Protein kinase C activation induces phosphatidylserine exposure on red blood cells

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 Ca(2+)-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 microM) encountered in diabetic patients. Surprisingly, methylglyoxal did not significantly increase cytosolic Ca(2+) concentration, and at concentrations up to 3 microM, 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.

Adenosine protects against suicidal erythrocyte death

Suicidal death of erythrocytes or eryptosis is characterized by cell shrinkage and cell membrane scrambling leading to phosphatidylserine exposure at the erythrocyte surface. The cell membrane scrambling is triggered by an increase in cytosolic Ca(2+) activity and activation of protein kinase C (PKC). Phosphatidylserine exposure fosters adherence of affected erythrocytes to the vascular wall. Thus, microcirculation in ischemic tissues may be impaired by the appearance of eryptotic erythrocytes. Ischemia leads to release of adenosine, which in most tissues leads to vasodilation and protects against cell injury. The present experiments explored whether adenosine influences mechanisms underlying eryptosis. Erythrocyte phosphatidylserine exposure was estimated from annexin V binding, cell volume from forward scatter and cytosolic Ca(2+) activity from Fluo3 fluorescence. Glucose depletion (for 24 or 48 h) significantly increased annexin binding and decreased forward scatter, effects partially reversed by adenosine. The protective effect of adenosine reached statistical significance (s.d.) at > =30 microM. Low Cl(-) solution (Cl(-) exchanged by gluconate for 24 h) similarly increased annexin binding and decreased forward scatter, effects again reversed by adenosine (s.d. at > or =10 and 30 microM, respectively). Similarly, phosphatase inhibitor okadaic acid (OA, 1 microM) and PKC activator phorbol 12-myristate-13-acetate (PMA, 3 microM) significantly enhanced annexin binding and decreased forward scatter. Adenosine significantly blunted the effects of OA and PMA on annexin V binding (s.d. at > or =30 and 10 microM, respectively) and the effect of OA on forward scatter (s.d. at > or =10 microM). In conclusion, adenosine inhibits eryptosis by a mechanism presumably effective downstream of PKC. The effect may participate in the maintenance of microcirculation in ischemic tissue.

Lysophosphatidic Acid Induces Thrombogenic Activity Through Phosphatidylserine Exposure and Procoagulant Microvesicle Generation in Human Erythrocytes

OBJECTIVE

Although erythrocytes have been suggested to play a role in blood clotting, mediated through phosphatidylserine (PS) exposure and/or PS-bearing microvesicle generation, an endogenous substance that triggers the membrane alterations leading to a procoagulant activity in erythrocytes has not been reported. We now demonstrated that lysophosphatidic acid (LPA), an important lipid mediator in various pathophysiological processes, induces PS exposure and procoagulant microvesicle generation in erythrocytes, which represent a biological significance resulting in induction of thrombogenic activity.

METHODS AND RESULTS

In human erythrocytes, LPA treatment resulted in PS exposure on remnant cells and PS-bearing microvesicle generation in a concentration-dependent manner. Consistent with the microvesicle generation, scanning electron microscopic study revealed that LPA treatment induced surface changes, alteration of normal discocytic shape into echinocytes followed by spherocytes. Surprisingly, chelation of intracellular calcium did not affect LPA-induced PS exposure and microvesicle generation. On the other hand, protein kinase C (PKC) inhibitors significantly reduced PS exposure and microvesicle generation induced by LPA, reflecting the role of calcium-independent PKC. Activation of PKC was confirmed by Western blot analysis showing translocation of calcium-independent isoform, PKCzeta, to erythrocyte membrane. The activity of flippase, which is important in the maintenance of membrane asymmetry, was also inhibited by LPA. Furthermore, LPA-exposed erythrocytes actually potentiated the thrombin generation as determined by prothrombinase assay and accelerated the coagulation process initiated by recombinant human tissue factor in plasma. The adherence of erythrocytes to endothelial cells, another important feature of thrombogenic process, was also stimulated by LPA treatment.

CONCLUSIONS

These results suggested that LPA-exposed erythrocytes could make an important contribution to thrombosis mediated through PS exposure and procoagulant microvesicle generation.

