Lack of constitutive activation of MAP kinase pathway in human acute myeloid leukemia cells with N-Ras mutation

Isoform-specific Ras signaling is growth factor dependent

HRAS, NRAS, and KRAS isoforms are almost identical proteins that are ubiquitously expressed and activate a common set of effectors. In vivo studies have revealed that they are not biologically redundant; however, the isoform specificity of Ras signaling remains poorly understood. Using a novel panel of isogenic SW48 cell lines endogenously expressing wild-type or G12V-mutated activated Ras isoforms, we have performed a detailed characterization of endogenous isoform-specific mutant Ras signaling. We find that despite displaying significant Ras activation, the downstream outputs of oncogenic Ras mutants are minimal in the absence of growth factor inputs. The lack of mutant KRAS-induced effector activation observed in SW48 cells appears to be representative of a broad panel of colon cancer cell lines harboring mutant KRAS. For MAP kinase pathway activation in KRAS-mutant cells, the requirement for coincident growth factor stimulation occurs at an early point in the Raf activation cycle. Finally, we find that Ras isoform-specific signaling was highly context dependent and did not conform to the dogma derived from ectopic expression studies.

Diabetic Retinopathy and Laser Therapy in Rats: A Protein-Protein Interaction Network Analysis

Introduction: Diabetic retinopathy (DR) is a serious microvascular complication of diabetes which can cause vision loss or blindness ultimately. Non enzymatic glycation of proteins leads to advanced glycation end products (AGEs) in DR. Since laser therapy is a well-established method, in this study, protein-protein interaction (PPI) network is applied for protein targets in DR disease in rats treated by laser. Methods: In this study, we focused on articles that investigated and compared the proteome profiles of DR rats with healthy control and also DR rats before and after laser therapy. The networks of related differentially expressed proteins were explored using Cytoscape version 3.3.0, the PPI analysis methods and ClueGO. Results: Analysis of PPI network of 37 related proteins to DR rats including 108 nodes, introduced 10 hub-bottleneck proteins and 5 concerned biochemical pathways. On the other hand, PPI analysis of related proteins to DR rats before and after laser therapy corresponded to 33 proteins and 2 biological pathways. Discussion: Centrality and cluster screening identified hub-bottelneck genes, including Aldoa, HSPD1, Pgam2, Mapk3, SLC2A4, Ctnnb1, Ywhab, HSPA8, GAPDH and Actb for DR rats versus healthy control and ENO1, Aldoa, GAPDH for DR samples after laser therapy.

CONCLUSION

Gene expression analysis of the DR samples treated via laser therapy provides a molecular evidence in support of the therapeutic effect of laser.

The value of genomics in dissecting the RAS-network and in guiding therapeutics for RAS-driven cancers

The rise in genomic knowledge over the past decade has revealed the molecular etiology of many diseases, and has identified intricate signaling network activity in human cancers. Genomics provides the opportunity to determine genome structure and capture the activity of thousands of molecular events concurrently, which is important for deciphering highly complex genetic diseases such as cancer. In this review, we focus on genomic efforts directed towards one of cancer's most frequently mutated networks, the RAS pathway. Genomic tools such as gene expression signatures and assessment of mutations across the RAS network enable the capture of RAS signaling complexity. Due to this high level of interaction and cross-talk within the network, efforts to target RAS signaling in the clinic have generally failed, and we currently lack the ability to directly inhibit the RAS protein with high efficacy. We propose that the use of gene expression data can identify effective treatments that broadly inhibit the RAS network as this approach measures pathway activity independent of mutation status or any single mechanism of activation. Here, we review the genomic studies that map the complexity of the RAS network in cancer, and that show how genomic measurements of RAS pathway activation can identify effective RAS inhibition strategies. We also address the challenges and future directions for treating RAS-driven tumors. In summary, genomic assessment of RAS signaling provides a level of complexity necessary to accurately map the network that matches the intricacy of RAS pathway interactions in cancer.

Activation of MEK2 is sufficient to induce skin papilloma formation in transgenic zebrafish

Background

Mutations in mitogen-activated protein kinase (MAPK) kinase 1 (MEK1) that occur during cell proliferation and tumor formation are well described. Information on the roles of MEK2 in these effects is still limited. We established a constitutive MEK2 transgenic zebrafish, Tg(krt14:MEK2S219D-GFP), to elucidate the role of MEK2 in skin tumor formation.

Results

We found that both constitutive MEK2 and MEK1 are able to phosphorylate the extracellular signal-regulated kinase 1 (ERK1) protein. Transient expression of constitutive MEK2 and MEK1 in the zebrafish epidermis induced papillary formation at 48 h post-fertilization, but no effects were observed due to the expression of MEK1, MEK2, or the dominant negative form of MEK2. The transgenic zebrafish, Tg(krt14:MEK2S219D-GFP), developed skin papillomas in the epidermis within 6 days post-fertilization (dpf). The phospho-ERK signal was detected in section of skin papillomas in an immunohistochemical experiment. Treatment with 50 μM of the MEK inhibitor, U0126, had significantly decreased the skin papilloma formation in Tg(krt14:MEK2S219D-GFP) zebrafish by 6 dpf. In vitro and in vivo proliferation assay in COS-1 cells and in Tg(krt14:MEK2S219D-GFP) transgenic fish show significantly increased cell number and Ki-67 signaling.

