Therapy with hydroxyurea is associated with reduced adhesion molecule gene and protein expression in sickle red cells with a concomitant reduction in adhesive properties

The Current Role of Hydroxyurea in the Treatment of Sickle Cell Anemia

Despite advancements in treatment of sickle cell disease (SCD), hydroxyurea, a ribonucleotide reductase inhibitor, remains the cornerstone of therapy. While its primary effect is the elevation of fetal hemoglobin (HbF), hydroxyurea's mechanisms of action are multifaceted. Hydroxyurea (HU) reduces leukocyte and platelet counts, decreases the expression of endothelial adhesion molecules CD36 and CD49d, and increases nitric oxide and cyclic nucleotide levels, which may facilitate vascular dilation and further HbF induction. Numerous studies have demonstrated that hydroxyurea therapy reduces the frequency of painful episodes, acute chest syndrome, and the need for erythrocyte transfusions and hospitalizations. Long-term use of hydroxyurea leads to reduced morbidity and mortality. Hydroxyurea should be initiated in children from 9 months of age, including asymptomatic individuals, and is recommended for adults experiencing pain crises that significantly interfere with daily activities or quality of life, as well as those with severe or recurrent vaso-occlusive crises, ACS, or severe symptomatic anemia. Hydroxyurea is not recommended during pregnancy or lactation due to potential teratogenic effects and transfer into breast milk. However, its use may be considered in high-risk patients, particularly during the second and third trimesters. Concerns about secondary tumor development have not been substantiated in long-term follow-up studies. Alternative therapies, including L-glutamine, crizanlizumab, and voxelotor, are not presently approved or available for clinical use in Europe.

Research in Sickle Cell Disease: From Bedside to Bench to Bedside

Sickle cell disease (SCD) is an exemplar of bidirectional translational research, starting with a remarkable astute observation of the abnormally shaped red blood cells that motivated decades of bench research that have now translated into new drugs and genetic therapies. Introduction of hydroxyurea (HU) therapy, the only SCD-modifying treatment for >30 years and now standard care, was initiated through another clinical observation by a pediatrician. While the clinical efficacy of HU is primarily due to its fetal hemoglobin (HbF) induction, the exact mechanism of how it increases HbF remains not fully understood. Unraveling of the molecular mechanism of how HU increases HbF has provided insights on the development of new HbF-reactivating agents in the pipeline. HU has other salutary effects, reduction of cellular adhesion to the vascular endothelium and inflammation, and dissecting these mechanisms has informed bench-both cellular and animal-research for development of the 3 recently approved agents: endari, voxelotor, and crizanlizumab; truly, a bidirectional bench to bedside translation. Decades of research to understand the mechanisms of fetal to adult hemoglobin have also culminated in promising anti-sickling genetic therapies and the first-in-human studies of reactivating an endogenous (γ-globin) gene HBG utilizing innovative genomic approaches.

Sickle cell vaso-occlusion: The dialectic between red cells and white cells

The pathophysiology of sickle cell anemia, a hereditary hemoglobinopathy, has fascinated clinicians and scientists alike since its description over 100 years ago. A single gene mutation in the HBB gene results in the production of abnormal hemoglobin (Hb) S, whose polymerization when deoxygenated alters the physiochemical properties of red blood cells, in turn triggering pan-cellular activation and pathological mechanisms that include hemolysis, vaso-occlusion, and ischemia-reperfusion to result in the varied and severe complications of the disease. Now widely regarded as an inflammatory disease, in recent years attention has included the role of leukocytes in vaso-occlusive processes in view of the part that these cells play in innate immune processes, their inherent ability to adhere to the endothelium when activated, and their sheer physical and potentially obstructive size. Here, we consider the role of sickle red blood cell populations in elucidating the importance of adhesion vis-a-vis polymerization in vaso-occlusion, review the direct adhesion of sickle red cells to the endothelium in vaso-occlusive processes, and discuss how red cell- and leukocyte-centered mechanisms are not mutually exclusive. Given the initial clinical success of crizanlizumab, a specific anti-P selectin therapy, we suggest that it is appropriate to take a holistic approach to understanding and exploring the complexity of vaso-occlusive mechanisms and the adhesive roles of the varied cell types, including endothelial cells, platelets, leukocytes, and red blood cells.

