Olivier M. Niemoeller

Stimulation of Suicidal Erythrocyte Death by Methylglyoxal

Diabetes increases the percentage of circulating erythrocytes exposing phosphatidylserine (PS) at the cell surface. PS-exposing erythrocytes are recognized, bound, engulfed and degraded by macrophages. Thus, PS exposure, a feature of suicidal erythrocyte death or eryptosis, accelerates clearance of affected erythrocytes from circulating blood. Moreover, PS-exposing erythrocytes bind to the vascular wall thus interfering with microcirculation. The present study explored mechanisms involved in the triggering of PS exposure by methylgloxal, an extra- and intracellular metabolite which is enhanced in diabetes. PS exposure, cell size and cytosolic Ca2+-activity after methylglyoxal treatment were measured by FACS analysis of annexin V binding, forward scatter and Fluo-3-fluorescence, respectively, and it was shown that the treatment significantly enhanced the percentage of PS-exposing erythrocytes at concentrations (0.3 µM) encountered in diabetic patients. Surprisingly, methylglyoxal did not significantly increase cytosolic Ca2+ concentration, and at concentrations up to 3 µM, did not decrease the forward scatter. Instead, exposure to methylglyoxal inhibited glycolysis thus decreasing ATP and GSH concentrations. In conclusion, methylglyoxal impairs energy production and anti-oxidative defense, effects contributing to the enhanced PS exposure of circulating erythrocytes and eventually resulting in anemia and deranged microcirculation.

Stimulation of Erythrocyte Phosphatidylserine Exposure by Paclitaxel

Side effects of cytostatic treatment include development of anemia resulting from either decreased generation or accelerated clearance of circulating erythrocytes. Recent experiments revealed a novel kind of stress-induced erythrocyte death, i.e. eryptosis, which is characterized by enhanced cytosolic Ca2+ levels, increased ceramide formation and exposure of phosphatidylserine at the cell surface. The present study explored whether cytostatic treatment with paclitaxel (Taxol®) triggers eryptosis. Blood was drawn from cancer patients before and after infusion of 175 mg/m2 Taxol®. The treatment significantly decreased the hematocrit and significantly increased the percentage of annexin-Vbinding erythrocytes in vivo (by 37%). In vitro incubation of human erythrocytes with 10 μM paclitaxel again significantly increased annexin-V-binding (by 129%) and augmented the increase of annexin-Vbinding following cellular stress. The enhanced phosphatidylserine exposure was not dependent on caspase-activity but paralleled by erythrocyte shrinkage, increase of cytosolic Ca2+ activity, ceramide formation and activation of calpain. Phosphatidylserine exposure was similarly induced by docetaxel but not by carboplatin or doxorubicin. Moreover, eryptosis was triggered by the Ca2+ ionophore ionomycin (10 μM). In mice, ionomycintreated eryptotic erythrocytes were rapidly cleared from circulating blood and sequestrated into the spleen. In conclusion, our data strongly suggest that paclitaxel-induced anemia is at least partially due to induction of eryptosis.

Influence of NO Synthase Inhibitor L-NAME on Parasitemia and Survival of Plasmodium berghei Infected Mice

Accelerated suicidal death or eryptosis of infected erythrocytes may delay development of parasitemia in malaria. Eryptosis is inhibited by nitric oxide (NO). The present study has been performed to explore, whether inhibition of NO synthase by L-NAME modifies the course of malaria. We show here that L-NAME (>10 µM) increased phosphatidylserine exposure of Plasmodium falciparum infected human erythrocytes, an effect significantly more marked than in noninfected human erythrocytes. We further show that parasitemia in Plasmodium berghei infected mice was significantly decreased (from 50% to 18% of circulating erythrocytes 20 days after infection) by addition of 1 mg/ml L-NAME to the drinking water. According to CFSE labelling L-NAME treatment accelerated the clearance of both, noninfected and infected, erythrocytes from circulating blood, but did not significantly extend the life span of infected animals. In conclusion, treatment with L-NAME shortens the life span of circulating erythrocytes and thus delays development of parasitemia during malaria.

