Estrous cycle dependence of nitrosomethylurea (NMU)-induced preneoplastic lesions in rat mammary gland

Long-term hormonal promotion overcomes genetic resistance to mammary cancer

It is well known that ovarian steroids estradiol and progesterone play a vital role in the development of mammary cancer. Here, using the genetically highly resistant Copenhagen rats we demonstrate that sustained exogenous treatment with estradiol and progesterone overcomes genetic resistance to mammary cancer. It has been demonstrated that Copenhagen rats develop preneoplastic lesions upon exposure to carcinogens. However, these preneoplastic lesions fail to progress to ductal carcinomas in situ or overt mammary carcinomas. The preneoplastic lesions eventually decrease in number and are absent by 60 days post-carcinogen treatment. In the present study, we exposed 7-week-old female Copenhagen rats to N-methyl-N-nitrosourea (MNU; 50mg/kg BW). Immediately after MNU treatment the rats were divided into the following groups: (1) control; (2) 30 mg estradiol 17β; (3) 30 mg progesterone; and (4) 30 mg estradiol 17β plus 30 mg progesterone. All hormone treatments were administered via individual silastic pellets for a period of 9 months post-carcinogen treatment. The control animals displayed a low incidence of mammary cancer (10%). Hormone treatments produced significantly higher incidences of mammary cancer, with estradiol at 50%, progesterone at 65% and estradiol plus progesterone at 90%. Hormone treatment sustained the growth of the lesions induced by MNU by increasing expression of Areg, Bcl-2, Ccnd-1 and Vegf genes, while decreasing expression of Bad, Bax, Casp 3, 8, 9 and p53 genes. Furthermore, hormone treatment increased CCND-1 and PARP proteins levels. The data clearly demonstrates that hormonal environment supports mammary cancer progression by increasing cell proliferation, and angiogenesis while inhibiting apoptosis.

Breast Cancer

It has been firmly established in epidemiological studies that early full-term pregnancy affords lifetime protection against the development of breast cancer. This phenomenon can be mimicked in rat and mouse models of mammary cancer in which the hormones estrogen and progesterone are given for 21 days. Carcinogen-induced proliferation is blocked as a consequence of hormone pretreatment. Among several genes implicated by molecular studies to be differentially expressed is the tumor suppressor gene p53. Both immunohistochemical and Western blot studies indicate that p53 protein expression is increased in hormone-pretreated mice and rats. The p53-regulated gene p21Cip1 is also increased concomitantly with p53. To test directly the causative role of p53 in conferring a protective phenotype, we examined the hormone-induced protective effect in BALB/c p53 null mammary epithelium. In the mammary epithelium, the absence of p53 gene expression abrogated the protective effect of prior pregnancy. The tumor incidence curves were superimposable in p53 null mammary epithelium that were treated with 7,12-dimethylbenzanthracene or pregnancy plus 7,12-dimethylbenzanthracene. These results demonstrate that p53 plays a pivotal role in hormone-induced protection and raises the question of the mechanisms by which the steroid hormones, estrogen and progesterone, functionally activate p53.

Vitamin D receptor (VDR) ablation alters carcinogen-induced tumorigenesis in mammary gland, epidermis and lymphoid tissues

The Vitamin D receptor (VDR) and its ligand, 1,25(OH)(2)D(3), regulate cell proliferation, differentiation and apoptosis in vitro, yet the physiological significance of this regulation is unclear. In these studies, we used VDR knockout (VDRKO) mice to examine the impact of VDR on chemical carcinogen-induced tumorigenesis in vivo. Wild type (WT) and VDRKO littermates were fed a high calcium diet to prevent disturbances in calcium homeostasis and were gavaged with dimethylbenzanthracence (DMBA) using a protocol designed to induce mammary tumors. Compared to WT littermates, VDRKO mice exhibited an increased incidence of mammary gland hyperplasia and a higher percentage of hormone independent tumors with squamous differentiation. VDR ablation also significantly enhanced tumor development in epidermis and lymphoid tissues, but did not affect tumor development in ovary, uterus, lung or liver. These data indicate that VDR ablation alters susceptibility to DMBA-induced carcinogenesis in a tissue specific fashion, and provide support that optimal VDR signaling may act to suppress tumorigenesis.

