CCAAT ENHANCER-BINDING PROTEIN (C EBP) AND AML1 (CBF-ALPHA-2) SYNERGISTICALLY ACTIVATE THE MACROPHAGE-COLONY-STIMULATING FACTOR-RECEPTOR PROMOTER/

Transcription factors play a key role in the development and different iation of specific lineages from multipotential progenitors, Identific ation of these regulators and determining the mechanism of how they ac tivate their target genes are important for understanding normal devel opment of monocytes and macrophages and the pathogenesis of a common f orm of adult acute leukemia, in which the differentiation of monocytic cells is blocked, Our previous work has shown that the monocyte-speci fic expression of the macrophage colony-stimulating factor (M-CSF) rec eptor is regulated by three transcription factors interacting with cri tical regions of the M-CSF receptor promoter, including PU.1 and AML1. PU,I is essential for myeloid cell development, while the AML1 gene i s involved in several common leukemia-related chromosome translocation s, although its role in hematopoiesis has not been fully identified, A long with AML1, a third factor, Mono A, interacts with a small region of the promoter which can function as a monocyte-specific enhancer whe n multimerized and linked to a heterologous basal promoter, Here, we d emonstrate by electrophoretic mobility shift assays with monocytic nuc lear extracts, COS-I cell-transfected factors, and specific antibodies that the monocyte-enriched factor Mono A is CCAAT enhancer-binding pr otein (C/EBP), C/EBP has been shown previously to be an important tran scription factor involved in hepatocyte and adipocyte differentiation; in hematopoietic cells, C/EBP is specifically expressed in myeloid ce lls, In vitro binding analysis reveals a physical interaction between C/EBP and AML1, Further transfection studies show that C/EBP and AML1 in concert with the AML1 heterodimer partner CBF beta synergistically activate M-CSF receptor by more than 60-fold, These results demonstrat e that C/EBP and AML1 are important factors for regulating a critical hematopoietic growth factor receptor, the M-CSF receptor, suggesting a mechanism of how the AML1 fusion protein could contribute to acute my eloid leukemia, Furthermore, they demonstrate physical and functional interactions between AML1 and C/EBP transcription factor family member s.