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Biomedical subjects

A G Turhan

Publications and source records attributed to A G Turhan.

At least 19 recordsLinked to original sources
PubMed2005

Osteopontin is upregulated by BCR-ABL.

Chronic myelogenous leukemia (CML) is characterized by its hallmark oncogene BCR-ABL and the progression from a chronic phase toward an acute leukemia, with a differentiation arrest of the leukemic clone. In the present study, we conducted a microarray analysis using an inducible model of BCR-ABL expression based on the TET-OFF system, and we found that osteopontin (OPN), a component of stem cell niche, is overexpressed in BCR-ABL-expressing cells. Studies using mutant forms of BCR-ABL demonstrated that the BCR-ABL-induced OPN overexpression was a tyrosine kinase-dependent event. Furthermore, OPN concentration was significantly increased in the serum of leukemic mice generated by transplantation of BCR-ABL-expressing bone marrow cells. Most importantly, a significant increase of OPN concentration was observed in the serum of CML patients as compared to controls. Overall these results show that OPN is deregulated by BCR-ABL oncogene and suggest that OPN could be involved in CML stem cell biology.

Animals↗
PubMed2004

Tyrosine kinase inhibitor AG1024 exerts antileukaemic effects on STI571-resistant Bcr-Abl expressing cells and decreases AKT phosphorylation.

Chronic myelogenous leukaemia (CML) is a clonal malignancy of the pluripotent haematopoietic stem cell, characterised by an uncontrolled proliferation and expansion of myeloid progenitors expressing a fusion oncogene, BCR-ABL, the molecular counterpart of the Ph1 chromosome. The tyrosine kinase (TK) activity of BCR-ABL is known to activate several major signalling pathways in malignant cells, including Ras, JAK/STAT and PI3K/Akt with evidence of proteasome-mediated degradation of other targets such as the DNA repair protein DNA-PKcs and cyclin-dependent kinases inhibitor p27. Targeting these abnormalities by blocking TK of BCR-ABL with STI571 provided a promising approach for the therapy of CML. The recent development of resistance to STI571 illustrates, however, that the use of other TK inhibitors could be of major interest for therapeutic purposes. To this end, the TK inhibitor Tyrphostin AG1024 was used to evaluate effect on regulation of BCR-ABL expression, inhibition of cell proliferation and tumour formation in vivo in human and murine BCR-ABL expressing cell lines. Tyrphostin AG1024 was shown to downregulate expression of BCR-ABL and P-Akt, and to upregulate DNA-PKcs expression. In addition, Tyrphostin AG1024 was able to inhibit cell proliferation, and delay tumour growth in vivo. Thus, AG1024 is able to interfere with three major targets of BCR-ABL in leukaemic cells. Interestingly, Tyrphostin AG1024 was also effective against cells resistant to STI571 by distinct mechanisms including Bcr-Abl mutation. Therefore, these data suggest that Tyrphostin AG1024 could represent the basis of a novel therapy for STI571 refractory CML.

Animals↗
PubMed2004

Evidence of ABL-kinase domain mutations in highly purified primitive stem cell populations of patients with chronic myelogenous leukemia.

To study the hierarchical levels of stem cell targets for ABL-kinase domain mutations in CML, highly purified CD34+CD38- and CD34+CD38+ cell populations and their LTC-IC-derived progeny were analyzed in four patients at diagnosis (n=1) or in advanced phases (n=3) of their disease. In the single patient with early phase CML who later developed an Imatinib Mesylate-resistance and a Y253H mutation, no mutation was detectable in purified cell fractions analyzed at diagnosis nor in their LTC-IC-derived progeny. In contrast, in three patients in advanced phase CML, ABL-kinase mutations demonstrated in peripheral blood cells by sequencing (Q252E and M351T) were detectable in the FACS-sorted cells and became amplified in the LTC-IC-derived progeny of the primitive cells. These findings demonstrate that in late CP or advanced CML, ABL-kinase mutations occur as an intraclonal event in the primitive Ph1+ stem cell compartments with progression of this clone towards IM-resistant blast phase.

Antineoplastic Agents↗
PubMed2004

Ex vivo expansion of megakaryocyte precursor cells in autologous stem cell transplantation for relapsed malignant lymphoma.

