Luminal hormone-responsive cells tune the regenerative remodeling of mammary glands in large mammals

The complexities of identifying a cell of origin for human prostate cancer

Breast cancer (BC) is the most diagnosed cancer in women with aging being the major risk factor. According to American Cancer Society (2024), an estimation of 310, 720 invasive BC and about 56, 500 cases of DCIS diagnosed among women in the USA, and the women die from BC are likely among age groups of 70 or older. Our previous report demonstrated that aging was associated with an increased number of basal epithelial cells (CD49fhiCD24low) and reduced luminal epithelial cells (CD49flowCD24hi) in mice. The CD49f (Integrin α6), a cell surface marker of mammary stem cells (MaSc), is enriched in the basal cell population of the mammary gland (MG). These cells have oligo-lineage differentiation potential, especially when transplanted into the cleared mammary fat pad (CFP). Integrin α6 has two variants with a different cytoplasmic domain, produced via alternative splicing (Isoforms: α6A & α6B). Recent scientific evidence suggests that α6B produced in transformed mammary cell lines was oncogenic. Previous studies from us and others showed CD49fhi aberrant stem cell-like cells in the luminal epithelium display more hyperplastic duct in the old mice, as well as tumor adjacent normal tissue from women with breast cancer. The mechanism underlying the expansion of CD49fhi cells during aging is unclear. It could be due to the proliferation of basal cells, or trans-differentiation of luminal cells. Henceforward, this study examines changes in the CD49f expression (and its isoforms) in the basal and luminal cell populations with aging in mice, hypothesizing that luminal cell attains basal cell signature by expressing more CD49f that contributes to hyperplasia in the aged mammary glands. To explore this hypothesis, we evaluated the gene and protein expression of Integrin α6 isoforms (α6A & α6B) across basal and luminal populations in FVB/NJ and C57Bl/6 aging mice using RT-PCR and Western blotting. Mammary cells were gated and sorted in FACS and used for RNA and protein studies. Our results revealed that aged basal and luminal cells express higher levels of ITGA6B mRNA transcript compared to ITGA6A in both the mice stains. Cell cycle analysis was performed using Hoechst to evaluate the cell cycle phase of each population using flow cytometry. The results showed that luminal cells from aged mice have a higher fraction of G2/M phase cells than those from young mice, suggesting an increased cell cycle in the aged luminal cells, which may be due to increased Integrin α6B. We are currently developing a method to measure Integrin α6B protein levels in young and aged mouse mammary luminal and basal cells. Our study provides the first evidence that aged CD49fhi cells, particularly those expressing α6B exhibit increased cell proliferation potential, revealing a potential target for breast cancer prevention. Citation Format: Thangarajeswari Mohan, Morgana McLaughlin, Jacob Mackay, Luzhe Sun. Deciphering the role of oncogenic integrin alpha 6B in aged mouse mammary epithelial cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 12.

Aging is the number one risk factor for breast cancer development. Increasing evidence suggests the potential of mammary stem cells (MaSCs) and their progenitors to generate certain types of breast cancers through neoplastic transformation. Our previous study has shown increased percentage of MaSC-enriched basal cell population (Lin-CD49fhighCD24med) and increased MaSC frequency during aging in murine models. On the other hand, a recent study from another group showed an increased frequency of CD49fhigh cells in the human luminal population (CD227+) during aging, indicating possible aberrant expression of CD49f in the aged luminal cells. However, how these age-related luminal cells with basal markers are generated and how they contribute to potential breast cancer development remains unknown. Here we apply bioinformatics analysis on Next Generation Whole Transcriptome Sequencing data of MaSC-enriched basal cell population (Lin-CD49fhighCD24med) and luminal progenitor-enriched cell population (Lin-CD49flowCD24high) of both young (4 to 6 months) and old (26 to 31 months) mouse mammary gland to test the hypothesis that age-associated increase of basal cell population and MaSCs may be due to the gain of basal cell markers and features by luminal cells. By Gene Set Enrichment Analysis (GSEA) we found a significant loss of basal cell and basal mammosphere signatures and a significant enrichment of luminal cell and luminal mammosphere signature in the old basal cell population and mammospheres in comparison with the young basal cell population and mammospheres. The core enrichment luminal genes from GSEA are able to cluster the old MaSC-enriched basal cell population as well as mammospheres closer to the cluster of luminal population than to the young basal population. These analyses indicate that aging may be associated with an expansion of aberrant MaSCs with both basal and luminal markers in mice, which may be the precursors of certain types of breast cancer. We are now studying the potential function of the basal-like luminal cells in the aged basal population. Citation Format: Gu X, Gao H, Wu A, Bandyopadhyay A, Dong Q, Sun L-Z. Transcriptome profiling reveals enrichment of luminal feature in aged basal cell population in murine mammary gland. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P6-04-03.

