Production of the Alzheimer amyloid beta protein by normal proteolytic processing

The 4-kilodalton (39 to 43 amino acids) amyloid beta protein (beta AP), which is deposited as amyloid in the brains of patients with Alzheimer's diseases, is derived from a large protein, the amyloid beta protein precursor (beta APP). Human mononuclear leukemic (K562) cells expressing a beta AP-bearing, carboxyl-terminal beta APP derivative released significant amounts of a soluble 4-kilodalton beta APP derivative essentially identical to the beta AP deposited in Alzheimer's disease. Human neuroblastoma (M17) cells transfected with constructs expressing full-length beta APP and M17 cells expressing only endogenous beta APP also released soluble 4-kilodalton beta AP, and a similar, if not identical, fragment was readily detected in cerebrospinal fluid from individuals with Alzheimer's disease and normal individuals. Thus cells normally produce and release soluble 4-kilodalton beta AP that is essentially identical to the 4-kilodalton beta AP deposited as insoluble amyloid fibrils in Alzheimer's disease.

Tumor refractoriness to anti-VEGF treatment is mediated by CD11b+Gr1+ myeloid cells

Vascular endothelial growth factor (VEGF) is an essential regulator of normal and abnormal blood vessel growth. A monoclonal antibody (mAb) that targets VEGF suppresses tumor growth in murine cancer models and human patients. We investigated cellular and molecular events that mediate refractoriness of tumors to anti-angiogenic therapy. Inherent anti-VEGF refractoriness is associated with infiltration of the tumor tissue by CD11b+Gr1+ myeloid cells. Recruitment of these myeloid cells is also sufficient to confer refractoriness. Combining anti-VEGF treatment with a mAb that targets myeloid cells inhibits growth of refractory tumors more effectively than anti-VEGF alone. Gene expression analysis in CD11b+Gr1+ cells isolated from the bone marrow of mice bearing refractory tumors reveals higher expression of a distinct set of genes known to be implicated in active mobilization and recruitment of myeloid cells. These findings indicate that, in our models, refractoriness to anti-VEGF treatment is determined by the ability of tumors to prime and recruit CD11b+Gr1+ cells.

Molecular cloning of the complementary DNA for human tumor necrosis factor

Tumor necrosis factor (TNF) is a soluble protein that causes damage to tumor cells but has no effect on normal cells. Human TNF was purified to apparent homogeneity as a 17.3-kilodalton protein from HL-60 leukemia cells and showed cytotoxic and cytostatic activities against various human tumor cell lines. The amino acid sequence was determined for the amino terminal end of the purified protein, and oligodeoxyribonucleotide probes were synthesized on the basis of this sequence. Complementary DNA (cDNA) encoding human TNF was cloned from induced HL-60 messenger RNA and was confirmed by hybrid-selection assay, direct expression in COS-7 cells, and nucleotide sequence analysis. The human TNF cDNA is 1585 base pairs in length and encodes a protein of 233 amino acids. The mature protein begins at residue 77, leaving a long leader sequence of 76 amino acids. Expression of high levels of human TNF in Escherichia coli was accomplished under control of the bacteriophage lambda PL promoter and gene N ribosome binding site.

Enhancement of hematopoietic stem cell repopulating capacity and self-renewal in the absence of the transcription factor C/EBP alpha

The transcription factor C/EBP alpha is required for granulopoiesis and frequently disrupted in human acute myeloid leukemia (AML). Here, we show disruption of C/EBP alpha blocks the transition from the common myeloid to the granulocyte/monocyte progenitor but is not required beyond this stage for terminal granulocyte maturation. C/EBP alpha-deficient hematopoietic stem cells (HSCs) have increased expression of Bmi-1 and enhanced competitive repopulating activity. Bone marrow in adult C/EBP alpha-deficient mice was filled with myeloblasts, similar to human AML, supporting the notion that disruption of C/EBP alpha cooperates with other events in the development of leukemia. Therefore, C/EBP alpha is not only essential for granulocyte development but, in addition, is a regulator of hematopoietic stem cell activity.

