Adenosine stimulates 5'-nucleotidase activity in rat mesangial cells via A2 receptors

Regulation of bone and cartilage by adenosine signaling

There is growing recognition that bone serves important endocrine and immunologic functions that are compromised in several disease states. While many factors are known to affect bone metabolism, recent attention has focused on investigating the role of purinergic signaling in bone formation and regulation. Adenosine is a purine nucleoside produced intracellularly and extracellularly in response to stimuli such as hypoxia and inflammation, which then interacts with P1 receptors. Numerous studies have suggested that these receptors play a pivotal role in osteoblast, osteoclast, and chondrocyte differentiation and function. This review discusses the various ways by which adenosine signaling contributes to bone and cartilage homeostasis, while incorporating potential therapeutic applications of these signaling pathways.

The pathophysiology of retinopathy of prematurity: an update of previous and recent knowledge

Retinopathy of prematurity (ROP) is a disease that can cause blindness in very low birthweight infants. The incidence of ROP is closely correlated with the weight and the gestational age at birth. Despite current therapies, ROP continues to be a highly debilitating disease. Our advancing knowledge of the pathogenesis of ROP has encouraged investigations into new antivasculogenic therapies. The purpose of this article is to review the findings on the pathophysiological mechanisms that contribute to the transition between the first and second phases of ROP and to investigate new potential therapies. Oxygen has been well characterized for the key role that it plays in retinal neoangiogenesis. Low or high levels of pO2 regulate the normal or abnormal production of hypoxia-inducible factor 1 and vascular endothelial growth factors (VEGF), which are the predominant regulators of retinal angiogenesis. Although low oxygen saturation appears to reduce the risk of severe ROP when carefully controlled within the first few weeks of life, the optimal level of saturation still remains uncertain. IGF-1 and Epo are fundamentally required during both phases of ROP, as alterations in their protein levels can modulate disease progression. Therefore, rhIGF-1 and rhEpo were tested for their abilities to prevent the loss of vasculature during the first phase of ROP, whereas anti-VEGF drugs were tested during the second phase. At present, previous hypotheses concerning ROP should be amended with new pathogenetic theories. Studies on the role of genetic components, nitric oxide, adenosine, apelin and β-adrenergic receptor have revealed new possibilities for the treatment of ROP. The genetic hypothesis that single-nucleotide polymorphisms within the β-ARs play an active role in the pathogenesis of ROP suggests the concept of disease prevention using β-blockers. In conclusion, all factors that can mediate the progression from the avascular to the proliferative phase might have significant implications for the further understanding and treatment of ROP.

Changes in Atrium and Thoracic Aorta Reactivity to Adenosinergic and Adrenergic Agonists in Experimental Hyperhomocysteinemia

We prepared diet-induced hyperhomocysteinemia (hHcy) in adult male Wistar rats and investigated the effects of hHcy on the adenosinergic and adrenergic responses in vitro and in vivo. The responsiveness of right atria from hHcy rats to the negative chronotropic effects of adenosine (Ado) was found to be significantly greater in hHcy rats than in controls. The pD2 value and maximum effect of Ado were significantly increased in 12-week hHcy right atria when compared with those from age-matched controls. The vasodilatory effect of Ado on rat thoracic aorta was also increased in hHcy rats. In the presence of dipyridamole, an Ado uptake inhibitor, the negative chronotropic and vasodilatory effects of Ado were significantly potentiated in the hHcy rats much more than in the control rats. In anesthetized rats, Ado and dipyridamole, given as a rapid bolus into the femoral artery, led to reduction in mean blood pressure and heart rate. This effect was significantly pronounced in hHcy rats when compared with control animals. Otherwise, hHcy atria were found to have increased responsiveness to the positive chronotropic response to isoproterenol, an beta-adrenoceptor agonist. However, there were no significant differences between two groups in the vasoconstrictor effects to phenylephrine, an alpha-adrenoceptor agonist. On the basis of these results, we concluded that hHcy rats were significantly more sensitive to the negative chronotropic and vasorelaxant effects of Ado, possibly because of accelerated cellular Ado uptake and/or a change in Ado receptor-G protein system. This change may be related with the increased responsiveness to beta-adrenergic agonists in hHcy rats, and might contribute to the harmful cardiac effects of hHcy.

