p38 MAPK activation and function following osmotic shock of erythrocytes

Apoptosis and eryptosis: similarities and differences

Eryptosis is a regulated cell death (RCD) of mature erythrocytes initially described as a counterpart of apoptosis for enucleated cells. However, over the recent years, a growing number of studies have emphasized certain differences between both cell death modalities. In this review paper, we underline the hallmarks of eryptosis and apoptosis and highlight resemblances and dissimilarities between both RCDs. We summarize and critically discuss differences in the impact of caspase-3, Ca2+ signaling, ROS signaling pathways, opposing roles of casein kinase 1α, protein kinase C, Janus kinase 3, cyclin-dependent kinase 4, and AMP-activated protein kinase to highlight a certain degree of divergence between apoptosis and eryptosis. This review emphasizes the crucial importance of further studies that focus on deepening our knowledge of cell death machinery and identifying novel differences between cell death of nucleated and enucleated cells. This might provide evidence that erythrocytes can be defined as viable entities capable of programmed cell destruction. Additionally, the revealed cell type-specific patterns in cell death can facilitate the development of cell death-modulating therapeutic agents.

Bufalin reprograms erythrocyte lifespan through p38 MAPK and Rac1 GTPase

Ample evidence indicates that bufalin (BFN), a cardiotonic steroid in Bufo toad toxin, possesses a potent anticancer activity mainly by stimulating apoptosis in cancer cells. Human red blood cells (RBCs) undergo eryptosis which contributes to a plethora of pathological conditions. No reports, however, have examined the potential toxicity of BFN to RBCs. This study aims to characterize the biochemical mechanisms governing the influence of BFN on the physiology and lifespan of RBCs. Isolated RBCs from healthy volunteers were exposed to anticancer concentrations of commercially available BFN from the skin of Bufo gargarizans (10-200 μM) for 24 h at 37 °C. Photometric assays were used to estimate hemolysis and hemolytic markers, and flow cytometry was used to detect eryptotic markers. Phosphatidylserine externalization was captured by fluorescein isothiocyante-labeled annexin V, cellular dimensions by light scatter patterns, and intracellular Ca2+ and reactive oxygen species (ROS) by fluorogenic dyes Fluo4/AM and 2',7'-dichlorodihydrofluorescein diacetate (H2DCFDA), respectively. BFN caused Ca2+-independent hemolysis and release of LDH, AST, CK, and K+, and increased annexin V-bound cells, cytosolic Ca2+, cell shrinkage, and ROS levels. BFN also disrupted Na+ and Mg2+ trafficking, and was sensitive to PEG 8000, sucrose, SB203580, and NSC 23766. In whole blood, BFN depleted hemoglobin stores, increased fragmented RBCs, and was selectively toxic to reticulocytes, lymphocytes, and platelets. In conclusion, BFN elicits premature RBC death, subject to regulation by p38 MAPK and Rac1 GTPase, and is detrimental to other peripheral blood cells. Altogether, these novel findings prompt cautious consideration of the toxin in anticancer therapy.

Rosmarinic Acid Elicits Calcium-Dependent and Sucrose-Sensitive Eryptosis and Hemolysis through p38 MAPK, CK1α, and PKC

BACKGROUND

Rosmarinic acid (RA) possesses promising anticancer potential, but further development of chemotherapeutic agents is hindered by their toxicity to off-target tissue. In particular, chemotherapy-related anemia is a major obstacle in cancer therapy, which may be aggravated by hemolysis and eryptosis. This work presents a toxicity assessment of RA in human RBCs and explores associated molecular mechanisms.

METHODS

RBCs isolated from healthy donors were treated with anticancer concentrations of RA (10-800 μM) for 24 h at 37 °C, and hemolysis and related markers were photometrically measured. Flow cytometry was used to detect canonical markers of eryptosis, including phosphatidylserine (PS) exposure by annexin-V-FITC, intracellular Ca2+ by Fluo4/AM, cell size by FSC, and oxidative stress by H2DCFDA. Ions and pH were assessed by an ion-selective electrode, while B12 was detected by chemiluminescence.

RESULTS

RA elicited concentration-dependent hemolysis with AST and LDH release but rescued the cells from hypotonic lysis at sub-hemolytic concentrations. RA also significantly increased annexin-V-positive cells, which was ameliorated by extracellular Ca2+ removal and isosmotic sucrose. Furthermore, a significant increase in Fluo4-positive cells and B12 content and a decrease in FSC and extracellular pH with KCl efflux were noted upon RA treatment. Hemolysis was augmented by blocking KCl efflux and was blunted by ATP, SB203580, staurosporin, D4476, isosmotic urea, and PEG 8000.

CONCLUSIONS

RA stimulates Ca2+-dependent and sucrose-sensitive hemolysis and eryptosis characterized by PS exposure, Ca2+ accumulation, loss of ionic regulation, and cell shrinkage. These toxic effects were mediated through energy deprivation, p38 MAPK, protein kinase C, and casein kinase 1α.

Molecular Mechanisms and Pathophysiological Significance of Eryptosis

Despite lacking the central apoptotic machinery, senescent or damaged RBCs can undergo an unusual apoptosis-like cell death, termed eryptosis. This premature death can be caused by, or a symptom of, a wide range of diseases. However, various adverse conditions, xenobiotics, and endogenous mediators have also been recognized as triggers and inhibitors of eryptosis. Eukaryotic RBCs are unique among their cell membrane distribution of phospholipids. The change in the RBC membrane composition of the outer leaflet occurs in a variety of diseases, including sickle cell disease, renal diseases, leukemia, Parkinson’s disease, and diabetes. Eryptotic erythrocytes exhibit various morphological alterations such as shrinkage, swelling, and increased granulation. Biochemical changes include cytosolic Ca2+ increase, oxidative stress, stimulation of caspases, metabolic exhaustion, and ceramide accumulation. Eryptosis is an effective mechanism for the elimination of dysfunctional erythrocytes due to senescence, infection, or injury to prevent hemolysis. Nevertheless, excessive eryptosis is associated with multiple pathologies, most notably anemia, abnormal microcirculation, and prothrombotic risk; all of which contribute to the pathogenesis of several diseases. In this review, we provide an overview of the molecular mechanisms, physiological and pathophysiological relevance of eryptosis, as well as the potential role of natural and synthetic compounds in modulating RBC survival and death.

Casein kinase 1α mediates eryptosis: a review

Eryptosis is a coordinated non-lytic cell death of erythrocytes characterized by cell shrinkage, cell membrane scrambling, Ca²⁺ influx, ceramide accumulation, oxidative stress, activation of calpain and caspases. Physiologically, it aims at removing damaged or aged erythrocytes from circulation. A plethora of diseases are associated with enhanced eryptosis, including metabolic diseases, cardiovascular pathology, renal and hepatic diseases, hematological disorders, systemic autoimmune pathology, and cancer. This makes eryptosis and eryptosis-regulating signaling pathways a target for therapeutic interventions. This review highlights the eryptotic signaling machinery containing several protein kinases and its small molecular inhibitors with a special emphasis on casein kinase 1α (CK1α), a serine/threonine protein kinase with a broad spectrum of activity. In this review article, we provide a critical analysis of the regulatory role of CK1α in eryptosis, highlight triggers of CK1α-mediated suicidal death of red blood cells, cover the knowledge gaps in understanding CK1α-driven eryptosis and discover the opportunity of CK1α-targeted pharmacological modulation of eryptosis. Moreover, we discuss the directions of future research focusing on uncovering crosstalks between CK1α and other eryptosis-regulating kinases and pathways.

A Mixture of Dietary Plant Sterols at Nutritional Relevant Serum Concentration Inhibits Extrinsic Pathway of Eryptosis Induced by Cigarette Smoke Extract

Cell death program of red blood cells (RBCs), called eryptosis, is characterized by activation of caspases and scrambling of membrane phospholipids with externalization of phosphatidylserine (PS). Excessive eryptosis confers a procoagulant phenotype and is implicated in impairment of microcirculation and increased prothrombotic risk. It has recently been reported that cigarette smokers have high levels of circulating eryptotic erythrocytes, and a possible contribution of eryptosis to the vaso-occlusive complications associated to cigarette smoke has been postulated. In this study, we demonstrate how a mixture of plant sterols (MPtS) consisting of β-sitosterol, campesterol and stigmasterol, at serum concentration reached after ingestion of a drink enriched with plant sterols, inhibits eryptosis induced by cigarette smoke extract (CSE). Isolated RBCs were exposed for 4 h to CSE (10-20% v/v). When RBCs were co-treated with CSE in the presence of 22 µM MPtS, a significant reduction of the measured hallmarks of apoptotic death like assembly of the death-inducing signaling complex (DISC), PS outsourced, ceramide production, cleaved forms of caspase 8/caspase 3, and phosphorylated p38 MAPK, was evident. The new beneficial properties of plant sterols on CSE-induced eryptosis presented in this work open new perspectives to prevent the negative physio-pathological events caused by the eryptotic red blood cells circulating in smokers.

