Sialic acid-binding immunoglobulin-like lectin 15 (Siglec-15) mediates periarticular bone loss, but not joint destruction, in murine antigen-induced arthritis

Involvement of Siglec-15 in regulating RAP1/RAC signaling in cytoskeletal remodeling in osteoclasts mediated by macrophage colony-stimulating factor

DNAX-associated protein 12 kD size (DAP12) is a dominant immunoreceptor tyrosine-based activation motif (ITAM)-signaling adaptor that activates costimulatory signals essential for osteoclastogenesis. Although several DAP12-associated receptors (DARs) have been identified in osteoclasts, including triggering receptor expressed on myeloid cells 2 (TREM-2), C-type lectin member 5 A (CLEC5A), and sialic acid-binding Ig-like lectin (Siglec)-15, their precise role in the development of osteoclasts and bone remodeling remain poorly understood. In this study, mice deficient in Trem-2, Clec5a, Siglec-15 were generated. In addition, mice double deficient in these DAR genes and FcεRI gamma chain (FcR)γ, an alternative ITAM adaptor to DAP12, were generated. Bone mass analysis was conducted on all mice. Notably, Siglec-15 deficient mice and Siglec-15/FcRγ double deficient mice exhibited mild and severe osteopetrosis respectively. In contrast, other DAR deficient mice showed normal bone phenotype. Likewise, osteoclasts from Siglec-15 deficient mice failed to form an actin ring, suggesting that Siglec-15 promotes bone resorption principally by modulating the cytoskeletal organization of osteoclasts. Furthermore, biochemical analysis revealed that Sigelc-15 activates macrophage colony-stimulating factor (M-CSF)-induced Ras-associated protein-1 (RAP1)/Ras-related C3 botulinum toxin substrate 1 (Rac1) pathway through formation of a complex with p130CAS and CrkII, leading to cytoskeletal remodeling of osteoclasts. Our data provide genetic and biochemical evidence that Siglec-15 facilitates M-CSF-induced cytoskeletal remodeling of the osteoclasts.

Glycobiology in osteoclast differentiation and function

Glycans, either alone or in complex with glycan-binding proteins, are essential structures that can regulate cell biology by mediating protein stability or receptor dimerization under physiological and pathological conditions. Certain glycans are ligands for lectins, which are carbohydrate-specific receptors. Bone is a complex tissue that provides mechanical support for muscles and joints, and the regulation of bone mass in mammals is governed by complex interplay between bone-forming cells, called osteoblasts, and bone-resorbing cells, called osteoclasts. Bone erosion occurs when bone resorption notably exceeds bone formation. Osteoclasts may be activated during cancer, leading to a range of symptoms, including bone pain, fracture, and spinal cord compression. Our understanding of the role of protein glycosylation in cells and tissues involved in osteoclastogenesis suggests that glycosylation-based treatments can be used in the management of diseases. The aims of this review are to clarify the process of bone resorption and investigate the signaling pathways mediated by glycosylation and their roles in osteoclast biology. Moreover, we aim to outline how the lessons learned about these approaches are paving the way for future glycobiology-focused therapeutics.

Roles of the Siglec family in bone and bone homeostasis

Tremendous progress has been seen in the study of the role of sialic acid binding im-munoglobulin type lectins (Siglecs) in osteoimmunology in the past two decades. Interest in Siglecs as immune checkpoints has grown from the recognition that Siglecs have relevance to human disease. Siglecs play important roles in inflammation and cancer, and play key roles in immune cell signaling. By recognizing common sialic acid containing glycans on glycoproteins and glycolipids as regulatory receptors for immune cell signals, Siglecs are expressed on most immune cells and play important roles in normal homeostasis and self-tolerance. In this review, we describe the role that the siglec family plays in bone and bone homeostasis, including the regulation of osteoclast differentiation as well as recent advances in inflammation, cancer and osteoporosis. Particular emphasis is placed on the relevant functions of Siglecs in self-tolerance and as pattern recognition receptors in immune responses, thereby potentially providing emerging strategies for the treatment of bone related diseases.

