The Hypoxia-Adenosine Link during Myocardial Ischemia-Reperfusion Injury

Abstract

Despite increasing availability and more successful interventional approaches to restore coronary reperfusion, myocardial ischemia-reperfusion injury is a substantial cause of morbidity and mortality worldwide. During myocardial ischemia, the myocardium becomes profoundly hypoxic, thus causing stabilization of hypoxia-inducible transcription factors (HIF). Stabilization of HIF leads to a transcriptional program that promotes adaptation to hypoxia and cellular survival. Transcriptional consequences of HIF stabilization include increases in extracellular production and signaling effects of adenosine. Extracellular adenosine functions as a signaling molecule via the activation of adenosine receptors. Several studies implicated adenosine signaling in cardioprotection, particularly through the activation of the Adora2a and Adora2b receptors. Adenosine receptor activation can lead to metabolic adaptation to enhance ischemia tolerance or dampen myocardial reperfusion injury via signaling events on immune cells. Many studies highlight that clinical strategies to target the hypoxia-adenosine link could be considered for clinical trials. This could be achieved by using pharmacologic HIF activators or by directly enhancing extracellular adenosine production or signaling as a therapy for patients with acute myocardial infarction, or undergoing cardiac surgery.

Keywords: A2A; A2B; Adora2a; Adora2b; CD39; CD73; ENT1; ENT2; adenosine; hypoxia.

Figure 1

Hypoxia increases extracellular adenosine during myocardial ischemia. In the context of hypoxia, different cell types such as inflammatory cells and platelets release large amounts of adenine nucleotides (particularly ATP or ADP). The ectonucleotidases CD39 and CD73 convert ADP/ATP to AMP and AMP to adenosine, respectively. Therefore, the level of extracellular adenosine during hypoxia or inflammation critically depends on the expression level and enzymatic activity of CD39 and CD73. Hypoxia promotes the induction of CD39 expression through SP1 signaling, and of CD73 expression through binding of the transcription factor hypoxia-inducible factor HIF1A to a hypoxia-response element (HRE) within the CD73 promoter. ATP: adenosine triphosphate; ADP: adenosine diphosphate; AMP: adenosine monophosphate.

Figure 2

HIF protects against myocardial ischemia-reperfusion injury through the modulation of adenosine receptor signaling events. Adenosine receptors belong to the G protein-coupled receptor family and are composed of different subunits: the Gs alpha subunits (Gαs) and the beta-gamma subunit complex (Gβ/γ). The adenosine receptors Adora2a and Adora2b have been identified as target genes of HIF. Under hypoxic conditions, Adora2a and Adora2b are transcriptionally induced by HIF2A and HIF1A, respectively. Activation of these receptors with their specific agonists showed reduced infarct size in murine models of myocardial ischemia-reperfusion injury, suggesting their role in mediating the cardioprotective effects of HIF. The cardioprotection provided is associated with the activation of Adora2a signaling on lymphocytes and Adora2b signaling on myeloid cells and cardiomyocytes. The red arrowhead denotes upregulation. A2_A: Adenosine A2a Receptor. A2_B: Adenosine A2b Receptor.

Figure 3

HIF coordinates alternative adenosine receptor signaling via increasing netrin-1 expression and signaling through Adora2b. During myocardial reperfusion injury, different types of inflammatory cells, such as neutrophils, monocytes, etc. infiltrate into the myocardial tissue. This further exacerbates tissue hypoxia and tissue damage. During reperfusion, the transcript and protein levels of Netrin-1 are robustly increased in patients with myocardial ischemia and in mice with myocardial IR injury. The increased expression of netrin-1 is mediated by HIF1A activity, which can bind to an HRE within the Netrin-1 promoter. The increased release of netrin-1 enhances Adora2b signaling by interacting with myeloid Adora2b in an autocrine manner, dampens the accumulation of inflammatory cells, and ultimately mediates cardioprotection against IR injury. The red arrowhead denotes increase, and the dark blue arrowhead denotes decrease. A2_B: Adenosine A2b Receptor. NTN1: Netrin-1.

Figure 4

HIF contributes to attenuated adenosine uptake, reduced adenosine metabolism and concomitant cardioprotection during myocardial ischemia-reperfusion injury. Equilibrative nucleoside transporters (ENTs) regulate the uptake of adenosine from the extracellular towards the intracellular compartment where the major routes of adenosine removal is based on phosphorylation to AMP via adenosine kinase, thereby modulating adenosine levels. During myocardial ischemia-reperfusion injury, HIF transcriptionally represses ENT1, ENT2 and adenosine kinase, leading to elevated extracellular adenosine levels. The inhibition of ENTs in mice with dipyridamole or global deletion of Ent1 showed decreased intracellular adenosine uptake and increased extracellular adenosine levels, ultimately exerting cardioprotective effects. These indicate the contribution of HIF-dependent repression of ENTs to adenosine-mediated cardioprotection. ENT: equilibrative nucleoside transporter; AK: adenosine kinase.

Figure 5

HIF2A induces AREG signaling in cardiac myocytes to provide cardioprotection. HIF2A contributes to cardioprotection during myocardial IR injury. The epithelial growth factor amphiregulin (AREG) has been identified as one of the target genes of HIF2A, which is significantly induced at both mRNA and protein levels in cardiomyocytes during hypoxia. HIF2A was also found to increase the expression of AREG receptor ERBB1 at the post-transcriptional level. These findings indicate HIF2A protects against myocardial IR injury through AREG signaling. ERBB: Epidermal growth factor receptor.