Autocrine signaling is a key regulatory element during osteoclastogenesis

Exploring the Role of Hormones and Cytokines in Osteoporosis Development

The disease of osteoporosis is characterized by impaired bone structure and an increased risk of fractures. There is a significant impact of cytokines and hormones on bone homeostasis and the diagnosis of osteoporosis. As defined by the World Health Organization (WHO), osteoporosis is defined as having a bone mineral density (BMD) that is 2.5 standard deviations (SD) or more below the average for young and healthy women (T score < -2.5 SD). Cytokines and hormones, particularly in the remodeling of bone between osteoclasts and osteoblasts, control the differentiation and activation of bone cells through cytokine networks and signaling pathways like the nuclear factor kappa-B ligand (RANKL)/the receptor of RANKL (RANK)/osteoprotegerin (OPG) axis, while estrogen, parathyroid hormones, testosterone, and calcitonin influence bone density and play significant roles in the treatment of osteoporosis. This review aims to examine the roles of cytokines and hormones in the pathophysiology of osteoporosis, evaluating current diagnostic methods, and highlighting new technologies that could help for early detection and treatment of osteoporosis.

Breast cancer cells promote osteoclast differentiation in an MRTF-dependent paracrine manner

Bone is a frequent site for breast cancer metastasis. Conditioning of the local tumor microenvironment (TME) through crosstalk between tumor cells and bone resident cells in the metastatic niche is a major driving force for bone colonization of breast cancer cells. The vast majority of breast cancer-associated metastasis is osteolytic in nature, and RANKL-induced differentiation of bone marrow-derived macrophages to osteoclasts (OCLs) is a key requirement for osteolytic metastatic growth of cancer cells. In this study, we demonstrate that breast cancer cell-secreted factors stimulate RANKL-induced OCL differentiation of BMDMs requiring the function of Myocardin-related transcription factor (MRTF) in tumor cells. This is partly attributed to the critical role of MRTF in maintaining the basal cellular expression of connective tissue growth factor (CTGF), a pro-osteoclastogenic matricellular factor known to promote bone metastasis in human breast cancer. Supporting these in vitro findings, bioinformatics analyses of multiple human breast cancer transcriptome datasets reveal a strong positive correlation between CTGF expression and MRTF gene signature further establishing the relevance of our findings in a human disease context. By Luminex analyses, we show that MRTF depletion in breast cancer cells has a broad impact on OCL-regulatory cell-secreted factors that extends beyond CTGF. These findings, taken together with demonstration of MRTF-dependence for bone colonization breast cancer cells in vivo, suggest that MRTF inhibition could be an effective strategy to diminish OCL formation and skeletal involvement in breast cancer. In summary, this study highlights a novel tumor-extrinsic function of MRTF relevant to breast cancer metastasis.

Navigating Post-Traumatic Osteoporosis: A Comprehensive Review of Epidemiology, Pathophysiology, Diagnosis, Treatment, and Future Directions

Post-traumatic osteoporosis (PTO) presents a significant challenge in clinical practice, characterized by demineralization and decreased skeletal integrity following severe traumatic injuries. This literature review manuscript addresses the knowledge gaps surrounding PTO, encompassing its epidemiology, pathophysiology, risk factors, diagnosis, treatment, prognosis, and future directions. This review emphasizes the complexity of the etiology of PTO, highlighting the dysregulation of biomineralization processes, inflammatory cytokine involvement, hormonal imbalances, glucocorticoid effects, vitamin D deficiency, and disuse osteoporosis. Moreover, it underscores the importance of multidisciplinary approaches for risk mitigation and advocates for improved diagnostic strategies to differentiate PTO from other musculoskeletal pathologies. This manuscript discusses various treatment modalities, including pharmacotherapy, dietary management, and physical rehabilitation, while also acknowledging the limited evidence on their long-term effectiveness and outcomes in PTO patients. Future directions in research are outlined, emphasizing the need for a deeper understanding of the molecular mechanisms underlying PTO and the evaluation of treatment strategies' efficacy. Overall, this review provides a comprehensive overview of PTO and highlights avenues for future investigation to enhance clinical management and patient outcomes.

A Machine Learning-Based Image Segmentation Method to Quantify In Vitro Osteoclast Culture Endpoints

Quantification of in vitro osteoclast cultures (e.g. cell number) often relies on manual counting methods. These approaches are labour intensive, time consuming and result in substantial inter- and intra-user variability. This study aimed to develop and validate an automated workflow to robustly quantify in vitro osteoclast cultures. Using ilastik, a machine learning-based image analysis software, images of tartrate resistant acid phosphatase-stained mouse osteoclasts cultured on dentine discs were used to train the ilastik-based algorithm. Assessment of algorithm training showed that osteoclast numbers strongly correlated between manual- and automatically quantified values (r = 0.87). Osteoclasts were consistently faithfully segmented by the model when visually compared to the original reflective light images. The ability of this method to detect changes in osteoclast number in response to different treatments was validated using zoledronate, ticagrelor, and co-culture with MCF7 breast cancer cells. Manual and automated counting methods detected a 70% reduction (p < 0.05) in osteoclast number, when cultured with 10 nM zoledronate and a dose-dependent decrease with 1-10 μM ticagrelor (p < 0.05). Co-culture with MCF7 cells increased osteoclast number by ≥ 50% irrespective of quantification method. Overall, an automated image segmentation and analysis workflow, which consistently and sensitively identified in vitro osteoclasts, was developed. Advantages of this workflow are (1) significantly reduction in user variability of endpoint measurements (93%) and analysis time (80%); (2) detection of osteoclasts cultured on different substrates from different species; and (3) easy to use and freely available to use along with tutorial resources.

The effect of cytokines on osteoblasts and osteoclasts in bone remodeling in osteoporosis: a review

The complicated connections and cross talk between the skeletal system and the immune system are attracting more attention, which is developing into the field of Osteoimmunology. In this field, cytokines that are among osteoblasts and osteoclasts play a critical role in bone remodeling, which is a pathological process in the pathogenesis and development of osteoporosis. Those cytokines include the tumor necrosis factor (TNF) family, the interleukin (IL) family, interferon (IFN), chemokines, and so on, most of which influence the bone microenvironment, osteoblasts, and osteoclasts. This review summarizes the effect of cytokines on osteoblasts and osteoclasts in bone remodeling in osteoporosis, aiming to providing the latest reference to the role of immunology in osteoporosis.

Tracking Targets of Dynamic Super-Enhancers in Vitro to Better Characterize Osteoclastogenesis and to Evaluate the Effect of Diuron on the Maturation of Human Bone Cells

BACKGROUND

Osteoclasts are major actors in the maintenance of bone homeostasis. The full functional maturation of osteoclasts from monocyte lineage cells is essential for the degradation of old/damaged bone matrix. Diuron is one of the most frequently encountered herbicides, particularly in water sources. However, despite a reported delayed ossification in vivo, its impact on bone cells remains largely unknown.

