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

Impacts of oxidative stress and anti-oxidants on the development, pathogenesis, and therapy of sickle cell disease: A comprehensive review

Sickle cell disease (SCD) is the most severe monogenic hemoglobinopathy caused by a single genetic mutation that leads to repeated polymerization and depolymerization of hemoglobin resulting in intravascular hemolysis, cell adhesion, vascular occlusion, and ischemia-reperfusion injury. Hemolysis causes oxidative damage indirectly by generating reactive oxygen species through various pathophysiological mechanisms, which include hemoglobin autoxidation, endothelial nitric oxide synthase uncoupling, reduced nitric oxide bioavailability, and elevated levels of asymmetric dimethylarginine. Red blood cells have a built-in anti-oxidant system that includes enzymes like sodium dismutase, catalase, and glutathione peroxidase, along with free radical scavenging molecules, such as vitamin C, vitamin E, and glutathione, which help them to fight oxidative damage. However, these anti-oxidants may not be sufficient to prevent the effects of oxidative stress in SCD patients. Therefore, in line with a recent FDA request that the focus to be placed on the development of innovative therapies for SCD that address the root cause of the disease, there is a need for therapies that target oxidative stress and restore redox balance in SCD patients. This review summarizes the current state of knowledge regarding the role of oxidative stress in SCD and the potential benefits of anti-oxidant therapies. It also discusses the challenges and limitations of these therapies and suggests future directions for research and development.

Interaction of carbonic anhydrase I released from red blood cells with human plasma in vitro

Red blood cells (RBCs) constitute ∼50% of the bloodstream and represent an important target for environmental pollutants and bacterial/viral infections, which can result in their rupture. In addition, diseases such as sickle cell anaemia and paroxysmal nocturnal haemoglobinuria can also result in the rupture of RBCs, which can be potentially life-threatening. With regard to the release of cytosolic metalloproteins from RBCs into the blood-organ system, the biochemical fate of haemoglobin is rather well understood, while comparatively little is known about another highly abundant Zn-metalloprotein, carbonic anhydrase (CA I). To gain insight into the interaction of CA I with human blood plasma constituents, we have employed a metallomics tool comprised of size-exclusion chromatography (SEC) coupled online with an inductively coupled plasma atomic emission spectrometer (ICP-AES), which allows to simultaneously observe all Cu, Fe, and Zn-metalloproteins. After the addition of CA I to human blood plasma incubated at 37°C, the SEC-ICP-AES analysis using phosphate buffered saline (pH 7.4) after 5 min, 1 h, and 2 h revealed that CA I eluted after all endogenous Zn-metalloproteins in the 30 kDa range. Matrix-assisted laser desorption-time of flight mass spectrometry analysis of the collected Zn-peak confirmed that CA I eluted from the column intact. Our in vitro results suggest that CA I released from RBCs to plasma remains free and may be actively involved in health-relevant adverse processes that unfold at the bloodstream-endothelial interface, including atherosclerosis and vision loss.

Accelerated atherosclerosis in beta-thalassemia

Children with beta-thalassemia (BT) present with an increase in carotid intima-medial thickness, an early sign suggestive of premature atherosclerosis. However, it is unknown if there is a direct relationship between BT and atherosclerotic disease. To evaluate this, wild-type (WT, littermates) and BT (Hbbth3/+) mice, both male and female, were placed on a 3-mo high-fat diet with low-density lipoprotein receptor suppression via overexpression of proprotein convertase subtilisin/kexin type 9 (PCSK9) gain-of-function mutation (D377Y). Mechanistically, we hypothesize that heme-mediated oxidative stress creates a proatherogenic environment in BT because BT is a hemolytic anemia that has increased free heme and exhausted hemopexin, heme's endogenous scavenger, in the vasculature. We evaluated the effect of hemopexin (HPX) therapy, mediated via an adeno-associated virus, to the progression of atherosclerosis in BT and a phenylhydrazine-induced model of intravascular hemolysis. In addition, we evaluated the effect of deferiprone (DFP)-mediated iron chelation in the progression of atherosclerosis in BT mice. Aortic en face and aortic root lesion area analysis revealed elevated plaque accumulation in both male and female BT mice compared with WT mice. Hemopexin therapy was able to decrease plaque accumulation in both BT mice and mice on our phenylhydrazine (PHZ)-induced model of hemolysis. DFP decreased atherosclerosis in BT mice but did not provide an additive benefit to HPX therapy. Our data demonstrate for the first time that the underlying pathophysiology of BT leads to accelerated atherosclerosis and shows that heme contributes to atherosclerotic plaque development in BT.NEW & NOTEWORTHY This work definitively shows for the first time that beta-thalassemia leads to accelerated atherosclerosis. We demonstrated that intravascular hemolysis is a prominent feature in beta-thalassemia and the resulting increases in free heme are mechanistically relevant. Adeno-associated virus (AAV)-hemopexin therapy led to decreased free heme and atherosclerotic plaque area in both beta-thalassemia and phenylhydrazine-treated mice. Deferiprone-mediated iron chelation led to deceased plaque accumulation in beta-thalassemia mice but provided no additive benefit to hemopexin therapy.

