A phosphatidyl-inositol-3-kinase-dependent anti-inflammatory pathway activated by Salmonella in epithelial cells

Differential Effects of Statins on Inflammatory Interleukin-8 and Antimicrobial Peptide Human Β-Defensin 2 Responses in Salmonella-Infected Intestinal Epithelial Cells

Alternative therapies are needed to reduce the use of antibiotics and incidence of drug-resistant Salmonellosis. Previous studies have revealed important roles of statins in regulating innate immunity. Therefore, we investigated the effects of statins on innate immunity in Salmonella-infected intestinal epithelial cells (IECs), which are involved in mucosal innate immunity. SW480 cells and Akt siRNA- or vitamin D receptor (VDR) siRNA-transfected SW480 cells were infected by wild-type S. Typhimurium strain SL1344 in the presence or absence of statins. The mRNA or protein expression was analyzed by real-time quantitative PCR or western blot analysis, respectively. Simvastatin or fluvastatin caused IL-8 (interleukin-8) suppression, but increased hBD-2 mRNA expression in Salmonella-infected SW480 cells. Both statins enhanced phosphorylated Akt and VDR expressions. Akt or VDR knockdown by siRNA counteracted the suppressive effect of simvastatin on IL-8 expression, whereas VDR knockdown diminished the enhanced hBD-2 expression in Salmonella-infected SW480 cells. Therefore, we observed differential regulation of statins on inflammatory IL-8 and anti-microbial hBD-2 expressions in Salmonella-infected IECs via PI3K/Akt signaling and VDR protein expression, respectively. The enhanced activity of antimicrobial peptides by statins in Salmonella-infected IECs could protect the host against infection, and modulation of pro-inflammatory responses could prevent the detrimental effects of overwhelming inflammation in the host.

The Role of Sphingolipids on Innate Immunity to Intestinal Salmonella Infection

Salmonella spp. remains a major public health problem for the whole world. To reduce the use of antimicrobial agents and drug-resistant Salmonella, a better strategy is to explore alternative therapy rather than to discover another antibiotic. Sphingolipid- and cholesterol-enriched lipid microdomains attract signaling proteins and orchestrate them toward cell signaling and membrane trafficking pathways. Recent studies have highlighted the crucial role of sphingolipids in the innate immunity against infecting pathogens. It is therefore mandatory to exploit the role of the membrane sphingolipids in the innate immunity of intestinal epithelia infected by this pathogen. In the present review, we focus on the role of sphingolipids in the innate immunity of intestinal epithelia against Salmonella infection, including adhesion, autophagy, bactericidal effect, barrier function, membrane trafficking, cytokine and antimicrobial peptide expression. The intervention of sphingolipid-enhanced foods to make our life healthy or pharmacological agents regulating sphingolipids is provided at the end.

Anti-inflammatory effect of resveratrol in human coronary arterial endothelial cells via induction of autophagy: implication for the treatment of Kawasaki disease

Background

Kawasaki disease (KD) is an acute febrile vasculitis in childhood, which is the leading cause of acquired heart disease in children. If untreated, KD can result in coronary aneurysms in 25% of patients, and even under intravenous immunoglobulin (IVIG) treatment, 10–20% of children will have IVIG resistance and increased risk of developing coronary arteritis complication. Additional therapies should be explored to decrease the incidence of coronary artery lesions and improve the prognosis in KD. Autophagy has been reported to play a critical role in a variety of heart diseases. Resveratrol (RSV) confers cardio protection during ischemia and reperfusion in rats via activation of autophagy. Serum TNF-alpha levels are elevated in KD, which might activate the endothelial cells to express intercellular adhesion molecule-1 (ICAM-1), vascular cellular adhesion molecule-1(VCAM-1), inducible nitric oxide synthase (iNOS) and IL-1β.

Methods

Human coronary arterial endothelial cells (HCAECs) were either untreated or treated by TNF-α 10 ng/ml for 2 h in the presence or absence of RSV or autophagy-related protein 16-like 1 (Atg16L1) siRNA. Total RNA was analyzed by real-time quantitative PCR for ICAM-1, VCAM-1, iNOS and IL-1β mRNA expressions. The involvement of autophagy proteins was investigated by Western blot.

Results

Pretreatment with resveratrol significantly inhibited TNF-α-induced ICAM-1, iNOS and IL-1β mRNA expression in HCAECs. Western blot revealed the enhanced autophagy proteins LC3B and Atg16L1 expression by RSV. The suppressive effects of RSV were obviously counteracted by Atg16L1 siRNA.

Conclusions

We demonstrated RSV had anti-inflammatory effects on HCAECs via induction of autophagy. Our results suggest that resveratrol may modulate the inflammatory response of coronary artery in KD and explore the role of autophagy in the pathogenesis and alternative therapy of coronary arterial lesions in KD.

