cAMP-adenosine pathway in the proximal tubule

Adenosine A2A receptor blocks the A 1 receptor inhibition of renal Na + transport and oxygen consumption

A high renal oxygen (O₂ ) need is primarily associated with the renal tubular O₂ consumption (VO₂ ) necessary for a high rate of sodium (Na⁺ ) transport. Limited O₂ availability leads to increased levels of adenosine, which regulates the kidney via activation of both A₁ and A_2A adenosine receptors (A1R and A2AR, respectively). The relative contributions of A1R and A2AR to the regulation of renal Na⁺ transport and VO₂ have not been determined. We demonstrated that A1R activation has a dose-dependent biphasic effect on both renal Na⁺ /H⁺ exchanger-3 (NHE3), a major player in Na⁺ transport, and VO₂ . Here, we report concentration-dependent effects of adenosine: less than 5 × 10^-7  M adenosine-stimulated NHE3 activity; between 5 × 10^-7  M and 10^-5  M adenosine-inhibited NHE3 activity; and greater than 10^-5  M adenosine reversed the change in NHE3 activity (returned to baseline). A1R activation mediated the activation and inhibition of NHE3 activity, whereas 10^-4  M adenosine had no effect on the NHE3 activity due to A2AR activation. The following occurred when A1R and A2AR were activated: (a) Blockade of the A2AR receptor restored the NHE3 inhibition mediated by A1R activation, (b) the NHE-dependent effect on VO₂ mediated by A1R activation became NHE independent, and (c) A2AR bound to A1R. In summary, A1R affects VO₂ via NHE-dependent mechanisms, whereas A2AR acts via NHE-independent mechanisms. When both A1R and A2AR are activated, the A2AR effect on NHE3 and VO₂ predominates, possibly via an A1R-A2AR protein interaction. A2AR-A1R heterodimerization is proposed as the molecular mechanism enabling the NHE-independent control of renal VO₂ .

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.

Adenosine contribution to normal renal physiology and chronic kidney disease

Adenosine is a nucleoside that is particularly interesting to many scientific and clinical communities as it has important physiological and pathophysiological roles in the kidney. The distribution of adenosine receptors has only recently been elucidated; therefore it is likely that more biological roles of this nucleoside will be unveiled in the near future. Since the discovery of the involvement of adenosine in renal vasoconstriction and regulation of local renin production, further evidence has shown that adenosine signaling is also involved in the tubuloglomerular feedback mechanism, sodium reabsorption and the adaptive response to acute insults, such as ischemia. However, the most interesting finding was the increased adenosine levels in chronic kidney diseases such as diabetic nephropathy and also in non-diabetic animal models of renal fibrosis. When adenosine is chronically increased its signaling via the adenosine receptors may change, switching to a state that induces renal damage and produces phenotypic changes in resident cells. This review discusses the physiological and pathophysiological roles of adenosine and pays special attention to the mechanisms associated with switching homeostatic nucleoside levels to increased adenosine production in kidneys affected by CKD.

Purines: forgotten mediators in traumatic brain injury

Recently, the topic of traumatic brain injury has gained attention in both the scientific community and lay press. Similarly, there have been exciting developments on multiple fronts in the area of neurochemistry specifically related to purine biology that are relevant to both neuroprotection and neurodegeneration. At the 2105 meeting of the National Neurotrauma Society, a session sponsored by the International Society for Neurochemistry featured three experts in the field of purine biology who discussed new developments that are germane to both the pathomechanisms of secondary injury and development of therapies for traumatic brain injury. This included presentations by Drs. Edwin Jackson on the novel 2',3'-cAMP pathway in neuroprotection, Detlev Boison on adenosine in post-traumatic seizures and epilepsy, and Michael Schwarzschild on the potential of urate to treat central nervous system injury. This mini review summarizes the important findings in these three areas and outlines future directions for the development of new purine-related therapies for traumatic brain injury and other forms of central nervous system injury. In this review, novel therapies based on three emerging areas of adenosine-related pathobiology in traumatic brain injury (TBI) were proposed, namely, therapies targeting 1) the 2',3'-cyclic adenosine monophosphate (cAMP) pathway, 2) adenosine deficiency after TBI, and 3) augmentation of urate after TBI.

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.

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.

In vivo hypoxic preconditioning protects from warm liver ischemia-reperfusion injury through the adenosine A2B receptor

BACKGROUND

Liver ischemia-reperfusion injury (IRI) is a known risk factor for the postoperative outcome of patients undergoing liver surgery/transplantation. Attempts to protect from organ damage require multidisciplinary strategies and are of emerging interest in view of patients with higher age and American Society of Anesthesiology status. Ischemic preconditioning has been successfully applied to prevent from IRI during liver resection/transplantation. Because even short periods of ischemia during preconditioning inevitably lead to hypoxia and formation of anti-inflammatory/cytoprotective acting adenosine, we reasoned that short nonischemic hypoxia also protects against hepatic IRI.

METHODS

Mice underwent hypoxic preconditioning (HPC) by breathing 10% oxygen for 10 min followed by 10 min of 21% oxygen before left liver lobe ischemia (45 min) and reperfusion (4 hr). The interactions of hypoxia→adenosine→adenosine receptors were tested by pharmacologic antagonism at adenosine receptor (AR) sites in wild-type mice and in mice with genetic deletions at the A1, A2A, A2B, and A3 ARs. Hepatocellular damage, inflammation, and metabolic effects were quantified by enzyme activities, cytokines, liver myeloperoxidase, blood adenosine, and tissue AMP, respectively.

RESULTS

Hepatoprotection by HPC was significant in wild-type and A1, A2A, and A3 AR knockout mice as quantified by lower alanine aminotransferase serum activities, cytokine levels, histologic damage scores, tissue myeloperoxidase concentrations, and preserved AMP concentrations. Protection by HPC was blunted in mice pretreated with the A2B AR antagonist MRS1754 or in A2B AR knockout mice.

