Role of prostaglandin E2 and its receptors in chronic liver disease.

Chronic liver disease (CLD) is a major global health burden in terms of growing morbidity and mortality. Although many conditions can cause CLD, leading to cirrhosis and hepatocellular carcinoma (HCC), viral hepatitis, drug-induced liver injury (DILI), alcoholic liver disease (ALD) and non-alcoholic fatty liver disease (NAFLD) are the most common culprits. Prostaglandin E2 (PGE2), produced in the liver, is an important lipid mediator derived from the ω-6 polyunsaturated fatty acid, arachidonic acid, and plays a critical role in hepatic homeostasis. The physiological effects of PGE2 are mediated through four classes of E-type prostaglandin (EP) receptors, namely EP1, EP2, EP3 and EP4. In recent years, an increasing number of studies has been done to clarify the effects of PGE2 and EP receptors in regulating liver function and the pathogenesis of CLD to create a new potential clinical impact. In this review, we overview the biosynthesis and regulation of PGE2 and discuss the role of its synthesizing enzymes and receptors in the maintenance of normal liver function and the development and progress of CLD. We also discuss the potential of the PGE2-EP receptors system in treating CLD with various etiologies.

Computational Insights into Prostaglandin E2 Ligand Binding and Activation of G-Protein-Coupled Receptors.

G-protein coupled receptors (GPCRs) are eukaryotic integral membrane proteins that regulate signal transduction cascade pathways implicated in a variety of human diseases and are consequently of interest as drug targets. For this reason, it is of interest to investigate the way in which specific ligands bind and trigger conformational changes in the receptor during activation and how this in turn modulates intracellular signaling. In the present study, we investigate the way in which the ligand Prostaglandin E2 interacts with three GPCRs in the E-prostanoid family: EP1, EP2, and EP3. We examine information transfer pathways based on long-time scale molecular dynamics simulations using transfer entropy and betweenness centrality to measure the physical transfer of information among residues in the system. We monitor specific residues involved in binding to the ligand and investigate how the information transfer behavior of these residues changes upon ligand binding. Our results provide key insights that enable a deeper understanding of EP activation and signal transduction functioning pathways at the molecular level, as well as enabling us to make some predictions about the activation pathway for the EP1 receptor, for which little structural information is currently available. Our results should advance ongoing efforts in the development of potential therapeutics targeting these receptors.

Targeting EP2 Receptor for Drug Discovery: Strengths, Weaknesses, Opportunities, and Threats (SWOT) Analysis.

Cyclooxygenase-1 and -2 (COX1 and COX2) derived endogenous ligand prostaglandin-E2 (PGE2) triggers several physiological and pathological conditions. It mediates signaling through four G-protein coupled receptors, EP1, EP2, EP3, and EP4. Among these, EP2 is expressed throughout the body including the brain and uterus. The functional role of EP2 has been extensively studied using EP2 gene knockout mice, cellular models, and selective small molecule agonists and antagonists for this receptor. The efficacy data from in vitro and in vivo animal models indicate that EP2 receptor is a major proinflammatory mediator with deleterious functions in a variety of diseases suggesting a path forward for EP2 inhibitors as the next generation of selective anti-inflammatory and antiproliferative agents. Interestingly in certain diseases, EP2 action is beneficial; therefore, EP2 agonists seem to be clinically useful. Here, we highlight the strengths, weaknesses, opportunities, and potential threats (SWOT analysis) for targeting EP2 receptor for therapeutic development for a variety of unmet clinical needs.

Tumor-derived prostaglandin E2 programs cDC1 dysfunction to impair intratumoral orchestration of anti-cancer T cell responses.

Type 1 conventional dendritic cells (cDC1s) are critical for anti-cancer immunity. Protective anti-cancer immunity is thought to require cDC1s to sustain T cell responses within tumors, but it is poorly understood how this function is regulated and whether its subversion contributes to immune evasion. Here, we show that tumor-derived prostaglandin E2 (PGE2) programmed a dysfunctional state in intratumoral cDC1s, disabling their ability to locally orchestrate anti-cancer CD8+ T cell responses. Mechanistically, cAMP signaling downstream of the PGE2-receptors EP2 and EP4 was responsible for the programming of cDC1 dysfunction, which depended on the loss of the transcription factor IRF8. Blockade of the PGE2-EP2/EP4-cDC1 axis prevented cDC1 dysfunction in tumors, locally reinvigorated anti-cancer CD8+ T cell responses, and achieved cancer immune control. In human cDC1s, PGE2-induced dysfunction is conserved and associated with poor cancer patient prognosis. Our findings reveal a cDC1-dependent intratumoral checkpoint for anti-cancer immunity that is targeted by PGE2 for immune evasion.