Combined DNA methyltransferase and histone deacetylase inhibition in the treatment of myeloid neoplasms

Optimal reexpression of most genes silenced through promoter methylation requires the sequential application of DNA methyltransferase inhibitors followed by histone deacetylase inhibitors in tumor cell cultures. Patients with myelodysplastic syndrome or acute myeloid leukemia (AML) were treated with the methyltransferase inhibitor 5-azacitidine (aza-CR) followed by the histone deacetylase inhibitor sodium phenylbutyrate. Major responses associated with cytogenetic complete response developed in patients receiving prolonged dosing schedules of aza-CR. Bisulfite sequencing of the p15 promoter in marrow DNA during the first cycle of treatment showed heterogeneous allelic demethylation in three responding patients, suggesting ongoing demethylation within the tumor clone, but no demethylation in two nonresponders. Six of six responding patients with pretreatment methylation of p15 or CDH-1 promoters reversed methylation during the first cycle of therapy (methylation-specific PCR), whereas none of six nonresponders showed any demethylation. Gene demethylation correlated with the area under the aza-CR plasma concentration-time curve. Administration of both drugs was associated with induction of acetylation of histones H3 and H4. This study provides the first demonstration that molecular mechanisms responsible for responses to DNA methyltransferase/histone deacetylase inhibitor combinations may include reversal of aberrant epigenetic gene silencing. The promising percentage of major hematologic responses justifies the testing of such combinations in prospective randomized trials.

Identity of Leu-19 (CD56) leukocyte differentiation antigen and neural cell adhesion molecule

Neural cell adhesion molecule (N-CAM) is a membrane glycoprotein expressed on neural and muscle tissues that is involved in homotypic adhesive interactions. We have demonstrated that N-CAM also is expressed on hematopoietic cells, and is recognized by the anti-Leu-19 mAb. Leu-19 is preferentially expressed on NK cells and T lymphocytes that mediate MHC-unrestricted cytotoxicity, but is also present on some myeloid leukemia cell lines. On NK cells, T cells, the KG1a.5 hematopoietic cell line, and a neuroblastoma cell line, Leu-19 is a approximately 140-kD polypeptide with N-linked carbohydrates and abundant sialic acid residues. Sequential immunoprecipitation and peptide mapping demonstrated that the Leu-19 and N-CAM molecules expressed on leukocyte and neuroblastoma cell lines are similar structures. These findings suggest that the Leu-19 antigen on leukocytes may be involved in cell adhesion, analogous to the function on N-CAM on neural cells.

Internal tandem duplication of the FLT3 gene is a novel modality of elongation mutation which causes constitutive activation of the product

An internal tandem duplication (ITD) of the FLT3 gene is found in nearly 20% of acute myeloid leukemia (AML) and 5% of myelodysplastic syndrome cases. Our serial studies on 51 samples with the FLT3 gene mutation indicated that the ITD was frequently (47/51) clustered in the tyrosine-rich stretch from codon 589 to 599 and rarely (3/51) in its downstream region, both of which are located within the juxtamembrane (JM) domain. One remaining sample had an insertion into the JM domain of nucleotides of unknown origin. To elucidate the biological relevance of the ITD or the insertion, we expressed various types of mutant FLT3 in Cos 7 cells. All mutant FLT3 studied were ligand-independently dimerized and their tyrosine residues were phosphorylated. The Y589 of FLT3 was essential for the phosphorylation in the wild FLT3, but a Y589F conversion did not affect the phosphorylation status of the mutant FLT3. These findings suggest that the elongation of the JM domain rather than increase of tyrosine residues causes gain-of-function of FLT3. Thus, ITD is a novel modality of somatic mutation which activates its product. Since the DNA corresponding to codon 593 to 602 potentially forms a palindromic intermediate, we propose that a DNA-replication error might be associated with generating the ITD of the FLT3 gene.

Transformation of myeloid progenitors by MLL oncoproteins is dependent on Hoxa7 and Hoxa9

Transcriptional deregulation through the production of dominant-acting chimeric transcription factors derived from chromosomal translocations is a common theme in the pathogenesis of acute leukemias; however, the essential target genes for acute leukemogenesis are unknown. We demonstrate here that primary myeloid progenitors immortalized by various MLL oncoproteins exhibit a characteristic Hoxa gene cluster expression profile, which reflects that preferentially expressed in the myeloid clonogenic progenitor fraction of normal bone marrow. Continued maintenance of this MLL-dependent Hoxa gene expression profile is associated with conditional MLL-associated myeloid immortalization. Moreover, Hoxa7 and Hoxa9 were specifically required for efficient in vitro myeloid immortalization by an MLL fusion protein but not other leukemogenic fusion proteins. Finally, in a bone marrow transduction/transplantation model, Hoxa9 is essential for MLL-dependent leukemogenesis in vivo, a primary requirement detected at the earliest stages of disease initiation. Thus, a genetic reliance on Hoxa7 and Hoxa9 in MLL-mediated transformation demonstrates a gain-of-function mechanism for MLL oncoproteins as upstream constitutive activators that promote myeloid transformation via a Hox-dependent mechanism.

