Impaired mammary gland development in Cyl-1(-/-) mice during pregnancy and lactation is epithelial cell autonomous

Progesterone signaling in uterine leiomyoma biology: Implications for potential targeted therapy

Uterine leiomyomas (ULs) are the most common benign smooth muscle cell steroid-dependent tumors that occur in women of reproductive age. Progesterone (P4) is a major hormone that promotes the ULs development and growth. P4 action in ULs is mediated mainly by its nuclear progesterone receptors (PGRs), although rapid non-genomic responses have also been observed. Data on the membrane progesterone receptors (mPRs) regulated signaling pathways in ULs in the available literature is still very limited. One of the essential characteristics of ULs is the excessive production of extracellular matrix (ECM). P4 has been shown to stimulate ECM production and collagen synthesis in ULs. Recent research demonstrated that, despite their benign nature, ULs may present with abnormal vasculature. P4 has been shown to regulate angiogenesis in ULs through the upregulation of vascular endothelial growth factor (VEGF) and by controlling the secretion of permeability factors. This review summarizes the key findings regarding the role of PGRs and mPRs in ULs, especially highlighting the potential ECM and angiogenesis modulation by P4. An increased understanding of this mechanistic role of nuclear and specifically mPRs in the biology of P4-modulated ECM and angiogenesis in the growth of ULs could turn out to be fundamental for developing effective targeted therapies for ULs.

Zfhx3 is essential for progesterone/progesterone receptor signaling to drive ductal side-branching and alveologenesis in mouse mammary glands

Progesterone (Pg)/progesterone receptor (PR) signaling drives mammary gland side-branching and alveologenesis, but the mechanisms through which Pg/PR signaling functions remain to be clarified. Using in vitro and in vivo models and histological and molecular analyses, we determined the role of Zfhx3 transcription factor in mammary gland development driven by Pg/PR signaling. Postnatal deletion of Zfhx3 in mouse mammary epithelial cells attenuated side-branching morphogenesis and alveologenesis. These effects were undetectable in the absence of Pg/PR signaling. During the estrus cycle, Zfhx3 expression corresponded to that of Pg, being at the highest level at the diestrus stage; Zfhx3 deletion inhibited mammary gland branching more potently at diestrus than estrus stage. Loss of Zfhx3 not only attenuated the expansion of stem/progenitor cells driven by Pg/PR signaling, but also impaired the function of Pg/PR signaling in the transcriptional activation of multiple genes. In addition, Pg/PR signaling significantly expanded PR- and Zfhx3-positive epithelial cells, and induced the physical association of ZFHX3 with PR. These findings establish Zfhx3 as an integral transcription factor of Pg/PR signaling in driving side-branching and alveologenesis during mammary gland development.

Human versus non-human sex steroid use in hormone replacement therapies part 1: Preclinical data

Prior to 2002, hormone replacement therapy (HRT) was considered to be an important component of postmenopausal healthcare. This was based on a plethora of basic, epidemiological and clinical studies demonstrating the health benefits of supplementation with human sex steroids. However, adverse findings from the Women's Health Initiative (WHI) studies that examined the 2 major forms of HRT in use in the US at that time - Premarin (conjugated equine estrogens; CEE) and Prempro (CEE + medroxyprogesterone acetate; MPA), cast a shadow over the use of any form of HRT. Here we review the biochemical and physiological differences between the non-human WHI study hormones - CEE and MPA, and their respective human counterparts 17β-estradiol (E₂) and progesterone (P₄). Preclinical data from the last 30 years demonstrate clear differences between human and non-human sex steroids on numerous molecular, physiological and functional parameters in brain, heart and reproductive tissue. In contrast to CEE supplementation, which is not always detrimental although certainly not as optimal as E₂ supplementation, MPA is clearly not equivalent to P₄, having detrimental effects on cognitive, cardiac and reproductive function. Moreover, unlike P₄, MPA is clearly antagonistic of the positive effects of E₂ and CEE on tissue function. These data indicate that minor chemical changes to human sex steroids result in physiologically distinct actions that are not optimal for tissue health and functioning.

Attenuation of Mammary Gland Dysplasia and Feeding Difficulties in Tabby Mice by Fetal Therapy

Hypohidrotic ectodermal dysplasias (HED) are hereditary differentiation disorders of multiple ectodermal structures including the mammary gland. The X-linked form of HED (XLHED) is caused by a lack of the secreted signaling molecule ectodysplasin A1 (EDA1) which is encoded by the gene EDA and belongs to the tumor necrosis factor (TNF) superfamily. Although male patients (hemizygous) are usually more severely affected by XLHED, heterozygous female carriers of an EDA mutation may also suffer from a variety of symptoms, in particular from abnormal development of their breasts. In Tabby mice, a well-studied animal model of XLHED, EDA1 is absent. We investigated the effects of prenatal administration of Fc-EDA, a recombinant EDA1 replacement protein, on mammary gland development in female Tabby mice. Intra-amniotic delivery of Fc-EDA to fetal animals resulted later in improved breastfeeding and thus promoted the growth of their offspring. In detail, such treatment led to a normalization of the nipple shape (protrusion, tapering) that facilitated sucking. Mammary glands of treated female Tabby mice also showed internal changes, including enhanced branching morphogenesis and ductal elongation. Our findings indicate that EDA receptor stimulation during development has a stable impact on later stages of mammary gland differentiation, including lactation, but also show that intra-amniotic administration of an EDA1 replacement protein to fetal Tabby mice partially corrects the mammary gland phenotype in female adult animals.

Binuclear Cells in the Lactating Mammary Gland: New Insights on an Old Concept?

In a recent paper (Rios et al. Nat Commun. 7:11400, 2016), it was reported that polyploid cells are frequent in lactating mammary tissues. This phenomenon was observed in mammary tissue sampled from five separate mammalian species. According to that report, these binucleated cells occur late in pregnancy and early in lactation. Unfortunately, this paper did not mention a number of earlier observations and findings that remain pertinent to this day (Banerjee et al. Life sciences Pt 2: Biochemistry, general and molecular biology. 10(15):867-77, 1971; Banerjee MR, Wagner JE. Biochem. Biophys. Res. Commun. 49(2):480-7, 1972). In these classical experiments, the authors demonstrated in vivo that DNA synthesis continued without commensurate cell division during late pregnancy and lactation, and that this DNA synthesis was imperative for functional differentiation of the mammary epithelium. Later studies showed that DNA synthesis was indispensable to the induction of milk protein production in explant cultures of mammary tissue from unprimed, nulliparous mice. This dependence on DNA synthesis in mammary explant cultures stimulated by lactogenic hormones was found to be dispensable following a single pregnancy. The absolute requirement for DNA synthesis in nulliparous mouse mammary explants stimulated to synthesize milk protein in vitro has remained unexplained, as has the need for DNA synthesis prior to the onset of lactation. From a historical perspective, it is more likely that binuclear secretory cells in the lactating mammary gland are a consequence of the DNA synthesis requirement for lactation, rather than an essential element.

