Pharmacological blockade of adenosine A2A receptors diminishes scarring

Inactivation of adenosine receptor 2A suppresses endothelial-to-mesenchymal transition and inhibits subretinal fibrosis in mice

Anti-vascular endothelial growth factor therapy has had a substantial impact on the treatment of choroidal neovascularization (CNV) in patients with neovascular age-related macular degeneration (nAMD), the leading cause of vision loss in older adults. Despite treatment, many patients with nAMD still develop severe and irreversible visual impairment because of the development of subretinal fibrosis. We recently reported the anti-inflammatory and antiangiogenic effects of inhibiting the gene encoding adenosine receptor 2A (Adora2a), which has been implicated in cardiovascular disease. Here, using two mouse models of subretinal fibrosis (mice with laser injury-induced CNV or mice with a deficiency in the very low-density lipoprotein receptor), we found that deletion of Adora2a either globally or specifically in endothelial cells reduced subretinal fibrosis independently of angiogenesis. We showed that Adora2a-dependent endothelial-to-mesenchymal transition contributed to the development of subretinal fibrosis in mice with laser injury-induced CNV. Deficiency of Adora2a in cultured mouse and human choroidal endothelial cells suppressed induction of the endothelial-to-mesenchymal transition. A metabolomics analysis of cultured human choroidal endothelial cells showed that ADORA2A knockdown with an siRNA reversed the increase in succinate because of decreased succinate dehydrogenase B expression under fibrotic conditions. Pharmacological inhibition of ADORA2A with a small-molecule KW6002 in both mouse models recapitulated the reduction in subretinal fibrosis observed in mice with genetic deletion of Adora2a. ADORA2A inhibition may be a therapeutic approach to treat subretinal fibrosis associated with nAMD.

Monitoring regulatory T cells as a prognostic marker in lung transplantation

Lung transplantation is the major surgical procedure, which restores normal lung functioning and provides years of life for patients suffering from major lung diseases. Lung transplant recipients are at high risk of primary graft dysfunction, and chronic lung allograft dysfunction (CLAD) in the form of bronchiolitis obliterative syndrome (BOS). Regulatory T cell (Treg) suppresses effector cells and clinical studies have demonstrated that Treg levels are altered in transplanted lung during BOS progression as compared to normal lung. Here, we discuss levels of Tregs/FOXP3 gene expression as a crucial prognostic biomarker of lung functions during CLAD progression in clinical lung transplant recipients. The review will also discuss Treg mediated immune tolerance, tissue repair, and therapeutic strategies for achieving in-vivo Treg expansion, which will be a potential therapeutic option to reduce inflammation-mediated graft injuries, taper the toxic side effects of ongoing immunosuppressants, and improve lung transplant survival rates.

International Union of Basic and Clinical Pharmacology. CXII: Adenosine Receptors: A Further Update

Our previous International Union of Basic and Clinical Pharmacology report on the nomenclature and classification of adenosine receptors (2011) contained a number of emerging developments with respect to this G protein-coupled receptor subfamily, including protein structure, protein oligomerization, protein diversity, and allosteric modulation by small molecules. Since then, a wealth of new data and results has been added, allowing us to explore novel concepts such as target binding kinetics and biased signaling of adenosine receptors, to examine a multitude of receptor structures and novel ligands, to gauge new pharmacology, and to evaluate clinical trials with adenosine receptor ligands. This review should therefore be considered a further update of our previous reports from 2001 and 2011. SIGNIFICANCE STATEMENT: Adenosine receptors (ARs) are of continuing interest for future treatment of chronic and acute disease conditions, including inflammatory diseases, neurodegenerative afflictions, and cancer. The design of AR agonists ("biased" or not) and antagonists is largely structure based now, thanks to the tremendous progress in AR structural biology. The A2A- and A2BAR appear to modulate the immune response in tumor biology. Many clinical trials for this indication are ongoing, whereas an A2AAR antagonist (istradefylline) has been approved as an anti-Parkinson agent.

