The extracellular cyclic AMP-adenosine pathway in renal physiology

cAMP-PKA/EPAC signaling and cancer: the interplay in tumor microenvironment

Cancer is a complex disease resulting from abnormal cell growth that is induced by a number of genetic and environmental factors. The tumor microenvironment (TME), which involves extracellular matrix, cancer-associated fibroblasts (CAF), tumor-infiltrating immune cells and angiogenesis, plays a critical role in tumor progression. Cyclic adenosine monophosphate (cAMP) is a second messenger that has pleiotropic effects on the TME. The downstream effectors of cAMP include cAMP-dependent protein kinase (PKA), exchange protein activated by cAMP (EPAC) and ion channels. While cAMP can activate PKA or EPAC and promote cancer cell growth, it can also inhibit cell proliferation and survival in context- and cancer type-dependent manner. Tumor-associated stromal cells, such as CAF and immune cells, can release cytokines and growth factors that either stimulate or inhibit cAMP production within the TME. Recent studies have shown that targeting cAMP signaling in the TME has therapeutic benefits in cancer. Small-molecule agents that inhibit adenylate cyclase and PKA have been shown to inhibit tumor growth. In addition, cAMP-elevating agents, such as forskolin, can not only induce cancer cell death, but also directly inhibit cell proliferation in some cancer types. In this review, we summarize current understanding of cAMP signaling in cancer biology and immunology and discuss the basis for its context-dependent dual role in oncogenesis. Understanding the precise mechanisms by which cAMP and the TME interact in cancer will be critical for the development of effective therapies. Future studies aimed at investigating the cAMP-cancer axis and its regulation in the TME may provide new insights into the underlying mechanisms of tumorigenesis and lead to the development of novel therapeutic strategies.

Total and Extracellular Vesicle cAMP Contents in Urine Are Associated with Autosomal Dominant Polycystic Kidney Disease (ADPKD) Progression

ADPKD is the most common genetic renal disease, characterized by the presence of multiple cysts which, through slow and gradual growth, lead to glomerular filtration rate (GFR) decline and end-stage renal disease. Cystic growth is associated with increased intracellular levels of 3',5'-cyclic adenosine monophosphate (cAMP). Extracellular vesicles (EVs) are proposed to participate in "remote sensing" by transporting different cargoes, but their relevance to ADPKD progression is poorly understood. This study aimed to determine whether cAMP is contained in urinary EVs and, if so, how total and/or EV cAMP contents participate in disease progression. Fourteen ADPKD patients, naïve for V2 receptor antagonism treatment, and seven controls were studied. Progression was evaluated by estimating GFR (eGFR) and height-adjusted total kidney volume (htTKV). Fresh morning urine was collected to determine cAMP by the competitive radioligand assay. Urine EVs were isolated using an adapted centrifugation method and characterized by electron microscopy, dynamic light scanning, flow cytometry with FITC CD63 labeling, protein and RNA content, and AQP2 and GAPDH mRNA detection. Total and EV cAMP was measurable in both control and patient urine samples. Total cAMP was significantly correlated with eGFR and its annual change but inversely correlated with htTKV. The cAMP-EVs showed a bimodal pattern with htTKV, increasing to ~1 L/m and falling at larger sizes. Our results demonstrate that urine cAMP correlates with ADPKD progression markers, and that its extracellular delivery by EVs could reflect the architectural disturbances of the organ.

Extracellular cAMP-Adenosine Pathway Signaling: A Potential Therapeutic Target in Chronic Inflammatory Airway Diseases

Adenosine is a purine nucleoside that, via activation of distinct G protein-coupled receptors, modulates inflammation and immune responses. Under pathological conditions and in response to inflammatory stimuli, extracellular ATP is released from damaged cells and is metabolized to extracellular adenosine. However, studies over the past 30 years provide strong evidence for another source of extracellular adenosine, namely the “cAMP-adenosine pathway.” The cAMP-adenosine pathway is a biochemical mechanism mediated by ATP-binding cassette transporters that facilitate cAMP efflux and by specific ectoenzymes that convert cAMP to AMP (ecto-PDEs) and AMP to adenosine (ecto-nucleotidases such as CD73). Importantly, the cAMP-adenosine pathway is operative in many cell types, including those of the airways. In airways, β₂-adrenoceptor agonists, which are used as bronchodilators for treatment of asthma and chronic respiratory diseases, stimulate cAMP efflux and thus trigger the extracellular cAMP-adenosine pathway leading to increased concentrations of extracellular adenosine in airways. In the airways, extracellular adenosine exerts pro-inflammatory effects and induces bronchoconstriction in patients with asthma and chronic obstructive pulmonary diseases. These considerations lead to the hypothesis that the cAMP-adenosine pathway attenuates the efficacy of β₂-adrenoceptor agonists. Indeed, our recent findings support this view. In this mini-review, we will highlight the potential role of the extracellular cAMP-adenosine pathway in chronic respiratory inflammatory disorders, and we will explore how extracellular cAMP could interfere with the regulatory effects of intracellular cAMP on airway smooth muscle and innate immune cell function. Finally, we will discuss therapeutic possibilities targeting the extracellular cAMP-adenosine pathway for treatment of these respiratory diseases.

Extracellular metabolism of 3',5'-cyclic AMP as a source of interstitial adenosine in the rat airways

In the respiratory tract, intracellular 3',5'-cAMP mediates smooth muscle relaxation triggered by the β₂-adrenoceptor/Gs protein/adenylyl cyclase axis. More recently, we have shown that β₂-adrenoceptor agonists also increase extracellular 3',5'-cAMP levels in isolated rat trachea, which leads to contraction of airway smooth muscle. In many other tissues, extracellular 3',5'-cAMP is metabolized by ectoenzymes to extracellular adenosine, a catabolic pathway that has never been addressed in airways. In order to evaluate the possible extracellular degradation of 3',5'-cAMP into 5'-AMP and adenosine in the airways, isolated rat tracheas were incubated with exogenous 3',5'-cAMP and the amount of 5'-AMP, adenosine and inosine (adenosine metabolite) produced was evaluated using ultraperformance liquid chromatography-tandem mass spectrometry. Incubation of tracheal tissue with 3',5'-cAMP induced a time- and concentration-dependent increase in 5'-AMP, adenosine and inosine in the medium. Importantly, IBMX (non-selective phosphodiesterase (PDE) inhibitor) and DPSPX (selective ecto-PDE inhibitor) reduced the extracellular conversion of 3',5'-cAMP to 5'-AMP. In addition, incubation of 3',5'-cAMP in the presence of AMPCP (inhibitor of ecto-5'-nucleotidase) increased extracellular levels of 5'-AMP while drastically reducing extracellular levels of adenosine and inosine. These results indicate that airways express an extracellular enzymatic system (ecto-phosphodiesterase, ecto-5'-nucleotidase and adenosine deaminase) that sequentially converts 3',5'-cAMP into 5'-AMP, adenosine and inosine. The observation that extracellular 3',5'-cAMP is a source of interstitial adenosine supports the idea that the extrusion and extracellular metabolism of 3',5'-cAMP has a role in respiratory physiology and pathophysiology.

The equilibrative nucleoside transporter ENT1 is critical for nucleotide homeostasis and optimal erythropoiesis

The tight regulation of intracellular nucleotides is critical for the self-renewal and lineage specification of hematopoietic stem cells (HSCs). Nucleosides are major metabolite precursors for nucleotide biosynthesis and their availability in HSCs is dependent on their transport through specific membrane transporters. However, the role of nucleoside transporters in the differentiation of HSCs to the erythroid lineage and in red cell biology remains to be fully defined. Here, we show that the absence of the equilibrative nucleoside transporter (ENT1) in human red blood cells with a rare Augustine-null blood type is associated with macrocytosis, anisopoikilocytosis, an abnormal nucleotide metabolome, and deregulated protein phosphorylation. A specific role for ENT1 in human erythropoiesis was demonstrated by a defective erythropoiesis of human CD34+ progenitors following short hairpin RNA-mediated knockdown of ENT1. Furthermore, genetic deletion of ENT1 in mice was associated with reduced erythroid progenitors in the bone marrow, anemia, and macrocytosis. Mechanistically, we found that ENT1-mediated adenosine transport is critical for cyclic adenosine monophosphate homeostasis and the regulation of erythroid transcription factors. Notably, genetic investigation of 2 ENT1null individuals demonstrated a compensation by a loss-of-function variant in the ABCC4 cyclic nucleotide exporter. Indeed, pharmacological inhibition of ABCC4 in Ent1-/- mice rescued erythropoiesis. Overall, our results highlight the importance of ENT1-mediated nucleotide metabolism in erythropoiesis.

