Enterotoxigenic Bacteroides fragilis (ETBF) is associated with noninvasive diarrheal diseases (1, 2), inflammatory bowel diseases (1), and colorectal cancers (3-5). B. fragilis enterotoxin (BFT), a virulence factor of ETBF, is responsible for these diseases (1). BFT interacts with a single layer of intestinal epithelial cells and can provoke signals that induce mucosal inflammation (1, 6-9).In mammalian cells, two genetically distinct isozymes of heme oxygenase (HO) have been clearly identified. HO-1 is inducible, whereas HO-2 is constitutively expressed. HO-1 catalyzes the degradation of free heme into carbon monoxide, biliverdin, and free iron (10, 11). Within mammalian cells, biliverdin reductase converts biliverdin to bilirubin. Pathogen-associated molecular patterns (PAMPs) such as lipopolysaccharide (LPS), lipoteichoic acid, and peptidoglycan, as well as several proinflammatory cytokines, can induce HO-1 expression (12). Upregulated HO-1 expression can lead to adaptive immune responses that protect cells from immunopathogenesis or stress damage (12, 13). In addition, HO-1 expression is involved in clearance of pathogenic bacteria and downregulation of inflammatory responses. For example, HO-1 deficiency not only results in inadequate pathogen clearance (14) but also promotes the development of necrotizing enterocolitis-like intestinal injury in mice (15). HO-1 and HO-1-induced carbon monoxide can ameliorate intestinal inflammation through promotion of bacterial clearance (16). The HO-1/carbon monoxide pathway also suppresses Toll-like receptor 4 (TLR4) signaling, leading to downregulation of proinflammatory signaling induced by stimulation with LPS (17). Based on these findings, we hypothesized that the induction of HO-1 may regulate inflammatory responses induced by BFT. However, there are no reports regarding BFT-induced HO-1 expression.Signals from transcription factors, including nuclear factor-B (NF-B), activator protein-1 (AP-1), and NF-E2-related factor 2 (Nrf2, or nuclear factor [erythroid-derived 2]-like 2 [NFE2L2]), regulate the expression of HO-1 (11). Stimulation of intestinal epithelial cells with BFT can activate NF-B and AP-1 signaling (6)(7)(8)(9)(18)(19)(20). We have previously demonstrated that exposure of intestinal epithelial cells to BFT results in delayed apoptosis, suggesting that protection of cells after BFT stimulation is related to the generation of signals that activate or suppress mucosal inflammation (21). These observations raise the possibility that signaling molecules that regulate HO-1 expression may be activated in BFTexposed cells. However, there is no evidence that BFT-induced signaling results in HO-1 induction in intestinal epithelial cells. We therefore investigated HO-1 induction in response to stimulation of intestinal epithelial cells with BFT. We found that a signaling pathway involving p38 mitogen-activated protein kinases (MAPKs)-IB kinase (IKK)-NF-B in intestinal epithelial cells is required for HO-1 induction following exposure to BFT.

Helicobacter pylori sheds outer membrane vesicles (OMVs) that contain many surface elements of bacteria. Dendritic cells (DCs) play a major role in directing the nature of adaptive immune responses against H. pylori, and heme oxygenase-1 (HO-1) has been implicated in regulating function of DCs. In addition, HO-1 is important for adaptive immunity and the stress response. Although H. pylori-derived OMVs may contribute to the pathogenesis of H. pylori infection, responses of DCs to OMVs have not been elucidated. In the present study, we investigated the role of H. pylori-derived crude OMVs in modulating the expression of HO-1 in DCs. Exposure of DCs to crude H. pylori OMVs upregulated HO-1 expression. Crude OMVs obtained from a cagA-negative isogenic mutant strain induced less HO-1 expression than OMVs obtained from a wild-type strain. Crude H. pylori OMVs activated signals of transcription factors such as NF-κB, AP-1, and Nrf2. Suppression of NF-κB or Nrf2 resulted in significant attenuation of crude OMV-induced HO-1 expression. Crude OMVs increased the phosphorylation of Akt and downstream target molecules of mammalian target of rapamycin (mTOR), such as S6 kinase 1 (S6K1). Suppression of Akt resulted in inhibition of crude OMV-induced Nrf2-dependent HO-1 expression. Furthermore, suppression of mTOR was associated with inhibition of IκB kinase (IKK), NF-κB, and HO-1 expression in crude OMV-exposed DCs. These results suggest that H. pylori-derived OMVs regulate HO-1 expression through two different pathways in DCs, Akt-Nrf2 and mTOR–IKK–NF-κB signaling. Following this induction, increased HO-1 expression in DCs may modulate inflammatory responses in H. pylori infection.