Identification of ERdj3 and OBF-1/BOB-1/OCA-B as direct targets of XBP-1 during plasma cell differentiation.

Plasma cell differentiation is accompanied by a modified unfolded protein response (UPR), which involves activation of the Ire1 and activating transcription factor 6 branches, but not the PKR-like endoplasmic reticulum kinase branch. Ire1-mediated splicing of XBP-1 (XBP-1(S)) is required for terminal differentiation, although the direct targets of XBP-1(S) in this process have not been identified. We demonstrate that XBP-1(S) binds to the promoter of ERdj3 in plasmacytoma cells and in LPS-stimulated primary splenic B cells, which corresponds to increased expression of ERdj3 transcripts in both cases. When small hairpin RNA was used to decrease XBP-1 expression in plasmacytoma lines, ERdj3 transcripts were concomitantly reduced. The accumulation of Ig gamma H chain protein was also diminished, but unexpectedly this occurred at the transcriptional level as opposed to effects on H chain stability. The decrease in H chain transcripts correlated with a reduction in mRNA encoding the H chain transcription factor, OBF-1/BOB-1/OCA-B. Chromatin immunoprecipitation experiments revealed that XBP-1(S) binds to the OBF-1/BOB-1/OCA-B promoter in the plasmacytoma line and in primary B cells not only during plasma cell differentiation, but also in response to classical UPR activation. Gel shift assays suggest that XBP-1(S) binding occurs through a UPR element conserved in both murine and human OBF-1/BOB-1/OCA-B promoters as opposed to endoplasmic reticulum stress response elements. Our studies are the first to identify direct downstream targets of XBP-1(S) during either plasma cell differentiation or the UPR. In addition, our data further define the XBP-1(S)-binding sequence and provide yet another role for this protein as a master regulator of plasma cell differentiation.