Aquaporin-8 expression is reduced in ileum and induced in colon of patients with ulcerative colitis

Abstract

Aim: To study susceptibility genes which may play a potential role in the pathogenesis and etiology of inflammatory bowel disease (IBD).

Methods: To identify potential susceptibility genes we performed global gene expression profiling in patients with IBD and control specimens. For determination of an intrinsic gene expression profile in ulcerative colitis (UC) and Crohn's disease (CD) compared to normal subjects, mucosal biopsies of non-inflamed regions of the colon and the terminal ileum were subjected to DNA microarray analysis. Real-time RT-PCR and immunohistochemistry were used for verification of selected regulated candidate genes and a genetic analysis was performed.

Results: We could show that aquaporin-8 (AQP8) mRNA and protein levels were significantly increased in the colon of UC patients compared to controls. Genetic analysis of the six exons and the promoter region of AQP8, however, revealed no mutations or polymorphisms in IBD patients.

Conclusion: Our results suggest that upregulation of AQP8 in the colon of UC patients represents a secondary phenomenon which may, due to altered water exchange of the distal intestinal mucosa, disturb the physiologic colonic mucus barrier and thus lead to chronic inflammation and ulceration.

Figure 1

AQP8 mRNA expression in biopsies of controls and UC patients. Real-time RT-PCR analysis was performed in ileal and colonic biopsy samples from control individuals and UC patients. TaqMan real-time RT-PCR results are expressed as relative gene expression referring to GAPDH expression as a housekeeping gene. The results from 8 individuals are combined. The extremal values, the mean value and the standard deviation are shown for each group.

Figure 2

A: Immunohisto-chemical detection of AQP8 in normal colonic mucosa. Some subepithelial mononuclear cells (MNC) in the lamina propria are positive (red immunostaining). Epithelial cells at the mucosal surface display very weak signals; B: Immunohistochemical detection of AQP8 in an inactive colonic area in a case with ulcerative colitis. A slight increase in staining intensity is apparent as compared with A. The intestinal epithelium as well as a subset of lamina propria MNCs are positive for AQP8; C: Immunohistochemical detection of AQP8 in an actively inflamed colonic area in a case with ulcerative colitis. A strong expression of AQP8 is apparent. Intestinal epithelial cells as well as numerous MNCs within the inflamed lamina propria are immunostained; D: Corresponding negative control to 2C with use of normal rabbit immunoglobulins as primary reagent. A-C: Alkaline phosphatase, original magnification x 40, respectively.