Differential regulation of the p70 S6 kinase pathway by interferon alpha (IFNalpha) and imatinib mesylate (STI571) in chronic myelogenous leukemia cells

Ribosomal Protein S6: A Potential Therapeutic Target against Cancer?

Ribosomal protein S6 (RPS6) is a component of the 40S small ribosomal subunit and participates in the control of mRNA translation. Additionally, phospho (p)-RPS6 has been recognized as a surrogate marker for the activated PI3K/AKT/mTORC1 pathway, which occurs in many cancer types. However, downstream mechanisms regulated by RPS6 or p-RPS remains elusive, and the therapeutic implication of RPS6 is underappreciated despite an approximately half a century history of research on this protein. In addition, substantial evidence from RPS6 knockdown experiments suggests the potential role of RPS6 in maintaining cancer cell proliferation. This motivates us to investigate the current knowledge of RPS6 functions in cancer. In this review article, we reviewed the current information about the transcriptional regulation, upstream regulators, and extra-ribosomal roles of RPS6, with a focus on its involvement in cancer. We also discussed the therapeutic potential of RPS6 in cancer.

Rapamycin enhanced the antitumor effects of doxorubicin in myelogenous leukemia K562 cells by downregulating the mTOR/p70S6K pathway

Chronic myelogenous leukemia (CML) is a common hematological malignancy. Some patients progressing to the blast phase develop chemotherapeutic drug resistance. In the authors' previous study, it was found that the mammalian target of rapamycin (mTOR) pathway was activated in CML and that rapamycin inhibited the proliferation of K562 cells. Targeting the mTOR pathway may be used in combination with chemotherapeutic drugs to enhance their efficacy and overcome multidrug resistance. The aim of the present study was to investigate the effects of rapamycin and doxorubicin on K562 cell proliferation following the combination treatment, and further focus on confirming whether rapamycin enhanced the antitumor effects of doxorubicin by downregulating the mTOR/ribosomal protein S6 kinase (p70S6K) pathway. It was found that rapamycin and doxorubicin significantly decreased the viability of K562 cells. The apoptotic cells were more frequently detected in rapamycin and doxorubicin treatment groups (25.50±1.25%). Both drugs decreased Bcl-2 and increased Bax expression in K562 cells. Rapamycin and doxorubicin also reduced the phosphorylation levels of mTOR and p70S6K. Meanwhile, p70S6K-targeting small interfering (si)RNA and doxorubicin inhibited cell proliferation and regulated key factors of the cell cycle. In addition, the exposure of cells to p70S6K siRNA and doxorubicin significantly increased cell apoptosis, as compared with single treatment. These results suggested that rapamycin could enhance the antitumor effects of doxorubicin on K562 cells by downregulating mTOR/p70S6K signaling. Targeting the mTOR/p70S6K pathway may be a new therapeutic approach for leukemia.

Interferon signaling in cancer. Non-canonical pathways and control of intracellular immune checkpoints

The interferons (IFNs) are cytokines with important antineoplastic and immune modulatory effects. These cytokines have been conserved through evolution as important elements of the immune surveillance against cancer. Despite this, defining their precise and specific roles in the generation of antitumor responses remains challenging. Emerging evidence suggests the existence of previously unknown roles for IFNs in the control of the immune response against cancer that may redefine our understanding on how these cytokines function. Beyond the engagement of classical JAK-STAT signaling pathways that promote transcription and expression of gene products, the IFNs engage multiple other signaling cascades to generate products that mediate biological responses and outcomes. There is recent emerging evidence indicating that IFNs control the expression of both traditional immune checkpoints like the PD-L1/PD1 axis, but also less well understood "intracellular" immune checkpoints whose targeting may define new approaches for the treatment of malignancies.

Sanguinarine suppresses growth and induces apoptosis in childhood acute lymphoblastic leukemia

Sanguinarine (Sang), a plant-derived compound isolated from the roots of Sanguinaria canadensis was evaluated for its potential pro-apoptotic effects in precursor B acute lymphoblastic leukemia (Pre-ALL) cell lines. Treatment of 697, REH, RS4;11, and SupB15 cell lines with Sang exhibited significant inhibition of cell viability via induction of apoptotic cell death. Sang-mediated apoptosis was found to be associated with the increased expression of proapoptotic bax with concomitant decrease of Bcl-2 expression leading to depolarization of mitochondria membrane resulting in loss of mitochondrial membrane potential (MMP). The reduced MMP caused the leakage in mitochondrial membrane and release of cytochrome c into the cytosol. The cytochrome c then mediates the activation of caspase-cascade and subsequently PARP cleavage. Furthermore, pretreatment with z-VAD-FMK, a pan-caspase inhibitor, abrogated Sang-induced inhibition of cell viability, induction of apoptosis. Sang treatment also reduced the phosphorylation of AKT and suppressed the expression of a number of anti-apoptotic genes such as cIAP1, cIAP2, and XIAP. Sang mediates its anti-cancer activity by generation of reactive oxygen species (ROS) due to depletion of glutathione level in leukemic cell lines. Pretreatment of these cells with N-acetyl cysteine (NAC) prevented Sang-induced depletion of glutathione level and mitochondrial-caspase-induced apoptosis. Finally, Sang treatment of Pre-ALL cell suppressed colony formation ability of these cells suggesting Sang has an anti-leukemic potential. Altogether, our data suggest that Sang is an efficient inducer of intrinsic apoptotic cell death via generation of ROS and exhibition of anti-leukemic effect in Pre-ALL cells raises the possibility to develop Sang as a therapeutic modality for the treatment and management of Pre-ALL.

Programmed cell death 4 and BCR-ABL fusion gene expression are negatively correlated in chronic myeloid leukemia

Programmed cell death 4 (PDCD4) is a tumor suppressor that inhibits carcinogenesis, tumor progression and invasion by preventing gene transcription and translation. Downregulation of PDCD4 expression has been identified in multiple types of human cancer, however, to date, the function of PDCD4 in leukemia has not been investigated. In the present study, PDCD4 mRNA and protein expression was investigated in 50 patients exhibiting various phases of chronic myeloid leukemia (CML) and 20 healthy individuals by reverse transcription-quantitative polymerase chain reaction and western blot analysis. PDCD4 expression and cell proliferation was also investigated following treatment with the tyrosine kinase inhibitor, imatinib, in K562 cells. The results demonstrated that PDCD4 mRNA and protein expression was decreased in all CML samples when compared with healthy controls, who expressed high levels of PDCD4 mRNA and protein. No significant differences in PDCD4 expression were identified between chronic phase, accelerated phase and blast phase CML patients. In addition, PDCD4 expression was negatively correlated with BCR-ABL gene expression (r=-0.6716; P<0.001). Furthermore, K562 cells treated with imatinib exhibited significantly enhanced PDCD4 expression. These results indicate that downregulation of PDCD4 expression may exhibit a critical function in the progression and malignant proliferation of human CML.

Targeting the mTOR Pathway in Leukemia

Optimal function of multiple intracellular signaling pathways is essential for normal regulation of cellular transcription, translation, growth, proliferation, and survival. Dysregulation or aberrant activation of such cascades can lead to inappropriate cell survival and abnormal cell proliferation in leukemia. Successful treatment of chronic myeloid leukemia (CML) with tyrosine kinase inhibitors targeting the BCR-ABL fusion gene is a prime example of effectively inhibiting intracellular signaling cascades. However, even in these patients resistance can develop via emergence of mutations or feedback activation of other pathways that cause refractory disease. Constitutive activation of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt)/mammalian target of rapamycin (mTOR) signaling pathway has been observed in different types of leukemia, including CML, acute myeloid leukemia, and acute lymphoblastic leukemia. Abnormal mTOR activity may contribute to chemotherapy resistance, while it may also be effectively targeted via molecular means and/or development of specific pharmacological inhibitors. This review discusses the role of PI3K/Akt/mTOR dysre-gulation in leukemia and summarizes the emergence of preliminary data for the development of novel therapeutic approaches. J. Cell. Biochem. 117: 1745-1752, 2016. © 2016 Wiley Periodicals, Inc.

Rapamycin combined with celecoxib enhanced antitumor effects of mono treatment on chronic myelogenous leukemia cells through downregulating mTOR pathway

Chronic myelogenous leukemia is a neoplasm of myeloid progenitor cells. We recently found that rapamycin could induce G0/G1 phase arrest and apoptosis and inhibit proliferation of K562 cells through inhibiting mammalian target of rapamycin (mTOR) pathway. However, whether rapamycin has synergistic effects with other drugs in chronic myelogenous leukemia (CML) therapies remain unclear. Therefore, we examined the effect of rapamycin combined with celecoxib on K562 cells in vitro. The survival rates showed a significant decrease in rapamycin + celecoxib treatment group. The combination treatment also increased the G0/G1 phase cells as compared to rapamycin or celecoxib treatment alone (P < 0.05), accompanied with the decreased population of S phase cells. Meanwhile, the rate of apoptosis was 15.87 ± 2.21 % in rapamycin + celecoxib treatment group, significantly higher than that in mono treatment group (P < 0.05). Western blot and reverse transcription PCR (RT-PCR) analysis showed that the expressions of mTOR, 4E-BP1, and p70S6K were all significantly decreased in K562 cells after rapamycin + celecoxib treatment (P < 0.05). In conclusion, rapamycin combined with celecoxib could induce cell cycle arrest and apoptosis and decrease the expressions of mTOR, 4E-BP1, and p70S6K. It suggested that the combination could enhance the antitumor effects of mono treatment on CML cells through downregulating mTOR pathway.

Beta interferon regulation of glucose metabolism is PI3K/Akt dependent and important for antiviral activity against coxsackievirus B3

An effective type I interferon (IFN)-mediated immune response requires the rapid expression of antiviral proteins that are necessary to inhibit viral replication and virus spread. We provide evidence that IFN-β regulates metabolic events important for the induction of a rapid antiviral response: IFN-β decreases the phosphorylation of AMP-activated protein kinase (AMPK), coincident with an increase in intracellular ATP. Our studies reveal a biphasic IFN-β-inducible uptake of glucose by cells, mediated by phosphatidylinositol 3-kinase (PI3K)/Akt, and IFN-β-inducible regulation of GLUT4 translocation to the cell surface. Additionally, we provide evidence that IFN-β-regulated glycolytic metabolism is important for the acute induction of an antiviral response during infection with coxsackievirus B3 (CVB3). Last, we demonstrate that the antidiabetic drug metformin enhances the antiviral potency of IFN-β against CVB3 both in vitro and in vivo. Taken together, these findings highlight an important role for IFN-β in modulating glucose metabolism during a virus infection and suggest that the use of metformin in combination with IFN-β during acute virus infection may result in enhanced antiviral responses.

IMPORTANCE

Type I interferons (IFN) are critical effectors of an antiviral response. These studies describe for the first time a role for IFN-β in regulating metabolism--glucose uptake and ATP production--to meet the energy requirements of a robust cellular antiviral response. Our data suggest that IFN-β regulates glucose metabolism mediated by signaling effectors similarly to activation by insulin. Interference with IFN-β-inducible glucose metabolism diminishes the antiviral response, whereas treatment with metformin, a drug that increases insulin sensitivity, enhances the antiviral potency of IFN-β.

Regulatory effects of sestrin 3 (SESN3) in BCR-ABL expressing cells

Chronic myeloid leukemia (CML) and Ph+ acute lymphoblastic leukemia (ALL) are characterized by the presence of the BCR-ABL oncoprotein, which leads to activation of a plethora of pro-mitogenic and pro-survival pathways, including the mTOR signaling cascade. We provide evidence that in BCR-ABL expressing cells, treatment with tyrosine kinase inhibitors (TKIs) results in upregulation of mRNA levels and protein expression of sestrin3 (SESN3), a unique cellular inhibitor of mTOR complex 1 (mTORC1). Such upregulation appears to be mediated by regulatory effects on mTOR, as catalytic inhibition of the mTOR kinase also induces SESN3. Catalytic mTOR inhibition also results in upregulation of SESN3 expression in cells harboring the TKI-insensitive T315I-BCR-ABL mutant, which is resistant to imatinib mesylate. Overexpression of SESN3 results in inhibitory effects on different Ph+ leukemic cell lines including KT-1-derived leukemic precursors, indicating that SESN3 mediates anti-leukemic responses in Ph+ cells. Altogether, our findings suggest the existence of a novel mechanism for the generation of antileukemic responses in CML cells, involving upregulation of SESN3 expression.

The Akt substrate Girdin is a regulator of insulin signaling in myoblast cells

Akt kinases are important mediators of the insulin signal, and some Akt substrates are directly involved in glucose homeostasis. Recently, Girdin has been described as an Akt substrate that is expressed ubiquitously in mammals. Cells overexpressing Girdin show an enhanced Akt activity. However, not much is known about Girdin's role in insulin signaling. We therefore analyzed the role of Girdin in primary human myotubes and found a correlation between Girdin expression and insulin sensitivity of the muscle biopsy donors, as measured by a hyperinsulinemic-euglycemic clamp. To understand this finding on a cellular level, we then investigated the function of Girdin in C2C12 mouse myoblasts. Girdin knock-down reduced Akt and insulin receptor substrate-1 phosphorylation. In contrast, stable overexpression of Girdin in C2C12 cells strikingly increased insulin sensitivity through a massive upregulation of the insulin receptor and enhanced tyrosine phosphorylation of insulin receptor substrate-1. Furthermore, Akt and c-Abl kinases were constitutively activated. To investigate medium-term insulin responses we measured glucose incorporation into glycogen. The Girdin overexpressing cells showed a high basal glycogen synthesis that peaked already at 1nM insulin. Taken together, we characterized Girdin as a new and major regulator of the insulin signal in myoblasts and skeletal muscle.

