Activation of cytosolic phospholipase A2 by platelet-derived growth factor is essential for cyclooxygenase-2-dependent prostaglandin E2 synthesis in mouse osteoblasts cultured with interleukin-1

Factors secreted by monosodium urate crystal-stimulated macrophages promote a proinflammatory state in osteoblasts: a potential indirect mechanism of bone erosion in gout

BACKGROUND

Tophi are lesions commonly present at sites of bone erosion in gout-affected joints. The tophus comprises a core of monosodium urate (MSU) crystals surrounded by soft tissue that contains macrophages and other immune cells. Previous studies found that MSU crystals directly reduce osteoblast viability and function. The aim of the current study was to determine the indirect, macrophage-mediated effects of MSU crystals on osteoblasts.

METHODS

Conditioned medium from the RAW264.7 mouse macrophage cell line cultured with MSU crystals was added to the MC3T3-E1 mouse osteoblastic cell line. Conditioned medium from the THP-1 human monocytic cell line cultured with MSU crystals was added to primary human osteoblasts (HOBs). Matrix mineralization was assessed by von Kossa staining. Gene expression was determined by real-time PCR, and concentrations of secreted factors were determined by enzyme-linked immunosorbent assay.

RESULTS

In MC3T3-E1 cells cultured for 13 days in an osteogenic medium, the expression of the osteoblast marker genes Col1a1, Runx2, Sp7, Bglap, Ibsp, and Dmp1 was inhibited by a conditioned medium from MSU crystal-stimulated RAW264.7 macrophages. Mineral staining of MC3T3-E1 cultures on day 21 confirmed the inhibition of osteoblast differentiation. In HOB cultures, the effect of 20 h incubation with a conditioned medium from MSU crystal-stimulated THP-1 monocytes on osteoblast gene expression was less consistent. Expression of the genes encoding cyclooxygenase-2 and IL-6 and secretion of the proinflammatory mediators PGE₂ and IL-6 were induced in MC3T3-E1 and HOBs incubated with conditioned medium from MSU crystal-stimulated macrophages/monocytes. However, inhibition of cyclooxygenase-2 activity and PGE₂ secretion from HOBs indicated that this pathway does not play a major role in mediating the indirect effects of MSU crystals in HOBs.

CONCLUSIONS

Factors secreted from macrophages stimulated by MSU crystals attenuate osteoblast differentiation and induce the expression and secretion of proinflammatory mediators from osteoblasts. We suggest that bone erosion in joints affected by gout results from a combination of direct and indirect effects of MSU crystals.

SM22α-lineage niche cells regulate intramembranous bone regeneration via PDGFRβ-triggered hydrogen sulfide production

Bone stromal cells are critical for bone homeostasis and regeneration. Growing evidence suggests that non-stem bone niche cells support bone homeostasis and regeneration via paracrine mechanisms, which remain to be elucidated. Here, we show that physiologically quiescent SM22α-lineage stromal cells expand after bone injury to regulate diverse processes of intramembranous bone regeneration. The majority of SM22α-lineage cells neither act as stem cells in vivo nor show their expression patterns. Dysfunction of SM22α-lineage niche cells induced by loss of platelet-derived growth factor receptor β (PDGFRβ) impairs bone repair. We further show that PDGFRβ-triggered hydrogen sulfide (H₂S) generation in SM22α-lineage niche cells facilitates osteogenesis and angiogenesis and suppresses overactive osteoclastogenesis. Collectively, these data demonstrate that non-stem SM22α-lineage niche cells support the niche for bone regeneration with a PDGFRβ/H₂S-dependent regulatory mechanism. Our findings provide further insight into non-stem bone stromal niche cell populations and niche-regulation strategy for bone repair.

Ca2+-independent phospholipase A2β-derived PGE2 contributes to osteogenesis

Bone modeling can be modulated by lipid signals such as arachidonic acid (AA) and its cyclooxygenase 2 (COX2) metabolite, prostaglandin E₂ (PGE₂), which are recognized mediators of optimal bone formation. Hydrolysis of AA from membrane glycerophospholipids is catalyzed by phospholipases A₂ (PLA₂s). We reported that mice deficient in the Ca²⁺- independent PLA₂beta (iPLA₂β), encoded by Pla2g6, exhibit a low bone phenotype, but the cause for this remains to be identified. Here, we examined the mechanistic and molecular roles of iPLA₂β in bone formation using bone marrow stromal cells and calvarial osteoblasts from WT and iPLA₂β-deficient mice, and the MC3T3-E1 osteoblast precursor cell line. Our data reveal that transcription of osteogenic factors (Bmp2, Alpl, and Runx2) and osteogenesis are decreased with iPLA₂β-deficiency. These outcomes are corroborated and recapitulated in WT cells treated with a selective inhibitor of iPLA₂ β (10 μM S-BEL), and rescued in iPLA₂β-deficient cells by additions of 10 μM PGE₂. Further, under osteogenic conditions we find that PGE₂ production is through iPLA₂β activity and that this leads to induction of Runx2 and iPLA₂β transcription. These findings reveal a strong link between osteogenesis and iPLA₂β-derived lipids and raise the intriguing possibility that iPLA₂β-derived PGE₂ participates in osteogenesis and in the regulation of Runx2 and also iPLA₂β.

Ketamine suppresses the substance P-induced production of IL-6 and IL-8 by human U373MG glioblastoma/astrocytoma cells

The neuropeptide substance P (SP) is an important mediator of neurogenic inflammation within the central and peripheral nervous systems. SP has been shown to induce the expression of pro-inflammatory cytokines implicated in the pathogenesis of several disorders of the human brain via the neurokinin-1 receptor (NK-1R). Ketamine, an intravenous anesthetic agent, functions as a competitive antagonist of the excitatory neurotransmission N-methyl-D‑aspartate (NMDA) receptor, and also antagonizes the NK-1R by interfering with the binding of SP. In the present study, we investigated the anti-inflammatory effects of ketamine on the SP-induced activation of a human astrocytoma cell line, U373MG, which expresses high levels of NK-1R. The results from our experiments indicated that ketamine suppressed the production of interleukin (IL)-6 and IL-8 by the U373MG cells. Furthermore, ketamine inhibited the SP-induced activation of extracellular signal‑regulated kinase (ERK)1/2, p38 mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB). Taken together, these observations suggest that ketamine may suppress the SP-induced activation (IL-6 and IL-8 production) of U373MG cells by inhibiting the phosphorylation of signaling molecules (namely ERK1/2, p38 MAPK and NF-κB), thereby exerting anti‑inflammatory effects. Thus, ketamine may modulate SP-induced inflammatory responses by NK-1R‑expressing cells through the suppression of signaling molecules (such as ERK1/2, p38 MAPK and NF-κB).

Signaling Pathways Regulating IL-1α-induced COX-2 Expression

Interleukin-1α(IL-1α) stimulates the production of prostaglandin E2 (PGE2) in odontogenic keratocyst fibroblasts. However, the signaling pathways remain obscure. In this study, we investigated IL-1αsignaling pathways that regulate cyclooxygenase-2 (COX-2) expression in odontogenic keratocyst fibroblasts. IL-1αincreased the expression of COX-2 mRNA and protein, and PGE2 secretion in the fibroblasts. IL-1αincreased the phosphorylation of extracellular signal-regulated protein kinase-1/2 (ERK1/2), p38 mitogen-activated protein kinase (MAPK), and c-Jun N-terminal kinase (JNK). PD-98059, SB-203580, SP-600125, and PDTC—which are inhibitors of ERK1/2, p38, JNK, and nuclear factor-κB (NF-κB), respectively—attenuated the IL-1α-induced COX-2 mRNA expression and activated protein kinase C PGE2 secretion. IL-1α(PKC), and PKC inhibitor staurosporine inhibited IL-1α-induced phosphorylation of ERK1/2, p38, and JNK, and decreased IL-1α-induced COX-2 mRNA expression. Thus, in odontogenic keratocyst fibroblasts, IL-1αmay stimulate COX-2 expression both through the PKC-dependent activation of ERK1/2, p38, and JNK signaling pathways, and through the NF-κB cascade.

