Stimulation of Wound Revascularization by Adenosine Receptor Activation - Extracellular ATP and Adenosine as Regulators of Endothelial Cell Function - page 102

receptors in promoting angiogenesis. The reduction in vascularity by paharmaclog-
ical blockade of A 2A receptors corresponded as well with a reduction in Evans blue
extravasation and in leukocyte accumulation after injection of carrageenan into the
airpouch (unpublished results).
In the mouse ischemic hind limb model, antagonism of adenosine receptors with
the non selective antagonist caffeine abrogated VEGF up-regulation induced by
local injection of the non selective agonist NECA into the hind limb and produced
a 46% reduction of neovascularization. Stimulation of adenosine receptors not only
contributed to the overall effect of hypoxia but also had additional actions in the reg-
ulation of angiogenic factors. Thus, adenosine actions appear to be complementary
to the direct effects of hypoxia rather than redundant [47].
6.4.1 Adenosine Receptor Activation Promotes Wound
Vasculogenesis
In addition to the classic mechanisms of angiogenesis, bone marrow derived
endothelial precursor cells, also referred to as angioblasts, have the ability to
incorporate into sites of active neovascularization in vivo, contributing to the devel-
opment of new vessels in the adult [8]. Adenosine A 2A receptors have been detected
in forming blood vessels and endothelial progenitor cells in the nerve fiber layer
of peripheral retina, both in neonatal dogs and during the vasoproliferative stage
of canine oxygen induced retinopathy [58]. We have shown that topical application
of an adenosine A 2A receptor agonist promotes endothelial precursor cells hom-
ing to newly forming vessels in wound granulation tissue. We inflicted two full
thickness wounds in mice that had undergone bone marrow transplantation with
whole non-fractionated bone marrow cells from transgenic mice that express the
marker gene green fluorescent protein (GFP) under the endothelial specific Tie2
promoter. Tie2 is a specific receptor only found in vascular endothelial cells there-
fore the recipient mice express GFP only in those vascular endothelial cells that
were originated in the bone marrow. Topical treatment with the selective adeno-
sine A 2A agonist CGS-21680 induced a three-fold increase in CD-31 positive,
GFP negative endothelial cells and a higher than 10-fold increase in GFP cells in
the granulation tissue of 3-day wounds [39]. Adenosine A 2A receptor activation
also increases dermal recruitment of other bone marrow originated and peripheral
blood circulating cells, fibrocytes, which migrate into injured tissue where they dis-
play fibroblast-like properties. In a bleomycin-induced model of dermal fibrosis,
increased fibrocytes recruitment to the dermis of bleomycin-treated mice was abro-
gated by genetic deletion or pharmacological blockade of adenosine A 2A receptors,
suggesting that this receptor subtype is implicated in bone marrow derived fibrocyte
recruitment [24].
Bone marrow-derived mesenchymal stem cells are characterized by their abil-
ity to form colonies comprising spindle-shaped cells deriving from a single cell,