Mammary gland neoplasia in long-term rodent studies

Epidemiology and pathophysiology of alcohol and breast cancer: Update 2012

AIMS

To update epidemiological data on alcohol and breast cancer, with special emphasis on light alcohol consumption, and to review mechanisms of alcohol mediated mammary carcinogenesis.

METHODS

For epidemiological data, in November 2011 we performed a literature search in various bibliographic databases, and we conducted a meta-analysis of data on light alcohol drinking. Relevant mechanistic studies were also reviewed to November 2011.

RESULTS

A significant increase of the order of 4% in the risk of breast cancer is already present at intakes of up to one alcoholic drink/day. Heavy alcohol consumption, defined as three or more drinks/day, is associated with an increased risk by 40-50%. This translates into up to 5% of breast cancers attributable to alcohol in northern Europe and North America for a total of approximately 50,000 alcohol-attributable cases of breast cancer worldwide. Up to 1-2% of breast cancers in Europe and North America are attributable to light drinking alone, given its larger prevalence in most female populations when compared with heavy drinking. Alcohol increases estrogen levels, and estrogens may exert its carcinogenic effect on breast tissue either via the ER or directly. Other mechanisms may include acetaldehyde, oxidative stress, epigenetic changes due to a disturbed methyl transfer and decreased retinoic acid concentrations associated with an altered cell cycle.

CONCLUSIONS

Women should not exceed one drink/day, and women at elevated risk for breast cancer should avoid alcohol or consume alcohol occasionally only.

Critical review of dose-response options for F344 rat mammary tumors for acrylamide - additional insights based on mode of action

Previous risk assessment reviews analyzed the potential for dietary acrylamide to increase breast cancer risk. Here, we critically review acrylamide animal bioassay data on mammary tumors for human relevance. We applied a systematic evaluation using reasonable standards of scientific certainty and a systematic weight of evidence (WOE) approach to evaluate several hypothesized modes of action (MOA), including (1) genotoxicity related to glycidamide formation and oxidative stress, (2) endocrine effects due to age-related hyperprolactinemia or secondary to neurotoxicity, and (3) epigenetic effects. We conclude that the appropriate approach for low-dose extrapolation of the rat mammary tumors can be narrowed to two options: (1) linear low-dose extrapolation (i.e., based on a MOA of mutagenicity from direct DNA interaction) from a point of departure (POD) for the combined incidence of adenomas and adenocarcinomas, since these tumor types are related; or (2) non-linear extrapolation, using uncertainty factors to estimate a Reference Dose (RfD) from a POD for tumor promotion derived using the combined fibroadenoma, adenoma and adenocarcinoma data. Non-linear extrapolation is used in the latter approach because these combined tumor types are unlikely to be exclusively caused by mutagenicity. Comparison of the WOE for each alternative MOA indicates that a non-linear approach (option 2) is more appropriate for evaluation of acrylamide-induced mammary tumors; a linear approach (option 1) is shown for comparison.

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.

Developmental stage determines estrogen receptor alpha expression and non-genomic mechanisms that control IGF-1 signaling and mammary proliferation in mice

Insulin like growth factor-1 (IGF-1) stimulates increased proliferation and survival of mammary epithelial cells and also promotes mammary tumorigenesis. To study the effects of IGF-1 on the mammary gland in vivo, we used BK5.IGF-1 transgenic (Tg) mice. In these mice, IGF-1 overexpression is controlled by the bovine keratin 5 promoter and recapitulates the paracrine exposure of breast epithelium to stromal IGF-1 that is seen in women. Studies have shown that BK5.IGF-1 Tg mice are more susceptible to mammary tumorigenesis than wild-type littermates. Investigation of the mechanisms underlying increased mammary cancer risk, reported here, revealed that IGF-1 preferentially activated the PI3K/Akt pathway in glands from prepubertal Tg mice, resulting in increased cyclin D1 expression and hyperplasia. However, in glands from postpubertal Tg mice, a pathway switch occurred and activation of the Ras/Raf/MAPK pathway predominated, without increased cyclin D1 expression or proliferation. We further showed that in prepubertal Tg glands, signaling was mediated by formation of an ERα/IRS-1 complex, which activated IRS-1 and directed signaling via the PI3K/Akt pathway. Conversely, in postpubertal Tg glands, reduced ERα expression failed to stimulate formation of the ERα/IRS-1 complex, allowing signaling to proceed via the alternate Ras/Raf/MAPK pathway. These in vivo data demonstrate that changes in ERα expression at different stages of development direct IGF-1 signaling and the resulting tissue responses. As ERα levels are elevated during the prepubertal and postmenopausal stages, these may represent windows of susceptibility during which increased IGF-1 exposure maximally enhances breast cancer risk.

Olive oil, an essential component of the Mediterranean diet, and breast cancer

Objective

The Mediterranean diet has been related to a lower risk of some chronic diseases, including cancer. We aim to gain insight into the effects of the main source of fat of this diet on breast cancer, the most common type of malignancy in women.

Design

Data from sixteen experimental series analysing the effects of dietary lipids on mammary carcinogenesis in an animal model, in the context of the international literature on the Mediterranean diet, olive oil and breast cancer risk.

Setting

Experimental and human data on the effects of olive oil and Mediterranean diet on breast cancer.

Subjects

An animal model of induced breast cancer and other human and experimental studies in the literature.

Results

Diets rich in extra virgin olive oil (EVOO) exert a negative modulatory effect on experimental breast cancer to a weak promoting effect, much lower than that obtained with a high-corn oil diet. EVOO confers to the mammary adenocarcinomas a clinical behaviour and morphological features compatible with low tumour aggressiveness. This differential effect, in relation to other dietary lipids, may be related to a lower effect on body weight and sexual maturation. In addition, EVOO induced different molecular changes in tumours, such as in the composition of cell membranes, activity of signalling proteins and gene expression. All these modifications could induce lower proliferation, higher apoptosis and lower DNA damage. These results, together with the favourable effect of olive oil reported in the literature when it is consumed in moderate quantities, suggest a beneficial influence of EVOO on breast cancer risk.

Conclusions

Consumption of EVOO in moderate quantities and throughout the lifetime appears to be a healthy choice and may favourably influence breast cancer risk.

