Effective and selective targeting of leukemia cells using a TORC1/2 kinase inhibitor

Akt-dependent glucose metabolism promotes Mcl-1 synthesis to maintain cell survival and resistance to Bcl-2 inhibition

Most cancer cells utilize aerobic glycolysis, and activation of the phosphoinositide 3-kinase/Akt/mTOR pathway can promote this metabolic program to render cells glucose dependent. Although manipulation of glucose metabolism may provide a means to specifically eliminate cancer cells, mechanistic links between cell metabolism and apoptosis remain poorly understood. Here, we examined the role and metabolic regulation of the antiapoptotic Bcl-2 family protein Mcl-1 in cell death upon inhibition of Akt-induced aerobic glycolysis. In the presence of adequate glucose, activated Akt prevented the loss of Mcl-1 expression and protected cells from growth factor deprivation-induced apoptosis. Mcl-1 associated with and inhibited the proapoptotic Bcl-2 family protein Bim, contributing to cell survival. However, suppression of glucose metabolism led to induction of Bim, decreased expression of Mcl-1, and apoptosis. The proapoptotic Bcl-2/Bcl-xL/Bcl-w inhibitor, ABT-737, shows clinical promise, but Mcl-1 upregulation can promote resistance. Importantly, inhibition of glucose metabolism or mTORC1 overcame Mcl-1-mediated resistance in diffuse large B cell leukemic cells. Together these data show that Mcl-1 protein synthesis is tightly controlled by metabolism and that manipulation of glucose metabolism may provide a mechanism to suppress Mcl-1 expression and sensitize cancer cells to apoptosis.

Frontier of epilepsy research - mTOR signaling pathway

Studies of epilepsy have mainly focused on the membrane proteins that control neuronal excitability. Recently, attention has been shifting to intracellular proteins and their interactions, signaling cascades and feedback regulation as they relate to epilepsy. The mTOR (mammalian target of rapamycin) signal transduction pathway, especially, has been suggested to play an important role in this regard. These pathways are involved in major physiological processes as well as in numerous pathological conditions. Here, involvement of the mTOR pathway in epilepsy will be reviewed by presenting; an overview of the pathway, a brief description of key signaling molecules, a summary of independent reports and possible implications of abnormalities of those molecules in epilepsy, a discussion of the lack of experimental data, and questions raised for the understanding its epileptogenic mechanism.

Utility of mTOR inhibition in hematologic malignancies

The mammalian target of rapamycin (mTOR) is an intracellular serine/threonine kinase that exists as a downstream component of numerous signaling pathways. The activation of mTOR results in the production of proteins involved in cell metabolism, growth, proliferation, and angiogenesis. Aberrant activation of mTOR signaling has been identified in a number of cancers, and targeted inhibition of mTOR has been successful in achieving tumor responses, prolonging progression-free survival, and increasing overall survival in various oncologic patient populations. In particular, persistent activation of mTOR signaling has been identified in cell lines and patient samples with leukemias, Hodgkin's lymphoma (HL), non-Hodgkin's lymphoma (NHL), multiple myeloma (MM), and Waldenström's macroglobulinemia (WM). In vitro and preclinical studies using agents that inhibit mTOR signaling have demonstrated cytostatic and cytotoxic effects in these hematologic malignancies, suggesting that mTOR is a rational target for therapy in these disease states. In addition, the combination of mTOR inhibitors with traditional therapies may help to overcome the development of resistance and may improve response rates over those seen with established regimens through synergistic or additive effects. Inhibitors of mTOR signaling currently are being investigated in clinical trials of hematologic malignancies as single agents and as components of combination regimens. Thus far, promising results have been seen with the application of mTOR inhibitors as single agents in patients with relapsed or refractory leukemia, HL, NHL, MM, and WM.

