Activation of P2Y but not P2X(4) nucleotide receptors causes elevation of [Ca2+]i in mammalian osteoclasts

Modulation of osteoblast differentiation and function by the P2X4 receptor

Bone cells are known to express multiple P2 receptor subtypes, and the functional effects of receptor activation have been described for many of these. One exception is the P2X4 receptor, which despite strong expression in osteoblasts and osteoclasts, has no defined functional activity. This study used the selective P2X4 receptor antagonists, 5-BDBD and PSB-12062, to investigate the role of this receptor in bone. Both antagonists (≥ 0.1 μM) dose-dependently decreased bone formation by 60-100%. This was accompanied by a ≤ 70% decrease in alkaline phosphatase activity, a ≤ 40% reduction in cell number, and a ≤ 80% increase in the number of adipocytes present in the culture. The analysis of gene expression showed that levels of osteoblast marker genes (e.g. Alpl, Bglap) were decreased in 5-BDBD treated cells. Conversely, expression of the adipogenic transcription factor PPARG was increased 10-fold. In osteoclasts, high doses of both antagonists were associated with a reduction in osteoclast formation and resorptive activity by ≤ 95% and ≤ 90%, respectively. Taken together, these data suggest that the P2X4 receptor plays a role in modulating bone cell function. In particular, it appears to influence osteoblast differentiation favouring the osteogenic lineage over the adipogenic lineage.

Hydrogen peroxide suppresses TRPM4 trafficking to the apical membrane in mouse cortical collecting duct principal cells

A Ca²⁺-activated nonselective cation channel (NSC_Ca) is found in principal cells of the mouse cortical collecting duct (CCD). However, the molecular identity of this channel remains unclear. We used mpkCCD_c14 cells, a mouse CCD principal cell line, to determine whether NSC_Ca represents the transient receptor potential (TRP) channel, the melastatin subfamily 4 (TRPM4). A Ca²⁺-sensitive single-channel current was observed in inside-out patches excised from the apical membrane of mpkCCD_c14 cells. Like TRPM4 channels found in other cell types, this channel has an equal permeability for Na⁺ and K⁺ and has a linear current-voltage relationship with a slope conductance of ~23 pS. The channel was inhibited by a specific TRPM4 inhibitor, 9-phenanthrol. Moreover, the frequency of observing this channel was dramatically decreased in TRPM4 knockdown mpkCCD_c14 cells. Unlike those previously reported in other cell types, the TRPM4 in mpkCCD_c14 cells was unable to be activated by hydrogen peroxide (H₂O₂). Conversely, after treatment with H₂O₂, TRPM4 density in the apical membrane of mpkCCD_c14 cells was significantly decreased. The channel in intact cell-attached patches was activated by ionomycin (a Ca²⁺ ionophore), but not by ATP (a purinergic P₂ receptor agonist). These data suggest that the NSC_Ca current previously described in CCD principal cells is actually carried through TRPM4 channels. However, the physiological role of this channel in the CCD remains to be further determined.

Attenuated RANKL-induced cytotoxicity by Portulaca oleracea ethanol extract enhances RANKL-mediated osteoclastogenesis

BACKGROUND

Portulaca oleracea (PO) has been widely used as traditional medicine because of its pharmacological activities. However, the effects of PO on osteoclasts that modulate bone homeostasis are still elusive.

METHODS

In this study, we examined the effects of PO ethanol extract (POEE) on receptor activator of nuclear factor-κB ligand (RANKL)-mediated Ca(2+) mobilization, nuclear factor of activated T-cell c1 (NFATc1) amplification, tartrate-resistant acid phosphatase-positive (TRAP+) multinucleated cell (MNC) formation, and cytotoxicity.

RESULTS

Our results demonstrated that POEE suppressed RANKL-induced Ca(2+) oscillations by inhibition of Ca(2+) release from internal Ca(2+) stores, resulting in reduction of NFATc1 amplification. Notably, POEE attenuated RANKL-mediated cytotoxicity and cleavage of polyadenosine 5'-diphosphate-ribose polymerase (PARP), resulted in enhanced formation of TRAP+ MNCs.

CONCLUSIONS

These results present in vitro effects of POEE on RANKL-mediated osteoclastogenesis and suggest the possible use of PO in treating bone disorders, such as osteopetrosis.

