Prostaglandin E2 induces expression of receptor activator of nuclear factor-kappa B ligand/osteoprotegrin ligand on pre-B cells: implications for accelerated osteoclastogenesis in estrogen deficiency

B cells play an important role in lipopolysaccharide-induced bone resorption

The host immune system, especially activated T cells, plays a crucial role in inflammatory bone resorption and osteoclastogenesis. Previously, we showed that T cells are involved in inflammatory bone resorption in vivo. However, little is known about whether B cells are involved in inflammatory bone resorption and how B cells take part in osteoclastogenesis. Therefore, the aim of this study was to examine whether B c ells truly influence inflammatory bone resorption in vivo. Alveolar bone resorption in normal mice, in SCID mice that lack both B and T cells, and in B cell-reconstituted SCID mice was compared histopathologically after repeated injections of lipopolysaccharide (LPS) into the gingiva. Furthermore, we examined whether the B cells that are stimulated by LPS are involved in osteoclastogenesis in vitro. As a result, the B cell-reconstituted SCID mice showed stronger inflammatory bone resorption than the SCID mice. Also, in vitro, LPS-stimulated B cells enhanced osteoclastogenesis and anti-tumor necrosis factor (TNF)-alpha antibody completely blocked osteoclastogenesis induced by LPS-stimulated B cells. These results suggest that B cells promote inflammatory bone resorption through TNF-alpha.

Bone loss in inflammatory arthritis: mechanisms and therapeutic approaches with bisphosphonates

The inflammatory process in rheumatoid arthritis provokes intense bone resorption, evidenced as bone erosions, juxta-articular osteopenia and generalized osteoporosis. These types of bone loss share a common pathogenesis, and the role of osteoclasts in focal bone erosion was verified in elegant animal studies. The tumour necrosis factor (TNF) family of cytokines and receptors--specifically TNF-alpha, RANKL, RANK and OPG--are dominant regulators of osteoclastic bone resorption in rheumatoid arthritis. The confirmation of the osteoclast mechanism provides new insight into the structural joint protection afforded by disease-modifying drugs and suggests innovative approaches to limit bone destruction. Emerging treatment strategies for bone disease in rheumatoid arthritis are the use of monoclonal antibodies to neutralize RANKL, and powerful bisphosphonates that target pathogenic osteoclasts.

Rheumatic diseases: the effects of inflammation on bone

Rheumatoid arthritis, juvenile idiopathic arthritis, the seronegative spondyloarthropathies including psoriatic arthritis, and systemic lupus erythematosus are all examples of rheumatic diseases in which inflammation is associated with skeletal pathology. Although some of the mechanisms of skeletal remodeling are shared among these diseases, each disease has a unique impact on articular bone or on the axial or appendicular skeleton. Studies in human disease and in animal models of arthritis have identified the osteoclast as the predominant cell type mediating bone loss in arthritis. Many of the cytokines and growth factors implicated in the inflammatory processes in rheumatic diseases have also been demonstrated to impact osteoclast differentiation and function either directly, by acting on cells of the osteoclast-lineage, or indirectly, by acting on other cell types to modulate expression of the key osteoclastogenic factor receptor activator of nuclear factor (NF) kappaB ligand (RANKL) and/or its inhibitor osteoprotegerin (OPG). Further elucidation of the mechanisms responsible for inflammation-induced bone loss will potentially lead to the identification of novel therapeutic strategies for the prevention of bone loss in these diseases. In this review, we provide an overview of the cell types, inflammatory mediators, and mechanisms that are implicated in bone loss and new bone formation in inflammatory joint diseases.

Liver-derived IGF-I is permissive for ovariectomy-induced trabecular bone loss

INTRODUCTION

Estrogen deficiency results in trabecular bone loss, associated with T-cell proliferation in the bone marrow. Insulin-like growth factor I (IGF-I) is involved in the regulation of both bone metabolism and lymphopoiesis. A major part of serum IGF-I is derived from the liver. The aim of the present study was to investigate the role of liver-derived IGF-I for ovariectomy (ovx)-induced trabecular bone loss.

MATERIALS AND METHODS

Mice with adult liver-specific IGF-I inactivation (LI-IGF-I-/-) and wild type mice (WT) were either ovx or sham operated. After 5 weeks, the skeletal phenotype was analyzed by pQCT and microCT. The bone marrow cellularity was analyzed using FACS technique, and mRNA levels were quantified using real-time PCR.

RESULTS

Ovx resulted in a pronounced reduction in trabecular bone mineral density (-52%, P < 0.001), number (-45%, P < 0.01) and thickness (-13%, P < 0.01) in WT mice while these bone parameters were unaffected by ovx in LI-IGF-I-/- mice. Furthermore, ovx increased the number of T-cells in the bone marrow of the femur in WT but not in LI-IGF-I-/- mice. Interleukin 7 (IL-7) has been reported to stimulate the formation and function of osteoclasts by inducing the expression of receptor activator of NF-kappaB ligand (RANKL) on T-cells. IL-7 mRNA levels and the RANKL/osteoprotegerin ratio in bone were increased by ovx in WT but not in LI-IGF-I-/- mice.

CONCLUSIONS

Liver-derived IGF-I is permissive for ovx-induced trabecular bone loss. Our studies indicate that IGF-I might exert this permissive action by modulation of the number of T-cells and the expression of IL-7, which in turn is of importance for the RANKL/OPG ratio and consequently osteoclastogenesis in the bone marrow.

Optimal compressive force induces bone formation via increasing bone sialoprotein and prostaglandin E2 production appropriately

Although orthodontic tooth movement can promote bone formation, the molecular mechanism that underlies this phenomenon is not fully understood. The purposes of this study were to determine how mechanical stress affects the osteogenic response of human osteoblastic cells (Saos-2), and also examine the optimal compression for osteogenesis in vitro. Saos-2 cells cultured with or without continuously compressive force (0.5 approximately 3.0 g/cm(2)). The expression of bone sialoprotein (BSP), osteopontin, and cyclooxygenase-2 (COX-2) were measured using real-time PCR, Western blot analysis and immunoassay. The calcium content in the mineralized nodules was determined using Calcium C-Test kit. Only one loading with 1.0 g/cm(2) of compressive force significantly increased the expression of BSP mRNA and protein, COX-2 mRNA expression and PGE(2) synthesis. Indomethacin, an inhibitor of PGE(2) synthesis, inhibited the compression-induced above phenomenon. Moreover, the conditioned medium from 1.0 g/cm(2) of compressive force apparently stimulated calcium content in mineralized nodules. This study demonstrates that an optimal compressive force stimulates in vitro mineralization by BSP synthesis through the autocrin action of PGE(2) production.

