Alterations in Wnt- and/or STAT3 signaling pathways and the immune microenvironment during metastatic progression

Metastatic breast cancer is an extremely complex disease with limited treatment options due to the lack of information about the major characteristics of metastatic disease. There is an urgent need, therefore, to understand the changes in cellular complexity and dynamics that occur during metastatic progression. In the current study, we analyzed the cellular and molecular differences between primary tumors and paired lung metastases using a syngeneic p53-null mammary tumor model of basal-like breast cancer. Distinct subpopulations driven by the Wnt- and/or STAT3 signaling pathways were detected in vivo using a lentiviral Wnt- and STAT3 signaling reporter system. A significant increase in the overlapping populations driven by both the Wnt- and STAT3 signaling pathways was observed in the lung metastases as compared to the primary tumors. Furthermore, the overlapping populations showed a higher metastatic potential relative to the other populations and pharmacological inhibition of both signaling pathways was shown to markedly reduce the metastatic lesions in established lung metastases. An analysis of the unique molecular features of the lung metastases revealed a significant association with immune response signatures. Specifically, Foxp3 gene expression was markedly increased and elevated levels of Foxp3 + Treg cells were detected in close proximity to lung metastases. Collectively, these studies illustrate the importance of analyzing intratumoral heterogeneity, changes in population dynamics, and the immune microenvironment during metastatic progression.

Abstract P2-02-14: Metabolic regulation and drug resistance in c-Src activated triple negative breast cancer

c-Src (Src) is a proto-oncogene involved in signaling that culminates in the control of multiple biological functions. Src is also one of the most frequently upregulated pathways in triple negative breast cancer (TNBC). Dysregulation of Src has been detected in TNBC and is strongly associated with tumor metastasis and poor prognosis. However, even after promising preclinical studies, Src inhibitors did not show major clinical advantage in unselected TNBC populations. We have previously published that metastatic TNBC has high energy-dependency to mitochondrial fatty acid beta-oxidation (FAO) and FAO activates Src by inducing autophosphorylation at Y419. However, our recent analysis suggests that as observed with the Src inhibitors, TNBC tumors treated with FAO inhibitors also develop drug-resistance and continue tumor growth. Evaluation of their drug resistance mechanism revealed that while short-term inhibition of FAO or Src induces autophagic and apoptotic cell deaths, long-term inhibition results in autophagy-mediated drug resistance and survival. Further analyses suggest that FAO and Src inhibitors activate mitogen-activated protein (MAP) kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway via the induction of cellular reactive oxygen species (ROS) in TNBC. Activated MEK/ERK then induces survival pathways for drug resistance and tumor survival. Validation of in vitro findings using in vivo TNBC models confirmed that combination of FAO/Src inhibitors with MEK/ERK inhibitors can provide significant benefit to overcome the therapeutic resistance of TNBC. These findings open-up new therapeutic opportunities to manage TNBC patients with currently non-targetable metastatic tumors.

Citation Format: Jung KH, Park JH, Sirupangi T, Jia D, Gandhi N, Pudakalakatti S, Elswood J, Porter W, Putluri N, Zhang XH-F, Chen X, Bhattacharya PK, Creighton CJ, Lewis MT, Rosen JM, Wong L-JC, Das GM, Osborne CK, Rimawi MF, Kaipparettu BA. Metabolic regulation and drug resistance in c-Src activated triple negative breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P2-02-14.

Overlap between functional abdominal pain disorders and organic diseases in children

Functional abdominal pain disorders are highly prevalent in children. These disorders can be present in isolation or combined with organic diseases, such as celiac disease and inflammatory bowel diseases. Intestinal inflammation (infectious and non-infectious) predisposes children to the development of visceral hypersensitivity that can manifest as functional abdominal pain disorders, including irritable bowel syndrome. The new onset of irritable bowel syndrome symptoms in a patient with an underlying organic disease, such as inflammatory bowel disease, is clinically challenging, given that the same symptomatology may represent a flare-up of the inflammatory bowel disease or an overlapping functional abdominal pain disorder. Similarly, irritable bowel syndrome symptoms in a child previously diagnosed with celiac disease may occur due to poorly controlled celiac disease or the overlap with a functional abdominal pain disorder. There is little research on the overlap of functional abdominal disorders with organic diseases in children. Studies suggest that the overlap between functional abdominal pain disorders and inflammatory bowel disease is more common in adults than in children. The causes for these differences in prevalence are unknown. Only a handful of studies have been published on the overlap between celiac disease and functional abdominal pain disorders in children. The present article provides a review of the literature on the overlap between celiac disease, inflammatory bowel disease, and functional abdominal pain disorders in children and establish comparisons with studies conducted on adults.

