Contributions of the Raf/MEK/ERK, PI3K/PTEN/Akt/mTOR and Jak/STAT pathways to leukemia

A role for PKR in hematologic malignancies

The double-stranded RNA-dependent kinase PKR has been described for many years as strictly a pro-apoptotic kinase. Recent data suggest that the main purpose of this kinase is damage control and repair following stress and, if all else fails, apoptosis. Aberrant activation of PKR has been reported in numerous neurodegenerative diseases and cancer. Although a subset of myelodysplastic syndromes (MDS) and chronic lymphocytic leukemia contain low levels of PKR expression and activity, elevated PKR activity and/or expression have been detected in a wide range of hematologic malignancies, from bone marrow failure disorders to acute leukemia. With the recent findings that cancers containing elevated PKR activity are highly sensitive to PKR inhibition, we explore the role of PKR in hematologic malignancies, signal transduction pathways affected by PKR, and how PKR may contribute to leukemic transformation.

Dominant roles of the Raf/MEK/ERK pathway in cell cycle progression, prevention of apoptosis and sensitivity to chemotherapeutic drugs

The effects of the Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR signaling pathways on cell cycle progression, gene expression, prevention of apoptosis and sensitivity to chemotherapeutic drugs were examined in FL/ΔAkt-1:ER*(Myr(+)) + ΔRaf-1:AR cells which are conditionally-transformed to grow in response to Raf-1 and Akt-1 activation by treatment with testosterone or tamoxifen respectively. In these cells we can compare the effects of normal cytokine vs. oncogene mediated signaling in the same cells by changing the culture conditions. Raf-1 was more effective than Akt-1 in inducing cell cycle progression and preventing apoptosis in the presence and absence of chemotherapeutic drugs. The normal cytokine for these cells, interleukin-3 induced/activated most downstream genes transiently, with the exception of p70S6K that was induced for prolonged periods of time. In contrast, most of the downstream genes induced by either the activate Raf-1 or Akt-1 oncogenes were induced for prolonged periods of time, documenting the differences between cytokine and oncogene mediated gene induction which has important therapeutic consequences. The FL/ΔAkt-1:ER*(Myr(+)) + ΔRaf-1:AR cells were sensitive to MEK and PI3K/mTOR inhibitors. Combining MEK and PI3K/mTOR inhibitors increased the induction of apoptosis. The effects of doxorubicin on the induction of apoptosis could be enhanced with MEK, PI3K and mTOR inhibitors. Targeting the Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR pathways may be an effective approach for therapeutic intervention in those cancers which have upstream mutations which result in activation of these pathways.

Compensatory activation of Akt in response to mTOR and Raf inhibitors - a rationale for dual-targeted therapy approaches in neuroendocrine tumor disease

Several studies have established a link between aberrant PI(3)K-Akt-mTOR- and Ras-Raf-MEK-Erk1/2 signaling and neuroendocrine tumor disease. In this study, we comparatively investigate the antitumor potential of novel small-molecule inhibitors targeting mTOR (RAD001), mTOR/PI(3)K (NVP-BEZ235) and Raf (Raf265) on human NET cell lines of heterogeneous origin. All inhibitors induced potent antitumor effects which involved the induction of apoptosis and G0/G1 arrest. However, the dual mTOR/PI(3)K inhibitor NVP-BEZ235 was more efficient compared to the single mTOR inhibitor RAD001. Consistently, NVP-BEZ235 prevented the negative feedback activation of Akt as observed after treatment with RAD001. Raf265 inhibited Erk1/2 phosphorylation but strongly induced Akt phosphorylation and VEGF secretion, suggesting the existence of a compensatory feedback loop on PI3K-Akt signaling. Finally, combined treatment with RAD001 or NVP-BEZ235 and Raf265 was more efficient than single treatment with either kinase inhibitor. Together, our data provide a rationale for dual targeting of PI(3)K-Akt-mTOR- and Ras-Raf-MEK-Erk1/2 signaling in NET disease.

