Relationship between organization of mammary tumors and the ability of tumor cells to replicate mammary tumor virus and to recognize growth-inhibitory contact signals in vitro

Local regulation of human breast xenograft models

Breast cancer studies implant human cancer cells under the renal capsule, subcutaneously, or orthotopically and often use estrogen supplementation and immune suppressants (etoposide) in xenograft mouse models. However, cell behavior is significantly impacted by signals from the local microenvironment. Therefore, we investigated how the combinatorial effect of the location of injection and procedural differences affected xenograft characteristics. Patient-derived breast cancer cells were injected into mouse abdominal or thoracic mammary glands +/- estrogen and/or etoposide pretreatment. Abdominal xenografts had increased tumor incidence and volume, and decreased latency (P < 0.001) compared to thoracic tumors. No statistically significant difference in tumor volume was found in abdominal xenografts treated +/- estrogen or etoposide; however, etoposide suppressed tumor volume in thoracic xenografts (P < 0.02). The combination of estrogen and etoposide significantly decreased tumor incidence in both sites. In addition, mice treated +/- estradiol were injected orthotopically or subcutaneously with well-characterized breast cancer cell lines (MCF7, ZR75-1, MDA MB-231, or MCF10Ca1h). Orthotopic injection increased tumor volume; growth varied with estrogen supplementation. Location also altered methylation status of several breast cancer-related gene promoters. Lastly, vascularization of orthotopic tumors was significantly enhanced compared to subcutaneous tumors. These data suggest that optimal xenograft success occurs with orthotopic abdominal injections and illustrate molecular details of the compelling influence of the local microenvironment on in vivo models.

Early increases in superantigen-specific Foxp3+ regulatory T cells during mouse mammary tumor virus infection

Mouse mammary tumor virus (MMTV) is a milk-borne betaretrovirus that has developed strategies to exploit and subvert the host immune system. Here, we show in a natural model of MMTV infection that the virus causes early and progressive increases in superantigen (SAg)-specific Foxp3(+) regulatory T cells (T(reg)) in Peyer's patches (PP). These increases were shown to be dependent on the presence of dendritic cells. CD4(+) CD25(+) T cells from the PP of infected mice preferentially suppress the proliferative response of T cells to SAg-expressing antigen-presenting cells ex vivo. We investigated the influence of the depletion of CD25(+) cells at different stages of the infection. When CD25(+) cells were depleted before MMTV infection, an increase in the number of PP SAg-cognate Foxp3(-) T cells was found at day 6 of infection. Since the SAg response is associated with viral amplification, the possibility exists that T(reg) cells attenuate the increase in viral load at the beginning of the infection. In contrast, depletion of CD25(+) cells once the initial SAg response has developed caused a lower viral load, suggesting that at later stages T(reg) cells may favor viral persistence. Thus, our results indicated that T(reg) cells play an important and complex role during MMTV infection.

Mouse Mammary Tumor Biology: A Short History

For over a century, mouse mammary tumor biology and the associated Mouse mammary tumor virus (MMTV) have served as the foundation for experimental cancer research, in general, and, in particular, experimental breast cancer research. Spontaneous mouse mammary tumors were the basis for studies of the natural history of neoplasia, oncogenic viruses, host responses, endocrinology, and neoplastic progression. However, lacking formal proof of a human mammary tumor virus, the preeminence of the mouse model faded in the 1980s. Since the late 1980s, genetically engineered mice (GEM) have proven extremely useful for studying breast cancer and have become the animal model for human breast cancer. Hundreds of mouse models of human breast cancer have been developed since the first demonstration, in 1984, that the mouse mammary gland could be molecularly targeted and used to test the oncogenicity of candidate human genes. Now, very few scientists can avoid using a mouse model to test the biology of their favorite gene. The GEM have attracted a new generation of molecular and cellular biologists eager to apply their skills to these surrogates of the human disease. Newcomers often enter the field without an appreciation of the origins of mouse mammary tumor biology and the basis for many of the prevailing concepts. Our purpose in writing this short history of mouse mammary tumor biology is to provide a historical perspective for the benefit of the newcomers. If Einstein was correct in that "we stand on the shoulders of giants," the neophytes should meet their giants.

