Adenosine receptors: G protein-mediated signalling and the role of accessory proteins

Role of A1 adenosine receptor in cardiovascular diseases: Bridging molecular mechanisms with therapeutic opportunities

Adenosine serves as a critical homeostatic regulator, exerting influence over physiological and pathological conditions in the cardiovascular system. During cellular stress, increased extracellular adenosine levels have been implicated in conferring cardioprotective effects through the activation of adenosine receptors with the A1 adenosine receptor subtype showing the highest expression in the heart. A1 adenosine receptor stimulation inhibits adenylyl cyclase activity via heterotrimeric Gi proteins, leading to the activation of distinct downstream effectors involved in cardiovascular homeostasis. While the comprehensive characterization of the pharmacological functions and intracellular signaling pathways associated with the A1 adenosine receptor subtype is still ongoing, this receptor is widely recognized as a crucial pharmacological target for the treatment of various states of cardiovascular diseases (CVDs). In this review, we focus on elucidating signal transduction of A1 adenosine receptor, particularly Gi protein-dependent and -independent pathways, and their relevance to cardiovascular protective effects as well as pathological consequences during cellular and tissue stresses in the cardiovascular system. Additionally, we provide comprehensive updates and detailed insights into a range of A1 adenosine receptor agonists and antagonists, detailing their development and evaluation through preclinical and clinical studies with a specific focus on their potential for the management of CVDs, especially heart diseases.

Extracellular purines in lung endothelial permeability and pulmonary diseases

The purinergic signaling system is an evolutionarily conserved and critical regulatory circuit that maintains homeostatic balance across various organ systems and cell types by providing compensatory responses to diverse pathologies. Despite cardiovascular diseases taking a leading position in human morbidity and mortality worldwide, pulmonary diseases represent significant health concerns as well. The endothelium of both pulmonary and systemic circulation (bronchial vessels) plays a pivotal role in maintaining lung tissue homeostasis by providing an active barrier and modulating adhesion and infiltration of inflammatory cells. However, investigations into purinergic regulation of lung endothelium have remained limited, despite widespread recognition of the role of extracellular nucleotides and adenosine in hypoxic, inflammatory, and immune responses within the pulmonary microenvironment. In this review, we provide an overview of the basic aspects of purinergic signaling in vascular endothelium and highlight recent studies focusing on pulmonary microvascular endothelial cells and endothelial cells from the pulmonary artery vasa vasorum. Through this compilation of research findings, we aim to shed light on the emerging insights into the purinergic modulation of pulmonary endothelial function and its implications for lung health and disease.

Clinical and Biologic Correlates of ADORA2A Transcriptomic Expression in Cancer

ADORA2A (adenosine A2a receptor) and ADORA2B propagate immunoregulatory signals, including restricting both innate and adaptive immunity, though recent data also suggest a tumor suppressor effect in certain settings. We evaluated the RNA expression from 514 tumors in a clinical-grade laboratory; 489 patients with advanced/metastatic disease had clinical outcome correlates. Transcript expression was standardized to internal housekeeping genes and ranked (0-100 scale) relative to 735 specimens from 35 different cancer types. Transcript abundance rank values were defined as "low/moderate" (0-74) or "high" (75-100) percentile RNA expression ranks. Overall, 20.8% of tumors had high ADORA2A (≥75 percentile RNA rank). The greatest proportion of high ADORA2A expressors was found in neuroendocrine and breast cancers and sarcomas, whereas the lowest was found in colorectal and ovarian cancers, albeit with patient-to-patient variability. In multivariable logistic regression analysis, there was a significant positive correlation between high ADORA2A RNA expression and a high expression of the immune checkpoint-related molecules PD-1 (p = 0.015), VISTA (p ≤ 0.001), CD38 (p = 0.031), and CD39 (p ≤ 0.001). In 217 immunotherapy-treated patients, high ADORA2A did not correlate significantly with progression-free (p = 0.51) or overall survival (OS) (p = 0.09) from the initiation of the checkpoint blockade. However, high versus not-high ADORA2A transcript expression correlated with longer OS from the time of advanced/metastatic disease (N = 489 patients; (HR 0.69 (95% CI 0.51-0.95) (p = 0.02)). Therefore, high ADORA2A transcript levels may be a favorable prognostic factor, unrelated to immunotherapy. Importantly, ascertaining co-expression patterns of ADORA2A with PD-1 and VISTA in individual tumors as a basis for the precision co-targeting of ADORA2A and these other checkpoint-related molecules warrants investigation in clinical trials.

Cryo-EM structure of the human adenosine A2B receptor-Gs signaling complex

The human adenosine A_2B receptor (A_2BR) is a class A G protein-coupled receptor that is involved in several major physiological and pathological processes throughout the body. A_2BR recognizes its ligands adenosine and NECA with relatively low affinity, but the detailed mechanism for its ligand recognition and signaling is still elusive. Here, we present two structures determined by cryo-electron microscopy of A_2BR bound to its agonists NECA and BAY60-6583, each coupled to an engineered G_s protein. The structures reveal conserved orthosteric binding pockets with subtle differences, whereas the selectivity or specificity can mainly be attributed to regions extended from the orthosteric pocket. We also found that BAY60-6583 occupies a secondary pocket, where residues V250^6.51 and N273^7.36 were two key determinants for its selectivity against A_2BR. This study offers a better understanding of ligand selectivity for the adenosine receptor family and provides a structural template for further development of A_2BR ligands for related diseases.

Deconstruction of a hypothalamic astrocyte-white adipocyte sympathetic axis that regulates lipolysis in mice

The role of non-neuronal glial cells in the regulation of adipose sympathetic nerve activity and adipocyte functions such as white adipose tissue lipid lipolysis is poorly understood. Here, we combine chemo/optogenetic manipulations of medio-basal hypothalamic astrocytes, real-time fiber photometry monitoring of white adipose tissue norepinephrine (NE) contents and nerve activities, electrophysiological recordings of local sympathetic inputs to inguinal white adipose tissue (iWAT), and adipose tissue lipid lipolytic assays to define the functional roles of hypothalamic astrocytes in the regulation of iWAT sympathetic outflow and lipolysis. Our results show that astrocyte stimulation elevates iWAT NE contents, excites sympathetic neural inputs and promotes lipolysis. Mechanistically, we find that sympathetic paravertebral ganglia (PG) partake in those astrocyte effects. We also find that astrocyte stimulation excites pro-opiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARH), and chemogenetic inhibition of POMC neurons blunts the effects induced by astrocyte stimulation. While we cannot exclude potential roles played by other cell populations such as microglia, our findings in this study reveal a central astrocyte-peripheral adipocyte axis modulating sympathetic drive to adipose tissues and adipocyte functions, one that might serve as a target for therapeutic intervention in the treatment of obesity.

