Low-density lipoprotein susceptibility to oxidation and cytotoxicity to endothelium in sickle cell anemia

Multiple inducers of endothelial NOS (eNOS) dysfunction in sickle cell disease

A characteristic aspect of the robust, systemic inflammatory state in sickle cell disease is dysfunction of endothelial nitric oxide synthase (eNOS). We identify 10 aberrant endothelial cell inputs, present in the specific sickle context, that are known to have the ability to cause eNOS dysfunction. These are: endothelial arginase depletion, asymmetric dimethylarginine, complement activation, endothelial glycocalyx degradation, free fatty acids, inflammatory mediators, microparticles, oxidized low density lipoproteins, reactive oxygen species, and Toll‐like receptor 4 signaling ligands. The effect of true eNOS dysfunction on clinical testing using flow‐mediated dilation can be simulated by two known examples of endothelial dysfunction mimicry (hemoglobin consumption of NO; and oxidation of smooth muscle cell soluble guanylate cyclase). This lends ambiguity to interpretation of such clinical testing. The presence of these multiple perturbing factors argues that a therapeutic approach targeting only a single injurious endothelial input (or either example of mimicry) would not be sufficiently efficacious. This would seem to argue for identifying therapeutics that directly protect eNOS function or application of multiple therapeutic approaches.

Vasculo-toxic and pro-inflammatory action of unbound haemoglobin, haem and iron in transfusion-dependent patients with haemolytic anaemias

Increasing evidence suggests that free haem and iron exert vasculo‐toxic and pro‐inflammatory effects by activating endothelial and immune cells. In the present retrospective study, we compared serum samples from transfusion‐dependent patients with β‐thalassaemia major and intermedia, hereditary spherocytosis and sickle cell disease (SCD). Haemolysis, transfusions and ineffective erythropoiesis contribute to haem and iron overload in haemolytic patients. In all cohorts we observed increased systemic haem and iron levels associated with scavenger depletion and toxic ‘free’ species formation. Endothelial dysfunction, oxidative stress and inflammation markers were significantly increased compared to healthy donors. In multivariable logistic regression analysis, oxidative stress markers remained significantly associated with both haem‐ and iron‐related parameters, while soluble vascular cell adhesion molecule 1 (sVCAM‐1), soluble endothelial selectin (sE‐selectin) and tumour necrosis factor α (TNFα) showed the strongest association with haem‐related parameters and soluble intercellular adhesion molecule 1 (sICAM‐1), sVCAM‐1, interleukin 6 (IL‐6) and vascular endothelial growth factor (VEGF) with iron‐related parameters. While hereditary spherocytosis was associated with the highest IL‐6 and TNFα levels, β‐thalassaemia major showed limited inflammation compared to SCD. The sVCAM1 increase was significantly lower in patients with SCD receiving exchange compared to simple transfusions. The present results support the involvement of free haem/iron species in the pathogenesis of vascular dysfunction and sterile inflammation in haemolytic diseases, irrespective of the underlying haemolytic mechanism, and highlight the potential therapeutic benefit of iron/haem scavenging therapies in these conditions.

Sickle Cell Disease: Role of Oxidative Stress and Antioxidant Therapy

Sickle cell disease (SCD) is the most common hereditary disorder of hemoglobin (Hb), which affects approximately a million people worldwide. It is characterized by a single nucleotide substitution in the β-globin gene, leading to the production of abnormal sickle hemoglobin (HbS) with multi-system consequences. HbS polymerization is the primary event in SCD. Repeated polymerization and depolymerization of Hb causes oxidative stress that plays a key role in the pathophysiology of hemolysis, vessel occlusion and the following organ damage in sickle cell patients. For this reason, reactive oxidizing species and the (end)-products of their oxidative reactions have been proposed as markers of both tissue pro-oxidant status and disease severity. Although more studies are needed to clarify their role, antioxidant agents have been shown to be effective in reducing pathological consequences of the disease by preventing oxidative damage in SCD, i.e., by decreasing the oxidant formation or repairing the induced damage. An improved understanding of oxidative stress will lead to targeted antioxidant therapies that should prevent or delay the development of organ complications in this patient population.

The multifaceted role of ischemia/reperfusion in sickle cell anemia

Sickle cell anemia is a unique disease dominated by hemolytic anemia and vaso-occlusive events. The latter trigger a version of ischemia/reperfusion (I/R) pathobiology that is singular in its origin, cyclicity, complexity, instability, perpetuity, and breadth of clinical consequences. Specific clinical features are probably attributable to local I/R injury (e.g., stroke syndromes) or remote organ injury (e.g., acute chest syndrome) or the systematization of inflammation (e.g., multifocal arteriopathy). Indeed, by fashioning an underlying template of endothelial dysfunction and vulnerability, the robust inflammatory systematization no doubt contributes to all sickle pathology. In this Review, we highlight I/R-targeting therapeutics shown to improve microvascular blood flow in sickle transgenic mice undergoing I/R, and we suggest how such insights might be translated into human therapeutic strategies.