A comprehensive review of hydroxyurea for β-haemoglobinopathies: the role revisited during COVID-19 pandemic

Background

Hydroxyurea is one of the earliest drugs that showed promise in the management of haemoglobinopathies that include β-thalassaemia and sickle cell disease. Despite this, many aspects of hydroxyurea are either unknown or understudied; specifically, its usefulness in β-thalassaemia major and haemoglobin E β-thalassaemia is unclear. However, during COVID-19 pandemic, it has become a valuable adjunct to transfusion therapy in patients with β-haemoglobinopathies. In this review, we aim to explore the available in vitro and in vivo mechanistic data and the clinical utility of hydroxyurea in β-haemoglobinopathies with a special emphasis on its usefulness during the COVID-19 pandemic.

Main body

Hydroxyurea is an S-phase-specific drug that reversibly inhibits ribonucleoside diphosphate reductase enzyme which catalyses an essential step in the DNA biosynthesis. In human erythroid cells, it induces the expression of γ-globin, a fetal globin gene that is suppressed after birth. Through several molecular pathways described in this review, hydroxyurea exerts many favourable effects on the haemoglobin content, red blood cell indices, ineffective erythropoiesis, and blood rheology in patients with β-haemoglobinopathies. Currently, it is recommended for sickle cell disease and non-transfusion dependent β-thalassaemia. A number of clinical trials are ongoing to evaluate its usefulness in transfusion dependent β-thalassaemia. During the COVID-19 pandemic, it was widely used as an adjunct to transfusion therapy due to limitations in the availability of blood and logistical disturbances. Thus, it has become clear that hydroxyurea could play a remarkable role in reducing transfusion requirements of patients with haemoglobinopathies, especially when donor blood is a limited resource.

Conclusion

Hydroxyurea is a well-tolerated oral drug which has been in use for many decades. Through its actions of reversible inhibition of ribonucleoside diphosphate reductase enzyme and fetal haemoglobin induction, it exerts many favourable effects on patients with β-haemoglobinopathies. It is currently approved for the treatment of sickle cell disease and non-transfusion dependent β-thalassaemia. Also, there are various observations to suggest that hydroxyurea is an important adjunct in the treatment of transfusion dependent β-thalassaemia which should be confirmed by randomised clinical trials.

MEK1/2 as a Therapeutic Target in Sickle Cell Disease

Identification of novel therapeutic targets has improved diagnostics and treatment of many diseases. Many innovative treatment strategies have been developed based on the newly identified biomarkers and key molecules. Most of the research focused on ways to manipulate signaling pathways by activating or suppressing them, validate new therapeutic targets for treatment, and epigenetic treatment of diseases. With the identification of aberrations in multiple growth pathways, the focus then shifted to the small molecules involved in these pathways for targeted therapy. In this communication/short review, we highlight the importance of identification of abnormal activation of the mitogen-activated protein kinase (MAPK), ERK1/2, and its upstream mediator MEK1/2, in erythrocytes in patients with sickle cell disease (SCD) critical for the adhesive interactions of these cells with the endothelium, and leukocytes promoting circulatory obstruction leading to tissue ischemia and infraction. We also discuss how targeting this signaling cascade with MEK1/2 inhibitors can reverse acute vasoocclusive crises in SCD.

Inflammation in sickle cell disease

The primary β-globin gene mutation that causes sickle cell disease (SCD) has significant pathophysiological consequences that result in hemolytic events and the induction of the inflammatory processes that ultimately lead to vaso-occlusion. In addition to their role in the initiation of the acute painful vaso-occlusive episodes that are characteristic of SCD, inflammatory processes are also key components of many of the complications of the disease including autosplenectomy, acute chest syndrome, pulmonary hypertension, leg ulcers, nephropathy and stroke. We, herein, discuss the events that trigger inflammation in the disease, as well as the mechanisms, inflammatory molecules and cells that propagate these inflammatory processes. Given the central role that inflammation plays in SCD pathophysiology, many of the therapeutic approaches currently under pre-clinical and clinical development for the treatment of SCD endeavor to counter aspects or specific molecules of these inflammatory processes and it is possible that, in the future, we will see anti-inflammatory drugs being used either together with, or in place of, hydroxyurea in those SCD patients for whom hematopoietic stem cell transplants and evolving gene therapies are not a viable option.

