Adenosine receptors in health and disease

Adenosine (Ado) is an endogenous nucleoside released from almost all cell types. It exerts neuroprotective and anti-inflammatory functions by acting through four receptor subtypes A1, A2A, A2B and A3 (ARs). These receptors differ in their affinity for Ado, in the type of G protein that they recruit and finally in the downstream signalling that are activated in target cells. The levels of Ado in the interstitial fluid are in the range of 20-200 nM, but many pathophysiological conditions such as asthma, neurodegenerative disorders, chronic inflammatory diseases and cancer are associated with changes in Ado levels. The primary aim of Ado is to reduce tissue injury and promote repair by different receptor-mediated mechanisms, including the increase of oxygen supply/demand ratio, anti-inflammatory effects and stimulation of angiogenesis. The investigation of ARs and their ligands is a rapidly growing field; there is extensive evidence for the involvement of ARs in the physiological regulation of several homeostatic processes and their implication in the ethiology of many diseases. The aim of this work was to analyse the expression of ARs and the signalling pathway, transcription factors and cytochines activated by them in different pathophysiological conditions linked to hypoxic and inflammatory conditions. At first, the role of ARs was studied in healthy and aneuploid pregnancies to understand reasons of spontaneous abortion (SA). It has been suggested that the causes of SA in aneuploidy are no different to those in euploidy, with the increased frequency in the former perhaps being ascribable to a genetically-determined imbalance in the mediators of placental perfusion and uterine contraction. So aneuploidy can be used as a model of this event. The ado transduction cascade appears to be disturbed in Trisomy 21 (TR21) through reduced expression of A2BAR and A1AR. These anomalies may be implicated in complications such as fetal growth restriction, malformation and/or SA, well known features of aneuploid pregnancies. Therefore A1AR and A2BAR could be potential biomarkers able to provide an early indication of SA risk and their stimulation may turn out to improve fetoplacental perfusion by increasing nitric oxide (NO) and vascular endothelial growth factor (VEGF). Next, the effects of ARs in glial cells under inflammatory conditions have been investigated. Glial cells, astrocytes and microglia, are important contributors to inflammatory immune responses and hypoxia-inducible factor 1 (HIF-1α) is the key transcription factor that is upregulated in response to hypoxia and inflammatory stimuli. Ado, through A1AR and A3AR activation, reduces lipopolysaccharide (LPS)-stimulated HIF-1α mRNA expression and protein accumulation by inhibiting LPS-triggered p42/p44 mitogen-activated protein kinase (p44/42 MAPK) and serine/threonine protein kinase (Akt) phosphorylation in normoxic and hypoxic conditions. This leads to an inhibition of genes involved in inflammation like Inducible Nitric Oxide Synthase (iNOS) and A2BARs, that are stimulated by LPS and further increased by LPS in concert with hypoxia, whilst does not affect angiogenesis and metabolic related genes, with the exception of glucose transporter 1 (GLUT1) and hexokinase 2 (HK2) that are stimulated by LPS in normoxia and reduced by A1AR and A3AR activation. These findings add a new molecular pathway activated by Ado in astrocytes to give a reduction of genes involved in inflammation and hypoxic injury that may cohexist in stroke, ischemia and other central nervous system (CNS) disorders.