Janus kinase 3 is expressed in erythrocytes, phosphorylated upon energy depletion and involved in the regulation of suicidal erythrocyte death

Effect of dermaseptin on erythrocytes

Dermaseptin, an antimicrobial peptide participating in the host defence against pathogens, interacts with the membrane of target cells, leading to membrane permeabilization and eventual cell lysis. Dermaseptin has previously been shown to trigger haemolysis. Prior to haemolysis, erythrocytes may enter suicidal death or eryptosis, which is characterized by cell shrinkage and by cell membrane scrambling leading to phosphatidylserine exposure at the erythrocyte surface. Triggers of eryptosis include increase in cytosolic Ca²⁺ activity [(Ca²⁺)](i) and formation of ceramide. This study explored whether dermaseptin modifies [Ca²⁺](i) and elicits eryptosis. Cell volume has been estimated from forward scatter, phosphatidylserine exposure from annexin-V binding, haemolysis from haemoglobin release, ceramide formation from binding of fluorescent antibodies and [Ca²⁺](i) from Fluo3-fluorescence. A 48-hr exposure to dermaseptin (50 μM) was followed by a significant increase in [Ca²⁺](i), a significant increase ceramide abundance, a significant decrease in forward scatter and a significant increase in annexin-V binding. The annexin-V binding after dermaseptin treatment was significantly blunted but not abrogated in the nominal absence of extracellular Ca²⁺. Dermaseptin triggers eryptosis, an effect at least partially due to entry of extracellular Ca²⁺.

Effect of miltefosine on erythrocytes

BACKGROUND

Miltefosine, an alkylphosphocholine drug with antiparasite, antibacterial, antifungal and antineoplastic potency, is the only oral drug that can be used to treat visceral and cutaneous leishmaniasis. The effect of miltefosine is at least partially due to triggering of apoptosis. Similar to apoptosis of nucleated cells, erythrocytes may enter suicidal death or eryptosis, which is characterized by cell shrinkage and by cell membrane scrambling with phosphatidylserine-exposure at the erythrocyte surface. Eryptosis may be triggered following increase of cytosolic Ca(2+)-level ([Ca(2+)]i). The present study explored, whether miltefosine elicits eryptosis.

METHODS

Cell volume has been estimated from forward scatter, phosphatidylserine-exposure from annexin-V-binding, hemolysis from hemoglobin release, [Ca(2+)]i from Fluo3-fluorescence.

RESULTS

A 48 h exposure to miltefosine (≥ 4.9 μM) was followed by significant decrease of forward scatter and significant increase of annexin-V-binding. The effect was paralleled by significant increase of [Ca(2+)]i. The annexin-V-binding following miltefosine treatment was significantly blunted in the nominal absence of extracellular Ca(2+).

CONCLUSION

Miltefosine stimulates eryptosis, an effect at least partially due to stimulation of Ca(2+) entry.

Effect of honokiol on erythrocytes

Honokiol ((3,5-di-(2-propenyl)-1,1-biphenyl-2,2-diol), a component of Magnolia officinalis, stimulates apoptosis and is thus considered for the treatment of malignancy. In analogy to apoptosis of nucleated cells, erythrocytes may enter eryptosis, a suicidal death characterized by cell shrinkage and by breakdown of cell membrane phosphatidylserine asymmetry with phosphatidylserine-exposure at the erythrocyte surface. Eryptosis may be triggered following increase of cytosolic Ca(2+)-activity ([Ca(2+)]i). The present study explored, whether honokiol elicits eryptosis. Cell volume has been estimated from forward scatter, phosphatidylserine-exposure from annexin V binding, hemolysis from hemoglobin release, [Ca(2+)]i from Fluo3-fluorescence, and ceramide from fluorescent antibodies. As a result, a 48 h exposure to honokiol was followed by a slight but significant increase of [Ca(2+)]i (15 μM), significant decrease of forward scatter (5 μM), significant increase of annexin-V-binding (5 μM) and significant increase of ceramide formation (15 μM). Honokiol further induced slight, but significant hemolysis. Honokiol (15 μM) induced annexin-V-binding was significantly blunted but not abrogated in the nominal absence of extracellular Ca(2+). In conclusion, honokiol triggers suicidal erythrocyte death or eryptosis, an effect at least in part due to stimulation of Ca(2+) entry and ceramide formation.

Carmustine-induced phosphatidylserine translocation in the erythrocyte membrane

The nitrosourea alkylating agent, carmustine, is used as chemotherapeutic drug in several malignancies. The substance triggers tumor cell apoptosis. Side effects of carmustine include myelotoxicity with anemia. At least in theory, anemia could partly be due to stimulation of eryptosis, the suicidal death of erythrocytes, 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 of cytosolic Ca²⁺ activity ([Ca²⁺]i). The present study tested whether carmustine triggers eryptosis. To this end [Ca²⁺]i was estimated from Fluo3 fluorescence, cell volume from forward scatter, phosphatidylserine exposure from annexin V binding, and hemolysis from hemoglobin release. As a result a 48 h exposure to carmustine (≥25 µM) significantly increased [Ca²⁺]i, decreased forward scatter and increased annexin V binding. The effect on annexin V binding was significantly blunted in the absence of extracellular Ca²⁺. In conclusion, carmustine stimulates eryptosis at least partially by increasing cytosolic Ca²⁺ activity.

Ca(2+)-dependent suicidal erythrocyte death following zearalenone exposure

Zearalenone, a cereal mycotoxin with mycoestrogen activity and effect on fertility, is known to trigger apoptosis of a variety of nucleated cell types including hematopoietic progenitor cells. In analogy to apoptosis of nucleated cells, eryptosis, the suicidal death of erythrocytes, leads to cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the erythrocyte surface. The most important stimulator of eryptosis is an increase in cytosolic Ca(2+) activity ([Ca(2+)]i). The present study explored whether zearalenone triggers eryptosis. Erythrocyte volume was estimated from forward scatter, phosphatidylserine exposure at the erythrocyte surface from annexin-V binding, hemolysis from hemoglobin release, and [Ca(2+)]i from Fluo3 fluorescence. A 48-h exposure to zearalenone (≥25 μM) was followed by a significant increase in [Ca(2+)]i and annexin-V binding, and a significant decrease in forward scatter. The effect on annexin-V binding was significantly blunted in the nominal absence of extracellular Ca(2+). Zearalenone stimulates the suicidal erythrocyte death, an effect at least partially due to stimulation of Ca(2+) entry.

