Aminoacyl-tRNA synthetase inhibition activates a pathway that branches from the canonical amino acid response in mammalian cells.

Signaling pathways that sense amino acid abundance are integral to tissue homeostasis and cellular defense. Our laboratory has previously shown that halofuginone (HF) inhibits the prolyl-tRNA synthetase catalytic activity of glutamyl-prolyl-tRNA synthetase (EPRS), thereby activating the amino acid response (AAR). We now show that HF treatment selectively inhibits inflammatory responses in diverse cell types and that these therapeutic benefits occur in cells that lack GCN2, the signature effector of the AAR. Depletion of arginine, histidine, or lysine from cultured fibroblast-like synoviocytes recapitulates key aspects of HF treatment, without utilizing GCN2 or mammalian target of rapamycin complex 1 pathway signaling. Like HF, the threonyl-tRNA synthetase inhibitor borrelidin suppresses the induction of tissue remodeling and inflammatory mediators in cytokine-stimulated fibroblast-like synoviocytes without GCN2, but both aminoacyl-tRNA synthetase (aaRS) inhibitors are sensitive to the removal of GCN1. GCN1, an upstream component of the AAR pathway, binds to ribosomes and is required for GCN2 activation. These observations indicate that aaRS inhibitors, like HF, can modulate inflammatory response without the AAR/GCN2 signaling cassette, and that GCN1 has a role that is distinct from its activation of GCN2. We propose that GCN1 participates in a previously unrecognized amino acid sensor pathway that branches from the canonical AAR.

CUX1 and IκBζ (NFKBIZ) mediate the synergistic inflammatory response to TNF and IL-17A in stromal fibroblasts.

The role of stromal fibroblasts in chronic inflammation is unfolding. In rheumatoid arthritis, leukocyte-derived cytokines TNF and IL-17A work together, activating fibroblasts to become a dominant source of the hallmark cytokine IL-6. However, IL-17A alone has minimal effect on fibroblasts. To identify key mediators of the synergistic response to TNF and IL-17A in human synovial fibroblasts, we performed time series, dose-response, and gene-silencing transcriptomics experiments. Here we show that in combination with TNF, IL-17A selectively induces a specific set of genes mediated by factors including cut-like homeobox 1 (CUX1) and IκBζ (NFKBIZ). In the promoters of CXCL1, CXCL2, and CXCL3, we found a putative CUX1-NF-κB binding motif not found elsewhere in the genome. CUX1 and NF-κB p65 mediate transcription of these genes independent of LIFR, STAT3, STAT4, and ELF3. Transcription of NFKBIZ, encoding the atypical IκB factor IκBζ, is IL-17A dose-dependent, and IκBζ only mediates the transcriptional response to TNF and IL-17A, but not to TNF alone. In fibroblasts, IL-17A response depends on CUX1 and IκBζ to engage the NF-κB complex to produce chemoattractants for neutrophil and monocyte recruitment.

Multiparameter Analysis Identifies Heterogeneity in Knee Osteoarthritis Synovial Responses.

OBJECTIVE: Synovial membrane inflammation is common in osteoarthritis (OA) and increases cartilage injury. However, synovial fluid and histology studies suggest that OA inflammatory responses are not homogeneous. Greater understanding of these responses may provide new insights into OA disease mechanisms. We undertook this study to develop a novel multiparameter approach to phenotype synovial responses in knee OA. METHODS: Cell composition and soluble protein production were measured by flow cytometry and multiplex enzyme-linked immunosorbent assay in synovium collected from OA patients undergoing knee replacement surgery (n = 35). RESULTS: Testing disaggregation conditions showed that aggressive digestion improved synovial cell yield and mesenchymal staining by flow cytometry, but it negatively impacted CD4+ T cell and CD56+ natural killer cell staining. Less aggressive digestion preserved these markers and showed highly variable T cell infiltration (range 0-43%; n = 32). Correlation analysis identified mesenchymal subpopulations associated with different nonmesenchymal populations, including macrophages and T cells (CD45+CD11b+HLA-DR+ myeloid cells with PDPN+CD73+CD90-CD34- mesenchymal cells [r = 0.65, P < 0.0001]; and CD45+CD3+ T cells with PDPN+CD73+CD90+CD34+ mesenchymal cells [r = 0.50, P = 0.003]). Interleukin-6 (IL-6) measured by flow cytometry strongly correlated with IL-6 released by ex vivo culture of synovial tissue (r = 0.59, P = 0.0012) and was highest in mesenchymal cells coexpressing CD90 and CD34. IL-6, IL-8, complement factor D, and IL-10 release correlated positively with tissue cellularity (P = 0.0042, P = 0.018, P = 0.0012, and P = 0.038, respectively). Additionally, increased CD8+ T cell numbers correlated with retinol binding protein 4 (P = 0.033). Finally, combining flow cytometry and multiplex data identified patient clusters with different types of inflammatory responses. CONCLUSION: We used a novel approach to analyze OA synovium, identifying patient-specific inflammatory clusters. Our findings indicate that phenotyping synovial inflammation may provide new insights into OA patient heterogeneity and biomarker development.