Targeting the Mammalian Target of Rapamycin (mTOR) in Cancer Therapy: Lessons from Past and Future Perspectives

Over the last decade, extensive studies have been made to understand the role played by the mammalian target of rapamycin (mTOR) in cancer. Knowledge in this field has been gained from discoveries in basic research as well as from observations made in patients treated with allosteric mTOR inhibitors such as rapamycin. Despite promising preclinical studies, targeting mTOR in cancer therapy has shown limited clinical benefits so far. However, recent findings have revealed the complexity of the functions of mTOR in cancer and have helped develop new strategies to improve the anticancer efficacy of mTOR inhibitors. In particular, a complex network between mTOR and other signaling pathways has been identified that influences the anticancer efficacy of mTOR inhibitors. In addition, an emerging role of mTOR in the tumor microenvironment has been suggested. In this review, we confront the major findings that have been made in the past, both in experimental settings as well as in clinical trials. We further review the strategies that have been designed to further improve the efficacy of therapies targeting mTOR.

The mammalian target of rapamycin pathway as a therapeutic target in multiple myeloma

The mammalian target of rapamycin (mTOR) is centrally located, linking proximal oncogenic cascades to critical downstream pathways that drive tumor growth. mTOR regulates such diverse functions as protein translation, proliferation, viability, autophagy, metabolism homeostasis, monitoring of energy reserves, and induction of angiogenesis. Given its fundamental role in tumorigenesis, it is not surprising that a huge effort is being made to develop mTOR inhibitors. The existence of feedback pathways that become activated subsequent to mTOR inhibition has complicated these efforts. However, the fact that mTOR exists in two separate complexes, TORC1 and TORC2, and rapalogs primarily inhibit only TORC1 and TORC2 is actually a key activator of AKT, has injected new energy into the quest to find inhibitors that can inhibit both complexes. In myeloma models, preclinical studies confirm the activity of rapalogs as well as newer TORC1/TORC2 inhibitors, and early phase clinical trials have begun. In addition, the recent finding of up-regulated myeloma cell expression of DEPTOR, an mTOR binding protein that restricts mTOR activity, suggests an additional future therapeutic target specific to the myeloma tumor model.

Targeting TOR dependence in cancer

A challenge in cancer therapy has been to identify targets whose function is essential for survival of malignant cells but not normal cells. This Perspective discusses recent evidence that novel inhibitors of the kinase TOR can provide an unprecedented balance of anti-cancer efficacy and tolerability.

The mTOR inhibitor, everolimus (RAD001), overcomes resistance to imatinib in quiescent Ph-positive acute lymphoblastic leukemia cells

In Ph-positive (Ph⁺) leukemia, the quiescent cell state is one of the reasons for resistance to the BCR-ABL-kinase inhibitor, imatinib. In order to examine the mechanisms of resistance due to quiescence and the effect of the mammalian target of rapamycin inhibitor, everolimus, for such a resistant population, we used Ph⁺ acute lymphoblastic leukemia patient cells serially xenotransplanted into NOD/SCID/IL2rγ^null (NOG) mice. Spleen cells from leukemic mice showed a higher percentage of slow-cycling G₀ cells in the CD34⁺CD38⁻ population compared with the CD34⁺CD38⁺ and CD34⁻ populations. After ex vivo imatinib treatment, more residual cells were observed in the CD34⁺CD38⁻ population than in the other populations. Although slow-cycling G₀ cells were insensitive to imatinib in spite of BCR-ABL and CrkL dephosphorylation, combination treatment with everolimus induced substantial cell death, including that of the CD34⁺CD38⁻ population, with p70-S6 K dephosphorylation and decrease of MCL-1 expression. The leukemic non-obese diabetic/severe combined immunodeficiency (NOD/SCID) mouse system with the in vivo combination treatment with imatinib and everolimus showed a decrease of tumor burden including CD34⁺ cells. These results imply that treatment with everolimus can overcome resistance to imatinib in Ph⁺ leukemia due to quiescence.

