Wound healing is accelerated by agonists of adenosine A2 (G alpha s-linked) receptors

Adenosine A(2A) receptors promote collagen production by a Fli1- and CTGF-mediated mechanism

Introduction

Adenosine, acting through the A_2A receptor, promotes tissue matrix production in the skin and the liver and induces the development of dermal fibrosis and cirrhosis in murine models. Since expression of A_2A receptors is increased in scleroderma fibroblasts, we examined the mechanisms by which the A_2A receptor produces its fibrogenic effects.

Methods

The effects of A_2A receptor ligation on the expression of the transcription factor, Fli1, a constitutive repressor for the synthesis of matrix proteins, such as collagen, is studied in dermal fibroblasts. Fli1 is also known to repress the transcription of CTGF/CCN2, and the effects of A_2A receptor stimulation on CTGF and TGF-β1 expression are also examined.

Results

A_2A receptor occupancy suppresses the expression of Fli1 by dermal fibroblasts. A_2A receptor activation induces the secretion of CTGF by dermal fibroblasts, and neutralization of CTGF abrogates the A_2A receptor-mediated enhancement of collagen type I production. A_2AR activation, however, resulted in a decrease in TGF-β1 protein release.

Conclusions

Our results suggest that Fli1 and CTGF are important mediators of the fibrogenic actions of adenosine and the use of small molecules such as adenosine A_2A receptor antagonists may be useful in the therapy of dermal fibrosis in diseases such as scleroderma.

Trophic effects of polynucleotides and hyaluronic acid in the healing of venous ulcers of the lower limbs: a clinical study

The aim of this study was to evaluate the results of treatment of venous lower limbs ulcers through the topical application of polynucleotides and hyaluronic acid gel (PNHA): Nucliaskin S™ (Mastelli srl, San Remo, Italy). This study was carried out in 39 consecutive patients who were randomly allocated to two groups: group I (20 patients) received treatment with PNHA (topical gel application two times a week, for a total of 6 weeks); group II (19 patients) received only hyaluronic acid (HA) topical application. All patients received a surgical debridement of the ulcerative lesions before topical treatment with PNHA or HA. Pre-treatment data indicated the area of ulceration. The number of healed ulcers and the variation in area of ulceration were considered as endpoints. The endpoints were observed after 45 days from the beginning of treatment. Complete wound healing occurred in 60% of limbs of group I and in 22% of those of group II patients. The average area reduction was 67% versus 34% in patients of group I and II, respectively. No side effects were recorded in both groups. Our experience shows that PNHA has an elevated trophic effect and speeds the healing rate of venous lower limb ulcers. This treatment may be a valid option in clinical practice.

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.

Molecular dynamics simulations of the adenosine A2a receptor in POPC and POPE lipid bilayers: effects of membrane on protein behavior

Analysis of 300 ns (ns) molecular dynamics (MD) simulations of an adenosine A2a receptor (A2a AR) model, conducted in triplicate, in 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and 1-palmitoyl-2-oleoylphosphatidylethanolamine (POPE) bilayers reveals significantly different protein dynamical behavior. Principal component analysis (PCA) shows that the dissimilarities stem from interhelical rather than intrahelical motions. The difference in the hydrophobic thicknesses of these simulated lipid bilayers is potentially a significant reason for the observed difference in results. The distinct lipid headgroups might also lead to different molecular interactions and hence different protein loop motions. Overall, the A2a AR shows higher mobility and flexibility in POPC as compared to POPE.

Adenosine 2A receptor promotes collagen production by human fibroblasts via pathways involving cyclic AMP and AKT but independent of Smad2/3

Activation of adenosine A2A receptor (A2AR) promotes fibrosis and collagen synthesis. However, the underlying mechanism is still unclear, not least because cAMP, its principal effector, has been found to inhibit TGFβ1-induced collagen synthesis. Here, we show that in primary normal human dermal fibroblasts, A2AR stimulation with CGS21680 elicits a modest cAMP increase (150 ± 12% of control; EC50 54.8 nM), which stimulates collagen1 (Col1) and collagen3 (Col3), but maximal cAMP resulting from direct activation of adenylyl cyclase by forskolin (15,689 ± 7038% of control; EC50 360.7 nM) inhibits Col1 and increases Col3. Similar to Col1 expression, fibroblast proliferation increased following physiological cAMP increases by CGS21680 but was inhibited by cAMP increases beyond the physiological range by forskolin. The A2AR-mediated increase of Col1 and Col3 was mediated by AKT, while Col3, but not Col1, expression was dependent on p38 and repressed by ERK. TGFβ1 induced phosphorylation of Smad2/3 and increased Col3 expression, which was prevented by Smad3 depletion. In contrast, CGS21680 did not activate Smad2/3, and Smad2/3 knockdown did not prevent CGS21680-induced Col1 or Col3 increases. Our results indicate that cAMP is a concentration-dependent switch for collagen production via noncanonical, AKT-dependent, Smad2/3-independent signaling. These observations explain the paradoxical effects of cAMP on collagen expression.

Intracellular ATP concentration contributes to the cytotoxic and cytoprotective effects of adenosine

Extracellular adenosine (Ade) interacts with cells by two pathways: by activating cell surface receptors at nanomolar/micromolar concentrations; and by interfering with the homeostasis of the intracellular nucleotide pool at millimolar concentrations. Ade shows both cytotoxic and cytoprotective effects; however, the underlying mechanisms remain unclear. In the present study, the effects of adenosine-mediated ATP on cell viability were investigated. Adenosine treatment was found to be cytoprotective in the low intracellular ATP state, but cytotoxic under the normal ATP state. Adenosine-mediated cytotoxicity and cytoprotection rely on adenosine-derived ATP formation, but not via the adenosine receptor pathway. Ade enhanced proteasome inhibition-induced cell death mediated by ATP generation. These data provide a new pathway by which adenosine exerts dual biological effects on cell viability, suggesting an important role for adenosine as an ATP precursor besides the adenosine receptor pathway.