Interactions between cadherin-11 and platelet-derived growth factor receptor-alpha signaling link cell adhesion and proliferation.

Cadherins are homophilic cell-to-cell adhesion molecules that help cells respond to environmental changes. Newly formed cadherin junctions are associated with increased cell phosphorylation, but the pathways driving this signaling response are largely unknown. Since cadherins have no intrinsic signaling activity, this phosphorylation must occur through interactions with other signaling molecules. We previously reported that cadherin-11 engagement activates joint synovial fibroblasts, promoting inflammatory and degradative pathways important in rheumatoid arthritis (RA) pathogenesis. Our objective in this study was to discover interacting partners that mediate cadherin-11 signaling. Protein array screening showed that cadherin-11 extracellular binding domains linked to an Fc domain (cad11Fc) induced platelet-derived growth factor (PDGFR)-α phosphorylation in synovial fibroblasts and glioblastoma cells. PDGFRs are growth factor receptor tyrosine kinases that promote cell proliferation, survival, and migration in mesodermally derived cells. Increased PDGFR activity is implicated in RA pathology and associates with poor prognosis in several cancers, including sarcoma and glioblastoma. PDGFRα activation by cadherin-11 signaling promoted fibroblast proliferation, a signaling pathway independent from cadherin-11-stimulated IL-6 or matrix metalloproteinase (MMP)-3 release. PDGFRα phosphorylation mediated most of the cad11Fc-induced phosphatidyl-3-kinase (PI3K)/Akt activation, but only part of the mitogen-activated protein kinase (MAPK) response. PDGFRα-dependent signaling did not require cell cadherin-11 expression. Rather, cad11Fc immunoprecipitated PDGFRα, indicating a direct interaction between cadherin-11 and PDGFRα extracellular domains. This study is the first to report an interaction between cadherin-11 and PDGFRα and adds to our growing understanding that cadherin-growth factor receptor interactions help balance the interplay between tissue growth and adhesion.

Functionally distinct disease-associated fibroblast subsets in rheumatoid arthritis.

Fibroblasts regulate tissue homeostasis, coordinate inflammatory responses, and mediate tissue damage. In rheumatoid arthritis (RA), synovial fibroblasts maintain chronic inflammation which leads to joint destruction. Little is known about fibroblast heterogeneity or if aberrations in fibroblast subsets relate to pathology. Here, we show functional and transcriptional differences between fibroblast subsets from human synovial tissues using bulk transcriptomics of targeted subpopulations and single-cell transcriptomics. We identify seven fibroblast subsets with distinct surface protein phenotypes, and collapse them into three subsets by integrating transcriptomic data. One fibroblast subset, characterized by the expression of proteins podoplanin, THY1 membrane glycoprotein and cadherin-11, but lacking CD34, is threefold expanded in patients with RA relative to patients with osteoarthritis. These fibroblasts localize to the perivascular zone in inflamed synovium, secrete proinflammatory cytokines, are proliferative, and have an in vitro phenotype characteristic of invasive cells. Our strategy may be used as a template to identify pathogenic stromal cellular subsets in other complex diseases.

Autocrine Loop Involving IL-6 Family Member LIF, LIF Receptor, and STAT4 Drives Sustained Fibroblast Production of Inflammatory Mediators.

