Nitrogen metabolism in sickle cell anemia: free amino acids in plasma and urine

A novel composition of endogenous metabolic modulators improves red blood cell properties in sickle cell disease

The most common forms of sickle cell disease (SCD) are sickle cell anemia (SCA; HbSS) and HbSC disease. In both, especially the more dense, dehydrated and adherent red blood cells (RBCs) with reduced deformability are prone to hemolysis and sickling, and thereby vaso-occlusion. Based on plasma amino acid profiling in SCD, a composition of 10 amino acids and derivatives (RCitNacQCarLKHVS; Axcella Therapeutics, USA), referred to as endogenous metabolic modulators (EMMs), was designed to target RBC metabolism. The effects of ex vivo treatment with the EMM composition on different RBC properties were studied in SCD (n = 9 SCA, n = 5 HbSC disease). Dose-dependent improvements were observed in RBC hydration assessed by hemocytometry (MCV, MCHC, dense RBCs) and osmotic gradient ektacytometry (Ohyper). Median (interquartile range [IQR]) increase in Ohyper compared to vehicle was 4.9% (4.0%-5.5%), 7.5% (6.9%-9.4%), and 12.8% (11.5%-14.0%) with increasing 20×, 40×, and 80X concentrations, respectively (all p < 0.0001). RBC deformability (EImax using oxygen gradient ektacytometry) increased by 8.1% (2.2%-12.1%; p = 0.0012), 9.6% (2.9%-15.1%; p = 0.0013), and 13.3% (5.7%-25.5%; p = 0.0007), respectively. Besides, RBC adhesion to subendothelial laminin decreased by 43% (6%-68%; p = 0.4324), 58% (48%-72%; p = 0.0185), and 71% (49%-82%; p = 0.0016), respectively. Together, these results provide a rationale for further studies with the EMM composition targeting multiple RBC properties in SCD.

Metabolomic profiling for dyslipidemia in pediatric patients with sickle cell disease, on behalf of the IHCC consortium

BACKGROUND

Previous study has shown that dyslipidemia is common in patients with Sickle cell disease (SCD) and is associated with more serious SCD complications.

METHODS

This study investigated systematically dyslipidemia in SCD using a state-of-art nuclear magnetic resonance (NMR) metabolomics platform, including 147 pediatric cases with SCD and 1234 controls without SCD. We examined 249 metabolomic biomarkers, including 98 biomarkers for lipoprotein subclasses, 70 biomarkers for relative lipoprotein lipid concentrations, plus biomarkers for fatty acids and phospholipids.

RESULTS

Specific patterns of hypolipoproteinemia and hypocholesterolemia in pediatric SCD were observed in lipoprotein subclasses other than larger VLDL subclasses. Triglycerides are not significantly changed in SCD, except increased relative concentrations in lipoprotein subclasses. Decreased plasma FFAs (including total-FA, SFA, PUFA, Omega-6, and linoleic acid) and decreased plasma phospholipids were observed in SCD.

CONCLUSION

This study scrutinized, for the first time, lipoprotein subclasses in pediatric patients with SCD, and identified SCD-specific dyslipidemia from altered lipoprotein metabolism. The findings of this study depict a broad panorama of lipid metabolism and nutrition in SCD, suggesting the potential of specific dietary supplementation of the deficient nutrients for the management of SCD.

Hydroxyurea improves nitric oxide bioavailability in humanized sickle cell mice

Despite advancements in disease management, sickle cell nephropathy, a major contributor to mortality and morbidity in patients, has limited therapeutic options. Previous studies indicate hydroxyurea, a commonly prescribed therapy for sickle cell disease (SCD), can reduce renal injury in SCD but the mechanisms are uncertain. Because SCD is associated with reduced nitric oxide (NO) bioavailability, we hypothesized that hydroxyurea treatment would improve NO bioavailability in the humanized sickle cell mouse. Humanized male 12-wk-old sickle (HbSS) and genetic control (HbAA) mice were treated with hydroxyurea or regular tap water for 2 wk before renal and systemic NO bioavailability as well as renal injury were assessed. Untreated HbSS mice exhibited increased proteinuria, elevated plasma endothelin-1 (ET-1), and reduced urine concentrating ability compared with HbAA mice. Hydroxyurea reduced proteinuria and plasma ET-1 levels in HbSS mice. Untreated HbSS mice had reduced plasma nitrite and elevated plasma arginase concentrations compared with HbAA mice. Hydroxyurea treatment augmented plasma nitrite and attenuated plasma arginase in HbSS mice. Renal vessels isolated from HbSS mice also had elevated nitric oxide synthase 3 (NOS3) and arginase 2 expression compared with untreated HbAA mice. Hydroxyurea treatment did not alter renal vascular NOS3, however, renal vascular arginase 2 expression was significantly reduced. These data support the hypothesis that hydroxyurea treatment augments renal and systemic NO bioavailability by reducing arginase activity as a potential mechanism for the improvement on renal injury seen in SCD mice.

Convergence of Inflammatory Pathways in Allergic Asthma and Sickle Cell Disease

The underlying pathologies of sickle cell disease and asthma share many characteristics in terms of respiratory inflammation. The principal mechanisms of pulmonary inflammation are largely distinct, but activation of common pathways downstream of the initial inflammatory triggers may lead to exacerbation of both disease states. The altered inflammatory landscape of these respiratory pathologies can differentially impact respiratory pathogen susceptibility in patients with sickle cell disease and asthma. How these two distinct diseases behave in a comorbid setting can further exacerbate pulmonary complications associated with both disease states and impact susceptibility to respiratory infection. This review will provide a concise overview of how asthma distinctly affects individuals with sickle cell disease and how pulmonary physiology and inflammation are impacted during comorbidity.

Correlation Between Soluble Endothelial Adhesion Molecules and Nitric Oxide Metabolites in Sickle Cell Disease

Nitric Oxide (NO) and soluble adhesion molecules are promising biomarkers, which predict endothelial dysfunction in sickle cell disease (SCD). Several studies have investigated the relationship between NO (as well as its metabolites) and endothelial adhesion molecules in SCD. However, these studies were done mainly in the developed world, and it is difficult to extrapolate the findings to SCD populations in other geographical regions such as Africa due to significant disparities in the results. The aim of the current study was to determine the correlation between levels of nitric oxide metabolites (NOx) and adhesion molecules in SCD patients in a tertiary hospital in Ghana. A case control cross-sectional study involving 100 SCD (made up of HbSS and HbSC patients) and 60 healthy controls was conducted. Concentrations of NOx and soluble endothelial adhesion molecules (ICAM-1, VCAM-1 and E-selectin) were measured in all the study participants (n = 160) by the Griess reagent system and enzyme-linked immunosorbent assay (ELISA). Correlation analysis was performed to determine a possible link between the variables. Levels of soluble adhesion molecules were higher in the HbSS patients. Correlation of NOx with ICAM-1 almost approached significance (r = 0.565, p = 0.058) in the HbSS patients. There were no correlations between NOx and E-selectin in both HbSS and HbSC patients. There were no significant correlations between NOx and VCAM-1 in all the study participants (p > 0.05). Of the soluble adhesion molecules, ICAM-1 showed a significant positive correlation with VCAM-1 in the HbSC patients. There were no significant differences between the adhesion molecules and the age of participants in the various study groups. Whether or not a significant correlation exists between NOx and soluble adhesion molecules may not depend on the sickle cell genotype. The expression of adhesion molecules may not depend on age.

