A β-catenin survival signal is required for normal lobular development in the mammary gland

Abstract

Research Article 1138 development has not been evaluated. However, a role for Wnt-4 has been suggested by rescue of Wnt-4-null mammary epithelial cells (MECs) by transplantation into the cleared fat pads of wild-type recipients (Brisken et al., 2000). In these studies, a delay in lobuloalveolar development was observed at mid-pregnancy, but by day 1 of lactation no differences were observed between the outgrowths of wild-type and Wnt-4 null MECs, presumably because of compensation by other family members. This problem of Wnt redundancy has been addressed in the current study by examining the requirement for β-catenin-mediated signalling as a convergence point for the canonical Wnt signal transduction pathway.β-Catenin, the vertebrate orthologue of Drosophila Armadillo (McCrea et al., 1991;Peifer et al., 1991), is a multifunctional protein, characterized by a stretch of arm repeats that are the sites of multiple protein-protein interactions (Huber et al., 1997;Peifer et al., 1994). β-Catenin binds to Ecadherin at the adherens junctions, modulates cadherindependent cell-cell adhesion (Barth et al., 1997;Steinberg and McNutt, 1999) and links the cadherin/catenin complex to the cortical actin cytoskeleton through the binding of α-catenin (Herrenknecht et al., 1991;Nagafuchi and Tsukita, 1994). Additionally, β-catenin plays a crucial role in the canonical Wnt signalling cascade. The intracellular Wnt signal is propagated from the membrane through Dishevelled (Yanagawa et al., 1995) to downregulate glycogen synthase kinase-3β (GSK-3β) and subsequently disrupt a protein complex that includes GSK-3β, adenomatous polyposis coli (APC), axin, and members of the ubiquitination/proteasome pathway (Easwaran et al., 1999a;Ikeda et al., 1998;Kikuchi, 1999;Kishida et al., 1998;Rubinfeld et al., 1996;Salomon et al., 1997). Disruption of this complex prevents the GSK-3β-dependent phosphorylation of β-catenin on specific N-terminal serine and threonine residues, and thus protects β-catenin protein from degradation through ubiquitin-mediated proteolysis (Easwaran et al., 1999a;Rubinfeld et al., 1996). The stabilized β-catenin protein can then be transported to the nucleus where it forms complexes with members of the T-cell factor (TCF)/Lef family of HMG-box transcription factors (Behrens et al., 1996; Huber et al., 1996;Molenaar et al., 1996). Together, β-catenin and TCF proteins comprise a bipartite transcripton factor in which TCF supplies the DNA binding moiety and β-catenin provides the transactivation domain (reviewed by Barker et al., 2000). This complex activates the transcription of target genes and, in some cases, relieves the repression activity of TCF alone (reviewed by Bienz, 1998). Thus, β-catenin plays crucial roles in both epithelial cell-cell adhesion, as well as in signal transduction.Additionally, β-catenin is a point of intersection and integration of several other signalling pathways. For example, the retinoic acid receptor RAR binds to β-catenin in a retinoic acid ligand-dependent manner, not only sequestering β...

Dissecting the roles of β-catenin and cyclin D1 during mammary development and neoplasia

“…Early divisions in the alveolar lineage involve stimulation of a ␤-catenin target that is not cyclin D1, whereas later divisions, and possibly further differentiation, depend completely on cyclin D1, which is up-regulated in response to ␤-catenin (2). The findings that mice expressing suppressors of ␤-catenin cannot sustain alveologenesis in cyclin D1 ϩ/ϩ mice (3,4) and that cyclin D1 Ϫ/Ϫ glands cannot progress even when ␤-catenin levels are augmented indicate that both stimuli are essential and act sequentially during pregnancy (Fig. 5).…”

“…These data suggest that ␤-catenin signaling determines alveolar cell fate, survival, and͞or proliferation. This concept is supported by the effects of overexpression of ␤-catenin activators (Wnt 1, 3, and 10b) and suppressors (axin and a dominant-negative ␤-catenin construct ␤-engrailed), which stimulate and impair alveolar development, respectively (3)(4)(5).…”

“…In addition, inappropriate up-regulation of cyclin D1 mRNA levels accompanies the various phenotypes induced in mice by MMTV-⌬N89␤-catenin. Moreover, cyclin D1-null mice and mice expressing the negative regulators of ␤-catenin signaling axin or ␤-engrailed show similar impairments in alveolar development (3,4,(7)(8)(9). Finally, in humans coincident elevations of ␤-catenin and cyclin D1 have been demonstrated in breast cancer and correlated with poor clinical outcome (10).…”

A noncoding RNA is a potential marker of cell fate during mammary gland development

“…HC11 cells (24), a clonal cell line of COMMA-D, were routinely maintained as previously described (37). For experiments requiring synchronization, normal growth medium was replaced with growth arrest medium (RPMI medium 1640 supplemented with 0.1% bovine calf serum).…”

“…Proliferation was monitored by BrdU staining as described (38). Apoptosis was detected by TUNEL staining (37) or by immunofluorescence using an antibody specific to the cleaved (active) form of caspase-3 (Santa Cruz Biotechnology).…”