Regulation of tight junctions during the epithelium-mesenchyme transition:direct repression of the gene expression of claudins/occludin by Snail

TLDR

Snail is a transcriptional repressor that drives epithelial‑mesenchymal transition and causes loss of epithelial polarity, while claudins and occludin are tight‑junction proteins essential for maintaining that polarity. The study aimed to determine how Snail regulates the promoter activity of claudins and occludin. Overexpression of Snail in cultured mouse epithelial cells induced EMT and repressed claudin and occludin expression at both protein and mRNA levels, and promoter analysis revealed multiple E‑box motifs. Snail binds directly to these E‑boxes, completely silencing claudin/occludin promoter activity, and EMT is accompanied by concurrent repression of E‑cadherin and tight‑junction genes.

Abstract

Snail is a transcription repressor that plays a central role in the epithelium-mesenchyme transition (EMT), by which epithelial cells lose their polarity. Claudins and occludin are integral membrane proteins localized at tight junctions, which are responsible for establishing and maintaining epithelial cell polarity. We examined the relationship between Snail and the promoter activity of claudins and occludin. When Snail was overexpressed in cultured mouse epithelial cells, EMT was induced with concomitant repression of the expression of claudins and occludin not only at the protein but also at the mRNA level. We then isolated the promoters of genes encoding claudins and occludin, in which multiple E-boxes were identified. Transfection experiments with various promoter constructs as well as electrophoretic mobility assays revealed that Snail binds directly to the E-boxes of the promoters of claudin/occludin genes, resulting in complete repression of their promoter activity. Because the gene encoding E-cadherin was also reported to be repressed by Snail, we concluded that EMT was associated with the simultaneous repression of the genes encoding E-cadherin and claudins/occludin (i.e. the expression of adherens and tight junction adhesion molecules, respectively).