Exposure to Iron Oxide Nanoparticles Coated with Phospholipid-Based Polymeric Micelles Induces Renal Transitory Biochemical and Histopathological Changes in Mice

The renal toxicity induced by the intravenously injected iron oxide nanoparticles (IONPs) encapsulated in phospholipid-based polymeric micelles was studied in CD1 mice for 2 weeks. Two doses of 5 and 15 mg of Fe/kg bodyweight of NPs or saline solution (control) were tested, and the levels of antioxidant enzyme activities, oxidative stress parameters, and the expressions of kidney fibrosis biomarkers were analyzed. The enzymatic activities of superoxide dismutase, catalase, glutathione peroxidase, glutathione-S-transferase, glutathione reductase, and glucose-6-phosphate dehydrogenase in the kidney were significantly decreased compared to the control in the first 3 days followed by a recovery up to 14 days. Concomitantly, a significant increase in lipid peroxidation (malondialdehyde) levels and a decrease in protein thiol groups were recorded. Moreover, increases in the expressions of T cell immunoglobulin and mucin domain 1 (TIM-1) and transforming growth factor-β (TGF-β) were observed in mouse tissue samples in the first week, which were more pronounced for the higher dose. The results suggested the role of oxidative stress as a mechanism for induced toxicity in mice kidneys after the IV administration of IONPs encapsulated in phospholipid-based polymeric micelles but also the capacity of the kidneys' defense systems to revert efficiently the biochemical modifications that were moderate and for short duration.

A novel secreted-cAMP pathway inhibits pulmonary hypertension via a feed-forward mechanism

AIMS

Cyclic adenosine monophosphate (cAMP) is the predominant intracellular second messenger that transduces signals from Gs-coupled receptors. Intriguingly, there is evidence from various cell types that an extracellular cAMP pathway is active in the extracellular space. Herein, we investigated the role of extracellular cAMP in the lung and examined whether it may act on pulmonary vascular cell proliferation and pulmonary vasculature remodelling in the pathogenesis of pulmonary hypertension (PH).

METHODS AND RESULTS

The expression of cyclic AMP-metabolizing enzymes was increased in lungs from patients with PH as well as in rats treated with monocrotaline and mice exposed to Sugen/hypoxia. We report that inhibition of the endogenous extracellular cAMP pathway exacerbated Sugen/hypoxia-induced lung remodelling. We found that application of extracellular cAMP induced an increase in intracellular cAMP levels and inhibited proliferation and migration of pulmonary vascular cells in vitro. Extracellular cAMP infusion in two in vivo PH models prevented and reversed pulmonary and cardiac remodelling associated with PH. Using protein expression analysis along with luciferase assays, we found that extracellular cAMP acts via the A2R/PKA/CREB/p53/Cyclin D1 pathway.

CONCLUSIONS

Taken together, our data reveal the presence of an extracellular cAMP pathway in pulmonary arteries that attempts to protect the lung during PH, and suggest targeting of the extracellular cAMP signalling pathway to limit pulmonary vascular remodelling and PH.

Apoptotic and anti-proliferative effect of guanosine and guanosine derivatives in HuT-78 T lymphoma cells

The effects of 100 μM of 3',5'-cGMP, cAMP, cCMP, and cUMP as well as of the corresponding membrane-permeant acetoxymethyl esters on anti-CD3-antibody (OKT3)-induced IL-2 production of HuT-78 cutaneous T cell lymphoma (Sézary lymphoma) cells were analyzed. Only 3',5'-cGMP significantly reduced IL-2 production. Flow cytometric analysis of apoptotic (propidium iodide/annexin V staining) and anti-proliferative (CFSE staining) effects revealed that 3',5'-cGMP concentrations > 50 μM strongly inhibited proliferation and promoted apoptosis of HuT-78 cells (cultured in the presence of αCD3 antibody). Similar effects were observed for the positional isomer 2',3'-cGMP and for 2',-GMP, 3'-GMP, 5'-GMP, and guanosine. By contrast, guanosine and guanosine-derived nucleotides had no cytotoxic effect on peripheral blood mononuclear cells (PBMCs) or acute lymphocytic leukemia (ALL) xenograft cells. The anti-proliferative and apoptotic effects of guanosine and guanosine-derived compounds on HuT-78 cells were completely eliminated by the nucleoside transport inhibitor NBMPR (S-(4-Nitrobenzyl)-6-thioinosine). By contrast, the ecto-phosphodiesterase inhibitor DPSPX (1,3-dipropyl-8-sulfophenylxanthine) and the CD73 ecto-5'-nucleotidase inhibitor AMP-CP (adenosine 5'-(α,β-methylene)diphosphate) were not protective. We hypothesize that HuT-78 cells metabolize guanosine-derived nucleotides to guanosine by yet unknown mechanisms. Guanosine then enters the cells by an NBMPR-sensitive nucleoside transporter and exerts cytotoxic effects. This transporter may be ENT1 because NBMPR counteracted guanosine cytotoxicity in HuT-78 cells with nanomolar efficacy (IC50 of 25-30 nM). Future studies should further clarify the mechanism of the observed effects and address the question, whether guanosine or guanosine-derived nucleotides may serve as adjuvants in the therapy of cancers that express appropriate nucleoside transporters and are sensitive to established nucleoside-derived cytostatic drugs.

Modulation of polycystic kidney disease by G-protein coupled receptors and cyclic AMP signaling

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a systemic disorder associated with polycystic liver disease (PLD) and other extrarenal manifestations, the most common monogenic cause of end-stage kidney disease, and a major burden for public health. Many studies have shown that alterations in G-protein and cAMP signaling play a central role in its pathogenesis. As for many other diseases (35% of all approved drugs target G-protein coupled receptors (GPCRs) or proteins functioning upstream or downstream from GPCRs), treatments targeting GPCR have shown effectiveness in slowing the rate of progression of ADPKD. Tolvaptan, a vasopressin V2 receptor antagonist is the first drug approved by regulatory agencies to treat rapidly progressive ADPKD. Long-acting somatostatin analogs have also been effective in slowing the rates of growth of polycystic kidneys and liver. Although no treatment has so far been able to prevent the development or stop the progression of the disease, these encouraging advances point to G-protein and cAMP signaling as a promising avenue of investigation that may lead to more effective and safe treatments. This will require a better understanding of the relevant GPCRs, G-proteins, cAMP effectors, and of the enzymes and A-kinase anchoring proteins controlling the compartmentalization of cAMP signaling. The purpose of this review is to provide an overview of general GPCR signaling; the function of polycystin-1 (PC1) as a putative atypical adhesion GPCR (aGPCR); the roles of PC1, polycystin-2 (PC2) and the PC1-PC2 complex in the regulation of calcium and cAMP signaling; the cross-talk of calcium and cAMP signaling in PKD; and GPCRs, adenylyl cyclases, cyclic nucleotide phosphodiesterases, and protein kinase A as therapeutic targets in ADPKD.

Functional insights into nucleotide-metabolizing ectoenzymes expressed by bone marrow-resident cells in patients with multiple myeloma

Human myeloma cells grow in a hypoxic acidic niche in the bone marrow. Cross talk among cellular components of this closed niche generates extracellular adenosine, which promotes tumor cell survival. This is achieved through the binding of adenosine to purinergic receptors into complexes that function as an autocrine/paracrine signal factor with immune regulatory activities that i) down-regulate the functions of most immune effector cells and ii) enhance the activity of cells that suppress anti-tumor immune responses, thus facilitating the escape of malignant myeloma cells from immune surveillance. Here we review recent findings confirming that the dominant phenotype for survival of tumor cells is that where the malignant cells have been metabolically reprogrammed for the generation of lactic acidosis in the bone marrow niche. Adenosine triphosphate and nicotinamide-adenine dinucleotide extruded from tumor cells, along with cyclic adenosine monophosphate, are the main intracellular energetic/messenger molecules that serve as leading substrates in the extracellular space for membrane-bound ectonucleotidases metabolizing purine nucleotides to signaling adenosine. Within this mechanistic framework, the adenosinergic substrate conversion can vary significantly according to the metabolic environment. Indeed, the neoplastic expansion of plasma cells exploits both enzymatic networks and hypoxic acidic conditions for migrating and homing to a protected niche and for evading the immune response. The expression of multiple specific adenosine receptors in the niche completes the profile of a complex regulatory framework whose signals modify multiple myeloma and host immune responses.