Frataxin deficiency increases cyclooxygenase 2 and prostaglandins in cell and animal models of Friedreich's ataxia

An inherited deficiency of the mitochondrial protein frataxin causes Friedreich's ataxia (FRDA); the mechanism by which this deficiency triggers neuro- and cardio-degeneration is unclear. Microarrays of neural tissue of animal models of the disease showed decreases in antioxidant genes, and increases in inflammatory genes. Cyclooxygenase (COX)-derived oxylipins are important mediators of inflammation. We measured oxylipin levels using tandem mass spectrometry and ELISAs in multiple cell and animal models of FRDA. Mass spectrometry revealed increases in concentrations of prostaglandins, thromboxane B2, 15-HETE and 11-HETE in cerebellar samples of knockin knockout mice. One possible explanation for the elevated oxylipins is that frataxin deficiency results in increased COX activity. While constitutive COX1 was unchanged, inducible COX2 expression was elevated over 1.35-fold (P < 0.05) in two Friedreich's mouse models and Friedreich's lymphocytes. Consistent with higher COX2 expression, its activity was also increased by 58% over controls. COX2 expression is driven by multiple transcription factors, including activator protein 1 and cAMP response element-binding protein, both of which were elevated over 1.52-fold in cerebella. Taken together, the results support the hypothesis that reduced expression of frataxin leads to elevation of COX2-mediated oxylipin synthesis stimulated by increases in transcription factors that respond to increased reactive oxygen species. These findings support a neuroinflammatory mechanism in FRDA, which has both pathomechanistic and therapeutic implications.

Functional interaction between cyclooxygenase-2 and p53 in response to an endogenous electrophile

Cyclooxygenase-2 (Cox-2) is rapidly expressed by various stimuli and plays a key role in conversion of free arachidonic acid to prostaglandins. We have previously identified 4-hydroxy-2-nonenal (HNE), a lipid peroxidation-derived electrophile, as the potent Cox-2 inducer in rat epithelial RL34 cells and revealed that the HNE-induced Cox-2 expression resulted from the stabilization of Cox-2 mRNA that is mediated by the p38 mitogen-activated protein kinase signaling pathway. In the present study, we investigated an alternative regulatory mechanism of Cox-2 expression mediated by a transcription factor p53. In addition, to characterize the causal role for Cox-2, we examined the effects of Cox-2 overexpression in RL34 cells. To examine whether the HNE-induced Cox-2 expression was mechanistically linked to the p53 expression, we analyzed changes in Cox-2 and p53 expression levels in response to HNE and observed that the Cox-2 levels were inversely correlated with the p53 levels. Down-regulation of p53 followed by the activation of a transcription factor Sp1 was suggested to be involved in the HNE-induced Cox-2 gene expression. To characterize the effect of Cox-2 expression in the cells, we established the Cox-2-overexpressing derivatives of RL34 cells by stable transfection with Cox-2 cDNA. An oligonucleotide microarray analysis revealed a dramatic down-regulation of the proteasome subunit RC1 in the Cox-2 overexpressed cells compared to the empty-vector transfected control cells. Consistent with the Cox-2-mediated down-regulation of proteasome, a moderate reduction of the proteasome activities was observed. This proteasome dysfunction mediated by the Cox-2 overproduction was associated with the enhanced accumulation of p53 and ubiquitinated proteins, leading to the enhanced sensitivity toward electrophiles. These results suggest the existence of a causal link between Cox-2 and p53, which may represent a toxic mechanism of electrophilic lipid peroxidation products.

Prostaglandin E2 regulates murine hematopoietic stem/progenitor cells directly via EP4 receptor and indirectly through mesenchymal progenitor cells

PGE2 signaling positively regulates hematopoietic stem cells both directly and via activation of a nonhematopoietic cell population. EP4 is a major receptor for the PGE2-mediated regulation of hematopoietic stem and progenitor cells.

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

Eicosanoids and adipokines in breast cancer: from molecular mechanisms to clinical considerations

Chronic inflammation is one of the foremost risk factors for different types of malignancies, including breast cancer. Additional risk factors of this pathology in postmenopausal women are weight gain, obesity, estrogen secretion, and an imbalance in the production of adipokines, such as leptin and adiponectin. Various signaling products of transcription factor, nuclear factor-kappaB, in particular inflammatory eicosanoids, reactive oxygen species (ROS), and cytokines, are thought to be involved in chronic inflammation-induced cancer. Together, these key components have an influence on inflammatory reactions in malignant tissue damage when their levels are deregulated endogenously. Prostaglandins (PGs) are well recognized in inflammation and cancer, and they are solely biosynthesized through cyclooxygenases (COXs) from arachidonic acid. Concurrently, ROS give rise to bioactive isoprostanes from arachidonic acid precursors that are also involved in acute and chronic inflammation, but their specific characteristics in breast cancer are less demonstrated. Higher aromatase activity, a cytochrome P-450 enzyme, is intimately connected to tumor growth in the breast through estrogen synthesis, and is interrelated to COXs that catalyze the formation of both inflammatory and anti-inflammatory PGs such as PGE(2), PGF(2α), PGD(2), and PGJ(2) synchronously under the influence of specific mediators and downstream enzymes. Some of the latter compounds upsurge the intracellular cyclic adenosine monophosphate concentration and appear to be associated with estrogen synthesis. This review discusses the role of COX- and ROS-catalyzed eicosanoids and adipokines in breast cancer, and therefore ranges from their molecular mechanisms to clinical aspects to understand the impact of inflammation.

Inhibitory effect of Zingiber cassumunar extracts on lipopolysaccharide-induced cyclooxygenase-2 and matrix metalloproteinase expression in human gingival fibroblasts

BACKGROUND AND OBJECTIVE

Lipopolysaccharides (LPS) induce the production of proinflammatory mediators such as prostaglandins and matrix metalloproteinases (MMPs) in human gingival fibroblasts (HGFs). Zingiber cassumunar is a medicinal plant that possesses anti-inflammatory properties. The aim of this study was to determine the effects of the Z. cassumunar extract on the expression of cyclooxygenase (COX)-1, COX-2 and MMP-2 in HGFs challenged with LPS.

MATERIAL AND METHODS

HGFs were treated with LPS in the presence or absence of Z. cassumunar extracts. The levels of expression of COX-1, COX-2 and MMP-2 mRNAs and of COX-1, COX-2 and MMP-2 proteins were detected by reverse transcription-polymerase chain reaction and western blotting, respectively. MMP-2 activities in cell-culture supernatants were determined using gelatin zymography. MAPK activation was evaluated by western blotting.

RESULTS

LPS treatment of HGFs resulted in the activation of ERK1/2, p38 and JNK. Z. cassumunar extracts significantly inhibited the phosphorylation of ERK1/2 and JNK in HGFs stimulated with LPS. A lesser inhibitory effect was observed for the phosphorylation of p38. RT-PCR and western blot analyses showed that Z. cassumunar extracts inhibited the LPS-induced expression of COX-2 mRNA and COX-2 protein, respectively, but not of COX-1 mRNA or COX-1 protein. Pretreatment of HGFs with Z. cassumunar also attenuated the induction of MMP-2 with LPS.

CONCLUSION

Our results indicate that Z. cassumunar extracts inhibit COX-2 and MMP-2 production by LPS-activated human gingival fibroblasts through blocking the proinflammatory signaling pathway involving ERK1/2, JNK and p38.

Toll-like receptor 2 heterodimers, TLR2/6 and TLR2/1 induce prostaglandin E production by osteoblasts, osteoclast formation and inflammatory periodontitis

TLR2 forms heterodimers with TLR1 and TLR6, and regulates host defense mechanisms against pathogens. We examined the role of TLR2 heterodimer signaling in osteoclast formation and inflammatory periodontitis. In co-cultures of mouse bone marrow cells and osteoblasts, a TLR2/6 ligand (diacylated lipopeptide designed from Gram-positive bacteria) markedly induced osteoclast formation. A TLR2/1 ligand (triacylated lipopeptide designed from Gram-negative bacteria) also induced osteoclast formation. The osteoclast formation induced by TLR2/6 and TLR2/1 ligands was completely suppressed by indomethacin. Osteoblasts expressed TLR1, 2, 4, and 6 mRNAs, and both TLR2/6 and TLR2/1 ligands induced the expression of COX-2, mPGES-1, and RANKL mRNA, as well as PGE production in osteoblasts. Both TLR2/6 and TLR2/1 ligands induced the resorption of mandibular alveolar bone in organ cultures, and elicited inflammatory periodontitis in vivo. Therefore, TLR2 heterodimer signaling may play a key role in PGE-mediated inflammatory bone loss in periodontal disease.