Low nitric oxide level is implicated in sickle cell disease and its complications in Ghana

BACKGROUND

Nitric oxide (NO) plays a fundamental role in maintaining normal vasomotor tone. Recent clinical and experimental data suggest that NO may play a role in the pathogenesis and therapy of sickle cell disease (SCD). The aim of this study was to determine NO metabolites (NOx) in SCD patients at steady state and in vaso-occlusive crisis (VOC), as well as those with hemolytic clinical sub-phenotype that includes leg ulcers and priapism.

METHODOLOGY

This was a case-control cross-sectional study conducted on a total of 694 subjects including 148 comparison group HbAA, 208 HbSS SCD patients in steady state, 82 HbSC SCD patients in steady state, 156 HbSS SCD patients in VOC, 34 HbSC SCD patients in VOC, 34 HbSS SCD patients in post VOC, 21 HbSS SCD patients with leg ulcer and 11 HbSS SCD patients with priapism, with age ranging from 15 to 65 years. Laboratory diagnosis of SCD was done at the Sickle Cell Clinic of the Korle-Bu Teaching Hospital. Plasma nitric oxide metabolites were measured using Griess reagent system by ELISA method.

RESULTS

Mean NOx of 59.66±0.75 µMol/L in the comparison group was significantly different from those in steady state (P=0.02). During VOC, there was a significant reduction in mean NOx levels to 6.08±0.81 µMol/L (P<0.001). Mean NOx levels were however, significantly higher (50.97±1.68 µMol/L) (P<0.001) in the immediate postcrisis period. The mean NOx levels in the leg ulcer (21.70±1.18 µMol/L) (P<0.001) and priapism (28.97±1.27 µMol/L) (P<0.001) patients were significantly low as compared to the SCD patients in the steady state and comparison group.

CONCLUSION

This study presents the first report on plasma NOx levels in SCD complication in Ghanaian SCD patients and confirms reduced plasma NOx levels in SCD patients in general.

Nitric oxide pathology and therapeutics in sickle cell disease

Sickle cell disease is caused by a mutant form of hemoglobin that polymerizes under hypoxic conditions which leads to red blood cell (RBC) distortion, calcium-influx mediated RBC dehydration, increased RBC adhesivity, reduced RBC deformability, increased RBC fragility, and hemolysis. These impairments in RBC structure and function result in multifaceted downstream pathology including inflammation, endothelial cell activation, platelet and leukocyte activation and adhesion, and thrombosis, all of which contribute vascular occlusion and substantial morbidity and mortality. Hemoglobin released upon RBC hemolysis scavenges nitric oxide (NO) and generates reactive oxygen species (ROS) and thereby decreases bioavailability of this important signaling molecule. As the endothelium-derived relaxing factor, NO acts as a vasodilator and also decreases platelet, leukocyte, and endothelial cell activation. Thus, low NO bioavailability contributes to pathology in sickle cell disease and its restoration could serve as an effective treatment. Despite its promise, clinical trials based on restoring NO bioavailability have so far been mainly disappointing. However, particular "NO donating" agents such as nitrite, which unlike some other NO donors can improve sickle RBC properties, may yet prove effective.

Precipitating factors and targeted therapies in combating the perils of sickle cell disease--- A special nutritional consideration

Nutritional research in sickle cell disease has been the focus in recent times owing to not only specific nutritional deficiencies, but also the improvements associated with less painful episodes. Though hydroxyurea remains the drug of choice, certain adverse health effects on long term supplementation makes room for researches of different compounds. Macro and micro nutrient deficiencies, along with vitamins, play an important role in not only meeting the calorific needs, but also reducing clinical complications and growth abnormalities. Symptoms of hyper protein metabolism, increased cell turnover, increased cardiac output, and appetite suppression due to enhanced cytokine production, might give us leads for better understanding of the mechanisms involved. Different nutritional approaches comprising of traditional herbal therapies, antioxidants, flavonoids, vitamins, minerals etc., reducing oxidative stress and blood aggregation, have been tried out to increase the health potential. Nutritional therapies may also serve complementary to the newer therapies using ozone, hematopoietic stem cell transplantation, antifungal medications, erythropoietin etc. Herein we try to present a holistic picture of the different patho-physiological mechanisms, and nutritional strategies adopted.

New strategies for the treatment of pulmonary hypertension in sickle cell disease : the rationale for arginine therapy

Nitric oxide (NO) is inactivated in sickle cell disease (SCD), while bioavailability of arginine, the substrate for NO synthesis, is diminished. Impaired NO bioavailability represents the central feature of endothelial dysfunction, and is a key factor in the pathophysiology of SCD. Inactivation of NO correlates with the hemolytic rate and is associated with erythrocyte release of cell-free hemoglobin and arginase during hemolysis. Accelerated consumption of NO is enhanced further by the inflammatory environment of oxidative stress that exists in SCD. Based upon its critical role in mediating vasodilation and cell growth, decreased NO bioavailability has also been implicated in the pathogenesis of pulmonary arterial hypertension (PHT). Secondary PHT is a common life-threatening complication of SCD that also occurs in most hereditary and chronic hemolytic disorders. Aberrant arginine metabolism contributes to endothelial dysfunction and PHT in SCD, and is strongly associated with prospective patient mortality. The central mechanism responsible for this metabolic disorder is enhanced arginine turnover, occurring secondary to enhanced plasma arginase activity. This is consistent with a growing appreciation of the role of excessive arginase activity in human diseases, including asthma and PHT. Decompartmentalization of hemoglobin into plasma consumes endothelial NO and thus drives a metabolic requirement for arginine, whose bioavailability is further limited by arginase activity. New treatments aimed at maximizing both arginine and NO bioavailability through arginase inhibition, suppression of hemolytic rate, or oral arginine supplementation may represent novel therapeutic strategies.

