Inflammatory cytokines for osteoclastogenesis

Bone homeostasis is maintained by a balance between bone resorption by osteoclasts and bone formation by osteoblasts. Osteoclast maturation requires stimulation by RANKL on osteoblasts and various stimuli. Pro-inflammatory cytokines such as IL-1 and TNF-alpha cause an imbalance in bone metabolism by favouring bone resorption via the induction of RANKL on osteoblasts and induction of osteoclast maturation. These inflammatory signals originate from the immune system, the largest source of cell-derived regulatory signals and such immunological signals to the bone induce osteoclast maturation, resulting in secondary osteoporosis. Actually, such phenomena mainly occur at the interface between proliferating synovium and bone tissue in rheumatoid arthritis (RA). Thus, therapeutic strategies for these conditions, an anti-TNF-alpha antibody, effective for treating RA disease activity, also reduce secondary osteoporosis and joint destruction.     

Osteoporosis associated with rheumatoid arthritis

Patients with rheumatoid arthritis(RA) develop both generalized and periarticular osteoporosis. Both of them are believed to be associated with increased production of inflammatory cytokines(TNF alpha, IL-1 beta, IL-6) and increased formation and activation of osteoclasts. Whether glucocorticoids work positively or negatively on generalized/periarticular osteoporosis is still controversial. RANKL has been shown to be expressed on T cells and fibroblast-like cells in the synovium, thus 'RANKL-RANK' pathway is likely to play an important role in periarticular osteoporosis and bone erosion as well as generalized osteoporosis. Among various therapies for generalized/periarticular osteoporosis in RA, anti-cytokine antibodies/antagonists and osteoclast inhibitors including bisphosphonates are promising.     

Rheumatoid arthritis and osteoporosis: trends in their treatments

Rheumatoid arthritis (RA) is a systemic autoimmune disease characterized by chronic synovitis and bone damages, which consist of joint destruction and systemic osteoporosis. During the pathological process, pro-inflammatory cytokines such as TNF-alpha are largely produced from inflamed synovium and cause activation of osteoclasts deviated from bone remodeling cycle, resulting in joint destruction. On the other side, systemic osteoporosis, mainly caused by glucocorticoid (GC) is often complicated in RA, in which GC decreases number of osteoblasts, reduces synthesis of bone matrix proteins from them and enhances bone resorption, resulting in impairment of bone remodeling. Thus, joint destruction and systemic osteoporosis are brought about by different mechanisms. However, recent treatment strategies have improved managements of RA-related joint destruction as well as GC-induced osteoporosis. Treatments using biologics including infliximab and etanercept, effective for treating RA disease activity, also reduce joint destruction. Also, bisphosphonate is well known to be effective for not only treatment but also prevention of GC-induced osteoporosis. Thus, it is a clinical trend that physicians treat joint destruction as an inflammatory disease and osteoporosis as a metabolic disease.     

Bisphosphonate therapy for postmenopausal osteoporosis.

Bisphosphonates are agents that are potentially useful for treatment of osteoporosis. They are antiresorptive agents, increasing bone mass by decreasing the frequency of osteoclast activation or the depth of osteoclast resorption, or both. Intermittent cyclical therapy with etidronate has been shown to be effective for postmenopausal osteoporosis in two controlled studies. Several second- and third- generation bisphosphonates are undergoing active clinical trials.     

Mechanism of the bone destruction in rheumatoid arthritis

Rheumatoid arthritis (RA) is characterized by the presence of inflammatory synovitis accompanied by cartilage and bone destruction. Histological examination of RA pannus shows a number of osteoclasts on the surface of the destructed bone. RA synovial tissues produce a variety of proinflammatory cytokines and growth factors that may increase osteoclast formation, activity, and/or survival. Synovial fibroblasts from RA patients express high levels of RANKL, which is essential for the differentiation of osteoclasts, and therefore, RANKL can be a good therapeutic target of joint destruction in RA.     

Bone disease related to rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disorder characterized by progressive bone destruction, in which proinflammatory cytokines such as tumor necrosis factor-alpha play essential roles. Recent studies have revealed an important involvement of osteoclasts in bone destruction of RA. In this review, I would like to explain the molecular mechanism of osteoclast development in RA, and propose the possibility of anti-osteoclast therapy to the disease.     