TNF-alpha and pathologic bone resorption

Chronic inflammatory bone diseases, such as rheumatoid arthritis, periodontal disease and aseptic periprosthetic osteolysis, are characterized by bone loss around affected joints and teeth caused by increased osteoclastic bone resorption. This resorption is mediated largely by the increased local production of pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNFa). These cytokines may induce resorption indirectly by affecting the production of the essential osteoclast differentiation factor, receptor activator of NF-kB ligand, and/or its soluble decoy receptor, osteoprotegerin, by osteoblast/stromal cells or directly by enhancing proliferation and/or activity of cells in the osteoclast lineage. The importance of TNFa in the pathogenesis of various forms of bone loss is supported by both experimental and clinical evidence. However, TNFa is not absolutely required for osteoclastogenesis, erosive arthritis, or osteolysis, as all these events could occur in the absence of TNFa and whether TNFa promotes osteoclast formation independently of RANK signaling is still a topic of debate. Here we review our current understanding of the mechanisms whereby TNFa increases osteoclastogenesis in vitro and in vivo.

Regulation of RANKL promoter activity is associated with histone remodeling in murine bone stromal cells

Receptor activator of NFkappa-B ligand (RANKL) is essential for osteoclast formation, function, and survival. Although RANKL mRNA and protein levels are modulated by 1,25(OH)2D3 and other osteoactive factors, regulatory mechanisms remain unclear. In this study, we show that 2 kb or 2 kb plus exon 1 of a RANKL promoter sequence conferred neither 1,25(OH)2D3 response nor tissue specificity. The histone deacetylase inhibitors trichostatin A (TSA) and sodium butyrate (SB), however, strongly increased RANKL promoter activity. A series of 5'-deleted RANKL promoter constructs from 2,020 to 110 bp showed fourfold increased activity after TSA treatment. TSA also dose dependently enhanced endogenous RANKL mRNA expression with 50 microM of TSA treatment causing equivalent RANKL expression to that seen with 1 nM 1,25(OH)2D3. Using a chromatin immunoprecipitation (ChIP) assay we showed that TSA significantly enhanced association of both acetylated histone H3 and H4 on the RANKL promoter, with H4 > H3. A similar increase in acetylated histone H4 on the RANKL gene locus was seen after 1,25(OH)2D3 treatment, but ChIP assay did not reveal localization of VDR/RXR heterodimers on the putative VDRE of the RANKL promoter. To explore the role of H4 acetylation of 1,25(OH)2D3 stimulated RANKL, we added both TSA and 1,25(OH)2D3 together. While the combination further increased acetylation of H4 on the RANKL locus, surprisingly, TSA inhibited 1,25(OH)2D3-induced RANKL mRNA expression by 70% at all doses of 1 ,25(OH)2D3 studied. These results suggest that TSA increases of endogenous expression of RANKL involve enhanced acetylation of histones on the proximal RANKL promoter. Preventing deacetylation, however, blocks 1,25(OH)2D3 action on this gene. Chromatin remodeling is therefore involved in RANKL expression.

Ovariectomy fails to augment bone resorption and marrow B lymphopoiesis in granulocyte colony-stimulating factor transgenic mice

The mechanism of pathological bone loss induced by estrogen deficiency has not been fully elucidated. It has been shown in recent animal studies that increased B lymphopoiesis induced by estrogen deficiency is involved in the mechanism of stimulated bone resorption. Mice transgenic for granulocyte colony-stimulating factor (G-CSF) (G-Tg) exhibit generalized osteopenia with an increase in osteoclast number and enhancement of bone resorption, which coexists with enhanced hematopoiesis. When ovariectomy was performed on G-Tg, it did not further reduce bone mass as revealed by radiography, dual-energy X-ray absorptiometry, and peripheral quantitative computed tomography. Ovariectomy increased the amount of colony-forming units of interleukin 7 (CFU-IL-7) by threefold in the marrow of normal mice in association with an increase in the number of B220-positive cells expressing the receptor activator of nuclear factor-kappaB ligand (RANKL). In contrast, the number of B220-positive cells expressing RANKL and CFU-IL-7 remarkably decreased in the marrow of G-Tg. Ovariectomy induced neither CFU-IL-7 nor B220-positive cells expressing RANKL in the marrow of G-Tg. Strong inhibition of B lymphopoiesis by G-CSF resulted in depletion of B cells expressing RANKL from the marrow, which may lead to resistance to bone loss due to ovariectomy. This observation suggests that B lymphopoiesis plays a possible role in bone loss in a condition of acute estrogen deficiency.

Deficiency of the G-protein alpha-subunit G(s)alpha in osteoblasts leads to differential effects on trabecular and cortical bone

The G-protein alpha-subunit G(s)alpha is required for the intracellular cAMP responses to hormones and other agonists. G(s)alpha is known to mediate the cAMP response to parathyroid hormone and other hormones and cytokines in bone and cartilage. To analyze the in vivo role of G(s)alpha signaling in osteoblasts, we developed mice with osteoblast/osteocyte-specific G(s)alpha deficiency (BGsKO) by mating G(s)alpha-floxed mice with collagen Ialpha1 promoter-Cre recombinase transgenic mice. Early skeletal development was normal in BGsKO mice, because formation of the initial cartilage template and bone collar was unaffected. The chondrocytic zones of the growth plates also appeared normal in BGsKO mice. BGsKO mice had a defect in the formation of the primary spongiosa with reduced immature osteoid (new bone formation) and overall length, which led to reduced trabecular bone volume. In contrast, cortical bone was thickened with narrowing of the bone marrow cavity. This was probably due to decreased cortical bone resorption, because osteoclasts were markedly reduced on the endosteal surface of cortical bone. In addition, the expression of alkaline phosphatase, an early osteoblastic differentiation marker, was normal, whereas the expression of the late osteoblast differentiation markers osteopontin and osteocalcin was reduced, suggesting that the number of mature osteoblasts in bone is reduced. Expression of the osteoclast-stimulating factor receptor activator of NF-kappaB ligand was also reduced. Overall, our findings have similarities to parathyroid hormone null mice and confirm that the differential effects of parathyroid hormone on trabecular and cortical bone are primarily mediated via G(s)alpha in osteoblasts. Osteoblast-specific G(s)alpha deficiency leads to reduced bone turnover.

Changes in receptor activator of nuclear factor-kappaB, and its ligand, osteoprotegerin, bone-type alkaline phosphatase, and tartrate-resistant acid phosphatase in ovariectomized rats

We investigated time-course changes in the expression of receptor activator of nuclear factor-kappaB (RANK), its ligand (RANKL), osteoprotegerin (OPG), bone-type alkaline phosphatase (BAP), and tartrate-resistant acid phosphatase (TRAP) in ovariectomized (OVX) rats. Samples of sera and coccyges were used for analysis of the enzyme activities and expression levels of proteins and mRNAs, and an immunohistochemical analysis was also performed. Serum BAP activity increased to 158.6% of the pre-operation value at 1 week after OVX, and then decreased to 38.7% at 8 weeks after OVX. On the other hand, the serum TRAP activity increased to 130.9% of the pre-operation level at 1 week after OVX, and was maintained at a high level, compared with the pre-operation level. The patterns of BAP and TRAP activity in the coccyges specimens were similar to those seen in the sera. The expression profiles of TRAP, RANK, and RANKL proteins in the coccyx specimens were similar to the pattern of serum TRAP activity, while the profiles of the BAP and OPG proteins were similar to the pattern of serum BAP activity in OVX rats. The changes in the mRNA expression levels of the osteogenic proteins were similar to those for protein expression. These biochemical changes in OVX rats were confirmed by immunohistochemical studies. Our results suggest that not only osteoclastogenesis accelerated but also osteoblastogenesis transiently increased during the early phase of osteoporosis.