TGFBR3 Polymorphisms (rs1805110 and rs7526590) Are Associated with Laboratory Biomarkers and Clinical Manifestations in Sickle Cell Anemia

Individuals with sickle cell anemia (SCA) present chronic anemia, hemolysis, an exacerbated inflammatory response, and heterogeneous clinical complications, which may be modulated by the transforming growth factor beta (TGF-β) pathway. Thus, we aimed to investigate polymorphisms (rs1805110 and rs7526590) of the transforming growth factor beta receptor III gene (TGFBR3) with regard to laboratory biomarkers and clinical manifestations in individuals with SCA. Hematological, biochemical, immunological, and genetic analyses were carried out, as well as serum endothelin-1 measurements. The minor allele (A) of the TGFBR3 rs1805110 polymorphism was associated with increased hemoglobin, hematocrit, reticulocyte counts, total cholesterol, low-density lipoprotein, uric acid, and endothelin levels, as well as decreased platelet distribution width (PDW) and the occurrence of bone alterations. The minor allele (T) of TGFBR3 rs7526590 was associated with increased red cell distribution width, PDW, alkaline phosphatase, aspartate aminotransferase, total and indirect bilirubin, and lactate dehydrogenase levels, as well as lower ferritin levels and the occurrence of leg ulcers. Our data suggest that the minor allele (A) of TGFBR3 rs1805110 is associated with inflammation and bone alterations, while the minor allele (T) of TGFBR3 rs7526590 is related to hemolysis and the occurrence of leg ulcers.

Haptoglobin Therapeutics and Compartmentalization of Cell-Free Hemoglobin Toxicity

Hemolysis and accumulation of cell-free hemoglobin (Hb) in the circulation or in confined tissue compartments such as the subarachnoid space is an important driver of disease. Haptoglobin is the Hb binding and clearance protein in human plasma and an efficient antagonist of Hb toxicity resulting from physiological red blood cell turnover. However, endogenous concentrations of haptoglobin are insufficient to provide protection against Hb-driven disease processes in conditions such as sickle cell anemia, sepsis, transfusion reactions, medical-device associated hemolysis, or after a subarachnoid hemorrhage. As a result, there is increasing interest in developing haptoglobin therapeutics to target 'toxic' cell-free Hb exposures. Here, we discuss key concepts of Hb toxicity and provide a perspective on the use of haptoglobin as a therapeutic protein.

7-Ketocholesterol in disease and aging

7-Ketocholesterol (7KC) is a toxic oxysterol that is associated with many diseases and disabilities of aging, as well as several orphan diseases. 7KC is the most common product of a reaction between cholesterol and oxygen radicals and is the most concentrated oxysterol found in the blood and arterial plaques of coronary artery disease patients as well as various other disease tissues and cell types. Unlike cholesterol, 7KC consistently shows cytotoxicity to cells and its physiological function in humans or other complex organisms is unknown. Oxysterols, particularly 7KC, have also been shown to diffuse through membranes where they affect receptor and enzymatic function. Here, we will explore the known and proposed mechanisms of pathologies that are associated with 7KC, as well speculate about the future of 7KC as a diagnostic and therapeutic target in medicine.

Polymorphisms in genes that affect the variation of lipid levels in a Brazilian pediatric population with sickle cell disease: rs662799 APOA5 and rs964184 ZPR1

This cross-sectional study investigated associations between SNPs in metabolizing lipid genes, alpha-thalassemia and laboratory parameters in two forms of sickle cell disease (SCD), sickle cell anemia (SCA) and hemoglobin SC disease (HbSC) in a pediatric population. Among the groups SCA and HbSC was found a higher proportion of increased triglycerides (TG) in SCA. High levels of TG were significantly associated with lower hemoglobin (p = 0.006) and HDL-C (p = 0.037), higher white blood cell count (p = 0.027), LDH (p = 0.004) and bilirubins (p < 0.05) in SCD. Patients with HDL-C ≤40 mg/dL had higher markers hemolytic levels. Therapy of HU significantly influenced several hematological and biochemical parameters but not lipid fractions. Genotypes of the APOA5 rs662799 were not associated with lipid levels. The G-risk allele rs964184/ZPRI ZNF259/ZPR1 gene (GC + GG genotypes) was associated with increased levels of TG in children ≥10 years old (p = 0.045) and the atherogenic ratio TG/HDL-C (p = 0.032) in SCD. The use of HU improves levels of hemolysis and inflammation markers in SCD with high TG and, while not interfering with lipid levels, seems to overlap the effect of the G-risk allele in on them. This study reported for the first time that rs964184 SNP could be a genetic modifier of TG in SCD.

