Stem cells are regulated by interplay between extrinsic factors and cell intrinsic regulatory pathways. During normal development and tissue homeostasis, these extrinsic factors are provided by cellular and extracellular elements which define the stem cell “niche”. There is increasing evidence that many tumors including breast cancer may be driven by a cellular subcomponent that displays stem cell properties. Although it is clear that the tumor microenvironment influences tumor growth and metastasis (28
), it is unclear whether these effects are mediated by CSCs. In this study we utilized in vitro
systems and mouse models to demonstrate an important role for bone marrow-derived MSCs in regulating breast CSCs. The use of mesenchymal cells and cancer cells both of human origin facilitated the study of cytokine interaction obviating known species differences in these factors. In fact, the significant facilitation of breast tumor growth by human MCs introduced into the mouse tibia may reflect these species differences. We demonstrate that the interaction between MSCs and CSCs is mediated by a positive feedback cytokine loop in which IL6 and CXCL7 play pivotal roles. This loop requires the simultaneous presence of both cell types but does not require cell-cell contact as demonstrated by transwell and conditioned medium experiments. Furthermore, we demonstrate that MCs, like cancer cells, are organized in a hierarchy in which primitive ALDH-expressing mesenchymal cells capable of self-renewal and multi-lineage differentiation interact with cancer cells to regulate CSC self-renewal. IL6 produced by cancer cells interacts with IL6 receptor and GP130 on ALDEFLUOR-positive MCs. IL6 mediated chemotaxis may facilitate the homing of MSCs to the sites of primary tumor growth, as well as inducing CXCL7 production by these cells. MSC-derived CXCL7 in turn, interacts with cancer cells through the CXCR2 receptor (29
), where it induces the synthesis of a number of cytokines including IL6 and IL8. These pleiotropic effects of CXCL7 are consistent with previous reports (29
). The expression of CXCL7 and its receptor CXCR2 have been shown to be increased in breast carcinomas (30
). Furthermore, CXCL7 transfection increased the invasive capacity of breast cancer cells (30
) consisted with the previously demonstrated increased invasive and metastatic properties of cancer stem cells (31
). We have previously shown that IL8 interacts with the CXCR1 receptor on cancer stem cells triggering their self-renewal and invasive properties (14
). IL6 also has been reported to be capable of regulating breast stem cells (24
) and colon cancer stem cells (25
). In addition to regulating CSCs, IL6 produced by cancer cells interacts with MSCs, further increasing their CXCL7 production generating a positive feedback loop (). We confirmed the functional importance of these interactions by demonstrating that MSCs accelerate breast tumor growth in NOD/SCID mice. Furthermore as was the case in vitro
, in this mouse model, this effect was mediated by ALDEFLUOR-positive MCs which were capable of increasing the CSC population in vivo
. The close apposition of ALDH1+ tumor cells and MSCs was also demonstrated in frozen sections of primary human breast cancers.
It is clear that the tumor microenvironment plays an important role in tumor growth and metastasis (8
). Previous elegant studies have suggested a role for mesenchymal stem cells in tumor metastasis, which is mediated by CCL5 (8
). In addition, researchers reported that IL1a produced by MSC mediates similar effects (33
) and that MSCs promote LTC-IC expansion (34
). Our studies extend these previous findings by demonstrating that MSCs regulate cancer cell behavior through their effect on cancer stem cells. On the other hand, MSCs have been shown to actually inhibit tumor growth in some models (35
). This suggests that the effects of MSCs on tumor growth are complex and may be context dependent.
The homing of bone marrow-derived mesenchymal stem cells to sites of tumor growth may rely on similar mechanisms to the homing of these cells to sites of tissue injury. Developing tumors may recruit MSCs from the bone marrow where they interact with and regenerate CSCs. If this is the case, then the development of strategies aimed at interfering with these pathways may provide a means of targeting cancer stem cells. Since these cells may mediate tumor growth and metastasis as well as contribute to treatment resistance, these strategies may lead to improved clinical outcomes for patients with advanced breast cancers.