Effect of bacterial peptidoglycan on erythrocyte death and adhesion to endothelial cells

Peptidoglycans, bacterial wall components, have previously been shown to trigger eryptosis, the suicidal erythrocyte death, characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Phosphatidylserine exposing erythrocytes adhere to the vascular wall at least partially by interaction of erythrocytic phosphatidylserine with endothelial CXC chemokine ligand 16 (CXCL16). The present study explored whether peptidoglycan exposure fosters the adhesion of erythrocytes to human umbilical vein endothelial cells (HUVEC). To this end, HUVEC were treated for 48 h with peptidoglycan (10 μg/ml) and CXCL16 abundance determined by confocal microscopy and FACS analysis. Moreover, human erythrocytes were exposed for 48 h to peptidoglycan (10 μg/ml) and phosphatidylserine exposure estimated from binding of fluorescent annexin-V, cell volume from forward scatter in FACS analysis and erythrocyte adhesion to human umbilical vein endothelial cells (HUVEC) from trapping of labeled erythrocytes in a flow chamber. As a result, bacterial peptidoglycan exposure was followed by increased CXCL16 expression in HUVEC as well as erythrocyte shrinkage, phosphatidylserine exposure and adhesion to HUVEC under flow conditions at arterial shear rates. The adhesion was significantly attenuated but not abrogated in the presence of either, erythrocyte phosphatidylserine-coating annexin-V (5 μl/ml) or CXCL16 neutralizing antibody directed against endothelial CXCL16 (4 μg/ml). In conclusion, exposure to peptidoglycan increases endothelial CXCL16 expression and leads to eryptosis followed by phosphatidylserine- and CXCL16-mediated adhesion of eryptotic erythrocytes to vascular endothelial cells.

Adhesion of klotho-deficient eryptotic erythrocytes to endothelial cells

AIM

Suicidal erythrocyte death or eryptosis is characterized by cell shrinkage and phosphatidylserine exposure at the cell surface. Eryptotic erythrocytes may adhere to the vascular wall by binding of phosphatidylserine to endothelial CXC chemokine ligand 16 (CXCL16). Triggers of eryptosis include osmotic shock or energy depletion. Susceptibility to eryptosis is modified by Klotho, a protein with profound effect on ageing and lifespan. Klotho deficiency leads to accelerated ageing and early death. The percentage of eryptotic erythrocytes is significantly larger in klotho-deficient mice (klotho(-/-) ) than in their wild-type littermates (klotho(+/+) ). The present study explored whether the accelerated eryptosis of klotho-deficient mice is paralleled by enhanced adhesion.

METHODS

Phosphatidylserine-exposing erythrocytes were identified by measurement of annexin V binding and adhesion to human umbilical vein endothelial cells (HUVEC) from trapping of labelled erythrocytes in a flow chamber.

RESULTS

Annexin V binding was higher in klotho(-/-) erythrocytes than in klotho(+/+) erythrocytes. Osmotic shock for 1 h (addition of 550 mm sucrose) and energy depletion (12-h glucose depletion) increased annexin V binding to values again significantly larger in klotho(-/-) erythrocytes than in klotho(+/+) erythrocytes. klotho(-/-) erythrocytes were particularly sensitive to osmotic shock. Both osmotic shock and energy depletion enhanced erythrocyte adhesion, an effect again more pronounced in klotho(-/-) erythrocytes than in klotho(+/+) erythrocytes. The adhesion was significantly decreased by coating of phospatidylserine with annexin V (5 μL mL(-1) ) or by coating of CXCL16 with neutralizing antibodies (4 μg mL(-1) ).

CONCLUSIONS

klotho(-/-) erythrocytes are particularly sensitive to osmotic shock, and enhanced eryptosis of klotho(-/-) erythrocytes is paralleled by enhanced adhesion to endothelial CXCL16.

Effect of chloride channel inhibitors on cytosolic Ca2+ levels and Ca2+-activated K+ (Gardos) channel activity in human red blood cells

DIDS, NPPB, tannic acid (TA) and AO1 are widely used inhibitors of Cl(-) channels. Some Cl(-) channel inhibitors (NPPB, DIDS, niflumic acid) were shown to affect phosphatidylserine (PS) scrambling and, thus, the life span of human red blood cells (hRBCs). Since a number of publications suggest Ca(2+) dependence of PS scrambling, we explored whether inhibitors of Cl(-) channels (DIDS, NPPB) or of Ca(2+)-activated Cl(-) channels (DIDS, NPPB, TA, AO1) modified intracellular free Ca(2+) concentration ([Ca(2+)]i) and activity of Ca(2+)-activated K(+) (Gardos) channel in hRBCs. According to Fluo-3 fluorescence in flow cytometry, a short treatment (15 min, +37 °C) with Cl(-) channels inhibitors decreased [Ca(2+)]i in the following order: TA > AO1 > DIDS > NPPB. According to forward scatter, the decrease of [Ca(2+)]i was accompanied by a slight but significant increase in cell volume following DIDS, NPPB and AO1 treatments. TA treatment resulted in cell shrinkage. According to whole-cell patch-clamp experiments, TA activated and NPPB and AO1 inhibited Gardos channels. The Cl(-) channel blockers further modified the alterations of [Ca(2+)]i following ATP depletion (glucose deprivation, iodoacetic acid, 6-inosine), oxidative stress (1 mM t-BHP) and treatment with Ca(2+) ionophore ionomycin (1 μM). The ability of the Cl(-) channel inhibitors to modulate PS scrambling did not correlate with their influence on [Ca(2+)]i as TA and AO1 had a particularly strong decreasing effect on [Ca(2+)]i but at the same time enhanced PS exposure. In conclusion, Cl(-) channel inhibitors affect Gardos channels, influence Ca(2+) homeostasis and induce PS exposure of hRBCs by Ca(2+)-independent mechanisms.