Higher values of triglycerides:HDL-cholesterol ratio hallmark disease severity in children and adolescents with sickle cell anemia

Dyslipidemia has been described in sickle cell anemia (SCA) but its association with increased disease severity is unknown. Here, we examined 55 children and adolescents with SCA as well as 41 healthy controls to test the association between the lipid profiles in peripheral blood and markers of hemolysis, inflammation, endothelial function, and SCA-related clinical outcomes. SCA patients exhibited lower levels of total cholesterol (P<0.001), low-density lipoprotein cholesterol (LDL-c) (P<0.001), and high-density lipoprotein cholesterol (HDL-c) (P<0.001), while displaying higher triglyceride (TG) levels and TG/HDL-c ratio values (P<0.001). TG/HDL-c values were positively correlated with lactate dehydrogenase (P=0.047), leukocyte count (P=0.006), and blood flow velocity in the right (P=0.02) and left (P=0.05) cerebral artery, while being negatively correlated with hemoglobin levels (P<0.04). Acute chest syndrome (ACS) and vaso-occlusive events (VOE) were more frequent in SCA patients exhibiting higher TG/HDL-c values (odds ratio: 3.77, P=0.027). Multivariate logistic regression analysis confirmed independent associations between elevated TG/HDL-c values and SCA. Thus, children and adolescents with SCA exhibited a lipid profile associated with hemolysis and inflammatory parameters, with increased risk of ACS and VOE. TG/HDL-c is a potential biomarker of severity of disease.

Oxidative Profile of Patients with Sickle Cell Disease

Oxidative stress plays a very significant role in the pathophysiology of sickle cell disease (SCD) and associated complications. Oxidative stress, which is often experienced by SCD patients as a result of continuous production of reactive oxygen species (ROS), may lead to endothelial dysfunction and acute inflammation. Antioxidant enzymes, such as superoxide dismutase (SOD) and catalase (CAT), often play a protective role. The current study aimed at determining the oxidative profile of persons with SCD at a tertiary hospital in Ghana. This was a case-control study involving 90 patients with SCD (34 HbSS patients at steady state, 30 HbSC at steady state, 15 HbSS with vaso-occlusive crisis, 11 HbSC with vaso-occlusive crisis), and 50 HbAA control group. Whole blood samples were collected from the study participants and analyzed for full blood counts. The blood samples were assayed for SOD and CAT as a measure of antioxidant defense, while lipid peroxidation was quantified as malondialdehyde (MDA). The results showed that the levels of SOD and CAT were significantly lower in SCD patients as compared to the control group. Patients with HbSS vaso-occlusive crisis had the lowest levels of SOD and CAT. The difference in SOD levels between HbSS at steady state and HbSC with vaso-occlusive crisis was, however, not significant (p = 0.228). The MDA level was significantly higher in SCD patients compared to the control group. This study concludes that the levels of various antioxidant enzymes (erythrocyte SOD and erythrocyte CAT) and oxidative marker (MDA) and are altered in SCD patients.

Depletion of haptoglobin and hemopexin promote hemoglobin-mediated lipoprotein oxidation in sickle cell disease

Intravascular sickling and lysis of red blood cells, a hallmark feature of sickle cell disease (SCD), releases hemoglobin (Hb) into the circulation. Increased cell-free Hb has been linked to vasculopathy and in vitro lipid oxidation. Scavenger plasma proteins haptoglobin (Hp) and hemopexin (Hpx) can attenuate cell-free Hb and total plasma heme lipid-oxidative capacity but are depleted in SCD. Here, we isolated lipids from BERK-SS mice, guinea pigs (GP) infused with heme-albumin, and patients with SCD undergoing regular exchange transfusion therapy and evaluated the level of lipid oxidation. Malondialdehyde formation, an end product of lipid peroxidation, was increased in BERK-SS mice, purified lipid fractions of the heme-albumin infused GP, and patients with SCD compared with controls. In humans, the extent of lipid oxidation was associated with the absence of Hp as well as decreased Hpx in plasma samples. Postmortem pulmonary tissue obtained from patients with SCD demonstrated oxidized LDL deposition in the pulmonary artery. The relationship between no Hp and low Hpx levels with greater LDL and HDL oxidation demonstrates the loss of protection against cell-free Hb and total plasma heme-mediated lipid oxidation and tissue injury in SCD. Strategies to protect against plasma lipid oxidation by cell-free Hb and total plasma heme (e.g., therapeutic Hp and Hpx replacement) may diminish the deleterious effects of cell-free Hb and total plasma heme toward the vascular system in SCD.

The role of carbon monoxide and heme oxygenase in the prevention of sickle cell disease vaso-occlusive crises

