Impaired bone resorption by lipopolysaccharide in vivo in mice deficient in the prostaglandin E receptor EP4 subtype

Prostaglandin E2 enhances osteoclastic differentiation of precursor cells through protein kinase A-dependent phosphorylation of TAK1

Prostaglandin E2 (PGE2) synergistically enhances the receptor activator for NF-kappa B ligand (RANKL)-induced osteoclastic differentiation of the precursor cells. Here we investigated the mechanisms of the stimulatory effect of PGE2 on osteoclast differentiation. PGE2 enhanced osteoclastic differentiation of RAW264.7 cells in the presence of RANKL through EP2 and EP4 prostanoid receptors. RANKL-induced degradation of I kappa B alpha and phosphorylation of p38 MAPK and c-Jun N-terminal kinase in RAW264.7 cells were up-regulated by PGE2 in a cAMP-dependent protein kinase A (PKA)-dependent manner, suggesting that EP2 and EP4 signals cross-talk with RANK signals. Transforming growth factor beta-activated kinase 1 (TAK1), an important MAPK kinase kinase in several cytokine signals, possesses a PKA recognition site at amino acids 409-412. PKA directly phosphorylated TAK1 in RAW264.7 cells transfected with wild-type TAK1 but not with the Ser412 --> Ala mutant TAK1. Ser412 --> Ala TAK1 served as a dominant-negative mutant in PKA-enhanced degradation of I kappa B alpha, phosphorylation of p38 MAPK, and PGE2-enhanced osteoclastic differentiation in RAW264.7 cells. Furthermore, forskolin enhanced tumor necrosis factor alpha-induced I kappa B alpha degradation, p38 MAPK phosphorylation, and osteoclastic differentiation in RAW264.7 cells. Ser412 --> Ala TAK1 abolished the stimulatory effects of forskolin on those cellular events induced by tumor necrosis factor alpha. Ser412 --> Ala TAK1 also inhibited the forskolin-induced up-regulation of interleukin 6 production in RAW264.7 cells treated with lipopolysaccharide. These results suggest that the phosphorylation of the Ser412 residue in TAK1 by PKA is essential for cAMP/PKA-induced up-regulation of osteoclastic differentiation and cytokine production in the precursor cells.

Lipopolysaccharide stimulates expression of osteoprotegerin and receptor activator of NF-kappa B ligand in periodontal ligament fibroblasts through the induction of interleukin-1 beta and tumor necrosis factor-alpha

Our recent work showed that human periodontal ligament fibroblasts (HPLF) secrete bioactive osteoprotegerin (OPG), which inhibits osteoclastic differentiation and activity. However, it is unknown how HPLF regulate bone metabolism in the presence of lipopolysaccharide (LPS), which is a cell component of gram-negative bacteria and a pathogen in inflammatory bone diseases such as periodontitis. The present study examined the effects of Escherichia coli LPS on the gene expression of interleukin-1 beta (IL-1 beta), tumor necrosis factor-alpha (TNF-alpha), OPG, and receptor activator of NF-kappa B ligand (RANKL) in HPLF using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis. In HPLF cultured with LPS for 48 h, expression of both OPG and RANKL mRNA was up-regulated, whereas for up to 24 h of stimulation, such up-regulation was not observed. However, LPS increased expression of IL-1 beta and TNF-alpha mRNA within 6 h of treatment. Moreover, in HPLF cultured with IL-1 beta or TNF-alpha, OPG and RANKL expression was induced within 12 h of culture. The administration of neutralizing antibodies against human IL-1 beta or TNF-alpha to LPS-treated cultures of HPLF inhibited the induction of OPG and RANKL expression. These suggest that LPS stimulates both OPG and RANKL expression in HPLF by up-regulating IL-1 beta and TNF-alpha. In addition, administration of conditioned medium (CM) from HPLF (HPLF-CM) stimulated with LPS for 48 h to mouse bone marrow culture failed to induce osteoclast-like cell (OCL) formation. When mouse spleen cells were cocultured with HPLF in the presence of LPS, OCL formation was completely blocked. Taken together, our results indicate that human periodontal ligament cells stimulated with LPS inhibit osteoclastogenesis by producing more effective OPG than RANKL via the induction of IL-1 beta and TNF-alpha.

