Evidence for P2-purinoceptors on human osteoblast-like cells

Metabolomic profiles of cartilage and bone reflect tissue type, radiography-confirmed osteoarthritis, and spatial location within the joint

Osteoarthritis is the most common chronic joint disease, characterized by the abnormal remodeling of joint tissues including articular cartilage and subchondral bone. However, there are currently no therapeutic drug targets to slow the progression of disease because disease pathogenesis is largely unknown. Thus, the goals of this study were to identify metabolic differences between articular cartilage and subchondral bone, compare the metabolic shifts in osteoarthritic grade III and IV tissues, and spatially map metabolic shifts across regions of osteoarthritic hip joints. Articular cartilage and subchondral bone from 9 human femoral heads were obtained after total joint arthroplasty, homogenized and metabolites were extracted for liquid chromatography-mass spectrometry analysis. Metabolomic profiling revealed that distinct metabolic endotypes exist between osteoarthritic tissues, late-stage grades, and regions of the diseased joint. The pathways that contributed the most to these differences between tissues were associated with lipid and amino acid metabolism. Differences between grades were associated with nucleotide, lipid, and sugar metabolism. Specific metabolic pathways such as glycosaminoglycan degradation and amino acid metabolism, were spatially constrained to more superior regions of the femoral head. These results suggest that radiography-confirmed grades III and IV osteoarthritis are associated with distinct global metabolic and that metabolic shifts are not uniform across the joint. The results of this study enhance our understanding of osteoarthritis pathogenesis and may lead to potential drug targets to slow, halt, or reverse tissue damage in late stages of osteoarthritis.

Extracellular purines and bone homeostasis

Maintenance of a healthy skeleton is highly dependent on an intricate regulation of bone metabolism, as changes in the balance between bone formation and bone resorption leads to bone loss, bone fragility and ultimately bone fractures. During the last three decades it has become increasingly evident that physiological release of purines in the extracellular space is imperative for bone homeostasis and is orchestrated via the network of purinoceptors. Adenosine derivatives are released locally in the skeleton either by the bone forming osteoblasts or the bone degrading osteoclasts actioned directly by processes like mechanical loading and indirectly by systemic hormones. Adenosine derivatives directly affect the bone cells by their action on the membranal receptors or have co-stimulatory actions with bone active hormones such as parathyroid hormone or the gut hormones. Any deviations leading to increased levels of extracellular adenosine derivatives in the bone tissue such as in pathologic situations, trigger complex pathways with opposing effects on tissue health as presented by studies involving a range of model organisms. Pathological conditions where skeletal purinergic signaling is affected are following tissue injury like microdamage and macroscopic fractures; and during inflammatory processes where nucleotides and nucleosides play an important part in the pathophysiological skeletal response. Moreover, adenosine derivatives also play an important role in the interaction between malignant cells and bone cells in several types of cancers involving the skeleton, such as but not limited to multiple myeloma and bone osteolysis. Much knowledge has been gained over the last decades. The net- resulting phenotype of adenosine derivatives in bone (including the ratio of ATP to Adenosine) is highly dependent on CD39 and CD73 enzymes together with the expression and activity of the specific receptors. Thus, each component is important in the physiological and pathophysiological processes in bone. Promising perspectives await in the future in treating skeletal disorders with medications targeting the individual components of the purinergic signaling pathway.

P2Y Receptors in Bone - Anabolic, Catabolic, or Both?

P2Y receptors, including eight subtypes, are G protein-coupled receptors that can be activated by extracellular nucleotides. Nearly all P2Y receptors are expressed in bone cells, suggesting their involvements in bone physiology and pathology. However, their exact roles in bone homeostasis are not entirely clear. Therefore, this mini review summarizes new research developments regarding individual P2Y receptors and their roles in bone biology, particularly detailing those which execute both anabolic and catabolic functions. This dual function has highlighted the conundrum of pharmacologically targeting these P2Y receptors in bone-wasting diseases. Further research in finding more precise targeting strategy, such as promoting anabolic effects via combining with physical exercise, should be prioritized.

Bone turnover is altered in transgenic rats overexpressing the P2Y2 purinergic receptor

It is now widely recognized that purinergic signaling plays an important role in the regulation of bone remodeling. One receptor subtype, which has been suggested to be involved in this regulation, is the P2Y2 receptor (P2Y2R). In the present study, we investigated the effect of P2Y2R overexpression on bone status and bone cell function using a transgenic rat. Three-month-old female transgenic Sprague Dawley rats overexpressing P2Y2R (P2Y2R-Tg) showed higher bone strength of the femoral neck. Histomorphometry showed increase in resorptive surfaces and reduction in mineralizing surfaces. Both mineral apposition rate and thickness of the endocortical osteoid layer were higher in the P2Y2R-Tg rats. μCT analysis showed reduced trabecular thickness and structural model index in P2Y2R-Tg rats. Femoral length was increased in the P2Y2R-Tg rats compared to Wt rats. In vitro, there was an increased formation of osteoclasts, but no change in total resorption in cultures from P2Y2R-Tg rats. The formation of mineralized nodules was significantly reduced in the osteoblastic cultures from P2Y2R-Tg rats. In conclusion, our study suggests that P2Y2R is involved in regulation of bone turnover, due to the effects on both osteoblasts and osteoclasts and that these effects might be relevant in the regulation of bone growth.

Lack of effect of adenosine on the function of rodent osteoblasts and osteoclasts in vitro

Extracellular ATP, signalling through P2 receptors, exerts well-documented effects on bone cells, inhibiting mineral deposition by osteoblasts and stimulating the formation and resorptive activity of osteoclasts. The aims of this study were to determine the potential osteotropic effects of adenosine, the hydrolysis product of ATP, on primary bone cells in vitro. We determined the effect of exogenous adenosine on (1) the growth, alkaline phosphatase (TNAP) activity and bone-forming ability of osteoblasts derived from the calvariae of neonatal rats and mice and the marrow of juvenile rats and (2) the formation and resorptive activity of osteoclasts from juvenile mouse marrow. Reverse transcription polymerase chain reaction (RT-PCR) analysis showed marked differences in the expression of P1 receptors in osteoblasts from different sources. Whilst mRNA for the A1 and A2B receptors was expressed by all primary osteoblasts, A2A receptor expression was limited to rat bone marrow and mouse calvarial osteoblasts and the A3 receptor to rat bone marrow osteoblasts. We found that adenosine had no detectable effects on cell growth, TNAP activity or bone formation by rodent osteoblasts in vitro. The analogue 2-chloroadenosine, which is hydrolysed more slowly than adenosine, had no effects on rat or mouse calvarial osteoblasts but increased TNAP activity and bone formation by rat bone marrow osteoblasts by 30-50 % at a concentration of 1 μM. Osteoclasts were found to express the A2A, A2B and A3 receptors; however, neither adenosine (≤100 μM) nor 2-chloroadenosine (≤10 μM) had any effect on the formation or resorptive activity of mouse osteoclasts in vitro. These results suggest that adenosine, unlike ATP, is not a major signalling molecule in the bone.