Sickle Cell Disease (SCD) is a painful, lifelong hemoglobinopathy inherited as a missense point mutation in the hemoglobin (Hb) beta-globin gene. This disease has significant impact on quality of life and mortality, thus a substantial medical need exists to reduce the vaso-occlusive crises which underlie the pathophysiology of the disease. The concept that a gaseous molecule may exert biological function has been well known for over one hundred years. Carbon monoxide (CO), although studied in SCD for over 50 years, has recently emerged as a powerful cytoprotective biological response modifier capable of regulating a host of physiologic and therapeutic processes that, at low concentrations, exerts key physiological functions in various models of tissue inflammation and injury. CO is physiologically generated by the metabolism of heme by the heme oxygenase enzymes and is measurable in blood. A substantial amount of preclinical and clinical data with CO have been generated, which provide compelling support for CO as a potential therapeutic in a number of pathological conditions. Data underlying the therapeutic mechanisms of CO, including in SCD, have been generated by a plethora of in vitro and preclinical studies including multiple SCD mouse models. These data show CO to have key signaling impacts on a host of metallo-enzymes as well as key modulating genes that in sum, result in significant anti-inflammatory, anti-oxidant and anti-apoptotic effects as well as vasodilation and anti-adhesion of cells to the endothelium resulting in preservation of vascular flow. CO may also have a role as an anti-polymerization HbS agent. In addition, considerable scientific data in the non-SCD literature provide evidence for a beneficial impact of CO on cerebrovascular complications, suggesting that in SCD, CO could potentially limit these highly problematic neurologic outcomes. Research is needed and hopefully forthcoming, to carefully elucidate the safety and benefits of this potential therapy across the age spectrum of patients impacted by the host of pathophysiological complications of this devastating disease.

Intravascular hemolysis and the pathophysiology of sickle cell disease

Hemolysis is a fundamental feature of sickle cell anemia that contributes to its pathophysiology and phenotypic variability. Decompartmentalized hemoglobin, arginase 1, asymmetric dimethylarginine, and adenine nucleotides are all products of hemolysis that promote vasomotor dysfunction, proliferative vasculopathy, and a multitude of clinical complications of pulmonary and systemic vasculopathy, including pulmonary hypertension, leg ulcers, priapism, chronic kidney disease, and large-artery ischemic stroke. Nitric oxide (NO) is inactivated by cell-free hemoglobin in a dioxygenation reaction that also oxidizes hemoglobin to methemoglobin, a non-oxygen-binding form of hemoglobin that readily loses heme. Circulating hemoglobin and heme represent erythrocytic danger-associated molecular pattern (eDAMP) molecules, which activate the innate immune system and endothelium to an inflammatory, proadhesive state that promotes sickle vaso-occlusion and acute lung injury in murine models of sickle cell disease. Intravascular hemolysis can impair NO bioavailability and cause oxidative stress, altering redox balance and amplifying physiological processes that govern blood flow, hemostasis, inflammation, and angiogenesis. These pathological responses promote regional vasoconstriction and subsequent blood vessel remodeling. Thus, intravascular hemolysis represents an intrinsic mechanism for human vascular disease that manifests clinical complications in sickle cell disease and other chronic hereditary or acquired hemolytic anemias.

Defective lipid metabolism in sickle cell anaemia subjects in vaso-occlusive crisis

Background:

Abnormal lipid homeostasis has been reported in sickle cell anaemia (SCA) as well as in other haematological disorders. However, there is little information on the lipid profile of SCA subjects in vaso-occlusive crisis (VOC). This study determined the lipid profile of adult SCA subjects in VOC and in steady state (SSCA).

Materials and Methods:

Fifty-eight (58) adults with HbSS (30 in steady state and 28 in vaso-occlusive crisis) and 24 age-matched healthy individuals with HbAA genotype were recruited into this study. Standard methods were used for the determination of blood pressure (BP), packed cell volume (PCV), total white blood cell count (WBC) and haemoglobin phenotype. After an overnight fast, 5 ml of venous blood was obtained from each SSCA and the controls while samples were collected upon admission in the VOC group. Plasma lipid profile was determined using enzymatic method. Differences between two groups were determined using independent Student's t-test or Man-Whitney U as appropriate. P-values less than 0.05 were considered significant.

Results:

Plasma total cholesterol (TC) and high density lipoprotein (HDL) were significantly lower while the ratio of triglyceride (TG) to HDL (TG/HDL) was significantly higher in SSCA compared with the controls. Low density lipoprotein (LDL) and TC were significantly lower in SCA subjects in VOC compared with controls. However, TC, TG, LDL and TG/HDL were significantly lower while HDL was significantly higher in VOC compared with SSCA.

Conclusion:

Sickle cell anaemia subjects have defective fasting lipid metabolism which becomes pronounced with VOC.

Atherogenesis and iron: from epidemiology to cellular level

Iron accumulates in human atherosclerotic lesions but whether it is a cause or simply a downstream consequence of the atheroma formation has been an open question for decades. According to the so called "iron hypothesis," iron is believed to be detrimental for the cardiovascular system, thus promoting atherosclerosis development and progression. Iron, in its catalytically active form, can participate in the generation of reactive oxygen species and induce lipid-peroxidation, triggering endothelial activation, smooth muscle cell proliferation and macrophage activation; all of these processes are considered to be proatherogenic. On the other hand, the observation that hemochromatotic patients, affected by life-long iron overload, do not show any increased incidence of atherosclerosis is perceived as the most convincing evidence against the "iron hypothesis." Epidemiological studies and data from animal models provided conflicting evidences about the role of iron in atherogenesis. Therefore, more careful studies are needed in which issues like the source and the compartmentalization of iron will be addressed. This review article summarizes what we have learnt about iron and atherosclerosis from epidemiological studies, animal models and cellular systems and highlights the rather contributory than innocent role of iron in atherogenesis.

