eNOS phosphorylation: A pivotal molecular switch in vasodilation and cardioprotection?

Nitric oxide (NO) is a ubiquitous signaling molecule that regulates various cellular physiological and pathological events in virtually all vital organs. The synthesis of NO is catalyzed primarily by three isoforms of NO synthase (NOS), which were originally named after the tissues in which these enzymes were first characterized and cloned (i.e. nNOS—neuronal, iNOS—inducible or inflammation-related, and eNOS—endothelial) [ [1] Lowenstein C.J. Michel T. What's in a name? eNOS and anaphylactic shock. J. Clin. Invest. 2006; 116: 2075-2078 Crossref PubMed Scopus (26) Google Scholar ]. Accumulating evidence suggests that all of the three NOS isozymes are expressed and distributed in various cell types in the cardiovascular system far beyond their initial identified cell types and locations. Besides the extensive studies on transcriptional control of NOS expression and the Ca2+-dependent activation of eNOS and nNOS, there has been an increasing recognition that post-translational modification of NOS may effectively modulate the enzyme activity which in turn tightly controls the cellular viability and functions through NO production at the right time, right location, and right amount [ [2] Kone B.C. Kuncewicz T. Zhang W. Yu Z.Y. Protein interactions with nitric oxide synthases: controlling the right time, the right place, and the right amount of nitric oxide. Am. J. Physiol.: Renal. Physiol. 2003; 285: F178-F190 Crossref PubMed Scopus (254) Google Scholar ]. Among these post-translational regulation mechanisms of eNOS, the kinase-mediated protein phosphorylation plays a critical role [ 3 Michel T. Li G.K. Busconi L. Phosphorylation and subcellular translocation of endothelial nitric oxide synthase. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 6252-6256 Crossref PubMed Scopus (313) Google Scholar , 4 Corson M.A. James N.L. Latta S.E. Nerem R.M. Berk B.C. Harrison D.G. Phosphorylation of endothelial nitric oxide synthase in response to fluid shear stress. Circ. Res. 1996; 79: 984-991 Crossref PubMed Scopus (412) Google Scholar ]. Several studies were simultaneously published in 1999, which provided direct evidence that eNOS activity may be substantially modulated through phosphorylation by Akt/PKB (protein kinase B) [ 5 Fulton D. Gratton J.P. McCabe T.J. Fontana J. Fujio Y. Walsh K. et al. Regulation of endothelium-derived nitric oxide production by the protein kinase Akt. Nature. 1999; 399: 597-601 Crossref PubMed Scopus (2263) Google Scholar , 6 Dimmeler S. Fleming I. Fisslthaler B. Hermann C. Busse R. Zeiher A.M. Activation of nitric oxide synthase in endothelial cells by Akt-dependent phosphorylation. Nature. 1999; 399: 601-605 Crossref PubMed Scopus (3082) Google Scholar , 7 Gallis B. Corthals G.L. Goodlett D.R. Ueba H. Kim F. Presnell S.R. et al. Identification of flow-dependent endothelial nitric-oxide synthase phosphorylation sites by mass spectrometry and regulation of phosphorylation and nitric oxide production by the phosphatidylinositol 3-kinase inhibitor LY294002. J. Biol. Chem. 1999; 274: 30101-30108 Abstract Full Text Full Text PDF PubMed Scopus (288) Google Scholar , 8 Michell B.J. Griffiths J.E. Mitchelhill K.I. Rodriguez-Crespo I. Tiganis T. Bozinovski S. et al. The Akt kinase signals directly to endothelial nitric oxide synthase. Curr. Biol. 1999; 9: 845-848 Abstract Full Text Full Text PDF PubMed Scopus (420) Google Scholar ] or AMPK (AMP-activated protein kinase) [ [9] Chen Z.P. Mitchelhill K.I. Michell B.J. Stapleton D. Rodriguez-Crespo I. Witters L.A. et al. AMP-activated protein kinase phosphorylation of endothelial NO synthase. FEBS Lett. 1999; 443: 285-289 Crossref PubMed Scopus (730) Google Scholar ]. More recently, there have been a number of studies on the regulatory role of eNOS phosphorylation in determining the enzyme activity, independently or interplaying with the subcellular localization of eNOS [ 10 Boo Y.C. Kim H.J. Song H. Fulton D. Sessa W. Jo H. Coordinated regulation of endothelial nitric oxide synthase activity by phosphorylation and subcellular localization. Free Radic. Biol. Med. 2006; 41: 144-153 Crossref PubMed Scopus (53) Google Scholar , 11 Fulton D. Gratton J.P. Sessa W.C. Post-translational control of endothelial nitric oxide synthase: why isn't calcium/calmodulin enough?. J. Pharmacol. Exp. Ther. 2001; 299: 818-824 PubMed Google Scholar ].