To evaluate the impact of ex vivo expanded megakaryocyte (MK) progenitors on high-dose chemotherapy-induced thrombocytopenia, we conducted a phase II study in 10 patients with relapsed lymphoma. Two fractions of peripheral blood progenitor cells (PBPC) were cryopreserved, one with enough cells for at least 2 x 10(6) CD34+ cells/kg and a second obtained after CD34+ selection. Ten days before autologous stem cell transplantation, the CD34+ fraction was cultured with MGDF+SCF for 10 days. After BEAM (BCNU, cyclophosphamide, cytarabine, and melphalan) chemotherapy, patients were reinfused with standard PBPC and ex vivo expanded cells. No toxicity was observed after reinfusion. The mean fold expansion was 9.27 for nucleated cells, 2 for CD34+ cells, 676 for CD41+ cells, and 627 for CD61+ cells. The median date of platelet transfusion independence was day 8 (range: 7-12). All patients received at least one platelet transfusion. In conclusion, ex vivo expansion of MK progenitors was feasible and safe, but this procedure did not prevent BEAM-induced thrombocytopenia. Future studies will determine if expansion of higher numbers of CD34+ cells towards the MK-differentiation pathway will translate into a functional effect in terms of shortening of BEAM-induced thrombocytopenia.

Antigens, CD34↗
PubMed2003

Multiple molecular mechanisms contribute to radiation sensitivity in mantle cell lymphoma.

Mantle cell lymphomas (MCL) are characterized by their aggressive behavior and poor response to chemotherapy regimens. We report here evidence of increased in vitro radiation sensitivity in two cell lines that we have generated from two MCL patients (UPN1 and UPN2). However, despite their increased radiation sensitivity, UPN2 cells were totally resistant to apoptotic cell death, whereas UPN1 cells underwent massive apoptosis 6 h after irradiation. The frequency of induced chromosomal abnormalities was higher in UPN1 as compared to UPN2. Distinct mechanisms have been found to contribute to this phenotype: a major telomere shortening (UPN1 and UPN2), deletion of one ATM allele and a point mutation in the remaining allele in UPN2, mutation of p53 gene (UPN1 and UPN2) with absence of functional p53 as revealed by functional yeast assays. After irradiation, Ku70 levels in UPN1 increased and decreased in UPN2, whereas in the same conditions, DNA-PKcs protein levels decreased in UPN1 and remained unchanged in UPN2. Thus, irradiation-induced apoptotic cell death can occur despite the nonfunctional status of p53 (UPN1), suggesting activation of a unique pathway in MCL cells for the induction of this event. Overall, our study demonstrates that MCL cells show increased radiation sensitivity, which can be the result of distinct molecular events. These findings could clinically be exploited to increase the dismal response rates of MCL patients to the current chemotherapy regimens.

Antigens, Nuclear↗
PubMed2003

Blastoid mantle cell lymphoma: evidence for nonrandom cytogenetic abnormalities additional to t(11;14) and generation of a mouse model.

Mantle cell lymphoma (MCL) is characterized by the t(11;14)(q13;q32), which is associated with cyclin D1 hyperexpression and a poor prognosis. MCL cases have been shown to progress to a more aggressive disease but the molecular events responsible of this phenomenon have not been determined. We have established two cell lines from the pleural effusions of two patients with MCL that we have used for further cytogenetic characterization to better define the incidence and nature of secondary chromosome abnormalities using multicolor fluorescence in situ hybridization, whole chromosome paint, and specific probes. Both cell lines grew independently without growth factors. Using CCND1/IGH-specific probes, patient UPN1 was found to have a masked t(11;14). Numerous and complex chromosomal abnormalities were found in both cell lines affecting chromosomes 2, 8, 13, 18, 22, X, and Y. These abnormalities included 8p losses, suggesting the presence of an anti-oncogene in this region, rearrangements of 8q24, MYC gene, and translocations involving 8, X, and Y chromosomes, which might be significant in the pathogenesis of MCL progression. The use of the cell lines (UPN1) allowed us to generate a mouse model of human MCL, mimicking a disseminated lymphoma and leading to the death of the animals in 4 weeks. This blastoid MCL model could be of major interest to determine molecular events involved in MCL progression, allowing isolation of involved genes and their functional characterization, and to study the effects of new chemotherapy regimens in mouse models.

Animals↗
PubMed2003

[Plasticity of adult stem cells].

