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 harboring a fluorescent reporter of WNT signaling, we discovered regulators of the intact pathway as well as those selectively required to sustain oncogenic signaling. This study (Lebensohn et al., eLife 2016) comprised seven genome-wide screens systematically interrogating the pathway, including screens for positive, negative, and attenuating regulators of ligand-induced signaling, as well as suppressor screens following disruption of key regulators commonly mutated in WNT-driven tumors, such as the tumor suppressor APC. A comparative analysis of the screens revealed new regulatory mechanisms for ligand reception, signal transduction and transcriptional activation. 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. Since the inception of my laboratory in October of 2018, our specific objective has been to understand new mechanisms of WNT signal transduction independent of changes in beta-catenin levels. Beta-catenin is the main transcriptional co-activator in the WNT pathway, and regulation of beta-catenin abundance by a multi-protein assembly called the destruction complex is considered the principal control point in the WNT signaling cascade. Binding of WNT ligands to their co-receptors triggers inactivation of the destruction complex, which results in accumulation of beta-catenin and expression of WNT target genes. Mutations in components of the destruction complex that lead to aberrant accumulation of beta-catenin are a major driver of tumorigenesis in many types of cancer. Identifying mechanisms that can revert constitutive WNT signaling in cells with compromised destruction complex function is therefore of great therapeutic interest. Genetic suppressor screens are a powerful way to reveal such mechanisms, as they can illuminate intricate connections in signaling networks. We previously reported a comprehensive, comparative forward genetic analysis of WNT signaling using a haploid human cell line containing a fluorescent reporter of WNT activity (Lebensohn et al., eLife 2016). In a suppressor screen to identify genes required for constitutive WNT signaling induced by loss of the destruction complex component casein kinase 1 alpha (CK1alpha), we identified HUWE1. HUWE1 encodes a HECT domain-containing ubiquitin ligase with diverse cellular functions, including regulation of cell proliferation and differentiation, apoptosis, DNA repair, and stress responses. We found that eliminating HUWE1 in cells lacking CK1alpha leads to an 80-90% reduction of WNT target gene expression, but surprisingly, only to a 20-30% reduction of beta-catenin protein abundance (Lebensohn et al., eLife 2016). These results suggest that HUWE1 regulates WNT signaling through additional mechanisms distinct from the control of beta-catenin protein degradation. We are testing this hypothesis and investigating the relevant molecular mechanisms through biochemical, genetic, proteomic and cell biological approaches.