Posttranscriptional regulation of the prostaglandin E receptor spliced-isoform EP3-γ and its implication in pancreatic ß-cell failure.

In Type 2 diabetes (T2D), elevated lipid levels have been suggested to contribute to insulin resistance and ß-cell dysfunction. We previously reported that the expression of the PGE2 receptor EP3 is elevated in islets of T2D individuals and is preferentially stimulated by palmitate, leading to ß-cell failure. The mouse EP3 receptor generates three isoforms by alternative splicing which differ in their C-terminal domain and are referred to as mEP3α, mEP3ß, and mEP3γ. We bring evidence that the expression of the mEP3γ isoform is elevated in islets of diabetic db/db mice and is selectively upregulated by palmitate. Specific knockdown of the mEP3γ isoform restores the expression of ß-cell-specific genes and rescues MIN6 cells from palmitate-induced dysfunction and apoptosis. This study indicates that palmitate stimulates the expression of the mEP3γ by a posttranscriptional mechanism, compared to the other spliced isoforms, and that the de novo synthesized ceramide plays an important role in FFA-induced mEP3γ expression in ß-cells. Moreover, induced levels of mEP3γ mRNA by palmitate or ceramide depend on p38 MAPK activation. Our findings suggest that mEP3γ gene expression is regulated at the posttranscriptional level and defines the EP3 signaling axis as an important pathway mediating ß-cell-impaired function and demise.

Prostaglandin E2 receptor Ptger4b regulates female-specific peptidergic neurons and female sexual receptivity in medaka.

In vertebrates, female receptivity to male courtship is highly dependent on ovarian secretion of estrogens and prostaglandins. We recently identified female-specific neurons in the medaka (Oryzias latipes) preoptic area that express Npba, a neuropeptide mediating female sexual receptivity, in response to ovarian estrogens. Here we show by transcriptomic analysis that these neurons express a multitude of neuropeptides, in addition to Npba, in an ovarian-dependent manner, and we thus termed them female-specific, sex steroid-responsive peptidergic (FeSP) neurons. Our results further revealed that FeSP neurons express a prostaglandin E2 receptor gene, ptger4b, in an ovarian estrogen-dependent manner. Behavioral and physiological examination of ptger4b-deficient female medaka found that they exhibit increased sexual receptivity while retaining normal ovarian function and that their FeSP neurons have reduced firing activity and impaired neuropeptide release. Collectively, this work provides evidence that prostaglandin E2/Ptger4b signaling mediates the estrogenic regulation of FeSP neuron activity and female sexual receptivity.

Prostaglandin E2 in neuroblastoma: Targeting synthesis or signaling?

Neuroblastoma (NB) is the most common pediatric extracranial solid tumor arising from neural crest cells of the developing sympathetic nervous system. Despite marked advances in cancer treatment, the survival rate of high-risk NB remains unsatisfactory. As a key pro-inflammatory mediator regulating tumor microenvironment, prostaglandin E2 (PGE2) promotes NB proliferation, angiogenesis, and immune evasion via acting on four G protein-coupled receptors, particularly the EP2 subtype. Recent studies have been vigorously focused on developing and evaluating compounds targeting PGE2-regulated tumor inflammation in animal models of NB. In this review, we revisit these translational efforts and examine the feasibility of pharmacological inhibition of enzymes responsible for PGE2 biosynthesis or its signaling receptors as emerging therapeutic strategies for NB. We also explore the potential downstream oncogenic pathways upon the activation of PGE2 receptors, aiming to bridge the knowledge gap between tumorigenesis and the role of elevated PGE2/EP2 signaling, which is widely observed in high-risk NBs.

Structural insights into the G protein selectivity revealed by the human EP3-Gi signaling complex.