Purinergic signaling in systemic sclerosis

Systemic sclerosis (SSc) is a chronic autoimmune rheumatic disease that involves numerous organs and presents major management challenges. The histopathologic hallmarks of SSc include vasculopathy, fibrosis and autoimmune phenomena involving both innate and adaptive immune systems. Purinergic signalling is a pathway that may be implicated in the pathophysiology of several of these disease manifestations. Extracellular purines are potent signalling mediators, which have been shown to be dysregulated in SSc. As examples, purines can exacerbate vasculopathy and provoke platelet dysfunction; as well as contributing to immune dysregulation. Elements of purinergic signalling further promote organ and tissue fibrosis in several disease models. Here, we provide an overview of extracellular purine metabolism in purinergic signalling and link disorders of these to the molecular pathology of SSc. We also discuss targeting the purinergic signalling and explore the translational applications for new therapeutic options in SSc.

The effects of polydeoxyribonucleotide on wound healing and tissue regeneration: a systematic review of the literature

Aim: The present study evaluated the effects of polydeoxyribonucleotide (PDRN) on tissue regeneration, paying special attention to the molecular mechanisms that underlie its tissue remodeling actions to better identify its effective therapeutic potential in wound healing. Materials & methods: Strategic searches were conducted through MEDLINE/PubMed, Google Scholar, Scopus, Web of Science and the Cochrane Central Register of Controlled Trials, from their earliest available dates to March 2020. The studies were included with the following eligibility criteria: studies evaluating tissue regeneration, and being an in vitro, in vivo and clinical study. Results: Out of more than 90 articles, 34 fulfilled the eligibility criteria. All data obtained proved the ability of PDRN in promoting a physiological tissue repair through salvage pathway and adenosine A2A receptor activation. Conclusion: Up to date PDRN has proved promising results in term of wound regeneration, healing time and absence of side effects.

Efficacy of Aeschynomene indica L. leaves for wound healing and isolation of active constituent

ETHNOPHARMACOLOGICAL RELEVANCE

In traditional Chinese medicine, the aerial parts of Aeschynomene indica L. (AIL) have been used for wound healing, and to treat urinary tract infection, hepatitis, enteritis, dysentery, nyctalopia, conjunctivitis, urticaria, and furuncle. However, no scientific investigation has been conducted on its wound healing potential.

AIM OF THE STUDY

To investigate the effects of AIL extract on wound healing, isolate the active constituent and reveal the possible mechanism of enhancing wound healing.

MATERIALS AND METHODS

The circular excision wound healing model was used to evaluate in vivo wound-healing activity. Hematoxylin and eosin staining was applied to assess inflammatory cells infiltration, angiogenesis, fibroblast proliferation, collagen synthesis, collagen remodeling, and skin appendages generation. Sirius red-picric acid staining was employed for quantitative analysis of the ratio of collagen I/III. Immunohistochemical staining for CD68, CCR7 (CD197), CD163, TGF-β1 and α-SMA was performed to determine macrophages phenotypes transition (M1-to-M2) and prove the scar-improving effect of AIL on wound healing.

RESULTS

We successfully isolated the active constituent (Sub-Fr0.2) for wound healing from AIL extract, circular excision wound healing experiment and hematoxylin & eosin staining showed Sub-Fr0.2 has a significant promoting effect on wound healing. Results of sirius red-picric acid staining demonstrated a reduced ratio of collagen I/III in the Sub-Fr0.2 group as compared with the vehicle group. Immunohistochemical staining for CD68, CCR7 (CD197), and CD163 in the Sub-Fr0.2 group exhibited an elevated speed of macrophages transiting from M1 phenotype to M2 phenotype, when compared with the vehicle group. Besides, the expression of TGF-β1 and α-SMA were inhibited on wounds treated with the ointment containing Sub-Fr0.2.

CONCLUSION

Leaves of AIL and its active constituent (Sub-Fr0.2) effectively promoted wound healing and reduced scar formation, this efficacy might be exerted by accelerating macrophages phenotypes transition and inhibiting TGF-β1 and α-SMA expression.

Cell-laden and orthogonal-multilayer tissue-engineered corneal stroma induced by a mechanical collagen microenvironment and transplantation in a rabbit model

The development of functional therapies for corneal repair and regeneration is a pressing issue. Corneal stroma provides the principal functions of the cornea. However, because of the highly organized nature of the stromal matrix, the attempts to reproduce corneal stroma might follow a scar model. Here, we have developed a protocol for the efficient generation of a cell-laden and orthogonal-multilayer tissue-engineered (TE) corneal stroma, which is induced by the mechanical effects of compressed collagen (CC) or stretched compressed collagen (SCC). Within SCC, with applied compression and force extension, collagen microfibres and corneal stromal cells (CSCs) are arranged orderly, while collagen fibres and CSCs in CC are randomly arranged. Dehydrated SCC has higher tensile strength than dehydrated CC. Hydrated SCC has similar transparency with hydrated native corneal stroma. Compared with those cultured on tissue culture plates (TCP), down-regulation of the genes and proteins of cytoskeleton, activation, proliferation, collagen and TRPV4, up-regulation of proteoglycans, gap junction proteins and TRPA1 are in CSCs of CC and SCC. Moreover, SCC and CC grafts displayed biocompatibility and integration with host corneal tissue after rabbit intra-corneal stromal transplantation by wk 6 under slit lamp microscopy, in vivo confocal microscopy and histological examination. The SCC model facilitates the construction of physiological feature TE corneal stroma, which serves as a foundation for physiological TE construction of other tissues.

STATEMENT OF SIGNIFICANCE

The development of functional therapies for corneal repair and regeneration is a pressing issue. Corneal stroma provides the principal functions of the cornea. Here, we have developed a protocol for the efficient generation of a cell-laden and orthogonal-multilayer tissue-engineered (TE) corneal stroma, which is induced by the mechanical effects of compressed collagen (CC) or stretched compressed collagen (SCC). These models facilitate the construction of physiological feature TE corneal stroma, which serves as a foundation for physiological TE construction of other tissues and helps to reverse fibrosis pathologies in general.

The antiviral drug tenofovir, an inhibitor of Pannexin-1-mediated ATP release, prevents liver and skin fibrosis by downregulating adenosine levels in the liver and skin

Background

Fibrosing diseases are a leading cause of morbidity and mortality worldwide and, therefore, there is a need for safe and effective antifibrotic therapies. Adenosine, generated extracellularly by the dephosphorylation of adenine nucleotides, ligates specific receptors which play a critical role in development of hepatic and dermal fibrosis. Results of recent clinical trials indicate that tenofovir, a widely used antiviral agent, reverses hepatic fibrosis/cirrhosis in patients with chronic hepatitis B infection. Belonging to the class of acyclic nucleoside phosphonates, tenofovir is an analogue of AMP. We tested the hypothesis that tenofovir has direct antifibrotic effects in vivo by interfering with adenosine pathways of fibrosis using two distinct models of adenosine and A2AR-mediated fibrosis.

Methods

Thioacetamide (100mg/kg IP)-treated mice were treated with vehicle, or tenofovir (75mg/kg, SubQ) (n = 5–10). Bleomycin (0.25U, SubQ)-treated mice were treated with vehicle or tenofovir (75mg/kg, IP) (n = 5–10). Adenosine levels were determined by HPLC, and ATP release was quantitated as luciferase-dependent bioluminescence. Skin breaking strength was analysed and H&E and picrosirus red-stained slides were imaged. Pannexin-1expression was knocked down following retroviral-mediated expression of of Pannexin-1-specific or scrambled siRNA.

Results

Treatment of mice with tenofovir diminished adenosine release from the skin of bleomycin-treated mice and the liver of thioacetamide-treated mice, models of diffuse skin fibrosis and hepatic cirrhosis, respectively. More importantly, tenofovir treatment diminished skin and liver fibrosis in these models. Tenofovir diminished extracellular adenosine concentrations by inhibiting, in a dose-dependent fashion, cellular ATP release but not in cells lacking Pannexin-1.

Conclusions

These studies suggest that tenofovir, a widely used antiviral agent, could be useful in the treatment of fibrosing diseases.

Searching Novel Therapeutic Targets for Scleroderma: P2X7-Receptor Is Up-regulated and Promotes a Fibrogenic Phenotype in Systemic Sclerosis Fibroblasts

