The role of CCAAT/enhancer binding protein beta2 in mammary epithelial cells

Abstract

C/EBPbeta is essential for mammary gland growth and development and has been associated with poor prognosis in breast cancer. Overexpression of C/EBPbeta2 in MCF10A cells, a model of normal mammary epithelial cells, results in a variety of cancer phenotypes including EMT and ErbB independence. IL1B is dramatically upregulated in MCF10A-C/EBPbeta2 cells but there is little, if any, processing to the mature 17 kD form. Although proIL1B has previously been considered to be biologically inactive, we demonstrate proIL1B is not only localized to the nucleus, but is also tightly associated with the chromatin. We show that proIL1B is bound at specific locations in the genome and is positioned in such a way to play a role in the cancer phenotypes observed in MCF10A-C/EBPbeta2 cells. Given that MCF10A cells overexpressing C/EBPbeta-2 are no longer dependent on ErbB signaling for survival, we sought to investigate whether aberrant C/EBPbeta-2 expression could contribute to the resistance of some breast cancers to ErbB targeted therapies, such as trastuzumab. C/EBPbeta2 was retrovirally introduced into two ErbB2 overexpressing cell lines, BT474 and HCC1954, which are sensitive to trastuzumab. These cells were then assayed for trastuzumab sensitivity and it was found that C/EBPbeta2 confers resistance to trastuzumab in both cell lines albeit by different mechanisms. In BT474 cells overexpression of C/EBPbeta2 was able to mediate resistance by the loss of ErbB2 and stem cell-like gene expression. C/EBPbeta2 overexpression in HCC1954 cells resulted in upregulation of proIL1B. The proIL1B in HCC1954-C/EBPbeta-2 cells was localized to the nucleus. Moreover, nuclear IL1B is detected in some human breast tumor samples. This study demonstrates the presence of nuclear proIL1B in transformed mammary epithelial cells providing the first evidence that IL1B may be a dual function cytokine with distinct secreted and nuclear functions.