Persistent parity-induced changes in growth factors, TGF-beta3, and differentiation in the rodent mammary gland

Dual role of pregnancy in breast cancer risk

Reproductive history is one of the strongest risk factors for breast cancer in women. Pregnancy can promote short-term breast cancer risk, but also reduce a woman's lifetime risk of breast cancer. Changes in hormone levels before and after pregnancy are one of the key factors in breast cancer risk. This article summarizes the changes in hormone levels before and after pregnancy, and the roles of hormones in mammary gland development and breast cancer progression. Other factors, such as changes in breast morphology and mammary gland differentiation, changes in the proportion of mammary stem cells (MaSCs), changes in the immune and inflammatory environment, and changes in lactation before and after pregnancy, also play key roles in the occurrence and development of breast cancer. This review discusses the dual effects and the potential mechanisms of pregnancy on breast cancer risk from the above aspects, which is helpful to understand the complexity of female breast cancer occurrence.

Effects of butyl benzyl phthalate exposure during pregnancy and lactation on the post-involution mammary gland

The mammary gland undergoes comprehensive reorganization during pregnancy, lactation, and subsequent involution. Following involution, the mammary gland has structural and functional differences compared to the gland of a nulliparous female. These parity-associated changes are regulated by hormones and may be vulnerable to endocrine-disrupting chemicals (EDCs). In this study, we evaluated the long-term effects of butyl benzyl phthalate (BBP), an estrogenic plasticizer, on the parous mouse mammary gland. Pregnant BALB/c mice were treated with 0, 3, 500, or 18000 µg/kg/day BBP throughout both pregnancy and the lactational period. The litters born to these females were evaluated for litter size and growth. The parous females were then kept for five weeks following weaning of the pups, during which period there was no exposure to BBP. After five weeks of post-weaning, mammary glands were collected and assessed for changes in histomorphology, steroid receptor expression, innate immune cell number, and gene expression. An unexposed age-matched nulliparous control was also evaluated as a comparator group. BBP increased male and female pup weight at puberty and female offspring in adulthood. BBP also altered innate immune cells in the post-involution mammary gland, reducing the effect of parity on macrophages. Lastly, BBP modestly increased mammary gland ductal complexity and periductal structure, but had no effect on expression of estrogen receptor, progesterone receptor, or a marker of proliferation. These results suggest that BBP may interfere with some effects of parity on the mouse mammary gland and induce weight gain in exposed offspring.

DNA methylation and gene expression changes in mouse mammary tissue during successive lactations: part II - the impact of lactation rank

Mastitis is among the main reasons women cease breastfeeding. In farm animals, mastitis results in significant economic losses and the premature culling of some animals. Nevertheless, the effect of inflammation on the mammary gland is not completely understood. This article discusses the changes to DNA methylation in mouse mammary tissue caused by lipopolysaccharide-induced inflammation after in vivo intramammary challenges and the differences in DNA methylation between 1^st and 2^nd lactations. Lactation rank induces 981 differential methylations of cytosines (DMCs) in mammary tissue. Inflammation in 1^st lactation compared to inflammation in 2^nd lactation results in the identification of 964 DMCs. When comparing inflammation in 1^st vs. 2^nd lactations with previous inflammation history, 2590 DMCs were identified. Moreover, Fluidigm PCR data show changes in the expression of several genes related to mammary function, epigenetic regulation, and the immune response. We show that the epigenetic regulation of two successive physiological lactations is not the same in terms of DNA methylation and that the effect of lactation rank on DNA methylation is stronger than that of the onset of inflammation. The conditions presented here show that few DMCs are shared between comparisons, suggesting a specific epigenetic response depending on lactation rank, the presence of inflammation, and even whether the cells had previously suffered inflammation. In the long term, this information could lead to a better understanding of the epigenetic regulation of lactation in both physiological and pathological conditions.Abbreviations: RRBS, reduced representation bisulphite sequencing; RT-qPCR, real-time quantitative polymerase chain reaction; MEC, mammary epithelial cells; MaSC, mammary stem cell; TSS, transcription start site; TTS, transcription termination site; UTR, untranslated region; SINE, short interspersed nuclear element; LINE, long interspersed nuclear element; CGI, CpG island; DEG, differentially expressed gene; DMC, differentially methylated cytosine; DMR, differentially methylated region; GO term, gene ontology term; MF, molecular function; BP, biological process.

Host, reproductive, and lifestyle factors in relation to quantitative histologic metrics of the normal breast

BACKGROUND

Emerging data indicate that variations in quantitative epithelial and stromal tissue composition and their relative abundance in benign breast biopsies independently impact risk of future invasive breast cancer. To gain further insights into breast cancer etiopathogenesis, we investigated associations between epidemiological factors and quantitative tissue composition metrics of the normal breast.

METHODS

The study participants were 4108 healthy women ages 18-75 years who voluntarily donated breast tissue to the US-based Susan G. Komen Tissue Bank (KTB; 2008-2019). Using high-accuracy machine learning algorithms, we quantified the percentage of epithelial, stromal, adipose, and fibroglandular tissue, as well as the proportion of fibroglandular tissue that is epithelium relative to stroma (i.e., epithelium-to-stroma proportion, ESP) on digitized hematoxylin and eosin (H&E)-stained normal breast biopsy specimens. Data on epidemiological factors were obtained from participants using a detailed questionnaire administered at the time of tissue donation. Associations between epidemiological factors and square root transformed tissue metrics were investigated using multivariable linear regression models.

RESULTS

With increasing age, the amount of stromal, epithelial, and fibroglandular tissue declined and adipose tissue increased, while that of ESP demonstrated a bimodal pattern. Several epidemiological factors were associated with individual tissue composition metrics, impacting ESP as a result. Compared with premenopausal women, postmenopausal women had lower ESP [β (95% Confidence Interval (CI)) = -0.28 (- 0.43, - 0.13); P < 0.001] with ESP peaks at 30-40 years and 60-70 years among pre- and postmenopausal women, respectively. Pregnancy [β (95%CI) vs nulligravid = 0.19 (0.08, 0.30); P < 0.001] and increasing number of live births (P-trend < 0.001) were positively associated with ESP, while breastfeeding was inversely associated with ESP [β (95%CI) vs no breastfeeding = -0.15 (- 0.29, - 0.01); P = 0.036]. A positive family history of breast cancer (FHBC) [β (95%CI) vs no FHBC = 0.14 (0.02-0.26); P = 0.02], being overweight or obese [β (95%CI) vs normal weight = 0.18 (0.06-0.30); P = 0.004 and 0.32 (0.21-0.44); P < 0.001, respectively], and Black race [β (95%CI) vs White = 0.12 (- 0.005, 0.25); P = 0.06] were positively associated with ESP.

CONCLUSION

Our findings revealed that cumulative exposure to etiological factors over the lifespan impacts normal breast tissue composition metrics, individually or jointly, to alter their dynamic equilibrium, with potential implications for breast cancer susceptibility and tumor etiologic heterogeneity.

Pleiotrophin and metabolic disorders: insights into its role in metabolism

Pleiotrophin (PTN) is a cytokine which has been for long studied at the level of the central nervous system, however few studies focus on its role in the peripheral organs. The main aim of this review is to summarize the state of the art of what is known up to date about pleiotrophin and its implications in the main metabolic organs. In summary, pleiotrophin promotes the proliferation of preadipocytes, pancreatic β cells, as well as cells during the mammary gland development. Moreover, this cytokine is important for the structural integrity of the liver and the neuromuscular junction in the skeletal muscle. From a metabolic point of view, pleiotrophin plays a key role in the maintenance of glucose and lipid as well as whole-body insulin homeostasis and favors oxidative metabolism in the skeletal muscle. All in all, this review proposes pleiotrophin as a druggable target to prevent from the development of insulin-resistance-related pathologies.

