Proliferative heterogeneity of murine epithelial cells in the adult mammary gland

A new insight into the impact of copy number variations on cell cycle deregulation of luminal-type breast cancer

Breast cancer is the most prevalent neoplasm in women. ER+ (Luminal subtype), representing over 70% of breast tumors, is a genetically diverse group. Structural and Numerical-Chromosomal instability initiates tumor development and is recognized as the primary driver of genetic alteration in luminal breast tumors. Genomic instability refers to the increased tendency of cancer cells to accumulate genomic alterations during cell proliferation. The cell cycle check-point response to constant and stable genomic alterations in tumor cells drives this process. The impact of CNV patterns and aneuploidies in cell cycle and proliferation perturbation has recently been highlighted by scientists in Luminal breast tumors. The impact of chromosomal instability on cancer therapy and prognosis is not a new concept. Still, the degree of emerging genomic instability leads to prognosis alteration following cell cycle deregulation by chromosomal instability could be predicted by CNVs-based reclassification of breast tumors. In this review, we try to explain the effect of CIN in the cell cycle that ended with genomic instability and altered prognosis and the impact of CIN in decision-making for a therapy strategy for patients with luminal breast cancer.

The oestrous cycle stage affects mammary tumour sensitivity to chemotherapy

The response of breast cancer to neoadjuvant chemotherapy (NAC) varies substantially, even when tumours belong to the same molecular or histological subtype1. Here we identify the oestrous cycle as an important contributor to this heterogeneity. In three mouse models of breast cancer, we show reduced responses to NAC when treatment is initiated during the dioestrus stage, when compared with initiation during the oestrus stage. Similar findings were observed in retrospective premenopausal cohorts of human patients. Mechanistically, the dioestrus stage exhibits systemic and localized changes, including (1) an increased number of cells undergoing epithelial-to-mesenchymal transition linked to chemoresistance2-4 and (2) decreased tumour vessel diameter, suggesting potential constraints to drug sensitivity and delivery. In addition, an elevated presence of macrophages, previously associated with chemoresistance induction5, characterizes the dioestrus phase. Whereas NAC disrupts the oestrous cycle, this elevated macrophage prevalence persists and depletion of macrophages mitigates the reduced therapy response observed when initiating treatment during dioestrus. Our data collectively demonstrate the oestrous cycle as a crucial infradian rhythm determining chemosensitivity, warranting future clinical studies to exploit optimal treatment initiation timing for enhanced chemotherapy outcomes.

Hormone-responsive progenitors have a unique identity and exhibit high motility during mammary morphogenesis

Hormone-receptor-positive (HR+) luminal cells largely mediate the response to estrogen and progesterone during mammary gland morphogenesis. However, there remains a lack of consensus on the precise nature of the precursor cells that maintain this essential HR+ lineage. Here we refine the identification of HR+ progenitors and demonstrate their unique regenerative capacity compared to mature HR+ cells. HR+ progenitors proliferate but do not expand, suggesting rapid differentiation. Subcellular resolution, 3D intravital microscopy was performed on terminal end buds (TEBs) during puberty to dissect the contribution of each luminal lineage. Surprisingly, HR+ TEB progenitors were highly elongated and motile compared to columnar HR- progenitors and static, conoid HR+ cells within ducts. This dynamic behavior was also observed in response to hormones. Development of an AI model for motility dynamics analysis highlighted stark behavioral changes in HR+ progenitors as they transitioned to mature cells. This work provides valuable insights into how progenitor behavior contributes to mammary morphogenesis.

RNA-Seq Analysis of Pubertal Mammary Epithelial Cells Reveals Novel n-3 Polyunsaturated Fatty Acid Transcriptomic Changes in the fat-1 Mouse Model

BACKGROUND

The early exposure of nutrients during pubertal mammary gland development may reduce the risk of developing breast cancer later in life. Anticancer n-3 polyunsaturated fatty acids (n-3 PUFA) are shown to modulate pubertal mammary gland development; however, the mechanisms of action remain unclear. Prior work focused on effects at the whole tissue level, and little is known at the cellular level, such as at the level of mammary epithelial cells (MECs), which are implicated in cancer development.

