Thermodynamics of A2B adenosine receptor binding discriminates agonistic from antagonistic behaviour

A2B adenosine receptor signaling and regulation

The A2B adenosine receptor (A2BR) is one of the four adenosine-activated G protein-coupled receptors. In addition to adenosine, protein kinase C (PKC) was recently found to activate the A2BR. The A2BR is coupled to both Gs and Gi, as well as Gq proteins in some cell types. Many primary cells and cell lines, such as bladder and breast cancer, bronchial smooth muscle, skeletal muscle, and fat cells, express the A2BR endogenously at high levels, suggesting its potentially important role in asthma, cancer, diabetes, and other conditions. The A2BR has been characterized as both pro- and anti-inflammatory, inducing cell type-dependent secretion of IL-6, IL-8, and IL-10. Theophylline and enprofylline have long been used for asthma treatment, although it is still not entirely clear if their A2BR antagonism contributes to their therapeutic effects or side effects. The A2BR is required in ischemic cardiac preconditioning by adenosine. Both A2BR and protein kinase C (PKC) contribute to cardioprotection, and both modes of A2BR signaling can be blocked by A2BR antagonists. Inhibitors of PKC and A2BR are in clinical cancer trials. Sulforaphane and other isothiocyanates from cruciferous vegetables such as broccoli and cauliflower have been reported to inhibit A2BR signaling via reaction with an intracellular A2BR cysteine residue (C210). A full, A2BR-selective agonist, critical to elucidate many controversial roles of the A2BR, is still not available, although agonist-bound A2BR structures have recently been reported.

In silico development of adenosine A2B receptor antagonists for sickle cell disease

Sickle cell disease (SCD) is a disease resulting from mutation in the globin portion of hemoglobin caused by the replacement of adenine for thymine in the codon of the β globin gene. In Brazil, SCD affects about 0.3% of the black and Caucasian population. Until now, there is no specific treatment and the available drugs have several serious adverse effects which makes the search for new drugs an emergently need. The use of computational techniques can accelerate the drug development process by prioritization of molecules with affinity against essential targets. Adenosine A2b receptor (rA2b) has been studied in SCD due to its relationship with red blood cells concentration of 2,3-diphosphoglycerate which reduces the hemoglobin affinity for oxygen (O₂), facilitating its availability for the tissues. Then, development of rA2b antagonists could be helpful for the treatment of SCD. However, there is still no 3D structure of rA2b and to overcome this limitation, homology modeling should be applied. In this scenario, this study aims to build a suitable 3D model of rA2b by SWISS MODEL and to evaluate the structural aspects of rA2b with known antagonists that may be useful for the identification of new potential antagonists by molecular dynamics on a lipid bilayer environment using GROMACS 5.1.4. The complexes with antagonists ZINC223070016 and ZINC17974526 interacted with key residues by hydrophobic contacts and hydrogen bonds which stabilized them at the rA2b binding site. This intermolecular profile can contribute to the development of more potent rA2b antagonists. Communicated by Ramaswamy H. Sarma.

A3 Adenosine Receptors as Modulators of Inflammation: From Medicinal Chemistry to Therapy

The A3 adenosine receptor (A3 AR) subtype is a novel, promising therapeutic target for inflammatory diseases, such as rheumatoid arthritis (RA) and psoriasis, as well as liver cancer. A3 AR is coupled to inhibition of adenylyl cyclase and regulation of mitogen-activated protein kinase (MAPK) pathways, leading to modulation of transcription. Furthermore, A3 AR affects functions of almost all immune cells and the proliferation of cancer cells. Numerous A3 AR agonists, partial agonists, antagonists, and allosteric modulators have been reported, and their structure-activity relationships (SARs) have been studied culminating in the development of potent and selective molecules with drug-like characteristics. The efficacy of nucleoside agonists may be suppressed to produce antagonists, by structural modification of the ribose moiety. Diverse classes of heterocycles have been discovered as selective A3 AR blockers, although with large species differences. Thus, as a result of intense basic research efforts, the outlook for development of A3 AR modulators for human therapeutics is encouraging. Two prototypical selective agonists, N6-(3-Iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA; CF101) and 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA; CF102), have progressed to advanced clinical trials. They were found safe and well tolerated in all preclinical and human clinical studies and showed promising results, particularly in psoriasis and RA, where the A3 AR is both a promising therapeutic target and a biologically predictive marker, suggesting a personalized medicine approach. Targeting the A3 AR may pave the way for safe and efficacious treatments for patient populations affected by inflammatory diseases, cancer, and other conditions.

