Mammary Tumor Development In Mice Research Articles

Tumor-associated antigens identified early in mouse mammary tumor development can be effective vaccine targets

Breast cancer vaccines composed of antigens identified by serological analysis of cDNA expression libraries (SEREX) induce antigen specific immune responses in patients but have had disappointing clinical benefits. While many attempts to modify the adjuvants and vaccine method have been tried, one issue not addressed was whether the SEREX tumor-associated antigens identified from late stages of disease were ideal targets. We questioned in the transgenic TgMMTV-neu mouse model whether the antigen repertoire is distinct between early and late stage breast cancer and whether the antigens identified via SEREX from transgenic mice with early or late stage tumors would elicit differential anti-tumor effects to address this question.Three early stage antigens, Pdhx, Stk39, and Otud6B, were identified from a SEREX screen of mice prior to development of palpable lesions. Formulated into a vaccine, each early antigen inhibited tumor growth (p < 0.0001). The antigens identified from mice with late stage tumors (Swap70, Gsn, and Arhgef2) were unable to inhibit tumor growth when used as vaccines (for example Gsn p = 0.26). Each of the three early stage antigens were essential for tumor survival in syngeneic mouse tumor cells and in human breast cancer cell lines across breast cancer subtypes. Silencing protein expression of the early antigens increased apoptosis (p < 0.0001 for all antigens in mouse and p < 0.05 for all antigens in human triple negative breast cancer) and decreased survival (p < 0.0001 for all antigens in mouse and human triple negative and HER2 positive breast cancer). Overexpression of the early stage antigens in women with breast cancer predicted worse prognosis (p = 0.03) while overexpression of late stage antigens did not impact prognosis (p = 0.09). These data suggest that antigens expressed earlier in breast tumor development and functionally relevant to breast tumor growth may be more effective targets for therapeutic breast cancer vaccines than antigens identified in later disease.

Abstract P5-07-03: The nuclear coactivator amplified in breast cancer 1 maintains tumor initiating cells during development of ductal carcinoma in situ

Abstract Introduction: Ductal Carcinoma In Situ (DCIS) has become the most diagnosed breast tumor type in North America. Amplified In Breast Cancer 1 (AIB1) is amplified in 5 to 10% of primary breast tumors. We have recently demonstrated that AIB1 causes development of DCIS lesions in the mammary gland of 30% of the AIB1 transgenic mice. Our laboratory has also shown that deletion of one allele of AIB1 prevents HER2-induced mammary tumor development in mice, suggesting a pivotal role of AIB1 during earlier stage of breast cancer. We hypothesize that AIB1-mediated pathways promote the transformation of the mammary epithelial cells and the alterations of the functional integrity of the mammary gland. By contrast, decrease in AIB1 functionality may result in prevented DCIS initiation and development. Results: We show by immunohistochemistry that AIB1 is expressed at low levels in normal breast but is highly expressed in DCIS lesions. We investigated the impact of decreasing AIB1 expression on the early transformed MCFDCIS cell line. MCFDCIS cells generate multiacinar disorganized structures with filling of the lumen, loss of polarization and escape from the proliferative suppression. Reduction of AIB1 in human MCFDCIS cells restored a more normal mammary acinar structure with basement membrane in 3D growth in Matrigel. Then, we studied the effect of regulating AIB1 on the histopathology development of the DCIS lesions both prior to DCIS development or in existing MCFDCIS lesions in a xenograft mouse model by subcutaneously injecting MCFDCIS cells deficient or not in AIB1 into nude mice. The MCFDCIS xenografted tumors obtained reproduced the human breast cancer progression and displayed multiple comedo DCIS-like lesions. Decreased levels of AIB1 in MCFDCIS cells, inhibited tumor growth and led to smaller, necrotic lesions. To investigate how AIB1 affects initiation and progression of DCIS, we compared global gene expression changes in MCFDCIS cells +/-AIB1 shRNA in vitro using cDNA array analysis. Among the most significant expression changes observed were genes that are differentially expressed in Breast Cancer Initiating Cells (BCIC) including CD24 and members of the NOTCH signaling pathway (DLL1 and DLL3). We demonstrated by flow cytometric and immunohistochemical analysis that AIB1 reduction in MCFDCIS cells was correlated with significant reduction in the CD24-/CD44+ BCIC population, and a decrease in myoepithelial progenitor cells in the DCIS lesions in vitro and in vivo. Loss of AIB1 in MCFDCIS cells was also accompanied by a loss of expression of NOTCH 2, 3 and 4, JAG2, HES1, GATA3, HER2 and HER3 in vivo. Conclusions: These data indicate that AIB1 plays a central role in the initiation and maintenance of DCIS and that reduction of AIB1 causes loss of BCIC, loss of components of the NOTCH, HER2 and HER3 signaling pathways and fewer DCIS myoepithelial progenitor cells in vivo. We propose that increased expression of AIB1, through maintenance of BCIC, facilitates formation of DCIS, a necessary step prior to development of invasive disease. Thus, targeting AIB1 may represent a new therapeutic paradigm especially for women with AIB1-positive primary tumor. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-07-03.