Immunolocalization of the prostaglandin E2 receptor subtypes in human bone tissue: differences in foetal, adult normal, osteoporotic and pagetic bone

PGE(2) is an important mediator of bone metabolism, but the precise localization of its receptors in human bone remains unknown. The present study used specific antibodies against EP(1), EP(2), EP(3) and EP(4) receptors for immunolocalization in normal, osteoporotic and pagetic human adult bone and in human foetal bone. No labelling was obtained for the EP(1) and EP(2) receptors. The EP(3) receptor was detected in foetal osteoclasts, osteoblasts and osteocytes, but only in osteoclasts and some osteoblasts from adult bone. The EP(4) receptor was detected in foetal osteoclasts, osteoblasts and osteocytes and in adult osteoclasts and osteoblasts, but not in adult osteocytes. Our results show differences in PGE(2) receptor expression in foetal and adult human bone but no difference in adult normal compared to pathologic bone. Finally, these results show that the distribution of EP receptors in human osteoblasts in bone corresponds in part to what we recently described in human osteoblasts in culture.

Effects of selective prostaglandin EP2 and EP4 receptor agonists on bone resorption and formation in fetal rat organ cultures

Prostaglandin E2 (PGE2) can stimulate bone resorption and formation through receptors which activate adenylyl cyclase. We examined the effects of selective EP2 and EP4 agonists (EP2A and EP4A) on the release of previously incorporated 45Ca from fetal rat long bones and the incorporation of [3H]-proline or [3H]-thymidine (TdR) in fetal rat calvaria to assess the relative effects of these selective agonists on bone formation and resorption. Only EP4A was effective in increasing 45Ca release. Both agonists increased [3H]-TdR incorporation and [3H]-proline incorporation into calvariae, particularly in the presence of cortisol.

Hard labour: bacterial infection of the skeleton

The skeleton is the largest mammalian organ system, containing a myriad of blood vessels, tissue surfaces and bone cells for bacterial colonization. Although rock-like, the skeleton is a dynamic structure that is undergoing constant remodelling. This is the result of the opposing actions of two key cells: the osteoblast, which produces bone, and the osteoclast, a multinucleate cell that 'eats' bone. It is not generally realized that the most prevalent chronic bacterial diseases of Homo sapiens afflict the skeleton. Several pathogens, and members of the normal microbiota, have evolved specific cellular and molecular mechanisms for invading bone, including its cellular constituents. The host cellular pathways that are activated and lead to destruction or loss of the bone matrix will be described.

Suppression of Osteoprotegerin Expression by Prostaglandin E2Is Crucially Involved in Lipopolysaccharide-Induced Osteoclast Formation

LPS is a potent stimulator of bone resorption in inflammatory diseases. The mechanism by which LPS induces osteoclastogenesis was studied in cocultures of mouse osteoblasts and bone marrow cells. LPS stimulated osteoclast formation and PGE(2) production in cocultures of mouse osteoblasts and bone marrow cells, and the stimulation was completely inhibited by NS398, a cyclooxygenase-2 inhibitor. Osteoblasts, but not bone marrow cells, produced PGE(2) in response to LPS. LPS-induced osteoclast formation was also inhibited by osteoprotegerin (OPG), a decoy receptor of receptor activator of NF-kappaB ligand (RANKL), but not by anti-mouse TNFR1 Ab or IL-1 receptor antagonist. LPS induced both stimulation of RANKL mRNA expression and inhibition of OPG mRNA expression in osteoblasts. NS398 blocked LPS-induced down-regulation of OPG mRNA expression, but not LPS-induced up-regulation of RANKL mRNA expression, suggesting that down-regulation of OPG expression by PGE(2) is involved in LPS-induced osteoclast formation in the cocultures. NS398 failed to inhibit LPS-induced osteoclastogenesis in cocultures containing OPG knockout mouse-derived osteoblasts. IL-1 also stimulated PGE(2) production in osteoblasts and osteoclast formation in the cocultures, and the stimulation was inhibited by NS398. As seen with LPS, NS398 failed to inhibit IL-1-induced osteoclast formation in cocultures with OPG-deficient osteoblasts. These results suggest that IL-1 as well as LPS stimulates osteoclastogenesis through two parallel events: direct enhancement of RANKL expression and suppression of OPG expression, which is mediated by PGE(2) production.