Erucylphosphohomocholine, the first intravenously applicable alkylphosphocholine, is cytotoxic to acute myelogenous leukemia cells through JNK- and PP2A-dependent mechanisms

Alkylphospholipids and alkylphosphocholines (APCs) are promising antitumor agents, which target the plasma membrane and affect multiple signal transduction networks. We investigated the therapeutic potential of erucylphosphohomocholine (ErPC3), the first intravenously applicable APC, in human acute myelogenous leukemia (AML) cells. ErPC3 was tested on AML cell lines, as well as AML primary cells. At short (6-12 h) incubation times, the drug blocked cells in G2/M phase of the cell cycle, whereas, at longer incubation times, it decreased survival and induced cell death by apoptosis. ErPC3 caused JNK 1/2 activation as well as ERK 1/2 dephosphorylation. Pharmacological inhibition of caspase-3 or a JNK 1/2 inhibitor peptide markedly reduced ErPC3 cytotoxicity. Protein phosphatase 2A downregulation by siRNA opposed ERK 1/2 dephosphorylation and blunted the cytotoxic effect of ErPC3. ErPC3 was cytotoxic to AML primary cells and reduced the clonogenic activity of CD34(+) leukemic cells. ErPC3 induced a significant apoptosis in the compartment (CD34(+) CD38(Low/Neg) CD123(+)) enriched in putative leukemia-initiating cells. This conclusion was supported by ErPC3 cytotoxicity on AML blasts showing high aldehyde dehydrogenase activity and on the side population of AML cell lines and blasts. These findings indicate that ErPC3 might be a promising therapeutic agent for the treatment of AML patients.

Targeting the PI3K/AKT/mTOR signaling network in acute myelogenous leukemia

BACKGROUND

The PI3K/Akt/mammalian target of rapamycin (mTOR) signaling pathway plays a central role in cell growth, proliferation and survival not only under physiological conditions but also in a variety of tumor cells. Therefore, the PI3K/Akt/mTOR axis may be a critical target for cancer therapy.

OBJECTIVE

This review discusses how PI3K/Akt/mTOR signaling network is constitutively active in acute myelogenous leukemia (AML), where it strongly influences proliferation, survival and drug-resistance of leukemic cells, and how effective targeting of this pathway with pharmacological inhibitors, used alone or in combination with existing drugs, may result in suppression of leukemic cell growth, including leukemic stem cells.

METHODS

We searched the literature for articles dealing with activation of this pathway in AML and highlighting the efficacy of small molecules directed against the PI3K/Akt/mTOR signaling cascade.

CONCLUSIONS

The limit of acceptable toxicity for standard chemotherapy has been reached in AML. Therefore, new therapeutic strategies are needed. Targeting the PI3K/Akt/mTOR signaling network with small molecule inhibitors, alone or in combinations with other drugs, may result in less toxic and more efficacious treatment of AML patients. Efforts to exploit selective inhibitors of the PI3K/Akt/mTOR pathway that show effectiveness and safety in the clinical setting are currently underway.

Cucurbitacin B induces apoptosis and S phase cell cycle arrest in BEL-7402 human hepatocellular carcinoma cells and is effective via oral administration

Cucurbitacin B is an anti-cancer drug candidate and its efficacy has been demonstrated in hepatocellular carcinoma (HCC). To explore its mechanism against HCC, BEL-7402 cells were treated with cucurbitacin B in vitro. Treatment with cucurbitacin B induced S phase arrest and apoptosis. The growth inhibition effect was associated with cyclin D1 and cdc-2 down regulations. Western blotting analysis of cell signaling molecules indicated that cucurbitacin B inhibited c-Raf activation without affecting STAT3 phosphorylation. Moreover, in vivo study demonstrated that cucurbitacin B is effective against BEL-7402 xenograft when administrated orally.

ABCG2: the key to chemoresistance in cancer stem cells?

Multi-drug chemoresistance remains one of the most common reasons for chemotherapy failure. The membrane transporter protein ABCG2/BCRP1 has been shown in vitro to effectively reduce the intracellular concentrations of several prominent anticancer chemotherapeutic agents such as mitoxantrone and doxorubicin. Intriguingly, cancer stem cells are known to be characterized by multi-drug chemoresistance. Taking into account that the ABCG2(+) subset of tumor cells are often enriched with cells with cancer stem-like phenotypes, it has been proposed that ABCG2 activity underlies the ability of cancer cells to regenerate post-chemotherapy. Furthermore, we also review evidence suggesting that tyrosine kinase inhibitors, including imatinib and gefitinib, are both direct and downstream inactivators of ABCG2 and, therefore, serve as candidates to reverse cancer stem cell chemoresistance and potentially target cancer stem cells.

Deregulation of Interferon Signaling in Malignant Cells

Interferons (IFNs) are a family of cytokines with potent antiproliferative, antiviral, and immunomodulatory properties. Much has been learned about IFNs and IFN-activated signaling cascades over the last 50 years. Due to their potent antitumor effects in vitro and in vivo, recombinant IFNs have been used extensively over the years, alone or in combination with other drugs, for the treatment of various malignancies. This review summarizes the current knowledge on IFN signaling components and pathways that are deregulated in human malignancies. The relevance of deregulation of IFN signaling pathways in defective innate immune surveillance and tumorigenesis are discussed.

