Docosahexaenoic and eicosapentaenoic acids increase prion formation in neuronal cells

BACKGROUND: The transmissible spongiform encephalopathies, otherwise known as prion diseases, occur following the conversion of the cellular prion protein (PrP(C)) to an alternatively folded, disease-associated isoform (PrP(Sc)). Recent studies suggest that this conversion occurs via a cholesterol-sensitive process, as cholesterol synthesis inhibitors reduced the formation of PrP(Sc )and delayed the clinical phase of scrapie infection. Since polyunsaturated fatty acids also reduced cellular cholesterol levels we tested their effects on PrP(Sc )formation in three prion-infected neuronal cell lines (ScGT1, ScN2a and SMB cells). RESULTS: We report that treatment with docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA) or the cholesterol synthesis inhibitor simvastatin reduced the amounts of free cholesterol in membrane extracts from prion-infected neuronal cells. Simvastatin reduced cholesterol production while DHA and EPA promoted the conversion of free cholesterol to cholesterol esters. Crucially, while simvastatin reduced PrP(Sc )formation, both DHA and EPA significantly increased the amounts of PrP(Sc )in these cells. Unlike simvastatin, the effects of DHA and EPA on PrP(Sc )content were not reversed by stimulation of cholesterol synthesis with mevalonate. Treatment of ScGT1 cells with DHA and EPA also increased activation of cytoplasmic phospholipase A(2 )and prostaglandin E(2 )production. Finally, treatment of neuronal cells with DHA and EPA increased the amounts of PrP(C )expressed at the cell surface and significantly increased the half-life of biotinylated PrP(C). CONCLUSION: We report that although treatment with DHA or EPA significantly reduced the free cholesterol content of prion-infected cells they significantly increased PrP(Sc )formation in three neuronal cell lines. DHA or EPA treatment of infected cells increased activation of phospholipase A(2), a key enzyme in PrP(Sc )formation, and altered the trafficking of PrP(C). PrP(C )expression at the cell surface, a putative site for the PrP(Sc )formation, was significantly increased, and the rate at which PrP(C )was degraded was reduced. Cholesterol depletion is seen as a potential therapeutic strategy for prion diseases. However, these results indicate that a greater understanding of the precise relationship between membrane cholesterol distribution, PrP(C )trafficking, cell activation and PrP(Sc )formation is required before cholesterol manipulation can be considered as a prion therapeutic.