EP2 and EP4 prostanoid receptor signaling

The EP(2) and EP(4) prostanoid receptors are two of the four subtypes of receptors for prostaglandin E(2) (PGE(2)). They are in the family of G-protein coupled receptors and both receptors were initially characterized as coupling to Gs and increasing intracellular cAMP formation. Recently, however, we have shown that both receptors can stimulate T-cell factor (Tcf) mediated transcriptional activity. The EP(2) receptor does this primarily through cAMP-dependent protein kinase (PKA), whereas the EP(4) utilizes phosphatidylinositol 3-kinase (PI3K) as well as PKA. In addition, we have shown that the EP(4) receptor, but not the EP(2), can activate the extracellular signal-regulated kinases (ERKs) 1 and 2 by way of PI3K leading to the induction of early growth response factor-1 (EGR-1), a transcription factor traditionally associated with wound healing. This induction of EGR-1 expression has significant implications concerning the potential role of PGE(2) in cancer and inflammatory disorders.

Contribution of membrane-associated prostaglandin E2 synthase to bone resorption

This study initially confirmed that, among prostaglandins (PGs) produced in bone, only PGE(2) has the potency to stimulate osteoclastogenesis and bone resorption in the mouse coculture system of osteoblasts and bone marrow cells. For the PGE(2) biosynthesis two isoforms of the terminal and specific enzymes, membrane-associated PGE(2) synthase (mPGES) and cytosolic PGES (cPGES) have recently been identified. In cultured mouse primary osteoblasts, both mPGES and cyclooxygenase-2 were induced by the bone resorptive cytokines interleukin-1, tumor necrosis factor-alpha, and fibroblast growth factor-2. Induction of mPGES was also seen in the mouse long bone and bone marrow in vivo by intraperitoneal injection of lipopolysaccharide. In contrast, cPGES was expressed constitutively both in vitro and in vivo without being affected by these stimuli. An antisense oligonucleotide blocking mPGES expression inhibited not only PGE(2) production, but also osteoclastogenesis and bone resorption stimulated by the cytokines, which was reversed by addition of exogenous PGE(2). We therefore conclude that mPGES, which is induced by and mediates the effects of bone resorptive stimuli, may make a target molecule for the treatment of bone resorptive disorders.

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.

Prostaglandin E2 induced the differentiation of osteoclasts in mouse osteoblast-depleted bone marrow cells

Prostaglandin (PG) E(2) is a known bone absorbing agent that acts on osteoblasts to facilitate osteoclastogenesis by increasing the secretion of RANKL. In the present study, we investigated the direct action of PGE(2) on osteoclastic progenitors that differentiate into TRAP-positive multinucleated cells. The hematopoietic stem cell obtained from murine bone marrow was purified by a Sephadex G-10 column, and cultured in the presence of CSF-1 and RANKL to facilitate cell differentiation. The introduction of low-density PGE(2) into the culture resulted in a drastic increase of TRAP-positive multinucleated cells, whereas the addition of high-density PGE(2) had the opposite effect. PCR analysis revealed increased level of EP3 mRNA in undifferentiated cells and reduced level after the development of osteoclast; EP1, EP2 and EP4 were constitutively expressed throughout the differentiation. Investigation of intracellular signaling verified that low-density PGE(2) suppressed PKA activity in undifferentiated cells, suggesting that PGE(2) acts on the osteoclastic cell lineage to facilitate cell differentiation by suppressing PKA in the presence of RANKL.

