Enhanced STAT5A expression renders cells less susceptible to imatinib-induced apoptosis. (A) FACS profiles of v-Abl⁺ cells ectopically expressing STAT5A and GFP (top profile) and vector control cells expressing only GFP (bottom profile) treated 24 hours with imatinib. The percentage of GFP⁺ cells and the dosage of imatinib administered are indicated. (B-C) Summary of the experiment shown in panel A repeated with 10 individual v-Abl⁺ (B) and 5 individual p185^BCR-ABL1+ (C) cell lines. (D) FACS analysis for propidium iodide (PI) uptake of v-Abl⁺ cell lines sorted for Stat5a-IRES-eGFP-expressing (S5a^high) and control (S5a^low) cell-treated imatinib for 48 hours (n = 3/group). STAT5A/B expression levels were determined by intracellular FACS staining and are shown in the inset. (E) Representative cytospins of S5a^high and S5a^low cells 48 hours after imatinib treatment. (F) v-ABL-transformed STCs were infected with pMSCv-Stat5a-IRES-eGFP and treated with imatinib, dasatinib, and nilotinib. The experiment was performed as outlined in panel A. The summary of 3 individually performed experiments is shown. (G) [³H]-Thymidine incorporation assay of Ba/F3 p210^BCR-ABL1 cells harboring a doxycycline-inducible dominant negative STAT5 (S5a^Δ749). [³H]-Thymidine uptake was measured 48 hours after induction of S5a^Δ749 expression and 24 hours after TKI treatment. The relative percentage of [³H]-thymidine uptake compared with untreated control cells (black bar) is shown (n ≥ 3/group). (B-E) Error bars represent mean ± SD. *P < .05. **P < .01. ***P < .001.