Enhanced protection from renal ischemia-reperfusion [correction of ischemia:reperfusion] injury with A(2A)-adenosine receptor activation and PDE 4 inhibition

Suppressed renoprotective purines in COVID-19 patients with acute kidney injury

Acute kidney injury (AKI) is common in patients hospitalized for COVID-19, complicating their clinical course and contributing to worse outcomes. Animal studies show that adenosine, inosine and guanosine protect the kidney against some types of AKI. However, until now there was no evidence in patients supporting the possibility that abnormally low kidney levels of adenosine, inosine and guanosine contribute to AKI. Here, we addressed the question as to whether these renoprotective purines are altered in the urine of COVID-19 patients with AKI. Purines were measured by employing ultra-high-performance liquid chromatography-tandem mass spectrometry with stable-isotope-labeled internal standards for each purine of interest. Compared with COVID-19 patients without AKI (n = 23), COVID-19 patients with AKI (n = 20) had significantly lower urine levels of adenosine (P < 0.0001), inosine (P = 0.0008), and guanosine (P = 0.0008) (medians reduced by 85%, 48% and 61%, respectively) and lower levels (P = 0.0003; median reduced by 67%) of the 2nd messenger for A2A and A2B adenosine receptors, i.e., 3',5'-cAMP. Moreover, in COVID-19 patients with AKI, urine levels of 8-aminoguanine (endogenous inhibitor of inosine and guanosine metabolism) were nearly abolished (P < 0.0001). In contrast, the "upstream" precursors of renoprotective purines, namely 5'-AMP and 5'-GMP, were not significantly altered in COVID-19 patients with AKI, suggesting defective conversion of these precursors by CD73 (converts 5'-AMP to adenosine and 5'-GMP to guanosine). These findings imply that an imbalance in renoprotective purines may contribute to AKI in COVID-19 patients and that pharmacotherapy targeted to restore levels of renoprotective purines may attenuate the risk of AKI in susceptible patients with COVID-19.

Human Recombinant Alkaline Phosphatase (Ilofotase Alfa) Protects Against Kidney Ischemia-Reperfusion Injury in Mice and Rats Through Adenosine Receptors

Adenosine triphosphate (ATP) released from injured or dying cells is a potent pro-inflammatory "danger" signal. Alkaline phosphatase (AP), an endogenous enzyme that de-phosphorylates extracellular ATP, likely plays an anti-inflammatory role in immune responses. We hypothesized that ilofotase alfa, a human recombinant AP, protects kidneys from ischemia-reperfusion injury (IRI), a model of acute kidney injury (AKI), by metabolizing extracellular ATP to adenosine, which is known to activate adenosine receptors. Ilofotase alfa (iv) with or without ZM241,385 (sc), a selective adenosine A_2A receptor (A_2AR) antagonist, was administered 1 h before bilateral IRI in WT, A_2AR KO (Adora2a^-/- ) or CD73^-/- mice. In additional studies recombinant alkaline phosphatase was given after IRI. In an AKI-on-chronic kidney disease (CKD) ischemic rat model, ilofotase alfa was given after the three instances of IRI and rats were followed for 56 days. Ilofotase alfa in a dose dependent manner decreased IRI in WT mice, an effect prevented by ZM241,385 and partially prevented in Adora2a^-/- mice. Enzymatically inactive ilofotase alfa was not protective. Ilofotase alfa rescued CD73^-/- mice, which lack a 5'-ectonucleotidase that dephosphorylates AMP to adenosine; ZM241,385 inhibited that protection. In both rats and mice ilofotase alfa ameliorated IRI when administered after injury, thus providing relevance for therapeutic dosing of ilofotase alfa following established AKI. In an AKI-on-CKD ischemic rat model, ilofotase alfa given after the third instance of IRI reduced injury. These results suggest that ilofotase alfa promotes production of adenosine from liberated ATP in injured kidney tissue, thereby amplifying endogenous mechanisms that can reverse tissue injury, in part through A_2AR-and non-A_2AR-dependent signaling pathways.

Cyclic nucleotide phosphodiesterase inhibition as a potential therapeutic target in renal ischemia reperfusion injury

AIMS

Ischemia/reperfusion (I/R) occurs in renal artery stenosis, partial nephrectomy and most commonly during kidney transplantation. It brings serious consequences such as DGF (Delayed Graft Function) or organ dysfunction leading to renal failure and ultimate death. There is no effective therapy to handle the consequences of Renal Ischemia/Reperfusion (I/R) injury. Cyclic nucleotides, cAMP and cGMP are the important second messengers that stimulate intracellular signal transduction for cell survival in response to growth factors and peptide hormones in normal tissues and in kidneys plays significant role that involves vascular tone regulation, inflammation and proliferation of parenchymal cells. Renal ischemia and subsequent reperfusion injury stimulate signal transduction pathways involved in oxidative stress, inflammation, alteration in renal blood flow leading to necrosis and apoptosis of renal cell.

MATERIALS AND METHODS

An extensive literature review of various search engines like PubMed, Medline, Bentham, Scopus, and EMBASE (Elsevier) databases was carried out. To understand the functioning of Phosphodiesterases (PDEs) and its pharmacological modulation in Renal Ischemia-Reperfusion Injury.

KEY FINDINGS

Current therapeutic options may not be enough to treat renal I/R injury in group of patients and therefore, the current review has discussed the general characteristics and physiology of PDEs and preclinical-studies defining the relationship between PDEs expression in renal injury due to I/R and its outcome on renal function.

SIGNIFICANCE

The role of PDE inhibitors in renal I/R injury and the clinical status of drugs for various renal diseases have been summarized in this review.

Dual Actions of A2A and A3 Adenosine Receptor Ligand Prevents Obstruction-Induced Kidney Fibrosis in Mice

Kidney fibrosis is the final outcome of chronic kidney disease (CKD). Adenosine plays a significant role in protection against cellular damage by activating four subtypes of adenosine receptors (ARs), A₁AR, A_2AAR, A_2BAR, and A₃AR. A_2AAR agonists protect against inflammation, and A₃AR antagonists effectively inhibit the formation of fibrosis. Here, we showed for the first time that LJ-4459, a newly synthesized dual-acting ligand that is an A_2AAR agonist and an A₃AR antagonist, prevents the progression of tubulointerstitial fibrosis. Unilateral ureteral obstruction (UUO) surgery was performed on 6-week-old male C57BL/6 mice. LJ-4459 (1 and 10 mg/kg) was orally administered for 7 days, started at 1 day before UUO surgery. Pretreatment with LJ-4459 improved kidney morphology and prevented the progression of tubular injury as shown by decreases in urinary kidney injury molecular-1 (KIM-1) and neutrophil gelatinase-associated lipocalin (NGAL) excretion. Obstruction-induced tubulointerstitial fibrosis was attenuated by LJ-4459, as shown by a decrease in fibrotic protein expression in the kidney. LJ-4459 also inhibited inflammation and oxidative stress in the obstructed kidney, with reduced macrophage infiltration, reduced levels of pro-inflammatory cytokines, as well as reduced levels of reactive oxygen species (ROS). These data demonstrate that LJ-4459 has potential as a therapeutic agent against the progression of tubulointerstitial fibrosis.

Cilomilast Ameliorates Renal Tubulointerstitial Fibrosis by Inhibiting the TGF-β1-Smad2/3 Signaling Pathway

Background: Renal tubulointerstitial fibrosis is the key pathological feature in chronic kidney diseases (CKDs) with no satisfactory therapies in clinic. Cilomilast is a second-generation, selective phosphodiesterase-4 inhibitor, but its role in renal tubulointerstitial fibrosis in CKD remains unclear.

Material and Methods: Cilomilast was applied to the mice with unilateral ureteric obstruction (UUO) and renal fibroblast cells (NRK-49F) stimulated by TGF-β1. Renal tubulointerstitial fibrosis and inflammation after UUO or TGF-β1 stimulation were examined by histology, Western blotting, real-time PCR and immunohistochemistry. KIM-1 and NGAL were detected to evaluate tubular injury in UUO mice.

Results:In vivo, immunohistochemistry and western blot data demonstrated that cilomilast treatment inhibited extracellular matrix deposition, profibrotic gene expression, and the inflammatory response. Furthermore, cilomilast prevented tubular injury in UUO mice, as manifested by reduced expression of KIM-1 and NGAL in the kidney. In vitro, cilomilast attenuated the activation of fibroblast cells stimulated by TGF-β1, as shown by the reduced expression of fibronectin, α-SMA, collagen I, and collagen III. Cilomilast also inhibited the activation of TGF-β1-Smad2/3 signaling in TGF-β1-treated fibroblast cells.

Conclusion: The findings of this study suggest that cilomilast is protective against renal tubulointerstitial fibrosis in CKD, possibly through the inhibition of TGF-β1-Smad2/3 signaling, indicating the translational potential of this drug in treating CKD.

Therapeutic targeting of neutrophil exocytosis

Dysregulation of neutrophil activation causes disease in humans. Neither global inhibition of neutrophil functions nor neutrophil depletion provides safe and/or effective therapeutic approaches. The role of neutrophil granule exocytosis in multiple steps leading to recruitment and cell injury led each of our laboratories to develop molecular inhibitors that interfere with specific molecular regulators of secretion. This review summarizes neutrophil granule formation and contents, the role granule cargo plays in neutrophil functional responses and neutrophil-mediated diseases, and the mechanisms of granule release that provide the rationale for development of our exocytosis inhibitors. We present evidence for the inhibition of granule exocytosis in vitro and in vivo by those inhibitors and summarize animal data indicating that inhibition of neutrophil exocytosis is a viable therapeutic strategy.