Prostaglandin receptors have been implicated in a wide range of functions, including inflammation, immune response, reproduction, and cancer. Our group has previously determined the crystal structure of the active-like EP3 bound to its endogenous agonist, prostaglandin E2. Here, we present the single-particle cryoelectron microscopy (cryo-EM) structure of the human EP3-Gi signaling complex at a resolution of 3.4 Å. The structure reveals the binding mode of Gi to EP3 and the structural changes induced in EP3 by Gi binding. In addition, we compare the structure of the EP3-Gi complex with other subtypes of prostaglandin receptors (EP2 and EP4) bound to Gs that have been previously reported and examine the differences in amino acid composition at the receptor-G protein interface. Mutational analysis reveals that the selectivity of the G protein depends on specific amino acid residues in the second intracellular loop and TM5.

Roles of EP Receptors in the Regulation of Fluid Balance and Blood Pressure.

Prostaglandin E2 (PGE2) is an important prostanoid expressing throughout the kidney and cardiovascular system. Despite the diverse effects on fluid metabolism and blood pressure, PGE2 is implicated in sustaining volume and hemodynamics homeostasis. PGE2 works through four distinct E-prostanoid (EP) receptors which are G protein-coupled receptors. To date, pharmacological specific antagonists and agonists of all four subtypes of EP receptors and genetic targeting knockout mice for each subtype have helped in uncoupling the diverse functions of PGE2 and discriminating the respective characteristics of each receptor. In this review, we summarized the functions of individual EP receptor subtypes in the renal and blood vessels and the molecular mechanism of PGE2-induced fluid metabolism and blood pressure homeostasis.

Identification and Characterization of Human Colorectal Cancer Cluster Predominantly Expressing EP3 Prostanoid Receptor Subtype.

Colorectal cancer (CRC) is one of the common types of cancer in humans. Prostaglandin E2 (PGE2) is a well-known mediator of colorectal cancer through stimulation of four E-type prostanoid (EP) receptor subtypes: EP1, EP2, EP3, and EP4 receptors. All subtypes of EP receptors are involved in CRC promotion or malignancy. However, the characteristics of CRC that highly expresses EP receptor subtypes have not been clarified. In the present study, we classified CRC from a cancer genomic database and identified CRC clusters which highly express EP receptor subtypes. Most of these clusters predominantly expressed one subtype of EP receptor and showed different gene expression patterns. Among them, we focused on the cluster highly expressing the EP3 receptor (CL-EP3). As the result of characterization of gene expression, CL-EP3 was characterized as: epithelial mesenchymal transition (EMT)-induced progressed cancer with activation of transforming growth factor-ß pathway, activation of hypoxia-inducible factor-1α, and suppression of runt-related transcription factor 3. Since we previously reported that EP3 receptor is involved in and induce colon cancer cell migration, EP3 receptor-expressing CRC may induce metastasis through these signaling pathways. Thus, the findings suggest the effectiveness of cancer clustering by gene expression of the EP receptor subtype to elucidate the mechanism of human CRC.

The EP3 and EP4 Receptor Subtypes both Mediate the Fever-producing Effects of Prostaglandin E2 in the Rostral Ventromedial Preoptic Area of the Hypothalamus in Rats.

This study aimed to re-examine the receptor subtype that mediates the fever-producing effects of prostaglandin E2 (PGE2) in the rostral ventromedial preoptic area (rvmPOA) of the hypothalamus. Among the four subtypes of PGE2 receptors (EP1, EP2, EP3, and EP4), EP3 receptor is crucially involved in the febrile effects of PGE2. However, it is possible for other subtypes of PGE2 receptor to contribute in the central mechanism of fever generation. Accordingly, effects of microinjection of PGE2 receptor subtype-specific agonists or antagonists were examined at the locus where a microinjection of a small amount (420 fmol) of PGE2 elicited prompt increases in the O2 consumption rate (VO2), heart rate, and colonic temperature (Tc) in the rvmPOA of urethane-chloralose-anesthetized rats. The EP3 agonist sulprostone mimicked, whereas its antagonist L-798,106 reduced, the febrile effects of PGE2 microinjected into the same site. Similarly, the EP4 agonist rivenprost mimicked, whereas its antagonist ONO-AE3-208 reduced, the effects of PGE2 microinjected into the same site. In contrast, microinjection of the EP1 agonist iloprost induced a very small increase in VO2 but did not have significant influences on the heart rate and Tc, whereas its antagonist, AH6809, did not affect the PGE2-induced responses. Microinjection of the EP2 agonist butaprost had no effects on the VO2, heart rate, and Tc. The results suggest that the EP3 and EP4 receptor subtypes are both involved in the fever generated by PGE2 in the rvmPOA.

