Extracellular ADP is a powerful osteolytic agent: evidence for signaling through the P2Y(1) receptor on bone cells

There is increasing evidence that extracellular nucleotides act on bone cel ls via P2 receptors, This study investigated the action of ADP and 2-methyl thioADP, a potent ADP analog with selectivity for the P2Y(1) receptor, on o steoclasts, the bone-resorbing multinuclear cells. Using three different as says, we show that ADP and 2-methylthioADP at nanomolar to submicromolar le vels caused up to fourfold to sixfold increases in osteoclastic bone resorp tion, On mature rat osteoclasts, cultured for 1 day on polished dentine dis ks, peak effects on resorption pit formation were observed between 20 nM an d 2 muM of ADP, The same concentrations of ADP also stimulated osteoclast a nd resorption pit formation in 10-day mouse marrow cultures on dentine disk s. In 3-day explant cultures of mouse calvarial bones, the stimulatory effe ct of ADP on osteoclast-mediated Ca2+ release was greatest at 5-50 muM and equivalent to the maximal effects of prostaglandin E-2. The ADP effects wer e blocked in a nontoxic manner by MRS 2179, a P2Y(1) receptor antagonist. U sing in situ hybridization and immunocytochemistry, we found evidence for P 2Y(1) receptor expression on both osteoclasts and osteoblasts; thus, ADP co uld exert its actions both directly on osteoclasts and indirectly via P2Y(1 ) receptors on osteoblasts, As a major ATP degradation product, ADP is a no vel stimulator of bone resorption that could help mediate inflammatory bone loss in vivo.