Inhibition of PDE4/PDE4B improves renal function and ameliorates inflammation in cisplatin-induced acute kidney injury

Nephrotoxicity is a known clinical complication of cisplatin that limits the use of this potent antitumor drug. Cyclic nucleotide phosphodiesterases (PDEs) play complex roles in physiology and pathology. PDE4, which is a member of the PDE family, has four subtypes (PDE4A-PDE4D), and PDE4B plays an important role in inflammation. Thus, in the present study, we investigated the effect of PDE4/PDE4B inhibition on renal function and inflammation in a cisplatin nephrotoxicity model. In mice, cisplatin enhanced mRNA and protein expression of PDE4B in renal tubules. After treatment with the PDE4 inhibitor cilomilast, cisplatin-induced renal dysfunction, renal tubular injury, tubular cell apoptosis, and inflammation were all improved. Next, after silencing PDE4B in vivo, we observed a protective effect against cisplatin nephrotoxicity similar to that of the PDE4 inhibitor. In vitro, cisplatin-induced renal tubular cell death was strikingly ameliorated by the PDE4 inhibitor and PDE4B knockdown along with the blockade of the inflammatory response. Considering the known roles of some cell survival pathways in antagonizing insults, we examined levels of PDE4-associated proteins sirtuin 1, phosphatidylinositol 3-kinase, and phosphorylated AKT in cisplatin-treated renal tubular cells with or without cilomilast treatment. Strikingly, cisplatin treatment downregulated the expression of the above proteins, and this effect was largely abolished by the PDE4 inhibitor. Together, these findings indicate the beneficial role of PDE4/PDE4B inhibition in treating cisplatin nephrotoxicity, possibly through antagonizing inflammation and restoring cell survival signaling pathways.

Description of a Novel Phosphodiesterase (PDE)-3 Inhibitor Protecting Mice From Ischemic Stroke Independent From Platelet Function

Supplemental Digital Content is available in the text.

Background and Purpose—

Acetylsalicylic acid and clopidogrel are the 2 main antithrombotic drugs for secondary prevention in patients with ischemic stroke (IS) without indication for anticoagulation. Because of their limited efficacy and potential side effects, novel antiplatelet agents are urgently needed. Cilostazol, a specific phosphodiesterase (PDE)-3 inhibitor, protected from IS in clinical studies comprising mainly Asian populations. Nevertheless, the detailed mechanistic role of PDE-3 inhibitors in IS pathophysiology is hardly understood. In this project, we analyzed the efficacy and pathophysiologic mechanisms of a novel and only recently described PDE-3 inhibitor (substance V) in a mouse model of focal cerebral ischemia.

Methods—

Focal cerebral ischemia was induced by transient middle cerebral artery occlusion in 6- to 8-week-old male C57Bl/6 wild-type mice receiving substance V or vehicle 1 hour after ischemia induction. Infarct volumes and functional outcomes were assessed between day 1 and day 7, and findings were validated by magnetic resonance imaging. Blood-brain barrier damage, as well as the extent of local inflammatory response and cell death, was determined.

Results—

Inhibition of PDE-3 by pharmacological blockade with substance V significantly reduced infarct volumes and improved neurological outcome on day 1 and 7 after experimental cerebral ischemia. Reduced blood-brain barrier damage, attenuated brain tissue inflammation, and decreased local cell death could be identified as potential mechanisms. PDE-3 inhibitor treatment did neither increase the number of intracerebral hemorrhages nor affect platelet function.

Conclusions—

The novel PDE-3 inhibitor substance V protected mice from IS independent from platelet function. Pharmaceutical inactivation of PDE-3 might become a promising therapeutic approach to combat IS via inhibition of thromboinflammatory mechanisms and stabilization of the blood-brain barrier.

PDE/cAMP/Epac/C/EBP-β Signaling Cascade Regulates Mitochondria Biogenesis of Tubular Epithelial Cells in Renal Fibrosis

AIMS

Cyclic adenosine 3'5'-monophosphate (cAMP) is a universal second messenger that plays an important role in intracellular signal transduction. cAMP is synthesized by adenylate cyclases from adenosine triphosphate and terminated by the phosphodiesterases (PDEs). In the present study, we investigated the role of the cAMP pathway in tubular epithelial cell mitochondrial biogenesis in the pathogenesis of renal fibrosis.

RESULTS

We found that the cAMP levels were decreased in fibrotic kidney tissues, and replenishing cAMP could ameliorate tubular atrophy and extracellular matrix deposition. The downregulation of cAMP was mainly attributed to the increased PDE4 expression in tubular epithelial cells. The inhibition of PDE4 by PDE4 siRNA or the specific inhibitor, rolipram, attenuated unilateral ureteral obstruction-induced renal interstitial fibrosis and transforming growth factor (TGF)-β1-stimulated primary tubular epithelial cell (PTC) damage. The Epac1/Rap1 pathway contributed to the main effect of cAMP on renal fibrosis. Rolipram could restore C/EBP-β and PGC-1α expression and protect the mitochondrial function and structure of PTCs under TGF-β1 stimulation. The antifibrotic role of rolipram in renal fibrosis relies on C/EBP-β and PGC-1α expression in tubular epithelial cells. Innovation and Conclusion: The results of the present study indicate that cAMP signaling regulates the mitochondrial biogenesis of tubular epithelial cells in renal fibrosis. Restoring cAMP by the PDE4 inhibitor rolipram may ameliorate renal fibrosis by targeting C/EBP-β/PGC1-α and mitochondrial biogenesis. Antioxid. Redox Signal. 29, 637-652.

Discovery and Roles of 2',3'-cAMP in Biological Systems

In 2009, investigators using ultra-performance liquid chromatography-tandem mass spectrometry to measure, by selected reaction monitoring, 3',5'-cAMP in the renal venous perfusate from isolated, perfused kidneys detected a large signal at the same m/z transition (330 → 136) as 3',5'-cAMP but at a different retention time. Follow-up experiments demonstrated that this signal was due to a positional isomer of 3',5'-cAMP, namely, 2',3'-cAMP. Soon thereafter, investigative teams reported the detection of 2',3'-cAMP and other 2',3'-cNMPs (2',3'-cGMP, 2',3'-cCMP, and 2',3'-cUMP) in biological systems ranging from bacteria to plants to animals to humans. Injury appears to be the major stimulus for the release of these unique noncanonical cNMPs, which likely are formed by the breakdown of RNA. In mammalian cells in culture, in intact rat and mouse kidneys, and in mouse brains in vivo, 2',3'-cAMP is metabolized to 2'-AMP and 3'-AMP; and these AMPs are subsequently converted to adenosine. In rat and mouse kidneys and mouse brains, injury releases 2',3'-cAMP, 2'-AMP, and 3'-AMP into the extracellular compartment; and in humans, traumatic brain injury is associated with large increases in 2',3'-cAMP, 2'-AMP, 3'-AMP, and adenosine in the cerebrospinal fluid. These findings motivate the extracellular 2',3'-cAMP-adenosine pathway hypothesis: intracellular production of 2',3'-cAMP → export of 2',3'-cAMP → extracellular metabolism of 2',3'-cAMP to 2'-AMP and 3'-AMP → extracellular metabolism of 2'-AMP and 3'-AMP to adenosine. Since 2',3'-cAMP has been shown to activate mitochondrial permeability transition pores (mPTPs) leading to apoptosis and necrosis and since adenosine is generally tissue protective, the extracellular 2',3'-cAMP-adenosine pathway may be a protective mechanism [i.e., removes 2',3'-cAMP (an intracellular toxin) and forms adenosine (a tissue protectant)]. This appears to be the case in the brain where deficiency in CNPase (the enzyme that metabolizes 2',3'-cAMP to 2-AMP) leads to increased susceptibility to brain injury and neurological diseases. Surprisingly, CNPase deficiency in the kidney actually protects against acute kidney injury, perhaps by preventing the formation of 2'-AMP (which turns out to be a renal vasoconstrictor) and by augmenting the mitophagy of damaged mitochondria. With regard to 2',3'-cNMPs and their downstream metabolites, there is no doubt much more to be discovered.

Dipyridamole attenuates ischemia reperfusion induced acute kidney injury through adenosinergic A1 and A2A receptor agonism in rats

Dipyridamole (DYP) is an anti-platelet agent with marked vasodilator, anti-oxidant, and anti-inflammatory activity. The present study investigated the role of adenosine receptors in DYP-mediated protection against ischemia reperfusion-induced acute kidney injury (AKI) in rats. The rats were subjected to bilateral renal ischemia for 40 min followed by reperfusion for 24 h. The renal damage induced by ischemia reperfusion injury (IRI) was assessed by measuring creatinine clearance, blood urea nitrogen, uric acid, plasma potassium, fractional excretion of sodium, and microproteinuria in rats. The oxidative stress in renal tissues was assessed by quantification of thiobarbituric acid-reactive substances, superoxide anion generation, and reduced glutathione level. The hematoxylin-eosin staining was carried out to observe histopathological changes in renal tissues. DYP (10 and 30 mg/kg, intraperitoneal, i.p.) was administered 30 min before subjecting the rats to renal IRI. In separate groups, caffeine (50 mg/kg, i.p.), an adenosinergic A1 and A2A receptor antagonist was administered with and without DYP treatment before subjecting the rats to renal IRI. The ischemia reperfusion-induced AKI was demonstrated by significant changes in serum as well as urinary parameters, enhanced oxidative stress, and histopathological changes in renal tissues. The administration of DYP demonstrated protection against AKI. The prior treatment with caffeine abolished DYP-mediated reno-protection suggesting role of A1 and A2A adenosine receptors in DYP-mediated reno-protection in rats. It is concluded that adenosine receptors find their definite involvement in DYP-mediated anti-oxidative and reno-protective effect against ischemia reperfusion-induced AKI.

