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

Simultaneous activation of multiple signal transduction pathways confers poor prognosis in acute myelogenous leukemia

Deregulation of signal transduction pathways (STPs) may promote leukemogenesis by conferring cell proliferation and survival advantages in acute myelogenous leukemia (AML). Several agents targeting STPs are under development; however, redundancy and cross-talk between STPs could activate multiple downstream effectors and this could negate the effect of single-target inhibition. The frequency of concurrent activation of multiple STPs in AML and the prognostic relevance of STP activation in AML are unknown. STP protein expression (PKCalpha, ERK2, pERK2, AKT, and pAKT) was measured by Western blot in samples from 188 patients with newly diagnosed, untreated AML. In univariate and multivariate analysis high levels of PKCalpha, ERK, pERK, and pAKT, but not AKT, were adverse factors for survival as was the combination variable PKCalpha-ERK2&pERK2-pAKT. Survival progressively decreased as the number of activated pathways increased. Patients were more likely to have none or all 3 pathways activated than was predicted based on the frequency of individual pathway activation, strongly suggesting that cross-activation occurred. Simultaneous activation of multiple STPs is common in AML and has a progressively worse adverse effect on prognosis. It is thus likely that only combinations of agents that target the multiply activated STPs will be beneficial for patients with AML.

Oh8dG induces G1 arrest in a human acute leukemia cell line by upregulating P21 and blocking the RAS to ERK signaling pathway

We reported previously that KG-1, a human acute leukemia cell line, has mutational loss of 8-oxoguanine (8-hydroxyguanine; oh8Gua) glycosylase 1 (OGG1) activity and undergoes apoptotic death after treatment with 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodeoxyguanosine, 8-hydroxydeoxyguanosine; oh8dG). In our present study, we further characterized the effects of oh8dG in KG-1 cells and found that, in addition to apoptosis, oh8dG induced the arrest of KG-1 at the G1 phase. Simultaneously, oh8dG-treated KG-1 showed an increase in the oh8Gua content of DNA, upregulation of p21 (an inhibitor of cdk), and Ras inactivation. Moreover, the upregulation of p21 was followed by the inactivations of cdk4 and cdk2, the hypophosphorylation of Rb, and a marked decline in the expression of c-myc (a gene regulated by E2F that is a transcription factor whose activity is suppressed when it is bound to hypophosphorylated Rb). Ras inactivation was also followed by the inactivation of ERK kinase (MEK) and the inactivation of AP-1, a downstream target of the Ras signaling pathway. The specific MEK inhibitors, PD98059 and U0126, also induced G1 arrest. These findings suggest that p21 upregulation and Ras inactivation contribute to G1 arrest. An increase of oh8Gua content in DNA does not seem to be a principal contributor to G1 arrest, however, because the kinetics of increases of oh8Gua content in DNA and of G1 cell number did not coincide. We report that oh8dG induces the arrest of KG-1 growth at the G1 phase mainly by upregulating p21 and inactivating Ras.

Quantifying ERK2–protein interactions by fluorescence anisotropy: PEA-15 inhibits ERK2 by blocking the binding of DEJL domains

While mitogen-activated protein kinase signaling pathways constitute highly regulated networks of protein-protein interactions, little quantitative information for these interactions is available. Here we highlight recent fluorescence anisotropy binding studies that focus on the interactions of ERK1 and ERK2 with PEA-15 (antiapoptotic phosphoprotein enriched in astrocytes-15 kDa), a small protein that sequesters ERK2 in the cytoplasm. The regulation of ERK2 by PEA-15 is appraised in the light of a simple equilibrium-binding model for reversible ERK2 nucleoplasmic-cytoplasmic shuttling, which elaborates on the theory of Burack and Shaw (J. Biol. Chem. 280, 3832-3837; 2005). Also highlighted is the recent observation that the peptide N-QKGKPRDLELPLSPSL-C, derived from the docking site for ERK/JNK and LEL (DEJL) in Elk-1, displaces PEA-15 from ERK2. It is proposed that the C-terminus of PEA-15 ((121)LXLXXXXKK(129)) is a reverse DEJL domain [which has a general consensus of R/K-phi(A)-X(3/4)-phi(B), where phi(A) and phi(B) are hydrophobic residues (Leu, Ile, or Val)], which mediates one arm of a bidentate PEA-15 interaction with ERK2. The notion that PEA-15 is a potent inhibitor of many ERK2-mediated phosphorylations, by virtue of its ability to block ERK2-DEJL domain interactions, is proposed.

Oncogenes as novel targets for cancer therapy (part II): Intermediate signaling molecules

This is the second part of a four-part review on potential therapeutic targeting of oncogenes. The previous part introduced the new technologies responsible for the advancement of oncogene identification, target validation, and drug design. Because of such advances, new specific and more efficient therapeutic agents can be developed for cancer. This part of the review continues the exploration of various oncogenes, which we have grouped within seven categories: growth factors, tyrosine kinases, intermediate signaling molecules, transcription factors, cell cycle regulators, DNA damage repair genes, and genes involved in apoptosis. Part I included a discussion of growth factors and tyrosine kinases. This portion of the review covers intermediate signaling molecules and the various strategies used to inhibit their expression or decrease their activities.

A phase I clinical trial of 12- O-tetradecanoylphorbol-13-acetate for patients with relapsed/refractory malignancies

Phorbol esters activate protein kinase C and modulate a variety of downstream cell signaling pathways. 12-O-tetradecanoylphorbol-13-acetate (TPA) is a phorbol ester that induces differentiation or apoptosis in a variety of cell lines at low concentrations. A phase I dose escalation trial of TPA was undertaken for patients with relapsed or refractory malignancies. The starting dose was 0.063 mg/m2 and most patients were treated with an intravenous infusion of TPA on days 1-5 and 8-12 followed by a 2-week rest period prior to retreatment. Thirty-five patients were treated. A biological assay was used to monitor levels of TPA-like activity in the blood after treatment. Serious adverse events included individual episodes of gross hematuria, a grand mal seizure, syncope, and hypotension. Many patients had transient fatigue, mild dyspnea, fever, rigors, and muscular aches shortly after the infusion. Dose-limiting toxicities included syncope and hypotension at a dose of 0.188 mg/m2. Only a single patient had evidence of tumor response. These studies establish 0.125 mg/m2 as the maximally tolerated dose when TPA is administered on this schedule.

