Survival of acute myeloid leukemia cells requires PI3 kinase activation

A phase Ib GOELAMS study of the mTOR inhibitor RAD001 in association with chemotherapy for AML patients in first relapse

The mTORC1 signaling pathway is constitutively activated in almost all acute myelogenous leukemia (AML) patients. We conducted a phase Ib trial combining RAD001 (everolimus), an allosteric inhibitor of mTORC1, and conventional chemotherapy, in AML patients under 65 years of age at first relapse (clinical trial NCT 01074086). Increasing doses of RAD001 from 10-70 mg were administrated orally on days 1 and 7 (d1 and d7) of a 3+7 daunorubicin+cytarabine conventional induction chemotherapy regimen. Twenty-eight patients were enrolled in this trial. The treatment was well tolerated with <10% toxicity, mainly involving the gastrointestinal tract and lungs. In this phase Ib trial, the RAD001 maximum tolerated dose was not reached at 70 mg. Sixty-eight percent of patients achieved CR, of which 14 received a double induction. Eight subsequently were intensified with allogeneic-stem cell transplant. Strong plasma inhibition of P-p70S6K was observed after RAD001 administration, still detectable at d7 (d7)at the 70 mg dosage. CR rates in patients with RAD001 areas under or above the curve median were 53% versus 85%. A 70 mg dose of RAD001 at d1 and d7 of an induction chemotherapy regimen for AML has acceptable toxicity and may improve treatment.

The hematopoietic stem cell niche--home for friend and foe?

The hematopoietic stem cell (HSC) niche is involved in the maintainance and regulation of quiescence, self-renewal and differentiation of hematopoietic stem cells and the fate of their progeny in mammals dealing with the daily stresses to the hematopoietic system. From the discovery that perturbations of the HSC niche can lead to hematopoietic disorders, we have now arrived at the prospect that the HSC niche may play a role in hematological malignancies and that this HSC niche may be a target for therapy. This review attempts to capture the discoveries of the last few years regarding the normal and malignant hematopoietic stem cell niche and possible ways to target this niche.

The evolution of the TOR pathway and its role in cancer

The target of rapamycin (TOR) pathway is highly conserved among eukaryotes and has evolved to couple nutrient sensing to cellular growth. TOR is found in two distinct signaling complexes in cells, TOR complex 1 (TORC1) and TOR complex 2 (TORC2). These complexes are differentially regulated and act as effectors for the generation of signals that drive diverse cellular processes such as growth, proliferation, protein synthesis, rearrangement of the cytoskeleton, autophagy, metabolism and survival. Mammalian TOR (mTOR) is very important for development in embryos, while in adult organisms it is linked to aging and lifespan effects. In humans, the mTOR pathway is implicated in the tumorigenesis of multiple cancer types and its deregulation is associated with familial cancer syndromes. Because of its high biological relevance, different therapeutic strategies have been developed to target this signaling cascade, resulting in the emergence of unique pharmacological inhibitors that are either already approved for use in clinical oncology or currently under preclinical or clinical development. Multimodal treatment strategies that simultaneously target multiple nodes of the pathway and/or negative feedback regulatory loops may ultimately provide the best therapeutic advantage in targeting this pathway for the treatment of malignancies.

The Development of Novel Therapies for the Treatment of Acute Myeloid Leukemia (AML)

Acute myeloid leukemia (AML) is nearly always a fatal malignancy. For the past 40 years, the standard of care remains a combination of cytarabine and an anthracycline known as 7 + 3. This treatment regimen is troubled by both low survival rates (10% at 5 years) and deaths due to toxicity. Substantial new laboratory findings over the past decade have identified many cellular pathways that contribute to leukemogenesis. These studies have led to the development of novel agents designed to target these pathways. Here we discuss the molecular underpinnings and clinical benefits of these novel treatment strategies. Most importantly these studies demonstrate that clinical response is best achieved by stratifying each patient based on a detailed understanding of their molecular abnormalities.

Casitas B-lineage lymphoma mutants activate AKT to induce transformation in cooperation with class III receptor tyrosine kinases

In addition to overexpression and the occurrence of activating mutations, receptors can be aberrantly activated by impaired downregulation. In this study, we show that an oncogenic mutant of the ubiquitin ligase casitas B-lineage lymphoma (CBL; CBLΔexon8), which is found in acute myeloid leukemia patients, predominantly cooperates with receptor tyrosine kinase (RTK) class III receptors (PDGFRA, PDGFRB, KIT, and FLT3), but not with non-class III RTKs or cytokine receptors, to induce IL-3-independent growth of Ba/F3 cells. In cells coexpressing RTK class III/CBLΔexon8, receptor internalization was delayed, and cells were protected from apoptosis after cytokine withdrawal. Ligand-stimulated Ba/F3 cells and acute myeloid leukemia cell lines coexpressing the CBL deletion mutant and FLT3 showed enhanced AKT phosphorylation. Combined pharmacologic inhibition of the PI3K/AKT pathway and FLT3 had an additive effect on cell proliferation. The transforming potential of the CBL mutant was completely abolished by the mutation of the CBL PTB domain and was decreased by the mutation of tyrosines 589 and 591 in the juxtamembrane domain of FLT3. A constitutively active AKT1 mutant (E17K) recapitulated the phenotype induced by the CBL deletion mutant in Ba/F3 cells. This study reveals FLT3-CBL interaction sites and the AKT pathway as critical mediators of transformation by oncogenic CBL mutants.

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.

