Bacterial induction of inducible nitric oxide synthase in cultured human intestinal epithelial cells

Regulation of nitric oxide production in cultured human T84 intestinal epithelial cells by nuclear factor-kappa B-dependent induction of inducible nitric oxide synthase after exposure to bacterial endotoxin

BACKGROUND

Intestinal epithelium is consistently in contact with lipopolysaccharide (LPS) produced by intraluminal microbes. LPS induces nitric oxide production in many rodent cells, but in human cells it is very differently regulated.

AIM

To test the hypothesis that exposure to LPS up-regulates nitric oxide synthesis in human intestinal epithelium.

METHODS AND RESULTS

LPS induced nitric oxide synthesis in T84 cells in a time- and dose-dependent manner whereas detectable amounts of peroxynitrite were not produced. A novel selective inducible nitric oxide synthase (iNOS) inhibitor 1400 W potently inhibited LPS-induced nitric oxide synthesis in T84 cells while dexamethasone was relatively ineffective. Nitric oxide production was sensitive to cycloheximide, indicating that it was dependent on de novo protein synthesis. Nuclear factor-kappa B (NF-kappa B) inhibitor pyrrolidinedithiocarbamate abolished iNOS and nitric oxide production. Nitric oxide synthesis was also suppressed by genistein (tyrosine kinase inhibitor) and PD 98059 (p44/42 MAP kinase inhibitor) but enhanced by SB 203580 (p38 MAP kinase inhibitor).

CONCLUSIONS

Intestinal epithelial cells express iNOS and produce nitric oxide in a nuclear factor-kappa B-dependent manner when exposed to LPS. The process is regulated by tyrosine kinases, and p44/42 and p38 MAP kinases. Because nitric oxide acts as an antimicrobial agent and immune modulator, these findings are implicated in the regulation of gut mucosal immunity.

Analysis by high density cDNA arrays of altered gene expression in human intestinal epithelial cells in response to infection with the invasive enteric bacteria Salmonella

Many clinically important enteric pathogens initiate disease by invading and passing through the intestinal epithelium, a process accompanied by increased epithelial expression of proinflammatory cytokines. To further define the role intestinal epithelial cells play in initiating and modulating the host response to infection with invasive bacteria, hybrid selection on high density cDNA arrays was used to characterize the mRNA expression profile of approximately 4,300 genes in human intestinal epithelial cells after infection with the prototypic invasive bacteria, Salmonella. Selected findings were further evaluated by reverse transcription-polymerase chain reaction, Northern blot analysis, and protein assays. Epithelial infection with Salmonella significantly up-regulated mRNA expression of a relatively small fraction of all genes tested. Of these, several cytokines (granulocyte colony-stimulating factor, inhibin A, Epstein-Barr virus-induced gene 3, interleukin-8, macrophage inflammatory protein-2alpha), kinases (TKT, Eck, HEK), transcription factors (interferon regulatory factor-1), and HLA class I were the most prominent. Furthermore, the transcription factor NF-kappaB is shown to be important for inducible mRNA expression for a broad group of genes tested. These findings expand the repertoire of known epithelial cell responses to infection with an invasive enteric pathogen. The results also show that evaluation of mRNA expression profiles by cDNA array analysis is a powerful approach to characterizing and understanding host-pathogen interactions.

The I kappa B/NF-kappa B system: a key determinant of mucosalinflammation and protection

The ubiquitous transcription factor NF-kappa B is a central regulator of the transcriptional activation of a number of genes involved in cell adhesion, immune and proinflammatory responses, apoptosis, differentiation, and growth. Induction of these genes in intestinal epithelial cells (IECs) by activated NF-kappa B profoundly influences mucosal inflammation and repair. NF-kappa B activation requires the removal of I kappa B from NF-kappa B by inducible proteolysis, which liberates this transcription factor for migration to the nucleus, where it binds to kappa B-regulatory elements and induces transcription. I kappa B alpha degradation is incomplete and delayed in IECs, resulting in buffered responses to luminal stimuli. The stimulatory environment partially determines whether the effect of NF-kappa B is protective or deleterious for the host. kappa B-dependent proinflammatory gene expression, particularly chemokines, major histocompatibility complex class II antigens, and adhesion molecules may be extremely important in early protective responses to mucosal pathogens but, when dysregulated, could lead to the development of chronic inflammation, as seen in inflammatory bowel diseases. The key role of NF-kappa B in regulating expression of a number of proinflammatory genes makes this protein an attractive target for selective therapeutic intervention.

