Interactions between normal mammary epithelial cells and mammary tumour cells in a model system

Benign mammary epithelial cells enhance the transformed phenotype of human breast cancer cells

Background

Recent research has yielded a wealth of data underscoring the key role of the cancer microenvironment, especially immune and stromal cells, in the progression of cancer and the development of metastases. However, the role of adjacent benign epithelial cells, which provide initial cell-cell contacts with cancer cells, in tumor progression has not been thoroughly examined. In this report we addressed the question whether benign MECs alter the transformed phenotype of human breast cancer cells.

Methods

We used both in vitro and in vivo co-cultivation approaches, whereby we mixed GFP-tagged MCF-10A cells (G2B-10A), as a model of benign mammary epithelial cells (MECs), and RFP-tagged MDA-MB-231-TIAS cells (R2-T1AS), as a model of breast cancer cells.

Results

The in vitro studies showed that G2B-10A cells increase the colony formation of R2-T1AS cells in both soft agar and clonogenicity assays. Conditioned media derived from G2B-10A cells enhanced colony formation of R2-T1AS cells, whereas prior paraformaldehyde (PFA) fixation of G2B-10A cells abrogated this enhancement effect. Moreover, two other models of benign MECs, MCF-12A and HuMECs, also enhanced R2-T1AS colony growth in soft agar and clonogenicity assays. These data reveal that factors secreted by benign MECs are responsible for the observed enhancement of the R2-T1AS transformed phenotype. To determine whether G2B-10A cells enhance the tumorigenic growth of co-injected R2-T1AS cells in vivo, we used the nude mouse xenograft assay. Co-injecting R2-T1AS cells with G2B-10A cells ± PFA-fixation, revealed that G2B-10A cells promoted a ~3-fold increase in tumor growth, irrespective of PFA pre-treatment. These results indicate that soluble factors secreted by G2B-10A cells play a less important role in promoting R2-T1AS tumorigenesis in vivo, and that additional components are operative in the nude mouse xenograft assay. Finally, using array analysis, we found that both live and PFA-fixed G2B-10A cells induced R2-T1AS cells to secrete specific cytokines (IL-6 and GM-CSF), suggesting that cell-cell contact activates R2-T1AS cells.

Conclusions

Taken together, these data shift our understanding of adjacent benign epithelial cells in the cancer process, from passive, noncontributory cells to an active and tumor-promoting vicinal cell population that may have significant effects early, when benign cells outnumber malignant cells.

Three-dimensional culture conditions lead to decreased radiation induced cytotoxicity in human mammary epithelial cells

For both targeted and non-targeted exposures, the cellular responses to ionizing radiation have predominantly been measured in two-dimensional monolayer cultures. Although convenient for biochemical analysis, the true interactions in vivo depend upon complex interactions between cells themselves and the surrounding extracellular matrix. This study directly compares the influence of culture conditions on radiation induced cytotoxicity following exposure to low-LET ionizing radiation. Using a three-dimensional (3D) human mammary epithelial tissue model, we have found a protective effect of 3D cell culture on cell survival after irradiation. The initial state of the cells (i.e., 2D versus 3D culture) at the time of irradiation does not alter survival, nor does the presence of extracellular matrix during and after exposure to dose, but long term culture in 3D which offers significant reduction in cytotoxicity at a given dose (e.g. approximately 4-fold increased survival at 5Gy). The cell cycle delay induced following exposure to 2 and 5Gy was almost identical between 2D and 3D culture conditions and cannot account for the observed differences in radiation responses. However the amount of apoptosis following radiation exposure is significantly decreased in 3D culture relative to the 2D monolayer after the same dose. A likely mechanism of the cytoprotective effect afforded by 3D culture conditions is the down regulation of radiation induced apoptosis in 3D structures.

Proteomics Demonstration That Normal Breast Epithelial Cells Can Induce Apoptosis of Breast Cancer Cells through Insulin-like Growth Factor-binding Protein-3 and Maspin

Normal breast epithelial cells are known to exert an apoptotic effect on breast cancer cells, resulting in a potential paracrine inhibition of breast tumor development. In this study we purified and characterized the apoptosis-inducing factors secreted by normal breast epithelial cells. Conditioned medium was concentrated by ultrafiltration and separated on reverse phase Sep-Pak C18 and HPLC. The proapoptotic activity of eluted fractions was tested on MCF-7 breast cancer cells, and nano-LC-nano-ESI-MS/MS allowed the identification of insulin-like growth factor-binding protein-3 (IGFBP-3) and maspin as the proapoptotic factors produced by normal breast epithelial cells. Western blot analysis of conditioned media confirmed the specific secretion of IGFBP-3 and maspin by normal cells but not by breast cancer cells. Immunodepletion of IGFBP-3 and maspin completely abolished the normal cell-induced apoptosis of cancer cells, and recombinant proteins reproduced the effect of normal cell-conditioned medium on apoptosis of breast cancer cells. Together our results indicated that normal breast epithelial cells can induce apoptosis of breast cancer cells through IGFBP-3 and maspin. These findings provide a molecular hypothesis for the long observed inhibitory effect of normal surrounding cells on breast cancer development.

