The various roles of ubiquitin in Wnt pathway regulation

Abstract

Wnt signaling mediates key developmental and homeostatic processes including stem cell maintenance, growth and cell fate specification, cell polarity and migration. Inappropriate activation of Wnt signaling is linked to a range of human disorders, most notably cancer and neurodegenerative diseases. In the Wnt/β-catenin cascade, signaling events converge on the regulation of ubiquitin-mediated degradation of the crucial transcriptional regulator β-catenin. The emerging mechanisms by which ubiquitin modification of proteins controls cellular pathways comprise both proteolytic and nonproteolytic functions. In nonproteolytic functions, ubiquitin acts as a signaling device in the control of protein activity, subcellular localization and complex formation. Here, we review and discuss recent developments that implicate ubiquitin-mediated mechanisms at multiple steps of Wnt pathway activation.

Figure 1

Ubiquitin-mediated regulation of the core Wnt pathway components APC, Axin and β-catenin. Binding of Wnt to its receptors Frizzled and Lrp5/6 induces recruitment of cytoplasmic proteins including Dvl and Axin. The resulting signaling complex inhibits the constitutive degradation of β-catenin, leading to its translocation to the nucleus (N) and activation of Tcf/Lef target genes. In the absence of Wnt, cytoplasmic β-catenin is phosphorylated by the Axin-APC-GSK3β-CK1 complex. Phospho-β-catenin is subjected to SCF^β-TrCP E3 ligase-mediated polyubiquitylation and proteasomal degradation. Both Axin and APC can undergo ubiquitin-mediated proteolysis and regulate each other's stability. Axin ubiquitylation and degradation is promoted by phosphorylation and poly-ADP-ribosylation (PARsylation). The CSN, which binds and stimulates SCF^β-TrCP activity, includes DUB enzyme Usp15 that protects APC from degradative ubiquitylation. An alternative Wnt-responsive degradation pathway for cytosolic β-catenin is mediated by Jade-1, a ubiquitin ligase that is found primarily in the nucleus and that depends on the tumor suppressor pVHL. Independent of Wnt signaling, cytosolic β-catenin levels are regulated by p53-activated Siah-1 in response to DNA damage. The pool of β-catenin that is associated to adherence junctions (AJ) can also be targeted for ubiquitin-mediated degradation, the involved E3 ligases are Hakai and Ozz-E3. DUB enzyme Trabid is a positive regulator of downstream Wnt-induced transcription and removes K63-linked polyubiquitylation of APC. Filled arrowheads point out enzyme-substrate connection; open arrowheads: positive regulation. Dashed lines indicate protein movement.

Figure 2

Ubiquitin in the regulation of upstream Wnt/β-catenin signaling. Upstream of the Axin-APC complex, Wnt signaling is regulated by several ubiquitin-mediated mechanisms. The cell surface levels of the receptors are a balance between biosynthesis and degradation. Monoubiquitylation and palmitolylation regulate Lrp6 exit from the endoplasmic reticulum (ER). Frizzled monoubiquitylation drives internalization into endosomal compartments (E) and degradation in lysosomes (L). DUB enzyme UBPY was found to inhibit lysosomal degradation of Fz and stimulate its recycling back to the plasma membrane (PM). The Fz-binding protein Dvl is regulated by ubiquitylation in at least two different ways. The E3 ligases KLHL12, NEDL1 and inversin are negative regulators of Wnt/β-catenin signaling by targeting Dvl for K48-linked ubiquitylation and subsequent degradation. Dvl can also be positively regulated by ubiquitylation through K63-linked polyubiquitylation of its DIX domain. Tumor suppressor and DUB enzyme CYLD negatively regulates Wnt/β-catenin signaling via inhibition of Dvl ubiquitylation.

Figure 3

Wnt signaling components regulate ubiquitylation. Axin acts as a subunit of a larger complex that stimulates SCF^Fbw7 E3 ligase-mediated degradation of c-Myc. Axin binds and coordinates MAP3K1 E3 ligase activity to drive β-catenin/TCF-mediated transcription. APC acts as a subunit of a SCF-like E3 ligase that includes Siah-1 and destabilizes cytosolic β-catenin upon genotoxic stress. In planar cell polarity signaling, asymmetric Prickle distribution is facilitated by its local ubiquitin-mediated proteolysis, through the action of a Fz-Dvl-Par-Smurf complex. Filled arrowheads point out enzyme-substrate; open arrowheads: positive regulation. Dashed lines indicate protein movement.