The role of purinergic signalling in the musculoskeletal system

Accumulating evidence now suggests that purinergic signalling exerts significant regulatory effects in the musculoskeletal system. In particular, it has emerged that extracellular nucleotides are key regulators of bone cell differentiation, survival and function. This review discusses our current understanding of the direct effects of purinergic signalling in bone, cartilage and muscle.

Ovarian cancer G protein-coupled receptor 1 is involved in acid-induced apoptosis of endplate chondrocytes in intervertebral discs

Ovarian cancer G protein-coupled receptor 1 (OGR1) has been shown to be a receptor for protons. We investigated the role of proton-sensing G protein-coupled receptors in the apoptosis of endplate chondrocytes induced by extracellular acid. The expression of proton-sensing G protein-coupled receptors was examined in rat lumbar endplate chondrocytes. Knockdown of OGR1 was achieved by transfecting chondrocytes with specific short hairpin RNA (shRNA) for OGR1. Apoptotic changes were evaluated by DNA fragmentation ELISA, electron microscopy, and flow cytometry. Intracellular calcium ([Ca(2+) ]i) was analyzed with laser scanning confocal microscopy. The mechanism of OGR1 in acid-induced apoptosis of endplate chondrocytes was also investigated. We found that OGR1 was predominantly expressed in rat endplate chondrocytes, and its expression was highly upregulated in response to acidosis. Knocking down OGR1 with shRNAs effectively attenuated acid-induced apoptosis of endplate chondrocytes and increased [Ca(2+) ]i. Blocking OGR1-mediated [Ca(2+) ]i elevation inhibited acid-induced calcium-sensitive proteases such as calpain and calcineurin, and also inhibited the activation of Bid, Bad, and Caspase 3 and cleavage of poly (ADP-ribose) polymerase (PARP). OGR1-mediated [Ca(2+) ]i elevation has a crucial role in apoptosis of endplate chondrocytes by regulating activation of calcium-sensitive proteases and their downstream signaling.

Purinergic signalling in the musculoskeletal system

It is now widely recognised that extracellular nucleotides, signalling via purinergic receptors, participate in numerous biological processes in most tissues. It has become evident that extracellular nucleotides have significant regulatory effects in the musculoskeletal system. In early development, ATP released from motor nerves along with acetylcholine acts as a cotransmitter in neuromuscular transmission; in mature animals, ATP functions as a neuromodulator. Purinergic receptors expressed by skeletal muscle and satellite cells play important pathophysiological roles in their development or repair. In many cell types, expression of purinergic receptors is often dependent on differentiation. For example, sequential expression of P2X5, P2Y1 and P2X2 receptors occurs during muscle regeneration in the mdx model of muscular dystrophy. In bone and cartilage cells, the functional effects of purinergic signalling appear to be largely negative. ATP stimulates the formation and activation of osteoclasts, the bone-destroying cells. Another role appears to be as a potent local inhibitor of mineralisation. In osteoblasts, the bone-forming cells, ATP acts via P2 receptors to limit bone mineralisation by inhibiting alkaline phosphatase expression and activity. Extracellular ATP additionally exerts significant effects on mineralisation via its hydrolysis product, pyrophosphate. Evidence now suggests that purinergic signalling is potentially important in several bone and joint disorders including osteoporosis, rheumatoid arthritis and cancers. Strategies for future musculoskeletal therapies might involve modulation of purinergic receptor function or of the ecto-nucleotidases responsible for ATP breakdown or ATP transport inhibitors.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

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.

Isolation and purification of rabbit osteoclasts

Newborn rabbits provide a useful and readily available source of authentic mature osteoclasts, which can be easily isolated directly from the long bones in relatively large numbers, compared to other rodents. Primary cultures of authentic rabbit osteoclasts on resorbable substrates in vitro are an ideal model of osteoclast behaviour in vivo, and for some studies may be preferable to osteoclast-like cells generated in vitro from bone marrow cultures or from human peripheral blood, for example in assessing osteoclast-mediated bone resorption independently of effects on osteoclast formation. Rabbits also provide a particularly useful model for determining the effects of pharmacological agents on osteoclasts in vivo, by isolating osteoclasts using immunomagnetic bead separation (with an antibody to α(V)β(3)) at the desired time following in vivo administration of the drug. Since osteoclasts are abundant in newborn rabbits, sufficient numbers of osteoclasts can be retrieved using this method for molecular and biochemical analyses.

