Rapid detection of phosphotyrosine proteins by flow cytometric analysis in Bcr-Abl-positive cells.

BACKGROUND: Constitutive tyrosine phosphorylation derived from Bcr-Abl kinase activity is the major characteristic of Bcr-Abl positive cells. In this study, we developed a method to detect the phosphotyrosine proteins by flow cytometry and we asked whether phosphorylation was affected by imatinib mesylate treatment. METHODS: Cells were treated or not with imatinib mesylate, fixed and permeabilized by PFA followed by saponin, then stained with anti-phosphotyrosine (p-tyr) monoclonal antibody and analyzed by flow cytometry. RESULTS: Optimal staining parameters were performed with p-tyr antibody using K562 and LAMA84 lines that displayed high levels of tyrosine phosphorylation as compared to the control line, HL60. Tyrosine phosphorylation was inhibited by imatinib in a dose-dependent manner, but not modified by other inhibitors demonstrating that the staining detected is specific to Bcr-Abl phosphorylation. The staining of imatinib-resistant cell lines such as the mutated BaF/Bcr-AblT315I cell line or resistant CML patient cells, showed that hyperphosphorylation was not affected by imatinib treatment. In one CML patient, our technique permitted us to detect a small hyperphosphorylated population resistant to imatinib that appeared hyperphosphorylated and amplified at the time of relapse. CONCLUSIONS: We have developed a flow cytometric technique presenting several advantages such as rapidity and sensitivity, which requires fewer cells than the Western blot.     

Inhibition of protein-tyrosine phosphatase 1B (PTP1B) mediates ubiquitination and degradation of Bcr-Abl protein

Chronic myelogenous leukemia (CML) is a myeloproliferative disorder characterized at the molecular level by the expression of Bcr-Abl, a chimeric protein with deregulated tyrosine kinase activity. The protein-tyrosine phosphatase 1B (PTP1B) is up-regulated in Bcr-Abl-expressing cells, suggesting a regulatory link between the two proteins. To investigate the interplay between these two proteins, we inhibited the activity of PTP1B in Bcr-Abl-expressing TonB.210 cells by either pharmacological or siRNA means and examined the effects of such inhibition on Bcr-Abl expression and function. Herein we describe a novel mechanism by which the phosphatase activity of PTP1B is required for Bcr-Abl protein stability. Inhibition of PTP1B elicits tyrosine phosphorylation of Bcr-Abl that triggers the degradation of Bcr-Abl through ubiquitination via the lysosomal pathway. The degradation of Bcr-Abl consequently inhibits tyrosine phosphorylation of Bcr-Abl substrates and the downstream production of intracellular reactive oxygen species. Furthermore, PTP1B inhibition reduces cell viability and the IC(50) of the Bcr-Abl inhibitor imatinib mesylate. Degradation of Bcr-Abl via PTP1B inhibition is also observed in human CML cell lines K562 and LAMA-84. These results suggest that inhibition of PTP1B may be a useful strategy to explore in the development of novel therapeutic agents for the treatment of CML, particularly because host drugs currently used in CML such as imatinib focus on inhibiting the kinase activity of Bcr-Abl.     

Protein-acrylamide copolymer hydrogels for array-based detection of tyrosine kinase activity from cell lysates

We describe the development of an array-based assay for the molecular level detection of tyrosine kinase activity directly from cellular extracts. Glutathione S-transferase-Crkl (GST-Crkl) fusion proteins are covalently immobilized into polyacrylamide gel pads via copolymerization of acrylic monomer and acrylic-functionalized GST-Crkl protein constructs on a polyacrylamide surface. The resulting hydrogels resist nonspecific protein adsorption, permitting quantitative and reproducible determination of Abl tyrosine kinase activity and inhibition, even in the presence of a complex cell lysate mixture. Half-maximal inhibition (IC50) values for imatinib mesylate inhibition of GST-Crkl (SH3) phosphorylation by v-Abl in a purified system and Bcr-Abl within a K562 cell lysate were determined to be 1.5 and 20 microM, respectively. Additionally, the protein-acrylamide copolymer arrays detected CML cell levels as low as 15% in a background of Bcr-Abl- leukemic cells and provided the framework for the parallel evaluation of six tyrosine kinase inhibitors. Such a system may have direct application to the detection and treatment of cancers resulting from upregulated tyrosine kinase activity, such as chronic myeloid leukemia (CML). These findings also establish a basis for screening tyrosine kinase inhibitors and provide a framework on which protein-protein interactions in other complex systems can be studied.     

