LEFTY1 Is a Dual-SMAD Inhibitor that Promotes Mammary Progenitor Growth and Tumorigenesis.

SMAD pathways govern epithelial proliferation, and transforming growth factor ß (TGF-ß and BMP signaling through SMAD members has distinct effects on mammary development and homeostasis. Here, we show that LEFTY1, a secreted inhibitor of NODAL/SMAD2 signaling, is produced by mammary progenitor cells and, concomitantly, suppresses SMAD2 and SMAD5 signaling to promote long-term proliferation of normal and malignant mammary epithelial cells. In contrast, BMP7, a NODAL antagonist with context-dependent functions, is produced by basal cells and restrains progenitor cell proliferation. In normal mouse epithelium, LEFTY1 expression in a subset of luminal cells and rare basal cells opposes BMP7 to promote ductal branching. LEFTY1 binds BMPR2 to suppress BMP7-induced activation of SMAD5, and this LEFTY1-BMPR2 interaction is specific to tumor-initiating cells in triple-negative breast cancer xenografts that rely on LEFTY1 for growth. These results suggest that LEFTY1 is an endogenous dual-SMAD inhibitor and that suppressing its function may represent a therapeutic vulnerability in breast cancer.

Single-cell transcriptional diversity is a hallmark of developmental potential.

Single-cell RNA sequencing (scRNA-seq) is a powerful approach for reconstructing cellular differentiation trajectories. However, inferring both the state and direction of differentiation is challenging. Here, we demonstrate a simple, yet robust, determinant of developmental potential-the number of expressed genes per cell-and leverage this measure of transcriptional diversity to develop a computational framework (CytoTRACE) for predicting differentiation states from scRNA-seq data. When applied to diverse tissue types and organisms, CytoTRACE outperformed previous methods and nearly 19,000 annotated gene sets for resolving 52 experimentally determined developmental trajectories. Additionally, it facilitated the identification of quiescent stem cells and revealed genes that contribute to breast tumorigenesis. This study thus establishes a key RNA-based feature of developmental potential and a platform for delineation of cellular hierarchies.

Serially transplantable mammary epithelial cells express the Thy-1 antigen.

BACKGROUND: Recent studies in murine mammary tissue have identified functionally distinct cell populations that may be isolated by surface phenotype or lineage tracing. Previous groups have shown that CD24medCD49fhigh cells enriched for long-lived mammary epithelial cells can be serially transplanted. METHODS: Flow cytometry-based enrichment of distinct phenotypic populations was assessed for their gene expression profiles and functional proliferative attributes in vitro and in vivo. RESULTS: Here, we show Thy-1 is differentially expressed in the CD24medCD49fhigh population, which allowed us to discern two functionally different populations. The Thy-1+CD24medCD49fhigh phenotype contained the majority of the serially transplantable epithelial cells. The Thy-1-CD24medCD49fhigh phenotype contains a rare progenitor population that is able to form primary mammary outgrowths with significantly decreased serial in vivo transplantation potential. CONCLUSIONS: Therefore, Thy-1 expression in the immature cell compartment is a useful tool to study the functional heterogeneity that drives mammary gland development and has implications for disease etiology.

Role of epithelial to mesenchymal transition associated genes in mammary gland regeneration and breast tumorigenesis.

Previous studies have proposed that epithelial to mesenchymal transition (EMT) in breast cancer cells regulates metastasis, stem cell properties and chemo-resistance; most studies were based on in vitro culture of cell lines and mouse transgenic cancer models. However, the identity and function of cells expressing EMT-associated genes in normal murine mammary gland homeostasis and human breast cancer still remains under debate. Using in vivo lineage tracing and triple negative breast cancer (TNBC) patient derived xenografts we demonstrate that the repopulating capacity in normal mammary epithelial cells and tumorigenic capacity in TNBC is independent of expression of EMT-associated genes. In breast cancer, while a subset of cells with epithelial and mesenchymal phenotypes have stem cell activity, in many cells that have lost epithelial characteristics with increased expression of mesenchymal genes, have decreased tumor-initiating capacity and plasticity. These findings have implications for the development of effective therapeutic agents targeting tumor-initiating cells.

Design of KDM4 Inhibitors with Antiproliferative Effects in Cancer Models.

Histone lysine demethylases (KDMs) play a vital role in the regulation of chromatin-related processes. Herein, we describe our discovery of a series of potent KDM4 inhibitors that are both cell permeable and antiproliferative in cancer models. The modulation of histone H3K9me3 and H3K36me3 upon compound treatment was verified by homogeneous time-resolved fluorescence assay and by mass spectroscopy detection. Optimization of the series using structure-based drug design led to compound 6 (QC6352), a potent KDM4 family inhibitor that is efficacious in breast and colon cancer PDX models.

Remodeling of endogenous mammary epithelium by breast cancer stem cells.

