagnostic and therapeutic targets for OA.

Hydrogen Sulfide: a Novel Immunoinflammatory Regulator in Rheumatoid Arthritis

Hydrogen sulfide (H₂S), an endogenous, gaseous, signaling transmitter, has been shown to have vasodilative, anti-oxidative, anti-inflammatory, and cytoprotective activities. Increasing evidence also indicates that H₂S can suppress the production of inflammatory mediators by immune cells, for example, T cells and macrophages. Inflammation is closely related to an immune response in several diseases such as rheumatoid arthritis (RA), multiple sclerosis (MS), systemic lupus erythematosus (SLE), and cancer. Considering these biological effects of H₂S, a potential role in the treatment of immune-related RA is being exploited. In the present review, we will provide an overview of the therapeutic potential of H₂S in RA treatment.

Porphyromonas gingivalis Peptidyl Arginine Deiminase Can Modulate Neutrophil Activity via Infection of Human Dental Stem Cells

Periodontitis (PD) is a widespread chronic inflammatory disease in the human population. Porphyromonas gingivalis is associated with PD and can citrullinate host proteins via P. gingivalis peptidyl arginine deiminase (PPAD). Here, we hypothesized that infection of human dental follicle stem cells (hDFSCs) with P. gingivalis and subsequent interaction with neutrophils will alter the neutrophil phenotype. To test this hypothesis, we established and analyzed a triple-culture system of neutrophils and hDFSCs primed with P. gingivalis. Mitogen-activated pathway blocking reagents were applied to gain insight into stem cell signaling after infection. Naïve hDFSCs do not influence the neutrophil phenotype. However, infection of hDFSCs with P. gingivalis prolongs the survival of neutrophils and increases their migration. These phenotypic changes depend on direct cellular contacts and PPAD expression by P. gingivalis. Active JNK and ERK pathways in primed hDFSCs are essential for the phenotypic changes in neutrophils. Collectively, our results confirm that P. gingivalis modifies hDFSCs, thereby causing an immune imbalance.

A network pharmacology approach to determine the synergetic mechanisms of herb couple for treating rheumatic arthritis

Purpose

The purpose of this study was to investigate the therapeutic mechanism(s) of Clematis chinensis Osbeck/Notopterygium incisum K.C. Ting ex H.T (CN).

Methods

A network pharmacology approach integrating prediction of ingredients, target exploration, network construction, module partition and pathway analysis was used.

Results

This approach successfully helped to identify 12 active ingredients of CN, interacting with 13 key targets (Akt1, STAT3, TNFsf13, TP53, EPHB2, IL-10, IL-6, TNF, MAPK8, IL-8, RELA, ROS1 and STAT4). Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis indicated that CN-regulated pathways were mainly classified into signal transduction and immune system.

Conclusion

The present work may help to illustrate the mechanism(s) of action of CN, and it may provide a better understanding of antirheumatic effects.

Effects of sodium selenite on c-Jun N-terminal kinase signalling pathway induced by oxidative stress in human chondrocytes and c-Jun N-terminal kinase expression in patients with Kashin-Beck disease, an endemic osteoarthritis

The c-Jun N-terminal kinases (JNK) are members of the mitogen-activated protein kinase family and are activated by environmental stress. Se plays an important role in the biological pathways by forming selenoprotein. Selenoproteins have been shown to exhibit a variety of biological functions including antioxidant functions and maintaining cellular redox balance, and compromise of such important proteins would lead to oxidative stress and apoptosis. We examined the expression levels of JNK in Kashin-Beck disease (KBD) patients, tested the potential protective effects of sodium selenite on tert-butyl hydroperoxide (tBHP)-induced oxidative injury and apoptosis in human chondrocytes as well as its underlying mechanism in this study. We produced an oxidative damage model induced by tBHP in C28/I2 human chondrocytes to test the essential anti-apoptosis effects of Se in vitro. The results indicated that the expression level of phosphorylated JNK was significantly increased in KBD patients. Cell apoptosis was increased and molecule expressions of the JNK signalling pathway were activated in the tBHP-injured chondrocytes. Na2SeO3 protected against tBHP-induced oxidative stress and apoptosis in cells by increasing cell viability, reducing reactive oxygen species generation, increasing Glutathione peroxidase (GPx) activity and down-regulating the JNK pathway. These results demonstrate that apoptosis induced by tBHP in chondrocytes might be mediated via up-regulation of the JNK pathway; Na2SeO3 has an effect of anti-apoptosis by down-regulating the JNK signalling pathway.

Identification and characterization of MKK7 as an upstream activator of JNK in Litopenaeus vannamei

Mitogen-activated protein kinase kinase 7 (MKK7) is a key signal transduction regulator in c-Jun N-terminal kinase (JNK) signaling pathway, which is involved in a wide range of physiological and pathological processes. In this study, we described the molecular cloning of a new member of MKK7 group from Litopenaeus vannamei named as LvMKK7. The full-length cDNA of LvMKK7 was 3093 bp in length, with an open reading frame (ORF) of 1440bp encoding a putative protein of 479 amino acids. LvMKK7 contained a conserved kinase domain of 261 amino acids in which there was a characteristic S-K-A-K-T motif as a potential target site of phosphorylation by MKKK. Moreover, subcellular localization showed LvMKK7 was located in both the cytoplasm and the nucleus of Drosophila S2 cells. Real-time PCR indicated that LvMKK7 was universally expressed in all tested tissues and its expression in hepatopancreas was responsive to the challenge of LPS, Poly (I:C), Vibrio parahaemolyticus, Staphhylococcus aureus and white spot syndrome virus (WSSV). In addition, co-immunoprecipitation assay demonstrated that LvJNK was phosphorylated and activated by LvMKK7, which suggested LvMKK7 was the upper regulator of LvJNK. Furthermore, RNAi-mediated knockdown of LvMKK7 enhanced the sensitivity of shrimps to V. parahaemolyticus infection. Overall, our results suggested that LvMKK7 may play important roles in the shrimp innate immunity.

Mechanisms of human lymphotoxin beta receptor activation on upregulation of CCL5/RANTES production

Human lymphotoxin-β receptor (LTβR), a member of the tumor necrosis factor receptor superfamily, plays an essential role in secondary lymphoid organ development, host defense, chemokine secretion, and apoptosis. In our study, LTβR activations by different stimulations were all found to induce RANTES secretion. Overexpression of LTβR or stimulation LTβR by ligands or agonistic antibody in human lung epithelial cells induced RANTES secretion However, the regulatory mechanism and the signaling cascade have not been fully elucidated. Therefore, the aim of this study was to elucidate the mechanism underlying LTβR-mediated RANTES production. Our study indicated that activation of JNK and ERK was important for the regulation of RANTES secretion. In addition, dominant negative mutants of ASK1, TAK1, and MEKK1 inhibited LTβR-induced RANTES expression. The dominant negative mutants of TRAF2, 3, and 5 also inhibited LTβR-mediated RANTES secretion. Chromatin immunoprecipitation analysis showed that LTβR activation induced the binding of c-Jun and NF-κB to the RANTES promoter. The results of this study show that LTβR activates ASK1, TAK1, and MEKK1 cascades via TRAF2, 3, and 5, resulting in the activation of JNK and ERK, which promotes the binding of c-Jun and NF-κB to the RANTES promoter, thereby increasing RANTES expression and secretion.

