Molecular regulation and pharmacological targeting of the ß-catenin destruction complex.

The ß-catenin destruction complex is a dynamic cytosolic multiprotein assembly that provides a key node in Wnt signalling regulation. The core components of the destruction complex comprise the scaffold proteins axin and adenomatous polyposis coli and the Ser/Thr kinases casein kinase 1 and glycogen synthase kinase 3. In unstimulated cells, the destruction complex efficiently drives degradation of the transcriptional coactivator ß-catenin, thereby preventing the activation of the Wnt/ß-catenin pathway. Mutational inactivation of the destruction complex is a major pathway in the pathogenesis of cancer. Here, we review recent insights in the regulation of the ß-catenin destruction complex, including newly identified interaction interfaces, regulatory elements and post-translationally controlled mechanisms. In addition, we discuss how mutations in core destruction complex components deregulate Wnt signalling via distinct mechanisms and how these findings open up potential therapeutic approaches to restore destruction complex activity in cancer cells. LINKED ARTICLES: This article is part of a themed section on WNT Signalling: Mechanisms and Therapeutic Opportunities. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.24/issuetoc.

Axin cancer mutants form nanoaggregates to rewire the Wnt signaling network.

Signaling cascades depend on scaffold proteins that regulate the assembly of multiprotein complexes. Missense mutations in scaffold proteins are frequent in human cancer, but their relevance and mode of action are poorly understood. Here we show that cancer point mutations in the scaffold protein Axin derail Wnt signaling and promote tumor growth in vivo through a gain-of-function mechanism. The effect is conserved for both the human and Drosophila proteins. Mutated Axin forms nonamyloid nanometer-scale aggregates decorated with disordered tentacles, which 'rewire' the Axin interactome. Importantly, the tumor-suppressor activity of both the human and Drosophila Axin cancer mutants is rescued by preventing aggregation of a single nonconserved segment. Our findings establish a new paradigm for misregulation of signaling in cancer and show that targeting aggregation-prone stretches in mutated scaffolds holds attractive potential for cancer treatment.

Lgr5-expressing cells are sufficient and necessary for postnatal mammary gland organogenesis.

Mammary epithelial stem cells are vital to tissue expansion and remodeling during various phases of postnatal mammary development. Basal mammary epithelial cells are enriched in Wnt-responsive cells and can reconstitute cleared mammary fat pads upon transplantation into mice. Lgr5 is a Wnt-regulated target gene and was identified as a major stem cell marker in the small intestine, colon, stomach, and hair follicle, as well as in kidney nephrons. Here, we demonstrate the outstanding regenerative potential of a rare population of Lgr5-expressing (Lgr5(+)) mammary epithelial cells (MECs). We found that Lgr5(+) cells reside within the basal population, are superior to other basal cells in regenerating functional mammary glands (MGs), are exceptionally efficient in reconstituting MGs from single cells, and exhibit regenerative capacity in serial transplantations. Loss-of-function and depletion experiments of Lgr5(+) cells from transplanted MECs or from pubertal MGs revealed that these cells are not only sufficient but also necessary for postnatal mammary organogenesis.