12-O-Tetradecanoylphorbol-13-acetate (TPA)-induced dual-specificity phosphatase expression and AML cell survival

12-O-Tetradecanoylphorbol-13-acetate (TPA) is being developed as a therapeutic agent by virtue of its being a potent modulator of signal transduction in pre-clinical models of AML [Strair RK, Schaar D, Goodell L, Aisner J, Chin KV, Eid J, et al. Administration of a phorbol ester to patients with hematological malignancies: preliminary results from a phase I clinical trial of 12-O-tetradecanoylphorbol-13-acetate. Clin Cancer Res 2002;8:2512-8]. In this report, we identify a subset of primary AML samples that undergoes apoptosis after exposure to TPA and demonstrate that TPA-induced cytotoxicity is associated with modulation of the ERK signaling pathway. Analysis of mitogen-activated protein kinase (MAPK) dual-specificity phosphatases (DUSP), as potential regulators of AML cell signaling, indicates that these genes are coordinately regulated and rapidly induced by TPA in primary AML cells. Therefore, TPA-induced primary AML cytotoxicity is associated with modulation of ERK signaling which may be partially mediated by regulation of phosphatase expression.

Genetics of Myeloid Malignancies: Pathogenetic and Clinical Implications

Myeloid malignancies are clonal disorders that are characterized by acquired somatic mutation in hematopoietic progenitors. Recent advances in our understanding of the genetic basis of myeloid malignancies have provided important insights into the pathogenesis of acute myeloid leukemia (AML) and myeloproliferative diseases (MPD) and have led to the development of novel therapeutic approaches. In this review, we describe our current state of understanding of the genetic basis of AML and MPD, with a specific focus on pathogenetic and therapeutic significance. Specific examples discussed include RAS mutations, KIT mutations, FLT3 mutations, and core binding factor rearrangements in AML, and JAK2 mutations in polycythemia vera, essential thrombocytosis, and chronic idiopathic myelofibrosis.

Combining prenylation inhibitors causes synergistic cytotoxicity, apoptosis and disruption of RAS-to-MAP kinase signalling in multiple myeloma cells

The high incidence of activating RAS mutations, coupled with accumulating evidence linking RAS to multiple myeloma (MM) pathogenesis, indicate that novel therapies utilising inhibitors of RAS prenylation and signalling may be successful in the management of this disease. While preclinical studies investigating prenylation inhibitors, such as lovastatin, farnesyltransferase inhibitors (FTI) and geranylgeranyltransferase inhibitors (GGTI), have been promising, recent phase I/II clinical trials with FTI R115777 were disappointing, suggesting resistance to FTI monotherapy. To address this issue, the effects of FTI, GGTI and lovastatin alone and in combination were analysed in MM cell lines and primary cells. FTI treatment blocked H-RAS processing, but was ineffective at inhibiting K- and N-RAS prenylation because of alternative geranylgeranylation of these isoforms. However, combinations of FTI and GGTI or lovastatin were found to synergistically inhibit MM cell proliferation, migration, K- and N-RAS processing, RAS-to-mitogen-activated protein kinase signalling and to induce apoptosis. In contrast to FTI, lovastatin and some GGTI were found to cause intracellular accumulation of Rho proteins. Our results suggest that clinical efficacy of prenylation inhibitors in MM are limited by alternative prenylation of several small G-proteins, such as RhoB, K- and N-RAS. Furthermore, strategies combining FTI with GGTI or statins may provide greater efficacy in MM treatment.

Identification of Ki23819, a highly potent inhibitor of kinase activity of mutant FLT3 receptor tyrosine kinase

Constitutively active internal tandem duplication (ITD) in the juxtamembrane domain of Fms-like tyrosine kinase 3 (FLT3), a type III receptor tyrosine kinase, is the most common molecular defect associated with acute myeloid leukemia. Its presence confers a poor outcome in patients with acute myeloid leukemia who receive conventional chemotherapy. FLT3-ITD has therefore been considered to be an attractive molecular target for a novel therapeutic modality. We describe here the identification and characterization of Ki23819 as a novel FLT3 inhibitor. Ki23819 suppressed proliferation and induced apoptosis of FLT3-ITD-expressing human leukemia cell lines. The growth-inhibitory effect of Ki23819 on MV4-11 cells was superior to that of SU11248, another FLT3 inhibitor (IC(50)<1 vs 3-10 nM). Ki23819 inhibited the autophosphorylation of FLT3-ITD more efficiently than that of wild-type FLT3. FLT3-ITD-dependent activation of the downstream signaling proteins ERK and STAT5 was also inhibited within similar concentration ranges. Thus, Ki23819 is a potent in vitro inhibitor of FLT3.

Inactivation of MAP kinase signalling in Myc transformed cells and rescue by LiCl inhibition of GSK3

c-Myc oncogene is an important regulator of cell cycle and apoptosis, and its dysregulated expression is associated with many malignancies. Myc is instrumental in directly or indirectly regulating the progression through the G1 phase and G1/S transition, and transformation by Myc results in perturbed cell cycle. Also contributory to the control of G1 is the Ras effector pathway Raf/MEK/ERK MAP kinase. Together with GSK3, ERK plays an important role in the critical hierarchical phosphorylation of S62/T58 controlling Myc protein levels. Therefore, our main aim was to examine the levels of MAPK in Myc transformed cells in light of the roles of ERK in cell cycle and control of Myc protein levels. We found that active forms of ERK were barely detectable in v-Myc (MC29) transformed cells. Furthermore, we could only detect reduced levels of activated ERK in c-Myc transformed cells compared to the non-transformed primary chick embryo fibroblast cells. The addition of LiCl inhibited GSK3 and successfully restored the levels of ERK in v-Myc and c-Myc transformed cells to those found in non-transformed cells. In addition, LiCl stabilised Myc protein in the non-transformed and c-Myc transformed cells but not in v-Myc transformed cells. These results can provide an important insight into the role of MAPK in the mechanism of Myc induced transformation and carcinogenesis.

A phase I trial of the dual farnesyltransferase and geranylgeranyltransferase inhibitor L-778,123 and radiotherapy for locally advanced pancreatic cancer

PURPOSE

Preclinical and clinical studies have demonstrated that inhibition of prenylation can radiosensitize cell lines with activation of Ras and produce clinical response in patients with cancer. The aim of this study was to determine the maximally tolerated dose of the dual farnesyltransferase and geranylgeranyltransferase I inhibitor L-778,123 in combination with radiotherapy for patients with locally advanced pancreatic cancer.

EXPERIMENTAL DESIGN

L-778,123 was given by continuous intravenous infusion with concomitant radiotherapy to 59.4 Gy in standard fractions. Two L-778,123 dose levels were tested: 280 mg/m2/day over weeks 1, 2, 4, and 5 for dose level 1; and 560 mg/m2/day over weeks 1, 2, 4, 5, and 7 for dose level 2.

