Purinergic transmitters inhibit bone formation by cultured osteoblasts

Glucose Metabolism in Osteoblasts in Healthy and Pathophysiological Conditions

Bone tissue in vertebrates is essential to performing movements, to protecting internal organs and to regulating calcium homeostasis. Moreover, bone has also been suggested to contribute to whole-body physiology as an endocrine organ, affecting male fertility; brain development and cognition; and glucose metabolism. A main determinant of bone quality is the constant remodeling carried out by osteoblasts and osteoclasts, a process consuming vast amounts of energy. In turn, clinical conditions associated with impaired glucose metabolism, including type I and type II diabetes and anorexia nervosa, are associated with impaired bone turnover. As osteoblasts are required for collagen synthesis and matrix mineralization, they represent one of the most important targets for pharmacological augmentation of bone mass. To fulfill their function, osteoblasts primarily utilize glucose through aerobic glycolysis, a process which is regulated by various molecular switches and generates adenosine triphosphate rapidly. In this regard, researchers have been investigating the complex processes of energy utilization in osteoblasts in recent years, not only to improve bone turnover in metabolic disease, but also to identify novel treatment options for primary bone diseases. This review focuses on the metabolism of glucose in osteoblasts in physiological and pathophysiological conditions.

Transcriptomic characterization of signaling pathways associated with osteoblastic differentiation of MC-3T3E1 cells

Bone remodeling involves the coordinated actions of osteoclasts, which resorb the calcified bony matrix, and osteoblasts, which refill erosion pits created by osteoclasts to restore skeletal integrity and adapt to changes in mechanical load. Osteoblasts are derived from pluripotent mesenchymal stem cell precursors, which undergo differentiation under the influence of a host of local and environmental cues. To characterize the autocrine/paracrine signaling networks associated with osteoblast maturation and function, we performed gene network analysis using complementary “agnostic” DNA microarray and “targeted” NanoString nCounter datasets derived from murine MC3T3-E1 cells induced to undergo synchronized osteoblastic differentiation in vitro. Pairwise datasets representing changes in gene expression associated with growth arrest (day 2 to 5 in culture), differentiation (day 5 to 10 in culture), and osteoblast maturation (day 10 to 28 in culture) were analyzed using Ingenuity Systems Pathways Analysis to generate predictions about signaling pathway activity based on the temporal sequence of changes in target gene expression. Our data indicate that some pathways involved in osteoblast differentiation, e.g. Wnt/β-catenin signaling, are most active early in the process, while others, e.g. TGFβ/BMP, cytokine/JAK-STAT and TNFα/RANKL signaling, increase in activity as differentiation progresses. Collectively, these pathways contribute to the sequential expression of genes involved in the synthesis and mineralization of extracellular matrix. These results provide insight into the temporal coordination and complex interplay between signaling networks controlling gene expression during osteoblast differentiation. A more complete understanding of these processes may aid the discovery of novel methods to promote osteoblast development for the treatment of conditions characterized by low bone mineral density.

Purinergic 2X7 receptor activation regulates WNT signaling in human mandibular-derived osteoblasts

OBJECTIVE

Purinergic 2X7 receptor (P2X7R) activation modulates in vitro mineralization by primary rat and human osteoblasts. However, the detailed mechanism of how P2X7R activation affects primary human osteoblasts remains unclear. The aim of this study was to investigate the effect of P2X7R activation on human mandibular-derived osteoblast (hMOB) differentiation.

DESIGN

Primary human osteoblasts were obtained from non-pathologic mandibular bone from healthy patients. The hMOBs were cultured in osteogenic medium with or without 0.5-5μM 2'(3')-O-(4-benzoyl) benzoyl-ATP (BzATP), a selective P2X7R agonist. The mRNA expression of osteogenic differentiation markers and WNT-signaling molecules was investigated by quantitative real time polymerase chain reaction. In vitro mineral deposition was determined by Alizarin Red S staining. Transfection of small interfering RNA was performed to confirm the effect of P2X7R activation. WNT/β-catenin signaling was detected by immunofluorescence staining for β-catenin.

RESULTS

BzATP inhibited osteogenic medium-induced RUNX2 and OSX mRNA expression in hMOBs. Moreover, BzATP significantly retarded in vitro mineralization. These findings indicated that BzATP/P2X7R activation inhibited hMOB differentiation. Interestingly, reduced WNT3A mRNA expression and blockage of osteogenic medium-induced β-catenin nuclear translocation were also found. These data suggested that WNT signaling might be a target of P2X7R-regulated osteogenic differentiation. Furthermore, when recombinant human WNT3A was added to the BzATP-treated group, it rescued the reduced RUNX2 and OSX expression, and in vitro mineralization.

CONCLUSION

Our results demonstrate that P2X7R activation by BzATP inhibits hMOB differentiation. This inhibitory effect was associated with inhibition of the WNT/β-catenin signaling pathway.

Endogenous adenosine maintains cartilage homeostasis and exogenous adenosine inhibits osteoarthritis progression

Osteoarthritis (OA) is characterized by cartilage destruction and chondrocytes have a central role in this process. With age and inflammation chondrocytes have reduced capacity to synthesize and maintain ATP, a molecule important for cartilage homeostasis. Here we show that concentrations of ATP and adenosine, its metabolite, fall after treatment of mouse chondrocytes and rat tibia explants with IL-1β, an inflammatory mediator thought to participate in OA pathogenesis. Mice lacking A2A adenosine receptor (A2AR) or ecto-5′nucleotidase (an enzyme that converts extracellular AMP to adenosine) develop spontaneous OA and chondrocytes lacking A2AR develop an ‘OA phenotype' with increased expression of Mmp13 and Col10a1. Adenosine replacement by intra-articular injection of liposomal suspensions containing adenosine prevents development of OA in rats. These results support the hypothesis that maintaining extracellular adenosine levels is an important homeostatic mechanism, loss of which contributes to the development of OA; targeting adenosine A2A receptors might treat or prevent OA.

Osteoarthritis (OA) is a debilitating and destructive joint disease for which disease modifying drugs are not available. Here the authors show that extracellular adenosine signalling via the A2AR receptor on chondrocytes is needed to prevent OA and that liposome-bound adenosine injection can treat the pathology in rats.

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.

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.

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.

Adenosine and bone metabolism

Bone is a dynamic organ that undergoes continuous remodeling while maintaining a balance between bone formation and resorption. Osteoblasts, which synthesize and mineralize new bone, and osteoclasts, the cells that resorb bone, act in concert to maintain bone homeostasis. In recent years, there has been increasing appreciation of purinergic regulation of bone metabolism. Adenosine, released locally, mediates its physiologic and pharmacologic actions via interactions with G protein-coupled receptors, and recent work has indicated that these receptors are involved in the regulation of osteoclast differentiation and function, as well as in osteoblast differentiation and bone formation. Moreover, adenosine receptors also regulate chondrocyte and cartilage homeostasis. These recent findings underscore the potential therapeutic importance of adenosine receptors in regulating bone physiology and pathology.

Isolation and culture of human osteoblasts

The skeleton is a dynamic organ that is constantly active throughout life. The highly coordinated actions of bone cells early in life determine the body's shape and form, whilst the constant remodelling (bone resorption followed by an equal amount of bone formation) during adulthood helps to maintain skeletal mass and repair microdamage. When the balance of bone resorption and bone formation becomes unequal, bone diseases, such as osteoporosis, occur. In order to develop drugs to combat bone disease, it is important to know the regulatory systems involved in normal bone formation and resorption. In this chapter, we concentrate on bone formation, providing a detailed guide to isolating and culturing primary human osteoblasts in bone explant cultures, as well as the methodology used to characterise and monitor the function of osteoblasts. In combination, these methods provide a powerful tool in bone cell biology and in the development of new novel treatments for bone disease.

