FUNCTIONAL-CHARACTERIZATION OF THE OCULAR PROSTAGLANDIN-F2-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 analo gs have been hypothesized to reduce intraocular pressure via relaxatio n 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 hum an 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 b e predominantly membrane-localized, as visualized by an FP-specific pe ptide antibody, and coupled to inositol phosphate formation when stabl y 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 inosito l phosphates via the hcb-FP in HEK 293 cells. Both ligands also stimul ated mitogenesis in NIH 3T3 cells. Although 12-iso-PGF(2) alpha caused a dose-dependent activation of the FP, it failed to activate the reco mbinant human prostacyclin receptor and caused only minimal activation of the thromboxane receptor isoforms stably expressed in HEK 293 cell s. 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, ac tivation of the FP. Consistent with these observations, only PGF(2) al pha and 12-iso-PGF(2) alpha caused rapid homologous desensitization of FP and also exhibited cross-desensitization, with PGF(2) alpha result ing in a maximum of similar to 60% desensitization. The human FP may t hus be activated specifically, by the free radical-catalyzed F-2 isopr ostane, 12-iso-PGF(2) alpha, in addition to the cyclooxygenase product , PGF(2) alpha. Incidental receptor activation by isoprostanes may com plement the actions of PGF(2) alpha in clinical syndromes where oxidan t stress and augmented prostaglandin biosynthesis coincide.