Epithelial properties of human colonic carcinoma cell line Caco-2: effect of secretagogues

A Simple Method to Test Mechanical Strain on Epithelial Cell Monolayers Using a 3D-Printed Stretcher

With the realization that mechanical forces mediate many biological processes and contribute to disease progression, researchers are focusing on developing new methods to understand the role of mechanotransduction in biological systems. Despite recent advances in stretching devices that analyze the effects of mechanical strain in vitro, there are still possibilities to develop new equipment. For example, many of these devices tend be expensive, whereas few have been designed to assess the effects of mechanical strain driven by the extracellular matrix (ECM) to epithelial cell monolayers and to cell-cell adhesion. In this chapter, we introduce a cost-efficient, user-friendly, 3D-printed stretching device that can be used to test the effects of mechanical strain on cultured epithelial cells. Evaluation of the device using speckle-tracking shows homogeneous strain distribution along the horizontal plane of membranes at 2.5% and 5% strains, supporting the reliability of the device. Since cell-cell junctions are mechanosensitive protein complexes, we hereby used this device to examine effects on cell-cell adhesion. For this, we used colon epithelial Caco2 cell monolayers that well-differentiate in culture and form mature adherens junctions. Subjecting Caco2 cells to 2.5% and 5% strain using our device resulted in significant reduction in the localization of the core adherens junction component E-cadherin at areas of cell-cell contact and its increased translocation to the cytoplasm, which in agreement with other methodologies showing that increased ECM-driven strain negatively affects cell-cell adhesion. In summary, we here present a new, cost-effective, homemade device that can be reliably used to examine effects of mechanical strain on epithelial cell monolayers and cell-cell adhesion, in vitro.

CFTR-mediated anion secretion across intestinal epithelium-like Caco-2 monolayer under PTH stimulation is dependent on intermediate conductance K+ channels

Parathyroid hormone (PTH), a pleiotropic hormone that maintains mineral homeostasis, is also essential for controlling pH balance and ion transport across renal and intestinal epithelia. Optimization of luminal pH is important for absorption of trace elements, e.g., calcium and phosphorus. We have previously demonstrated that PTH rapidly stimulated electrogenic [Formula: see text] secretion in intestinal epithelial-like Caco-2 monolayers, but the underlying cellular mechanism, contributions of other ions, particularly Cl^- and K⁺, and long-lasting responses are not completely understood. Herein, PTH and forskolin were confirmed to induce anion secretion, which peaked within 1-3 min (early phase), followed by an abrupt decay and plateau that lasted for 60 min (late phase). In both early and late phases, apical membrane capacitance was increased with a decrease in basolateral capacitance after PTH or forskolin exposure. PTH also induced a transient increase in apical conductance with a long-lasting decrease in basolateral conductance. Anion secretion in both phases was reduced under [Formula: see text]-free and/or Cl^--free conditions or after exposure to carbonic anhydrase inhibitor (acetazolamide), CFTR inhibitor (CFTRinh-172), Na⁺/H⁺ exchanger (NHE)-3 inhibitor (tenapanor), or K⁺ channel inhibitors (BaCl₂, clotrimazole, and TRAM-34; basolateral side), the latter of which suggested that PTH action was dependent on basolateral K⁺ recycling. Furthermore, early- and late-phase responses to PTH were diminished by inhibitors of PI3K (wortmannin and LY-294002) and PKA (PKI 14-22). In conclusion, PTH requires NHE3 and basolateral K⁺ channels to induce [Formula: see text] and Cl^- secretion, thus explaining how PTH regulated luminal pH balance and pH-dependent absorption of trace minerals.

Toxoplasma gondii Infection Promotes Epithelial Barrier Dysfunction of Caco-2 Cells

After oral infection, Toxoplasma gondii invades intestinal cells, induces breakdown of intestinal physiology and barrier functions, and causes intestinal pathology in some animal species. Although parasites' invasion into host cells is a known phenomenon, the effects of T. gondii infection in the intestinal barrier are still not well established. To evaluate morphological and physiological modifications on the colorectal adenocarcinoma-derived Caco-2 cell line during T. gondii infection, microvilli, tight junction integrity, and transepithelial electrical resistance (TEER) were investigated under infection. It was observed that the dextran uptake (endocytosis) and distribution were smaller in infected than in noninfected Caco-2 cells. The infection leads to the partial loss of microvilli at the cell surface. Claudin-1, zonula occludens-1 (ZO-1), and occludin expressions were colocalized by immunofluorescence and presented discontinuous net patterns in infected cells. Immunoblotting analysis at 24 hr postinfection revealed decreasing expression of occludin and ZO-1 proteins, whereas claudin-1 presented similar expression level compared with noninfected cells. T. gondii decreased TEER in Caco-2 cells 24 hr after infection. Our results suggest that T. gondii infection may lead to the loss of integrity of intestinal mucosa, resulting in impaired barrier function.

Pathogenesis of human diffusely adhering Escherichia coli expressing Afa/Dr adhesins (Afa/Dr DAEC): current insights and future challenges

The pathogenicity and clinical pertinence of diffusely adhering Escherichia coli expressing the Afa/Dr adhesins (Afa/Dr DAEC) in urinary tract infections (UTIs) and pregnancy complications are well established. In contrast, the implication of intestinal Afa/Dr DAEC in diarrhea is still under debate. These strains are age dependently involved in diarrhea in children, are apparently not involved in diarrhea in adults, and can also be asymptomatic intestinal microbiota strains in children and adult. This comprehensive review analyzes the epidemiology and diagnosis and highlights recent progress which has improved the understanding of Afa/Dr DAEC pathogenesis. Here, I summarize the roles of Afa/Dr DAEC virulence factors, including Afa/Dr adhesins, flagella, Sat toxin, and pks island products, in the development of specific mechanisms of pathogenicity. In intestinal epithelial polarized cells, the Afa/Dr adhesins trigger cell membrane receptor clustering and activation of the linked cell signaling pathways, promote structural and functional cell lesions and injuries in intestinal barrier, induce proinflammatory responses, create angiogenesis, instigate epithelial-mesenchymal transition-like events, and lead to pks-dependent DNA damage. UTI-associated Afa/Dr DAEC strains, following adhesin-membrane receptor cell interactions and activation of associated lipid raft-dependent cell signaling pathways, internalize in a microtubule-dependent manner within urinary tract epithelial cells, develop a particular intracellular lifestyle, and trigger a toxin-dependent cell detachment. In response to Afa/Dr DAEC infection, the host epithelial cells generate antibacterial defense responses. Finally, I discuss a hypothetical role of intestinal Afa/Dr DAEC strains that can act as "silent pathogens" with the capacity to emerge as "pathobionts" for the development of inflammatory bowel disease and intestinal carcinogenesis.

Antisecretory factor peptide AF-16 inhibits the secreted autotransporter toxin-stimulated transcellular and paracellular passages of fluid in cultured human enterocyte-like cells

Both the endogenous antisecretory factor (AF) protein and peptide AF-16, which has a sequence that matches that of the active N-terminal region of AF, inhibit the increase in the epithelial transport of fluid and electrolytes induced by bacterial toxins in animal and ex vivo models. We conducted a study to investigate the inhibitory effect of peptide AF-16 against the increase of transcellular passage and paracellular permeability promoted by the secreted autotransporter toxin (Sat) in a cultured cellular model of the human intestinal epithelial barrier. Peptide AF-16 produced a concentration-dependent inhibition of the Sat-induced increase in the formation of fluid domes, in the mucosal-to-serosal passage of D-[1-(14)C]mannitol, and in the rearrangements in the distribution and protein expression of the tight junction (TJ)-associated proteins ZO-1 and occludin in cultured human enterocyte-like Caco-2/TC7 cell monolayers. In addition, we show that peptide AF-16 also inhibits the cholera toxin-induced increase of transcellular passage and the Clostridium difficile toxin-induced effects on paracellular permeability and TJ protein organization in Caco-2/TC7 cell monolayers. Treatment of cell monolayers by the lipid raft disorganizer methyl-β-cyclodextrin abolished the inhibitory activity of peptide AF-16 at the transcellular passage level and did not modify the effect of the peptide at the paracellular level.

