Functional cooperation among Ras, STAT5, and phosphatidylinositol 3-kinase is required for full oncogenic activities of BCR/ABL in K562 cells

Phosphatidylinositol-3-kinase activation mediates proline-rich tyrosine kinase 2 phosphorylation and recruitment to β1-integrins in human CD34+ cells

OBJECTIVE

beta1-integrins mediate hematopoietic stem and progenitor cell homing and retention in the bone marrow (BM) and inhibit hematopoietic proliferation and differentiation. Having no intrinsic kinase activity, integrins recruit intracellular kinases, such as the focal adhesion kinase (FAK) or the related proline-rich tyrosine kinase 2 (PYK2), to initiate signal transduction. Phosphatidylinositol-3-kinase (PI3K), which is involved in beta1-integrin signaling in many cell types, is physically and functionally associated with FAK in anchorage-dependent cells. Because PYK2 is the principal focal adhesion kinase expressed in primary human CD34+ cells, we assessed its functional relationship with PI3K in CD34+ cells in response to integrin engagement.

METHODS

beta1-integrins on primary mobilized peripheral blood CD34+ cells and CD34+ KG1A cells were engaged by adhesion to fibronectin (FN) or by cross-linking with an anti-beta1 integrin antibody, respectively. PI3K activity and PYK2 phosphorylation were then assessed in the presence or absence of the PI3K inhibitor, wortmannin. Association between PI3K, PYK2, and the beta1-integrin subunit were also evaluated in co-immunoprecipitation experiments.

RESULTS

beta1-integrin engagement induced PI3K activation, which was required for, and temporally preceded, PYK2 phosphorylation, indicating that PI3K lies upstream of PYK2 in CD34+ cells. Furthermore, although PYK2 and PI3K were constitutively associated, interaction of the PYK2/PI3K complex with beta1-integrins required prior integrin engagement and PI3K activation.

CONCLUSION

Activation of PI3K following beta1-integrin engagement on human CD34+ cells results in subsequent phosphorylation of PYK2, and is required for the recruitment of the PI3K/PYK2 complex to beta1-integrins at the cell surface.

Identification of a cytokine-induced antiapoptotic molecule anamorsin essential for definitive hematopoiesis

Many growth factors and cytokines prevent apoptosis. Using an expression cloning method, we identified a novel antiapoptotic molecule named Anamorsin, which does not show any homology to known apoptosis regulatory molecules such as Bcl-2 family, caspase family, or signal transduction molecules. The expression of Anamorsin was completely dependent on stimulation with growth factors such as interleukin 3, stem cell factor, and thrombopoietin in factor-dependent hematopoietic cell lines, and forced expression of Anamorsin conferred resistance to apoptosis caused by growth factor deprivation in vitro. Furthermore, Anamorsin was found to act as an antiapoptotic molecule in vivo because Anamorsin^−/− mice die in late gestation due to defective definitive hematopoiesis in the fetal liver (FL). Although the number of hematopoietic stem/progenitor cells in the FL did not decrease in these mice, myeloid, and particularly erythroid colony formation in response to cytokines, was severely disrupted. Also, Anamorsin^−/− erythroid cells initiated apoptosis during terminal maturation. As for the mechanism of Anamorsin-mediated cell survival, a microarray analysis revealed that the expression of Bcl-xL and Jak2 was severely impaired in the FL of Anamorsin^−/− mice. Thus, Anamorsin is considered to be a necessary molecule for hematopoiesis that mediates antiapoptotic effects of various cytokines.

A Bcr/Abl-independent, Lyn-dependent Form of Imatinib Mesylate (STI-571) Resistance Is Associated with Altered Expression of Bcl-2

The relationship between the Src kinase Lyn and Bcl-2 expression was examined in chronic myelogenous leukemia cells (K562 and LAMA84) displaying a Bcr/Abl-independent form of imatinib mesylate resistance. K562-R and LAMA-R cells that were markedly resistant to induction of mitochondrial dysfunction (e.g. loss of mitochondrial membrane potential, Bax translocation, cytochrome c, and apoptosis-inducing factor release) and apoptosis by imatinib mesylate exhibited a pronounced reduction in expression of Bcr/Abl, Bcl-x(L), and STAT5 but a striking increase in levels of activated Lyn. Whereas basal expression of Bcl-2 protein was very low in parental cells, imatinib-resistant cells displayed a marked increase in Bcl-2 mRNA and/or protein levels. Treatment of LAMA-R cells with the Src kinase inhibitor PP2 significantly reduced Lyn activation as well as Bcl-2 mRNA and protein levels. Transient or stable transfection of LAMA84 or K562 cells with a constitutively active Lyn (Y508F), but not with a kinase-dead mutant (K275D), significantly increased Bcl-2 protein expression and protected cells from lethality of imatinib mesylate. Ectopic expression of Bcl-2 protected K562 and LAMA84 cells from imatinib mesylate- and PP2-mediated lethality. Conversely, interference with Bcl-2 function by co-administration of the small molecule Bcl-2 inhibitor HA14-1 or down-regulation of Bcl-2 expression by small interfering RNA or antisense strategies significantly increased mitochondrial dysfunction and apoptosis induced by imatinib mesylate and the topoisomerase inhibitor VP-16 in LAMA-R cells. In marked contrast, these interventions had little effect in parental LAMA84 cells that display low basal levels of Bcl-2. Together, these findings indicate that activation of Lyn in leukemia cells displaying a Bcr/Abl-independent form of imatinib mesylate resistance plays a functional role in Bcl-2 up-regulation and provide a theoretical basis for the development of therapeutic strategies targeting Bcl-2 in such a setting.