Membrane Lipid Alterations in Hemoglobinopathies

The red blood cell (RBC) membrane is a complex mixture of lipids and proteins. Hundreds of phospholipid molecular species spontaneously arrange themselves in a lipid bilayer and move rapidly in the plane as well as across the bilayer in a dynamic but highly organized fashion. Areas enriched in certain lipids determine proper protein function. Phospholipids are asymmetrically distributed across the lipid bilayer with phosphatidylserine (PS) exclusively on the inside. Both the composition and organization of the RBC membrane is well maintained. Alterations lead to apoptosis during erythropoiesis or early demise of the cell in the circulation. The mechanisms that govern the maintenance of the lipid bilayer are only recently being unraveled at the individual protein level. Oxidized lipids are rapidly repaired using fatty acids taken up from plasma to maintain membrane integrity. Several isoforms of a RBC acyl-Coenzyme A (CoA) synthase have been reported, as well as the first member of a family of lysophospholipid acylCoA acyltransferases. Phospholipid asymmetry is maintained by the recently identified RBC amino-phospholipid translocase. These enzymes, essential in maintaining membrane lipid organization, are affected by oxidant stress or an increase in cytosolic calcium. Normal lipid composition and organization is lost in subpopulations of RBC in hemoglobinopathies such as sickle cell disease and thalassemia. Despite elaborate antioxidant systems, lipids and membrane proteins, including those that maintain lipid organization, are damaged in these cells. This in turn leads to improper repair of damaged RBC membranes and altered interactions of RBCs with other blood cells and plasma components that play a role in the pathology that defines these disorders. The altered lipid bilayer in RBCs in hemoglobinopathies leads to premature removal (anemia) and imbalance in hemostasis, and plays a role in vaso-occlusive crisis in sickle cell disease. Lipid breakdown products of PS-exposing cells result in vascular dysfunction, including acute chest syndrome in sickle cell disease. In summary, altered membrane lipids play an important role in the pathology of hemoglobinopathies and characterization of the proteins involved in lipid turnover will elucidate the pathways that maintain plasma membrane organization and cellular viability.

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 Ca(2+) 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-V-binding erythrocytes in vivo (by 37%). In vitro incubation of human erythrocytes with 10 microM paclitaxel again significantly increased annexin-V-binding (by 129%) and augmented the increase of annexin-V-binding following cellular stress. The enhanced phosphatidylserine exposure was not dependent on caspase-activity but paralleled by erythrocyte shrinkage, increase of cytosolic Ca(2+) 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 Ca(2+) ionophore ionomycin (10 microM). In mice, ionomycin-treated 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.

Mechanisms and significance of eryptosis

Suicidal death of erythrocytes (eryptosis) is characterized by cell shrinkage, membrane blebbing, activation of proteases, and phosphatidylserine exposure at the outer membrane leaflet. Exposed phosphatidylserine is recognized by macrophages that engulf and degrade the affected cells. Eryptosis is triggered by erythrocyte injury after several stressors, including oxidative stress. Besides caspase activation after oxidative stress, two signaling pathways converge to trigger eryptosis: (a) formation of prostaglandin E(2) leads to activation of Ca(2+)-permeable cation channels, and (b) the phospholipase A(2)-mediated release of platelet-activating factor activates a sphingomyelinase, leading to formation of ceramide. Increased cytosolic Ca(2+) activity and enhanced ceramide levels lead to membrane scrambling with subsequent phosphatidylserine exposure. Moreover, Ca(2+) activates Ca(2+)-sensitive K(2+) channels, leading to cellular KCl loss and cell shrinkage. In addition, Ca(2+) stimulates the protease calpain, resulting in degradation of the cytoskeleton. Eryptosis is inhibited by erythropoietin, which thus extends the life span of circulating erythrocytes. Eryptosis may be a mechanism of defective erythrocytes to escape hemolysis. Conversely, excessive eryptosis favors the development of anemia. Conditions with excessive eryptosis include iron deficiency, lead or mercury intoxication, sickle cell anemia, thalassemia, glucose 6- phosphate dehydrogenase deficiency, malaria, and infection with hemolysin-forming pathogens.