Cytotoxic effect of hemin in colonic epithelial cell line: involvement of 18 kDa translocator protein (TSPO)

AIMS

The aim of this study is to investigate the effect of hemin in colonic epithelial cells (Caco-2) cell proliferation and if this effect was due to a direct modulation of 18-kDa translocator protein (TSPO) and/or heme oxygenase type 1 (HO-1).

MAIN METHODS

The main methods are as follow: cell proliferation and cell cytotoxic assays on Caco-2 cell lines treated with hemin in the presence or not of 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK 11195) and Sn-protoporphyrin IX (Sn-PPIX), and immunoblotting for TSPO and HO-1 protein analysis, siRNA directed against TSPO.

KEY FINDINGS

Hemin was shown to be toxic for the Caco-2 cell line in a concentration and time dependent manner. Although hemin was able to induce HO-1 in a dose dependent manner, a specific HO-1 inhibitor, Sn-PPIX, was unable to interfere with the effect of hemin on Caco-2 cells. Instead, PK 11195, a specific TSPO ligand, was able to counteract the effect of hemin, suggesting an important role of TSPO in the hemin activity. Cell viability assay further confirms the high cytotoxic effects exerted by hemin on Caco-2 cells expressing TSPO compared to the siRNA-TSPO targeted cells. In addition, hemin was able to decrease significantly the TSPO protein density in a dose dependent manner after 24h of incubation.

SIGNIFICANCE

The interaction and the consecutive down regulation of TSPO by hemin played an important role in the control of Caco-2 cell viability. The presented data suggest that TSPO might contribute to protect cells from potential toxic compounds such as free tetrapyrroles, candidating this receptor as a survival receptor protein.

Characterization of transferrin receptor-mediated endocytosis and cellular iron delivery of recombinant human serum transferrin from rice (Oryza sativa L.)

Background

Transferrin (TF) plays a critical physiological role in cellular iron delivery via the transferrin receptor (TFR)-mediated endocytosis pathway in nearly all eukaryotic organisms. Human serum TF (hTF) is extensively used as an iron-delivery vehicle in various mammalian cell cultures for production of therapeutic proteins, and is also being explored for use as a drug carrier to treat a number of diseases by employing its unique TFR-mediated endocytosis pathway. With the increasing concerns over the risk of transmission of infectious pathogenic agents of human plasma-derived TF, recombinant hTF is preferred to use for these applications. Here, we carry out comparative studies of the TFR binding, TFR-mediated endocytosis and cellular iron delivery of recombinant hTF from rice (rhTF), and evaluate its suitability for biopharmaceutical applications.

Result

Through a TFR competition binding affinity assay with HeLa human cervic carcinoma cells (CCL-2) and Caco-2 human colon carcinoma cells (HTB-37), we show that rhTF competes similarly as hTF to bind TFR, and both the TFR binding capacity and dissociation constant of rhTF are comparable to that of hTF. The endocytosis assay confirms that rhTF behaves similarly as hTF in the slow accumulation in enterocyte-like Caco-2 cells and the rapid recycling pathway in HeLa cells. The pulse-chase assay of rhTF in Caco-2 and HeLa cells further illustrates that rice-derived rhTF possesses the similar endocytosis and intracellular processing compared to hTF. The cell culture assays show that rhTF is functionally similar to hTF in the delivery of iron to two diverse mammalian cell lines, HL-60 human promyelocytic leukemia cells (CCL-240) and murine hybridoma cells derived from a Sp2/0-Ag14 myeloma fusion partner (HB-72), for supporting their proliferation, differentiation, and physiological function of antibody production.

Conclusion

The functional similarity between rice derived rhTF and native hTF in their cellular iron delivery, TFR binding, and TFR-mediated endocytosis and intracellular processing support that rice-derived rhTF can be used as a safe and animal-free alternative to serum hTF for bioprocessing and biopharmaceutical applications.

K-Ras and B-Raf oncogenes inhibit colon epithelial polarity establishment through up-regulation of c-myc

ERK-mediated up-regulation of c-myc by K-Ras or B-Raf oncogenes disrupts the establishment of apical/basolateral polarity independently of its effect on proliferation.

KRAS, BRAF, and PI3KCA are the most frequently mutated oncogenes in human colon cancer. To explore their effects on morphogenesis, we used the colon cancer–derived cell line Caco-2. When seeded in extracellular matrix, individual cells proliferate and generate hollow, polarized cysts. The expression of oncogenic phosphatidylinositol 3-kinase (PI3KCA H1047R) in Caco-2 has no effect, but K-Ras V12 or B-Raf V600E disrupts polarity and tight junctions and promotes hyperproliferation, resulting in large, filled structures. Inhibition of mitogen-activated protein/extracellular signal–regulated kinase (ERK) kinase blocks the disruption of morphology, as well as the increased levels of c-myc protein induced by K-Ras V12 and B-Raf V600E. Apical polarity is already established after the first cell division (two-cell stage) in Caco-2 three-dimensional cultures. This is disrupted by expression of K-Ras V12 or B-Raf V600E but can be rescued by ribonucleic acid interference–mediated depletion of c-myc. We conclude that ERK-mediated up-regulation of c-myc by K-Ras or B-Raf oncogenes disrupts the establishment of apical/basolateral polarity in colon epithelial cells independently of its effect on proliferation.

Campylobacter jejuni translocation across intestinal epithelial cells is facilitated by ganglioside-like lipooligosaccharide structures

Translocation across intestinal epithelial cells is an established pathogenic feature of the zoonotic bacterial species Campylobacter jejuni. The number of C. jejuni virulence factors known to be involved in translocation is limited. In the present study, we investigated whether sialylation of C. jejuni lipooligosaccharide (LOS) structures, generating human nerve ganglioside mimics, is important for intestinal epithelial translocation. We here show that C. jejuni isolates expressing ganglioside-like LOS bound in larger numbers to the Caco-2 intestinal epithelial cells than C. jejuni isolates lacking such structures. Next, we found that ganglioside-like LOS facilitated endocytosis of bacteria into Caco-2 cells, as visualized by quantitative microscopy using the early and late endosomal markers early endosome-associated protein 1 (EEA1), Rab5, and lysosome-associated membrane protein 1 (LAMP-1). This increased endocytosis was associated with larger numbers of surviving and translocating bacteria. Next, we found that two different intestinal epithelial cell lines (Caco-2 and T84) responded with an elevated secretion of the T-cell attractant CXCL10 to infection by ganglioside-like LOS-expressing C. jejuni isolates. We conclude that C. jejuni translocation across Caco-2 cells is facilitated by ganglioside-like LOS, which is of clinical relevance since C. jejuni ganglioside-like LOS-expressing isolates are linked with severe gastroenteritis and bloody stools in C. jejuni-infected patients.

Parathyroid hormone (PTH) rapidly enhances CFTR-mediated HCO₃⁻ secretion in intestinal epithelium-like Caco-2 monolayer: a novel ion regulatory action of PTH

Besides being a Ca²-regulating hormone, parathyroid hormone (PTH) has also been shown to regulate epithelial transport of certain ions, such as Cl, HCO₃, and Na, particularly in the kidney. Although the intestinal epithelium also expressed PTH receptors, little was known regarding its mechanism in the regulation of intestinal ion transport. We investigated the ion regulatory role of PTH in intestinal epithelium-like Caco-2 monolayer by Ussing chamber technique and alternating current impedance spectroscopy. It was found that Caco-2 cells rapidly responded to PTH within 1 min by increasing apical HCO₃- secretion. CFTR served as the principal route for PTH-stimulated apical HCOV efflux, which was abolished by various CFTR inhibitors, namely, NPPB, glycine hydrazide-101 (GlyH-101), and CFTRinh-172, as well as by small interfering RNA against CFTR. Concurrently, the plasma membrane resistance was decreased with no changes in the plasma membrane capacitance or paracellular permeability. HCOV was probably supplied by basolateral uptake via the electrogenic Na⁺-HCO₃⁻ cotransporter and by methazolamide-sensitive carbonic anhydrase, while the resulting intracellular H⁺ might be extruded by both apical and basolateral Na/H exchangers. Furthermore, the PTH-stimulated HCO₃-secretion was markedly reduced by protein kinase A (PKA) inhibitor (PKI 14-22 amide) and phosphoinositide 3-kinase (PI3K) inhibitors (wortmannin and LY-294002), but not by intracellular Ca²⁺ chelator (BAPTA-AM) or protein kinase C inhibitor (GF-109203X). In conclusion, the present study provided evidence that PTH directly and rapidly stimulated apical HCO₃- secretion through CFTR in PKA- and PI3K-dependent manner, which was a novel noncalciotropic, ion regulatory action of PTH in the intestinal epithelium.