Conclusion

Our data indicate that MEK2 is sufficient to induce epidermal papilloma formation through MAPK signaling in zebrafish, and this transgenic model can be used as a new platform for drug screening.

Constitutively active Ras negatively regulates Erk MAP kinase through induction of MAP kinase phosphatase 3 (MKP3) in NIH3T3 cells

The Ras/Raf/MEK/Erk signaling pathway is important for regulation of cell growth, proliferation, differentiation, survival, and apoptosis in response to a variety of extracellular stimuli. Lack of Erk MAPK activation is observed in several cancer cells despite active activation of Ras. However, little is known about the modulation of Erk1/2 activity by active Ras. Here, we show that overexpression of active H-Ras (H-RasG12R) in NIH3T3 fibroblasts impaired FGF2-induced Erk1/2 phosphorylation, as compared to wild-type cells. Northern blot analysis revealed that prolonged expression of active Ras increased MAP kinase phosphatase 3 (MKP3) mRNA expression, a negative regulator of Erk MAPK. Inhibition of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway abrogated active Ras-induced up-regulation of MKP3 expression, leading to the rescue of Erk1/2 phosphorylation. Our results demonstrated that the Ras/Raf/MEK/Erk signaling cascade is negatively regulated by the PI3K/Akt dependent transcriptional activation of the MKP3 gene.

Role of Ras/Raf/MEK/ERK signaling in physiological hematopoiesis and leukemia development

Recent research on hematological malignancies has shown that malignant cells often co-opt physiological pathways to promote their growth and development. Bone marrow homeostasis requires a fine balance between cellular differentiation and self-renewal; cell survival and apoptosis; and cellular proliferation and senescence. The Ras/Raf/MEK/ERK pathway has been shown to be important in regulating these biological functions. Moreover, the Ras/Raf/MEK/ERK pathway has been estimated to be mutated in 30% of all cancers, thus making it the focus of many scientific studies which have lead to a deeper understanding of cancer development and help to elucidate potential weaknesses that can be targeted by pharmacological agents [1]. In this review, we specifically focus on the role of this pathway in physiological hematopoiesis and how augmentation of the pathway may lead to hematopoietic malignancies. We also discuss the challenges and success of targeting this pathway.

Phosphatome profiling reveals PTPN2, PTPRJ and PTEN as potent negative regulators of PKB/Akt activation in Ras-mutated cancer cells

Oncogenic Ras mutations render the protein constitutively active and promote tumorigenesis via chronic stimulation of effector pathways. In A549 lung adenocarcinoma approx. 50% of the total Ras population is constitutively active, yet these cells display only weak activation of the effectors: ERK1/2 (extracellular-signal-regulated kinase 1/2) and Akt. In order to identify key negative regulators of oncogenic Ras signalling we performed a phosphatome RNAi (RNA interference) screen in A549 cells and ranked their effects on phosphorylation of Ser473 of Akt. As expected, the tumour suppressor PTEN (phosphatase and tensin homologue deleted on chromosome 10) emerged as a leading hit: knockdown elevated Akt activation to 70% of maximal generated by acute EGF (epidermal growth factor) stimulation. Importantly, we identified other phosphatases with similar potencies including PTPN2 (T-cell protein tyrosine phosphatase; also known as TC-PTP) and PTPRJ (protein tyrosine phosphatase receptor type J; also known as DEP-1/CD148). Potentiation of Akt phosphorylation by knockdown of PTEN or PTPRJ was contingent on the presence of oncogenic K-Ras. Our data reveal a synergy between oncogene function and the loss of a tumour suppressor within the same pathway that was necessary for full effector activation since each alone failed to elicit significant Akt phosphorylation. Taken together, these data reveal potent regulators of Akt signalling which contribute to ameliorating the consequences of oncogenic K-Ras activity.

Oncogenic HRAS mutations cause prolonged PI3K signaling in response to epidermal growth factor in fibroblasts of patients with Costello syndrome

Costello syndrome (CS) is a rare congenital disorder characterized by failure to thrive, craniofacial dysmorphisms, cardiac and skin abnormalities, mental retardation, and predisposition to malignancies. CS is caused by heterozygous gain-of-function mutations in HRAS that also occur as somatic alterations in human tumors. HRAS is one of the three classical RAS proteins and cycles between an active, GTP- and an inactive, GDP-bound conformation. We used primary human skin fibroblasts from patients with CS as a model system to study the functional consequences of HRAS mutations on endogenous signaling pathways. The GTP-bound form of HRAS was significantly enriched in CS compared with normal fibroblasts. Active HRAS is known to stimulate both the RAF-MEK-ERK and the PI3K-AKT signaling cascade. Phosphorylation of MEK and ERK was normal in CS fibroblasts under basal conditions and slightly prolonged after epidermal growth factor (EGF) stimulation. Interestingly, basal phosphorylation of AKT was increased yet more in CS fibroblasts. Moreover, AKT phosphorylation was diminished in the early and enhanced in the late phase of EGF stimulation. Taken together, these results document that CS-associated HRAS mutations result in prolonged signal flux in a ligand-dependent manner. Our data suggest that altered cellular response to growth factors rather than constitutive activation of HRAS downstream signaling molecules may contribute to some of the clinical features in patients with CS.

