Adenosine (P1) receptor signalling

Exogenous Adenosine Modulates Behaviors and Stress Response in Caenorhabditis elegans

Adenosine, a purine nucleoside with neuromodulatory actions, is part of the purinergic signaling system (PSS). Caenorhabditis elegans is a free-living nematode found in soil, used in biological research for its advantages as an alternative experimental model. Since there is a lack of evidence of adenosine's direct actions and the PSS's participation in this animal, such an investigation is necessary. In this research, we aimed to test the effects of acute and chronic adenosine at 1, 5, and 10 mM on nematode's behaviors, morphology, survival after stress conditions, and on pathways related to the response to oxidative stress (DAF-16/FOXO and SKN-1) and genes products downstream these pathways (SOD-3, HSP-16.2, and GCS-1). Acute or chronic adenosine did not alter the worms' morphology analyzed by the worms' length, width, and area, nor interfered with reproductive behavior. On the other hand, acute and chronic adenosine modulated the defecation rate, pharyngeal pumping rate, and locomotion, in addition, to interacting with stress response pathways in C. elegans. Adenosine interfered in the speed and mobility of the worms analyzed. In addition, both acute and chronic adenosine presented modulatory effects on oxidative stress response signaling. Acute adenosine prevented the heat-induced-increase of DAF-16 activation and SOD-3 levels, while chronic adenosine per se induced DAF-16 activation and prevented heat-induced-increase of HSP-16.2 and SKN-1 levels. Together, these results indicate that exogenous adenosine has physiological and biochemical effects on C. elegans and describes possible purinergic signaling in worms.

Role of interstitial ATP and adenosine in the regulation of renal hemodynamics and microvascular function

The role of adenosine in the regulation of renal hemodynamics and function has been studied extensively; however, another purine agent, ATP, is also gaining recognition for its paracrine role in the kidney. Adenosine and ATP bind to specific membrane-bound P1 and P2 purinoceptors, respectively, and initiate a variety of biological effects on renal microvascular tone, mesangial cell function, and renal epithelial transport. The purpose of this review is to summarize the potential roles of interstitial ATP and adenosine as regulators of renal hemodynamics and microcirculation. In vitro blood-perfused juxtamedullary nephron preparation was used to assess the roles of ATP and adenosine in the regulation of renal microvascular tone. This approach mimics the adventitial exposure of renal microvascular smooth muscle to ATP and adenosine synthesized locally and released into the interstitial fluid. ATP selectively vasoconstricts afferent but not efferent arterioles via P2X and P2Y receptors, whereas, adenosine vasoconstricts both vascular segments via activation of adenosine A(1) receptors. Furthermore, selective P2X and P2Y receptor stimulation increases intracellular calcium concentration in vascular smooth muscle cells that are freshly isolated from the preglomerular microvasculature. These data support the hypothesis that interstitial ATP plays a critical role in the control of renal microvascular function through mechanisms that are independent of adenosine receptors. We have recently developed a renal microdialysis method to determine the dynamics of ATP and adenosine levels in the renal cortical interstitium. In this review, we also summarize current knowledge pertaining to the alterations in renal interstitial ATP and adenosine in some pathophysiological conditions.

Adenosine A3 receptor-mediated regulation of p38 and extracellular-regulated kinase ERK1/2 via phosphatidylinositol-3′-kinase