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

Platelets and the Cybernetic Regulation of Ischemic Inflammatory Responses through PNC Formation Regulated by Extracellular Nucleotide Metabolism and Signaling

Ischemic events are associated with severe inflammation and are here referred to as ischemic inflammatory response (IIR). Recent studies identified the formation of platelet–neutrophil complexes (PNC) as key players in IIR. We investigated the role of extracellular platelet nucleotide signaling in the context of IIR and defined a cybernetic circle, including description of feedback loops. Cybernetic circles seek to integrate different levels of information to understand how biological systems function. Our study specifies the components of the cybernetic system of platelets in IIR and describes the theoretical progression of IIR passing the cybernetic cycle with positive and negative feedback loops based on nucleotide-dependent signaling and functional regulation. The cybernetic components and feedback loops were explored by cytometry, immunohistological staining, functional blocking antibodies, and ADP/ATP measurements. Using several ex vivo and in vivo approaches we confirmed cybernetic parameters, such as controller, sensor, and effector (VASP phosphorylation, P2Y₁₂, ADORAs and GPIIb/IIIa activity), as well as set points (ADP, adenosine) and interfering control and disturbance variables (ischemia). We demonstrate the impact of the regulated platelet–neutrophil complex (PNC) formation in blood and the resulting damage to the affected inflamed tissue. Taken together, extracellular nucleotide signaling, PNC formation, and tissue damage in IIR can be integrated in a controlled cybernetic circle of platelet function, as introduced through this study.

The role of adenosine A1 receptor on immune cells

BACKGROUND

Adenosine, acting as a regulator by mediating the activation of G protein-coupled adenosine receptor families (A₁, A₂A, A₂B, and A₃), plays an important role under physiological and pathological conditions. As the receptor with the highest affinity for adenosine, the role of adenosine A₁ receptor (A₁R)-mediated adenosine signaling pathway in the central nervous system has been well addressed. However, functions of A₁R on immune cells are less summarized. Considering that some immune cells express multiple types of adenosine receptors with distinct effects and varied density, exogenous adenosine of different concentrations may induce divergent immune cell functions.

MATERIALS AND METHODS

The literatures about the expression of A₁R and its regulation on immune cells and how it regulates the function of immune cells were searched on PubMed and Google Scholar.

CONCLUSION

In this review, we discussed the effects of A₁R on immune cells, including monocytes, macrophages, neutrophils, dendritic cells, and microglia, and focused on the role of A₁R in regulating immune cells in diseases, which may facilitate our understanding of the mechanisms by which adenosine affects immune cells through A₁R.

Characteristics of Zusanli Dorsal Root Ganglion Neurons in Rats and Their Receptor Mechanisms in Response to Adenosine

Neural systems play important roles in the functions of acupuncture. But the unclear structure and mechanism of acupoints hinder acupuncture standardization and cause the acupuncture effects to be varying or even paradoxical. It has been broadly assumed that the efficacy of acupuncture depends on the biological signals triggered at acupoints and passed up along neural systems. However, as the first station to transmit such signals, the characters of the dorsal root ganglia (DRG) neurons innervating acupoints are still not well elucidated. We adopted Zusanli (ST36) as a representative acupoint and found most DRG neurons innervating ST36 acupoint are middle-size neurons with a single spike firing pattern. This suggests that proprioceptive neurons take on greater possibility than small size nociceptive neurons do to mediate the acupuncture signals. Moreover, we found that adenosine injected into ST36 acupoints could dose- and acupoint-dependently mimic the analgesic effect of acupuncture. However, adenosine could not elicit action potentials in the acutely isolated ST36 DRG neurons, but it inhibited I_D currents and increased the areas of overshoots. Further, we found that 4 types of adenosine receptors were all expressed by ST36 DRG neurons, and A1, A2b, and A3 receptors were the principal reactors to adenosine. PERSPECTIVE: This study provides the major characteristics of ST36 DRG neurons, which will help to analyze the neural pathway of acupuncture signals. At the same time, these findings could provide a new possible therapy for pain relief, such as injecting adenosine or corresponding agonists into acupoints.

Stimulation of adenosine A1 receptor prevents oxidative injury in H9c2 cardiomyoblasts: Role of Gβγ-mediated Akt and ERK1/2 signaling

Oxidative stress causes cellular injury and damage in the heart primarily through apoptosis resulting in cardiac abnormalities such as heart failure and cardiomyopathy. During oxidative stress, stimulation of adenosine receptor (AR) has been shown to protect against oxidative damage due to their cytoprotective properties. However, the subtype specificity and signal transductions of adenosine A₁ receptor (A₁R) on cardiac protection during oxidative stress have remained elusive. In this study, we found that stimulation of A₁Rs with N⁶-cyclopentyladenosine (CPA), a specific A₁R agonist, attenuated the H₂O₂-induced intracellular and mitochondrial reactive oxygen species (ROS) production and apoptosis. In addition, A₁R stimulation upregulated the synthesis of antioxidant enzymes (catalase and GPx-1), antiapoptotic proteins (Bcl-2 and Bcl-xL), and mitochondria-related markers (UCP2 and UCP3). Blockades of Gβγ subunit of heterotrimeric Gα_i protein antagonized A₁R-mediated antioxidant and antiapoptotic effects, confirming the potential role of Gβγ subunit-mediated A₁R signaling. Additionally, cardioprotective effects of CPA mediated through PI3K/Akt- and ERK1/2-dependent signaling pathways. Thus, we propose that A₁R represents a promising therapeutic target for prevention of oxidative injury in the heart.

Therapeutic Potential of Highly Selective A3 Adenosine Receptor Ligands in the Central and Peripheral Nervous System

Ligands of the G_i protein-coupled adenosine A₃ receptor (A₃R) are receiving increasing interest as attractive therapeutic tools for the treatment of a number of pathological conditions of the central and peripheral nervous systems (CNS and PNS, respectively). Their safe pharmacological profiles emerging from clinical trials on different pathologies (e.g., rheumatoid arthritis, psoriasis and fatty liver diseases) confer a realistic translational potential to these compounds, thus encouraging the investigation of highly selective agonists and antagonists of A₃R. The present review summarizes information on the effect of latest-generation A₃R ligands, not yet available in commerce, obtained by using different in vitro and in vivo models of various PNS- or CNS-related disorders. This review places particular focus on brain ischemia insults and colitis, where the prototypical A₃R agonist, Cl-IB-MECA, and antagonist, MRS1523, have been used in research studies as reference compounds to explore the effects of latest-generation ligands on this receptor. The advantages and weaknesses of these compounds in terms of therapeutic potential are discussed.