RESULTS

There were no dose-limiting toxicities observed in the eight patients treated on dose level 1. Two of the four patients on dose level 2 experienced dose-limiting toxicities consisting of grade 3 diarrhea in one case and grade 3 gastrointestinal hemorrhage associated with grade 3 thrombocytopenia and neutropenia in the other case. Other common toxicities were mild neutropenia, dehydration, hyperglycemia, and nausea/vomiting. One patient on dose level 1 showed a partial response of 6 months in duration. Both reversible inhibition of HDJ2 farnesylation and radiosensitization of a study patient-derived cell line were demonstrated in the presence of L-778,123. K-RAS mutations were found in three of the four patients evaluated.

CONCLUSIONS

The combination of L-778,123 and radiotherapy at dose level 1 showed acceptable toxicity in patients with locally advanced pancreatic cancer. Radiosensitization of a patient-derived pancreatic cancer cell line was observed.

Arsenic trioxide (ATO) and MEK1 inhibition synergize to induce apoptosis in acute promyelocytic leukemia cells

Recent studies suggest that components of the prosurvival signal transduction pathways involving the Ras-mitogen-activated protein kinase (MAPK) can confer an aggressive, apoptosis-resistant phenotype to leukemia cells. In this study, we report that acute promyelocytic leukemia (APL) cells exploit the Ras-MAPK activation pathway to phosphorylate at Ser112 and to inactivate the proapoptotic protein Bad, delaying arsenic trioxide (ATO)-induced apoptosis. Both in APL cell line NB4 and in APL primary blasts, the inhibition of extracellular signal-regulated kinases 1/2 (ERK1/2) and Bad phosphorylation by MEK1 inhibitors enhanced apoptosis in ATO-treated cells. We isolated an arsenic-resistant NB4 subline (NB4-As(R)), which showed stronger ERK1/2 activity (2.7-fold increase) and Bad phosphorylation (2.4-fold increase) compared to parental NB4 cells in response to ATO treatment. Upon ATO exposure, both NB4 and NB4-As(R) cell lines doubled protein levels of the death antagonist Bcl-xL, but the amount of free Bcl-xL that did not heterodimerize with Bad was 1.8-fold greater in NB4-As(R) than in the parental line. MEK1 inhibitors dephosphorylated Bad and inhibited the ATO-induced increase of Bcl-xL, overcoming ATO resistance in NB4-As(R). These results may provide a rationale to develop combined or sequential MEK1 inhibitors plus ATO therapy in this clinical setting.

Microarray analysis reveals genetic pathways modulated by tipifarnib in acute myeloid leukemia

BACKGROUND

Farnesyl protein transferase inhibitors (FTIs) were originally developed to inhibit oncogenic ras, however it is now clear that there are several other potential targets for this drug class. The FTI tipifarnib (ZARNESTRA, R115777) has recently demonstrated clinical responses in adults with refractory and relapsed acute leukemias. This study was conducted to identify genetic markers and pathways that are regulated by tipifarnib in acute myeloid leukemia (AML).

METHODS

Tipifarnib-mediated gene expression changes in 3 AML cell lines and bone marrow samples from two patients with AML were analyzed on a cDNA microarray containing approximately 7000 human genes. Pathways associated with these expression changes were identified using the Ingenuity Pathway Analysis tool.

RESULTS

The expression analysis identified a common set of genes that were regulated by tipifarnib in three leukemic cell lines and in leukemic blast cells isolated from two patients who had been treated with tipifarnib. Association of modulated genes with biological functional groups identified several pathways affected by tipifarnib including cell signaling, cytoskeletal organization, immunity, and apoptosis. Gene expression changes were verified in a subset of genes using real time RT-PCR. Additionally, regulation of apoptotic genes was found to correlate with increased Annexin V staining in the THP-1 cell line but not in the HL-60 cell line.

CONCLUSIONS

The genetic networks derived from these studies illuminate some of the biological pathways affected by FTI treatment while providing a proof of principle for identifying candidate genes that might be used as surrogate biomarkers of drug activity.

The Hematopoietic Stem Cell in Myelodysplasia

The mechanisms underlying hematopoietic stem cell or progenitor cell abnormalities in myelodysplastic syndromes (MDSs) remain poorly characterized. Current evidence exists for multiple intrinsic and extrinsic influences upon the stem cell in these disorders. These influences are outlined in this review and include: stem cell characteristics in MDSs, as compared with those in acute myelogenous leukemia; the role of increased apoptosis; the role of signaling pathway abnormalities; the influences of immune modulation; and the effect of stromal cells and stromal cell cytokine production. Despite numerous studies that have examined these factors, how they converge to produce a situation in which accelerated proliferation and accelerated death occur simultaneously remains largely an unexplored area. It is anticipated that future studies that focus on well-characterized and purified progenitor populations in these disorders will elucidate the process by which ineffective hematopoiesis results from the influences of stem cell abnormalities versus abnormalities in the stem cell's microenvironmental and immunologic milieu.

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.

An investigation of the MEK/ERK inhibitor U0126 in acute myeloid leukemia

Blockade of mitogen-activated protein kinase kinase (MEK1/2), part of the extracellular signal-regulated kinase (ERK) or p44/42 mitogen-activated protein kinase (MAPK) pathway, has been shown in some instances to cause apoptosis in leukemic blast cells. This investigation examined the effect of the potent MEK/ERK inhibitor U0126 on apoptosis in acute myeloblastic leukemia (AML) cell lines, and acute leukemic and non-leukemic patient samples. The pro-apoptotic effect of the inhibitor varied across the five cell lines tested (KG1a, HEL, TF-1, MO7e, and THP-1) from highly significant induction of apoptosis to no apparent response. The pro-apoptotic effect of U0126 in the most sensitive cell line, KG1a, appeared to be related to its CD34 positivity. Three of five leukemic bone marrow samples showed considerable sensitivity to the inhibitor and a similar association with CD34 expression was evident. Interestingly, control marrow cells from six non-leukemic patients did not show a significant effect when exposed to U0126. These results suggest that this agent may offer a potential alternative to standard chemotherapy with a particular role in the most primitive types of leukemia, these often being the most resistant to standard chemotherapy.

