Regulation Of Bone Cell Function Research Articles

Messages from the Mineral: How Bone Cells Communicate with Other Tissues.

Bone is a highly dynamic tissue, and the constant actions of bone-forming and bone-resorbing cells are responsible for attaining peak bone mass, maintaining bone mass in the adults, and the subsequent bone loss with aging and menopause, as well as skeletal complications of diseases and drug side-effects. It is now accepted that the generation and activity of bone-forming osteoblasts and bone-resorbing osteoclasts is modulated by osteocytes, osteoblast-derived cells embedded in the bone matrix. The interaction among bone cells occurs through direct contact and via secreted molecules. In addition to the regulation of bone cell function, molecules released by these cells are also able to reach the circulation and have effects in other tissues and organs in healthy individuals. Moreover, bone cell products have also been associated with the establishment or progression of diseases, including cancer and muscle weakness. In this review, we will discuss the role of bone as an endocrine organ, and the effect of selected, osteoblast-, osteocyte-, and osteoclast-secreted molecules on other tissues.

Bone-Regulating MicroRNAs and Resistance Exercise: A Mini-Review

MicroRNAs (miRNA) are a class of short noncoding RNA that play important roles in controlling gene expression. Many miRNAs have been identified as being important regulators of bone cell function, thus affecting the bone remodeling processes. In addition to being expressed in specific tissues and exerting intracellular effects, miRNAs can enter the blood where they can be taken up by other tissues. These circulating miRNAs (c-miRNA) also have clinical significance as biomarkers of musculoskeletal diseases as they are tissue-specific, are stable and easily detectable, and require minimally invasive procedures. This mini-review discusses miRNAs with regulatory roles in bone metabolism and c-miRNA responses to acute bouts of resistance exercise. MiRNA responses (e.g., upregulation/downregulation of expression) vary depending on the resistance exercise protocol characteristics and the age of the participants. There are gaps in the literature that need to be addressed as most of the resistance exercise studies focused on miRNAs that regulate skeletal muscle in male participants.

The Effect of Amitriptyline and Sertraline on the Tooth Movement, Root Resorption and Alveolar Bone Remodeling After Load Application in Dogs

Introduction: Antidepressant drugs are the most commonly prescribed classes of pharmacologic agents in the public. The drugs have been shown to have a role in the regulation of bone cell function and as a result affecting the orthodontic tooth movements. The aim of this study was to determine the effect of Amitriptyline and Sertraline on the tooth movement, root resorption and alveolar bone remodeling after load application in dogs. Materials and Methods: In this experimental study, 9 male dogs were randomly divided into three groups, first group sertraline, second group Amitriptyline and the control group (normal saline). A nickel titanium spring (200 gr) was used between second premolar and canine after 1st premolar extraction. After 2 months, the reduction of distance between 2nd premolar and canine was measured. The percentages of root resorption and bone formation were determined. The data were analyzed using repeated measures analysis at significance level of 0.05. Results: In the three groups of amitriptyline, sertraline and control, the mean of teeth movement (p value = 0.483), external root resorption (p value = 0.608), total bone mineral density (p value = 0.078), bone formation percentage (p value = 0.616) and immature and lamellar bone formation (p value = 0.083), there was no statistical difference in any group and in premolar teeth Conclusions: The rate of tooth movement and the percentages of bone formation and root resorption in dogs decreased with systemic administration of amitriptyline and sertraline; although this reduction was not statistically significant in comparison with control group.

Mice lacking substance P have normal bone modeling but diminished bone formation, increased resorption, and accelerated osteopenia with aging.

Substance P (SP) is a sensory neuropeptide that is expressed by the neurons innervating bone. There is considerable evidence that SP can regulate bone cell function in vitro, but it is unclear whether SP modulates bone modeling or remodeling in vivo. To answer this question we characterized the bone phenotype of mice with deletion of the Tac1 gene expressing SP. The phenotypes of 2-month-old and 5-month-old SP deficient mice and their wildtype controls were characterized by using μCT imaging, static and dynamic bone histomorphometry, and urinary deoxypyridinoline cross-links (DPD) measurement. No differences in bone phenotypes were observed between the 2 strains at 2months of age. By 5months both the wildtype and SP deficient mice had developed cancellous osteopenia, but relative to the wild-type mice the SP deficient mice had significantly greater cancellous bone loss. The SP deficient mice also exhibited decreased bone formation, increased osteoclast number, and increased urinary DPD levels. Cortical defect early repair was delayed in 5-month-old mice lacking SP. Collectively, these findings indicate that SP signaling is not required for bone modeling, but SP signaling reduces age-related osteopenia and accelerates cortical defect reparation, data supporting the hypothesis that SP is an anabolic physiologic regulator of bone metabolism.

