Prostaglandin EP3 receptor

Prostaglandin EP₃ receptor (EP₃, 53kDa), is a prostaglandin receptor for prostaglandin E₂ (PGE₂) encoded by the human gene PTGER3; it is one of four identified EP receptors, the others being EP₁, EP₂, and EP₄, 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). EP has been implicated in various physiological and pathological responses.

Gene

The PTGER3 gene is located on human chromosome 1 at position p31.1 (i.e. 1p31.1), contains 10 exons, and codes for a G protein coupled receptor (GPCR) of the rhodopsin-like receptor family, Subfamily A14 (see rhodopsin-like receptors#Subfamily A14). PTGER3 codes for at least 8 different isoforms in humans, i.e. PTGER3-1 to PGGER3-8 (i.e., EP₃-1, EP₃-2, EP₃-3, EP₃-4, EP₃-5, EP₃-6, EP₃-7, and EP₃-8), while Ptger3 codes for at least 3 isoforms in mice, Ptger1-Ptger3 (i.e. Ep₃-α, Ep₃-β, and Ep₃-γ). These isoforms are variants made by Alternative splicing conducted at the 5'-end of DNA to form proteins that vary at or near their C-terminus. Since these isoforms different in their tissue expressions as well as the signaling pathways which they activate, they may vary in the functions that they perform. Further studies are needed to examine functional differences among these isoforms.

Expression

EP₃ is widely distributed in humans. Its protein and/or mRNA is expressed in kidney (i.e. glomeruli, Tamm-Horsfall protein negative late distal convoluted tubules, connecting segments, cortical and medullary collecting ducts, media and endothelial cells of arteries and arterioles); stomach (vascular smooth muscle and gastric fundus mucosal cells); thalamus (anterior, ventromedial, laterodorsal, paraventricular and central medial nuclei); intestinal mucosal epithelia at the apex of crypts; myometrium (stromal cells, endothelial cells, and, in pregnancy, placenta, chorion, and amnion); mouth gingival fibroblasts; and eye (corneal endothelium and keratocytes, trabecular cells, ciliary epithelium, and conjunctival and iridal stroma cells, and retinal Müller cells).

Ligands

Activating ligands

Standard prostanoids have the following relative efficacies in binding to and activating EP₃: PGE₂>PGF2α=PGI2>PGD2=TXA2. Prostglandin E₁ (PGE₁), which has one less double bond than PGE₂, has the same binding affinity and potency for EP₃ as PGE₂. PGE₂ has extreme high affinity (dissociation constant Kd=0.3 nM) for EP₃. Several synthetic compounds, e.g. sulprostone, SC-46275, MB-28767, and ONO-AE-248, bind to and stimulate with high potency EP₃ but unlike PGE₂ have the advantage of being highly 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 stomach ulcers in humans.

Inhibiting ligands

Numerous synthetic compounds have been found to be highly selective in binding to but not stimulating EP₃. These Receptor antagonist DG-O41, L798,106, and ONO-AE3-240, block EP₃ from responding to PGE₂ or other agonists of this receptor, including Sulprostone, ONO-AE-248 and TEI-3356. They are in development primarily as anti-thrombotics, i.e. drugs to treat pathological blood clotting in humans.

Mechanism of cell activation

EP₃ is classified as an inhibitory type of prostanoid receptor based on its ability, upon activation, to inhibit the activation of adenylyl cyclase stimulated by relaxant types of prostanoid receptors viz., prostaglandin DP, E2, and E4 receptors (see Prostaglandin receptors). When initially bound to PGE₂ or other of its agonists, it mobilizes G proteins containing various types of G proteins, depending upon the particular EP₃ isoform: EP_3α and EP_3β isoforms activate Gi alpha subunit (i.e. Gα_i)-G beta-gamma complexes (i.e. Gα_i)-G_{βγ}) complexes) as well as Gα₁₂-G_{βγ} complexes while the EP_3γ isoform activates in addition to and the Gα_i- G_{βγ} complexes Gα_i- G_{βγ} complexes. (G protein linkages for the other EP₃ isoforms have not been defined.) In consequence, complexes dissociate into Gα_i, Gα₁₂, G_s and G_{βγ} components which proceed to activate cell signaling pathways that lead functional responses viz., pathways that activate phospholipase C to convert cellular phospholipids to diacylglycerol which promotes the activation of certain isoforms of protein kinase C, pathways that elevated cellular cytosolic Ca²⁺ which thereby regulate Ca²⁺-sensitive cell signaling molecules, and pathways that inhibit adenylyl cyclase which thereby lowers cellular levels of cyclic adenosine monophosphate (cAMP) to reduce the activity of cAMP-dependent signaling molecules.

Functions

Studies using animals genetically engineered to lack EP₃ and supplemented by studies examining the actions of EP₃ receptor antagonists and agonists in animals as well as animal and human tissues indicate that this receptor serves various functions. However, an EP₃ receptor function found in these studies does not necessarily indicate that in does do in humans. For example, EP₃ receptor activation promotes duodenal secretion in mice; this function is mediated by EP₄ receptor activation in humans. EP receptor functions can vary with species and most of the functional studies cited here have not translated their animal and tissue models to humans.

Digestive system

The secretion of (bicarbonate anion) from Brunner's glands of the duodenum serves to neutralize the highly acidified digestive products released from the stomach and thereby prevents ulcerative damage to the small intestine. Activation of EP₃ and EP₄ receptors in mice stimulates this secretion but in humans activation of EP₄, not EP₃, appears responsible for this secretion. These two prostanoid receptors also stimulate intestinal mucous secretion, a function which may also act to reduce acidic damage to the duodenum.

Fever

EP₃-deficient mice as well as mice selectively deleted of EP₃ expression in the brain's median preoptic nucleus fail to develop fever in response to endotoxins (i.e. bacteria-derived lipopolysaccharide) or the host-derived regulator of body temperature, IL-1β. The ability of endotoxins and IL-1β but not that of PGE₂ to trigger fever is blocked by inhibitors of nitric oxide and PG₂. EP₃-deficient mice exhibit normal febrile responses to stress, interleukin-8, and macrophage inflammatory protein-1beta (MIP-1β). It is suggested that these findings indicate that a) activation of the EP₃ receptor suppresses the inhibitory tone that the preoptic hypothalamus has on thermogenic effector cells in the brain; b) endotoxin and IL-1β simulate the production of nitric oxide which in turn causes the production of PGE₂ and thereby the EP₃-dependent fever-producing; c) other factors such as stress, interleukin 8, and MIP-1β trigger fever independently of EP₃; and d) inhibition of the PGE₂-EP₃ pathway underlies the ability of aspirin and other Nonsteroidal anti-inflammatory drugs to reduce fever caused by inflammation in animals and, possibly, humans.

Allergy

In a mouse model of ovalbumin-induced asthma, a selective EP₃ agonist reduced airway cellularity, mucus, and bronchoconstriction responses to methacholine. In this model, EP₃-deficient mice, upon ovalbumin challenge, exhibited worsened allergic inflammation as measured by increased airway eosinophils, neutrophils, lymphocytes, and pro-allergic cytokines (i.e. interleukin 4, interleukin 5, and interleukin 13) as compared to wild type mice. EP₃ receptor-deficient mice and/or wild type mice treated with an EP₃ receptor agonist are similarly protected from allergic responses in models of allergic conjunctivitis and contact hypersensitivity. Thus, EP₃ appears to serve an important role in reducing allergic reactivity at least in mice.

Cough

Studies with mice, guinea pig, and human tissues and in guinea pigs indicate that PGE₂ operates through EP₃ to trigger cough responses. Its mechanism of action involves activation and/or sensitization of TRPV1 (as well as TRPA1) receptors, presumably by an indirect mechanism. Genetic polymorphism in the EP₃ receptor (rs11209716), has been associated with ACE inhibitor-induce cough in humans. The use of EP₃ receptor antagonists may warrant study for the treatment of chronic cough in humans.

Blood pressure

Activation of EP₃ receptors contracts vascular beds including rat mesentery artery, rat tail artery, guinea-pig aorta, rodent and human pulmonary artery, and murine renal and brain vasculature. Mice depleted of EP₃ are partially protected from brain injury consequential to experimentally induced cerebral ischemia. Furthermore, rodent studies indicate that agonist-induced activation of EP₃ in the brain by intra-cerebroventricular injection of PGE₂ or selective EP₃ agonist cause hypertension; a highly selective EP₃ receptor antagonist blocked this PGE2-induced response. These studies, which examine a sympatho-excitatory response (i.e. responses wherein brain excitation such as stroke raises blood pressure) suggest that certain hypertension responses in humans are mediated, at least in part, by EP₃.

Vascular permeability

Model studies indicate that PG₂ (but not specific antigens or IgE cross-linkage) stimulates mouse and human mast cells to release histamine by an EP₃-dependent mechanism. Furthermore, EP₃-deficient mice fail to develop increased capillary permeability and tissue swelling in response to EP₃ receptor agonists and the metabolic precursor to PGE₂, arachidonic acid. It is suggested, based on these and other less direct studies, that PGE₂-EP₃ signaling may be responsible for the skin swelling and edema provoked by topical 5-aminolaevulinic acid photodynamic therapy, contact with chemical irritants, infection with pathogens, and various skin disorders in humans.

