Alteration in protein kinase C activity and subcellular distribution in sickle erythrocytes

Ankyrin exposure induced by activated protein kinase C plays a potential role in erythrophagocytosis

BACKGROUND

In physiological and pathological conditions activated protein kinace C (PKC) has been observed in the erythrocytes. Externalization of ankyrin followed by Arg-Gly-Asp (RGD)/integrin recognition also triggers erythrophagocytosis. In the present study, to test whether activated PKC is associated with ankyrin exposure in erythrophagocytosis.

METHODS

Phorbol 12-myristate-13-acetate (PMA)-induced PKC activation and ankyrin phosphorylation were tested, and under different treatment conditions the subpopulation of erythrocytes with ankyrin exposure and the levels of intracellular calcium were analyzed by flow cytometry.

RESULTS

Results showed that treatment of erythrocytes with PMA in a calcium-containing buffer led to ankyrin exposure. In the absence of extracellular calcium, no ankyrin exposure was observed. PKC inhibition with calphostin C, a blocker of the PMA binding site, completely prevented the calcium entry, protein phosphorylation and ankyrin exposure. PKC inhibition with chelerythrine chloride, an inhibitor of the active site, diminished the level of ankyrin-exposing cells and ankyrin phosphorylation; however it even led to a higher percentage of cells with increased levels of calcium than with PMA treatment alone. Although PKC was activated and ankyrin phosphorylation occurred, no ankyrin exposure was observed in the absence of extracellular calcium.

CONCLUSION

Analyses of results suggested that PMA induces calcium influx into the erythrocytes, leading to the activation of calcium-dependent enzymes and the phosphorylation of membrane proteins, ultimately inducing ankyrin exposure and erythrophagocytosis. This study may provide insights into the molecular mechanisms of removing aged or diseased erythrocytes.

Activation of protein kinase C by phorbol ester increases red blood cell scramblase activity and external phosphatidylserine

Externalization of phosphatidylserine (PS) is thought to contribute to sickle cell disease (SCD) pathophysiology. The red blood cell (RBC) aminophospholipid translocase (APLT) mediates the transport of PS from the outer to the inner RBC membrane leaflet to maintain an asymmetric distribution of PL, while phospholipid scramblase (PLSCR) equilibrates PL across the RBC membrane, promoting PS externalization. We previously identified an association between PS externalization level and PLSCR activity in sickle RBC under basal conditions. Other studies showed that activation of protein kinase C (PKC) by PMA (phorbol-12-myristate-13-acetate) causes increased external PS on RBC. Therefore, we hypothesized that PMA-activated PKC stimulates PLSCR activity in RBC and thereby contributes to increased PS externalization. In the current studies, we show that PMA treatment causes immediate and variable PLSCR activation and subsequent PS externalization in control and sickle RBC. While TfR+ sickle reticulocytes display some endogenous PLSCR activity, we observed a robust activation of PLSCR in sickle reticulocytes treated with PMA. The PKC inhibitor, chelerythrine (Chel), significantly inhibited PMA-dependent PLSCR activation and PS externalization. Chel also inhibited endogenous PLSCR activity in sickle reticulocytes. These data provide evidence that PKC mediates PS externalization in RBC through activation of PLSCR.

Erythrocyte NADPH oxidase activity modulated by Rac GTPases, PKC, and plasma cytokines contributes to oxidative stress in sickle cell disease

