Nuclear Regulation of Wnt/β-Catenin Signaling: It's a Complex Situation

Wnt signaling is an evolutionarily conserved metazoan cell communication pathway required for proper animal development. Of the myriad of signaling events that have been ascribed to cellular activation by Wnt ligands, the canonical Wnt/β-catenin pathway has been the most studied and best understood. Misregulation of Wnt/β-catenin signaling has been implicated in developmental defects in the embryo and major diseases in the adult. Despite the latter, no drugs that inhibit the Wnt/β-catenin pathway have been approved by the FDA. In this review, we explore the least understood step in the Wnt/β-catenin pathway-nuclear regulation of Wnt target gene transcription. We initially describe our current understanding of the importation of β-catenin into the nucleus. We then focus on the mechanism of action of the major nuclear proteins implicated in driving gene transcription. Finally, we explore the concept of a nuclear Wnt enhanceosome and propose a modified model that describes the necessary components for the transcription of Wnt target genes.

Augmentation of Myc-Dependent Mitotic Gene Expression by the Pygopus2 Chromatin Effector

Mitotic segregation of chromosomes requires precise coordination of many factors, yet evidence is lacking as to how genes encoding these elements are transcriptionally controlled. Here, we found that the Pygopus (Pygo)2 chromatin effector is indispensable for expression of the MYC-dependent genes that regulate cancer cell division. Depletion of Pygo2 arrested SKOV-3 cells at metaphase, which resulted from the failure of chromosomes to capture spindle microtubules, a critical step for chromosomal biorientation and segregation. This observation was consistent with global chromatin association findings in HeLa S3 cells, revealing the enrichment of Pygo2 and MYC at promoters of biorientation and segmentation genes, at which Pygo2 maintained histone H3K27 acetylation. Immunoprecipitation and proximity ligation assays demonstrated MYC and Pygo2 interacting in nuclei, corroborated in a heterologous MYC-driven prostate cancer model that was distinct from Wnt/β-catenin signaling. Our evidence supports a role for Pygo2 as an essential component of MYC oncogenic activity required for mitosis.

Immunohistochemical Mapping of Bcl9 Using Two Antibodies that Recognize Different Epitopes Is Useful to Characterize Juvenile Development of Hepatocellular Carcinoma in Myanmar

B-cell lymphoma 9 (Bcl9) is the core component of Wnt/β-catenin signaling and overexpressed in nuclei of various tumors, including hepatocellular carcinoma (HCC). However, the extent of Bcl9 expression relative to HCC differentiation stage and its functional aspects are poorly understood. In this study, we examined the expression pattern of Bcl9 immunohistochemically, using two anti-Bcl9 antibodies; one was a conventional polyclonal-antibody (anti-Bcl9^ABC) against amino acid no.800-900 of human-Bcl9, while the other (anti-Bcl9^BIO) was against amino acid no.50-200, covering Pygopus-binding sites of Bcl9. Immunohistochemistry using anti-Bcl9^BIO demonstrated distinctive staining in the cytoplasm, while the anti-Bcl9^ABC signal was detected in both cytoplasm and nuclei of HCC cells, reflecting different states of Bcl9 function because Pygopus-binding to Bcl9 is essential to exert its function together with β-catenin in nucleus. Quantitative analysis revealed a significantly higher immunohistochemical-score by anti-Bcl9^BIO in normal liver comparing various differentiation grades of HCC (P < 0.004), whereas no significant difference was noted with anti-Bcl9^ABC. Interestingly, immunohistochemical-score of anti-Bcl9^BIO in patients aged < 40 years was significantly lower than that of ≥ 40 years group (P < 0.01). The results indicated that anti-Bcl9^BIO detected cytoplasmic Bcl9, which does not bind to Pygopus suggesting it could be a useful indicator for development of HCC in young Myanmar patients.

Pax6-dependent, but β-catenin-independent, function of Bcl9 proteins in mouse lens development

Bcl9 and Bcl9l (Bcl9/9l) encode Wnt signaling components that mediate the interaction between β-catenin and Pygo. Cantù et al. find that Bcl9/9l contribute in a Pygo-dependent, but β-catenin-independent, fashion to eye lens formation. Pax6, the master regulator of eye differentiation, directly activates Bcl9 and Bcl9l transcription.

Bcl9 and Bcl9l (Bcl9/9l) encode Wnt signaling components that mediate the interaction between β-catenin and Pygopus (Pygo) via two evolutionarily conserved domains, HD1 and HD2, respectively. We generated mouse strains lacking these domains to probe the β-catenin-dependent and β-catenin-independent roles of Bcl9/9l and Pygo during mouse development. While lens development is critically dependent on the presence of the HD1 domain, it is not affected by the lack of the HD2 domain, indicating that Bcl9/9l act in this context in a β-catenin-independent manner. Furthermore, we uncover a new regulatory circuit in which Pax6, the master regulator of eye development, directly activates Bcl9/9l transcription.

The Pygo2-H3K4me2/3 interaction is dispensable for mouse development and Wnt signaling-dependent transcription

Pygopus has been discovered as a fundamental Wnt signaling component in Drosophila. The mouse genome encodes two Pygopus homologs, Pygo1 and Pygo2. They serve as context-dependent β-catenin coactivators, with Pygo2 playing the more important role. All Pygo proteins share a highly conserved plant homology domain (PHD) that allows them to bind di- and trimethylated lysine 4 of histone H3 (H3K4me2/3). Despite the structural conservation of this domain, the relevance of histone binding for the role of Pygo2 as a Wnt signaling component and as a reader of chromatin modifications remains speculative. Here we generate a knock-in mouse line, homozygous for a Pygo2 mutant defective in chromatin binding. We show that even in the absence of the potentially redundant Pygo1, Pygo2 does not require the H3K4me2/3 binding activity to sustain its function during mouse development. Indeed, during tissue homeostasis, Wnt/β-catenin-dependent transcription is largely unaffected. However, the Pygo2-chromatin interaction is relevant in testes, where, importantly, Pygo2 binds in vivo to the chromatin in a PHD-dependent manner. Its presence on regulatory regions does not affect the transcription of nearby genes; rather, it is important for the recruitment of the histone acetyltransferase Gcn5 to chromatin, consistent with a testis-specific and Wnt-unrelated role for Pygo2 as a chromatin remodeler.