Of mice and women: a comparative tissue biology perspective of breast stem cells and differentiation.

Tissue based research requires a background in human and veterinary pathology, developmental biology, anatomy, as well as molecular and cellular biology. This type of comparative tissue biology (CTB) expertise is necessary to tackle some of the conceptual challenges in human breast stem cell research. It is our opinion that the scarcity of CTB expertise contributed to some erroneous interpretations in tissue based research, some of which are reviewed here in the context of breast stem cells. In this article we examine the dissimilarities between mouse and human mammary tissue and suggest how these may impact stem cell studies. In addition, we consider the differences between breast ducts vs. lobules and clarify how these affect the interpretation of results in stem cell research. Lastly, we introduce a new elaboration of normal epithelial cell types in human breast and discuss how this provides a clinically useful basis for breast cancer classification.

Modeling the breast cancer bone metastatic niche in complex three-dimensional cocultures.

Despite advances in early detection, prevention and treatment of breast cancer, the mortality of breast cancer patients did not decrease considerably in the last years. Metastatic breast cancer remains incurable. There is compelling evidence that dissemination of breast cancer cells at distant sites is an early event. At the time of detection and diagnosis, patients have disseminated breast cancer cells in the bone marrow. Only in half of these patients the disseminated cells proliferate and generate metastases, typically in 3-5 years for ER negative breast tumors and 10-15 years for ER positive breast tumors. In other patients metastases never develop. The ability to predict which patients will develop metastases and to devise strategies to interfere with this process hinges on understanding the mechanisms underlying growth at the metastatic site. In turn, this requires novel experimental systems that model in vitro the survival, dormancy and proliferation of disseminated cancer cells.We have established such experimental systems that model the bone microenvironment of the breast cancer metastatic niche. These systems are based on 3D complex cultures of human bone marrow stromal cells and breast cancer cell lines in collagen biomatrices. We identified conditions in which cancer cells are dormant, and conditions in which they proliferate and we validated the results in vivo. Dormant cancer cells were able to proliferate upon transfer into supportive microenvironment or upon manipulation of signaling pathways that control dormancy. These experimental systems will be instrumental in screening new compounds for metastasis studies and particularly in studying the pathways that control cellular dormancy. We provide in this chapter detailed protocols for these complex 3D coculture systems.

Models of breast morphogenesis based on localization of stem cells in the developing mammary lobule.

Characterization of normal breast stem cells is important for understanding their role in breast development and in breast cancer. However, the identity of these cells is a subject of controversy and their localization in the breast epithelium is not known. In this study, we utilized a novel approach to analyze the morphogenesis of mammary lobules, by combining one-dimensional theoretical models and computer-generated 3D fractals. Comparing predictions of these models with immunohistochemical analysis of tissue sections for candidate stem cell markers, we defined distinct areas where stem cells reside in the mammary lobule. An increased representation of stem cells was found in smaller, less developed lobules compared to larger, more mature lobules, with marked differences in the gland of nulliparous versus parous women and that of BRCA1/2 mutation carriers versus non-carriers.

Growth hormone is secreted by normal breast epithelium upon progesterone stimulation and increases proliferation of stem/progenitor cells.

Using in vitro and in vivo experimental systems and in situ analysis, we show that growth hormone (GH) is secreted locally by normal human mammary epithelial cells upon progesterone stimulation. GH increases proliferation of a subset of cells that express growth hormone receptor (GHR) and have functional properties of stem and early progenitor cells. In 72% of ductal carcinoma in situ lesions, an expansion of the cell population that expresses GHR was observed, suggesting that GH signaling may contribute to breast cancer development.

Aldehyde dehydrogenase and estrogen receptor define a hierarchy of cellular differentiation in the normal human mammary epithelium.