Two hits are better than one: targeting both phosphatidylinositol 3-kinase and mammalian target of rapamycin as a therapeutic strategy for acute leukemia treatment

Phosphatidylinositol 3-kinase (PI3K) and mammalian target of rapamycin (mTOR) are two key components of the PI3K/Akt/mTOR signaling pathway. This signal transduction cascade regulates a wide range of physiological cell processes, that include differentiation, proliferation, apoptosis, autophagy, metabolism, motility, and exocytosis. However, constitutively active PI3K/Akt/mTOR signaling characterizes many types of tumors where it negatively influences response to therapeutic treatments. Hence, targeting PI3K/Akt/mTOR signaling with small molecule inhibitors may improve cancer patient outcome. The PI3K/Akt/mTOR signaling cascade is overactive in acute leukemias, where it correlates with enhanced drug-resistance and poor prognosis. The catalytic sites of PI3K and mTOR share a high degree of sequence homology. This feature has allowed the synthesis of ATP-competitive compounds targeting the catalytic site of both kinases. In preclinical models, dual PI3K/mTOR inhibitors displayed a much stronger cytotoxicity against acute leukemia cells than either PI3K inhibitors or allosteric mTOR inhibitors, such as rapamycin. At variance with rapamycin, dual PI3K/mTOR inhibitors targeted both mTOR complex 1 and mTOR complex 2, and inhibited the rapamycin-resistant phosphorylation of eukaryotic initiation factor 4E-binding protein 1, resulting in a marked inhibition of oncogenic protein translation. Therefore, they strongly reduced cell proliferation and induced an important apoptotic response. Here, we reviewed the evidence documenting that dual PI3K/mTOR inhibitors may represent a promising option for future targeted therapies of acute leukemia patients.

AMPK in BCR-ABL expressing leukemias. Regulatory effects and therapeutic implications

The abnormal BCR-ABL oncoprotein is a constitutively active tyrosine kinase driving aberrant proliferation of transformed hematopoietic cells. BCR-ABL regulates activation of many mitogenic and pro-survival pathways, including the PI 3'K/AKT/mTOR pathway that controls various effectors and regulates initiation of mRNA translation in mammalian cells. Although tyrosine kinase inhibitors (TKIs) that target the ABL kinase domain have remarkable clinical activity and have dramatically changed the natural history of Ph+ leukemias, resistance to these agents also develops via a wide range of mechanisms. Efforts to target the PI3'K/AKT/mTOR signaling pathway using kinase inhibitors have been the focus of extensive ongoing investigations by several research groups. Here we review the effects of activation of the AMPK kinase, which regulates downstream targeting and inhibition of mTOR. The potential for future clinical-translational applications of AMPK activators such as AICAR, metformin and resveratrol for the treatment of chronic myelogenous leukemia (CML) and Ph+ acute lymphoblastic leukemia (ALL) are discussed.

Regulatory effects of programmed cell death 4 (PDCD4) protein in interferon (IFN)-stimulated gene expression and generation of type I IFN responses

The precise mechanisms by which the activation of interferon (IFN) receptors (IFNRs) ultimately controls mRNA translation of specific target genes to induce IFN-dependent biological responses remain ill defined. We provide evidence that IFN-α induces phosphorylation of programmed cell death 4 (PDCD4) protein on Ser67. This IFN-α-dependent phosphorylation is mediated by either the p70 S6 kinase (S6K) or the p90 ribosomal protein S6K (RSK) in a cell-type-specific manner. IFN-dependent phosphorylation of PDCD4 results in downregulation of PDCD4 protein levels as the phosphorylated form of PDCD4 interacts with the ubiquitin ligase β-TRCP (β-transducin repeat-containing protein) and undergoes degradation. This process facilitates IFN-induced eukaryotic translation initiation factor 4A (eIF4A) activity and binding to translation initiation factor eIF4G to promote mRNA translation. Our data establish that PDCD4 degradation ultimately facilitates expression of several ISG protein products that play important roles in the generation of IFN responses, including IFN-stimulated gene 15 (ISG15), p21(WAF1/CIP1), and Schlafen 5 (SLFN5). Moreover, engagement of the RSK/PDCD4 pathway by the type I IFNR is required for the suppressive effects of IFN-α on normal CD34(+) hematopoietic precursors and for antileukemic effects in vitro. Altogether, these findings provide evidence for a unique function of PDCD4 in the type I IFN system and indicate a key regulatory role for this protein in mRNA translation of ISGs and control of IFN responses.

The PI3K/PKB signaling module as key regulator of hematopoiesis: implications for therapeutic strategies in leukemia

An important mediator of cytokine signaling implicated in regulation of hematopoiesis is the PI3K/protein kinase B (PKB/c-Akt) signaling module. Constitutive activation of this signaling module has been observed in a large group of leukemias. Because activation of this signaling pathway has been demonstrated to be sufficient to induce hematologic malignancies and is thought to correlate with poor prognosis and enhanced drug resistance, it is considered to be a promising target for therapy. A high number of pharmacologic inhibitors directed against either individual or multiple components of this pathway have already been developed to improve therapy. In this review, the safety and efficacy of both single and dual-specificity inhibitors will be discussed as well as the potential of combination therapy with either inhibitors directed against other signal transduction molecules or classic chemotherapy.

Antileukemic effects of AMPK activators on BCR-ABL-expressing cells

The mammalian target of rapamycin (mTOR) signaling pathway plays a critical role in growth and survival of BCR-ABL transformed cells. AMPK kinase is a metabolic sensor that exhibits suppressive effects on the mTOR pathway and negatively regulates mTOR activity. We report that AMPK activators, such as metformin and 5-aminoimidazole-4-carboxamide ribonucleotide, suppress activation of the mTOR pathway in BCR-ABL-expressing cells. Treatment with these inhibitors results in potent suppression of chronic myeloid leukemia leukemic precursors and Ph(+) acute lymphoblastic leukemia cells, including cells expressing the T315I-BCR-ABL mutation. Altogether, our data suggest that AMPK is an attractive target for the treatment of BCR-ABL-expressing malignancies and raise the potential for use of AMPK activators in the treatment of refractory chronic myeloid leukemia and Ph(+) acute lymphoblastic leukemia.

Imatinib potentiates antitumor T cell responses in gastrointestinal stromal tumor through the inhibition of Ido

Imatinib mesylate targets mutated KIT oncoproteins in gastrointestinal stromal tumor (GIST) and produces a clinical response in 80% of patients. The mechanism is believed to depend predominantly on the inhibition of KIT-driven signals for tumor-cell survival and proliferation. Using a mouse model of spontaneous GIST, we found that the immune system contributes substantially to the antitumor effects of imatinib. Imatinib therapy activated CD8(+) T cells and induced regulatory T cell (T(reg) cell) apoptosis within the tumor by reducing tumor-cell expression of the immunosuppressive enzyme indoleamine 2,3-dioxygenase (Ido). Concurrent immunotherapy augmented the efficacy of imatinib in mouse GIST. In freshly obtained human GIST specimens, the T cell profile correlated with imatinib sensitivity and IDO expression. Thus, T cells are crucial to the antitumor effects of imatinib in GIST, and concomitant immunotherapy may further improve outcomes in human cancers treated with targeted agents.

Regulation of mammalian target of rapamycin and mitogen activated protein kinase pathways by BCR-ABL

A large body of evidence has established that BCR-ABL regulates engagement and activation of mammalian target of rapamycin (mTOR) and mitogen activated protein kinase (MAPK) signaling cascades. mTOR-mediated signals, as well as signals transduced by ERK, JNK, and p38 MAPK, are important components of the aberrant signaling induced by BCR-ABL. Such deregulation of mTOR or MAPK pathways contributes to BCR-ABL leukemogenesis, and their targeting with selective inhibitors provides an approach to enhance antileukemic responses and/or overcome leukemic cell resistance in chronic myeloid leukemia (CML) and Philadelphia chromosome-positive (Ph+) acute lymphoblastic leukemia (ALL). This review explores recent advances in our understanding of mTOR and MAPK signaling in BCR-ABL-expressing leukemias and discusses the potential therapeutic targeting of these pathways in CML and Ph+ ALL.