Enhancing the Anti-tumor Potency of a Novel Siglec-15 Antibody by Engineering its Fc-mediated Effector Functions

Siglec-15, an inhibitory immune checkpoint, is an emerging target in cancer immunotherapy. Blocking the function of Siglec-15 is an excellent strategy for cancer treatment and antibody blockade has been used to target Siglec-15. However, whether Fc-mediated effector functions contribute to the therapeutic effect of antibodies remains unclear. Herein, we generated a monoclonal antibody, 1-15D1, which had a high binding affinity with Siglec-15 and strongly activated T-cell immune response in vitro. Subsequently, the Fc-mediated effector functions of 1-15D1 were explored in a Siglec-15 humanized mouse model, and further improvement in antitumor efficacy was observed in the mouse IgG2a isotype group. Thus, we demonstrate that the antitumor effects of 1-15D1 were mediated via multiple factors. In addition to the T-cell immune response, 2 novel mechanisms were explored, including the internalization of the cell surface Siglec-15 and Fc-mediated effector functions. In conclusion, our studies not only provide a potential agent for the improvement of cancer immunotherapy but also suggest that a specific role of Fc-mediated immune regulation may improve the therapeutic potency of Siglec-15 monoclonal antibody.

Diversity of immune checkpoints in cancer immunotherapy

Finding effective treatments for cancer remains a challenge. Recent studies have found that the mechanisms of tumor evasion are becoming increasingly diverse, including abnormal expression of immune checkpoint molecules on different immune cells, in particular T cells, natural killer cells, macrophages and others. In this review, we discuss the checkpoint molecules with enhanced expression on these lymphocytes and their consequences on immune effector functions. Dissecting the diverse roles of immune checkpoints in different immune cells is crucial for a full understanding of immunotherapy using checkpoint inhibitors.

Significance of Siglec-15 expression in colorectal cancer: association with advanced disease stage and fewer tumor-infiltrating lymphocytes

Siglec-15, a novel immune suppressor, is upregulated in many human cancers. The aim of this study was to explore the expression of Siglec-15 in colorectal cancer (CRC), and investigate whether Siglec-15 could be a potential target for cancer immunotherapy in patients with CRC. We performed immunohistochemical analyses of Siglec-15 on a cohort of 805 patients with CRC and made comparisons between clinicopathological characteristics, PD-L1 expression, CD3, CD8, CD45RO tumor-infiltrating lymphocytes (TILs), and prognosis. We found that Siglec-15 expression was commonly detected in tumor cells (48.3%) and tumor-associated stromal cells (33.4%), and was more frequently observed than PD-L1 expression in tumor cells. In contrast, Siglec-15 expression was weakly and scarcely found in normal mucosa (13%). Siglec-15 overexpression in tumor cells was associated with advanced TNM stage (p = 0.020). Co-expression of Siglec-15 and PD-L1 in tumor cells was found in 14.4% of patients, and Siglec-15 expression was detected in almost half of PD-L1 negative cases. Elevated Siglec-15 expression in tumor and stromal cells was associated with sparser CD45RO and CD8 TILs (p = 0.035 and p = 0.004, respectively). The expression of Siglec-15 did not have prognostic significance. In summary, compared to PD-L1, Siglec-15 protein expression is more prevalent in CRC and is associated with advanced disease stage and fewer TILs. These findings support Siglec-15 as a potential cancer immunotherapy target, in addition to PD-1/PD-L1 inhibitors, in patients with CRC.

Unveiling the molecular features, relevant immune and clinical characteristics of SIGLEC15 in thyroid cancer

The groundbreaking research work about SIGLEC15 has raised it as a potential promising target in cancer immunotherapy. Unfortunately, the role of SIGLEC15 in thyroid carcinoma (THCA) remains obscure. Public and home multi-omics data were collected to investigate the role of SIGLEC15 in THCA in our study. SIGLEC15 was upregulated in THCA tumor tissue compared to nontumor tissue in both mRNA and protein levels; gene set enrichment analysis (GSEA) results showed that high SIGLEC15 mRNA expression was positively correlated to many immune pathways. Results of the examination of immunological landscape characteristics displayed high SIGLEC15 mRNA expression that mainly positively correlated with a large number of cancer immunity immunomodulators and pathways. In addition, upregulation of SIGLEC15 was positively correlated with an enhanced immune score, stromal score, and estimate score. However, higher SIGLEC15 mRNA also met high immune exhausted status. The majority of CpG methylation sites negatively correlated with SIGLEC15 mRNA expression. Analysis of clinical characteristics supported increased SIGLEC15 expression that was positively correlated with more extrathyroid extension and lymph node metastasis. We observed different single nucleotide variant (SNV) and copy number variation (CNV) patterns in high and low SIGLEC15 mRNA expression subgroups; some vital DNA damage repair deficiency scores addressed a negative correlation with SIGLEC15 mRNA expression. We also found that some commonly used chemotherapy drugs might be suitable for different SIGLEC15 mRNA expression subgroups. This study highlighted the vital role of SIGLEC15 in THCA. Targeting SIGLEC15 may offer a potential novel therapeutic opportunity for THCA patients. However, the detailed exact cellular mechanisms of SIGLEC15 in THCA still needed to be elucidated by further studies.