A systematic analysis of prostaglandin E2 type 3 receptor isoform signaling reveals isoform- and species-dependent L798106 Gαz-biased agonist responses.

Prostaglandins are bioactive lipids involved in many physiological and pathophysiological conditions, such as pain, atherosclerosis, type II diabetes, and parturition. Prostaglandin E2 (PGE2) activates four G protein-coupled receptors (GPCRs), named the PGE2 types 1-4 receptors (EP1-4), to elicit the intracellular signaling responsible for their physiological actions. There are more than twelve EP3 isoforms in humans that differ only by the sequence of their C-termini. However, the signaling mechanisms engaged by the various isoforms have never been clearly defined. In this study, we used a recently described BRET-based biosensor technology to define the signaling profiles for each of the human isoforms on a selection of signaling pathways using the agonists, PGE2 and sulprostone, and the purportedly EP3-specific antagonist L798106. We found that L798106 is a biased agonist of the Gαz pathway for some human EP3 isoforms, an effect that is not detected in the close ortholog mouse EP3 isoform α. We also found that the presence of a threonine residue at position 107 in the binding site of human EP3, which is a serine in most other species including mice, is important for L798106-mediated Gαz efficacy. Given the reported importance of EP3-Gαz signaling on the potential therapeutic efficacy of EP3 and since many preclinical studies for these mechanisms have been performed in rodents, this finding demonstrates the importance of determining a detailed signaling profile of ligands for different species and receptor isoforms, which constitutes an important step to better understand the therapeutic potential of the EP3.

Agonists of prostaglandin E2 receptors as potential first in class treatment for nephronophthisis and related ciliopathies.

Nephronophthisis (NPH) is an autosomal recessive tubulointerstitial nephropathy belonging to the ciliopathy disorders and known as the most common cause of hereditary end-stage renal disease in children. Yet, no curative treatment is available. The major gene, NPHP1, encodes a protein playing key functions at the primary cilium and cellular junctions. Using a medium-throughput drug-screen in NPHP1 knockdown cells, we identified 51 Food and Drug Administration-approved compounds by their ability to alleviate the cellular phenotypes associated with the loss of NPHP1; 11 compounds were further selected for their physicochemical properties. Among those compounds, prostaglandin E1 (PGE1) rescued ciliogenesis defects in immortalized patient NPHP1 urine-derived renal tubular cells, and improved ciliary and kidney phenotypes in our NPH zebrafish and Nphp1 knockout mouse models. Furthermore, Taprenepag, a nonprostanoid prostaglandin E2 receptor agonist, alleviated the severe retinopathy observed in Nphp1−/− mice. Finally, comparative transcriptomics allowed identification of key signaling pathways downstream PGE1, including cell cycle progression, extracellular matrix, adhesion, or actin cytoskeleton organization. In conclusion, using in vitro and in vivo models, we showed that prostaglandin E2 receptor agonists can ameliorate several of the pleotropic phenotypes caused by the absence of NPHP1; this opens their potential as a first therapeutic option for juvenile NPH-associated ciliopathies.

The unfavorable clinical outcome of COVID-19 in smokers is mediated by H3K4me3, H3K9me3 and H3K27me3 histone marks.

Smoking could predispose individuals to a more severe COVID-19 by upregulating a particular gene known as mdig, which is mediated through a number of well-known histone modifications. Smoking might regulate the transcription-activating H3K4me3 mark, along with the transcription-repressing H3K9me3 and H3K27me3 marks, in a way to favor SARS-CoV-2 entry by enhancing the expression of ACE2, NRP1 and NRP2, AT1R, CTSD and CTSL, PGE2 receptors 2-4, SLC6A20 and IL-6, all of which interact either directly or indirectly with important receptors, facilitating viral entry in COVID-19.

