Exploration of the co-regulation of the potential positive feedback partners estrogen receptor alpha and X-Box binding protein 1

X-Box Binding protein 1 (Xbp1) is a basic leucine zipper-containing transcription factor that plays a key role in the unfolded protein response (UPR), a cell program that is activated by misfolded proteins in the endoplasmic reticulum. After initiation of the UPR, Xbp1 mRNA is unconventionally spliced to generate a translational frameshift resulting in a larger, more transcriptionally active protein (Xbp1S) that enables cells to survive stressful environments. Xbp1 mRNA is up-regulated by estrogen in estrogen receptor a (ERalpha)-positive breast cancer cells. In exploring the relationship between ERalpha and Xbp1 we showed: (1) Anti-estrogen suppression of Xbp1 mRNA expression and splicing increased UPR-dependent apoptosis in ERalpha-positive cancer cells, as did siRNA knockdown of Xbp1 in the absence of anti-estrogens. Thus, breast cancer cells utilize high levels of spliced Xbp1 mRNA to resist the pro-apoptotic UPR-activating conditions, but this resistance is reduced by anti-estrogens in sensitive cells or by Xbp1 mRNA knockdown. (2) Xbp1 mRNA knockdown by specific siRNAs prevented estrogen-responsive growth of MCF7 cells and ERalpha-regulated gene expression, suggesting that Xbp1 is a critical cofactor for ERalpha-regulated transcription leading to growth, and Xbp1 over-expression could represent an early transformation step in breast cancer. (3) Continuous Xbp1S protein over-expression resulted in down-regulation of ERalpha expression, suppression of ERalpha-regulated gene expression, and inhibition of cell cycle progression. We propose that up-regulation of Xbp1 mRNA enables ERalpha-positive cancer cells to utilize estrogen as a growth factor and avoid apoptotic consequences of UPR-induction by production of Xbp1S protein through splicing of highly expressed (via the ERalpha) Xbp1 mRNA. Therefore, anti-estrogen treatment, or siRNA knockdown of Xbp1 prevents these cells from properly responding to the UPR, resulting in apoptosis. However, continuous activation of the UPR, and thus Xbp1S expression, due to conditions common to breast cancer tumors, such as hypoxia or glucose starvation, may enable tumor growth via alternative pathways independent of the ERalpha pathway, leading to the commonly seen tumor resistance to anti-estrogenic drugs. These results demonstrate a more complex interaction of Xbp1 and ERalpha than has been shown in previous studies and may represent critical transforming steps leading to common breast cancer.