Moderate hypoxia induces metabolic divergence in circulating monocytes and tissue resident macrophages from Berkeley sickle cell anemia mice

Introduction

Human and murine sickle cell disease (SCD) associated pulmonary hypertension (PH) is defined by hemolysis, nitric oxide depletion, inflammation, and thrombosis. Further, hemoglobin (Hb), heme, and iron accumulation are consistently observed in pulmonary adventitial macrophages at autopsy and in hypoxia driven rodent models of SCD, which show distribution of ferric and ferrous Hb as well as HO-1 and ferritin heavy chain. The anatomic localization of these macrophages is consistent with areas of significant vascular remodeling. However, their contributions toward progressive disease may include unique, but also common mechanisms, that overlap with idiopathic and other forms of pulmonary hypertension. These processes likely extend to the vasculature of other organs that are consistently impaired in advanced SCD.

Methods

To date, limited information is available on the metabolism of macrophages or monocytes isolated from lung, spleen, and peripheral blood in humans or murine models of SCD.

Results

Here we hypothesize that metabolism of macrophages and monocytes isolated from this triad of tissue differs between Berkley SCD mice exposed for ten weeks to moderate hypobaric hypoxia (simulated 8,000 ft, 15.4% O2) or normoxia (Denver altitude, 5000 ft) with normoxia exposed wild type mice evaluated as controls.

Discussion

This study represents an initial set of data that describes the metabolism in monocytes and macrophages isolated from moderately hypoxic SCD mice peripheral lung, spleen, and blood mononuclear cells.

Assessment of total and unbound cell-free heme in plasma of patients with sickle cell disease

Intravascular hemolysis results in the release of cell-free hemoglobin and heme in plasma. In sickle cell disease, the fragility of the sickle red blood cell leads to chronic hemolysis, which can contribute to oxidative damage and activation of inflammatory pathways. The scavenger proteins haptoglobin and hemopexin provide pathways to remove hemoglobin and heme, respectively, from the circulation. Heme also intercalates in membranes of blood cells and endothelial cells in the vasculature and associates with other plasma components such as albumin and lipoproteins. Hemopexin has a much higher affinity and can strip heme from the other pools and detoxify plasma from cell-free circulatory heme. However, due to chronic hemolysis, hemopexin is depleted in individuals with sickle cell disease. Thus, cell-free unbound heme is expected to accumulate in plasma. We developed a methodology for the accurate quantification of the fraction of heme, which is pathologically relevant in sickle cell disease, that does not appear to be sequestered to a plasma compartment. Our data show significant variation in the concentration of unbound heme, and rather unexpectedly, the size of the unbound fraction does not correlate to the degree of hemolysis, as measured by the concentration of bound heme. Very high heme concentrations (>150 µM) were obtained in some plasma with unbound concentrations that were several fold lower than in plasma with much lower hemolysis (<50 µM). These findings underscore the long-term effects of chronic hemolysis on the blood components and of the disruption of the essential equilibrium between release of hemoproteins/heme in the circulation and adaptative response of the scavenging/removal mechanisms. Understanding the clinical implications of this loss of response may provide insights into diagnostic and therapeutic targets in patients with sickle cell disease.

Hemolysis, free hemoglobin toxicity, and scavenger protein therapeutics

During hemolysis, erythrophagocytes dispose damaged red blood cells. This prevents the extracellular release of hemoglobin, detoxifies heme, and recycles iron in a linked metabolic pathway. Complementary to this process, haptoglobin and hemopexin scavenge and shuttle the red blood cell toxins hemoglobin and heme to cellular clearance. Pathological hemolysis outpaces macrophage capacity and scavenger synthesis across a diversity of diseases. This imbalance leads to hemoglobin-driven disease progression. To meet a void in treatment options, scavenger protein-based therapeutics are in clinical development.

Effects of sodium chloride and sodium tripolyphosphate on the prooxidant properties of hemoglobin in washed turkey muscle system

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MetHb in WTM acted as the most effective pro-oxidant, followed by hemin and oxyHb.

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The addition of NaCl significantly increased the oxyHb-mediated lipid oxidation.

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STPP inhibited oxyHb-mediated lipid oxidation.

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Formation of metHb and pH paly critical roles in oxyHb-mediated lipid oxidation.

This study examined the effects of sodium chloride (NaCl) and sodium tripolyphosphate (STPP) on lipid oxidation induced by oxyhemoglobin (oxyHb) in washed turkey muscle (WTM) model. To explore the reasons for observed effects, the pro-oxidant abilities of Hb derivatives (e.g., metHb, oxyHb, hemin, Fe²⁺, and Fe³⁺), pH change, and antioxidation of Hb in the presence of NaCl or STPP were also analyzed. The observed lipid oxidation capacity in WTM followed the order metHb > hemin > oxyHb > Fe²⁺ > Fe³⁺. Added Fe²⁺ accelerated auto-oxidation of oxyHb and oxyHb-mediated lipid oxidation. Hb auto-oxidation to metHb increased as the pH decreased from 6.6 to 5.0. NaCl promoted oxyHb-mediated lipid oxidation due to NaCl causing decreased pH value and increased formation of metHb. STPP inhibited oxyHb-mediated lipid oxidation and weakened the pro-oxidative effect of NaCl. This could be attributed to STPP increasing the pH, inactivating free iron, and inhibiting formation of metHb.

