Mechanisms modulating inflammatory osteolysis: a review with insights into therapeutic targets

Inflammatory osteolysis is a relatively frequent and incapacitating complication of rheumatoid arthritis and multiple other inflammation-associated bone diseases. It is thought to operate through an ultimate common pathway of accelerated osteoclast recruitment and activation under the control of cytokines produced in the inflammatory environment. Over the past decade, there have been major advances in our understanding of the mechanisms of osteoclastogenesis. It is now clear that the interaction of receptor activator NF-kappaB (RANK) and its ligand, RANKL, plays a central role in osteoclast formation and activity. Therefore, understanding osteoclastogenesis offers new pathways for potential therapeutic intervention in inflammatory osteolysis. The success of anti-tumor necrosis factor-alpha and interleukin-1 therapy highlights the central role that these specific cytokines play in this disease. This review outlines our current understanding of the mechanisms mediating inflammatory osteolysis and highlights potential therapeutic strategies.

Inhibition of monocyte chemoattractant protein-1 ameliorates rat adjuvant-induced arthritis

Chemokines, including RANTES/CCL5 and MCP-1/CCL2, are highly expressed in the joints of patients with rheumatoid arthritis, and they promote leukocyte migration into the synovial tissue. This study was conducted to determine whether the inhibition of RANTES and MCP-1 therapeutically was capable of ameliorating rat of adjuvant-induced arthritis (AIA). Postonset treatment of AIA using a novel inhibitor for endogenous MCP-1 (P8A-MCP-1) improved clinical signs of arthritis and histological scores measuring joint destruction, synovial lining, macrophage infiltration, and bone erosion. Using immunohistochemistry, ELISA, real-time RT-PCR, and Western blot analysis, we defined joint inflammation, bony erosion, monocyte migration, proinflammatory cytokines, and bone markers, and p-p38 levels were reduced in rat AIA treated with P8A-MCP-1. In contrast, neither the dominant-negative inhibitor for endogenous RANTES (44AANA47-RANTES) nor the CCR1/CCR5 receptor antagonist, methionylated-RANTES, had an effect on clinical signs of arthritis when administered after disease onset. Additionally, therapy with the combination of 44AANA47-RANTES plus P8A-MCP-1 did not ameliorate AIA beyond the effect observed using P8A-MCP-1 alone. Treatment with P8A-MCP-1 reduced joint TNF-alpha, IL-1beta, and vascular endothelial growth factor levels. P8A-MCP-1 also decreased p38 MAPK activation in the joint. Our results indicate that inhibition of MCP-1 with P8A-MCP-1 after the onset of clinically detectable disease ameliorates AIA and decreases macrophage accumulation, cytokine expression, and p38 MAPK activation within the joint.

Highly crosslinked vs conventional polyethylene particles--an in vitro comparison of biologic activities

Highly cross-linked polyethylenes (HXPEs) have been introduced to reduce wear after hip arthroplasty. The improved wear characteristics of HXPEs are well documented, but the relative biologic activity of HXPE and conventional polyethylene (CPE) particles remains unclear. Longevity (Zimmer, Warsaw, Ind; HXPE) and GUR 1050 (Zimmer; CPE) particles were isolated and characterized from a hip simulator and their in vitro inflammatory responses (tissue necrosis factor *, interleukin 1*, and vascular endothelial growth factor levels) were compared using macrophages. The average diameter of Longevity particles (0.111 microm) was smaller than CPE particles (0.196 microm), and both were predominantly round (granular appearance). The inflammatory response to HXPE and CPE was concentration-dependent. No statistically significant differences were noted at low (0.1 surface area ratio [SAR]) and intermediate (0.75 SAR) doses. At the highest dose tested (2.5 SAR), HXPE was significantly more inflammatory than CPE based on relative tissue necrosis factor alpha and vascular endothelial growth factor secretion levels. Further study is needed to determine if similar findings would be noted in vivo over a broad concentration range.

HMGB1 regulates RANKL-induced osteoclastogenesis in a manner dependent on RAGE

High-mobility group box 1 (HMGB1), a nonhistone nuclear protein, is released by macrophages into the extracellular milieu consequent to cellular activation. Extracellular HMGB1 has properties of a pro-inflammatory cytokine through its interaction with receptor for advanced glycation endproducts (RAGE) and/or toll-like receptors (TLR2 and TLR4). Although HMGB1 is highly expressed in macrophages and differentiating osteoclasts, its role in osteoclastogenesis remains largely unknown. In this report, we present evidence for a function of HMGB1 in this event. HMGB1 is released from macrophages in response to RANKL stimulation and is required for RANKL-induced osteoclastogenesis in vitro and in vivo. In addition, HMGB1, like other osteoclastogenic cytokines (e.g., TNFalpha), enhances RANKL-induced osteoclastogenesis in vivo and in vitro at subthreshold concentrations of RANKL, which alone would be insufficient. The role of HMGB1 in osteoclastogenesis is mediated, in large part, by its interaction with RAGE, an immunoglobin domain containing family receptor that plays an important role in osteoclast terminal differentiation and activation. HMGB1-RAGE signaling seems to be important in regulating actin cytoskeleton reorganization, thereby participating in RANKL-induced and integrin-dependent osteoclastogenesis. Taken together, these observations show a novel function of HMGB1 in osteoclastogenesis and provide a new link between inflammatory mechanisms and bone resorption.

Osteoimmunology: interactions of the bone and immune system

Bone and the immune system are both complex tissues that respectively regulate the skeleton and the body's response to invading pathogens. It has now become clear that these organ systems often interact in their function. This is particularly true for the development of immune cells in the bone marrow and for the function of bone cells in health and disease. Because these two disciplines developed independently, investigators in each don't always fully appreciate the significance that the other system has on the function of the tissue they are studying. This review is meant to provide a broad overview of the many ways that bone and immune cells interact so that a better understanding of the role that each plays in the development and function of the other can develop. It is hoped that an appreciation of the interactions of these two organ systems will lead to better therapeutics for diseases that affect either or both.

Dried plum polyphenols attenuate the detrimental effects of TNF-alpha on osteoblast function coincident with up-regulation of Runx2, Osterix and IGF-I

Previous studies have demonstrated that dried plums which contain high amounts of polyphenols can restore bone mass and structure, and significantly increase indices of bone formation. The purpose of this study was to determine how dried plum polyphenols influence osteoblast activity and mineralized nodule formation under normal and inflammatory conditions. MC3T3-E1 cells were plated and pretreated with dried plum polyphenols (0, 2.5, 5, 10 and 20 microg/ml) and 24 h later stimulated with TNF-alpha (0 or 1.0 ng/ml). The 5, 10 and 20 microg/ml doses of polyphenols significantly increased intracellular ALP activity under normal conditions at 7 and 14 days, and restored the TNF-alpha-induced suppression of intracellular ALP activity by 14 days (P<.001). Polyphenols also increased mineralized nodule formation under normal and inflammatory conditions. In the absence of TNF-alpha, 5 microg/ml of polyphenols significantly up-regulated the growth factor, IGF-I, compared to controls, and the 5 and 10 microg/ml doses increased the expression of lysyl oxidase involved in collagen crosslinking. TNF-alpha decreased the expression of Runx2, Osterix and IGF-I, and polyphenols restored their mRNA levels to that of the controls. Although TNF-alpha failed to alter lysyl oxidase at 18 h, the polyphenols up-regulated its expression (P<.05) in the presence of TNF-alpha. As expected, TNF-alpha up-regulated RANKL mRNA and polyphenols suppressed RANKL expression without altering OPG. Based on these findings, we conclude that dried plum polyphenols enhance osteoblast activity and function by up-regulating Runx2, Osterix and IGF-I and increasing lysyl oxidase expression, and at the same time attenuate osteoclastogenesis signaling.

