Stimulation of human umbilical vein endothelial cell proliferation by A2-adenosine and beta 2-adrenoceptors

Effects of Dual Purinoceptor-dependent Approach on Release of Vascular Endothelial Growth Factor From Human Microvascular Endothelial Cell (HMEC-1) and Endothelial Cell Condition

In the recent years, the awareness of the role purinergic signaling plays as a therapeutic target has increased considerably. The purinoceptor allows the action of extracellular nucleotides (P2 receptors) and intermediary products of their metabolism, such as adenosine (P1 receptors), regulating pivotal processes occurring in the cardiovascular system. This study focuses on a dual purinoreceptor-dependent approach, based on the activation of adenosine P1 receptors with the simultaneous inhibition of P2Y12 receptors that can be used as novel platelet inhibitors in antithrombotic therapy. Endothelial cells are directly exposed to the drugs circulating in the bloodstream. That is why effects of our concept on human microvascular endothelial cells (HMEC-1) were examined in in vitro studies, such as enzyme-linked immunosorbent assay and scratch assays. In response to adenosine receptor agonists, levels of secreted vascular endothelial growth factor varied. Two of them, 5'-N-ethylcarboxamidoadenosine and MRE0094 remarkably increased vascular endothelial growth factor release. The elevated levels were reduced when used together with the P2Y12 receptor antagonist. Also, rates of wound closure in a scratch assay were significantly reduced in these cases. The results suggest that the proposed treatment does not impair endothelial cell condition. In addition, it is suggested as a collateral benefit, namely solving the problem of excessive activation of endothelial cells during antiplatelet therapy.

Treprostinil reduces endothelial damage in murine sinusoidal obstruction syndrome

Abstract

Sinusoidal obstruction syndrome (SOS) is a major complication after hematopoietic stem cell transplantation and belongs to a group of diseases increasingly identified as transplant-related systemic endothelial disease. Administration of defibrotide affords some protection against SOS, but the effect is modest. Hence, there is unmet medical need justifying the preclinical search for alternative approaches. Prostaglandins exert protective actions on endothelial cells of various vascular beds. Here, we explored the therapeutic potential of the prostacyclin analog treprostinil to prevent SOS. Treprostinil acts via stimulation of IP, EP₂, and EP₄ receptors, which we detected in murine liver sinusoidal endothelial cells (LSECs). Busulfan-induced cell death was reduced when pretreated with treprostinil in vitro. In a murine in vivo model of SOS, concomitantly administered treprostinil caused lower liver weight-to-body weight ratios indicating liver protection. Histopathological changes were scored to assess damage to liver sinusoidal endothelial cells, to hepatocytes, and to the incipient fibrotic reaction. Treprostinil indeed reduced sinusoidal endothelial cell injury, but this did not translate into reduced liver cell necrosis or fibrosis. In summary, our observations provide evidence for a beneficial effect of treprostinil on damage to LSECs but unexpectedly treprostinil was revealed as a double-edged sword in SOS.

Key messages

Murine liver sinusoidal endothelial cells (LSECs) express prostanoid receptors.

Treprostinil reduces busulfan-induced cell death in vitro.

Treprostinil lowers liver weight-to-body weight ratios in mice.

Treprostinil positively affects LSECs in mice but not hepatic necrosis/fibrosis.

Dysregulated post-transcriptional control of COX-2 gene expression in gestational diabetic endothelial cells

BACKGROUND AND PURPOSE

Hyperglycaemic memory describes the progression of diabetic complications during subsequent periods of improved glycaemia. We addressed the hypothesis that transient hyperglycaemia causes aberrant COX-2 expression in HUVEC in response to IL-1β through the induction of long-lasting epigenetic changes involving microRNA-16 (miR-16), a post-transcriptional modulator of COX-2 expression.

EXPERIMENTAL APPROACH

Studies were performed on HUVEC collected from women with gestational diabetes mellitus (GDM) (dHUVEC) and normal women (nHUVEC).

KEY RESULTS

In dHUVEC treated with IL-1β, the expression of COX-2 mRNA and protein was enhanced and generation of prostanoids increased (the most abundant was the promitogenic PGF_2α ). COX-2 mRNA was more stable in dHUVEC and this was associated with miR-16 down-regulation and c-Myc induction (a suppressor of miR expression). dHUVEC showed increased proliferation in response to IL-1β, which was prevented by a COX-2 inhibitor and PGF_2α receptor antagonist. Comparable changes in COX-2 mRNA, miR-16 and c-Myc detected in dHUVEC were produced in nHUVEC exposed to transient high glucose and then stimulated with IL-1β under physiological glucose levels; superoxide anion production was enhanced under these experimental conditions.

CONCLUSIONS AND IMPLICATIONS

Our results describe a possible mechanism operating in GDM that links the enhanced superoxide anion production and epigenetic changes, associated with hyperglycaemic memory, to endothelial dysfunction through dysregulated post-transcriptional control of COX-2 gene expression in response to inflammatory stimuli. The association of conventional therapy for glycaemic control with agents affecting inflammatory responses and oxidative stress might lead to a more effective prevention of the complications associated with GDM.

Purinergic transmission in blood vessels

There are nineteen different receptor proteins for adenosine, adenine and uridine nucleotides, and nucleotide sugars, belonging to three families of G protein-coupled adenosine and P2Y receptors, and ionotropic P2X receptors. The majority are functionally expressed in blood vessels, as purinergic receptors in perivascular nerves, smooth muscle and endothelial cells, and roles in regulation of vascular contractility, immune function and growth have been identified. The endogenous ligands for purine receptors, ATP, ADP, UTP, UDP and adenosine, can be released from different cell types within the vasculature, as well as from circulating blood cells, including erythrocytes and platelets. Many purine receptors can be activated by two or more of the endogenous ligands. Further complexity arises because of interconversion between ligands, notably adenosine formation from the metabolism of ATP, leading to complex integrated responses through activation of different subtypes of purine receptors. The enzymes responsible for this conversion, ectonucleotidases, are present on the surface of smooth muscle and endothelial cells, and may be coreleased with neurotransmitters from nerves. What selectivity there is for the actions of purines/pyrimidines comes from differential expression of their receptors within the vasculature. P2X1 receptors mediate the vasocontractile actions of ATP released as a neurotransmitter with noradrenaline (NA) from sympathetic perivascular nerves, and are located on the vascular smooth muscle adjacent to the nerve varicosities, the sites of neurotransmitter release. The relative contribution of ATP and NA as functional cotransmitters varies with species, type and size of blood vessel, neuronal firing pattern, the tone/pressure of the blood vessel, and in ageing and disease. ATP is also a neurotransmitter in non-adrenergic non-cholinergic perivascular nerves and mediates vasorelaxation via smooth muscle P2Y-like receptors. ATP and adenosine can act as neuromodulators, with the most robust evidence being for prejunctional inhibition of neurotransmission via A1 adenosine receptors, but also prejunctional excitation and inhibition of neurotransmission via P2X and P2Y receptors, respectively. P2Y2, P2Y4 and P2Y6 receptors expressed on the vascular smooth muscle are coupled to vasocontraction, and may have a role in pathophysiological conditions, when purines are released from damaged cells, or when there is damage to the protective barrier that is the endothelium. Adenosine is released during hypoxia to increase blood flow via vasodilator A2A and A2B receptors expressed on the endothelium and smooth muscle. ATP is released from endothelial cells during hypoxia and shear stress and can act at P2Y and P2X4 receptors expressed on the endothelium to increase local blood flow. Activation of endothelial purine receptors leads to the release of nitric oxide, hyperpolarising factors and prostacyclin, which inhibits platelet aggregation and thus ensures patent blood flow. Vascular purine receptors also regulate endothelial and smooth muscle growth, and inflammation, and thus are involved in the underlying processes of a number of cardiovascular diseases.

Effects of isoprenaline on endothelial connexins and angiogenesis in a human endothelial cell culture system

Downregulation of endothelial connexins has been shown to result in impaired angiogenesis. Isoprenaline is known to upregulate Cx43 in cardiomyocytes. Effects of isoprenaline on endothelial connexins are unknown. We wanted to investigate whether isoprenaline might induce upregulation of connexins Cx37, Cx40, or Cx43 in human endothelial cells and whether it may promote angiogenesis. Human umbilical vein endothelial cells (HUVECs) were cultured until confluence (5 days) and subsequently seeded in Matrigel in vitro angiogenesis assays for 18 h. During the entire cell culture and angiogenesis period, cells were treated with vehicle or isoprenaline (100 nM). Finally, the resulting angiogenetic network was investigated (immuno)histologically. Moreover, expression of Cx37, Cx40, and Cx43 was determined by Western blot. In addition, we measured functional intercellular gap junction coupling by dye injection using patch clamp technique. Isoprenaline resulted in significantly enhanced expression of endothelial Cx43 and to a lower degree of Cx40 and Cx37. The number of coupling cells was significantly increased. Regarding angiogenesis, we observed significantly enhanced formation of branches and a higher complexity of the tube networks with more branches/length. Isoprenaline increases endothelial connexin expression and intercellular coupling and promotes tube formation.