INTRODUCTION: Although estrogen and progesterone play a key role in normal mammary development and in breast cancer, the potential for proliferation and lineage differentiation as well as origin of cells that express the estrogen receptor (ER) in normal breast epithelium are not known. Some evidence suggests that normal human mammary stem/progenitor cells are ER-, but the identity of these cells and the cellular hierarchy of breast epithelium are still subjects of controversy. It is likely that elucidation of these aspects will bring insight into the cellular origin of breast cancer subtypes. METHODS: We used fluorescence-activated cell sorting of primary human mammary epithelial cells along with in vitro and in vivo functional assays to examine the hierarchic relation between cells with aldehyde dehydrogenase enzymatic activity (ALDH+ cells) and ER+ cells in the normal human breast epithelium. We assessed the proliferation and lineage differentiation potential of these cells in vitro and in vivo. A gene reporter assay was used to separate live ER+ and ER- mammary epithelial cells. With shRNA-mediated knockdown, we investigated the role of ALDH isoforms in the functionality of mammary epithelial progenitor cells. RESULTS: We describe a cellular hierarchy in the normal human mammary gland in which ER-/ALDH+ cells with functional properties of stem/progenitor cells generate ER+ progenitor cells, which in turn give rise to cells of luminal lineage. We show that the ALDH1A1 isoform, through its function in the retinoic acid metabolism, affects the proliferation and/or early differentiation of stem/progenitor cells and is important for branching morphogenesis. CONCLUSIONS: This study presents direct evidence that ER+ cells are generated by ER-/ALDH+ stem/progenitor cells. We also show that ER+ cells are able to generate cell progeny of luminal lineage in vitro and in vivo. Loss of ALDH1A1 function impairs this process, as well as branching morphogenesis and clonogenicity in suspension culture. This latter effect is reversed by treatment with retinoic acid.

A novel model of dormancy for bone metastatic breast cancer cells.

Mortality of patients with breast cancer is due overwhelmingly to metastatic spread of the disease. Although dissemination is an early event in breast cancer, extended periods of cancer cell dormancy can result in long latency of metastasis development. Deciphering the mechanisms underlying cancer cell dormancy and subsequent growth at the metastatic site would facilitate development of strategies to interfere with these processes. A challenge in this undertaking has been the lack of models for cancer cell dormancy. We have established novel experimental systems that model the bone microenvironment of the breast cancer metastatic niche. These systems are based on 3D cocultures of breast cancer cells with cell types predominant in bone marrow. We identified conditions in which cancer cells are dormant and conditions in which they proliferate. Dormant cancer cells were able to proliferate upon transfer into supportive microenvironment or upon manipulation of signaling pathways that control dormancy. These experimental systems will be instrumental for metastasis studies, particularly the study of cellular dormancy.

Preclinical and clinical studies of gamma secretase inhibitors with docetaxel on human breast tumors.

PURPOSE: Accumulating evidence supports the existence of breast cancer stem cells (BCSC), which are characterized by their capacity to self-renew and divide indefinitely and resistance to conventional therapies. The Notch pathway is important for stem cell renewal and is a potential target for BCSC-directed therapy. EXPERIMENTAL DESIGN: Using human breast tumorgraft studies, we evaluated the impact of gamma secretase inhibitors (GSI) on the BCSC population and the efficacy of combining GSI with docetaxel treatment. The mouse experimental therapy paralleled a concurrent clinical trial in patients with advanced breast cancer, designed to determine the maximum-tolerated dose of the GSI, MK-0752, administered sequentially with docetaxel, and to evaluate BCSC markers in serial tumor biopsies. RESULTS: Treatment with GSI reduced BCSCs in MC1 and BCM-2147 tumorgrafts by inhibition of the Notch pathway. GSI enhanced the efficacy of docetaxel in preclinical studies. In the clinical trial, 30 patients with advanced breast cancer were treated with escalating doses of MK-0752 plus docetaxel. Clinically, meaningful doses of both drugs were possible with manageable toxicity and preliminary evidence of efficacy. A decrease in CD44(+)/CD24(-), ALDH(+), and mammosphere-forming efficiency were observed in tumors of patients undergoing serial biopsies. CONCLUSIONS: These preclinical data show that pharmacologic inhibition of the Notch pathway can reduce BCSCs in breast tumorgraft models. The clinical trial shows feasibility of combination GSI and chemotherapy, and together these results encourage further study of Notch pathway inhibitors in combination with chemotherapy in breast cancer.