Sulphydryl modifications alter scramblase activity in murine sickle cell disease

Increased oxidant stress has been suggested to play a role in the process of phosphatidylserine (PS) externalization in the red blood cells of sickle cell patients. Inhibition of the ATP-driven translocation from outer to inner monolayer (flippase) by sulphydryl modification has been established. The present study showed that phospholipid scrambling was also sensitive to protein sulphydryl modification. Treatment with N-ethylmaleimide lead to enhanced PS exposure and a lower Ca(++) requirement for scrambling. In contrast, pyridyldithioethylamine treatment inhibited PS exposure. Red blood cells from a murine model for sickle cell disease exhibited a reduced response to both reagents, suggestive of previous sulphydryl modifications to the protein(s) involved in phospholipid scrambling. We conclude that sulphydryl modifications to both scramblase and flippase underlie the enhanced formation of PS-exposing cells in sickle cell disease.

Suicidal death of erythrocytes in recurrent hemolytic uremic syndrome

Hemolytic uremic syndrome (HUS) is characterized by hemolytic anemia with fragmented erythrocytes, thrombocytopenia, and acute renal failure. Lack of complement inactivating factor H predisposes to the development of atypical HUS. Little is known about mechanisms linking complement activation with loss of erythrocyte integrity during HUS. Recent studies disclosed that increased cytosolic Ca2+ activity and cellular ceramide trigger programmed erythrocyte death or eryptosis, characterized by cell shrinkage and phosphatidylserine exposure at the erythrocyte surface. In the present study, we investigated whether eryptosis occurs during the course of HUS. To this end, erythrocytes from healthy volunteers were exposed to plasma from a patient with severe idiopathic recurrent HUS secondary to factor H depletion. Phosphatidylserine exposure (Annexin binding), cell volume (forward scatter), cytosolic Ca2+ activity (Fluo3 fluorescence), and ceramide formation [anti-ceramide antibody and enzymatic (diacylgycerol kinase) analysis] were determined. Exposure of erythrocytes to plasma from the patient, but not to plasma from healthy individuals, triggered Annexin binding. The effect of plasma on erythrocyte Annexin binding was abolished by plasmapheresis or filtration at 30 kDa. It was paralleled by formation of ceramide and increase of cytosolic Ca2+ activity. Enhanced Annexin binding of erythrocytes from healthy individuals was observed after exposure to plasma from three other patients with HUS. The proeryptotic effect of patient plasma was mimicked by exposure to the Ca2+ ionophore ionomycin, and eryptosis was potentiated in the presence of cell membrane-permeable C6-ceramide. Furthermore, in vitro complement activation similarly triggered erythrocyte phosphatidylserine exposure, an effect which was blunted by the addition of factor H. In conclusion, our present observations disclose a novel, pathophysiological, factor-H dependent mechanism leading to injury of erythrocytes during the course of hemolytic uremic syndrome.

Protein kinase C mediates erythrocyte "programmed cell death" following glucose depletion

Glucose depletion of erythrocytes leads to activation of Ca2+-permeable cation channels, Ca2+ entry, activation of a Ca2+-sensitive erythrocyte scramblase, and subsequent exposure of phosphatidylserine at the erythrocyte surface. Ca2+ entry into erythrocytes was previously shown to be stimulated by phorbol esters and to be inhibited by staurosporine and chelerythrine and is thus thought to be regulated by protein phosphorylation/dephosphorylation, presumably via protein kinase C (PKC) and the corresponding phosphoserine/threonine phosphatases. The present experiments explored whether PKC could contribute to effects of energy depletion on erythrocyte phosphatidylserine exposure and cell volume. Phosphatidylserine exposure was estimated from annexin binding and cell volume from forward scatter in fluorescence-activated cell sorter analysis. Removal of extracellular glucose led to depletion of cellular ATP, stimulated PKC activity, led to translocation of PKCalpha, enhanced serine phosphorylation of membrane proteins, decreased cell volume, and increased annexin binding, the latter effect being blunted but not abolished in the presence of 1 microM staurosporine or 50 nM calphostin C. The PKC stimulator phorbol-12-myristate-13-acetate (3 microM) and the phosphatase inhibitor okadaic acid (1-10 microM) mimicked the effect of glucose depletion and similarly led to translocation of PKCalpha and enhanced serine phosphorylation, increased annexin binding, and decreased forward scatter, the latter effects being abrogated by PKC inhibitor staurosporine (1 microM). Fluo-3 fluorescence measurements revealed that okadaic acid also enhanced erythrocyte Ca2+ activity. The present observations suggest that protein phosphorylation and dephosphorylation via PKC and the corresponding protein phosphatases contribute to phosphatidylserine exposure and cell shrinkage after energy depletion.