The Mammary Gland: Basic Structure and Molecular Signaling during Development

The mammary gland is a compound, branched tubuloalveolar structure and a major characteristic of mammals. The mammary gland has evolved from epidermal apocrine glands, the skin glands as an accessory reproductive organ to support postnatal survival of offspring by producing milk as a source of nutrition. The mammary gland development begins during embryogenesis as a rudimentary structure that grows into an elementary branched ductal tree and is embedded in one end of a larger mammary fat pad at birth. At the onset of ovarian function at puberty, the rudimentary ductal system undergoes dramatic morphogenetic change with ductal elongation and branching. During pregnancy, the alveolar differentiation and tertiary branching are completed, and during lactation, the mature milk-producing glands eventually develop. The early stages of mammary development are hormonal independent, whereas during puberty and pregnancy, mammary gland development is hormonal dependent. We highlight the current understanding of molecular regulators involved during different stages of mammary gland development.

Cellular dormancy in minimal residual disease following targeted therapy

BACKGROUND

Breast cancer mortality is principally due to tumor recurrence, which can occur following extended periods of clinical remission that may last decades. While clinical latency has been postulated to reflect the ability of residual tumor cells to persist in a dormant state, this hypothesis remains unproven since little is known about the biology of these cells. Consequently, defining the properties of residual tumor cells is an essential goal with important clinical implications for preventing recurrence and improving cancer outcomes.

METHODS

To identify conserved features of residual tumor cells, we modeled minimal residual disease using inducible transgenic mouse models for HER2/neu and Wnt1-driven tumorigenesis that recapitulate cardinal features of human breast cancer progression, as well as human breast cancer cell xenografts subjected to targeted therapy. Fluorescence-activated cell sorting was used to isolate tumor cells from primary tumors, residual lesions following oncogene blockade, and recurrent tumors to analyze gene expression signatures and evaluate tumor-initiating cell properties.

RESULTS

We demonstrate that residual tumor cells surviving oncogenic pathway inhibition at both local and distant sites exist in a state of cellular dormancy, despite adequate vascularization and the absence of adaptive immunity, and retain the ability to re-enter the cell cycle and give rise to recurrent tumors after extended latency periods. Compared to primary or recurrent tumor cells, dormant residual tumor cells possess unique features that are conserved across mouse models for human breast cancer driven by different oncogenes, and express a gene signature that is strongly associated with recurrence-free survival in breast cancer patients and similar to that of tumor cells in which dormancy is induced by the microenvironment. Although residual tumor cells in both the HER2/neu and Wnt1 models are enriched for phenotypic features associated with tumor-initiating cells, limiting dilution experiments revealed that residual tumor cells are not enriched for cells capable of giving rise to primary tumors, but are enriched for cells capable of giving rise to recurrent tumors, suggesting that tumor-initiating populations underlying primary tumorigenesis may be distinct from those that give rise to recurrence following therapy.

CONCLUSIONS

Residual cancer cells surviving targeted therapy reside in a well-vascularized, desmoplastic microenvironment at both local and distant sites. These cells exist in a state of cellular dormancy that bears little resemblance to primary or recurrent tumor cells, but shares similarities with cells in which dormancy is induced by microenvironmental cues. Our observations suggest that dormancy may be a conserved response to targeted therapy independent of the oncogenic pathway inhibited or properties of the primary tumor, that the mechanisms underlying dormancy at local and distant sites may be related, and that the dormant state represents a potential therapeutic target for preventing cancer recurrence.

Exposure to Propylparaben During Pregnancy and Lactation Induces Long-Term Alterations to the Mammary Gland in Mice

The mammary gland is a hormone sensitive organ that is susceptible to endocrine-disrupting chemicals (EDCs) during the vulnerable periods of parous reorganization (ie, pregnancy, lactation, and involution). Pregnancy is believed to have long-term protective effects against breast cancer development; however, it is unknown if EDCs can alter this effect. We examined the long-term effects of propylparaben, a common preservative used in personal care products and foods, with estrogenic properties, on the parous mouse mammary gland. Pregnant BALB/c mice were treated with 0, 20, 100, or 10 000 µg/kg/day propylparaben throughout pregnancy and lactation. Unexposed nulliparous females were also evaluated. Five weeks post-involution, mammary glands were collected and assessed for changes in histomorphology, hormone receptor expression, immune cell number, and gene expression. For several parameters of mammary gland morphology, propylparaben reduced the effects of parity. Propylparaben also increased proliferation, but not stem cell number, and induced modest alterations to expression of ERα-mediated genes. Finally, propylparaben altered the effect of parity on the number of several immune cell types in the mammary gland. These results suggest that propylparaben, at levels relevant to human exposure, can interfere with the effects of parity on the mouse mammary gland and induce long-term alterations to mammary gland structure. Future studies should address if propylparaben exposures negate the protective effects of pregnancy on mammary cancer development.

External and internal EGFR-activating signals drive mammary epithelial cells proliferation and viability

During puberty, the mammary gland undergoes an intense growth, dependent on the interplay between the Epidermal Growth Factor Receptor (EGFR) in the stroma and different mammary epithelial receptors. We hypothesize that EGFR expressed in the mammary epithelium also has a role in puberty and the epithelial cells can self-sustain by EGFR-mediated autocrine signaling. We adopted mammary cell lines from different species, as in vitro model for the epithelium, and we observed that EGFR-signaling positively affects their survival and proliferation. Once deprived of external growth factors, mammary cells still showed strong Erk 1/2 phosphorylation, abolished upon EGFR inhibition, coupled with a further reduction in survival and proliferation. Based on gene expression analysis, three EGFR-ligands (AREG, EREG and HBEGF) are likely to mediate this autocrine signaling. In conclusion, internal EGFR-activating signals sustain mammary epithelial cell proliferation and survival in vitro.

Cancer and Cardiovascular Risk in Women With Hypertensive Disorders of Pregnancy Carrying a Common IGF1R Variant

OBJECTIVE

To evaluate the impact of insulin-like growth factor 1 receptor variant rs2016347 on the risk for breast and nonbreast cancers and cardiovascular disease in women with a history of hypertensive disorders of pregnancy (HDP).

PATIENTS AND METHODS

This retrospective cohort study included all parous women in the UK Biobank with prior rs2016347 genotyping (N=204,155), with enrollment taking place from March 2006 to July 2010. History of HDP was self-reported, and outcomes included breast and all nonbreast cancers, hospital diagnoses of hypertension and cardiovascular disease, and direct blood pressure measurements.

RESULTS

Women with previous HDP had a higher risk for future hypertension and cardiovascular diagnoses, increased blood pressures, and lower risk for breast cancer compared with women without HDP, consistent with prior studies. Hazard ratios for all nonbreast cancers were unchanged. However, when taking genotype into account, HDP-positive women carrying at least 1 thymine (T) allele of rs2016347 had a lower risk for nonbreast cancer (hazard ratio, 0.59; 95% CI, 0.37 to 0.92; P=.02) and lower systolic blood pressure (-2.08±0.98 mm Hg; P=.03) compared with women with the guanine/guanine (GG) genotype with positive evidence of interaction (HDP:T allele) for both outcomes; P=.04 and P=.03, respectively.

CONCLUSION

Women who experience HDP and carry a T allele of rs2016347 have 41% lower risk for developing nonbreast cancer and a lower systolic blood pressure of 2.08 mm Hg when compared with those with the GG genotype, suggesting a possible role of the insulin-like growth factor 1 axis for both cardiovascular and cancer risk in women with HDP.

Physiologic changes in serotonin concentrations in breast milk during lactation

OBJECTIVES

Serotonin (5-hydroxytryptamine; 5-HT) plays an important role in milk volume homeostasis in the mammary glands during lactation, and 5-HT in milk also may affect infant development. The aim of this study was to investigate changes in 5-HT concentration in breast milk according to the duration of lactation and evaluate whether the 5-HT concentration varied before and after nursing.