Abstract DL-1: Leveraging Preclinical Models of Breast Cancer

Jeffrey M. Rosen1, Kevin Roarty1, Se Jin Kim1, Charles M. Perou2, Sendurai Mani3, Paul Ik Sun Kim4 and Xiang Zhang1,4

1Department of Molecular & Cellular Biology and 1,4Lester and Sue Smith Breast Center, Baylor College of Medicine, Houston, TX, 2University of North Carolina, Chapel Hill, NC and 3MD Anderson Cancer Center, Houston, TX

Breast cancer is a disease that displays both inter- and intra-tumoral heterogeneity. More than a decade ago we asked if a subpopulation of breast cancer stem cells might be resistant to chemotherapy and responsible for relapse in breast cancer patients given neoadjuvant therapy. These clinical studies provided evidence for population of chemotherapy-resistant breast cancer-stem cells. The gene expression signature derived from these comparisons overlapped with an identified “claudin-low” molecular subtype characterized by the low to absent expression of luminal differentiation markers, high enrichment for many EMT-associated genes, and immune-response genes. The claudin-low subtype also most closely resembled a subset mammary epithelial stem cells. The “claudin-low” signature was enriched in residual tumors remaining after either endocrine therapy or chemotherapy treatment. Double positive cells expressing mesenchymal and epithelial markers were enriched in resistant tumors. This suggests the presence of an intermediate or “partial EMT”, which has been proposed to be a hybrid E/M phenotype between the epithelial to mesenchymal transition. Recently we have developed a set of EMT/MET sensors to identify cells undergoing this transition, and we have used these to screen an FDA approved compound library. Genetically engineered mouse (GEM) syngeneic p53-null mammary tumor models that closely mimic several of the subtypes in human breast cancer are being used as preclinical models to study the response to both novel targeted therapies and chemotherapy. We have developed, extensively characterized and “credentialed” a bank of these tumors, which represent the different subtypes of human breast cancer including a subset of these tumors with a similar gene expression signature as the human claudin-low tumors. These murine claudin-low tumors showed high expression of EMT inducers, low expression of members of the miR-200 family and resistance to most standard-of-care therapies. Re-expression of miR-200 family members reversed EMT, decreased the CSC population and sensitized cells to chemotherapy. The ZEB1/miR-200 axis has additional, non-cell autonomous roles in cancer pathogenesis, and has recently been shown to affect immune recognition of cancer cells whereby ZEB1 suppression of the miR-200 family leads to upregulation of PD-L1, a direct miR-200 family target. Therefore, the EMT transition also may play a role in immunosuppression exacerbating treatment response. Accordingly, immunoprofiling has identified unique subsets of immunosuppressive neutrophils and macrophages in mice with the different subtypes of p53 null tumors. Hierarchical clustering revealed "extrinsic" subtypes of breast cancers in terms of their innate immune cell profiles. Finally, we have employed specific Wnt and Stat3 pathway reporters to better understand the signaling pathways involved in intratumoral heterogeneity and metastasis.

Citation Format: Rosen JM. Leveraging Preclinical Models of Breast Cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr DL-1.

