Functional characterization of the ER stress induced X-box-binding protein-1 (Xbp-1) in the porcine system

We found that the pXbp1 mRNA was the subject of the IRE1α-mediated unconventional splicing by ER stress. Knock-down of pXbp1 enhanced ER stress-mediated cell death in PEF cells. In adult organs, pXbp1 mRNA and protein were expressed and the spliced forms were detected.It was first found that the UPR mechanisms and the function of pXbp1 in the porcine system. These results indicate that pXbp1 plays an important role during the ER stress response like other animal systems and open a new opportunity for examining the UPR pathway in the porcine model system.Endoplasmic reticulum (ER) is an essential cellular compartment for protein synthesis and maturation [1]. The perturbation of ER functions such as disruption of Ca++ homeostasis, inhibition of protein glycosylation or disulfide bond formation, hypoxia, and bacteria infections results in the accumulation of unfolded or mis-folded proteins in ER lumen. To reduce the excessive mis-folded protein loading, cells trigger unfolded protein response (UPR) including the transient attenuation of protein translation, the degradation of mis-folded proteins, and the induction of molecular chaperones and protein folding enzymes [1]. In severe ER stress, UPR results in cell death through the activation of apoptotic pathways [2].Three distinct UPR signaling pathways exist in mammalian cells that include ER transmembrane inositol-requiring enzyme 1 (IRE1), PKR-like ER kinase (PERK), and activating transcription factor 6 (ATF6) pwthways [3]. The evolutionary conserved IRE1 in the UPR pathway plays as a Ser/Thr protein kinase and endoribonuclease [4,5]. Upon the activation of IRE1α by ER stress, the endonuclease domain of IRE1 splices the XBP1 mRNA and removes 26 base pairs from the full-length XBP1 mRNA by the unconventional splicing. This event results in the conversion of the premature unspliced XBP1 protein (XBP1u, 267 amino acids) to the spliced XBP1 protein (XBP1s, 371 amino acids) by the frame shift. XBP1s induces a subset of UPR