Heme triggers TLR4 signaling leading to endothelial cell activation and vaso-occlusion in murine sickle cell disease

Special delivery: microparticles convey heme

In this issue of Blood, Camus et al describe how microparticles derived from sickle erythrocytes can deliver heme to vascular endothelial cells, leading to their activation and injury, and promote vasoocclusion in sickle cell disease (SCD).

Red blood cell washing, nitrite therapy, and antiheme therapies prevent stored red blood cell toxicity after trauma-hemorrhage

Transfusion of stored red blood cells (RBCs) is associated with increased morbidity and mortality in trauma patients. Pro-oxidant, pro-inflammatory, and nitric oxide (NO) scavenging properties of stored RBCs are thought to underlie this association. In this study we determined the effects of RBC washing and nitrite and antiheme therapy on stored RBC-dependent toxicity in the setting of trauma-induced hemorrhage. A murine (C57BL/6) model of trauma-hemorrhage and resuscitation with 1 or 3 units of RBCs stored for 0-10 days was used. Tested variables included washing RBCs to remove lower MW components that scavenge NO, NO-repletion therapy using nitrite, or mitigation of free heme toxicity by heme scavenging or preventing TLR4 activation. Stored RBC toxicity was determined by assessment of acute lung injury indices (airway edema and inflammation) and survival. Transfusion with 5 day RBCs increased acute lung injury indexed by BAL protein and neutrophil accumulation. Washing 5 day RBCs prior to transfusion did not decrease this injury, whereas nitrite therapy did. Transfusion with 10 day RBCs elicited a more severe injury resulting in ~90% lethality, compared to <15% with 5 day RBCs. Both washing and nitrite therapy significantly protected against 10 day RBC-induced lethality, suggesting that washing may be protective when the injury stimulus is more severe. Finally, a spectral deconvolution assay was developed to simultaneously measure free heme and hemoglobin in stored RBC supernatants, which demonstrated significant increases of both in stored human and mouse RBCs. Transfusion with free heme partially recapitulated the toxicity mediated by stored RBCs. Furthermore, inhibition of TLR4 signaling, which is stimulated by heme, using TAK-242, or hemopexin-dependent sequestration of free heme significantly protected against both 5 day and 10 day mouse RBC-dependent toxicity. These data suggest that RBC washing, nitrite therapy, and/or antiheme and TLR4 strategies may prevent stored RBC toxicities.

Hemopexin and haptoglobin: allies against heme toxicity from hemoglobin not contenders

The goal here is to describe our current understanding of heme metabolism and the deleterious effects of "free" heme on immunological processes, endothelial function, systemic inflammation, and various end-organ tissues (e.g., kidney, lung, liver, etc.), with particular attention paid to the role of hemopexin (HPX). Because heme toxicity is the impetus for much of the pathology in sepsis, sickle cell disease (SCD), and other hemolytic conditions, the biological importance and clinical relevance of HPX, the predominant heme binding protein, is reinforced. A perspective on the function of HPX and haptoglobin (Hp) is presented, updating how these two proteins and their respective receptors act simultaneously to protect the body in clinical conditions that entail hemolysis and/or systemic intravascular (IVH) inflammation. Evidence from longitudinal studies in patients supports that HPX plays a Hp-independent role in genetic and non-genetic hemolytic diseases without the need for global Hp depletion. Evidence also supports that HPX has an important role in the prognosis of complex illnesses characterized predominantly by the presence of hemolysis, such as SCD, sepsis, hemolytic-uremic syndrome, and conditions involving IVH and extravascular hemolysis (EVH), such as that generated by extracorporeal circulation during cardiopulmonary bypass (CPB) and from blood transfusions. We propose that quantitating the amounts of plasma heme, HPX, Hb-Hp, heme-HPX, and heme-albumin levels in various disease states may aid in the diagnosis and treatment of the above-mentioned conditions, which is crucial to developing targeted plasma protein supplementation (i.e., "replenishment") therapies for patients with heme toxicity due to HPX depletion.

Complement System Part I - Molecular Mechanisms of Activation and Regulation

Complement is a complex innate immune surveillance system, playing a key role in defense against pathogens and in host homeostasis. The complement system is initiated by conformational changes in recognition molecular complexes upon sensing danger signals. The subsequent cascade of enzymatic reactions is tightly regulated to assure that complement is activated only at specific locations requiring defense against pathogens, thus avoiding host tissue damage. Here, we discuss the recent advances describing the molecular and structural basis of activation and regulation of the complement pathways and their implication on physiology and pathology. This article will review the mechanisms of activation of alternative, classical, and lectin pathways, the formation of C3 and C5 convertases, the action of anaphylatoxins, and the membrane-attack-complex. We will also discuss the importance of structure-function relationships using the example of atypical hemolytic uremic syndrome. Lastly, we will discuss the development and benefits of therapies using complement inhibitors.

Acute hemolytic vascular inflammatory processes are prevented by nitric oxide replacement or a single dose of hydroxyurea

