Proteasome inhibitors specifically induce the expression of the transcription factor ATF4. The indicated multiple myeloma cell lines were treated for up to 12 hours with 50 μM tunicamycin (TM), 30 μM melphalan (Mel), or the indicated concentration of bortezomib (BZ). Whole-cell lysates were prepared, and Western blots for ATF4 were performed. The blots were then stripped and reprobed for actin to confirm equal protein loading. Representative blots from 3 independent experiments are shown.