The effect of enamel matrix derivative (Emdogain®) on gene expression profiles of human primary alveolar bone cells

Emdogain® is frequently used in regenerative periodontal treatment. Understanding its effect on gene expression of bone cells would enable new products and pathways promoting bone formation to be established. The aim of the study was to analyse the effect of Emdogain® on expression profiles of human-derived bone cells with the help of the micro-array, and subsequent validation. Bone was harvested from non-smoking patients during dental implant surgery. After outgrowth, cells were cultured until subconfluence, treated for 24 h with either Emdogain® (100 µg/ml) or control medium, and subsequently RNA was isolated and micro-array was performed. The most important genes demonstrated by micro-array data were confirmed by qPCR and ELISA tests. Emdogain tipped the balance between genes expressed for bone formation and bone resorption towards a more anabolic effect, by interaction of the PGE2 pathway and inhibition of IL-7 production. In addition the results of the present study indicate that Emdogain possibly has an effect on gene expression for extracellular matrix formation of human bone cells, in particular on bone matrix formation and on proliferation and differentiation. With the micro-array and the subsequent validation, the genes possibly involved in Emdogain action on bone cells were identified. These results can contribute to establishing new products and pathways promoting bone formation.

Capsaicin, a TRPV1 Ligand, Suppresses Bone Resorption by Inhibiting the Prostaglandin E Production of Osteoblasts, and Attenuates the Inflammatory Bone Loss Induced by Lipopolysaccharide

Capsaicin, a transient receptor potential vanilloid type 1 (TRPV1) ligand, regulates nerve-related pain-sensitive signals, inflammation, and cancer growth. Capsaicin suppresses interleukin-1-induced osteoclast differentiation, but its roles in bone tissues and bone diseases are not known. This study examined the effects of capsaicin on inflammatory bone resorption and prostaglandin E (PGE) production induced by lipopolysaccharide (LPS) in vitro and on bone mass in LPS-treated mice in vivo. Capsaicin suppressed osteoclast formation, bone resorption, and PGE production induced by LPS in vitro. Capsaicin suppressed the expression of cyclooxygenase-2 (COX-2) and membrane-bound PGE synthase-1 (mPGES-1) mRNAs and PGE production induced by LPS in osteoblasts. Capsaicin may suppress PGE production by inhibiting the expression of COX-2 and mPGES-1 in osteoblasts and LPS-induced bone resorption by TRPV1 signals because osteoblasts express TRPV1. LPS treatment markedly induced bone loss in the femur in mice, and capsaicin significantly restored the inflammatory bone loss induced by LPS in mice. TRPV1 ligands like capsaicin may therefore be potentially useful as clinical drugs targeting bone diseases associated with inflammatory bone resorption.

Eicosanoid regulation of hematopoiesis and hematopoietic stem and progenitor trafficking

Hematopoietic stem cell (HSC) transplantation is a potentially curative treatment for numerous hematological malignancies. The transplant procedure as performed today takes advantage of HSC trafficking; either egress of HSC from the bone marrow to the peripheral blood, that is, mobilization, for acquisition of the hematopoietic graft, and/or trafficking of HSC from the peripheral blood to bone marrow niches in the recipient patient, that is HSC homing. Numerous studies, many of which are reviewed herein, have defined hematopoietic regulatory mechanisms mediated by the 20-carbon lipid family of eicosanoids, and recent evidence strongly supports a role for eicosanoids in regulation of hematopoietic trafficking, adding a new role whereby eicosanoids regulate hematopoiesis. Short-term exposure of HSC to the eicosanoid prostaglandin E(2) increases CXCR4 receptor expression, migration and in vivo homing of HSC. In contrast, cannabinoids reduce hematopoietic progenitor cell (HPC) CXCR4 expression and induce HPC mobilization when administered in vivo. Leukotrienes have been shown to alter CD34(+) cell adhesion, migration and regulate HSC proliferation, suggesting that eicosanoids have both opposing and complimentary roles in the regulation of hematopoiesis. As numerous FDA approved compounds regulate eicosanoid signaling or biosynthesis, the utility of eicosanoid-based therapeutic strategies to improve hematopoietic transplantation can be rapidly evaluated.

Staphylococcus aureus induces expression of receptor activator of NF-kappaB ligand and prostaglandin E2 in infected murine osteoblasts

Osteomyelitis is an inflammatory disease of the bone that is characterized by the presence of necrotic bone tissue and increased osteoclast activity. Staphylococcus aureus is responsible for approximately 80% of all cases of human osteomyelitis. While the disease is especially difficult to treat, the pathogenesis of S. aureus-induced osteomyelitis is poorly understood. Elucidating the molecular mechanisms by which S. aureus induces osteomyelitis could lead to a better understanding of the disease and its progression and development of new treatments. Osteoblasts can produce several soluble factors that serve to modulate the activity or formation of osteoclasts. Receptor activator of NF-kappaB ligand (RANK-L) and prostaglandin E(2) (PGE(2)) are two such molecules which can promote osteoclastogenesis and stimulate bone resorption. In addition, previous studies in our laboratory have shown that osteoblasts produce inflammatory cytokines, such as interleukin 6, following infection with S. aureus, which could induce COX-2 and in turn PGE(2), further modulating osteoclast recruitment and differentiation. Therefore, we hypothesized that following infection with S. aureus, osteoblasts will express increased levels of RANK-L and PGE(2). The results presented in this study provide evidence for the first time that RANK-L mRNA and protein and PGE(2) expression are upregulated in S. aureus-infected primary osteoblasts. In addition, through the use of the specific COX-2 inhibitor NS 398, we show that when PGE(2) production is inhibited, RANK-L production is decreased. These data suggest a mechanism whereby osteoblasts regulate the production of RANK-L during infection.

Age-related changes in bone morphology are accelerated in group VIA phospholipase A2 (iPLA2beta)-null mice

Phospholipases A(2) (PLA(2)) hydrolyze the sn-2 fatty acid substituent, such as arachidonic acid, from phospholipids, and arachidonate metabolites are recognized mediators of bone modeling. We have previously generated knockout (KO) mice lacking the group VIA PLA(2) (iPLA(2)beta), which participates in a variety of signaling events; iPLA(2)beta mRNA is expressed in bones of wild-type (WT) but not KO mice. Cortical bone size, trabecular bone volume, bone mineralizing surfaces, and bone strength are similar in WT and KO mice at 3 months and decline with age in both groups, but the decreases are more pronounced in KO mice. The lower bone mass phenotype observed in KO mice is not associated with an increase in osteoclast abundance/activity or a decrease in osteoblast density, but is accompanied by an increase in bone marrow fat. Relative to WT mice, undifferentiated bone marrow stromal cells (BMSCs) from KO mice express higher levels of PPAR-gamma and lower levels of Runx2 mRNA, and this correlates with increased adipogenesis and decreased osteogenesis in BMSCs from these mice. In summary, our studies indicate that age-related losses in bone mass and strength are accelerated in iPLA(2)beta-null mice. Because adipocytes and osteoblasts share a common mesenchymal stem cell origin, our findings suggest that absence of iPLA(2)beta causes abnormalities in osteoblast function and BMSC differentiation and identify a previously unrecognized role of iPLA(2)beta in bone formation.

Adenovirus type 5 exerts multiple effects on the expression and activity of cytosolic phospholipase A2, cyclooxygenase-2, and prostaglandin synthesis

In this study, we examine how infection of murine and human fibroblasts by adenovirus (Ad) serotype 5 (Ad5) affects the expression and activity of cytosolic phospholipase A2 (cPLA2), cyclooxygenase-2 (COX-2), and production of PGs. Our experiments showed that infection with Ad5 is accompanied by the rapid activation of cPLA2 and the cPLA2-dependent release of [3H]arachidonic acid ([3H]AA). Increased expression of COX-2 was also observed after Ad infection, as was production of PGE2 and PGI2. Later, however, as the infection progressed, release of [3H]AA and production of PGs stopped. Late-stage Ad5-infected cells also did not release [3H]AA or PGs following treatment with a panel of biologically diverse agents. Experiments with UV-inactivated virus confirmed that Ad infection is accompanied by the activation of a host-dependent response that is later inhibited by the virus. Investigations of the mechanism of suppression of the PG pathway by Ad5 did not reveal major effects on the expression or activity of cPLA2 or COX-2. We did note a change in the intracellular position of cPLA2 and found that cPLA2 did not translocate normally in infected cells, raising the possibility that Ad5 interferes with the PG pathway by interfering with the intracellular movement of cPLA2. Taken together, these data reveal dynamic interactions between Ad5 and the lipid mediator pathways of the host and highlight a novel mechanism by which Ad5 evades the host immune response. In addition, our results offer insight into the inflammatory response induced by many Ad vectors lacking early region gene products.