Transcriptomic profiling of the four adenosine receptors in human leukocytes of heart failure patients

In this study the transcriptomic profiling of adenosine receptors (ARs) in human leukocytes of heart failure (HF) patients as a function of clinical severity, assessing the possible changes with respect to healthy subjects (C), was evaluated. Total RNA was extracted from leukocytes of C (n = 8) and of HF patients (NYHA I-II n = 9; NYHA III-IV n = 14) with a PAXgene Blood RNA Kit. An increase as a function of clinical severity was observed in each AR (A₁R: C = 0.02 ± 0.009, NYHA I-II = 0.21 ± 0.09, NYHA III-IV = 3.6 ± 1.3, P = 0.03  C versus NYHA III-IV, P = 0.02 NYHA I-II versus NYHA III-IV; A_2aR: C = 0.2 ± 0.05, NYHA I-II = 0.19 ± 0.04, NYHA III-IV = 1.32 ± 0.33, P = 0.005  C versus NYHA III-IV, P = 0.003 NYHA I-II versus NYHA III-IV; A_2bR: C = 1.78 ± 0.36, NYHA I-II = 1.35 ± 0.29, NYHA III-IV = 4.07 ± 1.21, P = 0.03: NYHA I-II versus NYHA III-IV; A₃R: C = 0.76 ± 0.21, NYHA I-II = 0.94 ± 0.19, NYHA III-IV = 3.14 ± 0.77, P = 0.01  C versus NYHA III-IV and NYHA I-II versus NYHA III-IV, resp.). The mRNA expression of the ectonucleoside triphosphate diphosphohydrolase (CD39) and the ecto-5′-nucleotidase (CD73) were also evaluated. They resulted up-regulated. These findings show that components of adenosine metabolism and signalling are altered to promote adenosine production and signalling in HF patients. Thus, HF may benefit from adenosine-based drug therapy after confirmation by clinical trials.

Adenosine A2A receptor (A2AR) is a fine-tune regulator of the collagen1:collagen3 balance

Adenosine is a potent endogenous anti-inflammatory and immunosuppressive metabolite that is a potent modulator of tissue repair. However, the adenosine A2A receptor (A2AR)-mediated promotion of collagen synthesis is detrimental in settings such as scarring and scleroderma. The signaling cascade from A2AR stimulation to increased collagen production is complex and obscure, not least because cAMP and its downstream molecules PKA and Epac1 have been reported to inhibit collagen production. We therefore examined A2AR-stimulated signaling for collagen production by normal human dermal fibroblasts (NHDF). Collagen1 (Col1) and collagen3 (Col3) content after A2AR activation by CGS21680 was studied by western blotting. Contribution of PKA and Epac was analyzed by the PKA inhibitor PKI and by knockdowns of the PKA-Cα, -Cβ, -Cγ, Epac1, and Epac2. CGS21680 stimulates Col1 expression at significantly lower concentrations than those required to stimulate Col3 expression. A2AR stimulates Col1 expression by a PKA-dependent mechanism since PKA inhibition or PKA-Cα and -Cβ knockdown prevents A2AR-mediated Col1 increase. In contrast, A2AR represses Col3 via PKA but stimulates both Col1 and Col3 via an Epac2-dependent mechanism. A2AR stimulation with CGS21680 at 0.1 μM increased Col3 expression only upon PKA blockade. A2AR activation downstream signaling for Col1 and Col3 expression proceeds via two distinct pathways with varying sensitivity to cAMP activation; more highly cAMP-sensitive PKA activation stimulates Col1 expression, and less cAMP-sensitive Epac activation promotes both Col1 and Col3 expression. These observations may explain the dramatic change in Col1:Col3 ratio in hypertrophic and immature scars, where adenosine is present in higher concentrations than in normal skin.

The effects of polydeoxyribonucleotide on the survival of random pattern skin flaps in rats

Background

Partial or complete necrosis of a skin flap is a common problem. Polydeoxyribonucleotide (PDRN) can be extracted from trout sperm and used as a tissue repair agent. The aim of this study was to investigate whether PDRN could improve the survival of random pattern skin flaps in rats.

Methods

Twenty-two male Sprague-Dawley rats were randomly divided into two groups: the PDRN treatment group (n=11) and the control group (n=11). Caudally pedicled random pattern skin flaps were elevated on their dorsal skin and resutured. The treatment group received daily intraperitoneal administration of PDRN (8 mg/kg/day), and the control group received fluid vehicle (NaCl 0.9%, 8 mg/kg/day) from day 0 to day 6. On day 7, the flap survival was evaluated and the harvested tissue surrounding the demarcation line of the necrotic area was stained with H&E, anti-rat vascular endothelial cell growth factor (VEGF) antibody, and PECAM-1/CD31 antibody.

Results

The average necrotic area of the flap in the PDRN group was significantly smaller when compared with that of the control group. Histologic and immunohistochemical evaluation showed that granulation thickness score and VEGF-positive staining cells were marked higher in the PDRN group than in the control group. PECAM-1/CD31-positive microvascular densities were significantly higher in the PDRN group when compared with the control group.

Conclusions

This study confirms that PDRN improves the survival of random pattern skin flaps in rats. These results may represent a new therapeutic approach to enhancing flap viability and achieving faster wound repair.

Caffeine-induced endothelial cell death and the inhibition of angiogenesis

Numerous studies have shown that adenosine or adenosine agonists can stimulate angiogenesis. However, the effect of caffeine (a known adenosine receptor antagonist) on angiogenesis has not been previously studied. Accordingly, this study was undertaken to examine the effect of caffeine on angiogenesis and to clarify the mechanism involved. Chick chorioallantoic membrane assays were used to investigate the effect of caffeine on angiogenesis and proliferation assays using human umbilical vein endothelial cells (HUVECs), were used to study its effects on specific aspects of angiogenesis. The expressions of caspase-3 and Bcl-2 were examined by western blotting, immunofluorescence staining was used to identify HUVEC morphological changes, and fluorescence activated cell sorting (FACS) and DAPI staining were used to detect HUVEC apoptosis. Caffeine was found to inhibit blood vessel formation dose-dependently and to inhibit the proliferation of HUVECs time- and dose-dependently. FACS analysis and DAPI staining showed that inhibitory effect of caffeine on HUVEC proliferation was the result of apoptosis and the up-regulation of thrombospondin-1 (TSP-1). Furthermore, TSP-1 levels were down-regulated by NECA but were unaffected by CGS21680, indicating that caffeine regulated TSP-1 expression via adenosine A2B receptor. In addition, caffeine up-regulated caspase-3 and down-regulated Bcl-2 at the protein level. These results suggest that the inhibitory effect of caffeine on angiogenesis is associated, at least in part, with its induction of endothelial cell apoptosis, probably mediated by a caspase-3 dependent mechanism.