New Insights Into Monogenic Causes of Osteoporosis.

Osteoporosis, characterized by deteriorated bone microarchitecture and low bone mineral density, is a chronic skeletal disease with high worldwide prevalence. Osteoporosis related to aging is the most common form and causes significant morbidity and mortality. Rare, monogenic forms of osteoporosis have their onset usually in childhood or young adulthood and have specific phenotypic features and clinical course depending on the underlying cause. The most common form is osteogenesis imperfecta linked to mutations in COL1A1 and COL1A2, the two genes encoding type I collagen. However, in the past years, remarkable advancements in bone research have expanded our understanding of the intricacies behind bone metabolism and identified novel molecular mechanisms contributing to skeletal health and disease. Especially high-throughput sequencing techniques have made family-based studies an efficient way to identify single genes causative of rare monogenic forms of osteoporosis and these have yielded several novel genes that encode proteins partaking in type I collagen modification or regulating bone cell function directly. New forms of monogenic osteoporosis, such as autosomal dominant osteoporosis caused by WNT1 mutations or X-linked osteoporosis due to PLS3 mutations, have revealed previously unidentified bone-regulating proteins and clarified specific roles of bone cells, expanded our understanding of possible inheritance mechanisms and paces of disease progression, and highlighted the potential of monogenic bone diseases to extend beyond the skeletal tissue. The novel gene discoveries have introduced new challenges to the classification and diagnosis of monogenic osteoporosis, but also provided promising new molecular targets for development of pharmacotherapies. In this article we give an overview of the recent discoveries in the area of monogenic forms of osteoporosis, describing the key cellular mechanisms leading to skeletal fragility, the major recent research findings and the essential challenges and avenues in future diagnostics and treatments.

Role of IGF1 and EFN-EPH signaling in skeletal metabolism.

Insulin-like growth factor 1(IGF1) and ephrin ligand (EFN)-receptor (EPH) signaling are both crucial for bone cell function and skeletal development and maintenance. IGF1 signaling is the major mediator of growth hormone-induced bone growth, but a host of different signals and factors regulate IGF1 signaling at the systemic and local levels. Disruption of the Igf1 gene results in reduced peak bone mass in both experimental animal models and humans. Additionally, EFN-EPH signaling is a complex system which, particularly through cell-cell interactions, contributes to the development and differentiation of many bone cell types. Recent evidence has demonstrated several ways in which the IGF1 and EFN-EPH signaling pathways interact with and depend upon each other to regulate bone cell function. While much remains to be elucidated, the interaction between these two signaling pathways opens a vast array of new opportunities for investigation into the mechanisms of and potential therapies for skeletal conditions such as osteoporosis and fracture repair.

Activation of the P2Y2 receptor regulates bone cell function by enhancing ATP release.

Bone cells constitutively release ATP into the extracellular environment where it acts locally via P2 receptors to regulate bone cell function. Whilst P2Y2 receptor stimulation regulates bone mineralisation, the functional effects of this receptor in osteoclasts remain unknown. This investigation used the P2Y2 receptor knockout (P2Y2R-/- ) mouse model to investigate the role of this receptor in bone. MicroCT analysis of P2Y2R-/- mice demonstrated age-related increases in trabecular bone volume (≤48%), number (≤30%) and thickness (≤17%). In vitro P2Y2R-/- osteoblasts displayed a 3-fold increase in bone formation and alkaline phosphatase activity, whilst P2Y2R-/- osteoclasts exhibited a 65% reduction in resorptive activity. Serum cross-linked C-telopeptide levels (CTX, resorption marker) were also decreased (≤35%). The resorption defect in P2Y2R-/- osteoclasts was rescued by the addition of exogenous ATP, suggesting that an ATP deficit could be a key factor in the reduced function of these cells. In agreement, we found that basal ATP release was reduced up to 53% in P2Y2R-/- osteoclasts. The P2Y2 receptor agonists, UTP and 2-thioUTP, increased osteoclast activity and ATP release in wild-type but not in P2Y2R-/- cells. This indicates that the P2Y2 receptor may regulate osteoclast function indirectly by promoting ATP release. UTP and 2-thioUTP also stimulate ATP release from osteoblasts suggesting that the P2Y2 receptor exerts a similar function in these cells. Taken together, our findings are consistent with the notion that the primary action of P2Y2 receptor signalling in bone is to regulate extracellular ATP levels.