Adenosine receptors: Emerging non-opioids targets for pain medications

Physical and emotional pain deteriorates the quality of well-being. Also, numerous non-invasive and invasive treatments for diagnosed diseases such as cancer medications and surgical procedures cause various types of pain. Despite the multidisciplinary approaches available to manage pain, the unmet needs for medication with minimal side effects are substantial. Especially with the surge of opioid crisis during the last decades, non-opioid analgesics may reduce life-threatening overdosing and addictive liability. Although many clinical trials supported the potential potency of cannabis and cannabidiol (CBD) in pain management or treatment, the long-term benefits of cannabis or CBD are still not evident. At the same time, growing evidence shows the risk of overusing cannabis and CBD. Therefore, it is urgent to develop novel analgesic medications that minimize side effects. All four well-identified adenosine receptors, A1, A2A, A2B, and A3, are implicated in pain. Recently, a report demonstrated that an adenosine A1R-specific positive allosteric modulator (PAM) is a potent analgesic without noticeable side effects. Also, several A3R agonists are being considered as promising analgesic agent. However, the importance of adenosine in pain is relatively underestimated. To help readers understand, first, we will summarize the historical perspective of the adenosine system in preclinical and clinical studies. Then, we will discuss possible interactions of adenosine and opioids or the cannabis system focusing on pain. Overall, this review will provide the potential role of adenosine and adenosine receptors in pain treatment.

Caenorhabditis elegans provides an efficient drug screening platform for GNAO1-related disorders and highlights the potential role of caffeine in controlling dyskinesia

Dominant GNAO1 mutations cause an emerging group of childhood-onset neurological disorders characterized by developmental delay, intellectual disability, movement disorders, drug-resistant seizures and neurological deterioration. GNAO1 encodes the α-subunit of an inhibitory GTP/GDP-binding protein regulating ion channel activity and neurotransmitter release. The pathogenic mechanisms underlying GNAO1-related disorders remain largely elusive and there are no effective therapies. Here, we assessed the functional impact of two disease-causing variants associated with distinct clinical features, c.139A > G (p.S47G) and c.662C > A (p.A221D), using Caenorhabditis elegans as a model organism. The c.139A > G change was introduced into the orthologous position of the C. elegans gene via CRISPR/Cas9, whereas a knock-in strain carrying the p.A221D variant was already available. Like null mutants, homozygous knock-in animals showed increased egg laying and were hypersensitive to aldicarb, an inhibitor of acetylcholinesterase, suggesting excessive neurotransmitter release by different classes of motor neurons. Automated analysis of C. elegans locomotion indicated that goa-1 mutants move faster than control animals, with more frequent body bends and a higher reversal rate and display uncoordinated locomotion. Phenotypic profiling of heterozygous animals revealed a strong hypomorphic effect of both variants, with a partial dominant-negative activity for the p.A221D allele. Finally, caffeine was shown to rescue aberrant motor function in C. elegans harboring the goa-1 variants; this effect is mainly exerted through adenosine receptor antagonism. Overall, our findings establish a suitable platform for drug discovery, which may assist in accelerating the development of new therapies for this devastating condition, and highlight the potential role of caffeine in controlling GNAO1-related dyskinesia.

Myelin Defects in Niemann-Pick Type C Disease: Mechanisms and Possible Therapeutic Perspectives

Niemann–Pick type C (NPC) disease is a wide-spectrum clinical condition classified as a neurovisceral disorder affecting mainly the liver and the brain. It is caused by mutations in one of two genes, NPC1 and NPC2, coding for proteins located in the lysosomes. NPC proteins are deputed to transport cholesterol within lysosomes or between late endosome/lysosome systems and other cellular compartments, such as the endoplasmic reticulum and plasma membrane. The first trait of NPC is the accumulation of unesterified cholesterol and other lipids, like sphingosine and glycosphingolipids, in the late endosomal and lysosomal compartments, which causes the blockade of autophagic flux and the impairment of mitochondrial functions. In the brain, the main consequences of NPC are cerebellar neurodegeneration, neuroinflammation, and myelin defects. This review will focus on myelin defects and the pivotal importance of cholesterol for myelination and will offer an overview of the molecular targets and the pharmacological strategies so far proposed, or an object of clinical trials for NPC. Finally, it will summarize recent data on a new and promising pharmacological perspective involving A_2A adenosine receptor stimulation in genetic and pharmacological NPC dysmyelination models.

Adenosine: The common target between cancer immunotherapy and glaucoma in the eye

Adenosine, an endogenous purine nucleoside, is a well-known actor of the immune system and the inflammatory response both in physiologic and pathologic conditions. By acting upon particular, G-protein coupled adenosine receptors, i.e., A1, A2- a & b, and A3 receptors mediate a variety of intracellular and immunomodulatory actions. Several studies have elucidated Adenosine's effect and its up-and downstream molecules and enzymes on the anti-tumor response against several types of cancers. We have also targeted a couple of molecules to manipulate this pathway and get the immune system's desired response in our previous experiences. Besides, the outgrowth of the studies on ocular Adenosine in recent years has significantly enhanced the knowledge about Adenosine and its role in ocular immunology and the inflammatory response of the eye. Glaucoma is the second leading cause of blindness globally, and the recent application of Adenosine and its derivatives has shown the critical role of the adenosine pathway in its pathophysiology. However, despite a very promising background, the phase III clinical trial of Trabodenoson failed to achieve the non-inferiority goals of the study. In this review, we discuss different aspects of the abovementioned pathway in ophthalmology and ocular immunology; following a brief evaluation of the current immunotherapeutic strategies, we try to elucidate the links between cancer immunotherapy and glaucoma in order to introduce novel therapeutic targets for glaucoma.

ADORA1-driven brain-sympathetic neuro-adipose connections control body weight and adipose lipid metabolism

It is essential to elucidate brain-adipocyte interactions in order to tackle obesity and its comorbidities, as the precise control of brain-adipose tissue cross-talk is crucial for energy and glucose homeostasis. Recent studies show that in the peripheral adipose tissue, adenosine induces adipogenesis through peripheral adenosine A₁ receptor (pADORA₁) signaling; however, it remains unclear whether systemic and adipose tissue metabolism would also be under the control of central (c) ADORA₁ signaling. Here, we use tissue-specific pharmacology and metabolic tools to clarify the roles of cADORA₁ signaling in energy and adipocyte physiology. We found that cADORA₁ signaling reduces body weight while also inducing adipose tissue lipolysis. cADORA₁ signaling also increases adipose tissue sympathetic norepinephrine content. In contrast, pADORA₁ signaling facilitates a high-fat diet-induced obesity (DIO). We propose here a novel mechanism in which cADORA₁ and pADORA₁ signaling hinder and aggravate DIO, respectively.

