THE ROLE OF ARGINASE IN ENDOTHELIAL DYSFUNCTION Topic Editor Rudolf Lucas IMMUNOLOGY Frontiers in Immunology March 2015 The role of arginase in endothelial dysfunction 1 Frontiers in Physiology November 2014 | Energy metabolism | 1 ABOUT FRONTIERS Frontiers is more than just an open-access publisher of scholarly articles: it is a pioneering approach to the world of academia, radically improving the way scholarly research is managed. The grand vision of Frontiers is a world where all people have an equal opportunity to seek, share and generate knowledge. Frontiers provides immediate and permanent online open access to all its publications, but this alone is not enough to realize our grand goals. FRONTIERS JOURNAL SERIES The Frontiers Journal Series is a multi-tier and interdisciplinary set of open-access, online journals, promising a paradigm shift from the current review, selection and dissemination processes in academic publishing. All Frontiers journals are driven by researchers for researchers; therefore, they constitute a service to the scholarly community. At the same time, the Frontiers Journal Series operates on a revo- lutionary invention, the tiered publishing system, initially addressing specific communities of scholars, and gradually climbing up to broader public understanding, thus serving the interests of the lay society, too. DEDICATION TO QUALITY Each Frontiers article is a landmark of the highest quality, thanks to genuinely collaborative interac- tions between authors and review editors, who include some of the world’s best academicians. Research must be certified by peers before entering a stream of knowledge that may eventually reach the public - and shape society; therefore, Frontiers only applies the most rigorous and unbiased reviews. Frontiers revolutionizes research publishing by freely delivering the most outstanding research, evaluated with no bias from both the academic and social point of view. By applying the most advanced information technologies, Frontiers is catapulting scholarly publishing into a new generation. WHAT ARE FRONTIERS RESEARCH TOPICS? Frontiers Research Topics are very popular trademarks of the Frontiers Journals Series: they are collections of at least ten articles, all centered on a particular subject. With their unique mix of varied contributions from Original Research to Review Articles, Frontiers Research Topics unify the most influential researchers, the latest key findings and historical advances in a hot research area! Find out more on how to host your own Frontiers Research Topic or contribute to one as an author by contacting the Frontiers Editorial Office: researchtopics@frontiersin.org FRONTIERS COPYRIGHT STATEMENT © Copyright 2007-2014 Frontiers Media SA. All rights reserved. All content included on this site, such as text, graphics, logos, button icons, images, video/audio clips, downloads, data compilations and software, is the property of or is licensed to Frontiers Media SA (“Frontiers”) or its licensees and/or subcontractors. The copyright in the text of individual articles is the property of their respective authors, subject to a license granted to Frontiers. The compilation of articles constituting this e-book, wherever published, as well as the compilation of all other content on this site, is the exclusive property of Frontiers. For the conditions for downloading and copying of e-books from Frontiers’ website, please see the Terms for Website Use. If purchasing Frontiers e-books from other websites or sources, the conditions of the website concerned apply. Images and graphics not forming part of user-contributed materials may not be downloaded or copied without permission. Individual articles may be downloaded and reproduced in accordance with the principles of the CC-BY licence subject to any copyright or other notices. They may not be re-sold as an e-book. As author or other contributor you grant a CC-BY licence to others to reproduce your articles, including any graphics and third-party materials supplied by you, in accordance with the Conditions for Website Use and subject to any copyright notices which you include in connection with your articles and materials. All copyright, and all rights therein, are protected by national and international copyright laws. The above represents a summary only. For the full conditions see the Conditions for Authors and the Conditions for Website Use. ISSN 1664-8714 ISBN 978-2-88919-308-0 DOI 10.3389/978-2-88919-308-0 ISSN 1664-8714 ISBN 978-2-88919-440-7 DOI 10.3389/978-2-88919-440-7 2015 Frontiers in Immunology March 2015 The role of arginase in endothelial dysfunction 2 THE ROLE OF ARGINASE IN ENDOTHELIAL DYSFUNCTION Topic Editor: Rudolf Lucas, Medical College of Georgia, USA In recent years, an increasing number of manuscripts have been published addressing the deleterious role of arginase in endothelial dysfunction. ROS have been shown