Body composition and grip strength are improved in transgenic sickle mice fed a high-protein diet

Key pathophysiology of sickle cell anaemia includes compensatory erythropoiesis, vascular injury and chronic inflammation, which divert amino acids from tissue deposition for growth/weight gain and muscle formation. We hypothesised that sickle mice maintained on an isoenergetic diet with a high percentage of energy derived from protein (35 %), as opposed to a standard diet with 20 % of energy derived from protein, would improve body composition, bone mass and grip strength. Male Berkeley transgenic sickle mice (S; n 8-12) were fed either 20 % (S20) or 35 % (S35) diets for 3 months. Grip strength (BIOSEB meter) and body composition (dual-energy X-ray absorptiometry scan) were measured. After 3 months, control mice had the highest bone mineral density (BMD) and bone mineral content (BMC) (P < 0·005). S35 mice had the largest increase in grip strength. A two-way ANOVA of change in grip strength (P = 0·043) attributed this difference to genotype (P = 0·025) and a trend in type of diet (P = 0·067). l-Arginine (l-Arg) supplementation of the 20 % diet was explored, as a possible mechanism for improvement obtained with the 35 % diet. Townes transgenic sickle mice (TS; n 6-9) received 0·8, 1·6, 3·2 or 6·4 % l-Arg based on the same protocol and outcome measures used for the S mice. TS mice fed 1·6 % l-Arg for 3 months (TS1.6) had the highest weight gain, BMD, BMC and lean body mass compared with other groups. TS3.2 mice showed significantly more improvement in grip strength than TS0·8 and TS1.6 mice (P < 0·05). In conclusion, the high-protein diet improved body composition and grip strength. Outcomes observed with TS1.6 and TS3.2 mice, respectively, confirm the hypothesis and reveal l-Arg as part of the mechanism.

Endothelial Function and Vascular Properties in Children with Sickle Cell Disease

BACKGROUND

Sickle cell disease (SCD) is an inherited disorder characterized by recurrent painful crises with ischemia resulting from vascular occlusion. Adults with SCD have increased arterial stiffness and reduced flow-mediated dilation (FMD), due to impaired release of substances such as nitric oxide.

AIM

We aimed to assess the vascular properties of carotid and brachial arteries in children with SCD compared with a control group without cardiovascular risk factors.

METHODS

Thirty patients with SCD, mean age 12.3 ± 4.5 years, were prospectively enrolled. A control group was made up of 30 age- and gender-matched healthy subjects. Carotid intima-media thickness (IMT), cross-sectional compliance (CSC), cross-sectional distensibility (CSD), diastolic wall stress (DWS), incremental elastic modulus (Einc), and FMD were determined in both groups.

RESULTS

There was no significant difference in FMD between the two groups (8.2 ± 5.0% in the SCD group vs. 9.3 ± 4.2% in the control group, P = 0.15). There was no significant correlation between FMD and age, hemoglobin, LDH level, or transcranial Doppler findings. CSD was significantly elevated in the SCD group (0.96 ± 0.44 vs. 0.59 ± 0.21, P = 0.0002), whereas DWS and Einc were significantly lower in the SCD group. CSC did not differ significantly between the two groups.

CONCLUSIONS

Children with SCD have no marked endothelial dysfunction or change in arterial stiffness. These manifestations may be related to disease severity and duration. Changes may become evident later in life as the disease progresses.

Alterations of the arginine metabolome in sickle cell disease: a growing rationale for arginine therapy

Low global arginine bioavailability (GAB) is associated with numerous complications of SCD including early mortality. Mechanisms of arginine dysregulation involve a complex paradigm of excess activity of the arginine-consuming enzyme arginase, elevated levels of asymmetric dimethylarginine, altered intracellular arginine transport, and nitric oxide synthase dysfunction. Restoration of GAB through exogenous supplementation is therefore, a promising therapeutic target. Studies of arginine therapy demonstrate efficacy in treating patients with leg ulcers, pulmonary hypertension risk, and pain. Co-administration with hydroxyurea increases levels of nitrite and fetal hemoglobin. Addressing the alterations in the arginine metabolome may result in new strategies for treatment of SCD.

The effect of hemolysis on plasma oxidation and nitration in patients with sickle cell disease

This study aimed to determine the effect of haemolysis on plasma oxidation and nitration in sickle cell disease (SCD) patients. Blood was collected from haemoglobin (Hb)A volunteers and homozygous HbSS patients who had not received blood transfusions in the last 3 months. Haemolysis was characterised by low levels of haemoglobin and haptoglobin and high levels of reticulocyte, mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH), plasma cell-free haemoglobin, bilirubin, total lactate dehydrogenase (LDH) and dominance of LDH-1 isoenzyme. Plasma 8-isoprostane, protein carbonyl and nitrotyrosine levels were measured to evaluate oxidised lipids, oxidised and nitrated proteins, respectively. Plasma nitrite-nitrate levels were also determined to assess nitric oxide (NO) production in both SCD patients and controls. Markers of haemolysis were significantly evident in SCD patients compared to controls. Plasma 8-isoprostane, protein carbonyl and nitrotyrosine levels were markedly elevated in SCD patients compared to controls. Linear regression analysis revealed a significant inverse correlation between haemoglobin and reticulocyte counts and a significant positive correlation of plasma cell-free haemoglobin with protein carbonyl and nitrotyrosine levels. The obtained data shows that increased haemolysis in SCD increases plasma protein oxidation and nitration.

Acute chest syndrome: sickle cell disease

Acute chest syndrome (ACS) is a common complication and reason for hospital admission in patients with sickle cell disease (SCD). It is also the most common cause of death in this patient population. Most of the time, the trigger for ACS in an individual patient cannot be identified. However, although infection is the most common identifiable cause for ACS, other important triggers are vaso-occlusive crisis (VOC) and asthma. This comprehensive review will focus on the pathogenesis, clinical characteristics, complications and treatment available to manage ACS. But importantly, this review will highlight new possible etiologies, with the goal of improving oxygenation and, therefore, a reduction in sickling and lung damage in this patient population.

Effect of oral arginine supplementation on exhaled nitric oxide concentration in sickle cell anemia and acute chest syndrome

INTRODUCTION

Decreased exhaled nitric oxide levels (FE(NO)) have been described in patients with sickle cell disease (SCD) and a history of acute chest syndrome (ACS) when compared with non-ACS controls. Oral arginine supplementation has been shown to increase FE(NO) in healthy participants, but its effect in SCD patients is not known.

OBJECTIVE

To determine the effect of oral arginine intake on FENO in sickle cell patients with and without history of ACS, and in healthy controls.

HYPOTHESIS

No differences in the FE(NO) increase were seen in SCD patients with a history of ACS (ACS+) compared with healthy controls (HC) and SCD patients without history of ACS (ACS-).

MATERIALS AND METHODS

ACS+ (n=6), ACS- (n=9), and HC (n=7) patients were studied. At baseline, and after the administration of escalating doses of oral L-arginine (0.1, 0.2, and 0.4 g/kg), serial measurements were made of the following: FE(NO), plasma concentrations of arginine, ornithine, citrulline, aspartate, glutamate, arginine/ornithine ratio, nitrite, nitrate, heart rate (HR), respiratory rate (RR), blood pressure (BP), oxygen saturation (SpO2), forced expiratory volume in 1 second (FEV1), and forced vital capacity (FVC).

MAIN RESULTS

At baseline, FE(NO) did not differ among the groups. ACS- and ACS+ groups were deficient in arginine, and had decreased FEV1, FVC, and SaO2 when compared with HC patients. After arginine supplementation, FE(NO), arginine, ornithine, citrulline, nitrite, and the arginine/ornithine ratio increased similarly in all groups. Changes from baseline for HR, BP, SpO2, RR, FEV1, and FVC were minimal and similar in all groups.

