Four adenosine receptors have been cloned and characterized from several mammalian species. The receptors are named adenosine A(1), A(2A), A(2B), and A(3). The A(2A) and A(2B) receptors preferably interact with members of the G(s) family of G proteins and the A(1) and A(3) receptors with G(i/o) proteins. However, other G protein interactions have also been described. Adenosine is the preferred endogenous agonist at all these receptors, but inosine can also activate the A(3) receptor. The levels of adenosine seen under basal conditions are sufficient to cause some activation of all the receptors, at least where they are abundantly expressed. Adenosine levels during, e.g., ischemia can activate all receptors even when expressed in low abundance. Accordingly, experiments with receptor antagonists and mice with targeted disruption of adenosine A(1), A(2A), and A(3) expression reveal roles for these receptors under physiological and particularly pathophysiological conditions. There are pharmacological tools that can be used to classify A(1), A(2A), and A(3) receptors but few drugs that interact selectively with A(2B) receptors. Testable models of the interaction of these drugs with their receptors have been generated by site-directed mutagenesis and homology-based modelling. Both agonists and antagonists are being developed as potential drugs.

https://pharmrev.aspetjournals.org/content/pharmrev/53/4/527.full.pdf

International Union of Pharmacology. XXV. Nomenclature and Classification of Adenosine Receptors

The adenosine class of heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors (GPCRs) mediates the important role of extracellular adenosine in many physiological processes and is antagonized by caffeine. We have determined the crystal structure of the human A2A adenosine receptor, in complex with a high-affinity subtype-selective antagonist, ZM241385, to 2.6 angstrom resolution. Four disulfide bridges in the extracellular domain, combined with a subtle repacking of the transmembrane helices relative to the adrenergic and rhodopsin receptor structures, define a pocket distinct from that of other structurally determined GPCRs. The arrangement allows for the binding of the antagonist in an extended conformation, perpendicular to the membrane plane. The binding site highlights an integral role for the extracellular loops, together with the helical core, in ligand recognition by this class of GPCRs and suggests a role for ZM241385 in restricting the movement of a tryptophan residue important in the activation mechanism of the class A receptors.

/pdf/the-2-6-angstrom-crystal-structure-of-a-human-a2a-adenosine-2d7tik1aq3.pdf

The 2.6 Angstrom Crystal Structure of a Human A2A Adenosine Receptor Bound to an Antagonist.

ObjectivesIn view of the critical role of intracellular Ca2+-overload in the genesis of myocyte dysfunction and the ability of reactive oxygen species (ROS) to induce the intracellular Ca2+-overload, this article is concerned with analysis of the existing literature with respect to the role of oxida

Role of oxidative stress in cardiovascular diseases.

A constant supply of oxygen is indispensable for cardiac viability and function. However, the role of oxygen and oxygen-associated processes in the heart is complex, and they and can be either beneficial or contribute to cardiac dysfunction and death. As oxygen is a major determinant of cardiac gene expression, and a critical participant in the formation of ROS and numerous other cellular processes, consideration of its role in the heart is essential in understanding the pathogenesis of cardiac dysfunction.

/pdf/oxygen-oxidative-stress-hypoxia-and-heart-failure-5b9jc7hqrl.pdf

Oxygen, oxidative stress, hypoxia, and heart failure

https://www.thelancet.com/pdfs/journals/lancet/PIIS0140-6736(07)60601-1.pdf

Autosomal dominant polycystic kidney disease

Adenosine is a ubiquitous endogenous autacoid whose effects are triggered through the enrollment of four G protein-coupled receptors: A1, A2A, A2B, and A3. Due to the rapid generation of adenosine ...

Pharmacology of Adenosine Receptors: The State of the Art

Adenosine as a Multi-Signalling Guardian Angel in Human Diseases: When, Where and How Does it Exert its Protective Effects?

The A1, A2A, A2B and A3 G-protein-coupled cell surface adenosine receptors (ARs) are found to be upregulated in various tumor cells. Activation of the receptors by specific ligands, agonists or antagonists, modulates tumor growth via a range of signaling pathways. The A1AR was found to play a role in preventing the development of glioblastomas. This antitumor effect of the A1AR is mediated via tumor-associated microglial cells. Activation of the A2AAR results in inhibition of the immune response to tumors via suppression of T regulatory cell function and inhibition of natural killer cell cytotoxicity and tumor-specific CD4+/CD8+ activity. Therefore, it is suggested that pharmacological inhibition of A2AAR activation by specific antagonists may enhance immunotherapeutics in cancer therapy. Activation of the A2BAR plays a role in the development of tumors via upregulation of the expression levels of angiogenic factors in microvascular endothelial cells. In contrast, it was evident that activation of A2BAR results in inhibition of ERK1/2 phosphorylation and MAP kinase activity, which are involved in tumor cell growth signals. Finally, A3AR was found to be highly expressed in tumor cells and tissues while low expression levels were noted in normal cells or adjacent tissue. Receptor expression in the tumor tissues was directly correlated to disease severity. The high receptor expression in the tumors was attributed to overexpression of NF-κB, known to act as an A3AR transcription factor. Interestingly, high A3AR expression levels were found in peripheral blood mononuclear cells (PBMCs) derived from tumor-bearing animals and cancer patients, reflecting receptor status in the tumors. A3AR agonists were found to induce tumor growth inhibition, both in vitro and in vivo, via modulation of the Wnt and the NF-κB signaling pathways. Taken together, A3ARs that are abundantly expressed in tumor cells may be targeted by specific A3AR agonists, leading to tumor growth inhibition. The unique characteristics of these A3AR agonists make them attractive as drug candidates.

/pdf/adenosine-receptors-and-cancer-ev2ku9a9yf.pdf

Adenosine receptors and cancer.

3.1.3. Styrylxanthines 248 3.1.4. 9H-Purine derivatives 248 3.2. Pharmacology 248 3.3. Clinical Development and Patents 249 4. A2B Adenosine Receptor Antagonists 250 4.1. Chemistry 250 4.1.1. Xanthines 250 4.1.2. Pyrrolopyrimidines 251 4.2. Pharmacology 252 4.3. Clinical Development and Patents 253 5. A3 Adenosine Receptor Antagonists 253 5.1. Chemistry 254 5.1.1. Xanthines 254 5.1.2. 1,4-Dihydropyridine and Pyridines 255 5.1.3. Pyrazolo-triazolo-pyrimidines 255 5.1.4. Isoquinoline and Quinazoline Urea Analogues as Antagonists for the Human Adenosine A3 Receptor 256

Stefania Gessi

Papers

Pharmacology of Adenosine Receptors: The State of the Art

Adenosine as a Multi-Signalling Guardian Angel in Human Diseases: When, Where and How Does it Exert its Protective Effects?

Adenosine receptors and cancer.

Adenosine receptor antagonists: translating medicinal chemistry and pharmacology into clinical utility.

The A3 adenosine receptor: an enigmatic player in cell biology.