Abstract P4-05-04: Lack of the Effect of Mesenchymal Stem Cells on Breast Cancer Initiation and Growth in a Somatic Mouse Breast Cancer Model

Background: The impact of mesenchymal stem cells (MSCs) on breast cancer (BC) progression and growth remains controversial. Some studies using xenograft BC models in nude mice suggest that MSCs stimulate BC metastases, whereas other studies in syngeneic rat BC models suggest that MSCs suppress BC development and growth. In addition, whether MSCs have a role in BC initiation has not been tested. The objective of this study was to determine if bone marrow-derived MSCs affect BC initiation and progression in vitro and in clinically relevant somatic and synegeneic BC mouse models. Materials and Methods: MSCs were isolated from bone marrows of FVB wild-type mice, cultured, and characterized for their potential to differentiate into adipocytes and express flow-cytometric cell surface markers. MSC-conditioned medium (CM) was used to culture RCAS-Neu and RCAS-PyMT BC cell lines, derived from TVA-transgenic mice infected with an avian retroviral vector encoding Neu or polyoma middle T antigen (PyMT). Cell proliferation was analyzed with CellTiter 96 proliferation assay and 5-bromo-2-deoxyuridine labeling. The effect of RCAS-Neu and RCAS-PyMT cells and their CM on MSC migration was determined with Boyden chamber migration assay. In a somatic BC model, TVA-transgenic mice expressing the receptor for an avian retrovirus vector, RCAS, were infected with RCAS-PyMT vector by intraductal injection and treated with MSC (2x10 6 cells/mouse) by i.v. injection. Mice were observed for BC development by palpation. In a syngeneic BC model, RCAS-Neu BC cells were co-implanted with MSCs (5x10 5 /gland) at the ratios 1:0, 1:0.2, or 1:1 into the fat pad of FVB female mice. BC growth was monitored for 9 weeks. Results: CM from MSC did not significantly affect the proliferation of RCAS-Neu and RCAS-PyMT cells. However, RCAS-Neu and RCAS-PyMT cells and their CM induced morphologic changes in MSC and dramatically increased their motility. In somatic model, MSCs had no effect on BC initiation or growth with the mean tumor latency 27.5±7.5 days in MSC-treated mice and 29±5.5 days in control mice treated with PBS. In syngeneic BC model, there was no significant difference in the growth of RCAS-Neu alone or RCAS-Neu cells co-implanted with MSCs. Since BC in these models does not metastasize to distant organs, it could not be determined whether MSCs affect BC metastases. Discussion: Our results demonstrated that BC cell lines and their CM are able to induce MSCs migration and possibly differentiation. However, MSCs had no effect on BC initiation in a somatic BC model and on tumor growth in a syngeneic BC model. It is possible that the number of MSCs and schedule of MSC injection were not optimized or strong PyMT oncogene rapidly induced BC and overcame the effect of MSCs on BC formation. Experiments are ongoing to determine if multiple administrations of MSCs affect BC development induced by PyMT and Neu oncogenes. Lack of effect of MSCs in BC initiation and progression, if confirmed, will suggest that MSCs are safe for delivering novel antitumor agents for BC treatment. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P4-05-04.