Adenosine in Tissue Protection and Tissue Regeneration: TABLE 1

Adenosine A2A receptor regulates expression of vascular endothelial growth factor in feto-placental endothelium from normal and late-onset pre-eclamptic pregnancies

We aim to investigate whether A_2A/nitric oxide-mediated regulation of vascular endothelial growth factor (VEGF) expression is impaired in feto-placental endothelial cells from late-onset pre-eclampsia. Cultures of human umbilical vein endothelial cells (HUVECs) and human placental microvascular endothelial cells (hPMECs) from normal and pre-eclamptic pregnancies were used. Assays by using small interference RNA (siRNA) for A_2A were performed, and transfected cells were used for estimation of messenger RNA (mRNA) levels of VEGF, as well as for cell proliferation and angiogenesis in vitro. CGS-21680 (A_2A agonist, 24 h) increases HUVEC and hPMEC proliferation in a dose response manner. Furthermore, similar to CGS-21680, the nitric oxide donor, S-nitroso-N-acetyl-penicillamine oxide (SNAP), increased cell proliferation in a dose response manner (logEC₅₀ 10^-9.2 M). In hPMEC, CGS-21680 increased VEGF protein levels in both normal (∼1.5-fold) and pre-eclamptic pregnancies (∼1.2-fold), an effect blocked by the A_2A antagonist, ZM-241385 (10^-5 M) and the inhibitor of NO synthase, N _ω-nitro-L-arginine methyl ester hydrochloride (L-NAME). Subsequently, SNAP partially recovered cell proliferation and in vitro angiogenesis capacity of cells from normal pregnancies exposed to siRNA for A_2A. CGS-21680 also increased (∼1.5-fold) the level of VEGF mRNA in HUVEC from normal pregnancies, but not in pre-eclampsia. Additionally, transfection with siRNA for A_2A decrease (∼30 %) the level of mRNA for VEGF in normal pregnancy compared to untransfected cells, an effect partially reversed by co-incubation with SNAP. The A_2A-NO-VEGF pathway is present in endothelium from microcirculation and macrocirculation in both normal and pre-eclamptic pregnancies. However, NO signaling pathway seems to be impaired in HUVEC from pre-eclampsia.

Targeting adenosine receptors to prevent inflammatory skin diseases

Adenosine mediates its effects through activation of a family of four G-protein-coupled receptors, named A1 , A2A , A2B and A3 . This nucleoside plays an important role in immunity and inflammation, and the A2A adenosine receptor subtype has a key role in the inhibition of inflammatory processes besides promoting wound healing. In this issue of Experimental Dermatology, Arasa et al. show that the topical application of a selective A2A agonist, CGS 21680, to mouse skin reduced epidermal hyperplasia as well as skin inflammation, similarly to topical corticoids, without side effects like skin atrophy. Rigorously following up this work is important for the development of novel treatment strategies for chronic hyperproliferative inflammatory dermatoses, such as targeting the A2A adenosine receptor family.

Adenosine as a Multi-Signalling Guardian Angel in Human Diseases: When, Where and How Does it Exert its Protective Effects?

The importance of adenosine for human health cannot be overstated. Indeed, this ubiquitous nucleoside is an integral component of ATP, and regulates the function of every tissue and organ in the body. Acting via receptor-dependent and -independent mechanisms [the former mediated via four G-protein-coupled receptors (GPCRs), A1, A2A, A2B, and A3,], it has a significant role in protecting against cell damage in areas of increased tissue metabolism, and combating organ dysfunction in numerous pathological states. Accordingly, raised levels of adenosine have been demonstrated in epilepsy, ischaemia, pain, inflammation, and cancer, in which its behaviour can be likened to that of a guardian angel, even though there are instances in which overproduction of adenosine is pathological. In this review, we condense the current body of knowledge on the issue, highlighting when, where, and how adenosine exerts its protective effects in both the brain and the periphery.

Signaling pathways involving adenosine A2A and A2B receptors in wound healing and fibrosis

Collagen and matrix deposition by fibroblasts is an essential part of wound healing but also contributes to pathologic remodeling of organs leading to substantial morbidity and mortality. Adenosine, a small molecule generated extracellularly from adenine nucleotides as a result of direct stimulation, hypoxia, or injury, acts via a family of classical seven-pass G protein-coupled protein receptors, A2A and A2B, leading to generation of cAMP and activation of downstream targets such as PKA and Epac. These effectors, in turn, lead to fibroblast activation and collagen synthesis. The regulatory actions of these receptors likely involve multiple interconnected pathways, and one of the more interesting aspects of this regulation is opposing effects at different levels of cAMP generated. Additionally, adenosine signaling contributes to fibrosis in organ-specific ways and may have opposite effects in different organs. The development of drugs that selectively target these receptors and their signaling pathways will disrupt the pathogenesis of fibrosis and slow or arrest the progression of the important diseases they underlie.

Loss of CD73-mediated actin polymerization promotes endometrial tumor progression

Ecto-5'-nucleotidase (CD73) is central to the generation of extracellular adenosine. Previous studies have highlighted a detrimental role for extracellular adenosine in cancer, as it dampens T cell-mediated immune responses. Here, we determined that, in contrast to other cancers, CD73 is markedly downregulated in poorly differentiated and advanced-stage endometrial carcinoma compared with levels in normal endometrium and low-grade tumors. In murine models, CD73 deficiency led to a loss of endometrial epithelial barrier function, and pharmacological CD73 inhibition increased in vitro migration and invasion of endometrial carcinoma cells. Given that CD73-generated adenosine is central to regulating tissue protection and physiology in normal tissues, we hypothesized that CD73-generated adenosine in endometrial carcinoma induces an innate reflex to protect epithelial integrity. CD73 associated with cell-cell contacts, filopodia, and membrane zippers, indicative of involvement in cell-cell adhesion and actin polymerization-dependent processes. We determined that CD73-generated adenosine induces cortical actin polymerization via adenosine A1 receptor (A1R) induction of a Rho GTPase CDC42-dependent conformational change of the actin-related proteins 2 and 3 (ARP2/3) actin polymerization complex member N-WASP. Cortical F-actin elevation increased membrane E-cadherin, β-catenin, and Na(+)K(+) ATPase. Together, these findings reveal that CD73-generated adenosine promotes epithelial integrity and suggest why loss of CD73 in endometrial cancer allows for tumor progression. Moreover, our data indicate that the role of CD73 in cancer is more complex than previously described.

The Many Faces of the A2b Adenosine Receptor in Cardiovascular and Metabolic Diseases

Modulation of the low affinity adenosine receptor subtype, the A2b adenosine receptor (A2bAR), has gained interest as a therapeutic target in various pathologic areas associated with cardiovascular disease. The actions of the A2bAR are diverse and at times conflicting depending on cell and tissue type and the timing of activation or inhibition of the receptor. The A2bAR is a promising and exciting pharmacologic target, however, a thorough understanding of A2bAR action is necessary to reach the therapeutic potential of this receptor. This review will focus on the role of the A2bAR in various cardiovascular and metabolic pathologies in which the receptor is currently being studied. We will illustrate the complexities of A2bAR signaling and highlight areas of research with potential for therapeutic development.