Adhesion of annexin 7 deficient erythrocytes to endothelial cells

Annexin 7 deficiency has previously been shown to foster suicidal death of erythrocytes or eryptosis, which is triggered by increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) and characterized by cell shrinkage and cell membrane scrambling with subsequent phosphatidylserine exposure at the cell surface. Eryptosis following increase of [Ca(2+)](i) by Ca(2+) ionophore ionomycin, osmotic shock or energy depletion was more pronounced in erythrocytes from annexinA7-deficient mice (anxA7(-/-)) than in erythrocytes from wild type mice (anxA7(+/+)). As phosphatidylserine exposure is considered to mediate adhesion of erythrocytes to the vascular wall, the present study explored adhesion of erythrocytes from anx7(-/-) and anx7(+/+)-mice following increase of [Ca(2+)](i) by Ca(2+) ionophore ionomycin (1 µM for 30 min), hyperosmotic shock (addition of 550 mM sucrose for 2 hours) or energy depletion (removal of glucose for 12 hours). Phosphatidylserine exposing erythrocytes were identified by annexin V binding, cell volume estimated from forward scatter in FACS analysis and adhesion to human umbilical vein endothelial cells (HUVEC) utilizing a flow chamber. As a result, ionomycin, sucrose addition and glucose removal all triggered phosphatidylserine-exposure, decreased forward scatter and enhanced adhesion of erythrocytes to human umbilical vein endothelial cells (HUVEC), effects significantly more pronounced in anx7(-/-) than in anx7(+/+)-erythrocytes. Following ischemia, morphological renal injury was significantly higher in anx7(-/-) than in anx7(+/+)-mice. The present observations demonstrate that enhanced eryptosis of annexin7 deficient cells is paralleled by increased adhesion of erythrocytes to the vascular wall, an effect, which may impact on microcirculation during ischemia.

Stimulation of suicidal erythrocyte death by ipratropium bromide

BACKGROUND/AIMS

Ipratropium bromide, an anticholinergic agent widely used in obstructive lung disease, has previously been shown to trigger suicidal death of nucleated cells or apoptosis. Despite their lack of mitochondria and nuclei, key organelles in the execution of apoptosis, erythrocytes may similarly undergo suicidal cell death, which is characterized by cell shrinkage and by cell membrane scrambling with phosphatidylserine-exposure at the cell surface. Triggers of eryptosis include increase of cytosolic Ca(2+)-activity ([Ca(2+)](i)). The present study explored whether ipratropium bromide triggers eryptosis.

METHODS

[Ca Ca(2+)](i) was estimated utilizing Fluo3 fluorescence, cell volume from forward scatter, phosphatidylserine-exposure from annexin-V-binding, and hemolysis from hemoglobin release.

RESULTS

A 48 h exposure to ipratropium bromide (1 nM) significantly increased [Ca(2+)](i), decreased forward scatter and increased annexin-V-binding. Ipratropium bromide treatment was followed by slight but significant increase of hemolysis. Removal of extracellular Ca(2+) or inhibition of Ca(2+) permeable cation channels with amiloride (1 mM) virtually abolished cell membrane scrambling. Ca(2+) ionophore ionomycin (1 µM, 30 min) increased the percentage of phosphatidylserine exposing erythrocytes to similarly high levels in the absence and presence of ipratropium bromide (1 nM).

CONCLUSIONS

Ipratropium bromide triggers suicidal erythrocyte death or eryptosis, an effect mainly due to stimulation of Ca(2+)-entry.

Stimulation of suicidal erythrocyte death by fumagillin

Fumagillin, a cyclohexane isolated from fungus Aspergillus fumigatus, has anti-infective and anti-cancer potency. Fumagillin is at least partially effective by inducing suicidal death or apoptosis. In analogy to apoptosis of nucleated cells, eryptosis is the suicidal death of erythrocytes characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Stimulators of eryptosis include increase of cytosolic Ca(2+)-activity ([Ca(2+)](i)) and ceramide. The present study explored whether fumagillin (5-100 μM) could stimulate eryptosis. To this end, [Ca(2+)](i) was estimated from Fluo3 fluorescence, ceramide by utilizing specific antibodies, cell volume from forward scatter, phosphatidylserine exposure from annexin V binding and haemolysis from haemoglobin release. As a result, a 48-hr exposure to fumagillin significantly increased [Ca(2+)](i) (≥10 μM), enhanced ceramide abundance (100 μM), triggered annexin V binding (≥10 μM) and decreased forward scatter (≥10 μM). Fumagillin exposure was followed by slight but significant increase of haemolysis. Removal of extracellular Ca(2+) significantly blunted but did not abolish the effect of fumagillin (100 μM) on annexin V binding. The present observations disclose a novel effect of fumagillin, that is, stimulation of eryptosis, paralleled by Ca(2+) entry, ceramide formation, phosphatidylserine exposure and decrease of cell volume.

Enhanced Ca2+ entry, ceramide formation, and apoptotic death of erythrocytes triggered by plumbagin

Plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone, 1), a natural product from plants with potential anticancer potency, induces apoptosis. Mechanisms involved in 1-induced apoptosis include mitochondrial depolarization, inactivation of NF-κB, and altered expression of anti- and proapoptotic Bcl proteins. Similar to nucleated cells, erythrocytes may undergo suicidal death or eryptosis, which, like apoptosis, results in cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Triggers of eryptosis include increase of cytosolic Ca(2+) activity ([Ca(2+)]i) and ceramide formation. The present study explored whether 1 stimulates eryptosis. Cell volume was estimated from forward scatter, phosphatidylserine exposure from annexin-V-binding, hemolysis from hemoglobin release, [Ca(2+)]i from Fluo-3 fluorescence, and ceramide abundance utilizing antibodies. A 48 h exposure to 1 (2 μM) decreased forward scatter and increased annexin-V-binding significantly, events paralleled by increased [Ca(2+)]i and ceramide formation. Exposure to 1 was followed by a slight but significant increase of hemolysis. Removal of extracellular Ca(2+) slightly, but significantly blunted the effect of 1 (2 μM) on annexin-V-binding. The present observations demonstrate that 1 may trigger suicidal death of erythrocytes, cells devoid of mitochondria and nuclei.

Mitochondrial electron transport chain complex dysfunction in the colonic mucosa in ulcerative colitis

BACKGROUND

Ulcerative colitis (UC) is characterized by an energy deficiency state of the colonic epithelium. This study evaluated mitochondrial electron transport chain (ETC) complex activity in normal and disease mucosa in patients with UC. Alterations in ETC complexes were also investigated in experimental colitis in mice.

METHODS

Biopsies were obtained from macroscopically normal and diseased colonic mucosa of 43 patients with UC and 35 controls undergoing screening colonoscopy and ETC complex activity was assayed biochemically. ETC complex activities were also assayed in colonic epithelial cells isolated from Swiss albino mice with dextran sodium sulfate (DSS)-induced colitis at various stages of induction of colitis. Mucosal nitrite levels and protein carbonyl content were determined.

RESULTS

The activity of Complex II was significantly decreased in colonic biopsies from UC patients compared with controls, while activities of other mitochondrial complex were normal. Complex II activity was equally decreased in diseased and normal mucosa in UC; the degree of reduction did not correlate with clinical, endoscopic, or histological grading of disease activity. In DSS-fed mice, a reduction in activity of Complex IV and Complex II was observed. Activity of other complex was not affected. Administration of aminoguanidine, an inducible nitric oxide synthase (iNOS) inhibitor, attenuated all parameters of colitis as well as the reductions in Complex IV and Complex II activity.

