Hexavalent chromium-induced erythrocyte membrane phospholipid asymmetry

Hexavalent (VI) chromium is a global contaminant with cytotoxic activity. Chromium (VI) induces oxidative stress, inflammation, cell proliferation, malignant transformation and may trigger carcinogenesis and at the same time apoptosis. The toxic effects of chromium (VI) at least partially result from mitochondrial injury and DNA damage. Erythrocytes lack mitochondria and nuclei but may experience an apoptosis-like suicidal cell death, i.e. eryptosis, which is characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine exposure at the cell surface. Eryptosis may result from increase of cytosolic Ca²⁺ activity, ATP depletion and/or ceramide formation. The present study explored, whether chromium (VI) triggers eryptosis. Fluo-3-fluorescence was employed to determine cytosolic Ca²⁺-concentration, forward scatter to estimate cell volume, binding of fluorescent annexin V to detect phosphatidylserine exposure, hemoglobin concentration in the supernatant to quantify hemolysis, luciferin–luciferase to determine cytosolic ATP concentration and fluorescent anti-ceramide antibodies to uncover ceramide formation. A 48 h exposure to chromium (VI) (≥10 μM) significantly increased cytosolic Ca²⁺-concentration, decreased ATP concentration (20 μM), decreased forward scatter, increased annexin V-binding and increased (albeit to a much smaller extent) hemolysis. Chromium (VI) did not significantly modify ceramide formation. The effect of 20 μM chromium (VI) on annexin V binding was partially reversed in the nominal absence of Ca²⁺. The present observations disclose a novel effect of chromium (VI), i.e. Ca²⁺ entry and cytosolic ATP depletion in erythrocytes, effects resulting in eryptosis with cell shrinkage and cell membrane scrambling.