Induction of osteoclastogenesis and matrix metalloproteinase expression by the lipooligosaccharide of Treponema denticola

Alveolar bone destruction is a characteristic feature of periodontitis. Treponema denticola is known to be involved in periodontitis. To elucidate the role of T. denticola in alveolar bone destruction in periodontitis, the effects of lipooligosaccharide (LOS) from T. denticola on osteoclast formation and on expression of osteoclast differentiation factor (ODF) and osteoprotegerin (OPG) mRNAs were examined in a coculture system by using mouse calvaria and bone marrow cells. In addition, the effect of T. denticola LOS on expression of matrix metalloproteinases (MMPs), which are involved in bone resorption, was estimated in mouse calvaria-derived osteoblastic cells. When the mouse calvaria and bone marrow cells were challenged with LOS (0.1 to 10 micro g/ml) for 4 days, the number of tartrate-resistant acid phosphatase-positive multinucleated cells increased in a dose-dependent manner. The expression of ODF mRNA increased, while OPG mRNA expression decreased. Polymyxin B changed the effect of LOS (10 micro g/ml) on ODF and OPG mRNA expression to the control level. LOS (10 micro g/ml) stimulated prostaglandin E(2) (PGE(2)) production in the cocultures. Adding indomethacin, an inhibitor of prostaglandin synthesis, resulted in a reduction in the number of osteoclasts induced by LOS and eliminated the effect of T. denticola LOS on ODF and OPG mRNA expression. T. denticola LOS increased the levels of mRNAs encoding MMP-3, -8, -9, -10, -13, and -14. Expression of one of these mRNAs, MMP-9 mRNA, was significantly induced by T. denticola LOS. These findings suggest that LOS from T. denticola stimulates osteoclastogenesis and MMP expression. Up-regulation of ODF and down-regulation of OPG by a PGE(2)-dependent mechanism were involved in the osteoclastogenesis induced by T. denticola LOS.

Genetic susceptibility to chronic periodontal disease

Evidence suggests that there is a significant genetic component to susceptibility and resistance to chronic periodontal disease. Data from both clinical studies and studies using animal models are reviewed here. Also outlined are the genomic methods that are now available for identifying susceptibility and resistance loci.

The role of prostaglandin E2 receptors in the pathogenesis of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disorder leading to bone and cartilage destruction. A substantial body of evidence suggests that prostaglandin E2 (PGE2) contributes to the pathogenesis of RA, and nonsteroidal anti-inflammatory drugs, inhibitors of the synthesis of PGE2 and other prostanoids, continue to be used in the treatment of this disease. To begin to understand the mechanism by which prostaglandins modulate the pathophysiology of this disease, we examined mice lacking each of the four known PGE2 (EP) receptors after generation of collagen antibody-induced arthritis, an animal model of RA. Homozygous deletion of the EP1, EP2, or EP3 receptors did not affect the development of arthritis, whereas EP4 receptor-deficient mice showed decreased incidence and severity of disease. These animals also showed reduced inflammation as assessed by circulating IL-6 and serum amyloid A levels. Joint histopathology of EP4(-/-) animals revealed reduced bone destruction, proteoglycan loss, and type II collagen breakdown in cartilage compared with EP4(+/+) mice. Furthermore, liver and macrophages isolated from EP4(-/-) animals produced significantly less IL-1 beta and IL-6 than control samples. Thus, PGE2 contributes to disease progression at least in part by binding to the EP4 receptor. Antagonists of this receptor might therefore provide novel agents for the treatment of RA.

Prostaglandin E synthase

Prostaglandin E synthase (PGES), which converts cyclooxygenase (COX)-derived prostaglandin (PG)H2 to PGE2, occurs in multiple forms with distinct enzymatic properties, modes of expression, cellular and subcellular localizations and intracellular functions. Cytosolic PGES (cPGES) is a cytosolic protein that is constitutively expressed in a wide variety of cells and tissues and is associated with heat shock protein 90 (Hsp90). Membrane-associated PGES (mPGES), the expression of which is stimulus-inducible and is downregulated by anti-inflammatory glucocorticoids, is a perinuclear protein belonging to the microsomal glutathione S-transferase (GST) family. These two PGESs display distinct functional coupling with upstream COXs in cells; cPGES is predominantly coupled with the constitutive COX-1, whereas mPGES is preferentially linked with the inducible COX-2. Several cytosolic GSTs also have the capacity to convert PGH2 to PGE2 in vitro. Accumulating evidence has suggested that mPGES participates in various pathophysiological states in which COX-2 is involved, implying that mPGES represents a potential novel target for drug development.

