Williams-Beuren syndrome critical region-5/non-T-cell activation linker: a novel target gene of AML1/ETO

AML1/ETO and its function as a regulator of gene transcription via epigenetic mechanisms

The chromosomal translocation t(8;21) and the resulting oncofusion gene AML1/ETO have long served as a prototypical genetic lesion to model and understand leukemogenesis. In this review, we describe the wide-ranging role of AML1/ETO in AML leukemogenesis, with a particular focus on the aberrant epigenetic regulation of gene transcription driven by this AML-defining mutation. We begin by analyzing how structural changes secondary to distinct genomic breakpoints and splice changes, as well as posttranscriptional modifications, influence AML1/ETO protein function. Next, we characterize how AML1/ETO recruits chromatin-modifying enzymes to target genes and how the oncofusion protein alters chromatin marks, transcription factor binding, and gene expression. We explore the specific impact of these global changes in the epigenetic network facilitated by the AML1/ETO oncofusion on cellular processes and leukemic growth. Furthermore, we define the genetic landscape of AML1/ETO-positive AML, presenting the current literature concerning the incidence of cooperating mutations in genes such as KIT, FLT3, and NRAS. Finally, we outline how alterations in transcriptional regulation patterns create potential vulnerabilities that may be exploited by epigenetically active agents and other therapeutics.

Targeting miR-193a-AML1-ETO-β-catenin axis by melatonin suppresses the self-renewal of leukaemia stem cells in leukaemia with t (8;21) translocation

AML1‐ETO, the most common fusion oncoprotein by t (8;21) in acute myeloid leukaemia (AML), enhances hematopoietic self‐renewal and leukemogenesis. However, currently no specific therapies have been reported for t (8;21) AML patients as AML1‐ETO is still intractable as a pharmacological target. For this purpose, leukaemia cells and AML1‐ETO‐induced murine leukaemia model were used to investigate the degradation of AML1‐ETO by melatonin (MLT), synthesized and secreted by the pineal gland. MLT remarkedly decreased AML1‐ETO protein in leukemic cells. Meanwhile, MLT induced apoptosis, decreased proliferation and reduced colony formation. Furthermore, MLT reduced the expansion of human leukemic cells and extended the overall survival in U937T‐AML1‐ETO‐xenografted NSG mice. Most importantly, MLT reduced the infiltration of leukaemia blasts, decreased the frequency of leukaemia stem cells (LSCs) and prolonged the overall survival in AML1‐ETO‐induced murine leukaemia. Mechanistically, MLT increased the expression of miR‐193a, which inhibited AML1‐ETO expression via targeting its putative binding sites. Furthermore, MLT decreased the expression of β‐catenin, which is required for the self‐renewal of LSC and is the downstream of AML1‐ETO. Thus, MLT presents anti‐self‐renewal of LSC through miR‐193a‐AML1‐ETO‐β‐catenin axis. In conclusion, MLT might be a potential treatment for t (8;21) leukaemia by targeting AML1‐ETO oncoprotein.

The transmembrane adaptor protein NTAL limits mast cell chemotaxis toward prostaglandin E2

Chemotaxis of mast cells is one of the crucial steps in their development and function. Non–T cell activation linker (NTAL) is a transmembrane adaptor protein that inhibits the activation of mast cells and B cells in a phosphorylation-dependent manner. Here, we studied the role of NTAL in the migration of mouse mast cells stimulated by prostaglandin E2 (PGE2). Although PGE2 does not induce the tyrosine phosphorylation of NTAL, unlike IgE immune complex antigens, we found that loss of NTAL increased the chemotaxis of mast cells toward PGE2. Stimulation of mast cells that lacked NTAL with PGE2 enhanced the phosphorylation of AKT and the production of phosphatidylinositol 3,4,5-trisphosphate. In resting NTAL-deficient mast cells, phosphorylation of an inhibitory threonine in ERM family proteins accompanied increased activation of β1-containing integrins, which are features often associated with increased invasiveness in tumors. Rescue experiments indicated that only full-length, wild-type NTAL restored the chemotaxis of NTAL-deficient cells toward PGE2. Together, these data suggest that NTAL is a key inhibitor of mast cell chemotaxis toward PGE2, which may act through the RHOA/ERM/β1-integrin and PI3K/AKT axes.

