The Concise Guide to PHARMACOLOGY 2023/24: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2023/24 is the sixth in this series of biennial publications. The Concise Guide provides concise overviews, mostly in tabular format, of the key properties of approximately 1800 drug targets, and about 6000 interactions with about 3900 ligands. There is an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (https://www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. Although the Concise Guide constitutes almost 500 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/bph.16177. G protein-coupled receptors are one of the six major pharmacological targets into which the Guide is divided, with the others being: ion channels, nuclear hormone receptors, catalytic receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2023, and supersedes data presented in the 2021/22, 2019/20, 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2021/22 is the fifth in this series of biennial publications. The Concise Guide provides concise overviews, mostly in tabular format, of the key properties of nearly 1900 human drug targets with an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. Although the Concise Guide constitutes over 500 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/bph.15538. G protein-coupled receptors are one of the six major pharmacological targets into which the Guide is divided, with the others being: ion channels, nuclear hormone receptors, catalytic receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2021, and supersedes data presented in the 2019/20, 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2021/22 is the fifth in this series of biennial publications. The Concise Guide provides concise overviews, mostly in tabular format, of the key properties of nearly 1900 human drug targets with an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. Although the Concise Guide constitutes over 500 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/bph.15538. G protein-coupled receptors are one of the six major pharmacological targets into which the Guide is divided, with the others being: ion channels, nuclear hormone receptors, catalytic receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2021, and supersedes data presented in the 2019/20, 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

Lupin Peptide T9 (GQEQSHQDEGVIVR) Modulates the Mutant PCSK9D374Y Pathway: in vitro Characterization of its Dual Hypocholesterolemic Behavior

GQEQSHQDEGVIVR (T9) is a peptide originated by the tryptic digestion of lupin β-conglutin that is absorbed in human intestinal Caco-2 cells. A previous study has shown that T9 impairs the protein–protein interaction between mutant D374Y Proprotein Convertase Subtilisin/Kexin 9 (PCSK9^D374Y) and the low-density lipoprotein receptor (LDLR), thus exerting a hypocholesterolemic effect. Moreover, a bioinformatic study predicting that T9 may potentially act as an inhibitor of 3-hydroxy-3-methylglutaryl CoA reductase (HMGCoAR), has suggested a complementary cholesterol-lowering activity. The present study demonstrates that T9 inhibits in vitro the HMGCoAR functionality with an IC₅₀ value of 99.5 ± 0.56 µM. Through the inhibition of either HMGCoAR or PCSK9^D374Y activities, T9 enhances the LDLR protein levels leading to an improved ability of HepG2 cells transfected with the mutant PCSK9^D374Y-FLAG plasmid to uptake extracellular LDL with a final cholesterol-lowering effect. In addition, T9 modulates the PCSK9^D374Y signaling pathway in transfected HepG2 cells leading to a decrease of PCSK9^D374Y and HNF-1α protein levels. All these results indicate that the hypocholesterolemic effects of T9 are due to a dual mechanism of action involving either the modulation of the PCSK9^D374Y or LDLR pathways. This may represent an added value from a therapeutic point of view.

Steps towards Collective Sustainability in Biomedical Research

The optimism surrounding multistakeholder research initiatives does not match the clear view of policies that are needed to exploit the potential of these collaborations. Here we propose some action items that stem from the integration between research advancements with the perspectives of patient-advocacy organizations, academia, and industry.

Agonist-induced desensitization/resensitization of human G protein-coupled receptor 17: a functional cross-talk between purinergic and cysteinyl-leukotriene ligands

G protein-coupled receptor (GPR) 17 is a P2Y-like receptor that responds to both uracil nucleotides (as UDP-glucose) and cysteinyl-leukotrienes (cysLTs, as LTD(4)). By bioinformatic analysis, two distinct binding sites have been hypothesized to be present on GPR17, but little is known on their putative cross-regulation and on GPR17 desensitization/resensitization upon agonist exposure. In this study, we investigated in GPR17-expressing 1321N1 cells the cross-regulation between purinergic- and cysLT-mediated responses and analyzed GPR17 regulation after prolonged agonist exposure. Because GPR17 receptors couple to G(i) proteins and adenylyl cyclase inhibition, both guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPγS) binding and the cAMP assay have been used to investigate receptor functional activity. UDP-glucose was found to enhance LTD(4) potency in mediating activation of G proteins and vice versa, possibly through an allosteric mechanism. Both UDP-glucose and LTD(4) induced a time- and concentration-dependent GPR17 loss of response (homologous desensitization) with similar kinetics. GPR17 homologous desensitization was accompanied by internalization of receptors inside cells, which occurred in a time-dependent manner with similar kinetics for both agonists. Upon agonist removal, receptor resensitization occurred with the typical kinetics of G protein-coupled receptors. Finally, activation of GPR17 by UDP-glucose (but not vice versa) induced a partial heterologous desensitization of LTD(4)-mediated responses, suggesting that nucleotides have a hierarchy in producing desensitizing signals. These findings suggest a functional cross-talk between purinergic and cysLT ligands at GPR17. Because of the recently suggested key role of GPR17 in brain oligodendrogliogenesis and myelination, this cross-talk may have profound implications in fine-tuning cell responses to demyelinating and inflammatory conditions when these ligands accumulate at lesion sites.

Comparison and optimization of transient transfection methods at human astrocytoma cell line 1321N1

Gene delivery to eukaryotic cells is the technique to study the regulation of gene expression. Human astrocytoma cell line 1321N1 could be useful to study G-protein-coupled receptors (GPCRs). Different transient transfection methods, namely calcium phosphate, Lipofectamine, FuGENE, Arrest-In, and microporation (Microporator), were investigated. Results were analyzed by fluorescence-activated cell sorting and fluorescence microscope using green fluorescent protein (GFP) as a reporter gene. To verify the transfection efficiency of these techniques, the expression of human GPR17 gene (hgpr17) was analyzed by transcription quantitative polymerase chain reaction. Microporation resulted in the best method to promote enriched hgpr17 delivery into the human astrocytoma cell line.

Different properties of P2X(7) receptor in hippocampal and cortical astrocytes

P2X(7) receptor is a ligand-gated ion channel, which can induce the opening of large membrane pores. Here, we provide evidence that the receptor induces pore formation in astrocytes cultured from cortex, but not from the hippocampus. Furthermore, P2X(7) receptor activation promptly induces p38 mitogen-activated protein kinase (MAPK) phosphorylation in cortical but not in hippocampal astrocytes. Given the role of p38 MAPK activation in pore opening, these data suggest that defective coupling of the receptor to the enzyme could occur in hippocampal cultures. The different capabilities of the receptor to open membrane pores cause relevant functional consequences. Upon pore formation, caspase-1 is activated and pro-IL1-beta is cleaved and released extracellularly. The receptor stimulation does not result in interleukin-1beta secretion from hippocampal astrocytes, although the pro-cytokine is present in the cytosol of lipopolysaccharide-primed cultures. These results open the possibility that activation of P2X(7) receptors differently influences the neuroinflammatory processes in distinct brain regions.