OBJECTIVES

The objectives of this study were to first better characterize osteoclastogenesis by identifying genes that drive the differentiation of CD14+ monocyte progenitors into osteoclasts and to evaluate the toxicity of diuron on osteoblastic and osteoclastic differentiation in vitro.

METHODS

We performed chromatin immunoprecipitation (ChIP) against H3K27ac followed by ChIP-sequencing (ChIP-Seq) and RNA-sequencing (RNA-Seq) at different stages of differentiation of CD14+ monocytes into active osteoclasts. Differentially activated super-enhancers and their potential target genes were identified. Then to evaluate the toxicity of diuron on osteoblasts and osteoclasts, we performed RNA-Seq and functional tests during in vitro osteoblastic and osteoclastic differentiation by exposing cells to different concentrations of diuron.

RESULTS

The combinatorial study of the epigenetic and transcriptional remodeling taking place during differentiation has revealed a very dynamic epigenetic profile that supports the expression of genes vital for osteoclast differentiation and function. In total, we identified 122 genes induced by dynamic super-enhancers at late days. Our data suggest that high concentration of diuron (50μM) affects viability of mesenchymal stem cells (MSCs) in vitro associated with a decrease of bone mineralization. At a lower concentration (1μM), an inhibitory effect was observed in vitro on the number of osteoclasts derived from CD14+ monocytes without affecting cell viability. Among the diuron-affected genes, our analysis suggests a significant enrichment of genes targeted by pro-differentiation super-enhancers, with an odds ratio of 5.12 (ρ=2.59×10-5).

DISCUSSION

Exposure to high concentrations of diuron decreased the viability of MSCs and could therefore affect osteoblastic differentiation and bone mineralization. This pesticide also disrupted osteoclasts maturation by impairing the expression of cell-identity determining genes. Indeed, at sublethal concentrations, differences in the expression of these key genes were mild during the course of in vitro osteoclast differentiation. Taken together our results suggest that high exposure levels of diuron could have an effect on bone homeostasis. https://doi.org/10.1289/EHP11690.

Tobacco heating system has less impact on bone metabolism than cigarette smoke

Cigarette smoking promotes osteoclast activity, thus increasing the risk of secondary osteoporosis, leading to osteoporosis-associated fracture and impaired fracture healing. Heated tobacco products (HTP) are considered potential reduced-risk alternatives to cigarettes. However, their impact on bone metabolism remains to be elucidated. We developed an in vitro model that mimics in vivo bone cell interactions to comparatively evaluate the effects of HTPs and cigarette smoke on bone cell functionality and viability. We generated an in vitro coculture system with SCP-1 and THP-1 cells (1:8 ratio) cultured on a decellularized Saos-2 matrix with an optimized coculture medium. We found that, following acute or chronic exposure, particulate matter extract from the aerosol of an HTP, the Tobacco Heating System (THS), was less harmful to the bone coculture system than reference cigarette (1R6F) smoke extract. In the fracture healing model, cultures exposed to the THS extract maintained similar osteoclast activity and calcium deposits as control cultures. Conversely, smoke extract exposure promoted osteoclast activity, resulting in an osteoporotic environment, whose formation could be prevented by bisphosphonate coadministration. Thus, THS is potentially less harmful than cigarette smoke to bone cell differentiation and bone mineralization - both being crucial aspects during the reparative phase of fracture healing.

The forgotten key players in rheumatoid arthritis: IL-8 and IL-17 - Unmet needs and therapeutic perspectives

Despite the relevant advances in our understanding of the pathogenetic mechanisms regulating inflammation in rheumatoid arthritis (RA) and the development of effective therapeutics, to date, there is still a proportion of patients with RA who do not respond to treatment and end up progressing toward the development of joint damage, extra-articular complications, and disability. This is mainly due to the inter-individual heterogeneity of the molecular and cellular taxonomy of the synovial membrane, which represents the target tissue of RA inflammation. Tumor necrosis factor alpha (TNFα) and interleukin-6 (IL-6) are crucial key players in RA pathogenesis fueling the inflammatory cascade, as supported by experimental evidence derived from in vivo animal models and the effectiveness of biologic-Disease Modifying Anti-Rheumatic Drugs (b-DMARDs) in patients with RA. However, additional inflammatory soluble mediators such as IL-8 and IL-17 exert their pathogenetic actions promoting the detrimental activation of immune and stromal cells in RA synovial membrane, tendons, and extra-articular sites, as well as blood vessels and lungs, causing extra-articular complications, which might be excluded by the action of anti-TNFα and anti-IL6R targeted therapies. In this narrative review, we will discuss the role of IL-8 and IL-17 in promoting inflammation in multiple biological compartments (i.e., synovial membrane, blood vessels, and lung, respectively) in animal models of arthritis and patients with RA and how their selective targeting could improve the management of treatment resistance in patients.

Cytokine-mediated immunomodulation of osteoclastogenesis

Cytokines are an important set of proteins regulating bone homeostasis. In inflammation induced bone resorption, cytokines, such as RANKL, TNF-α, M-CSF, are indispensable for the differentiation and activation of resorption-driving osteoclasts, the process we know as osteoclastogenesis. On the other hand, immune system produces a number of regulatory cytokines, including IL-4, IL-10 and IFNs, and limits excessive activation of osteoclastogenesis and bone loss during inflammation. These unique properties make cytokines powerful targets as rheostat to maintain bone homeostasis and for potential immunotherapies of inflammatory bone diseases. In this review, we summarize recent advances in cytokine-mediated regulation of osteoclastogenesis and provide insights of potential translational impact of bench-side research into clinical treatment of bone disease.

Apoptotic mesenchymal stromal cells support osteoclastogenesis while inhibiting multinucleated giant cells formation in vitro

In bone regeneration induced by the combination of mesenchymal stromal cells (MSCs) and calcium-phosphate (CaP) materials, osteoclasts emerge as a pivotal cell linking inflammation and bone formation. Favorable outcomes are observed despite short-term engraftments of implanted MSCs, highlighting their major paracrine function and the possible implication of cell death in modulating their secretions. In this work, we focused on the communication from MSCs towards osteoclasts-like cells in vitro. MSCs seeded on a CaP biomaterial or undergoing induced apoptosis produced a conditioned media favoring the development of osteoclasts from human CD14+ monocytes. On the contrary, MSCs’ apoptotic secretion inhibited the development of inflammatory multinucleated giant cells formed after IL-4 stimulation. Components of MSCs’ secretome before and after apoptotic stress were compared using mass spectrometry-based quantitative proteomics and a complementary immunoassay for major cytokines. CXCR-1 and CXCR-2 ligands, primarily IL-8/CXCL-8 but also the growth-regulated proteins CXCL-1, -2 or -3, were suggested as the major players of MSCs’ pro-osteoclastic effect. These findings support the hypothesis that osteoclasts are key players in bone regeneration and suggest that apoptosis plays an important role in MSCs’ effectiveness.