Chronic myeloid leukemia stem cells and developing therapies

Chronic myeloid leukemia therapy has remarkably improved with the use of frontline BCR-ABL kinase inhibitors such that newly diagnosed patients have minimal disease manifestations or progression. Effective control of disease may also set the stage for eventual 'cure' of this leukemia. However, the existence of Philadelphia chromosome-positive leukemic cells that are unaffected by BCR-ABL inhibition represents a major barrier that may delay or prevent curative therapy with the current approaches. The most commonly reported mechanism of resistance to tyrosine kinase inhibitor-based therapies involves BCR-ABL gene mutations and amplification, but these changes may not be solely responsible for disease relapse when inhibitor-based therapies are curtailed. Therefore new targets may need to be defined before significant advancement in curative therapies is possible. Emerging evidence suggests that persistence of chronic myeloid leukemia stem cells or acquisition of stem cell-like characteristics prevents complete elimination of chronic myeloid leukemia by tyrosine kinase inhibition alone. This review focuses on several recently emerging concepts regarding the existence and characteristics of chronic myeloid leukemia stem cells. Definitions based on human primary cells and animal model studies are highlighted as are the potential signaling pathways associated with disease repopulating cells. Finally, several recently defined therapeutic targets and active compounds that have emerged from stem cell studies are described. Our goal is to provide an unbiased report on the current state of discovery within the chronic myeloid leukemia stem cell field and to orient the reader to emerging therapeutic targets and strategies that may lead to elimination of this leukemia.

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.

A comprehensive map of the mTOR signaling network

The mammalian target of rapamycin (mTOR) is a central regulator of cell growth and proliferation. mTOR signaling is frequently dysregulated in oncogenic cells, and thus an attractive target for anticancer therapy. Using CellDesigner, a modeling support software for graphical notation, we present herein a comprehensive map of the mTOR signaling network, which includes 964 species connected by 777 reactions. The map complies with both the systems biology markup language (SBML) and graphical notation (SBGN) for computational analysis and graphical representation, respectively. As captured in the mTOR map, we review and discuss our current understanding of the mTOR signaling network and highlight the impact of mTOR feedback and crosstalk regulations on drug-based cancer therapy. This map is available on the Payao platform, a Web 2.0 based community-wide interactive process for creating more accurate and information-rich databases. Thus, this comprehensive map of the mTOR network will serve as a tool to facilitate systems-level study of up-to-date mTOR network components and signaling events toward the discovery of novel regulatory processes and therapeutic strategies for cancer.

Targeting the translational apparatus to improve leukemia therapy: roles of the PI3K/PTEN/Akt/mTOR pathway

It has become apparent that regulation of protein translation is an important determinant in controlling cell growth and leukemic transformation. The phosphoinositide 3-kinase (PI3K)/phosphatase and tensin homologue deleted on chromosome ten (PTEN)/Akt/mammalian target of rapamycin (mTOR) pathway is often implicated in sensitivity and resistance to therapy. Dysregulated signaling through the PI3K/PTEN/Akt/mTOR pathway is often the result of genetic alterations in critical components in this pathway as well as mutations at upstream growth factor receptors. Furthermore, this pathway is activated by autocrine transformation mechanisms. PTEN is a critical tumor suppressor gene and its dysregulation results in the activation of Akt. PTEN is often mutated, silenced and is often haploinsufficient. The mTOR complex1 (mTORC1) regulates the assembly of the eukaryotic initiation factor4F complex, which is critical for the translation of mRNAs that are important for cell growth, prevention of apoptosis and transformation. These mRNAs have long 5'-untranslated regions that are G+C rich, rendering them difficult to translate. Elevated mTORC1 activity promotes the translation of these mRNAs via the phosphorylation of 4E-BP1. mTORC1 is a target of rapamycin and novel active-site inhibitors that directly target the TOR kinase activity. Although rapamycin and novel rapalogs are usually cytostatic and not cytotoxic for leukemic cells, novel inhibitors that target the kinase activities of PI3K and mTOR may prove more effective for leukemia therapy.

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.