Applications of Metabolomics for the Elucidation of Abiotic Stress Tolerance in Plants: A Special Focus on Osmotic Stress and Heavy Metal Toxicity

Plants undergo metabolic perturbations under various abiotic stress conditions; due to their sessile nature, the metabolic network of plants requires continuous reconfigurations in response to environmental stimuli to maintain homeostasis and combat stress. The comprehensive analysis of these metabolic features will thus give an overview of plant metabolic responses and strategies applied to mitigate the deleterious effects of stress conditions at a biochemical level. In recent years, the adoption of metabolomics studies has gained significant attention due to the growing technological advances in analytical biochemistry (plant metabolomics). The complexity of the plant biochemical landscape requires sophisticated, advanced analytical methods. As such, technological advancements in the field of metabolomics have been realized, aided much by the development and refinement of separatory techniques, including liquid and gas chromatography (LC and GC), often hyphenated to state-of-the-art detection instruments such as mass spectrometry (MS) or nuclear resonance magnetic (NMR) spectroscopy. Significant advances and developments in these techniques are briefly highlighted in this review. The enormous progress made thus far also comes with the dawn of the Internet of Things (IoT) and technology housed in machine learning (ML)-based computational tools for data acquisition, mining, and analysis in the 4IR era allowing for broader metabolic coverage and biological interpretation of the cellular status of plants under varying environmental conditions. Thus, scientists can paint a holistic and comprehensive roadmap and predictive models for metabolite-guided crop improvement. The current review outlines the application of metabolomics and related technological advances in elucidating plant responses to abiotic stress, mainly focusing on heavy metal toxicity and subsequent osmotic stress tolerance.

Cigarette Smoke Extract Induces p38 MAPK-Initiated, Fas-Mediated Eryptosis

Eryptosis is a physiological mechanism for the clearance of senescent or damaged erythrocytes by phagocytes. Excessive eryptosis is stimulated under several pathologies and associated with endothelial injury and thrombosis. Cigarette smoke (CS) is an established risk factor for vascular diseases and cigarette smokers have high-levels of eryptotic erythrocytes. This study, for the first time, investigates the mechanism by which CS damages red blood cells (RBCs). CS extract (CSE) from commercial cigarettes was prepared and standardized for nicotine content. Cytofluorimetric analysis demonstrated that treatment of human RBCs with CSE caused dose-dependent, phosphatidylserine externalization and cell shrinkage, hallmarks of apoptotic death. CSE did not affect cellular levels of Ca²⁺, reactive oxygen species (ROS) or glutathione (GSH). Immununoprecipitation and immunoblotting revealed the assembly of the death-inducing signaling complex (DISC) and oligomerization of Fas receptor as well as cleaved caspase-8 and caspase-3 within 6 h from the treatment. At the same time-interval, CSE elicited neutral sphyngomielinase (nSMase) activity-dependent ceramide formation and phosphorylation of p38 MAPK. Through specific inhibitors' nSMase, caspase-8 or p38 MAPK activities, we demonstrated that p38 MAPK activation is required for caspase-8-mediated eryptosis and that ceramide generation is initiator caspase-dependent. Finally, ex vivo analysis detected phosphorylated p38 MAPK (p-p38) and Fas-associated signaling complex in erythrocytes from cigarette smokers. In conclusion, our study demonstrates that CSE exposure induces in erythrocytes an extrinsic apoptotic pathway involving p38 MAPK-initiated DISC formation followed by activation of caspase-8/caspase-3 via ceramide formation.

Erythrocyte mitogen-activated protein kinases mediate hemolytic events under osmotic and oxidative stress and in hemolytic diseases

p38 MAPKs are key regulators of cellular adaptation to various stress stimuli, however, their role in mediating erythrocyte cell death and hemolysis is largely unknown. We hypothesized that activation of erythrocyte p38 MAPK is a common event in the stimulation of hemolysis, and that inhibition of p38 MAPK pathways could mitigate hemolysis in hemoglobinopathies. We exposed human erythrocytes to diamide-induced oxidative stress or to hypoosmotic shock in the presence or absence of p38 MAPK inhibitors (SCIO469, SB203580, CMPD1) and used immunoblotting to determine MAPK activity and to identify possible downstream effectors of p38 MAPK. We also evaluated the impact of p38 MAPK inhibitors on stress-induced hemolysis or hypoxia-induced sickling in erythrocytes from mouse models of sickle cell disease. We found that human erythrocytes express conventional MAPKs (MKK3, p38 MAPK, MAPKAPK2) and identified differential MAPK activation pathways in each stress condition. Specifically, p38 MAPK inhibition in diamide-treated erythrocytes was associated with decreased phosphorylation of Src tyrosine kinases and Band 3 protein. Conversely, hypoosmotic shock induced MAPKAPK2 and RSK2 phosphorylation, which was inhibited by SCIO469 or CMPD1. Relevant to hemoglobinopathies, sickle cell disease was associated with increased erythrocyte MKK3, p38 MAPK, and MAPKAPK2 expression and phosphorylation as compared with erythrocytes from healthy individuals. Furthermore, p38 MAPK inhibition was associated with decreased hemolysis in response to diamide treatments or osmotic shock, and with decreased erythrocyte sickling under experimental hypoxia. These findings provided insights into MAPK-mediated signaling pathways that regulate erythrocyte function and hemolysis in response to extracellular stressors or human diseases.

Urinary red blood cell-derived microparticles and phosphatidylserine-exposing red blood cells in glomerular and non-glomerular hematuria patients

BACKGROUND

Distinguishing glomerular hematuria (GH) from non-glomerular hematuria (NGH) is important for treating the cause of hematuria. We aimed to determine red blood cell-derived microparticles (RMPs) and phosphatidylserine (PS)-exposing red blood cells (RBCs) and evaluate their use for diagnosing GH and NGH patients.

METHODS

All patients received a physical assessment and urological examination. Dysmorphic RBCs (dRBCs) and acanthocytes were examined using a light microscope. The urinary RMPs and PS-exposing RBCs were determined using flow cytometry.

RESULTS

The ratio of RMPs to RBCs was higher in GH patients (n = 29) than in NGH patients (n = 29) (1.06 vs. 0.18). The value of the sum of the PS-exposing RBCs plus RMPs divided by the number of RBCs was higher in GH patients than in NGH patients (48.3% vs. 19.4%). The percentage of RBCs was higher in GH patients than in NGH patients (54.5% vs. 21.8%). Similarly, both the percentages of acanthocytes and of non-acanthocytes were higher in GH patients than in NGH patients (29% vs. 7.7% and 25.4% vs. 14.2%, respectively). The ROC-AUC of the number of PS-exposing RBCs plus RMPs divided by the number of RBCs was 0.9 (95% CI, 0.82-0.97), and the RMPs:RBCs ratio was 0.88 (95% CI, 0.79-0.98). The ROC-AUCs of the dRBCs and acanthocytes were 0.85 (95% CI, 0.78-0.95) and 0.88 (95% CI, 0.8-0.97), respectively.

CONCLUSIONS

Patients with GH have higher numbers of urinary RMPs and PS-exposing RBCs. These parameters have the potential to be predictive tools for classifying GH in the future.

Eryptosis as a New Insight in Malaria Pathogenesis

Eryptosis is a programmed cell death-like process that occurs in red blood cells. Although the red blood cells are anucleated, there are similarities between eryptosis and apoptosis, such as increased calcium efflux, calpain activation, phosphatidylserine exposure, cell blebbing and cell shrinkage. Eryptosis occurs physiologically in red blood cells, as a consequence of the natural senescence process of these cells, but it can also be stimulated in pathological situations such as metabolic syndromes, uremic syndromes, polycythemia vera, anemias such as sickle cell anemia and thalassemia, and infectious processes including Plasmodium infection. Infection-induced eryptosis is believed to contribute to damage caused by Plasmodium, but it’s still a topic of debate in the literature. In this review, we provided an overview of eryptosis mechanisms and its possible pathogenic role in malaria.

Anti-Eryptotic Activity of Food-Derived Phytochemicals and Natural Compounds

Human red blood cells (RBCs), senescent or damaged due to particular stress, can be removed by programmed suicidal death, a process called eryptosis. There are various molecular mechanisms underlying eryptosis. The most frequent is the increase in the cytoplasmic concentration of Ca²⁺ ions, later exposure of erythrocytes to oxidative stress, hyperosmotic shock, ceramide formation, stimulation of caspases, and energy depletion. Phosphatidylserine (PS) exposed by eryptotic RBCs due to interaction with endothelial CXC-Motiv-Chemokin-16/Scavenger-receptor, causes the RBCs to adhere to vascular wall with consequent damage to the microcirculation. Eryptosis can be triggered by various xenobiotics and endogenous molecules, such as high cholesterol levels. The possible diseases associated with eryptosis are various, including anemia, chronic kidney disease, liver failure, diabetes, hypertension, heart failure, thrombosis, obesity, metabolic syndrome, arthritis, and lupus. This review addresses and collates the existing ex vivo and animal studies on the inhibition of eryptosis by food-derived phytochemicals and natural compounds including phenolic compounds (PC), alkaloids, and other substances that could be a therapeutic and/or co-adjuvant option in eryptotic-driven disorders, especially if they are introduced through the diet.

