Tumor necrosis factor-alpha induces Cl- and K+ secretion in human distal colon driven by prostaglandin E2

Time-dependent effects of tumor necrosis factor α on Ca2+-dependent secretion in murine small intestinal organoids

Background

Intestinal organoids are stem cell-derived, 3D "mini-guts" with similar functions as the native intestinal epithelium such as electrolyte transport or establishment of an epithelial barrier. During intestinal inflammation, epithelial functions are dysregulated by proinflammatory cytokines like tumor necrosis factor α (TNFα) and other messengers from the immune system resulting in a loss of electrolytes and water due to an impaired epithelial barrier and higher net secretion.

Methods

A murine small intestinal organoid model was established to study (long-term) effects of TNFα on the intestinal epithelium in vitro using live imaging, immunohistochemical staining and qPCR.

Results

TNFα induced apoptosis in intestinal organoids as indicated by an increased number of cells with immunoreactivity for cleaved caspase 3. Furthermore, TNFα exposure led to swelling of the organoids which was inhibited by bumetanide and was concomitant with an upregulation of the bumetanide-sensitive Na+-K+-2Cl- symporter 1 (NKCC1) as shown by qPCR. Fura-2 imaging experiments revealed time-dependent changes in Ca2+ signaling consisting of a rise in the basal cytosolic Ca2+ concentration at day 1 and an increase of the carbachol-induced Ca2+ response after 3 days TNFα exposure. This was prevented by preincubation with La3+, an inhibitor of non-selective cation channels, or by using a Ca2+-free buffer indicating an enhancement of the Ca2+ influx from the extracellular side by the cytokine. No significant changes in cDNA levels of epithelial barrier proteins could be observed in the presence of TNFα.

Conclusion

Intestinal organoids are a useful tool to study the mechanism underlying the TNFα-induced secretion on enterocytes such as the regulation of NKCC1 expression or the modulation of cellular Ca2+ signaling.

Intestinal Immune Imbalance is an Alarm in the Development of IBD

Immune regulation plays a crucial role in human health and disease. Inflammatory bowel disease (IBD) is a chronic relapse bowel disease with an increasing incidence worldwide. Clinical treatments for IBD are limited and inefficient. However, the pathogenesis of immune-mediated IBD remains unclear. This review describes the activation of innate and adaptive immune functions by intestinal immune cells to regulate intestinal immune balance and maintain intestinal mucosal integrity. Changes in susceptible genes, autophagy, energy metabolism, and other factors interact in a complex manner with the immune system, eventually leading to intestinal immune imbalance and the onset of IBD. These events indicate that intestinal immune imbalance is an alarm for IBD development, further opening new possibilities for the unprecedented development of immunotherapy for IBD.

Diarrheal Mechanisms and the Role of Intestinal Barrier Dysfunction in Campylobacter Infections

Campylobacter enteritis is the most common cause of foodborne bacterial diarrhea in humans. Although various studies have been performed to clarify the pathomechanism in Campylobacter infection, the mechanism itself and bacterial virulence factors are yet not completely understood. The purpose of this chapter is to (i) give an overview on Campylobacter-induced diarrheal mechanisms, (ii) illustrate underlying barrier defects, (iii) explain the role of the mucosal immune response and (iv) weigh preventive and therapeutic approaches. Our present knowledge of pathogenetic and diarrheal mechanisms of Campylobacter jejuni is explained in the first part of this chapter. In the second part, the molecular basis for the Campylobacter-induced barrier dysfunction is compared with that of other species in the Campylobacter genus. The bacteria are capable of overcoming the intestinal epithelial barrier. The invasion into the intestinal mucosa is the initial step of the infection, followed by a second step, the epithelial barrier impairment. The extent of the impairment depends on various factors, including tight junction dysregulation and epithelial apoptosis. The disturbed intestinal epithelium leads to a loss of water and solutes, the leak flux type of diarrhea, and facilitates the uptake of harmful antigens, the leaky gut phenomenon. The barrier dysfunction is accompanied by increased pro-inflammatory cytokine secretion, which is partially responsible for the dysfunction. Moreover, cytokines also mediate ion channel dysregulation (e.g., epithelial sodium channel, ENaC), leading to another diarrheal mechanism, which is sodium malabsorption. Future perspectives of Campylobacter research are the clarification of molecular pathomechanisms and the characterization of therapeutic and preventive compounds to combat and prevent Campylobacter infections.

