**5. Conclusions**

258 Breast Cancer – Focusing Tumor Microenvironment, Stem Cells and Metastasis

transformation of human mammary epithelial cells by SV40 T-antigen and forced expression of EMT-inducing proteins had the potential to undergo adipogenic and chondrogenic differentiation. Also, these mesenchymal breast cancer cells were attracted to wounds and tumors, a feature typical for MSCs. The latter observation may shed a new light on a phenomenon called tumor-self seeding (Leung & Brugge, 2009). Tumor-self seeding describes the ability of metastasized cells to circulate back to the primary tumor. Chemoattraction to the primary tumor was shown to be driven by IL-6 and IL-8. As mentioned above, IL-6 is highly active on MSCs and triggers the production of a number of chemokines (Leung & Brugge, 2009). Based on these data, it is tempting to assume that MSCs are also generated from breast cancer cells (Fig. 1) and that these MSCs containing the mutations (and epigenetic changes) of the breast cancer cells they derived from play a role in breast cancer metastasis and tumor-self seeding. Nestin+ - MSCs have been reported to share with haematopoietic stem cells the same niche in the bone marrow (Mendez-Ferrer et al., 2010). This niche might also be available for breast cancer-derived MSCs and allow these cells to survive in this tissue. An exciting hypothesis would be to assume that, at least in some cases of breast cancer, dormancy (Pantel et al., 2009; Willis et al., 2010) is caused by breast cancer-derived MSCs that are caught in these niches. The niches could fulfill two functions in order to maintain dormancy, preventing the cells from proliferating while, at the same time, protecting them from death-inducing signals. The bone marrow seems to be an attractive tissue for circulating tumor cells to home and form micrometastasis which is an early event in breast cancer development (Pantel et al., 2009). The number of such disseminated breast cancer cells in bone has been linked to prognosis of breast cancer patients. MSCs may also play a role in this early entry of breast cancer cells into bone marrow (Corcoran et al., 2008). MSCs were shown to facilitate the migration of MCF-7 and T47D breast cancer cells across bone marrow endothelial cells *in vitro* and to be in close contact with bone-metastasized breast cancer cells *in vivo*. Evidence was presented that these MSC/breast cancer cell interactions may require the chemokine receptor CXCR4 as well as its ligand SDF-1/CXCL12. Hence, it might be possible that breast cancer-derived MSCs not only would be able to home to the bone marrow and induce tumor dormancy, but also to help other breast cancer cells to enter this tissue and form micrometastasis. The breast cancer cell may not be the only non-stem cell which may be able to convert to an MSC. Vascular endothelial cells have been shown to become MSC-like cells as well as displaying typical MSC features, such as the ability to differentiate to osteoblasts, chondrocytes and adipocytes, upon treatment with ALK2 (activin-like kinase-2), TGF2 or BMP4 (bone morphognetic protein-4) (Medici et al., 2010). This suggests that certain non-stem cells under certain conditions can be an additonal source for generating MSCs. These cells may have

different features compared to those MSCs that derived from the bone marrow.

Besides differentiating to osteoblast, chondrocytes and adipocytes, MSCs are able to convert to neural cells or to undergo transdifferentiation to different kinds of epithelial cells (Uccelli et al., 2008; Wislet-Gendebien et al., 2005). In tumors, MSCs can also differentiate to the carcinoma-associated fibroblasts (CAFs) (Mishra et al., 2008; Spaeth et al., 2009). These cells are different to normal fibroblasts/myofibroblasts in that they are able to stimulate tumor progression (Olumi et al., 1999) and show higher proliferative and migratory activity (Schor

**4.4 MSCs and carcinoma-associated fibroblasts** 

MSCs display an astounding plasticity and have shown to differentiate to cells as different as neurons and epithelial cells. The main function of MSCs is likely to promote tissue regeneration after injuries and, since tumors are probably wounds that never heal, also to support repair of tumoral lesions. However, tumors may misguide MSCs and "misuse" them for their "own benefit". Primary tumors may particularly profit from MSCs when they differentiate to tumor-promoting CAFs. MSCs may further facilitate breast cancer to metastasize by helping breast cancer cells to enter the bone marrow as well as by increasing the pool of metastasizing breast cancer stem cells. Most of the interactions between MSCs and tumor cells are mediated by cytokines as secreted by both cell types. Paracrine feedback mechanisms may further increase cytokine concentrations at places where these cells communicate with each other and may attract other cell types, such as macrophages, that are known to support tumor progression. To interfere with the interaction between MSCs and breast cancer cells treatments may be considered involving the inhibition of the activities of key cytokines, such as IL-6 (Liu & Wicha, 2010), which are important for both attraction of MSCs to breast cancer and expansion of the breast cancer stem cell pool by MSCs. On the other hand, there is also evidence that MSCs may have suppressive effects on breast cancer. Different sources from which MSCs were isolated may partially account for these contradictory results. Further studies are necessary to clarify this controversy, before conclusions can be drawn in terms of treatment of breast cancer patients. Certainly, when

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engineered to produce anti-tumor factors, MSCs possess anti-tumoral effects and may be used as trojan horses that enter and eradicate tumor cells. Drug-carrying MSCs may have a great advantage over "naked" drugs since they may deliver drugs more selectively and more efficiently at places where they are meant to act.
