Alternative signals to RAS for hematopoietic transformation by the BCR-ABL oncogene

In vivo eradication of human BCR/ABL-positive leukemia cells with an ABL kinase inhibitor

BACKGROUND

The leukemia cells of approximately 95% of patients with chronic myeloid leukemia and 30%-50% of adult patients with acute lymphoblastic leukemia express the Bcr/Abl oncoprotein, which is the product of a fusion gene created by a chromosomal translocation [(9:22) (q34;q11)]. This oncoprotein expresses a constitutive tyrosine kinase activity that is crucial for its cellular transforming activity. In this study, we evaluated the antineoplastic activity of CGP57148B, which is a competitive inhibitor of the Bcr/Abl tyrosine kinase.

METHODS

Nude mice were given an injection of the Bcr/Abl-positive human leukemia cell lines KU812 or MC3. Tumor-bearing mice were treated intraperitoneally or orally with CGP57148B according to three different schedules. In vitro drug wash-out experiments and in vivo molecular pharmacokinetic experiments were performed to optimize the in vivo treatment schedule.

RESULTS

Treatment schedules administering CGP57148B once or twice per day produced some inhibition of tumor growth, but no tumor-bearing mouse was cured. A single administration of CGP57148B caused substantial (>50%) but short-lived (2-5 hours) inhibition of Bcr/Abl kinase activity. On the basis of the results from in vitro wash-out experiments, 20-21 hours was defined as the duration of continuous exposure needed to block cell proliferation and to induce apoptosis in these two leukemia cell lines. A treatment regimen assuring the continuous block of the Bcr/Abl phosphorylating activity that was administered over an 11-day period cured 87%-100% of treated mice.

CONCLUSION

These data indicate that the continuous block of the oncogenic tyrosine kinase of Bcr/Abl protein is needed to produce important biologic effects in vivo.

Nucleophosmin-anaplastic lymphoma kinase of large-cell anaplastic lymphoma is a constitutively active tyrosine kinase that utilizes phospholipase C-gamma to mediate its mitogenicity

Large-cell anaplastic lymphoma is a subtype of non-Hodgkin's lymphoma characterized by the expression of CD30. More than half of these lymphomas have a chromosomal translocation, t(2;5), that leads to the expression of a hybrid protein comprised of the nucleolar phosphoprotein nucleophosmin (NPM) and the anaplastic lymphoma kinase (ALK). Here we show that transfection of the constitutively active tyrosine kinase NPM-ALK into Ba/F3 and Rat-1 cells leads to a transformed phenotype. Oncogenic tyrosine kinases transform cells by activating the mitogenic signal transduction pathways, e.g., by binding and activating SH2-containing signaling molecules. We found that NPM-ALK binds most specifically to the SH2 domains of phospholipase C-gamma (PLC-gamma) in vitro. Furthermore, we showed complex formation of NPM-ALK and PLC-gamma in vivo by coimmunoprecipitation experiments in large-cell anaplastic lymphoma cells. This complex formation leads to the tyrosine phosphorylation and activation of PLC-gamma, which can be corroborated by enhanced production of inositol phosphates (IPs) in NPM-ALK-expressing cells. By phosphopeptide competition experiments, we were able to identify the tyrosine residue on NPM-ALK responsible for interaction with PLC-gamma as Y664. Using site-directed mutagenesis, we constructed a comprehensive panel of tyrosine-to-phenylalanine NPM-ALK mutants, including NPM-ALK(Y664F). NPM-ALK(Y664F), when transfected into Ba/F3 cells, no longer forms complexes with PLC-gamma or leads to PLC-gamma phosphorylation and activation, as confirmed by low IP levels in these cells. Most interestingly, Ba/F3 and Rat-1 cells expressing NPM-ALK(Y664F) also show a biological phenotype in that they are not stably transformed. Overexpression of PLC-gamma can partially rescue the proliferative response of Ba/F3 cells to the NPM-ALK(Y664F) mutant. Thus, PLC-gamma is an important downstream target of NPM-ALK that contributes to its mitogenic activity and is likely to be important in the molecular pathogenesis of large-cell anaplastic lymphomas.

Efficient and Rapid Induction of a Chronic Myelogenous Leukemia-Like Myeloproliferative Disease in Mice Receiving P210 bcr/abl-Transduced Bone Marrow

Expression of the 210-kD bcr/abl fusion oncoprotein can cause a chronic myelogenous leukemia (CML)-like disease in mice receiving bone marrow cells transduced by bcr/abl-encoding retroviruses. However, previous methods failed to yield this disease at a frequency sufficient enough to allow for its use in the study of CML pathogenesis. To overcome this limitation, we have developed an efficient and reproducible method for inducing a CML-like disease in mice receiving P210 bcr/abl-transduced bone marrow cells. All mice receiving P210 bcr/abl-transduced bone marrow cells succumb to a myeloproliferative disease between 3 and 5 weeks after bone marrow transplantation. The myeloproliferative disease recapitulates many of the hallmarks of human CML and is characterized by high white blood cell counts and extensive extramedullary hematopoiesis in the spleen, liver, bone marrow, and lungs. Use of a retroviral vector coexpressing P210 bcr/abl and green fluorescent protein shows that the vast majority of bcr/abl-expressing cells are myeloid. Analysis of the proviral integration pattern shows that, in some mice, the myeloproliferative disease is clonal. In multiple mice, the CML-like disease has been transplantable, inducing a similar myeloproliferative syndrome within 1 month of transfer to sublethally irradiated syngeneic recipients. The disease in many of these mice has progressed to the development of acute lymphoma/leukemia resembling blast crisis. These results demonstrate that murine CML recapitulates important features of human CML. As such, it should be an excellent model for addressing specific issues relating to the pathogenesis and treatment of this disease.

Efficient and Rapid Induction of a Chronic Myelogenous Leukemia-Like Myeloproliferative Disease in Mice Receiving P210 bcr/abl-Transduced Bone Marrow

Expression of the 210-kD bcr/abl fusion oncoprotein can cause a chronic myelogenous leukemia (CML)-like disease in mice receiving bone marrow cells transduced by bcr/abl-encoding retroviruses. However, previous methods failed to yield this disease at a frequency sufficient enough to allow for its use in the study of CML pathogenesis. To overcome this limitation, we have developed an efficient and reproducible method for inducing a CML-like disease in mice receiving P210 bcr/abl-transduced bone marrow cells. All mice receiving P210 bcr/abl-transduced bone marrow cells succumb to a myeloproliferative disease between 3 and 5 weeks after bone marrow transplantation. The myeloproliferative disease recapitulates many of the hallmarks of human CML and is characterized by high white blood cell counts and extensive extramedullary hematopoiesis in the spleen, liver, bone marrow, and lungs. Use of a retroviral vector coexpressing P210 bcr/abl and green fluorescent protein shows that the vast majority of bcr/abl-expressing cells are myeloid. Analysis of the proviral integration pattern shows that, in some mice, the myeloproliferative disease is clonal. In multiple mice, the CML-like disease has been transplantable, inducing a similar myeloproliferative syndrome within 1 month of transfer to sublethally irradiated syngeneic recipients. The disease in many of these mice has progressed to the development of acute lymphoma/leukemia resembling blast crisis. These results demonstrate that murine CML recapitulates important features of human CML. As such, it should be an excellent model for addressing specific issues relating to the pathogenesis and treatment of this disease.

A DNA damage and stress inducible G protein-coupled receptor blocks cells in G2/M

Cell cycle progression is monitored by highly coordinated checkpoint machinery, which is activated to induce cell cycle arrest until defects like DNA damage are corrected. We have isolated an anti-proliferative cell cycle regulator named G2A (for G2 accumulation), which is predominantly expressed in immature T and B lymphocyte progenitors and is a member of the seven membrane-spanning G protein-coupled receptor family. G2A overexpression attenuates the transformation potential of BCR-ABL and other oncogenes, and leads to accumulation of cells at G2/M independently of p53 and c-Abl. G2A can be induced in lymphocytes and to a lesser extent in nonlymphocyte cell lines or tissues by multiple stimuli including different classes of DNA-damaging agents and serves as a response to damage and cellular stimulation which functions to slow cell cycle progression.