Association Between Bone Mineral Density and Autoantibodies in Patients With Rheumatoid Arthritis

Objective

Autoantibodies, such as anti–citrullinated protein antibodies (ACPAs), have been described as inducing bone loss in rheumatoid arthritis (RA), which can also be reflected by bone mineral density (BMD). We therefore examined the association between osteoporosis and autoantibodies in two independent RA cohorts.

Methods

Dual x‐ray absorptiometry (DXA) of the lumbar spine and left hip was performed in 408 Dutch patients with early RA during 5 years of follow‐up and in 198 Swedish patients with early RA during 10 years of follow‐up. The longitudinal effect of ACPAs and other autoantibodies on several BMD measures was assessed using generalized estimating equations.

Results

In the Dutch cohort, significantly lower BMD at baseline was observed in ACPA‐positive patients compared to ACPA‐negative patients, with an estimated marginal mean BMD in the left hip of 0.92 g/cm² (95% confidence interval [95% CI] 0.91–0.93) versus 0.95 g/cm² (95% CI 0.93–0.97) (P = 0.01). In line with this, significantly lower Z scores at baseline were noted in the ACPA‐positive group compared to the ACPA‐negative group (estimated marginal mean Z score in the left hip of 0.18 [95% CI 0.08–0.29] versus 0.48 [95% CI 0.33–0.63]) (P < 0.01). However, despite clear differences at baseline, ACPA positivity was not associated with greater decrease in absolute BMD or Z scores over time. Furthermore, there was no association between BMD and higher levels of ACPAs or other autoantibodies (rheumatoid factor and anti–carbamylated protein antibodies). In the Swedish cohort, ACPA‐positive patients tended to have a higher prevalence of osteopenia at baseline (P = 0.04), but again, ACPA positivity was not associated with an increased prevalence of osteopenia or osteoporosis over time.

Conclusion

The presence of ACPAs is associated with significantly lower BMD at baseline, but not with greater BMD loss over time in treated RA patients. These results suggest that ACPAs alone do not appear to contribute to bone loss after disease onset when disease activity is well‐managed.

Soybean Meal-Dependent Intestinal Inflammation Induces Different Patterns of Bone-Loss in Adult Zebrafish Scale

Inflammatory bowel disease have been linked to several health issues, including high risk of low bone mineral density. Danio rerio (zebrafish) is a good model to verify the effects of intestinal inflammation, since its gastrointestinal and immune systems are closely related to that of mammalians. Zebrafish is also a powerful model to study bone metabolism using the scale as the read-out model. Food strongly impacts zebrafish gut physiology, and it is well known that soybean meal induces intestinal inflammation. Adult zebrafish fed with defatted soybean meal (SBM) exhibited an intestinal inflammation evidenced by morphological alterations, inflammatory infiltrate, and increased mRNA expression of inflammatory cytokines (IL-1β, IL-6, IL-8, IL-10, TGFβ, TNF-α). The peak of acute intestinal inflammation, spanning between week 2 and 3, correlates with a transitory osteoporosis-like phenotype in the scale border. Later, a chronic inflammatory condition, associated with persistent IL-8 expression, correlates with the progression of resorption lacunae in the scale center. Both types of resorption lacunae were associated with intense osteoclastic tartrate-resistant acid phosphatase (TRAP) activity. After 3 weeks of SBM treatment, osteoclast activity decreased in the scale border but not in the center. At the same time, alkaline phosphatase (ALP) is activated in the border to repair the bone matrix. This model can contribute to elucidate in vivo the molecular mechanisms that links intestinal inflammation and bone metabolism in IBD.

3D Environment Is Required In Vitro to Demonstrate Altered Bone Metabolism Characteristic for Type 2 Diabetics

A large British study, with almost 3000 patients, identified diabetes as main risk factor for delayed and nonunion fracture healing, the treatment of which causes large costs for the health system. In the past years, much progress has been made to treat common complications in diabetics. However, there is still a lack of advanced strategies to treat diabetic bone diseases. To develop such therapeutic strategies, mechanisms leading to massive bone alterations in diabetics have to be well understood. We herein describe an in vitro model displaying bone metabolism frequently observed in diabetics. The model is based on osteoblastic SaOS-2 cells, which in direct coculture, stimulate THP-1 cells to form osteoclasts. While in conventional 2D cocultures formation of mineralized matrix is decreased under pre-/diabetic conditions, formation of mineralized matrix is increased in 3D cocultures. Furthermore, we demonstrate a matrix stability of the 3D carrier that is decreased under pre-/diabetic conditions, resembling the in vivo situation in type 2 diabetics. In summary, our results show that a 3D environment is required in this in vitro model to mimic alterations in bone metabolism characteristic for pre-/diabetes. The ability to measure both osteoblast and osteoclast function, and their effect on mineralization and stability of the 3D carrier offers the possibility to use this model also for other purposes, e.g., drug screenings.

Comprehensive Bioinformatics Analysis Reveals Hub Genes and Inflammation State of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is an autoimmune disease characterized by erosive arthritis, which has not been thoroughly cured yet, and standardized treatment is helpful for alleviating clinical symptoms. Here, various bioinformatics analysis tools were comprehensively utilized, aiming to identify critical biomarkers and possible pathogenesis of RA. Three gene expression datasets profiled by microarray were obtained from GEO database. Dataset GSE55235 and GSE55457 were merged for subsequent analyses. We identified differentially expressed genes (DEGs) in RStudio with limma package, performing functional enrichment analysis based on GSEA software and clusterProfiler package. Next, protein-protein interaction (PPI) network was set up through STRING database and Cytoscape. Moreover, CIBERSORT website was used to assess the inflammatory state of RA. Finally, we validated the candidate hub genes with dataset GSE77298. As a result, we identified 106 DEGs (72 upregulated and 34 downregulated genes). Through GO, KEGG, and GSEA analysis, we found that DEGs were mainly involved in immune response and inflammatory signaling pathway. With the help of Cytoscape software and MCODE plug-in, the most prominent subnetwork was screened out, containing 14 genes and 45 edges. For ROC curve analysis, eight genes with AUC >0.80 were considered as hub genes of RA. In conclusion, compared with healthy controls, the DEGs and their closely related biological functions were analyzed, and we held that chemokines and immune cells infiltration promote the progression of rheumatoid arthritis. Targeting the eight biomarkers we identified may be useful for the diagnosis and treatment of rheumatoid arthritis.

Molecular mechanisms and clinical management of cancer bone metastasis

As one of the most common metastatic sites of malignancies, bone has a unique microenvironment that allows metastatic tumor cells to grow and flourish. The fenestrated capillaries in the bone, bone matrix, and bone cells, including osteoblasts and osteoclasts, together maintain the homeostasis of the bone microenvironment. In contrast, tumor-derived factors act on bone components, leading to subsequent bone resorption or excessive bone formation. The various pathways involved also provide multiple targets for therapeutic strategies against bone metastases. In this review, we summarize the current understanding of the mechanism of bone metastases. Based on the general process of bone metastases, we specifically highlight the complex crosstalk between tumor cells and the bone microenvironment and the current management of cancer bone metastases.