Targeting the hypoxia-adenosinergic signaling pathway to improve the adoptive immunotherapy of cancer

The recent approval by the FDA of cancer vaccines and drugs that blockade immunological negative regulators has further enhanced interest in promising approaches of the immunotherapy of cancer. However, the disappointingly short life extension has also underscored the need to better understand the mechanisms that prevent tumor rejection and survival even after the blockade of immunological negative regulators. Here, we describe the implications of the "metabolism-based" immunosuppressive mechanism, where the local tissue hypoxia-driven accumulation of extracellular adenosine triggers suppression via A2 adenosine receptors on the surface of activated immune cells. This molecular pathway is of critical importance in mechanisms of immunosuppression in inflamed and cancerous tissue microenvironments. The protection of tumors by tumor-generated extracellular adenosine and A2 adenosine receptors could be the misguided application of the normal tissue-protecting mechanism that limits excessive collateral damage to vital organs during the anti-pathogen immune response. The overview of the current state of the art regarding the immunosuppressive effects of extracellular adenosine is followed by a historical perspective of studies focused on the elucidation of the physiological negative regulators that protect tissues of vital organs from excessive collateral damage, but, as a trade-off, may also weaken the anti-pathogen effector functions and negate the attempts of anti-tumor immune cells to destroy cancerous cells.

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.

Anti-gastritis and wound healing effects of Momordicae Semen extract and its active component

Momordicae Semen, Momordica cochinchinensis Springer (Cucurbitaceae), has long been known to effectively relieve boils, rheumatic pain, and hemorrhoids. In this study, we investigated whether Momordicae Semen extract (MSE) has anti-gastritis effects in various rodent models and also explored possible mechanisms for the gastroprotective effects of MSE. MSE provided remarkable protective effects, comparable to those of rebamipide, in ethanol- and diclofenac-induced acute gastritis. In addition, it has demonstrated protective effect in a Helicobacter pylori-insulted chronic gastritis model. MSE also showed wound healing effect on cutaneous injury of mice and stimulated calcitonin gene-related peptide and somatostatin receptors, which may be related to its anti-gastritis effects. In a single oral dose toxicity study, the approximate lethal dose of MSE was determined at >2000 mg/kg/day. The NOAEL was set to be 2000 mg/kg/day from the repeated oral dose toxicity study. Moreover, momordica saponin I, a major ingredient of MSE, treatment decreased gastric mucosa damage indices in the ethanol- and diclofenac-induced acute gastritis models. The results suggest that MSE could be a promising gastroprotective herbal medicine and momordica saponin I might be used as an active marker compound for MSE.

Pharmacological blockade of adenosine A2A receptors diminishes scarring

Adenosine A2A receptor (A2AR) stimulation promotes wound healing and is required for the development of fibrosis in murine models of scleroderma and cirrhosis. Nonetheless, the role of A2AR in the formation of scars following skin trauma has not been explored. Here, we examined the effect of pharmacological blockade of A2AR, with the selective adenosine A2AR-antagonist ZM241385 (2.5 mg/ml), in a murine model of scarring that mimics human scarring. We found that application of the selective adenosine A2AR antagonist ZM241385 decreased scar size and enhanced the tensile strength of the scar. Within the scar itself, collagen alignment and composition (marked reduction in collagen 3), but not periostin, biglycan, or fibronectin accumulation, was improved by application of ZM241385. Moreover, A2AR blockade reduced the number of myofibroblasts and angiogenesis but not macrophage infiltration in the scar. Taken together, our work strongly suggests that pharmacological A2AR blockade can be used to diminish scarring while improving the collagen composition and tensile strength of the healed wound.

Fibrocytes in health and disease

Fibrocytes, a group of bone marrow-derived mesenchymal progenitor cells, were first described in 1994 as fibroblast-like, peripheral blood cells that migrate to regions of tissue injury. These cells are unique in their expression of extracellular matrix proteins concomitantly with markers of hematopoietic and monocyte lineage. The involvement of fibrocytes and the specific role they play in the process of wound repair has been a focus of study since their initial description. Fibrocytes contribute to the healing repertoire via several mechanisms; they produce a combination of cytokines, chemokines, and growth factors to create a milieu favorable for repair to occur; they serve as antigen presenting cells (APCs); they contribute to wound closure; and, they promote angiogenesis. Furthermore, regulatory pathways involving serum amyloid P, leukocyte-specific protein 1, and adenosine A_2A receptors have emphasized the significant role that fibrocytes have in wound healing and fibrosis. The therapeutic targeting of fibrocytes holds promise for the augmentation of wound repair and the treatment of different fibrosing disorders.

Effects of dose ranging of adenosine infusion on electrocardiographic findings during and after general anesthesia

PURPOSE

To assess changes in the electrocardiogram (ECG) associated with intraoperative infusion of adenosine in patients undergoing open abdominal gynecological surgery.

METHODS

One hundred and sixty-six patients undergoing gynecological surgery were randomly assigned to receive one of four doses of adenosine infusion (25, 50, 100, or 200 μg/kg/min) or matching placebo. Study drug administration was started at skin incision and discontinued at end of surgery. A standardized general anesthetic regimen was used and adjusted based on hemodynamic and bispectral index values. Heart rate and rhythm variables, and PR, QRS, QT, and QTc intervals were recorded from 12-lead ECGs before anesthesia and immediately after patient arrival in the postanesthesia care unit. In addition, a rhythm strip was obtained during administration of the loading dose of the study drug. ECG variables were compared within and between groups. Incidence of ECG and hemodynamic abnormalities was recorded.

RESULTS

One hundred and fifty-one subjects had a full set of electrocardiographic data: placebo (n = 38), group adenosine 25 μg/kg/min (n = 31), group adenosine 50 μg/kg/min (n = 29), group adenosine 100 μg/kg/min (n = 28), and group adenosine 200 μg/kg/min (n = 25). Statistically significant postoperative QTc prolongation was observed in all study groups when compared with baseline except for the adenosine 200 μg/kg/min group. However, these changes from baseline were not different among the groups. There were also no significant differences in PR, QRS, and QT intervals between the treatment groups.

CONCLUSION

There was no difference in QTc prolongation following intraoperative administration of adenosine infusion compared with placebo during isoflurane general anesthesia. However, QTc prolongation is common following general anesthesia.