BCR/ABL-mediated leukemogenesis requires the activity of the small GTP-binding protein Rac

The phenotype of hematopoietic cells transformed by the BCR/ABL oncoprotein of the Philadelphia chromosome is characterized by growth factor-independent proliferation, reduced susceptibility to apoptosis, and altered adhesion and motility. The mechanisms underlying this phenotype are not fully understood, but there is evidence that some of the properties of BCR/ABL-expressing cells are dependent on the activation of downstream effector molecules such as RAS, PI-3k, and bcl-2. We show here that the small GTP-binding protein Rac is activated by BCR/ABL in a tyrosine kinase-dependent manner. Upon transfection with a vector carrying the dominant-negative N17Rac, BCR/ABL-expressing myeloid precursor 32Dcl3 cells retained the resistance to growth factor deprivation-induced apoptosis but showed a decrease in proliferative potential in the absence of interleukin-3 (IL-3) and markedly reduced invasive properties. Moreover, compared with BCR/ABL-expressing cells, fewer BCR/ABL plus N17Rac double transfectants were capable of homing to bone marrow and spleen. Consistent with these findings, survival of SCID mice injected with the BCR/ABL plus N17Rac double transfectants was markedly prolonged as compared with that of mice injected with BCR/ABL-expressing cells. Together, these data support the important role of a Rac-dependent pathway(s) controlling motility in BCR/ABL-mediated leukemogenesis.

Immune response to Philadelphia chromosome-positive acute lymphoblastic leukemia induced by expression of CD80, interleukin 2, and granulocyte-macrophage colony-stimulating factor

We examined the potential of generating an immune response against Philadelphia chromosome-positive acute lymphoblastic leukemia. The immunostimulatory molecules chosen for this study were the cytokines IL-2 and GM-CSF and the costimulatory ligand CD80 (B7.1). We used a murine model based on a BALB/c pre-B cell line, BM185wt, in which leukemia is induced by the p185 BCR-ABL oncogenic product, which reproduces Philadelphia chromosome-positive ALL. BM185wt cells were transduced with retroviral vectors and the transduced clones expressing mIL-2, mGM-CSF, or mCD80 were used for challenge. Expression of the immunomodulators by BM185 cells was correlated with delay in leukemia development in immunocompetent mice, but not in immunodeficient mice, indicating an immune response against the modified leukemia cells. Expression of CD80 caused leukemia rejection in 50% of the cohort, which was associated with the CD4+ and CD8+ T cell-dependent development of anti-leukemia cytotoxic T lymphocytes. Furthermore, mice surviving the BM185/CD80 challenge or preimmunized with irradiated BM185/CD80 cells developed an immune response against subsequent challenge with the parental leukemia. These studies provide evidence that immunotherapeutic approaches can be developed for the treatment of ALL.

Transformation of hematopoietic cell lines to growth-factor independence and induction of a fatal myelo- and lymphoproliferative disease in mice by retrovirally transduced TEL/JAK2 fusion genes

Recent reports have demonstrated fusion of the TEL gene on 12p13 to the JAK2 gene on 9p24 in human leukemias. Three variants have been identified that fuse the TEL pointed (PNT) domain to (i) the JAK2 JH1-kinase domain, (ii) part of and (iii) all of the JH2 pseudokinase domain. We report that all of the human TEL/JAK2 variants, and a human/mouse chimeric hTEL/mJAK2(JH1) fusion gene, transform the interleukin-3 (IL-3)-dependent murine hematopoietic cell line Ba/F3 to IL-3-independent growth. Transformation requires both the TEL PNT domain and JAK2 kinase activity. Furthermore, all TEL/JAK2 variants strongly activated STAT 5 by phosphotyrosine Western blots and by electrophoretic mobility shift assays (EMSA). Mice (n = 40) transplanted with bone marrow infected with the MSCV retrovirus containing either the hTEL/mJAK2(JH1) fusion or its human counterpart developed a fatal mixed myeloproliferative and T-cell lymphoproliferative disorder with a latency of 2-10 weeks. In contrast, mice transplanted with a TEL/JAK2 mutant lacking the TEL PNT domain (n = 10) or a kinase-inactive TEL/JAK2(JH1) mutant (n = 10) did not develop the disease. We conclude that all human TEL/JAK2 fusion variants are oncoproteins in vitro that strongly activate STAT 5, and cause lethal myelo- and lymphoproliferative syndromes in murine bone marrow transplant models of leukemia.

SLP-76-Cbl-Grb2-Shc Interactions in FcγRI Signaling

SLP-76 and Cbl are complex adapter proteins that have the capacity to bind to smaller adapter proteins, such as Grb2, which subsequently binds the nucleotide exchange protein Sos in the transmission of intracellular signals. SLP-76, Cbl, Shc, and Grb2 have been implicated in immunoreceptor tyrosine-based activation motif (ITAM) signaling, leading to activation of Ras. However, their mechanism of action has not been determined. To date, there have been no reports of SLP-76 involvement in FcγRI-receptor signaling and no data exist for an interaction between Cbl, Shc, and SLP-76 in vivo. We provide evidence that SLP-76, Cbl, and Shc are tyrosine phosphorylated on FcγRI-receptor stimulation and are associated with the adapter protein Grb2 in γ-interferon–differentiated U937 cells (U937IF). The interactions between SLP-76 and Cbl and SLP-76 and Grb2 are present in resting U937IF cells. However, the interaction between SLP-76 and Grb2 becomes augmented twofold on FcγRI-receptor aggregation. Our results provide the first evidence for a phosphorylation-dependent interaction between SLP-76 and Shc, induced at least 10-fold on FcγRI receptor stimulation. Our data indicate that a significant portion of a multimolecular complex containing Cbl, SLP-76, Shc, and Grb2 is distinct from a trimolecular complex containing the Ras guanine nucleotide exchanger Sos, Shc, and Grb2. FcγRI-induced tyrosine phosphorylation of SLP-76, Cbl, Shc, and the highly induced SLP-76-Shc interaction provide the first evidence that SLP-76 and Cbl are involved in FcγRI signaling and suggest a functional significance for these interactions in FcγRI signal relay in the control of Ras in myeloid cells.

© 1998 by The American Society of Hematology.

SLP-76-Cbl-Grb2-Shc Interactions in FcγRI Signaling

SLP-76 and Cbl are complex adapter proteins that have the capacity to bind to smaller adapter proteins, such as Grb2, which subsequently binds the nucleotide exchange protein Sos in the transmission of intracellular signals. SLP-76, Cbl, Shc, and Grb2 have been implicated in immunoreceptor tyrosine-based activation motif (ITAM) signaling, leading to activation of Ras. However, their mechanism of action has not been determined. To date, there have been no reports of SLP-76 involvement in FcγRI-receptor signaling and no data exist for an interaction between Cbl, Shc, and SLP-76 in vivo. We provide evidence that SLP-76, Cbl, and Shc are tyrosine phosphorylated on FcγRI-receptor stimulation and are associated with the adapter protein Grb2 in γ-interferon–differentiated U937 cells (U937IF). The interactions between SLP-76 and Cbl and SLP-76 and Grb2 are present in resting U937IF cells. However, the interaction between SLP-76 and Grb2 becomes augmented twofold on FcγRI-receptor aggregation. Our results provide the first evidence for a phosphorylation-dependent interaction between SLP-76 and Shc, induced at least 10-fold on FcγRI receptor stimulation. Our data indicate that a significant portion of a multimolecular complex containing Cbl, SLP-76, Shc, and Grb2 is distinct from a trimolecular complex containing the Ras guanine nucleotide exchanger Sos, Shc, and Grb2. FcγRI-induced tyrosine phosphorylation of SLP-76, Cbl, Shc, and the highly induced SLP-76-Shc interaction provide the first evidence that SLP-76 and Cbl are involved in FcγRI signaling and suggest a functional significance for these interactions in FcγRI signal relay in the control of Ras in myeloid cells.

© 1998 by The American Society of Hematology.

TLS/FUS, a pro-oncogene involved in multiple chromosomal translocations, is a novel regulator of BCR/ABL-mediated leukemogenesis

The leukemogenic potential of BCR/ABL oncoproteins depends on their tyrosine kinase activity and involves the activation of several downstream effectors, some of which are essential for cell transformation. Using electrophoretic mobility shift assays and Southwestern blot analyses with a double-stranded oligonucleotide containing a zinc finger consensus sequence, we identified a 68 kDa DNA-binding protein specifically induced by BCR/ABL. The peptide sequence of the affinity-purified protein was identical to that of the RNA-binding protein FUS (also called TLS). Binding activity of FUS required a functional BCR/ABL tyrosine kinase necessary to induce PKCbetaII-dependent FUS phosphorylation. Moreover, suppression of PKCbetaII activity in BCR/ABL-expressing cells by treatment with the PKCbetaII inhibitor CGP53353, or by expression of a dominant-negative PKCbetaII, markedly impaired the ability of FUS to bind DNA. Suppression of FUS expression in myeloid precursor 32Dcl3 cells transfected with a FUS antisense construct was associated with upregulation of the granulocyte-colony stimulating factor receptor (G-CSFR) and downregulation of interleukin-3 receptor (IL-3R) beta-chain expression, and accelerated G-CSF-stimulated differentiation. Downregulation of FUS expression in BCR/ABL-expressing 32Dcl3 cells was associated with suppression of growth factor-independent colony formation, restoration of G-CSF-induced granulocytic differentiation and reduced tumorigenic potential in vivo. Together, these results suggest that FUS might function as a regulator of BCR/ABL leukemogenesis, promoting growth factor independence and preventing differentiation via modulation of cytokine receptor expression.