Differential effects of exogenous androgen and an androgen receptor antagonist in the peri- and postpubertal murine mammary gland

There is emerging evidence that androgens inhibit proliferation of normal and malignant breast epithelial cells, but the actions of androgens in normal mammary gland morphogenesis are not well understood. In this study, we investigated whether development of the murine mammary gland could be altered by stimulating or suppressing androgen receptor (AR) signaling in vivo. Intact virgin female mice aged 5 wk (midpuberty) or 12 wk (postpuberty) were implanted with slow-release pellets containing either placebo, 5α-dihydrotestosterone (1.5 mg) or the AR antagonist flutamide (60 mg). Treatment with 5α-dihydrotestosterone from midpuberty to 12 wk of age-retarded ductal extension by 40% (P = 0.007), but treatment from 12-21 wk had no significant effect on gland morphology. In contrast, inhibition of AR signaling with flutamide from midpuberty had no effect on the mammary gland, but flutamide treatment from 12-21 wk increased ductal branching (P = 0.004) and proliferation (P = 0.03) of breast epithelial cells. The increased proliferation in flutamide-treated mice was not correlated with serum estradiol levels or estrogen receptor-α (ERα) expression. In control mice, the frequency and intensity of AR immunostaining in mammary epithelial cells was significantly increased in the 12- to 21-wk treatment group compared with the 5- to 12-wk group (P < 0.001). In contrast, no change in ERα occurred, resulting in a marked increase in the AR to ERα ratio from 0.56 (±0.12) to 1.47 (±0.10). Our findings indicate that androgen signaling influences development and structure of the adult mammary gland and that homeostasis between estrogen and androgen signaling in mature glands is critical to constrain the proliferative effects of estradiol.

Prepubertal daidzein exposure enhances mammary gland differentiation and regulates the expression of estrogen receptor-alpha and apoptotic proteins

Mechanism of chemoprevention by daidzein (500 μg/g bwt) was examined by injecting it subcutaneously at 16th, 18th, and 20th day postpartum, followed by counting of terminal end buds (TEBs), terminal ducts (TDs), and lobules and immunohistochemistry of ER-α, Bcl2, Bax, and caspase-3. DNA fragmentation was also analysed to measure the apoptosis level. Estradiol benzoate (EB) (500 ng/g bwt) and dimethyl sulphoxide (DMSO) were used as reference and vehicle, respectively. Observations show a significant enhancement of mammary gland differentiation at postnatal day 21 (PND21) as well as PND50. There was a significant decrease of ER-α expression at PND21 and increase in its expression at PND50, in daidzein-treated animals. The ratio of expression of Bcl-2 to Bax proteins increased at PND50 the same whereas, it decreased at PND50 due to daidzein. An increased expression of caspase-3 and DNA fragmentation was also seen due to daidzein at PND50. The mammary gland of EB-treated animals showed response a somewhat similar to that of daidzein-treated animals.

Pregnancy-induced changes in breast cancer risk

Breast cancer is the malignant disease most frequently diagnosed in women of all races and nationalities. Since the 1970s the worldwide incidence of this disease has increased 30-40% in postmenopausal women, in whom, paradoxically, the risk of developing breast cancer is significantly reduced by an early first full term pregnancy (FTP) as compared to nulliparous and late parous women. Although the cause of breast cancer is not known, the mechanisms mediating the protection conferred by an early FTP have been identified to reside in the breast itself, and to be modulated by endogenous and environmental exposures that might negatively affect this organ during specific windows in its development that extend from prenatal life until the first pregnancy. Soon after conception the embryo initiates the production of human chorionic gonadotropin (hCG), the glycoprotein hormone that is diagnostic of pregnancy. HCG in conjunction with ovarian steroid hormones primes the hypothalamic neuroendocrine system for maintaining the pregnancy. Higher levels of hCG during the first trimester of pregnancy have been associated with a reduction in maternal breast cancer incidence after age 50. In preclinical studies it has been demonstrated that both FTP and hCG treatment of virgin rats prevent the development of chemically-induced mammary tumors, a phenomenon mediated by the differentiation of the mammary gland epithelial cells prior to carcinogen exposure. Complete differentiation proceeds through complex morphological, physiological and molecular changes that occur during pregnancy and lactation, that ultimately result in increased DNA repair capabilities of the mammary epithelium, activation of genes controlling differentiation and programmed cell death and imprinting in the breast epithelium a specific and permanent genomic signature of pregnancy. This signature is indicative of a reduced breast cancer risk and serves as a molecular biomarker of differentiation for evaluating the potential use of chemopreventive agents.

Environmental exposures and mammary gland development: state of the science, public health implications, and research recommendations

OBJECTIVES

Perturbations in mammary gland (MG) development may increase risk for later adverse effects, including lactation impairment, gynecomastia (in males), and breast cancer. Animal studies indicate that exposure to hormonally active agents leads to this type of developmental effect and related later life susceptibilities. In this review we describe current science, public health issues, and research recommendations for evaluating MG development.

DATA SOURCES

The Mammary Gland Evaluation and Risk Assessment Workshop was convened in Oakland, California, USA, 16-17 November 2009, to integrate the expertise and perspectives of scientists, risk assessors, and public health advocates. Interviews were conducted with 18 experts, and seven laboratories conducted an MG slide evaluation exercise. Workshop participants discussed effects of gestational and early life exposures to hormonally active agents on MG development, the relationship of these developmental effects to lactation and cancer, the relative sensitivity of MG and other developmental end points, the relevance of animal models to humans, and methods for evaluating MG effects.

SYNTHESIS

Normal MG development and MG carcinogenesis demonstrate temporal, morphological, and mechanistic similarities among test animal species and humans. Diverse chemicals, including many not considered primarily estrogenic, alter MG development in rodents. Inconsistent reporting methods hinder comparison across studies, and relationships between altered development and effects on lactation or carcinogenesis are still being defined. In some studies, altered MG development is the most sensitive endocrine end point.

CONCLUSIONS

Early life environmental exposures can alter MG development, disrupt lactation, and increase susceptibility to breast cancer. Assessment of MG development should be incorporated in chemical test guidelines and risk assessment.

Protective effects of prepubertal genistein exposure on mammary tumorigenesis are dependent on BRCA1 expression

This study investigated whether prepubertal dietary exposure to genistein reduces mammary tumorigenesis by upregulating Brca1 expression in mice. Heterozygous Brca1(+/-) mice and their wild-type (WT) littermates were fed control AIN93G diet or 500 ppm genistein-supplemented AIN93G diet from postnatal day (PND) 15 to PND30 and then switched to AIN93G diet. Prepubertal dietary exposure to genistein reduced 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary incidence (P = 0.029) and aggressiveness of the tumors (P < 0.001) in the WT mice and upregulated the expression of Brca1 in their mammary glands (P = 0.04). In contrast, prepubertal genistein diet neither significantly reduced mammary tumorigenesis or tumor aggressivity nor increased Brca1 mRNA expression in the Brca1(+/-) mice. These results may be related to the opposing effects of prepubertal genistein diet on the expression of Rankl and CK5/CK18 ratio (marker of luminal epithelial cell differentiation) in the mammary gland and estrogen receptor (ER-α) and progesterone receptor (PgR) protein levels in the mammary tumor: these all were reduced in the WT mice or increased in Brca1(+/-) mice. Both the WT and Brca1(+/-) mice exhibited reduced levels of amphiregulin, CK5, and CK18, delayed ductal elongation and a reduction in terminal end bud number in the normal mammary gland, and reduced HER-2 protein levels in the mammary tumors; however, these effects were not sufficient to significantly reduce mammary tumorigenesis in Brca1(+/-) mice. Our results show that upregulation of Brca1 may be required for prepubertal dietary genistein exposure to reduce later mammary tumorigenesis, perhaps because in the absence of this upregulation, mice do not exhibit genistein-induced downregulation of ER-α, PgR, and Rankl.