Osteocyte-Related Cytokines Regulate Osteoclast Formation and Bone Resorption

The process of bone remodeling is the result of the regulated balance between bone cell populations, namely bone-forming osteoblasts, bone-resorbing osteoclasts, and the osteocyte, the mechanosensory cell type. Osteoclasts derived from the hematopoietic stem cell lineage are the principal cells involved in bone resorption. In osteolytic diseases such as rheumatoid arthritis, periodontitis, and osteoporosis, the balance is lost and changes in favor of bone resorption. Therefore, it is vital to elucidate the mechanisms of osteoclast formation and bone resorption. It has been reported that osteocytes express Receptor activator of nuclear factor κΒ ligand (RANKL), an essential factor for osteoclast formation. RANKL secreted by osteocytes is the most important factor for physiologically supported osteoclast formation in the developing skeleton and in pathological bone resorption such as experimental periodontal bone loss. TNF-α directly enhances RANKL expression in osteocytes and promotes osteoclast formation. Moreover, TNF-α enhances sclerostin expression in osteocytes, which also increases osteoclast formation. These findings suggest that osteocyte-related cytokines act directly to enhance osteoclast formation and bone resorption. In this review, we outline the most recent knowledge concerning bone resorption-related cytokines and discuss the osteocyte as the master regulator of bone resorption and effector in osteoclast formation.

Effect of Sevelamer on Calciprotein Particles in Hemodialysis Patients: The Sevelamer Versus Calcium to Reduce Fetuin-A-Containing Calciprotein Particles in Dialysis (SCaRF) Randomized Controlled Trial

Introduction

Calciprotein particles (CPPs) are potentially modifiable mediators of phosphate toxicity in patients with kidney disease. We compared the effects of calcium carbonate (CC) and the non–calcium-based phosphate binder sevelamer on CPP levels in patients undergoing hemodialysis (HD). We hypothesized that treatment with sevelamer would achieve greater reductions in amorphous calcium phosphate–containing CPP (CPP-1) and hydroxyapatite-containing CPP (CPP-2) owing to reduced calcium loading and anti-inflammatory pleiotropic effects.

Methods

We conducted an open-label, randomized controlled trial (RCT) in which 31 stable prevalent HD patients were allocated to receive either sevelamer hydrochloride (SH), sevelamer carbonate (SC), or CC for 24 weeks. Dual primary endpoints were the between groups differences in serum CPP-1 and CPP-2 levels at 24 weeks in SH + SC–treated versus CC-treated patients. Effects on aortic pulse wave velocity (aPWV), inflammatory cytokines (interleukin-6 and -8), and effects across individual treatment arms were also assessed.

Results

Serum CPP-1, but not CPP-2, levels were lower in those randomly assigned to the sevelamer (SH + SC) group compared with the CC group at 24 weeks (–70%, 95% confidence interval [CI] –90% to –15%, P = 0.02). In subgroup analysis, this effect was confined to those receiving SC (–83.4%, 95% CI –95.7% to –36.8%, P = 0.01). aPWV and interleukin-8 levels were also lower in those who received sevelamer compared with CC at 24 weeks (–2.0 m/s, 95% CI –2.9 to –1.1; –57%, 95% CI –73% to –30%, respectively, both P = 0.01). Conventional markers of mineral metabolism remained stable across all treatment groups.

Discussion

Compared with treatment with CC, use of sevelamer for 24 weeks was associated with lower serum CPP-1 levels and a reduction in aPWV and systemic inflammation.

Material-Dependent Formation and Degradation of Bone Matrix-Comparison of Two Cryogels

Cryogels represent ideal carriers for bone tissue engineering. We recently described the osteogenic potential of cryogels with different protein additives, e.g., platelet-rich plasma (PRP). However, these scaffolds raised concerns as different toxic substances are required for their preparation. Therefore, we developed another gelatin (GEL)-based cryogel. This study aimed to compare the two scaffolds regarding their physical characteristics and their influence on osteogenic and osteoclastic cells. Compared to the PRP scaffolds, GEL scaffolds had both larger pores and thicker walls, resulting in a lower connective density. PRP scaffolds, with crystalized calcium phosphates on the surface, were significantly stiffer but less mineralized than GEL scaffolds with hydroxyapatite incorporated within the matrix. The GEL scaffolds favored adherence and proliferation of the osteogenic SCP-1 and SaOS-2 cells. Macrophage colony-stimulating factor (M-CSF) and osteoprotegerin (OPG) levels seemed to be induced by GEL scaffolds. Levels of other osteoblast and osteoclast markers were comparable between the two scaffolds. After 14 days, mineral content and stiffness of the cryogels were increased by SCP-1 and SaOS-2 cells, especially of PRP scaffolds. THP-1 cell-derived osteoclastic cells only reduced mineral content and stiffness of PRP cryogels. In summary, both scaffolds present powerful advantages; however, the possibility to altered mineral content and stiffness may be decisive when it comes to using PRP or GEL scaffolds for bone tissue engineering.

Role of Altered Metabolic Microenvironment in Osteolytic Metastasis

Metastatic bone disease is generally incurable and leads to pathological fractures, pain, hypercalcemia, spinal cord compression and decreased mobility. The skeleton is the major site of bone metastases from solid cancers, including breast and prostate carcinoma. Bone metastasis is facilitated by activation of bone-resorbing osteoclasts, terminally differentiated multinucleated cells formed by fusion from monocytic precursors. Cancer cells are known to produce specific factors that stimulate osteoclast differentiation and function. Of interest, cancer cells are also known to alter their own bioenergetics increasing the use of glycolysis for their survival and function. Such change in energy utilization by cancer cells would result in altered levels of cell-permeable metabolites, including glucose, lactate, and pyruvate. Osteoclast resorption is energy-expensive, and we have previously demonstrated that during differentiation osteoclasts actively adapt to their bioenergetics microenvironment. We hypothesize that altered bioenergetics state of cancer cells will also modify the bioenergetics substrate availability for the tissue-resident bone cells, potentially creating a favorable milieu for pathological osteolysis. The goals of this review are to analyze how metastasizing cancer cells change the availability of energy substrates in bone microenvironment; and to assess how the altered bioenergetics may affect osteoclast differentiation and activity.

Interaction of Brucella abortus with Osteoclasts: a Step toward Understanding Osteoarticular Brucellosis and Vaccine Safety

Osteoarticular disease is a frequent complication of human brucellosis. Vaccination remains a critical component of brucellosis control, but there are currently no vaccines for use in humans, and no in vitro models for assessing the safety of candidate vaccines in reference to the development of bone lesions currently exist. While the effect of Brucella infection on osteoblasts has been extensively evaluated, little is known about the consequences of osteoclast infection. Murine bone marrow-derived macrophages were derived into mature osteoclasts and infected with B. abortus 2308, the vaccine strain S19, and attenuated mutants S19vjbR and B. abortus ΔvirB2 While B. abortus 2308 and S19 replicated inside mature osteoclasts, the attenuated mutants were progressively killed, behavior that mimics infection kinetics in macrophages. Interestingly, B. abortus 2308 impaired the growth of osteoclasts without reducing resorptive activity, while osteoclasts infected with B. abortus S19 and S19vjbR were significantly larger and exhibited enhanced resorption. None of the Brucella strains induced apoptosis or stimulated nitric oxide or lactose dehydrogenase production in mature osteoclasts. Finally, infection of macrophages or osteoclast precursors with B. abortus 2308 resulted in generation of smaller osteoclasts with decreased resorptive activity. Overall, Brucella exhibits similar growth characteristics in mature osteoclasts compared to the primary target cell, the macrophage, but is able to impair the maturation and alter the resorptive capacity of these cells. These results suggest that osteoclasts play an important role in osteoarticular brucellosis and could serve as a useful in vitro model for both analyzing host-pathogen interactions and assessing vaccine safety.