BLNK: a central linker protein in B cell activation

Linker or adapter proteins provide mechanisms by which receptors can amplify and regulate downstream effector proteins. We describe here the identification of a novel B cell linker protein, termed BLNK, that interfaces the B cell receptor-associated Syk tyrosine kinase with PLCgamma, the Vav guanine nucleotide exchange factor, and the Grb2 and Nck adapter proteins. Tyrosine phosphorylation of BLNK by Syk provides docking sites for these SH2-containing effector molecules that, in turn, permits the phosphorylation and/or activation of their respective signaling pathways. Hence, BLNK represents a central linker protein that bridges the B cell receptor-associated kinases with a multitude of signaling pathways and may regulate the biologic outcomes of B cell function and development.

Haemopoietic transformation by the TEL/ABL oncogene

Rare, novel forms of activated ABL kinase, the result of a fusion between TEL (or ETV6, a member of the ETS transcription factor family), and the non-receptor tyrosine kinase ABL, have been identified. We have analysed the TEL/ABL fusion protein (type A) cloned from an acute lymphoblastic leukaemia patient. In contrast to a second TEL/ABL fusion (type B) identified in two cases of myeloid leukaemia, the portion of TEL contained in the type A TEL/ABL fusion was smaller and did not contain a potential Grb2 binding site. The type A TEL/ABL cDNA we used in this study encoded a 155 kD protein with elevated tyrosine kinase activity and was responsible for the phosphorylation of a number of proteins in vivo. Its expression in factor-dependent murine haemopoietic precursor cells efficiently converted these cells to factor independence for both survival and growth. These cells continued to express high levels of myc mRNA after growth factor depletion. We also demonstrated that type A TEL/ABL self-associated in stably expressing haemopoietic cells. Although the TEL portion of the TEL/ABL fusion protein has no sequence similarity to that of BCR in the BCR/ABL protein, all forms of these fusion proteins contain a structure implicated in oligomerization. Our results support the conclusion that the protein interaction domain of BCR and TEL, but not the Grb2 binding site, are the important functional components in the activation of ABL kinase in haemopoietic discase.

Secondary Mutation Maintains the Transformed State in BaF3 Cells With Inducible BCR/ABL Expression

The BCR/ABL gene product of the Philadelphia (Ph) chromosome induces chronic myelogenous leukemia (CML). We generated a hematopoietic cell line, TonB210.1, with tetracycline-dependent BCR/ABL expression to investigate the pathways by which BCR/ABL transforms cells. TonB210.1 demonstrates conditional growth factor independence in tissue culture and rapidly forms tumors in mice fed the tetracycline analog doxycycline. The tumors regress completely upon doxycycline withdrawal, but ultimately reform in all animals. After a long latency, tumors also develop in animals never exposed to doxycycline. Subclones of TonB210.1 established from doxycycline-independent tumors demonstrate distinct mechanisms of transformation. Most subclones manifest increased basal levels of BCR/ABL expression; some have lost the capacity to augment expression upon induction, whereas others remain inducible. More interestingly, some subclones maintain tight conditional expression of BCR/ABL and are therefore transformed by secondary mechanisms that no longer require BCR/ABL expression. These subclones show constitutive phosphorylation of the STAT5 protein, suggesting that activating mutations have occurred upstream in the signaling pathway to STAT5. The tight conditional expression of BCR/ABL in the TonB210.1 cell line affords the opportunity to study several interesting aspects of the biology of BCR/ABL, including activation of critical signaling pathways and transcriptional programs, and its potential role in genomic instability.

Secondary Mutation Maintains the Transformed State in BaF3 Cells With Inducible BCR/ABL Expression

The BCR/ABL gene product of the Philadelphia (Ph) chromosome induces chronic myelogenous leukemia (CML). We generated a hematopoietic cell line, TonB210.1, with tetracycline-dependent BCR/ABL expression to investigate the pathways by which BCR/ABL transforms cells. TonB210.1 demonstrates conditional growth factor independence in tissue culture and rapidly forms tumors in mice fed the tetracycline analog doxycycline. The tumors regress completely upon doxycycline withdrawal, but ultimately reform in all animals. After a long latency, tumors also develop in animals never exposed to doxycycline. Subclones of TonB210.1 established from doxycycline-independent tumors demonstrate distinct mechanisms of transformation. Most subclones manifest increased basal levels of BCR/ABL expression; some have lost the capacity to augment expression upon induction, whereas others remain inducible. More interestingly, some subclones maintain tight conditional expression of BCR/ABL and are therefore transformed by secondary mechanisms that no longer require BCR/ABL expression. These subclones show constitutive phosphorylation of the STAT5 protein, suggesting that activating mutations have occurred upstream in the signaling pathway to STAT5. The tight conditional expression of BCR/ABL in the TonB210.1 cell line affords the opportunity to study several interesting aspects of the biology of BCR/ABL, including activation of critical signaling pathways and transcriptional programs, and its potential role in genomic instability.

Protein tyrosine phosphatase 1B antagonizes signalling by oncoprotein tyrosine kinase p210 bcr-abl in vivo

The p210 bcr-abl protein tyrosine kinase (PTK) appears to be directly responsible for the initial manifestations of chronic myelogenous leukemia (CML). In contrast to the extensive characterization of the PTK and its effects on cell function, relatively little is known about the nature of the protein tyrosine phosphatases (PTPs) that may modulate p210 bcr-abl-induced signalling. In this study, we have demonstrated that expression of PTP1B is enhanced specifically in various cells expressing p210 bcr-abl, including a cell line derived from a patient with CML. This effect on expression of PTP1B required the kinase activity of p210 bcr-abl and occurred rapidly, concomitant with maximal activation of a temperature-sensitive mutant of the PTK. The effect is apparently specific for PTP1B since, among several PTPs tested, we detected no change in the levels of TCPTP, the closest relative of PTP1B. We have developed a strategy for identification of physiological substrates of individual PTPs which utilizes substrate-trapping mutant forms of the enzymes that retain the ability to bind to substrate but fail to catalyze efficient dephosphorylation. We have observed association between a substrate-trapping mutant of PTP1B (PTP1B-D181A) and p210 bcr-abl, but not v-Abl, in a cellular context. Consistent with the trapping data, we observed dephosphorylation of p210 bcr-abl, but not v-Abl, by PTP1B in vivo. We have demonstrated that PTP1B inhibited binding of the adapter protein Grb2 to p210 bcr-abl and suppressed p210 bcr-abl-induced transcriptional activation that is dependent on Ras. These results illustrate selectivity in the effects of PTPs in a cellular context and suggest that PTP1B may function as a specific, negative regulator of p210 bcr-abl signalling in vivo.

A requirement for NF-kappaB activation in Bcr-Abl-mediated transformation

Bcr-Abl is a chimeric oncoprotein that is strongly implicated in acute lymphoblastic (ALL) and chronic myelogenous leukemias (CML). This deregulated tyrosine kinase selectively causes hematopoietic disorders resembling human leukemias in animal models and transforms fibroblasts and hematopoietic cells in culture. Bcr-Abl also protects cells from death induced on cytokine deprivation or exposure to DNA damaging agents. In addition, the antiapoptotic function of Bcr-Abl is thought to play a necessary role in hematopoietic transformation and potentially in leukemogenesis. The transcription factor NF-kappaB has been identified recently as an inhibitor of apoptosis and as a potential regulator of cellular transformation. This study shows that expression of Bcr-Abl leads to activation of NF-kappaB-dependent transcription by causing nuclear translocation of NF-kappaB as well as by increasing the transactivation function of the RelA/p65 subunit of NF-kappaB. Importantly, this activation is dependent on the tyrosine kinase activity of Bcr-Abl and partially requires Ras. The ability of Bcr-Abl to protect cytokine-dependent 32D myeloid cells from death induced by cytokine deprivation or DNA damage does not, however, require functional NF-kappaB. However, using a super-repressor form of IkappaBalpha, we show that NF-kappaB is required for Bcr-Abl-mediated tumorigenicity in nude mice and for transformation of primary bone marrow cells. This study implicates NF-kappaB as an important component of Bcr-Abl signaling. NF-kappaB-regulated genes, therefore, likely play a role in transformation by Bcr-Abl and thus in Bcr-Abl-associated human leukemias.

