Delayed involution of the mammary epithelium in BALB/c-p53null mice

Fur removal promotes an earlier expression of involution-related genes in mammary gland of lactating mice

Peak lactation occurs when milk production is at its highest. The factors limiting peak lactation performance have been subject of intense debate. Milk production at peak lactation appears limited by the capacity of lactating females to dissipate body heat generated as a by-product of processing food and producing milk. As a result, manipulations that enhance capacity to dissipate body heat (such as fur removal) increase peak milk production. We investigated the potential correlates of shaving-induced increases in peak milk production in laboratory mice. By transcriptomic profiling of the mammary gland, we searched for the mechanisms underlying experimentally increased milk production and its consequences for mother-young conflict over weaning, manifested by advanced or delayed involution of mammary gland. We demonstrated that shaving-induced increases in milk production were paradoxically linked to reduced expression of some milk synthesis-related genes. Moreover, the mammary glands of shaved mice had a gene expression profile indicative of earlier involution relative to unshaved mice. Once provided with enhanced capacity to dissipate body heat, shaved mice were likely to rear their young to independence faster than unshaved mothers.

A tumor-suppressive function for Notch3 in the parous mammary gland

Notch3 promotes mammary luminal cell specification and forced Notch3 activation can induce mammary tumor formation. However, recent studies suggest a tumor-suppressive role for Notch3. Here, we report on Notch3 expression and functional analysis in the mouse mammary gland. Notch3 is expressed in the luminal compartment throughout mammary gland development, but switches to basal cells with initiation of post-lactational involution. Deletion of Notch3 caused a decrease of Notch activation in luminal cells and diminished luminal progenitors at puberty, as well as reduced alveolar progenitors during pregnancy. Parous Notch3−/− mammary glands developed hyperplasia with accumulation of CD24hiCD49flo cells, some of which progressed to invasive tumors with luminal features. Notch3 deletion abolished Notch activation in basal cells during involution, accompanied by altered apoptosis and reduced brown adipocytes, leading to expansion of parity-identified mammary epithelial cells (PI-MECs). Interestingly, the postpartum microenvironment is required for the stem cell activity of Notch3−/− PI-MECs. Finally, high expression of NOTCH3 is associated with prolonged survival in patients with luminal breast cancer. These results highlight an unexpected tumor-suppressive function for Notch3 in the parous mammary gland through restriction of PI-MEC expansion.

Sporadic activation of an oxidative stress-dependent NRF2-p53 signaling network in breast epithelial spheroids and premalignancies

Breast and mammary epithelial cells experience different local environments during tissue development and tumorigenesis. Microenvironmental heterogeneity gives rise to distinct cell regulatory states whose identity and importance are just beginning to be appreciated. Cellular states diversify when clonal three-dimensional (3D) spheroids are cultured in basement membrane, and one such state is associated with stress tolerance and poor response to anticancer therapeutics. Here, we found that this state was jointly coordinated by the NRF2 and p53 pathways, which were costabilized by spontaneous oxidative stress within 3D cultures. Inhibition of NRF2 or p53 individually disrupted some of the transcripts defining the regulatory state but did not yield a notable phenotype in nontransformed breast epithelial cells. In contrast, combined perturbation prevented 3D growth in an oxidative stress-dependent manner. By integrating systems models of NRF2 and p53 signaling in a single oxidative stress network, we recapitulated these observations and made predictions about oxidative stress profiles during 3D growth. NRF2 and p53 signaling were similarly coordinated in normal breast epithelial tissue and hormone-negative ductal carcinoma in situ lesions but were uncoupled in triple-negative breast cancer (TNBC), a subtype in which p53 is usually mutated. Using the integrated model, we correlated the extent of this uncoupling in TNBC cell lines with the importance of NRF2 in the 3D growth of these cell lines and their predicted handling of oxidative stress. Our results point to an oxidative stress tolerance network that is important for single cells during glandular development and the early stages of breast cancer.

Laminin-binding integrins are essential for the maintenance of functional mammary secretory epithelium in lactation

Integrin dimers α3/β1, α6/β1 and α6/β4 are the mammary epithelial cell receptors for laminins, which are major components of the mammary basement membrane. The roles of specific basement membrane components and their integrin receptors in the regulation of functional gland development have not been analyzed in detail. To investigate the functions of laminin-binding integrins, we obtained mutant mice with mammary luminal cell-specific deficiencies of the α3 and α6 integrin chains generated by the Cre-Lox approach. During pregnancy, mutant mice displayed decreased luminal progenitor activity and retarded lobulo-alveolar development. Mammary glands appeared functional at the onset of lactation in mutant mice, however myoepithelial cell morphology was markedly altered, suggesting cellular compensation mechanisms involving cytoskeleton reorganization. Notably, lactation was not sustained in mutant females, and the glands underwent precocious involution. Inactivation of the p53 gene rescued the growth defects but did not restore lactogenesis in mutant mice. These results suggest that the p53 pathway is involved in the control of mammary cell proliferation and survival downstream of laminin-binding integrins and underline an essential role of cell interactions with laminin for lactogenic differentiation.

Metabolic Reprogramming in Breast Cancer and Its Therapeutic Implications

Current standard-of-care (SOC) therapy for breast cancer includes targeted therapies such as endocrine therapy for estrogen receptor-alpha (ERα) positive; anti-HER2 monoclonal antibodies for human epidermal growth factor receptor-2 (HER2)-enriched; and general chemotherapy for triple negative breast cancer (TNBC) subtypes. These therapies frequently fail due to acquired or inherent resistance. Altered metabolism has been recognized as one of the major mechanisms underlying therapeutic resistance. There are several cues that dictate metabolic reprogramming that also account for the tumors’ metabolic plasticity. For metabolic therapy to be efficacious there is a need to understand the metabolic underpinnings of the different subtypes of breast cancer as well as the role the SOC treatments play in targeting the metabolic phenotype. Understanding the mechanism will allow us to identify potential therapeutic vulnerabilities. There are some very interesting questions being tackled by researchers today as they pertain to altered metabolism in breast cancer. What are the metabolic differences between the different subtypes of breast cancer? Do cancer cells have a metabolic pathway preference based on the site and stage of metastasis? How do the cell-intrinsic and -extrinsic cues dictate the metabolic phenotype? How do the nucleus and mitochondria coordinately regulate metabolism? How does sensitivity or resistance to SOC affect metabolic reprogramming and vice-versa? This review addresses these issues along with the latest updates in the field of breast cancer metabolism.

Stromal Gas6 promotes the progression of premalignant mammary cells

Tumor progression is regulated by a complex interplay between neoplastic cells and the tumor microenvironment. Tumor associated macrophages have been shown to promote breast cancer progression in advanced disease and more recently, in early stage cancers. However, little is known about the macrophage-derived factors that promote tumor progression in early stage lesions. Using a p53-null model of early stage mammary tumor progression, we found that Gas6 is highly expressed in pre-invasive lesions associated with increased infiltrating macrophages, as compared to those with few recruited macrophages. We show that F4/80⁺CD11b⁺ macrophages produce Gas6 in premalignant lesions in vivo, and that macrophage-derived Gas6 induces a tumor-like phenotype ex vivo. Using a 3-D co-culture system, we show that macrophage-derived Gas6 activates its receptor Axl and downstream survival signals including Akt and STAT3, which was accompanied by altered E-cadherin expression to induce a malignant morphology. In vivo studies demonstrated that deletion of stromal Gas6 delays early stage progression and decreases tumor formation, while tumor growth in established tumors remains unaffected. These studies suggest that macrophage-derived Gas6 is a critical regulator of the transition from premalignant to invasive cancer, and may lead to the development of unique biomarkers of neoplastic progression for patients with early stage breast cancer, including ductal carcinoma in situ.

Reprogramming of the estrogen responsive transcriptome contributes to tamoxifen-dependent protection against tumorigenesis in the p53 null mammary epithelial cells

The tumor suppressor gene p53 is frequently mutated in human breast cancer and is a marker for poor prognosis and resistance to chemotherapy. Transplantation of p53 null mouse mammary epithelium into syngeneic wild-type mice leads to normal mammary gland development followed by spontaneous mammary tumors that recapitulate many of the phenotypic, molecular and genetic features of human breast cancer. Transient exposure of p53 null mice to the anti-estrogen, tamoxifen leads to sustained and robust protection against tumor development. However the mechanism underlying this anti-tumor activity remains poorly understood. Here we demonstrate that transient exposure to tamoxifen leads to a reduction in mammary ductal side-branching and epithelial cell proliferation after tamoxifen withdrawal. Global gene expression analysis showed that transient tamoxifen exposure leads to persistent changes in the expression of a subset of estrogen regulated gene signatures in mammary epithelial cells (MECs). Among these was the protein tyrosine phosphatase, non-receptor type 5 (Ptpn5). We show that Ptpn5 is a novel tamoxifen regulated target gene which is upregulated in MECs after transient tamoxifen exposure and displays tumor suppressor activity in human breast cancer cells. Further, PTPN5 expression is strongly associated with good clinical outcome in tamoxifen treated human breast cancer patients suggesting that PTPN5 may represent a novel biomarker of tamoxifen response in human breast cancer.

The role of MDM2 and MDM4 in breast cancer development and prevention

The major cause of death from breast cancer is not the primary tumour, but relapsing, drug-resistant, metastatic disease. Identifying factors that contribute to aggressive cancer offers important leads for therapy. Inherent defence against carcinogens depends on the individual molecular make-up of each person. Important molecular determinants of these responses are under the control of the mouse double minute (MDM) family: comprised of the proteins MDM2 and MDM4. In normal, healthy adult cells, the MDM family functions to critically regulate measured, cellular responses to stress and subsequent recovery. Proper function of the MDM family is vital for normal breast development, but also for preserving genomic fidelity. The MDM family members are best characterized for their negative regulation of the major tumour suppressor p53 to modulate stress responses. Their impact on other cellular regulators is emerging. Inappropriately elevated protein levels of the MDM family are highly associated with an increased risk of cancer incidence. Exploration of the MDM family members as cancer therapeutic targets is relevant for designing tailored anti-cancer treatments, but successful approaches must strategically consider the impact on both the target cancer and adjacent healthy cells and tissues. This review focuses on recent findings pertaining to the role of the MDM family in normal and malignant breast cells.

Macrophages promote the progression of premalignant mammary lesions to invasive cancer

Breast cancer initiation, progression and metastasis rely on a complex interplay between tumor cells and their surrounding microenvironment. Infiltrating immune cells, including macrophages, promote mammary tumor progression and metastasis; however, less is known about the role of macrophages in early stage lesions. In this study, we utilized a transplantable p53-null model of early progression to characterize the immune cell components of early stage lesions. We show that macrophages are recruited to ductal hyperplasias with a high tumor-forming potential where they are differentiated and polarized toward a tumor-promoting phenotype. These macrophages are a unique subset of macrophages, characterized by pro-inflammatory, anti-inflammatory and immunosuppressive factors. Macrophage ablation studies showed that macrophages are required for both early stage progression and primary tumor formation. These studies suggest that therapeutic targeting of tumor-promoting macrophages may not only be an effective strategy to block tumor progression and metastasis, but may also have critical implications for breast cancer prevention.

Dispersal of an ancient retroposon in the TP53 promoter of Bovidae: phylogeny, novel mechanisms, and potential implications for cow milk persistency

BACKGROUND

In recent years, the perception of transposable genetic elements has changed from "junk DNA" to a focus of interest when appearing near or inside genes. Bov-A2 is a short interspersed nuclear element (SINE) that was first found in Bovidae and later in other ruminants. This retroposon is mostly used as a marker for phylogenetic analysis.

RESULTS

We describe insertions of Bov-A2 in the promoter region of TP53, a key tumor suppressor gene that is indispensable for diverse developmental processes, in Antilopinae and Tragelaphini (belonging to the Bovinae subfamily). In Tragelaphini two Bov-A2 elements were inserted sequentially, whereas in 5 tribes of Antilopinae only one Bov-A2 element was inserted, in a different site and reverse orientation. The entrance site in both cases employed short palindromes that can form hairpin secondary structures. Interestingly, mutations that create or disrupt base pairing in the palindrome sequence dictated the presence or absence of Bov-A2, such as in the domestic cow and buffalo, which lack Bov-A2. Transcription factor binding site analysis revealed unique binding sites for STAT3 and NFκB within the Bov-A2 sequence, which together with TP53 itself are known to play a crucial role in mammary involution.

CONCLUSIONS

This report demonstrates how short palindromes serve as hot spots for Bov-A2 retroposon insertion into the mammalian genome. The strict correlation between point mutation in the palindromes and the presence/absence of Bov-A2 retroposon insertions, questions the use of singular insertion events as valid phylogenetic markers inside families. Bov-A2 insertion into the TP53 promoter in Antilopinae and Tragelaphini may not only provide a genetic network that regulates mammary involution, but can also answer the need for rapid mammary involution in Savanna antelopes after weaning, partially in response to predation stress. The absence of Bov-A2 in domestic bovids may constitute the molecular background for greater lactation persistency.

TP53 supports basal-like differentiation of mammary epithelial cells by preventing translocation of deltaNp63 into nucleoli

Multiple observations suggest a cell type-specific role for TP53 in mammary epithelia. We developed an in vitro assay, in which primary mouse mammary epithelial cells (mMECs) progressed from lumenal to basal-like phenotypes based on expression of Krt18 or ΔNp63, respectively. Such transition was markedly delayed in Trp53^−/− mMECs suggesting that Trp53 is required for specification of the basal, but not lumenal cells. Evidence from human basal-like cell lines suggests that TP53 may support the activity of ΔNp63 by preventing its translocation from nucleoplasm into nucleoli. In human lumenal cells, activation of TP53 by inhibiting MDM2 or BRCA1 restored the nucleoplasmic expression of ΔNp63. Trp53^−/− mMECs eventually lost epithelial features resulting in upregulation of MDM2 and translocation of ΔNp63 into nucleoli. We propose that TP63 may contribute to TP53-mediated oncogenic transformation of epithelial cells and shed light on tissue- and cell type-specific biases observed for TP53-related cancers.

p63 is a prosurvival factor in the adult mammary gland during post-lactational involution, affecting PI-MECs and ErbB2 tumorigenesis

In embryogenesis, p63 is essential to develop mammary glands. In the adult mammary gland, p63 is highly expressed in the basal cell layer that comprises myoepithelial and interspersed stem/progenitor cells, and has limited expression in luminal epithelial cells. In adult skin, p63 has a crucial role in the maintenance of epithelial stem cells. However, it is unclear whether p63 also has an equivalent role as a stem/progenitor cell factor in adult mammary epithelium. We show that p63 is essential in vivo for the survival and maintenance of parity-identified mammary epithelial cells (PI-MECs), a pregnancy-induced heterogeneous population that survives post-lactational involution and contain multipotent progenitors that give rise to alveoli and ducts in subsequent pregnancies. p63+/- glands are normal in virgin, pregnant and lactating states. Importantly, however, during the apoptotic phase of post-lactational involution p63+/- glands show a threefold increase in epithelial cell death, concomitant with increased activation of the oncostatin M/Stat3 and p53 pro-apoptotic pathways, which are responsible for this phase. Thus, p63 is a physiologic antagonist of these pathways specifically in this regressive stage. After the restructuring phase when involution is complete, mammary glands of p63+/- mice again exhibit normal epithelial architecture by conventional histology. However, using Rosa(LSL-LacZ);WAP-Cre transgenics (LSL-LacZ, lox-stop-lox β-galactosidase), a genetic in vivo labeling system for PI-MECs, we find that p63+/- glands have a 30% reduction in the number of PI-MEC progenitors and their derivatives. Importantly, PI-MECs are also cellular targets of pregnancy-promoted ErbB2 tumorigenesis. Consistent with their PI-MEC pool reduction, one-time pregnant p63+/- ErbB2 mice are partially protected from breast tumorigenesis, exhibiting extended tumor-free and overall survival, and reduced tumor multiplicity compared with their p63+/+ ErbB2 littermates. Conversely, in virgin ErbB2 mice p63 heterozygosity provides no survival advantage. In sum, our data establish that p63 is an important survival factor for pregnancy-identified PI-MEC progenitors in breast tissue in vivo.

