Adenosine as an Endogenous Mediator of Hypoxia for Induction of Vascular Endothelial Growth Factor mRNA in U-937 Cells

doi:10.1006/bbrc.1994.2462

Biochemical and Biophysical Research Communications

Volume 204, Issue 1, 15 October 1994, Pages 318-324

https://doi.org/10.1006/bbrc.1994.2462 Get rights and content

Abstract

Adenosine induced by hypoxia exerts various effects via different types of receptors. Recently, hypoxia was shown to be a strong inducer of vascular endothelial growth factor, a secreted endothelial cell specific mitogen. In this report, we studied on effects of adenosine on inducibility of VEGF and possible mediation of hypoxia for its induction in U-937 cells. Hypoxia induced expression of VEGF mRNA with an early peak at 1 hour. 5′-N-ethylcarboxamidoadenosine, an adenosine analog, strongly induced VEGF mRNA, which was inhibited by 3,7-dimethyl-1-propargylxanthine (DMPX), an A₂-antagonist. The hypoxic induction was inhibited by adenosine deaminase, 7-(β-hydroxyethyl)theophyline, a non-selective adenosine receptor antagonist and DMPX. These results suggest that the hypoxic induction of VEGF mRNA is mediated by adenosine via A₂-receptor in U-937 cells.

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Cited by (68)