Objectives: Systemic sclerosis (SSc) is a connective tissue disorder presenting fibrosis of the skin and internal organs, for which no effective treatments are currently available. Increasing evidence indicates that the P2X7 receptor (P2X7R), a nucleotide-gated ionotropic channel primarily involved in the inflammatory response, may also have a key role in the development of tissue fibrosis in different body districts. This study was aimed at investigating P2X7R expression and function in promoting a fibrogenic phenotype in dermal fibroblasts from SSc patients, also analyzing putative underlying mechanistic pathways. Methods: Fibroblasts were isolated by skin biopsy from 9 SSc patients and 8 healthy controls. P2X7R expression, and function (cytosolic free Ca²⁺ fluxes, α-smooth muscle actin [α-SMA] expression, cell migration, and collagen release) were studied. Moreover, the role of cytokine (interleukin-1β, interleukin-6) and connective tissue growth factor (CTGF) production, and extracellular signal-regulated kinases (ERK) activation in mediating P2X7R-dependent pro-fibrotic effects in SSc fibroblasts was evaluated. Results: P2X7R expression and Ca²⁺ permeability induced by the selective P2X7R agonist 2'-3'-O-(4-benzoylbenzoyl)ATP (BzATP) were markedly higher in SSc than control fibroblasts. Moreover, increased αSMA expression, cell migration, CTGF, and collagen release were observed in lipopolysaccharides-primed SSc fibroblasts after BzATP stimulation. While P2X7-induced cytokine changes did not affect collagen production, it was completely abrogated by inhibition of the ERK pathway. Conclusion: In SSc fibroblasts, P2X7R is overexpressed and its stimulation induces Ca²⁺-signaling activation and a fibrogenic phenotype characterized by increased migration and collagen production. These data point to the P2X7R as a potential, novel therapeutic target for controlling exaggerated collagen deposition and tissue fibrosis in patients with SSc.

Genome-wide DNA methylation profiling of the regenerative MRL/MpJ mouse and two normal strains

Aim: We aimed to identify the pivotal differences in the DNA methylation profiles between the regeneration capable MRL/MpJ mouse and reference mouse strains. Materials & methods: Global DNA methylation profiling was performed in ear pinnae, bone marrow, spleen, liver and heart from uninjured adult females of the MRL/MpJ and C57BL/6J and BALB/c. Results & conclusion: A number of differentially methylated regions (DMRs) distinguishing between the MRL/MpJ mouse and both references were identified. In the ear pinnae, the DMRs were enriched in genes associated with development, inflammation and apoptosis, and in binding sites of transcriptional modulator Smad1. Several DMRs overlapped previously mapped quantitative trait loci of regenerative capability. The results suggest potential epigenetic determinants of regenerative phenomenon.

Adenosine A2a Receptor Blockade Diminishes Wnt/β-Catenin Signaling in a Murine Model of Bleomycin-Induced Dermal Fibrosis

Adenosine A_2a receptor (A_2aR) stimulation promotes the synthesis of collagens I and III, and we have recently demonstrated that there is crosstalk between the A_2aR and WNT/β-catenin signaling pathway. In in vitro studies, A_2aR signaling for collagen III expression was mediated by WNT/β-catenin signaling in human dermal fibroblasts; we further verified whether the crosstalk between A_2aR and Wnt/β-catenin signaling was involved in diffuse dermal fibrosis in vivo. Wnt-signaling reporter mice (Tcf/Lef:H2B-GFP) were challenged with bleomycin and treated with the selective A_2aR antagonist istradefylline (KW6002) or vehicle. Dermal fibrosis was quantitated and nuclear translocation of β-catenin in fibroblasts was assessed by double-staining for Green fluorescent protein or dephosphorylated β-catenin or β-catenin phosphorylated at Ser552, and vimentin. KW6002 significantly reduced skin thickness, skinfold thickness, breaking tension, dermal hydroxyproline content, myofibroblast accumulation, and collagen alignment in bleomycin-induced dermal fibrosis. Also, there was increased expression of Tcf/Lef:H2B-GFP reporter in bleomycin-induced dermal fibrosis, an effect that was diminished by treatment with KW6002. Moreover, KW6002 significantly inhibited nuclear translocation of Tcf/Lef:H2B-GFP reporter, as well as dephosphorylated β-catenin and β-catenin phosphorylated at Ser552. Our work supports the hypothesis that pharmacologic blockade of A_2aR inhibits the WNT/β-catenin signaling pathway, contributing to its capacity to inhibit dermal fibrosis in diseases such as scleroderma.