to play a crucial role in arginase activation, which in turn leads to eNOS dysfunction. Frontiers in Immunology March 2015 The role of arginase in endothelial dysfunction 3 Table of Contents 05 Arginase in the vascular endothelium: friend or foe? Rudolf Lucas, David Fulton, Robert William Caldwell and Maritza J. Romero 07 The subcellular compartmentalization of arginine metabolizing enzymes and their role in endothelial dysfunction Feng Chen, Rudolf Lucas and David Fulton 12 Arginase: the emerging therapeutic target for vascular oxidative stress and inflammation Zhihong Yang and Xiu-Fen Ming 23 Therapeutic potential of the nitrite-generated NO pathway in vascular dysfunction Michael Madigan and Brian Zuckerbraun 32 Role of arginase in vessel wall remodeling William Durante 44 Arginase 1 mediates increased blood pressure and contributes to vascular endothelial dysfunction in deoxycorticosterone acetate-salt hypertension Haroldo A. Toque, Kenia P . Nunes, Modesto Rojas, Anil Bhatta, Lin Yao, Zhimin Xu, Maritza J. Romero, R. Clinton Webb, Ruth B. Caldwell and R. William Caldwell 54 Selective up-regulation of arginase-1 in coronary arteries of diabetic patients Zsolt Bagi, Attila Feher, Huijuan Dou and Zuzana Broskova 60 Arginase promotes skeletal muscle arteriolar endothelial dysfunction in diabetic rats Fruzsina K. Johnson, Robert A. Johnson, Kelly J. Peyton, Ahmad R. Shebib and William Durante 67 Vasomotor regulation of coronary microcirculation by oxidative stress: role of arginase Lih Kuo and Travis W. Hein 79 Arginase 1: an unexpected mediator of pulmonary capillary barrier dysfunction in models of acute lung injury Rudolf Lucas, Istvàn Czikora, Supriya Sridhar, Evgeny A. Zemskov, Aluya Oseghale, Sebastian Circo, Stephen D. Cederbaum, Trinad Chakraborty, David J. Fulton, Robert W. Caldwell and Maritza J. Romero 86 Arginase as a mediator of diabetic retinopathy Chintan Patel, Modesto Rojas, S. Priya Narayanan, Wenbo Zhang, Zhimin Xu, Tahira Lemtalsi, Kanjana Jittiporn, R. William Caldwell and Ruth B. Caldwell Frontiers in Immunology March 2015 The role of arginase in endothelial dysfunction 4 97 Development of novel arginase inhibitors for therapy of endothelial dysfunction Jochen Steppan, Daniel Nyhan and Dan E. Berkowitz 103 l -citrulline protects from kidney damage in type 1 diabetic mice Maritza J. Romero, Lin Yao, Supriya Sridhar, Anil Bhatta, Huijuan Dou, Ganesan Ramesh, Michael W. Brands, David M. Pollock, Ruth B. Caldwell, Stephen D. Cederbaum, C. Alvin Head, Zsolt Bagi, Rudolf Lucas and R. William Caldwell EDITORIAL published: 17 November 2014 doi: 10.3389/fimmu.2014.00589 Arginase in the vascular endothelium: friend or foe? Rudolf Lucas 1,2 *, David Fulton 1,2 , Robert William Caldwell 2 and Maritza J. Romero 1,2 1 Vascular Biology Center, Medical College of Georgia, Georgia Regents University, Augusta, GA, USA 2 Department of Pharmacology and Toxicology, Medical College of Georgia, Georgia Regents University, Augusta, GA, USA *Correspondence: rlucas@gru.edu Edited and reviewed by: Charles Dinarello, University of Colorado Health Sciences Center, USA Keywords: arginase inhibitors, eNOS, reactive oxygen species, microvascular permeability, impaired vasorelaxation, vessel wall remodeling, l-citrulline This special issue, entitled “the role of arginase in endothelial dys- function,” assembles original contributions (1–4), as well as timely reviews (5–12) broadly related to the deleterious activities of the manganese-containing enzyme arginase in the vascular endothe- lium. The arginase 1 isoform is cytosolic and is mainly localized in the liver, where it performs a crucial role in eliminating nitro- gen formed during amino acid and nucleotide metabolism via the urea cycle. Recent studies have demonstrated that it is also expressed in extra-hepatic tissues, including the vascular endothe- lium. Arginase 2 is a mitochondrial enzyme expressed in various cell types, including those in the kidneys (1) and the vascula- ture. In blood vessels, both isoforms likely play a role in the regulation of l -arginine homeostasis and the production of l - ornithine for subsequent polyamine and proline synthesis, which are involved in endothelial and smooth muscle cell proliferation and collagen deposition (13, 14). Polyamine and proline syn- thesis are crucial components of physiological and pathological vascular remodeling, a topic extensively discussed in this issue by Durante (5). Since both arginase isoforms are expressed in vascular endothelial cells, they can potentially interfere with the activity of endothelial nitric oxide synthase (eNOS), by “steal- ing away” the common substrate l -arginine, required to generate nitric oxide (NO) and l -citrulline. NO is a crucial mediator of endothelium-dependent vasorelaxation and restricts vascular growth and inflammation (14). Therefore, excessive activation of arginase by pathologic stimuli, including