Abstract 1404: The nuclear coactivator amplified in breast cancer 1 promotes the initiation and the development of ductal carcinoma in situ.

Abstract Introduction: Ductal Carcinoma In Situ (DCIS) has become the most diagnosed breast tumor type in North America. Amplified In Breast Cancer 1 (AIB1) is amplified in 5 to 10 % of primary breast tumors. We have recently demonstrated that AIB1 causes development of DCIS lesions in the mammary gland of 30% of the AIB1 transgenic mice (1). Our laboratory has also shown that deletion of one allele of AIB1 prevents HER2-induced mammary tumor development in mice (2), suggesting a pivotal role of AIB1 during earlier stage of breast cancer. We hypothesize that AIB1-mediated pathways promote the transformation of the mammary epithelial cells and the alterations of the functional integrity of the mammary gland. By contrast, decrease in AIB1 functionality may result in prevented DCIS initiation and arrested breast tumor progression. Results: We showed by immunohistochemistry that AIB1 is expressed at low levels in normal human breast. By contrast, we observed a strong cytoplasmic and/or moderate nuclear AIB1 protein expression in DCIS lesions in breast cancer patients tissues. We investigated the impact of decreasing AIB1 expression on the early transformed MCFDCIS cell line. MCFDCIS cells generate multiacinar disorganized structures with filling of the lumen, loss of polarization and escape from the proliferative suppression. Reduction of AIB1 in human MCFDCIS cells restored a more normal mammary acinar structure with basement membrane in 3D growth in MatrigelTM. Then, we studied the effect of regulating AIB1 on the histopathology development of the DCIS lesions in a xenograft mouse model by subcutaneously injecting MCFDCIS cells deficient or not in AIB1 into nude mice. The MCFDCIS xenografted tumors obtained reproduced the human breast cancer progression and displayed multiple comedo DCIS-like lesions. Decreased levels of AIB1 in MCFDCIS cells, either prior to DCIS development or in existing MCFDCIS lesions in vivo, inhibited tumor growth and led to smaller, and necrotic lesions. Conclusions: Altogether, these data suggest that increased expression of AIB1 facilitates the genesis and progression of DCIS to invasive disease. Thus, targeting AIB1 may represent a new therapeutic paradigm to prevent disease progression especially for women with AIB1-positive primary tumor. The results of this project will also serve as basis of human clinical studies that may define new predictive and prognostic biomarkers that recognize high-risk patients, and prevent over- or undertreatment of breast cancer patients. 1. Nakles et al. (2011). Mol Endocrinol. 25(4):549-63. 2. Fereshteh et al. (2008). Cancer Res. 68(10):3697-706. Citation Format: Virginie Ory, Elena Tassi, Luciane Cavalli, Francisco Saenz, Tabari Baker, Susette Mueller, Priscilla Furth, Anton Wellstein, Anna Riegel. The nuclear coactivator amplified in breast cancer 1 promotes the initiation and the development of ductal carcinoma in situ. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1404. doi:10.1158/1538-7445.AM2013-1404

Abstract 597: Response of reactive oxygen species to eicosapentaenoic and calorie restriction in relation to mammary tumor prevention