Hemolysis and consequent release of cell-free hemoglobin (CFHb) impair vascular nitric oxide (NO) bioavailability and cause oxidative and inflammatory processes. Hydroxyurea (HU), a common therapy for sickle cell disease (SCD), induces fetal Hb production and can act as an NO donor. We evaluated the acute inflammatory effects of intravenous water-induced hemolysis in C57BL/6 mice and determined the abilities of an NO donor, diethylamine NONOate (DEANO), and a single dose of HU to modulate this inflammation. Intravenous water induced acute hemolysis in C57BL/6 mice, attaining plasma Hb levels comparable to those observed in chimeric SCD mice. This hemolysis resulted in significant and rapid systemic inflammation and vascular leukocyte recruitment within 15 minutes, accompanied by NO metabolite generation. Administration of another potent NO scavenger (2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) to C57BL/6 mice induced similar alterations in leukocyte recruitment, whereas hemin-induced inflammation occurred over a longer time frame. Importantly, the acute inflammatory effects of water-induced hemolysis were abolished by the simultaneous administration of DEANO or HU, without altering CFHb, in an NO pathway-mediated manner. In vitro, HU partially reversed the Hb-mediated induction of endothelial proinflammatory cytokine secretion and adhesion molecule expression. In summary, pathophysiological levels of hemolysis trigger an immediate inflammatory response, possibly mediated by vascular NO consumption. HU presents beneficial anti-inflammatory effects by inhibiting rapid-onset hemolytic inflammation via an NO-dependent mechanism, independently of fetal Hb elevation. Data provide novel insights into mechanisms of hemolytic inflammation and further support perspectives for the use of HU as an acute treatment for SCD and other hemolytic disorders.

Hemoglobin-induced lung vascular oxidation, inflammation, and remodeling contribute to the progression of hypoxic pulmonary hypertension and is attenuated in rats with repeated-dose haptoglobin administration

Haptoglobin (Hp) is an approved treatment in Japan for trauma, burns, and massive transfusion-related hemolysis. Additional case reports suggest uses in other acute hemolytic events that lead to acute kidney injury. However, Hp's protective effects on the pulmonary vasculature have not been evaluated within the context of mitigating the consequences of chronic hemoglobin (Hb) exposure in the progression of pulmonary hypertension (PH) secondary to hemolytic diseases. This study was performed to assess the utility of chronic Hp therapy in a preclinical model of Hb and hypoxia-mediated PH. Rats were simultaneously exposed to chronic Hb infusion (35 mg per day) and hypobaric hypoxia for 5 weeks in the presence or absence of Hp treatment (90 mg/kg twice a week). Hp inhibited the Hb plus hypoxia-mediated nonheme iron accumulation in lung and heart tissue, pulmonary vascular inflammation and resistance, and right-ventricular hypertrophy, which suggests a positive impact on impeding the progression of PH. In addition, Hp therapy was associated with a reduction in critical mediators of PH, including lung adventitial macrophage population and endothelial ICAM-1 expression. By preventing Hb-mediated pathology, Hp infusions: (1) demonstrate a critical role for Hb in vascular remodeling associated with hypoxia and (2) suggest a novel therapy for chronic hemolysis-associated PH.

Circulating cell membrane microparticles transfer heme to endothelial cells and trigger vasoocclusions in sickle cell disease

Intravascular hemolysis describes the relocalization of heme and hemoglobin (Hb) from erythrocytes to plasma. We investigated the concept that erythrocyte membrane microparticles (MPs) concentrate cell-free heme in human hemolytic diseases, and that heme-laden MPs have a physiopathological impact. Up to one-third of cell-free heme in plasma from 47 patients with sickle cell disease (SCD) was sequestered in circulating MPs. Erythrocyte vesiculation in vitro produced MPs loaded with heme. In silico analysis predicted that externalized phosphatidylserine (PS) in MPs may associate with and help retain heme at the cell surface. Immunohistology identified Hb-laden MPs adherent to capillary endothelium in kidney biopsies from hyperalbuminuric SCD patients. In addition, heme-laden erythrocyte MPs adhered and transferred heme to cultured endothelial cells, inducing oxidative stress and apoptosis. In transgenic SAD mice, infusion of heme-laden MPs triggered rapid vasoocclusions in kidneys and compromised microvascular dilation ex vivo. These vascular effects were largely blocked by heme-scavenging hemopexin and by the PS antagonist annexin-a5, in vitro and in vivo. Adversely remodeled MPs carrying heme may thus be a source of oxidant stress for the endothelium, linking hemolysis to vascular injury. This pathway might provide new targets for the therapeutic preservation of vascular function in SCD.

Methemoglobin is an endogenous toll-like receptor 4 ligand-relevance to subarachnoid hemorrhage

Neuroinflammation is a well-recognized consequence of subarachnoid hemorrhage (SAH), and may be responsible for important complications of SAH. Signaling by Toll-like receptor 4 (TLR4)-mediated nuclear factor κB (NFκB) in microglia plays a critical role in neuronal damage after SAH. Three molecules derived from erythrocyte breakdown have been postulated to be endogenous TLR4 ligands: methemoglobin (metHgb), heme and hemin. However, poor water solubility of heme and hemin, and lipopolysaccharide (LPS) contamination have confounded our understanding of these molecules as endogenous TLR4 ligands. We used a 5-step process to obtain highly purified LPS-free metHgb, as confirmed by Fourier Transform Ion Cyclotron Resonance mass spectrometry and by the Limulus amebocyte lysate assay. Using this preparation, we show that metHgb is a TLR4 ligand at physiologically relevant concentrations. metHgb caused time- and dose-dependent secretion of the proinflammatory cytokine, tumor necrosis factor α (TNFα), from microglial and macrophage cell lines, with secretion inhibited by siRNA directed against TLR4, by the TLR4-specific inhibitors, Rs-LPS and TAK-242, and by anti-CD14 antibodies. Injection of purified LPS-free metHgb into the rat subarachnoid space induced microglial activation and TNFα upregulation. Together, our findings support the hypothesis that, following SAH, metHgb in the subarachnoid space can promote widespread TLR4-mediated neuroinflammation.