Bisphenol A increases mammary cancer risk in two distinct mouse models of breast cancer

Bisphenol A (BPA) is an industrial plasticizer that leaches from food containers during normal usage, leading to human exposure. Early and chronic exposure to endocrine-disrupting environmental contaminants such as BPA elevates the potential for long-term health consequences. We examined the impact of BPA exposure on fetal programming of mammary tumor susceptibility as well as its growth promoting effects on transformed breast cancer cells in vivo. Fetal mice were exposed to 0, 25, or 250 μg/kg BPA by oral gavage of pregnant dams. Offspring were subsequently treated with the known mammary carcinogen, 7,12-dimethylbenz[a]anthracene (DMBA). While no significant differences in postnatal mammary development were observed, both low- and high-dose BPA cohorts had a statistically significant increase in susceptibility to DMBA-induced tumors compared to vehicle-treated controls. To determine if BPA also promotes established tumor growth, MCF-7 human breast cancer cells were subcutaneously injected into flanks of ovariectomized NCR nu/nu female mice treated with BPA, 17beta-estradiol, or placebo alone or combined with tamoxifen. Both estradiol- and BPA-treated cohorts formed tumors by 7 wk post-transplantation, while no tumors were detected in the placebo cohort. Tamoxifen reversed the effects of estradiol and BPA. We conclude that BPA may increase mammary tumorigenesis through at least two mechanisms: molecular alteration of fetal glands without associated morphological changes and direct promotion of estrogen-dependent tumor cell growth. Both results indicate that exposure to BPA during various biological states increases the risk of developing mammary cancer in mice.

In Situ Phosphorylation of Akt and ERK1/2 in Rat Mammary Gland, Colon, and Liver Following Treatment with Human Insulin and IGF-1

High doses of insulin and the insulin analog AspB10 have been reported to increase mammary tumor incidence in female rats likely via receptor-mediated mechanisms, possibly involving enhanced IGF-1 receptor activation. However, insulin and IGF-1 receptor functionality and intracellular signaling in the rat mammary gland in vivo is essentially unexplored. The authors investigated the effect of a single subcutaneous dose of 600 nmol/kg human insulin or IGF-1 on Akt and ERK1/2 phosphorylation in rat liver, colon, and mammary gland. Rat tissues were examined by Western blotting and immunohistochemistry by phosphorylation-specific antibodies. Insulin as well as IGF-1 caused Akt phosphorylation in mammary epithelial cells, with myoepithelial and basal epithelial cells being most sensitive. IGF-1 caused stronger Akt phosphorylation than insulin in mammary gland epithelial cells. Phosphorylation of ERK1/2 was not influenced by insulin or IGF-1. Rather, in liver and mammary gland P-ERK1/2 appeared to correlate with estrous cycling, supporting that ERK1/2 has important physiological roles in these two organs. In short, these findings supported that the rat mammary gland epithelium expresses functional insulin and IGF-1 receptors and that phosphorylation of Akt as well as ERK1/2 may be of value in understanding the effects of exogenous insulin in the rat mammary gland and colon.

Use of imaging for investigation of suspected pulmonary embolism during pregnancy and the postpartum period

Pulmonary embolism (PE) is recognized as a leading cause of maternal mortality in the developed world; however, it is a very difficult diagnosis to make on clinical grounds, and in most cases imaging is required. Pregnancy is a recognized risk factor for venous thromboembolism, and symptoms of normal pregnancy including shortness of breath, tachycardia and leg swelling are included in clinical tools for risk stratification for PE in the non-pregnant population. This results in a very low threshold for imaging, despite concerns regarding the risk of exposure to ionizing radiation both for the fetus and the maternal breast. We reviewed the results of all ventilation/perfusion scans and computed tomography pulmonary angiograms performed in pregnant women at a single institution to identify how many of these tests were positive for PE, and which clinical features may identify a low-risk group. A total of 386 scans were performed to investigate 375 episodes of suspected PE, representing 1.3-1.5% of pregnant women. Fifteen patients were diagnosed with PE, giving an incidence of one in 2000 maternities. The only statistically significant factors associated with PE were smoking or the presence of multiple risk factors. Clinical features of tachycardia and leg swelling did not provide significant diagnostic value; however, the absence of pleuritic chest pain had a negative predictive value of 97.8%. Arterial blood gas and D-dimer were statistically different between those with and without PE but not to a clinically useful degree. Currently available clinical and laboratory tools are not adequate to exclude a diagnosis of PE in a pregnant patient, thus imaging is justified to exclude PE. Further longitudinal studies to identify a low-risk group who do not require imaging is vital.

Hormone-related risk factors and postmenopausal breast cancer among nulliparous versus parous women: An aggregated study

Nulliparity is an established breast cancer risk factor, particularly when compared with parity at young ages. The authors aggregated data from 4 US prospective studies (1979-2006) including 32,641 nulliparous (1,612 breast cancers) and 204,270 parous (8,180 breast cancers) women to examine the hypothesis that nulliparity may increase susceptibility to established postmenopausal breast cancer risk factors. The aggregated hazard ratio for nulliparous versus all parous women = 1.27 (95% confidence interval: 1.21, 1.34), and that for nulliparous versus women <25 years of age at first birth = 1.38 (95% confidence interval: 1.30, 1.46). Among nulliparous women, the hazard ratios for current menopausal hormone therapy use (vs. never use), body mass index ≥30 kg/m(2) (vs. <25 kg/m(2)), and weekly consumption of ≥7 alcoholic drinks (vs. none) ranged from 1.3 to 1.6. The hazard ratios did not differ by parity. In a model including all women, the joint association for each of these factors and nulliparity combined compared with first birth before age 25 years was an approximately 2-fold increased breast cancer risk. Although the baseline risk is higher for nulliparous women compared with parous women, these results suggest that the associations between hormone-related factors and breast cancer do not differ by parity.