The role of adenosine in chondrocyte death in murine osteoarthritis and in a murine chondrocyte cell line

OBJECTIVE

To investigate the role of adenosine in chondrocyte death in murine osteoarthritis (OA).

METHODS

5'-Nucleotidase (5'NT) generates adenosine. Enzyme activity was measured histochemically in normal murine and osteoarthritic STR/ort strain tibial cartilage. Adenosine-mediated cell death was investigated in MC615 chondrocyte cultures. Adenosine receptors (ARs) were assessed by reverse transcriptase polymerase chain reaction (RT-PCR). Cellular uptake of [(3)H] adenosine was measured with or without the inhibitor, nitrobenzylthioinosine (NBTI). Cell death was assessed by cell counting and DNA laddering following selective receptor stimulation, or after modulating adenosine metabolism with adenosine deaminase (ADA) or adenosine kinase (AK) inhibitors [erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) and Iodotubericidin (Itub)], or with homocysteine (HC). Markers of apoptosis were assessed by Western blotting. Cell studies were validated by incubating normal murine knee joints in a medium containing adenosine and metabolic inhibitors. Apoptotic chondrocytes were identified with the TUNEL reaction.

RESULTS

5'NT activity in STR/ort tibial cartilage increased with development of OA, especially close to OA lesions. Adenosine induced MC615 cell death in the presence of ADA inhibition (100 microM EHNA), or 1mM HC, or both. Adenosine uptake, mediated by NBTI-sensitive adenosine transporters, was required for cell death. ARs were expressed (A2b>A2a>A1) but were not involved in mediating cell death. Cell death involved the activation of caspase-3 and DNA fragmentation and was prevented by inhibiting caspase activity. However, neither caspase-8 nor caspase-9 was detected. Adenosine+EHNA induced chondrocyte apoptosis in normal murine knee joints.

CONCLUSION

Increased adenosine production may induce chondrocyte apoptosis and play a role in OA in STR/ort mice.

Endothelial and Microvascular Function

The endothelium is more than just a passive vessel lining. New advances have revealed and expanded the multifactorial role of the endothelium in the homeostatic regulation of the microvasculature, including control of primary hemostasis, blood coagulation and fibrinolysis, platelet and leukocyte interactions with the vessel wall, lipoprotein metabolism, presentation of histocompatibility antigens, regulation of vascular tone and growth, and regulation of blood pressure. It possesses numerous receptors and releases compounds that affect the regulation of vascular tone and contribute to vascular permeability. Many crucial vasoactive endogenous compounds are formed in the endothelial cells to control the functions of vascular smooth muscle cells and circulating blood cells. Gap junctions facilitate the exchange of metabolites, ions, and other messenger molecules among endothelial cells and smooth muscle cells, and regulate cell growth. Among the numerous regulatory systems affecting microvascular function are the cholinergic and adrenergic (α1, α2, and β) systems. Flow-metabolism coupling is affected by a variety of signaling systems, including adenosine, oxygen, carbon dioxide, lactate, nitric oxide, and others. Agents such as the angiotensin system and endothelin, as well as others, play a role in autoregulation (maintenance of constant flow in the face of changing pressure). All of these are discussed in detail.