Targeting the PI3K/AKT/mTOR signaling axis in children with hematologic malignancies

The phosphatidylinositiol 3-kinase (PI3K), AKT, mammalian target of rapamycin (mTOR) signaling pathway (PI3K/AKT/mTOR) is frequently dysregulated in disorders of cell growth and survival, including a number of pediatric hematologic malignancies. The pathway can be abnormally activated in childhood acute lymphoblastic leukemia (ALL), acute myelogenous leukemia (AML), and chronic myelogenous leukemia (CML), as well as in some pediatric lymphomas and lymphoproliferative disorders. Most commonly, this abnormal activation occurs as a consequence of constitutive activation of AKT, providing a compelling rationale to target this pathway in many of these conditions. A variety of agents, beginning with the rapamycin analogue (rapalog) sirolimus, have been used successfully to target this pathway in a number of pediatric hematologic malignancies. Rapalogs demonstrate significant preclinical activity against ALL, which has led to a number of clinical trials. Moreover, rapalogs can synergize with a number of conventional cytotoxic agents and overcome pathways of chemotherapeutic resistance for drugs commonly used in ALL treatment, including methotrexate and corticosteroids. Based on preclinical data, rapalogs are also being studied in AML, CML, and non-Hodgkin's lymphoma. Recently, significant progress has been made using rapalogs to treat pre-malignant lymphoproliferative disorders, including the autoimmune lymphoproliferative syndrome (ALPS); complete remissions in children with otherwise therapy-resistant disease have been seen. Rapalogs only block one component of the pathway (mTORC1), and newer agents are under preclinical and clinical development that can target different and often multiple protein kinases in the PI3K/AKT/mTOR pathway. Most of these agents have been tolerated in early-phase clinical trials. A number of PI3K inhibitors are under investigation. Of note, most of these also target other protein kinases. Newer agents are under development that target both mTORC1 and mTORC2, mTORC1 and PI3K, and the triad of PI3K, mTORC1, and mTORC2. Preclinical data suggest these dual- and multi-kinase inhibitors are more potent than rapalogs against many of the aforementioned hematologic malignancies. Two classes of AKT inhibitors are under development, the alkyl-lysophospholipids (APLs) and small molecule AKT inhibitors. Both classes have agents currently in clinical trials. A number of drugs are in development that target other components of the pathway, including eukaryotic translation initiation factor (eIF) 4E (eIF4E) and phosphoinositide-dependent protein kinase 1 (PDK1). Finally, a number of other key signaling pathways interact with PI3K/AKT/mTOR, including Notch, MNK, Syk, MAPK, and aurora kinase. These alternative pathways are being targeted alone and in combination with PI3K/AKT/mTOR inhibitors with promising preclinical results in pediatric hematologic malignancies. This review provides a comprehensive overview of the abnormalities in the PI3K/AKT/mTOR signaling pathway in pediatric hematologic malignancies, the agents that are used to target this pathway, and the results of preclinical and clinical trials, using those agents in childhood hematologic cancers.

New strategies in acute myeloid leukemia: redefining prognostic markers to guide therapy

Although standard therapy for AML has been relatively constant over the past 2 decades, this may be changing with enhanced technologies allowing for the classification of acute myeloid leukemia (AML) into molecularly distinct subsets. Some specific subsets of AML have an excellent prognosis in response to standard therapy, whereas the poor prognosis of AML associated with specific sets of mutations or chromosomal anomalies requires the development of new therapies. Elucidation of the molecular pathogenesis of AML has led to the development of therapies that affect signaling, apoptosis, protein and intermediate metabolism, the surface of the leukemia cell, leukemia cell/stromal interaction, and epigenetic regulation of gene expression.

Prognostic impact of δ-like ligand 4 and Notch1 in acute myeloid leukemia

Notch signaling plays a critical role in embryonic vascular development and tumor angiogenesis. The present study was conducted to investigate the prognostic role of the angiogenesis-related Notch ligand and the receptor in acute myeloid leukemia (AML) and assess whether their expression correlates with that of the vascular endothelial growth factor (VEGF) and angiopoietin (Ang)-2. Bone marrow mononuclear cells from 60 untreated AML patients and 40 healthy controls were obtained. Real-time RT-PCR was performed to evaluate the mRNA expression of δ-like ligand 4 (Dll4), Notch1, VEGF, VEGF receptor (VEGFR)-1, VEGFR-2, Ang-1, Ang-2 and Tie2. Western blot analysis was used to determine the protein levels of Dll4 and Notch1. The results demonstrated that Dll4, Notch1, VEGF, VEGFR-2 and Ang-2 expression were significantly higher in untreated AML patients than in the controls. Univariate analysis of factors associated with the overall survival showed a significantly shorter survival in patients with the unfavorable karyotype, higher Dll4 expression, higher Notch1 expression, higher VEGF expression or higher Ang-2 expression. Furthermore, multivariate analysis revealed that the karyotype and expression levels of Notch1, Dll4, VEGF and Ang-2 were independent prognostic factors for overall survival. Additionally, the prognostic value of Dll4 expression (but not Notch1) was more significant in the subgroup consisting of patients with intermediate-risk cytogenetics. Subgroup analysis showed that Notch1 and Dll4 expression levels had a prognostic impact on patients with high VEGF or Ang-2 levels. Taken together, our data provide evidence that the activation of the Notch pathway may indicate an unfavorable prognosis in AML. In particular, Dll4 may be a relevant prognostic marker in intermediate-risk AML.

Selective small molecule inhibitors of p110α and δ isoforms of phosphoinosityl-3-kinase are cytotoxic to human acute myeloid leukemia progenitors

The phosphoinosityl-3-kinase (PI3K) pathway is frequently constitutively active in blast cells from acute myeloid leukemia (AML) patients. RNA and protein from all four catalytic isoforms of PI3K (p110α, β, γ, and δ) were expressed in 38 AML samples, which also showed expression of phosphorylated Akt Ser473, indicating PI3K activation. Initial treatment of 12 AML samples with inhibitors targeting each of the four isoforms demonstrated that p110α and δ inhibition are more effective in killing AML blast colony-forming cells (CFC) than p110β or γ inhibition. In subsequent experiments, AML CFC from 46 patient samples were treated with the p110α and δ selective inhibitors, PI3Kα inhibitor 2 or PCN5603, and dose-dependent progenitor kill and inhibition of phosphorylated Akt Ser473 expression was observed. AML samples were more sensitive to PI3Kα inhibitor 2 and PCN5603 killing than normal bone marrow or normal peripheral blood CFC (median IC(50) for AML and normal CFCs treated with PI3Kα inhibitor 2, 1.8 and 4.3 μM, respectively, and for PCN5603, 1.9 and 6.2 μM, respectively). Furthermore, treatment of AML cells with PCN5603 also decreased survival of more primitive leukemia progenitors identified in long-term culture (AML long-term culture initiating cells), while less toxicity toward normal bone marrow long-term culture initiating cells was observed. Selective inhibition of the p110α and δ isoforms of PI3K kills AML progenitors while causing relative sparing of analogous normal cells.