CONCLUSIONS

Reduction in Complex II activity appears to be a specific change in UC, present in quiescent and active disease. Mitochondrial complex dysfunction occurs in DSS colitis in mice and appears to be mediated by nitric oxide.

Enhanced apoptotic death of erythrocytes induced by the mycotoxin ochratoxin A

BACKGROUND

The mycotoxin ochratoxin A, an agent responsible for endemic Balkan nephropathy is known to trigger apoptosis and thus being toxic to several organs including the kidney. The mechanisms involved in ochratoxin A induced apoptosis include oxidative stress. Sequelae of ochratoxin intoxication include anemia. Similar to apoptosis of nucleated cells, erythrocytes may undergo suicidal cell death or eryptosis, which is characterized by cell shrinkage and cell membrane scrambling resulting in phosphatidylserine-exposure at the cell surface. Eryptosis could be triggered by Ca2+ -entry through oxidant sensitive unspecificcation channels increasing cytosolic Ca2+ activity ([Ca2+]i). The Ca2+ -sensitivity of cell membrane scrambling could be enhanced and eryptosis thus triggered by ceramide. The removal of suicidal erythrocytes may lead to anemia. Moreover, eryptotic erythrocytes could adhere to the vascular wall thus impeding microcirculation. The present study explored, whether ochratoxin A stimulates eryptosis.

METHODS

Fluo3-fluorescence was utilized to determine [Ca2+]i, forward scatter to estimate cell volume, annexin-V-binding to identify phosphatidylserine-exposing cells, fluorescent antibodies to detect ceramide formation and hemoglobin release to quantify hemolysis. Moreover, adhesion to human vascular endothelial cells (HUVEC) was determined utilizing a flow chamber.

RESULTS

A 48 h exposure to ochratoxin A was followed by significant increase of Fluo3-fluorescencei (≥ 2.5 µM), increase of ceramide abundance (10 µM), decrease of forward scatter (≥ 5 µM) and increase of annexin-V-binding (≥ 2.5 µM). Ochratoxin A exposure slightly but significantly enhanced hemolysis (10 µM). Ochratoxin (10 µM) enhanced erythrocyte adhesion to HUVEC. Removal of extracellular Ca2+ significantly blunted, but did not abrogate ochratoxin A-induced annexin V binding.

CONCLUSIONS

Ochratoxin A triggers suicidal erythrocyte death or eryptosis, an effect partially but not fully due to stimulation of Ca2+ -entry.

Stimulation of suicidal death of erythrocytes by rifampicin

The antibiotic rifampicin is widely used in the treatment of tuberculosis. Side effects of rifampicin include hemolytic anemia. Loss of circulating erythrocytes resembling hemolytic anemia could result from stimulation of eryptosis, the suicidal erythrocyte death characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine (PS) exposure at the cell surface. Stimulators of eryptosis include increase of cytosolic Ca(2+) activity ([Ca(2+)](i)) and formation of ceramide. The present study explored, whether and, if so, how rifampicin triggers eryptosis. To this end, [Ca(2+)](i) was estimated from Fluo3 fluorescence, cell volume from forward scatter in flow cytometry, PS exposure from annexin binding, ceramide formation from binding of fluorescent antibodies and hemolysis from hemoglobin release. As a result, a 48 h exposure to rifampicin (≥ 24 μg/ml) significantly increased Fluo3 fluorescence, ceramide abundance and annexin binding, and significantly decreased forward scatter. Rifampicin triggered slight, but significant hemolysis. Removal of extracellular Ca(2+) significantly blunted, but did not fully abolish rifampicin induced annexin binding. In conclusion, exposure of human erythrocytes to rifampicin is followed by suicidal erythrocyte death or eryptosis, an effect at least partially due to increase of cytosolic Ca(2+) concentration and stimulation of ceramide formation.

Gossypol-induced suicidal erythrocyte death

Side effects of gossypol, a polyphenolic component of Gossypium, with male contraceptive, anticancer, antimicrobial and antiviral activities include anemia due to accelerated demise of erythrocytes. Erythrocytes may be cleared from circulating blood following apoptosis-like suicidal death or eryptosis. Hallmarks of eryptosis are cell shrinkage and cell membrane scrambling with subsequent phosphatidylserine-exposure at the cell surface. Stimulators of eryptosis include increase of cytosolic Ca(2+)-activity ([Ca(2+)](i)). The present study explored, whether gossypol stimulates eryptosis of human erythrocytes. Utilizing flow cytometry, [Ca(2+)](i) was estimated from Fluo-3 fluorescence, cell volume from forward scatter, phosphatidylserine-exposure from annexin-V-binding, and hemolysis from hemoglobin release. A 48 h exposure to gossypol (0.75 μM) significantly increased [Ca(2+)](i), decreased forward scatter and increased annexin-V-binding. Gossypol exposure was followed by a slight but significant increase of hemolysis. Removal of extracellular Ca(2+) significantly blunted the effect of gossypol (1 μM) on annexin-V-binding. The present observations reveal a novel effect of gossypol on human erythrocytes, which contributes to or even accounts for the triggering of anemia by this substance.

Sulindac sulfide--induced stimulation of eryptosis

BACKGROUND

Sulindac sulfide, a non-steroidal anti-inflammatory drug (NSAID), stimulates apoptosis of tumor cells and is thus effective against malignancy. In analogy to apoptosis of nucleated cells, erythrocytes may undergo eryptosis, an apoptosis-like suicidal erythrocyte death, characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine-exposure at the cell surface. Stimulators of eryptosis include increase of cytosolic Ca(2+)-activity ([Ca(2+)](i)) and ceramide formation. The present study explored, whether sulindac sulfide stimulates eryptosis.

METHODS

[Ca(2+)](i) was estimated from Fluo-3 fluorescence, cell volume from forward scatter, phosphatidylserine-exposure from binding of fluorescent annexin-V, hemolysis from hemoglobin release, and ceramide abundance utilizing fluorescent antibodies.

RESULTS

A 48 h exposure to sulindac sulfide (≤ 20 µM) was followed by significant increase of [Ca(2+)](i), enhanced ceramide abundance, decreased forward scatter and increased percentage of annexin-V-binding erythrocytes. Sulindac sulfide triggered slight but significant hemolysis. Removal of extracellular Ca(2+) significantly blunted, but did not abrogate the effect of sulindac sulfide (20 µM) on annexin-V-binding.

