Cardiac fibroblasts express the cAMP-adenosine pathway

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.

Cyclic adenosine monophosphate in acute ischemic stroke: some to update, more to explore

The development of effective treatment for ischemic stroke, which is a common cause of morbidity and mortality worldwide, remains an unmet goal because the current first-line treatment management interventional therapy has a strict time window and serious complications. In recent years, a growing body of evidence has shown that the elevation of intracellular and extracellular cyclic adenosine monophosphate (cAMP) alleviates brain damage after ischemic stroke by attenuating neuroinflammation in the central nervous system and peripheral immune system. In the central nervous system, upregulated intracellular cAMP signaling can alleviate immune-mediated damage by restoring neuronal morphology and function, inhibiting microglia migration and activation, stabilizing the membrane potential of astrocytes and improving the cellular functions of endothelial cells and oligodendrocytes. Enhancement of the extracellular cAMP signaling pathway can improve neurological function by activating the cAMP-adenosine pathway to reduce immune-mediated damage. In the peripheral immune system, cAMP can act on various immune cells to suppress peripheral immune function, which can alleviate the inflammatory response in the central nervous system and improve the prognosis of acute cerebral ischemic injury. Therefore, cAMP may play key roles in reducing post-stroke neuroinflammatory damage. The protective roles of the cAMP indicate that the cAMP enhancing drugs such as cAMP supplements, phosphodiesterase inhibitors, adenylate cyclase agonists, which are currently used in the treatment of heart and lung diseases. They are potentially able to be applied as a new therapeutic strategy in ischemic stroke. This review focuses on the immune-regulating roles and the clinical implication of cAMP in acute ischemic stroke.

Exchange protein activated by cyclic-adenosine monophosphate (Epac) regulates atrial fibroblast function and controls cardiac remodelling

Aims

Heart failure (HF) produces left atrial (LA)-selective fibrosis and promotes atrial fibrillation. HF also causes adrenergic activation, which contributes to remodelling via a variety of signalling molecules, including the exchange protein activated by cAMP (Epac). Here, we evaluate the effects of Epac1-signalling on LA fibroblast (FB) function and its potential role in HF-related atrial remodelling.

Methods and results

HF was induced in adult male mongrel dogs by ventricular tachypacing (VTP). Epac1-expression decreased in LA-FBs within 12 h (-3.9-fold) of VTP onset. The selective Epac activator, 8-pCPT (50 µM) reduced, whereas the Epac blocker ESI-09 (1 µM) enhanced, collagen expression in LA-FBs. Norepinephrine (1 µM) decreased Epac1-expression, an effect blocked by prazosin, and increased FB collagen production. The β-adrenoceptor (AR) agonist isoproterenol increased Epac1 expression, an effect antagonized by ICI (β2-AR-blocker), but not by CGP (β1-AR-blocker). β-AR-activation with isoproterenol decreased collagen expression, an effect mimicked by the β2-AR-agonist salbutamol and blocked by the Epac1-antagonist ESI-09. Transforming growth factor-β1, known to be activated in HF, suppressed Epac1 expression, an effect blocked by the Smad3-inhibitor SIS3. To evaluate effects on atrial fibrosis in vivo, mice subjected to myocardial infarction (MI) received the Epac-activator Sp-8-pCPT or vehicle for 2 weeks post-MI; Sp-8-pCPT diminished LA fibrosis and attenuated cardiac dysfunction.

Conclusions

HF reduces LA-FB Epac1 expression. Adrenergic activation has complex effects on FBs, with α-AR-activation suppressing Epac1-expression and increasing collagen expression, and β2-AR-activation having opposite effects. Epac1-activation reduces cardiac dysfunction and LA fibrosis post-MI. Thus, Epac1 signalling may be a novel target for the prevention of profibrillatory cardiac remodelling.