Potential of mTOR inhibitors as therapeutic agents in hematological malignancies

Despite significant advances in the treatment of hematological malignancies over the last decade, morbidity and mortality from these disorders remain high. New discoveries in the pathogenesis of these malignancies have led to better understanding of these diseases and new thinking in drug development. mTOR is a downstream effector of the PI3K/Akt (protein kinase B) signaling pathway that mediates cell survival and proliferation and is known to be deregulated in many cancers. Preclinical activity of mTOR inhibitors has been very promising in various hematological malignancies. Rapamycin analogs with relatively favorable pharmaceutical properties, including temsirolimus (CCI-779), everolimus (RAD001) and deforolimus (AP23573), are under clinical evaluations in patients with hematologic malignancies. They have shown encouraging results thus far and a favorable toxicity profile. Their utility, mainly as cytostatic agents, needs to be further explored in combination with pre-existing chemotherapeutic agents for various hematological malignancies.

Dual mTORC2/mTORC1 targeting results in potent suppressive effects on acute myeloid leukemia (AML) progenitors

PURPOSE

To determine whether mTORC2 and rapamycin-insensitive (RI)-mTORC1 complexes are present in acute myeloid leukemia (AML) cells and to examine the effects of dual mTORC2/mTORC1 inhibition on primitive AML leukemic progenitors.

EXPERIMENTAL DESIGN

Combinations of different experimental approaches were used, including immunoblotting to detect phosphorylated/activated forms of elements of the mTOR pathway in leukemic cell lines and primary AML blasts; cell-proliferation assays; direct assessment of mRNA translation in polysomal fractions of leukemic cells; and clonogenic assays in methylcellulose to evaluate leukemic progenitor-colony formation.

RESULTS

mTORC2 complexes are active in AML cells and play critical roles in leukemogenesis. RI-mTORC1 complexes are also formed and regulate the activity of the translational repressor 4E-BP1 in AML cells. OSI-027 blocks mTORC1 and mTORC2 activities and suppresses mRNA translation of cyclin D1 and other genes that mediate proliferative responses in AML cells. Moreover, OSI-027 acts as a potent suppressor of primitive leukemic precursors from AML patients and is much more effective than rapamycin in eliciting antileukemic effects in vitro.

CONCLUSIONS

Dual targeting of mTORC2 and mTORC1 results in potent suppressive effects on primitive leukemic progenitors from AML patients. Inhibition of the mTOR catalytic site with OSI-027 results in suppression of both mTORC2 and RI-mTORC1 complexes and elicits much more potent antileukemic responses than selective mTORC1 targeting with rapamycin.

Sphingosine kinase 1 overexpression is regulated by signaling through PI3K, AKT2, and mTOR in imatinib-resistant chronic myeloid leukemia cells

OBJECTIVE

As a better understanding of the molecular basis of carcinogenesis has emerged, oncogene-specific cell-signaling pathways have been successfully targeted to treat human malignances. Despite impressive advances in oncogene-directed therapeutics, genetic instability in cancer cells often manifest acquired resistance. This is particularly noted in the use of tyrosine kinase inhibitors therapies and not more evident than for chronic myeloid leukemia. Therefore, it is of great importance to understand the molecular mechanisms affecting cancer cell sensitivity and resistance to tyrosine kinase inhibitors.

MATERIALS AND METHODS

In this study, we used continuous exposure to stepwise increasing concentrations of imatinib (0.6-1 μM) to select imatinib-resistant K562 cells.

RESULTS

Expression of BCR-ABL increased both at RNA and protein levels in imatinib-resistant cell lines. Furthermore, expression levels of sphingosine kinase 1 (SphK1) were increased significantly in resistant cells, channeling sphingoid bases to the SphK1 pathway and activating sphingosine-1-phosphate-dependent tyrosine phosphorylation pathways that include the adaptor protein Crk. The partial inhibition of SphK1 activity by N,N-dimethylsphingosine or expression by small interfering RNA increased sensitivity to imatinib-induced apoptosis in resistant cells and returned BCR-ABL to baseline levels. To determine the resistance mechanism-induced SphK1 upregulation, we used pharmacological inhibitors of the phosphoinositide 3-kinase/AKT/mammalian target of rapamycin signaling pathway and observed robust downmodulation of SphK1 expression and activity when AKT2, but not AKT1 or AKT3, was suppressed.

CONCLUSIONS

These results demonstrate that SphK1 is upregulated in imatinib-resistant K562 cells by a pathway contingent on a phosphoinositide 3-kinase/AKT2/mammalian target of rapamycin signaling pathway. We propose that SphK1 plays an important role in development of acquired resistance to imatinib in chronic myeloid leukemia cell lines.

BCR-ABL1 kinase-dependent alteration of mRNA metabolism: potential alternatives for therapeutic intervention

The use of first- and second-generation tyrosine kinase inhibitors (TKIs) significantly improves prognosis for patients with early chronic phase chronic myeloid leukemia (CML) and efficiently counteracts leukemia in most patients with CML bearing a disease characterized by the expression of BCR-ABL1 mutants. However, the so-called 'tinib' TKIs (e.g. imatinib, nilotinib, dasatinib, and bosutinib) are both ineffective in patients who undergo blastic transformation and unable to eradicate CML at the stem cell level. This raises a few important questions. Is BCR-ABL1 expression and/or activity essential for blastic transformation? Is blastic transformation the result of genetic or epigenetic events that occur at the stem cell level which only become apparent in the granulocyte-macrophage progenitor (GMP) cell pool, or does it arise directly at the GMP level? As altered mRNA metabolism contributes to the phenotype of blast crisis CML progenitors (decreased translation of tumor suppressor genes and transcription factors essential for terminal differentiation and increased translation of anti-apoptotic genes), one attractive concept is to restore levels of these essential molecules to their normal levels. In this review, we discuss the mechanisms by which mRNA processing, translation, and degradation are deregulated in BCR-ABL1 myeloid blast crisis CML progenitors, and present encouraging results from studies with pharmacologic inhibitors which support their inclusion in the clinic.

Perspectives on inhibiting mTOR as a future treatment strategy for hematological malignancies

Mammalian target of rapamycin (mTOR) is a protein kinase implicated in the regulation of various cellular processes, including those required for tumor development, such as the initiation of mRNA translation, cell-cycle progression and cellular proliferation. In a wide range of hematological malignancies, the mTORC1 signaling pathway has been found to be deregulated and has been designed as a major target for tumor therapy. Given that pre-clinical studies have clearly established the therapeutic value of mTORC1 inhibition, numerous clinical trials of rapamycin and its derivates (rapalogs) are ongoing for treatment of these diseases. At this time, although disease stabilization and tumor regression have been observed, objective responses in some tumor types have been modest. Nevertheless, some of the mechanisms underlying cancer-cell resistance to rapamycin have now been described, thereby leading to the development of new strategy to efficiently target mTOR signaling in these diseases. In this review, we discuss the rationale for using mTOR inhibitors as novel therapies for a variety of hematological, malignancies with a focus on promising new perspectives for these approaches.

