Activation of P2X receptors induces apoptosis in human retinal pigment epithelium.

Extracellular adenosine triphosphate (ATP) acts as a key signaling molecule in numerous cellular processes and is viewed as an endogenous danger signal released in large quantities by cells after inflammation, oxidative stress, and cell injury.1,2 It activates a class of cell-surface nucleotide receptors termed P2 receptors that are further categorized into P2Y receptors and P2X receptors.3 P2 receptors are widely expressed in excitable and nonexcitable cells, where they play important functions.4–8 P2Y receptor expression and function have been reported in human, rat, bovine, and rabbit retinal pigment epithelium (RPE).9–18 Little is known of P2X receptors in the RPE. Ryan et al.13 suggested that in addition to P2Y receptors, cultured rat RPE cells exhibited functional P2X receptors. Dutot et al.19 showed that ATP and a selective P2X7 agonist 2′,3′-O-(4-benzoylbenzoyl)-ATP (BzATP) stimulated YO-PRO-1 dye uptake and confocal immunofluorescence microscopy detected P2X7 receptor protein in a human RPE cell line, ARPE-19 cells. Among seven P2X receptors, the P2X7 receptor is unique because it plays a critical role in oxidative stress, cell death, and inflammation as well as in several diseases such as Alzheimer's disease and kidney diseases.20,21 However, the role of this receptor in the RPE is unknown. Because oxidative stress, cell death, and inflammation are implicated in age-related macular degeneration (AMD) and apoptotic RPE death underlies AMD,22,23 we hypothesized that ATP may induce RPE apoptosis by activation of the P2X7 receptor. By combining molecular, functional, and pharmacologic approaches, we show that the P2X7 receptor is expressed in native and cultured human RPE and that activation of the P2X7 receptor induces both Ca2+ signaling and apoptosis in RPE cells.