Mechanical loading and ?12prostaglandin J2induce bone morphogenetic protein-2, peroxisome proliferator-activated receptor ?-1, and bone nodule formation in an osteoblastic cell line

BACKGROUND AND OBJECTIVE

We have previously reported that mechanical strain applied at a 1% level to an osteoblastic cell line induces the transcription of prostaglandin D2 synthase and increases the levels of prostaglandin D2 and its Delta12prostaglandin J2 metabolite. Mechanical strain also induces the expression of peroxisome proliferator-activated receptor gamma-1 and bone nodule formation. We hypothesized that mechanical load induces bone formation via Delta12prostaglandin J2-dependent synthesis of bone morphogenetic proteins. Our goal was to investigate the molecular events involved in osteogenesis induced by mechanical loading and Delta12prostaglandin J2, namely the induction of bone morphogenetic proteins and peroxisome proliferator-activated receptor gamma-1, a nuclear receptor for Delta12prostaglandin J2.

MATERIAL AND METHODS

Osteoblast monolayers were stretched for 1 h with a 1-h resting period and stretched for another hour at 1 Hz with 1% elongation. Cells were collected 0, 1, 6 and 16 h after stretching. Cyclooxygenase inhibitors and Delta12prostaglandin J2 were added in some experiments. Relative quantitative reverse transcriptase-polymerase chain reaction was used to examine whether the mRNA of bone morphogenetic protein-2, -4, -6, -7 and peroxisome proliferator-activated receptor gamma-1 was induced. Immunohistochemistry was used to evaluate bone morphogenetic protein expression in cells.

RESULTS

Mechanical strain significantly increased the mRNA expression of bone morphogenetic protein-2, -6, -7 and of peroxisome proliferator-activated receptor gamma-1, but not of bone morphogenetic protein-4. In stretched cells, bone morphogenetic protein-2 and peroxisome proliferator-activated receptor gamma-1 expression was blocked by cyclooxygenase inhibitors, but restored by exogenous Delta12prostaglandin J2. Delta12Prostaglandin J2 significantly enhanced bone nodule formation and bone morphogenetic protein-2 expression when added alone to resting osteoblasts.

CONCLUSION

These results suggest that the osteoblastic biomechanical pathways that trigger bone formation involve cyclooxygenase and prostaglandin D2 synthase activation, induction of Delta12prostaglandin J2 and its nuclear receptor, peroxisome proliferator-activated receptor gamma-1, and increased expression of bone morphogenetic protein-2. These data suggest that the Delta12prostaglandin J2/peroxisome proliferator-activated receptor gamma-1/bone morphogenetic protein-2 pathway plays an important role in osteogenesis.

Effect of a cyclooxygenase-2 inhibitor on interleukin-1beta-stimulated activation of the transcription factor nuclear factor-kappa B in human gingival fibroblasts

BACKGROUND

In previous work, the cyclooxygenase-2 inhibitor NS-398 inhibited interleukin (IL)-1beta-stimulated prostaglandin E(2) (PGE(2)) production almost completely while partially inhibiting IL-6 production in aggressive periodontitis (AgP) human gingival fibroblasts. PGE(2) and the transcription factor nuclear factor-kappa B (NF-kappaB) regulate IL-1beta-stimulated IL-6 production. Cytoplasmic NF-kappaB is bound to inhibitors (IkappaB proteins). IL-1beta initiates a cascade resulting in phosphorylation and degradation of IkappaB, allowing nuclear translocation of NF-kappaB and target gene activation. The purpose of this study was to determine whether NS-398 inhibited phosphorylation of IkappaB and NF-kappaB activation.

METHODS

AgP fibroblasts (1 to 2 x 10(6)) were exposed to IL-1beta (1 x 10(11)M) with or without NS-398 (10 nM) in serum-free medium. The NF-kappaB subunit p65 and phospho-IkappaBalpha were measured in whole cell, cytoplasmic, or nuclear extracts, using colorimetric assays. Enzyme-linked immunosorbent assays were used to measure PGE(2) and IL-6 production by 2.5 x 10(4) cells after exposure to IL-1beta with or without NS-398 in serum-free medium.

RESULTS

Consistent with previous results, NS-398 reduced IL-1beta-stimulated PGE(2) by approximately 98% (P <0.001) and IL-6 by approximately 65% (P <0.001). IL-1beta increased nuclear and cytoplasmic p65 ( approximately 8-fold [P <0.001] and approximately 2.5-fold [P <0.03], respectively) over control levels. NS-398 reduced IL-1beta-stimulated nuclear and cytoplasmic p65 to control levels. IL-1beta increased phospho-IkappaBalpha in whole cell extracts by a maximum of approximately 9.5 times (P = 0.0001), and this was inhibited significantly by NS-398 (P <or=0.008).

CONCLUSIONS

NS-398 inhibited NF-kappaB activation and nuclear p65 levels in human gingival fibroblasts. This seemed to be due to inhibition of the phosphorylation cascade resulting in formation of phospho-IkappaBalpha and free p65. NF-kappaB inhibition may be useful in treating inflammatory diseases such as AgP.

Arachidonic acid inhibits osteoblast differentiation through cytosolic phospholipase A2-dependent pathway

OBJECTIVE

Arachidonic acid, a precursor of prostaglandins (PGs), is released by phospholipase A2 (PLA2) and plays an important role in biological reactions. We examined the roles of arachidonic acid on the pathway of PG synthesis and osteoblast differentiation by using clone MC3T3-E1 cells.

MATERIALS AND METHODS

The effect of arachidonic acid was evaluated by the measurement of alkaline phosphatase activity, cells shape, production of arachidonic acid and the expression of cyclooxygenase (COX).

RESULTS

Arachidonic acid dose dependently decreased alkaline phosphatase activity and increased PGE2 production in MC3T3-E1 cells. The cell shape changed from polygonal to fibroblastic following treatment with arachidonic acid. These effects were recovered by the treatment of NS-398 and indomethacin. Arachidonic acid increased the expression of COX-2 mRNA and the PGE2 production. The exogenous arachidonic acid induced the release of cellular arachidonic acid in MC3T3-E1 cells. Moreover, methylarachidonyl fluorophosphonate suppressed the arachidonic acid release and the expression of COX-2 mRNA.

CONCLUSION

The present results indicate that exogenous arachidonic acid stimulated the activity of PLA2, leading to the new release of membranous arachidonic acid. The amplified arachidonic acid enhanced PGE2 production by COX-2, which inhibits the differentiation of MC3T3-E1 cells. Our results provide a new insight into the molecular mechanisms by which exogenous arachidonic acid plays a role as a paracrine/autocrine amplifier of PGE2 biosynthesis by coupling with PLA2 and COX-2.

Lipopolysaccharide enhances the production of nicotine-induced prostaglandin E2 by an increase in cyclooxygenase-2 expression in osteoblasts

Previous studies have indicated that lipopolysaccharide (LPS) from Gram-negative bacteria in plaque induces the release of prostaglandin E(2) (PGE(2)), which promotes alveolar bone resorption in periodontitis, and that tobacco smoking might be an important risk factor for the development and severity of periodontitis. We determined the effect of nicotine and LPS on alkaline phosphatase (ALPase) activity, PGE(2) production, and the expression of cyclooxygenase (COX-1, COX-2), PGE(2) receptors Ep1>4, and macrophage colony stimulating factor (M-CSF) in human osteoblastic Saos-2 cells. The cells were cultured with 10(-3) M nicotine in the presence of 0, 1, or 10 mug/ml LPS, or with LPS alone. ALPase activity decreased in cells cultured with nicotine or LPS alone, and decreased further in those cultured with both nicotine and LPS, whereas PGE(2) production significantly increased in the former and increased further in the latter. By itself, nicotine did not affect expression of COX-1, COX-2, any of the PGE(2) receptors, or M-CSF, but when both nicotine and LPS were present, expression of COX-2, Ep3, Ep4, and M-CSF increased significantly. Simultaneous addition of 10(-4) M indomethacin eliminated the effects of nicotine and LPS on ALPase activity, PGE(2) production, and M-CSF expression. Phosphorylation of protein kinase A was high in cells cultured with nicotine and LPS. These results suggest that LPS enhances the production of nicotine-induced PGE(2) by an increase in COX-2 expression in osteoblasts, that nicotine-LPS-induced PGE2 interacts with the osteoblast Ep4 receptor primarily in autocrine or paracrine mode, and that the nicotine-LPS-induced PGE(2) then decreases ALPase activity and increases M-CSF expression.