1 alpha,25-Dihydroxycholecalciferol and a human myeloid leukaemia cell line (HL-60)

Human promyelocytic leukaemia cells (HL-60) can be induced to differentiate into mature granulocytes in vitro by 1 alpha,25-dihydroxycholecalciferol [1 alpha,25(OH)2D3], the active form of cholecalciferol. The differentiation-associated properties, such as phagocytosis and C3 rosette formation, were induced by as little as 0.12 nM-1 alpha,25(OH)2D3, and, at 12 nM, about half of the cells exhibited differentiation on day 3 of incubation. Concomitantly the viable cell number was decreased to less than half of the control. Among various derivatives of cholecalciferol examined, 1 alpha,25(OH)2D3 and 1 alpha,24R-dihydroxycholecalciferol were the most potent in inducing differentiation, followed successively by 1 alpha,24S-dihydroxycholecalciferol, 1 alpha-hydroxycholecalciferol, 25-hydroxycholecalciferol and 24R,25-dihydroxycholecalciferol. A cytosol protein specifically bound to 1 alpha,25 (OH)2D3 was found in HL-60 cells. Its physical properties closely resembled those found in such target tissues as intestine and parathyroid glands. 1 alpha,25(OH)2D3 bound to the cytosol receptor was transferred quantitatively to the chromatin fraction. The specificity of various derivatives of cholecalciferol in inducing differentiation was well correlated with that of their association with the cytosol receptor. These results are compatible with the hypothesis that the active form of cholecalciferol induces differentiation of human myeloid leukaemia cells by a mechanism similar to that proposed for the classical concept of steroid hormone action.

NUP98-NSD1 links H3K36 methylation to Hox-A gene activation and leukaemogenesis

Nuclear receptor-binding SET domain protein 1 (NSD1) prototype is a family of mammalian histone methyltransferases (NSD1, NSD2/MMSET/WHSC1, NSD3/WHSC1L1) that are essential in development and are mutated in human acute myeloid leukemia (AML), overgrowth syndromes, multiple myeloma and lung cancers. In AML, the recurring t(5;11)(q35;p15.5) translocation fuses NSD1 to nucleoporin-98 (NUP98). Here, we present the first characterization of the transforming properties and molecular mechanisms of NUP98-NSD1. We demonstrate that NUP98-NSD1 induces AML in vivo, sustains self-renewal of myeloid stem cells in vitro, and enforces expression of the HoxA7, HoxA9, HoxA10 and Meis1 proto-oncogenes. Mechanistically, NUP98-NSD1 binds genomic elements adjacent to HoxA7 and HoxA9, maintains histone H3 Lys 36 (H3K36) methylation and histone acetylation, and prevents EZH2-mediated transcriptional repression of the Hox-A locus during differentiation. Deletion of the NUP98 FG-repeat domain, or mutations in NSD1 that inactivate the H3K36 methyltransferase activity or that prevent binding of NUP98-NSD1 to the Hox-A locus precluded both Hox-A gene activation and myeloid progenitor immortalization. We propose that NUP98-NSD1 prevents EZH2-mediated repression of Hox-A locus genes by colocalizing H3K36 methylation and histone acetylation at regulatory DNA elements. This report is the first to link deregulated H3K36 methylation to tumorigenesis and to link NSD1 to transcriptional regulation of the Hox-A locus.