Anti-Inflammatory Effects and Joint Protection in Collagen-Induced Arthritis after Treatment with IQ-1S, a Selective c-Jun N-Terminal Kinase Inhibitor

c-Jun N-terminal kinases (JNKs) participate in many physiologic and pathologic processes, including inflammatory diseases. We recently synthesized the sodium salt of IQ-1S (11H-indeno[1,2-b]quinoxalin-11-one oxime) and demonstrated that it is a high-affinity JNK inhibitor and inhibits murine delayed-type hypersensitivity. Here we show that IQ-1S is highly specific for JNK and that its neutral form is the most abundant species at physiologic pH. Molecular docking of the IQ-1S syn isomer into the JNK1 binding site gave the best pose, which corresponded to the position of cocrystallized JNK inhibitor SP600125 (1,9-pyrazoloanthrone). Evaluation of the therapeutic potential of IQ-1S showed that it inhibited matrix metalloproteinase 1 and 3 gene expression induced by interleukin-1β in human fibroblast-like synoviocytes and significantly attenuated development of murine collagen-induced arthritis (CIA). Treatment with IQ-1S either before or after induction of CIA resulted in decreased clinical scores, and joint sections from IQ-1S-treated CIA mice exhibited only mild signs of inflammation and minimal cartilage loss compared with those from control mice. Collagen II-specific antibody responses were also reduced by IQ-1S treatment. By contrast, the inactive ketone derivative 11H-indeno[1,2-b]quinoxalin-11-one had no effect on CIA clinical scores or collagen II-specific antibody titers. IQ-1S treatment also suppressed proinflammatory cytokine and chemokine levels in joints and lymph node cells. Finally, treatment with IQ-1S increased the number of Foxp3(+)CD4(+)CD25(+) regulatory T cells in lymph nodes. Thus, IQ-1S can reduce inflammation and cartilage loss associated with CIA and can serve as a small-molecule modulator for mechanistic studies of JNK function in rheumatoid arthritis.

Cellular targeting in autoimmunity

Many biologic agents that were first approved for the treatment of malignancies are now being actively investigated and used in a variety of autoimmune diseases such as rheumatoid arthritis (RA), antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis, systemic lupus erythematosus (SLE), and Sjogren's syndrome. The relatively recent advance of selective immune targeting has significantly changed the management of autoimmune disorders and in part can be attributed to the progress made in understanding effector cell function and their signaling pathways. In this review, we will discuss the recent FDA-approved biologic therapies that directly target immune cells as well as the most promising investigational drugs affecting immune cell function and signaling for the treatment of autoimmune disease.

c-Jun N-Terminal Kinase in Inflammation and Rheumatic Diseases

The c-Jun N-terminal kinases (JNKs) are members of the mitogen-activated protein kinase (MAPK) family and are activated by environmental stress. JNK is also activated by proinflammatory cytokines, such as TNF and IL-1, and Toll-like receptor ligands. This pathway, therefore, can act as a critical convergence point in immune system signaling for both adaptive and innate responses. Like other MAPKs, the JNKs are activated via the sequential activation of protein kinases that includes two dual-specificity MAP kinase kinases (MKK4 and MKK7) and multiple MAP kinase kinase kinases. MAPKs, including JNKs, can be deactivated by a specialized group of phosphatases, called MAP kinase phosphatases. JNK phosphorylates and regulates the activity of transcription factors other than c-Jun, including ATF2, Elk-1, p53 and c-Myc and non-transcription factors, such as members of the Bcl-2 family. The pathway plays a critical role in cell proliferation, apoptosis, angiogenesis and migration. In this review, an overview of the functions that are related to rheumatic diseases is presented. In addition, some diseases in which JNK participates will be highlighted.

Regulation of inflammatory arthritis by the upstream kinase mitogen activated protein kinase kinase 7 in the c-Jun N-terminal kinase pathway

Introduction

The c-Jun N-terminal kinase (JNK) is a key regulator of matrix metalloproteinase (MMP) and cytokine production in rheumatoid arthritis (RA) and JNK deficiency markedly protects mice in animal models of arthritis. Cytokine-induced JNK activation is strictly dependent on the mitogen-activated protein kinase kinase 7 (MKK7) in fibroblast-like synoviocytes (FLS). Therefore, we evaluated whether targeting MKK7 using anti-sense oligonucleotides (ASO) would decrease JNK activation and severity in K/BxN serum transfer arthritis.

Methods

Three 2'-O-methoxyethyl chimeric ASOs for MKK7 and control ASO were injected intravenously in normal C57BL/6 mice. PBS, control ASO or MKK7 ASO was injected from Day -8 to Day 10 in the passive K/BxN model. Ankle histology was evaluated using a semi-quantitative scoring system. Expression of MKK7 and JNK pathways was evaluated by quantitative PCR and Western blot analysis.

Results

MKK7 ASO decreased MKK7 mRNA and protein levels in ankles by about 40% in normal mice within three days. There was no effect of control ASO on MKK7 expression and MKK7 ASO did not affect MKK3, MKK4 or MKK6. Mice injected with MKK7 ASO had significantly less severe arthritis compared with control ASO (P < 0.01). Histologic evidence of synovial inflammation, bone erosion and cartilage damage was reduced in MKK7 ASO-treated mice (P < 0.01). MKK7 deficiency decreased phospho-JNK and phospho-c-Jun in ankle extracts (P < 0.05), but not phospho-MKK4. Interleukin-1beta (IL-1β), MMP3 and MMP13 gene expression in ankle joints were decreased by MKK7 ASO (P < 0.01).

Conclusions

MKK7 plays a critical regulatory role in the JNK pathway in a murine model of arthritis. Targeting MKK7 rather than JNK could provide site and event specificity when treating synovitis.

Reactive oxygen species control senescence-associated matrix metalloproteinase-1 through c-Jun-N-terminal kinase

The lifetime exposure of organisms to oxidative stress influences many aging processes which involve the turnover of the extracellular matrix. In this study, we identify the redox-responsive molecular signals that drive senescence-associated (SA) matrix metalloproteinase-1 (MMP-1) expression. Precise biochemical monitoring revealed that senescent fibroblasts increase steady-state (H(2)O(2)) 3.5-fold (13.7-48.6 pM) relative to young cells. Restricting H(2)O(2) production through low O(2) exposure or by antioxidant treatments prevented SA increases in MMP-1 expression. The H(2)O(2)-dependent control of SA MMP-1 is attributed to sustained JNK activation and c-jun recruitment to the MMP-1 promoter. SA JNK activation corresponds to increases and decreases in the levels of its activating kinase (MKK-4) and inhibitory phosphatase (MKP-1), respectively. Enforced MKP-1 expression negates SA increases in JNK phosphorylation and MMP-1 production. Overall, these studies define redox-sensitive signaling networks regulating SA MMP-1 expression and link the free radical theory of aging to initiation of aberrant matrix turnover.