mTOR kinase inhibitors as a treatment strategy in hematological malignancies

The mammalian target of rapamycin (mTOR) kinase is a key element of intracellular signal transduction, responsible for the regulation of cell growth and proliferation. Since abnormal activation of the mTOR pathway was found in several tumors, including human malignancies, it may be an attractive target for antineoplastic treatment. The first identified mTOR inhibitor was rapamycin (sirolimus). Subsequently, the most potent rapamycin analogues (rapalogues), such as everolimus, temsirolimus and deforolimus, have been developed. After encouraging preclinical experiments, several clinical trials testing the rapalogues in monotherapy or in combinations with other cytotoxic agents have been conducted in patients with hematological malignancies. Results of these studies, described in this review, indicate that inhibition of the mTOR pathway may be a very promising strategy of anti-tumor treatment in several types of lymphomas and leukemias. Recently, a second generation of more effective mTOR inhibitors has been developed. These are currently being assessed in preclinical, Phase I or I/II clinical studies.

Defining the role of sirolimus in the management of graft-versus-host disease: from prophylaxis to treatment

Graft-versus-host disease (GVHD) remains a major cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation (HSCT). Measures developed that have significantly reduced GVHD were also frequently associated with an increased risk of relapse. GVHD and graft-versus-tumor (GVT) effects are tightly linked, and balance between both reactions is difficult to achieve. To have an impact on the outcome and quality of life after HSCT, improvements in current strategies to prevent and treat GVHD while preserving the GVT effect are clearly needed. Sirolimus (rapamycin) is a lipophilic macrocytic lactone with immunosuppressive, antitumor, and antiviral properties. Because of its multiple modes of activities, it is being increasingly used in the management of GVHD. This review aims to summarize its mechanisms of action and potential advantages over other immunosuppressors and to analyze the most relevant studies investigating its role in both prevention and treatment of GVHD.

Activation of the AKT/cyclin D1/Cdk4 survival signaling pathway in radioresistant cancer stem cells

Radioresistance, which is a major cause of failure of radiotherapy (RT), is proposed as one of the intrinsic characteristics of cancer stem cells (CSCs) whose unique DNA damage response (DDR), efficient DNA repair and resistance to apoptosis are thought to confer the phenotype. We have isolated surviving CSCs by exposure to long-term fractionated radiation for 82 days from HepG2 and A172 cells (82FR-31NR cells). 82FR-31NR cells exhibited CSC properties, such as high expression of CSC marker CD133 and the ABC transporters (MDR1 and BCRP1), and high tumorigenic potential after transplantation into nude mice. The advantage of our isolated CSCs is that they can proliferate in as the same growth medium as that of parental cells without loss of CSC properties. Therefore, we can analyze DDR of non-stem cells and CSCs without any influences caused by different culture conditions. 82FR-31NR cells showed efficient DNA repair of radiation-induced DNA damage and radioresistance with activation of the AKT/cyclin D1 survival signaling pathway. In contrast, DNA damage persisted for a long time after irradiation in parental cells compared with isolated CSCs. Persisted DNA damage induced apoptosis in parental cells without activation of the AKT/cyclin D1 pathway. Therefore, inhibition of the AKT/cyclin D1 pathway by an AKT inhibitor, API-2, or cyclin D1 siRNA resulted in a loss of efficient DNA repair and radiosensitization of 82FR-31NR cells. Furthermore, knockdown of Cdk4 by its siRNA or a Cdk4 inhibitor was sufficient to suppress radioresistance of CSCs. In this study, we present a newly discovered DDR regarding the AKT/cyclin D1/Cdk4 pathway in response to radiation in CSCs. Combination of fractionated RT and reagents targeting the AKT/cyclin D1/Cdk4 pathway to eradicate CSCs would be effective therapeutic modality.

Analyzing gene expression profile in K562 cells exposed to sodium valproate using microarray combined with the connectivity map database

To explore the mechanism underlying antileukaemia effect of sodium valproate, the growth and survival of the K562 cell line were investigated. Global profiles of gene expression in K562 cells exposed to sodium valproate were assessed and validated. The differentially expressed genes identified were further used to query the connectivity map database to retrieve a ranked list of compounds that act on the same intracellular targets as sodium valproate. A significant increase in cell apoptosis and a change in gene expression profile were observed in valproate-exposed K562 cells. The significant enrichment analysis of gene ontology terms for the differentially expressed genes showed that these genes were involved in many important biological processes. Eight differentially expressed genes involved in apoptosis were verified by quantitative real-time PCR. The connectivity map analysis showed gene expression profile in K562 cells exposed to sodium valproate was most similar to that of HDACi and PI3K inhibitors, suggesting that sodium valproate might exert antileukaemic action by inhibiting HDAC as well as inhibiting PI3K pathway. In conclusion, our data might provide clues to elucidate the molecular and therapeutic potential of VPA in leukaemia treatment, and the connectivity map is a useful tool for exploring the molecular mechanism of drug action.

An overview on the role of FLT3-tyrosine kinase receptor in acute myeloid leukemia: biology and treatment

Hematopoiesis, the process by which the hematopoietic stem cells and progenitors differentiate into blood cells of various lineages, involves complex interactions of transcription factors that modulate the expression of downstream genes and mediate proliferation and differentiation signals. Despite the many controls that regulate hematopoiesis, mutations in the regulatory genes capable of promoting leukemogenesis may occur. The FLT3 gene encodes a tyrosine kinase receptor that plays a key role in controlling survival, proliferation and differentiation of hematopoietic cells. Mutations in this gene are critical in causing a deregulation of the delicate balance between cell proliferation and differentiation. In this review, we provide an update on the structure, synthesis and activation of the FLT3 receptor and the subsequent activation of multiple downstream signaling pathways. We also review activating FLT3 mutations that are frequently identified in acute myeloid leukemia, cause activation of more complex downstream signaling pathways and promote leukemogenesis. Finally, FLT3 has emerged as an important target for molecular therapy. We, therefore, report on some recent therapies directed against it.

