Parous rats regain high susceptibility to chemically induced mammary cancer after treatment with various mammotropic hormones

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.

Environmental exposures during windows of susceptibility for breast cancer: a framework for prevention research

Background

The long time from exposure to potentially harmful chemicals until breast cancer occurrence poses challenges for designing etiologic studies and for implementing successful prevention programs. Growing evidence from animal and human studies indicates that distinct time periods of heightened susceptibility to endocrine disruptors exist throughout the life course. The influence of environmental chemicals on breast cancer risk may be greater during several windows of susceptibility (WOS) in a woman’s life, including prenatal development, puberty, pregnancy, and the menopausal transition. These time windows are considered as specific periods of susceptibility for breast cancer because significant structural and functional changes occur in the mammary gland, as well as alterations in the mammary micro-environment and hormone signaling that may influence risk. Breast cancer research focused on these breast cancer WOS will accelerate understanding of disease etiology and prevention.

Main text

Despite the plausible heightened mechanistic influences of environmental chemicals on breast cancer risk during time periods of change in the mammary gland’s structure and function, most human studies of environmental chemicals are not focused on specific WOS. This article reviews studies conducted over the past few decades that have specifically addressed the effect of environmental chemicals and metals on breast cancer risk during at least one of these WOS. In addition to summarizing the broader evidence-base specific to WOS, we include discussion of the NIH-funded Breast Cancer and the Environment Research Program (BCERP) which included population-based and basic science research focused on specific WOS to evaluate associations between breast cancer risk and particular classes of endocrine-disrupting chemicals—including polycyclic aromatic hydrocarbons, perfluorinated compounds, polybrominated diphenyl ethers, and phenols—and metals. We outline ways in which ongoing transdisciplinary BCERP projects incorporate animal research and human epidemiologic studies in close partnership with community organizations and communication scientists to identify research priorities and effectively translate evidence-based findings to the public and policy makers.

Conclusions

An integrative model of breast cancer research is needed to determine the impact and mechanisms of action of endocrine disruptors at different WOS. By focusing on environmental chemical exposure during specific WOS, scientists and their community partners may identify when prevention efforts are likely to be most effective.

Effects of parity and serum prolactin levels on the incidence and regression of DMBA-induced tumors in OFA hr/hr rats

Prolactin (PRL) is a key player in the development of mammary cancer. We studied the effects of parity or hyperprolactinemia on mammary carcinogenesis in OFA hr/hr treated with 7,12-dimethylbenzanthracene. They were divided into three groups: nulliparous (Null), primiparous (PL, after pregnancy and lactation), and hyperprolactinemic rats (I, implanted in the arcuate nucleus with 17β-estradiol). The tumor incidence was similar in the three groups. However, a higher percentage of regressing tumors was evident in the PL group. Serum PRL, mammary development, and mammary β-casein content were higher in I rats compared to Null. The expression of hormone receptors was similar in the different groups. However, mammary tissue from PL rats bearing tumors had increased expression of PRL and estrogen alpha receptors compared to rats free of tumors. Our results suggest that serum PRL levels do not have relevance on the incidence of tumors, probably because the low levels of PRL in OFA rats are not further decreased by PL like in other strains. However, supraphysiological levels of PRL affect carcinogenesis. PL induces regression of the tumors due to the differentiation produced on the mammary cells. Alterations in the expression of hormonal receptors may be involved in progression and regression of tumors.

The effect of high gravidity on the carcinogenesis of mammary gland in TA2 mice

PROBLEM

Spontaneous breast cancer in Tientsin Albinao 2 (TA2) mice, like human pregnancy-associated breast cancer (PABC), often occurs in pregnancy and puerperium, especially in mice with high gravidity. We hypothesized that the dysfunction of cellular immunity caused by the increase of 17beta-estradiol (E2) and progesterone (P) might be one of the reasons for carcinogenesis of mammary gland.

