Phosphatidylserine metabolism modification is associated with manganese-induced apoptosis in PC12 cells

Introduction
In the workplace chronic exposure to high Manganese (Mn) levels can lead to Parkinson-type neurological disorders. In vitro Mn exposure induces apoptosis in a catecholaminergic cell line (PC12 cells), which is associated with generation of reactive oxygen species. Various apoptotic stimuli affect phosphatidylserine metabolism and oxidation contributes to its externalization on apoptotic cell surfaces. In the present study, we investigated the effect of MnCl2 on phosphatidylserine metabolism in PC12 cells.

Methods
PC12 cells were incubated (24-72 h) with or without MnCl2 (0.1 or 1 mM). Cell viability and apoptosis were evaluated by respectively the MTT assay and flow cytometry, using the annexin V-FITC apoptosis detection kit. PS metabolism was evaluated by adding [3H]serine 1h before the end of incubation; radioactivity in phosphatidylserine, phosphatidylethanolamine and phosphatidylcholine were then measured. Expression of phosphatidylserine-synthezing enzymes was evaluated by Western-Blotting.

Results
MnCl2 exposure induced cell death in a dose- and time- dependent manner that seems to happen, at least in part, by apoptosis. 24h treatment with 1 mM MnCl2 reduced phosphatidylserine radioactivity, which was further investigated by measuring phosphatidylserine conversion into phosphatidylethanolamine and phosphadylcholine. Newly synthesized phosphatidylserine was decarboxylated into phosphatidylethanolamine. Compared with radioactive phosphatidylserine, radioactive phosphatidylethanolamine was 80% in MnCl2-treated cells and 20% in controls. Total radioactivity into glycerophospholipids was lower in MnCl2-treated samples than in controls, confirming phosphatidylserine synthesis was reduced, in apparent contrast with increased expression of phosphatidylserine-synthesizing enzymes.

Discussion
MnCl2 reduced phosphatidylserine synthesis in PC12 cells and increased phosphatidylserine decarboxylation, a mitochondrial process, in conditions that did not induce significant apoptosis and release of cytochrome c from mitochondria. Thus, these effects could reflect early events in MnCl2-induced apoptosis. That greater expression of phosphatidylserine synthesizing enzymes might be a consequence of reduced phosphatidylserine levels in the membrane opens up interesting perspectives.