Cancer as development gone awry: the case for bisphenol-A as a carcinogen

The discovery of a rare clear cell carcinoma of the vagina in young women gestationally exposed to the estrogen diethylstilbestrol (DES) lent empirical support to the hypothesis that prenatal exposure to xenoestrogens might cause cancer. This fact contradicted two well-accepted notions: (i) mammalian development was merely the unfolding of a genetic program and (ii) only mutagenic agents could cause cancer. The ecological developmental biology (eco–devo) movement revitalized the concept of developmental plasticity through the occurrence of polyphenisms whereby a single genotype produces diverse phenotypes which are determined by environmental cues. Based on the principles of eco–devo and the tissue organization field theory of carcinogenesis, we hypothesized that developmental exposure to xenoestrogens increased the propensity to develop mammary cancer during adulthood. Bisphenol-A (BPA), a ubiquitous xenoestrogen, was chosen as a model for environmental estrogen exposure. In mice, perinatal exposure to environmentally relevant BPA levels induced alterations of the mammary gland architecture which manifested during fetal morphogenesis and throughout life, including the development of pre-neoplastic lesions. In rats, gestational exposure to BPA induced pre-neoplastic lesions and carcinoma in situ that manifested in adulthood in the absence of any additional treatment. Emerging epidemiological data reveal an increased incidence of breast cancer in women exposed to DES during gestation. Hence, both animal experiments and epidemiological data strengthen the hypothesis that fetal exposure to xenoestrogens may be an underlying cause of the increased incidence of breast cancer observed over the past 50 years.

The Effect of a Mediterranean Meal on Sprague Dawley Rats DMBA-Induced Mammary Tumors

The present study intents to find a possible protective role of a Mediterranean type meal on mammary carcinogenesis. Several factors have been associated with breast cancer risk, genetics and environment are the most pointed out in epidemiologic and experimental studies. Diet is an environmental factor that can promote or prevent disease, being responsible for almost 35% of total cancer cases. A total of 72 female rats 50 days old were randomly divided in three groups of 24 rats and housed 4 in each plastic cage in a holding room under constant conditions of 22 ± 2 °C, 55 ± 10% humidity and a 12 h light/dark cycle. All the animals were submitted to the administration of 20 mg of 7, 12 dimethylbenzanthracene (DMBA) in olive oil, by gavages, except group A. The same defined standard food was provided for all the animals in group A and B, supplemented with a Mediterranean meal in group C. All the animals were sacrificed by the end of 150 days. Total carcinoma number did not differ significantly between Groups B and C and there were not found any neoplastic lesions in Group A. Most tumors showed a mixed architectural pattern, with cribriform and papillary areas, comedocarcinoma and necrosis was only seen in Group B. Histopathologic analysis showed that Group C tumors had lower mitotic activity and Pattern Grades, but higher Nuclear Grades. Mediterranean diet type meal showed lower Pattern Grades and lower Mitotic count in spite of that a higher nuclear pleomorphism was also found. Even so, tumors from Group C were better differentiated which can indicate lower malignancy.

Changes in mammary gland morphology and breast cancer risk in rats

Studies in rodent models of breast cancer show that exposures to dietary/hormonal factors during the in utero and pubertal periods, when the mammary gland undergoes extensive modeling and re-modeling, alter susceptibility to carcinogen-induced mammary tumors. Similar findings have been described in humans: for example, high birthweight increases later risk of developing breast cancer, and dietary intake of soy during childhood decreases breast cancer risk. It is thought that these prenatal and postnatal dietary modifications induce persistent morphological changes in the mammary gland that in turn modify breast cancer risk later in life. These morphological changes likely reflect epigenetic modifications, such as changes in DNA methylation, histones and miRNA expression that then affect gene transcription . In this article we describe how changes in mammary gland morphology can predict mammary cancer risk in rats. Our protocol specifically describes how to dissect and remove the rat abdominal mammary gland and how to prepare mammary gland whole mounts. It also describes how to analyze mammary gland morphology according to three end-points (number of terminal end buds, epithelial elongation and differentiation) and to use the data to predict risk of developing mammary cancer.

Identification of stable and oestrus cycle-independent housekeeping genes in the rat mammary gland and other tissues

The function and development of the rat mammary gland is dependent on the oestrus cycle. Normalization of gene expression in mammary gland samples assessed by quantitative RT-PCR therefore requires housekeeping genes (HKGs) which are stably expressed during the oestrus cycle. mRNA expression of 10 HKGs was measured in the rat mammary gland at different phases of the oestrus cycle. In addition, mRNA expression of the HKGs was measured in a panel of other rat tissues comprising laser microdissected mammary gland alveolar lobules and interlobular connective tissue and macrodissected mammary gland, liver, skeletal muscle, colon and ovary samples. Expression and ranking of HKGs varied between tissues and oestrus cycle phases and several HKGs were necessary for normalization between samples. In the mammary gland samples, three HKGs (Sdha, Tbp, and Atp5b) were identified as the optimal combination of stably expressed genes across oestrus cycle phases. For normalization between samples from the entire panel of rat tissues, eight HKGs (Rps18, Eef1a1, B2m, Actb, Tbp, Hprt, Pgk1, and Sdha) were identified as the optimal combination. These HKGs are of general relevance for studies comparing gene expression between different rat tissues.

Ezetimibe reverses the inhibitory effects of dietary cholesterol on mammary tumorigenesis in rats

There are concerns regarding increased cancer incidence in patients treated with ezetimibe, an inhibitor of the absorption of dietary cholesterol. Here we tested the hypothesis that ezetimibe will accelerate mammary tumorigenesis in rats. The drug was administered at a dose of 1 ppm in an AIN-93G diet that contained 0.3% cholesterol. This experimental diet and control diets that contained either no additions or cholesterol or ezetimibe only, were fed to groups of 30 Sprague-Dawley rats 3 days after they were treated with 50 mg/kg methylnitrosourea (MNU). All rats were euthanized 22 weeks after MNU administration. Tumor multiplicity was significantly smaller in rats fed cholesterol than those fed no cholesterol (1.84 +/- 0.42 vs. 3.86 +/- 0.86 respectively, P < 0.05), but was significantly greater in the cholesterol/ezetimibe group than the group fed only cholesterol (3.48 +/- 0.59 vs. 1.84 +/- 0.42 respectively, P < 0.04). The average weight of tumors/rat was also significantly larger in the cholesterol/ezetimibe group than those fed cholesterol alone (5.67 +/- 1.15 vs. 2.56 +/- 0.71 respectively, P < 0.04). As expected, ezetimibe prevented the cholesterol raising effect of the dietary cholesterol. These results show that ezetimibe reverses the inhibitory effect of dietary cholesterol on the development of rat mammary tumors.

