{{DISPLAYTITLE:Prostaglandin EP4 receptor}} Prostaglandin E2 receptor 4 (EP4) is a prostaglandin receptor for prostaglandin E2 (PGE2) encoded by the PTGER4 gene in humans; it is one of four identified EP receptors, the others being EP1, EP2, and EP3, all of which bind with and mediate cellular responses to PGE2 and also, but generally with lesser affinity and responsiveness, certain other prostanoids (see Prostaglandin receptors). EP4 has been implicated in various physiological and pathological responses in animal models and humans. Gene The PTGER4 gene is located on human chromosome 5p13.1 at position p13.1 (i.e. 5p13.1), 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). Expression In humans, mRNA for EP4 has been detected by Northern blotting in the heart and small intestine and to lesser extents in lung, kidney, thymus, uterus, dorsal root ganglions, and brain. EP4 protein is found in humans as measured by immunochemistry in pulmonary veins; kidney glomeruli and Tunica media of kidney arteries; corpus cavernosum of the penis; carotid artery atherosclerotic plaques; Abdominal aorta aneurysms; corneal endothelium, corneal keratocytes, trabecular cells, ciliary epithelium, conjunctival stromal cells, and iridal stromal cells of the eye; and gingival fibroblasts. Ligands Activating ligands Standard prostanoids have the following relative efficacies in binding to and activating EP4: PGE2>PGF2α=PGI2>PGD2=TXA2. Prostaglandin E1 (PGE1), which has one less double bond than PGE2, has the same binding affinity and potency for EP4, both PGs having high affinity (Ki=3 nM) (http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=343). Several synthetic compounds, e.g. 1-hydroxy-PGE1, (ONO-4819), OOG-308, ONO-AE1-329, AGN205203, ONO-4819, CP-734,432m AE1-329, SC-19220, SC-51089, and EP4RAG bind to and stimulate EP4 but unlike PGE2 have the advantage of being selective for this receptor over other EP receptors and are relatively resistant to being metabolically degraded. They are in development as drugs for the potential treatment of various diseases including ulcerative colitis, Alzheimer's disease, osteoporosis, and certain cardiovascular diseases. Inhibiting ligands Inhibitory receptor antagonists for EP4, including grapiprant (CJ-023,423), ONO-AE3-208, GW627368X, AH23848, and ONO-AE2-227, are in development for possible clinical use as inhibitors of the progression of prostate, breast, colon, and lung cancers. Mechanism of cell activation EP4 is classified as a relaxant type of prostaglandin receptor based on its ability, upon activation, to relax the contraction of certain smooth muscle preparations and smooth muscle-containing tissues that have been pre-contracted by stimulation. When bound to PGE2 or other of its agonists, it mobilizes G