DIRECT EVIDENCE FOR A ROLE OF β‐CATENIN/LEF‐1 SIGNALING PATHWAY IN INDUCTION OF EMT

Epithelial‐mesenchymal transformation (EMT) is an important process in development that is characterized by loss of E‐cadherin, β‐catenin relocalization, and acquisition of elongated cell shape and ability to invade ECM. β‐catenin has been shown to activate LEF‐1 transcription during EMT induced in vitro by c‐Fos. Here, we ask whether or not LEF‐1 directly introduced into epithelial cells in an adenovirus construct can induce EMT. In normal epithelial cell lines, such as HCE and MDCK cells, that contain functional APC, nuclear β‐catenin induced by exogenous LEF‐1 is rapidly exported and EMT is not induced. Leptomycin‐B blocks β‐catenin nuclear export, but no EMT occurs due to toxicity. Addition of Wnt‐1 to normal epithelial cell lines stabilizes cytoplasmic β‐catenin that LEF‐1 then transports to nuclei, causing a small amount of EMT. Our experiments demonstrated, however, that overexpressed LEF‐1 upregulates nuclear β‐catenin and promotes dramatic EMT in DLD‐1 epithelial tumors that retain nuclear β‐catenin. This EMT is reversible if the LEF‐1 virus is removed. Thus, our results demonstrate that LEF‐1 can induce EMT directly when its transcription activity is activated by stable nuclear β‐catenin. Normal adult epithelial cells appear to use APC to keep β‐catenin out of the nucleus, thereby avoiding pathologies such as metastases due to LEF/β‐catenin‐induced EMT.

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