The different effects of probiotics treatment on Salmonella-induced interleukin-8 response in intestinal epithelia cells via PI3K/Akt and NOD2 expression

Salmonella spp. remains a major public health problem for the whole world. Intestinal epithelial cells serve as an essential component of the innate mucosal immune system to defend against Salmonella infection. A substantial amount of evidence has accumulated that probiotics can regulate interleukin 8 (IL-8) involved in innate immunity. However, the exact effect of probiotics on epithelial IL-8 response to Salmonella infection is not well understood. Therefore, we investigated the action of probiotics on Salmonella-infected Caco-2 cells and its novel mechanisms. Two probiotic strains were examined for Salmonella-induced IL-8 responses and regulating proteins using Caco-2 cell cultures. We demonstrated probiotic, either Lactobacillus rhamnosus GG or Bifidobacterium animalis subsp. lactis DSM10140, administered before Salmonella infection conferred significantly suppressive effect on Salmonella-induced IL-8 responses in Caco-2 cells, either in secreted protein or mRNA, via the PI3K/Akt signal pathway while probiotic administered after infection enhanced Salmonella-induced IL-8 responses via nucleotide-binding oligomerisation domain-containing protein 2 expression in membrane. These findings suggest that the different regulation of probiotics on Salmonella-induced IL-8 responses in Caco-2 cells according to the administered timing supports a rationale for the therapeutic use of probiotics in the treatment of Salmonella colitis and inflammatory bowel disease. This can explain the reported controversial effect of probiotics on these diseases.

The differential effects of 1,25-dihydroxyvitamin D3 on Salmonella-induced interleukin-8 and human beta-defensin-2 in intestinal epithelial cells

Salmonellosis or Salmonella, one of the most common food-borne diseases, remains a major public health problem worldwide. Intestinal epithelial cells (IECs) play an essential role in the mucosal innate immunity of the host to defend against the invasion of Salmonella by interleukin (IL)-8 and human β-defensin-2 (hBD-2). Accumulated research has unravelled important roles of vitamin D in the regulation of innate immunity. Therefore, we investigated the effects of 1,25-dihydroxyvitamin D3 (1,25D3) on Salmonella-induced innate immunity in IECs. We demonstrate that pretreatment of 1,25D3 results in suppression of Salmonella-induced IL-8 but enhancement of hBD-2, either protein secretion and mRNA expression, in IECs. Furthermore, 1,25D3 enhanced Salmonella-induced membranous recruitment of nucleotide oligomerization domain (NOD2) and its mRNA expression and activation of protein kinase B (Akt), a downstream effector of phosphoinositide 3-kinase (PI3K). Inhibition of the PI3K/Akt signal counteracted the suppressive effect of 1,25D3 on Salmonella-induced IL-8 expression, while knock-down of NOD2 by siRNA diminished the enhanced hBD-2 expression. These data suggest differential regulation of 1,25D3 on Salmonella-induced IL-8 and hBD-2 expression in IECs via PI3K/Akt signal and NOD2 protein expression, respectively. Active vitamin D-enhanced anti-microbial peptide in Salmonella-infected IECs protected the host against infection, while modulation of proinflammatory responses by active vitamin D prevented the host from the detrimental effects of overwhelming inflammation. Thus, active vitamin D-induced innate immunity in IECs enhances the host's protective mechanism, which may provide an alternative therapy for invasive Salmonella infection.

De Novo sphingolipid synthesis is essential for Salmonella-induced autophagy and human beta-defensin 2 expression in intestinal epithelial cells

BACKGROUND

Sphingolipids are important for innate immune response to eliminate infected pathogens and involved in autophagy. On the other hand, nucleotide-binding oligomerization domain-containing protein 2 (NOD2) served as an intracellular pattern recognition receptor to enhance host defense by inducing autophagy and the production of antimicrobial peptides, such as human beta-defensin-2 (hBD-2). However, the role of sphingolipids in Salmonella-induced autophagy and hBD-2 response in intestinal epithelial cells has not been previously elucidated.

METHODS

Salmonella typhimurium wild-type strain SL1344 was used to infect SW480, an intestinal epithelial cell. hBD-2 and interleukin-8 (IL-8) mRNA expressions were assessed in SW480 cells using RT-PCR, and intracellular signaling pathways and autophagy protein expression were analyzed by Western blot in SW480 cells in the presence or absence of inhibitors or transfected with siRNA.

RESULTS

We demonstrated that inhibition of de novo sphingolipid synthesis repressed the membrane recruitment of NOD2 and autophagy-related protein 16-like 1 (Atg16L1), suppressed Salmonella-induced autophagic protein LC3-II expression, and reduced NOD2-mediated hBD-2 response in Salmonella-infected SW480 cells. Contrasting to the utilization of membrane cholesterol on maintenance of Salmonella-containing vacuoles and anti-inflammation by Salmonella, sphingolipids act on epithelial defense against the invasive pathogen.