CTR9 encodes a key component of the human RNA polymerase II (RNAPII)-associated factor complex (hPAFc). CTR9 loss-of-function variants are predisposed to rare myeloid malignancies and Wilms tumors. Although CTR9 promotes the growth of estrogen receptor (ER)-positive breast cancer cells and controls the expression of 90% of ER-target genes, the roles of CTR9 in mammary gland development and breast cancer etiology in vivo remain unknown. About 0.1 % of breast cancer patients were found to harbor loss of function CTR9 mutations based on TCGA data. Analyzing normal mouse mammary gland scRNA-seq data reveals that CTR9 is highly expressed in luminal cells, which is confirmed by immunofluorescence staining in human breast tumors. To study the effects of the loss-of-function of CTR9 on mammary gland development, we generated a mammary gland-specific CTR9 knockout (KO) mouse using MMTV-Cre and CTR9 loxP system. CTR9 depletion from mammary epithelial cells was validated using qPCR, Western blot, and IHC. Loss of CTR9 in C57B/6 female mice led to increased duct branching and elongation during puberty and increased alveologenesis during pregnancy. This observation coincides with the detection of expanded mammary stem cells by flow cytometry in the 6-week-old mouse mammary glands. When treated with DMBA and progestin (MPA) to induce mammary tumors, both wild-type (WT) and CTR9 null mice began to develop tumors 6 weeks after the last DMBA treatment, but CTR9 null mice exhibited a markedly enhanced susceptibility to carcinogen/hormone-induced mammary tumors as compared with the WT counterparts. To detect changes during the early stage of tumorigenesis, we profiled the stem and luminal progenitor cell populations in the mammary gland 4-week after the last DMBA treatment. Remarkably, CTR9 depletion enhanced the stem cell and luminal progenitor populations while decreasing the mature luminal cells. CTR9 KO mammary gland had elevated Ki67 levels, decreased ER and PR, decreased mature luminal cells, and increased expression of KRT15, a luminal progenitor marker. The CTR9 null mammary gland expressed lower levels of KDM5A and higher levels of H3K4me3 as compared to the wild-type mice. Taken together, our findings establish that CTR9 depletion blocks the terminal differentiation of luminal cell lineage possibly through epigenetic regulation of histone methylation, and renders the mammary glands more susceptible to carcinogen/hormone-induced tumors. Citation Format: Gui Ma, Ang Gao, Mingshan Hu, Ngai Ting Chan, Yidan Wang, Wei Xu. CTR9 controls the mammary luminal lineage and susceptibility to carcinogen/hormone-induced breast tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4312.

