Probing biased/partial agonism at the G protein-coupled A(2B) adenosine receptor

Targeting G protein-coupled receptors for heart failure treatment

Heart failure remains a leading cause of morbidity and mortality worldwide. Current treatment for patients with heart failure include drugs targeting G protein-coupled receptors such as β-adrenoceptor antagonists (β-blockers) and angiotensin II type 1 receptor antagonists (or angiotensin II receptor blockers). However, many patients progress to advanced heart failure with persistent symptoms, despite treatment with available therapeutics that have been shown to reduce mortality and mortality. GPCR targets currently being explored for the development of novel heart failure therapeutics include adenosine receptor, formyl peptide receptor, relaxin/insulin-like family peptide receptor, vasopressin receptor, endothelin receptor and the glucagon-like peptide 1 receptor. Many GPCR drug candidates are limited by insufficient efficacy and/or dose-limiting unwanted effects. Understanding the current challenges hindering successful clinical translation and the potential to overcome existing limitations will facilitate the future development of novel heart failure therapeutics. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

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.

Single cell G-protein coupled receptor profiling of activated kidney fibroblasts expressing transcription factor 21

BACKGROUND AND PURPOSE

Activated fibroblasts deposit fibrotic matrix in chronic kidney disease (CKD) and G-protein coupled receptors (GPCRs) are the most druggable therapeutic targets. Here, we set out to establish a transcriptional profile that identifies activated kidney fibroblasts and the GPCRs that they express.

EXPERIMENTAL APPROACH

RNA sequencing and single cell qRT-PCR were performed on mouse kidneys after unilateral ureteral obstruction (UUO). Candidate expression was evaluated in mice with UUO or diabetes or injected with adriamycin or folic acid. Intervention studies were conducted in mice with diabetes or UUO. Correlative histology was performed in human kidney tissue.

KEY RESULTS

Transcription factor 21 (Tcf21)+ cells that expressed 2 or 3 of Postn, Acta2 and Pdgfra were highly enriched for fibrogenic genes and were defined as activated kidney fibroblasts. Tcf21+ α-smooth muscle actin (α-SMA)+ interstitial cells accumulated in kidneys of mice with UUO or diabetes or injected with adriamycin or folic acid, whereas renin-angiotensin system blockade attenuated increases in Tcf21 in diabetic mice. Fifty-six GPCRs were up-regulated in single Tcf21+ kidney fibroblasts, the most up-regulated being Adgra2 and S1pr3. Adenosine receptors, Adora2a/2b, were up-regulated in Tcf21+ fibroblasts and the adenosine receptor antagonist, caffeine decreased Tcf21 upregulation and kidney fibrosis in UUO mice. TCF21, ADGRA2, S1PR3 and ADORA2A/2B were each detectable in α-SMA+ interstitial cells in human kidney samples.

CONCLUSION AND IMPLICATIONS

Tcf21 is a marker of kidney fibroblasts that are enriched for fibrogenic genes in CKD. Further analysis of the GPCRs expressed by these cells may identify new targets for treating CKD.

LINKED ARTICLES

This article is part of a themed issue on Translational Advances in Fibrosis as a Therapeutic Target. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.22/issuetoc.

New paradigms in purinergic receptor ligand discovery

The discovery and clinical implementation of modulators of adenosine, P2Y and P2X receptors (comprising nineteen subtypes) have progressed dramatically in ∼50 years since Burnstock's definition of purinergic signaling. Although most clinical trials of selective ligands (agonists and antagonists) of certain purinergic receptors failed, there is a renewed impetus to redirect efforts to new disease conditions and the discovery of more selective or targeted compounds with potentially reduced side effects, such as biased GPCR agonists. The elucidation of new receptor and enzyme structures is steering rational design of potent and selective agonists, antagonists, allosteric modulators and inhibitors. A2A adenosine receptor (AR) antagonists are being applied to neurodegenerative conditions and cancer immunotherapy. A3AR agonists have potential for treating chronic inflammation (e.g. psoriasis), stroke and pain, as well as cancer. P2YR modulators are being considered for treating inflammation, metabolic disorders, acute kidney injury, cancer, pain and other conditions, often with an immune mechanism. ADP-activated P2Y12R antagonists are widely used as antithrombotic drugs, while their repurposing toward neuroinflammation is considered. P2X3 antagonists have been in clinical trials for chronic cough. P2X7 antagonists have been in clinical trials for inflammatory diseases and depression (compounds that penetrate the blood-brain barrier). Thus, purinergic signaling is now recognized as an immense regulatory system in the body for rebalancing tissues and organs under stress, which can be adjusted by drug intervention for therapeutic purposes. The lack of success of many previous clinical trials can be overcome given more advanced pharmacokinetic and pharmacodynamic approaches, including structure-based drug design, prodrugs and biased signaling. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

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.

