Signaling pathways induced by serine proteases to increase intestinal epithelial barrier function.

Changes in barrier function of the gastrointestinal tract are thought to contribute to the inflammatory bowel diseases Crohn's disease and ulcerative colitis. Previous work in our lab demonstrated that apical exposure of intestinal epithelial cell lines to serine proteases results in an increase in transepithelial electrical resistance (TER). However, the underlying mechanisms governing this response are unclear. We aimed to determine the requirement for proteolytic activity, epidermal growth factor receptor (EGFR) activation, and downstream intracellular signaling in initiating and maintaining enhanced barrier function following protease treatment using a canine intestinal epithelial cell line (SCBN). We also examined the role of phosphorylation of myosin regulatory light chain on the serine protease-induced increase in TER through. It was found that proteolytic activity of the serine proteases trypsin and matriptase is required to initiate and maintain the protease-mediated increase in TER. We also show that MMP-independent EGFR activation is essential to the sustained phase of the protease response, and that Src kinases may mediate EGFR transactivation. PI3-K and ERK1/2 signaling were important in reaching a maximal increase in TER following protease stimulation; however, their upstream activators are yet to be determined. CK2 inhibition prevented the increase in TER induced by serine proteases. The bradykinin B(2) receptor was not involved in the change in TER in response to serine proteases, and no change in phosphorylation of MLC was observed after trypsin or matriptase treatment. Taken together, our data show a requirement for ongoing proteolytic activity, EGFR transactivation, as well as downstream PI3-K, ERK1/2, and CK2 signaling in protease-mediated barrier enhancement of intestinal epithelial cells. The pathways mediating enhanced barrier function by proteases may be novel therapeutic targets for intestinal disorders characterized by disrupted epithelial barrier function.