Identification of Novel Chondroprotective Mediators in Resolving Inflammatory Exudates

We hypothesized that exudates collected at the beginning of the resolution phase of inflammation might be enriched for tissue protective molecules; thus an integrated cellular and molecular approach was applied to identify novel chondroprotective bioactions. Exudates were collected 6 h (inflammatory) and 24 h (resolving) following carrageenan-induced pleurisy in rats. The resolving exudate was subjected to gel filtration chromatography followed by proteomics, identifying 61 proteins. Fractions were added to C28/I2 chondrocytes, grown in micromasses, ions with or without IL-1β or osteoarthritic synovial fluids for 48 h. Three proteins were selected from the proteomic analysis, α1-antitrypsin (AAT), hemopexin (HX), and gelsolin (GSN), and tested against catabolic stimulation for their effects on glycosaminoglycan deposition as assessed by Alcian blue staining, and gene expression of key anabolic proteins by real-time PCR. In an in vivo model of inflammatory arthritis, cartilage integrity was determined histologically 48 h after intra-articular injection of AAT or GSN. The resolving exudate displayed protective activities on chondrocytes, using multiple readouts: these effects were retained in low m.w. fractions of the exudate (46.7% increase in glycosaminoglycan deposition; ∼20% upregulation of COL2A1 and aggrecan mRNA expression), which reversed the effect of IL-1β. Exogenous administration of HX, GSN, or AAT abrogated the effects of IL-1β and osteoarthritic synovial fluids on anabolic gene expression and increased glycosaminoglycan deposition. Intra-articular injection of AAT or GSN protected cartilage integrity in mice with inflammatory arthritis. In summary, the strategy for identification of novel chondroprotective activities in resolving exudates identified HX, GSN and AAT as potential leads for new drug discovery programs.

The role of 5-arylalkylamino- and 5-piperazino- moieties on the 7-aminopyrazolo[4,3-d]pyrimidine core in affecting adenosine A1 and A2A receptor affinity and selectivity profiles

New 7-amino-2-phenylpyrazolo[4,3-d]pyrimidine derivatives, substituted at the 5-position with aryl(alkyl)amino- and 4-substituted-piperazin-1-yl- moieties, were synthesized with the aim of targeting human (h) adenosine A₁ and/or A_2A receptor subtypes. On the whole, the novel derivatives 1–24 shared scarce or no affinities for the off-target hA_2B and hA₃ ARs. The 5-(4-hydroxyphenethylamino)- derivative 12 showed both good affinity (K_i = 150 nM) and the best selectivity for the hA_2A AR while the 5-benzylamino-substituted 5 displayed the best combined hA_2A (K_i = 123 nM) and A₁ AR affinity (K_i = 25 nM). The 5-phenethylamino moiety (compound 6) achieved nanomolar affinity (K_i = 11 nM) and good selectivity for the hA₁ AR. The 5-(N⁴-substituted-piperazin-1-yl) derivatives 15–24 bind the hA₁ AR subtype with affinities falling in the high nanomolar range. A structure-based molecular modeling study was conducted to rationalize the experimental binding data from a molecular point of view using both molecular docking studies and Interaction Energy Fingerprints (IEFs) analysis.

Potential Therapeutic Applications of Adenosine A2A Receptor Ligands and Opportunities for A2A Receptor Imaging

Adenosine A_2A receptors (A_2A Rs) are highly expressed in the human striatum, and at lower densities in the cerebral cortex, the hippocampus, and cells of the immune system. Antagonists of these receptors are potentially useful for the treatment of motor fluctuations, epilepsy, postischemic brain damage, or cognitive impairment, and for the control of an immune checkpoint during immunotherapy of cancer. A_2A R agonists may suppress transplant rejection and graft-versus-host disease; be used to treat inflammatory disorders such as asthma, inflammatory bowel disease, and rheumatoid arthritis; be locally applied to promote wound healing and be employed in a strategy for transient opening of the blood-brain barrier (BBB) so that therapeutic drugs and monoclonal antibodies can enter the brain. Increasing A_2A R signaling in adipose tissue is also a potential strategy to combat obesity. Several radioligands for positron emission tomography (PET) imaging of A_2A Rs have been developed in recent years. This review article presents a critical overview of the potential therapeutic applications of A_2A R ligands, the use of A_2A R imaging in drug development, and opportunities and limitations of PET imaging in future research.

