Effect of enforced MEK activation on SAHA/bortezomib-induced apoptosis in K562 cells. K562 cells were transiently transfected, and GFP-expressing cells were isolated using a Cytomation fluorescence-activated cell sorter as described in “Materials and methods.” Sorted cells transfected with either GFP alone or the GFP/constitutively active MEK1 fusion cDNA were cultured in drug-free medium for 5 hours, and then exposed to 1.5 μM SAHA ± 4.5 nM bortezomib for 36 hours, after which apoptosis was monitored by examining Wright Giemsa-stained cytospin preparations as described. Values represent the means ± SD for 2 separate determinations. ^*Significantly less than values for cells transfected with GFP alone; P < .05.