Sickle cell disease: renal manifestations and mechanisms

Sickle cell disease (SCD) substantially alters renal structure and function, and causes various renal syndromes and diseases. Such diverse renal outcomes reflect the uniquely complex vascular pathobiology of SCD and the propensity of red blood cells to sickle in the renal medulla because of its hypoxic, acidotic, and hyperosmolar conditions. Renal complications and involvement in sickle cell nephropathy (SCN) include altered haemodynamics, hypertrophy, assorted glomerulopathies, chronic kidney disease, acute kidney injury, impaired urinary concentrating ability, distal nephron dysfunction, haematuria, and increased risks of urinary tract infections and renal medullary carcinoma. SCN largely reflects an underlying vasculopathy characterized by cortical hyperperfusion, medullary hypoperfusion, and an increased, stress-induced vasoconstrictive response. Renal involvement is usually more severe in homozygous disease (sickle cell anaemia, HbSS) than in compound heterozygous types of SCD (for example HbSC and HbSβ(+)-thalassaemia), and is typically mild, albeit prevalent, in the heterozygous state (sickle cell trait, HbAS). Renal involvement contributes substantially to the diminished life expectancy of patients with SCD, accounting for 16-18% of mortality. As improved clinical care promotes survival into adulthood, SCN imposes a growing burden on both individual health and health system costs. This Review addresses the renal manifestations of SCD and focuses on their underlying mechanisms.

The fucosylation inhibitor, 2-fluorofucose, inhibits vaso-occlusion, leukocyte-endothelium interactions and NF-ĸB activation in transgenic sickle mice

2-Fluorofucose (2FF) blocks the fucosylation and the tethering of sialyl-Lewis^x tetrasaccharide and structural variants on leukocytes and red blood cells to P- and E-selectins on activated endothelial cell surfaces. Because P- and E-selectin are required for vaso-occlusion in murine sickle cell disease (SCD), we investigated whether 2FF would inhibit vaso-occlusion in SCD mice. Microvascular stasis was measured in subcutaneous venules in NY1DD and HbSS-Townes SCD mice with dorsal skin-fold chambers after infusion of hemoglobin or exposure to hypoxia/reoxygenation. 2FF in drinking water or administered by gavage inhibited stasis in sickle mice in a dose-responsive manner. Significant inhibitory effects on stasis were seen 1 day post-treatment. 2FF treatment of SCD mice also significantly reduced leukocyte rolling and adhesion along the vessel walls of SCD mice and the static adhesion of neutrophils and sickle red blood cells isolated from 2FF-treated SCD mice to resting and activated endothelial cells. Total white blood cell counts increased in response to 2FF. NF-ĸB activation and VCAM-1 and E-selectin expression were inhibited in the livers of SCD mice consistent with an overall decrease in vascular inflammation and ischemia-reperfusion physiology. Pretreatment with 2FF completely eliminated heme-induced lethality in HbSS-Townes mice, consistent with the observed anti-inflammatory and anti-adhesive properties of 2FF in SCD mice. These data suggest that 2FF may be beneficial for preventing or treating vaso-occlusive crises in SCD patients.

Dissection of the radical reactions linked to fetal hemoglobin reveals enhanced pseudoperoxidase activity

In the presence of excess hydrogen peroxide (H2O2), ferrous (Fe(+2)) human hemoglobin (Hb) (α2β2) undergoes a rapid conversion to a higher oxidation ferryl state (Fe(+4)) which rapidly autoreduces back to the ferric form (Fe(+3)) as H2O2 is consumed in the reaction. In the presence of additional H2O2 the ferric state can form both ferryl Hb and an associated protein radical in a pseudoperoxidative cycle that results in the loss of radicals and heme degradation. We examined whether adult HbA (β2α2) exhibits a different pseudoenzymatic activity than fetal Hb (γ2α2) due to the switch of γ to β subunits. Rapid mixing of the ferric forms of both proteins with excess H2O2 resulted in biphasic kinetic time courses that can be assigned to γ/β and α, respectively. Although there was a 1.5 fold increase in the fast reacting γ /β subunits the slower reacting phases (attributed to α subunits of both proteins) were essentially the same. However, the rate constant for the auto-reduction of ferryl back to ferric for both proteins was found to be 76% higher for HbF than HbA and in the presence of the mild reducing agent, ascorbate there was a 3-fold higher reduction rate in ferryl HbF as opposed to ferryl HbA. Using quantitative mass spectrometry in the presence of H2O2 we found oxidized γ/β Cys93, to be more abundantly present in HbA than HbF, whereas higher levels of nitrated β Tyr35 containing peptides were found in HbA samples treated with nitrite. The extraordinary stability of HbF reported here may explain the evolutionary advantage this protein may confer onto co-inherited hemoglobinopathies and can also be utilized in the engineering of oxidatively stable Hb-based oxygen carriers.

Excess of heme induces tissue factor-dependent activation of coagulation in mice

An excess of free heme is present in the blood during many types of hemolytic anemia. This has been linked to organ damage caused by heme-mediated oxidative stress and vascular inflammation. We investigated the mechanism of heme-induced coagulation activation in vivo. Heme caused coagulation activation in wild-type mice that was attenuated by an anti-tissue factor antibody and in mice expressing low levels of tissue factor. In contrast, neither factor XI deletion nor inhibition of factor XIIa-mediated factor XI activation reduced heme-induced coagulation activation, suggesting that the intrinsic coagulation pathway is not involved. We investigated the source of tissue factor in heme-induced coagulation activation. Heme increased the procoagulant activity of mouse macrophages and human PBMCs. Tissue factor-positive staining was observed on leukocytes isolated from the blood of heme-treated mice but not on endothelial cells in the lungs. Furthermore, heme increased vascular permeability in the mouse lungs, kidney and heart. Deletion of tissue factor from either myeloid cells, hematopoietic or endothelial cells, or inhibition of tissue factor expressed by non-hematopoietic cells did not reduce heme-induced coagulation activation. However, heme-induced activation of coagulation was abolished when both non-hematopoietic and hematopoietic cell tissue factor was inhibited. Finally, we demonstrated that coagulation activation was partially attenuated in sickle cell mice treated with recombinant hemopexin to neutralize free heme. Our results indicate that heme promotes tissue factor-dependent coagulation activation and induces tissue factor expression on leukocytes in vivo. We also demonstrated that free heme may contribute to thrombin generation in a mouse model of sickle cell disease.