[The advent of biosimilars: new rules to guarantee patient safety]

Biosimilars are medicinal products proposed as copies of biotech drugs whose patents have expired. Their entry into the therapeutic armamentarium entails knowledge of the rules established in Europe relevant to their use in clinical practice. In September 2007, an Italian panel of experts comprising two nephrologists, a clinical immunologist, an oncohematologist, a pharmacologist, and a hospital pharmacist examined the main features of biotech drugs and the issues faced by the regulatory authorities in the definition of a specific approval pathway for biosimilars in Europe. The panel of experts agreed that it is important to inform the medical and scientific community that biosimilars are not exact copies of their reference products; therefore the rules governing their clinical use are not the same as those established for biotech drugs in general. Patient safety should be the fundamental principle guiding therapeutic choices, and making these choices should be the prerogative of physicians.

Short-term TNF-Alpha treatment induced A2B adenosine receptor desensitization in human astroglial cells

Long-term glial cell treatment with the proinflammatory cytokine TNF-alpha has been demonstrated to increase the functional responsiveness of A(2B) adenosine receptors (A(2B) ARs), which in turn synergize with the cytokine inducing chronic astrogliosis. In the present study, we investigated the short-term effects of TNF-alpha on A(2B) AR functional responses in human astroglial cells (ADF), thus simulating the acute phase of cerebral damage which is characterized by both cytokine and adenosine high level release. Short-term TNF-alpha cell treatment caused A(2B) AR phosphorylation inducing, in turn, impairment in A(2B) AR-G protein coupling and cAMP production. These effects occurred in a time-dependent manner with a maximum following 3-h cell exposure. Moreover, we showed PKC intracellular kinase is mainly involved in the TNF-alpha-mediated regulation of A(2B) AR functional responses. The results may indicate the A(2B) AR functional impairment as a cell defense mechanism to counteract the A(2B) receptor-mediated effects during the acute phase of brain damage, underlying A(2B) AR as a target to modulate early inflammatory responses.

A(2b) receptor mediates adenosine inhibition of taurine efflux from pituicytes

BACKGROUND INFORMATION

Recent work suggests that part of the control of vasopressin output is mediated by taurine released from pituicytes, the astroglial cells of the neurohypophysis. Taurine release, in turn, is stimulated by hypotonic conditions and by vasopressin itself. As adenosine is generated from ATP co-released with vasopressin, it appeared important to study its effects on taurine efflux from pituicytes.

RESULTS

We measured radioactive efflux from cultured pituicytes and whole neurohypophyses pre-loaded with [(3)H]taurine. Cultured pituicytes were also used to study adenosine-receptor mRNA expression. Taurine efflux elicited by hypotonic shocks is approximately 30-50% smaller in the presence of 10 microM adenosine or 1 microM NECA (5'-N-ethylcarboxamidoadenosine). Both compounds also inhibited basal efflux in a manner that was not immediately reversible. Agonists of the adenosine A1-, A2a- or A3-receptor subtypes have no relevant effect on basal taurine release, and the A1-receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine) has no effect on the inhibition of release by NECA. In turn, the A2b-receptor antagonists MRS 1706 {N-(4-acetylphenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy]acetamide} or alloxazine partially reverse the inhibition of basal or hypotonicity-evoked efflux by NECA. Both A1- and A2b-receptor mRNAs are expressed in pituicytes, which is consistent with an A1-receptor-mediated effect on cell morphology and an A2b-receptor-mediated effect on taurine release. Forskolin and dibutyryl cAMP mimic the inhibitory effects of purinergics on basal taurine efflux, and the adenylate cyclase inhibitor DDA (2',5'-dideoxyadenosine) partially reverses the inhibition of the hypotonic response by NECA.Conclusions. Our results suggest that purinergic inhibition of taurine efflux from pituicytes operates through A2b receptors coupled to intracellular cAMP increase. They point to a possible modulation of neurohypophysial hormone output by endogenous adenosine released in either physiological or pathological situations.

Biological abnormalities of peripheral A2A receptors in a large representation of polyglutamine disorders and Huntington's disease stages

Huntington's disease is one of a group of hereditary neurodegenerative diseases characterized by a glutamine expansion (polyQ) in proteins which are expressed in various cell populations. In agreement with this widespread distribution, we have previously shown that A(2A) receptor signaling is affected in mouse brain as well as in peripheral blood cells from a small cohort of HD patients. Here we analyzed a total of 252 subjects, including 126 HD gene-positive individuals, from different clinical sites. Consistent with our previous data we show that A(2A) receptor B(max) values are robustly increased at all HD stages as well as in 32 pre-symptomatic subjects. We report that the same abnormality is present also in other polyQ but not in non-polyQ inherited neurological disorders. Finally, we demonstrate that the same peripheral cells exhibit an altered membrane fluidity, a finding that may explain the observed change in receptor density. We argue that the observed alteration in lymphocytes reflects the presence of the mutant protein, and we suggest that the measure of the A(2A) receptor binding activity might be of potential interest for a peripheral assessment of chemicals capable of interfering with the immediate toxic effects of the mutation.

Adenosine-induced cell death: evidence for receptor-mediated signalling

Adenosine modulates the proliferation, survival and apoptosis of many different cell types, ranging from epithelial, endothelial and smooth muscle cells, to cells of the immune and neural lineages. In this review, we critically discuss the available in vitro and in vivo data which support a role for adenosine in both development-associated apoptosis, and in diseases characterized by either pathologically increased cell death (e.g., ischemia, trauma and aging-associated neurodegeneration) or abnormally reduced spontaneous apoptosis (e.g., cancer). Particular emphasis is given to the possible role of extracellular adenosine receptors, since these may represent novel and attractive molecular targets for the pharmacological modulation of apoptosis. In some instances, adenosine-induced cell death has been demonstrated to require entry of the nucleoside inside cells; however, in many other cases, activation of specific adenosine extracellular receptors has been demonstrated. Of the four G protein-coupled adenosine receptors so far identified, the A2A and the A3 receptors have been specifically implicated in modulation of cell death. For the A3 receptor, results obtained by exposing both cardiomyocytes and brain astrocytes to graded concentrations of selective agonists suggest induction of both cell protection and cell death. Such opposite effects, which likely depend on the degree of receptor activation, may have important therapeutic implications in the pharmacological modulation of cardiac and brain ischemia. For the A2A receptor, recent intriguing data suggest a specific role in immune cell death and immunosuppression, which may be relevant to both adenosine-deaminase-immunodeficiency syndrome (a pathology characterized by accumulation of adenosine to toxic levels) and in tumors where induction of apoptosis via activation of specific extracellular receptors may be desirable. Finally, preliminary data suggest that, in a similar way to the adenosine-deaminase-immunodeficiency syndrome, the abnormal accumulation of adenosine in degenerative muscular diseases may contribute to muscle cell death. Although the role of adenosine receptors in this effect still remains to be determined, these data suggest that adenosine-induced apoptosis may also represent a novel pathogenic pathway in muscular dystrophies.

