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

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-beta peptide in inducing oxidative stress damage in white matter of AD, we investigated the effects of amyloid-beta peptide 25-35 (A beta) on proinflammatory cytokine tumor necrosis factor-alpha (TNF-alpha)-induced inducible nitric oxide synthase (NOS) in cultured oligodendrocytes (OLGs). Although A beta 25-35 by itself had little effect on NOS mRNA, protein, and nitrite production, it enhanced TNF-alpha-induced NOS expression and nitrite generation in OLGs. A beta, TNF-alpha, 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-alpha-induced NOS expression and nitrite generation. Moreover, the specific nSMase inhibitor, 3-O-methyl-sphingomyelin (3-OMS), inhibited iNOS expression and nitrite production induced by TNF-alpha. or by the combination of TNF-alpha and A. Overexpression of a truncated mutant of nSMase with a dominant negative function inhibited NOS mRNA production. 3-OMS also inhibited nuclear factor kappa B (NF-kappa B) binding activity induced by TNF-alpha or by the combination of TNF-a and A beta. These results suggest that neutral sphingomyelinase/ceramide pathway is required but may not be sufficient for NOS expression induced by TNF-alpha and the combination of TNF-alpha and A beta.