Inhibition of cyclooxygenase-2 and EP1 receptor antagonism reduces human colonic longitudinal muscle contractility in vitro

Prostaglandin E2 in the Tumor Microenvironment, a Convoluted Affair Mediated by EP Receptors 2 and 4

The involvement of the prostaglandin E2 (PGE2) system in cancer progression has long been recognized. PGE2 functions as an autocrine and paracrine signaling molecule with pleiotropic effects in the human body. High levels of intratumoral PGE2 and overexpression of the key metabolic enzymes of PGE2 have been observed and suggested to contribute to tumor progression. This has been claimed for different types of solid tumors, including, but not limited to, lung, breast, and colon cancer. PGE2 has direct effects on tumor cells and angiogenesis that are known to promote tumor development. However, one of the main mechanisms behind PGE2 driving cancerogenesis is currently thought to be anchored in suppressed antitumor immunity, thus providing possible therapeutic targets to be used in cancer immunotherapies. EP2 and EP4, two receptors for PGE2, are emerging as being the most relevant for this purpose. This review aims to summarize the known roles of PGE2 in the immune system and its functions within the tumor microenvironment. SIGNIFICANCE STATEMENT: Prostaglandin E2 (PGE2) has long been known to be a signaling molecule in cancer. Its presence in tumors has been repeatedly associated with disease progression. Elucidation of its effects on immunological components of the tumor microenvironment has highlighted the potential of PGE2 receptor antagonists in cancer treatment, particularly in combination with immune checkpoint inhibitor therapeutics. Adjuvant treatment could increase the response rates and the efficacy of immune-based therapies.

Prostaglandin and prostaglandin receptors: present and future promising therapeutic targets for pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH), Group 1 pulmonary hypertension (PH), is a type of pulmonary vascular disease characterized by abnormal contraction and remodeling of the pulmonary arterioles, manifested by pulmonary vascular resistance (PVR) and increased pulmonary arterial pressure, eventually leading to right heart failure or even death. The mechanisms involved in this process include inflammation, vascular matrix remodeling, endothelial cell apoptosis and proliferation, vasoconstriction, vascular smooth muscle cell proliferation and hypertrophy. In this study, we review the mechanisms of action of prostaglandins and their receptors in PAH.

MAIN BODY

PAH-targeted therapies, such as endothelin receptor antagonists, phosphodiesterase type 5 inhibitors, activators of soluble guanylate cyclase, prostacyclin, and prostacyclin analogs, improve PVR, mean pulmonary arterial pressure, and the six-minute walk distance, cardiac output and exercise capacity and are licensed for patients with PAH; however, they have not been shown to reduce mortality. Current treatments for PAH primarily focus on inhibiting excessive pulmonary vasoconstriction, however, vascular remodeling is recalcitrant to currently available therapies. Lung transplantation remains the definitive treatment for patients with PAH. Therefore, it is imperative to identify novel targets for improving pulmonary vascular remodeling in PAH. Studies have confirmed that prostaglandins and their receptors play important roles in the occurrence and development of PAH through vasoconstriction, vascular smooth muscle cell proliferation and migration, inflammation, and extracellular matrix remodeling.

CONCLUSION

Prostacyclin and related drugs have been used in the clinical treatment of PAH. Other prostaglandins also have the potential to treat PAH. This review provides ideas for the treatment of PAH and the discovery of new drug targets.

Increased role of E prostanoid receptor-3 in prostacyclin-evoked contractile activity of spontaneously hypertensive rat mesenteric resistance arteries

This study aimed to determine whether E prostanoid receptor-3 (EP3) is involved in prostacyclin (PGI₂)-evoked vasoconstrictor activity of resistance arteries and if so, how it changes under hypertensive conditions. Mesenteric resistance arteries from Wistar-Kyoto rats (WKYs) and spontaneously hypertensive rats (SHRs) were isolated for functional and biochemical studies. Here we show that in vessels from WKYs, PGI₂ or the endothelial muscarinic agonist ACh (which stimulates in vitro PGI₂ synthesis) evoked vasoconstrictor activity, which increased in SHRs. The thromboxane-prostanoid receptor (TP) antagonist SQ29548 partially removed the vasoconstrictor activity, and an increased contractile activity of PGI₂ resistant to SQ29548 was observed in SHRs. Interestingly, L798106, an antagonist of EP3 (whose expression was higher in SHRs than in WKYs), not only added to the effect of SQ29548 but also caused relaxation to PGI₂ more than that obtained with SQ29548. In accordance, EP3 deletion, which reduced PGI₂-evoked contraction, together with SQ29548 resulted in relaxation evoked by the agonist in mouse aortas. These results thus demonstrate an explicit involvement of EP3 in PGI₂-evoked vasoconstrictor activity in rat mesenteric resistance arteries and suggest that up-regulation of the receptor contributes significantly to the increased contractile activity evoked by PGI₂ under hypertensive conditions.

