A considerable body of circumstantial data suggests that cyclin D1 is an attractive candidate to mediate the effects of ␤-catenin in mammary tissue. To test the functional significance of these correlative findings, we investigated the genetic interaction between transcriptionally active ␤-catenin (⌬N89␤-catenin) and its target gene cyclin D1 in the mouse mammary gland during pubertal development, pregnancy, and tumorigenesis. Our data demonstrate that cyclin D1 is dispensable for the ⌬N89␤-catenin-stimulated initiation of alveologenesis in virgin females, for the de novo induction of alveoli in males, and for the formation of tumors. Indeed, lack of cyclin D1 accentuates and enhances these hyperplastic and tumorigenic ⌬N89␤-catenin phenotypes. Although alveologenesis is initiated by ⌬N89␤-catenin in a cyclin D1-independent fashion, up-regulation of cyclin D1 occurs in ⌬N89␤-catenin mice and its expression remains essential for the completion of alveolar development during the later stages of pregnancy. Thus, alveologenesis is a two-step process, and cyclin D1 activity during late alveologenesis cannot be replaced by the activity of other ␤-catenin target genes that successfully drive proliferation at earlier stages. mammary tumor ͉ Wnt ͉ cell adhesion ͉ cell cycle ͉ cadherins T he protein ␤-catenin is a multifunctional intracellular protein. In adherent cells it links cadherin cell-adhesion molecules to the actin cytoskeleton, thus playing an important role in stabilizing cell-cell adhesion. Additional pools of ␤-catenin shuttle between the cytoplasm and the nucleus and regulate the transcriptional capabilities of T cell factor͞lymphoid enhancement factor DNA-binding proteins (1).␤-Catenin's role in the canonical Wnt-signaling pathway has been investigated extensively, and recent studies have shown that it lies at the hub of several other major signaling pathways (ref. 1 and references therein). Many of these pathways stabilize ␤-catenin and enhance its transcriptional capability by preventing phosphorylation of the N-terminal domain. In the mammary gland, expression of N-terminally truncated, stabilized mouse mammary tumor virus (MMTV)-⌬N89␤-catenin causes precocious alveolar development in virgin mice and delayed involution and development of mammary tumors (2). 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 (3-5).Of the multiple target genes of ␤-catenin signaling identified so far (www.stanford.edu͞ϳrnusse͞pathways͞targets.html), the cell-cycle regulator cyclin D1 (referred to as cycD1 in the figures) has attracted particular interest. The phenotypic congruency among mice overexpressing or lacking the activity of cyclin D1 or ␤-catenin suggests a functional linkage between these proteins during normal mammar...

ABSTRACTβ-catenin and cyclin D1 have attracted considerable attention due to their proto-oncogenic roles in human cancer. The finding of cyclin D1 as a direct target gene of β-catenin in colon cancer cells led to the assumption that cyclin D1 upregulation is pivotal to β-catenin's oncogenicity. Our recent paper shows that this is not the case; cyclin D1 dampens the oncogenicity of activated β-catenin (MMTV-∆N89β-catenin). The relationships and dependencies of β-catenin and cyclin D1 point to distinct, essential and sequential roles during alveologenesis. These results support the concept that both β-catenin's and cyclin D1's actions are more sophisticated than simple acceleration of the cell cycle clock. These proteins are employed at critical junctures involving cell fate decisions that we speculate require specific types of cell cycle to traverse.