Targeting the dysregulated mammalian target of rapamycin pathway in organ transplantation: killing 2 birds with 1 stone

Dysregulation and hyperactivation of the mammalian target of rapamycin (mTOR) pathway define the molecular basis of the hamartoma syndromes, including Cowden syndrome, tuberous sclerosis complex (TSC)/lymphangioleiomyomatosis, and Peutz-Jeghers syndrome. Loss of the tumor suppressors phosphatase and tensin homolog (PTEN), TSC1, TSC2, and LKB1 results in uncontrolled growth of usually benign tumors in various organs that, however, frequently lead to organ failure. Therefore, organ transplantation is a common therapeutic option in distinct patients with hamartoma syndromes, especially those with TSC/lymphangioleiomyomatosis. mTOR inhibitors are currently used in allogeneic transplantation as immunosuppressants and for the treatment of a growing number of cancers with dysregulated mTOR/phosphoinositide 3-kinase pathway. This dual targeting provides the unique opportunity for mTOR inhibitors to affect hamartoma syndromes at the molecular level along with potent immunosuppression in transplanted individuals. Here, we review the molecular mechanisms of hamartoma syndromes and discuss the recent clinical progress in transplant patients with hamartomas. Combining the identification of novel molecular targets of the phosphoinositide 3-kinase/mTOR pathway with insights into the clinical effectiveness of current therapeutic strategies sets the stage for a broader translational potential essential for further progress both in the treatment of cancer and for transplantation.

Mechanisms of mTOR inhibitor resistance in cancer therapy

Mammalian target of rapamycin (mTOR) is a conserved serine/threonine kinase that regulates cell cycle progression, protein translation, metabolism, and cellular proliferation. The mTOR pathway promotes cell proliferation under energy or nutrient-rich conditions by increasing ribosomal biogenesis and protein synthesis. Since enhanced activity of the mTOR pathway is frequently observed in malignant cells, inhibition of this kinase has become an attractive strategy to treat cancer. Rapamycin and its analogs temsirolimus, everolimus, and ridaforolimus referred to as "rapalogs" have demonstrated promising efficacy against renal cell carcinoma and are under investigation for the treatment of other malignancies. However, the emergence of drug resistance may ultimately limit the utility of rapalog therapy. Here we summarize the known mechanisms of resistance to mTOR-inhibitor therapy and describe potential strategies to overcome these for the current agents that target this pathway.

Targeted inhibition of mTORC1 and mTORC2 by active-site mTOR inhibitors has cytotoxic effects in T-cell acute lymphoblastic leukemia

The mammalian Target Of Rapamycin (mTOR) serine/threonine kinase belongs to two multi-protein complexes, referred to as mTORC1 and mTORC2. mTOR-generated signals have critical roles in leukemic cell biology by controlling mRNA translation of genes that promote proliferation and survival. However, allosteric inhibition of mTORC1 by rapamycin has only modest effects in T-cell acute lymphoblastic leukemia (T-ALL). Recently, ATP-competitive inhibitors specific for the mTOR kinase active site have been developed. In this study, we have explored the therapeutic potential of active-site mTOR inhibitors against both T-ALL cell lines and primary samples from T-ALL patients displaying activation of mTORC1 and mTORC2. The inhibitors affected T-ALL cell viability by inducing cell-cycle arrest in G(0)/G(1) phase, apoptosis and autophagy. Western blot analysis demonstrated a Ser 473 Akt dephosphorylation (indicative of mTORC2 inhibition) and a dephosphorylation of mTORC1 downstream targets. Unlike rapamycin, we found a marked inhibition of mRNA translation in T-ALL cell lines treated with active-site mTOR inhibitors. The inhibitors strongly synergized with both vincristine and the Bcl-2 inhibitor, ABT-263. Remarkably, the drugs targeted a putative leukemia-initiating cell sub-population (CD34(+)/CD7(-)/CD4(-)) in patient samples. In conclusion, the inhibitors displayed remarkable anti-leukemic activity, which emphasizes their future development as clinical candidates for therapy in T-ALL.

