Effect of MEK inhibition on HD cell line viability. (A) Normal peripheral blood lymphocytes (PBLs) and HD cell lines were incubated with UO126 (25 μM) for 48 hours; UO126 had no significant effect on PBLs compared with DMSO alone, while it reduced the number of viable cells of all 4 HD cell lines by 30% to 50%. (B) With the use of the PI staining method, the antiproliferative effect of UO126 (25 μM for 48 hours) was found to be due to either cell-cycle arrest at the G₂M phase (percentages shown in the right upper area) or induction of apoptosis (percentages shown in the left upper area). The apoptotic activity of UO126 was most prominent in the HD-LM2, and to a lesser extent, the KM-H2 cell lines. Cell-cycle arrest at the G₂M phase was most prominent in the HD-MYZ, L-428, and KM-H2 cell lines. Data shown are representative of 3 independent experiments. Histogram analysis and statistics were generated by using Cellquest software. (C) Induction of apoptosis by UO126 in HD-LM2 cells was confirmed by dual staining with PI and TUNEL. Apoptotic cells are shown in the sub-G₀ area (arrowhead), and the percentage of apoptotic cells is shown. The upper 2 boxes show data with RPMI with and without terminal deoxynucleotidyl transferase (TdT). The lower panel shows results obtained after incubation with UO126 for 48 hours.