Cytoprotective Function of Heme Oxygenase 1 Induced by a Nitrated Cyclic Nucleotide Formed during Murine Salmonellosis

Signaling mechanisms of NO-mediated host defense are yet to be elucidated. In this study, we report a unique signal pathway for cytoprotection during Salmonella infection that involves heme oxygenase 1 (HO-1) induced by a nitrated cyclic nucleotide, 8-nitroguanosine 3',5'-cyclic monophosphate (8-nitro-cGMP). Wild-type C57BL/6 mice and C57BL/6 mice lacking inducible NO synthase (iNOS) were infected with Salmonella enterica serovar Typhimurium LT2. HO-1 was markedly up-regulated during the infection, the level being significantly higher in wild-type mice than in iNOS-deficient mice. HO-1 up-regulation was associated with 8-nitro-cGMP formation detected immunohistochemically in Salmonella-infected mouse liver and peritoneal macrophages. 8-Nitro-cGMP either exogenously added or formed endogenously induced HO-1 in cultured macrophages infected with Salmonella. HO-1 inhibition by polyethylene glycol-conjugated zinc-protoporphyrin IX impaired intracellular killing of bacteria in mouse liver and in both RAW 264 cells and peritoneal macrophages. Infection-associated apoptosis was also markedly increased in polyethylene glycol-conjugated zinc-protoporphyrin IX-treated mouse liver cells and cultured macrophages. This effect of HO-1 inhibition was further confirmed by using HO-1 short interfering RNA in peritoneal macrophages. Our results suggest that HO-1 induced by NO-mediated 8-nitro-cGMP formation contributes, via its potent cytoprotective function, to host defense during murine salmonellosis.