Extracellular adenosine (Ado) has been implicated as central signaling molecule during conditions of limited oxygen availability (hypoxia), regulating physiologic outcomes as diverse as vascular leak, leukocyte activation, and accumulation. Presently, the molecular mechanisms that elevate extracellular Ado during hypoxia are unclear. In the present study, we pursued the hypothesis that diminished uptake of Ado effectively enhances extracellular Ado signaling. Initial studies indicated that the half-life of Ado was increased by as much as fivefold after exposure of endothelia to hypoxia. Examination of expressional levels of the equilibrative nucleoside transporter (ENT)1 and ENT2 revealed a transcriptionally dependent decrease in mRNA, protein, and function in endothelia and epithelia. Examination of the ENT1 promoter identified a hypoxia inducible factor 1 (HIF-1)–dependent repression of ENT1 during hypoxia. Using in vitro and in vivo models of Ado signaling, we revealed that decreased Ado uptake promotes vascular barrier and dampens neutrophil tissue accumulation during hypoxia. Moreover, epithelial Hif1 α mutant animals displayed increased epithelial ENT1 expression. Together, these results identify transcriptional repression of ENT as an innate mechanism to elevate extracellular Ado during hypoxia.

Abstract-Nucleoside transporters (NTs) comprise 2 widely expressed families, the equilibrative nucleoside transporters (diffusion-limited channels) and concentrative nucleoside transporters (sodium-dependent transporters). Because of their anatomic position at the blood-tissue interface, vascular NTs are in an ideal position to influence vascular nucleoside levels, particularly adenosine, which among others plays an important role in tissue protection during acute injury. For example, endothelial NTs contribute to preserving the vascular integrity during conditions of limited oxygen availability (hypoxia). Indeed, hypoxia-inducible factor-1-dependent repression of NTs results in enhanced extracellular adenosine signaling and thus attenuates hypoxia-associated increases in vascular leakage. In addition, vascular NTs also contribute to cardiac ischemic preconditioning, coronary vasodilation, and inhibition of platelet aggregation. Moreover, vascular nucleoside uptake via NTs is important for nucleoside recovery, particularly in cells lacking de novo nucleotide synthesis pathways (erythrocytes, leukocytes). Taken together, vascular NTs are critical in modulating adenosine-mediated responses during conditions such as inflammation or hypoxia. (Arterioscler