Targeting the mTOR kinase domain: the second generation of mTOR inhibitors

The mTOR signaling pathway is dysregulated in ∼50% of all human malignancies and is a major cancer drug target. Although rapamycin analogs (rapalogs) have shown clinical efficacy in a subset of cancers, they do not fully exploit the antitumor potential of mTOR targeting. Because the mTOR kinase domain is important for rapamycin-sensitive and -insensitive functions, mTOR catalytic inhibitors have been developed recently as the second generation of anti-mTOR agents. Importantly, they have shown marked improvement of antitumor activity in vivo and in vitro. This review will detail the potential therapeutic value and issues of these novel antineoplastic agents, with emphasis placed on those that have already entered clinical trials.

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.

mTOR: from growth signal integration to cancer, diabetes and ageing

In all eukaryotes, the target of rapamycin (TOR) signalling pathway couples energy and nutrient abundance to the execution of cell growth and division, owing to the ability of TOR protein kinase to simultaneously sense energy, nutrients and stress and, in metazoans, growth factors. Mammalian TOR complex 1 (mTORC1) and mTORC2 exert their actions by regulating other important kinases, such as S6 kinase (S6K) and Akt. In the past few years, a significant advance in our understanding of the regulation and functions of mTOR has revealed the crucial involvement of this signalling pathway in the onset and progression of diabetes, cancer and ageing.

mTOR: from growth signal integration to cancer, diabetes and ageing

In all eukaryotes, the target of rapamycin (TOR) signalling pathway couples energy and nutrient abundance to the execution of cell growth and division, owing to the ability of TOR protein kinase to simultaneously sense energy, nutrients and stress and, in metazoans, growth factors. Mammalian TOR complex 1 (mTORC1) and mTORC2 exert their actions by regulating other important kinases, such as S6 kinase (S6K) and Akt. In the past few years, a significant advance in our understanding of the regulation and functions of mTOR has revealed the crucial involvement of this signalling pathway in the onset and progression of diabetes, cancer and ageing.

Constitutive reductions in mTOR alter cell size, immune cell development, and antibody production

Mammalian TOR (mTOR) regulates cell growth, proliferation, and migration. Because mTOR knock-outs are embryonic lethal, we generated a viable hypomorphic mouse by neo-insertion that partially disrupts mTOR transcription and creates a potential physiologic model of mTORC1/TORC2 inhibition. Homozygous knock-in mice exhibited reductions in body, organ, and cell size. Although reductions in most organ sizes were proportional to decreased body weight, spleens were disproportionately smaller. Decreases in the total number of T cells, particularly memory cells, and reduced responses to chemokines suggested alterations in T-cell homing/homeostasis. T-cell receptor-stimulated T cells proliferated less, produced lower cytokine levels, and expressed FoxP3. Decreased neutrophil numbers were also observed in the spleen, despite normal development and migration in the bone marrow. However, B-cell effects were most pronounced, with a partial block in B-cell development in the bone marrow, altered splenic populations, and decreases in proliferation, antibody production, and migration to chemokines. Moreover, increased AKT(Ser473) phosphorylation was observed in activated B cells, reminiscent of cancers treated with rapamycin, and was reduced by a DNA-pk inhibitor. Thus, mTOR is required for the maturation and differentiation of multiple immune cell lineages. These mice provide a novel platform for studying the consequences of constitutively reduced mTORC1/TORC2 activity.

The impact of PTEN regulation by CK2 on PI3K-dependent signaling and leukemia cell survival

Gene alterations affecting elements of PI3K signaling pathway do not appear to be sufficient to explain the extremely high frequency of PI3K signaling hyperactivation in leukemia. It has been known for long that PTEN phosphorylation at the C-terminal tail, in particular by CK2, contributes to the stabilization and simultaneous inhibition of this critical tumor suppressor. However, direct evidence of the involvement of this mechanism in cancer has been gathered only recently. It is now known that CK2-mediated posttranslational, non-deleting, inactivation of PTEN occurs in T-ALL, CLL and probably other leukemias and solid tumors. To explore this knowledge for therapeutic purposes remains one of the challenges ahead.

