The SIAH E3 ubiquitin ligases promote Wnt/β-catenin signaling through mediating Wnt-induced Axin degradation

Abstract

The Wnt/β-catenin signaling pathway plays essential roles in embryonic development and adult tissue homeostasis. Axin is a concentration-limiting factor responsible for the formation of the β-catenin destruction complex. Wnt signaling itself promotes the degradation of Axin. However, the underlying molecular mechanism and biological relevance of this targeting of Axin have not been elucidated. Here, we identify SIAH1/2 (SIAH) as the E3 ligase mediating Wnt-induced Axin degradation. SIAH proteins promote the ubiquitination and proteasomal degradation of Axin through interacting with a VxP motif in the GSK3-binding domain of Axin, and this function of SIAH is counteracted by GSK3 binding to Axin. Structural analysis reveals that the Axin segment responsible for SIAH binding is also involved in GSK3 binding but adopts distinct conformations in Axin/SIAH and Axin/GSK3 complexes. Knockout of SIAH1 blocks Wnt-induced Axin ubiquitination and attenuates Wnt-induced β-catenin stabilization. Our data suggest that Wnt-induced dissociation of the Axin/GSK3 complex allows SIAH to interact with Axin not associated with GSK3 and promote its degradation and that SIAH-mediated Axin degradation represents an important feed-forward mechanism to achieve sustained Wnt/β-catenin signaling.

Feedback control of Wnt signaling based on histidine cluster co-aggregation between Naked/NKD and Axin

“…The notion of Naked/NKD targeting its binding partners for autophagy is also consistent with earlier reports that Dishevelled can be degraded by autophagy via binding to p62 (Gao et al, 2010;Ma et al, 2015;Zhang et al, 2011b). Indeed, there appears to be an interaction network between Dishevelled, p62, Axin and aggregated Naked/NKD, implicating Naked/NKD HisC cores as adaptors between Wnt signalosomes and autophagy receptors, as an alternative route to the SIAH pathway (Ji et al, 2017) for the disposal of Axin and other signalosome components upon Wnt signalling. Since this route would involve the envelopment of Naked/NKD co-aggregates by autophagic membranes (Kirkin et al, 2009), it is worth noting that these co-aggregates may not inevitably reach lysosomes because of the endosomolytic properties of highly concentrated histidines (155-265 histidines per single Nkd1-Axin1 co-aggregate): these histidines are expected to become protonated under the mildly acidic conditions typical for endocytic compartments, which could cause swelling and rupture of autophagosomal membranes (Lo and Wang, 2008), thereby potentially allowing escape and recycling of Naked/NKD coaggregates.…”

“…The reduction of β-catenin signaling in NKD-deficient cells coincides roughly with the half-time (t1/2) of the known Wnt-induced destabilization of Axin1 (Ji et al, 2017;Yamamoto et al, 1999).…”

“…S8) are consistent with precocious termination of Wnt signaling by Axin which, as a result of its stabilization, resumes assembling active β-catenin destruction complexes prematurely. It therefore seems that Naked/NKD acts similarly to the ubiquitin E3 ligase SIAH which targets Axin for proteasomal degradation upon Wnt signaling (Ji et al, 2017). Indeed, the two factors may function redundantly to destabilize Axin, thereby maintaining Wnt signaling responses by preventing Axin from reestablishing quiescence too early.…”

“…It is therefore imperative that the levels of Dishevelled and Axin are tightly controlled in receiving cells. Indeed, Dishevelled and Axin are each recognized by specific ubiquitin E3 ligases that allow their individual targeting for proteasomal degradation (Angers et al, 2006;de Groot et al, 2014;Ji et al, 2017;Mund et al, 2015;Wei et al, 2012;Zhang et al, 2014;Zhang et al, 2011a). In addition, Dishevelled is also targeted for degradation by autophagy via association with the autophagy receptor p62 and the ubiquitin-like proteins LC3 or GABARAPL (Gao et al, 2010;Ma et al, 2015;Zhang et al, 2011b).…”

The role of ubiquitination in tumorigenesis and targeted drug discovery

“…For example, the E3 ligase RNF146 promotes ubiquitination-mediated proteasomal degradation of Axin on the basis of the PARsylation of Axin, 309,310 while SIAH binds to Axin and mediates its degradation, which amplifies the feedback regulation of the Wnt signaling pathway. 311 Unlike the degradation signal, SMURF1 inhibits its interaction with LRP5/6 by mediating the K29-linked polyubiquitination of Axin and negatively regulates the Wnt signaling pathway. 312 Notably, UPS also regulates the ubiquitination of different components in the Wnt signaling pathway.…”