Renal tubule necrosis and apoptosis modulation by A1 adenosine receptor expression

Medicinal Chemistry and Therapeutic Potential of Agonists, Antagonists and Allosteric Modulators of A1 Adenosine Receptor: Current Status and Perspectives

Adenosine is a purine nucleoside, responsible for the regulation of a wide range of physiological and pathophysiological conditions by binding with four G-protein-coupled receptors (GPCRs), namely A1, A2A, A2B and A3 adenosine receptors (ARs). In particular, A1 AR is ubiquitously present, mediating a variety of physiological processes throughout the body, thus represents a promising drug target for the management of various pathological conditions. Agonists of A1 AR are found to be useful for the treatment of atrial arrhythmia, angina, type-2 diabetes, glaucoma, neuropathic pain, epilepsy, depression and Huntington's disease, whereas antagonists are being investigated for the treatment of diuresis, congestive heart failure, asthma, COPD, anxiety and dementia. However, treatment with full A1 AR agonists has been associated with numerous challenges like cardiovascular side effects, off-target activation as well as desensitization of A1 AR leading to tachyphylaxis. In this regard, partial agonists of A1 AR have been found to be beneficial in enhancing insulin sensitivity and subsequently reducing blood glucose level, while avoiding severe CVS side effects and tachyphylaxis. Allosteric enhancer of A1 AR is found to be potent for the treatment of neuropathic pain, culminating the side effects related to off-target tissue activation of A1 AR. This review provides an overview of the medicinal chemistry and therapeutic potential of various agonists/partial agonists, antagonists and allosteric modulators of A1 AR, with a particular emphasis on their current status and future perspectives in clinical settings.

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.

A1 adenosine receptor attenuates intracerebral hemorrhage-induced secondary brain injury in rats by activating the P38-MAPKAP2-Hsp27 pathway

Background

This study was designed to determine the role of the A1 adenosine receptors in intracerebral hemorrhage (ICH)-induced secondary brain injury and the underlying mechanisms.

Methods

A collagenase-induced ICH model was established in Sprague–Dawley rats, and cultured primary rat cortical neurons were exposed to oxyhemoglobin at a concentration of 10 μM to mimic ICH in vitro. The A1 adenosine receptor agonist N(6)-cyclohexyladenosine and antagonist 8-phenyl-1,3-dipropylxanthine were used to study the role of A1 adenosine receptor in ICH-induced secondary brain injury, and antagonists of P38 and Hsp27 were used to study the underlying mechanisms of A1 adenosine receptor actions.

Results

The protein level of A1 adenosine receptor was significantly increased by ICH, while there was no significant change in protein levels of the other 3 adenosine receptors. In addition, the A1 adenosine receptor expression could be increased by N(6)-cyclohexyladenosine and decreased by 8-phenyl-1,3-dipropylxanthine under ICH conditions. Activation of the A1 adenosine receptor attenuated neuronal apoptosis in the subcortex, which was associated with increased phosphorylation of P38, MAPK, MAPKAP2, and Hsp27. Inhibition of the A1 adenosine receptor resulted in opposite effects. Finally, the neuroprotective effect of the A1 adenosine receptor agonist N(6)-cyclohexyladenosine was inhibited by antagonists of P38 and Hsp27.

Conclusions

This study demonstrates that activation of the A1 adenosine receptor by N(6)-cyclohexyladenosine could prevent ICH-induced secondary brain injury via the P38-MAPKAP2-Hsp27 pathway.

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.

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.

Global consequences of liver ischemia/reperfusion injury

Liver ischemia/reperfusion injury has been extensively studied during the last decades and has been implicated in the pathophysiology of many clinical entities following hepatic surgery and transplantation. Apart from its pivotal role in the pathogenesis of the organ's post reperfusion injury, it has also been proposed as an underlying mechanism responsible for the dysfunction and injury of other organs as well. It seems that liver ischemia and reperfusion represent an event with “global” consequences that influence the function of many remote organs including the lung, kidney, intestine, pancreas, adrenals, and myocardium among others. The molecular and clinical manifestation of these remote organs injury may lead to the multiple organ dysfunction syndrome, frequently encountered in these patients. Remote organ injury seems to be in part the result of the oxidative burst and the inflammatory response following reperfusion. The present paper aims to review the existing literature regarding the proposed mechanisms of remote organ injury after liver ischemia and reperfusion.

IL-11 is required for A1 adenosine receptor-mediated protection against ischemic AKI

A1 adenosine receptor activation ameliorates ischemic AKI through the induction of renal proximal tubular sphingosine kinase-1. However, systemic adverse effects may limit A1 adenosine receptor-based therapy for ischemic AKI, indicating a need to identify alternative therapeutic targets within this pathway. Here, we evaluated the function of renal proximal tubular IL-11, a clinically approved hematopoietic cytokine, in A1 adenosine receptor-mediated induction of sphingosine kinase-1 and renal protection. Treatment of human proximal tubule epithelial (HK-2) cells with a selective A1 adenosine receptor agonist, chloro-N(6)-cyclopentyladenosine (CCPA), induced the expression of IL-11 mRNA and protein in an extracellular signal-regulated kinase-dependent manner, and administration of CCPA in mice induced renal synthesis of IL-11. Pretreatment with CCPA protected against renal ischemia-reperfusion injury in wild-type mice, but not in IL-11 receptor-deficient mice. Administration of an IL-11-neutralizing antibody abolished the renal protection provided by CCPA. Similarly, CCPA did not induce renal IL-11 expression or protect against renal ischemia-reperfusion injury in mice lacking the renal proximal tubular A1 adenosine receptor. Finally, treatment with CCPA induced sphingosine kinase-1 in HK-2 cells and wild-type mice, but not in IL-11 receptor-deficient or renal proximal tubule A1 adenosine receptor-deficient mice. Taken together, these results suggest that induction of renal proximal tubule IL-11 is a critical intermediary in A1 adenosine receptor-mediated renal protection that warrants investigation as a novel therapeutic target for the treatment of ischemic AKI.