Lay abstract The role of smoking in development of several respiratory diseases has been clearly established. A significant proportion of these deleterious effects is mediated through epigenetic mechanisms, particularly histone modifications. Recent evidence indicates that smoking induces the expression of a mediator known as mdig, which in turn alters the transcription of several key proteins that have been implicated in development of COVID-19.

Differential Expression of Prostaglandin Receptors in Canine Uterus With Pyometra.

The objective of the study was to ascertain the role of prostaglandins Viz., PGE2 and PGF2α, and their respective receptors in the pathophysiology of canine pyometra. Normal (n = 6) and pyometra (n = 8) affected uterus were collected from bitches undergoing ovariohysterectomy. Pyometra was graded according to histopathological alterations. The levels of PGE2 and PGF2α were estimated in the endometrium. The differential expression in the mRNA of PGF2α receptor (FP) and PGE2 receptors (EP1, EP2, EP3, and EP4) were studied in the endometrium and myometrium of the pyometra-affected uterus. Normal uterus served as calibrator. Elevation of both PGE2 and PGF2α levels in the endometrium of pyometra-affected bitches was observed. The FP receptor gene in the endometrium and myometrium of pyometra-affected bitches was downregulated (P < .05). Out of all EP receptors, only EP2 receptor has shown upregulation in both endometrium and myometrium of pyometra affected uterus. EP3 receptor got downregulated in both endometrium and myometrium in pyometra. Thus, downregulation of FP, EP3 receptors in the myometrium reinforces the lack of contractility in pyometra-affected bitches favoring bacterial proliferation and subsequent pus accumulation. Moreover, upregulation of EP2 receptors in the pyometra bitches suggests the scope of selective pharmacological inhibition of EP2 receptors as an adjunct therapy in the treatment of pyometra.

The Signaling Pathway of PGE2 and Its Regulatory Role in T Cell Differentiation.

Prostaglandin E2 (PGE2) is a lipid mediator derived from the fatty acid arachidonic acid. As an essential inflammatory factor, PGE2 has a critical impact on immune regulation through the prostanoid E (EP) receptor pathway. T cells, including CD4+ and CD8+ T cell subsets, play crucial roles in the adaptive immune response. Previous studies have shown that PGE2 is involved in regulating CD4+ T cell differentiation and inflammatory cytokine production via the EP receptor pathway, thereby affecting the development of diseases mediated by CD4+ T cells. In this review, we summarize the signaling pathway of PGE2 and describe the relationship between PGE2 and T cell differentiation. Hence, this review may provide important evidence for immune therapies and may even promote the development of biomedicines.

Systems approach reveals distinct and shared signaling networks of the four PGE2 receptors in T cells.

Prostaglandin E2 (PGE2) promotes an immunosuppressive microenvironment in cancer, partly by signaling through four receptors (EP1, EP2, EP3, and EP4) on T cells. Here, we comprehensively characterized PGE2 signaling networks in helper, cytotoxic, and regulatory T cells using a phosphoproteomics and phosphoflow cytometry approach. We identified ~1500 PGE2-regulated phosphosites and several important EP1­4 signaling nodes, including PKC, CK2, PKA, PI3K, and Src. T cell subtypes exhibited distinct signaling pathways, with the strongest signaling in EP2-stimulated CD8+ cells. EP2 and EP4, both of which signal through Gαs, induced similar signaling outputs, but with distinct kinetics and intensity. Functional predictions from the observed phosphosite changes revealed PGE2 regulation of key cellular and immunological processes. Last, network modeling suggested signal integration between the receptors and a substantial contribution from G protein­independent signaling. This study offers a comprehensive view of the different PGE2-regulated phosphoproteomes in T cell subsets, providing a valuable resource for further research on this physiologically and pathophysiologically important signaling system.

Prostaglandin E2 Signaling Mediates Oenocytoid Immune Cell Function and Lysis, Limiting Bacteria and Plasmodium Oocyst Survival in Anopheles gambiae.

