PGE(2) receptors and their intracellular mechanisms in rabbit small intestine

Prostaglandin E2 receptors and their role in gastrointestinal motility - Potential therapeutic targets

Prostaglandin E2 (PGE2) is found throughout the gastrointestinal tract in a diverse variety of functions and roles. The recent discovery of four PGE2 receptor subtypes in intestinal muscle layers as well as in the enteric plexus has led to much interest in the study of their roles in gut motility. Gut dysmotility has been implicated in functional disease processes including irritable bowel syndrome (IBS) and slow transit constipation, and lubiprostone, a PGE2 derivative, has recently been licensed to treat both conditions. The diversity of actions of PGE2 in the intestinal tract is attributed to its differing effects on its downstream receptor types, as well as their varied distribution in the gut, in both health and disease. This review aims to identify the role and distribution of PGE2 receptors in the intestinal tract, and aims to elucidate their distinct role in gut motor function, with a specific focus on functional intestinal pathologies.

Involvement of different receptor subtypes in prostaglandin E2-induced contraction and relaxation in the lower esophageal sphincter and esophageal body

Prostaglandin E₂ (PGE₂) plays a role in the pathogenesis of gastro-esophageal reflux disease (GERD). There are 4 subtypes of PGE₂, PGE₂ receptor 1, 2, 3 and 4 (EP 1-4). In GERD patents, PGE₂, EP2 and EP4 are upregulated. However, the effects of PGE2 on esophageal motility remain elusive. We examined how PGE₂ regulates motility in the porcine circular smooth muscle of the lower esophageal sphincter (LES), and the circular and longitudinal smooth muscle of the esophagus body in organ bath. PGE₂ induced tonic relaxation in the LES and circular smooth muscle, but transient contraction in longitudinal smooth muscle. The relaxation of the LES and circular smooth muscle was similar in pattern and mechanism, but was much larger in the LES. The relaxation was completely blocked by a voltage-gated K⁺ channel blocker or 40 mM K⁺ depolarization, indicating the involvement of K⁺ channel. Longitudinal smooth muscle contraction was completely blocked by an L-type Ca²⁺ channel blocker, showing the contribution of Ca²⁺ movement. The involvement of the EP receptor in motility was examined with selective receptor agonists and antagonists. Activation of EP2 and EP4 caused relaxation in the LES and circular smooth muscle. Compatible with PGE₂, EP2 and EP4 agonists caused more significant relaxation in the LES than in circular smooth muscle. EP1 contributed to the longitudinal smooth muscle contraction. The different effects of PGE₂ in the LES, circular and longitudinal smooth muscle contributes to esophageal motility, their impairment might increase the amount and frequency of esophageal reflux.

TRPA1-expressing lamina propria mesenchymal cells regulate colonic motility

The physiological process of defecation is directly controlled by colorectal motility. The transient receptor potential ankyrin 1 (TRPA1) channel is expressed in small intestine enterochromaffin cells and is involved in gastrointestinal motility via serotonin release. In the colorectum, however, enterochromaffin cell localization is largely distinct from that in the small intestine. Here, we investigated the role of lower gastrointestinal tract TRPA1 in modulating colorectal motility. We found that in colonic tissue, TRPA1 is predominantly expressed in mesenchymal cells of the lamina propria, which are clearly distinct from those in the small intestine. These cells coexpressed COX1 and microsomal prostaglandin E synthase-1. Intracolonic administration of TRPA1 agonists induced colonic contraction, which was suppressed by a prostaglandin E2 (PGE2) receptor 1 antagonist. TRPA1 activation induced calcium influx and PGE2 release from cultured human fibroblastic cells. In dextran sulfate sodium-treated animals, both TRPA1 and its endogenous agonist were dramatically increased in the colonic lamina propria, accompanied by abnormal colorectal contractions. Abnormal colorectal contractions were significantly prevented by pharmacological and genetic inhibition of TRPA1. In conclusion, in the lower gastrointestinal tract, mesenchymal TRPA1 activation results in PGE2 release and consequently promotes colorectal contraction, representing what we believe is a novel physiological and inflammatory bowel disease-associated mechanism of gastrointestinal motility.