Molecular and chromosomal alterations: new therapies for relapsed acute myeloid leukemia

Acute myeloid leukemia (AML) remains the most common form of leukemia and the most common cause of leukemia death. Although conventional chemotherapy can cure between 25 and 45% of AML patients, the majority of patients die after relapse or of complications associated with treatment. Thus, more specific and less toxic treatments for AML patients are needed, especially for elderly patients. An indispensable prerequisite to investigate tailored approaches for AML is the recent progress in the understanding the molecular features that distinguish leukemia progenitors from normal hematopoietic counterparts and the identification of a variety of dysregulated molecular pathways. This in turn would allow the identification of tumor-specific characteristics that provide a rational basis for the development of more tailored, and hence potentially more effective and less toxic, therapeutic approaches. In this review, we describe some of the signaling pathways that are aberrantly regulated in AML, with a specific focus on their pathogenetic and therapeutic significance, and we examine some recent therapies directed against these targets, used in clinical trial for relapsed patients or unfit for conventional chemotherapy.

Ras isoform abundance and signalling in human cancer cell lines

The ubiquitously expressed major Ras isoforms: H-, K- and N-Ras, are highly conserved, yet exhibit different biological outputs. We have compared the relative efficiencies with which epidermal or hepatocyte growth factor activates Ras isoforms and the requirement for specific isoforms in the activation of downstream pathways. We find that the relative coupling efficiencies to each Ras isoform are conserved between stimuli. Furthermore, in both cases, inhibition of receptor endocytosis led to reduced N- and H-Ras activation, but K-Ras was unaffected. Acute knockdown of each isoform with siRNA allows endogenous Ras isoform function and abundance to be probed. This revealed that there is significant variation in the contribution of individual isoforms to total Ras across a panel of cancer cell lines although typically K> or =N>>H. Intriguingly, cancer cell lines where a significant fraction of endogenous Ras is oncogenically mutated showed attenuated activation of canonical Ras effector pathways. We profiled the contribution of each Ras isoform to the total Ras pool allowing interpretation of the effect of isoform-specific knockdown on signalling outcomes. In contrast to previous studies indicating preferential coupling of isoforms to Raf and PtdIns-3-kinase pathways, we find that endogenous Ras isoforms show no specific coupling to these major Ras pathways.

Ras proteins: paradigms for compartmentalised and isoform-specific signalling

Ras GTPases mediate a wide variety of cellular processes by converting a multitude of extracellular stimuli into specific biological responses including proliferation, differentiation and survival. In mammalian cells, three ras genes encode four Ras isoforms (H-Ras, K-Ras4A, K-Ras4B and N-Ras) that are highly homologous but functionally distinct. Differences between the isoforms, including their post-translational modifications and intracellular sorting, mean that Ras has emerged as an important model system of compartmentalised signalling and membrane biology. Ras isoforms in different subcellular locations are proposed to recruit distinct upstream and downstream accessory proteins and activate multiple signalling pathways. Here, we summarise data relating to isoform-specific signalling, its role in disease and the mechanisms promoting compartmentalised signalling. Further understanding of this field will reveal the role of Ras signalling in development, cellular homeostasis and cancer and may suggest new therapeutic approaches.

Overcoming resistance to molecularly targeted anticancer therapies: Rational drug combinations based on EGFR and MAPK inhibition for solid tumours and haematologic malignancies

Accumulating evidence suggests that cancer can be envisioned as a "signaling disease", in which alterations in the cellular genome affect the expression and/or function of oncogenes and tumour suppressor genes. This ultimately disrupts the physiologic transmission of biochemical signals that normally regulate cell growth, differentiation and programmed cell death (apoptosis). From a clinical standpoint, signal transduction inhibition as a therapeutic strategy for human malignancies has recently achieved remarkable success. However, as additional drugs move forward into the clinical arena, intrinsic and acquired resistance to "targeted" agents becomes an issue for their clinical utility. One way to overcome resistance to targeted agents is to identify genetic and epigenetic aberrations underlying sensitivity/resistance, thus enabling the selection of patients that will most likely benefit from a specific therapy. Since resistance often ensues as a result of the concomitant activation of multiple, often overlapping, signaling pathways, another possibility is to interfere with multiple, cross-talking pathways involved in growth and survival control in a rational, mechanism-based, fashion. These concepts may be usefully applied, among others, to agents that target two major signal transduction pathways: the one initiated by epidermal growth factor receptor (EGFR) signaling and the one converging on mitogen-activated protein kinase (MAPK) activation. Here, we review the molecular mechanisms of sensitivity/resistance to EGFR inhibitors, as well as the rationale for combining them with other targeted agents, in an attempt to overcome resistance. In the second part of the paper, we review MAPK-targeted agents, focusing on their therapeutic potential in haematologic malignancies, and examine the prospects for combinations of MAPK inhibitors with cytotoxic agents or other signal transduction-targeted agents to obtain synergistic anti-tumour effects.