MAPPINGS, a tool for network analysis of large phospho-signalling datasets: application to host erythrocyte response to Plasmodium infection

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The complexity of signal transduction networks in eukaryotic cells, superimposed to very large datasets generated by “omics” approaches (notably phosphor-proteomics), calls for tools to identify pathways that are mobilised under specific conditions, including infection by intracellular pathogens.

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This has become a bottleneck in various biology fields, from cancer through developmental biology to infectious diseases.

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We developed MAPPINGS, a computational tool to extract meaning from large phosphosignalling datasets, and used it to analyse host erythrocyte response to infection with malaria parasites, leading to the identification of host cell pathways that are activated by Plasmodium.

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MAPPINGS uses random walks to identify chains of phosphorylation events occurring much more or much less frequently than expected, and highlights pathways of phosphorylation that work synergistically, providing a rapid interpretation of the most critical pathways in any phosphosiganlling dataset.

Large datasets of phosphorylation interactions are constantly being generated, but deciphering the complex network structure hidden in these datasets remains challenging. Many phosphorylation interactions occurring in human cells have been identified and constitute the basis for the known phosphorylation interaction network. We overlayed onto this network phosphorylation datasets obtained from an antibody microarray approach aimed at determining changes in phospho-signalling of host erythrocytes, during infection with the malaria parasite Plasmodium falciparum. We designed a pathway analysis tool denoted MAPPINGS that uses random walks to identify chains of phosphorylation events occurring much more or much less frequently than expected. MAPPINGS highlights pathways of phosphorylation that work synergistically, providing a rapid interpretation of the most critical pathways in each dataset. MAPPINGS confirmed several signalling interactions previously shown to be modulated by infection, and revealed additional interactions which could form the basis of numerous future studies. The MAPPINGS analysis strategy described here is widely applicable to comparative phosphorylation datasets in any context, such as response of cells to infection, treatment, or comparison between differentiation stages of any cellular population.

Antiproliferative Wnt inhibitor wogonin prevents eryptosis following ionophoric challenge, hyperosmotic shock, oxidative stress, and metabolic deprivation

Anemia is a common complication of chemotherapy and may arise due to premature or suicidal death of red blood cells (RBCs). Prevention of RBC death thus lends itself as a promising strategy to counteract anemia. Wogonin (WGN; 5,7-dihydroxy-8-methoxyflavone) is a Wnt inhibitor derived from Scutellaria baicalensis plant with potent cytotoxic and antitumor potential. However, the nature of interaction of WGN with human RBCs is unknown. RBCs from healthy participants were exposed to different hemolytic and eryptotic stimuli for 24 or 48 hr at 37°C in the presence and absence of 100 μM WGN. Calcium overload was induced by 2 μM ionomycin, hyperosmotic shock with excessive sucrose, oxidative stress by 2-phenethyl isothiocyanate (PEITC), and metabolic deprivation by exclusion of glucose. Hemolysis was estimated from extracellular hemoglobin, phosphatidylserine (PS) exposure by Annexin V-FITC, intracellular calcium by Fluo4/AM, and oxidative stress by 2',7'-dichlorodihydrofluorescein diacetate (H₂ DCFDA). While WGN did not rescue the cells from the hemolytic activity of ionomycin, it reduced PS externalization by interfering with calcium influx under both ionomycin treatment and metabolic exhaustion. WGN also significantly inhibited PS exposure upon hyperosmotic shock, but the effect was independent of calcium entry. Moreover, WGN exhibited antihemolytic and anti-eryptotic activities against PEITC without appreciable reduction in ROS levels. Altogether, WGN prevents PEITC-induced hemolysis and suppresses eryptosis due to calcium accumulation, hyperosmotic shock, oxidative stress, and metabolic exhaustion. These novel insights may open new avenues into the therapeutic application of WGN for conditions in which anemia is commonly encountered, most notably cancer. PRACTICAL APPLICATIONS: The herbal supplement Sho-Saiko-To (Xiaochaihu-tang) is commonly prescribed to relieve symptoms of liver disease. Flavonoids from the herbal constituents of Sho-Saiko-To have demonstrated considerable anti-inflammatory, antioxidant, antimicrobial, antitumor, and immunomodulatory properties. In this work, we identify WGN, a major flavonoid in Sho-Saiko-To, as a novel inhibitor of hemolysis and eryptosis of human erythrocytes. Since inordinate erythrocyte death is a major obstacle in therapeutic drug development, our findings argue for the use of WGN as a natural alternative, either as a primary or an adjuvant drug, for a wide assortment of pathological conditions including cancer.

Bioymifi, a novel mimetic of TNF-related apoptosis-induced ligand (TRAIL), stimulates eryptosis

Tumor necrosis factor-related apoptosis-induced ligand (TRAIL) is a cytokine that initiates apoptosis upon binding to death receptor 5 (DR5) on cancer cells. Small molecule TRAIL mimetics have therefore been investigated as promising chemotherapeutic agents. Since anemia of chemotherapy is common, our goal is to investigate the hemolytic and eryptotic properties of novel DR5 agonist bioymifi (BMF) and identify the underlying molecular mechanisms. Whole blood (WB) was stimulated with 100 μM of BMF, whereas red blood cells (RBCs) were treated with 10-100 μM of BMF for 24 h at 37 °C. WB was analyzed for RBC, leukocyte, and platelet indices, while RBCs were examined for hemolysis by light absorbance of free hemoglobin, membrane scrambling by Annexin V-FITC, calcium by Fluo4/AM, cellular morphology by light scatter, and oxidative stress by 2',7'-dichlorodihydrofluorescein diacetate (H₂DCFDA) using flow cytometry. Caspase inhibitor Z-VAD-FMK, p38 inhibitor SB203580, casein kinase 1α inhibitor D4476, receptor-interacting protein 1 inhibitor necrostatin-2, reduced glutathione, or cyclooxygenase (COX) inhibitor aspirin were added accordingly. BMF exerted dose-responsive, calcium-independent hemolysis, reduced RBC hemoglobin, significantly increased Annexin V-, Fluo4-, and DCF-positive cells, along with a dual effect on forward and side light scatter. Notably, the cytotoxic potential of BMF was significantly mitigated upon pharmacological inhibition of p38. Furthermore, BMF exhibited selective toxicity to eosinophils and significantly diminished reticulocyte hemoglobin content. Altogether, these novel findings highlight the adverse outcomes of BMF exposure on RBC physiology and provide the first toxicological assessment of BMF as an antitumor agent.

Antileukemic activity of sulfoxide nutraceutical allicin against THP-1 cells is associated with premature phosphatidylserine exposure in human erythrocytes

Background

Allicin (ACN), a sulfoxide in freshly crushed garlic, is known for its diverse bioactive properties. Among the most notable effects of ACN is its antitumor activity against a wide array of cancer types. Thus, ACN may be a promising anticancer therapeutic. Nevertheless, chemotherapy-induced anemia is a major obstacle in cancer management with a prevalence of up to 70%. Although the pathophysiology behind it remains elusive, a number of medications known to cause anemia in patients have been shown to induce premature programmed cell death in red blood cells (RBCs) known as eryptosis. This study, thus, investigates the anticancer potential of ACN against THP-1 monocytic leukemia cells, its toxic effects on human RBCs, and delineate the underlying biochemical mechanisms.

Methods

Cytotoxicity was detected using the MTT assay, while hemoglobin leakage was used as a surrogate for hemolysis which was photometrically measured. Major eryptotic events were examined using flow cytometry with fluorescent probes. Phosphatidylserine (PS) exposure was detected by Annexin-V-FITC, cytosolic calcium with Fluo4/AM, and reactive oxygen species with H₂DCFDA.

Results

Our results show that ACN induces hemolysis in a dose-dependent fashion, which is significantly abrogated in absence of extracellular calcium. Moreover, ACN stimulates PS exposure, intracellular calcium overload, and oxidative stress. Using small-molecule inhibitors, we demonstrate that the pro-eryptotic activity of ACN is ameliorated in presence of zVAD(OMe)-FMK, SB203580, and D4476.

Conclusion

ACN possesses both hemolytic and eryptotic properties mediated through elevated intracellular calcium levels, oxidative stress, caspase, p38 MAPK, and CK1α.

Genetic disruption of KCC cotransporters in a mouse model of thalassemia intermedia

β-thalassemia (β-Thal) is caused by defective β-globin production leading to globin chain imbalance, aggregation of free alpha chain in developing erythroblasts, reticulocytes, and mature circulating red blood cells. The hypochromic thalassemic red cells exhibit increased cell dehydration in association with elevated K+ leak and increased K-Cl cotransport activity, each of which has been linked to globin chain imbalance and related oxidative stress. We therefore tested the effect of genetic inactivation of K-Cl cotransporters KCC1 and KCC3 in a mouse model of β-thalassemia intermedia. In the absence of these transporters, the anemia of β-Thal mice was ameliorated, in association with increased MCV and reductions in CHCM and hyperdense cells, as well as in spleen size. The resting K+ content of β-Thal red cells was greatly increased, and Thal-associated splenomegaly slightly decreased. Lack of KCC1 and KCC3 activity in Thal red cells reduced red cell density and improved β-Thal-associated osmotic fragility. We conclude that genetic inactivation of K-Cl cotransport can reverse red cell dehydration and partially attenuate the hematologic phenotype in a mouse model of β-thalassemia.

p38MAPK controls fibroblast growth factor 23 (FGF23) synthesis in UMR106-osteoblast-like cells and in IDG-SW3 osteocytes

BACKGROUND

p38 mitogen-activated protein kinase (p38MAPK) is a serine/threonine kinase activated by cellular stress stimuli including radiation, osmotic shock, and inflammation and influencing apoptosis, cell proliferation, and autophagy. Moreover, p38MAPK induces transcriptional activity of the transcription factor complex NFκB mediating multiple pro-inflammatory cellular responses. Fibroblast growth factor 23 (FGF23) is produced by bone cells, and regulates renal phosphate and vitamin D metabolism as a hormone. FGF23 expression is enhanced by NFκB. Here, we analyzed the relevance of p38MAPK activity for the production of FGF23.