The adenosine A(3) receptor generally couples to the G(i) class of heterotrimeric G proteins, thereby decreasing cAMP levels and also mediating signaling via release of betagamma subunits. Here we describe the central role of phosphatidylinositol-3'-kinase (PI3K) for adenosine A(3) receptor-induced intracellular signaling to the stress-activated protein kinase p38 and the extracellular signal-regulated protein kinases ERK1/2. We used Chinese hamster ovary cells expressing the human adenosine A(3) receptor, phospho-specific antibodies and different pharmacological tools to dissect the signaling pathways involving PI3K. The adenosine receptor agonist 5'N-ethylcarboxamidoadenosine induced a time- and dose-dependent increase in p38 and ERK1/2 phosphorylation, two signaling pathways that appeared also to be activated in the immortalized microglia cell line N13, which expressed endogenous adenosine A(3) receptors. The 5'N-ethylcarboxamidoadenosine-induced effects on p38 and ERK1/2 in CHO cells were blocked by pertussis toxin pretreatment and were sensitive to pharmacological inhibition of PI3K. In addition, inhibition of Rac/Cdc42, small GTPases of the Rho family, by clostridium toxin B, diminished p38 phosphorylation but did not affect ERK1/2. Furthermore, we identified the serine 727 site of signal transducer and activator of transcription STAT3 as a probable downstream target of ERK1/2, and thereby provide evidence that adenosine A(3) receptor mediated ERK1/2 activation has functional consequences.

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.

Adenosine A 2B receptors modulate cAMP levels and induce CREB but not ERK1/2 and p38 phosphorylation in rat skeletal muscle cells

The present study examined the existence of the adenosine A(1),A(2A), and A(2B) receptors and the effect of receptor activation on cAMP accumulation and protein phosphorylation in primary rat skeletal muscle cells. Presence of mRNA and protein for all three receptors was demonstrated in both cultured and adult rat skeletal muscle. NECA (10(-9)-10(-4)M) increased the cAMP concentration in cultured muscle cells with an EC(50) of (95% confidence interval)=15 (5.9-25.1) micro M, whereas CGS 21680 (10(-9)-10(-4)M) had no effect on cAMP accumulation. Concentrations of [R]-PIA below 10(-6)M had no effect on cAMP accumulation induced by either isoproterenol or forskolin. NECA resulted in phosphorylation of CREB with an EC(50) of (95% confidence interval)=1.7 (0.40-7.02) micro M, whereas ERK1/2 and p38 phosphorylation was unchanged. The results show that, although the A(1),A(2A), and A(2B) receptors are all present in skeletal muscle cells, the effect of adenosine on adenylyl cyclase activation and phosphorylation of CREB is mainly mediated via the adenosine A(2B) receptor.

Up-regulation of A(2A) adenosine receptors by proinflammatory cytokines in rat PC12 cells

The purpose of this study was to examine the regulation of A(2A) adenosine receptor (A(2A) AR) gene expression induced by proinflammatory cytokines in PC12 cells. The A(2A) AR mRNA levels were substantially increased following 3-48 hr PC12 cell treatment with interleukin 1 beta (500 unit/mL) or tumor necrosis factor alpha (1000 unit/mL), as revealed by RT-PCR analysis. In parallel, cell cytokine treatment induced an up-regulation of A(2A) receptor protein. Equilibrium radioligand binding studies on treated-cells showed a significant increase in maximum density of [3H] 2-(carboxyethylphenylethylamino) adenosine-5'-carboxamide binding sites, with no significant changes in the affinity constant value. The increase in A(2A) receptor density was also demonstrated by Western blot analysis. Interleukin 1 beta and tumor necrosis factor alpha effects on A(2A) AR mRNA and protein levels were detectable after 3 hr cytokine treatment and reached a maximum within 24 and 48 hr, respectively. These results demonstrated the existence of heterologous regulation of A(2A) ARs by proinflammatory cytokines. The biological significance of this regulation might be associated with modulating cellular activity in response to tissue damage associated with inflammatory mediator production.