BCR/ABL kinase inhibition by imatinib mesylate enhances MAP kinase activity in chronic myelogenous leukemia CD34+ cells

Chronic myelogenous leukemia (CML) results from malignant transformation of a primitive hematopoietic cell by the BCR/ABL oncogene. The breakpoint cluster region/ABL (BCR/ABL) tyrosine kinase inhibitor imatinib mesylate (imatinib) is highly effective in inducing remissions in CML. However, the effects of imatinib on intracellular signaling in primary progenitor cells are not well described. We show that imatinib exposure resulted in a significant dose-responsive reduction in BCR/ABL kinase activity in CML CD34+ cells. However, imatinib treatment resulted in an increase in activity of p42/44 mitogen-activated protein kinase (MAPK), an important downstream effector of BCR/ABL. Increased MAPK activity was growth factor dependent. Pharmacologic inhibition of MAPK using MAPK/extracellular signal–regulated kinase kinase–1/2 (MEK-1/2) inhibitors significantly reduced CML progenitor proliferation. Combined treatment with a MEK-1/2 inhibitor and imatinib significantly increased suppression of CML progenitors compared with either inhibitor alone. In contrast, imatinib treatment resulted in a small reduction in AKT activity. Combined treatment with a phosphatidylinositol-3 (PI-3) kinase inhibitor and imatinib significantly increased suppression of CML progenitor growth compared with either inhibitor alone. We conclude that inhibition of BCR/ABL kinase activity in CML progenitors by imatinib results in a growth factor-dependent compensatory increase in MAPK activity and in only partial inhibition of PI-3 kinase activity. These mechanisms may contribute to incomplete elimination of CML progenitors by imatinib. (Blood. 2004;103:3167-3174)

Downmodulation of ERK protein kinase activity inhibits VEGF secretion by human myeloma cells and myeloma-induced angiogenesis

The mitogen-activated protein (MAP) cascade leading to the activation of extracellular signal-regulated kinases 1/2 (ERK1/2) is critical for regulating myeloma cell growth; however, the relationship of ERK1/2 activity with vascular endothelial growth factor (VEGF) production and the effects of its downmodulation in myeloma cells are not elucidated. We found that the treatment with MAP/ERK kinase 1 (MEK1) inhibitors PD98059 or PD184352 produced a reduction of phosphorylated ERK1/2 (p-ERK1/2) levels in myeloma cells of more than 80% and prevented the increase of p-ERK1/2 induced by interleukin-6 (IL-6). MEK1 inhibitors also induced a significant inhibition of myeloma cell proliferation and blunted the stimulatory effect induced by IL-6. A significant inhibition of basal VEGF secretion by myeloma cells as well as a suppression of the stimulatory effect of IL-6 on VEGF was observed by either PD98059 or PD184352. Moreover, we also found that the PI3K kinase inhibitors, but not p38 MAPK inhibitors, reduced VEGF secretion by myeloma cells and increase the inhibitory effect of MEK1 inhibitors. In an 'in vitro' model of angiogenesis, we found that MEK1 inhibitors impair vessel formation induced by myeloma cells and restored by VEGF treatment, suggesting that the downmodulation of ERK1/2 activity reduces myeloma-induced angiogenesis by inhibiting VEGF secretion.

JAK/STAT, Raf/MEK/ERK, PI3K/Akt and BCR-ABL in cell cycle progression and leukemogenesis

The roles of the JAK/STAT, Raf/MEK/ERK and PI3K/Akt signal transduction pathways and the BCR-ABL oncoprotein in leukemogenesis and their importance in the regulation of cell cycle progression and apoptosis are discussed in this review. These pathways have evolved regulatory proteins, which serve to limit their proliferative and antiapoptotic effects. Small molecular weight cell membrane-permeable drugs that target these pathways have been developed for leukemia therapy. One such example is imatinib mesylate, which targets the BCR-ABL kinase as well as a few structurally related kinases. This drug has proven to be effective in the treatment of CML patients. However, leukemic cells have evolved mechanisms to become resistant to this drug. A means to combat drug resistance is to target other prominent signaling components involved in the pathway or to inhibit BCR-ABL by other mechanisms. Treatment of imatinib-resistant leukemia cells with drugs that target Ras (farnysyl transferase inhibitors) or with the protein destabilizer geldanamycin has proven to be a means to inhibit the growth of resistant cells. This review will tie together three important signal transduction pathways involved in the regulation of hematopoietic cell growth and indicate how their expression is dysregulated by the BCR-ABL oncoprotein.

Distinctive regulation and function of PI 3K/Akt and MAPKs in doxorubicin-induced apoptosis of human lung adenocarcinoma cells

Regulation and function of PI 3K/Akt and mitogen-activated protein kinases (MAPKs) in doxorubicin-induced cell death were investigated in human lung adenocarcinoma cells. Doxorubicin induced dose-dependent apoptosis of human lung adenocarcinoma NCI-H522 cells. Prior to cell death, both Akt and the MAPK family members (MAPKs: ERK1/2, JNK, and p38) were activated in response to the drug treatment. The kinetics of the inductions for Akt and MAPKs are, however, distinct. The activation of Akt was rapid and transient, activated within 30 min of drug addition, then declined after 3 h, whereas the activations of three MAPKs occurred later, 4 h after addition of the drug and sustained until cell death occurred. Inhibition of PI 3K/Akt activation had no effect on MAPKs' activation, suggesting that the two pathways are independently activated in response to the drug treatment. Inhibition of PI 3K/Akt and p38 accelerated and enhanced doxorubicin-induced cell death. On the contrary, inhibition of ERK1/2 or JNK had no apparent effect on the cell death. Taken together, these results suggest that PI 3K/Akt and MAPKs signaling pathways are all activated, but with distinct mechanisms, in response to doxorubicin treatment. Activation of PI 3K/Akt and p38 modulates apoptotic signal pathways and inhibits doxorubicin-induced cell death. These responses of tumor cells to cancer drug treatment may contribute to their drug resistance. Understanding of the mechanism and function of the responses will be beneficial for the development of novel therapeutic approaches for improvement of drug efficacy and circumvention of drug resistance.