Effect of cyclic mechanical stretch and titanium particles on prostaglandin E2 production by human macrophages in vitro

Early implant instability has been proposed as a critical factor in the onset and progression of aseptic loosening and periprosthetic osteolysis in total joint arthroplasties. Previous in vitro studies have reported that macrophages stimulated with cyclic mechanical strain release inflammatory mediators. Little is known, however, about the response of these cells to mechanical strain with particles, which is often a component of the physical environment of the cell. We therefore studied the production of prostaglandin E(2) (PGE(2)), an important mediator in aseptic loosening and periprosthetic osteolysis in total joint arthroplasties, for human macrophages treated with mechanical stretch alone, titanium particles alone, and mechanical stretch and particles combined. A combination of mechanical stretch and titanium particles resulted in a statistically synergistic elevation of levels of PGE(2) compared with the levels found with either stretch or particles alone. Exposure of human macrophages to mechanical stretch with particles upregulated the expression of cyclooxygenase (COX)-2 mRNA but not COX-1 mRNA, this expression resulting in a 97-fold increase in PGE(2) production compared to the nonstimulated cells. The current study is the first to investigate the effects of mechanical stretch with particles on cultured macrophages and include an investigation of the time course of PGE(2) production and COX-2 mRNA expression. Our results suggest that, while mechanical strain may be one of the primary factors responsible for macrophage activation and periprosthetic osteolysis, mechanical strain with particles load may contribute significantly to the osteolytic potential of macrophages in vitro. The synergistic effect observed between mechanical stretch and particles could accelerate implant loosening and implies that reduction in either cyclic mechanical strain or wear debris load would lead to a reduction of osteolysis.

Bone loss in inflammatory arthritis: mechanisms and treatment strategies

PURPOSE OF REVIEW

Focal bone loss in inflammatory arthritis begins early in the disease process and can contribute to patient morbidity. Current treatment strategies primarily target suppression of the inflammatory cascade with varying success in limiting the progression of focal bone destruction. This review outlines the current understanding of the mechanisms mediating inflammation-induced focal bone loss in rheumatoid arthritis and other inflammatory arthritides and highlights recent studies in animal models of arthritis that have contributed to our knowledge of this process.

RECENT FINDINGS

Bone-resorbing osteoclasts have been identified as important effector cells in inflammation-induced bone loss in both experimental animal models and human rheumatoid arthritis and psoriatic arthritis. The RANK/RANKL (receptor activator of nuclear factor-kappaB and RANK ligand) pathway has been shown to be essential for osteoclast differentiation in inflammatory arthritis. In addition, in vitro and in vivo studies have demonstrated that many cytokines and growth factors elaborated by inflamed synovial tissues may contribute to osteoclast differentiation and activation.

SUMMARY

Elucidation of the mechanisms mediating osteoclast differentiation and function has identified new pathways for potential targeted therapeutic intervention for focal bone loss in inflammatory arthritis. Challenges in the application of this approach are that therapies targeting the osteoclast would need to be used in combination with effective anti-inflammatory agents, and that pathways mediating osteoclast differentiation and function would need to remain at least partially functional to allow for continued skeletal remodeling.

Microrough implant surface topographies increase osteogenesis by reducing osteoclast formation and activity

Titanium implant surfaces with rough microtopographies exhibit increased pullout strength in vivo suggesting increased bone-to-implant contact. This is supported by in vitro studies showing that as surface microroughness increases, osteoblast proliferation decreases whereas differentiation increases. Differentiation is further enhanced on microrough surfaces by factors stimulating osteogenesis including 1alpha,25(OH)2D3. Levels of PGE2 and TGF-beta1 are increased in cultures grown on rough microtopographies; this surface effect is enhanced synergistically by 1alpha,25(OH)2D3-treatment. PGE2 and TGF-beta1 regulate osteoclasts as well as osteoblasts, suggesting that surface microtopography may modulate release of other factors from osteoblasts that regulate osteoclasts. To test this hypothesis, we examined the effects of substrate microarchitecture on production of osteoprotegerin (OPG) and receptor activator of nuclear factor kappa B ligand (RANKL), which have been identified as a key regulatory system of bone remodeling. We also examined the production of 1alpha,25(OH)2D3, which regulates osteoblast differentiation and osteoclastogenesis. MG63 osteoblast-like cells were grown on either tissue culture plastic or titanium disks of different surface microtopographies: PT (Ra < 0.2 microm), SLA (Ra = 4 microm), and TPS (Ra = 5 microm). At confluence, cultures were treated for 24 h with 0, 10(-8) M or 10(-7) M 1alpha,25(OH)2D3. RANKL and OPG were determined at the transcriptional level by RT-PCR and real time PCR and soluble RANKL, OPG and 1alpha,25(OH)2D3 in the conditioned media were measured using immunoassay kits. Cell number was reduced on SLA and TPS surfaces and 1alpha,25(OH)2D3 caused further decreases. OPG mRNA levels increased on rougher surfaces and 1alpha,25(OH)2D3 treatment caused a further synergistic increase. While the cells expressed RANKL mRNA, levels were low and independent of surface microtopography. OPG protein was greater when cells were grown on SLA and TPS. 1alpha,25(OH)2D3 increased OPG by 50% on the smooth Ti surface but on SLA, 10(-8) M 1alpha,25(OH)2D3 caused a 100% increase and 10(-7) M 1alpha,25(OH)2D3 increased OPG by 200%. On TPS 10(-7) M 1alpha,25(OH)2D3 increased OPG 350%. Soluble RANKL was not detected in the conditioned media of any of the cultures. 1alpha,25(OH)2D3 was produced endogenously and levels were positively correlated with surface roughness. Thus, on surfaces with rough microtopographies, osteoblasts secrete factors that enhance osteoblast differentiation while decreasing osteoclast formation and activity.

Systemic tumor necrosis factor alpha mediates an increase in peripheral CD11bhigh osteoclast precursors in tumor necrosis factor alpha-transgenic mice

OBJECTIVE

To investigate the mechanisms whereby tumor necrosis factor alpha (TNFalpha) increases osteoclastogenesis in vivo.

METHODS

TNFalpha-transgenic (TNF-Tg) and wild-type mice injected with TNFalpha were studied. In vitro osteoclastogenesis assays, monocyte colony-forming assays, and fluorescence-activated cell sorting were performed using splenocytes, peripheral blood mononuclear cells (PBMCs), and bone marrow cells to quantify and characterize osteoclast precursors (OCPs). Etanercept, a TNFalpha antagonist, was used to block TNFalpha activity in vivo. The effects of TNFalpha on proliferation, apoptosis, and differentiation of OCPs were assessed using 5-bromo-2'-deoxyuridine labeling, annexin V staining, and reverse transcriptase-polymerase chain reaction.