Influence ofKi-ras-driven oncogenic transformation on the protein network of murine fibroblasts

Ki-ras gene mutations that specifically occur in codons 12, 13 and 61 are involved in the carcinogenesis of acute myeloid leukemia, melanoma and different carcinomas. In order to define potential mutation-specific therapeutic targets, stable transfectants of NIH3T3 cells carrying different Ki-ras4B gene mutations were generated. Wild type Ki-ras transformants, mock transfectants and parental cells served as controls. These in vitro model systems were systematically analyzed for their protein expression pattern using two-dimensional gel electrophoresis followed by mass spectrometry and/or protein sequencing. Using this approach, a number of target molecules that are differentially but coordinately expressed in the ras transfectants were identified next to other proteins that exhibit a distinct regulation pattern in the different cell lines analyzed. The differentially expressed proteins predominantly belong to the families of cytoskeletal proteins, heat shock proteins, annexins, metabolic enzymes and oxidoreductases. Their validation was assessed by real-time quantitative RT-PCR and/or Western blot analysis. Our results suggest that the Ki-ras-transformed cells represent a powerful tool to study Ki-ras gene mutation-driven protein expression profiles. In addition, this approach allows the discovery of ras-associated cellular mechanisms, which might lead to the identification of physiological targets for pharmacological interventions of the treatment of Ki-ras-associated human tumors.

Detection of N-RAS and K-RAS in their active GTP-bound form in acute myeloid leukemia without activating RAS mutations

RAS genes, predominantly N-RAS and K-RAS, have been implicated in the pathogenesis of acute myeloid leukemia (AML), due to activating RAS mutations detectable in approximately 20% of AML patients. In the present study, RAS proteins were detected in their activated, GTP-bound form, in AML patients (n = 10) not expressing mutated forms of H-RAS, K-RAS and N-RAS. Further analysis revealed the simultaneous presence of N-RAS and K-RAS proteins in the GTP-bound state in seven out of 10 AML samples. In four out of 10 samples the levels of RAS-GTP were comparable to an AML cell line (TF-1) with an activating N-RAS mutation (Q61P). The detection of RAS-GTP in AML patients without RAS mutations further supports a functional role of RAS proteins in the pathogenesis of AML and may explain the observed effects of RAS inhibitors in some AML patients in the absence of activating RAS mutations.

Ral is both necessary and sufficient for the inhibition of myeloid differentiation mediated by Ras

Hyperactivation of Ras is one of the most common abnormalities in acute myeloid leukemia. In experimental models, Ras inhibits myeloid differentiation, which is characteristic of leukemia; however, the mechanism through which it disrupts hematopoiesis is poorly understood. In multipotent FDCP-mix cells, Ras inhibits terminal neutrophil differentiation, thereby indefinitely extending their proliferative potential. Ras also strongly promotes the sensitivity of these cells to granulocyte-macrophage colony-stimulating factor (GM-CSF). Using this model, we have dissected the signaling elements downstream of Ras to determine their relative contribution to the dysregulation of hematopoiesis. Cells expressing Ras mutants selectively activating Raf (Ras*T35S) or phosphatidylinositol 3-kinase (Ras*Y40C) did not significantly affect differentiation or proliferative capacity, whereas Ras*E37G (which selectively activates RalGEFs) perpetuated proliferation and blocked neutrophil development in a manner similar to that of Ras. Correspondingly, expression of constitutively active versions of these effectors confirmed the overriding importance of Ral guanine nucleotide exchange factors. Cells expressing Ras demonstrated hyperactivation of Ral, which itself was able to exactly mimic the phenotype of Ras, including hypersensitivity to GM-CSF. Conversely, dominant negative Ral promoted spontaneous neutrophil development. Ral, in turn, appears to influence differentiation through multiple effectors. These data show, for the first time, the importance of Ral in regulating differentiation and self-renewal in hematopoietic cells.

Mapping normal and cancer cell signalling networks: towards single-cell proteomics

Oncogenesis and tumour progression are supported by alterations in cell signalling. Using flow cytometry, it is now possible to track and analyse signalling events in individual cancer cells. Data from this type of analysis can be used to create a network map of signalling in each cell and to link specific signalling profiles with clinical outcomes. This form of 'single-cell proteomics' can identify pathways that are activated in therapy-resistant cells and can provide biomarkers for cancer diagnosis and for determining patient prognosis.

Rituximab (chimeric anti-CD20) sensitizes B-NHL cell lines to Fas-induced apoptosis