siRNA screening identifies differences in the Fanconi anemia pathway in BALB/c-Trp53+/- with susceptibility versus C57BL/6-Trp53+/- mice with resistance to mammary tumors

BALB/c mice heterozygous for Trp53 develop a high proportion of spontaneous mammary tumors, a phenotype distinct from other mouse strains. BALB/c-Trp53+/- female mice, thus, resemble the hereditary Li-Fraumeni syndrome (LFS) characterized by early-onset of breast cancer, even though LFS involves TP53 mutations, which may involve not only loss- but also gain-of-function. Previous analysis of tumors in BALB/c-Trp53+/- females showed frequent loss of heterozygosity involving the wild-type allele of Trp53 and displayed characteristics indicative of mitotic recombination. Critical involvement of DNA double-strand break (DSB) repair dysfunction, particularly of homologous recombination (HR), was also noticed in the etiology of human breast cancer. To better define functional alterations in BALB/c-Trp53+/- mice, we applied a fluorescence-based DSB repair assay on mouse embryonic fibroblasts (MEFs) from BALB/c-Trp53+/- versus C57BL/6J-Trp53+/- mice. This approach revealed deregulation of HR but not non-homologous end-joining (NHEJ) in BALB/c-Trp53+/-, which was further confirmed for mammary epithelial cells. Screening of a small interfering RNA-library targeting DSB repair, recombination, replication and signaling genes, identified 25 genes causing differences between homologous DSB repair in the two strains upon silencing. Interactome analysis of the hits revealed clustering of replication-related and fanconi anemia (FA)/breast cancer susceptibility (BRCA) genes. Further dissection of the functional change in BALB/c-Trp53+/- by immunofluorescence microscopy of nuclear 53BP1, Replication protein A (RPA) and Rad51 foci uncovered differences in crosslink and replication-associated repair. Chromosome breakage, G2 arrest and biochemical analyses indicated a FA pathway defect downstream of FancD2 associated with reduced levels of BRCA2. Consistent with polygenic models for BRCA, mammary carcinogenesis in BALB/c-Trp53+/- mice may, therefore, be promoted by a BRCA modifier allele in the FA pathway in the context of partial p53 loss-of-function.

Cooperativity of Rb, Brca1, and p53 in malignant breast cancer evolution

Breast cancers that are "triple-negative" for the clinical markers ESR1, PGR, and HER2 typically belong to the Basal-like molecular subtype. Defective Rb, p53, and Brca1 pathways are each associated with triple-negative and Basal-like subtypes. Our mouse genetic studies demonstrate that the combined inactivation of Rb and p53 pathways is sufficient to suppress the physiological cell death of mammary involution. Furthermore, concomitant inactivation of all three pathways in mammary epithelium has an additive effect on tumor latency and predisposes highly penetrant, metastatic adenocarcinomas. The tumors are poorly differentiated and have histologic features that are common among human Brca1-mutated tumors, including heterogeneous morphology, metaplasia, and necrosis. Gene expression analyses demonstrate that the tumors share attributes of both Basal-like and Claudin-low signatures, two molecular subtypes encompassed by the broader, triple-negative class defined by clinical markers.

Mammary gland development is delayed in mice deficient for aminopeptidase N

Development of the mammary gland requires the coordinated action of proteolytic enzymes during two phases of remodelling. Firstly, new ducts and side-branches thereof need to be established during pregnancy to generate an extensive ductal tree allowing the secretion and transport of milk. A second wave of remodelling occurs during mammary involution after weaning. We have analysed the role of the cell surface protease aminopeptidase N (Anpep, APN, CD13) during these processes using Anpep deficient and Anpep over-expressing mice. We find that APN deficiency significantly delays mammary gland morphogenesis during gestation. The defect is characterised by a reduction in alveolar buds and duct branching at mid-pregnancy. Conversely over-expression of Anpep leads to accelerated ductal development. This indicates that Anpep plays a critical role in the proteolytic remodelling of mammary tissue during adult mammary development.

Critical choices for modeling breast cancer in transgenic mouse models

Modeling breast cancer in the mouse has helped to better define the heterogeneity of human breast cancer. In the recent past, it has become evident that some limitations have restricted the potential benefits that can be achieved with this approach. In this review, we highlight some key points that should be taken into account when the mouse is used, with special emphasis on transgenic models.

The emerging role of p53 in stem cells

Among the hundreds of oncogenes and tumor suppressors that have been identified in the past 50 years, p53 is probably the best characterized; nevertheless, new functions are constantly being discovered. As a tumor suppressor, p53 regulates cellular responses to different stress stimuli by inducing reversible cell cycle arrest and DNA repair, or triggering senescence or apoptosis. Recent findings on the regulation of stem cell (SC) division and reprogramming suggest the intriguing possibility that p53 also carries out its tumor suppression function by regulating SC homeostasis. Specifically, p53 activation may counteract SC expansion by several emerging mechanisms including restriction of self-renewing divisions, inhibition of symmetric division and block of reprogramming of somatic/progenitor cells into SCs.

MiR-126-3p regulates progesterone receptors and involves development and lactation of mouse mammary gland

MicroRNAs (miRNAs) are small (18-22 nucleotide) non-coding, endogenous regulatory RNA molecules, and they regulate gene expression at the post-transcriptional level through binding to their target mRNAs by base-pairing and subsequently inducing either translational repression or mRNA destabilization by plants, animals, and some viruses. In this study, combining microarray techniques with qRT-PCR, we found that miR-126-3p expression showed significant difference in the mouse mammary cycle during pregnancy, particularly on transition from pregnancy to lactation. Bioinformatics were used to predict target gene of miR-126-3p, and luciferase activity assay to test it, it showed that the progesterone receptor (PGR) 3'UTR is directly targeted by miR-126-3p. In this study, mouse mammary epithelial cells as cell model in vitro were used to study the function of miR-126-3p. Using gene silencing and over-expression for miR-126-3p, the expression of PGR protein and the secretion of casein were detected by western blotting and HPLC, respectively. To determine whether miR-126-3p can affect mouse mammary epithelial cells viability, cells were analyzed by CASY-YY. In conclusion, PGR gene confirmed miR-126-3p target genes through luciferase activity and western blotting. And miR-126-3p could also inhibit proliferation of mouse mammary epithelial cells (P < 0.01) and expression of β-casein (P < 0.01), and down-regulate PGR protein (P < 0.05). Our results suggested that miR-126-3p inhibited expression of PGR protein level as well as the proliferation of mammary epithelial cells, therefore miR-126-3p could play an important role in the process of mammary gland development.

