Par-3 modulates intestinal epithelial barrier function through regulating intracellular trafficking of occludin and myosin light chain phosphorylation.

Tight junctions play a critical role in the maintenance of intestinal barrier function. Partitioning-defective protein 3 (Par-3) can regulate intestinal barrier function through the modulation of tight junction assembly and cell polarity. However, the mechanisms are still not fully understood. Adult C57BL/6 mice were treated with dextran sulfate sodium for 7 days, and segments of colon were harvested for immunofluorescent staining of Par-3. Caco-2 intestinal epithelial cells were treated with tumor necrosis factor α (TNF-α) for 24 h, and Par-3 expression was detected by Western blot analysis and immunofluorescence. Additionally, Caco-2 cells were treated with Par-3 small interfering RNA, and altered expression and subcellular localization of tight junction proteins were studied by Western blot analysis and immunofluorescence. Furthermore, the interaction between Par-3 and myosin light chain (MLC) was detected by immunoprecipitation. Par-3 was downregulated in murine dextran sulfate sodium induced acute inflammation and TNF-α-treated Caco-2 cells. Depletion of Par-3 expression by small interfering RNA delayed intestinal epithelial barrier development in Caco-2 cells. This regulation was due to the redistribution of the tight junction protein occludin rather than the altered total levels of tight junction proteins. Par-3 silencing blocked the trafficking of occludin from or through the Golgi complex to the cell surface, and dramatically induced occludin accumulated at the Golgi complex. Importantly, Par-3 can interact with MLC, and loss of Par-3 upregulated MLC kinase expression and MLC phosphorylation, which contributed to intestinal epithelial barrier dysfunction. These results indicate that Par-3 plays an important role in the modulation of intestinal barrier function by regulating delivery of occludin as well as suppression of MLC phosphorylation.