Abstract ES1-2: Paracrine signaling and intratumoral heterogeneity

Breast cancer is no longer considered a single disease, but instead comprises multiple subtypes with genetically and most likely epigenetically heterogeneous tumors composed of numerous clones. Both the hierarchical cancer stem cell (CSC) and clonal evolution models have been invoked to help explain this intratumoral heterogeneity. Several recent studies have helped define the functional interactions among the different cellular subpopulations necessary for the evolution of this complex ecosystem. These interactions involve paracrine interactions that include locally acting Wnt family members, reminiscent of the signaling pathways important for normal mammary gland development and stem cell self-renewal. In addition, they involve interactions between breast cancer cells and the immune microenvironment. In this presentation, we will discuss the interactions among various cell populations in both normal and tumor tissues. A better understanding of these interactions, especially in the metastatic setting, will be important for the development of improved combinatorial therapies designed to prevent relapse and to ultimately decrease mortality.

Citation Format: Rosen JM. Paracrine signaling and intratumoral heterogeneity [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr ES1-2.

Phosphorylation of serine 367 of FOXC2 by p38 regulates ZEB1 and breast cancer metastasis, without impacting primary tumor growth

Metastatic competence is contingent upon the aberrant activation of a latent embryonic program, known as the epithelial-mesenchymal transition (EMT), which bestows stem cell properties as well as migratory and invasive capabilities upon differentiated tumor cells. We recently identified the transcription factor FOXC2 as a downstream effector of multiple EMT programs, independent of the EMT-inducing stimulus, and as a key player linking EMT, stem cell traits and metastatic competence in breast cancer. As such, FOXC2 could serve as a potential therapeutic target to attenuate metastasis. However, as FOXC2 is a transcription factor, it is difficult to target by conventional means such as small-molecule inhibitors. Herein, we identify the serine/threonine-specific kinase p38 as a druggable upstream regulator of FOXC2 stability and function that elicits phosphorylation of FOXC2 at serine 367 (S367). Using an orthotopic syngeneic mouse tumor model, we make the striking observation that inhibition of p38-FOXC2 signaling selectively attenuates metastasis without impacting primary tumor growth. In this model, circulating tumor cell numbers are significantly reduced in mice treated with the p38 inhibitor SB203580, relative to vehicle-treated counterparts. Accordingly, genetic or pharmacological inhibition of p38 decreases FOXC2 protein levels, reverts the EMT phenotype and compromises stem cell attributes in vitro. We also identify the EMT-regulator ZEB1-known to directly repress E-cadherin/CDH1-as a downstream target of FOXC2, critically dependent on its activation by p38. Consistent with the notion that activation of the p38-FOXC2 signaling axis represents a critical juncture in the acquisition of metastatic competence, the phosphomimetic FOXC2(S367E) mutant is refractory to p38 inhibition both in vitro and in vivo, whereas the non-phosphorylatable FOXC2(S367A) mutant fails to elicit EMT and upregulate ZEB1. Collectively, our data demonstrate that FOXC2 regulates EMT, stem cell traits, ZEB1 expression and metastasis in a p38-dependent manner, and attest to the potential utility of p38 inhibitors as antimetastatic agents.

The Z-cad dual fluorescent sensor detects dynamic changes between the epithelial and mesenchymal cellular states

BACKGROUND

The epithelial to mesenchymal transition (EMT) has been implicated in metastasis and therapy resistance of carcinomas and can endow cancer cells with cancer stem cell (CSC) properties. The ability to detect cancer cells that are undergoing or have completed EMT has typically relied on the expression of cell surface antigens that correlate with an EMT/CSC phenotype. Alternatively these cells may be permanently marked through Cre-mediated recombination or through immunostaining of fixed cells. The EMT process is dynamic, and these existing methods cannot reveal such changes within live cells. The development of fluorescent sensors that mirror the dynamic EMT state by following the expression of bona fide EMT regulators in live cells would provide a valuable new tool for characterizing EMT. In addition, these sensors will allow direct observation of cellular plasticity with respect to the epithelial/mesenchymal state to enable more effective studies of EMT in cancer and development.