Lack of effects of postnatal exposure to a mixture of aryl hydrocarbon-receptor agonists on the development of methylnitrosourea-induced mammary tumors in sprague-dawley rats

There are concerns that early life exposure to organochlorines, including aryl hydrocarbon receptor (AhR) agonists, may lead to long-term effects and increase the risk of developing breast cancer. Our objective was to test if postnatal exposure to a mixture of 2,3,7,8-tetrachlorodibenzodioxin (TCDD)-like chemicals would modulate the development of methylnitrosourea (MNU)-induced mammary tumors. Females received by gavage a mixture containing 3 non-ortho-polychlorinated biphenyls (PCBs), 6 polychlorinated dibenzodioxins (PCDDs), and 7 polychlorinated dibenzofurans (PCDFs), at 1, 5, 10, 15, and 20d of age. The doses were equivalent to 0, 1, 10, 100, or 1000 times the amount ingested through breast milk by a human infant during its first 24 d of life. Subgroups of 1000 x reated rats and controls were sacrificed at 21 d of age for assessment of mammary-gland development, cell death, and proliferation. Mammary-tumor development was assessed in MNU (30 mg/kg body weight ip at 50 days of age)-induced rats pre-exposed to the mixture (MNU-0, MNU-1, MNU-10, MNU-100, MNU-1000). Rats were sacrificed when their mammary tumors reached 1 cm in diameter, or when the rats reached > or = 32 wk of age. Mammary-gland whole mounts were analyzed with all palpable and microscopic lesions (n = 1563) histologically classified and grouped as benign, intraductal proliferations, or malignant. There were no marked effects on age at onset of puberty (vaginal opening) and estrous cyclicity. Despite a significant decrease in proliferating cell nuclear antigen (PCNA)-positive mammary cells in 1000 x treated 21-d-old rats, there were no long-term dose-response effects on mammary-gland morphology and tumor development. In conclusion, postnatal exposure to the mixture of AhR agonists had no significant effects on the development of MNU-initiated mammary tumors.

Rat models of premalignant breast disease

While a number of agents have been shown to induce mammary carcinogenesis in the rat, premalignant stages of the disease have been best characterized in chemically-induced models, specifically those initiated by either 7,12 dimethylbenz[alpha]anthracene (DMBA) or 1-methyl-1-nitrosourea (MNU). In general, it appears that epithelial cells in mammary terminal end buds or terminal ductules are the targets of carcinogenic initiation, and that a series of morphologically identifiable steps are involved in the development of mammary carcinoma. The premalignant steps include ductal hyperplasia of the usual type and carcinoma in situ of the cribriform or comedo type; atypical ductal hyperplasia has not been reported. Thus the histogenesis of lesions occurring in chemically induced mammary carcinogenesis in the rat is similar to that observed in the human; although, the spectrum of lesions observed in the rat is limited. Opportunities to investigate the biological and molecular characteristics of premalignant breast disease in the rat are presented.

Estrogen mitogenic action. I. Demonstration of estrogen-dependent MTW9/PL2 carcinogen-induced rat mammary tumor cell growth in serum-supplemented culture and technical implications