Purinergic receptor stimulation decreases ischemic brain damage by energizing astrocyte mitochondria

As a leading cause of death in the world, cerebral ischemic stroke has limited treatment options. The lack of glucose and oxygen after stroke is particularly harmful in the brain because neuronal metabolism accounts for significantly more energy consumption per gram of body weight compared to other organs. Our laboratory has identified mitochondrial metabolism of astrocytes to be a key target for pharmacologic intervention, not only because astrocytes play a central role in regulating brain metabolism, but also because they are essential for neuronal health and support. Here we review current literature pertaining to the pathobiology of stroke, along with the role of astrocytes and metabolism in stroke. We also discuss our research, which has revealed that pharmacologic stimulation of metabotropic P2Y1 receptor signaling in astrocytes can increase mitochondrial energy production and also reduce damage after stroke.

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

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.

Purinergic signalling in bone

Purinergic signaling in bone was first proposed in the early 1990s with the observation that extracellular ATP could modulate events crucial to the normal functioning of bone cells. Since then the expression of nearly all the P2Y and P2X receptors by osteoblasts and osteoclasts has been reported, mediating multiple processes including cell proliferation, differentiation, function, and death. This review will highlight the most recent developments in the field of purinergic signaling in bone, with a special emphasis on recent work resulting from the European Framework 7 funded collaboration ATPBone, as well as Arthritis Research UK and Bone Research Society supported projects.

Purinergic signaling is required for fluid shear stress-induced NF-κB translocation in osteoblasts

Fluid shear stress regulates gene expression in osteoblasts, in part by activation of the transcription factor NF-κB. We examined whether this process was under the control of purinoceptor activation. MC3T3-E1 osteoblasts under static conditions expressed the NF-κB inhibitory protein IκBα and exhibited cytosolic localization of NF-κB. Under fluid shear stress, IκBα levels decreased, and concomitant nuclear localization of NF-κB was observed. Cells exposed to fluid shear stress in ATP-depleted medium exhibited no significant reduction in IκBα, and NF-κB remained within the cytosol. Similar results were found using oxidized ATP or Brilliant Blue G, P2X(7) receptor antagonists, indicating that the P2X(7) receptor is responsible for fluid shear-stress-induced IκBα degradation and nuclear accumulation of NF-κB. Pharmacologic blockage of the P2Y6 receptor also prevented shear-induced IκBα degradation. These phenomena involved neither ERK1/2 signaling nor autocrine activation by P2X(7)-generated lysophosphatidic acid. Our results suggest that fluid shear stress regulates NF-κB activity through the P2Y(6) and P2X(7) receptor.

Gap Junctions and Biophysical Regulation of Bone Cells

Communication between osteoblasts, osteoclasts, and osteocytes is integral to their ability to build and maintain the skeletal system and respond to physical signals. Various physiological mechanisms, including nerve communication, hormones, and cytokines, play an important role in this process. More recently, the important role of direct, cell-cell communication via gap junctions has been established. In this review, we demonstrate the integral role of gap junctional intercellular communication (GJIC) in skeletal physiology and bone cell mechanosensing.

H(2)O(2) modulates purinergic-dependent calcium signalling in osteoblast-like cells

Reactive oxygen species (ROS) have long been considered as toxic by-products of aerobic metabolism and appear involved in the pathogenesis of degenerative diseases. The physiological role of ROS as second messengers in cell signal transduction is, on the other hand, increasingly recognized. Here we investigated the effects of H(2)O(2) and extracellular nucleotides on calcium signalling in four osteoblastic cell lines. In the highly differentiated HOBIT cells, sensitive to nanomolar concentrations of ADP and UTP, millimolar H(2)O(2) induced oscillatory increases of the cytosolic calcium concentration followed by a steady and sustained calcium increase. Long lasting rhythmic calcium activity was induced by micromolar H(2)O(2) doses. The H(2)O(2)-induced calcium signals, due to both release from intracellular stores and influx from the extracellular milieu, were totally prevented by incubating the cells with the P2 receptor antagonist suramin or with the ATP/ADP hydrolyzing enzyme apyrase. In the osteosarcoma SaOS-2 cells micromolar H(2)O(2) failed to evoke calcium signals and millimolar H(2)O(2) induced a slowly developing calcium influx which was unaffected by suramin and apyrase. These cells responded to micromolar concentrations of ATP and ADP, but were largely insensitive to UTP. ROS 17/2.8 osteosarcoma cells were totally insensitive to ATP, ADP and UTP in keeping with the evidence that these cells lack functional purinergic receptors. In these cells, H(2)O(2) up to 1mM did not increase the cytosolic calcium concentration. In ROS/P2Y(2) cells, stably expressing the P2Y(2) receptor, spontaneous calcium oscillations were observed in 38% of the population and nanomolar concentration of extracellular ATP or UTP activated oscillations in quiescent cells. Spontaneous calcium signals were inhibited by suramin and apyrase. In these cells H(2)O(2) induced oscillatory calcium activity that was blocked by suramin and apyrase. The sensitivity of ROS/P2Y(2) cells to UTP decreased significantly in the presence of DTT, which was effective also in inhibiting spontaneous calcium oscillations. On the other hand, the membrane-impermeant thiol oxidant DTNB induced calcium oscillations that were inhibited by incubating the cells with suramin or apyrase. Since peroxide did not increase extracellular ATP in these cell lines, we propose that, in osteoblasts, mild oxidative conditions could activate purinergic signalling through the sensitization of P2Y(2) receptor.

ATP release mediates fluid flow-induced proliferation of human bone marrow stromal cells

Oscillatory fluid flow induced the vesicular release of ATP from human BMSCs that directly contributes to the induction of BMSC proliferation. Degrading extracellular nucleotides prevents fluid flow-induced increases in intracellular calcium concentration, the activation of calcineurin, and the nuclear translocation of NFAT.

INTRODUCTION

Regulation of bone cell activity by autocrine/paracrine factors is a well-established mechanism by which skeletal homeostasis is regulated by mechanical signals. The release of extracellular nucleotides in particular has been shown to induce many of the responses thought to be necessary for load-induced bone formation. In these studies, we examined the effect of oscillatory fluid flow on the release of ATP from bone marrow stromal cells (BMSCs) and the effect of ATP release on BMSC proliferation and intracellular calcium signaling pathways.

MATERIALS AND METHODS

BMSCs were exposed to oscillatory fluid flow, and the concentration of ATP in conditioned media samples was determined using a luciferin:luciferase-based reaction. Western blot analysis was used to examine the expression of purinergic receptors. Using pharmacological antagonists of gap junction hemichannels and vesicular trafficking, we studied the mechanism of ATP release from BMSCs. Apyrase was used to study the effect of extracellular nucleotides on intracellular calcium concentration, calcineurin activity, and nuclear factor of activated T cells (NFAT) nuclear translocation.