Targeting of the A2A adenosine receptor counteracts immunosuppression in vivo in a mouse model of chronic lymphocytic leukemia

Tumor immunosuppression is a major cause for treatment failure and disease relapse, both in solid tumors and leukemia. Local hypoxia is among the conditions that cause immunosuppression, acting at least in part through the upregulation of extracellular adenosine levels, which potently suppress T cell responses and skew macrophages towards an M2 phenotype. Hence, there is intense investigation to identify drugs that target this axis. By using the TCL1 adoptive transfer CLL mouse model, we show that adenosine production and signaling are upregulated in the hypoxic lymphoid niches, where intense colonization of leukemic cells occurs. This leads to a progressive remodeling of the immune system towards tolerance, with expansion of T regulatory cells (Tregs), loss of CD8+ T cell cytotoxicity and differentiation of murine macrophages towards the patrolling (M2-like) subset. In vivo administration of SCH58261, an inhibitor the A2A adenosine receptor, re-awakens T cell responses, while limiting Tregs expansion, and re-polarizes monocytes towards the inflammatory (M1-like) phenotype. These results show for the first time the in vivo contribution of adenosine signaling to immune tolerance in CLL, and the translational implication of drugs interrupting this pathway. Although the effects of SCH58261 on leukemic cells are limited, interfering with adenosine signaling may represent an appealing strategy for combination-based therapeutic approaches.

Adenosine integrates light and sleep signalling for the regulation of circadian timing in mice

The accumulation of adenosine is strongly correlated with the need for sleep and the detection of sleep pressure is antagonised by caffeine. Caffeine also affects the circadian timing system directly and independently of sleep physiology, but how caffeine mediates these effects upon the circadian clock is unclear. Here we identify an adenosine-based regulatory mechanism that allows sleep and circadian processes to interact for the optimisation of sleep/wake timing in mice. Adenosine encodes sleep history and this signal modulates circadian entrainment by light. Pharmacological and genetic approaches demonstrate that adenosine acts upon the circadian clockwork via adenosine A1/A2A receptor signalling through the activation of the Ca2+ -ERK-AP-1 and CREB/CRTC1-CRE pathways to regulate the clock genes Per1 and Per2. We show that these signalling pathways converge upon and inhibit the same pathways activated by light. Thus, circadian entrainment by light is systematically modulated on a daily basis by sleep history. These findings contribute to our understanding of how adenosine integrates signalling from both light and sleep to regulate circadian timing in mice.

Biased agonism at adenosine receptors

Adenosine modulates many aspects of human physiology and pathophysiology through binding to the adenosine family of G protein-coupled receptors, which are comprised of four subtypes, the A₁R, A_2AR, A_2BR and A₃R. Modulation of adenosine receptor function by exogenous agonists, antagonists and allosteric modulators can be beneficial for a number of conditions including cardiovascular disease, Parkinson's disease, and cancer. Unfortunately, many preclinical drug candidates targeting adenosine receptors have failed in clinical trials due to limited efficacy and/or severe on-target undesired effects. To overcome the key barriers typically encountered when transitioning adenosine receptor ligands into the clinic, research efforts have focussed on exploiting the phenomenon of biased agonism. Biased agonism provides the opportunity to develop ligands that favour therapeutic signalling pathways, whilst avoiding signalling associated with on-target undesired effects. Recent studies have begun to define the structure-function relationships that underpin adenosine receptor biased agonism and establish how this phenomenon can be harnessed therapeutically. In this review we describe the recent advancements made towards achieving therapeutically relevant biased agonism at adenosine receptors.

Adenosine A2A receptor inhibition reduces synaptic and cognitive hippocampal alterations in Fmr1 KO mice

In fragile X syndrome (FXS) the lack of the fragile X mental retardation protein (FMRP) leads to exacerbated signaling through the metabotropic glutamate receptors 5 (mGlu5Rs). The adenosine A_2A receptors (A_2ARs), modulators of neuronal damage, could play a role in FXS. A synaptic colocalization and a strong permissive interaction between A_2A and mGlu5 receptors in the hippocampus have been previously reported, suggesting that blocking A_2ARs might normalize the mGlu5R-mediated effects of FXS. To study the cross-talk between A_2A and mGlu5 receptors in the absence of FMRP, we performed extracellular electrophysiology experiments in hippocampal slices of Fmr1 KO mouse. The depression of field excitatory postsynaptic potential (fEPSPs) slope induced by the mGlu5R agonist CHPG was completely blocked by the A_2AR antagonist ZM241385 and strongly potentiated by the A_2AR agonist CGS21680, suggesting that the functional synergistic coupling between the two receptors could be increased in FXS. To verify if chronic A_2AR blockade could reverse the FXS phenotypes, we treated the Fmr1 KO mice with istradefylline, an A_2AR antagonist. We found that hippocampal DHPG-induced long-term depression (LTD), which is abnormally increased in FXS mice, was restored to the WT level. Furthermore, istradefylline corrected aberrant dendritic spine density, specific behavioral alterations, and overactive mTOR, TrkB, and STEP signaling in Fmr1 KO mice. Finally, we identified A_2AR mRNA as a target of FMRP. Our results show that the pharmacological blockade of A_2ARs partially restores some of the phenotypes of Fmr1 KO mice, both by reducing mGlu5R functioning and by acting on other A_2AR-related downstream targets.

CD26 expression and adenosine deaminase activity in regulatory T cells (Treg) and CD4(+) T effector cells in patients with head and neck squamous cell carcinoma

Adenosine deaminase (ADA) is responsible for the deamination of immunosuppressive adenosine to inosine. In human T lymphocytes, ADA is associated with dipeptidyl peptidase IV (CD26). ADA expression and activity were evaluated in regulatory T cells (Treg) and CD4(+) T effector cells (Teff) of patients with head and neck squamous cell cancer (HNSCC). CD4(+)CD39(+) and CD4(+)CD39(neg) T cells were isolated by single-cell sorting from the peripheral blood of 15 HNSCC patients and 15 healthy donors (NC). CD26/ADA expression in these cells was studied by multicolor flow cytometry, confocal microscopy, RT-PCR and immunohistochemistry in tumor tissues. ADA activity was evaluated by mass spectrometry, suppression of Teff proliferation in CFSE assays and cytokine production by Luminex. CD4(+)CD39(+) Treg had low and CD4(+)CD39(neg) Teff high CD26/ADA expression and ADA activity in NC or HNSCC. The frequency and suppressor activity of CD39(+)CD26(neg) Treg were elevated in patients relative to NC (p < 0.01). However, ADA activity in patients' CD4(+)CD39(neg) Teff was decreased (p < 0.05), resulting in extracellular adenosine accumulation. Also, patients' Teff were more sensitive to inhibitory signals delivered via adenosine receptors. IL-2, IL12 and INFγ upregulated ADA expression and activity in CD4(+)CD39(neg) Teff, whereas IL-10, PGE(2) and CADO downregulated it. The differentially expressed CD26/ADA can serve as surface markers for functionally-active CD39(+)CD26(neg) Treg.

