细胞外腺苷介导核苷转运体ENT1阻断cAMP/p-PKA信号通路抑制肝脏糖异生

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

糖尿病的关键特征是葡萄糖和脂质代谢紊乱。目前,糖尿病已成为沉重的全球疾病负担。越来越多的证据表明,腺苷系统在调节胰岛素和葡萄糖平衡中发挥关键作用。腺苷是一种重要的细胞代谢调节剂,通过激活G蛋白偶联受体和核苷转运体参与能量代谢、免疫调节、氧化应激等多个生理病理过程。然而,腺苷在肝脏糖异生调控中的角色尚未被阐明。该文从多层面验证了腺苷通过核苷转运体(equilibrative nucleoside transporter,ENT)转运入胞内后对胰高血糖素刺激引起的肝脏糖异生通路的影响。结果显示,在体内模型中,外源性腺苷显著抑制小鼠血糖升高。在细胞模型中,腺苷以剂量依赖的方式抑制肝脏糖异生进而降低葡萄糖输出水平,且无细胞毒性。肝脏组织及细胞中ENT广泛表达,其中1型核苷转运体(equilibrative nucleoside transporter 1,ENT1)介导了腺苷抑制的肝糖输出。此外,腺苷介导的糖异生抑制并非依赖于AMP依赖的蛋白激酶[adenosine 5’-monophosphate(AMP)-activated protein kinase,AMPK]通路的激活。最后发现,细胞外腺苷刺激后,细胞内的环磷酸腺苷(cyclic adenosine monophosphate,cAMP)浓度显著降低,磷酸化蛋白激酶A(phospho-protein kinase A,p-PKA)下游蛋白表达受抑制,细胞糖输出能力显著下降,该抑制作用可被ENT抑制剂有效逆转,但不能被AK抑制剂削弱。以上结果表明,细胞外腺苷通过ENT1转移入胞内后,抑制AC活性,从而抑制cAMP合成和p-PKA底物蛋白的表达,抑制肝脏糖异生功能,最终降低糖输出水平。

Diabetes mellitus,which is characterized by disorders of glucose and lipid metabolism,has become a heavy global disease burden.Increasing evidences suggest that the adenosine system plays a key role in regulating insulin and glucose homeostasis.Adenosine is an important regulator of cellular metabolism and is in-volved in several physiopathological processes such as energy metabolism,immune regulation,and oxidative stress through activation of G protein-coupled receptors and nucleoside transporters.However,the role of adenosine in the regulation 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(equilibrative nucleoside transporter)at multiple levels.The results showed that exogenous adenosine inhibited blood glucose elevation in mice.In a cellular model,adenosine inhibited hepatic gluconeogenesis and thereby reduced glucose output in a dose-dependent manner with minimal cytotoxicity.The ENT was widely expressed in liver tissues and cells,and the ENT1 mediated the hepatic glucose output inhibited by adenosine.Furthermore,adenosine-mediated inhibition of gluconeogenesis was not dependent on activation of the AMPK pathway.After extracellular adenosine stimulation,the intracellular cAMP concentration was significantly reduced,the expression of phosphorylated PKA downstream proteins was significantly inhibited,and the cellular glycolytic output capacity was significantly re-duced,and this inhibition could be attenuated by ENT inhibitors but not by adenosine kinase inhibitors.The results showed that the transfer of extracellular adenosine into the cell via the nucleoside transporter ENT inhibited adenyl-ate cyclase activity,which in turn inhibited cAMP synthesis and the expression of phosphorylated PKA substrate proteins,inhibited hepatic gluconeogenesis,and ultimately reduced glucose output.