Heme-stress activated NRF2 skews fate trajectories of bone marrow cells from dendritic cells towards red pulp-like macrophages in hemolytic anemia

Heme is an erythrocyte-derived toxin that drives disease progression in hemolytic anemias, such as sickle cell disease. During hemolysis, specialized bone marrow-derived macrophages with a high heme-metabolism capacity orchestrate disease adaptation by removing damaged erythrocytes and heme-protein complexes from the blood and supporting iron recycling for erythropoiesis. Since chronic heme-stress is noxious for macrophages, erythrophagocytes in the spleen are continuously replenished from bone marrow-derived progenitors. Here, we hypothesized that adaptation to heme stress progressively shifts differentiation trajectories of bone marrow progenitors to expand the capacity of heme-handling monocyte-derived macrophages at the expense of the homeostatic generation of dendritic cells, which emerge from shared myeloid precursors. This heme-induced redirection of differentiation trajectories may contribute to hemolysis-induced secondary immunodeficiency. We performed single-cell RNA-sequencing with directional RNA velocity analysis of GM-CSF-supplemented mouse bone marrow cultures to assess myeloid differentiation under heme stress. We found that heme-activated NRF2 signaling shifted the differentiation of bone marrow cells towards antioxidant, iron-recycling macrophages, suppressing the generation of dendritic cells in heme-exposed bone marrow cultures. Heme eliminated the capacity of GM-CSF-supplemented bone marrow cultures to activate antigen-specific CD4 T cells. The generation of functionally competent dendritic cells was restored by NRF2 loss. The heme-induced phenotype of macrophage expansion with concurrent dendritic cell depletion was reproduced in hemolytic mice with sickle cell disease and spherocytosis and associated with reduced dendritic cell functions in the spleen. Our data provide a novel mechanistic underpinning of hemolytic stress as a driver of hyposplenism-related secondary immunodeficiency.

Spatial transcriptome analysis defines heme as a hemopexin-targetable inflammatoxin in the brain

After intracranial hemorrhage, heme is released from cell-free hemoglobin. This red blood cell component may drive secondary brain injury at the hematoma‒brain interface. This study aimed to generate a spatially resolved map of transcriptome-wide gene expression changes in the heme-exposed brain and to define the potential therapeutic activity of the heme-binding protein, hemopexin. We stereotactically injected saline, heme, or heme‒hemopexin into the striatum of C57BL/6J mice. After 24 h, we elucidated the two-dimensional spatial transcriptome by sequencing 21760 tissue-covered features, at a mean transcript coverage of 3849 genes per feature. In parallel, we studied the extravasation of systemically administered fluorescein isothiocyanate labeled (FITC)-dextran, magnetic resonance imaging features indicative of focal edema and perfusion, and neurological functions as translational correlates of heme toxicity. We defined a cerebral heme-response signature by performing bidimensional differential gene expression analysis, based on unsupervised clustering and manual segmentation of sequenced features. Heme exerted a consistent and dose-dependent proinflammatory activity in the brain, which occurred at minimal exposures, below the toxicity threshold for the induction of vascular leakage. We found dose-dependent regional divergence of proinflammatory heme signaling pathways, consistent with reactive astrocytosis and microglial activation. Co-injection of heme with hemopexin attenuated heme-induced gene expression changes and preserved the homeostatic microglia signature. Hemopexin also prevented heme-induced disruption of the blood‒brain barrier and radiological and functional signals of heme injury in the brain. In conclusion, we defined heme as a potent inflammatoxin that may drive secondary brain injury after intracerebral hemorrhage. Co-administration of hemopexin attenuated the heme-derived toxic effects on a molecular, cellular, and functional level, suggesting a translational therapeutic strategy.

Influence of Haptoglobin Polymorphism on Stroke in Sickle Cell Disease Patients

This review outlines the current clinical research investigating how the haptoglobin (Hp) genetic polymorphism and stroke occurrence are implicated in sickle cell disease (SCD) pathophysiology. Hp is a blood serum glycoprotein responsible for binding and removing toxic free hemoglobin from the vasculature. The role of Hp in patients with SCD is critical in combating blood toxicity, inflammation, oxidative stress, and even stroke. Ischemic stroke occurs when a blocked vessel decreases oxygen delivery in the blood to cerebral tissue and is commonly associated with SCD. Due to the malformed red blood cells of sickle hemoglobin S, blockage of blood flow is much more prevalent in patients with SCD. This review is the first to evaluate the role of the Hp polymorphism in the incidence of stroke in patients with SCD. Overall, the data compiled in this review suggest that further studies should be conducted to reveal and evaluate potential clinical advancements for gene therapy and Hp infusions.