A3 adenosine receptors in human astrocytoma cells: agonist-mediated desensitization, internalization, and down-regulation

A(3) adenosine receptor activation has been previously demonstrated to result in both neuroprotective and neurodegenerative effects, depending upon specific pathophysiological conditions. This dual effect may depend on receptor regulation mechanisms that are able to change receptor availability and/or function. In the present study, we investigated desensitization, internalization, and down-regulation of native A(3) adenosine receptors in human astrocytoma cells after exposure to the agonist 2-chloro-N6-(3-iodobenzyl)-N-methyl-5'-carbamoyladenosine (Cl-IBMECA). Cl-IBMECA induced a concentration-dependent inhibition of adenylyl cyclase activity with an EC(50) value of 2.9 +/- 0.1 nM. The effect was suggested to be mediated by A(3) adenosine receptor subtype by the use of selective adenosine receptor antagonists. Cell treatment with pertussis toxin abolished Cl-IBMECA-mediated inhibition of adenylyl cyclase activity, evidencing an A(3) receptor coupling to inhibitory G protein. Short-term exposure to the agonist Cl-IBMECA (100 nM) caused rapid receptor desensitization, within 15 min. Agonist-induced desensitization was accompanied by receptor internalization: A(3) adenosine receptor internalized with rapid kinetics, within 30 min, after cell exposure to 100 nM Cl-IBMECA. The localization of A(3) adenosine receptors on the plasma membrane and in intracellular compartments was directly revealed by immunogold electron microscopy. After desensitization, the removal of agonist led to the restoration of A(3) adenosine receptor functioning through receptor recycling to the cell surface within 120 min. Prolonged agonist exposure (1-24 h) resulted in a marked down-regulation of A(3) adenosine receptors that reached 21.9 +/- 2.88% of control value after 24 h. After down-regulation, the recovery of receptor functioning was slow (24 h) and associated with the restoration of receptor levels close to control values. In conclusion, our results demonstrated that A(3) receptors, in astrocytoma cells, are regulated after short- and long-term agonist exposure.

Enhanced apoptosis of peripheral blood mononuclear cells in cardiac transplanted patients undergoing chronic immunosuppressive treatment

Apoptosis plays a major role in tissue transplantation because intact T-cell-apoptosis pathways are required for the induction of tolerance to allografts. Moreover, immunosuppressive agents commonly used in clinical transplantation medicine promote lymphocyte apoptosis inhibiting the expression and production of cytokines involved in lymphocyte survival. The aim of our study was to evaluate peripheral blood mononuclear cells (PBMC) spontaneous apoptosis in patients undergoing chronic immunosuppressive treatment after cardiac transplantation. PBMC obtained from patients (n = 31) and controls matched for age and sex (n = 25) were cultured for 72 h and apoptosis was evaluated by quantification of fragmented DNA, staining with Hoechst 33258 dye and annexin V binding. We also investigated Fas expression and FasL mRNA expression as well as the ability of an IgM anti-Fas antibody to induce apoptosis. Finally, we evaluated IL2 production induced by PHA and the ability of IL2 to prevent apoptosis. In patients, PBMC underwent enhanced spontaneous apoptosis in comparison with controls. However, we could not find any difference between patients and normals as regards the expression of Fas and of FasL mRNA, even if the cross-linking of the Fas molecule induced apoptosis in PBMC from patients, whereas it failed to induce cell death in normals. We also found that IL2 production was significantly decreased in patients and that the addition of IL2 to the culture medium reduced PBMC spontaneous apoptosis. Our findings suggest that in cardiac transplanted patients PBMC undergo enhanced spontaneous apoptosis, which may contribute to prevent allograft rejection.

Synthesis and pharmacological characterization of 2-(4-chloro-3-hydroxyphenyl)ethylamine and N,N-dialkyl derivatives as dopamine receptor ligands

2-(4-Chloro-3-hydroxyphenyl)ethylamine (4) and some derivatives were synthesized as dopamine (DA) receptor ligands. Amine 4 retains the dopaminergic pharmacophore 2-(3-hydroxyphenyl)-ethylamine, and the chlorine atom replaces the "para" hydroxyl group of DA. The derivatives 18a-e were obtained by introducing on the nitrogen of amine 4 the n-propyl and 2-phenylethyl or 3-phenylpropyl groups which can be accommodated by the D-2 receptor lipophilic sites 3C and pi 3, respectively. The affinity and selectivity of these compounds for D-1 and D-2 subtypes was determined in radioligand competition assays for the DA receptors of rat striatum membranes using [3H]SCH 23390 (D-1 selective) and [3H]spiperone (D-2 selective) as radioligands. The amine 4 shows about 7-fold lower affinity than DA for both sites and is not able to discriminate between the two subtypes of DA receptors. The introduction of two n-propyl groups (18a) on the nitrogen atom reduces by one-half and doubles the affinity for D-1 and D-2 binding sites, respectively. The substitution of an n-propyl group with different alkylphenyl groups, to give compounds 18b-e, increases the affinity for the D-2 subtype from 19-fold to 36-fold. These compounds have the same affinity at the D-2 site as the DA agonist N-n-propyl-N-(2-phenylethyl)-2-(3-hydroxyphenyl)-ethylamine (2a) and are about 20 times more selective than DA for this binding site. In the assay for D-2 receptor mediated inhibition of adenylate cyclase activity, all the tested compounds behaved as D-2 agonists; N-n-propyl-N-[2(4-hydroxyphenyl)ethyl]- (18d) and N-n-propyl-N-(2-phenyl-ethyl)-2-(4-chloro-3-hydroxyphenyl)ethylamine (18b) were more effective than DA or 2a. On the other hand, all compounds were less effective than DA in stimulation of adenylate cyclase activity in rat striatal homogenates, a kind of effect which is mediated by the D-1 subtype of DA receptors. These results suggest that the nitrogen substitution enhances the affinity and selectivity for the D-2 receptor. In the adenylate cyclase assay, the compounds behave as potent D-2 agonists.

Modulation of cyclooxygenase-2 and brain reactive astrogliosis by purinergic P2 receptors

Astroglial cells respond to trauma and ischemia with reactive gliosis, a reaction characterized by increased astrocytic proliferation and hypertrophy. Although beneficial to a certain extent, excessive gliosis may be detrimental, contributing to neuronal death in neurodegenerative diseases. We have tested the hypothesis that ATP may act as a trigger of reactive gliosis in an in vitro model (rat brain primary astrocytes) where reactive astrogliosis can be quantified as elongation of astrocytic processes. Challenge of cells with the ATP analog alpha,beta methyleneATP (alpha,beta meATP) resulted in concentration dependent elongation of astrocytic processes, an effect that was fully counteracted by the non-selective ATP/P2 receptor antagonists suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS). Signalling studies revealed that alpha,beta meATP-induced gliosis is mediated by a novel G-protein-coupled receptor (a P2Y receptor) coupled to an early release of arachidonic acid. Challenge of cells with alpha,beta meATP also resulted in an increase of inducible cyclooxygenase-2 (COX-2), the activity of which has been reported to be pathologically increased in a variety of neurodegenerative diseases characterized by inflammation and astrocytic activation. Induction of COX-2 by alpha,beta meATP was causally related to reactive astrogliosis, since the selective COX-2 inhibitor NS-398 prevented both the purine-induced elongation of astrocytic processes and the associated COX-2 increase. Preliminary data on the putative receptor-to-nucleus pathways responsible for purine-induced gliosis suggest that induction of the COX-2 gene may occur through the protein kinase C/mitogen activated protein kinase system, and may involve the formation of activated AP-1 transcription complexes. We speculate that antagonists selective at this novel P2Y receptor subtype may represent a novel class of neuroprotective agents able to slow down neurodegeneration by counteracting the inflammatory events contributing to neuronal cell death.