CONCLUSIONS

Our results offer mechanistic insights on the role of de novo sphingolipid synthesis in the innate immunity of intestinal epithelial cells to Salmonella infection. The pharmaceuticals enhancing or diet enriched with sphingolipids may induce the dual anti-bacterial mechanisms. The role of de novo sphingolipid synthesis on inflammatory bowel disease is deserved to be further investigated.

A specific A/T polymorphism in Western tyrosine phosphorylation B-motifs regulates Helicobacter pylori CagA epithelial cell interactions

Helicobacter pylori persistently colonizes the human stomach, with mixed roles in human health. The CagA protein, a key host-interaction factor, is translocated by a type IV secretion system into host epithelial cells, where its EPIYA tyrosine phosphorylation motifs (TPMs) are recognized by host cell kinases, leading to multiple host cell signaling cascades. The CagA TPMs have been described as type A, B, C or D, each with a specific conserved amino acid sequence surrounding EPIYA. Database searching revealed strong non-random distribution of the B-motifs (including EPIYA and EPIYT) in Western H. pylori isolates. In silico analysis of Western H. pylori CagA sequences provided evidence that the EPIYT B-TPMs are significantly less associated with gastric cancer than the EPIYA B-TPMs. By generating and using a phosphorylated CagA B-TPM-specific antibody, we demonstrated the phosphorylated state of the CagA B-TPM EPIYT during H. pylori co-culture with host cells. We also showed that within host cells, CagA interaction with phosphoinositol 3-kinase (PI3-kinase) was B-TPM tyrosine-phosphorylation-dependent, and the recombinant CagA with EPIYT B-TPM had higher affinity to PI3-kinase and enhanced induction of AKT than the isogenic CagA with EPIYA B-TPM. Structural modeling of the CagA B-TPM motif bound to PI3-kinase indicated that the threonine residue at the pY+1 position forms a side-chain hydrogen bond to N-417 of PI3-kinase, which cannot be formed by alanine. During co-culture with AGS cells, an H. pylori strain with a CagA EPIYT B-TPM had significantly attenuated induction of interleukin-8 and hummingbird phenotype, compared to the isogenic strain with B-TPM EPIYA. These results suggest that the A/T polymorphisms could regulate CagA activity through interfering with host signaling pathways related to carcinogenesis, thus influencing cancer risk.

Differential regulation of interleukin-8 and human beta-defensin 2 in Pseudomonas aeruginosa-infected intestinal epithelial cells

Background

The human opportunistic pathogen, Pseudomonas aeruginosa (P. aeruginosa) carries the highest case fatality rate of all gram-negative infections. Unfortunately, antimicrobial therapy has not been demonstrated to improve clinical outcome and the emergence of multidrug resistant P. aeruginosa has become a major concern in the hospital setting. Fever and diarrhea are the two most common initial symptoms in P. aeruginosa sepsis in previously healthy infants and children. This implies that intestinal epithelial cells in first contact with the pathogen may play an important role in innate immunity to P. aeruginosa infection. Human beta–defensins-2 (hBD-2) and interleukin-8 (IL-8) are crucial for host defense at mucosa but IL-8 may give rise to characteristic pathology of colitis.

Results

Pseudomonas aeruginosa strain PAO1 was used to infect SW480, an intestinal epithelial cell. IL-8 and hBD-2 mRNA expression and protein secretion were then assessed in SW480 cells using RT-PCR and enzyme-linked immunosorbent assay (ELISA), respectively. Intracellular signaling pathways and nucleotide-binding oligomerization domain (NOD) 1 protein expression were analyzed by Western blot in SW480 cells in the presence or absence of inhibitors or transfected with siRNA.

We demonstrate that prolonged infection by P. aeruginosa results in suppression of IL-8 but enhancement of hBD-2, either protein secretion and mRNA expression, in SW480 cells. Inhibitors of ERK suppressed but inhibitor of PI3K enhanced P. aeruginosa-induced IL-8 mRNA expression in SW480 cells while both signaling had no effect on P. aeruginosa-induced hBD-2 expression in SW480 cells. On the other hand, NOD 1 was illustrated to get involved in P. aeruginosa-induced hBD-2 mRNA expression and protein production in SW480 cells.

Conclusions

The P. aeruginosa-induced antimicrobial peptide in IECs continuously protect the host against prolonged infection, while modulation of proinflammatory responses prevents the host from the detrimental effects of overwhelming inflammation. Thus, P. aeruginosa-induced innate immunity in IECs represents a host protective mechanism, which may provide new insight into the pathogenesis of inflammatory bowel diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0275-6) contains supplementary material, which is available to authorized users.