A two-state model for the diffusion of the A2A adenosine receptor in hippocampal neurons: agonist-induced switch to slow mobility is modified by synapse-associated protein 102 (SAP102)

The A_2A receptor is a class A/rhodopsin-like G protein-coupled receptor. Coupling to its cognate protein, G_s, occurs via restricted collision coupling and is contingent on the presence of cholesterol. Agonist activation slows diffusion of the A_2A adenosine receptor in the lipid bilayer. We explored the contribution of the hydrophobic core and of the extended C terminus by examining diffusion of quantum dot-labeled receptor variants in dissociated hippocampal neurons. Single particle tracking of the A_2A receptor(1–311), which lacks the last 101 residues, revealed that agonist-induced confinement was abolished and that the agonist-induced decrease in diffusivity was reduced substantially. A fragment comprising the SH3 domain and the guanylate kinase domain of synapse-associated protein 102 (SAP102) was identified as a candidate interactor that bound to the A_2A receptor C terminus. Complex formation between the A_2A receptor and SAP102 was verified by coimmunoprecipitation and by tracking its impact on receptor diffusion. An analysis of all trajectories by a hidden Markov model was consistent with two diffusion states where agonist activation reduced the transition between the two states and, thus, promoted the accumulation of the A_2A receptor in the compartment with slow mobility. Overexpression of SAP102 precluded the access of the A_2A receptor to a compartment with restricted mobility. In contrast, a mutated A_2A receptor (with ³⁸³DVELL³⁸⁷ replaced by RVRAA) was insensitive to the action of SAP102. These observations show that the hydrophobic core per se does not fully account for the agonist-promoted change in mobility of the A_2A receptor. The extended carboxyl terminus allows for regulatory input by scaffolding molecules such as SAP102.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Differential effect of adenosine receptors on growth of human colon cancer HCT 116 and HT-29 cell lines

The study aimed to evaluate the impact of adenosine receptors (ARs) on human colon tumor cells (HCT 116, HT-29) growth and sensitivity to 5-Fluorouracil (5-FU) an anticancer chemotherapeutic drug. The exposure of cancer cells to a selective A(3)-AR agonist (IB-MECA) resulted in an increase in HT-29 cells number, whereas the number of HCT 116 cells decreased significantly. In the presence of IB-MECA (1 μM) the percentage of apoptotic HT-29 cells significantly decreased, whereas the number of apoptotic and necrotic HCT 116 cells increased by 3- and 2,5-fold, respectively. The application of a selective A(2A)-AR agonist resulted in the increased survival of HCT 116 cells, but not HT-29 cells. The blockade of A(2A)-AR with ZM 241385 (0.1 μM) significantly increased the cytotoxicity of 5-FU (1 μM) in HCT 116 cells but not in HT-29 cells. The suppression of A(3)-AR with MRS 1523 (1 μM) increased the sensitivity of HT-29 cells to 5-FU while response of HCT 116 cells to 5-FU decreased. The growth promoting effect of IB-MECA in HT-29 cells was associated with the decreased intracellular cAMP level, whereas IB-MECA growth inhibitory effect in HCT 116 cells was abolished by okadaic acid (2 nM) indicating the involvement of protein phosphatase PP2A.

Reengineering the collision coupling and diffusion mode of the A2A-adenosine receptor: palmitoylation in helix 8 relieves confinement

The A(2A)-adenosine receptor undergoes restricted collision coupling with its cognate G protein G(s) and lacks a palmitoylation site at the end of helix 8 in its intracellular C terminus. We explored the hypothesis that there was a causal link between the absence of a palmitoyl moiety and restricted collision coupling by introducing a palmitoylation site. The resulting mutant A(2A)-R309C receptor underwent palmitoylation as verified by both mass spectrometry and metabolic labeling. In contrast to the wild type A(2A) receptor, the concentration-response curve for agonist-induced cAMP accumulation was shifted to the left with increasing expression levels of A(2A)-R309C receptor, an observation consistent with collision coupling. Single particle tracking of quantum dot-labeled receptors confirmed that wild type and mutant A(2A) receptor differed in diffusivity and diffusion mode; agonist activation resulted in a decline in mean square displacement of both receptors, but the drop was substantially more pronounced for the wild type receptor. In addition, in the agonist-bound state, the wild type receptor was frequently subject to confinement events (estimated radius 110 nm). These were rarely seen with the palmitoylated A(2A)-R309C receptor, the preferred diffusion mode of which was a random walk in both the basal and the agonist-activated state. Taken together, the observations link restricted collision coupling to diffusion limits imposed by the absence of a palmitoyl moiety in the C terminus of the A(2A) receptor. The experiments allowed for visualizing local confinement of an agonist-activated G protein-coupled receptor in an area consistent with the dimensions of a lipid raft.

A novel role for an endothelial adrenergic receptor system in mediating catecholestradiol-induced proliferation of uterine artery endothelial cells

Sequential conversion of estradiol-17β to its biologically active catecholestradiols, 2-hydroxyestradiol (OHE(2)) and 4-OHE(2), contributes importantly to its angiogenic effects on uterine artery endothelial cells (UAECs) derived from pregnant, but not nonpregnant ewes via an estrogen receptor-independent mechanism. Because catecholestradiols and catecholamines exhibit structural similarities and have high affinity for α- and β-adrenergic receptors (ARs), we investigated whether the endothelial α- or β-ARs mediate catecholestradiol-induced proliferation of P-UAECs and whether catecholamines alter these responses. Western analyses revealed expression of specific AR subtypes in nonpregnant UAECs and P-UAECs, including α(2)-, β(2)-, and β(3)-ARs but not α(1)- and β(1)-ARs. Levels of β(2)-ARs and β(3)-ARs were unaltered by pregnancy, whereas α(2)-ARs were decreased. Norepinephrine and epinephrine increased P-UAEC, but not nonpregnant UAEC proliferation, and these effects were suppressed by propranolol (β-AR blocker) but not phentolamine (α-AR blocker). Catecholamines combinations with 2-OHE(2) or 4-OHE(2) enhanced P-UAEC mitogenesis. Catecholestradiol-induced P-UAEC proliferation was also inhibited by propranolol but not phentolamine. β(2)-AR and β(3)-AR antagonists (ICI 118 551and SR 59230A, respectively) abrogated the mitogenic effects of both 2-OHE(2) and 4-OHE(2). Stimulation of β(2)-ARs and β(3)-ARs using formoterol and BRL 37344 dose-dependently stimulated P-UAEC proliferation, which was abrogated by ICI 118 551 and SR 59230A, respectively. Proliferation effects of both catecholamines and catecholestradiols were only observed in P-UAECs (not nonpregnant UAECs) and were mediated via β(2)-ARs and β(3)-ARs. We demonstrate for the first time convergence of the endothelial AR and estrogenic systems in regulating endothelial proliferation, thus providing a distinct evolutionary advantage for modulating uterine perfusion during stressful pregnancies.

Up-regulation of vascular endothelial growth factor expression by adenosine through adenosine A2 receptors in the rat tongue treated with endotoxin

The main focus of the present investigation is to evaluate a differential effect of adenosine on the up-regulation of vascular endothelial growth factor (VEGF) expression through adenosine A(2) receptors in the rat tongue treated with endotoxin (lipopolysaccharide: LPS). Angiogenesis in the rat tongue treated with LPS/incomplete Freund's adjuvant (IFA) or endotoxin/IFA/adenosine A(2) receptor (A(2)R) antagonists was examined using immunohistochemistry for LYVE-1, ED1, ED2, OX6, langerin and VEGF, and real-time polymerase chain reaction (PCR) for VEGF. The distributional density of both blood vessels and OX6(+) cells was significantly increased at day 8 after injection of LPS/IFA. The immunoreactive products of VEGF were intensely labelled in the cytoplasm of various antigen presenting cells (APCs) including dendritic cells (DCs) with double-immunofluorescence technique. Increase in VEGF mRNA expression level, the occupancy ratio of blood vessels, and the number of ED1(+), ED2(+), OX6(+), and langerin(+) cells was inhibited in the injured tongue of rats as a consequence of the treatment with A(2)R antagonists. The present results indicate that the LPS-induced adenosine might promote angiogenesis by the up-regulation of VEGF expression in macrophages/DCs through A(2) receptors. This suggests that the synergistic interaction between toll-like receptor (TLR) and A(2) receptor signalling observed in vivo plays an important role in oral mucosal wound healing.