Neuronal Protein 3.1 Deficiency Leads to Reduced Cutaneous Scar Collagen Deposition and Tensile Strength due to Impaired Transforming Growth Factor-β1 to -β3 Translation

Neuronal protein 3.1 (P311), a conserved RNA-binding protein, represents the first documented protein known to stimulate transforming growth factor (TGF)-β1 to -β3 translation in vitro and in vivo. Because TGF-βs play critical roles in fibrogenesis, we initiated efforts to define the role of P311 in skin scar formation. Here, we show that P311 is up-regulated in skin wounds and in normal and hypertrophic scars. Genetic ablation of p311 resulted in a significant decrease in skin scar collagen deposition. Lentiviral transfer of P311 corrected the deficits, whereas down-regulation of P311 levels by lentiviral RNA interference reproduced the deficits seen in P311^-/- mice. The decrease in collagen deposition resulted in scars with reduced stiffness but also reduced scar tensile strength. In vitro studies using murine and human dermal fibroblasts showed that P311 stimulated TGF-β1 to -β3 translation, a process that involved eukaryotic translation initiation factor 3 subunit b as a P311 binding partner. This resulted in increased TGF-β levels/activity and increased collagen production. In addition, P311 induced dermal fibroblast activation and proliferation. Finally, exogenous TGF-β1 to -β3, each restituted the normal scar phenotype. These studies demonstrate that P311 is required for the production of normal cutaneous scars and place P311 immediately up-stream of TGF-βs in the process of fibrogenesis. Conditions that decrease P311 levels could result in less tensile scars, which could potentially lead to higher incidence of dehiscence after surgery.

Adenosine and adenosine receptors in the pathogenesis and treatment of rheumatic diseases

Adenosine, a nucleoside derived primarily from the extracellular hydrolysis of adenine nucleotides, is a potent regulator of inflammation. Adenosine mediates its effects on inflammatory cells by engaging one or more cell-surface receptors. The expression and function of adenosine receptors on different cell types change during the course of rheumatic diseases, such as rheumatoid arthritis (RA). Targeting adenosine receptors directly for the treatment of rheumatic diseases is currently under study; however, indirect targeting of adenosine receptors by enhancing adenosine levels at inflamed sites accounts for most of the anti-inflammatory effects of methotrexate, the anchor drug for the treatment of RA. In this Review, we discuss the regulation of extracellular adenosine levels and the role of adenosine in regulating the inflammatory and immune responses in rheumatic diseases such as RA, psoriasis and other types of inflammatory arthritis. In addition, adenosine and its receptors are involved in promoting fibrous matrix production in the skin and other organs, and the role of adenosine in fibrosis and fibrosing diseases is also discussed.

Adenosine A2A receptor promotes collagen type III synthesis via β-catenin activation in human dermal fibroblasts

BACKGROUND AND PURPOSE

Adenosine A_2A receptor stimulation promotes the synthesis of collagen type I and type III (Col1 and Col3), mediators of fibrosis and scarring. The A_2A receptor modulates collagen balance via cAMP/PKA/p38-MAPK/Akt pathways. Wnt signalling is important in fibrosis and the cAMP and Wnt pathways converge. Because the A_2A receptor is Gs-linked and increases cAMP, we determined whether A_2A receptors and Wnt signalling interact.

EXPERIMENTAL APPROACH

Total β-catenin, de-phosphorylated β-catenin (canonical activation, de-phospho β-catenin) and phosphorylated β-catenin at Ser⁵⁵² (non-canonical activation, p-Ser⁵⁵² β-catenin) levels were determined in primary human dermal fibroblasts, cytosol and nucleus, by western blot analysis and fluorescence microscopy, before and after stimulation by A_2A receptor-selective agonist CGS21680, with/without A_2A receptor-selective antagonist (SCH56261) pretreatment. β-Catenin was knocked down by transfection with scrambled-siRNA or specific-siRNA, and Col1 and Col3 levels determined by western blots.

KEY RESULTS

CGS21680 stimulation rapidly (15 min) increased cellular β-catenin levels. Both de-phospho β-catenin and p-Ser⁵⁵² β-catenin levels were also increased. CGS21680 stimulated the translocation of total de-phospho and p-Ser⁵⁵² β-catenin to the nucleus. A_2A receptor-stimulation increased Col1 synthesis similarly in β-catenin knockeddown and scrambled cells. However, β-catenin knockdown abolished the increase in Col3 synthesis induced in A_2A receptor-stimulated fibroblasts.

CONCLUSIONS AND IMPLICATIONS

A_2A receptor stimulation promotes Col3 synthesis via the activation of canonical and non-canonical β-catenin, consistent with a role for A_2A receptors in dermal fibrosis and scarring.