The critical role of membrane cholesterol in salmonella-induced autophagy in intestinal epithelial cells

It was previously observed that plasma membrane cholesterol plays a critical role in the Salmonella-induced phosphatidylinositol 3-kinase-dependent (PI3K)-dependent anti-inflammatory response in intestinal epithelial cells (IECs). The PI3K/Akt pathway is associated with autophagy which has emerged as a critical mechanism of host defense against several intracellular bacterial pathogens. Plasma membrane contributes directly to the formation of early Atg16L1-positive autophagosome precursors. Therefore, this study aimed to investigate the role of plasma membrane cholesterol on the Salmonella-induced autophagy in IECs. By using methyl-beta-cyclodextrin (MBCD), it was demonstrated that disruption of membrane cholesterol by MBCD enhanced NOD2 and Atg16L1 proteins expression in membrane, and autophagic LC3II proteins expression and LC3 punctae in Salmonella-infected Caco-2 cells, which was counteracted by Atg16L1 siRNA. Nucleotide-binding oligomerization domain-containing protein 2 (NOD2) siRNA enhanced the Salmonella-induced activation of Akt in Caco-2 cells. However, inhibitors of Akt or extracellular signal-regulated kinases (ERK) had no significant effect on Salmonella-induced autophagy Beclin 1 or LC3 proteins expression. In conclusion, our study suggests that cholesterol accumulation in the plasma membrane at the entry site of Salmonella results in the formation of Salmonella-containing vacuole (SCV) and decreased autophagy. Our results offer mechanistic insights on the critical role of membrane cholesterol in the pathogenesis of Salmonella infection in intestinal epithelial cells and the therapeutic potential of its antagonists.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

Regulation of Salmonella flagellin-induced interleukin-8 in intestinal epithelial cells by muramyl dipeptide

Toll-like receptor 5 (TLR5) and nucleotide-binding oligomerization domain 2 (Nod2) are two important pattern recognition receptors involved in innate immunity to invading pathogens. Flagellin, recognized by TLR5, is Salmonella's dominant pro-inflammatory determinant in intestinal epithelial cells (IECs). Nod2 has played a pivotal role in protecting against intestinal bacterial infection. Therefore the aim of the study is to investigate regulation of Salmonella flagellin-induced interleukin (IL)-8 (IL-8) in IECs by Nod2 agonist, muramyl dipeptide (MDP). We found that MDP by itself induced only a weak IL-8 secretion in Caco-2 cells. However, it did show synergistic enhancement on flagellin-induced IL-8 production in Caco-2 cells, possibly caused by flagellin-mediated enhanced Nod2 recruitment into cell membrane. By Western blot and siRNA, we showed ERK and NF-κB, Nod2 and Rip2 were involved in the synergistic effect of MDP. These findings suggested that the cooperation of TLR5 and Nod2 in IECs regulates inflammatory response to Salmonella infection.

The role of phosphoinositide 3-kinase signaling in intestinal inflammation

The phosphatidylinositol 3-kinase signaling pathway plays a central role in regulating the host inflammatory response. The net effect can either be pro- or anti-inflammatory depending on the system and cellular context studied. This paper focuses on phosphatidylinositol 3-kinase signaling in innate and adaptive immune cells of the intestinal mucosa. The role of phosphatidylinositol 3-kinase signaling in mouse models of inflammatory bowel disease is also discussed. With the development of new isoform specific inhibitors, we are beginning to understand the specific role of this complex pathway, in particular the role of the γ isoform in intestinal inflammation. Continued research on this complex pathway will enhance our understanding of its role and provide rationale for the design of new approaches to intervention in chronic inflammatory conditions such as inflammatory bowel disease.

Salmonella enteritidis Rck-mediated invasion requires activation of Rac1, which is dependent on the class I PI 3-kinases-Akt signaling pathway

The Salmonella outer membrane protein Rck mediates a Zipper-like entry mechanism controlled by Rac, the Arp2/3 complex, and actin polymerization. However, little is known about the early steps leading to Rac activation and Rck-mediated internalization. The use of pharmacological inhibitors or PI 3-kinase dominant-negative mutant induced more than 80% less invasion without affecting attachment. Moreover, Rck-mediated internalization caused an increase in the association of p85 with at least one tyrosine-phosphorylated protein, indicating that class I PI 3-kinase activity was stimulated. We also report that this PI 3-kinase activity is essential for Rac1 activation. However, Rac recruitment at the Rck-mediated entry site was independent of its activation. Using a pharmacological approach or Akt-knockout cells, we also demonstrated that Akt was phosphorylated in response to Rck-mediated internalization as demonstrated by immunoblotting analysis and that all three Akt isoforms were required during this process. Overall, our results describe a signaling pathway involving tyrosine phosphorylation, class I PI 3-kinase, Akt activation, and Rac activation, leading to Rck-dependent Zipper entry. The specificity of this signaling pathway with regard to that of the type 3 secretion system, which is the other invasion process of Salmonella, is discussed.