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.

The volatile anesthetic isoflurane induces ecto-5'-nucleotidase (CD73) to protect against renal ischemia and reperfusion injury

The volatile anesthetic isoflurane protects against renal ischemia and reperfusion injury by releasing renal tubular TGF-β1. Since adenosine is a powerful cytoprotective molecule, we tested whether TGF-β1 generated by isoflurane induces renal tubular ecto-5′-nucleotidase (CD73) and adenosine to protect against renal ischemia and reperfusion injury. Isoflurane induced new CD73 synthesis and increased adenosine generation in cultured kidney proximal tubule cells and in mouse kidney. Moreover, a TGF-β1 neutralizing antibody prevented isoflurane-mediated induction of CD73 activity. Mice anesthetized with isoflurane after renal ischemia and reperfusion had significantly reduced plasma creatinine and decreased renal tubular necrosis, neutrophil infiltration and apoptosis compared to pentobarbital-anesthetized mice. Isoflurane failed to protect against renal ischemia and reperfusion injury in CD73 deficient mice, in mice pretreated with a selective CD73 inhibitor or mice treated with an adenosine receptor antagonist. The TGF-β1 neutralizing antibody or the CD73 inhibitor attenuated isoflurane-mediated protection against HK-2 cell apoptosis. Thus, isoflurane causes TGF-β1-dependent induction of renal tubular CD73 and adenosine generation to protect against renal ischemia and reperfusion injury. Modulation of this pathway may have important therapeutic implications to reduce morbidity and mortality arising from ischemic acute kidney injury.

Paneth cell-mediated multiorgan dysfunction after acute kidney injury

Acute kidney injury (AKI) is frequently complicated by extrarenal multiorgan injury, including intestinal and hepatic dysfunction. In this study, we hypothesized that a discrete intestinal source of proinflammatory mediators drives multiorgan injury in response to AKI. After induction of AKI in mice by renal ischemia-reperfusion or bilateral nephrectomy, small intestinal Paneth cells increased the synthesis and release of IL-17A in conjunction with severe intestinal apoptosis and inflammation. We also detected significantly increased IL-17A in portal and systemic circulation after AKI. Intestinal macrophages appear to transport released Paneth cell granule constituents induced by AKI, away from the base of the crypts into the liver. Genetic or pharmacologic depletion of Paneth cells decreased small intestinal IL-17A secretion and plasma IL-17A levels significantly and attenuated intestinal, hepatic, and renal injury after AKI. Similarly, portal delivery of IL-17A in macrophage-depleted mice decreased markedly. In addition, intestinal, hepatic, and renal injury following AKI was attenuated without affecting intestinal IL-17A generation. In conclusion, AKI induces IL-17A synthesis and secretion by Paneth cells to initiate intestinal and hepatic injury by hepatic and systemic delivery of IL-17A by macrophages. Modulation of Paneth cell dysregulation may have therapeutic implications by reducing systemic complications arising from AKI.

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.

Interleukin-11 protects against renal ischemia and reperfusion injury

Renal ischemia reperfusion (IR) injury causes renal tubular necrosis, apoptosis, and inflammation leading to acute and chronic kidney dysfunction. IL-11 is a multifunctional hematopoietic cytokine clinically approved to treat chemotherapy-induced thrombocytopenia. Recent studies suggest that IL-11 also has potent antiapoptotic and antinecrotic properties. In this study, we tested the hypothesis that exogenous IL-11 protects against renal IR injury and determined the mechanisms involved in renal protection. Pretreatment with human recombinant IL-11 (HR IL-11) or with long-acting site-specific polyethylene glycol (PEG)-conjugated human IL-11 analog (PEGylated IL-11) produced partial but significant protection against renal IR injury in mice. In addition, HR IL-11 or PEGylated IL-11 given 30-60 min after IR also provided renal protection in mice. Significant reductions in renal tubular necrosis and neutrophil infiltration as well as tubular apoptosis were observed in mice treated with HR IL-11 or PEGylated IL-11. Furthermore, HR IL-11 or PEGylated IL-11 decreased both necrosis and apoptosis in human proximal tubule (HK-2) cells in culture. Mechanistically, IL-11 increased nuclear translocation of hypoxia-inducible factor-1α (HIF-1α) and induced sphingosine kinase-1 (SK1) expression and activity in HK-2 cells. Moreover, selective HIF-1α inhibitors blocked IL-11-mediated induction of SK1 in HK-2 cells. Finally, HR IL-11 or PEGylated IL-11 failed to protect against renal IR injury in SK1-deficient mice. Together, our data show powerful renal protective effects of exogenous IL-11 against IR injury by reducing necrosis, inflammation, and apoptosis through induction of SK1 via HIF-1α.

