Ability of Meis1 to induce apoptosis is dependent on the presence of the HD and of the Pbx-interaction motif. (A) Reh cells were transfected with murine Meis1a, murine Meis1d (lacking the HD), or murine Pbx1b alone or in all possible cotransfection combinations. Total amounts of DNA were kept constant by transfecting empty vectors with single transfections. Results are pooled means and standard deviations of 3 separate experiments consisting of 3 replicate samples. Transfection efficiency, 38%. (B) Reh cells were transfected with murine Meis1b, murine Meis1bΔM2 (lacking the Pbx-interaction motif), or murine Pbx1b alone or in all possible cotransfection combinations. Transfection efficiency, 36%. (C) Nuclear extracts from transiently transfected Reh cells were separated on a 10% SDS-PAGE gel, blotted onto PVDF membrane, and probed with a polyclonal Meis1 antibody (78-5) directed at an epitope at the Meis1 N-terminus. THP-1 nuclear extract was used as a positive control for Meis1 expression, whereas mock-transfected Reh nuclear lysate was used to indicate baseline expression in Reh cells. Cytoplasmic extracts were used for cultures transfected with Meis1bΔM2 because this protein was expected to remain in the cytoplasm. (D) Murine Meis1a was transfected into human Reh lymphoblastoma cells, human HL-60 promyelomonocytic cells, murine 32Dcl3 pre–B-cell leukemia, and murine NIH3T3 fibroblasts. Apoptosis was scored as previously described with the exception of NIH3T3 cells, for which detached cells in the media were collected and added to trypsinized cells for the purpose of trypan blue counting. (E) Murine Pbx1b was transfected into the 4 cell types described in panel D. (D-E) Transfection efficiencies: Reh, 39%; HL-60, 36%; 32D, 37%; NIH3T3, 32%