Modulation of monocyte signaling and pore formation in response to agonists of the nucleotide receptor P2X7

Previous reports about the nucleotide receptor P2X 7, which exhibits ion channel and pore-forming activity and is known to promote IL-1 processing, have centered largely on its role in macrophage function, whereas its participation in monocyte activity has been unclear. However, because extracellular ATP has been shown to af- fect monocytes with respect to IL-1 release, we hypothesized that the P2X 7 receptor is also present and functional in a subpopulation of blood mono- cytes. Flow cytometric analysis revealed that about 70% of monocytes isolated from normal human donors expressed the P2X 7 receptor. Activation of P2X 7 receptor-associated pore formation by the agonist BzATP resulted in a 9- to 15-fold increase in the uptake of the membrane-impermeant fluo- rescent dye YO-PRO, and this dye uptake is mark- edly inhibited by the P2X 7 receptor antagonists KN-62 and oATP. Evidence supporting the pres- ence of the functional P2X 7 receptor in monocytes also includes the observation that BzATP exposure results in a dose-dependent increase in the activa- tion of mitogen-activated protein kinases and the nuclear translocation of the transcription factor NF-B in human monocytes and in THP-1 human monocytic cells. Furthermore, treatment of mono- cytes with BzATP induced the expression of cyclo- oxygenase-2 (COX-2) and tissue factor, which are two important endpoints that have not been previ- ously shown to be regulated by nucleotide receptor action in monocytes. Together, these data indicate that a subpopulation of human monocytes express P2X 7 receptors that are functional with respect to pore formation, signal transduction, and mediator production, further supporting a key role for this nucleotide receptor in host immune responses. J. Leukoc. Biol. 72: 222-232; 2002.