Enhanced programmed cell death of iron-deficient erythrocytes

Exposure of erythrocytes to osmotic shock, oxidative stress, and energy depletion activates Cl--sensitive and Ca2+-permeable cation channels. Subsequent Ca2+ entry triggers eryptosis, characterized by erythrocyte shrinkage, membrane blebbing, and phosphatidylserine exposure all features typical for apoptotic death of nucleated cells. Erythrocytes exposing phosphatidylserine are recognized, bound, engulfed, and degraded by macrophages. Eryptosis thus fosters clearance of affected erythrocytes from circulating blood. Iron deficiency leads to anemia, in part by decreasing erythrocyte life span. In this study, phosphatidylserine exposure, cell size, and cytosolic Ca2+ were measured by FACS analysis of annexin-V binding, forward scatter, and Fluo-3 fluorescence, respectively. Erythrocytes from mice on control diet were compared with erythrocytes from mice exposed 10 weeks to iron-deficient diet. Iron deficiency significantly (P<0.001) enhanced erythrocyte annexin-V binding (from 2.4 to 3.7%), decreased forward scatter (from 544 to 393), and increased cytosolic Ca2+ concentration. 45Ca2+ flux measurements and patch clamp experiments revealed enhanced Ca2+ uptake (by 2.3-fold) and cation channel activity. The half-life of fluorescence-labeled, iron-deficient, or Ca2+-loaded erythrocytes was significantly reduced compared with control erythrocytes. Thus, the experiments reveal a novel mechanism triggered by iron deficiency, which presumably contributes to accelerated clearance of erythrocytes in iron deficiency anemia.

Erythrocyte adhesion receptors: blood group antigens and related molecules

During the second half of the 20th century, blood bankers quickly expanded our knowledge of human erythrocyte blood group antigens. By the dawn of the 21st century, several hundred blood group antigen polymorphisms had been identified. Hot on the heels of the serologists, membrane biochemists and molecular geneticists defined both the biochemical and genetic bases of most of these antigens. Perhaps to their surprise, this work has led to the discovery of functionally diverse and important membrane proteins expressed on the surface of red cells, including numerous adhesion molecules. Red cells express an unexpected number of such adhesion receptors, some of which contribute to human disease, as well as to normal red cell development. And perhaps most interestingly, study of these molecules has elucidated ways in which even mature red cells respond to external stimuli, such as adrenergic hormones.

Contributions of Ca2+ to galectin-1-induced exposure of phosphatidylserine on activated neutrophils

Apoptotic cells redistribute phosphatidylserine (PS) to the cell surface by both Ca(2+)-dependent and -independent mechanisms. Binding of dimeric galectin-1 (dGal-1) to glycoconjugates on N-formyl-Met-Leu-Phe (fMLP)-activated neutrophils exposes PS and facilitates neutrophil phagocytosis by macrophages, yet it does not initiate apoptosis. We asked whether dGal-1 initiated Ca(2+) fluxes that are required to redistribute PS to the surface of activated neutrophils. Prolonged occupancy by dGal-1 was required to maximally mobilize PS to the surfaces of fMLP-activated neutrophils. Like fMLP, dGal-1 rapidly elevated cytosolic Ca(2+) levels in Fluo-4-loaded neutrophils. An initial Ca(2+) mobilization from intracellular stores was followed by movement of extracellular Ca(2+) to the cytosolic compartment, with return to basal Ca(2+) levels within 10 min. Chelation of extracellular Ca(2+) did not prevent PS mobilization. Chelation of intracellular Ca(2+) revealed that fMLP and dGal-1 independently release Ca(2+) from intracellular stores that cooperate to induce optimal redistribution of PS. Ca(2+) mobilization by ionomycin did not permit dGal-1 to mobilize PS, indicating that fMLP initiated both Ca(2+)-dependent and -independent signals that facilitated dGal-1-induced exposure of PS. dGal-1 elevated cytosolic Ca(2+) and mobilized PS through a pathway that required action of Src kinases and phospholipase Cgamma. These results demonstrate that transient Ca(2+) fluxes contribute to a sustained redistribution of PS on neutrophils activated with fMLP and dGal-1.