Siglec15 Checkpoint Blockade for Simultaneous Immunochemotherapy and Osteolysis Inhibition in Lung Adenocarcinoma Spinal Metastasis via a Hollow Nanoplatform

Low responsiveness to anti-programmed death-1/programmed death-ligand 1 (anti-PD-1/PD-L1) for solid tumors indicates the presence of other immunosuppressive pathways. Siglec15, a newly discovered immune checkpoint, has been reported to repress immune responses in the tumor microenvironment (TME) and regulate osteoclast differentiation. However, the role of Siglec15 in the treatment for bone metastasis remains unclear. Herein, Siglec15 shows significantly higher expression in lung adenocarcinoma spinal metastasis (LUAD-SM) than in para-cancerous spinal tissues and primary LUAD. Subsequently, a TME-responsive hollow MnO₂ nanoplatform (H-M) loaded with Siglec15 siRNA and cisplatin (H-M@siS15/Cis) is developed, and the surface is modified with an aspartic acid octapeptide (Asp⁸ ), thus allowing H-M to target spinal metastasis. High drug-loading capacity, good biocompatibility, effective tumor accumulation, and efficient Siglec15 silencing are demonstrated. Furthermore, the nanoparticles could reverse immunosuppression caused by tumor cells and tumor-associated macrophages (TAMs) and inhibit osteoclast differentiation via Siglec15 downregulation in vitro. In a LUAD-SM mouse model, H-M@siS15/Cis-Asp⁸ exhibits superior therapeutic efficacy via synergetic immunochemotherapy and osteolysis inhibition. Taken together, this single nanoplatform reveals the therapeutic potential of the new immune checkpoint Siglec15 in LUAD-SM and provides a strategy to treat this disease.

Knocking down Siglec-15 in osteosarcoma cells inhibits proliferation while promoting apoptosis and pyroptosis by targeting the Siglec-15/STAT3/Bcl-2 pathway

PURPOSE

Sialic acid-bound immunoglobulin lectin 15 (Siglec-15) plays a crucial role in many kinds of tumors. The relationship between Siglec-15 and the prognosis of osteosarcoma patients and its role in the apoptosis and pyroptosis of osteosarcoma cells are not sufficiently understood. Our study aimed to investigate the function of Siglec-15 in osteosarcoma cells and its effect on tumor cell proliferation, apoptosis and pyroptosis.

MATERIALS AND METHODS

The Siglec-15 expression in pathological sections of osteosarcoma patients was analyzed and the overall survival time related to the expression of Siglec-15 was further analyzed. Next, we detected the expression of Siglec-15 in osteosarcoma cells and downregulated the expression of Siglec-15 by small interfering RNA (siRNA). The proliferation, apoptosis and pyroptosis of osteosarcoma cells were studied by proliferation and apoptosis kits and Western blotting. Furthermore, the Siglec-15 signaling pathway was examined, which may be involved in the observed cellular effects.

RESULTS

We demonstrated the expression of Siglec-15 in osteosarcoma cells. SiRNA-mediated downregulation of Siglec-15 was successful. We found that knockdown of Siglec-15 in osteosarcoma cell lines significantly inhibited proliferation while promoting apoptosis. Further investigation showed that the expression of proliferation-related proteins was downregulated and that apoptosis- and pyroptosis-related proteins were upregulated. In addition, we found that Siglec-15 may inhibit proliferation while inducing apoptosis and pyroptosis via the (Signal Transducer and Activator of Transcription 3) STAT3/Bcl-2 pathway in osteosarcoma.

CONCLUSIONS

In this study, we demonstrated that the ablation of Siglec-15 in osteosarcoma inhibited proliferation and promoted apoptosis and pyroptosis by targeting the Siglec-15/STAT3/Bcl-2 pathway.