The role of the STAT5 proteins in the proliferation and apoptosis of the CML and AML cells

OBJECTIVES

The STAT5 proteins are activated by many haematological cytokines and growth factors. They regulate cell cycle, apoptosis and proliferation of different cells via the influence on gene transcription. Because STAT5s are constitutively activated in certain haematooncologic diseases, they are suggested to play an important role in leukaemogenesis. However, the real function of these proteins in haematopoietic cell transformation and proliferation is not clear enough. The aim of this study was to evaluate the influence of suppression of STAT5A and STAT5B expression on the clonogenicity and apoptosis of the chronic myeloid leukaemia (CML) and acute myeloid leukaemia (AML) cells.

MATERIAL AND METHODS

Blast cells from 34 newly diagnosed patients with CML and AML were used in our experiments. Antisense oligodeoxynucleotides (ODNs) were applied to block STAT5A and STAT5B at the mRNA level and the RT-PCR method was used to study STAT5 mRNA expression in the cells after incubation with ODNs. Moreover, Western blot analysis of the STAT5 proteins was performed. The effect of ODN pretreatment on cell clonogenicity in methylocellulose cultures was examined according to the type of oligodeoxynucleotide and the time of exposure. The induction of apoptosis in cells was also estimated by the Annexin V/PI staining and the TUNEL method using flow cytometry.

RESULTS

Perturbation of STAT5 expression decreased proliferative potential of the CML and the AML blasts as well as enhanced their apoptosis (P < 0.05).

CONCLUSIONS

Our studies showed that the STAT5 proteins may be critical in the regulation of growth and apoptosis of the CML and AML leukaemic cells.

Phosphoinositide 3-kinase signaling is essential for ABL oncogene-mediated transformation of B-lineage cells

BCR-ABL and v-ABL are oncogenic forms of the Abl tyrosine kinase that can cause leukemias in mice and humans. ABL oncogenes trigger multiple signaling pathways whose contribution to transformation varies among cell types. Activation of phosphoinositide 3-kinase (PI3K) is essential for ABL-dependent proliferation and survival in some cell types, and global PI3K inhibitors can enhance the antileukemia effects of the Abl kinase inhibitor imatinib. Although a significant fraction of BCR-ABL-induced human leukemias are of B-cell origin, little is known about PI3K signaling mechanisms in B-lineage cells transformed by ABL oncogenes. Here we show that activation of class I(A) PI3K and downstream inactivation of FOXO transcription factors are essential for survival of murine pro/pre-B cells transformed by v-ABL or BCR-ABL. In addition, analysis of mice lacking individual PI3K genes indicates that products of the Pik3r1 gene contribute to transformation efficiency by BCR-ABL. These findings establish a role for PI3K signaling in B-lineage transformation by ABL oncogenes.

Apoptosis induced by molecular targeting therapy in hematological malignancies

Molecular targeting therapies for hematological malignant diseases such as monoclonal antibodies and small molecules have been reviewed. Imatinib mesylate (STI571) targets the tyrosine kinase activity of the BCR-ABL fusion protein in CML, and was superior to IFN-alpha plus low-dose cytarabine in newly diagnosed chronic-phase CML in a phase III randomized study. Imatinib induced apoptosis in BCR-ABL-positive cells in vitro, and activates several signaling pathways such as PI3K/Akt, STAT5 and Ras/MAPK. Combination therapies with imatinib and new strategies for downregulation of intracellular BCR-ABL protein levels have also been investigated from the phenomenon of resistance to imatinib. Anti-CD20 (rituximab) became the first monoclonal antibody approved for the treatment of a relapsed/refractory follicular/low-grade NHL and promising results were obtained from a phase III randomized study. Although antibody-dependent cell-mediated cytotoxicity and complement-mediated cytotoxicity are likely to be the major effectors of B-cell depletion in vivo, direct cytotoxicity by CD20 monoclonal antibody on B-cell lines in vitro has been reported. Anti-CD33 (Mylotarg) and FLT3 inhibitors for AML have also been used in clinical trials and signaling pathways induced by these agents are under intensive investigation. Arsenic trioxide, like all-TRANS-retinoic acid (ATRA), downregulates promyelocytic leukemia protein/retinoic acid receptor-alpha (PML/RARalpha) fusion protein and induced apoptosis in APL cells, and promising results were obtained from ATRA-resistant APL patients. Finally we show our promising in vitro and in vivo data of R-etodolac (a non-steroidal anti-inflammatory drug lacking cyclooxygenase inhibitor activity) against chronic lymphocytic leukemia (CLL) cells.