Evidence for coordinated induction and repression of ecto-5'-nucleotidase (CD73) and the A2a adenosine receptor in a human B cell line

In the human B cell line P493-6 two mitogenic signals, the Epstein-Barr virus nuclear antigen 2 (EBNA2) and myc, can be independently regulated by means of an estrogen receptor fusion construct or an inducible expression vector, respectively. Shut off of EBNA2, either in the presence or absence of myc, leads to a significant increase in enzymatic activity and surface expression of ecto-5'-nucleotidase (CD73) as well as an increased adenosine receptor response in cyclic AMP formation. Shut off of myc expression has a small additional positive effect on CD73 activity. Among the four different subtypes of adenosine receptors, the A2a receptor exclusively is subject to regulation in this system, which is substantiated by pharmacologic data (specific agonists and inhibitors), as well as on the mRNA level. With up-regulated CD73 and A2a, cells also respond to 5'-AMP with increased cyclic AMP formation. Turn on of EBNA2 has the reverse effect of repression of CD73 and A2a expression. The time course of both induction and repression of CD73 and A2a is rather slow.

Adenosine-induced apoptosis in glomerular mesangial cells

BACKGROUND

Mesangial cell apoptosis is a mechanism of resolution of glomerular hypercellularity in inflammatory forms of glomerular injury in which adenosine (ADO) was shown to play an anti-inflammatory role. This, and the observation that mesangial cell have ADO receptors prompted us to determine whether ADO induces mesangial cell apoptosis and to explore underlying mechanisms.

METHODS

Cultured mouse mesangial cell were incubated in the presence or absence of ADO or ADO receptor agonists (R-PIA, NECA, IB-MECA, CGS26180) or antagonists (DPCPX, DPSPX, MRS1191) for 48 hours. Cell death was assessed by trypan blue exclusion analysis. Apoptosis was assessed by DNA fragmentation, TUNEL staining and flow cytometry.

RESULTS

ADO and the A3 ADO receptor agonist IB-MECA induced mesangial cell death, which was markedly attenuated by the A3 receptor antagonist MRS1191. The A1 receptor agonist R-PIA, A2 receptor agonist NECA or the A2a receptor agonist CGS-12680 had no effect. The IB-MECA-induced mesangial cell death was due to apoptosis. This occurred via a cAMP independent mechanism. RT-PCR analysis revealed presence of A3, A1 and A2b but lack of A2a receptor transcripts in MC total RNA. Western blot analysis of mesangial cell lysates revealed expression the A3 receptor protein only.

CONCLUSION

The observations indicate that ADO induces mesangial cell apoptosis via stimulation of the A3 receptor.

Pharmacology of adenosine receptors in the vasculature

Adenosine is widely distributed in mammals. One of the primary roles of adenosine within the cardiovascular system is to directly control the functions of both cardiac and vascular tissues. Recently, there has been considerable interest in the subclassification of adenosine receptors. Characterization of a heterogeneous population of receptors for adenosine could provide an opportunity for the development of novel compounds of therapeutic value. Adenosine is released from cells as a result of metabolism, and its release can be increased dramatically from cells that are metabolically stressed. This implies that adenosine can be released from a variety of cells throughout the body, as a result of increased metabolic rates, in concentrations that can have a profound impact on blood vessel function and, consequently, blood flow. It is recognized that the actions of this nucleoside on the vasculature are most prominent when oxygen demand is high and there is a reduction in oxygen tension at the site in question. Therefore, it is not surprising that adenosine has been shown to be an important regulator of blood vessel tone under hypoxic conditions. Furthermore, the activation of adenosine receptors on blood vessels can result in relaxation and/or contractions. The nature of the response subsequent to the activation of adenosine receptors is primarily dependent on the type of blood vessel involved and basal tone. This review will focus on the characterization of subtypes of adenosine receptors in blood vessels, as well as the effect of the stimulation of adenosine receptors on the peripheral circulation.