[99mTc]-labelled interleukin-8 as a diagnostic tool compared to [18F]FDG and CT in an experimental porcine osteomyelitis model

Osteomyelitis (OM) is an important cause of morbidity and sometimes mortality in children and adults. Long-term complications can be reduced when treatment is initiated in an early phase. The diagnostic gold standard is microbial examination of a biopsy and current non-invasive imaging methods are not always optimal. [¹¹¹In]-leukocyte scintigraphy is recommended for peripheral OM, but is time-consuming and not recommended in children. [¹⁸F]FDG PET/CT is recommended for vertebral OM in adults, but has the disadvantage of false positive findings and a relatively high radiation exposure; the latter is a problem in children. [^99mTc]-based tracers are consequently preferred in children. We, therefore, aimed to find a [^99mTc]-marked tracer with high specificity and sensitivity for early detection of OM. Suppurating inflammatory lesions like OM caused by Staphylococcus aureus (S. aureus) will attract large numbers of neutrophils and macrophages. A preliminary study has shown that [^99m Tc]-labelled IL8 may be a possible candidate for imaging of peripheral OM. We investigated [^99mTc]IL8 scintigraphy in a juvenile pig model of peripheral OM and compared it with [¹⁸F]FDG PET/CT. The pigs were experimentally inoculated with S. aureus to induce OM and scanned one week later. We also examined leukocyte count, serum CRP and IL8, as well as performed histopathological and microbiological investigations. [ ^99m Tc]IL8 was easily and relatively quickly prepared and was shown to be suitable for visualization of OM lesions in peripheral bones detecting 70% compared to a 100% sensitivity of [¹⁸F]FDG PET/CT. [ ^99m Tc]IL8 is a promising candidate for detection of OM in peripheral bones in children.

Micro-structural bone changes are associated with broad-spectrum autoimmunity and predict the onset of rheumatoid arthritis

OBJECTIVE

To assess if microstructural bone lesions in RA at-risk individuals are related to the spectrum of anti-modified protein antibodies (AMPA) and affect the risk to develop RA.

METHODS

Cortical micro-channels (CoMiCs) as well as cortical and trabecular bone volumes were analyzed by high-resolution computed tomography in the hand joints of RA at-risk individuals. Anti-modified protein antibody (AMPA) response was profiled including reactivities against citrullinated proteins (vimentin, enolase, fibrinogen) as well as carbamylated and acetylated vimentin. All subjects were followed for the development of RA.

RESULTS

RA at-risk subjects (all N=74) with broad spectrum AMPA (6-8 reactivties) had significantly more severe microstructural changes (CoMiCs: 95±35/joint; total bone volume: 265±45 HA/cm3; trabecular bone volume: 176±42; cortical bone volume: 585±138) compared to subjects with moderate (3-5 reactivties; 79±30; 293±33; 195±32, 627±91, respectively) or narrow (1-2 reactivties; 47±20, 311±34, 211±30, 674±56, respectively) AMPA reactivity. Progressors to RA had significantly higher numbers of CoMiCs and lower bone volumes (CoMiCs: 103±30 vs. 71±35; total bone volume: 258±37 vs.295±34). Furthermore, progression to RA was high in subjects with broad (48%) vs. those with medium (26%) or narrow (0%) AMPA as well as in those with high (44%) vs. low (10%) CoMiCs.

CONCLUSION

Microstructural changes in RA at-risk individuals are associated with broad-spectrum autoimmunity and predict the onset of RA. These data support the concept of structural priming of joints by autoimmunity before the onset of the inflammatory phase of the disease.

The inflammatory response to bone infection - a review based on animal models and human patients

Bone infections are difficult to diagnose and treat, especially when a prosthetic joint replacement or implant is involved. Bone loss is a major complication of osteomyelitis, but the mechanism behind has mainly been investigated in cell cultures and has not been confirmed in human settings. Inflammation is important in initiating an appropriate immune response to invading pathogens. However, many of the signaling molecules used by the immune system can also modulate bone remodeling and contribute to bone resorption during osteomyelitis. Our current knowledge of the inflammatory response relies heavily on animal models as research based on human samples is scarce. Staphylococcus aureus is one of the most common causes of bone infections and is the pathogen of choice in animal models. The regulation of inflammatory genes during prosthetic joint infections and implant-associated osteomyelitis has only been studied in rodent models. It is important to consider the validity of an animal model when results are extrapolated to humans, and both bone composition and the immune system of pigs has been shown to be more similar to humans, than to rodents. Here in vivo studies on the inflammatory response to prosthetic joint infections and implant-associated osteomyelitis are reviewed.

The Role of Mast Cells in Bone Metabolism and Bone Disorders

Mast cells (MCs) are important sensor and effector cells of the immune system that are involved in many physiological and pathological conditions. Increasing evidence suggests that they also play an important role in bone metabolism and bone disorders. MCs are located in the bone marrow and secrete a wide spectrum of mediators, which can be rapidly released upon activation of mature MCs following their differentiation in mucosal or connective tissues. Many of these mediators can exert osteocatabolic effects by promoting osteoclast formation [e.g., histamine, tumor necrosis factor (TNF), interleukin-6 (IL-6)] and/or by inhibiting osteoblast activity (e.g., IL-1, TNF). By contrast, MCs could potentially act in an osteoprotective manner by stimulating osteoblasts (e.g., transforming growth factor-β) or reducing osteoclastogenesis (e.g., IL-12, interferon-γ). Experimental studies investigating MC functions in physiological bone turnover using MC-deficient mouse lines give contradictory results, reporting delayed or increased bone turnover or no influence depending on the mouse model used. By contrast, the involvement of MCs in various pathological conditions affecting bone is evident. MCs may contribute to the pathogenesis of primary and secondary osteoporosis as well as inflammatory disorders, including rheumatoid arthritis and osteoarthritis, because increased numbers of MCs were found in patients suffering from these diseases. The clinical observations could be largely confirmed in experimental studies using MC-deficient mouse models, which also provide mechanistic insights. MCs also regulate bone healing after fracture by influencing the inflammatory response toward the fracture, vascularization, bone formation, and callus remodeling by osteoclasts. This review summarizes the current view and understanding of the role of MCs on bone in both physiological and pathological conditions.