Plasma membrane cholesterol plays a critical role in the Salmonella-induced anti-inflammatory response in intestinal epithelial cells

Our recent study demonstrated that a phosphatidylinositol-3 kinase (PI3K)/Akt-dependent anti-inflammatory pathway was activated by Salmonella in intestinal epithelial cells. Salmonella virulence is dependent on the ability of the bacterium to invade nonphagocytic host cells and then survive and replicate within modified Salmonella-containing vacuoles where cholesterol accumulates. In addition, cholesterol in membrane lipid rafts is frequently a platform for the activation of downstream signaling pathways, including the PI3K/Akt pathway. However, the role of plasma membrane cholesterol in the Salmonella-induced anti-inflammatory response in intestinal epithelial cells has not been elucidated. Here, we show that the effect of plasma membrane cholesterol depletion on the inhibition of Akt activation allows sustained ERK activation and the subsequent upregulation of IL-8 expression. These results demonstrate that plasma membrane cholesterol plays a critical role in the PI3K-dependent anti-inflammatory pathway activated by Salmonella in intestinal epithelial cells.

Inhibition of cell proliferation by CD44: Akt is inactivated and EGR-1 is down-regulated

OBJECTIVE

CD44 is a transmembrane glycoprotein and can facilitate signal transduction by serving as a platform for molecular recruitment and assembly. A number of studies have suggested that CD44 can either positively or negatively regulate cell proliferation. The purpose of this study was to investigate how CD44 can inhibit cell proliferation.

MATERIALS AND METHODS

We engineered E6.1 Jurkat cells to express CD44. Importantly, these cells lack endogenous CD44 expression. Molecular pathways involved with cell proliferation were studied using RT(2)-PCR array, siRNA, Western blotting and by employing pharmacological inhibitors of ERK1/2, p38 and the PI3K/Akt pathways.

RESULTS

We found that CD44 expression significantly inhibited cell proliferation and down-regulated EGR-1 expression and EGR-1 targets cyclin D1 and cyclin D2. Transfection of control E6.1 Jurkat cells with EGR-1 siRNA also inhibited cell proliferation, confirming its role. Disruption of the PI3K/Akt pathway with pharmacological inhibitors reduced both EGR-1 expression and cell proliferation, recapitulating the properties of CD44 expressing cells. Akt was hypophosphorylated in cells expressing CD44 showing its potential role in negatively regulating Akt activation. Strikingly, constitutively active Akt rescued the proliferation defect showing requirement for active Akt, in our system.

CONCLUSION

Our results suggest a novel pathway by which CD44 inactivates Akt, down-regulates EGR-1 expression and inhibits cell proliferation.

Activation of proinflammatory response in human intestinal epithelial cells following Vibrio cholerae infection through PI3K/Akt pathway

Vibrio cholerae activates proinflammatory response in cultured intestinal epithelial cells. In this study, we demonstrate that V. cholerae O395 infection of intestinal epithelial cells results in the activation of Akt. Inhibition of Akt significantly decreases IL-1alpha, IL-6, and TNF-alpha production in V. cholerae infected Int407 cells. Analysis of the mechanisms of Akt influences on cytokine response demonstrates that Akt promotes NF-kappaB activation. We have extended these findings to show that Akt activation may be regulated by bacterial genes associated with virulence, adherence, or motility. Insertion mutants in the virulence genes coding for CtxA, ToxT, and OmpU of V. cholerae modulate the activation of PI3K/Akt signaling pathway, whereas an aflagellate non-motile mutant (O395FLAN) and a adherent and less motile mutant (O395Y3N/O395Y4N) of V. cholerae both show very significant down-regulation of Akt activity in Int407 cells. Together, these observations indicate that Akt promotes proinflammatory cytokine production by V. cholerae infected human intestinal epithelial cells through its influences on NF-kappaB.

Upregulation of Salmonella-induced IL-6 production in Caco-2 cells by PJ-34, PARP-1 inhibitor: involvement of PI3K, p38 MAPK, ERK, JNK, and NF-kappaB

Following Salmonella invasion, intestinal epithelial cells release a distinct array of proinflammatory cytokines. Interleukin (IL)-6 produced by enterocytes may have anti-inflammatory and cell-protective effects, and may counteract some of the injurious effects of sepsis and endotoxemia. Recent studies in a variety of rodent models of experimental colitis by using PJ-34, a potent poly (ADP-ribose) polymerase-1 (PARP-1) inhibitor, support the concept that the marked beneficial effect of PJ-34 can be exploited to treat human inflammatory diseases. The present study was to investigate the effect of PJ-34 on Salmonella-induced enterocyte IL-6 production and its mechanisms. We found that PJ-34 enhanced Salmonella-induced IL-6 production in Caco-2 cells, either secreted protein or mRNA expression. PJ-34 treatment enhanced the activity of NF-κB in Salmonella-infected Caco-2 cells. Besides, the involvement of PJ-34 in up-regulating IL-6 production in S. typhimurium-infected Caco-2 cells might be also through the ERK but not p38 MAPK, JNK or PI3K/Akt pathways, as demonstrated by Western blot of phosphorylated ERK, p38, JNK and Akt proteins. It suggests that PJ-34 may exert its protective effect on intestinal epithelial cells against invasive Salmonella infection by up-regulating IL-6 production through ERK and NF-κB but not P38 MAPK, JNK or PI3K/Akt signal pathways.