Semi-permeable membrane retention of synovial fluid lubricants hyaluronan and proteoglycan 4 for a biomimetic bioreactor

Synovial fluid (SF) contains lubricant macromolecules, hyaluronan (HA), and proteoglycan 4 (PRG4). The synovium not only contributes lubricants to SF through secretion by synoviocyte lining cells, but also concentrates lubricants in SF due to its semi-permeable nature. A membrane that recapitulates these synovium functions may be useful in a bioreactor system for generating a bioengineered fluid (BF) similar to native SF. The objectives were to analyze expanded polytetrafluoroethylene membranes with pore sizes of 50 nm, 90 nm, 170 nm, and 3 microm in terms of (1) HA and PRG4 secretion rates by adherent synoviocytes, and (2) the extent of HA and PRG4 retention with or without synoviocytes adherent on the membrane. Experiment 1: Synoviocytes were cultured on tissue culture (TC) plastic or membranes +/- IL-1beta + TGF-beta1 + TNF-alpha, a cytokine combination that stimulates lubricant synthesis. HA and PRG4 secretion rates were assessed by analysis of medium. Experiment 2: Bioreactors were fabricated to provide a BF compartment enclosed by membranes +/- adherent synoviocytes, and an external compartment of nutrient fluid (NF). A solution with HA (1 mg/mL, MW ranging from 30 to 4,000 kDa) or PRG4 (50 microg/mL) was added to the BF compartment, and HA and PRG4 loss into the NF compartment after 2, 8, and 24 h was determined. Lubricant loss kinetics were analyzed to estimate membrane permeability. Experiment 1: Cytokine-regulated HA and PRG4 secretion rates on membranes were comparable to those on TC plastic. Experiment 2: Transport of HA and PRG4 across membranes was lowest with 50 nm membranes and highest with 3 microm membranes, and transport of high MW HA was decreased by adherent synoviocytes (for 50 and 90 nm membranes). The permeability to HA mixtures for 50 nm membranes was approximately 20 x 10(-8) cm/s (- cells) and approximately 5 x 10(-8) cm/s (+ cells), for 90 nm membranes was approximately 35 x 10(-8) cm/s (- cells) and approximately 19 x 10(-8) cm/s (+ cells), for 170 nm membranes was approximately 74 x 10(-8) cm/s (+/- cells), and for 3 microm membranes was approximately 139 x 10(-8) cm/s (+/- cells). The permeability of 450 kDa HA was approximately 40x lower than that of 30 kDa HA for 50 nm membranes, but only approximately 2.5x lower for 3 microm membranes. The permeability of 4,000 kDa HA was approximately 250x lower than that of 30 kDa HA for 50 nm membranes, but only approximately 4x lower for 3 microm membranes. The permeability for PRG4 was approximately 4 x 10(-8) cm/s for 50 nm membranes, approximately 48 x 10(-8) cm/s for 90 nm membranes, approximately 144 x 10(-8) cm/s for 170 nm membranes, and approximately 336 x 10(-8) cm/s for 3 microm membranes. The associated loss across membranes after 24 h ranged from 3% to 92% for HA, and from 3% to 93% for PRG4. These results suggest that semi-permeable membranes may be used in a bioreactor system to modulate lubricant retention in a bioengineered SF, and that synoviocytes adherent on the membranes may serve as both a lubricant source and a barrier for lubricant transport.

Fibroblast-like synoviocytes: key effector cells in rheumatoid arthritis

Rheumatoid arthritis (RA) remains a significant unmet medical need despite significant therapeutic advances. The pathogenesis of RA is complex and includes many cell types, including T cells, B cells, and macrophages. Fibroblast-like synoviocytes (FLS) in the synovial intimal lining also play a key role by producing cytokines that perpetuate inflammation and proteases that contribute to cartilage destruction. Rheumatoid FLS develop a unique aggressive phenotype that increases invasiveness into the extracellular matrix and further exacerbates joint damage. Recent advances in understanding the biology of FLS, including their regulation regulate innate immune responses and activation of intracellular signaling mechanisms that control their behavior, provide novel insights into disease mechanisms. New agents that target FLS could potentially complement the current therapies without major deleterious effect on adaptive immune responses.

Blocking ERK-1/2 reduces tumor necrosis factor alpha-induced interleukin-18 bioactivity in rheumatoid arthritis synovial fibroblasts by induction of interleukin-18 binding protein A

OBJECTIVE

To examine the mechanism of regulation of interleukin-18 (IL-18) bioactivity by IL-18 binding protein (IL-18BP) induction.

METHODS

Levels of IL-18 and IL-18BPa in synovial fluid samples from patients with osteoarthritis (OA) or rheumatoid arthritis (RA) were determined by enzyme-linked immunosorbent assays (ELISAs), followed by calculation of free IL-18. IL-18 and IL-18BPa synthesis in RA synovial fibroblasts that had been treated with proinflammatory and antiinflammatory cytokines were assessed by quantitative real-time polymerase chain reaction and ELISA, respectively, followed by IL-18 bioactivity determination using KG-1 cells. Chemical signaling inhibitors were used for determination of the signal transduction pathways involved in IL-18BPa/IL-18 regulation. Tumor necrosis factor alpha (TNFalpha)-induced caspase 1 activity was determined by a colorimetric assay.

RESULTS

IL-18BPa was lower in RA synovial fluid than in OA synovial fluid (P < 0.05; n = 8), and free IL-18 was higher in RA synovial fluid than in OA synovial fluid. TNFalpha induced RA synovial fibroblast IL-18BPa and IL-18 in a time-dependent manner (P < 0.05). Evaluation of signaling pathways suggested that TNFalpha induced IL-18 production through the ERK-1/2, protein kinase Cdelta (PKCdelta), and Src pathways, whereas IL-18BPa synthesis was mediated through the NFkappaB, PKC, Src, and JNK pathways. Furthermore, addition of exogenous IL-18BPa-Fc reduced the RA synovial fibroblast phosphorylation of ERK-1/2 induced by TNFalpha.

CONCLUSION

These results suggest that IL-18BPa reduces IL-18 bioactivity induced by TNFalpha, by regulating the ERK-1/2 pathway in RA synovial fibroblasts. Targeting IL-18 bioactivity by induction or addition of IL-18BPa may provide another therapeutic option in the management of RA.

A peripheral neuroimmune link: glutamate agonists upregulate NMDA NR1 receptor mRNA and protein, vimentin, TNF-alpha, and RANTES in cultured human synoviocytes

Human primary and clonal synovial cells were incubated with glutamate receptor agonists to assess their modulating influence on glutamate receptors N-methyl-d-aspartate (NMDA) NR1 and NR2 and inflammatory cytokines to determine potential for paracrine or autocrine (neurocrine) upregulation of glutamate receptors, as has been shown for bone and chondrocytes. Clonal SW982 synoviocytes constitutively express vimentin, smooth muscle actin (SMA), and NMDA NR1 and NR2. Coincubation (6 h) with glutamate agonists NMDA (5 microM), and the NMDA NR1 glycine site activator (+/-)1-aminocyclopentane-cis-1,3-dicarboxylic acid (5 muM), significantly increases cellular mRNA and protein levels of glutamate receptors, as well as increasing vimentin, SMA, tumor necrosis factor-alpha, and RANTES (regulated on activation, normal T-cell expressed and secreted), assessed qualitatively and quantitatively with nucleotide amplification, image analysis of immunocytochemical staining, fluorescein-activated cell sorting, Western blotting, and immunoassays. Human primary synovial cells harvested from patients with arthritic conditions also constitutively expressed NMDA NR1 with increases after agonist treatment. Glutamate receptor agonist-induced increases were blocked by the noncompetitive glutamate antagonist MK-801 (8 microg/ml) and NR1 blocking antibody. Coincubation with glutamate agonists and phorbol 12-myristate 13-acetate, a protein kinase C activator, significantly enhanced mean levels of TNF-alpha and RANTES in SW982 cell supernatants compared with incubation with either agent alone. Increases were diminished with protein kinase inhibitor and NR1 blocking antibody. The functional activation of glutamate receptors on human synoviocytes establishes a neurogenic cell signaling link between neurotransmitter glutamate released from nerve terminals and target cells in the joint capsule. The influence of glutamate on subsequent release of cellular proinflammatory mediators in non-neural tissue for activation of downstream immune events supports a peripheral neuroimmune link in arthritis.