The osteoclast: friend or foe?

Bone is a dynamic organ constantly remodeled to support calcium homeostasis and structural needs. The osteoclast is the cell responsible for removing both the organic and inorganic components of bone. It is derived from hematopoietic progenitors in the macrophage lineage and differentiates in response to the tumor necrosis factor family cytokine receptor activator of NF kappa B ligand. alpha v beta 3 integrin mediates cell adhesion necessary for polarization and formation of an isolated, acidified resorptive microenvironment. Defects in osteoclast function, whether genetic or iatrogenic, may increase bone mass but lead to poor bone quality and a high fracture risk. Pathological stimulation of osteoclast formation and resorption occurs in postmenopausal osteoporosis, inflammatory arthritis, and metastasis of tumors to bone. In these diseases, osteoclast activity causes bone loss that leads to pain, deformity, and fracture. Thus, osteoclasts are critical for normal bone function, but their activity must be controlled.

Molecular mechanisms of inflammatory bone damage: emerging targets for therapy

Chronic inflammatory bone diseases, such as rheumatoid arthritis (RA), ankylosing spondylitis and periodontal disease, demonstrate the major impact of chronic inflammation on both bone metabolism and bone architecture. During the past decade, scientists have gained increasing insight into the link between inflammation and bone. As a result of new discoveries about the molecular mechanisms of inflammatory bone loss, several molecules have been identified that are attractive and novel targets for the treatment of inflammatory bone loss. These novel therapeutic approaches include anti-tumor necrosis factor (TNF)-alpha blocking agents, neutralizing antibodies against certain pro-inflammatory cytokines, such as interleukin (IL)-6 and IL-17, and a set of other promising targets that still require extensive research, such as the Wnt signaling network.

Lymphocyte-derived cytokines in inflammatory arthritis

Inflammatory bone loss is observed in a number of disorders including rheumatoid arthritis (RA), osteoporosis and periodontal disease. Lymphocytes are key components in the onset and exacerbation of autoimmune diseases and the cytokines produced by these cells have a powerful impact on disease progression. Many cytokines implicated in inflammation impact upon osteoclast (OCL) differentiation and function either directly or indirectly by modulating the relative expression of RANKL and OPG. This review highlights the contribution of lymphocyte-derived cytokines to the bone loss observed in RA and other autoimmune disorders. A greater understanding of the cytokines involved in these disorders will ultimately lead to the identification of novel therapeutic strategies for the prevention of bone loss in these diseases.

Highly conserved surface proteins of oral spirochetes as adhesins and potent inducers of proinflammatory and osteoclastogenic factors

Oral spirochetes include enormously heterogeneous Treponema species, and some have been implicated in the etiology of periodontitis. In this study, we characterized highly conserved surface proteins in four representative oral spirochetes (Treponema denticola, T. lecithinolyticum, T. maltophilum, and T. socranskii subsp. socranskii) that are homologs of T. pallidum Tp92, with opsonophagocytic potential and protective capacity against syphilis. Tp92 homologs of oral spirochetes had predicted signal peptides (20 to 31 amino acids) and molecular masses of 88 to 92 kDa for mature proteins. They showed amino acid sequence identities of 37.9 to 49.3% and similarities of 54.5 to 66.9% to Tp92. The sequence identities and similarities of Tp92 homologs of oral treponemes to one another were 41.6 to 71.6% and 59.9 to 85.6%, respectively. The tp92 gene homologs were successfully expressed in Escherichia coli, and the recombinant proteins were capable of binding to KB cells, an epithelial cell line, and inhibited the binding of the whole bacteria to the cells. Antiserum (the immunoglobulin G fraction) raised against a recombinant form of the T. denticola Tp92 homolog cross-reacted with homologs from three other species of treponemes. The Tp92 homologs stimulated various factors involved in inflammation and osteoclastogenesis, like interleukin-1beta (IL-1beta), tumor necrosis factor alpha, IL-6, prostaglandin E(2), and matrix metalloproteinase 9, in host cells like monocytes and fibroblasts. Our results demonstrate that Tp92 homologs of oral spirochetes are highly conserved and may play an important role in cell attachment, inflammation, and tissue destruction. The coexistence of various Treponema species in a single periodontal pocket and, therefore, the accumulation of multiple Tp92 homologs may amplify the pathological effect in periodontitis.

Polymethylmethacrylate particles stimulate bone resorption of mature osteoclasts in vitro

BACKGROUND

Interaction between wear particle debris and the cells at the implant-bone interface is an important contributory factor to periprosthetic bone loss seen in arthroplasties.

METHOD

To investigate the effect of this particle-induced response on different stages of osteoclast maturation, polymethylmethacrylate (PMMA) particles were added to a murine osteoclastogenic bone marrow cell culture system at either day 0, day 4, or day 8 of culture, which represented PMMA particle stimulation of precursor osteoclasts, mature osteoclasts, or end-stage osteoclasts, respectively. The number of TRAP-posi-tive multinucleate cells (MNCs) and the degree of bone resorption in culture were measured

RESULTS

Treatment of precursor osteoclasts with PMMA particles resulted in a statistically significant increase in TRAP-positive MNCs that persisted for 4 days, but there was no significant increase in bone resorption. Addition of particles to mature osteoclasts resulted in a significant increase in the number of TRAP-positive MNCs that lasted for 8 days, and also a significant increase in bone resorption. Treatment of end-stage osteoclasts with PMMA particles did not result in an increased number of TRAP-positive MNCs and there was no increase in bone resorption.

INTERPRETATION

Treatment of mature osteoclasts with PMMA particles resulted in an elevated number of TRAP-positive cells. This persisted over a longer period of time than at the other stages of osteoclast development, and there was also a greater increase in bone resorption.