METHODS

This pilot study examined the effects of lifelong n-3 PUFA exposure on the transcriptome by RNA-Seq in the isolated MECs of pubertal (6-8-week-old) female fat-1 transgenic mice capable of de novo n-3 PUFA synthesis. edgeR and DESeq2 were used separately for the differential expression analysis of RNA sequencing data followed by the Benjamani-Hochberg procedure for multiple testing correction.

RESULTS

Nine genes were found concordant and significantly different (p ≤ 0.05) by both the DESeq2 and edgeR methods. These genes were associated with multiple pathways, suggesting that n-3 PUFA stimulates estrogen-related signaling (Mlltl0, Galr3, and Nrip1) and a glycolytic profile (Soga1, Pdpr, and Uso1) while offering protective effects for immune and DNA damage responses (Glpd1, Garre1, and Rpa1) in MECs during puberty.

CONCLUSIONS

This pilot study highlights the utility of RNA-Seq to better understanding the mechanistic effects of specific nutrients such as n-3 PUFA in a cell-specific manner. Thus, further studies are warranted to investigate the cell-specific mechanisms by which n-3 PUFA influences pubertal mammary gland development and breast cancer risk later in life.

Intramammary Labeling of Epithelial Cell Division

Thymidine analogs such as ethynyl deoxyuridine (EdU) or bromodeoxyuridine (BrdU) can be used to label mitosis of mammary epithelial cells (MEC) and to quantify their proliferation. However, labeling cells in larger animals requires considerable amounts of chemical that can be costly and hazardous. We developed a strategy to infuse EdU into the mammary glands of ewes to directly label mitotic MEC. First, each udder half of nulliparous ewes (n = 2) received an intramammary infusion of one of four different concentrations of EdU (0, 0.1, 1.0 or 10 mM) which was compared to BrdU IV (5 mg/kg) 24 h later. Tissues were analyzed by immunofluorescent histochemistry to detect EdU, BrdU, and total MEC. Of the EdU doses tested, 10 mM EdU yielded the greatest labeling index, while a proportion of MEC were labeled by both EdU and BrdU. We next sought to establish whether intramammary labeling could detect the induction of mitosis after exposure to exogenous estrogen and progesterone (E + P). We first infused EdU (10 mM) into the right udder half of ewes (n = 6) at t 0, followed by thymidine (100 mM) 24 h later to prevent further labeling. Three ewes were then administered E + P for 5 d, while n = 3 ewes served as controls. On d 5, EdU was infused into the left udder half of all mammary glands alongside BrdU IV (5 mg/kg). By the time of necropsy 24 h later an average MEC labeling index of 2.9% resulted from EdU delivered at t 0. In the left half of the udder on d 5, CON glands had a final EdU labeling index of 3.4% while glands exposed to E + P had a labeling index of 4.6% (p = 0.05). The corresponding degree of labeling with BrdU was 5.6% in CON glands, and 12% following E + P (p < 0.001). Our findings reveal that intramammary labeling is an efficient and cost-effective method for single- and dual-labeling of cell division in the mammary glands.