Endocrine hormones and local signals during the development of the mouse mammary gland

Most of mammary gland development occurs postnatally under the control of female reproductive hormones, which in turn interact with other endocrine factors. While hormones impinge on many tissues and trigger very complex biological responses, tissue recombination experiments with hormone receptor-deficient mammary epithelia revealed eminent roles for estrogens, progesterone, and prolactin receptor (PrlR) signaling that are intrinsic to the mammary epithelium. A subset of the luminal mammary epithelial cells expresses the estrogen receptor α (ERα), the progesterone receptor (PR), and the PrlR and act as sensor cells. These cells convert the detected systemic signals into local signals that are developmental stage-dependent and may be direct, juxtacrine, or paracrine. This setup ensures that the original input is amplified and that the biological responses of multiple cell types can be coordinated. Some key mediators of hormone action have been identified such as Wnt, EGFR, IGFR, and RANK signaling. Multiple signaling pathways such as FGF, Hedgehog, and Notch signaling participate in driving different aspects of mammary gland development locally but how they link to the hormonal control remains to be elucidated. An increasing number of endocrine factors are appearing to have a role in mammary gland development, the adipose tissue is increasingly recognized to play a role in endocrine regulation, and a complex role of the immune system with multiple different cell types is being revealed. For further resources related to this article, please visit the WIREs website.

Estrogen receptor coregulators and pioneer factors: the orchestrators of mammary gland cell fate and development

The steroid hormone, 17β-estradiol (E2), plays critical role in various cellular processes such as cell proliferation, differentiation, migration and apoptosis, and is essential for reproduction and mammary gland development. E2 actions are mediated by two classical nuclear hormone receptors, estrogen receptor α and β (ERs). The activity of ERs depends on the coordinated activity of ligand binding, post-translational modifications (PTMs), and importantly the interaction with their partner proteins called “coregulators.” Because coregulators are proved to be crucial for ER transcriptional activity, and majority of breast cancers are ERα positive, an increased interest in the field has led to the identification of a large number of coregulators. In the last decade, gene knockout studies using mouse models provided impetus to our further understanding of the role of these coregulators in mammary gland development. Several coregulators appear to be critical for terminal end bud (TEB) formation, ductal branching and alveologenesis during mammary gland development. The emerging studies support that, coregulators along with the other ER partner proteins called “pioneer factors” together contribute significantly to E2 signaling and mammary cell fate. This review discusses emerging themes in coregulator and pioneer factor mediated action on ER functions, in particular their role in mammary gland cell fate and development.

RANKL/RANK - from bone physiology to breast cancer

RANK and its ligand RANKL are key molecules in bone metabolism and are critically involved in pathologic bone disorders. Deregulation of the RANK/RANKL system is for example a main reason for the development of postmenopausal osteoporosis, which affects millions of women worldwide. Another essential function of RANK and RANKL is the development of a functional lactating mammary gland during pregnancy. Sex hormones, in particular progesterone, induce RANKL expression resulting in proliferation of mammary epithelial cells. Moreover, RANK and RANKL have been shown to regulate mammary epithelial stem cells. RANK and RANKL were also identified as critical mechanism in the development of hormone-induced breast cancer and metastatic spread to bone. In this review, we will focus on the various RANK/RANKL functions ranging from bone physiology, immune regulation, and initiation of breast cancer.

D-type Cyclins are important downstream effectors of cytokine signaling that regulate the proliferation of normal and neoplastic mammary epithelial cells

In response to the ligand-mediated activation of cytokine receptors, cells decide whether to proliferate or to undergo differentiation. D-type Cyclins (Cyclin D1, D2, or D3) and their associated Cyclin-dependent kinases (CDK4, CDK6) connect signals from cytokines to the cell cycle machinery, and they propel cells through the G1 restriction point and into the S phase, after which growth factor stimulation is no longer essential to complete cell division. D-type Cyclins are upregulated in many human malignancies including breast cancer to promote an uncontrolled proliferation of cancer cells. After summarizing important aspects of the cytokine-mediated transcriptional regulation and the posttranslational modification of D-type Cyclins, this review will highlight the physiological significance of these cell cycle regulators during normal mammary gland development as well as the initiation and promotion of breast cancer. Although the vast majority of published reports focus almost exclusively on the role of Cyclin D1 in breast cancer, we summarize here previous and recent findings that demonstrate an important contribution of the remaining two members of this Cyclin family, in particular Cyclin D3, for the growth of ErbB2-associated breast cancer cells in humans and in mouse models. New data from genetically engineered models as well as the pharmacological inhibition of CDK4/6 suggest that targeting the combined functions of D-type Cyclins could be a suitable strategy for the treatment of ErbB2-positive and potentially other types of breast cancer.

Wnt signalling in murine postnatal mammary gland development

The mammary gland undergoes numerous developmental processes postnatally, from the elongation of the ductal tree-like structure to pregnancy-induced lobulo-alveolar development. Mammary epithelial stem cells have been suggested to be central to the control of enormous tissue expansion and remodelling during phases of mammary development. The Wnt signalling pathway plays a critical role in these biological steps and is suggested to be involved in the maintenance of the stem cell population. This review provides insight into recent findings on the activity of Wnt signalling during ductal and lobular mammary development and discusses the potential interplay between Wnt signals and mammary stem cells in mice.

Cdk2-null mice are resistant to ErbB-2-induced mammary tumorigenesis

The concept of targeting G(1) cyclin-dependent kinases (CDKs) in breast cancer treatments is supported by the fact that the genetic ablation of Cdk4 had minimal impacts on normal cell proliferation in majority of cell types, resulting in near-normal mouse development, whereas such loss of Cdk4 completely abrogated ErbB-2/neu-induced mammary tumorigenesis in mice. In most human breast cancer tissues, another G(1)-regulatory CDK, CDK2, is also hyperactivated by various mechanisms and is believed to be an important therapeutic target. In this report, we provide genetic evidence that CDK2 is essential for proliferation and oncogenesis of murine mammary epithelial cells. We observed that 87% of Cdk2-null mice were protected from ErbB-2-induced mammary tumorigenesis. Mouse embryonic fibroblasts isolated from Cdk2-null mouse showed resistance to various oncogene-induced transformation. Previously, we have reported that hemizygous loss of Cdc25A, the major activator of CDK2, can also protect mice from ErbB-2-induced mammary tumorigenesis [Cancer Res (2007) 67(14): 6605-11]. Thus, we propose that CDC25A-CDK2 pathway is critical for the oncogenic action of ErbB-2 in mammary epithelial cells, in a manner similar to Cyclin D1/CDK4 pathway.

Signal transducer and activator of transcription 5a mediates mammary ductal branching and proliferation in the nulliparous mouse

Signal transducer and activator of transcription (Stat)5a is a critical regulator of mammary gland development. Previous studies have focused on Stat5a's role in the late pregnant and lactating gland, and although active Stat5a is detectable in mammary epithelial cells in virgin mice, little is known about its role during early mammary gland development. In this report, we compare mammary gland morphology in pubertal and adult nulliparous wild-type and Stat5a-/- mice. The Stat5a-null mammary glands exhibited defects in secondary and side branching, providing evidence that Stat5a regulates these processes. In addition, Stat5a-/- mammary glands displayed an attenuated proliferative response to pregnancy levels of estrogen plus progesterone (E+P), suggesting that it plays an important role in early pregnancy. Finally, we examined one potential mediator of Stat5a's effects, receptor activator of nuclear factor-kappaB ligand (RANKL). Stat5a-/- mammary glands were defective in inducing RANKL in response to E+P treatment. In addition, regulation of several reported RANKL targets, including inhibitor of DNA binding 2 (Id2), cyclin D1, and the cyclin-dependent kinase inhibitor p21(Waf1/Cip1), was altered in Stat5a-/- mammary cells, suggesting that one or more of these proteins mediate the effects of Stat5a in E+P-treated mammary epithelial cells.

