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

The Superior Cytotoxicity of Dual Targeting of BCR/ABL and PI3K in K562 Cells: Proposing a Novel Therapeutic Potential for the Treatment of CML

Apart from BCR/ABL which is the main player in the pathogenesis of chronic myeloid leukemia (CML), the role of other signaling cascades should not be underestimated especially for the maintenance of leukemic cells survival. The results of the present study indicate that either an isoform-specific or a pan-PI3K inhibitor could potently reduce the survival of CML-derived K562 cells, shedding more light on the involvement of the PI3K axis in the pathogenesis of CML. Of particular interest, the importance of the PI3K pathway in this disease became more evident when we found that there was a more remarkable reduction in the viability of K562 cells when BKM120 was used in combination with imatinib. Moreover, BKM120 robustly enhanced the growth-suppressive effect of imatinib through p21-mediated induction of G2/M cell cycle arrest and induction of apoptotic cell death. Despite the favorable anti-survival effects of the drug combination, these agents failed to induce inhibitory effects on the expression of c-Myc and NF-κB anti-apoptotic target genes. However, the ability of combinational therapy in diminishing K562 cell survival was potentiated either in the presence of 10058-F4 (c-Myc inhibitor) or Bortezomib (proteasome inhibitor), suggestive of the role of both NF-κB and c-Myc in overshadowing the therapeutic value of drugs combination. Taken together, the results of this study showed that inhibition of the PI3K pathway is a suitable approach to enhance the therapeutic value of imatinib in the treatment of CML.

PI3K/mTOR dual-inhibition with VS-5584 enhances anti-leukemic efficacy of ponatinib in blasts and Ph-negative LSCs of chronic myeloid leukemia

Ponatinib is used for advanced treatment of chronic myeloid leukemia (CML), although low doses to prevent side effects do not suppress survival pathways and eradicate leukemia stem cells (LSCs). We evaluated the potential of ponatinib and PI3K/mTOR dual-inhibitor VS-5584 combination (PoVS) therapy to increase the anti-leukemic effects of ponatinib and investigated the underlying mechanisms at the molecular level. We measured the cytotoxicities of ponatinib, VS-5584, and PoVS (CCK-8 assay), and used the median-effect equation for combination analyses. We investigated the effects of inhibitory concentrations on apoptosis, cell viability and cell-cycle regulation (flow cytometry), protein levels (ELISA, Western blot), transcriptional activities (dual-luciferase reporter assay), gene expressions (qRT-PCR). VS-5584 exerted selective cytotoxic effects against CML and LSC cell lines. VS-5584 inhibited the PI3K/Akt/mTOR pathway, resulting in reduced cell viability, slightly induced caspase-independent apoptosis, prominent G₀/G₁ cell-cycle blockade that is not a consequence of quiescence. Normal hematopoietic stem cell line was the least affected. Moreover, ponatinib and VS-5584 mediated synergistic anti-leukemic effects on leukemic cells. VS-5584 reduced the ponatinib dose required to target leukemic cells. PoVS treatment inhibited PI3K/Akt/mTOR pathway more consistently than either of the two agents alone through reducing p-Akt, p-mTOR, p-S6K, p-PRAS40, p-S6. The subsequent downstream effects were an increase in C/EBP transcriptional activity and decreases in activities of E2F/DP1, Myc/Max, CREB, STAT3, NFκB, AP-1, Elk-1/SRF. Transcriptional regulation resulted in alterations in the expression levels of target mRNAs. Our results highlight PoVS can be a promising treatment strategy for eliminating CML cells and LSCs selectively, with the reduced ponatinib doses.

The application of BH3 mimetics in myeloid leukemias

Execution of the intrinsic apoptotic pathway is controlled by the BCL-2 proteins at the level of the mitochondrial outer membrane (MOM). This family of proteins consists of prosurvival (e.g., BCL-2, MCL-1) and proapoptotic (e.g., BIM, BAD, HRK) members, the functional balance of which dictates the activation of BAX and BAK. Once activated, BAX/BAK form pores in the MOM, resulting in cytochrome c release from the mitochondrial intermembrane space, leading to apoptosome formation, caspase activation, and cleavage of intracellular targets. This pathway is induced by cellular stress including DNA damage, cytokine and growth factor withdrawal, and chemotherapy/drug treatment. A well-documented defense of leukemia cells is to shift the balance of the BCL-2 family in favor of the prosurvival proteins to protect against such intra- and extracellular stimuli. Small molecule inhibitors targeting the prosurvival proteins, named 'BH3 mimetics', have come to the fore in recent years to treat hematological malignancies, both as single agents and in combination with standard-of-care therapies. The most significant example of these is the BCL-2-specific inhibitor venetoclax, given in combination with standard-of-care therapies with great success in AML in clinical trials. As the number and variety of available BH3 mimetics increases, and investigations into applying these novel inhibitors to treat myeloid leukemias continue apace the need to evaluate where we currently stand in this rapidly expanding field is clear.

Icaritin: A Novel Natural Candidate for Hematological Malignancies Therapy

Hematological malignancies including leukemia and lymphoma can severely impact human health. With the current therapies combined with chemotherapy, stem cell transplantation, radiotherapy, and immunotherapy, the prognosis of hematologic malignancies improved significantly. However, most hematological malignancies are still incurable. Therefore, research for novel treatment options was continuing with the natural product as one source. Icaritin is a compound extracted from a traditional Chinese herb, Epimedium Genus, and demonstrated an antitumor effect in various neoplasms including hematological malignancies such as leukemia, lymphoma, and multiple myeloma. In hematological malignancies, icaritin showed multiple cytotoxic effects to induce apoptosis, arrest the cell cycle, inhibit proliferation, promote differentiation, restrict metastasis and infiltration, and suppress the oncogenic virus. The proved underlying mechanisms of the cytotoxic effects of icaritin are different in various cell types of hematological malignancies but associated with the critical cell signal pathway, including PI3K/Akt, JAK/STAT3, and MAPK/ERK/JNK. Although the primary target of icaritin is still unspecified, the existing evidence indicates that icaritin is a potential novel therapeutic agent for neoplasms as with hematological malignancies. Here, in the field of hematology, we reviewed the reported activity of icaritin in hematologic malignancies and the underlying mechanisms and recognized icaritin as a candidate for therapy of hematological malignancies.