METHOD OF STUDY

We investigated the T lymphocyte subsets and the concentration of serum hormone and cytokines in cancer-bearing, pregnant or postpartum TA2 mice using flow cytometry, chemiluminescent immunoassay, and enzyme-linked immunosorbent assay (ELISA), respectively.

RESULTS

The number of T lymphocytes and the concentration of E2, P, interleukin-2 (IL-2), IL-4, and interferon-gamma (IFN-gamma) changed with the increase of pregnancy and puerperium. During four pregnancies, elevated E2 and P resulted in a decrease in the number of CD3(+), CD4(+) T lymphocytes, CD4(+)/CD8(+) ratio, and the concentration of IL-2, IL-4, and IFN-gamma. Data in the fourth pregnancy were the closest to those of cancer-bearing mice.

CONCLUSION

T lymphocyte subsets and concentration of IL-2, IL-4, and IFN-gamma are affected by E2 and P during multiple pregnancy and delivery to some degree, which may contribute to the genesis of spontaneous breast cancer in TA2 mice.

Decreasing hormonal promotion is key to breast cancer prevention

An early full-term pregnancy in women is highly protective against breast cancer. This protection can be mimicked by short-term treatment with estradiol plus progesterone in nulliparous rats. We determined the effect of long-term hormonal promotion following the protective short-term estradiol and progesterone treatment that mimics parity protection against mammary tumors. Rats were treated with N-methyl-N-nitrosourea before or after protective hormone treatment. In brief, the animals could be broadly classified into three categories. First, the controls that received no protective hormone treatment, second, the short-term protective hormone treated rats, and third, rats which received the short-term protective hormone treatment plus continuous treatment with estradiol or progesterone or a combination of estradiol and progesterone. Different doses of hormones were used for short-term protective and long-term promotion treatments. The experiments were terminated 9 months post carcinogen treatment. Mammary tumor incidence in all the short-term estradiol- plus progesterone-treated rats was significantly lower compared with controls. Short-term hormone treatment followed by long-term promotion resulted in an increased mammary tumor incidence compared with animals that received only the short-term treatment. Overall, the results demonstrate the importance of the promotional environment in mammary carcinogenesis indicating that the decreased promotional environment could be the reason for protection against mammary carcinogenesis.

Estrogenic effects of a Kampo formula, Tokishakuyakusan, in parous ovariectomized rats

Female hormone-dependent cancers and other diseases pose a serious health threat for women, and low-risk medicines against such cancers have not yet been discovered. The present study examines the effects of the traditional Chinese herbal mixture, Tokishakuyakusan (TS) and 17beta-estradiol on the uterus of parous ovariectomized rats. Uterine atrophy that causes a reduction in uterine tissue and the uterine cavity area, was induced by ovariectomy, and slightly recovered by the daily oral administration of TS for two weeks (1000 mg/kg body weight). TS restored the decreased plasma estradiol concentration due to ovariectomy. However the yeast two-hybrid assay showed that TS did not bind estrogen receptors alpha and beta and immunohistochemical staining revealed that 17beta-estradiol stimulated the protein expression of estrogen receptor alpha, progesterone receptor, c-fos and c-jun in the uterus, whereas TS did not. These results suggest that TS might be useful for treating menopausal syndromes among women, as well as for patients when hormone replacement therapy (HRT) with estrogen is contraindicated.

Induction of a novel histone deacetylase 1/c-Myc/Mnt/Max complex formation is implicated in parity-induced refractoriness to mammary carcinogenesis

Refractoriness to carcinogen-induced increases in epithelial cell proliferation is a very important characteristic of parous mammary glands. We found that N-methyl-N-nitrosourea (MNU)-induced proliferative burst in the mammary ductal epithelium was blocked in parous glands but not in age-matched virgin (AMV) glands. The inhibition of the proliferative burst in MNU-treated parous mammary glands coincided with the upregulation of Mnt, a Myc-suppressor, and the formation of histone deacetylase 1/Mnt/Max complexes that unexpectedly contained c-Myc. These complexes formed on the promoters of Myc targets, such as ornithine decarboxylase, cyclin D2, and transforming growth factor beta1 genes, in quiescent fibroblasts, and were disassembled in serum-stimulated cells. These results suggest that the complexes also function as transcription repressors of the growth-related Myc targets in MNU-treated parous mammary glands. Using the chemical mammary carcinogenesis model of human c-Ha-ras transgenic (Tg) rats, we confirmed that parity protected the mammary glands at the postinitiation phase of tumorigenesis. Although the incidence of 7,12-dimethylbenz[alpha]anthracene-induced palpable tumors was reduced from 61.5% in the AMV Tg rats to 28.5% in the parous animals, the incidence of early neoplastic lesions in the parous rats was the same as that in the AMV rats. Restriction fragment length polymorphism analysis detected mutations in the human c-Ha-ras gene in most of the normal-appearing parous Tg glands, as well as in the virgin glands. We propose that accelerated formation of HDAC1/c-Myc/Mnt/Max complexes in response to carcinogen exposure results in down-regulation of growth-related genes, leading to the refractoriness of parous mammary glands at the postinitiation phase of carcinogenesis.

Inhibition of mammary tumorigenesis by estrogen and progesterone in genetically engineered mice

Estrogen and progesterone play a critical role in normal and neoplastic development of the mammary gland. A long duration of estrogen and progesterone exposure is associated with increased breast cancer risk, and a short duration of the same doses of these hormones is associated with a reduced breast cancer risk. The protective effects of estrogen and progesterone have been extensively studied in animal models. Several studies have demonstrated that these hormones induce persistent and long-lasting alterations in gene expression in the mammary epithelial cells. In the experiments discussed herein, the protective effect of estrogen and progesterone is shown to occur in genetically engineered mice (the p53-null mammary gland). The protective effect is associated with a decrease in cell proliferation. The effects of hormones seem to manifest as a delay in premalignant progression. In the nontumor-bearing glands of hormone-treated mice, premalignant foci are present at the time the control glands are actively developing mammary tumors. If the hormone-treated cells are transplanted from the treated host to the untreated host, the cells resume their predetermined tumorigenic potential. The protective effect reflects both host-mediated factors (either stroma-determined or systemic factors) and mammary epithelial intrinsic changes. If normal, untreated p53 cells are transplanted into a host that has been previously treated with a short dose of hormones, the cells exhibit a significant delay in tumorigenesis. The relative contributions of host-mediated factors and mammary cell intrinsic factors remain to be determined. Current studies are moving this research area from the biological to the molecular realm and from the rodent models to human studies and offer the potential for directing prevention efforts at specific molecular targets.

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.

Hormone-induced protection of mammary tumorigenesis in genetically engineered mouse models

INTRODUCTION

The experiments reported here address the question of whether a short-term hormone treatment can prevent mammary tumorigenesis in two different genetically engineered mouse models.

METHODS

Two mouse models, the p53-null mammary epithelial transplant and the c-neu mouse, were exposed to estrogen and progesterone for 2 and 3 weeks, respectively, and followed for development of mammary tumors.

RESULTS

In the p53-null mammary transplant model, a 2-week exposure to estrogen and progesterone during the immediate post-pubertal stage (2 to 4 weeks after transplantation) of mammary development decreased mammary tumorigenesis by 70 to 88%. At 45 weeks after transplantation, analysis of whole mounts of the mammary outgrowths demonstrated the presence of premalignant hyperplasias in both control and hormone-treated glands, indicating that the hormone treatment strongly affects the rate of premalignant progression. One possible mechanism for the decrease in mammary tumorigenesis may be an altered proliferation activity as the bromodeoxyuridine labeling index was decreased by 85% in the mammary glands of hormone-treated mice. The same short-term exposure administered to mature mice at a time of premalignant development also decreased mammary tumorigenesis by 60%. A role for stroma and/or systemic mediated changes induced by the short-term hormone (estrogen/progesterone) treatment was demonstrated by an experiment in which the p53-null mammary epithelial cells were transplanted into the cleared mammary fat pads of previously treated mice. In such mice, the tumor-producing capabilities of the mammary cells were also decreased by 60% compared with the same cells transplanted into unexposed mice. In the second set of experiments using the activated Her-2/neu transgenic mouse model, short-term estradiol or estradiol plus progesterone treatment decreased mammary tumor incidence by 67% and 63%, and tumor multiplicity by 91% and 88%, respectively. The growth rate of tumors arising in the hormone-treated activated Her-2/neu mice was significantly lower than tumors arising in non-hormone treated mice.