Apoptosis-based dual molecular targeting by INNO-406, a second-generation Bcr-Abl inhibitor, and ABT-737, an inhibitor of antiapoptotic Bcl-2 proteins, against Bcr-Abl-positive leukemia

Bcr-Abl is the cause of Philadelphia-positive (Ph(+)) leukemias and also constitutes their principal therapeutic target, as exemplified by dramatic effects of imatinib mesylate. However, mono-targeting of Bcr-Abl does not always achieve complete leukemia eradication, and additional strategies those enable complete elimination of leukemic cells are desired to develop. Here we demonstrate that INNO-406, a much more active Bcr-Abl tyrosine kinase inhibitor than imatinib, augments the activities of several proapoptotic Bcl-2 homology (BH)3-only proteins (Bim, Bad, Bmf and Bik) and induces apoptosis in Ph(+) leukemia cells via Bcl-2 family-regulated intrinsic apoptosis pathway. ABT-737, an inhibitor of antiapoptotic Bcl-2 and Bcl-X(L), greatly enhanced the apoptosis by INNO-406, even in INNO-406-less sensitive cells with Bcr-Abl point mutations except T315I mutation. In contrast, co-treatment with INNO-406 and other pharmacologic inducers of those BH3-only proteins, such as 17-allylaminogeldanamycin, an heat shock protein-90 inhibitor, or PS-341, a proteasome inhibitor, did not further increase the BH3-only protein levels or sensitize leukemic cells to INNO-406-induced apoptosis, suggesting a limit to how much expression levels of BH3-only proteins can be increased by anticancer agents. Thus, double-barrelled molecular targeting for Bcr-Abl-driven oncogenic signaling and the cell protection by antiapoptotic Bcl-2 family proteins may be the rational therapeutic approach for eradicating Ph(+) leukemic cells.     

Erythropoietin promotes resistance against the Abl tyrosine kinase inhibitor imatinib (STI571) in K562 human leukemia cells

Chronic myeloid leukemia is characterized by the Philadelphia chromosome translocation that causes expression of Bcr-Abl, a deregulated tyrosine kinase. Imatinib mesylate (STI571, Gleevec), a therapeutically used inhibitor of Bcr-Abl, causes apoptosis of Bcr-Abl-positive cells. In the leukemia cell line K562, we observed spontaneous resistance to imatinib at very low frequencies when cells were exposed to the drug (1 micro M) for more than 4 weeks. Surprisingly, in the presence of erythropoietin (Epo), K562 cells were temporarily able to sustain proliferation in the presence of imatinib, and imatinib-resistant clones could be isolated with high frequencies. From such imatinib-resistant, Epo-dependent clones, sublines could be established that were resistant to imatinib in the absence of Epo. Mitogen-activated protein (MAP) kinase activity was inhibited by imatinib treatment but could be partially restored by Epo. Inhibition of MAP kinase or phosphatidylinositol 3-kinase blocked the protective effect of Epo. The data suggest that K562 cells acquire factor dependency under imatinib/Epo treatment, allowing them to escape from imatinib-induced, immediate cell death. This pool of cells provides the basis for the outgrowth of imatinib-resistant clones of unlimited proliferative capacity. Thus, Epo, an endogenous regulator of hematopoiesis, promotes the development of resistance to imatinib.     

