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

Therapeutic silencing of mTOR by systemically administered siRNA-loaded neutral liposomal nanoparticles inhibits DMBA-induced mammary carcinogenesis

BACKGROUND

Mammary carcinogenesis possesses great challenges due to the lack of effectiveness of the multiple therapeutic options available. Gene therapy-based cancer treatment strategy provides more targeting accuracy, fewer side effects, and higher therapeutic efficiency. Downregulation of the oncogene mTOR by mTOR-siRNA is an encouraging approach to reduce cancer progression. However, its employment as means of therapeutic strategy has been restricted due to the unavailability of a suitable delivery system.

METHODS

A suitable nanocarrier system made up of 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) has been developed to prevent degradation and for proficient delivery of siRNA. This was followed by in vitro and in vivo anti-breast cancer efficiency analysis of the mTOR siRNA-loaded neutral liposomal formulation (NL-mTOR-siRNA).

RESULTS

In our experiment, a profound reduction in MCF-7 cell growth, proliferation and invasion was ascertained following extensive downregulation of mTOR expression. NL-mTOR-siRNA suppressed tumour growth and restored morphological alterations of DMBA-induced breast cancer. In addition, neutral liposome enhanced accumulation of siRNA in mammary cancer tissues facilitating its deep cytosolic distribution within the tumour, which allows apoptosis thereby facilitating its anti-tumour potential.

CONCLUSION

Hence, the current study highlighted the augmented ground for therapies aiming toward cancerous cells to diminish mTOR expression by RNAi in managing mammary carcinoma.

Phosphoproteomics of primary AML patient samples reveals rationale for AKT combination therapy and p53 context to overcome selinexor resistance

Acute myeloid leukemia (AML) is a heterogeneous disease with variable patient responses to therapy. Selinexor, an inhibitor of nuclear export, has shown promising clinical activity for AML. To identify the molecular context for monotherapy sensitivity as well as rational drug combinations, we profile selinexor signaling responses using phosphoproteomics in primary AML patient samples and cell lines. Functional phosphosite scoring reveals that p53 function is required for selinexor sensitivity consistent with enhanced efficacy of selinexor in combination with the MDM2 inhibitor nutlin-3a. Moreover, combining selinexor with the AKT inhibitor MK-2206 overcomes dysregulated AKT-FOXO3 signaling in resistant cells, resulting in synergistic anti-proliferative effects. Using high-throughput spatial proteomics to profile subcellular compartments, we measure global proteome and phospho-proteome dynamics, providing direct evidence of nuclear translocation of FOXO3 upon combination treatment. Our data demonstrate the potential of phosphoproteomics and functional phosphorylation site scoring to successfully pinpoint key targetable signaling hubs for rational drug combinations.

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Phosphoproteomics with functional scoring uncovers context for selinexor sensitivity

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Functional p53 correlates with selinexor sensitivity, which is enhanced by nutlin-3a

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Dysregulated AKT-FOXO3 drives selinexor resistance, which is overcome with MK-2206

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Spatial proteomics reveals selinexor-induced nucleocytoplasmic protein shuttling

Strategic Developments & Future Perspective on Gene Therapy for Breast Cancer: Role of mTOR and Brk/ PTK6 as Molecular Targets

Breast cancer is a serious health issue and a major concern in biomedical research. Alteration in major signaling (viz. PI3K-AKT-mTOR, Ras-Raf-MEK-Erk, NF-kB, cyclin D1, JAK-STAT, Wnt, Notch, Hedgehog signaling and apoptotic pathway) contributes to the development of major subtypes of mammary carcinoma such as HER2 positive, TNBC, luminal A and B and normal-like breast cancer. Further, mutation and expression parameters of different genes involved in the growth and development of cells play an important role in the progress of different types of carcinoma, making gene therapy an emerging new therapeutic approach for the management of life-threatening diseases like cancer. The genetic targets (oncogenes and tumor suppressor genes) play a major role in the formation of a tumor. Brk/PTK6 and mTOR are two central molecules that are involved in the regulation of numerous signaling related to cell growth, proliferation, angiogenesis, survival, invasion, metastasis, apoptosis, and autophagy. Since these two proteins are highly upregulated in mammary carcinogenesis, this can be used as targeted genes for the treatment of breast cancer. However, not much work has been done on them. This review highlights the therapeutic significance of Brk and mTOR and their associated signaling in mammary carcinogenesis, which may provide a strategy to develop gene therapy for breast cancer management.

