STAT5A-mediated survival advantage is restricted to TKI-exposed cells and depends on Abelson kinase activity. (A) RT-PCR of S5a^high and S5a^low cells before and after treatment with 100nM dasatinib for 3 hours. Results were normalized by Gapdh mRNA expression. The fold change compared with untreated S5a^low cell mRNA levels is shown (n = 3/group). (B) Top panel: Intracellular pSTAT5 staining of S5a^low and S5a^high cells 24 hours after imatinib treatment as indicated. The dashed line indicates the predicted threshold of pSTAT5 level essential for survival. Bottom: FACS analysis for propidium iodide (PI) uptake 72 hours after imatinib treatment as indicated. Percentages of PI-positive cells are shown. (C) Dose-response curves determined by [³H]-thymidine incorporation of S5a^high and S5a^low cells treated with indicated substances for 24 hours. Control values in the absence of any substances were set to 100%. (D) Scheme showing the mechanism of high STAT5 protein levels decreasing imatinib response and how a dose escalation can help to overcome this resistance.