Negative feedback regulation of lipopolysaccharide-induced inducible nitric oxide synthase gene expression by heme oxygenase-1 induction in macrophages

Heme oxygenase-1 (HO-1) is induced under infectious diseases in macrophages. We performed experiments using various gene deficient mouse-derived macrophages to determine a detailed induction mechanism of HO-1 by lipopolysaccharide (LPS) and the functional role of HO-1 induction in macrophages. LPS (1 μg/mL) maximally induced inducible nitric oxide synthase (iNOS) and HO-1 mRNAs in wild-type (WT) macrophages at 6 h and 12 h after treatment, respectively, and liberated tumor necrosis factor α (TNFα) from WT macrophages. LPS also induced iNOS and HO-1 in TNFα(−/−) macrophages, but not in iNOS(−/−) macrophages. Interestingly, although LPS strongly induced iNOS, it failed to induce HO-1 almost completely in nuclear-factor erythroid 2-related factor 2 (Nrf2)(−/−) macrophages. The LPS-induced iNOS gene expression was suppressed by pretreatment with HO-1 inducers, hemin and Co-protoporphyrin (CoPP), but not with HO-1 inhibitor, Sn-protoporphyrin in WT macrophages. In the Nrf2(−/−) macrophages, the ability of CoPP to induce HO-1 and its inhibitory effect on the LPS-induced iNOS gene expression were lower than seen in WT macrophages. The present findings suggest that HO-1 is induced via NO-induced nuclear translocation of Nrf2, and the enzymatic function of HO-1 inhibits the overproduction of NO in macrophages.