Dual Kinase Targeting in Leukemia

Pharmacological cancer therapy is often based on the concurrent inhibition of different survival pathways to improve treatment outcomes and to reduce the risk of relapses. While this strategy is traditionally pursued only through the co-administration of several drugs, the recent development of multi-targeting drugs (i.e., compounds intrinsically able to simultaneously target several macromolecules involved in cancer onset) has had a dramatic impact on cancer treatment. This review focuses on the most recent developments in dual-kinase inhibitors used in acute myeloid leukemia (AML), chronic myelogenous leukemia (CML), and lymphoid tumors, giving details on preclinical studies as well as ongoing clinical trials. A brief overview of dual-targeting inhibitors (kinase/histone deacetylase (HDAC) and kinase/tubulin polymerization inhibitors) applied to leukemia is also given. Finally, the very recently developed Proteolysis Targeting Chimeras (PROTAC)-based kinase inhibitors are presented.

BIX-01294 enhanced chemotherapy effect in gastric cancer by inducing GSDME-mediated pyroptosis

Adjuvant chemotherapy in combination with surgery is expected to be a curative strategy for gastric cancer. However, drug resistance remains an obstacle in effective chemotherapy. Therefore, understanding the potential mechanisms of chemotherapy induced gastric cancer cell death is of great importance. We demonstrated that BIX-01294 (BIX) at low concentration could induce autophagic flux by converting LC3B-I to LC3B-II and directly activate autophagy associated cell death in gastric cancer cell lines at high concentration. BIX at low concentration could help obtain sensitivity of gastric cancer cells to chemotherapy with significantly reduced cell viability. Interestingly, BIX combined Cis (BIX + Cis) treated SGC-7901 cells display pyroptosis related cell death with large bubbles blown around the membrane, significantly decreased cell viability, elevated lactate dehydrogenase release and increased percentage of propidium iodide and Annexin-V double positive cells. Furthermore, the cleavage of gasdermin E (GSDME) and caspase-3 but not GSDMD was detected by immunoblotting and the knockout of GSDME switched pyroptosis into apoptosis in the BIX + Cis combined treated group. Furthermore, the deficiency of Beclin-1 to inhibit BIX induced autophagic flux completely blocked BIX + Cis combined treated induced cell pyroptosis related cell death. Additionally, BIX + Cis in vivo treatment could inhibit tumor growth, which could be reversed by the deficiency of Beclin-1 and be delayed by the deficiency of GSDME. In conclusion, our data was the first to reveal that BIX enhanced the anticancer chemotherapy effect by induced GSDME-mediated pyroptosis through the activation of autophagic flux in gastric cancer cells.

Targeting mTOR in Acute Lymphoblastic Leukemia

Acute Lymphoblastic Leukemia (ALL) is an aggressive hematologic disorder and constitutes approximately 25% of cancer diagnoses among children and teenagers. Pediatric patients have a favourable prognosis, with 5-years overall survival rates near 90%, while adult ALL still correlates with poorer survival. However, during the past few decades, the therapeutic outcome of adult ALL was significantly ameliorated, mainly due to intensive pediatric-based protocols of chemotherapy. Mammalian (or mechanistic) target of rapamycin (mTOR) is a conserved serine/threonine kinase belonging to the phosphatidylinositol 3-kinase (PI3K)-related kinase family (PIKK) and resides in two distinct signalling complexes named mTORC1, involved in mRNA translation and protein synthesis and mTORC2 that controls cell survival and migration. Moreover, both complexes are remarkably involved in metabolism regulation. Growing evidence reports that mTOR dysregulation is related to metastatic potential, cell proliferation and angiogenesis and given that PI3K/Akt/mTOR network activation is often associated with poor prognosis and chemoresistance in ALL, there is a constant need to discover novel inhibitors for ALL treatment. Here, the current knowledge of mTOR signalling and the development of anti-mTOR compounds are documented, reporting the most relevant results from both preclinical and clinical studies in ALL that have contributed significantly into their efficacy or failure.

