Hypoxia-inducible factor 1α stabilization by carbon monoxide results in cytoprotective preconditioning

The most salient feature of carbon monoxide (CO)-mediated cytoprotection is the suppression of inflammation and cell death. One of the important cellular targets of CO is the macrophage (m 4). Many studies have shown that exposure of m phi to CO results in the generation of an antiinflammatory phenotype; however, these reports have ignored the effect of CO alone on the cell before stimulation. Most investigations have focused on the actions of CO in modulating the response to noxious stimuli. We demonstrate here that exposure of m phi to CO results in a significant and transient burst of reactive oxygen species (ROS) arising from the mitochondria (mitochondria-deficient m(P do not respond to CO to produce ROS). The ROS promote rapid activation and stabilization of the transcription factor hypoxia-inducible factor 1 alpha (HIF-1 alpha), which regulates expression of genes involved in inflammation, metabolism, and cell survival. The increase in HIF-1 alpha expression induced by CO results in regulated expression of TGF-beta, a potent antiinflammatory cytokine. CO-induced HIF-1 alpha and TGF-beta expression are necessary to prevent anoxia/reoxygenation-induced apoptosis in m phi. Furthermore, blockade of HIF-1 alpha using RNA interference and HIF-1 alpha-cre-lox m phi resulted in a loss of TGF-beta expression and CO-induced protection. A similar mechanism of CO-induced protection was operational in vivo to protect against lung ischemia-reperfusion injury. Taken together, we conclude that CO conditions the m phi via a HIF-1 alpha and TGF-beta dependent mechanism and we elucidate the earliest events in m phi signaling that lead to and preserve cellular homeostasis at the site of injury.