CONCLUSIONS

Because liver protective effects of HPC are negated when the A2B receptor is nonfunctional, the hypoxia→adenosine→A2B receptor pathway plays a critical role in the prevention of warm IRI in vivo. Hypoxic activation of this pathway warrants use of selective A2B AR agonists or even intermittent hypoxia (e.g., in deceased organ donors) to protect from liver IRI.

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.

Endogenous production of extracellular adenosine by trabecular meshwork cells: potential role in outflow regulation

PURPOSE

To investigate the mechanisms for endogenous production of extracellular adenosine in trabecular meshwork (TM) cells and evaluate its physiological relevance to the regulation of aqueous humor outflow facility.

METHODS

Extra-cellular levels of adenosine monophosphate (AMP) and adenosine in porcine trabecular meshwork (PTM) cells treated with adenosine triphosphate (ATP), AMP, cAMP or forskolin with or without specific inhibitors of phosphodiesterases (IBMX) and CD73 (AMPCP) were determined by high-pressure liquid chromatography fluorometry. Extracellular adenosine was also evaluated in cell cultures subjected to cyclic mechanical stress (CMS) (20% stretching; 1 Hz) and after disruption of lipid rafts with methyl-β-cyclodextrin. Expression of CD39 and CD73 in porcine TM cells and tissue were examined by Q-PCR and Western blot. The effect of inhibition of CD73 on outflow facility was evaluated in perfused living mouse eyes.

RESULTS

PTM cells generated extracellular adenosine from extracellular ATP and AMP but not from extracellular cAMP. Increased intracellular cAMP mediated by forskolin led to a significant increase in extracellular adenosine production that was not prevented by IBMX. Inhibition of CD73 resulted, in all cases, in a significant decrease in extracellular adenosine. CMS induced a significant activation of extracellular adenosine production. Inhibition of CD73 activity with AMPCP in living mouse eyes resulted in a significant decrease in outflow facility.

CONCLUSIONS

These results support the concept that the extracellular adenosine pathway might play an important role in the homeostatic regulation of outflow resistance in the TM, and suggest a novel mechanism by which pathologic alteration of the TM, such as increased tissue rigidity, could lead to abnormal elevation of IOP in glaucoma.

Extracellular 2',3'-cAMP and 3',5'-cAMP stimulate proliferation of preglomerular vascular endothelial cells and renal epithelial cells

Kidneys release into the extracellular compartment 3',5'-cAMP and its positional isomer 2',3'-cAMP. The purpose of the present study was to investigate the metabolism of extracellular 2',3'-cAMP and 3',5'-cAMP in preglomular vascular endothelial and proximal tubular epithelial cells and to determine whether these cAMPs and their downstream metabolites affect cellular proliferation. In preglomerular vascular endothelial and proximal tubular epithelial cells, 1) extracellular 2',3'-cAMP increased extracellular levels of 3'-AMP and 2'-AMP, whereas extracellular 3',5'-cAMP increased extracellular levels of 5'-AMP; 2) extracellular 5'-AMP, 3'-AMP, and 2'-AMP increased extracellular adenosine; 3) α,β-methylene-adenosine-5'-diphosphate (CD73 inhibitor) prevented the 5'-AMP-induced increase in extracellular adenosine in preglomerular vascular endothelial cells, but did not affect the 5'-AMP-induced increase in extracellular adenosine in proximal tubular cells or the 3'-AMP-induced or 2'-AMP-induced increase in extracellular adenosine in either cell type; 4) extracellular 2',3'-cAMP, 3'-AMP, 2'-AMP, 3',5'-cAMP, 5'-AMP, and adenosine stimulated proliferation of both preglomerular vascular endothelial and proximal tubular cells; and 5) MRS-1754 (selective A(2B) receptor antagonist) abolished the progrowth effects of extracellular 2',3'-cAMP, 3'-AMP, 2'-AMP, 3',5'-cAMP, 5'-AMP, and adenosine in both cell types. Extracellular 2',3'-cAMP and 3',5'-cAMP stimulate proliferation of preglomerular vascular endothelial cells and proximal tubular cells. The mechanism by which the cAMPs increase cell proliferation entails 1) metabolism to their respective AMPs, 2) metabolism of their respective AMPs to adenosine (which for 5'-AMP in preglomerular vascular endothelial cells is mediated by CD73), and 3) activation of A(2B) receptors. Both extracellular 2',3'-cAMP and 3',5'-cAMP may help restore architecture of the preglomerular microcirculation and tubular system following kidney 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).

Expression of the 2',3'-cAMP-adenosine pathway in astrocytes and microglia

Many organs express the extracellular 3',5'-cAMP-adenosine pathway (conversion of extracellular 3',5'-cAMP to 5'-AMP and 5'-AMP to adenosine). Some organs release 2',3'-cAMP (isomer of 3',5'-cAMP) and convert extracellular 2',3'-cAMP to 2'- and 3'-AMP and convert these AMPs to adenosine (extracellular 2',3'-cAMP-adenosine pathway). As astrocytes and microglia are important participants in the response to brain injury and adenosine is an endogenous neuroprotectant, we investigated whether these extracellular cAMP-adenosine pathways exist in these cell types. 2',3'-, 3',5'-cAMP, 5'-, 3'-, and 2'-AMP were incubated with mouse primary astrocytes or primary microglia for 1 h and purine metabolites were measured in the medium by mass spectrometry. There was little evidence of a 3',5'-cAMP-adenosine pathway in either astrocytes or microglia. In contrast, both cell types converted 2',3'-cAMP to 2'- and 3'-AMP (with 2'-AMP being the predominant product). Although both cell types converted 2'- and 3'-AMP to adenosine, microglia were five- and sevenfold, respectively, more efficient than astrocytes in this regard. Inhibitor studies indicated that the conversion of 2',3'-cAMP to 2'-AMP was mediated by a different ecto-enzyme than that involved in the metabolism of 2',3'-cAMP to 3'-AMP and that although CD73 mediates the conversion of 5'-AMP to adenosine, an alternative ecto-enzyme metabolizes 2'- or 3'-AMP to adenosine.

