Cellular and biochemical response to tyrosine kinase inhibitors in imatinib-sensitive and -resistant cells. (A) K562 and K562R cells were treated with imatinib (left) or dasatinib (right) at the indicated concentration for 48 hours before assessment of cell viability as previously described.¹⁹  Each point represents the average plus or minus SEM (error bar) of 4 determinations. (Inset) Immunoblot analysis of BCR-ABL, Lyn, Src, Yes, Fyn, and actin in equal protein lysates from K562 and K562R cells. BCR-ABL sequencing did not detect mutations in the Abl kinase domain in either cell line (data not shown). (B) K562 (left) or K562R (right) cells were incubated with imatinib (5 μM) or dasatinib (0.5 μM) for 0, 2, or 24 hours before lysates were analyzed for tyrosine-phosphorylated (activated) BCR-ABL, Lyn, and CrkL. Lyn and PARP protein (uncleaved [uPARP] = 116 kDa or cleaved [cPARP] = 85 kDa) were also detected by immunoblotting. (C) K562 and K562R cells were left untreated (control) or electroporated (AMAXA) with control (Con) or Lyn siRNA and analyzed for activated Lyn (pY396-Lyn) or Lyn protein levels by immunoblotting after 48 hours. Lysates were also probed for PARP as a marker of apoptosis. Table 1 summarizes the effect of siRNA on viability in K562 and K562R cells. The results represent the average and standard deviation from 3 independent experiments.