Human placental adenosine receptor expression is elevated in preeclampsia and hypoxia increases expression of the A2A receptor

Placental hypoxia as a result of impaired trophoblast invasion is suggested to be involved in the pathophysiology of preeclampsia. Hypoxia is a potent stimulus for the release of adenosine, and the actions of adenosine are mediated through four adenosine receptors, A(1), A(2A), A(2B) and A(3). We investigated the presence, distribution and expression of adenosine receptor subtypes in the human placenta, the expression of the adenosine receptors in placentas from pregnancies complicated by preeclampsia, small for gestational age (SGA) infants and uncomplicated pregnancies, and the effect of hypoxia on placental adenosine receptor expression. Immunofluorescent microscopy localized A(1), A(2A), A(2B) and A(3) adenosine receptors to the syncytiotrophoblast, endothelial cells and myofibroblasts within the human placenta. Adenosine receptor protein and message expression levels were significantly higher in placentas from preeclamptic pregnancies with or without SGA infants, but not different in pregnancies with SGA infants alone. In vitro exposure of placental villous explants to hypoxia (2% oxygen) increased the expression of A(2A) adenosine receptor 50%. These data indicate that all four known adenosine receptors are expressed in the human placenta and adenosine receptor expression is significantly higher in pregnancies complicated by preeclampsia. These data are consistent with the hypothesis that differences in placental adenosine receptors may contribute to alterations in placental function in preeclampsia.

Adenosine receptors in wound healing, fibrosis and angiogenesis

Wound healing and tissue repair are critical processes, and adenosine, released from injured or ischemic tissues, plays an important role in promoting wound healing and tissue repair. Recent studies in genetically manipulated mice demonstrate that adenosine receptors are required for appropriate granulation tissue formation and in adequate wound healing. A(2A) and A(2B) adenosine receptors stimulate both of the critical functions in granulation tissue formation (i.e., new matrix production and angiogenesis), and the A(1) adenosine receptor (AR) may also contribute to new vessel formation. The effects of adenosine acting on these receptors is both direct and indirect, as AR activation suppresses antiangiogenic factor production by endothelial cells, promotes endothelial cell proliferation, and stimulates angiogenic factor production by endothelial cells and other cells present in the wound. Similarly, adenosine, acting at its receptors, stimulates collagen matrix formation directly. Like many other biological processes, AR-mediated promotion of tissue repair is critical for appropriate wound healing but may also contribute to pathogenic processes. Excessive tissue repair can lead to problems such as scarring and organ fibrosis and adenosine, and its receptors play a role in pathologic fibrosis as well. Here we review the evidence for the involvement of adenosine and its receptors in wound healing, tissue repair and fibrosis.

Adenosine A2A receptors play an active role in mouse bone marrow-derived mesenchymal stem cell development

Bone marrow-derived mesenchymal stem cells (BM-MSCs) play a role in wound healing and tissue repair and may also be useful for organ regeneration. As we have demonstrated previously that A(2A) adenosine receptors (A(2A)R) promote tissue repair and wound healing by stimulating local repair mechanisms and enhancing accumulation of endothelial progenitor cells, we investigated whether A(2A)R activation modulates BM-MSC proliferation and differentiation. BM-MSCs were isolated and cultured from A(2A)-deficient and ecto-5'nucleotidase (CD73)-deficient female mice; the MSCs were identified and quantified by a CFU-fibroblast (CFU-F) assay. Procollagen alpha2 type I expression was determined by Western blotting and immunocytochemistry. MSC-specific markers were examined in primary cells and third-passage cells by cytofluorography. PCR and real time-PCR were used to quantitate adenosine receptor and CD73 expression. There were significantly fewer CFU-Fs in cultures of BM-MSCs from A(2A)R knockout (KO) mice or BM-MSCs treated with the A(2A)R antagonist ZM241385, 1 microM. Similarly, there were significantly fewer procollagen alpha2 type I-positive MSCs in cultures from A(2A)R KO and antagonist-treated cultures as well. In late passage cells, there were significantly fewer MSCs from A(2A) KO mice expressing CD90, CD105, and procollagen type I (P<0.05 for all; n=3). These findings indicate that adenosine and adenosine A(2A)R play a critical role in promoting the proliferation and differentiation of mouse BM-MSCs.

The A(2A)-adenosine receptor: a GPCR with unique features?

The A(2A)-adenosine receptor is a prototypical G(s)-coupled receptor. However, the A(2A)-receptor has several structural and functional characteristics that make it unique. In contrast to the classical model of collision coupling described for the beta-adrenergic receptors, the A(2A)-receptor couples to adenylyl cyclase by restricted collision coupling and forms a tight complex with G(s). The mechanistic basis for this is not clear; restricted collision coupling may arise from the interaction of the receptor with additional proteins or due to the fact that G protein-coupling is confined to specialized membrane microdomains. The A(2A)-receptor has a long C-terminus (of >120 residues), which is for the most part dispensable for coupling to G(s). It was originally viewed as the docking site for kinases and the beta-arrestin family to initiate receptor desensitization and endocytosis. The A(2A)-receptor is, however, fairly resistant to agonist-induced internalization. Recently, the C-terminus has also been appreciated as a binding site for several additional 'accessory' proteins. Established interaction partners include alpha-actinin, ARNO, USP4 and translin-associated protein-X. In addition, the A(2A)-receptor has also been reported to form a heteromeric complex with the D(2)-dopamine receptor and the metabotropic glutamate receptor-5. It is clear that (i) this list cannot be exhaustive and (ii) that all these proteins cannot bind simultaneously to the receptor. There must be rules of engagement, which allow the receptor to elicit different biological responses, which depend on the cellular context and the nature of the concomitant signal(s). Thus, the receptor may function as a coincidence detector and a signal integrator.

Hydrogen peroxide-induced oxidative stress in MC3T3-E1 cells: The effects of glutamate and protection by purines

Glutamate has toxic effects on a number of tissues, partly by inducing toxic (e.g., oxidative) stress, whereas adenosine can be protective. Since there is evidence that glutamate and adenosine receptors are present in bone, we set out to study whether oxidative stress, induced by hydrogen peroxide (H2O2), affected viability in the MC3T3-E1 osteoblast-like cell line and whether treatment with adenosine receptor ligands attenuated this. Hydrogen peroxide (100 microM to 5 mM) reduced the viability of the MC3T3-E1 cells, while catalase reversed this cell loss and itself had some mitogenic effect. Superoxide dismutase (SOD) increased the number of viable cells alone but failed to modify significantly the effect of H2O2 treatments. Glutamate (100 microM, 1 mM) and NMDA (10 microM), applied alone for up to 1 h, had a mitogenic effect (P < 0.05). Adenosine A1 and A2A receptor agonists and antagonists at low and high concentrations showed some mitogenic effects when added singly, but only high concentrations of the agonists showed significant protection against cell death resulting from H2O2 treatments. Contributions from both apoptotic and necrotic pathways were implicated in the H2O2-induced cell loss as was demonstrated by the use of the caspase-3 inhibitor (Z-DEVD-fmk) and the PARP-1 inhibitor (DPQ). The results demonstrate that hydrogen peroxide was toxic to MC3T3-E1 cells, whereas glutamate was not and may even have a trophic influence. Adenosine and its receptors afforded some protection to osteoblasts against cellular death mediated partly by apoptosis and partly by necrosis.

Adenosine A2A receptors promote adenosine-stimulated wound healing in bronchial epithelial cells

Adenosine produces a wide variety of physiological effects through the activation of specific adenosine receptors (A(1), A(2A), A(2B), A(3)). Adenosine, acting particularly at the A(2A) adenosine receptor (A(2A)AR), is a potent endogenous anti-inflammatory agent and sensor of inflammatory tissue damage. The complete healing of wounds is the final step in a highly regulated response to injury. Recent studies on epidermal wounds have identified the A(2A)AR as the main adenosine receptor responsible for altering the kinetics of wound closure. We hypothesized that A(2A)AR promotes wound healing in bronchial epithelial cells (BECs). To test this hypothesis, the human BEC line BEAS-2B and bovine BECs (BBECs) were used. Real-time RT-PCR of RNA from unstimulated BEAS-2B cells revealed transcriptional expression of A(1), A(2A), A(2B) and A(3) receptors. Western blot analysis of lysates from BEAS-2B cells and BBECs detected a single band at 44.7 kDa in both the BECs, indicating the presence of A(2A)AR. In a wound healing model, we found that adenosine stimulated wound repair in cultured BBECs in a concentration-dependent manner, with an optimal closure rate observed between 4 and 6 h. Similarly, the A(2A)AR agonist 5'-(N-cyclopropyl)carboxamidoadenosine (CPCA) augmented wound closure, with a maximal closure rate occurring between 4 and 6 h. Inhibition of A(2A)AR with ZM-241385, a known A(2A)AR antagonist, impeded wound healing. In addition, ZM-241385 also attenuated adenosine-mediated wound repair. Kinase studies revealed that adenosine-stimulated airway repair activates PKA by ligating A(2A)AR. Collectively, the data suggest that the A(2A)AR is involved in BEC adenosine-stimulated wound healing and may prove useful in understanding purinergic-mediated actions on airway epithelial repair.