Two distinct mechanisms underlie progesterone-induced proliferation in the mammary gland

The mouse mammary gland develops postnatally under the control of female reproductive hormones. Estrogens and progesterone trigger morphogenesis by poorly understood mechanisms acting on a subset of mammary epithelial cells (MECs) that express their cognate receptors, estrogen receptor alpha (ERalpha) and progesterone receptor (PR). Here, we show that in the adult female, progesterone drives proliferation of MECs in two waves. The first, small wave, encompasses PR(+) cells and requires cyclin D1, the second, large wave, comprises mostly PR(-) cells and relies on the tumor necrosis factor (TNF) family member, receptor activator of NF-kappaB-ligand (RANKL). RANKL elicits proliferation by a paracrine mechanism. Ablation of RANKL in the mammary epithelium blocks progesterone-induced morphogenesis, and ectopic expression of RANKL in MECs completely rescues the PR(-/-) phenotype. Systemic administration of RANKL triggers proliferation in the absence of PR signaling, and injection of a RANK signaling inhibitor interferes with progesterone-induced proliferation. Thus, progesterone elicits proliferation by a cell-intrinsic and a, more important, paracrine mechanism.

ESCI award lecture: from a little mouse to rationale medicine for bone loss

Completion of the human genome is one of the many significant milestones in the new era of systems biology. The current phase of genomic studies is focused upon parsing this new found genetic data with respect to scientific interest, and economic and health impact applications. As the sequences are now available and whole genome single nucleotide polymorphism maps for multiple human diseases will be available with the advent of modern genomics, the big challenge is to determine the function of these genes in the context of the entire organism. The emphasis is therefore on functional genomic analysis that represents the new front-line and limiting factor for realizing potential benefits of genome-based science. Defined gene targeting has been proven to be particularly useful as loss of expression mutants can reveal essential functions of molecules and the pathogenesis of disease. Using gene-targeted mice, my group has over the years identified genes that control heart and lung functions; apoptosis; lymphocyte activation; cancer; pain; diabetes; fertility or wound healing . In this study, I would like to review our work on RANKL in more detail.

MMTV mouse models and the diagnostic values of MMTV-like sequences in human breast cancer

Mouse mammary tumor virus (MMTV) long terminal repeat (LTR)-driven transgenic mice are excellent models for breast cancer as they allow for the targeted expression of various oncogenes and growth factors in neoplastic transformation of mammary glands. Numerous MMTV-LTR-driven transgenic mouse models of breast cancer have been created in the past three decades, including MMTV-neu/ErbB2, cyclin D1, cyclin E, Ras, Myc, int-1 and c-rel. These transgenic mice develop mammary tumors with different latency, histology and invasiveness, reflecting the oncogenic pathways activated by the transgene. Recently, homologous sequences of the env gene of MMTV have been identified in approximately 40% of human breast cancers, but not in normal breast or other types of cancers, suggesting possible involvement of mammary tumor virus in human breast carcinogenesis. Accumulating evidence demonstrates the association of MMTV provirus with progesterone receptor, p53 mutations and advanced-stage breast cancer. Thus, the detection of MMTV-like sequences may have diagnostic value to predict the clinical outcome of breast cancer patients.

The APC tumor suppressor is required for epithelial integrity in the mouse mammary gland

Inactivation of the adenomatous polyposis coli (APC) tumor suppressor has been associated with mammary tumorigenesis in mouse models and through epidemiological studies of human breast cancers, but the normal role for APC in mammary development has not been thoroughly characterized. We report here that Apc(Min/+) mice containing one functional allele of Apc have severely disrupted lobuloalveolar development during pregnancy and lactation, time points at which Apc gene expression is at its highest levels in normal mice. This phenotype was accompanied by altered proliferation during pregnancy and involution, increased apoptosis throughout lactation, the formation of preneoplastic lesions and changes in specific genes associated with each of these processes. Neither modifications in beta-catenin localization, nor the expression of beta-catenin transcriptional target genes, were observed in Apc(Min/+) mammary tissues; however, tissues from lactating Apc(Min/+) mice had a significantly altered epithelial architecture, including disrupted localization of junctional proteins and polarization. Consistent with these findings, APC knockdown in non-transformed mouse mammary epithelial cells in vitro resulted in altered monolayer formation and proliferation without changes in beta-catenin-mediated transcription. These results suggest that APC expression is tightly regulated during mammary gland development and is required for normal mammary homeostasis and tumor suppression primarily through maintaining epithelial integrity.

Strain-specific differences in the mechanisms of progesterone regulation of murine mammary gland development

Progesterone (P) is required for normal mammary gland development, and is implicated in the etiology of mammary cancer in rodents and humans. We analyzed mammary gland developmental responses to P and estrogen (E) in two strains of mice (BALB/c and C57BL/6) that exhibit differences in ductal development at sexual maturity and alveologenesis during pregnancy. C57BL/6 mice exhibited reduced proliferative and morphological responses to P. Analysis of known mediators of sidebranching and alveologenesis revealed that reduced P-induced expression of P receptor isoform B and receptor activator of nuclear factor-kappaB ligand (RANKL), as well as altered expression and regulation of cyclin D1, CCAAT/enhancer binding protein beta, and the downstream effectors of RANKL, nuclear Id2 and p21, contribute significantly to the reduced P responsiveness of the C57BL/6 mammary gland. In contrast, E responsiveness was greater in C57BL/6 than in BALB/c glands. E may play a compensatory role in C57BL/6 alveologenesis through its effect on the induction and activation of signal transducer and activator of transcription 5a, a known regulator of RANKL. These observations suggest that in human populations with heterogeneous genetic backgrounds, individuals may respond differentially to the same hormone. Thus, genetic diversity may have a role in determining the effects of P in normal mammary development and tumorigenesis.

NDRG2 suppresses cell proliferation through down-regulation of AP-1 activity in human colon carcinoma cells

Recently, the anti-tumor activity of N-myc downstream-regulated gene 2 (NDRG2) was elucidated, but the molecular mechanism of how NDRG2 works as a tumor suppressor is not well known. To determine the function of NDRG2 as a tumor suppressor, we established stable cell lines expressing NDRG2 protein or its mutant forms, and studied their effects on tumor cell growth. Interestingly, constitutive expression of wild-type NDRG2 induced the growth retardation of SW620 colon carcinoma cells. Introduction of NDRG2 into SW620 cells induced the decrease of c-Jun phosphorylation at Ser63, followed by the attenuation of activator protein-1 (AP-1) function as a transcriptional activator. Subsequently, the down-regulation of cyclin D1, which is known as a major target for AP-1 transcription activator, resulted in cell cycle arrest at G1/S phase. Additionally, treatment of NDRG2-siRNA on NDRG2-expressing cells has induced the recovery of c-Jun phosphorylation and cyclin D1 expression. Cell proliferation of those cells was also increased compared with untreated cells. NDRG2 mutants of which the phosphorylation sites at C-terminal region were removed by deletion or site-directed mutagenesis have shown no effect on cyclin D1 expression and could not induce cell growth retardation. In conclusion, NDRG2 modulates intracellular signals to control cell cycle through the regulation of cyclin D1 expression via phosphorylation pathway.

