Antithrombotic effects of heme-degrading and heme-binding proteins

Induction of p16Ink4a Gene Expression in Heme Protein-Induced AKI and by Heme: Pathophysiologic Implications

Key Points

In heme protein–mediated AKI (HP-AKI), a senescence phenotype promptly occurs, and increased expression of p16Ink4a contributes to HP-AKI. Renal p16Ink4a expression is induced by hemoglobin, myoglobin, and heme in vivo and in renal epithelial cells exposed to heme in vitro . Impairing the binding or degradation of heme by hemopexin deficiency or heme oxygenase-1 deficiency, respectively, further upregulates p16Ink4a.

Background

Understanding the pathogenetic basis for AKI involves the study of ischemic and nephrotoxic models of AKI, the latter including heme protein–mediated AKI (HP-AKI). Recently, interest has grown regarding the role of senescence as a mechanism of kidney injury, including AKI. We examined whether senescence occurs in HP-AKI and potential inducers of and the role of a key driver of senescence, namely, p16Ink4a, in HP-AKI.

Methods

The long-established murine glycerol model of HP-AKI was used, and indices of senescence were examined. To evaluate the interaction of heme and p16Ink4a expression, murine models of genetic deficiency of hemopexin (HPX ) and heme oxygenase-1 (HO-1 ) were used. To determine the involvement of p16Ink4a in HP-AKI, the population of p16Ink4a-expressing cells was reduced using the INK-ATTAC model.

Results

Using multiple indices, a senescence phenotype appears in the kidney within hours after the induction of HP-AKI. This phenotype includes significant upregulation of p16Ink4a. p16Ink4a is upregulated in the kidney after the individual administration of myoglobin, hemoglobin, and heme, as well as in renal epithelial cells exposed to heme in vitro . Genetic deficiencies of HPX and HO-1 , which, independently, are expected to increase heme content in the kidney, exaggerate induction of p16Ink4a in the kidney and exacerbate HP-AKI, the latter shown in the present studies involving HPX −/− mice and in previous studies involving HO-1 −/− mice. Finally, reduction in the population of p16Ink4a-expressing cells in the kidney improves renal function in HP-AKI even within 24 hours.

Conclusions

The pathogenesis of HP-AKI involves senescence and the induction of p16Ink4a, the latter driven, in part, by hemoglobin, myoglobin, and heme.

Double-edged functions of hemopexin in hematological related diseases: from basic mechanisms to clinical application

It is now understood that hemolysis and the subsequent release of heme into circulation play a critical role in driving the progression of various diseases. Hemopexin (HPX), a heme-binding protein with the highest affinity for heme in plasma, serves as an effective antagonist against heme toxicity resulting from severe acute or chronic hemolysis. In the present study, changes in HPX concentration were characterized at different stages of hemolytic diseases, underscoring its potential as a biomarker for assessing disease progression and prognosis. In many heme overload-driven conditions, such as sickle cell disease, transfusion-induced hemolysis, and sepsis, endogenous HPX levels are often insufficient to provide protection. Consequently, there is growing interest in developing HPX therapeutics to mitigate toxic heme exposure. Strategies include HPX supplementation when endogenous levels are depleted and enhancing HPX's functionality through modifications, offering a potent defense against heme toxicity. It is worth noting that HPX may also exert deleterious effects under certain circumstances. This review aims to provide a comprehensive overview of HPX's roles in the progression and prognosis of hematological diseases. It highlights HPX-based clinical therapies for different hematological disorders, discusses advancements in HPX production and modification technologies, and offers a theoretical basis for the clinical application of HPX.

Heme Proteins and Kidney Injury: Beyond Rhabdomyolysis

Heme proteins, the stuff of life, represent an ingenious biologic strategy that capitalizes on the biochemical versatility of heme, and yet is one that avoids the inherent risks to cellular vitality posed by unfettered and promiscuously reactive heme. Heme proteins, however, may be a double-edged sword because they can damage the kidney in certain settings. Although such injury is often viewed mainly within the context of rhabdomyolysis and the nephrotoxicity of myoglobin, an increasing literature now attests to the fact that involvement of heme proteins in renal injury ranges well beyond the confines of this single disease (and its analog, hemolysis); indeed, through the release of the defining heme motif, destabilization of intracellular heme proteins may be a common pathway for acute kidney injury, in general, and irrespective of the underlying insult. This brief review outlines current understanding regarding processes underlying such heme protein-induced acute kidney injury (AKI) and chronic kidney disease (CKD). Topics covered include, among others, the basis for renal injury after the exposure of the kidney to and its incorporation of myoglobin and hemoglobin; auto-oxidation of myoglobin and hemoglobin; destabilization of heme proteins and the release of heme; heme/iron/oxidant pathways of renal injury; generation of reactive oxygen species and reactive nitrogen species by NOX, iNOS, and myeloperoxidase; and the role of circulating cell-free hemoglobin in AKI and CKD. Also covered are the characteristics of the kidney that render this organ uniquely vulnerable to injury after myolysis and hemolysis, and pathobiologic effects emanating from free, labile heme. Mechanisms that defend against the toxicity of heme proteins are discussed, and the review concludes by outlining the therapeutic strategies that have arisen from current understanding of mechanisms of renal injury caused by heme proteins and how such mechanisms may be interrupted.