Stimulation by toll-like receptors inhibits osteoclast differentiation

Osteoclasts, the cells capable of resorbing bone, are derived from hemopoietic precursor cells of monocyte-macrophage lineage. The same precursor cells can also give rise to macrophages and dendritic cells, which are essential for proper immune responses to various pathogens. Immune responses to microbial pathogens are often triggered because various microbial components induce the maturation and activation of immunoregulatory cells such as macrophages or dendritic cells by stimulating Toll-like receptors (TLRs). Since osteoclasts arise from the same precursors as macrophages, we tested whether TLRs play any role during osteoclast differentiation. We showed here that osteoclast precursors prepared from mouse bone marrow cells expressed all known murine TLRs (TLR1-TLR9). Moreover, various TLR ligands (e.g., peptidoglycan, poly(I:C) dsRNA, LPS, and CpG motif of unmethylated DNA, which act as ligands for TLR2, 3, 4, and 9, respectively) induced NF-kappa B activation and up-regulated TNF-alpha production in osteoclast precursor cells. Unexpectedly, however, TLR stimulation of osteoclast precursors by these microbial products strongly inhibited their differentiation into multinucleated, mature osteoclasts induced by TNF-related activation-induced cytokine. Rather, TLR stimulation maintained the phagocytic activity of osteoclast precursors in the presence of osteoclastogenic stimuli M-CSF and TNF-related activation-induced cytokine. Taken together, these results suggest that TLR stimulation of osteoclast precursors inhibits their differentiation into noninflammatory mature osteoclasts during microbial infection. This process favors immune responses and may be critical to prevent pathogenic effects of microbial invasion on bone.

Phospholipase A2 enzymes

Phospholipase A2 (PLA2) catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to liberate arachidonic acid (AA), a precursor of eicosanoids including prostaglandins and leukotrienes. The same reaction also produces lysophosholipids, which represent another class of lipid mediators. So far, at least 19 enzymes that possess PLA2 activity have been identified and cloned in mammals. The secretory PLA2 (sPLA2) family, in which 10 isozymes have been identified, consists of low-molecular weight, Ca2+-requiring secretory enzymes that have been implicated in a number of biological processes, such as modification of eicosanoid generation, inflammation, and host defense. The cytosolic PLA2 (cPLA2) family consists of three enzymes, among which cPLA2alpha has been paid much attention by researchers as an essential component of the initiation of AA metabolism. The activation of cPLA2alpha is tightly regulated by Ca2+ and phosphorylation. The Ca2+-independent PLA2 (iPLA2) family contains two enzymes and may play a major role in phospholipid remodeling. The platelet-activating factor (PAF) acetylhydrolase (PAF-AH) family contains four enzymes that exhibit unique substrate specificity toward PAF and/or oxidized phospholipids. Degradation of these bioactive phospholipids by PAF-AHs may lead to the termination of inflammatory reaction and atherosclerosis.

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.

The growth of malignant keratinocytes depends on signaling through the PGE(2) receptor EP1

Recent discoveries shed light on the importance of prostaglandin (PG) production in the development of skin cancer. Work by Fischer et al. demonstrates that skin tumor promotion caused by ultraviolet B radiation can be decreased by up to 89% by blocking cyclooxygenase-2 (COX-2) with the drug Celecoxib. A similar study showed that Celecoxib can decrease new tumor formation by 44% in mice that already have tumors. These studies demonstrate the importance of COX-2 and PGs in the development of squamous cell carcinoma. We have explored growth signaling in a model of skin tumor progression. Because changes in PG production have been implicated in skin carcinogenesis, we examined this pathway. We found that malignant cell lines secrete more prostaglandin E(2) (PGE(2)) than the parental cells. We observed increased expression of COX-1 and -2. We also found that these cells express the PGE(2) receptors EP1 and EP4. When the cells are grown in the presence of indomethacin, the growth rate of the malignant cells is decreased. This effect can be reversed by addition of PGE(2) or an EP1 agonist to the medium. Thus, we have shown that skin tumor cells depend in part on PGE(2) signaling through the EP1 prostanoid receptor for their in vitro growth.