Rituximab (chimeric anti-CD20 monoclonal antibodies) is currently being used in the treatment of B non-Hodgkin's lymphoma (NHL). We have recently reported that rituximab triggers and modifies various intracellular signaling pathways in NHL B-cell lines, resulting in reverting the chemoresistant phenotype to a sensitive phenotype. This study investigated whether rituximab also modifies intracellular signaling pathways resulting in the sensitization of NHL cells to Fas-induced apoptosis. Treatment of the Fas-resistant NHL cell lines (2F7, Ramos and Raji) with rituximab sensitized the cells to CH-11 (FasL agonist mAb)-induced apoptosis and synergy was achieved. Fas expression was upregulated by rituximab as early as 6 h post-treatment as determined by flow cytometry, reverse transcriptase-polymerase chain reaction and Western blot. Rituximab inhibited both the expression and activity of the transcription repressor Yin-Yang 1 (YY1) that negatively regulates Fas transcription. Inhibition of YY1 resulted in the upregulation of Fas expression and sensitization of the tumor cells to CH-11-induced apoptosis. The downregulation of YY1 expression was the result of rituximab-induced inhibition of both the p38 mitogen-activated protein kinase (MAPK) signaling pathway and constitutive nuclear factor kappa of B cells (NF-kappaB) activity. The involvement of NF-kappaB and YY1 in the regulation of Fas expression was corroborated by the use of Ramos cells with a dominant-active inhibitor of NF-kappaB (Ramos IkappaB-estrogen receptor (ER) mutant) and by silencing YY1 with YY1 siRNA, respectively. Further, the role of rituximab-mediated inhibition of the p38 MAPK/NF-kappaB/YY1 pathway in the regulation of Fas and sensitization to CH-11-induced apoptosis was validated by the use of specific chemical inhibitors of this pathway and which mimicked rituximab-mediated effects. These findings provide a novel mechanism of rituximab-mediated activity by sensitizing NHL cells to Fas-induced apoptosis.

Cellular and molecular signal transduction pathways modulated by rituximab (rituxan, anti-CD20 mAb) in non-Hodgkin's lymphoma: implications in chemosensitization and therapeutic intervention

The clinical application of rituximab (chimeric mouse anti-human CD20 mAb, Rituxan, IDEC-C2B8), alone and/or combined with chemotherapy, has significantly ameliorated the treatment outcome of patients with relapsed and refractory low-grade or follicular non-Hodgkin's lymphoma (NHL). The exact in vivo mechanisms of action of rituximab are not fully understood, although antibody-dependent cell-mediated cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and apoptosis have been suggested. We have proposed that modifications of the cellular signaling pathways by rituximab may be crucial for its clinical response. The B-cell restricted cell surface phosphoprotein CD20 is involved in many cellular signaling events including proliferation, activation, differentiation, and apoptosis upon crosslinking. Monomeric rituximab chemosensitizes drug-resistant NHL cells via selective downregulation of antiapoptotic factors through the type II mitochondrial apoptotic pathway. Several signaling pathways are affected by rituximab which are implicated in the underlying molecular mechanisms of chemosensitization. ARL (acquired immunodeficiency syndrome (AIDS)-related lymphoma) and non-ARL cell lines have been examined as in vitro model systems. In ARL, rituximab diminishes the activity of the p38MAPK signaling pathway resulting in inhibition of the interleukin (IL)-10/IL-10R autocrine/paracrine cytokine autoregulatory loop leading to the inhibition of constitutive STAT-3 activity and subsequent downregulation of Bcl-2 expression leading to chemosensitization. Rituximab upregulates Raf-1 kinase inhibitor protein (RKIP) expression in non-ARL cells. Through physical association with Raf-1 and nuclear factor kappaB (NF-kappa B)-inducing kinase (NIK), RKIP negatively regulates two major survival pathways, namely, the extracellular signal-regulated kinase1/2 (ERK1/2) and the NF-kappa B pathways, respectively. Downmodulation of the ERK1/2 and NF-kappa B pathways inhibits the transcriptional activity of AP-1 and NF-kappa B transcription factors, respectively, both of which lead to the downregulation of Bcl-(xL) (Bcl-2 related gene (long alternatively spliced variant of Bcl-x gene)) transcription and expression and sensitization to drug-induced apoptosis. Bcl-(xL)-overexpressing cells corroborated the pivotal role of Bcl-(xL) in chemosensitization. The specificity of rituximab-mediated signaling and functional effects were corroborated by the use of specific pharmacological inhibitors. Many patients do not respond and/or relapse and the mechanisms of unresponsiveness are unknown. Rituximab-resistant B-NHL clones were generated to investigate the acquired resistance to rituximab-mediated signaling, and chemosensitization. Resistant clones display different phenotypic, genetic and functional properties compared to wild-type cells. This review summarizes the data highlighting a novel role of rituximab as a signal-inducing antibody and as a chemosensitizing agent through negative regulation of major survival pathways. Studies presented herein also reveal several intracellular targets modified by rituximab, which can be exploited for therapeutic and prognostic purposes in the treatment of patients with rituximab- and drug-refractory NHL.

Blocking the Raf/MEK/ERK pathway sensitizes acute myelogenous leukemia cells to lovastatin-induced apoptosis