Abstract Exposure of lipid biomembranes to reactive oxygen species (ROS) results in the generation of a wide variety of lipid peroxidation products. Some of these products, including 4-hydroxy-2-nonenal (4HNE) can form protein adducts which may impact pathways associated with tumorigenesis. We have reported that calorie restriction protects against spontaneous mammary tumor development in mice and have recently shown that this is accompanied by a decrease in serum levels of lipid peroxidation products. Eicosapentaenoic acid (EPA) is known to inhibit mammary tumorigenesis; however, this type of highly unsaturated omega-3 fatty acid has also been shown to increase in vivo levels of lipid peroxidation products. In the present study, the interaction of calorie restriction and dietary EPA was investigated with respect to mammary tumor formation. Lipid peroxidation products of ROS were also measured. From 10 weeks of age MMTV-Her2/neu mice were fed a modified AIN-93M diet (10.2% kcal from fat) which had either 100% soybean oil (SO) as the fat source or a blend of EPA (72%) and SO (18%). Mice were further divided into groups which were ad libitum-fed, (AL); chronic calorie restricted (CCR, received 75% of AL) or intermittently calorie restricted (ICR, received 50% of AL for 3 weeks followed by 3 weeks at 100% of AL) until 60 weeks of age or until mammary tumor size required euthanasia. Mice were weighed and examined for mammary tumors weekly. Serum thiobarbituric acid reactive substances (TBARS) and 4-hydroxy-2-nonenal (4HNE)-protein adducts in mammary tissues were measured to assess the extent of lipid peroxidation. In both diet groups, mammary tumor incidence was highest in the AL group (SO 87%, EPA 63%). These groups also had the highest levels of serum TBARS and the AL-SO group had the highest level of mammary tissue 4HNE-protein adducts. The calorie restricted mice had lower levels of mammary tumor incidence (CCR-SO 47%, CCR-EPA 40%, ICR-SO 59%) with the lowest level in the ICR-EPA group (15%). The ICR-SO group had serum TBARS and mammary tissue 4HNE-protein adduct levels that were significantly lower than the AL-SO group (p&amp;lt;0.05 for both). In the EPA groups, there was no difference between the AL, CCR, and ICR groups in either serum TBARS or mammary tissue 4HNE-protein adducts. These results suggest that higher levels of lipid peroxidation products are associated with a higher incidence of mammary tumors. However, although the dietary combination of ICR and EPA resulted in the greatest amount of mammary tumor inhibition, this combination was not associated with the lowest level of lipid peroxidation products. The relationship between EPA, calorie restriction, and reactive oxygen species appears to be quite complex. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 597. doi:1538-7445.AM2012-597

Abstract 1602: Deletion of cyclooxygenase-2 in mouse mammary epithelial cells delays breast cancer onset

Abstract Cyclooxygenase 2 (COX-2) enzyme, which catalyzes the synthesis of several pro- and anti-tumorigenic prostanoids, is associated with more than 40% of breast cancers. Selective and non-selective COX-2 inhibitors were shown to decrease the risk of tumorigenesis and breast cancer recurrence. However, the precise role of COX-2 in the mammary epithelium, and the potential role of COX-1, remains ill-defined. This is in part because of infertility, renal pathology and short life-span that accompanies global deletion of the COX-2 gene in mice. To study the role and mechanisms of COX-2 actions in breast cancer, we engineered mice on a pure FVB/N background that lack COX-2 only in mammary epithelial cells (MECs). Mice transgenic for COX-2 exons 5 and 6, flanked by loxP sites (COX-2flox/flox), were crossed with mice expressing cre recombinase under control of the mouse mammary tumor virus (mmtv) promoter (Cremmtv) to generated the MEC-targeted deletion of COX-2. MECs harvested from wild type (WT; COX-2flox/flox) and knock-out (KO; Cremmtv COX-2flox/flox) mice were treated, in vitro, with or without a single dose of 5µg/ml lipopolysaccaride (LPS). KO MECs showed about 80% less COX-2 mRNA and protein expression. Prostaglandin (PG) E2 production was similarly reduced in KO MECs, despite unchanged levels of COX-1 mRNA and protein (n=5-7). We confirmed COX-2 as the principle source of PGE2 in MECs using inhibitors specific for COX-2 (rofecoxib) or COX-1 (FR122047) (n=6). Expression of COX-2 and COX-1 mRNA and protein, and PGE2 generation, were not significantly different in untreated or LPS-treated peritoneal macrophages obtained from WT and KO mice (n=5-6) confirming the specificity of the targeted COX-2 deletion to MECs. Unlike global COX-2 deficiency, mice lacking COX-2 in MECs were viable, fertile and disease-free making them highly suitable for cancer studies. To induce breast tumorigenesis, 6 week old virgin female WT and KO mice received medroxyprogesterone acetate implants (75 mg/pellet, 21 day release) subcutaneously followed, at 9 weeks of age, by 1mg/week of 7,12-dimethylbenz[a]anthracene (DMBA) administration for 4 weeks. This regime causes predominantly mammary tumor development in mice. Starting from week 13, mice were checked weekly for mammary tumors. The onset of breast cancer, denoted as the week of age during which a mammary tumor was palpated in the glands of a live mouse, was significantly delayed in KO mice compared to their WT counterparts (p=0.03); the median duration of tumor free period was 28 weeks for KO animals (n=36) versus 22 weeks for WT mice (n=20). These data indicate that targeted deletion of COX-2 in mammary epithelium affords significant protection against initiation and progression of breast tumorigenesis. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1602.