Hepatic Homeostasis of Metal Ions Following Acute Repeated Stress Exposure in Rats

Essential metals such as copper, iron, and zinc are cofactors in various biological processes including oxygen utilisation, cell growth, and biomolecular synthesis. The homeostasis of these essential metals is carefully controlled through a system of protein transporters involved in the uptake, storage, and secretion. Some metal ions can be transformed by processes including reduction/oxidation (redox) reactions, and correspondingly, the breakdown of metal ion homeostasis can lead to formation of reactive oxygen and nitrogen species. We have previously demonstrated rapid biochemical responses to stress involving alterations in the redox state to generate free radicals and the resultant oxidative stress. However, the effects of stress on redox-active metals including iron and copper and redox-inert zinc have not been well characterised. Therefore, this study aims to examine the changes in these essential metals following exposure to short-term repeated stress, and to further elucidate the alterations in metal homeostasis through expression analysis of different metal transporters. Outbred male Wistar rats were exposed to unrestrained (control), 1 day, or 3 days of 6 h restraint stress (n = 8 per group). After the respective stress treatment, blood and liver samples were collected for the analysis of biometal concentrations and relative gene expression of metal transporter and binding proteins. Exposure to repeated restraint stress was highly effective in causing hepatic redox imbalance. Stress was also shown to induce hepatic metal redistribution, while modulating the mRNA levels of key metal transporters. Overall, this study is the first to characterise the gene expression profile of metal homeostasis following stress and provide insight into the changes occurring prior to the onset of chronic stress conditions.

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

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

Hemopexin dosing improves cardiopulmonary dysfunction in murine sickle cell disease

Hemopexin (Hpx) is a crucial defense protein against heme liberated from degraded hemoglobin during hemolysis. High heme stress creates an imbalance in Hpx bioavailability, favoring heme accumulation and downstream pathophysiological responses leading to cardiopulmonary disease progression in sickle cell disease (SCD) patients. Here, we evaluated a model of murine SCD, which was designed to accelerate red blood cell sickling, pulmonary hypertension, right ventricular dysfunction, and exercise intolerance by exposure of the mice to moderate hypobaric hypoxia. The sequence of pathophysiology in this model tracks with circulatory heme accumulation, lipid oxidation, extensive remodeling of the pulmonary vasculature, and fibrosis. We hypothesized that Hpx replacement for an extended period would improve exercise tolerance measured by critical speed as a clinically meaningful therapeutic endpoint. Further, we sought to define the effects of Hpx on upstream cardiopulmonary function, histopathology, and tissue oxidation. Our data shows that tri-weekly administrations of Hpx for three months dose-dependently reduced heme exposure and pulmonary hypertension while improving cardiac pressure-volume relationships and exercise tolerance. Furthermore, Hpx administration dose-dependently attenuated pulmonary fibrosis and oxidative modifications in the lung and myocardium of the right ventricle. Observations in our SCD murine model are consistent with pulmonary vascular and right ventricular pathology at autopsy in SCD patients having suffered from severe pulmonary hypertension, right ventricular dysfunction, and sudden cardiac death. This study provides a translational evaluation supported by a rigorous outcome analysis demonstrating therapeutic proof-of-concept for Hpx replacement in SCD.

Extracellular Vesicles in Sickle Cell Disease: Plasma Concentration, Blood Cell Types Origin Distribution and Biological Properties

Prototype of monogenic disorder, sickle cell disease (SCD) is caused by a unique single mutation in the β-globin gene, leading to the production of the abnormal hemoglobin S (HbS). HbS polymerization in deoxygenated condition induces the sickling of red blood cells (RBCs), which become less deformable and more fragile, and thus prone to lysis. In addition to anemia, SCD patients may exhibit a plethora of clinical manifestations ranging from acute complications such as the frequent and debilitating painful vaso-occlusive crisis to chronic end organ damages. Several interrelated pathophysiological processes have been described, including impaired blood rheology, increased blood cell adhesion, coagulation, inflammation and enhanced oxidative stress among others. During the last two decades, it has been shown that extracellular vesicles (EVs), defined as cell-derived anucleated particles delimited by a lipid bilayer, and comprising small EVs (sEVs) and medium/large EVs (m/lEVs); are not only biomarkers but also subcellular actors in SCD pathophysiology. Plasma concentration of m/lEVs, originated mainly from RBCs and platelets (PLTs) but also from the other blood cell types, is higher in SCD patients than in healthy controls. The concentration and the density of externalized phosphatidylserine of those released from RBCs may vary according to clinical status (crisis vs. steady state) and treatment (hydroxyurea). Besides their procoagulant properties initially described, RBC-m/lEVs may promote inflammation through their effects on monocytes/macrophages and endothelial cells. Although less intensely studied, sEVs plasma concentration is increased in SCD and these EVs may cause endothelial damages. In addition, sEVs released from activated PLTs trigger PLT-neutrophil aggregation involved in lung vaso-occlusion in sickle mice. Altogether, these data clearly indicate that EVs are both biomarkers and bio-effectors in SCD, which deserve further studies.

