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Abstract

Constitutive activation of the Wnt signaling pathway is a root cause of many colon cancers. Activation of this pathway is caused by genetic mutations that stabilize the beta-catenin protein, allowing it to accumulate in the nucleus and form complexes with any member of the lymphoid enhancer factor (LEF1) and T-cell factor (TCF1, TCF3, TCF4) family of transcription factors (referred to collectively as LEF/TCFs) to activate transcription of target genes. Target genes such as MYC, CCND1, MMP7 and TCF7 (refs. 5-9) are normally expressed in colon tissue, so it has been proposed that abnormal expression levels or patterns imposed by beta-catenin/TCF complexes have a role in tumor progression. We report here that LEF1 is a new type of target gene ectopically activated in colon cancer. The pattern of this ectopic expression is unusual because it derives from selective activation of a promoter for a full-length LEF1 isoform that binds beta-catenin, but not a second, intronic promoter that drives expression of a dominant-negative isoform. beta-catenin/TCF complexes can activate the promoter for full-length LEF1, indicating that in cancer high levels of these complexes misregulate transcription to favor a positive feedback loop for Wnt signaling by inducing selective expression of full-length, beta-catenin-sensitive forms of LEF/TCFs.

A New β-Catenin-dependent Activation Domain in T Cell Factor

“…DNase I Footprinting-Partially purified recombinant LEF-1N and TCF-1E were used in standard DNase I footprinting assays as described previously (20). Equivalent amounts of protein were judged by SDS-PAGE and Coomassie staining.…”

“…Plasmid Construction-The LEF1 promoter luciferase reporter plasmid has been described previously (20). The EVR eukaryotic expression plasmid was used to express LEF and TCF coding sequences (34).…”

“…1C) of whole cell lysates from transfected cells were separated by electrophoresis on SDS-10% polyacrylamide gels. Blots of these gels were probed with either TCF-1 monoclonal antibody (anti-TCF-1, Upstate Biotechnology) or LEF-1N polyclonal rabbit antiserum at a 1:1000 dilution (20).…”

“…For example, although it is not expressed in the normal adult colon, the LEF1 gene is aberrantly expressed in the majority of sporadic colon cancers, in which Wnt signaling is constitutive (20). Although the underlying mechanism of aberrant LEF1 expression is not fully understood, only the promoter for full-length forms of LEF-1 is activated.…”

Activation of β-catenin signaling in differentiated mammary secretory cells induces transdifferentiation into epidermis and squamous metaplasias

“…Upon binding of Wnt proteins to the transmembrane Frizzled receptor, a signaling cascade is activated that leads to the stabilization of ␤-catenin and its interaction with the lymphoid enhancer transcription factors (LEFs) and T cell factors (Tcfs), followed by translocation of the entire complex to the nucleus (2,3). The roles of ␤-catenin in development have been investigated through genetic loss-and gainof-function experiments (4)(5)(6)(7), which demonstrated the importance of ␤-catenin in the determination of the anterior-posterior axis and mesoderm structures during fetal development, and its role in the determination of epidermal cell fate.…”