The role of activin in mammary gland development and oncogenesis

TGFβ contributes to mammary gland development and has paradoxical roles in breast cancer because it has both tumor suppressor and tumor promoter activity. Another member of the TGFβ superfamily, activin, also has roles in the developing mammary gland, but these functions, and the role of activin in breast cancer, are not well characterized. TGFβ and activin share the same intracellular signaling pathways, but divergence in their signaling pathways are suggested. The purpose of this review is to compare the spatial and temporal expression of TGFβ and activin during mammary gland development, with consideration given to their functions during each developmental period. We also review the contributions of TGFβ and activin to breast cancer resistance and susceptibility. Finally, we consider the systemic contributions of activin in regulating obesity and diabetes; and the impact this regulation has on breast cancer. Elevated levels of activin in serum during pregnancy and its influence on pregnancy associated breast cancer are also considered. We conclude that evidence demonstrates that activin has tumor suppressing potential, without definitive indication of tumor promoting activity in the mammary gland, making it a good target for development of therapeutics.

Noncanonical TGF-β signaling during mammary tumorigenesis

Breast cancer is a heterogeneous disease comprised of at least five major tumor subtypes that coalesce as the second leading cause of cancer death in women in the United States. Although metastasis clearly represents the most lethal characteristic of breast cancer, our understanding of the molecular mechanisms that govern this event remains inadequate. Clinically, ~30% of breast cancer patients diagnosed with early-stage disease undergo metastatic progression, an event that (a) severely limits treatment options, (b) typically results in chemoresistance and low response rates, and (c) greatly contributes to aggressive relapses and dismal survival rates. Transforming growth factor-β (TGF-β) is a pleiotropic cytokine that regulates all phases of postnatal mammary gland development, including branching morphogenesis, lactation, and involution. TGF-β also plays a prominent role in suppressing mammary tumorigenesis by preventing mammary epithelial cell (MEC) proliferation, or by inducing MEC apoptosis. Genetic and epigenetic events that transpire during mammary tumorigenesis conspire to circumvent the tumor suppressing activities of TGF-β, thereby permitting late-stage breast cancer cells to acquire invasive and metastatic phenotypes in response to TGF-β. Metastatic progression stimulated by TGF-β also relies on its ability to induce epithelial-mesenchymal transition (EMT) and the expansion of chemoresistant breast cancer stem cells. Precisely how this metamorphosis in TGF-β function comes about remains incompletely understood; however, recent findings indicate that the initiation of oncogenic TGF-β activity is contingent upon imbalances between its canonical and noncanonical signaling systems. Here we review the molecular and cellular contributions of noncanonical TGF-β effectors to mammary tumorigenesis and metastatic progression.

Wnt5a as an effector of TGFβ in mammary development and cancer

Wnt5a is a member of the Wingless-related/MMTV-integration family of secreted growth factors, which are involved in a wide range of cellular processes. Wnt signaling can be broadly divided into two categories the canonical, ß-catenin-dependent pathway and the non-canonical ß-catenin-independent pathway. Wnt5a is a non-canonical signaling member of the Wnt family. Loss of Wnt5a is associated with early relapse of invasive breast cancer, increased metastasis, and poor survival in humans. It has been shown that TGF-ß directly regulates expression of Wnt5a in mammary gland and that Wnt5a mediates the effects of TGF-ß on branching during mammary gland development. Here we review the evidence suggesting Wnt5a acts as an effector of TGF-ß actions in breast cancer. It is suggested that the tumor suppressive functions of TGF-ß involve Wnt5a-mediated antagonism of Wnt/ß-catenin signaling and limiting the stem cell population. Interactions between TGF-ß and Wnt5a in metastasis appear to be more complex, and may depend on specific cues from the microenvironment as well as activation of specific intracellular signaling pathways.