TRPV4 Channel Signaling in Macrophages Promotes Gastrointestinal Motility via Direct Effects on Smooth Muscle Cells

Intestinal macrophages are critical for gastrointestinal (GI) homeostasis, but our understanding of their role in regulating intestinal motility is incomplete. Here, we report that CX3C chemokine receptor 1-expressing muscularis macrophages (MMs) were required to maintain normal GI motility. MMs expressed the transient receptor potential vanilloid 4 (TRPV4) channel, which senses thermal, mechanical, and chemical cues. Selective pharmacologic inhibition of TRPV4 or conditional deletion of TRPV4 from macrophages decreased intestinal motility and was sufficient to reverse the GI hypermotility that is associated with chemotherapy treatment. Mechanistically, stimulation of MMs via TRPV4 promoted the release of prostaglandin E2 and elicited colon contraction in a paracrine manner via prostaglandin E receptor signaling in intestinal smooth muscle cells without input from the enteric nervous system. Collectively, our data identify TRPV4-expressing MMs as an essential component required for maintaining normal GI motility and provide potential drug targets for GI motility disorders.

Prostanoid EP3 receptor agonist sulprostone enhances pacemaker activity of colonic interstitial cells of Cajal

EP receptor activation by PGE2 regulates gastrointestinal motility by modulating smooth muscle contractility. Interstitial cells of Cajal (ICCs) are pacemaker cells that regulate smooth muscle activity. We aimed to determine effects of the EP3 receptor agonist sulprostone on pacemaker potentials in colonic ICCs. We performed a whole cell patch clamp, RT-PCR, and Ca²⁺ imaging in cultured ICCs from mouse colon. Sulprostone depolarized the membrane and increased pacemaker frequency. EP3 receptor antagonist blocked these sulprostone-induced effects. EP3 receptors were expressed in ANO1-positive ICCs. Phospholipase C inhibitor or Ca²⁺-ATPase inhibitor from the endoplasmic reticulum blocked the sulprostone-induced effects and sulprostone increased intracellular Ca²⁺ ([Ca²⁺]_i) oscillations. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blockers also suppressed the sulprostone-induced effects. Sulprostone enhanced pacemaker activity through EP3 receptors by activating HCN channels via the [Ca²⁺]_i release pathway. Therefore, EP3 receptor activation in ICCs may modulate colonic motility and could be a therapeutic target for enhancing colonic GI motility.

Eicosanoid receptors: Targets for the treatment of disrupted intestinal epithelial homeostasis

The importance of cyclooxygenase and lipoxygenase pathways and the consequent eicosanoid synthesis in the physiology and pathophysiology of the intestinal epithelium is currently being established. Each eicosanoid (prostanoid, leukotriene, hydroxyeicosatetraenoic acid) preferentially recognizes one or more receptors coupled to one or more signal-transduction processes. This overview focuses on the role of eicosanoid receptors in the maintenance of intestinal epithelium physiology through the control of proliferation/differentiation/apoptosis processes. Furthermore, it is reported that the role of these receptors on the regulation of the barrier function of the intestinal epithelium have arisen through the regulation of absorption/secretion processes, tight-junction state and the control of the intestinal immune response. Also, this review considers the implication of AA cascade in the disruption of epithelial homeostasis during inflammatory bowel diseases and colorectal cancer as well as the therapeutic values and potential of the eicosanoid receptors as novel targets for the treatments of the pathologies above mentioned.

Roles of Toll-Like Receptor 4, IκB Kinase, and the Proteasome in the Intestinal Alterations Caused by Sepsis

BACKGROUND

Lipopolysaccharide decreases intestinal contractility and induces the production of cytokines, which play an important role in the pathogenesis of sepsis.

AIM

The objective of the present study was to examine the role of Toll-like receptor 4, IκB kinase, and the proteasome in the intestinal alterations induced by lipopolysaccharide.

METHODS

Sepsis was induced in rabbits by intravenous injection of lipopolysaccharide. Contractility studies of rabbit duodenum were performed in an organ bath. Expressions of interleukin-1β, interleukin-6, interleukin-8, interleukin-10, IκB kinase-α, IκB kinase-β, IκB kinase-γ, and the proteasome mRNA were determined by RT-PCR on rabbit duodenum.