Iron, inflammation, and early death in adults with sickle cell disease

RATIONALE

Patients with sickle cell disease (SCD) have markers of chronic inflammation, but the mechanism of inflammation and its relevance to patient survival are unknown.

OBJECTIVE

To assess the relationship between iron, inflammation, and early death in SCD.

METHODS AND RESULTS

Using peripheral blood mononuclear cell transcriptome profile hierarchical clustering, we classified 24 patients and 10 controls in clusters with significantly different expression of genes known to be regulated by iron. Subsequent gene set enrichment analysis showed that many genes associated with the high iron cluster were involved in the toll-like receptor system (TLR4, TLR7, and TLR8) and inflammasome complex pathway (NLRP3, NLRC4, and CASP1). Quantitative PCR confirmed this classification and showed that ferritin light chain, TLR4, and interleukin-6 expression were >100-fold higher in patients than in controls (P<0.001). Further linking intracellular iron and inflammation, 14 SCD patients with a ferroportin Q248H variant that causes intracellular iron accumulation had significantly higher levels of interleukin-6 and C-reactive protein compared with 14 matched SCD patients with the wild-type allele (P<0.05). Finally, in a cohort of 412 patients followed for a median period of 47 months (interquartile range, 24-82), C-reactive protein was strongly and independently associated with early death (hazard ratio, 3.0; 95% confidence interval, 1.7-5.2; P<0.001).

CONCLUSIONS

Gene expression markers of high intracellular iron in patients with SCD are associated with markers of inflammation and mortality. The results support a model in which intracellular iron promotes inflammatory pathways, such as the TLR system and the inflammasome, identifying important new pathways for additional investigation.

Oxidative stress in preeclampsia and the role of free fetal hemoglobin

Preeclampsia is a leading cause of pregnancy complications and affects 3-7% of pregnant women. This review summarizes the current knowledge of a new potential etiology of the disease, with a special focus on hemoglobin-induced oxidative stress. Furthermore, we also suggest hemoglobin as a potential target for therapy. Gene and protein profiling studies have shown increased expression and accumulation of free fetal hemoglobin in the preeclamptic placenta. Predominantly due to oxidative damage to the placental barrier, fetal hemoglobin leaks over to the maternal circulation. Free hemoglobin and its metabolites are toxic in several ways; (a) ferrous hemoglobin (Fe(2+)) binds strongly to the vasodilator nitric oxide (NO) and reduces the availability of free NO, which results in vasoconstriction, (b) hemoglobin (Fe(2+)) with bound oxygen spontaneously generates free oxygen radicals, and (c) the heme groups create an inflammatory response by inducing activation of neutrophils and cytokine production. The endogenous protein α1-microglobulin, with radical and heme binding properties, has shown both ex vivo and in vivo to have the ability to counteract free hemoglobin-induced placental and kidney damage. Oxidative stress in general, and more specifically fetal hemoglobin-induced oxidative stress, could play a key role in the pathology of preeclampsia seen both in the placenta and ultimately in the maternal endothelium.

Morphine for the treatment of pain in sickle cell disease

Pain is a hallmark of sickle cell disease (SCD) and its treatment remains challenging. Opioids are the major family of analgesics that are commonly used for treating severe pain. However, these are not always effective and are associated with the liabilities of their own. The pharmacology and multiorgan side effects of opioids are rapidly emerging areas of investigation, but there remains a scarcity of clinical studies. Due to opioid-induced endothelial-, mast cell-, renal mesangial-, and epithelial-cell-specific effects and proinflammatory as well as growth influencing signaling, it is likely that when used for analgesia, opioids may have organ specific pathological effects. Experimental and clinical studies, even though extremely few, suggest that opioids may exacerbate existent organ damage and also stimulate pathologies of their own. Because of the recurrent and/or chronic use of large doses of opioids in SCD, it is critical to evaluate the role and contribution of opioids in many complications of SCD. The aim of this review is to initiate inquiry to develop strategies that may prevent the inadvertent effect of opioids on organ function in SCD, should it occur, without compromising analgesia.

Prevalence and gene characteristics of antibodies with cofactor-induced HIV-1 specificity

The healthy immune repertoire contains a fraction of antibodies that bind to various biologically relevant cofactors, including heme. Interaction of heme with some antibodies results in induction of new antigen binding specificities and acquisition of binding polyreactivity. In vivo, extracellular heme is released as a result of hemolysis or tissue damage; hence the post-translational acquisition of novel antigen specificities might play an important role in the diversification of the immunoglobulin repertoire and host defense. Here, we demonstrate that seronegative immune repertoires contain antibodies that gain reactivity to HIV-1 gp120 upon exposure to heme. Furthermore, a panel of human recombinant antibodies was cloned from different B cell subpopulations, and the prevalence of antibodies with cofactor-induced specificity for gp120 was determined. Our data reveal that upon exposure to heme, ∼24% of antibodies acquired binding specificity for divergent strains of HIV-1 gp120. Sequence analyses reveal that heme-sensitive antibodies do not differ in their repertoire of variable region genes and in most of the molecular features of their antigen-binding sites from antibodies that do not change their antigen binding specificity. However, antibodies with cofactor-induced gp120 specificity possess significantly lower numbers of somatic mutations in their variable region genes. This study contributes to the understanding of the significance of cofactor-binding antibodies in immunoglobulin repertoires and of the influence that the tissue microenvironment might have in shaping adaptive immune responses.

