• osteoclast;
  • NF-κB;
  • nucleotides;
  • P2X7;


Nucleotides, released in response to mechanical and other stimuli, act on P2 receptors in osteoclasts and other cell types. In vitro studies of osteoclasts from rabbits and P2X7 receptor-deficient mice revealed that P2X7 receptors couple to activation of the key transcription factor NF-κB.

Introduction: Osteoclasts express functional P2X4 and P2X7 receptors, which are ATP-gated cation channels. Knockout (KO) of the P2X7 receptor has revealed its role in regulating bone formation and resorption, but the underlying signals are not known. The transcription factor NF-κB plays a key role in the response of osteoclasts to RANKL and other cytokines. The aim of this study was to examine whether P2X receptors on osteoclasts signal through NF-κB.

Materials and Methods: Osteoclasts were isolated from neonatal rabbits or wildtype (WT) and P2X7 receptor KO mice. Immunofluorescence was used to detect the p65 subunit of NF-κB, which, on activation, translocates from the cytosol to the nuclei. The concentration of cytosolic free Ca2+ ([Ca2+]i) was monitored in single osteoclasts loaded with fura-2.

Results: In control samples, few rabbit osteoclasts demonstrated nuclear localization of NF-κB. Benzoyl-benzoyl-ATP (BzATP, a P2X7 agonist, 300 μM) induced nuclear translocation of NF-κB after 3 h in ∼45% of rabbit osteoclasts. In contrast, a low concentration of ATP (10 μM, sufficient to activate P2X4 and P2Y2, but not P2X7 receptors) did not induce nuclear translocation of NF-κB. Because BzATP activates multiple P2 receptors, we examined responses of osteoclasts derived from WT and P2X7 receptor KO mice. Treatment with BzATP for 30 minutes increased nuclear localization of NF-κB in osteoclasts from WT but not KO mice, showing involvement of P2X7 receptors. Both ATP (10 μM) and BzATP (300 μM) caused transient elevation of [Ca2+]i, indicating that rise of calcium alone is not sufficient to activate NF-κB. Pretreatment of rabbit osteoclasts with osteoprotegerin inhibited translocation of NF-κB induced by RANKL but not by BzATP, establishing that the effects of BzATP are independent of RANKL signaling.

Conclusion: These findings show that P2X7 nucleotide receptors couple to activation of NF-κB in osteoclasts. Thus, nucleotides, released at sites of inflammation or in response to mechanical stimuli, may act through NF-κB to regulate osteoclast formation and activity.