METHODS

Healthy nursing Japanese women who had a natural delivery or underwent a cesarean delivery at Iwate Medical University Hospital were included in this study.

RESULTS

The mean 5-HT concentration in milk was obtained from multiparous mothers 6 to 7 d after delivery (colostrum) and was significantly higher compared with primiparous mothers (24.3 ± 2.63 versus 18.5 ± 2.60 ng/mL). Additionally, mean 5-HT concentration increased with increasing lactation duration in primiparous women (colostrum: 18.5 ± 2.60; 1 mo postdelivery: 19.8 ± 2.46; 3 mo postdelivery: 22.7 ± 2.55 ng/mL); in particular, the mean 5-HT concentration in breast milk 3 mo after delivery was significantly higher than in colostrum. The mean 5-HT concentrations in breast milk in primiparous mothers immediately before nursing, 1 to 2 h after nursing, and immediately before the next nursing event were 23.6 ± 1.48, 22.82 ± 1.65, and 21.84 ± 1.31 ng/mL, respectively; mean 5-HT concentrations in multiparous women were 25.4 ± 1.65, 23.6 ± 2.20, or 22.4 ± 2.09 ng/mL, respectively. There was no significant difference in 5-HT concentrations at each time point between the groups.

CONCLUSION

This information may be useful in determining the role of 5-HT in breast milk on infant development and growth.

Pregnancy-Associated Breast Cancer: A Multidisciplinary Approach

The diagnosis of breast cancer (BC) during pregnancy is uncommon. It has varied among different studies from 1:10,000 to 1:3000 of all pregnancies, with a median age of 33 years. Pregnancy-associated BC represents a challenge in terms of clinical management to guarantee both maternal and fetal security in choosing the right treatment. This situation is complex and requires a multidisciplinary approach, including the surgeon, anesthesiologist, oncologist, radiotherapist, psychologist, and maternal-fetal medicine specialist. In the present review, we examined the management of pregnancy-associated BC, focusing on pathophysiologic background, risk factors, diagnosis, staging procedures, anesthesia, surgical management, and systemic treatment.

Pregnancy reprograms the epigenome of mammary epithelial cells and blocks the development of premalignant lesions

Pregnancy causes a series of cellular and molecular changes in mammary epithelial cells (MECs) of female adults. In addition, pregnancy can also modify the predisposition of rodent and human MECs to initiate oncogenesis. Here, we investigate how pregnancy reprograms enhancer chromatin in the mammary epithelium of mice and influences the transcriptional output of the oncogenic transcription factor cMYC. We find that pregnancy induces an expansion of the active cis-regulatory landscape of MECs, which influences the activation of pregnancy-related programs during re-exposure to pregnancy hormones in vivo and in vitro. Using inducible cMYC overexpression, we demonstrate that post-pregnancy MECs are resistant to the downstream molecular programs induced by cMYC, a response that blunts carcinoma initiation, but does not perturb the normal pregnancy-induced epigenomic landscape. cMYC overexpression drives post-pregnancy MECs into a senescence-like state, and perturbations of this state increase malignant phenotypic changes. Taken together, our findings provide further insight into the cell-autonomous signals in post-pregnancy MECs that underpin the regulation of gene expression, cellular activation, and resistance to malignant development.

Mammary epithelial cells are epigenetically modified during pregnancy, these changes can influence the pre-disposition to cancer. Here, the authors examine the epigenetic landscape of mammary epithelial cells pre and post pregnancy and identify changes to the epigenetic landscape, which can protect mice from Myc induced cancer.

Histology and Transcriptome Profiles of the Mammary Gland across Critical Windows of Development in Sprague Dawley Rats

Breast development occurs through well-defined stages representing 'windows of susceptibility' to adverse environmental exposures that potentially modify breast cancer risk. Systematic characterization of morphology and transcriptome during normal breast development lays the foundation of our understanding of cancer etiology. We examined mammary glands in female Sprague Dawley rats across six developmental stages - pre-pubertal, peri-pubertal, pubertal, lactation, adult parous and adult nulliparous. We investigated histology by Hematoxylin and Eosin and Mallory's Trichrome stain, proliferative and apoptotic rate by immunohistochemistry and whole-transcriptome by microarrays. We identified differentially expressed genes between adjacent developmental stages by linear models, underlying pathways by gene ontology analysis and gene networks and hubs active across developmental stages by coexpression network analysis. Mammary gland development was associated with large-scale changes in the transcriptome; particularly from pre-pubertal to peri-pubertal period and the lactation period were characterized by distinct patterns of gene expression with unique biological functions such as immune processes during pre-pubertal development and cholesterol biosynthesis during lactation. These changes were reflective of the shift in mammary gland histology, from a rudimentary organ during early stages to a secretory organ during lactation followed by regression with age. Hub genes within mammary gene networks included metabolic genes such as Pparg during the pre-pubertal stage and tight junction-related genes claudins and occludins in lactating mammary glands. Transcriptome profile paired with histology enhanced our understanding of mammary development, which is fundamental in understanding the etiologic mechanism of breast cancer, especially pertaining to windows of susceptibility to environmental exposures that may alter breast cancer risk.

Etiology of hormone receptor positive breast cancer differs by levels of histologic grade and proliferation

Limited epidemiological evidence suggests that the etiology of hormone receptor positive (HR+) breast cancer may differ by levels of histologic grade and proliferation. We pooled risk factor and pathology data on 5,905 HR+ breast cancer cases and 26,281 controls from 11 epidemiological studies. Proliferation was determined by centralized automated measures of KI67 in tissue microarrays. Odds ratios (OR), 95% confidence intervals (CI) and p-values for case-case and case-control comparisons for risk factors in relation to levels of grade and quartiles (Q1-Q4) of KI67 were estimated using polytomous logistic regression models. Case-case comparisons showed associations between nulliparity and high KI67 [OR (95% CI) for Q4 vs. Q1 = 1.54 (1.22, 1.95)]; obesity and high grade [grade 3 vs. 1 = 1.68 (1.31, 2.16)] and current use of combined hormone therapy (HT) and low grade [grade 3 vs. 1 = 0.27 (0.16, 0.44)] tumors. In case-control comparisons, nulliparity was associated with elevated risk of tumors with high but not low levels of proliferation [1.43 (1.14, 1.81) for KI67 Q4 vs. 0.83 (0.60, 1.14) for KI67 Q1]; obesity among women ≥50 years with high but not low grade tumors [1.55 (1.17, 2.06) for grade 3 vs. 0.88 (0.66, 1.16) for grade 1] and HT with low but not high grade tumors [3.07 (2.22, 4.23) for grade 1 vs. 0.85 (0.55, 1.30) for grade 3]. Menarcheal age and family history were similarly associated with HR+ tumors of different grade or KI67 levels. These findings provide insights into the etiologic heterogeneity of HR+ tumors.

Psychosocial Stress Exposure Disrupts Mammary Gland Development

Exposure to psychosocial stressors and ensuing stress physiology have been associated with spontaneous invasive mammary tumors in the Sprague-Dawley rat model of human breast cancer. Mammary gland (MG) development is a time when physiologic and environmental exposures influence breast cancer risk. However, the effect of psychosocial stress exposure on MG development remains unknown. Here, in the first comprehensive longitudinal study of MG development in nulliparous female rats (from puberty through young adulthood; 8-25 wks of age), we quantify the spatial gradient of differentiation within the MG of socially stressed (isolated) and control (grouped) rats. We then demonstrate that social isolation increased stress reactivity to everyday stressors, resulting in downregulation of glucocorticoid receptor (GR) expression in the MG epithelium. Surprisingly, given that chemical carcinogens increase MG cancer risk by preventing normal terminal end bud (TEB) differentiation, chronic isolation stress did not alter TEBs. Instead, isolation blunted MG growth and alveolobular differentiation and reduced epithelial cell proliferation in these structures. Social isolation also enhanced corpora luteal progesterone at all ages but reduced estrogenization only in early adulthood, a pattern that precludes modulated ovarian function as a sufficient mechanism for the effects of isolation on MG development. This longitudinal study of natural variation provides an integrated view of MG development and the importance of increased GR activation in nulliparous ductal growth and alveolobular differentiation. Thus, social isolation and its physiological sequelae disrupt MG growth and differentiation and suggest a contribution of stress exposure during puberty and young adulthood to the previously observed increase in invasive MG cancer observed in chronically socially-isolated adult Sprague-Dawley rats.

