Effect of MEK inhibition on p-ERK in HD cell lines. (A) Effect of the MEK inhibitor UO126 on the HD-LM2 cell line. UO126 inhibited ERK phosphorylation in a dose- and time-dependent manner. When the cells were incubated with UO126 for 24 hours, the maximal effect was observed at a dose of 25 μM (p-ERK1/2 panel). When the cells were incubated with 25 μM UO126, the effect was nearly complete after 30 minutes of incubation and was complete after 6 hours of incubation. (B) A similar effect was observed in the 3 remaining HD cell lines, with complete inhibition of p-ERK that lasted for as long as 48 hours.