The extracellular cAMP-adenosine pathway regulates expression of renal D1 dopamine receptors in diabetic rats

Activation of D1 dopamine receptors expressed in the kidneys promotes the excretion of sodium and regulates sodium levels during increases in dietary sodium intake. A decrease in the expression or function of D1 receptors results in increased sodium retention which can potentially lead to the development of hypertension. Studies have shown that in the absence of functional D1 receptors, in null mice, the systolic, diastolic, and mean arterial pressures are higher. Previous studies have shown that the expression and function of D1 receptors in the kidneys are decreased in animal models of diabetes. The mechanisms that down-regulate the expression of renal D1 receptor gene in diabetes are not well understood. Using primary renal cells and acutely isolated kidneys from the streptozotocin-induced rat diabetic model, we demonstrate that the renal D1 receptor expression is down-regulated by the extracellular cAMP-adenosine pathway in vitro and in vivo. In cultures of primary renal cells, a 3 mm, 60-h cAMP treatment down-regulated the expression of D1 receptors. In vivo, we determined that the plasma and urine cAMP levels as well as the expression of 5'-ectonucleotidase, tissue-nonspecific alkaline phosphatase, and adenosine A2a receptors are significantly increased in diabetic rats. Inhibitors of 5'-ectonucleotidase and tissue-nonspecific alkaline phosphatase, α,β-methyleneadenosine 5'-diphosphate, and levamisole, respectively, blocked the down-regulation of D1 receptors in the primary renal cells and in the kidney of diabetic animals. The results suggest that inhibitors of the extracellular cAMP-adenosine pathway reverse the down-regulation of renal D1 receptor in diabetes.

Extracellular cAMP-adenosine pathways in the mouse kidney

The renal extracellular 2',3'-cAMP-adenosine and 3',5'-cAMP-adenosine pathways (extracellular cAMPs→AMPs→adenosine) may contribute to renal adenosine production. Because mouse kidneys provide opportunities to investigate renal adenosine production in genetically modified kidneys, it is important to determine whether mouse kidneys express these cAMP-adenosine pathways. We administered (renal artery) 2',3'-cAMP and 3',5'-cAMP to isolated, perfused mouse kidneys and measured renal venous secretion rates of 2',3'-cAMP, 3',5'-cAMP, 2'-AMP, 3'-AMP, 5'-AMP, adenosine, and inosine. Arterial infusions of 2',3'-cAMP increased (P < 0.0001) the mean venous secretion of 2'-AMP (390-fold), 3'-AMP (497-fold), adenosine (18-fold), and inosine (adenosine metabolite; 7-fold), but they did not alter 5'-AMP secretion. Infusions of 3',5'-cAMP did not affect venous secretion of 2'-AMP or 3'-AMP, but they increased (P < 0.0001) secretion of 5'-AMP (5-fold), adenosine (17-fold), and inosine (6-fold). Energy depletion (metabolic inhibitors) increased the secretion of 2',3'-cAMP (8-fold, P = 0.0081), 2'-AMP (4-fold, P = 0.0028), 3'-AMP (4-fold, P = 0.0270), 5'-AMP (3-fold, P = 0.0662), adenosine (2-fold, P = 0.0317), and inosine (7-fold, P = 0.0071), but it did not increase 3',5'-cAMP secretion. The 2',3'-cAMP-adenosine pathway was quantitatively similar in CD73 -/- vs. +/+ kidneys. However, 3',5'-cAMP induced a 6.7-fold greater increase in 5'-AMP, an attenuated increase (61% reduction) in inosine and a similar increase in adenosine in CD73 -/- vs. CD73 +/+ kidneys. In mouse kidneys, 1) 2',3'-cAMP and 3',5'-cAMP are metabolized to their corresponding AMPs, which are subsequently metabolized to adenosine; 2) energy depletion activates the 2',3'-cAMP-adenosine, but not the 3',5'-cAMP-adenosine, pathway; and 3) although CD73 is involved in the 3',5'-AMP-adenosine pathway, alternative pathways of 5'-AMP metabolism and reduced metabolism of adenosine to inosine compensate for life-long deficiency of CD73.

Glomerular tubular balance is suppressed in adenosine type 1 receptor-deficient mice

Glomerular tubular balance maintains a stable fractional solute and fluid reabsorption in the proximal tubule over a range of glomerular filtration rates. The mediators of this process are unknown. We tested the hypothesis that adenosine, produced in proximal tubule cells acting on adenosine type 1 receptors (A(1)-AR) promotes Na(+) and fluid uptake and mediates glomerular tubular balance. Absolute proximal fluid reabsorption (J(v)) was measured by in vivo microperfusion in A(1)-AR knockout and wild-type mice during perfusion of the closed proximal tubule at 2-10 nl/min. J(v) increased with perfusate flow from 2-4 nl/min in both strains, but the fractional increase was lower in A(1)-AR(-/-) mice (A(1)-AR(+/+): 114% vs. A(1)-AR(-/-): 38%; P < 0.001), suggesting reduced glomerular tubular balance (GTB). At higher perfusion rates, J(v) increased modestly in both strains, indicating less GTB at higher flow. The physiological effects of reduced GTB in A(1)-AR(-/-) mice were assessed from the response to an acute volume load (1 ml/2 min). Na(+) excretion and urine flow increased 76 and 73% more in A(1)-AR(-/-) mice than A(1)-AR(+/+) over the following 30 min, accompanied by a higher proximal tubule flow (A(1)-AR(-/-): 6.9 ± 0.9 vs. A(1)-AR(+/+): 5.2 ± 0.6 nl/min; P < 0.05). The expression of the sodium-hydrogen exchanger 3 and sodium phosphate cotransporter-2 were similar between strains. In conclusion, GTB is dependent on adenosine acting on type 1 receptors in the proximal tubule. This may contribute to acute changes in Na(+) and fluid reabsorption.