Role of ERalpha in the differential response of Stat5a loss in susceptibility to mammary preneoplasia and DMBA-induced carcinogenesis

Deregulated estrogen signaling is evidently linked to breast cancer pathophysiology, although the role of signal transducer and activator of transcription (Stat)5a, integral to normal mammary gland development, is less clear. A mouse model of mammary epithelial cell-targeted deregulated estrogen receptor alpha (ERalpha) expression [conditional ERalpha in mammary epithelium (CERM)] was crossed with mice carrying a germ line deletion of Stat5a [Stat5a-/-] to investigate interactions between ERalpha and Stat5a in mammary tissue. CERM, CERM/Stat5a+/-, CERM/Stat5a-/-, Stat5a+/-, Stat5a-/- and wild-type (WT) mice were generated to test the roles of ERalpha and Stat5a on pubertal differentiation and cancer progression with and without exposure to the chemical carcinogen 7,12-dimethylbenz[a]anthracene (DMBA). Only CERM/Stat5a-/- mice demonstrated delayed pubertal terminal end bud differentiation. Without DMBA exposure, Stat5a loss abrogated ERalpha-initiated hyperplastic alveolar nodule (HAN) development and, similarly, Stat5a-/- mice did not develop HANs. However, although Stat5a loss still reduced ERalpha-initiated HAN prevalence following DMBA exposure, Stat5a loss without deregulated ERalpha was associated with an increased HAN prevalence compared with WT. Progression to ERalpha(+) and ERalpha(-) adenocarcinoma was found in all CERM-containing genotypes (CERM, CERM/Stat5a+/-, CERM/Stat5a-/-) and ERalpha(+) adenocarcinoma in the Stat5a-/- genotype. The mammary epithelial cell proliferative index was increased only in CERM mice independent of Stat5a loss. No differences in apoptotic indices were found. In summary, Stat5a cooperated with deregulated ERalpha in retarding pubertal mammary differentiation and contributed to ERalpha-initiated preneoplasia, but its loss did not prevent development of invasive cancer. Moreover, in the absence of deregulated ERalpha, Stat5a loss was associated with development of both HANs and invasive cancer following DMBA exposure.

The breast cancer and the environment research centers: transdisciplinary research on the role of the environment in breast cancer etiology

OBJECTIVES

We introduce and describe the Breast Cancer and the Environment Research Centers (BCERC), a research network with a transdisciplinary approach to elucidating the role of environmental factors in pubertal development as a window on breast cancer etiology. We describe the organization of four national centers integrated into the BCERC network.

DATA SOURCES

Investigators use a common conceptual framework based on multiple levels of biologic, behavioral, and social organization across the life span. The approach connects basic biologic studies with rodent models and tissue culture systems, a coordinated multicenter epidemiologic cohort study of prepubertal girls, and the integration of community members of breast cancer advocates as key members of the research team to comprise the network.

DATA EXTRACTION

Relevant literature is reviewed that describes current knowledge across levels of organization. Individual research questions and hypotheses in BCERC are driven by gaps in our knowledge that are presented at genetic, metabolic, cellular, individual, and environmental (physical and social) levels.

DATA SYNTHESIS

As data collection on the cohort, animal experiments, and analyses proceed, results will be synthesized through a transdisciplinary approach.

CONCLUSION

Center investigators are addressing a large number of specific research questions related to early pubertal onset, which is an established risk factor for breast cancer. BCERC research findings aimed at the primary prevention of breast cancer will be disseminated to the scientific community and to the public by breast cancer advocates, who have been integral members of the research process from its inception.

A review of the molecular mechanisms of chemically induced neoplasia in rat and mouse models in National Toxicology Program bioassays and their relevance to human cancer

Tumor response in the B6C3F1 mouse, F344 rat, and other animal models following exposure to various compounds provides evidence that people exposed to these or similar compounds may be at risk for developing cancer. Although tumors in rodents and humans are often morphologically similar, underlying mechanisms of tumorigenesis are often unknown and may be different between the species. Therefore, the relevance of an animal tumor response to human health would be better determined if the molecular pathogenesis were understood. The underlying molecular mechanisms leading to carcinogenesis are complex and involve multiple genetic and epigenetic events and other factors. To address the molecular pathogenesis of environmental carcinogens, the authors examine rodent tumors (e.g., lung, colon, mammary gland, skin, brain, mesothelioma) for alterations in cancer genes and epigenetic events that are associated with human cancer. National Toxicology Program (NTP) studies have identified several genetic alterations in chemically induced rodent neoplasms that are important in human cancer. Identification of such alterations in rodent models of chemical carcinogenesis caused by exposure to environmental contaminants, occupational chemicals, and other compounds lends further support that they are of potential human health risk. These studies also emphasize the importance of molecular evaluation of chemically induced rodent tumors for providing greater public health significance for NTP evaluated compounds.

TGF-beta3 and cancer: a review

With the development of growth factors and growth factor modulators as therapeutics for a range of disorders, it is prudent to consider whether modulating the growth factor profile in a tissue can influence tumour initiation or progression. As recombinant human TGF-beta3 (avotermin) is being developed for the improvement of scarring in the skin it is important to understand the role, if any, of this cytokine in tumour progression. Elevated levels of TGF-beta3 expression detected in late-stage tumours have linked this cytokine with tumourigenesis, although functional data to support a causative role are lacking. While it has proved tempting for researchers to interpret a 'correlation' as a 'cause' of disease, what has often been overlooked is the normal biological role of TGF-beta3 in processes that are often subverted in tumourigenesis. Clarifying the role of this cytokine is complicated by inappropriate extrapolation of the data relating to TGF-beta1 in tumourigenesis, despite marked differences in biology between the TGF-beta isoforms. Indeed, published studies have indicated that TGF-beta3 may actually play a protective role against tumourigenesis in a range of tissues including the skin, breast, oral and gastric mucosa. Based on currently available data it is reasonable to hypothesize that administration of acute low doses of exogenous TGF-beta3 is unlikely to influence tumour initiation or progression.

Normal Breast Physiology: The Reasons Hormonal Contraceptives and Induced Abortion Increase Breast-Cancer Risk

A woman gains protection from breast cancer by completing a full-term pregnancy. In utero, her offspring produce hormones that mature 85 percent of the mother's breast tissue into cancer-resistant breast tissue. If the pregnancy ends through an induced abortion or a premature birth before thirty-two weeks, the mother's breasts will have only partially matured, retaining even more cancer-susceptible breast tissue than when the pregnancy began. This increased amount of immature breast tissue will leave the mother with more sites for cancer initiation, thereby increasing her risk of breast cancer. Hormonal contraceptives increase breast-cancer risk by their proliferative effect on breast tissue and their direct carcinogenic effects on DNA. Hormonal contraceptives include estrogen-progestin combination drugs prescribed in any manner of delivery: orally, transdermally, vaginally, or intrauterine. This article provides the detailed physiology and data that elucidate the mechanisms through which induced abortion and hormonal contraceptives increase breast-cancer risk.