Overproduction of BCR-ABL induces apoptosis in imatinib mesylate-resistant cell lines

BACKGROUND

Imatinib mesylate, a BCR-ABL tyrosine kinase inhibitor, induces apoptosis in chronic myeloid leukemia cells. Resistance to imatinib is currently the most important concern of this treatment. One of the main mechanisms of this resistance is overexpression of BCR-ABL.

METHODS

In the current study, the authors investigated the correlation between BCR-ABL overexpression and apoptosis in BaF/BCR-ABL and LAMA84 cell lines resistant to imatinib suddenly deprived of the inhibitor, and compared with their sensitive counterpart.

RESULTS

Removal of imatinib from culture medium led to a decrease in Bcr-Abl protein expression by Day 5, which was sustained for > or = 3 weeks of imatinib deprivation. Apoptosis was observed after 3 days of imatinib deprivation in resistant lines accompanied by caspase activation, loss of membrane asymmetry (annexin V staining), and alteration of mitochondrial potential (dihexyloxacarbocyanine iodide [DiOC6]). Transient activation of the STAT5/Bcl-xL pathway and Akt kinase activity preceded these responses.

CONCLUSIONS

Thus, imatinib removal led to apoptosis of BCR-ABL-overexpressing leukemic cells, a phenomenon that could be exploited to sensitize imatinib-resistant cells to the cytotoxic effect of other drugs.

D cyclins in lymphocytes

BACKGROUND

D cyclins are essential for the progression of cells through the G1 phase of the cell cycle. There are three distinct D cyclins. Cyclin D1 has been shown to be expressed by many different types of cells but not by lymphocytes. Cyclins D2 and D3 have been found in lymphocytes.

METHODS

We used high-resolution enzymatic amplification staining technology in conjunction with flow cytometry and confocal microscopy and with immunoblotting to reassess the expression of the D cyclins in human lymphocytes.

RESULTS

Using high-resolution technology for flow cytometry, we found all three D cyclins in quiescent human peripheral blood lymphocytes. Cyclin D1 was expressed in quiescent and activated cells at levels commensurate with those of actively proliferating tumor cell lines. Cyclin D1 was functional inasmuch as it was complexed with CDK4. In the quiescent cells, cyclin D1 was expressed in the cytoplasm but, after activation, was found in the nucleus.

CONCLUSIONS

These findings demonstrate that lymphocytes express cyclin D1 and necessitate a reappraisal of the hypothesis that the D cyclins subsume redundant activities with tissue-specific expression.

Monitoring of apoptosis in intact single cells with photothermal microscope

BACKGROUND

The methods for detection of apoptosis, cannot as a rule be applied to intact single cells and do not allow monitoring of the cell population. The photothermal (PT) method was evaluated for detection and monitoring of apoptosis in single intact cells--lymphocytes and blasts.

METHODS

PT microscopy is based on optical registration of cell response to the thermal impact that is induced in a cell due to absorption of a short laser pulse (532 nm, 10 ns) by cellular hem-proteins. With no pretreatment of cells, optical detection of cellular response with this method may allow monitoring of apoptosis. The PT method was applied for in vitro studies of lymphocytes and K562 blast cells during drug-induced apoptosis. Dexamethasone in 10 microM concentration was used and cell properties were monitored for 6 h.

RESULTS

PT parameters of cells deviated from control after incubation with Dexamethasone. Control measurements with fluorescent microscopy (using the Annexin-V-FLUOS kit) verified the development of apoptosis in drug-treated cells, while there was no development of necrosis. Also necrotic cells had PT properties similar to those of control cells. The PT method allowed the detection of the influence of Dexamethasone at earlier stages (2-hr incubation) than the fluorescent technique (4 h).

CONCLUSIONS

Results obtained confirmed the applicability of PT microscopy for detection and monitoring of apoptosis in single intact cells. The sensitivity of the suggested method may exceed the sensitivity of fluorescent methods.

