DNA-binding domain of AML1, expressed in t(8;21) and t(3;21) myeloid leukemias, inhibits PEBP2/CBF DNA-binding but is not sufficient to transform 32D cl3 myeloid cells

Truncated AML1 proteins are predicted to be expressed from out-of-frame AML1 transcripts present in myeloid leukemia cells harboring t(8;21) and t(3;21). To test whether these proteins, consisting of almost exclusively an N-terminal AML1 DNA-binding domain, interfere with myeloid differentiation we expressed a similar truncated AML1 protein in 32D cl3 myeloid cells. In all clones examined, the ectopically expressed truncated AML1 protein prevented binding of endogenous PEBP2/CBFs to DNA, possibly by interacting with all available CBF beta subunits. However, compared to control clones, the 32D cl3 clones expressing truncated AML1 remained IL-3 dependent for survival, proliferated similarly in low and high concentrations of IL-3, and differentiated similarly upon transfer to G-CSF. Thus, truncated AML1 proteins may contribute to myeloid leukemogeneis by inhibiting PEBP2/CBF activities, although contributions from other oncoproteins are likely required as well

Roles of HIPK1 and HIPK2 in AML1- and p300-dependent transcription, hematopoiesis and blood vessel formation

Histone acetyltransferases (HATs) p300 and CREB-binding protein (CBP) function as co-activators for a variety of sequence-specific transcription factors, including AML1. Here, we report that homeodomain-interacting protein kinase-2 (HIPK2) forms a complex with AML1 and p300, and phosphorylates both AML1 and p300 to stimulate transcription activation as well as HAT activities. Phosphorylation of p300 is triggered by phosphorylated AML1 as well as by PU.1, c-MYB, c-JUN and c-FOS, and is inhibited by dominant-negative HIPK2. Phosphorylation of p300 and AML1 is impaired in Hipk1/2 double-deficient mouse embryos. Double-deficient mice exhibit defects in primitive/definitive hematopoiesis, vasculogenesis, angiogenesis and neural tube closure. These phenotypes are in part similar to those observed in p300- and CBP-deficient mice. HIPK2 also phosphorylates another co-activator, MOZ, in an AML1-dependent manner. We discuss a possible mechanism by which transcription factors could regulate local histone acetylation and transcription of their target genes

Downregulation of Bim, a Proapoptotic Relative of Bcl-2, Is a Pivotal Step in Cytokine-Initiated Survival Signaling in Murine Hematopoietic Progenitors

Two distinct signaling pathways regulate the survival of interleukin-3 (IL-3)-dependent hematopoietic progenitors. One originates from the membrane-proximal portion of the cytoplasmic domain of the IL-3 receptor (βc chain), which is shared by IL-3 and granulocyte-macrophage colony-stimulating factor and is involved in the regulation of Bcl-x(L) through activation of STAT5. The other pathway emanates from the distal region of the βc chain and overlaps with downstream signals from constitutively active Ras proteins. Although the latter pathway is indispensable for cell survival, its downstream targets remain largely undefined. Here we show that the expression of Bim, a member of the BH3-only subfamily of cell death activators, is downregulated by IL-3 signaling through either of two major Ras pathways: Raf/mitogen-activated protein kinase and the phosphatidylinositol 3-kinase/mammalian target of rapamycin. Akt/phosphokinase B does not appear to play a significant role in this regulatory cascade. Bim downregulation has important implications for cell survival, since enforced expression of this death activator at levels equivalent to those induced by cytokine withdrawal led to apoptosis even in the presence of IL-3. We conclude that Bim is a pivotal molecule in cytokine regulation of hematopoietic cell survival