Chronic inflammation has emerged as an important pathogenic mechanism in sickle cell disease (SCD). One component of this inflammatory response is oxidant stress mediated by reactive oxygen species (ROS) generated by leukocytes, endothelial cells, plasma enzymes, and sickle red blood cells (RBC). Sickle RBC ROS generation has been attributed to sickle hemoglobin auto-oxidation and Fenton chemistry reactions catalyzed by denatured heme moieties bound to the RBC membrane. In this study, we demonstrate that a significant part of ROS production in sickle cells is mediated enzymatically by NADPH oxidase, which is regulated by protein kinase C, Rac GTPase, and intracellular Ca(2+) signaling within the sickle RBC. Moreover, plasma from patients with SCD and isolated cytokines, such as transforming growth factor β1 and endothelin-1, enhance RBC NADPH oxidase activity and increase ROS generation. ROS-mediated damage to RBC membrane components is known to contribute to erythrocyte rigidity and fragility in SCD. Erythrocyte ROS generation, hemolysis, vaso-occlusion, and the inflammatory response to tissue damage may therefore act in a positive-feedback loop to drive the pathophysiology of sickle cell disease. These findings suggest a novel pathogenic mechanism in SCD and may offer new therapeutic targets to counteract inflammation and RBC rigidity and fragility in SCD.

Altered phosphorylation of cytoskeleton proteins in sickle red blood cells: the role of protein kinase C, Rac GTPases, and reactive oxygen species

The small Rho GTPases Rac1 and Rac2 regulate actin structures and mediate reactive oxygen species (ROS) production via NADPH oxidase in a variety of cells. We have demonstrated that deficiency of Rac1 and Rac2 GTPases in mice disrupts the normal hexagonal organization of the RBC cytoskeleton and reduces erythrocyte deformability. This is associated with increased phosphorylation of adducin at Ser-724, (corresponding to Ser-726 in human erythrocytes), a domain target of protein kinase C (PKC). PKC phosphorylates adducin and leads to decreased F-actin capping and dissociation of spectrin from actin, implicating a significant role of such phosphorylation in cytoskeletal remodeling. We evaluated adducin phosphorylation in erythrocytes from patients with sickle cell disease and found it consistently increased at Ser-726. In addition, ROS concentration is elevated in sickle erythrocytes by 150-250% compared to erythrocytes from normal control individuals. Here, we review previous studies demonstrating that altered phosphorylation of erythrocyte cytoskeletal proteins and increased ROS production result in disruption of cytoskeleton stability in healthy and sickle cell erythrocytes. We discuss in particular the known and potential roles of protein kinase C and the Rac GTPases in these two processes.

Current knowledge about the functional roles of phosphorylative changes of membrane proteins in normal and diseased red cells

With the advent of proteomic techniques the number of known post-translational modifications (PTMs) affecting red cell membrane proteins is rapidly growing but the understanding of their role under physiological and pathological conditions is incompletely established. The wide range of hereditary diseases affecting different red cell membrane functions and the membrane modifications induced by malaria parasite intracellular growth represent a unique opportunity to study PTMs in response to variable cellular stresses. In the present review, some of the major areas of interest in red cell membrane research have been considered as modifications of erythrocyte deformability and maintenance of the surface area, membrane transport alterations, and removal of diseased and senescent red cells. In all mentioned research areas the functional roles of PTMs are prevalently restricted to the phosphorylative changes of the more abundant membrane proteins. The insufficient information about the PTMs occurring in a large majority of the red membrane proteins and the general lack of mass spectrometry data evidence the need of new comprehensive, proteomic approaches to improve the understanding of the red cell membrane physiology.

Two Distinct Pathways Mediate the Formation of Intermediate Density Cells and Hyperdense Cells From Normal Density Sickle Red Blood Cells

