Prolactin Alters the Mammary Epithelial Hierarchy, Increasing Progenitors and Facilitating Ovarian Steroid Action

Exploring the Mechanism of Myrrh in the Treatment of Breast Cancer Based on Network Pharmacology and Cell Experiments

This study aimed to investigate the mechanism of action of myrrh in breast cancer (BC) treatment and identify its effective constituents. Data on the compounds and targets of myrrh were collected from the TCMSP, PubChem, and Swiss Target Prediction databases. BC-related targets were obtained from the Genecard database. A protein-protein interaction (PPI) analysis, gene ontology (GO) enrichment, and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis were conducted on the intersecting targets of the disease and drug. The key targets of myrrh in BC treatment were identified based on the PPI network. The active constituents of myrrh were determined through reverse-screening using the top 20 KEGG pathways. Macromolecular docking studies, molecular dynamic (MD) simulations, and cell assays were utilized to validate the active constituents and critical targets. Network pharmacology indicated that VEGFA, TP53, ESR1, EGFR, and AKT1 are key targets of myrrh. Pelargonidin chloride, Quercetin, and Naringenin were identified as the active constituents of myrrh. Macromolecular docking showed that Quercetin and Naringenin have strong docking capabilities with ESR1. The results of MD simulation experiments align with those of molecular docking experiments. Cell and western blot assays demonstrated that Quercetin and Naringenin could inhibit MCF-7 cells and significantly reduce the expression of ESR1 protein. The findings reveal the active constituents, key targets, and molecular mechanisms of myrrh in BC treatment, providing scientific evidence that supports the role of myrrh in BC therapy. Furthermore, the results suggest that network pharmacology predictions require experimental validation for reliability.

Prolactin: The Third Hormone in Breast Cancer

Prolactin coordinates with the ovarian steroids to orchestrate mammary development and lactation, culminating in nourishment and an increasingly appreciated array of other benefits for neonates. Its central activities in mammary epithelial growth and differentiation suggest that it plays a role(s) in breast cancer, but it has been challenging to identify its contributions, essential for incorporation into prevention and treatment approaches. Large prospective epidemiologic studies have linked higher prolactin exposure to increased risk, particularly for ER+ breast cancer in postmenopausal women. However, it has been more difficult to determine its actions and clinical consequences in established tumors. Here we review experimental data implicating multiple mechanisms by which prolactin may increase the risk of breast cancer. We then consider the evidence for role(s) of prolactin and its downstream signaling cascades in disease progression and treatment responses, and discuss how new approaches are beginning to illuminate the biology behind the seemingly conflicting epidemiologic and experimental studies of prolactin actions across diverse breast cancers.

NSG-Pro mouse model for uncovering resistance mechanisms and unique vulnerabilities in human luminal breast cancers

Most breast cancer deaths are caused by estrogen receptor-α–positive (ER+) disease. Preclinical progress is hampered by a shortage of therapy-naïve ER+ tumor models that recapitulate metastatic progression and clinically relevant therapy resistance. Human prolactin (hPRL) is a risk factor for primary and metastatic ER+ breast cancer. Because mouse prolactin fails to activate hPRL receptors, we developed a prolactin-humanized Nod-SCID-IL2Rγ (NSG) mouse (NSG-Pro) with physiological hPRL levels. Here, we show that NSG-Pro mice facilitate establishment of therapy-naïve, estrogen-dependent PDX tumors that progress to lethal metastatic disease. Preclinical trials provide first-in-mouse efficacy of pharmacological hPRL suppression on residual ER+ human breast cancer metastases and document divergent biology and drug responsiveness of tumors grown in NSG-Pro versus NSG mice. Oncogenomic analyses of PDX lines in NSG-Pro mice revealed clinically relevant therapy-resistance mechanisms and unexpected, potently actionable vulnerabilities such as DNA-repair aberrations. The NSG-Pro mouse unlocks previously inaccessible precision medicine approaches for ER+ breast cancers.