Singleminded-2s (Sim2s) promotes delayed involution of the mouse mammary gland through suppression of Stat3 and NFκB

Postlactational involution of the mammary gland provides a unique model to study breast cancer susceptibility and metastasis. We have shown that the short isoform of Singleminded-2s (Sim2s), a basic helix loop helix/PAS transcription factor, plays a role in promoting lactogenic differentiation, as well as maintaining mammary epithelial differentiation and malignancy. Sim2s is dynamically expressed during mammary gland development, with expression peaking during lactation, and decreasing in early involution. To determine the role of SIM2S in involution, we used transgenic mice expressing SIM2S under the mouse mammary tumor virus-Sim2s promoter. Overexpression of Sim2s in the mouse mammary gland resulted in delayed involution, indicated by a lower proportion of cleaved caspase-3-positive cells and slower reestablishment of the mammary fat pad. Immunohistochemical and quantitative RNA analysis showed a decrease in apoptotic markers and inflammatory response genes, and an increase in antiapoptotic genes, which were accompanied by inhibition of signal transducer and activator of transcription 3 activity. Microarray analysis confirmed that genes in the signal transducer and activator of transcription 3 signaling pathway were repressed by SIM2S expression, along with nuclear factor-κB and other key pathways involved in mammary gland development. Multiparous mouse mammary tumor virus-Sim2s females displayed a more differentiated phenotype compared with wild-type controls, characterized by enhanced β-casein expression and alveolar structures. Together, these results suggest a role for SIM2S in the normal involuting gland and identify potential downstream pathways regulated by SIM2S.

Role of JNK in a Trp53-dependent mouse model of breast cancer

The cJun NH2-terminal kinase (JNK) signal transduction pathway has been implicated in mammary carcinogenesis. To test the role of JNK, we examined the effect of ablation of the Jnk1 and Jnk2 genes in a Trp53-dependent model of breast cancer using BALB/c mice. We detected no defects in mammary gland development in virgin mice or during lactation and involution in control studies of Jnk1(-/-) and Jnk2(-/-) mice. In a Trp53(-/+) genetic background, mammary carcinomas were detected in 43% of control mice, 70% of Jnk1(-/-) mice, and 53% of Jnk2(-/-) mice. These data indicate that JNK1 and JNK2 are not essential for mammary carcinoma development in the Trp53(-/+) BALB/c model of breast cancer. In contrast, this analysis suggests that JNK may partially contribute to tumor suppression. This conclusion is consistent with the finding that tumor-free survival of JNK-deficient Trp53(-/+) mice was significantly reduced compared with control Trp53(-/+) mice. We conclude that JNK1 and JNK2 can act as suppressors of mammary tumor development.

The contribution of dynamic stromal remodeling during mammary development to breast carcinogenesis

Breast cancer is a heterogeneous disease whose prognosis varies depending upon the developmental stage of the breast tissue at diagnosis. Notably, breast cancers associated with pregnancy exhibit increased rates of metastasis and poorer long-term survival compared to those diagnosed after menopause. However, postmenopausal breast cancers associated with obesity exhibit a more aggressive behavior and confer decreased overall patient survival compared to those diagnosed in non-obese individuals. Since the mammary gland is a dynamic tissue that undergoes significant changes throughout a woman's lifetime, especially during pregnancy and following menopause, we present evidence to support the notion that changes occurring throughout development within the mammary stromal compartment may account for some of the biological differences in breast cancer subtypes and behaviors.

Skp2B attenuates p53 function by inhibiting prohibitin

The F-box protein Skp2 and its isoform Skp2B are both overexpressed in breast cancers. Skp2 alters the activity of p53 by inhibiting its interaction with p300 and by promoting p300 degradation. Here, we report that Skp2B also attenuates the activity of p53; however, this effect is independent of p300, suggesting that another mechanism might be involved. Prohibitin, a protein reported to activate p53, was isolated in a two-hybrid screen with the carboxy-terminal domain unique to Skp2B. We observed that prohibitin is a new substrate of Skp2B and that the degradation of prohibitin is responsible for the attenuated activity of p53 in cells overexpressing Skp2B. Furthermore, we show that the activity of p53 is reduced in the mammary glands of Skp2B transgenic mice. This study indicates that both Skp2 and Skp2B attenuate p53 activity through different pathways, suggesting that amplification of the Skp2 locus represents a powerful mechanism to attenuate p53 function in cancer.

Pathways contributing to development of spontaneous mammary tumors in BALB/c-Trp53+/- mice

Mutation and loss of function in p53 are common features among human breast cancers. Here we use BALB/c-Trp53+/- mice as a model to examine the sequence of events leading to mammary tumors. Mammary gland proliferation rates were similar in both BALB/c-Trp53+/- mice and wild-type controls. In addition, sporadic mammary hyperplasias were rare in BALB/c-Trp53+/- mice and not detectably different from those of wild-type controls. Among the 28 mammary tumors collected from BALB/c-Trp53+/- mice, loss of heterozygosity for Trp53 was detected in more than 90% of invasive mammary tumors. Transplantation of Trp53+/- ductal hyperplasias also indicated an association between loss of the wild-type allele of Trp53 and progression to invasive carcinomas. Therefore, loss of p53 function seems to be a rate-limiting step in progression. Moreover, expression of biomarkers such as estrogen receptor alpha, progesterone receptor, Her2/Neu, and activated Notch1 varied among mammary tumors, suggesting that multiple oncogenic lesions collaborate with loss of p53 function. Expression of biomarkers was retained when tumor fragments were transplanted to syngeneic hosts. Tumors expressing solely luminal or basal keratins were also observed (27 and 11%, respectively), but the largest class of tumors expressed both luminal and basal keratins (62%). Overall, this panel of transplantable tumors provides a resource for detailed evaluation of the cell lineages undergoing transformation and preclinical testing of therapeutic agents targeting a variety of oncogenic pathways including cancer stem cells.

Effects of reduced frequency of milk removal on gene expression in the bovine mammary gland

Regulation of milk synthesis and secretion is controlled mostly through local (intramammary) mechanisms. To gain insight into the molecular pathways comprising this response, an analysis of mammary gene expression was conducted in 12 lactating cows shifted from twice daily to once daily milking. Tissues were sampled by biopsy from adjacent mammary quarters of these animals during the two milking frequencies, allowing changes in gene expression to be assessed within each animal. Using bovine-specific, oligonucleotide arrays representing 21,495 unique transcripts, a range of differentially expressed genes were found as a result of less frequent milk removal, constituting transcripts and pathways related to apoptotic signaling (NF-kappaB, JUN, ATF3, IGFBP5, TNFSF12A) mechanical stress and epithelial tight junction synthesis (CYR61, CTGF, THBS1, CLDN4, CLDN8), and downregulated milk synthesis (LALBA, B4GALT1, UGP2, CSN2, GPAM, LPL). Quantitative real-time PCR was used to assess the expression of 13 genes in the study, and all 13 of these were correlated (P < 0.05) with values derived from array analysis. It can be concluded that the physiological changes that occur in the bovine mammary gland as a result of reduced milk removal frequency likely comprise the earliest stages of the involution response and that mechano-signal transduction cascades associated with udder distension may play a role in triggering these events.

Dual inactivation of Hus1 and p53 in the mouse mammary gland results in accumulation of damaged cells and impaired tissue regeneration

In response to DNA damage, checkpoint proteins halt cell cycle progression and promote repair or apoptosis, thereby preventing mutation accumulation and suppressing tumor development. The DNA damage checkpoint protein Hus1 associates with Rad9 and Rad1 to form the 9-1-1 complex, which localizes to DNA lesions and promotes DNA damage signaling and repair. Because complete inactivation of mouse Hus1 results in embryonic lethality, we developed a system for regulated Hus1 inactivation in the mammary gland to examine roles for Hus1 in tissue homeostasis and tumor suppression. Hus1 inactivation in the mammary epithelium resulted in genome damage that induced apoptosis and led to depletion of Hus1-null cells from the mammary gland. Conditional Hus1 knockout females retained grossly normal mammary gland morphology, suggesting compensation by cells that failed to undergo Cre-mediated Hus1 deletion. p53-deficiency delayed the clearance of Hus1-null cells from conditional Hus1 knockout mice and caused the accumulation of damaged, dying cells in the mammary gland. Notably, compensatory responses were impaired following combined Hus1 and p53 loss, resulting in aberrant mammary gland morphology and lactation defects. Overall, these results establish a requirement for Hus1 in the survival and proliferation of mammary epithelium and identify a role for p53 in mammary gland tissue regeneration and homeostasis.