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.

Purinergic signaling in scarring

Adenosine (ADO) and nucleotides such as ATP, ADP, and uridine 5'-triphosphate (UTP), among others, may serve as extracellular signaling molecules. These mediators activate specific cell-surface receptors-namely, purinergic 1 and 2 (P1 and P2)-to modulate crucial pathophysiological responses. Regulation of this process is maintained by nucleoside and nucleotide transporters, as well as the ectonucleotidases ectonucleoside triphosphate diphosphohydrolase [ENTPD; cluster of differentiation (CD)39] and ecto-5'-nucleotidase (5'-NT; CD73), among others. Cells involved in tissue repair, healing, and scarring respond to both ADO and ATP. Our recent investigations have shown that modulation of purinergic signaling regulates matrix deposition during tissue repair and fibrosis in several organs. Cells release adenine nucleotides into the extracellular space, where these mediators are converted by CD39 and CD73 into ADO, which is anti-inflammatory in the short term but may also promote dermal, heart, liver, and lung fibrosis with repetitive signaling under defined circumstances. Extracellular ATP stimulates cardiac fibroblast proliferation, lung inflammation, and fibrosis. P2Y2 (UTP/ATP) and P2Y6 [ADP/UTP/uridine 5'-diphosphate (UDP)] have been shown to have profibrotic effects, as well. Modulation of purinergic signaling represents a novel approach to preventing or diminishing fibrosis. We provide an overview of the current understanding of purinergic signaling in scarring and discuss its potential to prevent or decrease fibrosis.

Adenosine A2A receptor plays an important role in radiation-induced dermal injury

Ionizing radiation is a common therapeutic modality and following irradiation dermal changes, including fibrosis and atrophy, may lead to permanent changes. We have previously demonstrated that occupancy of A2A receptor (A2AR) stimulates collagen production, so we determined whether blockade or deletion of A2AR could prevent radiation-induced fibrosis. After targeted irradiation (40 Gy) of the skin of wild-type (WT) or A2AR knockout (A2ARKO) mice, the A2AR antagonist ZM241385 was applied daily for 28 d. In irradiated WT mice treated with the A2AR antagonist, there was a marked reduction in collagen content and skin thickness, and ZM241385 treatment reduced the number of myofibroblasts and angiogenesis. After irradiation, there is an increase in loosely packed collagen fibrils, which is significantly diminished by ZM241385. Irradiation also induced an increase in epidermal thickness, prevented by ZM241385, by increasing the number of proliferating keratinocytes. Similarly, in A2ARKO mice, the changes in collagen alignment, skin thickness, myofibroblast content, angiogenesis, and epidermal hyperplasia were markedly reduced following irradiation. Radiation-induced changes in the dermis and epidermis were accompanied by an infiltrate of T cells, which was prevented in both ZM241385-treated and A2ARKO mice. Radiation therapy is administered to a significant number of patients with cancer, and radiation reactions may limit this therapeutic modality. Our findings suggest that topical application of an A2AR antagonist prevents radiation dermatitis and may be useful in the prevention or amelioration of radiation changes in the skin.

History and perspectives of A2A adenosine receptor antagonists as potential therapeutic agents

Growing evidence emphasizes that the purine nucleoside adenosine plays an active role as a local regulator in different pathologies. Adenosine is a ubiquitous nucleoside involved in various physiological and pathological functions by stimulating A1 , A2A , A2B , and A3 adenosine receptors (ARs). At the present time, the role of A2A ARs is well known in physiological conditions and in a variety of pathologies, including inflammatory tissue damage and neurodegenerative disorders. In particular, the use of selective A2A antagonists has been reported to be potentially useful in the treatment of Parkinson's disease (PD). In this review, A2A AR signal transduction pathways, together with an analysis of the structure-activity relationships of A2A antagonists, and their corresponding pharmacological roles and therapeutic potential have been presented. The initial results from an emerging polypharmacological approach are also analyzed. This approach is based on the optimization of the affinity and/or functional activity of the examined compounds toward multiple targets, such as A1 /A2A ARs and monoamine oxidase-B (MAO-B), both closely implicated in the pathogenesis of PD.

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.

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.

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.