RESULTS AND CONCLUSIONS

Fluid flow exposure induced the flow rate-dependent release of ATP from BMSCs that was attenuated by treatment with monensin and N-ethylmaleimide, suggesting a vesicular mechanism. Treating BMSCs with ATP, but not other nucleotides, increased cellular proliferation. Moreover, extracellular ATP was a prerequisite for fluid flow-induced increases in intracellular calcium concentration, activation of calcineurin, the nuclear translocation of NFATc1, and proliferation. These data indicate that ATP regulates not only osteoblastic and osteocytic cell behavior but also that of mesenchymal precursors and support our hypothesis that similar mechanotransduction mechanisms are activated by fluid flow in these cell types.

P2 receptors in macrophage fusion and osteoclast formation

Cells of the mononuclear phagocyte lineage fuse to form multinucleated giant cells and osteoclasts. Several lines of evidence suggest that P2 receptors, in particular P2X₇, are involved in this process, although P2X₇ is not absolutely required for fusion because P2X₇-null mice form multinucleated osteoclasts. Extracellular ATP may be an important regulator of macrophage fusion.

Autocrine/paracrine stimulation of purinergic receptors in osteoblasts: contribution of vesicular ATP release

Extracellular nucleotides such as ATP and UTP are released in response to mechanical stimulation in different cell systems. It is becoming increasingly evident that ATP release plays a role in autocrine and paracrine stimulation of osteoblasts. Mechanical stimulation, as shear stress, membrane stretch or hypo-osmotic swelling, as well as oscillatory fluid flow, stimulates ATP release from different osteoblastic cell lines. Human osteoblast-like initial transfectant (HOBIT) cells release ATP in response to mechanical stimulation. In the present study, we show that HOBIT cells are activated by nanomolar levels of extracellular ATP, concentrations that can be detected under resting conditions and increase following hypotonic shock. Cell activation by hypotonic medium induced intracellular Ca2+ oscillations, and Egr-1 synthesis and DNA-binding activity. Quinacrine staining of living, resting cells revealed a granular fluorescence, typical of ATP-storing vesicles. Monensin prevented quinacrine staining and considerably inhibited hypotonic-induced ATP release. Finally, elevated levels of cytosolic Ca2+ activated massive ATP release and a dose-dependent loss of quinacrine granules. The contribution of a vesicular mechanism for ATP release is proposed to sustain paracrine osteoblast activation.

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.

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.

Mechanosensitivity of bone cells to oscillating fluid flow induced shear stress may be modulated by chemotransport

Fluid flow has been shown to be a potent physical stimulus in the regulation of bone cell metabolism. In addition to membrane shear stress, loading-induced fluid flow will enhance chemotransport due to convection or mass transport thereby affecting the biochemical environment surrounding the cell. This study investigated the role of oscillating fluid flow induced shear stress and chemotransport in cellular mechanotransduction mechanisms in bone. Intracellular calcium mobilization and prostaglandin E(2) (PGE(2)) production were studied with varying levels of shear stress and chemotransport. In this study MC3T3-E1 cells responded to oscillating fluid flow with both an increase in intracellular calcium concentration ([Ca(2+)](i)) and an increase in PGE(2) production. These fluid flow induced responses were modulated by chemotransport. The percentage of cells responding with an [Ca(2+)](i) oscillation increased with increasing flow rate, as did the production of PGE(2). In addition, depriving the cells of nutrients during fluid flow resulted in an inhibition of both [Ca(2+)](i) mobilization and PGE(2) production. These data suggest that depriving the cells of a yet to be determined biochemical factor in media affects the responsiveness of bone cells even at a constant peak shear stress. Chemotransport alone will not elicit a response, but it appears that sufficient nutrient supply or waste removal is needed for the response to oscillating fluid flow induced shear stress.

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.

ATP and UTP at low concentrations strongly inhibit bone formation by osteoblasts: a novel role for the P2Y2 receptor in bone remodeling

There is increasing evidence that extracellular nucleotides act on bone cells via multiple P2 receptors. The naturally-occurring ligand ATP is a potent agonist at all receptor subtypes, whereas ADP and UTP only act at specific receptor subtypes. We have reported that the formation and resorptive activity of rodent osteoclasts are stimulated powerfully by both extracellular ATP and its first degradation product, ADP, the latter acting at nanomolar concentrations, probably via the P2Y1 receptor subtype. In the present study, we investigated the actions of ATP, ADP, adenosine, and UTP on osteoblastic function. In 16-21 day cultures of primary rat calvarial osteoblasts, ADP and the selective P2Y1 agonist 2-methylthioADP were without effect on bone nodule formation at concentrations between 1 and 125 microM, as was adenosine. However, UTP, a P2Y2 and P2Y4 receptor agonist, known to be without effect on osteoclast function, strongly inhibited bone nodule formation at concentrations >or= 1 microM. ATP was inhibitory at >or= 10 microM. Rat osteoblasts express P2Y2, but not P2Y4 receptor mRNA, as determined by in situ hybridization. Thus, the low-dose effects of extracellular nucleotides on bone formation and bone resorption appear to be mediated via different P2Y receptor subtypes: ADP, signalling through the P2Y1 receptor on both osteoclasts and osteoblasts, is a powerful stimulator of osteoclast formation and activity, whereas UTP, signalling via the P2Y2 receptor on osteoblasts, blocks bone formation by osteoblasts. ATP, the 'universal' agonist, can simultaneously stimulate resorption and inhibit bone formation. These findings suggest that extracellular nucleotides could function locally as important negative modulators of bone metabolism, perhaps contributing to bone loss in a number of pathological states.

P2Y purinoceptors are responsible for oscillatory fluid flow-induced intracellular calcium mobilization in osteoblastic cells