G protein-coupled receptors as therapeutic targets for multiple sclerosis

G protein-coupled receptors (GPCRs) mediate most of our physiological responses to hormones, neurotransmitters and environmental stimulants. They are considered as the most successful therapeutic targets for a broad spectrum of diseases. Multiple sclerosis (MS) is an inflammatory disease that is characterized by immune-mediated demyelination and degeneration of the central nervous system (CNS). It is the leading cause of non-traumatic disability in young adults. Great progress has been made over the past few decades in understanding the pathogenesis of MS. Numerous data from animal and clinical studies indicate that many GPCRs are critically involved in various aspects of MS pathogenesis, including antigen presentation, cytokine production, T-cell differentiation, T-cell proliferation, T-cell invasion, etc. In this review, we summarize the recent findings regarding the expression or functional changes of GPCRs in MS patients or animal models, and the influences of GPCRs on disease severity upon genetic or pharmacological manipulations. Hopefully some of these findings will lead to the development of novel therapies for MS in the near future.

Anti-inflammatory effects of purine nucleosides, adenosine and inosine, in a mouse model of pleurisy: evidence for the role of adenosine A2 receptors

Adenosine and its metabolite, inosine, have been described as molecules that participate in regulation of inflammatory response. The aim of this study was to investigate the effect of adenosine and inosine in a mouse model of carrageenan-induced pleurisy as well as the participation of adenosine receptors in this response. Injection of carrageenan into the pleural cavity induced an acute inflammatory response characterized by leukocyte migration, pleural exudation, and increased release of interleukin-1β and tumor necrosis factor-α in pleural exudates. The treatment with adenosine (0.3-100 mg/kg, i.p.) and inosine (0.1-300 mg/kg, i.p.) 30 min before carrageenan injection reduced significantly all these parameters analyzed. Our results also demonstrated that A(2A) and A(2B) receptors seem to mediate the adenosine and inosine effects observed, since pretreatment with selective antagonists of adenosine A(2A) (ZM241385) and A(2B) (alloxazine) receptors, reverted the inhibitory effects of adenosine and inosine in pleural inflammation. The involvement of A(2) receptors was reinforced with adenosine receptor agonist CGS21680 treatment, since its anti-inflammatory effects were reversed completely and partially with ZM241385 and alloxazine injection, respectively. Moreover, the combined treatment with subeffective dose of adenosine (0.3 mg/kg) and inosine (1.0 mg/kg) induced a synergistic anti-inflammatory effect. Thus, based on these findings, we propose that inosine contributes with adenosine to exert anti-inflammatory effects in pleural inflammation, reinforcing the notion that endogenous nucleosides play an important role in controlling inflammatory diseases. This effect is likely mediated by the activation of adenosine A(2) subtype receptors and inhibition of production or release of pro-inflammatory cytokines.

Circulating fibrocytes--biology and mechanisms in wound healing and scar formation

Fibrocytes were first described in 1994 as fibroblast-like, peripheral blood cells. These bone marrow-derived mesenchymal progenitor cells migrate into regions of tissue injury. They are unique in their expression of hematopoietic and monocyte lineage markers and extracellular matrix proteins. Several studies have focused on the specific role of fibrocytes in the process of wound repair and tissue regeneration. We discuss herein the biology and mechanistic action of fibrocytes in wound healing, scar formation, and maintenance of tissue integrity. Fibrocytes synthesize and secrete different cytokines, chemokines, and growth factors, providing a wound milieu that supports tissue repair. They further promote angiogenesis and contribute to wound closure via pathways involving specific cytokines, leukocyte-specific protein-1, serum amyloid P, and adenosine A(2A) receptors. Fibrocytes are involved in inflammatory fibrotic processes in such diseases as systemic fibrosis, atherosclerosis, asthma, hypertrophic scarring, and keloid formation. Accumulating literature has emphasized the important role of fibrocytes in wound healing and fibrosis. Detailed mechanisms nevertheless remain to be investigated to elucidate the full therapeutic potential of fibrocytes in the treatment of fibrosing disorders and the enhancement of tissue repair.

Enhancement of tumor immunotherapy by deletion of the A2A adenosine receptor

The A(2A) adenosine receptor plays a critical and non-redundant role in suppressing inflammation at sites of hypoxia and tissue damage. The tumor microenvironment has high levels of adenosine as a result of hypoxia and ectopic expression of enzymes responsible for the generation of extracellular adenosine. Thus, we sought to determine the ability of A(2A) receptor null mice to immunologically reject tumors. We observed that mice lacking the A(2A) adenosine receptor showed significantly delayed growth of lymphoma cells when compared to WT mice. Furthermore, when immunized with a low dose of tumor or with an irradiated GM-CSF-secreting tumor vaccine, A(2A) receptor null mice showed significantly enhanced protection from a subsequent high-dose challenge from both immunogenic and poorly immunogenic tumor lines. This increase in protection was accompanied by an increase in the number of tumor-antigen-specific CD8 T cells at the vaccine-site draining lymph node. Finally, we found that A(2A) receptor null mice displayed more robust anti-tumor responses than WT mice when they were treated with a soluble B7-DC/Fc fusion protein designed to antagonize B7-H1-mediated co-inhibition. This combinatorial immunotherapy strategy could also be recapitulated with pharmacological A(2A) receptor blockade paired with B7-DC/Fc administration. In light of these data, we believe that blockade of the A(2A) adenosine receptor is an attractive target for tumor immunotherapy that synergizes with other immunomodulatory approaches currently in clinical trials.

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.

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.

Adenosine receptors and fibrosis: a translational review

Adenosine—a purine nucleoside generated extracellularly from adenine nucleotides released by cells as a result of direct stimulation, hypoxia, trauma, or metabolic stress—is a well-known physiologic and pharmacologic agent. Recent studies demonstrate that adenosine, acting at its receptors, promotes wound healing by stimulating both angiogenesis and matrix production. Subsequently, adenosine and its receptors have also been found to promote fibrosis (excess matrix production) in the skin, lungs, and liver, but to diminish cardiac fibrosis. A commonly ingested adenosine receptor antagonist, caffeine, blocks the development of hepatic fibrosis, an effect that likely explains the epidemiologic finding that coffee drinking, in a dose-dependent fashion, reduces the likelihood of death from liver disease. Accordingly, adenosine may be a good target for therapies that prevent fibrosis of the lungs, liver, and skin.