An overview of the mTOR pathway as a target in cancer therapy

INTRODUCTION

The mammalian target of rapamycin (mTOR) signaling cascade is a key regulatory pathway controlling initiation of mRNA translation in mammalian cells. The mTOR inhibitor rapamycin and its derivatives have shown potent antineoplastic activities in many preclinical models and clinical trials. First-generation mTOR inhibitors are now FDA-approved for the treatment of renal cell carcinoma.

AREAS COVERED

This article reviews the components of the mTOR pathway and their normal functions, highlighting the most common alterations in the pathway, seen in various human malignancies. It also discusses elements and effectors of this signaling cascade and reviews the therapeutic relevance of pharmacological inhibitors of the pathway in several malignancies, including lymphomas, leukemias, sarcomas, renal cell carcinoma, and breast cancer.

EXPERT OPINION

mTOR targeting is a highly promising therapeutic approach. First-generation mTOR inhibitors have already shown substantial activity in the treatment of certain tumors, while the emergence of second-generation catalytic mTOR inhibitors provides a better approach to target the pathway in malignant cells and has raised the potential for better clinical outcomes in the future.

Ursolic acid-induced apoptosis in K562 cells involving upregulation of PTEN gene expression and inactivation of the PI3K/Akt pathway

Ursolic acid (UA), a pentacyclic triterpenoid derived from a variety of medicinal plants, exhibits potent anticancer activity against many types of cancer cells. However, the anticancer mechanism of UA is not clearly understood. Suppression of phosphatase and a tensin homolog deleted on chromosome 10 (PTEN) gene expression leading to activation of the phosphatidylinositol-3-OH kinase (PI3K)/Akt pathway has been observed in many cancers including leukemia, making the PTEN gene and PI3K/Akt pathway a central target for cancer therapy. Here, we demonstrated that UA was able to inhibit growth, induce apoptosis in a human chronic myelogenous leukemia cell line (K562 cells) via upregulation of PTEN gene expression, inhibit Akt kinase activity, change mitochondrial transmembrane potential and reduce the release of cytochrome c and the activity of caspases. These results suggest that UA may elicit its strong antitumor effects via upregulation of the PTEN gene and inhibition of the PI3K/Akt pathway.

Testosterone exerts antiapoptotic effects against H2O2 in C2C12 skeletal muscle cells through the apoptotic intrinsic pathway

Experimental data indicate that apoptosis is activated in the aged skeletal muscle, contributing to sarcopenia. We have previously demonstrated that testosterone protects against hydrogen peroxide (H(2)O(2))-induced apoptosis in C2C12 muscle cells. Here we identified molecular events involved in the antiapoptotic effect of testosterone. At short times of exposure to H(2)O(2) cells exhibit a defense response but at longer treatment times cells undergo apoptosis. Incubation with testosterone prior to H(2)O(2) induces BAD inactivation, inhibition of poly(ADP-ribose) polymerase cleavage, and a decrease in BAX levels, and impedes the loss of mitochondrial membrane potential, suggesting that the hormone participates in the regulation of the apoptotic intrinsic pathway. Simultaneous treatment with testosterone, H(2)O(2), and the androgen receptor (AR) antagonist, flutamide, reduces the effects of the hormone, pointing to a possible participation of the AR in the antiapoptotic effect. The data presented allow us to begin to elucidate the mechanism by which the hormone prevents apoptosis in skeletal muscle.

Evi1 forms a bridge between the epigenetic machinery and signaling pathways

Recent studies have demonstrated the significance of the leukemia oncogene Evi1 as the regulator of hematopoietic stem cells and marker of poor clinical outcomes in myeloid malignancies. Evi1-mediated leukemogenic activities include a wide array of functions such as the induction of epigenetic modifications, transcriptional control, and regulation of signaling pathways. We have recently succeeded in comprehensively elucidating the oncogenic function of Evi1 in a model of the polycomb-Evi1-PTEN/AKT/mTOR axis. These results may provide us with novel therapeutic approaches to conquer the poor prognosis associated with Evi1-activated leukemia or other solid tumors with high Evi1 expression. Here, we review the current understanding of the role of Evi1 in controlling the development of leukemia and highlight potential modalities for targeting factors involved in Evi1-regulated signaling.

AKT1 induces caspase-mediated cleavage of the CDK inhibitor p27Kip1 during cell cycle progression in leukemia cells transformed by FLT3-ITD

p27Kip1 cleavage and caspase-3 regulate cell cycle in human myeloma cells and B cells, however regulation of p27Kip1 cleavage during the cell cycle is not known. In BaF3-FLT3-ITD cells, p27Kip1 undergoes C-terminal cleavage. Inhibition of the PI3K/AKT pathway is associated with decreased cleavage of p27Kip1 and G1 phase arrest. A caspase-3 inhibitor reduces p27Kip1 cleavage and inhibits cell proliferation. Knockdown shRNA against AKT1 reduces cleavage of p27Kip1, inhibits caspase-3 activation, and is associated with a delay in cell cycle progression. Taken together, these findings indicate that AKT1 induces caspase-mediated cleavage of p27Kip1, required for G1-S progression in FLT3-ITD cells.

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.

Class I phosphoinositide 3-kinases in normal and pathologic hematopoietic cells

Class I phosphoinositide 3-kinases which produce the D3-phosphoinositide second messenger phosphatidylinositol 3,4,5-trisphosphate in response to membrane receptors activation play a critical role in cell proliferation, survival, metabolism, and motility. These lipid kinases and the phosphatases regulating the level of D3-phosphoinositides have been an intense area of research these last two decades. The class I phosphoinositide 3-kinases signaling is found aberrantly activated in numerous human cancers, including in malignant hemopathies, and are important therapeutic targets for cancer therapy. Haematopoiesis is an ongoing process which generates the distinct blood cell types from a common hematopoietic stem cell through the action of a variety of cytokines. In the human adult hematopoiesis occurs primarily in the bone marrow, and defects in hematopoiesis result in diseases, such as anemia, thrombocytopenia, myeloproliferative syndromes, or leukemia. Here we give a brief overview of the role of class I phosphoinositide 3-kinases in hematopoietic stem cells, in hematopoietic lineage development and in leukemia, particularly in acute myeloid leukemia and summarize the potential therapeutic implications.