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.

Extracellular 2',3'-cAMP is a source of adenosine

We discovered that renal injury releases 2',3'-cAMP (positional isomer of 3',5'-cAMP) into the interstitium. This finding motivated a novel hypothesis: renal injury leads to activation of an extracellular 2',3'-cAMP-adenosine pathway (i.e. metabolism of extracellular 2',3'-cAMP to 3'-AMP and 2'-AMP, which are metabolized to adenosine, a retaliatory metabolite). In isolated rat kidneys, arterial infusions of 2',3'-cAMP (30 mumol/liter) increased the mean venous secretion of 3'-AMP (3,400-fold), 2'-AMP (26,000-fold), adenosine (53-fold), and inosine (adenosine metabolite, 30-fold). Renal injury with metabolic inhibitors increased the mean secretion of 2',3'-cAMP (29-fold), 3'-AMP (16-fold), 2'-AMP (10-fold), adenosine (4.2-fold), and inosine (6.1-fold) while slightly increasing 5'-AMP (2.4-fold). Arterial infusions of 2'-AMP and 3'-AMP increased secretion of adenosine and inosine similar to that achieved by 5'-AMP. Renal artery infusions of 2',3'-cAMP in vivo increased urinary excretion of 2'-AMP, 3'-AMP and adenosine, and infusions of 2'-AMP and 3'-AMP increased urinary excretion of adenosine as efficiently as 5'-AMP. The implications are that 1) in intact organs, 2'-AMP and 3'-AMP are converted to adenosine as efficiently as 5'-AMP (previously considered the most important adenosine precursor) and 2) because 2',3'-cAMP opens mitochondrial permeability transition pores, a pro-apoptotic/pro-necrotic process, conversion of 2',3'-cAMP to adenosine by the extracellular 2',3'-cAMP-adenosine pathway would protect tissues by reducing a pro-death factor (2',3'-cAMP) while increasing a retaliatory metabolite (adenosine).

Attenuation of renal ischemia-reperfusion injury by postconditioning involves adenosine receptor and protein kinase C activation

SUMMARY

Significant organ injury occurs after transplantation and reflow (i.e., reperfusion injury). Postconditioning (PoC), consisting of alternating periods of reperfusion and re-occlusion at onset of reperfusion, attenuates reperfusion injury in organs including heart and brain. We tested whether PoC attenuates renal ischemia-reperfusion (I/R) injury in the kidney by activating adenosine receptors (AR) and protein kinase C (PKC). The single kidney rat I/R model was used. Groups: (1) sham: time-matched surgical protocol only. In all others, the left renal artery (RA) was occluded for 45 min and reperfused for 24 h. (2) CONTROL: I/R with no intervention at R. All antagonists were administered 5 min before reperfusion. (3) PoC: I/R + four cycles of 45 s of R and 45 s of re-occlusion before full R. (4) PoC + ARi: PoC plus the AR antagonist 8-rho-(sulfophenyl) theophylline (8-SPT). (5) PoC + PKCi: PoC plus the PKC antagonist chelerythrine (Che). In shams, plasma blood urea nitrogen (BUN mg/dl) at 24 h averaged 23.2 +/- 5.3 and creatinine (Cr mg/dl) averaged 1.28 +/- 0.2. PoC reduced BUN (87.2 +/- 10 in CONTROL vs. 38.8 +/- 9, P = 0.001) and Cr (4.2 +/- 0.6 in CONTROL vs. 1.5 +/- 0.2, P < 0.001). 8-SPT and Che reversed renal protection indices after PoC. I/R increased apoptosis, which was reduced by PoC, which was reversed by 8-SPT and Che. Postconditioning attenuates renal I/R injury by adenosine receptor activation and PKC signaling.

Multidrug resistance protein 4 mediates cAMP efflux from rat preglomerular vascular smooth muscle cells

1. Previous studies have shown that stimulation of adenylyl cyclase in preglomerular vascular smooth muscle cells (PGVSMC) increases extracellular cAMP; however, the mechanism by which PGVSMC transport intracellular cAMP into the extracellular milieu is unknown. 2. We hypothesize that multidrug resistance protein (MRP) 4 is the primary transporter mediating efflux of intracellular cAMP from PGVSMC. 3. Both reverse transcription-polymerase chain reaction and real-time polymerase chain reaction detected MRP4 mRNA in PGVSMC in culture. Moreover, western blotting using an antibody specific for MRP4 gave rise to a 150 kDa signal, consistent with the presence of MRP4 protein in PGVSMC. 4. Specifically designed short interference (si) RNA reduced MRP4 mRNA expression by 71% (P = 0.0075) and MRP4 protein by 80% (P = 0.0004). 5. Isoproterenol (1 micromol/L) increased intracellular cAMP, which resulted in efflux of cAMP into the medium. The siRNA knockdown of MRP4 significantly reduced basal extracellular cAMP and nearly abolished isoproterenol-induced increases in extracellular cAMP (P = 0.0143, interaction between isoproterenol and MRP4 siRNA in two-factor analysis of variance). In isoproterenol-treated cells, MRP4 siRNA decreased the ratio of extracellular cAMP to intracellular cAMP by 72% (P = 0.0019). 6. We conclude that MRP4 is the dominant cAMP transporter in PGVSMC.