Helicobacter pylori induces apoptosis in gastric epithelial cells through inducible nitric oxide

BACKGROUND

Gastric mucosal injury by Helicobacter pylori has been suggested to be mediated by various cytokines induced by this organism. Nitric oxide (NO) is an important effector molecule involved in immune regulation and defence. To clarify the mechanisms by which H. pylori induces gastric mucosal cell injury, we examined whether H. pylori induces gastric epithelial death via NO production.

METHODS

Cytotoxic and non-cytotoxic strains of H. pylori were used. The death of MKN45 cells caused by H. pylori was examined by the 3-(4,5-dimethyl-thiazole-2yl)-2,5-diphenyl tetrazolium bromide (MTT) assays. Aminoguanidine was used to inhibit inducible nitric oxide synthase (iNOS) activity. Expression of iNOS mRNA was determined by the reverse transcriptase-polymerase chain reaction and the DNA fragmentation analysis was performed by using agarose gel electrophoresis.

RESULTS

The MTT assay revealed that neither viable H. pylori nor other components of the microorganism induced cell death. Both preincubation of MKN45 cells with interferon-gamma for 6 h and coculture with TNF-alpha significantly increased the cytotoxicity of H. pylori. Both cytotoxic and non-cytotoxic strains of H. pylori induced cell death. Expression of iNOS mRNA was observed in MKN45 cells at 6, 8 and 12 h after H. pylori inoculation. The cytotoxicity of H. pylori was inhibited by aminoguanidine and DNA fragmentation analysis showed that H. pylori induced apoptosis.

CONCLUSIONS

These findings suggested that viable H. pylori induces apoptosis of gastric epithelial cells via nitric oxide. Our study indicated that iNOS expression plays an important role in gastric cell injury.

Nitric oxide production by human intestinal epithelial cells and competition for arginine as potential determinants of host defense against the lumen-dwelling pathogen Giardia lamblia

Giardia lamblia infection of the human small intestine is a common protozoan cause of diarrheal disease worldwide. Although infection is luminal and generally self-limiting, and secretory Abs are thought to be important in host defense, other defense mechanisms probably affect the duration of infection and the severity of symptoms. Because intestinal epithelial cells produce NO, and its stable end products, nitrite and nitrate, are detectable mainly on the apical side, we tested the hypothesis that NO production may constitute a host defense against G. lamblia. Several NO donors, but not their control compounds, inhibited giardial growth without affecting viability, suggesting that NO is cytostatic rather than cytotoxic for G. lamblia. NO donors also inhibited giardial differentiation induced by modeling crucial environmental factors, i. e., encystation induced by bile and alkaline pH, and excystation in response to gastric pH followed by alkaline pH and protease. Despite the potent antigiardial activity of NO, G. lamblia is not simply a passive target for host-produced NO, but has strategies to evade this potential host defense. Thus, in models of human intestinal epithelium, G. lamblia inhibited epithelial NO production by consuming arginine, the crucial substrate used by epithelial NO synthase to form NO. These studies define NO and arginine as central components in a novel cross-talk between a luminal pathogen and host intestinal epithelium.

Gastroduodenal mucosal defense

Research performed in the laboratory and the clinic over the past several years has added to our understanding of the mechanisms that are operative in protecting the epithelial lining of the stomach and duodenum from injury and ulceration, most frequently caused by necrotic agents in the lumen. The defensive mechanism of the gastroduodenal mucosa comprises a series of physical, chemical, biologic, and immunologic barriers or mechanisms that act in concert to either prevent or limit cellular injury or transformation. The field of gastroduodenal defense can be subdivided into the following four areas: extracellular mucus barrier properties; membrane and ion transport properties; cellular factors promoting growth and restitution; and vascular, neural, and inflammatory factors ensuring optimal tissue perfusion and immune responsiveness, respectively. In addition, a great deal can be learned about gastroduodenal defense by studying the effects of ulcerogenic factors and conditions on the defensive mechanisms described here and specifically how they may be compromised by nonsteroidal anti-inflammatory drugs and Helicobacter pylori infection. This review presents interesting and noteworthy findings impacting on these properties contributing to gastroduodenal defense since the prior review article on this subject appearing in this journal.