Delineation of breast cancer cell hierarchy identifies the subset responsible for dormancy.

The bone marrow (BM) is a major organ of breast cancer (BC) dormancy and a common source of BC resurgence. Gap junctional intercellular communication (GJIC) between BC cells (BCCs) and BM stroma facilitates dormancy. This study reports on a hierarchy of BCCs with the most immature subset (Oct4(hi)/CD44(hi/med)/CD24(-/+)) demonstrating chemoresistance, dormancy, and stem cell properties: self-renewal, serial passaging ability, cycling quiescence, long doubling time, asymmetric division, high metastatic and invasive capability. In vitro and in vivo studies indicated that this subset was responsible for GJIC with BM stroma. Similar BCCs were detected in the blood of patients despite aggressive treatment and in a patient with a relatively large tumor but no lymph node involvement. In brief, these findings identified a novel BCC subset with stem cell properties, with preference for dormancy and in the circulation of patients. The findings establish a working cellular hierarchy of BCCs based on phenotype and functions.

International expert consensus on primary systemic therapy in the management of early breast cancer: highlights of the Fourth Symposium on Primary Systemic Therapy in the Management of Operable Breast Cancer, Cremona, Italy (2010).

A panel of international breast cancer experts formulated a declaration of consensus regarding many key issues in the use of primary systemic therapy (PST) either in clinical routine or research practice. The attainment of pathological complete response (pCR), defined as no residual invasive tumor in the surgical specimens both in breast and in axillary nodes, is one of the main goals of PST, and pCR can be used as the primary objective in prospective clinical trials. However, pCR is not a reliable endpoint with all treatment approaches, and alternatives such as Ki67 index of the residual invasive disease or after 2 weeks of PST are also potential endpoints. PST has several advantages: breast conservation and the unique opportunity to obtain information on the interaction between treatment and tumor biology. Changes in tumor biology after PST are an early phenomenon; so, an additional core biopsy performed after 14 days from treatment start should be considered in clinical trials.

Dicer-mediated upregulation of BCRP confers tamoxifen resistance in human breast cancer cells.

PURPOSE: Tamoxifen (Tam) is the most prescribed hormonal agent for treatment of estrogen receptor α (ERα)-positive breast cancer patients. Using microarray analysis, we observed that metastatic breast tumors resistant to Tam therapy had elevated levels of Dicer. EXPERIMENTAL DESIGN: We overexpressed Dicer in ERα-positive MCF-7 human breast cancer cells and observed a concomitant increase in expression of the breast cancer resistance protein (BCRP). We thus hypothesized that Tam resistance associated with Dicer overexpression in ERα-positive breast cancer cells may involve BCRP. We analyzed BCRP function in Dicer-overexpressing cells using growth in soft agar and mammosphere formation and evaluated intracellular Tam efflux. RESULTS: In the presence of Tam, Dicer-overexpressing cells formed resistant colonies in soft agar, and treatment with BCRP inhibitors restored Tam sensitivity. Tumor xenograft studies confirmed that Dicer-overexpressing cells were resistant to Tam in vivo. Tumors and distant metastases could be initiated with as few as five mammosphere cells from both vector and Dicer-overexpressing cells, indicating that the mammosphere assay selected for cells with enhanced tumor-initiating and metastatic capacity. Dicer-overexpressing cells with elevated levels of BCRP effluxed Tam more efficiently than control cells, and BCRP inhibitors were able to inhibit efflux. CONCLUSION: Dicer-overexpressing breast cancer cells enriched for cells with enhanced BCRP function. We hypothesize that it is this population which may be involved in the emergence of Tam-resistant growth. BCRP may be a novel clinical target to restore Tam sensitivity.

Xenotransplantation of breast cancers.

Three experimental systems based on mouse models are currently used to study breast cancer: transgenic mice, carcinogen-induced models, and xenografts of breast cancers. Each of these models has advantages and limitations. This chapter focuses on xenotransplantation of breast cancers and reviews the techniques used so far in establishing this model, the advantages and limitations compared to other experimental systems, and finally, the technical questions that remain to be answered.