Deficiency of polycystic kidney disease-1 gene (PKD1) expression increases A(3) adenosine receptors in human renal cells: implications for cAMP-dependent signalling and proliferation of PKD1-mutated cystic cells

Cyst growth and expansion in autosomal dominant polycystic kidney disease (ADPKD) has been attributed to numerous factors, including ATP, cAMP and adenosine signalling. Although the role of ATP and cAMP has been widely investigated in PKD1-deficient cells, no information is currently available on adenosine-mediated signalling. Here we investigate for the first time the impact of abnormalities of polycystin-1 (PC1) on the expression and functional activity of adenosine receptors, members of the G-protein-coupled receptor superfamily. Pharmacological, molecular and biochemical findings show that a siRNA-dependent PC1-depletion in HEK293 cells and a PKD1-nonsense mutation in cyst-derived cell lines result in increased expression of the A(3) adenosine receptor via an NFkB-dependent mechanism. Interestingly, A(3) adenosine receptor levels result higher in ADPKD than in normal renal tissues. Furthermore, the stimulation of this receptor subtype with the selective agonist Cl-IB-MECA causes a reduction in both cytosolic cAMP and cell proliferation in both PC1-deficient HEK293 cells and cystic cells. This reduction is associated with increased expression of p21(waf) and reduced activation not only of ERK1/2, but also of S6 kinase, the main target of mTOR signalling. In the light of these findings, the ability of Cl-IB-MECA to reduce disease progression in ADPKD should be further investigated. Moreover, our results suggest that NFkB, which is markedly activated in PC1-deficient and cystic cells, plays an important role in modulating A(3)AR expression in cystic cells.

Identification and quantification of 2',3'-cAMP release by the kidney

We recently developed a sensitive assay for 3',5'-cAMP using high-performance liquid chromatography-tandem mass spectrometry. Using this assay, we investigated the release of 3',5'-cAMP from isolated, perfused rat kidneys. To our surprise, we observed a dominant chromatographic peak that was because of an endogenous substance that had the same parent ion as 3',5'-cAMP and that fragmented to the same daughter ion (adenine) as 3',5'-cAMP. However, the retention time of this unknown was approximately 2.9 min, compared with 6.3 min for authentic 3',5'-cAMP. We hypothesized that the unknown substance was an isomer of 3',5'-cAMP. The unknown substance had the same retention time and mass spectral properties as authentic 2',3'-cAMP. Renal venous secretion of 2',3'-cAMP was greater in kidneys from 20-week-old genetically hypertensive rats compared with age-matched normotensive rats (12.49 +/- 2.14 versus 5.32 +/- 1.97 ng/min/g kidney weight, respectively; n = 18). Isoproterenol (1 microM; beta-adrenoceptor agonist) increased renal venous 3',5'-cAMP secretion (approximately 690% of control) but had no effect on 2',3'-cAMP production. In contrast, rapamycin (0.2 microM; activator of mRNA turnover) and iodoacetate + 2,4-dinitrophenol (50 microM; metabolic inhibitors) increased the renal venous secretion of 2',3'-cAMP (approximately 1000 and 4100% of control, respectively) while simultaneously decreasing the renal venous secretion of 3',5'-cAMP. In conclusion, 2',3'-cAMP is a naturally occurring isomer of 3',5'-cAMP that is: 1) not made by adenylyl cyclase; 2) released from kidneys into the extracellular compartment; 3) released more by kidneys from rats with long-standing hypertension; 4) derived from mRNA turnover; and 5) increased by energy depletion.

Cooperation of adenosine and prostaglandin E2 (PGE2) in amplification of cAMP-PKA signaling and immunosuppression

INTRODUCTION

We hypothesize that adenosine and PGE2 could have a complementary immunosuppressive effect that is mediated via common cAMP-PKA signaling.

MATERIALS AND METHODS

To test this hypothesis, the effect of adenosine and PGE2 on the cytotoxic activity and cytokine production of lymphokine activated killer (LAK) cells was investigated.

RESULTS

PGE2 and adenosine inhibited LAK cells cytotoxic activity and production of INF-gamma, GM-CSF and TNF-alpha. In combination they showed substantially higher inhibition than each modality used alone. Using agonists and antagonists specific for PGE2 and adenosine receptors we found that cooperation of PGE2 and adenosine in their inhibitory effects are mediated via EP2 and A2A receptors, respectively. LAK cells have 35-fold higher expression of EP2 than A2A. Combined PGE2 and adenosine treatment resulted in augmentation of cAMP production, PKA activity, CREB phosphorylation and inhibition of Akt phosphorylation. Wortmannin and LY294002 enhanced the suppressive effects of adenosine and PGE2. In contrast, Rp-8-Br-cAMPS, an inhibitor of PKA type I blocked their immunosuppressive effects, suggesting that the inhibitory effects of PGE2 and adenosine are mediated via common pathway with activation of cAMP-PKA and inhibition of Akt.

CONCLUSION

In comparison to other immunosuppressive molecules (TGF-beta and IL-10), adenosine and PGE2 are unique in their ability to inhibit the executive function of highly cytotoxic cells. High intratumor levels of adenosine and PGE2 could protect tumor from immune-mediated destruction by inactivation of the tumor infiltrating functionally active immune cells.

Adenosine and renal tubular function

PURPOSE OF REVIEW

Intrarenal adenosine is present in the cytoplasm of renal epithelial cells and in the extracellular space. Adenosine is generated at high levels in response to imbalance between energy demand and supply (e.g. increased tubular sodium chloride transport or hypoxia) and activates cell membrane adenosine receptors to affect renal vascular and tubular functions. Adenosine regulates renal sodium and water excretion via a myriad of effects on renal hemodynamic, glomerular filtration rate, renin secretion and direct effects on the renal tubule epithelium. This review examines the direct effects of adenosine on renal tubular epithelial transport in light of the most recent evidence and discusses some physiologic and pathophysiologic implications.

RECENT FINDINGS

Intrarenal adenosine affects proximal fluid and solute transport in a biphasic fashion. Under physiological conditions adenosine stimulates proximal tubular re-absorption, thus reducing the load delivered to the distal nephron. A supra-physiologic increase in adenosine such as in ischemia reduces reabsorption in the proximal tubule, thus reducing renal oxygen consumption.