Endogenous and Therapeutic Estrogens: Maestro Conductors of the Microenvironment of ER+ Breast Cancers

Estrogen receptor alpha (ERα) marks heterogeneous breast cancers which display a repertoire of somatic genomic mutations and an immune environment that differs from other breast cancer subtypes. These cancers also exhibit distinct biological behaviors; despite an overall better prognosis than HER2+ or triple negative breast cancers, disseminated dormant cells can lead to disease recurrence decades after the initial diagnosis and treatment. Estrogen is the best studied driver of these cancers, and antagonism or reduction of estrogen activity is the cornerstone of therapeutic approaches. In addition to reducing proliferation of ERα+ cancer cells, these treatments also alter signals to multiple other target cells in the environment, including immune cell subpopulations, cancer-associated fibroblasts, and endothelial cells via several distinct estrogen receptors. In this review, we update progress in our understanding of the stromal cells populating the microenvironments of primary and metastatic ER+ tumors, the effects of estrogen on tumor and stromal cells to modulate immune activity and the extracellular matrix, and net outcomes in experimental and clinical studies. We highlight new approaches that will illuminate the unique biology of these cancers, provide the foundation for developing new treatment and prevention strategies, and reduce mortality of this disease.

Single Nucleotide Polymorphism in Prolactin Gene Is Associated With Clinical Aggressiveness and Outcome of Canine Mammary Malignant Tumors

Prolactin (PRL) is a key hormone involved in canine mammary development and tumorigenesis. In this study, the influence of a single nucleotide polymorphism (SNP) in the PRL gene (rs23932236) on the clinicopathological parameters and survival of dogs with canine mammary tumors (CMTs) was investigated. A total of 206 female dogs with spontaneous mammary tumors were enrolled in this study and circulating blood cells were genotyped. This specific SNP was associated with larger size (>3 cm diameter) for malignant tumors (P = .036), tumors with infiltrative/invasive growth pattern (P = .010), vascular invasion (P = .006), and lymph node metastasis (P = .004). Carriers of the variant allele had a shorter overall survival compared to the wild-type population with an overall survival of 18.7 months and 22.7 months, respectively (P = .004). These findings suggest that SNP rs23932236 of canine PRL gene may be used as an indicator for the development of clinically aggressive forms of CMTs.

Prolactin synergizes with canonical Wnt signals to drive development of ER+ mammary tumors via activation of the Notch pathway

Prolactin (PRL) cooperates with other factors to orchestrate mammary development and lactation, and is epidemiologically linked to higher risk for breast cancer. However, how PRL collaborates with oncogenes to foster tumorigenesis and influence breast cancer phenotype is not well understood. To understand its interactions with canonical Wnt signals, which elevate mammary stem cell activity, we crossed heterozygous NRL-PRL mice with Apc^Min/+ mice and treated pubertal females with a single dose of mutagen. PRL in the context of Apc^Min/+ fueled a dramatic increase in tumor incidence in nulliparous mice, compared to Apc^Min/+ alone. Although carcinomas in both NRL-PRL/Apc^Min/+ and Apc^Min/+ females acquired a mutation in the remaining wildtype Apc allele and expressed abundant β-catenin, PRL-promoted tumors displayed higher levels of Notch-driven target genes and Notch-dependent cancer stem cell activity, compared to β-catenin-driven activity in Apc^Min/+ tumors. This PRL-induced shift to dominant Notch signals was evident in preneoplastic epithelial hyperplasias at 120 days of age. In NRL-PRL/Apc^Min/+ females, rapidly proliferating hyperplasias, characterized by β-catenin at cell junctions and high NOTCH1 expression, contrasted with slower growing lesions with nuclear β-catenin in Apc^Min/+ females. These studies demonstrate that PRL can powerfully modulate the incidence and phenotype of mammary tumors, shedding light on mechanisms whereby PRL elevates risk of breast cancer.

Prolactin: A hormone with diverse functions from mammary gland development to cancer metastasis

Prolactin has a rich mechanistic set of actions and signaling in order to elicit developmental effects in mammals. Historically, prolactin has been appreciated as an endocrine peptide hormone that is responsible for final, functional mammary gland development and lactation. Multiple signaling pathways impacted upon by the microenvironment contribute to cell function and differentiation. Endocrine, autocrine and paracrine signaling are now apparent in not only mammary development, but also in cancer, and involve multiple cell types including those of the immune system. Multiple ligands agonists are capable of binding to the prolactin receptor, potentially expanding receptor function. Prolactin has an important role not only in tumorigenesis of the breast, but also in a number of hormonally responsive cancers such as prostate, ovarian and endometrial cancer, as well as pancreatic and lung cancer. Although pituitary and extra-pituitary sources of prolactin such as the epithelium are important, stromal sourced prolactin is now also being recognized as an important factor in tumor progression, all of which potentially signal to multiple cell types in the tumor microenvironment. While prolactin has important roles in milk production including calcium and bone homeostasis, in the disease state it can also affect bone homeostasis. Prolactin also impacts metastatic cancer of the breast to modulate the bone microenvironment and promote bone damage. Prolactin has a fascinating contribution in both physiologic and pathologic settings of mammals.