Oxybenzone Alters Mammary Gland Morphology in Mice Exposed During Pregnancy and Lactation

Hormones and endocrine-disrupting chemicals are generally thought to have permanent “organizational” effects when exposures occur during development but not adulthood. Yet, an increasing number of studies have shown that pregnant females are disrupted by endocrine-disrupting chemical exposures, with some effects that are permanent. Here, we examined the long-term effects of exposure to oxybenzone, an estrogenic chemical found in sunscreen and personal care products, on the morphology of the mammary gland in mice exposed during pregnancy and lactation. Female mice were exposed to vehicle or 30, 212, or 3000 µg oxybenzone/kg/d, from pregnancy day 0 until weaning. A nulliparous group, receiving vehicle treatment, was also evaluated. Mammary glands were collected 5 weeks after involution for whole-mount, histological, immunohistochemical, and molecular analyses. Exposure to 3000 µg oxybenzone/kg/d induced permanent changes to ductal density that was significantly different from both the nulliparous and vehicle groups. The two highest doses of oxybenzone similarly induced an intermediate phenotype for expression of progesterone receptor. A monotonic, dose-dependent increase in cell proliferation was also observed in the oxybenzone-treated females, becoming statistically significant at the highest dose. Finally, oxybenzone exposure induced an intermediate phenotype for Esr1 expression in all oxybenzone-treated groups. These data suggest that oxybenzone, at doses relevant to human exposures, produces long-lasting alterations to mammary gland morphology and function. Further studies are needed to determine if exposure to this chemical during pregnancy and lactation will interfere with the known protection that pregnancy provides against breast cancer.

The serum protein profile of early parity which induces protection against breast cancer

Early parity reduces the risk of breast cancer in women while nulliparity and late parity increase the risk of breast cancer. In order to translate this protection to women where early pregnancy is not feasible, much work has focused on understanding how parity confers protection against breast cancer, the molecular mechanisms by which this occurs is still not well understood. Healthy parous and nulliparous women were recruited for this study. We assessed serum protein profiles of early parous, late parous, and nulliparous women using the Phospho Explorer antibody array. Significantly altered proteins identified were validated by Western blot analysis. In silico analysis was performed with the data obtained. Our findings indicate increased phosphorylation levels of CDK1, AKT1 and Epo-R increased cell cycle and cell proliferation in late/nulliparous women. Increased levels of LIMK1, paxillin, caveolin-1, and tyrosine hydroxylase in late/nulliparous women demonstrate enhanced cell stress while decreased activity of p-p53 and pRAD51 in late/nulliparous women indicates decreased apoptosis and increased genomic instability. Further, increased levels of pFAK, pCD3zeta, pSTAT5B, MAP3K8 in early parous women favor enhanced innate/adaptive immunity. Overall, we have identified a unique protein signature that is responsible for the decreased risk of breast cancer and these proteins can also serve as biomarkers to predict the risk of breast cancer.

Quantitation of TGF-β proteins in mouse tissues shows reciprocal changes in TGF-β1 and TGF-β3 in normal vs neoplastic mammary epithelium

Transforming growth factor-βs (TGF-βs) regulate tissue homeostasis, and their expression is perturbed in many diseases. The three isoforms (TGF-β1, -β2, and -β3) have similar bioactivities in vitro but show distinct activities in vivo. Little quantitative information exists for expression of TGF-β isoform proteins in physiology or disease. We developed an optimized method to quantitate protein levels of the three isoforms, using a Luminex® xMAP®-based multianalyte assay following acid-ethanol extraction of tissues. Analysis of multiple tissues and plasma from four strains of adult mice showed that TGF-β1 is the predominant isoform with TGF-β2 being ~10-fold lower. There were no sex-specific differences in isoform expression, but some tissues showed inter-strain variation, particularly for TGF-β2. The only adult tissue expressing appreciable TGF-β3 was the mammary gland, where its levels were comparable to TGF-β1. In situ hybridization showed the luminal epithelium as the major source of all TGF-β isoforms in the normal mammary gland. TGF-β1 protein was 3-8-fold higher in three murine mammary tumor models than in normal mammary gland, while TGF-β3 protein was 2-3-fold lower in tumors than normal tissue, suggesting reciprocal regulation of these isoforms in mammary tumorigenesis.

The protective effect of longer duration of breastfeeding against pregnancy-associated triple negative breast cancer

Parity associated breast cancer (PABC) often diagnosed within the 2-5 years after a full term pregnancy. PABC is usually present with more advanced, poorly differentiated, high-grade cancers that show shorter time to progression and often of the triple negative breast cancer (TNBC) subtype. Data from around the world show that pregnancy-associated TNBC is independently associated with poor survival, underscoring the impact of the pregnant breast microenvironment on the biology and consequently the prognosis of these tumors. Although it is not yet clear, a link between pregnancy-associated TNBCs and lack or shorter duration of breastfeeding (not pregnancy per se) has been proposed. Here, we present epidemiological and experimental evidence for the protective effect of longer duration of lactation against pregnancy-associated TNBCs, and propose a putative molecular mechanism for this protective effect and its effect in eliminating any potential TNBC precursors from the breast by the end of the natural breast involution.

Rat models of 17β-estradiol-induced mammary cancer reveal novel insights into breast cancer etiology and prevention

Numerous laboratory and epidemiologic studies strongly implicate endogenous and exogenous estrogens in the etiology of breast cancer. Data summarized herein suggest that the ACI rat model of 17β-estradiol (E2)-induced mammary cancer is unique among rodent models in the extent to which it faithfully reflects the etiology and biology of luminal types of breast cancer, which together constitute ~70% of all breast cancers. E2 drives cancer development in this model through mechanisms that are largely dependent upon estrogen receptors and require progesterone and its receptors. Moreover, mammary cancer development appears to be associated with generation of oxidative stress and can be modified by multiple dietary factors, several of which may attenuate the actions of reactive oxygen species. Studies of susceptible ACI rats and resistant COP or BN rats provide novel insights into the genetic bases of susceptibility and the biological processes regulated by genetic determinants of susceptibility. This review summarizes research progress resulting from use of these physiologically relevant rat models to advance understanding of breast cancer etiology and prevention.

Pregnancy and Breast Cancer

Breast cancer is the most commonly diagnosed type of cancer among women worldwide. The majority of breast cancers are sporadic and the etiology is not well understood. Several factors have been attributed to altering the risk of breast cancer. A full-term pregnancy is a crucial factor in altering the risk. Early full-term pregnancy has been shown to reduce the lifetime risk of breast cancer, while a later first full-term pregnancy increases breast cancer risk. Epidemiological and experimental data demonstrate that spontaneous or induced abortions do not significantly alter the risk of breast cancer. In this study, we briefly discuss the different types and stages of breast cancer, various risk factors, and potential mechanisms involved in early full-term pregnancy-induced protection against breast cancer. Understanding how early full-term pregnancy induces protection against breast cancer will help design innovative preventive and therapeutic strategies. This understanding can also help in the development of molecular biomarkers that can be of tremendous help in predicting the risk of breast cancer in the general population.