The IGF pathway is activated in insulinomas but downregulated in metastatic disease

Clinical and molecular studies have implicated epidermal growth factor receptor (EGFR), insulin-like growth factor (IGF) and target of rapamycin (mTOR) signaling pathways in the regulation of pancreatic neuroendocrine tumor (PanNET) growth. Interpretation and comparison of these studies is complex due to clinical and molecular tumor heterogeneity. We therefore focused in this study on insulinomas, which we examined for mRNA and protein expression of EGFR, IGF and mTOR signaling pathway components by quantitative real-time PCR (n=48) and immunohistochemistry (n=86). Findings were compared with normal pancreatic islets and correlated with histopathological data and clinical outcome. Insulinomas showed low EGFR and high IGF2 expression. IGFBP2, IGFBP3 and IGFBP6 mRNA levels were 2-4 folds higher than in islets. High protein expression of IGF2, IGF1R and INSR (in 51-92% of the tumors) and low to moderate expression of mTORC1 pathway proteins p-PS6k and p-4EBP1 (7-28% of the tumors) were observed. Correlations were found between 1) ERK1 mRNA expression and that of numerous IGF pathway genes, 2) p-ERK and IGF1R protein expression and 3) decrease of IGF pathway components and both metastatic disease and shorter 10 years disease free survival. In conclusion, our observations suggest that high expression of IGF signaling pathway components is a hallmark of insulinomas, but does not necessarily lead to increased mTOR signaling. Reduced expression of IGF pathway components may be an adverse prognostic factor in insulinomas.

5'-methylschweinfurthin G reduces chondrosarcoma tumor growth

New treatment options are urgently required in the field of chondrosarcoma, particularly of chondrosarcomas with a well-differentiated hyaline cartilage-like extracellular matrix (e.g., collagen II and proteoglycan-rich) phenotype, notoriously resistant to drug penetration, and having potential of progression towards higher grade. We investigated the feasibility of using 5'-methylschweinfurthin G (MeSG) as a tumor suppressor agent in the Swarm rat chondrosarcoma, an intermediate- to high-grade chondrosarcoma model, having a hyaline cartilage-like phenotype. Tumor cell culture studies were performed to identify their proliferative and cytotoxicity sensitivity to MeSG. Tumor burden mice were treated with MeSG and analyzed for tumor growth, morphology and regression. The chondrosarcoma tumor cells had a half maximum cytotoxicity concentration (IC₅₀ ) of 35 nM MeSG; approximately 300-fold less than freshly isolated rat chondrocytes (IC₅₀ of 11 µM). Multiple injections of MeSG (20 mg/kg, body weight) resulted in reduced/eliminated tumor growth over a 17-day period in mice, and an 83% reduction (p = 0.023) in tumor mass. Three out of ten MeSG treated mice had complete elimination of tumor. Tumors of treated mice had a decrease in chondrosarcoma cell proliferation (p = 0.012) and an increase in cell death (p = 0.030) compared with tumors of control mice. These findings in an animal model demonstrate the effectiveness of MeSG for treatment of rat chondrosarcomas, and may have the potential use as a therapeutic option for the difficult-to-treat intermediate-to high-grade hyaline cartilage-like chondrosarcoma. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1283-1293, 2018.

Dueling for dual inhibition: Means to enhance effectiveness of PI3K/Akt/mTOR inhibitors in AML

The phosphatidylinositol 3-kinase/protein kinase B (Akt)/mechanistic target of rapamycin (PI3K/Akt/mTOR) pathway is amplified in 60-80% of patients with acute myelogenous leukemia (AML). Since this complex pathway is crucial to cell functions such as growth, proliferation, and survival, inhibition of this pathway would be postulated to inhibit leukemia initiation and propagation. Inhibition of the mTORC1 pathway has met with limited success in AML due to multiple resistance mechanisms including direct insensitivity of the mTORC1 complex, feedback activation of the PI3k/Akt signaling network, insulin growth factor-1 (IGF-1) activation of PI3K, and others. This review explores the role of mTOR inhibition in AML, mechanisms of resistance, and means to improve outcomes through use of dual mTORC1/2 inhibitors or dual TORC/PI3K inhibitors. How these inhibitors interface with currently available therapies in AML will require additional preclinical experiments and conduct of well-designed clinical trials.

Targeting the mTOR Pathway in Leukemia

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

High DEPTOR expression correlates with poor prognosis in patients with esophageal squamous cell carcinoma

OBJECTIVE

The disheveled, Egl-10, and pleckstrin (DEP) domain containing mammalian target of rapamycin (mTOR)-interacting protein (DEPTOR) is a binding protein containing mTOR complex 1 (mTORC1), mTOR complex 2 (mTORC2), and an endogenous mTOR inhibitor. DEPTOR shows abnormal expressions in numerous types of solid tumors. However, how DEP-TOR is expressed in esophageal squamous cell carcinoma (ESCC) remains elusive.

