990 IL-6 Induced Increase in Intestinal Epithelial Tight Junction Permeability is Mediated by AP-1 Induced Up-Regulation of Claudin-2 Gene Activity

Abstract

Background: Defective intestinal epithelial tight junction (TJ) barrier, leading to increased antigenic penetration, has been recognized as an important pathogenic mechanism contributing to the development of intestinal inflammation in Crohn's disease. Interleukin-6 (IL-6) is a pro-inflammatory cytokine which is elevated in various inflammatory diseases including Crohn's disease. Previous studies have shown that IL-6 induced increase in intestinal epithelial TJ permeability is mediated in part by an increase in claudin-2 expression. However, the cellular andmolecular mechanisms that regulate the claudin-2 gene activity and TJ permeability remains unclear. Aim: The aim of this study was to delineate the molecular mechanisms involved in IL-6 induced increase in intestinal epithelial TJ permeability. The major focus of these studies was to elucidate the AP-1 regulation of claudin-2 expression and TJ barrier. Methods: Filter-grown Caco-2 cells were used for TJ permeability studies. Various molecular and biochemical methods were used to determine claudin-2 gene activity and expression. Results: 1) IL-6 caused a time-dependent drop in Caco-2 transepithelial resistance and a size-selective increase in paracellular flux to small permeability markers ( 4 A in radius. 2) IL-6 caused a timedependent increase in claudin-2 expression, and did not affect other TJ protein expression including occludin and claudin-1, 3, 4, or 8. 3) IL-6 up-regulation of claudin-2 expression was preceded by the activation of JNK pathway. Inhibition of JNK activation by inhibitor (SP600125) or siRNA prevented the IL-6 induced increase in claudin-2 expression and TJ permeability. 4) IL-6 induced activation of JNK pathway resulted in AP-1 activation. 5) A 3 kb of claudin-2 promoter region was identified by 5' RACE, and cloned into pGL3 basic vector. 6) Minimal promoter region was determined by progressively 5' deletion. Four AP1 binding sites were identified within this region. 7) IL-6 induced an increase in claudin2 promoter activity, mRNA and protein expression. 8) SiRNA of AP-1 inhibited the IL-6 induced increase in claudin-2 promoter activity, expression, and increase in TJ permeability. Conclusion: IL-6 induced increase in Caco-2 TJ permeability was mediated by JNK pathway regulated activation of claudin-2 gene. The JNK pathway induced activation of AP-1 led to a step-wise binding of AP-1 to claudin-2 promoter and activation of promoter activity, increase in claudin-2 gene and protein expression, and claudin-2 dependent increase in TJ permeability. Thus, these studies demonstrate a novel mechanism of intestinal TJ barrier regulation.