Bcr-Abl Exerts Its Antiapoptotic Effect Against Diverse Apoptotic Stimuli Through Blockage of Mitochondrial Release of Cytochrome C and Activation of Caspase-3

Differential modulation of apoptosis sensitivity in CD95 type I and type II cells

We have recently identified two different pathways of CD95-mediated apoptosis (Scaffidi, C., Fulda, S., Srinivasan, A., Feng, L., Friesen, C., Tomaselli, K. J., Debatin, K.-M., Krammer, P. H., and Peter, M. E. (1998) EMBO J. 17, 1675-1687). CD95-mediated apoptosis in type I cells is initiated by large amounts of active caspase-8 formed at the death-inducing signaling complex (DISC) followed by direct cleavage of caspase-3. In contrast, in type II cells very little DISC and small amounts of active caspase-8 sufficient to induce the apoptogenic activity of mitochondria are formed causing a profound activation of both caspase-8 and caspase-3. Only in type II cells can apoptosis be blocked by overexpressed Bcl-2 or Bcl-x(L). We now show that a number of apoptosis-inhibiting or -inducing stimuli only affect apoptosis in type II cells, indicating that they act on the mitochondrial branch of the CD95 pathway. These stimuli include the activation of protein kinase C, which inhibits CD95-mediated apoptosis resulting in a delayed cleavage of BID, and the induction of apoptosis by the ceramide analog C(2)-ceramide. In addition, we have identified the CD95 high expressing cell line Boe(R) as a CD95 apoptosis-resistant type II cell that can be sensitized by treatment with cycloheximide without affecting formation of the DISC. This also places the effects of cycloheximide in the mitochondrial branch of the type II CD95 pathway. In contrast, c-FLIP was found to block CD95-mediated apoptosis in both type I and type II cells, because it acts directly at the DISC of both types of cells.

Chronic myelogenous leukemia

Over the past year, new information has been reported on the biology and treatment of chronic myelogenous leukemia (CML). Chronic myelogenous leukemia is characterized by the breakpoint cluster region (BCR-ABL) chimeric gene, the product of which is p210BCR-ABL, a tyrosine kinase that gives hematopoietic cells the characteristics of excessive proliferation, resistance to physiologic apoptotic signals, and resistance to chemotherapy. Recently, investigators have attempted to 1) elucidate the mechanisms by which the BCR-ABL gene and its product initiate and maintain the malignant phenotype, 2) improve the use of the BCR-ABL gene as a diagnostic marker of disease, and 3) inhibit the expression of this gene as a therapeutic maneuver. Other investigators have tried to explain interferon's mechanism of action in the treatment of CML and to improve the safety and applicability of stem cell transplantation (SCT) as a therapy for CML.

A mechanism behind the antitumour effect of 6-diazo-5-oxo-L-norleucine (DON): disruption of mitochondria

6-diazo-5-oxo-L-norleucine (DON) exerts a growth inhibitory effect selectively on the neuroendocrine tumour cell line BON and is proposed as an antitumour drug. The mechanism behind this has not yet been clarified. In the present study, transmission electron microscopy was used for the assessment of changes in cellular organelles. Furthermore, the methylthiazolyldiphenyl tetrazolium (MTT) assay for mitochondrial enzymatic activity, a fluorescent marker (rhodamine 123) for mitochondrial integrity and [2-(11)C]-acetyl-carnitine which is a substrate of the tricarboxylic acid cycle of mitochondria were employed. The studies were performed in parallel in BON and in a neuroblastoma cell line LAN, with the cells grown as monolayers or as multicellular aggregates. Severe morphological changes of intracellular organelles were observed in BON aggregates treated with low-doses of DON. Especially striking was the disruption of mitochondrial internal membrane structures. Other features included the swelling of endoplasmic reticulum, autophagocytosis of secretory granules and nuclear condensation (apoptosis). In LAN cells, no ultrastructural changes were seen after DON treatment. The MTT assay indicated inhibition of mitochondrial enzymatic activity in BON cells but not in LAN cells after 5 h treatment with DON. The mitochondrial damage was also demonstrated as a reduced metabolism of [2-(11)C]-acetyl-carnitine. The observations revealed mitochondrial damage by DON treatment and suggest that the mitochondria might be a primary target for the antitumour effect in neuroendocrine cells.