bacterial toxins [e.g., LPS and pneumolysin (6)], pro-inflammatory cytokines [e.g., TNF (15)], reactive oxygen species (ROS) (7, 11), or hyperglycemia (1, 3, 4) can potentially induce severe endothelial dysfunction. This theory is, however, complicated by observations that even dur- ing dramatically increased arginase activity, the concentrations of l -arginine in endothelial cells remain sufficiently high to concep- tually support eNOS-mediated NO synthesis. As such, this has led to the suggestion that there is sub-cellular compartmentaliza- tion of l -arginine into poorly interchangeable intracellular pools, a topic discussed by Chen et al. in this issue (8). The review by Yang and Ming (7) gives an overview of the direct role of arginase in eNOS dysfunction. Indeed, a pathological increase in vascular arginase activity was shown to significantly contribute to “eNOS uncoupling,” a phenomenon observed in various vas- cular pathologies and in aging (16), during which the enzyme generates detrimental amounts of superoxide instead of the vaso- protective NO. Decreased NO bioavailability within the vessel wall induced by competitive utilization of l -arginine by arginase and “eNOS uncoupling” can be partially circumvented by a recently discovered alternative pathway of NO generation: the reduction of nitrate and nitrite, which is the focus of the review by Madigan and Zuckerbraun (9). Since large vessel endothelial cells are function- ally and morphologically distinct from microvascular endothelial cells (17), this review highlights studies on the role of arginase in both of these cell types. Using a DOCA salt-induced mouse model in wild type, Arg1 + / − Arg2 − / − mice, Toque et al. present original data demonstrating an important role of arginase 1 in impaired vasorelaxation in the aorta and in hypertension (2). Bagi et al. present a concise review on the upregulation of arginase 1 expression and its effects on eNOS function in coronary arteries from diabetic patients (10). Addressing the microvascular com- partments, Johnson et al. demonstrate that arginase is an essential mediator of skeletal muscle arteriolar endothelial dysfunction in diabetes, which may further compromise glucose utilization and facilitate the development of diabetes and hypertension (3). Acute pretreatment with l -arginine or with arginase inhibitors signif- icantly improved endothelial function in skeletal arterioles. Kuo and Hein review recent data on the role of arginase in the gener- ation of ROS and the subsequent vasomotor dysfunction of the coronary microcirculation upon angiotensin 2 receptor activa- tion (11). Lucas et al. discuss recent findings suggesting a role for arginase 1 in pneumolysin-induced pulmonary capillary bar- rier dysfunction (6), which involves impairment of eNOS function (18). They demonstrate that arginase inhibitors significantly pre- vent pneumolysin-induced barrier dysfunction, at least in part by preventing the loss of expression of the adherens junction protein, VE cadherin (6). Comparing streptozotocin-treated diabetic wild type with diabetic Arg1 + / − Arg2 − / − transgenic mice, Patel et al. demonstrate an important role for arginase in the pathogenesis of diabetic retinopathy and they extend these findings in vitro in high glucose-treated bovine retinal endothelial cells. Their results advance arginase as a potential therapeutic target for preserv- ing NO bioavailability, limiting oxidative stress, and preventing early signs of diabetic retinopathy (4). Although acute treatment with l -arginine was shown to significantly improve endothelial function in several experimental studies, including those by John- son et al. in this issue (3), a recent clinical trial in patients with acute myocardial infarction demonstrated that a 6-month chronic treatment with l -arginine in addition to standard postinfarct med- ications did not improve clinical outcomes. Moreover, there was an increased risk of death in older patients after infarction, which promoted the early termination of the trial (19). This study clearly indicates that the long-term treatment with l -arginine is not the optimal treatment for arginase-mediated endothelial dysfunction www.frontiersin.org November 2014 | Volume 5 | Article 589 | 5 Lucas et al. Arginase in the vascular endothelium and stresses the need for alternative strategies to inhibit arginase. Given the vital role that arginase plays in the detoxification of ammonia in the urea cycle, this suggests that chronic inhibi- tion using specific arginase inhibitors should produce severe side effects. Surprisingly, this does not happen, possibly because the expression of arginase in the liver is much higher than in the vas- culature (14). The review by Steppan et al. in this issue discusses the development of novel ABH-based arginase inhibitors, apart from the classically used boronic acid derivatives and moreover addresses potential