Neisseria gonorrhoeae survival during primary human cervical epithelial cell infection requires nitric oxide and is augmented by progesterone

Neisseria gonorrhoeae is an obligate human pathogen that causes gonorrhea. We have shown previously that complement receptor 3 and Akt kinase play important roles in mediating cervical infection. At present, there are limited data to indicate how hormonally induced changes to the mucosal epithelia of the female genital tract mediate the course of gonococcal disease. Hence, I have expanded upon previous work to investigate the interaction of gonococci with primary human cervical epithelial (pex) cells under the variable estrogen and progesterone concentrations likely to be encountered in vivo throughout the female menstrual cycle. My data indicated that the ability of gonococci to survive and to replicate within pex cells was increased under progesterone-predominant conditions. Using bacterial survival, immunological, and kinase assays, I show that progesterone functioned in an additive manner with gonococcal phospholipase D to augment Akt kinase activity. This, in turn, resulted in a parallel increase in nitric oxide synthase expression. Nitric oxide production by pex cells was dependent upon Akt activity and was increased under progesterone-predominant conditions. Whereas both inducible and endothelial nitric oxide synthase contributed to nitric oxide production, only inducible nitric oxide synthase activity promoted gonococcal survival within pex cells. Collectively, these data provide the first clues as to how steroid hormones potentially modulate the course of gonococcal disease in women. In addition, these data demonstrate that host-derived nitric oxide likely is not protective against gonococci, in vivo; rather, nitric oxide may be required to sustain cervical bacterial disease.

Cell invasion of Yersinia pseudotuberculosis by invasin and YadA requires protein kinase C, phospholipase C-gamma1 and Akt kinase

The outer membrane proteins YadA and invasin of Yersinia pseudotuberculosis promote invasion into mammalian cells through beta(1)-integrins and trigger the production of interleukin (IL)-8. FAK, c-Src and the PI3 kinase were previously found to be important for both YadA- and invasin-promoted uptake. Here, we demonstrate that two different downstream effectors of PI3 kinase, Akt and phospholipase Cgamma1 are required for efficient cell invasion. Inhibition of Akt or phospholipase C-gamma (PLC-gamma)1 by pharmaceutical agents as well as reduced expression of the isoforms Akt1 and Akt2, and of PLC-gamma1 by RNA interference decreased entry of YadA- and Inv-expressing bacteria significantly. In addition, we report that the conventional protein kinases C (PKC)alpha and -beta, positioned downstream of PLC-gamma1, are activated upon Inv- or YadA-promoted cell entry. They colocalize with intracellular bacteria and their depletion by siRNA treatment also resulted in a strong reduction of cell entry. In contrast, neither Akt nor PLC-gamma1, and the PKCs are essential for YadA- and Inv-mediated IL-8 synthesis and release. We conclude that YadA and invasin of Y. pseudotuberculosis both trigger similar signal transduction pathways during integrin-mediated phagocytosis into epithelial cells, which lead to the activation of Akt, PLC-gamma1, PKCalpha and -beta downstream of PI3 kinase, separate from the MAPK-dependent pathway that triggers IL-8 production.

Helicobacter pylori regulates cellular migration and apoptosis by activation of phosphatidylinositol 3-kinase signaling

Helicobacter pylori is the strongest identified risk factor for gastric adenocarcinoma. One H. pylori virulence constituent that augments cancer risk is the cag secretion system, which translocates CagA and peptidoglycan into host cells, eventuating in activation of signal transduction pathways. AKT is a target of phosphatidylinositol 3-kinase (PI3K) and is activated in gastric cancer, but the relationship between PI3K-AKT and H. pylori-induced cellular responses with carcinogenic potential remains unclear. We defined the molecular pathways mediating H. pylori-stimulated AKT activation and the biological consequences of these events in gastric epithelial cells. H. pylori enhanced PI3K-AKT signaling in a Src- and epidermal growth factor receptor-dependent manner, which was also mediated by a functional cag secretion system and peptidoglycan. PI3K activation attenuated apoptosis in response to infection and was required for H. pylori-induced cell migration. These results indicate that PI3K-AKT signaling regulates pathophysiologic responses to H. pylori that may lower the threshold for carcinogenesis.

