Functional coupling of adenosine A2a receptor to inhibition of the mitogen-activated protein kinase cascade in Chinese hamster ovary cells

Adenosine receptor signalling in Alzheimer’s disease

Alzheimer’s disease (AD) is the most common dementia in the elderly and its increasing prevalence presents treatment challenges. Despite a better understanding of the disease, the current mainstay of treatment cannot modify pathogenesis or effectively address the associated cognitive and memory deficits. Emerging evidence suggests adenosine G protein-coupled receptors (GPCRs) are promising therapeutic targets for Alzheimer’s disease. The adenosine A₁ and A_2A receptors are expressed in the human brain and have a proposed involvement in the pathogenesis of dementia. Targeting these receptors preclinically can mitigate pathogenic β-amyloid and tau neurotoxicity whilst improving cognition and memory. In this review, we provide an accessible summary of the literature on Alzheimer’s disease and the therapeutic potential of A₁ and A_2A receptors. Although there are no available medicines targeting these receptors approved for treating dementia, we provide insights into some novel strategies, including allosterism and the targeting of oligomers, which may increase drug discovery success and enhance the therapeutic response.

Role of Cardiac A2A Receptors Under Normal and Pathophysiological Conditions

This review presents an overview of cardiac A2A-adenosine receptors The localization of A2A-AR in the various cell types that encompass the heart and the role they play in force regulation in various mammalian species are depicted. The putative signal transduction systems of A2A-AR in cells in the living heart, as well as the known interactions of A2A-AR with membrane-bound receptors, will be addressed. The possible role that the receptors play in some relevant cardiac pathologies, such as persistent or transient ischemia, hypoxia, sepsis, hypertension, cardiac hypertrophy, and arrhythmias, will be reviewed. Moreover, the cardiac utility of A2A-AR as therapeutic targets for agonistic and antagonistic drugs will be discussed. Gaps in our knowledge about the cardiac function of A2A-AR and future research needs will be identified and formulated.

G protein-coupled receptors expressed and studied in yeast. The adenosine receptor as a prime example

G protein-coupled receptors (GPCRs) are the largest class of membrane proteins with around 800 members in the human genome/proteome. Extracellular signals such as hormones and neurotransmitters regulate various biological processes via GPCRs, with GPCRs being the bodily target of 30-40% of current drugs on the market. Complete identification and understanding of GPCR functionality will provide opportunities for novel drug discovery. Yeast expresses three different endogenous GPCRs regulating pheromone and sugar sensing, with the pheromone pathway offering perspectives for the characterization of heterologous GPCR signaling. Moreover, yeast offers a ''null" background for studies on mammalian GPCRs, including GPCR activation and signaling, ligand identification, and characterization of disease-related mutations. This review focuses on modifications of the yeast pheromone signaling pathway for functional GPCR studies, and on opportunities and usage of the yeast system as a platform for human GPCR studies. Finally, this review discusses in some further detail studies of adenosine receptors heterologously expressed in yeast, and what Geoff Burnstock thought of this approach.

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

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

Activation of adenosine A(2A) receptor reduces osteoclast formation via PKA- and ERK1/2-mediated suppression of NFκB nuclear translocation

BACKGROUND AND PURPOSE

We previously reported that adenosine, acting at adenosine A(2A) receptors (A(2A)R), inhibits osteoclast (OC) differentiation in vitro (A(2A)R activation OC formation reduces by half) and in vivo. For a better understanding how adenosine A(2A)R stimulation regulates OC differentiation, we dissected the signalling pathways involved in A(2A)R signalling.

EXPERIMENTAL APPROACH

OC differentiation was studied as TRAP+ multinucleated cells following M-CSF/RANKL stimulation of either primary murine bone marrow cells or the murine macrophage line, RAW264.7, in presence/absence of the A(2A)R agonist CGS21680, the A(2A)R antagonist ZM241385, PKA activators (8-Cl-cAMP 100 nM, 6-Bnz-cAMP) and the PKA inhibitor (PKI). cAMP was quantitated by EIA and PKA activity assays were carried out. Signalling events were studied in PKA knockdown (lentiviral shRNA for PKA) RAW264.7 cells (scrambled shRNA as control). OC marker expression was studied by RT-PCR.