Biomechanics and biorheology of red blood cells in sickle cell anemia

Sickle cell anemia (SCA) is an inherited blood disorder that causes painful crises due to vaso-occlusion of small blood vessels. The primary cause of the clinical phenotype of SCA is the intracellular polymerization of sickle hemoglobin resulting in sickling of red blood cells (RBCs) in deoxygenated conditions. In this review, we discuss the biomechanical and biorheological characteristics of sickle RBCs and sickle blood as well as their implications toward a better understanding of the pathophysiology and pathogenesis of SCA. Additionally, we highlight the adhesive heterogeneity of RBCs in SCA and their specific contribution to vaso-occlusive crisis.

Sickle cell disease biochip: a functional red blood cell adhesion assay for monitoring sickle cell disease

Sickle cell disease (SCD) afflicts millions of people worldwide and is associated with considerable morbidity and mortality. Chronic and acute vaso-occlusion are the clinical hallmarks of SCD and can result in pain crisis, widespread organ damage, and early movtality. Even though the molecular underpinnings of SCD were identified more than 60 years ago, there are no molecular or biophysical markers of disease severity that are feasibly measured in the clinic. Abnormal cellular adhesion to vascular endothelium is at the root of vaso-occlusion. However, cellular adhesion is not currently evaluated clinically. Here, we present a clinically applicable microfluidic device (SCD biochip) that allows serial quantitative evaluation of red blood cell (RBC) adhesion to endothelium-associated protein-immobilized microchannels, in a closed and preprocessing-free system. With the SCD biochip, we have analyzed blood samples from more than 100 subjects and have shown associations between the measured RBC adhesion to endothelium-associated proteins (fibronectin and laminin) and individual RBC characteristics, including hemoglobin content, fetal hemoglobin concentration, plasma lactate dehydrogenase level, and reticulocyte count. The SCD biochip is a functional adhesion assay, reflecting quantitative evaluation of RBC adhesion, which could be used at baseline, during crises, relative to various long-term complications, and before and after therapeutic interventions.

Influence of the Clinical Status on Stress Reticulocytes, CD 36 and CD 49d of SSFA 2 Homozygous Sickle Cell Patients Followed in Abidjan

Background and Objectives. Interactions between sickle cells involving CD 49d, CD36, and the vascular endothelium may initiate vasoocclusion leading to acute painful episodes and multiple organ failure. Materials and Methods. We selected 60 SS patients who had never been treated by hydroxyurea. We performed a total blood count. We identified with immunophenotyping by flow cytometry total reticulocytes their distribution according to the degree of maturity (mature, intermediate, very immature) and CD 36(+) and CD 49d(+) antigens. Stress reticulocytes corresponded to the sum of intermediate and immature cells. Results. Subjects in crisis had more total reticulocytes and very immature reticulocytes than subjects in stationary phase (P < 0.05). During the crisis, total CD 36(+) reticulocytes (214 870 ± 107 584/ μ L versus 148 878 ± 115 024/ μ L; P < 0.05) and the very immature CD 36(+) reticulocytes (28.9 ± 7.9% versus 23.0 ± 6.4%; P < 0.05) increased. The clinical status had no impact on CD 49d(+) reticulocytes. Conclusion. The rates of stress reticulocytes in general and those expressing CD 49d and CD 36 were very high. The clinical status had an influence on CD 36(+) reticulocytes. The expression of adhesion molecules is only one of the parameters involved in sickle cell disease crisis.

Emerging science of hydroxyurea therapy for pediatric sickle cell disease

Hydroxyurea (HU) is the sole approved pharmacological therapy for sickle cell disease (SCD). Higher levels of fetal hemoglobin (HbF) diminish deoxygenated sickle globin polymerization in vitro and clinically reduce the incidence of disease morbidities. Clinical and laboratory effects of HU largely result from induction of HbF expression, though to a highly variable extent. Baseline and HU-induced HbF expression are both inherited complex traits. In children with SCD, baseline HbF remains the best predictor of drug-induced levels, but this accounts for only a portion of the induction. A limited number of validated genetic loci are strongly associated with higher baseline HbF levels in SCD. For induced HbF levels, genetic approaches using candidate single-nucleotide polymorphisms (SNPs) have identified some of these same loci as being also associated with induction. However, SNP associations with induced HbF are only partially independent of baseline levels. Additional approaches to understanding the impact of HU on HbF and its other therapeutic effects on SCD include pharmacokinetic, gene expression-based, and epigenetic analyses in patients and through studies in existing murine models for SCD. Understanding the genetic and other factors underlying the variability in therapeutic effects of HU for pediatric SCD is critical for prospectively predicting good responders and for designing other effective therapies.

