Regulation of enteric functions by adenosine: pathophysiological and pharmacological implications

Multifluid Metabolomics Identifies Novel Biomarkers for Irritable Bowel Syndrome

Background/Objectives: Irritable bowel syndrome (IBS) is a complex disorder affecting 10% of the global population, but the underlying mechanisms remain poorly understood. By integrating multifluid metabolomics, we aimed to identify metabolite markers of IBS in a large population-based cohort. Methods: We included individuals from TwinsUK with and without IBS, ascertained using the Rome III criteria, and analysed serum (232 cases, 1707 controls), urine (185 cases, 1341 controls), and stool (186 cases, 1284 controls) metabolites (Metabolon Inc.). Results: After adjusting for covariates, and multiple testing, 44 unique metabolites (25 novel) were associated with IBS, including lipids, amino acids, and xenobiotics. Androsterone sulphate, a sulfated steroid hormone precursor, was associated with lower odds of IBS in both urine (0.69 [95% confidence interval = 0.56-0.85], p = 2.34 × 10-4) and serum (0.75 [0.63-0.90], p = 1.54 × 10-3. Moreover, suberate (C8-DC) was associated with higher odds of IBS in serum (1.36 [1.15-1.61]; p = 1.84 × 10-4) and lower odds of IBS in stool (0.76 [0.63-0.91]; p = 2.30 × 10-3). On the contrary, 32 metabolites appeared to be fluid-specific, including indole, 13-HODE + 9-HODE, pterin, bilirubin (E,Z or Z,Z), and urolithin. The remaining 10 metabolites were associated with IBS in one fluid with suggestive evidence (p < 0.05) in another fluid. Finally, we identified androgenic signalling, dicarboxylates, haemoglobin, and porphyrin metabolism to be significantly over-represented in individuals with IBS compared to controls. Conclusions: Our results highlight the utility of a multi-fluid approach in IBS research, revealing distinct metabolic signatures across biofluids.

New drug discovery and development from natural products: Advances and strategies

Natural products (NPs) have a long history as sources for drug discovery, more than half of approved drugs are related to NPs, which also exhibit multifaceted advantages in the clinical treatment of complex diseases. However, bioactivity screening of NPs, target identification, and design optimization require continuously improved strategies, the complexity of drug mechanism of action and the limitations of technological strategies pose numerous challenges to the development of new drugs. This review begins with an overview of bioactivity- and target-based drug development patterns for NPs, advances in NP screening and derivatization, and the advantages and problems of major targets such as genes and proteins. Then, target-based drugs as well as identification and validation methods are further discussed to elucidate their mechanism of action. Subsequently, the current status and development trend of the application of traditional and emerging technologies in drug discovery and development of NPs are systematically described. Finally, the collaborative strategy of multi-technology integration and multi-disciplinary intersection is emphasized for the challenges faced in the identification, optimization, activity evaluation, and clinical application of NPs. It is hoped to provide a systematic overview and inspiration for exploring new drugs from natural resources in the future.

Adenosine signaling as target in cardiovascular pharmacology

Increasing evidence demonstrated the relevance of adenosine system in the onset and development of cardiovascular diseases, such as hypertension, myocardial infarct, ischemia, hypertension, heart failure, and atherosclerosis. In this regard, intense research efforts are being focused on the characterization of the pathophysiological significance of adenosine, acting at its membrane receptors named A1, A2A, A2B, and A3 receptors, in cardiovascular diseases. The present review article provides an integrated and comprehensive overview about current clinical and pre-clinical evidence about the role of adenosine in the pathophysiology of cardiovascular diseases. Particular attention has been focused on current scientific evidence about the pharmacological ligands acting on adenosine pathway as useful tools to manage cardiovascular diseases.

Adenosine A2A Receptor Activation Regulates Niemann-Pick C1 Expression and Localization in Macrophages

Adenosine plays an important role in modulating immune cell function, particularly T cells and myeloid cells, such as macrophages and dendritic cells. Cell surface adenosine A_2A receptors (A_2AR) regulate the production of pro-inflammatory cytokines and chemokines, as well as the proliferation, differentiation, and migration of immune cells. In the present study, we expanded the A_2AR interactome and provided evidence for the interaction between the receptor and the Niemann-Pick type C intracellular cholesterol transporter 1 (NPC1) protein. The NPC1 protein was identified to interact with the C-terminal tail of A_2AR in RAW 264.7 and IPMФ cells by two independent and parallel proteomic approaches. The interaction between the NPC1 protein and the full-length A_2AR was further validated in HEK-293 cells that permanently express the receptor and RAW264.7 cells that endogenously express A_2AR. A_2AR activation reduces the expression of NPC1 mRNA and protein density in LPS-activated mouse IPMФ cells. Additionally, stimulation of A_2AR negatively regulates the cell surface expression of NPC1 in LPS-stimulated macrophages. Furthermore, stimulation of A_2AR also altered the density of lysosome-associated membrane protein 2 (LAMP2) and early endosome antigen 1 (EEA1), two endosomal markers associated with the NPC1 protein. Collectively, these results suggested a putative A_2AR-mediated regulation of NPC1 protein function in macrophages, potentially relevant for the Niemann-Pick type C disease when mutations in NPC1 protein result in the accumulation of cholesterol and other lipids in lysosomes.

Adenosine 2A receptor contributes to the facilitation of post-infectious irritable bowel syndrome by γδ T cells via the PKA/CREB/NF-κB signaling pathway

BACKGROUND

Immunological dysfunction-induced low-grade inflammation is regarded as one of the predominant pathogenetic mechanisms in post-infectious irritable bowel syndrome (PI-IBS). γδ T cells play a crucial role in innate and adaptive immunity. Adenosine receptors expressed on the surface of γδ T cells participate in intestinal inflammation and immunity regulation.

AIM

To investigate the role of γδ T cell regulated by adenosine 2A receptor (A2AR) in PI-IBS.

METHODS

The PI-IBS mouse model has been established with Trichinella spiralis (T. spiralis) infection. The intestinal A2AR and A2AR in γδ T cells were detected by immunohistochemistry, and the inflammatory cytokines were measured by western blot. The role of A2AR on the isolated γδ T cells, including proliferation, apoptosis, and cytokine production, were evaluated in vitro. Their A2AR expression was measured by western blot and reverse transcription polymerase chain reaction (RT-PCR). The animals were administered with A2AR agonist, or A2AR antagonist. Besides, γδ T cells were also injected back into the animals, and the parameters described above were examined, as well as the clinical features. Furthermore, the A2AR-associated signaling pathway molecules were assessed by western blot and RT-PCR.

RESULTS

PI-IBS mice exhibited elevated ATP content and A2AR expression (P < 0.05), and suppression of A2AR enhanced PI-IBS clinical characteristics, indicated by the abdominal withdrawal reflex and colon transportation test. PI-IBS was associated with an increase in intestinal T cells, and cytokine levels of interleukin-1 (IL-1), IL-6, IL-17A, and interferon-α (IFN-α). Also, γδ T cells expressed A2AR in vitro and generated IL-1, IL-6, IL-17A, and IFN-α, which can be controlled by A2AR agonist and antagonist. Mechanistic studies demonstrated that the A2AR antagonist improved the function of γδ T cells through the PKA/CREB/NF-κB signaling pathway.

CONCLUSION

Our results revealed that A2AR contributes to the facilitation of PI-IBS by regulating the function of γδ T cells via the PKA/CREB/NF-κB signaling pathway.