Breast cancer stem cells are regulated by mesenchymal stem cells through cytokine networks.

We have used in vitro and mouse xenograft models to examine the interaction between breast cancer stem cells (CSC) and bone marrow-derived mesenchymal stem cells (MSC). We show that both of these cell populations are organized in a cellular hierarchy in which primitive aldehyde dehydrogenase expressing mesenchymal cells regulate breast CSCs through cytokine loops involving IL6 and CXCL7. In NOD/SCID mice, labeled MSCs introduced into the tibia traffic to sites of growing breast tumor xenografts where they accelerated tumor growth by increasing the breast CSC population. With immunochemistry, we identified MSC-CSC niches in these tumor xenografts as well as in frozen sections from primary human breast cancers. Bone marrow-derived MSCs may accelerate human breast tumor growth by generating cytokine networks that regulate the CSC population.

MicroRNAs: shortcuts in dealing with molecular complexity?

Recent studies from Clarke's group published in the journal Cell indicate that miRNAs may be the elusive universal stem cell markers that the field of cancer stem cell biology has been seeking. Distinct profiles of miRNAs appear to reflect the state of cell differentiation not only in breast cancer cells, but also in normal mammary epithelial cells. Moreover, they are conserved across tissues and species. The authors of this work also show evidence that downregulation of miRNA-200c in normal and malignant breast stem cells and in embryonal carcinoma cells has functional relevance, being responsible for the proliferative potential of these cells in vitro and in vivo.

CXCR1 blockade selectively targets human breast cancer stem cells in vitro and in xenografts.

Recent evidence suggests that breast cancer and other solid tumors possess a rare population of cells capable of extensive self-renewal that contribute to metastasis and treatment resistance. We report here the development of a strategy to target these breast cancer stem cells (CSCs) through blockade of the IL-8 receptor CXCR1. CXCR1 blockade using either a CXCR1-specific blocking antibody or repertaxin, a small-molecule CXCR1 inhibitor, selectively depleted the CSC population in 2 human breast cancer cell lines in vitro. Furthermore, this was followed by the induction of massive apoptosis in the bulk tumor population via FASL/FAS signaling. The effects of CXCR1 blockade on CSC viability and on FASL production were mediated by the FAK/AKT/FOXO3A pathway. In addition, repertaxin was able to specifically target the CSC population in human breast cancer xenografts, retarding tumor growth and reducing metastasis. Our data therefore suggest that CXCR1 blockade may provide a novel means of targeting and eliminating breast CSCs.

Aldehyde dehydrogenase 1-positive cancer stem cells mediate metastasis and poor clinical outcome in inflammatory breast cancer.

PURPOSE: To examine the role of cancer stem cells (CSC) in mediating metastasis in inflammatory breast cancer (IBC) and the association of these cells with patient outcome in this aggressive type of breast cancer. EXPERIMENTAL DESIGN: CSCs were isolated from SUM149 and MARY-X, an IBC cell line and primary xenograft, by virtue of increased aldehyde dehydrogenase (ALDH) activity as assessed by the ALDEFLUOR assay. Invasion and metastasis of CSC populations were assessed by in vitro and mouse xenograft assays. Expression of ALDH1 was determined on a retrospective series of 109 IBC patients and this was correlated with histoclinical data. All statistical tests were two sided. Log-rank tests using Kaplan-Meier analysis were used to determine the correlation of ALDH1 expression with development of metastasis and patient outcome. RESULTS: Both in vitro and xenograft assays showed that invasion and metastasis in IBC are mediated by a cellular component that displays ALDH activity. Furthermore, expression of ALDH1 in IBC was an independent predictive factor for early metastasis and decreased survival in this patient population. CONCLUSIONS: These results suggest that the metastatic, aggressive behavior of IBC may be mediated by a CSC component that displays ALDH enzymatic activity. ALDH1 expression represents the first independent prognostic marker to predict metastasis and poor patient outcome in IBC. The results illustrate how stem cell research can translate into clinical practice in the IBC field.

Targeting breast stem cells with the cancer preventive compounds curcumin and piperine.