Modular Platform for the Development of Recombinant Hemoglobin Scavenger Biotherapeutics

Cell-free hemoglobin (Hb) is a driver of disease progression in conditions with intravascular or localized hemolysis. Genetic and acquired anemias or emergency medical conditions such as aneurysmal subarachnoid hemorrhage involve tissue Hb exposure. Haptoglobin (Hp) captures Hb in an irreversible protein complex and prevents its pathophysiological contributions to vascular nitric oxide depletion and tissue oxidation. Preclinical proof-of-concept studies suggest that human plasma-derived Hp is a promising therapeutic candidate for several Hb-driven diseases. Optimizing the efficacy and safety of Hb-targeting biotherapeutics may require structural and functional modifications for specific indications. Improved Hp variants could be designed to achieve the desired tissue distribution, metabolism, and elimination to target hemolytic disease states effectively. However, it is critical to ensure that these modifications maintain the function of Hp. Using transient mammalian gene expression of Hp combined with co-transfection of the pro-haptoglobin processing protease C1r-LP, we established a platform for generating recombinant Hp-variants. We designed an Hpβ-scaffold, which was expressed in this system at high levels as a monomeric unit (mini-Hp) while maintaining the key protective functions of Hp. We then used this Hpβ-scaffold as the basis to develop an initial proof-of-concept Hp fusion protein using human serum albumin as the fusion partner. Next, a hemopexin-Hp fusion protein with bispecific heme and Hb detoxification capacity was generated. Further, we developed a Hb scavenger devoid of CD163 scavenger receptor binding. The functions of these proteins were then characterized for Hb and heme-binding, binding of the Hp-Hb complexes with the clearance receptor CD163, antioxidant properties, and vascular nitric oxide sparing capacity. Our platform is designed to support the generation of innovative Hb scavenger biotherapeutics with novel modes of action and potentially improved formulation characteristics, function, and pharmacokinetics.

Ferryl Hemoglobin and Heme Induce A1-Microglobulin in Hemorrhaged Atherosclerotic Lesions with Inhibitory Function against Hemoglobin and Lipid Oxidation

Infiltration of red blood cells into atheromatous plaques and oxidation of hemoglobin (Hb) and lipoproteins are implicated in the pathogenesis of atherosclerosis. α₁-microglobulin (A1M) is a radical-scavenging and heme-binding protein. In this work, we examined the origin and role of A1M in human atherosclerotic lesions. Using immunohistochemistry, we observed a significant A1M immunoreactivity in atheromas and hemorrhaged plaques of carotid arteries in smooth muscle cells (SMCs) and macrophages. The most prominent expression was detected in macrophages of organized hemorrhage. To reveal a possible inducer of A1M expression in ruptured lesions, we exposed aortic endothelial cells (ECs), SMCs and macrophages to heme, Oxy- and FerrylHb. Both heme and FerrylHb, but not OxyHb, upregulated A1M mRNA expression in all cell types. Importantly, only FerrylHb induced A1M protein secretion in aortic ECs, SMCs and macrophages. To assess the possible function of A1M in ruptured lesions, we analyzed Hb oxidation and heme-catalyzed lipid peroxidation in the presence of A1M. We showed that recombinant A1M markedly inhibited Hb oxidation and heme-driven oxidative modification of low-density lipoproteins as well plaque lipids derived from atheromas. These results demonstrate the presence of A1M in atherosclerotic plaques and suggest its induction by heme and FerrylHb in the resident cells.

Critical Role of Hemopexin Mediated Cytoprotection in the Pathophysiology of Sickle Cell Disease

Circulating hemopexin is the primary protein responsible for the clearance of heme; therefore, it is a systemic combatant against deleterious inflammation and oxidative stress induced by the presence of free heme. This role of hemopexin is critical in hemolytic pathophysiology. In this review, we outline the current research regarding how the dynamic activity of hemopexin is implicated in sickle cell disease, which is characterized by a pathological aggregation of red blood cells and excessive hemolysis. This pathophysiology leads to symptoms such as acute kidney injury, vaso-occlusion, ischemic stroke, pain crises, and pulmonary hypertension exacerbated by the presence of free heme and hemoglobin. This review includes in vivo studies in mouse, rat, and guinea pig models of sickle cell disease, as well as studies in human samples. In summary, the current research indicates that hemopexin is likely protective against these symptoms and that rectifying depleted hemopexin in patients with sickle cell disease could improve or prevent the symptoms. The data compiled in this review suggest that further preclinical and clinical research should be conducted to uncover pathways of hemopexin in pathological states to evaluate its potential clinical function as both a biomarker and therapy for sickle cell disease and related hemoglobinopathies.

Toxic effects of cell-free hemoglobin on the microvascular endothelium: implications for pulmonary and nonpulmonary organ dysfunction

Levels of circulating cell-free hemoglobin are elevated during hemolytic and inflammatory diseases and contribute to organ dysfunction and severity of illness. Though several studies have investigated the contribution of hemoglobin to tissue injury, the precise signaling mechanisms of hemoglobin-mediated endothelial dysfunction in the lung and other organs are not yet completely understood. The purpose of this review is to highlight the knowledge gained thus far and the need for further investigation regarding hemoglobin-mediated endothelial inflammation and injury to develop novel therapeutic strategies targeting the damaging effects of cell-free hemoglobin.

