Geoffrey Warhurst

Interferon-{gamma} selectively increases epithelial permeability to large molecules by activating different populations of paracellular pores.

Impairment of the gut epithelial barrier by agents such as IFNγ may play a key role in the pathogenesis of inflammatory disorders by increasing the paracellular penetration of luminal macromolecules, potentially including bacterial antigens. Owing to limitations of current paracellular probes, little is known about the precise functional changes induced by IFNγ and how these relate to the development of increased macromolecular permeability. Here we investigate how IFNγ modulates this pathway in T84 monolayers using a novel profiling technique that resolves different populations of paracellular pores by simultaneous analysis of 24 permeability probes of defined molecular size. Two types of functional pore present in control monolayers, an abundant restrictive pore with a radius of ∼4.5 Å and a much larger but infrequent, non-restrictive pore, were differentially regulated by IFNγ. Incubation with IFNγ dose-dependently and reversibly increased the frequency of the non-restrictive pores while having no significant effect on the restrictive component. Cytokine-induced increases in β, the descriptor of the non-restrictive pore, correlated closely with increased permeability to large molecules (10 kDa) including E. coli-derived lipopolysaccharide, but not small (0.182 kDa) molecules. This effect was associated with changes in expression of the tight junction proteins occludin and claudin-1. These data suggest that IFNγ selectively increases the transepithelial flux of large molecules by activating specific pathways within the junctional pore. One hypothesis is that this process may be activated in the early stages of the inflammatory response, facilitating the passage of large and potentially antigenic molecules across the gut without gross disruption of the barrier to small molecules.

Glutamine deprivation facilitates tumour necrosis factor induced bacterial translocation in Caco-2 cells by depletion of enterocyte fuel substrate

Background and aims: Factors that induce luminal bacteria to cross the intestinal epithelium following injury remain poorly defined. The aim of this study was to investigate the interaction between glutamine metabolism, energy supply, and inflammatory mediators in determining the translocation of non-pathogenic bacteria across cultured enterocytes. Methods: The effect of tumour necrosis factor α (TNF-α) on translocation of Escherichia coli C25 across Caco-2 epithelial monolayers was studied in the presence of products and inhibitors of glutamine metabolism. Simultaneous measurements of transepithelial electrical resistance (TEER) and flux of lucifer yellow were used to assess effects on the paracellular pathway. Lactate dehydrogenase release was used to monitor enterocyte integrity. Imaging of monolayers in these experimental conditions was undertaken with transmission electron microscopy. Results: Exposure to basolateral TNF-α (20 ng/ml) for six hours induced translocation of E coli across Caco-2 but only if accompanied by simultaneous glutamine depletion (p<0.01). Translocation was inhibited by addition of glutamine for two hours (p<0.01) but not by an isonitrogenous mixture of non-glutamine containing amino acids. Inhibition of glutamine conversion to α-ketoglutarate, but not blockade of glutathione or polyamine synthesis, also induced translocation in the presence of TNF-α. Manipulations that induced bacterial translocation were associated with a marked reduction in enterocyte ATP levels. No effect of these treatments on paracellular permeability or lactate dehydrogenase release was observed. Conditions in which translocation occurred were associated with the presence of bacteria within enterocyte vacuoles but not the paracellular space. Conclusions: In inflammatory conditions, the availability of glutamine as an enterocyte fuel substrate is essential for the preservation of a functional barrier to microorganisms. In conditions of acute glutamine depletion, cytokine mediated bacterial translocation appears to be primarily a transcellular process.

Interferon γ induces differential upregulation of α and β chemokine secretion in colonic epithelial cell lines