Characterization, pharmacokinetics, and pharmacodynamics of anti-Siglec-15 antibody and its potency for treating osteoporosis and as follow-up treatment after parathyroid hormone use

Recent studies have established the idea that Siglec-15 is involved in osteoclast differentiation and/or function, and it is anticipated that therapies suppressing Siglec-15 function can be used to treat bone diseases such as osteoporosis. We have produced rat monoclonal anti-Siglec-15 antibody (32A1) and successively generated humanized monoclonal anti-Siglec-15 antibody (DS-1501a) from 32A1. Studies on the biological properties of DS-1501a showed its specific binding affinity to Siglec-15 and strong activity to inhibit osteoclastogenesis. 32A1 inhibited multinucleation of osteoclasts and bone resorption (pit formation) in cultured mouse bone marrow cells. 32A1 also inhibited pit formation in cultured human osteoclast precursor cells. Maximum serum concentration and serum exposure of DS-1501a in rats were increased in a dose-dependent manner after single subcutaneous or intravenous administration. Furthermore, single administration of DS-1501a significantly suppressed bone resorption markers with minimal effects on bone formation markers and suppressed the decrease in bone mineral density (BMD) of the lumbar vertebrae in ovariectomized (OVX) rats. In histological analysis, the osteoclasts distant from the chondro-osseous junction of the tibia tended to be flattened, shrunken, and functionally impaired in 32A1-treated rats, while alkaline phosphatase-positive osteoblasts were observed throughout the metaphyseal trabeculae. In addition, we compared the efficacy of 32A1 with that of alendronate (ALN) as follow-up medicine after treatment with parathyroid hormone (PTH) using mature established osteoporosis rats. The beneficial effect of PTH on bone turnover disappeared 8 weeks after discontinuing the treatment. The administration of 32A1 once every 4 weeks for 8 weeks suppressed bone resorption and bone formation when the treatment was switched from PTH to 32A1, leading to the maintenance of BMD and bone strength. Unlike with ALN, the onset of suppression of bone resorption with 32A1 was rapid, while the suppression of bone formation was mild. The improvement of bone mass, beneficial bone turnover balance, and suppression of osteoclast differentiation/multinucleation achieved by 32A1 were supported by histomorphometry. Notably, the effects of 32A1 on bone strength, not only structural (extrinsic) but also material (intrinsic) properties, were significantly greater than those of ALN. Since the effect of 32A1 on BMD was moderate, its effect on bone strength could not be fully explained by the increase in BMD. The beneficial balance of bone turnover caused by 32A1 might, at least in part, be responsible for the improvement in bone quality. This is the first report describing the effects of anti-Siglec-15 antibody in OVX rats; the findings suggest that this antibody could be an excellent candidate for treating osteoporosis, especially in continuation therapy after PTH treatment, due to its rapid action and unprecedented beneficial effects on bone quality.

Molecular structure, expression, and the emerging role of Siglec-15 in skeletal biology and cancer

Siglec-15, a Siglec family member and type-1 transmembrane protein, is expressed mainly in human macrophages and dendritic cells. It is comprised of a lysine-containing transmembrane domain, two extracellular immunoglobulin (Ig)-like domains and a short cytoplasmic domain. Siglec-15 is highly conserved in vertebrates and acts as an immunoreceptor. It exerts diverse functions on osteoclast physiology as well as the tumor microenvironment. Siglec-15 interacts with adapter protein DAP12 - Syk signaling pathway to regulate the RANKL/RANK-mediated PI3K, AKT, and ERK signaling pathways during osteoclast formation in vitro. Consistently, the lack of the Siglec-15 gene in mice leads to impaired osteoclast activity and osteopetrosis in vivo. In addition, Siglec-15 is expressed by tumor-associated macrophages (TAMs) and regulates the tumor microenvironment by activating the SYK/MAPK signaling pathway. Interestingly, Siglec-15 shares sequence homology to programmed death-ligand 1 (PD-L1) and has a potential immune-regulatory role in cancer immunology. Thus, Siglec-15 might also represent an alternative target for the treatment of cancers that do not respond to anti-PD-L1/PD-1 immunotherapy. Understanding the role of Siglec-15 in osteoclastogenesis and the tumor microenvironment will help us to develop new treatments for bone disorders and cancer.