The cancer stem cell hypothesis asserts that malignancies arise in tissue stem and/or progenitor cells through the dysregulation or acquisition of self-renewal. In order to determine whether the dietary polyphenols, curcumin, and piperine are able to modulate the self-renewal of normal and malignant breast stem cells, we examined the effects of these compounds on mammosphere formation, expression of the breast stem cell marker aldehyde dehydrogenase (ALDH), and Wnt signaling. Mammosphere formation assays were performed after curcumin, piperine, and control treatment in unsorted normal breast epithelial cells and normal stem and early progenitor cells, selected by ALDH positivity. Wnt signaling was examined using a Topflash assay. Both curcumin and piperine inhibited mammosphere formation, serial passaging, and percent of ALDH+ cells by 50% at 5 microM and completely at 10 microM concentration in normal and malignant breast cells. There was no effect on cellular differentiation. Wnt signaling was inhibited by both curcumin and piperine by 50% at 5 microM and completely at 10 microM. Curcumin and piperine separately, and in combination, inhibit breast stem cell self-renewal but do not cause toxicity to differentiated cells. These compounds could be potential cancer preventive agents. Mammosphere formation assays may be a quantifiable biomarker to assess cancer preventive agent efficacy and Wnt signaling assessment can be a mechanistic biomarker for use in human clinical trials.

Aldehyde dehydrogenase 1 is a marker for normal and malignant human colonic stem cells (SC) and tracks SC overpopulation during colon tumorigenesis.

Although the concept that cancers originate from stem cells (SC) is becoming scientifically accepted, mechanisms by which SC contribute to tumor initiation and progression are largely unknown. For colorectal cancer (CRC), investigation of this problem has been hindered by a paucity of specific markers for identification and isolation of SC from normal and malignant colon. Accordingly, aldehyde dehydrogenase 1 (ALDH1) was investigated as a possible marker for identifying colonic SC and for tracking them during cancer progression. Immunostaining showed that ALDH1(+) cells are sparse and limited to the normal crypt bottom, where SCs reside. During progression from normal epithelium to mutant (APC) epithelium to adenoma, ALDH1(+) cells increased in number and became distributed farther up the crypt. CD133(+) and CD44(+) cells, which are more numerous and broadly distributed in normal crypts, showed similar changes during tumorigenesis. Flow cytometric isolation of cancer cells based on enzymatic activity of ALDH (Aldefluor assay) and implantation of these cells in nonobese diabetic-severe combined immunodeficient mice (a) generated xenograft tumors (Aldefluor(-) cells did not), (b) generated them after implanting as few as 25 cells, and (c) generated them dose dependently. Further isolation of cancer cells using a second marker (CD44(+) or CD133(+) serially) only modestly increased enrichment based on tumor-initiating ability. Thus, ALDH1 seems to be a specific marker for identifying, isolating, and tracking human colonic SC during CRC development. These findings also support our original hypothesis, derived previously from mathematical modeling of crypt dynamics, that progressive colonic SC overpopulation occurs during colon tumorigenesis and drives CRC development.

Breast cancer cell lines contain functional cancer stem cells with metastatic capacity and a distinct molecular signature.

Tumors may be initiated and maintained by a cellular subcomponent that displays stem cell properties. We have used the expression of aldehyde dehydrogenase as assessed by the ALDEFLUOR assay to isolate and characterize cancer stem cell (CSC) populations in 33 cell lines derived from normal and malignant mammary tissue. Twenty-three of the 33 cell lines contained an ALDEFLUOR-positive population that displayed stem cell properties in vitro and in NOD/SCID xenografts. Gene expression profiling identified a 413-gene CSC profile that included genes known to play a role in stem cell function, as well as genes such as CXCR1/IL-8RA not previously known to play such a role. Recombinant interleukin-8 (IL-8) increased mammosphere formation and the ALDEFLUOR-positive population in breast cancer cell lines. Finally, we show that ALDEFLUOR-positive cells are responsible for mediating metastasis. These studies confirm the hierarchical organization of immortalized cell lines, establish techniques that can facilitate the characterization of regulatory pathways of CSCs, and identify potential stem cell markers and therapeutic targets.