In sickle cell anemia (SS), some red blood cells dehydrate, forming a hyperdense (HD) cell fraction (>1.114 g/mL; mean corpuscular hemoglobin concentration [MCHC], >46 g/dL) that contains many irreversibly sickled cells (ISCs), whereas other SS red blood cells dehydrate to an intermediate density (ID; 1.090 to 1.114 g/mL; MCHC, 36 to 46 g/dL). This study asks if the potassium-chloride cotransporter (K:Cl) and the calcium-dependent potassium channel [K(Ca2+)] are participants in the formation of one or both types of dense SS red blood cells. We induced sickling by exposing normal density (ND; 1.080 to 1.090 g/mL; MCHC, 32 to 36 g/dL) SS discocytes to repetitive oxygenation-deoxygenation (O-D) cycles in vitro. At physiologic Na+, K+, and Cl−, and 0.5 to 2 mmol/L Ca2+, the appearance of dense cells was time- and pH-dependent. O-D cycling at pH 7.4 in 5% CO2-equilibrated buffer generated only ID cells, whereas O-D cycling at pH 6.8 in 5% CO2-equilibrated buffer generated both ID and HD cells, the latter taking more than 8 hours to form. At 22 hours, 35% ± 17% of the parent ND cells were recovered in the ID fraction and 18% ± 11% in the HD fraction. Continuous deoxygenation (N2/5% CO2) at pH 6.8 generated both ID and HD cells, but many of these cells had multiple projections, clearly different from the morphology of endogenous dense cells and ISCs. Continuous oxygenation (air/5% CO2) at pH 6.8 resulted in less than 10% dense cell (ID + HD) formation. ATP depletion substantially increased HD cell formation and moderately decreased ID cell formation. HD cells formed after 22 hours of O-D cycling at pH 6.8 contained fewer F cells than did ID cells, suggesting that HD cell formation is particularly dependent on HbS polymerization. EGTA chelation of buffer Ca2+ inhibited HD but not ID cell formation, and increasing buffer Ca2+ from 0.5 to 2 mmol/L promoted HD but not ID cell formation in some SS patients. Substitution of nitrate for Cl− inhibited ID cell formation, as did inhibitors of the K:Cl cotransporter, okadaic acid, and [(dihydroindenyl) oxy]alkanoic acid (DIOA). Conversely, inhibitors of K(Ca2+), charybdotoxin and clotrimazole, inhibited HD cell formation. The combined use of K(Ca2+) and K:Cl inhibitors nearly eliminated dense cell (ID + HD cell) formation. In summary, dense cells formed by O-D cycling for 22 hours at pH 7.4 cycling are predominately the ID type, whereas dense cells formed by O-D cycling for 22 hours at pH 6.8 are both the ID and HD type, with the latter low in HbF, suggesting that HD cell formation has a greater dependency on HbS polymerization. A combination of K:Cl cotransport and the K(Ca2+) activities account for the majority of dense cells formed, and these pathways can be driven independently. We propose a model in which reversible sickling-induced K+ loss by K:Cl primarily generates ID cells and K+ loss by the K(Ca2+) channel primarily generates HD cells. These results imply that both pathways must be inhibited to completely prevent dense SS cell formation and have potential therapeutic implications.

Two Distinct Pathways Mediate the Formation of Intermediate Density Cells and Hyperdense Cells From Normal Density Sickle Red Blood Cells