Inflammasomes and the Maintenance of Hematopoietic Homeostasis: New Perspectives and Opportunities

Hematopoietic stem cells (HSCs) regularly produce various blood cells throughout life via their self-renewal, proliferation, and differentiation abilities. Most HSCs remain quiescent in the bone marrow (BM) and respond in a timely manner to either physiological or pathological cues, but the underlying mechanisms remain to be further elucidated. In the past few years, accumulating evidence has highlighted an intermediate role of inflammasome activation in hematopoietic maintenance, post-hematopoietic transplantation complications, and senescence. As a cytosolic protein complex, the inflammasome participates in immune responses by generating a caspase cascade and inducing cytokine secretion. This process is generally triggered by signals from purinergic receptors that integrate extracellular stimuli such as the metabolic factor ATP via P2 receptors. Furthermore, targeted modulation/inhibition of specific inflammasomes may help to maintain/restore adequate hematopoietic homeostasis. In this review, we will first summarize the possible relationships between inflammasome activation and homeostasis based on certain interesting phenomena. The cellular and molecular mechanism by which purinergic receptors integrate extracellular cues to activate inflammasomes inside HSCs will then be described. We will also discuss the therapeutic potential of targeting inflammasomes and their components in some diseases through pharmacological or genetic strategies.

Tumor Immunotherapy Using A2A Adenosine Receptor Antagonists

The A_2A adenosine receptor (A_2AAR) plays critical roles in human physiology and pathophysiology, which makes it an important drug target. Previous drug-discovery efforts targeting the A_2AAR have been focused on the use of A_2AAR antagonists for the treatment of Parkinson's disease. More recently, the A_2AAR has attracted additional attention for its roles in immuno-oncology, and a number of A_2AAR antagonists are currently used as lead compounds for antitumor drugs in both preclinical models and clinical trials. This review surveys recent advances in the development of A_2AAR antagonists for cancer immunotherapy. The therapeutic potential of representative A_2AAR antagonists is discussed based on both animal efficacy studies and clinical data.

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.

Blame the signaling: Role of cAMP for the resolution of inflammation

A complex intracellular signaling governs different cellular responses in inflammation. Extracellular stimuli are sensed, amplified, and transduced through a dynamic cellular network of messengers converting the first signal into a proper response: production of specific mediators, cell activation, survival, or death. Several overlapping pathways are coordinated to ensure specific and timely induction of inflammation to neutralize potential harms to the tissue. Ideally, the inflammatory response must be controlled and self-limited. Resolution of inflammation is an active process that culminates with termination of inflammation and restoration of tissue homeostasis. Comparably to the onset of inflammation, resolution responses are triggered by coordinated intracellular signaling pathways that transduce the message to the nucleus. However, the key messengers and pathways involved in signaling transduction for resolution are still poorly understood in comparison to the inflammatory network. cAMP has long been recognized as an inducer of anti-inflammatory responses and cAMP-dependent pathways have been extensively exploited pharmacologically to treat inflammatory diseases. Recently, cAMP has been pointed out as coordinator of key steps of resolution of inflammation. Here, we summarize the evidence for the role of cAMP at inducing important features of resolution of inflammation.

Extracellular adenosine enhances pulmonary artery vasa vasorum endothelial cell barrier function via Gi/ELMO1/Rac1/PKA-dependent signaling mechanisms

The vasa vasorum (VV), the microvascular network around large vessels, has been recognized as an important contributor to the pathological vascular remodeling in cardiovascular diseases. In bovine and rat models of hypoxic pulmonary hypertension (PH), we have previously shown that chronic hypoxia profoundly increased pulmonary artery (PA) VV permeability, associated with infiltration of inflammatory and progenitor cells in the arterial wall, perivascular inflammation, and structural vascular remodeling. Extracellular adenosine was shown to exhibit a barrier-protective effect on VV endothelial cells (VVEC) via cAMP-independent mechanisms, which involved adenosine A1 receptor-mediated activation of Gi-phosphoinositide 3-kinase-Akt pathway and actin cytoskeleton remodeling. Using VVEC isolated from the adventitia of calf PA, in this study we investigated in more detail the mechanisms linking Gi activation to downstream barrier protection pathways. Using a small-interference RNA (siRNA) technique and transendothelial electrical resistance assay, we found that the adaptor protein, engulfment and cell motility 1 (ELMO1), the tyrosine phosphatase Src homology region 2 domain-containing phosphatase-2, and atypical Gi- and Rac1-mediated protein kinase A activation are implicated in VVEC barrier enhancement. In contrast, the actin-interacting GTP-binding protein, girdin, and the p21-activated kinase 1 downstream target, LIM kinase, are not involved in this response. In addition, adenosine-dependent cytoskeletal rearrangement involves activation of cofilin and inactivation of ezrin-radixin-moesin regulatory cytoskeletal proteins, consistent with a barrier-protective mechanism. Collectively, our data indicate that targeting adenosine receptors and downstream barrier-protective pathways in VVEC may have a potential translational significance in developing pharmacological approach for the VV barrier protection in PH.

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.

Adenosine mediates functional and metabolic suppression of peripheral and tumor-infiltrating CD8+ T cells

Background

Several mechanisms are present in the tumor microenvironment (TME) to impair cytotoxic T cell responses potentially able to control tumor growth. Among these, the accumulation of adenosine (Ado) contributes to tumor progression and represents a promising immunotherapeutic target. Ado has been shown to impair T cell effector function, but the role and mechanisms employed by Ado/Ado receptors (AdoRs) in modulating human peripheral and tumor-infiltrating lymphocyte (TIL) function are still puzzling.

Methods

CD8⁺ T cell cytokine production following stimulation was quantified by intracellular staining and flow cytometry. The cytotoxic capacity of tumor infiltrating lymphocytes (TILs) was quantified by the chromium release assay following co-culture with autologous or anti-CD3-loaded tumor cell lines. The CD8⁺ T cell metabolic fitness was evaluated by the seahorse assay and by the quantification of 2-NBDG uptake and CD71/CD98 upregulation upon stimulation. The expression of AdoRs was assessed by RNA flow cytometry, a recently developed technology that we validated by semiquantitative RT-PCR (qRT-PCR), while the impact on T cell function was evaluated by the use of selective antagonists and agonists. The influence of Ado/AdoR on the PKA and mTOR pathways was evaluated by phosphoflow staining of p-CREB and p-S6, respectively, and validated by western blot.

Results

Here, we demonstrate that Ado signaling through the A2A receptor (A2AR) in human peripheral CD8⁺ T cells and TILs is responsible for the higher sensitivity to Ado-mediated suppression of T central memory cells. We confirmed that Ado is able to impair peripheral and tumor-expanded T cell effector functions, and we show for the first time its impact on metabolic fitness. The Ado-mediated immunosuppressive effects are mediated by increased PKA activation that results in impairment of the mTORC1 pathway.