Breast cancer is one of the most common cancers among women in the United States and it is the second leading cause of cancer deaths in this population. Roughly 232,000 women in the U.S. were diagnosed with malignant breast cancer in 2013 and approximately 40,000 of these patients will die. The majority of breast cancers originate in the lobular or ductal cells of the milk-producing glands. In these structures, there are two main cell types: the inner luminal cells, which are surrounded by basal myoepithelium. These cells are structurally distinct and are the precursors to variable forms of breast cancer. As such, it is important to study them not only separately but also together. Current methods for culturing human mammary epithelial cells select for those of a basal phenotype. Isolation of pure basal and luminal cell populations was accomplished by using antibody-linked magnetic beads to isolate essentially pure populations of both basal (CALLA/CD10) and luminal mammary (MUC-1/CD227) epithelial cells from organoid explant cultures. Organoids were isolated and plated on collagen I coated flasks in either DFCI (basal) or MBCM (luminal) media and cultured at 37°C in 5% CO2 to induce cell migration. The media was replenished every 2 - 3 days until the cells reached confluence. Cultured cells were harvested and immunopurified using a CALLA/CD10 or MUC-1/CD227 antibody and magnetic beads. Positively selected cells were seeded in tissue culture treated flasks and propagated with medium exchange every 2 - 3 days. Each cell population was analyzed using flow cytometry to determine their purity. The DFCI/CD10 cells were over 98% CD10 positive with very little MUC1 reactivity, suggesting that they are an essentially pure basal epithelial cell population. The cells isolated using MBCM/MUC1 appear to be a mixed population with 86% MUC1 positive luminal cells and the remainder being CD10 positive basal cells. A second round of immunopurification resulted in an isolated cell population that was greater than 95% MUC1 positive, suggesting a pure luminal epithelial cell population. This procedure has been used on several organoid preparations and has generated high cell yields in addition to high cell purity. These isolated cell types can now be tested individually for drug sensitivity. Citation Format: Steven Hoynowski. Isolation and characterization of human breast epithelial cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1199. doi:10.1158/1538-7445.AM2014-1199

Breast cancer is one of the most common cancers among women in the United States and it is the second leading cause of cancer deaths in this population. Roughly 232,000 women in the U.S. were diagnosed with malignant breast cancer in 2013 and approximately 40,000 of these patients will die. The majority of breast cancers originate in the lobular or ductal cells of the milk-producing glands. In these structures, there are two main cell types: the inner luminal cells, which are surrounded by basal myoepithelium. These cells are structurally distinct and are the precursors to variable forms of breast cancer. As such, it is important to study them not only separately but also together. Current methods for culturing human mammary epithelial cells select for those of a basal phenotype. Isolation of pure basal and luminal cell populations was accomplished by using antibody-linked magnetic beads to isolate essentially pure populations of both basal (CALLA/CD10) and luminal mammary (MUC1/CD227) epithelial cells from organoid explant cultures. Organoids were isolated and plated on collagen I coated flasks in either DFCI (basal) or MBCM (luminal) media and cultured at 37°C in 5% CO2 to induce cell migration. The media was replenished every 2 - 3 days until the cells reached confluence. Cultured cells were harvested and immunopurified using a CALLA/CD10 or MUC1/CD227 antibody and magnetic beads. Positively selected cells were seeded in tissue culture treated flasks and propagated with medium exchange every 2 - 3 days. Each cell population was analyzed using flow cytometry to determine their purity. The DFCI/CD10 cells were over 98% CD10 positive with very little MUC1 reactivity, suggesting that they are an essentially pure basal epithelial cell population. The cells isolated using MBCM/MUC1 appear to be a mixed population with 86% MUC1 positive luminal cells and the remainder being CD10 positive basal cells. A second round of immunopurification resulted in an isolated cell population that was greater than 95% MUC1 positive, suggesting a pure luminal epithelial cell population. This procedure has been used on several organoid preparations and has generated high cell yields in addition to high cell purity. These isolated cell types can now be tested individually for drug sensitivity. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):C107. Citation Format: Steven Hoynowski. Isolation and characterization of human breast epithelial cells. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr C107.

Breast cancer is one of the most common cancers among women in the United States and it is the second leading cause of cancer deaths in this population. Roughly 232,000 women in the U.S. were diagnosed with malignant breast cancer in 2013 and approximately 40,000 of these patients will die. The majority of breast cancers originate in the lobular or ductal cells of the milk‐producing glands. In these structures are two main cell types: luminal cells, which are surrounded by basal myoepithelium. These cells are structurally distinct and are the precursors to variable forms of breast cancer. As such, it is important to study them not only separately but also together. Current methods for culturing human mammary epithelial cells select for those of a basal phenotype. Isolation of pure basal and luminal cell populations was accomplished by using antibody‐linked magnetic beads to isolate essentially pure populations of both basal (CALLA/CD10) and luminal mammary (MUC‐1/CD227) epithelial cells from organoid explant cultures. Organoids were isolated and plated on collagen I coated flasks in either DFCI (basal) or MBCM (luminal) media and cultured at 37°C in 5% CO2 to induce cell migration. The media was replenished every 2 – 3 days until the cells reached confluence. Cultured cells were harvested and immunopurified using a CALLA/CD10 or MUC‐1/CD227 antibody and magnetic beads. Positively selected cells were seeded in tissue culture treated flasks and propagated with medium exchange every 2 – 3 days. A second round of immunopurification resulted in an isolated cell population that was greater than 95% MUC1 positive, suggesting a pure luminal epithelial cell population. These isolated cell types can now be tested individually for drug sensitivity.

