Purinergic P2X₇ receptor regulates lung surfactant secretion in a paracrine manner

Alveolar epithelium is composed of alveolar epithelial cells of type I (AEC I) and type II (AEC II). AEC II secrete lung surfactant by means of exocytosis. P2X₇ receptor (P2X₇R), a P2 purinergic receptor, has been implicated in the regulation of synaptic transmission and inflammation. Here, we report that P2X₇R, which is expressed in AEC I but not AEC II, is a novel mediator for the paracrine regulation of surfactant secretion in AEC II. In primary co-cultures of AEC I and AEC II benzoyl ATP (BzATP; an agonist of P2X₇R) increased surfactant secretion, which was blocked by the P2X₇R antagonist Brilliant Blue G. This effect was observed in AEC II cocultured with human embryonic kidney HEK-293 cells stably expressing rat P2X₇R, but not when co-cultured with AEC I in which P2X₇R was knocked down or in co-cultures of AEC I and AEC II isolated from P2X ₇R^-/- mice. BzATP-mediated secretion involved P2Y ₂ receptor signaling because it was reduced by the addition of the ATP scavengers apyrase and adenosine deaminase and the P2Y₂ receptor antagonist suramin. However, the stimulation with BzATP might also release other substances that potentially increase surfactant secretion as a greater stimulation of secretion was observed in AEC II incubated with BzATP when co-cultured with E10 or HEK-293-P2X₇R cells than with ATP alone. P2X₇R^-/- mice failed to increase surfactant secretion in response to hyperventilation, pointing to the physiological relevance of P2X₇R in maintaining surfactant homeostasis in the lung. These results suggest that the activation of P2X₇R increases surfactant secretion by releasing ATP from AEC I and subsequently stimulating P2Y ₂ receptors in AEC II.