The Extracellular cAMP-Adenosine Pathway in Airway Smooth Muscle

In the respiratory tract, intracellular cAMP has a key role in the smooth muscle relaxation induced by the β₂-adrenoceptor/Gs protein/adenylyl cyclase axis. In other tissues, cAMP also works as an extracellular messenger, after its efflux and interstitial conversion to adenosine by ectoenzymes. The aim of this study was to identify cAMP efflux and the "extracellular cAMP-adenosine pathway" in the airway smooth muscle. First, we tested the ability of β₂-adrenoceptor agonists formoterol or fenoterol to increase the extracellular cAMP in isolated tracheal rings from adult male Wistar rats. The effects of adenosine, cAMP, 8-Br-cAMP, fenoterol, or formoterol were also evaluated in the isometric contraction of control or carbachol (CCh) precontracted tracheas, normalized as the percentage of CCh-induced response. Fenoterol and formoterol induced 70%-80% relaxation and increased extracellular cAMP levels by up to 280%-450%. Although exogenous cAMP or adenosine evoked phasic contractions, the membrane-permeable cAMP analog 8-Br-cAMP induced relaxation of CCh-precontracted tracheas. The simultaneous inhibition of adenosine degradation/uptake with EHNA [erythro-9-(2-hydroxy-3-nonyl) adenine hydrochloride] plus uridine increased by 3-fold the maximum cAMP-induced contraction, whereas it was significantly reduced by AMPCP [adenosine 5'-(α,β-methylene)diphosphate; an ecto-5'-nucleotidase inhibitor], and by adenosine receptor antagonists CGS-15943 (nonselective) or DPCPX (8-cyclopentyl-1,3-dipropylxanthine) (A₁ selective). Finally, CGS-15943 shifted to the left the concentration-relaxation curve for fenoterol. In conclusion, our results show that airway smooth muscle expresses the extracellular cAMP-adenosine pathway associated with contracting effects mediated by A₁ receptors. The cAMP efflux triggered by fenoterol/formoterol indicates that the extracellular cAMP-adenosine pathway may play a role in balancing the relaxant effects of β₂-adrenoceptor agonists in airways, which may impact their bronchodilation effects.

Probenecid Inhibits α-Adrenergic Receptor-Mediated Vasoconstriction in the Human Leg Vasculature

Coordination of vascular smooth muscle cell tone in resistance arteries plays an essential role in the regulation of peripheral resistance and overall blood pressure. Recent observations in animals have provided evidence for a coupling between adrenoceptors and Panx1 (pannexin-1) channels in the regulation of sympathetic nervous control of peripheral vascular resistance and blood pressure; however, evidence for a functional coupling in humans is lacking. We determined Panx1 expression and effects of treatment with the pharmacological Panx1 channel inhibitor probenecid on the vasoconstrictor response to α1- and α2-adrenergic receptor stimulation in the human forearm and leg vasculature of young healthy male subjects (23±3 years). By use of immunolabeling and confocal microscopy, Panx1 channels were found to be expressed in vascular smooth muscle cells of arterioles in human leg skeletal muscle. Probenecid treatment increased (P<0.05) leg vascular conductance at baseline by ≈15% and attenuated (P<0.05) the leg vasoconstrictor response to arterial infusion of tyramine (α1- and α2-adrenergic receptor stimulation) by ≈15%, whereas the response to the α1-agonist phenylephrine was unchanged. Inhibition of α1-adrenoceptors prevented the probenecid-induced increase in baseline leg vascular conductance, but did not alter the effect of probenecid on the vascular response to tyramine. No differences with probenecid treatment were detected in the forearm. These observations provide the first line of evidence in humans for a functional role of Panx1 channels in setting resting tone via α1-adrenoceptors and in the constrictive effect of noradrenaline via α2-adrenoceptors, thereby contributing to the regulation of peripheral vascular resistance and blood pressure in humans.

Inactivation of Non-canonical Cyclic Nucleotides: Hydrolysis and Transport

This chapter addresses cNMP hydrolysis by phosphodiesterases (PDEs) and export by multidrug resistance associated proteins (MRPs). Both mechanisms are well-established for the canonical cNMPs, cAMP, and cGMP. Increasing evidence shows that non-canonical cNMPs (specifically cCMP, cUMP) are also PDE and MRP substrates. Hydrolysis of cUMP is achieved by PDE 3A, 3B, and 9A, which possibly explains the cUMP-degrading activities previously reported for heart, adipose tissue, and brain. Regarding cCMP, the only known "conventional" (class I) PDE that hydrolyzes cCMP is PDE7A. Older reports describe cCMP-degrading PDE-like activities in mammalian tissues, bacteria, and plants, but the molecular identity of these enzymes is not clear. High K _M and V _max values, insensitivity to common inhibitors, and unusually broad substrate specificities indicate that these activities probably do not represent class I PDEs. Moreover, the older results have to be interpreted with caution, since the historical analytical methods were not as reliable as modern highly sensitive and specific techniques like HPLC-MS/MS. Besides PDEs, the transporters MRP4 and 5 are of major importance for cAMP and cGMP disposal. Additionally, both MRPs also export cUMP, while cCMP is only exported by MRP5. Much less data are available for the non-canonical cNMPs, cIMP, cXMP, and cTMP. None of these cNMPs has been examined as MRP substrate. It was shown, however, that they are hydrolyzed by several conventional class I PDEs. Finally, this chapter reveals that there are still large gaps in our knowledge about PDE and MRP activities for canonical and non-canonical cNMPs. Future research should perform a comprehensive characterization of the known PDEs and MRPs with the physiologically most important cNMP substrates.

Holistic Methods for the Analysis of cNMP Effects

Cyclic nucleotide monophosphates (cNMPs) typify the archetype second messenger in living cells and serve as molecular switches with broad functionality. cAMP and cGMP are the best-described cNMPs; however, there is a growing body of evidence indicating that also cCMP and cUMP play a substantial role in signal transduction. Despite research efforts, to date, relatively little is known about the biology of these noncanonical cNMPs, which is due, at least in part, to methodological issues in the past entailing setbacks of the entire field. Only recently, with the use of state-of-the-art techniques, it was possible to revive noncanonical cNMP research. While high-sensitive detection methods disclosed relevant levels of cCMP and cUMP in mammalian cells, knowledge about the biological effectors and their physiological interplay is still incomplete. Holistic biophysical readouts capture cell responses label-free and in an unbiased fashion with the advantage to detect concealed aspects of cell signaling that are arduous to access via traditional biochemical assay approaches. In this chapter, we introduce the dynamic mass redistribution (DMR) technology to explore cell signaling beyond established receptor-controlled mechanisms. Both common and distinctive features in the signaling structure of cCMP and cUMP were identified. Moreover, the integrated response of whole live cells revealed a hitherto undisclosed additional effector of the noncanonical cNMPs. Future studies will show how holistic methods will become integrated into the methodological arsenal of contemporary cNMP research.

Renal Handling of Ketones in Response to Sodium-Glucose Cotransporter 2 Inhibition in Patients With Type 2 Diabetes

OBJECTIVE

Pharmacologically induced glycosuria elicits adaptive responses in glucose homeostasis and hormone release, including decrements in plasma glucose and insulin levels, increments in glucagon release, enhanced lipolysis, and stimulation of ketogenesis, resulting in an increase in ketonemia. We aimed at assessing the renal response to these changes.

RESEARCH DESIGN AND METHODS

We measured fasting and postmeal urinary excretion of glucose, β-hydroxybutyrate (β-HB), lactate, and sodium in 66 previously reported patients with type 2 diabetes and preserved renal function (estimated glomerular filtration rate ≥60 mL · min^-1 · 1.73 m^-2) and in control subjects without diabetes at baseline and following empagliflozin treatment.

