MEK inhibitor toxicity is higher in MAF-expressing MM cell lines and rescued by MAF reexpression. (A) MM cell lines were incubated with indicated concentrations of MEK inhibitor U0126, and cell number was enumerated by flow cytometry after 8 days. Live cells were normalized to solvent (dimethyl sulfoxide)–treated control. (B) Proapoptotic BIM is up-regulated after MEK inhibition. Western blot also shows decreased ERK phosphorylation and FOS expression. (C) MEK inhibitor-induced apoptosis was detected by VAD-FITC labeling of cleaved caspase 3 in MM cell lines treated with U0126 (10μM) at the indicated time points. (D) MM cell lines were incubated with either dexamethasone (24nM) or U0126 (10μM) for 7 days in the presence or absence of HS-5 bone marrow stromal cell line. Live cells were quantified by sodium 3-[1-[(phenylamino) carbonyl]-2H-tetrazolium hydroxide assay and normalized to untreated control cells (± SEM). (E) MAF transgene was stably and constitutively expressed in MM cell lines using an exogenous LTR promoter. Live cells were enumerated by flow cytometry for up to 11 days of MEK inhibition (U0126, 10μM). Shown are representative growth curves from at least 3 independent experiments.