The MTW9/PL cell line was established by our laboratory in culture from the carcinogen-induced hormone-responsive MT-W9A rat mammary tumor of a Wistar-Furth (W/Fu) rat. This tumor formed estrogen, androgen, and progesterone responsive tumors in W/Fu rats (Sirbasku, D. A., Cancer Res. 38:1154-1165; 1978). It was later used to derive the MTW9/PL2 cell population which was also estrogen-responsive in vivo (Danielpour, D., et al., In Vitro Cell. Dev. Biol. 24:42-52; 1988). In the study presented here, we describe serum-supplemented culture conditions in which the MTW9/PL2 cells demonstrate > or = 80-fold steroid hormone growth responses. All sera used were steroid hormone-depleted by charcoal-dextran treatment at 34 degrees C. The studies were done with horse serum as well as serum from other mammalian species. The growth of the MTW9/PL2 cells was biphasic in response to hormone-depleted serum. Concentrations of < or = 5% (v/v) promoted optimum growth. Above this concentration, serum was inhibitory. Concentrations > or = 40% (v/v) inhibited growth altogether. Addition of 1.0 x 10(-13)-1.0 x 10(-8) M 17beta,-estradiol (E2) reversed the inhibition completely. At 1.0 x 10(-8) M, estrone, estriol and diethylstilbestrol promoted growth as well as E2. Testosterone and dihydrotestosterone promoted growth only at > or = 10(-7) M. Progesterone was effective only at > or = 10(-6) M. Cortisol was ineffective. Labeled-hormone-binding analysis and Western immunoblotting documented that MTW9/PL2 cells had estrogen and progesterone receptors but not androgen or cortisol receptors. Estrogen treatment of MTW9/PL2 cells induced a concentration and time dependent increase in progesterone receptors. We conclude (1) the MTW9/PL2 population is the first highly steroid hormone-responsive rat mammary tumor cell line to be established in culture from a carcinogen-induced tumor, and (2) sera from a number of species including horse, rat and human contain an inhibitor which mediates estrogen sensitive MTW9/PL2 cell growth in culture.

Somatic mutation in the mammary gland: influence of time and estrus

A critical factor in the quantitation of mutation induction in vivo is the time interval between treatment and sampling. In order to study mutagenesis in the mammary epithelium, the cell type in which breast cancer arises, we have measured the manifestation time, the minimum time required for the maximum mutant frequency to be achieved, in this tissue. The F1 LacZ transgenic mice (Muta MousexSWR) were treated with N-ethyl-N-nitrosourea (ENU) at 50 mg/kg for five consecutive days and then sampled at 1, 2, 4, 6, 9, or 12 weeks after the last treatment. The LacZ- mutant frequency reached a maximum at 4 weeks post-treatment and did not vary significantly thereafter. Dlb-1- mutations in the small intestine reached a maximum at 2 weeks after treatment and did not vary significantly thereafter. Since the stage of estrus cycle during carcinogen exposure influences the mammary tumor incidence and latency, it was expected that it would also affect mutation induction. To test this, F1 LacZ mice in the estrus or di-estrus stage were treated with an acute dose of 250 mg/kg ENU and sampled 10-13 weeks post-treatment. No statistical difference between the two groups was found, indicating that the effect of estrus on carcinogenesis is not due to variation in the sensitivity of the stage of the mammary gland to mutation.

Induction of mammary carcinomas by N-methyl-N-nitrosourea in ovariectomized rats treated with epidermal growth factor

The importance of epidermal growth factor (EGF) in both normal and malignant mammary gland development are presented in these studies. Initial findings demonstrated that in the absence of ovarian hormones, EGF had a significant proliferative effect on mammary epithelial cells. To determine whether mammary epithelial cells grown with EGF, in the absence of ovarian hormones, could be transformed by N-methyl-N-nitrosourea (MNU), female ovariectomized Lewis rats were implanted with pellets containing EGF for 1 week and then treated with MNU for initiation. Two days after MNU treatment, ovaries were implanted and EGF pellets were removed from all ovariectomized groups in order to promote carcinogenesis. The mammary carcinoma incidence of the EGF-stimulated group (90%) was not significantly different from the intact group (100%). The mammary cancer morphology of EGF-treated carcinomas was either ductal carcinoma or cribriform adenocarcinoma, whereas intact animals developed mainly papillary and occasional cribriform carcinomas. Fifty-eight percent of the carcinomas from the EGF group were ovarian hormone-independent compared with 10% of carcinomas from the intact group. These results demonstrate that EGF-induced proliferation during initiation with MNU was sufficient to induce the transformation of mammary carcinomas in the absence of ovarian hormones. The hormonal dependency of these EGF-induced carcinomas were different compared with MNU-initiated mammary carcinomas in intact rats.

