Copyright © 2006, European Society of Cardiology
Endothelial nitric oxide synthase uncoupling: Is it a physiological mechanism of endothelium-dependent relaxation in cerebral artery?
Cardiovascular Division, Department of Internal Medicine, Kobe University Graduate School of Medicine, 7-5-1 Kusunoki-cho, Chuo-ku, Kobe, 650-0017, Japan
* Corresponding author. Tel.: +81 78 382 5840; fax: +81 78 382 5858. Email address: yokoyama{at}med.kobe-u.ac.jp
Received 31 October 2006; accepted 8 November 2006
See article by Drouin et al. [8] (pages 73–81) in this issue.
Nitric oxide (NO) is generated from the conversion of L -arginine to L-citrulline by endothelial nitric oxide synthase (eNOS), which requires Ca2+/calmodulin, FAD, FMN, and tetrahydrobiopterin (BH4) as cofactors. Chemical studies in vitro demonstrated that the catalytic mechanisms of NOS involve flavin-mediated electron transport from a flavin-containing reductase domain to a heme-containing oxygenase domain. Here, oxygen is reduced and incorporated into the guanidine group of L-arginine, giving rise to NO and L-citrulline. Calcium/calmodulin binding to NOS increases the rate of reduction of both flavins and the heme iron. Reduction of iron (III) to iron (II) facilitates oxygen binding to the heme group to form a transient ferrous–dioxygen complex. NOS can also produce superoxide under certain conditions. Superoxide is generated from the oxygenase domain by dissociation of the ferrous–dioxygen complex [1,2]. This state is referred to as the "uncoupled state of eNOS" (eNOS uncoupling). BH4 couples L-arginine oxidation to NADPH consumption and prevents dissociation of the ferrous–dioxygen complex. Although in vitro data using purified eNOS demonstrate that modulation of the BH4 concentration may regulate the ratio of superoxide to NO generated by eNOS, it is as yet unclear how the endothelium regulates the production of these radicals in vivo. Although there are some methods to detect and quantify free radicals superoxide as well as NO in chemical systems, their detection in vivo has potential limitations. NO has a variety of functions, but its action as the endothelium-derived relaxing factor (EDRF) is the most important for the maintenance of vascular homeostasis. Recently, it was revealed that under certain pathological circumstances, eNOS becomes dysfunctional and produces superoxide rather than NO [3]. The pathophysiological role of dysfunctional eNOS has attracted attention in vascular disorders, including atherosclerosis, hypertension, and diabetes mellitus [4–6].
Among a number of molecular mechanisms proposed for eNOS uncoupling, evidence indicates that BH4 may be a key molecule in the control of NO and superoxide generation, and consequently the formation of hydrogen peroxide and peroxynitrite by eNOS [7]. Thus, it is conceivable that BH4 may switch eNOS activity from NO to superoxide generation at two levels: decreases in BH4 binding affinity or decreases in cellular BH4 concentration. Further studies of mechanisms involved in control of vascular GTP cyclohydrolase I activity and expression as well as metabolism of BH4 in endothelial cells may provide explanation for the role of BH4 in eNOS uncoupling.
In this issue of Cardiovascular Research, Drouin et al. demonstrate that by using pharmacological inhibitors in pressurized cerebral arteries isolated from young, healthy mice, NO derived from eNOS contributes to vascular tone at rest and that H2O2 derived from eNOS activation plays a physiological role as a significant part of endothelium-dependent dilation induced by acetylcholine (Fig. 1) [8]. These observations are in accord with previous observations [9]. They also assessed fluorescence-associated oxygen free radicals (DCF-DA) and NO (DAF-2) production in these arterial preparations. The critical importance of eNOS-derived superoxide as an EDRF was confirmed by using cerebral arteries from eNOS knockout mice. They conclude that H2O2-dependent dilation in mouse cerebral arteries originating from eNOS activation appears to be due to a physiological eNOS uncoupling state.
|
There is increasing evidence that the endothelium produces a variety of reactive oxygen species (ROS), such as superoxide, hydrogen peroxide, and hydroxyl radical, as well as reactive nitrogen species (RNS), such as NO and peroxynitrite, in vascular cells. Each molecule has been shown to affect vascular tone directly or indirectly. Superoxide is generated in the endothelium by many sources, including eNOS and NADPH oxidase. In particular, it has been shown that H2O2 is formed from superoxide as a result of the various superoxide dismutases (SOD) and that Cu/ZnSOD is an important source of H2O2 produced in response to acetylcholine. The role of H2O2 as an EDRF or endothelium-derived hyperpolarizing factor (EDHF) in mouse and human mesenteric arteries and porcine coronary microvessels has been published previously [10–12]. H2O2-induced vascular relaxation was reported to be mediated by several mechanisms, including an activation of potassium channels. The interaction of ROS with other molecules such as NO and arachidonic acid may also affect vascular tone. Indeed, superoxide is thought to generate peroxynitrite, a potent ROS, by reaction with NO, which oxidizes BH4, leading to eNOS uncoupling. However, it is not clear what concentrations of peroxynitrite are produced endogenously within the endothelial cells and what role endogenously produced peroxynitrite plays in regulating cerebral vascular tone. The compartmentalization of the enzymatic sources of ROS generation and local concentrations of the catalytic enzymes, including cytosolic Cu/ZnSOD and substances such as superoxide and nitric oxide, will determine the species and amounts of ROS that are generated and involved in regulation of vascular tone.