Honokiol induces proteasomal degradation of AML1-ETO oncoprotein via increasing ubiquitin conjugase UbcH8 expression in leukemia

AML1-ETO is the most common oncoprotein leading to acute myeloid leukemia (AML), in which 5-year survival rate is only about 30%. However, currently there are no specific therapies for AML patients with AML1-ETO. Here, we report that AML1-ETO protein is rapidly degraded by Honokiol (HNK), a natural phenolic compound isolated from the plant Magnolia officinalis. HNK induced the degradation of AML1-ETO in a concentration- and time-dependent manner in leukemic cell lines and primary AML blasts with t(8;21) translocation. Mechanistically, HNK obviously increased the expression of UbcH8, an E2-conjugase for the degradation of AML1-ETO, through triggering accumulation of acetylated histones in the promoter region of UbcH8. Knockdown of UbcH8 by small hairpin RNAs (shRNAs) prevented HNK-induced degradation of AML-ETO, suggesting that UbcH8 plays a critical role in the degradation of AML1-ETO. HNK inhibited cell proliferation and induced apoptotic death without activation of caspase-3, which was reported to cleave and degrade AML1-ETO protein. Thus, HNK-induced degradation of AML1-ETO is independent of activation of caspase-3. Finally, HNK reduced the angiogenesis and migration in Kasumi-1-injected zebrafish, decreased xenograft tumor size in a xenograft leukemia mouse model, and prolonged the survival time in mouse C1498 AML model. Collectively, HNK might be a potential treatment for t(8;21) leukemia by targeting AML1-ETO oncoprotein.

The AML1/ETO target gene LAT2 interferes with differentiation of normal hematopoietic precursor cells

The adaptor protein linker activator of T-cells 2 (LAT2) is a known AML1/ETO target gene whose function during normal hematopoiesis is unknown. We addressed the role of LAT2 during erythroid and myeloid differentiation of normal human CD34+ hematopoietic cells. LAT2 is expressed at low levels in CD34+ cells and upregulated during cytokine-induced myeloid and erythroid differentiation. Forced LAT2 expression leads to a delay of erythroid and myeloid differentiation keeping CD34+ cells in a more immature state, whereas LAT2 knockdown accelerates differentiation. It is tempting to speculate that by affecting the differentiation capacity of normal hematopoietic progenitors, LAT2 may contribute to the pathogenesis of AML.

Redistribution of H3K27me3 and acetylated histone H4 upon exposure to azacitidine and decitabine results in de-repression of the AML1/ETO target gene IL3

Human acute myeloid leukemia is characterized by a block in maturation caused by genetic and epigenetic alterations. We studied the effects of low concentrations of the DNA methyltransferase (DNMT) inhibitors 5-azacitidine and decitabine on apoptosis and on chromatin remodeling in an AML1/ETO inducible model of human AML. While both DNMT inhibitors induced apoptosis, only azacitidine did so via caspase activation, possibly through its exclusive non-DNA depending effects. We evaluated histone marks for permissive chromatin, H3K4me3, and acetylated histone H4, and for non-permissive chromatin, H3K9me2, and H3K27me3, at the promoter of the IL3 gene, which is under the direct control of AML1/ETO and is critical for myeloid maturation. We observed that low concentrations of DNMT inhibitors induced a loss of H3K27me3 and gain of acetylated histone H4 at the IL3 promoter exclusively in AML1/ETO-positive cells, which was associated with transcriptional reactivation of the IL3 gene.

AML1/ETO sensitizes via TRAIL acute myeloid leukemia cells to the pro-apoptotic effects of hypoxia

We determined the effects of severe hypoxia (∼0.1% O₂) on acute myeloid leukemia cells expressing the AML1/ETO oncogene. Incubation of Kasumi-1 cells in hypoxia induced growth arrest, apoptosis and reduction of AML1/ETO protein expression. The conditional expression of AML1/ETO in U937-A/E cells showed that hypoxia induces marked apoptosis in AML1/ETO-expressing cells only, pointing to AML1/ETO as a factor predisposing cells to hypoxia-induced apoptosis. In AML1/ETO-expressing cells, hypoxia enhanced TRAIL expression and its proapoptotic effects. AML1/ETO was found to bind TRAIL promoter and induce TRAIL transcription, although TRAIL expression was restrained by a concomitant relative transcription block. In hypoxia, such a TRAIL repression was removed and an increase of TRAIL expression was induced. Finally, blocking anti-TRAIL antibodies markedly reduced (Kasumi-1 cells) or completely inhibited (U937-A/E cells) hypoxia-induced apoptosis. Taken together, these results indicated that hypoxia induces apoptosis in AML1/ETO-expressing cells via a TRAIL/caspase 8-dependent autocrine loop and that TRAIL is a key regulator of hypoxia-induced apoptosis in these cells.