Adenosine receptor subtype expression and activation influence the differentiation of mesenchymal stem cells to osteoblasts and adipocytes

Osteoblasts and adipocytes differentiate from a common precursor cell, the mesenchymal stem cell (MSC). Adenosine is known to signal via four adenosine receptor subtypes, and significantly, recent findings indicate that these may play a role in MSC differentiation. We therefore investigated adenosine receptor expression and activation during the differentiation of MSCs to osteoblasts and adipocytes. The A(2B) R was dominant in MSCs, and its expression and activity were transiently upregulated at early stages of osteoblastic differentiation. Both activation and overexpression of A(2B) R induced the expression of osteoblast-related genes [Runx2 and alkaline phosphatase (ALP)], as well as ALP activity, and stimulation increased osteoblast mineralization. The expression of A(2A) R was upregulated during later stages of osteoblastic differentiation, when its activation stimulated ALP activity. Differentiation of MSCs to adipocytes was accompanied by significant increases in A(1) R and A(2A) R expression, and their activation was associated with increased adipogenesis. Enhanced A(2A) R expression was sufficient to promote expression of adipocyte-related genes (PPARγ and C/EBPα), and its activation resulted in increased adipocytic differentiation and lipid accumulation. In contrast, the A(1) R was involved mainly in lipogenic activity of adipocytes rather than in their differentiation. These results show that adenosine receptors are differentially expressed and involved in lineage-specific differentiation of MSCs. We conclude, therefore, that fruitful strategies for treating diseases associated with an imbalance in the differentiation and function of these lineages should include targeting adenosine receptor signal pathways. Specifically, these research avenues will be useful in preventing or treating conditions with insufficient bone or excessive adipocyte formation.

Adenosine A2A receptor polymorphisms in Korean patients with systemic sclerosis

Adenosine A2A receptor (ADORA2A) regulates inflammation, promotes tissue repair and collagen production by human dermal fibroblasts. We investigated the genetic polymorphisms of ADORA2A in susceptibility to systemic sclerosis (SSc). We genotyped 142 Korean SSc patients and 150 controls for polymorphisms of -1751A/C (rs5996696) and 1976C/T (rs5751876), to cover the promoter and all exon sequences of ADORA2A in Koreans, using TaqMan fluorogenic 5' nuclease assay and single base primer extension assay. Neither -1751A/C nor 1976C/T polymorphism showed difference in the distribution of alleles or genotypes between patients and controls with allele frequency of 89.9% v 91.0% for -1751A (p=0.64) and 56.5% v 54.0% for 1976C (p=0.55). Our findings suggest that the role of ADORA2A in SSc may not be genetically related.

Adenosine A(2A) receptor activation prevents progressive kidney fibrosis in a model of immune-associated chronic inflammation

Crescentic glomerulonephritis (GN) in Wistar-Kyoto rats progresses to lethal kidney failure by macrophage (Mφ)-mediated mechanisms. Mφs in nephritic glomeruli express adenosine A(2A) receptors (A(2A)Rs), the activation of which suppresses inflammation. Here, we pharmacologically activated the A(2A)Rs with a selective agonist, CGS 21680, and inactivated them with a selective antagonist, ZM241385, to test the effects on established GN. When activation was delayed until antiglomerular basement membrane GN and extracellular matrix deposition were established, glomerular Mφ infiltration was reduced by 83%. There was also a marked improvement in glomerular lesion histology, as well as decreased proteinuria. A(2A)R activation significantly reduced type I, III, and IV collagen deposition, and E-cadherin expression was restored in association with a reduction of α-smooth muscle actin-positive myofibroblasts in the interstitium and glomeruli. In contrast, pharmacological inactivation of A(2A)Rs increased glomerular crescent formation, type I, III, and IV collagen expression, and enhanced E-cadherin loss. Activation of A(2A)Rs suppressed the expression of the Mφ-linked glomerular damage mediators, transforming growth factor-β, osteopontin-1, thrombospondin-1, and tissue inhibitor of metalloproteinase-1. Thus, A(2A)R activation can arrest GN and prevent progressive fibrosis in established pathological lesions.

Wound-healing properties of nut oil from Pouteria lucuma

BACKGROUND

Cell migration, angiogenesis, inflammation, and extracellular matrix remodeling are key events in wound healing. Natural products, including fatty acids (FAs), can accelerate wound healing by modulating the aforementioned events.

AIMS


 This study aims to evaluate the effect of lucuma (Pouteria lucuma O Kezte) nut oil (LNO) on fibroblasts migration, angiogenesis, inflammation, bacterial and fungal growth, and wound healing. Methods  GC-MS analysis of FAs methyl esters (FAMES) was used for chemical characterization of LNO. In vitro studies were carried out with LNO investigating the induction of cell migration, cytoskeleton remodeling of human fibroblasts, inhibition of LPS-induced nitric oxide production in macrophages, and antibacterial and antifungal effects. Two in vivo studies were carried out to study LNO's effect on angiogenesis and wound healing: (i) tail fin regeneration in transgenic zebrafish larvae expressing enhanced green fluorescent protein (EGFP) in vascular endothelial cells was used to study vessel sprouting and wound healing and (ii) the closure of wounds was evaluated in CD-1 mice after topical applications of LNO-containing formulations.

RESULTS

Lucuma nut oil is a mixture of FAs, 99.7% of which were characterized. Major components of LNO (w/w) are linoleic acid (38.9%), oleic acid (27.9%), palmitic acid (18.6%), stearic acid (8.9%), and γ linolenic acid (2.9%). In vitro studies showed that LNO significantly promoted migration and vinculin expression in human fibroblasts. LNO decreased LPS-induced nitric oxide production and did not display significant antibacterial or antifungal effects. LNO induced tail fin regeneration in transgenic zebrafish larvae 48 h after tail fin amputation and significantly accelerated cutaneous wound closure in CD-1 mice.

CONCLUSIONS

Natural FAs from P. lucuma nut promote skin regeneration and, thus, may have applications in medicine and skin care.

Wound healing effects of noni (Morinda citrifolia L.) leaves: a mechanism involving its PDGF/A2A receptor ligand binding and promotion of wound closure

Morinda citrifolia L. (Rubiaceae) commonly known as noni, has been used in Polynesia by traditional healers for the treatment of cuts, bruises and wounds. Our objective was to investigate the wound-healing mechanisms of the noni leaf. The investigations of its wound-healing mechanisms were carried out using fresh noni leaf juice (NLJ), noni leaf ethanol extract (NLEE) and its methanol (MFEE) and hexane (HFEE) fractions on the PDGF and A(2A) receptors in vitro and topically in mice. Fresh noni leaf juice showed significant affinity to PDGF receptors, and displayed 166% binding inhibition of the ligand binding to its receptors, while at the same concentration, it only had 7% inhibition of the ligand binding to the A(2A) receptors. NLEE, HFEE and MFEE showed significant affinity to A(2A) receptors, concentration dependently, with IC(50) values of 34.1, 42.9 and 86.7 μg/mL, respectively. However, MFEE significantly increased wound closure and reduced the half closure time in mice with a CT(50) of 5.4 ± 0.2 days compared with control (p < 0.05). These results suggest that noni leaf significantly accelerated wound healing in mice via its ligand binding to the PDGF and A(2A) receptors as its probable mechanisms of wound-healing and also support its traditional usage for wound-healing in Polynesia.