Synthesis of Bcl-2 in response to anthracycline treatment may contribute to an apoptosis-resistant phenotype in leukemic cell lines

BACKGROUND

Some forms of chemoresistance in leukemia may start from failure of tumour cells to successfully undergo apoptosis and Bcl-2 may play a role in this defect. Therefore, we evaluated the Bcl-2 content and synthesis in relation with the apoptotic potential in leukemic cell lines after anthracycline treatment.

METHODS

U937, HL60, and K562 cells and their drug resistant (DR) variants were treated with varying concentrations of Idarubicin (IDA). Apoptosis was evaluated by fluorescence microscopy after acridine orange staining. Bcl-2 and Bax content were evaluated either by flow cytometry after indirect immunolabelling or by Western blot.

RESULTS

High Bcl-2 contents were not related to a poor ability to undergo apoptosis in U937, HL60, K562 and their DR variants. IDA induced a concentration-dependent increase in Bcl-2 content in all cell lines as long as they do not perform apoptosis. Enhanced Bcl-2 expression was inhibited by cycloheximide, actinomycin D, or antisense oligonucleotide directed against bcl-2 mRNA. Bcl-2 expression was also increased in the resistant U937 variant after serum deprivation or C2-ceramide treatment. The synthesis of Bcl-2 led to an increased Bcl-2/Bax ratio solely in the cells with an apoptosis-resistance phenotype.

CONCLUSIONS

These data suggest that exposure to IDA induces Bcl-2 expression in leukemic cell lines, and that this mechanism could contribute to apoptosis resistance and participate in the acquisition of chemoresistance. They also confirm that the evolution of the Bcl-2/Bax ratio reflects apoptotic ability better than the steady state level of Bcl-2 expression.

Ceramide triggers p53-dependent apoptosis in genetically defined fibrosarcoma tumour cells

p53 mutations are among the most common genetic alterations in human cancer and are frequently described in intrinsic or acquired radio- and chemotherapy resistance. Radiation-induced cell kill is not only mediated by DNA damage but also by the activation of signal transduction cascades generated at the plasma membrane like the sphingomyelin pathway. We used genetically defined wild-type p53 or p53-deficient mouse fibrosarcoma cells to investigate the p53-dependence of tumour response upon activation of the sphingomyelin pathway. Treatment of the tumour cells with neutral sphingomyelinase drastically reduced the amount of wild-type p53 fibrosarcoma cell proliferation over 72 h in a clear dose-response (0.2-1.0 U ml(-1) nSMase). Sphingomyelinase had no effect on cell proliferation in tumour cells lacking p53. Similarly, cell proliferation was abolished by C2-ceramide (5-20 microM) only in wild-type p53 cells. FACS-analysis revealed that C2-ceramide induced massive p53-dependent apoptosis (40-50% after 12-24 h) and cell cycle analysis showed a transient G1 arrest in p53-deficient tumour cells 12-24 h after C2-ceramide exposure. These results suggest that ceramide-induced apoptosis in tumour cells can be dependent on the status of p53 and imply that p53 is also important for stress-induced apoptotic signal transduction cascades generated at the plasma membrane.

Ordering of ceramide formation, caspase activation, and mitochondrial changes during CD95- and DNA damage-induced apoptosis

To evaluate the role of ceramide (Cer) in apoptosis signaling, we examined Cer formation induced by CD95, etoposide, or gamma-radiation (IR) in relation to caspase activation and mitochondrial changes in Jurkat T cells. The Cer response to all three stimuli was mapped in between caspases sensitive to benzoyloxycarbonyl-VAD-fluoromethylketone (zVAD-fmk) and acetyl-DEVD-aldehyde (DEVD-CHO). Cer production was independent of nuclear fragmentation but associated with the occurrence of other aspects of the apoptotic morphology. Caspase-8 inhibition abrogated Cer formation and apoptosis induced by CD95 but did not affect the response to etoposide or IR, placing CD95-induced Cer formation downstream from caspase-8 and excluding a role for caspase-8 in the DNA damage pathways. CD95 signaling to the mitochondria required caspase-8, whereas cytochrome c release in response to DNA damage was caspase-independent. These results indicate that the caspases required for the Cer response to etoposide and IR reside at or downstream from the mitochondria. Bcl-2 overexpression abrogated the Cer response to etoposide and IR and reduced CD95-induced Cer accumulation. We conclude that the Cer response to DNA damage fully depends on mitochondrion-dependent caspases, whereas the response to CD95 partially relies on these caspases. Our data imply that Cer is not instrumental in the activation of inducer caspases or signaling to the mitochondria. Rather, Cer formation is associated with the execution phase of apoptosis.