Dual inhibition of PI3K and mTORC1/2 signaling by NVP-BEZ235 as a new therapeutic strategy for acute myeloid leukemia

PURPOSE

The growth and survival of acute myeloid leukemia (AML) cells are enhanced by the deregulation of signaling pathways such as phosphoinositide 3-kinase (PI3K)/Akt and mammalian target of rapamycin (mTOR). Major efforts have thus been made to develop molecules targeting these activated pathways. The mTOR serine/threonine kinase belongs to two separate complexes: mTORC1 and mTORC2. The mTORC1 pathway is rapamycin sensitive and controls protein translation through the phosphorylation of 4E-BP1 in most models. In AML, however, the translation process is deregulated and rapamycin resistant. Furthermore, the activity of PI3K/Akt and mTOR is closely related, as mTORC2 activates the oncogenic kinase Akt. We therefore tested, in this study, the antileukemic activity of the dual PI3K/mTOR ATP-competitive inhibitor NVP-BEZ235 compound (Novartis).

EXPERIMENTAL DESIGN

The activity of NVP-BEZ235 was tested in primary AML samples (n = 21) and human leukemic cell lines. The different signaling pathways were analyzed by Western blotting. The cap-dependent mRNA translation was studied by 7-methyl-GTP pull-down experiments, polysomal analysis, and [(3)H]leucine incorporation assays. The antileukemic activity of NVP-BEZ235 was tested by analyzing its effects on leukemic progenitor clonogenicity, blast cell proliferation, and survival.

RESULTS

The NVP-BEZ235 compound was found to inhibit PI3K and mTORC1 signaling and also mTORC2 activity. Furthermore, NVP-BEZ235 fully inhibits the rapamycin-resistant phosphorylation of 4E-BP1, resulting in a marked inhibition of protein translation in AML cells. Hence, NVP-BEZ235 reduces the proliferation rate and induces an important apoptotic response in AML cells without affecting normal CD34(+) survival.

CONCLUSIONS

Our results clearly show the antileukemic efficiency of the NVP-BEZ235 compound, which therefore represents a promising option for future AML therapies.

mTOR attenuates the inflammatory response in cardiomyocytes and prevents cardiac dysfunction in pathological hypertrophy

Previous studies have suggested that inhibition of the mammalian target of rapamycin (mTOR) by rapamycin suppresses myocardial hypertrophy. However, the role of mTOR in the progression of cardiac dysfunction in pathological hypertrophy has not been fully defined. Interestingly, recent reports indicate that the inflammatory response, which plays an important role in the development of heart failure, is enhanced by rapamycin under certain conditions. Our aim in this study was to determine the influence of mTOR on pathological hypertrophy and to assess whether cardiac mTOR regulates the inflammatory response. We generated transgenic mice with cardiac-specific overexpression of wild-type mTOR (mTOR-Tg). mTOR-Tg mice were protected against cardiac dysfunction following left ventricular pressure overload induced by transverse aortic constriction (TAC) (P < 0.01) and had significantly less interstitial fibrosis compared with littermate controls (WT) at 4 wk post-TAC (P < 0.01). In contrast, TAC caused cardiac dysfunction in WT. At 1 wk post-TAC, the proinflammatory cytokines interleukin (IL)-1β and IL-6 were significantly increased in WT mice but not in mTOR-Tg mice. To further characterize the effects of mTOR activation, we exposed HL-1 cardiomyocytes transfected with mTOR to lipopolysaccharide (LPS). mTOR overexpression suppressed LPS-induced secretion of IL-6 (P < 0.001), and the mTOR inhibitors rapamycin and PP242 abolished this inhibitory effect of mTOR. In addition, mTOR overexpression reduced NF-κB-regulated transcription in HL-1 cells. These data suggest that mTOR mitigates adverse outcomes of pressure overload and that this cardioprotective effect of mTOR is mediated by regulation of the inflammatory reaction.