RESULTS

Neomycin and polymyxin B (Toll-like receptor 4 inhibitors), IKK NBD peptide (IκB kinase complex inhibitor), and MG-132 (proteasome inhibitor) blocked partially the effects of lipopolysaccharide on the acetylcholine-, prostaglandin E2-, substance P-, and KCl-induced contractions in the longitudinal and circular smooth muscle of rabbit duodenum. Lipopolysaccharide increased the mRNA expression of interleukin-6 and interleukin-8 in duodenal tissue, and this effect was partly reversed by neomycin, polymyxin B, IKK NBD peptide, and MG-132. IκB kinase-α, IκB kinase-β, IκB kinase-γ, and the proteasome mRNA expressions was not affected by lipopolysaccharide treatment.

CONCLUSIONS

Toll-like receptor 4, the IκB kinase complex, and the proteasome could be therapeutic targets in the treatment of sepsis symptoms in the intestine.

Role of PGE2 in the colonic motility: PGE2 generates and enhances spontaneous contractions of longitudinal smooth muscle in the rat colon

The aim of this study was to determine which PGE2 receptors (EP1-4 receptors) influence colonic motility. Mucosa-free longitudinal smooth muscle strips of the rat middle colon spontaneously induced frequent phasic contractions (giant contractions, GCs) in vitro, and the GCs were almost completely abolished by a cyclooxygenase inhibitor, piroxicam, and by an EP3 receptor antagonist, ONO-AE3-240, but enhanced by tetrodotoxin (TTX). In the presence of piroxicam, exogenous PGE2, both ONO-AE-248 (EP3 agonist), and ONO-DI-004 (EP1 agonist) induced GC-like contractions, and increased the frequency and amplitude. These effects of EP receptor agonists were insensitive to TTX and ω-conotoxins. In immunohistochemistry, the EP1 and EP3 receptors were expressed in the longitudinal smooth muscle cells. These results suggest that the endogenous PGE2 spontaneously generates and enhances the frequent phasic contractions directly activating the EP1 and EP3 receptors expressed on longitudinal smooth muscle cells in the rat middle colon.

Preoperative short-term parenteral administration of polyunsaturated fatty acids ameliorates intestinal inflammation and postoperative ileus in rodents

PURPOSE

Abdominal surgery results in an inflammation of the intestinal muscularis externa (ME), subsequently leading to postoperative ileus (POI). Polyunsaturated fatty acids (PUFA) are known to modulate inflammation. The aim of this study was to analyze the effect of preoperative parenteral administration of marine (n-3) or soybean (n-6) PUFA lipid emulsions (PUFA-LE) on POI and tissue fatty acid profiles.

METHODS

Rodents underwent intestinal manipulation (IM) after 5 days of parenteral administration of 10-mL/kg body weight saline, (n-3), or (n-6) PUFA-LE. Sham animals received saline treatment without IM. In rats, postoperative inflammation was quantified by ME neutrophil levels and NO production in organ culture, and ME function was determined by an in vitro contractility measurement. Additionally, in vivo gastrointestinal transit (GIT) was analyzed in mice. Lipopolysaccharide-induced IL-6 expression of rat bone marrow-derived mononuclear cells and ME was analyzed. Fatty acids were measured by gas chromatography in rat blood, bone marrow cells, and ME.

RESULTS

The (n-3) PUFA-LE reduced neutrophil levels and NO production after IM and improved in vitro jejunal contractility and GIT time. The (n-6) PUFA-LE significantly reduced postoperative inflammation and tended to improve intestinal motility (P < 0.06). Interestingly, (n-6) PUFA-LE significantly reduced the levels of arachidonic acid in ME (-63%), while (n-3) PUFA-LE reduced arachidonic acid (-20%) and additionally raised EPA (+550%).

CONCLUSION

Short-term preoperative parenteral administration of (n-3) or (n-6) PUFA-LE significantly alters tissue-specific fatty acid profiles. Preoperative parenteral PUFA-LE supplementation, preferably by marine (n-3) PUFA, ameliorates postoperative intestinal inflammation and dysmotility and could be a promising therapeutic option in POI prophylaxis.

Regulation of smooth muscle contraction by the epithelium: role of prostaglandins

As an analog to the endothelium situated next to the vascular smooth muscle, the epithelium is emerging as an important regulator of smooth muscle contraction in many vital organs/tissues by interacting with other cell types and releasing epithelium-derived factors, among which prostaglandins have been demonstrated to play a versatile role in governing smooth muscle contraction essential to the physiological and pathophysiological processes in a wide range of organ systems.