The tumor suppressor p53 regulates polarity of self-renewing divisions in mammary stem cells

Stem-like cells may be integral to the development and maintenance of human cancers. Direct proof is still lacking, mainly because of our poor understanding of the biological differences between normal and cancer stem cells (SCs). Using the ErbB2 transgenic model of breast cancer, we found that self-renewing divisions of cancer SCs are more frequent than their normal counterparts, unlimited and symmetric, thus contributing to increasing numbers of SCs in tumoral tissues. SCs with targeted mutation of the tumor suppressor p53 possess the same self-renewal properties as cancer SCs, and their number increases progressively in the p53 null premalignant mammary gland. Pharmacological reactivation of p53 correlates with restoration of asymmetric divisions in cancer SCs and tumor growth reduction, without significant effects on additional cancer cells. These data demonstrate that p53 regulates polarity of cell division in mammary SCs and suggest that loss of p53 favors symmetric divisions of cancer SCs, contributing to tumor growth.

Mammary involution and breast cancer risk: transgenic models and clinical studies

Postlactational involution is the process following weaning during which the mammary gland undergoes massive cell death and tissue remodeling as it returns to the pre-pregnant state. Lobular involution is the process by which the breast epithelial tissue is gradually lost with aging of the mammary gland. While postlactational involution and lobular involution are distinct processes, recent studies have indicated that both are related to breast cancer development. Experiments using a variety of rodent models, as well as observations in human populations, suggest that deregulation of postlactational involution may act to facilitate tumor formation. By contrast, new human studies show that completion of lobular involution protects against subsequent breast cancer incidence.

The mammary progenitor marker CD61/beta3 integrin identifies cancer stem cells in mouse models of mammary tumorigenesis

The cells of origin and mechanisms that underpin tumor heterogeneity in breast cancer are poorly understood. Here, we have examined three mouse models of mammary tumorigenesis (MMTV-wnt-1, MMTV-neu, and p53(+/-)) for changes in their epithelial cell hierarchy during the preneoplastic and neoplastic stages of tumor progression. In preneoplastic tissue, only MMTV-wnt-1 mice showed a perturbation in their epithelial subpopulations. In addition to an expanded mammary stem cell pool, repopulating cells capable of yielding extensive mammary outgrowths in vivo were revealed in the committed luminal progenitor population. These findings indicate that wnt-1 activation induces the appearance of aberrant progenitor cells, and suggest that both mammary stem and progenitor cells can serve as the cellular targets of wnt-1-induced tumorigenesis. In tumors arising in MMTV-wnt-1 tumors, the luminal epithelial progenitor marker CD61/beta3 integrin identified a cancer stem cell (CSC) population that was highly enriched for tumorigenic capability relative to the CD61(-) subset. CD61 expression also defined a CSC subset in 50% of p53(+/-)-derived tumors. No CSCs, however, could be identified in the more homogeneous MMTV-neu/erbB2 model, suggesting an alternative model of tumorigenesis. Overall, our findings show the utility of the progenitor marker CD61 in the identification of CSCs that sustain specific mammary tumors.

Patterns of cell signaling pathway activation that characterize mammary development

Previous work has detailed the histological and biochemical changes associated with mammary development and remodeling. We have now made use of gene expression profiling, and in particular of the previously described signatures of cell signaling pathway activation, to explore the events associated with mammary gland development. We find that there is elevated E2F-specific pathway activity prior to lactation and relatively low levels of other important signaling pathways, such as RAS, MYC and SRC. Upon lactation and continuing into the involution phase, these patterns reverse with a dramatic increase in RAS, SRC and MYC pathway activity and a decline in E2F activity. At the end of involution, these patterns return to that of the adult non-lactating mammary gland. The importance of the changes in E2F pathway activity, particularly during the proliferative phase of mammary development, was confirmed through the analysis of mice deficient for various E2F proteins. Taken together, these results reveal a complex pattern of pathway activity in relation to the various phases of mammary gland development.

Prepubertal physical activity up-regulates estrogen receptor beta, BRCA1 and p53 mRNA expression in the rat mammary gland

Findings in BRCA1 mutation carriers suggest that physical activity, particularly during childhood, may be linked to a reduced risk of developing breast cancer. We investigated whether physical activity at puberty alters the expression of BRCA1 and two other tumor suppressor genes--p53 and estrogen receptor (ER)-beta--in rats. In addition, the effects on ER-alpha expression, mammary proliferation and functional epithelial differentiation were investigated as markers of altered mammary cancer risk in rats exposed to regular physical activity at puberty. Female Sprague Dawley rat pups were randomized to voluntary exercise, sham-exercise control and non-manipulated control groups. Treadmill training (20-25 m/min, 15% grade, 30 min/day, 5 days/week) started on postnatal day 14 and continued through day 32. Third thoracic mammary glands (n = 5 per group and age) were obtained at days 32, 48 and 100 and assessed for changes in morphology through wholemounts, and at 100 days cell proliferation by using Ki67 staining, protein levels of ER-alpha and ER-beta by immunohistochemistry, and mRNA expression levels of BRCA1, p53, ER-alpha and ER-beta by real-time PCR. Mammary glands of rats exposed to exercise during puberty contained fewer terminal end buds (TEBs) and a higher number of differentiated alveolar buds and lobules than the sham controls. However, cell proliferation was not significantly altered among the groups. ER-alpha protein levels were significantly reduced, while ER-beta levels were increased in the mammary ducts and lobular epithelial structures of 100-day old rays which were voluntarily exercised at puberty, compared to sham controls. ER-beta, BRCA1 and p53 mRNA levels were significantly higher in the mammary glands of 100-day-old exercised versus sham control rats. Pubertal physical activity reduced mammary epithelial targets for neoplastic transformation through epithelial differentiation and it also up-regulated tumor suppressor genes BRCA1, p53 and ER-beta, and reduced ER-alpha/ER-beta ratio in the mammary gland. It remains to be determined whether the up-regulation of BRCA1, and perhaps p53, explains the protective effect of childhood physical activity against breast cancer in women who carry a germline mutation in one of the BRCA1 alleles.

Altered mammary gland development in the p53+/m mouse, a model of accelerated aging

The tumor suppressor p53 is important for inhibiting the development of breast carcinomas. However, little is known about the effects of increased p53 activity on mammary gland development. Therefore, the effect of p53 dosage on mammary gland development was examined by utilizing the p53+/m mouse, a p53 mutant which exhibits increased wild-type p53 activity, increased tumor resistance, a shortened longevity, and a variety of accelerated aging phenotypes. Here we report that p53+/m virgin mice exhibit a defect in mammary gland ductal morphogenesis. Transplants of mammary epithelium into p53+/m recipient mice demonstrate decreased outgrowth of wild-type and p53+/m donor epithelium, suggesting systemic or stromal alterations in the p53+/m mouse. Supporting these data, p53+/m mice display decreased levels of serum IGF-1 and reduced IGF-1 signaling in virgin glands. The induction of pregnancy or treatment of p53+/m mice with estrogen, progesterone, estrogen and progesterone in combination, or IGF-1 stimulates ductal outgrowth, rescuing the p53+/m mammary phenotype. Serial mammary epithelium transplants demonstrate that p53+/m epithelium exhibits decreased transplant capabilities, suggesting early stem cell exhaustion. These data indicate that appropriate levels of p53 activity are important in regulating mammary gland ductal morphogenesis, in part through regulation of the IGF-1 pathway.