Imatinib-resistant CML cells have low ENT activity but maintain sensitivity to gemcitabine

Philadelphia chromosome-positive chronic myelogenus leukemia (CML) is widely treated with imatinib mesylate (imatinib), a potent inhibitor of the Bcr-Abl tyrosine kinase. However, resistance to this compound remains a concern. Current treatment approaches include combinations of imatinib with nucleoside analogs such as gemcitabine, which requires equilibrative nucleoside transporters (ENTs) for uptake, to overcome this resistance. Here we report that imatinib treatment decreased ENT1-dependent activity and mRNA expression. Although, imatinib-resistant cells showed decreased levels of both ENT1 and ENT2 activity and expression, these cells remained sensitive to gemcitabine, suggesting that nucleoside analogs can be used as adjunctive therapy.     

Translational Regulation of GPx-1 and GPx-4 by the mTOR Pathway

Glutathione peroxidase activity was previously determined to be elevated in lymphocytes obtained from patients treated with the Bcr-Abl kinase inhibitor imatinib mesylate. In order to expand upon this observation, the established chronic myelogenous leukemia cell lines KU812 and MEG-01 were treated with imatinib and the effect on several anti-oxidant proteins was determined. The levels of GPx-1 were significantly increased following treatment with imatinib. This increase was not due to altered steady-state mRNA levels, and appeared to be dependent on the expression of Bcr-Abl, as no increases were observed following imatinib treatment of cells that did not express the fusion protein. The nutrient-sensing signaling protein, mammalian target of rapamycin (mTOR), can be activated by Bcr-Abl and its activity regulates the translation of many different proteins. Treatment of those same cells used in the imatinib studies with rapamycin, an inhibitor of mTOR, resulted in elevated GPx-1 and GPx-4 protein levels independent of Bcr-Abl expression. These proteins all belong to the selenoprotein family of peptides that contain the UGA-encoded amino acid selenocysteine. Collectively, these data provide evidence of a novel means of regulating anti-oxidants of the selenoprotein family via the mTOR pathway.     

Assaying Bcr-Abl kinase activity and inhibition in whole cell extracts by phosphorylation of substrates immobilized on agarose beads

There is a current and increasing demand for simple, robust, nonradioactive assays of protein tyrosine kinase activity with applications for clinical diagnosis and high-throughput screening of potential molecularly targeted therapeutic agents. One significant challenge is to detect and measure the activity of specific kinases with key roles in cell signaling as an approach to distinguish normal cells from cancer cells and as a means of evaluating targeted drug efficacy and resistance in cancer cells. Here, we describe a method in which kinase substrates fused to glutathione-S-transferase and immobilized on glutathione agarose beads are phosphorylated, eluted, and then assayed to detect kinase activity. The activity of recombinant, purified c-Abl kinase or Bcr-Abl kinase in whole cell extracts can be detected with equivalent specificity, sensitivity, and reproducibility. Similarly, inhibition of recombinant c-Abl or Bcr-Abl in cells or cell extracts by imatinib mesylate and other Bcr-Abl targeted kinase inhibitors is readily assayed. This simple kinase assay is sufficiently straightforward and robust for use in clinical laboratories and is potentially adaptable to high-throughput assay formats.     

Gaining insights into the Bcr-Abl activity-independent mechanisms of resistance to imatinib mesylate in KCL22 cells: A comparative proteomic approach