SNORD-host RNA Zfas1 is a regulator of mammary development and a potential marker for breast cancer

Long noncoding RNAs (lncRNAs) are increasingly recognized to play major regulatory roles in development and disease. To identify novel regulators in breast biology, we identified differentially regulated lncRNAs during mouse mammary development. Among the highest and most differentially expressed was a transcript (Zfas1) antisense to the 5' end of the protein-coding gene Znfx1. In vivo, Zfas1 RNA is localized within the ducts and alveoli of the mammary gland. Zfas1 intronically hosts three previously undescribed C/D box snoRNAs (SNORDs): Snord12, Snord12b, and Snord12c. In contrast to the general assumption that noncoding SNORD-host transcripts function only as vehicles to generate snoRNAs, knockdown of Zfas1 in a mammary epithelial cell line resulted in increased cellular proliferation and differentiation, while not substantially altering the levels of the SNORDs. In support of an independent function, we also found that Zfas1 is extremely stable, with a half-life >16 h. Expression analysis of the SNORDs revealed these were expressed at different levels, likely a result of distinct structures conferring differential stability. While there is relatively low primary sequence conservation between Zfas1 and its syntenic human ortholog ZFAS1, their predicted secondary structures have similar features. Like Zfas1, ZFAS1 is highly expressed in the mammary gland and is down-regulated in breast tumors compared to normal tissue. We propose a functional role for Zfas1/ ZFAS1 in the regulation of alveolar development and epithelial cell differentiation in the mammary gland, which, together with its dysregulation in human breast cancer, suggests ZFAS1 as a putative tumor suppressor gene.

Phenotypic transition maps of 3D breast acini obtained by imaging-guided agent-based modeling

We introduce an agent-based model of epithelial cell morphogenesis to explore the complex interplay between apoptosis, proliferation, and polarization. By varying the activity levels of these mechanisms we derived phenotypic transition maps of normal and aberrant morphogenesis. These maps identify homeostatic ranges and morphologic stability conditions. The agent-based model was parameterized and validated using novel high-content image analysis of mammary acini morphogenesis in vitro with focus on time-dependent cell densities, proliferation and death rates, as well as acini morphologies. Model simulations reveal apoptosis being necessary and sufficient for initiating lumen formation, but cell polarization being the pivotal mechanism for maintaining physiological epithelium morphology and acini sphericity. Furthermore, simulations highlight that acinus growth arrest in normal acini can be achieved by controlling the fraction of proliferating cells. Interestingly, our simulations reveal a synergism between polarization and apoptosis in enhancing growth arrest. After validating the model with experimental data from a normal human breast line (MCF10A), the system was challenged to predict the growth of MCF10A where AKT-1 was overexpressed, leading to reduced apoptosis. As previously reported, this led to non growth-arrested acini, with very large sizes and partially filled lumen. However, surprisingly, image analysis revealed a much lower nuclear density than observed for normal acini. The growth kinetics indicates that these acini grew faster than the cells comprising it. The in silico model could not replicate this behavior, contradicting the classic paradigm that ductal carcinoma in situ is only the result of high proliferation and low apoptosis. Our simulations suggest that overexpression of AKT-1 must also perturb cell-cell and cell-ECM communication, reminding us that extracellular context can dictate cellular behavior.

Polypeptide N-acetylgalactosaminyltransferase 6 disrupts mammary acinar morphogenesis through O-glycosylation of fibronectin

A high expression of short and immature O-glycans is one of the prominent features of breast cancer cells, which would be attributed to the upregulated expression of glycosyltransferases. Therefore, a detailed elucidation of glycosyltransferases and their substrate(s) may improve our understandings for their roles in mammary carcinogenesis. Here we report that overexpression of polypeptide N-acetylgalactosaminyltransferase 6 (GALNT6), a glycosyltransferase involved in the initial step of O-glycosylation, has transformational potentials through disruptive acinar morphogenesis and cellular changes similar to epithelial-to-mesenchymal transition in normal mammary epithelial cell, MCF10A. As one of the critical O-glycan substrates, we identified fibronectin that was O-glycosylated in vivo and thereby stabilized by GALNT6. Because knockdown of fibronectin abrogated the disruptive proliferation caused by introduction of GALNT6 into epithelial cells, our findings suggest that GALNT6-fibronectin pathway should be a critical component for breast cancer development and progression.

