Stimulation of Wound Revascularization by Adenosine Receptor Activation - Extracellular ATP and Adenosine as Regulators of Endothelial Cell Function - page 97

recruited inflammatory cells, the subsequent up-regulation of A 2A and A 2B receptors
on endothelial cells and other inflammatory cells along with endogenous adenosine
release constitutes a feedback loop to suppress further inflammation and to promote
angiogenesis.
6.3 Adenosine Receptor Activation Promotes Angiogenesis
In Vitro
Since the early studies demonstrating the angiogenic effect of adenosine in heart
and skeletal muscle in rabbits [64] and the chick chorioallantoic membrane model
[10], much effort has been addressed to elucidate the mechanisms and the receptors
implicated in the adenosine-induced increase in vascularity. For instance, adenosine
is known to stimulate endothelial cell migration and tube formation in vitro regard-
less the vascular origin [18, 33, 59]. On the other hand, the effect of adenosine
on endothelial cell proliferation is more controversial and seems to depend on the
vascular bed of origin. Thus, adenosine is mitogenic for microvascular and venu-
lar coronary and umbilical endothelial cells [35, 54], but not for aortic endothelial
cells [59]. Similarly, the adenosine receptor subtypes and the intracellular mediators
involved in promoting angiogenesis are still being explored.
Former studies, using venular coronary microvascular endothelial cells and
adenosine analogues, suggested that the proliferative response to adenosine involves
a pertussis toxin-sensitive substrate as well as an increase in cAMP, indicating that
both Gi or Gs protein were involved [35]. However, as previously mentioned, most
endothelial cells express mainly adenosine A 2A and/or A 2B receptors, and both
receptors seem to participate of the effect of adenosine on endothelial cell prolifer-
ation, migration and release of regulatory agents such as VEGF. Studies carried out
in retinal endothelial cells clearly demonstrated the participation of A 2B adenosine
receptors on cell proliferation and migration, ERK activation, and, moreover, capil-
lary tube formation by a mechanism involving increased angiogenic growth factor
expression [18, 19]. In addition, analysis of the cloned human A 2B receptor identi-
fied a functional binding site for hypoxia-inducible factor (HIF) within the promoter.
Additional studies in dermal endothelial cells overexpressing full-length A 2B recep-
tors revealed functional phenotypes of increased barrier function and enhanced tube
formation amplified during hypoxia [28].
Activation of A 2A receptor in human umbilical vein endothelial cells promotes
endothelial cell migration and proliferation, and synthesis of the potent angiogenic
factor VEGF. The mitogenic action of the A 2A -adenosine receptor activation is
independent of protein Gs coupling, since direct activation of downstream effec-
tors in the cyclic AMP-signalling cascade not only failed to mimic the action of
receptor-activation, but even reduced cell proliferation. In contrast, activation of the
A 2A -adenosine receptor is associated with increased levels of GTP-bound p21(ras)
and MAP kinase activation, which is neither impaired following pretreatment of
the cells with pertussis toxin nor affected by inhibitors of typical protein kinase C
isoforms [53-55].