The A3 adenosine receptor induces cytoskeleton rearrangement in human astrocytoma cells via a specific action on Rho proteins

In previous studies, we have demonstrated that exposure of astroglial cells to A3 adenosine receptor agonists results in dual actions on cell survival, with "trophic" and antiapoptotic effects at nanomolar concentrations and induction of cell death at micromolar agonist concentrations. The protective actions of A3 agonists have been associated with a reinforcement of the actin cytoskeleton, which likely results in increased resistance of cells to cytotoxic stimuli. The molecular mechanisms at the basis of this effect and the signalling pathway(s) linking the A3 receptor to the actin cytoskeleton have never been elucidated. Based on previous literature data suggesting that the actin cytoskeleton is controlled by small GTP-binding proteins of the Rho family, in the study reported here we investigated the involvement of these proteins in the effects induced by A3 agonists on human astrocytoma ADF cells. The presence of the A3 adenosine receptor in these cells has been confirmed by immunoblotting analysis. As expected, exposure of human astrocytoma ADF cells to nanomolar concentrations of the selective A3 agonist 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (CI-IB-MECA) resulted in formation of thick actin positive stress fibers. Preexposure of cells to the C3B toxin that inactivates Rho-proteins completely prevented the actin changes induced by CI-IB-MECA. Exposure to the A3 agonist also resulted in significant reduction of Rho-GDI, an inhibitory protein known to maintain Rho proteins in their inactive state, suggesting a potentiation of Rho-mediated effects. This effect was fully counteracted by the concomitant exposure to the selective A3 receptor antagonist MRS1191. These results suggest that the reinforcement of the actin cytoskeleton induced by A3 receptor agonists is mediated by an interference with the activation/inactivation cycle of Rho proteins, which may, therefore, represent a biological target for the identification of novel neuroprotective strategies.

2-Chloro-adenosine induces a glutamate-dependent calcium response in C2C12 myotubes

Adenosine and its derivatives may induce acute changes, i.e., injury and death, in muscle cells. In the present work, we evaluated the intracellular calcium concentration in C2C12 myogenic cells differentiated in vitro to form myotubes and exposed to a metabolically stable analogue of adenosine, 2-chloro-adenosine. The compound was able to significantly modify ionic homeostasis by sensitizing muscle cells to the excitatory amino acid glutamate. A single exposure to glutamate led to a marked increase in intracellular calcium level. This is the first demonstration that adenosine analogues can regulate muscle cell integrity and function via an indirect increase of intracellular calcium ions.

Adenosine- and 2-chloro-adenosine-induced cytopathic effects on myoblastic cells and myotubes: involvement of different intracellular mechanisms

We recently suggested that, in muscular dystrophies, the excessive accumulation of adenosine as a result of an altered purine metabolism may contribute to progressive functional deterioration and muscle cell death. To verify this hypothesis, we have taken advantage of C2C12 myoblastic cells, which can be differentiated in vitro into multinucleated cells (myotubes). Exposure of both proliferating myoblasts and differentiated myotubes to adenosine or its metabolically-stable analog, 2-chloro-adenosine, resulted in apoptotic cell death and myotube disruption. Cytotoxicity by either nucleoside did not depend upon extracellular adenosine receptors, but, at least in part, by entry into cells via the membrane nitro-benzyl-thio-inosine-sensitive transporter. The adenosine kinase inhibitor, 5-iodotubercidin, prevented 2-chloro-adenosine-induced (but not adenosine-induced) effects, suggesting that an intracellular phosphorylation/activation reaction plays a key role in 2-chloro-adenosine-mediated cytotoxicity. Conversely, adenosine cytotoxicity was aggravated by the addition of homocysteine, suggesting that adenosine effects may be due to the accumulation of S-adenosyl-homocysteine, which blocks intracellular methylation-dependent reactions. Both nucleosides markedly disrupted the myotube structure via an effect on the actin cytoskeleton; however, also for myotubes, there were marked differences in the morphological alterations induced by these two nucleosides. These results show that adenosine and 2-chloro-adenosine induce apoptosis of myogenic cells via completely different metabolic pathways, and are consistent with the hypothesis that adenosine accumulation in dystrophic muscles may represent a novel pathogenetic pathway in muscle diseases.

A novel gliotic P2 receptor mediating cyclooxygenase-2 induction in rat and human astrocytes

In astrocytic cultures maintained in vitro, a brief challenge with the ATP analog alpha,beta methyleneATP (alpha,betameATP) results, 3 days later, in marked elongation of astrocytic processes, an event that resembles the astrocytic hypertrophy known to occur in vivo during reactive astrogliosis. alpha,beta meATP-induced effects were observed in primary astrocytes obtained from both rat striatum and cortex (a brain area highly involved in chronic neurodegenerative pathologies), as well as in human astrocytoma cells (ADF cells). Purine-induced gliosis could be reversed by the non-selective P2X/P2Y receptor antagonist pyridoxalphosphate-6-azophenyl-2', 4'-disulphonic acid (PPADS), but not by oxidized ATP (an antagonist of the P2X(7) receptor), in line with previous studies of our laboratory suggesting the involvement of a P2Y receptor subtype. Induction of reactive gliosis was preceded by increased expression of cyclooxygenase-2 (COX-2), an enzyme whose excessive activation has been implicated in both acute and chronic neurodegenerative diseases. The selective COX-2 inhibitor NS-398 prevented both purine-induced astrogliosis and the associated COX-2 induction, suggesting that inhibition of the transcription of the COX-2 gene may also contribute to the anti-inflammatory properties of this agent. Significant blockade of both alpha,beta meATP-mediated reactive gliosis and COX-2 induction was also observed with PPADS. These data suggest that COX-2 mediates P2Y receptor-induced reactive astrogliosis, and that antagonists selective for this receptor subtype may represent a novel class of anti-inflammatory agents of potential interest in acute and chronic neurological disorders characterized by an inflammatory component and reactive gliosis.

Apoptosis induced by 2-chloro-adenosine and 2-chloro-2'-deoxy-adenosine in a human astrocytoma cell line: differential mechanisms and possible clinical relevance

We have previously demonstrated that 2-chloro-adenosine (2-CA) can induce apoptosis of rat astroglial cells (Abbracchio et al. [1995] Biochem. Biophys. Res. Commun. 213:908-915). In the present study, we have characterized, for the first time, the effects induced on a human astrocytoma cell line (ADF cells) by both 2-CA and its related analog 2-chloro-2'-deoxy-adenosine (2-CdA, that is employed as anti-cancer agent in chronic lymphoid malignancies). Exposure of these cells to either adenosine analog resulted in time- and concentration-dependent apoptosis. Experiments with pharmacological agents known to interfere with adenosine receptors, its membrane transporter, and intracellular nucleoside kinases showed that: (i) cell death induced by either adenosine analog did not depend on extracellular adenosine receptors, but on a direct intracellular action; however, only in the case of 2-CA, was entry into cells mediated by the specific nitrobenzyl-tioinosine-sensitive transporter; (ii) for both adenosine analogs, induction of apoptosis required the phosphorylation/activation by specific intracellular nucleoside kinases, i.e., adenosine kinase for 2-CA, and deoxycytidine kinase for 2-CdA. In addition, only in the case of 2-CdA, was induction of apoptosis preceded by a block of cells at the G2/M phase of the cell cycle. Finally, at concentrations of either analog that killed about 80-90% of astrocytoma cells, a significantly lower effect on the viability of primary cortical neurons was observed. In conclusion, both adenosine analogs can trigger apoptosis of human astrocytoma cells, albeit with different mechanisms. This effect together with the relative sparing of neuronal cells, may have potential clinical implications for the therapy of tumors of glial origin.