Patients with sickle cell disease taking hydroxyurea in the Hemocentro Regional de Montes Claros

Background

The development of therapies for sickle cell disease has received special attention, particularly those that reduce the polymerization of hemoglobin S. Hydroxyurea is a commonly used medication because it has the ability to raise levels of fetal hemoglobin, decrease the frequency of vaso-occlusive episodes and thus improve the clinical course of sickle cell disease patients.

Objective

To study hematological data and the clinical profile of sickle cell disease patients taking hydroxyurea in a regional blood center.

Methods

From the charts of 20 patients with sickle cell anemia, the clinical outcomes and a number of hematological variables were analyzed before and during treatment with hydroxyurea.

Results

The patients' ages ranged from 6 to 41 years old, most were dark skinned and there was a predominance of women. The main symptom that defined whether patients were prescribed hydroxyurea was painful crises followed by hospitalizations. During treatment with hydroxyurea there were significant increases in hemoglobin, fetal hemoglobin, mean corpuscular volume and mean corpuscular hemoglobin. The reticulocyte and white blood cell counts dropped significantly with treatment. A positive correlation was found between fetal hemoglobin and mean corpuscular volume before and during treatment. Additionally, a correlation was found between the white blood cell and reticulocyte counts before treatment with hydroxyurea.

Conclusion

Most patients showed improvements with treatment as demonstrated by increases in hemoglobin, fetal hemoglobin and mean corpuscular volume, as well as by reductions in the reticulocyte and white blood cell counts. Clinically, more than 50% of patients had a significant reduction of events.

Information exploration system for sickle cell disease and repurposing of hydroxyfasudil

Background

Sickle cell disease (SCD) is a fatal monogenic disorder with no effective cure and thus high rates of morbidity and sequelae. Efforts toward discovery of disease modifying drugs and curative strategies can be augmented by leveraging the plethora of information contained in available biomedical literature. To facilitate research in this direction we have developed a resource, Dragon Exploration System for Sickle Cell Disease (DESSCD) (http://cbrc.kaust.edu.sa/desscd/) that aims to promote the easy exploration of SCD-related data.

Description

The Dragon Exploration System (DES), developed based on text mining and complemented by data mining, processed 419,612 MEDLINE abstracts retrieved from a PubMed query using SCD-related keywords. The processed SCD-related data has been made available via the DESSCD web query interface that enables: a/information retrieval using specified concepts, keywords and phrases, and b/the generation of inferred association networks and hypotheses. The usefulness of the system is demonstrated by: a/reproducing a known scientific fact, the “Sickle_Cell_Anemia–Hydroxyurea” association, and b/generating novel and plausible “Sickle_Cell_Anemia–Hydroxyfasudil” hypothesis. A PCT patent (PCT/US12/55042) has been filed for the latter drug repurposing for SCD treatment.

Conclusion

We developed the DESSCD resource dedicated to exploration of text-mined and data-mined information about SCD. No similar SCD-related resource exists. Thus, we anticipate that DESSCD will serve as a valuable tool for physicians and researchers interested in SCD.