Stromal cells prevent apoptosis of AML cells by up-regulation of anti-apoptotic proteins

The aim of this study was to study interactions between stromal bone marrow microenvironment and leukemic cells. We tested the hypothesis that stromal cells prevent apoptosis of AML cells by up-regulating anti-apoptotic proteins in leukemic blasts. In HL-60 and NB-4 cells, serum deprivation- and ara-C-induced apoptosis was diminished when cells were cocultured with murine MS-5 stromal cells (P < 0.02). This effect was reproduced with conditioned medium from MS-5 cells. Cocultivation with stromal cells induced Bcl-2 expression levels, both by PCR analysis and flow cytometry. In primary AML (n = 14), ara-C-induced apoptosis was significantly lower in cells cocultured with MS-5 cells than in controls (P < 0.001). This effect was partially preserved when leukemic cells were separated from stromal cells by a microporous insert (in 5/9 samples, P = 0.04). In addition, Bcl-2 levels were significantly higher in stroma-supported than in control CD34(+) AML cells (P < 0.01). Bcl-X(L) levels were higher in 5/7 samples grown on stromal layers. Of note, in AML patients resistant to induction chemotherapy (n = 6), Bcl-2 increased significantly after cultivation with stromal cells, but no such increase was noted in cells from chemotherapy-sensitive patients. In conclusion, MS-5 stromal cells prevented apoptosis in HL-60 cells and in primary AML blasts via modulation of Bcl-2 family proteins. The observed association of high Bcl-2 expression in stroma-supported AML blasts in vitro with resistance to chemotherapy in vivo suggests that the same mechanisms may be operational in vivo.

JAK2 mutation 1849G>T is rare in acute leukemias but can be found in CMML, Philadelphia chromosome-negative CML, and megakaryocytic leukemia

An activating 1849G>T mutation of JAK2 (Janus kinase 2) tyrosine kinase was recently described in chronic myeloproliferative disorders (MPDs). Its role in other hematologic neoplasms is unclear. We developed a quantitative pyrosequencing assay and analyzed 374 samples of hematologic neoplasms. The mutation was frequent in polycythemia vera (PV) (86%) and myelofibrosis (95%) but less prevalent in acute myeloid leukemia (AML) with an antecedent PV or myelofibrosis (5 [36%] of 14 patients). JAK2 mutation was also detected in 3 (19%) of 16 patients with Philadelphia-chromosome (Ph)-negative chronic myelogenous leukemia (CML), 2 (18%) of 11 patients with megakaryocytic AML, 7 (13%) of 52 patients with chronic myelomonocytic leukemia, and 1 (1%) of 68 patients with myelodysplastic syndromes. No mutation was found in Ph(+)CML (99 patients), AML M0-M6 (28 patients), or acute lymphoblastic leukemia (20 patients). We conclude that the JAK2 1849G>T mutation is common in Ph(-) MPD but not critical for transformation to the acute phase of these diseases and that it is generally rare in aggressive leukemias.

A comparison of azacitidine and decitabine activities in acute myeloid leukemia cell lines

BACKGROUND

The cytidine nucleoside analogs azacitidine (AZA) and decitabine (DAC) are used for the treatment of patients with myelodysplastic syndromes and acute myeloid leukemia (AML). Few non-clinical studies have directly compared the mechanisms of action of these agents in a head-to-head fashion, and the agents are often viewed as mechanistically similar DNA hypomethylating agents. To better understand the similarities and differences in mechanisms of these drugs, we compared their in vitro effects on several end points in human AML cell lines.

METHODOLOGY/PRINCIPAL FINDINGS

Both drugs effected DNA methyltransferase 1 depletion, DNA hypomethylation, and DNA damage induction, with DAC showing equivalent activity at concentrations 2- to 10-fold lower than AZA. At concentrations above 1 microM, AZA had a greater effect than DAC on reducing cell viability. Both drugs increased the sub-G1 fraction and apoptosis markers, with AZA decreasing all cell cycle phases and DAC causing an increase in G2-M. Total protein synthesis was reduced only by AZA, and drug-modulated gene expression profiles were largely non-overlapping.

CONCLUSIONS/SIGNIFICANCE

These data demonstrate shared mechanisms of action of AZA and DAC on DNA-mediated markers of activity, but distinctly different effects in their actions on cell viability, protein synthesis, cell cycle, and gene expression. The differential effects of AZA may be mediated by RNA incorporation, as the distribution of AZA in nucleic acid of KG-1a cells was 65:35, RNA:DNA.