CONCLUSION

Because these experiments were performed in model systems that mimic many essential elements of human breast cancer, the results strengthen the rationale for translating this prevention strategy to humans at high risk for developing breast cancer.

Autocrine growth hormone prevents lactogenic differentiation of mouse mammary epithelial cells

We have examined the expression, postnatal ontogeny, and localization of mouse GH (mGH) and its relative expression during pregnancy, lactation, and weaning in the mouse. mGH mRNA and protein was expressed predominantly in the epithelial component of the mammary gland, and maximal expression was observed during the pubertal period. Autocrine mGH expression dramatically decreased during late pregnancy and lactation. Concordantly, autocrine mGH expression is repressed during forced differentiation of mouse HC11 mammary epithelial cells in culture. Forced expression of mGH in HC11 cells abrogated lactogenic differentiation as indicated by reduced expression of beta-casein and reduced expression and loss of lateral epithelial localization of E-cadherin. Forced expression of mGH in mouse mammary epithelial cells increased cell survival and proliferation and consequently increased the size of mammary acinar-like structures formed in three-dimensional Matrigel. Thus, autocrine mGH expression in the mouse mammary epithelial cell is maximal at puberty and prevents mammary epithelial cell differentiation. Autocrine GH will therefore participate in mammary morphogenic processes at puberty.

Insulin-like growth factor (IGF)-I obliterates the pregnancy-associated protection against mammary carcinogenesis in rats: evidence that IGF-I enhances cancer progression through estrogen receptor-alpha activation via the mitogen-activated protein kinase pathway

Introduction

Pregnancy protects against breast cancer development in humans and rats. Parous rats have persistently reduced circulating levels of growth hormone, which may affect the activity of the growth hormone/insulin-like growth factor (IGF)-I axis. We investigated the effects of IGF-I on parity-associated protection against mammary cancer.

Methods

Three groups of rats were evaluated in the present study: IGF-I-treated parous rats; parous rats that did not receive IGF-I treatment; and age-matched virgin animals, which also did not receive IGF-I treatment. Approximately 60 days after N-methyl-N-nitrosourea injection, IGF-I treatment was discontinued and all of the animal groups were implanted with a silastic capsule containing 17β-estradiol and progesterone. The 17β-estradiol plus progesterone treatment continued for 135 days, after which the animals were killed.

Results

IGF-I treatment of parous rats increased mammary tumor incidence to 83%, as compared with 16% in parous rats treated with 17β-estradiol plus progesterone only. Tumor incidence and average number of tumors per animal did not differ between IGF-I-treated parous rats and age-matched virgin rats. At the time of N-methyl-N-nitrosourea exposure, DNA content was lowest but the α-lactalbumin concentration highest in the mammary glands of untreated parous rats in comparison with age-matched virgin and IGF-I-treated parous rats. The protein levels of estrogen receptor-α in the mammary gland was significantly higher in the age-matched virgin animals than in untreated parous and IGF-I-treated parous rats. Phosphorylation (activation) of the extracellular signal-regulated kinase-1/2 (ERK1/2) and expression of the progesterone receptor were both increased in IGF-I-treated parous rats, as compared with those in untreated parous and age-matched virgin rats. Expressions of cyclin D₁ and transforming growth factor-β₃ in the mammary gland were lower in the age-matched virgin rats than in the untreated parous and IGF-I-treated parous rats.

Conclusion

We argue that tumor initiation (transformation and fixation of mutations) may be similar in parous and age-matched virgin animals, suggesting that the main differences in tumor formation lie in differences in tumor progression caused by the altered hormonal environment associated with parity. Furthermore, we provide evidence supporting the notion that tumor growth promotion seen in IGF-I-treated parous rats is caused by activation of estrogen receptor-α via the Raf/Ras/mitogen-activated protein kinase cascade.

Association of increased estrogen receptor beta2 expression with parity-induced alterations in the rat mammary gland

In this study, we investigated the cellular and molecular events involved in parity-related alterations in mammary gland (MG) proliferation and differentiation. Rat MGs were removed on day 9 of either first (nulliparous), second (primiparous) or third (multiparous) pregnancy. Expression of steroid hormone receptors along with cellular biomarkers of proliferation and differentiation were quantified in all MG tissue compartments by immunohistochemistry. Wnt-4 (a Wingless-like morphogenic gene involved in MG development), ERbeta and ERbeta2 mRNA were evaluated by RT-PCR analysis. Serum levels of mammotrophic hormones were measured. In comparison to nulliparous and primiparous rats, multiparous animals exhibited decreased luminal cell proliferation and PR levels, whereas alpha-lactalbumin, ERalpha, ERbeta and ERbeta2 expression were increased. In myoepithelial cells, while parity induced a decrease in proliferative activity, subsequent pregnancies and lactations lead to an increased state of differentiation. Our results showed that at least two periods of pregnancy and lactation were necessary to modify the studied parameters. The lower proliferative activity and higher differentiation state of the multiparous MG are associated with both a decreased PR expression and increased ERalpha and ERbeta expression. Since ERbeta and/or ERbeta2 isoform expression was related to parity history, results suggest that the decreased proliferative activity and PR expression observed in the MG of multiparous animals may be associated with overexpression of ERbeta and/or the ERbeta2 isoform, thereby antagonizing the proliferative effects associated with ERalpha.

Progesterone prevents radiation-induced apoptosis in breast cancer cells

Sex steroid hormones play an essential role in the control of homeostasis in the mammary gland. Although the involvement of progesterone in cellular proliferation and differentiation is well established, its exact role in the control of cell death still remains unclear. As dysregulation of the apoptotic process plays an important role in the pathogenesis of breast cancer, we investigated the regulation of apoptosis by progesterone in various breast cancer cell lines. Our results show that progesterone treatment protects against radiation-induced apoptosis. This prevention appears to be mediated by the progesterone receptor and is unrelated to p53 status. There is also no correlation with the intrinsic hormonal effect on cell proliferation, as the presence of cells in a particular phase of the cell cycle. Surprisingly, progesterone partly allows bypassing of the irradiation-induced growth arrest in G(2)/M in PgR+ cells, leading to an increase in cell proliferation after irradiation. One consequence of this effect is a higher rate of chromosome damage in these proliferating progesterone-treated cells compared to what is observed in untreated irradiated cells. We propose that progesterone, by inhibiting apoptosis and promoting the proliferation of cells with DNA damage, potentially facilitates the emergence of genetic mutations that may play a role in malignant transformation.

The role of prolactin in mammary carcinoma

The contribution of prolactin (PRL) to the pathogenesis and progression of human breast cancer at the cellular, transgenic, and epidemiological levels is increasingly appreciated. Acting at the endocrine and autocrine/paracrine levels, PRL functions to stimulate the growth and motility of human breast cancer cells. The actions of this ligand are mediated by at least six recognized PRL receptor isoforms found on, or secreted by, human breast epithelium. The PRL/PRL receptor complex associates with and activates several signaling networks that are shared with other members of the cytokine receptor superfamily. Coupled with the recently identified intranuclear function of PRL, these networks are integrated into the in vitro and in vivo actions induced by ligand. These findings indicate that antagonists of PRL/PRL receptor interaction or PRL receptor-associated signal transduction may be of considerable utility in the treatment of human breast cancer.