KEY RESULTS

A(2A)R stimulation increased cAMP and PKA activity which and were reversed by addition of ZM241385. The direct PKA stimuli 8-Cl-cAMP and 6-Bnz-cAMP inhibited OC maturation whereas PKI increased OC differentiation. A(2A)R stimulation inhibited p50/p105 NFκB nuclear translocation in control but not in PKA KO cells. A(2A)R stimulation activated ERK1/2 by a PKA-dependent mechanism, an effect reversed by ZM241385, but not p38 and JNK activation. A(2A)R stimulation inhibited OC expression of differentiation markers by a PKA-mechanism.

CONCLUSIONS AND IMPLICATIONS

A(2A)R activation inhibits OC differentiation and regulates bone turnover via PKA-dependent inhibition of NFκB nuclear translocation, suggesting a mechanism by which adenosine could target bone destruction in inflammatory diseases like rheumatoid arthritis.

Adenosine A(2A) receptor ligation inhibits osteoclast formation

Adenosine is generated in increased concentrations at sites of injury/hypoxia and mediates a variety of physiological and pharmacological effects via G protein-coupled receptors (A(1), A(2A), A(2B), and A(3)). Because all adenosine receptors are expressed on osteoclasts, we determined the role of A(2A) receptor in the regulation of osteoclast differentiation. Differentiation and bone resorption were studied as the macrophage colony-stimulating factor-1-receptor activator of NF-κB ligand formation of multinucleated tartrate-resistant acid phosphatase (TRAP)-positive cells from primary murine bone marrow-derived precursors. A(2A) receptor and osteoclast marker expression levels were studied by RT-PCR. Cytokine secretion was assayed by enzyme-linked immunosorbent assay. In vivo examination of A(2A) knockout (KO)/control bones was determined by TRAP staining, micro-computed tomography, and electron microscopy. The A(2A) receptor agonist, CGS21680, inhibited osteoclast differentiation and function (half maximal inhibitory concentration, 50 nmol/L), increased the percentage of immature osteoclast precursors, and decreased IL-1β and tumor necrosis factor-α secretion, an effect that was reversed by the A(2A) antagonist, ZM241385. Cathepsin K and osteopontin mRNA expression increased in control and ZM241385-pretreated osteoclasts, and this was blocked by CGS21680. Micro-computed tomography of A(2A)KO mouse femurs showed a significantly decreased bone volume/trabecular bone volume ratio, decreased trabecular number, and increased trabecular space. A(2A)KO femurs showed an increased TRAP-positive osteoclast. Electron microscopy in A(2A)KO femurs showed marked osteoclast membrane folding and increased bone resorption. Thus, adenosine, acting via the A(2A) receptor, inhibits macrophage colony-stimulating factor-1-receptor activator of NF-κB ligand-stimulated osteoclast differentiation and may regulate bone turnover under conditions in which adenosine levels are elevated.

Diadenosines as FHIT-ness instructors

FHIT is a tumor suppressor gene that is frequently inactivated in human cancer. Although the Fhit protein is known to hydrolyze diadenosine triphosphate (Ap(3)A), this hydrolase activity is not required for Fhit-mediated oncosuppression. Indeed, the molecular mechanisms and the regulatory elements of Fhit oncosuppression are largely unknown. Here, we review physiological and pathological aspects of Fhit in the context of the Ap(n)A family of signaling molecules, as well as the involvement of Fhit in apoptosis and the cell cycle in cancer models. We also discuss recent findings of novel Fhit interactions that may lead to new hypotheses about biochemical mechanisms underlying the oncosuppressor activity of this gene.

Adenosine downregulates DPPIV on HT-29 colon cancer cells by stimulating protein tyrosine phosphatase(s) and reducing ERK1/2 activity via a novel pathway

The multifunctional cell-surface protein dipeptidyl peptidase IV (DPPIV/CD26) is aberrantly expressed in many cancers and plays a key role in tumorigenesis and metastasis. Its diverse cellular roles include modulation of chemokine activity by cleaving dipeptides from the chemokine NH(2)-terminus, perturbation of extracellular nucleoside metabolism by binding the ecto-enzyme adenosine deaminase, and interaction with the extracellular matrix by binding proteins such as collagen and fibronectin. We have recently shown that DPPIV can be downregulated from the cell surface of HT-29 colorectal carcinoma cells by adenosine, which is a metabolite that becomes concentrated in the extracellular fluid of hypoxic solid tumors. Most of the known responses to adenosine are mediated through four different subtypes of G protein-coupled adenosine receptors: A(1), A(2A), A(2B), and A(3). We report here that adenosine downregulation of DPPIV from the surface of HT-29 cells occurs independently of these classic receptor subtypes, and is mediated by a novel cell-surface mechanism that induces an increase in protein tyrosine phosphatase activity. The increase in protein tyrosine phosphatase activity leads to a decrease in the tyrosine phosphorylation of ERK1/2 MAP kinase that in turn links to the decline in DPPIV mRNA and protein. The downregulation of DPPIV occurs independently of changes in the activities of protein kinases A or C, phosphatidylinositol 3-kinase, other serine/threonine phosphatases, or the p38 or JNK MAP kinases. This novel action of adenosine has implications for our ability to manipulate adenosine-dependent events within the solid tumor microenvironment.