Adenosine receptors and renal ischaemia reperfusion injury

One of the frequent clinical complications that results in billions of dollars in healthcare costs annually in the United States is acute kidney injury (AKI). Ischaemia reperfusion (IR) injury is a major cause AKI. Unfortunately, no effective treatment or preventive measure for AKI exists. With increased surgical complexity coupled with increasing number of elderly, the incidence of AKI is becoming more frequent. Adenosine is a metabolic breakdown product of adenosine triphosphate (ATP) and contributes to the regulation of multiple physiological events. Extracellular adenosine activates four subtypes of adenosine receptors (AR) including A1 AR, A2 A AR, A2 B AR and A3 AR. In the kidney, adenosine regulates glomerular filtration rate, vascular tone, renin release and is an integrative part of tubular glomerular feedback signal to the afferent arterioles. In addition, each AR subtype powerfully modulates renal IR injury. The A1 AR activation protects against ischaemic insult by reducing apoptosis, necrosis and inflammation. Activation of A2 A AR protects against renal injury by modulating leucocyte-mediated inflammation as well as directly reducing renal tubular inflammation. Activation of A2 B AR acts via direct activation of renal parenchymal as well as renovascular receptors and is important in kidney preconditioning. Finally, activation of A3 AR exacerbates renal damage following renal IR injury while A3 AR antagonism attenuates renal damage following ischaemic insult. Latest body of research suggests that kidney AR modulation may be a promising approach to treat ischaemic AKI. This brief review focuses on the signalling pathways of adenosine in the kidney followed by the role for various AR modulations in protecting against ischaemic AKI.

Mechanistic studies of a novel mycophenolic acid-glucosamine conjugate that attenuates renal ischemia/reperfusion injury in rat

Renal ischemia/reperfusion (I/R) injury causes high mortality and morbidity during renal procedures, yet current drugs should be used at high doses or for long periods due to lack of tissue specificity. In previous work we described a novel mycophenolic acid-glucosamine conjugate (MGC) that targets the proximal tubule epithelium, where it efficiently reduces renal I/R injury in rats and promotes recovery from reperfusion. Here we perform mechanistic studies of MGC in rats that suggest that the conjugate works by repressing the activation of renal inosine-5'-monophosphate dehydrogenase 2 (IMPDH2), thereby inhibiting the proliferation and accumulation of lympholeukocytes in the proximal tubules. In addition, MGC appears to inhibit inflammation through various pathways, including inhibition of free oxygen radical production, upregulation of bone morphogenetic protein-7, and downregulation of complement protein 3, TLR 4, intracellular adhesion molecules in the endothelium, proinflammatory cytokines (e.g., TNF-α, IL-6, IL-1, TGF-β), and chemotactic cytokines [e.g., monocyte chemoattractant protein-1 (MCP-1) and IL-8]. These findings suggest that MGC specifically targets the proximal tubules and acts through numerous mechanisms to substantially mitigate I/R injury in rats; this conjugate may provide a more effective alternative to current combination therapy.

The guanosine-adenosine interaction exists in vivo

In cultured renal cells and isolated perfused kidneys, extracellular guanosine augments extracellular adenosine and inosine (the major renal metabolite of adenosine) levels by altering the extracellular disposition of these purines. The present study addressed whether this "guanosine-adenosine mechanism" exists in vivo. In rats (n = 15), intravenous infusions of adenosine (1 µmol/kg per minute) decreased mean arterial blood pressure (MABP) from 114 ± 4 to 83 ± 5 mm Hg, heart rate (HR) from 368 ± 11 to 323 ± 9 beats/min), and renal blood flow (RBF) from 6.2 ± 0.5 to 5.3 ± 0.6 ml/min). In rats (n = 15) pretreated with intravenous guanosine (10 µmol/kg per minute), intravenous adenosine (1 µmol/kg per minute) decreased MABP (from 109 ± 4 to 58 ± 5 mm Hg), HR (from 401 ± 10 to 264 ± 20 beats/min), and RBF (from 6.2 ± 0.7 to 1.7 ± 0.3). Two-factor analysis of variance (2F-ANOVA) revealed a significant interaction (P < 0.0001) between guanosine and adenosine for MABP, HR, and RBF. In control rats, the urinary excretion rate of endogenous inosine was 211 ± 103 ng/30 minutes (n = 9); however, in rats treated with intravenous guanosine (10 µmol/kg per minute), the excretion rate of inosine was 1995 ± 300 ng/30 minutes (n = 12; P < 0.0001 versus controls). Because adenosine inhibits inflammatory cytokine production, we also examined the effects of intravenous guanosine on endotoxemia-induced increases in tumor necrosis factor-α (TNF-α). In control rats (n = 7), lipopolysaccharide (LPS; Escherichia coli 026:B6 endotoxin; 30 mg/kg) increased plasma TNF-α from 164 ± 56 to 4082 ± 730 pg/ml, whereas in rats pretreated with intravenous guanosine (10 µmol/kg per minute; n = 6), LPS increased plasma TNF-α from 121 ± 45 to 1821 ± 413 pg/ml (2F-ANOVA interaction effect, P = 0.0022). We conclude that the guanosine-adenosine mechanism exists in vivo and that guanosine may be a useful therapeutic for reducing inflammation.

Role of 2',3'-cyclic nucleotide 3'-phosphodiesterase in the renal 2',3'-cAMP-adenosine pathway

Energy depletion increases the renal production of 2',3'-cAMP (a positional isomer of 3',5'-cAMP that opens mitochondrial permeability transition pores) and 2',3'-cAMP is converted to 2'-AMP and 3'-AMP, which in turn are metabolized to adenosine. Because the enzymes involved in this "2',3'-cAMP-adenosine pathway" are unknown, we examined whether 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) participates in the renal metabolism of 2',3'-cAMP. Western blotting and real-time PCR demonstrated expression of CNPase in rat glomerular mesangial, preglomerular vascular smooth muscle and endothelial, proximal tubular, thick ascending limb and collecting duct cells. Real-time PCR established the expression of CNPase in human glomerular mesangial, proximal tubular and vascular smooth muscle cells; and the level of expression of CNPase was greater than that for phosphodiesterase 4 (major enzyme for the metabolism of 3',5'-cAMP). Overexpression of CNPase in rat preglomerular vascular smooth muscle cells increased the metabolism of exogenous 2',3'-cAMP to 2'-AMP. Infusions of 2',3'-cAMP into isolated CNPase wild-type (+/+) kidneys increased renal venous 2'-AMP, and this response was diminished by 63% in CNPase knockout (-/-) kidneys, whereas the conversion of 3',5'-cAMP to 5'-AMP was similar in CNPase +/+ vs. -/- kidneys. In CNPase +/+ kidneys, energy depletion (metabolic poisons) increased kidney tissue levels of adenosine and its metabolites (inosine, hypoxanthine, xanthine, and uric acid) without accumulation of 2',3'-cAMP. In contrast, in CNPase -/- kidneys, energy depletion increased kidney tissue levels of 2',3'-cAMP and abolished the increase in adenosine and its metabolites. In conclusion, kidneys express CNPase, and renal CNPase mediates in part the renal 2',3'-cAMP-adenosine pathway.

Guanosine regulates adenosine levels in the kidney

In cell culture, extracellular guanosine increases extracellular adenosine by attenuating the disposition of extracellular adenosine (American Journal of Physiology – Cell Physiology 304: C406–C421, 2013). The goal of this investigation was to determine whether this “guanosine–adenosine mechanism” is operative in an intact organ. Twenty‐seven isolated, perfused mouse kidneys were subjected to metabolic poisons (iodoacetate plus 2,4‐dinitrophenol) to cause energy depletion and thereby stimulate renal adenosine production. Adenosine levels in the renal venous perfusate increased from a baseline of 36 ± 8 to 499 ± 96, 258 ± 50, and 71 ± 13 nmol/L at 15, 30, and 60 min, respectively, after administering metabolic poisons (% of basal; 1366 ± 229, 715 ± 128, and 206 ± 33, respectively). Changes in renal venous levels of guanosine closely mirrored the time course of changes in adenosine: baseline of 15 ± 2 to 157 ± 13, 121 ± 8, and 50 ± 5 nmol/L at 15, 30, and 60 min, respectively (% of basal; 1132 ± 104, 871 ± 59, and 400 ± 51, respectively). Freeze‐clamp experiments in 12 kidneys confirmed that metabolic poisons increased kidney tissue levels of adenosine and guanosine. In eight additional kidneys, we examined the ability of guanosine to reduce the renal clearance of exogenous adenosine; and these experiments revealed that guanosine significantly decreased the renal extraction of adenosine. Because guanosine is metabolized by purine nucleoside phosphorylase (PNPase), in another set of 16 kidneys we examined the effects of 8‐aminoguanine (PNPase inhibitor) on renal venous levels of adenosine and inosine (adenosine metabolite). Kidneys treated with 8‐aminoguanine showed a more robust increase in both adenosine and inosine in response to metabolic poisons. We conclude that in the intact kidney, guanosine regulates adenosine levels.