Growth effects of insulin and insulin analogues

The definition of mitogenic activity of insulin is controversial. Under physiological conditions, mitogenic refers to cell proliferation and tissue repair. In pathological conditions, it may refer to stimulation of tumour cells in pre-existing (undiagnosed) tumours. The in vitro investigations using benign and malignant cell lines compare proliferative activity of insulin molecules (animal, human and analogues). In these studies, inclusion of [B10-Asp] insulin would be a valuable link to the existing evidence on proliferation of mammary tissue in rodents. Animal and human insulin have growth promoting activity on spontaneously arising tumours (e.g. mammary tumours in rodents). They have no carcinogenic activity (cell transformation), and moreover insulin is not a co-carcinogen when evaluated in special toxicology. Mitogenicity (growth promoting activity) of insulin may be a problem in people with undiagnosed tumours, and may require definition of patient groups who would benefit from targeted monitoring.

Polymorphisms in estrogen biosynthesis and metabolism-related genes, ionizing radiation exposure, and risk of breast cancer among US radiologic technologists

Ionizing radiation-associated breast cancer risk appears to be modified by timing of reproductive events such as age at radiation exposure, parity, age at first live birth, and age at menopause. However, potential breast cancer risk modification of low to moderate radiation dose by polymorphic estrogen metabolism-related gene variants has not been routinely investigated. We assessed breast cancer risk of 12 candidate variants in 12 genes involved in steroid metabolism, catabolism, binding, or receptor functions in a study of 859 cases and 1,083 controls within the US radiologic technologists (USRT) cohort. Using cumulative breast dose estimates from a detailed assessment of occupational and personal diagnostic ionizing radiation exposure, we investigated the joint effects of genotype on the risk of breast cancer. In multivariate analyses, we observed a significantly decreased risk of breast cancer associated with the CYP3A4 M445T minor allele (rs4986910, OR = 0.3; 95% CI 0.1-0.9). We found a borderline increased breast cancer risk with having both minor alleles of CYP1B1 V432L (rs1056836, CC vs. GG, OR = 1.2; 95% CI 0.9-1.6). Assuming a recessive model, the minor allele of CYP1B1 V432L significantly increased the dose-response relationship between personal diagnostic X-ray exposure and breast cancer risk, adjusted for cumulative occupational radiation dose (p (interaction) = 0.03) and had a similar joint effect for cumulative occupational radiation dose adjusted for personal diagnostic X-ray exposure (p (interaction) = 0.06). We found suggestive evidence that common variants in selected estrogen metabolizing genes may modify the association between ionizing radiation exposure and breast cancer risk.

A comparison of the effects of soya isoflavonoids and fish oil on cell proliferation, apoptosis and the expression of oestrogen receptors alpha and beta in the mammary gland and colon of the rat

Isoflavonoids and fish oil may be protective against colorectal cancer, but the evidence in relation to breast cancer risk is ambiguous. In the present study, we have investigated the impact of soya-derived isoflavonoids and n-3 fatty acids from fish oil, both individually and in combination, on apoptosis, cell proliferation and oestrogen receptor (ER) expression in the colon and mammary gland of the rat. Female rats were fed diets high in n-3 fatty acids (80 g/kg diet) or soya protein (765 mg/kg diet isoflavones) for 2 weeks, and then killed before the removal of the colon and mammary glands. Cell proliferation and apoptosis were quantified morphologically in whole crypts and terminal end buds. The expressions of ERalpha and ERbeta were measured in colon tissue scrapes and the mammary gland. Fish oil significantly increased apoptosis and decreased mitosis in both tissues, an effect associated with a decrease in the expressions of ERalpha and ERbeta. Soya had no effect on apoptosis in either tissue, but reduced mitosis in the colon (P < 0.001) while increasing it in the mammary gland (P = 0.001). The changes in proliferation were associated with contrasting changes in the ER expression such that fish oil significantly decreased both ERbeta and ERalpha, while soya increased ERalpha and decreased ERbeta. The results may provide a novel mechanism by which n-3 fatty acids could reduce cancer risk, but the interpretation of the results in relation to soya consumption and breast cancer risk requires further investigation.

Does iron have a role in breast cancer?

Oestrogen and family history are two of the most important risk factors for breast cancer. However, these risk factors cannot explain the differences in the incidence and recurrence of breast cancer between premenopausal and postmenopausal women. In this paper I propose that, in premenopausal women, an iron deficiency caused by menstruation stabilises hypoxia inducible factor-1alpha, which increases the formation of vascular endothelial growth factor. This mechanism results in premenopausal women being more susceptible to angiogenesis and, consequently, leads to a high recurrence of breast cancer. Conversely, increased concentrations of iron in postmenopausal women, as a result of menstrual cessation, contribute to a high incidence of breast cancer via oxidative-stress pathways. Although the focus of this Personal View is on iron, this by no means negates the roles of other known risk factors in breast-cancer development. Characterisation of the role of iron in breast cancer could potentially benefit patients by decreasing recurrence and incidence and increasing overall survival.

Paracrine overexpression of insulin-like growth factor-1 enhances mammary tumorigenesis in vivo

Insulin-like growth factor-1 (IGF-1) stimulates proliferation, regulates tissue development, protects against apoptosis, and promotes the malignant phenotype in the breast and other organs. Some epidemiological studies have linked high circulating levels of IGF-1 with an increased risk of breast cancer. To study the role of IGF-1 in mammary tumorigenesis in vivo, we used transgenic mice in which overexpression of IGF-1 is under the control of the bovine keratin 5 (BK5) promoter and is directed to either the myoepithelial or basal cells in a variety of organs, including the mammary gland. This model closely recapitulates the paracrine exposure of breast epithelium to stromal IGF-1 seen in women. Histologically, mammary glands from transgenic mice were hyperplastic and highly vascularized. Mammary glands from prepubertal transgenic mice had significantly increased ductal proliferation compared with wild-type tissues, although this difference was not maintained after puberty. Transgenic mice also had increased susceptibility to mammary carcinogenesis, and 74% of the BK5.IGF-1 mice treated with 7,12-dimethylbenz[a]anthracene (20 microg/day) developed mammary tumors compared with 29% of the wild-type mice. Interestingly, 31% of the vehicle-treated BK5.IGF-1 animals, but none of the wild-type animals, spontaneously developed mammary cancer. The mammary tumors were moderately differentiated adenocarcinomas that expressed functional, nuclear estrogen receptor at both the protein and mRNA levels. These data support the hypothesis that tissue overexpression of IGF-1 stimulates mammary tumorigenesis.

