Red Blood Cell Function and Dysfunction: Redox Regulation, Nitric Oxide Metabolism, Anemia

Abstract

Significance: Recent clinical evidence identified anemia to be correlated with severe complications of cardiovascular disease (CVD) such as bleeding, thromboembolic events, stroke, hypertension, arrhythmias, and inflammation, particularly in elderly patients. The underlying mechanisms of these complications are largely unidentified. Recent Advances: Previously, red blood cells (RBCs) were considered exclusively as transporters of oxygen and nutrients to the tissues. More recent experimental evidence indicates that RBCs are important interorgan communication systems with additional functions, including participation in control of systemic nitric oxide metabolism, redox regulation, blood rheology, and viscosity. In this article, we aim to revise and discuss the potential impact of these noncanonical functions of RBCs and their dysfunction in the cardiovascular system and in anemia.

Critical issues: The mechanistic links between changes of RBC functional properties and cardiovascular complications related to anemia have not been untangled so far.

Future directions: To allow a better understanding of the complications associated with anemia in CVD, basic and translational science studies should be focused on identifying the role of noncanonical functions of RBCs in the cardiovascular system and on defining intrinsic and/or systemic dysfunction of RBCs in anemia and its relationship to CVD both in animal models and clinical settings. Antioxid. Redox Signal. 26, 718-742.

Keywords: RBC deformability; anemia; cardiovascular disease; hemolysis; nitric oxide; red blood cells; red cell eNOS.

FIG. 1.

RBC function and dysfunction: redox regulation, NO metabolism, and anemia. (A) Intrinsic RBC properties and function. Beside their canonical role in transport of gases and nutrients, RBCs are well equipped with redox buffer systems and are important modulators of NO metabolism. Their intrinsic mechanical properties allow them to deform/change their shape in response to changes in flow and to changes in vessel diameter, thus participating in control of blood rheology. (B) Effects of RBCs in blood. A second way for RBCs to control blood rheology is via their concentration (hematocrit), which critically defines blood viscosity and blood rheology. In addition, RBCs interact with PLTs resulting in a complex cell–cell communication involving membrane adhesion molecules, NO metabolism, and redox regulation. (C) Effects on systemic hemodynamics. In addition to control of vascular tone and cardiac function, intrinsic RBC properties and overall blood rheology are contributors to systemic vascular hemodynamics. (D) Anemia. RBC dysfunction mainly results in a number of anemic conditions, which are characterized by a decrease in blood Hb concentration and circulating number of RBCs. Redox dysregulation results mainly in hemolytic anemia and release of Hb, affecting redox metabolism and NO scavenging. Anemia affects systemic hemodynamics and myocardial performance. Furthermore, patients with CVD show disturbances in hemostasis and thromboembolism and increased mortality, which cannot be effectively treated by blood transfusion or substitution of ESAs. CVD, cardiovascular disease; ESA, erythropoiesis-stimulating agent; Hb, hemoglobin; NO, nitric oxide; PLT, platelet; RBC, red blood cell. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 2.

Redox regulation in RBCs. The occurring steady origin of ROS is shown by heme oxidation of oxyHb to metHb and the release of superoxide anions (autoxidation of Hb). The enzyme, SOD1, transforms the superoxide anions to hydrogen peroxide. Three main detoxyfication pathways exist in RBC, with GPx and Prx2 (but not Cat) pathways depending on reduced NADPH, which is synthesized by glucose uptake. Glucose is taken up mediated by the Glut-1 transporter and then gated to the glycolytic pathway. As a detour of the glycolytic pathway, glucose-6-phosphate is partly channeled into the pentose phosphate pathway producing the reduced form of NADP⁺. NADPH/glutathione (GSH)-dependent pathway: NADPH is needed as a substrate for glutathione reductase (GSH reductase) to recycle oxidized dimerized GSH (GSSG) back to reduced GSH. GSH itself is utilized by two enzymes: first, GPx for the direct breakdown of hydrogen peroxide, and second, GRx to reverse the consumption of ASC to DHA by the plasma membrane redox system and diffusing ROS. NADPH/Trx-dependent pathway: NADPH is also needed as a substrate by the Trx reductase, which keeps the cofactor Trx in the reduced state [Trx(SH)₂]. Trx(SH)₂ serves as an electron-delivering system to the membrane-associated Prx2 and thiols within the active center are oxidized during this process forming disulfide bridges (TrxS₂). NADPH-independent pathway: Hydrogen peroxide breakdown by pathway 3 is independent of NADPH and catalyzation occurs by Cat. ASC, ascorbic acid; Cat, catalase; DHA, dehydroascorbic acid; GPx, glutathione peroxidase; GRx, glutaredoxin; GSSG, glutathione disulfide; metHb, methemoglobin; NADPH, nicotinamide adenine dinucleotide phosphate; Prx2, peroxiredoxin 2; ROS, reactive oxygen species; Trx, thioredoxin. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 3.

