How Bone Marrow Mesenchymal Stem Cells Influence the Metastasis of Breast Cancer

Abstract

Mesenchymal stem cells (MSCs) are multipotent non-haematopoietic cells primarily originating from within the bone marrow. Physiologically they aid in the process of wound healing. However, MSCs are also recruited by cancer cells to the tumour stroma where they have been shown to alter cancer cell proliferation, angiogenesis and metastasis.

There is no clear answer as to whether bone marrow-MSCs (BM-MSCs) aid or inhibit the metastasis of breast cancer cells. Given that 600+ woman die each year in NZ from metastatic breast cancer the need for a better understanding as to what influences breast cancer metastasis is critical. The purpose of our study was to use in vitro techniques to better understand the metastatic processes that the secretome of BM-MSCs may influence, and to further determine whether BM-MSCs aid or inhibit breast cancer metastasis. Conditioned media (CM) was generated by culturing BM-MSCs in serum free media for 1 or 5 days. This was then used in various in vitro experiments that simulate the cellular events of metastasis. These include cell migration, cell adhesion and cell invasion. Two breast cancer cell lines were used, the MDA-MB-231 and MCF-7 cell lines. BM-MSC CM was found to significantly increase the migration of the MDA-MB-231 cell line up to 180 ± 4.0% of untreated control (p < 0.001) in the Boyden chamber cell motility model. BM-MSC CM also increased the migration of the MCF-7 cell line in this same model, although not significantly. In a scratch assay, MDA-MB-231 cell motility was enhanced and cells were visibly elongated by the BM-MSC CM. BM-MSC CM appeared to restore the adhesion of the MCF-7 cells to fibronectin, increasing adhesion from 53.8 ± 5.8% to 112.4 ± 18.4% of uncoated control (p < 0.05). Cell invasion was not significantly altered in either cell line. From these results, it was concluded that BM-MSC CM primarily acts to enhance breast cancer cell migration. This helps understand how MSCs enhance the metastatic spread of breast cancer cells as shown in previous in vivo studies. Additional to understanding the possible metastatic processes BM-MSCs influence, a major focus of our study was to investigate the validity of the in vitro experimental models. Cell viability was assessed and BM-MSCs were not found to significantly alter the viability of either cell line in a manner that would confound the results seen in the Boyden chamber studies. The relevance of using healthy MSCs in culture compared to those that would be found in the tumour microenvironment was also investigated. BM-MSCs were cultured in breast cancer CM and then used in the Boyden chamber the same way normal MSCs were. Activated MSCs were seen to significantly enhance the migration of both MDA-MB-231 and MCF-7 cells to 112 ± 0.4% and 189.5 ± 20.0% of normal MSC control (p < 0.01 and p < 0.05, respectively). This shows that healthy MSCs in culture are not necessarily representative of those found in the tumour microenvironment and that simple techniques, like the one developed in our study, designed to make MSCs more relevant to those in the tumour microenvironment, will help make in vitro studies more comparable to in vivo research.