Deficient expression of NCR in NK cells from acute myeloid leukemia: evolution during leukemia treatment and impact of leukemia cells in NCRdull phenotype induction

Natural killer (NK) cells play an important role in tumor-cell clearance, particularly against leukemia, as shown by killer cell inhibitory receptor (KIR)–mismatched allogeneic stem cell transplantation. Analysis of in vitro IL-2–expanded NK cells from patients with myelocytic/monocytic acute myeloid leukemia (AML-NK cells) has revealed poor cytolytic functions because of deficient expression of pivotal activation molecules—the natural cytotoxicity receptors (NCRs) NKp30, NKp44, and NKp46. To exclude the possibility that this observation was caused by the in vitro amplification of a small NCRdull population, we analyzed the AML-NK phenotype directly, without any in vitro expansion. We first confirmed that the NCRdull phenotype was not an in vitro artifact. Moreover, analysis of a large population of AML patients allowed us to demonstrate that phenotype was not restricted to a French-American-British (FAB) subtype and was not associated with a particular cytogenetic abnormality. Our longitudinal study of AML patients showed that the NCRdull phenotype was acquired during leukemia development because we observed its complete (for NKp46) or partial (for NKp30) reversibility in patients achieving complete remission (CR). Reversibility of the NCRdull phenotype after CR suggested that leukemia cells might be involved in NCR down-regulation. In agreement with this hypothesis, direct contact between leukemic blasts and NK cells (but not leukemia-cell supernatants) induced loss or decrease in NKp30 and NKp46 expression while impeding NKp44 induction by IL-2. We excluded the major implication of TGF-β in NCR down-regulation. Although the clinical antitumor value of NK cells is clearly demonstrated in allogeneic stem cell transplantation, the role of NK cells in autologous transplantation is not proved. Interestingly, we observed a correlation between the NCRdull phenotype and poor survival in AML patients, suggesting that NK-deficient activation caused by NCR down-regulation could play a role in patient outcome. The prognostic value of NCR expression is discussed, and pathophysiologic implication of the NCR phenotype will be further investigated in a larger study.

High stem cell frequency in acute myeloid leukemia at diagnosis predicts high minimal residual disease and poor survival

PURPOSE

In CD34-positive acute myeloid leukemia (AML), the leukemia-initiating event originates from the CD34(+)CD38(-) stem cell compartment. Survival of these cells after chemotherapy may lead to minimal residual disease (MRD) and subsequently to relapse. Therefore, the prognostic impact of stem cell frequency in CD34-positive AML was investigated.

EXPERIMENTAL DESIGN

First, the leukemogenic potential of unpurified CD34(+)CD38(-) cells, present among other cells, was investigated in vivo using nonobese diabetic/severe combined immunodeficient mice transplantation experiments. Second, we analyzed whether the CD34(+)CD38(-) compartment at diagnosis correlates with MRD frequency after chemotherapy and clinical outcome in 92 AML patients.

RESULTS

In vivo data showed that engraftment of AML blasts in nonobese diabetic/severe combined immunodeficient mice directly correlated with stem cell frequency of the graft. In patients, a high percentage of CD34(+)CD38(-) stem cells at diagnosis significantly correlated with a high MRD frequency, especially after the third course of chemotherapy. Also, it directly correlated with poor survival. In contrast, total CD34(+) percentage showed no such correlations.

CONCLUSIONS

Both in vivo data, as well as the correlation studies, show that AML stem cell frequency at diagnosis offers a new prognostic factor. From our data, it is tempting to hypothesize that a large CD34(+)CD38(-) population at diagnosis reflects a higher percentage of chemotherapy-resistant cells that will lead to the outgrowth of MRD, thereby affecting clinical outcome. Ultimately, future therapies should be directed toward malignant stem cells.

BET Bromodomain Inhibition Suppresses the Function of Hematopoietic Transcription Factors in Acute Myeloid Leukemia

The bromodomain and extraterminal (BET) protein BRD4 is a validated drug target in leukemia, yet its regulatory function in this disease is not well understood. Here, we show that BRD4 chromatin occupancy in acute myeloid leukemia closely correlates with the hematopoietic transcription factors (TFs) PU.1, FLI1, ERG, C/EBPα, C/EBPβ, and MYB at nucleosome-depleted enhancer and promoter regions. We provide evidence that these TFs, in conjunction with the lysine acetyltransferase activity of p300/CBP, facilitate BRD4 recruitment to their occupied sites to promote transcriptional activation. Chemical inhibition of BET bromodomains was found to suppress the functional output of each hematopoietic TF, thereby interfering with essential lineage-specific transcriptional circuits in this disease. These findings reveal a chromatin-based signaling cascade comprised of hematopoietic TFs, p300/CBP, and BRD4 that supports leukemia maintenance and is suppressed by BET bromodomain inhibition.