Sodium hydrosulfide inhibits the differentiation of osteoclast progenitor cells via NRF2-dependent mechanism

Hydrogen sulfide (H2S), which recently emerged as a potent regulator of tissues and organs, is broadly produced in mammalian cells but whether it can regulate bone cell function is still elusive. The main objective of this study was to establish the role of H2S in the regulation of human osteoclast differentiation and function. Sodium hydrosulfide (NaHS), a common H2S-donor, was administered in vitro to CD11b+ human monocytes, the pool of circulating osteoclasts precursors which are critically involved in osteoclast development and function in bone. NaHS dose-dependently decreased human osteoclast differentiation at concentrations which did not induce toxicity. The inhibition of human osteoclast differentiation was associated with a down-regulation in RANKL-dependent intracellular ROS levels in human pre-osteoclasts cells. Furthermore, NaHS up-regulated NRF2 protein expression, its nuclear translocation, and the transcription of the two key downstream antioxidant genes Peroxiredoxin-1 and NAD(P)H dehydrogenase quinone 1, suggesting that NRF2 activation may inhibit human osteoclast differentiation by activating a sustained antioxidant response in osteoclast progenitors; furthermore, NRF2 activators Sulforaphane and Tert-butylhydroquinone inhibited in vitro human osteoclast differentiation. Moreover, silencing NRF2 in human pre-osteoclasts totally abolished NaHS-mediated inhibition of osteoclastogenesis, suggesting that NRF2 is essential to the inhibitory function of NaHS in osteoclast development. Finally, we found that NaHS also downregulated the RANKL/OPG mRNA ratio in human mesenchymal stem cells, the key osteoclast-supporting cells. Our results suggest that NaHS shows a potential therapeutical role in erosive diseases of bone by regulating both direct and indirect mechanisms controlling the differentiation of circulating osteoclasts precursors.

The Correlation between Osteoporosis Occurrences in Both Schizophrenia and Parkinson’s Disease

Osteoporosis disease is a metabolic disorder in which bone mineral density (BMD) is lower than the normal threshold. Based on literature, it is known that schizophrenic patients due to consuming anti-psychotic drugs and Parkinson’s disease patients due to vitamin D deficiency and decrease in mobility are at the high risk of osteoporosis (1–4). On the other hand, it is observed that adenosine 5′-monophosphate (AMP)-activated protein kinase (AMPK) activity, which is regulating cellular energy homeostasis, is reduced in schizophrenia diseases (5). In addition, it is known that there is mitochondrial dysfunction in Parkinson’s disease that can be treated by AMPK, which is identified as a mitochondrial biogenesis (6–8). Recently, some findings show that AMPK plays an important role in bone metabolism. Besides, some in vitro studies revealing that AMPK modulators regulate bone cell function (9–13). Also, some studies show that deficiency of AMPK α and β subunits in mice causes bone loss in vivo (14). Based on the above mentioned points, it can be referred that AMPK deficiency in Parkinson’s disease and schizophrenia may lead to osteoporosis. This can be used as a goal in treatment of osteoporosis in those disorders. In another word, nowadays there are some drugs available for both diseases but they have some side effect, which may lead to osteoporosis; by considering the fact that AMPK deficiency may cause osteoporosis, new drugs can be provided with AMPK supplement to reduce the osteoporosis symptoms. Surely, experimental trials are needed to validate our hypothesis.