Nucleoside transporters and immunosuppressive adenosine signaling in the tumor microenvironment: Potential therapeutic opportunities

Adenosine compartmentalization has a profound impact on immune cell function by regulating adenosine localization and, therefore, extracellular signaling capabilities, which suppresses immune cell function in the tumor microenvironment. Nucleoside transporters, responsible for the translocation and cellular compartmentalization of hydrophilic adenosine, represent an understudied yet crucial component of adenosine disposition in the tumor microenvironment. In this review article, we will summarize what is known regarding nucleoside transporter's function within the purinome in relation to currently devised points of intervention (i.e., ectonucleotidases, adenosine receptors) for cancer immunotherapy, alterations in nucleoside transporter expression reported in cancer, and potential avenues for targeting of nucleoside transporters for the desired modulation of adenosine compartmentalization and action. Further, we put forward that nucleoside transporters are an unexplored therapeutic opportunity, and modulation of nucleoside transport processes could attenuate the pathogenic buildup of immunosuppressive adenosine in solid tumors, particularly those enriched with nucleoside transport proteins.

Pathophysiology roles for adenosine 2A receptor in obesity and related diseases

Obesity, a burgeoning worldwide health system challenge, is associated with several comorbidities, including non-alcoholic fatty liver disease (NAFLD), diabetes, atherosclerosis, and osteoarthritis, leading to serious problems to people's health. Adenosine is a critical extracellular signaling molecule that has essential functions in regulating most organ systems by binding to four G-protein-coupled adenosine receptors, denoted A₁ , A_2A , A_2B , and A₃ . Among the receptors, a growing body evidence highlights the key roles of the adenosine 2A receptor (A_2A R) in obesity and related diseases. In the current review, we summarize the effects of A_2A R in obesity and obesity-associated non-alcoholic fatty liver disease, diabetes, atherosclerosis, and osteoarthritis, to clarify the complicated impacts of A_2A R on obesity and related diseases.

The possible role of the nucleoside adenosine in countering skin aging: A review

Skin aging is a complex biological process. Skin aspect is considered as a sign of well-being and of beauty. In view of this, noninvasive and/or minimally invasive anti-aging strategies were developed. Adenosine, a well-known nucleoside, may play a role in skin rejuvenation. Adenosine receptors belong to the G protein-coupled receptors superfamily and are divided into four subtypes: A₁ , A_2A , A_2B , and A₃ . The adenosine receptors expressed by skin are mainly the A₁ and A_2A subtypes. In the hypodermis, adenosine through the A₁ receptor stimulates lipogenesis and adipogenesis. In the dermis, adenosine through the A_2A receptor subtype stimulates collagen production. Moreover, the nucleoside increases new DNA synthesis and subsequently protein synthesis in dermal cells. Activation of adenosine receptors by interacting with various skin layers may induce a decrease in the amount of wrinkles, roughness, dryness, and laxity. This article has reviewed the mechanisms through which adenosine modulates biological mechanisms in the skin tissues and the effect of preparations containing adenosine or its derivatives on the skin.

Adenosine metabolized from extracellular ATP promotes type 2 immunity through triggering A2BAR signaling in intestinal epithelial cells

Intestinal nematode parasites can cross the epithelial barrier, causing tissue damage and release of danger-associated molecular patterns (DAMPs) that may promote host protective type 2 immunity. We investigate whether adenosine binding to the A2B adenosine receptor (A2BAR) on intestinal epithelial cells (IECs) plays an important role. Specific blockade of IEC A2BAR inhibits the host protective memory response to the enteric helminth, Heligmosomoides polygyrus bakeri (Hpb), including disruption of granuloma development at the host-parasite interface. Memory T cell development is blocked during the primary response, and transcriptional analyses reveal profound impairment of IEC activation. Extracellular ATP is visualized 24 h after inoculation and is shown in CD39-deficient mice to be critical for the adenosine production mediating the initiation of type 2 immunity. Our studies indicate a potent adenosine-mediated IEC pathway that, along with the tuft cell circuit, is critical for the activation of type 2 immunity.

The Adenosine Pathway and Human Immunodeficiency Virus-Associated Inflammation

Human immunodeficiency virus (HIV) is associated with an increased risk of age-associated comorbidities and mortality compared to people without HIV. This has been attributed to HIV-associated chronic inflammation and immune activation despite viral suppression. The adenosine pathway is an established mechanism by which the body regulates persistent inflammation to limit tissue damage associated with inflammatory conditions. However, HIV infection is associated with derangements in the adenosine pathway that limits its ability to control HIV-associated inflammation. This article reviews the function of purinergic signaling and the role of the adenosine signaling pathway in HIV-associated chronic inflammation. This review also discusses the beneficial and potential detrimental effects of pharmacotherapeutic strategies targeting this pathway among people with HIV.

Effectiveness of acupuncture for the recovery of gastrointestinal function of patients with gastric cancer in the postoperative period: A protocol for systematic review and meta-analysis

BACKGROUND

Gastric cancer (GC) is the most common malignant tumors in the world and surgical resection remains the primary treatment for it. Postoperative patients often suffer from gastrointestinal dysfunction as the most common side effects of surgery for GC patients. Acupuncture has a regulatory effect on gastrointestinal function. We conducted this study to assess the effectiveness of acupuncture on the restoration of gastrointestinal function of postoperative patients with GC.

METHODS

Seven electronic databases will be searched from inception to November 2020 to identify any relevant study: Medline, Embase, Cochrane Central Register of Controlled Trials (CENTRAL), China National Knowledge Infrastructure (CNKI), Wanfang Database, Chinese Biomedical Literature Database (CBM), and Chinese Scientific Journal Database (VIP database). No restriction on time and language. The primary outcome measure will be the Time to First Flatus and secondary outcome measures include the time of first defecation and the quality of life (QOL) and the number of patients with abdominal distention. We will use EndNote V.9.1 to screen the eligible literature and the I2 statistic to assess heterogeneity in the included studies. The meta-analysis will be conducted using the Review Manager (RevMan) software (V.5.3).

RESULTS

Our study aims to systematically assess whether the pooled effects of currently available trials prove effects of acupuncture in improving gastrointestinal function of patients with GC in the postoperative period.

CONCLUSION

This study will conduct an evaluation about the efficacy of acupuncture for the recovery of gastrointestinal function of patients with GC in the postoperative period, making up for the lack of relevant clinical evidence.

INPLASY REGISTRATION NUMBER

INPLASY2020110066.

Adenosine Signaling in the Tumor Microenvironment

Adenosine, deriving from ATP released by dying cancer cells and then degradated in the tumor environment by CD39/CD73 enzyme axis, is linked to the generation of an immunosuppressed niche favoring the onset of neoplasia. Signals delivered by extracellular adenosine are detected and transduced by G-protein-coupled cell surface receptors, classified into four subtypes: A₁, A_2A, A_2B, and A₃. A critical role of this nucleoside is emerging in the modulation of several immune and nonimmune cells defining the tumor microenvironment, providing novel insights about the development of novel therapeutic strategies aimed at undermining the immune-privileged sites where cancer cells grow and proliferate.

Suppression of adenosine A2a receptors alleviates bladder overactivity and hyperalgesia in cyclophosphamide-induced cystitis by inhibiting TRPV1

Interstitial cystitis/bladder pain syndrome (IC/BPS) is a type of chronic bladder inflammation characterized by increased voiding frequency, urgency and pelvic pain. The sensitization of bladder afferents is widely regarded as one of the pathophysiological changes in the development of IC/BPS. There is evidence that adenosine A_2a receptors are involved in regulating the sensitization of sensory afferents. However, the effect of adenosine A_2a receptors on cystitis remains unknown. In the present study, a rat model of chronic cystitis was established by intraperitoneal injection with cyclophosphamide (CYP). Cystometry and behavioral tests were performed to investigate bladder micturition function and nociceptive pain. The rats with chronic cystitis showed symptoms of bladder overactivity, characterized by an increase in bladder voiding frequency and voiding pressure. CYP treatment significantly increased the expression of the A_2a receptor in bladder afferent fibers and dorsal root ganglion (DRG) neurons. The A_2a receptor antagonist ZM241385 prevented bladder overactivity and hyperalgesia elicited by CYP-induced cystitis. In addition, the A_2a receptor and TRPV1 were coexpressed on DRG neurons. The TRPV1 antagonist capsazepine blocked bladder overactivity induced by the A_2a receptor agonist CGS21680. In contrast, ZM241385 significantly inhibited the capsaicin-induced increase in intracellular calcium concentration in DRG neurons. These results suggest that suppression of adenosine A_2a receptors in bladder afferents alleviates bladder overactivity and hyperalgesia elicited by CYP-induced cystitis in rats by inhibiting TRPV1, indicating that the adenosine A_2a receptor in bladder afferents is a potential therapeutic target for the treatment of IC/BPS.