Characterization of the EP receptor types that mediate longitudinal smooth muscle contraction of human colon, mouse colon and mouse ileum

BACKGROUND

Prostaglandin E(2) (PGE(2) ) is an inflammatory mediator implicated in several gastrointestinal pathologies that affect normal intestinal transit. The aim was to establish the contribution of the four EP receptor types (EP(1-4) ), in human colon, that mediate PGE(2) -induced longitudinal smooth muscle contraction.

METHODS

Changes in isometric muscle tension of human colon, mouse colon and mouse ileum were measured in organ baths in response to receptor-specific agonists and antagonists. In addition, lidocaine was used to block neurogenic activity to investigate whether EP receptors were pre- or post-junctional.

KEY RESULTS

PGE(2) contracted longitudinal muscle from human and mouse colon and mouse ileum. These contractions were inhibited by the EP(1) receptor antagonist, EP(1) A in human colon, whereas a combination of EP(1) A and the EP(3) antagonist, L798106 inhibited agonist responses in both mouse preparations. The EP(3) agonist, sulprostone also increased muscle tension in both mouse tissues, and these responses were inhibited by lidocaine in the colon but not in the ileum. Although PGE(2) consistently contracted all three muscle preparations, butaprost decreased tension by activating smooth muscle EP(2) receptors in both colonic tissues. Alternatively, in mouse ileum, butaprost responses were lidocaine-sensitive, suggesting that it was activating prejunctional EP(2) receptors on inhibitory motor neurons. Conversely, EP(4) receptors were not functional in all the intestinal muscle preparations tested.

CONCLUSIONS & INFERENCES

PGE(2) -induced contraction of longitudinal smooth muscle is mediated by EP(1) receptors in human colon and by a combination of EP(1) and EP(3) receptors in mouse intestine, whereas EP(2) receptors modulate relaxation in all three preparations.

Intestinal effects of lipopolysaccharide in rabbit are mediated by cyclooxygenase-2 through p38 mitogen activated protein kinase

The mediators of the pathophysiological symptoms of septic shock are not completely understood. The intracellular signalling mechanisms of lipopolysaccharide (LPS)-induced effects need further investigation. This study investigates (1) the role of COX-2 in the effect of LPS on (a) the KCl, acetylcholine and prostaglandin E₂-induced contractions of rabbit duodenum and (b) the oxidative stress status in plasma and intestine and (2) the relationship between p38 MAPK and COX-2 expression in rabbit duodenum. Rabbits were injected i.v. with either (1) saline, (2) LPS, (3) SB203580, a p38 MAPK inhibitor, (4) SB203580+LPS, (5) NS-398, a COX-2 inhibitor or (6) NS-398+LPS. Contractility studies were performed by suspending pieces of duodenum in an organ bath in the direction of longitudinal and circular smooth muscle fibres. The formation of products of oxidative damage to proteins (carbonyls) and lipids [malondialdehyde (MDA) and 4-hydroxyalkenals (4-HDA)] was quantified in intestinal tissue and plasma. The protein expression of COX-2 was measured by western blot. The KCl, acetylcholine and prostaglandin E₂-induced contractions decreased with LPS. In addition, LPS increased the levels of carbonyls and MDA+4-HDA in plasma and duodenum as well as COX-2 expression in duodenal tissue. All these effects were blocked by NS-398. The p38 MAPK inhibitor SB203580 blocked the effect of LPS on COX-2 expression. These results suggest that the effect of LPS on KCl, acetylcholine and prostaglandin E₂-induced contractions in the rabbit duodenum and oxidative stress might be mediated by an increase in COX-2 expression through the p38 MAPK pathway.

Jack bean urease alters serotonin-induced effects on Rhodnius prolixus anterior midgut

Urease isoforms from jack bean seeds are toxic to insects, and this entomotoxic effect is mostly due to the release of a peptide by insect digestive enzymes. We previously demonstrated that jack bean urease (JBU) has antidiuretic effects on Rhodnius prolixus Malpighian tubules, decreasing the serotonin-stimulated secretion of fluid. Now, we evaluate the toxicity of the intact JBU and its effect on R. prolixus anterior midgut, to further elucidate the mechanism of action of JBU in insects. JBU decreases the serotonin-induced fluid transport by the anterior midgut in vitro when injected into the lumen. A decrease in the levels of cAMP is observed in tissues treated with JBU (in the presence of serotonin). JBU also causes a dose-dependent increase in the frequency of serotonin-induced contractions in the anterior midgut, but does not alter the frequency of spontaneous contractions. The cyclooxygenase inhibitor indomethacin and the prostaglandin antagonist AH6809 block JBU's potentiation of serotonin-induced contractions, indicating that prostaglandins might act as second messengers for JBU action. Prostaglandin E(2) (PGE(2)) increases the frequency of serotonin-induced contractions, again supporting the role of prostaglandins as second messengers for JBU action. JBU and PGE(2) increase cGMP levels in the anterior midgut, indicating that this molecule might also be part of the JBU pathway.