International Union of Basic and Clinical Pharmacology. CXII: Adenosine Receptors: A Further Update

Our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors (2011) contained a number of emerging developments with respect to this G protein-coupled receptor subfamily, including protein structure, protein oligomerization, protein diversity, and allosteric modulation by small molecules. Since then, a wealth of new data and results has been added, allowing us to explore novel concepts such as target binding kinetics and biased signaling of adenosine receptors, to examine a multitude of receptor structures and novel ligands, to gauge new pharmacology, and to evaluate clinical trials with adenosine receptor ligands. This review should therefore be considered a further update of our previous reports from 2001 and 2011. SIGNIFICANCE STATEMENT: Adenosine receptors (ARs) are of continuing interest for future treatment of chronic and acute disease conditions, including inflammatory diseases, neurodegenerative afflictions, and cancer. The design of AR agonists ("biased" or not) and antagonists is largely structure based now, thanks to the tremendous progress in AR structural biology. The A2A- and A2BAR appear to modulate the immune response in tumor biology. Many clinical trials for this indication are ongoing, whereas an A2AAR antagonist (istradefylline) has been approved as an anti-Parkinson agent.

Biased and allosteric modulation of bone cell-expressing G protein-coupled receptors as a novel approach to osteoporosis therapy

On the cellular level, osteoporosis (OP) is a result of imbalanced bone remodeling, in which osteoclastic bone resorption outcompetes osteoblastic bone formation. Currently available OP medications include both antiresorptive and bone-forming drugs. However, their long-term use in OP patients, mainly in postmenopausal women, is accompanied by severe side effects. Notably, the fundamental coupling between bone resorption and formation processes underlies the existence of an undesirable secondary outcome that bone anabolic or anti-resorptive drugs also reduce bone formation. This drawback requires the development of anti-OP drugs capable of selectively stimulating osteoblastogenesis and concomitantly reducing osteoclastogenesis. We propose that the application of small synthetic biased and allosteric modulators of bone cell receptors, which belong to the G-protein coupled receptors (GPCR) family, could be the key to resolving the undesired anti-OP drug selectivity. This approach is based on the capacity of these GPCR modulators, unlike the natural ligands, to trigger signaling pathways that promote beneficial effects on bone remodeling while blocking potentially deleterious effects. Under the settings of OP, an optimal anti-OP drug should provide fine-tuned regulation of downstream effects, for example, intermittent cyclic AMP (cAMP) elevation, preservation of Ca²⁺ balance, stimulation of osteoprotegerin (OPG) and estrogen production, suppression of sclerostin secretion, and/or preserved/enhanced canonical β-catenin/Wnt signaling pathway. As such, selective modulation of GPCRs involved in bone remodeling presents a promising approach in OP treatment. This review focuses on the evidence for the validity of our hypothesis.

Allosterism vs. Orthosterism: Recent Findings and Future Perspectives on A2B AR Physio-Pathological Implications

The development of GPCR (G-coupled protein receptor) allosteric modulators has attracted increasing interest in the last decades. The use of allosteric modulators in therapy offers several advantages with respect to orthosteric ones, as they can fine-tune the tissue responses to the endogenous agonist. Since the discovery of the first A₁ adenosine receptor (AR) allosteric modulator in 1990, several efforts have been made to develop more potent molecules as well as allosteric modulators for all adenosine receptor subtypes. There are four subtypes of AR: A₁, A_2A, A_2B, and A₃. Positive allosteric modulators of the A₁ AR have been proposed for the cure of pain. A₃ positive allosteric modulators are thought to be beneficial during inflammatory processes. More recently, A_2A and A_2B AR allosteric modulators have also been disclosed. The A_2B AR displays the lowest affinity for its endogenous ligand adenosine and is mainly activated as a consequence of tissue damage. The A_2B AR activation has been found to play a crucial role in chronic obstructive pulmonary disease, in the protection of the heart from ischemic injury, and in the process of bone formation. In this context, allosteric modulators of the A_2B AR may represent pharmacological tools useful to develop new therapeutic agents. Herein, we provide an up-to-date highlight of the recent findings and future perspectives in the field of orthosteric and allosteric A_2B AR ligands. Furthermore, we compare the use of orthosteric ligands with positive and negative allosteric modulators for the management of different pathological conditions.

Molecular and Pharmacological Characterization of the Interaction between Human Geranylgeranyltransferase Type I and Ras-Related Protein Rap1B

Geranylgeranyltransferase type-I (GGTase-I) represents an important drug target since it contributes to the function of many proteins that are involved in tumor development and metastasis. This led to the development of GGTase-I inhibitors as anti-cancer drugs blocking the protein function and membrane association of e.g., Rap subfamilies that are involved in cell differentiation and cell growth. In the present study, we developed a new NanoBiT assay to monitor the interaction of human GGTase-I and its substrate Rap1B. Different Rap1B prenylation-deficient mutants (C181G, C181S, and ΔCQLL) were designed and investigated for their interaction with GGTase-I. While the Rap1B mutants C181G and C181S still exhibited interaction with human GGTase-I, mutant ΔCQLL, lacking the entire CAAX motif (defined by a cysteine residue, two aliphatic residues, and the C-terminal residue), showed reduced interaction. Moreover, a specific, peptidomimetic and competitive CAAX inhibitor was able to block the interaction of Rap1B with GGTase-I. Furthermore, activation of both Gα_s-coupled human adenosine receptors, A_2A (A_2AAR) and A_2B (A_2BAR), increased the interaction between GGTase-I and Rap1B, probably representing a way to modulate prenylation and function of Rap1B. Thus, A_2AAR and A_2BAR antagonists might be promising candidates for therapeutic intervention for different types of cancer that overexpress Rap1B. Finally, the NanoBiT assay provides a tool to investigate the pharmacology of GGTase-I inhibitors.