Lysophosphatidic acid signals through multiple receptors in osteoclasts to elevate cytosolic calcium concentration, evoke retraction, and promote cell survival

Lysophosphatidic acid (LPA) is a bioactive phospholipid whose functions are mediated by multiple G protein-coupled receptors. We have shown that osteoblasts produce LPA, raising the possibility that it mediates intercellular signaling among osteoblasts and osteoclasts. Here we investigated the expression, signaling and function of LPA receptors in osteoclasts. Focal application of LPA elicited transient increases in cytosolic calcium concentration ([Ca(2+)](i)), with 50% of osteoclasts responding at approximately 400 nm LPA. LPA-induced elevation of [Ca(2+)](i) was blocked by pertussis toxin or the LPA(1/3) receptor antagonist VPC-32183. LPA caused sustained retraction of osteoclast lamellipodia and disrupted peripheral actin belts. Retraction was insensitive to VPC-32183 or pertussis toxin, indicating involvement of a distinct signaling pathway. In this regard, inhibition of Rho-associated kinase stimulated respreading after LPA-induced retraction. Real-time reverse transcription-PCR revealed transcripts encoding LPA(1) and to a lesser extent LPA(2), LPA(4), and LPA(5) receptor subtypes. LPA induced nuclear translocation of NFATc1 and enhanced osteoclast survival, effects that were blocked by VPC-32183 or by a specific peptide inhibitor of NFAT activation. LPA slightly reduced the resorptive activity of osteoclasts in vitro. Thus, LPA binds to at least two receptor subtypes on osteoclasts: LPA(1), which couples through G(i/o) to elevate [Ca(2+)](i), activate NFATc1, and promote survival, and a second receptor that likely couples through G(12/13) and Rho to evoke and maintain retraction through reorganization of the actin cytoskeleton. These findings reveal a signaling axis in bone through which LPA, produced by osteoblasts, acts on multiple receptor subtypes to induce pleiotropic effects on osteoclast activity and function.

Expression, signaling, and function of P2X7 receptors in bone

Nucleotides released from cells in response to mechanical stimulation or injury may serve as paracrine regulators of bone cell function. Extracellular nucleotides bind to multiple subtypes of P2 receptors on osteoblasts (the cells responsible for bone formation) and osteoclasts (cells with the unique ability to resorb mineralized tissues). Both cell lineages express the P2X7 receptor subtype. The skeletal phenotype of mice with targeted disruption of P2rx7 points to interesting roles for this receptor in the regulation of bone formation and resorption, as well as the response of the skeleton to mechanical stimulation. This paper reviews recent work on the expression of P2X7 receptors in bone, their associated signal transduction mechanisms and roles in regulating bone formation and resorption. Areas for future research in this field are also discussed.

Activation of P2X7 receptors causes isoform-specific translocation of protein kinase C in osteoclasts

Nucleotides, released in response to mechanical or inflammatory stimuli, signal through P2 nucleotide receptors in many cell types. Osteoclasts express P2X7 receptors (encoded by P2rx7) - Ca(2+)-permeable channels that are activated by high concentrations of extracellular ATP. Genetic disruption of P2rx7 leads to increased resorption and reduced skeletal response to mechanical stimuli. To investigate whether P2X7 receptors couple to activation of protein kinase C (PKC), RAW 264.7 cells were differentiated into multinucleated osteoclast-like cells and live-cell confocal imaging was used to localize enhanced green fluorescent protein (EGFP)-tagged PKC. Benzoylbenzoyl-ATP (BzATP; a P2X7 agonist) induced transient translocation of PKCalpha to the basolateral membrane. UTP or ATP (10 microM), which activate P2 receptors other than P2X7, failed to induce translocation. Moreover, BzATP failed to induce PKC translocation in osteoclasts derived from the bone marrow of P2rx7(-/-) mice, demonstrating specificity for P2X7. BzATP induced a transient rise of cytosolic Ca(2+), and removal of extracellular Ca(2+) abolished the translocation of PKCalpha that was induced by BzATP (but not by phorbol ester). We examined the isoform specificity of this response, and observed translocation of the Ca(2+)-dependent isoforms PKCalpha and PKCbetaI, but not the Ca(2+)-independent isoform PKCdelta. Thus, activation of P2X7 receptors specifically induces Ca(2+)-dependent translocation of PKC to the basolateral membrane domain of osteoclasts, an aspect of spatiotemporal signaling not previously recognized.