Characterization of breast cancer progression in the rat

The incidence of breast cancer is continuously increasing worldwide. This increasing trend is attributed partly to the fact that a considerable number of cases are related to environmental factors and partly to the little information available on the early changes that occur during mammary gland carcinogenesis. To characterize some of these early cellular changes, breast cancer was induced in female rats using a single intragastric dose of the environmental carcinogen 7,12-dimethylbenz[a]anthracene (DMBA; 80 mg/kg body weight). Mammary gland tissues of control and DMBA-treated rats were processed for routine histopathological examination and immunohistochemical analysis using an antibody specific for the proliferating cell nuclear antigen (PCNA). Microscopic examination of all mammary glands of DMBA-treated rats revealed a wide range of preneoplastic stages in addition to the well-characterized benign and malignant tumors that developed. The first stage was characterized by slightly dilated terminal ducts with accumulation of dead cells. This was designated the stage of cell death. Then, stages of hyperplasia, dysplasia, and carcinoma in situ followed. Immunohistochemical localization of PCNA in these preneoplastic lesions revealed an initial decrease followed by a gradual increase in the labeling index of PCNA. In conclusion, the DMBA-treated rats provide a useful model to dissect the early changes that occur during the multistep process of mammary gland carcinogenesis.

Lack of c-kit receptor promotes mammary tumors in N-nitrosomethylurea-treated Ws/Ws rats

BACKGROUND

c-kit is expressed in various cell types during development and it has been linked to the promotion of cellular migration, proliferation and/or survival of melanoblasts, hematopoietic progenitors and primordial germ cells. Several reports have proposed a role for the c-kit gene on carcinogenesis. Gain-of-function mutations are associated with diseases such as mastocytosis and gastrointestinal stromal tumors among others. However, very little is known about pathologies associated with loss-of-function mutations. Regarding breast cancer, c-kit protein and mRNA are highly expressed in normal breast but their expression decreases or is absent in the presence of breast cancer. We studied the role of c-kit in mammary carcinogenesis in the Ws/Ws rats carrying spontaneous lack-of-function mutation in the c-kit gene. Fifty day-old virgin female Ws/Ws rats and their wild type counterparts were injected with either 50 mg/kg body weight of the chemical carcinogen N-nitrosomethylurea or with vehicle. The animals were followed-up for 6 months. Fisher 344 rats were used as positive controls for tumor development.

RESULTS

Eleven weeks after treatment, palpable tumors were detected in the Ws/Ws rats. The tumor incidence was 80% in Ws/Ws rats, while no tumors were observed in the wild type rats (p = 0.006). Our data show that the lack of c-kit is permissive for the development of mammary tumor in Ws/Ws rats treated with carcinogen.

CONCLUSION

We conclude that the lack of c-kit may contribute to an imbalanced homeostatic state in the mammary gland either by affecting signaling between stroma and epithelium, or through the lack of mast cells.

Review of the etiology of breast cancer with special attention to organochlorines as potential endocrine disruptors

Breast cancer is the most frequently diagnosed cancer among Canadian women, accounting for about 30% of all new cancer cases each year. Although the incidence of breast cancer has increased over the past 50 years, the cause of this rise is unknown. Risk factors for breast cancer may be classified into four broad categories: (1) genetic/familial, (2) reproductive/hormonal, (3) lifestyle, and (4) environmental. Established risk factors for breast cancer include older age, later age at first full-term pregnancy, no full-term pregnancies, postmenopausal obesity, and genetic factors. However, these known risk factors cannot account for the majority of cases. In the early 1990s, it was suggested that exposure to some environmental chemicals such as organochlorine compounds may play a causal role in the etiology of breast cancer through estrogen-related pathways. The relationship between organochlorines and breast cancer risk has been studied extensively in the past decade and more, and at this point there is no clear evidence to support a causal role of most organochlorine pesticides in the etiology of human breast cancer, but more evidence is needed to assess risk associated with polychlorinated biphenyls (PCBs). Future studies need to consider the combined effects of exposures, concentrate on vulnerable groups such as those with higher levels of exposure, only consider exposures occurring during the most etiologically relevant time periods, and more thoroughly consider gene-environment interactions.

The plasticizer butyl benzyl phthalate induces genomic changes in rat mammary gland after neonatal/prepubertal exposure

BACKGROUND

Phthalate esters like n-butyl benzyl phthalate (BBP) are widely used plasticizers. BBP has shown endocrine-disrupting properties, thus having a potential effect on hormone-sensitive tissues. The aim of this study is to determine the effect of neonatal/prepubertal exposure (post-natal days 2-20) to BBP on maturation parameters and on the morphology, proliferative index and genomic signature of the rat mammary gland at different ages of development (21, 35, 50 and 100 days).

RESULTS

Here we show that exposure to BBP increased the uterine weight/body weight ratio at 21 days and decreased the body weight at time of vaginal opening. BBP did not induce significant changes on the morphology of the mammary gland, but increased proliferative index in terminal end buds at 35 days and in lobules 1 at several ages. Moreover, BBP had an effect on the genomic profile of the mammary gland mainly at the end of the exposure (21 days), becoming less prominent thereafter. By this age a significant number of genes related to proliferation and differentiation, communication and signal transduction were up-regulated in the glands of the exposed animals.

CONCLUSION

These results suggest that BBP has an effect in the gene expression profile of the mammary gland.

Prolactin, TNF alpha and nitric oxide expression in nitroso-N-methylurea-induced-mammary tumours

Background

The N-Nitrosomethylurea breast cancer model induced in rats is used for the study of carcinogenesis in mammary cancer, prostate, pancreas, etc. This model is very similar to human neoplastic disease.

Methods

The present experimental study was designed to assess whether metoclopramide administration has any effect on development of MNU-induced tumours, and evaluate the treatment of goserelin acetate on PRL, TNF alpha and NO expression. NMU was administered to female Wistar rats on 2 occasions (5 mg/100 g body w/rat). PRL and TNF alpha were performed by immune-assay. Nitric Oxide by semi automated-assay and ploidy analyses by flow cytometry.

Results

The administration of metoclopramide made the induction time shorter and increased the incidence and average of tumours per rat. Tumours development was inhibited by a goserelin chronic administration. The ploidy of adenocarcinoma was polyploid-aneuploid type (average S = 60%). It was higher basal PRL plasma levels in rats with NMU induced tumours than in basal controls without tumour (p < 0.001). The goserelin "in bolus" administration showed maximal inhibition of plasma PRL at 90 min. Plasmatic TNF alpha expression was inhibited at 60 min and also remained inhibited in tissue homogenate post chronic treatment (P < 0.0125). Plasmatic NO expression is higher in rats with induced tumours than healthy controls (P < 0.001). In tissue homogenate NO values were inhibited at 90 min (P < 0.01), as well during chronically goserelin treatment (P < 0.005).