CONCLUSION

Sulindac sulfide stimulates the suicidal death of erythrocytes or eryptosis, an effect paralleled by Ca(2+)-entry, ceramide formation, cell shrinkage and phosphatidylserine-exposure.

Withaferin A-stimulated Ca2+ entry, ceramide formation and suicidal death of erythrocytes

Withaferin A, a triterpenoid component from Withania somnifera, counteracts malignancy, an effect attributed to stimulation of apoptosis. Withaferin A is partially effective through induction of oxidative stress, altered gene expression and mitochondrial depolarization. Erythrocytes lack mitochondria and nuclei but may enter apoptosis-like eryptosis, a suicidal cell death characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Triggers of eryptosis include increase of cytosolic Ca(2+)-activity [Ca(2+)](i) following activation of oxidant-sensitive Ca(2+)-permeable cation channels, ceramide formation and/or ATP-depletion. The present study explored, whether withaferin A triggers eryptosis. To this end, [Ca(2+)](i) was estimated from Fluo3-fluorescence, cell volume from forward scatter, phosphatidylserine exposure from annexin-V-binding, hemolysis from hemoglobin release, oxidative stress from DCFDA-fluorescence and ceramide abundance utilizing antibodies. A 48 h exposure to withaferin A significantly decreased forward scatter (at ≥ 10 μM withaferin concentration) and increased [Ca(2+)](i) (≥ 5 μM), ROS-formation (≥ 10 μM) ceramide-formation ( ≥ 10 μM) as well as annexin-V-binding ( ≥ 5 μM). Withaferin A treatment was followed by slight but significant increase of hemolysis. Extracellular Ca(2+) removal, amiloride, and the antioxidant N-acetyl-l-cysteine significantly blunted withaferin A-triggered annexin-V-binding. The present observations reveal that withaferin A triggers suicidal erythrocyte death despite the absence of gene expression and key elements of apoptosis such as mitochondria.

Physiology and pathophysiology of eryptosis

SUMMARY

Suicidal erythrocyte death (eryptosis) is characterized by cell shrinkage, cell membrane blebbing, and cell membrane phospholipid scrambling with phosphatidylserine exposure at the cell surface. Eryptotic cells adhere to the vascular wall and are rapidly cleared from circulating blood. Eryptosis is stimulated by an increase in cytosolic Ca(2)+ activity, ceramide, hyperosmotic shock, oxidative stress, energy depletion, hyperthermia, and a wide variety of xenobiotics and endogenous substances. Inhibitors of eryptosis include erythropoietin and nitric oxide. Enhanced eryptosis is observed in diabetes, renal insufficiency, hemolytic uremic syndrome, sepsis, mycoplasma infection, malaria, iron deficiency, sickle cell anemia, beta-thalassemia, glucose-6-phosphate dehydrogenase-(G6PD) deficiency, hereditary spherocytosis, paroxysmal nocturnal hemoglobinuria, Wilson's disease, myelodysplastic syndrome, and phosphate depletion. Eryptosis is further enhanced in gene-targeted mice with deficient annexin 7, cGMP-dependent protein kinase type I (cGKI), AMP-activated protein kinase (AMPK), anion exchanger 1 (AE1), adenomatous polyposis coli (APC), and Klotho, as well as in mouse models of sickle cell anemia and thalassemia. Decreased eryptosis is observed in mice with deficient phosphoinositide-dependent kinase 1 (PDK1), platelet activating factor (PAF) receptor, transient receptor potential channel 6 (TRPC6), janus kinase 3 (JAK3), and taurine transporter (TAUT). Eryptosis may be a useful mechanism to remove defective erythrocytes prior to hemolysis. Excessive eryptosis may, however, compromise microcirculation and lead to anemia.

Sphingomyelinase-induced adhesion of eryptotic erythrocytes to endothelial cells

Eryptosis, the suicidal erythrocyte death, leads to cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Eryptotic erythrocytes adhere to the vascular wall by binding of phosphatidylserine to the CXC chemokine ligand 16 (CXCL16). Stimulators of eryptosis include increased cytosolic Ca(2+) activity, energy depletion, and activation of ceramide-producing sphingomyelinase. The present study explored whether sphingomyelinase triggers erythrocyte adhesion to endothelial cells. To this end, human erythrocytes were exposed for 6 h to bacterial sphingomyelinase (1-10 mU/ml) and phosphatidylserine exposure was estimated from fluorescent annexin-V-binding, cell volume from forward scatter in FACS-analysis, erythrocyte adhesion to human umbilical vein endothelial cells (HUVEC) from trapping of labeled erythrocytes in a flow chamber under flow conditions at arterial shear rates, and CXCL16 protein abundance utilizing Western blotting and FACS analysis of fluorescent antibody binding. As a result, sphingomyelinase (≥1 mU/ml) triggered cell shrinkage, phosphatidylserine exposure and erythrocyte adhesion to HUVEC, effects blunted by Ca(2+) removal. Adhesion was significantly blunted by phosphatidylserine-coating annexin-V (5 μl/ml), following addition of neutralizing antibodies against endothelial CXCL16 (4 μg/ml) and following silencing of the CXCL16 gene with small interfering RNA. Pretreatment of HUVEC with sphingomyelinase upregulated CXCL16 protein abundance. Six hours pretreatment of HUVEC with sphingomyelinase (10 mU/ml) or C6-ceramide (50 μM) augmented erythrocyte adhesion following a 30-min treatment with Ca(2+) ionophore ionomycin (1 μM) or following energy depletion by 48-h glucose removal. Thus exposure to sphingomyelinase or C6-ceramide triggers eryptosis followed by phosphatidylserine- and CXCL16-sensitive adhesion of eryptotic erythrocytes to HUVEC.

Inhibitory effect of furosemide on non-selective voltage-independent cation channels in human erythrocytes

BACKGROUND

Furosemide, a loop diuretic inhibiting the renal tubular Na(+),K(+),2Cl(-) cotransporter, has been shown to decrease cytosolic Ca(2+) concentration ([Ca(2+)](i)) in platelets and erythrocytes. [Ca(2+)](i) in erythrocytes is a function of Ca(2+) permeable cation channels. Activation of those channels e.g. by energy depletion or oxidative stress leads to increase of [Ca(2+)](i), which in turn triggers eryptosis, a suicidal erythrocyte death characterized by cell membrane scrambling. The present study was performed to explore whether furosemide influences the cation channels and thus influences eryptosis.

METHODS

Cation channel activity was determined by whole-cell patch clamp, [Ca(2+)](i) utilizing Fluo3 fluorescence and annexin V binding to estimate cell membrane scrambling with phosphatidylserine exposure.