A1 adenosine receptor allosteric enhancer PD-81723 protects against renal ischemia-reperfusion injury

Activation of A(1) adenosine receptors (ARs) protects against renal ischemia-reperfusion (I/R) injury by reducing necrosis, apoptosis, and inflammation. However, extrarenal side effects (bradycardia, hypotension, and sedation) may limit A(1)AR agonist therapy for ischemic acute kidney injury. Here, we hypothesized that an allosteric enhancer for A(1)AR (PD-81723) protects against renal I/R injury without the undesirable side effects of systemic A(1)AR activation by potentiating the cytoprotective effects of renal adenosine generated locally by ischemia. Pretreatment with PD-81723 produced dose-dependent protection against renal I/R injury in A(1)AR wild-type mice but not in A(1)AR-deficient mice. Significant reductions in renal tubular necrosis, neutrophil infiltration, and inflammation as well as tubular apoptosis were observed in A(1)AR wild-type mice treated with PD-81723. Furthermore, PD-81723 decreased apoptotic cell death in human proximal tubule (HK-2) cells in culture, which was attenuated by a specific A(1)AR antagonist (8-cyclopentyl-1,3-dipropylxanthine). Mechanistically, PD-81723 induced sphingosine kinase (SK)1 mRNA and protein expression in HK-2 cells and in the mouse kidney. Supporting a critical role of SK1 in A(1)AR allosteric enhancer-mediated renal protection against renal I/R injury, PD-81723 failed to protect SK1-deficient mice against renal I/R injury. Finally, proximal tubule sphingosine-1-phosphate type 1 receptors (S1P(1)Rs) are critical for PD-81723-induced renal protection, as mice selectively deficient in renal proximal tubule S1P(1)Rs (S1P(1)R(flox/flox) PEPCK(Cre/-) mice) were not protected against renal I/R injury with PD-81723 treatment. Taken together, our experiments demonstrate potent renal protection with PD-81723 against I/R injury by reducing necrosis, inflammation, and apoptosis through the induction of renal tubular SK1 and activation of proximal tubule S1P(1)Rs. Our findings imply that selectively enhancing A(1)AR activation by locally produced renal adenosine may be a clinically useful therapeutic option to attenuate ischemic acute kidney injury without systemic side effects.

Proximal tubule sphingosine kinase-1 has a critical role in A1 adenosine receptor-mediated renal protection from ischemia

Renal ischemia reperfusion injury is a major cause of acute kidney injury. We previously found that renal A1 adenosine receptor (A1AR) activation attenuated multiple cell death pathways including necrosis, apoptosis and inflammation. Here, we tested whether induction of cytoprotective sphingosine kinase (SK)-1 and sphingosine-1 phosphate (S1P) synthesis might be the mechanism of protection. A selective A1AR agonist (CCPA) increased the synthesis of S1P and selectively induced SK-1 in mouse kidney and HK-2 cells. This agonist failed to protect SK1-knockout but protected SK2-knockout mice against renal ischemia reperfusion injury indicating a critical role of SK1 in A1AR-mediated renal protection. Inhibition of SK prevented A1AR-mediated defense against necrosis and apoptosis in HK-2 cells. A selective S1P1R antagonist (W146) and global in vivo gene knockdown of S1P1Rs with small interfering RNA completely abolished the renal protection provided by CCPA. Mice selectively deficient in renal proximal tubule S1P1Rs (S1P1Rflox/flox PEPCKCre/−) were not protected against renal ischemia reperfusion injury by CCPA. Mechanistically, CCPA increased nuclear translocation of hypoxia inducible factor-1α in HK-2 cells and selective hypoxia inducible factor-1α inhibition blocked A1AR-mediated induction of SK1. Thus, proximal tubule SK-1 has a critical role in A1AR-mediated protection against renal ischemia reperfusion injury.

Adenosine and protection from acute kidney injury

PURPOSE OF REVIEW

Acute kidney injury (AKI) is a major clinical problem without effective therapy. Development of AKI among hospitalized patients drastically increases mortality and morbidity. With increases in complex surgical procedures together with a growing elderly population, the incidence of AKI is rising. Renal adenosine receptor manipulation may have great therapeutic potential in mitigating AKI. In this review, we discuss renal adenosine receptor biology and potential clinical therapies for AKI.

RECENT FINDINGS

The four adenosine receptor subtypes (A(1)AR, A(2A)AR, A(2B)AR, and A(3)AR) have diverse effects on the kidney. The pathophysiology of AKI may dictate the specific adenosine receptor subtype activation needed to produce renal protection. The A(1)AR activation in renal tubules and endothelial cells produces beneficial effects against ischemia and reperfusion injury by modulating metabolic demand, decreasing necrosis, apoptosis, and inflammation. The A(2A)AR protects against AKI by modulating leukocyte-mediated renal and systemic inflammation, whereas the A(2B)AR activation protects by direct activation of renal parenchymal adenosine receptors. In contrast, the A(1)AR antagonism may play a protective role in nephrotoxic AKI and radiocontrast induced nephropathy by reversing vascular constriction and inducing naturesis and diuresis. Furthermore, as the A(3)AR activation exacerbates apoptosis and tissue damage due to renal ischemia and reperfusion, selective A(3)AR antagonism may hold promise to attenuate renal ischemia and reperfusion injury. Finally, renal A(1)AR activation also protects against renal endothelial dysfunction caused by hepatic ischemia and reperfusion injury.

SUMMARY

Despite the current lack of therapies for the treatment and prevention of AKI, recent research suggests that modulation of renal adenosine receptors holds promise in treating AKI and extrarenal injury.

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).

