Imatinib mesylate resistance conferred by increased Bcr/Abl expression fails to protect K562 cells from apoptosis induced by coadministration of bortezomib and flavopiridol. (A) K562 (left) and imatinib mesylate-resistant K562-R (right) cells were treated with 8 nM bortezomib (Btzmb) ± 200 nM flavopiridol (FP) for 24 hours and 48 hours, after which the percentage of cells exhibiting apoptotic morphology was determined by evaluating Wright Giemsa-stained cytospin preparations. In parallel, K562 and K562-R cells were exposed to 1 μM imatinib mesylate (STI) for 48 hours to document imatinib mesylate resistance. Results represent the means ± SDs for 3 separate experiments performed in triplicate. (B) K562 and K562-R cells were treated for 24 hours as described for panel A, after which cells were lysed and subjected to Western blot using the indicated antibodies. CF indicates cleavage fragment. Each lane was loaded with 30 μg protein; blots were stripped and reprobed with antibodies to actin to ensure equal loading and transfer. An additional 2 studies yielded equivalent results.