Never say die: survival signaling in large granular lymphocyte leukemia

Large granular lymphocyte (LGL) leukemia is a rare disorder of mature cytotoxic T or natural killer cells. Large granular lymphocyte leukemia is characterized by the accumulation of cytotoxic cells in blood and infiltration in the bone marrow, liver, and spleen. Herein, we review clinical features of LGL leukemia. We focus our discussion on known survival signals believed to play a role in the pathogenesis of LGL leukemia and their potential therapeutic implications.

Exploiting cellular pathways to develop new treatment strategies for AML

The standard approaches to the treatment of acute myeloid leukemia (AML) have been predominantly based on cytarabine and anthracyclines. Yet, the outcomes associated with AML continue to be poor, especially for those patients who are older or carry higher-risk disease. In recent years, extensive research has led to the development and study of novel agents which target AML by diverse and varied mechanisms. Among these are targeted therapeutics such as kinase inhibitors and oligonucleotide constructs. These aim to suppress the production or activity of proteins, such as FLT3 and BCL2, among others, and thus disrupt related signaling cascades essential for leukemogenesis and proliferation. In addition, other agents like flavopiridol appear to target the myeloid blast by various mechanisms including suppression of cyclin-dependent kinases and interference with nucleotide synthesis. Another class of novel therapies includes inhibitors of histone deacetylase, which cause growth arrest and apoptosis through histone acetylation and resultant conformational changes. Clinical trials are now studying these and other agents alone and in combination with traditional cytotoxic therapies, with some encouraging results. In this review, we aim to provide a summary of the preclinical and clinical investigations of selected promising agents currently under study.

Role of the PI3K/AKT and mTOR signaling pathways in acute myeloid leukemia

The PI3K/AKT and mTOR signaling pathways are activated in acute myeloid leukemia, including in the more immature leukemic populations. Constitutive PI3K activation is detectable in 50% of acute myeloid leukemia samples whereas mTORC1 is activated in all cases of this disease. In leukemic cells, the PI3K activity relates to the expression of the p110delta isoform of class IA PI3K. Constitutive PI3K activation is the result of autocrine IGF-1/IGF-1R signaling in 70% of acute myeloid leukemia samples but specific inhibition of this pathway does not induce apoptosis. Specific inhibition of PI3K/AKT or mTORC1 alone in vitro has anti-leukemic effects which are essentially exerted via the suppression of proliferation. However, as mTORC1 activation is independent of PI3K/AKT in acute myeloid leukemia, dual PI3K and mTOR inhibitors may induce apoptosis in blast cells. Moreover, mTORC1 inhibition using sirolimus overactivates PI3K/AKT via the upregulation of IRS2 expression and by favoring IGF-1/IGF-1R autocrine signaling. Recent data also indicate that mTORC1 does not control protein translation in acute myeloid leukemia. These results open the way for the design of direct inhibitors of protein synthesis as novel acute myeloid leukemia therapies and also for the development of second generation mTOR inhibitors (the TORKinhibs).

Targeted therapy in T-cell malignancies: dysregulation of the cellular signaling pathways

T-cell malignancies, mainly known as T-cell acute lymphoblastic leukemia (T-ALL) and T-cell non-Hodgkin's lymphoma (T-NHL), are aggressive tumors. Although the clinical outcome of the patients has improved dramatically with combination chemotherapy, significant challenges remain, including understanding of the factors that contribute to the malignant behavior of these tumor cells and developing subsequently optimal targeted therapy. Aberrant cell signal transduction is generally involved in tumor progression and drug resistance. This review describes the pathogenetic role of multiple cellular signaling pathways in T-cell malignancies and the potential therapeutic strategies based on the modulation of these key signaling networks.

Automated large-scale control of gene regulatory networks

Controlling gene regulatory networks (GRNs) is an important and hard problem. As it is the case in all control problems, the curse of dimensionality is the main issue in real applications. It is possible that hundreds of genes may regulate one biological activity in an organism; this implies a huge state space, even in the case of Boolean models. This is also evident in the literature that shows that only models of small portions of the genome could be used in control applications. In this paper, we empower our framework for controlling GRNs by eliminating the need for expert knowledge to specify some crucial threshold that is necessary for producing effective results. Our framework is characterized by applying the factored Markov decision problem (FMDP) method to the control problem of GRNs. The FMDP is a suitable framework for large state spaces as it represents the probability distribution of state transitions using compact models so that more space and time efficient algorithms could be devised for solving control problems. We successfully mapped the GRN control problem to an FMDP and propose a model reduction algorithm that helps find approximate solutions for large networks by using existing FMDP solvers. The test results reported in this paper demonstrate the efficiency and effectiveness of the proposed approach.

