Symplekin is multifunctional protein localized to both the tight junction and the nucleus in polarized epithelial cells with known roles in mRNA polyadenylation, proliferation, differentiation and tumorigenesis. Our previous studies confirmed that symplekin participates in the assembly of tight junctions and determines the epithelial monolayers permeability as well as cellular polarity. We also observed deceased expression of symplekin in premalignant and malignant hepatocytes. However, the nuclear functions symplekin and its regulation mechanisms, including related signal pathways have not been systematically investigated. Besides, the expression regulation of symplekin has not been studied yet. In the present study, by the use of dedifferentiated culture of epithelial cells and symplekin depleted stable cell lines, we have demonstrated that nuclear accumulation of symplekin increased in dedifferentiated cells with up-regulated proliferation rate. The mRNA profile analysis suggested symplekin predominantly participated in cell cycle progression and regulated expression of many cyclins and associated CDKs. Moreover, symplekin could be co-immunoprecipitated with ERK1/2 and its phosphorylated level was higher in the nuclear, indicating symplekin might be a substrate for ERK1/2 kinase. Partial EMT features could be detected in symplekin depleted cells, which also verified the significance of symplekin in maintaining epithelial junctions and cell polarity. Furthermore, decreased expression of symplekin was observed in hepatocellular carcinomas. An 18 bp AT-rich sequence was newly found in the promoter with reduced transcriptional activity, and its heterozygous or homozygous status was related to the expressions of symplekin in cultured cells and hepatic tumor tissues. We also found opposite expression pattern of symplekin and miR-124 in vivo and in vitro, and the treatment of miR-124 mimics impaired symplekin expression in hepatic cells. These results suggest that expression of symplekin in hepatocellular carcinomas was regulated by the alternation of its promoter and microRNAs. In another aspect, we developed several new in vitro methods to study the relationships between H. pylori and the gastric epithelial cells using Boyden Chambers. We found that H. pylori infection could repress expression of different adhesion molecules and MUC1, and direct contact of H. pylori with the apical membrane of the cells resulted in the greatest increase in permeability compared to basal membrane binding or non-binding of H. pylori to the cells according to the degree of damage at the tight junctions. These results strongly indicate that H. Pylori dwelling on the apical surface of the gastrointestinal epithelium could directly induce serious injury of the mucosal barrier. Key words: symplekin, tight junction, cell differentiation, cell cycle, promoter, miR-124, H. pylori, mucosal barrier
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