Hydroxyurea responsiveness in β-thalassemic patients is determined by the stress response adaptation of erythroid progenitors and their differentiation propensity

β-thalassemia is caused by mutations in the β-globin locus resulting in loss of, or reduced, hemoglobin A (adult hemoglobin, HbA, α2β2) production. Hydroxyurea treatment increases fetal γ-globin (fetal hemoglobin, HbF, α2γ2) expression in postnatal life substituting for the missing adult β-globin and is, therefore, an attractive therapeutic approach. Patients treated with hydroxyurea fall into three categories: i) 'responders' who increase hemoglobin to therapeutic levels; (ii) 'moderate-responders' who increase hemoglobin levels but still need transfusions at longer intervals; and (iii) 'non-responders' who do not reach adequate hemoglobin levels and remain transfusion-dependent. The mechanisms underlying these differential responses remain largely unclear. We generated RNA expression profiles from erythroblast progenitors of 8 responder and 8 non-responder β-thalassemia patients. These profiles revealed that hydroxyurea treatment induced differential expression of many genes in cells from non-responders while it had little impact on cells from responders. Part of the gene program up-regulated by hydroxyurea in non-responders was already highly expressed in responders before hydroxyurea treatment. Baseline HbF expression was low in non-responders, and hydroxyurea treatment induced significant cell death. We conclude that cells from responders have adapted well to constitutive stress conditions and display a propensity to proceed to the erythroid differentiation program.

Heme oxygenase-1 deletion affects stress erythropoiesis

Background

Homeostatic erythropoiesis leads to the formation of mature red blood cells under non-stress conditions, and the production of new erythrocytes occurs as the need arises. In response to environmental stimuli, such as bone marrow transplantation, myelosuppression, or anemia, erythroid progenitors proliferate rapidly in a process referred to as stress erythropoiesis. We have previously demonstrated that heme oxygenase-1 (HO-1) deficiency leads to disrupted stress hematopoiesis. Here, we describe the specific effects of HO-1 deficiency on stress erythropoiesis.

Methodology/Principal Findings

We used a transplant model to induce stress conditions. In irradiated recipients that received hmox^+/− or hmox^+/+ bone marrow cells, we evaluated (i) the erythrocyte parameters in the peripheral blood; (ii) the staining intensity of CD71-, Ter119-, and CD49d-specific surface markers during erythroblast differentiation; (iii) the patterns of histological iron staining; and (iv) the number of Mac-1⁺-cells expressing TNF-α. In the spleens of mice that received hmox^+/− cells, we show (i) decreases in the proerythroblast, basophilic, and polychromatophilic erythroblast populations; (ii) increases in the insoluble iron levels and decreases in the soluble iron levels; (iii) increased numbers of Mac-1⁺-cells expressing TNF-α; and (iv) decreased levels of CD49d expression in the basophilic and polychromatophilic erythroblast populations.

Conclusions/Significance

As reflected by effects on secreted and cell surface proteins, HO-1 deletion likely affects stress erythropoiesis through the retention of erythroblasts in the erythroblastic islands of the spleen. Thus, HO-1 may serve as a therapeutic target for controlling erythropoiesis, and the dysregulation of HO-1 may be a predisposing condition for hematologic diseases.

The proteome of sickle cell disease: insights from exploratory proteomic profiling

The expanding realm of exploratory proteomics has added a unique dimension to the study of the complex pathophysiology involved in sickle cell disease. A review of proteomic studies published on sickle cell erythrocytes and plasma shows trends of upregulation of antioxidant proteins, an increase in cytoskeletal defects, an increase in protein repair and turnover components, a decrease in lipid raft proteins and apolipoprotein dysregulation. Many of these findings are consistent with the pathophysiology of sickle cell disease, including high oxidant burden, resulting in damage to cytoskeletal and other proteins, and erythrocyte rigidity. More unexpected findings, such as a decrease in lipid raft components and apolipoprotein dysregulation, offer previously unexplored targets for future investigation and potential therapeutic intervention. Exploratory proteomic profiling is a valuable source of hypothesis generation for the cellular and molecular pathophysiology of sickle cell disease.

Effects of hydroxyurea on monoclonal antibody production induced by anti-mIgG and LPS stimulation on murine B cell hybridomas

Chemical treatment with hydroxyurea (HU) has been selected as a simple and low cost strategy to generate a cell population enriched for the G1 phase. After the chemical treatment with HU, cells were stimulated with anti-mIgG to test if the positive effects of anti-mIgG on CD40 expression and specific IgG2a production rate were improved upon a cell population with a higher percentage of cells in G1 phase at the beginning of the cell culture. In addition, other treatments assayed in this work were the cell stimulation with Lipopolysaccharide (LPS) both before and after the HU treatment. It has been observed that the use of HU under conditions able to maintain the cells in viable state (0.1 mM for 20 h), has a negative effect on CD40 expression and specific IgG2a production rate induced by anti-mIgG. The positive effect of LPS on cell stimulation induced by anti-mIgG is reduced on cells treated with HU.