RANKL inhibition decreases the incidence of mammary adenocarcinomas in wild type (WT) and MMTV-RANK transgenic mice.

Abstract Abstract #4167 Purpose: RANK and its ligand (RANKL), key factors for bone remodeling and metastasis, are crucial for the development of mouse mammary gland during pregnancy. Upon treatment with hormone medroxyprogesterone (MPA) and a carcinogen (DMBA), transgenic mice overexpressing RANK in the mammary gland via the MMTV promoter exhibit a higher incidence of ductular hyperplasias, mammary intraepithelial neoplasias (MIN), and mammary adenocarcinomas than WT mice. This study assessed the expression of mouse RANK and RANKL in WT and MMTV-RANK during mammary tumor progression in this model and determined whether RANKL inhibition could inhibit mammary carcinogenesis.&amp;#x2028; Methods: At 6 weeks of age WT and MMTV-RANK transgenic mice were implanted with 90-day release 50mg MPA pellets that were replaced at expiration (weeks 19 and 32). DMBA was administered orally at weeks 9, 10, 12, and 13. The expression of RANK and RANKL was determined by immunohistochemistry (IHC). The specificity of the anti-mouse RANKL and anti-mouse RANK antibodies were verified by their lack of reactivity in RANKL -/- or RANK -/- mouse tissues, respectively. Simultaneous with the first DMBA treatment, mice were treated with RANK-Fc (10 mg/kg, 3x/week) or vehicle and the onset of mammary tumor formation was measured. Mammary tumor formation was determined by palpation and confirmed by histologic examination. Preneoplastic lesions were counted by whole mount analysis of the mammary gland at 4 weeks after the last DMBA treatment. Epithelial proliferation was measured by BrdU labeling of the mammary epithelium at multiple timepoints following the last DMBA treatment (2 days after the first DMBA treatment; 4 and 7 weeks following the last).&amp;#x2028; Results: In the MPA/DMBA-induced mammary tumor model, MMTV-RANK mice developed palpable mammary tumors earlier than WT mice with a median onset of 86 days for the MMTV-RANK cohort compared with 132 days for the WT mice. At early timepoints, the number of preneoplastic lesions and mammary epithelial proliferation were greater in MMTV-RANK mice relative to WT. IHC demonstrated that MPA increased the number of RANKL-positive epithelial cells and that RANKL was present in preneoplastic MIN lesions and adenocarcinomas in both WT and MMTV-RANK mice. RANK expression was observed in the epithelial component of hyperplasias, MIN lesions, and adenocarcinomas in both WT and MMTV-RANK mice. Inhibition of RANKL with RANK-Fc substantially decreased the presence of preneoplastic lesions and the proliferation of mammary epithelial cells in MMTV-RANK mice. In addition, treatment of either MMTV-RANK or WT mice with RANK-Fc delayed the onset of palpable mammary tumors. Treatment of the MMTV-RANK mice with RANK-Fc decreased the incidence of mammary adenocarcinoma.&amp;#x2028; Conclusion: These data suggest an operative role for RANKL in progesterone-dependent mouse mammary tumor development in both transgenic MMTV-RANK and WT mice and support additional studies to determine whether RANKL inhibition will delay tumor progression and metastases. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 4167.