Myofibroblast β2 adrenergic signaling amplifies cardiac hypertrophy in mice

Abnormal β-adrenergic signaling plays a central role in human heart failure. In mice, chronic β-adrenergic receptor (βAR) stimulation elicits cardiac hypertrophy. It has been reported that cultured cardiac fibroblasts express βAR; however, the functional in vivo requirement of βAR signaling in cardiac fibroblasts during the development of cardiac hypertrophy remains elusive. β2AR null mice exhibited attenuated hypertrophic responses to chronic βAR stimulation upon continuous infusion of an agonist, isoprenaline (ISO), compared to those in wildtype controls, suggesting that β2AR activation in the heart induces pro-hypertrophic effects in mice. Since β2AR signaling is protective in cardiomyocytes, we focused on β2AR signaling in cardiac myofibroblasts. To determine whether β2AR signaling in myofibroblasts affects cardiac hypertrophy, we generated myofibroblast-specific transgenic mice (TG) with the catalytic subunit of protein kinase A (PKAcα) using Cre-loxP system. Myofibroblast-specific PKAcα overexpression resulted in enhanced heart weight normalized to body weight ratio, associated with an enlargement of cardiomyocytes at 12 weeks of age, indicating that myofibroblast-specific activation of PKA mediates cardiac hypertrophy in mice. Neonatal rat cardiomyocytes stimulated with conditioned media from TG cardiac fibroblasts likewise exhibited significantly more growth than those from controls. Thus, β2AR signaling in myofibroblasts plays a substantial role in ISO-induced cardiac hypertrophy, possibly due to a paracrine effect. β2AR signaling in cardiac myofibroblasts may represent a promising target for development of novel therapies for cardiac hypertrophy.

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.

Glucagon-induced extracellular cAMP regulates hepatic lipid metabolism

Hormonal signals help to maintain glucose and lipid homeostasis in the liver during the periods of fasting. Glucagon, a pancreas-derived hormone induced by fasting, promotes gluconeogenesis through induction of intracellular cAMP production. Glucagon also stimulates hepatic fatty acid oxidation but the underlying mechanism is poorly characterized. Here we report that following the acute induction of gluconeogenic genes Glucose 6 phosphatase (G6Pase) and Phosphoenolpyruvate carboxykinase (Pepck) expression through cAMP-response element-binding protein (CREB), glucagon triggers a second delayed phase of fatty acid oxidation genes Acyl-coenzyme A oxidase (Aox) and Carnitine palmitoyltransferase 1a (Cpt1a) expression via extracellular cAMP. Increase in extracellular cAMP promotes PPARα activity through direct phosphorylation by AMP-activated protein kinase (AMPK), while inhibition of cAMP efflux greatly attenuates Aox and Cpt1a expression. Importantly, cAMP injection improves lipid homeostasis in fasted mice and obese mice, while inhibition of cAMP efflux deteriorates hepatic steatosis in fasted mice. Collectively, our results demonstrate the vital role of glucagon-stimulated extracellular cAMP in the regulation of hepatic lipid metabolism through AMPK-mediated PPARα activation. Therefore, strategies to improve cAMP efflux could serve as potential new tools to prevent obesity-associated hepatic steatosis.

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.

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.

Extracellular nucleotide regulation and signaling in cardiac fibrosis

Following myocardial infarction, purinergic nucleotides and nucleosides are released via non-specific and specific mechanisms in response to cellular activation, stress, or injury. These extracellular nucleotides are potent mediators of physiologic and pathologic responses, contributing to the inflammatory and fibrotic milieu within the injured myocardium. Via autocrine or paracrine signaling, cell-specific effects occur through differentially expressed purinergic receptors of the P2X, P2Y, and P1 families. Nucleotide activation of the ionotropic (ligand-gated) purine receptors (P2X) and several of the metabotropic (G-protein-coupled) purine receptors (P2Y) or adenosine activation of the P1 receptors can have profound effects on inflammatory cell function, fibroblast function, and cardiomyocyte function. Extracellular nucleotidases that hydrolyze released nucleotides regulate the magnitude and duration of purinergic signaling. While there are numerous studies on the role of the purinergic signaling pathway in cardiovascular disease, the extent to which the purinergic signaling pathway modulates cardiac fibrosis is incompletely understood. Here we provide an overview of the current understanding of how the purinergic signaling pathway modulates cardiac fibroblast function and myocardial fibrosis.

Adenosine Attenuates Human Coronary Artery Smooth Muscle Cell Proliferation by Inhibiting Multiple Signaling Pathways That Converge on Cyclin D

