Equilibrative nucleoside transporter 1 (ENT1) regulates postischemic blood flow during acute kidney injury in mice

Abstract

A complex biologic network regulates kidney perfusion under physiologic conditions. This system is profoundly perturbed following renal ischemia, a leading cause of acute kidney injury (AKI) -a life-threatening condition that frequently complicates the care of hospitalized patients. Therapeutic approaches to prevent and treat AKI are extremely limited. Better understanding of the molecular pathways promoting postischemic reflow could provide new candidate targets for AKI therapeutics. Due to its role in adapting tissues to hypoxia, we hypothesized that extracellular adenosine has a regulatory function in the postischemic control of renal perfusion. Consistent with the notion that equilibrative nucleoside transporters (ENTs) terminate adenosine signaling, we observed that pharmacologic ENT inhibition in mice elevated renal adenosine levels and dampened AKI. Deletion of the ENTs resulted in selective protection in Ent1 -/-mice. Comprehensive examination of adenosine receptor-knockout mice exposed to AKI demonstrated that renal protection by ENT inhibitors involves the A2B adenosine receptor. Indeed, crosstalk between renal Ent1 and Adora2b expressed on vascular endothelia effectively prevented a postischemic no-reflow phenomenon. These studies identify ENT1 and adenosine receptors as key to the process of reestablishing renal perfusion following ischemic AKI. If translatable from mice to humans, these data have important therapeutic implications. IntroductionAcute kidney injury (AKI) is clinically defined by an abrupt reduction in kidney function (e.g., a decrease in glomerular filtration rate [GFR]), occurring over a period of minutes to days. AKI is frequently caused by an obstruction of renal blood flow (renal ischemia) and represents an important cause of morbidity and mortality of patients (1-3). Indeed, a recent study revealed that only a mild increase (0.3 mg/dl) in the serum creatinine level is associated with a 70% greater risk of death than in patients without this increase (2, 3). Particularly for surgical patients, AKI represents a significant threat. For example, surgical procedures requiring cross-clamping of the aorta and renal vessels are associated with a rate of AKI of up to 30% (4). Similarly, AKI after cardiac surgery occurs in up to 10% of patients under normal circumstances and is associated with dramatic increases in mortality (5). In addition, patients with sepsis frequently go on to develop AKI, and the combination of moder-

Alkaline phosphatase protects against renal inflammation through dephosphorylation of lipopolysaccharide and adenosine triphosphate

“…Interestingly, LPS administration significantly decreased the urinary excretion of adenosine [placebo: 77 (50-82) pg adenosine 10 μg À1 creatinine, LPS: 17 (6-33) pg adenosine 10 μg À1 creatinine; P < 0.01], without affecting the excretion of cAMP, ATP, ADP and AMP (data not shown). This may suggest translocation of adenosine to the proximal tubule cells where it is taken up by equilibrative nucleoside transporter-1, thereby restoring intracellular ATP levels (Grenz et al, 2012;Weinberg and Venkatachalam, 2012). RecAP treatment had no effect on adenosine receptor gene expression (Table 2), or on urinary adenosine excretion [LPS + recAP: 11 (5-32) pg adenosine 10 μg À1 creatinine; P < 0.01 compared with placebo] compared with LPS alone.…”

Reduced adenosine A2a receptor–mediated efferent arteriolar vasodilation contributes to diabetes-induced glomerular hyperfiltration

“…36 However, adenosine is rapidly cleared by cellular reuptake mediated by basolateral equilibrative nucleoside transporters on the renal epithelia. 37 A potential explanation to the finding in the present study is that diabetes is associated with reduced intra-renal tissue concentrations of adenosine, and it therefore is a 'shortage of ligand' issue rather than a direct dysfunctional cellular signaling pathway. However, direct measurements of adenosine concentration in diabetics are missing but this speculation is supported by decreased 5 0 -NT activity in mesangial cells exposed to high glucose and increased adenosine deaminase activity in type 2 diabetic patients 38,39 Furthermore, because of increased proximal tubular reabsorption through sodiumglucose linked transporters, distal tubular Na þ K þ and Cl À concentrations are decreased, attenuating macula densa ATP release and vascular adenosine signaling through the normal function of the TGF mechanism.…”

Purinergic Signaling in Neutrophils During Inflammatory Diseases

“…In the process of signal termination, adenosine is transported from the extracellular space to the intracellular space. This transport involves balancing nucleoside transporters-diffusionlimited channels, allowing adenosine to pass freely through the cell membrane depending on its concentration gradient (48). Mice with balanced nucleoside transporter gene deletion survived, but their adenosine levels increased during disease, which assists in the protection of them during organ ischemia.…”