In sickle cell anemia (SS), some red blood cells dehydrate, forming a hyperdense (HD) cell fraction (>1.114 g/mL; mean corpuscular hemoglobin concentration [MCHC], >46 g/dL) that contains many irreversibly sickled cells (ISCs), whereas other SS red blood cells dehydrate to an intermediate density (ID; 1.090 to 1.114 g/mL; MCHC, 36 to 46 g/dL). This study asks if the potassium-chloride cotransporter (K:Cl) and the calcium-dependent potassium channel [K(Ca2+)] are participants in the formation of one or both types of dense SS red blood cells. We induced sickling by exposing normal density (ND; 1.080 to 1.090 g/mL; MCHC, 32 to 36 g/dL) SS discocytes to repetitive oxygenation-deoxygenation (O-D) cycles in vitro. At physiologic Na+, K+, and Cl−, and 0.5 to 2 mmol/L Ca2+, the appearance of dense cells was time- and pH-dependent. O-D cycling at pH 7.4 in 5% CO2-equilibrated buffer generated only ID cells, whereas O-D cycling at pH 6.8 in 5% CO2-equilibrated buffer generated both ID and HD cells, the latter taking more than 8 hours to form. At 22 hours, 35% ± 17% of the parent ND cells were recovered in the ID fraction and 18% ± 11% in the HD fraction. Continuous deoxygenation (N2/5% CO2) at pH 6.8 generated both ID and HD cells, but many of these cells had multiple projections, clearly different from the morphology of endogenous dense cells and ISCs. Continuous oxygenation (air/5% CO2) at pH 6.8 resulted in less than 10% dense cell (ID + HD) formation. ATP depletion substantially increased HD cell formation and moderately decreased ID cell formation. HD cells formed after 22 hours of O-D cycling at pH 6.8 contained fewer F cells than did ID cells, suggesting that HD cell formation is particularly dependent on HbS polymerization. EGTA chelation of buffer Ca2+ inhibited HD but not ID cell formation, and increasing buffer Ca2+ from 0.5 to 2 mmol/L promoted HD but not ID cell formation in some SS patients. Substitution of nitrate for Cl− inhibited ID cell formation, as did inhibitors of the K:Cl cotransporter, okadaic acid, and [(dihydroindenyl) oxy]alkanoic acid (DIOA). Conversely, inhibitors of K(Ca2+), charybdotoxin and clotrimazole, inhibited HD cell formation. The combined use of K(Ca2+) and K:Cl inhibitors nearly eliminated dense cell (ID + HD cell) formation. In summary, dense cells formed by O-D cycling for 22 hours at pH 7.4 cycling are predominately the ID type, whereas dense cells formed by O-D cycling for 22 hours at pH 6.8 are both the ID and HD type, with the latter low in HbF, suggesting that HD cell formation has a greater dependency on HbS polymerization. A combination of K:Cl cotransport and the K(Ca2+) activities account for the majority of dense cells formed, and these pathways can be driven independently. We propose a model in which reversible sickling-induced K+ loss by K:Cl primarily generates ID cells and K+ loss by the K(Ca2+) channel primarily generates HD cells. These results imply that both pathways must be inhibited to completely prevent dense SS cell formation and have potential therapeutic implications.

Effects of PKC alpha activation on Ca2+ pump and K(Ca) channel in deoxygenated sickle cells

We have previously shown that a pretreatment with phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), reduced deoxygenation-induced K+ loss and Ca2+ uptake and prevented cell dehydration in sickle anemia red blood cells (SS cells) (H. Fathallah, E. Coezy, R.-S. De Neef, M.-D. Hardy-Dessources, and F. Giraud. Blood 86: 1999-2007, 1995). The present study explores the detailed mechanism of this PMA-induced inhibition. The main findings are, first, the detection of PKC alpha and PKC zeta in normal red blood cells and the demonstration that both isoforms are expressed at higher levels in SS cells. The alpha-isoform only is translocated to the membrane and activated by PMA and by elevation of cytosolic Ca2+. Second, PMA is demonstrated to activate Ca2+ efflux in deoxygenated SS cells by a direct stimulation of the Ca2+ pump. PMA, moreover, inhibits deoxygenation-induced, charybdotoxin-sensitive K+ efflux in SS cells. This inhibition is partly indirect and explained by the reduced deoxygenation-induced rise in cytosolic Ca2+ resulting from Ca2+ pump stimulation. However, a significant inhibition of the Ca2+-activated K+ channels (K(Ca) channels) by PMA can also be demonstrated when the channels are activated by Ca2+ plus ionophore, under conditions in which the Ca2+ pump is operating near its maximal extrusion rate, but swamped by Ca2+ plus ionophore. The data thus suggest a PKC alpha-mediated phosphorylation both of the Ca2+ pump and of the K(Ca) channel or an auxiliary protein.