The goal of this study was to determine whether and how adenosine affects the proliferation of human coronary artery smooth muscle cells (HCASMCs). In HCASMCs, 2-chloroadenosine (stable adenosine analogue), but not N(6)-cyclopentyladenosine, CGS21680, or N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide, inhibited HCASMC proliferation (A2B receptor profile). 2-Chloroadenosine increased cAMP, reduced phosphorylation (activation) of ERK and Akt (protein kinases known to increase cyclin D expression and activity, respectively), and reduced levels of cyclin D1 (cyclin that promotes cell-cycle progression in G1). Moreover, 2-chloroadenosine inhibited expression of S-phase kinase-associated protein-2 (Skp2; promotes proteolysis of p27(Kip1)) and upregulated levels of p27(Kip1) (cell-cycle regulator that impairs cyclin D function). 2-Chloroadenosine also inhibited signaling downstream of cyclin D, including hyperphosphorylation of retinoblastoma protein and expression of cyclin A (S phase cyclin). Knockdown of A2B receptors prevented the effects of 2-chloroadenosine on ERK1/2, Akt, Skp2, p27(Kip1), cyclin D1, cyclin A, and proliferation. Likewise, inhibition of adenylyl cyclase and protein kinase A abrogated 2-chloroadenosine's inhibitory effects on Skp2 and stimulatory effects on p27(Kip1) and rescued HCASMCs from 2-chloroadenosine-mediated inhibition. Knockdown of p27(Kip1) also reversed the inhibitory effects of 2-chloroadenosine on HCASMC proliferation. In vivo, peri-arterial (rat carotid artery) 2-chloroadenosine (20 μmol/L for 7 days) downregulated vascular expression of Skp2, upregulated vascular expression of p27(Kip1), and reduced neointima hyperplasia by 71% (P<0.05; neointimal thickness: control, 37 424±18 371 pixels; treated, 10 352±2824 pixels). In conclusion, the adenosine/A2B receptor/cAMP/protein kinase A axis inhibits HCASMC proliferation by blocking multiple signaling pathways (ERK1/2, Akt, and Skp2) that converge at cyclin D, a key G1 cyclin that controls cell-cycle progression.

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.

2',3'-cAMP, 3'-AMP, 2'-AMP and adenosine inhibit TNF-α and CXCL10 production from activated primary murine microglia via A2A receptors

BACKGROUND

Some cells, tissues and organs release 2',3'-cAMP (a positional isomer of 3',5'-cAMP) and convert extracellular 2',3'-cAMP to 2'-AMP plus 3'-AMP and convert these AMPs to adenosine (called the extracellular 2',3'-cAMP-adenosine pathway). Recent studies show that microglia have an extracellular 2',3'-cAMP-adenosine pathway. The goal of the present study was to investigate whether the extracellular 2',3'-cAMP-adenosine pathway could have functional consequences on the production of cytokines/chemokines by activated microglia.

METHODS

Experiments were conducted in cultures of primary murine microglia. In the first experiment, the effect of 2',3'-cAMP, 3'-AMP, 2'-AMP and adenosine on LPS-induced TNF-α and CXCL10 production was determined. In the next experiment, the first protocol was replicated but with the addition of 1,3-dipropyl-8-p-sulfophenylxanthine (DPSPX) (0.1 μM; antagonist of adenosine receptors). The last experiment compared the ability of 2-chloro-N(6)-cyclopentyladenosine (CCPA) (10 μM; selective A1 agonist), 5'-N-ethylcarboxamide adenosine (NECA) (10 μM; agonist for all adenosine receptor subtypes) and CGS21680 (10 μM; selective A2A agonist) to inhibit LPS-induced TNF-α and CXCL10 production.

RESULTS

(1) 2',3'-cAMP, 3'-AMP, 2'-AMP and adenosine similarly inhibited LPS-induced TNF-α and CXCL10 production; (2) DPSPX nearly eliminated the inhibitory effects of 2',3'-cAMP, 3'-AMP, 2'-AMP and adenosine on LPS-induced TNF-α and CXCL10 production; (3) CCPA did not affect LPS-induced TNF-α and CXCL10; (4) NECA and CGS21680 similarly inhibited LPS-induced TNF-α and CXCL10 production.

CONCLUSIONS

2',3'-cAMP and its metabolites (3'-AMP, 2'-AMP and adenosine) inhibit LPS-induced TNF-α and CXCL10 production via A2A-receptor activation. Adenosine and its precursors, via A2A receptors, likely suppress TNF-α and CXCL10 production by activated microglia in brain 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.

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.

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.

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.

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.

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.

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.

Dipyridamole enhances ischaemia-induced arteriogenesis through an endocrine nitrite/nitric oxide-dependent pathway

AIMS

Anti-platelet agents, such as dipyridamole, have several clinical benefits for peripheral artery disease with the speculation of angiogenic potential that could preserve ischaemic tissue viability, yet the effect of dipyridamole on ischaemic arteriogenesis or angiogenesis is unknown. Here we test the hypothesis that dipyridamole therapy augments arteriolar vessel development and function during chronic ischaemia.