Induction of eryptosis by cyclosporine

Side effects of cyclosporine treatment include anemia. Most recent studies have found that anemia may be caused by triggering of suicidal erythrocyte death (eryptosis), i.e. activation of an erythrocyte scramblase and phosphatidylserine exposure at the erythrocyte surface. Phosphatidylserine exposing cells are rapidly cleared from circulating blood by phagocytosis. Stimulators of erythrocyte membrane scrambling include cytosolic Ca2+ and ceramide, which are increased by entry through Ca2+-permeable cation channels and by activation of a sphingomyelinase, respectively. The present study has been performed to test for an effect of cyclosporine on eryptosis. Erythrocytes from healthy volunteers were exposed to cyclosporine, and phosphatidylserine exposure (annexin V binding), cell volume (forward scatter), cytosolic Ca2+ activity (Fluo3-dependent fluorescence), ceramide formation (anti-ceramide-FITC antibody), and 45Ca2+ uptake were determined by flow cytometry and tracer flux measurements, respectively. Exposure of erythrocytes to cyclosporine triggered annexin V binding and significantly enhanced the increased annexin V binding both following glucose depletion and after hyperosmotic or isotonic cell shrinkage. However, cyclosporine significantly decreased cytosolic Ca2+ activity and did not stimulate 45Ca2+ uptake. Instead, cyclosporine transiently stimulated ceramide formation, decreased the cytosolic ATP concentration and potentiated the decline of cytosolic ATP concentration following glucose depletion. Elevated ceramide levels and ATP depletion, in turn, sensitize the erythrocytes for the eryptotic effects of Ca2+. The present observations may provide a mechanistic explanation for the anemia following treatment with this important immunosuppressive drug.

Ciglitazone and 15d-PGJ2 Induced Suicidal Erythrocyte Death

Side effects of peroxisome proliferator activated receptor gamma (PPARγ) agonists such as ciglitazone include anemia, which in theory could be due to decreased formation or premature death of erythrocytes. A form of suicidal erythrocyte death is eryptosis, which is characterized by cell shrinkage and by breakdown of phosphatidylserine asymmetry leading to phosphatidylserine exposure at the cell surface. Phosphatidylserine-exposing erythrocytes are recognized by macrophages, engulfed, degraded and thus cleared from circulating blood. Triggers of eryptosis include increase in intracellular Ca2+ concentration. The present study thus explored, whether the PPARγ agonist ciglitazone or the natural PPARγ ligand 15deoxy-delta12,14-prostaglandin J2 (15d-PGJ2) are capable to trigger eryptosis. Phosphatidylserine exposure was determined from annexin V binding and cell shrinkage from decrease of forward scatter of human erythrocytes in FACS analysis. Both, ciglitazone (≧ 5 µM) and 15d-PGJ2 (≧ 3 µM), within 24 hours increased phosphatidylserine exposure and at concentrations of 10 µM led to a significant loss of the cell volume. Ciglitazone further stimulated hemolysis, which, however, affected only a fraction of erythrocytes undergoing eryptosis. According to Fluo3 fluorescence of human erythrocytes, 10 µM ciglitazone or 15d-PGJ2 increased intracellular Ca2+ activity. In conclusion, ciglitazone and 15d-PGJ2 trigger eryptosis at least in part by an increase in the cytosolic Ca2+ concentration. The effect most likely contributes to the anemia observed following treatment with PPARγ agonists.

Studying mechanisms of eryptosis

Cellular stress leads to activation of erythrocyte cation channels with subsequent Ca2+ entry and stimulates a sphingomyelinase with subsequent formation of ceramide. Both signaling molecules then activate the death program of erythrocytes (eryptosis) which is characterized by phosphatidylserine exposure, cellular shrinkage, membrane blebbing and activation of death-inducing proteases. Some of the mediators accounting for activation of the erythrocyte death machinery, i. e. prostaglandin E2 (PGE2) and platelet activating factor (PAF), have been described and the respective signaling cascades disclosed. The present article outlines and discusses the methods which have been used to analyze erythrocyte death pathways. Furthermore, some of the pathophysiological implications of eryptosis signaling are delineated and the methods to screen for eryptosis defects in those conditions are presented. Needless to say that further research will be required to fully understand the mechanisms leading to suicidal red blood cell death and to elucidate the role of eryptosis during anemic complications.