Relationship between type II phosphatidylinositol 4-kinase activity and protein tyrosine phosphorylation in membranes from normal and sickle red cells

To assess the origin of the previously reported higher type II phosphatidylinositol 4-kinase (PtdIns 4-kinase) activity of sickle-red-cell membranes [Rhoda-Hardy-Dessources, M.D., de Neef, R.S., Mérault, G.& Giraud, F. (1993) Biochim. Biophs. Acta 1181, 90-96], we have investigated the possible involvement of protein kinase C and tyrosine kinases in the regulation of the lipid kinase activity. Both protein kinase activities were found to be markedly higher in membranes from the pathological cells. When isolated normal-red-cell or sickle-red-cell membranes were assayed, phosphatidylinositol phosphorylation activity was not significantly modified after phorbol ester modulation of protein kinase C. In contrast, stimulation (with sodium orthovanadate) or inhibiton (by tyrphostin) of tyrosine phosphorylation led respectively, to increased or decreased PtdIns 4-kinase activity in membranes from both cell types. Moreover, immunoprecipitations of membrane extracts from normal and sickle red cells types with anti-PtdIns 4-kinase antibody 4C5G, followed by immunoblotting with an anti-phosphotyrosine Ig, revealed a 56-kDa band migrating with PtdIns 4-kinase activity. Taken together, these findings indicate that PtdIns 4-Kinase in red blood cells is a phosphotyrosine-containing protein and could be regulated by a mechanism involving tyrosine phosphorylation, and the increase in PtdIns 4-Kinase activity of sickle-red-cell membranes is at least in part mediated by their intrinsic tyrosine kinase activity.

Phorbol 12-myristate 13-acetate-stimulated phosphorylation of erythrocyte membrane skeletal proteins is blocked by calpain inhibitors: possible role of protein kinase M

Human erythrocytes contain cytosolic protein kinase C (PKC) which, when activated by phorbol 12-myristate 13-acetate (PMA), induces the phosphorylation of the membrane skeletal proteins band 4.1, band 4.9 and adducin. We found that brief treatments of erythrocytes with PMA resulted in a decrease in cytosolic PKC content and in the transient appearance in the cytosol of a Ca(2+)- and phospholipid-independent 55 kDa fragment of PKC, called PKM. Prolonged treatment with PMA resulted in the complete and irreversible loss of erythrocyte PKC. To investigate the possible role of calpain in this process, the calpain inhibitors leupeptin and E-64 were sealed inside erythrocytes by reversible haemolysis. Both inhibitors prolonged the lifetime of PKC in PMA-treated cells, and leupeptin was shown to block the PMA-stimulated appearance of PKM in the cytosol. Significantly, leupeptin also completely blocked PMA-stimulated phosphorylation of membrane and cytosolic substrates. This effect was mimicked by other calpain inhibitors (MDL-28170 and calpain inhibitor I), but did not occur when other protease inhibitors such as phenylmethanesulphonyl fluoride, pepstatin A or chymostatin were used. In addition, the phosphorylation of exogenous histone sealed inside erythrocytes was also blocked by leupeptin. Immunoblotting showed that leupeptin did not prevent the PMA-induced translocation of PKC to the erythrocyte membrane. Thus inhibition of PKC phosphorylation of membrane skeletal proteins by calpain inhibitors was not due to inhibition of PKC translocation to the membrane. Our results suggest that PMA treatment of erythrocytes results in the translocation of PKC to the plasma membrane, followed by calpain-mediated cleavage of PKC to PKM. This cleavage, or some other leupeptin-inhibitable process, is a necessary step for the phosphorylation of membrane skeletal substrates, suggesting that the short-lived PKM may be responsible for membrane skeletal phosphorylation. Our results suggest a potential mechanism whereby erythrocyte PKC may be subject to continual down-regulation during the lifespan of the erythrocyte due to repeated activation events, possibly related to transient Ca2+ influx. Such down-regulation may play an important role in erythrocyte survival or pathophysiology.

Phosphoinositide phosphorylation and shape changes produced by phosmet-oxon in human erythrocytes

1. "In vitro" incubation of red blood cells with phosmetoxon induced crenated and invaginated forms. 2. [32P] phosphate incorporation was greater in membranes from erythrocytes exposed to 300 nM phosmetoxon for 10 min than in control cells. 3. The highest incorporation was for phosphatidylinositol (PI), followed by phosphatidylinositol phosphate (PIP) and phosphatidylinositolbiphosphate (PIP2). 4. An activation of phosphatidylinositol (PI) kinase was detected with 150 and 300 nM of the pesticide, while there was no change in poliphosphoinositides (PPI) phosphodiesterase activity. 5. Results suggest an association between changes in PI kinase activity, the phosphorylation cycle of phosphatidylinositols and alterations in erythrocyte morphology induced by phosmetoxon.