Haptoglobin attenuates hemoglobin-induced heme oxygenase-1 in renal proximal tubule cells and kidneys of a mouse model of sickle cell disease

Sickle cell disease (SCD), a hereditary hemolytic disorder is characterized by chronic hemolysis, oxidative stress, vaso-occlusion and end-organ damage. Hemolysis releases toxic cell-free hemoglobin (Hb) into circulation. Under physiologic conditions, plasma Hb binds to haptoglobin (Hp) and forms Hb-Hp dimers. The dimers bind to CD163 receptors on macrophages for further internalization and degradation. However, in SCD patients plasma Hp is depleted and free Hb is cleared primarily by proximal tubules of kidneys. Excess free Hb in plasma predisposes patients to renal damage. We hypothesized that administration of exogenous Hp reduces Hb-mediated renal damage. To test this hypothesis, human renal proximal tubular cells (HK-2) were exposed to HbA (50μM heme) for 24h. HbA increased the expression of heme oxygenase-1 (HO-1), an enzyme which degrades heme, reduces heme-mediated oxidative toxicity, and confers cytoprotection. Similarly, infusion of HbA (32μM heme/kg) induced HO-1 expression in kidneys of SCD mice. Immunohistochemistry confirmed the increased HO-1 expression in the proximal tubules of the kidney. Exogenous Hp attenuated the HbA-induced HO-1 expression in vitro and in SCD mice. Our results suggest that Hb-mediated oxidative toxicity may contribute to renal damage in SCD and that Hp treatment reduces heme/iron toxicity in the kidneys following hemolysis.

Advances in sickle cell therapies in the hydroxyurea era

In the hydroxyurea era, insights into mechanisms downstream of erythrocyte sickling have led to new therapeutic approaches for patients with sickle cell disease (SCD). Therapies have been developed that target vascular adhesion, inflammation and hemolysis, including innovative biologics directed against P-selectin and invariant natural killer T cells. Advances in hematopoietic stem cell transplant and gene therapy may also provide more opportunities for cures in the near future. Several clinical studies are underway to determine the safety and efficacy of these new treatments. Novel approaches to treat SCD are desperately needed, since current therapies are limited and rates of morbidity and mortality remain high.

Characterization of heme binding to recombinant α1-microglobulin

Background: Alpha-1-microglobulin (A1M), a small lipocalin protein found in plasma and tissues, has been identified as a heme¹ and radical scavenger that may participate in the mitigation of toxicities caused by degradation of hemoglobin. The objective of this work was to investigate heme interactions with A1M in vitro using various analytical techniques and to optimize analytical methodology suitable for rapid evaluation of the ligand binding properties of recombinant A1M versions.

Methods: To examine heme binding properties of A1M we utilized UV/Vis absorption spectroscopy, visible circular dichroism (CD), catalase-like activity, migration shift electrophoresis, and surface plasmon resonance (SPR), which was specifically developed for the assessment of His-tagged A1M.

Results: The results of this study confirm that A1M is a heme binding protein that can accommodate heme at more than one binding site and/or in coordination with different amino acid residues depending upon heme concentration and ligand-to-protein molar ratio. UV/Vis titration of A1M with heme revealed an unusually large bathochromic shift, up to 38 nm, observed for heme binding to a primary binding site. UV/Vis spectroscopy, visible CD and catalase-like activity suggested that heme is accommodated inside His-tagged (tgA1M) and tagless A1M (ntA1M) in a rather similar fashion although the His-tag is very likely involved into coordination with iron of the heme molecule. SPR data indicated kinetic rate constants and equilibrium binding constants with K_D values in a μM range.

Conclusions: This study provided experimental evidence of the A1M heme binding properties by aid of different techniques and suggested an analytical methodology for a rapid evaluation of ligand-binding properties of recombinant A1M versions, also suitable for other His-tagged proteins.

Vasculopathy and pulmonary hypertension in sickle cell disease

Sickle cell disease (SCD) is an autosomal recessive disorder in the gene encoding the β-chain of hemoglobin. Deoxygenation causes the mutant hemoglobin S to polymerize, resulting in rigid, adherent red blood cells that are entrapped in the microcirculation and hemolyze. Cardinal features include severe painful crises and episodic acute lung injury, called acute chest syndrome. This population, with age, develops chronic organ injury, such as chronic kidney disease and pulmonary hypertension. A major risk factor for developing chronic organ injury is hemolytic anemia, which releases red blood cell contents into the circulation. Cell free plasma hemoglobin, heme, and arginase 1 disrupt endothelial function, drive oxidative and inflammatory stress, and have recently been referred to as erythrocyte damage-associated molecular pattern molecules (eDAMPs). Studies suggest that in addition to effects of cell free plasma hemoglobin on scavenging nitric oxide (NO) and generating reactive oxygen species (ROS), heme released from plasma hemoglobin can bind to the toll-like receptor 4 to activate the innate immune system. Persistent intravascular hemolysis over decades leads to chronic vasculopathy, with ∼10% of patients developing pulmonary hypertension. Progressive obstruction of small pulmonary arterioles, increase in pulmonary vascular resistance, decreased cardiac output, and eventual right heart failure causes death in many patients with this complication. This review provides an overview of the pathobiology of hemolysis-mediated endothelial dysfunction and eDAMPs and a summary of our present understanding of diagnosis and management of pulmonary hypertension in sickle cell disease, including a review of recent American Thoracic Society (ATS) consensus guidelines for risk stratification and management.

