Effect of Peroxynitrite on Passive K+ Transport in Human Red Blood Cells

Heterogeneity of Red Blood Cells: Causes and Consequences

Mean values of hematological parameters are currently used in the clinical laboratory settings to characterize red blood cell properties. Those include red blood cell indices, osmotic fragility test, eosin 5-maleimide (EMA) test, and deformability assessment using ektacytometry to name a few. Diagnosis of hereditary red blood cell disorders is complemented by identification of mutations in distinct genes that are recognized "molecular causes of disease." The power of these measurements is clinically well-established. However, the evidence is growing that the available information is not enough to understand the determinants of severity of diseases and heterogeneity in manifestation of pathologies such as hereditary hemolytic anemias. This review focuses on an alternative approach to assess red blood cell properties based on heterogeneity of red blood cells and characterization of fractions of cells with similar properties such as density, hydration, membrane loss, redox state, Ca2+ levels, and morphology. Methodological approaches to detect variance of red blood cell properties will be presented. Causes of red blood cell heterogeneity include cell age, environmental stress as well as shear and metabolic stress, and multiple other factors. Heterogeneity of red blood cell properties is also promoted by pathological conditions that are not limited to the red blood cells disorders, but inflammatory state, metabolic diseases and cancer. Therapeutic interventions such as splenectomy and transfusion as well as drug administration also impact the variance in red blood cell properties. Based on the overview of the studies in this area, the possible applications of heterogeneity in red blood cell properties as prognostic and diagnostic marker commenting on the power and selectivity of such markers are discussed.

The effect of xanthine oxidase and hypoxanthine on the permeability of red cells from patients with sickle cell anemia

Red cells from patients with sickle cell anemia (SCA) are under greater oxidative challenge than those from normal individuals. We postulated that oxidants generated by xanthine oxidase (XO) and hypoxanthine (HO) contribute to the pathogenesis of SCA through altering solute permeability. Sickling, activities of the main red cell dehydration pathways (Psickle , Gardos channel, and KCl cotransporter [KCC]), and cell volume were measured at 100, 30, and 0 mmHg O2 , together with deoxygenation-induced nonelectrolyte hemolysis. Unexpectedly, XO/HO mixtures had mainly inhibitory effects on sickling, Psickle , and Gardos channel activities, while KCC activity and nonelectrolyte hemolysis were increased. Gardos channel activity was significantly elevated in red cells pharmacologically loaded with Ca2+ using the ionophore A23187, consistent with an effect on the transport system per se as well as via Ca2+ entry likely via the Psickle pathway. KCC activity is controlled by several pairs of conjugate protein kinases and phosphatases. Its activity, however, was also stimulated by XO/HO mixtures in red cells pretreated with N-ethylmaleimide (NEM), which is thought to prevent regulation via changes in protein phosphorylation, suggesting that the oxidants formed could also have direct effects on this transporter. In the presence of XO/HO, red cell volume was better maintained in deoxygenated red cells. Overall, the most notable effect of XO/HO mixtures was an increase in red cell fragility. These findings increase our understanding of the effects of oxidative challenge in SCA patients and are relevant to the behavior of red cells in vivo.

N-ethylmaleimide activates a Cl(-)-independent component of K(+) flux in mouse erythrocytes