Mechanisms that clear mutations drive field cancerization in mammary tissue

Oncogenic mutations are abundant in the tissues of healthy individuals, but rarely form tumours1-3. Yet, the underlying protection mechanisms are largely unknown. To resolve these mechanisms in mouse mammary tissue, we use lineage tracing to map the fate of wild-type and Brca1-/-;Trp53-/- cells, and find that both follow a similar pattern of loss and spread within ducts. Clonal analysis reveals that ducts consist of small repetitive units of self-renewing cells that give rise to short-lived descendants. This offers a first layer of protection as any descendants, including oncogenic mutant cells, are constantly lost, thereby limiting the spread of mutations to a single stem cell-descendant unit. Local tissue remodelling during consecutive oestrous cycles leads to the cooperative and stochastic loss and replacement of self-renewing cells. This process provides a second layer of protection, leading to the elimination of most mutant clones while enabling the minority that by chance survive to expand beyond the stem cell-descendant unit. This leads to fields of mutant cells spanning large parts of the epithelial network, predisposing it for transformation. Eventually, clone expansion becomes restrained by the geometry of the ducts, providing a third layer of protection. Together, these mechanisms act to eliminate most cells that acquire somatic mutations at the expense of driving the accelerated expansion of a minority of cells, which can colonize large areas, leading to field cancerization.

Atypical cell cycle regulation promotes mammary stem cell expansion during mammary development and tumourigenesis

BACKGROUND

The cell cycle of mammary stem cells must be tightly regulated to ensure normal homeostasis of the mammary gland to prevent abnormal proliferation and susceptibility to tumorigenesis. The atypical cell cycle regulator, Spy1 can override cell cycle checkpoints, including those activated by the tumour suppressor p53 which mediates mammary stem cell homeostasis. Spy1 has also been shown to promote expansion of select stem cell populations in other developmental systems. Spy1 protein is elevated during proliferative stages of mammary gland development, is found at higher levels in human breast cancers, and promotes susceptibility to mammary tumourigenesis when combined with loss of p53. We hypothesized that Spy1 cooperates with loss of p53 to increase susceptibility to tumour initiation due to changes in susceptible mammary stem cell populations during development and drives the formation of more aggressive stem like tumours.

METHODS

Using a transgenic mouse model driving expression of Spy1 within the mammary gland, mammary development and stemness were assessed. These mice were intercrossed with p53 null mice to study the tumourigenic properties of Spy1 driven p53 null tumours, as well as global changes in signaling via RNA sequencing analysis.

RESULTS

We show that elevated levels of Spy1 leads to expansion of mammary stem cells, even in the presence of p53, and an increase in mammary tumour formation. Spy1-driven tumours have an increased cancer stem cell population, decreased checkpoint signaling, and demonstrate an increase in therapy resistance. Loss of Spy1 decreases tumor onset and reduces the cancer stem cell population.

CONCLUSIONS

This data demonstrates the potential of Spy1 to expand mammary stem cell populations and contribute to the initiation and progression of aggressive, breast cancers with increased cancer stem cell populations.

Transcriptomic Analysis of Pubertal and Adult Virgin Mouse Mammary Epithelial and Stromal Cell Populations

Conflicting data exist as to how mammary epithelial cell proliferation changes during the reproductive cycle. To study the effect of endogenous hormone fluctuations on gene expression in the mouse mammary gland, we performed bulk RNAseq analyses of epithelial and stromal cell populations that were isolated either during puberty or at different stages of the adult virgin estrous cycle. Our data confirm prior findings that proliferative changes do not occur in every mouse in every cycle. We also show that during the estrous cycle the main gene expression changes occur in adipocytes and fibroblasts. Finally, we present a comprehensive overview of the Wnt gene expression landscape in different mammary gland cell types in pubertal and adult mice. This work contributes to understanding the effects of physiological hormone fluctuations and locally produced signaling molecules on gene expression changes in the mammary gland during the reproductive cycle and should be a useful resource for future studies investigating gene expression patterns in different cell types across different developmental timepoints.

