BAY 1143572 inhibits proliferation, induces apoptosis, and affects CDK9 signaling in ATL-derived or HTLV-1–transformed cell lines. (A) Viabilities of ATL-derived or HTLV-1–transformed cell lines in the presence of different concentrations of BAY 1143572 for 72 hours (ATN-1, MJ, MT-1, MT-2, TL-Om1, TL-Su, and MT-4) and 24 hours (S-YU) were assessed. S-YU cells were assessed in the presence of recombinant human IL-2 at a final concentration of 100 IU/mL. The IC₅₀ value (the concentration of inhibitor where the response is reduced by half the maximal value) for each line is indicated in each panel. The absolute IC₅₀ value (the concentration of an inhibitor that reduced cell survival to 50% of the untreated control value, with the highest observed viability [no inhibitor] defined as 100%, and the lowest viability defined as 0%) for each line is also indicated in parentheses below the IC₅₀ value. (B) ATL-derived or HTLV-1–transformed cell lines were treated with various concentrations of BAY 1143572 for 72 hours. Apoptosis was then analyzed by Annexin V and propidium iodide (PI; nuclear) staining and flow cytometry. BAY 1143572 concentrations are indicated above the panels, and the percentage of cells in each quadrant is indicated in or beside each quadrant. (C) ATN-1, TL-Om1, MT-4, and MJ cell lines were treated with the indicated concentrations of BAY 1143572 for 5 hours, and western blotting was performed. Blots were probed with antibodies to phospho-RNAPII (Ser2 of the CTD [Ser2]), phospho-RNAPII (Ser5), RNAPII (N-20), c-Myc, Mcl-1, and HTLV-1 Tax. Actin was used as a loading control.