Poorly regulated tissue remodeling results in increased breast cancer risk, yet how breast cancer stem cells (CSC) participate in remodeling is unknown. We performed in vivo imaging of changes in fluorescent, endogenous duct architecture as a metric for remodeling. First, we quantitatively imaged physiologic remodeling of primary branches of the developing and regenerating mammary tree. To assess CSC-specific remodeling events, we isolated CSC from MMTV-Wnt1 (mouse mammary tumor virus long-term repeat enhancer driving Wnt1 oncogene) breast tumors, a well studied model in which tissue remodeling affects tumorigenesis. We confirm that CSC drive tumorigenesis, suggesting a link between CSC and remodeling. We find that normal, regenerating, and developing gland maintain a specific branching pattern. In contrast, transplantation of CSC results in changes in the branching patterns of endogenous ducts while non-CSC do not. Specifically, in the presence of CSC, we identified an increased number of branches, branch points, ducts which have greater than 40 branches (5/33 for CSC and 0/39 for non-CSC), and histological evidence of increased branching. Moreover, we demonstrate that only CSC implants invade into surrounding stroma with structures similar to developing mammary ducts (nine for CSC and one for non-CSC). Overall, we demonstrate a novel approach for imaging physiologic and pathological remodeling. Furthermore, we identify unique, CSC-specific, remodeling events. Our data suggest that CSC interact with the microenvironment differently than non-CSC, and that this could eventually be a therapeutic approach for targeting CSC.

Identification of a cKit(+) colonic crypt base secretory cell that supports Lgr5(+) stem cells in mice.

BACKGROUND & AIMS: Paneth cells contribute to the small intestinal niche of Lgr5(+) stem cells. Although the colon also contains Lgr5(+) stem cells, it does not contain Paneth cells. We investigated the existence of colonic Paneth-like cells that have a distinct transcriptional signature and support Lgr5(+) stem cells. METHODS: We used multicolor fluorescence-activated cell sorting to isolate different subregions of colon crypts, based on known markers, from dissociated colonic epithelium of mice. We performed multiplexed single-cell gene expression analysis with quantitative reverse transcriptase polymerase chain reaction followed by hierarchical clustering analysis to characterize distinct cell types. We used immunostaining and fluorescence-activated cell sorting analyses with in vivo administration of a Notch inhibitor and in vitro organoid cultures to characterize different cell types. RESULTS: Multicolor fluorescence-activated cell sorting could isolate distinct regions of colonic crypts. Four major epithelial subtypes or transcriptional states were revealed by gene expression analysis of selected populations of single cells. One of these, the goblet cells, contained a distinct cKit/CD117(+) crypt base subpopulation that expressed Dll1, Dll4, and epidermal growth factor, similar to Paneth cells, which were also marked by cKit. In the colon, cKit(+) goblet cells were interdigitated with Lgr5(+) stem cells. In vivo, this colonic cKit(+) population was regulated by Notch signaling; administration of a γ-secretase inhibitor to mice increased the number of cKit(+) cells. When isolated from mouse colon, cKit(+) cells promoted formation of organoids from Lgr5(+) stem cells, which expressed Kitl/stem cell factor, the ligand for cKit. When organoids were depleted of cKit(+) cells using a toxin-conjugated antibody, organoid formation decreased. CONCLUSIONS: cKit marks small intestinal Paneth cells and a subset of colonic goblet cells that are regulated by Notch signaling and support Lgr5(+) stem cells.

Cancer stem cells from human breast tumors are involved in spontaneous metastases in orthotopic mouse models.

To examine the role of breast cancer stem cells (BCSCs) in metastasis, we generated human-in-mouse breast cancer orthotopic models using patient tumor specimens, labeled with optical reporter fusion genes. These models recapitulate human cancer features not captured with previous models, including spontaneous metastasis in particular, and provide a useful platform for studies of breast tumor initiation and progression. With noninvasive imaging approaches, as few as 10 cells of stably labeled BCSCs could be tracked in vivo, enabling studies of early tumor growth and spontaneous metastasis. These advances in BCSC imaging revealed that CD44(+) cells from both primary tumors and lung metastases are highly enriched for tumor-initiating cells. Our metastatic cancer models, combined with noninvasive imaging techniques, constitute an integrated approach that could be applied to dissect the molecular mechanisms underlying the dissemination of metastatic CSCs (MCSCs) and to explore therapeutic strategies targeting MCSCs in general or to evaluate individual patient tumor cells and predict response to therapy.

Downregulation of miRNA-200c links breast cancer stem cells with normal stem cells.

Human breast tumors contain a breast cancer stem cell (BCSC) population with properties reminiscent of normal stem cells. We found 37 microRNAs that were differentially expressed between human BCSCs and nontumorigenic cancer cells. Three clusters, miR-200c-141, miR-200b-200a-429, and miR-183-96-182 were downregulated in human BCSCs, normal human and murine mammary stem/progenitor cells, and embryonal carcinoma cells. Expression of BMI1, a known regulator of stem cell self-renewal, was modulated by miR-200c. miR-200c inhibited the clonal expansion of breast cancer cells and suppressed the growth of embryonal carcinoma cells in vitro. Most importantly, miR-200c strongly suppressed the ability of normal mammary stem cells to form mammary ducts and tumor formation driven by human BCSCs in vivo. The coordinated downregulation of three microRNA clusters and the similar functional regulation of clonal expansion by miR-200c provide a molecular link that connects BCSCs with normal stem cells.