Small interfering RNA against BCR-ABL transcripts sensitize mutated T315I cells to nilotinib

BACKGROUND

Selective inhibition of the BCR-ABL tyrosine kinase by RNA interference has been demonstrated in leukemic cells. We, therefore, evaluated specific BCR-ABL small interfering RNA silencing in BCR-ABL-positive cell lines, including those resistant to imatinib and particularly those with the T315I mutation.

DESIGN AND METHODS

The factor-independent 32Dp210 BCR-ABL oligoclonal cell lines and human imatinib-resistant BCR-ABL-positive cells from patients with leukemic disorders were investigated. The effects of BCR-ABL small interfering RNA or the combination of BCR-ABL small interfering RNA with imatinib and nilotinib were compared with those of the ABL inhibitors imatinib and nilotinib.

RESULTS

Co-administration of BCR-ABL small interfering RNA with imatinib or nilotinib dramatically reduced BCR-ABL expression in wild-type and mutated BCR-ABL cells and increased the lethal capacity. BCR-ABL small interfering RNA significantly induced apoptosis and inhibited proliferation in wild-type (P<0.0001) and mutated cells (H396P, T315I, P<0.0001) versus controls. Co-treatment with BCR-ABL small interfering RNA and imatinib or nilotinib resulted in increased inhibition of proliferation and induction of apoptosis in T315I cells as compared to imatinib or nilotinib alone (P<0.0001). Furthermore, the combination of BCR-ABL small interfering RNA with imatinib or nilotinib significantly (P<0.01) reversed multidrug resistance-1 gene-dependent resistance of mutated cells. In T315I cells BCR-ABL small interfering RNA with nilotinib had powerful effects on cell cycle distribution.

CONCLUSIONS

Our data suggest that silencing by BCR-ABL small interfering RNA combined with imatinib or nilotinib may be associated with an additive antileukemic activity against tyrosine kinase inhibitor-sensitive and resistant BCR-ABL cells, and might be an alternative approach to overcome BCR-ABL mutations.

Critical roles for mTORC2- and rapamycin-insensitive mTORC1-complexes in growth and survival of BCR-ABL-expressing leukemic cells

mTOR-generated signals play critical roles in growth of leukemic cells by controlling mRNA translation of genes that promote mitogenic responses. Despite extensive work on the functional relevance of rapamycin-sensitive mTORC1 complexes, much less is known on the roles of rapamycin-insensitive (RI) complexes, including mTORC2 and RI-mTORC1, in BCR-ABL-leukemogenesis. We provide evidence for the presence of mTORC2 complexes in BCR-ABL-transformed cells and identify phosphorylation of 4E-BP1 on Thr37/46 and Ser65 as RI-mTORC1 signals in primary chronic myelogenous leukemia (CML) cells. Our studies establish that a unique dual mTORC2/mTORC1 inhibitor, OSI-027, induces potent suppressive effects on primitive leukemic progenitors from CML patients and generates antileukemic responses in cells expressing the T315I-BCR-ABL mutation, which is refractory to all BCR-ABL kinase inhibitors currently in clinical use. Induction of apoptosis by OSI-027 appears to negatively correlate with induction of autophagy in some types of BCR-ABL transformed cells, as shown by the induction of autophagy during OSI-027-treatment and the potentiation of apoptosis by concomitant inhibition of such autophagy. Altogether, our studies establish critical roles for mTORC2 and RI-mTORC1 complexes in survival and growth of BCR-ABL cells and suggest that dual therapeutic targeting of such complexes may provide an approach to overcome leukemic cell resistance in CML and Ph+ ALL.

Activation of the p38 Map kinase pathway is essential for the antileukemic effects of dasatinib

Dasatinib, a dual Src/Abl tyrosine kinase inhibitor, has significant antileukemic effects against various imatinib mesylate-resistant BCR/ABL mutants. Despite well-documented inhibitory effects of dasatinib on BCR/ABL kinase, the exact downstream cellular events leading to generation of its potent antileukemic effects remain to be defined. We provide evidence that p38 Map kinase (MAPK) pathway is activated leading to increased upregulation of mixed lineage kinase 3, MKK3/6, MSK1, and Mapkapk2, upon treatment of BCR/ABL expressing cells with dasatinib, including cells expressing various imatinib-resistant mutants, except for T315I. Our data demonstrate that such dasatinib-dependent activation of p38 MAPK and its effectors plays a critical role in the generation of antileukemic responses, since pharmacological inhibition of p38 or siRNA-mediated knockdown of its expression reverse dasatinib-mediated apoptosis, cell cycle arrest, and anti-proliferative effects. p38 MAPK inhibition also reversed dasatinib-induced suppression of CML patient-derived leukemic colony-forming units progenitor growth in vitro, as well as BCR/ABL expressing KT-1 cell-derived leukemic progenitor growth. Altogether, our findings suggest a critical role for p38 MAPK pathway in the generation of antileukemic effects of dasatinib, and raise the possibility that development of novel means to enhance p38 MAPK activation in BCR/ABL expressing cells may be an approach to promote antileukemic responses and, possibly, reverse T315I mutation-mediated resistance.

Interferon-alpha-induced mTOR activation is an anti-hepatitis C virus signal via the phosphatidylinositol 3-kinase-Akt-independent pathway

OBJECT

The interferon-induced Jak-STAT signal alone is not sufficient to explain all the biological effects of IFN. The PI3-K pathways have emerged as a critical additional component of IFN-induced signaling. This study attempted to clarify that relationship between IFN-induced PI3-K-Akt-mTOR activity and anti-viral action.

RESULT

When the human normal hepatocyte derived cell line was treated with rapamycin (rapa) before accretion of IFN-alpha, tyrosine phosphorylation of STAT-1 was diminished. Pretreatment of rapa had an inhibitory effect on the IFN-alpha-induced expression of PKR and p48 in a dose dependent manner. Rapa inhibited the IFN-alpha inducible IFN-stimulated regulatory element luciferase activity in a dose-dependent manner. However, wortmannin, LY294002 and Akt inhibitor did not influence IFN-alpha inducible luciferase activity. To examine the effect of PI3-K-Akt-mTOR on the anti-HCV action of IFN-alpha, the full-length HCV replication system, OR6 cells were used. The pretreatment of rapa attenuated its anti-HCV replication effect in comparison to IFN-alpha alone, whereas the pretreatment with PI3-K inhibitors, wortmannin and LY294002 and Akt inhibitor did not influence IFN-induced anti-HCV replication.

CONCLUSION

IFN-induced mTOR activity, independent of PI3K and Akt, is the critical factor for its anti-HCV activity. Jak independent mTOR activity involved STAT-1 phosphorylation and nuclear location, and then PKR is expressed in hepatocytes.

Gambogic acid induces G0/G1 arrest and apoptosis involving inhibition of SRC-3 and inactivation of Akt pathway in K562 leukemia cells

Gambogic acid (GA), a major active component of gamboge, exhibits potent anticancer activity in many kinds of cancer cells. However, the anticancer mechanism of GA is not clearly understood. Here we showed that GA could cause growth inhibition, induce the G0/G1 phase cell cycle arrest and apoptosis in human chronic myelogenous leukemia cell line K562 cells. Since steroid receptor coactivator-3 (SRC-3), overexpressed in many human malignancies including leukemia, is a central target for cancer therapy, we also explored the effects of GA on SRC-3 and SRC-3-regulated gene products in K562. GA treatment downregulated the expression of SRC-3 and then inhibited the activity of Akt kinase and its downstream targets p70 S6 kinase 1 (S6K1) and glycogen synthase kinase 3beta (GSK3beta) without changes in total protein levels of these three proteins, which thus influenced the expression of the apoptosis related gene Bcl-2 in K562 cells. These results suggest that GA might exhibit its strong antitumor effects via the interruption of SRC-3.