K562 Cell Sensitization to 5-Fluorouracil- or Interferon-Alpha-Induced Apoptosis Via Cordycepin (3′-Deoxyadenosine): Fine Control of Cell Apoptosis Via Poly(A) Polymerase Upregulation

K562 cells represent a classical model for the study of drug resistance. Induction of apoptosis is accompanied by concomitant distinct modulations of poly(A) polymerase (PAP) and other proteins involved in mRNA maturation. Recent data suggest the involvement of mRNA stability in the induction of specific apoptosis pathways. In this study we used a specific polyadenylation inhibitor, cordycepin (3-deoxyadenosine), to investigate the involvement of polyadenylation in K562 cell apoptosis and drug resistance. The combination of cordycepin with either 5-fluorouracil or interferon-alpha sensitized chemoresistant K562 cells to apoptosis. This sensitization was followed by distinct PAP modulations before and after the appearance of characteristic apoptosis pointers (DNA laddering, DAPI staining, mitochondrial transmembrane potential). PAP modulations appeared essential for K562 sensitization. mRNA polyadenylation therefore seemed to be involved not only in apoptosis but also in drug resistance. Polyadenylation inhibition by cordycepin under certain conditions sensitized chemoresistant K562 cells to apoptosis and thus polyadenylation could prove to be a fine target for overcoming drug resistance.

Bcr-Abl regulates protein kinase Ciota (PKCiota) transcription via an Elk1 site in the PKCiota promoter

The protein kinase C (PKC) family of serine/threonine kinases plays an important role in numerous cancer signaling pathways, including those downstream of the bcr-abl oncogene. We demonstrated previously that atypical PKCiota is required for Bcr-Abl-mediated resistance of human K562 chronic myelogenous leukemia (CML) cells to Taxol-induced apoptosis. Here, we report that the pattern of PKC isozyme expression characteristic of CML cells is regulated by Bcr-Abl. When Bcr-Abl was expressed in Bcr-Abl-negative HL-60 promyelocytic leukemia cells, expression of the PKCbetaI, PKCbetaII, and PKCiota genes was induced, whereas expression of the PKCdelta gene was reduced to levels similar to those found in CML cells. Given the importance of PKCiota in Bcr-Abl-mediated transformation, we characterized the mechanism by which Bcr-Abl regulates PKCiota expression. A 1200-bp PKCiota promoter construct isolated from genomic DNA was highly active in Bcr-Abl-positive K562 cells and was activated when Bcr-Abl-negative cells were transfected with Bcr-Abl. Bcr-Abl-mediated induction of the PKCiota promoter was dependent upon MEK1/2 activity, but not phosphatidylinositol 3-kinase or p38 MAPK activity. Mutational analysis of the PKCiota promoter revealed a region between 97 and 114 bp upstream of the transcriptional start site that is responsible for Bcr-Abl-mediated regulation. Mutation of a consensus Elk1-binding site within this region abolished Bcr-Abl-mediated regulation. We conclude that Bcr-Abl regulates PKCiota expression through the MEK-dependent activation of an Elk1 element within the proximal PKCiota promoter. Our results indicate that Bcr-Abl-mediated transformation involves transcriptional activation of the PKCiota gene, which in turn is required for Bcr-Abl-mediated chemoresistance.

A novel mechanism for imatinib mesylate-induced cell death of BCR-ABL-positive human leukemic cells: caspase-independent, necrosis-like programmed cell death mediated by serine protease activity

Caspase-independent programmed cell death can exhibit either an apoptosis-like or a necrosis-like morphology. The ABL kinase inhibitor, imatinib mesylate, has been reported to induce apoptosis of BCR-ABL-positive cells in a caspase-dependent fashion. We investigated whether caspases alone were the mediators of imatinib mesylate-induced cell death. In contrast to previous reports, we found that a broad caspase inhibitor, zVAD-fmk, failed to prevent the death of imatinib mesylate-treated BCR-ABL-positive human leukemic cells. Moreover, zVAD-fmk-preincubated, imatinib mesylate-treated cells exhibited a necrosis-like morphology characterized by cellular pyknosis, cytoplasmic vacuolization, and the absence of nuclear signs of apoptosis. These cells manifested a loss of the mitochondrial transmembrane potential, indicating the mitochondrial involvement in this caspase-independent necrosis. We excluded the participation of several mitochondrial factors possibly involved in caspase-independent cell death such as apoptosis-inducing factor, endonuclease G, and reactive oxygen species. However, we observed the mitochondrial release of the serine protease Omi/HtrA2 into the cytosol of the cells treated with imatinib mesylate or zVAD-fmk plus imatinib mesylate. Furthermore, serine protease inhibitors prevented the caspase-independent necrosis. Taken together, our results suggest that imatinib mesylate induces a caspase-independent, necrosis-like programmed cell death mediated by the serine protease activity of Omi/HtrA2.