Receptor activator of nuclear factor kappaB ligand and osteoprotegerin regulation of bone remodeling in health and disease

Osteoclasts and osteoblasts dictate skeletal mass, structure, and strength via their respective roles in resorbing and forming bone. Bone remodeling is a spatially coordinated lifelong process whereby old bone is removed by osteoclasts and replaced by bone-forming osteoblasts. The refilling of resorption cavities is incomplete in many pathological states, which leads to a net loss of bone mass with each remodeling cycle. Postmenopausal osteoporosis and other conditions are associated with an increased rate of bone remodeling, which leads to accelerated bone loss and increased risk of fracture. Bone resorption is dependent on a cytokine known as RANKL (receptor activator of nuclear factor kappaB ligand), a TNF family member that is essential for osteoclast formation, activity, and survival in normal and pathological states of bone remodeling. The catabolic effects of RANKL are prevented by osteoprotegerin (OPG), a TNF receptor family member that binds RANKL and thereby prevents activation of its single cognate receptor called RANK. Osteoclast activity is likely to depend, at least in part, on the relative balance of RANKL and OPG. Studies in numerous animal models of bone disease show that RANKL inhibition leads to marked suppression of bone resorption and increases in cortical and cancellous bone volume, density, and strength. RANKL inhibitors also prevent focal bone loss that occurs in animal models of rheumatoid arthritis and bone metastasis. Clinical trials are exploring the effects of denosumab, a fully human anti-RANKL antibody, on bone loss in patients with osteoporosis, bone metastasis, myeloma, and rheumatoid arthritis.

Bavachalcone inhibits osteoclast differentiation through suppression of NFATc1 induction by RANKL

Osteoclasts are cells that have a specialized role for bone resorption and are responsible for many bone diseases such as osteoporosis. As herbal products are invaluable sources in discovery of compounds for new therapies, we sought to identify compounds efficacious in suppressing osteoclastogenesis from medicinal plants that have been implicated for treatment of osteoporotic conditions. Bavachalcone was isolated from Psoralea corylifolia, and its effects on osteoclast differentiation were evaluated with primary cultures of osteoclast precursor cells. In addition, the molecular mechanism of action was investigated. Bavachalcone inhibited osteoclast formation from precursor cells with the IC(50) of approximately 1.5 microg ml(-1). The activation of MEK, ERK, and Akt by receptor activator of nuclear factor kappaB ligand (RANKL), the osteoclast differentiation factor, was prominently reduced in the presence of bavachalcone. The induction of c-Fos and NFATc1, key transcription factors for osteoclastogenesis, by RANKL was also suppressed by bavachalcone. In conclusion, bavachalcone inhibits osteoclastogenesis by interfering with the ERK and Akt signaling pathways and the induction of c-Fos and NFATc1 during differentiation. Our results suggest that bavachalcone may be useful as a therapeutic drug for bone resorption-associated diseases.

VEGF-C, a lymphatic growth factor, is a RANKL target gene in osteoclasts that enhances osteoclastic bone resorption through an autocrine mechanism

Osteoclasts are bone-resorbing cells, but they also secrete and respond to cytokines. Here, we test the hypothesis that osteoclasts secrete the lymphatic growth factor, VEGF-C, to increase their resorptive activity. Osteoclasts and osteoclast precursors were generated by culturing splenocytes with macrophage colony-stimulating factor and RANKL from wild-type, NF-kappaBp50(-/-)/p52(-/-), and Src(-/-) mice. Expression of VEGFs was measured by real time reverse transcription-PCR, Western blotting, and immunostaining. The effect of VEGF-C signaling on osteoclast function was determined by osteoclastogenesis and pit assays. RANKL increased the expression of VEGF-C but not of other VEGFs in osteoclasts and their precursors. RANKL-induced VEGF-C expression was reduced in NF-kappaBp50(-/-)/p52(-/-) precursors or wild-type cells treated with an NF-kappaB inhibitor. VEGF-C directly stimulated RANKL-mediated bone resorption, which was reduced by the VEGF-C-specific receptor blocker, VEGFR3:Fc. Osteoclasts express VEGFR3, and VEGF-C stimulated Src phosphorylation in osteoclasts. VEGF-C-mediated bone resorption was abolished in Src(-/-) osteoclasts or cells treated with an Src inhibitor. We conclude that RANKL stimulates osteoclasts and their precursors to release VEGF-C through an NF-kappaB-dependent mechanism, indicating that VEGF-C is a new RANKL target gene in osteoclasts and functions as an autocrine factor regulating osteoclast activity.

Osteoclast precursor interaction with bone matrix induces osteoclast formation directly by an interleukin-1-mediated autocrine mechanism

Interleukin-1 (IL-1) and tumor necrosis factor (TNF) mediate bone resorption in a variety of diseases affecting bone. Like TNF, IL-1 is secreted by osteoclast precursors (OCPs), but unlike TNF, it does not induce osteoclast formation directly from OCPs in vitro. TNF induces IL-1 expression and activates c-Fos, a transcription factor required in OCPs for osteoclast formation. Here, we examined whether IL-1 can induce osteoclast formation directly from OCPs overexpressing c-Fos and whether interaction with bone matrix affects OCP cytokine expression. We infected OCPs with c-Fos or green fluorescent protein retrovirus, cultured them with macrophage colony-stimulating factor and IL-1 on bone slices or plastic dishes, and assessed osteoclast and resorption pit formation and expression of IL-1 by OCPs. We used a Transwell assay to determine whether OCPs secrete IL-1 when they interact with bone matrix. IL-1 induced osteoclast formation directly from c-Fos-expressing OCPs on plastic. c-Fos-expressing OCPs formed osteoclasts spontaneously on bone slices without addition of cytokines. OCPs on bone secreted IL-1, which induced osteoclast formation from c-Fos-expressing OCPs in the lower Transwell dishes. The bone matrix proteins dentin sialoprotein and osteopontin, but not transforming growth factor-beta, stimulated OCP expression of IL-1 and induced c-Fos-expressing OCP differentiation into osteoclasts. Osteoclasts eroding inflamed joints have higher c-Fos expression compared with osteoclasts inside bone. We conclude that OCPs expressing c-Fos may induce their differentiation directly into osteoclasts by an autocrine mechanism in which they produce IL-1 through interaction with bone matrix. TNF could induce c-Fos expression in OCPs at sites of inflammation in bone to promote this autocrine mechanism and thus amplify bone loss.

Pathogenesis of calciphylaxis: Hans Selye to nuclear factor kappa-B

The clinical syndrome of calciphylaxis is characterized by arteriolar medial calcification, thrombotic cutaneous ischemia, necrotic skin ulceration, and a high mortality rate. This review integrates calciphylaxis risk factors with the molecular processes governing osseous and extraosseous mineralization. As the pathogenesis of calciphylaxis is better understood, targeted therapies aimed at disease prevention and reversal will follow.

Inflammatory osteoclastogenesis can be induced by GM-CSF and activated under TNF immunity

In inflammatory arthritis such as RA, osteoclastic activity is severely enhanced. GM-CSF was reportedly elevated in synovial fluid, but is a strong inhibitor of osteoclastogenesis; here lies a contradiction. Our objective was to examine what type of osteoclasts generate and resorb bone with resistance to GM-CSF in an inflammatory joint. Monocyte-derived cells generated in GM-CSF were morphologically and immunophenotypically different from both the conventional DC and macrophage. They could differentiate into osteoclasts in the presence of RANKL + M-CSF, acquiring a stronger osteoclastic activity under TNF treatment. Furthermore, their differentiation was not inhibited by GM-CSF, while monocyte-derived osteoclast differentiation was completely inhibited. The resorption was suppressed by GM-CSF, and the existence of another osteoclastic pathway has been suggested. Our findings indicate another type of osteoclast exists in inflammatory arthritis.

