Sickle cell disease vasculopathy: A state of nitric oxide resistance

Abstract

Sickle cell disease (SCD) is a hereditary hemoglobinopathy characterized by microvascular vaso-occlusion with erythrocytes containing polymerized sickle (S) hemoglobin, erythrocyte hemolysis, vasculopathy, and both acute and chronic multiorgan injury. It is associated with steady state increases in plasma cell-free hemoglobin and overproduction of reactive oxygen species (ROS). Hereditary and acquired hemolytic conditions release into plasma hemoglobin and other erythrocyte components that scavenge endothelium-derived NO and metabolize its precursor arginine, impairing NO homeostasis. Overproduction of ROS, such as superoxide, by enzymatic (xanthine oxidase, NADPH oxidase, uncoupled eNOS) and nonenzymatic pathways (Fenton chemistry), promotes intravascular oxidant stress that can likewise disrupt NO homeostasis. The synergistic bioinactivation of NO by dioxygenation and oxidation reactions with cell-free plasma hemoglobin and ROS, respectively, is discussed as a mechanism for NO resistance in SCD vasculopathy. Human physiological and transgenic animal studies provide experimental evidence of cardiovascular and pulmonary resistance to NO donors and reduced NO bioavailability that is associated with vasoconstriction, decreased blood flow, platelet activation, increased endothelin-1 expression, and end-organ injury. Emerging epidemiological data now suggest that chronic intravascular hemolysis is associated with certain clinical complications: pulmonary hypertension, cutaneous leg ulcerations, priapism, and possibly stroke. New therapeutic strategies to limit intravascular hemolysis and ROS generation and increase NO bioavailability are discussed.

Introduction

Sickle cell disease (SCD) is a severe hemoglobinopathy that produces multisystem complications due to the expression of abnormal sickle hemoglobin (HbS), which arises from a single point mutation in the gene that encodes synthesis of beta globin, a subunit of the hemoglobin tetramer [1]. The most common type of SCD is sickle cell anemia (SCA), the homozygous expression of the defective gene that codes for HbS. The geographic origins of HbS lie in regions of the world with endemic malaria (i.e., Africa, South Asia, the Middle East, and around the Mediterranean) wherein the heterozygote condition (sickle trait) confers relative resistance to malaria and thus confers a survival advantage. SCD is the most common hemoglobinopathy in the world, due in large measure to the slave trade and migration out of Africa to the United States, Caribbean Islands, Northern Europe, and South and Central America. In addition to HbS homozygous expression, other variants include multiple compound heterozygotic states of HbS with other mutant hemoglobins (Table 1). SCD manifests itself as a chronic hemolytic disease with acute vaso-occlusive complications that require frequent hospitalizations and therefore represents a substantial burden to national healthcare systems [2], [3], [4], [5]. For example, an analysis published in 1997 estimated a rate of 100,000 SCD hospitalizations/year in the United States alone, which resulted in direct costs exceeding $475 million [6].

The hallmark of SCD is episodic vaso-occlusion resulting in pain and/or acute chest syndrome (ACS), the occurrence of which is unpredictable except for its association with certain triggers for HbS polymerization [7], [8], [9]. Such triggers include viral and bacterial infection, changes in temperature, menstruation, pregnancy, psychosocial stressors, and numerous other factors. HbS is much less soluble than normal hemoglobin (HbA) when deoxygenated and polymerizes into rigid fibers that cause erythrocytes to deform (i.e., sickle), become rigid, and occlude the microvasculature. HbS is also less stable than HbA when oxygenated and therefore autooxidizes at a faster rate, thereby producing enhanced concentrations of reactive oxygen species within the sickle erythrocyte [10]. The subclinical chronic oxidative stress that arises from HbS autooxidation has been implicated in the lipid peroxidation of sickle erythrocyte membranes and their cross-linked cytoskeletal proteins. This cellular damage has been linked to irreversibility of erythrocyte sickling and a shortened erythrocyte lifespan [11], [12]. In contrast to previous reports, Aslan et al. have shown that there are no significant differences in rates of superoxide and hydrogen peroxide production and basal levels of membrane lipid peroxidation content in HbA vs HbS erythrocytes [13]. Additionally, they have shown that HbA vs HbS erythrocytes display similar rates of NO consumption under both normoxic and polymerization-inducing hypoxic conditions [13]. These findings were noted in intact erythrocytes that possess a well-integrated network of oxidant defense mechanisms that evidently prevent HbS erythrocytes from becoming significant loci of ROS production and oxygen radical-dependent inactivation of vascular NO signaling. However, outside of the RBC where there is possible impairment of tissue free radical defense mechanisms, the inherent instability of HbS may pose a greater challenge to redox homeostasis. The possible contribution of decompartmentalized HbS to extraerythrocytic ROS that can scavenge vascular NO and destabilize erythrocyte membranes warrants investigation.

The clinical manifestations of SCD that arise are acute or sudden episodic complications, such as vaso-occlusive pain crisis, stroke, priapism (painful, persistent erection of the penis in the absence of sexual stimulation), the acute chest syndrome (a life-threatening lung injury syndrome caused by infection or bone marrow fat embolism that resembles pneumonia on chest X-ray), retinal hemorrhage, avascular necrosis (death of tissue due to depletion of blood supply), sepsis (presence of infectious organisms or their toxins in the blood or other tissues), splenic infarction (death of tissue in the spleen due to interruption of blood flow by sickled cells), etc. Complications develop over time and include pulmonary hypertension, cutaneous leg ulceration, erectile dysfunction from recurrent priapism, etc. By focusing on these adult acute disease manifestations of SCD, research has begun to elucidate aspects of SCD pathophysiology beyond HbS polymerization and the abnormal sickle erythrocyte. In this review, we will focus particularly on recent data concerning the role of oxidative stress in SCD and the vascular effects of hemolysis that lead to a progressive systemic and pulmonary vasculopathy.