# Prostaglandin EP1 receptor

Prostaglandin E$_2$ receptor 1 (EP$_1$) is a 42kDa prostaglandin receptor encoded by the PTGER1 gene. EP$_1$ is one of four identified EP receptors, EP$_1$, EP$_2$, EP$_3$, and EP$_4$ which bind with and mediate cellular responses principally to prostaglandin E$_2$) (PGE$_2$) and also but generally with lesser affinity and responsiveness to certain other prostanoids (see Prostaglandin receptors). Animal model studies have implicated EP$_1$ in various physiological and pathological responses. However, key differences in the distribution of EP$_1$ between these test animals and humans as well as other complicating issues make it difficult to establish the function(s) of this receptor in human health and disease.

## Gene

The PTGER$_1$ gene is located on human chromosome 19 at position p13.12 (i.e. 19p13.12), contains 2 introns and 3 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

Studies in mice, rats, and guinea pigs have found EP$_1$ Messenger RNA and protein to be expressed in the papillary collecting ducts of the kidney, in the kidney, lung, stomach, thalamus, and in the dorsal root ganglia neurons as well as several central nervous system sites. However, the expression of EP$_1$ In humans, its expression appears to be more limited: EP$_1$ receptors have been detected in human mast cells, pulmonary veins, keratinocytes, myometrium, and colon smooth muscle.

## Ligands
### Activating ligands

The following standard prostaglandins have the following relative potencies in binding to and activating EP$_1$: PGE$_2$≥PGE1>PGF2alpha>PGD2. The receptor binding affinity Dissociation constant K$_d$ (i.e. ligand concentration needed to bind with 50% of available EP$_1$ receptors) is ~20 nM and that of PGE1 ~40 for the mouse receptor and ~25 nM for PGE2 with the human receptor.
Because PGE$_2$ activates multiple prostanoid receptors and has a short half-life in vivo due to its rapidly metabolism in cells by omega oxidation and beta oxidation], metabolically resistant EP$_1$-selective activators are useful for the study of EP$_1$'s function and could be clinically useful for the treatment of certain diseases. Only one such agonist that is highly selective in stimulating EP$_1$ has been synthesized and identified, ONO-D1-OO4. This compound has a K$_i$ inhibitory binding value (see Biochemistry#Receptor/ligand binding affinity) of 150 nM compared to that of 25 nM for PGE$_2$ and is therefore ~5 times weaker than PGE$_2$.

### Inhibiting ligands

SC51322 (K$_i$=13.8 nM), GW-848687 (K$_i$=8.6 nM), ONO-8711, SC-19220, SC-51089, and several other synthetic compounds given in next cited reference are selective competitive antagonists for EP$_1$ that have been used for studies in animal models of human diseases. Carbacylin, 17-phenyltrinor PGE$_1$, and several other tested compounds are dual EP$_1$/EP$_3$ antagonists (most marketed prostanoid receptor antagonists exhibit poor receptor selectivity).

## Mechanism of cell activation

When initially bound to PGE$_2$ or other stimulating ligand, EP$_1$ mobilizes G proteins containing the Gq alpha subunit (Gαq/11)-G beta-gamma complex. These two subunits in turn stimulate the Phosphoinositide 3-kinase pathway that raises cellular cytosolic Ca$^{2+}$ levels thereby regulating Ca$^{2+}$-sensitive cell signal pathways which include, among several others, those that promote the activation of certain protein kinase C isoforms. Since, this rise in cytosolic Ca$^{2+}$ can also contract muscle cells, EP$_1$ has been classified as a contractile type of prostanoid receptor. The activation of protein kinases C feeds back to phosphorylate and thereby desensitizes the activated EP$_1$ receptor (see homologous desensitization but may also desensitize other types of prostanoid and non-prostanoid receptors (see heterologous desensitization). These desensitizations limit further EP$_1$ receptor activation within the cell. Concurrently with the mobilization of these pathways, ligand-activated EP$_1$ stimulates ERK, p38 mitogen-activated protein kinases, and CREB pathways that lead to cellular functional responses.

## Function

Studies using animals genetically engineered to lack EP$_1$ and supplemented by studies using treatment with EP$_1$ receptor antagonists and agonists indicate that this receptor serves several functions. 1) It mediates hyperalgesia due to EP1$_1$ receptors located in the central nervous system but suppresses pain perception due to E$_1$ located on dorsal root ganglia neurons in rats. Thus, PGE$_2$ causes increased pain perception when administered into the central nervous system but inhibits pain perception when administered systemically; 2) It promotes colon cancer development in Azoxymethane-induced and APC gene knockout mice. 3) It promotes hypertension in diabetic mice and spontaneously hypertensive rats. 4) It suppresses stress-induced impulsive behavior and social dysfunction in mice by suppressing the activation of Dopamine receptor D1 and Dopamine receptor D2 signaling. 5) It enhances the differentiation of uncommitted T cell lymphocytes to the Th1 cell phenotype and may thereby favor the development of inflammatory rather than allergic responses to immune stimulation in rodents. Studies with human cells indicate that EP$_1$ serves a similar function on T cells. 6) It may reduce expression of Sodium-glucose transport proteins in the apical membrane or cells of the intestinal mucosa in rodents. 7) It may be differentially involved in etiology of acute brain injuries. Pharmacological inhibition or genetic deletion of EP$_1$ receptor produce either beneficial or deleterious effects in rodent models of neurological disorders such as ischemic stroke, epileptic seizure, surgically induced brain injury and traumatic brain injury.

## Clinical studies

EP1 receptor antagonists have been studied clinically primarily to treat hyperalgesia. Numerous EP antagonists have been developed including SC51332, GW-848687X, a benzofuran-containing drug that have had some efficacy in treating various hyperalgesic syndromes in animal models. None have as yet been reported to be useful in humans.