The recent identification of mouse mammary stem cells (MaSCs) and progenitor subpopulations has enhanced the prospect of investigating the genetic control of their lineage specification and differentiation. Here we have explored the role of the Notch pathway within the mammary epithelial hierarchy. We show that knockdown of the canonical Notch effector Cbf-1 in the MaSC-enriched population results in increased stem cell activity in vivo as well as the formation of aberrant end buds, implying a role for endogenous Notch signaling in restricting MaSC expansion. Conversely, Notch was found to be preferentially activated in the ductal luminal epithelium in vivo and promoted commitment of MaSCs exclusively along the luminal lineage. Notably, constitutive Notch signaling specifically targeted luminal progenitor cells for expansion, leading to hyperplasia and tumorigenesis. These findings reveal key roles for Notch signaling in MaSCs and luminal cell commitment and further suggest that inappropriate Notch activation promotes the self-renewal and transformation of luminal progenitor cells.

Hormonal cues regulate mammary development, but the consequent transcriptional changes and cell fate decisions are largely undefined. We show that knockout of the prolactin-regulated Ets transcription factor Elf5 prevented formation of the secretory epithelium during pregnancy. Conversely, overexpression of Elf5 in an inducible transgenic model caused alveolar differentiation and milk secretion in virgin mice, disrupting ductal morphogenesis. CD61 + luminal progenitor cells accumulated in Elf5-deficient mammary glands and were diminished in glands with Elf5 overexpression. Thus Elf5 specifies the differentiation of CD61 + progenitors to establish the secretory alveolar lineage during pregnancy, providing a link between prolactin, transcriptional events, and alveolar development.Supplemental material is available at http://www.genesdev.org.

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Overexpression of the prosurvival protein BCL-2 is common in breast cancer. Here we have explored its role as a potential therapeutic target in this disease. BCL-2, its anti-apoptotic relatives MCL-1 and BCL-XL, and the proapoptotic BH3-only ligand BIM were found to be coexpressed at relatively high levels in a substantial proportion of heterogeneous breast tumors, including clinically aggressive basal-like cancers. To determine whether the BH3 mimetic ABT-737 that neutralizes BCL-2, BCL-XL, and BCL-W had potential efficacy in targeting BCL-2-expressing basal-like triplenegative tumors, we generated a panel of primary breast tumor xenografts in immunocompromised mice and treated recipients with either ABT-737, docetaxel, or a combination. Tumor response and overall survival were significantly improved by combination therapy, but only for tumor xenografts that expressed elevated levels of BCL-2. Treatment with ABT-737 alone was ineffective, suggesting that ABT-737 sensitizes the tumor cells to docetaxel. Combination therapy was accompanied by a marked increase in apoptosis and dissociation of BIM from BCL-2. Notably, BH3 mimetics also appeared effective in BCL-2-expressing xenograft lines that harbored p53 mutations. Our findings provide in vivo evidence that BH3 mimetics can be used to sensitize primary breast tumors to chemotherapy and further suggest that elevated BCL-2 expression constitutes a predictive response marker in breast cancer.ABT-263 | navitoclax | programmed cell death | mammary | small molecule inhibitor

The transcription factor ELF5 is responsible for gene expression patterning underlying molecular subtypes of breast cancer and may mediate acquired resistance to anti-estrogen therapy.