Deregulated expression of Cdc6 as BCR/ABL-dependent survival factor in chronic myeloid leukemia cells

Chronic myeloid leukemia is characterized by the presence of the reciprocal translocation t(9;22) and the BCR/ABL oncogene. The BCR/ABL oncogene activates multiple signaling pathways and involves the dysregulation of oncogenes during the progression of chronic myeloid leukemia. The cell division cycle protein 6, an essential regulator of DNA replication, is elevated in some human cancer cells. However, the expression of cell division cycle protein 6 in chronic myeloid leukemia and the underlying regulatory mechanism remain to be elucidated. In this study, our data showed that cell division cycle protein 6 expression was significantly upregulated in primary chronic myeloid leukemia cells and the chronic myeloid leukemia cell line K562 cells, as compared to the normal bone marrow mononuclear cells. BCR/ABL kinase inhibitor STI571 or BCR/ABL small interfering RNA could significantly downregulate cell division cycle protein 6 messenger RNA expression in K562 cells. Moreover, phosphoinositide 3-kinase/AKT pathway inhibitor LY294002 and Janus kinase/signal transducer and activator of transcription pathway inhibitor AG490 could downregulate cell division cycle protein 6 expression in K562 cells, but not RAS/mitogen-activated protein kinase pathway inhibitor PD98059 had such effect. Cell division cycle protein 6 gene silencing by small interfering RNA effectively resulted in decrease of proliferation, increase of apoptosis, and arrest of cell cycle in K562 cells. These findings have demonstrated that cell division cycle protein 6 overexpression may contribute to the high proliferation and low apoptosis in chronic myeloid leukemia cells and can be regulated by BCR/ABL signal transduction through downstream phosphoinositide 3-kinase/Akt and Janus kinase/signal transducer and activator of transcription pathways, suggesting cell division cycle protein 6 as a potential therapeutic target in chronic myeloid leukemia.

2-Hydroxypropyl-β-Cyclodextrin Acts as a Novel Anticancer Agent

2-Hydroxypropyl-β-cyclodextrin (HP-β-CyD) is a cyclic oligosaccharide that is widely used as an enabling excipient in pharmaceutical formulations, but also as a cholesterol modifier. HP-β-CyD has recently been approved for the treatment of Niemann-Pick Type C disease, a lysosomal lipid storage disorder, and is used in clinical practice. Since cholesterol accumulation and/or dysregulated cholesterol metabolism has been described in various malignancies, including leukemia, we hypothesized that HP-β-CyD itself might have anticancer effects. This study provides evidence that HP-β-CyD inhibits leukemic cell proliferation at physiologically available doses. First, we identified the potency of HP-β-CyD in vitro against various leukemic cell lines derived from acute myeloid leukemia (AML), acute lymphoblastic leukemia and chronic myeloid leukemia (CML). HP-β-CyD treatment reduced intracellular cholesterol resulting in significant leukemic cell growth inhibition through G₂/M cell-cycle arrest and apoptosis. Intraperitoneal injection of HP-β-CyD significantly improved survival in leukemia mouse models. Importantly, HP-β-CyD also showed anticancer effects against CML cells expressing a T315I BCR-ABL mutation (that confers resistance to most ABL tyrosine kinase inhibitors), and hypoxia-adapted CML cells that have characteristics of leukemic stem cells. In addition, colony forming ability of human primary AML and CML cells was inhibited by HP-β-CyD. Systemic administration of HP-β-CyD to mice had no significant adverse effects. These data suggest that HP-β-CyD is a promising anticancer agent regardless of disease or cellular characteristics.

Downregulation of miR-31, miR-155, and miR-564 in chronic myeloid leukemia cells

Background/Aims

MicroRNAs (miRNAs) are short non-coding regulatory RNAs that control gene expression and play an important role in cancer development and progression. However, little is known about the role of miRNAs in chronic myeloid leukemia (CML). Our objective is to decipher a miRNA expression signature associated with CML and to determine potential target genes and signaling pathways affected by these signature miRNAs.

Results

Using miRNA microarrays and miRNA real-time PCR we characterized the miRNAs expression profile of CML cell lines and patients in reference to non-CML cell lines and healthy blood. Of all miRNAs tested, miR-31, miR-155, and miR-564 were down-regulated in CML cells. Down-regulation of these miRNAs was dependent on BCR-ABL activity. We next analyzed predicted targets and affected pathways of the deregulated miRNAs. As expected, in K562 cells, the expression of several of these targets was inverted to that of the miRNA putatively regulating them. Reassuringly, the analysis identified CML as the main disease associated with these miRNAs. MAPK, ErbB, mammalian target of rapamycin (mTOR) and vascular endothelial growth factor (VEGF) were the main molecular pathways related with these expression patterns. Utilizing Venn diagrams we found appreciable overlap between the CML-related miRNAs and the signaling pathways-related miRNAs.

Conclusions

The miRNAs identified in this study might offer a pivotal role in CML. Nevertheless, while these data point to a central disease, the precise molecular pathway/s targeted by these miRNAs is variable implying a high level of complexity of miRNA target selection and regulation. These deregulated miRNAs highlight new candidate gene targets allowing for a better understanding of the molecular mechanism underlying the development of CML, and propose possible new avenues for therapeutic treatment.

Perifosine induces protective autophagy and upregulation of ATG5 in human chronic myelogenous leukemia cells in vitro

AIM

The efficacy of the Akt inhibitor perifosine against chronic myeloid leukemia (CML) cells and its mechanisms of action are unknown. In this study, the cytotoxic effects of perifosine on CML and acute myeloid leukemia (AML) cell lines were compared to elucidate the mechanisms underlying the differences.

METHODS

Human AML cell lines Kasumi-1 and HL-60, and the CML cell line K562 were used. Cell viability was quantitated using MTT assay. Apoptosis was determined using Annexin V-FITC/propidium iodide and Hoechst staining, which were followed by flow cytometry and fluorescence microscopy analysis, respectively. Caspase pathway activation and the expression of autophagy-related genes were examined using Western blot. Autophagy was studied using electron microscopy, the acridine orange staining method, and GFP-LC3 was examined with fluorescence microscopy.

RESULTS

In contrast to AML cell lines, the CML cell lines K562 and K562/G (an imatinib-insensitive CML cell line) were resistant to perifosine (2.5-20 μmol/L) in respect to inhibiting cell growth and inducing apoptosis. Perifosine (2.5, 5, and 10 μmol/L) inhibited Akt and its phosphorylation in AML cells, but not in CML cells. Treatment with perifosine (20 μmol/L) resulted in autophagy in CML cells as shown by the increased formation of acidic vesicular organelles and the accumulation of LC3-II. Treatment of CML cells with perifosine (5, 10, and 20 μmol/L) dose-dependently upregulated AGT5, but not Beclin 1 at the protein level. Furthermore, inhibition of autophagy by chloroquine (40 nmol/L) significantly suppressed the cell growth and induced apoptosis in CML cells treated with perifosine (20 μmol/L).