The Nuclear Receptor Coactivator Amplified in Breast Cancer-1 Is Required for Neu (ErbB2/HER2) Activation, Signaling, and Mammary Tumorigenesis in Mice

Overexpression of the oncogene amplified in breast cancer 1 (AIB1)/steroid receptor coactivator-3 (SRC-3) induces mammary tumorigenesis in mice. In breast cancer, high levels of AIB1/SRC-3 and the growth factor receptor HER2/neu predict resistance to endocrine therapy and poor outcome. However, a mechanistic relationship between AIB1/SRC-3 and HER2/neu in the development of breast cancer has not been shown. Here, we show that deletion of one allele of SRC-3 significantly delays Neu-induced mammary tumor development in mice. Homozygous deletion of SRC-3 in mice completely prevents Neu-induced tumor formation. By ages 3 to 4 months, Neu/SRC-3(+/-) mice exhibit a noticeable reduction in lateral side-bud formation, accompanied by reduced cellular levels of phosphorylated Neu compared with Neu/SRC-3(wt) mice. In Neu-induced tumors, high levels of SRC-3, phosphorylated Neu, cyclin D1, cyclin E, and proliferating cell nuclear antigen expression are observed, accompanied by activation of the AKT and c-Jun NH(2) kinase (JNK) signaling pathways. In comparison, phosphorylated Neu, cyclin D1, and cyclin E are significantly decreased in Neu/SRC-3(+/-) tumors, proliferation is reduced, and AKT and JNK activation is barely detectable. Our data indicate that AIB1/SRC-3 is required for HER2/neu oncogenic activity and for the phosphorylation and activation of the HER2/neu receptor. We predict that reducing AIB1/SRC-3 levels or activity in the mammary epithelium could potentiate therapies aimed at inhibiting HER2/neu signaling in breast cancer.

Effects of dietary fatty acids on breast and prostate cancers: evidence from in vitro experiments and animal studies

Linoleic acid, an n-6 polyunsaturated fatty acid, is essential for normal mammary tissue development, at least in part because it provides the metabolic precursor required for the biosynthesis of key eicosanoids. A similar requirement applies to the growth of estrogen-independent but apparently not to estrogen-dependent rodent mammary and human breast carcinoma cells in vitro. By way of lipoxygenase products, n-6 fatty acids also regulate expression of the invasive phenotype. High-fat, linoleic acid-rich diets promote chemically induced rat mammary carcinogenesis, virally induced mouse mammary tumor development, and the growth and metastasis of estrogen-independent human breast cancer cells in athymic nude mice. In contrast, saturated fatty acids have no discernible effects on mammary carcinogenesis or progression. Most mechanistic studies have focused on the cyclooxygenase and lipoxygenase products of n-6 fatty acid metabolism, and support is accumulating for interactions between these eicosanoids and growth factors and oncogenes. The investigation of dietary fatty acids in prostate cancer is at an early stage and has been handicapped by a lack of satisfactory animal models. However, there are indications that the n-6 fatty acids perform functions in experimental prostate cancer progression similar to those described for breast cancer.

Cell cyclins and cyclin-dependent kinase activities in mouse mammary tumor development.

Breast cancer in humans, as in mice and rats, is thought to be the result of sequential changes in the epithelial cells of the mammalian glands. This study examines the altered expression or activation of cell cycle related proteins in an in situ system composed of hyperplasia, preneoplasia and neoplasia of mouse mammary glands. The results showed a high level of cdc2/cdk2 kinase activities in tumors compared to hyperplasias which was independent of cdc2/cdk2 protein levels. Some of the cdk-associated proteins which are thought to regulate cdk kinase activity were examined in these tissues. Cyclin A was overexpressed in all hyperplasias irrespective of their tumorigenic potentials. However, a number of alterations in cyclin E protein were associated with cdk2 and its associated kinase activity during mammary tumorigenesis. First, the level of normal cyclin E (p50) expression was positively correlated with the tumorigenic potentials of different hyperplasia lines. Second, several cyclin E isoforms (p48, p43, p35, p34, p32) were detected only in tumor tissues. Third, a 2.3- and 8.3-fold increase in cyclin E-associated cdk2 kinase activity was present in highly tumorigenic hyperplasias and neoplasias respectively compared to the low tumorigenic hyperplasias. Polymorphic cell nuclear antigen (PCNA) protein bound to cdk2 was a better indicator for cell proliferation and cdk2 kinase activity than the PCNA labeling index. These results suggest a sequential pattern of multiple derangements in factors regulating cdk2 protein function during mammary tumorigenesis. High levels of cdk2 kinase activity are observed only in tumors and appear to be closely related to alterations in cyclin E protein expression.

