Background: Epithelia reveal an asymmetric distribution of proteins along the apico-basal axis. It is well established that transiently epithelial cells abolish their polar orientation, e.g. in growth or wound healing processes or - pathologically - in malignant transformation and invasiveness.
Polarity changes in inflammatory processes are less well established. Interleukin-13 (IL-13) is involved in the barrier defect found in ulcerative colitis (UC). Herein, we have characterized the polarity defects in intestinal epithelial cells and in intestinal biopsies of UC patients.
Methods: Mucosal explants (human sigmoid colon) were mounted to Ussing chamber to determine epithelial resistance (Re). Consecutively expression of polarity complexes were done (confocal microscopy after immunostaining: ZO-1, occludin, JAM-A, Pard3, Dlg1, F-actin, beta-catenin, E-cadherin). In vitro experiments included exposure of various IECs (Caco-2, T84, HT-29/B6) with IL-13. Measurement of transepithelial resistance (TER) was followed by sandwich assay to reveal macromolecular passage and LSM-assisted polarity complex analysis. Finally, effects of IL-13 on cell polarity were determined in 3D Caco2-cysts (Matrigel). In cyst measurement of the paracellular barrier was done with TMR-dextran3000.
Results: As expected we found a reduced Re in colonic mucosal explants of UC patients. Moreover, IL-13 induced a barrier defect in intestinal epithelial culture cells (significant reduction in TER in all intestinal epithelial cells; sandwich assay resolved macromolecular leaks; increased TMR-Dextran3000 permeability in the 3D cyst model).
In correspondence to the defective barrier we found an altered expression and localization of polarity complex proteins in intestinal biopsies, which was in some, but not all proteins associated with the extent of the inflammatory infiltration of the mucosa. As a surrogate of epithelial dyspolarity we also identified an increased number of aberrant cysts (i.e. disturbed lumenogenesis), which was associated with IL-13-caused alterations in the expression of polarity protein complexes in the 3D cyst model.
Conclusions: Presenting evidence for an L-13-induced defect in polarity proteins underscores the relevance of IL-13 for pathophysiology of inflammation in UC. Alterations in polarity might add to other mechanisms in generation of colitis-associated carcinomas.