Genetic mapping in mice identifies DMBT1 as a candidate modifier of mammary tumors and breast cancer risk

Low-penetrance breast cancer susceptibility alleles seem to play a significant role in breast cancer risk but are difficult to identify in human cohorts. A genetic screen of 176 N2 backcross progeny of two Trp53(+/-) strains, BALB/c and C57BL/6, which differ in their susceptibility to mammary tumors, identified a modifier of mammary tumor susceptibility in an approximately 25-Mb interval on mouse chromosome 7 (designated SuprMam1). Relative to heterozygotes, homozygosity for BALB/c alleles of SuprMam1 significantly decreased mammary tumor latency from 70.7 to 61.1 weeks and increased risk twofold (P = 0.002). Dmbt1 (deleted in malignant brain tumors 1) was identified as a candidate modifier gene within the SuprMam1 interval because it was differentially expressed in mammary tissues from BALB/c-Trp53(+/-) and C57BL/6-Trp53(+/-) mice. Dmbt1 mRNA and protein was reduced in mammary glands of the susceptible BALB/c mice. Immunohistochemical staining demonstrated that DMBT1 protein expression was also significantly reduced in normal breast tissue from women with breast cancer (staining score, 1.8; n = 46) compared with cancer-free controls (staining score, 3.9; n = 53; P < 0.0001). These experiments demonstrate the use of Trp53(+/-) mice as a sensitized background to screen for low-penetrance modifiers of cancer. The results identify a novel mammary tumor susceptibility locus in mice and support a role for DMBT1 in suppression of mammary tumors in both mice and women.

The fur seal-a model lactation phenotype to explore molecular factors involved in the initiation of apoptosis at involution

Mammary gland involution requires co-ordination of milk production, immune responses, apoptosis and remodeling. Initiation and progression of each of these components involves integral control by the mammary gland. Although cell-based culture models and genetically manipulated animals have shed light on these processes, the factors controlling each step in the involution cascade are still poorly understood. The fur seal displays a unique lactation phenotype. During the lactation cycle the mammary gland downregulates milk production and initiates an immune response but fails to initiate the apoptotic phase of involution, allowing the female fur seal to undertake long foraging trips of up to 28 days between suckling bouts. Upon return to shore the female continues feeding her pup following resumption of lactation and milk production. Expression profiling of genes involved in this lactation cycle provides valuable tools for investigation of the factors responsible for the initiation of apoptosis at involution.

Mammary gland involution as a multi-step process

Mammary gland involution is a highly complex multi-step process in which the lactating gland returns to a morphologically near pre-pregnant state. This developmental stage is characterized by a high degree of epithelial cell death, redevelopment of the mammary adipose tissue and tissue remodelling. Many factors involved have been described and these have been reviewed intensively in this journal (Furth, P. A., J. Mammary Gland Biol. Neoplasia, 4:123-127, 1999) and elsewhere. Microarray analysis technology has now not only allowed us to identify genes not previously associated with involution (Stein, T., Morris, J.S., Davis, C.R.,Weber-Hall, S.J., Duffy, M.A., Heath, V.J., et al., Breast Cancer Res., 6: R75-R91, 2004; Clarkson, R.W., Wayland, M.T., Lee, J., Freeman, T., Watson, C.J., Breast Cancer Res., 6: R92-R109, 2004; Clarkson, R.W., Watson, C.J., J. Mammary Gland Biol. Neoplasia, 8: 309-319, 2003), it has also enabled us to define multiple phases of the controlled regulatory response to forced weaning on the basis of their transcriptional profiles. This review provides a synthesis of published data, integrating the time course of transcriptional activity in the mouse mammary gland with a gene ontology approach to identify the pathways involved.

TBX3, the gene mutated in ulnar-mammary syndrome, promotes growth of mammary epithelial cells via repression of p19ARF, independently of p53

TBX3, the gene mutated in ulnar-mammary syndrome (UMS), is involved in the production of a transcription factor of the T-box family, known to inhibit transcription from the p14ARF (p19ARF in mouse) promoter in fibroblasts and to contribute to cell immortalization. One of the main features of the UMS phenotype is the severe hypoplasia of the breast, associated with haploinsufficiency of the TBX3 gene product. In mice homozygous for the targeted disruption of Tbx3, the mammary glands (MGs) are nearly absent from early stages of embryogenesis, whereas in heterozygous adults, the MGs show reduced ductal branching. All these data strongly suggest a specific role of TBX3 in promoting the growth of mammary epithelial cells (MECs), although direct evidence of this is lacking. Here, we provide data showing the growth-promoting function of Tbx3 in several models of MECs, in association with its ability to repress the ARF promoter. However, no effect of Tbx3 on cell differentiation or apoptosis has been observed. The growth promoting function also entails the down-regulation of p21 ( CIP1/WAF ) and an increase in cyclin D1 but is independent of p53 and Mdm2 cell-cycle regulatory proteins, as p53-null MECs show similar growth responses associated with the up- or down-regulation of Tbx3. This is the first direct evidence that the level of Tbx3 expression positively controls the proliferation of MECs via pathways alternative to Mdm2-p53.

Illuminating the center: mechanisms regulating lumen formation and maintenance in mammary morphogenesis

The lumens present in ductal structures are required for transport of fluids and air. Studies in model organisms and cells in culture suggest that lumens can be generated by multiple mechanisms including apoptosis of centrally located cells, and re-modeling of epithelia. Several studies point to a role for apoptosis during lumen formation in the mammary ducts. However, a role for other mechanisms during lumen formation in the mammary ducts is largely unexplored. Understanding how lumens are formed and maintained free of cells is of clinical importance because filling of the luminal space is associated with cancer and inflammation. Thus, further investigation can lead to new diagnostic and therapeutic opportunities.

Estrogen and progesterone regulate radiation-induced p53 activity in mammary epithelium through TGF-beta-dependent pathways

DNA damage normally induces p53 activity, but responses to ionizing radiation in the mammary epithelium vary among developmental stages. The following studies examined the hormones and growth factors that regulate radiation-responsiveness of p53 in mouse mammary epithelium. Immunoreactive p21/WAF1 and TUNEL staining were used as indicators of p53 activity following exposure to ionizing radiation. In ovariectomized mice, radiation-induced accumulation of p21/WAF1 was minimal in the mammary epithelial cells (<1%). Systemic injections of estrogen and progesterone (E+P) for 72 h were necessary to recover maximal expression of p21/WAF1 following ionizing radiation (55%). The effects of E+P on radiation-induced p21/WAF1 were p53-dependent as responses were absent in Trp53-/- mice. Though hormonal treatments stimulated increases in the proportion of cycling cells (PCNA-positive), this was not directly correlated with p53 activity. Whole organ cultures were used to determine whether E+P act directly upon the mammary gland. Treatment with E+P was sufficient to render p53 responsive to radiation, but TGF-beta-neutralizing antibodies blocked responsiveness. In the absence of E+P, TGF-beta1 alone did not alter p53 activity. These results demonstrate that estrogen and progesterone together with TGF-beta signaling are necessary for maintenance of p53 activity in the mammary epithelium.

Identification and analysis of Hic-5/ARA55 isoforms: Implications for integrin signaling and steroid hormone action

Hic-5/ARA55 is a LIM-only member of the paxillin superfamily. Conflicting reports have suggested that Hic-5/ARA55 can both repress and enhance a number of biological processes, including myogenesis and tumorigenesis. With two Hic-5 isoforms documented, we hypothesized that multiple Hic-5 isoforms may exist that have both overlapping and isoform-specific functions. To test this hypothesis, we performed an extensive analysis of Hic-5 transcripts in both cell lines and mouse tissues and found 12 distinct isoforms that fall into two sub-families. These isoforms are derived from both alternative splicing and alternative transcriptional start sites (TSS). Hic-5 expression is regulated in a temporally and spatially controlled manner in vivo. The identification of numerous Hic-5 isoforms suggests that Hic-5 subsumes a number of distinct roles in cells and may explain the range of biological responses attributed to Hic-5.

