Adenosine: an old drug newly discovered

Abstract

Over decades, anesthesiologists have used intravenous adenosine as mainstay therapy for diagnosing or treating supraventricular tachycardia in the perioperative setting. More recently, specific adenosine receptor therapeutics or gene-targeted mice deficient in extracellular adenosine production or individual adenosine receptors became available. These models enabled physicians and scientists to learn more about the biologic functions of extracellular nucleotide metabolism and adenosine signaling. Such functions include specific signaling effects through adenosine receptors expressed by many mammalian tissues; for example, vascular endothelia, myocytes, hepatocytes, intestinal epithelia, or immune cells. At present, pharmacological approaches to modulate extracellular adenosine signaling are evaluated for their potential use in perioperative medicine, including attenuation of acute lung injury; renal, intestinal, hepatic and myocardial ischemia; or vascular leakage. If these laboratory studies can be translated into clinical practice, adenosine receptor-based therapeutics may become an integral pharmacological component of daily anesthesiology practice.

Figure 1. Extracellular Adenosine Generation

Adenosine is an extracellular signaling molecule that is generated from its precursor molecules 5’-adenosine triphosphate (ATP) and 5’-adenosine monophosphate (AMP). This process consists of a two-step enzymatic reaction. Extracellular ATP released by multiple cell types (e.g. platelets, endothelia, epithelia or inflammatory cells) is rapidly converted to AMP by the ecto-apyrase (CD39). As second step in extracellular adenosine generation, AMP is converted by the 5’-ecto-nucleotdase (CD73) to adenosine. Thus, extracellular adenosine is available on the cell surface to activate its receptors.

Figure 2. Extracellular Adenosine Signaling

Extracellular adenosine exerts its biological effects through activation of adenosine receptors (ARs). At present, four different ARs have been identified: A1AR, A2AAR, A2BAR and A3AR. ARs are G-protein coupled receptors that utilize cyclic adenosine monophosphate (cAMP) as a second messenger. While signaling events through the A1 or A3AR dampen intracellular cAMP levels, activation of the A2A or A2BAR elevates cAMP levels. Biological examples for AR signaling events include adenosine-elicited heart-block for activation of the A1, vaso-dilation for the A2A, vascular barrier function for the A2B or murine mast-cell degranulation for the A3AR.

Figure 3. Extracellular Adenosine Uptake

Extracellular adenosine is taken up from the extracellular to the intracellular space via nucleoside transporters. Functionally, extracellular adenosine uptake is mainly achieved through “equilibrative nucleoside transporters”, ENT1 and ENT2. These transporters represent diffusion-limited channels that allow adenosine to freely cross the cell membrane following a concentration gradient.

Figure 4. Extracellular Adenosine Uptake at “Baseline” or during “Distress.”

Adenosine (A) can freely cross the cell membrane via equilibrative nucleoside transporters (ENTs). ENTs represent diffusion limited channels that allow free passage of adenosine through the cell membrane. During baseline conditions, only a small concentration gradient for adenosine across the cell membrane is present. Therefore, flux through ENTs is minimal. In contrast, extracellular adenosine concentrations are elevated during conditions of hypoxia, ischemia, or inflammation (“distress”). Under these conditions, adenosine flux through ENTs is directed mainly from the extracellular space towards the intracellular compartment. As long as flux through ENTs is directed from the outside towards the inside of the cell, inhibitors of ENTs (such as dipyridamole) or transcriptional mechanisms that repress ENTs will attenuate adenosine transport, and result in increased extracellular adenosine concentrations and signaling effects.

Figure 5. Intracellular Adenosine Metabolism

Following adenosine uptake via equilibrative nucleoside transporters (ENTs), intracellular adenosine is rapidly metabolized. Two competing pathways exist. Intracellular adenosine can be metabolized to inosine by the adenosine deaminase. Alternatively, adenosine is phosphorylated by the adenosine kinase to 5’-adenosine monophosphate (AMP).

Figure 6. Netrin-1 Dampens Hypoxia-Induced Inflammation by Enhancing Extracellular Adenosine Signaling

During hypoxia-elicited inflammation of mucosal organs such as the lungs or the intestine, the transcription factor hypoxia-inducible factor (HIF) coordinates the induction of netrin-1. While originally described as neuronal guidance molecule, recent studies implicate netrin-1 in the regulation of inflammatory responses. Here, epithelial-released netrin-1 dampens neutrophil accumulation in the hypoxic mucosa. This process involves activation of A2B adenosine receptor (AR)-dependent signaling pathways of neutrophils (“alternative AR activation”).

Figure 7. Consequences of Hypoxia on Adenosine Signaling Pathways

During situations of cellular distress (acute hypoxia, inflammation, ischemia reperfusion injury), hypoxia coordinates changes that lead to increases in extracellular signaling effects. These changes involve four mechanisms. First, extracellular adenosine production is enhanced through the transcriptional induction of the adenosine-producing enzymes ecto-apyrase (CD39; conversion of adenosine tri-phosphate, ATP, to adenosine mono-phosphate, AMP) and5’-ecto-nucleotidase (CD73; conversion of AMP to adenosine). In addition, adenosine effects are also enhanced on the receptor level. As such, hypoxia coordinates the selective induction of the A2B adenosine receptor (A2BAR). Moreover, hypoxia leads to transcriptional repression of equilibrative nucleoside transporters (ENTs), resulting in attenuated adenosine uptake, enhanced extracellular adenosine concentration and signaling. Finally, hypoxia also causes transcriptional repression of the adenosine kinase, the main enzyme for intracellular adenosine metabolism. Adenosine kinase catalyzes phosphorylation of adenosine to adenosine monophosphate (AMP). Hypoxia-dependent repression of adenosine kinase represents an additional hypoxia-elicited mechanism that enhances extracellular adenosine concentration and signaling during hypoxia.