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.

Role of Adenosine and Purinergic Receptors in Myocardial Infarction: Focus on Different Signal Transduction Pathways

Myocardial infarction (MI) is a dramatic event often caused by atherosclerotic plaque erosion or rupture and subsequent thrombotic occlusion of a coronary vessel. The low supply of oxygen and nutrients in the infarcted area may result in cardiomyocytes necrosis, replacement of intact myocardium with non-contractile fibrous tissue and left ventricular (LV) function impairment if blood flow is not quickly restored. In this review, we summarized the possible correlation between adenosine system, purinergic system and Wnt/β-catenin pathway and their role in the pathogenesis of cardiac damage following MI. In this context, several pathways are involved and, in particular, the adenosine receptors system shows different interactions between its members and purinergic receptors: their modulation might be effective not only for a normal functional recovery but also for the treatment of heart diseases, thus avoiding fibrosis, reducing infarcted area and limiting scaring. Similarly, it has been shown that Wnt/β catenin pathway is activated following myocardial injury and its unbalanced activation might promote cardiac fibrosis and, consequently, LV systolic function impairment. In this regard, the therapeutic benefits of Wnt inhibitors use were highlighted, thus demonstrating that Wnt/β-catenin pathway might be considered as a therapeutic target to prevent adverse LV remodeling and heart failure following MI.

Design and in vivo activity of A3 adenosine receptor agonist prodrugs

Prodrugs (MRS7422, MRS7476) of highly selective A3 adenosine receptor (AR) agonists Cl-IB-MECA and MRS5698, respectively, were synthesized by succinylation of the 2' and 3' hydroxyl groups, and the parent, active drug was shown to be readily liberated upon incubation with liver esterases. The prodrug MRS7476 had greatly increased aqueous solubility compared with parent MRS5698 and was fully efficacious and with a longer duration than MRS7422 in reversing mouse neuropathic pain (chronic constriction injury model, 3 μmol/kg, p.o.), a known A3AR effect. MRS7476 (5 mg/kg, p.o., twice daily) was found to protect against non-alcoholic steatohepatitis (NASH) in the STAM mouse model, indicated by the NAFLD activity score. Hepatocyte ballooning, IL-10 production, and liver histology were significantly normalized in the MRS7476-treated mice, but not liver fibrosis (no change in ACTA2 levels) or inflammation. Hepatic expression of ADORA3 in human NAFLD patients was 1.9-fold lower compared to normal controls. Adora3 expression determined by qPCR in primary mouse liver was associated with the stellate cells, and its mouse full body A3AR knockout worsened liver markers of inflammation and steatosis. Thus, we have introduced a reversible prodrug strategy that enables water solubility and in vivo activity of masked A3AR agonists in models of two disease conditions.

Focusing on Adenosine Receptors as a Potential Targeted Therapy in Human Diseases

Adenosine is involved in a range of physiological and pathological effects through membrane-bound receptors linked to G proteins. There are four subtypes of adenosine receptors, described as A1AR, A2AAR, A2BAR, and A3AR, which are the center of cAMP signal pathway-based drug development. Several types of agonists, partial agonists or antagonists, and allosteric substances have been synthesized from these receptors as new therapeutic drug candidates. Research efforts surrounding A1AR and A2AAR are perhaps the most enticing because of their concentration and affinity; however, as a consequence of distressing conditions, both A2BAR and A3AR levels might accumulate. This review focuses on the biological features of each adenosine receptor as the basis of ligand production and describes clinical studies of adenosine receptor-associated pharmaceuticals in human diseases.

Involvement of A2B Receptor in DNA Damage Response and Radiosensitizing Effect of A2B Receptor Antagonists on Mouse B16 Melanoma

It is therapeutically important to elucidate the factors involved in the radiation resistance of tumors. We previously showed that ATP is released from mouse melanoma B16 cells in response to γ-irradiation, but the role of adenosine, a metabolite of ATP, is still unclear. Here, we show that the adenosine A2B receptor is involved in DNA damage repair and radioresistance in mouse melanoma B16 cells. The DNA damage response after γ-irradiation was attenuated by pretreatment with A2B receptor antagonists, such as PSB603, while it was enhanced by pretreatment with A2B receptor agonists, such as BAY60-6583. γ-Irradiation decreased the cell survival rate, and pretreatment with PSB603 further reduced the survival rate. On the other hand, pretreatment with BAY60-6583 increased the cell survival rate after irradiation. The DNA damage response and the cell survival rate after γ-irradiation were both decreased in A2B-knockdown cells. In vivo experiments in mice confirmed that tumor growth was suppressed and delayed in the irradiated group pretreated with PSB603, compared with the irradiation-alone group. Our results indicate that adenosine A2B receptor contributes to radioresistance, and could be a new target for the development of agents to increase the efficacy of radiotherapy.