METHODS

The expression of DEPTOR in 220 cases of ESCC and non-cancerous adjacent tissues was detected by immunohistochemistry. DEPTOR levels in ESCC and paired normal tissue were quantified using reverse transcription-polymerase chain reaction and Western blot analysis to verify the immunohistochemical results. The relationship between DEPTOR expression and the clinicopathological features of ESCC was analyzed based on the results of immunohistochemistry. Finally, we analyzed the relationship between DEPTOR expression and the prognosis of patients with ESCC.

RESULTS

Immunohistochemical staining showed that the expression rate of DEPTOR in ESCC tissues was significantly increased. DEPTOR mRNA and protein expression was significantly higher in ESCC tissues than in normal adjacent esophageal squamous tissues. High DEPTOR expression was significantly correlated with regional lymph node status in the TNM stage of patients with ESCC. Kaplan-Meier survival curves showed that the rate of overall survival was significantly lower in patients with high DEPTOR expression than in those with low DEPTOR expression. Additionally, high DEPTOR expression was an independent prognostic predictor for ESCC patients.

CONCLUSION

High DEPTOR expression is an independent prognostic biomarker indicating a worse prognosis for patients with ESCC.

Tuberous Sclerosis: A New Frontier in Targeted Treatment of Autism

Tuberous sclerosis complex (TSC) is a genetic disorder with a high prevalence of autism spectrum disorder (ASD). Tremendous progress in understanding the pathogenesis of TSC has been made in recent years, along with initial trials of medical treatment aimed specifically at the underlying mechanism of the disorder. At the cellular level, loss of TSC1 or TSC2 results in upregulation of the mechanistic target of rapamycin (mTOR) pathway. At the circuitry level, TSC and mTOR play crucial roles in axonal, dendritic, and synaptic development and function. In this review, we discuss the molecular mechanism underlying TSC, and how this disease results in aberrant neural connectivity at multiple levels in the central nervous system, leading to ASD symptoms. We then review recent advances in mechanism-based treatments of TSC, and the promise that these treatments provide for future mechanism-based treatment of ASD. Because of these recent advances, TSC represents an ideal model for how to make progress in understanding and treating the mechanisms that underlie ASD in general.

Effect of puerarin on human choriocarcinoma cells

OBJECTIVE

To discuss the effect of puerarin on human choriocarcinoma cells.

METHODS

Survival rates under puerarin monotherapy, fluorouracil (5-FU) monotherapy and puerarin in combination with 5-FU were detected by MTT assay. Apoptotic morphology was observed with Hoechst 33258 staining. Apoptosis rates were detected with flow cytometry. Expressions of AKT, mechanistic target of rapamycin (mTOR), and P70S6K mRNAs and phosphorylated proteins were detected by RT-PCR and Western blot. Tumor-bearing mice were administered puerarin and puerarin+5-FU, and serum levels of β-human chorionic gonadotropin (β-HCG) were measured.

RESULTS

Proliferation inhibition and apoptosis rates of JEG-3 cells were positively correlated with puerarin concentration, which increased in the puerarin+5-FU group. Expression levels of AKT, mTOR, P70S6K mRNAs, and phosphorylated proteins decreased significantly after action of puerarin at different concentrations. With increasing puerarin concentration, expression of cleaved-caspase-3 in JEG-3 cells increased, whereas that of Bcl-2 decreased. Puerarin significantly inhibited tumor growth in choriocarcinoma-bearing SCID mice. Serum β-HCG levels were significantly lower than those of control group after administration. Magnitude of β-HCG decline was positively correlated with concentration..

CONCLUSION

Puerarin+5-FU inhibited proliferation of JEG-3 choriocarcinoma cells and promoted their apoptosis, being associated with the mTOR signaling pathway.

Targeting mTOR signaling pathways and related negative feedback loops for the treatment of acute myeloid leukemia

An accumulating understanding of the complex pathogenesis of acute myeloid leukemia (AML) continues to lead to promising therapeutic approaches. Among the key aberrant intracellular signaling pathways involved in AML, the phosphatidylinositol 3-kinase/AKT/mammalian target of rapamycin (PI3K/AKT/mTOR) axis is of major interest. This axis modulates a wide array of critical cellular functions, including proliferation, metabolism, and survival. Pharmacologic inhibitors of components of this pathway have been developed over the past decade, but none has an established role in the treatment of AML. This review will discuss the preclinical data and clinical results driving ongoing attempts to exploit the PI3K/AKT/mTOR pathway in patients with AML and address issues related to negative feedback loops that account for leukemic cell survival. Targeting the PI3K/AKT/mTOR pathway is of high interest for the treatment of AML, but combination therapies with other targeted agents may be needed to block negative feedback loops in leukemia cells.