TNF-α induces vectorial secretion of IL-8 in Caco-2 cells

INTRODUCTION

Intestinal epithelial cells represent an important component of innate immunity, with sophisticated responses to inflammatory stimuli. The manner in which intestinal epithelial cell polarity affects responses to inflammatory stimuli is largely unknown. We hypothesized that polarized intestinal epithelial cells exhibit a bidirectional inflammatory response dependent upon the location of the stimulus.

METHODS

Caco-2 cells were grown on semi-permeable inserts in a dual-compartment culture system and treated with tumor necrosis factor-α (TNF-α; 100 ng/ml) or serum-free media in the apical or basolateral chamber. Interleukin-8 (IL-8) production in each chamber was measured by enzyme-linked immunosorbent assay. To determine receptor specificity, anti-TNF receptor antibodies were added to the apical or basolateral chamber.

RESULTS

Basolateral stimulation with TNF-α resulted in increased apical and basolateral IL-8 production. Apical TNF-α stimulation resulted in increased apical, but not basolateral IL-8 production. Receptor blockade suggested TNF receptor 1 involvement on both apical and basolateral membranes, while TNF receptor 2 was only active on the apical membrane.

CONCLUSION

Polarized intestinal epithelial cells respond to TNF-α stimulation with focused, directional secretion of the proinflammatory cytokine IL-8. These findings are important because they suggest that intestinal epithelial cells are capable of organizing their response to inflammatory signals and producing inflammatory mediators in a bidirectional, vectorial fashion.

TCF4 and CDX2, major transcription factors for intestinal function, converge on the same cis-regulatory regions

Surprisingly few pathways signal between cells, raising questions about mechanisms for tissue-specific responses. In particular, Wnt ligands signal in many mammalian tissues, including the intestinal epithelium, where constitutive signaling causes cancer. Genome-wide analysis of DNA cis-regulatory regions bound by the intestine-restricted transcription factor CDX2 in colonic cells uncovered highly significant overrepresentation of sequences that bind TCF4, a transcriptional effector of intestinal Wnt signaling. Chromatin immunoprecipitation confirmed TCF4 occupancy at most such sites and co-occupancy of CDX2 and TCF4 across short distances. A region spanning the single nucleotide polymorphism rs6983267, which lies within a MYC enhancer and confers colorectal cancer risk in humans, represented one of many co-occupied sites. Co-occupancy correlated with intestine-specific gene expression and CDX2 loss reduced TCF4 binding. These results implicate CDX2 in directing TCF4 binding in intestinal cells. Co-occupancy of regulatory regions by signal-effector and tissue-restricted transcription factors may represent a general mechanism for ubiquitous signaling pathways to achieve tissue-specific outcomes.

Enhanced microscopic definition of Campylobacter jejuni 81-176 adherence to, invasion of, translocation across, and exocytosis from polarized human intestinal Caco-2 cells

Campylobacter jejuni-mediated pathogenesis involves gut adherence and translocation across intestinal cells. The current study was undertaken to examine the C. jejuni interaction with and translocation across differentiated Caco-2 cells to better understand Campylobacter's pathogenesis. The efficiency of C. jejuni 81-176 invasion of Caco-2 cells was two- to threefold less than the efficiency of invasion of INT407 cells. Adherence-invasion analyses indicated that C. jejuni 81-176 adhered to most INT407 cells but invaded only about two-thirds of the host cells over 2 h (two bacteria/cell). In contrast, only 11 to 17% of differentiated Caco-2 cells were observed to bind and internalize either C. jejuni strain 81-176 or NCTC 11168, and a small percentage of infected Caco-2 cells contained 5 to 20 internalized bacteria per cell after 2 h. Electron microscopy revealed that individual C. jejuni cells adhered to the tips of host cell microvilli via intimate flagellar contacts and by lateral bacterial binding to the sides of microvilli. Next, bacteria were observed to bind at the apical host membrane surface via presumed interactions at one pole of the bacterium and with host membrane protrusions located near intercellular junctions. The latter contacts apparently resulted in coordinated, localized plasma membrane invagination, causing simultaneous internalization of bacteria into an endosome. Passage of this Campylobacter endosome intracellularly from the apical surface to the basolateral surface occurred over time, and bacterial release apparently resulted from endosome-basolateral membrane fusion (i.e., exocytosis). Bacteria were found intercellularly below tight junctions at 60 min postinfection, but not at earlier times. This study revealed unique host cell adherence contacts, early endocytosis-specific structures, and a presumptive exocytosis component of the transcellular transcytosis route.

High-throughput screening for fatty acid uptake inhibitors in humanized yeast identifies atypical antipsychotic drugs that cause dyslipidemias

Fatty acids are implicated in the development of dyslipidemias, leading to type 2 diabetes and cardiovascular disease. We used a standardized small compound library to screen humanized yeast to identify compounds that inhibit fatty acid transport protein (FATP)-mediated fatty acid uptake into cells. This screening procedure used live yeast cells expressing human FATP2 to identify small compounds that reduced the import of a fluorescent fatty acid analog, 4,4-difluoro-5-methyl-4-bora-3a,4a-diaza-s-indacene-3-dodecanoic acid (C(1)-BODIPY-C(12)). The library used consisted of 2,080 compounds with known biological activities. Of these, approximately 1.8% reduced cell-associated C(1)-BODIPY-C(12) fluorescence and were selected as potential inhibitors of human FATP2-mediated fatty acid uptake. Based on secondary screens, 28 compounds were selected as potential fatty acid uptake inhibitors. Some compounds fell into four groups with similar structural features. The largest group was structurally related to a family of tricyclic, phenothiazine-derived drugs used to treat schizophrenia and related psychiatric disorders, which are also known to cause metabolic side effects, including hypertriglyceridemia. Potential hit compounds were studied for specificity of interaction with human FATP and efficacy in human Caco-2 cells. This study validates this screening system as useful to assess the impact of drugs in preclinical screening for fatty acid uptake.

Parallels between global transcriptional programs of polarizing Caco-2 intestinal epithelial cells in vitro and gene expression programs in normal colon and colon cancer

Posttranslational mechanisms are implicated in the development of epithelial cell polarity, but little is known about the patterns of gene expression and transcriptional regulation during this process. We characterized temporal patterns of gene expression during cell-cell adhesion-initiated polarization of cultured human Caco-2 cells, which develop structural and functional polarity resembling enterocytes in vivo. A distinctive switch in gene expression patterns occurred upon formation of cell-cell contacts. Comparison to gene expression patterns in normal human colon and colon tumors revealed that the pattern in proliferating, nonpolarized Caco-2 cells paralleled patterns seen in human colon cancer in vivo, including expression of genes involved in cell proliferation. The pattern switched in polarized Caco-2 cells to one more closely resembling that in normal colon tissue, indicating that regulation of transcription underlying Caco-2 cell polarization is similar to that during enterocyte differentiation in vivo. Surprisingly, the temporal program of gene expression in polarizing Caco-2 cells involved changes in signaling pathways (e.g., Wnt, Hh, BMP, FGF) in patterns similar to those during migration and differentiation of intestinal epithelial cells in vivo, despite the absence of morphogen gradients and interactions with stromal cells characteristic of enterocyte differentiation in situ. The full data set is available at http://microarray-pubs.stanford.edu/CACO2.