Liver X receptor ligands inhibit the lipopolysaccharide-induced expression of microsomal prostaglandin E synthase-1 and diminish prostaglandin E2 production in murine peritoneal macrophages

Microsomal prostaglandin E synthase (mPGES)-1, which is dramatically induced in macrophages by inflammatory stimuli such as lipopolysaccharide (LPS), catalyzes the conversion of cyclooxygenase-2 (COX-2) reaction product prostaglandin H(2) (PGH(2)) into prostaglandin E(2) (PGE(2)). The mPGES-1-derived PGE(2) is thought to help regulate inflammatory responses. On the other hand, excess PGE(2) derived from mPGES-1 contributes to the development of inflammatory diseases such as arthritis and inflammatory pain. Here, we examined the effects of liver X receptor (LXR) ligands on LPS-induced mPGES-1 expression in murine peritoneal macrophages. The LXR ligands 22(R)-hydroxycholesterol (22R-HC) and T0901317 reduced LPS-induced expression of mPGES-1 mRNA and mPGES-1 protein as well as that of COX-2 protein. However, LXR ligands did not influence the expression of microsomal PGES-2 (mPGES-2) or cytosolic PGES (cPGES) protein. Consequently, LXR ligands suppressed the production of PGE(2) in macrophages. These results suggest that LXR ligands diminish PGE(2) production by inhibiting the LPS-induced gene expression of the COX-2-mPGES-1 axis in LPS-activated macrophages.

Reciprocal cross talk between gonadotropin-releasing hormone (GnRH) and prostaglandin receptors regulates GnRH receptor expression and differential gonadotropin secretion

The asynchronous secretion of gonadotrope LH and FSH under the control of GnRH is crucial for ovarian cyclicity but the underlying mechanism is not fully resolved. Because prostaglandins (PG) are autocrine regulators in many tissues, we determined whether they have this role in gonadotropes. We first demonstrated that GnRH stimulates PG synthesis by induction of cyclooxygenase-2, via the protein kinase C/c-Src/phosphatidylinositol 3'-kinase/MAPK pathway in the LbetaT2 gonadotrope cell line. We then demonstrated that PGF(2alpha) and PGI2, but not PGE2 inhibited GnRH receptor expression by inhibition of phosphoinositide turnover. PGF(2alpha), but not PGI2 or PGE2, reduced GnRH-induction of LHbeta gene expression, but not the alpha-gonadotropin subunit or the FSHbeta subunit genes. The prostanoid receptors EP1, EP2, FP, and IP were expressed in rat gonadotropes. Incubations of rat pituitaries with PGF(2alpha), but not PGI2 or PGE2, inhibited GnRH-induced LH secretion, whereas the cyclooxygenase inhibitor, indomethacin, stimulated GnRH-induced LH secretion. None of these treatments had any effect on GnRH-induced FSH secretion. The findings have thus elaborated a novel GnRH signaling pathway mediated by PGF(2alpha)-FP and PGI2-IP, which acts through an autocrine/paracrine modality to limit autoregulation of the GnRH receptor and differentially inhibit LH and FSH release. These findings provide a mechanism for asynchronous LH and FSH secretions and suggest the use of combination therapies of GnRH and prostanoid analogs to treat infertility, diseases with unbalanced LH and FSH secretion and in hormone-dependent diseases such as prostatic cancer.

Membrane-bound prostaglandin E synthase-1-mediated prostaglandin E2 production by osteoblast plays a critical role in lipopolysaccharide-induced bone loss associated with inflammation

PGE(2) acts as a potent stimulator of bone resorption in several disorders including osteoarthritis and periodontitis. Three PGE synthases (PGES) were isolated for PGE(2) production, but which PGES has the major role in inflammatory bone resorption is still unclear. In this study, we examined the role of PGE(2) in LPS-induced bone resorption using membrane-bound PGES (mPGES)-1-deficient mice (mPges1(-/-)). In osteoblasts from wild-type mice, PGE(2) production was greatly stimulated by LPS following the expression of cyclooxygenase 2 and mPGES-1 mRNA, whereas no PGE(2) production was found in osteoblasts from mPges1(-/-). LPS administration reduced the bone volume in wild-type femur that was associated with an increased number of osteoclasts. In mPges1(-/-), however, LPS-induced bone loss was reduced. We next examined whether mPGES-1 deficiency could alter the alveolar bone loss in LPS-induced experimental periodontitis. LPS was injected into the lower gingiva and bone mineral density of alveolar bone was measured. LPS induced the loss of alveolar bone in wild-type, but not in mPges1(-/-) mice, suggesting an mPGES-1 deficiency resistant to LPS-induced periodontal bone resorption. To understand the pathway of LPS-induced PGE(2) production in osteoblast, we used C3H/HeJ mice with mutated tlr4. Osteoblasts from C3H/HeJ mice did not respond to LPS, and PGE(2) production was not altered at all. LPS-induced bone loss in the femur was also impaired in C3H/HeJ mice. Thus, LPS binds to TLR4 on osteoblasts that directly induce mPGES-1 expression for PGE(2) synthesis, leading to subsequent bone resorption. Therefore, mPGES-1 may provide a new target for the treatment of inflammatory bone disease.

Inhibitors of cyclo-oxygenase-2 and secretory phospholipase A2 preserve bone architecture following ovariectomy in adult rats

Epidemiological evidence and in vitro data suggest that COX-2 is a key regulator of accelerated remodeling. Accelerated states of osteoblast and osteoclast activity are regulated by prostaglandins in vitro, but experimental evidence for specific roles of cyclooxygenase-2 (COX-2) and secretory phospholipase A2 (sPLA2) in activated states of remodeling in vivo is lacking. The aim of this study was to determine the effect of specific inhibitors of sPLA2-IIa and COX-2 on bone remodeling activated by estrogen deficiency in adult female rats. One hundred and twenty-four adult female Wistar rats were ovariectomized (OVX) or sham-operated. Rats commenced treatment 14 days after surgery with either vehicle, a COX-2 inhibitor (DFU at 0.02 mg/kg/day and 2.0 mg/kg/day) or a sPLA2-group-IIa inhibitor (KH064 at 0.4 mg/kg/day and 4.0 mg/kg/day). Treatment continued daily until rats were sacrificed at 70 days or 98 days post-OVX. The right tibiae were harvested, fixed and embedded in methylmethacrylate for structural histomorphometric bone analysis at the proximal tibial metaphysis. The specific COX-2 or sPLA2 inhibitors prevented ovariectomy-induced (OVX-induced) decreases in trabecular connectivity (P<0.05); suppressed the acceleration of bone resorption; and maintained bone turnover at SHAM levels following OVX in the rat. The sPLA2 inhibitor significantly suppressed increases in osteoclast surface induced by OVX (P<0.05), while the effect of COX-2 inhibition was less marked. These findings demonstrate that inhibitors of COX-2 and sPLA2-IIa can effectively suppress OVX-induced bone loss in the adult rat by conserving trabecular bone mass and architecture through reduced bone remodeling and decreased resorptive activity. Moreover, we report an important role of sPLA2-IIa in osteoclastogenesis that may be independent of the COX-2 metabolic pathway in the OVX rat in vivo.

