β-Amyloid stimulation of inducible nitric-oxide synthase in astrocytes is interleukin-1β- and tumor necrosis factor-α (TNFα)-dependent, and involves a TNFα receptor-associated factor- and NFκB-inducing kinase- dependent signaling mechanism

In Alzheimer's disease, β-amyloid (Aβ) plaques are surrounded by activated astrocytes and microglia. A growing body of evidence suggests that these activated glia contribute to neurotoxicity through the induction of inflammatory cytokines such as interleukin (IL)-1β and tumor necrosis factor-α (TNFα) and the production of neurotoxic free radicals, mediated in part by the expression of inducible nitric-oxide synthase (iNOS). Here, we address the possibility that Aβ-stimulated iNOS expression might result from an initial induction of IL-1β and TNFα. We find that in Aβ-stimulated astrocyte cultures, IL-1β and TNFα production occur before iNOS production, new protein synthesis is required for increased iNOS mRNA levels, and the IL- 1 receptor antagonist IL-1ra can inhibit nitrite accumulation. Likewise, dominant-negative mutants of tumor necrosis factor-α receptor-associated factor (TRAF) 6, TRAF2, and NFκB-inducing kinase (NIK), intracellular proteins involved in IL-1 and TNFα receptor signaling cascades, inhibit Aβ- stimulated iNOS promoter activity. Our data suggest that Aβ stimulation of astrocyte iNOS is mediated in part by IL-1β and TNFα, and involves a TRAF6- , TRAF2-, and NIK-dependent signaling mechanism.