Pitfalls and challenges of the purinergic signaling cascade in obesity

Obesity is a worldwide health problem which have reached pandemic proportions, now also including low and middle-income countries. Excessive or abnormal fat deposition in the abdomen especially in the visceral compartment is tightly associated with a high metabolic risk for arterial hypertension, type II diabetes, cardiovascular diseases, musculoskeletal disorders (especially articular degeneration) and some cancers. Contrariwise, accumulation of fat in the subcutaneous compartment has been associated with a neutral metabolic impact, favoring a lower risk of insulin resistance. Obesity results more often from an avoidable imbalance between food consumption and energy expenditure. There are several recommended strategies for dealing with obesity, including pharmacological therapies, but their success remains incomplete and may not compensate the associated adverse effects. Purinergic signaling operated by ATP and its metabolite, adenosine, has attracted increasing attention in obesity. The extracellular levels of purines often reflect the energy status of a given cell population. Adenine nucleotides and nucleosides fine tuning control adipogenesis and mature adipocytes function via the activation of P2 and P1 purinoceptors, respectively. These features make the purinergic signaling cascade a putative target for therapeutic intervention in obesity and related metabolic syndromes. There are, however, gaps in our knowledge regarding the role of purines in adipocyte precursors differentiation and mature adipocytes functions, as well as their impact among distinct adipose tissue deposits (e.g. white vs. brown, visceral vs. subcutaneous), which warrants further investigations before translation to clinical trials can be made.

Adenosine A2A Receptor Antagonists for Cancer Immunotherapy

Currently, the most promising therapeutic modality for cancer treatment is the blockade of immune checkpoint pathways, which has revolutionized cancer therapy in the past 15 years. Strategies targeting and modulating adenosine A_2A receptor (A_2AR), an emerging alternative immune checkpoint, have shown the potential to produce significant therapeutic effects. In this review, we describe the immunosuppressive activities of A_2AR and A_2BR in the tumor microenvironment (TME), followed by a summary and discussion of the structure-activity relationship (SAR) of the A_2AR (and dual A_2AR/A_2BR) antagonists that have been experimentally confirmed to exert oncoimmunological effects. This review also provides an update on the compounds under clinical evaluation and insights into the ligand binding modes of the receptor.

The Adenosine System at the Crossroads of Intestinal Inflammation and Neoplasia

Adenosine is a purine nucleoside, resulting from the degradation of adenosine triphosphate (ATP). Under adverse conditions, including hypoxia, ischemia, inflammation, or cancer, the extracellular levels of adenosine increase significantly. Once released, adenosine activates cellular signaling pathways through the engagement of the four known G-protein-coupled receptors, adenosine A₁ receptor subtype (A₁), A_2A, A_2B, and A₃. These receptors, expressed virtually on all immune cells, mitigate all aspects of immune/inflammatory responses. These immunosuppressive effects contribute to blunt the exuberant inflammatory responses, shielding cells, and tissues from an excessive immune response and immune-mediated damage. However, a prolonged persistence of increased adenosine concentrations can be deleterious, participating in the creation of an immunosuppressed niche, ideal for neoplasia onset and development. Based on this evidence, the present review has been conceived to provide a comprehensive and critical overview of the involvement of adenosine system in shaping the molecular mechanisms underlying the enteric chronic inflammation and in promoting the generation of an immunosuppressive niche useful for the colorectal tumorigenesis.

Inhibition of A2B Adenosine Receptor Attenuates Intestinal Injury in a Rat Model of Necrotizing Enterocolitis

Necrotizing enterocolitis (NEC) is a lethal gastrointestinal tract disease that occurs in premature infants. Adenosine receptor A_2B (A_2BR) regulates the inflammation cytokine secretion and immune cell infiltration in the colonic pathophysiology conditions. In the present study, we aim to determine the roles of A_2BR in the development of NEC. A NEC rat model was established and treated with A_2BR agonist-BAY60-6583 or A_2BR antagonist-PSB1115. Animals in the control group were free from any interventions. Our results showed that the inhibition of A_2BR PSB1115 improved intestinal injury and inflammation in newborn NEC rats. The expression levels of caspase-3 and the ratio of apoptotic cells were upregulated in NEC rats, and these indices were downregulated after treating with PSB1115 but further upregulated by BAY60-6583. Meanwhile, a similar trend was also witnessed in the changes of MPO activities and proinflammatory cytokines including IL-6, IFN-γ, and TNF-α. However, the anti-inflammatory cytokine IL-10 in the NECP group was significantly higher than that in the NEC and NECB groups (p < 0.05, respectively). Moreover, the expression of Ki67 was significantly increased in the NECP group as compared with those of the NEC and the NECB groups (p < 0.05, respectively). Collectively, our study suggested that the inhibition of A_2BR attenuates NEC in the neonatal rat, at least partially through the modulation of inflammation and the induction of epithelial cell proliferation.

The therapeutic potential of targeting CD73 and CD73-derived adenosine in melanoma

The hypoxic environment of melanoma results in CD73 upregulation on the surface of various tumor microenvironment (TME) cells including tumor cells, stromal cells and infiltrated immune cells. Consequently, CD73 through both enzymatic and none enzymatic functions affect melanoma progression. Overaccumulation of CD73-derived adenosine through interaction with its four G coupled receptors (A₁AR, A_2AAR, A_2BAR, and A₃AR) mediate tumor growth, immune suppression, angiogenesis, and metastasis. This paper aims to comprehensively review the therapeutic potential of CD73 ectonucleotidase targeting in melanoma. To reach this goal, firstly, we summarize the structure, function, regulation, and clinical outcome of CD73 ectonucleotidase. Then, we depict the metabolism and signaling of CD73-derived adenosine along with its progressive role in development of melanoma. Furthermore, the therapeutic potentials of CD73 -adenosine axis targeting is assessed in both preclinical and clinical studies. Targeting CD73-derived adenosine via small molecule inhibitor or monoclonal antibodies studies especially in combination with immune checkpoint blockers including PD-1 and CTLA-4 have shown desirable results for management of melanoma in preclinical studies and several clinical trials have recently been started to evaluate the therapeutic potential of CD73-derived adenosine targeting in solid tumors. Indeed, targeting of CD73-derived adenosine signaling could be considered as a new therapeutic target in melanoma.

The Anti-Inflammatory and Pain-Relieving Effects of AR170, an Adenosine A3 Receptor Agonist, in a Rat Model of Colitis

The pharmacological activation of A₃ receptors has shown potential usefulness in the management of bowel inflammation. However, the role of these receptors in the control of visceral hypersensitivity in the presence of intestinal inflammation has not been investigated. The effects of AR170, a potent and selective A₃ receptor agonist, and dexamethasone (DEX) were tested in rats with 2,4-dinitrobenzene sulfonic acid (DNBS)-induced colitis to assess their tissue inflammatory parameters. The animals received AR170, DEX, or a vehicle intraperitoneally for 6 days, starting 1 day before the induction of colitis. Visceral pain was assessed by recording the abdominal responses to colorectal distension in animals with colitis. Colitis was associated with a decrease in body weight and an increase in spleen weight. The macroscopic damage score and tissue tumor necrosis factor (TNF), interleukin 1β (IL-1β), and myeloperoxidase (MPO) levels were also enhanced. AR170, but not DEX, improved body weight. Both drugs counteracted the increase in spleen weight, ameliorated macroscopic colonic damage, and decreased TNF, IL-1β, and MPO tissue levels. The enhanced visceromotor response (VMR) in rats with colitis was decreased via AR170 administration. In rats with colitis, AR170 counteracted colonic inflammatory cell infiltration and decreased pro-inflammatory cytokine levels, thereby relieving visceral hypersensitivity.