In cell culture, extracellular guanosine increases extracellular adenosine by attenuating the disposition of extracellular adenosine (American Journal of Physiology – Cell Physiology 304: C406–C421, 2013). The goal of this study was to determine whether the “guanosine–adenosine mechanism” is operative in an intact organ. In isolated, perfused mouse kidneys, inhibition of energy production induced changes in renal venous levels of guanosine that closely mirrored the time course of changes in adenosine, and freeze‐clamp experiments confirmed that metabolic poisons similarly increased kidney tissue levels of adenosine and guanosine. Moreover, exogenous guanosine significantly decreased the renal extraction of exogenous adenosine, and inhibition of purine nucleoside phosphorylase (metabolizes guanosine) augmented the effects of energy depletion on renal levels of both guanosine and adenosine. We conclude that in the intact kidney, guanosine regulates adenosine levels.

Adenosine kinase inhibition protects the kidney against streptozotocin-induced diabetes through anti-inflammatory and anti-oxidant mechanisms

Adenosine provides anti-inflammatory effects in cardiovascular disease via the activation of adenosine A2A receptors; however, the physiological effect of adenosine could be limited due to its phosphorylation by adenosine kinase. We hypothesized that inhibition of adenosine kinase exacerbates extracellular adenosine levels to reduce renal inflammation and injury in streptozotocin-induced diabetes. Diabetes was induced in male C57BL/6 mice by daily injection of streptozotocin (50mg/kg/day, i.p. for 5 days). Control and diabetic mice were then treated with the adenosine kinase inhibitor ABT702 (1.5mg/kg, i.p. two times a week for 8 weeks, n=7-8/group) or the vehicle (5% DMSO). ABT702 treatment reduced blood glucose level in diabetic mice (∼20%; P<0.05). ABT702 also reduced albuminuria and markers of glomerular injury, nephrinuria and podocalyxin excretion levels, in diabetic mice. Renal NADPH oxidase activity and urinary thiobarbituric acid reactive substances (TBARS) excretion, indices of oxidative stress, were also elevated in diabetic mice and ABT702 significantly reduced these changes. ABT702 increased renal endothelial nitric oxide synthase expression (eNOS) and nitrate/nitrite excretion levels in diabetic mice. In addition, the diabetic mice displayed an increase in renal macrophage infiltration, in association with increased renal NFκB activation. Importantly, treatment with ABT702 significantly reduced all these inflammatory parameters (P<0.05). Furthermore, ABT702 decreased glomerular permeability and inflammation and restored the decrease in glomerular occludin expression in vitro in high glucose treated human glomerular endothelial cells. Collectively, the results suggest that the reno-protective effects of ABT702 could be attributed to the reduction in renal inflammation and oxidative stress in diabetic mice.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

In vivo cardiovascular pharmacology of 2',3'-cAMP, 2'-AMP, and 3'-AMP in the rat

The naturally occurring purine 2',3'-cAMP is metabolized in vitro to 2'-AMP and 3'-AMP, which are subsequently metabolized to adenosine. Whether in vivo 2',3'-cAMP, 2'-AMP, or 3'-AMP are rapidly converted to adenosine and exert rapid effects via adenosine receptors is unknown. To address this question, we compared the cardiovascular and renal effects of 2',3'-cAMP, 2'-AMP, 3'-AMP, 3',5'-cAMP, 5'-AMP, and adenosine in vivo in the rat. Purines were infused intravenously while monitoring mean arterial blood pressure (MABP), heart rate (HR), cardiac output, and renal and mesenteric blood flows. Total peripheral (TPR), renal vascular (RVR), and mesenteric vascular (MVR) resistances were calculated. Urine was collected for determination of urine excretion rate [urine volume (UV)]. When sufficient urine was available, the sodium excretion rate (Na(+)ER) and glomerular filtration rate (GFR) were determined. 2',3'-cAMP, 2'-AMP, and 3'-AMP dose-dependently and profoundly reduced MABP, HR, TPR, and MVR with efficacy and potency similar to adenosine and 5'-AMP. These effects of 2',3'-cAMP, 2'-AMP, and 3'-AMP were attenuated by blockade of adenosine receptors with 1,3-dipropyl-8-(p-sulfophenyl)xanthine. 2',3'-cAMP, 2'-AMP, 3'-AMP, adenosine, and 5'-AMP variably affected RVR, but profoundly (nearly 100%) decreased UV at higher doses. GFR and Na(+)ER could be measured at the lower doses and were suppressed by 2',3'-cAMP, 2'-AMP, and 3'-AMP, but not by adenosine or 5'-AMP. 2',3'-cAMP increased urinary excretion rates of 2'-AMP, 3'-AMP, and adenosine. 3',5'-cAMP exerted no adverse hemodynamic effects yet increased urinary adenosine as efficiently as 2',3'-cAMP.

CONCLUSIONS

In vivo 2',3'-cAMP is rapidly converted to adenosine. Because both cAMPs increase adenosine in the urinary compartment, these agents may provide unique therapeutic opportunities.

Osthole ameliorates renal ischemia-reperfusion injury by inhibiting inflammatory response

INTRODUCTION

Renal ischemia-reperfusion (I/R) injury is a primary cause of acute renal failure that results in high mortality. This study aimed to investigate the effect of osthole, a natural coumarin derivative, on renal I/R injury in a rat model.

MATERIALS AND METHODS

Rats were randomly allocated to the sham operation + vehicle, I/R + vehicle, and I/R + osthole groups. Renal I/R injury was induced by clamping the left renal artery for 45 min followed by 12 h of reperfusion and a contralateral nephrectomy. Osthole (40 mg/kg) was intraperitoneally injected 30 min before inducing I/R. Renal function and histological damage were determined subsequently. Myeloperoxidase activity, monocyte/macrophage infiltration, as well as tumor necrosis factor-α, IL-1β, and activated p38 mitogen-activated protein kinase expression in kidneys were also assessed.

RESULTS

Osthole treatment significantly ameliorated I/R-induced renal functional and morphological injuries. Moreover, osthole treatment attenuated myeloperoxidase activity, monocyte/macrophage infiltration, and tumor necrosis factor-α, IL-1β, and activated p38 mitogen-activated protein kinase expression in kidneys.

CONCLUSIONS

Osthole treatment ameliorates renal I/R injury by inhibiting inflammatory responses in kidneys. Thus, osthole may represent a novel practical strategy to prevent renal I/R injury.

Adenosine, adenosine receptors and their role in glucose homeostasis and lipid metabolism

Adenosine is an endogenous metabolite that is released from all tissues and cells including liver, pancreas, muscle and fat, particularly under stress, intense exercise, or during cell damage. The role of adenosine in glucose homeostasis has been attributed to its ability to regulate, through its membrane receptors, processes such as insulin secretion, glucose release and clearance, glycogenolysis, and glycogenesis. Additionally, adenosine and its multiple receptors have been connected to lipid metabolism by augmenting insulin-mediated inhibition of lipolysis, and the subsequent increase in free fatty acids and glycerol levels. Furthermore, adenosine was reported to control liver cholesterol synthesis, consequently affecting plasma levels of cholesterol and triglycerides, and the amount of fat tissue. Alterations in the balance of glucose and lipid homeostasis have implications in both cardiovascular disease and diabetes. The ability of different adenosine receptors to activate and inhibit the same signaling cascades has made it challenging to study the influence of adenosine, adenosine analogs and their receptors in health and disease. This review focuses on the role and significance of different adenosine receptors in mediating the effect of adenosine on glucose and lipid homeostasis. J. Cell. Physiol. © 2013 Wiley Periodicals, Inc.

Targeting the hypoxia-adenosinergic signaling pathway to improve the adoptive immunotherapy of cancer

The recent approval by the FDA of cancer vaccines and drugs that blockade immunological negative regulators has further enhanced interest in promising approaches of the immunotherapy of cancer. However, the disappointingly short life extension has also underscored the need to better understand the mechanisms that prevent tumor rejection and survival even after the blockade of immunological negative regulators. Here, we describe the implications of the "metabolism-based" immunosuppressive mechanism, where the local tissue hypoxia-driven accumulation of extracellular adenosine triggers suppression via A2 adenosine receptors on the surface of activated immune cells. This molecular pathway is of critical importance in mechanisms of immunosuppression in inflamed and cancerous tissue microenvironments. The protection of tumors by tumor-generated extracellular adenosine and A2 adenosine receptors could be the misguided application of the normal tissue-protecting mechanism that limits excessive collateral damage to vital organs during the anti-pathogen immune response. The overview of the current state of the art regarding the immunosuppressive effects of extracellular adenosine is followed by a historical perspective of studies focused on the elucidation of the physiological negative regulators that protect tissues of vital organs from excessive collateral damage, but, as a trade-off, may also weaken the anti-pathogen effector functions and negate the attempts of anti-tumor immune cells to destroy cancerous cells.