RESULTS

With chronic (4 weeks) sodium-glucose cotransporter 2 inhibition, baseline fractional glucose excretion (<2%) rose to 38 ± 12% and 46 ± 11% (fasting vs. postmeal, respectively; P < 0.0001) over a range of BMIs (range 23-41 kg/m²) and creatinine clearance (65-168 mL · min^-1 · m^-2). Excretion of β-HB (median [interquartile range]: 0.08 [0.10] to 0.31 [0.43] µmol · min^-1), lactate (0.06 [0.06] to 0.28 [0.25] µmol · min^-1), and sodium (0.27 [0.22] to 0.36 [0.16] mEq · min^-1) all increased (P ≤ 0.001 for all) and were each positively related to glycosuria (P ≤ 0.001). These parameters changed in the same direction in subjects without diabetes, but changes were smaller than in the patients with diabetes. Although plasma N-terminal pro-B-type natriuretic peptide levels were unaltered, plasma erythropoietin concentrations increased by 31 (64)% (P = 0.0078).

CONCLUSIONS

We conclude that the sodium-glucose cotransporter 2 inhibitor-induced increase in β-HB is not because of reduced renal clearance but because of overproduction. The increased lactate excretion contributes to lower plasma lactate levels, whereas the increased natriuresis may help in normalizing the exchangeable sodium pool. Taken together, glucose loss through joint inhibition of glucose and sodium reabsorption in the proximal tubule induces multiple changes in renal metabolism.

Substrate-Dependence of Competitive Nucleotide Pyrophosphatase/Phosphodiesterase1 (NPP1) Inhibitors

Nucleotide pyrophosphatase/phosphodiesterase type 1 (NPP1) is a membrane glycoprotein involved in the hydrolysis of extracellular nucleotides. Its major substrate is ATP which is converted to AMP and diphosphate. NPP1 was proposed as a new therapeutic target in brain cancer and immuno-oncology. Several NPP1 inhibitors have been reported to date, most of which were evaluated vs. the artificial substrate p-nitrophenyl 5′-thymidine monophosphate (p-Nph-5′-TMP). Recently, we observed large discrepancies in inhibitory potencies for a class of competitive NPP1 inhibitors when tested vs. the artificial substrate p-Nph-5′-TMP as compared to the natural substrate ATP. Therefore, the goal of the present study was to investigate whether inhibitors of human NPP1 generally display substrate-dependent inhibitory potency. Systematic evaluation of nucleotidic as well as non-nucleotidic NPP1 inhibitors revealed significant differences in determined K_i values for competitive, but not for non- and un-competitive inhibitors when tested vs. the frequently used artificial substrate p-Nph-5′-TMP as compared to ATP. Allosteric modulation of NPP1 by p-Nph-5′-TMP may explain these discrepancies. Results obtained using the AMP derivative p-nitrophenyl 5′-adenosine monophosphate (p-Nph-5′-AMP) as an alternative artificial substrate correlated much better with those employing the natural substrate ATP.

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.

Influence of Sodium Cationization versus Protonation on the Gas-Phase Conformations and Glycosidic Bond Stabilities of 2'-Deoxyadenosine and Adenosine

The influence of noncovalent interactions with a sodium cation on the gas-phase structures and N-glycosidic bond stabilities of 2'-deoxyadenosine (dAdo) and adenosine (Ado), [dAdo+Na](+) and [Ado+Na](+), are probed via infrared multiple photon dissociation (IRMPD) action spectroscopy and energy-resolved collision-induced dissociation (ER-CID) experiments. ER-CID experiments are also performed on the protonated forms of these nucleosides, [dAdo+H](+) and [Ado+H](+), for comparison purposes. Complementary electronic structure calculations are performed to determine the structures and relative stabilities of the stable low-energy conformations of the sodium cationized nucleoside complexes and to predict their IR spectra. Comparison between the measured IRMPD action spectra and calculated IR spectra enables the conformations of the sodium cationized nucleosides present in the experiments to be elucidated. The influence of sodium cationization versus protonation on the structures and IR spectra is elucidated by comparison to IRMPD and theoretical results previously reported for the protonated forms of these nucleosides. The influence of sodium cationization versus protonation on the glycosidic bond stability of the adenine nucleosides is determined by comparison of the ER-CID behavior of these systems. All structures present in the experiments are found to involve tridentate binding of Na(+) to the N3, O4', and O5' atoms forming favorable 5- and 6-membered chelation rings, which requires that adenine rotate to a syn configuration. This mode of sodium cation binding results in moderate flexibility of the sugar moiety such that the sugar puckering of the conformations present varies between C2'-endo and O4'-endo. Sodium cationization is found to be less effective toward activating the N-glycosidic bond than protonation for both dAdo and Ado. Both the IRMPD yields and ER-CID behavior indicate that the 2'-hydroxyl substituent of Ado stabilizes the N-glycosidic bond relative to that of dAdo.

Exogenously administered adenosine attenuates renal damage in streptozotocin-induced diabetic rats

BACKGROUND

Diabetic nephropathy (DNP) is one of the most serious complications of diabetes mellitus (DM). In the present study, we investigated the potential of adenosine as a therapeutic candidate for preventing DNP.

METHODS

Twenty-one adult male rats were included in the study. Fourteen rats were administered a single dose of 60 mg/kg streptozotocin (STZ) to induce diabetes. Seven rats served as normal control group. Diabetic rats were randomly divided into two groups: one group was treated with 1 mL/kg saline/day (DM + saline) and the other group was treated with 5 mg/kg/day adenosine (DM + adenosine) for 6 weeks. After 6 weeks, biochemical parameters including urea, creatinine, blood urea nitrogen (BUN), kidney injury molecule-1 (KIM-1) and tumor necrosis factor-α (TNF-α) were measured in plasma samples. Also, kidneys were removed for histopathological assessment.

RESULTS

Both of plasma KIM-1 and TNF-α levels were significantly higher in DM + saline group compared to controls. However, treatment of diabetic rats with adenosine significantly decreased the plasma KIM-1 and TNF-α levels compared to DM + saline group. Significant histopathological changes were observed in diabetic rats whereas adenosine treatment effectively prevented these changes.

CONCLUSIONS

The findings of the present study suggest that adenosine may be a useful therapeutic agent for preventing DNP.

MRP4 (ABCC4) as a potential pharmacologic target for cardiovascular disease

This review focuses on multidrug resistance protein 4 (MRP4 or ABCC4) that has recently been shown to play a role in cAMP homeostasis, a key-pathway in vascular biology and in platelet functions. In vascular system, recent data provide evidence that inhibition of MRP4 prevents human coronary artery smooth muscle cell proliferation in vitro and in vivo, as well as human pulmonary artery smooth muscle cell proliferation in vitro and pulmonary hypertension in mice in vivo. In the heart, MRP4 silencing in adult rat ventricular myocytes results in an increase in intracellular cAMP levels leading to enhanced cardiomyocyte contractility. However, a prolonged inhibition of MRP4 can promote cardiac hypertrophy. In addition, secreted cAMP, through its metabolite adenosine, prevents adrenergically induced cardiac hypertrophy and fibrosis. Finally, MRP4 inhibition in platelets induces a moderate thrombopathy. The localization of MRP4 underlines the emerging concept of cAMP compartmentalization in platelets, which is a major regulatory mechanism in other cells. cAMP storage in platelet dense granules might limit the cAMP cytosolic concentration upon adenylate cyclase activation, a necessary step to induce platelet activation. In this review, we discuss the therapeutic potential of direct pharmacological inhibition of MRP4 in atherothrombotic disease, via its vasodilating and antiplatelet effects.

Increased levels of adenosine and ecto 5'-nucleotidase (CD73) activity precede renal alterations in experimental diabetic rats

The pathogenesis of diabetic nephropathy (DN) has not been clearly established, making diagnosis and patient management difficult. Recent studies using experimental diabetic models have implicated adenosine signaling with renal cells dysfunction. Therefore, the study of the biochemical mechanisms that regulate extracellular adenosine availability during DN is of emerging interest. Using streptozotocin-induced diabetic rats we demonstrated that urinary levels of adenosine were early increased. Further analyses showed an increased expression of the ecto 5'-nucleotidase (CD73), which hydrolyzes AMP to adenosine, at the renal proximal tubules and a higher enzymatic activity in tubule extracts. These changes precede the signs of diabetic kidney injury recognized by significant proteinuria, morphological alterations and the presence of the renal fibrosis markers alpha smooth muscle actin and fibronectin, collagen deposits and thickening of the glomerular basement membrane. In the proximal tubule cell line HK2 we identified TGF-β as a key modulator of CD73 activity. Importantly, the increased activity of CD73 could be screened in urinary sediments from diabetic rats. In conclusion, the increase of CD73 activity is a key component in the production of high levels of adenosine and emerges as a new tool for the early diagnosis of tubular injury in diabetic kidney disease.

