Clonal analysis of Notch1-expressing cells reveals the existence of unipotent stem cells that retain long-term plasticity in the embryonic mammary gland.

Recent lineage tracing studies have revealed that mammary gland homeostasis relies on unipotent stem cells. However, whether and when lineage restriction occurs during embryonic mammary development, and which signals orchestrate cell fate specification, remain unknown. Using a combination of in vivo clonal analysis with whole mount immunofluorescence and mathematical modelling of clonal dynamics, we found that embryonic multipotent mammary cells become lineage-restricted surprisingly early in development, with evidence for unipotency as early as E12.5 and no statistically discernable bipotency after E15.5. To gain insights into the mechanisms governing the switch from multipotency to unipotency, we used gain-of-function Notch1 mice and demonstrated that Notch activation cell autonomously dictates luminal cell fate specification to both embryonic and basally committed mammary cells. These functional studies have important implications for understanding the signals underlying cell plasticity and serve to clarify how reactivation of embryonic programs in adult cells can lead to cancer.Wellcome Trus

Breast stem cells and hormonal mechanisms in carcinogenesis

A woman's risk of breast cancer is strongly affected by her reproductive history. The hormonal milieu is also a key determinant of the course of the disease. Combining mouse genetics with tissue recombination techniques, we have established that the female reproductive hormones, estrogens, progesterone, and prolactin, act sequentially on the mammary epithelium to trigger distinct developmental steps. The hormones impinge directly on a subset of luminal mammary epithelial cells that express the respective hormone receptors and act as sensor cells translating and amplifying systemic signals into local stimuli. Local signaling is stage and age specific. During puberty, estrogens promote proliferation using the EGF family member, amphiregulin, as essential paracrine mediator. In adulthood, progesterone, rather than estrogen, is the major inducer of stem cell activation and cell proliferation of the mammary epithelium. Hormonal signaling modulates crucial developmental pathways that impinge on mammary stem cell populations, while Notch signaling, by inhibiting p63, is central to mammary cell fate determination. Cell proliferation occurs in two waves. The first results from direct stimulation of the small fraction of hormone receptor positive cells. It is followed by a second wave of progesterone-induced proliferation involving mostly hormone receptor negative cells, in which RANKL is a key mediator. A model in which repeated activation of paracrine signaling by progesterone with resulting stem cell activation promotes breast carcinogenesis is proposed

Epstein-Barr Virus Latent Membrane Protein 2 Effects on Epithelial Acinus Development Reveal Distinct Requirements for the PY and YEEA motifs

Epstein-Barr virus (EBV) is a gammaherpesvirus associated with numerous cancers, including the epithelial cancers nasopharyngeal carcinoma (NPC) and gastric carcinoma. The latent membrane protein 2 (LMP2) encoded by EBV is consistently detected in NPC tumors and promotes a malignant phenotype when expressed in epithelial cells by inducing transformation and migration and inhibiting differentiation. Grown in three dimensions (3D) on Matrigel, the nontumorigenic mammary epithelial cell line MCF10A forms hollow, spherical acinar structures that maintain normal glandular features. Expression of oncogenes in these cells allows for the study of multiple aspects of tumor development in a 3D culture system. This study sought to examine the effects of LMP2 on the generation of MCF10A acini. LMP2 expression induced abnormal acini that were large, misshapen, and filled, indicating that LMP2 induced proliferation, impaired cellular polarization, and induced resistance to cell death, leading to luminal filling. Induction of cell death resistance required the PY, immunoreceptor tyrosine activation motif (ITAM), and YEEA signaling domains of LMP2 and activation of the Src and Akt signaling pathways. The PY domain was required for the inhibition of anoikis and also the delayed proliferative arrest of the LMP2-expressing cells. In addition to directly altering acinus formation, expression of LMP2 also induced morphological and protein expression changes consistent with epithelial-mesenchymal transition (EMT) in a manner that required only the YEEA signaling motif of LMP2. These findings indicate that LMP2 has considerable transforming properties that are not evident in standard tissue culture and requires the ability of LMP2A to bind ubiquitin ligases and Src family kinases

Progesterone/RANKL is a major regulatory axis in the human breast.

Estrogens and progesterones are major drivers of breast development but also promote carcinogenesis in this organ. Yet, their respective roles and the mechanisms underlying their action in the human breast are unclear. Receptor activator of nuclear factor κB ligand (RANKL) has been identified as a pivotal paracrine mediator of progesterone function in mouse mammary gland development and mammary carcinogenesis. Whether the factor has the same role in humans is of clinical interest because an inhibitor for RANKL, denosumab, is already used for the treatment of bone disease and might benefit breast cancer patients. We show that progesterone receptor (PR) signaling failed to induce RANKL in PR(+) breast cancer cell lines and in dissociated, cultured breast epithelial cells. In clinical specimens from healthy donors and intact breast tissue microstructures, hormone response was maintained and RANKL expression was under progesterone control, which increased RNA stability. RANKL was sufficient to trigger cell proliferation and was required for progesterone-induced proliferation. The findings were validated in vivo where RANKL protein expression in the breast epithelium correlated with serum progesterone levels and the protein was expressed in a subset of luminal cells that express PR. Thus, important hormonal control mechanisms are conserved across species, making RANKL a potential target in breast cancer treatment and prevention