Hormone dependence of mammary tumors induced in rats by intraperitoneal NMU injection

The purpose of this work was to determine the hormone dependence of mammary tumors induced in Sprague-Dawley rats by three intraperitoneal injections of N-nitroso-N-methylurea at 50, 80, and 110 days of age. Two experimental designs were carried out: (a) Ten days before the first NMU injection, 130 rats were divided into 13 batches and randomly assigned to the following treatments: control, ovariectomy (OVX), tamoxifen (TAM), bromocriptine (BROM), haloperidol (HAL), estradiol (E2), progesterone (Pg), OVX + BROM, TAM + BROM, OVX + HAL, TAM + HAL, OVX + TAM, and E2 + BROM. After 150 days of treatment the following growth parameters were determined: latency period (LP), mean tumor number per rat (n/t), and tumor incidence (TI). LP was significantly increased (p < 0.05) only by Pg and TAM + BROM. The n/t was significantly decreased (p < 0.05) by all treatments except HAL. TI was significantly reduced by OVX, TAM, BROM, and their combinations, (b) Rats bearing ip-NMU-induced mammary tumors were divided into 7 batches and assigned to the following treatments: control, OVX, TAM, BROM, HAL, OVX + BROM, and TAM + BROM. Tumor growth was assessed up to 60 days of treatment; only OVX, TAM and their combination with BROM were able to produce tumor regression. These results support the essential role of E2 and prolactin in the promotion stage of carcinogenesis. However, for established tumors, growth becomes more independent from hormone influence, in particular from prolactin deprivation. We conclude that this model seems suitable for studying the mechanisms underlying the evasion of hormonal control of tumor growth.

Hormone dependent and independent mammary tumor development form N-methyl-N-nitrosourea-treated rat mammary epithelial cell xenografts in the nude mouse: multiple pathways and H-ras activation

A nude mouse mammary fat pad xenograft system was developed to examine hormone dependent and independent mammary tumorigenesis and progression from N-methyl-N-nitrosourea (MNU)-induced hyperplastic lesions. Ninety-one percent of transplanted mammary tumors grew, with an orthotopic preference, and maintained their hormone dependence, histopathology, and H-ras mutation frequency. Grafted mammary epithelial cells, from MNU-treated rats, developed normal; and hyperplastic outgrowths, representative of those found in the rat mammary gland after MNU-treatment. Hyperplasias developed into neoplasias that were both hormone dependent and independent. We demonstrate that hormone independent tumors can develop directly either from lobuloalveolar or ductal hyperplasias or from hormone dependent tumors. H-ras mutation was detected in mammary preneoplasias (4 lines) before they developed into tumors and was associated with an elevated tumorigenic potential. Our observations suggest that there are multiple histopathogenic pathways in the development and progression to hormone independent rat mammary tumors.

Hormones and mammary carcinogenesis in mice, rats, and humans: a unifying hypothesis

An attempt has been made to put forward a unifying hypothesis explaining the role hormones play in the genesis of mammary cancers of different phenotypes and genotypes in mice, rats, and humans. Most mammary cancers in these species originate in luminal mammary epithelial cells lining the mammary ducts and alveoli. These cancers are histopathologically diverse and are classified on the basis of growth requirements as hormone-dependent or hormone-independent tumors. In most strains of mice, mammary cancers at the time of detection are largely of the hormone-independent type; in rats, almost all mammary cancers are hormone-dependent, while humans have both phenotypes. In spite of these differences, in vivo studies show that hormones (ovarian and pituitary) are essential for luminal mammary epithelial cell proliferation and also for the development of mammary cancers of both hormone-independent and hormone-dependent types. This article, based on our extensive in vivo and in vivo studies and on current literature, proposes a model to explain the central role of hormones in the genesis of all types of mammary cancers. The model attempts to address the following questions: (i) how hormones regulate luminal mammary epithelial cell proliferation, (ii) why hormones are required for the genesis of mammary cancers of all phenotypes and genotypes, including those which are always classified as hormone-independent tumors, and (iii) why the three species (mouse, rat, and human) have consistently different ratios of hormone-dependent to hormone-independent tumors.