Crossing signals: bioactive lipids in the microvasculature

The primary function of the arterial microvasculature is to ensure that regional perfusion of blood flow is matched to the needs of the tissue bed. This critical physiological mechanism is tightly controlled and regulated by a variety of vasoactive compounds that are generated and released from the vascular endothelium. Although these substances are required for modulating vascular tone, they also influence the surrounding tissue and have an overall effect on vascular, as well as parenchymal, homeostasis. Bioactive lipids, fatty acid derivatives that exert their effects through signaling pathways, are included in the list of vasoactive compounds that modulate the microvasculature. Although lipids were identified as important vascular messengers over three decades ago, their specific role within the microvascular system is not well defined. Thorough understanding of these pathways and their regulation is not only essential to gain insight into their role in cardiovascular disease but is also important for preventing vascular dysfunction following cancer treatment, a rapidly growing problem in medical oncology. The purpose of this review is to discuss how biologically active lipids, specifically prostanoids, epoxyeicosatrienoic acids, sphingolipids, and lysophospholipids, contribute to vascular function and signaling within the endothelium. Methods for quantifying lipids will be briefly discussed, followed by an overview of the various lipid families. The cross talk in signaling between classes of lipids will be discussed in the context of vascular disease. Finally, the potential clinical implications of these lipid families will be highlighted.

Proposal for an Organ-Specific Chronic Inflammation–Remodeling–Carcinoma Sequence

An organ-specific chronic inflammation–remodeling–carcinoma sequence has been proposed, mainly for the alimentary tract. As representative diseases, gastroesophageal reflux disease, chronic gastritis and inflammatory bowel disease (ulcerative colitis and Crohn’s disease of the colitis type) were adopted for this discussion. Tissue remodeling is such an important part of tumorigenesis in this sequence that an organ-specific chronic inflammation–remodeling–carcinoma sequence has been proposed in detail. Chronic inflammation accelerates the cycle of tissue injury and regeneration; in other words, cell necrosis (or apoptosis) and proliferation result in tissue remodeling in long-standing cases of inflammation. Remodeling encompasses epithelial cell metaplasia and stromal fibrosis, and modifies epithelial–stromal cell interactions. Further, the accumulation of genetic, epigenetic and molecular changes—as well as morphologic disorganization—also occurs during tissue remodeling. The expression of mucosal tissue adapted to chronic inflammatory injury is thought to occur at an early stage. Subsequently, dysplasia and carcinoma develop on a background of remodeling due to continuous, active inflammation. Accordingly, organ-specific chronic inflammation should be ameliorated or well controlled with appropriate monitoring if complete healing is unachievable.

Organoid-based epithelial to mesenchymal transition (OEMT) model: from an intestinal fibrosis perspective

The current in vitro or in vivo intestinal fibrosis models have many limitations. Recent advancements in the isolation and culturing of organoids has led to development of various three-dimensional (3D) intestinal disease models with in vivo physiology. In this study, we generated an organoid-based epithelial to mesenchymal transition (OEMT) model, which could be used as a novel intestinal fibrosis model. Intestinal epithelial organoids (IEOs) were isolated and cultured from the small intestines of normal mice. IEOs were treated with transforming growth factor- β1 (TGF-β1) or Tumor necrosis factor-α (TNF-α) to evaluate their phenotypic change. Raw 264.7 cells (macrophage) stimulated with lipopolysaccharide were co-cultured with IEOs in growth media with or without TGF-β1. TGF-β1 alone slightly induced epithelial to mesenchymal transition (EMT) in the IEOs but mainly disrupted them. Macrophage released cytokines synergistically induced mesenchymal phenotypic changes in TGF-β1 stimulated intestinal organoids. TNF-α and TGF-β1 synergistically induced proliferation of mesenchymal cells as well as EMT in the IEOs. We generated a novel OEMT model based on our finding that TNF-α and TGF-β synergistically induce type 2 EMT in IEOs. This 3D EMT model with in vivo physiology could be used to study EMT associated intestinal fibrosis.