METHODS

Fgf23 expression was analyzed by qRT-PCR and FGF23 protein by ELISA in UMR106 osteoblast-like cells and in IDG-SW3 osteocytes.

RESULTS

Inhibition of p38MAPK with SB203580 or SB202190 significantly down-regulated Fgf23 expression and FGF23 protein expression. Conversely, p38MAPK activator anisomycin increased the abundance of Fgf23 mRNA. NFκB inhibitors wogonin and withaferin A abrogated the stimulatory effect of anisomycin on Fgf23 gene expression.

CONCLUSION

p38MAPK induces FGF23 formation, an effect at least in part dependent on NFκB activity.

Genetic deficiency of the tumor suppressor protein p53 influences erythrocyte survival

The transcription factor p53 suppresses tumor growth by inducing nucleated cell apoptosis and cycle arrest. Because of its influence on primitive erythroid cell differentiation and survival, p53 is an important determinant of erythropoiesis. However, the impact of p53 on the fate of erythrocytes, cells lacking nucleus and mitochondria, during their post-maturation phase in the circulation remained elusive. Erythrocyte survival may be compromised by suicidal erythrocyte death or eryptosis, which is hallmarked by phosphatidylserine translocation and stimulated by increase of cytosolic Ca²⁺ concentration. Here, we comparatively examined erythrocyte homeostasis in p53-mutant mice (Trp53^tm1Tyj/J) and in corresponding WT mice (C57BL/6J) by analyzing eryptosis and erythropoiesis. To this end, spontaneous cell membrane phosphatidylserine exposure and cytosolic Ca²⁺ concentration were higher in erythrocytes drawn from Trp53^tm1Tyj/J mice than from WT mice. Eryptosis induced by glucose deprivation, a pathophysiological cell stressor, was slightly, but significantly more prominent in erythrocytes drawn from Trp53^tm1Tyj/J mice as compared to WT mice. The loss of erythrocytes by eryptosis was fully compensated by enhanced erythropoiesis in Trp53^tm1Tyj/J mice, as reflected by increased reticulocytosis and abundance of erythroid precursor cells in the bone marrow. Accordingly, erythrocyte number, packed cell volume and hemoglobin were similar in Trp53^tm1Tyj/J and WT mice. Taken together, functional p53 deficiency enhances the turnover of circulating erythrocytes by parallel increase of eryptosis and stimulated compensatory erythropoiesis.

Disruption of erythrocyte membrane asymmetry by triclosan is preceded by calcium dysregulation and p38 MAPK and RIP1 stimulation

Triclosan (TCS) is a broad-spectrum antimicrobial used in personal care products, household items, and medical devices. Owing to its apoptotic potential against tumor cells, TCS has been proposed for the treatment of malignancy. A major complication of chemotherapy is anemia, which may result from direct erythrocyte hemolysis or premature cell death known as eryptosis. Similar to nucleated cells, eryptotic cells lose membrane asymmetry and Ca²⁺ regulation, and undergo oxidative stress, shrinkage, and activation of a host of kinases. In this report, we sought to examine the hemolytic and eryptotic potential of TCS and dissect the underlying mechanistic scenarios involved there in. Hemolysis was spectrophotometrically evaluated by the degree of hemoglobin release into the medium. Flow cytometry was utilized to detect phosphatidylserine (PS) exposure by annexin-V binding, intracellular Ca²⁺ by Fluo-3/AM fluorescence, and oxidative stress by 2-,7-dichlorodihydrofluorescin diacetate (DCFH₂-DA). Incubation of cells with 10-100 μM TCS for 1-4 h induced time- and dose-dependent hemolysis. Moreover, TCS significantly increased the percentage of eryptotic cells as evident by PS exposure (significantly enhanced annexin-V binding). Interestingly, TCS-induced eryptosis was preceded by elevated intracellular Ca²⁺ levels but was not associated with oxidative stress. Cotreatment of erythrocytes with 50 μM TCS and 50 μM SB203580 (p38 MAPK inhibitor), or 300 μM necrostatin-1 (receptor-interacting protein 1 (RIP1) inhibitor) significantly ameliorated TCS-induced PS externalization. We conclude that TCS is cytotoxic to erythrocytes by inducing hemolysis and stimulating premature death at least in part through Ca²⁺ mobilization, and p38 MAPK and RIP1 activation.

In vitro assessment of eryptotic potential of tetrabromobisphenol A and other bromophenolic flame retardants

Brominated flame retardants (BFRs) such as tetrabromobisphenol A (TBBPA) and tetrabromobisphenol S (TBBPS) as well as bromophenols, i.e. 2,4-dibromophenol (2,4-DBP), 2,4,6-tribromophenol (2,4,6-TBP) and pentabromophenol (PBP) have raised wide concerns due to their widespread occurrence in the environment and adverse effects observed in living organisms including human. The effect of BFRs on apoptosis of human erythrocytes has not been examined, that is why we have decided to assess eryptotic potential of these substances by determining changes in phosphatidylserine (PS) translocation, alterations in intracellular ROS and calcium ion levels, as well as caspase-3 and calpain activation in this cell type. It has been found that all BFRs studied even in the concentration of 0.001 μg/mL induced ROS formation. The compounds examined caused apoptosis by PS externalization and caspase-3 activation in human red blood cells. It has also been shown that calcium ions and calpain did not play a significant role in eryptosis induction by BFRs studied in human erythrocytes.

Manipulating Eryptosis of Human Red Blood Cells: A Novel Antimalarial Strategy?

Malaria is a major global health burden, affecting over 200 million people worldwide. Resistance against all currently available antimalarial drugs is a growing threat, and represents a major and long-standing obstacle to malaria eradication. Like many intracellular pathogens, Plasmodium parasites manipulate host cell signaling pathways, in particular programmed cell death pathways. Interference with apoptotic pathways by malaria parasites is documented in the mosquito and human liver stages of infection, but little is known about this phenomenon in the erythrocytic stages. Although mature erythrocytes have lost all organelles, they display a form of programmed cell death termed eryptosis. Numerous features of eryptosis resemble those of nucleated cell apoptosis, including surface exposure of phosphatidylserine, cell shrinkage and membrane ruffling. Upon invasion, Plasmodium parasites induce significant stress to the host erythrocyte, while delaying the onset of eryptosis. Many eryptotic inducers appear to have a beneficial effect on the course of malaria infection in murine models, but major gaps remain in our understanding of the underlying molecular mechanisms. All currently available antimalarial drugs have parasite-encoded targets, which facilitates the emergence of resistance through selection of mutations that prevent drug-target binding. Identifying host cell factors that play a key role in parasite survival will provide new perspectives for host-directed anti-malarial chemotherapy. This review focuses on the interrelationship between Plasmodium falciparum and the eryptosis of its host erythrocyte. We summarize the current knowledge in this area, highlight the different schools of thoughts and existing gaps in knowledge, and discuss future perspectives for host-directed therapies in the context of antimalarial drug discovery.

Oxidative stress, eryptosis and anemia: a pivotal mechanistic nexus in systemic diseases

The average lifespan of circulating erythrocytes usually exceeds hundred days. Prior to that, however, erythrocytes may be exposed to oxidative stress in the circulation which could cause injury and trigger their suicidal death or eryptosis. Oxidative stress activates Ca²⁺ -permeable nonselective cation channels in the cell membrane, thus, stimulating Ca²⁺ entry and subsequent cell membrane scrambling resulting in phosphatidylserine exposure and activation of Ca²⁺ -sensitive K⁺ channels leading to K⁺ exit, hyperpolarization, Cl^- exit, and ultimately cell shrinkage due to loss of KCl and osmotically driven water. While the mechanistic link between oxidative stress and anemia remains ill-defined, several diseases such as diabetes, hepatic failure, malignancy, chronic kidney disease and inflammation have been identified to display both increased oxidative stress as well as eryptosis. Recent compelling evidence suggests that oxidative stress is an important perpetrator in accelerating erythrocyte loss in different systemic conditions and an underlying mechanism for anemia associated with these pathological states. In the present review, we discuss the role of oxidative stress in reducing erythrocyte survival and provide novel insights into the possible use of antioxidants as putative antieryptotic and antianemic agents in a variety of systemic diseases.