Transcriptional modulation of genes encoding structural characteristics of differentiating enterocytes during development of a polarized epithelium in vitro

Although there is considerable evidence implicating posttranslational mechanisms in the development of epithelial cell polarity, little is known about the patterns of gene expression and transcriptional regulation during this process. We characterized the temporal program of gene expression during cell-cell adhesion-initiated polarization of human Caco-2 cells in tissue culture, which develop structural and functional polarity similar to that of enterocytes in vivo. A distinctive switch in gene expression patterns occurred upon formation of cell-cell contacts between neighboring cells. Expression of genes involved in cell proliferation was down-regulated concomitant with induction of genes necessary for functional specialization of polarized epithelial cells. Transcriptional up-regulation of these latter genes correlated with formation of important structural and functional features in enterocyte differentiation and establishment of structural and functional cell polarity; components of the apical microvilli were induced as the brush border formed during polarization; as barrier function was established, expression of tight junction transmembrane proteins peaked; transcripts encoding components of the apical, but not the basal-lateral trafficking machinery were increased during polarization. Coordinated expression of genes encoding components of functional cell structures were often observed indicating temporal control of expression and assembly of multiprotein complexes.

Accumulation of transferrin in Caco-2 cells: a possible mechanism of intestinal transferrin absorption

Transferrin receptor (TfR)-mediated endocytosis and transcytosis in enterocyte-like Caco-2 cells was investigated in order to elucidate the transport mechanism of orally administered Tf-fusion proteins. Cellular uptake and pulse chase studies were performed in Caco-2, MCF-7 and bladder carcinoma (5637) cells using 125I-labeled Tf (125I-Tf). Co-localization studies of Rab 11 and FITC-Tf endocytosed at either the apical or basolateral membrane were performed in polarized Caco-2 cells grown on Transwells, using confocal laser scanning microscopy (LSM510, Zeiss). Unlike in MCF-7 or 5637 cells, where rapid recycling of Tf was observed, a significant amount of endocytosed 125I-Tf accumulated in Caco-2 cells. This accumulation was especially noticeable with the internalization of 125I-Tf from the apical membrane of polarized Caco-2 cells. Confocal microscopy studies showed that apically, but not basolaterally, endocytosed FITC-Tf was delivered to a Rab11-positive compartment. Our results suggest that a significant amount of apically endocytosed Tf in intestinal epithelial cells is transported to a Rab11-positive compartment, possibly a late endosomal and slow recycling compartment. The Rab11-positive compartment may control the release of apically internalized Tf for either slow recycling to apical membrane or processing to transcytotic compartments.

IFN-gamma and TNF-alpha decrease serotonin transporter function and expression in Caco2 cells

Recent studies have shown that mucosal serotonin (5-HT) transporter (SERT) expression is decreased in animal models of colitis, as well as in the colonic mucosa of humans with ulcerative colitis and irritable bowel syndrome. Altered SERT function or expression may underlie the altered motility, secretion, and sensation seen in these inflammatory gut disorders. In an effort to elucidate possible mediators of SERT downregulation, we treated cultured colonic epithelial cells (Caco2) with conditioned medium from activated human lymphocytes. Application of the conditioned medium caused a decrease in fluoxetine-sensitive [(3)H]5-HT uptake. Individual proinflammatory agents were then tested for their ability to affect uptake. Cells were treated for 48 or 72 h with PGE(2) (10 microM), IFN-gamma (500 ng/ml), TNF-alpha (50 ng/ml), IL-12 (50 ng/ml), or the nitric oxide-releasing agent S-nitrosoglutathione (GSNO; 100 microM). [(3)H]5-HT uptake was then measured. Neither PGE nor IL-12 had any effect on [(3)H]5-HT uptake, and GSNO increased uptake. However, after 3-day incubation, both TNF-alpha and IFN-gamma elicited significant decreases in SERT function. Neither TNF-alpha nor IFN-gamma were cytotoxic when used for this period of time and at these concentrations. These two cytokines also induced decreases in SERT mRNA and protein levels. By altering SERT expression, TNF-alpha and IFN-gamma could contribute to the altered motility and expression seen in vivo in ulcerative colitis or irritable bowel syndrome.

The secreted autotransporter toxin, Sat, functions as a virulence factor in Afa/Dr diffusely adhering Escherichia coli by promoting lesions in tight junction of polarized epithelial cells

Afa/Dr diffusely adhering Escherichia coli (DAEC) strains are responsible for urinary tract and intestinal infections. Both in intestine and kidney, the epithelial cells forming epithelium are sealed by junctional domains. We provide evidence that the Secreted autotransporter toxin, Sat, belonging to the subfamily of serine protease autotransporters of Enterobacteriaceae (SPATEs), acts as a virulence factor in Afa/Dr DAEC by promoting lesions in the tight junctions (TJs) of polarized epithelial Caco-2/TC7 cells. Southern blot analysis reveals that the prototype strains of the subclass-1 and subclass-2 typical Afa/Dr DAEC strains, hybridize with a sat probe. Using the wild-type IH11128 strain, the recombinant E. coli AAEC185 strain that expresses Sat, the recombinant E. coli that expresses both Dr adhesin and Sat, we report that Sat in monolayers of cultured enterocyte-like Caco-2/TC7 cells, induces rearrangements of the TJs-associated proteins ZO-1, ZO-3 and occludin, and increases the formation of domes as the result of an increase in the paracellular permeability without affecting the transepithelial electrical resistance of the cell monolayers. Moreover, we observe that Sat-induced disassembly of TJs-associated proteins is dependent on the serine protease motif. Finally, an analysis of the prevalence of the sat gene in three collections of Afa/Dr DAEC strains collected from the stools of children with and without diarrhoea, and from the urine of patients with urinary tract infection (UTI) shows that: (i) the sat gene is highly prevalent in UTI-associated Afa/Dr DAEC strains (88% positive), (ii) the sat gene is generally absent from Afa/Dr DAEC strains collected from the stools of children without diarrhoea (16% positive); whereas (iii) it is present in about half of the strains collected from the stools of children with diarrhoea (46% positive).

Apoptotic process of porcine intestinal M cells

Membranous (M) cells of the follicle-associated epithelium (FAE) are believed to sample antigens from the gut lumen. However, the origin, differentiation mechanism, and cell death of M cells are still a matter of controversy. Therefore, we investigated the process of M cell differentiation and determined their fate in the intestine of three-way crossbred female pigs. We used anti-cytokeratin 18 and anti-PCNA antibodies to distinguish M cells and proliferative cells and performed immunohistochemistry, enzyme histochemistry, and scanning electron microscopy on fresh ileal Peyer's patches. Cell migration and apoptotic cells were detected by BrdU labeling and the TUNEL method, respectively. The turnover of the FAE was similar to that of the villi. M cells were mostly observed from the FAE crypt to the FAE periphery, but not in the FAE apex. As proliferative M cells (cytokeratin 18(+)/PCNA(+) cells) have previously been detected in the FAE crypt, porcine M cells may be directly derived from intestinal epithelial stem cells and committed as a distinct cell lineage in the crypts. M cells from the FAE periphery were unstained or only weakly stained for alkaline phosphatase, whereas cytokeratin 18(+)/alkaline phosphatase(+) cells lying near to the FAE apex showed a columnar shape similar to that of adjacent enterocytes. These data suggest that the committed M cells differentiate to mature M cells by contact with lymphocytes at the FAE periphery, and that they trans-differentiate to enterocytes and are finally excluded near the FAE apex.