Genetic inactivation of the adenosine A2A receptor attenuates pathologic but not developmental angiogenesis in the mouse retina

PURPOSE

The adenosine A(2A) receptor (A(2A)R) modulates normal vascularization and pathologic angiogenesis in many tissues and may contribute to the pathogenesis of retinopathy of prematurity (ROP) characterized by abnormal retinal vascularization in surviving premature infants. Here, the authors studied the effects of the genetic inactivation of A(2A)R on normal retinal vascularization and the development of pathologic angiogenesis in oxygen-induced retinopathy (OIR), an animal model of ROP.

METHODS

After exposure to 75% oxygen for 5 days (postnatal day [P] 7-P12) and subsequently to room air for the next 9 days (P13-P21), we evaluated retinal vascular morphology by ADPase staining in retinal whole mounts, retinal neovascularization response by histochemistry in serial retinal sections, and retinal VEGF gene expression by real-time PCR analysis in A(2A)R knockout (KO) mice and their wild-type (WT) littermates.

RESULTS

At P17, A(2A)R KO mice displayed attenuated OIR compared with WT littermates, as evidenced by reduced vaso-obliteration and areas of nonperfusion in the center of the retina, reduced pathologic angiogenesis as evident by decreased non-ganglion cells and neovascular nuclei, and inhibited hypoxia-induced retinal VEGF gene expression. Notably, the attenuation of pathologic angiogenesis by A(2A)R inactivation was selective for OIR because it did not affect normal retinal vascularization during postnatal development.

CONCLUSIONS

These findings provide the first evidence that A(2A)R is critical for the development of OIR and suggest a novel therapeutic approach of A(2A)R inactivation for ROP by selectively targeting pathologic but not developmental angiogenesis in the retina.

A2A adenosine receptor may allow expansion of T cells lacking effector functions in extracellular adenosine-rich microenvironments

Immunosuppressive signaling via the A2A adenosine receptor (A2AR) provokes a mechanism that protects inflamed tissues from excessive damage by immune cells. This mechanism is desirable not only for preventing uncontrolled tissue destruction by overactive immune responses, but also for protecting tumor tissues from antitumor immune responses. In aforementioned circumstances, T cell priming may occur in an environment containing high concentrations of extracellular adenosine. To examine qualitative changes in T cells activated in the presence of adenosine, we asked whether different functional responses of T cells are equally susceptible to A2AR agonists. In this study, we demonstrate that A2AR signaling during T cell activation strongly inhibited development of cytotoxicity and cytokine-producing activity in T cells, whereas the inhibition of T cell proliferation was only marginal. Both CD8(+) and CD4(+) T cells proliferated well in the presence of A2AR agonists, but their IFN-gamma-producing activities were susceptible to inhibition by cAMP-elevating A2AR. Importantly, the impaired effector functions were maintained in T cells even after removal of the A2AR agonist, reflecting T cell memory of the immunoregulatory effect of adenosine. Thus, although the adenosine-rich environment may allow for the expansion of T cells, the functional activation of T cells may be critically impaired. This physiological mechanism could explain the inefficiency of antitumor T cells in the tumor microenvironment.

Adenosine in fibrosis

Adenosine is an endogenous autocoid that regulates a multitude of bodily functions. Its anti-inflammatory actions are well known to rheumatologists since it mediates many of the anti-inflammatory effects of a number of antirheumatic drugs such as methotrexate. However, inflammatory and tissue regenerative responses are intricately linked, with wound healing being a prime example. It has only recently been appreciated that adenosine has a key role in tissue regenerative and fibrotic processes. An understanding of these processes may shed new light on potential therapeutic options in diseases such as scleroderma where tissue fibrosis features prominently.

Up-regulation of vascular endothelial growth factor expression by adenosine through adenosine A2 receptors in the rat tongue treated with endotoxin

The main focus of the present investigation is to evaluate a differential effect of adenosine on the up-regulation of vascular endothelial growth factor (VEGF) expression through adenosine A(2) receptors in the rat tongue treated with endotoxin (lipopolysaccharide: LPS). Angiogenesis in the rat tongue treated with LPS/incomplete Freund's adjuvant (IFA) or endotoxin/IFA/adenosine A(2) receptor (A(2)R) antagonists was examined using immunohistochemistry for LYVE-1, ED1, ED2, OX6, langerin and VEGF, and real-time polymerase chain reaction (PCR) for VEGF. The distributional density of both blood vessels and OX6(+) cells was significantly increased at day 8 after injection of LPS/IFA. The immunoreactive products of VEGF were intensely labelled in the cytoplasm of various antigen presenting cells (APCs) including dendritic cells (DCs) with double-immunofluorescence technique. Increase in VEGF mRNA expression level, the occupancy ratio of blood vessels, and the number of ED1(+), ED2(+), OX6(+), and langerin(+) cells was inhibited in the injured tongue of rats as a consequence of the treatment with A(2)R antagonists. The present results indicate that the LPS-induced adenosine might promote angiogenesis by the up-regulation of VEGF expression in macrophages/DCs through A(2) receptors. This suggests that the synergistic interaction between toll-like receptor (TLR) and A(2) receptor signalling observed in vivo plays an important role in oral mucosal wound healing.

The adenosinergic immunomodulatory drugs

Hypoxia-driven increase of extracellular adenosine in local tissue microenvironments of inflamed and cancerous tissues plays a critical role in the regulation of tissue destruction by activated immune cells. Accumulated data suggest that injection or consumption of A2A adenosine receptor (A2AR) antagonists may represent a drug treatment that diminishes adenosine-mediated immunosuppression. Since this, in turn, enhances the immune response, inhibition of adenosine-A2AR signaling may be a promising approach to enhance anti-tumor or anti-pathogen immune response. Patients with disorders characterized by excessive inflammation may be at risk to A2AR antagonists (e.g. caffeine) because of the effect to increase inflammatory damage secondary to enhanced immunity. On the other hand, enhancement of hypoxia-adenosinergic immunomodulatory pathways may be beneficial to prevent inflammatory tissue destruction.