RESULTS

OCP numbers were increased 4-7-fold in PBMCs and spleen, but not in bone marrow of TNF-Tg mice. The OCPs in spleen were in the CD11b(high) population and contained both c-Fms- and c-Fms+ cells. The increased number of OCPs correlated with the initiation of detectable TNFalpha in serum and the onset of inflammatory arthritis in TNF-Tg mice. Etanercept eliminated the increase in peripheral OCPs. TNFalpha did not affect proliferation, survival, or differentiation of CD11b(high) splenocytes in vivo or in vitro, but caused a rapid increase in CD11b+ cells in blood within 4 hours of a single injection and an accumulation of CD11b(high) OCPs in spleen after 3 days of multiple injections.

CONCLUSION

Systemic TNFalpha induces a marked increase in circulating OCPs that is reversible by anti-TNF therapy and may result from their mobilization from bone marrow. Our findings provide a new mechanism whereby TNFalpha stimulates osteoclastogenesis in patients with inflammatory arthritis, suggesting that CD11b+ PBMCs could be used to evaluate a patient's potential for erosive disease and the efficacy of anti-TNF therapy.

RANK signaling is not required for TNFalpha-mediated increase in CD11(hi) osteoclast precursors but is essential for mature osteoclast formation in TNFalpha-mediated inflammatory arthritis

To address the controversy of whether TNFalpha can compensate for RANKL in osteoclastogenesis in vivo, we used a TNFalpha-induced animal model of inflammatory arthritis and blocked RANKL/RANK signaling. TNFalpha increased osteoclast precursors available for RANK-dependent osteoclastogenesis. RANK signaling is not required for the TNFalpha-stimulated increase in CD11b(hi) osteoclast precursors but is essential for mature osteoclast formation.

INTRODUCTION

Although critical roles of TNFalpha in inflammatory arthritis and RANKL in bone resorption have been firmly established, a central controversy remains about the extent to which TNFalpha can compensate for RANKL during osteoclastogenesis and the stage at which RANK signaling is required for osteoclastogenesis. Here, we used the human TNFalpha transgenic mouse model (TNF-Tg) of erosive arthritis to determine if there are both RANK-dependent and -independent stages of osteoclastogenesis in TNFalpha-induced erosive arthritis.

MATERIALS AND METHODS

Osteoclastogenesis and osteoclast precursor (OCP) frequency were analyzed using histology, fluorescence-activated cell sorting (FACS), and cell culture from (1) TNF-Tg mice treated with the RANKL antagonist, RANK:Fc, or (2) TNF-Tg X RANK -/- mice generated by crossing TNF-Tg mice with RANK-/- mice.

RESULTS

Treatment of TNF-Tg mice, which have increased OCPs in their spleens, with RANK:Fc dramatically reduced osteoclast numbers on the surface of their arthritic joints and within their bones, but did not decrease CD11b(hi) OCP numbers in their spleens. Long-term RANK:Fc administration alleviated joint erosion. Furthermore, TNF-Tg x RANK -/- mice had severe osteopetrosis, no osteoclasts, and no joint erosion, but increased CD11b(hi) precursor numbers that failed to form mature osteoclasts in vitro.

CONCLUSION

RANK signaling is essential for mature osteoclast formation in TNFalpha-mediated inflammatory arthritis but not for the TNFalpha-induced increase in CD11b(hi) OCP that subsequently can differentiate into osteoclasts in inflamed joints.

Transforming growth factor-beta controls human osteoclastogenesis through the p38 MAPK and regulation of RANK expression

Although RANK-L is essential for osteoclast formation, factors such as transforming growth factor-beta (TGF-beta) are potent modulators of osteoclastogenic stimuli. To systematically investigate the role of TGF-beta in human osteoclastogenesis, monocytes were isolated from peripheral blood by three distinct approaches, resulting in either a lymphocyte-rich, a lymphocyte-poor, or a pure osteoclast precursor (CD14-positive) cell population. In each of these osteoclast precursor populations, the effect of TGF-beta on proliferation, TRAP activity, and bone resorption was investigated with respect to time and length of exposure. When using the highly pure CD14 osteoclast precursor cell population, the effect of TGF-beta was strongly dependent on the stage of osteoclast maturation. When monocytes were exposed to TGF-beta during the initial culture period (days 1-7), TRAP activity and bone resorption were increased by 40%, whereas the cell number was reduced by 25%. A similar decrease in cell number was observed when TGF-beta was present during the entire culture period (days 1-21), but in direct contrast, TRAP activity, cell fusion, cathepsin K, and matrix metalloproteinase (MMP)-9 expression as well as bone resorption were almost completely abrogated. Moreover, we found that latent TGF-beta was strongly activated by incubation with MMP-9 and suggest this to be a highly relevant mechanism for regulating osteoclast activity. To further investigate the molecular mechanism responsible for the divergent effects of continuous versus discontinuous exposure to TGF-beta, we examined RANK expression and p38 MAPK activation. We found the TGF-beta strongly induced p38 MAPK in monocytes, but not in mature osteoclasts, and that continuous exposure of TGF-beta to monocytes down-regulated RANK expression. The current results suggest that TGF-beta promotes human osteoclastogenesis in monocytes through stimulation of the p38 MAPK, whereas continuous exposure to TGF-beta abrogates osteoclastogenesis through down-regulation of RANK expression and therefore attenuation of RANK-RANK-L signaling.

Prostaglandin E2 stimulates bone sialoprotein (BSP) expression through cAMP and fibroblast growth factor 2 response elements in the proximal promoter of the rat BSP gene

Bone sialoprotein (BSP), an early marker of osteoblast differentiation, has been implicated in the nucleation of hydroxyapatite during de novo bone formation. Prostaglandin E2 (PGE2) has anabolic effects on proliferation and differentiation of osteoblasts via diverse signal transduction systems. Because PGE2 increases the proportion of functional osteoblasts in fetal rat calvarial cell cultures, we investigated the regulation of BSP, as an osteoblastic marker, by PGE2. Treatment of rat osteosarcoma UMR 106 cells with 3 microm, 300 nm, and 30 nm PGE2 increased the steady state levels of BSP mRNA about 2.7-, 2.5-, and 2.4-fold after 12 h. From transient transfection assays, the constructs including the promoter sequence of nucleotides (nt) -116 to +60 (pLUC3) were found to enhance transcriptional activity 3.8- and 2.2-fold treated with 3 microm and 30 nm PGE2 for 12 h. 2-bp mutations were made in an inverted CCAAT box (between nt -50 and -46), a cAMP response element (CRE; between nt -75 and -68), a fibroblast growth factor 2 response element (FRE; nt -92 to -85), and a pituitary-specific transcription factor-1 motif (between nt -111 and -105) within pLUC3 and pLUC7 constructs. Transcriptional stimulation by PGE2 was almost completed abrogated in constructs that included 2-bp mutations in either the CRE and FRE. In gel shift analyses an increased binding of nuclear extract components to double-stranded oligonucleotide probes containing CRE and FRE was observed following treatment with PGE2. These studies show that PGE2 induces BSP transcription in UMR 106 cells through juxtaposed CRE and FRE elements in the proximal promoter of the BSP gene.

