Functional characterization of the ocular prostaglandin F-2 alpha (PGF(2 alpha)) receptor - Activation by the isoprostane, 12-iso-PGF(2 alpha)

Prostaglandin F-2 alpha (PGF(2) alpha) is a product of cyclooxygenase-catalyzed metabolism of arachidonic acid. Recently, PGF(2) alpha analogs have been hypothesized to reduce intraocular pressure via relaxation of the ciliary muscle. To investigate the molecular basis of PGF(2) alpha receptor (FP) activation in the eye, we cloned the FP from a human ciliary body (hcb) cDNA library. The open reading frame of the hcb-FP cDNA was identical to the uterine FP cDNA, The hcb-FP appeared to be predominantly membrane-localized, as visualized by an FP-specific peptide antibody, and coupled to inositol phosphate formation when stably expressed in HEK 293 cells. Interestingly, the hcb-FP could also be activated by the F-2 isoprostane, 12-iso-PGF(2) alpha, in addition to its cognate ligand, PGF(2) alpha. 12 iso-PGF(2) alpha was less potent (EC50 = 5 mu M) than PGF(2) alpha (EC50 = 10 nM) in generating inositol phosphates via the hcb-FP in HEK 293 cells. Both ligands also stimulated mitogenesis in NIH 3T3 cells. Although 12-iso-PGF(2) alpha caused a dose-dependent activation of the FP, it failed to activate the recombinant human prostacyclin receptor and caused only minimal activation of the thromboxane receptor isoforms stably expressed in HEK 293 cells. Four additional F-2 isoprostanes, 8-iso-PGF(2) alpha, IPF2 alpha,-I, IPF2 alpha-III, and 9 beta,11 beta-PGF(2), caused trivial, or no, activation of the FP. Consistent with these observations, only PGF(2) alpha and 12-iso-PGF(2) alpha caused rapid homologous desensitization of FP and also exhibited cross-desensitization, with PGF(2) alpha resulting in a maximum of similar to 60% desensitization. The human FP may thus be activated specifically, by the free radical-catalyzed F-2 isoprostane, 12-iso-PGF(2) alpha, in addition to the cyclooxygenase product, PGF(2) alpha. Incidental receptor activation by isoprostanes may complement the actions of PGF(2) alpha in clinical syndromes where oxidant stress and augmented prostaglandin biosynthesis coincide.