Haptoglobin, hemopexin, and related defense pathways-basic science, clinical perspectives, and drug development

Hemolysis, which occurs in many disease states, can trigger a diverse pathophysiologic cascade that is related to the specific biochemical activities of free Hb and its porphyrin component heme. Normal erythropoiesis and concomitant removal of senescent red blood cells (RBC) from the circulation occurs at rates of approximately 2 × 10⁶ RBCs/second. Within this physiologic range of RBC turnover, a small fraction of hemoglobin (Hb) is released into plasma as free extracellular Hb. In humans, there is an efficient multicomponent system of Hb sequestration, oxidative neutralization and clearance. Haptoglobin (Hp) is the primary Hb-binding protein in human plasma, which attenuates the adverse biochemical and physiologic effects of extracellular Hb. The cellular receptor target of Hp is the monocyte/macrophage scavenger receptor, CD163. Following Hb-Hp binding to CD163, cellular internalization of the complex leads to globin and heme metabolism, which is followed by adaptive changes in antioxidant and iron metabolism pathways and macrophage phenotype polarization. When Hb is released from RBCs within the physiologic range of Hp, the potential deleterious effects of Hb are prevented. However, during hyper-hemolytic conditions or with chronic hemolysis, Hp is depleted and Hb readily distributes to tissues where it might be exposed to oxidative conditions. In such conditions, heme can be released from ferric Hb. The free heme can then accelerate tissue damage by promoting peroxidative reactions and activation of inflammatory cascades. Hemopexin (Hx) is another plasma glycoprotein able to bind heme with high affinity. Hx sequesters heme in an inert, non-toxic form and transports it to the liver for catabolism and excretion. In the present review we discuss the components of physiologic Hb/heme detoxification and their potential therapeutic application in a wide range of hemolytic conditions.

Sickle cell disease increases high mobility group box 1: a novel mechanism of inflammation

High mobility group box 1 (HMGB1) is a chromatin-binding protein that maintains DNA structure. On cellular activation or injury, HMGB1 is released from activated immune cells or necrotic tissues and acts as a damage-associated molecular pattern to activate Toll-like receptor 4 (TLR4). Little is known concerning HMGB1 release and TLR4 activity and their role in the pathology of inflammation of sickle cell disease (SCD). Circulating HMGB1 levels were increased in both humans and mice with SCD compared with controls. Furthermore, sickle plasma increased HMGB1-dependent TLR4 activity compared with control plasma. HMGB1 levels were further increased during acute sickling events (vasoocclusive crises in humans or hypoxia/reoxygenation injury in mice). Anti-HMGB1 neutralizing antibodies reduced the majority of sickle plasma-induced TLR4 activity both in vitro and in vivo. These findings show that HMGB1 is the major TLR4 ligand in SCD and likely plays a critical role in SCD-mediated inflammation.

New perspectives on the thrombotic complications of haemolysis

Many clinical settings are associated with haemolysis, from rare conditions, such as paroxysmal nocturnal haemoglobinuria, to common interventions, such as mechanical circulatory support and blood transfusion. The toxic effects of circulating free haemoglobin, haem, and iron are becoming increasingly understood and include an increased risk of thrombotic complications. This review summarizes the epidemiological evidence for an association between haemolysis and thrombosis and explores potential underlying mechanisms. New insights into the role haem plays in inflammatory signalling and in generating neutrophil extracellular traps (NETs) may provide useful strategies for managing pathological states associated with severe haemolysis. A better understanding of the toxic effects of haemolysis will result in better therapies to prevent the side effect of thrombosis.

Red blood cell, hemoglobin and heme in the progression of atherosclerosis

For decades plaque neovascularization was considered as an innocent feature of advanced atherosclerotic lesions, but nowadays growing evidence suggest that this process triggers plaque progression and vulnerability. Neovascularization is induced mostly by hypoxia, but the involvement of oxidative stress is also established. Because of inappropriate angiogenesis, neovessels are leaky and prone to rupture, leading to the extravasation of red blood cells (RBCs) within the plaque. RBCs, in the highly oxidative environment of the atherosclerotic lesions, tend to lyse quickly. Both RBC membrane and the released hemoglobin (Hb) possess atherogenic activities. Cholesterol content of RBC membrane contributes to lipid deposition and lipid core expansion upon intraplaque hemorrhage. Cell-free Hb is prone to oxidation, and the oxidation products possess pro-oxidant and pro-inflammatory activities. Defense and adaptation mechanisms evolved to cope with the deleterious effects of cell free Hb and heme. These rely on plasma proteins haptoglobin (Hp) and hemopexin (Hx) with the ability to scavenge and eliminate free Hb and heme form the circulation. The protective strategy is completed with the cellular heme oxygenase-1/ferritin system that becomes activated when Hp and Hx fail to control free Hb and heme-mediated stress. These protective molecules have pharmacological potential in diverse pathologies including atherosclerosis.

Hemolysis-induced lethality involves inflammasome activation by heme

The increase of extracellular heme is a hallmark of hemolysis or extensive cell damage. Heme has prooxidant, cytotoxic, and inflammatory effects, playing a central role in the pathogenesis of malaria, sepsis, and sickle cell disease. However, the mechanisms by which heme is sensed by innate immune cells contributing to these diseases are not fully characterized. We found that heme, but not porphyrins without iron, activated LPS-primed macrophages promoting the processing of IL-1β dependent on nucleotide-binding domain and leucine rich repeat containing family, pyrin domain containing 3 (NLRP3). The activation of NLRP3 by heme required spleen tyrosine kinase, NADPH oxidase-2, mitochondrial reactive oxygen species, and K(+) efflux, whereas it was independent of heme internalization, lysosomal damage, ATP release, the purinergic receptor P2X7, and cell death. Importantly, our results indicated the participation of macrophages, NLRP3 inflammasome components, and IL-1R in the lethality caused by sterile hemolysis. Thus, understanding the molecular pathways affected by heme in innate immune cells might prove useful to identify new therapeutic targets for diseases that have heme release.

