A new transcriptional pathway of RelB NF-kappa B activation that promotes breast cancer invasiveness is repressed by estrogen receptor alpha

Cytomegalovirus (CMV) is a beta-herpesvirus that latently infects a majority of adults worldwide. The CMV immediate-early 1 (IE1) protein was recently shown to induce transcription of the gene encoding the RelB Nuclear Factor-kappaB (NF-kappaB) subunit. Previous studies have shown that relB promoter activity is regulated, at least in part, through the binding of NF-kappaB dimers, p50/p65 to two sites within the promoter. Sequences mediating IE-1 induction of the relB promoter were mapped to -1694 to -1096 bp. Within it a functional activator protein 1 (AP-1) element was identified, which was able to bind c-Jun, Fra-2, and c-Fos AP-1 family members in IE1 transfected NIH 3T3 cells. Mutation of this site eliminated relB activation by IE1 in both NIH 3T3 and vascular smooth muscle cells (SMCs). Further, inhibition of c-Jun N-terminal kinase (JNK) activity, known to regulate AP-1 levels and activity, blocked IE1-mediated induction of relB promoter activity. Importantly, ectopic expression of c-Jun and Fra-2 induced relB promoter activity in synergy with p50/p65 NF-kappaB complexes. Thus, induction of relB gene transcription is mediated via activation of c-Jun/Fra-2 AP-1 transcription factors by a novel de novo pathway for RelB synthesis.; Since we recently detected RelB subunit expression in mouse mammary epithelial-derived cancer cells induced by either ectopic c-Rel NF-kappaB expression or carcinogen exposure, we tested the hypothesis that the de novo RelB pathway occurs in breast cancer. Here, we demonstrate constitutive de novo RelB synthesis is selectively active in estrogen receptor (ER)alpha negative breast cancer cells and promotes a more invasive phenotype. ERalpha signaling reduced levels of functional NF-kappaB and AP-1 and inhibited de novo RelB synthesis, leading to an inverse correlation between relB and ERalpha gene expression in human breast cancer tissues and cell lines. RelB promoted the more invasive phenotype of ERalpha negative cancer cells and survival of breast cancer cells from death mediated by gamma-radiation. Our work identified the anti-apoptotic protein Bcl-2 as an important downstream mediator of RelB function in promoting a mesenchymal phenotype. Thus, inhibition of de novo RelB synthesis represents a novel mechanism whereby ERalpha controls epithelial to mesenchymal transition (EMT) of breast cancer.