Adenosine signaling and the regulation of chronic lung disease

Chronic lung diseases such as asthma, chronic obstructive pulmonary disease and interstitial lung disease are characterized by inflammation and tissue remodeling processes that compromise pulmonary function. Adenosine is produced in the inflamed and damaged lung where it plays numerous roles in the regulation of inflammation and tissue remodeling. Extracellular adenosine serves as an autocrine and paracrine signaling molecule by engaging cell surface adenosine receptors. Preclinical and cellular studies suggest that adenosine plays an anti-inflammatory role in processes associated with acute lung disease, where activation of the A(2A)R and A(2B)R has promising implications for the treatment of these disorders. In contrast, there is growing evidence that adenosine signaling through the A(1)R, A(2B)R and A(3)R may serve pro-inflammatory and tissue remodeling functions in chronic lung diseases. This review discusses the current progress of research efforts and clinical trials aimed at understanding the complexities of these signaling pathway as they pertain to the development of treatment strategies for chronic lung diseases.

Adenosine receptor agonists for promotion of dermal wound healing

Wound healing is a dynamic and complex process that involves a well-coordinated, highly regulated series of events including inflammation, tissue formation, revascularization and tissue remodeling. However, this orderly sequence is impaired in certain pathophysiological conditions such as diabetes mellitus, venous insufficiency, chronic glucocorticoid use, aging and malnutrition. Together with proper wound care, promotion of the healing process is the primary objective in the management of chronic poorly healing wounds. Recent studies have demonstrated that A(2A) adenosine receptor agonists promote wound healing in normal and diabetic animals and one such agonist, Sonedenoson, is currently being evaluated as a prospective new therapy of diabetic foot ulcers. We will review the mechanisms by which adenosine receptor activation affects the function of the cells and tissues that participate in wound healing, emphasizing the potential beneficial impact of adenosine receptor agonists in diabetic impaired healing.

Adenosine A2A receptors play an active role in mouse bone marrow-derived mesenchymal stem cell development

Bone marrow-derived mesenchymal stem cells (BM-MSCs) play a role in wound healing and tissue repair and may also be useful for organ regeneration. As we have demonstrated previously that A(2A) adenosine receptors (A(2A)R) promote tissue repair and wound healing by stimulating local repair mechanisms and enhancing accumulation of endothelial progenitor cells, we investigated whether A(2A)R activation modulates BM-MSC proliferation and differentiation. BM-MSCs were isolated and cultured from A(2A)-deficient and ecto-5'nucleotidase (CD73)-deficient female mice; the MSCs were identified and quantified by a CFU-fibroblast (CFU-F) assay. Procollagen alpha2 type I expression was determined by Western blotting and immunocytochemistry. MSC-specific markers were examined in primary cells and third-passage cells by cytofluorography. PCR and real time-PCR were used to quantitate adenosine receptor and CD73 expression. There were significantly fewer CFU-Fs in cultures of BM-MSCs from A(2A)R knockout (KO) mice or BM-MSCs treated with the A(2A)R antagonist ZM241385, 1 microM. Similarly, there were significantly fewer procollagen alpha2 type I-positive MSCs in cultures from A(2A)R KO and antagonist-treated cultures as well. In late passage cells, there were significantly fewer MSCs from A(2A) KO mice expressing CD90, CD105, and procollagen type I (P<0.05 for all; n=3). These findings indicate that adenosine and adenosine A(2A)R play a critical role in promoting the proliferation and differentiation of mouse BM-MSCs.

T regulatory cells: hypoxia-adenosinergic suppression and re-direction of the immune response

T regulatory cells (Treg cells) suppress immune responses to maintain self tolerance, but they also protect cancerous tissues. I propose a model to potentially unify the diverse functions of Treg cells. This assumes that Treg cells provide a complementary immunological arm to a physiological tissue-protecting mechanism, driven by low oxygen tension (i.e. hypoxia) in inflamed or cancerous tissues. The cAMP-elevating A2A and A2B adenosine receptors, hypoxia inducible transcription factor 1alpha (HIF), the cAMP response element (CRE)- and hypoxia response element (HRE)-mediated transcription in Treg and effector cells have key roles in this model. Both the T cell receptor (TCR)-triggered- and HRE- and CRE-driven activities of Treg cells are required to achieve a maximal level of immune suppression.

Heme oxygenase-1 contributes to an alternative macrophage activation profile induced by apoptotic cell supernatants

Apoptotic cells (AC) are rapidly engulfed by professional phagocytes such as macrophages to avoid secondary necrosis and thus inflammation. Recognition of AC polarizes macrophages toward an anti-inflammatory phenotype, which shows homology to an alternatively activated M2 macrophage. However, mechanistic details provoking these phenotype alterations are incompletely understood. Here, we demonstrate a biphasic up-regulation of heme oxygenase-1 (HO-1), a protein that bears an antiapoptotic as well as an anti-inflammatory potential, in primary human macrophages, which were exposed to the supernatant of AC. Although the first phase of HO-1 induction at 6 h was accomplished by AC-derived sphingosine-1-phosphate (S1P) acting via S1P receptor 1, the second wave of HO-1 induction at 24 h was attributed to autocrine signaling of vascular endothelial growth factor A (VEGFA), whose expression and release were facilitated by S1P. Whereas VEGFA release from macrophages was signal transducer and activator of transcription (STAT) 1-dependent, vascular endothelial growth factor itself triggered STAT1/STAT3 heterodimer formation, which bound to and activated the HO-1 promoter. Knockdown of HO-1 proved its relevance in facilitating enhanced expression of the antiapoptotic proteins Bcl-2 and Bcl-X(L), as well as the anti-inflammatory adenosine receptor A(2A). These findings suggest that HO-1, which is induced by AC-derived S1P, is critically involved in macrophage polarization toward an M2 phenotype.

Adenosine receptors in wound healing, fibrosis and angiogenesis

Wound healing and tissue repair are critical processes, and adenosine, released from injured or ischemic tissues, plays an important role in promoting wound healing and tissue repair. Recent studies in genetically manipulated mice demonstrate that adenosine receptors are required for appropriate granulation tissue formation and in adequate wound healing. A(2A) and A(2B) adenosine receptors stimulate both of the critical functions in granulation tissue formation (i.e., new matrix production and angiogenesis), and the A(1) adenosine receptor (AR) may also contribute to new vessel formation. The effects of adenosine acting on these receptors is both direct and indirect, as AR activation suppresses antiangiogenic factor production by endothelial cells, promotes endothelial cell proliferation, and stimulates angiogenic factor production by endothelial cells and other cells present in the wound. Similarly, adenosine, acting at its receptors, stimulates collagen matrix formation directly. Like many other biological processes, AR-mediated promotion of tissue repair is critical for appropriate wound healing but may also contribute to pathogenic processes. Excessive tissue repair can lead to problems such as scarring and organ fibrosis and adenosine, and its receptors play a role in pathologic fibrosis as well. Here we review the evidence for the involvement of adenosine and its receptors in wound healing, tissue repair and fibrosis.