Purinergic receptors influence the differentiation of human mesenchymal stem cells

Adult stem cells, including adipose tissue-derived mesenchymal stem cells (MSCs) or ectomesenchymal dental follicle cells (DFCs), attract considerable attention for their potential to differentiate into lineages, which are of major interest in the field of Regenerative Medicine. Purinergic receptors exert a wide range of biological actions in many cell and tissue types through extracellular nucleotides. Little is known about P2 receptors in adult stem cells and changes in their expression levels during differentiation. All known P2 receptors have been investigated, and a variety of P2X and P2Y receptor subtypes were detected in MSCs. Studies investigating intracellular calcium levels on receptor stimulation demonstrated that the found P2 receptors are metabolically active. Interestingly, up- or downregulation of several P2 receptor subtypes at gene and protein level was observed during adipogenic and osteogenic differentiation, and the effect on differentiation was directly influenced by both the application of agonists/antagonists and apyrase-induced nucleotide cleavage. Here, we show for the first time that the combination of several P2 receptors plays a role in the differentiation of adult stem cells. The expression pattern of the P2 receptors, as well as their fate in differentiation, varies in stem cells of mesenchymal origin if compared with stem cells of ectomesenchymal origin. The subtypes P2X6, P2Y4, and P2Y14 seem to be pivotal regulators in MSC commitment, as they are regulated in both adipogenic and osteogenic differentiation of adipose tissue-derived stem cells and DFCs. These findings provide new insights into the differentiation processes and might reveal novel options to influence stem cell fate in future applications.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

Adenosine A1 receptors (A1Rs) play a critical role in osteoclast formation and function

Adenosine regulates a wide variety of physiological processes via interaction with one or more G-protein-coupled receptors (A(1)R, A(2A)R, A(2B)R, and A(3)R). Because A(1)R occupancy promotes fusion of human monocytes to form giant cells in vitro, we determined whether A(1)R occupancy similarly promotes osteoclast function and formation. Bone marrow cells (BMCs) were harvested from C57Bl/6 female mice or A(1)R-knockout mice and their wild-type (WT) littermates and differentiated into osteoclasts in the presence of colony stimulating factor-1 and receptor activator of NF-kappaB ligand in the presence or absence of the A(1)R antagonist 1,3-dipropyl-8-cyclopentyl xanthine (DPCPX). Osteoclast morphology was analyzed in tartrate-resistant acid phosphatase or F-actin-stained samples, and bone resorption was evaluated by toluidine blue staining of dentin. BMCs from A(1)R-knockout mice form fewer osteoclasts than BMCs from WT mice, and the A(1)R antagonist DPCPX inhibits osteoclast formation (IC(50)=1 nM), with altered morphology and reduced ability to resorb bone. A(1)R blockade increased ubiquitination and degradation of TRAF6 in RAW264.7 cells induced to differentiate into osteoclasts. These studies suggest a critical role for adenosine in bone homeostasis via interaction with adenosine A(1)R and further suggest that A(1)R may be a novel pharmacologic target to prevent the bone loss associated with inflammatory diseases and menopause.

Electrophysiological properties of a novel Ca(2+)-activated K(+) channel expressed in human osteoblasts

Intracellular Ca(2+) mobilization plays important roles in cell survival, proliferation, and differentiation of osteoblasts. In this study, we identified a novel type of Ca(2+)-activated K(+) channel in human osteoblasts and investigated its physiological roles. Using RT-PCR methods and single-channel analysis in the patch-clamp technique, we found that BK and IK channels were genetically expressed in human osteoblasts and had electrophysiological properties similar to those reported previously for the channels in other organs (conductance, voltage dependence, and sensitivity to intracellular Ca(2+)). Taking advantage of the fact that ATP induces elevation of the intracellular Ca(2+) concentration in human osteoblasts, we successfully demonstrated that ATP-induced hyperpolarization was effectively inhibited by the IK channel blockers charybdotoxin and clotrimazole and by a P2 purinergic receptor antagonist, suramin, but not by the BK channel blockers tetraethylammonium chloride and iberiotoxin under the current-clamp mode of whole-cell clamp. The present study is the first to demonstrate the electrophysiological properties and functional expression of IK channels in human osteoblasts, findings which suggest that IK channels are regulators of membrane potential that give rise to intracellular Ca(2+) mobilization by physiological stimulation.