Background—Production of chemoattractant factors by the intestinal epithelium may contribute to mucosal infiltration by inflammatory cells in inflammatory bowel disease. Secretion of the α chemokine interleukin 8 (IL-8), a neutrophil chemoattractant, has been widely studied, but little is known about epithelial secretion of β chemokines, which are preferentially involved in recruiting monocytes. Aims—To investigate the profiles of α and β chemokine secretion in colonic cell lines and their differential modulation by interferon γ (IFN-γ), a product of activated T lymphocytes and natural killer cells. Methods and results—HT29-19A, a model of the Cl− secretory crypt cell, exhibited a parallel secretion of the α chemokines IL-8 and GROα, which could be markedly upregulated by tumour necrosis factor α (TNF-α) and IL-1β. These cells showed no significant expression of the β chemokines RANTES (regulated upon activation T cell expressed and secreted), MIP-1α (macrophage inflammatory protein 1α), and MCP-1 (monocyte chemotactic protein 1) under these conditions, but IFN-γ in combination with TNF-α caused a dose dependent induction of RANTES and MCP-1 secretion. This was accompanied by a marked increase of RANTES mRNA. In contrast, IFN-γ had no significant effect on TNF-α stimulated IL-8 secretion. Caco-2 cells, with features more typical of villus absorptive cells, were relatively poor secretors of α chemokines but secreted high levels of MCP-1 in response to IL-1β. IFN-γ did not influence α or β chemokine secretion in these cells. Conclusions—These studies suggest that intestinal epithelial cells may produce chemokines capable of attracting both neutrophils and monocytes. The ability of IFN-γ to activate the expression of β chemokines preferentially could facilitate the development of chronic inflammatory infiltrates.

Bench-to-bedside review: the promise of rapid infection diagnosis during sepsis using polymerase chain reaction-based pathogen detection.

Early infection diagnosis as the cause of a patients systemic inflammatory syndrome is an important facet of sepsis care bundles aimed at saving lives. Microbiological culture provides the main route for infection diagnosis but by its nature cannot provide time-critical results that can impact on early management. Consequently, broad-spectrum and high-potency antibiotics are essential during the immediate management of suspected sepsis in critical care but are associated with the development of drug-resistant organisms and superinfections. Established molecular laboratory techniques based on polymerase chain reaction (PCR) technology can detect pathogen DNA rapidly and have been developed for translation into a clinical diagnostic setting. In the setting of sepsis in critical care, emerging commercial systems are now available for the analysis of whole blood within hours, with the presumed aim of adoption into the current care bundles. In this review, we consider the importance of early infection diagnosis in sepsis, how this is limited by culture approaches and how the emerging PCR methods are showing promise in early clinical observational studies. The strengths and weaknesses of culture and PCR pathogen detection in whole-blood samples will be highlighted and recommendations made for urgent appropriately powered diagnostic validation studies in advance of clinical effectiveness trials before these emerging PCR pathogen detection techniques can be considered for adoption in clinical practice.

Region-Dependent Modulation of Intestinal Permeability by Drug Efflux Transporters: In Vitro Studies in mdr1a(−/−) Mouse Intestine

Information on the extent to which xenobiotics interact with P-glycoprotein (PGP) during transit through the intestine is crucial in determining the influence of PGP on oral drug absorption. We have recently described a novel use of isolated ileum from PGP-deficient mdr1a(−/−) mice to resolve PGP- and non-PGP-dependent drug efflux and provide a definitive measure of intrinsic drug permeability without recourse to inhibitors (Stephens et al., 2002). The present study uses this approach to investigate the impact of PGP on intestinal permeability of paclitaxel and digoxin in different regions of the mouse intestine (jejunum, ileum, and proximal and distal colon). Absorption of paclitaxel and digoxin in tissues from wild-type mice was low and showed little regional variation. In contrast, absorption of both drugs was markedly higher in mdr1a(−/−) intestine, although the increase was highly region-dependent, with the ileum and distal colon showing the greatest effect and much smaller changes in the jejunum and proximal colon. These effects were accompanied by the abolition of paclitaxel and digoxin secretion in mdr1a(−/−) mice, suggesting that regional variations in intestinal permeability are masked by differential PGP expression, confirmed by immunoblotting studies. Propranolol permeability, which is not influenced by PGP, showed similar regional variation in both wild-type and mdr1a(−/−) tissues, suggesting that differences are at the level of transcellular permeability. These data suggest that the ileum and the distal colon are regions of relatively high transcellular permeability for xenobiotics that are compensated by enhanced expression of PGP.

