Versatility of BCR/ABL-expressing leukemic cells in circumventing proapoptotic BAD effects

Bcr-Abl signaling through the PI-3/S6 kinase pathway inhibits nuclear translocation of the transcription factor Bach2, which represses the antiapoptotic factor heme oxygenase-1

The malignant phenotype of chronic myeloid leukemia (CML) is due to the abnormal tyrosine kinase activity of the Bcr-Abl oncoprotein. We have previously reported that expression of the Bach2 transcription factor, which induces apoptosis in response to oxidative stress, is greatly reduced in CML cells. Because these cells are resistant to apoptosis, we tested whether Bach2 could also be regulated through posttranslational mechanisms that promote inhibition of the apoptotic response to mutagenic stimuli in CML. We found that Bach2 is phosphorylated on S521 via the phosphatidylinositol-3/S6 kinase pathway, and substitution of this site to alanine leads to nuclear accumulation of the protein, indicating that this phosphorylation is important for its subcellular localization. Ectopic expression of the S521 mutant imparts greater impairment to CML cell growth than the wild-type factor. Furthermore, we showed that Bach2 transcriptionally represses heme oxygenase-1, an antiapoptotic factor up-regulated in CML. Because CML cells are known to produce high levels of intracellular reactive oxygen species, overexpression of heme oxygenase-1 resulting from inhibition of Bach2 activity may contribute to their genomic instability and leukemic phenotype.

Bcr-Abl-mediated suppression of normal hematopoiesis in leukemia

A variety of experimental evidence including findings in various mouse models indicates that the BCR-ABL oncogene is the cause of chronic myeloid leukemia (CML). Since normal hematopoietic cells in marrow and spleen are replaced with proliferating leukemic blasts, we determined whether this is an active process mediated by the leukemia cells. The lipocalin 24p3 was reported to be secreted by mouse hematopoietic cells deprived of IL-3, resulting in apoptosis induction in a variety of hematopoietic cells including bone marrow cells. Here, we show that BCR-ABL+ mouse hematopoietic cells induced persistent expression and secretion of 24p3. Importantly, BCR-ABL+ hematopoietic cells were resistant to the apoptotic effects of 24p3. The expression of the Bcr-Abl oncoprotein and its tyrosine kinase were required for induction of 24p3 expression. Co-culture studies showed that BCR-ABL+ cells induced apoptosis in BCR-ABL negative cells. Antisense 24p3/siRNA expression reduced the level of 24p3 protein in both BCR-ABL+ cells and in conditioned medium (CM) obtained from these cells. CM from BCR-ABL+ cells expressing antisense 24p3/siRNA had reduced apoptotic activity for target cells; 24p3 antibody also reduced the apoptotic activity of the CM. Leukemic mice induced by BCR-ABL+ cells expressing either antisense 24p3 or 24p3 siRNA had increased levels of normal hematopoiesis and reduced invasion of leukemia cells in marrow and spleen tissues. These findings indicate that suppression of normal hematopoiesis in BCR-ABL-induced leukemia is an active process involving secretion of the cell death-inducing factor 24p3 by mouse leukemia cells, raising the possibility that similar factors are involved in BCR-ABL+ CML.

ABL oncogenes and phosphoinositide 3-kinase: mechanism of activation and downstream effectors

The BCR-ABL oncogene is responsible for most cases of chronic myelogenous leukemia and some acute lymphoblastic leukemias. The fusion protein encoded by BCR-ABL possesses an aberrantly regulated tyrosine kinase activity. Imatinib mesylate (Gleevec, STI-571) is an inhibitor of ABL tyrosine kinase activity that has been remarkably effective in slowing disease progression in patients with chronic phase chronic myelogenous leukemia, but the emergence of imatinib resistance underscores the need for additional therapies. Targeting signaling pathways activated by BCR-ABL is a promising approach for drug development. The study of signaling components downstream of BCR-ABL and the related murine oncogene v-Abl has revealed a complex web of signals that promote cell division and survival. Of these, activation of phosphoinositide 3-kinase (PI3K) has emerged as one of the essential signaling mechanisms in ABL leukemogenesis. This review describes molecular mechanisms by which PI3K is activated and the downstream PI3K effectors that propagate the signal to promote myeloid and lymphoid transformation. Of particular recent interest is the mammalian target of rapamycin, a PI3K-regulated kinase that regulates protein synthesis and contributes to leukemogenesis.

BCR/ABL regulates response to DNA damage: the role in resistance to genotoxic treatment and in genomic instability

BCR/ABL regulates cell proliferation, apoptosis, differentiation and adhesion. In addition, BCR/ABL can induce resistance to cytostatic drugs and irradiation by modulation of DNA repair mechanisms, cell cycle checkpoints and Bcl-2 protein family members. Upon DNA damage BCR/ABL not only enhances reparation of DNA lesions (e.g. homologous recombination repair), but also prolongs activation of cell cycle checkpoints (e.g. G2/M) providing more time for repair of otherwise lethal lesions. Moreover, by modification of anti-apoptotic members of the Bcl-2 family (e.g. upregulation of Bcl-x(L)) BCR/ABL provides a cytoplasmic 'umbrella' protecting mitochondria from the 'rain' of apoptotic signals coming from the damaged DNA in the nucleus, thus preventing release of cytochrome c and activation of caspases. The unrepaired and/or aberrantly repaired (but not lethal) DNA lesions resulting from spontaneous and/or drug-induced damage can accumulate in BCR/ABL-transformed cells leading to genomic instability and malignant progression of the disease. Inhibition of BCR/ABL kinase activity by STI571 (Gleevec, imatinib mesylate) reverses drug resistance and, in combination with standard chemotherapeutics can exert strong anti-leukemia effect.

The US3 protein kinase of herpes simplex virus 1 mediates the posttranslational modification of BAD and prevents BAD-induced programmed cell death in the absence of other viral proteins

Earlier studies have shown that the d120 mutant of herpes simplex virus 1, which lacks both copies of the alpha4 gene, induces apoptosis in all cell lines tested. In some cell lines d120-induced apoptosis, manifested by the release of cytochrome c, activation of caspase 3, and fragmentation of cellular DNA, is blocked by the overexpression of Bcl-2. In these cells viral protein kinase U(S)3 delivered in trans blocks apoptosis induced by the mutant virus at a premitochondrial stage. We report that the U(S)3 protein kinase targets the pro-apoptotic BAD member of the Bcl-2 family. Specifically, the U(S)3 protein kinase mediates a posttranslational modification of BAD and blocks its cleavage, which is reported to activate apoptosis. Thus, U(S)3 protein kinase is the sole viral protein required to block activation of caspase 3, prevent cleavage of poly(ADP-ribose) polymerase, and block fragmentation of cellular DNA induced by BAD.