Osteoimmunology and Bone Homeostasis: Relevance to Spondyloarthritis

The seronegative spondyloarthopathies (SpA) share certain common articular and peri-articular features that differ from rheumatoid arthritis (RA) and other forms of inflammatory arthritis. These include the tendency of the SpAs to involve the axial skeleton in addition to the diarthrodial joints, and the prominent involvement of the extra-articular entheses (sites of ligamentous and tendon insertion), which are not common sites of primary pathology in RA and other inflammatory arthropathies. The differential anatomic sites of bone pathology in the SpAs in comparison to the other forms of arthritis suggest that the underlying pathogenic processes and cellular and molecular mechanisms that account for the peri-articular bone pathology involve different underlying disease mechanisms. This review will highlight the molecular and cellular processes that are involved in the pathogenesis of the skeletal pathology in the SpAs, and provide evidence that many of the factors involved in regulation of bone cell function exhibit potent immune-regulatory activity, providing support for the general concept of osteoimmunology.

Inflammation-resolving mediator resolvin E1 inhibits osteoclast fusion. (169.6)

Abstract Resolution of inflammation is an active, highly coordinated process leading to the return of tissues to homeostasis. The ω-3 polyunsaturated fatty acid derivative RvE1 is a proresolving receptor agonist that has been demonstrated to protect from inflammatory bone loss by inhibiting osteoclast formation and bone resorption. It is unclear whether the actions of RvE1 in inflammatory bone disease are indirect through modulation of inflammation or the direct action of RvE1 on bone cells. Investigation of the direct impact of RvE1 treatment on the specific stages of osteoclast maturation in vitro revealed that RvE1 targets late stages of osteoclast maturation. Time-lapse vital microscopy revealed that RvE1 inhibits migration and fusion of osteoclast precursors. Western blot analysis revealed that RvE1 specifically down-regulates the pivotal osteoclast fusion protein DC-STAMP (dendritic cell specific transmembrane protein) through the leukotriene receptor BLT-1. RvE1 does not change NFATc1 induction or nuclear translocation; however, NFATc1 binding to the DC-STAMP promoter was inhibited by RvE1 treatment. The data suggest that RvE1 inhibits the binding of the transcription factor NFATc1 to the promoter of osteoclast fusion protein DC-STAMP, leading to DC-STAMP down-regulation and attenuation of osteoclast fusion. Our observations establish a dual role for inflammation resolution in innate immunity as well as bone preservation through the direct regulation of bone cell function.

The P2Y6 Receptor Stimulates Bone Resorption by Osteoclasts

Accumulating evidence indicates that extracellular nucleotides, signaling through P2 receptors, play a significant role in bone remodeling. Osteoclasts (the bone-resorbing cell) and osteoblasts (the bone-forming cell) display expression of the G protein-coupled P2Y(6) receptor, but the role of this receptor in modulating cell function is unclear. Here, we demonstrate that extracellular UDP, acting via P2Y(6) receptors, stimulates the formation of osteoclasts from precursor cells, while also enhancing the resorptive activity of mature osteoclasts. Furthermore, osteoclasts derived from P2Y(6) receptor-deficient (P2Y(6)R(-/-)) animals displayed defective function in vitro. Using dual energy x-ray absorptiometry scanning and microcomputed tomographic analysis we showed that P2Y(6)R(-/-) mice have increased bone mineral content, cortical bone volume, and cortical thickness in the long bones and spine, whereas trabecular bone parameters were unaffected. Histomorphometric analysis showed the perimeter of the bone occupied by osteoclasts on the endocortical and trabecular surfaces was decreased in P2Y(6)R(-/-) mice. Taken together these results show the P2Y(6) receptor may play an important role in the regulation of bone cell function in vivo.

Smurf control in bone cells

The homologous to the E6-associated protein carboxyl terminus (HECT) domain E3 ubiquitin ligase Smurf1 is the first E3 ligase to be implicated in regulating bone cell function. The involvement of Smurf1 in multiple signaling pathways and pathological conditions is presently an area of extensive scientific interest. This review highlights recent works exploring Smurf-regulated biological processes in bone cells and highlights recent discoveries surrounding the regulatory mechanisms modulating its catalytic activity and substrate recognition capability. Moreover, we discuss the relevance of targeting the HECT E3s through the development of small-molecule inhibitors as an anticancer therapeutic strategy.