Myokines: The endocrine coupling of skeletal muscle and bone

Bone and skeletal muscle are integrated organs and their coupling has been considered mainly a mechanical one in which bone serves as attachment site to muscle while muscle applies load to bone and regulates bone metabolism. However, skeletal muscle can affect bone homeostasis also in a non-mechanical fashion, i.e., through its endocrine activity. Being recognized as an endocrine organ itself, skeletal muscle secretes a panel of cytokines and proteins named myokines, synthesized and secreted by myocytes in response to muscle contraction. Myokines exert an autocrine function in regulating muscle metabolism as well as a paracrine/endocrine regulatory function on distant organs and tissues, such as bone, adipose tissue, brain and liver. Physical activity is the primary physiological stimulus for bone anabolism (and/or catabolism) through the production and secretion of myokines, such as IL-6, irisin, IGF-1, FGF2, beside the direct effect of loading. Importantly, exercise-induced myokine can exert an anti-inflammatory action that is able to counteract not only acute inflammation due to an infection, but also a condition of chronic low-grade inflammation raised as consequence of physical inactivity, aging or metabolic disorders (i.e., obesity, type 2 diabetes mellitus). In this review article, we will discuss the effects that some of the most studied exercise-induced myokines exert on bone formation and bone resorption, as well as a brief overview of the anti-inflammatory effects of myokines during the onset pathological conditions characterized by the development a systemic low-grade inflammation, such as sarcopenia, obesity and aging.

Coordination of Fusion and Trafficking of Pre-osteoclasts at the Marrow-Bone Interface

Fusion is the final osteoclast differentiation step leading to bone resorption. In healthy trabecular bone, osteoclast fusion is restricted to bone surfaces undergoing resorption, and necessarily requires site-specific recruitment of mononucleated pre-osteoclasts originating from the bone marrow. However, the spatiotemporal mechanism coordinating recruitment and fusion is poorly investigated. Herein we identify a collagen/vascular network as a likely structure supporting this mechanism. We therefore used multiplex immunohistochemistry and electron microscopy on human iliac crest bone samples, in combination with functional assays performed in vitro with osteoclasts generated from healthy blood donors. First, we found that putative pre-osteoclasts are in close vicinity of a network of collagen fibers associated with vessels and bone remodeling compartment canopies. Based on 3D-reconstructions of serial sections, we propose that this network may serve as roads leading pre-osteoclasts to resorption sites, as reported for cell migration in other tissues. Importantly, almost all these bone marrow pre-osteoclasts, but only some osteoclasts, express the collagen receptor OSCAR, which is reported to induce fusion competence. Furthermore, differentiating osteoclasts cultured on collagen compared to mineral show higher fusion rates, higher expression of fusogenic cytokines, and a CD47 plasma membrane distribution pattern reported to be typical of a pre-fusion state-thus collectively supporting collagen-induced fusion competence. Finally, these in vitro assays show that collagen induces high cell mobility. The present data lead to a model where collagen fibers/vasculature support the coordination between traffic and fusion of pre-osteoclasts, by serving as a physical road and inducing fusion competence as well as cell mobility.

Monosodium iodoacetate-induced monoarthritis develops differently in knee versus ankle joint in rats

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Ankle versus knee joint injection of MIA in rats resulted in different behavioural profiles.

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Levels of biochemical mediators differs between ankle and knee injection of MIA in rats.

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Histopathological analysis show different results after ankle versus knee injection of MIA in rats.

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The rat results mirror what has been found in human patients with osteoarthritis.

Objective

Disability and movement-related pain are major symptoms of joint disease, motivating the development of methods to quantify motor behaviour in rodent joint pain models. We compared effects on behaviour, assessed the levels of biochemical mediators and made a detailed histopathological evaluation after induction of rat monoiodoacetate (MIA) monoarthritis into the ankle or knee joint.

Design

Twenty-seven male Lewis rats were used. Before and up to 28 days after induction, they were tested for weight bearing during walking (dynamic), and standing (static), and for mechanical sensitivity. At termination synovial fluid was taken from ankle and/or knee joints for analysis of monocyte chemoattractant protein-1 (MCP-1), interleukin-6 (IL-6), macrophage inflammatory protein 3 alpha (MIP-3α), keratinocyte chemoattractant (KC)/human growth-regulated oncogene (GRO) and L(+)-lactate, and from separate rats joints were collected for histopathological assessment.

Results

MIA ankle joint injection gave a marked reduction of dynamic weight bearing during the first days, not seen in rats with knee joint injection. At three weeks, it was decreased in the group with knee injection, but not in those with ankle injection. However, the different injection sites caused similar reductions in static weight bearing during the early phase, which was normalized in the group with ankle injection but continued and was strengthened with time in the knee injected group. Histopathological assessment, biochemical mediators and joint swelling confirmed the disparate profiles.

Conclusions

This work shows that ankle versus knee joint injection of MIA resulted in different profiles in rats, which may mirror what has been found in human patients with osteoarthritis.

Collagen type I degradation fragments act through the collagen receptor LAIR-1 to provide a negative feedback for osteoclast formation

The major organic component of bone is collagen type I. Osteoclasts are terminally differentiated multinucleated cells of hematopoietic origin that are essential for physiological development of bone and teeth. We examined if osteoclast differentiation from murine bone marrow precursors is affected by collagen type I, or by its degradation products produced by human recombinant cathepsin K. Osteoclasts formation was dose-dependently inhibited in the presence of full length collagen type I or its 30-75 kDa degradation products added to the osteoclast differentiation media for the duration of an experiment. Collagen degradation fragments signaled through SH-2 phosphatases, inhibiting calcium signaling and NFATc1 translocation in osteoclast precursors. Osteoclasts and their precursors expressed a collagen receptor of leukocyte receptor complex family, LAIR-1. Importantly, collagen fragments failed to inhibit osteoclast formation from LAIR-1 deficient murine osteoclast precursors. This study demonstrates that collagen degradation fragments inhibit osteoclast formation acting through LAIR-1, providing a novel mechanism for the physiologically-relevant negative control of osteoclastogenesis.

Comparative analysis of bone regeneration behavior using recombinant human BMP-2 versus plasmid DNA of BMP-2

Bone regeneration and the osteoinductive capacity of implants are challenging issues in clinical medicine. Currently, recombinant growth factors and nonviral gene transfer are the most frequently investigated methods for bone growth enhancement, although the more favorable method remains unclear. There is a lack of knowledge in literature about the in vivo comparison of these methods for bone regeneration. BMP-2, which is the most commonly used growth factor for osteogenesis, was applied at its most efficient dose as a recombinant growth factor (rhBMP-2) and as a growth-factor-encoding copolymer protected gene vector (pBMP-2) in a critical size bone defect (CSD) model to determine the most suitable method for bone regeneration. CSDs were induced bilaterally in 32 Sprague-Dawley rats. RhBMP-2 (62.5 μg) or pBMP-2 (2.5 μg) was embedded in poly(d,l-)lactide-coated titanium discs. Survival times were set at 14, 28, 56, and 112 days. After euthanasia, samples were analyzed via micro-computed tomography, polychrome sequential fluorescent labeling, and immunohistochemistry. Whereas defects in both groups were bridged with new bone after 56 days, rhBMP-2 initially induced ectopic new bone formation that was later remodeled in an unorganized hypodense manner. In contrast, pBMP-2 led to slower but steady bone regeneration with physiological tissue morphology, as confirmed by high osteoblast activity shown by osteocalcin staining. CD68 and TRAP staining verified high osteoclast activity for the rhBMP-2 group. pBMP-2 successfully induced locally controlled physiological bone regeneration, whereas rhBMP-2 triggered rapid and ectopic but insufficient bone formation. Thus, nonviral gene transfer appears to be more favorable for clinical applications. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 163-173, 2019.