Immune cell location and function during post-natal mammary gland development

Post-natal mammary gland development requires complex interactions between the epithelial cells and various cell types within the stroma. Recent studies have illustrated the importance of immune cells and their mediators during the various stages of mammary gland development. However, the mechanisms by which these immune cells functionally contribute to mammary gland development are only beginning to be understood. This review provides an overview of the localization of immune cells within the mammary gland during the various stages of post-natal mammary gland development. Furthermore, recent studies are summarized that illustrate the mechanisms by which these cells are recruited to the mammary gland and their functional roles in mammary gland development.

Extracellular matrix composition reveals complex and dynamic stromal-epithelial interactions in the mammary gland

The mammary gland is an excellent model system to study the interplay between stroma and epithelial cells because of the gland's unique postnatal development and its distinct functional states. This review focuses on the contribution of the extracellular matrix (ECM) to stromal-epithelial interactions in the mammary gland. We describe how ECM physical properties, protein composition, and proteolytic state impact mammary gland architecture as well as provide instructive cues that influence the function of mammary epithelial cells during pubertal gland development and throughout adulthood. Further, based on recent proteomic analyses of mammary ECM, we describe known mammary ECM proteins and their potential functions, as well as describe several ECM proteins not previously recognized in this organ. ECM proteins are discussed in the context of the morphologically-distinct stromal subcompartments: the basal lamina, the intra- and interlobular stroma, and the fibrous connective tissue. Future studies aimed at in-depth qualitative and quantitative characterization of mammary ECM within these various subcompartments is required to better elucidate the function of ECM in normal as well as in pathological breast tissue.

The normal microenvironment directs mammary gland development

Normal development of the mammary gland is a multidimensional process that is controlled in part by its mammary microenvironment. The mammary microenvironment is a defined location that encompasses mammary somatic stem cells, neighboring signaling cells, the basement membrane and extracellular matrix, mammary fibroblasts as well as the intercellular signals produced and received by these cells. These dynamic signals take numerous forms including growth factors, steroids, cell-cell or cell-basement membrane physical interactions. Cellular growth and differentiation of the mammary gland throughout the developmental stages are regulated by changes in these signals and interactions. The purpose of this review is to summarize current information and research regarding the role of the mammary microenvironment during normal glandular development.

Linking changes in epithelial morphogenesis to cancer mutations using computational modeling

Most tumors arise from epithelial tissues, such as mammary glands and lobules, and their initiation is associated with the disruption of a finely defined epithelial architecture. Progression from intraductal to invasive tumors is related to genetic mutations that occur at a subcellular level but manifest themselves as functional and morphological changes at the cellular and tissue scales, respectively. Elevated proliferation and loss of epithelial polarization are the two most noticeable changes in cell phenotypes during this process. As a result, many three-dimensional cultures of tumorigenic clones show highly aberrant morphologies when compared to regular epithelial monolayers enclosing the hollow lumen (acini). In order to shed light on phenotypic changes associated with tumor cells, we applied the bio-mechanical IBCell model of normal epithelial morphogenesis quantitatively matched to data acquired from the non-tumorigenic human mammary cell line, MCF10A. We then used a high-throughput simulation study to reveal how modifications in model parameters influence changes in the simulated architecture. Three parameters have been considered in our study, which define cell sensitivity to proliferative, apoptotic and cell-ECM adhesive cues. By mapping experimental morphologies of four MCF10A-derived cell lines carrying different oncogenic mutations onto the model parameter space, we identified changes in cellular processes potentially underlying structural modifications of these mutants. As a case study, we focused on MCF10A cells expressing an oncogenic mutant HER2-YVMA to quantitatively assess changes in cell doubling time, cell apoptotic rate, and cell sensitivity to ECM accumulation when compared to the parental non-tumorigenic cell line. By mapping in vitro mutant morphologies onto in silico ones we have generated a means of linking the morphological and molecular scales via computational modeling. Thus, IBCell in combination with 3D acini cultures can form a computational/experimental platform for suggesting the relationship between the histopathology of neoplastic lesions and their underlying molecular defects.