Evaluation of bone and mineral metabolism in patients recently diagnosed with leprosy

This study was conducted to evaluate patients recently diagnosed with the tuberculoid and lepromatous forms of leprosy for bone mass, bone remodeling, and hormones related to mineral control. Eleven normal control individuals (CG) and 12 patients with leprosy (LG) matched for physical characteristics were submitted to evaluation of bone mass density (BMD) and to the determination of serum levels of PTH, 25-hydroxyvitamin D [25(OH)D], testosterone, LH, FSH, osteocalcin (OC), and urinary levels of deoxypyridinoline (DPD). The T score of lumbar spine and total radius (mean +/- SD) were significantly lower in leprosy patients (L1-L4: CG = -0.7 +/- 1.5 vs LG = -1.8 +/- 1.0 SD, P < 0.04, and total radius: CG = -1.43 +/- 0.6 vs LG = -2.1 +/- 0.8 SD, P <0.02), whereas no significant differences were observed in total hip or femoral neck T score. However, at all sites, the rate of low bone mass (T score < -1.0) was higher in LG (femoral neck: CG = 18% vs LG = 50%, total hip: CG = 27% vs LG = 42%). There was a significant difference in albumin and PTH levels between groups but not in serum 25(OH)D and OC levels or urinary DPD levels. The present results indicate that bone mass loss is an early event in leprosy patients and frequently is already present at diagnosis. Its etiopathogenesis is multifactorial, and further studies are needed to determine the most efficient way to prevent fractures in this condition. The data obtained in the present study need confirmation by the evaluation of a larger sample.

Estrogen deficiency, T cells and bone loss

Estrogen plays a fundamental role in the maintenance of skeletal homeostasis. Although estrogen is established to have direct effects on bone cells, animal studies have identified additional regulatory effects of estrogen centered at the level of the adaptive immune response. Furthermore, a potential role for reactive oxygen species has now been identified in both humans and animals. One of the major challenges has been to integrate a multitude of redundant pathways and cytokines, that all appear capable of playing a relevant role, into a global model of postmenopausal osteoporosis. This review presents our current understanding of the process of estrogen deficiency mediated bone destruction and explores some of the most recent findings and hypotheses to explain estrogen action in bone.

Lipopolysaccharide-induced bone resorption is increased in TNF type 2 receptor-deficient mice in vivo

The release of tumor necrosis factor (TNF)-alpha from macrophages upon stimulation of lipopolysaccharide (LPS) is a major etiological factor of inflammatory bone disease and elicits the effects through TNF receptors type 1 and 2. Given the importance of TNF-alpha action on osteoclastic bone resorption, the role of TNF type 2 receptor (TNFR2) on bone resorption has not been elucidated well so far. The purpose of this study is to investigate the role of TNFR2 on LPS-induced inflammatory bone resorption in vivo. LPS at 10 mg/kg (Re 595) was injected subcutaneously on calvariae of wild-type (WT), TNF type 1 receptor (TNFR1)-deficient (KO), and TNFR2 KO mice, killed on day 5 after the LPS injection. The calvarial bone mineral density (BMD) was significantly decreased by LPS compared to the vehicle-injected control in WT mice, but not in TNFR1 KO mice. Interestingly, the decrease of calvarial BMD and the increase of the osteoclast number by LPS in TNFR2 KO mice seemed to be more than those in WT mice. Furthermore, the significant decrease by LPS on the BMD of tibiae, femurs, and lumber vertebrae were observed only in TNFR2 KO mice. Histomorphometric analysis of tibiae showed the significant increases of osteoclast number and surface in the LPS-injected TNFR2 KO mice, and the levels of urinary deoxypyridinoline reflected these increases of bone resorption parameters. The present data indicate that TNFR1 is critical for bone resorption at the site of LPS injection and that TNFR2 might have a protective role on the LPS-induced inflammatory bone resorption process.

Rheumatoid arthritis fibroblast-like synoviocytes express BCMA and are stimulated by APRIL

OBJECTIVE

Fibroblast-like synoviocytes (FLS) are among the principal effector cells in the pathogenesis of rheumatoid arthritis (RA). This study was undertaken to examine the variety of stimulating effects of APRIL and its specific effect on FLS in the affected RA synovium.

METHODS

Synovium and serum samples were obtained from patients with RA, patients with osteoarthritis (OA), and healthy subjects. Soluble APRIL proteins were assayed by enzyme-linked immunosorbent assay. The relative gene expression of APRIL, BCMA, interleukin-6 (IL-6), tumor necrosis factor alpha (TNFalpha), IL-1beta, and RANKL was assessed in RA and OA FLS by polymerase chain reaction. Effects of APRIL on the production of proinflammatory cytokines and RANKL in RA FLS were investigated by flow cytometry and with the use of a BCMA-Fc fusion protein.

RESULTS

A significantly higher level of soluble APRIL was detected in RA serum compared with normal serum. Among the 3 receptors of APRIL tested, RA FLS expressed only BCMA, whereas OA FLS expressed none of the receptors. APRIL stimulated RA FLS, but not OA FLS, to produce IL-6, TNFalpha, IL-1beta, and APRIL itself. In addition, APRIL increased RA FLS expression of RANKL and also enhanced progression of the cell cycle of RA FLS. Neutralization of APRIL by the BCMA-Fc fusion protein attenuated all of these stimulating effects of APRIL on RA FLS.

CONCLUSION

RA FLS are stimulated by APRIL and express the APRIL receptor BCMA. These results provide evidence that APRIL is one of the main regulators in the pathogenesis of RA.

A p38 mitogen-activated protein kinase inhibitor arrests active alveolar bone loss in a rat periodontitis model

BACKGROUND

Gram-negative bacterial species, such as Actinobacillus actinomycetemcomitans, contain lipopolysaccharide (LPS) that initiates the innate immune system, resulting in inflammatory alveolar bone loss. LPS activates Toll-like receptors on membrane surfaces, stimulating many intracellular signaling cascades, including the p38 mitogen-activated protein kinase (MAPK). Activation of p38 signaling mediates inflammatory cytokine expression, contributing toward osteoclastogenesis and bone loss. The aim of this study was to determine whether the novel, orally active p38 MAPK inhibitor SD282 could arrest progression of LPS-induced alveolar bone destruction in rats.

METHODS

Three groups of female Sprague-Dawley rats received LPS injections to the palatal molar gingiva three times per week for 4 weeks to establish periodontitis. From weeks 5 through 8, two groups received the drug SD282 (N = 14) or 1% polyethylene glycol drug vehicle (N = 14) via oral gavage in addition to LPS injections. The third group continued to receive only LPS injections (N = 8). Microcomputed tomography was used to measure volumetric alveolar bone loss, expressed as bone volume fraction (BVF). Expression of interleukin (IL)-1 and -6 and tumor necrosis factor-alpha (TNF-alpha) was assessed by immunohistochemistry, and osteoclasts were enumerated by tartrate-resistant acid phosphatase staining.