The K-Cl cotransporters (KCCs) of mouse erythrocytes exhibit higher basal activity than those of human erythrocytes, but are similarly activated by cell swelling, by hypertonic urea, and by staurosporine. However, the dramatic stimulation of human erythroid KCCs by N-ethylmaleimide (NEM) is obscured in mouse erythrocytes by a prominent NEM-stimulated K(+) efflux that lacks Cl(-)-dependence. The NEM-sensitivity of Cl(-)-independent K(+) efflux of mouse erythrocytes is lower than that of KCC. The genetically engineered absence of the K-Cl cotransporters KCC3 and KCC1 from mouse erythrocytes does not modify Cl(-)-independent K(+) efflux. Mouse erythrocytes genetically devoid of the Gardos channel KCNN4 show increased NEM-sensitivity of both Cl(-)-independent K(+) efflux and K-Cl cotransport. The increased NEM-sensitivity and stimulation magnitude of Cl(-)-independent K(+) efflux in mouse erythrocytes expressing transgenic hypersickling human hemoglobin SAD (HbSAD) are independent of the presence of KCC3 and KCC1, but absence of KCNN4 reduces the stimulatory effect of HbSAD. NEM-stimulated Cl(-)-independent K(+) efflux of mouse red cells is insensitive to ouabain and bumetanide, but partially inhibited by chloroquine, barium, and amiloride. The NEM-stimulated activity is modestly reduced at pH6.0 but not significantly altered at pH8.0, and is abolished at 0°C. Although the molecular identity of this little-studied K(+) efflux pathway of mouse erythrocytes remains unknown, its potential role in the pathophysiology of sickle red cell dehydration will be important for the extrapolation of studies in mouse models of sickle cell disease to our understanding of humans with sickle cell anemia.

Peroxynitrite oxidizes erythrocyte membrane band 3 protein and diminishes its anion transport capacity

We describe an altered membrane band 3 protein-mediated anion transport in erythrocytes exposed to peroxynitrite, and relate the loss of anion transport to cell damage and to band 3 oxidative modifications. We found that peroxynitrite down-regulate anion transport in a dose dependent relation (100-300 micromoles/l). Hemoglobin oxidation was found at all peroxynitrite concentrations studied. A dose-dependent band 3 protein crosslinking and tyrosine nitration were also observed. Band 3 protein modifications were concomitant with a decrease in transport activity. ( - )-Epicatechin avoids band 3 protein nitration but barely affects its transport capacity, suggesting that both processes are unrelated. N-acetyl cysteine partially reverted the loss of band 3 transport capacity. It is concluded that peroxynitrite promotes a decrease in anion transport that is partially due to the reversible oxidation of band 3 cysteine residues. Additionally, band 3 tyrosine nitration seems not to be relevant for the loss of its anion transport capacity.

Peroxynitrite Increases Protein Phosphatase Activity and Promotes the Interaction of Phospholamban with Protein Phosphatase 2a in the Myocardium

High levels of peroxynitrite have been shown to decrease cardiomyocyte contraction through a reduction in phospholamban (PLB) phosphorylation. However, previous reports did not examine the direct effect of peroxynitrite on protein phosphatase activity in the myocardium or the role of specific phosphatases. Here we test the effect of the peroxynitrite donor SIN-1 on protein phosphatase activity in whole heart homogenates, as well as the interaction of PLB with protein phosphatase 1 (PP1) and 2a (PP2a). SIN-1 (200 μmol/L) induced a significant increase in protein phosphatase activity, which was alleviated with the specific PP1/PP2a inhibitor okadaic acid. Conversely, lower concentrations of SIN-1 and the nitric oxide donor spermine NONOate (300 μmol/L) were both without effect on phosphatase activity. We next examined the effect of SIN-1 on the interaction of PLB with PP1 and PP2a using co-immunoprecipitation, since okadaic acid inhibited the effects of SIN-1 in our current and previous studies. SIN-1 significantly increased the interaction of PLB with PP2a, but had no effect on the interaction between PLB and PP1. Urate, a peroxynitrite scavenger, inhibited the effects of SIN-1 on phosphatase activity and the interaction of PLB with PP2a, thus implicating peroxynitrite as the causal species. The results of this study provide further insight into the mechanism through which high levels of peroxynitrite serve to decrease PLB phosphorylation and myocardial contraction. Therefore, peroxynitrite signaling could play a key role in the contractile dysfunction manifested in heart failure where peroxynitrite production and protein phosphatase activity are increased and PLB phosphorylation is decreased.