Ceramide induces cytochrome c release from isolated mitochondria. Importance of mitochondrial redox state

In the present study we show that N-acetylsphingosine (C2-ceramide), N-hexanoylsphingosine (C6-ceramide), and, to a much lesser extent, C2-dihydroceramide induce cytochrome c (cyto c) release from isolated rat liver mitochondria. Ceramide-induced cyto c release is prevented by preincubation of mitochondria with a low concentration (40 nM) of Bcl-2. The release takes place when cyto c is oxidized but not when it is reduced. Upon cyto c loss, mitochondrial oxygen consumption, mitochondrial transmembrane potential (Delta Psi), and Ca2+ retention are diminished. Incubation with Bcl-2 prevents, and addition of cyto c reverses the alteration of these mitochondrial functions. In ATP-energized mitochondria, ceramides do not alter Delta Psi, neither when cyto c is oxidized nor when it is reduced, ruling out a nonspecific disturbance by ceramides of mitochondrial membrane integrity. Furthermore, ceramides decrease the reducibility of cyto c. We conclude that the apoptogenic properties of ceramides are in part mediated via their interaction with mitochondrial cyto c followed by its release and that the redox state of cyto c influences its detachment by ceramide from the inner mitochondrial membrane.

Periodic chronic myelogenous leukaemia: spectral analysis of blood cell counts and aetiological implications

Of 24 published clinical reports of periodic chronic myelogenous leukaemia (PCML), 21 had sufficient data to analyse for periodicity, and 12 showed significant periodicity (p < or = 0.05) using the Lomb periodogram. Leucocyte oscillations had periods T ranging from 37 to 83d. When data were also reported for platelets and reticulocytes there was no significant difference between their periods and those of the leucocytes. These data and their analysis provide strong circumstantial evidence for a haemopoietic stem cell origin of PCML. Namely, the known chromosomal changes in CML patients may, on occasion, also be accompanied by a destabilization resulting in an oscillatory efflux into the leucocyte, platelet and erythrocytic pathways.

The BCR-ABL oncoprotein potentially interacts with the xeroderma pigmentosum group B protein

The previously uncharacterized CDC24 homology domain of BCR, which is missing in the P185 BCR-ABL oncogene of Philadelphia chromosome (Ph1)-positive acute lymphocytic leukemia but is retained in P210 BCR-ABL of chronic myelogeneous leukemia, was found to bind to the xeroderma pigmentosum group B protein (XPB). The binding appeared to be required for XPB to be tyrosine-phosphorylated by BCR-ABL. The interaction not only reduced both the ATPase and the helicase activities of XPB purified in the baculovirus system but also impaired XPB-mediated cross-complementation of the repair deficiency in rodent UV-sensitive mutants of group 3. The persistent dysfunction of XPB may in part underlie genomic instability in blastic crisis.

Blood Cells With Reduced Mitochondrial Membrane Potential and Cytosolic Cytochrome C Can Survive and Maintain Clonogenicity Given Appropriate Signals to Suppress Apoptosis

