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Intestinal inflammatory disease or infection often results in the loss of the epithelial layer as a result mainly of the action of proteases, including the leucocyte serine proteinases (neutrophil elastase), lysosomal cathepsins and the matrix metalloproteinases from recruited inflammatory cells. Previous studies have shown that bronchial or intestinal epithelial cells (IEC) can respond to proteolytic attack by producing cytokines. In this study, we have determined the effect of protease treatment on interleukin-6 (IL-6) and monocyte chemoattractant protein-1 (MCP-1) production by IEC lines. Both neutrophil elastase and trypsin treatment induced elevated levels of mRNA for IL-6 in rat IEC-6 cells. Non-proteolytic detachment of the IEC-6 cells also induced elevated levels of IL-6 mRNA, suggesting that the effect was not caused by a specific protease or degradation product, but probably by an effect on cell shape or cell detachment. Similar results were seen with the IEC-18 cell line. Trypsin treatment of the IEC-6 cells also enhanced unstimulated and IL-1 beta costimulated IL-6 secretion, but not MCP-1 secretion or mRNA levels. Finally, nuclear levels of the CCAAT/enhancer binding protein-beta (C/EBP-beta) were rapidly enhanced after proteolytic detachment of the IEC-6 cells, suggesting a mechanism for the enhancement of IL-6 mRNA responses. These data indicate that epithelial cells can respond to proteolytic attack or cell detachment by producing IL-6, a cytokine with several anti-inflammatory and antiprotease effects, which may be important in moderating the loss of the epithelial layer by its effects on nearby epithelial or inflammatory cells.