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

Abstract

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. 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 μM). 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. Over the same concentration range at which rat osteoclast activation occurred (0.2‐2 μM), 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 μM) reduced or blocked osteoclast formation. Adenosine, a P1 receptor agonist, was without effect on either osteoclast activation or formation. 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.

Hypoxia is a major stimulator of osteoclast formation and bone resorption

“…Prostaglandins also stimulate osteoclast formation (Collins and Chambers, 1992;Lader and Flanagan, 1998), and commonly mediate hypoxic responses in other cell types, such as macrophages (Lewis et al, 1999). Secondly, hypoxia stimulates purine nucleotide release from endothelial cells (Bodin and Burnstock, 1995), and we have previously shown that ATP and ADP are powerful osteolytic agents, acting through P2 receptors on bone cells (Morrison et al, 1998;Hoebertz et al, 2000Hoebertz et al, , 2001; this mechanism could account for some of the resorptive action of hypoxia in intact bone but it remains to be investigated whether nucleotide release from other cell types in bone might also be enhanced by low PO 2 . Thirdly, one of the major effects of hypoxia on cells is to stimulate the production of potent angiogenic factors such as VEGF, tumor necrosis factor-a, and fibroblast growth factors (Lewis et al, 1999); these factors are also stimulators of the formation and/or function of osteoclasts (Simmons and Raisz, 1991;Nakagawa et al, 1999Nakagawa et al, , 2000Suda et al, 2001).…”

Purinergic signalling in the musculoskeletal system

“…ATP, acting via P2Y receptors, was shown to inhibit non-quantal ACh release at the neuromuscular junction of mouse [43,44]. Later, it was identified that activation of the P2Y 12 and P2Y 13 receptors, as well as adenosine A 1 receptors, inhibited ACh release from mammalian motor nerve terminals [45]. The role of P2Y receptors ATP is released during neurotransmission and, after breakdown to adenosine, acts on presynaptic P1 (A 1 ) receptors to inhibit release of ACh.…”

“…Originally, this effect was proposed to be mediated via the P2Y 2 receptor; however, in a follow-up study, UTP failed to stimulate resorption suggesting this was not the case [259]. Subsequently, ATP was found to stimulate the formation and activity of rodent osteoclasts, effects which were inhibited by apyrase or suramin [12]. These stimulatory actions on resorption were further increased when osteoclasts were first activated by culture in acidified medium [12].…”

“…Subsequently, ATP was found to stimulate the formation and activity of rodent osteoclasts, effects which were inhibited by apyrase or suramin [12]. These stimulatory actions on resorption were further increased when osteoclasts were first activated by culture in acidified medium [12]. Since the P2X2 receptor is the only P2 receptor subtype that requires extracellular acidification to show its full sensitivity to ATP [260], it was suggested that these pro-resorptive effects could involve the P2X2 receptor.…”

“…Initial studies demonstrated that extracellular nucleotides could induce increases in intracellular Ca 2+ in bone cells [7][8][9][10]. Later, it was shown that extracellular ATP could inhibit bone formation whilst promoting bone resorption [11,12]. There is now abundant evidence to show that extracellular nucleotides play a key role in modulating the differentiation, function and survival of bone cells.…”