There is a paradox offered up by the cancer stem cell hypothesis. How are the mixed populations that are characteristic of heterogeneous solid tumors maintained at constant proportion, given their high, and different, mitotic indices? In this study, we evaluate a well-characterized mouse model of human basaloid tumors (induced by the oncogene Wnt1), which comprise mixed populations of mammary epithelial cells resembling their normal basal and luminal counterparts. We show that these cell types are substantially inter-dependent, since the MMTV LTR drives expression of Wnt1 ligand in luminal cells, whereas the functional Wnt1-responsive receptor (Lrp5) is expressed by basal cells, and both molecules are necessary for tumor growth. There is a robust tumor initiating activity (tumor stem cell) in the basal cell population, which is associated with the ability to differentiate into luminal and basal cells, to regenerate the oncogenic paracrine signaling cell pair. However, we found an additional tumor stem cell activity in the luminal cell population. Knowing that tumors depend upon Wnt1-Lrp5, we hypothesized that this stem cell must express Lrp5, and found that indeed, all the stem cell activity could be retrieved from the Lrp5-positive cell population. Interestingly, this reflects post-transcriptional acquisition of Lrp5 protein expression in luminal cells. Furthermore, this plasticity of molecular expression is reflected in plasticity of cell fate determination. Thus, in vitro, Wnt1-expressing luminal cells retro-differentiate to basal cell types, and in vivo, tumors initiated with pure luminal cells reconstitute a robust basal cell subpopulation that is indistinguishable from the populations initiated by pure basal cells. We propose this is an important proof of concept, demonstrating that bipotential tumor stem cells are essential in tumors where oncogenic ligand-receptor pairs are separated into different cell types, and suggesting that Wnt-induced molecular and fate plasticity can close paracrine loops that are usually separated into distinct cell types.

Molecular subtypes of breast cancer are characterized by distinct patterns of gene expression that are predictive of outcome and response to therapy. The luminal breast cancer subtypes are defined by the expression of ER-alpha (ERα)-associated genes, many of which are directly responsive to the Transcription Factor Activator Protein 2C (TFAP2C). TFAP2C participates in a gene regulatory network controlling cell growth and differentiation during ectodermal development and regulating ESR1/ERα and other luminal cell-associated genes in breast cancer. TFAP2C has been established as a prognostic factor in human breast cancer, however, its role in the establishment and maintenance of the luminal cell phenotype during carcinogenesis and mammary gland development have remained elusive. Herein, we demonstrate a critical role for TFAP2C in maintaining the luminal phenotype in human breast cancer and in influencing the luminal cell phenotype during normal mammary development. Knockdown of TFAP2C in luminal breast carcinoma cells induced EMT with morphological and phenotypic changes characterized by a loss of luminal-associated gene expression and a concomitant gain of basal-associated gene expression. Conditional knockout of the mouse homolog of TFAP2C, Tcfap2c, in mouse mammary epithelium driven by MMTV-Cre promoted aberrant growth of the mammary tree leading to a reduction in the CD24hi/CD49fmid luminal cell population and concomitant gain of the CD24mid/CD49fhi basal cell population at maturity. Our results establish TFAP2C as a key transcriptional regulator for maintaining the luminal phenotype in human breast carcinoma. Furthermore, Tcfap2c influences development of the luminal cell type during mammary development. The data suggest that TFAP2C plays an important role in regulated luminal specific genes and may be a viable therapeutic target in breast cancer.

