Functions and Regulation of Erythrocyte Equlibrative Nucleoside Transporter 1 (ENT1) in Acute Hypoxia Mediated Tissue Injury

Using a nonbiased high throughput metabolomic screen, coupled with genetic and pharmacological approaches, recent studies demonstrated that excessive adenosine signaling through the A2B adenosine receptor triggers sickling by induction of 2,3-bisphosphoglycerate (2,3-BPG), an erythroid specific metabolite that induces O₂ release from hemoglobin. Adenosine is a signaling nucleoside that elicits numerous physiological and pathological effects by engaging membrane receptors. Notably, equlibrative nucleoside transporters (ENTs) on erythrocytes have been long speculated to regulate extracellular adenosine concentrations under hypoxic conditions. Thus, we hypothesize that ENT is likely a key molecule responsible for elevated circulating adenosine levels and protects tissues from hypoxia induced injury. To test this hypothesis, we first conducted in vivo Carbon-14 labeled adenosine (C14-Ado) injection and in vitro functional C14-Ado uptake assays. We found that erythrocyte plays a key role in regulation of circulating adenosine. We then conducted western blot analysis to compare expression profiles of ENTs on erythrocyte. We found that ENT1 is the major ENT expressed on both mouse and human erythrocytes. Using genetic approach, we successfully generated an erythrocyte ENT1 knockout mouse model. Using this genetic model and pharmacological approach combined with in vivo C14-Ado injection and in vitro C14-Ado uptake assay, we demonstrated that ENT1 1) is the major adenosine transporter in erythrocyte and 2) erythrocyte is the major cell type involved in regulating circulating adenosine levels through ENT1’s function. Using erythrocyte ENT1 knockout mouse model, we found that, during acute hypoxia treatment, the loss of erythrocyte ENT1 can cause faster increase in circulating adenosine level, subsequently promoting 2,3-BPG production, triggering oxygen release, and protecting acute hypoxia-mediated tissue injury. Mechanistically, we demonstrated that hypoxia regulates ENT1 activity through adenosine-ADORA2B-PKA signaling pathway. Overall, our studies demonstrate that 1) ENT1 is a major adenosine transporter expressed by erythrocytes and erythrocytes are the major cell type responsible for regulating circulating adenosine. 2) Hypoxia regulates ENT1 activity through adenosine-ADORA2B-PKA signaling pathway. 3) Inhibition or deletion of erythrocyte ENT1 results in enhanced adenosine-mediated 2,3-BPG induction and hemoglobin deoxygenation in RBCs when hypoxia is encountered. Thus, our findings suggest that erythrocyte ENT1 and ADORA2B are novel targets to prevent hypoxia-mediated tissue injury.