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 br ain metastases, as these cells have been shown to respond to extracell ular stimuli by producing many cytokines and growth factors that can m odulate tumor cell proliferation, growth, and/or metastases. To test t his hypothesis, we analyzed the responses of the human breast cancer c ell line MDA-MB-435 and its metastatic sublines to astrocyte primary c ultures from newborn rat cerebra. Astrocyte purity of the glial cell c ultures was demonstrated by glial fibrillary acidic protein and rat ne ural antigen-2 (Ran-2) immunopositive staining. The 435-Br1 cell line, which was derived from a brain metastases in a nude mouse, showed inc reased adhesion to astrocytes and enhanced growth in vitro in the pres ence of media from Con A-stimulated astrocytes, relative to the parent al MDA-MB-435 and the lung metastasis-derived variant 435-Lung2. Furth ermore, the growth-stimulatory effect was partially reversed by anti-I L-6, anti-transforming growth factor beta (anti-TGF beta), and anti-IG F-I antibodies, indicating that these metastatic cells use exogenous c ytokines as paracrine growth factors. In an attempt to elucidate the r ole of several biologic-response modifiers produced by astrocytes, we tested the responses of MDA-MB-435 cells to purified cytokines and gro wth factors. We found that the addition of recombinant human or mouse IL-6 produced a variety of responses in the different 435 metastatic v ariants. Furthermore, IL-6 receptor (IL-6R) expression was slightly in creased in the 435-Br1 cells, and exogenous IL-6 rescued 435-Br1 cells from apoptosis in serum-depleted cultures. No apoptotic protective ef fect was observed in either MDA-MB-435 parental cells or 435-Lung2 cel ls. 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 fact ors 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 bre ast cancer cells.