Schwann Cells Metabolize Extracellular 2',3'-cAMP to 2'-AMP

The 3',5'-cAMP-adenosine pathway (3',5'-cAMP→5'-AMP→adenosine) and the 2',3'-cAMP-adenosine pathway (2',3'-cAMP→2'-AMP/3'-AMP→adenosine) are active in the brain. Oligodendrocytes participate in the brain 2',3'-cAMP-adenosine pathway via their robust expression of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase; converts 2',3'-cAMP to 2'-AMP). Because Schwann cells also express CNPase, it is conceivable that the 2',3'-cAMP-adenosine pathway exists in the peripheral nervous system. To test this and to compare the 2',3'-cAMP-adenosine pathway to the 3',5'-cAMP-adenosine pathway in Schwann cells, we examined the metabolism of 2',3'-cAMP, 2'-AMP, 3'-AMP, 3',5'-cAMP, and 5'-AMP in primary rat Schwann cells in culture. Addition of 2',3'-cAMP (3, 10, and 30 µM) to Schwann cells increased levels of 2'-AMP in the medium from 0.006 ± 0.002 to 21 ± 2, 70 ± 3, and 187 ± 10 nM/µg protein, respectively; in contrast, Schwann cells had little ability to convert 2',3'-cAMP to 3'-AMP or 3',5'-cAMP to either 3'-AMP or 5'-AMP. Although Schwann cells slightly converted 2',3'-cAMP and 2'-AMP to adenosine, they did so at very modest rates (e.g., 5- and 3-fold, respectively, more slowly compared with our previously reported studies in oligodendrocytes). Using transected myelinated rat sciatic nerves in culture medium, we observed a time-related increase in endogenous intracellular 2',3'-cAMP and extracellular 2'-AMP. These findings indicate that Schwann cells do not have a robust 3',5'-cAMP-adenosine pathway but do have a 2',3'-cAMP-adenosine pathway; however, because the pathway mostly involves 2'-AMP formation rather than 3'-AMP, and because the conversion of 2'-AMP to adenosine is slow, metabolism of 2',3'-cAMP mostly results in the accumulation of 2'-AMP. Accumulation of 2'-AMP in peripheral nerves postinjury could have pathophysiological consequences.

New perspectives in signaling mediated by receptors coupled to stimulatory G protein: the emerging significance of cAMP efflux and extracellular cAMP-adenosine pathway

G protein-coupled receptors (GPCRs) linked to stimulatory G (Gs) proteins (GsPCRs) mediate increases in intracellular cyclic AMP as consequence of activation of nine adenylyl cyclases , which differ considerably in their cellular distribution and activation mechanisms. Once produced, cyclic AMP may act via distinct intracellular signaling effectors such as protein kinase A and the exchange proteins activated by cAMP (Epacs). More recently, attention has been focused on the efflux of cAMP through a specific transport system named multidrug resistance proteins that belongs to the ATP-binding cassette transporter superfamily. Outside the cell, cAMP is metabolized into adenosine, which is able to activate four distinct subtypes of adenosine receptors, members of the GPCR family: A₁, A_2A, A_2B, and A₃. Taking into account that this phenomenon occurs in numerous cell types, as consequence of GsPCR activation and increment in intracellular cAMP levels, in this review, we will discuss the impact of cAMP efflux and the extracellular cAMP-adenosine pathway on the regulation of GsPCR-induced cell response.

From canonical to non-canonical cyclic nucleotides as second messengers: pharmacological implications

This review summarizes our knowledge on the non-canonical cyclic nucleotides cCMP, cUMP, cIMP, cXMP and cTMP. We place the field into a historic context and discuss unresolved questions and future directions of research. We discuss the implications of non-canonical cyclic nucleotides for experimental and clinical pharmacology, focusing on bacterial infections, cardiovascular and neuropsychiatric disorders and reproduction medicine. The canonical cyclic purine nucleotides cAMP and cGMP fulfill the criteria of second messengers. (i) cAMP and cGMP are synthesized by specific generators, i.e. adenylyl and guanylyl cyclases, respectively. (ii) cAMP and cGMP activate specific effector proteins, e.g. protein kinases. (iii) cAMP and cGMP exert specific biological effects. (iv) The biological effects of cAMP and cGMP are terminated by phosphodiesterases and export. The effects of cAMP and cGMP are mimicked by (v) membrane-permeable cyclic nucleotide analogs and (vi) bacterial toxins. For decades, the existence and relevance of cCMP and cUMP have been controversial. Modern mass-spectrometric methods have unequivocally demonstrated the existence of cCMP and cUMP in mammalian cells. For both, cCMP and cUMP, the criteria for second messenger molecules are now fulfilled as well. There are specific patterns by which nucleotidyl cyclases generate cNMPs and how they are degraded and exported, resulting in unique cNMP signatures in biological systems. cNMP signaling systems, specifically at the level of soluble guanylyl cyclase, soluble adenylyl cyclase and ExoY from Pseudomonas aeruginosa are more promiscuous than previously appreciated. cUMP and cCMP are evolutionary new molecules, probably reflecting an adaption to signaling requirements in higher organisms.

cCMP and cUMP: emerging second messengers

The cyclic purine nucleotides cAMP and cGMP are established second messengers. By contrast, the existence of the cyclic pyrimidine nucleotides cytidine 3',5'-cyclic monophosphate (cCMP) and uridine 3',5'-cyclic monophosphate (cUMP) has been controversial for decades. The recent development of highly sensitive mass spectrometry (MS) methods allowed precise quantitation and unequivocal identification of cCMP and cUMP in cells. Importantly, cCMP and cUMP generators, effectors, cleaving enzymes, and transporters have now been identified. Here, I discuss evidence in support of cCMP and cUMP as bona fide second messengers, the emerging therapeutic implications of cCMP and cUMP signaling, and important unresolved questions for this field.

Cardiac myocyte-secreted cAMP exerts paracrine action via adenosine receptor activation

Acute stimulation of cardiac β-adrenoceptors is crucial to increasing cardiac function under stress; however, sustained β-adrenergic stimulation has been implicated in pathological myocardial remodeling and heart failure. Here, we have demonstrated that export of cAMP from cardiac myocytes is an intrinsic cardioprotective mechanism in response to cardiac stress. We report that infusion of cAMP into mice averted myocardial hypertrophy and fibrosis in a disease model of cardiac pressure overload. The protective effect of exogenous cAMP required adenosine receptor signaling. This observation led to the identification of a potent paracrine mechanism that is dependent on secreted cAMP. Specifically, FRET-based imaging of cAMP formation in primary cells and in myocardial tissue from murine hearts revealed that cardiomyocytes depend on the transporter ABCC4 to export cAMP as an extracellular signal. Extracellular cAMP, through its metabolite adenosine, reduced cardiomyocyte cAMP formation and hypertrophy by activating A1 adenosine receptors while delivering an antifibrotic signal to cardiac fibroblasts by A2 adenosine receptor activation. Together, our data reveal a paracrine role for secreted cAMP in intercellular signaling in the myocardium, and we postulate that secreted cAMP may also constitute an important signal in other tissues.

cCMP is a substrate for MRP5

The cyclic pyrimidine nucleotide cCMP has been suggested to serve as second messenger. However, phosphodiesterases studied so far do not hydrolyze cCMP. Therefore, we searched for alternative cCMP inactivation mechanisms. cCMP is a substrate for multidrug resistance protein 5, indicating that export from the cytosol into the extracellular space is an important inactivation mechanism for cCMP.