Comparison of HT29-18-C1 and Caco-2 cell lines as models for studying intestinal paracellular drug absorption

AbstractPurpose. To compare the permeability characteristics of HT29-18-C1 colonic epithelial cell line with Caco-2, an established model of intestinal drug transport. Methods. Cell lines were grown as epithelial monolayers. Permeability was measured over a range of transepithelial electrical resistance (Rt) using a group of drug compounds. Results. HT29-18-C1 develop Rt slowly when grown in culture, allowing permeability to be measured over a wide range (80–600 Ω·cm2). In contrast, Caco-2 monolayers rapidly develop Rt of ≈300 Ω·cm2 and require Ca2+-chelation to generate Rt equivalent to human intestine (60–120 Ω·cm2). Permeability of atenolol, ranitidine, cimetidine, hydrochlorothiazide and mannitol across HT29-18-C1 decreased 4–5 fold as Rt developed from 100–300 Ω·cm2 indicating they permeate via the paracellular route. In contrast, ondansetron showed no difference in permeability with changing Rt consistent with transcellular permeation. Permeability profiles across low Rt HT29-18C1 and pulse EGTA-treated Caco-2 monolayers were the same for all 5 paracellular drugs suggesting that transient Ca2+ removal does not alter selectivity of the tight junctions. Permeabilities of cimetidine, hydrochlorothiazide and atenolol across 100 Ω·cm2 HT29-18-C1 monolayers reflect more closely those reported for the human ileum in vivo than did mature Caco-2 monolayers. Conclusions. HT29-18-C1 monolayers can be used to study drug permeability at Rt values similar to human intestine without the need for Ca2+ chelation. As such, they offer a useful alternative to Caco-2 for modelling intestinal drug absorption.

Fatty acid-induced cholecystokinin secretion and changes in intracellular Ca2+in two enteroendocrine cell lines, STC-1 and GLUTag

1 Fatty acid‐induced cholecystokinin (CCK) secretion in humans and from the enteroendocrine cell line STC‐1 depends critically on acyl chain length. 2 Therefore we have characterized the relationship between acyl chain length and the potency of the fatty acid to induce CCK secretion and changes in intracellular Ca2+ concentration ([Ca2+]i) in two enteroendocrine cell lines (STC‐1 and GLUTag). We found that the potency of the fatty acid was directly proportional to its chain length and therefore inversely proportional to its solubility. 3 In both cell types, the fatty acid‐induced rise in [Ca2+]i in response to decanoic acid (C10), dodecanoic acid (C12) and tetradecanoic acid (C14) was significantly reduced in Ca2+‐free medium and largely blocked by nicardipine. Intracellular stores also contributed to the overall shape of the [Ca2+]i peak. Thus all the fatty acids tested caused the release of Ca2+ from stores and influx of extracellular Ca2+, presumably through L‐type calcium channels. 4 To probe the site of fatty acid action, we studied the distribution of 14C‐labelled dodecanoic acid. This label was rapidly and irreversibly accumulated by both cell types, where it became concentrated about 20‐fold. Confocal microscopy of a fluorescent analogue of dodecanoic acid clearly demonstrated that it entered the cytosol and was not merely partitioning in the cell membrane. These data indicate that an intracellular action for fatty acid‐induced CCK secretion cannot be eliminated. 5 Dodecanoic acid itself, and not a metabolite, is the agent responsible for triggering Ca2+ entry since a non‐metabolizable form of dodecanoic acid (2‐bromododecanoic acid) was also capable of inducing a rise in [Ca2+]i in both cell types. 6 In conclusion, the rise in [Ca2+]i in STC‐1 and GLUTag cells evoked by medium‐ to long‐chain fatty acids results from the triggering of a specific signalling pathway. Whether triggering occurs through activation of a membrane‐bound receptor or at an intracellular site remains to be clarified.

Characterization of apical potassium channels induced in rat distal colon during potassium adaptation.

1 Chronic dietary K+ loading stimulates an active K+ secretory process in rat distal colon, which involves an increase in the macroscopic apical K+ conductance of surface epithelial cells. In the present study, the abundance and characteristics of K+ channels constituting this enhanced apical K+ conductance were evaluated using patch clamp recording techniques.2. In isolated non‐polarized surface cells, K+ channels were seen in 9 of 90 (10%) cell‐attached patches in cells from control animals, and in 247 of 437 (57%) cell‐attached patches in cells from K+‐loaded animals, with a significant (P < 0.001) shift in distribution density. Similarly, recordings from cell‐attached patches of the apical membrane of surface cells surrounding the openings of distal colonic crypts revealed identical K+ channels in 1 of 11 (9%) patches in control animals, and in 9 of 13 (69%) patches in K+‐loaded animals. 3 In isolated surface cells and surface cells in situ, K+ channels had mean slope conductances of 209 ± 6 and 233 ± 14 pS, respectively, when inside‐out patches were bathed symmetrically in K2SO4 solution. The channels were sensitive to ‘cytosolic’ Ca2+ concentration, were voltage sensitive at ‘cytosolic’ Ca2+ concentrations encountered in colonic epithelial cells, and were inhibited by 1 mm quinidine, 20 mm TEA or 5 mm Ba2+ ions. 4 The data show that dietary K+ loading increases the abundance of Ca2+‐ and voltage‐sensitive large‐conductance K+ channels in the apical membrane of surface cells in rat distal colon. These channels constitute the enhanced macroscopic apical K+ conductance previously identified in these cells, and are likely to play a critical role in the active K+ secretory process that typifies this model of colonic K+ adaptation.