RESULTS

By 4 weeks, severe alveolar bone resorption was seen in LPS-injected animals. Administration of SD282 significantly blocked additional volumetric bone loss in the LPS-only versus LPS + SD282 groups (0.37 +/- 0.01 BVF versus 0.43 +/- 0.01 BVF; P < 0.01). Significant reductions in IL-1beta (P < 0.01 ), TNF-alpha (P < 0.05), and osteoclast formation (P < 0.01) occurred in the presence of SD282.

CONCLUSIONS

An orally active p38 MAPK inhibitor reduced LPS-induced inflammatory cytokine expression, osteoclastogenesis, and alveolar bone loss in rats. Within the limits of the current study, SD282 arrested periodontal disease progression, thus highlighting the therapeutic potential of this novel class of inhibitors.

Morphine treatment accelerates sarcoma-induced bone pain, bone loss, and spontaneous fracture in a murine model of bone cancer

Metastatic bone cancer causes severe pain that is primarily treated with opioids. A model of bone cancer pain in which the progression of cancer pain and bone destruction is tightly controlled was used to evaluate the effects of sustained morphine treatment. In cancer-treated mice, morphine enhanced, rather than diminished, spontaneous, and evoked pain; these effects were dose-dependent and naloxone-sensitive. SP and CGRP positive DRG cells did not differ between sarcoma or control mice, but were increased following morphine in both groups. Morphine increased ATF-3 expression only in DRG cells of sarcoma mice. Morphine did not alter tumor growth in vitro or tumor burden in vivo but accelerated sarcoma-induced bone destruction and doubled the incidence of spontaneous fracture in a dose- and naloxone-sensitive manner. Morphine increased osteoclast activity and upregulated IL-1 beta within the femurs of sarcoma-treated mice suggesting enhancement of sarcoma-induced osteolysis. These results indicate that sustained morphine increases pain, osteolysis, bone loss, and spontaneous fracture, as well as markers of neuronal damage in DRG cells and expression of pro-inflammatory cytokines. Morphine treatment may result in "add-on" mechanisms of pain beyond those engaged by sarcoma alone. While it is not known whether the present findings in this model of osteolytic sarcoma will generalize to other cancers or opioids, the data suggest a need for increased understanding of neurobiological consequences of prolonged opioid exposure which may allow improvements in the use of opiates in the effective management of cancer pain.

RANK ligand as a therapeutic target for bone metastases and multiple myeloma

Osteoclastic bone resorption is a critical component of skeletal complications of malignancy including fracture, bone pain, hypercalcemia, and spinal cord compression. Three proteins, RANKL, RANK, and OPG have been recently identified as key determinants of osteoclastogenesis and the regulation of bone resorption. Both RANKL and OPG can be aberrantly regulated in the cancer setting and function as important gatekeepers of tumor-induced osteolytic bone disease. RANKL-induced osteoclastogenesis not only mediates osteolytic bone disease, but also contributes to the pathogenesis of osteoblastic bone disease resulting from tumors. In addition, an important role was recently described for bone marrow derived RANKL to mediate the bone-specific tropism of RANK-expressing tumor cells. This manuscript will review how RANKL contributes to skeletal complications of cancer and the development of targeted, mechanism-based drugs that inhibit RANKL.

Cytokine expression pattern in compression and tension sides of the periodontal ligament during orthodontic tooth movement in humans

Orthodontic tooth movement is achieved by the remodeling of periodontal ligament (PDL) and alveolar bone in response to mechanical loading and is believed to be mediated by several host mediators, such as cytokines. By means of real-time polymerase chain reaction (PCR), we studied the pattern of expression of mRNA encoding several pro- and anti-inflammatory cytokines in relation to several extracellular matrix and bone remodeling markers, in tension (T) and compression (C) sides of the PDL of human teeth subjected to rapid maxillary expansion. The PDL of normal teeth was used as a control. The results showed that both T and C sides exhibited significantly higher expression of all targets when compared with controls, except for type I collagen (COL-I) and tissue inhibitor of metalloproteinase-1 (TIMP-1) on the C side. Comparing C and T sides, the C side exhibited higher expression of tumor necrosis factor-alpha (TNF-alpha), receptor activator of nuclear factor-kappaB ligand (RANKL), and matrix metalloproteinase-1 (MMP-1), whereas the T side presented higher expression of interleukin-10 (IL-10), TIMP-1, COL-I, osteoprotegerin (OPG), and osteocalcin (OCN). The expression of transforming growth factor-beta (TGF-beta) was similar in both C and T sides. Our data demonstrate a differential expression of pro- and anti-inflammatory cytokines in compressed and stretched PDL during orthodontic tooth movement.

Mitogen-activated protein kinases mediate interleukin-1?-induced receptor activator of nuclear factor-?B ligand expression in human periodontal ligament cells

BACKGROUND AND OBJECTIVE

Interleukin-1beta-stimulated receptor activator of nuclear factor-kappaB ligand (RANKL) expression in human periodontal ligament cells is partially mediated by endogenous prostaglandin E2, whereas mitogen-activated protein kinases (MAPKs) are implicated in regulating various interleukin-1-responsive genes. We investigated herein the involvement of MAPKs in interleukin-1beta-stimulated RANKL expression in human periodontal ligament cells.

MATERIAL AND METHODS

Human periodontal ligament cells were pretreated separately with specific inhibitors of MAPKs, including extracellular signal-regulated kinase, p38 MAPK and c-Jun N-terminal kinase, and subsequently treated with interleukin-1beta. Following each treatment, the phosphorylation of each MAPK, the expression of RANKL, and the production of prostaglandin E2 were determined. RANKL activity was evaluated using an assay to determine the survival of prefusion osteoclasts.

RESULTS

Interleukin-1beta induced RANKL expression at the mRNA and protein levels, as well as RANKL activity in human periodontal ligament cells. Interleukin-1beta also activated extracellular signal-regulated kinase, p38 MAPK, and c-Jun N-terminal kinase. Pretreatment with each MAPK inhibitor partially, but significantly, suppressed interleukin-1beta-induced RANKL expression and its activity, as well as prostaglandin E2 production.

CONCLUSION

In human periodontal ligament cells, three types of MAPK inhibitor may abrogate RANKL expression and activity induced by interleukin-1beta, directly or indirectly through partial suppression of prostaglandin E2 synthesis. In addition, extracellular signal-regulated kinase, p38 MAPK, and c-Jun N-terminal kinase signals may co-operatively mediate interleukin-1beta-stimulated RANKL expression and its activity in those cells.