Gadd45beta deficiency in rheumatoid arthritis: enhanced synovitis through JNK signaling

OBJECTIVE

JNK-mediated cell signaling plays a critical role in matrix metalloproteinase (MMP) expression and joint destruction in rheumatoid arthritis (RA). Gadd45beta, which is an NF-kappaB-regulated gene, was recently identified as an endogenous negative regulator of the JNK pathway, since it could block the upstream kinase MKK-7. This study was carried out to evaluate whether low Gadd45beta expression in RA enhances JNK activation and overproduction of MMPs in RA, and whether Gadd45beta deficiency increases arthritis severity in passive K/BxN murine arthritis.

METHODS

Activation of the NF-kappaB and JNK pathways and Gadd45beta expression were analyzed in human synovium and fibroblast-like synoviocytes (FLS) using quantitative polymerase chain reaction, immunoblotting, immunohistochemistry, electrophoretic mobility shift assay, and luciferase reporter constructs. Gadd45beta(-/-) and wild-type mice were evaluated in the K/BxN serum transfer model of inflammatory arthritis, and clinical signs of arthritis, osteoclast formation, and bone erosion were assessed.

RESULTS

Expression levels of the Gadd45beta gene and protein were unexpectedly low in human RA synovium despite abundant NF-kappaB activity. Forced Gadd45beta expression in human FLS attenuated tumor necrosis factor-induced signaling through the JNK pathway, reduced the activation of activator protein 1, and decreased the expression of MMP genes. Furthermore, Gadd45beta deficiency exacerbated K/BxN serum-induced arthritis in mice, dramatically increased signaling through the JNK pathway, elevated MMP3 and MMP13 gene expression in the mouse joints, and increased the synovial inflammation and number of osteoclasts.

CONCLUSION

Deficient Gadd45beta expression in RA can contribute to activation of JNK, exacerbate clinical arthritis, and augment joint destruction. This process can be mitigated by enhancing Gadd45beta expression or by inhibiting the activity of JNK or its upstream regulator, MKK-7.

Tumor necrosis factor alpha-induced interleukin-32 is positively regulated via the Syk/protein kinase Cdelta/JNK pathway in rheumatoid synovial fibroblasts

OBJECTIVE

Interleukin-32 (IL-32) is a recently discovered cytokine that appears to play a critical role in human rheumatoid arthritis (RA). It is highly expressed in synovium and fibroblast-like synoviocytes (FLS) from RA patients, but not in patients with osteoarthritis (OA). This study was undertaken to assess IL-32 levels in RA synovial fluid (SF) and to investigate the secretion and regulation of IL-32 in RA FLS.

METHODS

FLS and SF were obtained from the joints of RA patients. The secretion and expression of IL-32 and activation of signaling molecules were examined by enzyme-linked immunosorbent assay, immunoblotting, immunoprecipitation, reverse transcriptase-polymerase chain reaction, and small interfering RNA (siRNA) transfection.

RESULTS

IL-32 levels were high in RA SF compared with OA SF. Furthermore, RA FLS expressed and secreted IL-32 when stimulated with tumor necrosis factor alpha (TNFalpha). TNFalpha-induced expression of IL-32 was significantly suppressed, in a dose-dependent manner, by inhibitors of Syk, protein kinase Cdelta (PKCdelta), and JNK and by knockdown of these kinases and c-Jun with siRNA. We also observed that PKCdelta mediated the activation of JNK and c-Jun, and experiments using specific inhibitors and siRNA demonstrated that Syk was the upstream kinase for the activation of PKCdelta.

CONCLUSION

The present findings suggest that IL-32 may be a newly identified prognostic biomarker in RA, thereby adding valuable knowledge to the understanding of this disease. The results also demonstrate that the production of IL-32 in RA FLS is regulated by Syk/PKCdelta-mediated signaling events.

Synoviocyte innate immune responses: I. Differential regulation of interferon responses and the JNK pathway by MAPK kinases

JNK is a key regulator of matrix metalloproteinase production in rheumatoid arthritis. It is regulated by two upstream kinases known as MKK4 and MKK7. Previous studies demonstrated that only MKK7 is required for cytokine-mediated JNK activation and matrix metalloproteinase expression in cultured fibroblast-like synoviocytes (FLS). However, the functions of MKK4 and MKK7 in synoviocyte innate immune responses have not been determined. TNF, peptidoglycan (PGN), and LPS stimulation led to higher and more prolonged MKK7 phosphorylation compared with MKK4 in FLS. However, this pattern was reversed in poly(I-C) stimulated cells. siRNA knockdown studies showed that TNF, PGN, and LPS-induced JNK and c-Jun phosphorylation are MKK7 dependent, while poly(I-C) responses require both MKK4 and MKK7. Poly(I-C)-induced expression of IP-10, RANTES, and IFN-beta mRNA was decreased in MKK4- or MKK7-deficient FLS. However, MKK4 and MKK7 deficiency did not affect phosphorylation of IkappaB kinase-related kinases in the TLR3 signaling pathway. MKK7, but not MKK4 deficiency, significantly decreased poly(I-C)-mediated IRF3 dimerization, DNA binding, and IFN-sensitive response element-mediated gene transcription. These results were mimicked by the JNK inhibitor SP600125, indicating that JNK can directly phosphorylate IRF3. In contrast, deficiency of either MKK4 or MKK7 decreased AP-1 transcriptional activity. Therefore, JNK is differentially regulated by MKK4 and MKK7 depending on the stimulus. MKK7 is the primary activator of JNK in TNF, LPS, and PGN responses. However, TLR3 requires both MKK4 and MKK7, with the former activating c-Jun and the latter activating both c-Jun and IRF3 through JNK-dependent mechanisms.

MAPK phosphatases as novel targets for rheumatoid arthritis

BACKGROUND

Rheumatoid arthritis (RA) represents a challenge for therapeutic interventions due to complex inflammatory signalling pathways underlying its pathogenesis. The MAPK signalling network, a major effector limb of the inflammatory lesion, is an attractive therapeutic target. MAPK phosphatases (MKPs), endogenous negative regulators of MAPK signalling, have received increasing recognition as modulators of inflammatory and immune responses, and hence as a potential therapeutic avenue for RA.

OBJECTIVE

To present the rationale for therapeutically targeting MAPK signalling and explore the case for addressing MKP1 as a novel therapeutic strategy for RA.

METHODS

We summarise literature describing the importance of MAPK signalling in RA and review reports describing the roles of MKPs in modulating innate and adaptive immune responses. Finally we expand on the role of MKP1 in RA pathogenesis and explore data defining MKP1 as a mediator of glucocorticoid action.

CONCLUSION

MKP1 constitutes an exciting, novel potential therapeutic target for RA.

Regulation of cartilage formation and maturation by mitogen-activated protein kinase signaling

The majority of bones comprising the adult vertebrate skeleton are generated from hyaline cartilage templates that form during embryonic development. A process known as endochondral ossification is responsible for the conversion of these transient cartilage anlagen into mature, calcified bone. Endochondral ossification is a highly regulated, multistep cell specification program involving the initial differentiation of prechondrogenic mesenchymal cells into hyaline chondrocytes, terminal differentiation of hyaline chondrocytes into hypertrophic chondrocytes, and finally, apoptosis of hypertrophic chondrocytes followed by bone matrix deposition. Recently, extensive research has been carried out describing roles for the three major mitogen-activated protein kinase (MAPK) signaling pathways, the extracellular signal-regulated kinase 1/2 (ERK1/2), p38, and c-jun N-terminal kinase (JNK) pathways, in the successive stages of chondrogenic differentiation. In this review, we survey this research examining the involvement of ERK1/2, p38, and JNK pathway signaling in all aspects of the chondrogenic differentiation program from embryonic through postnatal stages of development. In addition, we summarize evidence from in vitro studies examining MAPK function in immortalized chondrogenic cell lines and adult mesenchymal stem cells. We also provide suggestions for future studies that may help ameliorate existing confusion concerning the specific roles of MAPK signaling at different stages of chondrogenesis.

Synoviocyte stimulation by the LFA-1-intercellular adhesion molecule-2-Ezrin-Akt pathway in rheumatoid arthritis

In rheumatoid arthritis (RA), the synovium is infiltrated by mononuclear cells that influence the proliferation and activation of fibroblast-like synoviocytes (FLS) through soluble mediators as well as cell-to-cell contact. To identify receptor-ligand pairs involved in this cross-talk, we cocultured T cells with FLS lines isolated from synovial tissues from RA patients. Coculture with T cells induced phosphorylation of Akt (Ser(473)) and its downstream mediators, GSK-3alpha/GSK-beta, FoxO1/3a, and mouse double minute-2, and enhanced FLS proliferation. T cell-mediated phospho-Akt up-regulation was unique for FLS as no such effect was observed upon interaction of T cells with dendritic cells and B cells. Akt activation was induced by all functional T cell subsets independent of MHC/Ag recognition and was also found with other leukocyte populations, suggesting the involvement of a common leukocyte cell surface molecule. Akt phosphorylation, enhanced in vitro FLS proliferation, and enhanced FLS IL-6 production was inhibited by blocking Abs to CD11a and ICAM-2 whereas Abs to ICAM-1 had a lesser effect. Selective involvement of the LFA-1-ICAM-2 pathway was confirmed by the finding of increased ezrin phosphorylation at Tyr(353) that is known to be downstream of ICAM-2 and supports cell survival through Akt activation. CD28(-) T cells, which are overrepresented in RA patients, have high CD11a cell surface expression and induce Akt phosphorylation in FLS more potently than their CD28(+) counterparts. These findings identify ICAM-2 as a potential therapeutic target to inhibit FLS activation in RA, allowing for a more selective intervention than broad LFA-1 inhibition.

Pro-inflammatory cytokine-induced SAPK/MAPK and JAK/STAT in rheumatoid arthritis and the new anti-depression drugs

BACKGROUND

Adult rheumatoid arthritis (RA) patients are frequently clinically depressed. Peripheral inflammation in RA may influence neurotransmitter metabolism, neuroendocrine function, synaptic plasticity, as well as growth factor production, which can modify neural circuitry and contribute to depression.

OBJECTIVE

A convergence between pro-inflammatory cytokine-induced synovial joint inflammation in RA and the effects of pro-inflammatory cytokines on the brain may occur through activation of the stress-activated/mitogen-activated protein kinases (SAPK/MAPK) and/or Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathways.

METHODS

The PubMed and Medlines databases were critically evaluated for evidence of SAPK/MAPK and/or JAK/STAT pathway activation in RA and depression.

RESULTS/CONCLUSION

Some novel anti-depression drugs that were employed in animal models of 'sickness behavior' and in human depression clinical trials suppressed clinical markers of inflammation, as well as SAPK/MAPK and/or JAK/STAT signaling in vitro. Modifying pro-inflammatory cytokine signaling pathways in the brain with antidepressants may also be useful in ameliorating peripheral inflammation in RA.

Regulation of the JNK pathway by TGF-beta activated kinase 1 in rheumatoid arthritis synoviocytes

c-Jun N-terminal kinase (JNK) contributes to metalloproteinase (MMP) gene expression and joint destruction in inflammatory arthritis. It is phosphorylated by at least two upstream kinases, the mitogen-activated protein kinase kinases (MEK) MKK4 and MKK7, which are, in turn, phosphorylated by MEK kinases (MEKKs). However, the MEKKs that are most relevant to JNK activation in synoviocytes have not been determined. These studies were designed to assess the hierarchy of upstream MEKKs, MEKK1, MEKK2, MEKK3, and transforming growth factor-β activated kinase (TAK)1, in rheumatoid arthritis (RA). Using either small interfering RNA (siRNA) knockdown or knockout fibroblast-like synoviocytes (FLSs), MEKK1, MEKK2, or MEKK3 deficiency (either alone or in combination) had no effect on IL-1β-stimulated phospho-JNK (P-JNK) induction or MMP expression. However, TAK1 deficiency significantly decreased P-JNK, P-MKK4 and P-MKK7 induction compared with scrambled control. TAK1 knockdown did not affect p38 activation. Kinase assays showed that TAK1 siRNA significantly suppressed JNK kinase function. In addition, MKK4 and MKK7 kinase activity were significantly decreased in TAK1 deficient FLSs. Electrophoretic mobility shift assays demonstrated a significant decrease in IL-1β induced AP-1 activation due to TAK1 knockdown. Quantitative PCR showed that TAK1 deficiency significantly decreased IL-1β-induced MMP3 gene expression and IL-6 protein expression. These results show that TAK1 is a critical pathway for IL-1β-induced activation of JNK and JNK-regulated gene expression in FLSs. In contrast to other cell lineages, MEKK1, MEKK2, and MEKK3 did not contribute to JNK phosphorylation in FLSs. The data identify TAK1 as a pivotal upstream kinase and potential therapeutic target to modulate synoviocyte activation in RA.

Central role of the MEK/ERK MAP kinase pathway in a mouse model of rheumatoid arthritis: potential proinflammatory mechanisms

OBJECTIVE

To evaluate the role of the MEK/ERK MAP kinase pathway in murine collagen-induced arthritis (CIA) using the selective MEK inhibitor PD184352. We examined the effects of the inhibitor in cytokine-stimulated synovial fibroblasts and in cytokine-induced arthritis in rabbits to investigate its antiinflammatory mechanisms.

METHODS

Murine CIA was used to assess the effects of the selective MEK inhibitor on paw edema, clinical scores, weight loss, histopathologic features, and joint levels of p-ERK. Western blotting and immunohistochemistry techniques were used to assess p-ERK in human and rabbit synovial fibroblasts and synovial tissue from rheumatoid arthritis (RA) patients. Interleukin-1alpha (IL-1alpha)-stimulated stromelysin production in rabbit synovial fibroblasts was assessed by enzyme-linked immunosorbent assay. A rabbit IL-1alpha-induced arthritis model was used to assess the effects of the inhibitor on IL-1alpha-induced MEK activity, stromelysin production, and cartilage degradation.

