Ubiquitylation of Nuclear Factors in Canonical Wnt-Mediated Transcription

Abstract

The Wnt-beta-catenin signal transduction pathway is essential for animal development and adult tissue homeostasis. Wnt signaling promotes stabilization of beta-catenin, which translocates to the nucleus and binds to TCF/LEF transcription factors and other cofactors to initiate a Wnt transcriptional program. For beta-catenin to regulate transcription of Wnt target genes, the transcriptional corepressor Groucho (also known as TLE) must be displaced on TCF and the chromatin structure of target promoters must be remodeled into a permissive state to allow binding of transcription factors. This chromatin remodeling process is facilitated by the SWItch/Sucrose Non-Fermentable (SWI/SNF) complex. The SWI/SNF complex displaces nucleosomes to promote the access of transcription factors to enhancers and promoters. Through multiple screening processes, we identified the deubiquitylase USP47 and the ubiquitin ligase Thyroid Hormone Receptor Interactor 12 (TRIP12) as positive regulators of Wnt signaling. We found that both USP47 and TRIP12 were required for Wnt signaling under multiple physiological contexts, indicating evolutionary conservation. In human cells, knockdown of USP47 and TRIP12 inhibited Wnt reporter activity and both proteins acted downstream of the beta-catenin destruction complex. USP47 interacted with TLE3, and TRIP12 interacted with Brahma-related gene-1 (BRG1), the central catalytic subunit of the SWI/SNF complex. However, neither USP47 nor TRIP12 altered the stability of their substrate proteins. Deubiquitylation of TLE by USP47 enhances the ability of beta-catenin to cycle on and off TCF, thereby helping to ensure that the expression of Wnt target genes continues only as long as the upstream signal is present. TRIP12 ubiquitylates BRG1 in the presence of Wnt and promotes its interaction with beta-catenin, thereby bringing SWI/SNF to Wnt target genes. Our data suggest a mechanism by which regulated ubiquitylation and deubiquitylation of transcriptional cofactors enhances the ability of beta-catenin to bind TCF and promote a Wnt transcriptional program.