We previously found that oscillatory fluid flow activated MC3T3-E1 osteoblastic cell Ca(2+)(i) mobilization via the inositol 1,4,5-trisphosphate pathway in the presence of 2% fetal bovine serum (FBS). However, the molecular mechanism of fluid flow-induced Ca(2+)(i) mobilization is unknown. In this study, we first demonstrated that oscillatory fluid flow in the absence of FBS failed to increase [Ca(2+)](i) in MC3T3-E1 cells. Apyrase (10 units/ml), which rapidly hydrolyzes 5' nucleotide triphosphates to monosphophates, prevented the fluid flow induced increases in [Ca(2+)](i) in the presence of FBS. Adding ATP or UTP to flow medium without FBS restored the ability of fluid flow to increase [Ca(2+)](i), suggesting that ATP or UTP may mediate the effect of fluid flow on [Ca(2+)](i). Furthermore, adenosine, ADP, UDP, or adenosine 5'-O-(3-thiotriphosphate) did not induce Ca(2+)(i) mobilization under oscillatory fluid flow without FBS. Pyridoxal phosphate 6-azophenyl-2,4'-disulfonic acid, an antagonist of P2X purinoceptors, did not alter the effect of fluid flow on the Ca(2+)(i) response, whereas pertussis toxin, a G(i/o)-protein inhibitor, inhibited fluid flow-induced increases in [Ca(2+)](i) in the presence of 2% FBS. Thus, by the process of elimination, our data suggest that P2Y purinoceptors (P2Y2 or P2Y4) are involved in the Ca(2+)(i) response to fluid flow. Finally, a decreased percentage of MC3T3-E1 osteoblastic cells treated with P2Y2 antisense oligodeoxynucleotides responded to fluid flow with an increase in [Ca(2+)](i), and an increased percentage of ROS 17/2.8 cells, which do not normally express P2Y2 purinoceptors, transfected with P2Y2 purinoceptors responded to fluid flow in the presence of 2% FBS, confirming that P2Y2 purinoceptors are responsible for oscillatory fluid flow-induced Ca(2+)(i) mobilization. Our findings shed new light of the molecular mechanisms responsible for oscillatory fluid flow-induced Ca(2+)(i) mobilization in osteoblastic cells.

Extracellular nucleotide signaling: a mechanism for integrating local and systemic responses in the activation of bone remodeling

Bone turnover occurs at discreet sites in the remodeling skeleton. The focal nature of this process indicates that local cues may facilitate the activation of bone cells by systemic factors. Nucleotides such as adenosine triphosphate (ATP) are locally released, short-lived, yet potent extracellular signaling molecules. These ligands act at a large family of receptors-the P2 receptors, which are subdivided into P2Y and P2X subtypes based on mechanism of signal transduction. Nucleotides enter the extracellular milieu via non-lytic and lytic mechanisms where they activate multiple P2 receptor types expressed by both osteoblasts and osteoclasts. In this review the release of ATP by bone cells is discussed in the context of activation of bone remodeling. We provide compelling evidence that nucleotides, acting via P2Y receptors, are potent potentiators of parathyroid hormone-induced signaling and transcriptional activation in osteoblasts. The provision of a mechanism to induce activation of osteoblasts above a threshold attained by systemic factors alone may facilitate focal remodeling and address the paradox of why systemic regulators like PTH exert effects at discreet sites.

Extracellular ADP is a powerful osteolytic agent: evidence for signaling through the P2Y(1) receptor on bone cells

There is increasing evidence that extracellular nucleotides act on bone cells via P2 receptors. This study investigated the action of ADP and 2-methylthioADP, a potent ADP analog with selectivity for the P2Y(1) receptor, on osteoclasts, the bone-resorbing multinuclear cells. Using three different assays, we show that ADP and 2-methylthioADP at nanomolar to submicromolar levels caused up to fourfold to sixfold increases in osteoclastic bone resorption. On mature rat osteoclasts, cultured for 1 day on polished dentine disks, peak effects on resorption pit formation were observed between 20 nM and 2 microM of ADP. The same concentrations of ADP also stimulated osteoclast and resorption pit formation in 10-day mouse marrow cultures on dentine disks. In 3-day explant cultures of mouse calvarial bones, the stimulatory effect of ADP on osteoclast-mediated Ca(2+) release was greatest at 5-50 microM and equivalent to the maximal effects of prostaglandin E(2). The ADP effects were blocked in a nontoxic manner by MRS 2179, a P2Y(1) receptor antagonist. Using in situ hybridization and immunocytochemistry, we found evidence for P2Y(1) receptor expression on both osteoclasts and osteoblasts; thus, ADP could exert its actions both directly on osteoclasts and indirectly via P2Y(1) receptors on osteoblasts. As a major ATP degradation product, ADP is a novel stimulator of bone resorption that could help mediate inflammatory bone loss in vivo.

Expression of a P2X7 receptor by a subpopulation of human osteoblasts

There is now conclusive evidence that extracellular nucleotides acting via cell surface P2 receptors are important local modulators of bone cell function. Multiple subtypes of P2 receptors have been localized to bone, where their activation modulates multiple processes including osteoblast proliferation, osteoblast-mediated bone formation, and osteoclast formation and resorptive capacity. Locally released nucleotides also have been shown to sensitize surrounding cells to the action of systemic factors such as parathyroid hormone (PTH). In nonskeletal tissue recent attention has focused on one particular P2 receptor, the P2X7 receptor (previously termed P2Z), and its ability to form nonselective aqueous pores in the plasma membrane on prolonged stimulation. Expression of this receptor originally was thought to be restricted to cells of hemopoietic origin, in which it has been implicated in cell fusion, apoptosis, and release of proinflammatory cytokines. However, recent reports have indicated expression of this receptor in cells of stromal origin. In this study, we investigated the expression of the P2X7 receptor in two human osteosarcoma cell lines, as well as several populations of primary human bone-derived cells (HBDCs) at the levels of messenger RNA (mRNA) and protein. We found that there is a subpopulation of osteoblasts that expresses the P2X7 receptor and that these receptors are functional as assessed by monitoring ethidium bromide uptake following pore formation. Inhibition of delayed lactate dehydrogenase (LDH) release in response to the specific agonist 2',3'-(4-benzoyl)-benzoyl-adenosine triphosphate (BzATP) by the nonspecific P2X receptor antagonist PPADS confirmed a receptor-mediated event. After treatment with BzATP SaOS-2 cells exhibited dramatic morphological changes consistent with those observed after P2X7-mediated apoptosis in hemopoietic cells. Dual staining with terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL) and a P2X7-specific monoclonal antibody confirmed the induction of apoptosis in osteoblasts expressing the P2X7 receptor. These data show for the first time the expression of functional P2X7 receptors in a subpopulation of osteoblasts, activation of which can result in ATP-mediated apoptosis.

Serum modulates the intracellular calcium response of primary cultured bone cells to shear flow

We investigated the effect of newborn bovine serum on the intracellular calcium [Ca(2+)](i) response of primary cultured bone cells stimulated by fluid flow. As it has been previously established that these cells exhibit [Ca(2+)](i) responses to fluid flow shear stress in saline media without growth factors or other chemically stimulatory factors, we hypothesized that the addition of serum to the flow medium would enhance the mechanosensitivity of the cells. We examined the effect of a short-term (10-15min) exposure of the cells to 2 and 10% serum prior to flow stimulation (pretreated) compared to not exposing the cells prior to flow stimulation (unpretreated). The cells were subjected to a well-defined, 90-s flow stimulus with shear stress levels ranging from 0.02 to 3.5Pa in a laminar flow chamber using a saline medium supplemented with 2 or 10% serum. For pretreatment, the serum concentration was the same from pre-flow to flow exposure. We observed a differential effect in the magnitude of the peak [Ca(2+)](i) response modulated by the concentration of serum in the pre-flow medium. Additionally, ATP-supplemented flow was examined as a comparison to the serum-supplemented flow and exhibited a similar trend in the peak [Ca(2+)](i) flow response that was dependent on ATP concentration and pre-flow exposure conditions. These findings demonstrate that under the conditions of this study, chemical agonist exposure can modulate the [Ca(2+)](i) response in bone cells subjected to fluid flow-induced shear stress.