Reduction of mitochondrial membrane potential (Ψm) and release of cytochrome c from mitochondria appear to be key events during apoptosis. Apoptosis was induced in IC.DP premast cells by the withdrawal of interleukin-3 (IL-3). Ψm decreased by 12 hours and cytochrome c was detected in the cytosol at 18 hours. Despite these changes in the mitochondria after 18 hours of IL-3 deprivation, clonogenicity was unaffected when IL-3 was replenished at 18 hours. Activation of v-Abl tyrosine kinase (v-Abl TK) in IC.DP cells before IL-3 depletion led to increased levels of Bcl-XL, prevented reduction of Ψm and the release of mitochondrial cytochrome c, and suppressed apoptosis. Activation of v-Abl TK 18 hours after withdrawal of IL-3 when ≤10% of the cells had died restored Ψm in the remaining cells. More than 40% of cells thus rescued by v-Abl TK between 18 and 42 hours could subsequently form colonies in the presence of IL-3. These data suggest that reduction in Ψm precedes loss of mitochondrial cytochrome c in IC.DP cells; that v-Abl TK activation, probably via upregulation of Bcl-XL, prevents loss of Ψm and blocks the release of cytochrome c from mitochondria; and that neither of these mitochondrial events is sufficient for commitment to apoptosis.

Blood Cells With Reduced Mitochondrial Membrane Potential and Cytosolic Cytochrome C Can Survive and Maintain Clonogenicity Given Appropriate Signals to Suppress Apoptosis

Reduction of mitochondrial membrane potential (Ψm) and release of cytochrome c from mitochondria appear to be key events during apoptosis. Apoptosis was induced in IC.DP premast cells by the withdrawal of interleukin-3 (IL-3). Ψm decreased by 12 hours and cytochrome c was detected in the cytosol at 18 hours. Despite these changes in the mitochondria after 18 hours of IL-3 deprivation, clonogenicity was unaffected when IL-3 was replenished at 18 hours. Activation of v-Abl tyrosine kinase (v-Abl TK) in IC.DP cells before IL-3 depletion led to increased levels of Bcl-XL, prevented reduction of Ψm and the release of mitochondrial cytochrome c, and suppressed apoptosis. Activation of v-Abl TK 18 hours after withdrawal of IL-3 when ≤10% of the cells had died restored Ψm in the remaining cells. More than 40% of cells thus rescued by v-Abl TK between 18 and 42 hours could subsequently form colonies in the presence of IL-3. These data suggest that reduction in Ψm precedes loss of mitochondrial cytochrome c in IC.DP cells; that v-Abl TK activation, probably via upregulation of Bcl-XL, prevents loss of Ψm and blocks the release of cytochrome c from mitochondria; and that neither of these mitochondrial events is sufficient for commitment to apoptosis.

Activation of the dsRNA-dependent protein kinase, PKR, induces apoptosis through FADD-mediated death signaling

The dsRNA-dependent protein kinase (PKR) is considered to play a key role in interferon-mediated host defense against viral infection and conceivably malignant transformation. To investigate further the mechanisms of PKR-induced growth inhibition, we have developed tetracycline-inducible murine cell lines that express wild-type PKR or a catalytically inactive PKR variant, PKRdelta6. Following induction, the growth of the wild-type PKR-expressing cells was similar to that of cells transfected with vector alone, while cells expressing PKRdelta6 became malignantly transformed. Significantly, treatment with dsRNA caused the wild-type PKR-overexpressing cells to undergo programed cell death while, conversely, cells expressing PKRdelta6 were completely resistant. Our studies demonstrated that activation of PKR induces the expression of members of the tumor necrosis factor receptor (TNFR) family, including Fas (CD95/Apo-1) and pro-apopotic Bax. In contrast, transcripts representing Fas, TNFR-1, FADD (Fas-associated death domain), FLICE, Bad and Bax were ablated in cells expressing PKRdelta6. The involvement of the death receptors in PKR-induced apoptosis was underscored by demonstrating that murine fibroblasts lacking FADD were almost completely resistant to dsRNA-mediated cell death. Thus, PKR, a key cellular target for viral repression, is a receptor/inducer for the induction of pro-apoptotic genes by dsRNA and probably functions in interferon-mediated host defense to trigger cell death in response to virus infection and perhaps tumorigenesis.