Characteristics of two basolateral potassium channel populations in human colonic crypts.

The basolateral membrane of human colonic crypt cells contains Ca2+ and cAMP activated, Ba2+ blockable, low conductance (23 pS) K+ channels, which probably play an important part in intestinal Cl- secretion. This study has defined more clearly the basolateral K+ conductive properties of human colonic crypts using patch clamp recording techniques. High conductance (138 pS) K+ channels were seen in 25% of patches (one or two channels per patch), and significantly inhibited by the addition of 5 mM Ba2+, 1 mM quinidine or 20 mM tetraethylammonium chloride (TEA) to the cytosolic side of excised inside-out patches, whereas 1 mM diphenylamine-2-carboxylic acid (DPC) had no effect. In contrast, clusters of the 23 pS K+ channel (two to six channels per patch) were present in > 75% of patches, and channel activity was inhibited by quinidine and DPC, but not by TEA. Activity of the 138 pS K+ channel in inside-out patches was abolished almost completely by removal of bath Ca2+, but in contrast with its effect on the 23 pS K+ channel, addition of 0.1 mM carbachol had no effect on the 138 pS K+ channel in cell attached patches. It is concluded that human colonic crypt cells possess two discrete basolateral K+ channel populations, which can be distinguished by their responses to K+ channel blockers, and their different sensitivities to changes in intracellular Ca2+ concentration.

Active hexose transport across cultured human Caco-2 cells: characterisation and influence of culture conditions.

Human Caco-2 cells (passage 80 to 100) were seeded onto collagen-coated Millipore filter assemblies and these were maintained in culture either (a) floated on the surface of the medium or (b) submerged within the body of the medium. Structural and functional assessments were made over a 30-day period. After seeding, all cells assumed a flattened, squamous configuration and rapidly became confluent. Cells submerged within the medium formed polarised monolayers with well developed junctional complexes, abundant apical microvilli and increasing levels of alkaline phosphatase activity. Cells grown floated on the surface of the medium formed complex multilayers in which polarisation was confined to the surface layer. Junctional complexes and apical microvilli were similar to those seen in submerged monolayers but alkaline phosphatase activities were higher. Transepithelial electrical resistance increased rapidly from day 1, as the layers became confluent. Electrical resistance was higher and short-circuit current and potential differences were lower across monolayers than across multilayers. After 10 days in culture, the addition of D-glucose to the apical bathing solution, of all cell layers, caused a rapid rise in short-circuit current and potential difference. These changes were sodium-dependent and phlorizin-sensitive. Galactose and 3-O-methylglucose induced similar changes and the affinity constants for these hexoses ranked in the order reported for rat jejunum (Km glucose 2.44 +/- 0.52 mM; Km galactose 8.05 +/- 1.33 mM; Km 3-O-methylglucose 22.0 +/- 5.2 mM). Culture conditions had a marked effect on hexose maximum transport rates (glucose Vmax: submerged 2.94 +/- 0.20 microA/cm2; floated 9.94 +/- 0.82 microA/cm2, P less than 0.05) but affinity constants were unchanged. Apical to basolateral mannitol fluxes, used as an index of paracellular permeability, decreased from day 1 to day 5 and then remained steady. Fluxes across monolayers and multilayers were not significantly different. We conclude that sodium-dependent hexose transport occurs in cultured Caco-2 cell layers grown on permeable supports. Culture conditions, however, have a marked effect on both cell layer structure and function, and should be an important factor when considering Caco-2 cells as an in vitro model of enterocyte function.