The role of sirtuin 2 activation by nicotinamide phosphoribosyltransferase in the aberrant proliferation and survival of myeloid leukemia cells

BACKGROUND

Inhibitors of nicotinamide phosphoribosyltransferase have recently been validated as therapeutic targets in leukemia, but the mechanism of leukemogenic transformation downstream of this enzyme is unclear.

DESIGN AND METHODS

Here, we evaluated whether nicotinamide phosphoribosyltransferase's effects on aberrant proliferation and survival of myeloid leukemic cells are dependent on sirtuin and delineated the downstream signaling pathways operating during this process.

RESULTS

We identified significant upregulation of sirtuin 2 and nicotinamide phosphoribosyltransferase levels in primary acute myeloid leukemia blasts compared to in hematopoietic progenitor cells from healthy individuals. Importantly, specific inhibition of nicotinamide phosphoribosyltransferase or sirtuin 2 significantly reduced proliferation and induced apoptosis in human acute myeloid leukemia cell lines and primary blasts. Intriguingly, we found that protein kinase B/AKT could be deacetylated by nicotinamide phosphoribosyltransferase and sirtuin 2. The anti-leukemic effects of the inhibition of nicotinamide phosphoribosyltransferase or sirtuin 2 were accompanied by acetylation of protein kinase B/AKT with subsequent inhibition by dephosphorylation. This leads to activation of glycogen synthase kinase-3 β via diminished phosphorylation and, ultimately, inactivation of β-catenin by phosphorylation.

CONCLUSIONS

Our results provide strong evidence that nicotinamide phosphoribosyltransferase and sirtuin 2 participate in the aberrant proliferation and survival of leukemic cells, and suggest that the protein kinase B/AKT/ glycogen synthase kinase-3 β/β-catenin pathway is a target for inhibition of nicotinamide phosphoribosyltransferase or sirtuin 2 and, thereby, leukemia cell proliferation.

Single-cell pharmacodynamic monitoring of S6 ribosomal protein phosphorylation in AML blasts during a clinical trial combining the mTOR inhibitor sirolimus and intensive chemotherapy

PURPOSE

Integration of signal transduction inhibitors into chemotherapy regimens generally has generally not led to anticipated increases in response and survival. However, it remains unclear whether this is because of inadequate or inconsistent inhibition of target or other complex biology. The mTOR signaling pathway is frequently activated in acute myelogenous leukemia (AML) and we previously showed the safety of combining the mTOR inhibitor, sirolimus, with mitoxantrone, etoposide, and cytarabine (MEC) chemotherapy. However, we did not reliably determine the extent of mTOR inhibition on that study. Here, we sought to develop an assay that allowed us to serially quantify the activation state of mTOR kinase during therapy.

EXPERIMENTAL DESIGN

To provide evidence of mTOR kinase activation and inhibition, we applied a validated whole blood fixation/permeabilization technique for flow cytometry to serially monitor S6 ribosomal protein (S6) phosphorylation in immunophenotypically identified AML blasts.

RESULTS

With this approach, we show activation of mTOR signaling in 8 of 10 subjects' samples (80%) and conclusively show inhibition of mTOR in the majority of subjects' tumor cell during therapy. Of note, S6 phosphorylation in AML blasts is heterogeneous and, in some cases, intrinsically resistant to rapamycin at clinically achieved concentrations.

CONCLUSIONS

The methodology described is rapid and reproducible. We show the feasibility of real-time, direct pharmacodynamic monitoring by flow cytometry during clinical trials combining intensive chemotherapy and signal transduction inhibitors. This approach greatly clarifies pharmacokinetic/pharmacodynamic relationships and has broad application to preclinical and clinical testing of drugs whose direct or downstream effects disrupt PI3K/AKT/mTOR signaling.

Targeting mTOR for the treatment of AML. New agents and new directions

Despite recent advances in the field, the treatment of patients with acute myeloid leukemia (AML) remains challenging and difficult. Although chemotherapeutic agents induce remissions in a large number of patients, many of them eventually relapse and die. A major goal for the development of new approaches for the treatment of AML is to enhance the antileukemic effects of standard chemotherapeutics and to design effective combinations targeting non-overlapping cellular pathways. The PI3K/Akt/mTOR signaling pathway plays a critical role in survival and growth of malignant cells and its targeting has been the focus of extensive work and research efforts over the last two decades. It now appears possible that a major limitation of the first generation of mTOR inhibitors can be overcome by a new class of catalytic inhibitors of mTOR. There is emerging evidence that such compounds target both TORC1 and TORC2 and elicit much more potent responses against early leukemic precursors in vitro. In addition, recent studies have shown that combinations of such agents with cytarabine result in enhanced antileukemic responses in vitro, raising the prospect and potential of use of these agents in combination regimens for the treatment of AML.

MK886 inhibits the proliferation of HL-60 leukemia cells by suppressing the expression of mPGES-1 and reducing prostaglandin E2 synthesis

Microsomal prostaglandin E synthase-1 (mPGES-1), an inducible enzyme that specifically catalyzes the conversion of prostaglandin H2 (PGH2) to prostaglandin E2 (PGE2), has been reported to be over-expressed in a variety of solid tumor cells and tissues, but not in normal tissues. Its association with leukemia, however, has not been fully investigated. Our study revealed, for the first time, that mPGES-1 is over-expressed in human acute myeloid leukemia HL-60 cells. Cytotoxicity assays and flow cytometry showed that MK886, an inhibitor of mPGES-1, inhibits proliferation of HL-60 cells and induces apoptosis in a dose- and time-dependent manner, which may result from down-regulation of mPGES-1 expression and PGE2 synthesis. Evaluation of mediators of apoptotic signaling revealed up-regulation of BAX expression and caspase-3 activity, as well as significant decreases in Bcl2 and P-Akt. We conclude that MK886 reduces the viability of leukemia HL-60 cells by reducing mPGES-1 expression and PGE2 synthesis in a dose-dependent manner, which strongly suggests that mPGES-1 inhibitors should be considered as promising candidates for leukemia treatment.

