Autocrine/paracrine IFN-alphabeta mediates the lipopolysaccharide-induced activation of transcription factor Stat1alpha in mouse macrophages: pivotal role of Stat1alpha in induction of the inducible nitric oxide synthase gene.

We have examined the role of Stat1alpha in the induction by LPS of the mouse inducible nitric oxide synthase (EC 1.14.13.39) gene. LPS induced both the tyrosine phosphorylation of Stat1alpha and the production of nitric oxide in a time- and dose-dependent manner. The phosphorylation of Stat1alpha elicited by LPS differed from that observed using IFN-gamma or IFN-beta, in that LPS induced less phosphorylated protein and the time course of induction was much delayed (2-4 h compared with 30 min). Cycloheximide inhibited LPS-mediated Stat1alpha phosphorylation. In addition, cell culture supernatants derived from macrophages treated with LPS for 4 h could be transferred to naive macrophage cultures resulting in rapid (30 min), rather than delayed (4 h), phosphorylation of Stat1alpha. Together, these results implicated an autocrine/paracrine effector protein(s) in the phosphorylation process. LPS stimulated phosphorylation of Stat1alpha in peritoneal macrophages derived from IFN-gamma-knockout mice, negating any possibility that IFN-gamma was the mediator. By contrast, neutralizing Ig raised against mouse IFN-alphabeta inhibited both the delayed LPS-mediated phosphorylation of Stat1alpha and the rapid induction of phosphorylation induced by supernatants from LPS-stimulated cultures. Collectively, these results show that LPS-induced IFN-alphabeta production, Stat1alpha activation, and nitrite accumulation closely parallel one another, suggesting that indirect activation of transcription factor Stat1alpha by IFN-alphabeta is a critical determinant of LPS-mediated inducible nitric oxide synthase gene expression.