Reconstructing sickle cell disease: a data-based analysis of the "hyperhemolysis paradigm" for pulmonary hypertension from the perspective of evidence-based medicine

The "hyperhemolytic paradigm" (HHP) posits that hemolysis in sickle disease sequentially and causally establishes increased cell-free plasma Hb, consumption of NO, a state of NO biodeficiency, endothelial dysfunction, and a high prevalence of pulmonary hypertension. The basic science underpinning this concept has added an important facet to the complexity of vascular pathobiology in sickle disease, and clinical research has identified worrisome clinical issues. However, this critique identifies and explains a number of significant concerns about the various HHP component tenets. In addressing these issues, this report presents: a very brief history of the HHP, an integrated synthesis of mechanisms underlying sickle hemolysis, a review of the evidentiary value of hemolysis biomarkers, an examination of evidence bearing on existence of a hyperhemolytic subgroup, and a series of questions that should naturally be applied to the HHP if it is examined using critical thinking skills, the fundamental basis of evidence-based medicine. The veracity of different HHP tenets is found to vary from true, to weakly supported, to demonstrably false. The thesis is developed that the HHP has misidentified the mechanism and clinical significance of its findings. The extant research questions identified by these analyses are delineated, and a conservative, evidence-based approach is suggested for application in clinical medicine.

Lipid levels in sickle-cell disease associated with haemolytic severity, vascular dysfunction and pulmonary hypertension

Pulmonary hypertension (PH) in sickle cell disease (SCD) is an emerging and important clinical problem. In a single-institution adult cohort of 365 patients, we investigated lipid and lipoprotein levels and their relationship to markers of intravascular haemolysis, vascular dysfunction and PH. In agreement with prior studies, we confirm significantly decreased plasma levels of total cholesterol, high-density lipoprotein-cholesterol (HDL-C) and low-density lipoprotein-cholesterol (LDL-C) in SCD versus ethnically-matched healthy controls. Several cholesterol parameters correlated significantly with markers of anaemia, but not endothelial activation or PH. More importantly, serum triglyceride levels were significantly elevated in SCD compared to controls. Elevated triglyceride levels correlated significantly with markers of haemolysis (lactate dehydrogenase and arginase; both P < 0.0005), endothelial activation (soluble E-selectin, P < 0.0001; soluble P-selectin, P = 0.02; soluble vascular cell adhesion molecule-1, P = 0.01), inflammation (leucocyte count, P = 0.0004; erythrocyte sedimentation rate, P = 0.02) and PH (amino-terminal brain natriuretic peptide, P = 0.002; prevalence of elevated tricuspid regurgitant velocity (TRV), P < 0.001). In a multivariate analysis, triglyceride levels correlated independently with elevated TRV (P = 0.002). Finally, forearm blood flow studies in adult patients with SCD demonstrated a significant association between increased triglyceride/HDL-C ratio and endothelial dysfunction (P < 0.05). These results characterize elevated plasma triglyceride levels as a potential risk factor for PH in SCD.

Heme degradation and vascular injury

Heme is an essential molecule in aerobic organisms. Heme consists of protoporphyrin IX and a ferrous (Fe(2+)) iron atom, which has high affinity for oxygen (O(2)). Hemoglobin, the major oxygen-carrying protein in blood, is the most abundant heme-protein in animals and humans. Hemoglobin consists of four globin subunits (alpha(2)beta(2)), with each subunit carrying a heme group. Ferrous (Fe(2+)) hemoglobin is easily oxidized in circulation to ferric (Fe(3+)) hemoglobin, which readily releases free hemin. Hemin is hydrophobic and intercalates into cell membranes. Hydrogen peroxide can split the heme ring and release "free" redox-active iron, which catalytically amplifies the production of reactive oxygen species. These oxidants can oxidize lipids, proteins, and DNA; activate cell-signaling pathways and oxidant-sensitive, proinflammatory transcription factors; alter protein expression; perturb membrane channels; and induce apoptosis and cell death. Heme-derived oxidants induce recruitment of leukocytes, platelets, and red blood cells to the vessel wall; oxidize low-density lipoproteins; and consume nitric oxide. Heme metabolism, extracellular and intracellular defenses against heme, and cellular cytoprotective adaptations are emphasized. Sickle cell disease, an archetypal example of hemolysis, heme-induced oxidative stress, and cytoprotective adaptation, is reviewed.

The Role of Nutrition in Sickle Cell Disease

Finding a widely available cure for sickle cell anemia (HbSS) still remains a challenge one hundred years after its discovery as a genetically inherited disease. However, growing interest in the nutritional problems of the disease has created a body of literature from researchers seeking nutritional alternatives as a means of decreasing morbidity and improving quality of life among HbSS patients. This review demonstrates that over the past 30 years the role of protein/energy deficiency in HbSS has been more clearly defined via direct measurements, leading to the concept of a relative shortage of nutrients for growth and development, despite apparently adequate dietary intakes. Although there is still a paucity of data supporting the efficacy of macronutrient supplementation, it is becoming clearer that recommended dietary allowances (RDAs) for the general population are insufficient for the sickle cell patient. A similar shortage is likely to be true for micronutrient deficiencies, including recent findings of vitamin D deficiency that may be associated with incomplete ossification and bone disease, which are well known complications of HbSS disease. We conclude that there is need for more effort and resources to be dedicated to research (including supplementation studies of larger sample size) aimed at establishing specific RDAs for HbSS patients, much like the specific RDAs developed for pregnancy and growth within the general population.