METHODS AND RESULTS

Mice were treated with 200 mg/kg dipyridamole twice daily to achieve therapeutic plasma levels (0.8-1.2 microg/mL). Chronic hindlimb ischaemia was induced by permanent femoral artery ligation followed by measurement of tissue perfusion using laser Doppler blood flow along with quantification of vascular density, cell proliferation, and activation of nitric oxide (NO) metabolism. Dipyridamole treatment quickly restored ischaemic hindlimb blood flow, increased vascular density and cell proliferation, and enhanced collateral artery perfusion compared with control treatments. The beneficial effects of dipyridamole on blood flow and vascular density were dependent on NO production as dipyridamole did not augment ischaemic tissue reperfusion, vascular density, or endothelial cell proliferation in endothelial NO synthase (eNOS)-deficient mice. Blood and tissue nitrite levels were significantly higher in dipyridamole-treated mice compared with controls and eNOS(-/-) mice, verifying increased NO production that was regulated in a PKA-dependent manner.

CONCLUSION

Dipyridamole augments nitrite/NO production, leading to enhanced arteriogenesis activity and blood perfusion in ischaemic limbs. Together, these data suggest that dipyridamole can augment ischaemic vessel function and restore blood flow, which may be beneficial in peripheral artery disease.

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.

Cardiac fibroblasts: at the heart of myocardial remodeling

Cardiac fibroblasts are the most prevalent cell type in the heart and play a key role in regulating normal myocardial function and in the adverse myocardial remodeling that occurs with hypertension, myocardial infarction and heart failure. Many of the functional effects of cardiac fibroblasts are mediated through differentiation to a myofibroblast phenotype that expresses contractile proteins and exhibits increased migratory, proliferative and secretory properties. Cardiac myofibroblasts respond to proinflammatory cytokines (e.g. TNFalpha, IL-1, IL-6, TGF-beta), vasoactive peptides (e.g. angiotensin II, endothelin-1, natriuretic peptides) and hormones (e.g. noradrenaline), the levels of which are increased in the remodeling heart. Their function is also modulated by mechanical stretch and changes in oxygen availability (e.g. ischaemia-reperfusion). Myofibroblast responses to such stimuli include changes in cell proliferation, cell migration, extracellular matrix metabolism and secretion of various bioactive molecules including cytokines, vasoactive peptides and growth factors. Several classes of commonly prescribed therapeutic agents for cardiovascular disease also exert pleiotropic effects on cardiac fibroblasts that may explain some of their beneficial outcomes on the remodeling heart. These include drugs for reducing hypertension (ACE inhibitors, angiotensin receptor blockers, beta-blockers), cholesterol levels (statins, fibrates) and insulin resistance (thiazolidinediones). In this review, we provide insight into the properties of cardiac fibroblasts that underscores their importance in the remodeling heart, including their origin, electrophysiological properties, role in matrix metabolism, functional responses to environmental stimuli and ability to secrete bioactive molecules. We also review the evidence suggesting that certain cardiovascular drugs can reduce myocardial remodeling specifically via modulatory effects on cardiac fibroblasts.

THE EXTRACELLULAR CARDIAC PURINE METABOLOME IN SEPSIS

The total cardiac purine metabolome includes all of the adenine and guanine nucleoside and nucleosides and related molecules involved throughout the intracellular and extracellular compartments and various cell types in the heart. In considering purines as molecules involved in autocrine and paracrine communication, effective interstitial concentrations of the nucleoside adenosine, or purine metabolites, are of greatest interest. These molecules arise from the complex interactions between cardiac-specific cell types, including fibroblasts and myocytes, and noncardiac cells, such as tissue-resident macrophages and other immune cells that have vascular access. In the interstitial environment, adenosine can regulate vascular resistance, contractile function, and immunochemical interactions. The breakdown of purines can produce reactive oxygen species that also influence autocrine and paracrine interactions. A central enzyme in this paradigm, adenosine deaminase, is a pivotal molecule in regulating the balance between pro-inflammatory and anti-inflammatory signaling cascades. A new role for adenosine deaminase as an allosteric regulator of relevant membrane proteins has yet to be explored in the heart.

A Polymorphism of the Gene Encoding AMPD1: Clinical Impact and Proposed Mechanisms in Congestive Heart Failure

A vast array of gene polymorphisms have been described, and further discovery of these gene variants will continue as the human genome is defined. Therefore, selection of a single polymorphism to investigate in relation to disease evolution or outcome must be motivated by specific physiologic, pathophysiologic, or epidemiologic associations. A significant gene polymorphism may result in an alteration of protein function, or be associated with disease incidence or outcome. Prevalence of the polymorphism in the general population is of extreme importance, as it must be common enough to warrant interest in its clinical impact. The polymorphism of the gene encoding for the enzyme adenosine monophosphate deaminase 1 results in an abnormal protein necessary in skeletal muscle metabolism. While its physiologic effects are not completely understood, it has been associated with improved morbidity and mortality in patients with cardiovascular disease.