[Advances on progesterone receptor gene]

The steroid hormone, progesterone, plays a key role in diverse events associated with female reproduction. In humans and other vertebrates, the biological activity of progesterone is mediated by modulation of the transcriptional activity of two progesterone receptors, PGR-A and PGR-B. This review introduced the structure, expression regulation and polymorphism of progesterone receptor gene. The relationship between progesterone receptor gene and reproductive function was also discussed in mammals.

Tumors caused by overexpression and forced activation of Stat5 in mammary epithelial cells of transgenic mice are parity-dependent and developed in aged, postestropausal females

In transgenic mice overexpressing Stat5 or a constitutively activated Stat5 variant (STAT5ca), we show for the first time that parity is required for the development of tumors in postestropausal females. Tumors were detected in glands of multiparous transgenic female mice after latency period of 14 months, but rarely in their age-matched virgin (AMV) counterparts. This period was not affected by distinguishable tumor pathologies and was not dependent upon transgenic Stat5 variant. To associate Stat5 deregulation, parity and the postestropausal tumor occurrence with mammary cancer formation, the activities of endogenous and transgenic Stat5 were measured in the glands of aged multiparous and AMV females. No differences in phosphorylated Stat5 (pStat5) levels were found between the 2 cohorts. However, promoter sequences comprising the Stat5 binding sites from the cyclin D1 or the bcl-x genes associate differentially with acetylated histone H4 in aged multiparous and AMV STAT5ca transgenic females. Individual epithelial cells varied greatly with respect to the presence of nuclear pStat5. A small subset of epithelial cells, in which pStat5 and cyclin D1 were co-expressed, was exclusively present in the multiparous glands. Changes in chromatin structure might persist past the reproductive life time of the multiparous mice and contribute to the transcription of the cyclin D1 gene by activated Stat5. This may cause the detectable expression of cyclin D1 and add to the process of tumorigenesis.

The pattern of beta-catenin responsiveness within the mammary gland is regulated by progesterone receptor

Experiments involving beta-catenin loss- and gain-of-function in the mammary gland have decisively demonstrated the role of this protein in normal alveologenesis. However, the relationship between hormonal and beta-catenin signaling has not been investigated. In this study, we demonstrate that activated beta-catenin rescues alveologenesis in progesterone receptor (PR; Pgr)-null mice during pregnancy. Two distinct subsets of mammary cells respond to expression of DeltaN89beta-catenin. Cells at ductal tips are inherently beta-catenin-responsive and form alveoli in the absence of PR. However, PR activity confers beta-catenin responsiveness to progenitor cells along the lateral ductal borders in the virgin gland. Once activated by beta-catenin, responding cells switch on an alveolar differentiation program that is indistinguishable from that observed in pregnancy and is curtailed by PR signaling.

Mammary epithelial-specific deletion of the focal adhesion kinase gene leads to severe lobulo-alveolar hypoplasia and secretory immaturity of the murine mammary gland

Integrin-mediated cell adhesion and signaling is required for mammary gland development and functions. As a major mediator of integrin signaling, focal adhesion kinase (FAK) has been implicated to play a role in the survival, proliferation, and differentiation of mammary epithelial cells in previously studies in vitro. To assess the role of FAK in vivo, we created mice in which FAK is selectively deleted in mammary epithelial cells. The mammary gland FAK conditional knock-out (MFCKO) mice are viable, fertile, and macroscopically indistinguishable from the control littermates. In virgin MFCKO mice, mammary ductal elongation is retarded at 5 weeks of age but reaches the full extent by 8 weeks of age compared with the control mice. However, the MFCKO females are unable to nurse their pups due to severe lobulo-alveolar hypoplasia and secretory immaturity during pregnancy and lactation. Analysis of the mammary epithelial cells in MFCKO mice showed reduced Erk phosphorylation, expression of cyclin D1, and a corresponding decrease in proliferative capability compared with the littermate controls. In addition, phosphorylation of STAT5 and expression of whey acidic protein are significantly reduced in the mammary glands of MFCKO mice, suggesting defective secretory maturation in these mice. Therefore, the combination of the severe lobulo-alveolar hypoplasia and defective secretory differentiation is responsible for the inability of the MFCKO females to nurse their pups. Together, these results provide strong support for a role of FAK in the mammary gland development and function in vivo.

The alveolar switch: coordinating the proliferative cues and cell fate decisions that drive the formation of lobuloalveoli from ductal epithelium

Massive tissue remodelling occurs within the mammary gland during pregnancy, resulting in the formation of lobuloalveoli that are capable of milk secretion. Endocrine signals generated predominantly by prolactin and progesterone operate the alveolar switch to initiate these developmental events. Here we review the current understanding of the components of the alveolar switch and conclude with an examination of the role of the ets transcription factor Elf5. We propose that Elf5 is a key regulator of the alveolar switch.

Overexpression of activated murine Notch1 and Notch3 in transgenic mice blocks mammary gland development and induces mammary tumors

The mouse mammary tumor virus (MMTV) provirus was found to target the Notch1 gene, producing insertional mutations in mammary tumors of MMTV/neu transgenic (Tg) mice. In these mammary tumors, the Notch1 gene is truncated upstream of the transmembrane domain, and the resulting Notch1 intracellular domain (Notch1(intra)), deleted of most extracellular sequences, is overexpressed. Although Notch1(intra) transforms mammary epithelial cells in vitro, its role in mammary gland tumor formation in vivo was not studied. Therefore, we generated MMTV/Notch1(intra) Tg mice that overexpress murine Notch1(intra) in the mammary glands. We observed that MMTV/Notch1(intra) Tg females were unable to feed their pups because of impaired ductal and lobulo-alveolar mammary gland development. This was associated with decreased proliferation of ductal and alveolar epithelial cells during rapid expansion at puberty and in early pregnancy, as well as decreased production of beta-casein. Notch1(intra) repressed expression of the beta-casein gene promoter, as assessed in vitro with a beta-casein/luciferase reporter construct. The MMTV/Notch1(intra) Tg females developed mammary gland tumors, confirming the oncogenic potential of Notch1(intra) in vivo. Furthermore, MMTV/Notch3(intra) Tg mice exhibited a very similar phenotype. Thus, these Tg mice represent novel models for studying the role of Notch1 or Notch3 in the development and transformation of the mammary gland.

Requirement for CDK4 kinase function in breast cancer

Cyclin D1 is overexpressed in the majority of human breast cancers. We previously found that mice lacking cyclin D1 are resistant to mammary carcinomas triggered by the ErbB-2 oncogene. In this study, we investigated which function of cyclin D1 is required for ErbB-2-driven mammary oncogenesis. We report that the ability of cyclin D1 to activate cyclin-dependent kinase CDK4 underlies the critical role for cyclin D1 in breast cancer formation. We also found that the continued presence of CDK4-associated kinase activity is required to maintain breast tumorigenesis. We analyzed primary human breast cancers and found high cyclin D1 levels in a subset (approximately 25%) of ErbB-2-overexpressing tumors. We propose that this subset of breast cancer patients might benefit from inhibiting CDK4 kinase.

