The SH3 Domain Contributes to BCR/ABL-Dependent Leukemogenesis In Vivo: Role in Adhesion, Invasion, and Homing

Aleukemic Extramedullary Blast Crisis as an Initial Presentation of Chronic Myeloid Leukemia with E1A3 BCR-ABL1 Fusion Transcript

Right neck swelling and pain occurred in a 49-year-old man. A Blood count showed a slight increase in platelet count without leukemoid reaction. After a biopsy of the cervical mass and bone marrow aspiration, a diagnosis of extramedullary blast crisis (EBC) of chronic myeloid leukemia (CML) was made. Fluorescence in situ hybridization (FISH) analysis showed a BCR-ABL1 fusion signal, but results of real-time polymerase chain reaction (RT-PCR) for major and minor BCR-ABL1 transcripts were negative. We identified a rare e1a3 BCR-ABL1 fusion transcript. Administration of dasatinib resulted in disappearance of the extramedullary tumor. This is the first reported case of CML-EBC with e1a3 transcript. An aleukemic extramedullary tumor can be the initial presentation of CML.

Activation of Abl tyrosine kinases promotes invasion of aggressive breast cancer cells

The Abl family of nonreceptor tyrosine kinases consists of two related proteins, c-Abl and Abl-related gene (Arg). Activated forms of the Abl kinases (BCR-Abl, Tel-Abl, and Tel-Arg) induce the development of human leukemia; it is not known, however, whether Abl kinases are activated in solid tumors or whether they contribute to tumor development or progression. Previously, we showed that Abl kinases are activated downstream of growth factor receptors, Src family kinases, and phospholipase Cgamma1 (PLCgamma1) in fibroblasts and influence growth factor-mediated proliferation, membrane ruffling, and migration. Growth factor receptors, Src kinases, and PLCgamma1 are deregulated in many solid tumors and drive tumor invasion and metastasis. In this study, we found that Abl kinases are constitutively activated, in highly invasive breast cancer cell lines, downstream of deregulated ErbB receptors and Src kinases. Furthermore, activation of Abl kinases promotes breast cancer cell invasion, as treatment of cells with the Abl kinase inhibitor, STI571, or silencing c-Abl and Arg expression with RNA interference dramatically inhibits Matrigel invasion. This is the first evidence that (a) Abl kinases are deregulated and activated in a nonhematopoietic cancer, (b) activation of Abl kinases in breast cancer cells occurs via a novel mechanism, and (c) constitutive activation of Abl kinases promotes invasion of breast cancer cells. These data suggest that pharmacologic inhibitors targeted against Abl kinases could potentially be useful in preventing breast cancer progression in tumors harboring activated Abl kinases.

Id1 transcription inhibitor-matrix metalloproteinase 9 axis enhances invasiveness of the breakpoint cluster region/abelson tyrosine kinase-transformed leukemia cells

Breakpoint cluster region/Abelson (BCR/ABL) tyrosine kinase enhances the ability of leukemia cells to infiltrate various organs. We show here that expression of the helix-loop-helix transcription factor Id1 is enhanced by BCR/ABL in a signal transducer and activator of transcription 5 (STAT5)-dependent manner. Enhanced expression of Id1 plays a key role in BCR/ABL-mediated cell invasion. Down-regulation of Id1 in BCR/ABL leukemia cells by the antisense cDNA significantly reduced their invasive capability through the Matrigel membrane and their ability to infiltrate hematopoietic and nonhematopoietic organs resulting in delayed leukemogenesis in mice. The Id1-promoted cell invasiveness was seemingly mediated by matrix metalloproteinase 9 (MMP9). Transactivation of MMP9 promoter in BCR/ABL cells was dependent on Id1 and abrogation of the MMP9 catalytic activity by a metalloproteinase inhibitor or blocking antibody decreased invasive capacity of leukemia cells. These data suggest that BCR/ABL-STAT5-Id1-MMP9 pathway may play a critical role in BCR/ABL-mediated leukemogenesis by enhancing invasiveness of leukemia cells.