Oxygen-dependent ion transport in erythrocytes

The present contribution reviews current knowledge of apparently oxygen-dependent ion transport in erythrocytes and presents modern hypotheses on their regulatory mechanisms and physiological roles. In addition to molecular oxygen as such, reactive oxygen species, nitric oxide, carbon monoxide, regional variations of cellular ATP and hydrogen sulphide may play a role in the regulation of transport, provided that they are affected by oxygen tension. It appears that the transporter molecules themselves do not have direct oxygen sensors. Thus, the oxygen level must be sensed elsewhere, and the effect transduced to the transporter. The possible pathways involved in the regulation of transport, including haemoglobin as a sensor, and phosphorylation/dephosphorylation reactions both in the transporter and its upstream effectors, are discussed.

Mechanisms and significance of cell volume regulation

Survival of human and animal cells requires avoidance of excessive alterations of cell volume. The osmolarity amassed by cellular accumulation of organic substances must be compensated by lowering cytosolic ion concentrations. The Na+/K+ ATPase extrudes Na+ in exchange for K+, which can permeate the cell membrane through K+ channels. K+ exit generates a cell-negative potential difference across the cell membrane, driving the exit of anions such as Cl-. The low cytosolic Cl- concentrations counterbalance the excess cellular osmolarity by organic substances. Cell volume regulation following cell swelling involves releasing ions through activation of K+ channels and/or anion channels, KCl-cotransport, or parallel activation of K+/H+ exchange and Cl-/HCO3- exchange. Cell volume regulation following cell shrinkage involves accumulation of ions through activation of Na+,K+,2Cl- cotransport, Na+/H+ exchange in parallel to Cl-/HCO3- exchange, or Na+ channels. The Na+ taken up is extruded by the Na+/K+ ATPase in exchange for K+. Shrunken cells further accumulate organic osmolytes such as sorbitol and glycerophosphorylcholine, and monomeric amino acids by altered metabolism and myoinositol (inositol), betaine, taurine, and amino acids by Na+ coupled transport. They release osmolytes during cell swelling. Challenges of cell volume homeostasis include transport, hormones, transmitters, and drugs. Moreover, alterations of cell volume participate in the machinery regulating cell proliferation and apoptotic cell death. Deranged cell volume regulation significantly contributes to the pathophysiology of several disorders such as liver insufficiency, diabetic ketoacidosis, hypercatabolism, fibrosing disease, sickle cell anemia, and infection.

Targeting of phospholamban by peroxynitrite decreases beta-adrenergic stimulation in cardiomyocytes

AIMS

Peroxynitrite production increases during the pathogenesis of numerous cardiac disorders (e.g. heart failure). However, limited studies have investigated the mechanism through which peroxynitrite exerts anti-adrenergic effects. Thus, the purpose of this study is to investigate the contribution of phospholamban (PLB), a critical excitation-contraction coupling protein, to the peroxynitrite-induced dysfunction.

METHODS AND RESULTS

Isolated myocytes from wild-type (WT, CF-1) and PLB knockout (PLB(-/-)) mice were stimulated at 1 Hz, and myocyte shortening and Ca(2+) transients were simultaneously recorded. PLB phosphorylation was measured via western blot. Myocytes were superfused with isoproterenol, a beta-adrenergic agonist, and SIN-1, a peroxynitrite donor. SIN-1 superfusion dramatically decreased isoproterenol-stimulated Ca(2+) transients and myocyte shortening in WT myocytes. These effects were inhibited upon addition of the peroxynitrite decomposition catalyst, FeTPPS. Surprisingly, SIN-1 had no functional effect on beta-adrenergic-stimulated PLB(-/-) myocytes. Western blot analyses revealed that SIN-1 significantly decreased isoproterenol-stimulated PLB(Ser16) phosphorylation. Experiments with the protein phosphatase inhibitor, okadaic acid, alleviated the SIN-1-induced functional effects and the decrease in PLB phosphorylation.

CONCLUSIONS

The peroxynitrite donor SIN-1 decreases beta-adrenergic stimulation by reducing PLB(Ser16) phosphorylation via protein phosphatase activation. This peroxynitrite-induced decrease in PLB phosphorylation may be a key mechanism in the beta-adrenergic dysfunction observed in many cardiomyopathies.

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.