The mesenchymal property of mouse mammary anlagen repopulating cell population is associated with its stemness

During early embryogenesis, mammary glands are derived from surface ectoderm and their morphogenesis is controlled by mammary stem cells (MaSCs) and epithelial-mesenchymal transition (EMT). Mammary anlagen stage (E13.5-15.5) is an important stage for fetal mice to achieve EMT dependent mammary morphogenesis. And the characteristics of mammary anlagen repopulating cell population (MaRC) should be identified for understanding its stemness at earlier embryonic stage. Here we quantify and characterize MaSCs proportion at mammary anlagen stage. Compared with adult mouse mammary gland, our data revealed that E14.5 mammary anlagen exhibit higher stem cell activities. Then we purified mammary anlagen cell populations depending on the expression levels of CD24 and CD49f in mouse mammary anlagen, and identified an unique MaRC population (Lin-CD24medCD49f+) by real-time PCR, transplantation and mammosphere forming assays. In addition, by comparing with adult MaSC (Lin-CD24+CD29hi) and differentiated mammary anlagen cells, we find that E14.5 mouse MaRC population exhibit gene expression programs related to mesenchymal properties. To further identify the cell types of E14.5 mouse MaRC population, the expressions of K8, K14, K18, e-cadherin, n-cadherin and vimentin in mammary anlagen Lin-CD24medCD49f + cells were detected by immunofluorescence assay. These findings verified that the undifferentiated E14.5 mouse MaRC population is a heterogeneous population with mesenchymal property, which is associated with cell stemness and mammary duct morphogenesis.

A Spontaneous Aggressive ERα+ Mammary Tumor Model Is Driven by Kras Activation

The NRL-PRL murine model, defined by mammary-selective transgenic rat prolactin ligand rPrl expression, establishes spontaneous ER+ mammary tumors in nulliparous females, mimicking the association between elevated prolactin (PRL) and risk for development of ER+ breast cancer in postmenopausal women. Whole-genome and exome sequencing in a discovery cohort (n = 5) of end-stage tumors revealed canonical activating mutations and copy number amplifications of Kras. The frequent mutations in this pathway were validated in an extension cohort, identifying activating Ras alterations in 79% of tumors (23 of 29). Transcriptome analyses over the course of oncogenesis revealed marked alterations associated with Ras activity in established tumors compared with preneoplastic tissues; in cell-intrinsic processes associated with mitosis, cell adhesion, and invasion; as well as in the surrounding tumor environment. These genomic analyses suggest that PRL induces a selective bottleneck for spontaneous Ras-driven tumors that may model a subset of aggressive clinical ER+ breast cancers.

Glycomics: Immunoglobulin G N-Glycosylation Associated with Mammary Gland Hyperplasia in Women

Mammary gland hyperplasia (MGH) is very common, especially among young and middle-aged women. New diagnostics and biomarkers for MGH are needed for rational clinical management and precision medicine. We report, in this study, new findings using a glycomics approach, with a focus on immunoglobulin G (IgG) N-glycosylation. A cross-sectional study was conducted in a community-based population sample in Beijing, China. A total of 387 participants 40-65 years of age were enrolled in this study, including 194 women with MGH (cases) and 193 women who had no MGH (controls). IgG N-glycans were characterized in the serum by ultra-performance liquid chromatography. The levels of the glycan peaks (GPs) GP2, GP5, GP6, and GP7 were lower in the MGH group compared with the control group, whereas GP14 was significantly higher in the MGH group (p < 0.05). A predictive model using GP5, GP21, and age was established and a receiver operating characteristic curve analysis was performed. The sensitivity and specificity of the model for MGH was 61.3% and 63.2%, respectively, likely owing to receptor mechanisms and/or inflammation regulation. To the best of our knowledge, this is the first study reporting on an association between IgG N-glycosylation and MGH. We suggest person-to-person variations in IgG N-glycans and their combination with multiomics biomarker strategies offer a promising avenue to identify novel diagnostics and individuals at increased risk of MGH.