Hemin induces active chloride secretion in Caco-2 cells

Enterocytes maintain fluid-electrolyte homeostasis by keeping a tight barrier and regulating ion channels. Carbon monoxide (CO), a product of heme degradation, modulates electrolyte transport in kidney and lung epithelium, but its role in regulating intestinal fluid-electrolyte homeostasis has not been studied. The major source of endogenous CO formation comes from the degradation of heme via heme oxygenase. We hypothesized that heme activates electrolyte transport in intestinal epithelial cells. Basolateral hemin treatment increased baseline Caco-2 cell short-circuit currents (I(sc)) twofold (control = 1.96 +/- 0.14 microA/cm(2) vs. hemin = 4.07 +/- 0.16 microA/cm(2), P < 0.01); apical hemin had no effect. Hemin-induced I(sc) was caused by Cl- secretion because it was inhibited in Cl- -free medium, with ouabain, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), or DIDS. Apical electrogenic Na+ channel inhibitor benzamil had no effect on hemin-induced I(sc). Hemin did not alter the ability of Caco-2 cells to respond maximally to forskolin, but a soluble guanylate cyclase inhibitor, [1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) inhibited the effects of hemin. A CO-releasing molecule, tricarbonyldichlororuthenium II, induced active Cl- secretion that was also inhibited with ODQ. We conclude that hemin induces active Cl- secretion in Caco-2 cells via a cGMP-dependent pathway. These effects are probably the consequence of CO formation. Heme and CO may be important regulators of intestinal fluid-electrolyte homeostasis.

Activation of apical CFTR and basolateral Ca(2+)-activated K+ channels by tetramethylpyrazine in Caco-2 cell line

We have previously demonstrated that tetramethylpyrazine (TMP) could stimulate colonic and pancreatic anion secretion. The present study investigated the signaling pathways and cellular mechanisms underlying the effect of TMP using human colonic Caco-2 cells, with permeabilized apical or basolateral membranes, in conjunction with Ussing chamber technique, intracellular cAMP and Ca2+ measurements as well as competitive RT-PCR for mRNA expression of cystic fibrosis transmembrane conductance regulator (CFTR) and Ca(2+)-dependent Cl- channels (CACC). Basolateral addition of TMP induced a short circuit current (I(SC)) response, which could be mimicked by forskolin and 3-isobutyl-1-methylxanthine (IBMX). Adenylate cyclase inhibitor, MDL12330A, and intracellular Ca2+ chelator, BAPTA-AM, significantly inhibited the TMP-induced I(SC). In basolateral membrane-permeabilized cells, TMP, as well as forskolin and IBMX, induced an I(SC) response, which was sensitive to MDL-12330A, H89, and specific channel blocker CFTR(inh-172), but insensitive to apical application of 4-4'-didsothiocyanostilbene-2, 2'-disulfonic acid (DIDS) and basolateral pretreatment with BAPTA-AM. In apical membrane-permeabilized cells, TMP, similar to forskolin and IBMX, produced a very small current increase, which was sensitive to K+ channel blockers, BaCl2 and tetraethylammonium (TEA), but not Chromanol 293B and charybdotoxin (ChTX), alone or combined. However, in intact Caco-2 monolayers, the TMP-induced I(SC) could be partially inhibited by ChTX. TMP (5 mM) could stimulate intracellular cAMP production. Intracellular Ca2+ was also increased by TMP (5 mM) in both Ca(2+)-containing and Ca(2+)-free bathing solutions. RT-PCR showed that the expression of CFTR in Caco-2 cells was 5.2 fold higher than that of Ca(2+)-activated Cl- channel (CACC). In conclusion, TMP stimulates Cl- secretion by activating cAMP and [Ca2+]i signaling pathways leading to subsequent activation of apical CFTR and basolateral K+ channels.

Cultured monolayers of the dog jejunum with the structural and functional properties resembling the normal epithelium

The development of a culture of the normal mammalian jejunum motivated this work. Isolated crypt cells of the dog jejunum were induced to form primary cultures on Snapwell filters. Up to seven subcultures were studied under the electron microscope and in Ussing chambers. Epithelial markers were identified by RT-PCR, Western blot, and immunofluorescent staining. Confluent monolayers exhibit a dense apical brush border, basolateral membrane infoldings, desmosomes, and tight junctions expressing zonula occludens-1, occludin-1, and claudin-3 and -4. In OptiMEM medium fortified with epidermal growth factor, hydrocortisone, and insulin, monolayer transepithelial voltage was -6.8 mV (apical side), transepithelial resistance was 1,050 Omega.cm(2), and short-circuit current (I(sc)) was 8.1 microA/cm(2). Transcellular and paracellular resistances were estimated as 14.8 and 1.1 kOmega.cm(2), respectively. Serosal ouabain reduced voltage and current toward zero, as did apical amiloride. The presence of mRNA of alpha-epithelial Na(+) channel (ENaC) was confirmed. Na-d-glucose cotransport was identified with an antibody to Na(+)-glucose cotransporter (SGLT) 1. The unidirectional mucosa-to-serosa Na(+) flux (19 nmol.min(-1).cm(-2)) was two times as large as the reverse flux, and net transepithelial Na(+) flux was nearly double the amiloride-sensitive I(sc). In plain Ringer solution, the amiloride-sensitive I(sc) went toward zero. Under these conditions plus mucosal amiloride, serosal dibutyryl-cAMP elicited a Cl(-)-dependent I(sc) consistent with the stimulation of transepithelial Cl(-) secretion. In conclusion, primary cultures and subcultures of the normal mammalian jejunum form polarized epithelial monolayers with 1) the properties of a leaky epithelium, 2) claudins specific to the jejunal tight junction, 3) transepithelial Na(+) absorption mediated in part by SGLT1 and ENaC, and 4) electrogenic Cl(-) secretion activated by cAMP.

sPAR-3, a splicing variant of PAR-3, shows cellular localization and an expression pattern different from that of PAR-3 during enterocyte polarization

PAR-3 (partitioning-defective) is a scaffold-like PDZ (postsynaptic density-95/discs large/zonula occludens-1) domain-containing protein that forms a complex with PAR-6 and atypical PKC, localizes to tight junctions, and contributes to the formation of functional tight junctions. There are several alternatively spliced isoforms of PAR-3, although their physiological significance remains unknown. In this study, we show that one of the major isoforms of PAR-3, sPAR-3, is predominantly expressed in the Caco-2 cells derived from colon carcinoma and is used as a model to investigate the events involved in the epithelial cell differentiation and cell polarity development. During the polarization of Caco-2 cells, the expression of PAR-3 increases as do those of other cell-cell junction proteins, whereas the expression of sPAR-3 decreases. Biochemical characterization revealed that sPAR-3 associates with atypical PKC, as does PAR-3. On the other hand, immunofluorescence microscopy revealed that sPAR-3 does not concentrate at the cell-cell contact region in fully polarized cells, whereas it concentrates at premature cell-cell junctions. This makes a contrast to PAR-3, which concentrates at tight junctions in fully polarized cells. These results provide evidence suggesting the difference in the role between sPAR-3 and PAR-3 in epithelial cells.

DNA microarray analysis of vitamin D-induced gene expression in a human colon carcinoma cell line

The full extent to which 1,25-dihydroxyvitamin D3 affects gene expression in human intestinal epithelial cells is unknown. We used oligonucleotide arrays to catalog vitamin D-induced changes in gene expression in Caco-2 cells, a human colon carcinoma cell line. Five paired sets of Caco-2 cell cultures were subjected to either control conditions or 1,25-dihydroxyvitamin D (10−7 mol/l × 24 h), and RNA was analyzed on an Affymetrix cDNA array containing 12,625 human sequences. Only 13 sequences representing 12 distinct genes exhibited statistically significant changes in expression of twofold or greater and were also called as “present” or “marginal” by the array-reading software in all five experiments. Genes regulated by 1,25-dihydroxyvitamin D included two previously known genes (25-hydroxyvitamin D-24-hydroxylase and amphiregulin) and 10 genes (sorcin, Gem, adaptin-γ, TIG1, CEACAM6, carbonic anhydrase XII, junB, ceruloplasmin, and two unidentified sequences) that were novel. We tested and independently confirmed the effect of 1,25-dihydroxyvitamin D on 11 of these genes by RT-PCR. Increased protein expression was tested and confirmed in two of the novel 1,25-dihydroxyvitamin D-regulated genes, ceruloplasmin and sorcin. The known function of these genes suggests that many of them could be involved in the antiproliferative effects of 1,25-dihydroxyvitamin D3.