Effects of Plant Sterols or β-Cryptoxanthin at Physiological Serum Concentrations on Suicidal Erythrocyte Death

The eryptotic and hemolytic effects of a phytosterol (PS) mixture (β-sitosterol, campesterol, stigmasterol) or β-cryptoxanthin (β-Cx) at physiological serum concentration and their effect against oxidative stress induced by tert-butylhydroperoxide (tBOOH) (75 and 300 μM) were evaluated. β-Cryptoxanthin produced an increase in eryptotic cells, cell volume, hemolysis, and glutathione depletion (GSH) without ROS overproduction and intracellular Ca²⁺ influx. Co-incubation of both bioactive compounds protected against β-Cx-induced eryptosis. Under tBOOH stress, PS prevented eryptosis, reducing Ca²⁺ influx, ROS overproduction and GSH depletion at 75 μM, and hemolysis at both tBOOH concentrations. β-Cryptoxanthin showed no cytoprotective effect. Co-incubation with both bioactive compounds completely prevented hemolysis and partially prevented eryptosis as well as GSH depletion induced by β-Cx plus tBOOH. Phytosterols at physiological serum concentrations help to prevent pro-eryptotic and hemolytic effects and are promising candidate compounds for ameliorating eryptosis-associated diseases.

Eryptosis in health and disease: A paradigm shift towards understanding the (patho)physiological implications of programmed cell death of erythrocytes

During the course of their natural ageing and upon injury, anucleate erythrocytes can undergo an unconventional apoptosis-like cell death, termed eryptosis. Eryptotic erythrocytes display a plethora of morphological alterations including volume reduction, membrane blebbing and breakdown of the membrane phospholipid asymmetry resulting in phosphatidylserine externalization which, in turn, mediates their phagocytic recognition and clearance from the circulation. Overall, the eryptosis machinery is tightly orchestrated by a wide array of endogenous mediators, ion channels, membrane receptors, and a host of intracellular signaling proteins. Enhanced eryptosis shortens the lifespan of circulating erythrocytes and confers a procoagulant phenotype; this phenomenon has been tangibly implicated in the pathogenesis of anemia, deranged microcirculation, and increased prothrombotic risk associated with a multitude of clinical conditions. Herein, we reviewed the molecular mechanisms dictating eryptosis and erythrophagocytosis and critically analyzed the current evidence leading to the pathophysiological ramifications of eryptotic cell death in the context of human disease.

Blunted apoptosis of erythrocytes in mice deficient in the heterotrimeric G-protein subunit Gαi2

Putative functions of the heterotrimeric G-protein subunit Gαi2-dependent signaling include ion channel regulation, cell differentiation, proliferation and apoptosis. Erythrocytes may, similar to apoptosis of nucleated cells, undergo eryptosis, characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine (PS) exposure. Eryptosis may be triggered by increased cytosolic Ca²⁺ activity and ceramide. In the present study, we show that Gαi2 is expressed in both murine and human erythrocytes and further examined the survival of erythrocytes drawn from Gαi2-deficient mice (Gαi2^−/−) and corresponding wild-type mice (Gαi2^+/+). Our data show that plasma erythropoietin levels, erythrocyte maturation markers, erythrocyte counts, hematocrit and hemoglobin concentration were similar in Gαi2^−/− and Gαi2^+/+ mice but the mean corpuscular volume was significantly larger in Gαi2^−/− mice. Spontaneous PS exposure of circulating Gαi2^−/− erythrocytes was significantly lower than that of circulating Gαi2^+/+ erythrocytes. PS exposure was significantly lower in Gαi2^−/− than in Gαi2^+/+ erythrocytes following ex vivo exposure to hyperosmotic shock, bacterial sphingomyelinase or C6 ceramide. Erythrocyte Gαi2 deficiency further attenuated hyperosmotic shock-induced increase of cytosolic Ca²⁺ activity and cell shrinkage. Moreover, Gαi2^−/− erythrocytes were more resistant to osmosensitive hemolysis as compared to Gαi2^+/+ erythrocytes. In conclusion, Gαi2 deficiency in erythrocytes confers partial protection against suicidal cell death.

Ceranib-2-induced suicidal erythrocyte death

Ceramide is known to trigger apoptosis of nucleated cells and eryptosis of erythrocytes. Eryptosis is characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine translocation to the erythrocyte surface. Besides ceramide, stimulators of eryptosis include increase of cytosolic Ca(2+) -activity ([Ca(2+) ]i ) and oxidative stress. Ceramide is degraded by acid ceramidase and inhibition of the enzyme similarly triggers apoptosis. The present study explored, whether ceramidase inhibitor Ceranib-2 induces eryptosis. Flow cytometry was employed to quantify phosphatidylserine-exposure at the cell surface from annexin-V-binding, cell volume from forward scatter, [Ca(2+) ]i from Fluo3-fluorescence, reactive oxygen species (ROS) from DCF dependent fluorescence, and ceramide abundance utilizing specific antibodies. Hemolysis was estimated from hemoglobin concentration in the supernatant. A 48 h exposure of human erythrocytes to Ceranib-2 significantly increased the percentage of annexin-V-binding cells (≥50 μM) and the percentage of hemolytic cells (≥10 μM) without significantly modifying forward scatter. Ceranib-2 significantly increased Fluo3-fluorescence, DCF fluorescence and ceramide abundance. The effect of Ceranib-2 on annexin-V-binding was not significantly blunted by removal of extracellular Ca(2+) . Ceranib-2 triggers phospholipid scrambling of the erythrocyte cell membrane, an effect at least in part due to increase of ceramide abundance and induction of oxidative stress, but not dependent on Ca(2+) entry. Copyright © 2016 John Wiley & Sons, Ltd.

Phosphatidylserine externalization and procoagulant activation of erythrocytes induced by Pseudomonas aeruginosa virulence factor pyocyanin

The opportunistic pathogen Pseudomonas aeruginosa causes a wide range of infections in multiple hosts by releasing an arsenal of virulence factors such as pyocyanin. Despite numerous reports on the pleiotropic cellular targets of pyocyanin toxicity in vivo, its impact on erythrocytes remains elusive. Erythrocytes undergo an apoptosis‐like cell death called eryptosis which is characterized by cell shrinkage and phosphatidylserine (PS) externalization; this process confers a procoagulant phenotype on erythrocytes as well as fosters their phagocytosis and subsequent clearance from the circulation. Herein, we demonstrate that P. aeruginosa pyocyanin‐elicited PS exposure and cell shrinkage in erythrocyte while preserving the membrane integrity. Mechanistically, exposure of erythrocytes to pyocyanin showed increased cytosolic Ca²⁺ activity as well as Ca²⁺‐dependent proteolytic processing of μ‐calpain. Pyocyanin further up‐regulated erythrocyte ceramide abundance and triggered the production of reactive oxygen species. Pyocyanin‐induced increased PS externalization in erythrocytes translated into enhanced prothrombin activation and fibrin generation in plasma. As judged by carboxyfluorescein succinimidyl‐ester labelling, pyocyanin‐treated erythrocytes were cleared faster from the murine circulation as compared to untreated erythrocytes. Furthermore, erythrocytes incubated in plasma from patients with P. aeruginosa sepsis showed increased PS exposure as compared to erythrocytes incubated in plasma from healthy donors. In conclusion, the present study discloses the eryptosis‐inducing effect of the virulence factor pyocyanin, thereby shedding light on a potentially important mechanism in the systemic complications of P. aeruginosa infection.

Bromfenvinphos induced suicidal death of human erythrocytes

The organophosphorus pesticide bromfenvinphos ((E,Z)-O,O-diethyl-O-[1-(2,4-dichlorophenyl)-2-bromovinyl] phosphate) has been shown to decrease hematocrit and hemoglobin levels in blood presumably by triggering oxidative stress of erythrocytes. Oxidative stress is known to activate erythrocytic Ca(2+) permeable unselective cation channels leading to Ca(2+) entry and increase of cytosolic Ca(2+) activity ([Ca(2+)]i), which in turn triggers eryptosis, the suicidal death characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine translocation to the erythrocyte surface. The present study explored, whether and how bromfenvinphos induces eryptosis. To this end, phosphatidylserine exposure at the cell surface was estimated from annexin-V-binding, cell volume from forward scatter, hemolysis from hemoglobin release, [Ca(2+)]i from Fluo3-fluorescence, and ROS formation from DCFDA dependent fluorescence. As a result, a 48hour exposure of human erythrocytes to bromfenvinphos (≥100μM) significantly increased the percentage of annexin-V-binding cells, significantly decreased forward scatter, significantly increased Fluo3-fluorescence, and significantly increased DCFDA fluorescence. The effect of bromfenvinphos on annexin-V-binding and forward scatter was significantly blunted, but not abolished by removal of extracellular Ca(2+). In conclusion, bromfenvinphos triggers cell shrinkage and phospholipid scrambling of the erythrocyte cell membrane, an effect in part due to stimulation of ROS formation and Ca(2+) entry.

Enhanced suicidal erythrocyte death in acute cardiac failure

BACKGROUND

A common complication of acute cardiac failure (AHF) is anaemia, which negatively influences the clinical outcome. Causes of anaemia include enhanced eryptosis, a suicidal erythrocyte death characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine translocation. Signalling triggering eryptosis include oxidative stress, increase of cytosolic Ca(2+) -activity ([Ca(2+) ]i ) and ceramide. The present study explored whether AHF is associated with accelerated eryptosis.