Targeting adenosinergic pathway and adenosine A2A receptor signaling for the treatment of COVID-19: A hypothesis

The most serious health issue today is the rapid outbreak of Coronavirus Disease 2019 (COVID-19). More than 6,973,427 confirmed cases were diagnosed in nearly 213 countries and territories around the world and two international conveyances, causing globally over 400,000 deaths. Epidemiology, risk factors, and clinical characteristics of COVID-19 patients have been identified, but the factors influencing the immune system against COVID-19 have not been well established. Upon infection or cell damage, high amounts of adenosine triphosphate (ATP) are released from damaged cells, which serve as mediators of inflammation through purinergic cell surface receptor signaling. As a protective mechanism to prevent excessive damage to host tissue, adenosine counteracts ATP's effects by adenosine receptor stimulation to suppress the pro-inflammatory response. Adenosine is seen as a major obstacle to the efficacy of immune therapies, and the adenosinergic axis components are critical therapeutic targets for cancer and microbial infections. Pharmacologic inhibitors or antibodies specific to adenosinergic pathway components or adenosine receptors in microbial and tumor therapy have shown efficacy in pre-clinical studies and are entering the clinical arena. In this review, we provide a novel hypothesis explaining the potential for improving the efficiency of innate and adaptive immune systems by targeting adenosinergic pathway components and adenosine A_2A receptor signaling for the treatment of COVID-19.

Glial A2B Adenosine Receptors Modulate Abnormal Tachykininergic Responses and Prevent Enteric Inflammation Associated with High Fat Diet-Induced Obesity

The role played by adenosine A_2B receptors (A_2BRs) in the regulation of enteric glial cell (EGC) functions remains unclear. This study was aimed at investigating the involvement of A_2BRs in the control of EGC functions in a model of obesity. C57BL/6 mice were fed with standard diet (SD) or high fat diet (HFD) for eight weeks. Colonic tachykininergic contractions were recorded in the presence of BAY60-6583 (A_2BRs agonist), MRS1754 (A_2BRs antagonist), and the gliotoxin fluorocitrate. Immunofluorescence distribution of HuC/D, S100β, and A_2BRs was assessed in whole mount preparations of colonic myenteric plexus. To mimic HFD, EGCs were incubated in vitro with palmitate (PA) and lipopolysaccharide (LPS), in the absence or in the presence of A_2BR ligands. Toll-like receptor 4 (TLR4) expression was assessed by Western blot analysis. Interleukin-1β (IL-1β), substance P (SP), and glial cell derived neurotrophic factor (GDNF) release were determined by enzyme-linked immunosorbent assay (ELISA) assays. MRS1754 enhanced electrically evoked tachykininergic contractions of colonic preparations from HFD mice. BAY60-6583 decreased the evoked tachykininergic contractions, with higher efficacy in HFD mice. Such effects were blunted upon incubation with fluorocitrate. In in vitro experiments on EGCs, PA and LPS increased TLR4 expression as well as IL-1β, GDNF, and SP release. Incubation with BAY60-6583 reduced TLR4 expression as well as IL-1β, GDNF, and SP release. Such effects were blunted by MRS1754. The present results suggest that A_2BRs, expressed on EGCs, participate in the modulation of enteric inflammation and altered tachykininergic responses associated with obesity, thus representing a potential therapeutic target.

Role and Function of Adenosine and its Receptors in Inflammation, Neuroinflammation, IBS, Autoimmune Inflammatory Disorders, Rheumatoid Arthritis and Psoriasis

The physiological effects of endogenous adenosine on various organ systems are very complex and numerous which are elicited upon activation of any of the four G-protein-coupled receptors (GPCRs) denoted as A1, A2A, A2B and A3 adenosine receptors (ARs). Several fused heterocyclic and non-xanthine derivatives are reported as a possible target for these receptors due to physiological problems and lack of selectivity of xanthine derivatives. In the present review, we have discussed the development of various new chemical entities as a target for these receptors. In addition, compounds acting on adenosine receptors can be utilized in treating diseases like inflammation, neuroinflammation, autoimmune and related diseases.

Medicinal Chemistry and Therapeutic Potential of Agonists, Antagonists and Allosteric Modulators of A1 Adenosine Receptor: Current Status and Perspectives

Adenosine is a purine nucleoside, responsible for the regulation of a wide range of physiological and pathophysiological conditions by binding with four G-protein-coupled receptors (GPCRs), namely A1, A2A, A2B and A3 adenosine receptors (ARs). In particular, A1 AR is ubiquitously present, mediating a variety of physiological processes throughout the body, thus represents a promising drug target for the management of various pathological conditions. Agonists of A1 AR are found to be useful for the treatment of atrial arrhythmia, angina, type-2 diabetes, glaucoma, neuropathic pain, epilepsy, depression and Huntington's disease, whereas antagonists are being investigated for the treatment of diuresis, congestive heart failure, asthma, COPD, anxiety and dementia. However, treatment with full A1 AR agonists has been associated with numerous challenges like cardiovascular side effects, off-target activation as well as desensitization of A1 AR leading to tachyphylaxis. In this regard, partial agonists of A1 AR have been found to be beneficial in enhancing insulin sensitivity and subsequently reducing blood glucose level, while avoiding severe CVS side effects and tachyphylaxis. Allosteric enhancer of A1 AR is found to be potent for the treatment of neuropathic pain, culminating the side effects related to off-target tissue activation of A1 AR. This review provides an overview of the medicinal chemistry and therapeutic potential of various agonists/partial agonists, antagonists and allosteric modulators of A1 AR, with a particular emphasis on their current status and future perspectives in clinical settings.

Therapeutic Potentials of A2B Adenosine Receptor Ligands: Current Status and Perspectives

BACKGROUND

Adenosine receptors (ARs) are classified as A1, A2A, A2B, and A3 subtypes belong to the superfamily of G-protein coupled receptors (GPCRs). More than 40% of modern medicines act through either activation or inhibition of signaling processes associated with GPCRs. In particular, A2B AR signaling pathways are implicated in asthma, inflammation, cancer, ischemic hyperfusion, diabetes mellitus, cardiovascular diseases, gastrointestinal disorders, and kidney disease.

METHODS

This article reviews different disease segments wherein A2B AR is implicated and discusses the potential role of subtype-selective A2B AR ligands in the management of such diseases or disorders. All the relevant publications on this topic are reviewed and presented scientifically.

RESULTS

This review provides an up-to-date highlight of the recent advances in the development of novel and selective A2B AR ligands and their therapeutic role in treating various disease conditions. A special focus has been given to the therapeutic potentials of selective A2B AR ligands in the management of airway inflammatory conditions and cancer.

CONCLUSIONS

This systematic review demonstrates the current status and perspectives of A2B AR ligands as therapeutically useful agents that would assist medicinal chemists and pharmacologists in discovering novel and subtype-selective A2B AR ligands as potential drug candidates.

Approaches for designing and discovering purinergic drugs for gastrointestinal diseases

INTRODUCTION

Purines finely modulate physiological motor, secretory, and sensory functions in the gastrointestinal tract. Their activity is mediated by the purinergic signaling machinery, including receptors and enzymes regulating their synthesis, release, and degradation. Several gastrointestinal dysfunctions are characterized by alterations affecting the purinergic system.