Fibroblasts are major contributors to and regulators of inflammation and dominant producers of interleukin-6 (IL-6) in inflammatory diseases like rheumatoid arthritis. Yet, compared to leukocytes, the regulation of inflammatory pathways in fibroblasts is largely unknown. Here, we report that analyses of genes coordinately upregulated with IL-6 pointed to STAT4 and leukemia inhibitory factor (LIF) as potentially linked. Gene silencing revealed that STAT4 was required for IL-6 transcription. STAT4 was recruited to the IL-6 promoter after fibroblast activation, and LIF receptor (LIFR) and STAT4 formed a molecular complex that, together with JAK1 and TYK2 kinases, controlled STAT4 activation. Importantly, a positive feedback loop involving autocrine LIF, LIFR, and STAT4 drove sustained IL-6 transcription. Besides IL-6, this autorine loop also drove the production of other key inflammatory factors including IL-8, granulocyte-colony stimulating factor (G-CSF), IL-33, IL-11, IL-1α, and IL-1ß. These findings define the transcriptional regulation of fibroblast-mediated inflammation as distinct from leukocytes.

Genetic polymorphism directs IL-6 expression in fibroblasts but not selected other cell types.

Interleukin (IL)-6 blockade is an effective treatment for rheumatoid arthritis (RA), and synovial fibroblasts are a major IL-6 producer in the inflamed joint. We found that human RA and osteoarthritis (OA) synovial fibroblasts derived from independent donors reproducibly segregated into low, medium, and high IL-6 producers, independent of stimulus, cell passage, or disease state. IL-6 expression pattern correlated strongly with total mRNA expression, not mRNA stability, suggesting transcriptional rather than posttranscriptional regulation. High-fibroblast IL-6 expression was significantly associated with the IL-6 proximal promoter single nucleotide polymorphism (SNP) rs1800795 minor allele (CC) genotype. In contrast, no association between this SNP and IL-6 production was detected in CD14(+) monocytes, another major producer of synovial IL-6. Luciferase expression assays confirmed that this SNP was associated with differential IL-6 expression in fibroblasts. To date, several association studies examining rs1800795 allele frequency and disease risk have reported seemingly conflicting results ranging from no association to association with either the major or minor allele across a spectrum of conditions, including cancer and autoimmune, cardiovascular, infectious, and metabolic diseases. This study points to a prominent contribution from promoter genetic variation in fibroblast IL-6 regulation, but not in other IL-6-producing cell types. We propose that some of the heterogeneity in these clinical studies likely reflects the cellular source of IL-6 in specific diseases, much of which may be produced by nonhematopoietic cells. These results highlight that functional analysis of disease-associated SNPs on gene expression and pathologic processes must consider variation in diverse cell types.

Evidence for cadherin-11 cleavage in the synovium and partial characterization of its mechanism.

INTRODUCTION: Engagement of the homotypic cell-to-cell adhesion molecule cadherin-11 on rheumatoid arthritis (RA) synovial fibroblasts with a chimeric molecule containing the cadherin-11 extracellular binding domain stimulated cytokine, chemokine, and matrix metalloproteinases (MMP) release, implicating cadherin-11 signaling in RA pathogenesis. The objective of this study was to determine if cadherin-11 extracellular domain fragments are found inside the joint and if a physiologic synovial fibroblast cleavage pathway releases those fragments. METHODS: Cadherin-11 cleavage fragments were detected by western blot in cell media or lysates. Cleavage was interrupted using chemical inhibitors or short-interfering RNA (siRNA) gene silencing. The amount of cadherin-11 fragments in synovial fluid was measured by western blot and ELISA. RESULTS: Soluble cadherin-11 extracellular fragments were detected in human synovial fluid at significantly higher levels in RA samples compared to osteoarthritis (OA) samples. A cadherin-11 N-terminal extracellular binding domain fragment was shed from synovial fibroblasts after ionomycin stimulation, followed by presenilin 1 (PSN1)-dependent regulated intramembrane proteolysis of the retained membrane-bound C-terminal fragments. In addition to ionomycin-induced calcium flux, tumor necrosis factor (TNF)-α also stimulated cleavage in both two- and three-dimensional fibroblast cultures. Although cadherin-11 extracellular domains were shed by a disintegrin and metalloproteinase (ADAM) 10 in several cell types, a novel ADAM- and metalloproteinase-independent activity mediated shedding in primary human fibroblasts. CONCLUSIONS: Cadherin-11 undergoes ectodomain shedding followed by regulated intramembrane proteolysis in synovial fibroblasts, triggered by a novel sheddase that generates extracelluar cadherin-11 fragments. Cadherin-11 fragments were enriched in RA synovial fluid, suggesting they may be a marker of synovial burden and may function to modify cadherin-11 interactions between synovial fibroblasts.

Clinical course and management of a consecutive series of patients with "healed temporal arteritis".

