[Host defense and oxidative stress signaling in bacterial infection
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Nitric oxide (NO) and reactive oxygen species (ROS) produced during infection are involved critically in host defense mechanisms. It is quite important to physiologically regulate ROS, such as superoxide, and NO. These reactive species produced in excess may cause oxidative damage of biological molecules. An important cytoprotective and antimicrobial function of NO and ROS is mediated by induction of heme oxygenase (HO)-1. The signaling mechanism of this HO-1 induction has remained unclear, however. We discovered in 2007 a unique second messenger, 8-nitroguanosine 3',5'-cyclic monophosphate (8-nitro-cGMP), that mediates electrophilic signal transduction during oxidative stress and other cellular redox signaling in general. 8-Nitro-cGMP is formed via guanine nitration with NO and ROS, and in fact, NO-dependent 8-nitro-cGMP formation and HO-1 induction were identified in Salmonella-infected mice. HO-1 induction was regulated solely by 8-nitro-cGMP formed in cells, and more important, its potent anti-apoptotic function was evident in such a Salmonella infection. 8-Nitro-cGMP has a potent cytoprotective function, of which signaling appears to be mediated via protein sulfhydryls to generate a post-translational modification called protein S-guanylation. 8-Nitro-cGMP specifically S-guanylates Keap1, a negative regulator of transcription factor Nrf2, which in turn up-regulates transcription of HO-1. Our recent study revealed that the autophagy might be involved in the 8-nitro-cGMP-dependent antimicrobial effect. The 8-nitro-cGMP signaling was also found to be regulated by reactive sulfur species that have superior antioxidant activity and unique signaling function. This review will discuss a new paradigm of the host defense that operates via formation of a unique cell signaling molecule, 8-nitro-cGMP, during microbial infections.