Biology of chronic myelogenous leukemia

This article reviews the biology of chronic myelogenous leukemia (CML) and its effect on the process of hematopoiesis. The relevance of the BCR-ABL fusion protein as well as murine models are also discussed. CML has been studied more extensively than any other malignancy, yet the correlation between the clinical symptoms of chronic phase CML and the BCR-ABL oncoprotein is poorly understood. Insights from recent efforts both to develop a good in vivo animal model and to characterize the effect of the BCR-ABL oncoprotein on relevant signal molecules may lead to a better understanding of the pathophysiology of chronic phase CML and, thereby, to the development of targeted therapeutic approaches.

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

To determine the possible role of the BCR/ABL oncoprotein SH3 domain in BCR/ABL-dependent leukemogenesis, we studied the biologic properties of a BCR/ABL SH3 deletion mutant (▵SH3 BCR/ABL) constitutively expressed in murine hematopoietic cells. ▵SH3 BCR/ABL was able to activate known BCR/ABL-dependent downstream effector molecules such as RAS, PI-3kinase, MAPK, JNK, MYC, JUN, STATs, and BCL-2. Moreover, expression of ▵SH3 BCR/ABL protected 32Dcl3 murine myeloid precursor cells from apoptosis, induced their growth factor-independent proliferation, and resulted in transformation of primary bone marrow cells in vitro. Unexpectedly, leukemic growth from cells expressing ▵SH3 BCR/ABL was significantly retarded in SCID mice compared with that of cells expressing the wild-type protein. In vitro and in vivo studies to determine the adhesive and invasive properties of ▵SH3 BCR/ABL-expressing cells showed their decreased interaction to collagen IV- and laminin-coated plates and their reduced capacity to invade the stroma and to seed the bone marrow and spleen. The decreased interaction with collagen type IV and laminin was consistent with a reduced expression of α2 integrin by ▵SH3 BCR/ABL-transfected 32Dcl3 cells. Moreover, as compared with wild-type BCR/ABL, which localizes primarily in the cytoskeletal/ membrane fraction, ▵SH3 BCR/ABL was more evenly distributed between the cytoskeleton/membrane and the cytosol compartments. Together, the data indicate that the SH3 domain of BCR/ABL is dispensable for in vitro transformation of hematopoietic cells but is essential for full leukemogenic potential in vivo.

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

To determine the possible role of the BCR/ABL oncoprotein SH3 domain in BCR/ABL-dependent leukemogenesis, we studied the biologic properties of a BCR/ABL SH3 deletion mutant (▵SH3 BCR/ABL) constitutively expressed in murine hematopoietic cells. ▵SH3 BCR/ABL was able to activate known BCR/ABL-dependent downstream effector molecules such as RAS, PI-3kinase, MAPK, JNK, MYC, JUN, STATs, and BCL-2. Moreover, expression of ▵SH3 BCR/ABL protected 32Dcl3 murine myeloid precursor cells from apoptosis, induced their growth factor-independent proliferation, and resulted in transformation of primary bone marrow cells in vitro. Unexpectedly, leukemic growth from cells expressing ▵SH3 BCR/ABL was significantly retarded in SCID mice compared with that of cells expressing the wild-type protein. In vitro and in vivo studies to determine the adhesive and invasive properties of ▵SH3 BCR/ABL-expressing cells showed their decreased interaction to collagen IV- and laminin-coated plates and their reduced capacity to invade the stroma and to seed the bone marrow and spleen. The decreased interaction with collagen type IV and laminin was consistent with a reduced expression of α2 integrin by ▵SH3 BCR/ABL-transfected 32Dcl3 cells. Moreover, as compared with wild-type BCR/ABL, which localizes primarily in the cytoskeletal/ membrane fraction, ▵SH3 BCR/ABL was more evenly distributed between the cytoskeleton/membrane and the cytosol compartments. Together, the data indicate that the SH3 domain of BCR/ABL is dispensable for in vitro transformation of hematopoietic cells but is essential for full leukemogenic potential in vivo.

Signal Transduction by the Philadelphia Chromosome-encoded BCR/ABL Oncoproteins: Therapeutic Implications for Chronic Myeloid Leukemia and Philadelphia-positive Acute Lymphoblastic Leukemia

The Philadelphia chromosomes characteristic of chronic myeloid leukemia (CML) and Philadelphia-positive acute lymphoblastic leukemia (ALL) encode chimeric protein tyrosine kinases (PTKs) derived by fusion of the normal BCR and ABL genes. The oncogenic properties of these BCR/ABL oncoproteins are dependent on their elevated PTK activity and on their ability to interact with multiple signal transduction systems. Here we summarize some of the key pathways which are activated by normal receptors with PTK activity and which modulate cell proliferation and survival. Next, we review some of the biochemical pathways initiated by BCR/ABL oncoproteins and discuss their possible relevance to the leukemic phenotype. We finally review experimental approaches designed to suppress signalling by BCR/ABL oncoproteins and discuss their potential therapeutic applications.

Molecular genetics of childhood leukemias

PURPOSE

This review summarizes the molecular genetics of childhood leukemias, with emphasis on pathogenesis and clinical applications.

DESIGN

We first describe the most common genetic events that occur in pediatric acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), and chronic myeloid leukemia (CML). We then illustrate how these molecular alterations may be used to alter therapy.

RESULTS

In childhood ALL, the TEL-AML1 fusion and hyperdiploidy are both associated with excellent treatment outcomes and therefore identify patients who may be candidates for less intensive therapy. In contrast, MLL gene rearrangements and the BCR-ABL fusion confer a poor prognosis; these patients may be best treated by allogeneic bone marrow transplantation in first remission.

CONCLUSIONS

Although clinical features are important prognostic indicators, genetic alterations of leukemic blasts may be better predictors of outcome for acute leukemia patients. We therefore favor risk-adapted therapy based on classification schemes that incorporate both genetic and clinical features.

The Src family kinase Hck interacts with Bcr-Abl by a kinase-independent mechanism and phosphorylates the Grb2-binding site of Bcr

bcr-abl, the oncogene causing chronic myeloid leukemia, encodes a fusion protein with constitutively active tyrosine kinase and transforming capacity in hematopoietic cells. Various intracellular signaling intermediates become activated and/or associate by/with Bcr-Abl, including the Src family kinase Hck. To elucidate some of the structural requirements and functional consequences of the association of Bcr-Abl with Hck, their interaction was investigated in transiently transfected COS7 cells. Neither the complex formation of Hck kinase with Bcr-Abl nor the activation of Hck by Bcr-Abl was dependent on the Abl kinase activity. Both inactivating point mutations of Hck and dephosphorylation of Hck enhanced its complex formation with Bcr-Abl, indicating that their physical interaction was negatively regulated by Hck (auto)phosphorylation. Finally, experiments with a series of kinase negative Bcr-Abl mutants showed that Hck phosphorylated Bcr-Abl and induced the binding of Grb2 to Tyr177 of Bcr-Abl. Taken together, our results suggest that Bcr-Abl preferentially binds inactive forms of Hck by an Abl kinase-independent mechanism. This physical interaction stimulates the Hck tyrosine kinase, which may then phosphorylate the Grb2-binding site in Bcr-Abl.