The proteasome inhibitor bortezomib interacts synergistically with histone deacetylase inhibitors to induce apoptosis in Bcr/Abl+ cells sensitive and resistant to STI571

Interactions between the proteasome inhibitor bortezomib and histone deacetylase inhibitors (HDIs) have been examined in Bcr/Abl+ human leukemia cells (K562 and LAMA 84). Coexposure of cells (24-48 hours) to minimally toxic concentrations of bortezomib + either suberoylanilide hydroxamic acid (SAHA) or sodium butyrate (SB) resulted in a striking increase in mitochondrial injury, caspase activation, and apoptosis, reflected by caspases-3 and -8 cleavage and poly(adenosine diphosphate-ribose) polymerase (PARP) degradation. These events were accompanied by down-regulation of the Raf-1/mitogen-induced extracellular kinase (MEK)/extracellular signal-related kinase (ERK) pathway as well as diminished expression of Bcr/Abl and cyclin D1, cleavage of p21CIP1 and phosphorylation of the retinoblastoma protein (pRb), and induction of the stress-related kinases Jun kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). Transient transfection of cells with a constitutively active MEK construct significantly protected them from bortezomib/SAHA-mediated lethality. Coadministration of bortezomib and SAHA resulted in increased reactive oxygen species (ROS) generation and diminished nuclear factor kappa B (NF-kappa B) activation; moreover, the free radical scavenger L-N-acetylcyteine (LNAC) blocked bortezomib/SAHA-related ROS generation, induction of JNK and p21CIP1, and apoptosis. Lastly, this regimen potently induced apoptosis in STI571 (imatinib mesylate)-resistant K562 cells and CD34+ mononuclear cells obtained from a patient with STI571-resistant disease, as well as in Bcr/Abl- leukemia cells (eg, HL-60, U937, Jurkat). Together, these findings raise the possibility that combined proteasome/histone deacetylase inhibition may represent a novel strategy in leukemia, including apoptosis-resistant Bcr/Abl+ hematologic malignancies.

The influence of STAT5 antisense oligonucleotides on the proliferation and apoptosis of selected human leukaemic cell lines

The signal transducers and activators of transcription--STAT5A and STAT5B--take part in the regulation of many essential physiopathological processes. They influence the cell cycle, apoptosis and the proliferation of different types of cell lines. The STAT5 proteins are induced in response to multiple haematopoietic cytokines. Because they are constitutively active in certain haemato-oncologic diseases, it is also suggested that they play an important role in leukaemogenesis. However, function of these proteins in haematopoietic cell transformation and proliferation is not clear. The aim of this study was to evaluate the impact of perturbation of STAT5 expression [using oligodeoxynucleotide (ODN) against STAT5 mRNA], on the clonogenicity and survival of selected human leukaemic cell lines, HEL, HL-60, K562, TF-1. We analysed the effect of ODN pre-treatment on the cell clonogenicity in methylcellulose cultures according to the time and the temperature of exposure. Moreover, we attempted to estimate apoptosis induced in examined cells, by flow cytometry using combined Annexin V-PI staining and the TUNEL method. We also applied the RT-PCR method to analyse Bax and Bcl-xL gene expression. We found that the perturbation of STAT5 expression with antisense oligonucleotides caused a decrease in the proliferative potential of human K562 and TF-1 cell lines. Also, we observed higher induction of apoptotic cell death in the K562 and TF-1 cells incubated with the antisense STAT5A ODNs. We did not notice any impact of ODNs on the HL-60 and HEL cells. Our studies using STAT5 antisense oligonucleotides showed that these proteins may be critical in the regulation of growth and apoptosis of some types of leukaemic blasts.

Involvement of nitric oxide in farnesyltransferase inhibitor-mediated apoptosis in chronic myeloid leukemia cells

The mechanism of action of farnesyltransferase inhibitors (FTIs) has not been fully clarified. We investigated the cytotoxic effects of various FTIs in chronic myeloid leukemia (CML), using LAMA cells and marrow cells from 40 CML patients in chronic phase. FTI-mediated cytotoxic effect was observed in LAMA cells and in 65% of primary CML cells, whereas marrow cells from controls were only weakly affected. Cytotoxic effects were partially related to enhanced apoptosis; however, Fas-receptor (FasR) and Fas-ligand (FasL) expression were not modified by FTIs. Susceptibility to FTI-mediated inhibition did not correlate with FasR/FasL expression in CD34+ CML cells. Moreover, intra-cellular activation of caspase-1 and -8 were not altered by FTIs, and their blockade did not reverse FTI toxicity. However, we observed FTI-induced activation of caspase-3, and its inhibition partially reverted FTI-induced apoptosis. FTIs did not modulate bcl2, bclxL, and bclxS expression, whereas they increased inducible nitric oxide (iNOS) mRNA and protein levels, resulting in higher NO production. Furthermore, C3 exoenzyme, a Rho inhibitor, significantly increased iNOS expression in CML cells, suggesting that FTIs may up-regulate NO formation at least partially through FTI-mediated inhibition of Rho. We conclude that FTIs induce selective apoptosis in CML cells via activation of iNOS and caspase-3.

