New paradigm of host defense against intracellular pathogens by nitric oxide

Nitric oxide (NO) produced by inducible NO synthase (iNOS) during infection plays a crucial role in host defense mechanisms, via its antimicrobial and cytoprotective activities. Infection of Salmonella typhimurium in mice induces excessive production of NO, as a host defense response. We found much greater bacterial growth and apoptotic changes in iNOS-deficient (iNOS^-/-) mice than in wild-type mice. However, the mechanism of NO-mediated cytoprotection during Salmonella infection remained unclear. An important signaling mechanism induced by NO is heme oxygenase (HO)-1, a significant cytoprotective molecule produced by oxidative stress. Thus, we sought to clarify NO-dependent cytoprotective and antimicrobial host defense, with a particular focus on the signaling mechanism of HO-1 induction. We recently discovered a nitrated cyclic nucleotide, 8-nitroguanosine 3',5'-cyclic monophosphate (8-nitro-cGMP), which is formed via NO possibly with reactive oxygen species. We observed strong immunoreactivity for 8-nitro-cGMP in Salmonella -infected wild-type mouse liver and peritoneal macrophages in culture but not in iNOS^-/- mouse liver and macrophages. Moreover, a higher apoptosis was observed in iNOS^-/- macrophages compared with wild-type macrophages after Salmonella infection, but the difference was nullified when iNOS^-/- cells were treated with 8-nitro-cGMP. Finally, authentic 8-nitro-Cgmp induced HO-1 in cultured macrophages infected with Salmonella. The signaling function of 8-nitro-cGMP appears to be mediated by its unique reaction with the sulfhydryl group of cysteine, thus forming a protein- S-cGMP adduct, which is an important mechanism of post-translational modification of proteins called protein S-guanylation. More importantly, we found 8-nitro-cGMP-dependent S-guanylation of Keap1, a regulatory protein of transcription factor Nrf2, which regulates the transcription of HO-1. In this review, we focus on a unique mechanism of NO-mediated host defense via formation of a novel signaling molecule, 8-nitro-cGMP in microbial infections.