Nitric Oxide Participates in the Recovery of Normal Jejunal Epithelial Ion Transport Following Exposure to the Superantigen, Staphylococcus aureus Enterotoxin B

Bacterial superantigens (SAgs) are potent T cell activators. Mice treated 4 h previously with the SAg, Staphylococcus aureus enterotoxin B (SEB), display reduced ion transport (assessed by short circuit current) responses to prosecretory stimuli, which normalize 24 h posttreatment. Here, mice were treated with SEB alone or in combination with an inhibitor of the inducible form of NO synthase (iNOS), l-NIL. Subsequently, jejunal iNOS expression was detected by immunohistochemistry, ion transport was evaluated in Ussing chambers, and serum levels of TNF-α and IFN-γ were measured by ELISA. SEB-treated mice had increased epithelial iNOS immunoreactivity, and numerous iNOS-positive CD3+ T cells occurred in their mucosa and submucosa. Concomitant treatment with l-NIL did not affect the reduced short circuit current responsiveness to electrical nerve stimulation or the prosecretory agents, carbachol and forskolin, that occurred 4 h post-SEB (5 μg) treatment. However, Isc responses in l-NIL- plus SEB-treated mice were still significantly reduced 24 h posttreatment, indicating a role for NO in the restoration of normal ion transport following exposure to SAgs. The prolongation of epithelial ion transport abnormalities correlated with elevated serum levels of TNF-α and IFN-γ in mice treated 24 h previously with l-NIL plus SEB compared with those in controls and SEB-only-treated mice. Additionally, mice treated with l-NIL plus SEB and TNF-α- or IFN-γ-neutralizing Abs displayed normal jejunal ion transport characteristics 24 h posttreatment. We conclude that NO mobilization is important in the homeostatic recovery response following immune stimulation by SAgs and that the beneficial effect of NO in this model system is probably via regulation of TNF-α and IFN-γ production.

Regulation of induction of nitric oxide synthase and the inhibitory actions of dexamethasone in the human intestinal epithelial cell line, Caco-2: influence of cell differentiation

1. The inducible isoform of nitric oxide synthase (iNOS) may be involved in the pathogenesis of inflammatory bowel disease. Using the human intestinal epithelial cell line, Caco-2, iNOS expression, regulation and sensitivity to the glucocorticoid, dexamethasone after cytokine exposure and its relationship to the degree of differentiation has been studied. 2. NOS activity, assessed by NO2- and NO3- release, was time-dependently increased after exposure to interferon gamma alone or in combination with interleukin-1beta and tumour necrosis factor alpha. 3. Cytokine-induced iNOS activity was increased with days in culture over 20 days and number of passages, suggesting iNOS up-regulation during enterocyte-like differentiation. This activity was inhibited by the selective iNOS inhibitor 1400 W (0.1 - 100 microM). In addition, iNOS protein induction was confirmed by Western blot. 4. Actinomycin D (5 microg ml(-1) inhibited cytokine-induced iNOS activity, protein expression and mRNA level. Pyrrolidine dithiocarbamate (PDTC: 10 - 200 microM) and 3,4 dichloroisocoumarin (0.1 - 100 microM) reduced cytokine-induced iNOS activity and protein expression at both day 10 and 15 after confluence. PDTC also decreased iNOS mRNA levels, suggesting NF-kappaB involvement in its transcription at these times. 5. The tyrphostins A25 and B42 reduced cytokine-induced iNOS activity at both day 10 and 15 after confluence, indicating the JAK-2 kinase is also involved at these times. The tyrphostins also reduced the iNOS protein expression. 6. Dexamethasone (0.1 - 10 microM, for 24 h) reduced cytokine-induced iNOS activity at day 15 and 20 after cell confluence, but not at day 5 or 10. 7. Dexamethasone (5 microM) decreased cytokine-induced iNOS protein expression at day 10 as well as at day 15 after confluence. 8. These findings indicate that iNOS induction and its inhibition by dexamethasone in this human intestinal epithelial cell line is dependent on the degree of differentiation.