Involvement of Akt kinase in the action of STI571 on chronic myelogenous leukemia cells

To elucidate the role of mitogen-activated protein kinases (MAPKs) and Akt kinase in leukemogenesis caused by the breakpoint cluster region (BCR)-Abelson (ABL) tyrosine kinase oncoprotein, we examined the activities of MAPKs and Akt kinase and their roles in the action of STI571, a specific inhibitor of BCR-ABL tyrosine kinase, in chronic myelogenous leukemia (CML) cells. We found that extracellular signal-regulated kinase (ERK) 1/2 and Akt kinase are constitutively active in the chronic phase of CML, blast crisis of CML, and the CML-derived K562 cell line. Both interferon-alpha and STI571 suppressed ERK1/2 activity in K562 cells. In contrast, Akt kinase activity was inhibited only by STI571. K562 cell proliferation was markedly suppressed by LY294002, a specific inhibitor of PI3K/Akt kinase, and STI571 but not by PD98059, a specific inhibitor of MEK1/2. In addition, caspase-3 was activated by treatment of cells with STI571 and LY294002 but not with PD98059. These data indicate that Akt kinase may play a role in the proliferation of CML leukemia cells and the action of STI571. Primary leukemia cells from patients with CML blast crisis did not show inhibition of ERK1/2 or Akt kinase activity and were resistant to caspase-3-associated apoptosis after treatment with STI571. These findings suggest that STI571 does not effectively block signaling molecules downstream of the BCR-ABL tyrosine kinase in some cases of CML blast crisis.

FAK overexpression upregulates cyclin D3 and enhances cell proliferation via the PKC and PI3-kinase-Akt pathways

We previously demonstrated that FAK-transfected HL-60 (HL-60/FAK) cells exhibit anti-apoptotic capacity. Here, we report that HL-60/FAK cells proliferate much faster than vector-transfected control (HL-60/Vect) cells with a 1.5-fold faster doubling time. This observation prompted us to investigate the mechanism of how HL-60/FAK cells augment cell proliferation. Since a protein kinase C (PKC) inhibitor, chelerythrine, or a PI3-kinase inhibitor, LY294002, suppressed cell proliferation effectively, both PKC and PI-3-kinase pathways are presumed to be involved in the cell proliferation. Among cyclins and CDKs, cyclin D3 expression was particularly prominent in the HL-60/FAK cells. Among PKC family, particularly PKCalpha, beta and eta isoforms were activated and directly associated with FAK in HL-60/FAK cells. We assumed that FAK activates PKC and PI3-kinase-Akt pathway, which resulted in marked induction of cyclin D3 expression and CDK activity.

Molecular mechanisms of transformation by the BCR-ABL oncogene

The BCR-ABL oncogene is generated by the Philadelphia chromosome (Ph) translocation, fusing the BCR gene to the ABL gene. The BCR-ABL fusion protein has elevated ABL tyrosine kinase activity that is critical for transformation of hematopoietic cells. Chronic myelogenous leukemia (CML) cells transformed by BCR-ABL show reduced growth factor requirements and apoptosis, as well as enhanced viability and altered adhesion. The elevated ABL kinase activity leads to chronic activation of signaling pathways that are required for all aspects of transformation. Progression of the disease from chronic phase to blast crisis correlates with additional cytogenetic alterations that are likely to contribute to the failure of traditional therapy. This review describes molecular mechanisms that are thought to be important for transformation by the BCR-ABL oncoprotein and points at pathways for targeted drug development in the treatment of CML.

Anti-apoptotic signaling by the interleukin-2 receptor reveals a function for cytoplasmic tyrosine residues within the common gamma (gamma c) receptor subunit