Adenosine and glutamate extracellular concentrations and mitogen-activated protein kinases in the striatum of Huntington transgenic mice. Selective antagonism of adenosine A2A receptors reduces transmitter outflow

The basal ganglia and deep layers of cerebral cortex neurodegeneration typically characterize the postmortem brain of Huntington disease (HD) patients. In this study, we employed 10- to 11-week-old transgenic HD mice (R6/2 line), in which the striatal adenosine extracellular levels, measured using the microdialysis technique, are significantly increased in comparison to wild-type mice. An increase in striatal adenosine is probably a precocious index of mitochondrial dysfunction that is described in both the postmortem brain of HD patients and transgenic mice striatal cells. The adenosine increase is matched by activation of the p38 mitogen-activated protein kinase (MAPK) in the striatal neurons of R6/2 mouse but not in the cortex. This result indicates that p38 MAPK is a correlate of striatal damage and suggests a role for p38 in the striatal neuron suffering and apoptosis described in this disease. The selective adenosine A(2A) receptor antagonist SCH 58261, administered through microdialysis fiber into the striatum, significantly decreases the outflow of glutamate in R6/2 mice. Antagonism of adenosine A(2A) receptors might be regarded as potentially useful in the treatment of this disease to control striatal excitotoxicity.

The G(s)-coupled adenosine A(2B) receptor recruits divergent pathways to regulate ERK1/2 and p38

Adenosine A(2B) receptors have been suggested to influence cell differentiation and proliferation. Human adenosine A(2B) receptors expressed in Chinese hamster ovary cells mediate phosphorylation and activation of the extracellular signal-regulated kinase (ERK1/2). Already low concentrations of agonists such as 5'-N-ethylcarboxamidoadenosine (NECA) are effective. Phosphorylation of the stress-activated protein kinase p38 was also potently induced by NECA (EC(50) 18.5 nM). These NECA-induced effects were mimicked by forskolin and 8-Br-cAMP. Inhibition of cAMP-dependent protein kinase (PKA) using H89 (N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide)) blocked phosphorylation of the cAMP response element-binding protein (CREB) and p38, but did not decrease NECA-induced ERK1/2 phosphorylation. NECA activated the small GTPase Rap1, and this was also not blocked by H89. Inhibition of phosphatidylinositol-3'-kinase (PI3K) by wortmannin inhibited adenosine A(2B) receptor-mediated ERK1/2 phosphorylation and activation of Rap1, without affecting CREB and p38 phosphorylation. A(2B) receptor-stimulated protein kinase B phosphorylation was sensitive to wortmannin, but not to H89. Thus, stimulation of adenosine A(2B) receptors activates both ERK1/2 and p38 via cAMP, but the downstream pathways are markedly different. ERK1/2 activation was dependent on PI3K but not on PKA. p38 activation by NECA was instead independent of PI3K but required cAMP and PKA. The potent activation of both MAPKs suggests a physiological role.

A glance at adenosine receptors: novel target for antitumor therapy

Adenosine can be released from a variety of cells throughout the body, as the result of increased metabolic rates, in concentrations that can have a profound impact on the vasculature, immunoescaping, and growth of tumor masses. It is recognized that the concentrations of this nucleoside are increased in cancer tissues. Therefore, it is not surprising that adenosine has been shown to be a crucial factor in determining the cell progression pathway, either during apoptosis or during cytostatic state. From the perspective of cancer, the most important question then may be "Can activation and/or blockade of the pathways downstream of the adenosine receptor contribute to tumor development?" Rigorous examinations of the role of adenosine in in vivo and in vitro systems need to be investigated. The present review therefore proposes multiple adenosine-sustained ways that could prime tumor development together with the critical combinatorial role played by adenosine receptors in taking a choice between proliferation and death. This review proposes that adenosine acts as a potent regulator of normal and tumor cell growth. It is hypothesized that this effect is dependent on extracellular adenosine concentrations, cell surface expression of different adenosine receptor subtypes, and signal transduction mechanisms activated following the binding of specific agonists. We venture to suggest that the clarification of the role of adenosine and its receptors in cancer development may hold great promise for the treatment of chemotherapy in patients affected by malignancies.