Inhibition of sphingosine 1-phosphate receptor 2 protects against renal ischemia-reperfusion injury

Activation of the sphingosine 1-phosphate receptor 1 (S1P(1)R) protects against renal ischemia-reperfusion (IR) injury and inflammation, but the role of other members of this receptor family in modulating renal IR injury is unknown. We found that a selective S1P(2)R antagonist protected against renal IR injury in a dose-dependent manner. Consistent with this observation, both S1P(2)R-deficient mice and wild-type mice treated with S1P(2)R small interfering RNA had reduced renal injury after IR. In contrast, a selective S1P(2)R agonist exacerbated renal IR injury. The S1P(2)R antagonist increased sphingosine kinase-1 (SK1) expression via Rho kinase signaling in renal proximal tubules; the S1P(2)R agonist decreased SK1. S1P(2)R antagonism failed to protect the kidneys of SK1-deficient mice or wild-type mice pretreated with an SK1 inhibitor or an S1P(1)R antagonist, suggesting that the renoprotection conferred by S1P(2)R antagonism results from pathways involving activation of S1P(1)R by SK1. In cultured human proximal tubule (HK-2) cells, the S1P(2)R antagonist selectively upregulated SK1 and attenuated both H(2)O(2)-induced necrosis and TNF-α/cycloheximide-induced apoptosis; the S1P(2)R agonist had the opposite effects. In addition, increased nuclear hypoxia inducible factor-1α was critical in mediating the renoprotective effects of S1P(2)R inhibition. Finally, induction of SK1 and S1P(2)R in response to renal IR and S1P(2)R antagonism occurred selectively in renal proximal tubule cells but not in renal endothelial cells. Taken together, these data suggest that S1P(2)R may be a therapeutic target to attenuate the effects of renal IR injury.

Sphingosine kinase 1 protects against renal ischemia-reperfusion injury in mice by sphingosine-1-phosphate1 receptor activation

The roles of sphingosine kinases SK1 and SK2 in ischemia-reperfusion injury have not been fully elucidated since studies have found beneficial effects of SK1 while others showed no role in this injury. To help resolve this, we used SK1 or SK2 knockout mice and confirmed that renal ischemia-reperfusion injury induced SK1, but not SK2, in the kidneys. Furthermore, knockout or pharmacological inhibition of SK1 increased injury after renal ischemia-reperfusion injury. In contrast, lack of SK2 conferred renal protection following injury. In addition, we used lentiviral gene delivery to selectively express enhanced green fluorescent protein (EGFP) or human SK1 coexpressed with EGFP (EGFP-huSK1) in the kidney. Mice with kidney-specific overexpression of EGFP-huSK1 had significantly improved renal function with lower plasma creatinine, renal necrosis, apoptosis, and inflammation. Moreover, EGFP-huSK1 overexpression in cultured human proximal tubule (HK-2) cells protected against peroxide-induced necrosis. Selective overexpression of EGFP-huSK1 led to increased HSP27 mRNA and protein expression in vivo and in vitro. Functional protection as well as induction of HSP27 with EGFP-huSK1 overexpression in vivo was blocked with sphingosine-1-phosphate-1 receptor(1) (S1P(1)) antagonism. Thus, our findings suggest that SK1 is renoprotective by S1P(1) activation and perhaps HSP27 induction. Kidney-specific expression of SK1 through lentiviral delivery may be a viable therapeutic option to attenuate renal ischemia-reperfusion injury.

Paneth cell-derived interleukin-17A causes multiorgan dysfunction after hepatic ischemia and reperfusion injury

Hepatic ischemia and reperfusion (IR) injury is a major clinical problem that leads to frequent extrahepatic complications including intestinal dysfunction and acute kidney injury (AKI). In this study we aimed to determine the mechanisms of hepatic IR-induced extrahepatic organ dysfunction. Mice subjected to 60 minutes of hepatic IR not only developed severe hepatic injury but also developed significant AKI and small intestinal injury. Hepatic IR induced small intestinal Paneth cell degranulation and increased interleukin-17A (IL-17A) levels in portal vein plasma and small intestine. We also detected increased levels of IL-17A messenger RNA (mRNA) and protein in Paneth cells after hepatic IR with laser capture dissection. IL-17A-neutralizing antibody treatment or genetic deletion of either IL-17A or IL-17A receptors significantly protected against hepatic IR-induced acute liver, kidney, and intestinal injury. Leukocyte IL-17A does not contribute to organ injury, as infusion of wildtype splenocytes failed to exacerbate liver and kidney injury in IL-17A-deficient mice after hepatic IR. Depletion of Paneth cell numbers by pharmacological (with dithizone) or genetic intervention (SOX9 flox/flox Villin cre+/- mice) significantly attenuated intestinal, hepatic, and renal injury following liver IR. Finally, depletion of Paneth cell numbers significantly decreased small intestinal IL-17A release and plasma IL-17A levels after liver IR.

CONCLUSION

Taken together, the results show that Paneth cell-derived IL-17A plays a critical role in hepatic IR injury and extrahepatic organ dysfunction. Modulation of Paneth cell dysregulation may have therapeutic implications by reducing systemic complications arising from hepatic IR.

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.

The NLRP3 inflammasome promotes renal inflammation and contributes to CKD

Inflammation significantly contributes to the progression of chronic kidney disease (CKD). Inflammasome-dependent cytokines, such as IL-1β and IL-18, play a role in CKD, but their regulation during renal injury is unknown. Here, we analyzed the processing of caspase-1, IL-1β, and IL-18 after unilateral ureteral obstruction (UUO) in mice, which suggested activation of the Nlrp3 inflammasome during renal injury. Compared with wild-type mice, Nlrp3(-/-) mice had less tubular injury, inflammation, and fibrosis after UUO, associated with a reduction in caspase-1 activation and maturation of IL-1β and IL-18; these data confirm that the Nlrp3 inflammasome upregulates these cytokines in the kidney during injury. Bone marrow chimeras revealed that Nlrp3 mediates the injurious/inflammatory processes in both hematopoietic and nonhematopoietic cellular compartments. In tissue from human renal biopsies, a wide variety of nondiabetic kidney diseases exhibited increased expression of NLRP3 mRNA, which correlated with renal function. Taken together, these results strongly support a role for NLRP3 in renal injury and identify the inflammasome as a possible therapeutic target in the treatment of patients with progressive CKD.

Immunohistochemical study of heat shock protein 27 with respect to survival and regeneration of proximal tubular cells after uranyl acetate-induced acute tubular injury in rats

This study examined the possible role of heat shock protein 27 (HSP27) expression in the survival and regeneration of proximal tubule (PT) cells after acute tubular injury. Rats were injected with a low (0.2 mg/kg) or high (4 mg/kg) dose of uranyl acetate (UA) to induce renal injury. Renal tissues were immunostained for HSP27, focal adhesion kinase (FAK), and bromodeoxyuridine (BrdU), and stained by the TUNEL method. Low-dose UA induced focal PT depletion in the proximal three-quarters of the S3 segment. Here, cells became sporadically positive for cytoplasmic HSP27 in association with FAK+, and almost all BrdU+ early regenerating cells were positive for HSP27 from days 2 to 3. High-dose UA induced severe PT depletion in the proximal three-quarters of S3, and a small number of PT cells became positive for HSP27 as early as day 2. BrdU+, early regenerating cells were restricted to the distal quarter of S3 from days 2 to 3, with or without HSP27 staining and with FAK. In both groups, HSP+ PT cells and BrdU+ cells peaked in number at day 5. The PT cells showed reduced HSP27 accumulation by day 7 as they differentiated, but remained immunopositive for FAK. TUNEL+ apoptotic cells were immunonegative for both HSP27 and FAK. Cytoplasmic HSP27 accumulation in PT cells seems to contribute to PT survival and transition from PT cell proliferation to differentiation. When PT cells are severely impaired, distinct cells in the distal areas of S3 could undergo cell cycle progression without HSP27 accumulation.

Selective renal overexpression of human heat shock protein 27 reduces renal ischemia-reperfusion injury in mice

We have previously shown that exogenous and endogenous A(1) adenosine receptor (A(1)AR) activation protected against renal ischemia-reperfusion (IR) injury in mice by induction and phosphorylation of heat shock protein 27 (HSP27). With global overexpression of HSP27 in mice, however, there was a paradoxical increase in systemic inflammation with increased renal injury after an ischemic insult due to increased NK1.1 cytotoxicity. In this study, we hypothesized that selective renal expression of HSP27 in mice would improve renal function and reduce injury after IR. Mice were subjected to renal IR injury 2 days after intrarenal injection of saline or a lentiviral construct encoding enhanced green fluorescent protein (EGFP) or human HSP27 coexpressing EGFP (EGFP-huHSP27). Mice with kidney-specific reconstitution of huHSP27 had significantly lower plasma creatinine, renal necrosis, apoptosis, and inflammation as demonstrated by decreased proinflammatory cytokine mRNA induction and neutrophil infiltration. In addition, there was better preservation of the proximal tubule epithelial filamentous (F)-actin cytoskeleton in the huHSP27-reconstituted groups than in the control groups. Furthermore, huHSP27 overexpression led to increased colocalization with F-actin in renal proximal tubules. Taken together, these findings have important clinical implications, as they imply that kidney-specific expression of HSP27 through lentiviral delivery is a viable therapeutic option in attenuating the effects of renal IR.

Isoflurane protects human kidney proximal tubule cells against necrosis via sphingosine kinase and sphingosine-1-phosphate generation

BACKGROUND/AIMS

We previously showed that the inhalational anesthetic isoflurane protects against renal ischemia reperfusion injury in part via sphingosine kinase (SK)-mediated synthesis of sphingosine-1-phosphate (S1P). In this study, we tested the hypothesis that isoflurane directly targets renal proximal tubule cells via SK activation, S1P synthesis and activation of S1P receptors to initiate cytoprotective signaling.

METHODS AND RESULTS

Isoflurane-mediated phosphorylation of extracellular signal-regulated kinase (ERK) and Akt and induction of HSP70 in human kidney proximal tubule (HK-2) cells were inhibited by dimethylsphingosine (DMS), an SK inhibitor, and VPC23019, an S1P(1/3) receptor selective antagonist, in HK-2 cells. A selective S1P(1) receptor agonist, SEW2781, mimicked isoflurane-induced phosphorylation of ERK and Akt and induction of HSP70. Moreover, isoflurane-mediated protection against H(2)O(2)-induced necrosis of HK-2 cells was significantly attenuated by an S1P(1/3) receptor antagonist, VPC23019, and by SK inhibitors DMS or 4-[[4- (4-chlorophenyl)-2-thiazolyl]amino]phenol. Finally, overexpression of the SK1 enzyme in HK-2 cells protected against H(2)O(2)-induced necrosis.

CONCLUSIONS

Collectively, our study demonstrates that S1P released via isoflurane-mediated SK1 stimulation produces direct anti-necrotic effects probably via S1P(1) receptor-mediated cytoprotective signaling (ERK/Akt phosphorylation and HSP70 induction) in HK-2 cells. Our findings may help to unravel the cellular signaling pathways of volatile anesthetic-mediated renal protection and lead to new therapeutic applications of volatile anesthetics during the perioperative period.