Imatinib mesylate is a potent inhibitor of Bcr-Abl tyrosine kinase, an oncoprotein that plays a key role in the development of chronic myeloid leukemia. Consequently, imatinib is used as front-line therapy for this disease. A major concern in imatinib treatment is the emergence of resistance to the drug. Here we used the imatinib-resistant KCL22R and imatinib-sensitive KCL22S cells in which none of the known resistance mechanisms has been detected and hence novel Bcr-Abl activity-independent mechanisms could be envisaged. We characterized proteins that were differentially expressed between the KCL22R and KCL22S cells. Using two-dimensional differential gel electrophoresis coupled with mass spectrometry and Western blot analysis we identified 51 differentially expressed proteins: 27 were over-expressed and 24 were under-expressed in KCL22R versus KCL22S cells. Several of these proteins are likely to be involved in such survival mechanisms as modulation of redox balance and activation of anti-apoptotic pathways mediated by NF-κB and Ras-MAPK signaling. The data reported may be useful for further studies on mechanisms of imatinib resistance and for the screening of biomarkers to develop new combinatorial therapeutic approaches.     

Knocking Down Low Molecular Weight Protein Tyrosine Phosphatase (LMW-PTP) Reverts Chemoresistance through Inactivation of Src and Bcr-Abl Proteins

The development of multidrug resistance (MDR) limits the efficacy of continuous chemotherapeutic treatment in chronic myelogenous leukemia (CML). Low molecular weight protein tyrosine phosphatase (LMW-PTP) is up-regulated in several cancers and has been associated to poor prognosis. This prompted us to investigate the involvement of LMW-PTP in MDR. In this study, we investigated the role of LMW-PTP in a chemoresistant CML cell line, Lucena-1. Our results showed that LMW-PTP is highly expressed and 7-fold more active in Lucena-1 cells compared to K562 cells, the non-resistant cell line. Knocking down LMW-PTP in Lucena-1 cells reverted chemoresistance to vincristine and imatinib mesylate, followed by a decrease of Src and Bcr-Abl phosphorylation at the activating sites, inactivating both kinases. On the other hand, overexpression of LMW-PTP in K562 cells led to chemoresistance to vincristine. Our findings describe, for the first time, that LMW-PTP cooperates with MDR phenotype, at least in part, through maintaining Src and Bcr-Abl kinases in more active statuses. These findings suggest that inhibition of LMW-PTP may be a useful strategy for the development of therapies for multidrug resistant CML.     

Design and synthesis of 3-substituted benzamide derivatives as Bcr-Abl kinase inhibitors

A series of 3-substituted benzamide derivatives structurally related to STI-571 (imatinib mesylate), a Bcr-Abl tyrosine kinase inhibitor used to treat chronic myeloid leukemia (CML), was prepared and evaluated for antiproliferative activity against the Bcr-Abl-positive leukemia cell line K562. About ten 3-halogenated and 3-trifluoromethylated benzamide derivatives were identified as highly potent Bcr-Abl kinase inhibitors. One of these, NS-187 (9b), is a promising new candidate Bcr-Abl inhibitor for the therapy of STI-571-resistant chronic myeloid leukemia.     

Targeting the kinase activity of the BCR-ABL fusion protein in patients with chronic myeloid leukemia

Imatinib mesylate is a major advance in the therapy of patients with chronic myelogenous leukemia (CML). Imatinib mesylate binds to the inactive conformation of BCR-ABL tyrosine kinase suppressing the Philadelphia chromosome positive clone in CML. Clinical studies have yielded impressive results in all phases of CML. With higher rates of complete cytogenetic response with imatinib, molecular monitoring of disease is now advisable in assessing response and determining prognosis. Emergence of resistance to imatinib may be manifest at the hematologic, cytogenetic, or molecular levels in patients who remain in chronic phase, or may be evidenced by the development of more advanced CML phases. Resistance and eventual clinical failure of imatinib occurs in most patients with blastic phase disease. Resistance may occur at the level of Bcr-Abl, with reduction or loss of imatinib effectiveness as a kinase inhibitor, or, despite retention of its inhibitory ability, with changes in the ability to deliver an effective dose at the cellular level, and/or, the leukemia becoming less dependent on Bcr-Abl. The various mechanisms underlying these differing, non-mutually exclusive, mechanisms of resistance must be understood to develop corresponding therapeutic remedies. We review the current data on imatinib in CML, the criteria for diagnosis of imatinib resistance, and the mechanisms that underlie such resistance in CML.     