Glutamine preserves protein synthesis and paracellular permeability in Caco-2 cells submitted to "luminal fasting"

This study used polarized cell line Caco-2 as a model of human enterocytes to determine: 1) whether deprivation of nutrients on the apical (luminal) side of the epithelium (fasting) alters protein synthesis in enterocytes; 2) if so, whether glutamine can attenuate the effects of fasting; and 3) whether the effects of glutamine depend on its route (i.e., apical vs. basolateral) of supply. Caco-2 cells were submitted to nutrient deprivation on the apical side to mimic the effects of fasting, whereas the basolateral side of the epithelium remained exposed to regular medium. Cells were then incubated with [2H3]leucine with or without glutamine, and the fractional synthesis rate (FSR) of total cell protein was determined from [2H3]leucine enrichments in protein-bound and intracellular free leucine measured by gas chromatography/mass spectrometry. A 24-h apical nutrient deprivation (luminal fasting) was associated with a decline in intracellular glutamine, glutamate, and glutathione concentrations (-38, -40, and -40%, respectively), protein FSR (-20%), and a rise in passage of dextran, an index of transepithelial permeability. In fasted cells, basolateral or luminal glutamine supplementation did not alter the glutathione pool, but it restored protein FSR and improved permeability. The effects of glutamine were abolished by 6-diazo-oxo-l-norleucine, an inhibitor of glutaminase, and was mimicked by glutamate. We conclude that in Caco-2 cells, protein synthesis depends on nutrient supply on the apical side, and glutamine regardless of the route of supply corrects some of the deleterious effects of fasting in a model of human enterocytes through its deamidation into glutamate.

Substance P (NK(1)) receptor expression by human colonic epithelial cell line Caco-2

The peptide substance P (SP) is known to take part in the regulation of the Cl(-)-dependent secretion in the animal and human colonic mucosa. However, no conclusive evidence for the expression of the functional tachykinin NK(1) receptor has been found in the human colonic epithelial cells. Using the reverse transcription-polymerase chain reaction (RT-PCR) method we could detect the transcripts of the NK(1) receptor in the human colonic epithelial cell line Caco-2. Furthermore, we characterized the mechanism of substance P-induced intracellular signaling in Caco-2 cells. While substance P had no effect on intracellular calcium concentration as measured by fura-2 AM, it induced the activation of the mitogen-activated protein kinases (MAPKs) in a time- and dose-dependent manner. Surprisingly, the peptide NK(1) receptor antagonist [D-Pro(2), D-Trp(7,9)]SP stimulated the activity of MAPKs in the same manner as substance P. In contrast, the specific nonpeptide NK(1) receptor antagonist CP-96,345 clearly abolished the effect of substance P and [D-Pro(2), D-Trp(7,9)]SP on MAPK activity. CP-96,345 itself did not increase the activity of MAPKs. Thus, we provide the first evidence that a functional NK(1) receptor is expressed in the human colonic epithelial cell line Caco-2. The results show that in Caco-2 cells the peptide antagonist [D-Pro(2), D-Trp(7,9)]SP acts as a NK(1) receptor agonist in contrast to the nonpeptide antagonist CP-96,345.

Lactobacillus plantarum reduces the in vitro secretory response of intestinal epithelial cells to enteropathogenic Escherichia coli infection

OBJECTIVE

Enteropathogenic (EPEC) is a Gram-negative bacillus that causes diarrhea. Secretory responses of intestinal epithelial cells can be seen after EPEC infection. Probiotics, which are live bacteria that have proven benefit to the host, play a role in the treatment and prevention of the different enteric pathogens. The goals of the study were to determine whether the probiotic agent (LBP) strain 299v alters the secretory changes seen in EPEC infection and, if so, what underlying mechanism is possible.

METHODS

Caco-2 cell monolayers were rapidly infected with EPEC strain E2348/69 and immediately mounted in Ussing chambers. The monolayers were exposed to LBP before, after, and simultaneously with EPEC infection. Short circuit current (Isc) was measured in the Ussing chamber.

RESULTS

EPEC infection caused an increase in short circuit current that was reduced by preincubation with LBP ( < 0.01). No direct bactericidal effect was observed, but LBP reduced the attachment of EPEC to Caco-2 cells.

CONCLUSION

LBP can play an important role in reducing the secretory change in response to EPEC infection, possibly through inhibition of its binding. However, the presence of the probiotic agent before the infection is necessary. In this setting, its role is more preventive rather than therapeutic.

Reactive nitrogen species modulate the effects of rhein, an active component of senna laxatives, on human epithelium in vitro

BACKGROUND

Senna laxatives are used worldwide. However, their misuse can lead to chronic mucosal inflammation with the accumulation of pigment-laden leukocytes and may cause colon cells to undergo apoptosis. This study explores the mechanisms by which rhein, an active component of senna, acts on a human intestinal cell line to induce ion secretion, apoptosis, and indirect chemotaxis of polymorphonuclear leukocytes.

METHODS

Human colonic adenocarcinoma (CaCo-2) monolayer cells, in the presence or in the absence of rhein, were used to monitor the production of reactive nitrogen species using the Griess reaction. Modified Ussing chambers were used to study electrolyte secretion. The capacity to recruit human polymorphonuclear leukocytes was evaluated using masked well chemotaxis chambers. Rhein-induced apoptosis was investigated by counting apoptotic nuclei stained with Hoechst 33258 dye.

RESULTS

Rhein caused a dose-dependent increase in short-circuit current that was abolished in chloride-free bathing buffer or by preincubating with 100 micromol/L NG-nitro-L-arginine (L-NAME) methyl ester. The concentration that maximally stimulated intestinal secretion, 50 micromol/L rhein, induced nitrate production. Supernatants obtained from CaCo-2 cultures after incubation with 50 micromol/L rhein stimulated a time-dependent polymorphonuclear leukocytes chemotaxis that was significantly decreased with 100 micromol/L L-NAME, whereas rhein per se was not active. Neutralizing antibodies anti-interleukin-8 (IL-8) and anti-ENA78 also inhibited chemotaxis. Overnight rhein incubation produced an increased number of apoptotic cells in the culture supernatant that was significantly decreased by preincubation with 100 micromol/L L-NAME. Light-degraded rhein had no effects on CaCo-2 monolayers.

CONCLUSIONS

The integrity of rhein is crucial to generating nitric oxide, which mediates, with different time courses, ion secretion, chemotaxis, and apoptosis of human-derived cells.

Distribution of ClC-2 chloride channel in rat and human epithelial tissues

The ubiquitous ClC-2 Cl(-) channel is thought to contribute to epithelial Cl(-) secretion, but the distribution of the ClC-2 protein in human epithelia has not been investigated. We have studied the distribution of ClC-2 in adult human and rat intestine and airways by immunoblotting and confocal microscopy. In the rat, ClC-2 was present in the lateral membranes of villus enterocytes and was predominant at the basolateral membranes of luminal colon enterocytes. The expression pattern of ClC-2 in the human intestine differed significantly, because ClC-2 was mainly detected in a supranuclear compartment of colon cells. We found significant expression of ClC-2 at the apex of ciliated cells in both rat and human airways. These results show that the distribution of ClC-2 in airways is consistent with participation of ClC-2 channels in Cl(-) secretion and indicate that extrapolation of results from studies of ClC-2 function in rat intestine to human intestine is not straightforward.

