2'-AMP and 3'-AMP inhibit proliferation of preglomerular vascular smooth muscle cells and glomerular mesangial cells via A2B receptors

2',3'-cGMP exists in vivo and comprises a 2',3'-cGMP-guanosine pathway

The discovery in 2009 that 2',3'-cAMP exists in biological systems was rapidly followed by identification of 2',3'-cGMP in cell and tissue extracts. To determine whether 2',3'-cGMP exists in mammals under physiological conditions, we used ultraperformance LC-MS/MS to measure 2',3'-cAMP and 2',3'-cGMP in timed urine collections (via direct bladder cannulation) from 25 anesthetized mice. Urinary excretion rates (means ± SE) of 2',3'-cAMP (15.5 ± 1.8 ng/30 min) and 2',3'-cGMP (17.9 ± 1.9 ng/30 min) were similar. Mice also excreted 2'-AMP (3.6 ± 1.1 ng/20 min) and 3'-AMP (9.5 ± 1.2 ng/min), hydrolysis products of 2',3'-cAMP, and 2'-GMP (4.7 ± 1.7 ng/30 min) and 3'-GMP (12.5 ± 1.8 ng/30 min), hydrolysis products of 2',3'-cGMP. To validate that the chromatographic signals were from these endogenous noncanonical nucleotides, we repeated these experiments in mice (n = 18) lacking 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), an enzyme known to convert 2',3'-cyclic nucleotides to their corresponding 2'-nucleotides. In CNPase-knockout mice, urinary excretions of 2',3'-cAMP, 3'-AMP, 2',3'-cGMP, and 3'-GMP were increased, while urinary excretions of 2'-AMP and 2'-GMP were decreased. Infusions of exogenous 2',3'-cAMP increased urinary excretion of 2',3'-cAMP, 2'-AMP, 3'-AMP, and adenosine, whereas infusions of exogenous 2',3'-cGMP increased excretion of 2',3'-cGMP, 2'-GMP, 3'-GMP, and guanosine. Together, these data suggest the endogenous existence of not only a 2',3'-cAMP-adenosine pathway (2',3'-cAMP → 2'-AMP/3'-AMP → adenosine), which was previously identified, but also a 2',3'-cGMP-guanosine pathway (2',3'-cGMP → 2'-GMP/3'-GMP → guanosine), observed here for the first time. Because it is well known that adenosine and guanosine protect tissues from injury, our data support the concept that both pathways may work together to protect tissues from injury.

Oxidative stress induces release of 2'-AMP from microglia

BACKGROUND

Microglia metabolize exogenous 2'-AMP and 3'-AMP (non-canonical nucleotides) to adenosine and exogenous 2'-AMP and 3'-AMP (via conversion to adenosine) inhibit the production of inflammatory cytokines by microglia. This suggests that if microglia release endogenous 2'-AMP and/or 3'-AMP in response to injurious stimuli, this would complete an autocrine/paracrine mechanism that attenuates the over-activation of microglia during brain injury. Here we investigated in microglia (and for comparison astrocytes and neurons) the effects of injurious stimuli on extracellular and intracellular levels of 2',3'-cAMP (2'-AMP and 3'-AMP precursor), 2'-AMP, and 3'-AMP.

METHODS

Experiments were conducted in primary cultures of rat microglia, astrocytes, and neurons. Cells were exposed to oxygen/glucose deprivation, iodoacetate plus 2,4-dinitrophenol (metabolic inhibitors), glutamate, or H₂O₂ for one hour, and extracellular and intracellular 2',3'-cAMP, 2'-AMP, and 3'-AMP were measured by UPLC-MS/MS.

KEY RESULTS

In microglia, H₂O₂ increased extracellular levels of 2'-AMP, but not 3'-AMP, by ∼16-fold (from 0.17 ± 0.11 to 2.78 ± 0.27 ng/10⁶ cells; n = 13; mean ± SEM; P < 0.000005). H₂O₂ also induced oxidative changes in cellular proteins as detected by an increased number of carbonyl groups in protein side chains. In contrast, oxygen/glucose deprivation, metabolic inhibitors, or glutamate had no effect on either extracellular 2'-AMP or 3'-AMP levels. In astrocytes and neurons, none of the injurious stimuli increased extracellular 2'-AMP or 3'-AMP.