Interferon-dependent engagement of eukaryotic initiation factor 4B via S6 kinase (S6K)- and ribosomal protein S6K-mediated signals

Although the roles of Jak-Stat pathways in type I and II interferon (IFN)-dependent transcriptional regulation are well established, the precise mechanisms of mRNA translation for IFN-sensitive genes remain to be defined. We examined the effects of IFNs on the phosphorylation/activation of eukaryotic translation initiation factor 4B (eIF4B). Our data show that eIF4B is phosphorylated on Ser422 during treatment of sensitive cells with alpha IFN (IFN-alpha) or IFN-gamma. Such phosphorylation is regulated, in a cell type-specific manner, by either the p70 S6 kinase (S6K) or the p90 ribosomal protein S6K (RSK) and results in enhanced interaction of the protein with eIF3A (p170/eIF3A) and increased associated ATPase activity. Our data also demonstrate that IFN-inducible eIF4B activity and IFN-stimulated gene 15 protein (ISG15) or IFN-gamma-inducible chemokine CXCL-10 protein expression are diminished in S6k1/S6k2 double-knockout mouse embryonic fibroblasts. In addition, IFN-alpha-inducible ISG15 protein expression is blocked by eIF4B or eIF3A knockdown, establishing a requirement for these proteins in mRNA translation/protein expression by IFNs. Importantly, the generation of IFN-dependent growth inhibitory effects on primitive leukemic progenitors is dependent on activation of the S6K/eIF4B or RSK/eIF4B pathway. Taken together, our findings establish critical roles for S6K and RSK in the induction of IFN-dependent biological effects and define a key regulatory role for eIF4B as a common mediator and integrator of IFN-generated signals from these kinases.

Activation of protein kinase C{eta} by type I interferons

Type I interferons (IFNs) are cytokines with diverse biological properties, including antiviral, growth inhibitory, and immunomodulatory effects. Although several signaling pathways are activated during engagement of the type I IFN receptor and participate in the induction of IFN responses, the mechanisms of generation of specific signals for distinct biological effects remain to be elucidated. We provide evidence that a novel member of the protein kinase C (PKC) family of proteins is rapidly phosphorylated and activated during engagement of the type I IFN receptor. In contrast to other members of the PKC family that are also regulated by IFN receptors, PKCeta does not regulate IFN-inducible transcription of interferon-stimulated genes or generation of antiviral responses. However, its function promotes cell cycle arrest and is essential for the generation of the suppressive effects of IFNalpha on normal and leukemic human myeloid (colony-forming unit-granulocyte macrophage) bone marrow progenitors. Altogether, our studies establish PKCeta as a unique element in IFN signaling that plays a key and essential role in the generation of the regulatory effects of type I IFNs on normal and leukemic hematopoiesis.

Economic burden of follicular non-Hodgkin's lymphoma

Follicular non-Hodgkin's lymphoma (FNHL), a slow-growing cancer of the immune system, constitutes about 15-30% of all incident non-Hodgkin's lymphoma in developed countries. Its incidence is rising worldwide. Patients can live many years, but FNHL is considered incurable. We systematically reviewed the English-language MEDLINE-indexed and non-indexed economic literature published in the past 10 years on FNHL, identifying 23 primary economic studies. The economic burden of FNHL is significant, but available data are generally limited to retrospective considerations of hospital-based direct treatment costs, with little information available regarding societal cost of illness. Most direct cost information originates from the US, with one estimate of $US36 000 for the per-patient incremental cost of FNHL care during the first year following diagnosis. The most studied treatment is rituximab, which may offer similar overall costs to fludarabine considering higher resource use with fludarabine complications. Nearly all cost-effectiveness models identified by this review evaluated rituximab for relapsed/refractory FNHL responding to chemotherapy induction. Rituximab is supported as a cost-effective addition to standard chemotherapy by two models in the UK and one in the US, as maintenance therapy instead of stem-cell transplant by one UK model, and as maintenance therapy instead of observation alone by one model each in France, Spain and Canada. The UK National Institute for Health and Clinical Excellence updated guidance on rituximab in February 2008, concluding that it is cost effective when added to induction chemotherapy, and when used as maintenance therapy. No studies of per-patient or national indirect costs of illness were identified, with the only study of indirect costs a Canadian survey documenting lost work productivity. Across all study types identified by our review, the most common focus was on the direct costs of rituximab. As new treatments for FNHL come to market, more real-life cost data are imperative to calculate their relative cost effectiveness.

Hypersensitivity of Ph-positive lymphoid cell lines to rapamycin: Possible clinical application of mTOR inhibitor

The BCR/ABL tyrosine kinase inhibitor, imatinib mesylate, has shown substantial effects in chronic myelogenous leukemia (CML) and Ph-positive acute lymphoblastic leukemia (Ph(+)ALL). However, most patients relapse after an initial clinical response, indicating that drug resistance is a major problem in patients on imatinib. It is a serious problem that effective treatment choices to T315I, in the ABL kinase domain that shows a strong tolerance in imatinib do not exist clinically. In this study, we propose a new therapeutic approach to Ph(+)ALL with the T315I. Here, we report that the serine/threonine kinase mTOR (the mammalian target of rapamycin) inhibitor, rapamycin, inhibits the growth of not only the Bcr-Abl-positive lymphoid leukemic cell line, SU-Ph2, established from Ph(+)ALL patients, but also the imatinib-resistant cell line, SU/SR, that has acquired T315I. Rapamycin significantly inhibits cell growth in both these cell lines, and easily induces apoptosis at the same dose, thereby acting as an immunosuppressive agent. Our result suggested that the mTOR-signaling pathway has become an important therapeutic target for Ph-positive leukemias in the future, and at the same time, it is also becoming a very effective tool for the treatment of Ph(+)ALL with T315I.

Suppression of interferon (IFN)-inducible genes and IFN-mediated functional responses in BCR-ABL-expressing cells

The interferons (IFNs) are cytokines that play key roles in host defense against viral infections and immune surveillance against cancer. We report that BCR-ABL transformation of hematopoietic cells results in suppression of IFN-dependent responses, including transcription of IFN-inducible genes and generation of IFN-mediated antiviral effects. BCR-ABL transformation suppresses expression of several IFN-regulated genes containing IFN-sensitive response element (ISRE) or GAS elements in their promoters, including Isg15, Irf1, Irf9, and Ifit2 (interferon-induced protein with tetratricopeptide repeats 2). Suppression of transcription of ISRE-containing genes is also seen in cells expressing various BCR-ABL kinase domain mutants, including T315I, H396P, Y253F, and E255K, but not kinase-defective BCR-ABL. Such effects are associated with impaired IFN-dependent phosphorylation of Stat1 on Tyr(701) and Stat3 on Tyr(705) and defective binding of Stat complexes to ISRE or GAS elements. Beyond suppression of Stat activities, BCR-ABL inhibits IFN-inducible phosphorylation/activation of the p38 MAPK, suggesting a dual mechanism by which this abnormal fusion protein blocks IFN transcriptional responses. The inhibitory activities of BCR-ABL ultimately result in impaired IFNalpha-mediated protection against encephalomyocarditis virus infection and reversal of IFN-dependent growth suppression. Altogether, our data provide evidence for a novel mechanism by which BCR-ABL impairs host defenses and promotes malignant transformation, involving dual suppression of IFN-activated signaling pathways.

