Heme oxygenase-1 upregulation improves lipopolysaccharide-induced acute lung injury involving suppression of macrophage migration inhibitory factor

Although studies have demonstrated that heme oxygenase-1 (HO-1) prevents leukocyte infiltration and organ damage following LPS challenge, the mechanisms involved in this protection are incompletely understood. Macrophage migration inhibitory factor (MIF) is thought to play a pivotal role in modulation of inflammatory and immune response through upregulation of TLR4 expression. Activation of TLR4 results in the production of proinflammatory mediators including MIF, which induce neutrophils recruitment and subsequent tissue insults. We hypothesized that HO-1 mediates its salutary effects in lipopolysaccharide (LPS)-induced inflammatory lung injury via downregulation of MIF through modulation of TLR4-induced proinflammatory mediator production. Compared with wild-type cells, MIF-knockdown macrophages in vitro are hyporesponsive to LPS stimulation, as shown by a profound reduction in TLR4 expression and TNF-α production. In the murine model of LPS-induced acute lung injury, administration of CoPP, a potent HO-1 inducer, leaded to a significant reduction in LPS-induced pulmonary edema, leucocytes influx, myeloperoxidase activity as well as histopathologic insults. Most strikingly, pretreatment with CoPP markedly decreased the expression of TLR4 and MIF in lung tissues in response to LPS challenge. These findings herein suggest that the cytoprotective functions of HO-1 in LPS-induced lung injury are associated with negative regulation of lung MIF and TLR4-induced inflammatory response.