Extracellular Adenosine Inhibits Liver Gluconeogenesis via AdenosineTransporter ENT1 by Blocking of cAMP/p-PKA Signaling Pathway

Diabetes mellitus, which is characterized by disorders of glucose and lipid metabolism, hasbecome a heavy global disease burden. Increasing evidences suggest that the adenosine system plays a key role inregulating insulin and glucose homeostasis. Adenosine is an important regulator of cellular metabolism and is involved in several physiopathological processes such as energy metabolism, immune regulation, and oxidative stressthrough activation of G protein-coupled receptors and nucleoside transporters. However, the role of adenosine in theregulation of hepatic gluconeogenesis has not been elucidated. This article verified the regulatory effect of adenosine on glucagon stimulated hepatic gluconeogenesis pathway after its transfer into the cytosol via ENT (equilibrativenucleoside transporter) at multiple levels. The results showed that exogenous adenosine inhibited blood glucoseelevation in mice. In a cellular model, adenosine inhibited hepatic gluconeogenesis and thereby reduced glucoseoutput in a dose-dependent manner with minimal cytotoxicity. The ENT was widely expressed in liver tissues andcells, and the ENT1 mediated the hepatic glucose output inhibited by adenosine. Furthermore, adenosine-mediatedinhibition of gluconeogenesis was not dependent on activation of the AMPK pathway. After extracellular adenosinestimulation, the intracellular cAMP concentration was significantly reduced, the expression of phosphorylated PKAdownstream proteins was significantly inhibited, and the cellular glycolytic output capacity was significantly reduced, and this inhibition could be attenuated by ENT inhibitors but not by adenosine kinase inhibitors. The resultsshowed that the transfer of extracellular adenosine into the cell via the nucleoside transporter ENT inhibited adenylate cyclase activity, which in turn inhibited cAMP synthesis and the expression of phosphorylated PKA substrateproteins, inhibited hepatic gluconeogenesis, and ultimately reduced glucose output.