A2B Adenosine Receptor and Cancer

There are four subtypes of adenosine receptors (ARs), named A₁, A_2A, A_2B and A₃, all of which are G protein-coupled receptors (GPCRs). Locally produced adenosine is a suppressant in anti-tumor immune surveillance. The A_2BAR, coupled to both Gαs and Gαi G proteins, is one of the several GPCRs that are expressed in a significantly higher level in certain cancer tissues, in comparison to adjacent normal tissues. There is growing evidence that the A_2BAR plays an important role in tumor cell proliferation, angiogenesis, metastasis, and immune suppression. Thus, A_2BAR antagonists are novel, potentially attractive anticancer agents. Several antagonists targeting A_2BAR are currently in clinical trials for various types of cancers. In this review, we first describe the signaling, agonists, and antagonists of the A_2BAR. We further discuss the role of the A_2BAR in the progression of various cancers, and the rationale of using A_2BAR antagonists in cancer therapy.

Functional characterization of a novel adenosine A2B receptor agonist on short-term plasticity and synaptic inhibition during oxygen and glucose deprivation in the rat CA1 hippocampus

Adenosine is an endogenous neuromodulator exerting its biological functions via four receptor subtypes, A₁, A_2A, A_2B, and A₃. A_2B receptors (A_2BRs) are expressed at hippocampal level where they are known to inhibit paired pulse facilitation (PPF), whose reduction reflects an increase in presynaptic glutamate release. The effect of A_2BRs on PPF is known to be sensitive not only to A_2BR blockade but also to the A₁R antagonist DPCPX, indicating that it involves A₁R activation. In this study we provide the first functional characterization of the newly synthesized non-nucleoside like A_2BR agonist P453, belonging to the amino-3,5-dicyanopyridine series. By extracellular electrophysiological recordings, we demonstrated that P453 mimicked the effect of the prototypical A_2BR agonist BAY60-6583 in decreasing PPF at Schaffer collateral-CA1 synapses in rat acute hippocampal slices. This effect was prevented by two different A_2BR antagonists, PSB603 and MRS1754, and by the A₁R antagonist DPCPX. We also investigated the functional role of A_2BR during a 2 min of oxygen and glucose deprivation (OGD) insult, known to produce a reversible fEPSP inhibition due to adenosine A₁R activation. We found that P453 and BAY60-6583 significantly delayed the onset of fEPSP reduction induced by OGD and the effect was blocked by PSB603. We conclude that P453 is a functional A_2BR agonist whose activation decreases PPF by increasing glutamate release at presynaptic terminals and delays A₁R-mediated fEPSP inhibition during a 2-minute OGD insult.

Historical and Current Adenosine Receptor Agonists in Preclinical and Clinical Development

Adenosine receptors (ARs) function in the body’s response to conditions of pathology and stress associated with a functional imbalance, such as in the supply and demand of energy/oxygen/nutrients. Extracellular adenosine concentrations vary widely to raise or lower the basal activation of four subtypes of ARs. Endogenous adenosine can correct an energy imbalance during hypoxia and other stress, for example, by slowing the heart rate by A₁AR activation or increasing the blood supply to heart muscle by the A_2AAR. Moreover, exogenous AR agonists, antagonists, or allosteric modulators can be applied for therapeutic benefit, and medicinal chemists working toward that goal have reported thousands of such agents. Thus, numerous clinical trials have ensued, using promising agents to modulate adenosinergic signaling, most of which have not succeeded. Currently, short-acting, parenteral agonists, adenosine and Regadenoson, are the only AR agonists approved for human use. However, new concepts and compounds are currently being developed and applied toward preclinical and clinical evaluation, and initial results are encouraging. This review focuses on key compounds as AR agonists and positive allosteric modulators (PAMs) for disease treatment or diagnosis. AR agonists for treating inflammation, pain, cancer, non-alcoholic steatohepatitis, angina, sickle cell disease, ischemic conditions and diabetes have been under development. Multiple clinical trials with two A₃AR agonists are ongoing.

The adenosine A2B G protein-coupled receptor: Recent advances and therapeutic implications

The adenosine A_2B receptor (A_2BAR) is one of four adenosine receptor subtypes belonging to the Class A family of G protein-coupled receptors (GPCRs). Until recently, the A_2BAR remained poorly characterised, in part due to its relatively low affinity for the endogenous agonist adenosine and therefore presumed minor physiological significance. However, the substantial increase in extracellular adenosine concentration, the sensitisation of the receptor and the upregulation of A_2BAR expression under conditions of hypoxia and inflammation, suggest the A_2BAR as an exciting therapeutic target in a variety of pathological disease states. Here we discuss the pharmacology of the A_2BAR and outline its role in pathophysiology including ischaemia-reperfusion injury, fibrosis, inflammation and cancer.