Platelet activation biomarkers in Berkeley sickle cell mice and the response to prasugrel

Vaso-occlusive crisis (VOC) is a common complication that occurs in sickle cell disease (SCD) patients. Although underlying mechanisms of VOC remain unclear, platelet activation has been associated with VOC. In the present study, plasma adenine nucleotide measurements using LC-ESI-MS/MS showed that plasma ADP in the Berkeley murine model of SCD was significantly higher (applox. 2.7-fold increase) compared with control mice. Assessment of platelet activation markers using flow cytometry indicated that in SCD mice at steady state (8 weeks old), circulating platelets were partially activated and this tended to increase with age (15 weeks old). The administration of prasugrel, a thienopiridyl P2Y12 antagonist, did not affect the activation state of circulating platelets suggesting P2Y12 independent mechanism of activation. In this murine SCD model, ex vivo addition of ADP or PAR4 TRAP resulted in further platelet activation as assessed by expression of activated GPIIb/IIIa and P-selectin both at 8 and 15 weeks. In 15 weeks old SCD mice, agonist-induced increases in activation markers were enhanced compared to control mice. Oral administration of prasugrel effectively inhibited ex vivo platelet activation consistent with clinical data in patients with SCD. In conclusion, in the Berkeley murine model of SCD, we found evidence of basal and agonist-stimulated platelet activation which could in part be attenuated by prasugrel. These data are consistent with observations made in patients with SCD and suggest possible utility of this murine model and prasugrel therapy in exploring treatment options for patients with SCD.

Quantitative intravital two-photon excitation microscopy reveals absence of pulmonary vaso-occlusion in unchallenged Sickle Cell Disease mice

Sickle cell disease (SCD) is a genetic disorder that leads to red blood cell (RBC) sickling, hemolysis and the upregulation of adhesion molecules on sickle RBCs. Chronic hemolysis in SCD results in a hyper-inflammatory state characterized by activation of circulating leukocytes, platelets and endothelial cells even in the absence of a crisis. A crisis in SCD is often triggered by an inflammatory stimulus and can lead to the acute chest syndrome (ACS), which is a type of lung injury and a leading cause of mortality among SCD patients. Although it is believed that pulmonary vaso-occlusion could be the phenomenon contributing to the development of ACS, the role of vaso-occlusion in ACS remains elusive. Intravital imaging of the cremaster microcirculation in SCD mice has been instrumental in establishing the role of neutrophil-RBC-endothelium interactions in systemic vaso-occlusion; however, such studies, although warranted, have never been done in the pulmonary microcirculation of SCD mice. Here, we show that two-photon excitation fluorescence microscopy can be used to perform quantitative analysis of neutrophil and RBC trafficking in the pulmonary microcirculation of SCD mice. We provide the experimental approach that enables microscopic observations under physiological conditions and use it to show that RBC and neutrophil trafficking is comparable in SCD and control mice in the absence of an inflammatory stimulus. The intravital imaging scheme proposed in this study can be useful in elucidating the cellular and molecular mechanism of pulmonary vaso-occlusion in SCD mice following an inflammatory stimulus.

Heme on innate immunity and inflammation

Heme is an essential molecule expressed ubiquitously all through our tissues. Heme plays major functions in cellular physiology and metabolism as the prosthetic group of diverse proteins. Once released from cells and from hemeproteins free heme causes oxidative damage and inflammation, thus acting as a prototypic damage-associated molecular pattern. In this context, free heme is a critical component of the pathological process of sterile and infectious hemolytic conditions including malaria, hemolytic anemias, ischemia-reperfusion, and hemorrhage. The plasma scavenger proteins hemopexin and albumin reduce heme toxicity and are responsible for transporting free heme to intracellular compartments where it is catabolized by heme-oxygenase enzymes. Upon hemolysis or severe cellular damage the serum capacity to scavenge heme may saturate and increase free heme to sufficient amounts to cause tissue damage in various organs. The mechanism by which heme causes reactive oxygen generation, activation of cells of the innate immune system and cell death are not fully understood. Although heme can directly promote lipid peroxidation by its iron atom, heme can also induce reactive oxygen species generation and production of inflammatory mediators through the activation of selective signaling pathways. Heme activates innate immune cells such as macrophages and neutrophils through activation of innate immune receptors. The importance of these events has been demonstrated in infectious and non-infectious diseases models. In this review, we will discuss the mechanisms behind heme-induced cytotoxicity and inflammation and the consequences of these events on different tissues and diseases.

Atherogenesis and iron: from epidemiology to cellular level

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

H-ferritin ferroxidase induces cytoprotective pathways and inhibits microvascular stasis in transgenic sickle mice