mTOR Signaling in Protein Translation Regulation: Implications in Cancer Genesis and Therapeutic Interventions

mTOR is a central nutrient sensor that signals a cell to grow and proliferate. Through distinct protein complexes it regulates different levels of available cellular energy substrates required for cell growth. One of the important functions of the complex is to maintain available amino acid pool by regulating protein translation. Dysregulation of mTOR pathway leads to aberrant protein translation which manifests into various pathological states. Our review focuses on the role mTOR signaling plays in protein translation and its physiological role. It also throws some light on available data that show translation dysregulation as a cause of pathological complexities like cancer and the available drugs that target the pathway for cancer treatment.

Intersection of mTOR and STAT signaling in immunity

Optimal regulation of immune networks is essential for the generation of effective immune responses, and defects in such networks can lead to immunodeficiency while uncontrolled responses can result in autoimmune disorders. mTOR and STAT signaling cascades are key regulators of the differentiation and function of cells of the immune system. Both pathways act as sensors and transducers of environmental stimuli, and recent evidence has revealed points of crosstalk between these pathways, highlighting synergistic regulation of immune cell differentiation and function. We review here the current understanding of mTOR and STAT interactions in T cells and innate immune cells, and discuss potential mechanisms underlying these events. We further outline models for the intersection of these pathways in the regulation of immunity and highlight important areas for future research.

Phospho-mTOR in non-tumour and tumour bladder urothelium: Pattern of expression and impact on urothelial bladder cancer patients

Urothelial bladder carcinoma (UBC) is heterogeneous in its pathology and clinical behaviour. Evaluation of prognostic and predictive biomarkers is necessary, in order to produce personalised treatment options. The present study used immunohistochemistry to evaluate UBC sections containing tumour and non-tumour areas from 76 patients, for the detection of p-mTOR, CD31 and D2-40 (blood and lymphatic vessels identification, respectively). Of the non-tumour and tumour sections, 36 and 20% were scored positive for p-mTOR expression, respectively. Immunoexpression was observed in umbrella cells from non-tumour urothelium, in all cell layers from non-muscle-invasive (NMI) tumours (including expression in superficial cells), and in spots of cells from muscle-invasive (MI) tumours. Positive expression decreased from non-tumour to tumour urothelium, and from pT1/pTis to pT3/pT4 tumours; however, the few pT3/pT4 positive cases had worse survival rates, with 5-year disease-free survival being significantly lower. Angiogenesis occurrence was impaired in pT3/pT4 tumours that did not express p-mTOR. In conclusion, p-mTOR expression in non-tumour umbrella cells is likely a reflection of their metabolic plasticity, and extension to the inner layers of the urothelium in NMI tumours is consistent with an enhanced malignant potential. The expression in cell spots in a few MI tumours and absence of expression in the remaining tumours is intriguing and requires further research. Additional studies regarding the up- and downstream effectors of the mTOR pathway should be conducted.

Therapeutic potential of targeting mTOR in T-cell acute lymphoblastic leukemia (review)

T-cell acute lymphoblastic leukemia (T-ALL) is a heterogeneous neoplastic disorder of immature hematopoietic precursors committed to the T-cell lineage. T-ALL comprises about 15% of pediatric and 25% of adult ALL cases. Even if the prognosis of T-ALL has improved especially in the childhood due to the use of new intensified treatment protocols, the outcome of relapsed patients who are resistant to conventional chemotherapeutic drugs or who relapse is still poor. For this reason, there is a need for novel and less toxic targeted therapies against signaling pathways aberrantly activated in T-ALL, such as the phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR). Small molecules designed to target key components of this signaling axis have proven their efficacy both in vitro and in vivo in pre-clinical settings of T-ALL. In particular, different classes of mTOR inhibitors have been disclosed by pharmaceutical companies, and they are currently being tested in clinical trials for treating T-ALL patients. One of the most promising approaches for the treatment of T-ALL seems to be the combination of mTOR inhibitors with traditional chemotherapeutic agents. This could lead to a lower drug dosage that may circumvent the systemic side effects of chemotherapeutics. In this review, we focus on the different classes of mTOR inhibitors that will possibly have an impact on the therapeutic arsenal we have at our disposal against T-ALL.