Pathophisiology of sickle cell disease and new drugs for the treatment

A homozygous mutation in the gene for β globin, a subunit of adult hemoglobin A (HbA), is the proximate cause of sickle cell disease (SCD). Sickle hemoglobin (HbS) shows peculiar biochemical properties, which lead to polymerizing when deoxygenated. HbS polymerization is associated with a reduction in cell ion and water content (cell dehydration), increased red cell density which further accelerate HbS polymerization. Dense, dehydrated erythrocytes are likely to undergo instant polymerization in conditions of mild hypoxia due to their high HbS concentration, and HbS polymers may be formed under normal oxygen pressure. Pathophysiological studies have shown that the dense, dehydrated red cells may play a central role in acute and chronic clinical manifestations of sickle cell disease, in which intravascular sickling in capillaries and small vessels leads to vaso-occlusion and impaired blood flow in a variety of organs and tissue. The persistent membrane damage associated with HbS polymerization also favors the generation of distorted rigid cells and further contributes to vaso-occlusive crisis (VOCs) and cell destruction in the peripheral circulation. These damaged, dense sickle red cells also show a loss of phospholipid asymmetry with externalization of phosphatidylserine (PS), which is believed to play a significant role in promoting macrophage recognition with removal of erythrocytes (erythrophagocytosis). Vaso-occlusive events in the microcirculation result from a complex scenario involving the interactions between different cell types, including dense, dehydrated sickle cells, reticulocytes, abnormally activated endothelial cells, leukocytes, platelets and plasma factors such as cytokine and oxidized pro-inflammatory lipids. Hydroxycarbamide (hydroxyurea) is currently the only drug approved for chronic administration in adult patients with sickle cell disease to prevent acute painful crises and reduce the incidence of transfusion and acute chest crises. Here, we will focus on consolidated and experimental therapeutic strategies for the treatment of sickle cell disease, including:

agents which reduce or prevent sickle cell dehydration

agents which reduce sickle cell-endothelial adhesive events

nitric oxide (NO) or NO-related compounds

anti-oxidant agents

Correction of the abnormalities ranging from membrane cation transport pathways to red cell-endothelial adhesive events, might constitute new pharmacological targets for treating sickle cell disease.

Elimination of lipid peroxide during hemodialysis

BACKGROUND/AIMS

This study is aimed to show the antioxidative effect of hemodialysis (HD) by demonstrating the elimination of toxic lipid peroxides.

METHODS

Blood samples were obtained from patients on regular maintenance HD before and 15, 30, 60, 120 and 240 min after the start of each HD session. Plasma cholesteryl ester hydroperoxide (CE-OOH), phosphatidylcholine hydroperoxide (PC-OOH), and eliminators of lipid peroxides (LOOH) such as apolipoprotein A-I (apoA-I) and lecithin:cholesterol acyltransferase (LCAT) were investigated. The hydroxyl radical scavenging activity was measured for the evaluation of the pro-oxidative side.

RESULTS

CE-OOH and PC-OOH were elevated in patients with chronic kidney disease both on and not on HD, while these values were much higher in HD patients. CE-OOH quickly dropped during the first 30 min of HD, then gradually decreased until 240 min. CE-OOH concentrations were related to those of apoA-I. In contrast, PC-OOH showed an increase 30 min after the start of HD, a change which resembled that of LCAT and was the reverse of the hydroxyl radical scavenging activity.

CONCLUSION

These results demonstrate the antioxidative action through CE-OOH elimination involving apoA-I. The pro- and antioxidative effects of HD on LOOH are not uniform but PC-OOH is mainly influenced prooxidatively.

Polynitroxyl albumin inhibits inflammation and vasoocclusion in transgenic sickle mice

Individuals with sickle-cell disease (SCD) and transgenic sickle mice expressing human betaS globin exhibit enhanced reactive oxygen species (ROS) production, vascular inflammation, and episodic vasoocclusion. We hypothesize that reduction of ROS will reduce endothelial-cell activation and adhesion-molecule expression, thereby inhibiting vasoocclusion. To test this hypothesis, we measured endothelial-cell activation, adhesion-molecule expression, and vasoocclusion in sickle mice after administering i.v. polynitroxyl albumin (PNA), a superoxide dismutase and catalase mimetic. Untreated sickle mice, compared with normal mice, showed increased activation of nuclear factor-kappaB (NF-kappaB), an oxidant-sensitive transcription factor, in their lungs, livers, and skin. NF-kappaB activation was increased further in the livers and skin of sickle but not normal mice after hypoxia-reoxygenation. IV administration of PNA inhibited NF-kappaB activation by 60% (P < .01) in the lungs and by 33% (P < .05) in the livers of sickle mice after hypoxia-reoxygenation. PNA also reduced the expression of vascular cell-adhesion molecule-1 (VCAM-1) by 57% in lung (P < .05) and by 33% in liver (P < .05) and reduced the expression of intercellular-adhesion molecule-1 (ICAM-1) by 40% in lung (P < .05) and by 53% in liver (P < .05). PNA inhibited a hypoxia-reoxygenation-induced increase in leukocyte rolling (P < .01) and adhesion (P < .05) in venules of the dorsal skin. Most importantly, PNA completely inhibited hypoxia-reoxygenation-induced vasoocclusion (P < .001). Control albumin had no effect on NF-kappaB, VCAM-1, ICAM-1, rolling, adhesion, or vasoocclusion. We speculate that therapies to reduce oxidative stress will inhibit inflammation and vasoocclusion in SCD.