RESULTS

In the CIA model, PD184352 inhibited paw edema and clinical arthritis scores in a dose-dependent manner. Disease-induced weight loss and histopathologic changes were also significantly improved by treatment. Inhibition of disease-induced p-ERK levels in the joints was seen with the inhibitor. Levels of p-ERK in the synovium were higher in RA patients than in normal individuals. PD184352 reduced IL-1alpha-induced p-ERK levels in human RA synovial fibroblasts. The production of p-ERK and stromelysin was also inhibited in IL-1alpha-stimulated rabbit synovial fibroblasts. We observed IL-1alpha-induced p-ERK in the synovial lining, subsynovial vasculature, and articular chondrocytes. IL-1alpha-induced stromelysin production and proteoglycan loss from the articular cartilage were reduced by PD184352.

CONCLUSION

These data demonstrate the inhibition of murine CIA by PD184352, support the hypothesis that antiinflammatory activity contributes to the mechanism of action of the inhibitor, and suggest that a selective inhibitor may effectively treat RA and other inflammatory disorders.

Fibroblast activation protein is expressed by rheumatoid myofibroblast-like synoviocytes

Fibroblast activation protein (FAP), as described so far, is a type II cell surface serine protease expressed by fibroblastic cells in areas of active tissue remodelling such as tumour stroma or healing wounds. We investigated the expression of FAP by fibroblast-like synoviocytes (FLSs) and compared the synovial expression pattern in rheumatoid arthritis (RA) and osteoarthritis (OA) patients. Synovial tissue from diseased joints of 20 patients, 10 patients with refractory RA and 10 patients with end-stage OA, was collected during routine surgery. As a result, FLSs from intensively inflamed synovial tissues of refractory RA expressed FAP at high density. Moreover, FAP expression was co-localised with matrix metalloproteinases (MMP-1 and MMP-13) and CD44 splice variants v3 and v7/8 known to play a major role in the concert of extracellular matrix degradation. The pattern of signals appeared to constitute a characteristic feature of FLSs involved in rheumatoid arthritic joint-destructive processes. These FAP-expressing FLSs with a phenotype of smooth muscle actin-positive myofibroblasts were located in the lining layer of the synovium and differ distinctly from Thy-1-expressing and non-proliferating fibroblasts of the articular matrix. The intensity of FAP-specific staining in synovial tissue from patients with RA was found to be different when compared with end-stage OA. Because expression of FAP by RA FLSs has not been described before, the findings of this study highlight a novel element in cartilage and bone destruction of arthritic joints. Moreover, the specific expression pattern qualifies FAP as a therapeutic target for inhibiting the destructive potential of fibroblast-like synovial cells.

Immunomodulatory effects of statins and autoimmune rheumatic diseases: novel intracellular mechanism involved

Inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, known as statins, are the most commonly prescribed agents for the treatment of hypercholesterolemia. However, the effects of statins may extend beyond their influences on serum cholesterol levels resulting in cholesterol-independent or pleiotropic effects. Clinical, animal and in vitro studies suggest that statins have additional clinical uses because of their anti-inflammatory and immunomodulatory effects, in part due to their capacity to interfere with the mevalonate pathway and inhibit prenylation of Rho family GTPases. This review focuses on the molecular mechanisms of the anti-inflammatory and immunomodulatory effects of statins. In base to all these information, we suggest that statins could have similar inhibitory effects on MAPKs pathways in cells from RA patients, including osteoclasts and fibroblasts.

Mitogen activated protein kinase inhibitors: where are we now and where are we going?

Orally bioavailable compounds that target key intracellular signalling molecules are receiving increasing attention for the treatment of rheumatic diseases. The mitogen activated protein (MAP) kinases are especially attractive because they regulate both cytokine production and cytokine action. The MAP kinases are expressed and activated in rheumatoid arthritis (RA) synovium. Preclinical studies using MAP kinase inhibitors are very effective in animal models of arthritis, supporting their potential utility in human disease. Although the available data suggest a rationale for MAP kinase blockade, development of drugs has been hampered by toxicity and limited efficacy. Alternative strategies, such as targeting other kinases in the cascade or development of allosteric inhibitors have been proposed. These approaches might permit effective use of MAP kinase inhibitors for the treatment of rheumatic and immune-mediated diseases.

Activation of MKK4 (SEK1), JNK, and c-Jun in labial salivary infiltrating T cells in patients with Sjögren's syndrome

We wanted to determine via immunohistochemistry, whether or not c-Jun NH2-terminal kinase (JNK) cascade is activated in labial salivary infiltrating T cells in patients with Sjögren's syndrome (SS). Six patients with primary SS were selected for this study. Phosphorylation of mitogen-activated protein kinase (MAPK) kinase 4 (MKK4) (SEK1), JNK, and c-Jun in salivary infiltrating T cells was studied using immunohistochemistry assay, including mirror section technique. Phosphorylated forms of MKK4, JNK, and c-Jun were detected in salivary infiltrating mononuclear cells. Expression of phosphorylated JNK was found in both CD4+ T cells and CD8+ T cells. Moreover, co-expression of phosphorylated JNK and c-Jun was demonstrated in the mirror sections. The results of this study suggest that the JNK cascade is activated in salivary infiltrating CD4+ T cells and CD8+ T cells in SS patients, which appears to contribute to the inflammatory salivary microenvironment of SS.

Regulation of JNK by MKK-7 in fibroblast-like synoviocytes

OBJECTIVE

JNK regulates matrix metalloproteinase (MMP) gene expression and joint destruction in rheumatoid arthritis (RA). Previous studies demonstrated that the 2 upstream MAPK kinases (MKK-4 and MKK-7) are phosphorylated in RA synovium and form a complex with JNK in fibroblast-like synoviocytes (FLS). However, the functional hierarchy of MKK-4 and MKK-7 in FLS has not been determined. We determined the relative contributions of these MKKs by evaluating the effect of MKK-4 and MKK-7 gene knockdown in cultured FLS.

METHODS

FLS were transfected with MKK-4 and/or MKK-7 small interfering RNA, and protein levels were determined by immunoblotting. After stimulation with interleukin-1/beta (IL-1beta), tumor necrosis factor alpha(TNFalpha, or anisomycin, kinase function was determined by in vitro kinase assay. Activator protein 1 (AP-1) binding and transcriptional activity were determined by electrophoretic mobility shift assay and AP-1-luciferase promoter assay, respectively. MMP-3 expression was determined by enzyme-linked immunosorbent assay and quantitative polymerase chain reaction.

RESULTS

IL-1beta-induced JNK phosphorylation was dependent on MKK-7 but not on MKK-4; however, anisomycin-activated JNK required both kinases. In vitro kinase assay demonstrated that IL-1beta-or TNFalpha induced JNK activity was only MKK-7 dependent, while anisomycin-activated JNK was both MKK-4 and MKK-7 dependent. IL-1beta-induced AP-1 binding activity and AP-1-driven gene expression were strictly MKK-7 dependent. Finally, MMP-3 production only required MKK-7, and there was no effect of MKK-4 deficiency.