Analysis of a missense variant of the human N-formyl peptide receptor that is associated with agonist-independent β-arrestin association and indices of inflammation

Formyl-Met-Leu-Phe (fMLP) is a potent chemoattractant molecule released from both bacteria and damaged mitochondria that activates fMLP receptors (FPR) leading to neutrophil chemotaxis, degranulation and superoxide production. A common missense single nucleotide polymorphism in the human FPR1 gene at nucleotide c.32C>T results in the amino-acid substitution, p.I11T, in the FPR1 extracellular amino-terminus. The minor (c.32T) allele frequencies were 0.25, 0.27, 0.25, 0.15 and 0.14 in healthy Caucasian, African, East Indian, Chinese and Native Canadian individuals, respectively. In subjects homozygous for the p.T11 allele, we find elevated serum concentrations of C-reactive protein, increased absolute counts of blood leukocytes and neutrophils, and erythrocyte sedimentation rates. When expressed in HEK 293 and RBL-2H3 cells a substantial proportion of FPR1 p.I11T variant is retained intracellularly and agonist-independent internalization of the FPR1 p.I11T variant, but not the wild-type FPR1, is constitutively associated with beta-arrestin2-GFP in vesicles. Moreover, basal N-acetyl-D-glucosaminidase release is increased in primary neutrophils isolated from subjects either heterozygous or homozygous for the FPR1 p.T11 allele. Taken together, the data suggest an increased receptor activity and phenotypic expression of increased inflammatory indices in subjects with the p.T11 allele.

P2Y6 nucleotide receptors activate NF-kappaB and increase survival of osteoclasts

Nucleotides, released from cells during inflammation and by mechanical stimulation, act through the P2 family of nucleotide receptors. Previous studies have demonstrated the expression of P2Y1 and P2Y2 receptors in osteoclasts. The aim of this study was to determine whether osteoclast P2Y receptors signal through NF-kappaB, a key transcription factor regulating osteoclastogenesis. Immunofluorescence was used to detect the p65 subunit of NF-kappaB, which upon activation translocates from the cytosol to nuclei. Low levels of NF-kappaB activation were observed in untreated rabbit osteoclasts and in those exposed to 2-methylthio ADP (P2Y1 agonist) or ATP or UTP (P2Y2 agonists). In contrast, UDP or INS48823 (P2Y6 agonists) induced a significant increase in the number of cells exhibiting NF-kappaB activation, a process sensitive to the proteasome inhibitor lactacystin. In osteoclasts purified by micromanipulation, reverse transcription-PCR revealed the presence of P2Y1, P2Y2, and P2Y6 receptor transcripts, and application of agonists for these receptors induced the transient rise of cytosolic calcium. Treatment of rat osteoclasts with UDP or INS48823, but not 2-methylthio ADP or UTP, increased osteoclast survival. Osteoprotegerin (a decoy receptor for RANK ligand) did not significantly alter the effects of UDP on NF-kappaB localization or osteoclast survival, consistent with a direct action. Moreover, SN50 (cell-permeable peptide inhibitor of NF-kappaB) suppressed the enhancement of cell survival induced by UDP and INS48823. Our findings demonstrate the presence of functional P2Y6 receptors in osteoclasts. Thus, nucleotides, following their release at sites of inflammation and mechanical stimulation, can act through P2Y6 receptors to initiate NF-kappaB signaling and enhance osteoclast survival.

Convergent signaling by acidosis and receptor activator of NF-kappaB ligand (RANKL) on the calcium/calcineurin/NFAT pathway in osteoclasts