Creation and characterization of temperature-sensitive mutants of the lck tyrosine protein kinase.

Temperature-sensitive mutants of the lck tyrosine protein kinase were created by the introduction of mutations known to cause temperature sensitivity of the v-src tyrosine protein kinase of Rous sarcoma virus. p56lck activated by mutation of the regulatory site of tyrosine phosphorylation, Tyr-505, to Phe transforms fibroblasts in culture. Mutations identical to those responsible for the temperature-sensitive phenotypes of the tsNY68 and tsNY72-4 v-src mutants rendered this activated lck gene temperature sensitive for both morphological transformation and induction of growth in soft agar. The mutant proteins were incapable of cellular transformation at the nonpermissive temperature in part because of failure of the lck protein to accumulate to normal levels. Morphological transformation of fibroblasts was detectable within 24 h of a shift of cells to the permissive temperature and was essentially complete in 48 to 72 h. These mutants should prove useful for the study of the function of the lck kinase in hematopoietic cells.     

Role of protein tyrosine kinase inhibitors in cancer therapeutics

Protein tyrosine kinases (PTKs) are critical in regulating cell growth and differentiation and are deeply involved in several cancers. PTK-inhibitors are mainly ATP-site directed and are finding use in the treatment of several cancers, and more than 30 such agents are now in phase I-III clinical trials. The present review focuses mainly on the development of PTK inhibitors in clinical trials, with special emphasis on imatinib mesylate, a rationally designed, potent oral anticancer agent and selective inhibitor for Abl tyrosine kinase, including Bcr-Abl, C-kit and platelet-derived growth factor-receptor tyrosine kinases, which has been implicated in several malignancies, including chronic myeloid leukemia and gastrointestinal stromal tumour.     

CCN3: a key growth regulator in Chronic Myeloid Leukaemia

Chronic Myeloid Leukaemia (CML) is characterized by expression of the constitutively active Bcr-Abl tyrosine kinase. We have shown previously that the negative growth regulator, CCN3, is down-regulated as a result of Bcr-Abl kinase activity and that CCN3 has a reciprocal relationship of expression with BCR-ABL. We now show that CCN3 confers growth regulation in CML cells by causing growth inhibition and regaining sensitivity to the induction of apoptosis. The mode of CCN3 induced growth regulation was investigated in K562 CML cells using gene transfection and treatment with recombinant CCN3. Both strategies showed CCN3 regulated CML cell growth by reducing colony formation capacity, increasing apoptosis and reducing ERK phosphorylation. K562 cells stably transfected to express CCN3 showed enhanced apoptosis in response to treatment with the tyrosine kinase inhibitor, imatinib. Whilst CCN3 expression was low or undetectable in CML stem cells, primary CD34+ CML progenitors were responsive to treatment with recombinant CCN3. This study shows that CCN3 is an important growth regulator in haematopoiesis, abrogation of CCN3 expression enhances BCR-ABL dependent leukaemogenesis. CCN3 restores growth regulation, regains sensitivity to the induction of apoptosis and enhances imatinib cell kill in CML cells. CCN3 may provide an additional therapeutic strategy in the management of CML.     

Fc epsilon RI-induced protein tyrosine phosphorylation of pp72 in rat basophilic leukemia cells (RBL-2H3). Evidence for a novel signal transduction pathway unrelated to G protein activation and phosphatidylinositol hydrolysis.

