Abstract A79: Regulation of the breast tumor microenvironment by interferon regulatory factor 5 (IRF5).
Authors: Pimenta Erica M. , Barnes Betsy J. 
Published: August 2014

Interferon Regulatory Factor 5 (IRF5) is a member of the IRF family of proteins which predominately serve as immune regulators in response to extracellular pathogens. IRF5 has also been characterized as a tumor suppressor gene where loss of its expression has been observed in a variety of tumor types including blood cancers, breast and colon cancer. In cell types that constitutively express IRF5, such as immune and epithelial cells, it generally resides in the cytoplasm. Once activated by a stimulus that induces post-translational modification(s) of IRF5, it undergoes dimerization and translocation to the nucleus. In the nucleus, IRF5 acts as a transcription factor binding to specific DNA sequences in the promoter regions of target genes known as interferon-sensitive response elements (ISRE) or interferon regulatory elements (IRE). Upon binding to these elements, IRF5 can positively regulate transcription of its target genes, such as pro-inflammatory cytokines interleukin (IL)-6, tumor necrosis factor (TNF)-α, IL-12, and interferon (IFN)-α, or negatively regulate targets such as IL-10. Most of our current understanding of IRF5 expression and function comes from examining its role in the immune system.

Recent data from our lab has shown that IRF5 is a new tumor suppressor gene in breast cancer that controls metastasis in part by regulating expression of the chemokine receptor CXCR4. We detected a graded decrease in IRF5 expression that correlated with advanced disease stage in patients with ductal carcinoma in situ (DCIS), and almost complete loss of IRF5 expression in nearly all of the breast tissues examined from patients diagnosed with invasive ductal carcinoma (IDC). IRF5 expression was easily detectable in mammary epithelial cells from healthy donors and patients with atypical ductal hyperplasia (ADH). Interestingly, in these same tumor specimens from IDC patients, we noticed that a large percentage of infiltrating immune cells expressed high levels of IRF5. Together, these findings suggest at least two distinct functions for IRF5 in the breast tumor microenvironment: 1) in the tumor cell itself and 2) in the infiltrating immune cells of IDC patients. We hypothesize that it is the initial loss of IRF5 expression in mammary epithelial cells of patients with DCIS that reprograms the tumor microenvironment to allow for tumor progression and metastasis.

To begin to understand why loss of IRF5 in breast tumor cells allows for disease progression, we worked primarily in an in vitro 3D culture model of a highly invasive human breast cancer cell line, MDA-MB-231, which does not express detectable levels of IRF5. A population of MDA-MB-231 cells were retrovirally infected with either an empty vector or IRF5 pBabe plasmid (EV-231 or IRF5-231) and were used to assess cell proliferation, migration, invasion, chemotaxis, and gene expression profiles. In particular, a focused tumor cytokine array was performed to measure differences in chemokine and chemokine receptor expression between EV-231 and IRF5-231 cells grown in 3D culture. Co-culture of these cells with immune cell types allowed for characterization of an immune response.

This work demonstrates that loss of IRF5 in breast cancer cell lines allows for increased tumor cell migration/invasion and significantly alters the expression of several genes important in the regulation of immune cell trafficking and activation. The data supports an otherwise unknown role for IRF5 as an immune regulator in epithelial cells. The ability of IRF5 to slow mammary cell growth and metastasis, as well as shape the tumor-immune microenvironment, points to the importance of studying this new gene regulator in mammary epithelial cells.

Citation Format: Erica M. Pimenta, Betsy J. Barnes. Regulation of the breast tumor microenvironment by interferon regulatory factor 5 (IRF5). [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology: Multidisciplinary Science Driving Basic and Clinical Advances; Dec 2-5, 2012; Miami, FL. Philadelphia (PA): AACR; Cancer Res 2013;73(1 Suppl):Abstract nr A79.