The proliferative phase of mammary alveolar morphogenesis is initiated during early pregnancy by rising levels of serum prolactin and progesterone, establishing a program of gene expression that is ultimately responsible for the development of the lobuloalveoli and the onset of lactation. To explore this largely unknown genetic program, we constructed transcript profiles derived from transplanted mammary glands formed by recombination of prolactin receptor (Prlr) knockout or wild-type mammary epithelium with wild-type mammary stroma. Comparison with profiles derived from prolactin-treated Scp2 mammary epithelial cells produced a small set of commonly prolactin-regulated genes that included the negative regulator of cytokine signaling, Socs2 (suppressor of cytokine signaling 2), and the ets transcription factor, E74-like factor 5 (Elf5). Homozygous null mutation of Socs2 rescued the failure of lactation and reduction of mammary signal transducer and activator of transcription 5 phosphorylation that characterizes Prlr heterozygous mice, demonstrating that mammary Socs2 is a key regulator of the prolactin-signaling pathway. Reexpression of Elf5 in Prlr nullizygous mammary epithelium restored lobuloalveolar development and milk production, demonstrating that Elf5 is a transcription factor capable of substituting for prolactin signaling. Thus, Socs2 and Elf5 are key members of the set of prolactin-regulated genes that mediate prolactin-driven mammary development.

Massive tissue remodelling occurs within the mammary gland during pregnancy, resulting in the formation of lobuloalveoli that are capable of milk secretion. Endocrine signals generated predominantly by prolactin and progesterone operate the alveolar switch to initiate these developmental events. Here we review the current understanding of the components of the alveolar switch and conclude with an examination of the role of the ets transcription factor Elf5. We propose that Elf5 is a key regulator of the alveolar switch. Introduction: the alveolar switchMassive tissue remodelling within the mammary gland during pregnancy results in the formation of the secretory lobuloalveolar units in preparation for lactation. The initial proliferative phase of alveolar morphogenesis is instigated by an increase in the level of serum prolactin (Prl) and progesterone (Pg) [1]. These hormones activate the alveolar switch, a genetic program that coordinates changes in mammary epithelial cell proliferation, migration, differentiation and deletion within the many tissue types of the mammary gland. Here we review our current understanding of the genetic program controlling alveolar morphogenesis, using the mouse as a model of the human breast [2]. We then examine the role played by the ets transcription factor Elf5 in coordinating this program in epithelial cells, and propose that Elf5 is a central component of the alveolar switch. Tissue remodelling during pregnancyThe most striking aspect of mammary development during pregnancy is massive tissue remodelling. During the alveolar morphogenesis phase [3], rapid and global proliferation of the epithelial cells occurs within the ductal branches and developing alveoli. This increases both epithelial cell number and epithelial surface area, actions essential for sufficient milk production during lactation. Cell differentiation becomes dominant from mid-pregnancy as the gland moves into the secretory initiation phase [3]. The developing alveoli cleave and the alveolar cells become polarised and form a spherelike single layer of epithelial cells that envelopes a circular lumen, connected to the ductal network via a single small duct. Each individual alveolus is surrounded by a basket-like architecture of contractile myo-epithelial cells. The myoepithelium of the alveoli is discontinuous so that the luminal cells directly contact the underlying basement membrane, which forms part of the extracellular matrix. Some cells of the ductal network also contact the basement membrane. Contact is required for complete lobuloalveolar differentiation [4,5], seen morphologically by the appearance of lipid droplets [6] and by the initiation of gene expression in a defined order [7]. Nearing parturition, alveolar tight junctions close and milk and colostrum proteins move into the alveolar lumen, in preparation for active milk secretion post-partum, which marks the onset of the secretory activation phase [8] (Figure 1).The epithelial expansion is paralleled by equally dramatic changes in other tissu...

Mammary morphogenesis is orchestrated with other reproductive events by pituitary-driven changes to the systemic hormone environment, initiating the formation of a mammary ductal network during puberty and the addition of secretory alveoli during pregnancy. Prolactin is the major driver of development during pregnancy via regulation of ovarian progesterone production (in many species) and direct effects on mammary epithelial cells (in all species). Together these hormones regulate two aspects of development that are the subject of intense interest: (1) a genomic regulatory network that integrates many additional spatial and temporal cues to control gene expression and (2), the activity of a stem and progenitor cell hierarchy. Amalgamation of these two aspects will increase our understanding of cell proliferation and differentiation within the mammary gland, with clear application to our attempts to control breast cancer. Here we focus on providing an over-view of prolactin action during development of the model murine mammary gland.