Gene Therapy for Chronic Myelogenous Leukemia (CML): A Retroviral Vector That Renders Hematopoietic Progenitors Methotrexate-Resistant and CML Progenitors Functionally Normal and Nontumorigenic In Vivo

Chronic myelogenous leukemia (CML) is a malignant disease of the human hematopoietic stem cell caused by the BCR/ABL gene rearrangement. The only curative therapy is allogeneic transplantation. Although autologous transplants may prolong survival, most patients relapse because of disease persisting in the host and in the graft. Continued administration of chemotherapy after transplant could reduce the incidence of relapse provided that the autograft can be protected by transfer of a drug-resistance gene. However, CML autografts will almost certainly contain malignant stem cells that will also be rendered drug-resistant. The presence of the BCR/ABL oncoprotein is necessary and sufficient for malignant transformation seen in CML. We thus hypothesized that transfer of a vector that combines a drug-resistance gene with anti-BCR/ABL antisense (AS) sequences may allow for posttransplant chemotherapy to decrease persistent disease while rendering inadvertently transduced CML stem and progenitor cells functionally normal. We constructed a retroviral vector, LasBD, that combines the methotrexate (MTX)-resistant tyrosine-22 dihydrofolate-reductase (tyr22-DHFR) gene and AS sequences directed at the b3a2 BCR/ABL breakpoint. b3a2 BCR/ABL containing 32D and MO7e cells were transduced with LasBD and selected in MTX for 14 days. Expression of the AS sequences reduced BCR/ABL mRNA and p210BCR/ABL protein levels by 6- to 10-fold in most cells. This subsequently led to the restoration of normal function of BCR/ABL cDNA+ cells: they grew significantly slower in the presence of interleukin-3 (IL-3); they underwent apoptotic cell death when cultured without IL-3; and they had restored expression and function of adhesion receptors. These effects were specific, because LasBD-containing AS sequences directed at the b3a2 BCR/ABL breakpoint did not affect p190BCR/ABL-containing cells. LasBD also rendered 20% to 30% of primary Ph− and Ph+ CD34+ cells MTX-resistant and decreased BCR/ABL mRNA levels in MTX resistant Ph+ CD34+ cells by 10-fold. Expression of the MTX-resistant DHFR gene and the AS sequences has been stable for at least 1 year in vitro and for more than 70 days in vivo. Finally, LasBD decreased tumorigenicity of 32DBCR/ABL cells in vivo by 3 to 4 logs. In conclusion, the tyr22-DHFR gene in the LasBD vector can protect normal hematopoietic cells from MTX-mediated toxicity, whereas the AS sequences in LasBD can suppress expression of the BCR/ABL gene and restore normal function of BCR/ABL cDNA-containing cells. The LasBD vector may therefore prove to be an extremely useful adjunct in autologous transplantation for CML.

Gene Therapy for Chronic Myelogenous Leukemia (CML): A Retroviral Vector That Renders Hematopoietic Progenitors Methotrexate-Resistant and CML Progenitors Functionally Normal and Nontumorigenic In Vivo

Chronic myelogenous leukemia (CML) is a malignant disease of the human hematopoietic stem cell caused by the BCR/ABL gene rearrangement. The only curative therapy is allogeneic transplantation. Although autologous transplants may prolong survival, most patients relapse because of disease persisting in the host and in the graft. Continued administration of chemotherapy after transplant could reduce the incidence of relapse provided that the autograft can be protected by transfer of a drug-resistance gene. However, CML autografts will almost certainly contain malignant stem cells that will also be rendered drug-resistant. The presence of the BCR/ABL oncoprotein is necessary and sufficient for malignant transformation seen in CML. We thus hypothesized that transfer of a vector that combines a drug-resistance gene with anti-BCR/ABL antisense (AS) sequences may allow for posttransplant chemotherapy to decrease persistent disease while rendering inadvertently transduced CML stem and progenitor cells functionally normal. We constructed a retroviral vector, LasBD, that combines the methotrexate (MTX)-resistant tyrosine-22 dihydrofolate-reductase (tyr22-DHFR) gene and AS sequences directed at the b3a2 BCR/ABL breakpoint. b3a2 BCR/ABL containing 32D and MO7e cells were transduced with LasBD and selected in MTX for 14 days. Expression of the AS sequences reduced BCR/ABL mRNA and p210BCR/ABL protein levels by 6- to 10-fold in most cells. This subsequently led to the restoration of normal function of BCR/ABL cDNA+ cells: they grew significantly slower in the presence of interleukin-3 (IL-3); they underwent apoptotic cell death when cultured without IL-3; and they had restored expression and function of adhesion receptors. These effects were specific, because LasBD-containing AS sequences directed at the b3a2 BCR/ABL breakpoint did not affect p190BCR/ABL-containing cells. LasBD also rendered 20% to 30% of primary Ph− and Ph+ CD34+ cells MTX-resistant and decreased BCR/ABL mRNA levels in MTX resistant Ph+ CD34+ cells by 10-fold. Expression of the MTX-resistant DHFR gene and the AS sequences has been stable for at least 1 year in vitro and for more than 70 days in vivo. Finally, LasBD decreased tumorigenicity of 32DBCR/ABL cells in vivo by 3 to 4 logs. In conclusion, the tyr22-DHFR gene in the LasBD vector can protect normal hematopoietic cells from MTX-mediated toxicity, whereas the AS sequences in LasBD can suppress expression of the BCR/ABL gene and restore normal function of BCR/ABL cDNA-containing cells. The LasBD vector may therefore prove to be an extremely useful adjunct in autologous transplantation for CML.

Stem cells in chronic myelogenous leukemia

This article discusses briefly the molecular consequences of the BCR-ABL fusion gene. It then reviews the current evidence supporting the notion that chronic myelogenous leukemia in its chronic phase is a clonal, hematopoietic, stem cell disease in which malignant hematopoietic stem and progenitor cells respond to "normal" external proliferation and differentiation stimuli, but in which such responses are altered owing to defects in the stem and progenitor cells as a result of the BCR-ABL oncogene.

Inhibition of the ABL kinase activity blocks the proliferation of BCR/ABL+ leukemic cells and induces apoptosis

The BCR/ABL fusion protein transforms myeloid stem cells. Both chronic myelogenous leukemias (CML) and a subset of acute lymphoblastic leukemias (ALL) are associated with the expression of BCR/ABL proteins. This knowledge has not yet been translated into any specific tool to control ABL driven neoplastic cells growth. CGP57148B is an ATP-competitive inhibitor of the ABL protein kinase; it has been shown to inhibit the kinase activity of ABL both in vitro and in vivo and to inhibit the growth of v-abl and bcr/abl transfectants, as well as the in vitro formation of bone marrow (BM)-derived colonies in the presence of growth factors in some CML patients. These studies were performed to investigate the activity of CGP57148B on the spontaneous proliferation of both fresh and cultured, leukemic and normal, BCR/ABL positive and negative cells, and to study its mechanism of action. Six cell lines derived from BCR/ABL+ leukemias (K562, BV173, KCL22, KU812, MC3, LAMA84), thirteen BCR/ABL negative lines, both neoplastic (KG1, SU-DHL-1, U937, Daudi, NB4, NB4.306) and derived from normal cells (PHA blasts, LAK, fibroblasts, LCL, renal epithelial cells, endothelial cells, CD34(+) cells), and 14 fresh leukemic samples were tested using a tritiated thymidine uptake assay. The in vivo phosphorylation of the BCR/ABL protein was evaluated by western blot, while apoptosis was detected by the annexin V/propidium binding test. The induction of differentiation was assayed by immunofluorescence using multiple antibodies. All six BCR/ABL+ lines showed a dose dependent inhibition of their spontaneous proliferative rate, which was not accompanied by differentiation. The treatment caused, within minutes, dephosphorylation of the BCR/ABL protein, followed in 16-24 hours by a decrease in cycling cells and induction of apoptosis. No significant inhibition of DNA synthesis was observed in any BCR/ABL negative normal or neoplastic line at concentrations </=3 microM, with the exception of fibroblasts and CD34 cells. Proliferation inhibition was observed also when using fresh samples obtained from two Ph+ ALL and 12 consecutive CML patients. Induction of apoptosis was observed in these samples too. The activity of CGP57148B can be monitored in ex vivo isolated or cultured cells using a simple and reproducible assay, without the need for exogenously added growth factors. This molecule possibly exerts its effects through the inhibition of the kinase activity of BCR/ABL and the subsequent initiation of apoptosis, without inducing cell differentiation. Some normal cells are also affected. These data support the use of CGP57148B in initial clinical studies; possible toxic effects on BM and fibroblast-derived cells will have to be closely monitored. The in vivo monitoring of patients will have to be focused on the induction of apoptosis in leukemic cells.

Transformation of hematopoietic cells by BCR/ABL requires activation of a PI-3k/Akt-dependent pathway

The BCR/ABL oncogenic tyrosine kinase activates phosphatidylinositol 3-kinase (PI-3k) by a mechanism that requires binding of BCR/ABL to p85, the regulatory subunit of PI-3k, and an intact BCR/ABL SH2 domain. SH2 domain BCR/ABL mutants deficient in PI-3k activation failed to stimulate Akt kinase, a recently identified PI-3k downstream effector with oncogenic potential, but did activate p21 RAS and p70 S6 kinase. The PI-3k/Akt pathway is essential for BCR/ABL leukemogenesis as indicated by experiments demonstrating that wortmannin, a PI-3k specific inhibitor at low concentrations, suppressed BCR/ABL-dependent colony formation of murine marrow cells, and that a kinase-deficient Akt mutant with dominant-negative activity inhibited BCR/ABL-dependent transformation of murine bone marrow cells in vitro and suppressed leukemia development in SCID mice. In complementation assays using mouse marrow progenitor cells, the ability of transformation-defective SH2 domain BCR/ABL mutants to induce growth factor-independent colony formation and leukemia in SCID mice was markedly enhanced by expression of constitutively active Akt. In retrovirally infected mouse marrow cells, the BCR/ABL mutant lacking the SH2 domain was unable to upregulate the expression of c-Myc and Bcl-2; in contrast, expression of a constitutively active Akt mutant induced Bcl-2 and c-Myc expression, and stimulated the transcription activation function of c-Myc. Together, these data demonstrate the requirement for the BCR/ABL SH2 domain in PI-3k activation and document the essential role of the PI-3k/Akt pathway in BCR/ABL leukemogenesis.