Recently, we demonstrated that aggregation of the high affinity IgE receptor in rat basophilic leukemia (RBL-2H3) cells results in rapid tyrosine phosphorylation of a 72-kDa protein (pp72). Here we investigated the relationship of pp72 phosphorylation to guanine nucleotide-binding protein (G protein) activation and phosphatidylinositol hydrolysis. The activation of G proteins by NaF in intact cells or by guanosine 5'-O-(3-thiotriphosphate) in streptolysin O-permeabilized cells induced both phosphatidylinositol hydrolysis and histamine release without tyrosine phosphorylation of pp72. Similarly, in RBL-2H3 cells expressing the G protein-coupled muscarinic acetylcholine receptor, carbachol activated phospholipase C and induced secretion without concomitant pp72 phosphorylation. Therefore, pp72 phosphorylation was not induced by G protein activation or as a consequence of phosphatidylinositol hydrolysis. To investigate whether pp72 tyrosine phosphorylation precedes the activation of phospholipase C, we studied the effect of the tyrosine kinase inhibitor genistein. Preincubation of cells with genistein decreased, in parallel, antigen-induced tyrosine phosphorylation of pp72 (IC50 = 34 micrograms/ml) and histamine release (IC50 = 31 micrograms/ml). However, genistein at concentrations of up to 60 micrograms/ml did not inhibit phosphatidylinositol hydrolysis nor did it change the amount of the secondary messenger inositol (1,4,5)-triphosphate. Previous observations showed that there was no pp72 tyrosine phosphorylation after activation of protein kinase C or after an increase in intracellular calcium. Taken together, these results suggest that pp72 tyrosine phosphorylation represents a distinct, independent signaling pathway induced specifically by aggregation of the Fc epsilon RI.     

The two major imatinib resistance mutations E255K and T315I enhance the activity of BCR/ABL fusion kinase

The resistance to the tyrosine kinase inhibitor imatinib in BCR/ABL-positive leukemias is mostly associated with mutations in the kinase domain of BCR/ABL, which include the most prevalent mutations E255K and T315I. Intriguingly, these mutations have also been identified in some patients before imatinib treatment. Here we examined the effects of these mutations on the kinase activity of a BCR/ABL kinase domain construct that also contained the SH3 and SH2 domains. When expressed in COS7 cells, the BCR/ABL construct with either E255K or T315I exhibited not only the resistance to imatinib but also the increase in activity to induce autophosphorylation as well as tyrosine phosphorylation of various cellular proteins, which included STAT5. The mutant kinases also showed increased activities in in vitro kinase assays. These results raise a possibility that the major imatinib resistance mutations E255K and T315I may confer the growth advantage on leukemic cells to expand in the absence of selective pressure from imatinib treatment.     

High-affinity IgE receptor-mediated stimulation of rat basophilic leukemia (RBL-2H3) cells induces early and late protein-tyrosine phosphorylations.

We reported previously that stimulation of RBL-2H3 cells through the high-affinity IgE receptor resulted in tyrosine phosphorylation of a 72-kDa protein (pp72) that was coupled to signal transduction. In the present study, although pp72 tyrosine phosphorylation was induced only by antigen triggering, stimulation of RBL-2H3 cells by either antigen or the calcium-ionophore A23187 led to increased tyrosine phosphorylation of a 110-kDa protein (pp110). This tyrosine phosphorylated protein was also observed when RBL-2H3 cells were transfected with the G protein-coupled m3 muscarinic receptor and then stimulated to secrete with carbachol. In contrast to tyrosine phosphorylation of pp72, antigen-induced pp110 tyrosine phosphorylation required extracellular calcium, was absent in cells depleted of protein kinase C, and was detected between 1 and 5 min after stimulation. The protein-tyrosine kinase inhibitor genistein blocked both histamine release and tyrosine phosphorylation induced by A23187. Altogether, the data suggest a role for pp110 in secretion. However, protein kinase C activation induced pp110 tyrosine phosphorylation but not histamine release demonstrating that pp110 tyrosine phosphorylation alone is not sufficient for degranulation. We conclude that tyrosine phosphorylation of pp72 is associated with the early steps of IgE receptor-generated signaling, whereas pp110 tyrosine phosphorylation occurs secondary to calcium influx and protein kinase C activation.