Expression and functions of cluster of differentiation 9 and 81 in rat mammary epithelial cells

Cluster of differentiation (CD) 9 and CD81 are closely-related members of the tetraspanin family that consist of four-transmembrane domain proteins. Cd9 and Cd81 are highly expressed in breast cancer cells; however, their expression in healthy mammary glands is unclear. In this study, we performed quantitative real-time PCR to analyze the expression levels of Cd9 and Cd81. Histological techniques were employed to identify Cd9- and Cd81-expressing cells in rat mammary glands during pregnancy and lactation. It was observed that Cd9 and Cd81 were expressed in the mammary glands, and their expression levels correlated with mammary gland development. To identify cells expressing Cd9 and Cd81 in the mammary glands, we performed double immunohistochemical staining for CD9 and CD81, prolactin receptor long form, estrogen receptor alpha, or Ki67. The results showed that CD9 and CD81 were co-expressed in proliferating mammary epithelial cells. Next, we attempted to isolate CD9-positive epithelial cells from the mammary gland using pluriBead cell-separation technology based on antibody-mediated binding of cells to beads of different sizes, followed by isolation using sieves with different mesh sizes. We successfully isolated CD9-positive epithelial cells with 96.8% purity. In addition, we observed that small-interfering RNAs against Cd9 and Cd81 inhibited estrogen-induced proliferation of CD9-positive mammary epithelial cells. Our current findings may provide novel insights into the proliferation of mammary epithelial cells during pregnancy and lactation as well as in pathological processes associated with breast cancer.

Effects of Hydroxytyrosol against Lipopolysaccharide-Induced Inflammation and Oxidative Stress in Bovine Mammary Epithelial Cells: A Natural Therapeutic Tool for Bovine Mastitis

Background: Bovine mastitis is a growing health problem, affecting both welfare of dairy cattle and milk production. It often leads to chronic infections, disturbing the quality of milk and resulting in cow death. Thus, it has a great economic impact for breeders. Methods: In this study, we evaluated the protective effect of hydroxytyrosol—a natural molecule which is the major constituent of many phyto-complexes—in an in vitro model of mastitis induced by LPS (1μg/mL). Results: Our results showed that hydroxytyrosol (10 and 25 μM) was able to prevent the oxidative stress induced by LPS (intracellular ROS, GSH and NOX-1) and the consequently inflammatory response (TNF-α, IL-1β and IL-6). The protective effect of hydroxytyrosol is also related to the enhancement of endogenous antioxidant systems (Nrf2, HO-1, NQO-1 and Txnrd1). Moreover, hydroxytyrosol showed an important protective effect on cell functionality (α-casein S1, α-casein S2 and β-casein). Conclusions: Taken together, our results showed a significant protective effect of hydroxytyrosol on oxidative stress and inflammatory response in MAC-T cells. Thus, we indicated a possible important therapeutic role for hydroxytyrosol in the prevention or management of bovine mastitis.

Characterization of microRNA profiles in the mammary gland tissue of dairy goats at the late lactation, dry period and late gestation stages

MicroRNAs (miRNAs) play an important role in regulating mammary gland development and lactation. We previously analyzed miRNA expression profiles in Laoshan dairy goat mammary glands at the early (20 d postpartum), peak (90 d postpartum) and late lactation (210 d postpartum) stages. To further enrich and clarify the miRNA expression profiles during the lactation physiological cycle, we sequenced miRNAs in the mammary gland tissues of Laoshan dairy goats at three newly selected stages: the late lactation (240 d postpartum), dry period (300 d postpartum) and late gestation (140 d after mating) stages. We obtained 4038 miRNAs and 385 important miRNA families, including mir-10, let-7 and mir-9. We also identified 754 differentially expressed miRNAs in the mammary gland tissue at the 3 different stages and 6 groups of miRNA clusters that had unique expression patterns. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses showed that GO terms such as mammary gland development (GO:0030879) and mammary gland morphogenesis (GO:0060443) and important signaling pathways, including the insulin signaling pathway (chx04910), hippo signaling pathway (chx04390) and estrogen signaling pathway (chx04915), were enriched. We screened miRNAs and potential target genes that may be involved in the regulation of lactation, mammary gland growth and differentiation, cell apoptosis, and substance transport and synthesis and detected the expression patterns of important genes at the three stages. These miRNAs and critical target genes may be important factors for mammary gland development and lactation regulation and potentially valuable molecular markers, which may provide a theoretical reference for further investigation of mammary gland physiology.