SUMMARY

Intrarenal adenosine and its receptors have important regulatory functions in the renal epithelium. A complete understanding of this autocrine/paracrine system holds great potential for novel therapeutic strategies, such as the use of nucleoside analogues for reno-protection in renal ischemia.

Purinergic signaling in the lumen of a normal nephron and in remodeled PKD encapsulated cysts

The nephron is the functional unit of the kidney. Blood and plasma are continually filtered within the glomeruli that begin each nephron. Adenosine 5' triphosphate (ATP) and its metabolites are freely filtered by each glomerulus and enter the lumen of each nephron beginning at the proximal convoluted tubule (PCT). Flow rate, osmolality, and other mechanical or chemical stimuli for ATP secretion are present in each nephron segment. These ATP-release stimuli are also different in each nephron segment due to water or salt permeability or impermeability along different luminal membranes of the cells that line each nephron segment. Each of the above stimuli can trigger additional ATP release into the lumen of a nephron segment. Each nephron-lining epithelial cell is a potential source of secreted ATP. Together with filtered ATP and its metabolites derived from the glomerulus, secreted ATP and adenosine derived from cells along the nephron are likely the principal two of several nucleotide and nucleoside candidates for renal autocrine and paracrine ligands within the tubular fluid of the nephron. This minireview discusses the first principles of purinergic signaling as they relate to the nephron and the urinary bladder. The review discusses how the lumen of a renal tubule presents an ideal purinergic signaling microenvironment. The review also illustrates how remodeled and encapsulated cysts in autosomal dominant polycystic kidney disease (ADPKD) and remodeled pseudocysts in autosomal recessive PKD (ARPKD) of the renal collecting duct likely create an even more ideal microenvironment for purinergic signaling. Once trapped in these closed microenvironments, purinergic signaling becomes chronic and likely plays a significant epigenetic and detrimental role in the secondary progression of PKD, once the remodeling of the renal tissue has begun. In PKD cystic microenvironments, we argue that normal purinergic signaling within the lumen of the nephron provides detrimental acceleration of ADPKD once remodeling is complete.

Cyclic AMP in rat ileum: evidence for the presence of an extracellular cyclic AMP-adenosine pathway

BACKGROUND & AIMS

Extracellular adenosine plays a relevant role in regulating intestinal motility and preventing inflammatory processes. Adenosine 3',5'-cyclic monophosphate (cAMP) extruded from cells may be converted to adenosine monophosphate and then to adenosine by ecto-phosphodiesterase and CD73/ecto-5'nucleotidase, respectively, thus representing a source of adenosine. Our purpose was to assess the existence of a functional extracellular cAMP-adenosine pathway in intestinal tissue, obtaining evidence for CD73 expression and evaluating the effect of cAMP on ileum motility.

METHODS

The formation of cAMP metabolites in rat ileum strips incubated with exogenous cAMP or [(3)H]cAMP was monitored by high-performance liquid chromatography. CD73 was detected by immunoprecipitation and immunofluorescence. The functional activity of exogenous cAMP on ileum strips was recorded by measuring tension changes.

RESULTS

In ileum strips, the generation of cAMP-derived adenosine monophosphate, adenosine, and inosine was time and concentration dependent and was blocked by phosphodiesterase or CD73 inhibitors in a manner consistent with exogenous cAMP being processed through the extracellular cAMP-adenosine pathway. Accordingly, [(3)H]cAMP uptake in ileum strips was negligible. Immunofluorescence revealed CD73 surface expression on intestinal smooth muscle cells and intact smooth muscle. Exogenous cAMP concentration-dependently increased ileum muscle tension partially inhibited by adenosine inactivation or receptor blockade. Forskolin-stimulated endogenous cAMP induced concentration-dependent ileum relaxations.

CONCLUSIONS

A functioning extracellular cAMP-adenosine pathway featuring CD73 expression is present in rat ileum and affects intestinal motility. Extracellular cAMP may therefore act on intestinal muscle both directly by binding to specific smooth muscle cell membrane sites and indirectly through its degradation products.

cAMP: fuel for extracellular adenosine formation?

It is well known that cAMP, an important intracellular second messenger, is released from many cells upon adenylate cyclase stimulation. Cell surface bound phosphodiesterase together with ecto-5'-nucleotidase may convert the extracellular cAMP to adenosine, which may stimulate in a paracrine and/or autocrine manner cells expressing P1 receptors. In this issue of the British Journal of Pharmacology, Chiavegatti et al. demonstrate the existence of an extracellular cAMP-adenosine cascade in skeletal muscle cells which suggests a link between adrenergic stimulation of contraction, elevated cAMP formation and release and exercise hyperaemia.

Regulation of renal ectophosphodiesterase by protein kinase C and sodium diet

Kidneys metabolize arterial cAMP to adenosine by the sequential actions of ectophosphodiesterase (cAMP --> AMP) and ecto-5'-nucleotidase (AMP --> adenosine). In this study, we demonstrated that etheno-AMP (fluorescent AMP analog) is nearly completely converted to etheno-adenosine during a single pass through the isolated, perfused rat kidney indicating that ecto-5'-nucleotidase is not rate limiting. Therefore, we examined the regulation of ectophosphodiesterase. In 17 control kidneys pretreated with alpha,beta-methylene-adenosine-5'-diphosphate (inhibitor of ecto-5'-nucleotidase to prevent AMP metabolism; 100 microM), addition of cAMP (10 microM) to the perfusate increased renal venous AMP from 0.6 +/- 0.2 to 3.5 +/- 0.5 nmol/min/g. Pretreatment of kidneys with phorbol 12-myristate 13-acetate (protein kinase C activator; 7.5 nM) increased renal vascular resistance and significantly augmented the cAMP-induced increase in renal venous AMP (from 0.8 +/- 0.2 to 5.2 +/- 0.7 nmol/min/g with cAMP). Pretreatment of kidneys with bisindolymaleimide I (protein kinase C inhibitor; 3 microM) abrogated the effects of phorbol 12-myristate 13-acetate on both renovascular resistance and cAMP conversion to AMP. Compared with kidneys from rats fed a high-sodium diet (3.15%) for 1 week, in kidneys from rats fed a low-sodium diet (0.03%) the conversion of cAMP to AMP was attenuated (high sodium, from 1.0 +/- 0.1 to 4.6 +/- 0.4 nmol/min/g with cAMP; low sodium, from 0.5 +/- 0.04 to 2.6 +/- 0.04 nmol/min/g with cAMP). We conclude that the renal vasculature efficiently converts AMP to adenosine and that metabolism of cAMP to AMP is rate limiting and regulated acutely by protein kinase C and chronically by sodium intake.