The statin family of drugs are well-established inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase and are used clinically in the control of hypercholesterolemia. Recent evidence, from ourselves and others, shows that statins can also trigger tumor-specific apoptosis by blocking protein geranylgeranylation. We and others have proposed that statins disrupt localization and function of geranylgeranylated proteins responsible for activating signal transduction pathways essential for the growth and/or survival of transformed cells. To explore this further, we have investigated whether the mitogen-activated protein kinase (MAPK) signaling cascades play a role in regulating statin-induced apoptosis. Cells derived from acute myelogenous leukemia (AML) are used as our model system. We show that p38 and c-Jun NH2-terminal kinase/stress-activated kinase MAPK pathways are not altered during lovastatin-induced apoptosis. By contrast, exposure of primary and established AML cells to statins results in significant disruption of basal extracellular signal-regulated kinase (ERK) 1/2 phosphorylation. Addition of geranylgeranyl PPi reverses statin-induced loss of ERK1/2 phosphorylation and apoptosis. By establishing and evaluating the inducible Raf-1:ER system in AML cells, we show that constitutive activation of the Raf/MAPK kinase (MEK)/ERK pathway significantly represses but does not completely block lovastatin-induced apoptosis. Our results strongly suggest statins trigger apoptosis by regulating several signaling pathways, including the Raf/MEK/ERK pathway. Indeed, down-regulation of the Raf/MEK/ERK pathway potentiates statin-induced apoptosis because exposure to the MEK1 inhibitor PD98059 sensitizes AML cells to low, physiologically achievable concentrations of lovastatin. Our study suggests that lovastatin, alone or in combination with a MEK1 inhibitor, may represent a new and immediately available therapeutic approach to combat tumors with activated ERK1/2, such as AML.

Subcellular Localization Determines the Protective Effects of Activated ERK2 against Distinct Apoptogenic Stimuli in Myeloid Leukemia Cells

ERKs, mitogen-activated protein kinases, are well characterized as key mediators in the conveyance of signals that promote cell survival in cells of hemopoietic origin, a key factor in the upbringing of leukemogenesis. It is also well known that ERKs phosphorylate a wide array of substrates distributed throughout distinct cellular locations such as the nucleus, cytoplasm, and cell periphery, but the relative contribution of these compartmentalized signal components to the overall survival signal generated by activation of ERKs has yet to be established. To this end, we have utilized constitutively activated forms of ERK2, whose expression is restricted to the nucleus or to the cytoplasm, to investigate the consequences of compartmentalized activation of ERK in the survival of chronic myelogenous leukemia cells subjected to distinct apoptogenic stimuli. We show that cytoplasmic ERK2 activity protected against apoptosis caused by prolonged serum starvation, whereas ERK2 activation restricted to the nucleus antagonized apoptosis induced by the Bcr-Abl inhibitor STI571. On the other hand, neither cytoplasmic nor nuclear ERK2 activities were effective in counteracting apoptosis induced by UV light. These results demonstrate that the protective effects of ERK2 against defined apoptogenic stimuli are strictly dependent on the cellular localization where ERK activation takes place. Furthermore, we present evidence suggesting that the complex I kappa B-NF kappa B participates on ERK2-mediated survival mechanisms, in a fashion dependent on the cellular location where ERK2 is active and on the causative apoptogenic stimulus.

Rituximab inhibits p38 MAPK activity in 2F7 B NHL and decreases IL-10 transcription: pivotal role of p38 MAPK in drug resistance

We have recently reported that Rituximab (anti-CD20) sensitizes drug-resistant 2F7 and 10C9 B Non-Hodgkin's lymphoma (NHL) cell lines to the apoptotic effects of various chemotherapeutic drugs by downregulation of IL-10 and Bcl-2 expression. The mechanism by which Rituximab induces downregulation of IL-10 was examined. We hypothesized that Rituximab may inhibit p38 MAPK activity that regulates IL-10 expression via Sp1. Treatment of 2F7 cells with Rituximab or the p38 inhibitor SB203580 inhibited the constitutive p38 MAPK activity and resulted in the inhibition of Sp1, IL-10, STAT3, and Bcl-2. Inhibition of the Src-family PTKs, Lyn, and Src-family PTKs upstream signaling molecules of the p38MAPK pathway, by PP2, a specific Src-family kinase inhibitor, resulted in the inhibition of p38MAPK and IL-10 expression. In addition to p38 MAPK, Rituximab also inhibited NF-kappaB activity. Inhibition of the Src PTKs, MAPK, and NF-kappaB activities by Rituximab or by specific chemical inhibitors sensitized the cells to CDDP-mediated apoptosis. The above signaling-mediated effects by Rituximab were observed with similar kinetics beginning at 1 h following treatment. Thus, altogether, these results demonstrate that signaling by Rituximab results in the inhibition of the p38MAPK pathway, which in turn inhibits the transcription of IL-10 via Sp1. Inhibition of the IL-10 autocrine/paracrine loop results in the inhibition of STAT3 activity and, consequently, inhibition of Bcl-2 expression and sensitization to drugs-apoptosis. Further, Rituximab-mediated signaling identifies several new intracellular targets in NHL that may be of potential therapeutic interest for the development of new drugs in the treatment of drug-refractory NHL tumor cells.

Map kinase signaling pathways and hematologic malignancies

Mitogen-activated protein (Map) kinases are widely expressed serine-threonine kinases that mediate important regulatory signals in the cell. Three major groups of Map kinases exist: the p38 Map kinase family, the extracellular signal-regulated kinase (Erk) family, and the c-Jun NH2-terminal kinase (JNK) family. The members of the different Map kinase groups participate in the generation of various cellular responses, including gene transcription, induction of cell death or maintenance of cell survival, malignant transformation, and regulation of cell-cycle progression. Depending on the specific family isoform involved and the cellular context, Map kinase pathways can mediate signals that either promote or suppress the growth of malignant hematopoietic cells. Over the last few years, extensive work by several groups has established that Map kinase pathways play critical roles in the pathogenesis of various hematologic malignancies, providing new molecular targets for future therapeutic approaches. In this review, the involvement of various Map kinase pathways in the pathophysiology of hematologic malignances is summarized and the clinical implications of the recent advances in the field are discussed.

