Astrocyte-derived cytokines contribute to the metastatic brain specificity of breast cancer cells

The occurrence of breast cancer metastases is preferential to certain organs. Astrocytes may play an important role in the development of brain metastases, as these cells have been shown to respond to extracellular stimuli by producing many cytokines and growth factors that can modulate tumor cell proliferation, growth, and/or metastases. To test this hypothesis, we analyzed the responses of the human breast cancer cell line MDA-MB-435 and its metastatic sublines to astrocyte primary cultures from newborn rat cerebra. Astrocyte purity of the glial cell cultures was demonstrated by glial fibrillary acidic protein and rat neural antigen-2 (Ran-2) immunopositive staining. The 435-Br1 cell line, which was derived from a brain metastases in a nude mouse, showed increased adhesion to astrocytes and enhanced growth in vitro in the presence of media from Con A-stimulated astrocytes, relative to the parental MDA-MB-435 and the lung metastasis-derived variant 435-Lung2. Furthermore, the growth-stimulatory effect was partially reversed by anti-lL-6, anti-transforming growth factor β (anti-TGFβ), and anti-IGF-i antibodies, indicating that these metastatic cells use exogenous cytokines as paracrine growth factors. In an attempt to elucidate the role of several biologic-response modifiers produced by astrocytes, we tested the responses of MDA-MB-435 cells to purified cytokines and growth factors. We found that the addition of recombinant human or mouse IL-6 produced a variety of responses in the different 435 metastatic variants. Furthermore, IL-6 receptor (IL-6R) expression was slightly increased in the 435-Br1 cells, and exogenous IL-6 rescued 435-Br1 cells from apoptosis in serum-depleted cultures. No apoptotic protective effect was observed in either MDA-MB-435 parental cells or 435-Lung2 cells. Thus, responses to exogenous IL-6 might determine the differences among these metastatic variants by extending cell survival of selected subpopulations, giving them the opportunity to respond to growth factors or other favorable conditions that might be present. These results suggest that cytokines produced by glial cells in vivo may contribute, in a paracrine manner, to the development of brain metastases by breast cancer cells.