p53 controls the plasticity of mammary luminal progenitor cells downstream of Met signaling.

BACKGROUND: The adult mammary epithelium is composed of basal and luminal cells. The luminal lineage comprises two major cell populations, positive and negative for estrogen and progesterone receptors (ER and PR, respectively), both containing clonogenic progenitor cells. Deregulated ER/PR- luminal progenitor cells are suspected to be at the origin of basal-type triple-negative (TNBC) breast cancers, a subtype frequently associated with loss of P53 function and MET signaling hyperactivation. Using mouse models, we recently reported that p53 restricts luminal progenitor cell amplification whereas paracrine Met activation stimulates their growth and favors a luminal-to-basal switch. Here, we analyzed how these two critical pathways interact to control luminal progenitor function. METHODS: We have (i) established and analyzed the gene expression profile of luminal progenitors isolated by ICAM-1, a robust surface marker we previously identified; (ii) purified luminal progenitors from p53-deficient and p53-proficient mouse mammary epithelium to compare their functional and molecular characteristics; and (iii) analyzed their response to HGF, the major Met ligand, in three-dimensional cultures. RESULTS: We found that luminal progenitors, compared to non-clonogenic luminal cells, overexpress Trp53 and numerous p53 target genes. In vivo, loss of Trp53 induced the expansion of luminal progenitors, affecting expression of several important p53 target genes including those encoding negative regulators of cell cycle progression. Consistently, p53-deficient luminal progenitors displayed increased proliferative and self-renewal activities in culture. However, they did not exhibit perturbed expression of luminal-specific markers and major regulators, such as Hey1, Elf5, and Gata3. Moreover, although expressing Met at higher level than p53-proficient luminal progenitors, p53-deficient luminal progenitors failed to acquire basal-specific features when stimulated by HGF, showing that p53 promotes the plastic behavior of luminal progenitors downstream of Met activation. CONCLUSIONS: Our study reveals a crosstalk between Met- and p53-mediated signaling pathways in the regulation of luminal progenitor function. In particular, it shows that neither p53 loss alone nor p53 loss combined with Met signaling activation caused an early detectable cell fate alteration in luminal progenitors. Conceivably, additional events are required to confer basal-specific characteristics to luminal-derived TNBCs.

Prolactin-induced prostate tumorigenesis links sustained Stat5 signaling with the amplification of basal/stem cells and emergence of putative luminal progenitors.

Current androgen ablation therapies for prostate cancer are initially successful, but the frequent development of castration resistance urges the generation of alternative therapies and represents an important health concern. Prolactin/signal transducer and activator of transcription 5 (STAT5) signaling is emerging as a putative target for alternative treatment for prostate cancer. However, mechanistic data for its role in development or progression of prostate tumors are scarce. In vivo mouse studies found that local prolactin induced the amplification of prostate epithelial basal/stem cells. Because these cells are proposed cells of origin for prostate cancer and disease recurrence, we looked further into this amplification. Our results indicated that sustained Stat5 activation was associated with the occurrence of abnormal basal/stem cell clusters in prostate epithelium of prostate-specific prolactin-transgenic mice. Analysis of epithelial areas containing these clusters found high proliferation, Stat5 activation, and expression of stem cell antigen 1. Furthermore, enhanced prolactin signaling also led to amplification of a luminal cell population that was positive for stem cell antigen 1. These cells may originate from amplified basal/stem cells and might represent important progenitors for tumor development in prostate epithelium. These data provide a deeper understanding of the initial stages of prostate tumorigenesis induced by prolactin to help determine whether this hormone or its downstream messengers could be useful targets for prostate cancer treatment in the future.

Somatic loss of p53 leads to stem/progenitor cell amplification in both mammary epithelial compartments, basal and luminal.