Crosstalk between STAT5 activation and PI3K/AKT functions in normal and transformed mammary epithelial cells

Janus kinases (JAKs) and signal transducers and activators of transcription (STATs) have been shown to function downstream of several peptide hormones and cytokines that are required for postnatal development and secretory function of the mammary gland. As part of an extended network, these signal transducers can engage in crosstalk with other pathways to facilitate synergistic, and sometimes antagonistic, actions of different growth factors. Specifically, signaling through the JAK2/STAT5 cascade has been demonstrated to be indispensable for the specification, proliferation, differentiation, and survival of secretory mammary epithelial cells. Following a concise description of major cellular programs in mammary gland development and the role of growth factors that rely on JAK/STAT signaling to orchestrate these programs, this review highlights the significance of active STAT5 and its crosstalk with the PI3 kinase and AKT1 for mediating the proliferation of alveolar progenitors and survival of their functionally differentiated descendants in the mammary gland. Based on its ability to provide self-sufficiency in growth signals that are also capable of overriding intrinsic cell death programs, persistently active STAT5 can serve as a potent oncoprotein that contributes to the genesis of breast cancer. Recent experimental evidence demonstrated that, similar to normal developmental programs, oncogenic functions of STAT5 rely on molecular crosstalk with PI3K/AKT signaling for the initiation, and in some instances the progression, of breast cancer. The multitude by which STATs can interact with individual mediators of the PI3K/AKT signaling cascade may provide novel avenues for targeting signaling nodes within molecular networks that are crucial for the survival of cancer cells.

New paradigms for the Hedgehog signaling network in mammary gland development and breast Cancer

The Hedgehog signaling network regulates organogenesis, cell fate, proliferation, survival, and stem cell self-renewal in many mammalian tissues. Aberrant activation of the Hedgehog signaling network is present in ~25% of all cancers, including breast. Altered expression of Hedgehog network genes in the mammary gland can elicit phenotypes at many stages of development. However, synthesizing a cohesive mechanistic model of signaling at different stages of development has been difficult. Emerging data suggest that this difficulty is due, in part, to non-canonical and tissue compartment-specific (i.e., epithelial, versus stromal, versus systemic) functions of Hedgehog network components. With respect to systemic functions, Hedgehog network genes regulate development of endocrine organs that impinge on mammary gland development extrinsically. These new observations offer insight into previously conflicting data, and have bearing on the potential for anti-Hedgehog therapeutics in the treatment of breast cancer.

Mammary stem cells and parity-induced breast cancer protection- new insights

Parity (childbearing) significantly decreases a woman's risk of breast cancer and the protective effect is greater if the woman is younger and has more children. The mechanism/s of parity-induced protection are not known. Although several factors are postulated to play a role, we discuss how a reduction in the number of mammary stem cells (MaSCs) may lead to a reduction in breast cancer risk in parous women. Firstly we review the epidemiology linking childbearing to reduced breast cancer risk and discuss how additional births, a young age at first full term birth, and breastfeeding impact the protection. We then detail the mouse and human studies implicating MaSC in parity induced protection and the in-vivo work being performed in mice to directly investigate the effect of parity on MaSC. Finally we discuss the transplant and lineage tracing experiments assessing MaSC activity according to parity and the need to define if MaSC are indeed more carcinogen sensitive than mature mammary epithelial cells. Continuing and future studies attempting to define the parity induced mechanisms will aid in the development of preventative therapies.

Potential Mechanisms underlying the Protective Effect of Pregnancy against Breast Cancer: A Focus on the IGF Pathway

A first full-term birth at an early age protects women against breast cancer by reducing lifetime risk by up to 50%. The underlying mechanism resulting in this protective effect remains unclear, but many avenues have been investigated, including lobular differentiation, cell fate, and stromal composition. A single pregnancy at an early age protects women for 30-40 years, and this long-term protection is likely regulated by a relatively stable yet still modifiable method, such as epigenetic reprograming. Long-lasting epigenetic modifications have been shown to be induced by pregnancy and to target the IGF pathway. Understanding how an early first full-term pregnancy protects against breast cancer and the role of epigenetic reprograming of the IGF system may aid in developing new preventative strategies for young healthy women in the future.

Reproductive history and breast cancer prevention

The hormonal milieu of an early full-term pregnancy induces lobular development, completing the cycle of differentiation of the breast. This process induces a specific genomic signature in the mammary gland that is represented by the stem cell containing a heterochomatin condensed nucleus (HTN). Even though differentiation significantly reduces cell proliferation in the mammary gland, the mammary epithelium remains capable of responding with proliferation to given stimuli, such as a new pregnancy. The stem cell HTN is able to metabolize the carcinogen and repair the induced DNA damage more efficiently than the stem cell containing an euchromatinic structure (EUN), as it has been demonstrated in the rodent experimental system. The basic biological concept is that pregnancy shifts the stem cell EUN to the stem cell HTN that is refractory to carcinogenesis. Data generated by the use of cDNA micro array techniques have allowed to demonstrate that while lobular development regressed after pregnancy and lactation, programmed cell death genes, DNA repair genes, chromatin remodeling, transcription factors and immune-surveillance gene transcripts all of these genes are upregulated and are part of the genomic signature of pregnancy that is associated with the preventive effect of this physiological process.

Relation between parity and pregnancy-related hormones and breast cancer control

SUMMARY 

Epidemiological research has indicated the beneficial effects of full-term pregnancy at an early age for a reduction in breast cancer risk. Experimental data have shown that pregnancy and pregnancy-related hormones, such as estrogen plus progesterone, estrogen alone and human chorionic gonadotropin, are involved in parity-induced protection. Pregnancy and short-duration treatment of a young host with pregnancy-related hormones to mimic the pregnancy environment provide mammary cancer protection by making cells refractory to carcinogenic stimuli and causing growth arrest and programmed cell death. Experimental data concerning pregnancy and pregnancy-related hormones are reviewed in relation to intrinsic subtypes of mammary cancer.

The role of cytokines in breast cancer development and progression

Cytokines are highly inducible, secretory proteins that mediate intercellular communication in the immune system. They are grouped into several protein families that are referred to as tumor necrosis factors, interleukins, interferons, and colony-stimulating factors. In recent years, it has become clear that some of these proteins as well as their receptors are produced in the organisms under physiological and pathological conditions. The exact initiation process of breast cancer is unknown, although several hypotheses have emerged. Inflammation has been proposed as an important player in tumor initiation, promotion, angiogenesis, and metastasis, all phenomena in which cytokines are prominent players. The data here suggest that cytokines play an important role in the regulation of both induction and protection in breast cancer. This knowledge could be fundamental for the proposal of new therapeutic approaches to particularly breast cancer and other cancer-related disorders.

Biology of breast cancer during pregnancy using genomic profiling

Breast cancer during pregnancy is rare and is associated with relatively poor prognosis. No information is available on its biological features at the genomic level. Using a dataset of 54 pregnant and 113 non-pregnant breast cancer patients, we evaluated the pattern of hot spot somatic mutations and did transcriptomic profiling using Sequenom and Affymetrix respectively. We performed gene set enrichment analysis to evaluate the pathways associated with diagnosis during pregnancy. We also evaluated the expression of selected cancer-related genes in pregnant and non-pregnant patients and correlated the results with changes occurring in the normal breast using a pregnant murine model. We finally investigated aberrations associated with disease-free survival (DFS). No significant differences in mutations were observed. Of the total number of patients, 18.6% of pregnant and 23% of non-pregnant patients had a PIK3CA mutation. Around 30% of tumors were basal, with no differences in the distribution of breast cancer molecular subtypes between pregnant and non-pregnant patients. Two pathways were enriched in tumors diagnosed during pregnancy: the G protein-coupled receptor pathway and the serotonin receptor pathway (FDR <0.0001). Tumors diagnosed during pregnancy had higher expression of PD1 (PDCD1; P=0.015), PDL1 (CD274; P=0.014), and gene sets related to SRC (P=0.004), IGF1 (P=0.032), and β-catenin (P=0.019). Their expression increased almost linearly throughout gestation when evaluated on the normal breast using a pregnant mouse model underscoring the potential effect of the breast microenvironment on tumor phenotype. No genes were associated with DFS in a multivariate model, which could be due to low statistical power. Diagnosis during pregnancy impacts the breast cancer transcriptome including potential cancer targets.

