Abstract P5-04-14: Loss of interferon regulatory factor 5 expression in breast cancer alters the tumor microenvironment to promote metastasis

Staining of over 300 human breast cancer tissue samples demonstrates that over 75% of invasive ductal carcinomas, regardless of subtype, have lost expression of interferon regulatory factor 5 (IRF5). Loss of IRF5 correlates with higher tumor grades, indicating that it may be an important biomarker for disease progression and metastasis. IRF5 is an important transcriptional regulator in immune cells, yet its role in the immune response and other cellular processes in ductal epithelial cells has never been examined. Levels of IRF5 are lowest in human breast cancer cell lines that are most invasive. Re-introduction of IRF5 expression in a xenograft mouse model of breast cancer decreased the number and size of primary tumors as well as completely abrogated tumor spread. Since breast cancer survival is dependent on hindering tumor spread, the focus of this study is to elucidate the mechanisms by which IRF5 inhibits metastasis. Through cell based assays we found that overexpression of IRF5 in MDA-MB-231 cells inhibits cell movement by 50%. We also characterized the protein domain necessary for this function, and found that it is not dependent on the DNA binding domain. This is the first evidence of a non-transcriptional role for IRF5 and may explain its cytoplasmic localization in normal ductal epithelial cells. A more obvious aspect of metastatic inhibition by IRF5 comes from its role as an immune regulator. IRF5 positively regulates the expression of many proteins that are considered potent activators of an inflammatory immune response. To gain insight into the cytokines and receptors that IRF5 regulates, we isolated MDA-MB-231 cells that stably express IRF5 from 3D acini and performed a focused microarray analysis of gene expression. We found that cells expressing IRF5 had significantly elevated levels of immune stimulating genes such as CXCL5, CXCL13, IL-33, and IFNg among many others. In transwell assays using IRF5 positive or negative tumor conditioned media, we found that IRF5 positive media caused increased trafficking of human lymphocytes. These data indicate that signaling from IRF5 positive and negative tumor cells is distinct and that IRF5 may play a role in shaping the tumor-immune microenvironment. We postulate that a loss of IRF5 in breast tumor hinders a favorable immune response and allows cell movement beyond the basement membrane. View this table: Genes upregulated by IRF5 in MDA-MB-231 The ability of IRF5 to inhibit cell migration within the tumor cell itself and alter the surrounding microenvironment so drastically offers some explanation as to why tumor progression depends on its downregulation. With the advent of immunotherapy, the IRF5 status of a breast tumor may not only be important in determining metastatic potential, but also in determining the most appropriate immunotherapy since we expect that IRF5 positive and negative tumors will require altered treatment regimens. Further characterizing how IRF5 shapes the tumor microenvironment is essential for the development of clinically relevant immunotherapy strategies. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-04-14.