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).

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.

cAMP-adenosine pathway in the proximal tubule

The "extracellular cAMP-adenosine pathway" refers to the conversion of cAMP to AMP by ecto-phosphodiesterase, followed by metabolism of AMP to adenosine by ecto-5'-nucleotidase, with all the steps occurring in the extracellular compartment. This study investigated whether the extracellular cAMP-adenosine pathway exists in proximal tubules. Freshly isolated proximal tubules rapidly converted basolaterally administered cAMP to AMP and adenosine. Proximal tubular cells in culture (first passage) rapidly converted apically administered cAMP to AMP and adenosine. In both freshly isolated proximal tubules and cultured proximal tubular cells, conversion of cAMP to AMP and adenosine was affected by a broad-spectrum phosphodiesterase inhibitor (3-isobutyl-1-methylxanthine), an ecto-phosphodiesterase inhibitor (1,3-dipropyl-8-p-sulfophenylxanthine), and a blocker of ecto-5'-nucleotidase (alpha,beta-methyleneadenosine-5'-diphosphate) in a manner consistent with exogenous cAMP being processed by the extracellular cAMP-adenosine pathway. In cultured proximal tubular cells, but not freshly isolated proximal tubules, stimulation of adenylyl cyclase increased extracellular concentrations of cAMP, AMP, and adenosine plus inosine, and these changes were also modulated by the inhibitors in a manner consistent with the extracellular cAMP-adenosine pathway. Conversion of renal interstitial (basolateral) cAMP and AMP to adenosine in vivo was shown by microdialysis coupled with ion trap mass spectrometry. Western blot analysis showed A1, A2A, and A3 receptors on both apical and basolateral proximal tubular membranes, with A1 and A2A receptors more highly expressed on basolateral compared with apical membranes. We conclude that cAMP that reaches either the apical or basolateral membranes of proximal tubular cells is converted in part to adenosine that has ready access to adenosine receptors.

Activation of beta3 adrenergic receptor decreases DNA synthesis in human skin fibroblasts via cyclic GMP/nitric oxide pathway

BACKGROUND

Evidences have shown that beta1 and beta2 adrenoceptors co-exist in human fibroblasts, but it is not yet clear the functional expression of beta3 adrenoceptor in these cells. The aim of this study was to investigate the expression and biological effect of beta3 adrenoceptor activation in human skin fibroblast and the different signaling pathways involved in its effect.

METHODS

For this purpose in vitro cultures of human skin fibroblast were established from human foreskin and grown in Dulbecco's modified Eagle's medium. The effect of ZD 7114 (beta3 agonist) on cell DNA synthesis, radioligand binding assay, cyclic GMP and cyclic AMP accumulation and nitric oxide synthase (NOS) activity were evaluated.

RESULTS

3H-CGP binding to human fibroblast membranes was a saturable process to a single class of binding site. The equilibrium parameters were: Kd 20+/-3 pM and Bmax 222+/-19 fmol/mg protein. Ki values showed that these cells express a high number of beta(3)adrenoceptor subtypes. ZD 7114 stimulation of beta3 adrenoceptor exerts a concentration-dependent inhibition of DNA synthesis and cAMP accumulation with parallel increase in NOS activity that led to cGMP accumulation. All these effects were blocked by the beta3 adrenoceptor antagonist (SR 59230A). The effect of ZD 7114 on DNA synthesis significantly correlated with its action either on cAMP or NOS-cGMP signaling system. Inhibitors of NOS activity and NO-sensitive guanylate cyclase prevented the inhibitory effect of ZD 7114 on DNA synthesis. In addition, the beta3 adrenoceptor-dependent increase in cGMP and activation of NOS were blocked by the inhibition of phospholipase C (PLC), calcium/calmodulin (CaM), endothelial NOS activity and cGMP accumulation.

CONCLUSIONS

beta3 adrenoceptor activation exerts inhibitory effect on human fibroblast DNA synthesis as a result of the activation of NO-cGMP pathway and the inhibition of adenylate cyclase activity. The mechanism appears to occurs secondarily to stimulation of PLC and CaM. This in turn triggers cascade reaction leading to increase production of NO-cGMP with decrease in cAMP accumulation.