AREAS COVERED

The authors provide an overview on the purinergic receptor signaling machinery, the molecules and proteins involved, and a summary of medicinal chemistry efforts aimed at developing novel compounds able to modulate the activity of each player involved in this machinery. The involvement of purinergic signaling in gastrointestinal motor, secretory, and sensory functions and dysfunctions, and the potential therapeutic applications of purinergic signaling modulators, are then described.

EXPERT OPINION

A number of preclinical and clinical studies demonstrate that the pharmacological manipulation of purinergic signaling represents a viable way to counteract several gastrointestinal diseases. At present, the paucity of purinergic therapies is related to the lack of receptor-subtype-specific agonists and antagonists that are effective in vivo. In this regard, the development of novel therapeutic strategies should be focused to include tools able to control the P1 and P2 receptor expression as well as modulators of the breakdown or transport of purines.

Adenosine deaminase inhibition

Adenosine deaminase is a critical enzyme in purine metabolism that regulates intra and extracellular adenosine concentrations by converting it to inosine. Adenosine is an important purine that regulates numerous physiological functions by interacting with its receptors. Adenosine and consequently adenosine deaminase can have pro or anti-inflammatory effects on tissues depending on how much time has passed from the start of the injury. In addition, an increase in adenosine deaminase activity has been reported for various diseases and the significant effect of deaminase inhibition on the clinical course of different diseases has been reported. However, the use of inhibitors is limited to only a few medical indications. Data on the increase of adenosine deaminase activity in different diseases and the impact of its inhibition in various cases have been collected and are discussed in this review. Overall, the evidence shows that many studies have been done to introduce inhibitors, however, in vivo studies have been much less than in vitro, and often have not been expanded for clinical use.

Role of purines in regulation of metabolic reprogramming

Purines, among most influential molecules, are reported to have essential biological function by regulating various cell types. A large number of studies have led to the discovery of many biological functions of the purine nucleotides such as ATP, ADP, and adenosine, as signaling molecules that engage G protein-coupled or ligand-gated ion channel receptors. The role of purines in the regulation of cellular functions at the gene or protein level has been well documented. With the advances in multiomics, including those from metabolomic and bioinformatic analyses, metabolic reprogramming was identified as a key mechanism involved in the regulation of cellular function under physiological or pathological conditions. Recent studies suggest that purines or purine-derived products contribute to important regulatory functions in many fundamental biological and pathological processes related to metabolic reprogramming. Therefore, this review summarizes the role and potential mechanism of purines in the regulation of metabolic reprogramming. In particular, the molecular mechanisms of extracellular purine- and intracellular purine-mediated metabolic regulation in various cells during disease development are discussed. In summary, our review provides an extensive resource for studying the regulatory role of purines in metabolic reprogramming and sheds light on the utilization of the corresponding peptides or proteins for disease diagnosis and therapy.

Adenosine receptor activation in the Th17 autoimmune responses of experimental autoimmune uveitis

Th17-type autoreactive T cells contribute to pathogenicity in autoimmune diseases, including autoimmune uveitis. However, the mechanisms of regulation of Th17 cell activities remain unsolved and are likely to be tissue- and disease specific. In this review, we have summarized our studies from the murine model of experimental autoimmune uveitis (EAU). The resultsdemonstrate that γδ T cells have a regulatory effect on Th17 response. The regulatory effects of γδ T cells depend on their action state. Activated γδ T cells express significantly high levels of adenosine receptor A2 (A2AR) but low CD73. Both molecules are crucially involved in adenosine generation, thus modifying T cell responses. While the increased expression of A2AR-allows activated γδ T cells to bind adenosine more effectively than other immune cells, the decreased CD73 restricts their ability to convert AMP to adenosine. Adenosine affects Th1 and Th17 autoimmune responses differently. Its activation of γδ T cells shifts the Th1/Th17 balance towards the Th17 autoreactivity.

The relatively selective cyclooxygenase-2 inhibitor nimesulide: What's going on?

Nimesulide is a relatively selective cyclooxygenase (COX)-2 inhibitor, non-steroidal anti-inflammatory drug; it has been discovered in 1971 and firstly commercialized in Italy in 1985. There is much evidence that the pharmacological profile of nimesulide is peculiar and not shared with the other COX-2 selective inhibitors, suggesting that other molecular mechanisms besides inhibition of COX-2 derived prostaglandins are involved. Similarly, experimental data suggest that the gastrointestinal safety of nimesulide cannot be ascribed only to a COX-1 sparing effect. On the inflammatory process, the efficacy of nimesulide is dependent upon a wide spectrum of actions, due to the combination of effects on immune and non-immune cells. Early data demonstrated a central role for cyclic AMP (cAMP) in the anti-inflammatory effect of nimesulide; more recently, we have shown the involvement of the pathway ecto-5'-nucleotidase/adenosine A_2A receptor. To date, the molecular mechanism(s) that confers uniqueness to nimesulide have not yet been defined. To go inside the mechanism of action of an existing drug, such as nimesulide, would be helpful to refine its therapeutic use but also to identify new targets for novel therapeutic anti-inflammatory approach. Here, we focus on accumulated evidence for a peculiar pharmacological profile of nimesulide.

Pharmacology of Adenosine Receptors: The State of the Art

Adenosine is a ubiquitous endogenous autacoid whose effects are triggered through the enrollment of four G protein-coupled receptors: A₁, A_2A, A_2B, and A₃. Due to the rapid generation of adenosine from cellular metabolism, and the widespread distribution of its receptor subtypes in almost all organs and tissues, this nucleoside induces a multitude of physiopathological effects, regulating central nervous, cardiovascular, peripheral, and immune systems. It is becoming clear that the expression patterns of adenosine receptors vary among cell types, lending weight to the idea that they may be both markers of pathologies and useful targets for novel drugs. This review offers an overview of current knowledge on adenosine receptors, including their characteristic structural features, molecular interactions and cellular functions, as well as their essential roles in pain, cancer, and neurodegenerative, inflammatory, and autoimmune diseases. Finally, we highlight the latest findings on molecules capable of targeting adenosine receptors and report which stage of drug development they have reached.

Therapeutic potency of pharmacological adenosine receptors agonist/antagonist on cancer cell apoptosis in tumor microenvironment, current status, and perspectives

The hypoxic niche of tumor leads to a tremendous increase in the extracellular adenosine concentration through alteration of adenosine metabolism in the tumor microenvironment (TME). This consequently affects cancer progression, local immune responses, and apoptosis of tumor cells. Regulatory effect of adenosine on apoptosis in TME depends on the cancer cell type, pharmacological characteristics of adenosine receptor subtypes, and the adenosine concentration in the tumor niche. Exploiting specific pharmacological adenosine receptor agonist and antagonist inducing apoptosis in cancer cells can be considered as a proper procedure to control cancer progression. This review summarizes the regulatory role of adenosine in cancer cell apoptosis for a better understanding, and hence better management of the disease.

A2AR Antagonism with CPI-444 Induces Antitumor Responses and Augments Efficacy to Anti-PD-(L)1 and Anti-CTLA-4 in Preclinical Models

Adenosine signaling through A2A receptors (A2AR) expressed on immune cells suppresses antitumor immunity. CPI-444 is a potent, selective, oral A2AR antagonist. Blockade of A2AR with CPI-444 restored T-cell signaling, IL2, and IFNγ production that were suppressed by adenosine analogues in vitro CPI-444 treatment led to dose-dependent inhibition of tumor growth in multiple syngeneic mouse tumor models. Concentrations of extracellular adenosine in the tumor microenvironment, measured using microdialysis, were approximately 100-150 nmol/L and were higher than corresponding subcutaneous tissue. Combining CPI-444 with anti-PD-L1 or anti-CTLA-4 treatment eliminated tumors in up to 90% of treated mice, including restoration of immune responses in models that incompletely responded to anti-PD-L1 or anti-CTLA-4 monotherapy. Tumor growth was fully inhibited when mice with cleared tumors were later rechallenged, indicating that CPI-444 induced systemic antitumor immune memory. CD8⁺ T-cell depletion abrogated the efficacy of CPI-444 with and without anti-PD-L1 treatment, demonstrating a role for CD8⁺ T cells in mediating primary and secondary immune responses. The antitumor efficacy of CPI-444 with and without anti-PD-L1 was associated with increased T-cell activation, a compensatory increase in CD73 expression, and induction of a Th1 gene expression signature consistent with immune activation. These results suggest a broad role for adenosine-mediated immunosuppression in tumors and justify the further evaluation of CPI-444 as a therapeutic agent in patients with solid tumors. Cancer Immunol Res; 6(10); 1136-49. ©2018 AACR.