Brain adenosine receptors as targets for therapeutic intervention in neurodegenerative diseases

Adenosine acts as a neurotransmitter in the brain through the activation of four specific G-protein-coupled receptors (the A1, A2A, A2B, and A3 receptors). The A1 receptor has long been known to mediate neuroprotection, mostly by blockade of Ca2+ influx, which results in inhibition of glutamate release and reduction of its excitatory effects at a postsynaptic level. However, the development of selective A1 receptor agonists as antiischemic agents has been hampered by their major cardiovascular side effects. More recently, apparently deleterious effects have been reported following the activation of other adenosine receptor subtypes, namely, the A2A and the A3 receptors. In particular, selective A2A receptor antagonists have been demonstrated to markedly reduce cell death associated with brain ischemia in the rat, suggesting that the cerebral A2A receptor may indeed contribute to the development of ischemic damage. The beneficial effects evoked by A2A antagonists may be due to blockade of presynaptic A2A receptors (which are stimulatory on glutamate release) and/or to inhibition of A2A receptor-mediated activation of microglial cells. Even more puzzling data have been reported for the A3 receptor subtype, which can indeed mediate both cell protection and cell death, simply depending upon the degree of receptor activation and/or specific pathophysiological conditions. In particular, a mild subthreshold activation of this receptor has been associated with a reinforcement of the cytoskeleton and reduction of spontaneous apoptosis, which may play a role in "ischemic preconditioning" of the brain, according to which a short ischemic period may protect the brain from a subsequent, sustained ischemic insult that would be lethal. In contrast, a robust and prolonged activation of the A3 receptor has been shown to trigger cell death by either necrosis or apoptosis. Such apparently opposing actions may be reconciled by hypothesizing that adenosine-mediated cell killing during ischemia may be aimed at isolating the most damaged areas to favor those parts of the brain that still retain a chance for functional recovery. In fact, both A3 receptor-mediated cell death and A2A receptor-mediated actions may be viewed as an attempt to selectively kill irreversibly damaged cells in the "core" ischemic area, in order to save space and energy for the surrounding live cells in the "pneumbra" area. Hence, the pharmacological modulation of the A2A and A3 receptors via selective ligands may represent a novel strategy in the therapeutic approach to pathologies characterized by acute or chronic neurodegenerative events.

Activation of the A3 adenosine receptor affects cell cycle progression and cell growth

The A3 adenosine receptor has been implicated in modulation of cell growth. As a first step to the characterization of the underlying mechanisms, we exposed Chinese hamster ovary (CHO) cells transfected with the human A3 receptor (A3R-CHO) to selective A3 receptor ligands. At micromolar concentrations, the A3 agonists N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (IB-MECA) and its 2-chloro derivative Cl-IB-MECA reduced cell number, with no effects on either parental CHO cells (not expressing any adenosine receptor), or CHO cells transfected with the human A1 receptor. Cl-IB-MECA also reduced cell number in the human HEK293 cell line transfected with the human A3 receptor cDNA as opposed to the respective untransfected wild-type cells. In A3R-CHO, agonist-induced effects were antagonized by nanomolar concentrations of A3 antagonists, including the triazoloquinazoline derivative MRS 1220, the dihydropyridine derivative MRS 1191, and the triazolonaphthyridine derivative L-249,313. A3 agonist-induced effects were not due to modulation of cell adhesion, nor to necrosis or apoptosis. Growth curves revealed highly impeded growth, and flow-cytometric analysis showed markedly reduced bromodeoxyuridine incorporation into nuclei. The effect on cell cycle was completely antagonized by MRS1191. Hence, activation of the human A3 receptor in A3R-CHO results in markedly impaired cell cycle progression, suggesting an important role for this adenosine receptor subtype in cell cycle regulation and cell growth.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

European Stroke Prevention Study-2 results: serendipitous demonstration of neuroprotection induced by endogenous adenosine accumulation?

In patients with prior stroke or transient ischaemic attack, anti-platelet treatment with dipyridamole substantially reduced stroke recurrence, with a beneficial effect comparable to and additive with that induced by aspirin (the European Stroke Prevention Study-2). Eugenio Picano and Maria Abbracchio present here a platelet-independent hypothesis, according to which cardiovascular and neuroprotective actions achieved by dipyridamole through chronic elevation of endogenous adenosine levels may have contributed to the therapeutic success of this study.

The A3 adenosine receptor mediates cell spreading, reorganization of actin cytoskeleton, and distribution of Bcl-XL: studies in human astroglioma cells

The pathophysiological role of the adenosine A3 receptor in the central nervous system is largely unknown. We have investigated the effects of the selective A3 receptor agonist 2-chloro-N6-(3-iodobenzyl)-adenosine, Cl-IB-MECA, in cells of the astroglial lineage (human astrocytoma ADF cells). A marked reorganization of the cytoskeleton, with appearance of stress fibers and numerous cell protrusions, was found following exposure of cells to low (nM) concentrations of Cl-IB-MECA. These "trophic" effects were accompanied by induction of the expression of Rho, a small GTP-binding protein, which was virtually absent in control cells, and by changes of the intracellular distribution of the antiapoptotic protein Bcl-XL, that, in agonist-exposed cells, became specifically associated to cell protrusions. This is the first demonstration that the intracellular organization of Bcl-XL can be modulated by the activation of a G-protein-coupled membrane receptor, such as the A3 adenosine receptor. Moreover, modulation of the astrocytic cytoskeleton by adenosine may have intriguing implications in both nervous system development and in the response of the brain to trauma and ischemia.

Actin cytoskeleton as a target for 2-chloro adenosine: evidence for induction of apoptosis in C2C12 myoblastic cells

Recent results for various cell types have shown that adenosine is capable of inducing relevant cytophatological alterations which can eventually lead to apoptosis. No data are available regarding the involvement of adenosine in apoptosis of muscle cells. In this work, we studied the effect of the relatively hydrolysis-resistant adenosine analog 2-chloro adenosine on a cultured myogenic cell line, C2C12, which is able to differentiate in vitro, leading to the formation of syncythia, giant multinucleated cells called myotubes. Results indicated that 2-chloro adenosine induces apoptotic cell death in both myoblasts and myotubes; this was preceded by a derangement of actin microfilaments, which represent the main cytoskeletal component and play a pivotal role in the physiology of such cell line. The time-dependency of cytoskeletal alterations suggested a causal relationship between these changes and the induction of apoptosis. These results implicate adenosine as an endogenous regulator of apoptosis in muscle cells and validate this cell model system as a useful tool for studying human muscle cell pathologies.

