267 Introduction The pseudostratified mammary epithelium comprises a luminal layer of secretory cells and a basal layer of myoepithelial cells that are responsible for milk expulsion during lactation. Owing to their specific location, the myoepithelial cells can integrate multiple signals from the luminal cells, the underlying basement membrane and the connective tissue, thereby playing a central role in the control of epithelial-stromal interactions in the mammary gland. However, apart from their contractile function, the myoepithelial cells remain poorly studied, and currently very little is known about their role in mammary development and the mechanisms that control their phenotype. Like other basal epithelial cells, mammary myoepithelial cells express basal keratins (in particular, K5 and K14) and P- cadherin, and, in addition, contractile smooth muscle (SM) proteins, such as α-SM-actin. By contrast, luminal cells express K8 and K18, which are characteristic of simple epithelia. The origin of the two major mammary epithelial cell lineages remains poorly understood. Serial transplantation experiments have suggested that the adult mouse mammary epithelium harbors long-lived, bipotent and lineage-restricted progenitors with a high proliferative potential (Smalley and Ashworth, 2003; Smith and Boulanger, 2003). Moreover, a technique that allows the maintenance of early mammary progenitors in vitro was recently developed (Dontu et al., 2003). However, the location of the progenitor cell population in the mammary epithelium and its phenotypic characteristics remain to be established. Ultrastructural studies revealed a candidate cell type – undifferentiated ‘pale’ or ‘light’ cells resting on the basement membrane or the suprabasal surface of myoepithelial cells (Smith and Boulanger, 2003; Smith and Medina, 1988). These cells are rare, but they are present at all stages of mammary development and are distributed throughout the mammary tree. Welm et al. have shown that the cell population expressing Sca1, a presumable progenitor cell marker, is located in the luminal layer of mammary gland (Welm et al., 2002). Similarly, the experiments performed in vitro with separated luminal and myoepithelial cells have suggested that bipotent mammary precursor cells belong to the Wnt/β-catenin signaling pathway is involved in the maintenance of the progenitor cell population in the skin, intestine and other tissues, and its aberrant activation caused by stabilization of β-catenin contributes to tumorigenesis. In the mammary gland, constitutive activation of Wnt/β-catenin signaling in luminal secretory cells results in precocious lobuloalveolar differentiation and induces adenocarcinomas, whereas the impact of this signaling pathway on the function of the second major mammary epithelial cell lineage, the basal myoepithelial cells, has not been analyzed. We have used the keratin (K) 5 promoter to target the expression of stabilized N- terminally truncated β-catenin to the basal cell layer of mouse mammary epithelium. The transgenic mice presented an abnormal mammary phenotype: precocious lateral bud formation, increased proliferation and premature differentiation of luminal epithelium in pregnancy, persistent proliferation in lactation and accelerated involution. Precocious development in pregnancy was accompanied by increased Myc and cyclin D1 transcript levels, and a shift in p63 variant expression towards the ∆Np63 form. The expression of ECM- degrading proteinases and their inhibitors was altered in pregnancy and involution. Nulliparous transgenic females developed mammary hyperplasia that comprised undifferentiated basal (K5/14-positive, K8- and α-smooth muscle-actin-negative) cells. Multiparous mice, in addition, developed invasive basal-type carcinomas. Thus, activation of β-catenin signaling in basal mammary epithelial cells affects the entire process of mammary gland development and induces amplification of basal-type cells that lack lineage markers, presumably, a subpopulation of mammary progenitors able to give rise to tumors. Key words: β-catenin, Progenitor cell, Mammary gland, Basal epithelial cell, Hyperplasia, Mouse Summary Targeted activation of β-catenin signaling in basal mammary epithelial cells affects mammary development and leads to hyperplasia Jérôme Teulière 1 , Marisa M. Faraldo 1 , Marie-Ange Deugnier 1 , Michael Shtutman 2 , Avri Ben-Ze’ev 2 , Jean Paul Thiery 1 and Marina A. Glukhova 1, * 1 UMR 144 CNRS-Institut Curie, Institut Curie, Section de Recherche, 26 rue d’Ulm, 75248, Paris, Cedex 05, France 2 Department of Molecular Cell Biology, The Weizmann Institute of Science, Rehovot, 76100, Israel *Author for correspondence (e-mail: marina.glukhova@curie.fr) Accepted 12 November 2004 Development 132, 267-277 Published by The Company of Biologists 2005 doi:10.1242/dev.01583 Research article Development Development