Parity-related molecular signatures and breast cancer subtypes by estrogen receptor status

INTRODUCTION

Relationships of parity with breast cancer risk are complex. Parity is associated with decreased risk of postmenopausal hormone receptor-positive breast tumors, but may increase risk for basal-like breast cancers and early-onset tumors. Characterizing parity-related gene expression patterns in normal breast and breast tumor tissues may improve understanding of the biological mechanisms underlying this complex pattern of risk.

METHODS

We developed a parity signature by analyzing microRNA microarray data from 130 reduction mammoplasty (RM) patients (54 nulliparous and 76 parous). This parity signature, together with published parity signatures, was evaluated in gene expression data from 150 paired tumors and adjacent benign breast tissues from the Polish Breast Cancer Study, both overall and by tumor estrogen receptor (ER) status.

RESULTS

We identified 251 genes significantly upregulated by parity status in RM patients (parous versus nulliparous; false discovery rate = 0.008), including genes in immune, inflammation and wound response pathways. This parity signature was significantly enriched in normal and tumor tissues of parous breast cancer patients, specifically in ER-positive tumors.

CONCLUSIONS

Our data corroborate epidemiologic data, suggesting that the etiology and pathogenesis of breast cancers vary by ER status, which may have implications for developing prevention strategies for these tumors.

The genomic signature of breast cancer prevention

The breast of parous postmenopausal women exhibits a specific signature that has been induced by a full term pregnancy. This signature is centered in chromatin remodeling and the epigenetic changes induced by methylation of specific genes which are important regulatory pathways induced by pregnancy. Through the analysis of the genes found to be differentially methylated between women of varying parity, multiple positions at which beta-catenin production and use is inhibited were recognized. The biological importance of the pathways identified in this specific population cannot be sufficiently emphasized because they could represent a safeguard mechanism mediating the protection of the breast conferred by full term pregnancy.

Mammary gland development

The mammary gland develops through several distinct stages. The first transpires in the embryo as the ectoderm forms a mammary line that resolves into placodes. Regulated by epithelial–mesenchymal interactions, the placodes descend into the underlying mesenchyme and produce the rudimentary ductal structure of the gland present at birth. Subsequent stages of development—pubertal growth, pregnancy, lactation, and involution—occur postnatally under the regulation of hormones. Puberty initiates branching morphogenesis, which requires growth hormone (GH) and estrogen, as well as insulin-like growth factor 1 (IGF1), to create a ductal tree that fills the fat pad. Upon pregnancy, the combined actions of progesterone and prolactin generate alveoli, which secrete milk during lactation. Lack of demand for milk at weaning initiates the process of involution whereby the gland is remodeled back to its prepregnancy state. These processes require numerous signaling pathways that have distinct regulatory functions at different stages of gland development. Signaling pathways also regulate a specialized subpopulation of mammary stem cells that fuel the dramatic changes in the gland occurring with each pregnancy. Our knowledge of mammary gland development and mammary stem cell biology has significantly contributed to our understanding of breast cancer and has advanced the discovery of therapies to treat this disease.

Association of cellular and molecular responses in the rat mammary gland to 17β-estradiol with susceptibility to mammary cancer

BACKGROUND

We are using ACI and BN rats, which differ markedly in their susceptibility to 17β-estradiol (E2)-induced mammary cancer, to identify genetic variants and environmental factors that determine mammary cancer susceptibility. The objective of this study was to characterize the cellular and molecular responses to E2 in the mammary glands of ACI and BN rats to identify qualitative and quantitative phenotypes that associate with and/or may confer differences in susceptibility to mammary cancer.

METHODS

Female ACI and BN rats were treated with E2 for 1, 3 or 12 weeks. Mammary gland morphology and histology were examined by whole mount and hematoxylin and eosin (H&E) staining. Cell proliferation and epithelial density were evaluated by quantitative immunohistochemistry. Apoptosis was evaluated by quantitative western blotting and flow cytometry. Mammary gland differentiation was examined by immunohistochemistry. Gene expression was evaluated by microarray, qRT-PCR and quantitative western blotting assays. Extracellular matrix (ECM) associated collagen was evaluated by Picrosirius Red staining and Second Harmonic Generation (SHG) microscopy.

RESULTS

The luminal epithelium of ACI rats exhibited a rapid and sustained proliferative response to E2. By contrast, the proliferative response exhibited by the mammary epithelium of BN rats was restrained and transitory. Moreover, the epithelium of BN rats appeared to undergo differentiation in response to E2, as evidenced by production of milk proteins as well as luminal ectasia and associated changes in the ECM. Marked differences in expression of genes that encode proteins with well-defined roles in mammary gland development (Pgr, Wnt4, Tnfsf11, Prlr, Stat5a, Areg, Gata3), differentiation and milk production (Lcn2, Spp1), regulation of extracellular environment (Mmp7, Mmp9), and cell-cell or cell-ECM interactions (Cd44, Cd24, Cd52) were observed.

CONCLUSIONS

We propose that these cellular and molecular phenotypes are heritable and may underlie, at least in part, the differences in mammary cancer susceptibility exhibited by ACI and BN rats.

Exposure to excess estradiol or leptin during pregnancy increases mammary cancer risk and prevents parity-induced protective genomic changes in rats

Using a preclinical model, we investigated whether excess estradiol (E2) or leptin during pregnancy affects maternal mammary tumorigenesis in rats initiated by administering carcinogen 7,12-dimethylbenz(a)anthracene (DMBA) on day 50. Two weeks later, rats were mated, and pregnant dams were treated daily with 10 μg of 17β-estradiol, 15 μg of leptin, or vehicle from gestation day 8 to 19. Tumor development was assessed separately during weeks 1 to 12 and 13 to 22 after DMBA administration, because pregnancy is known to induce a transient increase in breast cancer risk, followed by a persistent reduction. Parous rats developed less (32%) mammary tumors than nulliparous rats (59%, P < 0.001), and the majority (93%) of tumors in the parous rats appeared before week 13 (vs. 41% in nulliparous rats), indicating that pregnancy induced a transient increase in breast cancer risk. Parous rats exposed to leptin (final tumor incidence 65%) or E2 (45%) during pregnancy developed mammary tumors throughout the tumor-monitoring period, similar to nulliparous control rats, and the incidence was significantly higher in both the leptin- and E2-exposed dams after week 12 than in the vehicle-exposed parous dams (P < 0.001). The mammary glands of the exposed parous rats contained significantly more proliferating cells (P < 0.001). In addition, the E2- or leptin-treated parous rats did not exhibit the protective genomic signature induced by pregnancy and seen in the parous control rats. Specifically, these rats exhibited downregulation of genes involved in differentiation and immune functions and upregulation of genes involved in angiogenesis, growth, and epithelial-to-mesenchymal transition.

Molecular profiling of human mammary gland links breast cancer risk to a p27(+) cell population with progenitor characteristics

Early full-term pregnancy is one of the most effective natural protections against breast cancer. To investigate this effect, we have characterized the global gene expression and epigenetic profiles of multiple cell types from normal breast tissue of nulliparous and parous women and carriers of BRCA1 or BRCA2 mutations. We found significant differences in CD44(+) progenitor cells, where the levels of many stem cell-related genes and pathways, including the cell-cycle regulator p27, are lower in parous women without BRCA1/BRCA2 mutations. We also noted a significant reduction in the frequency of CD44(+)p27(+) cells in parous women and showed, using explant cultures, that parity-related signaling pathways play a role in regulating the number of p27(+) cells and their proliferation. Our results suggest that pathways controlling p27(+) mammary epithelial cells and the numbers of these cells relate to breast cancer risk and can be explored for cancer risk assessment and prevention.