Lipopolysaccharide stimulates the production of prostaglandin E2 and the receptor Ep4 in osteoblasts

Previous studies have indicated that one of the causes of alveolar bone destruction with periodontitis is lipopolysaccharide (LPS) from the cell wall of gram-negative bacteria in plaque, and that prostaglandin E(2) (PGE(2)) is one of the bone resorption factors that stimulate osteoclast formation through an intercellular interaction between osteoblasts and osteoclast precursors. The present study was undertaken to determine the effect of LPS on cell growth, alkaline phosphatase (ALPase) activity, the production of PGE(2), and the expression of receptors by PGE(2), cyclooxygenase (COX)-1, and COX-2, using human osteosarcoma cell line Saos-2 as osteoblasts. The cells were cultured with 0, 1, or 10 microg mL(-1) of LPS for up to 14 days. The production of PGE(2) and the gene expression of COX-1, COX-2, and PGE(2) receptors, including Ep1, Ep2, Ep3, and Ep4, were determined using enzyme-linked immunosorbent assay (ELISA) and real-time reverse transcription-polymerase chain reaction (real-time RT-PCR), respectively. With the addition of LPS, cell growth and ALPase activity decreased by day 5 of the culture, while PGE(2) production increased in a dose-dependent manner throughout the entire 14-day culture period. LPS-reduced ALP activity and LPS-induced PGE(2) production returned to the control level by the addition simultaneously with indomethacin. The expression of COX-1, Ep1, Ep2, and Ep3 receptors decreased on day 14 of the culture, whereas the expression of COX-2 and Ep4 receptors increased significantly with the addition of LPS. These results suggest that LPS promotes PGE(2) production by increasing the expression of COX-2, and that LPS promotes the production of Ep4 receptors in osteoblasts. These results also indicate that LPS-induced PGE(2) may combine with osteoblast Ep4 receptors in autocrine or paracrine modes, and may promote the formation of osteoclasts.

Coupling between cyclooxygenases and terminal prostanoid synthases

Biosynthesis of prostanoids is regulated by three sequential enzymatic steps, namely phospholipase A2, cyclooxygenase (COX), and terminal prostanoid synthase. Recent evidence suggests that lineage-specific terminal prostanoid synthases, including prostaglandin (PG) E2, PGD2, PGF2alpha, PGI2, and thromboxane synthases, show distinct functional coupling with upstream COX isozymes, COX-1 and COX-2. This can account, at least in part, for segregated utilization of the two COX isozymes in distinct phases of PG-biosynthetic responses. In terms of their localization and COX preference, terminal prostanoid synthases are classified into three categories: (i) the perinuclear enzymes that prefer COX-2, (ii) the cytosolic enzyme that prefers COX-1, and (iii) the translocating enzyme that utilizes both COXs depending on the stimulus. Additionally, altered supply of arachidonic acid by phospholipase A2s significantly affects the efficiency of COX-terminal prostanoid synthase coupling. In this review, we summarize our recent understanding of the coupling profiles between the two COXs and various terminal prostanoid synthases.

Prostaglandin E Synthase, a Terminal Enzyme for Prostaglandin E2 Biosynthesis

Biosynthesis of prostanoids is regulated by three sequential enzymatic steps, namely phospholipase A2 enzymes, cyclooxygenase (COX) enzymes, and various lineagespecific terminal prostanoid synthases. Prostaglandin E synthase (PGES), which isomerizes COX-derived PGH2 specifically to PGE2, occurs in multiple forms with distinct enzymatic properties, expressions, localizations and functions. Two of them are membrane-bound enzymes and have been designated as mPGES-1 and mPGES-2. mPGES-1 is a perinuclear protein that is markedly induced by proinflammatory stimuli, is down-regulated by antiinflammatory glucocorticoids, and is functionally coupled with COX-2 in marked preference to COX-1. Recent gene targeting studies of mPGES-1 have revealed that this enzyme represents a novel target for anti-inflammatory and anti-cancer drugs. mPGES-2 is synthesized as a Golgi membrane-associated protein, and the proteolytic removal of the N-terminal hydrophobic domain leads to the formation of a mature cytosolic enzyme. This enzyme is rather constitutively expressed in various cells and tissues and is functionally coupled with both COX-1 and COX-2. Cytosolic PGES (cPGES) is constitutively expressed in a wide variety of cells and is functionally linked to COX-1 to promote immediate PGE2 production. This review highlights the latest understanding of the expression, regulation and functions of these three PGES enzymes.

Transcriptional induction of cyclooxygenase-2 in osteoclast precursors is involved in RANKL-induced osteoclastogenesis

Regulation of osteoclast differentiation is key to understanding the pathogenesis and to developing treatments for bone diseases such as osteoporosis. To gain insight into the mechanism of the receptor activator of nuclear factor (NF)–κB ligand (RANKL)–specific induction of the osteoclast differentiation program, we took a suppression-subtractive hybridization screening approach to identify genes specifically induced via the RANKL-Rac1 signaling pathway. Among identified targets, we show that RANKL selectively induces cyclooxygenase (COX) 2 expression via Rac1 that results in turn in production of prostaglandin E2 (PGE2) in RAW 264.7 cells. By using transient transfection assays, we found that the –233/–206 region of the COX-2 promoter gene was critical for RANKL-induced promoter activity. This RANKL-responsive region contained an NF-κB site that, when mutated, completely abolished the induction of NF-κB DNA-binding activity by RANKL. Blockade of COX-2 by celecoxib inhibits differentiation of bone marrow-derived monocyte/macrophage precursor cells (BMMs) into tartrate-resistant acid phosphatase-positive (TRAP+) osteoclastic cells. This inhibition can be rescued by the addition of exogenous PGE2, suggesting that COX-2–dependent PGE2 induction by RANKL in osteoclast precursors is required for osteoclast differentiation.

The effect of benzo(a)pyrene on porcine urinary bladder epithelial cells analyzed for the expression of selected genes and cellular toxicological endpoints

Consumption of tobacco products is the most relevant risk factor for the development of bladder cancer beside occupational contributions. In order to investigate mechanisms of tobacco smoke components in bladder carcinogenesis we have introduced a primary epithelial cell culture system derived from porcine urinary bladder as a suitable representative for the corresponding human tissue under physiological conditions. Two independent readouts were selected as markers for genotoxic events. Changes in the expression level of several toxicologically relevant genes should serve as indicators for early response, while classical genotoxic endpoints monitored manifested damages. Here, we present the first results of our study with benzo(a)pyrene (BaP) as a member of polycyclic aromatic hydrocarbons (PAHs) found in tobacco smoke. Cells treated with BaP show a dramatic increase in the expression of CYP1A1 that appears to be both indicator of and contributor for BaP toxicity. Genes coding for other proteins relevant in xenobiotic metabolism, signal transduction or tumor suppression show moderate effects or no enhancement of their expression levels. Comet assay and micronucleus test did show a significant, dose-dependent increase in DNA damages or aberrations after cell division. While these effects are conforming to the response at the mRNA expression level, they are less pronounced and require rather higher dosages of the chemical.

RAW264.7 cells lack prostaglandin-dependent autoregulation of tumor necrosis factor-alpha secretion

Studies of the response of RAW264.7 cells (RAW) to lipopolysaccharide (LPS) were carried out to determine why these cells do not demonstrate the prostaglandin (PG)-dependent autocrine regulation of tumor necrosis factor-alpha (TNF-alpha) secretion observed in primary resident peritoneal macrophages (RPMs). The major cyclooxygenase (COX) product of LPS-stimulated RAW was PGD2, with lesser amounts of PGE2. LPS-treated RAW produced PGs more slowly and reached their maximal PG synthetic rate later than did LPS-treated RPMs, as a result of lower constitutive COX-1 expression and a slower rate of COX-2 induction. Cytosolic phospholipase A2 and levels of free arachidonic acid were similar in RAW and RPMs. In contrast to RPMs, LPS-treated RAW produced high quantities of TNF-alpha, which were not altered in the presence of COX inhibitors. This failure of endogenous PGs to suppress TNF-alpha secretion was explained by the absence of the prostaglandin D2 receptor and the low levels of PGE2 produced during the first 2 h of the LPS response. These studies demonstrate that autocrine regulation of TNF-alpha secretion in response to LPS is greatly facilitated by a COX-1-mediated rapid accumulation of PGs as well by a correspondence between the PGs produced and the receptors expressed by the cells.