Bradykinin-induced Ca2+ signaling in human subcutaneous fibroblasts involves ATP release via hemichannels leading to P2Y12 receptors activation

Background

Chronic musculoskeletal pain involves connective tissue remodeling triggered by inflammatory mediators, such as bradykinin. Fibroblast cells signaling involve changes in intracellular Ca²⁺ ([Ca²⁺]_i). ATP has been related to connective tissue mechanotransduction, remodeling and chronic inflammatory pain, via P2 purinoceptors activation. Here, we investigated the involvement of ATP in bradykinin-induced Ca²⁺ signals in human subcutaneous fibroblasts.

Results

Bradykinin, via B₂ receptors, caused an abrupt rise in [Ca²⁺]_i to a peak that declined to a plateau, which concentration remained constant until washout. The plateau phase was absent in Ca²⁺-free medium; [Ca²⁺]_i signal was substantially reduced after depleting intracellular Ca²⁺ stores with thapsigargin. Extracellular ATP inactivation with apyrase decreased the [Ca²⁺]_i plateau. Human subcutaneous fibroblasts respond to bradykinin by releasing ATP via connexin and pannexin hemichannels, since blockade of connexins, with 2-octanol or carbenoxolone, and pannexin-1, with ¹⁰Panx, attenuated bradykinin-induced [Ca²⁺]_i plateau, whereas inhibitors of vesicular exocytosis, such as brefeldin A and bafilomycin A1, were inactive. The kinetics of extracellular ATP catabolism favors ADP accumulation in human fibroblast cultures. Inhibition of ectonucleotidase activity and, thus, ADP formation from released ATP with POM-1 or by Mg²⁺ removal from media reduced bradykinin-induced [Ca²⁺]_i plateau. Selective blockade of the ADP-sensitive P2Y₁₂ receptor with AR-C66096 attenuated bradykinin [Ca²⁺]_i plateau, whereas the P2Y₁ and P2Y₁₃ receptor antagonists, respectively MRS 2179 and MRS 2211, were inactive. Human fibroblasts exhibited immunoreactivity against connexin-43, pannexin-1 and P2Y₁₂ receptor.

Conclusions

Bradykinin induces ATP release from human subcutaneous fibroblasts via connexin and pannexin-1-containing hemichannels leading to [Ca²⁺]_i mobilization through the cooperation of B₂ and P2Y₁₂ receptors.

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.

Angiogenesis and scar formation in healing wounds

PURPOSE OF REVIEW

One well described feature of wound healing is the ingrowth of new capillaries or angiogenesis. At its peak, the capillary content in healing wounds may reach three or more times that of normal uninjured tissue. This new vasculature is required to restore oxygenation and allow the growth of new tissue to fill the wound space. This review examines the assumption that a capillary content in excess of normal density is essential for adequate healing.

RECENT FINDINGS

The regulation of wound angiogenesis has been demonstrated to involve both proangiogenic and antiangiogenic stimuli, with the level of capillary growth reliant upon both sets of factors. Several studies now show that normal skin wounds heal adequately even when the angiogenic response is artificially reduced. In normal skin, a reduction of capillary growth to a level consistent with normal tissue does not affect wound closure and may even lead to highly favorable long term healing outcomes.

SUMMARY

The angiogenic response in normal wounds may exceed what is needed for optimal repair.

Targeted therapies for systemic sclerosis

Pathogenic processes that underlie the development and progression of systemic sclerosis (SSc) are being defined in preclinical, clinical and genetic studies. Important evidence of interplay between the vasculature, connective tissue and specialized epithelial structures is emerging, and abnormalities of both the innate and adaptive immune systems have been identified. In this context, information regarding pivotal mediators, pathways or cell types that could be targets for therapeutic intervention, and that might offer potential for true disease modification, is accruing. Precedent for the regression of some aspects of the pathology has been set in clinical studies showing that potential exists to improve tissue structure and function as well as to prevent disease progression. This article reviews the concept of targeted therapies and considers potential pathways and processes that might be attenuated by therapeutic intervention in SSc. As well as improving outcomes, such approaches will undoubtedly provide information about pathogenesis. The concept of translational medicine is especially relevant in SSc, and we anticipate that the elusive goal of an effective antifibrotic treatment will emerge from one of the several clinical trials currently underway or planned in this disease. Therapeutic advances in SSc would have implications and potential beyond autoimmune rheumatic diseases.