Mammary epithelium comprises a layer of luminal cells and a basal myoepithelial cell layer. Both mammary epithelial compartments, basal and luminal, contain stem and progenitor cells, but only basal cells are capable of gland regeneration upon transplantation. Aberrant expansion of stem/progenitor cell populations is considered to contribute to breast tumorigenesis. Germline deletions of p53 in humans and mice confer a predisposition to tumors, and stem cell frequency is abnormally high in the mammary epithelium of p53-deficient mice. However, it is unknown whether stem/progenitor cell amplification occurs in both, basal and luminal cell populations in p53-deficient mammary tissue. We used a conditional gene deletion approach to study the role of p53 in stem/progenitor cells residing in the mammary luminal and basal layers. Using two- and three-dimensional cell culture assays, we showed that p53 loss led to the expansion of clonogenic stem/progenitor cells in both mammary epithelial cell layers. Moreover, following p53 deletion, luminal and basal stem/progenitor cells acquired a capacity for unlimited propagation in mammosphere culture. Furthermore, limiting dilution and serial transplantation assays revealed amplification and enhanced self-renewal in the basal regenerating cell population of p53-deficient mammary epithelium. Our data suggest that the increase in stem/progenitor cell activity may be, at least, partially mediated by the Notch pathway. Taken together, these results strongly indicate that p53 restricts the propagation and self-renewal of stem/progenitor cells in both layers of the mammary epithelium providing further insight into the impact of p53 loss in breast cancerogenesis.

Myc is required for ß-catenin-mediated mammary stem cell amplification and tumorigenesis.

BACKGROUND: Basal-like breast cancer is a heterogeneous disease characterized by the expression of basal cell markers, no estrogen or progesterone receptor expression and a lack of HER2 overexpression. Recent studies have linked activation of the Wnt/ß-catenin pathway, and its downstream target, Myc, to basal-like breast cancer. Transgenic mice K5ΔNßcat previously generated by our team present a constitutive activation of Wnt/ß-catenin signaling in the basal myoepithelial cell layer, resulting in focal mammary hyperplasias that progress to invasive carcinomas. Mammary lesions developed by K5ΔNßcat mice consist essentially of basal epithelial cells that, in contrast to mammary myoepithelium, do not express smooth muscle markers. METHODS: Microarray analysis was used to compare K5ΔNßcat mouse tumors to human breast tumors, mammary cancer cell lines and the tumors developed in other mouse models. Cre-Lox approach was employed to delete Myc from the mammary basal cell layer of K5ΔNßcat mice. Stem cell amplification in K5ΔNßcat mouse mammary epithelium was assessed with 3D-culture and transplantation assays. RESULTS: Histological and microarray analyses of the mammary lesions of K5ΔNßcat females revealed their high similarity to a subset of basal-like human breast tumors with squamous differentiation. As in human basal-like carcinomas, the Myc pathway appeared to be activated in the mammary lesions of K5ΔNßcat mice. We found that a basal cell population with stem/progenitor characteristics was amplified in K5ΔNßcat mouse preneoplastic glands. Finally, the deletion of Myc from the mammary basal layer of K5ΔNßcat mice not only abolished the regenerative capacity of basal epithelial cells, but, in addition, completely prevented the tumorigenesis. CONCLUSIONS: These results strongly indicate that ß-catenin-induced stem cell amplification and tumorigenesis rely ultimately on the Myc pathway activation and reinforce the hypothesis that basal stem/progenitor cells may be at the origin of a subset of basal-like breast tumors.

The proto-oncogene Myc is essential for mammary stem cell function.

The mammary epithelium comprises two major cell lineages: basal and luminal. Basal cells (BCs) isolated from the mammary epithelium and transplanted into the mouse mammary fat pad cleared from the endogenous epithelium regenerate the mammary gland, strongly suggesting that the basal epithelial compartment harbors a long-lived cell population with multipotent stem cell potential. The luminal cell layer is devoid of the regenerative potential, but it contains cells with clonogenic capacity, the luminal progenitors. Mammary BCs and luminal progenitors express high levels of the transcription factor Myc. Here, we show that deletion of Myc from mammary basal epithelial cells led to impaired stem cell self-renewal as evaluated by limiting dilution and serial transplantation assays. Luminal progenitor population was significantly diminished in mutant epithelium suggesting control by the BC layer. Colony formation assay performed with isolated BCs showed that clonogenic capacity was abolished by Myc deletion. Moreover, transplanted BCs depleted of Myc failed to produce epithelial outgrowths. Stimulation with ovarian hormones estrogen (E) and progesterone (P) partially rescued the repopulation capacity of Myc-depleted BCs; however, the Myc-deficient mammary epithelium developed in response to E/P treatment lacked stem and progenitor cells. This study provides the first evidence that in the mammary gland, Myc has an essential nonredundant function in the maintenance of the self-renewing multipotent stem cell population responsible for the regenerative capacity of the mammary epithelium and is required downstream from ovarian hormones, for the control of mammary stem and progenitor cell functions.

Systemic GDF11 attenuates depression-like phenotype in aged mice via stimulation of neuronal autophagy.

Cognitive decline and mood disorders increase in frequency with age. Many efforts are focused on the identification of molecules and pathways to treat these conditions. Here, we demonstrate that systemic administration of growth differentiation factor 11 (GDF11) in aged mice improves memory and alleviates senescence and depression-like symptoms in a neurogenesis-independent manner. Mechanistically, GDF11 acts directly on hippocampal neurons to enhance neuronal activity via stimulation of autophagy. Transcriptomic and biochemical analyses of these neurons reveal that GDF11 reduces the activity of mammalian target of rapamycin (mTOR), a master regulator of autophagy. Using a murine model of corticosterone-induced depression-like phenotype, we also show that GDF11 attenuates the depressive-like behavior of young mice. Analysis of sera from young adults with major depressive disorder (MDD) reveals reduced GDF11 levels. These findings identify mechanistic pathways related to GDF11 action in the brain and uncover an unknown role for GDF11 as an antidepressant candidate and biomarker.

Amphiregulin mediates non-cell-autonomous effect of senescence on reprogramming.

Cellular senescence is an irreversible growth arrest with a dynamic secretome, termed the senescence-associated secretory phenotype (SASP). Senescence is a cell-intrinsic barrier for reprogramming, whereas the SASP facilitates cell fate conversion in non-senescent cells. However, the mechanisms by which reprogramming-induced senescence regulates cell plasticity are not well understood. Here, we investigate how the heterogeneity of paracrine senescence impacts reprogramming. We show that senescence promotes in vitro reprogramming in a stress-dependent manner. Unbiased proteomics identifies a catalog of SASP factors involved in the cell fate conversion. Amphiregulin (AREG), frequently secreted by senescent cells, promotes in vitro reprogramming by accelerating proliferation and the mesenchymal-epithelial transition via EGFR signaling. AREG treatment diminishes the negative effect of donor age on reprogramming. Finally, AREG enhances in vivo reprogramming in skeletal muscle. Hence, various SASP factors can facilitate cellular plasticity to promote reprogramming and tissue repair.

A Robust Mammary Organoid System to Model Lactation and Involution-like Processes.

The mammary gland is a highly dynamic tissue that changes throughout reproductive life, including growth during puberty and repetitive cycles of pregnancy and involution. Mammary gland tumors represent the most common cancer diagnosed in women worldwide. Studying the regulatory mechanisms of mammary gland development is essential for understanding how dysregulation can lead to breast cancer initiation and progression. Three-dimensional (3D) mammary organoids offer many exciting possibilities for the study of tissue development and breast cancer. In the present protocol derived from Sumbal et al., we describe a straightforward 3D organoid system for the study of lactation and involution ex vivo. We use primary and passaged mouse mammary organoids stimulated with fibroblast growth factor 2 (FGF2) and prolactin to model the three cycles of mouse mammary gland lactation and involution processes. This 3D organoid model represents a valuable tool to study late postnatal mammary gland development and breast cancer, in particular postpartum-associated breast cancer. Graphic abstract: Mammary gland organoid isolation and culture procedures.

The crosstalk between cellular reprogramming and senescence in aging and regeneration.

Aging is associated with diminished regenerative capacity and increased risk of chronic diseases. There is now compelling evidence suggests that aging process is reversible. Besides metabolic modification and systematic factors, both senescence elimination and cellular reprogramming showed beneficial effects on tissue regeneration and rejuvenation. Here we review recent studies on the interplay between cellular senescence and reprogramming. We discuss how both strategies could impact aging process and the possibility of combine them for more efficient regeneration and rejuvenation.

Primary Mammary Organoid Model of Lactation and Involution.

Mammary gland development occurs mainly after birth and is composed of three successive stages: puberty, pregnancy and lactation, and involution. These developmental stages are associated with major tissue remodeling, including extensive changes in mammary epithelium, as well as surrounding stroma. Three-dimensional (3D) mammary organoid culture has become an important tool in mammary gland biology and enabled invaluable discoveries on pubertal mammary branching morphogenesis and breast cancer. However, a suitable 3D organoid model recapitulating key aspects of lactation and involution has been missing. Here, we describe a robust and straightforward mouse mammary organoid system modeling lactation and involution-like process, which can be applied to study mechanisms of physiological mammary gland lactation and involution as well as pregnancy-associated breast cancer.