Platelet-derived growth factor-induced arachidonic acid release for enhancement of prostaglandin E(2) synthesis in human gingival fibroblasts pretreated with interleukin-1beta

Platelet-derived growth factor (PDGF) is a biological mediator for connective tissue cells and plays a critical role in a wide variety of physiological and pathological processes. We here investigated the effect of PDGF on arachidonic acid release and prostaglandin E(2) (PGE(2)) synthesis in human gingival fibroblasts (HGF). PDGF induced arachidonic acid release in a time- and dose-dependent manner, and simultaneously induced a transient increase in intracellular Ca(2+) concentration ([Ca(2+)](i)), but less provoked PGE(2) release and cyclooxygenase-2 (COX-2) mRNA expression. When [Ca(2+)](i) was increased by Ca(2+)-mobilizing reagents, arachidonic acid release was increased. The PDGF-induced arachidonic acid release and increase in [Ca(2+)](i) were prevented by a tyrosine kinase inhibitor. On the other hand, in the HGF pre-stimulated with interleukin-1beta (IL-1beta), PDGF clearly increased PGE(2) release. The PDGF-induced PGE(2) release was inhibited by a tyrosine kinase inhibitor. In the HGF pretreated with IL-1beta, arachidonic acid strongly enhanced PGE(2) release and COX-2 mRNA expression. These results suggest that PDGF stimulates arachidonic acid release by the increase in [Ca(2+)](i) via tyrosine kinase activation, and which contributes to PGE(2) production via COX-2 expression in HGF primed with IL-1beta.

Platelet-derived growth factor-AA increases IL-1beta and IL-8 expression and activates NF-kappaB in rheumatoid fibroblast-like synoviocytes

The effect of platelet-derived growth factor (PDGF)-AA on the inflammation in rheumatoid arthritis (RA) and osteoarthritis (OA) was investigated using cultured fibroblast-like synoviocytes (FLS) obtained from RA and OA patients as well as control nonarthritic (NA) individuals. PDGF-AA increased the mRNA and protein expressions of proinflammatory cytokines, interleukin (IL)-1beta and IL-8 in RA FLS. Biological activity of IL-1 in the culture supernatant of RA FLS was also increased by PDGF-AA stimulation. Interestingly, PDGF-AA synergized with tumour necrosis factor (TNF)-alpha to upregulate the protein expressions of IL-1beta and IL-8. PDGF-induced enhancement of the IL-1beta and IL-8 mRNA expressions was also observed in OA FLS. However, the expression of these proinflammatory cytokines in NA FLS did not change by PDGF treatment, suggesting that the inflammatory condition might have modified the biological effects of PDGF. In accordance with the enhanced expression of inflammatory cytokines, the activity of nuclear factor kappaB was also induced in response to PDGF-AA in RA FLS. These results suggest that PDGF-AA plays an important role in the progression of RA inflammation, and inhibiting PDGF activity may be useful for the effective RA treatment.

Involvement of mitogen-activated protein kinases and protein kinase C in cadmium-induced prostaglandin E2 production in primary mouse osteoblastic cells

We previously reported that cadmium (Cd) induced prostaglandin E2 (PGE2) biosynthesis through the activation of cytosolic phospholipase A2 (cPLA2) and induction of cyclooxygenase 2 (COX-2) in primary mouse osteoblastic cells. In the present study, we further investigated the mechanism of PGE2 production by Cd focusing on the main mitogen-activated protein kinase (MAPK) subfamilies that mediate prostaglandin synthesis, extracellular signal-regulated kinase (ERK1/2 MAPK), c-jun-amino-terminal kinase (JNK MAPK) and p38 MAPK, and protein kinase C (PKC) which is activated by Cd in several kinds of cells. Cd at 2 microM and above stimulated PGE2 production in osteoblastic cells and its production was inhibited by the kinase-specific inhibitors PD98059, SB203580, curcumin, and calphostin C. Calphostin C also inhibited the production of PGE2 by phorbol 12-myristate 13-acetate (PMA), which is a potent activator of PKC. PD98059 inhibited PGE2 production stimulated by PMA as well as Cd, indicating that activation of PKC by ERK1/2 MAPK was necessary for Cd-stimulated PGE2 production. Moreover, Cd stimulated the phosphorylation of these three MAPKs, and inhibition of the phosphorylation of ERK1/2 MAPK by calphostin C was also observed. On the other hand, Cd was found to phosphorylate cPLA2 and the phosphorylation was inhibited by PD98059, indicating that cPLA2 was activated by Cd through ERK1/2 MAPK and released arachidonic acid (AA), a substrate of COX-2, from membranous phospholipids. From these results, it was suggested that activation of each of the ERK1/2, p38, and JNK MAPK cascades in addition to that of PKC and cPLA2 played an important role in the Cd-stimulated biosynthesis of PGE2 in mouse osteoblastic cells.

Regulatory effects of interleukin-1beta and prostaglandin E2 on expression of receptor activator of nuclear factor-kappaB ligand in human periodontal ligament cells

BACKGROUND

Receptor activator of nuclear factor-kappaB ligand (RANKL), which is expressed on the cell membrane of osteoblasts/stromal cells, stimulates osteoclastogenesis. We investigated the regulatory effects of interleukin-1beta (IL-1beta) and prostaglandin E2 (PGE2) on expression of RANKL in human periodontal ligament (HPDL) cells and the mechanisms involved in the PGE2 effect.

METHODS

The HPDL cells were treated with IL-1beta, alone or in combination with indomethacin (INDO) or NS398, a cyclooxygenase-2 (COX-2) inhibitor. The HPDL cells were also pretreated with H89, a protein kinase A (PKA) inhibitor or GF109203X, a protein kinase C (PKC) inhibitor and subsequently treated with PGE2, PGE receptor (EP)2 agonist, EP4 agonist, forskolin, dibutyryl cAMP (db-cAMP), or 3-(isobutyl)-1-methylxantine (IBMX). After each treatment, expression of EP2, EP4, or RANKL mRNA was analyzed by reverse transcription-polymerase chain reaction and Southern hybridization. Expression of RANKL protein was detected by Western blotting, and cAMP accumulation was determined using a cAMP enzyme immunoassay kit.

RESULTS

IL-1beta stimulated the expression of RANKL at messenger RNA (mRNA) and protein levels in HPDL cells. Endogenous PGE2 partially mediated the IL-1beta-induced RANKL mRNA expression. Exogenously added PGE2 also stimulated RANKL expression at mRNA and protein levels in the cells. The PGE2-stimulated RANKL expression was mediated by EP2/4 and cAMP-dependent PKA, while PKC was possibly involved in the PGE2 action.

CONCLUSION

Human periodontal ligament cells activated with inflammatory factors such as IL-1beta and PGE2 may directly stimulate osteoclastogenesis through RANKL, which is stimulated to express by these factors.

Transcriptional regulation of genes for enzymes of the prostaglandin biosynthetic pathway

Numerous studies over the years have demonstrated changes in prostaglandin (PG) levels in intrauterine tissues in association with labour, and PG administration has long been used to induce delivery. While it is now widely accepted that PGs play a major role in human parturition, the complex regulation of their levels is still being elucidated, with the focus on the transcriptional control of the enzymes responsible for the various steps in PG biosynthesis and catabolism.

Regulation of bone morphogenetic protein-2 expression by endogenous prostaglandin E2 in human mesenchymal stem cells

Cyclooxygenase (COX)-2 is generally known as an inducible enzyme, and it produces arachidonic acid to prostaglandin E2 (PGE2), which modulates bone metabolism. Here, we investigated the expression and role of COX isomers in human mesenchymal stem cells. Human mesenchymal stem cells constitutively expressed COX-2 as well as COX-1, and secretion of PGE2 was completely inhibited by NS-398, a specific inhibitor of COX-2. Levels of secreted PGE2 were strikingly higher in human mesenchymal stem cells than in osteoblastic cells differentiated from the mesenchymal cells. This higher production of PGE2 in mesenchymal stem cells was due to higher expression of membrane-associated PGE synthase (mPGES) regulated by early growth response factor-1 (Egr-1). Treatment of human mesenchymal stem cells with NS-398 suppressed expression of bone morphogenetic protein-2 (BMP-2). The suppression of BMP-2 by NS-398 was abrogated by an EP4 receptor agonist as well as by PGE2. Moreover, BMP-2 expression was suppressed by an EP4 receptor antagonist. These data indicate that PGE2 produced by COX-2 increases BMP-2 expression via binding the EP4 receptor.