Treponema denticola does not induce production of common innate immune mediators from primary gingival epithelial cells

It has been hypothesized that the neutrophil chemoattractant interleukin-8 (IL-8) forms a gradient in the oral cavity, with the highest concentration of IL-8 produced closest to the bacterial biofilm. In periodontitis, this gradient is disrupted, impairing neutrophil chemotaxis to diseased sites. Treponema denticola is prominently associated with periodontal disease, yet little is known about its ability to modulate the production of inflammatory mediators by epithelial cells. Others have shown that dentilisin, the major outer membrane protease of T. denticola, degrades IL-8 in vitro. We now provide evidence that T. denticola also fails to induce IL-8 production from primary gingival epithelial cells (PGEC). The lack of IL-8 production is not explained by IL-8 degradation, because a protease mutant that does not degrade IL-8 does not induce IL-8 production with these stimuli either. The lack of innate immune mediator production may be a more global phenomenon because T. denticola fails to induce IL-6 or intercellular adhesion molecule 1 production from PGEC. T. denticola also fails to induce transcription of IL-8 and human beta-defensin-2 messenger RNA. The lack of immune mediator production is not explained by the failure of T. denticola to interact with Toll-like receptor 2 (TLR-2), as T. denticola stimulates nuclear factor-kappaB nuclear translocation in TLR-2-transfected HEK293 cells. Not only can T. denticola degrade the IL-8 present in the periodontal lesion, but this organism also fails to induce IL-8 production by PGEC. The lack of an epithelial cell response to T. denticola may contribute to the pathogenesis of periodontitis by failing to trigger chemotaxis of neutrophils into the periodontal pocket.

PAR2 triggers IL-8 release via MEK/ERK and PI3-kinase/Akt pathways in GI epithelial cells

Proteinase-activated receptor-2 (PAR2) plays pro-inflammatory roles in many organs including the gastrointestinal (GI) tract. To clarify the downstream pro-inflammatory signaling of PAR2 in the GI tract, we examined interleukin-8 (IL-8) release and the underlying cellular signaling following PAR2 stimulation in human colorectal cancer-derived HCT-15 cells and human gastric adenocarcinoma-derived MKN-45 cells. A PAR2-activating peptide, but not a PAR2-inactive scrambled peptide or a PAR1- activating peptide, caused IL-8 release in these GI epithelial cells. The PAR2-triggered IL-8 release was suppressed by inhibitors of MEK (U0126) or PI3-kinase (LY294002), and PAR2 stimulation indeed activated the downstream kinases, ERK and Akt. U0126 blocked the phosphorylation of ERK, but not Akt, and LY294002 blocked the phosphorylation of Akt, but not ERK. Together, PAR2 triggers IL-8 release via two independent signaling pathways, MEK/ERK and PI3-kinase/Akt, suggesting a role of PAR2 as a pro-inflammatory receptor in the GI tract.

Tumor suppressor Foxo3a is involved in the regulation of lipopolysaccharide-induced interleukin-8 in intestinal HT-29 cells

Enteric bacteria and their products play an important role in intestinal inflammation; however, the complete mechanisms are not elucidated yet. Tumor suppressor Foxo3a regulates gene expression in the nucleus, and its translocation to the cytosol leads to inactivation. Proximally, Foxo3a is regulated by different pathways including the phosphoinositide 3-kinase (PI3K) pathway. The aim of this study was to determine the effect of bacterial infection on Foxo3a in intestinal epithelial cells and to examine the contribution of Foxo3a in intestinal inflammation. Bacterial lipopolysaccharide (LPS) and infection with mouse pathogen Citrobacter rodentium induce translocation of the nuclear Foxo3a into the cytosol, where it degrades in human HT-29 and mouse CMT-93 cells. In colonic epithelia of healthy mice, Foxo3a is localized in the epithelia at the bottom of the crypts in both the nucleus and the cytosol, while in C. rodentium-infected colon Foxo3a is expressed along the crypts and located mainly in the cytosol, suggesting its inactivation. LPS utilized the PI3K pathway to inhibit Foxo3a. Additionally, inhibition of PI3K attenuated LPS-induced proinflammatory interleukin-8 (IL-8). LPS-induced IL-8 is increased in HT-29 cells with silenced Foxo3a. Moreover, in HT-29 cells with silenced Foxo3a, the amount of IkappaBalpha, an NF-kappaB inhibitor, is decreased. In conclusion, LPS and bacterial infection inactivate Foxo3a in intestinal epithelia via the PI3K pathway and inactivated Foxo3a leads to the upregulation of IL-8 by suppressing inhibitory IkappaBalpha.

Growth control in the Salmonella-containing vacuole

Salmonella enterica is an intracellular bacterial pathogen that inhabits membrane-bound vacuoles of eukaryotic cells. Coined as the 'Salmonella-containing vacuole' (SCV), this compartment has been studied for two decades as a replicative niche. Recent findings reveal, however, marked differences in the lifestyle of bacteria enclosed in the SCV of varied host cell types. In fibroblasts, the emerging view supports a model of bacteria facing in the SCV a 'to grow' or 'not to grow' dilemma, which is solved by entering in a dormancy-like state. Fine-tuning of host cell defense/survival routes, drastic metabolic shift down, adaptation to hypoxia conditions, and attenuation of own virulence systems emerge as strategies used by Salmonella to intentionally reduce the growth rate inside the SCV.