Conclusions

Our findings unveil A2AR/PKA/mTORC1 as the main Ado signaling pathway impairing the immune competence of peripheral T cells and TILs. Thus, p-CREB and p-S6 may represent useful pharmacodynamic and efficacy biomarkers of immunotherapies targeting Ado. The effect of Ado on T cell metabolic fitness reinforces the importance of the adenosinergic pathway as a target for next-generation immunotherapy.

Electronic supplementary material

The online version of this article (10.1186/s40425-019-0719-5) contains supplementary material, which is available to authorized users.

Multiple adenosine receptor subtypes stimulate wound healing in human EA.hy926 endothelial cells

Wound healing is an important outcome of tissue damage and can be stimulated by adenosine released from cells during events such as tissue injury, ischaemia or tumour growth. The aim of this research was to determine the potency and efficacy of adenosine A₁, A_2A and A_2B receptor agonists on the rate of wound healing and cell proliferation in human EA.hy926 endothelial cells. Real-time PCR data showed that only adenosine A₁, A_2A and A_2B receptor mRNA were expressed in this cell line. All three adenosine receptor agonists, CPA, CGS21680 and NECA, significantly increased the rate of wound healing in human EAhy926 endothelial cells with the following order of potency CGS21680>CPA>NECA and efficacy CPA>NECA>CGS21680. The selective adenosine A₁, A_2A and A_2B receptor antagonists, DPCPX, ZM241385 and MRS1754 (all at 10 nM), reversed the effects of their respective agonists. EAhy926 endothelial cell proliferation was also significantly increased with the adenosine A₁ and A_2B receptor agonists, CPA and NECA. Western blot analysis demonstrated that adenosine A_2A and A₁ receptor protein levels were highly expressed compared with the adenosine A_2B receptors in the EAhy926 endothelial cell lines. While all three adenosine A₁, A_2A and A_2B receptor subtypes contribute to cell proliferation and wound healing in human EAhy926 endothelial cells, treatments selectively targeting receptor subtypes may further enhance wound healing.

Caffeine Inhibits NLRP3 Inflammasome Activation by Suppressing MAPK/NF-κB and A2aR Signaling in LPS-Induced THP-1 Macrophages

Excessive inflammation induced by various risk factors is associated with the development of bronchopulmonary dysplasia (BPD). Caffeine exerts potent anti-inflammatory effects as a clinical preventive medicine for BPD. Recently, NLRP3 inflammasome activation has been demonstrated to be essential for the pathogenesis of BPD. In the present study, we aimed to investigate the effects of caffeine on NLRP3 inflammasome activation in LPS-induced THP-1 macrophages and to explore the underlying the detailed mechanism. We found that caffeine significantly reduced NLRP3 expression, ASC speck formation, and caspase 1 cleavage and therefore decreased IL-1β and IL-18 secretion in THP-1 macrophages. Caffeine also markedly decreased the phosphorylation levels of MAPK and NF-κB pathway members, further suppressing the translocation of NF-κB in THP-1 macrophages. Moreover, silencing of the caffeine-antagonized adenosine A2a receptor (A2aR) significantly decreased cleaved caspase 1 expression in THP-1 macrophages by reducing ROS production. Given these findings, we conclude that caffeine inhibits NLRP3 inflammasome activation by suppressing MAPK/NF-κB signaling and A2aR-associated ROS production in LPS-induced THP-1 macrophages.

Pro-inflammatory Effect of Downregulated CD73 Expression in EAE Astrocytes

CD73, an ectonucleotidase, participates in the regulation of immune responses by controlling the conversion of extracellular AMP to adenosine. In this study, we investigated whether any type of brain cells, especially neuroglia cells, exhibit altered CD73 expression, localization or activity upon experimental autoimmune uveitis (EAU) induction and whether altered CD73 manipulates the activation of effector T cells that interact with such cell types. First, the amount of cell membrane-exposed CD73 was detected by flow cytometry in various types of brain cells collected from either naïve or EAE mice. Compared to that in astrocytes from naïve control mice, the amount of membrane-bound CD73 was significantly decreased in astrocytes from EAE mice, while no significant differences were detected in other cell types. Thereafter, wild-type and CD73-/- astrocytes were used to study whether CD73 influences the function of inflammatory astrocytes, such as the production of cytokines/chemokines and the activation of effector T cells that interact with astrocytes. The results indicated that the addition of exogenous AMP significantly inhibited cytokine/chemokine production by wild type astrocytes but had no effect on CD73-/- astrocytes and that the effect of AMP was almost completely blocked by the addition of either a CD73 inhibitor (APCP) or an adenosine receptor A1 subtype (ARA1) antagonist (DPCPX). Although the addition of AMP did not affect CD73-/- astrocytes, the addition of adenosine successfully inhibited their cytokine/chemokine production. The antigen-specific interaction of astrocytes with invading CD4 cells caused CD73 downregulation in astrocytes from mice that underwent EAE induction. Collectively, our findings support the conclusion that, upon EAE induction, likely due to an interaction with invading CD4+ cells, astrocytes lose most of their membrane-localized CD73; this inhibits the generation of adenosine in the local microenvironment. As adenosine has anti-inflammatory effects on astrocytes and CNS-infiltrating effector T cells in EAE, the downregulation of CD73 in astrocytes may be considered a pro-inflammatory process for facilitating the pathogenesis of EAE.

Adenosine Induces EBV Lytic Reactivation through ADORA1 in EBV-Associated Gastric Carcinoma

Cordyceps species are known to contain numerous bioactive compounds, including cordycepin. Extracts of Cordyceps militaris (CME) are used in diverse medicinal purposes because of their bioactive components. Cordycepin, one of the active components of CME, exhibits anti-proliferative, pro-apoptotic, and anti-inflammatory effects. Cordycepin structurally differs from adenosine in that its ribose lacks an oxygen atom at the 3′ position. We previously reported that cordycepin suppresses Epstein–Barr virus (EBV) gene expression and lytic replication in EBV-associated gastric carcinoma (EBVaGC). However, other studies reported that cordycepin induces EBV gene expression and lytic reactivation. Thus, it was reasonable to clarify the bioactive effects of CME bioactive compounds on the EBV life cycle. We first confirmed that CME preferentially induces EBV gene expression and lytic reactivation; second, we determined that adenosine in CME induces EBV gene expression and lytic reactivation; third, we discovered that the adenosine A1 receptor (ADORA1) is required for adenosine to initiate signaling for upregulating BZLF1, which encodes for a key EBV regulator (Zta) of the EBV lytic cycle; finally, we showed that BZLF1 upregulation by adenosine leads to delayed tumor development in the EBVaGC xenograft mouse model. Taken together, these results suggest that adenosine is an EBV lytic cycle inducer that inhibits EBVaGC development.

