ATP and UTP at low concentrations strongly inhibit bone formation by osteoblasts:: A novel role for the P2Y2 receptor in bone remodeling

There is increasing evidence that extracellular nucleotides act on bone cells via multiple P2 receptors. The naturally-occurring ligand ATP is a potent agonist at all receptor subtypes, whereas ADP and UTP only act at specific receptor subtypes. We have reported that the formation and resorptive activity of rodent osteoclasts are stimulated powerfully by both extracellular ATP and its first degradation product, ADP, the latter acting at nanomolar concentrations, probably via the P2Y(1) receptor subtype. In the present study, we investigated the actions of ATP, ADP, adenosine, and UTP on osteoblastic function. In 16-21 day cultures of primary rat calvarial osteoblasts, ADP and the selective P2Y(1) agonist 2-methylthioADP were without effect on bone nodule formation at concentrations between 1 and 125 muM, as was adenosine. However, UTP, a P2Y(2) and P2Y(4) receptor agonist, known to be without effect on osteoclast function, strongly inhibited bone nodule formation at concentrations greater than or equal to 1 muM. ATP was inhibitory at greater than or equal to 10 muM. Rat osteoblasts express P2Y(2), but not P2Y(4) receptor mRNA, as determined by in situ hybridization. Thus, the low-dose effects of extracellular nucleotides on bond formation and bone resorption appear to be mediated via different P2Y receptor subtypes: ADP, signalling through the P2Y(1) receptor on both osteoclasts and osteoblasts, is a powerful stimulator of osteoclast formation and activity, whereas UTP, signalling via the P2Y(2) receptor on osteoblasts, blocks bone formation by osteoblasts. ATP, the 'universal' agonist, can simultaneously stimulate resorption and inhibit bone formation. These findings suggest that extracellular nucleotides could function locally as important negative modulators of bone metabolism, perhaps contributing to bone loss in a number of pathological states.