Systemic acidosis has detrimental effects on the skeleton, and local acidosis coincides with bone destruction in inflammatory and metastatic diseases. Acidification dramatically enhances osteoclastic resorption, although the underlying mechanism has remained elusive. We investigated the effect of acidosis on the osteoclastogenic transcription factor NFATc1, which upon dephosphorylation translocates from the cytoplasm to nuclei. Lowering extracellular pH dramatically increased accumulation of NFATc1 in nuclei of rat and rabbit osteoclasts to levels comparable with those induced by the proresorptive cytokine receptor activator of NF-kappaB ligand (RANKL). Activation of NFATc1 by RANKL was mediated by means of prolonged stimulation of the Ca2+/calmodulin-dependent protein phosphatase, calcineurin. In contrast, NFATc1 activation by acidosis involved stimulation of calcineurin and suppression of NFATc1 inactivation. Acidosis, like RANKL, induced transient elevation of cytosolic free Ca2+ concentration ([Ca2+]i), which persisted in Ca2+-free media and was abolished by inhibition of phospholipase C or depletion of intracellular Ca2+ stores. Real-time-PCR of osteoclast-like cells generated from RAW 264.7 cells revealed high levels of expression of ovarian cancer G protein-coupled receptor 1, which links extracellular acidification to elevation of [Ca2+]i. In addition, the calcineurin inhibitor cyclosporin A suppressed the stimulatory effect of acidification on resorption, implicating NFAT in mediating the actions of acidosis on osteoclast activity. In summary, acidification and RANKL induce signals in osteoclasts that converge on the Ca2+/calcineurin/NFAT pathway. Acidosis acts directly on osteoclasts to activate NFATc1 and stimulate resorption.

Extracellular nucleotides act through P2X7 receptors to activate NF-kappaB in osteoclasts

Nucleotides, released in response to mechanical and other stimuli, act on P2 receptors in osteoclasts and other cell types. In vitro studies of osteoclasts from rabbits and P2X7 receptor-deficient mice revealed that P2X7 receptors couple to activation of the key transcription factor NF-kappaB.

INTRODUCTION

Osteoclasts express functional P2X4 and P2X7 receptors, which are ATP-gated cation channels. Knockout (KO) of the P2X7 receptor has revealed its role in regulating bone formation and resorption, but the underlying signals are not known. The transcription factor NF-kappaB plays a key role in the response of osteoclasts to RANKL and other cytokines. The aim of this study was to examine whether P2X receptors on osteoclasts signal through NF-kappaB.

MATERIALS AND METHODS

Osteoclasts were isolated from neonatal rabbits or wildtype (WT) and P2X7 receptor KO mice. Immunofluorescence was used to detect the p65 subunit of NF-kappaB, which, on activation, translocates from the cytosol to the nuclei. The concentration of cytosolic free Ca2+ ([Ca2+]i) was monitored in single osteoclasts loaded with fura-2.

RESULTS

In control samples, few rabbit osteoclasts demonstrated nuclear localization of NF-kappaB. Benzoyl-benzoyl-ATP (BzATP, a P2X7 agonist, 300 microM) induced nuclear translocation of NF-kappaB after 3 h in approximately 45% of rabbit osteoclasts. In contrast, a low concentration of ATP (10 microM, sufficient to activate P2X4 and P2Y2, but not P2X7 receptors) did not induce nuclear translocation of NF-kappaB. Because BzATP activates multiple P2 receptors, we examined responses of osteoclasts derived from WT and P2X7 receptor KO mice. Treatment with BzATP for 30 minutes increased nuclear localization of NF-kappaB in osteoclasts from WT but not KO mice, showing involvement of P2X7 receptors. Both ATP (10 microM) and BzATP (300 microM) caused transient elevation of [Ca2+]i, indicating that rise of calcium alone is not sufficient to activate NF-kappaB. Pretreatment of rabbit osteoclasts with osteoprotegerin inhibited translocation of NF-kappaB induced by RANKL but not by BzATP, establishing that the effects of BzATP are independent of RANKL signaling.

CONCLUSION

These findings show that P2X7 nucleotide receptors couple to activation of NF-kappaB in osteoclasts. Thus, nucleotides, released at sites of inflammation or in response to mechanical stimuli, may act through NF-kappaB to regulate osteoclast formation and activity.

Emerging neuroskeletal signalling pathways: a review

Recent work has demonstrated that neurotransmitters, signalling molecules primarily associated with the nervous system, can have profound effects on the skeleton. Bone cells express a broad range of neurotransmitter receptors and transporters, and respond to receptor activation by initiating diverse intracellular signalling pathways, which modulate cellular function. Evidence of neuronal innervation in skeletal tissues, neurotransmitter release directly from bone cells and functional effects of pharmacological manipulation support the existence of a complex and functionally significant neurotransmitter-mediated signalling network in bone. This review aims to concisely summarise our current understanding of how neurotransmitters affect the skeletal system, focusing on their origin, cellular targets and functional effects in bone.