Advanced glycation end-products in sickle cell anaemia

Tissue accumulation of advanced glycation end-products (AGEs) has been implicated in the oxidant-induced vascular pathology of diabetes and other diseases. Because homozygous sickle cell anaemia (SCA) is a state of oxidative stress, we tested the hypothesis that circulating AGE levels are elevated in SCA. Blood was obtained from age- and race-matched children classified as either non-sickle cell controls, SCA without vaso-occlusive crisis (SCA - VOC), or SCA with vaso-occlusive crisis (SCA + VOC). Plasma and red blood cell (RBC) AGE levels were measured by immunoassay. RBC levels of reduced (GSH) and oxidized (GSSG) glutathione were measured by capillary electrophoresis as an indicator of endogenous antioxidant status. The results showed that plasma AGE levels and the rate of RBC AGE accumulation were significantly higher in patients with SCA compared with controls. GSH was not different between groups but was significantly inversely correlated with plasma AGEs in both controls and patients with SCA. GSSG was significantly lower and GSH/GSSG higher in SCA + VOC patients, suggesting that GSH/GSSG might be an objective indicator of acute VOC or a risk factor for VOC. We conclude that circulating AGE levels are strongly influenced by endogenous antioxidant status and may play a role in the vascular pathology of SCA.

Differential gene expression in pulmonary artery endothelial cells exposed to sickle cell plasma

Clinical variability in sickle cell disease (SCD) suggests a role for extra-erythrocytic factors in the pathogenesis of vasoocclusion. We hypothesized that endothelial cell (EC) dysfunction, one possible modifier of disease variability, results from induction of phenotypic changes by circulating factors. Accordingly, we analyzed gene expression in cultured human pulmonary artery ECs (HPAEC) exposed to plasma from 1) sickle acute chest syndrome (ACS) patients, 2) SCD patients at steady state, 3) normal volunteers, and 4) serum-free media, using whole genome microarrays (U133A-B GeneChip, Affymetrix). Data were analyzed by Bayesian analysis of differential gene expression (BADGE). Differential expression was defined by the probability of >1.5 fold change in signal intensity greater than 0.999 and a predicted score of 70-100, measured by cross-validation. Compared with normal plasma, plasma from SCD patients (steady state) resulted in differential expression of 50 genes in HPAEC. Of these genes, molecules involved in cholesterol biosynthesis and lipid transport, the cellular stress response, and extracellular matrix proteins were most prominent. Another 58 genes were differentially expressed in HPAEC exposed to plasma from ACS patients. The pattern of altered gene expression suggests that plasma from SCD patients induces an EC phenotype which is anti-apoptotic and favors cholesterol biosynthesis. An altered EC phenotype elicited by SCD plasma may contribute to the pathogenesis of sickle vasoocclusion.

Generation of free radical by interaction of iron with thiols in human plasma and its possible significance

It has been reported that iron can generate reactive oxygen species (ROS) with thiols. In this study, we examined the interaction of iron with thiols in plasma and the generation of ROS. In human plasma, unlike with Fe(3+), treatment with Fe(2+) increased lucigenin-enhanced chemiluminescence in a concentration-dependent manner, and this was inhibited by superoxide dismutase. Boiling of plasma did not affect chemiluminescence generation induced by Fe(2+). Thiol depletion in plasma by pretreatment with N-ethylmaleimide (NEM) decreased chemiluminescence significantly. Consistent with these findings, albumin, the major thiol contributor in plasma, also generated ROS with Fe(2+). Treatment with Fe(2+) resulted in significant reduction of oxygen radical absorbance capacity (ORAC value) in plasma followed by an increase in low-density lipoprotein (LDL) oxidation. These results suggest that generation of ROS by nonenzymatic reaction of Fe(2+) with plasma thiols could lead to reduction of total antioxidant capacity in plasma, thereby enhancing susceptibility of plasma LDL to oxidation under iron overload conditions.