Selective intrarenal human A1 adenosine receptor overexpression reduces acute liver and kidney injury after hepatic ischemia reperfusion in mice

Acute kidney injury (AKI) is frequent after liver ischemia reperfusion (IR) can potentiate liver injury and is often complicated by subsequent multiorgan dysfunction syndrome. AKI because of liver IR is characterized by early renal endothelial cell apoptosis and impaired vascular integrity with subsequent neutrophil infiltration, proximal tubule necrosis/inflammation, and filamentous (F) actin disintegration. We tested whether selective renal overexpression of human A(1) adenosine receptors (huA(1)AR) protects against both liver and kidney injury sustained after liver IR. Mice were subjected to liver IR or to sham surgery 48 h after unilateral intrarenal injection of lentivirus encoding enhanced green fluorescent protein (EGFP) or EGFP-huA(1)AR. Intrarenal lentiviral gene delivery caused a robust transgene expression in the injected kidney without significant expression in the contralateral kidney or in the liver. Mice injected with EGFP-huA(1)AR lentivirus were protected against hepatic IR-induced liver and kidney injury with reduced necrosis, inflammation, and apoptosis, and better preserved F-actin and vascular permeability compared with mice injected with EGFP lentivirus. Importantly, we show that removing the EGFP-huA(1)AR lentivirus-injected kidney before hepatic ischemia abolished both renal and hepatic protection after liver IR showing that the overexpression of huA(1)AR in the injected kidney has a crucial role in protecting the kidney and liver after liver IR. Therefore, our findings show that protecting the kidney reduces liver IR injury and selective overexpression of cytoprotective A(1)ARs in the kidney leads to protection of both liver and kidney after hepatic IR.

Human heat shock protein 27 overexpressing mice are protected against hepatic ischemia and reperfusion injury

BACKGROUND

Hepatic ischemia reperfusion injury (IRI) is a major clinical problem during the perioperative period and occurs frequently after major hepatic resection or liver transplantation. Our laboratory previously demonstrated that exogenous A1 adenosine receptor activation protects against renal IRI by upregulation and phosphorylation of heat shock protein 27 (HSP27).

METHODS

This study used mice overexpressing human HSP27 (huHSP27 OE) to determine whether these mice are protected against liver IRI.

RESULTS

After hepatic IR, the huHSP27 OE mice had significant protection against liver injury (reduced alanine transferase) and necrosis (hematoxylin-eosin staining) compared with the HSP27 WT mice. The huHSP27 OE mice also showed less induction of proinflammatory messenger RNA MIP-2, reduced neutrophil infiltration, and decreased apoptosis (caspase 3 fragmentation and DNA laddering) compared with the HSP27 WT mice. Finally, the huHSP27 OE mice showed significantly less disruption of filamentous actin in hepatocytes and bile canaliculi of the ischemic lobes compared with the HSP27 WT mice. Depletion of Kupffer cells with gadolinium chloride provided significant protection against liver IRI in HSP27 WT mice but not in huHSP27 OE mice suggesting that the overexpression of huHSP27 in the Kupffer cells may be responsible for the hepatic protection observed in huHSP27 OE mice.

CONCLUSIONS

Our results show that the overexpression of huHSP27 in Kupffer cells of the liver may be responsible for the protection against hepatic IRI in vivo by reducing necrosis and apoptosis and by stabilizing F-actin with subsequent reductions in inflammation and proinflammatory neutrophil infiltration. Harnessing the mechanisms of cytoprotection with HSP27 may lead to new therapies for the management of perioperative hepatic IRI.

Kidney-specific reconstitution of the A1 adenosine receptor in A1 adenosine receptor knockout mice reduces renal ischemia-reperfusion injury

Genetic deletion of the adenosine A1 receptor (A1AR) increased renal injury following ischemia-reperfusion injury suggesting that receptor activation is protective in vivo. Here we tested this hypothesis by expressing the human-A(1)AR in A(1)AR knockout mice. Renal ischemia-reperfusion was induced in knockout mice 2 days after intrarenal injection of saline or a lentivirus encoding enhanced green fluorescent protein (EGFP) or EGFP-human-A(1)AR. We found that the latter procedure induced a robust expression of the reporter protein in the kidneys of knockout mice. Mice with kidney-specific human-A(1)AR reconstitution had significantly lower plasma creatinine, tubular necrosis, apoptosis, and tubular inflammation as evidenced by decreased leukocyte infiltration, pro-inflammatory cytokine, and intercellular adhesion molecule-1 expression in the kidney following injury compared to mice injected with saline or the control lentivirus. Additionally, there were marked disruptions of the proximal tubule epithelial filamentous (F)-actin cytoskeleton in both sets of control mice upon renal injury, whereas the reconstituted mice had better preservation of the renal tubule actin cytoskeleton, which co-localized with the human-A(1)ARs. Consistent with reduced renal injury, there was a significant increase in heat shock protein-27 expression, also co-localizing with the preserved F-actin cytoskeleton. Our findings suggest that selective expression of cytoprotective A(1)ARs in the kidney can attenuate renal injury.