Siglec-15 on Osteoclasts Is Crucial for Bone Erosion in Serum-Transfer Arthritis

Siglec-15 is a conserved sialic acid-binding Ig-like lectin, which is expressed on osteoclasts. Deficiency of Siglec-15 leads to an impaired osteoclast development, resulting in a mild osteopetrotic phenotype. The role of Siglec-15 in arthritis is still largely unclear. To address this, we generated Siglec-15 knockout mice and analyzed them in a mouse arthritis model. We could show that Siglec-15 is directly involved in pathologic bone erosion in the K/BxN serum-transfer arthritis model. Histological analyses of joint destruction provided evidence for a significant reduction in bone erosion area and osteoclast numbers in Siglec-15^-/- mice, whereas the inflammation area and cartilage destruction was comparable to wild-type mice. Thus, Siglec-15 on osteoclasts has a crucial function for bone erosion during arthritis. In addition, we generated a new monoclonal anti-Siglec-15 Ab to clarify its expression pattern on immune cells. Whereas this Ab demonstrated an almost exclusive Siglec-15 expression on murine osteoclasts and hardly any other expression on various other immune cell types, human Siglec-15 was more broadly expressed on human myeloid cells, including human osteoclasts. Taken together, our findings show a role of Siglec-15 as a regulator of pathologic bone resorption in arthritis and highlight its potential as a target for future therapies, as Siglec-15 blocking Abs are available.

Medicinal plants used against various inflammatory biomarkers for the management of rheumatoid arthritis

OBJECTIVES

Rheumatoid arthritis is a chronic autoimmune disease manifested clinically by polyarthralgia associated with joint dysfunction triggering the antibodies targeting against the self-neoepitopes determined by autoimmune responses associated with chronic arthritic attacks. The activation of macrophages and other defence cells in response to self-epitopes as biomarkers in RA provides a better understanding of pathogenesis of disease and has led to the development of novel therapeutic approaches acting as potent inhibitors of these cells.

KEY FINDINGS

The current review retrieved the various medicinal plants possessing an active phytoconstituents with anti-inflammatory and antioxidant properties, which tends to be effective alternative approach over the synthetic drugs concerned with high toxic effects. The current available literature provided an evident data concluding that the active constituents like fatty acids, flavonoids, terpenes and sesquiterpene lactones attenuate the RA symptoms by targeting the inflammatory biomarkers involved in the pathogenesis of RA.

SUMMARY

Despite the various synthetic treatment approaches targeting immune cells, cytokines improved the quality of life but still the drug management is challenging due to toxic and chronic teratogenic effects with anti-arthritic drugs. The current review has elaborated the selected traditionally used herbal medicinal plants with phytoconstituents possessing anti-inflammatory activity by suppressing the inflammatory biomarkers with lesser side effects and providing the future exploration of natural drug therapy for rheumatoid arthritis.

Siglecs that Associate with DAP12

Siglecs are a family of transmembrane receptor-like glycan-recognition proteins expressed primarily on leukocytes. Majority of Siglecs have an intracellular sequence motif called immunoreceptor tyrosine-based inhibitory motif (ITIM) and associate with Src homology region 2 domain-containing tyrosine phosphatase-1 (SHP-1), and negatively regulate tyrosine phosphorylation-mediated intracellular signaling events. On the other hand, some Siglecs have a positively charged amino acid residue in the transmembrane domain and associate with DNAX activation protein of 12 kDa (DAP12), which in turn recruits spleen tyrosine kinase (Syk). These DAP12-associated Siglecs play diverse functions. For example, Siglec-15 is conserved throughout vertebrate evolution and plays a role in bone homeostasis by regulating osteoclast development and function. Human Siglec-14 and -16 have inhibitory counterparts (Siglec-5 and -11, respectively), which show extremely high sequence similarity with them at the extracellular domain but interact with SHP-1. The DAP12-associated Siglec in such "paired receptor" configuration counteracts the pathogens that exploit the inhibitory counterpart. Polymorphisms (mutations) that render DAP12-associated inactive Siglecs are found in humans, and some of these appear to be associated with sensitivity or resistance of human hosts to bacterially induced conditions. Studies of mouse Siglec-H have revealed complex and intriguing functions it plays in regulating adaptive immunity. Many questions remain unanswered, and further molecular and genetic studies of DAP12-associated Siglecs will yield valuable insights with translational relevance.