Suppression of programmed cell death 4 (PDCD4) protein expression by BCR-ABL-regulated engagement of the mTOR/p70 S6 kinase pathway

There is accumulating evidence that mammalian target of rapamycin (mTOR)-activated pathways play important roles in cell growth and survival of BCR-ABL-transformed cells. We have previously shown that the mTOR/p70 S6 kinase (p70 S6K) pathway is constitutively activated in BCR-ABL transformed cells and that inhibition of BCR-ABL kinase activity by imatinib mesylate abrogates such activation. We now provide evidence for the existence of a novel regulatory mechanism by which BCR-ABL promotes cell proliferation, involving p70 S6K-mediated suppression of expression of programmed cell death 4 (PDCD4), a tumor suppressor protein that acts as an inhibitor of cap-dependent translation by blocking the translation initiation factor eIF4A. Our data also establish that second generation BCR-ABL kinase inhibitors block activation of p70 S6K and downstream engagement of the S6 ribosomal protein in BCR-ABL transformed cells. Moreover, PDCD4 protein expression is up-regulated by inhibition of the BCR-ABL kinase in K562 cells and BaF3/BCR-ABL transfectants, suggesting a mechanism for the generation of the proapoptotic effects of such inhibitors. Knockdown of PDCD4 expression results in reversal of the suppressive effects of nilotinib and imatinib mesylate on leukemic progenitor colony formation, suggesting an important role for this protein in the generation of antileukemic responses. Altogether, our studies identify a novel mechanism by which BCR-ABL may promote leukemic cell growth, involving sequential engagement of the mTOR/p70 S6K pathway and downstream suppression of PDCD4 expression.

Evaluation of antileukaemic effects of rapamycin in patients with imatinib-resistant chronic myeloid leukaemia

BACKGROUND

Recent data suggest that the mammalian target of rapamycin (mTOR) is involved in the regulation of growth of neoplastic cells in chronic myeloid leukaemia (CML).

PATIENTS AND METHODS

We treated six patients with imatinib-resistant CML in haematological relapse (leukocytes > 20,000 microL(-1)) with rapamycin at 2 mg per os daily for 14 consecutive days, with dose-adjustment allowed to reach a target rapamycin serum concentration of 10-20 pg mL(-1).

RESULTS

A major leukocyte response with decrease to less than 10,000 microL(-1) was obtained in two patients, and a minor transient response was seen in two other patients. In responding patients, we also observed a decrease in vascular endothelial growth factor (VEGF) mRNA levels in circulating leukaemic cells. Side effects during rapamycin treatment were mild in most patients. In one patient, pneumonia developed. Rapamycin was also found to counteract growth of CML cells in vitro as determined by (3)H-thymidine incorporation. Moreover, rapamycin inhibited the growth of Ba/F3 cells exhibiting various imatinib-resistant mutants of BCR/ABL, including the T315I variant that exhibits resistance against most currently available BCR/ABL kinase inhibitors.

CONCLUSIONS

Rapamycin shows antileukaemic effects in imatinib-resistant CML in vitro and in vivo. Larger trials with rapamycin or rapamycin-derivatives in combination with other targeted drugs are warranted to further determine clinical efficacy in CML.

AIDS-associated Kaposi's sarcoma: is there still a role for interferon alfa?

Interferon alfa (IFNalpha) was one of the first agents to be used therapeutically in AIDS-associated Kaposi's sarcoma (KS) more than 25 years ago, and induces tumor regression in a subset of patients. Although much has been learned about the clinical role of IFNalpha in KS treatment, little is currently known about the mechanism(s) by which IFNalpha causes KS regression. This is despite a growing understanding of both KS pathogenesis and relevant IFNalpha activities. To a large extent other agents have supplanted IFNalpha as treatments for KS, but there may still remain a therapeutic role for IFNalpha, possibly in combination with other agents targeting angiogenesis and/or HHV-8-encoded human gene homologs that encode proteins involved in cell cycle regulation and signaling.

BCR-ABL Activity Is Critical for the Immunogenicity of Chronic Myelogenous Leukemia Cells

Chronic myelogenous leukemia (CML) is a myeloproliferative disorder caused by excessive granulopoiesis due to the formation of the constitutively active tyrosine kinase BCR-ABL. An effective drug against CML is imatinib mesylate, a tyrosine kinase inhibitor acting on Abl kinases, c-KIT, and platelet-derived growth factor receptor. Recently, a study revealed that patients treated with imatinib showed impaired CTL responses compared with patients treated with IFN-alpha, which might be due to a treatment-induced reduction in immunogenicity of CML cells or immunosuppressive effects. In our study, we found that inhibition of BCR-ABL leads to a down-regulation of immunogenic antigens on the CML cells in response to imatinib treatment, which results in the inhibition of CML-directed immune responses. By treating CML cells with imatinib, we could show that the resulting inhibition of BCR-ABL leads to a decreased expression of tumor antigens, including survivin, adipophilin, hTERT, WT-1, Bcl-x(L), and Bcl-2 in correlation to a decreased development of CML-specific CTLs. In contrast, this reduction in immunogenicity was not observed when a CML cell line resistant to the inhibitory effects of imatinib was used, but could be confirmed by transfection with specific small interfering RNA against BCR-ABL or imatinib treatment of primary CML cells.

A novel mechanism for Bcr-Abl action: Bcr-Abl-mediated induction of the eIF4F translation initiation complex and mRNA translation

The oncogenic kinase Bcr-Abl is thought to cause chronic myelogenous leukemia (CML) by altering the transcription of specific genes with growth- and survival-promoting functions. Recently, Bcr-Abl has also been shown to activate an important regulator of protein synthesis, the mammalian target of rapamycin (mTOR), which suggests that dysregulated translation may also contribute to CML pathogenesis. In this study, we found that both Bcr-Abl and the rapamycin-sensitive mTORC1 complex contribute to the phosphorylation (inactivation) of 4E-BP1, an inhibitor of the eIF4E translation initiation factor. Experiments with rapamycin and the Bcr-Abl inhibitor, imatinib mesylate, in Bcr-Abl-expressing cell lines and primary CML cells indicated that Bcr-Abl and mTORC1 induced formation of the translation initiation complex, eIF4F. This was characterized by reduced 4E-BP1 binding and increased eIF4G binding to eIF4E, two events that lead to the assembly of eIF4F. One target transcript is cyclin D3, which is regulated in Bcr-Abl-expressing cells by both Bcr-Abl and mTORC1 in a translational manner. In addition, the combination of imatinib and rapamycin was found to act synergistically against committed CML progenitors from chronic and blast phase patients. These experiments establish a novel mechanism of action for Bcr-Abl, and they provide insights into the modes of action of imatinib mesylate and rapamycin in treatment of CML. They also suggest that aberrant cap-dependent mRNA translation may be a therapeutic target in Bcr-Abl-driven malignancies.

