MEK inhibition sensitizes to spontaneous and drug-induced apoptosis. (a) OCI-AML3 cells were cultured in the presence of DMSO or PD98059 (20 μM) and stained for annexin V binding at the indicated times. Matched DMSO control at 96 hours is shown (C). Results are representative of one of three independent experiments. (b) Primary AML blasts were cultured in the presence of DMSO or PD98059 at the indicated doses for 96 hours. Apoptosis was then measured as percentage of cells with hypodiploid DNA content. Results are expressed as the net apoptosis induction (percentage of apoptosis in treated cells minus percentage of apoptosis in DMSO-treated cells) and represent the mean ± SD of the results obtained in three different patient samples (patients 3, 14, and 16). (c) AML cell lines were cultured for 96 hours in the presence of DMSO (white bars), 20 μM PD98059 (hatched bars), 1 μM ATRA (gray bars), or a combination of PD98059 and ATRA (black bars), and stained for annexin V binding. Results are representative of one of three independent experiments. (d) HL-60 cells were exposed to 10 μM Ara-C, washed, and cultured in the presence of DMSO (gray bars) or 20 μM PD98059 (black bars). White bars and hatched bars represent DMSO- and PD98059-treated HL-60 cells, respectively, in the absence of prior exposure to Ara-C. At the indicated times, cells were stained for annexin V binding. *DMSO-treated cultures were discontinued after 144 hours due to overgrowth. Results are representative of one of three independent experiments.