Combination of the histone deacetylase inhibitor LBH589 and the hsp90 inhibitor 17-AAG is highly active against human CML-BC cells and AML cells with activating mutation of FLT-3

Present studies show that LBH589, a novel cinnamic hydroxamic acid analog histone deacetylase inhibitor, induces acetylation of histone H3 and H4 and of heat shock protein 90 (hsp90), increases p21 levels, as well as induces cell-cycle G(1) phase accumulation and apoptosis of the human chronic myeloid leukemia blast crisis (CML-BC) K562 cells and acute leukemia MV4-11 cells with the activating length mutation of FLT-3. In MV4-11 cells, this was associated with marked attenuation of the protein levels of p-FLT-3, FLT-3, p-AKT, and p-ERK1/2. In K562 cells, exposure to LBH589 attenuated Bcr-Abl, p-AKT, and p-ERK1/2. Treatment with LBH589 inhibited the DNA binding activity of signal transducers and activators of transcription 5 (STAT5) in both K562 and MV4-11 cells. The hsp90 inhibitor 17-allyl-amino-demethoxy geldanamycin (17-AAG) also induced polyubiquitylation and proteasomal degradation of FLT-3 and Bcr-Abl by reducing their chaperone association with hsp90. Cotreatment with LBH589 and 17-AAG exerted synergistic apoptosis of MV4-11 and K562 cells. In the imatinib mesylate (IM)-refractory leukemia cells expressing Bcr-Abl with the T315I mutation, treatment with the combination attenuated the levels of the mutant Bcr-Abl and induced apoptosis. Finally, cotreatment with LBH589 and 17-AAG also induced more apoptosis of IM-resistant primary CML-BC and acute myeloid leukemia (AML) cells (with activating mutation of FLT-3) than treatment with either agent alone.

A phase I study of intravenous LBH589, a novel cinnamic hydroxamic acid analogue histone deacetylase inhibitor, in patients with refractory hematologic malignancies

PURPOSE

LBH589 is a novel histone deacetylase inhibitor that inhibits proliferation and induces apoptosis in tumor cell lines. In this phase I study, LBH589 was administered i.v. as a 30-minute infusion on days 1 to 7 of a 21-day cycle.

EXPERIMENTAL DESIGN

Fifteen patients (median age, 63 years; range, 42-87 years) with acute myeloid leukemia (13 patients), acute lymphocytic leukemia (1 patient), or myelodysplastic syndrome (1 patient) were treated with LBH589 at the following dose levels (mg/m(2)): 4.8 (3 patients), 7.2 (3 patients), 9.0 (1 patient), 11.5 (3 patient), and 14.0 (5 patients). The levels of histone acetylation were measured using quantitative flow cytometry and plasma LBH589 concentrations were assayed.

RESULTS

Four dose-limiting toxicities (grade 3 QTcF prolongation) were observed, four at 14.0 mg/m(2) and one at 11.5 mg/m(2). QTcF prolongation was asymptomatic and reversed on LBH589 discontinuation. Other potentially LBH589-related toxicities included nausea (40%), diarrhea (33%), vomiting (33%), hypokalemia (27%), loss of appetite (13%), and thrombocytopenia (13%). In 8 of 11 patients with peripheral blasts, transient reductions occurred with a rebound following the 7-day treatment period. H3 acetylation increase was significant in B-cells (CD19(+); P = 0.02) and blasts (CD34(+); P = 0.04). The increase in H2B acetylation was highest in CD19(+) and CD34(+) cells [3.8-fold (P = 0.01) and 4.4-fold (P = 0.03), respectively]. The median acetylation of histones H2B and H3 in CD34(+) and CD19(+) cells significantly increased on therapy as did apoptosis in CD14(+) cells. Area under the curve increased proportionally with dose with a terminal half-life of approximately 11 hours.

CONCLUSION

Intravenous administration of LBH589 was well tolerated at doses <11.5 mg/m(2) with consistent transient antileukemic and biological effects.