Bcr-Abl regulates protein kinase Ciota (PKCiota) transcription via an Elk1 site in the PKCiota promoter

The protein kinase C (PKC) family of serine/threonine kinases plays an important role in numerous cancer signaling pathways, including those downstream of the bcr-abl oncogene. We demonstrated previously that atypical PKCiota is required for Bcr-Abl-mediated resistance of human K562 chronic myelogenous leukemia (CML) cells to Taxol-induced apoptosis. Here, we report that the pattern of PKC isozyme expression characteristic of CML cells is regulated by Bcr-Abl. When Bcr-Abl was expressed in Bcr-Abl-negative HL-60 promyelocytic leukemia cells, expression of the PKCbetaI, PKCbetaII, and PKCiota genes was induced, whereas expression of the PKCdelta gene was reduced to levels similar to those found in CML cells. Given the importance of PKCiota in Bcr-Abl-mediated transformation, we characterized the mechanism by which Bcr-Abl regulates PKCiota expression. A 1200-bp PKCiota promoter construct isolated from genomic DNA was highly active in Bcr-Abl-positive K562 cells and was activated when Bcr-Abl-negative cells were transfected with Bcr-Abl. Bcr-Abl-mediated induction of the PKCiota promoter was dependent upon MEK1/2 activity, but not phosphatidylinositol 3-kinase or p38 MAPK activity. Mutational analysis of the PKCiota promoter revealed a region between 97 and 114 bp upstream of the transcriptional start site that is responsible for Bcr-Abl-mediated regulation. Mutation of a consensus Elk1-binding site within this region abolished Bcr-Abl-mediated regulation. We conclude that Bcr-Abl regulates PKCiota expression through the MEK-dependent activation of an Elk1 element within the proximal PKCiota promoter. Our results indicate that Bcr-Abl-mediated transformation involves transcriptional activation of the PKCiota gene, which in turn is required for Bcr-Abl-mediated chemoresistance.

Farnesyltransferase inhibitors in hematologic malignancies: new horizons in therapy

Farnesyltransferase inhibitors (FTIs) are small-molecule inhibitors that selectively inhibit farnesylation of a number of intracellular substrate proteins such as Ras. Preclinical work has revealed their ability to effectively inhibit tumor growth across a wide range of malignant phenotypes. Many hematologic malignancies appear to be reasonable disease targets, in that they express relevant biologic targets, such as Ras, mitogen-activated protein kinase (MAPK), AKT, and others that may depend on farnesyl protein transferase (FTase) activity to promote proliferation and survival. A host of phase 1 trials have been recently launched to assess the applicability of FTIs in hematologic malignancies, many of which demonstrate effective enzyme target inhibition, low toxicity, and some clinical responses. As a result, phase 2 trials have been initiated in a variety of hematologic malignancies and disease settings to further validate clinical activity and to identify downstream signal transduction targets that may be modified by these agents. It is anticipated that these studies will serve to define the optimal roles of FTIs in patients with hematologic malignancies and provide insight into effective methods by which to combine FTIs with other agents.

Downmodulation of ERK activity inhibits the proliferation and induces the apoptosis of primary acute myelogenous leukemia blasts

MAP kinase/ERK kinase (MEK)-extracellular signal-regulated kinase (ERK) kinases are frequently activated in acute myelogenous leukemia (AML), and can have prosurvival function. The purpose of this study was to induce downmodulation of MEK-ERK activation in AML primary blasts in order to detect the effect on cell cycle progression and on the apoptosis of leukemic cells. We investigated 14 cases of AML with high ERK 1/2 activity and four cases with undetectable or very low activity. After 24 h incubation of the AML blasts with high ERK activity using PD98059 (New England BioLabs, Beverly, MA, USA), a selective inhibitor of MEK1 phosphorylation, at concentrations of 20 and 40 microM, we observed a strong decrease in the levels of ERK1/2 activity. A significant decrease of blast cell proliferation compared with untreated controls was found. In contrast, the proliferation of blast cells that expressed low or undetectable levels of ERK activity was not inhibited. Time-course analysis demonstrated that the downmodulation of MEK1/2, ERK1 and ERK2 dual-phosphorylation was evident even after 3 h of treatment with 20 and 40 microM. The cleavage of poly(ADP-ribose) polymerase (PARP), an early sign of apoptosis, appeared after 18 h of PD98059 treatment at concentrations of 20 and 40 microM in eight of the 14 cases. After 24 h of treatment, cleaved PARP appeared in all 14 cases. Time-course analysis of cell cycle progression and apoptosis showed that PD98059 induced a G1-phase accumulation with low or undetectable levels of apoptosis after 24 h incubation; after 48 and 72 h incubation, a significant increase of apoptosis was observed. Thus, the primary effect of ERK downmodulation was a cell cycle arrest followed by the apoptosis of a significant percentage of the leukemic blasts. The preclinical model of leukemia treatment reported in this paper makes further comment with regard to MEK1 inhibition as a useful antileukemic target, and encourages the conducting of in vivo studies and clinical investigations.

Involvement of nitric oxide in farnesyltransferase inhibitor-mediated apoptosis in chronic myeloid leukemia cells

The mechanism of action of farnesyltransferase inhibitors (FTIs) has not been fully clarified. We investigated the cytotoxic effects of various FTIs in chronic myeloid leukemia (CML), using LAMA cells and marrow cells from 40 CML patients in chronic phase. FTI-mediated cytotoxic effect was observed in LAMA cells and in 65% of primary CML cells, whereas marrow cells from controls were only weakly affected. Cytotoxic effects were partially related to enhanced apoptosis; however, Fas-receptor (FasR) and Fas-ligand (FasL) expression were not modified by FTIs. Susceptibility to FTI-mediated inhibition did not correlate with FasR/FasL expression in CD34+ CML cells. Moreover, intra-cellular activation of caspase-1 and -8 were not altered by FTIs, and their blockade did not reverse FTI toxicity. However, we observed FTI-induced activation of caspase-3, and its inhibition partially reverted FTI-induced apoptosis. FTIs did not modulate bcl2, bclxL, and bclxS expression, whereas they increased inducible nitric oxide (iNOS) mRNA and protein levels, resulting in higher NO production. Furthermore, C3 exoenzyme, a Rho inhibitor, significantly increased iNOS expression in CML cells, suggesting that FTIs may up-regulate NO formation at least partially through FTI-mediated inhibition of Rho. We conclude that FTIs induce selective apoptosis in CML cells via activation of iNOS and caspase-3.