Target of rapamycin signaling in leukemia and lymphoma

Growth factors and many oncogenes activate the lipid kinase phosphoinositide 3-kinase (PI3K), initiating a signaling cascade that includes the protein kinases AKT and target of rapamycin (TOR). The PI3K/AKT/TOR signaling pathway is a significant contributor to disease in various human cancers, including hematologic malignancies. Here we discuss different strategies to inhibit TOR for the treatment of leukemia, lymphoma, and myeloma. The TOR enzyme exists in two complexes in cells, TORC1 and TORC2. The majority of preclinical and clinical efforts to target TOR have involved using rapamycin and its analogs (rapalogs), which suppress TORC1 only partially and do not acutely inhibit TORC2. A new class of small molecules targeting the ATP-binding site of the TOR kinase, termed active-site TOR inhibitors (asTORi), achieves greater inhibition of both TOR complexes, resulting in broader suppression of the PI3K/AKT/TOR signaling network. Preclinical evidence suggests that asTORi have greater efficacy than rapalogs in Philadelphia chromosome-positive acute lymphoblastic leukemia and in T-cell lymphoma. These agents also show greater tolerability in animal models relative to rapalogs or inhibitors of PI3K. These findings encourage broader evaluation of asTORi efficacy in acute myeloid leukemia, B-cell lymphoma, myeloma, and other blood cancers.

Leukemias induced by altered TRK-signaling are sensitive to mTOR inhibitors in preclinical models

Rapamycin is a potent allosteric mTORC1 inhibitor with clinical applications as an anticancer agent. However, only a fraction of cancer patients responds to the drug, and no biomarkers are available to predict tumor sensitivity. Recently, we and others have obtained evidence for potential involvement of tropomyosin-related kinase (TRK) receptor protein tyrosine kinases (TRKA, TRKB, TRKC) in leukemia. In the present study, we tested the therapeutic effect of Rapamycin and its analog RAD001 on altered TRK-induced leukemia in a murine model. Daily treatment with Rapamycin (2 mg/kg) or RAD001 (1 mg/kg) significantly prolonged the survival of treated animals (n = 40) compared with the placebo group. Consistently, both mTOR and S6 proteins were strongly dephosphorylated in vitro and in vivo after treatment with Rapamycin or RAD001. However, Rapamycin did not completely inhibit mTORC1-dependent phosphorylation of 4E-BP1. With exception of one mouse showing slight reactivation of Akt after treatment, no reactivation of MAPK or Akt pathways was observed in other resistant tumors. Interestingly, leukemic cells isolated from a Rapamycin-resistant mouse were still highly sensitive to Rapamycin in vitro. Our findings suggest that altered TRK signaling may be a good predictor of tumor sensitivity to mTOR inhibition and that pathways other than MAPK and Akt exist that may trigger resistance of leukemic cells to Rapamycin in vivo.

Chemoprevention meets glucose control

The report by Memmott et al. (beginning on page 1066 in this issue of the journal) assessing the efficacy of the antidiabetes drug metformin in a mouse model of lung carcinogenesis suggests protective effects via two possible avenues: Decreased circulating insulin and insulin-like growth factor levels and energy stress leading to inhibition of mammalian target of rapamycin signaling. These potential mechanisms are discussed in this perspective, as are their implications for cancer prevention and therapy.

Targeting paediatric acute lymphoblastic leukaemia: novel therapies currently in development

Modifications to the treatment of acute lymphoblastic leukaemia (ALL) in children have led to a dramatic increase in survival in the past 40 years. Despite this success, a significant subset of paediatric leukaemia patients either relapse or fail to ever achieve a complete remission. Additionally, some patients necessitate treatment with intensified chemotherapy regimens due to clinical or laboratory findings which identify them as high risk. These patients are unlikely to respond to further minor adjustments to the dosing or timing of administration of the same chemotherapy medications. Many novel targeted therapies for the treatment of childhood ALL provide potential mechanisms to further improve cure rates, and provide the possibility of minimizing toxicity to non-malignant cells, given their specificity to malignant cell phenotypes. This article explores many of the potential targeted therapies in varying stages of development, from those currently in clinical trials to those still being refined in the research laboratory.

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.