Oligomerization of the ABL tyrosine kinase by the Ets protein TEL in human leukemia

TEL is a member of the Ets family of transcription factors which are frequently rearranged in human leukemia. The mechanism of TEL-mediated transformation, however, is unknown. We report the cloning and characterization of a chromosomal translocation associated with acute myeloid leukemia which fuses TEL to the ABL tyrosine kinase. The TEL-ABL fusion confers growth factor-independent growth to the marine hematopoietic cell line Ba/F3 and transforms Rat-1 fibroblasts and primary murine bone marrow cells. TEL-ABL is constitutively tyrosine phosphorylated and localizes to the cytoskeleton. A TEL-ABL mutant containing an ABL kinase-inactivating mutation is not constitutively phosphorylated and is nontransforming but retains cytoskeletal localization. However, constitutive phosphorylation, cytoskeletal localization, and transformation are all dependent upon a highly conserved region of TEL termed the helix-loop-helix (HLH) domain. TEL-ABL formed HLH-dependent homo-oligomers in vitro, a process critical for tyrosine kinase activation. These experiments suggest that oligomerization of TEL-ABL mediated by the TEL HLH domain is required for tyrosine kinase activation, cytoskeletal localization, and transformation. These data also suggest that oligomerization of Ets proteins through the highly conserved HLH domain may represent a previously unrecognized phenomenon.

High EVI1 expression predicts poor survival in acute myeloid leukemia: a study of 319 de novo AML patients

The proto-oncogene EVI1 encodes a DNA binding protein and is located on chromosome 3q26. The gene is aberrantly expressed in acute myeloid leukemia (AML) patients carrying 3q26 abnormalities. Two mRNAs are transcribed from this locus: EVI1 and a fusion of EVI1 with MDS1 (MDS1-EVI1), a gene located 5' of EVI1. The purpose of this study was to investigate which of the 2 gene products is involved in transformation in human AML. To discriminate between EVI1 and MDS1-EVI1 transcripts, distinct real-time quantitative polymerase chain reaction (PCR) assays were developed. Patients with 3q26 abnormalities often showed high EVI1 and MDS1-EVI1 expression. In a cohort of 319 AML patients, 4 subgroups could be distinguished: EVI1(+) and MDS1-EVI1(-) (6 patients; group I), EVI1(+) and MDS1-EVI1(+) (26 patients; group II), EVI1(-) and MDS1-EVI1(+) (12 patients; group III), and EVI1(-) and MDS1-EVI1(-) (275 patients; group IV). The only 4 patients with a 3q26 aberration belonged to groups I and II. Interestingly, high EVI1 and not MDS1-EVI1 expression was associated with unfavorable karyotypes (eg, -7/7q-) or complex karyotypes. Moreover, a significant correlation was observed between EVI1 expression and 11q23 aberrations (mixed lineage leukemia [MLL] gene involvement). Patients from groups I and II had significantly shorter overall and event-free survival than patients in groups III and IV. Our data demonstrate that high EVI1 expression is an independent poor prognostic marker within the intermediate- risk karyotypic group.

The folate receptor: what does it promise in tissue-targeted therapeutics?

For over a decade the folate receptor has been intensively investigated as a means for tumor-specific delivery of a broad range of experimental therapies including several conceptually new treatments. Despite a few set backs in clinical trials, the literature is replete with encouraging in vitro and pre-clinical studies of gynecological and other tumors and more therapeutic approaches are ready for clinical testing. Recent studies have added myelogenous leukemias to the list of candidate cancers for FR-targeted therapies. Each approach faces unique challenges in translation that could be addressed through a mechanistic understanding of the function and expression of the receptor in the appropriate experimental systems and by improvements in the technology. This review discusses FR in the context of positive recent developments in broad areas of FR-targeted therapy and attempts to highlight its potential and the anticipated challenges.

Phosphoinositide 3-kinase/Akt signaling pathway and its therapeutical implications for human acute myeloid leukemia

The phosphoinositide 3-kinase (PI3K)/Akt signaling pathway is crucial to many aspects of cell growth, survival and apoptosis, and its constitutive activation has been implicated in the both the pathogenesis and the progression of a wide variety of neoplasias. Hence, this pathway is an attractive target for the development of novel anticancer strategies. Recent studies showed that PI3K/Akt signaling is frequently activated in acute myeloid leukemia (AML) patient blasts and strongly contributes to proliferation, survival and drug resistance of these cells. Upregulation of the PI3K/Akt network in AML may be due to several reasons, including FLT3, Ras or c-Kit mutations. Small molecules designed to selectively target key components of this signal transduction cascade induce apoptosis and/or markedly increase conventional drug sensitivity of AML blasts in vitro. Thus, inhibitory molecules are currently being developed for clinical use either as single agents or in combination with conventional therapies. However, the PI3K/Akt pathway is important for many physiological cellular functions and, in particular, for insulin signaling, so that its blockade in vivo might cause severe systemic side effects. In this review, we summarize the existing knowledge about PI3K/Akt signaling in AML cells and we examine the rationale for targeting this fundamental signal transduction network by means of selective pharmacological inhibitors.