Effect of adenosine in extracellular matrix synthesis in human and rat mesangial cells

Adenosine (ADO) is an intermediary metabolite of adenosine trisphosphate degradation and a vasoactive mediator. We showed previously that ADO induces contraction and proliferation in rat mesangial cells by a mechanism involving A1 and A2 receptors. The studies concerning the effect of ADO on extracellular matrix (ECM) accumulation in mesangial cells are scarce. The purpose of our study was to evaluate the effect of ADO and the effect of the selective stimulation of A1 and A2 ADO receptors on the expression of ECM components fibronectin and collagen type I, in human and rat renal mesangial cells. Cultured human and rat renal mesangial cells were subjected to selective stimulation of A1 and A2 ADO receptors for 24 and 48 h. Fibronectin and collagen type I expression was evaluated by Western blot; total collagen synthesis was measured by [3H]-proline incorporation into collagen proteins. ADO, A1 and A2 receptor stimulation induce increases in fibronectin expression in rat mesangial cells, and A1 receptor stimulation partially inhibits fibronectin expression in serum-stimulated rat mesangial cells, without any effect in human mesangial cells. A2 receptor stimulation reduces collagen type I expression in serum-stimulated mesangial cells. Neither ADO nor A1 or A2 receptor stimulation induce significant changes in total collagen synthesis. These data suggest that ADO is not a major regulator of ECM synthesis in rat and human mesangial cells.

A1 adenosine receptor upregulation accompanies decreasing myocardial adenosine levels in mice with left ventricular dysfunction

BACKGROUND

It is well known that adenosine levels are increased during ischemia and protect the heart during ischemia/reperfusion. However, less is known about the role of adenosine-adenosine receptor (AR) pathways in hearts with left ventricular dilation and dysfunction. Therefore, we assessed adenosine levels and selective AR expression in transgenic mice with left ventricular systolic dysfunction secondary to overexpression of tumor necrosis factor-alpha (TNF 1.6).

METHODS AND RESULTS

Cardiac adenosine levels were reduced by 70% at 3 and 6 weeks of age in TNF 1.6 mice. This change was accompanied by a 4-fold increase in the levels of A1-AR and a 50% reduction in the levels of A2A-AR. That the increase in A1-AR density was of physiological significance was shown by the fact that chronotropic responsiveness to the A1-AR selective agonist 2-chloro-N6-cyclopentanyladenosine was enhanced in the TNF 1.6 mice. Similar changes in adenosine levels were found in 2 other models of heart failure, mice overexpressing calsequestrin and mice after chronic pressure overload, suggesting that the changes in adenosine-AR signaling were secondary to myocardial dysfunction rather than to TNF overexpression.

CONCLUSIONS

Cardiac dysfunction secondary to the overexpression of TNF is associated with marked alterations in myocardial levels of adenosine and ARs. Modulation of the myocardial adenosine system and its signaling pathways may be a novel therapeutic target in patients with heart failure.

The pancreatohepatorenal cAMP-adenosine mechanism

Stimulation of adenylyl cyclase causes cellular efflux of cAMP, and cAMP (unlike adenosine) is stable in blood. Therefore, it is conceivable that cAMP could function as a circulating adenosine prohormone by local target-organ conversion of distally released cAMP to adenosine via the sequential actions of ectophosphodiesterase and ecto-5'-nucleotidase (cAMP==> AMP==> adenosine; called the cAMP-adenosine pathway). A possible specific representation of this general concept is the pancreatohepatorenal cAMP-adenosine mechanism. The pancreas secretes glucagon into the portal circulation, and glucagon is a stimulant of hepatic adenylyl cyclase. Therefore, we hypothesize that the pancreas, via glucagon, stimulates hepatic cAMP production, which provides circulating cAMP for conversion to adenosine in the kidney via the cAMP-adenosine pathway. In normal rats, intravenous cAMP increased urinary and renal interstitial (assessed by renal microdialysis) cAMP and adenosine. Intraportal infusions of glucagon increased plasma cAMP 10-fold, it did not affect plasma adenosine, and it increased urinary and renal interstitial cAMP and adenosine. Local renal interstitial blockade (by adding inhibitors directly to the microdialysis perfusate) of ectophosphodiesterase (using 3-isobutyl-1-methylxanthine or 1,3-dipropyl-8-p-sulfophenylxanthine) or ecto-5'-nucleotidase (using alpha,beta-methyleneadenosine-5'-diphosphate) prevented the cAMP-induced and glucagon-induced increases in renal interstitial adenosine, but not cAMP. In ZSF1 rats with the metabolic syndrome, an oral glucose load increased plasma glucagon and urinary cAMP and adenosine excretion. We conclude that circulating cAMP is a substrate for local conversion to adenosine via the cAMP-adenosine pathway. A specific manifestation of this is the pancreatohepatorenal cAMP-adenosine mechanism (pancreas==> portal glucagon==> liver==> circulating cAMP==> kidney==> local cAMP-adenosine pathway).