Siglecs as Immune Cell Checkpoints in Disease

Sialic acid-binding immunoglobulin-type lectins (Siglecs) are expressed on the majority of white blood cells of the immune system and play critical roles in immune cell signaling. Through recognition of sialic acid-containing glycans as ligands, they help the immune system distinguish between self and nonself. Because of their restricted cell type expression and roles as checkpoints in immune cell responses in human diseases such as cancer, asthma, allergy, neurodegeneration, and autoimmune diseases they have gained attention as targets for therapeutic interventions. In this review we describe the Siglec family, its roles in regulation of immune cell signaling, current efforts to define its roles in disease processes, and approaches to target Siglecs for treatment of human disease.

Siglec-15: a potential regulator of osteoporosis, cancer, and infectious diseases

Siglec-15 is a member of the Siglec family of glycan-recognition proteins, primarily expressed on a subset of myeloid cells. Siglec-15 has been known to be involved in osteoclast differentiation, and is considered to be a potential therapeutic target for osteoporosis. Recent studies revealed unexpected roles of Siglec-15 in microbial infection and the cancer microenvironment, expanding the potential pathophysiological roles of Siglec-15. Chemical biology has advanced our understanding of the nature of Siglec-15 ligands, but the exact nature of Siglec-15 ligand depends on the biological context, leaving plenty of room for further exploration.

Siglec-15 as an immune suppressor and potential target for normalization cancer immunotherapy

Overexpression of the B7-H1 (PD-L1) molecule in the tumor microenvironment (TME) is a major immune evasion mechanism in some patients with cancer, and antibody blockade of the B7-H1/PD-1 interaction can normalize compromised immunity without excessive side-effects. Using a genome-scale T cell activity array, we identified Siglec-15 as a critical immune suppressor. While only expressed on some myeloid cells normally, Siglec-15 is broadly upregulated on human cancer cells and tumor-infiltrating myeloid cells, and its expression is mutually exclusive to B7-H1, partially due to its induction by macrophage colony-stimulating factor and downregulation by IFN-γ. We demonstrate that Siglec-15 suppresses antigen-specific T cell responses in vitro and in vivo. Genetic ablation or antibody blockade of Siglec-15 amplifies anti-tumor immunity in the TME and inhibits tumor growth in some mouse models. Taken together, our results support Siglec-15 as a potential target for normalization cancer immunotherapy.

Therapeutic effect of Parmotrema tinctorum against complete Freund's adjuvant-induced arthritis in rats and identification of novel Isophthalic ester derivative

The present study reveals the anti-arthritic potential of traditionally used Parmotrema tinctorum (Pt) on experimental rats and purification of novel Isophthalic ester derivative. Arthritis was induced in rats using Freund's complete adjuvant (CFA) and subsequently treated with Pt extract (100 & 200 mg/kg.b.w). Assessment of antiarthritic activity was carried out using paw volume, arthritic score, haematological, biochemical, tissue antioxidant, histopathology and radiological analyses of ankle joints. The results revealed that continuous administration of Pt reduces the complication associated with arthritis by inhibiting the edema formation and arthritic score significantly (P < 0.05). The altered changes in biochemical parameters were brought back with an improvement in free radical scavenging ability after treatment with Pt significantly. Further purification of Pt using conventional column chromatography led to the isolation of four compounds and the structure of these isolated compounds were elucidated on the basis of spectral data's FT-IR, ¹H NMR, ¹³C NMR, DEPT-NMR spectroscopy, COSY & HSQC-NMR spectroscopy and LC-MS. The spectral data revealed that the three compounds were found to be Methyl-γ-Orsellinate, Atranorin, and usnic acid (CI-III) along with a novel secondary metabolite, (C-IV)4-Hydroxy-5-methyl-isophthalicacid3-(3,4-dihydroxy-5-methyl-phenyl)ester (C₁₆H₁₄O₇,318.1amu). The present study reveals that Parmotrema tinctorum reduces complications associated with arthritis and the compounds were isolated for the first time.