MicroRNA-128b suppresses tumor growth and promotes apoptosis by targeting A2bR in gastric cancer

MicroRNAs (miRNAs) play crucial roles in the development and progression of human cancers, including gastric cancer (GC). The discovery of miRNAs may provide a new and powerful tool for studying the mechanism, diagnosis, and treatment of GC. In this study, we aimed to investigate the role and mechanism of miR-128b in the development and progression of GC. Quantitative real-time PCR (qRT-PCR) was used to measure the expression level of miR-128b in GC tissues and cell lines. We found that miR-128b was significantly down-regulated in GC tissues and cell lines. In addition, over-expression of miR-128b inhibited GC cell proliferation, migration and invasion of GC cells in vitro. Gain-of-function in vitro experiments further showed that the miR-128b mimic significantly promoted GC cell apoptosis. Subsequent dual-luciferase reporter assay identified one of the proto-oncogene A2bR as direct target of miR-128b. Therefore, our results indicate that miR-128b is a proto-oncogene miRNA that can suppresses GC proliferation and migration through down-regulation of the oncogene gene A2bR. Taken together, our results indicate that miR-128b could serve as a potential diagnostic biomarker and therapeutic option for human GC in the near future.

Beneficial and detrimental role of adenosine signaling in diseases and therapy

Adenosine is a major signaling nucleoside that orchestrates cellular and tissue adaptation under energy depletion and ischemic/hypoxic conditions by activation of four G protein-coupled receptors (GPCR). The regulation and generation of extracellular adenosine in response to stress are critical in tissue protection. Both mouse and human studies reported that extracellular adenosine signaling plays a beneficial role during acute states. However, prolonged excess extracellular adenosine is detrimental and contributes to the development and progression of various chronic diseases. In recent years, substantial progress has been made to understand the role of adenosine signaling in different conditions and to clarify its significance during the course of disease progression in various organs. These efforts have and will identify potential therapeutic possibilities for protection of tissue injury at acute stage by upregulation of adenosine signaling or attenuation of chronic disease progression by downregulation of adenosine signaling. This review is to summarize current progress and the importance of adenosine signaling in different disease stages and its potential therapeutic effects.

Mechanisms of sensorineural cell damage, death and survival in the cochlea

The majority of acquired hearing loss, including presbycusis, is caused by irreversible damage to the sensorineural tissues of the cochlea. This article reviews the intracellular mechanisms that contribute to sensorineural damage in the cochlea, as well as the survival signaling pathways that can provide endogenous protection and tissue rescue. These data have primarily been generated in hearing loss not directly related to age. However, there is evidence that similar mechanisms operate in presbycusis. Moreover, accumulation of damage from other causes can contribute to age-related hearing loss (ARHL). Potential therapeutic interventions to balance opposing but interconnected cell damage and survival pathways, such as antioxidants, anti-apoptotics, and pro-inflammatory cytokine inhibitors, are also discussed.

Effects of A2BR on the biological behavior of mouse renal fibroblasts during hypoxia

Fibroblasts are the effector cells of collagen secretion in renal interstitial fibrosis (RIF), and their proliferation and activation are essential for the development of RIF. Hypoxic ischemia in local tissues has been identified in chronic kidney diseases (CKDs), with adenosine (ADO) as a key signaling molecule. The current study investigated the association between ADO and the biological behavior of renal fibroblasts by establishing an in vitro hypoxia cell model. This aimed to provide experimental evidence for the prevention and treatment of RIF. NIH3T3 fibroblasts were exposed to hypoxia, and the subtypes of the ADO receptor (AR) on the cell surface were identified by a TaqMan probe‑based assay. Cells were divided into the following four groups: i) Control; ii) 5'‑N‑ethylcarboxamidoadenosine (NECA); iii) PT, NECA + 8‑phenyltheophylline (PT); and iv) MRS, NECA + N‑(4‑cyanophenyl)‑2‑[4‑(2,3,6,7‑tetrahydro‑2,6‑dioxo‑1,3‑dipropyl‑1H‑purin‑8‑yl)phenoxy]‑acetamide (MRS1754). The mRNA levels of transforming growth factor‑β1 (TGF‑β1), procollagen α1 (I) and α‑smooth muscle actin (α‑SMA) were measured following 24, 48, and 72 h of hypoxia. Cell proliferation was evaluated by a 3‑(4,5‑dimethylthiazol‑2‑yl)‑2,5‑diphenyltetrazolium bromide assay at 0, 12, 24, 48 and 72 h. The results demonstrated that A2BR was the predominant AR subtype present in hypoxia‑stimulated fibroblasts. NECA significantly induced fibroblast proliferation and upregulated the expression of TGF‑β1, procollagen α1 (I) and α‑SMA mRNA, while 8‑PT and MRS1754 inhibited fibroblast proliferation and downregulated the expression of TGF‑β1, procollagen α1 (I) and α‑SMA mRNA. The blockage of A2BR in hypoxia significantly inhibited the proliferation and activation of fibroblasts, and reduced the production of profibrotic cytokines, thus preventing the generation and development of fibrosis.

Increased adenosine levels contribute to ischemic kidney fibrosis in the unilateral ureteral obstruction model

Renal interstitial fibrosis (RIF) occurs as a result of chronic kidney disease (CKD) and is a common pathway leading to end-stage renal failure. Renal tissue hypoxia and ischemia are present during CKD. Adenosine (ADO) is an important signaling molecule induced under ischemic and hypoxic conditions. In the present study, the association between ADO and RIF was investigated using a mouse model, with the aim of obtaining important information relevant to the prevention and treatment of RIF. A unilateral ureteral obstruction (UUO) model of RIF was established in mice. A total of 44 male mice were randomly divided into sham, model and intervention groups, and samples were collected on days 1, 3, 7, and 14 after modeling. These were collected to detect hypoxia and changes in ADO concentration in obstructed renal tissue as well as to analyze the pathological changes and degree of RIF in the renal tissue. Changes in the levels of collagen deposition and profibrogenic factors in renal tissues were analyzed following intervention with an ADO receptor blocker. Following the UUO procedure, continuous hypoxia was present in the obstructed renal tissue, accompanied by an increased ADO concentration. Tubular injury and interstitial fibrosis progressively increased over time following the UUO procedure. The mRNA expression levels of tissue tumor growth factor β₁ (TGF-β₁) and α₁(I) procollagen were significantly increased. Subsequent to the ADO pathway being blocked by 8-(p-sulfophenyl)-theophylline, tubular injury and interstitial fibrosis were reduced and the expression of related cytokines was decreased. Increased ADO levels were induced by hypoxia, causing the development of RIF. Following the blocking of the ADO pathway, renal damage was deferred and renal functions were protected.