Epithelial-mesenchymal transition in cancer: parallels between normal development and tumor progression

From the earliest stages of embryonic development, cells of epithelial and mesenchymal origin contribute to the structure and function of developing organs. However, these phenotypes are not always permanent, and instead, under the appropriate conditions, epithelial and mesenchymal cells convert between these two phenotypes. These processes, termed Epithelial-Mesenchymal Transition (EMT), or the reverse Mesenchymal-Epithelial Transition (MET), are required for complex body patterning and morphogenesis. In addition, epithelial plasticity and the acquisition of invasive properties without the full commitment to a mesenchymal phenotype are critical in development, particularly during branching morphogenesis in the mammary gland. Recent work in cancer has identified an analogous plasticity of cellular phenotypes whereby epithelial cancer cells acquire mesenchymal features that permit escape from the primary tumor. Because local invasion is thought to be a necessary first step in metastatic dissemination, EMT and epithelial plasticity are hypothesized to contribute to tumor progression. Similarities between developmental and oncogenic EMT have led to the identification of common contributing pathways, suggesting that the reactivation of developmental pathways in breast and other cancers contributes to tumor progression. For example, developmental EMT regulators including Snail/Slug, Twist, Six1, and Cripto, along with developmental signaling pathways including TGF-beta and Wnt/beta-catenin, are misexpressed in breast cancer and correlate with poor clinical outcomes. This review focuses on the parallels between epithelial plasticity/EMT in the mammary gland and other organs during development, and on a selection of developmental EMT regulators that are misexpressed specifically during breast cancer.

The epigenetic landscape of mammary gland development and functional differentiation

Most of the development and functional differentiation in the mammary gland occur after birth. Epigenetics is defined as the stable alterations in gene expression potential that arise during development and proliferation. Epigenetic changes are mediated at the biochemical level by the chromatin conformation initiated by DNA methylation, histone variants, post-translational modifications of histones, non-histone chromatin proteins, and non-coding RNAs. Epigenetics plays a key role in development. However, very little is known about its role in the developing mammary gland or how it might integrate the many signalling pathways involved in mammary gland development and function that have been discovered during the past few decades. An inverse relationship between marks of closed (DNA methylation) or open chromatin (DnaseI hypersensitivity, certain histone modifications) and milk protein gene expression has been documented. Recent studies have shown that during development and functional differentiation, both global and local chromatin changes occur. Locally, chromatin at distal regulatory elements and promoters of milk protein genes gains a more open conformation. Furthermore, changes occur both in looping between regulatory elements and attachment to nuclear matrix. These changes are induced by developmental signals and environmental conditions. Additionally, distinct epigenetic patterns have been identified in mammary gland stem and progenitor cell sub-populations. Together, these findings suggest that epigenetics plays a role in mammary development and function. With the new tools for epigenomics developed in recent years, we now can begin to establish a framework for the role of epigenetics in mammary gland development and disease.

[Comparative observation on acupoint catgut embedding and electroacupuncture in treatment of hyperplasia of mammary glands]

OBJECTIVE

To observe the effect difference between the catgut embedding method and electroacupuncture in treatment of hyperplasia of mammary glands.

METHODS

Eighty-six cases were divided into a catgut embedding group and an electroacupuncture group according to their treatment order, including 43 cases in each group. Both groups were treated with such points as Danzhong (CV 17), Qihai (CV 6), Jianjing (GB 21), Qihu (ST 13) etc. The treatment in the catgut embedding group was carried out every 1-2 weeks and 8 times as a course of treatment; the treatment in the electroacupuncture group was carried out every other day, 24 times as a course of treatment; the effects were analyzed respectively at the end of the first course and 3 months after the end of the second course of treatment.