Codonopsis lanceolata polysaccharide ameliorates high-fat diet induced-postpartum hypogalactia via stimulating prolactin receptor-mediated Jak2/Stat5 signaling

This study aims to investigate the ameliorative effect of Codonopsis lanceolata polysaccharide (PCL) on mice with hypogalatia induced by a high-fat diet (HFD) and the potential underlying mechanism. We found that oral administration of PCL demonstrated significant benefits in countering the negative effects of HFD, including weight gain, hepatic steatosis, mesenteric adipocyte hypertrophy, and abnormal glucose/lipid metabolism. In addition, PCL improved mammary gland development and enhanced lactogenesis performance. Histologically, PCL ameliorated the retardation of ductal growth, reduced mammary fat pad thickness, improved the incomplete linear encapsulation of luminal epithelium and myoepithelium, and increased the proliferation of mammary epithelial cells. Flow cytometry analysis showed that PCL mitigated the detrimental effects of HFD on mammary gland development by promoting the proliferation and differentiation of mammary epithelial cells. Mechanistic studies revealed that PCL upregulated the levels of prolactin (PRL) and its receptor (PRLR) in the mammary gland, activated JAK2/STAT5 signaling pathway, and increased the expression of p63, ERBB4, and NRG1. Overall, PCL can ameliorate HFD-induced hypogalactia by activating PRLR-mediated JAK2/STAT5 signaling. Our findings offer a methodological and theoretical foundation for investigating the functional constituents of traditional Chinese medicine in the treatment of hypogalactia.

Single-cell and spatial analyses reveal a tradeoff between murine mammary proliferation and lineage programs associated with endocrine cues

Although distinct epithelial cell types have been distinguished in glandular tissues such as the mammary gland, the extent of heterogeneity within each cell type and the degree of endocrine control of this diversity across development are incompletely understood. By combining mass cytometry and cyclic immunofluorescence, we define a rich array of murine mammary epithelial cell subtypes associated with puberty, the estrous cycle, and sex. These subtypes are differentially proliferative and spatially segregate distinctly in adult versus pubescent glands. Further, we identify systematic suppression of lineage programs at the protein and RNA levels as a common feature of mammary epithelial expansion during puberty, the estrous cycle, and gestation and uncover a pervasive enrichment of ribosomal protein genes in luminal cells elicited specifically during progesterone-dominant expansionary periods. Collectively, these data expand our knowledge of murine mammary epithelial heterogeneity and connect endocrine-driven epithelial expansion with lineage suppression.

Differential DNA damage repair and PARP inhibitor vulnerability of the mammary epithelial lineages

It is widely assumed that all normal somatic cells can equally perform homologous recombination (HR) and non-homologous end joining in the DNA damage response (DDR). Here, we show that the DDR in normal mammary gland inherently depends on the epithelial cell lineage identity. Bioinformatics, post-irradiation DNA damage repair kinetics, and clonogenic assays demonstrated luminal lineage exhibiting a more pronounced DDR and HR repair compared to the basal lineage. Consequently, basal progenitors were far more sensitive to poly(ADP-ribose) polymerase inhibitors (PARPis) in both mouse and human mammary epithelium. Furthermore, PARPi sensitivity of murine and human breast cancer cell lines as well as patient-derived xenografts correlated with their molecular resemblance to the mammary progenitor lineages. Thus, mammary epithelial cells are intrinsically divergent in their DNA damage repair capacity and PARPi vulnerability, potentially influencing the clinical utility of this targeted therapy.

A transcriptional response to replication stress selectively expands a subset of Brca2-mutant mammary epithelial cells

Germline BRCA2 mutation carriers frequently develop luminal-like breast cancers, but it remains unclear how BRCA2 mutations affect mammary epithelial subpopulations. Here, we report that monoallelic Brca2mut/WT mammary organoids subjected to replication stress activate a transcriptional response that selectively expands Brca2mut/WT luminal cells lacking hormone receptor expression (HR-). While CyTOF analyses reveal comparable epithelial compositions among wildtype and Brca2mut/WT mammary glands, Brca2mut/WT HR- luminal cells exhibit greater organoid formation and preferentially survive and expand under replication stress. ScRNA-seq analysis corroborates the expansion of HR- luminal cells which express elevated transcript levels of Tetraspanin-8 (Tspan8) and Thrsp, plus pathways implicated in replication stress survival including Type I interferon responses. Notably, CRISPR/Cas9-mediated deletion of Tspan8 or Thrsp prevents Brca2mut/WT HR- luminal cell expansion. Our findings indicate that Brca2mut/WT cells activate a transcriptional response after replication stress that preferentially favours outgrowth of HR- luminal cells through the expression of interferon-responsive and mammary alveolar genes.

Macrophages maintain mammary stem cell activity and mammary homeostasis via TNF-α-PI3K-Cdk1/Cyclin B1 axis

Adult stem cell niche is a special environment composed of a variety stromal cells and signals, which cooperatively regulate tissue development and homeostasis. It is of great interest to study the role of immune cells in niche. Here, we show that mammary resident macrophages regulate mammary epithelium cell division and mammary development through TNF-α-Cdk1/Cyclin B1 axis. In vivo, depletion of macrophages reduces the number of mammary basal cells and mammary stem cells (MaSCs), while increases mammary luminal cells. In vitro, we establish a three-dimensional culture system in which mammary basal cells are co-cultured with macrophages, and interestingly, macrophage co-culture promotes the formation of branched functional mammary organoids. Moreover, TNF-α produced by macrophages activates the intracellular PI3K/Cdk1/Cyclin B1 signaling in mammary cells, thereby maintaining the activity of MaSCs and the formation of mammary organoids. Together, these findings reveal the functional significance of macrophageal niche and intracellular PI3K/Cdk1/Cyclin B1 axis for maintaining MaSC activity and mammary homeostasis.

ATM suppresses c-Myc overexpression in the mammary epithelium in response to estrogen

ATM gene mutation carriers are predisposed to estrogen-receptor-positive breast cancer (BC). ATM prevents BC oncogenesis by activating p53 in every cell; however, much remains unknown about tissue-specific oncogenesis after ATM loss. Here, we report that ATM controls the early transcriptional response to estrogens. This response depends on topoisomerase II (TOP2), which generates TOP2-DNA double-strand break (DSB) complexes and rejoins the breaks. When TOP2-mediated ligation fails, ATM facilitates DSB repair. After estrogen exposure, TOP2-dependent DSBs arise at the c-MYC enhancer in human BC cells, and their defective repair changes the activation profile of enhancers and induces the overexpression of many genes, including the c-MYC oncogene. CRISPR/Cas9 cleavage at the enhancer also causes c-MYC overexpression, indicating that this DSB causes c-MYC overexpression. Estrogen treatment induced c-Myc protein overexpression in mammary epithelial cells of ATM-deficient mice. In conclusion, ATM suppresses the c-Myc-driven proliferative effects of estrogens, possibly explaining such tissue-specific oncogenesis.

Dedifferentiation-mediated stem cell niche maintenance in early-stage ductal carcinoma in situ progression: insights from a multiscale modeling study

We present a multiscale agent-based model of ductal carcinoma in situ (DCIS) to study how key phenotypic and signaling pathways are involved in the early stages of disease progression. The model includes a phenotypic hierarchy, and key endocrine and paracrine signaling pathways, and simulates cancer ductal growth in a 3D lattice-free domain. In particular, by considering stochastic cell dedifferentiation plasticity, the model allows for study of how dedifferentiation to a more stem-like phenotype plays key roles in the maintenance of cancer stem cell populations and disease progression. Through extensive parameter perturbation studies, we have quantified and ranked how DCIS is sensitive to perturbations in several key mechanisms that are instrumental to early disease development. Our studies reveal that long-term maintenance of multipotent stem-like cell niches within the tumor are dependent on cell dedifferentiation plasticity, and that disease progression will become arrested due to dilution of the multipotent stem-like population in the absence of dedifferentiation. We have identified dedifferentiation rates necessary to maintain biologically relevant multipotent cell populations, and also explored quantitative relationships between dedifferentiation rates and disease progression rates, which may potentially help to optimize the efficacy of emerging anti-cancer stem cell therapeutics.

Trichodermin Induces G0/G1 Cell Cycle Arrest by Inhibiting c-Myc in Ovarian Cancer Cells and Tumor Xenograft-Bearing Mice

Ovarian cancer is a fatal gynecological cancer because of a lack of early diagnosis, which often relapses as chemoresistant. Trichodermin, a trichothecene first isolated from Trichoderma viride, is an inhibitor of eukaryotic protein synthesis. However, whether trichodermin is able to suppress ovarian cancer or not was unclear. In this study, trichodermin (0.5 µM or greater) significantly decreased the proliferation of two ovarian cancer cell lines A2780/CP70 and OVCAR-3. Normal ovarian IOSE 346 cells were much less susceptible to trichodermin than the cancer cell lines. Trichodermin predominantly inhibited ovarian cancer cells by inducing G0/G1 cell cycle arrest rather than apoptosis. Trichodermin decreased the expression of cyclin D1, CDK4, CDK2, retinoblastoma protein, Cdc25A, and c-Myc but showed little effect on the expression of p21^Waf1/Cip1, p27^Kip1, or p16^Ink4a. c-Myc was a key target of trichodermin. Trichodermin regulated the expression of Cdc25A and its downstream proteins via c-Myc. Overexpression of c-Myc attenuated trichodermin's anti-ovarian cancer activity. In addition, trichodermin decelerated tumor growth in BALB/c nude mice, proving its effectiveness in vivo. These findings suggested that trichodermin has the potential to contribute to the treatment of ovarian cancer.

Characterization of Gene Expression Signatures for the Identification of Cellular Heterogeneity in the Developing Mammary Gland

The developing mammary gland depends on several transcription-dependent networks to define cellular identities and differentiation trajectories. Recent technological advancements that allow for single-cell profiling of gene expression have provided an initial picture into the epithelial cellular heterogeneity across the diverse stages of gland maturation. Still, a deeper dive into expanded molecular signatures would improve our understanding of the diversity of mammary epithelial and non-epithelial cellular populations across different tissue developmental stages, mouse strains and mammalian species. Here, we combined differential mammary gland fractionation approaches and transcriptional profiles obtained from FACS-isolated mammary cells to improve our definitions of mammary-resident, cellular identities at the single-cell level. Our approach yielded a series of expression signatures that illustrate the heterogeneity of mammary epithelial cells, specifically those of the luminal fate, and uncovered transcriptional changes to their lineage-defined, cellular states that are induced during gland development. Our analysis also provided molecular signatures that identified non-epithelial mammary cells, including adipocytes, fibroblasts and rare immune cells. Lastly, we extended our study to elucidate expression signatures of human, breast-resident cells, a strategy that allowed for the cross-species comparison of mammary epithelial identities. Collectively, our approach improved the existing signatures of normal mammary epithelial cells, as well as elucidated the diversity of non-epithelial cells in murine and human breast tissue. Our study provides a useful resource for future studies that use single-cell molecular profiling strategies to understand normal and malignant breast development.

An Intravital Microscopy Toolbox to Study Mammary Gland Dynamics from Cellular Level to Organ Scale