Lubiprostone Increases Small Intestinal Smooth Muscle Contractions Through a Prostaglandin E Receptor 1 (EP1)-mediated Pathway

Background/Aims

Lubiprostone, a chloride channel type 2 (ClC-2) activator, was thought to treat constipation by enhancing intestinal secretion. It has been associated with increased intestinal transit and delayed gastric emptying. Structurally similar to prostones with up to 54% prostaglandin E₂ activity on prostaglandin E receptor 1 (EP₁), lubiprostone may also exert EP₁-mediated procontractile effect on intestinal smooth muscles. We investigated lubiprostone's effects on intestinal smooth muscle contractions and pyloric sphincter tone.

Methods

Isolated murine small intestinal (longitudinal and circular) and pyloric tissues were mounted in organ baths with modified Krebs solution for isometric recording. Basal muscle tension and response to electrical field stimulation (EFS; 2 ms pulses/10 V/6 Hz/30 sec train) were measured with lubiprostone (10^-10-10^-5 M) ± EP₁ antagonist. Significance was established using Student t test and P < 0.05.

Results

Lubiprostone had no effect on the basal tension or EFS-induced contractions of longitudinal muscles. With circular muscles, lubiprostone caused a dose-dependent increase in EFS-induced contractions (2.11 ± 0.88 to 4.43 ± 1.38 N/g, P = 0.020) that was inhibited by pretreatment with EP₁ antagonist (1.69 ± 0.70 vs. 4.43 ± 1.38 N/g, P = 0.030). Lubiprostone had no effect on circular muscle basal tension, but it induced a dose-dependent increase in pyloric basal tone (1.07 ± 0.01 to 1.97 ± 0.86 fold increase, P < 0.05) that was inhibited by EP₁ antagonist.

Conclusions

In mice, lubiprostone caused a dose-dependent and EP₁-mediated increase in contractility of circular but not longitudinal small intestinal smooth muscles, and in basal tone of the pylorus. These findings suggest another mechanism for lubiprostone's observed clinical effects on gastrointestinal motility.

Cannabinoid CB2 receptor activation attenuates cytokine-evoked mucosal damage in a human colonic explant model without changing epithelial permeability

Cannabinoid receptor activation is protective in animal colitis models. We sought to investigate if cannabinoids attenuated colitis-like tissue damage in human colonic specimens, with the hypothesis that cannabinoids would be protective in a cytokine-driven model of human colonic mucosal damage. Healthy human colonic mucosa was incubated with pro-inflammatory cytokines TNF-α and IL-1β to elicit colitis-like tissue damage. The cytokine-driven increase in scored crypt and mucosal damage and lymphocyte density was attenuated with concomitant hydrocortisone pretreatment. The cannabinoid receptor 2 (CB2) receptor-selective agonist JWH-015 significantly reduced colitis scores following cytokine incubation, as evidenced by a reduction in mucosal crypt and luminal epithelial damage and lymphocyte density in the lamina propria. The effect of JWH-015 was reversed in the presence of the CB2 receptor inverse agonist JTE-907. Anandamide was also protective in the cytokine-incubated explant colitis model in a manner reversible with JTE-907, while CB1 receptor agonism with ACEA was without effect. TNF-α and IL-1β together evoked an increase in paracellular epithelial permeability in Caco-2 cell monolayers over 48h of incubation. However, neither CB2 nor CB1 receptor activation altered the cytokine-evoked increase in permeability. These findings support a discrete role for CB2 receptors in the attenuation of detrimental pro-inflammatory cytokine-mediated mucosal damage in the human colon without directly affecting mucosal epithelial barrier function.

Prophylactic and therapeutic benefits of COX-2 inhibitor on motility dysfunction in bowel obstruction: roles of PGE₂ and EP receptors