AML1-ETO targets and suppresses cathepsin G, a serine protease, which is able to degrade AML1-ETO in t(8;21) acute myeloid leukemia

Although the significance of cathepsin G (CTSG) in host defense has been intensively investigated, little is known about its potential roles in granulopoiesis or leukemogenesis. We report here that CTSG is directly targeted and suppressed by AML1-ETO in t(8;21) acute myeloid leukemia (AML). Luciferase assays demonstrate that the CTSG promoter is strongly transactivated by AML1 and the AML1-dependent transactivation is suppressed by AML1-ETO. We also define a novel regulatory mechanism by which AML1-ETO-mediated transrepression requires both AML1-ETO and AML1 binding at adjacent sites, instead of the replacement of AML1 by AML1-ETO, and wild-type AML1 binding is a prerequisite for the repressive effect caused by AML1-ETO. Further evidence shows that CTSG, as a hematopoietic serine protease, can degrade AML1-ETO both in vitro and in vivo. Restoration of CTSG induces partial differentiation, growth inhibition and apoptosis in AML1-ETO-positive cells. In addition to t(8;21) AML, CTSG downregulation is observed in AML patients with other cytogenetic/genetic abnormalities that potentially interrupt normal AML1 function, that is, inv(16) and EVI1 overexpression. Thus, the targeting and suppression of CTSG by AML1-ETO in t(8;21) AML may provide a mechanism for leukemia cells to escape from the intracellular surveillance system by preventing degradation of foreign proteins.

The HDAC class I-specific inhibitor entinostat (MS-275) effectively relieves epigenetic silencing of the LAT2 gene mediated by AML1/ETO

The chromosomal translocation (8;21) fuses the hematopoietic transcription factor AML1 (RUNX1) with ETO (RUNX1T1, MTG8), resulting in the leukemia-specific chimeric protein AML1/ETO. This fusion protein has been implicated in epigenetic silencing, recruiting histone deacetylases (HDACs) and DNA methyltransferases to target promoters. Previously, we have identified a novel in vivo AML1/ETO target gene, LAT2 (NTAL/LAB/WBSCR5), which is involved in FcɛR I, c-Kit, B-cell and T-cell receptor signalling. We have now addressed the molecular mechanisms of AML1/ETO-mediated LAT2 repression. In Kasumi-1 cells, where AML1/ETO bound to the LAT2 gene, small interfering RNA (siRNA)-mediated AML1/ETO depletion caused upregulation of LAT2, suggesting a possible direct mechanism of repression. Expression of AML1/ETO was associated with a decrease in acetylation of histones H3, H3K9 and H4, and an increase in H3K9 and H3K27 trimethylation. The class I-specific HDAC inhibitors entinostat (MS-275) and mocetinostat (MGCD0103) induced LAT2 expression specifically in AML1/ETO-expressing cells, resulting in induction of several activating histone marks on the LAT2 gene, including trimethylation of histone H3K4. The combination of entinostat and decitabine increased acetylation of histones H3 and H4, as well as LAT2 mRNA expression, in an at least additive fashion. In conclusion, several repressive histone modifications mark the LAT2 gene in the presence of AML1/ETO, and LAT2 gene derepression is achieved by pharmacological inhibition of HDACs.