Transglutaminase activity regulates differentiation, migration and fusion of osteoclasts via affecting actin dynamics

Osteoclasts, bone resorbing cells, derive from monocyte/macrophage cell lineage. Increased osteoclast activity is responsible for bone destruction in diseases such as osteoporosis, periodontitis and rheumatoid arthritis. Transglutaminases (TGs), protein crosslinking enzymes, were recently found involved in osteoclastogenesis in vivo, however their mechanisms of action have remained unknown. In this study, we have investigated the role of TG activity in osteoclastogenesis in vitro using four TG inhibitors, NC9, Z006, T101, and monodansyl cadaverine. Our results showed that all TG inhibitors were capable of blocking the entire osteoclastogenesis process. The most potent of the inhibitors, NC9 when added to cultures at different phases of osteoclastogenesis, inhibited differentiation, migration, and fusion of pre-osteoclasts as well as resorption activity of mature osteoclasts. Further investigation into the mechanisms revealed that NC9 increased RhoA levels and blocked podosome belt formation suggesting that TG activity regulates actin dynamics in pre-osteoclasts. The inhibitory effect of NC9 on osteoclastogenesis as well as podosome belt formation was completely reversed with a Rho-family inhibitor Exoenzyme C3. Microtubule architecture, acetylation, and detyrosination of α-tubulin were not affected. Finally, we demonstrated that macrophages and osteoclasts expressed mRNA of three TGs:TG1, TG2, and Factor XIII-A which were all differentially regulated in these cells during differentiation. Immunofluoresence microscopic analysis showed that all three enzymes co-localized to podosomes in osteoclasts. Taken together, our data suggests that TG activity regulates differentiation, migration and fusion of osteoclasts via affecting actin dynamics and that this may involve contribution from all three TG enzymes.

Regulation of Osteoclast Differentiation by Cytokine Networks

Cytokines play a pivotal role in maintaining bone homeostasis. Osteoclasts (OCs), the sole bone resorbing cells, are regulated by numerous cytokines. Macrophage colony-stimulating factor and receptor activator of NF-κB ligand play a central role in OC differentiation, which is also termed osteoclastogenesis. Osteoclastogenic cytokines, including tumor necrosis factor-α, IL-1, IL-6, IL-7, IL-8, IL-11, IL-15, IL-17, IL-23, and IL-34, promote OC differentiation, whereas anti-osteoclastogenic cytokines, including interferon (IFN)-α, IFN-β, IFN-γ, IL-3, IL-4, IL-10, IL-12, IL-27, and IL-33, downregulate OC differentiation. Therefore, dynamic regulation of osteoclastogenic and anti-osteoclastogenic cytokines is important in maintaining the balance between bone-resorbing OCs and bone-forming osteoblasts (OBs), which eventually affects bone integrity. This review outlines the osteoclastogenic and anti-osteoclastogenic properties of cytokines with regard to osteoimmunology, and summarizes our current understanding of the roles these cytokines play in osteoclastogenesis.

Oral Biofilms from Symbiotic to Pathogenic Interactions and Associated Disease -Connection of Periodontitis and Rheumatic Arthritis by Peptidylarginine Deiminase

A wide range of bacterial species are harbored in the oral cavity, with the resulting complex network of interactions between the microbiome and host contributing to physiological as well as pathological conditions at both local and systemic levels. Bacterial communities inhabit the oral cavity as primary niches in a symbiotic manner and form dental biofilm in a stepwise process. However, excessive formation of biofilm in combination with a corresponding deregulated immune response leads to intra-oral diseases, such as dental caries, gingivitis, and periodontitis. Moreover, oral commensal bacteria, which are classified as so-called “pathobionts” according to a now widely accepted terminology, were recently shown to be present in extra-oral lesions with distinct bacterial species found to be involved in the onset of various pathophysiological conditions, including cancer, atherosclerosis, chronic infective endocarditis, and rheumatoid arthritis. The present review focuses on oral pathobionts as commensal and healthy members of oral biofilms that can turn into initiators of disease. We will shed light on the processes involved in dental biofilm formation and also provide an overview of the interactions of P. gingivalis, as one of the most prominent oral pathobionts, with host cells, including epithelial cells, phagocytes, and dental stem cells present in dental tissues. Notably, a previously unknown interaction of P. gingivalis bacteria with human stem cells that has impact on human immune response is discussed. In addition to this very specific interaction, the present review summarizes current knowledge regarding the immunomodulatory effect of P. gingivalis and other oral pathobionts, members of the oral microbiome, that pave the way for systemic and chronic diseases, thereby showing a link between periodontitis and rheumatoid arthritis.

Regulation of Osteoclast Growth and Fusion by mTOR/raptor and mTOR/rictor/Akt

Osteoclasts are giant bone cells formed by fusion from monocytes and uniquely capable of a complete destruction of mineralized tissues. Previously, we have demonstrated that in energy-rich environment not only osteoclast fusion index (the number of nuclei each osteoclast contains), but also cytoplasm volume per single nucleus was increased. The goal of this study was to investigate the regulation of metabolic sensor mTOR during osteoclast differentiation in energy-rich environment simulated by addition of pyruvate. We have found that in the presence of pyruvate, the proportion of mTOR associated with raptor increased, while mTOR-rictor-mediated Akt phosphorylation decreased. Inhibition of mTOR with rapamycin (10 nM) significantly interfered with all aspects of osteoclastogenesis. However, rapamycin at 1 nM, which preferentially targets mTOR-raptor complex, was only effective in control cultures, while in the presence of pyruvate osteoclast fusion index was successfully increased. Inhibition of Akt drastically reduced osteoclast fusion, however in energy-rich environment, osteoclasts of comparable size were formed through increased cytoplasm growth. These data suggest that mTOR-rictor mediated Akt signaling regulates osteoclast fusion, while mTOR-raptor regulation of protein translation contributes to fusion-independent cytoplasm growth. We demonstrate that depending on the bioenergetics microenvironment osteoclastogenesis can adjust to occur through preferential multinucleation or through cell growth, implying that attaining large cell size is part of the osteoclast differentiation program.