Characterization of the Ca2+ responses evoked by ATP and other nucleotides in mammalian brain astrocytes

1. This study was aimed at characterizing ATP-induced rises in cytosolic free calcium ion, [Ca2+]i, in a population of rat striatal astrocytes loaded with the fluorescent Ca2+ probe Fura2, by means of fluorescence spectrometry. 2. ATP triggered a fast and transient elevation of [Ca2+]i in a concentration-dependent manner. The responses of the purine analogues 2-methylthio-ATP (2-meSATP), adenosine-5'-O-(2-thiodiphosphate) (ADP beta S), as well as uridine-5'-triphosphate (UTP) resembled that of ATP, while alpha, beta-methylene-ATP (alpha, beta-meATP) and beta, gamma-methylene-ATP (beta, gamma-meATP) were totally ineffective. 3. Suramin (50 microM) had only a minor effect on the ATP response, whereas pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (5 microM) significantly depressed the maximum response. 4. Extracellular Ca2+ did not contribute to the observed [Ca2+]i rise: removing calcium from the extracellular medium (with 1 mM EGTA) or blocking its influx by means of either Ni2+ (1 mM) or Mn2+ (1 mM) did not modify the nucleotide responses. 5. Furthermore, after preincubation with 10 microM thapsigargin, the nucleotide-evoked [Ca2+]i increments were completely abolished. In contrast, 10 mM caffeine did not affect the responses, suggesting that thapsigargin-, but not caffeine/ryanodine-sensitive stores are involved. 6. Both application of the G-protein blocker guanosine-5'-O-(2-thiodiphosphate) (GDP beta S) (1 mM) and preincubation with pertussis toxin (PTx) (350 ng ml-1) partially inhibited the nucleotide-mediated responses. Moreover, the phospholipase C (PLC) inhibitor U-73122, but not its inactive stereoisomer U-73343 (5 microM), significantly reduced the ATP-evoked [Ca2+]i rise. 7. In conclusion, our results suggest that, in rat striatal astrocytes, ATP-elicited elevation of [Ca2+]i is due solely to release from intracellular stores and is mediated by a G-protein-linked P2Y receptor, partially sensitive to PTx and coupled to PLC.

Characterization of the signalling pathways involved in ATP and basic fibroblast growth factor-induced astrogliosis

1. A brief challenge of rat astrocytes with either alpha, beta-methyleneATP (alpha, beta-meATP) or basic fibroblast growth factor (bFGF) resulted, three days later, in morphological differentiation of cells, as shown by marked elongation of astrocytic processes. The P2 receptor antagonist suramin prevented alpha, beta-meATP- but not bFGF-induced astrocytic elongation. Similar effects on astrocytic elongation were also observed with ATP and other P2 receptor agonists (beta, gamma meATP, ADP beta S, 2meSATP and, to a lesser extent, UTP). 2. Pertussis toxin completely abolished alpha, beta-meATP- but not bFGF-induced effects. No effects were exerted by alpha, beta-meATP on cyclic AMP production; similarly, neomycin had no effects on elogation of processes induced by the purine analogue, suggesting that adenylyl cyclase and phospholipase C are probably not involved in alpha, beta-meATP-induced effects (see also the accompanying paper by Centemeri et al., 1997). The tyrosine-kinase inhibitor genistein greatly reduced bFGF- but not alpha, beta-meATP-induced astrocytic elongation. 3. Challenge of cultures with alpha, beta-meATP rapidly and concentration-dependently increased [3H]-arachidonic acid (AA) release from cells, suggesting that activation of phospholipase A2 (PLA2) may be involved in the long-term functional effects evoked by purine analogues. Consistently, exogenously added AA markedly elongated astrocytic processes. Moreover, various PLA2 inhibitors (e.g. mepacrine and dexamethasone) prevented both the early alpha, beta-meATP-induced [3H]-AA release and/or the associated long-term morphological changes, without affecting the astrocytic elongation induced by bFGF. Finally, the protein kinase C (PKC) inhibitor H7 fully abolished alpha, beta-meATP- but not bFGF-induced effects. 4. Both alpha, beta-meATP and bFGF rapidly and transiently induced the nuclear accumulation of Fos and Jun. Both c-fos and c-jun induction by the purine analogue could be fully prevented by pretreatment with suramin. In contrast, the effects of bFGF were unaffected by this P2 receptor antagonist. 5. It was concluded that alpha, beta-meATP- and bFGF-morphological differentiation of astrocytes occurs via independent transductional pathways. For the purine analogue, signalling involves a Gi/G(o) protein-coupled P2Y-receptor which may be linked to activation of PLA2 (involvement of an arachidonate-sensitive PKC is speculated); for bFGF, a tyrosine kinase receptor is involved. Both pathways merge on some common intracellular target, as suggested by induction of primary response genes, which in turn may regulate late response genes mediating long-term phenotypic changes of astroglial cells. 6. These findings implicate P2 receptors as novel targets for the pharmacological regulation of reactive astrogliosis, which has intriguing implications in nervous system diseases characterized by degenerative events.

Adenosine A3 receptor agonists protect HL-60 and U-937 cells from apoptosis induced by A3 antagonists

The effects of novel, selective adenosine (ADO) A3 receptor antagonists of diverse structure on cells of the human HL-60 leukemia and U-937 lymphoma cell lines were examined. Both 3-ethyl 5-benzyl 2-methyl-6-phenyl-4-phenylethynyl-1,4-(+/-)-dihydropyridine-3, 5-dicarboxylate (MRS 1191, 0.5 microM) and 6-carboxy-methyl-5, 9-dihydro-9-methyl-2-phenyl-[1,2,4]-triazolo [5,1-a][2,7]naphthyridine (L-249313, 0.5 microM) induced apoptotic cell death and expression of bak protein. Low concentrations of the A3 receptor agonist 2-chloro-N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide (Cl-IB-MECA, 10 nM or 1 microM) protected against antagonist-induced cell death. At concentrations > or = 10 microM, the agonist alone produced apoptosis and bak expression in various cell lines. It is suggested that there exists a tonic low level of A3 receptor activation, possibly induced by release of endogenous adenosine, that results in cell protection.

Different pathways of apoptosis revealed by 2-chloro-adenosine and deoxy-D-ribose in mammalian astroglial cells

Both the adenosine analogue 2-chloro-adenosine (2-CA) and the reducing sugar deoxy-D-ribose (dRib) induce apoptosis of astroglial cells in rat brain primary cultures (Abbracchio et al.: Biochem Biophys Res Commun 213:908-915, 1995). The present study was undertaken to elucidate by both morphological and cytofluorimetric analyses the intracellular mechanism(s) involved in induction of apoptosis by these two agents. The poly(ADP-ribose)polymerase (PARP) inhibitor 3-aminobenzamide did not prevent either 2-CA- or dRib-induced cell death, suggesting that activation of PARP is not critically important for induction of apoptosis in astrocytes. The radical scavenger N-acetyl-cysteine (NAC) strongly inhibited dRib- but not 2-CA-induced cell death, suggesting a differential role for radical formation in apoptosis by these two agents. A time-dependent increase of cells with depolarized mitochondria was observed in dRib-, and to a lesser extent, in 2-CA-treated cultures. NAC also prevented dRib- but not 2-CA-induced mitochondrial changes. We conclude that, in mammalian astrocytes, apoptosis can proceed through diverse and multiple pathways, depending upon the apoptotic stimulus. For dRib, apoptosis likely proceeds through generation of radicals and mitochondrial involvement. An adenosine extracellular receptor linked to an as yet unidentified signaling pathway may instead mediate 2-CA-induced cell death, which may have intriguing implications for both nervous system development and brain response to trauma and ischemia.

