Inhibitor of DNA binding/differentiation 2 induced by hypoxia promotes synovial fibroblast-dependent osteoclastogenesis

Tissue microenvironment dictates inflammation and disease activity in rheumatoid arthritis

The recent advance in treatments for rheumatoid arthritis (RA) has significantly improved the prognosis of RA patients. However, these novel therapies do not work well for all RA patients. The unmet need suggests that the current understanding about how inflammatory response arises and progresses in RA is limited. Recent accumulating evidence reveals an important role for the tissue microenvironment in the pathogenesis of RA. The synovium, the main tissue where the RA activity occurs, is composed by a unique extracellular matrix (ECM) and residing cells. The ECM molecules provide environmental signals that determine programmed site-specific cell behavior. Improved understanding of the tissue microenvironment, especially how the synovial architecture, ECM molecules, and site-specific cell behavior promote chronic inflammation and tissue destruction, will enhance deciphering the pathogenesis of RA. Moreover, in-depth analysis of tissue microenvironment will allow us to identify potential therapeutic targets. Research is now undertaken to explore potential candidates, both cellular and ECM molecules, to develop novel therapies. This article reviews recent advances in knowledge about how changes in cellular and ECM factors within the tissue microenvironment result in propagation of chronic inflammation in RA.

Fibroblast pathology in inflammatory joint disease

Rheumatoid arthritis is an immune-mediated inflammatory disease in which fibroblasts contribute to both joint damage and inflammation. Fibroblasts are a major cell constituent of the lining of the joint cavity called the synovial membrane. Under resting conditions, fibroblasts have an important role in maintaining joint homeostasis, producing extracellular matrix and joint lubricants. In contrast, during joint inflammation, fibroblasts contribute to disease pathology by producing pathogenic levels of inflammatory mediators that drive the recruitment and retention of inflammatory cells within the joint. Recent advances in single-cell profiling techniques have transformed our ability to examine fibroblast biology, leading to the identification of specific fibroblast subsets, defining a previously underappreciated heterogeneity of disease-associated fibroblast populations. These studies are challenging the previously held dogma that fibroblasts are homogeneous and are providing unique insights into their role in inflammatory joint pathology. In this review, we discuss the recent advances in our understanding of how fibroblast heterogeneity contributes to joint pathology in rheumatoid arthritis. Finally, we address how these insights could lead to the development of novel therapies that directly target selective populations of fibroblasts in the future.

Location, location, location: how the tissue microenvironment affects inflammation in RA

Current treatments for rheumatoid arthritis (RA) do not work well for a large proportion of patients, or at all in some individuals, and cannot cure or prevent this disease. One major obstacle to developing better drugs is a lack of complete understanding of how inflammatory joint disease arises and progresses. Emerging evidence indicates an important role for the tissue microenvironment in the pathogenesis of RA. Each tissue is made up of cells surrounded and supported by a unique extracellular matrix (ECM). These complex molecular networks define tissue architecture and provide environmental signals that programme site-specific cell behaviour. In the synovium, a main site of disease activity in RA, positional and disease stage-specific cellular diversity exist. Improved understanding of the architecture of the synovium from gross anatomy to the single-cell level, in parallel with evidence demonstrating how the synovial ECM is vital for synovial homeostasis and how dysregulated signals from the ECM promote chronic inflammation and tissue destruction in the RA joint, has opened up new ways of thinking about the pathogenesis of RA. These new ideas provide novel therapeutic approaches for patients with difficult-to-treat disease and could also be used in disease prevention.

Intra-articular etanercept attenuates pain and hypoxia from TMJ loading in the rat

Mechanical overloading of the temporomandibular joint (TMJ) and biochemical changes, like inflammation and hypoxia, contribute to cartilage degeneration and pain associated with osteoarthritis (OA). Yet, how overloading contributes to early dysregulation of chondrocytes is not understood, limiting the development of diagnostics and treatments for TMJ OA. Hypoxia-inducible factors (HIF)-1α/2α in chondrocytes were evaluated at Days 8 and 15 in a rat TMJ pain model induced by jaw loading (1 h/day for 7 days) using immunohistochemistry and compared between cases that induce persistent (3.5 N), acute (2 N), or no (0 N) sensitivity. Hypoxia was measured on Day 8 by immunolabeling of the tracer EF5 and 18 F-EF5 PET imaging. To assess the role of tumor necrosis factor (TNF) in painful TMJ loading, intra-articular etanercept was given before loading. Orofacial sensitivity was evaluated during and after loading. Facial grimace, TNF-α, HIF-2α, and hypoxia levels in the TMJ were measured after loading. HIF-2α was elevated (P = .03) after 3.5 N loading at Day 8, but HIF-1α was unchanged. EF5 uptake increased on Day 8 in the 3.5 N group (P < .048) by tissue assay and 18 F-EF5 PET. At Day 8, both HIF-2α (P = .01) and EF5 uptake (P = .005) were correlated with loading magnitude. Etanercept attenuated sensitivity (P < .01) and the facial grimace on Day 7 (P = .01). It also reduced (P < .01) HIF-2α and EF5 uptake on Day 8; but TNF-α levels were not different from controls at that time. Findings suggest that TMJ loading that induces persistent sensitivity upregulates the catabolic factor HIF-2α and reduces oxygen levels in the cartilage, which may be TNF-driven.