The effect of tai chi chuan on bone remodeling and cytokines among breast cancer survivors: A feasibility trial

9610 Background: Weight-bearing exercise may slow the rate of bone loss associated with breast cancer treatment. The purpose of this study is to determine the effect of tai chi chuan (TCC) on bone health, as measured by the changes in the levels of bone resorption and bone formation. This study also aimed to investigate whether changes in bone health were correlated with growth and inflammation markers that serve as regulators of bone cell function. Methods: Female patients (N=16) who completed treatment for breast cancer within the past 30 months were randomly assigned to either the TCC group or the psycho-educational support group without exercise (ST) for 60 minutes, three times a week for a period of 12 weeks. Serum levels of bone resorption (N-telopeptides of type I collagen; NTx) and bone formation (bone specific alkaline phosphatase; BAP) were determined by ELISA at baseline and post-intervention. Using validated methods, a bone remodeling index (BRI) was calculated from levels of NTx and BAP. In addition, pre- and post-intervention levels of insulin-like growth factor binding protein 1 (IGFBP-1) and interleukin-2 (IL-2), markers associated with excessive bone resorption, were measured. Lastly, levels of interleukin-6 (IL-6), believed to enhance bone formation, were measured at both pre- and post-intervention. Results: ANCOVA analyses demonstrated that survivors in the TCC group experienced a greater increase in bone remodeling than those in the ST group (Δ BRITCC=1.6 vs Δ BRIST=0.2; p=0.04). All correlations were determined by Pearson's correlation coefficients. IGFBP-1 was negatively correlated with increasing bone remodeling levels (r=-0.43, p=0.14). IL-2 was also negatively correlated with increasing bone remodeling levels (r=-0.35, p=0.24). IL-6 was positively correlated with increasing bone remodeling levels (r=0.69, p=0.01). Conclusions: This pilot study suggests that TCC has positive effects on bone remodeling through changes in growth and inflammation factors that regulate bone cell function. A larger, more definitive trial examining the influence of TCC on bone remodeling is warranted. Funding: Sally Schindel Cone and R25 CA102618 No significant financial relationships to disclose.

Nucleotides and regulation of bone cell function

Nucleotides and regulation of bone cell function

Hypoxia stimulates vesicular ATP release from rat osteoblasts

Many neuronal and non-neuronal cell types release ATP in a controlled manner. After release, extracellular ATP (or, following hydrolysis, ADP) acts on cells in a paracrine manner via P2 receptors. Extracellular nucleotides are now thought to play an important role in the regulation of bone cell function. ATP (and ADP), acting via the P2Y(1) receptor, stimulate osteoclast formation and activity, whilst P2Y(2) receptor stimulation by ATP (or UTP) inhibits bone mineralization by osteoblasts. We found that rat calvarial osteoblasts released ATP constitutively, in a differentiation-dependent manner, with mature, bone-forming osteoblasts releasing up to sevenfold more ATP than undifferentiated, proliferating cells. The inhibitors of vesicular exocytosis, monensin, and N-ethylmaleimide (1-1,000 microM) inhibited basal ATP release by up to 99%. The presence of granular ATP-filled vesicles within the osteoblast cytoplasm was demonstrated by quinacrine staining. Exposure to hypoxia (2% O(2)) for up to 3 min increased ATP release from osteoblasts up to 2.5-fold without affecting cell viability. Peak concentrations of ATP released into culture medium were >1 microM, which equates with concentrations known to exert significant effects on osteoblast and osteoclast function. Monensin and N-ethylmaleimide (100 microM) attenuated the hypoxia-induced ATP release by up to 80%. Depletion of quinacrine-stained vesicles was also apparent after hypoxic stimulation, indicating that ATP release had taken place. These data suggest that vesicular exocytosis is a key mediator of ATP release from osteoblasts, in biologically significant amounts. Moreover, increased extracellular ATP levels following acute exposure to low O(2) could influence local purinergic signaling and affect the balance between bone formation and bone resorption.