Extracellular ATP and adenosine: The Yin and Yang in immune responses?
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Citation Excerpt :
On the other hand, adenosine facilitates the release of IL-10, an anti-inflammatory cytokine from monocytes (Hasko et al., 1996, 2007; Koscso et al., 2012). Adenosine also induces the production of VEGF, a potent inducer of angiogenesis and vascular permeability (Semenza, 2003; Hashimoto et al., 1994). Many of these effects are regulated by the A2 receptors (Feoktistov and Biaggioni, 1997; Bouma et al., 1994; Hashimoto et al., 1994; Leibovich et al., 2002).
Extracellular adenosine 5′-triphosphate (ATP) and adenosine molecules are intimately involved in immune responses. ATP is mostly a pro-inflammatory molecule and is released during hypoxic condition and by necrotic cells, as well as by activated immune cells and endothelial cells. However, under certain conditions, for instance at low concentrations or at prolonged exposure, ATP may also have anti-inflammatory properties. Extracellular ATP can activate both P2X and P2Y purinergic receptors. Extracellular ATP can be hydrolyzed into adenosine in a two-step enzymatic process involving the ectonucleotidases CD39 (ecto-apyrase) and CD73. These enzymes are expressed by many cell types, including endothelial cells and immune cells.
The counterpart of ATP is adenosine, which is produced by breakdown of intra- or extracellular ATP. Adenosine has mainly anti-inflammatory effects by binding to the adenosine, or P1, receptors (A1, A2A, A2B, and A3). These receptors are also expressed in many cells, including immune cells. The final effect of ATP and adenosine in immune responses depends on the fine regulatory balance between the 2 molecules. In the present review, we will discuss the current knowledge on the role of these 2 molecules in the immune responses.
Adenosine, adenosine receptors and glaucoma: An updated overview
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Thus, it has been suggested that an A1 agonist/A2A antagonist mix as a new strategy for ischemic damage prevention in the retina. Moreover, hypoxia-evoked adenosine accumulation increases the expression of vascular endothelial growth factor in retinal cells by an A2 receptor mechanism [151,152]. On the other hand, it has been shown that the A1 receptor plays a key role in the inhibitory effect of ischemic preconditioning on leukocyte rolling during retinal ischemia-reperfusion in rat [153].
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Evidence suggested that the adenosine system is one of the potential target systems for therapeutic approaches in glaucoma.
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VEGF stimulates wound healing through angiogenesis, but likely promotes collagen deposition and epithelialization as well. Further study of the molecule by utilizing the protein itself, or novel forms of delivery such as gene therapy, will increase its therapeutic possibilities to accelerate closure of a chronic wound.
Nucleotides in ocular secretions: Their role in ocular physiology
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Shaping of monocyte and macrophage function by adenosine receptors
2007, Pharmacology and Therapeutics
Adenosine is an endogenous purine nucleoside that, following its release into the extracellular space, binds to specific adenosine receptors expressed on the cell surface. Adenosine appears in the extracellular space under metabolically stressful conditions, which are associated with ischemia, inflammation, and cell damage. There are 4 types of adenosine receptors (A₁, A_2A, A_2B and A₃) and all adenosine receptors are members of the G protein-coupled family of receptors. Adenosine receptors are expressed on monocytes and macrophages and through these receptors adenosine modulates monocyte and macrophage function. Since monocytes and macrophages are activated by the same danger signals that cause accumulation of extracellular adenosine, adenosine receptors expressed on macrophages represent a sensor system that provide monocytes and macrophages with information about the stressful environment. Adenosine receptors, thus, allow monocytes and macrophages to fine-tune their responses to stressful stimuli. Here, we review the consequences of adenosine receptor activation on monocyte/macrophage function. We will detail the effect of stimulating the various adenosine receptor subtypes on macrophage differentiation/proliferation, phagocytosis, and tissue factor (TF) expression. We will also summarize our knowledge of how adenosine impacts the production of extracellular mediators secreted by monocytes and macrophages in response to toll-like receptor (TLR) ligands and other inflammatory stimuli. Specifically, we will delineate how adenosine affects the production of superoxide, nitric oxide (NO), tumor necrosis factor-α, interleukin (IL)-12, IL-10, and vascular endothelial growth factor (VEGF). A deeper insight into the regulation of monocyte and macrophage function by adenosine receptors should assist in developing new therapies for inflammatory diseases.
Synergistic up-regulation of vascular endothelial growth factor expression in murine macrophages by adenosine A<inf>2A</inf> receptor agonists and endotoxin
2002, American Journal of Pathology
Citation Excerpt :
In canine myocardial vascular smooth muscle cells, adenosine A2 agonists also stimulated VEGF protein levels by 51 to 132% and VEGF mRNA levels by 44 to 90%.36 In a human macrophage cell line (U937 cells), adenosine A2 receptor antagonists inhibited hypoxic induction of VEGF mRNA, suggesting that in these cells hypoxic induction of VEGF mRNA is mediated, at least in part, by endogenously released adenosine.41 Consistent with the capacity of A2A receptor occupancy to promote VEGF expression, NECA also up-regulated VEGF mRNA in U937 cells.
Under normoxic conditions, macrophages from C57BL mice produce low levels of vascular endothelial growth factor (VEGF). Hypoxia stimulates VEGF expression by ∼500%; interferon-γ (IFN-γ) with endotoxin [lipopolysaccharide (LPS)] also stimulates VEGF expression by ∼50 to 150% in an inducible nitric oxide synthase (iNOS)-dependent manner. Treatment of normoxic macrophages with 5′-N-ethyl-carboxamido-adenosine (NECA), a nonselective adenosine A₂ receptor agonist, or with 2-[p-(2-carboxyethyl)-phenylethyl amino]-5′-N-ethyl-carboxamido-adenosine (CGS21680), a specific adenosine A_2A receptor agonist, modestly increases VEGF expression, whereas 2-chloro-N⁶-cyclopentyl adenosine (CCPA), an adenosine A₁ agonist, does not. Treatment with LPS (0 to 1000 ng/ml), or with IFN-γ (0 to 300 U/ml), does not affect VEGF expression. In the presence of LPS (EC₅₀ < 10 ng/ml), but not of IFN-γ, both NECA and CGS21680 synergistically up-regulate VEGF expression by as much as 10-fold. This VEGF is biologically active in vivo in the rat corneal bioassay of angiogenesis. Inhibitors of iNOS do not affect this synergistic induction of VEGF, and macrophages from iNOS−/− mice produce similar levels of VEGF as wild-type mice, indicating that NO does not play a role in this induction. Under hypoxic conditions, VEGF expression is slightly increased by adenosine receptor agonists but adenosine A₂ or A₁ receptor antagonists 3,7-dimethyl-1-propargyl xanthine (DMPX), ZM241385, and 8-cyclopentyl-1,3-dipropylxanthine (DCPCX) do not modulate VEGF expression. VEGF expression is also not reduced in hypoxic macrophages from A₃−/− and A_2A−/− mice. Thus, VEGF expression by hypoxic macrophages does not seem to depend on endogenously released or exogenous adenosine. VEGF expression is strongly up-regulated by LPS/NECA in macrophages from A₃−/− but not A_2A−/− mice, confirming the role of adenosine A_2A receptors in this pathway. LPS with NECA strongly up-regulates VEGF expression by macrophages from C₃H/HeN mice (with intact Tlr4 receptors), but not by macrophages from C₃H/HeJ mice (with mutated, functionally inactive Tlr4 receptors), implicating signaling through the Tlr4 pathway in this synergistic up-regulation. Finally, Western blot analysis of adenosine A_2A receptor expression indicated that the synergistic interaction of LPS with A_2A receptor agonists does not involve up-regulation of A_2A receptors by LPS. These results indicate that in murine macrophages there is a novel pathway regulating VEGF production, that involves the synergistic interaction of adenosine A_2A receptor agonists through A_2A receptors with LPS through the Tlr4 pathway, resulting in the strong up-regulation of VEGF expression by macrophages in a hypoxia- and NO-independent manner.

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Regular ArticleAdenosine as an Endogenous Mediator of Hypoxia for Induction of Vascular Endothelial Growth Factor mRNA in U-937 Cells

Abstract

Regular Article
Adenosine as an Endogenous Mediator of Hypoxia for Induction of Vascular Endothelial Growth Factor mRNA in U-937 Cells