MATERIALS AND METHODS

Erythrocytes were drawn from healthy volunteers (n = 10) and patients hospitalized for AHF (n = 22). Phosphatidylserine exposure was estimated from annexin-V-binding, cell volume from forward scatter, [Ca(2+) ]i from Fluo3-fluorescence, ceramide abundance utilizing specific antibodies and reactive oxygen species (ROS) abundance from 2',7'-dichlorodihydrofluorescein diacetate (DCFDA) fluorescence, as determined by flow cytometry.

RESULTS

In AHF-patients, haemoglobin concentration (11·5 ± 0·5 g/dL), and haematocrit (35·6 ± 1·2%) were significantly lower than haemoglobin concentration (14·1 ± 0·4 g/dL), and haematocrit (40·1 ± 1·0%) in healthy volunteers, even though reticulocyte number was significantly higher in AHF patients (2·3 ± 0·3%) than in healthy volunteers (1·1 ± 0·2%). The percentage of erythrocytes exposing phosphatidylserine was significantly higher in AHF patients (1·8 ± 0·1%) than in healthy volunteers (1·2 ± 0·2%). The forward scatter was significantly lower and the ROS abundance significantly larger in AHF patients than in healthy volunteers. In erythrocytes drawn from healthy volunteers, phosphatidylserine and ROS abundance was increased to significantly higher values following a 24 h treatment with plasma from AHF patients than with plasma from healthy volunteers.

CONCLUSION

AHF leads to anaemia despite increased reticulocyte number and at least partially due to enhanced eryptosis. Underlying mechanisms include oxidative stress imposed by a plasma borne component.

Accelerated apoptotic death and in vivo turnover of erythrocytes in mice lacking functional mitogen- and stress-activated kinase MSK1/2

The mitogen- and stress-activated kinase MSK1/2 plays a decisive role in apoptosis. In analogy to apoptosis of nucleated cells, suicidal erythrocyte death called eryptosis is characterized by cell shrinkage and cell membrane scrambling leading to phosphatidylserine (PS) externalization. Here, we explored whether MSK1/2 participates in the regulation of eryptosis. To this end, erythrocytes were isolated from mice lacking functional MSK1/2 (msk^−/−) and corresponding wild-type mice (msk^+/+). Blood count, hematocrit, hemoglobin concentration and mean erythrocyte volume were similar in both msk^−/− and msk^+/+ mice, but reticulocyte count was significantly increased in msk^−/− mice. Cell membrane PS exposure was similar in untreated msk^−/− and msk^+/+ erythrocytes, but was enhanced by pathophysiological cell stressors ex vivo such as hyperosmotic shock or energy depletion to significantly higher levels in msk^−/− erythrocytes than in msk^+/+ erythrocytes. Cell shrinkage following hyperosmotic shock and energy depletion, as well as hemolysis following decrease of extracellular osmolarity was more pronounced in msk^−/− erythrocytes. The in vivo clearance of autologously-infused CFSE-labeled erythrocytes from circulating blood was faster in msk^−/− mice. The spleens from msk^−/− mice contained a significantly greater number of PS-exposing erythrocytes than spleens from msk^+/+ mice. The present observations point to accelerated eryptosis and subsequent clearance of erythrocytes leading to enhanced erythrocyte turnover in MSK1/2-deficient mice.

Identification of signalling cascades involved in red blood cell shrinkage and vesiculation

Even though red blood cell (RBC) vesiculation is a well-documented phenomenon, notably in the context of RBC aging and blood transfusion, the exact signalling pathways and kinases involved in this process remain largely unknown. We have established a screening method for RBC vesicle shedding using the Ca²⁺ ionophore ionomycin which is a rapid and efficient method to promote vesiculation. In order to identify novel pathways stimulating vesiculation in RBC, we screened two libraries: the Library of Pharmacologically Active Compounds (LOPAC) and the Selleckchem Kinase Inhibitor Library for their effects on RBC from healthy donors. We investigated compounds triggering vesiculation and compounds inhibiting vesiculation induced by ionomycin. We identified 12 LOPAC compounds, nine kinase inhibitors and one kinase activator which induced RBC shrinkage and vesiculation. Thus, we discovered several novel pathways involved in vesiculation including G protein-coupled receptor (GPCR) signalling, the phosphoinositide 3-kinase (PI3K)–Akt (protein kinase B) pathway, the Jak–STAT (Janus kinase–signal transducer and activator of transcription) pathway and the Raf–MEK (mitogen-activated protein kinase kinase)–ERK (extracellular signal-regulated kinase) pathway. Moreover, we demonstrated a link between casein kinase 2 (CK2) and RBC shrinkage via regulation of the Gardos channel activity. In addition, our data showed that inhibition of several kinases with unknown functions in mature RBC, including Alk (anaplastic lymphoma kinase) kinase and vascular endothelial growth factor receptor 2 (VEGFR-2), induced RBC shrinkage and vesiculation.

After screening two libraries of small bioactive molecules and kinase inhibitors, we identified several signalling pathways to be involved in red blood cell (RBC) shrinkage and vesiculation. These include the Jak (Janus kinase)–STAT (signal transducer and activator of transcription) pathway, phosphoinositide 3-kinase (PI3K)–Akt pathway, the Raf–MEK (mitogen-activated protein kinase kinase)–ERK (extracellular signal-regulated kinase) pathway and GPCR (G protein-coupled receptor) signalling.

Triggers, inhibitors, mechanisms, and significance of eryptosis: the suicidal erythrocyte death

Suicidal erythrocyte death or eryptosis is characterized by erythrocyte shrinkage, cell membrane blebbing, and cell membrane scrambling with phosphatidylserine translocation to the erythrocyte surface. Triggers of eryptosis include Ca(2+) entry, ceramide formation, stimulation of caspases, calpain activation, energy depletion, oxidative stress, and dysregulation of several kinases. Eryptosis is triggered by a wide variety of xenobiotics. It is inhibited by several xenobiotics and endogenous molecules including NO and erythropoietin. The susceptibility of erythrocytes to eryptosis increases with erythrocyte age. Phosphatidylserine exposing erythrocytes adhere to the vascular wall by binding to endothelial CXC-Motiv-Chemokin-16/Scavenger-receptor for phosphatidylserine and oxidized low density lipoprotein (CXCL16). Phosphatidylserine exposing erythrocytes are further engulfed by phagocytosing cells and are thus rapidly cleared from circulating blood. Eryptosis eliminates infected or defective erythrocytes thus counteracting parasitemia in malaria and preventing detrimental hemolysis of defective cells. Excessive eryptosis, however, may lead to anemia and may interfere with microcirculation. Enhanced eryptosis contributes to the pathophysiology of several clinical disorders including metabolic syndrome and diabetes, malignancy, cardiac and renal insufficiency, hemolytic uremic syndrome, sepsis, mycoplasma infection, malaria, iron deficiency, sickle cell anemia, thalassemia, glucose 6-phosphate dehydrogenase deficiency, and Wilson's disease. Facilitating or inhibiting eryptosis may be a therapeutic option in those disorders.

Mechanisms and pathophysiological significance of eryptosis, the suicidal erythrocyte death

Eryptosis, the suicidal erythrocyte death characterized by cell shrinkage and cell membrane scrambling, is stimulated by Ca(2+) entry through Ca(2+)-permeable, PGE2-activated cation channels, by ceramide, caspases, calpain, complement, hyperosmotic shock, energy depletion, oxidative stress, and deranged activity of several kinases (e.g. AMPK, GK, PAK2, CK1α, JAK3, PKC, p38-MAPK). Eryptosis is triggered by intoxication, malignancy, hepatic failure, diabetes, chronic renal insufficiency, hemolytic uremic syndrome, dehydration, phosphate depletion, fever, sepsis, mycoplasma infection, malaria, iron deficiency, sickle cell anemia, thalassemia, glucose 6-phosphate dehydrogenase deficiency, and Wilson's disease. Eryptosis may precede and protect against hemolysis but by the same token result in anemia and deranged microcirculation.

Conjugated bilirubin triggers anemia by inducing erythrocyte death

Hepatic failure is commonly associated with anemia, which may result from gastrointestinal bleeding, vitamin deficiency, or liver-damaging diseases, such as infection and alcohol intoxication. At least in theory, anemia during hepatic failure may result from accelerated clearance of circulating erythrocytes. Here we show that bile duct ligation (BDL) in mice leads to severe anemia despite increased reticulocyte numbers. Bilirubin stimulated suicidal death of human erythrocytes. Mechanistically, bilirubin triggered rapid Ca(2+) influx, sphingomyelinase activation, formation of ceramide, and subsequent translocation of phosphatidylserine to the erythrocyte surface. Consistent with our in vitro and in vivo findings, incubation of erythrocytes in serum from patients with liver disease induced suicidal death of erythrocytes in relation to their plasma bilirubin concentration. Consistently, patients with hyperbilirubinemia had significantly lower erythrocyte and significantly higher reticulocyte counts compared to patients with low bilirubin levels.