Hypoxic regulation of osteoclast differentiation and bone resorption activity

Bone integrity is maintained throughout life via the homeostatic actions of bone cells, namely, osteoclasts, which resorb bone, and osteoblasts, which produce bone. Disruption of this balance in favor of osteoclast activation results in pathological bone loss, which occurs in conditions including osteoporosis, rheumatoid arthritis, primary bone cancer, and cancer metastasis to bone. Hypoxia also plays a major role in these conditions, where it is associated with disease progression and poor prognosis. In recent years, considerable interest has arisen in the mechanisms whereby hypoxia and the hypoxia-inducible transcription factors, HIF-1α and HIF-2α, affect bone remodeling and bone pathologies. This review summarizes the current evidence for hypoxia-mediated regulation of osteoclast differentiation and bone resorption activity. Role(s) of HIF and HIF target genes in the formation of multinucleated osteoclasts from cells of the monocyte-macrophage lineage and in the activation of bone resorption by mature osteoclasts will be discussed. Specific attention will be paid to hypoxic metabolism and generation of ATP by osteoclasts. Hypoxia-driven increases in both glycolytic flux and mitochondrial metabolic activity, along with consequent generation of mitochondrial reactive oxygen species, have been found to be essential for osteoclast formation and resorption activity. Finally, evidence for the use of HIF inhibitors as potential therapeutic agents targeting bone resorption in osteolytic disease will be discussed.

Hyperbaric oxygen therapy suppresses osteoclast formation and bone resorption

The cellular and molecular mechanism through which hyperbaric oxygen therapy (HBO) improves osteonecrosis (ON) is unclear. The present study therefore examined the effect of HBO, pressure and hyperoxia on RANKL-induced osteoclast formation in RAW 264.7 cells and human peripheral blood monocytes (PBMC). Daily exposure to HBO (2.4 ATA, 97% O2 , 90 min), hyperbaric pressure (2.4 ATA, 8.8% O2 , 90 min) or normobaric hyperoxia (1 ATA, 95% O2 , 90 min) significantly decreased RANKL-induced osteoclast formation and bone resorption in normoxic conditions. HBO had a more pronounced anti-osteoclastic effect than hyperoxia or pressure alone and also directly inhibited osteoclast formation and resorption in hypoxic conditions a hallmark of many osteolytic skeletal disorders. The suppressive action of HBO was at least in part mediated through a reduction in RANK, NFATc1, and Dc-STAMP expression and inhibition of hypoxia-induced HIF-1α mRNA and protein expression. This data provides mechanistic evidence supporting the use of HBO as an adjunctive therapy to prevent osteoclast formation and bone loss associated with low oxygen partial pressure.

Dual functions of histone-lysine N-methyltransferase Setdb1 protein at promyelocytic leukemia-nuclear body (PML-NB): maintaining PML-NB structure and regulating the expression of its associated genes

Setdb1/Eset is a histone H3 lysine 9 (H3K9)-specific methyltransferase that associates with various transcription factors to regulate gene expression via chromatin remodeling. Here, we report that Setdb1 associates with promyelocytic leukemia (Pml) protein from the early stage of mouse development and is a constitutive member of promyelocytic leukemia (PML)-nuclear bodies (PML-NBs) that have been linked to many cellular processes such as apoptosis, DNA damage responses, and transcriptional regulation. Arsenic treatment, which induces Pml degradation, caused Setdb1 signals to disappear. Setdb1 knockdown resulted in dismantlement of PML-NBs. Immunoprecipitation results demonstrated physical interactions between Setdb1 and Pml. Chromatin immunoprecipitation revealed that, within the frame of PML-NBs, Setdb1 binds the promoter of Id2 and suppresses its expression through installing H3K9 methylation. Our findings suggest that Setdb1 performs dual, but inseparable, functions at PML-NBs to maintain the structural integrity of PML-NBs and to control PML-NB-associated genes transcriptionally.