RESULTS

We generated a lentiviral-based, dual fluorescent reporter system, designated as the Z-cad dual sensor, comprising destabilized green fluorescent protein containing the ZEB1 3' UTR and red fluorescent protein driven by the E-cadherin (CDH1) promoter. Using this sensor, we robustly detected EMT and mesenchymal to epithelial transition (MET) in breast cancer cells by flow cytometry and fluorescence microscopy. Importantly, we observed dynamic changes in cellular populations undergoing MET. Additionally, we used the Z-cad sensor to identify and isolate minor subpopulations of cells displaying mesenchymal properties within a population comprising predominately epithelial-like cells. The Z-cad dual sensor identified cells with CSC-like properties more effectively than either the ZEB1 3' UTR or E-cadherin sensor alone.

CONCLUSIONS

The Z-cad dual sensor effectively reports the activities of two factors critical in determining the epithelial/mesenchymal state of carcinoma cells. The ability of this stably integrating dual sensor system to detect dynamic fluctuations between these two states through live cell imaging offers a significant improvement over existing methods and helps facilitate the study of EMT/MET plasticity in response to different stimuli and in cancer pathogenesis. Finally, the versatile Z-cad sensor can be adapted to a variety of in vitro or in vivo systems to elucidate whether EMT/MET contributes to normal and disease phenotypes.

Expression of miR-200c in claudin-low breast cancer alters stem cell functionality, enhances chemosensitivity and reduces metastatic potential

Claudin-low tumors are a highly aggressive breast cancer subtype with no targeted treatments and a clinically documented resistance to chemotherapy. They are significantly enriched in cancer stem cells (CSCs), which makes claudin-low tumor models particularly attractive for studying CSC behavior and developing novel approaches to minimize CSC therapy resistance. One proposed mechanism by which CSCs arise is via an epithelial-mesenchymal transition (EMT), and reversal of this process may provide a potential therapeutic approach for increasing tumor chemosensitivity. Therefore, we investigated the role of known EMT regulators, miR-200 family of microRNAs in controlling the epithelial state, stem-like properties, and therapeutic response in an in vivo primary, syngeneic p53^null claudin-low tumor model that is normally deficient in miR-200 expression. Using an inducible lentiviral approach, we expressed the miR-200c cluster in this model and found that it changed the epithelial state, and consequently, impeded CSC behavior in these mesenchymal tumors. Moreover, these state changes were accompanied by a decrease in proliferation and an increase in the differentiation status. miR-200c expression also forced a significant reorganization of tumor architecture, affecting important cellular processes involved in cell-cell contact, cell adhesion, and motility. Accordingly, induced miR200c expression significantly enhanced the chemosensitivity and decreased the metastatic potential of this p53^null claudin-low tumor model. Collectively, our data suggest that miR-200c expression in claudin-low tumors offers a potential therapeutic application to disrupt the EMT program on multiple fronts in this mesenchymal tumor subtype, by altering tumor growth, chemosensitivity, and metastatic potential in vivo.

Abstract P4-07-02: MicroRNA 204 mediated negative regulation of the IGF2R promotes breast cancer progression and is a potential mechanism driving breast cancer disparity