Extracellular 2',3'-cAMP-adenosine pathway in proximal tubular, thick ascending limb, and collecting duct epithelial cells

In a previous study, we demonstrated that human proximal tubular epithelial cells obtained from a commercial source metabolized extracellular 2',3'-cAMP to 2'-AMP and 3'-AMP and extracellular 2'-AMP and 3'-AMP to adenosine (the extracellular 2',3'-cAMP-adenosine pathway; extracellular 2',3'-cAMP → 2'-AMP + 3'-AMP → adenosine). The purpose of this study was to investigate the metabolism of extracellular 2',3'-cAMP in proximal tubular vs. thick ascending limb vs. collecting duct epithelial cells freshly isolated from their corresponding nephron segments obtained from rat kidneys. In epithelial cells from all three nephron segments, 1) extracellular 2',3'-cAMP was metabolized to 2'-AMP and 3'-AMP, with 2'-AMP > 3'-AMP, 2) the metabolism of extracellular 2',3'-cAMP to 2'-AMP and 3'-AMP was not inhibited by either 3-isobutyl-1-methylxanthine (phosphodiesterase inhibitor) or 1,3-dipropyl-8-p-sulfophenylxanthine (ecto-phosphodiesterase inhibitor), 3) extracellular 2',3'-cAMP increased extracellular adenosine levels, 4) 3'-AMP and 2'-AMP were metabolized to adenosine with an efficiency similar to that of 5'-AMP, and 5) the metabolism of 5'-AMP, 3'-AMP, and 2'-AMP was not inhibited by α,β-methylene-adenosine-5'-diphosphate (CD73 inhibitor). These results support the conclusion that renal epithelial cells all along the nephron can metabolize extracellular 2',3'-cAMP to 2'-AMP and 3'-AMP and can efficiently metabolize extracellular 2'-AMP and 3'-AMP to adenosine and that the metabolic enzymes involved are not the classical phosphodiesterases nor ecto-5'-nucleotidase (CD73). Because 2',3'-cAMP is released by injury and because previous studies demonstrate that the extracellular 2',3'-cAMP-adenosine pathway stimulates epithelial cell proliferation via adenosine A(2B) receptors, the present results suggest that the extracellular 2',3'-cAMP-adenosine pathway may help restore epithelial cells along the nephron following kidney injury.

Specific deletion of glycogen synthase kinase-3β in the renal proximal tubule protects against acute nephrotoxic injury in mice

Renal proximal tubular damage and repair are hallmarks of acute kidney injury. Because glycogen synthase kinase-3β (GSK-3β) is an important cellular regulator of survival and proliferation, we determined its role during injury and recovery of proximal tubules in a mercuric chloride-induced nephrotoxic model of acute kidney injury. Renal proximal tubule-specific GSK-3β knockout mice exposed to mercuric chloride had improved survival and renal function compared to wild type mice. Apoptosis, measured by TUNEL staining, Bax activation, and caspase 3 cleavage were all reduced in the knockout mice. The restoration of renal structure, function, and cell proliferation was also accelerated in the GSK-3β knockout mice. This enhanced repair, evidenced by increased Ki-67 and BrdU staining, along with increased cyclin D1 and c-myc levels, was recapitulated by treatment of wild type mice with the small-molecule GSK-3 inhibitor TDZD-8 following injury. This confirmed that hastened repair in the knockout mice was not merely due to lower initial injury levels. Thus, inhibition of GSK-3β prior to nephrotoxic insult protects from renal injury. Such treatment after acute kidney injury may accelerate repair and regeneration.

Pathophysiology of acute kidney injury

Acute kidney injury (AKI) is the leading cause of nephrology consultation and is associated with high mortality rates. The primary causes of AKI include ischemia, hypoxia, or nephrotoxicity. An underlying feature is a rapid decline in glomerular filtration rate (GFR) usually associated with decreases in renal blood flow. Inflammation represents an important additional component of AKI leading to the extension phase of injury, which may be associated with insensitivity to vasodilator therapy. It is suggested that targeting the extension phase represents an area potential of treatment with the greatest possible impact. The underlying basis of renal injury appears to be impaired energetics of the highly metabolically active nephron segments (i.e., proximal tubules and thick ascending limb) in the renal outer medulla, which can trigger conversion from transient hypoxia to intrinsic renal failure. Injury to kidney cells can be lethal or sublethal. Sublethal injury represents an important component in AKI, as it may profoundly influence GFR and renal blood flow. The nature of the recovery response is mediated by the degree to which sublethal cells can restore normal function and promote regeneration. The successful recovery from AKI depends on the degree to which these repair processes ensue and these may be compromised in elderly or chronic kidney disease (CKD) patients. Recent data suggest that AKI represents a potential link to CKD in surviving patients. Finally, earlier diagnosis of AKI represents an important area in treating patients with AKI that has spawned increased awareness of the potential that biomarkers of AKI may play in the future.

The effects of tadalafil on renal ischemia reperfusion injury: an experimental study

Many pharmacological agents were investigated for the prevention of renal ischemic reperfusion (I/R) injury as well as the phosphodiesterase (PDE) inhibitors. The aim of the study was to examine the possible renoprotective effect of a member in this family, tadalafil (Td) on I/R injury. Thirty-six Spraque Dawley rats were allocated to six groups as; control, sham, ischemia (I), ischemia/reperfusion (I/R), Td pretreatment ischemia (Td/I) and Td pretreatment ischemia/reperfusion (Td/IR) groups. Right nephrectomy was performed in all groups. Td was dissolved in saline solution and given as a single dose (1mg/kg) through an orogastrictube 60 min before the operation in the Td pretreatment groups. In ischemia group the left renal pedicle was occluded for 45 minutes and after than underwent left nephrectomy. In I/R group left renal pedicle was occluded for 45 minutes, reperfused for 1hour and after then underwent nephrectomy. The left kidneys were evaluated after standard laboratory procedures with regard to tubular morphology, and leukocyte infiltration. The data were analyzed by using Kruskal-Wallis test to determine differences among the groups. A p value of < 0.05 was considered significant. Renal tubular damage was significant increased in the ischemia and I/R group (Groups III and IV) when compared to those in the sham group (Group II), (p = 0.004, 0.004, respectively). Tubular damage, in the Td pretreatment ischemia (Td/I) (Group V) and Td pretreatment ischemia/reperfusion (Td/IR) (Group VI) were less than that in the ischemia group (Group III) (p= 0.010, p= 0.025, respectively). Td administration prior to the renal I/R injury attenuated these morphological disarrangements, which were observed in renal I/R. Tubular necrosis, which may be considered as an important issue of the developing renal injury, was also completely prevented with Td administration.

The 2',3'-cAMP-adenosine pathway

Our recent studies employing HPLC-tandem mass spectrometry to analyze venous perfusate from isolated, perfused kidneys demonstrate that intact kidneys produce and release into the extracellular compartment 2',3'-cAMP, a positional isomer of the second messenger 3',5'-cAMP. To our knowledge, this represents the first detection of 2',3'-cAMP in any cell/tissue/organ/organism. Nuclear magnetic resonance experiments with isolated RNases and experiments in isolated, perfused kidneys suggest that 2',3'-cAMP likely arises from RNase-mediated transphosphorylation of mRNA. Both in vitro and in vivo kidney experiments demonstrate that extracellular 2',3'-cAMP is efficiently metabolized to 2'-AMP and 3'-AMP, both of which can be further metabolized to adenosine. This sequence of reactions is called the 2',3'-cAMP-adenosine pathway (2',3'-cAMP → 2'-AMP/3'-AMP → adenosine). Experiments in rat and mouse kidneys show that metabolic poisons increase extracellular levels of 2',3'-cAMP, 2'-AMP, 3'-AMP, and adenosine; however, little is known regarding the pharmacology of 2',3'-cAMP, 2'-AMP, and 3'-AMP. What is known is that 2',3'-cAMP facilitates activation of mitochondrial permeability transition pores, a process that can lead to apoptosis and necrosis, and inhibits proliferation of vascular smooth muscle cells and glomerular mesangial cells. In summary, there is mounting evidence that at least some types of cellular injury, by triggering mRNA degradation, engage the 2',3'-cAMP-adenosine pathway, and therefore this pathway should be added to the list of biochemical pathways that produce adenosine. Although speculative, it is possible that the 2',3'-cAMP-adenosine pathway may protect against some forms of acute organ injury, for example acute kidney injury, by both removing an intracellular toxin (2',3'-cAMP) and increasing an extracellular renoprotectant (adenosine).

Interplay of hypoxia and A2B adenosine receptors in tissue protection

That adenosine signaling can elicit adaptive tissue responses during conditions of limited oxygen availability (hypoxia) is a long-suspected notion that recently gained general acceptance from genetic and pharmacologic studies of the adenosine signaling pathway. As hypoxia and inflammation share an interdependent relationship, these studies have demonstrated that adenosine signaling events can be targeted to dampen hypoxia-induced inflammation. Here, we build on the hypothesis that particularly the A(2B) adenosine receptor (ADORA(2B)) plays a central role in tissue adaptation to hypoxia. In fact, the ADORA(2B) requires higher adenosine concentrations than any of the other adenosine receptors. However, during conditions of hypoxia or ischemia, the hypoxia-elicited rise in extracellular adenosine is sufficient to activate the ADORA(2B). Moreover, several studies have demonstrated very robust induction of the ADORA(2B) elicited by transcriptional mechanisms involving hypoxia-dependent signaling pathways and the transcription factor "hypoxia-induced factor" 1. In the present chapter, genetic and pharmacologic evidence is presented to support our hypothesis of a tissue protective role of ADORA(2B) signaling during hypoxic conditions, including hypoxia-elicited vascular leakage, organ ischemia, or acute lung injury. All these disease models are characterized by hypoxia-elicited tissue inflammation. As such, the ADORA(2B) has emerged as a therapeutic target for dampening hypoxia-induced inflammation and tissue adaptation to limited oxygen availability.