The interleukin-2 receptor (IL-2R) is composed of one affinity-modulating subunit (IL-2Ralpha) and two essential signaling subunits (IL-2Rbeta and gammac). Although most known signaling events are mediated through tyrosine residues located within IL-2Rbeta, no functions have yet been ascribed to gammac tyrosine residues. In this study, we describe a role for gammac tyrosines in anti-apoptotic signal transduction. We have shown previously that a tyrosine-deficient IL-2Rbeta chain paired with wild type gammac stimulated enhancement of bcl-2 mRNA in IL-2-dependent T cells, but it was not determined which region of the IL-2R or which pathway was activated to direct this signaling response. Here we show that up-regulation of Bcl-2 by an IL-2R lacking IL-2Rbeta tyrosine residues leads to increased cell survival after cytokine deprivation; strikingly, this survival signal does not occur in the absence of gammac tyrosine residues. These gammac-dependent signals are revealed only in the absence of IL-2Rbeta tyrosines, indicating that the IL-2R engages at least two distinct signaling pathways to regulate apoptosis and Bcl-2 expression. Mechanistically, the gammac-dependent signal requires activation of Janus kinases 1 and 3 and is sensitive to wortmannin, implicating phosphatidylinositol 3-kinase. Consistent with involvement of phosphatidylinositol 3-kinase, Akt can be activated via tyrosine residues on gammac. Thus, gammac mediates an anti-apoptotic signaling pathway through Akt which cooperates with signals from its partner chain, IL-2Rbeta.

Naturally occurring dominant-negative Stat5 suppresses transcriptional activity of estrogen receptors and induces apoptosis in T47D breast cancer cells

Signal transducer and activator of transcription (Stat) 5 regulates growth, differentiation, and survival of mammary and hematopoietic cells. The role of Stat5 in breast cancer has not been established, although Stat5 is critical for some hematopoietic malignancies. We detected for the first time that Stat5b is constitutively activated in human breast cancer cell lines, and analysed the role of Stat5 in estrogen receptor(ER)-positive breast cancer cell lines using dominant-negative variants of Stat5. Two distinct carboxyl-truncated Stat5a derivatives were generated. Stat5aDelta740 corresponded to a naturally occurring alternative splice variant, and Stat5aDelta713 was analogous to an 80 kDa Stat5a product of a nuclear protease. Stat5aDelta740 and Stat5aDelta713 displayed comparable dominant-negative properties and suppressed transcriptional activity of wild-type Stat5a and Stat5b equally well. Cotransfection experiments revealed that Stat5aDelta740 completely blocked transcriptional activity of endogenous estrogen receptor in T47D and MCF7 cells, and of both ER alpha and ER beta in COS-7 cells. Stat5aDelta740 was selected for adenoviral delivery, and high-efficiency expression of tyrosine phosphorylated Stat5aDelta740 was achieved in infected cells. Adenoviral-mediated Stat5aDelta740 induced apoptosis in T47D cells but not in caspase-3-negative MCF7 cells. The present study indicates that overexpression of a dominant-negative variant of Stat5 suppresses ER transcriptional activity and induces apoptosis in estrogen-responsive breast cancer tissue culture cells.

Involvement of PI3K/Akt pathway in cell cycle progression, apoptosis, and neoplastic transformation: a target for cancer chemotherapy

The PI3K/Akt signal transduction cascade has been investigated extensively for its roles in oncogenic transformation. Initial studies implicated both PI3K and Akt in prevention of apoptosis. However, more recent evidence has also associated this pathway with regulation of cell cycle progression. Uncovering the signaling network spanning from extracellular environment to the nucleus should illuminate biochemical events contributing to malignant transformation. Here, we discuss PI3K/Akt-mediated signal transduction including its mechanisms of activation, signal transducing molecules, and effects on gene expression that contribute to tumorigenesis. Effects of PI3K/Akt signaling on important proteins controlling cellular proliferation are emphasized. These targets include cyclins, cyclin-dependent kinases, and cyclin-dependent kinase inhibitors. Furthermore, strategies used to inhibit the PI3K/Akt pathway are presented. The potential for cancer treatment with agents inhibiting this pathway is also addressed.

BCR-ABL independence and LYN kinase overexpression in chronic myelogenous leukemia cells selected for resistance to STI571

Clinical studies have shown that the tyrosine kinase inhibitor STI571 effectively controls BCR-ABL-positive chronic myelogenous leukemia (CML). However, disease progression while on STI571 therapy has been reported, suggesting de novo or intrinsic resistance to BCR-ABL-targeted therapy. To investigate possible mediators of acquired STI571 resistance, K562 cells resistant to 5 microM STI571 (K562-R) were cloned and compared to the parental cell population. K562-R cells had reduced BCR-ABL expression and limited activation of BCR-ABL signaling cascades (Stat 5, CrkL, MAPK). STI571 failed to activate caspase cascades or to suppress expression of survival genes (bcl-xL) in resistant cells. Gene sequencing and tyrosine kinase activity measurements demonstrated that K562-R cells retained wild-type and active BCR-ABL tyrosine kinase that was inhibitable by in vitro incubation with STI571, suggesting that BCR-ABL was not coupled to proliferation or survival of K562-R cells. The src-related kinase LYN was highly overexpressed and activated in K562-R cells, and its inhibition reduced proliferation and survival of K562-R cells while having limited effects of K562 cells. Specimens taken from patients with advanced CML that progressed on STI571 therapy also were analyzed for LYN kinase expression, and they were found to be elevated to a level similar to that of K562-R cells. Comparison of samples from patients taken prior to and following STI571 failure suggested that expression and/or activation of LYN/HCK occurs during disease progression. Together, these results suggest that acquired STI571 resistance may be associated with BCR-ABL independence and mediated in part through overexpression of other tyrosine kinases.