RESULTS

The total effective rate was 69.8% (30/43) in the catgut embedding group and 88.4% (38/43) in the electroacupuncture group after the first course of treatment, there was significant difference between the two groups (P<0.05); the cured-markedly effective rate of catgut embedding group was 87.8% (36/41) compared with 69.7% (23/33) in the electroacupuncture group after the two courses of treatment, which was significant difference (P<0.01).

CONCLUSION

Both the catgut embedding and electroacupuncture therapies have good clinical effects in treatment of hyperplasia of mammary glands. The therapeutic effect is better in the electroacupuncture group after the first therapeutic course while that of acupoint catgut embedding is better after two therapeutic courses.

Hormones and progeny of breast tumor cells

The rudimentary human glandular breast, with the approach of puberty, starts to grow both at glandular and stromal sites. Full differentiation is a gradual process and takes many years, and is only fully attained by pregnancy. The risk of breast cancer is inversely related to parity. Women during adolescence have the highest susceptibility to breast cancer development. This appears to be the period when the mammary gland has the highest number of stem cells. Stem cells may represent important targets for transformational events. Immunohistochemistry allows for identification of the lineage-specific precursor glandular and myoepithelial cells and their differentiated progeny. Both estrogen receptor subtypes are found in epithelial cells of alveoli and ducts as well as in stromal cells. Immunophenotypia of benign proliferative breast disease favors a fundamentally different epithelial composition from that of most malignant epithelial proliferations such as atypical ductal hyperplasia, ductal carcinoma in situ, lobular neoplasia and invasive breast carcinoma. Immunophenotypical characterization of these lesions assists in distinguishing benign from malignant disease. Based on the observation of bilateral risks and frequent multifocality with atypical ductal hyperplasia, atypical lobular hyperplasia and lobular carcinoma in situ, it is suggested that these may represent risk factors as well as precursors. One should, however, realize that ductal as well as lobular premalignant breast lesions ultimately arise from stem cells in the terminal duct lobular units. Estrogen receptor-beta (ERbeta)-positive and ERalpha-negative expression characterizes the highest levels of proliferative cancer cell activity. Point mutations and alterations of co-activators and co-repressors will also determine hormone sensitivity. There is evidence for different genetic pathways in the development of ductal carcinoma in situ and lobular carcinoma in situ. While they share recurrent 16q losses, a second hit in the E-cadherin gene explains the advent of lobular lesions and their common existence with the primary ductal type. Based on our immunocytochemical observations, the most likely target cell of malignant transformation is the Ck18/18-positive and ER-negative transient cell of normal breast epithelium. Pregnancy confers a different genomic imprint to breast epithelial stem cells. Further elucidation of this mechanism may assist in developing appropriate means of breast cancer prevention. This paper is the Pieter van Keep Memorial Lecture given by Professor Hermann Schneider at the 11th World Congress on the Menopause, October 18-22, 2005, in Buenos Aires, Argentina.

[Nongenomic mechanisms of progesterone]

Rapid, independent of transcriptional effects of progesterone have been observed in various types of cells, tissues and species. In some biological systems, these nongenomic actions and associated with them signal transduction pathways are characterized in detail at molecular level. This review summarizes findings concerning the role of progestins in the regulation of such physiological functions and processes as meiotic maturation of fish and amphibian oocytes; growth and proliferation of normal and transformed cells of mammary gland; contraction of myometrium; survival and functional activity of granulose cells; sperm capacitation, acrosome reaction and hypermotility; immune function of T lymphocytes; survival and function of brain cells. The participation of several types of receptor proteins in the nongenomic progesterone signaling is discussed. They include the classic nuclear progesterone receptor, PR, the membrane progestin receptor, mPR, the progesterone membrane receptor component, PGMRC, the oxytocin receptor, OTR, and the GABA receptor, GABA(A).