Bile deficiency induces changes in intestinal Cl(-) and HCO3 (-) secretions in mice

AIMS

Biliary tract obstruction is a common clinical lesion. However, the effect of biliary tract obstruction on intestinal secretion is poorly understood. In this study, we made an investigation on intestinal HCO3 (-) and Cl(-) secretions in an experimental model of murine biliary duct ligation.

METHODS

Murine intestinal mucosal HCO3 (-) and Cl(-) secretions were examined in vitro in Ussing chambers by pH-stat and short-circuit current (Isc ) techniques. The mRNA and protein expressions of the cystic fibrosis transmembrane conductance regulator (CFTR) and the Na(+) -K(+) -2Cl(-) cotransporter (NKCC1) were analysed by real-time PCR, western blot and immunohistochemistry.

RESULTS

Basal Cl(-) secretion and forskolin-stimulated duodenal and jejunal mucosal HCO3 (-) and Cl(-) secretions in mice with common biliary duct ligation were markedly elevated, compared with controls (P < 0.05 and P < 0.01). Further experiments showed that basal Cl(-) secretion and forskolin-stimulated duodenal and jejunal mucosal HCO3 (-) and Cl(-) secretions in mice with external bile drainage were also markedly elevated. CFTRinh -172 inhibited forskolin-stimulated HCO3 (-) and Cl(-) secretions. The mRNA and protein expression levels of CFTR and NKCC1 in the intestinal mucosa with both biliary duct ligation and external bile drainage were markedly higher than those in controls (P < 0.001). Bile acid administration restored the changes in function and expression of CFTR and NKCC1 in the intestinal mucosa.

CONCLUSION

Bile deficiency in the intestine up-regulates the expressions of intestinal mucosal CFTR and NKCC1 and enhances intestinal mucosal HCO3 (-) and Cl(-) secretion capacity, which contributes to the understanding of intestinal physiological function for patients with biliary duct obstruction.

Vaginal microbicides to prevent human immunodeficiency virus infection in women: perspectives on the female genital tract, sexual maturity and mucosal inflammation

Topically applied vaginal microbicides to protect against human immunodeficiency (HIV) virus infection offer an important female-controlled prevention strategy. Microbicides have been in development for more than 2 decades, and have included various agents that disrupt cellular and microbial membranes (surfactants), restore the natural acidic protective pH of the vagina (acid buffers), and those that interfere with interactions between HIV envelope proteins and cellular receptors (anionic polymers). Although none of these candidate microbicides have shown significant protection against HIV in clinical trials, a topical gel, including the antiretroviral drug tenofovir (TFV) 1% was the first microbicide to be tested to show some protection against HIV infection. This review explores the effect of female genital tract biology and anatomy, mucosal inflammation, and age on the effectiveness of microbicides to prevent HIV infection.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

T cell activation alters intestinal structure and function

Treatment with anti-CD3 antibody (anti-CD3) causes transient diarrhea. In this issue of the JCI, Clayburgh et al. show that, in jejunum of mice injected with anti-CD3 or with TNF, fluid accumulation and changes in epithelial phenotype develop, the latter including an increase in the passive permeability to proteins, smaller solutes, and water and the endocytosis of the brush border Na+/H+ exchanger, thereby inhibiting Na+ absorption (a second cytokine, LIGHT, has the former effect, but not the latter) (see the related article beginning on page 2682). These phenotypic changes, by themselves, do not, however, explain increased fluid secretion. Since active anion secretion is not stimulated (in fact it is inhibited), a non-epithelial cell-mediated driving force must be present--most likely an increase in interstitial pressure due to an effect of TNF on capillary permeability, smooth muscle contractility, or both.