CONCLUSIONS

In contrast to our earlier study, ACS+ patients had similar FE(NO) values when compared with ACS- and HC patients. All SCD patients were arginine deficient at baseline and showed impairment in respiratory physiology when compared with HC patients. After arginine supplementation, FE(NO) concentration increased in all groups to a similar degree, and lung function and physiologic parameters were minimally affected. The physiologic significance of alterations in FE(NO) in SCD patients and its relationship to ACS predilection requires further delineation.

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.

Pharmacotherapy in sickle cell disease--state of the art and future prospects

In the last decade, the care of patients with sickle cell disease (SCD) has undergone important advances with better understanding of disease pathophysiology and improvement in standards of care, especially among paediatric patients. Although many new drugs are currently being investigated and are at different stages of development, the pace of drug discovery and utilization has been slow and suboptimal. Hydroxycarbamide (hydroxyurea) has been investigated and utilized for at least two decades. Hydroxycarbamide's efficacy has been demonstrated, albeit with different levels of evidence, in paediatric and adult populations, and yet clinician and patient acceptance and use have been far from ideal. In this review we discuss the current usage of hydroxycarbamide and its possible future indications in SCD, as well as the use of new compounds that have very different mechanisms of action, which may prove safe and efficacious when used alone or in combination in patients with SCD.

Nitric oxide and arginine dysregulation: a novel pathway to pulmonary hypertension in hemolytic disorders

Secondary pulmonary hypertension (PH) is emerging as one of the leading causes of mortality and morbidity in patients with hemolytic anemias such as sickle cell disease (SCD) and thalassemia. Impaired nitric oxide (NO) bioavailability represents the central feature of endothelial dysfunction, and is a major factor in the pathophysiology of PH. Inactivation of NO correlates with hemolytic rate and is associated with the erythrocyte release of cell-free hemoglobin, which consumes NO directly, and the simultaneous release of the arginine-metabolizing enzyme arginase, which limits bioavailability of the NO synthase substrate arginine during the process of intravascular hemolysis. Rapid consumption of NO is accelerated by oxygen radicals that exists in both SCD and thalassemia. A dysregulation of arginine metabolism contributes to endothelial dysfunction and PH in SCD, and is strongly associated with prospective patient mortality. The central mechanism responsible for this metabolic disorder is enhanced arginine turnover, occurring secondary to enhanced plasma arginase activity. This is consistent with a growing appreciation of the role of excessive arginase activity in human diseases, including asthma and pulmonary arterial hypertension. New treatments aimed at improving arginine and NO bioavailability through arginase inhibition, suppression of hemolytic rate, oral arginine supplementation, or use of NO donors represent potential therapeutic strategies for this common pulmonary complication of hemolytic disorders.

Novel therapies in sickle cell disease

Despite an increased understanding of the pathophysiology of sickle cell disease (SCD), there remains a paucity of available agents for the prevention and treatment of specific SCD-related complications. Recently, there has been significant progress in the development of novel drugs for this disease. These agents, which increase the production of fetal hemoglobin, improve red blood cell hydration, increase the availability of nitric oxide and possess anti-inflammatory effects, are in varying stages of clinical development. With the complex pathophysiology of SCD, it is unlikely that a single agent will prevent or treat all the sequelae of this disease. As a result, patients may benefit from treatment with a combination of agents that possess different mechanisms of action. This overview discusses selected novel agents that appear promising in SCD.

Novel therapies in sickle cell disease

Despite an increased understanding of the pathophysiology of sickle cell disease (SCD), there remains a paucity of available agents for the prevention and treatment of specific SCD-related complications. Recently, there has been significant progress in the development of novel drugs for this disease. These agents, which increase the production of fetal hemoglobin, improve red blood cell hydration, increase the availability of nitric oxide and possess anti-inflammatory effects, are in varying stages of clinical development. With the complex pathophysiology of SCD, it is unlikely that a single agent will prevent or treat all the sequelae of this disease. As a result, patients may benefit from treatment with a combination of agents that possess different mechanisms of action. This overview discusses selected novel agents that appear promising in SCD.

C-reactive protein and interleukin-6 are decreased in transgenic sickle cell mice fed a high protein diet

Sickle cell disease is associated with hypermetabolism and a consequent shortage of substrates for normal growth and healthy immune response. The protein:energy ratio is a major determinant of dietary adequacy; the requirement for optimal growth of control mice is 20% of energy from dietary protein. This study investigated the efficacy of increased dietary protein for improving weight gain and reducing inflammation in the Berkeley sickle cell mouse model (S). The study examined the effect of diet on weight gain and circulating levels of 2 inflammatory proteins, C-reactive protein (CRP), and cytokine interleukin-6 (IL-6). Male C57BL/6 (C) control (n = 8) and S mice (n = 8) were randomized at weaning to 40 d of isoenergetic diets containing 20% (normal) and 35% (high) of energy from protein (C20, C35, S20, S35), replacing dextrin. Rate of weight gain was calculated and plasma CRP and IL-6 concentrations determined by ELISA. Liver mRNA expression of these proteins was measured by real-time PCR and L-arginase by colorimetric assay. S35 mice tended to gain weight more rapidly than S20 mice (P = 0.06) and more rapidly than C35 mice (P < 0.01). Circulating CRP and IL-6 levels were also lower in S35 mice than in S20 mice (P < 0.05), as was liver CRP mRNA expression (P < 0.01). These results demonstrate that introducing a high protein diet at weaning attenuates the steady-state inflammation in this S mouse model. Dietary L-arginine availability was investigated as a possible mechanism for increased nitric oxide production and consequent reduced inflammation.

Nitric oxide metabolism and the acute chest syndrome of sickle cell anemia

OBJECTIVE

To review the role of endothelial dysfunction and nitric oxide metabolism in the pathogenesis of the acute chest syndrome.

DATA SOURCE

A thorough literature search of PubMed for publications relevant to acute chest syndrome and nitric oxide metabolism in sickle cell disease was performed using search terms that included acute chest syndrome, sickle cell disease, nitric oxide metabolism, arginine, nitrite, nitrate, exhaled nitric oxide, nitric oxide synthase, and oxidant injury. We identified randomized controlled trials, case reports, editorials, and review articles from English-language and non-English-language studies of adult, pediatric, animal, and human subjects that describe the pathophysiology of acute chest syndrome, the biology of nitric oxide relevant to the pathophysiology of sickle cell disease, and the evidence for the role of endothelial dysfunction and abnormal nitric oxide metabolism in acute chest syndrome. We identified and reviewed 350 publications by the initial search and subsequent bibliography review. The articles most pertinent to the topic of this article were selected to support the discussion.

RESULTS

Acute chest syndrome is the leading cause of acute respiratory system dysfunction and a leading cause of morbidity and mortality among patients with sickle cell disease. Evidence is available to support decreased nitric oxide production, increased nitric oxide consumption, and abnormal metabolism of nitric oxide in patients with acute chest syndrome. Moreover, substrate availability is disturbed, and alternate pathways for substrate and nitric oxide metabolism exist.