Bisphosphonates: focus on inflammation and bone loss

Bisphosphonates are pharmacological compounds that have been used for the prevention and treatment of several pathological conditions including osteoporosis, primary hyperparathyroidism, osteogenesis imperfecta, and other conditions characterized by bone fragility. Many studies have been performed to date to analyze their effects on inflammation and bone remodelling and related pathologies. The aim of this review is, starting from a background on inflammatory processes and bone remodelling, to give an update on the use of bisphosphonates, outlining the possible side effects and proposing new trends for the future. Starting from a brief introduction on inflammation and bone remodelling, we collect and analyze studies involving the use of bisphosphonates for treatment of inflammatory conditions and pathologies characterized by bone loss. Selected articles, including reviews, published between 1976 and 2011, were chosen from Pubmed/Medline on the basis of their content. Bisphosphonates exert a selective activity on inflammation and bone remodelling and related pathologies, which are characterized by an excess in bone resorption. They improve not only skeletal defects, but also general symptoms. Bisphosphonates have found clinical application preventing and treating osteoporosis, osteitis deformans (Paget's disease of bone), bone metastasis (with or without hypercalcaemia), multiple myeloma, primary hyperparathyroidism, osteogenesis imperfecta, and other conditions that feature bone fragility. Further clinical studies involving larger cohorts are needed to optimize the dosage and length of therapy for each of these agents in each clinical field in order to be able to maximize their properties concerning modulation of inflammation and bone remodelling. In the near future, although "old" bisphosphonates will reach the end of their patent life, "new" bisphosphonates will be designed to specifically target a pathological condition.     

Induction of osteoclastogenesis and bone loss by human autoantibodies against citrullinated vimentin

Autoimmunity is complicated by bone loss. In human rheumatoid arthritis (RA), the most severe inflammatory joint disease, autoantibodies against citrullinated proteins are among the strongest risk factors for bone destruction. We therefore hypothesized that these autoantibodies directly influence bone metabolism. Here, we found a strong and specific association between autoantibodies against citrullinated proteins and serum markers for osteoclast-mediated bone resorption in RA patients. Moreover, human osteoclasts expressed enzymes eliciting protein citrullination, and specific N-terminal citrullination of vimentin was induced during osteoclast differentiation. Affinity-purified human autoantibodies against mutated citrullinated vimentin (MCV) not only bound to osteoclast surfaces, but also led to robust induction of osteoclastogenesis and bone-resorptive activity. Adoptive transfer of purified human MCV autoantibodies into mice induced osteopenia and increased osteoclastogenesis. This effect was based on the inducible release of TNF-α from osteoclast precursors and the subsequent increase of osteoclast precursor cell numbers with enhanced expression of activation and growth factor receptors. Our data thus suggest that autoantibody formation in response to citrullinated vimentin directly induces bone loss, providing a link between the adaptive immune system and bone.     

Involvement of ICAM-1 in bone metabolism: a potential target in the treatment of bone diseases?

Bone diseases such as osteoporosis, osteoarthritis and rheumatoid arthritis (RA) affect a great proportion of individuals, with debilitating consequences in terms of pain and progressive limitation of function. Existing treatment of these pathologies has been unable to alter the natural evolution of the disease and, as such, a clearer understanding of the pathophysiology is necessary in order to generate new treatment alternatives. One therapeutic strategy could involve the targeting of intercellular adhesion molecule-1 (ICAM-1; CD54). In bone, ICAM-1 is expressed at the surface of osteoblasts (Obs) and its counter-receptor, leukocyte function-associated antigen-1 (LFA-1; CD11a), at the surface of osteoclast (Oc) precursors. ICAM-1 blockade between the Ob and the pre-Oc results in an inhibition of Oc recruitment and a modulation of inflammation, which could potentially help in controlling disease activity in bone pathologies. So far, clinical studies on ICAM-1 blockade in bone diseases have been limited to RA. A better understanding of the implication of this adhesion molecule in Ob/Oc interactions and inflammatory mediation in the bone pathological state, however, is needed. As new discoveries on the role of this adhesion molecule are being reported, ICAM-1 could become a potential target for other bone diseases as well.     

Involvement of ICAM-1 in bone metabolism: a potential target in the treatment of bone diseases?