CONCLUSIONS

Oxidative stress (but not oxygen/glucose deprivation, energy deprivation, or excitotoxicity) induces microglia (but not astrocytes or neurons) to release 2'-AMP, but not 3'-AMP. The 2',3'-cAMP/2'-AMP/adenosine pathway mechanism may serve to prevent over-activation of microglia in response to oxidative stress.

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.

The Adenosinergic System as a Therapeutic Target in the Vasculature: New Ligands and Challenges

Adenosine is an adenine base purine with actions as a modulator of neurotransmission, smooth muscle contraction, and immune response in several systems of the human body, including the cardiovascular system. In the vasculature, four P1-receptors or adenosine receptors—A₁, A_2A, A_2B and A₃—have been identified. Adenosine receptors are membrane G-protein receptors that trigger their actions through several signaling pathways and present differential affinity requirements. Adenosine is an endogenous ligand whose extracellular levels can reach concentrations high enough to activate the adenosine receptors. This nucleoside is a product of enzymatic breakdown of extra and intracellular adenine nucleotides and also of S-adenosylhomocysteine. Adenosine availability is also dependent on the activity of nucleoside transporters (NTs). The interplay between NTs and adenosine receptors’ activities are debated and a particular attention is given to the paramount importance of the disruption of this interplay in vascular pathophysiology, namely in hypertension., The integration of important functional aspects of individual adenosine receptor pharmacology (such as in vasoconstriction/vasodilation) and morphological features (within the three vascular layers) in vessels will be discussed, hopefully clarifying the importance of adenosine receptors/NTs for modulating peripheral mesenteric vascular resistance. In recent years, an increase interest in purine physiology/pharmacology has led to the development of new ligands for adenosine receptors. Some of them have been patented as having promising therapeutic activities and some have been chosen to undergo on clinical trials. Increased levels of endogenous adenosine near a specific subtype can lead to its activation, constituting an indirect receptor targeting approach either by inhibition of NT or, alternatively, by increasing the activity of enzymes responsible for ATP breakdown. These findings highlight the putative role of adenosinergic players as attractive therapeutic targets for cardiovascular pathologies, namely hypertension, heart failure or stroke. Nevertheless, several aspects are still to be explored, creating new challenges to be addressed in future studies, particularly the development of strategies able to circumvent the predicted side effects of these therapies.

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.

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.

Purinergic transmission in blood vessels

There are nineteen different receptor proteins for adenosine, adenine and uridine nucleotides, and nucleotide sugars, belonging to three families of G protein-coupled adenosine and P2Y receptors, and ionotropic P2X receptors. The majority are functionally expressed in blood vessels, as purinergic receptors in perivascular nerves, smooth muscle and endothelial cells, and roles in regulation of vascular contractility, immune function and growth have been identified. The endogenous ligands for purine receptors, ATP, ADP, UTP, UDP and adenosine, can be released from different cell types within the vasculature, as well as from circulating blood cells, including erythrocytes and platelets. Many purine receptors can be activated by two or more of the endogenous ligands. Further complexity arises because of interconversion between ligands, notably adenosine formation from the metabolism of ATP, leading to complex integrated responses through activation of different subtypes of purine receptors. The enzymes responsible for this conversion, ectonucleotidases, are present on the surface of smooth muscle and endothelial cells, and may be coreleased with neurotransmitters from nerves. What selectivity there is for the actions of purines/pyrimidines comes from differential expression of their receptors within the vasculature. P2X1 receptors mediate the vasocontractile actions of ATP released as a neurotransmitter with noradrenaline (NA) from sympathetic perivascular nerves, and are located on the vascular smooth muscle adjacent to the nerve varicosities, the sites of neurotransmitter release. The relative contribution of ATP and NA as functional cotransmitters varies with species, type and size of blood vessel, neuronal firing pattern, the tone/pressure of the blood vessel, and in ageing and disease. ATP is also a neurotransmitter in non-adrenergic non-cholinergic perivascular nerves and mediates vasorelaxation via smooth muscle P2Y-like receptors. ATP and adenosine can act as neuromodulators, with the most robust evidence being for prejunctional inhibition of neurotransmission via A1 adenosine receptors, but also prejunctional excitation and inhibition of neurotransmission via P2X and P2Y receptors, respectively. P2Y2, P2Y4 and P2Y6 receptors expressed on the vascular smooth muscle are coupled to vasocontraction, and may have a role in pathophysiological conditions, when purines are released from damaged cells, or when there is damage to the protective barrier that is the endothelium. Adenosine is released during hypoxia to increase blood flow via vasodilator A2A and A2B receptors expressed on the endothelium and smooth muscle. ATP is released from endothelial cells during hypoxia and shear stress and can act at P2Y and P2X4 receptors expressed on the endothelium to increase local blood flow. Activation of endothelial purine receptors leads to the release of nitric oxide, hyperpolarising factors and prostacyclin, which inhibits platelet aggregation and thus ensures patent blood flow. Vascular purine receptors also regulate endothelial and smooth muscle growth, and inflammation, and thus are involved in the underlying processes of a number of cardiovascular diseases.

