Establishment of two hormone-responsive mouse mammary carcinoma cell lines derived from a metastatic mammary tumor

PTEN/PIK3CA genes are frequently mutated in spontaneous and medroxyprogesterone acetate-accelerated 7,12-dimethylbenz(a)anthracene-induced mammary tumours of tree shrews

Tree shrew has increasingly become an attractive experimental animal model for human diseases, particularly for breast cancer due to spontaneous breast tumours and their close relationship to primates and by extension to humans. However, neither normal mammary glands nor breast tumours have been well characterised in the Chinese tree shrew (Tupaia belangeri chinensis). In this study, normal mammary glands from four different developmental stages and 18 spontaneous breast tumours were analysed. Haematoxylin and eosin (H&E) staining and immunohistochemistry (IHC) showed that normal mammary gland morphology and structures of tree shrews were quite similar to those found in humans. Spontaneous breast tumours of tree shrews were identified as being intraductal papilloma, papillary carcinoma, and invasive ductal carcinoma with or without lung metastasis. To further analyse breast cancer tumours among tree shrews, 40 3-4 month-old female tree shrews were orally administrated 20 mg 7,12-dimethylbenz(a)anthracene (DMBA) or peanut oil thrice, and then, 15 of these DMBA administrated tree shrews were implanted with medroxyprogesterone acetate (MPA) pellets. DMBA was shown to induce breast tumours (12%) while the addition of MPA increased the tumour incidence (50%). Of these, three induced breast tumours were intraductal papillary carcinomas and one was invasive ductal carcinoma (IDC). The PTEN/PIK3CA (phosphatase and tensin homologue/phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha), but not TP53 and GATA3, genes are frequently mutated in breast tumours, and the PTEN/PIK3CA gene mutation status correlated with the expression of pAKT in tree shrew breast tumours. These results suggest that tree shrews may be a promising animal model for a subset of human breast cancers with PTEN/PIK3CA gene mutations.

STAT1-deficient mice spontaneously develop estrogen receptor α-positive luminal mammary carcinomas

INTRODUCTION

Although breast cancers expressing estrogen receptor-α (ERα) and progesterone receptors (PR) are the most common form of mammary malignancy in humans, it has been difficult to develop a suitable mouse model showing similar steroid hormone responsiveness. STAT transcription factors play critical roles in mammary gland tumorigenesis, but the precise role of STAT1 remains unclear. Herein, we show that a subset of human breast cancers display reduced STAT1 expression and that mice lacking STAT1 surprisingly develop ERα+/PR+ mammary tumors.

METHODS

We used a combination of approaches, including histological examination, gene targeted mice, gene expression analysis, tumor transplantaion, and immunophenotyping, to pursue this study.

RESULTS

Forty-five percent (37/83) of human ERα+ and 22% (17/78) of ERα- breast cancers display undetectable or low levels of STAT1 expression in neoplastic cells. In contrast, STAT1 expression is elevated in epithelial cells of normal breast tissues adjacent to the malignant lesions, suggesting that STAT1 is selectively downregulated in the tumor cells during tumor progression. Interestingly, the expression levels of STAT1 in the tumor-infiltrating stromal cells remain elevated, indicating that single-cell resolution analysis of STAT1 level in primary breast cancer biopsies is necessary for accurate assessment. Female mice lacking functional STAT1 spontaneously develop mammary adenocarcinomas that comprise > 90% ERα+/PR+ tumor cells, and depend on estrogen for tumor engraftment and progression. Phenotypic marker analyses demonstrate that STAT1-/- mammary tumors arise from luminal epithelial cells, but not myoepithelial cells. In addition, the molecular signature of the STAT1-/- mammary tumors overlaps closely to that of human luminal breast cancers. Finally, introduction of wildtype STAT1, but not a STAT1 mutant lacking the critical Tyr701 residue, into STAT1-/- mammary tumor cells results in apoptosis, demonstrating that the tumor suppressor function of STAT1 is cell-autonomous and requires its transcriptional activity.

CONCLUSIONS

Our findings demonstrate that STAT1 suppresses mammary tumor formation and its expression is frequently lost during breast cancer progression. Spontaneous mammary tumors that develop in STAT1-/- mice closely recapitulate the progression, ovarian hormone responsiveness, and molecular characteristics of human luminal breast cancer, the most common subtype of human breast neoplasms, and thus represent a valuable platform for testing novel treatments and detection modalities.