Redox Balance in β-Thalassemia and Sickle Cell Disease: A Love and Hate Relationship

β-thalassemia and sickle cell disease (SCD) are inherited hemoglobinopathies that result in both quantitative and qualitative variations in the β-globin chain. These in turn lead to instability in the generated hemoglobin (Hb) or to a globin chain imbalance that affects the oxidative environment both intracellularly and extracellularly. While oxidative stress is not among the primary etiologies of β-thalassemia and SCD, it plays a significant role in the pathogenesis of these diseases. Different mechanisms exist behind the development of oxidative stress; the result of which is cytotoxicity, causing the oxidation of cellular components that can eventually lead to cell death and organ damage. In this review, we summarize the mechanisms of oxidative stress development in β-thalassemia and SCD and describe the current and potential antioxidant therapeutic strategies. Finally, we discuss the role of targeted therapy in achieving an optimal redox balance.

The spike protein of SARS-CoV-2 induces heme oxygenase-1: Pathophysiologic implications

BACKGROUND

Acute kidney injury (AKI) is both a consequence and determinant of outcomes in COVID-19. The kidney is one of the major organs infected by the causative virus, SARS-CoV-2. Viral entry into cells requires the viral spike protein, and both the virus and its spike protein appear in the urine of COVID-19 patients with AKI. We examined the effects of transfecting the viral spike protein of SARS-CoV-2 in kidney cell lines.

METHODS

HEK293, HEK293-ACE2⁺ (stably overexpressing ACE2), and Vero E6 cells having endogenous ACE2 were transfected with SARS-CoV-2 spike or control plasmid. Assessment of gene and protein expression, and syncytia formation was performed, and the effects of quercetin on syncytia formation examined.

FINDINGS

Spike transfection in HEK293-ACE2⁺ cells caused syncytia formation, cellular sloughing, and focal denudation of the cell monolayer; transfection in Vero E6 cells also caused syncytia formation. Spike expression upregulated potentially nephrotoxic genes (TNF-α, MCP-1, and ICAM1). Spike upregulated the cytoprotective gene HO-1 and relevant signaling pathways (p-Akt, p-STAT3, and p-p38). Quercetin, an HO-1 inducer, reduced syncytia formation and spike protein expression.

INTERPRETATION

The major conclusions of the study are: 1) Spike protein expression in kidney cells provides a relevant model for the study of maladaptive and adaptive responses germane to AKI in COVID-19; 2) such spike protein expression upregulates HO-1; and 3) quercetin, an HO-1 inducer, may provide a clinically relevant/feasible protective strategy in AKI occurring in the setting of COVID-19.

FUNDING

R01-DK119167 (KAN), R01-AI100911 (JPG), P30-DK079337; R01-DK059600 (AA).

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.

Revisiting the interaction of heme with hemopexin

In hemolytic disorders, erythrocyte lysis results in massive release of hemoglobin and, subsequently, toxic heme. Hemopexin is the major protective factor against heme toxicity in human blood and currently considered for therapeutic use. It has been widely accepted that hemopexin binds heme with extraordinarily high affinity of <1 pM in a 1:1 ratio. However, several lines of evidence point to a higher stoichiometry and lower affinity than determined 50 years ago. Here, we re-analyzed these data. SPR and UV/Vis spectroscopy were used to monitor the interaction of heme with the human protein. The heme-binding sites of hemopexin were characterized using hemopexin-derived peptide models and competitive displacement assays. We obtained a K _D value of 0.32 ± 0.04 nM and the ratio for the interaction was determined to be 1:1 at low heme concentrations and at least 2:1 (heme:hemopexin) at high concentrations. We were able to identify two yet unknown potential heme-binding sites on hemopexin. Furthermore, molecular modelling with a newly created homology model of human hemopexin suggested a possible recruiting mechanism by which heme could consecutively bind several histidine residues on its way into the binding pocket. Our findings have direct implications for the potential administration of hemopexin in hemolytic disorders.