Toll-like receptor 4-dependent activation of dendritic cells by a retrovirus

Mouse mammary tumor virus (MMTV) is a milk-borne retrovirus that exploits the adaptive immune system. It has recently been shown that MMTV activates B cells via Toll-like receptor 4 (TLR4), a molecule involved in innate immune responses. Here, we show that direct virus binding to TLR4 induced maturation of bone marrow-derived dendritic cells and up-regulated expression of the MMTV entry receptor (CD71) on these cells. In vivo, MMTV increased the number of dendritic cells in neonatal Peyer's patches and their expression of CD71; both these effects were dependent on TLR4. Thus, retroviral signaling through TLRs plays a critical role in dendritic-cell participation during infection.

Increased yield of mouse mammary tumor virus (MMTV) by cultivation of monolayer-derived mammary tumor cells in suspension

The MJY-alpha epithelial-like mammary tumor cell line was adapted for cultivation in suspension using a shaker culture technique. Replication of suspension (MJY-beta) cells was more sensitive than monolayer cells to decreases in the concentration of serum in the medium. Comparison of amino acid incorporation and lactate production rates revealed additional differences between monolayer and suspension cultures. In addition, growth in suspension resulted in 10- to 400-fold increases in mouse mammary tumor virus (MMTV) production by the mammary tumor cells. Increases in MMTV yield were detected within 48 h of culture initiation and MMTV production remained elevated throughout 20 cell passages in suspension. Exposure of MJY-beta cells to 14 microM hydrocortisone further increased MMTV yield two- to five-fold. The MJY-beta suspension cultures demonstrated that these epithelial-like cells do not require attachment to a solid substrate for replication or for MMTV production. Loss of structural polarization associated with growth as a monolayer resulted in stimulation of MMTV production greater than and independent of steroid exposure.

Hormone-dependent enhancement of neoplastic phenotype expression by MuMTV-infected heterologous cells, and MuMTV-induced cellular antigens

A new property, i.e. acquisition of insulin (insulin + dexamethasone)-mediated enhancement of anchorage independence by heterologous MuMTV-infected cells, is described. Hormone-mediated anchorage-independent growth enhancement in MuMTV-infected cells is followed by expression of a cell-surface antigen, recognized by allogeneic (C3H----AKR) anti-Thy 1.2 antigen serum. The absorption test and the use of monoclonal antibody to the Thy 1.2 antigen showed that hormone-dependent antigen on the cell surfaces of experimentally and naturally MTV-infected cells was different from the Thy 1.2 antigen itself. Utilization of anti-p53 ("protein of transformation") monoclonal antibody enabled us to identify this cell membrane antigen as p53.

Serum-independent modulation of hemicyst formation by dissolved oxygen in postconfluent epithelial monolayers

Hemicyst formation is considered a manifestation of either transepithelial solute and fluid movement or secretory activity in culture. This study shows that hemicyst formation in postconfluent monolayers of rhesus monkey kidney (LLC-MK2) cells is modulated by the dissolved oxygen concentration (PO2) of the culture medium. Either daily replacement of serum-free medium or displacement of the gas phase with 18% vol/vol O2 (initial medium PO2 = 125 to 135 mm Hg) enhances formation of hemicysts. Use of 30% O2 (medium PO2 approximately equal to 175 mm Hg) does not further increase the incidence, but neither 10% O2 (medium PO2 = 90 to 95 mm Hg) nor 1% O2 (medium PO2 = 35 to 50 mm Hg), the approximate range of dissolved oxygen values in blood, supports hemicyst formation unless cultures are gently rocked to disrupt diffusion gradients. Phase photomicrography of living cultures shows that the surface of a turgid hemicyst is furrowed, and cinephotomicrography reveals that the walls vibrate subtly. When hypoxic conditions (0 to 1% O2) are introduced this vibration ceases within 2 to 3 h, whereas collapse and disappearance of turgid hemicysts requires 18 to 20 h, seems virtually synchronous, and is reversible. Hemicysts form in a broad osmotic range, and increased electrolyte concentration increases the incidence. Hemicysts persist in locally dense areas when cell-free strips are etched in the postconfluent monolayer; no DNA synthesis is detected under these conditions, but two-dimensional cell spreading into the denuded area is seen along the edge of the wound. We conclude that the dissolved oxygen supply in the cellular microenvironment modulates functional expression by differentiated kidney epithelial cells in culture and that increased electrolyte concentration also enhances expression of this phenotypic marker.

