Quantitative studies on the effect of prostacyclin on freshly isolated rat osteoclasts in culture

Calcium sensing and cell signaling processes in the local regulation of osteoclastic bone resorption

The skeletal matrix in terrestrial vertebrates undergoes continual cycles of removal and replacement in the processes of bone growth, repair and remodeling. The osteoclast is uniquely important in bone resorption and thus is implicated in the pathogenesis of clinically important bone and joint diseases. Activated osteoclasts form a resorptive hemivacuole with the bone surface into which they release both acid and osteoclastic lysosomal hydrolases. This article reviews cell physiological studies of the local mechanisms that regulate the resorptive process. These used in vitro methods for the isolation, culture and direct study of the properties of neonatal rat osteoclasts. They demonstrated that both local microvascular agents and products of the bone resorptive process such as ambient Ca2+ could complement longer-range systemic regulatory mechanisms such as those that might be exerted through calcitonin (CT). Thus elevated extracellular [Ca2+], or applications of surrogate divalent cation agonists for Ca2+, inhibited bone resorptive activity and produced parallel increases in cytosolic [Ca2+], cell retraction and longer-term inhibition of enzyme release in isolated rat osteoclasts. These changes showed specificity, inactivation, and voltage-dependent properties that implicated a cell surface Ca2+ receptor (CaR) sensitive to millimolar extracellular [Ca2+]. Pharmacological, biophysical and immunochemical evidence implicated a ryanodine-receptor (RyR) type II isoform in this process and localized it to a unique, surface membrane site, with an outward-facing channel-forming domain. Such a surface RyR might function either directly or indirectly in the process of extracellular [Ca2+] sensing and in turn be modulated by cyclic adenosine diphosphate ribose (cADPr) produced by the ADP-ribosyl cyclase, CD38. The review finishes by speculating about possible detailed models for these transduction events and their possible interactions with other systemic mechanisms involved in Ca2+ homeostasis as well as the possible role of the RyR-based signaling mechanisms in longer-term cell regulatory processes.

Activation of IP and EP(3) receptors alters cAMP-dependent cell migration

Migration of vascular smooth cells from the media to the intima essentially contributes to neointima formation after percutaneous transluminal angioplasty and stent implantation. The stable prostacyclin mimetic iloprost has been shown to inhibit neointima formation in experimental restenosis, but it is currently unknown whether this may be caused by an antimigratory effect. Hence, the present study analyses (i) the influence of G(s)-coupled prostacyclin (IP) receptors on cell migration and (ii) verifies whether EP(3) receptors with opposite (i.e., G(i)) coupling may conversely stimulate cell migration. In a modified Boyden chamber model, it was shown that iloprost dose-dependently inhibits the migration of primary human arterial smooth muscle cells, which constitutively express the IP receptor. On the other hand, human arterial smooth muscle cell migration was stimulated by the EP(3) receptor agonist M&B 28.767. To independently study the effects of these receptors, IP or EP(3) receptors were stably overexpressed in chinese hamster ovary cells (CHO-IP and CHO-EP(3)). Chemotaxis of CHO cells transfected with G(s)-coupled IP receptors was concentration-dependently inhibited by iloprost (2-100 nM), while there was no effect of iloprost on mock-transfected CHO. By contrast, CHO-cells that overexpressed EP(3) receptors showed a significant, concentration dependent (1-100 nM) increase of cell migration in presence of the selective EP(3) agonist M&B 28.767. It is concluded that the prostacyclin mimetic iloprost inhibits vascular cell migration, which probably depends on a G(s)-mediated increase of intracellular cAMP. EP(3) receptors conversely stimulate CHO migration.

Cloning and function of rabbit peroxisome proliferator-activated receptor delta/beta in mature osteoclasts

Osteoclasts modulate bone resorption under physiological and pathological conditions. Previously, we showed that both estrogens and retinoids regulated osteoclastic bone resorption and postulated that such regulation was directly mediated through their cognate receptors expressed in mature osteoclasts. In this study, we searched for expression of other members of the nuclear hormone receptor superfamily in osteoclasts. Using the low stringency homologous hybridization method, we isolated the peroxisome proliferator-activated receptor delta/beta (PPARdelta/beta) cDNA from mature rabbit osteoclasts. Northern blot analysis showed that PPARdelta/beta mRNA was highly expressed in highly enriched rabbit osteoclasts. Carbaprostacyclin, a prostacyclin analogue known to be a ligand for PPARdelta/beta, significantly induced both bone-resorbing activities of isolated mature rabbit osteoclasts and mRNA expression of the cathepsin K, carbonic anhydrase type II, and tartrate-resistant acid phosphatase genes in these cells. Moreover, the carbaprostacyclin-induced bone resorption was completely blocked by an antisense phosphothiorate oligodeoxynucleotide of PPARdelta/beta but not by the sense phosphothiorate oligodeoxynucleotide of the same DNA sequence. Our results suggest that PPARdelta/beta may be involved in direct modulation of osteoclastic bone resorption.