Historical and Current Adenosine Receptor Agonists in Preclinical and Clinical Development

Adenosine receptors (ARs) function in the body's response to conditions of pathology and stress associated with a functional imbalance, such as in the supply and demand of energy/oxygen/nutrients. Extracellular adenosine concentrations vary widely to raise or lower the basal activation of four subtypes of ARs. Endogenous adenosine can correct an energy imbalance during hypoxia and other stress, for example, by slowing the heart rate by A₁AR activation or increasing the blood supply to heart muscle by the A_2AAR. Moreover, exogenous AR agonists, antagonists, or allosteric modulators can be applied for therapeutic benefit, and medicinal chemists working toward that goal have reported thousands of such agents. Thus, numerous clinical trials have ensued, using promising agents to modulate adenosinergic signaling, most of which have not succeeded. Currently, short-acting, parenteral agonists, adenosine and Regadenoson, are the only AR agonists approved for human use. However, new concepts and compounds are currently being developed and applied toward preclinical and clinical evaluation, and initial results are encouraging. This review focuses on key compounds as AR agonists and positive allosteric modulators (PAMs) for disease treatment or diagnosis. AR agonists for treating inflammation, pain, cancer, non-alcoholic steatohepatitis, angina, sickle cell disease, ischemic conditions and diabetes have been under development. Multiple clinical trials with two A₃AR agonists are ongoing.

Probing structure-activity relationship in β-arrestin2 recruitment of diversely substituted adenosine derivatives

In the adenosine receptor (AR) subfamily of G protein-coupled receptors (GPCRs), biased agonism has been described for the human A₁AR, A_2BAR and A₃AR. While diverse A₃AR agonists have been evaluated for receptor binding and G_i-mediated cAMP signalling, the β-arrestin2 (βarr2) pathway has been left largely unexplored. We screened nineteen diverse adenosine derivatives for βarr2 recruitment using a stable hA₃AR-NanoBit^®-βarr2 HEK293T cell line. Their activity profiles were compared with a cAMP accumulation assay in stable hA₃AR CHO cells. Structural features linked to βarr2 activation were further investigated by the evaluation of an additional ten A₃AR ligands. The A₃AR-selective reference agonist 2-Cl-IB-MECA, which is a full agonist in terms of cAMP inhibition, only showed partial agonist behaviour in βarr2 recruitment. Highly A₃AR-selective (N)-methanocarba 5'-uronamide adenosine derivatives displayed higher potency in both cAMP signalling and βarr2 recruitment than reference agonists NECA and 2-Cl-IB-MECA. Their A₃AR-preferred conformation tolerates C2-position substitutions, for increased βarr2 efficacy, better than the flexible scaffolds of ribose derivatives. The different amino functionalities in the adenosine scaffold of these derivatives each seem to be important for signalling as well. In conclusion, we have provided insights into ligand features that can help to guide the future therapeutic development of biased A₃AR ligands with respect to G-protein and βarr2 signalling.

New paradigms in adenosine receptor pharmacology: allostery, oligomerization and biased agonism

Adenosine receptors are a family of GPCRs containing four subtypes (A₁ , A_2A , A_2B and A₃ receptors), all of which bind the ubiquitous nucleoside adenosine. These receptors play an important role in physiology and pathophysiology and therefore represent attractive drug targets for a range of conditions. The theoretical framework surrounding drug action at adenosine receptors now extends beyond the notion of prototypical agonism and antagonism to encompass more complex pharmacological concepts. New paradigms include allostery, in which ligands bind a topographically distinct receptor site from that of the endogenous agonist, homomeric or heteromeric interactions across receptor oligomers and biased agonism, that is, ligand-dependent differential intracellular signalling. This review provides a concise overview of allostery, oligomerization and biased agonism at adenosine receptors and outlines how these paradigms may enhance future drug discovery endeavours focussed on the development of novel therapeutic agents acting at adenosine receptors.

LINKED ARTICLES

This article is part of a themed section on Molecular Pharmacology of GPCRs. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.21/issuetoc.

Tritium-labeled agonists as tools for studying adenosine A2B receptors

A selective agonist radioligand for A_2B adenosine receptors (A_2BARs) is currently not available. Such a tool would be useful for labeling the active conformation of the receptors. Therefore, we prepared BAY 60-6583, a potent and functionally selective A_2BAR (partial) agonist, in a tritium-labeled form. Despite extensive efforts, however, we have not been able to establish a radioligand binding assay using [³H]BAY 60-6583. This is probably due to its high non-specific binding and its moderate affinity, which had previously been overestimated based on functional data. As an alternative, we evaluated the non-selective A_2BAR agonist [³H]NECA for its potential to label A_2BARs. [³H]NECA showed specific, saturable, and reversible binding to membrane preparations of Chinese hamster ovary (CHO) or human embryonic kidney (HEK) cells stably expressing human, rat, or mouse A_2BARs. In competition binding experiments, the AR agonists 2-chloroadenosine (CADO) and NECA displayed significantly higher affinity when tested versus [³H]NECA than versus the A_2B-antagonist radioligand [³H]PSB-603 while structurally diverse AR antagonists showed the opposite effects. Although BAY 60-6583 is an A_2BAR agonist, it displayed higher affinity versus [³H]PSB-603 than versus [³H]NECA. These results indicate that nucleoside and non-nucleoside agonists are binding to very different conformations of the A_2BAR. In conclusion, [³H]NECA is currently the only useful radioligand for determining the affinity of ligands for an active A_2BAR conformation.