Targeting TORC2 in multiple myeloma with a new mTOR kinase inhibitor

Although preclinical work with rapalogs suggests potential in treatment of multiple myeloma (MM), they have been less successful clinically. These drugs allostearically inhibit the mammalian target of rapamycin kinase primarily curtailing activity of the target of rapamycin complex (TORC)1. To assess if the mammalian target of rapamycin within the TORC2 complex could be a better target in MM, we tested a new agent, pp242, which prevents activation of TORC2 as well as TORC1. Although comparable to rapamycin against phosphorylation of the TORC1 substrates p70S6kinase and 4E-BP-1, pp242 could also inhibit phosphorylation of AKT on serine 473, a TORC2 substrate, while rapamycin was ineffective. pp242 was also more effective than rapamycin in achieving cytoreduction and apoptosis in MM cells. In addition, pp242 was an effective agent against primary MM cells in vitro and growth of 8226 cells in mice. Knockdown of the TORC2 complex protein, rictor, was deleterious to MM cells further supporting TORC2 as the critical target for pp242. TORC2 activation was frequently identified in primary specimens by immunostaining for AKT phosphorylation on serine 473. Potential mechanisms of up-regulated TORC2 activity in MM were stimulation with interleukin-6 or insulin-like growth factor 1, and phosphatase and tensin homolog or RAS alterations. Combining pp242 with bortezomib led to synergistic anti-MM effects. These results support TORC2 as a therapeutic target in MM.

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.

Effective targeting of STAT5-mediated survival in myeloproliferative neoplasms using ABT-737 combined with rapamycin

Signal transducer and activator of transcription-5 (STAT5) is a critical transcription factor for normal hematopoiesis and its sustained activation is associated with hematologic malignancy. A persistently active mutant of STAT5 (STAT5a^S711F) associates with Grb2 associated binding protein 2 (Gab2) in myeloid leukemias and promotes growth in vitro through AKT activation. Here we have retrovirally transduced wild-type or Gab2^−/− mouse bone marrow cells expressing STAT5a^S711F and transplanted into irradiated recipient mice to test an in vivo myeloproliferative disease (MPD) model. To target Gab2-independent AKT/mTOR activation, wild-type mice were treated separately with rapamycin. In either case, mice lacking Gab2 or treated with rapamycin displayed attenuated myeloid hyperplasia and modestly improved survival, but the effects were not cytotoxic and were reversible. To improve upon this approach, in vitro targeting of STAT5-mediated AKT/mTOR using rapamycin was combined with inhibition of the STAT5 direct target genes bcl-2 and bcl-X_L using ABT-737. Striking synergy with both drugs was observed in mouse BaF3 cells expressing STAT5a^S711F, TEL-JAK2, or BCR-ABL and in the relatively single agent-resistant human BCR-ABL positive K562 cell line. Therefore, targeting distinct STAT5 mediated survival signals, e.g. bcl-2/bcl-X_L and AKT/mTOR may be an effective therapeutic approach for human myeloproliferative neoplasms.

Allosteric and ATP-competitive kinase inhibitors of mTOR for cancer treatment

Over the past few years a number of components of the PI3K/mTOR pathway have been the subject of intense drug discovery activities both in pharmaceutical companies and in academia. This review article summarizes progress made in the identification and development of allosteric and ATP-competitive kinase inhibitors of mTOR and their potential therapeutic use in oncology.

Targeting mTOR: prospects for mTOR complex 2 inhibitors in cancer therapy

Small molecule inhibitors that selectively target cancer cells and not normal cells would be valuable anti-cancer therapeutics. The mammalian target of rapamycin complex 2 (mTORC2) is emerging as a promising candidate target for such an inhibitor. Recent studies in cancer biology indicate that mTORC2 activity is essential for the transformation and vitality of a number of cancer cell types, but in many normal cells, mTORC2 activity is less essential. These studies are intensifying interest in developing inhibitors that specifically target mTORC2. However, there are many open questions regarding the function and regulation of mTORC2 and its function in both normal and cancer cells. Here, we summarize exciting new research into the biology of mTORC2 signaling and highlight the current state and future prospects for mTOR-targeted therapy.