Blood clotting

Activation of EP₃ receptors on the blood platelets of mice, monkeys, and humans enhances their aggregation, degranulation, and blood clot-promoting responsiveness to a wide array of physiological (e.g. thrombin) and pathological (e.g. atheromatous plaques. (In contrast, activation of either the EP₂ or EP₃ receptor inhibits platelet activation) Inhibition of EP₃ with the selective EP₃ receptor antagonist, DG-041, has been shown to prevent blood clotting but not to alter hemostasis or blood loss in mice and in inhibit platelet activation responses in human whole blood while not prolonging bleeding times when given to human volunteers. The drug has been proposed to be of potential clinical use for the prevention of blood clotting while causing little or no bleeding tendencies.

Pain

EP₃ deficient mice exhibit significant reductions in: hyperalgesic writhing (i.e. squirming) responses to acetic acid administration; acute but not chronic Herpes simplex infection-induced pain; and HIV-1 Envelope glycoprotein GP120 intrathecal injection-induced tactile allodynia. Furthermore, a selective EP₃ agonist, ONO-AE-248, induces hyperalgesia pain in wild type but not EP₃-deficient mice. While pain perception is a complex phenomenon involving multiple causes and multiple receptors including EP₂, EP₁, LTB₄, bradykinin, nerve growth factor, and other receptors, these studies indicate that EP₃ receptors contribute to the perception of at least certain types of pain in mice and may also do so in humans.

Cancer

Studies of the direct effects of EP₃ receptor activation on cancer in animal and tissue models give contradictory results suggesting that this receptor does not play an important role in Carcinogenesis. However, some studies suggest an indirect pro-carcinogenic function for the EP₃ receptor: The growth and metastasis of implanted Lewis lung carcinoma cells, a mouse lung cancer cell line, is suppressed in EP₃ receptor deficient mice. This effect was associated with a reduction in the levels of Vascular endothelial growth factor and matrix metalloproteinase-9 expression in the tumor's stroma; expression of the pro-lymphangiogenic growth factor VEGF-C and its receptor, VEGFR3; and a tumor-associated angiogenesis and lymphangiogenesis.

Clinical significance

Therapeutics

Many drugs that act on EP₃ and, often, other prostaglandin receptors, are in clinical use. A partial list of these includes:

• Misoprostol, an EP₃ and EP₄ receptor agonist, is in clinical use to prevent ulcers, to induce labor in pregnancy, medical abortion, and late miscarriage, and to prevent and treat postpartum bleeding (see Misoprostol).
• Sulprostone, relatively selective EP₃ receptor agonist with a weak ability to stimulate the EP₁ receptor is in clinical use for inducing medical abortion and ending pregnancy after fetal death (see Sulprostone).
• Iloprost activates EP₂, EP₃, and EP₄ receptors; it is in clinical use to treat diseases involving pathological constriction of blood vessels such as pulmonary hypertension, Raynauds disease, and scleroderma. Presumably, Iloprost works by stimulating EP₂, and EP₄ receptors which have vasodilation actions.

Other drugs are in various stages of clinical development or have been proposed to be tested for clinical development. A sampling of these includes:

• Enprostil, which binds to and activates primarily the EP₃ receptor, was found in a prospective multicenter randomized controlled trial conducted in Japan to significantly improve the effects of cimetidine in treating gastric ulcer. It is considered to be an efficient and safe treatment for gastric and duodenal ulcers.
• ONO-9054 (Sepetoprost), a dual an EP₃/Prostaglandin F receptor agonist, is in phase 1 clinical trial studies for the treatment of ocular hypertension and open-angle glaucoma.
• DG-041, a highly selective EP₃ antagonist, has been proposed to warrant further study as anti-thrombosis agent.
• GR 63799X, MB-28767, ONO-AE-248, and TEI-3356 are putative EP₃ receptor-selective agonists that have been proposed to warrant further study to treat and/or prevent various types of cardiovascular diseases.

Genomic studies

The single nucleotide polymorphism (SNP) in the PTGER3, rs977214 A/G variant has been associated with an increase in pre-term births in two populations of European ancestry; the SNP variant -1709T>A in PTGER3 has been associated with aspirin-exacerbated respiratory disease in a Korean population; and 6 SNP variants have been associated with development of the Steven Johnson syndrome and its more severe form, toxic epidermal necrolysis, in a Japanese population.

See also

• Eicosanoid receptor
• Prostaglandin E2 receptor 1 (EP1)
• Prostaglandin E2 receptor 2 (EP2)
• Prostaglandin E2 receptor 4 (EP4)

References

Further reading

External links