Breast cancer (BC) is a worldwide health issue as it represents the leading cause of cancer in women and the second leading cause of cancer-related mortality in women, with an increasing incidence. In the US, African American (AA) women have a significantly higher rate of mortality due to BC compared to Caucasian American (CA) women. It is becoming increasingly apparent that racial disparity in cancer exists due to molecular differences in tumor biology as well as, or in addition to, socioeconomic and standard of care issues. Sparse information exists regarding the molecular mechanisms that promote BC health disparity therefore, a greater understanding of the risk factors and biological links associated with BC, will significantly impact AA women. Our studies have identified a race specific mechanistic link between microRNA-204 (miR-204) and the Insulin-like Growth Factor-2 Receptor. MicroRNAs are small non-coding RNAs that function to inhibit gene expression through translational repression. Our published studies identified miR-204 as a novel oncomir and we recently identified IGF2R as a direct target. The IGF2R has been proposed to be a tumor suppressor gene in several cancers including breast cancer. We show that over-expression of miR-204 results in a decrease in IGF2R protein levels. We propose that the inhibition of IGF2R allows IGF-2 to bind the IGF1R leading to hyperactivation of this pathway which results in increased proliferation, migration and invasion, processes that are required for tumor progression. Indeed, we show that miR-204 expression results in the activation of the IGF1R/IRS-1/ERK pathway together with an increase in proliferation, migration and invasion. Published studies have shown that exogenous expression of activated IGF1R increases migration in non-transformed breast cells. We have shown that exogenous expression of miR-204 also increases migration in these cells. However, when miR-204 and IGF1R are both exogenously expressed no additional increase in migration was observed, suggesting that mIR-204 mediated increase in migration is through activation of the IGF1R pathway. Reduced IGF2R expression correlates with poor patient prognosis in BC patients and a recent study showed significantly higher levels of IGF2R in CA compared to AA tumor samples, suggesting that decreased IGF2R expression may contribute to BC disparity. We examined serum from BC patients and found elevated levels of miR-204 in AA compared to CA women. Circulating miRNAs can serve as accessible biomarkers for diagnosis & prognosis and may define a novel area of potential therapeutic intervention to reduce BC disparity. Our data also suggest that miR-204 mediated inhibition of IGF2R could be a potential biological mechanism driving BC disparity. A major hurdle to the identification of biological mechanisms conferring cancer health disparity is a lack of suitable experimental models with which to investigate race specific differences in tumor biology. We have developed a unique inducible miR-204 transgenic mouse model to define in vivo the oncogenic potential of miR-204 and the mechanism and functional consequences of IGF2R loss.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-07-02.

P190A RhoGAP is required for mammary gland development

P190A and p190B Rho GTPase activating proteins (GAPs) are essential genes that have distinct, but overlapping roles in the developing nervous system. Previous studies from our laboratory demonstrated that p190B is required for mammary gland morphogenesis, and we hypothesized that p190A might have a distinct role in the developing mammary gland. To test this hypothesis, we examined mammary gland development in p190A-deficient mice. P190A expression was detected by in situ hybridization in the developing E14.5day embryonic mammary bud and within the ducts, terminal end buds (TEBs), and surrounding stroma of the developing virgin mammary gland. In contrast to previous results with p190B, examination of p190A heterozygous mammary glands demonstrated that p190A deficiency disrupted TEB morphology, but did not significantly delay ductal outgrowth indicating haploinsufficiency for TEB development. To examine the effects of homozygous deletion of p190A, embryonic mammary buds were rescued by transplantation into the cleared fat pads of SCID/Beige mice. Complete loss of p190A function inhibited ductal outgrowth in comparison to wildtype transplants (51% vs. 94% fat pad filled). In addition, the transplantation take rate of p190A deficient whole gland transplants from E18.5 embryos was significantly reduced compared to wildtype transplants (31% vs. 90%, respectively). These results suggest that p190A function in both the epithelium and stroma is required for mammary gland development. Immunostaining for p63 demonstrated that the myoepithelial cell layer is disrupted in the p190A deficient glands, which may result from the defective cell adhesion between the cap and body cell layers detected in the TEBs. The number of estrogen- and progesterone receptor-positive cells, as well as the expression levels of these receptors was increased in p190A deficient outgrowths. These data suggest that p190A is required in both the epithelial and stromal compartments for ductal outgrowth and that it may play a role in mammary epithelial cell differentiation.

Therapeutic resistance and tumor-initiation: Molecular pathways involved in breast cancer stem cell self-renewal