High salt diet exacerbates vascular contraction in the absence of adenosine A₂A receptor

High salt (4% NaCl, HS) diet modulates adenosine-induced vascular response through adenosine A(2A) receptor (A(2A)AR). Evidence suggests that A(2A)AR stimulates cyp450-epoxygenases, leading to epoxyeicosatrienoic acids (EETs) generation. The aim of this study was to understand the vascular reactivity to HS and underlying signaling mechanism in the presence or absence of A(2A)AR. Therefore, we hypothesized that HS enhances adenosine-induced relaxation through EETs in A(2A)AR⁺/⁺, but exaggerates contraction in A(2A)AR⁻/⁻. Organ bath and Western blot experiments were conducted in HS and normal salt (NS, 0.18% NaCl)-fed A(2A)AR⁺/⁺ and A(2A)AR⁻/⁻ mice aorta. HS produced concentration-dependent relaxation to non-selective adenosine analog, NECA in A(2A)AR⁺/⁺, whereas contraction was observed in A(2A)AR⁻/⁻ mice and this was attenuated by A₁AR antagonist (DPCPX). CGS 21680 (selective A(2A)AR agonist) enhanced relaxation in HS-A(2A)AR⁺/⁺ versus NS-A(2A)AR⁺/⁺, which was blocked by EETs antagonist (14,15-EEZE). Compared with NS, HS significantly upregulated the expression of vasodilators A(2A)AR and cyp2c29, whereas vasoconstrictors A₁AR and cyp4a in A(2A)AR⁺/⁺ were downregulated. In A(2A)AR⁻/⁻ mice, however, HS significantly downregulated the expression of cyp2c29, whereas A₁AR and cyp4a were upregulated compared with A(2A)AR⁺/⁺ mice. Hence, our data suggest that in A(2A)AR⁺/⁺, HS enhances A(2A)AR-induced relaxation through increased cyp-expoxygenases-derived EETs and decreased A₁AR levels, whereas in A(2A)AR⁻/⁻, HS exaggerates contraction through decreased cyp-epoxygenases and increased A₁AR levels.

Adenosine amine congener as a cochlear rescue agent

We have previously shown that adenosine amine congener (ADAC), a selective A₁ adenosine receptor agonist, can ameliorate noise- and cisplatin-induced cochlear injury. Here we demonstrate the dose-dependent rescue effects of ADAC on noise-induced cochlear injury in a rat model and establish the time window for treatment. Methods. ADAC (25–300 μg/kg) was administered intraperitoneally to Wistar rats (8–10 weeks old) at intervals (6–72 hours) after exposure to traumatic noise (8–16 kHz, 110 dB sound pressure level, 2 hours). Hearing sensitivity was assessed using auditory brainstem responses (ABR) before and 12 days after noise exposure. Pharmacokinetic studies investigated ADAC concentrations in plasma after systemic (intravenous) administration. Results. ADAC was most effective in the first 24 hours after noise exposure at doses >50 μg/kg, providing up to 21 dB protection (averaged across 8–28 kHz). Pharmacokinetic studies demonstrated a short (5 min) half-life of ADAC in plasma after intravenous administration without detection of degradation products. Conclusion. Our data show that ADAC mitigates noise-induced hearing loss in a dose- and time-dependent manner, but further studies are required to establish its translation as a clinical otological treatment.

Purine-metabolizing ectoenzymes control IL-8 production in human colon HT-29 cells

Interleukin-8 (IL-8) plays key roles in both chronic inflammatory diseases and tumor modulation. We previously observed that IL-8 secretion and function can be modulated by nucleotide (P2) receptors. Here we investigated whether IL-8 release by intestinal epithelial HT-29 cells, a cancer cell line, is modulated by extracellular nucleotide metabolism. We first identified that HT-29 cells regulated adenosine and adenine nucleotide concentration at their surface by the expression of the ectoenzymes NTPDase2, ecto-5′-nucleotidase, and adenylate kinase. The expression of the ectoenzymes was evaluated by RT-PCR, qPCR, and immunoblotting, and their activity was analyzed by RP-HPLC of the products and by detection of P_i produced from the hydrolysis of ATP, ADP, and AMP. In response to poly (I:C), with or without ATP and/or ADP, HT-29 cells released IL-8 and this secretion was modulated by the presence of NTPDase2 and adenylate kinase. Taken together, these results demonstrate the presence of 3 ectoenzymes at the surface of HT-29 cells that control nucleotide levels and adenosine production (NTPDase2, ecto-5′-nucleotidase and adenylate kinase) and that P2 receptor-mediated signaling controls IL-8 release in HT-29 cells which is modulated by the presence of NTPDase2 and adenylate kinase.

Extracellular generation of adenosine by the ectonucleotidases CD39 and CD73 promotes dermal fibrosis

Adenosine has an important role in inflammation and tissue remodeling and promotes dermal fibrosis by adenosine receptor (A2AR) activation. Adenosine may be formed intracellularly from adenine nucleotides or extracellularly through sequential phosphohydrolysis of released ATP by nucleoside triphosphate diphosphohydrolase (CD39) and ecto-5'-nucleotidase (CD73). Because the role of these ecto-enzymes in fibrosis appears to be tissue specific, we determined whether these ectonucleotidases were directly involved in diffuse dermal fibrosis. Wild-type and mice globally deficient in CD39 knockout (CD39KO), CD73 (CD73KO), or both (CD39/CD73DKO) were challenged with bleomycin. Extracellular adenosine levels and dermal fibrosis were quantitated. Adenosine release from skin cultured ex vivo was increased in wild-type mice after bleomycin treatment but remained low in skin from CD39KO, CD73KO, or CD39/CD73DKO bleomycin-treated mice. Deletion of CD39 and/or CD73 decreased the collagen content, and prevented skin thickening and tensile strength increase after bleomycin challenge. Decreased dermal fibrotic features were associated with reduced expression of the profibrotic mediators, transforming growth factor-β1 and connective tissue growth factor, and diminished myofibroblast population in CD39- and/or CD73-deficient mice. Our work supports the hypothesis that extracellular adenosine, generated in tandem by ecto-enzymes CD39 and CD73, promotes dermal fibrogenesis. We suggest that biochemical or biological inhibitors of CD39 and/or CD73 may hold promise in the treatment of dermal fibrosis in diseases such as scleroderma.

Two new adenosine receptor antagonists for the treatment of Parkinson's disease: istradefylline versus tozadenant

INTRODUCTION

Adenosine A2A receptors are localized in the brain, mainly within the caudate and putamen nuclei of the basal ganglia. Their activation leads to stimulation of the 'indirect' pathway. Conversely, administration of A2A receptor antagonists leads to inhibition of this pathway, which was translated into reduced hypomotility in several animal models of parkinsonism.

AREAS COVERED

In this review, the effects of two A2A receptor antagonists, istradefylline and tozadenant, on parkinsonian symptoms in animal and humans will be discussed.

EXPERT OPINION

Animal studies have shown potent antiparkinsonian effects for several A2A receptor antagonists, including istradefylline. In clinical trials, istradefylline reduced OFF time when administered with levodopa, but results are inconclusive. Results with tozadenant are scarce. Modification of thalamic blood flow compatible with reduced inhibition was noted in one small trial, followed by a significant reduction in OFF time in a larger one. Therefore, both drugs show promising efficacy for the reduction of OFF time in levodopa-treated Parkinson's disease patients, but further research is needed in order to obtain definitive conclusions.