Role of RBCs in systemic NO metabolism. NO is a vasodilator produced by eNOS enzymes in ECs and mediates vasodilation by activating the soluble guanylate cylase in VSMCs. NO is inactivated by RBCs by reaction with oxyHb forming metHb and nitrate. In the plasma, tissues and RBCs, NO can be oxidized to NO₂⁻ or NO₃⁻ and to other metabolites. Under hypoxic conditions, NO₂⁻ can react with deoxyHb to form NO, which was proposed to mediate hypoxic vasodilation and to inhibit platelet aggregation. Additionally, hypoxic ATP release by RBCs was also proposed to induce vasodilation by activating eNOS in the endothelium. ATP, adenosine triphosphate; ECs, endothelial cells; eNOS, endothelial nitric oxide synthase; sGC, soluble guanylate cyclase; VSMC, vascular smooth muscle cell. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 4.

RBC shape, deformation, and mechanical properties. (A) RBCs can deform from the normal biconcave disc shape due to changes in flow conditions in the blood stream. Deformability of a single cell is primarily determined by viscosity of the cytoplasm and flexibility of the cytoskeleton. Intracellular viscosity is mostly due to two factors, the concentration of Hb (the most abundant protein in the RBC) and water content due to osmolarity balancing effects. The membrane shape is affected by the cytoskeleton scaffold, which is formed by spectrin links between membrane-associated protein complexes, comprising ankyrin, band 3, and actin, among other proteins. (B) Modes of deformation of RBCs. There are four distinct ways that RBCs can deform while traveling in the bloodstream. Elongation of the discoid shape happens in response to shear stress along the elongation axis. Membrane tanktreading is movement of the membrane around the discoid shape without significant changes in the shape of the whole cell. This movement has been shown to force the RBC into the center of the vessel, promoting blood flow. Swinging is membrane tanktreading combined with small oscillations in the direction of the principal axis of the ellipsoid shape. Finally, end-over-end tumbling of RBCs is possible and promotes turbulent flow of cells within the blood stream. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 5.

Classification of Anemia. Anemic conditions can be classified according to their (A) morphology/size of the circulating RBCs, i.e., normocytic, macrocytic, and microcytic anemia, assessed as the mean corpuscular volume (MCV); (B) the concentration of Hb in RBCs, i.e., normochromic, hypochromic, and hyperchromic anemia, mainly determined by MCH; or (C) etiopathology, i.e., due to (1.) defects in erythropoiesis (i.e., due to a lack of iron or EPO); (2.) increased hemolysis or RBC degradation in the circulation (which may have a number of different intra- or extraerythrocytic reasons); (3.) acute or chronic bleeding; or (4.) a disorder of cell distribution with an increased RBC uptake by the reticuloendothelial system of the spleen. The cytological characteristics of anemia may correspond to multiple etiologies. For example, hypochromic microcytic anemia can be caused by either lack of iron or increased hemolysis or chronic blood loss. EPO, erythropoietin; MCH, mean corpuscular hemoglobin; MCV, mean corpuscular volume. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars

FIG. 6.

RBC turnover and pathological changes in anemia. (1.) Erythropoiesis occurs in the bone marrow, where blood cell precursors mature in ∼7 days. Iron, nutrient, or EPO deficiency, or other disorders of the bone marrow, negatively affect erythropoiesis leading to anemia. (2.) Hemolysis, e.g., due to SCD, reduces the amount of functional circulating RBCs. (3.) Blood loss, due to repeated blood draws or a traumatic injury, can also lead to anemic conditions until new RBCs are formed. (4.) Hypersplenism can diminish the amount of circulating cells by increased RBC removal from the circulation. EPO, erythropoietin; SCD, sickle cell disease. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars