Prolonged endoplasmic reticulum stress promotes mislocalization of immunoglobulins to the cytoplasm

Signal peptide-dependent insertion of newly synthesized proteins into the endoplasmic reticulum (ER) is a multi-step process, whose fidelity varies with the identity of the protein and the cell type. ER translocation of prions is sensitive to conditions of acute ER stress in a manner that pre-emptively prevents their aggregation and proteo-toxicity. While this has been documented for extreme ER stress conditions and for a special type of proteins, the impact of chronic ER stress on protein translocation in general has not been well characterized. The unfolded protein response (UPR) is a cytoprotective signaling pathway activated by ER stress. The transcription factor X-box-binding protein 1 (XBP-1) is a key element of the mammalian UPR, which is activated in response to ER stress. Deletion of XBP-1 generates constitutive chronic ER stress conditions. Chronic ER stress can also be produced pharmacologically, for example by prolonged treatment with proteasome inhibitors, which abrogates XBP-1 activation. We tested the impact of chronic ER stress on protein insertion into the ER with special emphasis on antibody secreting cells (ASCs), as these cells cope physiologically with prolonged stress conditions. We show that XBP-1 in plasmablasts and fibroblasts controls the ER translocation of US2, a viral-encoded protein with a priori poor insertion efficiency. Using monoclonal antibodies that preferentially recognize ER-mis-inserted micro Ig chains we demonstrate that prolonged treatment of plasmablasts with proteasome inhibitors, as well as deletion of XBP-1, impaired the translocation of mu chains to the ER. Our data suggest that ASCs under prolonged ER stress conditions endure cytoplasmic mislocalization of Ig proteins. This mislocalization may further explain the exquisite sensitivity of multiple myeloma to proteasome inhibitors.