Interactions of PPAR-alpha and adenosine receptors in hypoxia-induced angiogenesis

Hypoxia and adenosine are known to upregulate angiogenesis; however, the role of peroxisome proliferator-activated receptor alpha (PPARα) in angiogenesis is controversial. Using transgenic Tg(fli-1:EGFP) zebrafish embryos, interactions of PPARα and adenosine receptors in angiogenesis were evaluated under hypoxic conditions. Epifluorescent microscopy was used to assess angiogenesis by counting the number of intersegmental (ISV) and dorsal longitudinal anastomotic vessel (DLAV) at 28 h post-fertilization (hpf). Hypoxia (6h) stimulated angiogenesis as the number of ISV and DLAV increased by 18-fold (p<0.01) and 100 ± 8% (p<0.001), respectively, at 28 hpf. Under normoxic and hypoxic conditions, WY-14643 (10 μM), a PPARα activator, stimulated angiogenesis at 28 hpf, while MK-886 (0.5 μM), an antagonist of PPARα, attenuated these effects. Compared to normoxic condition, adenosine receptor activation with NECA (10 μM) promoted angiogenesis more effectively under hypoxic conditions. Involvement of A2B receptor was implied in hypoxia-induced angiogenesis as MRS-1706 (10nM), a selective A2B antagonist attenuated NECA (10 μM)-induced angiogenesis. NECA- or WY-14643-induced angiogenesis was also inhibited by miconazole (0.1 μM), an inhibitor of epoxygenase dependent production of eicosatrienoic acid (EET) epoxide. Thus, we conclude that: activation of PPARα promoted angiogenesis just as activation of A2B receptors through an epoxide dependent mechanism.

Demyelination initiated by oligodendrocyte apoptosis through enhancing endoplasmic reticulum-mitochondria interactions and Id2 expression after compressed spinal cord injury in rats

BACKGROUND

Demyelination is one of the most important pathological factors of spinal cord injury. Oligodendrocyte apoptosis is involved in triggering demyelination. However, fewer reports on pathological changes and mechanism of demyelination have been presented from compressed spinal cord injury (CSCI). The relative effect of oligodendrocyte apoptosis on CSCI-induced demyelination and the mechanism of apoptosis remain unclear.

AIMS

In this study, a custom-designed model of CSCI was used to determine whether or not demyelination and oligodendrocyte apoptosis occur after CSCI. The pathological changes in axonal myelinated fibers were investigated by osmic acid staining and transmission electron microscopy. Myelin basic protein (MBP), which is used in myelin formation in the central nervous system, was detected by immunofluorescence and Western blot assays. Oligodendrocyte apoptosis was revealed by in situ terminal-deoxytransferase-mediated dUTP nick-end labeling. To analyze the mechanism of oligodendrocyte apoptosis, we detected caspase-12 [a representative of endoplasmic reticulum (ER) stress], cytochrome c (an apoptotic factor and hallmark of mitochondria), and inhibitor of DNA binding 2 (Id2, an oligodendrocyte lineage gene) by immunofluorescence and Western blot assays.

RESULTS

The custom-designed model of CSCI was successfully established. The rats were spastic, paralyzed, and incontinent. The Basso, Beattie, and Bresnahan (BBB) locomotor rating scale scores were decreased as time passed. The compressed spinal cord slices were ischemic. Myelin sheaths became swollen and degenerative; these sheaths were broken down as time passed after CSCI. MBP expression was downregulated after CSCI and consistent with the degree of demyelination. Oligodendrocyte apoptosis occurred at 1 day after CSCI and increased as caspase-12 expression was enhanced and cytochrome c was released. Id2 was distributed widely in the white matter. Id2 expression increased with time after CSCI.

CONCLUSION

Demyelination occurred after CSCI and might be partly caused by oligodendrocyte apoptosis, which was positively correlated with ER-mitochondria interactions and enhanced Id2 expression after CSCI in rats.

Identification of a potent endothelium-derived angiogenic factor

The secretion of angiogenic factors by vascular endothelial cells is one of the key mechanisms of angiogenesis. Here we report on the isolation of a new potent angiogenic factor, diuridine tetraphosphate (Up4U) from the secretome of human endothelial cells. The angiogenic effect of the endothelial secretome was partially reduced after incubation with alkaline phosphatase and abolished in the presence of suramin. In one fraction, purified to homogeneity by reversed phase and affinity chromatography, Up4U was identified by MALDI-LIFT-fragment-mass-spectrometry, enzymatic cleavage analysis and retention-time comparison. Beside a strong angiogenic effect on the yolk sac membrane and the developing rat embryo itself, Up4U increased the proliferation rate of endothelial cells and, in the presence of PDGF, of vascular smooth muscle cells. Up4U stimulated the migration rate of endothelial cells via P2Y2-receptors, increased the ability of endothelial cells to form capillary-like tubes and acts as a potent inducer of sprouting angiogenesis originating from gel-embedded EC spheroids. Endothelial cells released Up4U after stimulation with shear stress. Mean total plasma Up4U concentrations of healthy subjects (N=6) were sufficient to induce angiogenic and proliferative effects (1.34 ± 0.26 nmol L(-1)). In conclusion, Up4U is a novel strong human endothelium-derived angiogenic factor.

Adenosine and its receptors as therapeutic targets: An overview

The main goal of the authors is to present an overview of adenosine and its receptors, which are G-protein coupled receptors. The four known adenosine receptor subtypes are discussed along with the therapeutic potential indicating that these receptors can serve as targets for various dreadful diseases.

Contributions of neutrophils to resolution of mucosal inflammation

Neutrophil (PMN) recruitment from the blood stream into surrounding tissues involves a regulated series of events central to acute responses in host defense. Accumulation of PMN within mucosal tissues has historically been considered pathognomonic features of both acute and chronic inflammatory conditions. Historically, PMNs have been deemed necessary but detrimental when recruited, given the potential for tissue damage that results from a variety of mechanisms. Recent work, however, has altered our preconceived notions of PMN contributions to inflammatory processes. In particular, significant evidence implicates a central role for the PMN in triggering inflammatory resolution. Such mechanisms involve both metabolic and biochemical crosstalk pathways during the intimate interactions of PMN with other cell types at inflammatory sites. Here, we highlight several recent examples of how PMN coordinate the resolution of ongoing inflammation, with a particular focus on the gastrointestinal mucosa.

Adenosine signaling pathways as potential therapeutic targets in respiratory disease

INTRODUCTION

Adenosine receptors (ARs) and their differential pattern of expression modulate a series of pleiotropic activities that are known to contribute to the control of inflammation, remodeling, and tissue repair. Consequently, pharmacological manipulation of adenosine signaling pathway is of great interest and is currently exploited as a therapeutic target for a number of respiratory diseases with several molecules with agonist and antagonist activities against known ARs being developed for the treatment of different conditions of the respiratory system.

AREAS COVERED

Herein, we will review the rational basis leading to the development of novel therapies for asthma, chronic obstructive pulmonary disease (COPD), interstitial lung disease (ILD), pulmonary arterial hypertension (PAH), and cystic fibrosis. Their most recent clinical development will be also discussed.

EXPERT OPINION

Advances in our understanding of the pathogenetic role of adenosine in respiratory diseases may be soon translated into effective treatment options. In consideration of the complex interplay driven by the different pattern of receptor distribution and/or affinity of the four known AR subtypes in specific cell types at different stages of the disease, it is likely that combination of selective antagonist/agonists for different AR subtypes will be required to obtain reasonable clinical efficacy. Alternatively, controlling the factors involved in driving adenosine concentrations in the tissue may be also of great significance.

Adenosine kinase: exploitation for therapeutic gain

Adenosine kinase (ADK; EC 2.7.1.20) is an evolutionarily conserved phosphotransferase that converts the purine ribonucleoside adenosine into 5'-adenosine-monophosphate. This enzymatic reaction plays a fundamental role in determining the tone of adenosine, which fulfills essential functions as a homeostatic and metabolic regulator in all living systems. Adenosine not only activates specific signaling pathways by activation of four types of adenosine receptors but it is also a primordial metabolite and regulator of biochemical enzyme reactions that couple to bioenergetic and epigenetic functions. By regulating adenosine, ADK can thus be identified as an upstream regulator of complex homeostatic and metabolic networks. Not surprisingly, ADK dysfunction is involved in several pathologies, including diabetes, epilepsy, and cancer. Consequently, ADK emerges as a rational therapeutic target, and adenosine-regulating drugs have been tested extensively. In recent attempts to improve specificity of treatment, localized therapies have been developed to augment adenosine signaling at sites of injury or pathology; those approaches include transplantation of stem cells with deletions of ADK or the use of gene therapy vectors to downregulate ADK expression. More recently, the first human mutations in ADK have been described, and novel findings suggest an unexpected role of ADK in a wider range of pathologies. ADK-regulating strategies thus represent innovative therapeutic opportunities to reconstruct network homeostasis in a multitude of conditions. This review will provide a comprehensive overview of the genetics, biochemistry, and pharmacology of ADK and will then focus on pathologies and therapeutic interventions. Challenges to translate ADK-based therapies into clinical use will be discussed critically.

Elevated ecto-5'-nucleotidase-mediated increased renal adenosine signaling via A2B adenosine receptor contributes to chronic hypertension

RATIONALE

Hypertension is the most prevalent life-threatening disease worldwide and is frequently associated with chronic kidney disease (CKD). However, the molecular basis underlying hypertensive CKD is not fully understood.

OBJECTIVE

We sought to identify specific factors and signaling pathways that contribute to hypertensive CKD and thereby exacerbate disease progression.

METHODS AND RESULTS

Using high-throughput quantitative reverse-transcription polymerase chain reaction profiling, we discovered that the expression level of 5'-ectonucleotidase (CD73), a key enzyme that produces extracellular adenosine, was significantly increased in the kidneys of angiotensin II-infused mice, an animal model of hypertensive nephropathy. Genetic and pharmacological studies in mice revealed that elevated CD73-mediated excess renal adenosine preferentially induced A2B adenosine receptor (ADORA2B) production and that enhanced kidney ADORA2B signaling contributes to angiotensin II-induced hypertension. Similarly, in humans, we found that CD73 and ADORA2B levels were significantly elevated in the kidneys of CKD patients compared with normal individuals and were further elevated in hypertensive CKD patients. These findings led us to further discover that elevated renal CD73 contributes to excess adenosine signaling via ADORA2B activation that directly stimulates endothelin-1 production in a hypoxia-inducible factor-α-dependent manner and underlies the pathogenesis of the disease. Finally, we revealed that hypoxia-inducible factor-α is an important factor responsible for angiotensin II-induced CD73 and ADORA2B expression at the transcriptional level.

CONCLUSIONS

Overall, our studies reveal that angiotensin II-induced renal CD73 promotes the production of renal adenosine that is a prominent driver of hypertensive CKD by enhanced ADORA2B signaling-mediated endothelin-1 induction in a hypoxia-inducible factor-α-dependent manner. The inhibition of excess adenosine-mediated ADORA2B signaling represents a novel therapeutic target for the disease.

Purinergic signalling during sterile liver injury

The liver plays a vital role in the organism, and thousands of patients suffer and even die from hepatic complications every year. Viral hepatitis is one of the most important causes of liver-related pathological processes. However, sterile liver diseases, such as drug-induced liver injury, cirrhosis and fibrosis, are still a worldwide concern and contribute significantly to liver transplantation statistics. During hepatocyte death, several genuine intracellular contents are released to the interstitium, where they will trigger inflammatory responses that may boost organ injury. Intracellular purines are key molecules to several metabolic pathways and regulate cell bioenergetics. However, seminal studies in early 70s revealed that purines may also participate in cell-to-cell communication, and more recent data have unequivocally demonstrated that the purinergic signalling plays a key role in the recognition of cell functionality by neighbouring cells and also by the immune system. This new body of knowledge has pointed out that several promising therapeutic opportunities may rely on the modulation of purine release and sensing during diseases. Here, we review the most recent data on the physiological roles of purinergic signalling and how its imbalance may contribute to injury progression during sterile liver injury.

Adenosine A₂A receptor agonist-mediated increase in donor-derived regulatory T cells suppresses development of graft-versus-host disease

Graft-versus-host disease (GVHD) remains a significant complication of allogeneic transplantation. We previously reported that the adenosine A(2A) receptor (A(2A)R) specific agonist, ATL146e, decreases the incidence and severity of GVHD in a mouse transplant model. There is increasing interest in treatments that increase CD4(+)CD25(high)Foxp3(+) regulatory T cells (Tregs) to suppress GVHD. Our current study found in vitro that A(2A)R selective agonists enhanced TGF-β-induced generation of mouse Tregs 2.3- to 3-fold. We demonstrated in vivo suppression of GVHD with specific A(2A)R agonists in two different murine GVHD transplant models associated with profound increases in both circulating and target tissue Tregs of donor origin. Three different A(2A)R agonists of differing potency, ATL146e, ATL370, and ATL1223, all significantly inhibited GVHD-associated weight loss and mortality. At the same time, Tregs shown to be of donor origin increased 5.1- to 7.4-fold in spleen, 2.7- to 4.6-fold in peripheral blood, 2.3- to 4.7-fold in colon, and 3.8- to 4.6-fold in skin. We conclude that specific activation of A(2A)R inhibits acute GVHD through an increase of donor-derived Tregs. Furthermore, the increased presence of Tregs in target tissues (colon and skin) of A(2A)R-specific agonist-treated mice is likely the mechanistic basis for the anti-inflammatory effect preventing acute GVHD.

Adenosine A2A receptor activation reduces recurrence and mortality from Clostridium difficile infection in mice following vancomycin treatment

Background

Activation of the A_2A adenosine receptor (A_2AAR) decreases production of inflammatory cytokines, prevents C. difficile toxin A-induced enteritis and, in combination with antibiotics, increases survival from sepsis in mice. We investigated whether A_2AAR activation improves and A_2AAR deletion worsens outcomes in a murine model of C. difficile (strain VPI10463) infection (CDI).

Methods

C57BL/6 mice were pretreated with an antibiotic cocktail prior to infection and then treated with vancomycin with or without an A_2AAR agonist. A_2AAR^-/- and littermate wild-type (WT) mice were similarly infected, and IFNγ and TNFα were measured at peak of and recovery from infection.

Results

Infected, untreated mice rapidly lost weight, developed diarrhea, and had mortality rates of 50-60%. Infected mice treated with vancomycin had less weight loss and diarrhea during antibiotic treatment but mortality increased to near 100% after discontinuation of antibiotics. Infected mice treated with both vancomycin and an A_2AAR agonist, either ATL370 or ATL1222, had minimal weight loss and better long-term survival than mice treated with vancomycin alone. A_2AAR KO mice were more susceptible than WT mice to death from CDI. Increases in cecal IFNγ and blood TNFα were pronounced in the absence of A_2AARs.

Conclusion

In a murine model of CDI, vancomycin treatment resulted in reduced weight loss and diarrhea during acute infection, but high recurrence and late-onset death, with overall mortality being worse than untreated infected controls. The administration of vancomycin plus an A_2AAR agonist reduced inflammation and improved survival rates, suggesting a possible benefit of A_2AAR agonists in the management of CDI to prevent recurrent disease.

Adenosine A(2A) receptor activation supports an atheroprotective cholesterol balance in human macrophages and endothelial cells

The adenosine A(2A) receptor (A(2A)R) plays an important role in the regulation of inflammatory and immune responses. Our previous work has demonstrated that A(2A)R agonists exhibit atheroprotective effects by increasing expression of reverse cholesterol transport proteins in cultured human macrophages. This study explores the impact of pharmacologic activation/inhibition and gene silencing of A(2A)R on cholesterol homeostasis in both THP-1 human monocytes/macrophages and primary human aortic endothelial cells (HAEC). THP-1 human monocytes/macrophages and HAEC exposed to the A(2A)R-specific agonist ATL313 exhibited upregulation of proteins responsible for cholesterol efflux: the ABCA1 and G1 transporters. Further, activation of A(2A)R led to upregulation of the cholesterol metabolizing enzyme P450 27-hydroxylase, accompanied by intracellular changes in level of oxysterols. We demonstrate that anti-atherogenic properties of A(2A)R activation are not limited to the regulation of lipid efflux in vasculature, but include protection from lipid overload in macrophages, particularly via suppression of the CD36 scavenger receptor. The reduced lipid accumulation manifests directly as a diminution in foam cell transformation. In THP-1 macrophages, either A(2A)R pharmacological blockade or gene silencing promote lipid accumulation and enhance foam cell transformation. Our pre-clinical data provides evidence suggesting that A(2A)R stimulation by ATL313 has the potential to be a viable therapeutic strategy for cardiovascular disease prevention, particularly in patients with elevated risk due to immune/inflammatory disorders.

Autoregulation of thromboinflammation on biomaterial surfaces by a multicomponent therapeutic coating

Activation of the thrombotic and complement systems is the main recognition and effector mechanisms in the multiple adverse biological responses triggered when biomaterials or therapeutic cells come into blood contact. We have created a surface which is auto-protective to human innate immunity by combining three fundamentally different strategies, all developed by us previously, which have been shown to induce substantial, but incomplete hemocompatibility when used separately. In summary, we have conjugated a factor H–binding peptide; and an ADP-degrading enzyme; using a PEG linker on both material and cellular surfaces. When exposed to human whole blood, factor H was specifically recruited to the modified surfaces and inhibited complement attack. In addition, activation of platelets and coagulation was efficiently attenuated, by degrading ADP. Thus, by inhibiting thromboinflammation using a multicomponent approach, we have created a hybrid surface with the potential to greatly reduce incompatibility reactions involving biomaterials and transplantation.

Design and application of locally delivered agonists of the adenosine A(2A) receptor

The broad spectrum anti-inflammatory actions of adenosine A(2A) receptor agonists are well described. The wide distribution of this receptor, however, suggests that the therapeutic potential of these agents is likely to reside in topical treatments to avoid systemic side effects associated with oral administration. Adenosine A(2A) receptor agonists have been assessed as topical agents: GW328267X (GSK; allergic rhinitis and asthma), UK-432097 (Pfizer; chronic obstructive pulmonary disease [COPD]) and Sonedenoson (MRE0094, King Pharmaceuticals; wound healing). All trials failed to achieve effects against the desired clinical end points. This broad-based review will discuss general principles of chemical design of topically applied agents and potential therapeutic topical applications of current adenosine A(2A) receptor agonists. Potential factors contributing to the lack of efficacy in the above clinical trials will be discussed together with design principles, which may influence efficacy in disease states. Our analysis suggests that adenosine A(2A) receptor agonists have a wide therapeutic potential as topical agents in a wide variety of diseases, such as neutrophil-dependent lung diseases (acute lung injury, exacerbations in asthma and COPD), allergic rhinitis, glaucoma and wound repair. Factors that will influence topical activity include formulation, tissue retention, compound potency, receptor kinetics and pharmacokinetics.

Targeting adenosine receptors in the development of cardiovascular therapeutics

Adenosine receptor stimulation has negative inotropic and dromotropic actions, reduces cardiac ischemia-reperfusion injury and remodeling, and prevents cardiac arrhythmias. In the vasculature, adenosine modulates vascular tone, reduces infiltration of inflammatory cells and generation of foam cells, and may prevent the development of atherosclerosis as a result. Modulation of insulin sensitivity may further add to the anti-atherosclerotic properties of adenosine signaling. In the kidney, adenosine plays an important role in tubuloglomerular feedback and modulates tubular sodium reabsorption. The challenge is to take advantage of the beneficial actions of adenosine signaling while preventing its potential adverse effects, such as salt retention and sympathoexcitation. Drugs that interfere with adenosine formation and elimination or drugs that allosterically enhance specific adenosine receptors seem to be most promising to meet this challenge.

A2 adenosine receptors and vascular pathologies

Cardiovascular disease, a leading cause of death and morbidity, is regulated, among various factors, by inflammation. The level of the metabolite adenosine is augmented under stress, including inflammatory, hypoxic, or injurious events. Adenosine has been shown to affect various physiological and pathological processes, largely through 1 or more of its 4 types of receptors: the A1 and A3 adenylyl cyclase inhibitory receptors and the A2A and A2B adenylyl cyclase stimulatory receptors. This article focuses on reviewing common and distinct effects of the 2 A2-type adenosine receptors on vascular disease and the mechanisms involved. Understanding the pathogenesis of vascular disease mediated by these receptors is important to the development of therapeutics and to the prevention and management of disease.

Therapeutic potential of A2 and A3 adenosine receptor: a review of novel patented ligands

INTRODUCTION

Adenosine exerts its effects by interacting with G-protein coupled receptors (GPCR) namely A(1), A(2A), A(2B) and A(3), respectively. These are involved in several diseases, for example and most importantly, Parkinson's disease, ischemia and inflammation. There is high interest in the development of potent and selective ligands for these adenosine receptor (AR) subtypes, primarily for their therapeutic potential but also as pharmacological tools in receptor studies.

AREAS COVERED

This paper concentrates on reviewing the therapeutic potential of A(2) and A(3) ARs, which represent the most interesting subtypes of recent years. A general description of each receptor is reported with novel agonist and antagonist structures, patented in 2008 - 2011. PubMed and Free Patents Online databases were principally used to collect all the material.

EXPERT OPINION

In the past years, by modulating A(2) and A(3)ARs, several new possible therapeutic applications were discovered. For this reason, research concerning AR ligands is still of great interest. In particular, few potent and selective A(2B) agonists and antagonists are actually reported and a clear SAR (structure-activity relationship) profile lacks for this AR subtype. At the A(3)AR, allosteric modulation may prevent problems related to the high difference between rat and human orthosteric sites and simplify the preclinical studies on A(3)AR.