Expression, signaling, and function of P2X7 receptors in bone

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

Pharmacological Topics of Bone Metabolism: Preface

This Forum Minireview contains the proceedings of the symposium at the 80th Annual meeting of The Japanese Pharmacological Society (March 27, 2007). With the growing incidence of fragility fractures, bone loss and osteoporosis have become active areas of research in clinical and experimental pharmacology. The symposium aimed to summarize recent advances and trends in elucidating the pathophysiological significance of bone metabolism and considered the important targets for drug development strategies. In the symposium, the clinical and basic research discussed may provide new types of therapeutic drugs and contribute to the development of treatments for the pathologies associated with modifications of bone remodeling such as osteoporosis. Suzuki et al. described the recent progress in diagnosis of these disorders and showed that treatments with new drugs seem to have some efficacy to reduce osteoporotic fracture. They summarized the present pharmacological management of osteoporosis by estrogen, selective estrogen receptor modulators (SERMs), bisphosphonate, cathepsin K inhibitors, the modulator of RANKRANKL system, parathyrioid hormone, and strontium ranelate. Then, on the basis of these evidences, the future perspectives are demonstrated. Takarada and Yoneda identified a glutamatergic signaling in bone similar to that of the central nervous system and demonstrated that glutamate receptors can regulate bone cell function, suggesting an important role for this neuromediator in bone metabolism. The neural modulation in bone may, therefore, present a major therapeutic impact for treatment of pathologies associated with modifications of bone remodeling such as osteoporosis. Togari and Arai demonstrated the increased sympathetic nervous activity causes bone loss via increase in bone resorption. The increase is based on the stimulation of both osteoclast formation and osteoclast activity. These effects are associated with β2-adrenergic activity toward both osteoblastic and osteoclastic cells. Such findings indicate that β-blockers may be effective against osteoporosis, in which case there is increased sympathetic activity. Woo et al. discovered new types of antiresorptive agents, such as reveromycin A, destruxins, statins, FK506, cyclosporine A, symbioimine, and prodigiosins, by screening for microbial natural compounds that regulate osteoclast differentiation, function, and /or survival. They discuss the cellular and molecular mechanisms of the action of these compounds on osteoclasts. Shinoda et al. reported that TRK-530, a novel synthetic bisphosphonate showing both anti-inflammatory and anti–bone-resorbing effects, efficiently prevented experimentally induced alveolar bone resorption in animals with periodontitis, and this compound could be effective for the treatment of diseases with excessive bone resorption accompanied by inflammation. They suggest that TRK-530 might be useful for the treatment of alveolar bone loss in periodontitis. Finally, we would be happy if these reviews can stimulate further theoretical debate and be carried to the bench towards new directions of osteoporosis research.

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.

Serotonin and fluoxetine modulate bone cell function in vitro

Recent studies have proposed a role for serotonin and its transporter in regulation of bone cell function. In the present study, we examined the in vitro effects of serotonin and the serotonin transporter inhibitor fluoxetine "Prozac" on osteoblasts and osteoclasts. Human mononuclear cells were differentiated into osteoclasts in the presence of serotonin or fluoxetine. Both compounds affected the total number of differentiated osteoclasts as well as bone resorption in a bell-shaped manner. RT-PCR on the human osteoclasts demonstrated several serotonin receptors, the serotonin transporter, and the rate-limiting enzyme in serotonin synthesis, tryptophan hydroxylase 1 (Tph1). Tph1 expression was also found in murine osteoblasts and osteoclasts, indicating an ability to produce serotonin. In murine pre-osteoclasts (RAW264.7), serotonin as well as fluoxetine affected proliferation and NFkappaB activity in a biphasic manner. Proliferation of human mesenchymal stem cells (MSC) and primary osteoblasts (NHO), and 5-HT2A receptor expression was enhanced by serotonin. Fluoxetine stimulated proliferation of MSC and murine preosteoblasts (MC3T3-E1) in nM concentrations, microM concentrations were inhibitory. The effect of fluoxetine seemed direct, probably through 5-HT2 receptors. Serotonin-induced proliferation of MC3T3-E1 cells was inhibited by the PKC inhibitor (GF109203) and was also markedly reduced when antagonists of the serotonin receptors 5-HT2B/C or 5-HT2A/C were added. Serotonin increased osteoprotegerin (OPG) and decreased receptor activator of NF-kappaB ligand (RANKL) secretion from osteoblasts, suggesting a role in osteoblast-induced inhibition of osteoclast differentiation, whereas fluoxetine had the opposite effect. This study further describes possible mechanisms by which serotonin and the serotonin transporter can affect bone cell function.