CONCLUSIONS

Abnormalities of nitric oxide metabolism are prevalent during acute illness and baseline health in patients with sickle cell disease. Further investigation is needed to understand the clinical significance of aberrant nitric oxide metabolism as well as the potential for therapeutic manipulation of the arginine-nitric oxide pathway in patients with sickle cell disease.

Redox-dependent impairment of vascular function in sickle cell disease

The vascular pathophysiology of sickle cell disease (SCD) is influenced by many factors, including adhesiveness of red and white blood cells to endothelium, increased coagulation, and homeostatic perturbation. The vascular endothelium is central to disease pathogenesis because it displays adhesion molecules for blood cells, balances procoagulant and anticoagulant properties of the vessel wall, and regulates vascular homeostasis by synthesizing vasoconstricting and vasodilating substances. The occurrence of intermittent vascular occlusion in SCD leads to reperfusion injury associated with granulocyte accumulation and enhanced production of reactive oxygen species. The participation of nitric oxide (NO) in oxidative reactions causes a reduction in NO bioavailability and contributes to vascular dysfunction in SCD. Therapeutic strategies designed to counteract endothelial, inflammatory, and oxidative abnormalities may reduce the frequency of hospitalization and blood transfusion, the incidence of pain, and the occurrence of acute chest syndrome and pulmonary hypertension in patients with SCD.

Sickle cell disease and nitric oxide: a paradigm shift?

Traditionally the pathophysiology of sickle cell disease is thought to result from the polymerization of hemoglobin S in red cells, under hypoxic conditions, resulting in the occlusion of blood vessels. Adhesion of cells to the venular endothelium also appears to play a role. Recent studies have also suggested that in addition to the polymerization of hemoglobin S in the red blood cell, a deficiency of the endogenous vasodilator, nitric oxide may be involved. Hemoglobin released as a result of hemolysis rapidly consumes nitric oxide resulting in a whole program of events that inhibit blood flow. Therapies directed at decreasing the destruction of nitric oxide, increasing the production of nitric oxide, or amplifying the nitric oxide response may prove beneficial.

Levels of soluble endothelium-derived adhesion molecules in patients with sickle cell disease are associated with pulmonary hypertension, organ dysfunction, and mortality

Endothelial cell adhesion molecules orchestrate the recruitment and binding of inflammatory cells to vascular endothelium. With endothelial dysfunction and vascular injury, the levels of endothelial bound and soluble adhesion molecules increase. Such expression is modulated by nitric oxide (NO), and in patients with sickle cell disease (SCD), these levels are inversely associated with measures of NO bioavailability. To further evaluate the role of endothelial dysfunction in a population study of SCD, we have measured the levels of soluble endothelium-derived adhesion molecules in the plasma specimens of 160 adult patients with SCD during steady state. Consistent with a link between endothelial dysfunction and end-organ disease, we found that higher levels of soluble vascular cell adhesion molecule-1 (sVCAM-1) were associated with markers indicating renal dysfunction and hepatic impairment. Analysis of soluble intercellular cell adhesion molecule-1 (sICAM-1), sE-selectin and sP-selectin levels indicated partially overlapping associations with sVCAM-1, with an additional association with inflammatory stress and triglyceride levels. Importantly, increased soluble adhesion molecule expression correlated with severity of pulmonary hypertension, a clinical manifestation of endothelial dysfunction. Soluble VCAM-1, ICAM-1, and E-selectin were independently associated with the risk of mortality in this cohort. Our data are consistent with steady state levels of soluble adhesion molecules as markers of pulmonary hypertension and risk of death.

Arginine supplementation improves rotorod performance in sickle transgenic mice

Patients with sickle cell disease (SCD) have been shown to have impaired visual-motor speed and coordination. Sensorimotor deficits in mice can be investigated by motor coordination tests that require whole body movements such as the rotorod. A sickle transgenic mouse model (S+S-Antilles) that expresses human alpha, human beta(S) and human beta(S-Antilles), is homozygous for the mouse beta(major) deletion, and has low plasma arginine was compared to control C57BL/6J mice and S+S-Antilles mice supplemented with 5% arginine on the rotorod. The rotorod consists of a 2.5 cm diameter, grooved rod turning at constant acceleration, requiring postural adjustments on the part of the mice to maintain equilibrium. C57BL mice on Purina mouse chow had an average latency to fall of S+S-Antilles mice on Purina mouse chow had an average of 127+/-56 s S+S-Antilles mice after 5% arginine supplementation had a mean latency of Arginine may improve rotorod performance in sickle transgenic mice by increasing NO synthesis thereby improving vasodilatation and blood flow with reversal of ischemia in brain and/or muscle. In conclusion, impaired rotorod performance in sickle transgenic mice presents an opportunity to apply this simple task to provide an efficient method to screen some types of therapeutic regimens for efficacy in SCD.

Oral Glutamine Supplementation Decreases Resting Energy Expenditure in Children and Adolescents With Sickle Cell Anemia

OBJECTIVES

To determine the effects of orally administered glutamine on the resting energy expenditure (REE) and nutritional status of children and adolescents with sickle cell anemia.

METHODS

Twenty-seven children and adolescents (13 boys, 14 girls), 5.2 to 17.9 years old (median 11.0 years), received orally administered glutamine (600 mg/kg per day) for 24 weeks. Measures of REE and other nutritional parameters were compared at baseline and 24 weeks.

RESULTS

After 24 weeks, the patients' median REE (kcal/d) decreased by 6% (P = 0.053) as indicated by the Harris Benedict equations and by 5% (P = 0.049) as indicated by the modified equations. Patients with less than 90% ideal body weight had even greater declines in REE after 24 weeks (P < 0.03 and 0.02, respectively). Improvements in nutrition parameters and in two amino acids in the plasma were observed.

CONCLUSIONS

After 24 weeks of orally administered glutamine, children and adolescents with sickle cell anemia had a decrease in REE and improvement in nutritional parameters. Those who were underweight had a greater decrease in REE than those of normal body weight. Lowering REE may be an effective way to improve the growth of these children and adolescents.

Differential gene expression in the kidney of sickle cell transgenic mice: upregulated genes

The S+S-Antilles transgenic mouse used in this study has renal defects similar to those seen in sickle cell anemia patients: congested glomeruli, medullary fibrosis, renal enlargement, vasoocclusion, and a urine concentrating defect. We used gene expression microarrays to identify genes highly up-regulated in the kidneys of these mice and validated their expression by real-time PCR. Kidney hypoxia, as demonstrated by the presence of deoxyhemoglobin, was detected by blood oxygen dependent magnetic resonance imaging (BOLD-MRI). Some of the up-regulated genes included cytochrome P450 4a14, glutathione-S-transferase alpha-1, mitochondrial hydroxymethylglutaryl CoA synthase, cytokine inducible SH-2 containing protein, retinol dehydrogenase type III, arginase II, glycolate oxidase, Na/K ATPase, renin-1, and alkaline phosphatase 2. An increase in enzyme activity was also demonstrated for one of the up-regulated genes (arginase II). These genes can be integrated into several different pathophysiological processes: a hypoxia cascade, a replacement cascade, or an ameliorating cascade, one or all of which may explain the phenotype of this disease. We conclude that microarray technology is a powerful tool to identify genes involved in renal disease in sickle cell anemia and that the identification of various metabolic pathways may open new avenues for therapeutic interventions.

