Regulation and function of adenosine deaminase in mice

Focusing on Adenosine Receptors as a Potential Targeted Therapy in Human Diseases

Adenosine is involved in a range of physiological and pathological effects through membrane-bound receptors linked to G proteins. There are four subtypes of adenosine receptors, described as A1AR, A2AAR, A2BAR, and A3AR, which are the center of cAMP signal pathway-based drug development. Several types of agonists, partial agonists or antagonists, and allosteric substances have been synthesized from these receptors as new therapeutic drug candidates. Research efforts surrounding A1AR and A2AAR are perhaps the most enticing because of their concentration and affinity; however, as a consequence of distressing conditions, both A2BAR and A3AR levels might accumulate. This review focuses on the biological features of each adenosine receptor as the basis of ligand production and describes clinical studies of adenosine receptor-associated pharmaceuticals in human diseases.

Adenosine A2B Receptor: From Cell Biology to Human Diseases

Extracellular adenosine is a ubiquitous signaling molecule that modulates a wide array of biological processes. Recently, significant advances have been made in our understanding of A2B adenosine receptor (A2BAR). In this review, we first summarize some of the general characteristics of A2BAR, and then we describe the multiple binding partners of the receptor, such as newly identified α-actinin-1 and p105, and discuss how these associated proteins could modulate A2BAR's functions, including certain seemingly paradoxical functions of the receptor. Growing evidence indicates a critical role of A2BAR in cancer, renal disease, and diabetes, in addition to its importance in the regulation of vascular diseases, and lung disease. Here, we also discuss the role of A2BAR in cancer, renal disease, and diabetes and the potential of the receptor as a target for treating these three diseases.

Identification of A3 adenosine receptor agonists as novel non-narcotic analgesics

Chronic pain negatively impacts the quality of life in a variety of patient populations. The current therapeutic repertoire is inadequate in managing patient pain and warrants the development of new therapeutics. Adenosine and its four cognate receptors (A1 , A2A , A2B and A3 ) have important roles in physiological and pathophysiological states, including chronic pain. Preclinical and clinical studies have revealed that while adenosine and agonists of the A1 and A2A receptors have antinociceptive properties, their therapeutic utility is limited by adverse cardiovascular side effects. In contrast, our understanding of the A3 receptor is only in its infancy, but exciting preclinical observations of A3 receptor antinociception, which have been bolstered by clinical trials of A3 receptor agonists in other disease states, suggest pain relief without cardiovascular side effects and with sufficient tolerability. Our goal herein is to briefly discuss adenosine and its receptors in the context of pathological pain and to consider the current data regarding A3 receptor-mediated antinociception. We will highlight recent findings regarding the impact of the A3 receptor on pain pathways and examine the current state of selective A3 receptor agonists used for these studies. The adenosine-to-A3 receptor pathway represents an important endogenous system that can be targeted to provide safe, effective pain relief from chronic pain.

Pain and Poppies: The Good, the Bad, and the Ugly of Opioid Analgesics

Treating pain is one of the most difficult challenges in medicine and a key facet of disease management. The isolation of morphine by Friedrich Sertürner in 1804 added an essential pharmacological tool in the treatment of pain and spawned the discovery of a new class of drugs known collectively as opioid analgesics. Revered for their potent pain-relieving effects, even Morpheus the god of dreams could not have dreamt that his opium tincture would be both a gift and a burden to humankind. To date, morphine and other opioids remain essential analgesics for alleviating pain. However, their use is plagued by major side effects, such as analgesic tolerance (diminished pain-relieving effects), hyperalgesia (increased pain sensitivity), and drug dependence. This review highlights recent advances in understanding the key causes of these adverse effects and explores the effect of chronic pain on opioid reward.

SIGNIFICANCE STATEMENT

Chronic pain is pervasive and afflicts >100 million Americans. Treating pain in these individuals is notoriously difficult and often requires opioids, one of the most powerful and effective classes of drugs used for controlling pain. However, their use is plagued by major side effects, such as a loss of pain-relieving effects (analgesic tolerance), paradoxical pain (hyperalgesia), and addiction. Despite the potential side effects, opioids remain the pharmacological cornerstone of modern pain therapy. This review highlights recent breakthroughs in understanding the key causes of these adverse effects and explores the cellular control of opioid systems in reward and aversion. The findings will challenge traditional views of the good, the bad, and the ugly of opioids.