Adenosine stimulation of the proliferation of colorectal carcinoma cell lines. Roles of cell density and adenosine metabolism

Adenosine is a purine nucleoside which is present at micromolar concentrations in the extracellular fluid of solid cancers as a result of tissue hypoxia. Adenosine acts to promote tumor survival by inhibiting the cell-mediated anti-tumor immune response. However, its role in modulating proliferation of the tumor cell population is unclear. Differing results have been obtained using adenosine analogues or by interfering with adenosine metabolism. We examined the effect of adenosine itself on DNA synthesis and cell growth in six different human and mouse colorectal carcinoma cell lines, from different sites and at different stages of differentiation. Adenosine given as a single dose consistently stimulated DNA synthesis and cell proliferation in all cell lines tested, with an EC(50) of 3.8-30 microM and a maximum stimulation being reached at 10-100 microM. AMP and ATP also stimulated cell proliferation at similar doses. The stimulation by adenosine varied depending upon the culture cell density, with the greatest mitogenic effect at subconfluent densities. Adenosine was metabolized by cellular adenosine deaminase and adenosine kinase. The half-life (t(1/2)) for the decline in adenosine concentration in the medium following a single addition was between 40 min and 3 hr depending on the cell line and culture conditions. The rate of production of endogenous adenosine was low under normoxic culture conditions. Continuous dosing of cultures with adenosine to provide a steady-state concentration showed that proliferation could be stimulated by low micromolar concentrations of adenosine. We conclude that adenosine is stimulatory to the growth of human colorectal carcinoma cells at concentrations present within the tumor extracellular environment.

A glance at adenosine receptors: novel target for antitumor therapy

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

Reduction in preretinal neovascularization by ribozymes that cleave the A2B adenosine receptor mRNA

Adenosine modulates a variety of cellular functions by interacting with specific cell surface G protein-coupled receptors (A1, A2A, A2B, and A3) and is a potential mediator of angiogenesis through the A2B receptor. The lack of a potent, selective A2B receptor inhibitor has hampered its characterization. Our goal was to design a hammerhead ribozyme that would specifically cleave the A2B receptor mRNA and examine its effect on retinal angiogenesis. Ribozymes specific for the mouse and human A2B receptor mRNAs were designed and cloned in expression plasmids. Human embryonic kidney (HEK) 293 cells were transfected with these plasmids and A2B receptor mRNA levels were determined by quantitative real-time RT-PCR. Human retinal endothelial cells (HRECs) were also transfected and cell migration was examined. The effects of these ribozymes on the levels of preretinal neovascularization were determined using a neonatal mouse model of oxygen-induced retinopathy (OIR). We produced a ribozyme with a Vmax of 515+/-125 pmol/min and a Kcat of 36.1+/-8.3 min(-1) (P< or =1x10(-5)). Transfection of HEK293 cells with the plasmid expressing the ribozyme reduced A2B receptor mRNA levels by 45+/-4.8% (P=5.1x10(-5)). Transfection of HRECs reduced NECA-stimulated migration of cells by 47.3+/-1.2% (P=7x10(-4)). Intraocular injection of the constructs into the mouse model reduced preretinal neovascularization by 53.5+/-8.2% (P=4.5x10(-5)). Our results suggest that the A2B receptor ribozyme will provide a tool for the selective inhibition of this receptor and provide further support for the role of A2B receptor in retinal angiogenesis.

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 promotes wound healing and mediates angiogenesis in response to tissue injury via occupancy of A(2A) receptors

Recent evidence indicates that topical application of adenosine A(2A) receptor agonists, unlike growth factors, increases the rate at which wounds close in normal animals and promotes wound healing in diabetic animals as well as growth factors, yet neither the specific adenosine receptor involved nor the mechanism(s) by which adenosine receptor occupancy promotes wound healing have been fully established. To determine which adenosine receptor is involved and whether adenosine receptor-mediated stimulation of angiogenesis plays a role in promotion of wound closure we compared the effect of topical application of the adenosine receptor agonist CGS-21680 (2-p-[2-carboxyethyl]phenethyl-amino-5'-N-ethylcarboxamido-adenosine) on wound closure and angiogenesis in adenosine A(2A) receptor knockout mice and their wild-type littermates. There was no change in the rate of wound closure in the A(2A) receptor knockout mice compared to their wild-type littermates although granulation tissue formation was nonhomogeneous and there seemed to be greater inflammation at the base of the wound. Topical application of CGS-21680 increased the rate of wound closure and increased the number of microvessels in the wounds of wild-type mice but did not affect the rate of wound closure in A(2A) receptor knockout mice. Similarly, in a model of internal trauma and repair (murine air pouch model), endogenously produced adenosine released into areas of internal tissue injury stimulates angiogenesis because there was a marked reduction in blood vessels in the walls of healing air pouches of A(2A) receptor knockout mice compared to their wild-type controls. Inflammatory vascular leakage and leukocyte accumulation in the inflamed air pouch were similarly reduced in the A(2A) receptor knockout mice reflecting the reduced vascularity. Thus, targeting the adenosine A(2A) receptor is a novel approach to promoting wound healing and angiogenesis in normal individuals and those suffering from chronic wounds.

Adenosine-regulated cell proliferation in pituitary folliculostellate and endocrine cells: differential roles for the A(1) and A(2B) adenosine receptors

A(1) and A(2) adenosine receptors have been identified in the pituitary gland, but the cell type(s) on which they are located and their effects on pituitary cell growth are not known. Therefore, we analyzed the expression of A(1) and A(2) receptors in primary rat anterior pituitary cells, two pituitary folliculostellate (TtT/GF and Tpit/F1) and two pituitary endocrine (GH(3) and AtT20) cell lines, and compared their effects on cell proliferation. In anterior pituitary and folliculostellate cells, adenosine and adenosine receptor agonists (5'-N-ethylcarboxamidoadenosine, a universal agonist, and CGS 21680, an A(2A) receptor agonist) stimulated cAMP levels with a rank order of potency that indicates the presence of functional A(2B) receptors. This stimulation, however, was not observed in either GH(3) or AtT20 cells, where adenosine and the A(1) receptor agonist 2-chloro-N(6)-cyclopentyladenosine inhibited VIP/forskolin-stimulated cAMP production. Expression of A(2B) and A(1) receptors in the folliculostellate cells and that of the A(1) receptor in the endocrine cells were confirmed by RT-PCR, immunocytochemistry, and ligand binding. Adenosine and 5'-N-ethylcarboxamidoadenosine dose-dependently (10 nM to 10 microM) stimulated growth in the folliculostellate, but not in the endocrine, cells, whereas in the latter, 100 microM adenosine and 2-chloro-N(6)-cyclopentyladenosine inhibited cell proliferation by slowing cell cycle progression. These data highlight the differential expression of A(1) and A(2B) adenosine receptors in pituitary cells and provide evidence for opposing effects of adenosine on pituitary folliculostellate and endocrine cell growth.

Purinergic signaling and vascular cell proliferation and death

Evidence for the role of purinergic signaling (via P1 and P2Y receptors) in the proliferation of vascular smooth muscle and endothelial cells is reviewed. The involvement of the mitogen-activated protein kinase second-messenger cascade in this action is clearly implicated, although details of the precise intracellular pathways involved still remain to be determined. Synergistic actions of purines and pyrimidines with growth factors occur in promoting cell proliferation. Interaction between purinergic signaling for vascular cell proliferation and cell death mediated by P2X7 receptors is discussed. There is evidence of the release of ATP from endothelial cells, platelets, and sympathetic nerves as well as from damaged cells in atherosclerosis, hypertension, restenosis, and ischemia; furthermore, there is evidence that vascular smooth muscle and endothelial cells proliferate in these pathological conditions. Thus, the involvement of ATP and its breakdown product, adenosine, is implicated; it is hoped that with the development of selective P1 (A2) and P2Y receptor agonists and antagonists, new therapeutic strategies will be explored.