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

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

Agonistic Anti-CD40 Antibody Triggers an Acute Liver Crisis With Systemic Inflammation in Humanized Sickle Cell Disease Mice

Sickle cell disease (SCD) is an inherited hemolytic disorder, defined by a point mutation in the β-globin gene. Stress conditions such as infection, inflammation, dehydration, and hypoxia trigger erythrocyte sickling. Sickled red blood cells (RBCs) hemolyze more rapidly, show impaired deformability, and increased adhesive properties to the endothelium. In a proinflammatory, pro-coagulative environment with preexisting endothelial dysfunction, sickled RBCs promote vascular occlusion. Hepatobiliary involvement related to the sickling process, such as an acute sickle hepatic crisis, is observed in about 10% of acute sickle cell crisis incidents. In mice, ligation of CD40 with an agonistic antibody leads to a macrophage activation in the liver, triggering a sequence of systemic inflammation, endothelial cell activation, thrombosis, and focal ischemia. We found that anti-CD40 antibody injection in sickle cell mice induces a systemic inflammatory and hemodynamic response with accelerated hemolysis, extensive vaso-occlusion, and large ischemic infarctions in the liver mimicking an acute hepatic crisis. Administration of the tumor necrosis factor-α (TNF-α) blocker, etanercept, and the heme scavenger protein, hemopexin attenuated end-organ damage. These data collectively suggest that anti-CD40 administration offers a novel acute liver crisis model in humanized sickle mice, allowing for evaluation of therapeutic proof-of-concept.

Biomarker Discovery of Acute Coronary Syndrome Using Proteomic Approach

Acute coronary syndrome (ACS) is a condition in which the coronary artery supplying blood to the heart is infarcted via formation of a plaque and thrombus, resulting in abnormal blood supply and high mortality and morbidity. Therefore, the prompt and efficient diagnosis of ACS and the need for new ACS diagnostic biomarkers are important. In this study, we aimed to identify new ACS diagnostic biomarkers with high sensitivity and specificity using a proteomic approach. A discovery set with samples from 20 patients with ACS and 20 healthy controls was analyzed using mass spectrometry. Among the proteins identified, those showing a significant difference between each group were selected. Functional analysis of these proteins was conducted to confirm their association with functions in the diseased state. To determine ACS diagnostic biomarkers, standard peptides of the selected protein candidates from the discovery set were quantified, and these protein candidates were validated in a validation set consisting of the sera of 50 patients with ACS and 50 healthy controls. We showed that hemopexin, leucine-rich α-2-glycoprotein, and vitronectin levels were upregulated, whereas fibronectin level was downregulated, in patients with ACS. Thus, the use of these biomarkers may increase the accuracy of ACS diagnosis.

Sickle Cell Disease: Role of Oxidative Stress and Antioxidant Therapy

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

The Worst Things in Life are Free: The Role of Free Heme in Sickle Cell Disease

Hemolysis is a pathological feature of several diseases of diverse etiology such as hereditary anemias, malaria, and sepsis. A major complication of hemolysis involves the release of large quantities of hemoglobin into the blood circulation and the subsequent generation of harmful metabolites like labile heme. Protective mechanisms like haptoglobin-hemoglobin and hemopexin-heme binding, and heme oxygenase-1 enzymatic degradation of heme limit the toxicity of the hemolysis-related molecules. The capacity of these protective systems is exceeded in hemolytic diseases, resulting in high residual levels of hemolysis products in the circulation, which pose a great oxidative and proinflammatory risk. Sickle cell disease (SCD) features a prominent hemolytic anemia which impacts the phenotypic variability and disease severity. Not only is circulating heme a potent oxidative molecule, but it can act as an erythrocytic danger-associated molecular pattern (eDAMP) molecule which contributes to a proinflammatory state, promoting sickle complications such as vaso-occlusion and acute lung injury. Exposure to extracellular heme in SCD can also augment the expression of placental growth factor (PlGF) and interleukin-6 (IL-6), with important consequences to enthothelin-1 (ET-1) secretion and pulmonary hypertension, and potentially the development of renal and cardiac dysfunction. This review focuses on heme-induced mechanisms that are implicated in disease pathways, mainly in SCD. A special emphasis is given to heme-induced PlGF and IL-6 related mechanisms and their role in SCD disease progression.

Human Plasma and Recombinant Hemopexins: Heme Binding Revisited

Plasma hemopexin (HPX) is the key antioxidant protein of the endogenous clearance pathway that limits the deleterious effects of heme released from hemoglobin and myoglobin (the term “heme” is used in this article to denote both the ferrous and ferric forms). During intra-vascular hemolysis, heme partitioning to protein and lipid increases as the plasma concentration of HPX declines. Therefore, the development of HPX as a replacement therapy during high heme stress could be a relevant intervention for hemolytic disorders. A logical approach to enhance HPX yield involves recombinant production strategies from human cell lines. The present study focuses on a biophysical assessment of heme binding to recombinant human HPX (rhHPX) produced in the Expi293F^TM (HEK293) cell system. In this report, we examine rhHPX in comparison with plasma HPX using a systematic analysis of protein structural and functional characteristics related to heme binding. Analysis of rhHPX by UV/Vis absorption spectroscopy, circular dichroism (CD), size-exclusion chromatography (SEC)-HPLC, and catalase-like activity demonstrated a similarity to HPX fractionated from plasma. In particular, the titration of HPX apo-protein(s) with heme was performed for the first time using a wide range of heme concentrations to model HPX–heme interactions to approximate physiological conditions (from extremely low to more than two-fold heme molar excess over the protein). The CD titration data showed an induced bisignate CD Soret band pattern typical for plasma and rhHPX versions at low heme-to-protein molar ratios and demonstrated that further titration is dependent on the amount of protein-bound heme to the extent that the arising opposite CD couplet results in a complete inversion of the observed CD pattern. The data generated in this study suggest more than one binding site in both plasma and rhHPX. Furthermore, our study provides a useful analytical platform for the detailed characterization of HPX–heme interactions and potentially novel HPX fusion constructs.