Hypoxia-induced deoxycytidine kinase expression contributes to apoptosis in chronic lung disease

Chronic obstructive pulmonary disease (COPD) is characterized by persistent inflammation and tissue remodeling and is a leading cause of death in the United States. Increased apoptosis of pulmonary epithelial cells is thought to play a role in COPD development and progression. Identification of signaling pathways resulting in increased apoptosis in COPD can be used in the development of novel therapeutic interventions. Deoxyadenosine (dAdo) is a DNA breakdown product that amplifies lymphocyte apoptosis by being phosphorylated to deoxyadenosine triphosphate (dATP). dAdo is maintained at low levels by adenosine deaminase (ADA). This study demonstrated that mice lacking ADA developed COPD manifestations in association with elevated dAdo and dATP levels and increased apoptosis in the lung. Deoxycitidine kinase (DCK), a major enzyme for dAdo phosphorylation, was up-regulated in mouse and human airway epithelial cells in association with air-space enlargement. Hypoxia was identified as a novel regulator of DCK, and inhibition of DCK resulted in diminished dAdo-mediated apoptosis in the lungs. Our results suggest that activating the dAdo-DCK-dATP pathway directly results in increased apoptosis in the lungs of mice with air-space enlargement and suggests a novel therapeutic target for the treatment of COPD.

Adenosine receptors and inflammation

Extracellular adenosine is produced in a coordinated manner from cells following cellular challenge or tissue injury. Once produced, it serves as an autocrine- and paracrine-signaling molecule through its interactions with seven-membrane-spanning G-protein-coupled adenosine receptors. These signaling pathways have widespread physiological and pathophysiological functions. Immune cells express adenosine receptors and respond to adenosine or adenosine agonists in diverse manners. Extensive in vitro and in vivo studies have identified potent anti-inflammatory functions for all of the adenosine receptors on many different inflammatory cells and in various inflammatory disease processes. In addition, specific proinflammatory functions have also been ascribed to adenosine receptor activation. The potent effects of adenosine signaling on the regulation of inflammation suggest that targeting specific adenosine receptor activation or inactivation using selective agonists and antagonists could have important therapeutic implications in numerous diseases. This review is designed to summarize the current status of adenosine receptor signaling in various inflammatory cells and in models of inflammation, with an emphasis on the advancement of adenosine-based therapeutics to treat inflammatory disorders.

A3 adenosine receptor signaling contributes to airway mucin secretion after allergen challenge

Mucin hypersecretion is a prominent feature of obstructive airway diseases such as asthma. Clara cells conditionally produce mucin in response to inflammatory signals in a process termed mucous metaplasia. This can be followed by mucin secretion stimulated by various signaling molecules. The cellular and molecular mechanisms that regulate mucin production and secretion are not well understood. Adenosine is a signaling nucleoside that has been implicated in airway diseases in which mucus obstruction is prominent. Furthermore, the A(3) adenosine receptor (A(3)AR) is upregulated in mucin-producing goblet cells of the airway, thereby implicating it in processes involved in mucous cell biology. Here we use genetic approaches to investigate the contribution of A(3)AR signaling to mucus production and secretion in a mouse model of allergen-induced pulmonary disease. We found that the degree of mucin production in response to allergen is similar in wild-type and A(3)AR-deficient mice, and that overexpression of this receptor in Clara cells neither induces mucin production itself, nor enhances mucin production in response to allergen challenge. Collectively, these experiments demonstrate that the A(3)AR is neither necessary nor sufficient for mucous cell metaplasia. In contrast to the lack of effect on mucin production, agonist-induced mucin secretion was increased in goblet cells overexpressing the A(3)AR, and was absent in A(3)AR-deficient mice. Thus, the A(3)AR contributes to mucin secretion in allergen-induced metaplasia. Signaling through this receptor may contribute to mucus airway obstruction seen in pulmonary disorders in which adenosine levels are elevated.

Adenosine Deaminase Deficiency: Metabolic Basis of Immune Deficiency and Pulmonary Inflammation

Genetic deficiencies in the purine catabolic enzyme adenosine deaminase (ADA) in humans results primarily in a severe lymphopenia and immunodeficiency that can lead to the death of affected individuals early in life. The metabolic basis of the immunodeficiency is likely related to the sensitivity of lymphocytes to the accumulation of the ADA substrates adenosine and 2'-deoxyadenosine. Investigations using ADA-deficient mice have provided compelling evidence to support the hypothesis that T and B cells are sensitive to increased concentrations of 2'-deoxyadenosine that kill cells through mechanisms that involve the accumulation of dATP and the induction of apoptosis. In addition to effects on the developing immune system, ADA-deficient humans exhibit phenotypes in other physiological systems including the renal, neural, skeletal, and pulmonary systems. ADA-deficient mice develop similar abnormalities that are dependent on the accumulation of adenosine and 2'-deoxyadenosine. Detailed analysis of the pulmonary insufficiency seen in ADA-deficient mice suggests that the accumulation of adenosine in the lung can directly access cellular signaling pathways that lead to the development and exacerbation of chronic lung disease. The ability of adenosine to regulate aspects of chronic lung disease is likely mediated by specific interactions with adenosine receptor subtypes on key regulatory cells. Thus, the examination of ADA deficiency has identified the importance of purinergic signaling during lymphoid development and in the regulation of aspects of chronic lung disease.