Epac-Rap signaling reduces cellular stress and ischemia-induced kidney failure

Renal ischemia-reperfusion injury is associated with the loss of tubular epithelial cell-cell and cell-matrix interactions which contribute to renal failure. The Epac-Rap signaling pathway is a potent regulator of cell-cell and cell-matrix adhesion. The cyclic AMP analogue 8-pCPT-2'-O-Me-cAMP has been shown to selectively activate Epac, whereas the addition of an acetoxymethyl (AM) ester to 8-pCPT-2'-O-Me-cAMP enhanced in vitro cellular uptake. Here we demonstrate that pharmacological activation of Epac-Rap signaling using acetoxymethyl-8-pCPT-2'-O-Me-cAMP preserves cell adhesions during hypoxia in vitro, maintaining the barrier function of the epithelial monolayer. Intrarenal administration in vivo of 8-pCPT-2'-O-Me-cAMP also reduced renal failure in a mouse model for ischemia-reperfusion injury. This was accompanied by decreased expression of the tubular cell stress marker clusterin-α, and lateral expression of β-catenin after ischemia indicative of sustained tubular barrier function. Our study emphasizes the undervalued importance of maintaining tubular epithelial cell adhesion in renal ischemia and demonstrates the potential of pharmacological modulation of cell adhesion as a new therapeutic strategy to reduce the extent of injury in kidney disease and transplantation.

Targeting iNKT cells for the treatment of sickle cell disease

Sickle cell disease (SCD) causes widely disseminated vaso-occlusive episodes. Building on evidence implicating invariant NKT (iNKT) cells in the pathogenesis of ischemia/reperfusion injury, recent studies demonstrate that blockade of iNKT cell activation in mice with SCD reduces pulmonary inflammation and injury. In patients with SCD, iNKT cells in blood are increased in absolute number and activated in comparison to healthy controls. iNKT cell activation is reduced by agonists of adenosine 2A receptors (A(2A)Rs) such as the clinically approved coronary vasodilator, regadenoson. An ongoing multi-center, dose-finding and safety trial of infused regadenoson, has been initiated and is providing preliminary data about its safety and efficacy to treat SCD. Very high accumulation of adenosine may have deleterious effects in SCD through activation of adenosine 2B receptors that are insensitive to regadenoson. Future possible therapeutic approaches for treating SCD include selective A(2B)R antagonists and antibodies that deplete iNKT cells.

Regulation of leukocyte function by adenosine receptors

The immune system responds to cues in the microenvironment to make acute and chronic adaptations in response to inflammation and injury. Locally produced purine nucleotides and adenosine provide receptor-mediated signaling to all bone-marrow derived cells of the immune system to modulate their responses. This review summarizes recent advances in our understanding of the effects of adenosine signaling through G protein-coupled adenosine receptors on cells of the immune system. Adenosine A(2A) receptors (A(2A)Rs) have a generally suppressive effect on the activation of immune cells. Moreover, their transcription is strongly induced by signals that activate macrophages or dendritic cells through toll-like receptors, or T cells through T cell receptors. A(2A)R induction is responsible for producing a gradual dissipation of inflammatory responses. A(2A)R activation is particularly effective in limiting the activation of invariant NKT (iNKT) cells that play a central role in acute reperfusion injury. A(2A) agonists have clinical promise for the treatment of vaso-occlusive tissue injury. Blockade of A(2A) receptors may be useful to enhance immune-mediated killing of cancer cells. A(2B)R expression also is transcriptionally regulated by hypoxia, cytokines, and oxygen radicals. Acute A(2B)R activation attenuates the production of proinflammatory cytokines from macrophages, but sustained activation facilitates macrophage and dendritic cell remodeling and the production of acute phase proteins and angiogenic factors that may participate in evoking insulin resistance and tissue fibrosis. A(2B)R activation also influences macrophage and neutrophil function by influencing expression of the anti-inflammatory netrin receptor, UNC5B. The therapeutic significance of adenosine-mediated effects on the immune system is discussed.

Targeting of adenosine receptors in ischemia-reperfusion injury

IMPORTANCE OF THE FIELD

Ischemia-reperfusion (IR) injury is a common problem after transplantation as well as myocardial infarction and stroke. IR initiates an inflammatory response leading to rapid tissue damage. Adenosine, produced in response to IR, is generally considered a protective signaling molecule and elicits its physiological responses through four distinct adenosine receptors. The short half-life, lack of specificity and rapid metabolism limits the use of adenosine as a therapeutic agent. Thus, intense research efforts have focused on the synthesis and implementation of specific adenosine receptor agonists and antagonists as potential therapeutic agents for a variety of inflammatory conditions including IR injury.

AREAS COVERED IN THIS REVIEW

Current knowledge on IR injury with a focus on lung, heart and kidney and studies that have advanced our understanding of the role of adenosine receptors and the therapeutic potential of adenosine receptor agonists and antagonists for the prevention of IR injury.

WHAT THE READER WILL GAIN

Insight into the role of adenosine receptor signaling in IR injury.

TAKE HOME MESSAGE

No therapies are currently available that specifically target IR injury; however, targeting of specific adenosine receptors may offer therapeutic strategies in this regard.

Olprinone attenuates the acute inflammatory response and apoptosis after spinal cord trauma in mice

Background

Olprinone hydrochloride is a newly developed compound that selectively inhibits PDE type III and is characterized by several properties, including positive inotropic effects, peripheral vasodilatory effects, and a bronchodilator effect. In clinical settings, olprinone is commonly used to treat congestive cardiac failure, due to its inotropic and vasodilating effects. The mechanism of these cardiac effects is attributed to increased cellular concentrations of cAMP. The aim of the present study was to evaluate the pharmacological action of olprinone on the secondary damage in experimental spinal cord injury (SCI) in mice.

Methodology/Principal Findings

Traumatic SCI is characterized by an immediate, irreversible loss of tissue at the lesion site, as well as a secondary expansion of tissue damage over time. Although secondary injury should be preventable, no effective treatment options currently exist for patients with SCI. Spinal cord trauma was induced in mice by the application of vascular clips (force of 24 g) to the dura via a four-level T5–T8 laminectomy. SCI in mice resulted in severe trauma characterized by edema, neutrophil infiltration, and production of inflammatory mediators, tissue damage, apoptosis, and locomotor disturbance. Olprinone treatment (0.2 mg/kg, i.p.) 1 and 6 h after the SCI significantly reduced: (1) the degree of spinal cord inflammation and tissue injury (histological score), (2) neutrophil infiltration (myeloperoxidase activity), (3) nitrotyrosine formation, (4) pro-inflammatory cytokines, (5) NF-κB expression, (6) p-ERK1/2 and p38 expression and (7) apoptosis (TUNEL staining, FAS ligand, Bax and Bcl-2 expression). Moreover, olprinone significantly ameliorated the recovery of hind-limb function (evaluated by motor recovery score).

Conclusions/Significance

Taken together, our results clearly demonstrate that olprinone treatment reduces the development of inflammation and tissue injury associated with spinal cord trauma.

Effect of olprinone, a phosphodiesterase III inhibitor, on hepatic ischemia-reperfusion injury in rats

I/R injury is the main cause for hepatic dysfunction and failure after liver transplantation and liver resection. Therefore, reduction of I/R injury is the most important goal to improve the outcome of these procedures. Olprinone is a newly developed selective phosphodiesterase III inhibitor, which has been reported to ameliorate renal I/R injury in rats. However, no clear evidence for the actions of olprinone on inflammatory response after hepatic I/R injury has been disclosed thus far. Our study was designed to evaluate the action of olprinone on the hepatic I/R injury in rats. Olprinone increased the cyclic adenosine monophosphate level in injured liver tissue and ameliorated the liver injury after hepatic I/R. Moreover, olprinone suppressed the activation of p38 mitogen-activated protein kinase, c-Jun N-terminal kinase, and nuclear factor-kappaB, cytokine production (TNF-alpha, IL-6, and cytokine-induced neutrophil chemoattractant factor 1), and intercellular adhesion molecule 1 expression in liver after hepatic I/R. These observations suggest that olprinone protects liver against I/R injury via the elevation of cyclic adenosine monophosphate level and suppression of intercellular adhesion molecule 1 expression and cytokine production (TNF-alpha, IL-6, and cytokine-induced neutrophil chemoattractant factor 1), possibly by interfering with the signaling pathways of p38 mitogen-activated protein kinase, c-Jun N-terminal kinase, and nuclear factor-kappaB in rats.

Apart from the other members of PDE inhibitors' family, enoximone does not enhance renal ischemic reperfusion injury: the effects of enoximone on renal ischemia reperfusion

Many pharmacological agents were investigated for the prevention of renal ischemic reperfusion (IR) injury as well as the phosphodiesterase (PDE) inhibitors. The aim of the study was to examine the possible renoprotective effect of enoximone as a member of this family on IR injury. Thirty-six Wistar-Albino rats were allocated to six groups. Sham (S) and control groups (E1, E2) only received 0.09% NaCl, 5 mg/kg and 10 mg/kg enoximone via caudal caval vein, respectively. In ischemia (I) and treatment groups (IE1, IE2), the rats were subjected to bilateral renal artery occlusion and were given 0.09% NaCl, 5 mg/kg and 10 mg/kg enoximone in the same route, respectively. Bilateral kidneys were removed at the sixth hour of laparotomy for histopathological and biochemical analysis, such as superoxide dismutase, myeloperoxidase, malonyldialdehyde, and nitric oxide end products. Blood samples were taken in order to evaluate renal function tests. The data were analyzed by using one-way analysis of variance, and p < .05 was considered to be statistically significant. The worst results were achieved in ischemia group (p < .05). Treatments groups showed nearly similar findings with this group (p < .05). There was no significant difference between control and sham groups. In this study, we found that apart from the other members of the PDE inhibitors' family, enoximone did not contribute to the attenuation of IR injury of kidney.