Conclusion

The increase of blood PRL levels in NMU-induced rats may be an indicator of a poor prognosis of mammary cancer evolution. The metoclopramide administration accelerates tumour growth. However goserelin administration achieves regression in tumour development associated to inhibition PRL, TNF alpha and NO expression.

Charles River Sprague Dawley rats lack early age-dependent susceptibility to DMBA-induced mammary carcinogenesis

Developmental stages of mammary glands influence their susceptibility to initiating events related to carcinogenesis. The "window of susceptibility" to mammary carcinogenesis is classically defined as the time in early puberty when the mammary gland morphology is most sensitive to initiation events. Administration of the polyaromatic hydrocarbon, 7,12-dimethylbenz(a)anthracene (DMBA), in a single oral dose yields maximal mammary tumor formation when administered in this "window". We examined the DMBA treated mammary glands, precursor lesions, and morphology of the uninvolved mammary epithelium for the first 100 days of life for Charles River Sprague Dawley CD(R) IGS. Our goal was to determine the DMBA dose at which 50% of the rats (IC50) developed carcinoma in situ (CIS) within three months of dosing. Here we demonstrate, rather than the classical U-shaped dose curve in which there is maximum sensitivity for DMBA at 50 days, there is an increasing degree of sensitivity with age in the CD(R) IGS rat. Additionally, we report that vehicle-treated animals developed mammary CIS without any known initiator, and 100 day virgin animals demonstrated lactational changes, independent of DMBA exposure or dose. Lastly, we demonstrate this strain of virgin female rats has elevated pituitary prolactin immunoreactivity independent of the level of mammary differentiation. We conclude this strain of Charles River Sprague Dawley rats has prolactin-induced pituitary stimulation, and therefore, the window of susceptibility for mammary tumorigenesis is absent.

Estrogen receptor-alpha expression in the mammary epithelium is required for ductal and alveolar morphogenesis in mice

The estrogen receptor-alpha (ERalpha) is a critical transcription factor that regulates epithelial cell proliferation and ductal morphogenesis during postnatal mammary gland development. Tissue recombination and transplantation studies using the first generation of ERalpha knockout (ERKO) mice suggested that this steroid hormone receptor is required in the mammary stroma that subsequently exerts its effect on the epithelium through additional paracrine signaling events. A more detailed analysis revealed that ERKO mice produce a truncated ERalpha protein with detectable transactivation activity, and it is likely that this functional ERalpha variant has masked the biological significance of this steroid receptor in the mammary epithelium. In this article, we describe the generation a Cre-lox-based conditional knockout of the ERalpha gene to study the biological function of this steroid receptor in the epithelial compartment at defined stages of mammary gland development. The mouse mammary tumor virus (MMTV)-Cre-mediated, epithelial-specific ablation of exon 3 of the ERalpha gene in virgin mice severely impaired ductal elongation and side branching. The conditional knockout resulted in ablation of the ERalpha protein, and the progesterone receptor (PR), whose expression is under the control of ERalpha, was largely absent. The whey acidic protein (WAP)-Cre-mediated deletion of ERalpha during successive gestation cycles resulted in a loss of ductal side-branching and lobuloalveolar structures, ductal dilation, and decreased proliferation of alveolar progenitors. These abnormalities compromised milk production and led to malnourishment of the offspring by the second lactation. These observations suggest that ERalpha expression in the mammary epithelium is essential for normal ductal morphogenesis during puberty and alveologenesis during pregnancy and lactation.

HER-2/neu x aromatase double transgenic mice model: the effects of aromatase overexpression on mammary tumorigenesis

A majority of breast cancers are hormone-responsive, and require estrogen for growth, and respond to hormonal therapy that blocks estrogen receptor action. Breast tumors with low levels of or completely lacking estrogen receptor fail to respond to antiestrogen therapy yet require estrogen for tumor initiation. To address the importance of local estrogen in oncogene-mediated breast tumorigenesis, we have crossed MMTV-aromatase with MMTV-HER2/neu and examined the incidence of breast cancer in double transgenic mice in comparison with parental strains. Double transgenic mice show normal mammary development and express both transgenes at similar levels to that of parental strains. Tumor incidence in double transgenic mice (<5%) decreased compared to HER2/neu mice (>65%). In addition to a significant decrease in tumorigenesis, these mice expressed ERalpha as well as high levels of ERbeta along with decreased levels of cyclin D1 and phosphorylated pRb among other changes. Furthermore, experiments using THC (ERalpha-agonist and ERbeta-antagonist) clearly demonstrate the critical role of ERbeta in HER2/neu-mediated tumorigenesis. These studies provide the first genetic evidence that estrogen receptor, mainly ERbeta than ERalpha and its dependent changes play an important role in regulating mammary tumorigenesis. These findings provide further evidence for development and testing of novel therapeutic approaches based on selective regulation of estrogen receptors (ERalpha and beta)-dependent actions for the treatment and prevention of breast cancers.

Chemicals causing mammary gland tumors in animals signal new directions for epidemiology, chemicals testing, and risk assessment for breast cancer prevention

Identifying chemical carcinogens in animal studies is currently the primary means of anticipating cancer effects in humans. Animal studies to evaluate potential chemical carcinogenicity are particularly important for breast cancer because environmental and occupational epidemiologic research is sparse. Chemicals that increased mammary gland tumors in animal studies were compiled from the International Agency for Research on Cancer (IARC), the U.S. National Toxicology Program (NTP), and other sources. Summary assessments of the carcinogenic potential for each chemical and potentially exposed populations were also compiled. In all, 216 chemicals were identified that have been associated with increases in mammary gland tumors in at least 1 study. These include industrial chemicals, chlorinated solvents, products of combustion, pesticides, dyes, radiation, drinking water disinfection byproducts, pharmaceuticals and hormones, natural products, and research chemicals. Twenty-nine are produced in the U.S. at >1 million pounds/year; 35 are air pollutants, 25 have involved occupational exposures to >5000 women, and 73 have been present in consumer products or as contaminants of food. Thus, exposure is widespread. Nearly all of the chemicals were mutagenic and most caused tumors in multiple organs and species; these characteristics are generally believed to indicate likely carcinogenicity in humans. To our knowledge, this is the most comprehensive list developed of animal mammary gland carcinogens and, along with associated data, is publicly available at URL: www.silentspring.org/sciencereview and at URL: www.komen.org/environment. Valuable information from cancer bioassays is not well utilized in risk assessment and regulatory processes, suggesting a need to strengthen chemicals testing and risk assessment as tools for breast cancer prevention.