Systemic GDF11 stimulates the secretion of adiponectin and induces a calorie restriction-like phenotype in aged mice.

Aging is a negative regulator of general homeostasis, tissue function, and regeneration. Changes in organismal energy levels and physiology, through systemic manipulations such as calorie restriction and young blood infusion, can regenerate tissue activity and increase lifespan in aged mice. However, whether these two systemic manipulations could be linked has never been investigated. Here, we report that systemic GDF11 triggers a calorie restriction-like phenotype without affecting appetite or GDF15 levels in the blood, restores the insulin/IGF-1 signaling pathway, and stimulates adiponectin secretion from white adipose tissue by direct action on adipocytes, while repairing neurogenesis in the aged brain. These findings suggest that GDF11 has a pleiotropic effect on an organismal level and that it could be a linking mechanism of rejuvenation between heterochronic parabiosis and calorie restriction. As such, GDF11 could be considered as an important therapeutic candidate for age-related neurodegenerative and metabolic disorders.

Productive Infection of Mouse Mammary Glands and Human Mammary Epithelial Cells by Zika Virus.

Zika virus (ZIKV) belongs to the large category of arboviruses. Surprisingly, several human-to-human transmissions of ZIKV have been notified, either following sexual intercourse or from the mother to fetus during pregnancy. Importantly, high viral loads have been detected in the human breast milk of infected mothers, and the existence of breastfeeding as a new mode of mother-to-child transmission of ZIKV was recently hypothesized. However, the maternal origin of infectious particles in breast milk is currently unknown. Here, we show that ZIKV disseminates to the mammary glands of infected mice after both systemic and local exposure with differential kinetics. Ex vivo, we demonstrate that primary human mammary epithelial cells were sensitive and permissive to ZIKV infection in this study. Moreover, by using in vitro models, we prove that mammary luminal- and myoepithelial-phenotype cell lines are both able to produce important virus progeny after ZIKV exposure. Our data suggest that the dissemination of ZIKV to the mammary glands and subsequent infection of the mammary epithelium could be one mechanism of viral excretion in human breast milk.

Deciphering the Mammary Stem Cell Niche: A Role for Laminin-Binding Integrins.

Integrins, which bind laminin, a major component of the mammary basement membrane, are strongly expressed in basal stem cell-enriched populations, but their role in controlling mammary stem cell function remains unclear. We found that stem cell activity, as evaluated in transplantation and mammosphere assays, was reduced in mammary basal cells depleted of laminin receptors containing α3- and α6-integrin subunits. This was accompanied by low MDM2 levels, p53 stabilization, and diminished proliferative capacity. Importantly, disruption of p53 function restored the clonogenicity of α3/α6-integrin-depleted mammary basal stem cells, while inhibition of RHO or myosin II, leading to decreased p53 activity, rescued the mammosphere formation. These data suggest that α3/α6-integrin-mediated adhesion plays an essential role in controlling the proliferative potential of mammary basal stem/progenitor cells through myosin II-mediated regulation of p53 and indicate that laminins might be important components of the mammary stem cell niche.

Evaluation of Injury-induced Senescence and In Vivo Reprogramming in the Skeletal Muscle.

Cellular senescence is a stress response that is characterized by a stable cellular growth arrest, which is important for many physiological and pathological processes, such as cancer and ageing. Recently, senescence has also been implicated in tissue repair and regeneration. Therefore, it has become increasingly critical to identify senescent cells in vivo. Senescence-associated ß-galactosidase (SA-ß-Gal) assay is the most widely used assay to detect senescent cells both in culture and in vivo. This assay is based on the increased lysosomal contents in the senescent cells, which allows the histochemical detection of lysosomal ß-galactosidase activity at suboptimum pH (6 or 5.5). In comparison with other assays, such as flow cytometry, this allows the identification of senescent cells in their resident environment, which offers valuable information such as the location relating to the tissue architecture, the morphology, and the possibility of coupling with other markers via immunohistochemistry (IHC). The major limitation of the SA-ß-Gal assay is the requirement of fresh or frozen samples. Here, we present a detailed protocol to understand how cellular senescence promotes cellular plasticity and tissue regeneration in vivo. We use SA-ß-Gal to detect senescent cells in the skeletal muscle upon injury, which is a well-established system to study tissue regeneration. Moreover, we use IHC to detect Nanog, a marker of pluripotent stem cells, in a transgenic mouse model. This protocol enables us to examine and quantify cellular senescence in the context of induced cellular plasticity and in vivo reprogramming.