Changes in ion transport in inflammatory disease

Ion transport is essential for maintenance of transmembranous and transcellular electric potential, fluid transport and cellular volume. Disturbance of ion transport has been associated with cellular dysfunction, intra and extracellular edema and abnormalities of epithelial surface liquid volume. There is increasing evidence that conditions characterized by an intense local or systemic inflammatory response are associated with abnormal ion transport. This abnormal ion transport has been involved in the pathogenesis of conditions like hypovolemia due to fluid losses, hyponatremia and hypokalemia in diarrhoeal diseases, electrolyte abnormalities in pyelonephritis of early infancy, septicemia induced pulmonary edema, and in hypersecretion and edema induced by inflammatory reactions of the mucosa of the upper respiratory tract. Components of membranous ion transport systems, which have been shown to undergo a change in function during an inflammatory response include the sodium potassium ATPase, the epithelial sodium channel, the Cystic Fibrosis Transmembrane Conductance Regulator and calcium activated chloride channels and the sodium potassium chloride co-transporter. Inflammatory mediators, which influence ion transport are tumor necrosis factor, gamma interferon, interleukins, transforming growth factor, leukotrienes and bradykinin. They trigger the release of specific messengers like prostaglandins, nitric oxide and histamine which alter ion transport system function through specific receptors, intracellular second messengers and protein kinases. This review summarizes data on in vivo measurements of changes in ion transport in acute inflammatory conditions and in vitro studies, which have explored the underlying mechanisms. Potential interventions directed at a correction of the observed abnormalities are discussed.

Probiotics and commensals reverse TNF-alpha- and IFN-gamma-induced dysfunction in human intestinal epithelial cells

BACKGROUND & AIMS

Commensal bacteria are crucial for the development of the mucosal immune system. Probiotics are commensals with special characteristics and may protect mucosal surfaces against pathogens. Pathogens cause significant phenotypic alterations in infected epithelial cells, and probiotics reverse these deleterious responses. We hypothesized that probiotics and/or commensals may also reverse epithelial damage produced by cytokines.

METHODS

Human intestinal epithelial cells were exposed basolaterally to interferon (IFN)-gamma (10(3) U/mL) or tumor necrosis factor (TNF)-alpha (10 ng/mL) for up to 48 hours and assessed for ion transport, transepithelial resistance (TER), and epithelial permeability in the presence or absence of probiotics (Streptococcus thermophilus [ST] and Lactobacillus acidophilus [LA]), or the commensal, Bacteroides thetaiotaomicron (BT).

RESULTS

Agonist-stimulated chloride secretion was inhibited by IFN-gamma, an effect prevented by ST/LA or BT. The ability of ST/LA or BT to restore Cl(-) secretion was blocked by inhibitors of p38 MAPK, ERK1, 2, and PI3K. The cystic fibrosis transmembrane conductance regulator (CFTR) and the NKCC1 cotransporter were down-regulated by IFN-gamma, and ST/LA pretreatment reversed this effect. Both TNF-alpha and IFN-gamma significantly reduced TER and increased epithelial permeability, effects prevented by ST/LA or BT. A Janus kinase (JAK) inhibitor synergistically potentiated effects of ST/LA or BT on TER and permeability, but p38, ERK1, 2, or PI3K inhibition did not. Finally, only probiotic-treated epithelial cells exposed to cytokines showed reduced activation of SOCS3 and STAT1,3.

CONCLUSIONS

Deleterious effects of TNF-alpha and IFN-gamma on epithelial function are prevented by probiotic, and to a lesser extent, commensal pretreatment. These data extend the spectrum of effects of such bacteria on intestinal epithelial function and may justify their use in inflammatory disorders.

PGE2 exerts dose-dependent opposite effects on net water and chloride absorption from the rat colon

This work investigated the effect of different doses of PGE2 on net water and Cl- absorption from the rat colon, using an in situ perfusion technique. PGE2 exerted opposite effects at different concentrations. Net water and Cl- absorption was significantly reduced at low doses with a minimum at 0.4 microg/100g BW, and significantly elevated at high doses with an observed maximal effect at 21 microg/100g BW. At low doses, PGE2 increased in superficial cells, the activity of the Na+-K+ ATPase and the protein expression of the Na+K+2Cl- cotransporter, but reduced them in crypt cells. Thus, the reduction in net water and Cl- absorption was ascribed to an increase in secretion by surface cells that masked absorptive processes. At high doses, PGE2 increased significantly the activity of the Na+-K+ ATPase in superficial cells only, and was without any effect on the protein expression of the cotransporter and the pump in both surface and crypt cells. The observed increase in net water and Cl- absorption was attributed in this case to an increase in absorptive processes with no effect on secretion.

