Abstract
Background
In inflammatory bowel diseases (IBD), epithelial barrier defects occur as a consequence of chronic inflammation. Recent research suggested that cell polarity alterations may be upstream of barrier defects and additionally play a role in IBD-associated carcinogenesis (colitis-associated and small intestinal carcinoma). Par4 is a gene encoding a protein crucial in the development of cell polarity. LKB1, its human homologue, is mutated in Peutz–Jeghers syndrome (PJS), a genetic condition characterised by a higher risk of epithelial cancers. While the pivotal role of Par4/LKB1 in the development of epithelial cell polarity is established, its involvement in IBD-associated barrier defects and carcinogenesis is yet to be defined.
Methods
Endoscopic bowel mucosa samples from patients with Crohn’s disease, ulcerative colitis, PJS and controls were analysed to assess expression and localisation of Par4/LKB1. Cryosections were immunostained for Par4/LKB-1 along with a diverse set of markers of cell polarity and intercellular adhesion. The analysis was performed by confocal laser scanning microscopy in order to compare the expression of mucosal as well as the subcellular localisation of Par4/LKB1 in the gut mucosa. A quantitative analysis of protein expression with western blotting was performed as well. The function of Par4/LKB1 in intestinal epithelial cells (IEC) was evaluated by means of a Par4-deficient IEC model as well as in an IEC Par4-overexpression model. Moreover, a preliminary assessment of the possible role of Par4/LKB1-related polarity processes in IBD-associated carcinomas was performed through scanning genetic data from colitis-associated carcinomas for potential Par4-related mutations and altered Par4/LKB1 expression.
Results
The immunofluorescent staining allowed visualisation of intracellular expression of Par4 in epithelia from PJS, IBD patients and controls. In PJS-polyps, despite the alteration of regular tissue architecture typical of these lesions, the polarity of epithelial cells was maintained - contrary to control tissue, a punctate pattern of the Par4 staining was shown. In IBD tissue, no relevant differences in Par4 expression at confocal microscopy, as well as a quantitative assessment, were observed as compared with controls. No differences were observed in PJS or IBD samples as regards the expression of cell polarity markers such as Par3, CD71, crb3 or adhesion molecules such as ZO-1, e-cadherin, occludin, JAM-A. As regards the IEC model, we observed that Caco-2/BBe cells overexpressing Par4/LKB1 showed enhanced Par4/LKB1 cytosolic staining at immunofluorescence. In this model, a defective epithelial polarity and organisation on permeable supports could be observed in Caco2/BBe cysts in parallel with reduced expression of native LKB1 as seen in western blots. Preliminary analysis of data from our colitis-associated carcinomas database suggested various pathways potentially involving Par4/LKB1, yet no direct analysis of tissue samples for Par4-related mutations was performed yet.
Conclusions
Altered expression of Par4 in epithelial cells may influence the development of a functional epithelial barrier as suggested by IEC models. Par4/LKB1 as a known oncosuppressor gene is involved in a number of pathways potentially relevant for carcinogenesis in IBD; however, a direct connection between Par4-related alteration of the epithelial barrier and carcinogenesis is yet to be established.
Molecular Pathophysiology of Epithelial Barrier Dysfunction in Inflammatory Bowel Diseases