Cytoprotective effects of adenosine and inosine in an in vitro model of acute tubular necrosis

BACKGROUND AND PURPOSE

We have established an in vitro model of acute tubular necrosis in rat kidney tubular cells, using combined oxygen-glucose deprivation (COGD) and screened a library of 1280 pharmacologically active compounds for cytoprotective effects.

EXPERIMENTAL APPROACH

We used in vitro cell-based, high throughput, screening, with cells subjected to COGD using hypoxia chambers, followed by re-oxygenation. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and the Alamar Blue assay measured mitochondrial respiration and the lactate dehydrogenase assay was used to indicate cell death. ATP levels were measured using a luminometric assay.

KEY RESULTS

Adenosine markedly reduced cellular injury, with maximal cytoprotective effect at 100 microM and an EC(50) value of 14 microM. Inosine was also found to be cytoprotective. The selective A(3) adenosine receptor antagonist MRS 1523 attenuated the protective effects of adenosine and inosine, while an A(3) adenosine receptor agonist provided a partial protective effect. Adenosine deaminase inhibition attenuated the cytoprotective effect of adenosine but not of inosine during COGD. Inhibition of adenosine kinase reduced the protective effects of both adenosine and inosine during COGD. Pretreatment of the cells with adenosine or inosine markedly protected against the fall in cellular ATP content in the cells subjected to COGD.

CONCLUSIONS AND IMPLICATIONS

The cytoprotection elicited by adenosine and inosine in a model of renal ischaemia involved both interactions with cell surface adenosine receptors on renal tubular epithelial cells and intracellular metabolism and conversion of adenosine to ATP.

Adenosine A(2A) agonist administration improves islet transplant outcome: Evidence for the role of innate immunity in islet graft rejection

Activation of adenosine A(2A) receptors inhibits inflammation in ischemia/reperfusion injury, and protects against cell damage at the injury site. Following transplantation 50% of islets die due to inflammation and apoptosis. This study investigated the effects of adenosine A(2A) receptor agonists (ATL146e and ATL313) on glucose-stimulated insulin secretion (GSIS) in vitro and transplanted murine syngeneic islet function in vivo. Compared to vehicle controls, ATL146e (100 nM) decreased insulin stimulation index [SI, (insulin)(high glucose)/(insulin)(low glucose)] (2.36 +/- 0.22 vs. 3.75 +/- 0.45; n = 9; p < 0.05). Coculture of islets with syngeneic leukocytes reduced SI (1.41 +/- 0.17; p < 0.05), and this was restored by ATL treatment (2.57 +/- 0.18; NS). Addition of a selective A(2A)AR antagonist abrogated ATL's protective effect, reducing SI (1.11 +/- 0.42). ATL treatment of A(2A)AR(+/+) islet/A(2A)AR(-/-) leukocyte cocultures failed to protect islet function (SI), implicating leukocytes as likely targets of A(2A)AR agonists. Diabetic recipient C57BL/6 mice (streptozotocin; 250 mg/kg, IP) received islet transplants to either the renal subcapsular or hepatic-intraportal site. Recipient mice receiving ATL therapy (ATL 146e or ATL313, 60 ng/kg/min, IP) achieved normoglycemia more rapidly than untreated recipients. Histological examination of grafts suggested reduced cellular necrosis, fibrosis, and lymphocyte infiltration in agonist-treated animals. Administration of adenosine A(2A) receptor agonists (ATL146e or ATL313) improves in vitro GSIS by an effect on leukocytes, and improves survival and functional engraftment of transplanted islets by inhibiting inflammatory islet damage in the peritransplant period, suggesting a potentially significant new strategy for reducing inflammatory islet loss in clinical transplantation.

Rolipram, a specific type-4 phosphodiesterase inhibitor, inhibits cyclophosphamide-induced haemorrhagic cystitis in rats

OBJECTIVE

To investigate the protective roles of type 4 phosphodiesterase (PDE4) inhibitor in cyclophosphamide (CYP)-induced haemorrhagic cystitis, as the PDE4 inhibitor has anti-inflammatory effects but its characterization is still unknown in urinary tract diseases.

MATERIALS AND METHODS

In female Sprague-Dawley rats, CYP was administered intraperitoneally and bladders were harvested 24 h after CYP injection. In another group, rolipram as a PDE4 inhibitor was administered before CYP treatment. The effects and mechanisms of CYP with/without rolipram pretreatment were evaluated by microscopic features, bladder wet weight, myeloperoxidase (MPO) activity, nitric oxide (NO)-metabolite production and expression levels of inflammation-related genes.

RESULTS

CYP injection resulted in severe cystitis. Pretreatment with rolipram significantly reduced the increase in bladder wet weight and MPO activity, and ameliorated histological inflammatory changes caused by CYP. The levels of inflammation-related transcripts including inducible NO synthase (iNOS), interleukin-1beta and tumour necrosis factor-alpha, induced by CYP, were down-regulated significantly by pretreatment with rolipram. Also, rolipram reduced the NO-metabolite production and iNOS protein expression in the immunohistochemical examination.

CONCLUSION

These results indicate that rolipram can attenuate the development of CYP-induced cystitis in rats by suppressing cytokine production and iNOS induction. Thus, treatment with PDE4 inhibitor has potential clinical implications of the prevention of bladder inflammatory diseases.

Resolving the conundrum of islet transplantation by linking metabolic dysregulation, inflammation, and immune regulation

Although type 1 diabetes cannot be prevented or reversed, replacement of insulin production by transplantation of the pancreas or pancreatic islets represents a definitive solution. At present, transplantation can restore euglycemia, but this restoration is short-lived, requires islets from multiple donors, and necessitates lifelong immunosuppression. An emerging paradigm in transplantation and autoimmunity indicates that systemic inflammation contributes to tissue injury while disrupting immune tolerance. We identify multiple barriers to successful islet transplantation, each of which either contributes to the inflammatory state or is augmented by it. To optimize islet transplantation for diabetes reversal, we suggest that targeting these interacting barriers and the accompanying inflammation may represent an improved approach to achieve successful clinical islet transplantation by enhancing islet survival, regeneration or neogenesis potential, and tolerance induction. Overall, we consider the proinflammatory effects of important technical, immunological, and metabolic barriers including: 1) islet isolation and transplantation, including selection of implantation site; 2) recurrent autoimmunity, alloimmune rejection, and unique features of the autoimmune-prone immune system; and 3) the deranged metabolism of the islet transplant recipient. Consideration of these themes reveals that each is interrelated to and exacerbated by the other and that this connection is mediated by a systemic inflammatory state. This inflammatory state may form the central barrier to successful islet transplantation. Overall, there remains substantial promise in islet transplantation with several avenues of ongoing promising research. This review focuses on interactions between the technical, immunological, and metabolic barriers that must be overcome to optimize the success of this important therapeutic approach.

The reno-vascular A2B adenosine receptor protects the kidney from ischemia

BACKGROUND

Acute renal failure from ischemia significantly contributes to morbidity and mortality in clinical settings, and strategies to improve renal resistance to ischemia are urgently needed. Here, we identified a novel pathway of renal protection from ischemia using ischemic preconditioning (IP).

METHODS AND FINDINGS

For this purpose, we utilized a recently developed model of renal ischemia and IP via a hanging weight system that allows repeated and atraumatic occlusion of the renal artery in mice, followed by measurements of specific parameters or renal functions. Studies in gene-targeted mice for each individual adenosine receptor (AR) confirmed renal protection by IP in A1(-/-), A2A(-/-), or A3AR(-/-) mice. In contrast, protection from ischemia was abolished in A2BAR(-/-) mice. This protection was associated with corresponding changes in tissue inflammation and nitric oxide production. In accordance, the A2BAR-antagonist PSB1115 blocked renal protection by IP, while treatment with the selective A2BAR-agonist BAY 60-6583 dramatically improved renal function and histology following ischemia alone. Using an A2BAR-reporter model, we found exclusive expression of A2BARs within the reno-vasculature. Studies using A2BAR bone-marrow chimera conferred kidney protection selectively to renal A2BARs.

CONCLUSIONS

These results identify the A2BAR as a novel therapeutic target for providing potent protection from renal ischemia.

Adenosine A2A receptor antagonists: blockade of adenosinergic effects and T regulatory cells

The intensity and duration of host responses are determined by protective mechanisms that control tissue injury by dampening down inflammation. Adenosine generation and consequent effects, mediated via A2A adenosine receptors (A2AR) on effector cells, play a critical role in the pathophysiological modulation of these responses in vivo. Adenosine is both released by hypoxic cells/tissues and is also generated from extracellular nucleotides by ecto-enzymes e.g. CD39 (ENTPD1) and CD73 that are expressed by the vasculature and immune cells, in particular by T regulatory cell. In general, these adenosinergic mechanisms minimize the extent of collateral damage to host tissues during the course of inflammatory reactions. However, induction of suppressive pathways might also cause escape of pathogens and permit dissemination. In addition, adenosinergic responses may inhibit immune responses while enhancing vascular angiogenic responses to malignant cells that promote tumor growth. Novel drugs that block A2AR-adenosinergic effects and/or adenosine generation have the potential to boost pathogen destruction and to selectively destroy malignant tissues. In the latter instance, future treatment modalities might include novel 'anti-adenosinergic' approaches that augment immune clearance of malignant cells and block permissive angiogenesis. This review addresses several possible pharmacological modalities to block adenosinergic pathways and speculates on their future application together with impacts on human disease.