Signalling from adenosine receptors to mitogen-activated protein kinases

The purine nucleoside adenosine acts via four distinct adenosine receptor subtypes: the adenosine A(1), A(2A), A(2B), and A(3) receptor. They are all G protein-coupled receptors (GPCR) coupling to classical second messenger pathways such as modulation of cAMP production or the phospholipase C (PLC) pathway. In addition, they couple to mitogen-activated protein kinases (MAPK), which could give them a role in cell growth, survival, death and differentiation. Although each of the adenosine receptors can activate one or more of the MAPKs, the mechanisms appear to differ substantially, both between receptor subtypes in the same cell type and between the same receptor in different cell types.

P2Y(AC)(-)-receptor agonists enhance the proliferation of rat C6 glioma cells through activation of the p42/44 mitogen-activated protein kinase

1. Extracellularly added P(1),P(3)-di(adenosine-5') triphosphate (Ap(3)A), P(1),P(4)-di(adenosine-5') tetraphosphate (Ap(4)A), ATP, ADP, AMP and adenosine are growth inhibitory for rat C6 glioma cells. Analysis of nucleotide hydrolysis and the use of nucleotidase inhibitors demonstrated that the latter inhibition is due to hydrolysis of the nucleotides to adenosine. 2. Agonists of the P2Y(AC)(-)-receptor enhance the growth of C6 cells if their hydrolysis to adenosine is inhibited by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). In these conditions, the potency to stimulate cell growth parallels the ranking of the receptor agonists, i.e. 2-methylthioadenosine-5'-diphosphate (2MeSADP)>Ap(3)A>Ap(4)A. ATP and ADP are still hydrolysed in the presence of PPADS and have no proliferative effect on C6 cells. 3. The enhanced growth is due to a P2Y(AC)(-)-receptor-mediated activation of p42/44 mitogen-activated protein kinase (MAPK) as shown by immunoblotting and protein kinase assays for active MAPK and the use of the MAPK/extracellular signal-regulated kinase kinase (MEK) inhibitor PD98059. 4. The UTP-induced enhancement of the growth of C6 cells is due to activation of MAPK by a PPADS sensitive nucleotide receptor. 5. In conclusion, the effect of nucleotides on the growth of C6 cells is determined by ecto-nucleotidases and by activation of nucleotide receptors. Hydrolysis of nucleotides to adenosine induces growth inhibition while inhibition of the hydrolysis of agonists of the P2Y(AC)(-)-receptor enhances cell growth by activation of MAPK.

Activation of extracellular signal-regulated kinases (ERK 44/42) by chlorpyrifos oxon in Chinese hamster ovary cells

Acetylcholinesterase inhibition explains most but not all of the toxicological manifestations of exposure to the major organophosphorus insecticide chlorpyrifos (CP) and its metabolically activated form chlorpyrifos oxon (CPO); CPO is also reported to interact with muscarinic acetylcholine receptors and alter secondary messenger status. We find that CP and CPO activate extracellular signal-regulated kinases (ERK 44/42) in both wild-type (CHOK1) and human muscarinic receptor-expressing Chinese hamster ovary cells (CHO-M2). The degree of ERK 44/42 activation on treatment with 50 microM CPO for 40 minutes is 2- to 3-fold compared with control cells and is both concentration- and time-dependent. CP is at least 2-fold less potent than CPO as an activator of ERK 44/42 and the hydrolysis products 3,5,6-trichloropyridinol and diethyl phosphate are not activators. ERK 44/42 activation by CPO is insensitive to the protein kinase A inhibitor H-89, but is completely abolished by the phosphatidylinositol 3-kinase (P13-K) inhibitor wortmannin, the protein kinase C (PKC) inhibitor GF-109203X, and the mitogen-activated extracellular signal-regulated protein kinase kinase (MEK) inhibitor PD 098059. Therefore, CPO activates the ERK 44/42 signaling cascade in CHOK1 cells via a pathway dependent on P13-K, PKC, and MEK but not requiring PKA or the human M2 muscarinic receptor. In summary we find that CPO activates a mammalian signal transduction cascade involved in cell growth and differentiation. This occurs through a pathway common to growth factors and mitogens, consistent with a receptor-mediated event. However, CPO may also inhibit an enzyme involved in signal transduction. The specific target of CPO leading to the activation of ERK 44/42 and the potential effects of this activation on cell function remain to be determined.