Hydroxyurea and arginine therapy: impact on nitric oxide production in sickle cell disease

PURPOSE

Recent data suggest that hydroxyurea (HU) increases the production of nitric oxide (NO), a potent vasodilator. NO is normally metabolized from l-Arginine (Arg). However, in vitro and animal experiments suggest that HU is the NO donor itself. In contrast, a recent study indicates that nitric oxide synthase (NOS) may play a role. Since adults with sickle cell disease (SCD) are Arg-deficient, Arg availability may limit the ability of HU to maximally impact NO production if an NOS mechanism is involved. The authors have previously shown that Arg supplementation alone induces a paradoxical decrease in NO metabolite (NO(x)) production.

METHODS

The authors studied the effects of HU and Arg supplementation on NO(x) production. HU alone or HU + Arg was administered to patients with SCD at steady state, and sequential levels of Arg, serum NO(x) and exhaled NO were followed over 4 hours.

RESULTS

After HU + Arg, all patients demonstrated a significant increase in serum NO(x) production within 2 hours. When the same patients were treated with HU alone (5.1 +/- 2 micromol/L), a mixed response occurred. NO(x) levels increased in four patients and decreased in one patient (-23.3 micromol/L).

CONCLUSIONS

While Arg alone does not increase serum NO(x) production in SCD patients at steady state, it does when given together with HU. Hence, co-administration of Arg with HU may augment the NO(x) response in SCD and improve utilization of Arg in patients at steady state.

Decreased exhaled nitric oxide in sickle cell disease: relationship with chronic lung involvement

A deficiency in airway nitric oxide (NO) could contribute to pulmonary vaso-occlusion in sickle cell disease (SCD). We measured the fractional expired concentration of NO (FE(NO)) by chemiluminescence during a slow vital capacity maneuver against a positive pressure of 16 cm H(2)O at an expiratory flow rate of 50 mL/sec in 44 stable ambulatory adults with SCD and 30 healthy controls. A history of acute chest syndrome was present in 29 patients, and 22 complained of dyspnea. Mean +/- SD FE(NO) was significantly reduced in the SCD group compared with controls (14.8 +/- 8.4 vs. 24.9 +/- 13.5 ppb, P < 0.001). SCD patients with dyspnea had lower FE(NO) than those without dyspnea (10.1 +/- 5.7 vs. 19.6 +/- 8 ppb, P < 0.001) and those with a history of ACS had lower values than those no episodes of ACS (13.0 +/- 8.3 vs. 18.4 +/- 7.6 ppb, P < 0.05). There was a weak correlation between FE(NO) and percent-predicted DLCO (r = 0.4, P = 0.02) among the SCD patients. We conclude that exhaled NO is reduced in adults with SCD, and this may play a role in the pathogenesis of acute chest syndrome and chronic sickle cell lung disease.

An emerging role for nitric oxide in sickle cell disease vascular homeostasis and therapy

Nitric oxide participates in the compensatory response to chronic vascular injury in patients with sickle cell disease. The authors have found reductions of basal and stimulated nitric oxide production and responses to exogenous nitric oxide in male patients with sickle cell disease. Gender differences in nitric oxide bioavailability are probably caused in part by the protective effects of ovarian estrogen on nitric oxide synthase expression and activity in women. Further, in men, and likely all patients during vaso-occlusive crisis and the acute chest syndrome, nitric oxide is destroyed by increased circulating plasma hemoglobin and superoxide. The combined effects of inhaled nitric oxide gas of improving pulmonary ventilation to perfusion matching and hemodynamics, reducing alveolar and systemic inflammation, and inhibiting circulating plasma hemoglobin (and thus restoring peripheral nitric oxide bioavailability) may modulate the course of the disease, including the frequency and severity of vaso-occlusive crises and acute chest syndrome episodes. Possible effects of chronic nitric oxide-based therapies on erythrocyte density, pulmonary artery pressures, and fetal hemoglobin induction deserve study.

Divergent nitric oxide bioavailability in men and women with sickle cell disease

BACKGROUND

Although reduced endothelial nitric oxide (NO) bioavailability has been demonstrated in arteriosclerotic vascular disease, the integrity of this system in sickle cell disease remains uncertain.

METHODS AND RESULTS

We measured forearm blood flow in 21 patients with sickle cell disease (hemoglobin SS genotype) and 18 black control subjects before and after intra-arterial infusions of acetylcholine, nitroprusside, and the NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA). Endothelium-dependent vasodilation, measured by the percent increase in flow induced by acetylcholine infusion, was significantly greater than in controls (252+/-37% for patients versus 134+/-24% for controls; P<0.0001). However, there was a large sex difference in blood flow responses between female and male patients (340+/-46% versus 173+/-41%; P=0.035). Similarly, basal NO bioactivity, as measured by the percent decrease in flow induced by L-NMMA, was depressed in male compared with female patients (-17+/-5% versus -34+/-4%; P=0.01), as was the response to nitroprusside (86+/-21% versus 171+/-22%; P=0.008). L-NMMA reduced the blood flow response to acetylcholine in women, but not in men. Sex differences in vascular cell adhesion molecule-1 were appreciated, with significant correlations between levels of soluble vascular cell adhesion molecule-1 and blood flow responses to L-NMMA and nitroprusside (r=0.53, P=0.004 and r=-0.66, P<0.001, respectively).

CONCLUSIONS

NO bioavailability and NO responsiveness are greater in women than in men with sickle cell disease and determines adhesion molecule expression. Endothelium-dependent blood flows are largely non-NO mediated in male patients. These results provide a possible mechanism for reported sex differences in sickle cell disease morbidity and mortality and provide a basis for novel pharmacological interventions.

The metabolic basis of arginine nutrition and pharmacotherapy

As an essential precursor for the synthesis of proteins and other molecules with enormous biological importance (including nitric oxide, urea, ornithine, proline, polyamines, glutamate, creatine, agmatine, and dimethylarginines), arginine displays remarkable metabolic and regulatory versatility. Evidence available to date provides a sound reason to classify arginine as an essential amino acid for young mammals (including parenterally fed human infants) and as a conditionally essential amino acid for adults under such conditions as trauma, burn injury, massive small-bowel resection, and renal failure. Arginine administration reverses endothelial dysfunction, enhances wound healing, prevents the early stages of tumorigenesis, and improves cardiovascular, reproductive, pulmonary, renal, digestive, and immune functions. Arginine or its effective precursor citrulline may hold great promise as a nutritional or pharmacotherapeutic treatment for a wide array of human diseases.