Potentiation of cytokine induction of group IIA phospholipase A(2) in rat mesangial cells by ATP and adenosine via the A2A adenosine receptor

1. In rat mesangial cells extracellular nucleotides were found to increase arachidonic acid release by a cytosolic phospholipase A(2) through the P2Y(2) purinergic receptor. 2. In this study we investigated the effects of ATP and UTP on interleukin-1ss (IL-1ss)-induced mRNA expression and activity of group IIA phospholipase A(2) (sPLA(2)-IIA) in rat mesangial cells. 3. Treatment of cells for 24 h with extracellular ATP potentiated IL-1ss-stimulated sPLA(2)-IIA induction, whereas UTP had no effect. 4. We obtained the following evidence that the P2Y(2) receptor is not involved in the potentiation of sPLA(2)-IIA induction: (i) ATP-gamma-S had no enhancing effect; (ii) suramin, a P(2) receptor antagonist, did not inhibit ATP-mediated potentiation; (iii) inhibition of degradation of extracellular nucleotides by the 5'-ectonucleotidase inhibitor AOPCP did not enhance sPLA(2)-IIA induction and (iv) adenosine deaminase treatment completely abolished the ATP-mediated potentiation of sPLA(2)-IIA induction. 5. In contrast, treatment of mesangial cells with adenosine or the A2A receptor agonist CGS 21680 mimicked the effects of ATP in enhancing IL-1ss-stimulated sPLA(2)-IIA induction, whereas the specific A2A receptor antagonist ZM 241385 completely abolished the potentiating effect of ATP or adenosine. 6. The protein kinase A inhibitor Rp-8-Br-cyclic AMPS dose-dependently inhibited the enhancing effect of ATP or adenosine indicating the participation of an adenosine receptor-mediated cyclic AMP-dependent signalling pathway. 7. These data indicate that ATP mediates proinflammatory long-term effects in rat mesangial cells via its degradation product adenosine through the A2A receptor resulting in potentiation of sPLA(2)-IIA induction.

Regulation of proliferation of LLC-MK2 cells by nucleosides and nucleotides: the role of ecto-enzymes

1. Using the incorporation of [methyl-3H]thymidine as a proliferation marker, the effects of various nucleosides and nucleotides on endothelial LLC-MK2 cells were studied. We found that ATP, ADP, AMP and adenosine in concentrations of 10 microM or higher stimulate the proliferation of these cells. 2. Inhibition of ecto-ATPase (EC 3.6.1.15), 5'-nucleotidase (EC 3.1.3.5) or alkaline phosphatase (EC 3.1.3.1) significantly diminished the stimulatory effect of ATP, indicating that the effect is primarily caused by adenosine and not by adenine nucleotides. Also, the effect depends only on extracellular nucleosides, since inhibition of nucleoside uptake by dipyridamole has no influence on proliferation. 3. Other purine nucleotides and nucleosides (ITP, GTP, inosine and guanosine) also stimulate cell proliferation, while pyrimidine nucleotides and nucleosides (CTP, UTP, cytidine and uridine) inhibit proliferation. Furthermore, the simultaneous presence of adenosine and any of the other purine nucleosides is not entirely additive in its effect on cell proliferation. At the same time any pyrimidine nucleoside, when added together with adenosine, has the same inhibitory effect as the pyrimidine nucleoside alone. 4. Apparently these proliferative effects are neither caused by any pharmacologically known P1-purinoceptor, nor are they mediated by cyclic AMP, cyclic GMP, or D-myo-inositol 1,4,5-trisphosphate as second messenger, nor by extracellular Ca2+. 5. Therefore, we conclude that various purine and pyrimidine nucleosides can influence the proliferation of LLC-MK2 cells by acting on putative purinergic and pyrimidinergic receptors not previously described.

A2 adenosine receptors in human glomerular mesangial cells

A2 adenosine receptors were characterized in human glomerular mesangial cells using [3H]5'-N-ethylcarboxamidoadenosine (NECA) as a tracer. There was a single group of receptor sites with a KD of 184 nM, and a number of sites of 317 fmol/mg of cell protein. Adenosine agonists increased 5'-nucleotidase activity via A2 receptor stimulation. The specific A2 agonist-NECA, at 0.1 and 1 micron, was a potent inhibitor of DNA synthesis.