OBJECTIVE: To describe the clinical course and management of patients with a pathologic diagnosis of "healed" giant cell arteritis (GCA), and to determine whether previously published histological descriptions of healed arteritis can identify patients with a greater likelihood of clinically significant arteritis. METHODS: All temporal artery biopsy reports between 1994 and 2003 were examined for a diagnosis of "healed arteritis." Two rheumatologists abstracted the medical record for presenting features, physical findings, comorbid conditions, and data on treatment and outcomes. One pathologist, blinded to the clinical data, reviewed all specimens and reinterpreted the biopsies according to published histological descriptions of healed arteritis. RESULTS: Forty-seven patients with an initial pathologic diagnosis of healed arteritis were identified. In 54% of these patients, corticosteroid therapy did not change after the diagnosis of healed arteritis was documented in the pathology report. Seventy percent were ultimately treated with no corticosteroids or low-moderate corticosteroid regimens. Only 32% of the initial cases were confirmed upon review of the biopsies using standardized histological criteria. Patients with confirmed healed arteritis were more likely to have a documented history of polymyalgia rheumatica/GCA and a longer duration of corticosteroid treatment before biopsy. These patients were not more likely to have adverse outcomes. CONCLUSION: In this case series, the diagnosis of healed arteritis had little effect on treatment decisions. In most cases, the initial pathologic diagnosis of healed arteritis was not confirmed when biopsies were reviewed by a single pathologist using uniform histological criteria.

Modulation of matrix metalloproteinase production by rheumatoid arthritis synovial fibroblasts after cadherin 11 engagement.

OBJECTIVE: Cadherin 11 is a homophilic cell-to-cell adhesion molecule expressed on joint synovial fibroblasts. Absence of cadherin 11 in a mouse rheumatoid arthritis (RA) model led to striking reductions in cartilage erosion. Matrix metalloproteinases (MMPs) are enzymes expressed by synovial fibroblasts important for cartilage erosion. The objective of this study was to determine if synovial fibroblast MMP production is regulated by cadherin 11. METHODS: To mimic cadherin 11 engagement, human RA synovial fibroblasts were stimulated with a chimeric construct consisting of the cadherin 11 extracellular domain linked to the human IgG1 Fc domain (Cad-11-Fc). Effects on MMP production were measured by enzyme-linked immunosorbent assay, quantitative reverse transcription-polymerase chain reaction analysis, and immunoblotting. RESULTS: Human Cad-11-Fc up-regulated MMP-1 and MMP-3 protein production by RA synovial fibroblasts, both alone and in synergy with tumor necrosis factor α. This up-regulation required cell cadherin 11 engagement, since a mutant Cad-11-Fc with reduced binding affinity stimulated significantly less MMP production. Also, short hairpin RNA (shRNA) cadherin 11 silencing almost completely inhibited Cad-11-Fc-induced MMP expression. Cad-11-Fc stimulation increased RA synovial fibroblast MMP messenger RNA levels. It also increased the phosphorylation of the MAPKs JNK, ERK, and p38 kinase, the phosphorylation of NF-κB p65, and the nuclear translocation of activator protein 1 transcription factor. MAPK and NF-κB inhibitors partially blocked RA synovial fibroblast MMP expression. CONCLUSION: Cadherin 11 engagement stimulates increased synthesis of several MMPs by RA synovial fibroblasts in a MAPK- and NF-κB-dependent manner. These results underscore the existence of a pathway by which cadherin 11 regulates MMP production and has important implications for joint destruction in RA.

Cadherin-11 regulates fibroblast inflammation.

Fibroblasts are important participants in inflammation. Although not leukocytes, their capacity to produce cytokines, chemokines, and other inflammatory factors locally in tissues suggests that they can contribute to inflammatory diseases. For example, fibroblasts in a rheumatoid arthritis (RA) joint are a dominant source of IL-6 and RANKL in the synovium, both of which are therapeutic targets for inflammation and bone erosion. Previously, we found that fibroblasts can be targeted by mAb directed against cadherin-11 (cad-11), a mesenchymal cadherin that fibroblasts selectively express. Targeting cad-11 significantly reduced inflammation as assessed by joint swelling and clinical inflammation scores. However, the mechanism by which anti-cad-11 reduced inflammation was not known. Here, we show that cad-11 engagement induces synovial fibroblasts to secret proinflammatory cytokines including IL-6. Cad-11 engagement strongly synergized with TNF-α and IL-1ß in the induction of IL-6. Importantly, cad-11 activated MAP kinases and NF-κB for IL-6 induction. IL-6 levels in ankles of inflamed joints were reduced in cad-11 mutant mice compared to wild-type mice with inflammatory arthritis. Thus, we suggest that cad-11 modulates synovial fibroblasts to evoke inflammatory factors that may contribute to the inflammatory process in RA.