Until recently, adults stem cells, defined by their self-renewal and differentiation abilities, were thought to be tissue-specific. This concept has been challenged by bone marrow transplantation experiments in mice, demonstrating generation of cells of different phenotype after transplantation of marrow or muscle cells. The term "plasticity" has been coined to explain this phenomenon which could be due to the persistence in adult tissues, of stem cells with multidifferentiation ability or to the "transdifferentiation" ability of some adult cells committed to differentiation, under the influence of unknown environmental cues. The relationship of the cells at the origin of the stem cells plasticity with a new type of mesodermal cell designed under the term of "multipotent adult progenitor cell" (MAPC) remains to be determined. The discovery of this latter is a major advance in this field as the MAPC have isolated from the adult bone marrow and presents certain characteristics of embryonic stem cells with the demonstration of their totipotency towards many tissues, including hematopoiesis. The discovery of the adult stem cell plasticity phenomenon in general, represents a major change in our concepts of stem and developmental biology and possibly the basis for the development of future cell therapy protocols.

Adult↗
PubMed2001

BCR-ABL down-regulates the DNA repair protein DNA-PKcs.

This study demonstrates in both stable and inducible BCR-ABL-expressing hematopoietic cells a down-regulation of the major mammalian DNA repair protein DNA-PKcs by BCR-ABL. Similar results were found in BCR-ABL CD34(+) cells from patients with chronic myelogenous leukemia (CML). DNA-PKcs down-regulation is a proteasome-dependent degradation that requires tyrosine kinase activity and is associated with a marked DNA repair deficiency along with increased sensitivity to ionizing radiation. The conjunction of a major DNA repair deficiency and a resistance to apoptosis, both induced by BCR-ABL, provides a new mechanism to explain how secondary genetic alterations can accumulate in CML, eventually leading to blast crisis. The down-regulation of DNA-PKcs was reversible in CD34(+) CML cells suggesting that this approach might offer a novel and powerful therapeutic strategy in this disease, especially to delay the blast crisis. (Blood. 2001;97:2084-2090)

Acetylcysteine↗
PubMed2001

Rapid generation of a tetracycline-inducible BCR-ABL defective retrovirus using a single autoregulatory retroviral cassette.

The development of chronic myelogenous leukemia (CML) models in mice using an inducible BCR-ABL gene has been hampered by the requirement of sequential expression of tTA (Tet repressor-VP16 fusion protein) and Tet-OP sequences in the same cells after separate transfection. This double transfection strategy is time consuming as it requires screening of many hundreds of individual clones and cannot be applied to primary hematopoietic cells. To generate a tetracycline-inducible BCR-ABL retrovirus, we have subcloned BCR-ABL p210 cDNA in the SIN-Retro-TET vector, which allows regulated expression of a gene of interest in a single autoregulatory cassette, containing both tTA and Tet OP sequences. Retroviral particles were obtained by transfecting the SIN-BCR-ABL p210 construct into the 293 cells and by VSVG pseudotyping. To determine the functionality of the retrovirus, the IL-3-dependent murine Ba/F3 cell line was retrovirally transduced and clones were grown in the absence of both IL-3 (to select for transformed cells) and a tetracycline analog, doxycycline (to induce BCR-ABL expression). Using this technique, polyclonal Ba/F3 cells and several growth factor-independent Ba/F3 clones expressing BCR-ABL were obtained within 2-3 weeks. A single dose of doxycycline added to the medium (1 microg/ml), induced in different clones, a reduction of BCR-ABL protein levels by 60-90% at 24 h, leading to cell death in the absence of IL-3. In several individual clones, BCR-ABL expression was further reduced to become almost undetectable at 48 h. The doxycycline-regulated BCR-ABL expression was stable, as many clones maintained in culture for >8 months showed a persistent inhibitory response to doxycycline addition in the medium. In in vivo experiments, subcutaneous injection of 2 x 10(6) Ba/F3-SIN p210 cells in nude mice induced visible tumors in 2 weeks and all established tumors completely regressed upon addition of doxycycline in the drinking water (200 microg/ml). To determine the functionality of the inducible BCR-ABL retrovirus in vivo, primary Lin- bone marrow cells were transduced with SIN-p210 and transplanted in lethally irradiated mice. All transplanted mice had successful hematopoietic reconstitution and BCR-ABL integration was found in the peripheral blood of seven out of 14 mice available for long-term analysis (>6 months). However, despite evidence of retrovirus-mediated gene transfer, there was no evidence of leukemia, due either to low viral titers or to the relative inefficiency of the minimal CMV promoter in primary hematopoietic cells. Thus, these results demonstrate for the first time, to our knowledge, the feasibility to generate an inducible BCR-ABL retrovirus in a single step, in the context of an immortalized cell line. Our data suggest that with further improvements of the retrovirus-mediated gene transfer technology, it might be possible to generate inducible leukemia models in mice by the use of single retroviral constructs.