Extracellular cyclic AMP-adenosine pathway in isolated adipocytes and adipose tissue

OBJECTIVE

Our goal was to evaluate the presence and lipolytic impact of the extracellular cyclic adenosine monophosphate (AMP)-adenosine pathway in adipose tissue.

RESEARCH METHODS AND PROCEDURES

Sixteen miniature Yucatan swine (Sus scrofa) were used for these in vitro and in situ experiments. Four microdialysis probes were implanted into subcutaneous adipose tissue and perfused at 2 microL/min with Ringer's solution containing no addition, varying levels of cyclic AMP, 10 microM isoproterenol, or 10 microM isoproterenol plus 1 mM alpha,beta-methylene adenosine 5'-diphosphate (AMPCP), a 5'-nucleotidase inhibitor. Dialysate was assayed for AMP, adenosine, inosine, hypoxanthine, and glycerol. Freshly isolated adipocytes were incubated with buffer, 1 microM isoproterenol, or 1 microM isoproterenol plus 0.1 mM AMPCP, and extracellular levels of AMP, adenosine, inosine, hypoxanthine, and glycerol were measured.

RESULTS

Perfusion of adipose tissue with exogenous cyclic AMP caused a significant increase in AMP and adenosine appearance. Perfusion with AMPCP, in the presence or absence of isoproterenol, significantly increased the levels of AMP and glycerol, whereas it significantly reduced the level of adenosine and its metabolites. However, the AMPCP-provoked increase in lipolysis observed in situ and in vitro was not temporally associated with a decrease in adenosine.

DISCUSSION

These data suggest the existence of a cyclic AMP-adenosine pathway in adipocytes and adipose tissue. The role of this pathway in the regulation of lipolysis remains to be clarified.

Plasma membrane-bound cyclic AMP phosphodiesterase activity in 3T3-L1 adipocytes

Plasma membranes were isolated from 3T3-L1 adipocytes. Plasma membrane phosphodiesterase (PM-PDE) was measured in the presence of 5 microM cilostamide. Time course and cAMP dose response ranging from 0 to 2 microM were measured. PM-PDE remained linear up to 20 min. Non-linear curve fitting analysis showed that the low Km cAMP dose data fit a two component curve significantly better than a one component curve, indicating that there are two iso-forms of PDE in the plasma membrane of 3T3-L1 adipocytes, similar to swine adipocytes. The Km and Vmax values for this two component curve were Km1=0.12 microM, Vmax1=3.08 pmol min(-1) mg(-1) protein, and Km2=3.67 microM, Vmax2=83.8 pmol min(-1) mg(-1) protein. Inhibitors of PDE1, PDE2 and PDE5 failed to inhibit PM-PDE, as observed in swine adipocyte plasma membranes. However, PDE4 inhibitors were three-fold more effective at inhibiting PDE in 3T3-L1 PM compared to swine adipocyte PM. One mM 1, 3-dipropyl-8-p-sulfophenylxanthine (DPSPX) inhibited PM-PDE by approximately 75% in both preparations. These data demonstrate that PM-PDE is distinct from microsomal membrane PDE and may be responsible for extracellular cAMP metabolism to AMP in 3T3-L1 adipocytes.

Functional effects of enhancing or silencing adenosine A2b receptors in cardiac fibroblasts

Cardiac fibroblasts (CF) express adenosine (ADO) receptors, and pharmacological evidence suggests the possible involvement of the A2 (A2a and A2b) receptor (A2aR and A2bR) subtypes in inhibiting cell functions involved in fibrosis. The main objective of this study was to define the contributions of A2a and/or A2b receptors in modulating ADO-induced decreases in CF functions. For this purpose, CF were either treated pharmacologically or had the A2aR or A2bR levels modified through the use of recombinant adenovirus or siRNA. The assessment of mRNA expression in adult rat CF yielded evidence for A1R, A2bR, A2a), and A3R. Endogenously or exogenously enhanced ADO significantly inhibits CF proliferation, collagen, and protein synthesis. A2R and A2aR agonists, although capable of inhibiting CF protein and collagen synthesis, were unable to define the contributions derived from A2aR or A2bR. Overexpression of A2bR in CF yielded significant decreases in basal levels of collagen and protein synthesis and correlated with increases in cAMP levels. However, at higher doses of ADO receptor agonists, significant increases in protein and collagen synthesis were observed. CF with underexpression of A2bR yielded increases in protein and collagen synthesis. In contrast, A2aR underexpression did not modify ADO-induced decreases in CF protein or collagen synthesis. In conclusion, results derived from the molecular manipulation of receptor levels indicate that A2bR are critically involved in ADO-mediated inhibition of CF functions.