Caffeine treatment disturbs the angiogenesis of zebrafish embryos

Caffeine is a widely consumed substance that occurs in numerous dietary sources, but teratogenic effects of caffeine intake during embryonic development are still not clear. In the present study, we used the zebrafish as a model to assess caffeine-induced toxicity on embryonic vascular development. A green fluorescent vascular endothelium transgenic line, Tg(fli1:egfp), was utilized for the sensitive detection of vascular development, including vasculo- and angiogenesis. Caffeine-treated embryos showed no defects in vasculogenesis, but revealed dose-dependent (250-350 ppm) developmental defects in intersegmental vessels, dorsal longitudinal anastomotic vessels, and subintestinal vein sprouting. Further, real-time polymerase chain reaction analysis of caffeine-treated embryos showed an upregulation of nrp1a along with a downregulation of sema3aa and sema3c. In conclusion, caffeine treatment induces defects of angiogenesis in zebrafish embryos.

Partial adenosine A1 receptor agonists for cardiovascular therapies

Adenosine, a purine nucleoside, is present in all cells in tightly regulated concentrations. It has many different physiological effects in the whole body and in the heart. Adenosine activates four G protein-coupled receptors A1, A2a, A2b, and A3. Activation of myocardial A1 receptors has been shown to inhibit a variety of myocardial pathologies associated with ischemia and reperfusion injury, including stunning, arrhythmogenesis, coronary and ventricular dysfunction, acute myocardial infarction, apoptosis, and chronic heart failure, implying several options for new cardiovascular therapies for diseases, like angina pectoris, control of cardiac rhythm, ischemic injury during an acute coronary syndrome, or heart failure. However, the main issue of using full A1 receptor agonists in such indications is the broad physiologic spectrum of cardiac and extracardiac effects. Desired A1 receptor-mediated protective and regenerative cardiovascular effects might be counter-regulated by unintended side effects when considering full A1 receptor agonists. These effects can be overcome by partial A1 agonists. Partial A1 agonists can be used to trigger only some of the physiological responses of receptor activation depending on endogenous adenosine levels and on receptor reserve in different tissues. CV-Therapeutics reported the identification of a partial A1 receptor agonist CVT-3619, and recently, another partial A1 receptor agonist VCP28 was published. Both compounds are adenosine derivatives. Adenosine-like A1 receptor agonists often have the drawback of a short half-life and low bioavailability, making them not suitable for chronic oral therapy. We identified the first non-adenosine-like partial A1 receptor agonist(s) with pharmacokinetics optimal for oral once daily treatment and characterized the qualities of the partial character of the A1 receptor agonist(s) in preclinical and clinical studies.

Cytotoxic purine nucleoside analogues bind to A1, A2A, and A3 adenosine receptors

Fludarabine, clofarabine, and cladribine are anticancer agents which are analogues of the purine nucleoside adenosine. These agents have been associated with cardiac and neurological toxicities. Because these agents are analogues of adenosine, they may act through adenosine receptors to elicit their toxic effects. The objective of this study was to evaluate the ability of cytotoxic nucleoside analogues to bind and activate adenosine receptor subtypes (A(1), A(2A), A(2B), and A(3)). Radioligand binding studies utilizing Chinese hamster ovary cells, stably transfected with adenosine A(1), A(2A), or A(3) receptor subtype, were used to assess the binding affinities of these compounds, whereas adenylyl cyclase activity was used to assess the binding to A(2B) receptors. Clofarabine and cladribine both bound to the A(2A) receptor with a K (i) of 17 and 15 μM, respectively. Clofarabine was the only adenosine analogue to bind to the A(3) receptor with a K (i) of 10 μM, and none of these compounds bound to the A(2B) receptor. Results show that clofarabine, cladribine, and fludarabine bind to the A(1) receptor. In addition, clofarabine, cladribine, and fludarabine were A(1) agonists (IC(50) 3.1, 30, and 30 μM, respectively). Neither pyrimidine nucleoside analogues gemcitabine nor cytarabine associated with any of the adenosine receptor subtypes (K (i) > 100μM). This is the first report of an interaction between all adenosine receptor subtypes and chemotherapeutic nucleoside analogues commonly used in the treatment of cancer. Therefore, activation of these receptors may be at least one mechanism through which fludarabine-associated toxicity occurs.

Recovery of the Cell Cycle Inhibition in CCl(4)-Induced Cirrhosis by the Adenosine Derivative IFC-305

Introduction. Cirrhosis is a chronic degenerative illness characterized by changes in normal liver architecture, failure of hepatic function, and impairment of proliferative activity. The aim of this study is to know how IFC-305 compound induces proliferation of the liver during reversion of cirrhosis. Methods. Once cirrhosis has been installed by CCl(4) treatment for 10 weeks in male Wistar rats, they were divided into four groups: two received saline and two received the compound; all were euthanized at 5 and 10 weeks of treatment. Liver homogenate, mitochondria, and nucleus were used to measure cyclins, CDKs, and cell cycle regulatory proteins PCNA, pRb, p53, E2F, p21, p27, HGF, liver ATP, and mitochondrial function. Results. Diminution and small changes were observed in the studied proteins in the cirrhotic animals without treatment. The IFC-305-treated rats showed a clear increase in most of the proteins studied mainly in PCNA and CDK6, and a marked increased in ATP and mitochondrial function. Discussion/Conclusion. IFC-305 induces a recovery of the cell cycle inhibition promoting recovery of DNA damage through the action of PCNA and p53. The increase in energy and preservation of mitochondrial function contribute to recovering the proliferative function.

A2b adenosine receptor regulates hyperlipidemia and atherosclerosis

BACKGROUND

The cAMP-elevating A(2b) adenosine receptor (A(2b)AR) controls inflammation via its expression in bone marrow cells.

METHODS AND RESULTS

Atherosclerosis induced by a high-fat diet in apolipoprotein E-deficient mice was more pronounced in the absence of the A(2b)AR. Bone marrow transplantation experiments indicated that A(2b)AR bone marrow cell signals alone were not sufficient to elicit this effect. Intriguingly, liver expression of the A(2b)AR in wild-type mice was vastly augmented by a high-fat diet, raising the possibility that this upregulation is of functional significance. A(2b)AR genetic ablation led to elevated levels of liver and plasma cholesterol and triglycerides and to fatty liver pathology typical of steatosis, assessed by enzymatic assays and analysis of liver sections. Western blotting and quantitative polymerase chain reaction revealed elevated expression of the following molecules in the liver of A(2b)AR-null mice: the transcription factor sterol regulatory element binding protein-1 (SREBP-1) and its 2 downstream targets and regulators of lipogenesis, acetyl CoA carboxylase and fatty acid synthase. Pharmacological activation or inhibition of A(2b)AR in primary hepatocytes confirmed the regulation of SREBP-1 by this receptor. A(2b)AR-mediated changes in cAMP were found to regulate levels of the transcriptionally active form of SREBP-1. Finally, adenovirally mediated restoration of the A(2b)AR in the liver of A(2b)AR-null mice reduced the lipid profile and atherosclerosis. Similarly, in vivo administration of the A(2b)AR ligand BAY 60-6853 in control mice on a high-fat diet reduced the lipid profile and atherosclerosis.

CONCLUSION

This study provides the first evidence that the A(2b)AR regulates liver SREBP-1, hyperlipidemia, and atherosclerosis, suggesting that this receptor may be an effective therapeutic target.

