Levine Yehoshua C, Li Gordon K, Michel Thomas
Cardiovascular Division, Brigham and Women's Hospital, Harvard Medical School, 75 Francis Street, Boston, MA 02115, USA.
J Biol Chem. 2007 Jul 13;282(28):20351-64. doi: 10.1074/jbc.M702182200. Epub 2007 May 22.
The endothelial isoform of nitric-oxide synthase (eNOS), a key determinant of vascular homeostasis, is a calcium/calmodulin-dependent phosphoprotein regulated by diverse cell surface receptors. Vascular endothelial growth factor (VEGF) and sphingosine 1-phosphate (S1P) stimulate eNOS activity through Akt/phosphoinositide 3-kinase and calcium-dependent pathways. AMP-activated protein kinase (AMPK) also activates eNOS in endothelial cells; however, the molecular mechanisms linking agonist-mediated AMPK regulation with eNOS activation remain incompletely understood. We studied the role of AMPK in VEGF- and S1P-mediated eNOS activation and found that both agonists led to a striking increase in AMPK phosphorylation in pathways involving the calcium/calmodulin-dependent protein kinase kinase beta. Treatment with tyrosine kinase inhibitors or the phosphoinositide 3-kinase inhibitor wortmannin demonstrated differential effects of VEGF versus S1P. Small interfering RNA (siRNA)-mediated knockdown of AMPKalpha1or Akt1 impaired the stimulatory effects of both VEGF and S1P on eNOS activation. AMPKalpha1 knockdown impaired agonist-mediated Akt phosphorylation, whereas Akt1 knockdown did not affect AMPK activation, thus suggesting that AMPK lies upstream of Akt in the pathway leading from receptor activation to eNOS stimulation. Importantly, we found that siRNA-mediated knockdown of AMPKalpha1 abrogates agonist-mediated activation of the small GTPase Rac1. Conversely, siRNA-mediated knockdown of Rac1 decreased the agonist-mediated phosphorylation of AMPK substrates without affecting that of AMPK, implicating Rac1 as a molecular link between AMPK and Akt in agonist-mediated eNOS activation. Finally, siRNA-mediated knockdown of caveolin-1 significantly enhanced AMPK phosphorylation, suggesting that AMPK is negatively regulated by caveolin-1. Taken together, these results suggest that VEGF and S1P differentially regulate AMPK and establish a central role for an agonist-modulated AMPK --> Rac1 --> Akt axis in the control of eNOS in endothelial cells.
一氧化氮合酶（eNOS）的内皮亚型是血管稳态的关键决定因素，是一种受多种细胞表面受体调节的钙/钙调蛋白依赖性磷蛋白。血管内皮生长因子（VEGF）和鞘氨醇-1-磷酸（S1P）通过Akt/磷酸肌醇3-激酶和钙依赖性途径刺激eNOS活性。AMP激活的蛋白激酶（AMPK）也在内皮细胞中激活eNOS；然而，将激动剂介导的AMPK调节与eNOS激活联系起来的分子机制仍未完全明了。我们研究了AMPK在VEGF和S1P介导的eNOS激活中的作用，发现这两种激动剂在涉及钙/钙调蛋白依赖性蛋白激酶激酶β的途径中均导致AMPK磷酸化显著增加。用酪氨酸激酶抑制剂或磷酸肌醇3-激酶抑制剂渥曼青霉素处理显示出VEGF与S1P的不同作用。小干扰RNA（siRNA）介导的AMPKα1或Akt1敲低削弱了VEGF和S1P对eNOS激活的刺激作用。AMPKα1敲低损害了激动剂介导的Akt磷酸化，而Akt1敲低不影响AMPK激活，因此表明在从受体激活到eNOS刺激的途径中AMPK位于Akt的上游。重要的是，我们发现siRNA介导的AMPKα1敲低消除了激动剂介导的小GTP酶Rac1的激活。相反，siRNA介导的Rac1敲低降低了激动剂介导的AMPK底物的磷酸化，而不影响AMPK的磷酸化，这表明Rac1是激动剂介导的eNOS激活中AMPK和Akt之间的分子联系。最后，siRNA介导的小窝蛋白-1敲低显著增强了AMPK磷酸化，表明AMPK受小窝蛋白-1负调控。综上所述，这些结果表明VEGF和S1P对AMPK的调节存在差异，并在控制内皮细胞中eNOS方面确立了激动剂调节的AMPK→Rac1→Akt轴的核心作用。