The extracellular cyclic AMP-adenosine pathway in renal physiology

Many cell types in the kidney express adenosine receptors, and adenosine has multiple effects on renal function. Although adenosine is produced within the kidney by several biochemical reactions, recent studies support a novel mechanism for renal adenosine production, the extracellular cAMP-adenosine pathway. This extracellular cAMP-adenosine pathway is initiated by efflux of cAMP from cells following activation of adenylyl cyclase. Extracellular cAMP is then converted to adenosine by the serial actions of ecto-phosphodiesterase and ecto-5'-nucleotidase. When extracellular cAMP is converted to adenosine near the biophase of cAMP production and efflux, this local extracellular cAMP-adenosine pathway permits tight coupling of the site of adenosine production to the site of adenosine receptors. cAMP in renal compartments may also be formed by tissues/organs remote from the kidney. For example, stimulation of hepatic adenylyl cyclase by the pancreatic hormone glucagon increases circulating cAMP, which is filtered at the glomerulus and concentrated in the tubular lumen as water is extracted from the ultrafiltrate. Conversion of hepatic-derived cAMP to adenosine in the kidney completes a pancreatohepatorenal cAMP-adenosine pathway that may serve as an endocrine link between the pancreas, liver, and kidney.

Acute adenosinergic cardioprotection in ischemic-reperfused hearts

Cells of the cardiovascular system generate and release purine nucleoside adenosine in increasing quantities when constituent cells are "stressed" or subjected to injurious stimuli. This increased adenosine can interact with surface receptors in myocardial, vascular, fibroblast, and inflammatory cells to modulate cellular function and phenotype. Additionally, adenosine is rapidly reincorporated back into 5'-AMP to maintain the adenine nucleotide pool. Via these receptor-dependent and independent (metabolic) paths, adenosine can substantially modify the acute response to ischemic insult, in addition to generating a more sustained ischemia-tolerant phenotype (preconditioning). However, the molecular basis for acute adenosinergic cardioprotection remains incompletely understood and may well differ from more widely studied preconditioning. Here we review current knowledge and some controversies regarding acute cardioprotection via adenosine and adenosine receptor activation.

Adenosine biosynthesis in the collecting duct

Adenosine regulates tubular transport in collecting ducts (CDs); however, the sources of adenosine that modulate ion transport in CDs are unknown. The extracellular cAMP-adenosine pathway refers to the conversion of cAMP to AMP by ectophosphodiesterase, followed by metabolism of AMP to adenosine by ecto-5'-nucleotidase, with all steps occurring in the extracellular compartment. The goal of this study was to assess whether the extracellular cAMP-adenosine pathway exists in CDs. Studies were conducted in both freshly isolated CDs and in CD cells in culture (first passage) that were derived from isolated CDs. Purity of CDs was confirmed by microscopy, by Western blotting for aquaporin-1, aquaporin-2, bumetanide-sensitive cotransporter type 1, and thiazide-sensitive cotransporter; and by reverse transcription-polymerase chain reaction for adenosine receptors. Both freshly isolated CDs and CD cells in culture converted exogenous cAMP to AMP and adenosine. In both freshly isolated CDs and CD cells in culture, conversion of cAMP to AMP and adenosine was affected by a broad-spectrum phosphodiesterase inhibitor (3-isobutyl-1-methylxanthine), an ectophosphodiesterase inhibitor (1,3-dipropyl-8-p-sulfophenylxanthine), and a blocker of ecto-5'-nucleotidase (alpha,beta-methylene-adenosine-5'-diphosphate) in a manner consistent with exogenous cAMP being processed by the extracellular cAMP-adenosine pathway. In CD cells in culture, stimulation of adenylyl cyclase increased extracellular concentrations of cAMP, AMP, and adenosine, and these changes were also modulated by the aforementioned inhibitors in a manner consistent with the extracellular cAMP-adenosine pathway. In conclusion, the extracellular cAMP-adenosine pathway is an important source of adenosine in CDs.