Translational regulation by the p210 BCR/ABL oncoprotein

The ability of oncogenic proteins to regulate the rate of translation of specific mRNA subsets may be a rapid and efficient mechanism to modulate the levels and, in many cases, the activity of the corresponding proteins. In the past few years, we have identified several RNA binding proteins with translation regulatory activity whose expression is markedly activated in the blast crisis of chronic myelogenous leukemia, which represents the most malignant disease stage. Perturbation of the activity of some RNA binding proteins suppresses the leukemogenic potential of BCR/ABL-expressing cells. Most importantly, we have identified some of the targets of these RNA binding proteins. Two of these targets, c/ebp alpha and mdm2 mRNAs, are directly relevant for the altered differentiation and survival of leukemic cells. The identification of mRNA targets translationally regulated by RNA binding proteins overexpressed in tumor cells may lead to the development of therapeutic strategies aimed at modulating the translation rate of specific mRNAs.

Chronic myelogenous leukemia molecular signature

To obtain comprehensive information about the genes involved in BCR/ABL-dependent leukemogenesis, samples from 15 chronic myelogenous leukemia (CML) patients and seven normal donors were analysed using a cDNA microarray assay. After subtraction of the artificial, random or cross-hybridization signals, data about 5315 genes have been effectively analysed in all samples. The assay revealed >/=4-fold difference in the average expression of 263 genes in all CML samples when compared to normal counterparts, with 148 genes being upregulated and 115 being downregulated. Differentially expressed genes include those associated with BCR/ABL-induced abnormalities in signal transduction, gene transactivation, cell cycle, apoptosis, adhesion, DNA repair, differentiation, metabolism and malignant progression. Interestingly, CML-blast crisis cells in peripheral blood differ from those from bone marrow, indicating major changes in gene expression profiles upon entering into the bloodstream. Moreover, BCR/ABL modulates expression of genes, which are involved in regulation of chromosome/chromatin/DNA dynamics during S and M cell cycle phase. Moreover, the ability of CML cells to recognize and respond to a pathogen infection may be compromised. Altogether, this work provides a large body of information regarding gene expression profiles associated with CML and also represents a source of potential targets for CML therapeutics.

Selective pyrrolo-pyrimidine inhibitors reveal a necessary role for Src family kinases in Bcr-Abl signal transduction and oncogenesis

Chronic myelogenous leukemia (CML) is defined by the presence of the Philadelphia (Ph) chromosome, which results in the expression of the 210 kDa Bcr-Abl tyrosine kinase. Bcr-Abl constitutively activates several signaling proteins important for the proliferation and survival of myeloid progenitors, including the Src family kinases Hck and Lyn, the Stat5 transcription factor and upstream components of the Ras/Erk pathway. Recently, we found that kinase-defective Hck blocks Bcr-Abl-induced transformation of DAGM myeloid leukemia cells to cytokine independence, suggesting that activation of the Src kinase family may be essential to oncogenic signaling by Bcr-Abl. To investigate the contribution of Src kinases to Bcr-Abl signaling in vivo, we used the pyrrolo-pyrimidine Src kinase inhibitors PP2 and A-419259. Treatment of the Ph+ CML cell lines K-562 and Meg-01 with either compound resulted in growth arrest and induction of apoptosis, while the Ph- leukemia cell lines TF-1 and HEL were unaffected over the same concentration ranges. Suppression of Ph+ cell growth by PP2 and A-419259 correlated with a decrease in Src kinase autophosphorylation. Both inhibitors blocked Stat5 and Erk activation, consistent with the suppressive effects of the compounds on survival and proliferation. In contrast, the phosphotyrosine content of Bcr-Abl and its endogenous substrate CrkL was unchanged at inhibitor concentrations that induced apoptosis, blocked oncogenic signaling and inhibited Src kinases. These data implicate the Src kinase family in Stat5 and Erk activation downstream of Bcr-Abl, and identify myeloid-specific Src kinases as potential drug targets in CML.