Tamoxifen inhibition of estrogen receptor-alpha-negative mouse mammary tumorigenesis

Tamoxifen reduces the relative risk of breast cancer developing from specific premalignant lesions. Many breast cancers that arise after tamoxifen treatment are estrogen receptor-alpha (ER-alpha)-negative, although premalignant lesions such as atypical ductal hyperplasia are highly ER-alpha-positive. The p53 null mouse mammary epithelial transplant model is characterized by ER-alpha-positive premalignant lesions that give rise to both ER-alpha-positive and ER-alpha-negative tumors. Given this progression from ER-alpha-positive to ER-alpha-negative lesions, we tested the ability of tamoxifen to block or delay mammary tumorigenesis in several versions of this model. In groups 1 and 2, p53 null normal mammary epithelial transplants were maintained in virgin mice. In groups 3 to 5, the p53 null and mammary transplants were maintained in mice continuously exposed to high levels of progesterone. In groups 6 and 7, transplants of the premalignant outgrowth line PN8a were maintained in virgin mice. Tamoxifen blocked estrogen signaling in these mice as evidenced by decreases in progesterone-induced lateral branching and epithelial proliferation in the mammary epithelium. Tamoxifen did not alter the elevated levels of progesterone in the blood while significantly reducing the circulating level of prolactin. Tamoxifen reduced tumor incidence in p53 null normal mammary epithelial transplants maintained in virgin mice from 55% to 5% and in progesterone-stimulated mice from 81% to 21%. The majority of the resultant tumors were ER-alpha-negative. Tamoxifen also significantly delayed tumorigenesis in the ER-alpha-positive high premalignant line PN8a from 100% to 75%. These results show that tamoxifen delays the emergence of ER-alpha-negative tumors if given early in premalignant progression.

p53 mediates a default programme of mammary gland involution in the absence of STAT3

Previous studies have demonstrated a proapoptotic role for the transcription factor STAT3 in involuting murine mammary epithelium, resulting in delayed involution and lower levels of apoptosis in the STAT3 null gland relative to wild-type controls. As p53 was implicated in the eventual involution of the STAT3 null gland, we examined the effect of STAT3 loss in the mammary gland in a p53 null background. Combined loss of STAT3 and p53 severely perturbed involution, with hyperdelayed loss of epithelium and reappearance of adipocytes. The early apoptotic response was almost completely abrogated, although elevated levels of delayed apoptosis persisted at days 6, 17 and 4 weeks of involution in STAT3-p53 doubly null mammary glands. A 5.7-fold upregulation of the cyclin-dependent kinase inhibitor p21Waf1 at 3 days of involution in STAT3 null glands was abolished in STAT3-p53 doubly null glands -- suggesting that the critical factor triggering delayed involution in the STAT3 null gland is a p53-dependent rise in p21Waf1 levels around day 3 of involution. Further, STAT3-p53 doubly null glands showed significantly higher levels of proliferation compared to STAT3 or p53 singly null (or wild-type) glands at days 6, 17 and 4 weeks of involution. Combined loss of STAT3 and p53 therefore results in hyperdelayed involution, demonstrating their synergistic physiological roles in normal involution. This inappropriate retention of p53-deficient cells may represent a novel mechanism of tumour predisposition.

p19ARF determines the balance between normal cell proliferation rate and apoptosis during mammary gland development

This study demonstrated, for the first time, the following events related to p19(ARF) involvement in mammary gland development: 1) Progesterone appears to regulate p19(ARF) in normal mammary gland during pregnancy. 2) p19(ARF) expression levels increased sixfold during pregnancy, and the protein level plateaus during lactation. 3) During involution, p19(ARF) protein level remained at high levels at 2 and 8 days of involution and then, declined sharply at day 15. Absence of p19(ARF) in mammary epithelial cells leads to two major changes, 1) a delay in the early phase of involution concomitant with downregulation of p21(Cip1) and decrease in apoptosis, and 2) p19(ARF) null cells are immortal in vivo measured by serial transplantion, which is partly attributed to complete absence of p21(Cip1) compared with WT cells. Although, p19(ARF) is dispensable in mammary alveologenesis, as evidenced by normal differentiation in the mammary gland of pregnant p19(ARF) null mice, the upregulation of p19(ARF) by progesterone in the WT cells and the weakness of p21(Cip1) in mammary epithelial cells lacking p19(ARF) strongly suggest that the functional role(s) of p19(ARF) in mammary gland development is critical to sustain normal cell proliferation rate during pregnancy and normal apoptosis in involution possibly through the p53-dependent pathway.

Hormone-induced chromosomal instability in p53-null mammary epithelium

The absence of p53 function increases risk for spontaneous tumorigenesis in the mammary gland. Hormonal stimulation enhances tumor risk in p53-null mammary epithelial cells as well as the incidence of aneuploidy. Aneuploidy appears in normal p53-null mammary epithelial cells within 5 weeks of hormone stimulation. Experiments reported herein assessed a possible mechanism of hormone-induced aneuploidy. Hormones increased DNA synthesis equally between wild-type (WT) and p53-null mammary epithelial cells. There were two distinct responses in p53-null cells to hormone exposure. First, Western blot analysis demonstrated that the levels of two proteins involved in regulating sister chromatid separation and the spindle checkpoint, Mad2 and separase (ESPL1) were increased in null compared with WT cells. In contrast, the levels of securin and Rad21 proteins were not increased in hormone-stimulated p53-null compared with WT cells. ESPL1 RNA was also increased in p53-null mouse mammary cells in vivo by 18 h of hormone stimulation and in human breast MCF7 cells in monolayer culture by 8 h of hormone stimulation. Furthermore, both promoters contained p53 and steroid hormone response elements. Mad2 protein was increased as a consequence of the absence of p53 function. The increase in Mad2 protein was observed also at the cellular level by immunohistochemistry. Second, hormones increased gene amplication in the distal arm of chromosome 2, as shown by comparative genomic hybridization. These results support the hypothesis that hormone stimulation acts to increase aneuploidy by several mechanisms. First, by increasing mitogenesis in the absence of the p53 checkpoint in G2, hormones allow the accumulation of cells that have experienced chromosome missegregation. Second, the absolute rate of chromosome missegregation may be increased by alterations in the levels of two proteins, separase and Mad2, which are important for maintaining chromosomal segregation and the normal spindle checkpoint during mitosis.

Loss of Heterozygosity Occurs via Mitotic Recombination in Trp53+/− Mice and Associates with Mammary Tumor Susceptibility of the BALB/c Strain

Loss of heterozygosity (LOH) occurs commonly in cancers causing disruption of tumor suppressor genes and promoting tumor progression. BALB/c-Trp53(+/-) mice are a model of Li-Fraumeni syndrome, exhibiting a high frequency of mammary tumors and other tumor types seen in patients. However, the frequency of mammary tumors and LOH differs among strains of Trp53(+/-) mice, with mammary tumors occurring only on a BALB/c genetic background and showing a high frequency of LOH, whereas Trp53(+/-) mice on a 129/Sv or (C57BL/6 x 129/Sv) mixed background have a very low frequency of mammary tumors and show LOH for Trp53 in only approximately 50% of tumors. We have performed studies on tumors from Trp53(+/-) mice of several genetic backgrounds to examine the mechanism of LOH in BALB/c-Trp53(+/-) mammary tumors. By Southern blotting, 96% (24 of 25) of BALB/c-Trp53(+/-) mammary tumors displayed LOH for Trp53. Karyotype analysis indicated that cells lacking one copy of chromosome 11 were present in all five mammary tumors analyzed but were not always the dominant population. Comparative genomic hybridization analysis of these five tumors indicated either loss or retention of the entire chromosome 11. Thus chromosome loss or deletions within chromosome 11 do not account for the LOH observed by Southern blotting. Simple sequence length polymorphism analysis of (C57BL/6 x BALB/c) F1-Trp53(+/-) mammary tumors showed that LOH occurred over multiple loci and that a combination of maternal and paternal alleles were retained, indicating that mitotic recombination is the most likely mechanism of LOH. Nonmammary tumors of BALB/c mice also showed a high frequency of LOH (22 of 26, 85%) indicating it was not a mammary tumor specific phenomenon but rather a feature of the BALB/c strain. In (C57BL/6 x BALB/c) F1-Trp53(+/-) mice LOH was observed in 93% (13 of 14) of tumors, indicating that the high frequency of LOH was a dominant genetic trait. Thus the high frequency of LOH for Trp53 in BALB/c-Trp53(+/-) mammary tumors occurs via mitotic recombination and is a dominant genetic trait that associates with the occurrence of mammary tumors in (C57BL/6 x BALB/c) F1-Trp53(+/-) mice. These results further implicate double-strand DNA break repair machinery as important contributors to mammary tumorigenesis.