Synthetic progestins induce growth and metastasis of BT-474 human breast cancer xenografts in nude mice

OBJECTIVE

Previous studies have shown that sequential exposure to estrogen and progesterone or medroxyprogesterone acetate (MPA) stimulates vascularization and promotes the progression of BT-474 and T47-D human breast cancer cell xenografts in nude mice (Liang et al, Cancer Res 2007, 67:9929). In this follow-up study, the effects of progesterone, MPA, norgestrel (N-EL), and norethindrone (N-ONE) on BT-474 xenograft tumors were compared in the context of several different hormonal environments. N-EL and N-ONE were included in the study because synthetic progestins vary considerably in their biological effects and the effects of these two progestins on the growth of human tumor xenografts are not known.

METHODS

Estradiol-supplemented intact and ovariectomized immunodeficient mice were implanted with BT-474 cells. Progestin pellets were implanted simultaneously with estradiol pellets either 2 days before tumor cell injection (ie, combined) or 5 days after tumor cell injections (ie, sequentially).

RESULTS

Progestins stimulated the growth of BT-474 xenograft tumors independent of exposure timing and protocol, MPA stimulated the growth of BT-474 xenograft tumors in ovariectomized mice, and progestins stimulated vascular endothelial growth factor elaboration and increased tumor vascularity. Progestins also increased lymph node metastasis of BT-474 cells. Therefore, progestins, including N-EL and N-ONE, induce the progression of breast cancer xenografts in nude mice and promote tumor metastasis.

CONCLUSIONS

These observations suggest that women who ingest progestins for hormone therapy or oral contraception could be more at risk for developing breast cancer because of proliferation of existing latent tumor cells. Such risks should be considered in the clinical setting.

Transcriptional response of the murine mammary gland to acute progesterone exposure

Our mechanistic understanding of progesterone's involvement in murine mammary morphogenesis and tumorigenesis is dependent on defining effector pathways responsible for transducing the progesterone signal into a morphogenetic response. Toward this goal, microarray methods were applied to the murine mammary gland to identify novel downstream gene targets of progesterone. Consistent with a tissue undergoing epithelial expansion, mining of the progesterone-responsive transcriptome revealed the up-regulation of functional gene classes involved in epithelial proliferation and survival. Reassuringly, signaling pathways previously reported to be responsive to progesterone were also identified. Mining this informational resource for rapidly induced genes, we identified "inhibitor of differentiation 4" (Id4) as a new molecular target acutely induced by progesterone exposure. Mammary Id4 is transiently induced during early pregnancy and colocalizes with progesterone receptor (PR) expression, suggesting that Id4 mediates the early events of PR-dependent mammary morphogenesis. Chromatin immunoprecipitation assay detecting direct recruitment of ligand occupied PR to the Id4 promoter supports this proposal. Given that Id4 is a member of the Id family of transcriptional regulators that have been linked to the maintenance of proliferative status and tumorigenesis, the establishment of a mechanistic link between PR signaling and Id4 promises to furnish a wider conceptual framework with which to advance our understanding of normal and abnormal mammary epithelial responses to progestins. In sum, the progesterone-responsive transcriptome described herein not only reinforces the importance of progesterone in mammary epithelial expansion but also represents an invaluable information resource with which to identify novel signaling paradigms for mammary PR action.

The relevance of mouse models to understanding the development and progression of human breast cancer

Mouse modeling of human breast cancer has developed tremendously over the past ten years. Human breast cancer is characterized by enormous biological diversity and, collectively, the new models have come much closer to encompassing this diversity. They have provided a deeper understanding of the fundamental events that mediate the initiation, development, and progression of breast cancer, and they offer new opportunities to develop and test strategies to treat and, perhaps, even prevent the disease. This chapter reviews the historical development of mouse models of breast cancer and highlights some of their major strengths, weaknesses, and contributions.

Rat models of bone metastases

Bone metastases occur frequently in patients with advanced breast or prostate cancer. Bone metastases can be predominantly osteolytic, osteoblastic or mixed. Studies with animal models allow advances in understanding the molecular basis for bone metastases and provide new targets for therapy. Several animal models have been developed in rat with different pathophysiologies; they required injection or implantation of neoplastic cells into orthotopic locations, bones or the left ventricle of the heart. Several specific strains of rat have an increased incidence of spontaneous tumors. Carcinomas can be induced by either chemicals or physical agents. However, the most used and convenient way to induce bone metastases is a syngeneic transmission. MAT-Ly-Lu cells have been used in several models using Copenhagen rats to induce osteoblastic bone lesions. PA-III cells derived from Pollard tumors can also produce a combination of osteolytic and osteoblastic reactions at the site of transplantation. Osteolytic bone lesions can be obtained with an injection of Walker cells. The use of 13762 or c-SST2 cells allows also leads to osteolysis. Human xenografts can only be used in nude animals. It is essential to validate and correctly interpret the lesions in several models of bone metastasis. No animal model is sufficient by itself to represent the clinical findings observed in humans. The use of models developed in different species should be more predictive and bring a beam of arguments for a better knowledge of pathophysiological and therapeutic mechanisms.