Effects of Dietary Supplementation of Lauric Acid on Lactation Function, Mammary Gland Development, and Serum Lipid Metabolites in Lactating Mice

Simple Summary

Milk secreted from mammary glands is an important nutrition source for offspring after parturition. Mammary gland development and lactation ability have important effects on the growth and health of the offspring. Many studies have demonstrated that external factors, including the environment and nutrition influence the development of mammary glands. Lauric acid is a fatty acid that has many nutritional and physiological properties. In this study, we investigated the effects of dietary supplementation of lauric acid on lactation function and mammary gland development in lactating mice. We found that dietary supplementation of lauric acid during lactation might enhance the mammary development to promote the lactation function of mice. Through the study of mice, we hoped that the results could be applied to animal feed development and animal breeding production.

Abstract

Our previous studies demonstrated that lauric acid (LA) stimulated mammary gland development during puberty. However, the roles of LA on lactation in mice remain indeterminate. Thus, the aim of this study was to investigate the effects of dietary LA supplementation on lactation functioning and to study the potential mechanisms during lactation. in vivo, there was no effect of 1% LA dietary supplementation during lactation on the feed intake or body weight of breast-feeding mice. However, maternal LA supplementation significantly expanded the number of mammary gland alveoli of mice during lactation and the average body weight of the offspring, suggesting that LA supplementation enhanced the development and lactation function of the mammary glands. in vitro, 100 μM of LA significantly increased the content of triglycerides (TG) in the cell supernatant of induced HC11 cells, however, with no effect on the expression of the genes associated with fatty acid synthesis. LA also activated the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) signaling pathway. LA dietary supplementation significantly expanded the serum levels of lipid metabolites, including sphingomyelin and other metabolites with the sn-2 position of C12 and sn-1 position of C18 in the TG of the lactating mice. Taken together, dietary supplementation of LA during lactation could promote the lactation function of mice, which might be related to increasing the development of the mammary glands and alternation of serum lipid metabolites. These findings provided more theoretical and experimental basis for the application of lauric acid in the development of mammary glands and lactation function of lactating animals.

Moringa Extract Attenuates Inflammatory Responses and Increases Gene Expression of Casein in Bovine Mammary Epithelial Cells

Bovine mastitis is a common inflammatory disease in the udder of dairy cows that causes economic loss to dairy industries. The development of alternative strategies, especially the utilization of natural products, e.g. Moringa oleifera, has gained a lot of interests. The objective of the current study was to investigate the protective effects of moringa extract (ME) in bovine mammary epithelial cells (MAC-T) in in vitro settings. Radical scavenging capacities and anti-inflammatory properties of ME were examined using lipopolysaccharide (LPS)-challenged MAC-T cells. ME showed significant radical scavenging activities. In addition, ME decreased reactive oxygen species produced by LPS in cells. ME also attenuated inflammatory cyclooxygenase-2 expression induced by LPS by down-regulating NF-κB signaling cascade. Moreover, ME ameliorated LPS-induced pro-inflammatory cytokines including tumor necrosis factor-, interleukin-1, and interleukin-6. Furthermore, ME up-regulated mRNA expression levels of heme oxygenase-1, NAD(P)H: quinone oxidoreductase-1, and thioredoxin reductase 1. Importantly, ME promoted differentiated MAC-T cells by increasing mRNA expression levels of α-casein S1, α-casein S2, and β-casein. In conclusion, ME has beneficial effects in bovine mammary epithelial cells through its anti-inflammatory, antioxidant, and casein production properties. Our study provides evidence that ME could be a good candidate for a feed supplement to decrease inflammatory responses due to bovine mastitis.

