XBP1 is critical to protect cells from endoplasmic reticulum stress: evidence from Site-2 protease-deficient Chinese hamster ovary cells.

XBP1 is a transcription factor downstream of IRE1, a transmembrane protein in the endoplasmic reticulum (ER) which functions as a sensor and transducer of ER stress. XBP1 mRNA is constitutively expressed at a low level as an intron-containing precursor mRNA (unspliced mRNA), which is subject to IRE1-mediated splicing reaction upon ER stress to produce the active form of XBP1, pXBP1(S). Because the XBP1 promoter carries a perfect ER stress-response element, namely, the cis-acting element responsible for the induction of ER chaperones, and XBP1 mRNA is induced in response to ER stress with a time course similar to that of ER chaperone mRNAs, it is conjectured that transcription factor ATF6, activated immediately upon ER stress, induces the transcription of not only ER chaperone genes but also of XBP1 gene, such that pXBP1(S) produced by the splicing of an increased level of XBP1 mRNA escapes from proteasome-mediated degradation. Here, we examined this notion by determining the induction of XBP1 mRNA and pXBP1(S) in mutant Chinese hamster ovary (M19) cells deficient in Site-2 protease, which executes the last step of ER stress-induced activation of ATF6. We found that the induction of XBP1 mRNA and pXBP1(S) was greatly reduced in M19 cells as compared with wild-type cells, leading to a marked reduction in the extent of induction of XBP1-target gene. M19 cells were much more sensitive to ER stress than wild-type cells. Importantly, overexpression of XBP1 unspliced mRNA in M19 cells reversed all of these phenotypes. We concluded that ATF6-mediated induction of XBP1 mRNA is important to the production of pXBP1(S), activation of XBP1-target genes, and protection of cells from ER stress.