E3 ubiquitin ligase Mule targets β-catenin under conditions of hyperactive Wnt signaling

Abstract

Wnt signaling, named after the secreted proteins that bind to cell surface receptors to activate the pathway, plays critical roles both in embryonic development and the maintenance of homeostasis in many adult tissues. Two particularly important cellular programs orchestrated by Wnt signaling are proliferation and stem cell selfrenewal. Constitutive activation of the Wnt pathway resulting from mutation or improper modulation of pathway components contributes to cancer development in various tissues. Colon cancers frequently bear inactivating mutations of the adenomatous polyposis coli (APC) gene, whose product is an important component of the destruction complex that regulates β-catenin levels. Stabilization and nuclear localization of β-catenin result in the expression of a panel of Wnt target genes. We previously showed that Mule/ Huwe1/Arf-BP1 (Mule) controls murine intestinal stem and progenitor cell proliferation by modulating the Wnt pathway via c-Myc. Here we extend our investigation of Mule's influence on oncogenesis by showing that Mule interacts directly with β-catenin and targets it for degradation under conditions of hyperactive Wnt signaling. Our findings suggest that Mule uses various mechanisms to finetune the Wnt pathway and provides multiple safeguards against tumorigenesis.β-catenin | Mule | Wnt signaling | stem cells | colorectal cancer A ctivation of the Wnt pathway, named after the secreted proteins that bind to cell surface receptors to activate the pathway, plays a critical role in stem cell self-renewal, and thus the homeostasis of normal mammalian tissues. However, constitutive activation of Wnt signaling has been implicated in a broad range of cancers, including melanoma, hepatocellular carcinoma, and cancers of the prostate, thyroid, ovary, and breast (1-3). Perhaps the best-studied malignancy arising from the loss of regulation of the Wnt signaling pathway is colorectal cancer, which develops through a multistage process driven by the progressive accumulation of genetic mutations (4). The key initial activating mutation occurs in components of the Wnt pathway, but most commonly in APC.Canonical Wnt signaling involves a relay of protein interactions that serve to transmit a signal from the extracellular space to the plasma membrane. This signal is amplified in the cytoplasm and then directed to the nucleus to culminate in the activation of the Wnt target gene program. In molecular terms, when the glycoprotein Wnt is secreted into the extracellular space, it binds to a cell's Frizzled receptors in conjunction with the low-density lipoprotein receptorrelated proteins 5 and 6 (LRP5/6). The engagement of these receptors recruits the scaffolding protein Dishevelled (Dvl), leading to LRP5/6 phosphorylation and consequent recruitment of the scaffolding protein, Axin2 and glycogen synthase kinase 3 beta (GSK-3β). This recruitment results in disruption of the "destruction complex" that contains APC and normally binds to the signal transduction protein β-catenin to trigger its degradation. On Wn...

USP20 positively regulates tumorigenesis and chemoresistance through β-catenin stabilization

“…For example, TRIM33 ubiquitinated β-catenin and induced its degradation in a GSK-3β or β-TrCP-independent manner [33]. Another E3 ligase, Mule directly ubiquitinated non phosphorylated β-catenin in vitro [34]. Furthermore, depletion of Mule upregulated β-catenin level in HCT116 cell (β-catenin phosphorylation mutant in one allele) and APC min intestinal organoids [34].…”

“…Another E3 ligase, Mule directly ubiquitinated non phosphorylated β-catenin in vitro [34]. Furthermore, depletion of Mule upregulated β-catenin level in HCT116 cell (β-catenin phosphorylation mutant in one allele) and APC min intestinal organoids [34]. We next tested whether USP20 has an effect on β-catenin ubiquitination in an APCand GSK3β-β-TrCP-independent manner.…”

Histone methyltransferase SETD2 modulates alternative splicing to inhibit intestinal tumorigenesis

“…The Wnt pathway involves various feedback loops that balance the opposing processes of cell proliferation and differentiation. Studies indicate that, even in the presence of heterozygousactivating mutations downstream of the FRZ receptor, mutationdriven Wnt-signaling activation can be further enhanced in APC min/+ mice and cells (40)(41)(42)(43)(44)(45). Our results highlighted that SETD2 fine-tuned Wnt signaling to safeguard ISCs or progenitor cells in the intestine, whereas downregulation facilitated inherently more proliferative and progenitor traits in ISCs in the Apc-mutated background ( Figure 8E).…”

Mesenchymal Stem Cells Promote Hepatocarcinogenesis via lncRNA–MUF Interaction with ANXA2 and miR-34a

“…Through RNA pulldown and mass spectrometry analyses, ANXA2 was found to bind with lncRNA-MUF, and the binding led to upregulated b-catenin and the EMT process. Upregulation of b-catenin occurs in several cancers such as colorectal cancer (41), breast cancer (42), and liver cancer (43). Canonical Wnt signaling depends on b-catenin/ GSK-3b complex conformation and phosphorylation by GSK-3b, leading to b-catenin degradation (44,45).…”