Adenosine receptors: G protein-mediated signalling and the role of accessory proteins

Ever since the discovery of the effects of adenosine in the circulation, adenosine receptors continue to represent a promising drug target. Firstly, this is due to the fact that the receptors are expressed in a large variety of cells; in particular, the actions of adenosine (or, respectively, of the antagonistic methylxanthines) in the central nervous system, in the circulation, on immune cells and on other tissues can be beneficial in certain disorders. Secondly, there exists a large number of ligands, which have been generated by introducing several modifications in the structure of the lead compounds (adenosine and methylxanthine), some of them highly specific. Four adenosine receptor subtypes have been identified by molecular cloning; they belong to the family of G protein-coupled receptors, which transfer signals by activating heterotrimeric G proteins. It has been appreciated recently that accessory proteins impinge on the receptor/G protein interaction and thus modulate the signalling reaction. These accessory components may be thought as adaptors that redirect the signalling pathway to elicit a cell-specific response. Here, we review the recent literature on adenosine receptors and place a focus on the role of accessory proteins in the organisation of adenosine receptor signalling. These components have been involved in receptor sorting, in the control of signal amplification and in the temporal regulation of receptor activity, while the existence of others is postulated on the basis of atypical cellular reactions elicited by receptor activation.

A(2B) adenosine receptors stimulate growth of porcine and rat arterial endothelial cells

The goal of this study was to determine which adenosine receptor subtype mediates growth stimulation by adenosine in arterial endothelial cells. In porcine coronary artery and rat aortic endothelial cells, 2-chloroadenosine (Cl-Ad), a metabolically stable analog of adenosine, stimulated DNA synthesis ((3)H-thymidine incorporation), cellular proliferation (cell number), collagen synthesis ((3)H-proline incorporation), and cell migration. The growth effects of adenosine and Cl-Ad were mimicked by the adenosine receptor agonist 5'-N-methylcarboxamidoadenosine but not by the adenosine receptor agonists N(6)-cyclopentyladenosine, 4-aminobenzyl-5'-N-methylcarboxamidoadenosine or CGS21680, an agonist profile consistent with an A(2B) receptor-mediated effect. The adenosine receptor antagonists KF17837 and 1,3-dipropyl-8-p-sulfophenylxanthine but not 8-cyclopentyl-1,3-dipropylxanthine blocked the growth-stimulatory effects of Cl-Ad and 5'-N-methylcarboxamidoadenosine, an antagonist profile consistent with an A(2) receptor-mediated action. Treatment of endothelial cells with erythro-9-(2-hydroxy-3-nonyl) adenine plus iodotubericidin (inhibitors of adenosine deaminase and adenosine kinase, respectively) induced endothelial cell growth, and these effects were blocked by 1,3-dipropyl-8-p-sulfophenylxanthine and KF17837 but not 8-cyclopentyl-1,3-dipropylxanthine, suggesting that endothelial cell-derived adenosine induces growth via A(2) receptors. The growth-stimulatory effects of Cl-Ad, 5'-N-methylcarboxamidoadenosine, and erythro-9-(2-hydroxy-3-nonyl) adenine plus iodotubericidin were abolished by antisense but not scrambled or sense oligonucleotides to the A(2B) receptor. Our findings strongly support the hypothesis that adenosine induces endothelial cell growth by activating A(2B) receptors. Thus, A(2B) receptors may play a critical role in regulating vascular remodeling associated with endothelial cell proliferation in angiogenesis, collateral vessel development, and recovery after vascular injury. Pharmacological or molecular biological activation of A(2B) receptors may be useful in modulating vascular remodeling.

Adenosine A2A receptor agonists promote more rapid wound healing than recombinant human platelet-derived growth factor (Becaplermin gel)

Animal studies of the topical application of adenosine A2A receptor agonists show that it promotes wound closure. To further confirm the efficacy of adenosine A2A receptor agonists as promoters of wound healing, we compared the effect of MRE0094, a novel selective adenosine A2A receptor agonist, to CGS-21680, a reference selective adenosine A2A receptor agonist, as well as to recombinant human platelet-derived growth factor (0.01% Becaplermin gel), an agent currently used to promote healing of diabetic ulcers, on wound closure in healthy BALB/C mice. Wounds (approximately 12 mm diameter) were created on the dorsum of mice (two per mouse) and then treated daily with vehicle, 0.01% Becaplermin gel, or different doses of the adenosine A2A receptor agonists. The wound margins were traced onto plastic sheets, and the wound areas were digitized, quantitated, and compared. We found that application of MRE0094 (1 microg/wound and 10 microg/wound) and CGS-21680 (1 microg/wound and 5 microg/wound) achieved 50% wound closure significantly more rapidly than control application (day 1.9, 1.9, 3.5, 3.2, respectively, versus control day 4, p < 0.05 ANOVA). Surprisingly, neither higher nor lower concentrations of CGS-21680 affected the rate of wound closure, as compared to control. In contrast, Becaplermin gel did not increase the rate at which wounds closed (50% closure by day 7.2, p = NS versus control). These data confirm our prior observations that adenosine A2A receptor agonists promote wound closure, and they suggest that these agents may be as effective if not more effective than Becaplermin gel for the treatment of poorly healing wounds.

Molecular action of methotrexate in inflammatory diseases

Despite the recent introduction of biological response modifiers and potent new small-molecule antirheumatic drugs, the efficacy of methotrexate is nearly unsurpassed in the treatment of inflammatory arthritis. Although methotrexate was first introduced as an antiproliferative agent that inhibits the synthesis of purines and pyrimidines for the therapy of malignancies, it is now clear that many of the anti-inflammatory effects of methotrexate are mediated by adenosine. This nucleoside, acting at one or more of its receptors, is a potent endogenous anti-inflammatory mediator. In confirmation of this mechanism of action, recent studies in both animals and patients suggest that adenosine-receptor antagonists, among which is caffeine, reverse or prevent the anti-inflammatory effects of methotrexate.

Nebivolol induces calcium-independent signaling in endothelial cells by a possible beta-adrenergic pathway

Nebivolol is a highly selective beta1-adrenoreceptor-blocking agent with a peculiar pharmacodynamic profile. It has peripheral acute vasodilating properties that are mediated by modulation of the endogenous production of nitric oxide. In this study we analyzed the different signaling pathways implicated in the response of human umbilical vein endothelial cells to nebivolol. Its effect on endothelial transduction pathways was determined by assaying phospholipase C and A2 activities and cyclic adenosine monophosphate (AMP) production. Variations in intracellular calcium concentration were also measured. Our results showed that nebivolol activates a calcium-independent transduction pathway that implicates an increase in adenylate cyclase and phospholipase A2 activity. Beta1- or beta2-Adrenoreceptor antagonists do not inhibit the action of nebivolol. However, its action on cyclic AMP production is inhibited by bupranolol, a beta1-3-adrenoreceptor antagonist, and S-(-)-cyanopindolol, a selective beta3-adrenoreceptor antagonist. Nebivolol also dose-dependently increased nitrite production. This effect was inhibited by bupranolol, suggesting that the possible action of nebivolol on beta3-adrenoreceptor is involved in its vasodilating properties. This study suggests that nebivolol could behave as a beta3-adrenoreceptor agonist and induce some calcium-independent pathways implicating phospholipase A2 and adenylate cyclase. This agonistic activity of nebivolol seems to be responsible for its endothelium-dependent vasodilating activity.

Adenosine A(2A) receptor stimulation reduces inflammation and neointimal growth in a murine carotid ligation model

Endothelial activation and leukocyte recruitment are early events in atherosclerosis and the vascular response to injury. Adenosine has anti-inflammatory effects on leukocytes and endothelial cells mediated through its A(2A) receptor. We tested the hypothesis that A(2A) activation would reduce inflammation and neointimal formation in a murine carotid ligation model. Before injury, mice were randomized to a 7-day subcutaneous infusion of a specific A(2A) receptor agonist (ATL-146e, 0.004 microg/kg per minute), vehicle control, ATL-146e plus ZM241385 (a selective A(2A) antagonist), or ZM241385 alone. Leukocyte recruitment and adhesion molecule expression were assessed at early time points, and the neointimal area was measured at 14 and 28 days after injury. Compared with control mice, ATL-146e-treated mice had significantly less neutrophil and macrophage recruitment and vascular cell adhesion molecule-1, intercellular adhesion molecule-1, and P-selectin expression in the first 7 days after injury. Neointimal area was markedly and persistently reduced by 80% at 14 and 28 days, despite termination of ATL infusion at 7 days. ATL-146e+ZM241385-treated and ZM241385-treated animals had neointimal areas similar to those of control animals, confirming that the observed effects of ATL-146e were mediated specifically by the A(2A) receptor. These data demonstrate that novel stimulation of adenosine A(2A) receptors can inhibit early inflammatory processes that are important in neointimal formation after vascular injury.