The mammary gland consists of epithelial ducts that stretch out in a tree-like structure through a stromal fat pad. The ducts are formed by two layers, the inner layer composed of luminal cells and an outer layer of basal (myoepithelial) cells. Despite this relatively simple anatomy, the precise cellular composition and hierarchies are still ill-defined. Recent efforts to enrich, isolate, and characterize the different mammary epithelial cell compartments only used a handful of markers to define and trace cell populations. Therefore, there is a need for an unbiased and comprehensive description of mammary epithelial cells within the gland at different developmental stages. To this end we herein use single-cell RNA sequencing to determine the gene expression profile of individual cells across four adult developmental stages; nulliparous, mid-gestation, lactation, and post-weaning (full natural involution). Our data from over 23,000 individual cells identify distinct mammary epithelial cell populations and allow their hierarchical structure across development to be charted. The luminal compartment showed a differentiation structure with a common progenitor that can give rise to intermediate progenitors of both alveolar and hormone-sensing luminal cells. In contrast, we captured only few cells that were transitioning between basal and luminal cells, suggesting that the basal compartment is less involved in maintaining luminal cells in adult tissue homeostasis. Interestingly, the transcriptional profile of some cell types appeared to be more affected by gestation and lactation. For example, our analysis revealed a cluster of luminal progenitor cells in post-involution glands, which is distinct from progenitors found in nulliparous glands. The post-involution progenitor cells share all the luminal progenitor characteristics with their nulliparous counterparts but were marked by higher expression of genes involved in milk synthesis and the immune response, suggesting that these cells maintain a memory of having undergone a full pregnancy cycle. This is especially interesting in the light of the protective effect of early pregnancies on breast cancers. The data also showed that only few clusters could be fully characterized by a single marker gene. We argue instead that the epithelial cells—especially in the luminal compartment—should rather be conceptualized as being part of a continuous spectrum of differentiation. This view highlights the plasticity of the tissue and might help to explain some of the conflicting results from lineage tracing studies. Our work provides a foundation for understanding the cellular aspects of the developmental biology of the mammary gland. This is vital to understand the early steps of tumor development. We hope that our dataset can also be mined to help to gain insights into the cells-of-origin for various breast cancers. This abstract is also being presented as Poster A66. Citation Format: Karsten Bach, Sara Pensa, David J. Adams, John C. Marioni, Walid T. Khaled. Differentiation dynamics of the developing mammary gland revealed by single-cell RNA-sequencing [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr PR04.

The classification of patient tumors by clinical subtype has gained wide acceptance due to the implications for prognosis and treatment. However, recent studies have cast doubt on previously advocated normal mammary origins of these subtypes. Thus, the link between the normal mammary gland and mammary tumors is more complex than expected. C-Jun N-Terminal Kinase-2 (JNK2) is a protein that is involved in numerous developmental processes and our previous work has shown it to be important for DNA damage response in mammary tumors. In attempt to gain insight into the link between mammary development and tumorigenesis, we compared normal mammary glands of JNK2 knockout (jnk2ko) mice to jnk2ko mammary tumors expressing or lacking wildtype p53 (p53ko). These studies showed that jnk2ko glands possess 35% fewer basal cells (p=0.0078) with a corresponding increase in luminal epithelial cell populations (p=0.100). This luminal response is corroborated by in vitro 3D assays of primary mammary epithelial cells (MECs) where luminal cell differentiation is normalized by inhibition of Notch signaling. Expression notch-1, a well-known regulator of MEC differentiation, is increased jnk2ko mammary glands. Increased expression of the Notch-1 target gene, hes-1, was also seen (p=0.005). Histology revealed that increased expression of active Notch-1 is localized to the mammary stem cell niche, the terminal end bud. Similar to the normal gland, jnk2ko mammary tumors possessing wildtype p53 exhibit decreased proportions of basal cells (p=0.0002) and increased proportions of luminal cells (p=0.0411) relative to wildtype. Jnk2ko cell lines derived from these tumors show decreased expression of notch-1 (p=0.0018) and hes-1 (p=0.0602) following introduction of JNK2. Luciferase assays comparing activity of the notch-1 promoter to a notch-1 promoter with mutated p53 response elements revealed a dependence of increased notch-1 promoter activity in jnk2ko cells on the p53 response element. P53ko tumor cells, by contrast, do not exhibit alterations in notch-1 promoter activity in the absence of p53 response elements, regardless of JNK2 status. QPCR showed that loss of JNK2 in normal mammary glands and tumors causes increased p53 expression—thus providing a potential mechanism. In support that Notch upregulation in the absence of JNK2 is dependent upon p53, normal glands lacking p53 show no differences in lineage differentiation. P53ko tumors also show no differences in basal lineage differentiation, however, increases in luminal differentiation are maintained in the absence of JNK2. Consistent with increased luminal differentiation, jnk2ko caused decreased expression of markers involved in the epithelial to mesenchymal transition phenotype. This data suggests that JNK2 is important not only for lineage differentiation in normal mammary glands, but in mammary tumors and that the effect is dependent on both Notch1 and p53. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr PD08-01.