A Comprehensive Review of Immunoreceptor Regulation of Osteoclasts

Osteoclasts require coordinated co-stimulation by several signaling pathways to initiate and regulate their cellular differentiation. Receptor activator for NF-κB ligand (RANKL or TNFSF11), a tumor necrosis factor (TNF) superfamily member, is the master cytokine required for osteoclastogenesis with essential co-stimulatory signals mediated by immunoreceptor tyrosine-based activation motif (ITAM)-signaling adaptors, DNAX-associated protein 12 kDa size (DAP12) and FcεRI gamma chain (FcRγ). The ITAM-signaling adaptors do not have an extracellular ligand-binding domain and, therefore, must pair with ligand-binding immunoreceptors to interact with their extracellular environment. DAP12 pairs with a number of different immunoreceptors including triggering receptor expressed on myeloid cells 2 (TREM2), myeloid DAP12-associated lectin (MDL-1), and sialic acid-binding immunoglobulin-type lectin 15 (Siglec-15); while FcRγ pairs with a different set of receptors including osteoclast-specific activating receptor (OSCAR), paired immunoglobulin receptor A (PIR-A), and Fc receptors. The ligands for many of these receptors in the bone microenvironment remain unknown. Here, we will review immunoreceptors known to pair with either DAP12 or FcRγ that have been shown to regulate osteoclastogenesis. Co-stimulation and the effects of ITAM-signaling have turned out to be complex, and now include paradoxical findings that ITAM-signaling adaptor-associated receptors can inhibit osteoclastogenesis and immunoreceptor tyrosine-based inhibitory motif (ITIM) receptors can promote osteoclastogenesis. Thus, co-stimulation of osteoclastogenesis continues to reveal additional complexities that are important in the regulatory mechanisms that seek to maintain bone homeostasis.

Therapeutic effects of Caragana pruinosa Kom. roots extract on type II collagen-induced arthritis in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Caragana pruinosa Kom. is a deciduous shrub belonging to the genus of Caragana (Leguminosae), and Caragana plants exhibit a wide range of interesting pharmacological properties including anti-inflammatory, analgesic, and anti-arthritis activity, etc.

AIM OF THE STUDY

This study was aimed to investigate the anti-arthritic effect of 80% EtOH extract from the roots of C. pruinosa (ERCP) on arthritis and explore the potential pharmacological mechanism.

MATERIALS AND METHODS

After collagen induced arthritis (CIA) were established in rats, the animals were orally administered with ERCP (130, 260 and 520mg/kg) for 30 days. During the treatment, the rats' body weights, arthritis indices and paw volumes were measured every 5 days. Subsequently, rats' blood samples were collected to determine TNF-α, IL-1β, IL-6, IL-10, and C-reactive protein (CRP) contents in serum. Then, rats were sacrificed and the hind paws and knee joints were collected for histopathological examination.

RESULTS

Our results indicated that ERCP significantly suppressed the inflammatory reactions and destructions in joints and synovial tissues. ERCP inhibited the paw swelling and arthritis index in CIA rats. Additionally, it decreased the levels of pro-inflammatory cytokines (TNF-α, IL-1β and IL-6) and CRP, whereas increased that of IL-10.

CONCLUSION

Our results suggested ERCP has significant anti-arthritic effect on CIA rats, and the pharmacological mechanisms are related to the down-regulation of TNF-α, IL-1β, IL-6 and CRP and the up-regulation of IL-10.

Mediators of inflammation and bone remodeling in rheumatic disease

Remodeling of bone is a continuous process that occurs throughout life. Under normal physiologic conditions, bone-resorbing osteoclasts and bone-forming osteoblasts are tightly coupled and regulated to ensure proper balance, such that there is no net change in bone mass. However, inflammation perturbs normal bone homeostasis. The impact of inflammation on bone is dependent upon the anatomic site affected, cell types, factors and cytokines present in the local microenvironment, and local mechanical forces. Cytokines are central to the pathogenesis of inflammation-induced bone loss and contribute to the uncoupling of osteoclast-mediated bone resorption and osteoblast-mediated bone formation, thereby disrupting normal remodeling. In this review, we will discuss the effects of cytokines on bone in two settings, rheumatoid arthritis and spondyloarthritis, a disease category that includes ankylosing spondylitis, psoriatic arthritis, reactive arthritis, inflammatory bowel disease, and juvenile onset spondyloarthropathy. The outcome for bone in these disease settings is quite different, and an understanding of the pathogenic mechanisms leading to the net impact on bone has been essential in developing new therapeutic approaches to bone health in these diseases.