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Background: Recent evidence supports the existance of a rare subpopulation of ‘cancer stem cells‘ (CSCs) which is chemoresistant and capable of self-renewal and tumor-initiation, resulting in relapse and metastases. We hypothesized that residual breast tumors after conventional chemotherapy (CTx) are enriched for CSCs bearing CD44+/CD24- markers, and show increased self-renewal as demonstrated by mammosphere (MS) forming assays. Molecular pathways like Notch, Wnt, and the polycomb family that regulate normal mammary self-renewal may be in aberrant in CSCs. Methods: Paired breast cancer biopsies from 35 patients were obtained before and after 12 weeks of neoadjuvant CTx (docetaxel 100 mg/m2 or Adriamycin/Cytoxan 60/600 mg/m2, 4 cycles, q3weeks), digested by collagenase, stained with CD24/CD44/lineage antibodies, and analyzed by flow cytometry. MS assays were performed to measure self-renewal ability. Gene expression, using the Affymetrix U133 GeneChip platform, of cancer cells bearing CD44+/CD24- markers vs. all other sorted cells, and between secondary cancer MS vs. the primary bulk invasive cancers were analyzed. Results: CD44+/CD24- cells increased from a median of 4.8% to 14.8% after CTx (p<0.005). Increased self-renewal was demonstrated by an increase in MS capacity after CTx (p=0.03), with a positive correlation between the number of CD44+/24- cells and MS assays (R=0.8, p<0.05). Common molecular pathways shared by CD44+/CD24- cells and MS show increased expression in normal self-renewal pathways - polycomb family (PCGF5), Notch (MAML2), FOXP1, and BBX. In addition, genes governing alternative splicing were increased, including a non-coding RNA (MALAT1) of unknown function, and RNA splicing factors (SFRS3, SFRS21P, SFRS4). Conclusions: Our results with an increase in cells bearing stem cell markers, and increased MS formation of residual tumors provide the first strong clinical evidence for the existance of therapy-resistant cancer stem cells. Post-transcriptional regulation may play a crucial role in modifying gene function involved in cancer stem cell self-renewal. Clinical trials targeting these newly identified pathways may eradicate residual disease and improved cure rates for many breast cancer patients.

[Table: see text]

Isolation and characterization of functional mammary gland stem cells

Significant advances in the stem-cell biology of several tissues, including the mammary gland, have occurred over the past several years. Recent progress on stem-cell fate determination, molecular markers, signalling pathways and niche interactions in haematopoietic, neuronal and muscle tissue may provide parallel insight into the biology of mammary epithelial stem cells. Taking advantage of approaches similar to those employed to isolate and characterize haematopoietic and epidermal stem cells, we have identified a mammary epithelial cell population with several stem/progenitor cell qualities. In this article, we review some recent data on mammary epithelial stem/progenitor cells in genetically engineered mouse models. We also discuss several potential molecular markers, including stem-cell antigen-1 (Sca-1), which may be useful for both the isolation of functional mammary epithelial stem/progenitor cells and the analysis of tumour aetiology and phenotype in genetically engineered mouse models. In different transgenic mammary tumour models, Sca-1 expression levels, as well as several other putative markers of progenitors including keratin-6, possess dramatically altered expression profiles. These data suggest that the heterogeneity of mouse models of breast cancer may partially reflect the selection or expansion of different progenitors.

Developmental and hormonal signals dramatically alter the localization and abundance of insulin receptor substrate proteins in the mammary gland

Insulin receptor substrates (IRS) are central integrators of hormone, cytokine, and growth factor signaling. IRS proteins can be phosphorylated by a number of signaling pathways critical to normal mammary gland development. Studies in transgenic mice that overexpress IGF-I in the mammary gland suggested that IRS expression is important in the regulation of normal postlactational mammary involution. The goal of these studies was to examine IRS expression in the mouse mammary gland and determine the importance of IRS-1 to mammary development in the virgin mouse. IRS-1 and -2 show distinct patterns of protein expression in the virgin mouse mammary gland, and protein abundance is dramatically increased during pregnancy and lactation, but rapidly lost during involution. Consistent with hormone regulation, IRS-1 protein levels are reduced by ovariectomy, induced by combined treatment with estrogen and progesterone, and vary considerably throughout the estrous cycle. These changes occur without similar changes in mRNA levels, suggesting posttranscriptional control. Mammary glands from IRS-1 null mice have smaller fat pads than wild-type controls, but this reduction is proportional to the overall reduction in body size. Development of the mammary duct (terminal endbuds and branch points) is not altered by the loss of IRS-1, and pregnancy-induced proliferation is not changed. These data indicate that IRS undergo complex developmental and hormonal regulation in the mammary gland, and that IRS-1 is more likely to regulate mammary function in lactating mice than in virgin or pregnant mice.