CONCLUSION

These data indicate that only MKK-7 is required for JNK activation in FLS after cytokine stimulation; however, other forms of cellular stress utilize MKK-4. Thus, JNK function might be modulated by targeting MKK-7 to suppress cytokine-mediated FLS activation while leaving other stress responses intact.

A novel spleen tyrosine kinase inhibitor blocks c-Jun N-terminal kinase-mediated gene expression in synoviocytes

Spleen tyrosine kinase (Syk) is a key regulator of cell signaling induced by cytokines or Fc receptor engagement. However, the role of Syk in rheumatoid arthritis (RA) is not known yet. We investigated the pathways activated by Syk in tumor necrosis factor-alpha (TNFalpha)-stimulated fibroblast-like synoviocytes (FLS) using the novel Syk inhibitor N4-(2,2-dimethyl-3-oxo-4H-pyrid[1,4]oxazin-6-yl)-5-fluoro-N2-(3,4,5-trimethoxyphenyl)-2,4-pyrimidinediamine (R406). Using immunohistochemistry, Syk was detected in RA synovial tissue (ST), primarily in the synovial intimal lining. Western blot analysis demonstrated significantly greater amounts of phospho-Syk expression in RA ST compared with osteoarthritis ST. The kinase was expressed and functionally activated by TNFalpha in FLS and was blocked by R406. Western blot analysis demonstrated that Syk inhibition by R406 markedly suppressed TNFalpha-induced c-Jun N-terminal kinase (JNK) phosphorylation in FLS, with a modest decrease in extracellular signal-regulated kinase phosphorylation. Surprisingly, p38 activation was not affected by R406. The Syk inhibitor also decreased TNFalpha-induced mitogen-activated protein kinase kinase (MKK) 4 phosphorylation but not MKK3 and MKK6 phosphorylation, which is consistent with its selective sparing of p38. The connection between Syk and JNK was confirmed by demonstrating decreased phospho-c-Jun protein expression and complete inhibition of JNK function in R406-treated cells. R406 also suppressed downstream actions of JNK, as determined by activator protein 1 binding, as well as matrix metalloproteinase 3 gene expression. These data demonstrate that Syk activation plays an essential role in TNFalpha-induced cytokine and matrix metalloproteinase production in RA FLS, especially by suppressing activation of the JNK pathway.

MAPK signalling in rheumatoid joint destruction: can we unravel the puzzle?

Mitogen-activated protein kinases (MAPKs) have been associated with the pathogenesis of rheumatoid arthritis (RA), but the individual contributions of the three MAPK family members are incompletely understood. Although previous data have established a role for c-Jun N-terminal kinase (JNK) and extracellular signal-related kinase (ERK) in different animal models of arthritis, most recent data indicate that the stable activation of p38 MAPK and in part of ERK significantly contributes to destructive arthritis in mice transgenic for human tumour necrosis factor-alpha. These data highlight the complexity of MAPK signalling in arthritis and provide a basis for the design of novel strategies to treat human RA.

Tumour necrosis factor activates the mitogen-activated protein kinases p38alpha and ERK in the synovial membrane in vivo

Tumour necrosis factor (TNF) is considered to be a major factor in chronic synovial inflammation and is an inducer of mitogen-activated protein kinase (MAPK) signalling. In the present study we investigated the ability of TNF to activate MAPKs in the synovial membrane in vivo. We studied human TNF transgenic mice – an in vivo model of TNF-induced arthritis – to examine phosphorylation of extracellular signal-regulated kinase (ERK), c-Jun amino terminal kinase (JNK) and p38MAPKα in the inflamed joints by means of immunoblot and immunohistochemistry. In addition, the effects of systemic blockade of TNF, IL-1 and receptor activator of nuclear factor-κB (RANK) ligand on the activation of MAPKs were assessed. In vivo, overexpression of TNF induced activation of p38MAPKα and ERK in the synovial membrane, whereas activation of JNK was less pronounced and rarely observed on immunohistochemical analysis. Activated p38MAPKα was predominantly found in synovial macrophages, whereas ERK activation was present in both synovial macrophages and fibroblasts. T and B lymphocytes did not exhibit major activation of any of the three MAPKs. Systemic blockade of TNF reduced activation of p38MAPKα and ERK, whereas inhibition of IL-1 only affected p38MAPKα and blockade of RANK ligand did not result in any decrease in MAPK activation in the synovial membrane. These data indicate that TNF preferentially activates p38MAPKα and ERK in synovial membrane exposed to TNF. This not only suggests that targeted inhibition of p38MAPKα and ERK is a feasible strategy for blocking TNF-mediated effects on joints, but it also shows that even currently available methods to block TNF effectively reduce activation of these two MAPKs.

Regulation of c-Jun phosphorylation by the I kappa B kinase-epsilon complex in fibroblast-like synoviocytes

Rheumatoid arthritis (RA) causes a symmetric, inflammatory polyarthritis that results in joint destruction and significant disability. Signaling pathways that regulate the production of cytokines and destructive enzymes have been implicated in its pathogenesis and represent potential therapeutic targets. The IkappaB kinase (IKK)-related kinase, IKKepsilon/IKKi, which plays a pivotal role in regulating antiviral gene transcription, is constitutively expressed by cultured fibroblast-like synoviocytes (FLS) and could participate in the pathogenesis of RA. In the current studies we demonstrate that IKKepsilon protein is expressed in RA and osteoarthritis synovium and that the protein is found primarily in the synovial intimal lining. Functional studies in cultured FLS showed that IKKepsilon kinase activity is rapidly induced by cytokines, although IkappaB phosphorylation is significantly less compared with IKK2. Because NF-kappaB activation is similar in wild-type and IKKepsilon knockout murine FLS, studies were performed to identify an alternative substrate for IKKepsilon. Interestingly, c-Jun is a more efficient substrate for IKKepsilon immunocomplexes in human FLS and this activity appears to be independent of JNK. The functional relevance of IKKepsilon was examined using murine IKKepsilon(-/-) cultured FLS. IL-1-, TNF-alpha-, and LPS-mediated induction of matrix metalloproteinases, MMP3 and MMP13, is significantly decreased in the IKKepsilon(-/-) cells. These data suggest a novel role for the IKKepsilon complex in synovial inflammation, extracellular matrix destruction, and activation of the viral program and innate immune response in RA.

The fibroblast-like synovial cell in rheumatoid arthritis: a key player in inflammation and joint destruction

Although multiple cell types are present in the rheumatoid joint, the fibroblast-like synovial cell (FLS) is among the most prominent. It is now appreciated that the FLS is not only space-filling, but is directly responsible for cartilage destruction, and also drives both inflammation and autoimmunity. In this article, we consider the normal role of the FLS in healthy joints, and review evidence that implicates the FLS as a central player in the propagation of rheumatoid arthritis.