Structure-activity relationship (SAR) studies of 3-(2-amino-ethyl)-5-(4-ethoxy-benzylidene)-thiazolidine-2,4-dione: development of potential substrate-specific ERK1/2 inhibitors

A series of analogs of 3-(2-amino-ethyl)-5-(4-ethoxy-benzylidene)-thiazolidine-2,4-dione, a putative substrate-specific ERK1/2 inhibitor, were synthesized and biologically characterized in human leukemia U937 cells to define its pharmacophore. It was discovered that shift of ethoxy substitution from the 4- to the 2-position on the phenyl ring significantly improved functional activities of inhibiting cell proliferation and inducing apoptosis. This may provide access to a new lead for developing ERK1/2 substrate-specific inhibitors.

PKR activity is required for acute leukemic cell maintenance and growth: a role for PKR-mediated phosphatase activity to regulate GSK-3 phosphorylation

Recent reports demonstrate that PKR is constitutively active in a variety of tumors and is required for tumor maintenance and growth. Here we report acute leukemia cell lines contain elevated levels of p-T451 PKR and PKR activity as compared to normal controls. Inhibition of PKR with a specific inhibitor, as well as overexpression of a dominant-negative PKR, inhibited cell proliferation and induced cell death. Interestingly, PKR inhibition using the specific inhibitor resulted in a time-dependent augmentation of AKT S473 and GSK-3alpha S21 phosphorylation, which was confirmed in patient samples. Increased phosphorylation of AKT and GSK-3alpha was not dependent on PI3K activity. PKR inhibition augmented levels of p-S473 AKT and p-S21/9 GSK-3alpha/beta in the presence of the PI3K inhibitor, LY294002, but was unable to augment GSK-3alpha or beta phosphorylation in the presence of the AKT inhibitor, A443654. Pre-treatment with the PKR inhibitor blocked the ability of A443654 and LY294002 to promote phosphorylation of eIF2alpha, indicating the mechanism leading to AKT phosphorylation and activation did not require eIF2alpha phosphorylation. The effects of PKR inhibition on AKT and GSK-3 phosphorylation were found to be, in part, PP2A-dependent. These data indicate that, in acute leukemia cell lines, constitutive basal activity of PKR is required for leukemic cell homeostasis and growth and functions as a negative regulator of AKT, thereby increasing the pool of potentially active GSK-3.

Relative resistance of SGK1 knockout mice against chemical carcinogenesis

The serum and glucocorticoid inducible kinase SGK1 was originally cloned from mammary tumor cells. SGK1 was found to be up-regulated in a variety of tumors, but down-regulated in several distinct tumors. Thus, evidence for a role of SGK1 in tumor growth remained conflicting. According to in vitro observations, SGK1 is up-regulated by the oncogene beta-catenin and negatively regulates the proapoptotic transcription factor FOXO3a, which in turn stimulates transcription of the Bcl2-interacting mediator BIM. This study aimed to define the role of SGK1 in colon carcinoma in vivo. SGK1 knockout mice (sgk1(-/-)) and their wild type littermates (sgk1(+/+)) were subjected to chemical cancerogenesis (intraperitoneal injection of 20 mg/kg 1,2-dimethylhydrazine followed by three cycles of 30 g/L synthetic dextran sulfate sodium for 7 days). Moreover, SGK1 was silenced in HEK293 cells. FOXO3a and BIM protein abundance was determined by Western blotting and immunohistochemistry. Following chemical cancerogenesis, sgk1(-/-)mice developed significantly less colonic tumors than sgk1(+/+)mice. According to Western blotting and immunohistochemistry, SGK1 deficiency enhanced the expression of FOXO3a and BIM both, in vitro and in vivo. SGK1 deficiency counteracts the development of colonic tumors, an effect at least in part due to up-regulation of FOXO3a and BIM.