Animals↗
PubMed2001

Efficient ex vivo expansion of NOD/SCID-repopulating cells with lympho-myeloid potential in hematopoietic grafts of children with solid tumors.

INTRODUCTION: The ex vivo expansion of hematopoietic grafts could be an important therapeutic tool for accelerating hematopoietic recovery after administration of high-dose chemotherapy regimens. The fate of the long-term repopulating cells during the ex vivo manipulation of grafts is a critical issue and will ultimately define the clinical applicability of this technology to hematopoietic transplantation. MATERIALS AND METHODS: To study the effects of a clinically applicable ex vivo expansion protocol in the proliferative potential of the most primitive human hematopoietic cells, both LTC-IC and NOD/SCID-RC assays were used to determine LTC-IC and NOD/SCID-RC contents of hematopoietic grafts, both before and after expansion (SCF, IL-3, PEG-MGDF Flt3-L and 5% AB serum), in four children with non-hematological malignancies. RESULTS: The mean percentage of CD34+ cells after expansion was 16%. The numbers of nucleated cells increased 20-fold with a mean three-fold increase in the numbers of CD34+ cells during the expansion period. The CFC content of the samples showed a mean 11-fold increase (range: 5-17) after ex vivo expansion. The primitive hematopoietic stem cell content of the expanded cell fraction evaluated by LTC-IC assays was found to be increased in two patients out of three, with maintenance of the LTC-IC frequency in the third patient. The NOD/SCID-RC potential, evaluated in five experiments from four patients using 109 mice injected 5-6 weeks earlier with human hematopoietic cells, increased from a mean percentage of 36% (range: 7-75%) before expansion, to a mean percentage of 70% (range: 37-100%) after expansion (P < 0.00001). The frequency of NOD/SCID-RC calculated with pooled data from all patients was 1/80,000 at day 0 and 1/40,000 after seven days of culture. The full phenotypic analysis of human hematopoietic cells obtained in NOD/SCID mice injected with expanded cells showed the presence of significant numbers of CD34+, CD19+ and CD15+ cells, suggesting the persistent lympho-myeloid potential of the expanded hematopoietic cells. CONCLUSION: Our results suggest that efficient expansion of NOD/SCID-RC with lympho-myeloid potential can be achieved not only in cord blood or normal marrow as previously reported, but also in hematopoietic grafts obtained from children exposed to high-dose chemotherapy.

Animals↗
PubMed2001

BCR-ABL fails to inhibit apoptosis in U937 myelomonocytic cells expressing a carboxyl-terminal truncated STAT5.

Recent experimental data suggest that one of the major effects of BCR-ABL gene expression in hematopoietic cells is the inhibition of apoptosis. Although the exact mechanisms of this phenomenon are not clear, it is thought to be related to the fact that BCR-ABL induces several signalling pathways also activated by growth factors. In order to determine the anti-apoptotic role of BCR-ABL in a hematopoietic cell line and to by-pass the influence of cytokine-dependence, BCR-ABL gene was expressed in the autonomously growing myelomonocytic U937 cell line using retroviral vectors. There was no resistance to apoptosis induced by either serum deprivation or different doses of etoposide in any U937 clones expressing BCR-ABL protein. In addition to serum deprivation and etoposide, BCR-ABL-expressing clones were not protected from apoptosis induced by TNF, ceramide-C2 and FAS-cross-linking. BCL2 expression was absent in U937 cells and BAX levels were identical between Neo and BCR-ABL clones. To further investigate the mechanisms of this phenomenon, band-shift assays were performed to detect activation of STAT molecules. No constitutive activation of STATs was detected in either NeoR or BCR-ABL-U937 cells, although both IFN-gamma and GM-CSF activated STAT1 and STAT5, respectively, with similar kinetics in both NeoR and BCR-ABL-U937 cells. In addition, the GM-CSF-induced-STAT5 activation was found to be weakened in all clones expressing BCR-ABL. In both control NeoR and BCR-ABL-transfected clones, band-shift assays revealed the presence of an abnormal truncated STAT5 recognized only by an anti-N-terminal but not by an anti-C-Terminal STAT5 antibody. These findings suggest a possible link between the absence of anti-apoptotic potential of BCR-ABL and abnormalities of the STAT5 pathway, including, absence of constitutive activation of STAT5, inhibition of GM-CSF-induced STAT5 activation and expression of a carboxyl-terminal-truncated STAT5.

