Prostaglandin E2 receptor 4 (EP4) is aprostaglandin receptor forprostaglandin E2 (PGE2) encoded by thePTGER4gene in humans.[5] 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 otherprostanoids (seeProstaglandin receptors). EP4 has been implicated in various physiological and pathological responses in animal models and humans.[6]
The PTGER4 gene is located on human chromosome 5p13.1 at position p13.1 (i.e. 5p13.1), contains 7 exons, and codes for aG protein-coupled receptor (GPCR) of the rhodopsin-like receptor family, Subfamily A14 (seerhodopsin-like receptors#Subfamily A14).[7][8]
In humans,mRNA for EP4 has been detected bynorthern 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 byimmunochemistry in pulmonary veins; kidneyglomeruli andtunica media of kidney arteries;corpus cavernosum of the penis;carotid arteryatherosclerotic plaques;Abdominal aortaaneurysms;corneal endothelium, cornealkeratocytes,trabecular cells,ciliary epithelium,conjunctivalstromal cells, andiridalstromal cells of the eye; andgingival fibroblasts.[9][10][11]
Standardprostanoids have the following relative efficacies in binding to and activating EP4: PGE2>PGF2α=PGI2>PGD2=TXA2.Prostaglandin E1 (PGE1), which has one lessdouble 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).[12] Several synthetic compounds, e.g. 1-hydroxy-PGE1, rivenprost (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 includingulcerative colitis,Alzheimer's disease,osteoporosis, and certaincardiovascular diseases.[13]
Inhibitoryreceptor 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.[13]
EP4 is classified as a relaxant type ofprostaglandin 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.[6] When bound to PGE2 or other of its agonists, it mobilizesG proteins containing theGs alpha subunit (i.e. Gαs)-G beta-gammaes (i.e. Gβγ) complex. The complex then dissociate into its Gαs and Gβγ components which act to regulatecell signaling pathways. In particular, Gαs stimulatesadenyl cyclase to raise cellular levels ofcAMP; cAMP activatesPKA, akinase which in turn activates signaling molecules, in particular, the transcription factor,CREB. Activated CREB stimulates the expression of genes such asc-fos,somatostatin, andcorticotropin-releasing hormone that regulatecellular proliferation,cellular differentiation, cellular survival, andangiogenesis. EP4 activation of G proteins also activatePI3K/AKT/mTOR,ERK, andp38 MAPK pathways. Activation of ERK induces expression ofEGR1, a transcription factor which controls transcription of genes involved incellular differentiation andmitogenesis. EP4 also interacts with Prostaglandin E receptor 4-associated protein (EPRAP) to inhibit phosphorylation of theproteasome protein,p105, thereby suppressing a cells ability to activatenuclear factor kappa B, a transcription factor that controls genes coding for cytokines and other elements that regulate inflammation, cell growth, and cell survival (seeNF-κB#Structure). The activation of these pathways lead to variety of different types of functional responses depending on cell type, the pathways available in different cell types, and numerous other factors; EP4 activation may therefore have diverse effects on cell function depending on these factors.[6][14] In many respects, EP4 actions resemble those of another type of another relaxant prostanoid receptor,EP2 but differs from the contractile prostanoid receptors,EP1 andEP3 receptors which mobilize G proteins containing theGαq-Gβγ complex.[15][16]
Following its activation, EP4 undergoeshomologous desensitization. That is, EP4 becomes insensitive to further activation and internalizes. This effect limits the duration and extent to which EP4 can stimulate cells. Agents which activate certain isoforms ofprotein kinase C can also desensitize EP4 by a process termedheterologous desensitization.[16]
Studies using animals genetically engineered to lack EP4 and supplemented by studies examining the actions of EP4 receptor antagonists and agonists in animals as well as animal and human tissues indicate that this receptor serves various functions. However, an EP4 receptor function found in these studies does not necessarily indicate that in does so in humans since EP receptor functions can vary between species.[14]
EP4 plays a critical role in postnatal closure of theductus arteriosus as defined in mice lacking a functional gene for this receptor, i.e. EP4(-/-) mice (seeKnockout mouse). About 95% of EP4(-/-) mice die within 3 days of birth due to the pulmonary congestion and heart failure caused by a patent ductus arteriosus. The ductus operates in the fetus to shunt blood from the pulmonary artery to the proximal descending aorta thereby allowing blood from the heart's right ventricle to bypass the fetus's non-functioning lungs. The ductus must close at birth to allow blood flow into the lungs. In mice, this is accomplished by turning off the mechanism which maintains the ductus's patency. Continuous activation of EP4 by PGE2 keeps the ductus open in thefetus; at birth, however, levels of EP4 and PGE2 in the smooth muscle cells and media in mouse ductus fall. This closes the ductus thereby establishing normal post-fetal circulation of blood through the lungs. Based on studies using EPreceptor agonists andreceptor antagonists, EP2 in mice and, at least in lambs, EP3 may play minor parts in maintaining patency of the ductus.