Control of adenosine transport by hypoxia.

The extracellular accumulation of the nucleoside adenosine is one of the first steps in a protective auto/ paracrine signaling cascade aimed at limiting cellular damage in response to adverse conditions including hypoxia or ischemia.1 This adenosine acts as a signal molecule that is able to mediate numerous physiological and metabolic effects that could be beneficial to hypoxic cells including vasodilation, stimulation of glycogen breakdown to provide glucose for ATP production via anaerobic glycolysis and reduction of neuronal excitability as well as neurotransmitter release to reduce neuronal energy requirements.2 These intracellular effects of adenosine are mediated by 4 subtypes of G-protein–coupled adenosine receptors (AR) (A1, A2A, A2B, and A3) which differ in their expression profiles in defined cell types, the type of G-proteins to which they are coupled and their sensitivity to control by receptor phosphorylation.3 Endothelial cells predominantly express the A2A and A2BAR subtypes. The expression of both subtypes is regulated by hypoxia, such that low pO2 induces A2BAR expression, which promotes the expression of angiogenic factors and reduction of A2AAR expression.4 The expression of the AR is further stimulated by increased extracellular adenosine during hypoxia.3 Adenosine released from the endothelium during systemic hypoxia acts back on AR on the endothelium to increase the synthesis of nitric oxide (NO) which then causes vasodilatation.5 Therefore, the regulation of extracellular adenosine levels is critical for the interaction of adenosine with its receptors and subsequent responses that modify cell function in response to hypoxia. Adenosine can be generated and metabolized both intraand extracellularly (Figure). Cytosolic 5 and membranebound ecto-5 -AMP nucleotidases (CD73) produce adenosine from AMP. Alternatively, adenosine can be generated from hydrolysis of S-adensylhomocysteine by S-adenosylhomocysteine hydrolase.6 On the other hand, adenosine deaminase which is widely distributed in many cells and tissues converts adenosine to inosine, whereas adenosine kinase catalyzes the formation of AMP from adenosine.6 Hypoxia leads to the breakdown of ATP resulting in nucleotide catabolism predominantly via dephosphorylation of AMP by 5 -nucleotidases.3 Hypoxia can upregulate an adenine nucleotide-metabolizing ecto-enzyme cascade comprising ecto-ATP apyrase (CD39) and CD73, whereby de novo synthesis of functional active CD73 is dependent on the hypoxia-inducible transcription factor HIF-1 .7 Gene targeting studies in mice demonstrated that functional CD39 and CD73 are necessary to maintain endothelial barrier function and to prevent vascular leakage after hypoxia.8 In addition, extracellular adenosine concentrations may be further potentiated by preventing reutilization through hypoxic inhibition of adenosine kinase and adenosine deaminase.3,9 Thus, hypoxia appears to induce a program which shifts the cellular phenotype toward an increase in intracellular adenosine. Importantly, adenosine flux across the membrane depends on the concentration gradient between extraand intracellular nucleoside levels.6 In human umbilical vein endothelial cells (HUVEC), adenosine is generated continuously but is immediately recycled via adenosine kinase thus leaving the cytosolic adenosine concentrations low and allowing adenosine uptake.1 The capacity to take up adenosine from the extracellular space plays a prominent role in adenosine homeostasis.10 Because adenosine is not lipophilic, release and uptake of adenosine requires nucleoside membrane transport which is conducted by 2 families of unrelated nucleoside transporter proteins. Active sodium-dependent nucleoside transport is found primarily in specialized epithelial tissues and is mediated by members of the concentrative nucleoside transporter family (SLC28). Passive nucleoside transport processes are ubiquitous and are mediated by members of the equilibrative nucleoside transporter (ENT) family (SLC29).11 ENTs are bidirectional, allowing adenosine release and uptake by facilitated diffusion along its concentration gradient. The human and rodent genomes encode 4 ENT isoforms, designated ENT1–4. The best characterized of these isoforms, ENT1 and ENT2, have broad selectivity and have been classified on the basis of their sensitivity to inhibition by nitrobenzylmercaptopurine riboside (NBMPR). The importance of ENTs in controlling adenosine levels is apparent from coronary vasodilatory and cardioprotectant pharmacological agents such as dipyridamole which inhibit ENTs thereby preventing reuptake of adenosine and potentiating AR activation.11 In this issue of Circulation Research, Casanello et al12 investigated the role of hypoxia in the regulation of ENT expression and activity in HUVEC. In contrast to other studies routinely exposing HUVEC to 20% O2, normoxia was here set to 5% O2 to model the physiological oxygen concentration in the umbilical vein and to 1% to 2% O2 for hypoxic incubations. Local changes in pO2 are important determinants controlling umbilical vein tone thereby regulating blood flow from the placenta to the fetus.13 Whereas hypoxia induces vasodilation, pO2 levels exceeding 5% O2 The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From Experimental Pediatric Cardiology, Department of Pediatric Cardiology and Congenital Heart Disease, German Heart Center Munich at the TU Munich, Germany. Correspondence to Dr Agnes Görlach, Experimental Pediatric Cardiology, Department of Pediatric Cardiology and Congenital Heart Disease, German Heart Center Munich at the TU Munich, Lazarettstr. 36, 80636 Munich, Germany. [email protected] (Circ Res. 2005;97:1-3.) © 2005 American Heart Association, Inc.