XBP-1 Increases ERα Transcriptional Activity Through Regulation Of Large-scale Chromatin Unfolding

X-box binding protein 1 (XBP-1) plays a critical role in unfolded protein responsesignaling pathway (UPR) and regulate a wide range of cellular responses, includingproliferation, differentiation, migration and apoptosis. It has two splicing variants: XBP-1Sand XBP-1U, both have transcriptional activity. However, how XBP-1 regulatesintracellular events remains largely unknown. XBP-1 has transcriptional activity suggest that it may play its role through regulatechromatin structure. Here, we report that wide type XBP-1S and XBP-1U protein recruitand induce large-scale chromatin unfolding by targeting XBP-1 to an amplified lacoperator-containing chromosome region in mammalian cells. The transactive domain ofXBP-1S and XBP-1U also can induce large scale chromatin unfolding, but mutants ofXBP-1 lack of transactive domain can not induce chromain unfolding. This unfoldingactivity maps to the subdomains within the transactivition domain of XBP-1. So we inferthis domain interact with some other proteins to induce the transcriptional activity ofXBP-1. Using yeast two-hybrid screen, XBP-1 was isolated from a human mammary library,with breast cancer relating gene COBRA1 as bait. This study suggests that XBP-1 mayinteract with COBRA1. GST pull-down assay showed that wide XBP-1S and XBP-1Uboth bound to COBRA1,The binding domain of COBRA1 with XBP-1S maps to its135-335 amino acids, and with XBP-1U maps to its 135-335 and 285-480 amino acids. Lacoperator-containing luciferase reporter assay was used to determine the effects ofCOBRA1 on the transcription activity of XBP-1S and XBP-1U, the result suggest thatCOBRA1 can increase the transcription activity of XBP-1S and XBP-1U, but not tomutants without DNA binding domain. Co-immunoprecipitation assay also show thatXBP-1S and XBP-1U both bound to COBRA1 in vivo. The binding of XBP-1U toCOBRA1 was stronger than that of XBP-1S to COBRA1. Large-scale chromatin unfoldingassay show that XBP-1S and XBP-1U induce chromatin unfolding map with the regionhaving COBRA1 expression, but not with mutants. These results showed that COBRA1may participate XBP-1-regulating pathway. Our laboratory has testified that XBP-1 can interact with estrogen receptor α (ERα),and increase ERα transcription activity in ligand-independent manner, but the mechanismremains unknown. On the basis of report that ERα can induce large scale chromatinunfolding without its ligand—estrogen. We construct lac-operator and ERα fusion plasmidand transfect it with XBP-1, and find that wide type XBP-1 can enhance ERα induce largescale chromatin unfolding activity. When XBP-1 lacks its DNA binding domain, thisactivity disappeared. Based on the principle that ERα can bind with estrogen responseelement (ERE), we detect how XBP-1 influences the transcription activity of ERα, and wefound wide type XBP-1 can enhance ERα transcriptional activity. These data show a newfunction of XBP-1 that it regulates chromatin unfolding may be the essential hypothesis toincrease the transcriptional activity of ERα, it provides a new research method to healbreast cancer.