Expression of P2 receptors in bone and cultured bone cells

Extracellular nucleotides acting through P2 receptors elicit a wide range of responses in many cell types. There is increasing evidence that adenosine triphosphate (ATP) may function as an important local messenger in bone and cartilage. In this study, we used immunocytochemistry, employing novel polyclonal antibodies against P2X(1-7) receptors, and in situ hybridization, using oligonucleotide probes corresponding to P2X(2,4) and P2Y(2,4) messenger RNAs (mRNAs), to localize P2 receptors on undecalcified bone sections and on cultured osteoblasts and osteoclasts. We provide the first direct evidence that the P2X(2) receptor subtype is expressed on osteoclasts, osteoblasts, and chondrocytes. We also obtained evidence for the expression of P2X(5) and P2Y(2) receptors on osteoblasts and chondrocytes, and for P2X(4) and P2X(7) receptors on osteoclasts. Our results confirm earlier reports of P2Y(2) and P2X(4) expression in human osteoclastoma and rabbit osteoclasts, respectively, and are consistent with ATP responses observed on bone cells using electrophysiological techniques. Our novel finding that P2X(2) is expressed by osteoclasts is of particular interest. P2X(2) is the only P2 receptor subtype that requires extracellular acidification to show its full sensitivity to ATP, and our recent functional studies have shown that the stimulatory action of ATP on resorption pit formation by mature osteoclasts is amplified greatly at low pH. These findings point to fundamental new mechanisms for the local modulation of bone resorption.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

P2Y2 receptors are expressed by human osteoclasts of giant cell tumor but do not mediate ATP-induced bone resorption

Extracellular nucleotides acting through P2 receptors elicit a range of responses in many cell types. Previously, we have cloned the G-protein coupled P2Y2 receptor from a human osteoclastoma complementary deoxyribonucleic acid (cDNA) library and demonstrated its expression by reverse transcription linked (RT)-PCR and Southern analysis in a number of skeletal tissues, including a purified population of giant cells. In this study we have localized the expression of P2Y2 receptor transcripts to osteoclasts of giant cell tumor of bone by in situ hybridization. In osteoblasts and other cell types, the P2Y2 receptor is coupled to Ins(1,4,5)P3-mediated Ca2+ release from intracellular stores. In this study, the P2Y2 receptor agonists adenosine triphosphate (ATP) and uridine triphosphate (UTP) did not increase cytosolic free calcium concentration ([Ca2+]i) in giant cells isolated from osteoclastoma, while the G-protein coupled calcium sensing receptor agonist, Ni2+, elevated [Ca2+]i in the same cells. These data indicate that P2Y2 receptor transcripts expressed by giant cells are not presented at the surface of cells as functional receptors, or alternatively, functional receptors are coupled to an effector other than [Ca2+]i. ATPgammaS (10 micromol/L), but not UTP (10 micromol/L), significantly stimulated resorption by an enriched giant cell population. These results indicate that ATP-induced effects on resorption, following direct osteoclastic activation, are mediated by a P2 receptor other than the P2Y2 subtype. Nucleotides, released locally in the bone microenvironment in response to acute trauma or transient physical stress, will interact with a complement of P2 receptors expressed by both osteoclasts and osteoblasts to influence the remodeling process.

Differential purinergic receptor signalling in osteoclasts and osteoblastic cells

There are differences between osteoclasts and osteoblastic cells in their cytosolic calcium responses to purinergic receptor activation. Application of 50 or 100 microM extracellular ATP inhibits the calcium response to a second application of ATP in osteoblastic rat osteosarcoma UMR 106 cells, but not in rabbit osteoclasts. This shows that there is adaptation to the extracellular, ATP in osteoclasts, but not in the UMR 106 cells. Extracellular washing of the UMR 106 cells restores the calcium response to ATP partially but not completely, indicating that there is a purinergic receptor activation-induced desensitisation of the receptor or its linked signalling pathways. In contrast to these results, if extracellular UTP is applied first, application of ATP produces no calcium response in osteoclasts, with or without washing, while in the UMR 106 cells there is some response to the ATP, which is greatly enhanced by washing. This indicates that UTP induces a complete desensitisation of the purinergic receptor/calcium signalling system in osteoclasts, but not in the osteoblastic cells, in which there is simply competition between UTP and ATP for the same receptors. Taken together, these results demonstrate that ATP and UTP could differentially regulate osteoblasts and osteoclasts.

Purinergic transmitters inhibit bone formation by cultured osteoblasts

Adenosine triphosphate (ATP) and other purinoceptor agonists cause a transient rise in [Ca2+]i in cultured osteoblast-like cells and have a mitogenic effect, as does parathyroid hormone (PTH), and there is evidence that ATP and PTH can act synergistically on osteoblasts. The likelihood that nucleotides, acting through purinoceptors, are important local factors in bone remodeling is therefore considerable. However, their effect on bone formation is unknown. We recently developed a culture system in which appositional bone formation occurs only in narrow grooves cut in a substratum. We have used this as an assay to measure the effects of ATP (50 and 500 mumol/L), ATP gamma S (20 mumol/L), 2-MeSATP (2 and 20 mumol/L), uridine triphosphate (UTP) (0.2, 2, and 20 mumol/L), adenosine (20 mumol/L), bovine PTH (0.25 and 0.5 IU/mL), rat PTH1-34 (10(-8) and 10(-7) mol/L), and rat PTHrP1-40 (10(-9) and 10(-8) mol/L) on bone formation by rat calvarial osteoblasts. The culture medium was renewed 3 times/week (every 2 or 3 days), and the number of bone loci and length and area of Alizarin red-stained mineralized bone formed in the grooves of each specimen in 16-29 days were measured. Compared with controls, ATP gamma S, 2-MeSATP, and ATP reduced the amount of bone formed in a 2-3 week culture period. Adenosine had no effect, and UTP either had no effect or at 2 mumol/L stimulated bone formation. PTH and PTHrP completely abolished bone formation in 4 week cultures. Our findings are consistent with evidence for more than one P2 purinoceptor subtype in bone, and show for the first time that the effect of ATP on appositional bone formation by osteoblasts in vitro is, like PTH and PTHrP, inhibitory.