Purinoceptors in the Central Nervous System

New exciting developments on the occurrence and functional role of purinoceptors in mammalian brain were presented at the session "Purinoceptors in the central nervous system" chaired by Flaminio Cattabeni and Tom Dunwiddie at the Purines '96 international conference. The focus of the session were topics of recent interest, including the sources and mechanisms involved in ATP and adenosine release during physiological neurotransmission in hippocampus, the brain expression of the recently cloned P2 receptors, and the role of the various adenosine receptor subtypes in brain protection from neurodegeneration associated with trauma-, ischemia-and excessive excitatory amino acid neurotransmission. New important insights into the mechanisms responsible for the formation and release of adenosine into the extracellular space were provided by data obtained by Dunwiddie and coworkers in hippocampal pyramidal neurons. These data may have functional implications for the role of purines in modulation of synaptic plasticity and long-term potentiation in this brain area, and hence in cognitive functions. Buell provided an updated overview on the cloning, molecular characteristics and brain expression of various ligand-gated P2X purinoceptors; although the functional role of these receptors in mammalian brain still awaits elucidation, their widespread distribution in the nervous system strongly suggests that ATP-mediated events are more prevalent and important in brain than expected. Pedata presented data on the functional interrelationships between adenosine and glutamate in the brain, and also provided evidence for alterations of the reciprocal regulation between these two systems in aged brain, which may have important implications for both ischemia-and trauma-associated neurodegenerative events and senescence-associated cognitive impairment. Finally, von Lubitz provided novel data on the molecular mechanisms likely to be at the basis of the brain protective effects associated with the chronic stimulation of the adenosine A3 receptor, further confirming that this receptor represents a crucial target for the development of new antiischemic and antineurodegenerative therapeutic agents.

Trophic actions of extracellular nucleotides and nucleosides on glial and neuronal cells

In addition to their well-established roles as neurotransmitters and neuromodulators, growing evidence suggests that nucleotides and nucleosides might also act as trophic factors in both the central and peripheral nervous systems. Specific extracellular receptor subtypes for these compounds are expressed on neurons, glial and endothelial cells, where they mediate strikingly different effects. These range from induction of cell differentiation and apoptosis, mitogenesis and morphogenetic changes, to stimulation of synthesis or release, or both, of cytokines and neurotrophic factors, both under physiological and pathological conditions. Nucleotides and nucleosides might be involved in the regulation of development and plasticity of the nervous system, and in the pathophysiology of neurodegenerative disorders. Receptors for nucleotides and nucleosides could represent a novel target for the development of therapeutic strategies to treat incurable diseases of the nervous system, including trauma- and ischemia-associated neurodegeneration, demyelinating and aging-associated cognitive disorders.

G protein-dependent activation of phospholipase C by adenosine A3 receptors in rat brain

The recently cloned G protein-coupled adenosine A3 receptor has been proposed to play a role in the pathophysiology of cerebral ischemia. Because phospholipase C activation occurs as a very early response to brain ischemia, we evaluated the ability of A3- selective and nonselective adenosine analogues to elicit phosphoinositide hydrolysis. In myo-[3H]inositol-labeled rat striatal and hippocampal slices, A3 agonists stimulated formation of [3H]inositol phosphates in a concentration-dependent manner. In striatum, the potency order was 2-chloro-N6-(3-iodobenzyl)- adenosine-5'-N-methyluronamide > or = N6-(3-iodobenzyl)- adenosine-5'-N-methyluronamide >> N-methyl-1,3-di-n-butylxanthine-7-beta-D-ribofuronamide > or = 5'-N-ethylcarboxamidoadenosine > or = N6-2-(4-aminophenyl)-ethyladenosine > N6-(p-sulfophenyl)-adenosine = 1,3-dibutylxanthine-7- riboside, which is identical to the potency order in binding studies at cloned rat A3 receptors. Stimulation of phospholipase C activity was abolished by guanosine-5'-O-(2-thiodiphosphate), confirming the involvement of a G protein-coupled receptor. Activation of phospholipase C was higher in the striatum than in the hippocampus, consistent with A3 receptor densities. Stimulation of phospholipase C activity by adenosine analogues was only modestly antagonized by xanthine derivatives and at much higher concentrations than needed for blocking adenosine A1, A2A, and A2b receptors. In the presence of an A1/A2 antagonist, a selective A3 in rat striation. Thus, stimulation of phospholipase C activity agonist only weakly inhibited forskolin-stimulated adenylyl cyclase activity represents a principal transduction mechanism for A3 receptors in mammalian brain, and perhaps A3 receptor-mediated increases of inositol phosphates in the ischemic brain contribute to neurodegeneration by raising intracellular calcium levels.

Effects of ATP analogues and basic fibroblast growth factor on astroglial cell differentiation in primary cultures of rat striatum

We have used primary cultures of rat striatum to study the effects of ATP analogues on the elongation of astrocytic processes, a parameter of astroglial cell differentiation. Parallel studies were performed with basic fibroblast growth factor, a known regulator of astroglial cell function. After three days in culture, both the growth factor and alpha beta-methylene-ATP induced dramatic increases in the mean length of astrocytic processes/cell. For both agents, effects were dose-dependent. The effect of alpha beta-methylene-ATP was antagonized by the trypanoside suramin and mimicked by 2-methyl-thio-ATP, suggesting the involvement of a suramin-sensitive P2-purinoceptor. Neither an additive nor a synergistic effect between alpha beta-methylene-ATP and basic fibroblast growth factor on the elongation of processes was detected in cultures exposed to both agents. Indeed, an inhibition with respect to the effects induced by either agent alone was recorded, suggesting that the growth factor and the purine analogue can modulate astrocytic differentiation by activation of common intracellular pathways. It is concluded that, like basic fibroblast growth factor, ATP can promote the maturation of astrocytes towards a more differentiated phenotype characterized by the presence of longer astrocytic processes. These findings might have interesting implications for astroglial cell differentiation during brain development and for ischemia- and trauma-associated hypergliosis.

A novel action for adenosine: apoptosis of astroglial cells in rat brain primary cultures

2-chloro-adenosine induced apoptosis of astroglial cells in rat brain cultures, as shown by flow cytometry and morphological analysis. The adenosine analogue was far more potent than several previously characterized sugars (including 2-deoxy-D-ribose and D-ribose, the sugar moiety of 2-chloro-adenosine), which trigger apoptosis in a variety of cell-lines [8-10], suggesting that the effects of 2-chloro-adenosine are only partially dictated by its sugar moiety. Nevertheless, 2-chloro-adenosine and 2-deoxy-D-ribose attenuated each other's cell death when used in combination, suggesting the involvement of common intracellular mechanisms. It is suggested that 2-chloro-adenosine may induce apoptosis via a yet-to-be identified adenosine receptor, which may have intriguing implications for both nervous system development and brain response to trauma and ischemia.