Selective A(2A) adenosine receptor activation reduces skin pressure ulcer formation and inflammation

Activation of A(2A) adenosine receptors (A(2A)-AR) by ATL-146e (formerly DWH-146e) prevents inflammatory cell activation and adhesion. Recurrent ischemia-reperfusion (I/R) of the skin results in pressure ulcer formation, a major clinical problem. ATL-146e was evaluated in a novel reproducible rat model of pressure ulcer. A 9-cm(2) region of dorsal rat skin was cyclically compressed at 50 mmHg using a surgically implanted metal plate and an overlying magnet to generate reproducible tissue necrosis. Osmotic minipumps were implanted into 24 rats divided into four equal groups to infuse vehicle (control), ATL-146e (0.004 microg x kg(-1) x min(-1)), ATL-146e plus an equimolar concentration of A(2A) antagonist, ZM-241385, or ZM-241385 alone. Each group received 10 I/R cycles. In non-I/R-treated skin, ATL-146e has no effect on blood flow. I/R-treated skin of the ATL-146e group compared with the vehicle group had 65% less necrotic area, 31% less inhibition of average skin blood flow, and fewer extravasated leukocytes (23 +/- 3 vs. 49 +/- 6 per 500 microm(2)). These data suggest that ATL-146e, acting via an A(2A)-AR, reduces leukocyte infiltration and is a potent prophylactic for I/R injury in skin.

Lack of interaction between alpha(2)-autoreceptors and prejunctional receptors mediating a facilitatory effect on noradrenaline release

The present study was undertaken to investigate the effect of alpha(2)-autoreceptor blockade on the facilitatory influence exerted by activation of beta -, A(2A)-adenosine- and angiotensin II receptors. Segments of a rat-tail artery, previously incubated with(3)H-noradrenaline, were subjected to electrical stimulation. The influence of isoprenaline, the compound CGS21680 and angiotensin II on the overflow of tritium evoked by electrical stimulation was checked before and after alpha(2)-adrenoceptor blockade. All the agonists used caused concentration-dependent increases of tritium overflow, the maximal effect representing increases of 44.2, 27.4 and 41.2% for isoprenaline, CGS21680 and angiotensin II, respectively. In the presence of alpha(2)-adrenoceptor blockade by phenoxybenzamine ( 1 microm) or yohimbine (33 or 100 nm), the facilitatory influence of isoprenaline, CGS21680 and angiotensin II was not significantly changed. Since this facilitatory influence, which involves the activation of G(s)- or G(q)-proteins, was not enhanced by alpha(2)-adrenoceptor blockade, it is concluded that the enhancement of the negative modulation resulting from activation of A(1)-adenosine-, muscarine- and kappa -receptors, as previously shown, should be due to the fact that the involved systems share signal transduction mechanisms, or at least G-proteins.

The role of adenosine receptors in the action of theophylline on human peripheral blood mononuclear cells from healthy and asthmatic subjects

1. The aim of the present study was to investigate the role of adenosine A2b receptors in the anti-proliferative action of theophylline in human peripheral blood mononuclear cells (HPBMC) from healthy and asthmatic subjects. 2. Theophylline significantly inhibited PHA-induced proliferation of HPBMC from both healthy and asthmatic donors but only at relatively high concentrations at 1 mM (P<0.05). Enprophylline, a drug which also acts as a non-selective phosphodiesterase (PDE) inhibitor and is a selective A2b receptor antagonist, had no significant effect on proliferation of cells from either group at concentrations up to 10 microM (P>0.05; n=6). 3. Adenosine deaminase (2 u ml(-1)), which metabolizes adenosine, had no significant effect on PHA-induced HPBMC proliferation over a range of concentrations (0 - 8 microg ml(-1)) in cells from either healthy or asthmatic subjects. 4. The adenosine receptor agonists N(6)-cyclopentyladenosine (CPA, A1-selective) and 5'-N-ethylcarboxamidoadenosine (NECA, A1/A2) produced a small but significant inhibition of PHA-induced proliferation of HPBMC from healthy and asthmatic subjects (10 microM, P<0.05; n=6). In contrast, 5'-N-ethylcarboxamido-2-[4-(2-]carboxyethyl)phenethyl]adenosine (CGS21680, A2a-selective) was without significant effect (P>0.05; n=6). 5. The adenosine receptor antagonist alloxazine (A2b-selective) had no significant effect, while 8(3-chlorostyryl)caffeine,(CSC, A2a-selective) significantly inhibited PHA-induced proliferation of HPBMC from both groups (P<0.05; n=6). 6. Our results suggest that endogenous or exogenous adenosine has little effect on the proliferation of HPBMC obtained from healthy or asthmatic subjects. Thus it would appear that the effect of high concentrations of theophylline is not related to adenosine receptor antagonism.

Adenosine modulates cell growth in baby hamster kidney (BHK) cells

Adenosine is known to modulate cell growth in a variety of mammalian cells either via the activation of receptors or through metabolism. We investigated the effect of adenosine on Baby Hamster Kidney (BHK) cell growth and attempted to determine its mechanism of modulation. In wild-type BHK cells, adenosine evoked a biphasic response in which a low concentration of adenosine (1-5 microM) produced an inhibition of colony formation but at higher concentrations (up to 50 microM) this inhibition was progressively reversed. However, no biphasic response was observed in an "adenosine kinase" deficient BHK mutant, "5a", which suggests that adenosine kinase plays an important role in the modulation of growth response to adenosine. Adenosine receptors did not appear to have a role in regulating cell growth of BHK cells. Specific A1 and A2 receptor antagonists were unable to reverse the effect of adenosine on cell growth. Even though a specific A3 adenosine receptor antagonist MRS-1220 partly reversed the inhibition in colony formation at 1 microM adenosine, it also affected the transport of adenosine. Thus adenosine transport and metabolism appears to play the major role in this modulation of cell growth as 5'-amino-5'-deoxyadenosine, an adenosine kinase inhibitor, reversed the inhibition of cell growth observed at 1 microM adenosine. These results, taken together, would suggest that adenosine modulates cell growth in BHK mainly through its transport and metabolism to adenine nucleotides.

A(2B) receptors mediate antimitogenesis in vascular smooth muscle cells

Adenosine inhibits growth of vascular smooth muscle cells. The goals of this study were to determine which adenosine receptor subtype mediates the antimitogenic effects of adenosine and to investigate the signal transduction mechanisms involved. In rat aortic vascular smooth muscle cells, platelet-derived growth factor-BB (PDGF-BB) (25 ng/mL) stimulated DNA synthesis ([(3)H]thymidine incorporation), cellular proliferation (cell number), collagen synthesis ([(3)H]proline incorporation), total protein synthesis ([(3)H]leucine incorporation), and mitogen-activated protein (MAP) kinase activity. The adenosine receptor agonists 2-chloroadenosine and 5'-N-methylcarboxamidoadenosine, but not N(6)-cyclopentyladenosine or CGS21680, inhibited the growth effects of PDGF-BB, an agonist profile consistent with an A(2B) receptor-mediated effect. The adenosine receptor antagonists KF17837 and 1,3-dipropyl-8-p-sulfophenylxanthine, but not 8-cyclopentyl-1, 3-dipropylxanthine, blocked the growth-inhibitory effects of 2-chloroadenosine and 5'-N-methylcarboxamidoadenosine, an antagonist profile consistent with an A(2) receptor-mediated effect. Antisense, but not sense or scrambled, oligonucleotides to the A(2B) receptor stimulated basal and PDGF-induced DNA synthesis, cell proliferation, and MAP kinase activity. Moreover, the growth-inhibitory effects of 2-chloroadenosine, 5'-N-methylcarboxamidoadenosine, and erythro-9-(2-hydroxy-3-nonyl) adenine plus iodotubericidin (inhibitors of adenosine deaminase and adenosine kinase, respectively) were abolished by antisense, but not scrambled or sense, oligonucleotides to the A(2B) receptor. Our findings strongly support the hypothesis that adenosine causes inhibition of vascular smooth muscle cell growth by activating A(2B) receptors coupled to inhibition of MAP kinase activity. Pharmacological or molecular biological activation of A(2B) receptors may prevent vascular remodeling associated with hypertension, atherosclerosis, and restenosis following balloon angioplasty.