Human monocytes respond to extracellular cAMP through A2A and A2B adenosine receptors

In this study, we test the hypothesis that cAMP, acting as an extracellular mediator, affects the physiology and function of human myeloid cells. The cAMP is a second messenger recognized as a universal regulator of several cellular functions in different organisms. Many studies have shown that extracellular cAMP exerts regulatory functions, acting as first mediator in multiple tissues. However, the impact of extracellular cAMP on cells of the immune system has not been fully investigated. We found that human monocytes exposed to extracellular cAMP exhibit higher expression of CD14 and lower amount of MHC class I and class II molecules. When cAMP-treated monocytes are exposed to proinflammatory stimuli, they exhibit an increased production of IL-6 and IL-10 and a lower amount of TNF-α and IL-12 compared with control cells, resembling the features of the alternative-activated macrophages or M2 macrophages. In addition, we show that extracellular cAMP affects monocyte differentiation into DCs, promoting the induction of cells displaying an activated, macrophage-like phenotype with reduced capacity of polarized, naive CD4(+) T cells into IFN-γ-producing lymphocytes compared with control cells. The effects of extracellular cAMP on monocytes are mediated by CD73 ecto-5'-nucleotidase and A2A and A2B adenosine receptors, as selective antagonists could reverse its effects. Of note, the expression of CD73 molecules has been found on the membrane of a small population of CD14(+)CD16(+) monocytes. These findings suggest that an extracellular cAMP-adenosine pathway is active in cells of the immune systems.

Computational model for the regulation of extracellular ATP and adenosine in airway epithelia

Extracellular nucleotides are key components of the signaling network regulating airway clearance. They are released by the epithelium into the airway surface liquid (ASL) to stimulate cilia beating activity, mucus secretion and airway hydration. Understanding the factors affecting their availability for purinoceptor activation is an important step toward the development of new therapies for obstructive lung diseases. This chapter presents a mathematical model developed to gain predictive insights into the regulation of ASL nucleotide concentrations on human airway epithelia. The parameters were estimated from experimental data collected on polarized primary cultures of human nasal and bronchial epithelial cells. This model reproduces major experimental observations: (1) the independence of steady-state nucleotide concentrations on ASL height, (2) the impact of selective ectonucleotidase inhibitors on their steady-state ASL concentrations, (3) the changes in ASL composition caused by mechanical stress mimicking normal breathing, (4) and the differences in steady-state concentrations existing between nasal and bronchial epithelia. In addition, this model launched the study of nucleotide release into uncharted territories, which led to the discovery that airway epithelia release, not only ATP, but also ADP and AMP. This study shows that computational modeling, coupled to experimental validation, provides a powerful approach for the identification of key therapeutic targets for the improvement of airway clearance in obstructive respiratory diseases.

A novel adenosine precursor 2',3'-cyclic adenosine monophosphate inhibits formation of post-surgical adhesions

BACKGROUND

Intraperitoneal adenosine reduces abdominal adhesions. However, because of the ultra-short half-life and low solubility of adenosine, optimal efficacy requires multiple dosing.

AIM

Here, we compared the ability of potential adenosine prodrugs to inhibit post-surgical abdominal adhesions after a single intraperitoneal dose.

METHODS

Abdominal adhesions were induced in mice using an electric toothbrush to damage the cecum. Also, 20 μL of 95 % ethanol was applied to the cecum to cause chemically induced injury. After injury, mice received intraperitoneally either saline (n = 18) or near-solubility limit of adenosine (23 mmol/L; n = 12); 5'-adenosine monophosphate (75 mmol/L; n = 11); 3'-adenosine monophosphate (75 mmol/L; n = 12); 2'-adenosine monophosphate (75 mmol/L; n = 12); 3',5'-cyclic adenosine monophosphate (75 mmol/L; n = 19); or 2',3'-cyclic adenosine monophosphate (75 mmol/L; n = 20). After 2 weeks, adhesion formation was scored by an observer blinded to the treatments. In a second study, intraperitoneal adenosine levels were measured using tandem mass spectrometry for 3 h after instillation of 2',3'-cyclic adenosine monophosphate (75 mmol/L) into the abdomen.

RESULTS

The order of efficacy for attenuating adhesion formation was: 2',3'-cyclic adenosine monophosphate > 3',5'-cyclic adenosine monophosphate ≈ adenosine > 5'-adenosine monophosphate ≈ 3'-adenosine monophosphate ≈ 2'-adenosine monophosphate. The groups were compared using a one-factor analysis of variance, and the overall p value for differences between groups was p < 0.000001. Intraperitoneal administration of 2',3'-cAMP yielded pharmacologically relevant levels of adenosine in the abdominal cavity for >3 h.

CONCLUSION

Administration of 2',3'-cyclic adenosine monophosphate into the surgical field is a unique, convenient and effective method of preventing post-surgical adhesions by acting as an adenosine prodrug.

Role of CNPase in the oligodendrocytic extracellular 2',3'-cAMP-adenosine pathway

Extracellular adenosine 3',5'-cyclic monophosphate (3',5'-cAMP) is an endogenous source of localized adenosine production in many organs. Recent studies suggest that extracellular 2',3'-cAMP (positional isomer of 3',5'-cAMP) is also a source of adenosine, particularly in the brain in vivo post-injury. Moreover, in vitro studies show that both microglia and astrocytes can convert extracellular 2',3'-cAMP to adenosine. Here, we examined the ability of primary mouse oligodendrocytes and neurons to metabolize extracellular 2',3'-cAMP and their respective adenosine monophosphates (2'-AMP and 3'-AMP). Cells were also isolated from mice deficient in 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase). Oligodendrocytes metabolized 2',3'-cAMP to 2'-AMP with 10-fold greater efficiency than did neurons (and also more than previously examined microglia and astrocytes); whereas, the production of 3'-AMP was minimal in both oligodendrocytes and neurons. The production of 2'-AMP from 2',3'-cAMP was reduced by 65% in CNPase -/- versus CNPase +/+ oligodendrocytes. Oligodendrocytes also converted 2'-AMP to adenosine, and this was also attenuated in CNPase -/- oligodendrocytes. Inhibition of classic 3',5'-cAMP-3'-phosphodiesterases with 3-isobutyl-1-methylxanthine did not block metabolism of 2',3'-cAMP to 2'-AMP and inhibition of classic ecto-5'-nucleotidase (CD73) with α,β-methylene-adenosine-5'-diphosphate did not attenuate the conversion of 2'-AMP to adenosine. These studies demonstrate that oligodendrocytes express the extracellular 2',3'-cAMP-adenosine pathway (2',3'-cAMP → 2'-AMP → adenosine). This pathway is more robustly expressed in oligodendrocytes than in all other CNS cell types because CNPase is the predominant enzyme that metabolizes 2',3'-cAMP to 2-AMP in CNS cells. By reducing levels of 2',3'-cAMP (a mitochondrial toxin) and increasing levels of adenosine (a neuroprotectant), oligodendrocytes may protect axons from injury.

Dissociations in the effects of β2-adrenergic receptor agonists on cAMP formation and superoxide production in human neutrophils: support for the concept of functional selectivity

In neutrophils, activation of the β2-adrenergic receptor (β2AR), a Gs-coupled receptor, inhibits inflammatory responses, which could be therapeutically exploited. The aim of this study was to evaluate the effects of various β2AR ligands on adenosine-3',5'-cyclic monophosphate (cAMP) accumulation and N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP)-induced superoxide anion (O2(•-)) production in human neutrophils and to probe the concept of ligand-specific receptor conformations (also referred to as functional selectivity or biased signaling) in a native cell system. This is an important question because so far, evidence for functional selectivity has been predominantly obtained with recombinant systems, due to the inherent difficulties to genetically manipulate human native cells. cAMP concentration was determined by HPLC/tandem mass spectrometry, and O2(•-) formation was assessed by superoxide dismutase-inhibitable reduction of ferricytochrome c. β2AR agonists were generally more potent in inhibiting fMLP-induced O2(•-) production than in stimulating cAMP accumulation. (-)-Ephedrine and dichloroisoproterenol were devoid of any agonistic activity in the cAMP assay, but partially inhibited fMLP-induced O2(•-) production. Moreover, (-)-adrenaline was equi-efficacious in both assays whereas the efficacy of salbutamol was more than two-fold higher in the O2(•-) assay. Functional selectivity was visualized by deviations of ligand potencies and efficacies from linear correlations for various parameters. We obtained no evidence for involvement of protein kinase A in the inhibition of fMLP-induced O2(•-) production after β2AR-stimulation although cAMP-increasing substances inhibited O2(•-) production. Taken together, our data corroborate the concept of ligand-specific receptor conformations with unique signaling capabilities in native human cells and suggest that the β2AR inhibits O2(•-) production in a cAMP-independent manner.

Compartmentalization of cyclic nucleotide signaling: a question of when, where, and why?