Use of mTOR inhibitors in the treatment of malignancies

INTRODUCTION

mTOR and its effectors are central regulators of cellular metabolism in malignant cells and control mRNA translation that ultimately leads to generation of mitogenic proteins. Efforts to target this pathway have been ongoing for over a decade and have had a substantial impact in the management of certain patients with solid tumors. Although activity of mTOR inhibitors has been established in several trials, inability to predict responses remains a limiting factor for the successful incorporation of these agents in the treatment of a variety of malignancies.

AREAS COVERED

Recent clinical findings are discussed and studies focused on advanced phase development of mTOR inhibitors are summarized. The emergence of precision medicine approaches and the effects that such approaches may have on prospective selection of patients for treatment with mTOR inhibitors are discussed. Also, potential approaches and future prospects to improve responses to mTOR inhibitors by modulating other parallel mitogenic pathways essential for malignant cell proliferation are discussed.

EXPERT OPINION

Selective targeting of the mTOR pathway offers significant clinical advantage in subsets of patients with diverse malignancies. Approaches to enhance responses by concomitant targeting of resistance pathways and/or predict responses via identification of molecular markers should substantially impact this area in the near and distant future.

PI3K and cancer: lessons, challenges and opportunities

The central role of phosphoinositide 3-kinase (PI3K) activation in tumour cell biology has prompted a sizeable effort to target PI3K and/or downstream kinases such as AKT and mammalian target of rapamycin (mTOR) in cancer. However, emerging clinical data show limited single-agent activity of inhibitors targeting PI3K, AKT or mTOR at tolerated doses. One exception is the response to PI3Kδ inhibitors in chronic lymphocytic leukaemia, where a combination of cell-intrinsic and -extrinsic activities drive efficacy. Here, we review key challenges and opportunities for the clinical development of inhibitors targeting the PI3K-AKT-mTOR pathway. Through a greater focus on patient selection, increased understanding of immune modulation and strategic application of rational combinations, it should be possible to realize the potential of this promising class of targeted anticancer agents.

PI3K/mTOR pathway inhibitors sensitize chronic myeloid leukemia stem cells to nilotinib and restore the response of progenitors to nilotinib in the presence of stem cell factor

Nilotinib is a second-generation tyrosine kinase inhibitor, designed to specifically inhibit break-point cluster region (BCR)-Abelson (ABL) and developed to treat chronic myeloid leukemia (CML) in patients showing a resistance to imatinib. We previously demonstrated that nilotinib-induced apoptosis was reduced by stem cell factor (SCF) addition. Here, the SCF-activated survival pathway was investigated. BCR-ABL expression was accompanied by the activation of the SCF receptor: c-KIT. Nilotinib inhibited this activation that was restored by SCF binding. Parallel variations were observed for mammaliam target of rapamycin (mTOR) kinase and mTOR complex 1 substrate S6K. The inhibition of mTORC1 restored the response of BCR-ABL cell lines to nilotinib in the presence of SCF. PI3K inhibition restored nilotinib-induced apoptosis. On hematopoietic progenitors from CML patient's bone marrows, mTORC1 inhibition also restored nilotinib sensitivity in the presence of SCF, confirming its involvement in SCF-activated survival pathway. However, this pathway seems not to be involved in the nilotinib-induced resistance of the CML stem cell population. Conversely, PI3K inhibition sensitized both CML progenitors and stem cells to nilotinib, suggesting that, downstream PI3K, two different kinase pathways are activated in CML progenitor and stem cell populations.

Acute myeloid leukemia: potential for new therapeutic approaches targeting mRNA translation pathways

Despite advances in molecular research related to acute myeloid leukemia (AML) and a better understanding of the mechanisms of leukemogenesis and pathophysiology of the disease, the pharmacological agents used in the treatment of AML have remained essentially unchanged for the last three decades. Advances in the clinical management of AML patients have been achieved by defining better molecular prognostic markers, but there remains a need for new targeted drugs that disrupt non-overlapping pathways in leukemia cells. The mTOR cellular cascade is critical for cell metabolism, growth, proliferation and survival. Extensive preclinical work suggests that targeting mTOR may provide a powerful approach to block AML precursor cells, while other findings suggest enhanced antileukemic effects by combining mTOR inhibitors with traditional chemotherapy. Such combinations may increase antileukemic responses further, offering unique ways to overcome leukemic cell resistance and to eliminate primitive leukemic precursors.

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.