Purinergic receptors in neurogenic processes

Neurogenesis is a process of generating functional neurons, which occurs during embryonic and adult stages in mammals. While neurogenesis during development phase is characterized by intensive proliferation activity in all regions of the brain to form the architecture and neural function of the nervous system, adult neurogenesis occurs with less intensity in two brain regions and is involved in the maintenance of neurogenic niches, local repair, memory and cognitive functions in the hippocampus. Taking such differences into account, the understanding of molecular mechanisms involved in cell differentiation in developmental stages and maintenance of the nervous system is an important research target. Although embryonic and adult neurogenesis presents several differences, signaling through purinergic receptors participates in this process throughout life. For instance, while embryonic neurogenesis involves P2X7 receptor down-regulation and calcium waves triggered by P2Y1 receptor stimulation, adult neurogenesis may be enhanced by increased activity of A_2A and P2Y1 receptors and impaired by A₁, P2Y13 and P2X7 receptor stimulation.

Differential mechanisms of adenosine- and ATPγS-induced microvascular endothelial barrier strengthening

Maintenance of the endothelial cell (EC) barrier is critical to vascular homeostasis and a loss of barrier integrity results in increased vascular permeability. While the mechanisms that govern increased EC permeability have been under intense investigation over the past several decades, the processes regulating the preservation/restoration of the EC barrier remain poorly understood. Herein we show that the extracellular purines, adenosine (Ado) and adenosine 5'-[γ-thio]-triphosphate (ATPγS) can strengthen the barrier function of human lung microvascular EC (HLMVEC). This ability involves protein kinase A (PKA) activation and decreases in myosin light chain 20 (MLC20) phosphorylation secondary to the involvement of MLC phosphatase (MLCP). In contrast to Ado, ATPγS-induced PKA activation is accompanied by a modest, but significant decrease in cyclic adenosine monophosphate (cAMP) levels supporting the existence of an unconventional cAMP-independent pathway of PKA activation. Furthermore, ATPγS-induced EC barrier strengthening does not involve the Rap guanine nucleotide exchange factor 3 (EPAC1) which is directly activated by cAMP but is instead dependent upon PKA-anchor protein 2 (AKAP2) expression. We also found that AKAP2 can directly interact with the myosin phosphatase-targeting protein MYPT1 and that depletion of AKAP2 abolished ATPγS-induced increases in transendothelial electrical resistance. Ado-induced strengthening of the HLMVEC barrier required the coordinated activation of PKA and EPAC1 in a cAMP-dependent manner. In summary, ATPγS-induced enhancement of the EC barrier is EPAC1-independent and is instead mediated by activation of PKA which is then guided by AKAP2, in a cAMP-independent mechanism, to activate MLCP which dephosphorylates MLC20 resulting in reduced EC contraction and preservation.

Functional insights into nucleotide-metabolizing ectoenzymes expressed by bone marrow-resident cells in patients with multiple myeloma

Human myeloma cells grow in a hypoxic acidic niche in the bone marrow. Cross talk among cellular components of this closed niche generates extracellular adenosine, which promotes tumor cell survival. This is achieved through the binding of adenosine to purinergic receptors into complexes that function as an autocrine/paracrine signal factor with immune regulatory activities that i) down-regulate the functions of most immune effector cells and ii) enhance the activity of cells that suppress anti-tumor immune responses, thus facilitating the escape of malignant myeloma cells from immune surveillance. Here we review recent findings confirming that the dominant phenotype for survival of tumor cells is that where the malignant cells have been metabolically reprogrammed for the generation of lactic acidosis in the bone marrow niche. Adenosine triphosphate and nicotinamide-adenine dinucleotide extruded from tumor cells, along with cyclic adenosine monophosphate, are the main intracellular energetic/messenger molecules that serve as leading substrates in the extracellular space for membrane-bound ectonucleotidases metabolizing purine nucleotides to signaling adenosine. Within this mechanistic framework, the adenosinergic substrate conversion can vary significantly according to the metabolic environment. Indeed, the neoplastic expansion of plasma cells exploits both enzymatic networks and hypoxic acidic conditions for migrating and homing to a protected niche and for evading the immune response. The expression of multiple specific adenosine receptors in the niche completes the profile of a complex regulatory framework whose signals modify multiple myeloma and host immune responses.

Computer-aided Drug Design Applied to Parkinson Targets

Background

Parkinson’s disease (PD) is a progressive neurodegenerative disorder characterized by debilitating motor deficits, as well as autonomic problems, cognitive declines, changes in affect and sleep disturbances. Although the scientific community has performed great efforts in the study of PD, and from the most diverse points of view, the disease remains incurable. The exact mechanism underlying its progression is unclear, but oxidative stress, mitochondrial dysfunction and inflammation are thought to play major roles in the etiology.

Objective

Current pharmacological therapies for the treatment of Parkinson’s disease are mostly inadequate, and new therapeutic agents are much needed.

Methods

In this review, recent advances in computer-aided drug design for the rational design of new compounds against Parkinson disease; using methods such as Quantitative Structure-Activity Relationships (QSAR), molecular docking, molecular dynamics and pharmacophore modeling are discussed.

Results

In this review, four targets were selected: the enzyme monoamine oxidase, dopamine agonists, acetylcholine receptors, and adenosine receptors.

Conclusion

Computer aided-drug design enables the creation of theoretical models that can be used in a large database to virtually screen for and identify novel candidate molecules.

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.

Adenosinergic signaling as a target for natural killer cell immunotherapy

Purinergic signaling through adenosine plays a key role in immune regulation. Hypoxia-driven accumulation of extracellular adenosine results in the generation of an immunosuppressive niche that fuels tumor development. Such immunometabolic modulation has shown to be a promising therapeutic target through blockade of adenosine receptors which mediate adenosine's immunosuppressive function, or cancer-associated ectonucleotidases CD39 and CD73 that catalyze the synthesis of adenosine. Adenosinergic signaling heavily implicates natural killer cells through both direct and indirect effects on their cytolytic activity, expression of cytotoxic granules, interferon-γ, and activating receptors. Continuing work has uncovered multiple checkpoints linked to adenosine within the purinergic signaling cascade as contributing to immune evasion from NK cell effector function. Here, we discuss these checkpoints and the recent body of work that focuses on adenosinergic signaling as a target for natural killer cell of cancer.