Prostaglandin E2-EP1 and EP2 receptor signaling promotes apical junctional complex disassembly of Caco-2 human colorectal cancer cells

BACKGROUND

The apical junctional complex (AJC) is a dynamic structure responsible to maintain epithelial cell-cell adhesions and it plays important functions such as, polarity, mechanical integrity, and cell signaling. Alteration of this complex during pathological events leads to an impaired epithelial barrier by perturbation of the cell-cell adhesion system. Although clinical and experimental data indicate that prostaglandin E(2) (PGE2) plays a critical function in promoting cell motility and cancer progression, little is known concerning its role in AJC disassembly, an event that takes place at the beginning of colorectal tumorigenesis. Using Caco-2 cells, a cell line derived from human colorectal cancer, we investigated the effects of prostaglandin E(2) (PGE(2)) treatment on AJC assembly and function.

RESULTS

Exposition of Caco-2 cells to PGE(2) promoted differential alteration of AJC protein distribution, as evidenced by immunofluorescence and immunoblotting analysis and impairs the barrier function, as seen by a decrease in the transepithelial electric resistance and an increase in the permeability to ruthenium red marker. We demonstrated the involvement of EP1 and EP2 prostaglandin E(2) receptor subtypes in the modulation of the AJC disassembly caused by prostanoid. Furthermore, pharmacological inhibition of protein kinase-C, but not PKA and p38MAPK significantly prevented the PGE(2) effects on the AJC disassembly.

CONCLUSION

Our findings strongly suggest a central role of Prostaglandin E2-EP1 and EP2 receptor signaling to mediate AJC disassembly through a mechanism that involves PKC and claudin-1 as important target for the TJ-related effects in human colorectal cancer cells (Caco-2).

Experimental endometriosis in the rat is correlated with colonic motor function alterations but not with bacterial load

Endometriosis commonly presents with symptoms that mimic chronic gastrointestinal disorders. The authors used the autotransplantion model of endometriosis in rats to investigate the possible underlying mechanisms. After the rats were killed, the presence of endometriotic vesicles, colonic inflammation, and white blood cell (WBC) numbers in the peritoneal fluid was determined. Sections of colon and of jejunum were collected for measurement of myeloperoxidase (MPO) activity and bacterial counts, and isometric recording in response to acetylcholine was measured in segments of longitudinal and circular smooth muscle. Experimental animals had significantly more colonic damage, MPO activity, and WBC numbers than controls did. There was no significant difference in the total bacterial load; however, experimental animals demonstrated an increased tension in the longitudinal muscle, which correlated with WBC numbers and colonic damage. In summary, this study presents evidence for a significant effect of peritoneal endometriosis on colonic function and integrity, which may help explain the gastrointestinal symptoms associated with this disease.

Prostaglandin E2 receptor distribution and function in the gastrointestinal tract

Prostaglandin E2 (PGE2) is one of the most important biologically active prostanoids found throughout the gastrointestinal tract. Despite the fact that PGE2 regulates many physiological functions of the gut including mucosal protection, gastrointestinal secretion and motility, it is implicated in the pathophysiology of inflammatory bowel diseases (IBD) and colorectal neoplasia. The varied biological functions exerted by PGE2 are through the pharmacologically distinct, G-protein coupled plasma membrane receptors termed EP receptors. Disruptions of various prostanoid receptor genes have helped in unravelling the physiological functions of these receptors. To date, all four subtypes of EP receptors have been individually knocked out in mice and various phenotypes have been reported for each subtype. Similarly, in vitro and in vivo studies using EP receptor agonists and antagonists have helped in uncoupling the diverse functions of PGE2 signalling involving distinct EP receptors in the gut. In this review, we will summarize and conceptualize the salient features of EP receptor subtypes, their regional functions in the gut and how expressions of EP receptors are altered during disease states.