CONCLUSION

Our results show that CML cell lines were resistant to the Akt inhibitor perifosine in vitro, which is due to perifosine-induced protective autophagy and upregulation of ATG5.

Icaritin shows potent anti-leukemia activity on chronic myeloid leukemia in vitro and in vivo by regulating MAPK/ERK/JNK and JAK2/STAT3 /AKT signalings

Purpose

To explore the effects of Icaritin on chronic myeloid leukemia (CML) cells and underlying mechanisms.

Method

CML cells were incubated with various concentration of Icaritin for 48 hours, the cell proliferation was analyzed by MTT and the apoptosis was assessed with Annexin V and Hoechst 33258 staining. Cell hemoglobinization was determined. Western blotting was used to evaluate the expressions of MAPK/ERK/JNK signal pathway and Jak-2/Phorpho-Stat3/Phorsph-Akt network-related protein. NOD-SCID nude mice were applied to demonstrate the anti-leukemia effect of Icaritin in vivo.

Results

Icaritin potently inhibited proliferation of K562 cells (IC50 was 8 µM) and primary CML cells (IC50 was 13.4 µM for CML-CP and 18 µM for CML-BC), induced CML cells apoptosis and promoted the erythroid differentiation of K562 cells with time-dependent manner. Furthermore, Icaritin was able to suppress the growth of primary CD34+ leukemia cells (CML) and Imatinib-resistant cells, and to induce apoptosis. In mouse leukemia model, Icaritin could prolong lifespan of NOD-SCID nude mice inoculated with K562 cells as effective as Imatinib without suppression of bone marrow. Icaritin could up-regulate phospho-JNK or phospho-C-Jun and down-regulate phospho-ERK, phospho-P-38, Jak-2, phospho-Stat3 and phospho-Akt expression with dose- or time-dependent manner. Icaritin had no influence both on c-Abl and phospho-c-Abl protein expression and mRNA levels of Bcr/Abl.

Conclusion

Icaritin from Chinese herb medicine may be a potential anti-CML agent with low adverse effect. The mechanism of anti-leukemia for Icaritin is involved in the regulation of Bcr/Abl downstream signaling. Icaritin may be useful for an alternative therapeutic choice of Imatinib-resistant forms of CML.

GILZ inhibits the mTORC2/AKT pathway in BCR-ABL(+) cells

The malignant phenotype of chronic myeloid leukemia (CML) is due to the abnormal tyrosine kinase activity of the BCR-ABL oncoprotein, which signals several downstream cell survival pathways, including phosphoinositide 3-kinase/AKT, signal transducer and activator of transcription 5 and extracellular signal-regulated kinase 1/2. In patients with CML, tyrosine kinase inhibitors (TKIs) are used to suppress the BCR-ABL tyrosine kinase, resulting in impressive response rates. However, resistance can occur, especially in acute-phase CML, through various mechanisms. Here, we show that the glucocorticoid-induced leucine zipper protein (GILZ) modulates imatinib and dasatinib resistance and suppresses tumor growth by inactivating the mammalian target of rapamycin complex-2 (mTORC2)/AKT signaling pathway. In mouse and human models, GILZ binds to mTORC2, but not to mTORC1, inhibiting phosphorylation of AKT (at Ser473) and activating FoxO3a-mediated transcription of the pro-apoptotic protein Bim; these results demonstrate that GILZ is a key inhibitor of the mTORC2 pathway. Furthermore, CD34⁺ stem cells isolated from relapsing CML patients underwent apoptosis and showed inhibition of mTORC2 after incubation with glucocorticoids and imatinib. Our findings provide new mechanistic insights into the role of mTORC2 in BCR-ABL⁺ cells and indicate that regulation by GILZ may influence TKI sensitivity.

Modulation of Akt and ERK1/2 pathways by resveratrol in chronic myelogenous leukemia (CML) cells results in the downregulation of Hsp70

Background

Resveratrol is known to downregulate the high endogenous level of Heat shock protein 70 (Hsp70) in Chronic Myelogenous Leukemia (CML) K562 cells and induce apoptosis. Since Heat Shock Factor 1 (HSF1) controls transcription of Hsp70, we wanted to probe the signaling pathways responsible for transcriptional activation of HSF1.

Methodology/Principal Findings

Cells exposed to 40µM Resveratrol rapidly abolished serine473 phosphorylation of Akt and significantly reduced its kinase activity. Inactivation of Akt pathway by Resveratrol subsequently blocked serine9 phosphorylation of Gsk3β. Active non-phosphorylated Gsk3β rendered HSF1 transcriptionally inactive and reduced Hsp70 production. Blocking PI3K/Akt activity also demonstrated similar effects on Hsp70 comparable to Resveratrol. Inactivation of Gsk3β activity by inhibitors SB261763 or LiCl upregulated Hsp70. Resveratrol significantly modulated ERK1/2 activity as evident from hyper phosphorylation at T302/Y304 residues and simultaneous upregulation in kinase activity. Blocking ERK1/2 activation resulted in induction of Hsp70. Therefore, increase in ERK1/2 activity by Resveratrol provided another negative influence on Hsp70 levels through negative regulation of HSF1 activity. 17-allylamino-17-demethoxygeldanamycin (17AAG), a drug that inhibits Hsp90 chaperone and degrades its client protein Akt concomitantly elevated Hsp70 levels by promoting nuclear translocation of HSF1 from the cytosol. This effect is predominantly due to inhibition of both Akt and ERK1/2 activation by 17AAG. Simultaneously treating K562 with Resveratrol and 17AAG maintained phosho-ERK1/2 levels close to untreated controls demonstrating their opposite effects on ERK1/2 pathway. Resveratrol was found not to interfere with Bcr-Abl activation in K562 cells.

Conclusion/Significance

Thus our study comprehensively illustrates that Resveratrol acts downstream of Bcr-Abl and inhibits Akt activity but stimulates ERK1/2 activity. This brings down the transcriptional activity of HSF1 and Hsp70 production in K562 cells. Additionally, Resveratrol can be used in combination with chemotherapeutic agents such as 17AAG, an Hsp90 inhibitor reported to induce Hsp70 and hence compromise its chemotherapeutic potential.

Effect of imatinib on the signal transduction cascade regulating telomerase activity in K562 (BCR-ABL-positive) cells sensitive and resistant to imatinib

OBJECTIVE

Imatinib mesylate (IM) is a tyrosine kinase inhibitor selective for BCR-ABL and indicated for the treatment of chronic myeloid leukemia. It has recently been demonstrated that IM also targets other cellular components. Considering the significant role of telomerase in malignant transformation, we studied the effect of IM on telomerase activity (TA) and regulation in BCR-ABL-positive and -negative cells, sensitive and resistant to IM.