The biphasic response of rat vesical smooth muscle to ATP

1. Adenosine-5'-triphosphate (ATP) is known to exert a variety of biological effects via the activation of either ionotropic P2x- or G-protein coupled P2Y-purinoceptor subtypes. In this study the effects induced by ATP and ATP analogues on rat bladder strips were characterized at resting tone and in carbachol-prestimulated tissues. 2. ATP exerted a clear concentration-dependent biphasic response, which was maximal at 1 mM concentration and was characterized by an immediate and transient contraction, followed by a slower sustained relaxation. The receptor mediating contraction was susceptible to desensitization by ATP and by the ATP analogue, alpha,beta-methyleneATP (alpha,beta-meATP) showing the typical features of the P2x-purinoceptor; conversely, ATP-evoked relaxation did not undergo tachyphylaxis following either ATP or alpha,beta-meATP. 3. The slower and sustained relaxant phase seemed to be due to activation of P2Y-purinoceptors, based on responses obtained with the P2Y agonist, 2-methyl-thioATP (2-meSATP) and, more importantly, based on the clear involvement of the G-proteins. In fact, the G-protein activator, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) significantly potentiated and the G-protein blocking agent, guanosine 5'-O-(2-thio-diphosphate) (GDP beta S) completely abolished the ATP-induced relaxation. No effects were exerted by these two G-protein modulators on the ATP-induced contraction. 4. The relaxant component of the ATP response of bladder tissue was not significantly influenced by nitro-benzyl-thioinosine (NBTI) or by 8-phenyltheophylline (8-PT), suggesting that the contribution of the ATP metabolite adenosine to this response was negligible. Moreover, relaxation evoked by ATP and by the adenosine analogue, 5'-N-ethylcarboxamidoadenosine (NECA) was additive.5. Suramin was unable to modify either the relaxant or the contractile responses of bladder strips to ATP. However, when tested on the concentration-response curve to the slowly hydrolysable P2x-agonist alpha,beta-meATP, a rightward shift was detected, suggesting that ATP contractile responses are mediated by suramine-sensitive P2x-purinoceptors.6. Uridine-5'-triphosphate (UTP) only induced a rapid and concentration-dependent contraction of the rat bladder preparation, which was not desensitized by pre-exposure to alpha,beta-meATP, suggesting that UTP responses were not mediated by the 'classical' P2X-purinoceptor.7. It is therefore concluded that both P2x- and P2y-purinoceptors, which mediate ATP-induced contraction and relaxation, respectively, are present in rat bladder. Moreover, removal of epithelium did not affect ATP-elicited contraction, whereas ATP-induced relaxation was significantly augmented. These data suggest that P2x- and P2Y- purinoceptors are localized in smooth muscle cells and that the relaxant response is probably modulated by excitatory factor(s) released by epithelial cells.

Modulation of astroglial cell proliferation by analogues of adenosine and ATP in primary cultures of rat striatum

We have studied the possible purinoceptor-mediated modulation of astroglial cell proliferation in neuron-glia primary cultures obtained from rat corpus striatum. Cultures were grown for three days in the presence of either 2-chloro-adenosine or alpha beta-methylene-ATP (which behave as agonists of adenosine/P1 and ATP/P2 purinoceptors, respectively), and then immunostained with an antibody to glial fibrillary acidic protein. 2-Chloro-adenosine decreased and alpha beta-methylene-ATP increased the number of astroglial cells in culture. For both derivatives, the effect was dose-dependent. The effect of alpha beta-methylene-ATP was antagonized by the trypanoside suramin, suggesting the involvement of a suramin-sensitive P2 purinoceptor, whereas the effect of 2-chloro-adenosine was not reversed by the P1 purinoceptor antagonist p-sulphonyl-phenyl-theophylline, implying the activation of a xanthine-insensitive adenosine purinoceptor subtype. In order to evaluate the extent of astrocyte proliferation in the presence of these two analogues, some cultures were incubated with bromodeoxyuridine for 24 h before fixing, and then double-immunostained for glial fibrillary acidic protein and bromodeoxyuridine. The percentage of bromodeoxyuridine positive astrocytes was significantly increased after exposure to both agents. It is therefore concluded that purines can modulate astroglial cells in opposite ways, inducing decreases or increases of cell number by activation of P1 and P2 purinoceptors, respectively. For the P2 purinoceptor-mediated effect, there was a quantitative correlation between the percentage of bromodeoxyuridine positive astrocytes and the cell number. For the P1 purinoceptor-mediated effect, no apparent correlation between these two parameters was found. This suggests the activation of independent effects, which involve other mechanisms besides the stimulation of DNA synthesis, and which eventually result in a reduction of cell number. The possible relevance of these findings to in vivo regulation of astrocyte cell function as well as in trauma- and ischaemia-associated hypergliosis is discussed.

Purinoceptors: are there families of P2X and P2Y purinoceptors?

There has been an exponential growth in interest in purinoceptors since the potent effects of purines were first reported in 1929 and purinoceptors defined in 1978. A distinction between P1 (adenosine) and P2 (ATP/ADP) purinoceptors was recognized at that time and later, A1 and A2, as well as P2x and P2y subclasses of P1 and P2 purinoceptors were also defined. However, in recent years, many new subclasses have been claimed, particularly for the receptors to nucleotides, including P2t, P2z, P2u(n) and P2D, and there is some confusion now about how to incorporate additional discoveries concerning the responses of different tissues to purines. The studies beginning to appear defining the molecular structure of P2-purinoceptor subtypes are clearly going to be important in resolving this problem, as well as the introduction of new compounds that can discriminate pharmacologically between subtypes. Thus, in this review, on the basis of this new data and after a detailed analysis of the literature, we propose that: (1) P2X(ligand-gated) and P2Y(G-protein-coupled) purinoceptor families are established; (2) four subclasses of P2X-purinoceptor can be identified (P2X1-P2X4) to date; (3) the variously named P2-purinoceptors that are G-protein-coupled should be incorporated into numbered subclasses of the P2Y family. Thus: P2Y1 represents the recently cloned P2Y receptor (clone 803) from chick brain; P2Y2 represents the recently cloned P2u (or P2n) receptor from neuroblastoma, human epithelial and rat heart cells; P2Y3 represents the recently cloned P2Y receptor (clone 103) from chick brain that resembles the former P2t receptor; P2Y4-P2Y6 represent subclasses based on agonist potencies of newly synthesised analogues; P2Y7 represents the former P2D receptor for dinucleotides. This new framework for P2 purinoceptors would be fully consistent with what is emerging for the receptors to other major transmitters, such as acetylcholine, gamma-aminobutyric acid, glutamate and serotonin, where two main receptor families have been recognised, one mediating fast receptor responses directly linked to an ion channel, the other mediating slower responses through G-proteins. We fully expect discussion on the numbering of the different receptor subtypes within the P2X and P2Y families, but believe that this new way of defining receptors for nucleotides, based on agonist potency order, transduction mechanisms and molecular structure, will give a more ordered and logical approach to accommodating new findings. Moreover, based on the extensive literature analysis that led to this proposal, we suggest that the development of selective antagonists for the different P2-purinoceptor subtypes is now highly desirable, particularly for therapeutic purposes.