We examined our hypothesis that heme-oxygenase-1 (HO-1)-derived carbon monoxide (CO) inhibits the release of highmobility group box 1 (HMGB1) in RAW264.7 cells activated with lipopolysaccharide (LPS) in vitro and in LPS-or cecal ligation and puncture (CLP)-induced septic mice in vivo, so that HO-1 induction or CO improves survival of sepsis in rodents. We found that pretreatment with HO-1 inducers (hemin, cobalt protoporphyrin IX) or transfection of HO-1 significantly inhibited HMGB1 release, which was blocked by HO-1 small interfering RNA, in cells activated by LPS. Carbon monoxide-releasing molecule 2 (CORM-2) but not bilirubin or deferoxamine inhibited HMGB1 release in LPS-activated macrophages. Oxyhemoglobin reversed the effect of HO-1 inducers on HMGB1 release. Translocation of HMGB1 from nucleus to cytosol was significantly inhibited by HO-1 inducers, CORM-2, or HO-1 transfection. Neutralizing antibodies to tumor necrosis factor (TNF)-␣, interleukin (IL)-1␤, interferon-␤, and N -nitro-L-arginine methyl ester hydrochloride but not N-[2-(cyclohexyloxyl)-4-nitrophenyl]-methane sulfonamide (NS-398) significantly inhibited HMGB1 release in LPS-activated cells. Production of TNF-␣, IL-1␤, and IFN-␤ was significantly reduced by pretreatment of HO-1 inducers, CORM-2, or HO-1 transfection in LPS-activated cells. Plasma levels of HMGB1 in mice challenged with LPS or CLP were significantly reduced by the administration of HO-1 inducers or CORM-2, which was accompanied by either reduction (pretreatment) or no change (delayed administration) of serum TNF-␣ and IL-1␤ levels. Regardless of pretreatment or delayed administration, CORM-2 and hemin rescued mice from lethal endotoxemia and sepsis induced by LPS or CLP. Taken together, we concluded that HO-1-derived CO reduces HMGB1 release in LPS-activated cells and LPS-or CLP-induced animal model of sepsis.Sepsis is defined as a systemic inflammatory response syndrome from a microbial infection that results from excessive stimulation of the host immune system by pathogen components to produce various proinflammatory cytokines, and their overproduction causes systemic inflammation that can lead to the lethal multiple organ damage (Oberholzer et al., 2001). High-mobility group box 1 (HMGB1) is a chromatin-binding protein that participates in maintaining nucleosome structure and regulation of gene transcription (Landsman and Bustin, 1993). Various evidence indicated that HMGB1 is a necessary and sufficient late mediator of severe sepsis (Wang et al., 1999;Yang et al., 2004). Once released, HMGB1 can bind to cell-surface receptors, such as the receptor for advanced glycation end products and Toll-like receptors 2 and 4, and mediate various cellular responses, chemotactic cell movement, and release of proinflammatory Article, publication date, and citation information can be found at http://molpharm.aspetjournals.org. doi:10.1124/mol.109.055137.ABBREVIATIONS: HMGB1, high-mobility group box 1; HO-1, heme-oxygenase-1; CORM-2, carbon monoxide-releasing molecule II; LPS, lip...