Bortezomib, in contrast to Hsp90 inhibitors or TM and TG, exhibits a delayed ability to splice XBP1 and induces a progressive transcriptional upregulation of CHOP. (A) Treatment of myeloma cells over a 24-hour period with bortezomib induced late splicing of XBP1, as demonstrated by reverse-transcription PCR. (B) Levels of the proapoptotic factor CHOP were determined by real-time Taqman PCR. (C) Immunoblotting of nuclear fractions of cell lysates demonstrated the increase in cleaved ATF6α. Representative cell line data on U266 are shown. All experiments were repeated in triplicate.