Therapeutic Potential of Carbon Monoxide (CO) and Hydrogen Sulfide (H2S) in Hemolytic and Hemorrhagic Vascular Disorders-Interaction between the Heme Oxygenase and H2S-Producing Systems

Over the past decades, substantial work has established that hemoglobin oxidation and heme release play a pivotal role in hemolytic/hemorrhagic disorders. Recent reports have shown that oxidized hemoglobins, globin-derived peptides, and heme trigger diverse biological responses, such as toll-like receptor 4 activation with inflammatory response, reprogramming of cellular metabolism, differentiation, stress, and even death. Here, we discuss these cellular responses with particular focus on their mechanisms that are linked to the pathological consequences of hemorrhage and hemolysis. In recent years, endogenous gasotransmitters, such as carbon monoxide (CO) and hydrogen sulfide (H₂S), have gained a lot of interest in connection with various human pathologies. Thus, many CO and H₂S-releasing molecules have been developed and applied in various human disorders, including hemolytic and hemorrhagic diseases. Here, we discuss our current understanding of oxidized hemoglobin and heme-induced cell and tissue damage with particular focus on inflammation, cellular metabolism and differentiation, and endoplasmic reticulum stress in hemolytic/hemorrhagic human diseases, and the potential beneficial role of CO and H₂S in these pathologies. More detailed mechanistic insights into the complex pathology of hemolytic/hemorrhagic diseases through heme oxygenase-1/CO as well as H₂S pathways would reveal new therapeutic approaches that can be exploited for clinical benefit.

Heme Induces IL-6 and Cardiac Hypertrophy Genes Transcripts in Sickle Cell Mice

Emerging data indicate that free heme promotes inflammation in many different disease settings, including in sickle cell disease (SCD). Although free heme, proinflammatory cytokines, and cardiac hypertrophy are co-existing features of SCD, no mechanistic links between these features have been demonstrated. We now report significantly higher levels of IL-6 mRNA and protein in hearts of the Townes sickle cell disease (SS) mice (2.9-fold, p ≤ 0.05) than control mice expressing normal human hemoglobin (AA). We find that experimental administration of heme 50 μmoles/kg body weight induces IL-6 expression directly in vivo and induces gene expression markers of cardiac hypertrophy in SS mice. We administered heme intravenously and found that within three hours plasma IL-6 protein significantly increased in SS mice compared to AA mice (3248 ± 275 vs. 2384 ± 255 pg/ml, p ≤ 0.05). In the heart, heme induced a 15-fold increase in IL-6 transcript in SS mice heart compared to controls. Heme simultaneously induced other markers of cardiac stress and hypertrophy, including atrial natriuretic factor (Nppa; 14-fold, p ≤ 0.05) and beta myosin heavy chain (Myh7; 8-fold, p ≤ 0.05) in SS mice. Our experiments in Nrf2-deficient mice indicate that the cardiac IL-6 response to heme does not require Nrf2, the usual mediator of transcriptional response to heme for heme detoxification by heme oxygenase-1. These data are the first to show heme-induced IL-6 expression in vivo, suggesting that hemolysis may play a role in the elevated IL-6 and cardiac hypertrophy seen in patients and mice with SCD. Our results align with published evidence from rodents and humans without SCD that suggest a causal relationship between IL-6 and cardiac hypertrophy.

Haptoglobin Therapeutics and Compartmentalization of Cell-Free Hemoglobin Toxicity

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

What Is Next in This "Age" of Heme-Driven Pathology and Protection by Hemopexin? An Update and Links with Iron

This review provides a synopsis of the published literature over the past two years on the heme-binding protein hemopexin (HPX), with some background information on the biochemistry of the HPX system. One focus is on the mechanisms of heme-driven pathology in the context of heme and iron homeostasis in human health and disease. The heme-binding protein hemopexin is a multi-functional protectant against hemoglobin (Hb)-derived heme toxicity as well as mitigating heme-mediated effects on immune cells, endothelial cells, and stem cells that collectively contribute to driving inflammation, perturbing vascular hemostasis and blood–brain barrier function. Heme toxicity, which may lead to iron toxicity, is recognized increasingly in a wide range of conditions involving hemolysis and immune system activation and, in this review, we highlight some newly identified actions of heme and hemopexin especially in situations where normal processes fail to maintain heme and iron homeostasis. Finally, we present preliminary data showing that the cytokine IL-6 cross talks with activation of the c-Jun N-terminal kinase pathway in response to heme-hemopexin in models of hepatocytes. This indicates another level of complexity in the cell responses to elevated heme via the HPX system when the immune system is activated and/or in the presence of inflammation.