Factors involved in leukaemogenesis and haemopoiesis

This review describes the chromosomal abnormalities in T-cell acute lymphoblastic leukaemia (ALL) which result in the over-expression of the gene SCL, which encodes a helix-loop-helix transcription factor. Also described are how gene targeting studies have revealed a key role for SCL in normal haemopoiesis. Next, the BCR-ABL fusion protein, seen in chronic myeloid leukaemia (CML) and in some patients with ALL, is discussed. Finally, the involvement of members of the core-binding factor (CBF) gene family in leukaemogenesis are described. Members of this gene family are involved in the generation of fusion proteins as a result of t(8;21) and inv(16), the most common translocations associated with acute myeloid leukaemia (AML). They provide a useful model of the way in which aberrant transcriptional function, brought about through genetic alterations, can modify haemopoietic development.

Distinct tyrosine autophosphorylation sites negatively and positively modulate neu-mediated transformation

A number of cytoplasmic signaling molecules are thought to mediate mitogenic signaling from the activated Neu receptor tyrosine kinase through binding specific phosphotyrosine residues located within the intracellular portion of Neu/c-ErbB-2. An activated neu oncogene containing tyrosine-to-phenylalanine substitutions at each of the known autophosphorylation sites was generated and assessed for its specific transforming potential in Rat1 and NIH 3T3 fibroblasts. Mutation of these sites resulted in a dramatic impairment of the transforming potential of neu. To assess the role of these tyrosine phosphorylation sites in cellular transformation, the transforming potential of a series of mutants in which individual tyrosine residues were restored to this transformation-debilitated neu mutant was evaluated. Reversion of any one of four mutated sites to tyrosine residues restored wild-type transforming activity. While each of these transforming mutants displayed Ras-dependent signaling, the transforming activity of two of these mutants was correlated with their ability to bind either the GRB2 or SHC adapter molecules that couple receptor tyrosine kinases to the Ras signaling pathway. By contrast, restoration of a tyrosine residue located at position 1028 completely suppressed the basal transforming activity of this mutated neu molecule or other transforming neu molecules which possessed single tyrosine residues. These data argue that the transforming potential of activated neu is mediated both by positive and negative regulatory tyrosine phosphorylation sites.

Inhibition of Grb2 and Crkl proteins results in growth inhibition of Philadelphia chromosome positive leukemic cells

The Bcr-Abl oncoprotein is necessary for the growth of Philadelphia chromosome positive (Ph+) leukemic cells. The Bcr-Abl protein has been found to bind to SH2/SH3-containing adaptor proteins such as Grb2 and Crkl, and these complexes are believed to activate various signaling pathways. Grb2 and Crkl are important for the Bcr-Abl-mediated transformation of rat fibroblasts and murine hematopoietic cells. We have used liposomes to deliver nuclease-resistant antisense oligonucleotides (oligos) that are specific for the GRB2 or CRKL mRNA to leukemic cells to specifically downregulate Grb2 or Crkl protein expression. We found that by downregulating Grb2 or Crkl protein expression, Grb2 or Crkl antisense oligos could selectively inhibit the growth of Bcr-Abl positive cells, but not that of Bcr-Abl negative cells. Our data, together with other investigators' data, strongly indicate that Grb2 and Crkl are vital for the maintenance of cell growth in Ph+ leukemias.

Inhibition of the Raf-1 kinase by cyclic AMP agonists causes apoptosis of v-abl-transformed cells

Here we investigate the role of the Raf-1 kinase in transformation by the v-abl oncogene. Raf-1 can activate a transforming signalling cascade comprising the consecutive activation of Mek and extracellular-signal-regulated kinases (Erks). In v-abl-transformed cells the endogenous Raf-1 protein was phosphorylated on tyrosine and displayed high constitutive kinase activity. The activities of the Erks were constitutively elevated in both v-raf- and v-abl-transformed cells. In both cell types the activities of Raf-1 and v-raf were almost completely suppressed after activation of the cyclic AMP-dependent kinase (protein kinase A [PKA]), whereas the v-abl kinase was not affected. Raf inhibition substantially diminished the activities of Erks in v-raf-transformed cells but not in v-abl-transformed cells, indicating that v-abl can activate Erks by a Raf-1-independent pathway. PKA activation induced apoptosis in v-abl-transformed cells while reverting v-raf transformation without severe cytopathic effects. Overexpression of Raf-1 in v-abl-transformed cells partially protected the cells from apoptosis induced by PKA activation. In contrast to PKA activators, a Mek inhibitor did not induce apoptosis. The diverse biological responses correlated with the status of c-myc gene expression. v-abl-transformed cells featured high constitutive levels of expression of c-myc, which were not reduced following PKA activation. Myc activation has been previously shown to be essential for transformation by oncogenic Abl proteins. Using estrogen-regulated c-myc and temperature-sensitive Raf-1 mutants, we found that Raf-1 activation could protect cells from c-myc-induced apoptosis. In conclusion, these results suggest (i) that Raf-1 participates in v-abl transformation via an Erk-independent pathway by providing a survival signal which complements c-myc in transformation, and (ii) that cAMP agonists might become useful for the treatment of malignancies where abl oncogenes are involved, such as chronic myeloid leukemias.

Regulation of the oncogenic activity of BCR-ABL by a tightly bound substrate protein RIN1

RIN1 was originally identified by its ability to physically bind to and interfere with activated Ras in yeast. Paradoxically, RIN1 potentiates the oncogenic activity of the BCR-ABL tyrosine kinase in hematopoietic cells and dramatically accelerates BCR-ABL-induced leukemias in mice. RIN1 rescues BCR-ABL mutants for transformation in a manner distinguishable from the cell cycle regulators c-Myc and cyclin D1 and the Ras connector Shc. These biological effects require tyrosine phosphorylation of RIN1 and binding of RIN1 to the Abl-SH2 and SH3 domains. RIN1 is tyrosine phosphorylated and is associated with BCR-ABL in human and murine leukemic cells. RIN1 exemplifies a new class of effector molecules dependent on the concerted action of the SH3, SH2, and catalytic domains of a cytoplasmic tyrosine kinase.

Signal transduction pathways involved in BCR-ABL transformation

BCR-ABL is an oncogenic fusion gene found in patients with chronic myelogenous leukaemia (CML) and acute lymphocytic leukaemia whose oncogenic potential has been demonstrated using in vitro and in vivo model systems. Current research efforts are focused on defining the mechanism by which BCR-ABL transforms cells, with a view toward applying insights from these studies to the treatment of CML patients. BCR-ABL contains tyrosine residues, an SH2 domain, an SH3 domain, and proline-rich sequences. The presence of so many protein-protein interaction domains raises the possibility of multiple contacts with cellular signal transduction pathways. Indeed, BCR-ABL is reported to bind and/or phosphorylate more than 20 proteins. Many of these can be directly linked to signal transduction pathways based on defined roles in other systems, but others have no known function. As the list of such proteins grows, it is critical to define the role of each in the leukaemogenic activity of BCR-ABL. This review summarizes current views of the mechanism of BCR-ABL transformation with emphasis on the substrates and signal transduction pathways affected by its tyrosine kinase activity.

The chimeric BCR-ABL gene

The 1982 discovery that in chronic myeloid leukaemia (CML) the ABL proto-oncogene is translocated to the BCR gene located on chromosome 22 initiated many studies on the structural organization and function of these genes. The nucleotide sequence of the entire BCR and major parts of the ABL gene has now been determined. However, the actual cause of the fusion of BCR with ABL remains essentially unknown. Mouse models have been helpful to unravel the normal cellular function of BCR and ABL, as well the activity of BCR-ABL, although a single mechanism explaining the transforming activity of the latter has not been discovered. The cause of progression of the disease remains unknown, and no single genetic abnormality has been linked to the blast phase of CML. Much has been learned concerning the molecular biology of CML, but answers to the fundamental questions above may be expected in the coming years in parallel to increasing knowledge of genome structure, signal transduction and cell cycle control.