Adenosine inhibits IL-12 and TNF-[alpha] production via adenosine A2a receptor-dependent and independent mechanisms

Interleukin 12 (IL-12) is a crucial cytokine in the regulation of T helper 1 vs. T helper 2 immune responses. In the present study, we investigated the effect of the endogenous purine nucleoside adenosine on the production of IL-12. In mouse macrophages, adenosine suppressed IL-12 production. Although the order of potency of adenosine receptor agonists suggested the involvement of A2a receptors, data obtained with A2a receptor-deficient mice showed that the adenosine suppression of IL-12 and even TNF-alpha production is only partly mediated by A2a receptor ligation. Studies with adenosine receptor antagonists or the adenosine uptake blocker dipyridamole showed that adenosine released endogenously also decreases IL-12. Although adenosine increases IL-10 production, the inhibition of IL-12 production is independent of the increased IL-10. The mechanism of action of adenosine was not associated with alterations of the activation of the p38 and p42/p44 mitogen-activated protein kinases or the phosphorylation of the c-Jun terminal kinase. Adenosine failed to affect steady-state levels of either IL-12 p35 or p40 mRNA, but augmented IL-10 mRNA levels. In summary, adenosine inhibits IL-12 production via various adenosine receptors. These results support the notion that adenosine-based therapies might be useful in certain autoimmune and/or inflammatory diseases.

Central adenosine A(2A) receptors: an overview

Recent advances in molecular biology, biochemistry, cell biology and behavioral pharmacology together with the development of more selective ligands to the various adenosine receptors have increased our understanding of the functioning of central adenosine A(2A) receptors. The A(2A) receptor is one of four adenosine receptors found in the brain. Its expression is highest in striatum, nucleus accumbens and olfactory tubercles, although it also occurs in neurons and microglia in most other brain regions. The receptor has seven transmembrane domains and couples via Gs to adenyl cyclase stimulation. Antagonistic interactions between A(2A) receptors and dopamine D(2) receptors have been described, as stimulation of the A(2A) receptor leads to a reduction in the affinity of D(2) receptors for D(2) receptor agonists. The A(2A) receptor is thought to play a role in a number of physiological responses and pathological conditions. Indeed, A(2A) receptor antagonists may be useful for the treatment of acute and chronic neurodegenerative disorders such as cerebral ischemia or Parkinson's disease. A(2A) receptor agonists may treat certain types of seizures or sleep disorders. This review discusses the characteristics, distribution, pharmacochemical properties and regulation of central A(2A) receptors, as well as A(2A) receptor-mediated behavioural responses and their potential role in various neuropsychiatric disorders.

Ca2+ dependence of the response of three adenosine type receptors in rat hepatocytes

The effect of three different receptor-specific adenosine agonists on the rate of ureagenesis by isolated rat hepatocytes and the dependence on the external free Ca2+ concentration ([Ca2+]e) were investigated. In the presence of high [Ca2+]e all adenosine receptor agonists increased ureagenesis to similar levels. However, with low [Ca2+]e the effects of each agonist varied as follows: (i) the adenosine A1 receptor agonist, 2-chloro-N6-cyclopentyl-adenosine, increased ureagenesis depending partially on [Ca2+]e, (ii) the adenosine receptor A2 agonist, 2-p-(-2-carboxy-ethyl) phenethylamino-5'-N-ethylcarboxyamido adenosine hydrochloride, increased ureagenesis independently of [Ca2+]e and (iii) in contrast, the adenosine receptor A3 agonist N6-2-(-4-aminophenyl) ethyladenosine, increased ureagenesis only in the presence of high [Ca2+]e. The adenosine receptor A1 antagonist, 1-allyl-3,7-dimethyl-8-phenyl xanthine, inhibited the effect of the adenosine receptor A1 agonist on ureagenesis, but not the effect of the adenosine A2 or A3 receptor agonists. The adenosine A2 receptor antagonist, 3,7-dimethyl-1-propargylxanthine, inhibited only the effect of the adenosine A2 receptor agonist. Thus, in addition to A1 and A2 type adenosine receptors, rat hepatocytes possess an A3-like adenosine receptor which responds to the addition of an adenosine A3 agonist by accelerating ureagenesis a [Ca2+]e dependent manner. Moreover, it was observed that in the presence of extracellular Ca2+ each agonist increased [Ca2+]i and this effect was inhibited by the appropriate specific antagonist.