Adenosine receptor activation induces vascular endothelial growth factor in human retinal endothelial cells

Adenosine, released in increased amounts by hypoxic tissues, is thought to be an angiogenic factor that links altered cellular metabolism caused by oxygen deprivation to compensatory angiogenesis. Adenosine interacts with 4 subtypes of G protein-coupled receptors, termed A(1), A(2A), A(2B), and A(3). We investigated whether adenosine causes proliferation of human retinal endothelial cells (HRECs) and synthesis of vascular endothelial growth factor (VEGF) and, if so, which adenosine receptor subtype mediates these effects. The nonselective adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA), in a concentration-dependent manner, increased both VEGF mRNA and protein expression by HRECs, as well as proliferation. This proliferative effect of NECA was inhibited by the addition of anti-human VEGF antibody. NECA also increased insulin-like growth factor-I and basic fibroblast growth factor mRNA expression in a time-dependent manner and cAMP accumulation in these cells. In contrast, neither the A(1) agonist N(6)-cyclopentyladenosine nor the A(2A) agonist 2-p-(2-carboxyethyl) phenethylamino-NECA caused any of the above effects of NECA. The effects of NECA were not significantly attenuated by either the A(2A) antagonist SCH58261 or the A(1) antagonist 8-cyclopentyl-1, 3-dipropylxanthine. However, the nonselective adenosine receptor antagonist xanthine amine congener completely inhibited the effects of NECA. Addition of antisense oligonucleotide complementary to A(2B) adenosine receptor mRNA inhibited VEGF protein production by HRECs after NECA stimulation. Thus, the A(2B) adenosine receptor subtype appears to mediate the actions of adenosine to increase growth factor production, cAMP content, and cell proliferation of HRECs. Adenosine activates the A(2B) adenosine receptor in HRECs, which may lead to neovascularization by a mechanism involving increased angiogenic growth factor expression.

Activation of mitogen-activated protein kinase by the A(2A)-adenosine receptor via a rap1-dependent and via a p21(ras)-dependent pathway

The A(2A)-adenosine receptor, a prototypical G(s)-coupled receptor, activates mitogen-activated protein (MAP) kinase in a manner independent of cAMP in primary human endothelial cells. In order to delineate signaling pathways that link the receptor to the regulation of MAP kinase, the human A(2A) receptor was heterologously expressed in Chinese hamster ovary (CHO) and HEK293 cells. In both cell lines, A(2A) agonist-mediated cAMP accumulation was accompanied by activation of the small G protein rap1. However, rap1 mediates A(2A) receptor-dependent activation of MAP kinase only in CHO cells, the signaling cascade being composed of G(s), adenylyl cyclase, rap1, and the p68 isoform of B-raf. This isoform was absent in HEK293 cells. Contrary to CHO cells, in HEK293 cells activation of MAP kinase by A(2A) agonists was not mimicked by 8-bromo-cAMP, was independent of Galpha(s), and was associated with activation of p21(ras). Accordingly, overexpression of the inactive S17N mutant of p21(ras) and of a dominant negative version of mSos (the exchange factor of p21(ras)) blocked MAP kinase stimulation by the A(2A) receptor in HEK 293 but not in CHO cells. In spite of the close homology between p21(ras) and rap1, the S17N mutant of rap1 was not dominant negative because (i) overexpression of rap1(S17N) failed to inhibit A(2A) receptor-dependent MAP kinase activation, (ii) rap1(S17N) was recovered in the active form with a GST fusion protein comprising the rap1-binding domain of ralGDS after A(2A) receptor activation, and (iii) A(2A) agonists promoted the association of rap1(S17N) with the 68-kDa isoform of B-raf in CHO cells. We conclude that the A(2A) receptor has the capacity two activate MAP kinase via at least two signaling pathways, which depend on two distinct small G proteins, namely p21(ras) and rap1. Our observations also show that the S17N version of rap1 cannot be assumed a priori to act as a dominant negative interfering mutant.

Treatment with sulfasalazine or sulfapyridine, but not 5-aminosalicyclic acid, inhibits basic fibroblast growth factor-induced endothelial cell chemotaxis

OBJECTIVE

Rheumatoid arthritis (RA) is characterized by leukocyte recruitment and angiogenesis. We investigated the effects of sulfasalazine (SSZ) and its metabolites, sulfapyridine (SP) and 5-aminosalicylic acid (5-ASA), on components of angiogenesis, namely, endothelial cell (EC) chemotaxis and proliferation, as well as on EC chemokine and soluble adhesion molecule expression.

METHODS

SSZ, SP, and 5-ASA were assayed for their effects on basic fibroblast growth factor (bFGF)-induced human dermal microvascular endothelial cell (HMVEC) chemotaxis and proliferation. EC were plated on Matrigel to assess the effect of SSZ on EC tube formation. Enzyme-linked immunosorbent assays were performed to determine changes in HMVEC production of interleukin-8 (IL-8), monocyte chemoattractant protein-1 (MCP-1), growth-related oncogene alpha (GROalpha), epithelial neutrophil-activating peptide 78 (ENA-78), soluble E-selectin (sE-selectin), and soluble intercellular adhesion molecule 1 (sICAM-1) upon treatment with SSZ or its metabolites.

RESULTS

HMVEC incubated with SSZ or SP exhibited reduced bFGF-induced chemotaxis (59%, [n = 7] and 22%, [n = 3], respectively) (P<0.05). SSZ and SP decreased basal HMVEC proliferation, while 5-ASA increased proliferation (P<0.05; [n = 5]). SSZ decreased bFGF-induced HMVEC proliferation (P<0.05 [n = 5]). SSZ inhibited phorbol 12-myristate 13-acetate-induced HMVEC tube formation (P<0.05; [minimum n = 5]). Tumor necrosis factor alpha-stimulated HMVEC shedding of sICAM-1 was reduced by incubation with either SSZ (19%) or 5-ASA (23%) (P<0.05; [n = 6]). SP inhibited cytokine-stimulated HMVEC expression of IL-8 and MCP-1 (P<0.05; [n = 4]). Neither SSZ nor its metabolites had any effect on HMVEC production of sE-selectin, GROalpha, or ENA-78.

CONCLUSION

These results demonstrate that SSZ and its metabolite SP may affect the pathogenesis of RA by inhibiting EC chemotaxis, proliferation, tube formation, and expression of sICAM-1, IL-8, and MCP-1.

Polydeoxyribonucleotides enhance the proliferation of human skin fibroblasts: involvement of A2 purinergic receptor subtypes

It is well-known that nucleotides, nucleosides and purine/pyrimidine bases enhance cell proliferation in vitro. Nevertheless, the molecular mechanisms involved in this mitogenic activity is still controversial, since these compounds are reported both to synergize with growth factor, and to act directly on purinergic receptor inducing per se a proliferative response. It was suggested that cell growth enhancement could be mediated by the A2 purinergic receptor activation. Here we report that a polydeoxyribonucleotide (PDRN) and adenosine are able to increase, the growth rate of human skin fibroblasts in primary cultures. The proliferative activity exerted by PDRN was significantly counteracted by the A2 antagonist 3, 7-Dimethyl-1-propargylxanthine (DMPX), but not by the A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (PD 116,948, DPCPX). Accordingly, the trophic action of PDRN was mimicked by the A2 agonist N6-[2-(3,5-Dimethoxyphenyl)-2-(methylphenyl)-ethyl]adenosine (DPMA), while the A1 agonist N6-Cyclopenthyladenosine (CPA) did not show any effect. In microfluorimetric studies, we observed that PDRN and adenosine increased the concentration of cytosolic calcium ions. The PDRN-evoked calcium rise was dose-dependent and DMPX sensitive. Taken together, our results suggest that PDRN may operate as a pro-drug providing the cultured cells with an effective amount of mitogenic deoxyribonucleotides, deoxyribonucleosides and bases; moreover, cell proliferation enhancement that has been induced by PDRN seems to be mediated, at least in part, by the activation of purinergic receptors of the A2 subtype.

A2B adenosine and P2Y2 receptors stimulate mitogen-activated protein kinase in human embryonic kidney-293 cells. cross-talk between cyclic AMP and protein kinase c pathways

Mitogen-activated protein kinase (MAPK) cascades underlie long-term mitogenic, morphogenic, and secretory activities of purinergic receptors. In HEK-293 cells, N-ethylcarboxamidoadenosine (NECA) activates endogenous A2BARs that signal through Gs and Gq/11. UTP activates P2Y2 receptors and signals only through Gq/11. The MAPK isoforms, extracellular-signal regulated kinase 1/2 (ERK), are activated by NECA and UTP. H-89 blocks ERK activation by forskolin, but weakly affects the response to NECA or UTP. ERK activation by NECA or UTP is unaffected by a tyrosine kinase inhibitor (genistein), attenuated by a phospholipase C inhibitor (U73122), and is abolished by a MEK inhibitor (PD098059) or dominant negative Ras. Inhibition of protein kinase C (PKC) by GF 109203X failed to block ERK activation by NECA or UTP, however, another PKC inhibitor, Ro 31-8220, which unlike GF 109203X, can block the zeta-isoform, and prevents UTP- but not NECA-induced ERK activation. In the presence of forskolin, Ro 31-8220 loses its ability to block UTP-stimulated ERK activation. PKA has opposing effects on B-Raf and c-Raf-1, both of which are found in HEK-293 cells. The data are explained by a model in which ERK activity is modulated by differential effects of PKC zeta and PKA on Raf isoforms.

