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Involvement of macrophages in breast tumor growth and development is complex. The role of proteases in this process has not yet been clearly elucidated. We have previously established that media conditioned by MDA-MB-231 human breast carcinoma cells stimulate the proteolytic activity of U937 human monocytic cells. The aims of our present study were to: 1) determine whether active monocytes/macrophages affect growth and invasiveness of breast carcinoma cells; 2) identify potential regulatory molecules secreted by both the breast carcinoma cells and active monocytes/macrophages; and 3) characterize the molecular pathways involved in growth and invasion. MDA-MB-231 cells were cultured on Matrigel-coated coverslips alone or in cocultures with unstimulated or stimulated U937 cells. U937 cells were stimulated (activated) by incubation with MDA-MB-231 conditioned media. The MDA-MB-231 cells formed spheroids on the Matrigel and the size of the spheroids was larger when cocultured with activated U937 cells. Furthermore, invasion of MDA-MB-231 cells through Matrigel was enhanced by media conditioned by activated U937 cells. We analyzed the media conditioned by the monocytes/macrophages and breast carcinoma cells for growth factors and cytokines using a cytokine antibody array technique. A prominent cytokine secreted by both the active monocytes/macrophages and the breast carcinoma cells was IL-6, a cytokine previously shown to activate the cysteine protease cathepsin B in myotubes. Therefore, we tested the effect of IL-6 on invasiveness of the MDA-MB-231 cells. IL-6 at a concentration of 1 ng/ml enhanced the invasiveness of the breast carcinoma cells comparably to the media conditioned by active monocytes/macrophages. We have previously shown that media conditioned by MDA-MB-231 cells induces expression, secretion and activity of cathepsin B in U937 cells and secretion of MMP-2 and -9 from U937 cells. We are presently determining the effect of media conditioned by unstimulated or stimulated U937 cells on the proteolytic profile of the MDA-MB-231 cells. IL-6 is highly expressed by MDA-MB-231 cells and monocytes/macrophages activated by MDA-MB-231 conditioned media and therefore our working hypothesis is that IL-6 may exert autocrine and paracrine actions on tumor and stromal cells in the tumor microenviroment via proteases.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]