Gab1 transduces PI3K-mediated erythropoietin signals to the Erk pathway and regulates erythropoietin-dependent proliferation and survival of erythroid cells

In this study, we examined the biological functions of Gab1 in erythropoietin receptor (EPOR)-mediated signaling in vivo. Knockdown of Gab1 by the introduction of the Gab1 siRNA expression vector into F-36P human erythroleukemia (F-36P-Gab1-siRNA) cells resulted in a reduction of cell proliferation and survival in response to EPO. EPO-induced activation of Erk1/2 but not of Akt was significantly suppressed in F-36P-Gab1-siRNA cells compared with mock-transfected F-36P cells. The co-immunoprecipitation experiments revealed an EPO-enhanced association of Gab1 with the Grb2-SOS1 complex and SHP-2 in F-36P cells. A selective inhibitor of phosphatidylinositol 3-kinase (PI3K) LY294002 and short interfering RNA (siRNA) duplexes targeting the p85 regulatory subunit of PI3K (p85-siRNA) independently suppressed tyrosine phosphorylation of Gab1; its association with Grb2, SHP-2 and p85; and the activation of Erk in EPO-treated F-36P cells. LY294002 inhibited EPO-induced tyrosine phosphorylation of Gab1 and its association with Grb2 in human primary EPO-sensitive erythroid cells. The co-immunoprecipitation experiments using the Jak inhibitor AG490 or siRNA duplexes targeting Jak2 and in vitro binding experiments demonstrated that Jak2 regulated Gab1-mediated Erk activation through tyrosine phosphorylation of Gab1. Taken together, these results suggest that Gab1 couples PI3K-mediated EPO signals with the Ras/Erk pathway and that Gab1 plays an important role in EPOR-mediated signal transduction involved in the proliferation and survival of erythroid cells.

Developing curcumin into a viable therapeutic for lymphoma

BACKGROUND

Emerging evidence suggests that natural plant ingredients have played an important role in the healthcare of many countries. Several of these natural plant products possess therapeutic potential for various diseases including cancer. Curcumin is the pigment of turmeric, a well-known chemopreventive agent that has been shown to suppress the proliferation of a wide variety of tumor cells, including lymphoma. Curcumin has been shown to have cancer chemopreventive potential against a variety of tumors via targeting key survival pathways that are aberrantly activated in cancer cells.

METHODS

This review discusses therapeutic potential of curcumin in malignancies of lymphoma as well as therapeutic implications of the recent advances in the field.

RESULTS/CONCLUSION

Dietary-compound curcumin hardwires to multiple cellular processes. Suppression of cell proliferation, induction of apoptosis, and inhibition of metastasis are considered to be the major mechanisms underlying its anticancer properties.

CCL5 promotes proliferation of MCF-7 cells through mTOR-dependent mRNA translation

The proliferative capacity of cancer cells is regulated by factors intrinsic to cancer cells and by secreted factors in the microenvironment. Here, we investigated the proto-oncogenic potential of the chemokine receptor, CCR5, in MCF-7 breast cancer cell lines. At physiological levels, CCL5, a ligand for CCR5, enhanced MCF-7.CCR5 proliferation. Treatment with the mTOR inhibitor, rapamycin, inhibited this CCL5-inducible proliferation. Because mTOR directly modulates mRNA translation, we investigated whether CCL5 activation of CCR5 leads to increased translation. CCL5 induced the formation of the eIF4F translation initiation complex through an mTOR-dependent process. Indeed, CCL5 initiated mRNA translation, shown by an increase in high-molecular-weight polysomes. Specifically, we show that CCL5 mediated a rapid up-regulation of protein expression for cyclin D1, c-Myc and Dad-1, without affecting their mRNA levels. Taken together, we describe a mechanism by which CCL5 influences translation of rapamycin-sensitive mRNAs, thereby providing CCR5-positive breast cancer cells with a proliferative advantage.

ERK2, but not ERK1, mediates acquired and "de novo" resistance to imatinib mesylate: implication for CML therapy

Resistance to Imatinib Mesylate (IM) is a major problem in Chronic Myelogenous Leukaemia management. Most of the studies about resistance have focused on point mutations on BCR/ABL. However, other types of resistance that do not imply mutations in BCR/ABL have been also described. In the present report we aim to study the role of several MAPK in IM resistance not associate to BCR/ABL mutations. Therefore we used an experimental system of resistant cell lines generated by co-culturing with IM (K562, Lama 84) as well as primary material from resistant and responder patient without BCR/ABL mutations. Here we demonstrate that Erk5 and p38MAPK signaling pathways are not implicated in the acquired resistance phenotype. However, Erk2, but not Erk1, is critical for the acquired resistance to IM. In fact, Bcr/Abl activates preferentially Erk2 in transient transfection in a dose dependent fashion through the c-Abl part of the chimeric protein. Finally, we present evidences demonstrating how constitutive activation of Erk2 is a de novo mechanism of resistance to IM. In summary our data support the use of therapeutic approaches based on Erk2 inhibition, which could be added to the therapeutic armamentarium to fight CML, especially when IM resistance develops secondary to Erk2 activation.

Galpha(i1) and Galpha(i3) are required for epidermal growth factor-mediated activation of the Akt-mTORC1 pathway

The precise mechanism whereby epidermal growth factor (EGF) activates the serine-threonine kinase Akt and the mammalian target of rapamycin (mTOR) complex 1 (mTORC1) remains elusive. Here, we report that the alpha subunits of the heterotrimeric guanine nucleotide-binding proteins (G proteins) Galpha(i1) and Galpha(i3) are critical for this activation process. Both Galpha(i1) and Galpha(i3) formed complexes with growth factor receptor binding 2 (Grb2)-associated binding protein 1 (Gab1) and the EGF receptor (EGFR) and were required for the phosphorylation of Gab1 and its subsequent interaction with the p85 subunit of phosphatidylinositol 3-kinase in response to EGF. Loss of Galpha(i1) and Galpha(i3) severely impaired the activation of Akt and of p70 S6 kinase and 4E-BP1, downstream targets of mTORC1, in response to EGF, heparin-binding EGF-like growth factor, and transforming growth factor alpha, but not insulin, insulin-like growth factor, or platelet-derived growth factor. In addition, ablation of Galpha(i1) and Galpha(i3) largely inhibited EGF-induced cell growth, migration, and survival and the accumulation of cyclin D1. Overall, this study suggests that Galpha(i1) and Galpha(i3) lie downstream of EGFR, but upstream of Gab1-mediated activation of Akt and mTORC1, thus revealing a role for Galpha(i) proteins in mediating EGFR signaling.

Dual inhibition of class IA phosphatidylinositol 3-kinase and mammalian target of rapamycin as a new therapeutic option for T-cell acute lymphoblastic leukemia

Recent investigations have documented that constitutively activated phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling is a common feature of T-cell acute lymphoblastic leukemia (T-ALL), where it strongly influences growth and survival. These findings lend compelling weight for the application of PI3K/Akt/mTOR inhibitors in T-ALL. However, our knowledge of PI3K/Akt/mTOR signaling in T-ALL is limited and it is not clear whether it could be an effective target for innovative therapeutic strategies. Here, we have analyzed the therapeutic potential of the dual PI3K/mTOR inhibitor PI-103, a small synthetic molecule of the pyridofuropyrimidine class, on both T-ALL cell lines and patient samples, which displayed constitutive activation of PI3K/Akt/mTOR signaling. PI-103 inhibited the growth of T-ALL cells, including 170-kDa P-glycoprotein overexpressing cells. PI-103 cytotoxicity was independent of p53 gene status. PI-103 was more potent than inhibitors that are selective only for PI3K (Wortmannin, LY294002) or for mTOR (rapamycin). PI-103 induced G(0)-G(1) phase cell cycle arrest and apoptosis, which was characterized by activation of caspase-3 and caspase-9. PI-103 caused Akt dephosphorylation, accompanied by dephosphorylation of the Akt downstream target, glycogen synthase kinase-3beta. Also, mTOR downstream targets were dephosphorylated in response to PI-103, including p70S6 kinase, ribosomal S6 protein, and 4E-BP1. PI-103 strongly synergized with vincristine. These findings indicate that multitargeted therapy toward PI3K and mTOR alone or with existing drugs may serve as an efficient treatment toward T-ALL cells, which require up-regulation of PI3K/Akt/mTOR signaling for their survival and growth.

High Erk-1 activation and Gadd45a expression as prognostic markers in high risk pediatric haemolymphoproliferative diseases

