Stimulation of Wound Revascularization by Adenosine Receptor Activation - Extracellular ATP and Adenosine as Regulators of Endothelial Cell Function - page 99

antagonists. Moreover, in the presence of antibodies to thrombospondin-1 and its
receptor CD36, adenosine A 2A receptor agonists failed to promote tube forma-
tion by endothelial cell in Matrigel R

, indicating that the angiogenic effects of
adenosine A 2A receptor activation are, at least in part, caused by the suppression
of thrombospondin-1 secretion [9]. Thus, although elevated expression of VEGF
might account for the angiogenic effects of adenosine, it is more likely that adeno-
sine also stimulates angiogenesis via other secondary angiogenic and antiangiogenic
mediators or by way of an intracellular action [2].

6.4 Adenosine A 2A Receptor Activation Stimulates both
Angiogenesis and Vasculogenesis In Vivo

Different experimental in vivo models confirmed the role of adenosine in blood
vessel formation, where other cells, besides endothelial cells, are implicated in
tissue neovascularization by releasing a variety of soluble regulatory cytokines,
chemokines, growth factors, proteolytic enzymes and extracellulr matrix proteins,
which activate local endothelial cells and facilitate endothelial progenitor cell hom-
ing [41, 44] . Thus, adenosine A 2A receptors activation stimulates macrophage
production of VEGF synergistically with the Toll-like receptor 4 (TLR4) pathway
[29] and in human mast cells, A 2B receptors mediate IL-8 and bFGF expression and
A 3 receptors may mediate the expression of angiopoietin-2 [15].
Adenosine levels are elevated during normal retinal vascular development in
neonatal dog by the activity of the ecto-5 -nucleotidase. In the canine model of
oxygen-induced retinopathy, exposure of the developing retina to high oxygen
haults vascular development, process known as vaso-obliteration. During this stage,
5 -nucleotidase is dramatically reduced, resulting in a sharp decline in adenosine.
When animals are returned to room air, systemic levels of oxygen return to nor-
mal but the retina is hypoxic because of the lack of blood vessels. At this time,
5 -nucleotidase activity and adenosine levels are elevated well beyond normal levels.
This stage is the vasoproliferative stage and A 2A receptor immunoreactivity was sig-
nificantly elevated at the edge of forming vasculature [58]. In a similar model using
mice, daily intraperitoneal injections for 5 days of pharmacologically relevant doses
of a non selective adenosine receptor antagonist or the A 2B selective antagonists
3-N-propylxanthine (enprofylline) and 3-isobutyl-8-pyrrolidinoxanthine (IPDX)
significantly reduced retinal neovascularization. By contrast, neither A 1 nor A 2A
selective antagonists had an effect on neovascularization [36]. Further proof of the
relevance of adenosine A 2B receptors in angiogenesis has been demonstrated by
the intra-ocular injection of ribozymes designed to specifically cleave A 2B receptor
mRNA, which significantly inhibited pre-retinal neovascularisation [3].
We have also demonstrated that adenosine A 2A receptor activation stimulates
neovascularization in vivo. Topical application of agonists for adenosine A 2A recep-
tors increased the rate of wound closure and the number of microvessels in the
granulation tissue of wounds in wild-type but not A 2A deficient mice. Moreover,
A 2A receptor deficient mice manifest fewer vessels in their wound granulation