Mechanisms leading from systemic autoimmunity to joint-specific disease in rheumatoid arthritis

A key unanswered question in the pathophysiology of rheumatoid arthritis (RA) is how systemic autoimmunity progresses to joint-specific inflammation. In patients with seropositive RA (that is, characterized by the presence of autoantibodies) evidence is accumulating that immunity against post-translationally modified (such as citrullinated) autoantigens might be triggered in mucosal organs, such as the lung, long before the first signs of inflammation are seen in the joints. However, the mechanism by which systemic autoimmunity specifically homes to the joint and bone compartment, thereby triggering inflammation, remains elusive. This Review summarizes potential pathways involved in this joint-homing mechanism, focusing particularly on osteoclasts as the primary targets of anti-citrullinated protein antibodies (ACPAs) in the bone and joint compartment. Osteoclasts are dependent on citrullinating enzymes for their normal differentiation and are unique in displaying citrullinated antigens on their cell surface in a non-inflamed state. The binding of ACPAs to osteoclasts releases the chemokine IL-8, leading to bone erosion and pain. This process initiates a chain of events that could lead to attraction and activation of neutrophils, resulting in a complex series of proinflammatory processes in the synovium, eventually leading to RA.

Urinary interleukin-6 as a predictor of radiographic progression in rheumatoid arthritis: A 3-year evaluation

Previously, we demonstrated that the urine proteome signature of patients with rheumatoid arthritis (RA) reflects inflammation-related cellular processes. Here, we measured interleukin (IL)-6, IL-8, and chemokine ligand 2 (CCL2) concentrations in the urine of RA patients and prospectively investigated their role in predicting RA activity and prognosis. One hundred seventy-three RA patients and 62 non-RA controls were recruited. Urinary IL-6, CCL2, and IL-8 levels were elevated in RA patients and correlated well with disease activity. Urinary IL-6 level at presentation was an independent risk factor of radiographic progression at 1 and 3 years. High urinary IL-6 level increased the risk ratio of radiographic progression by 2.9-fold, which was comparable to high serum CRP. Moreover, combination of urinary IL-6 and serum CRP measures synergistically increased the predictability of radiographic progression. In a subgroup with normal ESR, patients with the highest tertile of urinary IL-6 were at 6.4-fold greater risk of radiographic progression. Conclusively, high urinary IL-6 level at presentation is an independent risk factor for radiographic progression of RA, reflecting disease activity. Urinary IL-6 in combination with serum CRP may be a useful parameter for estimating RA prognosis.

Human Monocyte-Derived Osteoclasts Are Targeted by Staphylococcal Pore-Forming Toxins and Superantigens

Staphylococcus aureus is the leading cause of bone and joint infections (BJIs). Staphylococcal pathogenesis involves numerous virulence factors including secreted toxins such as pore-forming toxins (PFTs) and superantigens. The role of these toxins on BJI outcome is largely unknown. In particular, few studies have examined how osteoclasts, the bone-resorbing cells, respond to exposure to staphylococcal PFTs and superantigens. We investigated the direct impact of recombinant staphylococcal toxins on human primary mature monocyte-derived osteoclasts, in terms of cytotoxicity and cell activation with cell death and bone resorption assays, using macrophages of the corresponding donors as a reference. Monocyte-derived osteoclasts displayed similar toxin susceptibility profiles compared to macrophages. Specifically, we demonstrated that the Panton-Valentine leukocidin, known as one of the most powerful PFT which lyses myeloid cells after binding to the C5a receptor, was able to induce the death of osteoclasts. The archetypal superantigen TSST-1 was not cytotoxic but enhanced the bone resorption activity of osteoclasts, suggesting a novel mechanism by which superantigen-producing S. aureus can accelerate the destruction of bone tissue during BJI. Altogether, our data indicate that the diverse clinical presentations of BJIs could be related, at least partly, to the toxin profiles of S. aureus isolates involved in these severe infections.

Identification of a novel chemokine-dependent molecular mechanism underlying rheumatoid arthritis-associated autoantibody-mediated bone loss

Objectives

Rheumatoid arthritis (RA)-specific anti-citrullinated protein/peptide antibodies (ACPAs) appear before disease onset and are associated with bone destruction. We aimed to dissect the role of ACPAs in osteoclast (OC) activation and to identify key cellular mediators in this process.

Methods

Polyclonal ACPA were isolated from the synovial fluid (SF) and peripheral blood of patients with RA. Monoclonal ACPAs were isolated from single SF B-cells of patients with RA. OCs were developed from blood cell precursors with or without ACPAs. We analysed expression of citrullinated targets and peptidylarginine deiminases (PAD) enzymes by immunohistochemistry and cell supernatants by cytometric bead array. The effect of an anti-interleukin (IL)-8 neutralising antibody and a pan-PAD inhibitor was tested in the OC cultures. Monoclonal ACPAs were injected into mice and bone structure was analysed by micro-CT before and after CXCR1/2 blocking with reparixin.

Results

Protein citrullination by PADs is essential for OC differentiation. Polyclonal ACPAs enhance OC differentiation through a PAD-dependent IL-8-mediated autocrine loop that is completely abolished by IL-8 neutralisation. Some, but not all, human monoclonal ACPAs derived from single SF B-cells of patients with RA and exhibiting distinct epitope specificities promote OC differentiation in cell cultures. Transfer of the monoclonal ACPAs into mice induced bone loss that was completely reversed by the IL-8 antagonist reparixin.

Conclusions

We provide novel insights into the key role of citrullination and PAD enzymes during OC differentiation and ACPA-induced OC activation. Our findings suggest that IL8-dependent OC activation may constitute an early event in the initiation of the joint specific inflammation in ACPA-positive RA.

Marrow adipocyte-derived CXCL1 and CXCL2 contribute to osteolysis in metastatic prostate cancer

Increased bone marrow adiposity is a common feature of advanced age, obesity and associated metabolic pathologies. Augmented numbers of marrow adipocytes positively correlate with dysregulated bone remodeling, also a well-established complication of metastatic disease. We have shown previously that marrow adiposity accelerates prostate tumor progression in the skeleton and promotes extensive destruction of the bone; however, the factors behind adipocyte-driven osteolysis in the skeletal tumor microenvironment are not currently known. In this study, utilizing in vivo diet-induced models of bone marrow adiposity, we reveal evidence for positive correlation between increased marrow fat content, bone degradation by ARCaP(M) and PC3 prostate tumors, and augmented levels of host-derived CXCL1 and CXCL2, ligands of CXCR2 receptor. We show by in vitro osteoclastogenesis assays that media conditioned by bone marrow adipocytes is a significant source of CXCL1 and CXCL2 proteins. We also demonstrate that both the adipocyte-conditioned media and the recombinant CXCL1 and CXCL2 ligands efficiently accelerate osteoclast maturation, a process that can be blocked by neutralizing antibodies to each of the chemokines. We further confirm the contribution of CXCR2 signaling axis to adiposity-driven osteoclastogenesis by blocking fat cell-induced osteoclast differentiation with CXCR2 antagonist or neutralizing antibodies. Together, our results link CXCL1 and CXCL2 chemokines with bone marrow adiposity and implicate CXCR2 signaling in promoting effects of marrow fat on progression of skeletal tumors in bone.