The antirheumatic drug leflunomide inhibits osteoclastogenesis by interfering with receptor activator of NF-κB ligand-stimulated induction of nuclear factor of activated T cells c1

OBJECTIVE

Suppression of bone destruction is required as part of an effective therapeutic strategy for autoimmune arthritis. Although numerous antirheumatic drugs are in clinical use, little is known about whether they inhibit bone destruction by acting on activated T cells or other cell types, such as bone-resorbing osteoclasts. This study was undertaken to determine whether leflunomide has a direct action on the osteoclast lineage and to gain insights into the molecular basis for the bone-protective effect of leflunomide.

METHODS

The direct effect of leflunomide on osteoclast differentiation was investigated using an in vitro culture system of bone marrow monocyte/macrophages stimulated with receptor activator of NF-kappa B ligand (RANKL) and macrophage colony-stimulating factor. The molecular mechanism of the inhibition was analyzed by genome-wide screening. The T cell-independent effect of leflunomide was examined in rag-2(-/-) mice.

RESULTS

Leflunomide blocked de novo pyrimidine synthesis and RANKL-induced calcium signaling in osteoclast precursor cells in vitro; hence, the induction of nuclear factor of activated T cells c1 (NF-ATc1) was strongly inhibited. The inhibition of this pathway is central to the action of leflunomide, since the inhibition was overcome by ectopic expression of NF-ATc1 in the precursor cells. Leflunomide suppressed endotoxin-induced inflammatory bone destruction even in rag-2(-/-) mice.

CONCLUSION

Leflunomide has a direct inhibitory effect on RANKL-mediated osteoclast differentiation by inhibiting the induction of NF-ATc1, the master switch regulator for osteoclast differentiation. Our study suggests that the direct inhibitory action of leflunomide on osteoclast differentiation constitutes an important aspect in the amelioration of bone destruction, and that the RANKL-dependent NF-ATc1 induction pathway is a promising target for pharmacologic intervention in arthritic bone destruction.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Distinct intracellular Ca2+ response to extracellular adenosine triphosphate in pancreatic beta-cells in rats and mice

Extracellular adenosine triphosphate (ATP) has distinct effects on insulin secretion from pancreatic beta-cells between rats and mice. Using a confocal microscope, we compared changes between rats and mice in cytosolic free calcium concentration ([Ca2+]c) in pancreatic beta-cells stimulated by extracellular ATP. Extracellular ATP (50 microM) induced calcium release from intracellular calcium stores by activating P2Y receptors in both rat and mouse beta-cells. The intracellular calcium release stimulated by extracellular ATP is significantly smaller in amplitude and longer in duration in rat beta-cells than in mouse. In response to extracellular ATP, rat beta-cells activate store-operated calcium entry following intracellular calcium release. This response is lacking in mouse beta-cells. Rat and mouse beta-cells both responded to 9 mM glucose by increasing [Ca2+]c. This increase, however, was pronounced only in the rat beta-cells. In 9 mM glucose, extracellular ATP induced a pronounced calcium release above the increased level of [Ca2+]c in rat beta-cells. In mouse beta-cells, however, extracellular ATP did not exhibit calcium release on top of the increased level of [Ca2+]c in 9 mM glucose. These results demonstrate distinct responses between rat and mouse beta-cells to extracellular ATP under the condition of low and high glucose. Considering that extracellular ATP inhibits insulin secretion from mouse beta-cells but stimulates insulin secretion from rat beta-cells, we suggest that store-operated Ca2+ entry may be related to exocytosis in pancreatic rat beta-cells.

