LOW ENVIRONMENTAL PH PARTICIPATES IN THE INDUCTION OF NO SYNTHASE TYPE-II IN MACROPHAGES

Stimulation of macrophages with endotoxin and/or cytokines is responsi ble for the expression of the inducible isoform of nitric oxide syntha se (iNOS). Because macrophages are exposed to low pH within the microe nvironment of inflammatory lesions, the potential role of low pH as an additional regulator of iNOS was investigated. Substitution of the cu lture medium of rat peritoneal macrophages at pH 7.4 with medium at pH 7.0 upregulated iNOS activity, as reflected by a 2.5-fold increase in nitrite accumulation. The increase in iNOS activity was associated wi th a similar increase in iNOS mRNA expression. Low environmental pH-in duced iNOS gene expression involved the activation of nuclear factor-k appa B (NF-kappa B) transcription factor since [1] exposure of macroph ages to low environmental pH increased NF-kappa B binding activity in the nucleus, and [2] treatment of macrophages with pyrrolidine dithioc arbamate or n-acetyl-leucinyl-leucinyl-norleucinal, two drugs preventi ng NF-kappa B translocation to the nucleus, canceled low pH-induced ni trite accumulation. The overall mechanism required the synthesis of tu mor necrosis factor-alpha (TNF-alpha). Indeed, [1] elevated TNF-alpha bioactivity was observed in the medium of macrophages exposed to pH 7. 0, and [2] incubation of macrophages with a neutralizing anti-TNF-alph a antibody impaired both NF-kappa B activation and nitrite accumulatio n in response to acid challenge. In summary, exposure of macrophages t o acidic microenvironment in inflammatory lesions leads to the upregul ation of iNOS activity through the activation of NF-kappa B.