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.

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

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

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.

Purinergic signaling and blood vessels in health and disease

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

Antiproliferative effects of selective adenosine receptor agonists and antagonists on human lymphocytes: evidence for receptor-independent mechanisms

The effects of standard adenosine receptor (AR) agonists and antagonists on the proliferation of human T lymphocytes, unstimulated and phytohemagglutinin-stimulated human peripheral blood lymphocytes (PBL), and Jurkat T cells were investigated. Real-time PCR measurements confirmed the presence of all four AR subtypes on the investigated cells, although at different expression levels. A2A ARs were predominantly expressed in PBL and further upregulated upon stimulation, while malignant Jurkat T cells showed high expression levels of A1, A2A, and A2B ARs. Cell proliferation was measured by [(3)H]-thymidine incorporation assays. Several ligands, including the subtype-selective agonists CPA (A1), BAY60-6583 (A2B), and IB-MECA (A3), and the antagonists PSB-36 (A1), MSX-2 (A2A), and PSB-10 (A3) significantly inhibited cell proliferation at micromolar concentrations, which were about three orders of magnitude higher than their AR affinities. In contrast, further investigated AR ligands, including the agonists NECA (nonselective) and CGS21680 (A2A), and the antagonists preladenant (SCH-420814, A2A), PSB-1115 (A2B), and PSB-603 (A2B) showed no or only minor effects on lymphocyte proliferation. The anti-proliferative effects of the AR agonists could not be blocked by the corresponding antagonists. The non-selective AR antagonist caffeine stimulated phytohemagglutinin-activated PBL with an EC50 value of 104 μM. This is the first study to compare a complete set of commonly used AR ligands for all subtypes on lymphocyte proliferation. Our results strongly suggest that these compounds induce an inhibition of lymphocyte proliferation and cell death through AR-independent mechanisms.

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.

Regulation of Cell Proliferation by the Guanosine-Adenosine Mechanism: Role of Adenosine Receptors

A recent study (American Journal of Physiology – Cell Physiology 304:C406–C421, 2013) suggests that extracellular guanosine increases extracellular adenosine by modifying the disposition of extracellular adenosine (“guanosine–adenosine mechanism”) and that the guanosine–adenosine mechanism is not mediated by classical adenosine transport systems (SLC28 and SLC29 families) nor by classical adenosine-metabolizing enzymes. The present investigation had two aims (1) to test the hypothesis that the “guanosine–adenosine mechanism” affects cell proliferation; and (2) to determine whether the transporters SLC19A1, SLC19A2, SLC19A3, or SLC22A2 (known to carrier guanosine analogs) might be responsible for the guanosine–adenosine mechanism. In the absence of added adenosine, guanosine had little effect on the proliferation of coronary artery vascular smooth muscle cells (vascular conduit cells) or preglomerular vascular smooth muscle cells (vascular resistance cells). However, in the presence of added adenosine (3 or 10 μmol/L), guanosine (10–100 μmol/L) decreased proliferation of both cell types, thus resulting in a highly significant (P < 0.000001) interaction between guanosine and adenosine on cell proliferation. The guanosine–adenosine interaction on cell proliferation was abolished by 1,3-dipropyl-8-(p-sulfophenyl)xanthine (adenosine receptor antagonist). Guanosine (30 μmol/L) increased extracellular levels of adenosine when adenosine (3 μmol/L) was added to the medium. This effect was not reproduced by high concentrations of methotrexate (100 μmol/L), thiamine (1000 μmol/L), chloroquine (1000 μmol/L), or acyclovir (10,000 μmol/L), archetypal substrates for SLC19A1, SLC19A2, SLC19A3, and SLC22A2, respectively; and guanosine still increased adenosine levels in the presence of these compounds. In conclusion, the guanosine–adenosine mechanism affects cell proliferation and is not mediated by SLC19A1, SLC19A2, SLC19A3, or SLC22A2.