disadvantages of developing isoform-specific inhibitors for macrophage function (12). As an alternative to l - arginine therapies, l -citrulline is a product of NO synthases that can be recycled to l -arginine. Romero et al. presents data demon- strating a potent protective action of l -citrulline in streptozotocin- induced diabetic nephropathy in mice, accompanied by lower albuminuria and kidney fibrosis (1). l -citrulline also restored the NO/ROS balance and barrier function in high glucose-treated monolayers of human glomerular endothelial cells. Intriguingly, l -citrulline promoted the sustained elevation of arginase 2 expres- sion/activity in proximal tubule epithelium of kidneys from mice. This was mediated, at least in part, via a previously unappreci- ated immunomodulatory ability to significantly increase plasma levels of the anti-inflammatory cytokine IL-10 (20, 21). Given the weight of experimental evidence to date, there are little doubts that arginase plays a significant role in limiting l -arginine utiliza- tion by eNOS and compromising endothelial function. Currently, acute l -arginine and arginase inhibitors are effective in improv- ing endothelial function. However, given the important contri- butions of arginase to vital metabolic functions, the long-term consequences of arginase inhibition remain uncertain. Improved selectivity could perhaps be obtained by elucidating and targeting the molecular mechanisms that lead to specific upregulation of arginases in the vascular compartment. REFERENCES 1. Romero MJ, Yao L, Sridhar S, Bhatta A, Dou H, Ramesh G, et al. l -Citrulline protects from kidney damage in type 1 diabetic mice. Front Immunol (2013) 4 :480. doi:10.3389/fimmu.2013.00480 2. Toque HA, Nunes KP, Rojas M, Bhatta A, Yao L, Xu Z, et al. Arginase 1 mediates increased blood pressure and contributes to vascular endothelial dysfunction in deoxycorticosterone acetate-salt hypertension. Front Immunol (2013) 4 :219. doi:10.3389/fimmu.2013.00219 3. Johnson FK, Johnson RA, Peyton KJ, Shebib AR, Durante W. Arginase pro- motes skeletal muscle arteriolar endothelial dysfunction in diabetic rats. Front Immunol (2013) 4 :119. doi:10.3389/fimmu.2013.00119 4. Patel C, Rojas M, Narayanan SP, Zhang W, Xu Z, Lemtalsi T, et al. Arginase as a mediator of diabetic retinopathy. Front Immunol (2013) 4 :173. doi:10.3389/ fimmu.2013.00173 5. Durante W. Role of arginase in vessel wall remodeling. Front Immunol (2013) 4 :111. doi:10.3389/fimmu.2013.00111 6. Lucas R, Czikora I, Sridhar S, Zemskov EA, Oseghale A, Circo S, et al. Arginase 1: an unexpected mediator of pulmonary capillary barrier dysfunction in models of acute lung injury. Front Immunol (2013) 4 :228. doi:10.3389/fimmu.2013.00228 7. Yang Z, Ming XF. Arginase: the emerging therapeutic target for vascular oxida- tive stress and inflammation. Front Immunol (2013) 4 :149. doi:10.3389/fimmu. 2013.00149 8. Chen F, Lucas R, Fulton D. The subcellular compartmentalization of arginine metabolizing enzymes and their role in endothelial dysfunction. Front Immunol (2013) 4 :184. doi:10.3389/fimmu.2013.00184 9. Madigan M, Zuckerbraun B. Therapeutic potential of the nitrite-generated NO pathway in vascular dysfunction. Front Immunol (2013) 4 :174. doi:10.3389/ fimmu.2013.00174 10. Bagi Z, Feher A, Dou H, Broskova Z. Selective up-regulation of arginase- 1 in coronary arteries of diabetic patients. Front Immunol (2013) 4 :293. doi:10.3389/fimmu.2013.00293 11. Kuo L, Hein TW. Vasomotor regulation of coronary microcirculation by oxida- tive stress: role of arginase. Front Immunol (2013) 4 :237. doi:10.3389/fimmu. 2013.00237 12. Steppan J, Nyhan D, Berkowitz DE. Development of novel arginase inhibitors for therapy of endothelial dysfunction. Front Immunol (2013) 4 :278. doi:10.3389/ fimmu.2013.00278 13. Li H, Meininger CJ, Hawker JR Jr, Haynes TE, Kepka-Lenhart D, Mistry SK, et al. Regulatory role of arginase I and II in nitric oxide, polyamine, and proline syn- theses in endothelial cells. Am J Physiol Endocrinol Metab (2001) 280 (1):E75–82. 14. Pernow J, Jung C. Arginase as a potential target in the treatment of cardiovas- cular disease: reversal of arginine steal? Cardiovasc Res (2013) 98 (3):334–43. doi:10.1093/cvr/cvt036 15. Gao X, Xu X, Belmadani S, Park Y, Tang Z, Feldman AM, et al. TNF- alpha contributes to endothelial dysfunction by upregulating arginase in ischemia/reperfusion injury. Arterioscler Thromb Vasc Biol (2007) 27 (6):1269–75. doi:10.1161/ATVBAHA.107.142521 16. Katusic ZS. Mechanisms of endothelial dysfunction induced by aging: role of arginase I. Circ Res (2007) 101 (7):640–1. doi:10.1161/CIRCRESAHA.107. 162701 17. Lucas R, Lou J, Morel DR, Ricou B, Suter PM, Grau GE. TNF receptors in the microvascular pathology of acute respiratory distress syndrome and cerebral malaria. J Leukoc Biol (1997) 61 (5):551–8. 18. Predescu D, Predescu S, Shimizu J, Miyawaki-Shimizu K, Malik AB. Con- stitutive eNOS-derived nitric oxide is a determinant of endothelial junc- tional integrity. Am J Physiol Lung Cell Mol Physiol (2005) 289 :L371–81. doi:10.1152/ajplung.00175.2004 19. Schulman SP, Becker LC, Kass DA, Champion HC, Terrin ML, Forman S, et al. l -arginine therapy in acute myocardial infarction: the vascular interaction with age in myocardial infarction (VINTAGE MI) randomized clinical trial. JAMA (2006) 295 (1):58–64. doi:10.1001/jama.295.1.58 20. Ochoa JB, Bernard AC, O’Brien WE, Griffen MM, Maley ME, Rockich AK, et al. Arginase I expression and activity in human mononuclear cells after injury. Ann Surg (2001) 233 (3):393–9. doi:10.1097/00000658-200103000-00014 21. Liu Y, Gardner CR, Laskin JD, Laskin DL. Classical and alternative activation of rat hepatic sinusoidal endothelial cells by inflammatory stimuli. Exp Mol Pathol (2013) 94 (1):160–7. doi:10.1016/j.yexmp.2012.10.015 Conflict of Interest Statement: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Received: 07 October 2014; accepted: 04 November 2014; published online: 17 November 2014. Citation: Lucas R, Fulton D, Caldwell RW and Romero MJ (2014) Arginase in the vascular endothelium: friend or foe? Front. Immunol. 5 :589. doi: 10.3389/fimmu.2014.00589 This article was submitted to Inflammation, a section of the journal Frontiers in Immunology. Copyright © 2014 Lucas, Fulton, Caldwell and Romero. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. Frontiers in Immunology | Inflammation November 2014 | Volume 5 | Article 589 | 6 MINI REVIEW ARTICLE published: 09 July 2013 doi: 10.3389/fimmu.2013.00184 The subcellular compartmentalization of arginine metabolizing enzymes and their role in endothelial dysfunction Feng Chen 1 , Rudolf Lucas 1,2 and David Fulton 1,2 * 1 Vascular Biology Center, Georgia Regents University, Augusta, GA, USA 2 Department of Pharmacology, Georgia Regents University, Augusta, GA, USA Edited by: Charles Dinarello, University of Colorado Health Sciences Center, USA Reviewed by: Sungwoo Ryoo, Kangwon National University, South Korea Mark James Crabtree, University of Oxford, UK *Correspondence: David Fulton, Vascular Biology Center, Georgia Regents University, 1459 Laney Walker Bvld., Augusta, GA 30912, USA e-mail: dfulton@gru.edu The endothelial production of nitric oxide (NO) mediates endothelium-dependent vasore- laxation and restrains vascular inflammation, smooth muscle cell proliferation, and platelet aggregation. Impaired production of NO is a hallmark of endothelial dysfunction and pro- motes the development of cardiovascular disease. In endothelial cells, NO is generated by endothelial nitric oxide synthase (eNOS) through the conversion of its substrate, l-arginine to l-citrulline. Reduced access to l-arginine has been proposed as a major mechanism under- lying reduced eNOS activity and NO production in cardiovascular disease. The arginases (Arg1 and Arg2) metabolize l-arginine to generate l-ornithine and urea and increased expres- sion of arginase has been proposed as a mechanism of reduced eNOS activity secondary to the depletion of l-arginine. Indeed, supplemental l-arginine and suppression of arginase activity has been shown to improve endothelium-dependent relaxation and ameliorate car- diovascular disease. However, this simple relationship is complicated by observations that l-arginine concentrations in endothelial cells remain sufficiently high to support NO synthe- sis. Accordingly, the subcellular compartmentalization of intracellular l-arginine into poorly interchangeable pools has been proposed to allow for the local depletion of pools or pockets of l-arginine. In agreement with this, there is considerable evidence supporting the impor- tance of the subcellular localization of l-arginine metabolizing enzymes. In endothelial cells in vitro and in vivo , eNOS is found in discrete intracellular locations and the capacity to gen- erate NO is heavily influenced by its localization inside the cell. Arg1 and Arg2 also reside in different subcellular environments and are thought to differentially influence endothe- lial function. The plasma membrane solute transporter, CAT-1 and the arginine recycling enzyme, arginosuccinate lyase, co-localize with eNOS and facilitate NO release. Herein, we highlight the importance of the subcellular location of eNOS and arginine transporting and metabolizing enzymes to NO release and cardiovascular disease. Keywords: eNOS, l-arginine, nitric, arginase, CAT-1, ASL, ASS, l-citrulline ENDOTHELIAL DYSFUNCTION The past three decades have provided unprecedented gains in our understanding of vascular biology. It is now hard to conceive of a time when the vascular endothelium was thought to be a simple barrier, an inert layer of cells lining the lumen of blood vessels. However this was the prevailing view prior to 1981 and the world of vascular biology was irrevocably changed with Furch- gott’s discovery of an