Genetic replacement of cyclin D1 function in mouse development by cyclin D2

D cyclins (D1, D2, and D3) are components of the core cell cycle machinery in mammalian cells. It is unclear whether each of the D cyclins performs unique, tissue-specific functions or the three proteins have virtually identical functions and differ mainly in their pattern of expression. We previously generated mice lacking cyclin D1, and we observed that these animals displayed hypoplastic retinas and underdeveloped mammary glands and a presented developmental neurological abnormality. We now asked whether the specific requirement for cyclin D1 in these tissues reflected a unique pattern of D cyclin expression or the presence of specialized functions for cyclin D1 in cyclin D1-dependent compartments. We generated a knock-in strain of mice expressing cyclin D2 in place of D1. Cyclin D2 was able to drive nearly normal development of retinas and mammary glands, and it partially replaced cyclin D1's function in neurological development. We conclude that the differences between these two D cyclins lie mostly in the tissue-specific pattern of their expression. However, we propose that subtle differences between the two D cyclins do exist and they may allow D cyclins to function in a highly optimized fashion. We reason that the acquisition of multiple D cyclins may allow mammalian cells to drive optimal proliferation of a diverse array of cell types.

Effects of Neonatal Exposure to Diethylstilbestrol, Tamoxifen, and Toremifene on the BALB/c Mouse Mammary Gland1

In this study, we compared the long-term effects of neonatal exposure to diethylstilbestrol (DES, 0.0125-50 microg), tamoxifen (TAM, 0.0125-50 microg), and toremifene (TOR, 53 microg) on mammary gland development and differentiation. Allometric growth of the mammary ducts was stimulated by neonatal DES exposure (12.5 microg) and impaired by exposure to TAM (25 microg). Neonatal treatment with high doses of DES resulted in mammary ducts that displayed extensive dilatation and precocious lactogenesis in postpubertal, nulliparous females. Initiation of this precocious differentiation coincided with the absence of corpora lutea, increased levels of serum prolactin (PRL), and the induction of Prl mRNA expression within the mammary glands. Neonatal exposure to 1.25 microg TAM increased alveolar development in postpubertal, nulliparous females similar to that recorded in females treated with low doses of DES. Lower doses of TAM did not affect alveolar development, whereas branching morphogenesis and alveolar development were impaired by higher doses. Increased alveolar development in females exposed to 1.25 microg TAM was associated with elevated serum progesterone (P) and increased alveolar development in response to exogenous P. Taken together, our findings demonstrate that neonatal exposure to both DES and TAM exerts long-lasting effects on the proliferation and differentiation of the mammary glands in female BALB/c, primarily as the result of endocrine disruption.

Reproductive tissue selective actions of progesterone receptors

The steroid hormone, progesterone, plays a central coordinate role in diverse events associated with female reproduction. In humans and other vertebrates, the biological activity of progesterone is mediated by modulation of the transcriptional activity of two progesterone receptors, PR-A and PR-B. These receptors arise from the same gene and exhibit both overlapping and distinct transcriptional activities in vitro. To delineate the individual roles of PR-A and PR-B in vivo, we have generated mouse models in which expression of a single PR isoform has been ablated. Analysis of the reproductive phenotypes of these mice has indicated that PR-A and PR-B mediate mostly distinct but partially overlapping reproductive responses to progesterone. While selective ablation of the PR-A protein (PR-A knockout mice, PRAKO mice) shows normal mammary gland response to progesterone but severe uterine hyperplasia and ovarian abnormalities, ablation of PR-B protein (PRBKO mice) does not affect biological responses of the ovary or uterus to progesterone but results in reduced pregnancy-associated mammary gland morphogenesis. The distinct tissue-specific reproductive responses to progesterone exhibited by these isoforms are due to regulation of distinct subsets of progesterone-dependent target genes by the individual PR isoforms. This review will summarize our current understanding of the selective contribution of PR isoforms to the cellular and molecular actions of progesterone in reproductive tissues.

Progesterone receptors induce cell cycle progression via activation of mitogen-activated protein kinases

Progestins induce proliferation of breast cancer cells and are implicated in the development of breast cancer. The effects of progestins are mediated by progesterone receptors (PRs), although it is unclear whether proliferative effects are delivered through activities as ligand-activated transcription factors or via activation of cytoplasmic kinases. We report that progestin induces S phase entry of T47D cells stably expressing either wild-type (wt) PR-B or a transcriptionally impaired PR-B harboring a point mutation at Ser294, a ligand-dependent and MAPK consensus phosphorylation site (S294A). Both wt and S294A PR are capable of activating p42/p44 MAPKs and promoting proliferation. However, cells expressing wt, but not S294A PR, exhibited enhanced proliferation in response to combined epidermal growth factor and progestin. S phase progression correlated with up-regulation of cyclin D1. The PR antagonist RU486 also induced MAPK activation, increased cyclin D1 expression, and stimulated S phase entry, which was blocked by inhibition of either p42/p44 or p38 MAPKs, whereas proliferation induced by R5020 was sensitive only to p42/p44 MAPK inhibition. MCF-7 cells stably expressing a mutant PR unable to bind c-Src and activate MAPK failed to support progestin-induced proliferation. These data suggest that PR mediate cell cycle progression primarily through activation of cytoplasmic kinases and independently of direct regulation of transcription, whereas the coordinate regulation of both aspects of PR action are required for enhanced proliferation in response to progestins in the presence of growth factors. Targeting the ability of steroid receptors to activate MAPKs may be beneficial for breast cancer patients.

Metastasis-associated protein 1 deregulation causes inappropriate mammary gland development and tumorigenesis

Emerging data suggest that metastasis-associated protein 1 (MTA1) represses ligand-dependent transactivation functions of estrogen receptor-alpha in cultured breast cancer cells and that MTA1 is upregulated in human breast tumors. However, the role of MTA1 in tumorigenesis in a physiologically relevant animal system remains unknown. To reveal the role of MTA1 in mammary gland development, transgenic mice expressing MTA1 under the control of the mouse mammary tumor virus promoter long terminal repeat were generated. Unexpectedly, we found that mammary glands of these virgin transgenic mice exhibited extensive side branching and precocious differentiation because of increased proliferation of ductal and alveolar epithelial cells. Mammary glands of virgin transgenic mice resemble those from wild-type mice in mid-pregnancy and inappropriately express beta-casein, cyclin D1 and beta-catenin protein. Increased ductal growth was also observed in the glands of ovariectomized female mice, as well as of transgenic male mice. MTA1 dysregulation in mammary epithelium and cancer cells triggered downregulation of the progesterone receptor-B isoform and upregulation of the progesterone receptor-A isoform, resulting in an imbalance in the native ratio of progesterone receptor A and B isoforms. MTA1 transgene also increased the expression of progesterone receptor-A target genes Bcl-XL (Bcl2l1) and cyclin D1 in mammary gland of virgin mice, and, subsequently, produced a delayed involution. Remarkably, 30% of MTA1 transgenic females developed focal hyperplastic nodules, and about 7% exhibited mammary tumors within 18 months. These studies establish, for the first time, a potential role of MTA1 in mammary gland development and tumorigenesis. The underlying mechanism involves the upregulation of progesterone receptor A and its targets, Bcl-XL and cyclin D1.