Identification of a haptoglobin-hemoglobin complex in human blood plasma

Blood plasma metalloproteins that contain copper (Cu), iron (Fe), zinc (Zn) and/or other metals/metalloids are potential disease biomarkers because the bloodstream is in permanent contact with organs. Their quantification and/or the presence of additional metal-entities or the absence of certain metalloproteins in blood plasma (e.g. in Wilson's disease) may provide insight into the dyshomeostasis of the corresponding metal (s) to gain insight into disease processes. The first step in investigating if the determination of plasma metalloproteins is useful for the diagnosis of diseases is their definitive qualitative identification. To this end, we have added individual highly pure Cu, Fe or Zn-containing metalloproteins to plasma (healthy volunteer) and analyzed this mixture by size-exclusion chromatography (SEC) coupled to an inductively coupled plasma atomic spectrometer (ICP-AES), simultaneously monitoring the emission lines of Cu, Fe and Zn. The results clearly identified ceruloplasmin (Cp), holo-transferrin (hTf), and α₂-macroglobulin (α₂M), which verifies our previous assignments. Interestingly, another major Fe-peak in plasma was identified as a haptoglobin (Hp)-hemoglobin (Hb) complex. This Hp-Hb complex is formed after Hb, which is released during the hemolysis of erythrocytes, binds to the plasma protein Hp. The Hp-Hb complex formation is known to be one of the strongest interactions in biochemistry (K_d≈1pmol/L) and is critical because it prevents kidney toxicity of free Hb. Hence, the simultaneous determination of Cp, hTf, α₂M and the Hp-Hb complex in plasma in <25min has the potential to provide new insight into disease processes associated with the bioinorganic chemistry of Cu, Fe and Zn.

An Hb-mediated circulating macrophage contributing to pulmonary vascular remodeling in sickle cell disease

Circulating macrophages recruited to the lung contribute to pulmonary vascular remodeling in various forms of pulmonary hypertension (PH). In this study we investigated a macrophage phenotype characterized by intracellular iron accumulation and expression of antioxidant (HO-1), vasoactive (ET-1), and proinflammatory (IL-6) mediators observed in the lung tissue of deceased sickle cell disease (SCD) patients with diagnosed PH. To this end, we evaluated an established rat model of group 5 PH that is simultaneously exposed to free hemoglobin (Hb) and hypobaric hypoxia (HX). Here, we tested the hypothesis that pulmonary vascular remodeling observed in human SCD with concomitant PH could be replicated and mechanistically driven in our rat model by a similar macrophage phenotype with iron accumulation and expression of a similar mixture of antioxidant (HO-1), vasoactive (ET-1), and inflammatory (IL-6) proteins. Our data suggest phenotypic similarities between pulmonary perivascular macrophages in our rat model and human SCD with PH, indicating a potentially novel maladaptive immune response to concomitant bouts of Hb and HX exposure. Moreover, by knocking out circulating macrophages with gadolinium trichloride (GdCl3), the response to combined Hb and hypobaric HX was significantly attenuated in rats, suggesting a critical role for macrophages in the exacerbation of SCD PH.

S100A8 acts as an autocrine priming signal for heme-induced human Mϕ pro-inflammatory responses in hemolytic inflammation

Intravascular hemolysis, in addition to reducing red cell counts, incurs extensive vascular inflammation and oxidative stress. One product of hemolysis, heme, is a potent danger associated molecular pattern (DAMP), activating leukocytes and inducing cytokine expression and processing, among other pro-inflammatory effects. We explored pathways by which heme-induced inflammation may be amplified under sterile conditions. Incubation of human Mϕs, differentiated from CD14⁺ cells, with heme induced time- and concentration-dependent gene and protein expression of S100A8, a myeloid cell-derived alarmin. Human Mϕ stimulation with recombinant S100A8, in turn, induced robust pro-IL-1β expression that was dependent upon NF-κB activation, gene transcription, and partially dependent upon TLR4-mediated signaling. Moreover, heme itself stimulated significant Mϕ pro-IL-1β gene and protein expression via an S100A8-mediated mechanism and greatly amplified S100A8-driven NLRP3 inflammasome-mediated IL-1β secretion. In vivo, induction of acute intravascular hemolysis in mice induced a rapid elevation of plasma S100A8 that could be abolished by hemopexin, a heme scavenger. Finally, plasma S100A8 levels were found to be significantly elevated in patients with the inherited hemolytic anemia, sickle cell anemia, when compared with levels in healthy individuals. In conclusion, we demonstrate that hemolytic processes are associated with S100A8 generation and that some of the inflammatory effects of heme may be amplified by autocrine S100A8 production. Findings suggest a mechanism by which hemolytic inflammation could be propagated via leukocyte priming by endogenous proteins, even in sterile inflammatory environments such as those that occur in the hemolytic diseases. S100A8 may represent a therapeutic target for reducing inflammation in hemolytic disorders.