Arachidonic acid release induced by extracellular ATP in osteoblasts: role of phospholipase D

In a previous study, we have shown that extracellular ATP stimulates Ca2+ influx resulting in the release of arachidonic acid (AA) and prostaglandin E2 (PGE2) synthesis in osteoblast-like MC3T3-E1 cells. In addition, we have recently reported that extracellular ATP stimulates phosphatidylcholine hydrolysis by phospholipase D (PLD) independently from the activation of protein kinase C in these cells. It is well recognized that phosphatidylcholine is hydrolysed by PLD, generating phosphatidic acid, which can be further degraded by phosphatidic acid phosphohydrolase to diacylglycerol (DG). In the present study, we investigated the role of PLD activation in the extracellular ATP-induced AA release and PGE2 synthesis in osteoblast-like MC3T3-E1 cells. Extracellular ATP stimulated AA release dose-dependently in the range between 0.1 and 1 mM. Propranolol, which is known to inhibit phosphatidic acid phosphohydrolase, significantly inhibited the AA release induced by extracellular ATP in a dose-dependent manner in the range between 100 and 300 microM. 1,6-Bis-(cyclohexyloximinocarbonylamino)-hexane (RHC-80267), a selective inhibitor of DG lipase, significantly suppressed the AA release induced by extracellular ATP. Both the pretreatment of propranolol and RHC-80267 also inhibited the extracellular ATP-induced PGE2 synthesis. These results strongly suggest that the AA release induced by extracellular ATP is mediated at least in part by phosphatidylcholine hydrolysis by PLD in osteoblast-like cells.

Cloning of P2Y6 cDNAs and identification of a pseudogene: comparison of P2Y receptor subtype expression in bone and brain tissues

Cellular responses to ATP/UTP and analogs are mediated by G-protein coupled P2Y receptors and have been proposed to play a role in the regulation of bone metabolism. Using a degenerate PCR approach on MG-63 cell cDNA we found PCR fragments coding for human P2Y1 and a new receptor, P2Y6. cDNA cloning of the P2Y6 receptor identified three cDNA isoforms. Two contained the same contiguous ORFs but differed in their 5' UTRs and may therefore originate by alternative splicing whereas the third represents a pseudogene. Analysis of P2Y receptor subtype expression in human bone and the osteoblastic cell lines OHS-4 and MG-63 by RT-PCR showed that all known human P2Y receptor subtypes (P2Y1, P2Y2, P2Y4, P2Y6, and P2Y7) were expressed. In contrast, analysis of brain-derived cell lines suggests that a selective expression of P2Y receptor subtypes occurs in brain tissue.

Extracellular ATP modulates [Ca2+]i in retinoic acid-treated embryonic chondrocytes

When treated with low doses of retinoic acid (RA), cephalic chondrocytes of the chick embryonic sternum mature and express phenotypic characteristics of postmitotic hypertrophic cells. In concert with these maturation-dependent changes, cells release adenine nucleotides into the culture medium. To ascertain if these compounds modulate chondrocyte function, we challenged chondrocytes with nucleotides and measured one determinant of the signal transduction pathway, intracellular Ca2+ concentration ([Ca2+]i). In the presence of micromolar concentrations of ATP, there was a dose-dependent elevation in chondrocyte [Ca2+]i; ADP caused a small but significant rise in the peak [Ca2+]i response. We found that the change in the [Ca2+]i response is linked to retinoid-dependent maturation of chondrocytes. Thus the [Ca2+]i rise was dependent on the RA concentration and treatment time. Immature caudal chondrocytes, cells that were not affected by RA, were used as control cells for this study. When treated with ATP, these cells did not exhibit a [Ca2+]i response. Although the purinergic subtype receptor was not characterized, the observation that cells responded to ATP and ADP but were refractory to AMP and adenosine suggested that P2 purinoceptors were expressed by chondrocytes. Because, during the same culture period, chondrocytes exhibited many of the unique characteristics of the terminally differentiated cell, the acquisition of purinergic receptors represents a new feature associated with expression of the mature phenotype. Finally, to ascertain if the ATP-dependent response was due to release of Ca2+ from intracellular stores, cells were treated with thapsigargin. Since this compound significantly reduced the [Ca2+]i signal, we concluded that the ATP response is mediated by release of cation, from the endoplasmic reticulum.

Evidence of ectokinase-mediated phosphorylation of osteopontin and bone sialoprotein by osteoblasts during bone formation in vitro

Osteopontin (OPN) and bone sialoprotein (BSP) are phosphorylated glycoproteins that, together with osteonectin/secreted protein, acidic, rich in cysteine (SPARC) and osteocalcin, comprise the major non-collagen proteins of bone. Although phosphorylation of OPN and BSP, which is known to influence the biological properties of these proteins, has been shown to occur intracellularly, recent studies have demonstrated ectokinase activity in bone cell populations [Mikuni-Takagaki, Kakai, Satoyoshi, Kawano, Suzuki, Kawase and Saito (1995) J. Bone Miner. Res. 10, 231-241]. To determine whether OPN and BSP are phosphorylated by ectokinase activity we have used [gamma-32P]ATP and [gamma-32P]GTP as cell-impenetrable phosphate donors to analyse for ectokinase activity in osteoblastic UMR106.06 cells and fetal rat calvarial cells (FRCCs). By pulse-labelling confluent cells with radiolabelled nucleotides, the phosphorylation of endogenous and exogenously added OPN and BSP was demonstrated together with the labelling of a number of cell surface proteins. These phosphorylation reactions were inhibited by a cell-impermeable ectokinase inhibitor, K252b, and cell surface phosphorylation was also inhibited by exogenously added OPN and BSP substrates, indicating competition for the ectokinase enzyme. However, phosphorylation of OPN and BSP, both of which can mediate cell attachment through Arg-Gly-Asp (RGD) motifs, was not inhibited by an RGD peptide, suggesting that binding of OPN and BSP to cell surface integrins is not required. In similar experiments, ectokinase-mediated phosphorylation of OPN and BSP was demonstrated during mineralized tissue formation by FRCCs in vitro. These studies demonstrate that OPN and BSP secreted by bone cells are phosphorylated by a casein kinase II-like ectokinase present on the surface of osteoblastic cells.

Effects of extracellular nucleotides on single cells and populations of human osteoblasts: contribution of cell heterogeneity to relative potencies

1. Human osteoblasts responded to the application of extracellular nucleotides, acting at P2-receptors, with increases in cytosolic free calcium concentration ([Ca2+]i). 2. In populations of human osteoblasts, adenosine 5'-diphosphate (ADP) evoked a rise in [Ca2+]i with less than 40% of the amplitude of that induced by adenosine 5'-triphosphate (ATP). 3. ATP and uridine 5'-triphosphate (UTP) were applied to single human osteoblasts and induced [Ca2+]i rises of comparable amplitude in every cell tested. 4. However, from the results of single cell studies with ADP (and 2-methylthioATP (2-meSATP)) two groups of cells were delineated; one group responded to ADP (or 2-meSATP) with a rise in [Ca2+]i indistinguishable from that evoked by ATP; whereas the second group failed completely to respond to ADP (or 2-meSATP). 5. Therefore heterogeneity of receptor expression exists within this population of human osteoblasts. The limited distribution of the ADP-responsive receptor underlies the small response to ADP, compared with ATP, recorded in populations of human osteoblasts. This heterogeneity may reflect differences in the differentiation status of individual cells.

