1,2) Macrophages use a wide variety of pattern recognition receptors including Toll-like receptors (TLRs) that bind to these agents.3,4) Such receptors recognize various microbial components. 5) Once TLRs sense the presence of invading pathogens, their engagement with them activates macrophages, which then produce and release various sets of effector molecules aimed at destroying the foreign agents. Reactive oxygen species (ROS), such as superoxide anion (O 2 − ), hydrogen peroxide (H 2 O 2 ), and nitric oxide (NO), are their front line effector molecules.6-11) These highly diffusive products exert strong cytotoxic activities against micro-organisms and many neighboring cells, including against the macrophages themselves. However, macrophages and most of the aerobic host cells can protect themselves by enhancing the expression of heme oxygenase (HO). HO, a cellular antioxidant, is one of the key enzymes catalyzing the degradation of heme-containing molecules to biliverdin, free iron, and carbon monoxide.12,13) Three HO isozymes, including HO-1 (HSP32), HO-2, and HO-3, have been identified 14) ; although HO-3 may be a pseudo gene derived from HO-2 transcripts. 15)HO-1 is an inducible enzyme, whereas HO-2 is constitutive one.16) HO-1 is induced by various molecules and environmental factors, such as heme, radiation, cytokine, oxidative, and heat stresses. [17][18][19][20][21] Previous studies reported that lipopolysaccharide (LPS), a major component of the outer membrane of Gram-negative bacteria, also induces the expression of both the HO-1 gene and its protein in macrophages.22-24) Exposure of HO-1-deficient mice to LPS leads to increased hepatocellular necrosis, increased splenic proinflammatory cytokine secretion, and higher mortality of septic shock when compared with wild-type animals. 25,26) These studies suggest that HO-1 plays an important role in reducing the deleterious increase in oxidative damage. Srisook et al. reported that both O 2 − and NO are related to HO-1 induction in macrophages by using an O 2 − generator and a NO donor. 27) However, it is not easy to study the role of O 2 − and NO separately in HO-1 induction in activated macrophages which produce both of these molecules simultaneously after LPS-treatment. Thus, among ROS, the important regulators of HO-1 in the activated macrophages have not been clear. The aim of the present work was to identify the regulator of HO-1 induction in activated macrophages. In this study, we sought to identify the regulator of HO-1 induction of the activated macrophages by using 2 types of macrophages, i.e., cells of the murine macrophage-like cell line J774.1/JA-4 and their LPS-resistant mutant, LPS1916. 7) This mutant cell line is characterized by a specific defect in the induction of ROS after LPS treatment. Our recent study reported that LPS1916 cells have the phenotype of mal-expression of CD14, a key molecule required for LPS-induced activation of cells, on the macrophage cell surface, leading to reduced responses to LPS. 28) Interestingly, this defect i...