ability of the endothelium to direct changes in vasomotor function (1). In the time since, the depth and pace of research to understand the myriad functions of the endothelium has been remarkable. Not the least of these has been the discov- ery of endothelial nitric oxide synthase (eNOS) (2–4), an enzyme selectively expressed in the endothelial cells with the ability to generate nitric oxide (NO) and thus regulate blood vessel tone (5). Dysfunction of the vascular endothelium is considered to be the harbinger of cardiovascular disease and precedes the develop- ment of overt symptoms (6, 7). Given the importance of eNOS and endogenous NO production to endothelial function, it is not surprising that considerable effort has been focused on the mech- anisms influencing eNOS activity in cardiovascular disease. The primary enzymatic function of eNOS is to catalyze the NADPH- dependent conversion of l -arginine into NO, a process shared by the two other NOS isoforms (8). Once formed, NO has an expan- sive array of cellular targets both locally in the endothelium to influence inflammatory signaling, metabolism, exocytosis, prolif- eration, motility, and survival, but also in adjacent cells such as vascular smooth muscle cells to decrease vasomotor tone, prolif- eration and migration, and in platelets to suppress aggregation (9). Loss of these functions promotes increased inflammation, thrombosis, high blood pressure, and vascular cell proliferation, processes that are intimately involved in the development of cardiovascular disease. L-ARGININE Because of the obligatory role of l -arginine in NO synthesis, considerable attention has been focused on the importance of www.frontiersin.org July 2013 | Volume 4 | Article 184 | 7 Chen et al. Subcellular regulation of arginine l -arginine availability in the vascular production of NO. Fuel- ing this interest were early studies reporting that l -arginine could directly stimulate EDRF/NO synthesis (10–12) and that compro- mised endothelial function in cardiovascular disease states could be improved by supplementation with l -arginine both in animals (13–17), healthy humans (18) and those with high cholesterol (19–21), cardiac transplantation (22), peripheral artery disease (23), pulmonary hypertension (24), and angina (25). Consider- able evidence pointed toward l -arginine deficiency being a major rate limiting step in the synthesis of NO. However, the affinity of eNOS for l -arginine is low ( ∼ 2–3 μ M) (26) and the amount of l -arginine in endothelial cells is hundreds of times higher ( ∼ 840 μ M) (27) suggesting that a substrate deficiency was an unlikely unitary cause of eNOS dysfunction and that additional mechanisms of dysfunction must exist. SUBCELLULAR LOCALIZATION OF eNOS The co-translational N -myristoylation (glycine 2) and post- translational cysteine palmitoylation of eNOS (cysteines 15 and 26) enable membrane binding and the discrete subcellular target- ing (28). In the endothelial cell, eNOS can be found predominantly localized to the perinuclear Golgi (29) and microdomains of the plasma membrane, including caveolae and lipid rafts (30, 31). eNOS has also been reported in other compartments, such as the mitochondria, the nucleus and the cytoskeleton (32, 33). The importance of location to eNOS function and cellular NO release was first demonstrated by mutations that prevent both myristoyla- tion and palmitoylation resulting in an enzyme that is catalytically competent in activity assays outside the cell, but exhibits dra- matically reduced capacity to generate NO in intact cells (29). Furthermore, the relative activity of eNOS varies depending on its intracellular location with the highest activity observed from eNOS at the plasma membrane, followed by outer membranes of the cis-Golgi and very low activity in the cytosol, nucleus, and mitochondria (32, 34, 35). Given the dramatic ability of subcellular location to influence eNOS activity and NO release, it is not sur- prising that compartmentalization has been proposed as a major mechanism by which the local concentration of l -arginine can influence NO release (36). L-ARGININE TRANSPORTERS The concentration of l -arginine in human plasma is ∼ 100– 200 μ M (37) and higher concentrations, up to 840 μ M (27) can be found within the endothelial cell reflecting the existence of trans- port processes. A number of distinct transmembrane transporters exist on the plasma membrane of endothelial cells that mediate the predominantly sodium independent import of l -arginine via y + and y + l transporters. The major genes involved in y + import are CAT-1 (SLC7A1) and CAT-2 (SLC7A2), whereas for y + L import, LAT1 (SLC7A7 and SLC3A2) and LAT2 (SLC7A6 and SLC3A2) are required (36). The presence of CAT-1 in plasmalemma caveolae and the ability of extracellular l -arginine to stimulate NO release in cells with abundant l -arginine levels has led to the hypoth- esis