MKL1 promotes human megakaryocytic differentiation. (A) Validation of 3 HEL cell clones (clone numbers 5, 8, and 9) using Western blot analysis of MKL1 expression in the absence (left) and presence (right) of Dox for 2 days. MKL1-His protein is detected by anti-His antibody. (B) Based on analysis of Wright-Giemsa–stained cytospins, MKL1 increases the percentage of mature megakaryocytes in response to TPA over 4 days (P < .05). (C) Representative data showing ploidy of TPA-stimulated HEL/MKL1 cells or control cells after 4 days of differentiation with (solid line) or without (tinted) Dox. MKL1 increases the ploidy of cells exposed to TPA. Note increased 8N and 16N peaks in HEL/MKL1 cells. (D) Morphology of HEL cells treated with TPA and/or Dox as indicated. MKL1 has no effect on cell morphology in the absence of TPA (compare untreated with Dox), and promotes adhesion and spreading when cells are exposed to TPA (compare TPA with TPA + Dox). (E,F) The effects of enforced expression of MKL1 in human CD34⁺ cells during megakaryocytic differentiation. (E) The figures shown are gated for GFP-positive cells infected with pCCL and pCCL-MKL1 virus. Average percentage-positive cells for CD61, CD41a, and CD42b expression plus or minus SD of 3 independent experiments. *P < .05 from pCCL versus pCCL-MKL1 in the respective immunophenotype group. (F) Ploidy distribution of CD41a⁺ fraction in pCCL and pCCL-MKL1–infected cells plus or minus SD of 3 independent experiments. *P < .05 from pCCL versus pCCL-MKL1 in the respective ploidy group.