Extracellular levels of adenosine increase during hypoxia. While acute increases in adenosine are important to counterbalance excessive inflammation or vascular leakage, chronically elevated adenosine levels may be toxic. Thus, we reasoned that clearance mechanisms might exist to offset deleterious influences of chronically elevated adenosine. Guided by microarray results revealing induction of endothelial adenosine deaminase (ADA) mRNA in hypoxia, we used in vitro and in vivo models of adenosine signaling, confirming induction of ADA protein and activity. Further studies in human endothelia revealed that ADA-complexing protein CD26 is coordinately induced by hypoxia, effectively localizing ADA activity at the endothelial cell surface. Moreover, ADA surface binding was effectively blocked with glycoprotein 120 (gp120) treatment, a protein known to specifically compete for ADA-CD26 binding. IntroductionPhysiologic adaptation and pathophysiologic response to hypoxia are currently areas of intense investigation, and several reports suggest that both transcriptional and metabolic pathways may contribute to a broad range of diseases. 1 For example, during episodes of hypoxia/ischemia, polymorphonuclear leukocytes (PMNs) are mobilized from the intravascular space to the interstitium, 2,3 and such responses may contribute significantly to tissue damage during subsequent reperfusion. 4 Emigration of PMNs through the endothelial barrier is associated with a disruption of tissue barriers, creating the potential for vascular fluid leakage and subsequent edema formation. 5,6 At the same time, extracellular nucleotide metabolites (particularly adenosine) may function as endogenous anti-inflammatory mediators during hypoxia. 1,3,4,6,7 Vascular adenosine signaling during hypoxia has been implicated, dampening pathophysiologic changes related to increased tissue permeability, accumulation of inflammatory cells, and transcriptional induction of proinflammatory cytokines during hypoxia. 1,8 Several lines of evidence support this assertion: first, adenosine receptors are widely expressed on vascular endothelial cells, and have been studied for their capacity to modulate inflammation; 1,3,5 second, murine models of inflammation and/or hypoxia provide evidence for adenosine receptor signaling as a mechanism for regulating hypoxia responses in vivo. Indeed, mice genetically deficient in surface enzymes necessary for adenosine generation (ecto-apyrase, CD39 [conversion of ATP to AMP] and ecto-5Ј-nucleotidase, CD73 [conversion of AMP to adenosine]) show increased hypoxia-associated tissue damage and vascular leak syndrome during hypoxia. 5,6 Third, hypoxia accompanies the normal inflammatory response [9][10][11] and is associated with significantly increased levels of adenosine. 12,13 The exact source(s) of adenosine are not well defined, but likely include a combination of increased intracellular metabolism and amplified extracellular phosphohydrolysis of adenine nucleotides via surface ectonucleotidases. 5,6 In addition, recent stud...