ATP-mediated mineralization of MC3T3-E1 osteoblast cultures

While bone is hypomineralized in hypophosphatemia patients and in tissue-nonspecific alkaline phosphatase (Tnsalp)-deficient mice, the extensive mineralization that nevertheless occurs suggests involvement of other phosphatases in providing phosphate ions for mineral deposition. Although the source of phosphate liberated by these phosphatases is unknown, pyrophosphate, ATP, pyridoxal-5'-phosphate (PLP) and phoshoethanolamine (PEA) are likely candidates. In this study, we have induced mineralization of MC3T3-E1 osteoblast cultures using ATP, and have investigated potential phosphatases involved in this mineralization process. MC3T3-E1 osteoblasts were cultured for 12 days and treated either with beta-glycerophosphate (betaGP) or ATP. Matrix and mineral deposition was examined by biochemical, cytochemical, ultrastructural and X-ray microanalytical methods. ATP added at levels of 4-5 mM resulted in mineral deposition similar to that following conventional treatment with betaGP. Collagen levels were similarly normal in ATP-mineralized cultures and transmission electron microscopy and X-ray microanalysis confirmed hydroxyapatite mineral deposition along the collagen fibrils in the ECM. Phosphate release from 4 mM ATP into the medium was rapid and resulted in approximately twice the phosphate levels than after release from 10 mM betaGP. ATP treatment did not affect mineralization by altering the expression of mineral-regulating genes such as Enpp1, Ank, and Mgp, nor phosphatase genes indicating that ATP induces mineralization by serving as a phosphate source for mineral deposition. Levamisole, an inhibitor of TNSALP, completely blocked mineralization in betaGP-treated cultures, but had minor effects on ATP-mediated mineralization, indicating that other phosphatases such as plasma membrane Ca2+ transport ATPase 1 (PMCA1) and transglutaminase 2 (TG2) are contributing to ATP hydrolysis. To examine their involvement in ATP-mediated mineralization, the inhibitors cystamine (TG2 inhibitor) and ortho-vanadate (PMCA inhibitor) were added to the cultures - both inhibitors significantly reduced mineralization whereas suppression of the phosphate release by ortho-vanadate was minor comparing to other two inhibitors. The contribution of PMCA1 to mineralization may occur through pumping of calcium towards calcification sites and TG2 can likely act as an ATPase in the ECM. Unlike the GTPase activity of TG2, its ATPase function was resistant to calcium, demonstrating the potential for participation in ATP hydrolysis and mineral deposition within the ECM at elevated calcium concentrations.

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.

Extracellular nucleotides block bone mineralization in vitro: evidence for dual inhibitory mechanisms involving both P2Y2 receptors and pyrophosphate

Extracellular nucleotides, signaling through P2 receptors, may act as local regulators of bone cell function. We investigated the effects of nucleotide agonists [ATP, ADP, uridine triphosphate (UTP), and uridine diphosphate] and pyrophosphate (PPi, a key physiological inhibitor of mineralization) on the deposition and mineralization of collagenous matrix by primary osteoblasts derived from rat calvariae. Our results show that extracellular ATP, UTP, and PPi strongly and selectively blocked the mineralization of matrix nodules; ADP and uridine diphosphate were without effect. Significant inhibition of mineralization occurred in the presence of relatively low concentrations of ATP, UTP, or PPi (1-10 microm), without affecting production of fibrillar or soluble collagen. In cultures treated with 10 microm ATP or UTP, the expression and activity of alkaline phosphatase, which promotes mineralization by hydrolyzing PPi, was inhibited. The potent inhibitory actions of ATP and UTP on bone mineralization are consistent pharmacologically with mediation by the P2Y(2) receptor, which is strongly expressed by mature osteoblasts. In support of this notion, we found 9-17% increases in bone mineral content of hindlimbs of P2Y(2)-deficient mice. We also found that osteoblasts express ectonucleotide phosphodiesterase/pyrophosphatase-1, an ectonucleotidase that hydrolyzes nucleotide triphosphates to yield PPi, and that addition of 10 microm ATP or UTP to osteoblast cultures generated 2 microm PPi within 10 min. Thus, a component of the profound inhibitory action of ATP and UTP on bone mineralization could be mediated directly by PPi, independently of P2 receptors.

Osteoblast responses to nucleotides increase during differentiation

Accumulating evidence suggests that extracellular nucleotides, signaling through P2 receptors, play a role in modulating bone cell function. ATP and ADP stimulate osteoclastic resorption, while ATP and UTP are powerful inhibitors of bone formation by osteoblasts. We investigated changes in the expression of P2 receptors with cell differentiation in primary osteoblast cultures. Rat calvarial osteoblasts, cultured for up to 10 days, were loaded with the intracellular Ca(2+)-sensing fluorophore, Fluo-4 AM, and a fluorescence imaging plate reader was used to measure responses to nucleotide agonists. Peak responses occurred within 20 s and were evoked by ATP or UTP at concentrations as low as 2 microM. Osteoblast number doubled between day 4 and 10 of culture, but the peak intracellular Ca(2+) response to ATP or UTP increased up to 6-fold over the same period, indicating that osteoblast responsiveness to nucleotides increases as cell differentiation proceeds. The approximate order of potency for the most active nucleotide agonists at day 8 of culture was ATP > UTP and ATPgammaS > ADP > UDP, consistent with the expression of functional P2Y(2), P2X(2), P2Y(4), P2Y(1) and P2Y(6) receptors. Smaller responses were elicited by 2-MeSATP, Bz-ATP and alpha,beta-meATP, additionally suggesting the presence of functional P2X(1), P2X(3), P2X(5) and P2X(7) receptors. Expression of mRNA for the ATP- and UTP-selective P2Y(2) receptor increased strongly between day 6 and 15 in primary rat osteoblasts, whereas mRNAs for the P2Y(4) (also ATP/UTP selective) and P2Y(6) (UDP/UTP selective) receptors were highly expressed at intermediate time points. In contrast, mRNA for the cell-proliferation-associated P2X(5) receptor decreased to undetectable as osteoblasts matured, but mRNA for the cell-death-associated P2X(7) receptor was detected at all time points. Similar trends were evident using immunostaining and Western blotting for P2 receptors. Exposure to 10 muM ATP or UTP during days 10-14 of culture was sufficient to cause near-total blockade of the 'trabecular' bone nodules formed by osteoblasts; however, UDP and ADP were without effect. Our results show that there is a shift from P2X to P2Y expression during differentiation in culture, with mature osteoblasts preferentially expressing the P2Y(2) receptor and to a lesser extent P2Y(4) and P2Y(6) receptors. Taken together, these data suggest that the P2Y(2) receptor, and possibly the P2Y(4) receptor, could function as 'off-switches' for mineralized bone formation.

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.

ATP-stimulated interleukin-6 synthesis through P2Y receptors on human osteoblasts

We investigated the effect of extracellular adenosine triphosphate (ATP) on the production of interleukin (IL)-6, whose molecules are capable of stimulating the development of osteoclasts from their hematopoietic precursors as well as are involved in signal transduction systems in human osteoblastic SaM-1 cells. These human osteoblasts constitutively expressed P2X4, P2X5, P2X6, P2Y2, P2Y5, and P2Y6 purinergic receptors. ATP increased gene- and protein-expression of IL-6 in SaM-1 cells. The expression of the IL-6 mRNA was maximal at 1h, and the increase in IL-6 synthesis in response to ATP (10-100 microM) occurred in a concentration-dependent manner. Over the same concentration range of the nucleotide that was effective for IL-6 synthesis, ATP caused an increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)), which increase was inhibited by pretreatment with suramin, a P2Y receptor antagonist, or 2-aminoethoxydiphenyl borate (2-APB), an inositol 1,4,5-trisphosphate receptor blocker, but not by the extracellular Ca(2+)-chelating agent EGTA. The pretreatment of SaM-1 cells with suramin or 2-APB also inhibited the increase in IL-6 synthesis in response to ATP. These findings suggest that extracellular ATP-induced IL-6 synthesis occurs through P2Y receptors and mobilization of Ca(2+) from internal stores in human osteoblastic cells.

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.

Extracellular ATP stimulates the early growth response protein 1 (Egr-1) via a protein kinase C-dependent pathway in the human osteoblastic HOBIT cell line

Extracellular nucleotides exert an important role in controlling cell physiology by activating intracellular signalling cascades. Osteoblast HOBIT cells express P2Y(1) and P2Y(2) G-protein-coupled receptors, and respond to extracellular ATP by increasing cytosolic calcium concentrations. Early growth response protein 1 (Egr-1) is a C(2)H(2)-zinc-finger-containing transcriptional regulator responsible for the activation of several genes involved in the control of cell proliferation and apoptosis, and is thought to have a central role in osteoblast biology. We show that ATP treatment of HOBIT cells increases Egr-1 protein levels and binding activity via a mechanism involving a Ca(2+)-independent protein kinase C isoform. Moreover, hypotonic stress and increased medium turbulence, by inducing ATP release, result in a similar effect on Egr-1. Increased levels of Egr-1 protein expression and activity are achieved at very early times after stimulation (5 min), possibly accounting for a rapid way for changing the osteoblast gene-expression profile. A target gene for Egr-1 that is fundamental in osteoblast physiology, COL1A2, is up-regulated by ATP stimulation of HOBIT cells in a timescale that is compatible with that of Egr-1 activation.

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.

Novel therapeutic targets in osteoporosis

Osteoporosis is a common condition in which significant bone loss occurs resulting in an increased risk of sustaining fractures. Several licensed therapies are available to treat this condition, which suffer from several disadvantages including limited efficacy, high cost and poor long-term patient adherence as a consequence of significant side effects and inconvenient methods of administration. A wide range of therapeutic targets have been developed to provide a basis for developing newer therapies which overcome these limitations. These can be subdivided into those that are primarily directed towards inhibiting osteoclast-dependent bone resorption and those that stimulate osteoblastic bone formation. Targets can be grouped as follows: systemic factors such as steroid and peptide hormones; local factors produced in bone involved in osteoblast and osteoclastic regulation; and cellular targets such as cell membrane receptors and attachment proteins, cellular enzymes and nuclear transcription factors. To date, only a small proportion of these targets have yielded novel compounds to have entered clinical trials. However, it is anticipated that these will provide the basis for significant numbers of new therapies for osteoporosis in the foreseeable future.

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.

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.

Parathyroid hormone potentiates nucleotide-induced [Ca2+]i release in rat osteoblasts independently of Gq activation or cyclic monophosphate accumulation. A mechanism for localizing systemic responses in bone

The regulation of tissue turnover requires the coordinated activity of both local and systemic factors. Nucleotides exist transiently in the extracellular environment, where they serve as ligands to P2 receptors. Here we report that the localized release of these nucleotides can sensitize osteoblasts to the activity of systemic factors. We have investigated the ability of parathyroid hormone (PTH), a principal regulator of bone resorption and formation, to potentiate signals arising from nucleotide stimulation of UMR-106 clonal rat osteoblasts. PTH receptor activation alone did not lead to [Ca(2+)](i) elevation in these cells, indicating no G(q) coupling, however, activation of G(q)-coupled P2Y(1) receptors resulted in characteristic [Ca(2+)](i) release. PTH potentiated this nucleotide-induced Ca(2+) release, independently of Ca(2+) influx. PTH-(1-31), which activates only G(s), mimicked the actions of PTH-(1-34), whereas PTH-(3-34), which only activates G(q), was unable to potentiate nucleotide-induced [Ca(2+)](i) release. Despite this coupling of the PTHR to G(s), cAMP accumulation or protein kinase A activation did not contribute to the potentiation. 3-Isobutyl-1-methylxanthine, but not forskolin effectively potentiated nucleotide-induced [Ca(2+)](i) release, however, further experiments proved that cyclic monophosphates were not involved in the potentiation mechanism. Costimulation of UMR-106 cells with P2Y(1) agonists and PTH led to increased levels of cAMP response element-binding protein phosphorylation and a synergistic effect was observed on endogenous c-fos gene expression following costimulation. In fact the calcium responsive Ca/cAMP response element of the c-fos promoter alone was effective at driving this synergistic gene expression. These findings demonstrate that nucleotides can provide a targeted response to systemic factors, such as PTH, and have important implications for PTH-induced signaling in bone.

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.

Bioenergetics and mitochondrial transmembrane potential during differentiation of cultured osteoblasts

To evaluate the relationship between osteoblast differentiation and bioenergetics, cultured primary osteoblasts from fetal rat calvaria were grown in medium supplemented with ascorbate to induce differentiation. Before ascorbate treatment, the rate of glucose consumption was 320 nmol. h(-1). 10(6) cells(-1), respiration was 40 nmol. h(-1). 10(6) cells(-1), and the ratio of lactate production to glucose consumption was approximately 2, indicating that glycolysis was the main energy source for immature osteoblasts. Ascorbate treatment for 14 days led to a fourfold increase in respiration, a threefold increase in ATP production, and a fivefold increase in ATP content compared with that shown in immature cells. Confocal imaging of mitochondria stained with a transmembrane potential-sensitive vital dye showed that mature cells possessed abundant amounts of high-transmembrane-potential mitochondria, which were concentrated near the culture medium-facing surface. Acute treatment of mature osteoblasts with metabolic inhibitors showed that the rate of glycolysis rose to maintain the cellular energy supply constant. Thus progressive differentiation coincided with changes in cellular metabolism and mitochondrial activity, which are likely to play key roles in osteoblast function.