Apoptosis regulation in the mammary gland

Epithelial apoptosis has a key role in the development and function of the mammary gland. It is involved with the formation of ducts during puberty and is required to remove excess epithelial cells after lactation so that the gland can be prepared for future pregnancies. Deregulated apoptosis contributes to malignant progression in the genesis of breast cancer. Since epithelial cell apoptosis in the lactating mammary gland can be synchronised by forced weaning, it has been possible to undertake biochemical analysis of the pathways involved. Together with the targeted overexpression or deletion of candidate genes, these approaches have provided a unique insight into the complex mechanisms of apoptosis regulation in vivo. This review explores what is currently known about the triggers for apoptosis in the normal mammary gland, and how they link with the intrinsic apoptotic machinery.

Comparison of mammary gland involution between 129S1 and C57BL/6 inbred mouse strains: differential regulation of Bcl2a1, Trp53, Cebpb, and Cebpd expression

Genetic engineering has made the mouse an invaluable tool to address the function of individual genes in a targeted manner. Over the last decade it has become apparent that the genetic mouse strain background can significantly influence the phenotype of an engineered mouse. Therefore, it is essential to characterize the biology of the different wild-type background strains. In this study, we have compared mouse mammary gland involution in the 129S1 and C57BL/6 inbred strains and report significant differences at the molecular level with differential expression of Bcl2a1 (Bfl1), Trp53 (p53), Cebpb (C/EBP beta), and Cebpd (C/EBP delta). The C57BL/6 strain exhibits dynamic responses with induction of Trp53 and Cebpd and concomitant downregulation of Bcl2a1 during the first phase of involution. In contrast, expression of these genes does not change significantly in 129S1 mice. During the second phase, C57BL/6 glands contain more Cebpb than 129S1 glands. Nevertheless, involution proceeds morphologically with similar kinetics in both strains. The data demonstrate that the genetic response of mammary tissue varies significantly between 129S1 and C57BL/6. These results may provide a basis for the interpretation of strain-specific phenotypes in engineered mice and underline the importance of pure strains for large-scale expression studies with mutant mice.

Cell cycle genes in a mouse mammary hyperplasia model

Human mammary epithelial cells emerge spontaneously from senescence, exhibiting eroding telomeric sequences, and ultimately enter crisis to generate the type of chromosomal abnormalities seen in early stages of breast cancer. In a mouse mammary tumor model, the spontaneous escape of senescence can be observed as an increase in DNA synthesis that is reflected by alterations in the cell cycle profile and increases in the expression levels and activities of cell cycle molecular components. This review provides an overview of gene alterations in the cell cycle components in mouse mammary hyperplasia.

Epithelial cell cycling predicts p53 responsiveness to γ-irradiation during post-natal mammary gland development

The tumor suppressor gene, TP53, plays a major role in surveillance and repair of radiation-induced DNA damage. In multiple cell types, including mammary epithelial cells, abrogation of p53 (encoded by Trp53) function is associated with increased tumorigenesis. We examined γ-irradiated BALB/c-Trp53+/+ and -Trp53–/– female mice at five stages of post-natal mammary gland development to determine whether radiation-induced p53 activity is developmentally regulated. Our results show that p53-mediated responses are attenuated in glands from irradiated virgin and lactating mice, as measured by induction of p21/WAF1 (encoded by Cdkn1a) and apoptosis, while irradiated early- and mid-pregnancy glands exhibit robust p53 activity. There is a strong correlation between p53-mediated apoptosis and the degree of cellular proliferation, independent of the level of differentiation. In vivo, proliferation is intimately influenced by steroid hormones. To determine whether steroid hormones directly modulate p53 activity, whole organ cultures of mammary glands were induced to proliferate using estrogen plus progesterone or epidermal growth factor plus transforming growth factor-α and p53 responses to γ-irradiation were measured. Regardless of mitogens used, proliferating mammary epithelial cells show comparable p53 responses to γ-irradiation, including expression of nuclear p53 and p21/WAF1 and increased levels of apoptosis, compared to non-proliferating irradiated control cultures. Our study suggests that differences in radiation-induced p53 activity during post-natal mammary gland development are influenced by the proliferative state of the gland, and may be mediated indirectly by the mitogenic actions of steroid hormones in vivo.

Regulation of apoptosis during mammary involution by the p53 tumor suppressor gene

Regulation and functions of the p53 tumor suppressor gene have been studied extensively with respect to its critical role in maintaining the stability of genomic DNA following genotoxic insults. However, p53 is also induced by physiologic stimuli resulting in cell cycle arrest and apoptosis. In other situations, the activity of p53 must be repressed to prevent inappropriate removal of cells. The mammary gland provides a valuable system in which to study the mechanisms by which the expression and biological responses to p53 can be regulated under a variety of physiological circumstances. The pro-apoptotic role of p53 in the secretory mammary epithelium may be especially relevant to lactation in livestock. We have utilized p53-deficient mice to establish the molecular targets of p53 in the mammary gland and biological consequences when it is absent. The p21/WAF1 gene (Cdkn1a) is a transcriptional target gene of the p53 protein that responds to elevated levels of p53 during milk stasis providing an endogenous reporter of p53 activity. Abrogation of p53 resulted in delayed involution of the mammary epithelium, demonstrating the physiological role of p53 in regulating involution. Though delayed, stromal proteases were induced in the mammary gland by 5 d postweaning, providing a p53-independent mechanism that resulted in removal of the residual secretory epithelium. These processes can be interrupted by treatment with hydrocortisone. These data establish p53 as a physiological regulator of involution that acts to rapidly initiate apoptosis in the secretory epithelium in response to stress signals, but also indicate the presence of compensatory pathways to effect involution. Additional mechanisms involving intracellular stress signaling pathways (e.g., Stat3) and stromal-mediated pathways have been identified and, together with p53 pathways, may be used to identify animals with greater persistency of lactation.

Knockout and transgenic mice of Trp53: what have we learned about p53 in breast cancer?

The human p53 tumor suppressor gene TP53 is mutated at a high frequency in sporadic breast cancer, and Li-Fraumeni syndrome patients who carry germline mutations in one TP53 allele have a high incidence of breast cancer. In the 10 years since the first knockout of the mouse p53 tumor suppressor gene (designated Trp53) was published, much has been learned about the contribution of p53 to biology and tumor suppression in the breast through the use of p53 transgenic and knockout mice. The original mice deficient in p53 showed no mammary gland phenotype. However, studies using BALB/c-Trp53-deficient mice have demonstrated a delayed involution phenotype and a mammary tumor phenotype. Together with other studies of mutant p53 transgenes and p53 bitransgenics, a greater understanding has been gained of the role of p53 in involution, of the regulation of p53 activity by hormones, of the effect of mouse strain and modifier genes on tumor phenotype, and of the cooperation between p53 and other oncogenic pathways, chemical carcinogens and hormonal stimulation in mammary tumorigenesis. Both p53 transgenic and knockout mice are important in vivo tools for understanding breast cancer, and are yet to be exploited for developing therapeutic strategies in breast cancer.