New therapeutic targets and drugs in non-Hodgkin's lymphoma

PURPOSE OF REVIEW

Although enormous progress has been made in treating non-Hodgkin's lymphoma (NHL), and some patients can be cured with combination immunochemotherapy, patients with relapsed and refractory lymphoma often succumb to their disease. Advances in our understanding of lymphoma biology and molecular pathogenesis are yielding new therapeutic targets.

RECENT FINDINGS

This article reviews NHL biology and describes how our understanding of molecular pathogenesis is leading to the discovery of many therapeutic targets, including the cell signaling and cell cycle regulatory proteins, pro-apoptotic family members, the B-cell antigen receptor (BCR), and histone deacetylase. Recent preclinical and clinical data with inhibitors of phosphatidylinositol 3-kinase, AKT, mammalian target of rapamycin, histone deacetylase, bcl-2, and the Bruton's tyrosine kinase, a pivotal enzyme in the BCR pathway, are discussed.

SUMMARY

Understanding these novel targets in the context of NHL biology will bring new therapies and allow us to develop new therapeutic platforms for the treatment of relapsed and refractory NHL, and will hopefully improve the clinical outcome for these patients.

Extinction models for cancer stem cell therapy

Cells with stem cell-like properties are now viewed as initiating and sustaining many cancers. This suggests that cancer can be cured by driving these cancer stem cells to extinction. The problem with this strategy is that ordinary stem cells are apt to be killed in the process. This paper sets bounds on the killing differential (difference between death rates of cancer stem cells and normal stem cells) that must exist for the survival of an adequate number of normal stem cells. Our main tools are birth-death Markov chains in continuous time. In this framework, we investigate the extinction times of cancer stem cells and normal stem cells. Application of extreme value theory from mathematical statistics yields an accurate asymptotic distribution and corresponding moments for both extinction times. We compare these distributions for the two cell populations as a function of the killing rates. Perhaps a more telling comparison involves the number of normal stem cells NH at the extinction time of the cancer stem cells. Conditioning on the asymptotic time to extinction of the cancer stem cells allows us to calculate the asymptotic mean and variance of NH. The full distribution of NH can be retrieved by the finite Fourier transform and, in some parameter regimes, by an eigenfunction expansion. Finally, we discuss the impact of quiescence (the resting state) on stem cell dynamics. Quiescence can act as a sanctuary for cancer stem cells and imperils the proposed therapy. We approach the complication of quiescence via multitype branching process models and stochastic simulation. Improvements to the τ-leaping method of stochastic simulation make it a versatile tool in this context. We conclude that the proposed therapy must target quiescent cancer stem cells as well as actively dividing cancer stem cells. The current cancer models demonstrate the virtue of attacking the same quantitative questions from a variety of modeling, mathematical, and computational perspectives.

mTOR signaling in disease

The target of rapamycin (TOR) is a highly conserved serine/threonine kinase and a central controller of cell growth, metabolism and aging. Mammalian TOR (mTOR) is activated in response to nutrients, growth factors and cellular energy. Dysregulated mTOR signaling has been implicated in major disease. Here we review recent findings on the role of mTOR in cancer, metabolic disorders, neurological diseases, and inflammation.

The in vitro and in vivo effects of human umbilical cord mesenchymal stem cells on the growth of breast cancer cells

The purpose of the study was to detect the effect and possible mechanism of human umbilical cord mesenchymal stem cells (hUCMSCs) on the in vitro and in vivo growth of stem cells isolated from primary human breast cancer cells and cell lines MDA-MB-231 and MCF-7. Primary human breast cancer cells and MDA-MB-231 and MCF-7 cells were sorted in vitro using flow cytometry, and the ESA+, CD44+, CD24-/low cells were isolated as breast cancer stem cells (CSCs). The inhibitory effect of hUCMSCs on CSCs was examined using the Cell Counting Kit-8 cell proliferation and soft agar colony formation assay. In vivo tumor inhibition was studied using a severe combined immunodeficient xenograft mouse model transplanted with MDA-MB-231 breast CSCs. The expression of phosphoinositide 3-kinase (PI3K) and AKT was examined in the xenograft tumors using immunohistochemistry. The number of colonies formed by breast CSCs co-cultured with hUCMSCs at the bottom of soft agar was significantly lower than those formed by the control group (P < 0.01). Compared with the control group, the CSCs co-cultured with hUCMSCs showed a higher number of cells in the G2-M phase (P < 0.05) and an increased number of apoptotic cells (P < 0.01). The mice in the medium- and high-concentration hUCMSC treatment groups exhibited clearly reduced tumor volume and tumor weight, compared with the control group (P < 0.01). Compared with the saline group, the xenograft tumor tissues from the mice treated with different concentrations of hUCMSCs showed significantly reduced levels of PI3K and AKT proteins (P < 0.001). In conclusion, hUCMSC significantly inhibited the growth of breast CSCs in vitro and in vivo. The underlying mechanism is likely related to cell cycle arrest, induction of tumor cell apoptosis, and suppressed activities of PI3K and AKT protein kinases.