Automated microfluidic chromatin immunoprecipitation from 2,000 cells.

Chromatin immunoprecipitation (ChIP) is a powerful assay used to probe DNA-protein interactions. Traditional methods of implementing this assay are lengthy, cumbersome and require a large number of cells, making it difficult to study rare cell types such as certain cancer and stem cells. We have designed a microfluidic device to perform sensitive ChIP analysis on low cell numbers in a rapid, automated fashion while preserving the specificity of the assay. Comparing ChIP results for two modified histone protein targets, we showed our automated microfluidic ChIP (AutoChIP) from 2,000 cells to be comparable to that of conventional ChIP methods using 50,000-500,000 cells. This technology may provide a solution to the need for a high sensitivity, rapid, and automated ChIP assay, and in doing so facilitate the use of ChIP for many interesting and valuable applications.

Association of reactive oxygen species levels and radioresistance in cancer stem cells.

The metabolism of oxygen, although central to life, produces reactive oxygen species (ROS) that have been implicated in processes as diverse as cancer, cardiovascular disease and ageing. It has recently been shown that central nervous system stem cells and haematopoietic stem cells and early progenitors contain lower levels of ROS than their more mature progeny, and that these differences are critical for maintaining stem cell function. We proposed that epithelial tissue stem cells and their cancer stem cell (CSC) counterparts may also share this property. Here we show that normal mammary epithelial stem cells contain lower concentrations of ROS than their more mature progeny cells. Notably, subsets of CSCs in some human and murine breast tumours contain lower ROS levels than corresponding non-tumorigenic cells (NTCs). Consistent with ROS being critical mediators of ionizing-radiation-induced cell killing, CSCs in these tumours develop less DNA damage and are preferentially spared after irradiation compared to NTCs. Lower ROS levels in CSCs are associated with increased expression of free radical scavenging systems. Pharmacological depletion of ROS scavengers in CSCs markedly decreases their clonogenicity and results in radiosensitization. These results indicate that, similar to normal tissue stem cells, subsets of CSCs in some tumours contain lower ROS levels and enhanced ROS defences compared to their non-tumorigenic progeny, which may contribute to tumour radioresistance.

The biology of cancer stem cells.

Cancers originally develop from normal cells that gain the ability to proliferate aberrantly and eventually turn malignant. These cancerous cells then grow clonally into tumors and eventually have the potential to metastasize. A central question in cancer biology is, which cells can be transformed to form tumors? Recent studies elucidated the presence of cancer stem cells that have the exclusive ability to regenerate tumors. These cancer stem cells share many characteristics with normal stem cells, including self-renewal and differentiation. With the growing evidence that cancer stem cells exist in a wide array of tumors, it is becoming increasingly important to understand the molecular mechanisms that regulate self-renewal and differentiation because corruption of genes involved in these pathways likely participates in tumor growth. This new paradigm of oncogenesis has been validated in a growing list of tumors. Studies of normal and cancer stem cells from the same tissue have shed light on the ontogeny of tumors. That signaling pathways such as Bmi1 and Wnt have similar effects in normal and cancer stem cell self-renewal suggests that common molecular pathways regulate both populations. Understanding the biology of cancer stem cells will contribute to the identification of molecular targets important for future therapies.

Novel chromosomally encoded multidrug efflux transporter MdeA in Staphylococcus aureus.

Antibiotic efflux is an important mechanism of resistance in pathogenic bacteria. Here we describe the identification and characterization of a novel chromosomally encoded multidrug resistance efflux protein in Staphylococcus aureus, MdeA (multidrug efflux A). MdeA was identified from screening an S. aureus open reading frame expression library for resistance to antibiotic compounds. When overexpressed, MdeA confers resistance on S. aureus to a range of quaternary ammonium compounds and antibiotics, but not fluoroquinolones. MdeA is a 52-kDa protein with 14 predicted transmembrane segments. It belongs to the major facilitator superfamily and is most closely related, among known efflux proteins, to LmrB of Bacillus subtilis and EmrB of Escherichia coli. Overexpression of mdeA in S. aureus reduced ethidium bromide uptake and enhanced its efflux, which could be inhibited by reserpine and abolished by an uncoupler. The mdeA promoter was identified by primer extension. Spontaneous mutants selected for increased resistance to an MdeA substrate had undergone mutations in the promoter for mdeA, and their mdeA transcription levels were increased by as much as 15-fold. The mdeA gene was present in the genomes of all six strains of S. aureus examined. Uncharacterized homologs of MdeA were present elsewhere in the S. aureus genome, but their overexpression did not mediate resistance to the antibacterials tested. However, MdeA homologs were identified in other bacteria, including Bacillus anthracis, some of which were shown to be functional orthologs of MdeA.