The correlation between tissue differentiation and production of mammary tumor virus (MTV) in transplanted murine mammary tumors. Electron microscopic observations

The spontaneous mammary tumors of the NMRI mouse are well developed microcystic adenocarcinomas. Serial isologous transplantation of the tumors results in nearly complete dedifferentiation to a solid tumor, in which only electron-microscopically rudimentary acinus-like microlumina can be observed. The adenocarcinomas produce A and B particles in abundance, with the A particles appearing intracellularly in the adluminal cytoplasmic regions of the epithelial cells in association with typical cellular structures and the B particles being restricted to closed extracellular compartments such as vacuoles or acini alone. The loss of alveolar organization in the solid tumors is followed by an almost complete reduction in mature B particles, while A particles are still regularly observed and appear to be less reduced in number. This suggests that the production of extracellular B particles is dependent upon the secretory activity of the tumor cells and that in nonsecreting cells it is predominantly a late step in virus release that is inhibited, not the synthesis of intracellular precursors.

The capacity of ovarian cells of the postnatal rat to reorganize into histiotypic structures

Cell-reorganization experiments in vitro were performed with dissociated rat ovaries at different ages of postnatal development, namely newborn, 8-10, 15-22, and 90-day-old. Ovarian cells consistently aggregated into follicularlike structures. Follicular organization ion vitro is comparable to the ovarian histology of the respective age. The histogenic properties conserved by ovarian cells are considered to be related with the morphogenetic processes steadily occurring in the ovary.

Differentiation of a rat mammary cell line in vitro

We have studied the development of fusiform (probably related to myoepithelial) cells in Rama 25 cultures [Bennett, D. C. Peachey, L. A., Durbin, H. & Rudland, P. S. (1978) Cell 15, 283--298]; we show that they are generated from special differentiated structures (projections) that contain a rapidly differentiating cell type (F-precursor cells). Clonal sublines isolated from projections develop in several directions under both environmental and genetic control. Some types of differentiation are reversible; others are irreversible. The various cell occurring in vitro may correspond to specific cell types in vivo.

Regulation of mouse mammary tumor virus gene expression by glucocorticoid hormones

Several laboratories have documented that glucocorticoid hormones markedly stimulate the expression of mouse mammary tumor virus genes in a variety of mouse mammary tumor cells and in infected heterologous cells. The effect of the hormone appears to be a rapid and specific augmentation of the synthesis of viral RNA, mediated by interaction with glucocorticoid receptor proteins. The availability of virus-specific reagents and recent developments in the molecular biology of RNA tumor viruses now permit a highly refined analysis of hormonal regulation in this experimental system.

Markers to distinguish normal and neoplastic mammary epithelial cells in vitro: comparison of saturation density, morphology and concanavalin A reactivity

Normal and premalignant mouse mammary epithelial cells can be prepared in high yields by collagenase dissociation of minced glands followed by a brief, differential centrifugation to remove contaminating fibroblasts and fat cells. The major difficulties in preparing pure cultures in quantity are 1) incomplete dissociation of gland material, and 2) cell death during enzymatic digestion. These problems are eliminated by careful selection of collagenases for dissociation. Normal and premalignant mammary epithelial cells are morphologically indistinguishable from malignant mouse mammary epithelial cells in primary monolayer cultures. In addition, the growth rates and saturation densities achieved by normal mammary epithelial cells are indistinguishable from those of malignant mammary epithelial cells in primary culture. In both cases, a monolayer of cells is preserved with no evidence of focal overgrowth. Malignant adenocarcinoma mammary cells can however be distinguished from normal mammary epithelial cells by virtue of differences in their surface interactions with concanavalin A. A hemadsorption assay using Con-A-coated erythrocytes was the most sensitive indicator for these differences. In hemadsorption assays malignant mammary epithelial cells were half-maximally reactive with 2.5 mug/ml concanavalin A, while normal cells were completely unreactive even at concanavalin A concentrations five-times higher. Premalignant mammary epithelial cells were as reactive as malignant mammary epithelial cells in the hemadsorption assays. Hemadsorption of malignant cells was observed in primary and secondary cultures of epithelium as well as in cell lines. Malignant cells forming mammary adenocarcinomas were as highly reactive as malignant cells forming scirrhous carcinomas. Malignant cells not releasing mammary tumor virus (MuMTV) were as reactive as cells releasing that virus. Adsorption of concanavalin-A-coated erythrocytes to normal mammary epithelial cells could be induced by brief treatment of cell monolayers with hyaluronidase. Exposure of active sites was not affected with either trypsin or collagenase. Our results show that while the growth of malignant cells does not serve to distinguish them from normal cells in monolayer culture, surface changes do exist which can be identified by differences in concanavalin A reactivity. Since the earliest transformants identifiable in vivo (premalignant) have undergone conversion of the surface marker, concanavalin-A-mediated hemadsorption provides a sensitive measure for mammary epithelial cell transformants in vitro.

