NSC606985 inhibits the formation of CFU-GM and BFU-E of bone marrow cells from patients with AML and a sketch of mechanisms of NSC606985 action in leukemic cells. (A) BM mononuclear cells were cultured for colony assay as described in “Patients, materials, and methods.” In cases 1 to 4, the number of colonies per well in vehicle control was 33, 16, 40, and 20, respectively, for CFU-GMs. For BFU-E, the number of colonies per well in vehicle control was 37 and 20, respectively, while BM cells from case 2 and case 4 patients did not form BFU-Es. The values represent the inhibition % of colony formation compared with control. (B) Representative images of CFU-GMs and BFU-Es for the case-3 patient. Images were visualized with an Olympus IMT microscope equipped with a 40 ×/0.5 numerical aperture objective lens (Olympus), and were captured through a SPOT RT camera and SPOT software. (C) An illustration of the potential mechanisms of NSC606985 action in leukemic cells. NSC606985 at nanomolar concentrations activates PKCδ that leads to mitochondrial ΔΨm loss and activation of caspase-3, resulting in apoptosis. These events were completely blocked by PKCδ-specific inhibitor rottlerin. However, caspase-3 inhibitor Z-DEVD-fmk only partially blocks NSC606985-induced apoptosis, indicating that caspase-3–independent mechanisms are also involved in the event. The activated caspase-3 has a positive feedback to proteolytic activation of PKCδ. The inhibition of cell proliferation by NSC606985 may involve independent mechanisms from cell apoptosis. Solid lines indicate complete blockage; dashed lines, partial blockage.