Dietary plasma protein affects the immune response of weaned rats challenged with S. aureus Superantigen B

The aim of this study was to determine the potential modulatory effects of diets supplemented with spray-dried animal plasma (SDAP) or immunoglobulin concentrates (IC) on the immune response of rats challenged with Staphylococcus aureus enterotoxin B (SEB). Lewis rats were fed diets containing 80 g of SDAP/kg diet, 22.7 g of IC/kg diet, or milk proteins (Control diet) from postnatal d 21 (weaning) for 14 d. On d 30 and 33, rats were given SEB (0.5 mg/kg body weight; i.p.). Organized gut-associated lymphoid tissue (GALT) populations, intestinal secretion, mucosal and serum immunoglobulin concentrations, and neutrophil infiltration were studied. On d 35, blood was collected under anesthesia and samples of intestinal mucosa, Peyer's patches, mesenteric lymph nodes (MLN), and spleen were taken. SEB increased the water content of feces, which was prevented by diets containing either SDAP (P < 0.002) or IC (P < 0.001), indicating that plasma protein-supplemented diets can reverse the SEB-induced secretory response. In Peyer's patches, the diet containing SDAP partially prevented the SEB-induced increase in T lymphocytes (P < 0.1) and reduced the percentage of activated T helper cells (P < 0.05). In MLN, activated T lymphocytes were increased by SEB but they were not affected by diet. No effects of SEB or dietary supplementation on mucosal IgA and serum IgA and IgG were observed. The effects of SDAP supplementation on the lymphocyte populations of GALT in rats challenged with SEB support the view that SDAP can modulate the immune response and suggest that plasma protein supplementation can prevent GALT from possible activation by luminal bacterial superantigens.

First Encounter: How Pathogens Compromise Epithelial Transport

Pathogenic organisms trigger numerous signaling pathways that ultimately lead to drastic changes in physiological functions. Apart from altering structure and function of the epithelial tight junction barrier and activating inflammatory cascades, they induce changes in fluid and electrolyte transport. Pathogens do so by activating or by inhibiting ion channels and transporters, and the result might be to their benefit or to their disadvantage.

TNF-alpha hyperpolarizes membrane potential and potentiates the response to nicotinic receptor stimulation in cultured rat myenteric neurones

AIMS

Tumour necrosis factor-alpha (TNF-alpha) plays a central role in the pathophysiology of inflammatory bowel disease. The present experiments were designed to characterize the action of this cytokine on enteric neurones.

METHODS

Myenteric ganglia from newborn rats were treated for 20 h with TNF-alpha (100 ng mL(-1)) and studied with the patch-clamp technique.

RESULTS

Control neurones showed a membrane potential of -34.6 +/- 2.2 mV (n = 22), whereas TNF-alpha-treated cells exhibited a membrane potential of -50.8 +/- 3.5 mV (n = 25). The depolarization evoked by carbachol (50 microm) was potentiated from 5.2 +/- 0.7 mV (n = 6) in control neurones to 27.5 +/- 2.0 mV (n = 10) in TNF-alpha-treated cells. This effect was mimicked by 1,1-dimethyl-4-phenylpiperazinium iodide, but not by bethanechol. The changes in basal membrane potential and in the nicotinic receptor response were suppressed by the non-selective cyclooxygenase (COX) inhibitor indomethacin (10 microm), and the COX II-specific inhibitor, nimesulide (100 microm), whereas the COX-I selective inhibitor SC-560 (5 microm) and the proteintyrosinekinase inhibitor genistein (50 microm) only partially inhibited the action of TNF-alpha. Staining of the ganglionic cells with an antibody against the transcription factor STAT5 revealed that TNF-alpha induced a nuclear translocation of STAT5 in non-neuronal cells.

CONCLUSION

TNF-alpha changes the electrophysiological properties of myenteric neurones via cyclooxygenase metabolites and protein tyrosine phosphorylation; the cells primarily responding to the cytokine seem to be non-neuronal cells in the ganglion culture, which respond with a nuclear STAT5 translocation suggesting an action on gene transcription.

