[XBP-1 enhances the transcriptional activity of estrogen receptor alpha].

The estrogen receptor (ERalpha) is a member of a large superfamily of nuclear receptors that regulates the transcription of estrogen-responsive genes. Several recent studies have demonstrated that human X-box binding protein 1 (XBP-1) mRNA expression is associated with ERalpha status in breast tumors. More recently, two forms of XBP-1 were identified due to their unique splicing. The two splicing variants of XBP-1 were designated XBP-1S and XBP-1U, respectively. In this study, the coding sequences of XBP-1S and XBP-1U were cloned respectively into the expression vector pcDNA3 harboring FLAG epitope, generating the recombinant plasmids pcDNA3-FLAG-XBP-1S and pcDNA3-FLAG-XBP-1U. Western blot analysis showed that both XBP-1S and XBP-1U were expressed in mammalian cells. To determine the effects of XBP-1S and XBP-1U on the transcriptional activity of ERalpha, MDA-MB-231 breast cancer cells were cotransfected with the expression vectors for ERalpha and either pcDNA3-FLAG-XBP-1S or pcDNA3-FLAG-XBP-1U. The results indicated that XBP-1S and XBP-1U enhanced ERalpha-mediated transcriptional activities in a hormone-independent manner. GST pull-down assay showed that both XBP-1S and XBP-1U interacted with ERalpha. These data suggest that XBP-1S and XBP-1U may play an important role in breast cancer growth and progression through ERalpha signaling.