Activation of mammalian target of rapamycin and the p70 S6 kinase by arsenic trioxide in BCR-ABL-expressing cells

Arsenic trioxide (As(2)O(3)) exhibits important antitumor activities in vitro and in vivo, but the precise mechanisms by which it induces its effects are not known. We provide evidence that during treatment of BCR-ABL-expressing cells with As(2)O(3), there is activation of a cellular pathway involving the p70 S6 kinase (p70S6K). Our data show that p70S6K is rapidly phosphorylated on Thr(421) and Ser(424) and is activated in an As(2)O(3)-inducible manner. The mammalian target of rapamycin (mTOR) is also phosphorylated/activated in an As(2)O(3)-inducible manner, and its activity is required for downstream engagement of p70S6K. p70S6K subsequently phosphorylates the S6 ribosomal protein on Ser(235)/Ser(236) and Ser(240)/Ser(244) to promote initiation of mRNA translation. Treatment of chronic myelogenous leukemia-derived cell lines with As(2)O(3) also results in phosphorylation of the 4E-BP1 repressor of mRNA translation on Thr(37)/Thr(46) and Thr(70), sites required for its deactivation and its dissociation from the eukaryotic initiation factor 4E complex to allow cap-dependent mRNA translation. In studies to determine the functional relevance of this pathway, we found that inhibition of mTOR and downstream cascades enhances induction of apoptosis by As(2)O(3). Consistent with this, the mTOR inhibitor rapamycin strongly potentiated As(2)O(3)-mediated suppression of primitive leukemic progenitors from the bone marrow of chronic myelogenous leukemia patients. Altogether, our data show that the mTOR/p70S6K pathway is activated in a negative feedback regulatory manner in response to As(2)O(3) in BCR-ABL-transformed cells and plays a key regulatory role in the induction of anti-leukemic responses.

Regulatory Effects of Mammalian Target of Rapamycin-activated Pathways in Type I and II Interferon Signaling

The mechanisms regulating initiation of mRNA translation for the generation of protein products that mediate interferon (IFN) responses are largely unknown. We have previously shown that both Type I and II IFNs engage the mammalian target of rapamycin (mTOR), resulting in downstream phosphorylation and deactivation of the translational repressor 4E-BP1 (eIF4E-binding protein 1). In the current study, we provide direct evidence that such regulation of 4E-BP1 by IFNalpha or IFNgamma results in sequential dissociation of 4E-BP1 from eukaryotic initiation factor-4E and subsequent formation of a functional complex between eukaryotic initiation factor-4E and eukaryotic initiation factor-4G, to allow initiation of mRNA translation. We also demonstrate that the induction of key IFNalpha- or IFNgamma-inducible proteins (ISG15 (interferon-stimulated gene 15) and CXCL10) that mediate IFN responses are enhanced in 4E-BP1 (4E-BP1(-/-)) knockout MEFs, as compared with wild-type 4E-BP1(+/+) MEFs. On the other hand, IFN-dependent transcriptional regulation of the Isg15 and Cxcl10 genes is intact in the absence of 4E-BP1, as determined by real time reverse transcriptase-PCR assays and promoter assays for ISRE and GAS, establishing that 4E-BP1 plays a selective negative regulatory role in IFN-induced mRNA translation. Interestingly, the induction of expression of ISG15 and CXCL10 proteins by IFNs was also strongly enhanced in cells lacking expression of the tuberin (TSC2(-/-)) or hamartin (TSC1(-/-)) genes, consistent with the known negative regulatory effect of the TSC1-TSC2 complex on mTOR activation. In other work, we demonstrate that the induction of an IFN-dependent antiviral response is strongly enhanced in cells lacking expression of 4E-BP1 and TSC2, demonstrating that these elements of the IFN-activated mTOR pathway exhibit important regulatory effects in the generation of IFN responses. Taken altogether, our data suggest an important role for mTOR-dependent pathways in IFN signaling and identify 4E-BP1 and TSC1-TSC2 as key components in the generation of IFN-dependent biological responses.

Bcr-Abl signaling through the PI-3/S6 kinase pathway inhibits nuclear translocation of the transcription factor Bach2, which represses the antiapoptotic factor heme oxygenase-1

The malignant phenotype of chronic myeloid leukemia (CML) is due to the abnormal tyrosine kinase activity of the Bcr-Abl oncoprotein. We have previously reported that expression of the Bach2 transcription factor, which induces apoptosis in response to oxidative stress, is greatly reduced in CML cells. Because these cells are resistant to apoptosis, we tested whether Bach2 could also be regulated through posttranslational mechanisms that promote inhibition of the apoptotic response to mutagenic stimuli in CML. We found that Bach2 is phosphorylated on S521 via the phosphatidylinositol-3/S6 kinase pathway, and substitution of this site to alanine leads to nuclear accumulation of the protein, indicating that this phosphorylation is important for its subcellular localization. Ectopic expression of the S521 mutant imparts greater impairment to CML cell growth than the wild-type factor. Furthermore, we showed that Bach2 transcriptionally represses heme oxygenase-1, an antiapoptotic factor up-regulated in CML. Because CML cells are known to produce high levels of intracellular reactive oxygen species, overexpression of heme oxygenase-1 resulting from inhibition of Bach2 activity may contribute to their genomic instability and leukemic phenotype.

Investigating mammalian target of rapamycin inhibitors for their anticancer properties

The mammalian target of rapamycin (mTOR) is a key element of the PI3KAkt (protein kinase B) signalling pathway, responsible for the regulation of cell growth and proliferation. There are two main downstream messengers of the mTOR kinase, eukaryotic initiation factor 4E-binding protein-1 and the 40S ribosomal protein S6 kinase 1, that control translation and cell-cycle progression. Abnormal activation of the mTOR pathway occurs frequently in numerous human malignancies; therefore, mTOR represents an attractive target for anticancer drug development. Rapamycin and its analogues CCI-779, RAD-001 and AP-23573 are known specific inhibitors of the mTOR kinase. Several clinical Phase I/II trials showed their activity in solid tumours and haematological malignancies. Moreover, inhibitors of mTOR were found to synergise with some cytostatics or other biological agents, which seems to be a promising direction for future strategies of antitumour treatment.

The p38 mitogen-activated protein kinase pathway in interferon signal transduction

Interferons (IFNs) are cytokines that regulate a variety of biologic effects, including cellular antiviral responses, inhibition of proliferation, induction of differentiation, and immunoregulation, via different mechanisms. In order to mediate such pleiotropic effects, IFNs trigger numerous signaling events. One way for IFNs to regulate cellular functions is through activation of mitogen-activated protein (MAP) kinases. Three major cascades of MAP kinases are known. The c-Jun NH(2)-terminal kinase (JNK) cascade, the extracellular signal-regulated kinase (ERK) cascade, and the p38 MAP kinase cascade. ERK and p38 MAP kinases are activated in response to type I IFNs and participate in the regulation of cellular responses. In this review we discuss recent findings on the role of the p38 MAP kinase pathway and its function in mediating IFN-dependent biologic effects. We further dissect and discuss the roles of upstream and downstream components of the p38 MAP kinase in the control of cellular responses triggered by IFNs.