NF-κB and Estrogen Receptor α Interactions: Differential Function in Estrogen Receptor-Negative and -Positive Hormone-independent Breast Cancer Cells

Estrogen receptor (ER)-positive breast cancer cells have low levels of constitutive NF-kappa B activity while ER negative (-) cells and hormone-independent cells have relatively high constitutive levels of NF-kappa B activity. In this study, we have examined the aspects of mutual repression between the ER alpha and NF-kappa B proteins in ER+ and ER- hormone-in dependent cells. Ectopic expression of the ERa reduced cell numbers in ER+ and ER- breast cancer cell lines while NF-kappa B-binding activity and the expression Of Several NF-kappa B-regulated proteins were reduced in ER- cells. ER overexpression in ER+/E2-independent LCC1 cells only weakly inhibited the predominant p50 NF-kappa B. GST-ER alpha fusion protein pull downs and in vivo co-immunoprecipitations of NF-kappa B:ER alpha complexes showed that the ER alpha interacts with p50 and p65 in vitro and in vivo. Inhibition of NF-kappa B increased the expression of diverse E2-regulated proteins. p50 differentially associated directly with the ER:ERE complex in LCC1 and MCF-7 cells by supershift analysis while p65 antibody reduced ER alpha:ERE complexes in the absence of a supershift. ChIP analysis demonstrated that NF-kappa B proteins are present on an endogenous ERE. Together these results demonstrate that the ER and NF-kappa B undergo mutual repression, which may explain, in part, why expression of the ER alpha in ER-cells does not confer growth signaling. Secondly, the acquisition of E2-independence in ER+ cells is associated with predominantly p50:p50 NF-kappa B, which may reflect alterations in the ER in these cells. Since the p50 homodimer is less sensitive to the presence of the ER, this may allow for the activation of both pathways in the same cell. J. Cell. Biochem. 107: 448-459, 2009. (C) 2009 Wiley-Liss, Inc.