Regulation of inducible nitric oxide synthase messenger RNA expression and nitric oxide production by lipopolysaccharide in vivo: the roles of macrophages, endogenous IFN-gamma, and TNF receptor-1-mediated signaling.

To evaluate potential roles for macrophages, IFN-gamma, and TNF receptor 1 (TNFR1) in the regulation of LPS-induced inducible nitric oxide synthase (iNOS) mRNA expression, we used a model of macrophage depletion as well as IFN-gamma (GKO) and TNFR1 (TNFR1 -/-) knockout mice. LPS-induced iNOS mRNA in spleen was ablated in both macrophage-depleted and GKO mice. In livers of macrophage-depleted mice, LPS-induced iNOS mRNA was reduced by 55 to 85%, with the most profound reductions detected 6 and 8 h after LPS injection. In GKO mice, peak iNOS mRNA expression in liver (3 h) was unaffected by the loss of endogenous IFN-gamma. By 6 to 12 h after LPS challenge, however, hepatic LPS-induced iNOS mRNA and serum nitrate/nitrite levels were reduced substantially in GKO mice. Residual LPS-induced iNOS mRNA in livers of GKO mice was nearly ablated by macrophage depletion, indicating that induction of iNOS mRNA in liver requires both endogenous IFN-gamma and either macrophages and/or macrophage-derived factors. TNFR1-mediated signaling was involved in the induction of LPS-induced iNOS mRNA in liver at 3 and 6 h, but not in its maintenance at 8 h. Conversely, induction of iNOS mRNA in spleen by LPS was independent of TNFR1-mediated signaling. Our results indicate that macrophages and/or their secreted products, endogenous IFN-gamma production, and TNFR1-mediated signaling play key roles in the in vivo regulation of iNOS mRNA expression and that the extent of their involvement is both time and organ specific.