The effects of P2X7 receptor antagonists on the formation and function of human osteoclasts in vitro.

The P2X7 receptor (P2X7R) has been implicated in the process of multinucleation and cell fusion. We have previously demonstrated that blockade of P2X7Rs on osteoclast precursors using a blocking antibody inhibited multinucleated osteoclast formation in vitro, but that P2X7R KO mice maintain the ability to form multinucleated osteoclasts. This apparent contradiction of the role the P2X7R plays in multinucleation has prompted us to examine the effect of the most commonly used and recently available P2X7R antagonists on osteoclast formation and function. When added to recombinant RANKL and M-CSF human blood monocytes cultures, all but one compound, decreased the formation and function of multinucleated TRAP-positive osteoclasts in a concentration-dependent manner. These data provide further evidence for the role of the P2X7R in the formation of functional human multinucleated osteoclasts and highlight the importance of selection of antagonists for use in long-term experiments.

Involvement of adenosine 5'-triphosphate in ultrasound-induced fracture repair.

Ultrasound (US) accelerates fracture healing; however, the mechanism of this effect remains unclear. Adenosine 5'-triphosphate (ATP) stimulates bone remodeling and is released constitutively from intact osteoblasts; this is a process that is enhanced after mechanical stimulation. We hypothesized that ATP release from osteoblasts is increased after US stimulation and that this leads to accelerated fracture healing. US was applied to SaOS-2 human osteoblasts and the concentration of ATP in the cell culture medium was determined. Cell proliferation and gene expression were subsequently investigated. Increased concentrations of ATP were detected in the culture medium of US-treated cells and both ATP and US stimulation caused increased receptor activator of nuclear factor-kappa B ligand (RANKL), decreased osteoprotegerin expression and increased cell proliferation by SaOS-2 cells. These findings indicate that US causes ATP release by osteoblasts in vitro and that this may contribute to accelerated fracture healing by enhancing osteoblast proliferation and increasing RANKL expression and decreasing osteoprotegerin expression by osteoblasts to promote osteoclastogenesis.

Human keratinocytes release ATP and utilize three mechanisms for nucleotide interconversion at the cell surface.

Nucleotide activation of P2 receptors is important in autocrine and paracrine regulation in many tissues. In the epidermis, nucleotides are involved in proliferation, differentiation, and apoptosis. In this study, we have used a combination of luciferin-luciferase luminometry, pharmacological inhibitors, and confocal microscopy to demonstrate that HaCaT keratinocytes release ATP into the culture medium, and that there are three mechanisms for nucleotide interconversion, resulting in ATP generation at the cell surface. Addition of ADP, GTP, or UTP to culture medium elevated the ATP concentration. ADP to ATP conversion was inhibited by diadenosine pentaphosphate, oligomycin, and UDP, suggesting the involvement of cell surface adenylate kinase, F(1)F(0) ATP synthase, and nucleoside diphosphokinase (NDPK), respectively, which was supported by immunohistochemistry. Simultaneous addition of ADP and GTP elevated ATP above that for each nucleotide alone indicating that GTP acts as a phosphate donor. However, the activity of NDPK, F(1)F(0) ATP synthase or the forward reaction of adenylate kinase could not fully account for the culture medium ATP content. We postulate that this discrepancy is due to the reverse reaction of adenylate kinase utilizing AMP. In normal human skin, F(1)F(0) ATP synthase and NDPK were differentially localized, with mitochondrial expression in the basal layer, and cell surface expression in the differentiated layers. We and others have previously demonstrated that keratinocytes express multiple P2 receptors. In this study we now identify the potential sources of extracellular ATP required to activate these receptors and provide better understanding of the role of nucleotides in normal epidermal homeostasis and wound healing.

P2 receptors in bone--modulation of osteoclast formation and activity via P2X7 activation.

The concept that adenosine triphosphate (ATP) can act as an extracellular signaling molecule via interactions with specific purinergic receptors to mediate a wide variety of processes as diverse as neurotransmission (Edwards et al., 1992), inflammation (Perregaux et al., 1994), apoptosis (Chow et al., 1997), and bone remodelling (Jones et al., 1997; Morrison et al., 1998) is now widely accepted. Since the early work of Burnstock (Burnstock, 1972), the number of characterized P2 receptors responsive to extracellular nucleotides has increased dramatically. It is now known that both osteoblasts and osteoclasts express multiple P2 receptor subtypes, and the increasing number of nucleotide-induced effects reported to occur in bone serves to highlight the importance of these receptors in the bone microenvironment and the bone remodeling processes. In this article we will review work from our laboratory, and others, that has established nucleotides and P2 receptors as important signaling molecules in bone. In particular, we will focus on the expression of P2 receptors by osteoclasts and, more specifically, the P2X7 receptor and its paradoxical role in osteoclast function.

Receptor activator for nuclear factor kappaB ligand and osteoprotegerin: regulators of bone physiology and immune responses/potential therapeutic agents and biochemical markers.

The discovery of receptor activator for nuclear factor kappaB ligand (RANKL) and osteoprotegerin (OPG) as the fundamental factors in controlling osteoclast formation and activation has led to a greater understanding of bone biology over the past few years. Here we discuss the role of these molecules in immunology and skeletal remodelling and assess their involvement in diseases of bones and joints, including rheumatoid arthritis, Paget's disease, post-menopausal osteoporosis and malignant bone diseases. OPG has been identified as a potential anabolic agent for treating conditions in which there is net bone loss and is currently in Phase I clinical trials. This review examines the current evidence indicating that OPG increases bone mass, and discusses other possible beneficial effects of OPG, such as inhibition of tumour growth and relief from bone cancer pain. OPG can be measured in human serum, and numerous studies have suggested that increased or decreased serum concentrations of this molecule can indicate the existence of remodelling disorders. Here we discuss how abnormal serum OPG concentrations could potentially be used to indicate imbalances of bone resorption and formation. The possible applications of serum OPG concentration as a marker for non-skeletal disease conditions are also considered.