Role of mitogen-activated protein kinases in influenza virus induction of prostaglandin E2 from arachidonic acid in bronchial epithelial cells

BACKGROUND

Influenza virus (IV) infection causes airway inflammation; however, it has not been determined whether IV infection could catabolize arachidonic acid cascade in airway epithelial cells. In addition, the responsible intracellular signalling molecules that catabolize arachidonic acid cascade have not been determined.

OBJECTIVE

In the present study, to clarify these issues, we examined the cyclooxygenase (COX) expression, cytosolic phospholipase A2 (cPLA2) phosphorylation and prostaglandin E2 (PGE2) release in human bronchial epithelial cells (BEC) upon IV infection, and the role of mitogen-activated protein kinase (MAPK) including extracellular signal-regulated kinase (ERK), p38 MAPK and c-Jun-NH2-terminal kinase (JNK) in catabolizing arachidonic acid cascade in BEC.

METHODS

COX-2 expression, phosphorylation of cPLA2 and phosphorylation of ERK, JNK and p38 MAPK were determined by Western blot. The concentrations of PGE2 were determined by ELISA. PD 98059 as a specific inhibitor of MAPK kinase-1 (MEK-1), an up-stream kinase of ERK, SB 203580 as a specific inhibitor of p38 MAPK and CEP-11004 as a specific inhibitor of JNK cascade were used to investigate the role of ERK, p38 MAPK and JNK in catabolizing arachidonic acid cascade in BEC.

RESULTS

The results showed that (1) IV infection increases COX-2 expression, cPLA2 phosphorylation and PGE2 release, (2) ERK, p38 MAPK and JNK were phosphorylated, (3) CEP-11004 and PD 98059 predominantly attenuated COX-2 expression and cPLA2 phosphorylation, respectively, (4) SB 203580 did not remarkably affect COX-2 expression and cPLA2 phosphorylation, and (5) each inhibitor dose-dependently attenuated PGE2 release by various extents.

CONCLUSION

These results indicate that IV infection activates three distinct MAPKs, ERK, p38 MAPK and JNK, to participate to various extents in the induction of PGE2 synthesis from arachidonic acid in BEC.

Activation of p38 mitogen-activated protein kinase in spinal microglia is a critical link in inflammation-induced spinal pain processing

We examined the effect of p38 mitogen-activated protein kinase (MAPK) inhibitors in models of nociception and correlated this effect with localization and expression levels of p38 MAPK in spinal cord. There was a rapid increase in phosphorylated p38 MAPK in spinal cord following intrathecal administration of substance P or intradermal injection of formalin. Immunocytochemistry revealed that phosphorylated p38 MAPK-immunoreactive cells were predominantly present in laminae I-IV of the dorsal horn. Double-staining with markers for neurons, microglia, astrocytes and oligodendrocytes unexpectedly revealed co-localization with microglia but not with neurons or other glia. Pretreatment with p38 MAPK inhibitors (SB20358 or SD-282) had no effect on acute thermal thresholds. However, they attenuated hyperalgesia in several nociceptive models associated with spinal sensitization including direct spinal activation (intrathecal substance P) and peripheral tissue inflammation (intraplantar formalin or carrageenan). Spinal sensitization, manifested by enhanced expression of cyclo-oxygenase-2 and inflammation-induced appearance of Fos-positive neurons, was blocked by pretreatment, but not post-treatment, with p38 MAPK inhibitors. Taken together, these results indicate that spinal p38 MAPK is involved in inflammation-induced pain and that activated spinal microglia play a direct role in spinal nociceptive processing.

Phospholipase A(2) isoforms: a perspective

Several new PLA(2)s have been identified based on their nucleotide gene sequences. They were classified mainly into three groups: cytosolic PLA(2) (cPLA(2)), secretary PLA(2) (sPLA(2)), and intracellular PLA(2) (iPLA(2)). They differ from each other in terms of substrate specificity, Ca(2+) requirement and lipid modification. The questions that still remain to be addressed are the subcellular localization and differential regulation of the isoforms in various cell types and under different physiological conditions. It is required to identify the downstream events that occur upon PLA(2) activation, particularly target protein or metabolic pathway for liberated arachidonic acid or other fatty acids. Understanding the same will greatly help in the development of potent and specific pharmacological modulators that can be used for basic research and clinical applications. The information of the human and other genomes of PLA(2)s, combined with the use of proteomics and genetically manipulated mouse models of different diseases, will illuminate us about the specific and potentially overlapping roles of individual phospholipases as mediators of physiological and pathological processes. Hopefully, such understanding will enable the development of specific agents aimed at decreasing the potential contribution of individual secretary phospholipases to vascular diseases. The signaling cascades involved in the activation of cPLA(2) by mitogen activated protein kinases (MAPKs) is now evident. It has been demonstrated that p44 MAPK phosphorylates cPLA(2) and increases its activity in cells and tissues. The phosphorylation of cPLA(2) at ser505 occurs before the increase in intracellular Ca(2+) that facilitate the binding of the lipid binding domain of cPLA(2) to phospholipids, promoting its translocation to cellular membranes and AA release. Recently, a negative feed back loop for cPLA(2) activation by MAPK has been proposed. If PLA(2) activation in a given model depends on PKC, PKA, cAMP, or MAPK then inhibition of these phosphorylating enzymes may alter activities of PLA(2) isoforms during cellular injury. Understanding the signaling pathways involved in the activation/deactivation of PLA(2) during cellular injury will point to key events that can be used to prevent the cellular injury. Furthermore, to date, there is limited information available regarding the regulation of iPLA(2) or sPLA(2) by these pathways.

An essential role of cytosolic phospholipase A2alpha in prostaglandin E2-mediated bone resorption associated with inflammation

Prostaglandin E (PGE)2 produced by osteoblasts acts as a potent stimulator of bone resorption. Inflammatory bone loss is accompanied by osteoclast formation induced by bone-resorbing cytokines, but the mechanism of PGE2 production and bone resorption in vivo is not fully understood. Using cytosolic phospholipase A2alpha (cPLA2alpha)-null mice, we examined the role of cPLA2alpha in PGE2 synthesis and bone resorption. In bone marrow cultures, interleukin (IL)-1 markedly stimulated PGE2 production and osteoclast formation in wild-type mice, but not in cPLA2alpha-null mice. Osteoblastic bone marrow stromal cells induced the expression of cyclooxygenase (COX)-2 and membrane-bound PGE2 synthase (mPGES) in response to IL-1 and lipopolysaccharide (LPS) to produce PGE2. Osteoblastic stromal cells collected from cPLA2alpha-null mice also induced the expression of COX-2 and mPGES by IL-1 and LPS, but could not produce PGE2 due to the lack of arachidonic acid release. LPS administration to wild-type mice reduced femoral bone mineral density by increased bone resorption. In cPLA2alpha-null mice, however, LPS-induced bone loss could not be observed at all. Here, we show that cPLA2alpha plays a key role in PGE production by osteoblasts and in osteoclastic bone resorption, and suggest a new approach to inflammatory bone disease by inhibiting cPLA2alpha.

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.

Role of RANKL-induced osteoclast formation and MMP-dependent matrix degradation in bone destruction by breast cancer metastasis

Bone metastasis of breast cancer induces severe osteolysis with increased bone resorption. Osteoclast differentiation regulated by the receptor activator of NF-kappaB ligand (RANKL) in osteoblasts and matrix degradation induced by matrix metalloproteinases (MMPs) are thought to be involved in the process of bone resorption. When nude mice were inoculated with human breast cancer cells, MDA-MB-231(MDA-231), numerous osteoclasts resorbed bone and the degradation of the bone matrix markedly progressed in the femur and tibia with metastasis of the MDA-231 tumour. The expression of RANKL, MMP-13 and membrane-type 1-MMP mRNA was markedly elevated in bone with metastasis. When MDA-231 cells were cocultured with mouse calvaria, MDA-231 markedly induced bone resorption measured by calcium release from the calvaria, and the expression of RANKL, MMP-2 and MMP-13 was elevated in the calvaria after the coculture. The separation of MDA-231 from the calvaria using filter insert showed decreased bone resorption, suggesting that cell-to-cell interaction is essential for cancer-induced bone resorption. Adding MDA-231 cells to bone marrow cultures markedly induced osteoclast formation, and the expression of RANKL in osteoblasts was enhanced by contact with the cell surface of MDA-231 cells. These results indicate that RANKL-induced osteoclast formation and MMP-dependent matrix degradation are associated with osteolysis because of bone metastasis of breast cancer.