JAK2 inhibition induces growth inhibition and apoptosis in cells with mutated JAK2. (A) Dose-dependent growth inhibition detected by 3-(4,5 dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl-2-(4-sulfophenyl)-2H-tetrazolium assay. HEL cells (HEL), SET-2 cells (SET-2), CHRF 288-11 cells (CHRF), and K562 cells (K562) were plated in 96-well plates and treated with increasing doses of JAK inhibitor I (range, 0-10μM) for 48 hours. Data are mean ± SD of at least 4 independent experiments comparing the JAK2 mutated cell lines with the bcr-abl⁺ cell line K562. JAK2 mutated cell lines versus K562 cells: ***P < .001. (B) Annexin V apoptosis assay. The cells were treated with increasing JAK inhibitor I concentrations (0, 0.3, 1, and 3μM) for 24 hours. Then, cells were harvested, stained with for annexin V and propidium iodide, and analyzed. Data are results from a representative experiment repeated 3 times with similar results. (C) Modulation of signaling after JAK inhibitor I treatment in HEL cells. The cells were treated with 3μM JAK inhibitor I for 0, 6, 12, and 24 hours. Cells were harvested, and extracts were analyzed by Western blotting.