The Punicalagin Metabolites Ellagic Acid and Urolithin A Exert Different Strengthening and Anti-Inflammatory Effects on Tight Junction-Mediated Intestinal Barrier Function In Vitro

Ulcerative colitis (UC) is characterized by a Th2 immune response with inflammation and epithelial barrier dysfunction. So far, Th2 cytokines have not been shown to directly influence epithelial barrier function. Lamina propria mononuclear cells (LPMCs) were stimulated and interleukin (IL)-13 was measured by enzyme-linked immunosorbent assay. Functional IL-13 and IL-4 effects were studied on HT-29/B6 colonic epithelial cells in Ussing chambers and by conductance scanning. Apoptosis was detected by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling assays. IL-13/IL-4 receptors were analyzed by reverse-transcription polymerase chain reaction and immunofluorescence. Western blotting combined with immunofluorescence was used to detect tight junction proteins. Furthermore, restitution velocity was measured. Finally, mucosal biopsy specimens from patients with UC were compared with cultured cells for these features. LPMCs from patients with UC produced large amounts of IL-13 (985 +/- 73 pg/mL), much more than from controls or patients with Crohn's disease. IL-13Ralpha1 and IL-4Ralpha receptors were present in HT-29/B6 cells and colonic epithelial cells of control patients and patients with UC. IL-13 had a dose-dependent effect on transepithelial resistance of HT-29/B6 monolayers (reduction to 60% +/- 4%), whereas IL-4 had no effect. This was due to an increased number of apoptotic cells (5.6-fold +/- 0.9-fold) and an increased expression of the pore-forming tight junction protein claudin-2 to 295% +/- 37%, both of which contributed equally. Finally, epithelial restitution velocity decreased from 15.1 +/- 0.6 to 10.6 +/- 0.5 microm/h after treatment with IL-13. Parallel changes were observed in human samples, with an increase in claudin-2 expression to 956% +/- 252%. IL-13 was identified as an important effector cytokine in UC that impairs epithelial barrier function by affecting epithelial apoptosis, tight junctions, and restitution velocity. Show more

Tight junctions seal the paracellular pathway of epithelia but, in leaky tissues, also exhibit specific permeability. In order to characterize the contribution of claudin-2 to barrier and permeability properties of the tight junction in detail, we studied two strains of Madin-Darby canine kidney cells (MDCK-C7 and MDCK-C11) with different tight junctional permeabilities. Monolayers of C7 cells exhibited a high transepithelial resistance (>1 kOhms cm(2)), compared with C11 cells (<100 Ohms cm(2)). Genuine expression of claudin-1 and claudin-2, but not of occludin or claudin-3, was reciprocal to transepithelial resistance. However, confocal microscopy revealed a marked subjunctional localization of claudin-1 in C11 cells, indicating that claudin-1 is not functionally related to the low tight junctional resistance of C11 cells. Strain MDCK-C7, which endogenously does not express junctional claudin-2, was transfected with claudin-2 cDNA. In transfected cells, but not in vector controls, the protein was detected in colocalization with junctional occludin by means of immunohistochemical analyses. Overexpression of claudin-2 in the originally tight epithelium with claudin-2 cDNA resulted in a 5.6-fold higher paracellular conductivity and relative ion permeabilities of Na(+) identical with 1, K(+)=1.02, NMDG(+)=0.79, choline(+)=0.71, Cl(-)=0.12, Br(-)=0.10 (vector control, 1:1.04:0.95:0.94:0.85:0.83). By contrast, fluxes of (radioactively labeled) mannitol and lactulose and (fluorescence labeled) 4 kDa dextran were not changed. Hence, with regular Ringer's, Na(+) conductivity was 0.2 mS cm(-2) in vector controls and 1.7 mS cm(-2) in claudin-2-transfected cells, while Cl(-) conductivity was 0.2 mS cm(-2) in both cells. Thus, presence of junctional claudin-2 causes the formation of cation-selective channels sufficient to transform a 'tight' tight junction into a leaky one. Show more

Claudins (Cldn) are essential membrane proteins of tight junctions (TJs), which form the paracellular permselective barrier. They are produced by a multi-gene family of 24 reported members in mouse and human. Based on a comprehensive search combined with phylogenetic analyses, we identified three novel claudins (claudin-25, -26, and -27). Quantitative RT-PCR revealed that the three novel claudins were expressed in a tissue- and/or developmental stage-dependent manner. Claudins-25 and -26, but not claudin-27, were immunofluorescently localized to TJs when exogenously expressed in cultured MDCK and Eph epithelial cell lines. These findings expand the claudin family to include at least 27 members. Copyright © 2011 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved. Show more