Arsenic disulfide synergizes with the phosphoinositide 3-kinase inhibitor PI-103 to eradicate acute myeloid leukemia stem cells by inducing differentiation

Although dramatic clinical success has been achieved in acute promyelocytic leukemia (APL), the success of differentiating agents has not been reproduced in non-APL leukemia. A key barrier to the clinical success of arsenic is that it is not potent enough to achieve a clinical benefit at physiologically tolerable concentrations by targeting the leukemia cell differentiation pathway alone. We explored a novel combination approach to enhance the eradication of leukemia stem cells (LSCs) by arsenic in non-APL leukemia. In the present study, phosphatidylinositol 3-kinase /AKT/mammalian target of rapamycin (mTOR) phosphorylation was strengthened after As(2)S(2) exposure in leukemia cell lines and stem/progenitor cells, but not in cord blood mononuclear cells (CBMCs). propidium iodide-103, the dual PI3K/mTOR inhibitor, effectively inhibited the transient activation of the PI3K/AKT/mTOR pathway by As(2)S(2). The synergistic killing and differentiation induction effects on non-APL leukemia cells were examined both in vitro and in vivo. Eradication of non-APL LSCs was determined using the nonobese diabetic/severe combined immunodeficiency mouse model. We found that a combined As(2)S(2)/PI-103 treatment synergized strongly to kill non-APL leukemia cells and promote their differentiation in vitro. Furthermore, the combined As(2)S(2)/PI-103 treatment effectively reduced leukemia cell repopulation and eradicated non-APL LSCs partially via induction of differentiation while sparing normal hematopoietic stem cells. Taken together, these findings suggest that induction of the PI3K/AKT/mTOR pathway could provide a protective response to offset the antitumor efficacy of As(2)S(2). Targeting the PI3K/AKT/mTOR pathway in combination with As(2)S(2) could be exploited as a novel strategy to enhance the differentiation and killing of non-APL LSCs.

Eukaryotic translation initiation factor 4E binding protein 1 (4EBP-1) function is suppressed by Src and protein phosphatase 2A (PP2A) on extracellular matrix

Human lung fibroblasts utilize integrins to attach and proliferate on type I collagen. β1 integrin is the major integrin subunit for this attachment. Integrins coordinate cellular responses to cell-cell and cell-extracellular matrix interactions that regulate a variety of biological processes. Although β1 integrin-mediated signaling pathways in lung fibroblasts have been studied, a detailed molecular mechanism regulating translational control of gene expression by 4EBP-1 is not understood. 4EBP-1 inhibits cap-dependent translation by binding to the eIF4E translation initiation factor. We found that when lung fibroblasts attach to collagen via β1 integrin, high Src activity suppresses 4EBP-1 expression via PP2A, and the decrease of 4EBP-1 is due to protein degradation. The inhibition of Src activity dramatically increases PP2A and 4EBP-1 expression. Furthermore ectopic expression of PP2A, or PP2A silencing using PP2A siRNA confirmed that 4EBP-1 is regulated by PP2A. In addition, we found that 4EBP-1 inhibition by fibroblast attachment to collagen increases cap-dependent translation. Our study showed that when lung fibroblasts are attached to collagen matrix, the β1 integrin/Src/PP2A-mediated 4EBP-1 regulatory pathway is activated. We suggest that β1 integrin-mediated signaling pathway may be a crucial event in regulating fibroblast translational control machinery on collagen matrix.

The phosphatidylinositol 3-kinase/Akt/mTOR signaling network as a therapeutic target in acute myelogenous leukemia patients

The phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling axis plays a central role in cell proliferation, growth, and survival under physiological conditions. However, aberrant PI3K/Akt/mTOR signaling has been implicated in many human cancers, including acute myelogenous leukemia (AML). Therefore, the PI3K/Akt/mTOR network is considered as a validated target for innovative cancer therapy. The limit of acceptable toxicity for standard polychemotherapy has been reached in AML. Novel therapeutic strategies are therefore needed. This review highlights how the PI3K/Akt/mTOR signaling axis is constitutively active in AML patients, where it affects survival, proliferation, and drug-resistance of leukemic cells including leukemic stem cells. Effective targeting of this pathway with small molecule kinase inhibitors, employed alone or in combination with other drugs, could result in the suppression of leukemic cell growth. Furthermore, targeting the PI3K/Akt/mTOR signaling network with small pharmacological inhibitors, employed either alone or in combinations with other drugs, may result in less toxic and more efficacious treatment of AML patients. Efforts to exploit pharmacological inhibitors of the PI3K/Akt/mTOR cascade which show efficacy and safety in the clinical setting are now underway.

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.

Inhibiting the mTOR pathway synergistically enhances cytotoxicity in ovarian cancer cells induced by etoposide through upregulation of c-Jun

PURPOSE

The mTOR pathway is thought to be a central regulator of proliferation and survival of cells. Rapamycin and its analogs are undergoing clinical trials in patients with epithelial ovarian cancer. This study aimed to assess the potential to use rapamycin and anticancer agents in combination for first- and second-line chemotherapy to treat ovarian cancer.

EXPERIMENTAL DESIGN

We used six ovarian serous adenocarcinoma cell lines (KF, KOC-2S, SHIN-3, SK-OV-3, TU-OS-3, and TU-OS-4) in this study. We treated the cells with rapamycin and anticancer agents, then assessed cell viability, apoptosis, and the expression of protein in apoptotic pathways and molecules downstream of the mTOR signaling pathways. We also investigated the effect of these drug combinations on survival in nude mouse xenograft models.

RESULTS

Synergistic effects were observed in five cell lines from the combination of etoposide and rapamycin. However, we observed antagonistic effects when rapamycin was combined with gemcitabine, cisplatin, or paclitaxel on more than two cell lines. Rapamycin dramatically enhanced apoptosis induced by etoposide and the expression of cleaved caspase 9. This effect was associated with upregulation of phosphorylated c-Jun and downregulation of Bcl-xL. The synergistic interaction of rapamycin and etoposide was lower when the c-Jun pathway was suppressed by a c-Jun N-terminal kinase inhibitor (SP600125). Finally, treating nude mice with rapamycin and etoposide significantly prolonged survival in the model mice with ovarian cancer xenografts.

CONCLUSIONS

Chemotherapy with rapamycin and etoposide combined is worth exploring as a treatment modality for women with epithelial ovarian cancer.

Proliferation signal inhibitors and post-transplant malignancies in heart transplantation: practical clinical management questions

Although malignancy is a major threat to long-term survival of heart transplant (HT) recipients, clear strategies to manage immunosuppression in these patients are lacking. Several lines of evidences support the hypothesis of an anticancer effect of proliferation signal inhibitors (PSIs: mammalian target of rapamycin [mTOR] inhibitors) in HT recipients. This property may arise from PSI's ability to replace immunosuppressive therapies that promote cancer progression, such as calcineurin inhibitors or azathioprine, and/or through their direct biological actions in preventing tumor development and progression. Given the lack of randomized studies specifically exploring these issues in the transplant setting, a collaborative group reviewed current literature and personal clinical experience to reach a consensus aimed to provide practical guidance for the clinical conduct in HT recipients with malignancy, or at high risk of malignancy, with a special focus on advice relevant to potential role of PSIs.