CONCLUSION

Bilirubin triggers suicidal erythrocyte death, thus contributing to anemia during liver disease.

Triggering of programmed erythrocyte death by alantolactone

The sesquiterpene alantolactone counteracts malignancy, an effect at least in part due to stimulation of suicidal death or apoptosis of tumor cells. Signaling of alantolactone induced apoptosis involves altered gene expression and mitochondrial depolarization. Erythrocytes lack mitochondria and nuclei but may enter suicidal death or eryptosis, which is characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the erythrocyte surface. Cellular mechanisms involved in triggering of eryptosis include increase of cytosolic Ca²⁺-activity ([Ca²⁺]_i) and oxidative stress. The present study explored, whether alantolactone stimulates eryptosis. To this end, erythrocyte volume was estimated from forward scatter, phosphatidylserine-exposure at the erythrocyte surface from FITC-annexin-V-binding, [Ca²⁺]_i from Fluo3-fluorescence, ceramide abundance from binding of fluorescent antibodies, and oxidative stress from 2',7'-dichlorodihydrofluorescein-diacetate (DCFDA) fluorescence. As a result, a 48 h exposure of human erythrocytes to alantolactone (≥20 μM) significantly decreased erythrocyte forward scatter and increased the percentage of annexin-V-binding cells. Alantolactone significantly increased Fluo3 fluorescence (60 μM), ceramide abundance (60 μM) and DCFDA fluorescence (≥40 μM). The effect of alantolactone (60 μM) on annexin-V-binding was not significantly modified by removal of extracellular Ca²⁺. In conclusion, alantolactone stimulates suicidal erythrocyte death or eryptosis, an effect paralleled by increase of [Ca²⁺]_i, ceramide abundance and oxidative stress.

Enhanced eryptosis following juglone exposure

Juglone, a quinone isolated from Juglans mandshurica Maxim, has previously been shown to be effective against malignancy. The effect is at least partially due to stimulation of suicidal death or apoptosis of tumour cells. On the other hand, juglone has been shown to counteract apoptosis, for example, of neurons. In analogy to apoptosis of nucleated cells, erythrocytes may enter eryptosis, a suicidal death characterized by cell shrinkage and breakdown of phosphatidylserine asymmetry of the cell membrane with phosphatidylserine exposure at the erythrocyte surface. Stimulators of eryptosis include increase in cytosolic Ca(2+) activity [(Ca(2+) )i]. This study explored whether juglone stimulates eryptosis. To this end, erythrocyte volume was estimated from forward scatter, phosphatidylserine exposure at the erythrocyte surface from FITC annexin V binding, ceramide abundance from binding of fluorescent antibodies in flow cytometry and cytosolic ATP with a luciferin-luciferase-based assay. As a result, a 24-hr exposure of human erythrocytes to juglone (5 μM) significantly decreased erythrocyte forward scatter. Juglone (1-5 μM) significantly increased the percentage of annexin V binding cells. Juglone (5 μM) significantly increased ceramide abundance at the erythrocyte surface and decreased erythrocyte ATP concentration. The effect of juglone (10 μM) on annexin V binding was slightly but significantly blunted by removal of extracellular Ca(2+) and by addition of protein kinase C (PKC) inhibitor staurosporine (1 μM). In conclusion, juglone stimulates suicidal erythrocyte death or eryptosis at least in part by upregulation of ceramide abundance, energy depletion and activation of PKC.

Reduction of tendon adhesions following administration of Adaprev, a hypertonic solution of mannose-6-phosphate: mechanism of action studies

Repaired tendons may be complicated by progressive fibrosis, causing adhesion formation or tendon softening leading to tendon rupture and subsequent reduced range of motion. There are few therapies available which improve the gliding of damaged tendons in the hand. We investigate the role of Mannose 6-phosphate (M6P) in a 600 mM hypertonic solution (Adaprev) on tendon adhesion formation in vivo using a mouse model of severed tendon in conjunction with analysis of collagen synthesis, cellular proliferation and receptors involved in TGF beta signalling. Cytotoxicity was assessed by measuring tissue residency, mechanical strength and cell viability of tendons after treatment with Adaprev. To elicit potential modes of action, in vitro and ex vivo studies were performed investigating phosphorylation of p38, cell migration and proliferation. Adaprev treatment significantly (p<0.05) reduced the development of adhesions and improved collagen organisation without reducing overall collagen synthesis following tendon injury in vivo. The bioavailability of Adaprev saw a 40% reduction at the site of administration over 45 minutes and tendon fibroblasts tolerated up to 120 minutes of exposure without significant loss of cell viability or tensile strength. These favourable effects were independent of CI-MPR and TGF-β signalling and possibly highlight a novel mechanism of action related to cellular stress demonstrated by phosphorylation of p38. The effect of treatment reduced tendon fibroblast migration and transiently halted tendon fibroblast proliferation in vitro and ex vivo. Our studies demonstrate that the primary mode of action for Adaprev is potentially via a physical, non-chemical, hyperosmotic effect.

Aristolochic acid induced suicidal erythrocyte death

BACKGROUND/AIMS

Aristolochic Acid, a component of Aristolochia plants, has been shown to cause acute kidney injury, renal aristolochic acid nephropathy, Balkan endemic nephropathy, and urothelial carcinoma. Aristolochic acid nephropathy may be associated with severe anemia. The anemia could theoretically be due to stimulation of eryptosis, the suicidal death of erythrocytes characterized by cell shrinkage and cell membrane scrambling with translocation of phosphatidylserine to the erythrocyte cell membrane surface. Signalling involved in the stimulation of eryptosis include increase of cytosolic Ca(2+)-activity ([Ca(2+)]i) and formation of ceramide.

METHODS

Cell volume was estimated from forward scatter, phosphatidylserine-exposure from annexin V binding, [Ca(2+)]i from Fluo3 fluorescence, and ceramide abundance from binding of fluorescent antibodies in flow cytometry.

RESULTS

A 48 hours exposure to Aristolochic Acid (≥ 75 µg/ml) was followed by a significant decrease of forward scatter and increase of annexin-V-binding. The effects were paralleled by a significant increase of [Ca(2+)]i and significantly blunted, but not abrogated by removal of extracellular Ca(2+). Aristolochic Acid further significantly increased ceramide abundance.

CONCLUSIONS

Aristolochic Acid triggers eryptosis, an effect at least in part due to entry of extracellular Ca(2+) and ceramide formation.

Piperlongumine-induced phosphatidylserine translocation in the erythrocyte membrane

BACKGROUND

Piperlongumine, a component of Piper longum fruit, is considered as a treatment for malignancy. It is effective by inducing apoptosis. Mechanisms involved in the apoptotic action of piperlongumine include oxidative stress and activation of p38 kinase. In analogy to apoptosis of nucleated cells, erythrocytes may undergo eryptosis, the suicidal death of erythrocytes characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine-exposure at the erythrocyte surface. Signaling involved in eryptosis include increase of cytosolic Ca²⁺-activity ([Ca²⁺]i), formation of ceramide, oxidative stress and activation of p38 kinase.

METHODS

Cell volume was estimated from forward scatter, phosphatidylserine-exposure from annexin V binding, [Ca²⁺]i from Fluo3 fluorescence, reactive oxygen species from 2',7'-dichlorodihydrofluorescein-diacetate fluorescence, and ceramide abundance from binding of fluorescent antibodies in flow cytometry.

RESULTS

A 48 h exposure to piperlongumine (30 µM) was followed by significant decrease of forward scatter and increase of annexin-V-binding. Piperlongumine did not significantly modify [Ca²⁺]i and the effect was not dependent on presence of extracellular Ca²⁺. Piperlongumine significantly increased ROS formation and ceramide abundance.

CONCLUSIONS

Piperlongumine triggers cell membrane scrambling, an effect independent from entry of extracellular Ca²⁺ but at least partially due to ROS and ceramide formation.

In vitro sensitization of erythrocytes to programmed cell death following baicalein treatment

The polyphenolic flavonoid Baicalein has been shown to trigger suicidal death or apoptosis of tumor cells and is thus considered for the prevention and treatment of malignancy. Similar to apoptosis of nucleated cells, erythrocytes may enter eryptosis, the suicidal erythrocyte death characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine translocation to the erythrocyte surface. Stimulators of eryptosis include increase of cytosolic Ca²⁺-activity ([Ca²⁺]_i) and ceramide. The present study explored whether Baicalein stimulates eryptosis. To this end, forward scatter was taken for measurement of cell volume, annexin-V-binding for phosphatidylserine-exposure, Fluo3 fluorescence for [Ca²⁺]_i and fluorescent antibodies for ceramide abundance. As a result, a 48 h exposure of human erythrocytes to Baicalein was followed by significant decrease of forward scatter (≥10 µM), significant increase of the percentage of annexin-V-binding cells (≥25 µM), significant increase of [Ca²⁺]_i (50 µM) and significant increase of ceramide abundance (50 µM). The effect of Baicalein (50 µM) on annexin-V-binding was significantly blunted but not abrogated by removal of extracellular Ca²⁺. In conclusion, at the concentrations employed, Baicalein stimulates suicidal erythrocyte death or eryptosis, an effect at least in part due to the combined effects of Ca²⁺ entry and ceramide formation.