Phosphorylated Forms of Activated Caspases Are Present in Cytosol From HL-60 Cells During Etoposide-Induced Apoptosis

Treatment of HL-60 human leukemia cells with etoposide induces apoptotic cell death and activation of at least 18 electrophoretically distinct cysteine-dependent aspartate-directed protease (caspase) isoforms, several of which differ only in their isoelectric points. The purpose of the present study was to determine whether activated caspases are phosphorylated. Phosphatase treatment of cytosolic extracts containing active caspases followed by affinity labeling with N-(N-benzyloxycarbonylglutamyl-N-biotinyllysyl)aspartic acid [(2,6-dimethylbenzoyl)oxy] methyl ketone (Z-EK(bio)D-aomk) showed a mobility shift in several of the labeled species, suggesting that phosphorylated forms of these enzymes are present in the extracts. Metabolic labeling with 32P followed by etoposide treatment and subsequent affinity purification of affinity-labeled caspases confirmed that at least three caspase species were phosphorylated. To detect effects of the phosphorylation on enzymatic activity, caspase-mediated cleavage of aspartylglutamylvalinylaspartyl-7-amino-4-trifluoromethylcoumarin (DEVD-AFC) and poly(ADP-ribose) polymerase (PARP) by phosphorylated and dephosphorylated extracts was measured. No significant changes in Km or vmax were detected using DEVD-AFC. In contrast, a slight, but significant enhancement of PARP cleavage was observed in dephosphorylated extracts, suggesting that phosphorylation of active caspases could have an inhibitory effect on enzyme activity. These observations, which provide the first evidence that caspases are phosphoproteins, suggest that caspases may be targets for some of the growing list of protein kinases that are involved in apoptotic events.

© 1998 by The American Society of Hematology.

Phosphorylated Forms of Activated Caspases Are Present in Cytosol From HL-60 Cells During Etoposide-Induced Apoptosis

Treatment of HL-60 human leukemia cells with etoposide induces apoptotic cell death and activation of at least 18 electrophoretically distinct cysteine-dependent aspartate-directed protease (caspase) isoforms, several of which differ only in their isoelectric points. The purpose of the present study was to determine whether activated caspases are phosphorylated. Phosphatase treatment of cytosolic extracts containing active caspases followed by affinity labeling with N-(N-benzyloxycarbonylglutamyl-N-biotinyllysyl)aspartic acid [(2,6-dimethylbenzoyl)oxy] methyl ketone (Z-EK(bio)D-aomk) showed a mobility shift in several of the labeled species, suggesting that phosphorylated forms of these enzymes are present in the extracts. Metabolic labeling with 32P followed by etoposide treatment and subsequent affinity purification of affinity-labeled caspases confirmed that at least three caspase species were phosphorylated. To detect effects of the phosphorylation on enzymatic activity, caspase-mediated cleavage of aspartylglutamylvalinylaspartyl-7-amino-4-trifluoromethylcoumarin (DEVD-AFC) and poly(ADP-ribose) polymerase (PARP) by phosphorylated and dephosphorylated extracts was measured. No significant changes in Km or vmax were detected using DEVD-AFC. In contrast, a slight, but significant enhancement of PARP cleavage was observed in dephosphorylated extracts, suggesting that phosphorylation of active caspases could have an inhibitory effect on enzyme activity. These observations, which provide the first evidence that caspases are phosphoproteins, suggest that caspases may be targets for some of the growing list of protein kinases that are involved in apoptotic events.

© 1998 by The American Society of Hematology.

Cell death induced by topoisomerase-targeted drugs: more questions than answers

Chemotherapeutic agents that target topoisomerase I and II set into motion a series of biochemical changes that culminate in cell death, but only under some conditions. The realization that stabilization of covalent topoisomerase-DNA complexes is not sufficient to insure cell death has prompted investigators to examine various aspects of the drug-induced death process itself. Several discrete steps along this pathway have been identified, including (a) the processing of stabilized cleavage complexes into frank DNA strand breaks; (b) sensing of the DNA damage, leading to activation of stress-associated signaling pathways and cell cycle arrest; and (c) activation of a preexisting group of enzymes and enzyme precursors, typified by the cysteine-dependent aspartate-directed proteases (caspases), that catalyze the relatively orderly biochemical cascade of terminal events known as apoptosis. The present review discusses the evidence that these steps occur after treatment with etoposide or camptothecin, the two prototypic topoisomerase poisons that are commonly studied. As in any emerging area, a large number of questions remain to be answered about the process of cell death induced by topoisomerase-directed drugs.