Molecular targets in acute myelogenous leukemia

Acute myeloid leukemia (AML) remains the most common form of leukemia and the most common cause of leukemia death. Although conventional chemotherapy can cure between 25 and 45% of AML patients, most patients will either die of relapse or die from the complications associated with treatment. Thus, more specific and less toxic treatments for AML patients are needed. Recently, a small molecular inhibitor (STI571 or Gleevec) that targets the BCR-ABL gene was found to have a dramatic clinical effect in patients with chronic myelogenous leukemia (CML). These results have encouraged investigators to search for additional small molecular inhibitors and other targeted therapies that may be applicable to other forms of leukemia. In this review, we examine some of the signaling pathways that are aberrantly regulated in AML, focusing on the tyrosine kinase/RAS/MAP kinase and JAK/STAT pathways. After reviewing these two pathways, we explore some of the targeted therapies directed at these pathways that are under development for AML, many of which are already in clinical trials.

Mevastatin can increase toxicity in primary AMLs exposed to standard therapeutic agents, but statin efficacy is not simply associated with ras hotspot mutations or overexpression

The 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase is a rate-limiting enzyme in the mevalonate biochemical pathway and HMG-CoA reductase inhibitors (statins) show toxicity for certain tumors, including acute myeloid leukemia (AML). This toxicity has been attributed to statin inhibition of Ras isoprenylation in tumors like AML where oncogenic ras mutations and/or overexpression are common. We show that mevastatin kills certain AML cell lines and is more toxic to a majority of primary AML cell samples than to myeloid cells in bone marrow (BM) samples from normal donors, and that mevastatin can produce more than additive kill with standard chemotherapeutics. Mevastatin reduces Ras membrane localization, but statin sensitivity in primary AML cells is not consistently associated with ras mutations nor with Ras overexpression, suggesting that another mevalonate pathway by-product(s) is the statin target in at least some AMLs.

Targeting the Raf kinase cascade in cancer therapy--novel molecular targets and therapeutic strategies

The mitogen-activated protein kinases (MAPKs) are a group of signal transducers with oncogenic potential in an assortment of cell types. Dysregulated signalling from any of the members of this family has been shown to result in development of human malignancies. Consequently, the collective goal of the scientific community is to inhibit aberrant signalling initiated from these molecules whilst minimising toxicity associated with such inhibition. This review covers events responsible for MAPK activation in detail, with an emphasis placed upon possible points of pharmacological intervention. A discussion addressing numerous chemotherapeutic approaches that have been developed over the previous decade for MAPK inhibition is also included. In addition, emphasis is placed upon the various arrays of kinase inhibitors, small molecule inhibitors, competitive inhibitors, nucleic acid aptamers and other molecules which have been proven effective in prevention of MAPK signalling. Finally, the potential therapeutic promise of many of these compounds is addressed in a manner that encompasses the complexities of MAPK signal transduction, in addition to concerns surrounding the development of drug resistance.

CT53518, a novel selective FLT3 antagonist for the treatment of acute myelogenous leukemia (AML)

Up to 30% of acute myelogenous leukemia (AML) patients harbor an activating internal tandem duplication (ITD) within the juxtamembrane domain of the FLT3 receptor, suggesting that it may be a target for kinase inhibitor therapy. For this purpose we have developed CT53518, a potent antagonist that inhibits FLT3, platelet-derived growth factor receptor (PDGFR), and c-Kit (IC(50) approximately 200 nM), while other tyrosine or serine/threonine kinases were not significantly inhibited. In Ba/F3 cells expressing different FLT3-ITD mutants, CT53518 inhibited IL-3-independent cell growth and FLT3-ITD autophosphorylation with an IC(50) of 10-100 nM. In human FLT3-ITD-positive AML cell lines, CT53518 induced apoptosis and inhibited FLT3-ITD phosphorylation, cellular proliferation, and signaling through the MAP kinase and PI3 kinase pathways. Therapeutic efficacy of CT53518 was demonstrated both in a nude mouse model and in a murine bone marrow transplant model of FLT3-ITD-induced disease.

The Raf/MEK/ERK signal transduction cascade as a target for chemotherapeutic intervention in leukemia

The Raf/MEK/ERK (MAPK) signal transduction cascade is a vital mediator of a number of cellular fates including growth, proliferation and survival, among others. The focus of this review centers on the MAPK signal transduction pathway, its mechanisms of activation, downstream mediators of signaling, and the transcription factors that ultimately alter gene expression. Furthermore, negative regulators of this cascade, including phosphatases, are discussed with an emphasis placed upon chemotherapeutic intervention at various points along the pathway. In addition, mounting evidence suggests that the PI3K/Akt pathway may play a role in the effects elicited via MAPK signaling; as such, potential interactions and their possible cellular ramifications are discussed.