STAT5 and Oct-1 form a stable complex that modulates cyclin D1 expression

Signal transducer and activator of transcription 5 (STAT5) is activated by numerous cytokines that control blood cell development. STAT5 was also shown to actively participate in leukemogenesis. Among the target genes involved in cell growth, STAT5 had been shown to activate cyclin D1 gene expression. We now show that thrombopoietin-dependent activation of the cyclin D1 promoter depends on the integrity of a new bipartite proximal element that specifically binds STAT5A and -B transcription factors. We demonstrate that the stable recruitment of STAT5 to this element in vitro requires the integrity of an adjacent octamer element that constitutively binds the ubiquitous POU homeodomain protein Oct-1. We observe that cytokine-activated STAT5 and Oct-1 form a unique complex with the cyclin D1 promoter sequence. We find that STAT5 interacts with Oct-1 in vivo, following activation by different cytokines in various cellular contexts. This interaction involves a small motif in the carboxy-terminal region of STAT5 which, remarkably, is similar to an Oct-1 POU-interacting motif present in two well-known partners of Oct-1, namely, OBF-1/Bob and SNAP190. Our data offer new insights into the transcriptional regulation of the key cell cycle regulator cyclin D1 and emphasize the active roles of both STAT5 and Oct-1 in this process.

Tissue microarray analyses of G1/S-regulatory proteins in ductal carcinoma in situ of the breast indicate that low cyclin D1 is associated with local recurrence

Ductal carcinoma in situ (DCIS) of the breast constitutes about 10% of all diagnosed breast cancers and, despite surgical removal, it may recur, either as DCIS or invasive breast cancer. Nuclear grade and growth pattern according to Andersen et al as well as surgical margins are factors that have been used to predict local recurrence, but ideally a set of tumour-specific factors should be identified and used as prognostic markers. Many cell cycle regulatory gene products have been shown to be involved in the formation of tumours and are either oncogenes or suppressor genes and involved in key processes in the transformation. We therefore characterised the cell cycle regulators cyclin E, cyclin D1, p27 and p16 in a material of DCIS cases arranged in a tissue microarray. With a manual tissue arrayer, 52% of the initial 177 DCIS samples were successfully targeted allowing immunohistochemical analyses of all four proteins in 92 cases of DCIS. As also observed in invasive breast cancer, there was a trend indicating that DCIS cases with high cyclin D1 were cyclin E low and oestrogen receptor-positive, whereas cyclin E high DCIS cases were cyclin D1 low and oestrogen receptor-negative. For the 64 patients that did not receive postoperative radiotherapy, there were 16 local recurrences (eight DCIS and eight invasive breast cancer) during a mean follow-up time of 63 months. Cyclin E, p27 or p16 were not associated with local recurrence, but interestingly cyclin D1 was significantly and inversely associated with local recurrence, both using univariate and multivariate analyses. In summary, using a tissue array approach we have shown that cyclin D1, besides growth pattern, is a prognostic marker for local recurrence in DCIS.

Role of the CDK inhibitor p27 (Kip1) in mammary development and carcinogenesis: insights from knockout mice

The cyclin-dependent kinase inhibitor p27 (Kip1) is an important cell cycle regulatory gene in breast cancer, and decreased p27 expression is associated with poor prognosis. Some investigations of its role in mammary development have demonstrated reduced cyclin D1 expression and consequent lack of lobuloalveolar development, but others have found increased cyclin E-Cdk2 activity and increased proliferation balanced by increased apoptosis. It is unclear at present why these apparently divergent results have been obtained. Mice with reduced p27 gene dosage alone do not develop mammary carcinomas but do display substantially shorter tumor latency upon overexpression of erbB2, consistent with a role for p27 as a mammary tumor suppressor gene. In this review we summarize these and other data addressing the role of p27 in normal mammary epithelium and experimental models of mammary carcinogenesis.

The cyclin-dependent kinase inhibitor p27 (Kip1) regulates both DNA synthesis and apoptosis in mammary epithelium but is not required for its functional development during pregnancy

Decreased expression of the cyclin-dependent kinase (CDK) inhibitor p27(Kip1) is common in breast cancer and is associated with poor prognosis. p27 is also an important mediator of steroidal regulation of cell cycle progression. We have therefore investigated the role of p27 in mammary epithelial cell proliferation. Examination of the two major functions of p27, assembly of cyclin D1-Cdk4 complexes and inhibition of Cdk2 activity, revealed that cyclin D1-Cdk4 complex formation was not impaired in p27-/- mammary epithelial cells in primary culture. However, cyclin E-Cdk2 activity was increased approximately 3-fold, indicating that the CDK inhibitory function of p27 is important in mammary epithelial cells. Increased epithelial DNA synthesis was observed during pregnancy in p27-/- mammary gland transplants, but this was paralleled by increased apoptosis. During pregnancy and at parturition, development and differentiation of p27+/+ and p27-/- mammary tissue were indistinguishable. These results demonstrate a role for p27 in both the proliferation and survival of mammary epithelial cells. However, the absence of morphological and cellular defects in p27-/- mammary tissue during pregnancy raises the possibility that loss of p27 in breast cancer may not confer an overall growth advantage unless apoptosis is also impaired.

Receptor activator of NF-kappaB ligand induction via Jak2 and Stat5a in mammary epithelial cells

Prolactin (PRL) is the primary hormone that, in conjunction with local factors, leads to lobuloalveolar development during pregnancy. Recently, receptor activator of NF-kappaB ligand (RANKL) has been identified as one of the effector molecules essential for lobuloalveolar development. The molecular mechanisms by which PRL may induce RANKL expression have not been carefully examined. Here we report that RANKL expression in the mammary gland is developmentally regulated and dependent on PRL and progesterone, whereas its receptor RANK (receptor activator of NF-kappaB) and decoy receptor osteoprotegerin (OPG) are constitutively expressed at all stages in both normal (PRL+/-) and prolactin knockout (PRL-/-) mice. In vitro, PRL markedly increased RANKL expression in primary mammary epithelial cells and RANKL-luciferase reporter activity in CHOD6 cells, which constitutively express the PRL receptor. We identified a gamma-interferon activation sequence (GAS) in the region between residues -965 to -725 of the RANKL promoter, which conferred a PRL response. Using dominant negative mutants of recombinant Jak2 and Stat5 in CHOD6 cells, and by reconstituting the Jak2/Stat5 pathway in COS7 cells, we determined that Jak2 and Stat5a are essential for the PRL-induced RANKL expression in mammary gland.

Progesterone-dependent regulation of female reproductive activity by two distinct progesterone receptor isoforms

The steroid hormone, progesterone, is a central coordinator of all aspects of female reproductive activity. The physiological effects of progesterone are mediated by interaction of the hormone with specific intracellular progesterone receptors (PRs) that are expressed from a single gene as two protein isoforms and that are members of the nuclear receptor superfamily of transcription factors. Analysis of the structural and functional relationships of each isoform using in vitro systems has demonstrated that the PR-A and PR-B proteins have different transcription activation properties when liganded to progesterone. More recently, selective ablation of the PR-A and PR-B proteins in mice had facilitated examination of the contribution of the individual PR isoforms to the pleiotropic reproductive activities of progesterone. Analysis of the phenotypic consequences of these mutations on female reproductive function has provided proof of concept that the distinct transcriptional responses to PR-A and PR-B observed in cell-based transactivation assays are reflected in a distinct tissue-selective contribution of the individual isoforms to the reproductive activities of progesterone. In PR-A knock-out mice, in which the expression of the PR-A isoform is selectively ablated (PRAKO), the PR-B isoform functions in a tissue-specific manner to mediate a subset of the reproductive functions of PRs. Ablation of PR-A does not affect response of the mammary gland or thymus to progesterone but results in severe abnormalities in ovarian and uterine function leading to female infertility. More recent studies using PR-B knock-out (PRBKO) mice have shown that ablation of PR-B does not affect either ovarian, uterine or thymic responses to progesterone but results in reduced mammary ductal morphogenesis and alveologenesis during pregnancy. Thus, PR-A is both necessary and sufficient to elicit the progesterone-dependent reproductive responses necessary for female fertility, while the PR-B isoform is required to elicit normal proliferative and differentiative responses of the mammary gland to progesterone. This review will summarize our current understanding of the selective contribution of the two PR isoforms to progesterone action.