Signal transducer and activator of transcription (Stat) 5 controls the proliferation and differentiation of mammary alveolar epithelium

Functional development of mammary epithelium during pregnancy depends on prolactin signaling. However, the underlying molecular and cellular events are not fully understood. We examined the specific contributions of the prolactin receptor (PrlR) and the signal transducers and activators of transcription 5a and 5b (referred to as Stat5) in the formation and differentiation of mammary alveolar epithelium. PrlR- and Stat5-null mammary epithelia were transplanted into wild-type hosts, and pregnancy-mediated development was investigated at a histological and molecular level. Stat5-null mammary epithelium developed ducts but failed to form alveoli, and no milk protein gene expression was observed. In contrast, PrlR-null epithelium formed alveoli-like structures with small open lumina. Electron microscopy revealed undifferentiated features of organelles and a perturbation of cell-cell contacts in PrlR- and Stat5-null epithelia. Expression of NKCC1, an Na-K-Cl cotransporter characteristic for ductal epithelia, and ZO-1, a protein associated with tight junction, were maintained in the alveoli-like structures of PrlR- and Stat5-null epithelia. In contrast, the Na-Pi cotransporter Npt2b, and the gap junction component connexin 32, usually expressed in secretory epithelia, were undetectable in PrlR- and Stat5-null mice. These data demonstrate that signaling via the PrlR and Stat5 is critical for the proliferation and differentiation of mammary alveoli during pregnancy.

Persistent changes in gene expression induced by estrogen and progesterone in the rat mammary gland

Epidemiological studies have consistently shown that an early full-term pregnancy is protective against breast cancer. We hypothesize that the hormonal milieu that is present during pregnancy results in persistent changes in the pattern of gene expression in the mammary gland, leading to permanent changes in cell fate that determine the subsequent proliferative response of the gland. To investigate this hypothesis, we have used suppression subtractive hybridization to identify genes that are persistently up-regulated in the glands of E- and progesterone (P)-treated Wistar-Furth rats 28 d after steroid hormone treatment compared with age-matched virgins. Using this approach, a number of genes displaying persistent altered expression in response to previous treatment with E and P were identified. Two markers have been characterized in greater detail: RbAp46 and a novel gene that specifies a noncoding RNA (designated G.B7). Both were persistently up-regulated in the lobules of the regressed gland and required previous treatment with both E and P for maximal persistent expression. RbAp46 has been implicated in a number of complexes involving chromatin remodeling, suggesting a mechanism whereby epigenetic factors responsible for persistent changes in gene expression may be related to the determination of cell fate. These results provide the first support at the molecular level for the hypothesis that hormone-induced persistent changes in gene expression are present in the involuted mammary gland.

Differential interactions of specific nuclear factor I isoforms with the glucocorticoid receptor and STAT5 in the cooperative regulation of WAP gene transcription

The distal region (-830 to -720 bp) of the rat whey acidic protein (WAP) gene contains a composite response element (CoRE), which has been demonstrated previously to confer mammary gland-specific and hormonally regulated WAP gene expression. Point mutations in the binding sites for specific transcription factors present within this CoRE have demonstrated the importance of both nuclear factor I (NFI) and STAT5 as well as cooperative interactions with the glucocorticoid receptor (GR) in the regulation of WAP gene expression in the mammary gland of transgenic mice. This study reports the characterization of NFI gene expression during mammary gland development and the identification and cloning of specific NFI isoforms (NFI-A4, NFI-B2, and NFI-X1) from the mouse mammary gland during lactation. Some but not all of these NFI isoforms synergistically activate WAP gene transcription in cooperation with GR and STAT5, as determined using transient cotransfection assays in JEG-3 cells. On both the WAP CoRE and the mouse mammary tumor virus long terminal repeat promoter, the NFI-B isoform preferentially activated gene transcription in cooperation with STAT5A and GR. In contrast, the NFI-A isoform suppressed GR and STAT cooperativity at the WAP CoRE. Finally, unlike their interaction with the NFI consensus binding site in the adenovirus promoter, the DNA-binding specificities of the three NFI isoforms to the palindromic NFI site in the WAP CoRE were not identical, which may partially explain the failure of the NFI-A isoform to cooperate with GR and STAT5A.