The Impact of Purinergic System Enzymes on Noncommunicable, Neurological, and Degenerative Diseases

Evidences show that purinergic signaling is involved in processes associated with health and disease, including noncommunicable, neurological, and degenerative diseases. These diseases strike from children to elderly and are generally characterized by progressive deterioration of cells, eventually leading to tissue or organ degeneration. These pathological conditions can be associated with disturbance in the signaling mediated by nucleotides and nucleosides of adenine, in expression or activity of extracellular ectonucleotidases and in activation of P2X and P2Y receptors. Among the best known of these diseases are atherosclerosis, hypertension, cancer, epilepsy, Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS). The currently available treatments present limited effectiveness and are mostly palliative. This review aims to present the role of purinergic signaling highlighting the ectonucleotidases E-NTPDase, E-NPP, E-5'-nucleotidase, and adenosine deaminase in noncommunicable, neurological, and degenerative diseases associated with the cardiovascular and central nervous systems and cancer. In conclusion, changes in the activity of ectonucleotidases were verified in all reviewed diseases. Although the role of ectonucleotidases still remains to be further investigated, evidences reviewed here can contribute to a better understanding of the molecular mechanisms of highly complex diseases, which majorly impact on patients' quality of life.

Adenosinergic signaling inhibits oxalate transport by human intestinal Caco2-BBE cells through the A2B adenosine receptor

Most kidney stones (KS) are composed of calcium oxalate, and small increases in urine oxalate affect the stone risk. Intestinal oxalate secretion mediated by anion exchanger SLC26A6 (PAT1) plays a crucial role in limiting net absorption of ingested oxalate, thereby preventing hyperoxaluria and related KS, reflecting the importance of understanding regulation of intestinal oxalate transport. We previously showed that ATP and UTP inhibit oxalate transport by human intestinal Caco2-BBE cells (C2). Since ATP is rapidly degraded to adenosine (ADO), we examined whether intestinal oxalate transport is regulated by ADO. We measured [¹⁴C]oxalate uptake in the presence of an outward Cl gradient as an assay of Cl-oxalate exchange activity, ≥49% of which is PAT1-mediated in C2 cells. We found that ADO significantly inhibited oxalate transport by C2 cells, an effect completely blocked by the nonselective ADO receptor antagonist 8- p-sulfophenyltheophylline. ADO also significantly inhibited oxalate efflux by C2 cells, which is important since PAT1 mediates oxalate efflux in vivo. Using pharmacological antagonists and A_2B adenosine receptor (A_2B AR) siRNA knockdown studies, we observed that ADO inhibits oxalate transport through the A_2B AR, phospholipase C, and PKC. ADO inhibits oxalate transport by reducing PAT1 surface expression as shown by biotinylation studies. We conclude that ADO inhibits oxalate transport by lowering PAT1 surface expression in C2 cells through signaling pathways including the A_2B AR, PKC, and phospholipase C. Given higher ADO levels and overexpression of the A_2B AR in inflammatory bowel disease (IBD), our findings have potential relevance to pathophysiology of IBD-associated hyperoxaluria and related KS.

Toward an effective peripheral visceral analgesic: responding to the national opioid crisis

This minireiew summarizes recent new developments in visceral analgesics. This promising field is important, as a new approach to address abdominal pain with peripheral visceral analgesics is considered a key approach to addressing the current opioid crisis. Some of the novel compounds address peripheral pain mechanisms through modulation of opioid receptors via biased ligands, nociceptin/orphanin FQ opioid peptide (NOP) receptor, or dual action on NOP and μ-opioid receptor, buprenorphine and morphiceptin analogs. Other compounds target nonopioid mechanisms, including cannabinoid (CB2), N-methyl-d-aspartate, calcitonin gene-related peptide, estrogen, and adenosine A_2B receptors and transient receptor potential (TRP) channels (TRPV1, TRPV4, and TRPM8). Although current evidence is based predominantly on animal models of visceral pain, early human studies also support the evidence from the basic and animal research. This augurs well for the development of nonaddictive, visceral analgesics for treatment of chronic abdominal pain, an unmet clinical need.

New potent and selective A1 adenosine receptor antagonists as potential tools for the treatment of gastrointestinal diseases

The synthesis of 9-alkyl substituted adenine derivatives presenting aromatic groups and cycloalkyl rings in 8- and N⁶-position, respectively, is reported. The compounds were tested with radioligand binding studies showing, in some cases, a low nanomolar A₁ adenosine receptor affinity and a very good selectivity versus the other adenosine receptor subtypes. Functional assays at human adenosine receptors and at a mouse ileum tissue preparation clearly demonstrate the antagonist profile of these molecules, with inhibitory potency at nanomolar level. A molecular modeling study, consisting in docking analysis at the recently reported A₁ adenosine receptor crystal structure, was performed for the interpretation of the obtained pharmacological results. The N⁶-cyclopentyl-9-methyl-8-phenyladenine (17), resulting the most active derivative of the series (K_i = 2.8 nM and IC₅₀ = 14 nM), was also very efficacious in counteracting the effect of the agonist CCPA on mouse ileum contractility. This new compound represents a tool for the development of new agents for the treatment of intestinal diseases as constipation and postoperative ileus.

Adenosine A2A Receptor Agonist, 2-p-(2-Carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine Hydrochloride Hydrate, Inhibits Inflammation and Increases Fibroblast Growth Factor-2 Tissue Expression in Carrageenan-Induced Rat Paw Edema

Adenosine is the final product of ATP metabolism, mainly derived from the action of 5'-nucleotidase cleavage of AMP. Cellular production of adenosine is greatly enhanced in inflamed tissues, ischemic tissues, and under hypoxia, where ATP is released from damaged cells. Much evidence has been accumulated on adenosine anti-inflammatory effects mediated through A2A receptor activation; A2A adenosine receptor has also been shown to play a role in matrix deposition and wound healing in a damaged tissue, contributing to dermal tissue protection and repair. Fibroblast growth factor-2 (FGF-2) is a powerful mitogen for fibroblast; it is expressed by several inflammatory cell types and plays a pivotal role in angiogenesis, wound healing, gastric ulcer protection. Human recombinant FGF-2 has been shown to have anti-inflammatory effects. The purpose of the present work was to investigate on the anti-inflammatory effect of systemic administration of the adenosine A2A agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride hydrate (CGS21680) in the rat model of carrageenan-induced paw edema. We found that CGS21680 inhibits inflammation induced by carrageenan injection into the rat paw, and this effect is associated to the local reduction of cytokine levels and dermal increase of FGF-2 expression. Our results suggest that FGF-2 might be involved in the anti-inflammatory and tissue protective effect due to A2A receptor activation.