Bone diseases such as osteoporosis, osteoarthritis and rheumatoid arthritis (RA) affect a great proportion of individuals, with debilitating consequences in terms of pain and progressive limitation of function. Existing treatment of these pathologies has been unable to alter the natural evolution of the disease and, as such, a clearer understanding of the pathophysiology is necessary in order to generate new treatment alternatives. One therapeutic strategy could involve the targeting of intercellular adhesion molecule-1 (ICAM-1; CD54). In bone, ICAM-1 is expressed at the surface of osteoblasts (Obs) and its counter-receptor, leukocyte function-associated antigen-1 (LFA-1; CD11a), at the surface of osteoclast (Oc) precursors. ICAM-1 blockade between the Ob and the pre-Oc results in an inhibition of Oc recruitment and a modulation of inflammation, which could potentially help in controlling disease activity in bone pathologies. So far, clinical studies on ICAM-1 blockade in bone diseases have been limited to RA. A better understanding of the implication of this adhesion molecule in Ob/Oc interactions and inflammatory mediation in the bone pathological state, however, is needed. As new discoveries on the role of this adhesion molecule are being reported, ICAM-1 could become a potential target for other bone diseases as well.     

Absence of platelet-activating factor receptor protects mice from osteoporosis following ovariectomy

While platelet-activating factor (PAF) is produced in various diseases associated with bone resorption, its functions in bone metabolism remain unknown. Using PAF receptor-deficient mice, we evaluated the role of PAF in the development of bone resorption following ovariectomy, a model of postmenopausal osteoporosis. Through observations of bone mineral density and histomorphometric parameters, it was found that bone resorption was markedly attenuated in PAF receptor-deficient mice, indicating that PAF links estrogen depletion and osteoporosis in vivo. Osteoclasts expressed higher amounts of the enzymes required for PAF biosynthesis than osteoblasts. TNF-alpha and IL-1beta increased the acetyl-coenzyme A:lyso-PAF acetyltransferase activity in osteoclasts. Osteoclasts, but not osteoblasts, expressed the functional PAF receptor. PAF receptor stimulation prolonged the survival of osteoclasts in vitro. Furthermore, osteoclasts treated with a PAF receptor antagonist, and also those from PAF receptor-deficient mice, showed reductions in survival rate and Ca resorption activity. Consistently, in organ cultures, bone resorption was significantly suppressed by a PAF receptor antagonist treatment or genetic PAF receptor deficiency. Thus, these results suggest that, through the inflammatory cytokines, estrogen depletion enhances PAF production as a unique autocrine factor for osteoclast functions. Inhibition of PAF function might pave the way for a new strategy to prevent postmenopausal bone loss without disturbing osteoblast functions.     

Parathyroid hormone and lipopolysaccharide induce murine osteoblast-like cells to secrete a cytokine indistinguishable from granulocyte-macrophage colony-stimulating factor.

Osteoblasts are the cells responsible for the secretion of collagen and ultimately the formation of new bone. These cells have also been shown to regulate osteoclast activity by the secretion of cytokines, which remain to be defined. In an attempt to identify these unknown cytokines, we have induced primary murine osteoblasts with two bone active agents, parathyroid hormone (PTH) and lipopolysaccharide (LPS) and analyzed the conditioned media (CM) for the presence of specific cytokines. Analysis of the CM was accomplished by functional, biochemical, and serological techniques. The data indicate that both PTH and LPS are capable of inducing the osteoblasts to secrete a cytokine, which by all of the techniques used, is indistinguishable from granulocyte-macrophage colony-stimulating factor (GM-CSF). Secretion of GM-CSF is not constitutive and requires active induction. Production of the cytokine is dependent on the dose of PTH or LPS added. It has been demonstrated that the addition of GM-CSF to bone marrow cultures results in the formation of increased numbers of osteoclasts. Therefore, these data suggest that osteoblasts not only participate in bone remodeling by formation of new matrix but may regulate osteoclast activity indirectly by their ability to regulate hematopoiesis.     

Inflammatory arthritis increases mouse osteoclast precursors with myeloid suppressor function

Increased osteoclastic bone resorption leads to periarticular erosions and systemic osteoporosis in RA patients. Although a great deal is known about how osteoclasts differentiate from precursors and resorb bone, the identity of an osteoclast precursor (OCP) population in vivo and its regulatory role in RA remains elusive. Here, we report the identification of a CD11b^–/loLy6C^hi BM population with OCP activity in vitro and in vivo. These cells, which can be distinguished from previously characterized precursors in the myeloid lineage, display features of both M1 and M2 monocytes and expand in inflammatory arthritis models. Surprisingly, in one mouse model of RA (adoptive transfer of SKG arthritis), cotransfer of OCP with SKG CD4⁺ T cells diminished inflammatory arthritis. Similar to monocytic myeloid-derived suppressor cells (M-MDSCs), OCPs suppressed CD4⁺ and CD8⁺ T cell proliferation in vitro through the production of NO. This study identifies a BM myeloid precursor population with osteoclastic and T cell–suppressive activity that is expanded in inflammatory arthritis. Therapeutic strategies that prevent the development of OCPs into mature bone-resorbing cells could simultaneously prevent bone resorption and generate an antiinflammatory milieu in the RA joint.     