Studies on activated cell-signaling pathways responsible for neoplastic transformation are numerous in solid tumors and in adult leukemias. Despite of positive results in the evolution of pediatric hematopoietic neoplasias, there are some high-risk subtypes at worse prognosis. The aim of this study was to asses the expression and activation status of crucial proteins involved in cell-signaling pathways in order to identify molecular alterations responsible for the proliferation and/or escape from apoptosis of leukemic blasts. The quantitative and qualitative expression and activation of Erk-1, c-Jun, Caspase8, and Gadd45a was analyzed, by immunocytochemical (ICC) and western blotting methods, in bone marrow blasts of 72 patients affected by acute myeloid leukemia (AML), T-cell acute lymphoblastic leukemia (ALL) and stage IV non-Hodgkin Lymphoma (NHL). We found an upregulation of Erk-1, Caspase8, c-Jun, and Gadd45a proteins with a constitutive activation in 95.8%, 91.7%, 86.2%, 83.4% of analyzed specimens, respectively. It is worth noting that all AML patients showed an upregulation of all proteins studied and the high expression of GADD45a was associated to the lowest DFS median (p = 0.04). On univariate analysis, only Erk-1 phosphorylation status was found to be correlated with a significantly shorter 5-years DFS in all disease subgroups (p = 0.033) and the lowest DFS median in ALL/NHL subgroup (p = 0.04). Moreover, the simultaneous activation of multiple kinases, as we found for c-Jun and Erk-1 (r = 0.26; p = 0.025), might synergistically enhance survival and proliferation potential of leukemic cells. These results demonstrate an involvement of these proteins in survival of blast cells and, consequently, on relapse percentages of the different subgroups of patients.

Expression of sprouty2 inhibits B-cell proliferation and is epigenetically silenced in mouse and human B-cell lymphomas

B-cell lymphoma is the most common immune system malignancy. TCL1 transgenic mice (TCL1-tg), in which TCL1 is ectopically expressed in mature lymphocytes, develop multiple B- and T-cell leukemia and lymphoma subtypes, supporting an oncogenic role for TCL1 that probably involves AKT and MAPK-ERK signaling pathway augmentation. Additional, largely unknown genetic and epigenetic alterations cooperate with TCL1 during lymphoma progression. We examined DNA methylation patterns in TCL1-tg B-cell tumors to discover tumor-associated epigenetic changes, and identified hypermethylation of sprouty2 (Spry2). Sprouty proteins are context-dependent negative or positive regulators of MAPK-ERK pathway signaling, but their role(s) in B-cell physiology or pathology are unknown. Here we show that repression of Spry2 expression in TCL1-tg mouse and human B-cell lymphomas and cell lines is associated with dense DNA hypermethylation and was reversed by inhibition of DNA methylation. Spry2 expression was induced in normal splenic B cells by CD40/B-cell receptor costimulation and regulated a negative feedback loop that repressed MAPK-ERK signaling and decreased B-cell viability. Conversely, loss of Spry2 function hyperactivated MAPK-ERK signaling and caused increased B-cell proliferation. Combined, these results implicate epigenetic silencing of Spry2 expression in B lymphoma progression and suggest it as a companion lesion to ectopic TCL1 expression in enhancing MAPK-ERK pathway signaling.

Upsides and downsides to polarity and asymmetric cell division in leukemia

The notion that polarity regulators can act as tumor suppressors in epithelial cells is now well accepted. The function of these proteins in lymphocytes is less well explored, and their possible function as suppressors of leukemia has had little attention so far. We review the literature on lymphocyte polarity and the growing recognition that polarity proteins have an important function in lymphocyte function. We then describe molecular relationships between the polarity network and signaling pathways that have been implicated in leukemogenesis, which suggest mechanisms by which the polarity network might impact on leukemogenesis. We particularly focus on the possibility that disruption of polarity might alter asymmetric cell division (ACD), and that this might be a leukemia-initiating event. We also explore the converse possibility that leukemic stem cells might be produced or maintained by ACD, and therefore that Dlg, Scribble and Lgl might be important regulators of this process.

Role and regulation of autophagy in cancer

Autophagy is an evolutionarily conserved process whereby cytoplasm and cellular organelles are degraded in lysosomes for amino acid and energy recycling. Autophagy is a survival pathway activated in response to nutrient deprivation and other stressful stimuli, such as metabolic stress and exposure to anticancer drugs. However, autophagy may also result in cell death, if it proceeds to completion. Defective autophagy is implicated in tumorigenesis, as the essential autophagy regulator beclin 1 is monoallelically deleted in human breast, ovarian and prostate cancers, and beclin 1(+/-) mice are tumor-prone. How autophagy suppresses tumorigenesis is under intense investigation. Cell-autonomous mechanisms, involving protection of genome integrity and stability, and a non-cell-autonomous mechanism, involving suppression of necrosis and inflammation, have been discovered so far. The role of autophagy in treatment responsiveness is also complex. Autophagy inhibition concurrently with chemotherapy or radiotherapy has emerged as a novel approach in cancer treatment, as autophagy-competent tumor cells depend on autophagy for survival under drug- and radiation-induced stress. Alternatively, autophagy stimulation and preservation of cellular fitness by maintenance of protein and organelle quality control, suppression of DNA damage and genomic instability, and limitation of necrosis-associated inflammation may play a critical role in cancer prevention.

