Potential Regulatory Effects of Corticotropin-Releasing Factor on Tight Junction-Related Intestinal Epithelial Permeability are Partially Mediated by CK8 Upregulation
Background/Aims: Intestinal permeability and stress have been implicated in the pathophysiology of irritable bowel syndrome (IBS). Cytokeratin 8 (CK8), for the first time, has been shown to mediate corticotropin-releasing factor (CRF)-induced changes in intestinal permeability in animal models of IBS. In this study, we investigated the regulatory effects of CRF on the permeability of human intestinal epithelial cells through the CK8-mediated tight junction. Methods: The expression levels of corticotropin-releasing factor receptor 1 (CRFR1) and corticotropin-releasing factor receptor 2 (CRFR2) on the HT29 cell surface were determined by immunofluorescence, RT-PCR, and Western blotting. After treatment with 100 nM CRF for 72 h, the translocation of FITC-labelled dextran was measured in a transwell chamber; the structural changes of tight junctions were observed under transmission electron microscopy; the expression levels of CK8, F-actin and tight junction proteins ZO-1, claudin-1, and occludin were detected by immunoblotting and immunofluorescence. The activity of RhoA was detected by immunoprecipitation. Furthermore, the effects of CRF on intestinal epithelial permeability were examined in CK8-silenced HT29 cells, which were constructed by shRNA interference. Results: CRF treatment increased FITC-labelled dextran permeability, caused the opening of tight junctions, induced increased fluorescence intensity of CK8 and decreased the intensities of ZO-1, claudin-1, and occludin, together with structural disruption. The expression levels of F-actin, occludin, claudin-1, and ZO-1 were downregulated. RhoA activity peaked at 30 min after CRF treatment. CRF-induced increased permeability, and downregulation of claudin-1 and occludin were not blocked by CK8 silencing. Nevertheless, CK8 silencing blocked the effects of CRF regarding the decrease in the expression levels of F-action and ZO-1 and increase in RhoA activity. Conclusion: CRF may increase intestinal epithelial permeability by upregulating CK8 expression, activating the RhoA signalling pathway, promoting intestinal epithelial actin remodelling, and decreasing the expression of the tight junction protein ZO-1. Other CK8-independent pathways may be involved in the downregulation of claudin-1 and occludin, which might also contribute to increased intestinal epithelial permeability.