Dissecting the roles of beta-catenin and cyclin D1 during mammary development and neoplasia

A considerable body of circumstantial data suggests that cyclin D1 is an attractive candidate to mediate the effects of beta-catenin in mammary tissue. To test the functional significance of these correlative findings, we investigated the genetic interaction between transcriptionally active beta-catenin (DeltaN89beta-catenin) and its target gene cyclin D1 in the mouse mammary gland during pubertal development, pregnancy, and tumorigenesis. Our data demonstrate that cyclin D1 is dispensable for the DeltaN89beta-catenin-stimulated initiation of alveologenesis in virgin females, for the de novo induction of alveoli in males, and for the formation of tumors. Indeed, lack of cyclin D1 accentuates and enhances these hyperplastic and tumorigenic DeltaN89beta-catenin phenotypes. Although alveologenesis is initiated by DeltaN89beta-catenin in a cyclin D1-independent fashion, up-regulation of cyclin D1 occurs in DeltaN89beta-catenin mice and its expression remains essential for the completion of alveolar development during the later stages of pregnancy. Thus, alveologenesis is a two-step process, and cyclin D1 activity during late alveologenesis cannot be replaced by the activity of other beta-catenin target genes that successfully drive proliferation at earlier stages.

Defective mammary gland morphogenesis in mice lacking the progesterone receptor B isoform

Progesterone (P) regulates female reproduction via two nuclear receptors, PR-A and PR-B. Although both receptors display overlapping and distinct transcription regulatory properties, their individual physiological roles are unclear. To address the physiological role of PR-A, we generated a mouse model in which expression of PR-B was specifically ablated (PRBKO-/-). We show that selective activation of PR-A in PRBKO-/- mice is sufficient to elicit normal ovarian and uterine responses to P but results in reduced mammary gland morphogenesis. In the absence of PR-B, pregnancy-associated ductal sidebranching and lobuloalveolar development are markedly reduced due to decreased ductal and alveolar epithelial cell proliferation and decreased survival of alveolar epithelium. In an effort to elucidate the molecular genetic signaling pathways that are differentially regulated by PRs in the mammary gland, we have identified receptor activator of nuclear factor kappa B ligand (RANKL) as a paracrine mediator of P-dependent alveologenesis. Further, we demonstrate that the defects in PRBKO-/- mice are associated with an inability of PR-A to activate the RANKL signaling pathway in response to P. Our data indicate that functional interaction between PR-A and PR-B is not required for reproductive activity and that selective modulation of PR-A activity by progestin agonists may have a protective effect against both uterine and mammary gland hyperplasias.

Alternative mammary oncogenic pathways are induced by D-type cyclins; MMTV-cyclin D3 transgenic mice develop squamous cell carcinoma

The three human D-type cyclins, cyclin D1, D2 and D3 share the ability to bind to and activate cdk4 and 6. MMTV-cyclin D1 transgenic mice develop mainly adenocarcinoma, while MMTV-cyclin D2 mice show a lack of alveologenesis during pregnancy and only develop carcinoma at low frequency. The effect of cyclin D3 overexpression in mammary glands remains hitherto unknown. We generated MMTV-cyclin D3 transgenic mice and report here that they develop exclusively squamous cell carcinoma. We show that although cyclin D3 transgene expression was detected early in puberty, postnatal development and mammary gland proliferation were normal in virgin animals. In contrast, multiparous mice develop multiple foci of abnormal growth that correspond to various stages of squamous metaplasia. Therefore, our results support a role for cyclin D3 in squamous differentiation. In addition, we found that p16 expression during involution is abolished, while p27 expression increased in MMTV-cyclin D3 mice, two modifications that have been reported in the other MMTV-D-type cyclin transgenic models. Our observations indicate that despite biochemical redundancy in vitro and in vivo, D-type cyclins promote distinct oncogenic pathways.

Using gene expression arrays to elucidate transcriptional profiles underlying prolactin function

Prolactin is an ancient hormone, with different functions in many species. The binding of prolactin to its receptor, a member of the cytokine receptor superfamily, results in the activation of different intracellular signaling pathways, such as JAK2/STAT5, MAP kinase, and PI3K/AKT. How prolactin elicits so many different biological responses remains unclear. Recently, microarray technology has been applied to identify prolactin target genes in different systems. Here, we attempt to summarize and compare the available data. Our comparison of the genes reported to be transcriptionally regulated by prolactin indicates that there are few genes in common between the different tissues. Among the organs studied, mammary and prostate glands displayed the largest number of overlaps in putative prolactin target genes. Some of the candidates have been implicated in tumorigenesis. The relevance and validation of microarray data, as well as comparison of the results obtained by different groups, will be discussed.

Mammary physiology and milk secretion

The presence of drugs or other potentially toxic materials in milk is an obvious public health risk, especially to infants and neonates. There is also increasing concern that human breast cancer is principally epigenetic in origin and results from environmentally produced lesions. Little is known about the mechanisms by which toxic substances enter milk or mammary tissue but knowledge of these processes is important to toxicologists and researchers involved in drug design and metabolism. Five general pathways have been described for transport of proteins, lipids, ions, nutrients and water into milk. Four of these pathways are transcellular, involving transport across at least two membrane barriers; the fifth is paracellular and allows direct exchange of interstitial and milk components. Solute transport by these pathways is mediated by a diverse, and complex array of transport and secretory processes that are regulated by hormonal, developmental, and physiological factors. Current research is beginning to define the mechanisms underlying some of these processes, however the regulation and coordination of solute transport mechanisms remains poorly understood. In this article we review our current understanding of the normal solute transport and secretory processes involved in milk production, and discuss potential regulatory mechanisms.

Beta-catenin and Tcfs in mammary development and cancer

Beta-catenin regulates cell-cell adhesion and transduces signals from many pathways to regulate the transcriptional activities of Tcf/Lef DNA binding factors. Gene ablation and transgenic expression studies strongly support the concept that beta-catenin together with Lef/Tcf factors act as a switch to determine cell fate and promote cell survival and proliferation at several stages during mammary gland development. Mice expressing the negative regulator of Wnt/beta-catenin signaling (K14-Dkk) fail to form mammary buds, and those lacking Lef-1 show an early arrest in this process at stage E13.5. Stabilized deltaN89beta-catenin initiates precocious alveologenesis during pubertal development, and negative regulators of endogenous beta-catenin signaling suppress normal alveologenesis during pregnancy. Stabilized beta-catenin induces hyperplasia and mammary tumors in mice. Each of the beta-catenin-induced phenotypes is accompanied by upregulation of the target genes cyclin D1 and c-myc. Cyclin D1, however, is dispensable for tumor formation and the initiation of alveologenesis but is essential for later alveolar expansion.