Effect ofAlkanna albugam root on LDL oxidation. A comparative study with species of the Lamiaceae family

The role of natural products in preventing diseases, such as atherosclerosis, is being recognized increasingly. In this study, the in vitro antioxidant activity of the polar extract obtained from Alkanna albugam root, belonging to the Boraginaceae family, against LDL oxidation, was investigated and compared with the respective activity of Greek Mentha piperita, Rosmarinus officinalis and Sideritis clandestina, belonging to the Lamiaceae family. Of the extracts tested, only those from Alkanna albugam root and Mentha piperita exhibited statistically significant activity. Weaker activity was shown by the extracts obtained from Rosmarinus officinalis and Sideritis clandestina.

The neurotoxic effect of sickle cell hemoglobin

A growing body of experimental evidence suggests that the oxidative neurotoxicity of hemoglobin A may contribute to neuronal loss after CNS hemorrhage. Several hemoglobin variants, including hemoglobin S, are more potent oxidants in cell-free systems. However, despite the increased incidence of hemorrhagic stroke associated with sickle cell disease, little is known of the effect of hemoglobin S on cells of neural origin. In the present study, its toxicity was quantified and directly compared with that of hemoglobin A in murine cortical cell cultures. Reactive oxygen species production, as assessed by cellular fluorescence after treatment with dihydrorhodamine 123, was significantly increased by exposure to 10 microM hemoglobin S for 2-4 h. Neuronal death, as measured by propidium iodide staining and lactate dehydrogenase release, commenced at 4 h; for a 20-h exposure, the EC50 was approximately 0.71 microm. Glial cells were not injured. Cell death was completely blocked by iron chelation with deferoxamine or phenanthroline. Direct comparison of sister cultures exposed to either hemoglobin A or hemoglobin S revealed a similar amount of cell injury in both groups. A significant difference was consistently observed only after treatment with 1 microM hemoglobin for 20 h, which resulted in death of approximately one third more neurons with hemoglobin S than with hemoglobin A. The results of this study suggest that sickle cell hemoglobin is neurotoxic at physiologically relevant concentrations. This toxicity is iron-dependent, oxidative, and quantitatively similar to that produced by hemoglobin A.

Endothelial cell expression of tissue factor in sickle mice is augmented by hypoxia/reoxygenation and inhibited by lovastatin

Abnormal tissue factor (TF) expression has been demonstrated on blood monocytes and circulating endothelial cells in humans with sickle cell anemia. We have now studied sickle transgenic mice to help define the biology of endothelial TF expression in sickle disease. Using immunostaining of tissue sections, we find that this is confined almost exclusively to the pulmonary veins. About 15% and 13% of these exhibit TF-positive endothelium in the wild-type normal mouse and the normal human hemoglobin (HbA)–expressing control transgenic mouse, respectively. The mild sickle mouse is indistinguishable from normal (∼ 14% positive), but TF expression is significantly elevated in the moderate and severe mouse models of sickle disease (∼ 29% and ∼ 41% positive, respectively). Exposure of the mild sickle mouse to hypoxia for 3 hours, followed by reoxygenation, converted its TF expression phenotype to that of the severe sickle mouse (∼ 36% positive). Pretreatment with lovastatin eliminated excessive expression of TF in the posthypoxic mild sickle mouse (∼ 16% positive) and in the more severe mouse at ambient air (∼ 21% positive). In addition to identifying tissue expression of endothelial TF in the sickle lung, these studies implicate reperfusion injury physiology in its expression and suggest a rationale for use of statins in sickle disease.

The iron hypothesis of atherosclerosis and its clinical impact

The iron hypothesis as an alternative explanation for the gender difference in the incidence and mortality of atherosclerosis has provoked increased debates and public health concerns. In this review we summarize the historical and recent literature on the iron hypothesis and discuss several related clinical issues and their implications. Apart from misconstruction of study populations, lack of a good method to reflect the iron contents of tissues may be the major factor for causing inconsistent results from epidemiological studies. Published data from 11 countries clearly indicate that the mortality from cardiovascular diseases is correlated with liver iron. We propose that redox-active iron in tissue is the atherogenic portion of total iron stores. Recently developed magnetic resonance imaging techniques in combination with Fe chelators may allow future studies to examine this component of body iron in lesions and the whole body. Several clinical situations characterized by increased iron stores have been proposed as 'human models' suitable for further tests of the iron hypothesis. Patients with end-stage renal disease may be the most unique cohort, having significant increases in their iron stores, low-density lipoprotein (LDL) oxidation, and cardiovascular events. Other patient groups may be well suited for specific studies of different atherogenic events. With a better understanding of iron-driven oxidative damage, well controlled and effectively designed studies on these models will finally bring us to the truth of the iron hypothesis.

Transfusion practice for patients with sickle cell disease

The practice of transfusion for patients with sickle cell disease is changing. Diminished childhood mortality is resulting in greater patient longevity and a higher prevalence of chronic systemic complications. Many of these chronic complications, as well as several acute ones, are treatable with erythrocyte transfusion. Proactive or preventive uses for transfusion are being considered for selected patients. Immunologic phenomena, hemosiderosis, and risk for transmission of infectious agents are the three most noticeable risks associated with repetitive transfusion. Evolving work with alternative oxygen carriers and surface modification of erythrocytes is also promising and relevant for patients with sickle cell disease.