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Anemia contributes to cardiovascular disease through reductions in nitric oxide

2016; American Physiological Society; Volume: 122; Issue: 2 Linguagem: Inglês

10.1152/japplphysiol.00995.2015

8750-7587

Felicita Andreotti, Giulio Coluzzi, Teodosio Pafundi, Teresa Rio, Eliano Pio Navarese, Filippo Crea, Massimo Pistolesi, Attilio Maseri, Charles H. Hennekens,

Nitric Oxide and Endothelin Effects

ViewpointAnemia contributes to cardiovascular disease through reductions in nitric oxideFelicita Andreotti, Giulio Coluzzi, Teodosio Pafundi, Teresa Rio, Eliano Pio Navarese, Filippo Crea, Massimo Pistolesi, Attilio Maseri, and Charles H. HennekensFelicita AndreottiInstitute of Cardiology, Catholic University Hospital, Rome, Italy; , Giulio ColuzziInstitute of Cardiology, Catholic University Hospital, Rome, Italy; , Teodosio PafundiInstitute of Cardiology, Catholic University Hospital, Rome, Italy; , Teresa RioInstitute of Cardiology, Catholic University Hospital, Rome, Italy; , Eliano Pio NavareseDepartment of Cardiology, Dusseldorf, Germany; , Filippo CreaInstitute of Cardiology, Catholic University Hospital, Rome, Italy; , Massimo PistolesiSection of Respiratory Medicine, Department of Experimental and Clinical Medicine, University of Florence, Italy; , Attilio MaseriHeart Care Foundation, Florence, Italy; and , and Charles H. HennekensCharles E. Schmidt College of Medicine, Florida Atlantic University, Boca Raton, FloridaPublished Online:15 Feb 2017https://doi.org/10.1152/japplphysiol.00995.2015This is the final version - click for previous versionMoreSectionsPDF (302 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat anemia is significantly associated with cardiovascular disease (CVD) after adjustment for other risk factors in patients with established disease as well as in the general population (21, 23). The main proposed mechanisms linking anemia to CVD involve the reduction of oxygen delivery to myocardial as well as systemic tissues, the compensatory increase in stroke volume/heart rate to maintain adequate oxygen delivery, the development of secondary left ventricular remodeling/hypertrophy, and the presence of underlying kidney or inflammatory diseases (21, 23). During acute myocardial infarction, anemia affects up to 20–35% of patients, in whom each 1 g/dl decrement in Hb increases the risk of cardiovascular death at 30 days by 20% independently of other risk factors (21). In a prospective cohort study of apparently healthy men and women aged 45–64 yr, the Atherosclerosis Risk in Communities study, those with anemia experienced a 41% increased risk of CVD during a 6-yr follow-up after adjustment for other risk factors (23).Of the several mechanisms that have been proposed (21, 23), to the best of our knowledge, none has considered nitric oxide (NO). NO is a labile diatomic gas that diffuses rapidly in tissues. Its production is catalyzed by a family of NO synthases that facilitate the reaction of l-arginine with oxygen to yield NO and citrulline (3, 15, 30). NO synthases are activated by several cofactors, including tetrahydrobiopterin and calmodulin, and are present in many tissues, including erythrocytes (29, 30). NO synthesis is inhibited experimentally by l-n-methylarginine and endogenously by asymmetric dimethylarginine (9, 30). Other sources of NO include S-nitrosothiols and nitrate/nitrite(NO2−) (30). The best characterized pathway of NO bioactivity is its binding to soluble guanylyl cyclase to produce cyclic guanosine monophosphate (cGMP) from guanosine triphosphate. cGMP in turn activates protein kinases and phosphodiesterases that mediate vascular smooth muscle relaxation, platelet and leucocyte inhibition, and cell growth and differentiation (Fig. 1). Through these mechanisms, NO provides protection against CVD (2, 11, 15, 30). Intracellular Hb is an oxygen carrier and donor, but also a circulating reservoir of NO (3). In this manuscript we propose the viewpoint that anemia contributes to CVD through reductions in NO as well as oxygen. Specifically, when Hb declines, the donation of NO into the tissues is compromised, thereby adversely affecting the fundamental physiologic process of erythrocyte-mediated NO systemic hypoxic vasodilatation (3, 7, 15, 22, 24, 29).Fig. 1.Nitric oxide generation and physiological effects. Nitric oxide activates soluble guanylyl cyclase, leading to generation of cyclic GMP, responsible for vascular smooth muscle relaxation, platelet inhibition, and suppression of leucocyte recruitment. Nitric oxide also generates S-nitrosoproteins that promote sarco/endoplasmic reticulum calcium ATPase (SERCA), calcium depletion, and relaxation.Download figureDownload PowerPointFig. 