Prostaglandin F receptor (FP) is a receptor belonging to the prostaglandin (PG) group of receptors. FP binds to and mediates the biological actions of prostaglandin F2α (PGF2α). It is encoded in humans by the PTGFR gene.
Gene The PTGFR gene is located on human chromosome 1 at position p31.1 (i.e. 1p31.1), contains 7 exons, and codes for a G protein coupled receptor (GPCR) of the rhodopsin-like receptor family, Subfamily A14 (see rhodopsin-like receptors#Subfamily A14). PTGFR is expressed as two alternatively spliced transcript variants encoding different isoforms, FPA and FPB, which have different C-terminal lengths. MicroRNA miR-590-3p binds to the Three prime untranslated region of the FP gene to repress its translation. miR-590-3p thus appears to be a negative regulator of FP expression in various cell types.
Expression In humans, FP mRNA and/or protein is highly expressed in the uterine myometrium; throughout the eye (endothelium and smooth muscle cells of blood vessels of the iris), ciliary body and choroid plexus; ciliary muscle (circular muscle, collagenous connective tissues; sclera; and ovarian (follicles and corpus luteum). Studies in mice indicate that FP mRNA and/or protein is expressed in diverse tissues including the kidney (distal tubules), uterus, and ovary (Luteal cells of corpus luteum.
Ligands
Activating ligands The FP receptor is the least selective of the prostenoid receptors in that it is responsive to PGD2 and to a lesser extent PGE2 at concentrations close to those of PGF2α. Standard prostanoids have the following relative efficacies as receptor ligands in binding to and activating FP: PGF2α>PGD2>PGE2>PGI2=TXA2. In typical binding studies, PGF2α has one-half maximal binding and cell stimulating actions at ~1 nanomolar whereas PGD2 and PGE2 are ~5- to 10-fold and 10-100-fold weaker than this. The synthetic analogs that like PGF2α act as selective receptor agonists of FP viz., cloprostenol, flupostenol, latanoprost, and tafluprost (acid form) have FP binding affinities and stimulating potencies similar to PGF2α while others as enprostil, sulprostone, U46619, carbacyclin, and iloprost are considerably weaker FP agonists. Fluprostenol is a widely used clinically as a selective FP receptor agonist; latanoprost is a suitable substitute.
Inhibiting ligands Currently, there are no selective receptor antagonists for FP.
Mechanism of cell activation FP is classified as a contractile type of prostenoid receptor based on its ability, upon activation, to contract certain smooth muscle preparations and smooth muscle-containing tissues such as those of the uterus. When bound to PGF2α or other of its agonists, FP mobilizes primarily G proteins containing the Gq alpha subunit bound to of the Gq-Gβγ complex(i.e. Gqβγ). Gqβγ then dissociate into its Gq and Gβγ components which act to regulate cell signaling pathways. In particular, Gq stimulates cell signal pathways involving a) phospholipase C/IP3/cell Ca2+ mobilization/diacylglycerol/protein kinase Cs; calmodulin-modulated myosin light chain kinase; RAF/MEK/Mitogen-activated protein kinases; PKC/Ca2+/Calcineurin/Nuclear factor of activated T-cells; and the EGF cellular receptor. In certain cells, activation of FP also stimulates G12/G13-Gβγ G proteins to activate the Rho family of GTPases signaling proteins and Gi-Gβγ G proteins to activateRaf/MEK/mitogen-activated kinase pathways.
Functions Studies using animals genetically engineered to lack FP and examining the actions of EP4 receptor agonists in animals as well as animal and human tissues indicate that this receptor serves various functions. It has been regarded as the most successful therapeutic target among the 9 prostanoid receptors.
Eye Animal and human studies have found that the stimulation of FP receptors located on Ciliary muscle and trabecular meshwork cells of the eye widens the drainage channels (termed the uveoscleral pathway) that they form. This increases the outflow of aqueous humor from the anterior chamber of the eye through Schlemm's canal to outside of the eyeball. The increase in aqueous humor outflow triggered by FP receptor activation reduces Intraocular pressure and underlies the widespread usage of FP receptor agonists to treat glaucoma. László Z. Bitó is credited with making critical studies to define this intraocular pressure-relieving pathway. Three FP receptor agonists are approved for clinical use in the USA viz., travoprost, latanoprost, and bimatoprost, and two additional agonists are prescribed in Europe and Asia viz., unoprostone and tafluprost.
Hair growth Since FP receptors are expresses in human dermal papillae and the use of FP agonists to treat glaucoma has as a side-effect an increase in eyelash growth, it has been suggested that FP agonists may be useful for treating baldness. This is supported by studies in the stump-tailed Macaque primate model of androgen-induced scalp alopecia which have found that the FP agonist, latanoprost, promotes scalp hair growth. These studies have not yet been translated into baldness therapy in humans.
Reproduction FP receptor activation contributes to the regression of the corpus luteum and thereby the estrous cycle in many species of farm animals. However, it does not make these contributions in mice and its contribution to these functions in humans is controversial. The receptor has been in use as a target for decades to regulate the estrous cycle as well as to induce labor in pregnant farm animals FP gene knockout in female mice blocks parturition. That is, these FP-/- mice fail to enter labor even if induced by oxytocin due to a failure in copus luteum regression and consequential failure to stop secreting progesterone (declining progesterone levels trigger labor). Studies with monkey and human tissues allow that FP receptors may have a similar function in humans.
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