Mutant N-RAS induces erythroid lineage dysplasia in human CD34+ cells

RAS mutations arise at high frequency (20-40%) in both acute myeloid leukemia and myelodysplastic syndrome (which is considered to be a manifestation of preleukemic disease). In each case, mutations arise predominantly at the N-RAS locus. These observations suggest a fundamental role for this oncogene in leukemogenesis. However, despite its obvious significance, little is known of how this key oncogene may subvert the process of hematopoiesis in human cells. Using CD34+ progenitor cells, we have modeled the preleukemic state by infecting these cells with amphotropic retrovirus expressing mutant N-RAS together with the selectable marker gene lacZ. Expression of the lacZ gene product, beta-galactosidase, allows direct identification and study of N-RAS-expressing cells by incubating infected cultures with a fluorogenic substrate for beta-galactosidase, which gives rise to a fluorescent signal within the infected cells. By using multiparameter flow cytometry, we have studied the ability of CD34+ cells expressing mutant N-RAS to undergo erythroid differentiation induced by erythropoietin. By this means, we have found that erythroid progenitor cells expressing mutant N-RAS exhibit a proliferative defect resulting in an increased cell doubling time and a decrease in the proportion of cells in S + G2M phase of the cell cycle. This is linked to a slowing in the rate of differentiation as determined by comparative cell-surface marker analysis and ultimate failure of the differentiation program at the late-erythroblast stage of development. The dyserythropoiesis was also linked to an increased tendency of the RAS-expressing cells to undergo programmed cell death during their differentiation program. This erythroid lineage dysplasia recapitulates one of the most common features of myelodysplastic syndrome, and for the first time provides a causative link between mutational activation of N-RAS and the pathogenesis of preleukemia.

Tyrosine phosphorylation sites at amino acids 239 and 240 of Shc are involved in epidermal growth factor-induced mitogenic signaling that is distinct from Ras/mitogen-activated protein kinase activation

Epidermal growth factor (EGF) induces tyrosine phosphorylation of the Shc adapter protein, which plays an important role in EGF-stimulated mitogenesis. Shc stimulates Ras/mitogen-activated protein kinase (MAPK) through forming a complex with Grb2 at the phosphorylated tyrosine (Y) residue 317. In this study, we identified novel phosphorylation sites of Shc, at Y239 and Y240. To define the Shc pathway further, we used NIH 3T3 cells expressing the previously characterized mutant EGF receptor (EGF-R) which lacks all known autophosphorylation sites but retains EGF-stimulated mitogenesis with selective phosphorylation of Shc. We constructed wild-type (WT) or mutant Shc cDNAs in which Y317 or/and Y239 and Y240 are replaced with phenylalanine (F) and introduced them into NIH 3T3 cells expressing WT or mutant EGF-R. In the WT EGF-R-expressing cells, the Y239/240/317F Shc, but not Y317F or Y239/240F Shc, decreased EGF-stimulated cell growth. In the mutant EGF-R-expressing cells, Y317F Shc or Y239/240F Shc decreased EGF-stimulated cell growth significantly, though Y317F was a little more potent than Y239/240F. Although cells expressing the Y317F Shc hardly activated MAPK in response to EGF, cells expressing the Y239/240F Shc fully activated MAPK. In contrast, Y239/240F Shc, but not Y317F Shc, reduced the EGF-induced c-myc message. These results suggest that Shc activates two distinct signaling pathways, Y317 to Ras/MAPK and Y239 and Y240 to another pathway including Myc, and that both are involved in EGF-induced mitogenic signaling.

Activation of hematopoietic growth factor signal transduction pathways by the human oncogene BCR/ABL

BCR/ABL is a human chimeric oncogene that causes chronic myelogenous leukemia (CML). The BCR/ABL oncogene is generated from the Philadelphia chromosome (Ph) translocation, t(9;22)(q34;q11), and creates a constitutively active tyrosine kinase. There is clonal expansion of hematopoietic stem cells of several different lineages in CML. CML patients in stable phase usually have high white blood counts and immature cells of granulocytic lineages. Stable phase CML evolves to a more aggressive phase typically within 3.5-5 years, where differentiation is blocked and acute leukemia ensues. The transition of CML stable phase to blast phase is reflected in the loss of growth factor requirement of CML cells and correlates with additional cytogenetic alterations. Some biological effects reported in primary CML cells include reduced apoptosis and altered adhesion to fibronectin; however, the cells are dependent on hematopoietic growth factors. On a molecular level, the BCR/ABL translocation is well characterized. However, the actual mechanism of transformation by the BCR/ABL oncogene of hematopoietic cells is largely unknown. Enhancement of the c-ABL tyrosine kinase activity in BCR/ABL appears to be crucial for transformation. This tyrosine kinase activity leads to activation of several signal transduction pathways that are also utilized by hematopoietic growth factors, including steel factor, thrombopoietin, interleukin-3, and granulocyte/macrophage-colony stimulating factor. In several model systems, BCR/ABL has overlapping biological effects with hematopoietic growth factors, and transformation of hematopoietic growth factor-dependent cell lines leads to growth factor independence. In this review, we will describe the molecular and biological abnormalities in CML and several signal transduction mechanisms utilized by BCR/ABL as compared to hematopoietic growth factors.

Induction of c-myc transcription by the v-Abl tyrosine kinase requires Ras, Raf1, and cyclin-dependent kinases

v-Abl is an oncogenic form of the c-Abl nonreceptor tyrosine kinase. v-Abl induces transcription of c-myc, and c-Myc function is a necessary but not sufficient component of the v-Abl transformation program. Previously we showed that the E2F site in the c-myc promoter is a v-Abl response element and that v-Abl appears to induce c-myc by initiating a phosphorylation cascade that ultimately activates E2F-binding proteins. In this work we have investigated the signaling pathway between the v-Abl tyrosine kinase and activated E2F proteins. We show that the Ras GTPase and Raf1 serine/threonine kinase are required in this pathway. However, in contrast to other aspects of v-Abl signaling, induction of c-myc transcription is independent of the Rac GTPase. Our results also establish a requirement for activated cyclin-dependent kinases (cdks), as v-Abl-dependent induction of c-myc transcription is blocked by cdk inhibitor p21 and induction of c-myc is accompanied by activation of cdk2 and cdk4. Finally, we show that v-Abl-dependent induction of c-myc is accompanied by hyperphosphorylation of pRb, p107, and p130. On the basis of these data, we propose a model for the signaling path from v-Abl to c-myc.

A coiled-coil tetramerization domain of BCR-ABL is essential for the interactions of SH2-containing signal transduction molecules

BCR-ABL is a chimeric oncoprotein that exhibits deregulated tyrosine kinase activity and is implicated in the pathogenesis of Philadelphia chromosome (Ph1)-positive leukemia. We have previously shown SH2-containing phosphotyrosine phosphatase SHP-2 forms stable complexes with BCR-ABL and Grb2 in BCR-ABL-transformed cells (Tauchi, T., Feng, G. S., Shen, R., Song, H. Y., Donner, D., Pawson, T., and Broxmeyer, H. E. (1994) J. Biol. Chem. 269, 15381-15387). To elucidate the structural requirement of BCR-ABL for the interactions with SH2-containing signaling molecules, we examined a series of BCR-ABL mutants which include the Grb2 binding site-deleted BCR-ABL (1-63 BCR/ABL), the tetramerization domain-deleted BCR-ABL (64-509 BCR/ABL), and the SH2 domain-deleted BCR-ABL (BCR/ABL deltaSH2). These BCR-ABL mutants were previously shown to reduce the transforming activity in fibroblasts. We found that the tetramerization domain-deleted BCR-ABL did not induce the tyrosine phosphorylation of SHP-2 and the interactions of BCR-ABL, SHP-2, and Grb2. In vitro kinase assays have also shown that the tetramerization domain-deleted BCR-ABL mutant did not phosphorylate GST-SHP-2 in vitro. SHP-2 was co-immunoprecipitated with phosphatidylinositol 3-kinase in BCR/ABL p210-transformed cells; however, this interaction was not observed in the tetramerization domain-deleted BCR-ABL mutant. Therefore the tetramerization domain of BCR-ABL is essential for interactions of these downstream molecules.