MATERIALS AND METHODS

Through combining telomeric repeat amplification protocol for detecting TA, reverse transcription polymerase chain reaction and Western blots for detecting RNA and protein levels of telomerase regulating proteins and fluorescence-activated cell sorting analysis, we showed that IM targets telomerase and the signal transduction cascade upstream of it.

RESULTS

IM significantly inhibited TA in BCR-ABL-positive and -negative cells and in chronic myeloid leukemia patients. TA inhibition was also observed in BCR-ABL positive cells resistant to IM at drug concentrations that did not lead to a reduction in BCR-ABL expression. In addition, a reduction in phosphorylated AKT and phosphorylated PDK-1 was also detected following IM incubation.

CONCLUSIONS

We demonstrate an inhibitory effect of IM on TA and on the AKT/PDK pathway. Because this effect was observed in cell expressing the BCR-ABL protein as well as cells not expressing it, and in cells sensitive as well as resistant to IM, it is reasonable to assume that the inhibitory effect of IM on TA is not mediated through known IM targets. The results of this study show that cells resistant to IM with regard to its effect on BCR-ABL could still be sensitive to IM treatment regarding other cellular components.

Compound 48, a novel dual PPAR alpha/gamma ligand, inhibits the growth of human CML cell lines and enhances the anticancer-effects of imatinib

Compound 48 (C48) is a novel dual ligand for peroxisome proliferator-activated receptor alpha and gamma (PPAR alpha/gamma). Culture of imatinib-sensitive and -resistant CML cell lines with C48 resulted in a strong growth inhibition which associated with G0/G1 cell cycle arrest. However, it showed no obvious toxicity to normal CD34(+) hematopoietic stem cells. Decrease of pSTATs and pAKT were noticed suggesting that interference of AKT and STATs signaling may be the mechanisms for the effects of PPAR alpha/gamma ligands. Of more clinical importance, this ligand strongly enhanced the anticancer-effects of imatinib. Overall, our data suggest that the PPAR alpha/gamma ligands may have potentials in the treatment of CML in an adjuvant setting either before or after the development of imatinib resistance.

Combi-targeting concept: an optimized single-molecule dual-targeting model for the treatment of chronic myelogenous leukemia

Blockade of Bcr-Abl by the inhibitor Imatinib has proven efficacious in the therapy of chronic myelogenous leukemia (CML). However resistance to the drug emerges at the advanced phases of the disease. Therefore, novel therapy models remained to be designed. We have developed a novel dual targeted agent termed "combi-molecule" designed to not only block Bcr-Abl but also damage DNA. ZRF1, the first optimized prototype of the approach, was "programmed" to degrade into another inhibitor ZRF0 plus a methyl diazonium species. It was approximately 2-fold stronger Abl tyrosine kinase inhibitor than Imatinib and a more potent DNA-damaging agent than Temodal. In the p53 wild-type Mo7p210 cells, the potency of ZRF1 was approximately 1,000-fold superior to that of the equieffective combinations of Imatinib plus Temodal. More importantly, its superior potency over Imatinib was more pronounced in Bcr-Abl-positive cells coexpressing wild-type p53. Studies to rationalize these results showed that, through its Bcr-Abl inhibitory function, it down-regulated p53. However, sufficient level of the latter protein was available for transactivating p21 and Bax, which are required for cell cycle arrest and apoptosis. The results suggest that, in p53 wild-type cells, apoptosis is induced not only through Bcr-Abl inhibition but also through the p53-controlled DNA-damaging pathway, leading to an additive effect that translates into enhanced cell death. The study conclusively showed that p53 is a major determinant for the cytotoxic advantages of the novel combi-molecular approach in CML, a disease in which 70% to 85% of all the cases express wild-type p53.

Growth inhibition by STI571 in combination with radiation in human chronic myelogenous leukemia K562 cells

Altered radiation responses by STI571 (Imatinib, Glivec), a specific inhibitor of the tyrosine kinase activity of Bcr-Abl, was assessed in K562 chronic myelogenous leukemia cells using growth inhibition and colony formation assays. Flow cytometry, Western blotting, and microscope observation were used to determine cell cycle redistribution, erythroid differentiation, apoptosis, necrosis, senescence, and expression and phosphorylation of effectors downstream from Bcr-Abl as endpoints. STI571 (> or =24-h contact) retarded the growth of K562 cells and elicited reduction in the G(2)-phase content due to an efficient arrest in early S phase rather than to the disruption of the G(2) checkpoint as confirmed by analysis of Lyn and CDK1 phosphorylation. STI571 brought about the inhibitory dephosphorylation of Bcr-Abl and STAT5, but the expression of DNA-PKcs and Rad51 was unaffected and the interaction between radiation and STI571 was strictly additive with regard to induction of apoptosis. Overall STI571 interacted cooperatively with radiation to retard the growth of K562 cells but did not affect intrinsic radiosensitivity. However, STI571 and radiation acted antagonistically with each other with regard to induction of senescence and erythroid differentiation.

Sphingosine kinase-1 is a downstream regulator of imatinib-induced apoptosis in chronic myeloid leukemia cells

We examined the involvement of sphingosine kinase-1 (SphK1), which governs the ceramide/sphingosine-1-phosphate balance, in susceptibility to imatinib of either sensitive or resistant chronic myeloid leukemia cells. Imatinib-sensitive LAMA84-s displayed marked SphK1 inhibition coupled with increased content of ceramide and decreased pro-survival sphingosine-1-phosphate. Conversely, no changes in the sphingolipid metabolism were observed in LAMA84-r treated with imatinib. Overcoming imatinib resistance in LAMA84-r with farnesyltransferase or MEK/ERK inhibitors as well as with cytosine arabinoside led to SphK1 inhibition. Overexpression of SphK1 in LAMA84-s cells impaired apoptosis and inhibited the effects of imatinib on caspase-3 activation, cytochrome c and Smac release from mitochondria through modulation of Bim, Bcl-xL and Mcl-1 expression. Pharmacological inhibition of SphK1 with F-12509a or its silencing by siRNA induced apoptosis of both imatinib-sensitive and -resistant cells, suggesting that SphK1 inhibition was critical for apoptosis signaling. We also show that imatinib-sensitive and -resistant primary cells from chronic myeloid leukemia patients can be successfully killed in vitro by the F-12509a inhibitor. These results uncover the involvement of SphK1 in regulating imatinib-induced apoptosis and establish that SphK1 is a downstream effector of the Bcr-Abl/Ras/ERK pathway inhibited by imatinib but upstream regulator of Bcl-2 family members.

