Amyloid‐β peptide enhances tumor necrosis factor‐α‐induced iNOS through neutral sphingomyelinase/ceramide pathway in oligodendrocytes

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Abstract
Although accumulating evidence demonstrates that white matter degeneration contributes to pathology in Alzheimer's disease (AD), the underlying mechanisms are unknown. In order to study the roles of the amyloid‐β peptide in inducing oxidative stress damage in white matter of AD, we investigated the effects of amyloid‐β peptide 25–35 (Aβ) on proinflammatory cytokine tumor necrosis factor‐α (TNF‐α)‐induced inducible nitric oxide synthase (iNOS) in cultured oligodendrocytes (OLGs). Although Aβ 25–35 by itself had little effect on iNOS mRNA, protein, and nitrite production, it enhanced TNF‐α‐induced iNOS expression and nitrite generation in OLGs. Aβ, TNF‐α, or the combination of both, increased neutral sphingomyelinase (nSMase) activity, but not acidic sphingomyelinase (aSMase) activity, leading to ceramide accumulation. Cell permeable C2‐ceramide enhanced TNF‐α‐induced iNOS expression and nitrite generation. Moreover, the specific nSMase inhibitor, 3‐O‐methyl‐sphingomyelin (3‐OMS), inhibited iNOS expression and nitrite production induced by TNF‐α or by the combination of TNF‐α and Aβ. Overexpression of a truncated mutant of nSMase with a dominant negative function inhibited iNOS mRNA production. 3‐OMS also inhibited nuclear factor κB (NF‐κB) binding activity induced by TNF‐α or by the combination of TNF‐α and Aβ. These results suggest that neutral sphingomyelinase/ceramide pathway is required but may not be sufficient for iNOS expression induced by TNF‐α and the combination of TNF‐α and Aβ.