A2A receptors in inflammation and injury: lessons learned from transgenic animals

Adenosine regulates the function of the innate and adaptive immune systems through targeting virtually every cell type that is involved in orchestrating an immune/inflammatory response. Of the four adenosine receptors (A(1), A(2A), A(2B), A(3)), A(2A) receptors have taken center stage as the primary anti-inflammatory effectors of extracellular adenosine. This broad, anti-inflammatory effect of A(2A) receptor activation is a result of the predominant expression of A(2A) receptors on monocytes/macrophages, dendritic cells, mast cells, neutrophils, endothelial cells, eosinophils, epithelial cells, as well as lymphocytes, NK cells, and NKT cells. A(2A) receptor activation inhibits early and late events occurring during an immune response, which include antigen presentation, costimulation, immune cell trafficking, immune cell proliferation, proinflammatory cytokine production, and cytotoxicity. In addition to limiting inflammation, A(2A) receptors participate in tissue remodeling and reparation. Consistent with their multifaceted, immunoregulatory action on immune cells, A(2A) receptors have been shown to impact the course of a wide spectrum of ischemic, autoimmune, infectious, and allergic diseases. Here, we review the regulatory roles of A(2A) receptors in immune/inflammatory diseases of various organs, including heart, lung, gut, liver, kidney, joints, and brain, as well as the role of A(2A) receptors in regulating multiple organ failure and sepsis.

Activation of adenosine 2A receptors preserves structure and function of podocytes

Adenosine 2A receptor (A(2A)R) activation was recently shown to be renoprotective in diabetic nephropathy. A(2A)R are found in glomeruli and have been shown to associate with the podocyte cytoskeletal protein alpha-actinin-4, but the effect of their activation on podocyte structure and function is unknown. Podocyte injury was induced in C57BL/6 mice with puromycin aminonucleoside, and the selective A(2A)R agonist ATL313 was found to attenuate the resulting albuminuria and foot process fusion. The selective A(2A)R antagonist ZM241385 reversed the effects of ATL313. In vitro, A(2A)R mRNA and protein were expressed in a conditionally immortalized podocyte cell line, and A(2A)R-like immunoreactivity co-localized with the actin cytoskeleton. Treatment with ATL313 also blocked the increased podocyte permeability to albumin and disruption of the actin cytoskeleton that accompanied puromycin aminonucleoside-induced injury in vitro. ATL313 was ineffective, however, in the presence of the A(2A)R antagonist and in A(2A)R-deficient podocytes. It was concluded that A(2A)R activation reduces glomerular proteinuria, at least in part, by preserving the normal structure of podocyte foot processes, slit diaphragms, and actin cytoskeleton.

Novel pharmacological approaches to the treatment of renal ischemia-reperfusion injury: a comprehensive review

Renal ischemia-reperfusion (I-R) contributes to the development of ischemic acute renal failure (ARF). Multi-factorial processes are involved in the development and progression of renal I-R injury with the generation of reactive oxygen species, nitric oxide and peroxynitrite, and the decline of antioxidant protection playing major roles, leading to dysfunction, injury, and death of the cells of the kidney. Renal inflammation, involving cytokine/adhesion molecule cascades with recruitment, activation, and diapedesis of circulating leukocytes is also implicated. Clinically, renal I-R occurs in a variety of medical and surgical settings and is responsible for the development of acute tubular necrosis (a characteristic feature of ischemic ARF), e.g., in renal transplantation where I-R of the kidney directly influences graft and patient survival. The cellular mechanisms involved in the development of renal I-R injury have been targeted by several pharmacological interventions. However, although showing promise in experimental models of renal I-R injury and ischemic ARF, they have not proved successful in the clinical setting (e.g., atrial natriuretic peptide, low-dose dopamine). This review highlights recent pharmacological developments, which have shown particular promise against experimental renal I-R injury and ischemic ARF, including novel antioxidants and antioxidant enzyme mimetics, nitric oxide and nitric oxide synthase inhibitors, erythropoietin, peroxisome-proliferator-activated receptor agonists, inhibitors of poly(ADP-ribose) polymerase, carbon monoxide-releasing molecules, statins, and adenosine. Novel approaches such as recent research involving combination therapies and the potential of non-pharmacological strategies are also considered.

Therapeutic anti-inflammatory effects of myeloid cell adenosine receptor A2a stimulation in lipopolysaccharide-induced lung injury

To determine the role of the adenosine receptor A2a in a murine model of LPS-induced lung injury, migration of polymorphonuclear leukocytes (PMNs) into the different compartments of the lung was determined by flow cytometry, microvascular permeability was assessed by the extravasation of Evans blue, and the release of chemotactic cytokines into the alveolar airspace was determined by ELISA. Measurements were performed in wild-type and A2a gene-deficient mice (A2a(-/-)). To differentiate the role of A2a on hemopoietic and nonhemopoietic cells, we created chimeric mice by transfer of bone marrow (BM) between wild-type and A2a(-/-) mice and used mice that lacked A2a expression selectively on myeloid cells (A2a(flox/flox) x LysM-cre). A specific A2a receptor agonist (ATL202) was used to evaluate its potential to reduce lung injury in vivo. In wild-type mice, therapeutic treatment with ATL202 reduced LPS-induced PMN recruitment, and release of cytokines. Pretreatment, but not posttreatment, also reduced Evans blue extravasation. In the BM chimeric mice lacking A2a on BM-derived cells, PMN migration into the alveolar space was increased by approximately 50%. These findings were confirmed in A2a(flox/flox) x LysM-cre mice. ATL202 was only effective when A2a was present on BM-derived cells. A2a agonists may be effective at curbing inflammatory lung tissue damage.

THE ADENOSINE 2A RECEPTOR AGONIST ATL-146E ATTENUATES EXPERIMENTAL POSTHEMORRHAGIC VASOSPASM

OBJECTIVE

Selective adenosine 2A receptor agonists, such as ATL-146e, are known to be potent anti-inflammatory agents devoid of systemic side effects and have been used clinically in a number of disease states. However, adenosine 2A receptor agonists have not been studied in the treatment of cerebral vasospasm after subarachnoid hemorrhage. The present study investigated the efficacy of ATL-146e in the prevention of leukocyte infiltration and attenuation of posthemorrhagic vasospasm.

METHODS

The rodent femoral artery model of vasospasm was used. Forty male Sprague-Dawley rats were randomly assigned to four different groups (vehicle, 1 ng/kg/min, 10 ng/kg/min, or 100 ng/kg/min ATL-146e administered via subcutaneous osmotic minipump). Vasospasm was evaluated at posthemorrhage Day 8 (period of peak constriction) by calculating the lumen cross-sectional area (expressed as percent change in luminal area: ratio of blood-exposed vessel to normal saline-exposed vessel) and radial wall thickness. Immunostaining with anti-CD45 monoclonal antibody to detect leukocytes was used to evaluate localized inflammation.

RESULTS

Significant vasospasm was noted in the vehicle-treated (blood-exposed) control group (78.5%, P < 0.001; expressed as a ratio of luminal area of the saline [no blood] control), but not in the ATL-146e-treated groups (lumen ratio to control: 105.0, 83.4, and 91.3% for the 1, 10, and 100 ng/kg/min groups, respectively). Additionally, infiltration of inflammatory cells was reduced significantly and radial wall thickness was decreased in the ATL-146e-treated groups compared with the vehicle-treated control group.

CONCLUSION

Selective activation of the adenosine 2A receptor with ATL-146e prevented posthemorrhagic vasospasm and reduced leukocyte infiltration in this experimental model. This agent is worthy of further investigation and lends credence to the hypothesis supporting a role for inflammation in the pathogenesis of cerebral vasospasm after subarachnoid hemorrhage.

Contribution of E-NTPDase1 (CD39) to renal protection from ischemia-reperfusion injury

Previous studies showed increased extracellular nucleotides during renal ischemia-reperfusion. While nucleotides represent the main source for extracellular adenosine and adenosine signaling contributes to renal protection from ischemia, we hypothesized a role for ecto-nucleoside-triphosphate-diphosphohydrolases (E-NTPDases) in renal protection. We used a model of murine ischemia-reperfusion and in situ ischemic preconditioning (IP) via a hanging weight system for atraumatic renal artery occlusion. Initial studies with a nonspecific inhibitor of E-NTPDases (POM-1) revealed inhibition of renal protection by IP. We next pursued transcriptional responses of E-NTPDases (E-NTPDase1-3, and 8) to renal IP, and found a robust and selective induction of E-NTPDase1/CD39 transcript and protein. Moreover, based on clearance studies, plasma electrolytes, and renal tubular histology, IP protection was abolished in gene-targeted mice for cd39 whereas increased renal adenosine content with IP was attenuated. Furthermore, administration of apyrase reconstituted renal protection by IP in cd39-/- mice. Finally, apyrase treatment of wild-type mice resulted in increased renal adenosine concentrations and a similar degree of renal protection from ischemia as IP treatment. Taken together, these data identify CD39-dependent nucleotide phosphohydrolysis in renal protection. Moreover, the present studies suggest apyrase treatment as a novel pharmacological approach to renal diseases precipitated by limited oxygen availability.

Pharmacologic treatment of acute kidney injury: why drugs haven't worked and what is on the horizon

Current strategies to limit the extent of injury in acute renal failure are based on extensive studies that identified cellular and molecular mechanisms of acute kidney injury. Despite successes in various animal models, translation to human studies has failed or studies are inconclusive. This review describes past failures and barriers to successful clinical trials. It also focuses on promising preclinical studies using novel compounds that currently are in or close to human investigation. Implementation of previous or novel compounds in well-designed clinical trials provides hope for the successful treatment of this devastating disorder.