Chromatin-specific regulation of LEF-1-β-catenin transcription activation and inhibition in vitro

Transcriptional activation of Wnt/Wg-responsive genes requires the stabilization and nuclear accumulation of beta -catenin, a dedicated coactivator of LEF/TCF enhancer-binding proteins. Here we report that recombinant beta -catenin strongly enhances binding and transactivation by LEF-1 on chromatin templates in vitro. Interestingly, different LEF-1 isoforms vary in their ability to bind nucleosomal templates in the absence of beta -catenin, owing to N-terminal residues that repress binding to chromatin, but not nonchromatin, templates. Transcriptional activation in vitro requires both the armadillo (ARM) repeats and the C terminus of beta -catenin, whereas the phosphorylated N terminus is inhibitory to transcription. A fragment spanning the C terminus (CT) and ARM repeats 11 and 12 (CT-ARM), but not the CT alone, functions as a dominant negative inhibitor of LEF-1-beta -cat activity in vitro and can block ATP-dependent binding of the complex to chromatin. LEF-1-beta -cat transactivation in vitro was also repressed by inhibitor of beta -catenin and Tcf-4 (ICAT), a physiological inhibitor of Wnt/Wg signaling that interacts with ARM repeats 11 and 12, and by the nonsteroidal anti-inflammatory compound, sulindac. None of these transcription inhibitors (CT-ARM, ICAT, or sulindac) could disrupt the LEF-1-beta -cat complex after it was stably bound to chromatin. We conclude that the CT-ARM region of beta -catenin functions as a chromatin-specific activation domain, and that several inhibitors of the Wnt/Wg pathway directly modulate LEF-1-beta -cat activity on chromatin.