Pathogenesis and treatment of graft-versus-host disease after bone marrow transplant

Stem cell transplantation is being used to treat a number of hematologic malignancies as well as hematologic and immune deficiency states. The ages of patients being offered this therapy and the donor marrow sources have been expanded. One persistent problem has been graft-versus-host disease (GVHD). This article reviews the basic biology of GVHD, clinical manifestations of acute and chronic GVHD, prophylaxis and treatment of acute GVHD, and treatment of chronic GVHD.

Dual and selective antagonism of neurokinin NK(1) and NK(2) receptor-mediated responses in human colon mucosa

The neurokinin (NK) receptors, NK(1) and NK(2), which are activated by substance P (SP) and NKA, have been identified as potential therapeutic targets in irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD). Here we have investigated the effects of a novel dual NK(1) and NK(2) receptor antagonist, namely DNK333 upon responses elicited by [Sar(9), Met(O(2))(11)]-SP (SMSP) and [betaAla(8)]-NKA(4-10) in isolated human colon mucosa mounted in Ussing chambers. A selective NK(1) receptor antagonist, SR140333 and NK(2) receptor antagonist, SR48968 have been tested for comparison. Additions of SMSP (100 nM) or [betaAla(8)]-NKA(4-10) (100 nM) increased basal short-circuit current and responses to both peptides were inhibited by DNK333, while SR140333 only inhibited SMSP and SR48968 blocked only [betaAla(8)]-NKA(4-10) responses. SR140333 did not attenuate [betaAla(8)]-NKA(4-10) effects and SR48968 had no effect upon SMSP responses. Carbachol (1 micro M) responses were not altered by any of the three NK antagonists. We conclude that activation of either NK(1) or NK(2) receptors can stimulate epithelial ion transport in human colon mucosa and that the novel dual antagonist, DNK333 may be of potential therapeutic interest in the treatment of IBD and IBS.

T cell activation causes diarrhea by increasing intestinal permeability and inhibiting epithelial Na+/K+-ATPase

Inflammatory bowel disease (IBD) is associated with mucosal T cell activation and diarrhea. We found that T cell activation with anti-CD3 mAb induces profound diarrhea in mice. Diarrhea was quantified by intestinal weight-to-length (wt/l) ratios, mucosal Na(+)/K(+)-ATPase activity was determined and ion transport changes were measured in Ussing chambers. Anti-CD3 mAb increased jejunal wt/l ratios by more than 50% at 3 hours, returning to base line after 6 hours. Fluid accumulation was significantly reduced in TNF receptor-1 (TNFR-1(-/-)), but not IFN-gamma knockout mice. Anti-CD3 mAb decreased mucosal Na(+)/K(+)-ATPase activity, which was blocked by anti-TNF mAb and occurred to a lesser degree in TNFR-1(-/-) mice. Neither alpha nor beta subunits of Na(+)/K(+)-ATPase decreased in abundance at 3 hours. Intestinal tissue from anti-CD3-treated mice exhibited increased permeability to mannitol at 1 hour and decreases in electroneutral Na(+) absorption, Na(+)-dependent glucose absorption, and cAMP-stimulated anion secretion at 3 hours. Furthermore, enteral fluid accumulation was observed in CFTR(-/-) mice, indicating a minor role of active anion secretion. These data suggest that diarrhea in IBD is due to TNF-mediated malabsorption rather than to secretory processes. T cell activation induces luminal fluid accumulation by increasing mucosal permeability and reducing epithelial Na(+)/K(+)-ATPase activity leading to decreased intestinal Na(+) and water absorption.

Substance P-evoked Cl(-) secretion in guinea pig distal colonic epithelia: interaction with PGE(2)