Acute kidney injury after hepatic ischemia and reperfusion injury in mice

Hepatic ischemia reperfusion (IR) is the leading cause of acute liver failure (ALF) during the perioperative period and patients with ALF frequently develop acute kidney injury (AKI). There is no effective therapy for AKI associated with ALF because pathomechanisms are incompletely characterized, in part due to the lack of an animal model. In this study, we characterize a novel murine model of AKI following hepatic IR. Mice subjected to approximately 70% liver IR not only developed acute liver dysfunction, but also developed severe AKI 24 h after liver injury. Mice subjected to liver IR developed histological changes of acute tubular injury including focal proximal tubular cell necrosis involving the S3 segment, cortical tubular ectasia, focal tubular simplification and granular bile/heme cast formation. In addition, there was focal interstitial edema and hyperplasia of the juxtaglomerular apparatus. Inflammatory changes in the kidney after hepatic IR included neutrophil infiltration of the interstitium and upregulation of several proinflammatory mRNAs (tumor necrosis factor-alpha, keratinocyte-derived cytokine, monocyte chemotactic protein-1, macrophage inflammatory protein-2, intercellular adhesion molecule-1). In addition, marked renal endothelial cell apoptosis was detected involving peritubular interstitial capillaries, accompanied by increased renal vascular permeability. Finally, there was severe disruption of renal proximal tubule epithelial filamentous-actin. Our results show that AKI rapidly and reproducibly develops in mice after hepatic IR and is characterized by renal tubular necrosis, inflammatory changes and interstitial capillary endothelial apoptosis. Our murine model of AKI after liver injury closely mimics human AKI associated with ALF and may be useful in delineating the mechanisms and potential therapies for this common clinical condition.

Adenosine receptors and inflammation

Extracellular adenosine is produced in a coordinated manner from cells following cellular challenge or tissue injury. Once produced, it serves as an autocrine- and paracrine-signaling molecule through its interactions with seven-membrane-spanning G-protein-coupled adenosine receptors. These signaling pathways have widespread physiological and pathophysiological functions. Immune cells express adenosine receptors and respond to adenosine or adenosine agonists in diverse manners. Extensive in vitro and in vivo studies have identified potent anti-inflammatory functions for all of the adenosine receptors on many different inflammatory cells and in various inflammatory disease processes. In addition, specific proinflammatory functions have also been ascribed to adenosine receptor activation. The potent effects of adenosine signaling on the regulation of inflammation suggest that targeting specific adenosine receptor activation or inactivation using selective agonists and antagonists could have important therapeutic implications in numerous diseases. This review is designed to summarize the current status of adenosine receptor signaling in various inflammatory cells and in models of inflammation, with an emphasis on the advancement of adenosine-based therapeutics to treat inflammatory disorders.

Adenosine receptors and the kidney

The autacoid, adenosine, is present in the normoxic kidney and generated in the cytosol as well as at extracellular sites. The rate of adenosine formation is enhanced when the rate of ATP hydrolysis prevails over the rate of ATP synthesis during increased tubular transport work or during oxygen deficiency. Extracellular adenosine acts on adenosine receptor subtypes (A(1), A(2A), A(2B), and A(3)) in the cell membranes to affect vascular and tubular functions. Adenosine lowers glomerular filtration rate by constricting afferent arterioles, especially in superficial nephrons, and thus lowers the salt load and transport work of the kidney consistent with the concept of metabolic control of organ function. In contrast, it leads to vasodilation in the deep cortex and the semihypoxic medulla, and exerts differential effects on NaCl transport along the tubular and collecting duct system. These vascular and tubular effects point to a prominent role of adenosine and its receptors in the intrarenal metabolic regulation of kidney function, and, together with its role in inflammatory processes, form the basis for potential therapeutic approaches in radiocontrast media-induced acute renal failure, ischemia reperfusion injury, and in patients with cardiorenal failure.

Mechanisms of induction of adenosine receptor genes and its functional significance

Adenosine is a metabolite generated and released from cells, particularly under injury or stress. It elicits protective or damaging responses via signaling through the adenosine receptors, including the adenylyl cyclase inhibitory A(1) and A(3), and the adenylyl cyclase stimulatory A(2A) and A(2B). Multiple adenosine receptor types, including stimulatory and inhibitory, can be found in the same cell, suggesting that a careful balance of adenosine receptor expression in a particular cell is necessary for a specific adenosine-induced response. This balance could be controlled by differential expression of the adenosine receptor genes under different stimuli. Here, we have reviewed an array of studies that have characterized basal or induced expression of the adenosine receptors and common as well as distinct mechanisms of effect, in hopes that ongoing studies on this topic will further elucidate detailed mechanisms of adenosine receptor regulation, leading to potential therapeutic applications.

Mice that overexpress human heat shock protein 27 have increased renal injury following ischemia reperfusion

We previously showed that activation of the A1 adenosine receptor protected the kidney against ischemia-reperfusion injury by induction and phosphorylation of heat shock protein 27 (HSP27). Here, we used mice that overexpress human HSP27 (huHSP27) to determine if kidneys from these mice were protected against injury. Proximal tubule cells cultured from the transgenic mice had increased resistance to peroxide-induced necrosis compared to cells from wild-type mice. However, after renal ischemic injury, HSP27 transgenic mice had decreased renal function compared to wild-type mice, along with increased renal expression of mRNAs of pro-inflammatory cytokines (TNF-alpha, ICAM-1, MCP-1) and increased plasma and kidney keratinocyte-derived cytokine. Following ischemic injury, neutrophils infiltrated the kidneys earlier in the transgenic mice. Flow cytometric analysis of lymphocyte subsets showed that those isolated from the kidneys of transgenic mice had increased CD3(+), CD4(+), CD8(+), and NK1.1(+) cells 3 h after injury. When splenocytes or NK1.1(+) cells were isolated from transgenic mice and adoptively transferred into wild-type mice there was increased renal injury. Further, depletion of lymphocytes by splenectomy or neutralization of NK1.1(+) cells resulted in improved renal function in the transgenic mice following reperfusion. Our study shows that induction of HSP27 in renal tubular cells protects against necrosis in vitro, but its systemic increase counteracts this protection by exacerbating renal and systemic inflammation in vivo.