Is the adenosine A2B 'biased' receptor a valuable target for the treatment of pulmonary arterial hypertension?

Pulmonary arterial hypertension (PAH) is a maladaptive disorder characterized by increased pulmonary vascular resistance leading to right ventricular failure and death. Adenosine released by injured tissues, such as the lung and heart, influences tissue remodeling through the activation of adenosine receptors. Evidence regarding activation of the low-affinity A_2BAR by adenosine points towards pivotal roles of this receptor in processes associated with both acute and chronic lung diseases. Conflicting results exist concerning the beneficial or detrimental roles of the A_2B 'biased' receptor in right ventricular failure secondary to PAH. In this review, we discuss the pros and cons of manipulating A_2BARs as a putative therapeutic target in PAH.

On the G protein-coupling selectivity of the native A2B adenosine receptor

A2B adenosine receptor (A2BAR) activation induces Gs-dependent cyclic AMP accumulation. However, A2BAR G protein-coupling to other signaling events, e.g. ERK1/2 and calcium, is not well documented. We explored Gi, Gq/11 and Gs coupling in 1321 N1 astrocytoma, HEK293, and T24 bladder cancer cells endogenously expressing human A2BAR, using NECA or nonnucleoside BAY60-6583 as agonist, selective Gi, Gs and Gq/11 blockers, and CRISPR/Cas9-based Gq- and Gs-null HEK293 cells. In HEK293 cells, A2BAR-mediated ERK1/2 activity occurred via both Gi and Gs, but not Gq/11. However, HEK293 cell calcium mobilization was completely blocked by Gq/11 inhibitor UBO-QIC and by Gq/11 knockout. In T24 cells, Gi was solely responsible for A2BAR-mediated ERK1/2 stimulation, and Gs suppressed ERK1/2 activity. A2BAR-mediated intracellular calcium mobilization in T24 cells was mainly via Gi, although Gs may also play a role, but Gq/11 is not involved. In 1321 N1 astrocytoma cells A2BAR activation suppressed rather than stimulated ERK1/2 activity. The ERK1/2 activity decrease was reversed by Gs downregulation using cholera toxin, but potentiated by Gi inhibitor pertussis toxin, and UBO-QIC had no effect. EPACs played an important role in A2BAR-mediated ERK1/2 signaling in all three cells. Thus, A2BAR may: couple to the same downstream pathway via different G proteins in different cell types; activate different downstream events via different G proteins in the same cell type; activate Gi and Gs, which have opposing or synergistic roles in different cell types/signaling pathways. The findings, relevant to drug discovery, address some reported controversial roles of A2BAR and could apply to signaling mechanisms in other GPCRs.

The aminopyridine-3,5-dicarbonitrile core for the design of new non-nucleoside-like agonists of the human adenosine A2B receptor

A new series of amino-3,5-dicyanopyridines (3-28) as analogues of the adenosine hA_2B receptor agonist BAY60-6583 (compound 1) was synthesized. All the compounds that interact with the hA_2B adenosine receptor display EC₅₀ values in the range 9-350 nM behaving as partial agonists, with the only exception being the 2-{[4-(4-acetamidophenyl)-6-amino-3,5-dicyanopyridin-2-yl]thio}acetamide (8) which shows a full agonist profile. Moreover, the 2-[(1H-imidazol-2-yl)methylthio)]-6-amino-4-(4-cyclopropylmethoxy-phenyl)pyridine-3,5-dicarbonitrile (15) turns out to be 3-fold more active than 1 although less selective. This result can be considered a real breakthrough due to the currently limited number of non-adenosine hA_2B AR agonists reported in literature. To simulate the binding mode of nucleoside and non-nucleoside agonists at the hA_2B AR, molecular docking studies were performed at homology models of this AR subtype developed by using two crystal structures of agonist-bound A_2A AR as templates. These investigations allowed us to represent a hypothetical binding mode of hA_2B receptor agonists belonging to the amino-3,5-dicyanopyridine series and to rationalize the observed SAR.

Human Adenosine A2A Receptor: Molecular Mechanism of Ligand Binding and Activation

Adenosine receptors (ARs) comprise the P1 class of purinergic receptors and belong to the largest family of integral membrane proteins in the human genome, the G protein-coupled receptors (GPCRs). ARs are classified into four subtypes, A₁, A_2A, A_2B, and A₃, which are all activated by extracellular adenosine, and play central roles in a broad range of physiological processes, including sleep regulation, angiogenesis and modulation of the immune system. ARs are potential therapeutic targets in a variety of pathophysiological conditions, including sleep disorders, cancer, and dementia, which has made them important targets for structural biology. Over a decade of research and innovation has culminated with the publication of more than 30 crystal structures of the human adenosine A_2A receptor (A_2AR), making it one of the best structurally characterized GPCRs at the atomic level. In this review we analyze the structural data reported for A_2AR that described for the first time the binding of mode of antagonists, including newly developed drug candidates, synthetic and endogenous agonists, sodium ions and an engineered G protein. These structures have revealed the key conformational changes induced upon agonist and G protein binding that are central to signal transduction by A_2AR, and have highlighted both similarities and differences in the activation mechanism of this receptor compared to other class A GPCRs. Finally, comparison of A_2AR with the recently solved structures of A₁R has provided the first structural insight into the molecular determinants of ligand binding specificity in different AR subtypes.