Integrins traffic rapidly via circular dorsal ruffles and macropinocytosis during stimulated cell migration.

During cell migration, integrins are redistributed from focal adhesions undergoing disassembly at the cell's trailing edges to new focal adhesions assembling at leading edges. The initial step of integrin redistribution is thought to require clathrin-mediated endocytosis. However, whether clathrin-mediated endocytosis functions in different contexts, such as basal versus stimulated migration, has not been determined. In this paper, we examine the spatial and temporal redistribution of integrins from focal adhesions upon stimulation by growth factors. Four-dimensional confocal live-cell imaging along with functional analysis reveals that surface integrins do not undergo significant endocytosis at ventral focal adhesions upon cell stimulation with the platelet-derived growth factor. Rather, they abruptly redistribute to dorsal circular ruffles, where they are internalized through macropinocytosis. The internalized integrins then transit through recycling endosomal compartments to repopulate newly formed focal adhesions on the ventral surface. These findings explain why integrins have long been observed to redistribute through both surface-based and internal routes and identify a new function for macropinocytosis during growth factor-induced cell migration.

The role and therapeutic implications of fibroblast-like synoviocytes in inflammation and cartilage erosion in rheumatoid arthritis.

SUMMARY: Fibroblast-like synoviocytes (FLS) are resident mesenchymal cells of synovial joints that have been recognized to play an increasingly important role in the pathogenesis of rheumatoid arthritis (RA). Activation of FLS in the setting of RA leads to the production of a broad array of cell surface and soluble mediators that help to recruit, retain, and activate both cells of the immune system and resident joint cells, leading to the promotion of ongoing inflammation and tissue destruction. The ability of FLS to stimulate both inflammation and tissue damage suggests that this cell type may be a unique target for the treatment of inflammatory arthritis. Greater understanding of how FLS are activated and how they interact with other cells in the RA synovium may provide insights that allow development of novel agents for RA therapy.

Cadherin-11 in synovial lining formation and pathology in arthritis.

The normal synovium forms a membrane at the edges of joints and provides lubrication and nutrients for the cartilage. In rheumatoid arthritis, the synovium is the site of inflammation, and it participates in an organized tissue response that damages cartilage and bone. We identified cadherin-11 as essential for the development of the synovium. Cadherin-11-deficient mice have a hypoplastic synovial lining, display a disorganized synovial reaction to inflammation, and are resistant to inflammatory arthritis. Cadherin-11 therapeutics prevent and reduce arthritis in mouse models. Thus, synovial cadherin-11 determines the behavior of synovial cells in their proinflammatory and destructive tissue response in inflammatory arthritis.

Rituximab as therapy for refractory polymyositis and dermatomyositis.

We describe response to rituximab treatment of refractory inflammatory myopathy. Three patients with long-standing polymyositis (PM) or dermatomyositis (DM) poorly responsive to prednisone combined with several immunosuppressants were given intravenous rituximab 1,000 mg on Days 0 and 14. Prior to rituximab, each had significant proximal weakness with creatine phosphokinase (CPK) elevation to>3 times the normal upper limit (range 789-3,123 U/l). Patients were receiving prednisone plus methotrexate (MTX) or azathioprine. CPK decrease was observed 1 month post-infusion, with normalization of levels averaging 4.6 months (range 2.6-7.7 mo). Muscle strength improved in all, with strength returning to normal in 2. Average daily prednisone dose decreased from 16.7 mg (range 10-20 mg) to 4 mg (range 0-7 mg) after infusion. MTX dose was tapered by 50% in 2 patients. The third patient eventually discontinued all additional therapies. Percentage of CD19+ cells in each were suppressed at 0-1% 5 to 6 months after infusion (normal 5-21%). Elevated CPK with return of clinical symptoms occurred in 2 patients 6 and 10 months post-infusion, requiring rituximab retreatment. CD19+ cells remained suppressed at 1% in one patient, but were almost normal at 4% in the other. The third patient remains disease-free 12 months after initial treatment, even though her CD19+ cells are now normal at 8%. Thus, short-term beneficial effects with rituximab were observed in patients with DM and PM. However, the need for retreatment did not correlate with levels of CD19+ cells.