Androgens and the breast

Androgens have important physiological effects in women while at the same time they may be implicated in breast cancer pathologies. However, data on the effects of androgens on mammary epithelial proliferation and/or breast cancer incidence are not in full agreement. We performed a literature review evaluating current clinical, genetic and epidemiological data regarding the role of androgens in mammary growth and neoplasia. Epidemiological studies appear to have significant methodological limitations and thus provide inconclusive results. The study of molecular defects involving androgenic pathways in breast cancer is still in its infancy. Clinical and nonhuman primate studies suggest that androgens inhibit mammary epithelial proliferation and breast growth while conventional estrogen treatment suppresses endogenous androgens. Abundant clinical evidence suggests that androgens normally inhibit mammary epithelial proliferation and breast growth. Suppression of androgens using conventional estrogen treatment may thus enhance estrogenic breast stimulation and possibly breast cancer risk. Addition of testosterone to the usual hormone therapy regimen may diminish the estrogen/progestin increase in breast cancer risk but the impact of this combined use on mammary gland homeostasis still needs evaluation.

The impact of transgenic IGF-IR overexpression on mammary development and tumorigenesis

Insulin-like growth factor-I and -II (IGF-I and IGF-II) and/or the type I insulin-like growth factor receptor (IGF-IR) have been implicated in a number of human tumors including breast cancer. However, despite being implicated in breast cancer for approximately 25 years and given that transgenic technology has been available for about the same period of time, it is surprising that transgenic mice overexpressing the IGF-IR in the mammary gland have only recently been characterized. This review will describe the effects of IGF-IR overexpression on mammary ductal morphogenesis and mammary tumorigenesis in the two available transgenic models.

Insulin receptor substrates (IRSs) and breast tumorigenesis

Insulin receptor substrate (IRS)-1 and IRS-2 are adaptor proteins in the insulin-like growth factor I (IGF-I)/IGF-I receptor (IGF-IR) pathway that mediate cell proliferation, migration, and survival. In addition to their role as scaffolding proteins in the cytoplasm, they are able to translocate into the nucleus and regulate gene transcription. IRS levels are developmentally and hormonally regulated in the normal mammary gland and both are essential for normal mammary gland bud formation and lactation. Both IRS-1 and IRS-2 are transforming oncogenes, and induce transformation and metastasis in vitro and in vivo. In breast cancer IRSs have unique functions, with IRS-1 being mainly involved in cell proliferation and survival, whereas IRS-2 has clear roles in cell migration and metastasis. In this review we will discuss the roles of IRSs in mammary gland development and breast cancer.

Role of insulin-like growth factor binding proteins in mammary gland development

Insulin-like growth factors (IGFs) play an important role in mammary gland development and their effects are, in turn, influenced by a family of 6 IGF-binding proteins (IGFBPs). The IGFBPs are expressed in time- and tissue-specific fashion during the periods of rapid growth and involution of the mammary gland. The precise roles of these proteins in vivo have, however, been difficult to determine. This review examines the indirect evidence (evolution, chromosomal location and roles in lower life-forms) the evidence from in vitro studies and the attempts to examine their roles in vivo, using IGFBP-deficient and over-expression models. Evidence exists for a role of the IGFBPs in inhibition of the survival effects of IGFs as well as in IGF-enhancing effects from in vitro studies. The location of the IGFBPs, often associated with the extracellular matrix, suggests roles as a reservoir of IGFs or as a potential barrier, restricting access of IGFs to distinct cellular compartments. We also discuss the relative importance of IGF-dependent versus IGF-independent effects. IGF-independent effects include nuclear localization, activation of proteases and interaction with a variety of extracellular matrix and cell surface proteins. Finally, we examine the increasing evidence for the IGFBPs to be considered as part of a larger family of extracellular matrix proteins involved in morphogenesis and tissue re-modeling.