Apoptosis↗
PubMed2000

The leukaemic oncoproteins Bcr-Abl and Tel-Abl (ETV6/Abl) have altered substrate preferences and activate similar intracellular signalling pathways.

Inappropriate activation of Abl family kinases plays a crucial role in different human leukaemias. In addition to the well known oncoproteins p190Bcr-Abl and p210Bcr-Abl, Tel-Abl, a novel fusion protein resulting from a different chromosomal translocation, has recently been described. In this study, the kinase specificities of the Bcr-Abl and Tel-Abl proteins were compared to the physiological Abl family kinases c-Abl and Arg (abl related gene). Using short peptides which correspond to the target epitopes in known substrate proteins of Abl family kinases, we found a higher catalytic promiscuity of Bcr-Abl and Tel-Abl. Similar to Bcr-Abl, Tel-Abl was found in complexes with the adapter protein CRKL. In addition, c-Crk II and CRKL are tyrosine phosphorylated and complexed with numerous other tyrosine phosphorylated proteins in Tel-Abl expressing Ba/F3 cells. GTPase analysis with a Ras-GTP-specific precipitation assay showed constitutive elevation of GTP-loaded Ras in cells expressing the leukaemic Abl proteins. The mitogenic MAPK/Erk kinases as well as Akt/PKB, a kinase implicated to negatively regulate apoptosis, were also constitutively activated by both Bcr-Abl and Tel-Abl. The results indicate that the leukaemic Abl-fusion proteins have catalytic specificities different from the normal kinases c-Abl and Arg and that Tel-Abl is capable to activate at least some pathways which are also upregulated by Bcr-Abl.

3T3 Cells↗
PubMed2000

Biological effects induced by variable levels of BCR-ABL protein in the pluripotent hematopoietic cell line UT-7.

There is currently no satisfactory model allowing analysis of dose-effect relationships of BCR-ABL proteins in human hematopoietic cells. To study comparatively the proliferative, differentiative and anti-apoptotic actions of different levels of BCR-ABL proteins in the context of the same cellular background, we have introduced the BCR-ABL gene into the GM-CSF-dependent pluripotent human cell line UT-7. Individual clones expressing BCR-ABL were analyzed by Western blots. After normalization to equivalent levels of endogenous ABL protein, 14 clones always grown in GM-CSF were found to express low but variable levels of BCR-ABL whereas two clones selected in the absence of GM-CSF expressed very high levels of BCR-ABL. All low-level BCR-ABL expressing clones exhibited a behavior similar to that of the GM-CSF-dependent parental cells as they ceased to proliferate upon growth factor deprivation and showed a strong proliferative response upon GM-CSF addition. One out of 14 clones showed progressive GM-CSF independence during culture over several weeks and was found to have a significant increase of BCR-ABL expression at that time. The resistance of this clone (E8-2) to different apoptotic stimuli was found to be increased as compared to its low BCR-ABL-expressing counterpart (E8-1) and similar to that observed in clones with very high levels of BCR-ABL (UT-7/9 and UT-7/11) which were totally resistant to apoptotic stimuli. When injected into nude mice, parental UT-7 cells and clones with low-level of BCR-ABL were not tumorigenic over 10 weeks of observation whereas UT-7 clones with high levels of BCR-ABL (UT-7/9, UT-7/11 and UT-7/E8-2) induced aggressive tumors in 2-4 weeks with a significant correlation between the amount of BCR-ABL protein and the rate of tumor growth. In conclusion, the establishment of an in vitro and in vivo CML model using UT-7 cells suggests for the first time in human cells, that the fully transformed phenotype induced by BCR-ABL requires high levels of BCR-ABL expression. These findings suggest that variable levels of BCR-ABL in primary patient cells could also be responsible for the different phenotypic features seen in chronic and acute phases of CML, such as the differentiation ability induced by growth factors.

Animals↗
PubMed1999

Mutator phenotype of BCR--ABL transfected Ba/F3 cell lines and its association with enhanced expression of DNA polymerase beta.