Preciseness of cellular behavior depends upon how an extracellular cue mobilizes a correct orchestra of cellular messengers and effector proteins spatially and temporally. This concept, termed compartmentalization of cellular signaling, is now known to form the molecular basis of many aspects of cellular behavior in health and disease. The cyclic nucleotides cyclic adenosine monophosphate and cyclic guanosine monophosphate are ubiquitous cellular messengers that can be compartmentalized in three ways: first, by their physical containment; second, by formation of multiple protein signaling complexes; and third, by their selective depletion. Compartmentalized cyclic nucleotide signaling is a very prevalent response among all cell types. In order to understand how it becomes relevant to cellular behavior, it is important to know how it is executed in cells to regulate physiological responses and, also, how its execution or dysregulation can lead to a pathophysiological condition, which forms the scope of the presented review.

Cyclic nucleotide compartmentalization: contributions of phosphodiesterases and ATP-binding cassette transporters

Cyclic nucleotides [e.g., cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP)] are ubiquitous second messengers that affect multiple cell functions from maturation of the egg to cell division, growth, differentiation, and death. The concentration of cAMP can be regulated by processes within membrane domains (local regulation) as well as throughout a cell (global regulation). The phosphodiesterases (PDEs) that degrade cAMP have well-known roles in both these processes. It has recently been discovered that ATP-binding cassette (ABC) transporters contribute to both local and global regulation of cAMP. This regulation may require the formation of macromolecular complexes. Some of these transporters are ubiquitously expressed, whereas others are more tissue restricted. Because some PDE inhibitors are also ABC transporter inhibitors, it is conceivable that the therapeutic benefits of their use result from the combined inhibition of both PDEs and ABC transporters. Deciphering the individual contributions of PDEs and ABC transporters to such drug effects may lead to improved therapeutic benefits.

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.

Autoimmunity in CD73/Ecto-5'-nucleotidase deficient mice induces renal injury

Extracellular adenosine formed by 5'-ectonucleotidase (CD73) is involved in tubulo-glomerular feedback in the kidney but is also known to be an important immune modulator. Since CD73(-/-)mutant mice exhibit a vascular proinflammatory phenotype, we asked whether long term lack of CD73 causes inflammation related kidney pathologies. CD73(-/-)mice (13 weeks old) showed significantly increased low molecule proteinuria compared to C57BL6 wild type controls (4.8 ≥ 0.52 vs. 2.9 ± 0.54 mg/24 h, p<0.03). Total proteinuria increased to 5.97 ± 0.78 vs. 2.55 ± 0.35 mg/24 h at 30 weeks (p<0.01) whereas creatinine clearance decreased (0.161 ± 0.02 vs. 0.224 ± 0.02 ml/min). We observed autoimmune inflammation in CD73(-/-)mice with glomerulitis and peritubular capillaritis, showing glomerular deposition of IgG and C3 and enhanced presence of CD11b, CD8, CD25 as well as GR-1-positive cells in the interstitium. Vascular inflammation was associated with enhanced serum levels of the cytokines IL-18 and TNF-α as well as VEGF and the chemokine MIP-2 (CXCL-2) in CD73(-/-)mice, whereas chemokines and cytokines in the kidney tissue were unaltered or reduced. In CD73(-/-)mice glomeruli, we found a reduced number of podocytes and endothelial fenestrations, increased capillaries per glomeruli, endotheliosis and enhanced tubular fibrosis. Our results show that adult CD73(-/-)mice exhibit spontaneous proteinuria and renal functional deterioration even without exogenous stress factors. We have identified an autoimmune inflammatory phenotype comprising the glomerular endothelium, leading to glomeruli inflammation and injury and to a cellular infiltrate of the renal interstitium. Thus, long term lack of CD73 reduced renal function and is associated with autoimmune inflammation.

Contribution of the extracellular cAMP-adenosine pathway to dual coupling of β2-adrenoceptors to Gs and Gi proteins in mouse skeletal muscle

β(2)-Adrenoceptor (β(2)-AR) agonists increase skeletal muscle contractile force via activation of G(s) protein/adenylyl cyclases (AC) and increased generation of cAMP. Herein, we evaluated the possible dual coupling of β(2)-AR to G(s) and G(i) proteins and the influence of the β(2)-AR/G(s)-G(i)/cAMP signaling cascade on skeletal muscle contraction. Assuming that the increment of intracellular cAMP is followed by cAMP efflux and extracellular generation of adenosine, the contribution of the extracellular cAMP-adenosine pathway on the β(2)-AR inotropic response was also addressed. The effects of clenbuterol/fenoterol (β(2)-AR agonists), forskolin (AC activator), cAMP/8-bromo-cAMP, and adenosine were evaluated on isometric contractility of mouse diaphragm muscle induced by supramaximal direct electrical stimulation (0.1 Hz, 2 ms duration). Clenbuterol/fenoterol (10-1000 μM), 1 μM forskolin, and 20 μM rolipram induced transient positive inotropic effects that peaked 30 min after stimulation onset, declining to 10 to 20% of peak levels in 30 min. The late descending phase of the β(2)-AR agonist inotropic effect was mimicked by either cAMP or adenosine and abolished by preincubation of diaphragm with pertussis toxin (PTX) (G(i) signaling inhibitor) or the organic anion transporter inhibitor probenecid, indicating a delayed coupling of β(2)-AR to G(i) protein which depends on cAMP efflux. Remarkably, the PTX-sensitive β(2)-AR inotropic effect was inhibited by the A(1) adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine and ecto-5'-phosphodiesterase inhibitor α,β-methyleneadenosine 5'-diphosphate sodium salt, indicating that β(2)-AR coupling to G(i) is indirect and dependent on A(1) receptor activation. The involvement of the extracellular cAMP-adenosine pathway in β(2)-AR signaling would provide a negative feedback loop that may limit stimulatory G protein-coupled receptor positive inotropism and potential deleterious effects of excessive contractile response.

Regulation of SGLT expression and localization through Epac/PKA-dependent caveolin-1 and F-actin activation in renal proximal tubule cells

This study demonstrated that exchange proteins directly activated by cAMP (Epac) and protein kinase A (PKA) by 8-bromo (8-Br)-adenosine 3',5'-cyclic monophosphate (cAMP) stimulated [(14)C]-α-methyl-D-glucopyranoside (α-MG) uptake through increased sodium-glucose cotransporters (SGLTs) expression and translocation to lipid rafts in renal proximal tubule cells (PTCs). In PTCs, SGLTs were colocalized with lipid raft caveolin-1 (cav-1), disrupted by methyl-β-cyclodextrin (MβCD). Selective activators of Epac or PKA, 8-Br-cAMP, and forskolin stimulated expressions of SGLTs and α-MG uptake in PTCs. In addition, 8-Br-cAMP-induced PKA and Epac activation increased phosphorylation of extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (MAPK), and nuclear factor kappa B (NF-κB), which were involved in expressions of SGLTs. Furthermore, 8-Br-cAMP stimulated SGLTs translocation to lipid rafts via filamentous actin (F-actin) organization, which was blocked by cytochalasin D. In addition, cav-1 and SGLTs stimulated by 8-Br-cAMP were detected in lipid rafts, which were blocked by cytochalasin D. Furthermore, 8-Br-cAMP-induced SGLTs translocation and α-MG uptake were attenuated by inhibition of cav-1 activation with cav-1 small interfering RNA (siRNA) and inhibition of F-actin organization with TRIO and F-actin binding protein (TRIOBP). In conclusion, 8-Br-cAMP stimulated α-MG uptake via Epac and PKA-dependent SGLTs expression and trafficking through cav-1 and F-actin in PTCs.

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.