Advanced bone formation in grooves in vitro is not restricted to calcified biological materials

The aim of this in vitro study was to investigate whether the phenomenon of advanced bone formation in grooves (Bone 18, 115, 1995) is restricted to calcified biological materials. Osteoblasts were released from neonatal rat calvaria by enzyme digestion and cultured in EMEM and 10% FCS. At confluence, they were seeded on to dentine, bone, plastic, titanium, or silicon, which had been grooved using a water-cooled, diamond-edged, slow-speed or high-speed circular saw or reciprocating wire saw, or a rotating dental burr. Cultures were continued for 14-21 days, with a few extended for up to 7 weeks. Osteoblasts were also cultured on grooved dentine and plastic with or without added Stanozolol for 18 days, and bone formation assayed by measuring the total length of bone formed in the grooves in each specimen. Bone formation always occurred first within the grooves and was appositional. It formed on both calcified biological and nonbiological substrates, but developed consistently earlier on the biological substrates, and conformed to both the main grooves and the secondary finer grooving within them. Surface features at scales ranging from the millimeter to nanometer therefore influence the development of bone in vitro and possibly in vivo. The described site-induced bone formation system is valuable as an in vitro assay for biomaterial and pharmaceutical research.

PTH(1-34) suppresses appositional bone formation by cultured rat cranial osteoblasts

Parathyroid hormone (PTH) provokes cell division of osteoblasts in vitro and is anabolic in vivo when administered intermittently. We studied the effect on bone formation in vitro by rat primary calvarial osteoblasts of the short-term addition of rat PTH(1-34) (10(-12), 10(-9), and 10(-7) mol/L) to the medium. Our aim was to determine whether intermittent PTH(1-34) would promote osteogenesis in an experimental system designed to promote appositional bone formation. Unexpectedly, whether the exposure to PTH(1-34) was for 24 h only on days 3 or 12, or 24 h on both days 3 and 12, or a pulse for 6 h every 48 h, we observed suppression of bone formation even at the lowest concentration of added PTH(1-34). Our finding that cells which are of the osteoblastic phenotype can be entirely prevented from making appositional bone by high concentrations of PTH(1-34) for 1 day in a week, and have their osteogenic capacity suppressed by lower concentrations even when the PTH(1-34) is administered in brief pulses every other day, suggests that pulses of PTH administered in vivo may not increase bone formation by already differentiated osteoblasts.

ATP is a potent stimulator of the activation and formation of rodent osteoclasts

1. There is increasing evidence that extracellular ATP acts directly on bone cells via P2 receptors. In normal rat osteoclasts, ATP activates both non-selective cation channels and Ca2+-dependent K+ channels. In this study we investigated the action of ATP on the formation of osteoclasts and on the ultimate function of these cells, namely resorption pit formation. 2. We found that ATP stimulated resorption pit formation up to 5.6-fold when osteoclast-containing bone cell populations from neonatal rats were cultured for 26 h on ivory discs, with a maximum effect occurring at relatively low concentrations (0.2-2 microM). The stimulatory effect of ATP was amplified greatly when osteoclasts were activated by culture in acidified media (pH 6.9-7.0). Pit formation by acid-activated osteoclasts in the absence of ATP was inhibited by apyrase, an ecto-ATPase and by suramin, an antagonist of P2 receptors. 3. Over the same concentration range at which rat osteoclast activation occurred (0.2-2 microM), ATP also enhanced osteoclast formation in 10 day mouse marrow cultures, by up to 3.3-fold, with corresponding increases in resorption pit formation. Higher concentrations of ATP (20-200 microM) reduced or blocked osteoclast formation. Adenosine, a P1 receptor agonist, was without effect on either osteoclast activation or formation. 4. These results suggest that low levels of extracellular ATP may play a fundamental role in modulating both the resorptive function and formation of mammalian osteoclasts.