Regulation of the c-jun Gene in p210 BCR-ABL Transformed Cells Corresponds With Activity of JNK, the c-jun N-Terminal Kinase

Activity of the c-jun N-terminal kinase (JNK) has been shown in hematopoietic cells transformed by p210 BCR-ABL. However, analysis has not been reported for hematopoietic cells on the consequences of this activity for c-jun promoter regulation within its distinctive proximal 8-base consensus CRE-like element, an element linked to JNK-mediated increase in c-jun transcription. In the present study, regulation of the proximal c-jun promoter was studied in murine myeloid cells transformed by p210 BCR-ABL. Promoter regulation in p210 BCR-ABL transformed cells was compared with regulation of the promoter in nontransformed interleukin-3 (IL-3)–dependent parental cells. The composition of nuclear AP-1 proteins contained within cells with p210 BCR-ABL, and their binding to the c-jun promoter proximal CRE-like element, was compared with the composition and binding of AP-1 proteins in IL-3–treated parental cells without p210 BCR-ABL. The present analysis found fivefold increased c-jun transcription occurring in p210 BCR-ABL transformed murine myeloid cells possessing a corresponding magnitude of increased kinase activity of JNK, compared with IL-3–stimulated parental cells. Augmented JNK activity was accompanied by increased nuclear abundance of c-jun and c-fos proteins that bound specifically to the proximal c-jun promoter CRE element. Also, representative human leukemic cell lines expressing p210 BCR-ABL and possessing abundant kinase activity of JNK, when compared with parental cells that were deficient in JNK activity, had increased c-jun and c-fosproteins. Finally, to show the relevance of these observations in model systems, we studied blast cells from patients with Philadelphia chromosome–positive acute leukemic transformation, and observed comparable activities of JNK catalysis and c-jun/AP-1 protein relative to the cell lines that possessed p210 BCR-ABL and JNK activity. These studies provide a basis for investigating the set of downstream genes which augmented c-jun/AP-1 activity enlists in the process of transformation by p210 BCR-ABL.

Regulation of the c-jun Gene in p210 BCR-ABL Transformed Cells Corresponds With Activity of JNK, the c-jun N-Terminal Kinase

Activity of the c-jun N-terminal kinase (JNK) has been shown in hematopoietic cells transformed by p210 BCR-ABL. However, analysis has not been reported for hematopoietic cells on the consequences of this activity for c-jun promoter regulation within its distinctive proximal 8-base consensus CRE-like element, an element linked to JNK-mediated increase in c-jun transcription. In the present study, regulation of the proximal c-jun promoter was studied in murine myeloid cells transformed by p210 BCR-ABL. Promoter regulation in p210 BCR-ABL transformed cells was compared with regulation of the promoter in nontransformed interleukin-3 (IL-3)–dependent parental cells. The composition of nuclear AP-1 proteins contained within cells with p210 BCR-ABL, and their binding to the c-jun promoter proximal CRE-like element, was compared with the composition and binding of AP-1 proteins in IL-3–treated parental cells without p210 BCR-ABL. The present analysis found fivefold increased c-jun transcription occurring in p210 BCR-ABL transformed murine myeloid cells possessing a corresponding magnitude of increased kinase activity of JNK, compared with IL-3–stimulated parental cells. Augmented JNK activity was accompanied by increased nuclear abundance of c-jun and c-fos proteins that bound specifically to the proximal c-jun promoter CRE element. Also, representative human leukemic cell lines expressing p210 BCR-ABL and possessing abundant kinase activity of JNK, when compared with parental cells that were deficient in JNK activity, had increased c-jun and c-fosproteins. Finally, to show the relevance of these observations in model systems, we studied blast cells from patients with Philadelphia chromosome–positive acute leukemic transformation, and observed comparable activities of JNK catalysis and c-jun/AP-1 protein relative to the cell lines that possessed p210 BCR-ABL and JNK activity. These studies provide a basis for investigating the set of downstream genes which augmented c-jun/AP-1 activity enlists in the process of transformation by p210 BCR-ABL.

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.