Intercellular adhesion molecule-4 and CD36 are implicated in the abnormal adhesiveness of sickle cell SAD mouse erythrocytes to endothelium

BACKGROUND

Abnormal adhesiveness of red blood cells to endothelium has been implicated in vaso-occlusive crisis of sickle cell disease. The present study examined whether the SAD mouse model exhibits the same abnormalities of red blood cell adhesion as those found in human sickle cell disease.

DESIGN AND METHODS

The repertoire of adhesive molecules on murine erythrocytes and bEnd.3 microvascular endothelial cells was determined by flow cytometry using monoclonal antibodies or by western blotting. Adhesion was investigated in dynamic conditions and measured at different shear stresses.

RESULTS

CD36, CD47 and intercellular adhesion molecular-4, but not Lutheran blood group antigen/basal cell adhesion molecule, are present on mouse mature erythrocytes. alpha(4)beta(1) are not expressed on SAD and wild type reticulocytes. Endothelial bEnd.3 cells express alpha(V)beta(3), alpha(4)beta(1), CD47, vascular cell adhesion molecule-1, and Lutheran blood group antigen/basal cell adhesion molecule, but not CD36. Adhesion of SAD red cells is: (i) 2- to 3-fold higher than that of wild type red cells; (ii) further increased on platelet activating factor-activated endothelium; (iii) not stimulated by epinephrine; (iv) inhibited after treating the endothelium with a peptide reproducing one of the binding sequences of mouse intercellular adhesion molecular-4, or with mon-oclonal antibody against murine alpha(v) integrin; and (v) inhibited after pretreatment of red blood cells with anti-mouse CD36 monoclonal antibodies. The combination of treatments with intercellular adhesion molecular-4 peptide and anti-CD36 monoclonal antibodies eliminates excess adhesion of SAD red cells. The phosphorylation state of intercellular adhesion molecular-4 and CD36 is probably not involved in the over-adhesiveness of SAD erythrocytes.

CONCLUSIONS

Intercellular adhesion molecular-4/alpha(v)beta(3) and CD36/thrombospondin interactions might contribute to the abnormally high adhesiveness of SAD red cells. The SAD mouse is a valuable animal model for investigating adhesion processes of sickle cell disease.

Inhibition of caspase-dependent spontaneous apoptosis via a cAMP-protein kinase A dependent pathway in neutrophils from sickle cell disease patients

Sickle cell disease (SCD) is a chronic inflammatory condition characterized by high leucocyte counts, altered cytokine levels and endothelial cell injury. As the removal of inflammatory cells by apoptosis is fundamental for the resolution of inflammation, we aimed to determine whether the leucocyte apoptotic process is altered in SCD. Neutrophils from SCD individuals showed an inhibition of spontaneous apoptosis when cultured in vitro, in the presence of autologous serum for 20 h. Intracellular cyclic adenosine monophosphate (cAMP) levels were approximately twofold increased in SCD neutrophils; possible cAMP-upregulating factors present in SCD serum include interleukin-8, granulocyte-macrophage colony-stimulating factor and prostaglandin. Accordingly, co-incubation of SCD neutrophils with KT5720, a cAMP-dependent protein kinase (PKA) inhibitor, abrogated increased SCD neutrophil survival. Caspase-3 activity was also significantly diminished in SCD neutrophils cultured for 16 h and this activity was restored when cells were co-incubated with KT5720. BIRC2 (encoding cellular inhibitor of apoptosis protein 1, cIAP(1)), MCL1 and BAX expression were unaltered in SCD neutrophils; however, BIRC3 (encoding the caspase inhibitor, cIAP(2)), was expressed at significantly higher levels. Thus, we report an inhibition of spontaneous SCD neutrophil apoptosis that appears to be mediated by upregulated cAMP-PKA signalling and decreased caspase activity. Increased neutrophil survival may have significant consequences in SCD; contributing to leucocytosis, tissue damage and exacerbation of the chronic inflammatory state.