Adenosine Receptors as a Biological Pathway for the Anti-Inflammatory and Beneficial Effects of Low Frequency Low Energy Pulsed Electromagnetic Fields

Several studies explored the biological effects of low frequency low energy pulsed electromagnetic fields (PEMFs) on human body reporting different functional changes. Much research activity has focused on the mechanisms of interaction between PEMFs and membrane receptors such as the involvement of adenosine receptors (ARs). In particular, PEMF exposure mediates a significant upregulation of A_2A and A₃ARs expressed in various cells or tissues involving a reduction in most of the proinflammatory cytokines. Of particular interest is the observation that PEMFs, acting as modulators of adenosine, are able to increase the functionality of the endogenous agonist. By reviewing the scientific literature on joint cells, a double role for PEMFs could be hypothesized in vitro by stimulating cell proliferation, colonization of the scaffold, and production of tissue matrix. Another effect could be obtained in vivo after surgical implantation of the construct by favoring the anabolic activities of the implanted cells and surrounding tissues and protecting the construct from the catabolic effects of the inflammatory status. Moreover, a protective involvement of PEMFs on hypoxia damage in neuron-like cells and an anti-inflammatory effect in microglial cells have suggested the hypothesis of a positive impact of this noninvasive biophysical stimulus.

GPCR-radioligand binding assays

Radioligand binding assays provide sensitive and quantitative information about guanine nucleotide protein G protein-coupled receptor (GPCR) expression and affinity for a wide variety of ligands, making them essential for drug structure-activity studies and basic GPCR research. Three basic radioligand binding protocols, saturation, indirect (competition, displacement, or modulation), and kinetic binding assays, are used to assess GPCR expression (Bmax), equilibrium dissociation constants for radioligands (Kd) and nonradioactive ligands (Ki), association and dissociation rates, and to distinguish competitive and allosteric mechanisms of GPCR-ligand interactions. Nonspecific radioligand binding may be mitigated by appropriate choices of reaction conditions. Radioligand depletion (bound radioactivity >10% of total radioligand), which compromises accuracy of Kd and Ki measurements, can be limited by adjusting receptor concentration and appropriate radioligand choice. Accurate Kd and Ki values in saturation and indirect binding assays depend on binding equilibrium. Equilibration time for high-affinity ligands, with slow dissociation rates, may require much extended incubation times or increased incubation temperature.

The A3 adenosine receptor: history and perspectives

By general consensus, the omnipresent purine nucleoside adenosine is considered a major regulator of local tissue function, especially when energy supply fails to meet cellular energy demand. Adenosine mediation involves activation of a family of four G protein-coupled adenosine receptors (ARs): A(1), A(2)A, A(2)B, and A(3). The A(3) adenosine receptor (A(3)AR) is the only adenosine subtype to be overexpressed in inflammatory and cancer cells, thus making it a potential target for therapy. Originally isolated as an orphan receptor, A(3)AR presented a twofold nature under different pathophysiologic conditions: it appeared to be protective/harmful under ischemic conditions, pro/anti-inflammatory, and pro/antitumoral depending on the systems investigated. Until recently, the greatest and most intriguing challenge has been to understand whether, and in which cases, selective A(3) agonists or antagonists would be the best choice. Today, the choice has been made and A(3)AR agonists are now under clinical development for some disorders including rheumatoid arthritis, psoriasis, glaucoma, and hepatocellular carcinoma. More specifically, the interest and relevance of these new agents derives from clinical data demonstrating that A(3)AR agonists are both effective and safe. Thus, it will become apparent in the present review that purine scientists do seem to be getting closer to their goal: the incorporation of adenosine ligands into drugs with the ability to save lives and improve human health.

Differential thermodynamic driving force of first- and second-generation antihistamines to determine their binding affinity for human H1 receptors

Differential binding sites for first- and second-generation antihistamines were indicated on the basis of the crystal structure of human histamine H1 receptors. In this study, we evaluated differences between the thermodynamic driving forces of first- and second-generation antihistamines for human H1 receptors and their structural determinants. The binding enthalpy and entropy of 20 antihistamines were estimated with the van't Hoff equation using their dissociation constants obtained from their displacement curves against the binding of [(3)H]mepyramine to membrane preparations of Chinese hamster ovary cells expressing human H1 receptors at various temperatures from 4°C to 37°C. Structural determinants of antihistamines for their thermodynamic binding properties were assessed by quantitative structure-activity relationship (QSAR) analyses. We found that entropy-dependent binding was more evident in second- than first-generation antihistamines, resulting in enthalpy-entropy compensation between the binding forces of first- and second-generation antihistamines. QSAR analyses indicated that enthalpy-entropy compensation was determined by the sum of degrees, maximal electrostatic potentials, water-accessible surface area and hydrogen binding acceptor count of antihistamines to regulate their affinity for receptors. In conclusion, it was revealed that entropy-dependent hydrophobic interaction was more important in the binding of second-generation antihistamines, even though the hydrophilicity of second-generation antihistamines is generally increased. Furthermore, their structural determinants responsible for enthalpy-entropy compensation were explored by QSAR analyses. These findings may contribute to understanding the fundamental mechanisms of how the affinity of ligands for their receptors is regulated.