Innate mechanisms of epithelial host defense: spotlight on intestine

The single layer of epithelial cells lining the intestinal tract is charged with a most difficult task: protecting the underlying biological compartments from both the normal commensal flora that reside within the intestinal lumen as well as the uninvited pathogens. To such an end, the intestinal epithelial cells are equipped with a panoply of defense mechanisms, both constitutive and inducible. This review focuses only on those defense mechanisms that are initiated and executed by the intestinal epithelial cell. Fitting these strict criteria are three major categories of epithelial host defense: enhanced salt and water secretion, expression of antimicrobial proteins and peptides, and production of intestinal mucins. Each of these areas is discussed in this review.

Nitric oxide inhibits heterologous CFTR expression in polarized epithelial cells

Nitric oxide (. NO) has been implicated in a wide range of autocrine and paracrine signaling mechanisms. Herein, we assessed the role of exogenous. NO in the modulation of heterologous gene expression in polarized kidney epithelial cells (LLC-PK(1)) that were stably transduced with a cDNA encoding human wild-type cystic fibrosis transmembrane conductance regulator (CFTR) under the control of a heavy metal-sensitive metallothionein promoter (LLC-PK(1)-WTCFTR). Exposure of these cells to 125 microM DETA NONOate at 37 degrees C for 24 h (a chemical. NO donor) diminished Zn(2+)-induced and uninduced CFTR protein levels by 43.3 +/- 5.1 and 34.4 +/- 17.1% from their corresponding control values, respectively. These changes did not occur if red blood cells, effective scavengers of. NO, were added to the medium. Exposure to. NO did not alter lactate dehydrogenase release in the medium or the extent of apoptosis. Coculturing LLC-PK(1)-WTCFTR cells with murine fibroblasts that were stably transduced with the human inducible. NO synthase cDNA gene also inhibited CFTR protein expression in a manner that was antagonized by 1 mM N(G)-monomethyl-L-arginine in the medium. Pretreatment of LLC-PK(1)-WTCFTR with ODQ, an inhibitor of guanylyl cyclase, did not affect the ability of. NO to inhibit heterologous CFTR expression; furthermore, 8-bromo-cGMP had no effect on heterologous CFTR expression. These data indicate that. NO impairs the heterologous expression of CFTR in epithelial cells at the protein level via cGMP-independent mechanisms.

Differential regulation of inducible nitric oxide synthase gene expression by ethanol in the human intestinal epithelial cell line DLD-1

We have examined the regulation of inducible nitric oxide synthase (iNOS) gene expression by ethanol in monolayers of DLD-1 cells, an epithelial cell line derived from human intestinal adenocarcinoma. Optimum induction of iNOS mRNA in these cells was obtained with IFN-gamma and IL-1beta treatment, while further addition of TNF-alpha did not have significant effect. In a set of experiments to study ethanol effects, DLD-1 monolayers were pretreated with ethanol for 24 h and were then treated with IFN-gamma + IL-1beta for an additional 24 h. Cells pretreated with ethanol showed decreased iNOS mRNA levels, indicating that ethanol may inhibit cytokine-induced iNOS transcription or affect mRNA destabilization. The suppression was ethanol-dose dependent with an IC50 of 50 mM. In another set of experiments to study ethanol effects, DLD-1 monolayers were pretreated with 66 mM ethanol for 24 h. These cells showed significant upregulation of IL-1beta mRNA and protein as detected in the supernatants. Aliquoted supernatants from these cells (i.e., conditioned media) were added to naive DLD-1 monolayers together with IFN-gamma. Conditioned medium from ethanol-treated cells increased the IFN-gamma-induced iNOS mRNA of naive cells by threefold. Two different effects of ethanol are now reported: (a) ethanol inhibits IFN-gamma + IL-1beta-induced iNOS mRNA of the same DLD-1 cells and (b) ethanol induces cellular paracrine signals by releasing IL-1beta into the medium, which in combination with IFN-gamma increases iNOS mRNA levels of the recipient naive DLD-1 cells. Because IFN-gamma and IL-1beta are produced by intestinal immune cells, these findings may have implications for differential in vivo regulation of epithelial iNOS genes by ethanol, depending on the inflammatory and immune status of the host.

