Expression, regulation, and function of P2X4 purinergic receptor in human cervical epithelial cells

Micromolar concentrations of ATP stimulate biphasic change in transepithelial conductance across CaSki cultures on filters, an acute transient increase (phase I response; triggered by P2Y(2) receptor and mediated by calcium mobilization-dependent cell volume decrease) followed by a slower decrease in permeability (phase II response). Phase II response is mediated by augmented calcium influx and protein kinase C-dependent increase in tight junctional resistance. The objective of the study was to determine the role of P2X(4) receptor as a mediator of phase II response. Human cervical epithelial cells express P2X(4) receptor mRNA (1.4-, 2.2-, and 4.4-kb isoforms by Northern blot analysis) and P2X(4) protein. Depletion of vitamin A reversibly downregulated P2X(4) receptor mRNA and protein and ATP-induced calcium influx. Depletion of vitamin A abrogated phase II response, and the effect could be partially reversed only with retinoic acid receptor (RAR)-selective retinoids but not retinoid X receptor (RXR) agonists. Depletion of vitamin A also abrogated protein kinase C increase in tight junctional resistance, and the effect could not be reversed with retinoids. Depletion of vitamin A also abrogated phase I increase in permeability and reversibly downregulated P2Y(2) receptor mRNA and ATP-induced calcium mobilization. However, in contrast to phase II response, both RAR and RXR agonists could fully reverse those effects. These results suggest that phase II response is mediated by a P2X(4) receptor mechanism.