NF-κB2 (p100) limits TNF-α–induced osteoclastogenesis

Bone undergoes a continuous cycle of renewal, and osteoclasts — the cells responsible for bone resorption — play a pivotal role in bone homeostasis. This resorption is largely mediated by inflammatory cytokines such as TNF-α. In this issue of the JCI, Yao et al. demonstrate that the NF-κB precursor protein NF-κB2 (p100) acts as a negative regulator of osteoclastogenesis (see the related article beginning on page 3024). TNF-α induced a sustained accumulation of p100 in osteoclast precursors, and TNF-α–induced osteoclast formation was markedly increased in Nfkb2^–/– mice. They also found that TNF receptor–associated factor 3 (TRAF3) is involved in the posttranslational regulation of p100 expression. These results suggest that blockade of the processing of p100 is a novel strategy to treat TNF-α–related bone diseases such as RA. Osteoclasts are primarily implicated in physiologic and pathologic bone resorption. They are derived from hematopoietic stem cells, and their differentiation is critically regulated by supporting cells and cytokines. The important cytokines regulating osteoclast differentiation are M-CSF and RANKL (1). In particular, the central role of RANKL, which belongs to the TNF-α superfamily, has been demonstrated by the fact that the targeted disruption of Rankl or its receptor Rank induced severe osteopetrosis (increased bone density) in mice due to the complete lack of osteoclasts, while knockout of osteoprotegerin, a natural antagonist of RANKL, conversely caused marked bone loss (2). The remarkable clinical success of anti–TNF-α therapies such as anti–TNF-α antibody and soluble TNF receptor has established a critical role of TNF-α in inflammatory diseases such as RA and Crohn disease (3). Anti–TNF-α strategies not only ameliorate the inflammatory conditions in these disorders but also suppress bone erosion in RA, indicating an essential role of TNF-α in the pathologic bone destruction. However, in spite of such strong clinical evidence, the relationship between TNF-α and RANKL signaling in osteoclast development is not necessarily clear. The osteoclastogenic effect of TNF-α independent of RANKL has been particularly controversial. Although several studies have demonstrated that TNF-α directly promotes osteoclast formation in vitro in the absence of RANKL (4–6), the ability of TNF-α–induced osteoclast formation is limited, and the administration of TNF-α does not induce osteoclast formation in Rank-deficient mice in vivo (7). This may be at least partly because RANK, but not TNF receptor 1 or TNF receptor 2, recruits the adaptor molecule TNF receptor–associated factor 6 (TRAF6), which is essential for osteoclast development. However, the possibility that molecule(s) induced by TNF-α negatively regulate osteoclast differentiation has not been excluded.     

Circulating levels of osteoclast activating cytokines, interleukin-11 and transforming growth factor-beta2, as valuable biomarkers for the assessment of bone turnover in postmenopausal osteoporosis

The objective of this study was to evaluate the role of osteoclast activating cytokines, interleukin-11 (IL-11) and transforming growth factor-beta2 (TGF-beta2) in the assessment of bone turnover in postmenopausal osteoporosis (PO). Eighty postmenopausal osteoporotic women with lumbar spine bone mineral densities (BMD) as measured by DEXA that were more than 2.5 SD below the normal mean of healthy women (controls), participated in this study. Various therapeutic modalities (hormone replacement therapy, HRT, alendronate, calcitonin and 1alpha-hydroxyvitamin D (alfacalcidol) were administered for 12 months to 4 groups of postmenopausal osteoporotic patients. Fasting blood samples and two hour urine samples were collected from control subjects and from patients before and after treatment. Serum samples were assayed for IL-11, TGF-beta2, osteocalcin (OC) and bone alkaline phosphatase (B-ALP), whereas urine samples were assayed for N-telopeptide for type I collagen (NTX) and deoxypyridinoline (DPyr). The results demonstrated a significant increase of both IL-11 and TGF-beta2 in postmenopausal osteoporosis. Positive correlations exist between TGF-beta2 or IL-11 and markers of bone resorption (NTX and DPyr). Moreover, there was a significant positive correlation between TGF-beta2 and IL-11. Therapeutic modalities enhancing bone formation and/or with antiresorptive effect revealed a significant decrease in markers of bone resorption, formation and osteoclast activating cytokines, indicating a decrease in bone turnover. The decrease of IL-11 and TGF-beta2 may be attributed to a drug inhibitory effect of these cytokines on enhancing osteoblast mediated osteoid degradation. In conclusion, both serum IL-11 and TGF-beta2 determinations may be considered as biomarkers for the assessment of bone turnover and for monitoring antiresorptive therapy in postmenopausal osteoporosis.     