MEK/ERK pathway is aberrantly active in Hodgkin disease: a signaling pathway shared by CD30, CD40, and RANK that regulates cell proliferation and survival

The mitogen-activated protein kinase (MAPK) (also called extracellular signal-regulated kinase [ERK]) pathway has been implicated in malignant transformation and in the regulation of cellular growth and proliferation of several tumor types, but its expression and function in Hodgkin disease (HD) are unknown. We report here that the active phosphorylated form of MAPK/ERK is aberrantly expressed in cultured and primary HD cells. Inhibition of the upstream MAPK kinase (also called MEK) by the small molecule UO126 inhibited the phosphorylation of ERK and demonstrated a dose- and time-dependent antiproliferative activity in HD cell lines. UO126 modulated the levels of several intracellular proteins including B-cell lymphoma protein 2 (Bcl-2), myeloid cell leukemia-1 (Mcl-1) and caspase 8 homolog FLICE-inhibitory protein (cFLIP), and induced G2M cell-cycle arrest or apoptosis. Furthermore, UO126 potentiated the activity of apoliprotein 2/tumor necrosis factor-related apoptosis-inducing ligand (APO2L/TRAIL) and chemotherapy-induced cell death. Activation of CD30, CD40, and receptor activator of nuclear kappabeta (RANK) receptors in HD cells by their respective ligands increased ERK phosphorylation above the basal level and promoted HD cell survival. UO126 inhibited basal and ligand-induced ERK phosphorylation, and inhibited ligand-induced cell survival of HD cell lines. These findings provide a proof-of-principle that inhibition of the MEK/ERK pathway may have therapeutic value in HD.

Therapeutic efficacy of prenylation inhibitors in the treatment of myeloid leukemia

Farnesyltransferase inhibitors (FTIs) represent a new class of anticancer agents that specifically target post-translational farnesylation of various proteins that mediate several cellular processes such as signal transduction, growth, differentiation, angiogenesis and apoptosis. These compounds were originally designed to block oncogenic RAS-induced tumor growth by impeding RAS localization to the membrane, but it is now evident that FTIs also affect processing of several other proteins. The need for novel therapies in myeloid leukemia is underscored by the high rate of treatment failure due to high incidences of relapse- and treatment-related toxicities. As RAS deregulation is important in the pathogenesis of myeloid leukemias, targeting of RAS signaling may provide a new therapeutic strategy. Several FTIs (eg BMS-214662, L-778,123, R-115777 and SCH66336) have entered phase I and phase II clinical trials in myeloid leukemias. This review discusses recent clinical results, potential combination therapies, mechanisms of resistance and the clinical challenges of toxicities associated with prenylation inhibitors.

Synergistic cytotoxic effects in myeloid leukemia cells upon cotreatment with farnesyltransferase and geranylgeranyl transferase-I inhibitors

As deregulation of RAS signaling is important in the pathogenesis of myeloid leukemias, molecular targeting of RAS signaling may be a promising therapeutic strategy. Farnesyl transferase inhibitors (FTIs) are the most promising class of these new cancer therapeutics. Several FTIs have entered phase II clinical trials in acute myeloid leukemia (AML). Since geranylgeranylation of K-RAS and N-RAS in the presence of FTIs may represent an important mechanism of FTI resistance, 6 geranylgeranyl transferase-I inhibitors (GGTIs) were screened alone and in combination with FTI for growth inhibition of myeloid leukemia cells. Significant growth inhibition (>70%) in myeloid cell lines was observed for GGTI-286 (9/19), GGTI-298 (14/19), GGTI-2147 (16/19) and FTI L-744,832 (17/17). GGTI treatment of NB-4 cells resulted in an accumulation of cells in G(0)/G(1), whereas FTI L-744,832 primarily caused an increase in G(2)/M. FTI and GGTIs both induced apoptosis. In all cases, FTI/GGTI cotreatment led to synergistic cytotoxic effects in both myeloid cell lines (5/5) and primary AML cells (6/6). This synergy coincided with increased apoptosis. FTI/GGTI cotreatment caused an accumulation of unprocessed N-RAS and inactive N-RAS-RAF complexes. Our results suggest that alternative geranylgeranylation of N-RAS may represent an important mechanism of resistance to FTI monotherapy in myeloid leukemia cells.

Signal transduction mediated by the Ras/Raf/MEK/ERK pathway from cytokine receptors to transcription factors: potential targeting for therapeutic intervention

The Ras/Raf/Mitogen-activated protein kinase/ERK kinase (MEK)/extracellular-signal-regulated kinase (ERK) cascade couples signals from cell surface receptors to transcription factors, which regulate gene expression. Depending upon the stimulus and cell type, this pathway can transmit signals, which result in the prevention or induction of apoptosis or cell cycle progression. Thus, it is an appropriate pathway to target for therapeutic intervention. This pathway becomes more complex daily, as there are multiple members of the kinase and transcription factor families, which can be activated or inactivated by protein phosphorylation. The diversity of signals transduced by this pathway is increased, as different family members heterodimerize to transmit different signals. Furthermore, additional signal transduction pathways interact with the Raf/MEK/ERK pathway to regulate positively or negatively its activity, or to alter the phosphorylation status of downstream targets. Abnormal activation of this pathway occurs in leukemia because of mutations at Ras as well as genes in other pathways (eg PI3K, PTEN, Akt), which serve to regulate its activity. Dysregulation of this pathway can result in autocrine transformation of hematopoietic cells since cytokine genes such as interleukin-3 and granulocyte/macrophage colony-stimulating factor contain the transacting binding sites for the transcription factors regulated by this pathway. Inhibitors of Ras, Raf, MEK and some downstream targets have been developed and many are currently in clinical trials. This review will summarize our current understanding of the Ras/Raf/MEK/ERK signal transduction pathway and the downstream transcription factors. The prospects of targeting this pathway for therapeutic intervention in leukemia and other cancers will be evaluated.

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.

Chronic myeloid leukemia

Chronic myeloid leukemia (CML) was the first human malignancy to be associated with a specific genetic lesion, the Philadelphia chromosome, harboring the BCR-ABL oncogene. Since then, it has become a paradigm for the discovery of molecular mechanisms and targeted therapeutic approaches in the field of hematologic neoplasias. The past 5 years or so have been particularly fruitful in the dissection of the signal transduction pathways abnormally activated in CML and in the translation of this knowledge to clinical practice. In this report, we discuss the biological basis for such translation and highlight the current and potential tools for the effective treatment of CML patients. The first part presents a review of the basic concepts on the biology of CML and their application to the design of targeted therapy. The mechanisms of action of the molecular-specific drugs currently used in clinical trials are discussed, with emphasis on the description of the most promising new compounds that are enhancing the potential for effective alternative or combination chemotherapy in CML. In the following section, we explain how molecular monitoring of response to imatinib mesylate in patients with CML can be used as a guide to clinical management. In particular, we discuss the relative value of regular quantitative RT/PCR and cytogenetic analyses, how responding patients should be monitored and managed, and how to investigate patients who are refractory or become resistant to imatinib treatment. In the last part of this report, a discussion on the possibility of managing CML with patient-specific strategies is presented. We review the current treatment options, highlight the factors impacting on decision making, discuss the range of possibilities for future therapeutic strategies and propose a systematic approach for individualizing treatment for patients in different disease categories.