Intestinal innate immunity and the pathogenesis of Salmonella enteritis

Acute gastroenteritis caused by Salmonella typhimurium infection is a clinical problem with significant public health impact. The availability of several experimental models of this condition has allowed detailed investigation of the cellular and molecular interactions involved in its pathogenesis. Such studies have shed light on the roles played by bacterial virulence factors and host innate immune mechanisms in the development of intestinal inflammation.

Shiga toxin produced by enterohemorrhagic Escherichia coli inhibits PI3K/NF-kappaB signaling pathway in globotriaosylceramide-3-negative human intestinal epithelial cells

Shiga toxin (Stx) produced by enterohemorrhagic Escherichia coli (EHEC) binds to endothelial cells expressing globotriaosylceramide-3 (Gb-3) and induces cell death by inhibiting translation. Nonetheless, the effects of Stx on human enterocytes, which lacks receptor Gb-3, remain less known. In this study, we questioned whether EHEC-derived Stx may modulate cellular signalization in the Gb-3-negative human epithelial cell line T84. Stx produced by EHEC was fixed and internalized by the cells. A weak activation of NF-kappaB was observed in T84 cells after EHEC infection. Cells infected with an isogenic mutant lacking stx1 and stx2, the genes encoding Stx, displayed an increased NF-kappaB DNA-binding activity. Consequently, the NF-kappaB-dependent CCL20 and IL-8 gene transcription and chemokine production were enhanced in T84 cells infected with the Stx mutant in comparison to the wild-type strain. Investigating the mechanism by which Stx modulates NF-kappaB activation, we showed that the PI3K/Akt signaling pathway was not induced by EHEC but was enhanced by the strain lacking Stx. Pharmacological inhibition of the PI3K/Akt signalization in EHEC DeltaStx-infected T84 cells yielded to a complete decrease of NF-kappaB activation and CCL20 and IL-8 mRNA expression. This demonstrates that the induction of the PI3K/Akt/NF-kappaB pathway is potentially induced by EHEC, but is inhibited by Stx in Gb-3-negative epithelial cells. Thus, Stx is an unrecognized modulator of the innate immune response of human enterocytes.

Flagellin-induced tolerance of the Toll-like receptor 5 signaling pathway in polarized intestinal epithelial cells

Salmonella typhimurium is a gram-negative enteric pathogen that invades the mucosal epithelium and is associated with diarrheal illness in humans. Flagellin from S. typhimurium and other gram-negative bacteria has been shown to be the predominant proinflammatory mediator through activation of the basolateral Toll-like receptor 5 (TLR5). Recent evidence has shown that prior exposure can render immune cells tolerant to subsequent challenges by TLR ligands. Accordingly, we examined whether prior exposure to purified flagellin would render human intestinal epithelial cells insensitive to future contact. We found that flagellin-induced tolerance is common to polarized epithelial cells and prevents further activation of proinflammatory signaling cascades by both purified flagellin and Salmonella bacteria but does not affect TNF-alpha stimulation of the same pathways. Flagellin tolerance is a rapid process that does not require protein synthesis, and that occurs within 1 to 2 h of flagellin exposure. Prolonged flagellin exposure blocks activation of the NF-kappaB, MAPK, and phosphoinositol 3-kinase signaling pathways and results in the internalization of a fraction of the basolateral TLR5 without affecting the polarity or total expression of TLR5. After removal of flagellin, cells require more than 24 h to fully recover their ability to mount a normal proinflammatory response. We have found that activation of phosphoinositol 3-kinase and Akt by flagellin has a small damping effect in the early stages of flagellin signaling but is not responsible for tolerance. Our study indicates that inhibition of TLR5-associated IL-1 receptor-associated kinase-4 activity occurs during the development of flagellin tolerance and is likely to be the cause of tolerance.

Neisseria gonorrhoeae PLD directly interacts with Akt kinase upon infection of primary, human, cervical epithelial cells

Neisseria gonorrhoeae secrets a phospholipase D (NgPLD), which augments complement receptor 3 (CR3)-mediated invasion of cervical epithelial cells. To elucidate the signalling pathways triggered with gonococcus CR3-engagement and the putative function of NgPLD in these events, we analysed the contribution of the phosphoinositide-Akt pathway to cervical infection. Our data indicated that Akt plays a critical role in cervical infection. Inhibition of myosin light chain kinase, PtdIns(4,5)P2, and Akt functions resulted in decreased gonococcus invasion of primary, human, cervical epithelial cells as well as Akt kinase activity. Akt activity was similarly impaired when cervical cells were challenged with NgPLD-mutant gonococci. Conversely, the PI3-kinase inhibitor, LY294002, enhanced gonococcal invasion of, and Akt activity within, primary cervical cells. We demonstrated that NgPLD directly binds to the Akt PH domain and can compete with a natural Akt ligand, PtdIns(3,4,5)P3, for Akt binding. Collectively, our data suggested that NgPLD augments gonococcus invasion of cervical epithelia by interacting with Akt kinase in a PI3-kinase-independent manner, which results in subversion of normal cervical cell signalling.