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Background & Aims:: In this study, we determined the signal transduction and functional consequences after ligand-specific activation of the fractalkine receptor CX3CR1 in human intestinal epithelial cells. Methods:: CX3CR1 expression in human colonic tissues and intestinal epithelial cell lines was determined by immunohistochemistry, immunoblotting, and reverse-transcription polymerase chain reaction. The regulation of mitogenactivated protein kinase (MAPK) activation was assessed by immunoblotting. Regulation of chemokine messenger RNA (mRNA) expression was determined by Northern blotting. NF-@kB and p53 activation was assessed by electromobility shift assays. Results:: Fractalkine mediated the MEK-1 and G@ai-dependent but phosphatidylinositol-3-kinase-independent activation of extracellular signal-regulated kinase-MAPK. Fractalkine activated NF-@kB and p53 resulting in interleukin 8 and fractalkine mRNA expression. CX3CR1-mediated activation of intestinal epithelial cells was able to induce migration of human neutrophils into but not through the intestinal epithelial cell monolayer. Conclusions:: CX3CR1 mediates distinct functional responses in intestinal epithelial cells, which include the autocrine regulation of cell-survival signals and activation of immune modulators, indicating a role of CX3CR1 in host defense mechanisms originating from the intestinal epithelium.

Background & Aims: To determine the functional role of immune mediators in the formation of the intestinal barrier, we have examined the regulation of claudin expression by interleukin (IL)-17 in human intestinal epithelial cells. Methods: Expression of claudins, extracellular signal-related (ERK) mitogen-activated protein kinases (MAPKs), and activated ERK MAPKs was determined by immunoblotting. Claudin membrane association was assessed by immunohistochemistry and claudin messenger RNA expression by Northern blot analysis. Intestinal epithelial barrier function was characterized through transepithelial electrical resistance and mannitol tracer flux. Results: IL-17 induced the development of a paracellular barrier of T84 cell monolayers. Inhibition of ERK activation with the MEK inhibitor PD98059 blocked IL-17 as well as basal development of tight junctions in T84 cells. IL-17 induced formation of tight junctions correlated with up-regulation of claudin-1 and claudin-2 gene transcription. Inhibition of MEK reduced the activated and basal expression of claudin-2 messenger RNA and protein expression. Functional MEK was required for the expression and membrane association of claudin-2 but not claudin-1 in T84 cells. Conclusions: MEK activity is required for claudin-mediated formation of tight junctions. IL-17 is able to regulate the intestinal barrier through the ERK MAPK pathway. GASTROENTEROLOGY 2000;118:1001-1011