It seems likely that the role of H2O2 as an EDRF or EDHF is not a ubiquitous pathway in the vasculature, but is selective for only some vasoactive stimuli as well as some vascular-specified beds. There is increasing evidence that endothelial cells differ between vascular beds and between species, although endothelial cells throughout the vasculature share basic characteristics of its structure and function. Further studies are required to clarify the molecular mechanisms responsible for the different modes of vascular tone regulation in different vascular beds. More information will help to localize the source of ROS generation within endothelial cells coupled to the selective activation of signaling systems that are involved in regulating vascular tone. The development of new methods to detect ROS and RNS generation and assess vascular tone regulation in vivo should provide a unique, new insight into the validation of a critical role of ROS in cardiovascular function and disease [13,14].
 |
References |
|---|
 Top References |
|---|
- Rodriguez-Crespo I., Gerber N.C., Ortiz de Montellano P.R. Endothelial nitric-oxide synthase. Expression in Escherichia coli, spectroscopic characterization, and role of tetrahydrobiopterin in dimer formation. J Biol Chem (1996) 271:11462–11467.
[Abstract/Free Full Text] - Vasquez-Vivar J., Kalyanaraman B., Martasek P., Hogg N., Masters B.S., Karoui H., et al. Superoxide generation by endothelial nitric oxide synthase: the influence of cofactors. Proc Natl Acad Sci U S A (1998) 95:9220–9225.
[Abstract/Free Full Text] - Kawashima S., Yokoyama M. Dysfunction of endothelial nitric oxide synthase and atherosclerosis. Arterioscler Thromb Vasc Biol (2004) 24:998–1005.
[Abstract/Free Full Text] - Ozaki M., Kawashima S., Yamashita T., Hirase T., Namiki M., Inoue N., et al. Overexpression of endothelial nitric oxide synthase accelerates atherosclerotic lesion formation in apoE-deficient mice. J Clin Invest (2002) 110:331–340.[CrossRef][Web of Science][Medline]
- Yokoyama M. Oxidant stress and atherosclerosis. Curr Opin Pharmacol (2004) 4:110–115.[CrossRef][Web of Science][Medline]
- Verhaar M.C., Westerweel P.E., van Zonneveld A.J., Rabelink T.J. Free radical production by dysfunctional eNOS. Heart (2004) 90:494–495.
[Free Full Text] - Alp N.J., Channon K.M. Regulation of endothelial nitric oxide synthase by tetrahydrobiopterin in vascular disease. Arterioscler Thromb Vasc Biol (2004) 24:413–420.
[Abstract/Free Full Text] - Drouin E., Thorin-Trescases N., Hamel E., Falck J.R., Thorin E. Endothelial nitric oxide synthase activation leads to dilatory H2O2 production in mouse cerebral arteries. Cardiovasc Res (2007) 73:73–81. 10.1016/j.cardiores.2006.10.005.
[Abstract/Free Full Text] - Faraci F.M. Reactive oxygen species: influence on cerebral vascular tone. J Appl Physiol (2006) 100:739–743.
[Abstract/Free Full Text] - Matoba T., Shimokawa H., Nakashima M., Hirakawa Y., Mukai Y., Hirano K., et al. Hydrogen peroxide is an endothelium-derived hyperpolarizing factor in mice. J Clin Invest (2000) 106:1521–1530.[Web of Science][Medline]
- Shimokawa H., Morikawa K. Hydrogen peroxide is an endothelium-derived hyperpolarizing factor in animals and humans. J Mol Cell Cardiol (2005) 39:725–732.[CrossRef][Web of Science][Medline]
- Cai H. Hydrogen peroxide regulation of endothelial function: origins, mechanisms, and consequences. Cardiovasc Res (2005) 68:26–36.
[Abstract/Free Full Text] - Yamato M., Egashira T., Utsumi H. Application of in vivo ESR spectroscopy to measurement of cerebrovascular ROS generation in stroke. Free Radic Biol Med (2003) 35:1619–1631.[CrossRef][Web of Science][Medline]
- Kidoguchi K., Tamaki M., Mizobe T., Koyama J., Kondoh T., Kohmura E., et al. In vivo X-ray angiography in the mouse brain using synchrotron radiation. Stroke (2006) 37:1856–1861.
[Abstract/Free Full Text]
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  E. N. Dedkova and L. A. Blatter Characteristics and function of cardiac mitochondrial nitric oxide synthase J. Physiol., February 15, 2009; 587(4): 851 - 872. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||