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The BCR/ABL oncogene is responsible for the phenotype of Philadelphia chromosome-positive (Ph+) leukemia. BCR/ABL exhibits an aberrant ABL-tyrosine kinase activity. The treatment of advanced Ph+ leukemia with selective ABL-kinase inhibitors such as Imatinib, Nilotinib and Dasatinib is initially effective but rapidly followed by resistance mainly because of specific mutations in BCR/ABL. Tetramerization of ABL through the N-terminal coiled-coil region (CC) of BCR is essential for the ABL-kinase activation. Targeting the CC-domain forces BCR/ABL into a monomeric conformation reduces its kinase activity and increases the sensitivity for Imatinib. We show that (i) targeting the tetramerization by a peptide representing the Helix-2 of the CC efficiently reduced the autophosphorylation of both unmutated and mutated BCR/ABL; (ii) Helix-2 inhibited the transformation potential of BCR/ABL independently of the presence of mutations; and (iii) Helix-2 efficiently cooperated with Imatinib as revealed by their effects on the transformation potential and the factor-independence related to BCR/ABL with the exception of mutant T315I. These findings support earlier observations that BCR/ABL harboring the T315I mutation have a transformation potential that is at least partially independent of its kinase activity. These data provide evidence that the inhibition of tetramerization inhibits BCR/ABL-mediated transformation and can contribute to overcome Imatinib-resistance., ((c) 2008 Wiley-Liss, Inc.)

Background and Objectives: Stem cells play an important role in the pathogenesis and maintenance of most malignant tumors. Acute myeloid leukemia (AML) is a stem cell disease. The inefficient targeting of the leukemic stem cells (LSC) is considered responsible for relapse after the induction of complete hematologic remission (CR) in AML. Acute promyelocytic leukemia (APL) is a subtype of AML characterized by the t(15;17) translocation and expression of the PML/RARalpha fusion protein. Treatment of APL with all-trans retinoic acid (ATRA) induces CR, but not molecular remission (CMR), because the fusion transcript remains detectable, followed by relapse within a few months. Arsenic induces high rates of CR and CMR followed by a long relapse-free survival (RFS). Here we compared the effects of ATRA and arsenic on PML/RARalpha-positive stem cell compartments., Design and Methods: As models for the PML/RARalpha-positive LSC we used: (i) Sca1+/lin- murine HSC retrovirally transduced with PML/RARalpha; (ii) LSC from mice with PML/RARalpha-positive leukemia; (iii) the side population of the APL cell line NB4., Results: In contrast to ATRA, arsenic abolishes the aberrant stem cell capacity of PML/RARalpha-positive stem cells. Arsenic had no apparent influence on the proliferation of PML/RARalpha-positive stem cells, whereas ATRA greatly increased the proliferation of these cells. Furthermore ATRA induces proliferation of APL-derived stem cells, whereas arsenic inhibits their growth., Interpretations and Conclusions: Taken together our data suggest a relationship between the capacity of a compound to target the leukemia-initiating cell and its ability to induce long relapse-free survival. These data strongly support the importance of efficient LSC-targeting for the outcome of patients with leukemia.

Since the 19th century, arsenic (As2O3) has been used in the treatment of chronic myelogenous leukemia (CML) characterized by the t(9;22) translocation. As2O3 induces complete remissions in patients with acute promyelocytic leukemia. The response to As2O3 is genetically determined by the t(15;17)-or the t(9;22)-specific fusion proteins PML/RARalpha or BCR/ABL. The PML portion of PML/RARalpha is crucial for the sensitivity to As2O3. PML is nearly entirely contained in PML/RARalpha. PML is upregulated by oncogenic RAS in primary fibroblasts. The aberrant kinase activity of BCR/ABL leads to constitutive activation of RAS. Therefore, we hypothesized that BCR/ABL could increase sensitivity to As2O2-induced apoptosis by modifying PML expression. To disclose the mechanism of As2O3-induced apoptosis in PML/RARalpha- and BCR/ABL-expressing cells, we focused on the role of PML for As2O3-induced cell death. Here we report that (i) sensitivity to As2O3-induced apoptosis of U937 cells can be increased either by overexpression of PML, or by conditional expression of activated RAS; (ii) also the expression of the t(8;21)-related AML-1/ETO increased sensitivity to As2O3-induced apoptosis; (iii) both BCR/ABL and AML-1/ETO activated RAS and modified the PML expression pattern; (iv) the expression of either BCR/ABL or AML-1/ETO rendered U937 cells sensitive to interferon alpha-induced apoptosis. In summary, these data suggest a crucial role of factors able to upregulate PML for As2O2-induced cell death.