Injury-Induced Senescence Enables In Vivo Reprogramming in Skeletal Muscle.

In vivo reprogramming is a promising approach for tissue regeneration in response to injury. Several examples of in vivo reprogramming have been reported in a variety of lineages, but some including skeletal muscle have so far proven refractory. Here, we show that acute and chronic injury enables transcription-factor-mediated reprogramming in skeletal muscle. Lineage tracing indicates that this response frequently originates from Pax7+ muscle stem cells. Injury is associated with accumulation of senescent cells, and advanced aging or local irradiation further enhanced in vivo reprogramming, while selective elimination of senescent cells reduced reprogramming efficiency. The effect of senescence appears to be, at least in part, due to the release of interleukin 6 (IL-6), suggesting a potential link with the senescence-associated secretory phenotype. Collectively, our findings highlight a beneficial paracrine effect of injury-induced senescence on cellular plasticity, which will be important for devising strategies for reprogramming-based tissue repair.

Gain-of-function Prolactin Receptor Variants Are Not Associated With Breast Cancer and Multiple Fibroadenoma Risk.

CONTEXT: In a cohort of 95 women with multiple breast fibroadenomas (MFAs), we recently identified patients harboring germline heterozygous variants of the prolactin receptor (PRLR) exhibiting constitutive activity (PRLRI146L and PRLRI176V). OBJECTIVE: This study sought to better delineate the potential role of PRLR gain-of-function variants in benign and malignant mammary tumorigenesis. DESIGN: This was an observational study and transgenic mouse model analysis. SETTING: The study took place at the Department of Endocrinology, Reproductive Disorders and Rare Gynecologic Diseases, Pitié Salpêtrière, Paris, and Inserm Unit 1151, Paris. PATIENTS OR OTHER PARTICIPANTS: We generated a second MFA cohort (n = 71) as well as a group of control subjects (n = 496) and a cohort of women with breast cancer (n = 119). We also generated two transgenic mouse models carrying the coding sequences of human PRLRI146L or PRLRWT. INTERVENTION: We aimed to determine the prevalence of PRLR variants in these three populations and to uncover any association of the latter with specific tumor pattern, especially in patients with breast cancer. RESULTS: This study did not highlight a higher prevalence of PRLR variants in the MFA group and in the breast cancer group compared with control subjects. Transgenic mice expressing PRLRI146L exhibited very mild histological mammary phenotype but tumors were never observed. CONCLUSION: PRLRI146L and PRLRI176V variants are not associated with breast cancer or MFA risk. However, one cannot exclude that low but sustained PRLR signaling may facilitate or contribute to pathological development driven by oncogenic pathways. Long-term patient follow-up should help to address this issue.

Paracrine Met signaling triggers epithelial-mesenchymal transition in mammary luminal progenitors, affecting their fate.

HGF/Met signaling has recently been associated with basal-type breast cancers, which are thought to originate from progenitor cells residing in the luminal compartment of the mammary epithelium. We found that ICAM-1 efficiently marks mammary luminal progenitors comprising hormone receptor-positive and receptor-negative cells, presumably ductal and alveolar progenitors. Both cell populations strongly express Met, while HGF is produced by stromal and basal myoepithelial cells. We show that persistent HGF treatment stimulates the clonogenic activity of ICAM1-positive luminal progenitors, controlling their survival and proliferation, and leads to the expression of basal cell characteristics, including stem cell potential. This is accompanied by the induction of Snai1 and Snai2, two major transcription factors triggering epithelial-mesenchymal transition, the repression of the luminal-regulatory genes Elf5 and Hey1, and claudin down-regulation. Our data strongly indicate that paracrine Met signaling can control the function of luminal progenitors and modulate their fate during mammary development and tumorigenesis.