Distinct Signaling Patterns of Allosteric Antagonism at the P2Y1 Receptor

Traditionally, G protein-coupled receptor antagonists are classified as competitive or noncompetitive and surmountable or insurmountable based on functional antagonism. P2Y₁ receptor (P2Y₁R) structures showed two antagonists binding to two spatially distinct sites: nucleotide MRS2500 (orthosteric, contacting the helical bundle) and urea BPTU (allosteric, on the external receptor surface). However, the nature of their P2Y₁R antagonism has not been characterized. Here we characterized BPTU antagonism at various signaling pathways activated by structurally diverse agonists. BPTU rightward shifted the concentration-response curves of both 2-methylthioadenosine 5'-diphosphate trisodium salt and MRS2365 (5'-diphosphates) in some signaling events, such as extracellular signal-regulated kinase 1/2 and label free, in a parallel manner without affecting the maximum agonist effect (E_max) but antagonized insurmountably (suppressed agonist E_max) in signaling events such as guanosine 5'-3-O-(thio)triphosphate binding and β-arrestin2 recruitment. However, with dinucleotide Ap4A as an agonist, BPTU suppressed the E_max insurmountably in all signaling pathways. By comparison, MRS2500 behaved as surmountable antagonist rightward-shifting concentration-response curves of all three agonists in a parallel manner for all signaling pathways measured. Thus, we demonstrated a previously undocumented phenomenon that P2Y₁R antagonism patterns could vary in different signaling pathways, which could be related to conformational selection, signaling amplification, and probe dependence. This phenomenon may apply generally to other receptors considering that antagonism by a specific ligand is often not compared at multiple signaling pathways. Thus, antagonism can be surmountable or insurmountable depending on the signaling pathways measured and the agonists used, which should be of broad relevance to drug discovery and disease treatment.

Ocular Purine Receptors as Drug Targets in the Eye

Agonists and antagonists of various subtypes of G protein coupled adenosine receptors (ARs), P2Y receptors (P2YRs), and ATP-gated P2X receptor ion channels (P2XRs) are under consideration as agents for the treatment of ocular diseases, including glaucoma and dry eye. Numerous nucleoside and nonnucleoside modulators of the receptors are available as research tools and potential therapeutic molecules. Three of the 4 subtypes of ARs have been exploited with clinical candidate molecules for treatment of the eye: A1, A2A, and A3. An A1AR agonist is in clinical trials for glaucoma, A2AAR reduces neuroinflammation, A3AR protects retinal ganglion cells from apoptosis, and both A3AR agonists and antagonists had been reported to lower intraocular pressure (IOP). Extracellular concentrations of endogenous nucleotides, including dinucleoside polyphosphates, are increased in pathological states, activating P2Y and P2XRs throughout the eye. P2YR agonists, including P2Y2 and P2Y6, lower IOP. Antagonists of the P2X7R prevent the ATP-induced neuronal apoptosis in the retina. Thus, modulators of the purinome in the eye might be a source of new therapies for ocular diseases.

The hybrid molecule, VCP746, is a potent adenosine A2B receptor agonist that stimulates anti-fibrotic signalling

We have recently described the rationally-designed adenosine receptor agonist, 4-(5-amino-4-benzoyl-3-(3-(trifluoromethyl)phenyl)thiophen-2-yl)-N-(6-(9-((2R,3R,4S,5R)-3,4-dihydroxy-5-(hydroxylmethyl)tetrahydro-furan-2-yl)-9H-purin-6-ylamino)hexyl)benzamide (VCP746), a hybrid molecule consisting of an adenosine moiety linked to an adenosine A1 receptor (A1AR) allosteric modulator moiety. At the A1AR, VCP746 mediated cardioprotection in the absence of haemodynamic side effects such as bradycardia. The current study has now identified VCP746 as an important pharmacological tool for the adenosine A2B receptor (A2BAR). The binding and function of VCP746 at the A2BAR was rigorously characterised in a heterologous expression system, in addition to examination of its anti-fibrotic signalling in cardiac- and renal-derived cells. In FlpInCHO cells stably expressing the human A2BAR, VCP746 was a high affinity, high potency A2BAR agonist that stimulated Gs- and Gq-mediated signal transduction, with an apparent lack of system bias relative to prototypical A2BAR agonists. The distinct agonist profile may result from an atypical binding mode of VCP746 at the A2BAR, which was consistent with a bivalent mechanism of receptor interaction. In isolated neonatal rat cardiac fibroblasts (NCF), VCP746 stimulated potent inhibition of both TGF-β1- and angiotensin II-mediated collagen synthesis. Similar attenuation of TGF-β1-mediated collagen synthesis was observed in renal mesangial cells (RMC). The anti-fibrotic signalling mediated by VCP746 in NCF and RMC was selectively reversed in the presence of an A2BAR antagonist. Thus, we believe, VCP746 represents an important tool to further investigate the role of the A2BAR in cardiac (patho)physiology.