Interaction between substance P (SP) and PGE(2) on Cl(-) secretion in the guinea pig distal colonic epithelia was investigated. A short-circuit current (I(sc)) was measured as an index of ion transport. Mucosa preparations deprived of muscle and submucosa of distal colon were mounted in the Ussing flux chamber and treated with TTX and piroxicam to remove the influences of neuronal activity and endogenous PG synthesis, respectively. Although SP (10(-7) M) itself evoked little increase in I(sc), exogenous PGE(2) concentration dependently enhanced the response of SP. The effect of PGE(2) on the SP-evoked response was mimicked by forskolin and 8-bromoadenosine cAMP. Depletion of Ca2+ from the bathing solution reduced the PGE(2)-dependent response of SP. Effects of PGE(2), SP, and SP in the presence of PGE(2) on intracellular Ca2+ concentration ([Ca2+](i)) in isolated crypt cells were measured by the confocal microscope fluorescence imaging system. SP, but not PGE(2), temporally evoked an increase in [Ca2+](i) but declined to the baseline within 3 min. A return of the SP-evoked increase in [Ca2+](i) was slower in the presence of PGE(2) than SP alone. These results suggest that PGE(2) synergistically enhances SP-evoked Cl(-) secretion via an interaction between the intracellular cAMP and [Ca2+](i) in the epithelial cells. In conclusion, SP and PGE(2) could cooperatively induce massive Cl(-) secretion in guinea pig distal colon at epithelial levels.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Inhibition of attaching and effacing lesion formation following enteropathogenic Escherichia coli and Shiga toxin-producing E. coli infection

Enteropathogenic Escherichia coli (EPEC) and Shiga toxin-producing E. coli (STEC) induce cytoskeletal changes in infected epithelial cells. To further characterize host cytosolic responses to infection, a series of specific cell-signaling inhibitors were employed. Initial bacterial adhesion to HEp-2 epithelial cells was not reduced, whereas alpha-actinin accumulation in infected cells was blocked by a phosphoinositide-specific phospholipase C inhibitor (ET-18-OCH3), phosphoinositide 3-kinase inhibitors (wortmannin and LY294002), and a 5-lipoxygenase inhibitor, nordihydroguaretic acid. A cyclooxygenase-2 inhibitor (NS-398), however, did not block alpha-actinin reorganization in response to EPEC and STEC infections. Understanding signal transduction responses to enteric pathogens could provide the basis for the development of novel therapeutic strategies.

Prostanoids stimulate K secretion and Cl secretion in guinea pig distal colon via distinct pathways

Short-circuit current (I(sc)) and transepithelial conductance (Gt) were measured in guinea pig distal colonic mucosa isolated from submucosa and underlying muscle layers. Indomethacin (2 microM) and NS-398 (2 microM) were added to suppress endogenous production of prostanoids. Serosal addition of PGE2 (10 nM) stimulated negative I(sc) consistent with K secretion, and concentrations >30 nM stimulated positive I(sc) consistent with Cl secretion. PGE2 also stimulated Gt at low and high concentrations. Dose responses to prostanoids specific for EP prostanoid receptors were consistent with stimulating K secretion through EP2 receptors, based on a rank order potency (from EC50 values) of PGE2 (1.9 nM) > 11-deoxy-PGE1 (8.3 nM) > 19(R)-hydroxy-PGE2 (13.9 nM) > butaprost (67 nM) > 17-phenyl-trinor-PGE2 (307 nM) >> sulprostone (>10 microM). An isoprostane, 8-iso-PGE2, stimulated K secretion with an EC50 of 33 nM. Cl secretory response was stimulated by PGD2 and BW-245C, a DP prostanoid receptor-specific agonist: BW-245C (15 nM) > PGD2 (30 nM) > PGE2 (203 nM). Agonists specific for FP, IP, and TP prostanoid receptors were ineffective in stimulating I(sc) and Gt at concentrations <1 microM. These results indicate that PGE2 stimulated electrogenic K secretion through activation of EP2 receptors and electrogenic KCl secretion through activation of DP receptors. Thus stimulation of Cl secretion in vivo would occur either via physiological concentrations of PGD2 (<100 nM) or pathophysiological concentrations of PGE2 (>100 nM) that could occur during inflammatory conditions.