We reported previously that mechanical stretch in rat colonic obstruction induces cyclooxygenase (COX)-2 expression in smooth muscle cells. The aims of the present study were to investigate whether in vivo treatment with COX-2 inhibitor has prophylactic and therapeutic effects on motility dysfunction in colon obstruction, and if so what are the underlying mechanisms. Partial colon obstruction was induced with a silicon band in the distal colon of 6-8-wk-old Sprague-Dawley rats; obstruction was maintained for 3 days or 7 days. Daily administration of COX-2 inhibitor NS-398 (5 mg/kg) or vehicle was started before or after the induction of obstruction to study its prophylactic and therapeutic effects, respectively. The smooth muscle contractility was significantly suppressed, and colonic transit rate was slower in colonic obstruction. Prophylactic treatment with NS-398 significantly prevented the impairments of colonic transit and smooth muscle contractility and attenuated fecal collection in the occluded colons. When NS-398 was administered therapeutically 3 days after the initiation of obstruction, the muscle contractility and colonic transit still improved on day 7. Obstruction led to marked increase of COX-2 expression and prostaglandin E(2) (PGE(2)) synthesis. Exogenous PGE(2) decreased colonic smooth muscle contractility. All four PGE(2) E-prostanoid receptor types (EP1 to EP4) were detected in rat colonic muscularis externa. Treatments with EP1 and EP3 antagonists suppressed muscle contractility in control tissue but did not improve contractility in obstruction tissue. On the contrary, the EP2 and EP4 antagonists did not affect control tissue but significantly restored muscle contractility in obstruction. We concluded that our study shows that COX-2 inhibitor has prophylactic and therapeutic benefits for motility dysfunction in bowel obstruction. PGE(2) and its receptors EP2 and EP4 are involved in the motility dysfunction in obstruction, whereas EP1 and EP3 mediate PGE(2) regulation of colonic smooth muscle contractile function in normal state.

International Union of Basic and Clinical Pharmacology. LXXXIII: classification of prostanoid receptors, updating 15 years of progress

It is now more than 15 years since the molecular structures of the major prostanoid receptors were elucidated. Since then, substantial progress has been achieved with respect to distribution and function, signal transduction mechanisms, and the design of agonists and antagonists (http://www.iuphar-db.org/DATABASE/FamilyIntroductionForward?familyId=58). This review systematically details these advances. More recent developments in prostanoid receptor research are included. The DP(2) receptor, also termed CRTH2, has little structural resemblance to DP(1) and other receptors described in the original prostanoid receptor classification. DP(2) receptors are more closely related to chemoattractant receptors. Prostanoid receptors have also been found to heterodimerize with other prostanoid receptor subtypes and nonprostanoids. This may extend signal transduction pathways and create new ligand recognition sites: prostacyclin/thromboxane A(2) heterodimeric receptors for 8-epi-prostaglandin E(2), wild-type/alternative (alt4) heterodimers for the prostaglandin FP receptor for bimatoprost and the prostamides. It is anticipated that the 15 years of research progress described herein will lead to novel therapeutic entities.

An agonist sensitive, quick and simple cell-based signaling assay for determination of ligands mimicking prostaglandin E2 or E1 activity through subtype EP1 receptor: Suitable for high throughput screening

BACKGROUND

Conventionally the active ingredients in herbal extracts are separated into individual components, by fractionation, desalting, and followed by high-performance liquid chromatography (HPLC). In this study we have tried to directly screen water-soluble fractions of herbs with potential active ingredients before purification or extraction. We propose that the herbal extracts mimicking prostaglandin E(1) (PGE(1)) and E(2) (PGE(2)) can be identified in the water-soluble non-purified fraction. PGE(1) is a potent anti-inflammatory molecule used for treating peripheral vascular diseases while PGE(2) is an inflammatory molecule.

METHODS

We used cell-based assays (CytoFluor multi-well plate reader and fluorescence microscopy) in which a calcium signal was generated by the recombinant EP(1) receptor stably expressed in HEK293 cells (human embryonic kidney). PGE(1) and PGE(2) were tested for their ability to generate a calcium signal. Ninety-six water soluble fractions of Treasures of the east (single Chinese herb dietary supplements) were screened.

RESULTS

After screening, the top ten stimulators were identified. The identified herbs were then desalted and the calcium fluorescent signal reconfirmed using fluorescence microscopy. Among these top ten agonists identified, seven stimulated the calcium signaling (1-40 μM concentration) using fluorescence microscopy.

CONCLUSIONS

Fluorescence microscopy and multi-well plate readers can be used as a target specific method for screening water soluble fractions with active ingredients at a very early stage, before purification. Our future work consists of purifying and separating the active ingredients and repeating fluorescence microscopy. Under ordinary circumstances we would have to purify the compounds first and then test all the extracts from 96 herbs. Conventionally, for screening natural product libraries, the procedure followed is the automated separation of all constituents into individual components using fractionation and high performance liquid chromatography. We, however, demonstrated that the active ingredients of the herbal extracts can be tested before purification using an agonist sensitive, quick and simple cell-based signaling assay for ligands mimicking the agonists, PGE(1) and PGE(2).

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.