Modeling Philadelphia chromosome positive leukemias

The Ph chromosome has been genetically linked to CML and ALL. Its chimeric fusion gene product, BCR-ABL, can generate leukemia in mice. This review will discuss selected model systems developed to study BCR-ABL induced leukemia and focuses on what we have learned about the human disease from these models. Five main experimental approaches will be discussed including: (i) Reconstitution of mice with bone marrow cells retrovirally transduced with BCR-ABL; (ii) Transgenic mice expressing BCR-ABL; (iii) Knock-in mice with BCR-ABL expression driven from the endogenous bcr locus; (iv) Development of CML-like disease in mice with loss of function mutations in heterologous genes; and (v) ES in vitro hematopoietic differentiation coupled with regulated BCR-ABL expression.

Progressive changes in the leukemogenic signaling in BCR/ABL-transformed cells

Our previous study indicated that BCR/ABL SH2 domain and BCR/ABL SH3 domain+SH2 domain complex are required for immediate activation of the phosphatidylinositol-3 kinase PI-3k)--> Akt serine/threonine kinase pathway and of the signal transducer and activator of transcription 5 (STAT5), respectively, in hematopoietic cells. We show here that the defect in activation of PI-3k/Akt by BCR/ABL DeltaSH2 mutant (SH2 domain deleted) and of STAT5 by BCR/ABL DeltaSH3+DeltaSH2 mutant (SH3 and SH2 domains deleted) is not permanent and both Akt and STAT5 could be 're-activated' by in vitro culture. This phenomenon was responsible for increased resistance to apoptosis, growth factor-independent proliferation and leukemogenesis in SCID mice. Incubation of cells with BCR/ABL tyrosine kinase inhibitor STI571 abrogated the 're-activation' of Akt or STAT5 by BCR/ABL SH3+SH2 mutants in some clones, in the others Akt and STAT5 activation became independent on BCR/ABL kinase activity. The immediate upstream activators of Akt and STAT5 such as PI-3k and Jak-2 were also activated. In addition, the common beta subunit of IL-3/IL-5/GM-CSF receptor was tyrosine phosphorylated in the clones in which 're-activation' was dependent on the BCR/ABL kinase activity. These results suggested that 're-activation' of Akt and STAT5, in the absence of functional BCR/ABL SH3+SH2 domains, may be achieved by two different mechanisms: (i) BCR/ABL kinase-dependent activation of alternative pathway(s) and (ii) additional genetic changes stimulating Akt and STAT5 independently of BCR/ABL. Oncogene (2000) 19, 4117 - 4124

STATs in oncogenesis

Since their discovery as key mediators of cytokine signaling, considerable progress has been made in defining the structure-function relationships of Signal Transducers and Activators of Transcription (STATs). In addition to their central roles in normal cell signaling, recent studies have demonstrated that diverse oncoproteins can activate specific STATs (particularly Stat3 and Stat5) and that constitutively-activated STAT signaling directly contributes to oncogenesis. Furthermore, extensive surveys of primary tumors and cell lines derived from tumors indicate that inappropriate activation of specific STATs occurs with surprisingly high frequency in a wide variety of human cancers. Together, these findings provide compelling evidence that aberrant STAT activation associated with oncogenesis is not merely adventitious but instead contributes to the process of malignant transformation. These studies are beginning to reveal the molecular mechanisms leading to STAT activation in the context of oncogenesis, and candidate genes regulated by STATs that may contribute to oncogenesis are being identified. Recent studies suggest that activated STAT signaling participates in oncogenesis by stimulating cell proliferation and preventing apoptosis. This review presents the evidence for critical roles of STATs in oncogenesis and discusses the potential for development of novel cancer therapies based on mechanistic understanding of STAT signaling. Oncogene (2000).