Abnormal alveolar development associated with elevated adenine nucleosides

Adenosine signaling has been characterized in various physiologic systems, but little is known about the role of adenosine signaling in lung development. Alveogenesis and microvascular maturation are the final stages in lung development in mammals. Alveogenesis in the mouse begins on Postnatal Day 5, when the process of secondary septation plays a pivotal role in the expansion of the alveolar sacs and microvascular maturation. Adenosine deaminase null mice (ADA-/-) exhibit abnormalities in alveogenesis in association with elevated lung adenosine levels. Large-scale gene expression analysis of ADA-/- lungs using oligonucleotide-based microarrays revealed novel relationships between gene expression patterns and elevated lung adenosine during the stages of alveolar maturation. Genes regulating apoptosis, proliferation, and vascular development were shown to be altered, and decreased cell proliferation in association with increased alveolar type II cell apoptosis was shown to contribute to abnormal secondary septation in these mice. ADA enzyme therapy allowed for normal patterns of apoptosis, proliferation, and alveolar development in association with prevention of adenosine elevations. These findings were correlated with the presence of adenosine receptors in the developing lung, suggesting the involvement of receptor signaling. These studies provide evidence that elevated lung adenosine can lead to abnormal alveogenesis by disrupting patterns of cell proliferation and apoptosis.

Adenosine mediates IL-13-induced inflammation and remodeling in the lung and interacts in an IL-13-adenosine amplification pathway

IL-13 is an important mediator of inflammation and remodeling. We hypothesized that adenosine accumulation, alterations in adenosine receptors, and adenosine-IL-13 autoinduction are critical events in IL-13-induced pathologies. To test this, we characterized the effects of IL-13 overexpression on the levels of adenosine, adenosine deaminase (ADA) activity, and adenosine receptors in the murine lung. We also determined whether adenosine induced IL-13 in lungs from ADA-null mice. IL-13 induced an inflammatory and remodeling response that caused respiratory failure and death. During this response, IL-13 caused a progressive increase in adenosine accumulation, inhibited ADA activity and mRNA accumulation, and augmented the expression of the A1, A2B, and A3 but not the A2A adenosine receptors. ADA enzyme therapy diminished the IL-13-induced increase in adenosine, inhibited IL-13-induced inflammation, chemokine elaboration, fibrosis, and alveolar destruction, and prolonged the survival of IL-13-transgenic animals. In addition, IL-13 was strongly induced by adenosine in ADA-null mice. These findings demonstrate that adenosine and adenosine signaling contribute to and influence the severity of IL-13-induced tissue responses. They also demonstrate that IL-13 and adenosine stimulate one another in an amplification pathway that may contribute to the nature, severity, progression, and/or chronicity of IL-13 and/or Th2-mediated disorders.

High-level activation by a duodenum-specific enhancer requires functional GATA binding sites

The purine metabolic gene adenosine deaminase (ADA) is expressed at high levels in a well-defined spatiotemporal pattern in the villous epithelium of proximal small intestine. A duodenum-specific enhancer module responsible for this expression pattern has been identified in the second intron of the human ADA gene. It has previously been shown that binding of the factor PDX-1 is essential for function of this enhancer. The studies presented here examine the proposed roles of GATA factors in the enhancer. Site-directed mutagenesis of the enhancer's GATA binding sites crippled enhancer function in 10 lines of transgenic mice, with 9 of the lines demonstrating <1% of normal activity. Detailed studies along the longitudinal axis of mouse small intestine indicate that GATA-4 and GATA-5 mRNA levels display a reciprocal pattern, with low levels of GATA-6 throughout. Interestingly, gel shift studies with duodenal nuclear extracts showed binding only by GATA-4.