RESULTS

A 45 min exposure to furosemide (10 and 100 µM) slightly, but significantly decreased cation channel activity and [Ca(2+)](i) in human erythrocytes drawn from healthy individuals. ATP-depletion (> 3 hours, +37°C, 6 mM ionosine and 6 mM iodoacetic acid) enhanced the non-selective cation channel activity, increased [Ca(2+)](i) and triggered cell membrane scrambling, effects significantly blunted by furosemide (10 - 100 µM). Oxidative stress by exposure to tert-butylhydroperoxide (0.1 -1 mM) similarly enhanced the non-selective cation channels activity, increased [Ca(2+)](i) and triggered cell membrane scrambling, effects again significantly blunted by furosemide (10 - 100 µM).

CONCLUSIONS

The present study shows for the first time that the loop diuretic furosemide applied at micromolar concentrations (10 - 100 µM) inhibits non-selective cation channel activity in and eryptosis of human erythrocytes.

Enhanced erythrocyte membrane exposure of phosphatidylserine following sorafenib treatment: an in vivo and in vitro study

BACKGROUND

Sorafenib (Nexavar(®)), a polytyrosine kinase inhibitor, stimulates apoptosis and is thus widely used for chemotherapy in hepatocellular carcinoma (HCC). Hematological side effects of Nexavar(®) chemotherapy include anemia. Erythrocytes may undergo apoptosis-like suicidal death or eryptosis, which is characterized by cell shrinkage and phosphatidylserine-exposure at the cell surface. Signaling leading to eryptosis include increase in cytosolic Ca(2+)activity ([Ca(2+)](i)), formation of ceramide, ATP-depletion and oxidative stress. The present study explored, whether sorafenib triggers eryptosis in vitro and in vivo.

METHODS

[Ca(2+)](i )was estimated from Fluo3-fluorescence, cell volume from forward scatter, phosphatidylserine-exposure from annexin-V-binding, hemolysis from hemoglobin release, ceramide with antibody binding-dependent fluorescence, cytosolic ATP with a luciferin-luciferase-based assay, and oxidative stress from 2',7' dichlorodihydrofluorescein diacetate (DCFDA) fluorescence.

RESULTS

A 48 h exposure of erythrocytes to sorafenib (≥0.5 µM) significantly increased Fluo 3 fluorescence, decreased forward scatter, increased annexin-V-binding and triggered slight hemolysis (≥5 µM), but did not significantly modify ceramide abundance and cytosolic ATP. Sorafenib treatment significantly enhanced DCFDA-fluorescence and the reducing agents N-acetyl-L-cysteine and tiron significantly blunted sorafenib-induced phosphatidylserine exposure. Nexavar(®) chemotherapy in HCC patients significantly enhanced the number of phosphatidylserine-exposing erythrocytes.

CONCLUSIONS

The present observations disclose novel effects of sorafenib, i.e. stimulation of suicidal erythrocyte death or eryptosis, which may contribute to the pathogenesis of anemia in Nexavar(®)-based chemotherapy.

Inhibition of Ca(2+) entry and suicidal erythrocyte death by naringin

Naringin is a dietary flavonoid from citrus fruits with antioxidant and antiapoptotic activity. Similar to apoptosis of nucleated cells, suicidal death of erythrocytes or eryptosis is paralleled by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the erythrocyte surface. Eryptosis is triggered by increased cytosolic Ca(2+) activity, e.g. following energy depletion or oxidative stress. The present study thus explored whether naringin interferes with eryptosis. To this end, the cytosolic Ca(2+) concentration was estimated from Fluo3 fluorescence, phosphatidylserine exposure from annexin-V-binding and cell volume from forward scatter in FACS analysis. As a result, energy depletion (48 h glucose removal) and oxidative stress (30 min exposure to 0.3 mM tert-butylhydroperoxide) increased Fluo-3 fluorescence, decreased the erythrocyte forward scatter and enhanced the percentage of annexin-V-binding erythrocytes. Naringin (up to 40 µM) did not significantly modify Fluo-3 fluorescence, erythrocyte forward scatter or annexin-V-binding in the presence of glucose and absence of oxidative stress. Naringin, however, significantly blunted the effect of glucose depletion and oxidative stress on Fluo-3 fluorescence, erythrocyte forward scatter or annexin-V-binding. In conclusion, naringin blunts the increase of cytosolic Ca(2+) concentration, the shrinkage, the cell membrane scrambling and thus the suicidal death of erythrocytes following energy depletion or oxidative stress.

Decreased redox-sensitive erythrocyte cation channel activity in aquaporin 9-deficient mice

Survival of the malaria pathogen Plasmodium falciparum in host erythrocytes requires the opening of new permeability pathways (NPPs) in the host cell membrane, accomplishing entry of nutrients, exit of metabolic waste products such as lactate and movement of inorganic ions such as Cl⁻, Na⁺ and Ca²⁺. The molecular identity of NPPs has remained largely elusive but presumably involves several channels, which partially can be activated by oxidative stress in uninfected erythrocytes. One NPP candidate is aquaporin 9 (AQP9), a glycerol-permeable water channel expressed in erythrocytes. Gene-targeted mice lacking functional AQP9 (aqp⁻/⁻) survive infection with the malaria pathogen Plasmodium berghei better than their wild-type littermates (aqp9⁺/⁺). In the present study whole-cell patch-clamp recordings were performed to explore whether ion channel activity is different in erythrocytes from aqp⁻/⁻ and aqp9⁺/⁺ mice. As a result, the cation conductance (K⁺ > Na⁺ > Ca²⁺ ≫ NMDG⁺) was significantly lower in erythrocytes from aqp⁻/⁻ than in erythrocytes from aqp9⁺/⁺ mice. Oxidative stress by exposure for 15-30 min to 1 mM H₂O₂ or 1 mM tert-butyl-hydroperoxide enhanced the cation conductance and increased cytosolic Ca²⁺ concentration, effects significantly less pronounced in erythrocytes from aqp⁻/⁻ than in erythrocytes from aqp9⁺/⁺ mice. In conclusion, lack of AQP9 decreases the cation conductance of erythrocytes, an effect that possibly participates in the altered susceptibility of AQP9-deficient mice to infection with P. berghei.