Intestinal trefoil factor binds to intestinal epithelial cells and induces nitric oxide production: priming and enhancing effects of mucin

Intestinal trefoil factor (ITF or TFF3), NO and epithelium-associated mucin have important roles in sustaining mucosal integrity in the gastrointestinal tract. In the present study we examined ITF-binding molecules on IEC-18 cells (an intestinal epithelial cell line) with the use of flow cytometry and localized these molecules on the cell surface by confocal microscopy. Furthermore, we studied the interaction of mucin and ITF and their co-operative effect on NO production by the epithelium. Stimulation of cells with mucin (5 mg/ml) for 90 min resulted in a 5-fold increase in ITF binding. Treatment of IEC-18 cells with actinomycin D or cycloheximide attenuated mucin-enhanced ITF binding. Ligand blot analysis confirmed the induction of ITF-binding protein in IEC-18 cells by mucin. These results indicate that transcriptional and translational mechanisms are involved in the effect of mucin. Treatment with ITF overnight resulted in a low level of nitrite production by the cells, a 5-fold increase over control, in a concentration-dependent manner. ITF-induced NO production was attenuated by 1400W, a selective type II nitric oxide synthase (NOS2) inhibitor. By immunoblotting we found that NOS2 was up-regulated by ITF treatment. Priming IEC-18 cells with mucin for 90 min enhanced the effect of ITF on NO production, suggesting that the up-regulation of ITF-binding molecules by mucin might be physiologically relevant. Taken together, these observations indicate (1) that ITF-binding molecules that are up-regulated by mucin exist on the intestinal epithelial surface, and (2) that ITF modulates epithelial NO production via the NOS2 pathway, which is enhanced by mucin.

Intestinal trefoil factor binds to intestinal epithelial cells and induces nitric oxide production: priming and enhancing effects of mucin

Intestinal trefoil factor (ITF or TFF3), NO and epithelium-associated mucin have important roles in sustaining mucosal integrity in the gastrointestinal tract. In the present study we examined ITF-binding molecules on IEC-18 cells (an intestinal epithelial cell line) with the use of flow cytometry and localized these molecules on the cell surface by confocal microscopy. Furthermore, we studied the interaction of mucin and ITF and their co-operative effect on NO production by the epithelium. Stimulation of cells with mucin (5 mg/ml) for 90 min resulted in a 5-fold increase in ITF binding. Treatment of IEC-18 cells with actinomycin D or cycloheximide attenuated mucin-enhanced ITF binding. Ligand blot analysis confirmed the induction of ITF-binding protein in IEC-18 cells by mucin. These results indicate that transcriptional and translational mechanisms are involved in the effect of mucin. Treatment with ITF overnight resulted in a low level of nitrite production by the cells, a 5-fold increase over control, in a concentration-dependent manner. ITF-induced NO production was attenuated by 1400W, a selective type II nitric oxide synthase (NOS2) inhibitor. By immunoblotting we found that NOS2 was up-regulated by ITF treatment. Priming IEC-18 cells with mucin for 90 min enhanced the effect of ITF on NO production, suggesting that the up-regulation of ITF-binding molecules by mucin might be physiologically relevant. Taken together, these observations indicate (1) that ITF-binding molecules that are up-regulated by mucin exist on the intestinal epithelial surface, and (2) that ITF modulates epithelial NO production via the NOS2 pathway, which is enhanced by mucin.