Transcription factor gene AP-2 gamma essential for early murine development

Transcription factor gene AP-2 gamma belongs to a family of four closely related genes. AP-2 gamma had been implicated in multiple functions during proliferation and differentiation based on its expression pattern in trophoblast, neural crest, and ectoderm cells in murine embryos. In order to address the question of the role of AP-2 gamma during mammalian development, we generated mice harboring a disrupted AP-2 gamma allele. AP-2 gamma heterozygous mice are viable and display reduced body sizes at birth but are fertile. Mice deficient for AP-2 gamma, however, are growth retarded and die at days 7 to 9 of embryonic development. Immunohistochemical analysis revealed that the trophectodermal cells that are found to express AP-2 gamma fail to proliferate, leading to failure of labyrinth layer formation. As a consequence, the developing embryo suffers from malnutrition and dies. Analysis of embryo cultures suggests that AP-2 gamma is also implicated in the regulation of the adenosine deaminase (ADA) gene, a gene involved in purine metabolism found expressed at the maternal-fetal interface. Therefore, AP-2 gamma seems to be required in early embryonic development because it regulates the genetic programs controlling proliferation and differentiation of extraembryonic trophectodermal cells.

Gene expression profiling in inflammatory airway disease associated with elevated adenosine

Adenosine has been implicated as a modulator of inflammatory processes central to asthma. However, the molecular mechanisms involved are poorly understood. We used Atlas mouse cDNA arrays to analyze differential gene expression in association with lung inflammation resulting from elevated adenosine in adenosine deaminase (ADA)-deficient mice. We report that of the 1,176 genes on the array, the expression patterns of 280 genes were consistently altered. Of these genes, the steady-state levels of 93 genes were upregulated and 29 were downregulated. We also show that lowering adenosine levels with ADA enzyme therapy has striking effects on gene expression that may be associated with resolution of pulmonary eosinophilia. In addition, we confirmed the nucleic acid and protein expression of vascular endothelial growth factor and monocyte chemoattractant protein-3, two candidate genes that may be regulated by adenosine. In conclusion, high-throughput profiling of gene expression by cDNA array hybridization has provided an overview of critical regulatory genes involved in airway inflammation in ADA-deficient mice. These mice will serve as a useful in vivo model for characterizing molecular mechanisms of adenosine-mediated lung damage.

A duodenum-specific enhancer regulates expression along three axes in the small intestine

Adenosine deaminase (ADA) is expressed at high levels in the epithelium of proximal small intestine. Transgenic mice were used to characterize the regulatory region governing this activation. A duodenum-specific enhancer is located in intron 2 of the human ADA gene at the central site among a cluster of seven DNase I-hypersensitive sites present in duodenal DNA. Flanking DNA, including the remaining hypersensitive sites, is required for consistent high-level enhancer function. The enhancer activates expression in a pattern identical to endogenous ADA along both the anterior-posterior axis of the small intestine and the crypt-villus differentiation axis of the intestinal epithelium. Timing of activation by the central enhancer mimics endogenous mouse ADA activation, occurring at 2-3 wk of age. However, two upstream DNA segments, one proximal and one distal, collaborate to change enhancer activation to a perinatal time point. Studies with duodenal nuclear extracts identified five distinct DNase I footprints within the enhancer. Protected regions encompass six putative binding sites for the transcription factor PDX-1, as well as proposed CDX, hepatocyte nuclear factor-4, and GATA-type sites.

Transcription factor AP-2gamma regulates murine adenosine deaminase gene expression during placental development

Trophoblast cells are specialized extra-embryonic cells present only in eutherian mammals. They play a major role in the implantation and placentation processes. To understand better the molecular mechanisms that control the development and function of trophoblast cells, we sought to identify the transcription factors that regulate murine adenosine deaminase (ADA) gene expression in the placenta. Here we report a detailed characterization of a placenta-specific footprinting region (FP1) in the Ada placental regulatory element. The sequence of FP1 was mapped by DNase I footprinting and was found to match a consensus AP-2 transcription factor-binding site. Electrophoretic mobility shift assays demonstrated that FP1 interacted with AP-2-like proteins. Further analysis using AP-2 antibody confirmed that AP-2 protein was indeed present in the placenta and bound to FP1. Mutation at the AP-2 site in FP1 abolished the ability of the Ada placental regulatory element to bind AP-2 proteins and failed to target chloramphenicol acetyltransferase reporter gene expression to placentas in transgenic mice, indicating that AP-2 is required for Ada expression in the placenta. In addition, RNase protection assays demonstrated that AP-2gamma was the predominant AP-2 family member expressed in the placenta. In situ hybridization analysis revealed that AP-2gamma expression was enriched in the trophoblast lineage throughout development, suggesting that AP-2gamma may be critical for trophoblast development and differentiation.