Role of adenosine signaling in the pathogenesis of head and neck cancer

The concentrations of adenosine may increase under ischemic conditions in the tumor microenvironment, and then it enters the systemic circulation. Adenosine controls cancer progression and responses to therapy by regulating angiogenesis, cell survival, apoptosis, cell proliferation, and metastases in tumors. Hence, adenosine metabolism, adenosine-generating enzymes, and adenosine signaling are potentially novel therapeutic targets in a wide range of pathological conditions, including cerebral and cardiac ischemic diseases, inflammatory disorders, immunomodulatory disorders, and, of special interest in this review, cancer. This review summarizes the role of adenosine in the pathogenesis of head and neck cancer for a better understanding of how this may be applied to treating this type of cancer.

Current Nondopaminergic Therapeutic Options for Motor Symptoms of Parkinson's Disease

Objective:

The aim of this study was to summarize recent studies on nondopaminergic options for the treatment of motor symptoms in Parkinson's disease (PD).

Data Sources:

Papers in English published in PubMed, Cochrane, and Ovid Nursing databases between January 1988 and November 2016 were searched using the following keywords: PD, nondopaminergic therapy, adenosine, glutamatergic, adrenergic, serotoninergic, histaminic, and iron chelator. We also reviewed the ongoing clinical trials in the website of clinicaltrials.gov.

Study Selection:

Articles related to the nondopaminergic treatment of motor symptoms in PD were selected for this review.

Results:

PD is conventionally treated with dopamine replacement strategies, which are effective in the early stages of PD. Long-term use of levodopa could result in motor complications. Recent studies revealed that nondopaminergic systems such as adenosine, glutamatergic, adrenergic, serotoninergic, histaminic, and iron chelator pathways could include potential therapeutic targets for motor symptoms, including motor fluctuations, levodopa-induced dyskinesia, and gait disorders. Some nondopaminergic drugs, such as istradefylline and amantadine, are currently used clinically, while most such drugs are in preclinical testing stages. Transitioning of these agents into clinically beneficial strategies requires reliable evaluation since several agents have failed to show consistent results despite positive findings at the preclinical level.

Conclusions:

Targeting nondopaminergic transmission could improve some motor symptoms in PD, especially the discomfort of dyskinesia. Although nondopaminergic treatments show great potential in PD treatment as an adjunct therapy to levodopa, further investigation is required to ensure their success.

Caffeine Suppresses the Activation of Hepatic Stellate Cells cAMP-Independently by Antagonizing Adenosine Receptors

During liver injury, hepatic stellate cells (HSCs) are activated by various cytokines and transdifferentiated into myofibroblast-like activated HSCs, which produce collagen, a major source of liver fibrosis. Therefore, the suppression of HSC activation is regarded as a therapeutic target for liver fibrosis. Several epidemiological reports have revealed that caffeine intake decreases the risk of liver disease. In this study, therefore, we investigated the effect of caffeine on the activation of primary HSCs isolated from mice. Caffeine suppressed the activation of HSC in a concentration-dependent manner. BAPTA-AM, an intracellular Ca²⁺ chelator, had no effect on the caffeine-induced suppression of HSC activation. None of the isoform-selective inhibitors of phosphodiesterase1 to 5 affected changes in the morphology of HSC during activation, whereas CGS-15943, an adenosine receptor antagonist, inhibited them. Caffeine had no effect on intracellular cAMP level or on the phosphorylation of extracellular signal-regulated kinase (ERK)1/2. In contrast, caffeine significantly decreased the phosphorylation of Akt1. These results suggest that caffeine inhibits HSC activation by antagonizing adenosine receptors, leading to Akt1 signaling activation.

Epac is required for exogenous and endogenous stimulation of adenosine A2B receptor for inhibition of angiotensin II-induced collagen synthesis and myofibroblast differentiation

Angiotensin II (Ang II) plays an important role on the pathogenesis of cardiac fibrosis. Prolong and overstimulation of angiotensin II type 1 receptor with Ang II-induced collagen synthesis and myofibroblast differentiation in cardiac fibroblasts, leading to cardiac fibrosis. Although adenosine and its analogues are known to have cardioprotective effects, the mechanistic by which adenosine A₂ receptors (A₂Rs) inhibit Ang II-induced cardiac fibrosis is not clearly understood. In the present study, we examined the effects of exogenous adenosine and endogenous adenosine on Ang II-induced collagen and myofibroblast differentiation determined by α-smooth muscle action (α-SMA) overexpression and their underlying signal transduction. Elevation of endogenous adenosine levels resulted in the inhibition of Ang II-induced collagen type I and III and α-SMA synthesis in cardiac fibroblasts. Moreover, treatment with exogenous adenosine which selectively stimulated A₂Rs also suppressed Ang II-induced collagen synthesis and α-SMA production. These antifibrotic effects of both endogenous and exogenous adenosines are mediated through the A_2B receptor (A_2BR) subtype. Stimulation of A_2BR exhibited antifibrotic effects via the cAMP-dependent and Epac-dependent pathways. Our results provide new mechanistic insights regarding the role for cAMP and Epac on A_2BR-mediated antifibrotic effects. Thus, A_2BR is one of the potential therapeutic targets against cardiac fibrosis.

Stimulation of Adenosine A2B Receptor Inhibits Endothelin-1-Induced Cardiac Fibroblast Proliferation and α-Smooth Muscle Actin Synthesis Through the cAMP/Epac/PI3K/Akt-Signaling Pathway

Background and Purpose: Cardiac fibrosis is characterized by an increase in fibroblast proliferation, overproduction of extracellular matrix proteins, and the formation of myofibroblast that express α-smooth muscle actin (α-SMA). Endothelin-1 (ET-1) is involved in the pathogenesis of cardiac fibrosis. Overstimulation of endothelin receptors induced cell proliferation, collagen synthesis, and α-SMA expression in cardiac fibroblasts. Although adenosine was shown to have cardioprotective effects, the molecular mechanisms by which adenosine A₂ receptor inhibit ET-1-induced fibroblast proliferation and α-SMA expression in cardiac fibroblasts are not clearly identified.

Experimental Approach: This study aimed at evaluating the mechanisms of cardioprotective effects of adenosine receptor agonist in rat cardiac fibroblast by measurement of cell proliferation, and mRNA and protein levels of α-SMA.

Key results: Stimulation of adenosine subtype 2B (A_2B) receptor resulted in the inhibition of ET-1-induced fibroblast proliferation, and a reduction of ET-1-induced α-SMA expression that is dependent on cAMP/Epac/PI3K/Akt signaling pathways in cardiac fibroblasts. The data in this study confirm a critical role for Epac signaling on A_2B receptor-mediated inhibition of ET-1-induced cardiac fibrosis via PI3K and Akt activation.

Conclusion and Implications: This is the first work reporting a novel signaling pathway for the inhibition of ET-1-induced cardiac fibrosis mediated through the A_2B receptor. Thus, A_2B receptor agonists represent a promising perspective as therapeutic targets for the prevention of cardiac fibrosis.