Role of the immune system in the pathophysiology of postmenopausal osteoporosis

The immune system plays a critical role in the pathophysiology of postmenopausal osteoporosis. Estrogen deficiency induces mild increase in production of proinflammatory cytokines, such as IL-1, IL-6, and TNF-alpha, systemically and locally, which promotes osteoclastogenesis in the bone marrow. The most important cytokine in the context of estrogen deficiency-induced bone loss has been shown to be TNF-alpha produced by bone marrow T-lymphocytes that are stimulated through a complex mechanism involving antigen-presenting cells and various cytokines including IL-7, IFN-gamma, and TGF-beta. Proinflammatory cytokines also suppress osteoblastogenesis, while the crosstalk between T-lymphocytes and marrow stromal cells regulates the osteoclastogenic activity of the latter. Counteracting these interactions could be a novel strategy for osteoporosis treatment.     

A bio-assay for effectors of osteoclast differentiation in serum from patients with bone disease

Osteoclast differentiation and activity, and hence bone loss, depend on two opposing cytokines. Receptor activator of NF-(kappa)B ligand (RANKL) produced by osteoblasts and T-cells stimulates, while osteoprotegerin inhibits. Both of these cytokines are found in serum. Our aim was to develop a functional assay for any factors present in human serum that can affect osteoclast differentiation and to assess whether any such factors vary in diseases in which bone loss occurs. METHODS: Using a culture model of osteoclast differentiation in the presence of macrophage colony stimulating factor and soluble RANKL, we have measured the effects of different human sera on osteoclast differentiation. The production of a marker enzyme for the osteoclast, tartrate-resistant acid phosphatase (TRAP), was used to follow osteoclast differentiation. RESULTS: In general, human serum stimulates osteoclast differentiation as indicated by TRAP activity, but in patients with low bone density this stimulation was attenuated. Sera from 40 female subjects with low bone mineral density showed significantly lower TRAP cell differentiation activity than sera from the healthy female controls. CONCLUSION: We describe a functional bio-assay for factors in human serum which can affect osteoclast differentiation. This assay may have application in monitoring the effects of therapy in bone disease.     

New therapeutic agents for the treatment of bone diseases

Bisphosphonates have demonstrated important clinical benefits for patients with malignant bone disease, metabolic bone diseases, such as Paget's disease, and postmenopausal osteoporosis. The introduction of nitrogen-containing bisphosphonates with high affinity for hydroxyapatite in bone represents an important advancement. These agents are now a standard of care for osteoporosis, Paget's disease, osteogenesis imperfecta, primary bone lesions from multiple myeloma and bone metastases from breast cancer. Moreover, the recent clinical development of zoledronic acid (4 mg by 15-minute intravenous infusion) has expanded the benefits of bisphosphonate therapy to patients with bone metastases from any solid tumour. Bisphosphonates are also being investigated at present for the prevention of bone loss resulting from cancer therapy. In addition, a variety of novel biologic agents, receptor activator of nuclear factor-kappaB (RANK) ligand antibodies, osteoprotegerin and cathepsin K inhibitors are being investigated at present for the treatment of malignant bone disease. The management of bone health is an important area of active research, and the armamentarium and role of bone-specific therapies continue to expand.     

Prospects of treatment using interferon for bone diseases

Osteoblasts and osteoclasts produce a variety of cytokines to maintain bone homeostasis. One of the most important cytokines, receptor activator of nuclear factor-IkappaB ligand (RANKL), is essential for osteoclastogenesis. Recently, it was shown that activated T cells promote osteoclastogenesis through RANKL expression and also negatively affect osteoclastogenesis through interferon (IFN)-gamma production. Additionally, it was revealed that IFN-beta was involved in osteoclast regulation by signaling cross-talk with RANKL and that it contributed to the maintenance of normal bone mass. These studies indicate that there are complex regulatory interactions between bone-remodeling cells and immune cells, which depend on the balance between RANKL and IFN. Thus, the interaction between T cells and bone cells could be physiologically critical for the maintenance of normal bone metabolism, and IFN might be an attractive cytokine for use in therapy for bone disease in pathological bone resorptive conditions such as rheumatoid arthritis, osteoporosis, osteomyelitis and bone metastasis of cancers.