Role of constitutively activated protein tyrosine kinases in malignant myeloproliferative disorders: an update

Modern molecular technology helped identify more than 10 protein tyrosine kinases related to myeloid malignancies, which allowed the development of small molecule inhibitors targeting deregulated protein tyrosine kinase activity. Protein tyrosine kinase deregulation can occur as a consequence of fusion gene formation because of chromosomal translocations, or as distinct gain-of-function point mutations. Although the tyrosine kinase inhibitor imatinib mesylate (Gleevec) targeting the ABL protein tyrosine kinase has revolutionized current chronic myeloid leukemia therapy, it became rapidly evident that overcoming the multiple cellular resistance mechanisms will be very challenging. To develop efficient therapeutic alternatives, one must understand the complex signal transduction mechanisms involved in transformation by deregulated protein tyrosine kinases. This article reviews the most recently identified molecular mechanisms involved in cell transformation by the BCR/ABL protein tyrosine kinase fusion and presents new members of the increasing family of deregulated protein tyrosine kinases involved in myeloproliferative disorders. In addition, the article discusses new, promising small molecule protein tyrosine kinase inhibitors and the molecular mechanism that may lead to resistance to these drugs. Finally, the article highlights putative alternative strategies that could be used to block signal transduction pathways of deregulated protein tyrosine kinase activity.

Inhibition of Bcr-Abl kinase activity by PD180970 blocks constitutive activation of Stat5 and growth of CML cells

Chronic myelogenous leukemia (CML) is a myeloproliferative disease characterized by the BCR-ABL genetic translocation and constitutive activation of the Abl tyrosine kinase. Among members of the Signal Transducers and Activators of Transcription (STAT) family of transcription factors, Stat5 is activated by the Bcr-Abl kinase and is implicated in the pathogenesis of CML. We recently identified PD180970 as a new and highly potent inhibitor of Bcr-Abl kinase. In this study, we show that blocking Bcr-Abl kinase activity using PD180970 in the human K562 CML cell line resulted in inhibition of Stat5 DNA-binding activity with an IC(50) of 5 nM. Furthermore, abrogation of Abl kinase-mediated Stat5 activation suppressed cell proliferation and induced apoptosis in K562 cells, but not in the Bcr-Abl-negative myeloid cell lines, HEL 92.1.7 and HL-60. Dominant-negative Stat5 protein expressed from a vaccinia virus vector also induced apoptosis of K562 cells, consistent with earlier studies that demonstrated an essential role of Stat5 signaling in growth and survival of CML cells. RNA and protein analyses revealed several candidate target genes of Stat5, including Bcl-x, Mcl-1, c-Myc and cyclin D2, which were down-regulated after treatment with PD180970. In addition, PD180970 inhibited Stat5 DNA-binding activity in cultured primary leukemic cells derived from CML patients. To detect activated Stat5 in CML patient specimens, we developed an immunocytochemical assay that can be used as a molecular end-point assay to monitor inhibition of Bcr-Abl signaling. Moreover, PD180970 blocked Stat5 signaling and induced apoptosis of STI-571 (Gleevec, Imatinib)-resistant Bcr-Abl-positive cells. Together, these results suggest that the mechanism of action of PD180970 involves inhibition of Bcr-Abl-mediated Stat5 signaling and provide further evidence that compounds in this structural class may represent potential therapeutic agents for CML.

The Src family kinase Hck couples BCR/ABL to STAT5 activation in myeloid leukemia cells

Signal transducer and activator of transcription 5 (STAT5) is constitutively activated by BCR/ABL, the oncogenic tyrosine kinase responsible for chronic myelogenous leukemia. The mechanism of BCR/ABL-mediated STAT5 activation is unknown. We show here that the BCR/ABL SH3 and SH2 domains interact with hematopoietic cell kinase (Hck), leading to the stimulation of Hck catalytic activity. Active Hck phosphorylated STAT5B on Tyr699, which represents an essential step in STAT5B stimulation. Moreover, a kinase-dead Hck mutant and Hck inhibitor PP2 abrogated BCR/ABL-dependent activation of STAT5 and elevation of expression of STAT5 downstream effectors A1 and pim-1. These data identify a novel BCR/ABL-Hck-STAT5 signaling pathway, which plays an important role in BCR/ABL-mediated transformation of myeloid cells.