Molecular signature of the putative stem/progenitor cells committed to the development of the bovine mammary gland at puberty

Milk production is highly dependent on the extensive development of the mammary epithelium, which occurs during puberty. It is therefore essential to distinguish the epithelial cells committed to development from the related epithelial hierarchy. Using cell phenotyping and sorting, we highlighted four cell sub-populations within the bovine mammary gland at puberty. The CD49_f^highCD24^neg cells expressing CD10, KRT14, vimentin and PROCR corresponded to cells committed to the basal lineage. The CD49_f^low sub-population contained two cell subsets (CD49_f^lowCD24^neg and CD49_f^lowCD24^pos). Both subsets expressed hormone receptors including ER, PR and PRLR, as well as ALDH1 activity but only the CD49_f^lowCD24^pos subset expressed ELF5. These data indicated that the CD49_f^low sub-population is mainly composed of cells displaying a luminal phenotype and that this population comprises two luminal cell subsets, namely the CD24^neg and CD24^pos cells, likely committed to ductal and alveolar lineage, respectively. The putative mammary stem cell (MaSC) fraction was recovered in the CD49_f^highCD24^pos sub-population which were shown to form mammospheres in vitro. These cells differentially expressed CD10, KRT14 and KRT7, suggesting the existence of several putative MaSC sub-fractions. In-depth characterization of these epithelial sub-populations provides new insights into the bovine mammary epithelial cell lineage and suggests a common developmental lineage in mammals.

Prolactin signaling drives tumorigenesis in human high grade serous ovarian cancer cells and in a spontaneous fallopian tube derived model

The pathways responsible for tumorigenesis of high grade serous ovarian cancer (HGSOC) from the fallopian tube epithelium (FTE) are still poorly understood. A human prolactin (PRL) like gene, Prl2c2 was amplified >100 fold in a spontaneous model of FTE-derived ovarian cancer (MOE^high - murine oviductal epithelium high passage). Prl2c2 stable knockdown in MOE^high cells demonstrated a significant reduction in cell proliferation, 2-dimensional foci, anchorage independent growth, and blocked tumor formation. The overall survival of ovarian cancer patients from transcriptome analysis of 1868 samples was lower when abundant PRL and prolactin receptors (PRL-R) were expressed. A HGSOC cell line (OVCAR3) and a tumorigenic human FTE cell line (FT33-Tag-Myc) were treated with recombinant PRL and a significant increase in cellular proliferation was detected. A CRISPR/Cas9 mediated PRL-R deletion in OVCAR3 and FT33-Tag-Myc cells demonstrated significant reduction in cell proliferation and eliminated tumor growth using the OVCAR3 model. PRL was found to phosphorylate STAT5, m-TOR and ERK in ovarian cancer cells. This study identified Prl2c2 as a driver of tumorigenesis in a spontaneous model and confirmed that prolactin signaling supports tumorigenesis in high grade serous ovarian cancer.

Antiestrogen Therapy Increases Plasticity and Cancer Stemness of Prolactin-Induced ERα+ Mammary Carcinomas

Although antiestrogen therapies are successful in many patients with estrogen receptor alpha-positive (ERα⁺) breast cancer, 25% to 40% fail to respond. Although multiple mechanisms underlie evasion of these treatments, including tumor heterogeneity and drug-resistant cancer stem cells (CSC), further investigations have been limited by the paucity of preclinical ERα⁺ tumor models. Here, we examined a mouse model of prolactin-induced aggressive ERα⁺ breast cancer, which mimics the epidemiologic link between prolactin exposure and increased risk for metastatic ERα⁺ tumors. Like a subset of ERα⁺ patient cancers, the prolactin-induced adenocarcinomas contained two major tumor subpopulations that expressed markers of normal luminal and basal epithelial cells. CSC activity was distributed equally across these two tumor subpopulations. Treatment with the selective estrogen receptor downregulator (SERD), ICI 182,780 (ICI), did not slow tumor growth, but induced adaptive responses in CSC activity, increased markers of plasticity including target gene reporters of Wnt/Notch signaling and epithelial-mesenchymal transition, and increased double-positive (K8/K5) cells. In primary tumorsphere cultures, ICI stimulated CSC self-renewal and was able to overcome the dependence of self-renewal upon Wnt or Notch signaling individually, but not together. Our findings demonstrate that treatment of aggressive mixed lineage ERα⁺ breast cancers with a SERD does not inhibit growth, but rather evokes tumor cell plasticity and regenerative CSC activity, predicting likely negative impacts on patient tumors with these characteristics.Significance: This study suggests that treatment of a subset of ERα⁺ breast cancers with antiestrogen therapies may not only fail to slow growth but also promote aggressive behavior by evoking tumor cell plasticity and regenerative CSC activity. Cancer Res; 78(7); 1672-84. ©2018 AACR.