Intermittent force induces high RANKL expression in human periodontal ligament cells

Intermittent compressive force stimulates bone resorption in orthodontic treatment. This study examined the molecular mechanism in human periodontal ligament (PDL) cells stimulated by an intermittent force. PDL cells were subjected to compressive force (2.0 or 5.0 g/cm(2)) for 2-4 days. Continuous or intermittent force was applied all day or for 8 hrs per day, respectively. At days 3 and 4, cell damage was less with intermittent force than with continuous force. At day 4, RANKL and IL-1beta expressions were greater with intermittent force than with continuous force. An IL-1 receptor antagonist inhibited the compressive force-induced RANKL expression. These findings indicate that IL-1beta is an autocrine factor regulating compressive force-induced RANKL expression in PDL cells, and that intermittent force can effectively induce RANKL in PDL cells with less cell damage.

Knochenstoffwechsel

In order to accommodate individual load, the skeletal system is in a continual state of change. Bone metabolism guarantees optimal bone structure. The osteoblasts are responsible for the synthesis and the osteoclasts for resorption of the bone. A finely adjusted interplay between molecular mechanisms leads, via cytokines, hormones and growth factors, to an homeostasis in bone metabolism. Disturbances of this process lead via increased bone resorption to osteoporosis, and via increased synthesis to osteopetrosis. This contribution describes the known molecular mechanisms in this remodelling process.

Cytokines in the pathogenesis of rheumatoid arthritis

Cytokines regulate a broad range of inflammatory processes that are implicated in the pathogenesis of rheumatoid arthritis. In rheumatoid joints, it is well known that an imbalance between pro- and anti-inflammatory cytokine activities favours the induction of autoimmunity, chronic inflammation and thereby joint damage. However, it remains less clear how cytokines are organized within a hierarchical regulatory network, and therefore which cytokines may be the best targets for clinical intervention a priori. Here, we discuss the crucial effector function of cytokines in the immunological processes that are central to the pathogenesis of rheumatoid arthritis.

TNF-induced structural joint damage is mediated by IL-1

Blocking TNF effectively inhibits inflammation and structural damage in human rheumatoid arthritis (RA). However, so far it is unclear whether the effect of TNF is a direct one or indirect on up-regulation of other mediators. IL-1 may be one of these candidates because it has a central role in animal models of arthritis, and inhibition of IL-1 is used as a therapy of human RA. We removed the effects of IL-1 from a TNF-mediated inflammatory joint disease by crossing IL-1alpha and beta-deficient mice (IL-1-/-) with arthritic human TNF-transgenic (hTNFtg) mice. Development of synovial inflammation was almost unaffected on IL-1 deficiency, but bone erosion and osteoclast formation were significantly reduced in IL-1-/-hTNFtg mice, compared with hTNFtg mice based on an intrinsic differentiation defect of IL-1-deficient monocytes. Most dramatically, however, cartilage damage was absent in IL-1-/-hTNFtg mice. Chimera studies revealed that protection of cartilage is based on the loss of IL-1 on hematopoietic, but not mesenchymal, cells, leading to decreased expression of ADAMTS-5 and MMP-3. These data show that TNF-mediated cartilage damage is completely and TNF-mediated bone damage is partially dependent on IL-1, suggesting that IL-1 is a crucial mediator for inflammatory cartilage and bone degradation.

A Triterpene Glycoside from Black Cohosh that Inhibits Osteoclastogenesis by Modulating RANKL and TNFα Signaling Pathways

Osteoporosis is a major age-related source of morbidity and mortality. Increased bone resorption mediated by osteoclasts is central to its pathogenesis. Cytokines, particularly RANKL and TNFalpha, are often increased under pathologic conditions, leading to enhanced osteoclastogenesis. Black cohosh (Actaea/Cimicifuga racemosa L), a popular herbal supplement for the treatment of menopausal symptoms, was recently shown to have the beneficial effect of preventing bone loss. Here, we demonstrate that 25-acetylcimigenol xylopyranoside (ACCX), a triterpenoid glycoside isolated from black cohosh, potently blocks in vitro osteoclastogenesis induced by either RANKL or TNFalpha. This blockage of osteoclastogenesis elicited by ACCX results from abrogation of the NF-kappaB and ERK pathways induced by either RANKL or TNFalpha, respectively. Importantly, this compound attenuates TNFalpha-induced bone loss in vivo. Therefore, ACCX represents a potential lead for the development of a new class of antiosteoporosis agents.

The biology of aseptic osteolysis

Total hip arthroplasty is one of the most commonly performed and successful elective orthopaedic procedures. However, numerous failure mechanisms limit the long-term success including aseptic osteolysis, aseptic loosening, infection, and implant instability. Aseptic osteolysis and subsequent implant failure occur because of a chronic inflammatory response to implant-derived wear particles. To reduce particulate debris and their consequences, implants have had numerous design modifications including high-molecular-weight polyethylene sockets and noncemented implants that rely on bone ingrowth for fixation. Surgical techniques have improved cementation with the use of medullary plugs, cement guns, lavage of the canal, pressurization, centralization of the stem, and reduction in cement porosity. Despite these advances, aseptic osteolysis continues to limit implant longevity. Numerous proinflammatory cytokines, such as interleukin-1, interleukin-6, tumor necrosis factor-alpha, and prostaglandin E2, have proosteoclastogenic effects in response to implant-derived wear particles. However, none of these cytokines represents a final common pathway for the process of particle-induced osteoclast differentiation and maturation. Recent work has identified the fundamental role of the RANKL-RANK-NF-kappaB pathway not only in osteoclastogenesis but also in the development and function the immune system. Thus, the immune system and skeletal homeostasis may be linked in the process of osteoclastogenesis and osteolysis.

Osteochondral repair in synovial joints

PURPOSE OF REVIEW

One of the major challenges in rheumatology remains the induction of osteochondral repair in synovial joints. Remarkable progress has been made in controlling the inflammatory pathways of chronic synovitis and tissue damage in rheumatoid arthritis and spondyloarthropathy. Here, we provide an overview of the current knowledge on the mechanisms involved in osteochondral repair in degenerative joint diseases, as well as in immune mediated inflammatory arthritides, with special emphasis on tumor necrosis factor alpha and IL-1.

RECENT FINDINGS

Homeostasis of articular cartilage and subchondral bone are essential for maintaining the integrity of osteochondral structures within synovial joints. This is achieved by the regulation of a delicate balance between anabolic and catabolic signals. In articular cartilage one cell type, the chondrocyte, is responsible for regulation of homeostasis. In bone, however, two distinct cell types, osteoblasts and osteoclasts, are responsible for anabolic and catabolic pathways, respectively. In inflammatory joint disorders, this tight regulation is profoundly dysregulated, with tumor necrosis factor alpha acting as an important catalyst of a disturbed homeostasis, together with IL-1. Targeting these cytokines may restore the intrinsic repair capacity of osteochondral structures.

SUMMARY

To restore catabolic cytokine balances appears to be a suitable strategy to promote osteochondral repair.

RANKL Upregulation Associated With Periodontitis andPorphyromonas gingivalis

BACKGROUND

Receptor activator of nuclear factor-kappa B (NF-kappaB) ligand (RANKL) and osteoprotegerin (OPG) are critical for homeostatic control of osteoclast activity, suggesting their vital roles in the progression of bone loss in periodontitis. In this study, the expression of RANKL and OPG mRNA and the relationship between these factors and periodontopathic bacteria in periodontal tissue were studied.