Hemolysis, oxidative stress, inflammation, vaso-occlusion, and organ infarction are hallmarks of sickle cell disease (SCD). We have previously shown that increases in heme oxygenase-1 (HO-1) activity detoxify heme and inhibit vaso-occlusion in transgenic mouse models of SCD. HO-1 releases Fe(2+) from heme, and the ferritin heavy chain (FHC) ferroxidase oxidizes Fe(2+) to catalytically inactive Fe(3+) inside ferritin. FHC overexpression has been shown to be cytoprotective. In this study, we hypothesized that overexpression of FHC and its ferroxidase activity will inhibit inflammation and microvascular stasis in transgenic SCD mice in response to plasma hemoglobin. We utilized a Sleeping Beauty (SB) transposase plasmid to deliver a human wild-type-ferritin heavy chain (wt-hFHC) transposable element by hydrodynamic tail vein injections into NY1DD SCD mice. Control SCD mice were infused with the same volume of lactated Ringer's solution (LRS) or a human triple missense FHC (ms-hFHC) plasmid with no ferroxidase activity. 8 weeks later, LRS-injected mice had ~40% microvascular stasis (% non-flowing venules) 1 h after infusion of stroma-free hemoglobin, while mice overexpressing wt-hFHC had only 5% stasis (p < 0.05), and ms-hFHC mice had 33% stasis suggesting vascular protection by ferroxidase active wt-hFHC. The wt-hFHC SCD mice had marked increases in splenic hFHC mRNA and hepatic hFHC protein, ferritin light chain (FLC), 5-aminolevulinic acid synthase (ALAS), heme content, ferroportin, nuclear factor erythroid 2-related factor 2 (Nrf2), and HO-1 activity and protein. There was also a decrease in hepatic activated nuclear factor-kappa B (NF-κB) phospho-p65 and vascular cell adhesion molecule-1 (VCAM-1). Inhibition of HO-1 activity with tin protoporphyrin demonstrated HO-1 was not essential for the protection by wt-hFHC. We conclude that wt-hFHC ferroxidase activity enhances cytoprotective Nrf2-regulated proteins including HO-1, thereby resulting in decreased NF-κB-activation, adhesion molecules, and microvascular stasis in transgenic SCD mice.

Not simply misshapen red cells: multimolecular and cellular events in sickle vaso-occlusion

Thromboinflammatory diseases result from the interactions of vascular endothelial cells, inflammatory cells, and platelets with cellular adhesion molecules, plasma proteins, and lipids. Tipping the balance toward a prothrombotic, proinflammatory phenotype results from multicellular activation signals. In this issue of the JCI, Li et al. explore the regulation of heterotypic neutrophil-platelet contacts in response to TNF-α-induced venular inflammation with relevance to sickle cell disease (SCD).

Blood substitutes: why haven't we been more successful?

Persistent safety concerns have stalled the development of viable hemoglobin (Hb)-based oxygen carriers (HBOCs). HBOCs have several advantages over human blood, including availability, long-term storage, and lack of infectious risk. The basis of HBOC toxicity is poorly understood, however, several mechanisms have been suggested, including Hb extravasation across the blood vessel wall, scavenging of endothelial nitric oxide (NO), oversupply of oxygen, and heme-mediated oxidative side reactions. Although there are some in vitro and limited animal studies supporting these mechanisms, heme-mediated reactivity appears to provide an alternative path that can explain some of the observed pathophysiological changes. Moreover, recent mechanistic and animal studies support a role for globin and heme scavengers in controlling oxidative toxicity associated with Hb infusion.

Redox properties of human hemoglobin in complex with fractionated dimeric and polymeric human haptoglobin

Haptoglobin (Hp) is an abundant and conserved plasma glycoprotein, which binds acellular adult hemoglobin (Hb) dimers with high affinity and facilitates their rapid clearance from circulation after hemolysis. Humans possess three main phenotypes of Hp, designated Hp 1-1, Hp 2-1, and Hp 2-2. These variants exhibit diverse structural configurations and have been reported to be functionally nonequivalent. We have investigated the functional and redox properties of Hb-Hp complexes prepared using commercially fractionated Hp and found that all forms exhibit similar behavior. The rate of Hb dimer binding to Hp occurs with bimolecular rate constants of ~0.9 μM(-1) s(-1), irrespective of the type of Hp assayed. Although Hp binding does accelerate the observed rate of HbO2 autoxidation by dissociating Hb tetramers into dimers, the rate observed for these bound dimers is three- to fourfold slower than that of Hb dimers free in solution. Co-incubation of ferric Hb with any form of Hp inhibits heme loss to below detectable levels. Intrinsic redox potentials (E1/2) of the ferric/ferrous pair of each Hb-Hp complex are similar, varying from +54 to +59 mV (vs NHE), and are essentially the same as reported by us previously for Hb-Hp complexes prepared from unfractionated Hp. All Hb-Hp complexes generate similar high amounts of ferryl Hb after exposure to hydrogen peroxide. Electron paramagnetic resonance data indicate that the yields of protein-based radicals during this process are approximately 4 to 5% and are unaffected by the variant of Hp assayed. These data indicate that the Hp fractions examined are equivalent to one another with respect to Hb binding and associated stability and redox properties and that this result should be taken into account in the design of phenotype-specific Hp therapeutics aimed at countering Hb-mediated vascular disease.

Heme-induced neutrophil extracellular traps contribute to the pathogenesis of sickle cell disease

Sickle cell disease (SCD) is characterized by recurring episodes of vascular occlusion in which neutrophil activation plays a major role. The disease is associated with chronic hemolysis with elevated cell-free hemoglobin and heme. The ensuing depletion of heme scavenger proteins leads to nonspecific heme uptake and heme-catalyzed generation of reactive oxygen species. Here, we have identified a novel role for heme in the induction of neutrophil extracellular trap (NET) formation in SCD. NETs are decondensed chromatin decorated by granular enzymes and are released by activated neutrophils. In humanized SCD mice, we have detected NETs in the lungs and soluble NET components in plasma. The presence of NETs was associated with hypothermia and death of these mice, which could be prevented and delayed, respectively, by dismantling NETs with DNase I treatment. We have identified heme as the plasma factor that stimulates neutrophils to release NETs in vitro and in vivo. Increasing or decreasing plasma heme concentrations can induce or prevent, respectively, in vivo NET formation, indicating that heme plays a crucial role in stimulating NET release in SCD. Our results thus suggest that NETs significantly contribute to SCD pathogenesis and can serve as a therapeutic target for treating SCD.