Characterization of renal ecto-phosphodiesterase

In kidneys, stimulation of adenylyl cyclase causes egress of cAMP, conversion of cAMP to AMP by ecto-phosphodiesterase, and metabolism of AMP to adenosine by ecto-5'-nucleotidase. Although much is known about ecto-5'-nucleotidase, the renal ecto-phosphodiesterase remains uncharacterized. We administered cAMP (10 microM in the perfusate) to 12 different groups of perfused kidneys. AMP was measured in perfusate using ion trap mass spectrometry. In control kidneys (n=19), basal renal secretion rate of AMP was 0.49+/-0.08 and increased to 3.0+/-0.2 nmol AMP/g kidney weight/min during administration of cAMP. A broad-spectrum phosphodiesterase (PDE) inhibitor (1,3-isobutyl-1-methylxanthine, 300 microM, n=6) and an ecto-phosphodiesterase inhibitor (1,3-dipropyl-8-p-sulfophenylxanthine, 1 mM, n=6) significantly attenuated cAMP-induced AMP secretion by 60 and 74%, respectively. Blockade of PDE1 (8-methoxymethyl-3-isobutyl-1-methylxanthine, 100 microM), PDE2 [erythro-9-(2-hydroxy-3-nonyl)adenine, 30 microM], PDE3 (milrinone, 10 microM; cGMP, 10 microM), PDE4 (Ro 20-1724 [4-(3-butoxy-4-methoxybenzyl)imidazolidin-2-one], 100 microM), PDE5 and PDE6 (zaprinast, 30 microM), and PDE7 [BRL-50481 (5-nitro-2,N,N-trimethylbenzenesulfonamide), 10 microM] did not alter renal ecto-phosphodiesterase activity. Administration of a concentration (100 microM) of dipyridamole that blocks PDE8 inhibited ecto-phosphodiesterase activity (by 44%). However, a lower concentration of dipyridamole (3 microM) that blocks PDE9, PDE10, and PDE11, but not PDE8, did not inhibit ecto-phosphodiesterase activity. These data support the conclusion that renal ecto-phosphodiesterase activity is not mediated by PDE1, PDE2, PDE3, PDE4, PDE5, PDE6, PDE7, PDE9, PDE10, or PDE11 and is inhibited by high concentrations of dipyridamole. Ecto-phosphodiesterase has some pharmacological characteristics similar to PDE8.

Extracellular cAMP inhibits D1 dopamine receptor expression in CAD catecholaminergic cells via A2a adenosine receptors

The expression of D1 dopamine (DA) receptor gene is regulated during development, aging, and pathophysiology. The extracellular factors and signaling mechanisms that modulate the expression of D1 DA receptor have not been well characterized. Here, we present novel evidence that endogenous D1 DA receptor expression is inhibited by extracellular cAMP in the Cath.A Derived (CAD) catecholaminergic neuronal cell line. CAD cells express the multi-drug resistance protein 5 transporters and secrete cAMP. Addition of exogenous cAMP decreases D1 receptor mRNA and protein greater than fourfold in 24 h. The cAMP-induced decrease of D1 receptor mRNA levels is blocked by cGMP and by 1,3-dipropyl-8-(p-sulfo-phenyl)xanthine, an inhibitor of ecto-phosphodiestrase. Extracellular AMP, a metabolite of cAMP, also independently decreased D1 receptor mRNA levels. Inhibitors of ecto-nucleotidases, alpha,beta-methyleneadenosine 5'-di-phosphate and GMP, completely blocked the decrease of D1 receptor mRNA by extracellular cAMP, but only partially blocked the decrease induced by extracellular AMP. Levamisole, an inhibitor of tissue non-specific alkaline phosphatase, completely blocked the AMP-induced decrease of D1 receptor mRNA. The extracellular cAMP, AMP, and adenosine (ADO)-induced decrease in D1 receptor mRNA expression are mediated by A2a ADO receptor subtype. The results suggest a novel molecular mechanism linking activation of A2a ADO receptors with inhibition of D1 DA receptor expression.

The extracellular cAMP-adenosine pathway significantly contributes to the in vivo production of adenosine

The extracellular cAMP-adenosine pathway is the cellular egress of cAMP followed by extracellular conversion of cAMP to adenosine by the sequential actions of ecto-phosphodiesterase and ecto-5'-nucleotidase. Although detailed studies in isolated organs, tissues, and cells provide evidence for an extracellular cAMP-adenosine pathway, whether this mechanism contributes significantly to adenosine production in vivo is unclear. 1,3-Dipropyl-8-p-sulfophenylxanthine is restricted to the extracellular compartment due to a negative charge at physiological pH and, at high concentrations (> or =0.1 mM), blocks ecto-phosphodiesterase. Here, we show that administration of 1,3-dipropyl-8-p-sulfophenylxanthine at a dose that provided concentrations in plasma and urine of approximately 0.3 and 6 mM, respectively, inhibited urinary adenosine excretion. In Sprague-Dawley rats i.v., 1,3-dipropyl-8-p-sulfophenylxanthine (10 mg + 0.15 mg/min) significantly decreased by 48 and 39% the urinary excretion of adenosine (from 3.57 +/- 0.38 to 1.87 +/- 0.14 nmol/30 min; p = 0.0003) and the ratio of urinary adenosine to cAMP (from 0.93 +/- 0.08 to 0.57 +/- 0.06; p = 0.0044), respectively, without altering blood pressure, renal blood flow, or glomerular filtration rate. Although 1,3-dipropyl-8-p-sulfophenylxanthine transiently increased urine volume and sodium excretion, these effects subsided, yet adenosine excretion remained reduced. Thus, changes in systemic and renal hemodynamics and excretory function could not account for the effects of 1,3-dipropyl-8-p-sulfophenylxanthine on adenosine excretion. Additional experiments showed that 1,3-dipropyl-8-p-sulfophenylxanthine, as in Sprague-Dawley rats, significantly attenuated adenosine excretion and the ratio of urinary adenosine to cAMP in both Wistar-Kyoto rats and spontaneously hypertensive rats. We conclude that the extracellular cAMP-adenosine pathway significantly contributes to the in vivo production of adenosine.