Bmi1- Is a SALL4 Target Gene in Hematopoeitic and Leukemic Cells.

Bmi-1 and SALL4 are putative oncogenes modulating stem cell pluripotency and playing a role in leukemogenesis. Previously, we report that SALL4, a zinc finger transcription factor, is constitutively expressed in human acute myeloid leukemia (AML) and induces AML in transgenic mice. Here we demonstrate that transcription from the Bmi-1 promoter is strikingly activated by SALL4. The SALL4 responsiveness of the Bmi-1 promoter exhibits dose-dependent activation in a luciferase reporter gene assay. We mapped the N-terminal domain of SALL4 responsible for activation of the Bmi-1 promoter. Chromatin immunoprecipitation from a myeloid stem cell line, 32D demonstrated that SALL4 bound directly to the Bmi-1 promoter in a region involving SALL4 stimulation. The over-expression of SALL4 in the 32D cell line up-regulated Bmi-1expression and was associated with increased proliferation and inhibition of apoptosis. Deletion of one copy of SALL4 by the gene target in the mouse marrow significantly reduced Bmi-1 expression. SALL4 up-regulated Bmi-1 expression in SALL4 transgenic mice and the up-regulated levels of Bmi-1 in SALL4 transgenic mice correlated to that of disease progression from normal, to preleukemic and leukemic stages. Similar to Bmi-1, SALL4 was expressed highly in hematopoietic cells (HSCs) and was down-regulated as differentiation proceeds. These findings, when taken together, reveal a novel link between SALL4 and Bmi-1 in regulating self-renewal of normal and leukemic stem cells.