On the selectivity of the Gαq inhibitor UBO-QIC: A comparison with the Gαi inhibitor pertussis toxin

Gαq inhibitor UBO-QIC (FR900359) is becoming an important pharmacological tool, but its selectivity against other G proteins and their subunits, especially βγ, has not been well characterized. We examined UBO-QIC's effect on diverse signaling pathways mediated via various G protein-coupled receptors (GPCRs) and G protein subunits by comparison with known Gαi inhibitor pertussis toxin. As expected, UBO-QIC inhibited Gαq signaling in all assay systems examined. However, other non-Gαq-events, e.g. Gβγ-mediated intracellular calcium release and inositol phosphate production, following activation of Gi-coupled A1 adenosine and M2 muscarinic acetylcholine receptors, were also blocked by low concentrations of UBO-QIC, indicating that its effect is not limited to Gαq. Thus, UBO-QIC also inhibits Gβγ-mediated signaling similarly to pertussis toxin, although UBO-QIC does not affect Gαi-mediated inhibition or Gαs-mediated stimulation of adenylyl cyclase activity. However, the blockade by UBO-QIC of GPCR signaling, such as carbachol- or adenosine-mediated calcium or inositol phosphate increases, does not always indicate inhibition of Gαq-mediated events, as the βγ subunits released from Gi proteins following the activation of Gi-coupled receptors, e.g. M2 and A1Rs, may produce similar signaling events. Furthermore, UBO-QIC completely inhibited Akt signaling, but only partially blocked ERK1/2 activity stimulated by the Gq-coupled P2Y1R. Thus, we have revealed new aspects of the pharmacological interactions of UBO-QIC.

New paradigms in GPCR drug discovery

G protein-coupled receptors (GPCRs) remain a major domain of pharmaceutical discovery. The identification of GPCR lead compounds and their optimization are now structure-based, thanks to advances in X-ray crystallography, molecular modeling, protein engineering and biophysical techniques. In silico screening provides useful hit molecules. New pharmacological approaches to tuning the pleotropic action of GPCRs include: allosteric modulators, biased ligands, GPCR heterodimer-targeted compounds, manipulation of polypharmacology, receptor antibodies and tailoring of drug molecules to fit GPCR pharmacogenomics. Measurements of kinetics and drug efficacy are factors influencing clinical success. With the exception of inhibitors of GPCR kinases, targeting of intracellular GPCR signaling or receptor cycling for therapeutic purposes remains a futuristic concept. New assay approaches are more efficient and multidimensional: cell-based, label-free, fluorescence-based assays, and biosensors. Tailoring GPCR drugs to a patient's genetic background is now being considered. Chemoinformatic tools can predict ADME-tox properties. New imaging technology visualizes drug action in vivo. Thus, there is reason to be optimistic that new technology for GPCR ligand discovery will help reverse the current narrowing of the pharmaceutical pipeline.

Adenosine A2b receptors control A1 receptor-mediated inhibition of synaptic transmission in the mouse hippocampus

Adenosine is a neuromodulator mostly acting through A1 (inhibitory) and A2A (excitatory) receptors in the brain. A2B receptors (A(2B)R) are G(s/q)--protein-coupled receptors with low expression in the brain. As A(2B)R function is largely unknown, we have now explored their role in the mouse hippocampus. We performed electrophysiological extracellular recordings in mouse hippocampal slices, and immunological analysis of nerve terminals and glutamate release in hippocampal slices and synaptosomes. Additionally, A(2B)R-knockout (A(2B)R-KO) and C57/BL6 mice were submitted to a behavioural test battery (open field, elevated plus-maze, Y-maze). The A(2B)R agonist BAY60-6583 (300 nM) decreased the paired-pulse stimulation ratio, an effect prevented by the A(2B)R antagonist MRS 1754 (200 nM) and abrogated in A(2B)R-KO mice. Accordingly, A(2B)R immunoreactivity was present in 73 ± 5% of glutamatergic nerve terminals, i.e. those immunopositive for vesicular glutamate transporters. Furthermore, BAY 60-6583 attenuated the A(1)R control of synaptic transmission, both the A(1)R inhibition caused by 2-chloroadenosine (0.1-1 μM) and the disinhibition caused by the A(1)R antagonist DPCPX (100 nM), both effects prevented by MRS 1754 and abrogated in A(2B)R-KO mice. BAY 60-6583 decreased glutamate release in slices and also attenuated the A(1)R inhibition (CPA 100 nM). A(2B)R-KO mice displayed a modified exploratory behaviour with an increased time in the central areas of the open field, elevated plus-maze and the Y-maze and no alteration of locomotion, anxiety or working memory. We conclude that A(2B)R are present in hippocampal glutamatergic terminals where they counteract the predominant A(1)R-mediated inhibition of synaptic transmission, impacting on exploratory behaviour.