A direct interaction between oncogenic Ha-Ras and phosphatidylinositol 3-kinase is not required for Ha-Ras-dependent transformation of epithelial cells

Cells expressing oncogenic Ras proteins transmit a complex set of signals that ultimately result in constitutive activation of signaling molecules, culminating in unregulated cellular function. Although the role of oncogenic Ras in a variety of cellular responses including transformation, cell survival, differentiation, and migration is well documented, the direct Ras/effector interactions that contribute to the different Ras biological end points have not been as clearly defined. Observations by other groups in which Ras-dependent transformation can be blocked by expression of either dominant negative forms of Phosphatidylinositol (PI) 3-kinase or PTEN, a 3-phosphoinositide-specific phosphatase, support an essential role for PI 3-kinase and its lipid products in the transformation process. These observations coupled with the in vitro observations that the catalytic subunits of PI 3-kinase, the p110 isoforms, bind directly to Ras-GTP foster the implication that a direct interaction between an oncogenic Ras protein and PI 3-kinase are causal in the oncogenicity of mutant Ras proteins. Using an activated Ha-Ras protein (Y64G/Y71G/F156L) that fails to interact with PI 3-kinase, we demonstrate that oncogenic Ha-Ras does not require a direct interaction with PI 3-kinase to support anchorage-independent growth of IEC-6 epithelial cells. We do find, however, that IEC-6 cells expressing an oncogenic Ha-Ras protein that no longer binds PI 3-kinase are greatly impaired in their ability to migrate toward fibronectin.

Cell-cycle-dependent activation of mitogen-activated protein kinase kinase (MEK-1/2) in myeloid leukemia cell lines and induction of growth inhibition and apoptosis by inhibitors of RAS signaling

Disruption of the RAS-to-mitogen-activated protein kinase (MAPK/ERK) signaling pathway, either directly through activating RAS gene mutations or indirectly through other genetic aberrations, plays an important role in the molecular pathogenesis of myeloid leukemias. Constitutive activation of ERK-1/2 and MEK-1/2, which elicit oncogenic transformation in fibroblasts, has recently been observed in acute myeloid leukemias (AML). In this study, the activation of the RAS-to-MAPK cascade in 14 AML and 5 chronic myeloid leukemia (CML) cell lines is examined and correlated with the effects of a panel of 9 RAS signaling inhibitors on cell viability, colony formation, cell-cycle progression, and induction of apoptosis. Activation of MEK, ERK, and the transcription factors CREB-1, ATF-1, and c-Myc is demonstrated in the majority of the cell lines (9 of 14 AML and 2 of 5 CML cell lines). Although activation of the ERK cascade did not always correlate with the presence of activating RAS mutations or BCR-Abl, it is linked to the G0/G1 and the G2/M phase of the cell cycle. In contrast to most inhibitors (eg, B581, Cys-4-Abs-Met, FPT-2, FTI-276, and FTS), a significant growth inhibition was only observed for FTI-277 (19 of 19), FPT-3 (10 of 19), and the MEK inhibitors U0126 (19 of 19) and PD098059 (8 of 19). Treatment of NB-4 cells with FTI-277 primarily resulted in a G2/M block, whereas treatment with FPT-3 and U0126 led to induction of apoptosis. FTI-277 revealed strong toxicity toward normal purified CD34+ cells. The results suggest differences in the mechanisms of action and support a potential therapeutic usefulness of these inhibitors in the treatment of myeloid leukemias.

Pancreatic tumor cells with mutant K-ras suppress ERK activity by MEK-dependent induction of MAP kinase phosphatase-2

Activating mutations within the K-ras gene occur in a high percentage of human pancreatic carcinomas. We reported previously that the presence of oncogenic, activated K-ras in human pancreatic carcinoma cell lines did not result in constitutive activation of the extracellular signal-regulated kinases (ERK1 and ERK2). In the present study, we further characterized the ERK signaling pathway in pancreatic tumor cell lines in order to determine whether the ERK pathway is subject to a compensatory downregulation. We found that the attenuation of serum-induced ERK activation was not due to a delay in the kinetics of ERK phosphorylation. Treatment with the tyrosine phosphatase inhibitor orthovanadate increased the level of ERK phosphorylation, implicating a vanadate-sensitive tyrosine phosphatase in the negative regulation of ERK. Furthermore, expression of a dual specificity phosphatase capable of inactivating ERK known as mitogen-activated protein (MAP) kinase phosphatase-2 (MKP-2) was elevated in most of the pancreatic tumor cell lines and correlated with the presence of active MAP kinase kinase (MEK). Taken together, these results suggest that pancreatic tumor cells expressing oncogenic K-ras compensate, in part, by upregulating the expression of MKP-2 to repress the ERK signaling pathway.

The Raf signal transduction cascade as a target for chemotherapeutic intervention in growth factor-responsive tumors

This review focuses on the Ras-Raf-mitogen-activated protein kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) signal transduction pathway and the consequences of its unregulation in the development of cancer. The roles of some of the cell membrane receptors involved in the activation of this pathway, the G-protein Ras, the Raf, MEK and ERK kinases, the phosphatases that regulate these kinases, as well as the downstream transcription factors that become activated, are discussed. The roles of the Ras-Raf-MEK-ERK pathway in the regulation of apoptosis and cell cycle progression are also analyzed. In addition, potential targets for pharmacological intervention in growth factor-responsive cells are evaluated.