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

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

CONCLUSIONS

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

Tamm-Horsfall protein-associated nucleotides in patients with interstitial cystitis

WHAT'S KNOWN ON THE SUBJECT? AND WHAT DOES THE STUDY ADD?: The nucleotides associated with Tamm-Horsfall protein (THP) identified in this study are 2'(3') isomers, which are uncommon and very little is known about their biochemical pathway and role in interstitial cystitis (IC). The current study provides additional evidence of our earlier finding that THP is abnormal in IC patients. This can adversely affect THP's protective function, and suggests that THP plays a role in the pathogenesis of the disease.

OBJECTIVE

To identify and characterise Tamm-Horsfall protein (THP)-associated metabolites present in patients with interstitial cystitis (IC). Identification of these metabolites would give us insight into the complex interaction of THP with urinary metabolites and its effect on the protective function of THP.

PATIENTS, SUBJECTS AND METHODS

THP was isolated from the urine specimens of 146 patients with IC and 87 control subjects, and was analysed for total sialic acid (SA) content by 1,2-diamino-4,5-methylenedioxybenzene. 2HCl (DMB)-high performance liquid chromatography (HPLC). THP-associated metabolites were isolated by Superdex-200 size-exclusion chromatography (SEC) as a second peak (SP2) and the SP2 was further fractionated into six major components by C18 reverse phase-HPLC. Ion-pair HPLC analysis was performed to identify and quantify THP-associated metabolites. The metabolite structures were confirmed by reversed-phase HPLC combined with electrospray ionization (ESI), liquid chromatography-tandem mass spectrometry (LC-MS and LC-MS/MS) and by high-resolution ESI-time of flight mass spectrometry (HR-ESI-TOFMS).

RESULTS

The THP-associated metabolites (SP2) were more prevalent in patients with IC than in the controls (40.4% vs 12.6%, P < 0.001) as determined by Superdex-200 SEC. Superdex-200 SEC showed higher SP2 content in patients with IC than in controls, as determined by area under the peak/100 μg of THP. The mean (sem), for patients was 84.3 (8.1) vs 18.0 (2.4) milli-absorption unit*min, for controls (P < 0.001). Total SA content of THP was much lower in SP2-positive patients with IC than those who were SP2-negative. The mean (sem) was 41.6 (3.2) vs 92.6 (2.2) nmol/mg THP, respectively (P < 0.001). Ion-pair HPLC identified SP2 metabolites as nucleotides, namely 5'-CMP, 3'-UMP, 3'-AMP, 3'-GMP, 2'-AMP and 2'-GMP. The total nucleotide content of SP2 was significantly higher in patients with IC than in controls. The mean (sem) was 25.3 (2.9) vs 2.2 (0.2) μg/mg THP, respectively (P < 0.001). LC-MS and LC-MS/MS confirmed the nucleotides based on retention time on HPLC, and mass to charge ratio (m/z) of the parent ion and the daughter ions. HR-ESI-TOFMS gave further confimation of the nucleotide sturctures with high mass accuracy.

CONCLUSIONS

In the THP of patients with IC, there is a direct correlation between reduced SA levels and high prevalence of nucleotides associated with it. The THP of patients with IC has a much higher content of these nucleotides than control, and these unique nucleotide isomers identified are very consistent in all SP2-positive patients with IC, suggesting biological significance. This study provides additional evidence that THP is abnormal in patients with IC.