METHODS

Gingival tissue and subgingival plaque samples were collected from 15 patients with chronic periodontitis and 15 periodontally healthy subjects. RNA was extracted from the tissue and subjected to reverse transcription-polymerase chain reaction (RT-PCR) using primers specific for RANKL or OPG. Beta-actin was amplified as a control to ensure equal loading. The intensity of RT-PCR products was analyzed by a densitometer in proportion to the intensity of beta-actin. The numbers of Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans were determined by quantitative real-time PCR.

RESULTS

Our results showed increased levels of RANKL mRNA in chronic periodontitis tissues. The RANKL/OPG expression ratio was significantly higher in the periodontitis group compared to the healthy control group (P = 0.001). Interestingly, the expression of RANKL (r = 0.64; P <0.001), but not OPG (r = -0.24; P = 0.20), was significantly correlated with increased numbers of P. gingivalis. A. actinomycetemcomitans was detected in only 6.7% of all sites.

CONCLUSIONS

Chronic periodontitis was associated with RANKL mRNA upregulation and increased RANKL/OPG mRNA expression ratio. In addition, our data showed for the first time to our knowledge an association between upregulated RANKL levels and the number of P. gingivalis in clinically obtained periodontal tissues.

Inflammation-induced bone loss: can it be prevented?

Inflammatory synovitis induces profound bone loss and OCLs are the instrument of this destruction. TNF blockers have an established role in the prevention of inflammatory bone loss in RA; however, not all patients respond to anti-TNF therapy and side effects may prevent long-term treatment in others. The B-cell--depleting antibody rituximab and the T-cell costimulation blocker abatacept are emerging as major treatment options for patients who are resistant to anti-TNF [96,97]. Proof-of-concept studies demonstrate that targeting RANK-mediated osteoclastogenesis prevents inflammatory bone loss and clinical application has only just begun. The efficacy of RANKL inhibition has been witnessed in trials of Denosumab, and RANKL-neutralizing antibodies are likely to become the treatment of choice for blocking RANKL in RA [77,78]. A major limitation of RANKL antagonism is that it does not treat synovitis. Therefore, anti-RANKL therapy most likely will be used in the context of MTX therapy. There is uncertainty about the possible extraskeletal adverse effects of long-term effects of long-term RANKL blockade. In particular, anti-RANKL therapy could jeopardize dendritic cell function or survival. The demonstrable role of OCLs in inflammation-induced bone loss also invites a reconsideration of the new BPs for bone protection [98]. Studies of ZA in preclinical models indicate that bone protection is comparable to that afforded by OPG. One possible caveat is that intravenous BPs are linked to jaw osteonecrosis [99], although the incidence is confined mainly to intensive treatment in the oncology setting. Although pulsed PTH stimulated bone formation in arthritic models, it has yet to be proven clinically in the context of powerful OCL inhibition with TNF or RANKL antagonists. With strategies that normalize OCL numbers, clinicians are poised to accomplish effective prevention of inflammation-induced bone loss.

How does joint remodeling work?: new insights in the molecular regulation of the architecture of joints

Remodeling of joints is a key feature of inflammatory and degenerative joint disease. Bone erosion, cartilage degeneration and growth of bony spurs termed osteophytes are key features of structural joint pathology in the course of arthritis, which lead to impairment of joint function. Understanding their molecular mechanisms is essential to tailor targeted therapeutic approaches to protect joint architecture from inflammatory and mechanical stress. This addendum summarizes the new insights in the molecular regulation of bone formation in the joint and its relation to bone resorption. It describes how inflammatory cytokines impair bone formation and block the repair response of joints towards inflammatory stimuli. It particularly points out the key role of Dickkopf-1 protein, a regulator of the Wingless signaling and inhibitor of bone formation. This new link between inflammation and bone formation is also crucial for explaining the generation of osteophytes, bony spurs along joints, which are characterized by new bone and cartilage formation. This mechanism is largely dependent on an activation of wingless protein signaling and can lead to complete joint fusion. This addendum summarized the current concepts of joint remodeling in the limelight of these new findings.

Osteoclasts: what do they do and how do they do it?

As Americans live longer, degenerative skeletal diseases, such as osteoporosis, become increasingly prevalent. Regardless of cause, osteoporosis reflects a relative enhancement of osteoclast activity. Thus, this unique bone resorptive cell is a prominent therapeutic target. A number of key observations provide insights into the mechanisms by which precursors commit to the osteoclast phenotype and how the mature cell degrades bone. The osteoclast is a member of the monocyte/macrophage family that differentiates under the aegis of two critical cytokines, namely RANK ligand and M-CSF. Tumor necrosis factor (TNF)-alpha also promotes osteoclastogenesis, particularly in states of inflammatory osteolysis such as that attending rheumatoid arthritis. Once differentiated, the osteoclast forms an intimate relationship with the bone surface via the alphavbeta3 integrin, which transmits matrix-derived, cytoskeleton-organizing, signals. These integrin-transmitted signals include activation of the associated proteins, c-src, syk, Vav3, and Rho GTPases. The organized cytoskeleton generates an isolated microenvironment between the cell's plasma membrane and the bone surface in which matrix mineral is mobilized by the acidic milieu and organic matrix is degraded by the lysosomal protease, cathepsin K. This review focuses on these and other molecules that mediate osteoclast differentiation or function and thus serve as candidate anti-osteoporosis therapeutic targets.

The RANK/RANKL/OPG triad in cancer-induced bone diseases

The maintenance of skeletal integrity in a healthy individual requires a balanced regulation of the processes of bone formation, mediated by osteoblasts, and bone resorption, mediated by osteoclasts. This balanced process of bone remodeling becomes co-opted in the skeleton by tumor cells and this dramatically accelerates the process of remodeling and disrupts the normal equilibrium resulting in a spectrum of osteolytic to osteoblastic bone lesions. Certain tumor types, such as breast and prostate, frequently metastasize to the bone. It is now widely understood that the molecular triad--receptor activator of NF-kappaB ligand (RANKL), its receptor RANK, and the endogenous soluble RANKL inhibitor, osteoprotegerin (OPG)--play direct and essential roles in the formation, function, and survival of osteoclasts. Osteoclastic bone resorption contributes to the majority of skeletal sequelae, or skeletal-related events (SREs), in patients with bone metastases. In addition, osteoclastic bone resorption also contributes to the establishment of tumors in the skeleton. Therefore, blocking osteoclast activity and differentiation via RANKL inhibition may not only provide a beneficial treatment for skeletal complications of malignancy, but may also prevent bone metastases. In this review, we will first describe the operative role of osteoclasts and the RANK/RANKL/OPG triad in the pathophysiology of cancer-induced bone diseases, specifically solid tumor metastases to the bone. Secondly, we will describe a therapeutic approach that specifically targets the RANKL molecule.