2.Erythrocyte-mediated NO-dependent systemic hypoxic vasodilatation. As Hb is deoxygenated in the systemic microcirculation, it switches to a tense structure that triggers the release of nitric oxide (NO) from the cysteine thiols of S-nitroso-Hb to the erythrocyte membrane and surrounding tissues. Unloaded deoxyHb reloads local NO through high affinity heme iron binding, renewing the NO cycle.Download figureDownload PowerPointHb as NO Carrier and DonorDuring inspiration, oxygen enters the pulmonary microcirculation and saturates Hb. Specifically, the Hb tetramer loads oxygen on its heme iron atoms and assumes a relaxed (R) conformation; in parallel, the NO bound to heme iron or deriving from nitrite (NO2−) is transferred to inwardly oriented cysteine residues in position 93 (Cysβ93) to form S-nitroso-Hb (3, 7, 25, 29). As blood deoxygenates in the systemic microcirculation, Hb switches to a tense (T) structure that orients Cysβ93 toward the protein surface and triggers NO release from S-nitroso-Hb (24) (Fig. 2). In parallel, the heme iron atoms reload NO with high affinity (Fig. 2). In the normally ventilated lungs, the iron-bound NO moves again within seconds to produce S-nitroso-Hb (7), thus renewing the NO cycle within the red cell. Erythrocyte-mediated NO-dependent systemic hypoxic vasodilatation has been demonstrated in vivo in the coronary circulation of dogs and mice (20, 29).The export of NO out of erythrocytes may involve reduction of nitrite (NO2−) to NO by deoxyHb itself (7, 22, 30)—particularly at the erythrocyte submembrane as it traverses the arteriole (26)—generating intermediate species from nitrite (NO2−) that diffuse out to form extracellular NO or S-nitrosothiols (SNOs) (7, 22, 26, 30). In addition or alternatively, there may be binding of S-nitroso-Hb to the cytoplasmic domain of transmembrane band 3 protein (or anion exchanger AE1) (30), favoring NO transfer to other SNOs, such as low mass S-nitrosoglutathione and AE1 itself (25). The role of S-nitroso-Hb as a primary transport mechanism for NO has been debated, because mice expressing human Hb in which the Cysβ93 residue had been replaced by alanine seemed to maintain erythrocyte-mediated hypoxic vasodilatation (25). More recent evidence, however, confirms the essential role of Cysβ93 in response to hypoxia (13, 29). Of note, Cysβ93 is one of three amino acids in Hb strictly conserved in all mammals and birds (29). Other heme-containing proteins such as myoglobin, neuroglobin, and cytoglobin also react with nitrite to form NO (14).Systemic Vasodilatation vs. Pulmonary Vasoconstriction in Response to HypoxiaSystemic microcirculatory dilatation occurs physiologically under multiple stimuli that include metabolites (e.g., bradykinin, histamine, ACh, substance P), physical forces (shear stress, exercise), growth factors, hormones, and hypoxia (30). Hypoxic vasoconstriction in the pulmonary circulation seems in apparent contrast to systemic hypoxic vasodilatation. The former, however, is a direct response of pulmonary vascular smooth muscle cells to alveolar hypoxia, through increased intracellular calcium and rho kinase-mediated calcium sensitization, independently of Hb-NO interactions; such vasoconstriction prevents gas in poorly ventilated alveoli from lowering the arterial oxygen pressure (1, 27). In the microcirculation of poorly ventilated hypoxic alveoli, Hb is not subjected to oxygenation and NO remains bound to the high affinity iron of the heme group. Conversely, in the normoxic well-ventilated lung, Hb is exposed to oxygen and the latter, given the even higher affinity of heme for oxygen than for NO, displaces NO from the heme group onto Cysβ93.Vascular and Metabolic Effects of NONO released by deoxyHb in systemic capillary beds can diffuse across several cell diameters into arteriolar smooth muscle cells, platelets, and endothelial cells. Therein, NO binds to the heme group of soluble guanylyl cyclase inducing this cyclase to generate cGMP, activate protein kinase G, and reduce intracellular calcium concentrations (30) (Fig. 1). Smooth muscle relaxation, suppression of platelet adhesion/degranulation/aggregation, downregulation of endothelial and platelet P-selectin expression, and prevention of leukocyte rolling ensue (2, 11, 15, 24, 30) (Fig. 1).Moreover, independently of cGMP, NO may modify protein cysteine thiols (S-nitrosylation) in endothelium, lymphocytes, cardiomyocytes, platelets, and vascular smooth muscle cells, causing, among other effects, delayed apoptosis, proangiogenesis, anti-inflammatory effects, and platelet inhibition (7, 15, 30). NO pathways are also involved in insulin, insulin-like growth factor 1, and erythropoietin signaling, as well as in progenitor cell production required for vascular/myocardial repair (9, 10, 15, 18). Activation of phosphoinositol-3-kinase by insulin increases endothelial NO-synthase gene expression (10).Clinical Evidence of Hb-Mediated VasodilatationData in humans support the role of Hb-related NO vasodilatation. For example, systemic hypoxic vasodilatation is impaired with transfused blood and is reversed by S-nitroso-Hb repletion (20). Adverse effects of blood transfusion have been attributed to depletion of NO bioactivity in banked blood (20). Although rigorous reversibility studies in humans are lacking, considerable circumstantial evidence is available.In patients with stable IHD, low Hb concentrations are associated with reduced NO bioavailability, assessed as plasma nitrites/nitrates (5), and with reduced circulating progenitor cells, assessed as CD34+ cells (4), independently of age, sex, or indices of inflammation (5). The CD34 surface antigen identifies