Co-expression with BCR induces activation of the FES tyrosine kinase and phosphorylation of specific N-terminal BCR tyrosine residues

The human BCR gene encodes a protein with serine/threonine kinase activity and regulatory domains for the small G-proteins RAC and CDC42. Previous work in our laboratory has established that BCR is a substrate for c-FES, a non-receptor tyrosine kinase linked to myeloid growth and differentiation. Tyrosine phosphorylation led to the association of BCR with the RAS guanine nucleotide exchange complex GRB2-SOS in vivo via the GRB2 SH2 domain, linking BCR to RAS signaling (Maru, Y., Peters, K. L., Afar, D. E. H., Shibuya, M., Witte, O. N., and Smithgall, T. E. (1995) Mol. Cell. Biol. 15, 835-842). In the present study, we demonstrate that BCR Tyr-246 and at least one of the closely spaced tyrosine residues, Tyr-279, Tyr-283, and Tyr-289 (3Y cluster), are phosphorylated by FES both in vitro and in 32Pi-labeled cells. Mutagenesis of BCR Tyr-177 to Phe completely abolished FES-induced BCR binding to the GRB2 SH2 domain, identifying Tyr-177 as an additional phosphorylation site for FES. Co-expression of BCR and FES in human 293T cells stimulated the tyrosine autophosphorylation of FES. By contrast, tyrosine phosphorylation of BCR by FES suppressed BCR serine/threonine kinase activity toward the 14-3-3 protein and BCR substrate, BAP-1. These data show that tyrosine phosphorylation by FES affects the interaction of BCR with multiple signaling partners and suggest a general role for BCR in non-receptor protein-tyrosine kinase regulation and signal transduction.

Signal transduction-based strategies for the treatment of chronic myelogenous leukemia

Recent studies of the BCR-ABL fusion protein, the product of the oncogene responsible for chronic myelogenous leukemia, have identified a number of signal transduction pathways that are activated by this tyrosine kinase. In some cases, these pathways are critical mediators of the growth stimulatory effects of the oncogene on hemopoietic cells. This knowledge has been translated into therapeutic strategies that directly target BCR-ABL or the signaling pathways that BCR-ABL activates. Promising results in animal models have led to the design of Phase I clinical trials, which are in progress or will be under way shortly. These studies are among the first to target a specific genetic abnormality in human cancer.

BCR/ABL regulation of PI-3 kinase activity

The ability of BCR-ABL oncoproteins to induce leukemic transformation of hematopoietic cells depends on their tyrosine kinase activity, which is essential for recruitment and activation of multiple pathways that transduce oncogenic signals. Although it is unknown yet whether activation of PI 3-kinase is required for transformation, the colony-forming ability of Philadelphia cells is dependent on PI 3-kinase activity, as indicated by the results of studies using a number of strategies to interfere with the synthesis and/or the function of the regulatory and catalytic subunits of this kinase. In particular, wortmannin, a specific PI 3-kinase inhibitor, preferentially affected colony formation of Philadelphia cells over that of normal marrow hematopoietic progenitors. The mechanism(s) of such effects are unknown, but PI 3-kinase inhibitors may represent a novel class of therapeutic agents for the ex vivo and/or in vivo treatment of Philadelphia leukemias.

The CRKL adaptor protein transforms fibroblasts and functions in transformation by the BCR-ABL oncogene

The CRKL adaptor protein was recently identified as a substrate for the BCR-ABL tyrosine kinase in patients with chronic myelogenous leukemia, but its function is unknown. Here we report that CRKL is phosphorylated when overexpressed, activates RAS and JUN kinase signaling pathways, and transforms fibroblasts in a RAS-dependent fashion. We examined the potential role of CRKL in BCR-ABL function by deleting the CRKL binding site in BCR-ABL. This mutant BCR-ABL protein shows a 50% reduction in fibroblast transforming activity. The GRB2 adaptor protein has previously been implicated in this pathway, presumably linking BCR-ABL to RAS. To address the relative roles of CRKL and GRB2 in this system, we compared BCR-ABL mutants with defects in binding to one or both adaptors. Whereas each single mutant showed a 2-3-fold loss in transforming activity, the double mutant showed a 15-fold reduction, suggesting that GRB2 and CRKL both contribute to BCR-ABL transformation. These results demonstrate the oncogenic potential of CRKL and provide functional evidence that CRKL plays a role in fibroblast transformation by BCR-ABL in conjunction with other adaptor proteins.

Oligomerization of the ABL tyrosine kinase by the Ets protein TEL in human leukemia

TEL is a member of the Ets family of transcription factors which are frequently rearranged in human leukemia. The mechanism of TEL-mediated transformation, however, is unknown. We report the cloning and characterization of a chromosomal translocation associated with acute myeloid leukemia which fuses TEL to the ABL tyrosine kinase. The TEL-ABL fusion confers growth factor-independent growth to the marine hematopoietic cell line Ba/F3 and transforms Rat-1 fibroblasts and primary murine bone marrow cells. TEL-ABL is constitutively tyrosine phosphorylated and localizes to the cytoskeleton. A TEL-ABL mutant containing an ABL kinase-inactivating mutation is not constitutively phosphorylated and is nontransforming but retains cytoskeletal localization. However, constitutive phosphorylation, cytoskeletal localization, and transformation are all dependent upon a highly conserved region of TEL termed the helix-loop-helix (HLH) domain. TEL-ABL formed HLH-dependent homo-oligomers in vitro, a process critical for tyrosine kinase activation. These experiments suggest that oligomerization of TEL-ABL mediated by the TEL HLH domain is required for tyrosine kinase activation, cytoskeletal localization, and transformation. These data also suggest that oligomerization of Ets proteins through the highly conserved HLH domain may represent a previously unrecognized phenomenon.

The dimerization property of glutathione S-transferase partially reactivates Bcr-Abl lacking the oligomerization domain

Bcr-Abl oncoproteins are responsible for the pathogenesis of human leukemias with a reciprocal chromosome translocation t(9;22). The amino-terminal Bcr sequence has a potential to form a homotetramer (tetramer domain), and destructions of the tetramer domain cause a complete loss of biological activities in Bcr-Abl. Here we show that Bcr-Abl in which the tetramer domain is replaced with glutathione S-transferase (GST) with a dimerizing ability (GST/Bcr-Abl-(Delta1-160)) can no longer induce an interleukin-3 (IL-3) independence in Ba/F3 cells or transform mouse bone marrow cells but still retains by 30-40% the ability to transform Rat1 cells. Compared with the wild type Bcr-Abl, autophosphorylation of GST/Bcr-Abl-(Delta1-160) in vivo was reduced by more than 50%. The Grb-2 binding to GST/Bcr-Abl-(Delta1-160) was 50% reduced in Rat1 cells and undetectable in Ba/F3 cells. In Rat1 cells expressing GST/Bcr-Abl-(Delta1-160), phosphotyrosine contents of p62 and Shc were 70% decreased.

Fibroblast growth factor-2 suppression of tumor necrosis factor alpha-mediated apoptosis requires Ras and the activation of mitogen-activated protein kinase

Treatment of L929 cells with tumor necrosis factor alpha (TNFalpha) activates a programmed cell death pathway resulting in apoptosis. We investigated the intracellular signaling pathways activated in L929 cells by TNFalpha. TNFalpha robustly activates Jun kinase (JNK), a member of the mitogen-activated protein kinase (MAPK) family. In addition, p42(MAPK) is activated, but a 10-fold greater concentration of TNFalpha was required for substantial MAPK activation than was needed for maximal JNK stimulation. Simultaneous treatment of L929 cells with fibroblast growth factor (FGF-2) significantly reduced the apoptotic response to TNFalpha. FGF-2 substantially activated the Raf/MEK/MAPK (where MEK is mitogen-activated protein kinase kinase) pathway but did not affect TNFalpha activation of JNK. These results indicate that although JNK may play an important role in transmitting the TNFalpha signal from the cell surface to the nucleus, activation of the JNK pathway is not sufficient to induce apoptosis. Expression of dominant-negative Asn-17 Ras in L929 cells diminished the FGF-2 stimulation of p42(MAPK) and eliminated the protective effect of FGF-2. Asn-17 Ras expression did not affect JNK activity and had no effect on TNFalpha activation of JNK. Pharmacological inhibition of MEK-1 activity by incubation of cells with the compound PD 098059 blocked p42(MAPK) activation and FGF-2 protection against apoptosis. Interestingly, activated Val-12 Ras expression substantially enhanced TNFalpha-mediated apoptosis in L929 cells, but Val-12 Ras did not constitutively activate MAPK in L929 cells and FGF-2 partially protected Val-12 Ras-expressing cells from TNFalpha-mediated apoptosis. Our data indicate that activation of the MAPK pathway mediates an FGF-2 protective effect against apoptosis and highlights the important role that integration of multiple intracellular signaling pathways plays in the regulation of cell growth and death.