2',3'-cAMP, 3'-AMP, and 2'-AMP inhibit human aortic and coronary vascular smooth muscle cell proliferation via A2B receptors

Rat vascular smooth muscle cells (VSMCs) from renal microvessels metabolize 2',3'-cAMP to 2'-AMP and 3'-AMP, and these AMPs are converted to adenosine that inhibits microvascular VSMC proliferation via A(2B) receptors. The goal of this study was to test whether this mechanism also exists in VSMCs from conduit arteries and whether it is similarly expressed in human vs. rat VSMCs. Incubation of rat and human aortic VSMCs with 2',3'-cAMP concentration-dependently increased levels of 2'-AMP and 3'-AMP in the medium, with a similar absolute increase in 2'-AMP vs. 3'-AMP. In contrast, in human coronary VSMCs, 2',3'-cAMP increased 2'-AMP levels yet had little effect on 3'-AMP levels. In all cell types, 2',3'-cAMP increased levels of adenosine, but not 5'-AMP, and 2',3'-AMP inhibited cell proliferation. Antagonism of A(2B) receptors (MRS-1754), but not A(1) (1,3-dipropyl-8-cyclopentylxanthine), A(2A) (SCH-58261), or A(3) (VUF-5574) receptors, attenuated the antiproliferative effects of 2',3'-cAMP. In all cell types, 2'-AMP, 3'-AMP, and 5'-AMP increased adenosine levels, and inhibition of ecto-5'-nucleotidase blocked this effect of 5'-AMP but not that of 2'-AMP nor 3'-AMP. Also, 2'-AMP, 3'-AMP, and 5'-AMP, like 2',3'-cAMP, exerted antiproliferative effects that were abolished by antagonism of A(2B) receptors with MRS-1754. In conclusion, VSMCs from conduit arteries metabolize 2',3'-cAMP to AMPs, which are metabolized to adenosine. In rat and human aortic VSMCs, both 2'-AMP and 3'-AMP are involved in this process, whereas, in human coronary VSMCs, 2',3'-cAMP is mainly converted to 2'-AMP. Because adenosine inhibits VSMC proliferation via A(2B) receptors, local vascular production of 2',3'-cAMP may protect conduit arteries from atherosclerosis.

Inactivation of multidrug resistance proteins disrupts both cellular extrusion and intracellular degradation of cAMP

In addition to xenobiotics and several other endogenous metabolites, multidrug-resistance proteins (MRPs) extrude the second-messenger cAMP from various cells. Pharmacological and/or genetic inactivation of MRPs has been shown to augment intracellular cAMP signaling, an effect assumed to be a direct consequence of the blockade of cAMP extrusion. Here we provide evidence that the augmented intracellular cAMP levels are not due exclusively to the prevention of cAMP efflux because MRP inactivation is also associated with reduced cAMP degradation by phosphodiesterases (PDEs). Several prototypical MRP inhibitors block PDE activity at concentrations widely used to inhibit MRPs. Their dose-dependent effects in several paradigms of cAMP signaling are more consistent with their potency in inhibiting PDEs than MRPs. Moreover, genetic manipulation of MRP expression results in concomitant changes in PDE activity and protein levels, thus affecting cAMP degradation in parallel with cAMP efflux. These findings suggest that the effects of MRP inactivation on intracellular cAMP levels reported previously may be due in part to reduced degradation by PDEs and identify MRP-dependent transport mechanisms as novel regulators of cellular PDE expression levels. Mathematical simulations of cAMP signaling predict that selective ablation of MRP-dependent cAMP efflux per se does not affect bulk cytosolic cAMP levels, but may control cAMP levels in restricted submembrane compartments that are defined by small volume, high MRP activity, limited PDE activity, and limited exchange of cAMP with the bulk-cytosolic cAMP pool. Whether this regulation occurs in cells remains to be confirmed experimentally under conditions that do not affect PDE activity.

Extracellular 2,3-cyclic adenosine monophosphate is a potent inhibitor of preglomerular vascular smooth muscle cell and mesangial cell growth [corrected]

Recently we discovered that intact kidneys release into the extracellular compartment 2',3'-cAMP (a positional isomer of 3',5'-cAMP with unknown pharmacology) and metabolize 2',3'-cAMP to 2'-AMP, 3'-AMP, and adenosine. Because adenosine inhibits growth of vascular smooth muscle cells and mesangial cells, we tested the hypothesis that extracellular 2',3'-cAMP attenuates growth of preglomerular vascular smooth muscle and mesangial cells via production of adenosine. For comparison, all of the experiments were performed with both 2',3'-cAMP and 3',5'-cAMP. In study 1, 2',3'-cAMP, 3',5'-cAMP, 5'-AMP, 3'-AMP, or 2'-AMP was incubated with cells and purines measured in the medium by mass spectrometry. Both preglomerular vascular smooth muscle and mesangial cells metabolized 3',5'-cAMP to 5'-AMP and adenosine; 5'-AMP to adenosine; 2',3'-cAMP to 2'-AMP, 3'-AMP, and adenosine; and 2'-AMP and 3'-AMP to adenosine. 3-Isobutyl-1-methylxanthine (phosphodiesterase inhibitor) and 1,3-dipropyl-8-p-sulfophenylxanthine (ecto-phosphodiesterase inhibitor) blocked conversion of 3',5'-cAMP to 5'-AMP and adenosine, and alpha,beta-methylene-adenosine-5'-diphosphate (CD73 inhibitor) blocked conversion of 5'-AMP to adenosine. These enzyme inhibitors had little effect on metabolism of 2',3'-cAMP, 2'-AMP, or 3'-AMP. For study 2, 2',3'-cAMP and 3',5'-cAMP profoundly inhibited proliferation (thymidine incorporation and cell number) of both cell types, with 2',3'-cAMP more potent than 3',5'-cAMP. Antagonism of A(2B) receptors (MRS-1724), but not A(1) (1,3-dipropyl-8-cyclopentylxanthine), A(2A) (SCH-58261), or A(3) (VUF-5574) receptors, attenuated the growth inhibitory effects of 2',3'-cAMP and 3',5'-cAMP. Extracellular 2',3'-cAMP inhibits growth of preglomerular vascular smooth muscle and mesangial cells more profoundly than does 3',5'-cAMP. Although both cAMPs inhibit growth in part via conversion to adenosine followed by A(2B) receptor activation, their metabolism is mediated by different enzymes.

cAMP efflux from human trophoblast cell lines: a role for multidrug resistance protein (MRP)1 transporter

Cyclic adenosine 3'-5'-monophosphate (cAMP) is a second messenger, which exerts an important role in the control of human first-trimester trophoblast functions. In the present study we demonstrate the existence of a mechanism that is able to extrude cAMP from trophoblast-derived cell lines, and show evidence indicating the involvement of multidrug resistance protein (MRP) 1, a transporter belonging to the ATP-binding cassette family, in cAMP egress. MRP1 is expressed in trophoblast cell lines and cAMP efflux is highly reduced by the MRP1 inhibitor, MK-571. In addition, interleukin-1beta and estrone are able to enhance MRP1 gene expression and influence extracellular cAMP concentration. The occurrence of a MRP1-dependent cAMP efflux is also shown in human first-trimester placenta explants. Extracellular cAMP could represent a source for adenosine formation, which in turn could regulate cAMP-dependent responses in placental tissue. Evidence is provided that adenosine receptor subtypes are present and functional in human trophoblast-derived cells. A role for cAMP egress mechanism in the fine modulation of the nucleotide homeostasis is therefore suggested.

CFTR chloride channel in the apical compartments: spatiotemporal coupling to its interacting partners

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-regulated chloride channel located primarily at the apical or luminal surfaces of epithelial cells in the airway, intestine, pancreas, kidney, sweat gland, as well as male reproductive tract, where it plays a crucial role in transepithelial fluid homeostasis. CFTR dysfunction can be detrimental and may result in life-threatening disorders. CFTR hypofunctioning because of genetic defects leads to cystic fibrosis, the most common lethal genetic disease in Caucasians, whereas CFTR hyperfunctioning resulting from various infections evokes secretory diarrhea, the leading cause of mortality in early childhood. Therefore, maintaining a dynamic balance between CFTR up-regulating processes and CFTR down-regulating processes is essential for maintaining fluid and body homeostasis. Accumulating evidence suggests that protein-protein interactions play a critical role in the fine-tuned regulation of CFTR function. A growing number of proteins have been reported to interact directly or indirectly with CFTR chloride channel, suggesting that CFTR might be coupled spatially and temporally to a wide variety of interacting partners including ion channels, receptors, transporters, scaffolding proteins, enzyme molecules, signaling molecules, and effectors. Most interactions occur primarily between the opposing terminal tails (amino or carboxyl) of CFTR protein and its binding partners, either directly or mediated through various PDZ scaffolding proteins. These dynamic interactions impact the channel function, as well as localization and processing of CFTR protein within cells. This article reviews the most recent progress and findings about the interactions between CFTR and its binding partners through PDZ scaffolding proteins, as well as the spatiotemporal regulation of CFTR-containing macromolecular signaling complexes in the apical compartments of polarized cells lining the secretory epithelia.

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).