The role of adenosinergic pathway in human autoimmune diseases

Autoimmune diseases are characterized by the abnormal immune response against self-tissue, which are caused by the failure of nature immune homeostasis. Nature immune homeostasis represents the normal state of appropriate immune response to nonself-antigen and unresponsiveness to self-antigens. In normal situation, immune homeostasis is regulated by immunosuppressive signal and immunostimulating signal together. Accumulating data have demonstrated that the adenosinergic pathway played key roles in immune suppression and shield body from an excessive inflammatory response. The deficiency of adenosinergic pathway results in the imbalance between the pro- and anti-inflammatory activities. Thus, researchers pay much attention to the role of adenosinergic pathway in autoimmune diseases development. To date, accumulating data have suggested an important role of adenosinergic pathway-related molecules (i.e., CD39, CD73, ADA, adenosine receptors, etc.) in many types of human autoimmune diseases. More importantly, these findings have presented potential value of adenosinergic pathway analysis to be used for autoimmune diseases diagnosis, monitoring and treatment. In this review, we will provide a comprehensive description of the role of adenosinergic pathway in human autoimmune diseases.

Purinergic Receptors in Adipose Tissue As Potential Targets in Metabolic Disorders

Extracellular nucleosides and nucleotides, such as adenosine and adenosine triphosphate (ATP), are involved in many physiological and pathological processes in adipose tissue (AT). It is becoming accepted that, in addition to the well-established sympathetic and hormonal system, purinergic receptors contribute significantly to regulation of adipocyte functions. Several receptor subtypes for both adenosine (P1) and ATP (P2X and P2Y) have been characterized in white adipocytes (WA) and brown adipocytes (BA). The effects mediated by adenosine and ATP on adipocytes are multiple and often differing, depending on specific receptors activated. Using a variety of agonists, antagonists and transgenic animals it has been demonstrated that adenosine and P2 receptors are involved in lipolysis, lipogenesis, adipokines secretion, glucose uptake, adipogenesis, cell proliferation, inflammation, and other processes. Given their central role in regulating many AT functions, purinergic receptors are considered potential therapeutic targets in different pathological conditions, such as obesity and type-2 diabetes. To achieve this goal, specific and potent P1 and P2 receptors activators and inhibitors are being developed and show promising results. However, more insight is needed into the function of P2 receptors in brown and beige adipocytes and their potential role in thermogenesis. This review aims at summarizing current knowledge on the patho-/physiological role of P1, P2X, and P2Y receptors in WA and BA and their potential exploitation for pharmacological intervention. Furthermore, we analyze impact of purinergic signaling in AT - in health and metabolic diseases.

Anti-inflammatory effect of a novel locally acting A2A receptor agonist in a rat model of oxazolone-induced colitis

Adenosine represents a powerful modulating factor, which has been shown to orchestrate the scope, duration, and remission of the inflammatory response through the activation of four specific receptors, classified as A₁, A_2A, A_2B, and A₃, all being widely expressed in a variety of immune cells. Several selective A_2A receptor agonists have displayed anti-inflammatory effects, through the suppression of IL-12, TNF, and IFN-γ production by monocytes and lymphocytes, in the setting of chronic intestinal inflammation. However, the therapeutic application of A_2A receptor agonists remains hindered by the risk of serious cardiovascular adverse effects arising from the wide systemic distribution of A_2A receptors. The present study focused on evaluating the anti-inflammatory effects of the novel poorly absorbed A_2A receptor agonist PSB-0777 in a rat model of oxazolone-induced colitis as well as to evaluate its cardiovascular adverse effects, paying particular attention to the onset of hypotension, one of the main adverse effects associated with the systemic pharmacological activation of A_2A receptors. Colitis was associated with decreased body weight, an enhanced microscopic damage score and increased levels of colonic myeloperoxidase (MPO). PSB-0777, but not dexamethasone, improved body weight. PSB-0777 and dexamethasone ameliorated microscopic indexes of inflammation and reduced MPO levels. The beneficial effects of PSB-0777 on inflammatory parameters were prevented by the pharmacological blockade of A_2A receptors. No adverse cardiovascular events were observed upon PSB-0777 administration. The novel A_2A receptor agonist PSB-0777 could represent the base for the development of innovative pharmacological entities able to act in an event-specific and site-specific manner.

Adenosine A2B Receptors: An Optional Target for the Management of Irritable Bowel Syndrome with Diarrhea?

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder, with the characteristic symptoms of chronic abdominal pain and altered bowel habits (diarrhea, constipation, or both). IBS is a highly prevalent condition, which negatively affects quality of life and is a significant burden on global healthcare costs. Although many pharmacological medicines have been proposed to treat IBS, including those targeting receptors, channels, and chemical mediators related to visceral hypersensitivity, successful pharmacotherapy for the disease has not been established. Visceral hypersensitivity plays an important role in IBS pathogenesis. Immune activation is observed in diarrhea-predominant patients with IBS and contributes to the development of visceral hypersensitivity. Adenosine is a chemical mediator that regulates many physiological processes, including inflammation and nociception. Among its receptors, the adenosine A_2B receptor regulates intestinal secretion, motor function, and the immune response. We recently demonstrated that the adenosine A_2B receptor is involved in visceral hypersensitivity in animal models of IBS. In this review, we discuss the possibility of the adenosine A_2B receptor as a novel therapeutic target for IBS.

Adenosine: Direct and Indirect Actions on Gastric Acid Secretion

Composed by a molecule of adenine and a molecule of ribose, adenosine is a paradigm of recyclable nucleoside with a multiplicity of functions that occupies a privileged position in the metabolic and regulatory contexts. Adenosine is formed continuously in intracellular and extracellular locations of all tissues. Extracellular adenosine is a signaling molecule, able to modulate a vast range of physiologic responses in many cells and organs, including digestive organs. The adenosine A1, A2A, A2B, and A3 receptors are P1 purinergic receptors, G protein-coupled proteins implicated in tissue protection. This review is focused on gastric acid secretion, a process centered on the parietal cell of the stomach, which contains large amounts of H⁺/K⁺-ATPase, the proton pump responsible for proton extrusion during acid secretion. Gastric acid secretion is regulated by an extensive collection of neural stimuli and endocrine and paracrine agents, which act either directly at membrane receptors of the parietal cell or indirectly through other regulatory cells of the gastric mucosa, as well as mechanic and chemic stimuli. In this review, after briefly introducing these points, we condense the current body of knowledge about the modulating action of adenosine on the pathophysiology of gastric acid secretion and update its significance based on recent findings in gastric mucosa and parietal cells in humans and animal models.

Colonic motor dysfunctions in a mouse model of high-fat diet-induced obesity: an involvement of A2B adenosine receptors

Adenosine A_2B receptors (A_2BR) regulate several enteric functions. However, their implication in the pathophysiology of intestinal dysmotility associated with high-fat diet (HFD)-induced obesity has not been elucidated. We investigated the expression of A_2BR in mouse colon and their role in the mechanisms underlying the development of enteric dysmotility associated with obesity. Wild-type C57BL/6J mice were fed with HFD (60% kcal from fat) or normocaloric diet (NCD; 18% kcal from fat) for 8 weeks. Colonic A_2BR localization was examined by immunofluorescence. The role of A_2BR in the control of colonic motility was examined in functional experiments on longitudinal muscle preparations (LMPs). In NCD mice, A_2BR were predominantly located in myenteric neurons; in HFD animals, their expression increased throughout the neuromuscular layer. Functionally, the A_2BR antagonist MRS1754 enhanced electrically induced NK₁-mediated tachykininergic contractions in LMPs from HFD mice, while it was less effective in tissues from NCD mice. The A_2B receptor agonist BAY 60-6583 decreased colonic tachykininergic contractions in LMPs, with higher efficacy in preparations from obese mice. Both A_2BR ligands did not affect contractions elicited by exogenous substance P. Obesity is related with a condition of colonic inflammation, leading to an increase of A_2BR expression. A_2BR, modulating the activity of excitatory tachykininergic nerves, participate to the enteric dysmotility associated with obesity.