Mammalian target of rapamycin as a target in hematological malignancies

The mammalian target of rapamycin (mTOR) regulates protein synthesis in addition to cell growth and cell proliferation. Elucidation of the roles of the phosphatidylinositol 3-kinase (PI3K)/Akt/mTOR pathway in the regulation of the pathogenesis of hematological neoplasms has led to the development and clinical evaluation of agents targeting this pathway for the treatment of leukemia and lymphomas. Clinical trials conducted to date have shown modest responses to mTOR inhibition in patients with various hematological malignancies. Novel agents that simultaneously target mTOR complex 2 (mTORC2) or AKT in addition to mTOR complex 1 (mTORC1) may offer an opportunity to improve therapeutic efficacy.

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.

Melatonin protects against apoptotic and autophagic cell death in C2C12 murine myoblast cells

In this study, we investigated whether or not melatonin inhibits apoptotic and autophagic cell death in C2C12 murine myoblast cells. Treatment of cells with S-nitroso-N-acetylpenicillamine (SNAP), an NO donor, was shown to induce cell death, and treatment with melatonin (100 μm) significantly attenuated the occurrence of NO-induced cell death. Decreased p-Akt expression in response to NO was also arrested by melatonin. Under these conditions, p-Bad (Ser 136) expression increased with melatonin treatment prior to NO treatment. Treatment with Akt inhibitors (LY 294002, wortmannin) plus melatonin reduced p-Akt expression. Compared with NO treatment, Bcl-2 expression increased with melatonin treatment, while Bax expression was inhibited by melatonin treatment. Expression of catalase and Mn-superoxide dismutase (SOD) was elevated with melatonin treatment, whereas Cu/Zn-SOD expression decreased with melatonin, lower than NO treatment, respectively. Next, we investigated the question of whether or not melatonin may restrain autophagic cell death in C2C12 cells. Nutrient starvation induced a rise in expression of the microtubule-associated protein 1 light chain 3 (LC3)-II; however, melatonin treatment suppressed LC3-II expression by nutrient deprivation. Expression of Bcl-2, Bax, catalase, and Cu/Zn-SODs coincided with results of apoptotic cell death. Together, these results suggest that melatonin protects against apoptotic and autophagic cell death through the common pathway resulted in the increment of Bcl-2 expression and the reduction of Bax expression in C2C12 murine myoblast cells.

Regulation of the expression of death receptors and their ligands by melatonin in haematological cancer cell lines and in leukaemia cells from patients

Incorporation of new therapeutic agents remains as a major challenge for treatment of patients with malignant haematological disorders. Melatonin is an indolamine without relevant side effects. It has been shown previously to exhibit synergism with several chemotherapeutic drugs in Ewing sarcoma cells by potentiating the extrinsic pathway of apoptosis. It also sensitizes human glioma cells against TRAIL by increasing DR5 expression. Here, we report the induction of cell death by melatonin in several human malignant haematological cell lines through the activation of the extrinsic pathway of apoptosis. Such activation was mediated by the increase in the expression of the death receptors Fas, DR4 and DR5 and their ligands Fas L and TRAIL, with a remarkable rise in the expression of Fas and Fas L. The cytotoxic effect and the increase in Fas and Fas L were dependent on Akt activation. Results were corroborated in blasts from bone marrow and peripheral blood of acute myeloid leukaemia patients, where melatonin induced cell death and increased both Fas and Fas L expressions. We conclude that melatonin may be considered as a potential antileukaemic agent and its therapeutic use, either alone or in combination with current chemotherapeutic drugs, should be taken into consideration for further research.

Acute myeloid leukemia stem cells: seek and destroy

Most adult patients with acute myeloid leukemia (AML) die from their disease. Relapses are frequent even after aggressive multiagent chemotherapy and allogeneic stem cell transplantation. AML is a biologically heterogeneous disease, characterized by frequent cytogenetic abnormalities and an increasing spectrum of genetic mutations and molecular aberrations. Laboratory data suggest that AML originates from a rare population of cells, termed leukemic stem cells (LSCs) or leukemia-initiating cells, which are capable of self-renewal, proliferation and differentiation. These cells may persist after treatment and are probably responsible for disease relapse. This review will describe bench and translational research in LSCs and discuss how the data should be used to change the direction of developmental therapeutics and clinical trials in AML.

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.

A phase I/II study of imatinib plus reinduction therapy for c-kit-positive relapsed/refractory acute myeloid leukemia: inhibition of Akt activation correlates with complete response

This phase I/II study evaluated imatinib as a c-kit inhibitor combined with mitoxantrone, etoposide and cytarabine therapy for patients with primary refractory or relapsed c-kit+ acute myeloid leukemia (AML). Imatinib was escalated through three dose levels in successive six patient cohorts. The combination was well tolerated up to 400 mg/day imatinib. Of 21 patients treated at this dose, 13 (62%) achieved complete response (CR), 7 (33%) were non-responders and one died during induction. The CR rate was 80% in patients with standard-risk karyotype versus 33% in patients with adverse karyotype. The CR rate for primary non-responders was 6/14 (43%) versus 7/7 (100%) for relapsed patients. AML blasts from peripheral blood were assayed for phosphorylated Akt (pAkt) and phosphorylated ERK (pERK) by flow cytometry before to and after imatinib dosing. Of eight patients achieving CR with reinduction, seven demonstrated marked (≥60%) pAkt inhibition with imatinib therapy. In contrast, all the six non-responders to reinduction demonstrated <60% pAkt inhibition (P=0.005). There was no correlation between pERK inhibition and response to therapy. These results indicate that lack of pAkt inhibition in vivo is associated with resistance to reinduction therapy using this regimen. Further studies using agents that are able to inhibit Akt more effectively are warranted.