Receptor activator of NF-[kappa]B ligand arrests bone growth and promotes cortical bone resorption in growing rats

Receptor activator of NF-kappaB ligand (RANKL), produced by osteoblastic lineage cells and activated T cells, is an essential factor for osteoclast differentiation, activation, and survival. Therefore, RANKL is a focal point of therapies targeting bone diseases where there is an imbalance of bone metabolism in favor of bone resorption. The present study assesses the effects of exogenous RANKL on growing bone. RANKL (100 microg x kg-1x day-1 for 7 days) administered to Sprague-Dawley weanling rats caused major deficits in growth, appearance, and bone mineral densities (BMD). Urinary deoxypyridinoline crosslinks, a measure of bone turnover, were higher in the RANKL-treated rats (P = 0.031), and the bone mineral content was lower (P < 0.001). The final BMD in the RANKL-treated rats was lower (P = 0.039) than in the control rats (19 +/- 7 vs. 38 +/- 5 mg/cm3). Moreover, calculated cortical bone density in each bone slice (total BMD - trabecular BMD) indicated there was only 5% cortical bone remaining in RANKL-treated rats. We conclude that therapies targeting RANKL are likely to have effects on cortical as well as trabecular bone density.

Dietary n-3 fatty acids decrease osteoclastogenesis and loss of bone mass in ovariectomized mice

The mechanisms of action of dietary fish oil (FO) on osteoporosis are not fully understood. This study showed FO decreased bone loss in ovariectomized mice because of inhibition of osteoclastogenesis. This finding supports a beneficial effect of FO on the attenuation of osteoporosis.

INTRODUCTION

Consumption of fish or n-3 fatty acids protects against cardiovascular and autoimmune disorders. Beneficial effects on bone mineral density have also been reported in rats and humans, but the precise mechanisms involved have not been described.

METHODS

Sham and ovariectomized (OVX) mice were fed diets containing either 5% corn oil (CO) or 5% fish oil (FO). Bone mineral density was analyzed by DXA. The serum lipid profile was analyzed by gas chromatography. Receptor activator of NF-kappaB ligand (RANKL) expression and cytokine production in activated T-cells were analyzed by flow cytometry and ELISA, respectively. Osteoclasts were generated by culturing bone marrow (BM) cells with 1,25(OH)2D3. NF-kappaB activation in BM macrophages was measured by an electrophoretic mobility shift assay.

RESULTS AND CONCLUSION

Plasma lipid C16:1n6, C20:5n3, and C22:6n3 were significantly increased and C20:4n6 and C18:2n6 decreased in FO-fed mice. Significantly increased bone mineral density loss (20% in distal left femur and 22.6% in lumbar vertebrae) was observed in OVX mice fed CO, whereas FO-fed mice showed only 10% and no change, respectively. Bone mineral density loss was correlated with increased RANKL expression in activated CD4+ T-cells from CO-fed OVX mice, but there was no change in FO-fed mice. Selected n-3 fatty acids (docosahexaenoic acid [DHA] and eicosapentaenoic acid [EPA]) added in vitro caused a significant decrease in TRACP activity and TRACP+ multinuclear cell formation from BM cells compared with selected n-6 fatty acids (linoleic acid [LA] and arachidonic acid [AA]). DHA and EPA also inhibited BM macrophage NF-kappaB activation induced by RANKL in vitro. TNF-alpha, interleukin (IL)-2, and interferon (IFN)-gamma concentrations from both sham and OVX FO-fed mice were decreased in the culture medium of splenocytes, and interleukin-6 was decreased in sham-operated FO-fed mice. In conclusion, inhibition of osteoclast generation and activation may be one of the mechanisms by which dietary n-3 fatty acids reduce bone loss in OVX mice.

Protective role of n-3 lipids and soy protein in osteoporosis

It is well established that bone loss due to estrogen deficiency after menopause is greater in women consuming higher quantities of animal protein than in women consuming vegetable protein, particularly soy protein. Besides the dietary protein source altering bone loss, it has also been postulated recently that the source of a higher n-6/n-3 ratio in dietary oils is implicated in causing osteoporosis. Both animal and human studies have indicated that an increased intake of n-6 fatty acids from vegetable oils elevates prostaglandin E(2) levels as well as pro-inflammatory cytokines such as IL-1, IL-6 and TNF-alpha. Interestingly, it has been found that lack of estrogen also increases the production of these cytokines by immune cells and thereby activates osteoclasts during the peri-menopausal period. We speculated that the use of n-3 fatty acids and soy protein, which are known to act as anti-inflammatory and down regulate pro-inflammatory cytokines, may also protect against bone loss by decreasing osteoclast activation and bone resorption. Similar to the results of others, our ongoing studies indeed show that the bone loss in ovariectomized mice is significantly attenuated by feeding diets enriched with either fish oil or soy protein when compared to corn oil and casein-fed mice. One of the mechanisms appears to be decreasing the activation of receptor activator of NF-kappaB ligand (RANKL) on T cells, which has been found to increase osteoclast activation along with increasing pro-inflammatory cytokines in OVX mice. Since hormone replacement therapy has been found to cause adverse effects, further both animal and human studies are required with moderate soy protein and fish oil supplements in understanding the mechanisms involved in altering immune function and bone loss during menopause in women and aging in men.

Soybean isoflavones inhibit tumor necrosis factor-alpha-induced apoptosis and the production of interleukin-6 and prostaglandin E2 in osteoblastic cells

The effects of individual soybean isoflavones, genistein (4',5,7-trihydroxyisoflavone) and daidzein (4',7-dihydroxyisoflavone), on tumor necrosis factor-alpha (TNF-alpha)-induced apoptosis and the production of local factors in osteoblastic cells has been investigated. Soybean isoflavones increased DNA synthesis and the number of viable cells. When cells were treated with TNF-alpha, the number of viable cells dose-dependently decreased. The decrease in cell number caused by TNF-alpha treatment was due to apoptosis, which was confirmed by TUNEL and cell death ELISA analyses. Soybean isoflavones inhibited apoptosis of osteoblastic cells subjected to TNF-alpha treatment. MC3T3-E1 osteoblastic cells secrete interleukin-6 (IL-6), interleukin-1beta (IL-1beta), nitric oxide (NO) and prostaglandin E(2) (PGE(2)) constitutively, but at low levels. Soybean isoflavones had no effect on the constitutive production of these local factors. When cells were treated with TNF-alpha (10(-10)M), the production of IL-6 and PGE(2), but not that of IL-1beta and NO, significantly increased. Treatment with soybean isoflavones (10(-5)M), in the presence of TNF-alpha (10(-10)M), for 48 h inhibited production of IL-6 and PGE(2), suggesting the antiresorptive action of soy phytoestrogen may be mediated by decreases in these local factors. The findings of this study thus suggest that soybean isoflavones may promote the function of osteoblastic cells and play an important role in bone remodeling.