Adenosine receptors and asthma

The pathophysiological processes underlying respiratory diseases like asthma are complex, resulting in an overwhelming choice of potential targets for the novel treatment of this disease. Despite this complexity, asthmatic subjects are uniquely sensitive to a range of substances like adenosine, thought to act indirectly to evoke changes in respiratory mechanics and in the underlying pathology, and thereby to offer novel insights into the pathophysiology of this disease. Adenosine is of particular interest because this substance is produced endogenously by many cells during hypoxia, stress, allergic stimulation, and exercise. Extracellular adenosine can be measured in significant concentrations within the airways; can be shown to activate adenosine receptor (AR) subtypes on lung resident cells and migrating inflammatory cells, thereby altering their function, and could therefore play a significant role in this disease. Many preclinical in vitro and in vivo studies have documented the roles of the various AR subtypes in regulating cell function and how they might have a beneficial impact in disease models. Agonists and antagonists of some of these receptor subtypes have been developed and have progressed to clinical studies in order to evaluate their potential as novel antiasthma drugs. In this chapter, we will highlight the roles of adenosine and AR subtypes in many of the characteristic features of asthma: airway obstruction, inflammation, bronchial hyperresponsiveness and remodeling. We will also discuss the merit of targeting each receptor subtype in the development of novel antiasthma drugs.

Adenosine A2A receptor blockade or deletion diminishes fibrocyte accumulation in the skin in a murine model of scleroderma, bleomycin-induced fibrosis

Peripheral blood fibrocytes are a newly identified circulating leukocyte subpopulation that migrates into injured tissue where it may display fibroblast-like properties and participate in wound healing and fibrosis of skin and other organs. Previous studies in our lab demonstrated that A(2A) receptor-deficient and A(2A) antagonist-treated mice were protected from developing bleomycin-induced dermal fibrosis, thus the aim of this study was to determine whether the adenosine A(2A) receptor regulates recruitment of fibrocytes to the dermis in this bleomycin-induced model of dermal fibrosis. Sections of skin from normal mice and bleomycin-treated wild type, A(2A) knockout and A(2A) antagonist-treated mice were stained for Procollagen alpha2 Type I and CD34 and the double stained cells, fibrocytes, were counted in the tissue sections. There were more fibrocytes in the dermis of bleomycin-treated mice than normal mice and the increase was abrogated by deletion or blockade of adenosine A(2A) receptors. Because fibrocytes play a central role in tissue fibrosis these results suggest that diminished adenosine A(2A) receptor-mediated recruitment of fibrocytes into tissue may play a role in the pathogenesis of fibrosing diseases of the skin. Moreover, these results provide further evidence that adenosine A(2A) receptors may represent a new target for the treatment of such fibrosing diseases as scleroderma or nephrogenic fibrosing dermopathy.

Adenosine receptors and asthma in humans

According to an executive summary of the GINA dissemination committee report, it is now estimated that approximately 300 million people (5% of the global population or 1 in 20 persons) have asthma. Despite the scientific progress made over the past several decades toward improving our understanding of the pathophysiology of asthma, there is still a great need for improved therapies, particularly oral therapies that enhance patient compliance and that target new mechanisms of action. Adenosine is an important signalling molecule in human asthma. By acting on extracellular G-protein-coupled ARs on a number of different cell types important in the pathophysiology of human asthma, adenosine affects bronchial reactivity, inflammation and airway remodelling. Four AR subtypes (A(1), A(2a), A(2b) and A(3)) have been cloned in humans, are expressed in the lung, and are all targets for drug development for human asthma. This review summarizes what is known about these AR subtypes and their function in human asthma as well as the pros and cons of therapeutic approaches to these AR targets. A number of molecules with high affinity and high selectivity for the human AR subtypes have entered clinical trials or are poised to enter clinical trials as anti-asthma treatments. With the availability of these molecules for testing in humans, the function of ARs in human asthma, as well as the safety and efficacy of approaches to the different AR targets, can now be determined.

Polydeoxyribonucleotide (PDRN) restores blood flow in an experimental model of peripheral artery occlusive disease

OBJECTIVE

This study investigated whether polydeoxyribonucleotide (PDRN) may be efficacious in the treatment of peripheral artery occlusive diseases, which are a major cause of morbidity in Western countries and still lack standardized treatment.

METHODS

We investigated the effects of PDRN, a mixture of deoxyribonucleotides, in an experimental model of hind limb ischemia (HLI) in rats to stimulate vascular endothelial growth factor (VEGF)-A production and to avoid critical ischemia. The femoral artery was excised to induce HLI. Sham-operated on rats (sham HLI) were used as controls. Animals were treated daily with intraperitoneal PDRN (8 mg/kg) or its vehicle. Animals were euthanized at day 7, 14, and 21 after the evaluation of blood flow by laser Doppler. Dissected muscles were used to measure VEGF-A messenger RNA (mRNA) and protein expression, to evaluate edema, and to assess histologic damage.

RESULTS

Administration of PDRN dramatically increased VEGF mRNA throughout the study (day 14: HLI, 7 +/- 2.2 n-fold/beta-actin; HLI + PDRN, 13.3 +/- 3.8 n-fold/beta-actin; P < .0001) and protein expression (HLI, 11 +/- 3.4 integrated intensity; HLI + PDRN, 16 +/- 3.8 integrated intensity; P < .0001). The compound stimulated revascularization, as confirmed by blood flow restoration (P < .005 vs HLI + vehicle), and blunted the histologic damage and the degree of edema. PDRN did not modify VEGF-A expression and blood flow in sham HLI animals. Furthermore, the concomitant administration of 3,7-dimethyl-1-propargilxanthine (DMPX), a selective adenosine A(2A) receptor antagonist, abolished the positive effects of PDRN, confirming that PDRN acts through this receptor.

CONCLUSION

These results led us to hypothesize a role for PDRN in treating peripheral artery occlusive diseases.