bone marrow progenitor cells, including endothelial progenitors that are considered relevant to vascular regeneration and repair (4, 9). Reduced numbers of circulating CD34+ cells predict CVD outcomes (4).Impaired tissue blood flow in patients with diabetes mellitus or in patients with sickle cell anemia has been associated with altered levels of erythrocyte S-nitroso-Hb. Altered S-nitroso-Hb was related to glycated Hb in diabetes and to HbS in sickle cells and was considered relevant to the microcirculatory complications of these disorders (29).On the other hand, high-altitude polycythemic individuals, compared with sea-level subjects, have increased basal cardiac output and forearm blood flow as well as increased blood concentrations of Hb, S-nitroso-Hb, and NO products (12, 16).Among patients with chronic obstructive pulmonary disease (COPD), those with polycythemia show increased baseline brachial artery diameters, with similar wall shear stress despite increased viscosity, compared with normocythemic COPD patients (8). The increased baseline arterial diameters, cardiac output, and systemic blood flow in the polycythemic patients are attributed to the greater amounts of NO released by the higher Hb concentrations (6).Both flow-mediated dilatation (FMD) and ACh-induced vasodilatation have been consistently inversely related to Hb concentrations (17, 19, 28). Indeed increased Hb concentration may upregulate baseline Hb-NO dilatation with diminished further dilator potential. This view is consistent with the close inverse relation found in healthy subjects between baseline radial artery diameter and radial FMD (17).ConclusionsHb is among the most abundant blood proteins, with concentrations greater than two times those of albumin and ~40 times those of fibrinogen. NO is a highly diffusible short-lived vasculoprotective gas with a high affinity reaction with Hb’s heme. Rate limiting steps of the Hb-NO interaction are likely the Hb concentration itself, as well as the NO generation and NO diffusing capacities. By limiting Hb’s potential to release bioactive NO in the systemic microcirculation, reduced Hb levels may contribute to CVD.Our hypothesis does not clash with any of the other proposed mechanisms linking anemia to CVD, but rather illustrates an additional potential mechanism, intimately associated with Hb’s function in the respiratory cycle, as carrier and donor of both oxygen and NO. Given the crucial role of the microcirculation, the elusive nature of NO, and the uncertainties surrounding the actual clinical role of Hb as carrier and donor of NO, further research is necessary. For example, relevant information may derive from the assessment of forearm blood flow adjusted to changes in blood pressure, heart rate, and viscosity in blood donors before and after donation or in polycythemic patients before and after phlebotomy in the presence and absence of the NO synthase inhibitor l-n-methylarginine. In the meanwhile, it is our viewpoint that a plausible interpretation of the data is that anemia contributes to CVD through reduced NO as well as reduced oxygen bioavailability.DISCLOSURESF.A. reports receiving lecture/consultation fees from Amgen, Bayer, Boehringer Ingelheim, BMS-Pfizer, and Daiichi Sankyo. C.H.H. reports that he is funded by the Charles E. Schmidt College of Medicine at Florida Atlantic University; serves as an independent scientist in an advisory role to investigators and sponsors as: Chair or Member of Data and Safety Monitoring Boards for Amgen, AstraZeneca, Bayer, Bristol Myers-Squibb, British Heart Foundation, Cadila, Canadian Institutes of Health Research, DalCor, Genzyme, Lilly, Regeneron, Sanofi, Sunovion and the Wellcome Foundation; to Aralez/Pozen, the United States (U.S.) Food and Drug Administration, UpToDate, and legal counsel for Pfizer and Takeda; receives royalties for authorship or editorship of 3 textbooks and as coinventor on patents for inflammatory markers and CV disease that are held by Brigham and Women’s Hospital; has an investment management relationship with the West-Bacon Group within SunTrust Investment Services, which has discretionary investment authority and does not own any common or preferred stock in any pharmaceutical or medical device company.AUTHOR CONTRIBUTIONSF.A., A.M., and G.C. conceived and designed research; F.A., T.P., and T.R. prepared figures; F.A. and G.C. drafted manuscript; F.A., T.P., T.R., F.C., M.P., and C.H.H. edited and revised manuscript; F.A., G.C., T.P., T.R., E.P.N., F.C., M.P., A.M., and C.H.H. approved final version of manuscript.REFERENCES1. 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Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation Cited ByCommentaries on Viewpoint: Anemia contributes to cardiovascular disease through reductions in nitric oxide15 February 2017 | Journal of Applied Physiology, Vol. 122, No. 2 More from this issue > Volume 122Issue 2February 2017Pages 414-417 Copyright & PermissionsCopyright © 2017 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.00995.2015PubMed27687564History Received 30 November 2015 Accepted 27 September 2016 Published online 15 February 2017 Published in print 1 February 2017 Metrics