The Taste of Caffeine

Many people avidly consume foods and drinks containing caffeine, despite its bitter taste. Here, we review what is known about caffeine as a bitter taste stimulus. Topics include caffeine's action on the canonical bitter taste receptor pathway and caffeine's action on noncanonical receptor-dependent and -independent pathways in taste cells. Two conclusions are that (1) caffeine is a poor prototypical bitter taste stimulus because it acts on bitter taste receptor-independent pathways, and (2) caffeinated products most likely stimulate "taste" receptors in nongustatory cells. This review is relevant for taste researchers, manufacturers of caffeinated products, and caffeine consumers.

Mechanosensory Signaling in Enterochromaffin Cells and 5-HT Release: Potential Implications for Gut Inflammation

Enterochromaffin (EC) cells synthesize 95% of the body 5-HT and release 5-HT in response to mechanical or chemical stimulation. EC cell 5-HT has physiological effects on gut motility, secretion and visceral sensation. Abnormal regulation of 5-HT occurs in gastrointestinal disorders and Inflammatory Bowel Diseases (IBD) where 5-HT may represent a key player in the pathogenesis of intestinal inflammation. The focus of this review is on mechanism(s) involved in EC cell "mechanosensation" and critical gaps in our knowledge for future research. Much of our knowledge and concepts are from a human BON cell model of EC, although more recent work has included other cell lines, native EC cells from mouse and human and intact mucosa. EC cells are "mechanosensors" that respond to physical forces generated during peristaltic activity by translating the mechanical stimulus (MS) into an intracellular biochemical response leading to 5-HT and ATP release. The emerging picture of mechanosensation includes Piezo 2 channels, caveolin-rich microdomains, and tight regulation of 5-HT release by purines. The "purinergic hypothesis" is that MS releases purines to act in an autocrine/paracrine manner to activate excitatory (P2Y₁, P2Y₄, P2Y₆, and A_2A/A_2B) or inhibitory (P2Y₁₂, A₁, and A₃) receptors to regulate 5-HT release. MS activates a P2Y₁/G_αq/PLC/IP₃-IP₃R/SERCA Ca²⁺signaling pathway, an A_2A/A_2B-Gs/AC/cAMP-PKA signaling pathway, an ATP-gated P2X₃ channel, and an inhibitory P2Y₁₂-G_i/o/AC-cAMP pathway. In human IBD, P2X₃ is down regulated and A_2B is up regulated in EC cells, but the pathophysiological consequences of abnormal mechanosensory or purinergic 5-HT signaling remain unknown. EC cell mechanosensation remains poorly understood.

A Purinergic Trail for Metastases

Nucleotides and nucleosides have emerged as important modulators of tumor biology. Recently acquired evidence shows that, when these molecules are released by cancer cells or surrounding tissues, they act as potent prometastatic factors, favoring tumor cell migration and tissue colonization. Therefore, nucleotides and nucleosides should be considered as a new class of prometastatic factors. In this review, we focus on the prometastatic roles of nucleotides and discuss future applications of purinergic signaling modulation in view of antimetastatic therapies.

Differential activation of afferent neuronal and inflammatory pathways during small bowel obstruction (SBO)

BACKGROUND

Small bowel obstruction (SBO) is a potentially life-threatening condition which may be caused by a variety of pathologies such as postoperative adhesions or malignant diseases. Little is known on alterations in gut physiology during SBO, although its comprehension is essential to improve treatment which may help to prevent subsequent organ failure prior to surgical resolution. We aimed to investigate afferent nerve sensitivity and intestinal inflammatory response during SBO to identify possible targets of treatment.

METHODS

C57Bl6 mice were anesthetized, and a midline laparotomy was performed. A small bowel loop was ligated 5 cm proximal to ileo-cecal valve to induce SBO. Control animals received a sham midline laparotomy. SBO animals and controls were sacrificed after 3, 9, or 24 h (each n = 6). A dilated segment of small intestine located 1.5 cm oral to the ligature was prepared for multi-unit mesenteric afferent nerve recordings in vitro. Histological assessment of leukocyte infiltration was performed by myeloperoxidase (MPO). Pro-inflammatory cytokine expression was quantified by RT-PCR. Data are mean ± SEM.

KEY RESULTS

Afferent firing to serosal 5-HT (500 μM) peaked at 3.9 ± 0.2 impulse/s 24 h after induction of SBO compared to 2.4 ± 0.1 impulse/s in sham controls (p < 0.05). Serosal bradykinin (0.5 μM) led to an increase in peak afferent firing of 5.3 ± 0.5 impulse/s in 24 h SBO animals compared to 3.5 ± 0.2 impulse/s in sham controls (p < 0.05). No differences in 5-HT and BK sensitivity were observed in 3 and 9 h SBO animals compared to controls. Continuous mechanical ramp distension of the intestinal loop was followed by a pressure-dependent rise in afferent nerve discharge that was reduced in 3 h SBO animals compared to sham controls (p < 0.05). MPO stains showed a rise in leukocyte infiltration of the intestine in SBO animals at 9 and 24 h (p < 0.05). Il-6 but not TNF-a gene expression was increased at 9 and 24 h in SBO animals compared to sham controls (p < 0.05).

CONCLUSIONS & INFERENCES

Afferent nerve sensitivity is increased 24 h after induction of SBO. SBO led to a delayed onset intestinal inflammatory response. Inflammatory mediators released during this inflammatory response may be responsible for a later increase in afferent sensitivity. Agents with anti-inflammatory action may, therefore, have a beneficial effect during SBO and may subsequently help to prevent possible organ dysfunction.

Adenosine-generating ovarian cancer cells attract myeloid cells which differentiate into adenosine-generating tumor associated macrophages - a self-amplifying, CD39- and CD73-dependent mechanism for tumor immune escape

BACKGROUND

Ovarian cancer (OvCA) tissues show abundant expression of the ectonucleotidases CD39 and CD73 which generate immunomodulatory adenosine, thereby inhibiting cytotoxic lymphocytes. Little, however, is known about the effect of adenosine on myeloid cells. Considering that tumor associated macrophages (TAM) and myeloid-derived suppressor cells (MDSC) constitute up to 20 % of OvCA tissue, we investigated the effect of adenosine on myeloid cells and explored a possible contribution of myeloid cells to adenosine generation in vitro and ex vivo.

METHODS

Monocytes were used as human blood-derived myeloid cells. After co-incubation with SK-OV-3 or OAW-42 OvCA cells, monocyte migration was determined in transwell assays. For conversion into M2-polarized "TAM-like" macrophages, monocytes were co-incubated with OAW-42 cells. Ex vivo TAMs were obtained from OvCA ascites. Macrophage phenotypes were investigated by intracellular staining for IL-10 and IL-12. CD39 and CD73 expression were assessed by FACS analysis both on in vitro-induced TAM-like macrophages and on ascites-derived ex situ-TAMs. Myeloid cells in solid tumor tissue were analyzed by immunohistochemistry. Generation of biologically active adenosine by TAM-like macrophages was measured in luciferase-based reporter assays. Functional effects of adenosine were investigated in proliferation-experiments with CD4(+) T cells and specific inhibitors.

RESULTS

When CD39 or CD73 activity on OvCA cells were blocked, the migration of monocytes towards OvCA cells was significantly decreased. In vivo, myeloid cells in solid ovarian cancer tissue were found to express CD39 whereas CD73 was mainly detected on stromal fibroblasts. Ex situ-TAMs and in vitro differentiated TAM-like cells, however, upregulated the expression of CD39 and CD73 compared to monocytes or M1 macrophages. Expression of ectonucleotidases also translated into increased levels of biologically active adenosine. Accordingly, co-incubation with these TAMs suppressed CD4(+) T cell proliferation which could be rescued via blockade of CD39 or CD73.

CONCLUSION

Adenosine generated by OvCA cells likely contributes to the recruitment of TAMs which further amplify adenosine-dependent immunosuppression via additional ectonucleotidase activity. In solid ovarian cancer tissue, TAMs express CD39 while CD73 is found on stromal fibroblasts. Accordingly, small molecule inhibitors of CD39 or CD73 could improve immune responses in ovarian cancer.