Characterization of phosphoinositide kinases in normal and sickle anaemia red cells

PtdIns and PtdInsP kinases from normal erythrocyte (AA) membranes and sickle cell anaemia erythrocyte (SS) membranes have been characterized. PtdIns kinase was studied in native membranes under conditions in which PtdInsP kinase and PtdInsP phosphatase do not express any activity. Kinetic analysis of the AA and SS PtdIns kinases indicate similar Km values for PtdIns and ATP but higher Vmax values for SS PtdIns kinase. PtdInsP kinase was partially purified from erythrocyte ghosts by NaCl extraction. The kinetic parameters of PtdInsP kinase determined under these conditions were similar in AA and SS NaCl extracts. These data suggest the presence of some effector of PtdIns kinase in SS cell membranes, resulting in a greater activity of the enzyme. This leads consequently, to increase the PtdIns4P pool and to activate PtdInsP kinase, in agreement with our previous observations of a greater [32P]Pi incorporation in both polyphosphoinositides in SS cells relatively to AA cells.

Sickle cell disease pathophysiology

The primary pathophysiological event in the erythrocytes of individuals with the various sickle syndromes is the intracellular aggregation or polymerization of sickle haemoglobin (HbS). The extent of polymerization is determined by the intracellular haemoglobin composition (% HbS and % HbS A, A2 and F), concentration (MCHC and % of dense cells) and oxygen saturation, as well as minor factors such as intracellular pH and DPG concentration. Intracellular HbS polymerization leads to a marked decrease in the flexibility or rheological properties of the sickle erythrocytes and obstruction in various microcirculatory beds, as well as chronic anaemia. Other abnormalities in the properties of the sickle erythrocytes, including membrane abnormalities, changes in ion fluxes and volume and endothelial adhesion, result from acute and chronic oxygen-linked polymerization events and may, in turn, modify polymerization. However, within a good approximation, many aspects of sickle cell disease pathophysiology--for example variations in anaemia among the different sickle syndromes--can be explained in terms of differences in polymerization tendency. Thus, the effects of alpha-thalassaemia can be explained with reference to changes in MCHC and syndromes with high HbF are understandable in terms of the sparing effect of HbF on polymerization. Recent therapeutic approaches to sickle cell disease focus on attempts to reduce intracellular HbS polymerization by altering the haemoglobin molecules, erythrocyte properties, or the distribution of intracellular haemoglobin species. The last, through pharmacological elevation of HbF, has become the central focus of much laboratory and clinical research in recent years. Agents such as hydroxyurea (with or without recombinant erythropoietin) and butyrate compounds elevate HbF (and reduce HbS) in a majority of sickle erythrocytes, thus decreasing intracellular polymerization. Current prospective protocols are designed to see if these changes cause clinical improvement at acceptable doses. Other treatment strategies, including bone marrow transplantation and possible gene replacement therapies, are also under active clinical or laboratory investigation.

Altered erythrocyte protein kinase C activity and membrane protein phosphorylation in chronic myelogenous leukemia

The membrane protein kinase C (PKC) content was found to be higher in erythrocytes form patients suffering from chronic myelogenous leukemia (CML) compared to normal erythrocytes. PKC activity was also higher in the cytosol and after translocation to the membrane, as assessed by histone phosphorylation. The increased PKC activity in CML erythrocytes was associated with abnormal phosphorylation of protein 4.1. Since phosphorylation-dephosphorylation mechanisms are likely candidates for controlling membrane protein associations, the altered PKC activity may be one of the factors responsible for altered thermal sensitivity and mechanical stability of CML erythrocytes.