Review article: the potential role of nitric oxide in chronic inflammatory bowel disorders

The aetiology of the chronic inflammatory bowel diseases-ulcerative colitis and Crohn's disease-as well as 'microscopic colitis'-both collagenous (COC) and lymphocytic colitis (LC)-remains unknown. Autoimmune mechanisms, cytokine polymorphism, commensal bacteria, infectious agents and vascular impairment have all been proposed as playing important roles in the pathogenesis of this spectrum of diseases. A variety of proinflammatory mediators, including tumour necrosis factor alpha, interleukin-1beta, interferon gamma, leukotriene B4 and platelet activating factor, promote the adherence of phagocytes to the venular endothelium and extravasation of these cells into the colonic mucosa. In addition to large amounts of nitric oxide (NO), injurious peroxynitrite may be formed in the epithelium by the inducible nitric oxide synthase (iNOS), which is considered to elicit cytotoxicity by the generation of superoxide with reduced L-arginine availability. In active ulcerative colitis, and to a lesser extent in Crohn's disease, a greatly increased production of NO has been demonstrated by indirect and direct measurements. Surprisingly, even higher rates of production have been observed in COC-a condition which is never associated with injurious inflammation. The latter observation favours the notion that NO promotes mucosal integrity. Further evidence for a protective role of NO in chronic inflammatory bowel disorders is provided by the observation of increased susceptibility to the induction of experi mental colitis in 'knock-out' mice deficient in iNOS. Selective inhibitors of iNOS activity, as well as topical L-arginine, may therefore prove beneficial in inflammatory bowel disease by reducing the production of superoxide by iNOS, while only the former option may be expected to reduce diarrhoea in chronic inflammatory bowel disorders. Clearly, further experimental work needs to be done before testing topical L-arginine in human inflammatory bowel disease.

Activation of apoptotic caspase-3 and nitric oxide synthase-2 in gastric mucosal injury induced by indomethacin

BACKGROUND

Apoptosis is the process of programmed cell death characterized by a series of distinct biochemical and morphologic changes involving activation of the caspase protease, cascade, which remains under the regulatory control of nitric oxide. In this study we investigated the activity of a key apoptotic protease, caspase-3, and the expression of inducible nitric oxide synthase (NOS-2) associated with gastric epithelial cell apoptosis during indomethacin-induced gastric mucosal injury.

METHODS

The experiments were conducted with groups of rats subjected to intragastric administration of 60 mg/kg indomethacin or the vehicle. After 2 h the animals were killed and their gastric mucosal tissue used for macroscopic assessment, assays of epithelial cell apoptosis, and the measurement of caspase-3 and NOS-2 activities.

RESULTS

Indomethacin caused extensive multiple hemorrhagic lesions accompanied by marked enhancement in epithelial cell apoptosis, a 3.9-fold increase in mucosal expression of caspase-3 activity, and an 11.9-fold induction in NOS-2. Moreover, the mucosal expression of NOS-2 showed a positive correlation with the extent of changes induced in caspase-3 activity.

CONCLUSIONS

The results implicate caspase-3 in the process of indomethacin-induced gastric epithelial cell apoptosis and point towards participation of NOS-2 in the amplification of the cell death signaling cascade, hence contributing to the extent of mucosal injury.

Enteroinvasive bacteria directly activate expression of iNOS and NO production in human colon epithelial cells

In these studies, we investigated whether bacterial infection of human colon epithelial cells is a sufficient stimulus to upregulate epithelial cell expression of inducible nitric oxide synthase (iNOS) and nitric oxide (NO) production. Human colon epithelial cells (Caco-2 and HT-29) rapidly upregulated iNOS mRNA and protein expression and NO production after infection with enteroinvasive Escherichia coli, Salmonella dublin, or Shigella flexneri but not after infection with noninvasive E. coli or an invasion-deficient mutant of S. dublin. Bacterial infection in the absence of added cytokines was as potent or more potent a stimulus of iNOS expression and NO production as stimulation of cells with combinations of cytokines known to strongly upregulate this epithelial cell response. Enteroinvasive E. coli increased epithelial NO production to a greater extent than S. dublin, although S. dublin was a stronger stimulus of epithelial cell interleukin-8 (IL-8) production. After enteroinvasive E. coli infection of polarized epithelial cell monolayers, nitrite, a stable NO end product, was released predominately into the apical compartment early after infection, whereas IL-8 was released in parallel into the basolateral compartment. These studies suggest NO and/or its redox products are an important component of the intestinal epithelial cell response to microbial infection.