Effect of dipyridamole on glomerular mesangial cell ecto-5'-nucleotidase expression

Although dipyridamole has been extensively studied as an anti-aggregating agent, its mechanism of action has not been elucidated. Cultured mesangial cells were treated with dipyridamole 1-100 microM from 6-72 h. Ecto-5'-nucleotidase activity approximately doubled (from 115 +/- 11 to 226 +/- 14 nmol/min/mg) after treatment with 100 microM dipyridamole for 72 h. This effect was concentration- and time-dependent. Cycloheximide, an inhibitor of protein synthesis, did not alter basal 5'-nucleotidase activity. However, it prevented stimulation by 5 microM dipyridamole. Adenosine availability at the receptor sites was increased by dipyridamole and S-(p-nitrobenzyl)-6-thioinosine (NBTI), which inhibit adenosine uptake into the cell. Addition of dipyridamole or NBTI to the adenosine-treated mesangial cells produced an additive increase in ecto-5'-nucleotidase activity. Dipyridamole, through its effect on extracellular adenosine and ecto-5'-nucleotidase, may have an influence upon regulation of the glomerular microcirculation.

Distribution of ecto-5'-nucleotidase in the rat liver: effect of anaemia

In the kidney a striking parallel exists between the expression of ecto-5'-nucleotidase and of erythropoietin by renal fibroblasts. It was therefore hypothesized that the expression of ecto-5'-nucleotidase in fibroblasts might be controlled by oxygen tension. In order to test this hypothesis, we examined the distribution of the enzyme in a tissue which displays a defined zonation in respect to oxygen tension, namely in the liver; anaemia was used in order to exaggerate this zonation. The distribution of ecto-5'-nucleotidase was investigated by light and electron microscopy using enzyme and immunohistochemical methods in the livers of healthy and of anaemic rats. Anaemia was produced by haemolysis combined with X-ray irradiation. The enzyme was detected in the bile canaliculi, in the connective tissue of the portal triads and of the central veins, and in fat-storing cells probably corresponding to a special form of fibroblasts. In healthy animals the perisinusoidal ecto-5'-nucleotidase activity was slightly higher in the pericentral than in the periportal area of the acinus whereas the inverse was observed for the staining of bile canaliculi. Anaemia provoked an increase of ecto-5'-nucleotidase in fat-storing cells in the pericentral zone of the acinus and in fibroblasts around the central veins, resulting in steepended gradients along the sinusoids. The intralobular gradient of ecto-5'-nucleotidase in perisinusoidal cells and the effect thereon of anaemia suggest that the expression of the ecto-5'-nucleotidase might be directly or indirectly controlled by local oxygen tension.

Effect of dopamine on ecto-5'-nucleotidase expression in human glomerular mesangial cells

Ecto-5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) of mesangial cells may be the main source of adenosine within the glomerulus, and thus essential in the regulation of glomerular microcirculation. c-AMP and c-AMP-stimulating agents were found to induce ecto-5'-nucleotidase of mesangial cells. Dopamine is a catecholamine known to increase c-AMP levels in mesangial cells. We have studied the effect of dopamine on ecto-5'-nucleotidase expression and DNA synthesis of glomerular mesangial cells in culture. Human mesangial cells were exposed to dopamine in the concentration range from 0.1 microM- to 1 mM, for 6-72 h. Ecto-5'-nucleotidase activity of human mesangial cells increased from 118.6 +/- 7.7 to 171 +/- 12 nmol/min/mg in a 72 h culture. This effect was time- and dose-dependent. Cycloheximide, an inhibitor of protein synthesis did not modify basal 5'-nucleotidase activity but it suppressed the stimulatory effect of 10 microM dopamine. DNA synthesis of human mesangial cells, studied after exposure of these cells to the same concentrations of dopamine used in the 5'-nucleotidase stimulation, was inhibited, being also dose dependent. These results indicate that dopamine induces ecto-5'-nucleotidase and inhibits DNA synthesis of cultured human mesangial cells. This action of dopamine on glomerular mesangial cells may be important in the regulation of glomerular hemodynamics.