pVHL regulates protein stability of the TCF/LEF transcription factor family via ubiquitin-independent proteasomal degradation

The Wnt/β-catenin signaling pathway plays key roles in development and adult tissue homeostasis by controlling cell proliferation and cell fate decisions. TCF/LEF transcription factors play a pivotal role in this pathway, acting as repressors by recruiting co-repressors in the absence of Wnt signals, and as activators via β-catenin binding in the presence of Wnt signaling. While progress has been made in our understanding of Wnt signaling regulation, the underlying mechanism that regulates the protein stability of the TCF/LEF family is far less clear. Using cultured cells and zebrafish as in vitro and in vivo models, we demonstrated that the von Hippel-Lindau protein (pVHL), the substrate recognition component of an E3 ubiquitin ligase complex, regulates the stability of TCF/LEF proteins. Unexpectedly, pVHL directly binds to TCF/LEF and promotes their proteasomal degradation independent of its E3 ubiquitin ligase activity. Notably, a human homologue of pVHL, the pVHL-like protein (pVHLL), which lacks the capability to assemble an E3 ligase complex with Elongin B/C, RBX1, and CUL2, similarly downregulates TCF/LEF protein levels. Knockout of vhl in zebrafish embryos leads to a reduction of dorsal habenular neurons and this effect is upstream of dorsal habenular neurons phenotype in tcf7l2-null mutants. Our study uncovers a previously unknown mechanism for the protein stability regulation of TCF/LEF transcription factors and demonstrates that pVHL contains a 26S proteasome binding domain that drives ubiquitin-independent proteasomal degradation. These findings provide new insights into the ubiquitin-independent function of pVHL and uncover a novel mechanistic regulation of Wnt/β-catenin signaling.