Clostridium difficile toxin A causes acute colitis associated with intense infiltrating neutrophils. Although dendritic cells (DCs) play an important role in the regulation of inflammation, little is known about the effects of toxin A on the maturation and neutrophil-attracting chemokine expression in DCs. This study investigated whether C. difficile toxin A could influence the maturation of mouse bone-marrow-derived DCs and chemokine CXCL2 expression. Toxin A increased the DC maturation which was closely related to CXCL2 upregulation. Concurrently, toxin A activated the signals of p65/p50 nuclear factor kappa B (NF-kappaB) heterodimers and phospho-I kappa B kinase (IKK) in DCs. The increased DC maturation, CXCL2 expression, and neutrophil chemoattraction were significantly downregulated in the NF-kappaB knockout mice. In addition, toxin A activated the phosphorylated signals of mitogen-activated protein kinases (MAPKs), such as ERK, p38, and JNK. Of all three MAPK signals, p38 MAPK was significantly related to DC maturation. Thus, suppression of p38 activity using SB203580 and siRNA transfection resulted in the significant reduction of IKK activity, DC maturation, and CXCL2 upregulation by toxin A. These results suggest that p38 MAPK may lead to the activation of IKK and NF-kappaB signaling, resulting in enhanced DC maturation and CXCL2 expression in response to C. difficile toxin A stimulation.

Enterotoxigenic Bacteroides fragilis (ETBF) produces an approximately 20-kDa heat-labile enterotoxin (BFT) that plays an essential role in mucosal inflammation. Although spontaneous disappearance of ETBF infection is common, little information is available on regulated expression of antibacterial factors in response to BFT stimulation. This study investigates the role of BFT in human β-defensin 2 (hBD-2) induction from intestinal epithelial cells. Stimulation of HT-29 and Caco-2 intestinal epithelial cell lines with BFT resulted in the induction of hBD-2. Activation of a reporter gene for hBD-2 was dependent on the presence of NF-κB binding sites. In contrast, suppression of AP-1 did not affect hBD-2 expression in BFT-stimulated cells. Inhibition of p38 mitogen-activated protein kinase (MAPK) using SB203580 and small interfering RNA (siRNA) transfection resulted in a significant reduction in BFT-induced IκB kinase (IKK)/NF-κB activation and hBD-2 expression. Our results suggest that a pathway including p38 MAPK, IKK, and NF-κB activation is required for hBD-2 induction in intestinal epithelial cells exposed to BFT, and may be involved in the host defense following infection with ETBF.

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.

Enterotoxigenic Bacteroides fragilis (ETBF) produces a ∼20-kDa heat-labile enterotoxin (BFT) that plays an essential role in mucosal inflammation. Although a variety of inflammatory cells is found at ETBF-infected sites, little is known about leukocyte adhesion in response to BFT stimulation. We investigated whether BFT affected the expression of ICAM-1 and monocytic adhesion to endothelial cells (ECs). Stimulation of HUVECs and rat aortic ECs with BFT resulted in the induction of ICAM-1 expression. Upregulation of ICAM-1 was dependent on the activation of IκB kinase (IKK) and NF-κB signaling. In contrast, suppression of AP-1 did not affect ICAM-1 expression in BFT-stimulated cells. Suppression of NF-κB activity in HUVECs significantly reduced monocytic adhesion, indicating that ICAM-1 expression is indispensable for BFT-induced adhesion of monocytes to the endothelium. Inhibition of JNK resulted in a significant attenuation of BFT-induced ICAM-1 expression in ECs. Moreover, inhibition of aldose reductase significantly reduced JNK-dependent IKK/NF-κB activation, ICAM-1 expression, and adhesion of monocytes to HUVECs. These results suggest that a signaling pathway involving aldose reductase, JNK, IKK, and NF-κB is required for ICAM-1 induction in ECs exposed to BFT, and may be involved in the leukocyte–adhesion cascade following infection with ETBF.

Toxoplasma gondii infection results in an infiltration of immune cells. The mechanisms responsible for triggering inflammatory cell infiltration in T. gondii infection are not fully understood. We report that T. gondii-infected HeLa cells induced nuclear factor-kappa B (NF-kappaB) activation and increased the expression of interleukin-8 (IL-8) and monocyte chemotactic protein-1 (MCP-1) mRNA. An inhibitor of NF-kappaB activation, calpain-1 inhibitor, blocked the chemokine secretion induced by live T. gondii. Activation of the IL-8 and NF-kappaB transcriptional reporters was suppressed in cells co-transfected with IkappaB kinase beta and the IkappaBalpha super-repressor plasmids. Moreover, the addition of IL-1alpha increased NF-kappaB activation and IL-8 mRNA expression in T. gondii-infected HeLa cells. These results suggest that NF-kappaB is a central regulator of the chemokine response in T. gondii-infected human epithelial cells and that chemokine IL-8 and MCP-1 secretion might be involved in the pathogenesis of T. gondii, via the recruitment of neutrophils, monocytes, and lymphocytes.