that l -arginine exists in poorly interchangeable subcellular compartments and reaches eNOS in sufficient concentrations via metabolite channeling (36). While there is suggestive data for the existence of these pools (38), direct evidence and a mechanism for l -arginine sequestration is lacking. A further wrinkle to this story is that the cationic amino acid transporter, CAT-1 can stimulate eNOS activity via direct binding rather than delivering abundant l -arginine to its catalytic doorstep (39). L-ARGININE RECYCLING Endothelial cells can maintain their l -arginine levels despite the continuous release of NO (40), suggesting the existence of mech- anisms to recover substrate. Indeed, l -citrulline, the byproduct of eNOS-dependent NO generation, can be converted back to l - arginine via the sequential actions of arginosuccinate synthase (ASS) and arginosuccinate lyase (ASL) (41). The co-localization of ASS/ASL with eNOS in plasma membrane caveolae suggests that l -arginine recycling from l -citrulline is a significant source of NO (42, 43). The importance of this pathway is revealed by impaired endothelium-dependent NO generation and increased blood pressure in humans and mice with ASL deficiency (44). Not only do ASL and ASS co-localize with eNOS, but they have been shown to bind directly and regulate eNOS activ- ity (44). A caveat to these studies is that only a fraction of eNOS is present in plasma membrane caveolae and significant amounts of eNOS can be found on endomembranes such as the Golgi. It is not yet known if eNOS at the Golgi or other organelles are regulated by arginine regulatory enzymes in the same way as the plasma membrane/caveolae bound eNOS. l - arginine can also be generated from the breakdown of pro- teins via both proteosomal and lysosomal pathways, which lib- erates l -arginine (45). The breakdown of proteins also liberates asymmetric methylated arginines, monomethylarginine (MMA), and asymmetric dimethylarginine (ADMA) which are potent substrate inhibitors of eNOS activity. The methylation of pro- teins is increased in cardiovascular disease, providing a source for the increased levels of MMA and ADMA via proteolysis (46). Methylated arginines are metabolized by the dimethylargi- nine dimethylaminohydrolases (DDAH1 and DDAH2). DDAH is found primarily in the cytosol (47) although there are reports of expression in the mitochondria (47) and nucleus (48). Accu- mulation of asymmetric methylated arginines results in a degree of eNOS-inhibition that is proportional to the ratio of l - arginine/methylated arginine. The inhibition of eNOS can be relieved by supplementation with l -arginine leading to increased production of NO and improvement of endothelial function (49). ARGINASES Arginase I and Arginase II are homologous genes encoded by different chromosomes that share the catalytic function of con- verting l -arginine into urea and ornithine (50). A significant difference between Arginase I and Arginase II is their distinct sub- cellular distribution, with Arginase I detected predominantly in the cytosol and Arginase II within the mitochondria (51, 52). As enzymes that consume the substrate for eNOS, l -arginine, the arginases have been proposed as endogenous antagonists of eNOS. Increased expression and activity of Arginase I have been implicated in numerous cardiovascular diseases including diabetic retinopathy, asthma, coronary artery dysfunction during renovas- cular hypertension, and sickle cell disease (53–57) and Arginase Frontiers in Immunology | Inflammation July 2013 | Volume 4 | Article 184 | 8 Chen et al. Subcellular regulation of arginine II has been shown to be specifically increased in retinopathy of prematurity, human pulmonary arterial endothelial cells during hypertension, atherosclerosis, and in diabetic renal injury (57– 60). Numerous studies have shown that increased expression of arginase correlates with impaired NO synthesis and that inhi- bition of arginase increases NO production (53, 61, 62). How- ever, this seemingly simple relationship between eNOS and the arginases is complicated by enzyme kinetics and l -arginine con- centrations. The affinity of eNOS for arginine is relatively high ( K m = 3 μ M), the affinity of arginase for l -arginine relatively low (2 mM) and the concentrations of intracellular l -arginine (300– 800 μ M) sufficient to support near maximal eNOS activity. Two explanations have been proposed to explain the inhibitory actions of arginase, one is the 1000-fold higher enzyme activity ( V max ) and the other, the compartmentalization and regional deficiency of l -arginine (63). Vascular dysfunction achieved through the arginase-mediated depletion of l -arginine can be reversed with l -arginine supplementation (64) but this also drives increased arginase activity. L-ARGININE SUPPLEMENTATION The preceding evidence has emphasized the important role l - arginine plays in the maintenance