IGF ligand and receptor regulation of mammary development

The insulin-like growth factors, IGF-I and IGF-II, have endocrine as well as autocrine-paracrine actions on tissue growth. Both IGF ligands are expressed within developing mammary tissue throughout postnatal stages with specific sites of expression in the epithelial and stromal compartments. The elucidation of circulating versus local actions and of epithelial versus stromal actions of IGFs in stimulating mammary epithelial development has been the focus of several laboratories. The recent studies addressing IGF ligand function provide support for the hypotheses that (1) the diverse sites of IGF expression may mediate different cellular outcomes, and (2) IGF-I and IGF-II are distinctly regulated and have diverse functions in mammary development. The mechanisms for IGF function likely are mediated, in part, through diverse IGF signaling receptors. The local actions of the IGF ligands and receptors as revealed through recent publications are the focus of this review.

Role of autophagy in mammary gland development

Autophagy is a highly conserved catabolic process responsible for degradation and recycling of long-lived proteins and organelles by lysosomes. This degradative pathway, together with proteasome system is particularly important during development and under certain environmental stress conditions. This review summarizes the latest achievements of studies aiming to explore the role of autophagy in development and differentiation of eukaryotic cells. It shows the importance of this process in the development of lower eukaryotic organisms such as Dicyostelium discoideum, and Caenorhabditis elegans, as well as functions of autophagy and autophagy related genes (Atg) in development and differentiation of higher eukaryotic organisms. The review is focused on the results of studies conducted on mammary gland, as it is a good model for studying the mechanisms controlling higher eukaryotic organisms' development. Studies have shown that autophagy is involved in the removal of epithelial cells during formation of alveolar structures, indicating its role in mammogenesis. There are also evidences of involvement of Atg's in epithelial tumors development. Context dependent manipulations of autophagic pathways may create more effective anticancer therapies in the future.

IGF-I, GH, and sex steroid effects in normal mammary gland development

Although the pubertal surge of estrogen is the immediate stimulus to mammary development, the action of estrogen depends upon the presence of pituitary growth hormone and the ability of GH to stimulate production of IGF-I in the mammary gland. Growth hormone binds to its receptor in the mammary fat pad, after which production of IGF-I mRNA and IGF-I protein occurs. It is likely that IGF-I then works through paracrine means to stimulate formation of TEBs, which then form ducts by bifurcating or trifurcating and extending through the mammary fat pad. By the time pubertal development is complete a tree-like structure of branching ducts fills the rodent mammary fat pad. In addition to requiring IGF-I in order to act, estradiol also directly synergizes with IGF-I to enhance formation of TEBs and ductal morphogenesis. Together they increase IRS-1 phosphorylation and cell proliferation, and inhibit apoptosis. In fact, the entire process of ductal morphogenesis, in oophorectomized IGF-I(-/-) knockout female mice, can occur as a result of the combined actions of estradiol and IGF-I. IGF-I also permits progesterone action in the mammary gland. Together they have been shown to stimulate a form of ductal morphogenesis, which is anatomically different from the kind induced by IGF-I and estradiol. Although both progesterone and estradiol synergize with IGF-I by increasing IGF-I action parameters, there must be other, as yet unknown mechanisms that account for the anatomical differences in the different forms of ductal morphogenesis observed (hyperplasia in response to IGF-I plus estradiol and single layered ducts in response to IGF-I plus progesterone).

Minireview: hormones and mammary cell fate--what will I become when I grow up?

Systemic hormones are key regulators of postnatal mammary gland development and play an important role in the etiology and treatment of breast cancer. Mammary ductal morphogenesis is controlled by circulating hormones, and these same hormones are also critical mediators of mammary stem cell fate decisions. Recent studies have helped further our understanding of the origin, specification, and fate of mammary stem cells during postnatal development. Here we review recent studies on the involvement of hormone receptors and several transcription factors in mammary stem/progenitor cell differentiation and lineage commitment.