Cell and molecular biology of the novel protein tyrosine-phosphatase-interacting protein 51

This chapter examines the current state of knowledge about the expression profile, as well as biochemical properties and biological functions of the evolutionarily conserved protein PTPIP51. PTPIP51 is apparently expressed in splice variants and shows a particularly high expression in epithelia, skeletal muscle, placenta, and germ cells, as well as during mammalian development and in cancer. PTPIP51 is an in vitro substrate of Src- and protein kinase A, the PTP1B/TCPTP protein tyrosine phosphatases and interacts with 14-3-3 proteins, the Nuf2 kinetochore protein, the ninein-interacting CGI-99 protein, diacylglycerol kinase alpha, and also with itself forming dimers and trimers. Although the precise cellular function remains to be elucidated, the current data implicate PTPIP51 in signaling cascades mediating proliferation, differentiation, apoptosis, and motility.

Targeting the leukemic stem cell: the Holy Grail of leukemia therapy

Since the discovery of leukemic stem cells (LSCs) over a decade ago, many of their critical biological properties have been elucidated, including their distinct replicative properties, cell surface phenotypes, their increased resistance to chemotherapeutic drugs and the involvement of growth-promoting chromosomal translocations. Of particular importance is their ability to transfer malignancy to non-obese diabetic-severe combined immunodeficient (NOD-SCID) mice. Furthermore, numerous studies demonstrate that acute myeloid leukemia arises from mutations at the level of stem cell, and chronic myeloid leukemia is also a stem cell disease. In this review, we will evaluate the main characteristics of LSCs elucidated in several well-documented leukemias. In addition, we will discuss points of therapeutic intervention. Promising therapeutic approaches include the targeting of key signal transduction pathways (for example, PI3K, Rac and Wnt) with small-molecule inhibitors and specific cell surface molecules (for example, CD33, CD44 and CD123), with effective cytotoxic antibodies. Also, statins, which are already widely therapeutically used for a variety of diseases, show potential in targeting LSCs. In addition, drugs that inhibit ATP-binding cassette transporter proteins are being extensively studied, as they are important in drug resistance-a frequent characteristic of LSCs. Although the specific targeting of LSCs is a relatively new field, it is a highly promising battleground that may reveal the Holy Grail of cancer therapy.

Involvement of p53 and Raf/MEK/ERK pathways in hematopoietic drug resistance

A cytokine-dependent (FL5.12), drug-sensitive, p53 wild type (WT) and a doxorubicin-resistant derivative line (FL/Doxo) were used to determine the mechanisms that could result in drug resistance of early hematopoietic precursor cells. Drug resistance was associated with decreased p53 induction after doxorubicin treatment, which was due to a higher level of proteasomal degradation of p53. Dominant-negative (DN) p53 genes increased the resistance to chemotherapeutic drugs, MDM-2 and MEK inhibitors, further substantiating the role of p53 in therapeutic sensitivity. The involvement of signal transduction and apoptotic pathways was examined, as drug resistance did not appear to be due to increased drug efflux. Drug-resistant FL/Doxo cells had higher levels of activated Raf/MEK/ERK signaling and decreased induction of apoptosis when cultured in the presence of doxorubicin than drug-sensitive FL5.12 cells. Introduction of DN MEK1 increased drug sensitivity, whereas constitutively active (CA) MEK1 or conditionally active BRAF augmented resistance, documenting the importance of the Raf/MEK/ERK pathway in drug resistance. MEK inhibitors synergized with chemotherapeutic drugs to reduce the IC(50). Thus the p53 and Raf/MEK/ERK pathways play key roles in drug sensitivity. Targeting these pathways may be effective in certain drug-resistant leukemias that are WT at p53.

Targeting prostate cancer based on signal transduction and cell cycle pathways

Prostate cancer remains a leading cause of death in men despite increased capacity to diagnose at earlier stages. After prostate cancer has become hormone independent, which often occurs after hormonal ablation therapies, it is difficult to effectively treat. Prostate cancer may arise from mutations and dysregulation of various genes involved in regulation signal transduction (e.g., PTEN, Akt, etc.,) and the cell cycle (e.g., p53, p21(Cip1), p27(Kip1), Rb, etc.,). This review focuses on the aberrant interactions of signal transduction and cell cycle genes products and how they can contribute to prostate cancer and alter therapeutic effectiveness.