Parity induces differentiation and reduces Wnt/Notch signaling ratio and proliferation potential of basal stem/progenitor cells isolated from mouse mammary epithelium

Introduction

Early pregnancy has a strong protective effect against breast cancer in humans and rodents, but the underlying mechanism is unknown. Because breast cancers are thought to arise from specific cell subpopulations of mammary epithelia, we studied the effect of parity on the transcriptome and the differentiation/proliferation potential of specific luminal and basal mammary cells in mice.

Methods

Mammary epithelial cell subpopulations (luminal Sca1^-, luminal Sca1⁺, basal stem/progenitor, and basal myoepithelial cells) were isolated by flow cytometry from parous and age-matched virgin mice and examined by using a combination of unbiased genomics, bioinformatics, in vitro colony formation, and in vivo limiting dilution transplantation assays. Specific findings were further investigated with immunohistochemistry in entire glands of parous and age-matched virgin mice.

Results

Transcriptome analysis revealed an upregulation of differentiation genes and a marked decrease in the Wnt/Notch signaling ratio in basal stem/progenitor cells of parous mice. Separate bioinformatics analyses showed reduced activity for the canonical Wnt transcription factor LEF1/TCF7 and increased activity for the Wnt repressor TCF3. This finding was specific for basal stem/progenitor cells and was associated with downregulation of potentially carcinogenic pathways and a reduction in the proliferation potential of this cell subpopulation in vitro and in vivo. As a possible mechanism for decreased Wnt signaling in basal stem/progenitor cells, we found a more than threefold reduction in the expression of the secreted Wnt ligand Wnt4 in total mammary cells from parous mice, which corresponded to a similar decrease in the proportion of Wnt4-secreting and estrogen/progesterone receptor-positive cells. Because recombinant Wnt4 rescued the proliferation defect of basal stem/progenitor cells in vitro, reduced Wnt4 secretion appears to be causally related to parity-induced alterations of basal stem/progenitor cell properties in mice.

Conclusions

By revealing that parity induces differentiation and downregulates the Wnt/Notch signaling ratio and the in vitro and in vivo proliferation potential of basal stem/progenitor cells in mice, our study sheds light on the long-term consequences of an early pregnancy. Furthermore, it opens the door to future studies assessing whether inhibitors of the Wnt pathway may be used to mimic the parity-induced protective effect against breast cancer.

Exposures to synthetic estrogens at different times during the life, and their effect on breast cancer risk

Women are using estrogens for many purposes, such as to prevent pregnancy or miscarriage, or to treat menopausal symptoms. Estrogens also have been used to treat breast cancer which seems puzzling, since there is convincing evidence to support a link between high lifetime estrogen exposure and increased breast cancer risk. In this review, we discuss the findings that maternal exposure to the synthetic estrogen diethylstilbestrol during pregnancy increases breast cancer risk in both exposed mothers and their daughters. In addition, we review data regarding the use of estrogens in oral contraceptives and as postmenopausal hormone therapy and discuss the opposing effects on breast cancer risk based upon timing of exposure. We place particular emphasis on studies investigating how maternal estrogenic exposures during pregnancy increase breast cancer risk among daughters. New data suggest that these exposures induce epigenetic modifications in the mammary gland and germ cells, thereby causing an inheritable increase in breast cancer risk for multiple generations.

Differential expression of cancer-associated proteins in breastmilk

Breast cancer that develops during or shortly after pregnancy is frequently more aggressive than cancer diagnosed at other times in a woman's life. To better understand the patterns of cancer-related protein expression in the breasts of lactating women, we determined the differences in total and individual protein expression in milk based on (a) three time points during lactation (early, mid, and late), (b) length of lactation, and (c) parity. Breastmilk was collected from 72 healthy lactating women within 10 days of starting lactation (transitional [T]), 2 months after lactation started, and during breast weaning (W). Sixteen proteins whose expression is altered in breast cancer (11 kallikreins [KLKs], basic fibroblast growth factor [bFGF], YKL-40, neutrophil gelatinase-associated lipocalin, and transforming growth factor [TGF] β1 and β2) were evaluated. The concentration of total milk protein decreased over time (p<0.01 at 2 months and W compared with T). After we controlled for total protein, KLK6 and TGFβ2 significantly increased, and bFGF decreased from T to W. Neither length of nursing nor parity significantly influenced individual protein expression at the W time point. On the other hand, length of nursing did influence the difference in KLK6, -7, and -8 expression between the W and T time points. Total milk protein concentration is lower in the mid and late phases of nursing. Biomarker differences between T and W milk samples in KLK6, TGFβ2, and bFGF are consistent with a protective effect of nursing.

Epigenetic modifications unlock the milk protein gene loci during mouse mammary gland development and differentiation

BACKGROUND

Unlike other tissues, development and differentiation of the mammary gland occur mostly after birth. The roles of systemic hormones and local growth factors important for this development and functional differentiation are well-studied. In other tissues, it has been shown that chromatin organization plays a key role in transcriptional regulation and underlies epigenetic regulation during development and differentiation. However, the role of chromatin organization in mammary gland development and differentiation is less well-defined. Here, we have studied the changes in chromatin organization at the milk protein gene loci (casein, whey acidic protein, and others) in the mouse mammary gland before and after functional differentiation.

METHODOLOGY/PRINCIPAL FINDINGS

Distal regulatory elements within the casein gene cluster and whey acidic protein gene region have an open chromatin organization after pubertal development, while proximal promoters only gain open-chromatin marks during pregnancy in conjunction with the major induction of their expression. In contrast, other milk protein genes, such as alpha-lactalbumin, already have an open chromatin organization in the mature virgin gland. Changes in chromatin organization in the casein gene cluster region that are present after puberty persisted after lactation has ceased, while the changes which occurred during pregnancy at the gene promoters were not maintained. In general, mammary gland expressed genes and their regulatory elements exhibit developmental stage- and tissue-specific chromatin organization.

CONCLUSIONS/SIGNIFICANCE

A progressive gain of epigenetic marks indicative of open/active chromatin on genes marking functional differentiation accompanies the development of the mammary gland. These results support a model in which a chromatin organization is established during pubertal development that is then poised to respond to the systemic hormonal signals of pregnancy and lactation to achieve the full functional capacity of the mammary gland.

Breast arterial calcification is associated with reproductive factors in asymptomatic postmenopausal women

OBJECTIVE

The etiology of breast arterial calcification (BAC) is not well understood. We examined reproductive history and cardiovascular disease (CVD) risk factor associations with the presence of detectable BAC in asymptomatic postmenopausal women.

METHODS

Reproductive history and CVD risk factors were obtained in 240 asymptomatic postmenopausal women from a community-based research study who had a screening mammogram within 2 years of their participation in the study. The mammograms were reviewed for the presence of detectable BAC. Age-adjusted logistic regression models were fit to assess the association between each risk factor and the presence of BAC. Multiple variable logistic regression models were used to identify the most parsimonious model for the presence of BAC.

RESULTS

The prevalence of BAC increased with increased age (p < 0.0001). The most parsimonious logistic regression model for BAC presence included age at time of examination, increased parity (p = 0.01), earlier age at first birth (p = 0.002), weight, and an age-by-weight interaction term (p = 0.004). Older women with a smaller body size had a higher probability of having BAC than women of the same age with a larger body size.

CONCLUSIONS

The presence or absence of BAC at mammography may provide an assessment of a postmenopausal woman's lifetime estrogen exposure and indicate women who could be at risk for hormonally related conditions.