Selective pyrrolo-pyrimidine inhibitors reveal a necessary role for Src family kinases in Bcr-Abl signal transduction and oncogenesis

Chronic myelogenous leukemia (CML) is defined by the presence of the Philadelphia (Ph) chromosome, which results in the expression of the 210 kDa Bcr-Abl tyrosine kinase. Bcr-Abl constitutively activates several signaling proteins important for the proliferation and survival of myeloid progenitors, including the Src family kinases Hck and Lyn, the Stat5 transcription factor and upstream components of the Ras/Erk pathway. Recently, we found that kinase-defective Hck blocks Bcr-Abl-induced transformation of DAGM myeloid leukemia cells to cytokine independence, suggesting that activation of the Src kinase family may be essential to oncogenic signaling by Bcr-Abl. To investigate the contribution of Src kinases to Bcr-Abl signaling in vivo, we used the pyrrolo-pyrimidine Src kinase inhibitors PP2 and A-419259. Treatment of the Ph+ CML cell lines K-562 and Meg-01 with either compound resulted in growth arrest and induction of apoptosis, while the Ph- leukemia cell lines TF-1 and HEL were unaffected over the same concentration ranges. Suppression of Ph+ cell growth by PP2 and A-419259 correlated with a decrease in Src kinase autophosphorylation. Both inhibitors blocked Stat5 and Erk activation, consistent with the suppressive effects of the compounds on survival and proliferation. In contrast, the phosphotyrosine content of Bcr-Abl and its endogenous substrate CrkL was unchanged at inhibitor concentrations that induced apoptosis, blocked oncogenic signaling and inhibited Src kinases. These data implicate the Src kinase family in Stat5 and Erk activation downstream of Bcr-Abl, and identify myeloid-specific Src kinases as potential drug targets in CML.

SU5416 and SU5614 inhibit kinase activity of wild-type and mutant FLT3 receptor tyrosine kinase

Internal tandem duplication (ITD) in the juxtamembrane portion of Fms-like tyrosine kinase 3 (FLT3), a type III receptor tyrosine kinase (RTK), is the most common molecular defect associated with acute myeloid leukemia (AML). The high prevalence of this activating mutation makes it a potential target for molecularly based therapy. Indolinone tyrosine kinase inhibitors have known activity against KIT, another member of the type III RTK family. Given the conserved homology between members of this family, we postulated that the activity of some KIT inhibitors would extend to FLT3. We used various leukemic cell lines (BaF3, MV 4-11, RS 4;11) to test the activity of indolinone compounds against the FLT3 kinase activity of both wild-type (WT) and ITD isoforms. Both SU5416 and SU5614 were capable of inhibiting autophosphorylation of ITD and WT FLT3 (SU5416 concentration that inhibits 50% [IC(50)], 100 nM; and SU5614 IC(50) 10 nM). FLT3-dependent activation of the downstream signaling proteins mitogen-activated protein kinase (MAPK) and signal transducer and activator of transcription 5 (STAT5) was also inhibited by treatment in the same concentration ranges. FLT3 inhibition by SU5416 and SU5614 resulted in reduced proliferation (IC(50), 250 nM and 100 nM, respectively) and induction of apoptosis of FLT3 ITD-positive leukemic cell lines. Treatment of these cells with an alternative growth factor (granulocyte-macrophage colony-stimulating factor [GM-CSF]) restored MAPK signaling and cellular proliferation, demonstrating specificity of the observed inhibitory effects. We conclude that SU5416 and SU5614 are potent inhibitors of FLT3. Our finding that inhibition of FLT3 induces apoptosis of leukemic cells supports the feasibility of targeting FLT3 as a novel treatment strategy for AML.

Mdr1 gene expression and mutations in Ras proto-oncogenes in acute myeloid leukemia

Resistance to cytotoxic therapy and development of refractory disease in acute myeloid leukemia (AML) is frequently associated with the expression of mdr1/P-gp. In the last years many potential signaling pathways leading to modulation of mdr1 expression have been described. Thus, it has been assumed that activated Ras may influence mdr1 expression. This activation can be realized by mutations in the Ras oncogene leading to constitutive signaling. Ras mutations are observed in many human cancers, including AML. Recently, we could show a negative correlation between Ras mutations and mdr1 expression in blast samples of AML patients. Taking this up the potential possibilities of Ras influence on mdr1 activity and their implications on treatment outcome in AML are discussed.

Manumycin inhibits farnesyltransferase and induces apoptosis of drug-resistant interleukin 6-producing myeloma cells

Interleukin 6 (IL-6) is an important survival and growth factor for myeloma cells and exerts its effects by activating several transduction pathways, including the Ras cascade. As farnesylation of the activated Ras oncogene product by protein farnesyltransferase (FTase) is a critical step for Ras functional activity, FTase has emerged as a potential target for the development of new anti-cancer agents. Based on our previous demonstration that IL-6-producing myeloma cells are refractory to drug-induced apoptosis, we have analysed the effect of manumycin, a natural FTase inhibitor, on IL-6-producing myeloma cells resistant to Fas-, dexamethasone- and doxorubicin-induced apoptosis. Treatment of myeloma cells with manumycin prevented cell proliferation and induced apoptosis. Western blotting experiments demonstrated that this effect was related to inhibition of the post-translational Ras processing.Further analysis showed that manumycin-induced apoptosis involved caspase-3. Activation of caspase-3, in fact, was observed in 6 h-treated myeloma cells expressing Apo 2.7 antigen, the marker of early apoptosis, whereas their treatment with cell-permeable DEVD-fmk, that irreversibly inhibits caspase-3 activity, prevented their apoptosis. Over-expression of caspase-3 was also demonstrated by reverse transcription-polymerase chain reaction. Finally, over-expression of Bcl-2 and its homologue Bcl-xL was observed in manumycin-treated cells as well as in control myeloma cells, implying that the Bcl-2 family is not involved. FTase inhibitors may thus be proposed as a potential pharmacological weapon, as they block the Ras pathway and induce the apoptosis of drug-resistant IL-6-producing myeloma cells.

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.