Overexpression of transcription factor AP-2alpha suppresses mammary gland growth and morphogenesis

AP-2 transcription factors are key regulators of mouse embryonic development. Aberrant expression of these genes has also been linked to the progression of human breast cancer. Here, we have investigated the role of the AP-2 gene family in the postnatal maturation of the mouse mammary gland. Analysis of AP-2 RNA and protein levels demonstrates that these genes are expressed in the mammary glands of virgin and pregnant mice. Subsequently, AP-2 expression declines during lactation and then is reactivated during involution. The AP-2alpha and AP-2gamma proteins are localized in the ductal epithelium, as well as in the terminal end buds, suggesting that they may influence growth of the ductal network. We have tested this hypothesis by targeting AP-2alpha expression to the mouse mammary gland using the MMTV promoter. Our studies indicate that overexpression of AP-2alpha inhibits mammary gland growth and morphogenesis, and this coincides with a rise in PTHrP expression. Alveolar budding is severely curtailed in transgenic virgin mice, while lobuloalveolar development and functional differentiation are inhibited during pregnancy and lactation, respectively. Our studies strongly support a role for the AP-2 proteins in regulating the proliferation and differentiation of mammary gland epithelial cells in both mouse and human.

Progesterone receptors in mammary gland development and tumorigenesis

The steroid hormone, progesterone (P), is a central coordinator of all aspects of female reproductive activity and plays a key role in pregnancy-associated mammary gland morphogenesis and mammary tumorigenesis. The effects of P on the mammary gland are mediated by two structurally and functionally distinct nuclear receptors PR-A and PR-B that arise from a single gene. Null mutation of both receptors in PR knockout (PRKO) mice has demonstrated a critical role for PRs in mediating pregnancy-associated mammary ductal branching and lobuloalveolar differentiation and in initiation of mammary tumors in response to carcinogen. Analysis of the molecular genetic pathways disrupted in PRKO mice has recently yielded important insights into the molecular mechanisms of regulation of mammary gland morphogenesis by PRs. In addition to its essential role in regulating proliferative and differentiative responses of the adult mammary gland during pregnancy, P plays a critical role in the protection against mammary tumorigenesis afforded by early parity. Thus, the effects of P on postnatal developmental plasticity of the mammary gland differ between young and adult glands. This review will summarize recent advances in our understanding of 1) the molecular mechanisms by which PRs mediate pregnancy-associated mammary gland morphogenesis, 2) the role of PRs in mediating tumorigenic responses of the adult mammary gland to carcinogen, and 3) the role of P in long-term protection of the juvenile mammary gland against tumorigenesis. In addition, we will summarize recent insights into the isoform selective contributions to some of these activities of PRs obtained from comparative analysis of P-dependent mammary gland development in PR isoform specific knockout mice lacking either the PR-A (PRAKO) or PR-B (PRBKO).

CCAAT/enhancer binding protein beta (C/EBPbeta)-2 transforms normal mammary epithelial cells and induces epithelial to mesenchymal transition in culture

The transcription factor CCAAT/enhancer binding protein (C/EBP)beta is critical for normal growth and differentiation of the mammary gland. The intronless C/EBPbeta gene encodes a single mRNA that produces three protein isoforms, C/EBPbeta-1, -2, and -3, which share a common basic-leucine zipper domain at their C-terminus, but are distinguished at their N-termini by the in-frame methionine codon used to initiate translation. Although C/EBPbeta-1 and -2 are both transactivators, they likely perform distinct functions in mammary epithelial cells. C/EBPbeta-1 is the only isoform detected in normal human mammary tissue. In breast cancer cell lines, C/EBPbeta-1 is absent, and the C/EBPbeta-2 transactivator is expressed. Moreover, our data suggest that C/EBPbeta-2 is upregulated in human primary breast tumors. To assess C/EBPbeta-2's ability to participate in the transformation process, we generated recombinant retrovirus selectively encoding epitope-tagged C/EBPbeta-2. Strikingly, 10 days after infecting a normal human mammary epithelial cell line (MCF10A) with C/EBPbeta-2 virus, transformed subcultures were readily generated. Specifically, C/EBPbeta-2-overexpressing MCF10A cells form foci, gain anchorage independence, express markers associated with having undergone an epithelial-to-mesenchymal transition, and acquire an invasive phenotype. These studies, and our previous observations, provide supportive evidence that deregulated expression of C/EBPbeta-2 contributes to malignant conversion of the human breast.

IKKalpha regulates mitogenic signaling through transcriptional induction of cyclin D1 via Tcf

The Wnt/beta-catenin/Tcf and IkappaB/NF-kappaB cascades are independent pathways involved in cell cycle control, cellular differentiation, and inflammation. Constitutive Wnt/beta-catenin signaling occurs in certain cancers from mutation of components of the pathway and from activating growth factor receptors, including RON and MET. The resulting accumulation of cytoplasmic and nuclear beta-catenin interacts with the Tcf/LEF transcription factors to induce target genes. The IkappaB kinase complex (IKK) that phosphorylates IkappaB contains IKKalpha, IKKbeta, and IKKgamma. Here we show that the cyclin D1 gene functions as a point of convergence between the Wnt/beta-catenin and IkappaB pathways in mitogenic signaling. Mitogenic induction of G(1)-S phase progression and cyclin D1 expression was PI3K dependent, and cyclin D1(-/-) cells showed reduced PI3K-dependent S-phase entry. PI3K-dependent induction of cyclin D1 was blocked by inhibitors of PI3K/Akt/IkappaB/IKKalpha or beta-catenin signaling. A single Tcf site in the cyclin D1 promoter was required for induction by PI3K or IKKalpha. In IKKalpha(-/-) cells, mitogen-induced DNA synthesis, and expression of Tcf-responsive genes was reduced. Reintroduction of IKKalpha restored normal mitogen induction of cyclin D1 through a Tcf site. In IKKalpha(-/-) cells, beta-catenin phosphorylation was decreased and purified IKKalpha was sufficient for phosphorylation of beta-catenin through its N-terminus in vitro. Because IKKalpha but not IKKbeta induced cyclin D1 expression through Tcf activity, these studies indicate that the relative levels of IKKalpha and IKKbeta may alter their substrate and signaling specificities to regulate mitogen-induced DNA synthesis through distinct mechanisms.

The role of the ubiquitination-proteasome pathway in breast cancer: use of mouse models for analyzing ubiquitination processes

Turnover of several regulatory proteins results from targeted destruction via ubiquitination and subsequent degradation through the proteosome. The timely and irreversible degradation of critical regulators is essential for normal cellular function. The precise biochemical mechanisms that are involved in protein turnover by ubiquitin-mediated degradation have been elucidated using in vitro assays and cell culture systems. However, pathways that lead to ubiquitination of critical regulatory proteins in vivo are more complex, and have both temporal and tissue-specific differences. In vivo models will allow identification of substrates and enzymes of the ubiquitin-proteosome pathway that play important roles in selected tissues and diseases. In addition, assessment of the therapeutic efficacy of drugs designed to inhibit or enhance protein turnover by ubiquitination requires in vivo models. In the present review we describe selected examples of transgenic and knockout models of proteins that are known either to be regulated by ubiquitin-mediated degradation or to have a catalytic function in this process, and to play an important role in breast cancer. We outline the functions of these proteins in vivo and focus on knowledge gained in the comparison of in vivo behavior predicted from cell-free in vitro data or from experiments conducted in cell culture systems.