GATA-3 and the regulation of the mammary luminal cell fate

The unmasking of novel unipotent stem cells in the mammary gland

BACKGROUND: A role for EZH2 in stem cell maintenance was recently reported. Our lab has previously demonstrated that overexpression of EZH2 accelerates the initiation of ErbB2-induced mammary tumors. To understand underlying mechanism, we investigated the role of EZH2 in regulation of stem cells and luminal progenitor cells utilizing xenotransplantation studies of preneoplastic mammary glands of EZH2+; neu and EZH2 wt; neu transgenic mice. METHODS: Generation of crossing EZH2+; neu and EZH2 wt; neu transgenic mice was described previously. Mouse handling and experimental procedures were conducted in accordance with the NIH Guide for the Care and Use of Laboratory Animals and were approved by the IACUC of the UM. Stem cells (Lin-/CD49f+/ESA-) and luminal progenitor cells (Lin-/CD49f+/ESA+) were isolated from the hyperplastic mammary glands of 8 weeks old EZH2+; neu and EZH2 wt; neu mice by flow cytometry (n=15 per group). Stem cells and luminal progenitor cells were injected into cleared fat pads of 3 weeks old FVB mice. After 5 weeks the mammary glands were subjected to flow cytometry and histology. RESULTS: Histologic study showed that only the stem cells, but not the luminal progenitor cells, extracted from EZH2+; neu and EZH2 wt; neu transgenic mice were able to form mammary outgrowths. Outgrowths of EZH2+;neu mice had significantly increased epithelial proliferation and increased numbers of terminal end buds (p<0.05) compared to EZH2 wt;neu outgrowths. EZH2 +;neu outgrowths had significant upregulation of Ki67 and pSTAT5 proteins. Flow cytometry of the outgrowths revealed that while all cell populations were increased, EZH2 induced a more pronounced expansion of the stem cell and luminal progenitor cell populations. CONCLUSIONS: Our data show that EZH2 overexpression in ErbB2 mice induces an expansion of the multipotent mammary stem cell population with the ability to regenerate the cellular heterogeneity of the mammary gland and develop hyperplastic outgrowths. We found that EZH2 promotes hyperproliferation and upregulation of STAT5 phosphorylation in the mammary epithelium in vivo. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5208. doi:1538-7445.AM2012-5208

<div>Abstract<p>It is not well understood how paracrine communication between basal and luminal cell populations in the mammary gland affects tumorigenesis. During ErbB2-induced mammary tumorigenesis, enriched mammary stem cells that represent a subpopulation of basal cells exhibit enhanced tumorigenic capacity compared with the corresponding luminal progenitors. Transcript profiling of tumors derived from basal and luminal tumor-initiating cells (TIC) revealed preferential loss of the noncanonical Wnt ligand WNT5A in basal TIC-derived tumors. Heterozygous loss of <i>WNT5A</i> was correlated with shorter survival of breast cancer patients. In a mouse model of ErbB2-induced breast cancer, <i>Wnt5a</i> heterozygosity promoted tumor multiplicity and pulmonary metastasis. As a TGFβ substrate, luminal cell-produced WNT5A induced a feed-forward loop to activate SMAD2 in a RYK and TGFβR1-dependent manner to limit the expansion of basal TIC in a paracrine fashion, a potential explanation for the suppressive effect of WNT5A in mammary tumorigenesis. Our results identify the WNT5A/RYK module as a spatial regulator of the TGFβ–SMAD signaling pathway in the context of mammary gland development and carcinogenesis, offering a new perspective on tumor suppression provided by basal–luminal cross-talk in normal mammary tissue. <i>Cancer Res; 75(10); 1972–82. ©2015 AACR</i>.</p></div>