Human-specific bacterial pore-forming toxins induce programmed necrosis in erythrocytes

A subgroup of the cholesterol-dependent cytolysin (CDC) family of pore-forming toxins (PFTs) has an unusually narrow host range due to a requirement for binding to human CD59 (hCD59), a glycosylphosphatidylinositol (GPI)-linked complement regulatory molecule. hCD59-specific CDCs are produced by several organisms that inhabit human mucosal surfaces and can act as pathogens, including Gardnerella vaginalis and Streptococcus intermedius. The consequences and potential selective advantages of such PFT host limitation have remained unknown. Here, we demonstrate that, in addition to species restriction, PFT ligation of hCD59 triggers a previously unrecognized pathway for programmed necrosis in primary erythrocytes (red blood cells [RBCs]) from humans and transgenic mice expressing hCD59. Because they lack nuclei and mitochondria, RBCs have typically been thought to possess limited capacity to undergo programmed cell death. RBC programmed necrosis shares key molecular factors with nucleated cell necroptosis, including dependence on Fas/FasL signaling and RIP1 phosphorylation, necrosome assembly, and restriction by caspase-8. Death due to programmed necrosis in RBCs is executed by acid sphingomyelinase-dependent ceramide formation, NADPH oxidase- and iron-dependent reactive oxygen species formation, and glycolytic formation of advanced glycation end products. Bacterial PFTs that are hCD59 independent do not induce RBC programmed necrosis. RBC programmed necrosis is biochemically distinct from eryptosis, the only other known programmed cell death pathway in mature RBCs. Importantly, RBC programmed necrosis enhances the growth of PFT-producing pathogens during exposure to primary RBCs, consistent with a role for such signaling in microbial growth and pathogenesis.

IMPORTANCE

In this work, we provide the first description of a new form of programmed cell death in erythrocytes (RBCs) that occurs as a consequence of cellular attack by human-specific bacterial toxins. By defining a new RBC death pathway that shares important components with necroptosis, a programmed necrosis module that occurs in nucleated cells, these findings expand our understanding of RBC biology and RBC-pathogen interactions. In addition, our work provides a link between cholesterol-dependent cytolysin (CDC) host restriction and promotion of bacterial growth in the presence of RBCs, which may provide a selective advantage to human-associated bacterial strains that elaborate such toxins and a potential explanation for the narrowing of host range observed in this toxin family.

Stimulation of erythrocyte cell membrane scrambling by nystatin

The antifungal ionophore nystatin dissipates the Na(+) and K(+) gradients across the cell membrane, leading to cellular gain of Na(+) and cellular loss of K(+) . The increase of cellular Na(+) concentration may result in Ca(2+) accumulation in exchange for Na(+) . Increase of cytosolic Ca(2+) activity ([Ca(2+) ]i ) and loss of cellular K(+) foster apoptosis-like suicidal erythrocyte death or eryptosis, which is characterised by cell shrinkage and cell membrane scrambling leading to phosphatidylserine exposure at the erythrocyte surface. The present study explored whether nystatin stimulates eryptosis. Cell volume was estimated from forward scatter (FSC), phosphatidylserine exposure from annexin V binding and [Ca(2+) ]i from Fluo3-fluorescence in flow cytometry. A 48-hr exposure to nystatin (15 μg/ml) was followed by a significant increase of [Ca(2+) ]i , a significant increase of annexin V binding and a significant decrease of FSC. The annexin V binding after nystatin treatment was significantly blunted in the nominal absence of extracellular Ca(2+) . Partial replacement of extracellular Na(+) with extracellular K(+) blunted the nystatin-induced erythrocyte shrinkage but increased [Ca(2+) ]i and annexin V binding. Nystatin triggers cell membrane scrambling, an effect at least partially due to entry of extracellular Ca(2+) .

Oxidative stress and suicidal erythrocyte death

SIGNIFICANCE

Eryptosis, the suicidal erythrocyte death, is characterized by cell shrinkage, membrane blebbing, and phosphatidylserine translocation to the outer membrane leaflet. Phosphatidylserine at the erythrocyte surface binds endothelial CXCL16/SR-PSOX (CXC-Motiv-Chemokin-16/Scavenger-receptor-for-phosphatidylserine-and-oxidized-low-density-lipoprotein) and fosters engulfment of affected erythrocytes by phagocytosing cells. Eryptosis serves to eliminate infected or defective erythrocytes, but excessive eryptosis may lead to anemia and may interfere with microcirculation. Clinical conditions with excessive eryptosis include diabetes, chronic renal failure, hemolytic uremic syndrome, sepsis, malaria, iron deficiency, sickle cell anemia, thalassemia, glucose 6-phosphate dehydrogenase deficiency, glutamate cysteine ligase modulator deficiency, and Wilson's disease.

RECENT ADVANCES

Eryptosis is triggered by a wide variety of xenobiotics and other injuries such as oxidative stress. Signaling of eryptosis includes prostaglandin E₂ formation with subsequent activation of Ca(2+)-permeable cation channels, Ca(2+) entry, activation of Ca(2+)-sensitive K(+) channels, and cell membrane scrambling, as well as phospholipase A2 stimulation with release of platelet-activating factor, sphingomyelinase activation, and ceramide formation. Eryptosis may involve stimulation of caspases and calpain with subsequent degradation of the cytoskeleton. It is regulated by AMP-activated kinase, cGMP-dependent protein kinase, Janus-activated kinase 3, casein kinase 1α, p38 kinase, and p21-activated kinase 2. It is inhibited by erythropoietin, antioxidants, and further small molecules.

CRITICAL ISSUES

It remains uncertain for most disorders whether eryptosis is rather beneficial because it precedes and thus prevents hemolysis or whether it is harmful because of induction of anemia and impairment of microcirculation.

FUTURE DIRECTIONS

This will address the significance of eryptosis, further mechanisms underlying eryptosis, and additional pharmacological tools fostering or inhibiting eryptosis.

In vitro induction of erythrocyte phosphatidylserine translocation by the natural naphthoquinone shikonin

Shikonin, the most important component of Lithospermum erythrorhizon, has previously been shown to exert antioxidant, anti-inflammatory, antithrombotic, antiviral, antimicrobial and anticancer effects. The anticancer effect has been attributed to the stimulation of suicidal cell death or apoptosis. Similar to the apoptosis of nucleated cells, erythrocytes may experience eryptosis, the suicidal erythrocyte death characterized by cell shrinkage and by phosphatidylserine translocation to the erythrocyte surface. Triggers of eryptosis include the increase of cytosolic Ca²⁺-activity ([Ca²⁺]_i) and ceramide formation. The present study explored whether Shikonin stimulates eryptosis. To this end, Fluo 3 fluorescence was measured to quantify [Ca²⁺]_i, forward scatter to estimate cell volume, annexin V binding to identify phosphatidylserine-exposing erythrocytes, hemoglobin release to determine hemolysis and antibodies to quantify ceramide abundance. As a result, a 48 h exposure of human erythrocytes to Shikonin (1 µM) significantly increased [Ca²⁺]_i, increased ceramide abundance, decreased forward scatter and increased annexin V binding. The effect of Shikonin (1 µM) on annexin V binding was significantly blunted, but not abolished by the removal of extracellular Ca²⁺. In conclusion, Shikonin stimulates suicidal erythrocyte death or eryptosis, an effect at least partially due to the stimulation of Ca²⁺ entry and ceramide formation.

Triggering of erythrocyte cell membrane scrambling by salinomycin

Salinomycin, a polyether ionophore antibiotic effective against a variety of pathogens, has been shown to trigger apoptosis of cancer cells and cancer stem cells. The substance is thus considered for the treatment of malignancy. Salinomycin compromises tumour cell survival at least in part by interference with mitochondrial function. Erythrocytes lack mitochondria but may undergo apoptosis-like suicidal cell death or eryptosis, which is characterized by scrambling of the cell membrane with phosphatidylserine exposure at the erythrocyte surface. Signalling involved in the triggering of eryptosis includes activation of oxidant-sensitive Ca(2+) permeable cation channels with subsequent increase in cytosolic Ca(2+) activity ([Ca(2+)]i). This study explored whether salinomycin stimulates eryptosis. Phosphatidylserine-exposing erythrocytes were identified by measurement of annexin-V binding, cell volume was estimated from forward scatter, haemolysis determined from haemoglobin release, [Ca(2+)]i quantified utilizing Fluo3-fluorescence and oxidative stress from 2',7' dichlorodihydrofluorescein diacetate (DCFDA) fluorescence in flow cytometry. A 48-hr exposure to salinomycin (5-100 nM) was followed by a significant increase in Fluo3-fluorescence, DCFDA fluorescence and annexin-V binding, as well as a significant decrease in forward scatter (at 5-10 nM, but not at 50 and 100 nM). The annexin-V binding after salinomycin treatment was significantly blunted but not abrogated in the nominal absence of extracellular Ca(2+) or in the presence of antioxidant n-acetyl cysteine (1 mM). Salinomycin triggers cell membrane scrambling, an effect at least partially due to oxidative stress and entry of extracellular Ca(2+).