Endocrine hormones and local signals during the development of the mouse mammary gland

Most of mammary gland development occurs postnatally under the control of female reproductive hormones, which in turn interact with other endocrine factors. While hormones impinge on many tissues and trigger very complex biological responses, tissue recombination experiments with hormone receptor-deficient mammary epithelia revealed eminent roles for estrogens, progesterone, and prolactin receptor (PrlR) signaling that are intrinsic to the mammary epithelium. A subset of the luminal mammary epithelial cells expresses the estrogen receptor α (ERα), the progesterone receptor (PR), and the PrlR and act as sensor cells. These cells convert the detected systemic signals into local signals that are developmental stage-dependent and may be direct, juxtacrine, or paracrine. This setup ensures that the original input is amplified and that the biological responses of multiple cell types can be coordinated. Some key mediators of hormone action have been identified such as Wnt, EGFR, IGFR, and RANK signaling. Multiple signaling pathways such as FGF, Hedgehog, and Notch signaling participate in driving different aspects of mammary gland development locally but how they link to the hormonal control remains to be elucidated. An increasing number of endocrine factors are appearing to have a role in mammary gland development, the adipose tissue is increasingly recognized to play a role in endocrine regulation, and a complex role of the immune system with multiple different cell types is being revealed. For further resources related to this article, please visit the WIREs website. Conflict of interest: The authors have declared no conflicts of interest for this article.

Mechanisms underlying cell-fate determination in the mammary gland development

Duje Buric

Notch signaling pathway is an important developmental pathway and has been implicated in both mammary gland development and tumorigenesis. Several studies investigating Notch signaling in mouse mammary gland implied that Notch signaling is an important factor in the determination of the luminal cell type. Clinical studies on breast tumors as well as studies in transgenic mouse models overexpressing active Notch receptors suggested an oncogenic function of Notch in the mammary gland connecting it to the poor breast cancer prognosis. However the physiological role of Notch signaling and its downstream mechanisms remain unclear. Analysis of Transgenic-EGFP Notch reporter mouse mammary epithelium revealed that Notch signaling is active specifically in a subset of hormone receptor positive cells. More sensitive FACS analysis consistently shows EGFP expression in HR+ luminal cells and reveals a weaker signal in a subset of basal (CD24lo) cells. Additionally mRNA analysis revealed that Notch active luminal cells are expressing Wnt4 ligand. To unveil the role of the Notch signaling in the Wnt4 expressing HR+ cells, we conditionally deleted RBP-Jκ, Notch signaling mediator, in Wnt4 expressing subpopulation of hormone receptor positive cells via Wnt4Cre. Abrogation of RBP-Jκ resulted in loss of progesterone receptor expression in 90% of cells bearing the RBP-Jκ deletion while estrogen receptor expression remained intact, implicating RBP-Jκ in regulation of progesterone receptor expression. To test whether Notch signaling regulates PR expression in the entire HR+ population, we analyzed mammary epithelium in which RBP-Jκ has been conditionally deleted via MMTV-Cre in progenitor cells revealing the presence of a subpopulation of luminal hormone receptor positive cells that differentiate independently of Notch signaling. Time directed deletion of RBP-Jκ via intraductally injected Adeno-Cre virus into the adult mouse mammary ductal system through the nipple additionally confirmed that RBP-Jκ is required for progesterone receptor expression. Chromatin immunoprecipitation assay showed that RBP-Jκ binds two out of four putative RBPJκ binding sites in the progesterone receptor promoter. Inhibition of Notch signaling in vivo via intraductal injection of γ-secretase inhibitor, DAPT, resulted in reduced progesterone receptor expression strongly suggesting that Notch-related RBP-Jκ signaling is responsible for PR expression. In this project we are proposing that there are two different populations of HR+ cells: one defined as Wnt4 expressing subpopulation of HR+ cells in which PR expression is dependent of Notch signaling, and another one defined as Wnt4 non expressing subpopulation of HR+ cells which is independent of Notch signaling. Since Notch signaling is able to replace ER signaling and activate ER target genes in the endocrine therapy resistant cell lines, Wnt4 expressing population of HR+ luminal cells might play a crucial role in the acquiring of the resistance.

EPFL2015

90 YEARS OF PROGESTERONE Progesterone receptor signaling in the normal breast and its implications for cancer

Cathrin Brisken, Valentina Scabia

Progesterone is considered as the pregnancy hormone and acts on many different target tissues. Progesterone receptor (PR) signaling is important for normal development and the physiologic function of the breast and impinges on breast carcinogenesis. Both systemically and locally, in the breast epithelium, there are multiple layers of complexity to progesterone action, many of which have been revealed through experiments in mice. The hormone acts via its receptor expressed in a subset of cells, the sensor cells, in the breast epithelium with different signaling outcomes in individual cells eliciting distinct cell-intrinsic and paracrine signaling involving different mediators for different intercellular interactions. PR expression itself is developmentally regulated and the biological outcome of PR signaling depends on the developmental stage of the mammary gland and the endocrine context. During both puberty and adulthood PR activates stem and progenitor cells through Wnt4-driven activation of the myoepithelium with downstream Adamts18-induced changes in extracellualr matrix (ECM) / basal membrane (BM). During estrous cycling and pregnancy, the hormone drives a major cell expansion through Rankl. At all stages, PR signaling is closely tied to estrogen receptor a (ER) signaling. As the PR itself is a target gene of ER, the complex interactions are experimentally difficult to dissect and still poorly understood. Ex vivo models of the human breast and studies on biopsy samples show that major signaling axes are conserved across species. New intraductal xenograft models hold promise to provide a better understanding of PR signaling in the normal breast epithelium and in breast cancer development in the near future.

BIOSCIENTIFICA LTD2020

Mechanisms underlying cell-fate determination in the mammary gland development

Duje Buric

EPFL2015