[6][17][18] These studies also appear relevant to humans:nonsteroidal anti-inflammatory drugs, particularlyindomethacin, are used to reduce prostaglandin production and thereby close the ductus in neonates, infants, and older patients withPatent ductus arteriosus; furthermore, prostaglandins or their analogs are used to keep the ductus open in neonates with congenital heart defects such asTransposition of the great arteries until corrective surgery can be performed (seeDuctus arteriosus#Disorder: Patent ductus arteriosus).[6]
To allow further studies of EP4 function, colonies obtained by cross-breeding the 5% of mice surviving EP4 deletion are used.[6]
Activation of EP4 suppresses the production ofIL-12p70 and increasesIL-23 thereby promoting development ofIL-17-producingTh17 cells, a subset of pro-inflammatoryT helper cells that serves to maintain mucosal barriers, clear mucosal surfaces of pathogens, and contribute to autoimmune and inflammatory disorders. Its activation also:a) supports the development ofRegulatory T cells (i.e. suppressor T cells that modulate the immune system to maintain tolerance to self-antigens and prevent autoimmune disease);b) stimulateDendritic cells (i.e.antigen-presenting cells located primarily in the skin and mucous membranes) to mature, migrate, and direct the early stage of immune responses;c) inhibit antibody-producingB cells from proliferating;d) suppresses the development ofAtherosclerosis plaques by promoting the death (i.e.apoptosis) of plaque-bound pro-inflammatorymacrophages;e) increases the survival of neurons in an inflammation-based model ofAlzheimer's disease;f) increases localarteriole andcapillary blood flow to cause, for example, site-specific signs of inflammation such as redness, heat, and swelling in rodent models; andg) suppresses sensoryDorsal root ganglion neurons from signaling inflammation-induced pain (i.e.allodynia andhyperalgesia) and has been used successfully to block the osteoarthritis pain in dogs.[6][15][19][13]
EP4 receptors are highly expressed in the small intestine and colon. Mice lacking this receptor or treated with a selective EP4 antagonist proved to be far more susceptible to the development of dextran sodium sulphate (DSS)-inducedcolitis and to be protected from developing the colitis by pre-treatment with EP4-selective agonists (ONO-AE1-734 and AGN205203). The DDS-inflicted lesions were associated with defective colon mucosa barrier function along with the overexpression of genes mediating inflammatory responses and under-expression of genes involved in mucosal repair and remodeling. EP4 thus appears to serve anti-inflammatory and protective functions in the colon and agonists of this receptor may be useful for treatinginflammatory bowel diseases such asulcerative colitis.[19] Activation of EP4 stimulatesduodenum epithelial cells to secrete bicarbonate (HCO3-) in mice and humans; this response neutralizes the acidic fluid flowing from the stomach thereby contributing to the process of intestinal ulcer healing. Activators of this receptor therefore may useful as anti-ulcer drugs.[14]
Studies in mice found that the PGE2-EP4 pathway inducesosteoclast (i.e. cells responsible for bone absorption) to differentiate from precursor cells and is required forIL-1beta-,Tumor necrosis factor alpha-, andbasic fibroblast growth factor-induced osteoclast formation; bone taken from EP4(-/-) mice to re-absorb bone when induced to do so and the infusion of PGE2 into mice failed to stimulate bone absorption. Furthermore, the infusion of selective EP4 agonists into mice stimulated increases in the number of bone osteoclasts andosteoblasts as well as increases in bone density. These studies indicate that the EP4 receptor mediates bone remolding in mice and, it is suggested, other animals including humans.[6]
In mice, EP4 receptor agonists reduce the acute rejection of transplanted hearts, prolong the survival of heart-transplanted animals, and reduce cardiac damage in a model ofischemic reperfusion injury but also stimulatecardiac hypertrophy accompanied by poor cardiac function. EP4 receptor-depleted mice exhibit more severe cardiac damage in experimental models ofmyocardial infarction andischemic reperfusion injury but also develop cardiac hypertrophy with poor cardiac function.[11] Cardiac specific EP4 deficiency usingSite-specific recombination by theCre recombinase method to inactivate EP4 only in cardiac muscle causes a somewhat different form of cardiac disease,dilated cardiomyopathy, that develops within 23–33 weeks after birth in mice.[6] These studies are interpreted as indicating that EP4 plays both protective and damaging roles in the heart with the protective effects of EP4 due at least in part to its ability to suppress inflammation.