Role of protein kinase C alpha, Arf, and cytoplasmic calcium transients in phospholipase D activation by sodium fluoride in osteoblast-like cells

The effect of fluoride on phospholipase D (PLD) activation was studied using in vitro culture of Saos-2, MG-63 osteosarcoma cells, and normal osteoblast-like cells derived from human bone explants. Millimolar concentrations of NaF induced a significant accumulation of phosphatidylethanol (PEt) in Saos-2 cells but not in MG-63 and normal osteoblast-like cells. PLD activation was evident at 15 mM and concentration-dependent up to 50 mM. This stimulation was inhibited by deferoxamine, a chelator of Al3+, suggesting that PLD activation involves fluoride-sensitive G proteins. A good correlation was found between the levels of intracellular free Ca2+ and the activation of PLD. The time courses of the two responses were nearly identical. The ability of NaF to induce both responses was largely dependent on the presence of extracellular calcium. The calcium ionophore A23187 reproduced the effect of NaF, and this effect was antagonized by EGTA, suggesting that PLD activation was, at least in part, a calcium-regulated event. Phorbol 12-myristate 13-acetate (PMA) also stimulated PLD activity in human bone cells. Protein kinase C alpha (PKC alpha) and epsilon were expressed in Saos-2 cells. Acute pretreatment of cells with PMA reduced concomitantly the amounts of PKC alpha, but not of PKC epsilon, and the subsequent activation of PLD elicited by PKC activators. The PLD response to NaF was not attenuated but rather enhanced by down-regulation of PKC alpha. Therefore, PKC-alpha-induced PLD activation is unlikely to mediate the effect of NaF. Moreover, PMA and NaF showed a supraadditive effect on PLD activation in Saos-2 cells. This stimulation, in contrast to NaF alone, was not reduced by EGTA. Hence, mobilization of calcium by NaF cannot account for the enhanced PLD activation in response to PMA stimulation. Membrane Arf and RhoA contents were assessed by Western immunoblot analyses. Membranes derived from NaF-stimulated Saos-2 cells contained more Arf and RhoA when compared with membranes derived from control or PMA-stimulated cells. Translocation of the small GTPases was calcium-independent. We conclude that PLD activation by NaF in Saos-2 cells includes a fluoride-sensitive G protein, increases in the levels of intracellular calcium, and Arf/RhoA redistribution to membranes. The results also indicate that the NaF-induced Arf/RhoA translocation exerts in concert with PMA-activated PKC alpha a synergistic effect on the activation of PLD in Saos-2 cells.

Adenine nucleotide metabolism by chondrocytes in vitro: role of ATP in chondrocyte maturation and matrix mineralization

The objective of the investigation was to explore the notion that chondrocytes in the growth plate secrete nucleotides and that these compounds are used to regulate cell maturation and matrix mineralization. Chondrocytes were isolated from the cephalic region of chick embryo sterna and maintained in culture until confluent. To promote expression of the mature phenotype, cultures were then treated with retinoic acid. During the culture period, medium was removed and analyzed for nucleotides using a modified reverse-phase high-performance liquid chromatography (HPLC) procedure. We found that culture medium, conditioned by the chondrocytes, contained significant quantities of nucleotides. Moreover, the nucleotide concentrations were similar in magnitude to levels reported for media conditioned by other cell types. In terms of species, adenosine diphosphate (ADP) was the major nucleotide present in the conditioned medium; adenosine monophosphate (AMP) was present, but at a lower concentration than ADP. To examine the possibility that adenosine triphosphate (ATP) was released by the cultured chondrocytes, but was rapidly degraded into ADP and AMP, we examined the kinetics of ATP breakdown by chondrocytes. We found that chondrocytes degraded over 70% of exogenous ATP within 15 minutes. Similar experiments performed with ADP and AMP indicated that these nucleotides were also degraded by the cells, but at a slower rate than ATP. To determine whether the extracellular nucleotides modulate cartilage development, we examined the effect of exogenous ATP on four major determinants of chondrocyte function: alkaline phosphatase activity, cell proliferation rate, anaerobic metabolism, and mineral deposition. We found that ATP caused only minimum alterations in cell number and alkaline phosphatase activity; however, it increased the lactate content of the medium probably by stimulating anaerobic glycolysis. We noted that ATP had a significant effect on the amount and type of mineral deposited into chondrocyte cultures. Compared with untreated controls, ATP stimulated formation of a small amount of poorly crystallized calcium phosphate. The results of the study show for the first time that chondrocytes release nucleotides into the extracellular milieu. Although they are rapidly degraded, they serve to regulate both mineral formation and energy metabolism.

Calcium-induced relocation of annexins IV and V in the human osteosarcoma cell line MG-63

In cell culture, human osteoblasts and the osteosarcoma cell line MG-63 express annexins I, II, IV, V and VI. Small proportions of annexins IV and V are lost from MG-63 cells into the culture medium in a sedimentable form. however, the bulk of these annexins is intracellular. In non-confluent cells 3 days after passaging, annexin IV and annexin V are strongly present throughout the nucleus and are also present in the cytoplasm. On elevation of the intracellular calcium concentration with the lonophore ionomycin, the intranuclear pools of annexin IV in 38 +/- 4% of cells and annexin V in 70 +/- 5% of cells show relocation to the nuclear membrane within 40 s. Extracellular ATP, which causes a transient increase in the cytosolic free calcium concentration by acting at P2-purinoceptors, also causes relocation of the intranuclear pool of annexin IV in 22 +/- 4% of cells and of annexin V in 38 +/- 8% of cells. After stimulation no significant reversal of the relocation is observed. Elevation of intracellular calcium with ionophore and ATP also causes relocation of the cytoplasmic pools of annexins IV and V. The results support a role for annexins at cellular membranes in response to elevation of cytosolic calcium levels.

Identification and cloning of human P2U purinoceptor present in osteoclastoma, bone, and osteoblasts

Extracellular ATP acting through purinoceptors may be an important factor in the modulation of bone turnover. In this study we cloned and sequenced the P2U purinoceptor from osteoclastoma, confirming the recently published human sequence. Furthermore, by the reverse transcriptase-linked polymerase chain reaction (RT-PCR) and Southern blotting we demonstrated expression of P2U receptor mRNA in bone, primary cultures of human bone-derived cells, and two osteosarcoma cell lines, Saos2 and Te85. P2U receptor transcripts were identified in alkaline phosphatase-positive human bone-derived cells isolated by flow cytometry providing strong evidence for the expression of the P2U purinoceptor in mature osteoblasts. P2U receptor transcripts were also detected in a purified giant cell population isolated from osteoclastoma, indicating that this receptor is also expressed by osteoclasts. These data suggest that purinergic agonists may play a role in the regulation of bone metabolism.