P2 receptors in regulation of renal microvascular function

In the last 10-15 years, interest in the physiological role of P2 receptors has grown rapidly. Cellular, tissue, and organ responses to P2 receptor activation have been described in numerous in vivo and in vitro models. The purpose of this review is to provide an update of the recent advances made in determining the involvement of P2 receptors in the control of renal hemodynamics and the renal microcirculation. Special attention will be paid to work published in the last 5-6 years directed at understanding the role of P2 receptors in the physiological control of renal microvascular function. Several investigators have begun to evaluate the effects of P2 receptor activation on renal microvascular function across several species. In vivo and in vitro evidence consistently supports the hypothesis that P2 receptor activation by locally released extracellular nucleotides influences microvascular function. Extracellular nucleotides selectively influence preglomerular resistance without having an effect on postglomerular tone. P2 receptor inactivation blocks autoregulatory behavior whereas responsiveness to other vasoconstrictor agonists is retained. P2 receptor stimulation activates multiple intracellular signal transduction pathways in preglomerular smooth muscle cells and mesangial cells. Renal microvascular cells and mesangial cells express multiple subtypes of P2 receptors; however, the specific role each plays in regulating vascular and mesangial cell function remains unclear. Accordingly, the results of studies performed to date provide strong support for the hypothesis that P2 receptors are important contributors to the physiological regulation of renal microvascular and/or glomerular function.

Absence of the adenosine A(2A) receptor or its chronic blockade decrease ethanol withdrawal-induced seizures in mice

Considering the existing interactions between ethanol and adenosine, the influence of the genetic impairment of the adenosine A(2A) receptor has been examined upon the seizures occurring at the cessation of chronic ethanol intake or 'ethanol withdrawal' in male mice. Acute clearance of ethanol did not differ between adenosine A(2A) receptor knockout and wild-type mice. Mice were exposed for 10 days to a diet consisting of a milky chocolate drink that contained increasing concentrations (1.8, 3.6 and 6.3% v/v) of ethanol. Adenosine A(2A) receptor knockout mice ingested similar amounts of the fluid, either containing alcohol or not, as did the controls. The severity of handling-induced convulsions during withdrawal was significantly reduced in the adenosine A(2A) receptor knockout mice as compared with their wild-type controls. The selective adenosine A(2A) receptor antagonist ZM 241385 (20 mg/kg) also significantly attenuated the intensity of withdrawal-induced seizures occurring in wild-type male mice when intraperitoneally administered twice daily during the last 5 days of the forced alcohol intake. These results suggest that selective adenosine A(2A) receptor antagonists may be useful in the treatment of alcohol withdrawal.

Mechanism of adenosine-induced vasodilation in rat diaphragm microcirculation

The mechanism of adenosine-induced vasodilation in rat diaphragm microcirculation was investigated using laser Doppler flowmetry. Adenosine (10(-5), 3.2 x 10(-5), and 10(-4) M), the nonselective adenosine agonist 5'-N-ethylcarboxamido-adenosine (NECA) (10(-8)-10(-7) M), the specific A(2A) agonist 2-p-(2-carboxyethyl)phenyl-amino-5'-N-ethyl carboxamidoadenosine (CGS-21680) (10(-8)-10(-7) M), and the adenosine agonist with higher A(1)-receptor affinity, R-N(6)-phenylisopropyladenosine (R-PIA) (10(-7), 3.2 x 10(-7), and 10(-6) M) elicited a similar degree of incremental increase of microcirculatory flow in a dose-dependent manner. The ATP-dependent potassium (K(ATP)) channel blocker glibenclamide (3.2 x 10(-6) M) significantly attenuated the vasodilation effects of these agonists. Adenosine-induced vasodilation could be significantly attenuated by the nonselective adenosine antagonist 8-(p-sulfophenyl)-theophylline (3 x 10(-5) M) or the selective A(2A) antagonist 4-(2-[7-amino-2-(2-furyl)[1,2, 4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl) phenol (ZM-241385, 10(-6) M), but not by the selective A(1) antagonist 8-cyclopentyl-1, 3-dipropylxanthine (5 x 10(-8) M). Adenylate cyclase inhibitor N-(cis-2-phenyl-cyclopentyl) azacyclotridecan-2-imine-hydrochloride (MDL-12330A, 10(-5)M) effectively suppressed the vasodilator response of adenosine and forskolin. These results suggest that adenosine-induced vasodilation in rat diaphragm microcirculation is mediated through the stimulation of A(2A) receptors, which are coupled to adenylate cyclase activation and opening of the K(ATP) channel.