Permeability of human HT-29/B6 colonic epithelium as a function of apoptosis

1. The barrier function of colonic epithelia is challenged by apoptotic loss of enterocytes. In monolayers of human colonic HT-29/B6 cells, apoptosis induced by camptothecin was assessed by poly-(ADP-ribose)-polymerase (PARP) cleavage, histone ELISA and DNA-specific fluorochrome staining (with 4',6'-diamidino-2'-phenylindoladihydrochloride (DAPI)). Epithelial barrier function was studied in Ussing chambers by measuring transepithelial conductivity and unidirectional tracer fluxes. The ion permeability associated with single cell apoptoses was investigated with the conductance scanning technique. 2. The spontaneous rate of apoptotic cells was 3.5 +/- 0.3 % with an overall epithelial conductivity of 3.2 +/- 0.1 mS cm(-2). Camptothecin induced a time- and dose-dependent increase of apoptosis and permeability. With 20 microg ml(-1) of camptothecin for 48 h, apoptosis increased 4.1-fold to 14.3 +/- 1.5 % and the conductivity doubled to 6.4 +/- 1.0 mS cm(-2). 3. While 3H-mannitol flux increased 3.8-fold and 3H-lactulose flux increased 2.6-fold, the flux of 3H-polyethylene glycol 4000 remained unchanged. Hence, the higher permeability was limited to molecules < 4000 Da. 4. The local epithelial conductivity was higher at the sites of apoptosis than in non-apoptotic areas. With camptothecin the leaks associated with apoptosis became more numerous and more conductive, while in non-apoptotic areas the conductivity remained at control level. Hence, the camptothecin-induced increase in epithelial conductivity reflected the opening of apoptotic leaks and thus the results described, for the first time, epithelial permeability as a function of apoptosis only. 5. The conductivity of apoptotic leaks contributed 5.5 % to the epithelial conductivity of controls and 60 % to the conductivity of monolayers treated with 20 microg ml(-1) of camptothecin. Thus apoptosis increased the contribution of paracellular pathways to the overall epithelial permeability. Under control conditions the paracellular conductivity (G(para)) was smaller than the transcellular (G(trans)), but with 12 % apoptosis, G(para) exceeded G(trans). By definition, the epithelium became 'leaky'.

The envelope glycoprotein of simian immunodeficiency virus contains an enterotoxin domain

By the use of a mouse model, the enteropathic effects of the simian immunodeficiency virus (SIV) surface unit (SU) envelope glycoprotein were explored. Purified SU (0.01-0.45 nmol) was administered intraperitoneally to 6- to 8-day-old mouse pups and induced a dose-dependent diarrheal response. Surgical introduction of SU into adult mouse intestinal loops revealed fluid accumulation without histological alterations and SU-treated unstripped intestinal mucosa induced chloride (Cl(-)) secretory currents in Ussing chambers. Similarly to rotavirus NSP4, the first described viral enterotoxin, SU induced a transient increase in intracellular calcium levels and increased inositol 1,4,5-triphosphate (IP(3)) levels in HT-29 cells. These data indicate the calcium response is mediated by IP(3). The presence of diarrhea and fluid accumulation within intestinal loops in the absence of histological alterations and induction of Cl(-) secretory currents demonstrate that SIV contains an enterotoxic domain localized within SU and is the second viral enterotoxin described.

TNF-alpha potentiates the ion secretion induced by muscarinic receptor activation in HT29cl.19A cells

Chronic gastrointestinal diseases such as ulcerative colitis and Crohn's disease are characterized by severe diarrhea. Mucosal biopsies of these patients show enhanced levels of cytokines, secreted by infiltrated inflammatory cells. In this study, we investigated the effect of the cytokine tumor necrosis factor-alpha (TNF-alpha) on ion secretion in human intestinal epithelial cells. The conventional microelectrode technique in the cell line HT29cl. 19A was used, which allows for simultaneous measurements of transepithelial potential difference and intracellular potential difference across the apical membrane. Preincubation (2-78 h) with 10 ng/ml TNF-alpha did not change basal secretory activity. However, the secretory response to the muscarinic receptor agonist carbachol was strongly increased after exposure to TNF-alpha. Application of the protein kinase C (PKC) inhibitor GF 109203X (bisindolylmaleimide I) inhibited the response to carbachol as well as the TNF-alpha-potentiated response, indicating that PKC mediates the effect of carbachol in this cell line. Propranolol, a substance that inhibits the phospholipase D (PLD) pathway, strongly reduced the response to muscarinic stimulation and its potentiation by TNF-alpha. The results indicate that activation of PLD is involved in ion secretion induced by muscarinic receptor activation and that TNF-alpha can potentiate this pathway.