Mouse mammary hyperplasias and neoplasias exhibit different patterns of cyclins D1 and D2 binding to cdk4.

Deregulated expression of G1 cyclins D1 and D2 is a feature of some neoplasias. This study examined the altered expression of D1 and D2 cyclins, both the total pool and as associated with cdk4 and cdk2, at different stages of mouse mammary tumorigenesis. Three different mammary hyperplastic outgrowth lines, TM2, TM10 and TM12, and their respective tumors were examined. Increasing levels of the cyclin D1 protein pool, D1 binding to cdk4 and cdk2 and cdk4 kinase activity were closely correlated with tumorigenesis. In constrast, cyclin D2 binding to cdk4 was predominant in hyperplasias and much less in tumors, where cyclin D1 became predominant. However, the cyclin D2 pool showed increases of 15-65 times in hyperplasias compared with normal gland and further increases of 11-15 times in two of three different tumors. The message level for cyclin D1 increased only 2-3 times in tumors compared with normal gland. Cyclin D2 mRNA was highest in normal tissue and decreased only marginally in tumors. These results suggest that cyclin D2 functions uniquely from cyclin D1 in the early stages of mouse mammary tumor development. Cyclin D2 bound to cdk4 may act to guarantee a low level of kinase activity in hyperplasias and may be an attempt to direct the mammary epithelial cells through differentiation rather than proliferation. This interaction may be one of the negative regulatory mechanisms in the early stages in mouse mammary tumor development, until cyclin D1 totally replaces cyclin D2 binding to cdk4, which would activate the high levels of cdk4 kinase activity observed in neoplasias.

Gene expression screening for specific genes associated with mouse mammary tumor development

The preneoplastic hyperplastic alveolar nodule is a frequent and well-characterized precursor to mammary tumors in the mouse. Although the biological characteristics of the preneoplastic state have been understood for many years, the molecular alterations associated with preneoplasia and tumorigenicity are unknown. We applied the technique of differential display of mRNA to two closely matched cell populations of mammary preneoplasias that differed only in their tumorigenic potential. Two mRNAs were isolated that were overexpressed only in tumorigenic preneoplasias and in tumors and not in normal pregnant mammary gland or in nontumorigenic preneoplasias. Partial nucleotide sequencing indicated that one of the mRNAs had not yet been described, whereas the second mRNA was highly homologous to a relatively uncharacteristic gene termed pT-2. These results illustrate the utility of the differential display method for isolating and identifying uniquely expressed genes from tissues maintained in the microenvironment where tumors arise naturally.

Bioactivity of C3H and RIII Mammary Tumor Viruses in Virgin Female BALB/c Mice: Brief Communication 2

The bioactivities of C3H and RIII mammary tumor virus (MuMTV) in virgin female BALB/c mice differed. The average number of mammary hyperplastic alveolar nodules per mouse after noduligenic tests was 20.1 in BALB/cfC3H and 10.7 in BALB/cfRIII females. Spontaneous mammary tumor incidence after 20 months of observation was 47.5% in BALB/cfC3H and 14.6% in BALB/cfRIII females (P less than 0.01). The frequency of lung metastases in mammary tumor-bearing mice was 63.1% in BALB/cfC3H and 16.6% in BALB/cfRIII females (P less than 0.05), although the clinical duration of mammary tumors was the same. These data demonstrated a lower bioactivity of RIII MuMTV when compared to C3H MuMTV in BALB/c mice and suggested that the causative virus may control all the steps of mouse mammary tumor development, including metastasis.

Mammary Tumor Development in Mice Ovariectomized at Birth

Mammary Tumor Development in Mice Ovariectomized at Birth