Osteogenic differentiation of human ligament fibroblasts induced by conditioned medium of osteoclast-like cells

Osteoclasts secrete factors that may promote mesenchymal stem cell mineralization in vitro. Fibroblasts are the most common cells in connective tissue and are involved in the process of exotic ossification in many diseases such as ankylosing spondylitis. The purpose of this study was to investigate whether osteoclast-like cells would induce the osteogenic differentiation of fibroblasts in vitro. In the present study, osteoclast-like cells (OLCs) were generated by CD14(+) cells from human peripheral blood. Fibroblasts were primarily cultured from spinal ligaments. After treatment with conditioned medium of OLCs, the level of alkaline phosphatase (ALP) and mineralization of fibroblasts increased significantly. cDNA microarray analysis identified a set of differentially expressed mRNA associated with signal transduction, cell differentiation, and bone formation, and microarray analysis of microRNA expression profiles revealed a group of microRNAs, including hsa-miR-20a, hsa-miR-300, hsa-miR-185, hsa-miR-30d, hsa-miR-320a, hsamiR- 130b, hsa-miR-33a, hsa-miR-155, and hsa-miR-222, that were significantly downregulated. These microRNAs were predicted to have an inhibitory effect on genes associated with osteogenic differentiation, such as BMP2, Osteocalcin, and Runx2. The current results suggest that osteoclasts might induce the osteogenic differentiation of fibroblasts in vitro and that miRNA may play an important role in regulation of the cell-cell interaction between osteoclasts and fibroblasts.

Negative regulation of osteoclastogenesis and bone resorption by cytokines and transcriptional repressors

Bone remodeling in physiological and pathological conditions represents a balance between bone resorption mediated by osteoclasts and bone formation by osteoblasts. Bone resorption is tightly and dynamically regulated by multiple mediators, including cytokines that act directly on osteoclasts and their precursors, or indirectly by modulating osteoblast lineage cells that in turn regulate osteoclast differentiation. The critical role of cytokines in inducing and promoting osteoclast differentiation, function and survival is covered by the accompanying review by Zwerina and colleagues. Recently, it has become clear that negative regulation of osteoclastogenesis and bone resorption by inflammatory factors and cytokines, downstream signaling pathways, and a newly described network of transcriptional repressors plays a key role in bone homeostasis by fine tuning bone remodeling and restraining excessive bone resorption in inflammatory settings. In this review we discuss negative regulators of osteoclastogenesis and mechanisms by which these factors suppress bone resorption.

IL-33 shifts the balance from osteoclast to alternatively activated macrophage differentiation and protects from TNF-alpha-mediated bone loss

IL-33 is a new member of the IL-1 family, which plays a crucial role in inflammatory response, enhancing the differentiation of dendritic cells and alternatively activated macrophages (AAM). Based on the evidence of IL-33 expression in bone, we hypothesized that IL-33 may shift the balance from osteoclast to AAM differentiation and protect from inflammatory bone loss. Using transgenic mice overexpressing human TNF, which develop spontaneous joint inflammation and cartilage destruction, we show that administration of IL-33 or an IL-33R (ST2L) agonistic Ab inhibited cartilage destruction, systemic bone loss, and osteoclast differentiation. Reconstitution of irradiated hTNFtg mice with ST2(-/-) bone marrow led to more bone loss compared with the chimeras with ST2(+/+) bone marrow, demonstrating an important endogenous role of the IL-33/ST2L pathway in bone turnover. The protective effect of IL-33 on bone was accompanied by a significant increase of antiosteoclastogenic cytokines (GM-CSF, IL-4, and IFN-γ) in the serum. In vitro IL-33 directly inhibits mouse and human M-CSF/receptor activator for NF-κB ligand-driven osteoclast differentiation. IL-33 acts directly on murine osteoclast precursors, shifting their differentiation toward CD206(+) AAMs via GM-CSF in an autocrine fashion. Thus, we show in this study that IL-33 is an important bone-protecting cytokine and may be of therapeutic benefit in treating bone resorption.

Rheumatoid arthritis progression mediated by activated synovial fibroblasts

Rheumatoid arthritis (RA) is a chronic inflammatory disease characterized by synovial hyperplasia and progressive joint destruction. Rheumatoid arthritis synovial fibroblasts (RASFs) are leading cells in joint erosion and contribute actively to inflammation. RASFs show an activated phenotype that is independent of the inflammatory environment and requires the combination of several factors. Although new aspects regarding RASF activation via matrix degradation products, epigenetic modifications, inflammatory factors, Toll-like receptor (TLR) activation and others have recently been uncovered, the primary pathophysiological processes in early arthritis leading to permanent activation are mostly unknown. Here, we review new findings regarding RASF activation and their altered behavior that contribute to matrix destruction and inflammation as well as their potential to spread RA.