CXCR4 up-regulation by imatinib induces chronic myelogenous leukemia (CML) cell migration to bone marrow stroma and promotes survival of quiescent CML cells

Chronic myelogenous leukemia (CML) is driven by constitutively activated Bcr-Abl tyrosine kinase, which causes the defective adhesion of CML cells to bone marrow stroma. The overexpression of p210Bcr-Abl was reported to down-regulate CXCR4 expression, and this is associated with the cell migration defects in CML. We proposed that tyrosine kinase inhibitors, imatinib or INNO-406, may restore CXCR4 expression and cause the migration of CML cells to bone marrow microenvironment niches, which in turn results in acquisition of stroma-mediated chemoresistance of CML progenitor cells. In KBM5 and K562 cells, imatinib, INNO-406, or IFN-alpha increased CXCR4 expression and migration. This increase in CXCR4 levels on CML progenitor cells was likewise found in samples from CML patients treated with imatinib or IFN-alpha. Imatinib induced G0-G1 cell cycle block in CML cells, which was further enhanced in a mesenchymal stem cell (MSC) coculture system. MSC coculture protected KBM-5 cells from imatinib-induced cell death. These antiapoptotic effects were abrogated by the CXCR4 antagonist AMD3465 or by inhibitor of integrin-linked kinase QLT0267. Altogether, these findings suggest that the up-regulation of CXCR4 by imatinib promotes migration of CML cells to bone marrow stroma, causing the G0-G1 cell cycle arrest and hence ensuring the survival of quiescent CML progenitor cells.

Targeting 14-3-3 sensitizes native and mutant BCR-ABL to inhibition with U0126, rapamycin and Bcl-2 inhibitor GX15-070

Small molecule tyrosine kinase inhibitors, such as imatinib, are effective therapies for BCR-ABL-mediated human leukemias. However, clinical drug resistance occurs, which warrants development of alternative and/or complementary therapeutic strategies to target critical downstream signaling molecules. We recently demonstrated that disrupting 14-3-3/ligand association by a peptide-based 14-3-3 competitive antagonist R18 induces significant apoptosis, partially through reactivation of AKT-inhibited proapoptotic FOXO3a, in FGFR1 fusion-transformed hematopoietic cells. Here, we report that targeting 14-3-3 by R18 effectively induced significant apoptosis in Ba/F3 and K562 cells expressing BCR-ABL, similarly through liberation and reactivation of FOXO3a. Moreover, R18 sensitized BCR-ABL-transformed cells to inhibition with MEK1 inhibitor U0126, Bcl-2 inhibitor GX15-070, or mTOR inhibitor rapamycin. Treatment with these reagents potentiated R18-induced reactivation of proapoptotic FOXO3a with enhanced expression of downstream transcription targets p27(kip1) and Bim1. Furthermore, R18-induced apoptotic cell death in cells expressing diverse imatinib-resistant BCR-ABL mutants, including T315I. This inhibition was enhanced by R18 in combination with U0126 and rapamycin. Thus, our findings suggest that targeting 14-3-3 may potentiate the effects of conventional therapy for BCR-ABL-associated hematopoietic malignancies, and overcome drug resistance.

Imatinib-resistant K562 cells are more sensitive to celecoxib, a selective COX-2 inhibitor: role of COX-2 and MDR-1

Selective inhibition of the BCR/ABL tyrosine kinase by imatinib (STI571, Glivec/Gleevec) is the therapeutic strategy in patients with chronic myelogenous leukemia (CML). Despite significant hematologic and cytogenetic responses with imatinib, mainly due to the mutations in the Abl kinase domain, resistance occurs in patients with advanced disease. In the present study on imatinib-resistant K562 cells (IR-K562), however, no such mutations in the Abl kinase domain were observed. Further studies revealed the over-expression of COX-2 and MDR-1 in IR-K562 cells suggesting the possible involvement of COX-2 in the development of resistance to imatinib. So, we sought to examine the effect of celecoxib, a selective COX-2 inhibitor, on IR-K562 cells. The results clearly indicate that celecoxib is more effective in IR-K562 cells with a lower IC50 value of 10 microM compared to an IC50 value of 40 microM in K562 cells. This increase in the sensitivity of IR-K562 cells towards celecoxib suggests that the development of resistance in IR-K562 cells is COX-2 dependent. Further studies revealed down-regulation of MDR-1 by celecoxib and a decline in p-Akt levels. Celecoxib-induced apoptosis of IR-K562 cells led to release of cytochrome c, PARP cleavage and decreased Bcl2/Bax ratio. Also, celecoxib at 1 microM concentration induced apoptosis in IR-K562 cells synergistically with imatinib by reducing the IC50 value of imatinib from 10 to 6 microM. In conclusion, the present study indicates over-expression of COX-2 and MDR-1 in IR-K562 cells and celecoxib, a COX-2 specific inhibitor, induces apoptosis by inhibiting COX-2 and down-regulating MDR-1 expression through Akt/p-Akt signaling pathway.

Potentiation of antileukemic therapies by the dual PI3K/PDK-1 inhibitor, BAG956: effects on BCR-ABL- and mutant FLT3-expressing cells

Mediators of PI3K/AKT signaling have been implicated in chronic myeloid leukemia (CML) and acute myeloid leukemia (AML). Studies have shown that inhibitors of PI3K/AKT signaling, such as wortmannin and LY294002, are able to inhibit CML and AML cell proliferation and synergize with targeted tyrosine kinase inhibitors. We investigated the ability of BAG956, a dual PI3K/PDK-1 inhibitor, to be used in combination with inhibitors of BCR-ABL and mutant FLT3, as well as with the mTOR inhibitor, rapamycin, and the rapamycin derivative, RAD001. BAG956 was shown to block AKT phosphorylation induced by BCR-ABL-, and induce apoptosis of BCR-ABL-expressing cell lines and patient bone marrow cells at concentrations that also inhibit PI3K signaling. Enhancement of the inhibitory effects of the tyrosine kinase inhibitors, imatinib and nilotinib, by BAG956 was demonstrated against BCR-ABL expressing cells both in vitro and in vivo. We have also shown that BAG956 is effective against mutant FLT3-expressing cell lines and AML patient bone marrow cells. Enhancement of the inhibitory effects of the tyrosine kinase inhibitor, PKC412, by BAG956 was demonstrated against mutant FLT3-expressing cells. Finally, BAG956 and rapamycin/RAD001 were shown to combine in a nonantagonistic fashion against BCR-ABL- and mutant FLT3-expressing cells both in vitro and in vivo.

Optimization of novel combi-molecules: Identification of balanced and mixed bcr-abl/DNA targeting properties

Steps toward the identification of combi-molecules with strong abl tyrosine kinase (TK) inhibitory property and significant DNA damaging potential are described. The optimized combi-molecule 13a was shown to induce approximately twofold stronger abl TK inhibitory activity than Gleevec and high levels of DNA damage in chronic myelogenous leukemic cells.