Cytogenetic findings, Trp53 mutations, and hormone responsiveness in a medroxyprogesterone acetate induced murine breast cancer model

Medroxyprogesterone acetate (MPA)-induced mammary carcinomas express high levels of estrogen (ER) and progesterone receptors (PR) and when transplanted in syngeneic mice they show a progestin-dependent (PD) growth pattern. By successive transplantation, progestin-independent (PI) variants were generated and showed a different response to antihormone therapy. A diploid chromosome number (2n=40) was found in three of five PD tumors, with numbers in the triploid to tetraploid range in the other two. Some PI tumors were diploid, but most were aneuploid (8 of 12 tumors). The most frequent alterations found in PD and PI tumors were gains of chromosomes 3, 4, and 6 and losses of chromosomes 16 and X. Chromosomes 4 and 7 were involved in translocations in three of the four tumor families studied. single-strand conformation polymorphism analysis revealed a point mutation on the Trp53 gene in one of the PD tumors; this showed a stable diploid karyotype, suggesting that mutated Trp53 is not uniquely involved in chromosome instability. We have shown that hormone independence may be acquired without changes in ploidy, suggesting that the increase in ploidy is favored by successive transplantation. In our model, diploid tumors responded to hormone treatment but aneuploid tumors were either responsive or not.

Estrogen or antiprogestin treatment induces complete regression of pulmonary and axillary metastases in an experimental model of breast cancer progression

In this paper we demonstrate, using the C7-2-HI metastatic transplantable ductal mammary tumor, that endocrine therapy can induce complete regression of spontaneous lymph node and lung metastases in a mouse model of breast cancer progression. This tumor expresses high levels of estrogen and progesterone receptors and shows a high incidence of early axillary lymph nodes and lung metastases; using this model we had previously shown complete tumor regression of subcutaneous implants. Interestingly, although the metastases showed a more differentiated histology as compared with the primary growth, they underwent complete regression when treated with estrogens or antiprogestins. This phenomenon was associated with sustained cytostasis and apoptosis accompanied by increases in p21 and p27 expression and early tissue remodeling. These results highlight the essential role of PR in regulating cell proliferation in this model as well as its possible use as therapeutic target.

Isolation of a stromal cell line from an early passage of a mouse mammary tumor line: A model for stromal parenchymal interactions

We have developed a murine mammary tumor cell line, MC4-L4, and after 15 passages, a spindle-shaped population became evident. The cuboidal cells, MC4-L4E, cloned by limit dilution, proved to be epithelial tumor cells. When inoculated in syngeneic mice, they gave rise to invasive metastatic carcinomas expressing estrogen and progesterone receptors. These tumors regressed after anti-progestin treatment and stopped growing after 17-beta-estradiol administration. In vitro, they were insensitive to medroxyprogesterone acetate (MPA), 17-beta-estradiol, and EGF and were inhibited by TGFbeta1. They expressed mutated p53 and estrogen receptors alpha; progesterone receptors were undetectable. Cells were polyploid and shared the same four common marker chromosomes present in the parental tumor in addition to an exclusive marker. Spindle-shaped cells, MC4-L4F, were selected by differential attachment and detachment and proved to be non-epithelial non-tumorigenic cells. They were cytokeratin negative, showed mesenchymal features by electron microscopy, differentiated to adipocytes when treated with an adipogenic cocktail, were stimulated by TGFbeta1 and EGF, showed a wild-type p53, and did not exhibit the marker chromosomes of the parental tumor. Although they expressed estrogen receptors alpha, they were insensitive to 17-beta-estradiol in proliferation assays. Co-cultures of both cell types had a synergic effect on progesterone receptors expression and on cell proliferation, being the epithelial cells, the most responsive ones, and 17-beta-estradiol increased cell proliferation only in co-cultures. Cytogenetic studies and data on p53 mutations rule out the possibility of an epithelial mesenchymal transition. Their unique characteristics make them an excellent model to be used in studies of epithelial-stromal interactions in the context of hormone responsiveness in hormone related tumors.