Chronic myelogenous leukemia (CML) is characterized by the Philadelphia chromosome resulting from the translocation t(9-22) producing the chimeric 190 and 210 kDa BCR-ABL fusion proteins. Evolution of the CML to the more agressive acute myelogenous leukemia (AML) is accompanied by increased cellular proliferation and genomic instability at the cytogenetic level. We hypothezised that genomic instability at the nucleotide level and spontaneous error in DNA replication may also contribute to the evolution of CML to AML. Murine Ba/F3 cell line was transfected with the p190 and p210-encoding BCR-ABL oncogenes, and spontaneous mutation frequency at the Na-K-ATPase and the hypoxanthine guanine phosphoribosyl transferase (HPRT) loci were measured. A significant 3-5-fold increase in mutation frequency for the transfected cells relative to the untransfected control cells was found. Furthermore, we observed that BCR-ABL transfection induced an overexpression of DNA polymerase beta, the most inaccurate of the mammalian DNA polymerases, as well as an increase in its activity, suggesting that inaccuracy of DNA replication may account for the observed mutator phenotype. These data suggest that the Philadelphia abnormality confers a mutator phenotype and may have implications for the potential role of DNA polymerase beta in this process.

Animals↗
PubMed1999

Unfractionated peripheral blood stem cell autografts and CD(34+)-enriched autografts have similar long-term culture initiating capacity in multiple myeloma.

CD(34+)-enriched peripheral blood stem cells (PBSC) are increasingly being used as an autograft in patients with multiple myeloma (MM). The rationale for the use of the CD34+-enriched fraction in MM is the ability to obtain a graft with a significant reduction of contamination by plasma cells. However, the effect of such a manipulation on the proliferating potential of the engrafted cells is not known. We wished to study, as part of a randomized trial comparing the outcome in MM patients transplanted with either CD(34+)-enriched cells or unfractionated PBSC, the primitive hematopoietic cell content of the autografts using long-term culture initiating cell (LTC-IC) assays in 7 MM patients. In 3 patients CD(34+)cell-enriched fraction was compared to unfractionated PBSC whereas in the remaining 4 patients the LTC-IC assay was performed on total PBSC. The mean percentage of CD34+ cells of the CD34+ selected fraction in three patients was 82% (range 71%-96%) whereas the same percentage in PBSC varied from 0.6% to 10% in 4 patients (mean: 4.2%). Out of three patients transplanted with CD34+ cell fraction, two patients were found to have a very similar LTC-IC generating potential in their CD34+ versus PBSC fractions as this was assessed by the clonogenic cell output at week+5 per 10(4) CD34+ cells initiating the culture (PBSC: 92 and 168 and CD34+ fraction: 102 and 16, respectively) whereas one patient had a slightly different values (PBSC: 51 and CD34+ fraction: 103). When the PBSC fraction was compared in all 7 patients, the LTC-IC generation potential was very heterogenous, varying from 1.4 to 168. To determine if the selection procedure influences the numbers of LTC-IC's in both fractions, we have performed limiting dilution assays to determine both the frequency of distribution of hematopoietic colonies and the frequency of LTC-IC's in two patients. The frequency of distribution of hematopoietic colonies was linear in both CD34+ and PBSC fractions as was the frequency of LTC-IC when the corrections were made with regard to the CD34+ cell-content of the cultures (1/20). Our results indicate that the CD34+ selection procedure used in all three patients (Ceprate) is not deleterious for the generation of LTC-IC's and these findings support the rationale for the use of this procedure in multiple for the purposes of tumor depletion.

Adult↗
PubMed1998

BCR-ABL delays apoptosis upstream of procaspase-3 activation.

The p210(bcr-abl) protein was shown to inhibit apoptosis induced by DNA damaging agents. Apoptotic DNA fragmentation is delayed in the bcr-abl+ K562 and KCL-22 compared with the bcr-abl- U937 and HL-60 cell lines when treated with etoposide concentrations that induce similar DNA damage in the four cell lines. By the use of a cell-free system, we show that nuclei from untreated cells that express p210(bcr-abl) remain sensitive to apoptotic DNA fragmentation induced by triton-soluble extracts from p210(bcr-abl-) cells treated with etoposide. In the four tested cell lines, apoptotic DNA fragmentation is associated with a decreased expression of procaspase-3 (CPP32/Yama/apopain) and its cleavage into a p17 active fragment, whereas the long isoform of procaspase-2 (ICH-1L) remains unchanged and the poly(adenosine diphosphate-ribose)polymerase protein is cleaved. These events are delayed in bcr-abl+ compared with bcr-abl- cell lines. The role of p210(bcr-abl) in this delay is confirmed by comparing the effect of etoposide on the granulocyte-macrophage colony-stimulating factor (GM-CSF)-dependent UT7 cells and the bcr-abl-transfected GM-CSF-independent UT7/9 clone. We conclude that the cytosolic pathway that leads to apoptotic DNA fragmentation in etoposide-treated leukemic cells is delayed upstream of procaspase-3-mediated events in bcr-abl+ cell lines.

Apoptosis↗