Lipid-derived signaling molecules known as eicosanoids have integral roles in mediating immune and inflammatory processes across metazoans. This includes the function of prostaglandins and their cognate G protein-coupled receptors (GPCRs) to employ their immunological actions. In insects, prostaglandins have been implicated in the regulation of both cellular and humoral immune responses, yet in arthropods of medical importance, studies have been limited. Here, we describe a prostaglandin E2 receptor (AgPGE2R) in the mosquito Anopheles gambiae and demonstrate that its expression is most abundant in oenocytoid immune cell populations. Through the administration of prostaglandin E2 (PGE2) and AgPGE2R-silencing, we demonstrate that prostaglandin E2 signaling regulates a subset of prophenoloxidases (PPOs) and antimicrobial peptides (AMPs) that are strongly expressed in populations of oenocytoids. We demonstrate that PGE2 signaling via the AgPGE2R significantly limits both bacterial replication and Plasmodium oocyst survival. Additional experiments establish that PGE2 treatment increases phenoloxidase (PO) activity through the increased expression of PPO1 and PPO3, genes essential to anti-Plasmodium immune responses that promote oocyst killing. We also provide evidence that the mechanisms of PGE2 signaling are concentration-dependent, where high concentrations of PGE2 promote oenocytoid lysis, negating the protective effects of lower concentrations of PGE2 on anti-Plasmodium immunity. Taken together, our results provide new insights into the role of PGE2 signaling on immune cell function and its contributions to mosquito innate immunity that promote pathogen killing.

Molecular characterization of the prostaglandin E receptor subtypes 2a and 4b and their expression patterns during embryogenesis in zebrafish.

The molecular expression profiles of zebrafish ep2a and ep4b have not been defined to date. Phylogenetic trees of EP2a and EP4b in zebrafish and other species revealed that human EP4 and zebrafish EP4b were more closely related than EP2a. Zebrafish EP2a is a 281 amino acid protein which shares high identity with that of human (43%), mouse (44%), rat (43%), dog (44%), cattle (41%), and chicken (41%). Zebrafish EP4b encoded a 497 amino acid precursor with high amino acid identity to that of mammals, including human (57%), mouse (54%), rat (55%), dog (55%), cattle (56%), and chicken (54%). Whole-mount in situ hybridization revealed that ep2a was robustly expressed in the anterior four somites at the 10-somites stages, but was absent in the somites at 19 hpf. It was observed again in the pronephric duct at 24 hpf, in the intermediate cell mass located in the trunk, and in the rostral blood island at 30 hpf. Ep2a was also expressed in the notochord at 48 hpf. During somitogenesis, ep4b was highly expressed in the eyes, somites, and the trunk neural crest. From 30 to 48 hpf, ep4b could be detected in the posterior cardinal vein and the neighboring inner cell mass. From these data we conclude that ep2a and ep4b are conserved in vertebrates and that the presence of ep2a and ep4b transcripts during developmental stages infers their role during early zebrafish larval development. In addition, the variable expression of the two receptor isoforms was strongly suggestive of divergent roles of molecular regulation.

[Research advances of prostaglandin E2 synthases and receptors in cardiovascular diseases].

Prostaglandin E2 (PGE2) is an important lipid mediator derived from arachidonic acid. It is widely distributed in various tissues and involved in numerous physiological and pathophysiological processes. Based on the inhibition of inflammatory PGE2 production, non-steroidal anti-inflammatory drugs (NSAIDs) are considered as the most commonly used drugs to treat pain and inflammation. However, clinical trials have revealed that NSAIDs, especially cyclooxygenase-2 (COX-2) selective inhibitors, may predispose patients to a remarkably increased cardiovascular risk, including hypertension, myocardial infarction, and heart failure. This promotes scientists to develop new drugs to not only afford pain relief but also have cardiovascular efficacy. Microsomal prostaglandin E synthase-1 (mPGES-1), the key terminal enzyme catalyzing the synthesis of inflammatory PGE2, and the four PGE2 receptors (EP1-4) have gained more attention as the promising alternative drug targets for the development of novel NSAIDs. The role of mPGES-1 and EP receptors in cardiovascular diseases also has been widely studied. In this review, we highlight the most recent advances from our and other studies on the role of PGE2, particularly mPGES-1 and the four PGE2 receptors, in cardiovascular diseases.