7,12-DMBA-induced rat leukemia: a review with insights into future research

7,12-Dimethylbenz[a]anthracene (DMBA) elicits leukemia in Long-Evans rats (LE). This leukemia is mostly erythroblastic and 30% of leukemias have total and partial trisomy of #2 chromosome and the rest have diploid karyotype. The common duplication site is in 2q26-q34 and N-ras gene is located in 2q34. 7,8,12-Trimethylbenz[a]anthracene (TMBA) also induces similar leukemias. These leukemias reveal a highly specific mutation of N-ras gene as in human leukemias. N-ras mutation is induced 48h after DMBA treatment. Wild type N-ras allele is frequently lost in diploid leukemias but not in trisomy type. Therefore, a gene dosage problem related to the mutant N-ras gene is involved in development of leukemia. Some secondary genetic rearrangements involving abl and H-ras are also observed in cultured leukemia cells. DMBA-induced chromosome aberrations as well as leukemia are enhanced by erythropoietin and blocked by Sudan III given prior to DMBA treatment. This leukemia will provide an important tool for chemical carcinogenesis and leukemia studies.

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

Epidemiological studies have repeatedly demonstrated that women who undergo an early first full-term pregnancy have a significantly reduced lifetime risk of breast cancer. Similarly, rodents that have previously undergone a full-term pregnancy are highly resistant to carcinogen-induced breast cancer compared with age-matched nulliparous controls. Little progress has been made, however, toward understanding the biological basis of this phenomenon. We have used DNA microarrays to identify a panel of 38 differentially expressed genes that reproducibly distinguishes, in a blinded manner, between the nulliparous and parous states of the mammary gland in multiple strains of mice and rats. We find that parity results in the persistent down-regulation of multiple genes encoding growth factors, such as amphiregulin, pleiotrophin, and IGF-1, as well as the persistent up-regulation of the growth-inhibitory molecule, TGF-beta3, and several of its transcriptional targets. Our studies further indicate that parity results in a persistent increase in the differentiated state of the mammary gland as well as lifelong changes in the hematopoietic cell types resident within the gland. These findings define a developmental state of the mammary gland that is refractory to carcinogenesis and suggest novel hypotheses for the mechanisms by which parity may modulate breast cancer risk.

Synergistic tumor promoter effects of estrone and progesterone in methylnitrosourea-induced rat mammary cancer

Tumor promoter effects of steroid hormones were tested in ovariectomized rats treated with the tumor initiator, methylnitrosourea. Continuous treatment with low doses of estrone induced tumor in 55% of experimental animals while intact control animals had a 96-100% incidence. Although progesterone alone produced no tumors in ovariectomized, methylnitrosourea-treated animals, estrone and progesterone acted synergistically, inducing tumorigenesis in 88% of treated rats. These results point to an important interaction between estrogens and progestins in promoting mammary tumorigenesis.

Hormonal regulation of mammary differentiation and milk secretion

The endocrine system coordinates development of the mammary gland with reproductive development and the demand of the offspring for milk. Three categories of hormones are involved. The levels of the reproductive hormones, estrogen, progesterone, placental lactogen, prolactin, and oxytocin, change during reproductive development or function and act directly on the mammary gland to bring about developmental changes or coordinate milk delivery to the offspring. Metabolic hormones, whose main role is to regulate metabolic responses to nutrient intake or stress, often have direct effects on the mammary gland as well. The important hormones in this regard are growth hormone, corticosteroids, thyroid hormone, and insulin. A third category of hormones has recently been recognized, mammary hormones. It currently includes growth hormone, prolactin, PTHrP, and leptin. Because a full-term pregnancy in early life is associated with a reduction in breast carcinogenesis, an understanding of the mechanisms by which these hormones bring about secretory differentiation may offer clues to the prevention of breast cancer.