A Novel MCL1 Inhibitor Combined with Venetoclax Rescues Venetoclax-Resistant Acute Myelogenous Leukemia

Suppression of apoptosis by expression of antiapoptotic BCL2 family members is a hallmark of acute myeloblastic leukemia (AML). Induced myeloid leukemia cell differentiation protein (MCL1), an antiapoptotic BCL2 family member, is commonly upregulated in AML cells and is often a primary mode of resistance to treatment with the BCL2 inhibitor venetoclax. Here, we describe VU661013, a novel, potent, selective MCL1 inhibitor that destabilizes BIM/MCL1 association, leads to apoptosis in AML, and is active in venetoclax-resistant cells and patient-derived xenografts. In addition, VU661013 was safely combined with venetoclax for synergy in murine models of AML. Importantly, BH3 profiling of patient samples and drug-sensitivity testing ex vivo accurately predicted cellular responses to selective inhibitors of MCL1 or BCL2 and showed benefit of the combination. Taken together, these data suggest a strategy of rationally using BCL2 and MCL1 inhibitors in sequence or in combination in AML clinical trials. SIGNIFICANCE: Targeting antiapoptotic proteins in AML is a key therapeutic strategy, and MCL1 is a critical antiapoptotic oncoprotein. Armed with novel MCL1 inhibitors and the potent BCL2 inhibitor venetoclax, it may be possible to selectively induce apoptosis by combining or thoughtfully sequencing these inhibitors based on a rational evaluation of AML.See related commentary by Leber et al., p. 1511.This article is highlighted in the In This Issue feature, p. 1494.

Wnt5a inhibits B cell proliferation and functions as a tumor suppressor in hematopoietic tissue

Wnt5a is a member of the Wnt family of secreted glycoproteins that play essential organizing roles in development. Similar to other Wnt members, Wnt5a can upregulate cell proliferation and has been proposed to have oncogenic function. Here we report that Wnt5a signals through the noncanonical Wnt/Ca++ pathway to suppress cyclin D1 expression and negatively regulate B cell proliferation in a cell-autonomous manner. Wnt5a hemizygous mice develop myeloid leukemias and B cell lymphomas that are clonal in origin and display loss of Wnt5a function in tumor tissues. Furthermore, analysis of human primary leukemias reveals deletion of the WNT5A gene and/or loss of WNT5A expression in a majority of the patient samples. These results demonstrate that Wnt5a suppresses hematopoietic malignancies.

Abrogation of nuclear receptors Nr4a3 and Nr4a1 leads to development of acute myeloid leukemia

Nur77 (NR4A1) and Nor-1 (NR4A3) are highly homologous orphan nuclear receptors that regulate the transcription of overlapping target genes. The transcriptional activity of both proteins is regulated in a ligand-independent manner by cell- and stimulus-specific gene induction and protein phosphorylation. Nor-1 and Nur77 have been implicated in a variety of cellular processes, including the transduction of hormonal, inflammatory, mitogenic, apoptotic and differentiative signals. Cellular responses to these proteins suggest that they may function as homeostatic regulators of proliferation, apoptosis and differentiation, and thus may regulate cellular susceptibility to tumorigenesis. Their physiological functions, however, remain poorly understood. Here we describe a previously unsuspected function of Nor-1 and Nur77-as critical tumor suppressors of myeloid leukemogenesis. The abrogation of these proteins in mice led to rapidly lethal acute myeloid leukemia (AML), involving abnormal expansion of hematopoietic stem cells (HSCs) and myeloid progenitors, decreased expression of the AP-1 transcription factors JunB and c-Jun and defective extrinsic apoptotic (Fas-L and TRAIL) signaling. We found that downregulation of NR4A3 ( NOR-1 ) and NR4A1 ( NUR77 ) was a common feature in leukemic blasts from human AML patients, irrespective of karyotype. Thus Nor-1 and Nur77 may provide potential targets for therapeutic intervention in AML.