EP4 receptor-depleted mice exhibit slower weight gain; reduced adiposity upon high fat diet challenge; and shortened life span. These deficiencies are associated with disrupted lipid metabolism due to impaired triglyceride clearance; this impaired triglyceride clearance may underlie the cited deficiencies.[11][20]
The EP4 receptor is over-expressed in human prostate cancer tissue and a selective EP4-receptor antagonist inhibits the growth andmetastasis of human prostate cancer cellxenografts. An EP4 receptor antagonist as well EP4Gene knockdown inhibit the in vitro proliferation and invasiveness of human breast cancer cells. And, gene knockdown of EP4 inhibit themetastasis of murine breast cancer cells in a mouse model of induced breast cancer. PGE2 stimulates the in vitro growth of humannon-small cell lung cancer while an antagonist of EP4 or EP4 gene knockdown inhibits this growth. These results indicate that the stimulation of EP4 promotes the growth of various types of cancer cells and therefore may play a role in the progression of certain types of human cancer.[13]
EP44 receptors are expressed in thecochlea of the inner ear. Pre- and post-treatment of guinea pigs with an EP4 agonist significantly attenuated threshold shifts of auditory brain stem responses and significantly reduced the loss of outer hair cells caused by prior noise exposure. These findings indicate that EP4 is involved in mechanisms for prostaglandin E(1) actions on the cochlea, and local EP4 agonist treatment may be a means for attenuating noise-induced hearing lose.[21][6]
A selective EP4 antagonists significantly reducedcorneal neovascularization in rats caused byoxygen-induced retinopathy or laser-induced choroidal neovascularization. This result suggests that EP4 activation contributes to corneal neovascularization and that EP4 antagonists may be useful for treating neovascular eye disease.[6]
Clinicaltranslational research studies using EP4 stimulators (i.e. agonists) or inhibitors (i.e. antagonists) that have been conducted or are underway include:
Single nucleotide polymorphism (SNP) A/G variant rs10440635[24] close to thePTGER4 gene on human chromosome 5 has been associated with an increased incidence ofAnkylosing spondylitis in a population recruited from the United Kingdom, Australia, and Canada. Ankylosing spondylitis is a chronic inflammatory disease involving excessive bone deposition in theVertebral column and increased expression of EP4 at vertebral column sites of involvement. Thus, excessive EP4 activation may contribute to the pathological bone remodeling and deposition found in ankylosing spondylitis and the rs10440635 variant may predispose to this disease by influencing EP4's production or expression pattern.[25][26]
The GGgenotype at -1254G>A inPTGER4 is associated with thenon-steroidal anti-inflammatory drug (NSAID)-exacerbated cutaneous disease (NECD). NECD is a non-allergichypersensitivity reaction involving the acute development ofwheals andangioedema in response to NSAID consumption in individuals with a history of chronicurticarial. The G allele at the -1254 position leads to lowerPTGER4 gene promoter function, lower levels of EP4, and presumably thereby less of the anti-inflammatory effects of EP4.[27]
SeveralPTGER4 gene variations have been associated with inflammatory bowel disease:a)Meta-analysis ofGenome-wide association studies found that SNP variant rs11742570[28] containing a C/T single-nucleotide variation inPTGER4 is associated with an increased incidence ofCrohn's disease;b) rs4495224,[29] an A/C SNP variant, and rs7720838,[30] both of which are projected to be binding sites inPTERG4 for the transcription factor,NF-κB, have been associated with Crohn's disease in three independent cohorts with the association between rs7720838 and Crohn's disease being replicated in other populations; andc) certainalleles in 5p13.1, aGene desert close toPTGER4, correlate with the expression levels of EP4 as well as with the development of Crohn's disease.[27]
The A/T SNP variant, rs4434423,[31] in the5'-untranslated region ofPTGER4 has been associated with and increase rate of primary graft dysfunction in a multicentered cohort study of graph recipients of different ethnicities.[27]
This article incorporates text from theUnited States National Library of Medicine, which is in thepublic domain.