Tumor signaling to the bone marrow changes the phenotype of monocytes and pulmonary macrophages during urethane-induced primary lung tumorigenesis in A/J mice

Little is known about how the composition of stromal cells within the lung cancer microenvironment varies during tumor progression. We examined by immunohistochemistry each of six different stromal cell populations during the development of chemically induced primary lung cancer in mice. Blood vessels were seen even in microscopic lesions, and their numbers increased with tumor size. Neutrophils infiltrated the alveoli of tumor-bearing lungs and within the periphery of macroscopic adenomas and adenocarcinomas. The numbers of peritumoral lymphocytes and macrophages increased during oncogeny, but quantitative changes in mast cells and fibroblasts were not evident. Because macrophage depletion reduces tumor growth and these cells are thus important to tumorigenesis, we also investigated their phenotype. Pulmonary macrophages expressed arginase I (subtype M2) but not inducible nitric-oxide synthase in lungs with premalignant lesions, whereas macrophages in carcinoma-bearing lungs expressed inducible nitric-oxide synthase (subtype M1) but not arginase I. Local pulmonary stimuli did not seem responsible for this shift in macrophage activation state because monocytes still residing within the bone marrow adopted these expression patterns before entering the circulation, presumably in response to tumor-derived signals. These biochemical markers of macrophage activation states would have diagnostic and/or therapeutic value if analogous systemic shifts occur in humans.

Normal breast epithelial cells induce apoptosis of breast cancer cells via Fas signaling

Fas/Fas ligand (Fas L) death pathway is an important mediator of apoptosis. Deregulation of Fas pathway is reported to be involved in the immune escape of breast cancer and the resistance to anti-cancer drugs. In this study, we demonstrated that conditioned medium by normal breast epithelial cells (NBEC-CM) induced apoptosis of MCF-7 and T-47D Fas-sensitive cells but had no effect on MDA-MB-231 Fas-resistant cells. Inhibition of PI3 kinase or NF-kappaB by specific inhibitors or transient transfections restored the sensitivity of MDA-MB-231 cells to NBEC-induced apoptosis. Moreover, the constitutive activation of NF-kappaB was controlled by PI3 kinase because inhibition of PI3 kinase reduced NF-kappaB activity. Inducible activation of NF-kappaB rendered MCF-7 cells resistant to NBEC-CM- and Fas agonist antibody-triggered apoptosis. Therefore, constitutive or inducible activation of PI3 kinase and/or NF-kappaB in breast cancer cells rendered them resistant to NBEC-triggered apoptosis. In addition, Fas neutralizing antibody and dominant negative Fas abolished NBEC-triggered apoptosis. Western blot and confocal microscopy analysis showed an increase of membrane Fas/Fas L when cells were induced into apoptotis by NBEC-CM. Taken together, these data show that NBEC induced apoptosis in breast cancer cells via Fas signaling.

The lung tumor promoter, butylated hydroxytoluene (BHT), causes chronic inflammation in promotion-sensitive BALB/cByJ mice but not in promotion-resistant CXB4 mice

An inflammatory response accompanies the reversible pneumotoxicity caused by butylated hydroxytoluene (BHT) administration to mice. Lung tumor formation is promoted by BHT administration following an initiating agent in BALB/cByJ mice, but not in CXB4 mice. To assess the contribution of inflammation to this differential susceptibility, we quantitatively characterized inflammation after one 150 mg/kg body weight, followed by three weekly 200 mg/kg ip injections of BHT into male mice of both strains. This examination included inflammatory cell infiltrate and protein contents in bronchoalveolar lavage (BAL) fluid, cyclooxygenase (COX)-1 and COX-2 expression in lung extracts, and PGE(2) and PGI(2) production by isolated bronchiolar Clara cells. BAL macrophage and lymphocyte numbers increased in BALB mice (P<0.0007 and 0.02, respectively), as did BAL protein content (P<0.05), COX-1 and COX-2 expression (P<0.05 for each), and PGI(2) production (P<0.05); conversely, these indices were not perturbed by BHT in CXB4 mice. BALB mice fed aspirin (400 mg/kg of chow) for two weeks prior to BHT treatment had reduced inflammatory cell infiltration. Our results support a hypothesis that resistance to BHT-induced inflammation in CXB4 mice accounts, at least in part, for the lack of effect of BHT on lung tumor multiplicity in this strain.

Normal breast epithelial cells induce apoptosis of MCF-7 breast cancer cells through a p53-mediated pathway

Cancer development depends not only on the nature of the tumor cells themselves but also on the regulatory effects of various normal cells. The present study was performed to better understand the mechanism by which normal breast epithelial cells (NBEC) can control the growth of MCF-7 breast cancer cells. When MCF-7 cells were treated with NBEC conditioned medium, cell growth was inhibited in a concentration-dependent manner. This inhibition was due to an induction of apoptosis without any change in cell cycle progression. The induction of apoptosis was correlated with increased levels of p53, p21(waf1) and decreased levels of bcl-2. Transient transfections of MCF-7 cells with two p53 cDNA constructs demonstrated the induction of apoptosis was mediated by endogenous p53. Taken together, our results indicate that NBEC inhibit the growth of MCF-7 breast cancer cells by inducing apoptosis in them via endogenous p53.