P2X4, P2Y1 and P2Y2 receptors on rat alveolar macrophages

ATP receptors present on rat alveolar macrophages (NR8383 cells) were identified by recordings of membrane current, measurements of intracellular calcium, RT-PCR and immunocytochemistry. In whole-cell recordings with a sodium-based internal solution, ATP evoked an inward current at -60 mV. This reversed at 0 mV. The EC50 for ATP was 18 microM in normal external solution (calcium 2 mm, magnesium 1 mm). The currents evoked by 2',3-O-(4-benzoyl)benzoyl-ATP were about five-fold smaller than those observed with ATP. ADP, UTP and alphabeta-methylene-ATP (alphabetameATP) (up to 100 microM) had no effect. ATP-evoked currents were potentiated up to ten-fold by ivermectin and were unaffected by suramin (30-100 microM), pyridoxal-phosphate-6-azophenyl-(2,4-sulphonic acid) (30-100 microM), and brilliant blue G (1 microM). In whole-cell recordings with a potassium-based internal solution and low EGTA (0.01 mm), ATP evoked an inward current at -60 mV that was followed by larger outward current. ADP and UTP (1-100 microM) evoked only outward currents; these reversed polarity at the potassium equilibrium potential and were blocked by apamin (10 nm). Outward currents were also blocked by the phospholipase C inhibitor U73122 (1 microM), and they were not seen with higher intracellular EGTA (10 mm). Suramin (30 microM) blocked the outward currents evoked by ATP and UTP, but not that evoked by ADP. PPADS (10 microM) blocked the ADP-evoked outward current without altering the ATP or UTP currents. RT-PCR showed transcripts for P2X subunits 1, 4 and 7 (not 2, 3, 5, 6) and P2Y receptors 1, 2, 4 and 12 (not 6). Immunocytochemistry showed strong P2X4 receptor expression partly associated with the membrane, weak P2X7 staining that was not associated with the cell membrane, and no P2X1 receptor immunoreactivity. We conclude that rat alveolar macrophages express (probably homomeric) P2X4 receptors, but find no evidence for other functional P2X subtypes. The P2Y receptors are most likely P2Y1 and P2Y2 and these couple through phospholipase C to an increase in intracellular calcium and the opening of SK type potassium channels.

Regulation of bone resorption and formation by purines and pyrimidines

Growing evidence suggests that extracellular nucleotides, signalling through P2 receptors, might play important roles in the regulation of bone and cartilage metabolism. ATP and other nucleotides can exert impressive stimulatory effects on the formation and activity of osteoclasts (bone-resorbing cells) in addition to inhibiting bone formation by osteoblasts. In this review, the current understanding of the actions of nucleotides on skeletal cells and the probable receptor subtypes involved are discussed.

RANK ligand-induced elevation of cytosolic Ca2+ accelerates nuclear translocation of nuclear factor kappa B in osteoclasts

RANK ligand (RANKL) induces activation of NFkappaB, enhancing the formation, resorptive activity, and survival of osteoclasts. Ca(2+) transduces many signaling events, however, it is not known whether the actions of RANKL involve Ca(2+) signaling. We investigated the effects of RANKL on rat osteoclasts using microspectrofluorimetry and patch clamp. RANKL induced transient elevation of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) to maxima 220 nm above basal, resulting in activation of Ca(2+)-dependent K(+) current. RANKL elevated [Ca(2+)](i) in Ca(2+)-containing and Ca(2+)-free media, and responses were prevented by the phospholipase C inhibitor. Suppression of [Ca(2+)](i) elevation using the intracellular Ca(2+) chelator 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) abolished the ability of RANKL to enhance osteoclast survival. Using immunofluorescence, NFkappaB was found predominantly in the cytosol of untreated osteoclasts. RANKL induced transient translocation of NFkappaB to the nuclei, which was maximal at 15 min. or BAPTA delayed nuclear translocation of NFkappaB. Delays were also observed upon inhibition of calcineurin or protein kinase C. We conclude that RANKL acts through phospholipase C to release Ca(2+) from intracellular stores, accelerating nuclear translocation of NFkappaB and promoting osteoclast survival. Such cross-talk between NFkappaB and Ca(2+) signaling provides a novel mechanism for the temporal regulation of gene expression in osteoclasts and other cell types.

Ca2+ signaling mediated by IP3-dependent Ca2+ releasing and store-operated Ca2+ channels in rat odontoblasts