Contribution of the orphan nuclear receptor Nur77 to the apoptotic action of IGFBP-3

Tumor suppression by insulin-like growth factor-binding protein-3 (IGFBP-3) has been demonstrated to occur via insulin-like growth factor-dependent and -independent mechanisms in vitro and in vivo. We have recently described IGFBP-3-induced mitochondrial translocation of the nuclear receptors RXRalpha/Nur77 in the induction of prostate cancer (CaP) cell apoptosis. Herein, we demonstrate that IGFBP-3 and Nur77 associate in the cytoplasmic compartment in 22RV1 CaP cells. Nur77 is a major component of IGFBP-3-induced apoptosis as shown by utilizing mouse embryonic fibroblasts (MEFs) derived from Nur77 wild-type and knockout (KO) mice. However, dose-response experiments revealed that a small component of IGFBP-3-induced apoptosis is Nur77 independent. Reintroduction of Nur77 into Nur77 KO MEFs restores full responsiveness to IGFBP-3. IGFBP-3 induces phosphorylation of Jun N-terminal kinase and inhibition of Akt phosphorylation and activity, which have been associated with Nur77 translocation. Finally, IGFBP-3 administration to CaP xenografts on SCID mice induced apoptosis and translocated Nur77 out of the nucleus. Taken together, our results verify an important role for the orphan nuclear receptor Nur77 in the apoptotic actions of IGFBP-3.

N-Benzyladriamycin-14-valerate (AD 198) cytotoxicty circumvents Bcr-Abl anti-apoptotic signaling in human leukemia cells and also potentiates imatinib cytotoxicity

Bcr-Abl activity in chronic myelogenous leukemia (CML) results in dysregulated cell proliferation and resistance against multiple cytotoxic agents due to the constitutive activation of proliferative signaling pathways. Currently, the most effective treatment of CML is the inhibition of Bcr-Abl activity by imatinib mesylate (Gleevec). Imatinib efficacy is limited by development of resistance through either expression of Bcr-Abl variants that bind imatinib less avidly, increased expression of Bcr-Abl, or expression of multidrug transport proteins. N-Benzyladriamycin-14-valerate (AD 198) is a novel antitumor PKC activating agent that triggers rapid apoptosis through PKC-delta activation and mitochondrial depolarization in a manner that is unaffected by Bcl-2 expression. We demonstrate that Bcr-Abl expression does not confer resistance to AD 198. Further, AD 198 rapidly induces Erk1/2 and STAT5 phosphorylation prior to cytochrome c release from mitochondria, indicating that proliferative pathways are active even as drug-treated cells undergo apoptosis. At sub-cytotoxic doses, AD 198 and its cellular metabolite, N-benzyladriamycin (AD 288) sensitize CML cells to imatinib through a supra-additive reduction in the level of Bcr-Abl protein expression. These results suggest that AD 198 is an effective treatment for CML both in combination with imatinib and alone against imatinib-resistant CML cells.

FOXO3a induces differentiation of Bcr-Abl-transformed cells through transcriptional down-regulation of Id1

Leukemic transformation often requires activation of protein kinase B (PKB/c-Akt) and is characterized by increased proliferation, decreased apoptosis, and a differentiation block. PKB phosphorylates and inactivates members of the FOXO subfamily of Forkhead transcription factors. It has been suggested that hyperactivation of PKB maintains the leukemic phenotype through actively repressing FOXO-mediated regulation of specific genes. We have found expression of the transcriptional repressor Id1 (inhibitor of DNA binding 1) to be abrogated by FOXO3a activation. Inhibition of PKB activation or growth factor deprivation also resulted in strong down-regulation of Id1 promoter activity, Id1 mRNA, and protein expression. Id1 is highly expressed in Bcr-Abl-transformed K562 cells, correlating with high PKB activation and FOXO3a phosphorylation. Inhibition of Bcr-Abl by the chemical inhibitor STI571 resulted in activation of FOXO3a and down-regulation of Id1 expression. By performing chromatin immunoprecipitation assays and promoter-mutation analysis, we demonstrate that FOXO3a acts as a transcriptional repressor by directly binding to the Id1 promoter. STI571 treatment, or expression of constitutively active FOXO3a, resulted in erythroid differentiation of K562 cells, which was inhibited by ectopic expression of Id1. Taken together our data strongly suggest that high expression of Id1, through PKB-mediated inhibition of FOXO3a, is critical for maintenance of the leukemic phenotype.

Complex interaction of BCRP/ABCG2 and imatinib in BCR-ABL-expressing cells: BCRP-mediated resistance to imatinib is attenuated by imatinib-induced reduction of BCRP expression

Imatinib, a potent tyrosine kinase inhibitor, is effluxed from cells by the breast cancer resistance protein (BCRP/ABCG2), yet published studies to date fail to demonstrate resistance to imatinib cytotoxicity in BCRP-overexpressing cells in vitro. We investigated cellular resistance to imatinib in BCR-ABL-expressing cells transduced and selected to overexpress BCRP (K562/BCRP-MX10). These cells exhibited a 2- to 3-fold increase in resistance to imatinib (P < .05) and a 7- to 12-fold increase in resistance to mitoxantrone, a known BCRP substrate. Resistance to imatinib was completely abolished by the specific BCRP inhibitor fumitremorgin C. Studies of the mechanism of the diminished resistance to imatinib compared with mitoxantrone revealed that imatinib decreased the expression of BCRP in K562/BCRP-MX10 cells without affecting mRNA levels. BCRP levels in cells that do not express BCR-ABL were not affected by imatinib. Loss of BCRP expression was accompanied by imatinib-induced reduction of phosphorylated Akt in the BCRP-expressing K562 cells. The phosphoinositol-3 kinase (PI3K) inhibitor LY294002 also decreased BCRP levels in K562/BCRP-MX10 cells. These studies show that BCRP causes measurable imatinib resistance, but this effect is attenuated by imatinib-mediated inhibition of BCR-ABL, which in turn downregulates overall BCRP levels posttranscriptionally via the PI3K-Akt pathway.

