Wilson disease is caused by accumulation of Cu(2+) in cells, which results in liver cirrhosis and, occasionally, anemia. Here, we show that Cu(2+) triggers hepatocyte apoptosis through activation of acid sphingomyelinase (Asm) and release of ceramide. Genetic deficiency or pharmacological inhibition of Asm prevented Cu(2+)-induced hepatocyte apoptosis and protected rats, genetically prone to develop Wilson disease, from acute hepatocyte death, liver failure and early death. Cu(2+) induced the secretion of activated Asm from leukocytes, leading to ceramide release in and phosphatidylserine exposure on erythrocytes, events also prevented by inhibition of Asm. Phosphatidylserine exposure resulted in immediate clearance of affected erythrocytes from the blood in mice. Accordingly, individuals with Wilson disease showed elevated plasma levels of Asm, and displayed a constitutive increase of ceramide- and phosphatidylserine-positive erythrocytes. Our data suggest a previously unidentified mechanism for liver cirrhosis and anemia in Wilson disease.
depletion of erythrocytes leads to activation of Ca 2ϩ -permeable cation channels, Ca 2ϩ entry, activation of a Ca 2ϩ -sensitive erythrocyte scramblase, and subsequent exposure of phosphatidylserine at the erythrocyte surface. Ca 2ϩ 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 PKC␣, 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 M staurosporine or 50 nM calphostin C. The PKC stimulator phorbol-12-myristate-13-acetate (3 M) and the phosphatase inhibitor okadaic acid (1-10 M) mimicked the effect of glucose depletion and similarly led to translocation of PKC␣ and enhanced serine phosphorylation, increased annexin binding, and decreased forward scatter, the latter effects being abrogated by PKC inhibitor staurosporine (1 M). Fluo-3 fluorescence measurements revealed that okadaic acid also enhanced erythrocyte Ca 2ϩ 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. cell volume; eryptosis; calcium; okadaic acid; staurosporine AS SHOWN RECENTLY, erythrocytes exposed to oxidative stress, osmotic shock, or glucose depletion activate a Ca 2ϩ -permeable cation channel (22,29,30,32)
Osmotic erythrocyte shrinkage leads to activation of cation channels with subsequent Ca2+ entry and stimulates a sphingomyelinase with subsequent formation of ceramide. Ca2+ and ceramide then activate a scramblase leading to breakdown of phosphatidylserine asymmetry of the cell membrane. The mediators accounting for activation of erythrocyte sphingomyelinase and phosphatidylserine exposure remained elusive. The study demonstrates that platelet-activating factor (PAF) is released from erythrocytes upon hyperosmotic cell shrinkage. The experiments further disclose the presence of PAF receptors in erythrocytes and show that PAF stimulates the breakdown of sphingomyelin and the release of ceramide from erythrocytes at isotonic conditions. PAF further triggers cell shrinkage (decrease of forward scatter) and phosphatidylserine exposure (annexin binding) of erythrocytes. The stimulation of annexin-binding is blunted by a genetic knockout of PAF receptors, by the PAF receptor antagonist ABT491 or by inhibition of sphingomyelinase with urea. In conclusion, PAF activates an erythrocyte sphingomyelinase and the then formed ceramide leads to the activation of scramblase with subsequent phosphatidylserine exposure.
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.
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