Heme Degradation in Pathophysiology of and Countermeasures to Inflammation-Associated Disease

The class of tetrapyrrol "coordination complexes" called hemes are prosthetic group components of metalloproteins including hemoglobin, which provide functionality to these physiologically essential macromolecules by reversibly binding diatomic gasses, notably O₂, which complexes to ferrous (reduced/Fe(II)) iron within the heme porphyrin ring of hemoglobin in a pH- and PCO₂-dependent manner-thus allowing their transport and delivery to anatomic sites of their function. Here, pathologies associated with aberrant heme degradation are explored in the context of their underlying mechanisms and emerging medical countermeasures developed using heme oxygenase (HO), its major degradative enzyme and bioactive metabolites produced by HO activity. Tissue deposits of heme accumulate as a result of the removal of senescent or damaged erythrocytes from circulation by splenic macrophages, which destroy the cells and internal proteins, including hemoglobin, leaving free heme to accumulate, posing a significant toxicogenic challenge. In humans, HO uses NADPH as a reducing agent, along with molecular oxygen, to degrade heme into carbon monoxide (CO), free ferrous iron (FeII), which is sequestered by ferritin protein, and biliverdin, subsequently metabolized to bilirubin, a potent inhibitor of oxidative stress-mediated tissue damage. CO acts as a cellular messenger and augments vasodilation. Nevertheless, disease- or trauma-associated oxidative stressors sufficiently intense to overwhelm HO may trigger or exacerbate a wide range of diseases, including cardiovascular and neurologic syndromes. Here, strategies are described for counteracting the effects of aberrant heme degradation, with a particular focus on "bioflavonoids" as HO inducers, shown to cause amelioration of severe inflammatory diseases.

Heme oxygenase-1 modulation: A potential therapeutic target for COVID-19 and associated complications

Coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to infect hundred thousands of people every day worldwide. Since it is a novel virus, research continues to update the possible therapeutic targets when new evidence regarding COVID-19 are gathered. This article presents an evidence-based hypothesis that activating the heme oxygenase-1 (HO-1) pathway is a potential target for COVID-19. Interferons (IFNs) have broad-spectrum antiviral activity including against SARS-CoV-2. Induction of HO-1 and increase in the heme catabolism end-product confer antiviral activity. IFN activation results in inhibition of viral replication in various viral infections. COVID-19 induced inflammation as well as acute respiratory distress syndrome (ARDS), and coagulopathies are now known major causes of mortality. A protective role of HO-1 induction in inflammation, inflammation-induced coagulation, and ARDS has been reported. Based on an association of HO-1 promoter polymorphisms and disease severity, we propose an evaluation of the status of these polymorphisms in COVID-19 patients who become severely ill. If an association is established, it might be helpful in identifying patients at high risk. Hence, we hypothesize that HO-1 pathway activation could be a therapeutic strategy against COVID-19 and associated complications.

Thromboinflammatory mechanisms in sickle cell disease - challenging the hemostatic balance

Sickle cell disease (SCD) is an inherited hemoglobinopathy that is caused by the presence of abnormal hemoglobin S (HbS) in red blood cells, leading to alterations in red cell properties and shape, as the result of HbS dexoygenation and subsequent polymerization. SCD pathophysiology is characterized by chronic inflammatory processes, triggered by hemolytic and vaso-occlusive events, which lead to the varied complications, organ damage and elevated mortality seen in individuals with the disease. In association with activation of the endothelium and leukocytes, hemostatic alterations and thrombotic events are well-documented in SCD. Here we discuss the role for inflammatory pathways in modulating coagulation and inducing platelet activation in SCD, due to tissue factor activation, adhesion molecule expression, inflammatory mediator production and the induction of innate immune responses, amongst other mechanisms. Thromboinflammatory pathways may play a significant role in some of the major complications of SCD, such as stroke, venous thromboembolism and possibly acute chest syndrome, besides exacerbating the chronic inflammation and cellular interactions that trigger vaso-occlusion, ischemia-reperfusion processes, and eventually organ damage.

Targeting the Heme-Heme Oxygenase System to Prevent Severe Complications Following COVID-19 Infections

SARS-CoV-2 is causing a pandemic resulting in high morbidity and mortality. COVID-19 patients suffering from acute respiratory distress syndrome (ARDS) are often critically ill and show lung injury and hemolysis. Heme is a prosthetic moiety crucial for the function of a wide variety of heme-proteins, including hemoglobin and cytochromes. However, injury-derived free heme promotes adhesion molecule expression, leukocyte recruitment, vascular permeabilization, platelet activation, complement activation, thrombosis, and fibrosis. Heme can be degraded by the anti-inflammatory enzyme heme oxygenase (HO) generating biliverdin/bilirubin, iron/ferritin, and carbon monoxide. We therefore postulate that free heme contributes to many of the inflammatory phenomena witnessed in critically ill COVID-19 patients, whilst induction of HO-1 or harnessing heme may provide protection. HO-activity not only degrades injurious heme, but its effector molecules possess also potent salutary anti-oxidative and anti-inflammatory properties. Until a vaccine against SARS-CoV-2 becomes available, we need to explore novel strategies to attenuate the pro-inflammatory, pro-thrombotic, and pro-fibrotic consequences of SARS-CoV-2 leading to morbidity and mortality. The heme-HO system represents an interesting target for novel "proof of concept" studies in the context of COVID-19.