Imatinib mesylate (STI571) abrogates the resistance to doxorubicin in human K562 chronic myeloid leukemia cells by inhibition of BCR/ABL kinase-mediated DNA repair

Imatinib mesylate (STI571), a specific inhibitor of BCR/ABL tyrosine kinase, exhibits potent antileukemic effects in the treatment of chronic myelogenous leukemia (CML). However, the precise mechanism by which inhibition of BCR/ABL activity results in pharmacological responses remains unknown. BCR/ABL-positive human K562 CML cells resistant to doxorubicin (K562DoxR) and their sensitive counterparts (K562DoxS) were used to determine the mechanism by which the STI571 inhibitor may overcome drug resistance. K562 wild type cells and CCRF-CEM lymphoblastic leukemia cells without BCR/ABL were used as controls. The STI571 specificity was examined by use of murine pro-B lymphoid Baf3 cells with or without BCR/ABL kinase expression. We examined kinetics of DNA repair after cell treatment with doxorubicin in the presence or absence of STI571 by the alkaline comet assay. The MTT assay was used to estimate resistance against doxorubicin and Western blot analysis with Crk-L antibody was performed to evaluate BCR/ABL kinase inhibition by STI571. We provide evidence that treatment of CML-derived BCR/ABL-expressing leukemia K562 cells with STI571 results in the inhibition of DNA repair and abrogation of the resistance of these cells to doxorubicin. We found that doxorubicin-resistant K562DoxR cells exhibited accelerated kinetics of DNA repair compared with doxorubicin-sensitive K562DoxS cells. Inhibition of BCR/ABL kinase in K562DoxR cells with 1 microM STI571 decreased the kinetics of DNA repair and abrogated drug resistance. The results suggest that STI571-mediated inhibition of BCR/ABL kinase activity can affect the effectiveness of the DNA-repair pathways, which in turn may enhance drug sensitivity of leukemia cells.

The survival kinases Akt and Pim as potential pharmacological targets

The Akt and Pim kinases are cytoplasmic serine/threonine kinases that control programmed cell death by phosphorylating substrates that regulate both apoptosis and cellular metabolism. The PI3K-dependent activation of the Akt kinases and the JAK/STAT-dependent induction of the Pim kinases are examples of partially overlapping survival kinase pathways. Pharmacological manipulation of such kinases could have a major impact on the treatment of a wide variety of human diseases including cancer, inflammatory disorders, and ischemic diseases.

AKT crystal structure and AKT-specific inhibitors

AKT kinases are attractive targets for small molecule drug discovery because of their key role in tumor cell survival/proliferation and their overexpression/activation in many human cancers. This review summarizes studies that support the rationale for targeting AKT kinases in new drug discovery efforts. Structural features of AKT kinase in its inactive and active states, as determined by crystal structure analysis, are described. Recent efforts in the development and biological evaluation of small molecule inhibitors of AKT, and the challenges remaining are summarized. Inhibitors targeting the ATP binding site, PH domain and protein substrate binding site, as well as isoform selective allosteric inhibitors are reviewed. Structure-based design using PKA mutants as surrogates and computer modeling in the discovery of selective inhibitors is discussed. The issues and challenges facing the development of different classes of inhibitors as therapeutics are also discussed.

Tyrosine kinase inhibitor STI571 (Imatinib) cooperates with wild-type p53 on K562 cell line to enhance its proapoptotic effects

In order to ascertain whether p53 has a role in chronic myeloid leukemia hematopoietic progenitor response to the innovative tyrosine kinase inhibitor STI571 (Imatinib), we overexpressed a wild type (wt) p53 construct in the K562 cell line, generated from a human blast crisis and lacking endogenous p53. Wt p53 overexpression was associated with a significant reduction of bcr-abl expression levels resulting, at least in part, from post-transcriptional events affecting the stability of p210 bcr-abl fusion protein. Moreover, we demonstrated that p53 overexpression enhances the commitment to the apoptotic death fate of K562 following its in vitro exposure to 1 microM STI571. Multiple mechanisms are involved in p53 impact on K562 survival: Most importantly, we found that a greater reduction of bcr-abl transcription by STI571 was associated with the overexpression of wt p53. Further studies are required to elucidate the mechanisms involved in the transcriptional repression of bcr-abl by STI571 and p53 and in their synergic effects on the clonal hematopoiesis of chronic myeloid leukemia.

IL-2 increases human telomerase reverse transcriptase activity transcriptionally and posttranslationally through phosphatidylinositol 3'-kinase/Akt, heat shock protein 90, and mammalian target of rapamycin in transformed NK cells

Human telomerase activity is induced by Ag receptor ligation in T and B cells. However, it is unknown whether telomerase activity is increased in association with activation and proliferation of NK cells. We found that telomerase activity in a human NK cell line (NK-92), which requires IL-2 for proliferation, was increased within 24 h after stimulation with IL-2. Levels of human telomerase reverse transcriptase (hTERT) mRNA and protein correlated with telomerase activity. ERK1/2 and Akt kinase (Akt) were activated by IL-2 stimulation. LY294002, an inhibitor of PI3K, abolished expression of hTERT mRNA and protein expression and abolished hTERT activity, whereas PD98059, which inhibits MEK1/2 and thus ERK1/2, had no effect. In addition, radicicol, an inhibitor of heat shock protein 90 (Hsp90), and rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), blocked IL-2-induced hTERT activity and nuclear translocation of hTERT but not hTERT mRNA expression. hTERT was coimmunoprecipitated with Akt, Hsp90, mTOR, and p70 S6 kinase (S6K), suggesting that these molecules form a physical complex. Immunoprecipitates of Akt, Hsp90, mTOR, and S6K from IL-2-stimulated NK-92 cells contained telomerase activity. Furthermore, the findings that Hsp90 and mTOR immunoprecipitates from primary samples contained telomerase activity are consistent with the results from NK-92 cells. These results indicate that IL-2 stimulation induces hTERT activation and that the mechanism of IL-2-induced hTERT activation involves transcriptional or posttranslational regulation through the pathway including PI3K/Akt, Hsp90, mTOR, and S6K in NK cells.

Synergistic interactions between imatinib mesylate and the novel phosphoinositide-dependent kinase-1 inhibitor OSU-03012 in overcoming imatinib mesylate resistance

Resistance to the Ableson protein tyrosine (Abl) kinase inhibitor imatinib mesylate has become a critical issue for patients in advanced phases of chronic myelogenous leukemia. Imatinib-resistant tumor cells develop, in part, as a result of point mutations within the Abl kinase domain. As protein kinase B (Akt) plays a pivotal role in Abl oncogene-mediated cell survival, we hypothesize that concurrent inhibition of Akt will sensitize resistant cells to the residual apoptotic activity of imatinib mesylate, thereby overcoming the resistance. Here, we examined the effect of OSU-03012, a celecoxib-derived phosphoinositide-dependent kinase-1 (PDK-1) inhibitor, on imatinib mesylate-induced apoptosis in 2 clinically relevant breakpoint cluster region (Bcr)-Abl mutant cell lines, Ba/F3p210(E255K) and Ba/F3p210(T315I). The 50% inhibitory concentration (IC50) values of imatinib mesylate to inhibit the proliferation of Ba/F3p210(E255K) and Ba/F3p210(T315I) were 14 +/- 4 and 30 +/- 2 microM, respectively. There was no cross-resistance to OSU-03012 in these mutant cells with an IC50 of 5 microM irrespective of mutations. Nevertheless, in the presence of OSU-03012 the susceptibility of these mutant cells to imatinib-induced apoptosis was significantly enhanced. This synergistic action was, at least in part, mediated through the concerted effect on phospho-Akt. Together these data provide a novel therapeutic strategy to overcome imatinib mesylate resistance, especially with the Abl mutant T315I.

