TAT–Bcl-xL protein is efficiently taken up by LSK cells and inhibits apoptosis. (A) WT LSK cells were treated with dextran sulfate for 30 min and TAT–Bcl-xL for 2 h (1.5 µM). Protein content was determined 48 h later by flow cytometry using an anti–Bcl-xL antibody. (B) Localization of TAT–Bcl-xL protein in LSK cells was determined 24 h after protein transduction by fluorescence microscopy and shows TAT–Bcl-xL on mitochondria and in the cytoplasm. Bars, 5 µm. (C and D) LSK cells transduced with TAT–Bcl-xL protein were subjected to 1 µg/ml etoposide (C) or cytokine withdrawal (D). Specific apoptosis was determined by flow cytometry at indicated time points. Bars represent means of n = 4 independent experiments ± SEM. Significant p-values (Mann–Whitney test): (C) Untreated vs. TAT–Bcl-xL: P = 0.005 on day 2 and P = 0.01 at day 4 and 6; Buffer vs. TAT–Bcl-xL: P = 0.02 at day 2, 4, and 6; (D) Untreated vs. TAT–Bcl-xL: P = 0.005 at day 2 and 8, P = 0.01 at day 4, P = 0.02 at day 6; Buffer vs. TAT–Bcl-xL: P = 0.02 at day 2, P = 0.03 at day 4 and 6. (E) TAT–Bcl-xL–treated LSK cells were cultured in the presence or absence of cytokines. At the indicated time points, TAT–Bcl-xL–positive cells were determined by intracellular flow cytometry. Bars represent means of n = 4 independent experiments ± SEM.