Comparison of energy prediction equations with measured resting energy expenditure in children with sickle cell anemia

OBJECTIVE

To determine the accuracy of energy prediction equations when compared with measured resting energy expenditure (REE) in children with sickle cell anemia. To develop a modified equation that more accurately estimates the energy needs of children with sickle cell anemia and to cross-validate these on a different set of patients (test patients).

DESIGN

REE was measured in children using indirect calorimetry and compared with predicted values using the Harris-Benedict and the Food and Agriculture Organization/World Health Organization/United Nations University equations (WHO).

SUBJECTS/SETTING

Eighteen patients participated in the original sample that compared predicted with measured energy expenditure. The modified equations were developed using the original 18 patients. A test population of 20 different patients was used to validate the modified equations.

STATISTICAL ANALYSIS

Wilcoxon signed-rank test was performed to compare measured with predicted REE. The correlation analysis method and multiple linear regression method were used to develop 2 modified versions for the Harris-Benedict and WHO prediction equations.

RESULTS

When compared with the mean predicted REE using the Harris-Benedict and WHO equations, the mean measured REE was 14% and 12% greater than both (P=.005 and P=.014, respectively). Two modified equations were developed from the Harris-Benedict and WHO equations. Based on the data from the test patients, the mean measured REE was 15% greater than the mean predicted REE based on the Harris-Benedict and WHO equations (P=.0001 for both). When the modified Harris-Benedict and WHO equations were used, there was almost no difference in the mean measured REE and the mean predicted REE (mean difference using Harris-Benedict = 14, P = .9273; mean difference using WHO = -13, P = .6215).

CONCLUSION

Both energy prediction equations underestimated REE in children with sickle cell anemia. The 2 modified versions of the energy prediction equations that we propose predicted the energy needs of these children much more accurately; however, the modified equations need to be validated through application to other children with sickle cell anemia.

Energy intake and resting metabolic rate in preschool Jamaican children with homozygous sickle cell disease

BACKGROUND

A relative energy deficiency consequent to a high resting metabolic rate (RMR) may contribute to growth impairment in persons with homozygous (SS genotype) sickle cell disease (SCD). The growth deficit in SCD emerges at an early age, but few studies have addressed the adequacy of energy intake relative to RMR in young children.

OBJECTIVE

Our objective was to test the hypothesis that energy intake relative to RMR is lower in children with SCD than in control subjects.

DESIGN

The dietary intake of 41 children with SCD and 31 control subjects with a normal hemoglobin genotype (AA) aged 3-6 y was assessed by weighing all food consumed during 3 d. RMR was determined with the use of indirect calorimetry.

RESULTS

The RMR in the children with SCD ( +/- SD: 5.47 +/- 0.93 MJ/d) was higher than that in the control subjects (5.19 +/- 1.3 MJ/d) after adjustment for sex and weight (P = 0.04). Energy intake did not differ significantly between the 2 genotype groups. The ratio of energy intake to RMR was lower in the children with SCD ( +/- SD: 1.13 +/- 0.33) than in the control subjects (1.35 +/- 0.38) after adjustment for sex and weight (P = 0.005).

CONCLUSIONS

Prepubertal children with SCD fail to compensate for their higher RMR by increasing their energy intake. This observation is consistent with a hypothesis of a relative energy deficiency in SCD.

Mechanisms of dysfunction of the nitric oxide pathway in vascular diseases

Vascular nitric oxide (NO) is involved in many physiologic and pathophysiologic processes throughout the body. Many vascular diseases have a reduction in the activity of endothelium-derived NO as an important component involved in the initiation and/or progression of the disease. It is now known that there are multiple mechanisms for this reduction in NO activity with one or more mechanisms operating depending on the specific condition or stage of a disease. In other instances, the therapy for certain diseases is responsible for the reduction in NO activity and contributes to the acceleration of vascular disease. This review details the known mechanisms of dysfunction of the NO pathway leading to vascular diseases, which provides the rationale for why certain therapies can improve while other therapies adversely affect vascular health.

Arginine supplementation of sickle transgenic mice reduces red cell density and Gardos channel activity

Nitric oxide (NO), essential for maintaining vascular tone, is produced from arginine by nitric oxide synthase. Plasma arginine levels are low in sickle cell anemia, and it is reported here that low plasma arginine is also found in our sickle transgenic mouse model that expresses human alpha, human beta(S), and human beta(S-Antilles) and is homozygous for the mouse beta(major) deletion (S+S-Antilles). S+S-Antilles mice were supplemented with a 4-fold increase in arginine that was maintained for several months. Mean corpuscular hemoglobin concentration (MCHC) decreased and the percent high-density red cells was reduced. Deoxy K(+) efflux is characteristic of red cells in sickle cell disease and contributes to the disease process by increasing the MCHC and rendering the cells more susceptible to polymer formation. This flux versus the room air flux was reduced in S+S-Antilles red cells from an average value of 1.6 +/- 0.3 mmol per liter of red cells x minute (FU) in nonsupplemented mice to 0.9 +/- 0.3 FU (n = 4, P < .02, paired t test) in supplemented mice. In room air, V(max) of the Ca(++)-activated K(+) channel (Gardos) was reduced from 4.1 +/- 0.6 FU (off diet) to 2.6 +/- 0.4 FU (n = 7 and 8, P < .04, t test) in arginine-supplemented mice versus clotrimazole. In conclusion, the major mechanism by which arginine supplementation reduces red cell density (MCHC) in S+S-Antilles mice is by inhibiting the Ca(++)-activated K(+) channel.

Low exhaled nitric oxide and a polymorphism in the NOS I gene is associated with acute chest syndrome

Abnormalities of nitric oxide metabolism have been implicated in the pathogenesis of acute chest syndrome in subjects with sickle cell anemia. It is not known whether exhaled nitric oxide levels (FE(NO)) are abnormal in children with a history of the acute chest syndrome (ACS). We compared FE(NO), plasma nitric oxide metabolites (NO(x)), serum arginine and citrulline levels, and the number of AAT repeats in intron 20 of NOS I in subjects with sickle cell disease (SCD) and a history of at least one episode of ACS (ACS(+), n = 13), subjects with SCD and no prior history of ACS (ACS(-), n = 7), and healthy children (HC, n = 6). Mean +/- SD FE(NO) (ppb) was lower in ACS(+) than in ACS(-) and HC: (10.4 +/- 4.3 versus 23.4 +/- 6.1 p = 0.002] and 30.4 +/- 15.8 [p = 0.0001], respectively). Plasma NO(x) (microM) were similar in all three groups (37.3 +/- 19.4, 33.0 +/- 13.2, 44.7 +/- 7.8, respectively). Arginine and citrulline levels (microM) did not differ between ACS(+) and ACS(-) groups. Spirometric data revealed a mildly diminished FEV(1) and FVC in ACS(+) that was statistically different from HC but not ACS(-): (FEV(1) as % of predicted for ACS(+), ACS(-), and HC; 83 +/- 17 versus 87 +/- 16 versus 102 +/- 16, respectively, p < 0.05 between ACS(+) and HC). The level of FE(NO) was significantly associated with the sum of AAT repeats in intron 20 of NOS I gene alleles. The correlation coefficient (r) was 0.62 (p < 0.005). We conclude that FE(NO) levels are significantly reduced in subjects who have a history of ACS and that the FE(NO) levels are significantly correlated with the number of NOS I AAT repeats. FE(NO) is a sensitive marker and may be a predictor of ACS prone children.

