Regulated Extracellular Nucleotide Metabolism and Function at the Mucosa - Extracellular ATP and Adenosine as Regulators of Endothelial Cell Function - page 128

HIF-1 and Sp1 under hypoxic conditions [31]. It remains unclear whether hypoxia-
mediated increases in CD39 are due to HIF-induced Sp1 or through a more complex
transcriptional coordination.
While metabolic control of adenosine generation at sites of tissue hypoxia /
inflammation is now well established, it is only recently appreciated that subse-
quent signaling through adenosine receptors can also be influenced by low pO 2
environments. A number of studies have strongly implicated enhanced adenosine
A2B receptor (A2BR) signaling by hypoxia and inflammation within the mucosa
[15, 25, 26, 34-36, 64]. As an interesting caveat, it was recently shown that the neu-
ronal guidance molecule netrin-1 binds to and activates A2BR as a mechanism of
attenuating inflammation within the mucosa [52]. With regard to adenosine recep-
tor regulation, analysis of the cloned human A2BR promoter identified a functional
hypoxia-responsive region, including a functional binding site for hypoxia-inducible
factor (HIF) within the A2BR promoter [36]. Further studies examining HIF-
1alpha DNA binding and HIF-1alpha gain and loss of function confirmed strong
dependence of A2BR induction by HIF-1alpha in vitro and in vivo mouse mod-
els. Additional studies in endothelia over-expressing full-length A2BR revealed
functional phenotypes of increased endothelial barrier function and enhanced angio-
genesis [36]. More recently, studies in pulmonary endothelial cells revealed that
the A2AR is induced selectively through HIF-2-dependent mechanisms. Adenoviral
vector over-expression and siRNA-mediated repression of HIF-2alpha demonstrated
prominent upregulation of A2AR that correlated with increased angiogenesis and
increased A2AR in lung tumor samples [1]. Together, these studies indicate the cen-
tral role of hypoxia in mediating a heightened capacity for extracellular nucleotide
metabolism, coupled with an adaptive physiological response through the regulation
of adenosine receptors.
8.5 Therapeutic Considerations for Targeting Nucleotide
Metabolism
The design and implementation of adenosine receptor agonists and antagonists is
currently an area of intense investigation [8, 11]. While drugs targeting adenosine
receptors hold great promise in a variety of diseases, specificity and pharmacody-
namics have been significant challenges. An alternative to modulating adenosine
receptor signaling may be pharmacologically regulate extracellular nucleotide
metabolism.
One strategy may be to enhance the rate of extracellular adenosine formation
using soluble nucleotidases. Some work has been done in animal models using sol-
uble 5 nucleotidase and apyrase. Administration of 5 -nucleotidase has been shown
to be beneficial in a number of experimental scenarios. For example, enzyme admin-
istration promotes vascular barrier function and decreases neutrophil accumulation
in inflammatory models [18, 69]. A significant limitation for this line of work has
been the identification of a reliable source of purified protein. Indeed, one of the