Novel mechanisms of calcium handling by the osteoclast: A review-hypothesis

The osteoclast is a cell that is unique in its ability to resorb bone and, in doing so, becomes exposed to unusually high millimolar Ca2+ concentrations. It is generally accepted that, during resorption, osteoclasts can "sense" changes in their ambient Ca2+ concentration. This triggers a sharp cytosolic Ca2+ increase through both Ca2+ release and Ca2+ influx. The change in cytosolic Ca2+ is transduced finally into inhibition of bone resorption. It has been shown that a type 2 ryanodine receptor isoform, expressed uniquely in the plasma membrane, functions as a Ca2+ influx channel and possibly as a Ca2+ sensor. Ryanodine receptors are ordinarily Ca2+ release channels that have a microsomal membrane location in a wide variety of eukaryotic cells, including the osteoclasts. However, only recently has it become obvious that ryanodine receptors are also expressed in osteoclast nuclear membranes, at which site they probably gate nucleoplasmic Ca2+ influx. Nucleoplasmic Ca2+ in turn regulates key nuclear processes, including gene expression and apoptosis. Here, we review the potential mechanisms underlying the recognition, movement, and effects of Ca2+ in the osteoclast. We will also speculate on the general biological significance of the unique processes used by the osteoclast to handle high Ca2+ loads during bone resorption.

Emerging insights into the role of calcium ions in osteoclast regulation

Osteoclasts are exposed to unusually high, millimolar, Ca2+ concentrations and can "sense" changes in their ambient Ca2+ concentration during resorption. This results in a sharp cystolic Ca2+ increase through both Ca2+ release and Ca2+ influx. The rise in cystolic Ca2+ is transduced finally into an inhibition of bone resorption. We have shown that a type 2 ryanodine receptor isoform, expressed uniquely in the osteoblast plasma membrane, functions as a Ca2+ influx channel, and possibly as a Ca2+ sensor. Ryanodine receptors are ordinarily microsomal membrane Ca2+ release channels. They have only recently been shown to be expressed a other sites, including nuclear membranes. At the latter site, ryanodine receptors gate nucleoplasmic Ca2+ influx. Nucleoplasmic Ca2+, in turn, regulates key nuclear processes, including gene expression and apoptosis. Here, we review potential mechanisms underlying the recognition, movement, and actions of Ca2+ in the osteoclast.

Mode of action of interleukin-6 on mature osteoclasts. Novel interactions with extracellular Ca2+ sensing in the regulation of osteoclastic bone resorption

We describe a physiologically significant mechanism through which interleukin-6 (IL-6) and a rising ambient Ca2+ interact to regulate osteoclastic bone resorption. VOXEL-based confocal microscopy of nonpermeabilized osteoclasts incubated with anti- IL-6 receptor antibodies revealed intense, strictly peripheral plasma membrane fluorescence. IL-6 receptor expression in single osteoclasts was confirmed by in situ reverse transcriptase PCR histochemistry. IL-6 (5 ng/l to 10 microg/l), but not IL-11 (10 and 100 microg/l), reversed the inhibition of osteoclastic bone resorption induced by high extracellular Ca2+ (15 mM). The IL-6 effect was abrogated by excess soluble IL-6 receptor (500 microg/l). Additionally, IL-6 (5 pg/l to 10 microg/l) inhibited cytosolic Ca2+ signals triggered by high Ca2+ or Ni2+. In separate experiments, osteoclasts incubated in 10 mM Ca2+ or on bone released more IL-6 than those in 1.25 mM Ca2+. Furthermore, IL-6 mRNA histostaining was more intense in osteoclasts in 10 or 20 mM Ca2+ than cells in 1.25 mM Ca2+. Similarly, IL-6 receptor mRNA histostaining was increased in osteoclasts incubated in 5 or 10 mM Ca2+. Thus, while high Ca2+ enhances IL-6 secretion, the released IL-6 attenuates Ca2+ sensing and reverses inhibition of resorption by Ca2+. Such an autocrine-paracrine loop may sustain osteoclastic activity in the face of an inhibitory Ca2+ level generated locally during resorption.