Critical roles of threonine 187 phosphorylation in cellular stress-induced rapid and transient activation of transforming growth factor-beta-activated kinase 1 (TAK1) in a signaling complex containing TAK1-binding protein TAB1 and TAB2

Transforming growth factor-beta-activated kinase 1 (TAK1) mitogen-activated protein kinase kinase kinase has been shown to be activated by cellular stresses including tumor necrosis factor-alpha (TNF-alpha). Here, we characterized the molecular mechanisms of cellular stress-induced TAK1 activation, focusing mainly on the phosphorylation of TAK1 at Thr-187 and Ser-192 in the activation loop. Thr-187 and Ser-192 are conserved among species from Caenorhabditis elegans to human, and their replacement with Ala resulted in inactivation of TAK1. Immunoblotting with a novel phospho-TAK1 antibody revealed that TNF-alpha significantly induced the phosphorylation of endogenous TAK1 at Thr-187, and subsequently the phosphorylated forms of TAK1 rapidly disappeared. Intermolecular autophosphorylation of Thr-187 was essential for TAK1 activation. RNA interference and overexpression experiments demonstrated that TAK1-binding protein TAB1 and TAB2 were involved in the phosphorylation of TAK1, but they regulated TAK1 phosphorylation differentially. Furthermore, SB203580 and p38alpha small interfering RNA enhanced TNF-alpha-induced Thr-187 phosphorylation as well as TAK1 kinase activity, indicating that the phosphorylation is affected by p38alpha/TAB1/TAB2-mediated feedback control of TAK1. These results indicate critical roles of Thr-187 phosphorylation in the stress-induced rapid and transient activation of TAK1 in a signaling complex containing TAB1 and TAB2.

Signal transduction in rheumatoid arthritis

PURPOSE

Signal transduction pathways are the intracellular mechanism by which cells respond and adapt to environmental stress. Understanding the critical networks in diseases like rheumatoid arthritis can potentially identify novel therapeutic targets.

RECENT FINDINGS

Dissecting the complex pathways involved in rheumatoid synovitis, including mitogen-activated protein kinases, NF-kB, tumor suppressors, Janus kinases, the signal transducer and activator of transcription, suppressors of cytokine stimulation, and toll-like receptors may lead to new approaches to inflammatory arthritis. For instance, targeting NF-kB via IkB kinase 2 with specific inhibitors may block an array of proinflammatory cytokines that contribute to synovitis. Inhibition of Janus kinases and p38 could block metalloproteinase expression and protect the extracellular matrix. Overexpression of suppressors of cytokine stimulation and inhibition of signal transducer and activator of transcription are additional approaches that have demonstrated efficacy in animal models of arthritis. Tumor suppressor proteins and cell cycle inhibitors represent additional targets with unexpected anti-inflammatory activities. Recent evidence also suggests that targeting toll-like receptors may regulate cytokine expression in rheumatoid arthritis.

SUMMARY

Multiple signal transduction pathways have been implicated in rheumatoid arthritis, and preclinical models have confirmed the therapeutic potential of small molecule inhibitors. Orally bioavailable inhibitors of the mitogen-activated protein kinase and NF-kB pathways have been designed and are currently being evaluated. Many other pathways could be targeted and offer new therapeutic options for rheumatoid arthritis.

Expression and activation of mitogen-activated protein kinase kinases-3 and -6 in rheumatoid arthritis

The p38 mitogen-activated protein (MAP) kinase signal transduction pathway regulates the production of interleukin-1 and tumor necrosis factor-alpha. p38 kinase inhibitors are effective in animal models of arthritis and are currently being developed in rheumatoid arthritis (RA). However, little is known about the upstream kinases that control the activation of p38 in RA synovium. In vitro studies previously identified the MAP kinase kinases (MAPKKs) MKK3 and MKK6 as the primary regulators of p38 phosphorylation and activation. To investigate a potential role for MKK3 and MKK6 in RA, we evaluated their expression and regulation in RA synovium and cultured fibroblast-like synoviocytes (FLS). Immunohistochemistry demonstrated that MKK3 and MKK6 are expressed in RA and osteoarthritis (OA) synovium. Digital image analysis showed no significant differences between OA and RA with regard to expression or distribution. However, phosphorylated MKK3/6 expression was significantly higher in RA synovium and was localized to the sublining mononuclear cells and the intimal lining. Actin-normalized Western blot analysis of synovial tissue lysates confirmed the increased expression of phosphorylated MKK3/6 in RA. Western blot analysis demonstrated constitutive expression of MKK3 and MKK6 in RA and OA FLS. Phospho-MKK3 levels were low in medium-treated FLS, but were rapidly increased by interleukin-1 and tumor necrosis factor-alpha, although phospho-MKK6 levels only modestly increased. p38 co-immunoprecipitated with MKK3 and MKK6 from cytokine-stimulated FLS and the complex phosphorylated activating transcription factor-2 in an in vitro kinase assay. These data are the first documentation of MKK3 and MKK6 activation in human inflammatory disease. By forming a complex with p38 in synovial tissue and FLS, these kinases can potentially be targeted to regulate the production of proinflammatory cytokine production in inflamed synovium.

Regulation of c-Jun N-Terminal Kinase by MEKK-2 and Mitogen-Activated Protein Kinase Kinase Kinases in Rheumatoid Arthritis

The mitogen-activated protein kinase (MAPK) c-Jun N-terminal kinase (JNK) is a critical regulator of collagenase-1 production in rheumatoid arthritis (RA). The MAPKs are regulated by upstream kinases, including MAPK kinases (MAPKKs) and MAPK kinase kinases (MAP3Ks). The present study was designed to evaluate the expression and regulation of the JNK pathway by MAP3K in arthritis. RT-PCR studies of MAP3K gene expression in RA and osteoarthritis synovial tissue demonstrated mitogen-activated protein kinase/ERK kinase kinase (MEKK) 1, MEKK2, apoptosis-signal regulating kinase-1, TGF-beta activated kinase 1 (TAK1) gene expression while only trace amounts of MEKK3, MEKK4, and MLK3 mRNA were detected. Western blot analysis demonstrated immunoreactive MEKK2, TAK1, and trace amounts of MEKK3 but not MEKK1 or apoptosis-signal regulating kinase-1. Analysis of MAP3K mRNA in cultured fibroblast-like synoviocytes (FLS) showed that all of the MAP3Ks examined were expressed. Western blot analysis of FLS demonstrated that MEKK1, MEKK2, and TAK1 were readily detectable and were subsequently the focus of functional studies. In vitro kinase assays using MEKK2 immunoprecipitates demonstrated that IL-1 increased MEKK2-mediated phosphorylation of the key MAPKKs that activate JNK (MAPK kinase (MKK)4 and MKK7). Furthermore, MEKK2 immunoprecipitates activated c-Jun in an IL-1 dependent manner and this activity was inhibited by the selective JNK inhibitor SP600125. Of interest, MEKK1 immunoprecipitates from IL-1-stimulated FLS appeared to activate c-Jun through the JNK pathway and TAK1 activation of c-Jun was dependent on JNK, ERK, and p38. These data indicate that MEKK2 is a potent activator of the JNK pathway in FLS and that signal complexes including MEKK2, MKK4, MKK7, and/or JNK are potential therapeutic targets in RA.

Signal transduction networks in rheumatoid arthritis

Signal transduction pathways regulate cellular responses to stress and play a critical role in inflammation. The complexity and specificity of signalling mechanisms represent major hurdles for developing effective, safe therapeutic interventions that target specific molecules. One approach is to dissect the pathways methodically to determine their hierarchy in various cell types and diseases. This approach contributed to the identification and prioritisation of specific kinases that regulate NF-kappa B and the mitogen activated protein (MAP) kinase cascade as especially attractive targets. Although significant issues remain with regard to the discovery of truly selective kinase inhibitors, the risks that accompany inhibition of fundamental signal transduction mechanisms can potentially be decreased by careful dissection of the pathways and rational target selection.