Effects of glutamine deprivation on protein synthesis in a model of human enterocytes in culture

To assess the effect of glutamine availability on rates of protein synthesis in human enterocytes, Caco-2 cells were grown until differentiation and then submitted to glutamine deprivation produced by exposure to glutamine-free medium or methionine sulfoximine [L-S-[3-amino-3-carboxypropyl]-S-methylsulfoximine (MSO)], a glutamine synthetase inhibitor. Cells were then incubated with (2)H(3)-labeled leucine with or without glutamine, and the fractional synthesis rate (FSR) of total cell protein was determined from (2)H(3)-labeled enrichments in protein-bound and intracellular free leucine measured by gas chromatography-mass spectrometry. Both protein FSR (28 +/- 1.5%/day) and intracellular glutamine concentration (6.1 +/- 0.6 micromol/g protein) remained unaltered when cells were grown in glutamine-free medium. In contrast, MSO treatment resulted in a dramatic reduction in protein synthesis (4.6 +/- 0.6 vs. 20.2 +/- 0.8%/day, P < 0.01). Supplementation with 0.5-2 mM glutamine for 4 h after MSO incubation, but not with glycine nor glutamate, restored protein FSR to control values (24 +/- 1%/day). These results demonstrate that in Caco-2 cells, 1) de novo glutamine synthesis is highly active, since it can maintain intracellular glutamine pool during glutamine deprivation, 2) inhibition of glutamine synthesis is associated with reduced protein synthesis, and 3) when glutamine synthesis is depressed, exogenous glutamine restores normal intestinal FSR. Due to the limitations intrinsic to the use of a cell line as an experimental model, the physiological relevance of these findings for the human intestine in vivo remains to be determined.

Rabbit M cells and dome enterocytes are distinct cell lineages

We have studied the M cell origin and differentiation pathway in rabbit gut-associated lymphoid tissues. Micro-dissected domes and epithelium isolated by ethylene diamine tetra acetic acid detachment allowed us to view the whole epithelial surface from the bottom of crypts to the top of domes. We used monoclonal antibodies specific to the apex of either M cells or dome enterocytes, lectins, and antibodies to vimentin in appendix, distal Peyer’s patches and caecal patches.

The earliest vimentin-labeled M cells were observed in the BrdU-positive proliferative zone of dome-associated crypts. Gradual differentiation of the M cell vimentin cytoskeleton started at this site to progressively give rise to the first pocket-forming M cells in the upper dome. Therefore, these mitotic cells of the crypts appear as the direct precursors of M cells. In addition to an early appearance of M cell markers, a regular mosaic-like relative distribution of M cells and dome enterocytes was already detected in the vicinity of crypts, similar to that observed on the lateral surface of domes where functional M cells lie. This constant distribution implies that there is no trans-differentiation of enterocytes to M cells along the crypt-dome axis. Together, these observations provide very strong evidence in favor of an early commitment in crypts of M cell and enterocyte distinct lineages.

A primary culture of guinea pig gallbladder epithelial cells that is responsive to secretagogues

We have developed a cell culture of guinea pig gallbladder epithelial cells with which to study ion transport. When grown on permeable supports, the cultured epithelia developed a transepithelial resistance (R(t)) of approximately 500 Omega. cm(2). The epithelial cell origin of the cell culture was further confirmed by immunocytochemical localization of cytokeratin. Ionomycin and forskolin increased transepithelial voltage and short-circuit current (I(sc)) and decreased R(t). The response to ionomycin was transient, whereas that to forskolin was sustained. Both were attenuated by replacement of Cl(-) and/or HCO(3)(-). Mucosal addition of the anion transport inhibitors DIDS or diphenylamine-2-carboxylic acid (DPC) blocked the response to ionomycin. The response to forskolin was blocked by DPC but not by DIDS. Ionomycin, but not forskolin, increased intracellular Ca(2+) concentration in fura 2-loaded cells. PGE(2), histamine, vasoactive intestinal polypeptide, and secretin elicited a sustained increase in I(sc). Responses to ATP and CCK were transient. Thus cultured guinea pig gallbladder epithelia display the range of responses observed in the native tissue and are an appropriate model for studies of ion transport in gallbladder and intestinal epithelia.

Role of CFTR in the colon

In contrast to the airways, the defects in colonic function in cystic fibrosis (CF) patients are closely related to the defect in CFTR. The gastrointestinal phenotype of CF transgenic mice closely resembles the phenotype in CF patients, which clearly indicates the crucial role of CFTR in colonic Cl- secretion and the absence of an effective compensation. In the colon, stimulation of CFTR Cl- channels involves cAMP- or cGMP-dependent phosphorylation. Exocytosis is not involved. Activation of CFTR leads to coactivation of basolateral KVLQT1-type K+ channels and inhibition of luminal Na+ channels (ENaC). In contrast to cultured cells, Ca2+ does not activate luminal Cl- channels in intact enterocytes. It activates basolateral SK4-type K+ channels and luminal K+ channels, which provide additional driving force for Cl- exit. The magnitude of Cl- secretion, however, completely depends on the presence of at least a residual CFTR function in the luminal membrane. These findings have been clearly demonstrated by Ussing chamber experiments in colon epithelium biopsies of CF and normal individuals: Colonic Cl- secretion in CF patients is variable and reflects the genotype; a complete defect of CFTR is paralleled by the absence of Cl- secretion and unmasks Ca(2+)-regulated K+ channels in the luminal membrane; overabsorption of Na+ in CF reflects the absence of ENaC inhibition by CFTR; and the functional status of CF colon can be mimicked by the complete suppression of cAMP stimulation in enterocytes of healthy individuals.

Activation of transepithelial ion transport by secretin in human intestinal Caco-2 cells

Secretin stimulates bicarbonate secretion from pancreatic duct cells, but what influence secretin exerts on intestinal tissues remains to be clarified. The aim of this study is to examine effects of secretin on ion transport in intestinal epithelial Caco-2 cells. We mounted monolayers of Caco-2 cells grown on permeable supports for 21-28 d in a Ussing chamber and measured short-circuit currents (I(sc)). Addition of secretin (5-100 nM) to the basolateral solution dose-dependently induced biphasic increases of I(sc) (transient and sustained phase). Dibutyryl cyclic AMP (200 microM), forskolin (10 microM), and 3-isobutyl-1-methylxanthine (IBMX, 1 mM) also induced I(sc) responses similar to the administration of secretin. Addition of 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB, 100 microM) or benzamil (100 microM) to the apical solution markedly reduced the secretin-induced I(sc) increase in the transient phase. A selective antagonist of cAMP-dependent protein kinase (PKA), N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89, 1 microM), and a membrane permeable Ca(2+) chelator, 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) (BAPTA/AM, 10 microM) reduced the secretin-induced I(sc). Basolateral addition of 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, 1 mM) suppressed the sustained phase I(sc) increase. Secretin also induced alkalinization of the apical solution (DeltapH, 0.053 +/- 0.013). The alkalinization did not occur when DIDS (1 mM) was added to the basolateral solution or Na(+) was removed from the solutions. Taken together, our observations suggest: (1) secretin stimulates a benzamil-sensitive Na(+) influx and an NPPB-sensitive Cl(-) efflux across the apical membrane through PKA-dependent and Ca(2+)-sensitive pathways; and (2) secretin also induces alkalinization of the apical solution through the activation of a DIDS-sensitive Na(+)-HCO(3)(-) cotransport in the basolateral membrane of Caco-2 cells.

Interruption of transmembrane signaling as a novel antisecretory strategy to treat enterotoxigenic diarrhea

Bacteria that produce heat-stable enterotoxins (STs), a leading cause of secretory diarrhea, are a major cause of morbidity and mortality worldwide. ST stimulates guanylyl cyclase C (GCC) and accumulation of intracellular cyclic GMP ([cGMP]i), which opens the cystic fibrosis transmembrane conductance regulator (CFTR)-related chloride channel, triggering intestinal secretion. Although the signaling cascade mediating ST-induced diarrhea is well characterized, antisecretory therapy targeting this pathway has not been developed. 2-ChloroATP (2ClATP) and its cell-permeant precursor, 2-chloroadenosine (2ClAdo), disrupt ST-dependent signaling in intestinal cells. However, whether the ability to disrupt guanylyl cyclase signaling translates into effective antisecretory therapy remains untested. In this study, the efficacy of 2ClAdo to prevent ST-induced water secretion by human intestinal cells was examined. In Caco-2 human intestinal cells, ST increased [cGMP]i, induced a chloride current, and stimulated net basolateral-to-apical water secretion. This effect on chloride current and water secretion was mimicked by the cell-permeant analog of cGMP, 8-bromo-cGMP. Treatment of Caco-2 cells with 2ClAdo prevented ST-induced increases in [cGMP]i, chloride current and water secretion. Inhibition of the downstream consequences of ST-GCC interaction reflects proximal disruption of cGMP production because 8-bromo-cGMP stimulated chloride current and water secretion in 2ClAdo-treated cells. Thus, this study demonstrates that disruption of guanylyl cyclase signaling is an effective strategy for antisecretory therapy and provides the basis for developing mechanism-based treatments for enterotoxigenic diarrhea.