Occluding junctions and cell behavior in primary cultures of normal and neoplastic mammary gland cells

Cells dissociated from normal prelactating mouse mammary glands or from spontaneous mammary adenocarcinomas, when grown at high density on an impermeable substrate, form nonproliferating, confluent, epithelial pavements in which turgid, blister-like domes appear as a result of fluid accumulation beneath the cell layer. To compare the structure of the fluid-segregating cell associations in normal and tumor cell cultures with that of lactating gland in vivo, we have examined such cultures alive and in thick and thin sections and freeze-fracture replicas. Pavement cells in all cases are polarized toward the bulk medium as a lumen equivalent, with microvilli and continuous, well-developed occluding junctions at this surface. Between the pavement and the substrate are other cells, of parenchymal or stromal origin, scattered or in loose piles; these sequestered cells are relatively unpolarized and never possess occluding junctions. Small gap junctions have been found in the pavement layer, and desmosomes may link epithelial cells in any location. Under the culture conditions used, development of the epithelial secretory apparatus is not demonstrable; normal and neoplastic cells do not differ consistently in any property examined. A dome's roof is merely a raised part of the epithelial pavement and does not differ from the latter in either cell or junction structure. We suggest that dome formation demonstrates the persistence of some transport functions and of the capacity to form effective occluding junctions. These basic epithelial properties can survive both neoplastic transformation and transition to culture.

An Effective Immunodepressive Treatment for Demonstrating in Allogeneic Mice the Tumorigenic Properties of Cells Grown in Vitro

Short intensive treatment with N-diazoacetylglycine amide (DGA) before the inoculum, followed by prolonged daily administration of L-aspàraginase (Asnase), was tested for its ability to elicit tumorigenic properties of fibroblast-like cells cultured in vitro. With this treatment progressive tumor growth was obtained in allogeneic mice injected with cells of a transformed subline. Results show that combined use of DGA and Asnase affords a higher probability of proving in vivo the tumorigenic properties of injected cells than in newborn or X-irradiated recipients. Experimental data indicate that L-asparagine depletion does not inhibit the in vitro growth of fibroblast-like cells.

Studies of lipid class and fatty acid profiles of rat mammary tumors induced by 7, 12-dimethylbenz(a)anthracene

The lipid class and fatty acid composition profiles of mammary glands of female rats fed a nutritionally adequate diet are compared to those of tumors induced in the mammary glands by intravenous injection of dimethylbenz(a)anthracene of animals fed the same diet. Ca. 95% of the lipids of the mammary glands of the control group of animals consisted of triglycerides; glycolipids and phospholipids were present in only minor amounts. In contrast, the lipids of the mammary tumors contained much lower amounts of neutral lipids and higher concentrations of phospholipids. The glycolipid fraction was a minor component of both tissues but differed greatly in composition. The composition of the phospholipid and neutral lipid fractions, particulary the latter, of the mammary tumors also differed from that of the mammary glands of the control animals. The neutral lipids of the tumor tissues contained elevated levels of free fatty acids and cholesterol and much lower concentrations of triglyceride compared to the mammary gland lipids. Differences also were observed in the fatty acid composition of tumor and mammary gland lipid. The greatest differences occurred in the concentrations of polyunsaturated fatty acids which were generally much higher in the tumor lipids.

Structure of the mouse mammary tumor virus: polypeptides and glycoproteins

The polypeptide and glycoprotein compositions of the mouse mammary tumor virus virion from primary monolayer cultures of BALB/cfC3H mouse mammary tumor cells were studied by polyacrylamide gel electrophoresis by using internal and external labeling and Coomassie blue and periodic acid Schiff (PAS) staining. Twelve polypeptides were reproducibly resolved by the combined methods. Five major polypeptides were demonstrable with estimated molecular weights of 52,000, 36,000, 28,000, 14,000, and 10,000. Seven minor polypeptides were also consistently detected and had estimated molecular weights of 70,000, 60,000, 46,000, 38,000, 30,000, 22,000, and 17,000. Carbohydrate was associated with five of these polypeptides as measured by PAS stain or [(3)H] glucosamine labeling, or both. These glycoproteins had estimated molecular weights of 70,000, 60,000, 52,000, 36,000 and 10,000. The majority of the PAS stain and glucosamine was found in the 52,000 and 36,000 dalton peaks.