Role of RANK ligand in mediating increased bone resorption in early postmenopausal women

Studies in rodents have implicated various cytokines as paracrine mediators of increased osteoclastogenesis during estrogen deficiency, but increases in RANKL, the final effector of osteoclastogenesis, have not been demonstrated. Thus, we isolated bone marrow mononuclear cells expressing RANKL on their surfaces by two-color flow cytometry using FITC-conjugated osteoprotegerin-Fc (OPG-Fc-FITC) as a probe. The cells were characterized as preosteoblastic marrow stromal cells (MSCs), T lymphocytes, or B lymphocytes by using Ab's against bone alkaline phosphatase (BAP), CD3, and CD20, respectively, in 12 premenopausal women (Group A), 12 early postmenopausal women (Group B), and 12 age-matched, estrogen-treated postmenopausal women (Group C). Fluorescence intensity of OPG-Fc-FITC, an index of the surface concentration of RANKL per cell, was increased in Group B over Groups A and C by two- to threefold for MSCs, T cells, B cells, and total RANKL-expressing cells. Moreover, in the merged groups, RANKL expression per cell correlated directly with the bone resorption markers, serum C-terminal telopeptide of type I collagen and urine N-telopeptide of type I collagen, in all three cell types and inversely with serum 17beta-estradiol for total RANKL-expressing cells. The data suggest that upregulation of RANKL on bone marrow cells is an important determinant of increased bone resorption induced by estrogen deficiency.

Osteoclastogenesis-related antigen, a novel molecule on mouse stromal cells, regulates osteoclastogenesis

Osteoclastogenesis is regulated by RANKL expressed on stromal cells. In this study, we sought to isolate a new surface molecule regulating osteoclastogenesis on stromal cells by generating monoclonal antibodies. A rat was immunized with the mouse stromal cell line, TSB13, which can support osteoclastogenesis, and a monoclonal antibody, A15-1, was obtained. A15-1 bound to a surface antigen on TSB13 cells, termed osteoclastogenesis-related antigen (OCRA), and immunoprecipitation with this antibody revealed that OCRA was a 220-kDa molecule. By means of flow cytometry, the A15-1 antigen (OCRA) was found to be expressed on various mesenchymal cell lines but not on hematopoietic cell lines, and the expression level of OCRA on the TSB13 cells was slightly increased by treatment with 1alpha,25(OH)2D3. When osteoclast progenitors and TSB13 cells were co-cultured in the presence of 1alpha,25(OH)2D3, the addition of A15-1 inhibited osteoclast differentiation in a dose-dependent manner; however, no significant inhibition of soluble RANKL-induced osteoclastogenesis was observed, suggesting that A15-1 inhibited only stromal cell-dependent osteoclastogenesis. The same inhibitory effect of A15-1 was also observed when primary bone marrow-derived stromal cells were used. The osteoclastogenesis-promoting effects of other osteotropic factors, such as parathyroid hormone (PTH) and interleukin (IL)-1beta, were also inhibited by A15-1. Time-course analysis of osteoclast differentiation in vitro indicated that the initial 2 days of treatment with A15-1 was sufficient for inhibition, suggesting that A15-1 inhibits the early stages of osteoclast differentiation. Finally, we investigated the in vivo effects of A15-1 on PTH-induced hypercalcemia in mice. Treatment with A15-1 significantly decreased the osteoclast surface in the PTH-administered mice. Taken together, our data indicate that OCRA, a novel A15-1-detected antigen, regulates stromal cell-dependent osteoclastogenesis.

RANK-Fc: a therapeutic antagonist for RANK-L in myeloma

BACKGROUND

Severe bone destruction due to inappropriate osteoclastogenesis is a prominent feature of multiple myeloma (MM). MM increases bone loss by disrupting the checks that normally control signaling by receptor activator of nuclear factor kappaB ligand (RANK-L, also called TRANCE [tumor necrosis factor-related, activation-induced cytokine], osteoprotegerin ligand [OPG-L], osteoclast differentiation factor [ODF], and tumor necrosis factor superfamily member 11 [TNFSF11]), a TNF-family cytokine required for osteoclast differentiation and activation. RANK-L binds to its functional receptor RANK (TNF receptor superfamily member 11a [TNF RSF11a]) to stimulate osteoclastogenesis. Osteotropic cytokines regulate this process by controlling bone marrow stromal expression of RANK-L. Further control over osteoclastogenesis is maintained by regulated expression of osteoprotegerin (OPG, also called osteoclastogenesis inhibitory factor and TNFRSF11b), a soluble decoy receptor for RANK-L. In normal bone marrow, abundant stores of OPG in stroma, megakaryocytes, and myeloid cells provide a natural buffer against increased RANK-L. MM disrupts these controls by increasing expression of RANK-L and decreasing expression of OPG. Concurrent deregulation of RANK-L and OPG expression is found in bone marrow biopsies from patients with MM but not in specimens from patients with non-MM hematologic malignancies.

METHODS

RANK-Fc is a recombinant RANK-L antagonist that is formed by fusing the extracellular domain of RANK to the Fc portion of human immunoglobulin G(1) (hIgG(1)). In vitro, addition of RANK-Fc virtually eliminates the formation of osteoclasts in cocultures of MM with bone marrow and osteoblast/stromal cells. The severe combined immunodeficiency (SCID)/ARH77 mouse model and the SCID-hu-MM mouse model of human MM were used to assess the ability of RANK-Fc to block the development of MM-induced bone disease in vivo. Mice received either RANK-Fc or hIgG(1) 200 microg intravenously three times per week.

RESULTS

RANK-Fc limited bone destruction in both the SCID/ARH-77 model and the SCID-hu-MM model. Administration of RANK-Fc also caused a marked reduction in tumor burden and serum paraprotein in SCID-hu-MM mice that was associated with the restoration of OPG and a reduction in RANK-L expression in the xenograft.

CONCLUSIONS

MM-induced bone destruction requires increased RANK-L expression and is facilitated by a concurrent reduction in OPG, a natural decoy receptor for RANK-L. Administration of the RANK-L antagonist RANK-Fc limits MM-induced osteoclastogenesis, development of bone disease, and MM tumor progression.

IL-7 induces bone loss in vivo by induction of receptor activator of nuclear factor kappa B ligand and tumor necrosis factor alpha from T cells

IL-7, a powerful lymphopoietic cytokine, is elevated in rheumatoid arthritis (RA) and known to induce bone loss when administered in vivo. IL-7 has been suggested to induce bone loss, in part, by stimulating the proliferation of B220(+) cells, a population capable of acting as early osteoclast (OC) precursors. However, the mechanism by which IL-7 leads to differentiation of precursors into mature OCs remains unknown. We previously reported that, in vitro, IL-7 up-regulated T cell cytokines including receptor activator of nuclear factor kappaB ligand (RANKL). To demonstrate the importance of T cells to the bone-wasting effect of IL-7 in vivo, we have now examined IL-7-induced bone loss in T cell-deficient nude mice. We show that T cell-replete mice undergo significant osteoclastic bone loss after IL-7 administration, concurrent with induction of RANKL and tumor necrosis factor alpha (TNF-alpha) secretion by splenic T cells. In contrast, nude mice were resistant to IL-7-induced bone loss and showed no detectable increase in either RANKL or TNF-alpha, despite an up-regulation of B220(+) cells. Importantly, T cell adoptive transfer into nude mice restored IL-7-induced bone loss, and RANKL and TNF-alpha secretion, demonstrating that T cells are essential mediators of IL-7-induced bone loss in vivo.