Regulation of intracellular cyclic AMP in skeletal muscle cells involves the efflux of cyclic nucleotide to the extracellular compartment

(1) This report analyses the intracellular and extracellular accumulation of cyclic AMP in primary rat skeletal muscle cultures, after direct and receptor-dependent stimulation of adenylyl cyclase (AC). (2) Isoprenaline, calcitonin gene-related peptide (CGRP) and forskolin induced a transient increase in the intracellular cyclic AMP that peaked 5 min after onset stimulation. (3) Under stimulation with isoprenaline or CGRP, the intracellular cyclic AMP initial rise was followed by an exponential decline, reaching 46 and 52% of peak levels in 10 min, respectively. (4) Conversely, the forskolin-dependent accumulation of intracellular cyclic AMP decreased slowly and linearly, reaching 49% of the peak level in 30 min. (5) The loss of intracellular cyclic AMP from peak levels, induced by direct or receptor-induced activation of AC, was followed by an increase in the extracellular cyclic AMP. (6) This effect was independent on PDEs, since it was obtained in the presence of 3-isobutyl-1-methylxanthine (IBMX). (7) Besides, in isoprenaline treated cells, the beta-adrenoceptor antagonist propranolol reduced both intra- and extracellular accumulation of cyclic AMP, whereas the organic anion transporter inhibitor probenecid reduced exclusively the extracellular accumulation. (8) Together our data show that direct or receptor-dependent activation of skeletal muscle AC results in a transient increase in the intracellular cyclic AMP, despite the continuous presence of the stimulus. The temporal declining of intracellular cyclic AMP was not dependent on the cyclic AMP breakdown but associated to the efflux of cyclic nucleotide to the extracellular compartment, by an active transport since it was prevented by probenecid.

Attenuation of cAMP accumulation in adult rat cardiac fibroblasts by IL-1beta and NO: role of cGMP-stimulated PDE2

Treatment of cultured adult rat cardiac fibroblasts with interleukin-1beta (IL-1beta) induces the inducible nitric oxide synthase (iNOS) expression, increases nitric oxide (NO) and cGMP production, and attenuates cAMP accumulation in response to isoproterenol by ~50%. Reduced cAMP accumulation is due to NO production: the effect is mimicked by NO donors and prevented by N(G)-monomethyl-L-arginine, an NOS inhibitor. Effects of NO are not restricted to the beta-adrenergic response; the response to forskolin is similarly diminished. NO donors only slightly (12%) decrease forskolin-stimulated adenylyl cyclase (AC) activity in cardiac fibroblast plasma membranes, suggesting that the main effect of NO is not a direct one on AC. An inhibitor of soluble guanylyl cyclase inhibits the effects of IL-1beta and NO donors; inhibition of cGMP-dependent protein kinase is without effect. 3-Isobutyl-1-methylxanthine, a nonspecific phosphodiesterase (PDE) inhibitor, and erythro-9-(2-hydroxy-3-nonyl)adenine, a specific inhibitor of the cGMP-stimulated PDE (PDE2), completely restore cAMP accumulation in sodium nitroprusside-treated fibroblasts and largely reverse the attenuated response in IL-1beta-treated fibroblasts. Although NO reportedly acts by reducing AC activity in some cells, in cardiac fibroblasts NO production decreases cAMP accumulation largely by the cGMP-mediated activation of PDE2.

Role of the extracellular cAMP-adenosine pathway in renal physiology

Adenosine exerts physiologically significant receptor-mediated effects on renal function. For example, adenosine participates in the regulation of preglomerular and postglomerular vascular resistances, glomerular filtration rate, renin release, epithelial transport, intrarenal inflammation, and growth of mesangial and vascular smooth muscle cells. It is important, therefore, to understand the mechanisms that generate extracellular adenosine within the kidney. In addition to three "classic" pathways of adenosine biosynthesis, contemporary studies are revealing a novel mechanism for renal adenosine production termed the "extracellular cAMP-adenosine pathway." The extracellular cAMP-adenosine pathway is defined as the egress of cAMP from cells during activation of adenylyl cyclase, followed by the extracellular conversion of cAMP to adenosine by the serial actions of ecto-phosphodiesterase and ecto-5'-nucleotidase. This mechanism of extracellular adenosine production may provide hormonal control of adenosine levels in the cell-surface biophase in which adenosine receptors reside. Tight coupling of the site of adenosine production to the site of adenosine receptors would permit a low-capacity mechanism of adenosine biosynthesis to have a large impact on adenosine receptor activation. The purposes of this review are to summarize the physiological roles of adenosine in the kidney; to describe the classic pathways of renal adenosine biosynthesis; to review the evidence for the existence of the extracellular cAMP-adenosine pathway; and to describe possible physiological roles of the extracellular cAMP-adenosine pathway, with particular emphasis on the kidney.