PD166326, a novel tyrosine kinase inhibitor, has greater antileukemic activity than imatinib mesylate in a murine model of chronic myeloid leukemia

Imatinib mesylate is highly effective in newly diagnosed chronic myeloid leukemia (CML), but BCR/ABL (breakpoint cluster region/abelson murine leukemia)-positive progenitors persist in most patients with CML treated with imatinib mesylate, indicating the need for novel therapeutic approaches. In this study, we have used the murine CML-like myeloproliferative disorder as a platform to characterize the pharmacokinetic, signal transduction, and antileukemic properties of PD166326, one of the most potent members of the pyridopyrimidine class of protein tyrosine kinase inhibitors. In mice with the CML-like disease, PD166326 rapidly inhibited Bcr/Abl kinase activity after a single oral dose and demonstrated marked antileukemic activity in vivo. Seventy percent of PD166326-treated mice achieved a white blood cell (WBC) count less than 20.0 x 10(9)/L (20,000/microL) at necropsy, compared with only 8% of imatinib mesylate-treated animals. Further, two thirds of PD166326-treated animals had complete resolution of splenomegaly, compared with none of the imatinib mesylate-treated animals. Consistent with its more potent antileukemic effect in vivo, PD166326 was also superior to imatinib mesylate in inhibiting the constitutive tyrosine phosphorylation of numerous leukemia-cell proteins, including the src family member Lyn. PD166326 also prolonged the survival of mice with imatinib mesylate-resistant CML induced by the Bcr/Abl mutants P210/H396P and P210/M351T. Altogether, these findings demonstrate the potential of more potent Bcr/Abl inhibitors to provide more effective antileukemic activity. Clinical development of PD166326 or a related analog may lead to more effective drugs for the treatment of de novo and imatinib mesylate-resistant CML.

Roles of Bim in apoptosis of normal and Bcr-Abl-expressing hematopoietic progenitors

Bcr-Abl kinase is known to reverse apoptosis of cytokine-dependent cells due to cytokine deprivation, although it has been controversial whether chronic myeloid leukemia (CML) progenitors have the potential to survive under conditions in which there are limited amounts of cytokines. Here we demonstrate that early hematopoietic progenitors (Sca-1(+) c-Kit(+) Lin(-)) isolated from normal mice rapidly undergo apoptosis in the absence of cytokines. In these cells, the expression of Bim, a proapoptotic relative of Bcl-2 which plays a key role in the cytokine-mediated survival system, is induced. In contrast, those cells isolated from our previously established CML model mice resist apoptosis in cytokine-free medium without the induction of Bim expression, and these effects are reversed by the Abl-specific kinase inhibitor imatinib mesylate. In addition, the expression levels of Bim are uniformly low in cell lines established from patients in the blast crisis phase of CML, and imatinib induced Bim in these cells. Moreover, small interfering RNA that reduces the expression level of Bim effectively rescues CML cells from apoptosis caused by imatinib. These findings suggest that Bim plays an important role in the apoptosis of early hematopoietic progenitors and that Bcr-Abl supports cell survival in part through downregulation of this cell death activator.

Hemangiopoietin inhibits apoptosis of MO7e leukemia cells through phosphatidylinositol 3-kinase-AKT pathway

Hemangiopoietin (HAPO) is a growth factor that significantly stimulates proliferation and survival of the primitive cells of hematopoietic and endothelial lineages. To determine the mechanism of action of HAPO, the anti-apoptotic activity and signal transduction pathway of HAPO were investigated using a factor-dependent leukemia cell line, the MO7e cells. Recombinant human HAPO (rhHAPO) was produced in Escherichia coli and purified by a series of column chromatography with a purity of more than 95%. rhHAPO significantly supported the survival of MO7e cells after deprivation of granulocyte-macrophage colony stimulating factor and activated phosphatidylinositol 3-kinase (PI3K). When the MO7e cells were treated with two specific inhibitors to PI3K (LY294002 or wortmannin), a significant loss of cell viability with evidence of apoptosis was observed. Moreover, the protein kinase B (Akt), one of the downstream effectors of PI3K-dependent survival signaling, was activated in HAPO-stimulated MO7e cells. Phosphorylation of Akt at serine 473 and its downstream molecular Bad at serine 136 was induced by HAPO, but was blocked by two PI3K inhibitors, LY294002 and wortmannin. In addition, HAPO inhibited caspase-3 activities and poly(ADP-ribose) polymerase degradation. Such an effect of HAPO was also significantly blocked by either LY294002 or wortmannin. These results indicate that HAPO protects MO7e cells from apoptotic death through a PI3K-Akt pathway.

Role of the p38 mitogen-activated protein kinase pathway in the generation of the effects of imatinib mesylate (STI571) in BCR-ABL-expressing cells

Imatinib mesylate (STI571), a specific inhibitor of the BCR-ABL tyrosine kinase, exhibits potent antileukemic effects in vitro and in vivo. Despite the well established role of STI571 in the treatment of chronic myelogenous leukemia, the precise mechanisms by which inhibition of BCR-ABL tyrosine kinase activity results in generation of antileukemic responses remain unknown. In the present study we provide evidence that treatment of CML-derived BCR-ABL-expressing leukemia cells with STI571 results in activation of the p38 mitogen-activated protein (MAP) kinase signaling pathway. Our data indicate that STI571 induces phosphorylation of the p38 and activation of its kinase domain, in KT-1 cells and other BCR-ABL-expressing cell lines. We also identify the kinases MAP kinase-activated protein kinase-2 and Msk1 as two downstream effectors of p38, activated during inhibition of BCR-ABL activity by STI571. Importantly, pharmacological inhibition of p38 reverses the growth inhibitory effects of STI571 on primary leukemic colony-forming unit granulocyte/macrophage progenitors from patients with CML. Altogether, our data establish that activation of the p38 MAP kinase signaling cascade plays an important role in the generation of the effects of STI571 on BCR-ABL-expressing cells. They also suggest that, in addition to activation of mitogenic pathways, BCR-ABL promotes leukemogenesis by suppressing the function of growth inhibitory signaling cascades.