Pleiotrophin (PTN) expression and function and in the mouse mammary gland and mammary epithelial cells

Expression of the heparin-binding growth factor, pleiotrophin (PTN) in the mammary gland has been reported but its function during mammary gland development is not known. We examined the expression of PTN and its receptor ALK (Anaplastic Lymphoma Kinase) at various stages of mouse mammary gland development and found that their expression in epithelial cells is regulated in parallel during pregnancy. A 30-fold downregulation of PTN mRNA expression was observed during mid-pregnancy when the mammary gland undergoes lobular-alveolar differentiation. After weaning of pups, PTN expression was restored although baseline expression of PTN was reduced significantly in mammary glands of mice that had undergone multiple pregnancies. We found PTN expressed in epithelial cells of the mammary gland and thus used a monoclonal anti-PTN blocking antibody to elucidate its function in cultured mammary epithelial cells (MECs) as well as during gland development. Real-time impedance monitoring of MECs growth, migration and invasion during anti-PTN blocking antibody treatment showed that MECs motility and invasion but not proliferation depend on the activity of endogenous PTN. Increased number of mammospheres with laminin deposition after anti-PTN blocking antibody treatment of MECs in 3D culture and expression of progenitor markers suggest that the endogenously expressed PTN inhibits the expansion and differentiation of epithelial progenitor cells by disrupting cell-matrix adhesion. In vivo, PTN activity was found to inhibit ductal outgrowth and branching via the inhibition of phospho ERK1/2 signaling in the mammary epithelial cells. We conclude that PTN delays the maturation of the mammary gland by maintaining mammary epithelial cells in a progenitor phenotype and by inhibiting their differentiation during mammary gland development.

Defining the genomic signature of the parous breast

Background

It is accepted that a woman's lifetime risk of developing breast cancer after menopause is reduced by early full term pregnancy and multiparity. This phenomenon is thought to be associated with the development and differentiation of the breast during pregnancy.

Methods

In order to understand the underlying molecular mechanisms of pregnancy induced breast cancer protection, we profiled and compared the transcriptomes of normal breast tissue biopsies from 71 parous (P) and 42 nulliparous (NP) healthy postmenopausal women using Affymetrix Human Genome U133 Plus 2.0 arrays. To validate the results, we performed real time PCR and immunohistochemistry.

Results

We identified 305 differentially expressed probesets (208 distinct genes). Of these, 267 probesets were up- and 38 down-regulated in parous breast samples; bioinformatics analysis using gene ontology enrichment revealed that up-regulated genes in the parous breast represented biological processes involving differentiation and development, anchoring of epithelial cells to the basement membrane, hemidesmosome and cell-substrate junction assembly, mRNA and RNA metabolic processes and RNA splicing machinery. The down-regulated genes represented biological processes that comprised cell proliferation, regulation of IGF-like growth factor receptor signaling, somatic stem cell maintenance, muscle cell differentiation and apoptosis.

Conclusions

This study suggests that the differentiation of the breast imprints a genomic signature that is centered in the mRNA processing reactome. These findings indicate that pregnancy may induce a safeguard mechanism at post-transcriptional level that maintains the fidelity of the transcriptional process.

Pregnancy-induced chromatin remodeling in the breast of postmenopausal women

Early pregnancy and multiparity are known to reduce the risk of women to develop breast cancer at menopause. Based on the knowledge that the differentiation of the breast induced by the hormones of pregnancy plays a major role in this protection, this work was performed with the purpose of identifying what differentiation-associated molecular changes persist in the breast until menopause. Core needle biopsies (CNB) obtained from the breast of 42 nulliparous (NP) and 71 parous (P) postmenopausal women were analyzed in morphology, immunocytochemistry and gene expression. Whereas in the NP breast, nuclei of epithelial cells were large and euchromatic, in the P breast they were small and hyperchromatic, showing strong methylation of histone 3 at lysine 9 and 27. Transcriptomic analysis performed using Affymetrix HG_U133 oligonucleotide arrays revealed that in CNB of the P breast, there were 267 upregulated probesets that comprised genes controlling chromatin organization, transcription regulation, splicing machinery, mRNA processing and noncoding elements including XIST. We concluded that the differentiation process induced by pregnancy is centered in chromatin remodeling and in the mRNA processing reactome, both of which emerge as important regulatory pathways. These are indicative of a safeguard step that maintains the fidelity of the transcription process, becoming the ultimate mechanism mediating the protection of the breast conferred by full-term pregnancy.

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.

Mammary stem cells and their regulation by steroid hormones

Sustained exposure to estrogen and progesterone is a well-established risk factor for breast cancer. These hormones play a central role in the female reproductive cycle, in which they control morphogenesis of the mammary gland during puberty, ovulatory cycles and pregnancy. Mouse mammary stem cells (MaSCs) have recently been discovered to be highly responsive to female hormones, despite lacking expression of the estrogen and progesterone receptors. The inhibition of MaSCs by hormone receptor antagonists further suggests that these cells contribute to oncogenesis. Identification of paracrine mediators of hormone signaling to MaSCs may lead to the development of novel inhibitors that drive MaSCs into a more quiescent state. In this context, inhibition of the receptor activator of NF-κB/receptor activator of NF-κB ligand signaling pathway has profound implications for the prevention of breast cancer.

Leukocytes in mammary development and cancer

Leukocytes, of both the innate and adaptive lineages, are normal cellular components of all tissues. These important cells not only are critical for regulating normal tissue homeostasis, but also are significant paracrine regulators of all physiologic and pathologic tissue repair processes. This article summarizes recent insights regarding the trophic roles of leukocytes at each stage of mammary gland development and during cancer development, with a focus on Murids and humans.

From milk to malignancy: the role of mammary stem cells in development, pregnancy and breast cancer

Adult stem cells of the mammary gland (MaSCs) are a highly dynamic population of cells that are responsible for the generation of the gland during puberty and its expansion during pregnancy. In recent years significant advances have been made in understanding how these cells are regulated during these developmentally important processes both in humans and in mice. Understanding how MaSCs are regulated is becoming a particularly important area of research, given that they may be particularly susceptible targets for transformation in breast cancer. Here, we summarize the identification of MaSCs, how they are regulated and the evidence for their serving as the origins of breast cancer. In particular, we focus on how changes in MaSC populations may explain both the increased risk of developing aggressive ER/PR(-) breast cancer shortly after pregnancy and the long-term decreased risk of developing ER/PR(+) tumors.

Assessment of mammographic density before and after first full-term pregnancy

Mammographic density (MD) has consistently been found as one of the strongest breast cancer risk factors. In our study, both qualitative and quantitative density measurements were performed in a hospital-based group of premenopausal women before and after first full-term pregnancy providing an opportunity for direct evaluation of the effects of one pregnancy on MD. Mammograms were obtained from 23 women before and after first full-term pregnancy and from 28 nulliparous controls. MD was determined by a standard qualitative assessment method using the Breast Imaging Reporting and Data System, and a quantitative computer-based threshold method (0-100%). The mean age at mammography before and after pregnancy was 31 and 34 years, respectively, with a mean difference of 40 months between mammographies. The quantitative density assessment showed a significant reduction in relative MD after pregnancy of 12 percentage points (8.6-15.4), compared with 3.1 (0.0-6.2) in the nulliparous control group (P<0.001). A reduction in MD of more than 10% was seen in 52% of the patients, compared with 18% of the controls. The qualitative density assessment confirmed a reduction in MD after pregnancy by one Breast Imaging Reporting and Data System category (P=0.02). This longitudinal study showed that MD can be influenced by one full-term pregnancy. This effect was seen with both quantitative and qualitative assessment methods. It may be hypothesized that breast cancer risk reduction associated with pregnancy is mediated through a direct reduction of MD, and MD assessment might be incorporated in individualizing risk assessment and prevention.