4673

Inflammatory breast cancer (IBC), the most aggressive form of breast cancer, is characterized by rapid progression, clumping of tumor cells within lymphatics and distant metastasis. The role of monocytes/macrophages in progression of IBC is not understood. Our working hypothesis is that in vivo circulating monocytes potentiate the rapid progression of IBC. Here, we have analyzed interactions between SUM149 IBC cells and U937 monocytic cells in vitro, using U937 cells that were naive (i.e., untreated) or tumor-educated (i.e., cultured in media conditioned by SUM149 cells). SUM149 cells alone or in coculture were grown on the reconstituted basement membrane Cultrex. SUM149 cells alone formed spheroids, whereas in coculture with either naive or tumor-educated U937 cells the SUM-149 cells formed branched structures. In a live cell proteolysis assay, we observed more degradation of DQ-collagen IV by SUM149 cells that were cocultured with naive U937 cells than with tumor-educated U937 cells. Moreover, media conditioned by naive U937 cells enhanced invasion of SUM149 cells through Matrigel-coated filters more than did media conditioned by tumor-educated U937 cells. We also investigated the effects of IBC:monocyte interactions on the expression of two proteins implicated in progression of IBC: E-cadherin and caveolin-1, the main structural protein of caveolae. Both E-cadherin and caveolin-1 were expressed in SUM149 cells. Expression of E-cadherin was decreased in SUM149 cells cocultured with naive U937 cells or grown in media conditioned by naive U937 cells, whereas expression of caveolin-1 was increased in SUM149 cells grown under these conditions. In contrast, expression of E-cadherin or caveolin-1 was not altered in SUM149 cells cocultured with tumor-educated U937 cells or their conditioned media. E-cadherin can be degraded by cathepsin B, a cysteine protease upregulated in macrophages within lung metastases in the MMTV-PyMT mammary cancer model and localized to caveolae on the surface of tumor cells. We observed that cathepsin B secretion was increased when SUM149 cells were cultured in media conditioned by naive U937 cells, yet not when SUM149 cells were cultured in media conditioned by tumor-educated U937 cells. Our results reveal that interactions between SUM149 IBC cells and U937 monocytic cells increase invasion and proteolysis by the IBC cells. We speculate that this is the result of increased secretion of cathepsin B, increased association of cathepsin B with caveolae on the surface of IBC cells and the resulting degradation of E-cadherin by caveolae-associated cathepsin B.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA