eNOS at a glance.

Endothelium-derived nitric oxide (NO) is a critical regulator of cardiovascular homeostasis. Endothelial nitric oxide synthase (eNOS or NOS3)-derived NO is an endogenous vasodilatory gas that continually regulates the diameter of blood vessels and maintains an anti-proliferative and anti-apoptotic environment in the vessel wall. Initially thought to be a simple, calmodulin (CaM) regulated enzyme, it is clear that eNOS has evolved to be tightly controlled by coand post-translational lipid modifications, phosphorylation on multiple residues and regulated proteinprotein interactions (Fulton et al., 2001). Various extracellular signals can promote NO release from endothelial cells Physiologically, endothelial cells are exposed to the hemodynamic forces of blood including laminar shear stress. Shear stress, via G proteins (Gs), can activate several signal transduction pathways, including the phosphoinoside 3-kinase (PI3K) and adenylate cyclase (AC) pathways, leading to eNOS activation via phosphorylation of serine residues (S617 and S1179 for Akt, and S635 and S1179 for PKA), which promote eNOS activation. Shear stress also increases S116 phosphorylation; however, the kinase responsible for this phosphorylation and the function of S116 are not well understood (Boo and Jo, 2003). Additional stimuli, such as vascular endothelial growth factor (VEGF), estrogen, sphingosine 1phosphate (S-1-P) and bradykinin, can bind to their cognate receptors and also stimulate PI3K/Akt. However, they also activate phospholipase C-γ (PLC-γ) to increase cytoplasmic calcium and diacylglycerol (DAG) levels. The increase in cytoplasmic calcium levels activates CaM, which binds to the canonical CaM-binding domain in eNOS to promote the alignment of the oxygenase and reductase domains of eNOS, leading to efficient NO synthesis. In addition, CaM can activate CaM kinase II, which may phosphorylate eNOS on S1179. Increases in DAG levels can activate PKC to phosphorylate T497, which may negatively regulate eNOS or influence its coupling. Finally, metabolic stress triggering the breakdown of ATP will stimulate AMP kinase (AMPK) to phosphorylate eNOS on S1179 (Chen et al., 1999; Dimmeler et al., 1998; Fleming et al., 2001; Fulton et al., 1999; Harris et al., 2001; Haynes et al., 2000; Igarashi and Michel, 2001; Lin et al., 2003; Michell et al., 2002; Morales-Ruiz et al., 2001; Simoncini et al., 2000).