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

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

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

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

The brain in vivo expresses the 2',3'-cAMP-adenosine pathway

Although multiple biochemical pathways produce adenosine, studies suggest that the 2',3'-cAMP-adenosine pathway (2',3'-cAMP→2'-AMP/3'-AMP→adenosine) contributes to adenosine production in some cells/tissues/organs. To determine whether the 2',3'-cAMP-adenosine pathway exists in vivo in the brain, we delivered to the brain (gray matter and white matter separately) via the inflow perfusate of a microdialysis probe either 2',3'-cAMP, 3',5'-cAMP, 2'-AMP, 3'-AMP, or 5'-AMP and measured the recovered metabolites in the microdialysis outflow perfusate with mass spectrometry. In both gray and white matter, 2',3'-cAMP increased 2'-AMP, 3'-AMP and adenosine, and 3',5'-cAMP increased 5'-AMP and adenosine. In both brain regions, 2'-AMP, 3-AMP and 5'-AMP were converted to adenosine. Microdialysis experiments in 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase) wild-type mice demonstrated that traumatic brain injury (controlled cortical impact model) activated the brain 2',3'-cAMP-adenosine pathway; similar experiments in CNPase knockout mice indicated that CNPase was involved in the metabolism of endogenous 2',3'-cAMP to 2'-AMP and to adenosine. In CSF from traumatic brain injury patients, 2',3'-cAMP was significantly increased in the initial 12 h after injury and strongly correlated with CSF levels of 2'-AMP, 3'-AMP, adenosine and inosine. We conclude that in vivo, 2',3'-cAMP is converted to 2'-AMP/3'-AMP, and these AMPs are metabolized to adenosine. This pathway exists endogenously in both mice and humans.

Adenosine A₂B receptor agonism inhibits neointimal lesion development after arterial injury in apolipoprotein E-deficient mice

OBJECTIVE

The A(2B) adenosine receptor (A(2B)R) is highly expressed in macrophages and vascular smooth muscle cells and has been established as an important regulator of inflammation and vascular adhesion. Recently, it has been demonstrated that A(2B)R deficiency enhances neointimal lesion formation after vascular injury. Therefore, we hypothesize that A(2B)R agonism protects against injury-induced intimal hyperplasia.

METHODS AND RESULTS

Apolipoprotein E-deficient mice were fed a Western-type diet for 1 week, after which the left common carotid artery was denuded. Mice were treated with the A(2B) receptor agonist BAY60-6583 or vehicle control for 18 days. Interestingly, lumen stenosis as defined by the neointima/lumen ratio was inhibited by treatment with the A(2B) receptor agonist, caused by reduced smooth muscle cell proliferation. Collagen content was significantly increased in the BAY60-6583-treated mice, whereas macrophage content remained unchanged. In vitro, vascular smooth muscle cell proliferation decreased dose dependently whereas collagen content of cultured smooth muscle cells was increased by BAY60-6583.

CONCLUSIONS

Our data show that activation of the adenosine A(2B) receptor protects against vascular injury, while it also enhances plaque stability as indicated by increased collagen content. These outcomes thus point to A(2B) receptor agonism as a new therapeutic approach in the prevention of restenosis.

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).

Inactivation of CD73 promotes atherogenesis in apolipoprotein E-deficient mice

AIMS

CD73 (ecto-5'-nucleotidase) is expressed by a broad range of immune cells and attenuates inflammation in several acute disease models. This study therefore explored the role of CD73-derived adenosine in a model of chronic vascular inflammation such as atherogenesis.

METHODS AND RESULTS

CD73(-/-) mice were backcrossed into the apolipoprotein E (ApoE(-/-)) background. In CD73(-/-)/ApoE(-/-) double mutants, atherosclerotic lesion formation was increased by ∼50% compared with ApoE(-/-). However, the cellular composition and extracellular matrix of the plaques did not differ. Surprisingly, we found significant activity and expression of CD73 in the plaque of ApoE(-/-) mice which increased over time. CD73 co-localized with macrophages, Tregs, and cells of mesenchymal origin. Genome-wide microarray analysis of the aorta lacking CD73 revealed upregulation of endothelin-1 (Edn1) mRNA together with changes of genes in lipid metabolism and the Wnt and nuclear factor kappa B pathways. Measurement of plasma levels verified the upregulation of Edn1 in CD73(-/-) and double mutants. Plasma triglycerides (TG) were also found to be significantly elevated in the CD73(-/-)/ApoE(-/-) mice compared with ApoE(-/-) controls.

CONCLUSION

Lack of CD73 promotes atherogenesis most likely by de-inhibition of resident macrophages and T cells. Elevated Edn1 and TG levels may have contributed. This establishes CD73-derived adenosine as a direct or indirect regulator of atherogenesis.