Induction of mammary tumors in rat by intraperitoneal injection of NMU: histopathology and estral cycle influence

In order to obtain an experimental model we induced mammary tumors in female Sprague-Dawley rats. The carcinogen N-nitroso-N-methylurea (NMU) was injected intraperitoneally (i.p.) at doses of 50 mg/kg body weight when animals were 50, 80 and 110 days old. Tumor sizes were measured with a caliper and their growth parameters and histopathological properties were tested. For 100 rats, 88.4% of developed lesions were ductal carcinomas, histologically classified as 52.8% cribiform variety, 30.6% solid carcinoma. Metastases in liver, spleen and lung were present. Other primary tumors were detected with low incidence. The influence of the rat estrous cycle during the first exposure to intraperitoneal NMU injection was studied. The latency period in estrus, proestrus and diestrus was 82 +/- 15, 77 +/- 18 and 79 +/- 18 days, respectively. Tumor incidence was significantly higher in estrus (95.2%) than proestrus (71.4%) or diestrus (77.4), (P < 0.01). Mean number or tumors per animal was similar among the three groups (4.4 +/- 3.2, 3.8 +/- 3.6, 3.2 +/- 1.8). The procedure described appears to be the simplest method for inducing experimental mammary tumors in rats.

Estrous cycle modulation of O6-alkylguanine-DNA alkyltransferase expression in rat mammary epithelial cells

N-methyl-N-nitrosourea (MNU)-induced rat mammary tumor incidence and tumor number per rat, is directly correlated with an increase in the circulating level of estrogen(s) at the time of carcinogen administration and subsequent mammary epithelial O6-methylguanine content. We report that, expression of O6-alkyltransferase (AGT) is also regulated by reproductive hormones in a tissue specific manner. The level of mammary epithelial cell AGT activity on estrus (0.47 pmol/mg protein) and proestrus (0.32) was significantly higher than on metestrus (0.14) (P < 0.05, estrus vs. metestrus). However, no change was observed in liver AGT activity (0.52 pmol/mg protein). In contrast, the mean level of AGT protein was not significantly different between tumors from rats injected with MNU on different days of the estrous cycle. In conclusion, the different tumor biologies resulting from carcinogen injection on different days of the estrous cycle may be partially explained by variation in levels of DNA repair activity. However, the cells in the resulting tumors did not continue an obligatory differential expression of the AGT activity consistent with their stage of initiation.

Influence of the estrous cycle at gamma-ray exposure on radiation-induced mammary tumorigenesis in virgin rats

Wistar rats received whole body irradiation with 260 cGy gamma-rays at 10 a.m. of individual phases of their estrous cycle and then had diethylstilbestrol pellets implanted for 1 year. When the radiation was given during di-estrus II, the highest incidence (73.3%) of mammary tumorigenesis was observed, and mean latency until the first tumor appearance was 8.5 +/- 0.7 months. Also, rats irradiated on estrus had significantly lower incidence (35.3%) of mammary tumors than those irradiated on pro-estrus and di-estrus II, but there was no significant difference in latency period. Iball's indices of total mammary tumors, fibroadenoma plus adenocarcinoma, were 14.3 +/- 0.9, 28.8 +/- 2.1, 23.3 +/- 0.8 and 12.2 +/- 0.2 in rats irradiated during di-estrus I, di-estrus II, pro-estrus and estrus respectively. The percentage of adenocarcinomas was comparatively uniform (25.0 to 34.8%) throughout the various phases of the estrous cycles. Also, Iball's indices calculated from adenocarcinoma indicated no significant differences between all groups. From our results, the highest incidence of mammary tumors arose in rats after irradiation at di-estrus II with minimum level of prolactin in serum. We discuss different mechanisms of radiation-induced and chemical-carcinogen-induced tumorigenesis of mammary glands.