Impact of Celastrol on the proliferation of primary AML cells and normal hematopoietic progenitors. (A) Effect of Celastrol in mouse models of AML. As outlined schematically at the top and on the left, lineage-negative cells from the bone marrow of healthy mice and primary leukemic blasts from mice were subjected to colony formation assays in the absence or presence of 0.5 μM Celastrol. Leukemias were generated by transduction of progenitor cells with Myc/Bcl2 or MLL/AF9 retroviruses. MLL/AF9-derived leukemic blasts were sorted into c-Kit–negative (bulk blast cells) and c-Kit–positive cells (leukemic stem cells) before colony formation assays were performed. Equal numbers of cells were plated with DMSO (black bars) or with Celastrol (gray bars) in each case. Columns show the relative colony number, normalized to the colony number of the DMSO control. Asterisks indicate statistical significance (**P < .01, Student t test). (B) Influence of Celastrol on primary human cells. Colony formation assay of 4 batches of CD34⁺ hematopoietic progenitor cells from healthy donors and leukemic blasts from 7 different AML patients. Equal numbers of cells were plated in the absence (black bars) or presence (gray bars) of 0.5 μM Celastrol. Bars show colony numbers as percent of DMSO-treated controls. The numbers above the black bars indicate the average colony number for each control sample.