Activation of protein kinase C inhibits ATP-induced [Ca2+]i elevation in rat osteoblastic cells: selective effects on P2Y and P2U signaling pathways

Extracellular ATP elicits transient elevation of cytosolic free Ca2+ concentration ([Ca2+]i) in osteoblasts through interaction with more than one subtype of cell surface P2-purinoceptor. Elevation of [Ca2+]i arises, at least in part, by release of Ca2+ from intracellular stores. In the present study, we investigated the possible roles of protein kinase C (PKC) in regulating these signaling pathways. [Ca2+]i of indo-1-loaded UMR-106 osteoblastic cells was monitored by spectrofluorimetry. In the absence of extracellular Ca2+, ATP (100 microM) induced transient elevation of [Ca2+]i to a peak 57 +/- 7 nM above basal levels (31 +/- 2 nM, means +/- S.E.M., n = 25). Exposure of cells to the PKC activator 12-O-tetradecanoyl-beta-phorbol 13-acetate (TPA, 100 nM) for 2 min significantly reduced the amplitude of the ATP response to 13 +/- 4 nM (n = 11), without altering basal [Ca2+]i. Inhibition was half-maximal at approximately 1 nM TPA. The Ca2+ response to ATP was also inhibited by the PKC activators 1,2-dioctanoyl-sn-glycerol or 4 beta-phorbol 12,13-dibutyrate, but not by the control compounds 4 alpha-phorbol or 4 alpha-phorbol 12,13-didecanoate. Furthermore, exposure of cells to the protein kinase inhibitors H-7 or staurosporine for 10 min significantly attenuated the inhibitory effect of TPA. However, these protein kinase inhibitors did not prolong the [Ca2+]i response to ATP alone, indicating that activation of PKC does not account for the transient nature of this response. When the effects of other nucleotides were examined, TPA was found to cause significantly greater inhibition of the response to the P2Y-receptor agonists, ADP and 2-methylthioATP, than the response to the P2U-receptor agonist, UTP. These data indicate that activation of PKC selectively inhibits the P2Y signaling pathway in osteoblastic cells. In vivo, endocrine or paracrine factors, acting through PKC, may regulate the responsiveness of osteoblasts to extracellular nucleotides.

Mechanisms of ATP-induced Ca2+ signaling in osteoclasts

We investigate the mechanisms underlying the intracellular calcium pulse that occurs in response to extracellular adenosine triphosphate (ATP) in osteoclasts. We find that pre-loading of GDP-beta-S abolishes the response in Ca(2+)-free medium, demonstrating an internal release of Ca2+ via a pathway that involves a G protein. GDP-beta-S does not block in normal Ca(2+)-containing medium, suggesting that ATP also induces a Ca2+ influx across the cell membrane. We confirmed this using the Mn2+ quenching technique, which shows significant opening of Ca2+ channels. We find a smaller response to adenosine diphosphate (ADP) and 2-methylthio-ATP (2-MeSATP), but no response to beta, gamma-methylene-ATP (AMP-PCP), adenosine monophosphate (AMP) or uridine triphosphate (UTP). Prior application of AMP and UTP, but not AMP-PCP, blocks the response to ATP. Our results indicate that the receptor is a P2 subtype that is not characteristic of any previously reported P2 receptor or combination of P2 receptors.

Permeabilization of cells of hemopoietic origin by extracellular ATP4-: elimination of osteoclasts, macrophages, and their precursors from isolated bone cell populations and fetal bone rudiments

Skeletal tissues contain, apart from cells of the osteogenic and chondrogenic lineage, cells of hemopoietic origin, e.g., macrophages, osteoclasts, and their precursors. In the present study we examined the sensitivity for extracellular ATP4- of the above-mentioned cell types in freshly isolated, bone-derived cell populations and in explanted fetal metatarsal bones. Cells of hemopoietic origin reacted to the presence of ATP4- with an increased permeability for impermeant cytotoxic molecules, e.g., ethidium bromide (EB), thiocyanate (KSCN), and an increased non-ion selective membrane conductance. As a consequence, these cells could be killed by a short treatment with adenosine-5' triphosphate (ATP)+KSCN. On the other hand, cells of nonhemopoietic origin (e.g., osteoblasts, chondrocytes) were found to be insensitive to ATP4- in this respect. These cells survived the treatment without apparent damage to their alkaline phosphatase activities, osteogenic potentials, and osteoclast induction capacities. The elimination of the endogenous cells of hemopoietic origin from bone tissue or cell populations derived therefrom offers the possibility to study the properties and functions of osteogenic or chondrogenic cells without interference by the presence of cells of hemopoietic origin. It also allows the study of interactions between osteogenic cells and selected cell populations of hemopoietic origin in coculture experiments.

Prostaglandin E2 is a potential mediator of extracellular ATP action in osteoblast-like cells

Extracellular ATP dose dependently stimulated 45Ca2+ influx even in the presence of nifedipine, a Ca2+ antagonist that inhibits voltage-dependent Ca2+ channel, in osteoblast-like MC3T3-E1 cells. ATP stimulated arachidonic acid release and the synthesis of prostaglandin E2 (PGE2). However, the ATP-induced arachidonic acid release was significantly reduced by chelating extracellular Ca2+ with EGTA. On the other hand, ATP induced DNA synthesis of these cells in a dose-dependent manner in the range between 1 microM and 1 mM. The pretreatment with indomethacin, a cyclooxygenase inhibitor, suppressed both ATP-induced PGE2 synthesis and DNA synthesis in these cells. The inhibitory effect by 50 microM indomethacin on the DNA synthesis was reversed by adding 10 microM PGE2. These results strongly suggest that extracellular ATP stimulates Ca2+ influx resulting in the release of arachidonic acid in osteoblast-like cells and that extracellular ATP-induced proliferative effect is mediated, at least in part, by ATP-stimulated PGE2 synthesis.

Osteoblast-like cells have a variable mixed population of purino/nucleotide receptors

Osteoblast-like UMR 106.06 cells respond to extracellular application of nucleotides with a fast intracellular calcium pulse (latency of about 20 s, half-width of about 10 s), as measured with fluo-3 on a confocal laser scanning system. Cross-inhibition experiments at 50 microM show that, on a cell population basis, adenosine triphosphate (ATP) strongly inhibits the effect of uridine triphosphate (UTP) or 2-methylthio-ATP (2-MeSATP) applied within 2 min after the end of the ATP-induced pulse, while prior application of UTP or 2-MeSATP only weakly inhibits the ATP effect, and UTP and 2-MeSATP weakly inhibit each other. Furthermore, there are clear differences in cross-inhibition between individual cells. Our measurements provide strong evidence that these cells have at least two types of purino/nucleotide receptors, probably P2y and P2u, with a proportion that varies between individual cells.