Hormonal control of p53 and chemoprevention

Improvements in the detection and treatment of breast cancer have dramatically altered its clinical course and outcome. However, prevention of breast cancer remains an elusive goal. Parity, age of menarche, and age at menopause are major risk factors drawing attention to the important role of the endocrine system in determining the risk of breast cancer, while heritable breast cancer susceptibility syndromes have implicated tumor suppressor genes as important targets. Recent work demonstrating hormonal modulation of the p53 tumor suppressor pathway draws together these established determinants of risk to provide a model of developmental susceptibility to breast cancer. In this model, the mammary epithelium is rendered susceptible due to impaired p53 activity during specific periods of mammary gland development, but specific endocrine stimuli serve to activate p53 function and to mitigate this risk. The results focus attention on p53 as a molecular target for therapies to reduce the risk of breast cancer.

Identification of rat mammary tumor-1 gene (RMT-1), which is highly expressed in rat mammary tumors

Full-term pregnancy early in life results in a permanent reduction in lifetime breast cancer risk in women. Parous rats and mice are also refractory to chemical carcinogenesis. Therefore, investigation of the differences between mammary glands from virgin and parous rats would provide valuable information regarding the protective effects of early full-term pregnancy. In this report, we examined the gene expression patterns in mammary glands from virgin and parous Lewis rats. Using differential display technology, a novel 4.2 kb cDNA, designated rat mammary tumor-1 (RMT-1) was isolated. Northern blot analysis of RMT-1 showed that RMT-1 expression was higher in the pre-pubertal and pubertal stages during rat mammary gland development while it was down-regulated in mammary glands from mature virgin and parous rats. RMT-1 expression was highest in rat mammary cancers compared with either the mammary glands of virgin or parous rats. At the Northern blot sensitivity level, RMT-1 expression was found only in the mammary gland. Northern blot analysis also showed that the expression of this gene was found in 74% of N-methyl-nitrosourea (MNU)-induced mammary cancers while it was not found in MNU-induced cancers from other organs. The examination of the RMT-1 gene structure revealed that it consists of five exons spanning 5.9 kb. Using fluorescence in situ hybridization, the gene was localized on rat chromosome 1 band q 43-51. The present data show that there is a correlation between high RMT-1 expression and rat mammary carcinogenesis or decreased RMT-1 expression and parity associated refractoriness to chemically induced mammary carcinogenesis. However, whether or not RMT-1 gene has a functional role in these processes remains to be investigated.

Short-term exposure to pregnancy levels of estrogen prevents mammary carcinogenesis

It is well established that pregnancy early in life reduces the risk of breast cancer in women and that this effect is universal. This phenomenon of parity protection against mammary cancer is also observed in rodents. Earlier studies have demonstrated that short-term administration of estradiol (E) in combination with progesterone mimics the protective effect of parity in rats. In this study, the lowest effective E dosage for preventing mammary cancer was determined. Rats were injected with N-methyl-N-nitrosourea at 7 weeks of age; 2 weeks later, the rats were subjected to sustained treatment with 20 microg, 100 microg, 200 microg, or 30 mg of E in silastic capsules for 3 weeks. Treatments with 100 microg, 200 microg, and 30 mg of E resulted in serum levels of E equivalent to those of pregnancy and were highly effective in preventing mammary cancer. E treatment (20 microg) did not result in pregnancy levels of E and was not effective in reducing the mammary cancer incidence. In another set of experiments, we determined the effect of different durations of E with or without progesterone treatments on mammary carcinogenesis. These experiments indicate that a period as short as one-third the period of gestation is sufficient to induce protection against mammary carcinogenesis. The pioneering aspect of our study in contrast to long-term estrogen exposure, which is thought to increase the risk of breast cancer, is that short-term sustained treatments with pregnancy levels of E can induce protection against frank mammary cancer.