Carbon monoxide-sensitive apoptotic death of erythrocytes

Carbon monoxide (CO) intoxication severely interferes with the oxygen-transporting function of haemoglobin. Beyond that, CO participates in the regulation of apoptosis. CO could be generated from CO-releasing molecules (CORM), such as the tricarbonyl-dichlororuthenium (II) dimer (CORM-2), which is presently considered for the treatment of vascular dysfunction, inflammation, tissue ischaemia and organ rejection. CORM-2 is at least partially effective by modifying gene expression and mitochondrial potential. Erythrocytes lack nuclei and mitochondria but may undergo suicidal cell death or eryptosis, characterized by cell shrinkage and phospholipid scrambling of the cell membrane. Eryptosis is triggered by the increase in cytosolic Ca²⁺ activity ([Ca²⁺](i)). The present study explored whether CORM-2 influences eryptosis. To this end, [Ca²⁺](i) was estimated from Fluo-3-fluorescence, cell volume from forward scatter, phospholipid scrambling from annexin-V-binding and haemolysis from haemoglobin release. CO-binding haemoglobin (COHb) was estimated utilizing a blood gas analyser. As a result, exposure of erythrocytes for 24 hr to CORM-2 (≥5 μM) significantly increased COHb, [Ca²⁺](i) , forward scatter, annexin-V-binding and haemolysis. Annexin-V-binding was significantly blunted by 100% oxygen and was virtually abolished in the nominal absence of Ca²⁺. In conclusion, CORM-2 stimulates cell membrane scrambling of erythrocytes, an effect largely due to Ca²⁺ entry and partially reversed by O₂.

Induction of apoptotic erythrocyte death by rotenone

The pesticide rotenone stimulates apoptosis and rotenone intoxication has been considered a cause of Parkinson's disease. Rotenone further sensitizes tumor cells to cytotoxic drugs. The apoptotic effect of rotenone is at least partially due to mitochondrial injury. Even though lacking mitochondria and nuclei, erythrocytes may undergo eryptosis, an apoptosis-like suicidal death characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine-exposure at the cell surface. Triggers of eryptosis include increase of cytosolic Ca(2+)-activity ([Ca(2+)](i)) and enhanced ceramide formation. The present study explored, whether rotenone elicits eryptosis. To this end, [Ca(2+)](i) was estimated utilizing Fluo3-fluorescence, cell volume from forward scatter, phosphatidylserine-exposure from annexin-V-binding, ceramide utilizing fluorescence antibodies and hemolysis from hemoglobin release. A 48 h exposure to rotenone significantly increased Fluo3-fluorescence(i) (≥1 μM), increased ceramide abundance (10 μM), decreased forward scatter (≥2.5 μM) and increased annexin-V-binding (≥ 1 μM). Rotenone exposure was further followed by slight but significant hemolysis. Rotenone-induced cell membrane scrambling was significantly blunted, but not completely abrogated by removal of extracellular Ca(2+). The present observations disclose a novel effect of rotenone, i.e. triggering of erythrocyte shrinkage and cell membrane scrambling, an effect paralleled by and partially dependent on Ca(2+)-entry.

Killing me softly - suicidal erythrocyte death

Similar to nucleated cells, erythrocytes may undergo suicidal death or eryptosis, which is characterized by cell shrinkage, cell membrane blebbing and cell membrane phospholipid scrambling. Eryptotic cells are removed and thus prevented from undergoing hemolysis. Eryptosis is stimulated by Ca(2+) following Ca(2+) entry through unspecific cation channels. Ca(2+) sensitivity is enhanced by ceramide, a product of acid sphingomyelinase. Eryptosis is triggered by hyperosmolarity, oxidative stress, energy depletion, hyperthermia and a wide variety of xenobiotics and endogenous substances. Eryptosis is inhibited by nitric oxide, catecholamines and a variety of further small molecules. Erythropoietin counteracts eryptosis in part by inhibiting the Ca(2+)-permeable cation channels but by the same token may foster formation of erythrocytes, which are particularly sensitive to eryptotic stimuli. Eryptosis is triggered in several clinical conditions such as iron deficiency, diabetes, renal insufficiency, myelodysplastic syndrome, phosphate depletion, sepsis, haemolytic uremic syndrome, mycoplasma infection, malaria, sickle-cell anemia, beta-thalassemia, glucose-6-phosphate dehydrogenase-(G6PD)-deficiency, hereditary spherocytosis, paroxysmal nocturnal hemoglobinuria, and Wilson's disease. Enhanced eryptosis is observed in mice with deficient annexin 7, cGMP-dependent protein kinase type I (cGKI), AMP-activated protein kinase AMPK, anion exchanger AE1, adenomatous polyposis coli APC and Klotho as well as in mouse models of sickle cell anemia and thalassemia. Eryptosis is decreased in mice with deficient phosphoinositide dependent kinase PDK1, platelet activating factor receptor, transient receptor potential channel TRPC6, janus kinase JAK3 or taurine transporter TAUT. If accelerated eryptosis is not compensated by enhanced erythropoiesis, clinically relevant anemia develops. Eryptotic erythrocytes may further bind to endothelial cells and thus impede microcirculation.

The impact of erythrocyte age on eryptosis

Mature, circulating erythrocytes undergo senescence, which limits their life span to approximately 120 d. Upon injury, erythrocytes may undergo suicidal erythrocyte death or eryptosis, which may accelerate senescence and shorten their survival. Eryptosis is defined as cell shrinkage and exposure of phosphatidylserine at the cell surface. Triggers of eryptosis include oxidative stress. The present study addresses the impact of erythrocyte age on the relative susceptibility to eryptosis. Erythrocytes were separated into five fractions, based on age-associated differences in density and volume. Cell membrane scrambling was estimated from binding of annexin V to phosphatidylserine at the erythrocyte surface, the cell volume from forward scatter, and the Ca(2+) level from Fluo-3-dependent fluorescence. In addition, glutathione (GSH) concentrations were measured by an enzymatic/colourimetric method. After 48 h incubation in Ringer solution, Annexin V binding increased significantly with erythrocyte age. The differences were not accompanied by altered GSH concentrations, but were reversed by addition of the antioxidant N-acetyl-L-cysteine in vitro. Also, N-acetyl-L-cysteine significantly prolonged the half-life of circulating mouse erythrocytes in vivo. Thus, the susceptibility to eryptosis increases with the age of the erythrocytes, and this effect is at least partially due to enhanced sensitivity to oxidative stress.