Fcgamma receptors directly mediate cartilage, but not bone, destruction in murine antigen-induced arthritis: uncoupling of cartilage damage from bone erosion and joint inflammation

OBJECTIVE

To determine the relationship between synovial inflammation and the concomitant occurrence of cartilage and bone erosion during conditions of variable inflammation using various Fcgamma receptor knockout (FcgammaR(-/-)) mice.

METHODS

Antigen-induced arthritis (AIA) was introduced in the knee joints of various FcgammaR(-/-) mice and wild-type controls. Joint inflammation and cartilage and bone destruction levels were determined by histologic analysis. Cathepsin K, RANKL, and osteoprotegerin (OPG) levels were detected by immunolocalization.

RESULTS

In FcgammaRIIb(-/-) mice, which lack the inhibiting Fcgamma receptor IIb, levels of joint inflammation and cartilage and bone destruction were significantly higher (infiltrate 93%, exudate 200%, cartilage 100%, bone 156%). AIA in mice lacking activating FcgammaR types I, III, and IV, but not FcgammaRIIb (FcR gamma-chain(-/-) mice), prevented cartilage destruction completely. In contrast, levels of bone erosion and joint inflammation were comparable with their wild-type controls. Of great interest, in arthritic mice lacking activating FcgammaR types I, II, and III, but not IV (FcgammaRI/II/III(-/-) mice), levels of joint inflammation were highly elevated (infiltrate and exudate, 100% and 188%, respectively). Cartilage destruction levels were decreased by 92%, whereas bone erosion was increased by 200%. Cathepsin K, a crucial mediator of osteoclasts, showed a strong correlation with the amount of inflammation but not with the amount of activating FcgammaR, which was low in osteoclasts. RANKL, but not OPG, levels were higher in the inflammatory cells of arthritic knee joints of FcgammaRI/II/III(-/-) mice versus wild-type mice.

CONCLUSION

Activating FcgammaR are crucial in mediating cartilage destruction independently of joint inflammation. In contrast, FcgammaR are not directly involved in bone erosion. Indirectly, FcgammaR drive bone destruction by regulating joint inflammation.

Identification of genes related to mechanical stress in human periodontal ligament cells using microarray analysis

BACKGROUND AND OBJECTIVE

Differential expression of genes in human periodontal ligament (PDL) under mechanical stress, such as orthodontic force, is thought to be involved in the remodeling of PDL cells and periodontal tissues. However, little is known about the genes expressed in PDL cells under mechanical stress.

MATERIAL AND METHODS

We employed microarray analysis to assess, in a comprehensive manner, the gene expression profiles in PDL cells compressed by a static force using an in vitro three-dimensional culture system. Six genes were selected and validated by quantitative real-time polymerase chain reaction analysis, consistent with the microarray data.

RESULTS

The microarray data revealed that 108 of 30,000 genes tested were differentially expressed by mechanical force loading. Among them, 85 genes were up-regulated by mechanical stress, while 23 genes were down-regulated, judging by the thresholds of a two-fold increase/decrease compared with the controls. Thirty-two of the up-regulated and eight of the down-regulated genes, well-characterized in protein function, were involved in numerous biological processes including cell communication, cell signaling, cell cycle, stress response, and calcium release. However, several genes differentially expressed in our microarray data have not been well defined as stress-response molecules.

CONCLUSION

Our microarray is the first to show the gene profile in PDL cells caused by mechanical stress; however, further studies to clarify the physiological function of these molecules in PDL cells are required.

SHIP1 negatively regulates proliferation of osteoclast precursors via Akt-dependent alterations in D-type cyclins and p27

Osteoclasts arise from macrophage progenitors in bone marrow (BMMs) as a consequence of signaling events elicited by M-CSF and receptor activator of NF-kappaB ligand, acting on their unique receptors, via c-Fms and receptor activator of NF-kappaB. Both receptors activate the PI3K and MAPK pathways, which promote cell proliferation and survival. SHIP1 is essential for normal bone homeostasis, as mice lacking the protein exhibit osteoporosis resulting from increased numbers of hyper-resorptive osteoclasts. In this study, we show that BMMs from SHIP1 null mice respond to M-CSF, but not receptor activator of NF-kappaB ligand, by increasing Akt activation. In consequence, there are up-regulation of D-type cyclins, down-regulation of the cyclin-dependent kinase inhibitor p27, and, therefore, increased phosphorylation of the retinoblastoma protein and cell proliferation. Surprisingly, cell survival of wild-type and knockout BMMs is unaltered. Finally, osteoclastogenesis and periarticular bone erosions are markedly increased in SHIP1(-/-) mice with inflammatory arthritis, a condition characterized by increased M-CSF expression. The SHIP1/Akt pathway therefore suppresses bone loss in pathological states associated with an excess of the cytokine.

p38 MAPK as a potential therapeutic target for inflammatory osteolysis

Inflammatory osteolysis is a relatively frequent and incapacitating complication of rheumatoid arthritis and other inflammatory diseases, and is induced by accelerated osteoclast recruitment and activation in bone under the aegis of cytokines produced in the inflammatory environment. The success of antitumor necrosis factor-alpha and interleukin-1 therapy in correcting this condition highlights the central role of these cytokines in this process. Recent years have witnessed a revolution in understanding the molecular mechanism and pathogenesis of this family of diseases. It is now clear that p38 mitogen-activated protein kinase plays an essential role in the production of proinflammatory cytokines and cytokine-induced osteoclastogenesis, thus providing a potential therapeutic target for prevention of pathologic bone loss.

The dual role of p55 tumour necrosis factor-alpha receptor in Actinobacillus actinomycetemcomitans-induced experimental periodontitis: host protection and tissue destruction

Inflammatory immune reactions in response to periodontopathogens are thought to protect the host against infection, but may trigger periodontal destruction. Thus, we examined the mechanisms by which the proinflammatory cytokine tumour necrosis factor (TNF)-alpha modulates the outcome of Actinobacillus actinomycetemcomitans-induced periodontal disease in mice. Our results showed that TNF-alpha receptor p55-deficient mice [p55TNF-knock-out (KO)] developed a less severe periodontitis in response to A. actinomycetemcomitans infection, characterized by significantly less alveolar bone loss and inflammatory reaction. Real-time polymerase chain reaction (PCR) demonstrated that levels of chemokines (CXCL1, 3 and 10; CCL3 and 5) and their receptors (CXCR2 and 3, CCR5) were lower in p55TNF-KO mice, as were matrix metalloproteinase (MMP)-1, 2 and 9 and receptor activator of nuclear factor kB ligand (RANKL) mRNA levels. However, the absence of the TNF-alpha p55 results in an impairment of protective immunity to A. actinomycetemcomitans infection, characterized by increased bacterial load and higher levels of C-reactive protein during the course of disease. Such impaired host response may be the result of the reduced chemoattraction of lymphocytes, neutrophils and macrophages, and reduced inducible nitric oxide synthase expression (iNOS) and myeloperoxidase (MPO) production in periodontal tissues of p55 TNF-KO mice. Our results demonstrate the mechanisms involved determining periodontal disease severity by TNF-alpha receptor p55, and its role in providing immune protection to A. actinomycetemcomitans periodontal infection.