Regulation of the Erk2-Elk1 signaling pathway and megakaryocytic differentiation of Bcr-Abl(+) K562 leukemic cells by Gab2

In the blast crisis phase of chronic myelogenous leukemia (CML), Bcr-Abl(+) myeloblasts fail to undergo terminal maturation. The extracellular signal-regulated kinase (Erk) mitogen-activated protein (MAP) kinase has been shown to mediate terminal differentiation of myeloid cells. Interestingly, Bcr-Abl(+) CML cell lines established from blast crisis were found to have low Erk MAP kinase activity. In this study, we analyzed the role of the Gab2 docking protein in regulation of the Erk MAP kinase in Bcr-Abl(+) K562 human CML cells. Overexpression of Gab2 in K562 cells resulted in transcriptional activation of the c-fos serum response element (SRE) promoter, whereas overexpression of SHP2, Grb2, and CrkL had no effect. Activation of the c-fos SRE transcriptional activity by Gab2 required tyrosine 604, which is a SHP2 docking site on Gab2, and the SHP2 tyrosine phosphatase activity. Elk1, c-Jun, and CHOP trans-reporting assays indicated that overexpression of Gab2 selectively activated the Erk2-Elk1 signaling pathway. To determine cellular consequences of elevating the Gab2 level in K562 cells, stable cell lines for doxycycline-inducible expression of the wild-type Gab2 (Gab2WT) and an SHP2-binding defective Gab2 (Gab2Tyr604Phe) were established. Analysis of these cell lines indicated that induction of Gab2WT expression, but not Gab2Tyr604Phe expression, led to Erk activation, growth arrest, cell spreading, and enlargement; expression of megakaryocyte/platelet lineage-specific integrins alphaIIb/beta3 (CD41/CD61); and upregulation of RNA for megakaryocyte/platelet proteins. All of these changes are characteristics of megakaryocytic differentiation. Together, these results reveal Gab2 as a limiting signaling component for Erk MAP kinase activation and terminal differentiation of K562 CML cells.

Downregulation of c-fos gene transcription in cells transformed by E1A and cHa-ras oncogenes: a role of sustained activation of MAP/ERK kinase cascade and of inactive chromatin structure at c-fos promoter

REF cells transformed by oncogenes E1A and cHa-ras reveal high and constitutive DNA-binding activity of AP-1 factor lacking in c-Fos protein. Consistently, the transcription of c-fos gene has been found to be downregulated. To elucidate the mechanisms of c-fos downregulation in E1A+cHa-ras transformants, we studied the levels of activity of ERK, JNK/SAPK and p38 kinases and phosphorylation state of Elk-1 transcription factor involved in regulation of c-fos gene. Using two approaches, Western blot analysis with phospho-specific antibodies to MAP kinases and in vitro kinase assay with specific substrates, we show here that ectopic expression of E1A and ras oncogenes leads to a sustained activation of ERK and p38 kinases, whereas JNK/SAPK kinase activity is similar to that in non-transformed REF52 cells. Due to sustained activity of the MAP kinase cascades, Elk-1 transcription factor is being phosphorylated even in serum-starved E1A+cHa-ras cells; moreover, serum does not additionally increase phosphorylation of Elk-1, which is predominant TCF protein bound to SRE region of c-fos gene promoter in these cells. Although the amount of ternary complexes SRE/SRF/TCF estimated by EMSA was similar both in serum-starved and serum-stimulated transformed cells, serum addition still caused a modest activation of c-fos gene transcription at the level of 20% to normal REF cells. In attempt to determine how serum caused the stimulatory effect, we found that PD98059, an inhibitor of MEK/ERK kinase cascade, completely suppressed serum-induced c-fos transcription both in REF and E1A+cHa-ras cells, implicating the ERK as primary kinase for c-fos transcription in these cells. In contrast, SB203580, an inhibitor of p38 kinase, augmented noticeably serum-stimulated transcription of c-fos gene in REF cells, implying the involvement of p38 kinase in negative regulation of c-fos. Furthermore, sodium butyrate, an inhibitor of histone deacetylase activity, was capable of activating c-fos transcription both in serum-stimulated and even in serum-starved E1A+cHa-ras cells. Conversely, serum-starved REF cells fail to respond to sodium butyrate treatment by c-fos activation confirming necessity of prior Elk-1 phosphorylation. Taken together, these data suggest that downregulation of c-fos in E1A+cHa-ras cells seems to occur due to a maintenance of a refractory state that arises in normal REF cells after serum-stimulation. The refractory state of c-fos in E1A+cHa-ras cells is likely a consequence of Ras-induced sustained activation of MAPK (ERK) cascade and persistent phosphorylation of TCF (Elk-1) bound to SRE. Combination of these events eventually does contribute to formation of an inactive chromatin structure at c-fos promoter mediated through recruitment of histone deacetylase activity.

Orchestration of multiple arrays of signal cross-talk and combinatorial interactions for maturation and cell death: another vision of t(15;17) preleukemic blast and APL-cell maturation

Despite intensive molecular biology investigations over the past 10 years, and an important breakthrough on how PML-RARalpha, the fusion protein resulting from t(15;17), can alter RARalpha and PML functions, no definitive views on how leukemia is generated and by what mechanism(s) the normal phenotype is restored, are yet available. 'Resistances' to pharmacological levels of all-trans-retinoic acid (ATRA) have been observed in experimental in vivo and in vitro models. In this review, we emphasize the key role played by signal cross-talk for both normal and neoplastic hemopoiesis. After an overview of reported experimental data on APL-cell maturation and apoptosis, we apply our current knowledge on signaling pathways to underline those which might generate signal cross-talks. The design of biological models suitable to decipher the integration of signal cross-talks at the transcriptional level should be our first priority today, to generate some realistic therapeutic approaches After 'Ten Years of Molecular APL', we still know very little about how the disease develops and how effective medicines work.

Therapeutic targeting of the MEK/MAPK signal transduction module in acute myeloid leukemia

The mitogen-activated protein kinase (MAPK) pathway regulates growth and survival of many cell types, and its constitutive activation has been implicated in the pathogenesis of a variety of malignancies. In this study we demonstrate that small-molecule MEK inhibitors (PD98059 and PD184352) profoundly impair cell growth and survival of acute myeloid leukemia (AML) cell lines and primary samples with constitutive MAPK activation. These agents abrogate the clonogenicity of leukemic cells but have minimal effects on normal hematopoietic progenitors. MEK blockade also results in sensitization to spontaneous and drug-induced apoptosis. At a molecular level, these effects correlate with modulation of the expression of cyclin-dependent kinase inhibitors (p27(Kip1) and p21(Waf1/CIP1)) and antiapoptotic proteins of the inhibitor of apoptosis proteins (IAP) and Bcl-2 families. Interruption of constitutive MEK/MAPK signaling therefore represents a promising therapeutic strategy in AML.