Adenosine and cAMP signalling skew human dendritic cell differentiation towards a tolerogenic phenotype with defective CD8(+) T-cell priming capacity

Multiple endogenous mechanisms that regulate immune and inflammatory processes contribute to the maintenance of peripheral tolerance and prevent chronic inflammation in mammals. Yet pathogens and tumours are able to exploit these homeostatic pathways to foster immunosuppressive microenvironments and evade immune surveillance. The release of adenosine in the extracellular space contributes to these phenomena by exerting a broad range of immunomodulatory effects. Here we document the influence of adenosine receptor triggering on human dendritic cell differentiation and functions. We show that the expression of several immunomodulatory proteins and myeloid/monocytic lineage markers was affected by adenosine receptors and the cAMP pathway. These changes were reminiscent of the phenotype associated with tolerogenic dendritic cells and, functionally, translated into a defective capacity to prime CD8(+) T-cells with a common tumour antigen in vitro. These results establish a novel mechanism by which adenosine hampers CD8(+) T-cell immunity via dendritic cells that may contribute to peripheral tolerance as well as to the establishment of immunosuppressive microenvironments relevant to tumour biology.

Entropy-enthalpy transduction caused by conformational shifts can obscure the forces driving protein-ligand binding

Molecular dynamics simulations of unprecedented duration now can provide new insights into biomolecular mechanisms. Analysis of a 1-ms molecular dynamics simulation of the small protein bovine pancreatic trypsin inhibitor reveals that its main conformations have different thermodynamic profiles and that perturbation of a single geometric variable, such as a torsion angle or interresidue distance, can select for occupancy of one or another conformational state. These results establish the basis for a mechanism that we term entropy-enthalpy transduction (EET), in which the thermodynamic character of a local perturbation, such as enthalpic binding of a small molecule, is camouflaged by the thermodynamics of a global conformational change induced by the perturbation, such as a switch into a high-entropy conformational state. It is noted that EET could occur in many systems, making measured entropies and enthalpies of folding and binding unreliable indicators of actual thermodynamic driving forces. The same mechanism might also account for the high experimental variance of measured enthalpies and entropies relative to free energies in some calorimetric studies. Finally, EET may be the physical mechanism underlying many cases of entropy-enthalpy compensation.

Ligand binding assays at equilibrium: validation and interpretation

The focus of this review paper is factors affecting data interpretation in ligand binding assays under equilibrium conditions. Protocols for determining K(d) (the equilibrium dissociation constant) and K(dA) (the equilibrium inhibitor constant) for receptor ligands are discussed. The basic theory describing the interaction of a radiotracer and an unlabelled competitor ligand with a receptor is developed. Inappropriate experimental design may result in ligand depletion and non-attainment of equilibrium, distorting the calculation of K(d) and K(dA) . Strategies, both theoretical and practical, will be given to avoid and correct such errors, thus leading to the determination of reliable values for these constants. In determining K(dA) from competition binding studies, two additional concepts are discussed. First, the necessity to measure an adequate specific binding signal from the bound radiotracer ligand limits the range of affinity constants that can be measured: a particular set of assay conditions may lead to an upper limit on the apparent affinity of unlabelled ligands. Second, an extension of the basic assay methodology can indicate whether the interaction between the tracer and a test ligand is mediated by a competitive or an allosteric mechanism. Finally, the review ends with a discussion of two factors that are often overlooked: buffer composition and the temperature at which the assay is conducted, and the impact these can have on affinity measurements and the understanding of drug interactions.

Modulation of metalloproteinase-9 in U87MG glioblastoma cells by A3 adenosine receptors

In this work, we investigated the biological functions of adenosine (ado) in metalloproteinase-9 (MMP-9) regulation in U87MG human glioblastoma cells. The nucleoside was able to increase both MMP-9 mRNA and protein levels through A3 receptors activation. We revealed that A3 receptor stimulation induced an increase of MMP-9 protein levels in cellular extracts of U87MG cells by phosphorylation of extracellular signal-regulated protein kinases (ERK1/2), c-Jun N-terminal kinase/stress-activated protein kinase (pJNK/SAPK), protein kinase B (Akt/PKB) and finally activator protein 1 (AP-1). A3 receptor activation stimulated also an increase of extracellular MMP-9 in the supernatants from U87MG glioblastoma cells. Finally, the Matrigel invasion assay demonstrated that A3 receptors, by inducing an increase in MMP-9 levels, was responsible for an increase of glioblastoma cells invasion. Collectively, these results suggest that ado, through A3 receptors activation, modulates MMP-9 protein levels and plays a role in increasing invasion of U87MG cells.