ATP as an osteoclast regulator?

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.

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 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.

Regulation of osteoclast activity

Osteoclasts are the primary cell type responsible for bone resorption. This paper reviews many of the known regulators of osteoclast activity, including hormones, cytokines, ions, and arachidonic acid metabolites. Most of the hormones and cytokines that inhibit osteoclast activity act directly on the osteoclasts. In contrast, most of the hormones and cytokines that stimulate osteoclast activity act indirectly through osteoblasts. Particularly interesting in this regard are agents that directly inhibit activity of highly purified osteoclasts yet stimulate activity of osteoclasts that are co-cultured with osteoblasts. Recent studies have demonstrated that the primary mechanism by which bone resorptive agents stimulate osteoclast activity indirectly is likely to be up-regulation of production of osteoclast differentiation factor/osteoprotegerin ligand (ODF/OPGL) by the osteoblasts. In addition to discussing regulators of osteoclast activity per se, this paper also reviews the role of osteoclast apoptosis to limit the extent of bone resorption.

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.