In the phospholipase-C (PLC) signaling system, Ca2+ is mobilized from intracellular Ca2+ stores by an action of inositol 1,4,5-trisphosphate (IP3). The depletion of IP3-sensitive Ca2+ stores activates a store-operated Ca2+ entry (SOCE). However, no direct evidence has been obtained about these signaling pathways in odontoblasts. In this study, we investigate the characteristics of the SOCE and IP3-mediated Ca2+ mobilizations in rat odontoblasts using fura-2 microfluorometry and a nystatin-perforated patch-clamp technique. In the absence of extracellular Ca2+ ([Ca2+]o), thapsigargin (TG) evoked a transient rise in intracellular Ca2+ concentration ([Ca2+]i). After TG treatment to deplete the store, the subsequent application of Ca2+ resulted in a rapid rise in [Ca2+]i caused by SOCE. In the absence of TG treatment, no SOCE was evoked. The Ca2+ influx was dependent on [Ca2+]o (KD = 1.29 mM) and was blocked by an IP3 receptor inhibitor, 2-aminoethoxydiphenyl borate (2-APB), as well as La3+ in a concentration-dependent manner (IC50 = 26 microM). In TG-treated cells, an elevation of [Ca2+]o from 0 to 2.5 mM elicited an inwardly rectifying current at hyperpolarizing potentials with a positive reversal potential. The currents were selective for Ca2+ over the other divalent cations (Ca2+ > Ba2+ > Sr2+ >> Mn2+). In the absence of [Ca2+]o, carbachol, bradykinin, and 2-methylthioadenosine 5'triphosphate activated Ca2+ release from the store; these were inhibited by 2-APB. These results indicate that odontoblasts possessed Ca2+ signaling pathways through the activation of store-operated Ca2+ channels by the depletion of intracellular Ca2+ stores and through the IP3-induced Ca2+ release activated by PLC-coupled receptors.

Extracellular nucleotides inhibit growth of human oesophageal cancer cells via P2Y(2)-receptors

Extracellular ATP is known to inhibit growth of various tumours by activating specific purinergic receptors (P2-receptors). Since the therapy of advanced oesophageal cancer is unsatisfying, new therapeutic approaches are mandatory. Here, we investigated the functional expression and potential antiproliferative effects of P2-purinergic receptors in human oesophageal cancer cells. Prolonged incubation of primary cell cultures of human oesophageal cancers as well as of the squamous oesophageal cancer cell line Kyse-140 with ATP or its stable analogue ATP gamma S dose-dependently inhibited cell proliferation. This was due to both an induction of apoptosis and cell cycle arrest. The expression of P2-receptors was examined by RT-PCR, immunocytochemistry, and [Ca(2+)](i)-imaging. Application of various extracellular nucleotides dose-dependently increased [Ca(2+)](i). The rank order of potency was ATP=UTP>ATP gamma S>ADP=UDP. 2-methylthio-ATP and alpha,beta-methylene-ATP had no effects on [Ca(2+)](i). Complete cross-desensitization between ATP and UTP was observed. Moreover, the phospholipase C inhibitor U73122 dose-dependently reduced the ATP triggered [Ca(2+)](i) signal. The pharmacological features strongly suggest the functional expression of G-protein coupled P2Y(2)-receptors in oesophageal squamous cancer cells. P2Y(2)-receptors are involved in the antiproliferative actions of extracellular nucleotides. Thus, P2Y(2)-receptors are promising target proteins for innovative approaches in oesophageal cancer therapy.

Activity-dependent development of P2X7 current and Ca2+ entry in rabbit osteoclasts

Bone remodeling is regulated by local factors and modulated by mechanical stimuli. Mechanical stimulation can cause release of ATP, an agent that stimulates osteoclastic resorption at low concentrations and inhibits at high concentrations. We examined whether osteoclasts express P2X(7) receptors, which are activated by high concentrations of ATP and can behave as ion channels or cause the formation of membrane pores. Rabbit osteoclasts were studied using patch clamp techniques. Successive or prolonged applications of 2'- & 3'-O-(4-benzoylbenzoyl)-ATP (BzATP, a relatively potent P2X(7) agonist) or high concentrations of ATP caused the development of a slowly deactivating inward current. The underlying channel was permeable only to small cations, ruling out pore formation. Divalent cations reduced current magnitude, consistent with the presence of P2X(7) receptors, a finding confirmed in rat osteoclasts by immunocytochemistry. Successive applications of BzATP also elicited [Ca(2+)](i) elevations that required extracellular Ca(2+). The BzATP-induced current and the rise of [Ca(2+)](i) were temporally associated, and both were inhibited by PPADS, a P2X(7) antagonist. This study demonstrates that high concentrations of ATP activate P2X(7) receptors and provides the first functional evidence for an extracellular ligand-gated Ca(2+) influx pathway in osteoclasts. ATP released in response to mechanical stimuli may act through P2X(7) receptors to inhibit osteoclastic resorption.