of endothelial and cardiovas- cular function and is supported by studies showing that at least in the short-term; l -arginine supplementation can increase endothe- lial function and mitigate disease. However, more recent evidence suggests that chronic long term supplementation offers little ben- efit and may instead be harmful (65). The reasons for this are not well understood and likely to be numerous. Chronic exposure to high levels of NO can desensitize NO signaling, impair l -arginine import and increase vascular lesions and mortality (66–69). In contrast, inhibition of endogenous NO can increase sensitivity to NO donors and collectively this suggests that there is pushback when “pushing” NO signaling. Chronic supplementation with l - arginine can also influence other pathways including the greater activation of iNOS (70) which unlike eNOS, is primarily con- strained by substrate availability, and the increased expression and catalytic activity of the arginases due to their higher K m A consequence of increased arginase activity is the production of ornithine and attendant elevation of l -proline and the polyamines which can promote cell proliferation and maladaptive vascular remodeling (71). CONCLUSION l -Arginine is a semi-essential amino acid with a number of important roles in the endothelium including the ability to drive NO production. The compartmentalization of arginine metab- olizing and transporting enzymes has important ramifications for endothelial function and cardiovascular health. l -arginine transporters and recycling enzymes have been found in the same intracellular location as eNOS, and some have been found to directly bind eNOS. However, whether this proximity is necessary for providing eNOS with ready access to l -arginine is question- able. Catalytically inactive forms of ASL and substrate-inhibition of CAT-1 do not prevent the ability of these enzymes/transporters to stimulate NO release and this suggests they instead play a struc- tural role in the activation of eNOS. The arginases, which compete for and metabolize l -arginine, particularly when l -arginine is in high abundance, do not reside in the same intracellular locations as eNOS (and presumably do not physically associate) and thus are unlikely to exclusively regulate l -arginine content in pools accessi- ble to eNOS. Instead a more important role of the arginases may be to generate l -proline and polyamines that can negatively impact endothelial and vascular function. The accumulation of asym- metric methylated arginines occurs at the major sites of protein degradation, the proteasome and lysosome, and like the cytosolic DDAH, they are not thought to be in close proximity to eNOS. Frequently underappreciated is the important role eNOS subcel- lular location has on NO release. Targeting eNOS to the plasma membrane supports the highest levels of NO production followed by the Golgi and the cytoplasm (35). Rendering eNOS insensi- tive to calcium overrides the effects of intracellular location on eNOS activity and suggests that local calcium and not l -arginine, is the major determinant of efficient NO release (32, 72). While compartmentalization may not be a critical mechanisms by which l -arginine influences eNOS activity, its ability to increase NO release is well documented. However, the failure of supplemen- tal l -arginine to improve cardiovascular health may be considered another lesson learned of why too much of a good thing can be bad. Chronic high levels of NO can result in the refractoriness of its targets to respond and is well documented in vascular smooth muscle. Mechanisms that temporarily restrict eNOS activity such as caveolin-1 or the location of eNOS on membranes of the Golgi enable efficient production of NO in the right amount at the right time for the right response. REFERENCES 1. Furchgott RF, Zawadski JV. The obligatory role of the endothelial cells in the relaxation of arterial smooth muscle by acetylcholine. Nature (1980) 288 :373–6. doi:10.1038/2883 73a0 2. Nishida K, Harrison DG, Navas JP, Fisher AA, Dockery SP, Uematsu M, et al. Molecular cloning and characterization of the consti- tutive bovine aortic endothe- lial cell nitric oxide synthase. J Clin Invest (1992) 90 (5):2092–6. doi:10.1172/JCI116092 3. Lamas S, Marsden PA, Li GK, Tempst P, Michel T. Endothelial nitric oxide synthase: molecular cloning and characterization of a distinct constitutive enzyme isoform. Proc Natl Acad Sci U S A (1992) 89 (14):6348–52. doi:10.1073/pnas.89.14.6348 4. Sessa WC, Harrison JK, Bar- ber CM, Zeng D, Durieux ME, D’Angelo DD, et al. Molecular cloning and expression of a cDNA encoding endothelial cell nitric oxide synthase. J Biol Chem (1992) 267 (22):15274–6. 5. Förstermann U, Sessa WC. Nitric oxide synthases: regulation and function. Eur Heart J (2012) 33 (7):829–37. doi:10.1093/ eurheartj/ehr304 837a-837d, 6. Celermajer DS, Sorensen KE, Gooch VM, S