The architecture of the mouse mammary gland is highly dynamic and constantly remodeled during pubertal development and estrous cycle-driven sprouting and regression of alveolar side branches. During each of these developmental stages, turnover is driven by distinct subsets of mammary epithelial cells. Extensive previous research has shed light on the unique morphological and cell biological characteristics of each stage. However, technological shortcomings failed to capture the dynamics and single-cell contributions to mammary remodeling. Here, we developed in vivo imaging strategies to follow the same mammary ducts over time and quantify the dynamics of mammary gland growth and remodeling from single-cell level to organ scale. Using a combination of intravital microscopy and genetic reporter systems we show how proliferative heterogeneity drives ductal morphogenesis during different developmental stages. To visualize pubertal growth at the cellular level, we performed long-term time-lapse imaging of extending terminal end buds through a mammary imaging window. We show that single-cells within the terminal end buds are extremely motile and continuously exchange position whilst the duct is elongating. To visualize short-term remodeling in the adult mammary gland at the single cell level, we performed multi-day intravital imaging in photoconvertible Kikume Green-Red mice and fluorescent ubiquitination-based cell cycle indicator mice. We demonstrate that the contribution of single-cells to estrous-driven remodeling is highly variable between cells in the same micro-environment. To assess the effects of this dynamic proliferative contribution on the long-term stability of tissue architecture, we developed a repeated skin flap method to assess mammary gland morphology by intravital microscopy over extended time spans for up to six months. Interestingly, in contrast to the short-term dynamic remodeling, the long-term morphology of the mammary gland remains remarkably stable. Together, our tool box of imaging strategies allows to identify and map transient and continuing dynamics of single cells to the architecture of the mammary gland.

GDNF drives rapid tubule morphogenesis in a novel 3D in vitro model for ADPKD

Cystogenesis is a morphological consequence of numerous genetic diseases of the epithelium. In the kidney, the pathogenic mechanisms underlying the program of altered cell and tubule morphology are obscured by secondary effects of cyst expansion. Here, we developed a new 3D tubuloid system to isolate the rapid changes in protein localization and gene expression that correlate with altered cell and tubule morphology during cyst initiation. Mouse renal tubule fragments were pulsed with a cell differentiation cocktail including glial-derived neurotrophic factor (GDNF) to yield collecting duct-like tubuloid structures with appropriate polarity, primary cilia, and gene expression. Using the 3D tubuloid model with an inducible Pkd2 knockout system allowed the tracking of morphological, protein, and genetic changes during cyst formation. Within hours of inactivation of Pkd2 and loss of polycystin-2, we observed significant progression in tubuloid to cyst morphology that correlated with 35 differentially expressed genes, many related to cell junctions, matrix interactions, and cell morphology previously implicated in cystogenesis.This article has an associated First Person interview with the first author of the paper.

Mammary stem cells and progenitors: targeting the roots of breast cancer for prevention

Breast cancer prevention is daunting, yet not an unsurmountable goal. Mammary stem and progenitors have been proposed as the cells-of-origin in breast cancer. Here, we present the concept of limiting these breast cancer precursors as a risk reduction approach in high-risk women. A wealth of information now exists for phenotypic and functional characterization of mammary stem and progenitor cells in mouse and human. Recent work has also revealed the hormonal regulation of stem/progenitor dynamics as well as intrinsic lineage distinctions between mammary epithelial populations. Leveraging these insights, molecular marker-guided chemoprevention is an achievable reality.

Identifying the murine mammary cell target of metformin exposure

The heterogeneity of breast cancer makes current therapies challenging. Metformin, the anti-diabetic drug, has shown promising anti-cancer activities in epidemiological studies and breast cancer models. Yet, how metformin alters the normal adult breast tissue remains elusive. We demonstrate metformin intake at a clinically relevant dose impacts the hormone receptor positive (HR+) luminal cells in the normal murine mammary gland. Metformin decreases total cell number, progenitor capacity and specifically reduces DNA damage in normal HR+ luminal cells, decreases oxygen consumption rate and increases cell cycle length of luminal cells. HR+ luminal cells demonstrate the lowest levels of mitochondrial respiration and capacity to handle oxidative stress compared to the other fractions, suggesting their intrinsic susceptibility to long-term metformin exposure. Uncovering HR+ luminal cells in the normal mammary gland as the major cell target of metformin exposure could identify patients that would most benefit from repurposing this anti-diabetic drug for cancer prevention/therapy purposes.