SET antagonism activates PP2A, induces apoptosis, and reduces Mcl-1 levels. (A) SET was knocked down by shRNA to SET after lentiviral transduction in 32D:BCR/Abl cell cultures or treatment with 1μM COG449 followed by lysis with NP40 lysis buffer. PP2A was immunoprecipitated and assayed with the PP2A immunoprecipitation assay kit (Upstate Biotechnology), with the exception of using DiFMUP as a fluorescent substrate. The line for each sample represents the phosphate release from a given sample after subtraction of the OA-inhibited control reactions (P < .05). (B) 32D:BCR/Abl cells were treated with the indicated compounds for 30 minutes followed by lysis with NP40 lysis buffer. PP2A was immunoprecipitated and assayed as in panel A (P < .01). (C) CLL cells were treated with COG449 followed by annexin-V and propidium iodide staining to assess apoptosis and death. (D) Freshly isolated human CLL cells were incubated with the indicated concentrations of COG449 for 24 hours. Cells were lysed, protein lysates subjected to PAGE, and immunoblotted to quantify the Mcl-1 and β-actin ratio. *P < .01 (relative to control). **P < .001 (relative to control).