New regulatory mechanisms of WNT signaling in development, stem cells and cancer

Division Of Basic Sciences - Nci

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Abstract

Through forward genetics screens in haploid human cells, we discovered new regulators of ligand-induced WNT signaling and regulators selectively required to sustain hyperactive, oncogenic WNT signaling (Lebensohn et al., eLife 2016). Our current goal is to elucidate the molecular underpinnings of these new regulatory mechanisms, understand their physiological functions and evaluate their potential as therapeutic targets. The following sub-projects are aimed at elucidating some of these new regulatory mechanisms and evaluating their therapeutic potential. 1) The transcription factor TFAP4 is a new positive regulator of WNT signaling (Lebensohn et al., eLife 2016). We found that TFAP4 regulates b-catenin abundance, the principal transcriptional co-activator in the WNT pathway, and is a limiting component for WNT/b-catenin signaling activity. Furthermore, excess TFAP4 can promote ectopic activation of WNT signaling during Xenopus laevis development, causing the formation of a secondary body axis. Interestingly, TFAP4 expression is tightly linked to malignancy in gastrointestinal cancers. We are dissecting the molecular mechanism by which TFAP4 regulates b-catenin and evaluating its potential as a therapeutic target. 2) A new function of the b-catenin destruction complex regulating WNT signaling through the ubiquitin ligase HUWE1. HUWE1 is a HECT-domain ubiquitin ligase involved in dozens of cellular processes through the ubiquitination of diverse substrates. In our unbiased forward genetic screen for mediators of hyperactive WNT signaling induced by loss of the b-catenin destruction complex kinase casein kinase 1a, we identified HUWE1 as a positive regulator of the WNT pathway (Lebensohn et al., eLife 2016). We have now found that HUWE1 promotes WNT/b-catenin signaling by regulating both b-catenin abundance (McKenna et al. bioRxiv 2024) and sub-cellular localization. The second mechanism is independent of the control of b-catenin protein stability and can shift the balance between two distinct cellular functions of b-catenin - one as a transcriptional co-activator in the WNT pathway and the other as a structural component of adherens junctions involved in cell-cell adhesion. Control of b-catenin localization and WNT signaling by HUWE1 requires its ubiquitin ligase activity and a subset of b-catenin destruction complex components. These results reveal a new role for some destruction complex components in regulating b-catenin sub-cellular localization and WNT signaling downstream of HUWE1, distinct from their established activity in controlling b-catenin stability. We are further investigating the molecular mechanisms by which HUWE1, acting through the destruction complex, regulates b-catenin subcellular localization and function.