| Kinase Assay | FLT3 phosphorylation: Leukemia cells are washed in phosphate-buffered saline (PBS), then lysed by resuspending the cells in lysis buffer (20 mM Tris pH 7.4, 100 mM NaCl, 1% Igepal, 1 mM EDTA, 2 mM NaVO4, plus Complete protease inhibitor KW-2449 for 30 minutes while rocking. The extract is clarified by centrifugation at 1.6 × 104?g and the supernatant is assayed for protein (Bio-Rad). A 50-μg aliquot is removed as a whole-cell lysate for analysis of STAT5, and the remainder is used for immunoprecipitation with anti-FLT3. Anti-FLT3 antibody is added to the extract for overnight incubation, then protein A sepharose is added for 2 additional hours. Separate sodium dodecyl sulfate–polyacrylamide electrophoresis (SDS-PAGE) gels for whole-cell lysate and immunoprecipates are run in parallel. After transfer to Immobilon membranes, immunoblotting is performed with antiphosphotyrosine antibody (4 g10) to detect phosphorylated FLT3 or, for the whole-cell lysate gels, with a rat monoclonal antibody against phosphorylated STAT5 (residue Y694) then stripped and reprobed with anti-FLT3 antibody to measure total FLT3. Proteins are visualized using chemiluminescence, exposed on Kodak BioMax XAR film, developed, and scanned using a Bio-Rad GS800 densitometer. The concentration of KW-2449 for which the phosphorylation of FLT3 or STAT5 is inhibited to 50% of its baseline (IC50) is determined using linear regression analysis of the dose response curves. For direct analysis of FLT3 and STAT5 in circulating blasts, peripheral blood is collected in heparinized tubes and promptly chilled on ice. Samples are centrifuged for 10 minutes at 900 g, at 4 °C. The plasma is removed and stored frozen at ?80 °C. The buffy coat is carefully transferred to ice-cold PBS, layered onto chilled Ficoll-Hypaque, and centrifuged for 5 minutes at 600 g, at 4 °C. All subsequent steps are carried out at 4 °C. Mononuclear cells are collected and washed rapidly once in red blood cell lysis buffer (0.155 M NH4Cl, 0.01 M KHCO3, 0.1 mM EDTA), then washed once in PBS. Cells are then lysed as described for FLT3 and STAT5 analysis. |
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