Protein kinase CK1α regulates erythrocyte survival

Protein kinase CK1 (casein kinase 1) isoforms are involved in the regulation of various physiological functions including apoptosis. The specific CK1 inhibitor D4476 may either inhibit or foster apoptosis. Similar to apoptosis of nucleated cells, eryptosis, the suicidal death of erythrocytes, is paralleled by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Triggers of eryptosis include increase of cytosolic Ca(2+) activity following energy depletion (removal of glucose) or oxidative stress (exposure to the oxidant tert-butyl hydroperoxide [TBOOH]). Western blotting was utilized to verify that erythrocytes express the protein kinase CK1α, and FACS analysis to determine whether the CK1 inhibitor D4476 and CK1α activator pyrvinium pamoate modify forward scatter (reflecting cell volume), annexin V binding (reflecting phosphatidylserine exposure), and Fluo3 fluorescence (reflecting cytosolic Ca(2+) activity). As a result, both, human and murine erythrocytes express CK1 isoform α. Glucose depletion (48 hours) and exposure to 0.3 mM TBOOH (30 minutes) both decreased forward scatter, increased annexin V binding and increased Fluo3 fluorescence. CK1 inhibitor D4476 (10 μM) significantly blunted the decrease in forward scatter, the increase in annexin V binding and the increase in Fluo 3 fluorescence. (R)-DRF053, another CK1 inhibitor, similarly blunted the increase in annexin V binding upon glucose depletion. The CK1α specific activator pyrvinium pamoate (10 μM) significantly enhanced the increase in annexin V binding and Fluo3 fluorescence upon glucose depletion and TBOOH exposure. In the presence of glucose, pyrvinium pamoate slightly but significantly increased Fluo3 fluorescence. In conclusion, CK1 isoform α participates in the regulation of erythrocyte programmed cell death by modulating cytosolic Ca(2+) activity.

Dynamic adhesion of eryptotic erythrocytes to endothelial cells via CXCL16/SR-PSOX

Suicidal death of erythrocytes, or eryptosis, is characterized by cell shrinkage and cell membrane scrambling leading to phosphatidylserine exposure at the cell surface. Eryptosis is triggered by increase of cytosolic Ca2+activity, which may result from treatment with the Ca2+ionophore ionomycin or from energy depletion by removal of glucose. The present study tested the hypothesis that phosphatidylserine exposure at the erythrocyte surface fosters adherence to endothelial cells of the vascular wall under flow conditions at arterial shear rates and that binding of eryptotic cells to endothelial cells is mediated by the transmembrane CXC chemokine ligand 16 (CXCL16). To this end, human erythrocytes were exposed to energy depletion (for 48 h) or treated with the Ca2+ionophore ionomycin (1 μM for 30 min). Phosphatidylserine exposure was quantified utilizing annexin-V binding, cell volume was estimated from forward scatter in FACS analysis, and erythrocyte adhesion to human vascular endothelial cells (HUVEC) was determined in a flow chamber model. As a result, both, ionomycin and glucose depletion, triggered eryptosis and enhanced the percentage of erythrocytes adhering to HUVEC under flow conditions at arterial shear rates. The adhesion was significantly blunted in the presence of erythrocyte phosphatidylserine-coating annexin-V (5 μl/ml), of a neutralizing antibody against endothelial CXCL16 (4 μg/ml), and following silencing of endothelial CXCL16 with small interfering RNA. The present observations demonstrate that eryptotic erythrocytes adhere to endothelial cells of the vascular wall in part by interaction of phosphatidylserine exposed at the erythrocyte surface with endothelial CXCL16.

Mechanisms and significance of eryptosis, the suicidal death of erythrocytes

Eryptosis, the suicidal death of erythrocytes, is characterized by erythrocyte shrinkage, blebbing, and phospholipid scrambling of the cell membrane. Eryptosis is triggered by increased cytosolic Ca(2+) activity, which may result from Ca(2+) entry through PGE(2)-activated Ca(2+)-permeable cation channels. The Ca(2+) sensitivity of the scrambling machinery is enhanced by ceramide, which is formed by an acid sphingomyelinase, an enzyme stimulated by platelet-activating factor. Eryptosis is enhanced in a variety of clinical conditions such as sickle-cell anemia, β-thalassemia, glucose-6-phosphate dehydrogenase deficiency, hereditary spherocytosis, paroxysmal nocturnal hemoglobinuria, myelodysplastic syndrome, phosphate depletion, iron deficiency, sepsis, hemolytic uremic syndrome, renal insufficiency, diabetes, malaria, mycoplasma infection, and Wilson's disease. Eryptosis is enhanced in mouse models of sickle cell anemia and thalassemia, as well as in mice lacking functional annexin 7, cGMP-dependent protein kinase type I, AMP-activated protein kinase, Janus kinase 3, anion exchanger 1, adenomatous polyposis coli, or Klotho. Eryptosis is triggered by osmotic shock, oxidative stress, energy depletion, hyperthermia, and a myriad of small molecules. Eryptosis is inhibited by a variety of substances including nitric oxide and catecholamines. Erythropoietin counteracts eryptosis in part by inhibiting the Ca(2+)-permeable cation channels. Excessive erythropoietin concentrations lead, however, to formation of erythrocytes, which are particularly sensitive to eryptotic stimuli. Accelerated eryptosis may be compensated by enhanced erythropoiesis, which is apparent from reticulocytosis. If the compensation is not sufficient, clinically relevant anemia develops. Beyond that, adhesion of eryptotic erythrocytes to the vascular wall may lead to impairment of microcirculation.

Apigenin-induced suicidal erythrocyte death

Apigenin, a flavone in fruits and vegetables, stimulates apoptosis and thus counteracts cancerogenesis. Erythrocytes may similarly undergo suicidal cell death or eryptosis, characterized by cell shrinkage and phosphatidylserine exposure at the cell surface. Triggers of eryptosis include increase of cytosolic Ca(2+) activity ([Ca(2+)](i)), ceramide formation and ATP depletion. The present study explored the effect of apigenin on eryptosis. [Ca(2+)](i) was estimated from Fluo3-fluorescence, cell volume from forward scatter, phosphatidylserine exposure from annexin V binding, hemolysis from hemoglobin release, ceramide utilizing antibodies, and cytosolic ATP with luciferin-luciferase. A 48 h exposure to apigenin significantly increased [Ca(2+)](i) (≥ 1 μM), increased ceramide formation (15 μM), decreased ATP concentration (15 μM), decreased forward scatter (≥ 1 μM), and increased annexin V binding (≥ 5 μM) but did not significantly modify hemolysis. The effect of 15 μM apigenin on annexin V binding was blunted by Ca(2+) removal. The present observations reveal novel effects of apigenin, i.e. stimulation of Ca(2+) entry, ceramide formation and ATP depletion in erythrocytes with subsequent triggering of suicidal erythrocyte death, paralleled by cell shrinkage and phosphatidylserine exposure.