Polydeoxyribonucleotide restores blood flow in an experimental model of ischemic skin flaps

BACKGROUND

Ischemia is a major factor contributing to failure of skin flap surgery, which is routinely used for coverage of wounds to prevent infection and to restore form and function. An emerging concept is that adenosine A(2A) receptors can improve tissue oxygenation by stimulating angiogenesis, likely through vascular endothelial growth factor (VEGF). This study assessed the ability of polydeoxyribonucleotide (PDRN) to restore blood flow and improve wound healing, acting through the A(2A) receptor, in a rat model of ischemic skin flaps.

METHODS

The H-shaped double-flap model was used in male Sprague-Dawley rats. After surgical procedures, the animals were randomized to receive intraperitoneal PDRN (8 mg/kg) or vehicle (NaCl 0.9%). Rats were euthanized 3, 5, and 10 days after skin injury, after the evaluation of skin perfusion by laser Doppler. The wounds underwent histologic analysis and were measured for VEGF messenger RNA and protein expression, hypoxia inducible factor-1-α (HIF-1α), and inducible nitric oxide synthase (iNOS) protein expression, and nitrite content.

RESULTS

Blood flow markedly increased in blood flow in ischemic flaps treated with PDRN, with a complete recovery starting from day 5 (ischemic flap + vehicle, 1.80 ± 0.25; ischemic flap + PDRN, 2.46 ± 0.25; P < .001). Administration of PDRN enhanced the expression of VEGF (ischemic flap + vehicle, 5.3 ± 0.6; ischemic flap + PDRN, 6.2 ± 0.5; P < .01) at day 5, and iNOS (ischemic flap + vehicle, 3.9 ± 0.6; ischemic flap + PDRN, 5.3 ± 1; P < .01), but reduced HIF-1α expression (ischemic flap + vehicle, 7 ± 1.1; ischemic flap + PDRN, 4.8 ± 0.5; P < .05) at day 3. Histologically, the PDRN-treated group showed complete re-epithelialization and well-formed granulation tissue rich in fibroblasts.

CONCLUSIONS

These results suggest that PDRN restores blood flow and tissue architecture, probably by modulating HIF-1α and VEGF expression, and may be an effective therapeutic approach in improving healing of ischemic skin flaps.

Adenosine A₂A agonist improves lung function during ex vivo lung perfusion

BACKGROUND

Ex vivo lung perfusion (EVLP) is a novel technique than can be used to assess and potentially repair marginal lungs that may otherwise be rejected for transplantation. Adenosine has been shown to protect against pulmonary ischemia-reperfusion (IR) injury through its A(2A) receptor. We hypothesized that combining EVLP with adenosine A(2A) receptor agonist treatment would enhance lung functional quality and increase donor lung use.

METHODS

Eight bilateral pig lungs were harvested and flushed with cold Perfadex (Vitrolife, Englewood, CO). After 14 hours of storage at 4°C, EVLP was performed for 5 hours on 2 explanted lung groups: (1) control group lungs (n = 4) were perfused with Steen Solution (Vitrolife) and dimethyl sulfoxide and (2) treated group lungs (n = 4) received 10 μM CGS21680, a selective A(2A) receptor agonist, in a Steen solution-primed circuit. Lung histologic features, tissue cytokines, gas analysis, and pulmonary function were compared between groups.

RESULTS

Treated lungs demonstrated significantly less edema as reflected by wet-dry weight ratio (6.6 versus 5.2; p < 0.03) and confirmed by histologic examination. In addition, treated lung demonstrated significantly lower levels of interferon-γ (IFN- γ) (45.1 versus 88.5; p < 0.05). Other measured tissue cytokine levels (interleukin [IL]-1β, IL-6, and IL-8) were lower in the treatment group, but values failed to reach statistical significance. The oxygenation index was improved in the treated group (1.5 versus 2.3; p < 0.01) as was mean airway pressure (10.3 versus 13; p < 0.009).

CONCLUSIONS

Combined use of adenosine A(2A) agonist and EVLP significantly attenuates the inflammatory response in acutely injured lungs after IR and enhances pulmonary function. This combination may improve donor lung quality and could increase the donor lung pool for transplantation.

Adenosine receptor targeting in health and disease

INTRODUCTION

The adenosine receptors A(1), A(2A), A(2B) and A(3) are important and ubiquitous mediators of cellular signaling that play vital roles in protecting tissues and organs from damage. In particular, adenosine triggers tissue protection and repair by different receptor-mediated mechanisms, including increasing the oxygen supply:demand ratio, pre-conditioning, anti-inflammatory effects and the stimulation of angiogenesis.

AREAS COVERED

The state of the art of the role of adenosine receptors which have been proposed as targets for drug design and discovery, in health and disease, and an overview of the ligands for these receptors in clinical development.

EXPERT OPINION

Selective ligands of A(1), A(2A), A(2B) and A(3) adenosine receptors are likely to find applications in the treatment of pain, ischemic conditions, glaucoma, asthma, arthritis, cancer and other disorders in which inflammation is a feature. The aim of this review is to provide an overview of the present knowledge regarding the role of these adenosine receptors in health and disease.

Elevated uric acid correlates with wound severity

Chronic venous leg ulcers are a major health issue and represent an often overlooked area of biomedical research. Nevertheless, it is becoming increasingly evident that new approaches to enhance healing outcomes may arise through better understanding the processes involved in the formation of chronic wounds. We have for the first time shown that the terminal purine catabolite uric acid (UA) is elevated in wound fluid (WF) from chronic venous leg ulcers with relative concentrations correlating with wound chronicity. We have also shown a corresponding depletion in UA precursors, including adenosine, with increased wound severity. Further, we have shown that xanthine oxidase, the only enzyme in humans that catalyses the production of UA in conjunction with a burst of free radicals, is active in chronic WF. Taken together, this provides compelling evidence that xanthine oxidase may play a critical role in the formation of chronic wounds by prolonging the inflammatory process.

Interplay of hypoxia and A2B adenosine receptors in tissue protection

That adenosine signaling can elicit adaptive tissue responses during conditions of limited oxygen availability (hypoxia) is a long-suspected notion that recently gained general acceptance from genetic and pharmacologic studies of the adenosine signaling pathway. As hypoxia and inflammation share an interdependent relationship, these studies have demonstrated that adenosine signaling events can be targeted to dampen hypoxia-induced inflammation. Here, we build on the hypothesis that particularly the A(2B) adenosine receptor (ADORA(2B)) plays a central role in tissue adaptation to hypoxia. In fact, the ADORA(2B) requires higher adenosine concentrations than any of the other adenosine receptors. However, during conditions of hypoxia or ischemia, the hypoxia-elicited rise in extracellular adenosine is sufficient to activate the ADORA(2B). Moreover, several studies have demonstrated very robust induction of the ADORA(2B) elicited by transcriptional mechanisms involving hypoxia-dependent signaling pathways and the transcription factor "hypoxia-induced factor" 1. In the present chapter, genetic and pharmacologic evidence is presented to support our hypothesis of a tissue protective role of ADORA(2B) signaling during hypoxic conditions, including hypoxia-elicited vascular leakage, organ ischemia, or acute lung injury. All these disease models are characterized by hypoxia-elicited tissue inflammation. As such, the ADORA(2B) has emerged as a therapeutic target for dampening hypoxia-induced inflammation and tissue adaptation to limited oxygen availability.