Osteoclastogenic potential of bone marrow cells increases with age in elderly women with fracture

The most reliable explanation for decreasing bone mass in elderly women is an imbalance of osteoclastic resorption and osteoblastic formation resulting from a relative increase in osteoclastic resorption. However, it is not clear whether an increase in osteoclastic bone resorption with age is due to increased osteoclast formation or to osteoclastic bone resorption activity. In this study, using a human bone marrow culture system, we attempt to clarify the increase in osteoclast formation with age. The mononuclear cell-rich fraction from bone marrow, obtained from the proximal region of the femur from female elderly patients with fracture, were cultured for 14 days in the presence of 1,25 dihydroxyvitamin D(3). Tartrate-resistant acid phosphatase-positive multinucleated cells were counted as osteoclasts. In our investigation, human osteoclast formation in the bone marrow culture increased with age in elderly women (age 64-96 years). The osteoclast formation was positively correlated with macrophage-colony stimulation factor and prostaglandin E(2) production in bone marrow culture. Also, osteoclast formation ex vivo was negatively correlated with bone mineral density of the lumbar spine (L2-L4). The above results indicate that the osteoclastogenic potential of bone marrow cells increases with aging in elderly women with fracture, and suggest that a decrease in bone mass of elderly women may be due to an increase in osteoclast population associated with aging.

Assessment of periodontal conditions and systemic disease in older subjects. I. Focus on osteoporosis

BACKGROUND

Osteoporosis (OPOR) is a common chronic disease, especially in older women. Patients are often unaware of the condition until they experience bone fractures. Studies have suggested that OPOR and periodontitis are associated diseases and exaggerated by cytokine activity. Panoramic radiography (PMX) allows studies of mandibular cortical index (MCI), which is potentially diagnostic for OPOR.

AIMS

i). To study the prevalence of self-reported history of OPOR in an older, ethnically diverse population, ii). to assess the agreement between PMX/MCI findings and self-reported OPOR, and iii). to assess the likelihood of having both a self-reported history of OPOR and a diagnosis of periodontitis.

MATERIALS AND METHODS

PMX and medical history were obtained from 1084 subjects aged 60-75 (mean age 67.6, SD +/- 4.7). Of the films, 90.3% were useful for analysis. PMXs were studied using MCI. The PMXs were used to grade subjects as not having periodontitis or with one of three grades of periodontitis severity.

RESULTS

A positive MCI was found in 38.9% of the subjects, in contrast to 8.2% self-reported OPOR. The intraclass correlation between MCI and self-reported OPOR was 0.20 (P < 0.01). The likelihood of an association between OPOR and MCI was 2.6 (95%CI: 1.6, 4.1, P < 0.001). Subjects with self-reported OPOR and a positive MCI had worse periodontal conditions (P < 0.01). The Mantel-Haentzel odds ratio for OPOR and periodontitis was 1.8 (95%CI: 1.2, 2.5, P < 0.001).

CONCLUSIONS

The prevalence of positive MCI was high and consistent with epidemiological studies, but only partly consistent with a self-reported history of osteoporosis with a higher prevalence of positive MCI in Chinese women. Horizontal alveolar bone loss is associated with both positive self-reported OPOR and MCI.

RANK-L and RANK: T cells, bone loss, and mammalian evolution

TNF and TNFR family proteins play important roles in the control of cell death, proliferation, autoimmunity, the function of immune cells, or the organogenesis of lymphoid organs. Recently, novel members of this large family have been identified that have critical functions in immunity and that couple lymphoid cells with other organ systems such as bone morphogenesis and mammary gland formation in pregnancy. The TNF-family molecule RANK-L (RANK-L, TRANCE, ODF) and its receptor RANK are key regulators of bone remodeling, and they are essential for the development and activation of osteoclasts. Intriguingly, RANK-L/RANK interactions also regulate T cell/dendritic cell communications, dendritic cell survival, and lymph node formation; T cell-derived RANK-L can mediate bone loss in arthritis and periodontal disease. Moreover, RANK-L and RANK are expressed in mammary gland epithelial cells, and they control the development of a lactating mammary gland during pregnancy and the propagation of mammalian species. Modulation of these systems provides us with a unique opportunity to design novel therapeutics to inhibit bone loss in arthritis, periodontal disease, and osteoporosis.

Vector-averaged gravity regulates gene expression of receptor activator of NF-kappaB (RANK) ligand and osteoprotegerin in bone marrow stromal cells via cyclic AMP/protein kinase A pathway

Bone loss due to unloading of the skeleton may be caused by an acceleration of osteoclastic bone resorption as well as a decline of osteoblastic bone formation. Recently, two molecular species that play important roles in osteoclastogenesis were discovered: (i) the receptor activator of NF-kappaB ligand (RANKL)/osteoprotegerin (OPG) ligand/osteoclast differentiation factor induces osteoclastogenesis; and (ii) the OPG/osteoclastogenesis inhibitory factor potently inhibits osteoclastogenesis. To investigate the effects of gravity on gene expression of RANKL and OPG, a mouse bone marrow-derived stromal cell line, ST2, was cultured on a single axis clinostat, which generates a vector-averaged gravity environment. Northern blot analysis revealed that RANKL mRNA was increased, whereas that of OPG decreased. The clinostat culture also caused an increase in intracellular cyclic (cAMP) level. Both forskolin and dibutyryl-cAMP mimicked the regulation of RANKL and OPG transcription in clinostat culture. These modulations of gene expression in clinostat culture were blocked by a protein kinase A (PKA) inhibitor, H89, but not by a cyclooxygenase inhibitor, indomethacin. The enhancement of RANKL gene expression under clinostat culture and its inhibition by H89 were confirmed by a reporter assay with the murine RANKL 5'-flanking region. These results suggest that modulations of RANKL and OPG expression in stromal cells might be one of the causes of bone loss during skeletal unloading. An elevation of intracellular cAMP level caused through an as yet undetermined pathway is involved in modulation of RANKL and OPG expression during clinostat culture.

Effects of prostaglandin E2 on gene expression in primary osteoblastic cells from prostaglandin receptor knockout mice

Recent studies have shown that stimulation of osteoclastogenesis in cocultures of osteoblasts and spleen cells in response to prostaglandin E2 (PGE2) is markedly decreased when the osteoblasts are derived from cells lacking either the EP2 or the EP4 receptor. Induction of osteoclast formation requires upregulation of receptor activator of nuclear factor-kappaB ligand (RANKL) on cells of the osteoblastic lineage, which then binds to the RANK receptor on cells of the osteoclast lineage. Osteoprotegerin (OPG) is a decoy receptor for RANKL that can block its interaction with RANK. In addition, macrophage-colony stimulating factor (M-CSF) is essential for osteoclast formation. Finally, PGE2 can increase interleukin-6 (IL-6), which may further enhance osteoclastogenesis. To study the relative influence of the EP2 and EP4 receptors on response of these factors to PGE2, we examined mRNA levels for RANKL, OPG, M-CSF, and IL-6 in primary osteoblastic cell cultures derived from two lines of EP2 knockout mice (EP2-/-) and one line of EP4 knockout mice (EP4-/-) and the relevant wild-type controls (EP2+/+ and EP4+/+). The responses of cells from wild-type animals of all three lines were similar. After PGE2 treatment, RANKL mRNA levels were increased at 2 h, and this was sustained over 72 h. Basal RANKL expression was moderately reduced in EP2-/- cells and markedly reduced in EP4-/- cells. PGE2 increased RANKL mRNA in EP2-/- cells and EP4-/- cells, but the levels were significantly reduced compared with wild-type cells. There were no consistent changes in expression of M-CSF or OPG in the different genotypes or with PGE2 treatment. IL-6 mRNA was variably increased by PGE2 in both wild-type and knockout cells, although the absolute levels were somewhat lower in both EP2-/- and EP4 -/- cultures. Parathyroid hormone (PTH) increased RANKL and IL-6 and decreased OPG mRNA levels similarly in both wild-type and EP2-/- or EP4-/- cells. The major defect in the response to PGE2 in animals lacking either EP2 or EP4 receptors is a reduction in basal and stimulated RANKL levels. Loss of EP4 receptor appears to have a greater effect on basal RANKL expression than EP2.