Cl- transport in an immortalized human epithelial cell line (NCM460) derived from the normal transverse colon

Cells of a newly described, immortalized, epithelial, human transverse colonic cell line, NCM460, reach approximately 90% confluence on plastic and develop transepithelial resistances of 120-250 Omega . cm2 on porous substrates. Its utility as a model for the transverse human colon was validated by comparing second messenger-mediated Cl- transport, using the fluorescent probe 6-methoxy-quinolyl acetoethyl ester, in NCM460 cells and colonocytes isolated from human transverse crypts. Basal Cl- influx was increased (P < 0.01) by PGE1 (1 microM), forskolin (1 microM), 8-bromoadenosine 3'5'-cyclic monophosphate (100 microM), heat-stable Escherichia coli enterotoxin (STa; 1 microM), 8-bromoguanosine 3'5'-cyclic monophosphate (100 microM), histamine (1 microM), and phorbol 12,13-dibutyrate (1 microM) in both cell types. The Cl- channel blocker diphenylamine 2-carboxylic acid (50 microM) and the Na+-K+-2Cl- cotransport inhibitor furosemide (1 microM), but not the K+ channel blocker Ba2+ (3 mM), inhibited these Cl- permeabilities. These cells possess transcripts for cystic fibrosis transmembrane conductance regulator, Na+-K+-2Cl- cotransporter, STa receptor, and intestine-specific cGMP-dependent protein kinase II. Thus cAMP-, cGMP-, and Ca2+-dependent secretagogues act on NCM460 and primary colonocytes to stimulate Cl- transport. This validates the utility of NCM460 as a model for transverse colonic crypts and is the first demonstration of a colonic cell line whose origin is known.

Characterization of furazolidone apical-related effects to human polarized intestinal cells

In studying the effects of furazolidone (FZ) on the human intestinal Caco-2 cell line grown on microporous membrane, we have previously demonstrated a higher toxicity when the compound was administered at the apical (AP) side than at the basolateral (BL) side. Moreover, we have also shown the production, in the intact cells, of a nitroanion radical from FZ by a cytochrome c P450 reductase. The aim of the present study was to investigate which specific cell structures and functions are involved in the observed domain-related toxicity. The relevance of alterations in integrity and selective properties of the intestinal barrier as first-pass site for ingested molecules is also discussed. We have confirmed that, as expected, the Caco-2 cells are protected from FZ injury by a specific inhibitor of the cytochrome c P450 reductase, and we have shown that this protection is more active on the apical side of the cells. In sublethal conditions, FZ causes increased permeability to 3H-mannitol and, to a different extent, to 3H-inulin. Again the effect is higher when the cells are apically exposed. We have thus examined the tight junctions morphology: a disruption of the apical perijunctional actin-bound cytoskeleton was detected by rhodamine-phalloidin staining and microtubule disorganization by antitubulin fluoresceinated antibodies. Again, the effect was more evident when the cells were apically treated with FZ. Preferential transport and accumulation of the compound by active transport mechanisms could be excluded, since transport of FZ was linear and no intracellular accumulation was detected either from the AP and or the BL sides. All together these results may suggest that the AP formation of the active metabolite and its possible reactivity with SH groups of perijunctional microfilaments could be responsible of the higher FZ apical toxicity. This study shows that polarized differentiated cells are very interesting in vitro models to investigate specific cellular domains as targets of toxic effects and to detect subtle changes that may be induced, in absence of cell death, in specialized epithelial layers.

Campylobacter jejuni cytolethal distending toxin causes a G2-phase cell cycle block

Cytolethal distending toxin (CDT) from the diarrheagenic bacterium Campylobacter jejuni was shown to cause a rapid and specific cell cycle arrest in HeLa and Caco-2 cells. Within 24 h of treatment, CDT caused HeLa cells to arrest with a 4N DNA content, indicative of cells in G2 or early M phase. Immunofluorescence studies indicated that the arrested cells had not entered M phase, since no evidence of tubulin reorganization or chromatin condensation was visible. CDT treatment was also shown to cause HeLa cells to accumulate the inactive, tyrosine-phosphorylated form of CDC2. These results indicated that CDT treatment results in a failure to activate CDC2, which leads to cell cycle arrest in G2. This mechanism of action is novel for a bacterial toxin and provides a model for the generation of diarrheal disease by C. jejuni and other diarrheagenic bacteria that produce CDT.

Identification of a human enterocyte lipoxin A4 receptor that is regulated by interleukin (IL)-13 and interferon gamma and inhibits tumor necrosis factor alpha-induced IL-8 release

Epithelial cells of the alimentary tract play a central role in mucosal immunophysiology. Pathogens and/or agonists that interact with mucosal surfaces often elicit epithelial responses that upregulate inflammation. Therefore, it was of interest to explore potential epithelial targeted antiinflammatory signals. Here we identified and sequenced a human enterocyte lipoxin (LX) A4 [5(S), 6(R),15(S)-trihydroxy-7,9,13-trans-11-cis eicosatetraenoic acid] receptor, and demonstrate that transcription of this receptor was controlled by cytokines, of which lymphocyte-derived interleukin (IL)-13 and interferon gamma were the most potent. When lipoxins and LXA4 stable analogs were evaluated for enterocyte functional as well as immune responses, lipoxins sharply inhibited TNF-alpha-induced IL-8 release but did not alter either barrier function or agonist-stimulated chloride secretion. 15R/S-methyl-LXA4 and 16-phenoxy-LXA4 each attenuated (IC50 approximately 10 nM) IL-8 release. Cyclooxygenase (COX) II is emerging as an important component in wound healing and proliferation in intestinal epithelia and when acetylated by acetylsalicylic acid (aspirin) initiates the biosynthesis of a LXA4 receptor ligand. We therefore determined whether colonic cell lines (HT-29 Cl.19A, Caco-2, or T84) express the COX II isozyme. Results for RT-PCR and Western blot analysis showed that COX I as well as an IL-1beta- and TNF-alpha-inducible COX II are expressed in HT-29 Cl.19A. In addition, aspirin-treated enterocytes generated 15R-HETE, a precursor of 15-epi-LXA4 biosynthesis, whose potent bioactions were mimicked by the stable analog 15R/S-methyl-LXA4. Taken together, these results identify an endogenous pathway for downregulating mucosal inflammatory events and suggest a potential therapeutic benefit for LXA4 stable analogs.

Differential expression and function of PACAP and VIP receptors in four human colonic adenocarcinoma cell lines

Human colonic adenocarcinoma cell lines have conserved several features of the native tissue. Among these is the expression of cell surface receptors for hormones and neurotransmitters that may be involved in the regulation of proliferation and differentiation processes in these cancer cells. Here, we confirm that high-affinity binding sites for the Vasoactive Intestinal Polypeptide (VIP) and for the VIP analogue Pituitary Adenylate-Cyclase Activating Polypeptide (PACAP), were expressed in 4 human colonic adenocarcinoma cell lines, HT29, SW403, DLD-1 and Caco-2, that spontaneously displayed variable phenotypic properties in culture. We demonstrated that after long-term treatments, VIP and PACAP significantly reduced cell proliferation in the 4 cell lines and modulated intracellular cAMP and cGMP levels. Furthermore, conspicuous differences were observed from one cell type to another concerning expression of the receptor subsets or the effects of the neuropeptides on cell growth and on cyclic nucleotides production.