Extracellular ATP: a growth factor for vascular smooth muscle cells

1. Extracellular adenosine triphosphate (ATP) is mitogenic for vascular smooth muscle cells (VSMC) and stimulates several events that are important for cell proliferation: DNA synthesis, protein synthesis, increase of cell number, immediate early genes, cell-cycle progression, and tyrosine phosphorylation. 2. Receptor characterization indicates mitogenic effects of both P2U and P2Y receptors. The P2X receptor is lost in cultured VSMC and is not involved. Several related biological substances such as UTP, ITP, GTP, AP4A, ADP, and UDP are also mitogenic. 3. Signal transduction is mediated via Gq-proteins, phospholipase C beta, phospholipase D, diacyl glycerol, protein kinase C alpha, delta, Raf-1, MEK, and MAPK. 4. ATP acts synergistically with polypeptide growth factors (PDGF, bFGF, IGF-1, EGF, insulin) and growth factors acting via G-protein-coupled receptors (noradrenaline, neuropeptide Y, 5-hydroxytryptamine, angiotensin II, endothelin-1). 5. The mitogenic effects have been demonstrated in rat, porcine, and bovine VSMC and cells from human coronary arteries, aorta, and subcutaneous arteries and veins. 6. The trophic effects on VSMC and the abundant sources for extracellular ATP in the vessel wall make a pathophysiological role probable in the development of atherosclerosis, neointima-formation after angioplasty, and possibly hypertension.

The cyclin-dependent kinase inhibitor p21cip1 mediates the growth inhibitory effect of phorbol esters in human venous endothelial cells

Long-term application of the phorbol ester phorbol 12,13-dibutyrate (PDBu) inhibits the proliferation of human venous endothelial cells. The cyclin-dependent kinase inhibitor p21cip1 is a potential candidate mediating the PDBu-induced delayed entry of the cells into S-phase (by approximately 10 h when compared with cells stimulated with basic fibroblast growth factor (bFGF)). Levels of p21cip1 (protein and mRNA) rapidly rise (within approximately 2 h) in endothelial cells treated with the active isomer beta-PDBu, but not with alpha-PDBu; this effect is blocked by the mitogen-activated protein kinase kinase-1 (Mek1) inhibitor PD098059 and by the protein kinase C (PKC) antagonists GF109203X and rottlerin (selective for PKC-delta), but not Gö 6976 (selective for Ca2+-dependent PKC isoforms). Rapamycin blocks the PDBu-induced accumulation of p21cip1 (but not of the cognate mRNA), indicating an action of PKC on p21(cip1) mRNA translation. If endothelial cells are recruited into the cell cycle by bFGF, p21cip1 mRNA and protein levels rise initially (within 2 h) and decline subsequently such that p21cip1 drops to a minimum prior to the initiation of DNA synthesis (i.e. after approximately 12 h). In bFGF-stimulated cells, changes in p21cip1 mRNA and protein are strictly linked. In contrast, the levels of p21cip1 mRNA decline substantially (>10 h) before the protein decreases in PDBu-stimulated cells. Thus, PKC (presumably PKC-delta) regulates the amounts of p21cip1 in endothelial cells at the level of mRNA accumulation and translation, leading to a rapid and robust induction; following persistent PKC activation, p21(cip1) remains elevated despite reduced mRNA levels, indicating an enhanced stability of the protein. The bFGF-mediated increase in p21cip1 is blocked by the Mek1 inhibitor, but not by GF109203X; hence, in endothelial cells, induction of p21cip1 by PKC- and growth factor-dependent signaling is achieved by distinct pathways that converge and require activation of the mitogen-activated protein kinase cascade. The beta-PDBu-induced delayed S-phase entry and drop in p21cip1 are reversed if GF109203X is added 4 h after beta-PDBu to prevent persistent PKC activation. These observations indicate a cause and effect relation between sustained p21cip1 elevations and the delay in S-phase entry induced by beta-PDBu.

Wound healing is accelerated by agonists of adenosine A2 (G alpha s-linked) receptors

The complete healing of wounds is the final step in a highly regulated response to injury. Although many of the molecular mediators and cellular events of healing are known, their manipulation for the enhancement and acceleration of wound closure has not proven practical as yet. We and others have established that adenosine is a potent regulator of the inflammatory response, which is a component of wound healing. We now report that ligation of the G alpha s-linked adenosine receptors on the cells of an artificial wound dramatically alters the kinetics of wound closure. Excisional wound closure in normal, healthy mice was significantly accelerated by topical application of the specific A2A receptor agonist CGS-21680 (50% closure by day 2 in A2 receptor antagonists. In rats rendered diabetic (streptozotocin-induced diabetes mellitus) wound healing was impaired as compared to nondiabetic rats; CGS-21680 significantly increased the rate of wound healing in both nondiabetic and diabetic rats. Indeed, the rate of wound healing in the CGS-21680-treated diabetic rats was greater than or equal to that observed in untreated normal rats. These results appear to constitute the first evidence that a small molecule, such as an adenosine receptor agonist, accelerates wound healing in both normal animals and in animals with impaired wound healing.

Stimulation of the mitogen-activated protein kinase via the A2A-adenosine receptor in primary human endothelial cells

Adenosine exerts a mitogenic effect on human endothelial cells via stimulation of the A2A-adenosine receptor. This effect can also be elicited by the beta2-adrenergic receptor but is not mimicked by elevation of intracellular cAMP levels. In the present work, we report that stimulation of the A2A-adenosine receptor and of the beta2-adrenergic receptor activates mitogen-activated protein kinase (MAP kinase) in human endothelial cells based on the following criteria: adenosine analogues and beta-adrenergic agonists cause an (i) increase in tyrosine phosphorylation of the p42 isoform and to a lesser extent of the p44 isoform of MAP kinase and (ii) stimulate the phosphorylation of myelin basic protein by MAP kinase; (iii) this is accompanied by a redistribution of the enzyme to the perinuclear region. Pretreatment of the cells with cholera toxin (to down-regulate Gsalpha) abolishes activation of MAP kinase by isoproterenol but not that induced by adenosine analogues. In addition, MAP kinase stimulation via the A2A-adenosine receptor is neither impaired following pretreatment of the cells with pertussis toxin (to block Gi-dependent pathways) nor affected by GF109203X (1 microM; to inhibit typical protein kinase C isoforms) nor by a monoclonal antibody, which blocks epidermal growth factor-dependent signaling. In contrast, MAP kinase activation is blocked by PD 098059, an inhibitor of MAP kinase kinase 1 (MEK1) activation, which also blunts the A2A-adenosine receptor-mediated increase in [3H]thymidine incorporation. Activation of the A2A-adenosine receptor is associated with increased levels of GTP-bound p21(ras). Thus, our experiments define stimulation of MAP kinase as the candidate cellular target mediating the mitogenic action of the A2A-adenosine receptor on primary human endothelial cells; the signaling pathway operates via p21(ras) and MEK1 but is independent of Gi, Gs, and the typical protein kinase C isoforms. This implies an additional G protein which links this prototypical Gs-coupled receptor to the MAP kinase cascade.

The role of angiotensin receptors in cardiovascular diseases

As an antihypertensive regimen, angiotensin I-converting enzyme (ACE) inhibition appears to have an antiproliferative cardiovascular effect that is not caused by blood pressure reduction alone. On the other hand, ACE inhibition has been shown to induce neocapillarization in hypertrophied myocardium. The possible mechanisms behind these beneficial cardiovascular effects of ACE inhibition are the suppression of angiotensin II formation and the potentiation of bradykinin. Angiotensin II receptor antagonism appears to have a similar antiproliferative effect on myocardium and vascular smooth muscle as ACE inhibition. This suggests that the antiproliferative action of both regimens is due only to the reduction of the pressor and growth effects of angiotensin II, or that both regimens have an additional, similarly effective antiproliferative action. Recently, knowledge about angiotensin II receptors has almost exponentially expanded. The two main classes of angiotensin II receptors, type 1 and 2 (AT1 and AT2), have been shown to belong to the same receptor family. However, their signal transduction and function seem to differ totally. The function and signal transduction of AT1 are to a large extent known. All the well-known physiological and pathophysiological effects of angiotensin II have been attributed to AT1. On the other hand, AT2 has quite recently been shown to mediate antiproliferation and differentiation at least in some tissues and cells, e.g. in vascular endothelial cells and some cells of neuronal origin. This review highlights the recent findings on angiotensin II receptors, and discusses the mechanisms behind the beneficial cardiovascular effects of interfering with the renin-angiotensin system.