Changes in proinflammatory cytokine activity after menopause

There is now a large body of evidence suggesting that the decline in ovarian function with menopause is associated with spontaneous increases in proinflammatory cytokines. The cytokines that have obtained the most attention are IL-1, IL-6, and TNF-alpha. The exact mechanisms by which estrogen interferes with cytokine activity are still incompletely known but may potentially include interactions of the ER with other transcription factors, modulation of nitric oxide activity, antioxidative effects, plasma membrane actions, and changes in immune cell function. Experimental and clinical studies strongly support a link between the increased state of proinflammatory cytokine activity and postmenopausal bone loss. Preliminary evidence suggests that these changes also might be relevant to vascular homeostasis and the development of atherosclerosis. Better knowledge of the mechanisms and the time course of these interactions may open new avenues for the prevention and treatment of some of the most prevalent and important disorders in postmenopausal women.

Differential expression of bone matrix regulatory proteins in human atherosclerotic plaques

In the present study, we examined the expression of regulators of bone formation and osteoclastogenesis in human atherosclerosis because accumulating evidence suggests that atherosclerotic calcification shares features with bone calcification. The most striking finding of this study was the constitutive immunoreactivity of matrix Gla protein, osteocalcin, and bone sialoprotein in nondiseased aortas and the absence of bone morphogenetic protein (BMP)-2, BMP-4, osteopontin, and osteonectin in nondiseased aortas and early atherosclerotic lesions. When atherosclerotic plaques demonstrated calcification or bone formation, BMP-2, BMP-4, osteopontin, and osteonectin were upregulated. Interestingly, this upregulation was associated with a sustained immunoreactivity of matrix Gla protein, osteocalcin, and bone sialoprotein. The 2 modulators of osteoclastogenesis (osteoprotegerin [OPG] and its ligand, OPGL) were present in the nondiseased vessel wall and in early atherosclerotic lesions. In advanced calcified lesions, OPG was present in bone structures, whereas OPGL was only present in the extracellular matrix surrounding calcium deposits. The observed expression patterns suggest a tight regulation of the expression of bone matrix regulatory proteins during human atherogenesis. The expression pattern of both OPG and OPGL during atherogenesis might suggest a regulatory role of these proteins not only in osteoclastogenesis but also in atherosclerotic calcification.

Nutraceutical fatty acids as biochemical and molecular modulators of skeletal biology

Several systemic hormones and localized growth factors coordinate events of bone formation and resorption to support bone growth in the young and maintain bone mineral content in the adult. Some of the more important factors produced in the bone microenvironment that impact skeletal biology include prostaglandins, cytokines, and insulin-like growth factors. Dietary fat sources that exert potent biological effects on the skeletal tissues belong to the omega-6 and omega-3 families of essential fatty acids. Specific long-chain polyunsaturated fatty acids (PUFA) belonging to these families are substrates for prostanoids that influence the differentiation and activity of cells in bone and cartilage tissues. These PUFA appear to alter prostanoid formation, cell-to-cell signaling processes, and impact transcription factors in vivo. Hence, these biologically active PUFA can be called nutraceutical fatty acids. This review highlights the role of nutraceutical fatty acids on bone metabolism and joint disease. The recent discovery of transcription factors controlling osteoblast function, and soluble proteins directing osteoclastogenesis and osteoblastogenesis offer new research opportunities for studying nutraceutical fatty acids in skeletal biology.

Connection between B lymphocyte and osteoclast differentiation pathways

Osteoclasts differentiate from the hemopoietic monocyte/macrophage cell lineage in bone marrow through cell-cell interactions between osteoclast progenitors and stromal/osteoblastic cells. Here we show another osteoclast differentiation pathway closely connected with B lymphocyte differentiation. Recently the TNF family molecule osteoclast differentiation factor/receptor activator of NF-kappaB ligand (ODF/RANKL) was identified as a key membrane-associated factor regulating osteoclast differentiation. We demonstrate that B-lymphoid lineage cells are a major source of endogenous ODF/RANKL in bone marrow and support osteoclast differentiation in vitro. In addition, B-lymphoid lineage cells in earlier developmental stages may hold a potential to differentiate into osteoclasts when stimulated with M-CSF and soluble ODF/RANKL in vitro. B-lymphoid lineage cells may participate in osteoclastogenesis in two ways: they 1) express ODF/RANKL to support osteoclast differentiation, and 2) serve themselves as osteoclast progenitors. Consistent with these observations in vitro, a decrease in osteoclasts is associated with a decrease in B-lymphoid cells in klotho mutant mice (KL(-/-)), a mouse model for human aging that exhibits reduced turnover during bone metabolism, rather than a decrease in the differentiation potential of osteoclast progenitors. Taken together, B-lymphoid lineage cells may affect the pathophysiology of bone disorders through regulating osteoclastogenesis.

Omega-3 polyunsaturated fatty acids and skeletal health

This minireview on skeletal biology describes the actions of prostaglandins and cytokines involved in the local regulation of bone metabolism, it documents the role of lipids in bone biology, and it presents relationships between fatty acids and other factors that impact skeletal metabolism. The data presented herein show consistent and reproducible beneficial effects of omega-3 (n-3) fatty acids on bone metabolism and bone/joint diseases. Polyunsaturated fatty acids modulate eicosanoid biosynthesis in numerous tissues and cell types, alter signal transduction, and influence gene expression. These effects have not been explored in the skeletal system. Future research on n-3 fatty acids in bone biology should focus on the following two aspects. First, the further elucidation of how n-3 fatty acids alter biochemical and molecular processes involved in bone modeling and bone cell differentiation, and second, the evaluation of the potential pharmaceutical applications of these nutraceutical fatty acids in maintaining bone mineral status and controlling inflammatory bone/joint diseases.

Lipids as modulators of bone remodelling

Bone remodelling processes are regulated by systemic hormones and a multitude of local and systemic factors, including prostaglandins, cytokines, and growth factors. Dietary fatty acids and their derivatives (eicosanoids) have been a recent focus of investigation on bone and cartilage metabolism. Specific fatty acids are recognized modulators of eicosanoid biosynthesis, signal transduction, and gene expression. The actions of polyunsaturated fatty acids have not been extensively examined in the skeletal system. Promising research on fatty acids and bone remodelling should evaluate the potential effects on pathways for osteoclastogenesis and osteoblastogenesis.