Nitric oxide therapy in sickle cell disease

Recent clinical and experimental data suggest that nitric oxide (NO) may play a role in the pathogenesis and therapy of sickle cell disease. NO, a soluble gas continuously synthesized in endothelial cells by the NO synthase (NOS) enzyme systems, regulates basal vascular tone and endothelial function, and maintains blood oxygenation via hypoxic pulmonary vasoconstriction and reduced shunt physiology. These vital homeostatic processes may be impaired in sickle cell disease and contribute to its pathogenesis. Therapeutic NO inhalation exerts significant direct effects on the pulmonary vasculature to reduce pulmonary pressures and increase oxygenation that may prove beneficial in acute chest syndrome and secondary pulmonary hypertension. Delivery of NO bound to hemoglobin or in plasma may improve blood flow and hemoglobin saturation, and thus reduce ischemia-reperfusion injury. Other NO-related effects on adhesion molecule expression and fetal hemoglobin induction are of interest. While direct evidence for a clinical benefit of NO therapy in sickle cell disease has not been reported, studies are underway to determine if inhaled NO will reduce the substantial morbidity and mortality suffered by these patients.

Glutathione in disease

Altered glutathione metabolism in association with increased oxidative stress has been implicated in the pathogenesis of many diseases. However, whether strategies aimed at restoring glutathione concentration and homeostasis are effective in ameliorating or modifying the natural history of these states is unknown. In this review we discuss the pathogenic role for altered glutathione metabolism in such diseases as protein energy malnutrition, seizures, Alzheimer's disease, Parkinson's disease, sickle cell anaemia, chronic diseases associated with ageing and the infected state. In addition, we discuss the efficacy of glutathione precursors in restoring glutathione homeostasis both in vitro and in vivo.

Patterns of arginine and nitric oxide in patients with sickle cell disease with vaso-occlusive crisis and acute chest syndrome

PURPOSE

Our objective was to evaluate L-arginine and nitric oxide metabolite (NOx) levels in children with sickle cell disease (SCD) at steady-state and during vaso-occlusive crisis (VOC). Because alterations in nitric oxide production may have an important role in the pathophysiology of SCD, our second aim was to determine if a relationship exists between these levels and vaso-occlusive crisis (VOC).

PATIENTS AND METHODS

Plasma L-arginine and serum NOx levels were examined in 36 patients with SCD with 39 episodes of VOC and 10 children with SCD at steady-state. Daily levels were obtained in children requiring hospitalization.

RESULTS

Steady-state L-arginine levels were normal in children with SCD. L-arginine levels were low, however, in children with VOC (37.4 +/- 2.7 vs. 53.6 +/- 4.6 micromol/L; P = 0.008) but returned to baseline during hospitalization. In contrast, NOx levels were normal at presentation but decreased during hospitalization for both patients with VOC and patients with acute chest syndrome (ACS) (21.1 +/- 2.0, 17.4 +/- 2.4, and 12.3 +/- 1.6 micromol/L, respectively; P < 0.05). In the patients with VOC who had ACS develop, L-arginine decreased to the lowest levels at the time of the ACS diagnosis, correlating with decreasing NOx levels.

CONCLUSION

These data suggest that there may be a relationship between the L-arginine-nitric oxide pathway and vaso-occlusion in SCD. Low arginine levels during VOC could reflect a state of acute substrate depletion that results in a decrease in nitric oxide production.

Arginine therapy: a novel strategy to induce nitric oxide production in sickle cell disease

To determine the effects of L-arginine (L-Arg) supplementation on nitric oxide metabolite (NOx) production, oral L-Arg was given to normal controls, sickle cell disease (SCD) patients at steady state and SCD patients hospitalized with a vaso-occlusive crisis (VOC). L-Arg (0.1 g/kg) increased NOx formation by 18.8 +/- 68% in normal controls, whereas steady-state SCD patients demonstrated a paradoxical decrease in NOx of -16.7 +/- 4% (P = 0.004). In contrast, patients with VOC demonstrated a dramatic increase in NOx production by +77.7 +/- 103%, a response that was dose dependent. L-Arg appears to be the rate-limiting step in NOx production during VOC. Oral arginine may therefore benefit SCD patients by inducing an increase in NO production during VOC.

Reactive species in sickle cell disease

The red cell is a relatively abundant locus of both free radical generation and reaction. Erythrocytes have a high content of unsaturated membrane lipids, a rich oxygen supply and are densely packed with redox-active hemoglobin residues. In response, red cells have a highly evolved and well-integrated network of oxidant defense mechanisms that lend an ability to withstand oxidative stress. In the case of congenital hemoglobin mutations that underlie sickle cell disease, they become very susceptible to free radical-mediated injury by virtue of enhanced endogenous rates of production of reactive species and impairment of tissue free radical defense mechanisms. In sickle cell disease, a combination of these susceptibility factors are hypothesized to lead to an overall impairment of vascular function, in large part due to loss of "bioactive" nitric oxide via the free radical-mediated consumption of this vasoactive molecule.

Inhibition of nitric oxide synthase ameliorates cellular injury in sickle cell mouse kidneys

BACKGROUND

In previous studies of transgenic sickle cell mice, increased renal expression of inducible nitric oxide synthase (iNOS) and endothelial cell isoform of NOS (EcNOS) was found by Western blot and immunohistochemistry. In addition, putative evidence of peroxynitrite (ONOO-) formation was found in the form of positive immunostaining and immunoblot for nitrotyrosine. Apoptosis was also detected by DNA strand breakage and TUNEL assay. The present study was carried out to examine the role of NO/ONOO- in mediating renal tubular cell apoptosis in sickle cell mouse kidneys.

METHODS

Mercaptoethylguanidine (MEG), a compound that selectively inhibits iNOS and also is a scavenger of ONOO-, was administered intraperitoneally over a five-day period to control and betas mice. Immunohistochemistry of iNOS and nitrotyrosine, DNA electrophoresis, ApoTACS assay for apoptosis, and Western blot of poly(ADP-ribose) polymerase (PARP) were carried out.

RESULTS

MEG administration virtually eliminated renal immunostaining of iNOS and nitrotyrosine and prevented DNA strand breakage. In addition, Western blot analysis of PARP, a nuclear DNA-reparative enzyme activated in response to DNA strand breakage, was found to be cleavaged in hypoxic betas mice, but was partially protected in MEG-treated betas hypoxic mice. Finally, apoptosis was markedly reduced by MEG in betas hypoxic mice.

CONCLUSIONS

These observations provide evidence that NO and/or ONOO- are responsible for initiating cell damage, which leads to apoptosis in sickle cell mouse kidneys.