HSP27 is involved in the pathogenesis of kidney tubulointerstitial fibrosis

We hypothesized that heat shock protein 27 (HSP27), a small heat shock protein with actin-remodeling properties, is involved in the pathogenesis of kidney tubulointerstitial fibrosis. We first examined its expression in the rat unilateral ureteral obstruction (UUO) model of kidney fibrosis and epithelial-to-mesenchymal transition (EMT). Immunoblot analyses showed that UUO resulted in significant upregulation of TGF-beta1, alpha-smooth muscle actin (alpha-SMA), total and phosphorylated HSP27, and phosphorylated p38MAPK. Immunofluorescence studies showed that HSP27 costained with TGF-beta1, alpha-SMA, and E-cadherin in areas of tubulointerstitial injury. We next attempted to translate these studies in an in vitro model of EMT using rat proximal tubular epithelial cells (NRK52E). TGF-beta1 (20 ng/ml) treatment resulted in EMT (upregulation of alpha-SMA and downregulation of E-cadherin) and significant upregulation of total and phosphorylated HSP27 and p38MAPK after 3 days. Real-time PCR analyses showed that HSP27, vimentin, and fibronectin increased whereas E-cadherin mRNA levels decreased. Double-staining immunofluorescence studies showed intracytoplasmic colocalization of HSP27 with both F-actin and E-cadherin in cells undergoing EMT. HSP27 overexpression by transient transfection significantly increased E-cadherin while decreasing E-cadherin repressor Snail levels. In aggregate, these studies show that HSP27 is involved in the pathogenesis of TGF-beta1-induced EMT and chronic tubulointerstitial fibrosis. HSP27 overexpression may delay injury by upregulating E-cadherin through downregulation of Snail.

Endogenous A1 adenosine receptors protect against hepatic ischemia reperfusion injury in mice

Hepatic ischemia reperfusion (IR) injury is a major clinical problem during the perioperative period and occurs frequently after major hepatic resection or liver transplantation. Exogenous and endogenous A(1) adenosine receptor (A(1)AR) activation protects against renal IR injury. In this study, we questioned whether exogenous and endogenous A(1)AR activation protects against hepatic IR injury in vivo. A(1)AR wild-type (WT) or knockout mice were subjected to 60 minutes of partial hepatic IR. Some animals were treated with a selective A(1)AR agonist, 2-chloro-N(6)-cyclopentyladenosine (CCPA; 0.1 mg/kg), or a selective A(1)AR antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.4 mg/kg), 15 minutes before hepatic ischemia. Twenty-four hours after hepatic IR, the A(1) knockout mice and DPCPX-treated A(1) wild-type (A(1)WT) mice developed significantly worse liver injury (alanine aminotransferase, liver necrosis, neutrophil infiltration, and apoptosis) compared to A(1)AR WT mice. However, the selective A(1)AR agonist CCPA failed to protect against hepatic IR injury in A(1)WT mice. Our results show that the endogenous A(1)ARs protect against hepatic IR injury in vivo by primarily reducing apoptosis and necrosis with subsequent reductions in proinflammatory neutrophil infiltration. However, in contrast to the kidneys, in which exogenous A(1)AR activation protected against IR injury, exogenous A(1)AR activation failed to protect against liver injury after IR. We conclude that endogenous A(1)AR activation prevents worsened murine liver IR injury primarily by reducing necrotic and apoptotic cell death. Harnessing the mechanisms of cytoprotection with endogenous A(1)AR activation may lead to new therapies for perioperative hepatic IR injury.

Acute and delayed renal protection against renal ischemia and reperfusion injury with A1adenosine receptors

We showed previously that activation of A1adenosine receptors (AR) protects against renal ischemia-reperfusion (IR) injury in rats and mice. In the heart, transient A1AR activation produces biphasic protective effects: acute protection wanes after several hours but protective effects return 24–72 h later (second window of protection). In this study, we determined whether A1AR activation produces delayed renal protection and elucidated the mechanisms of acute and delayed renal protection. A1AR wild-type mice were subjected to 30-min renal ischemia and 24 h of reperfusion to produce acute renal failure. Pretreatment with a selective A1AR agonist 2-chloro- N6-cyclopentyladenosine (CCPA; 0.1 mg/kg bolus ip) either 15 min or 24 h before renal ischemia protected against renal IR injury and reduced renal corticomedullary necrosis, apoptosis, and inflammation. Transient A1AR activation led to phosphorylation of extracellular signal-regulated protein kinase mitogen-activated protein kinase (ERK MAPK), Akt, and heat shock protein 27 (HSP27). Moreover, induction of HSP27 and Akt occurred with CCPA treatment. Inhibition of PKC with chelerythrine prevented acute but not delayed renal protection with A1AR activation. Moreover, deletion of PI3Kγ or inhibition of Akt, but not inhibition of ERK, prevented delayed and acute renal protection with A1AR activation. Inhibition of Gi/owith pertussis toxin obliterated both acute and delayed A1AR-mediated renal protection. In contrast to renal protection with delayed ischemic preconditioning, nitric oxide synthase activity was not induced with delayed A1AR-mediated renal protection. Therefore, transient activation of renal A1AR led to acute as well as delayed protective effects against renal IR injury via distinct signaling pathways.