Regulation of the adaptor molecule LAT2, an in vivo target gene of AML1/ETO (RUNX1/RUNX1T1), during myeloid differentiation

The leukaemia-specific fusion oncoprotein RUNX1/RUNX1T1 (AML1/ETO), resulting from the chromosomal translocation (8;21) in acute myeloid leukaemia (AML), imposes a striking genotype-phenotype relationship upon this distinct subtype of AML, which is mediated by multiple, co-ordinate downstream effects induced by this chimeric transcription factor. We previously identified the LAT2 gene, encoding the adaptor molecule LAT2 (NTAL, LAB), which is phosphorylated by KIT and has a role in mast cell and B-cell activation, as a target of the repressor activity of RUNX1/RUNX1T1. These results were confirmed and extended by demonstrating downregulation of the LAT2 protein in response to conditional RUNX1/RUNX1T1 expression, and its absence in primary AML with the t(8;21). In contrast, in a cohort of 43 AML patients, higher levels of LAT2 were associated with myelomonocytic features. Differentiation of HL-60 and NB4 cells towards granulocytes by all trans-retinoic acid (ATRA) resulted in downregulation of LAT2; conversely, it was upregulated during phorbol ester-induced monocytic differentiation of HL-60 cells. Forced expression of LAT2 in Kasumi-1 cells resulted in a striking block of ATRA- and phorbol ester-induced differentiation, implicating disturbances of the graded expression of this adaptor molecule in the maturation block of myeloid leukaemia cells.

AML1/ETO proteins control POU4F1/BRN3A expression and function in t(8;21) acute myeloid leukemia

A variety of genetic lesions, including chromosomal translocations, internal tandem duplications, and mutations, have been described in acute myeloid leukemia (AML). Expression profiling has shown that chromosomal translocations, in particular, are associated with distinctive patterns of gene expression. AML exhibiting the translocation t(8;21), which fuses the AML1 and ETO genes, has such a characteristic expression profile. One gene whose expression is highly correlated with the presence of the AML1/ETO fusion is POU4F1, which encodes the POU homeodomain transcription factor BRN3A. Here we show using specific siRNA in t(8;21) cells and overexpression studies in progenitor cells that AML1/ETO promotes expression of POU4F1/BRN3A. This effect requires DNA-binding function of AML1/ETO, and accordingly, AML1/ETO is bound to the POU4F1 locus in t(8;21) cells. Functionally, whereas overexpression of Brn3a in murine hematopoietic progenitor cells induces terminal myeloid differentiation, coexpression of AML1/ETO or AML1/ETO9a blocks this effect. Furthermore, Brn3a reduction by shRNA impairs AML1/ETO-induced immortalization of murine progenitors. In summary, we identify POU4F1/BRN3A as a novel potential upregulated AML1/ETO target gene whose dramatically high expression may cooperate with AML1/ETO in t(8;21) cells.

Multi-sites cleavage of leukemogenic AML1-ETO fusion protein by caspase-3 and its contribution to increased apoptotic sensitivity

Leukemia-associated fusion protein AML1-ETO is a product of the chromosome translocation (8;21) frequently occurred in acute myeloid leukemia (AML). The fusion oncoprotein blocks leukemic cell differentiation, and it also induces growth arrest with increased sensitivity to apoptosis induction. Such dichotomous functions make it difficult to clarify the role of AML1-ETO in leukemogenesis. Here, we systematically showed that constitutively and overexpressed AML1-ETO protein was cleaved to four fragments of 70, 49, 40 and 25 kDa by activated caspase-3 during apoptosis induction by extrinsic mitochondrial and death receptor signaling pathways. The in vitro proteolytic system combined with MALDI-TOF/TOF mass spectrometer confirmed that AML1-ETO and wild-type ETO but not RUNX1 (AML1) proteins were direct substrates of apoptosis executioner caspase-3. Site-directed mutagenesis analyses identified two nonclassical aspartates (TMPD188 and LLLD368) as caspase-3-targeted sites in the AML1-ETO sequence. When these two aspartates were mutated into alanines, more intriguingly, the apoptosis-amplified action of AML1-ETO induction completely disappeared, while inducible expression of the caspase-3-cleaved 70 kDa fragment of AML1-ETO after tetracycline removal is sufficient to enhance apoptotic sensitivity. Further investigations on the potential in vivo effects of such a cleavage and its possible role in leukemogenesis would provide new insights for understanding the biology and treatment of AML1-ETO-associated leukemia.

Selective anti-leukaemic activity of low-dose histone deacetylase inhibitor ITF2357 on AML1/ETO-positive cells

We analysed the in vitro effects of a new hydroxamate derivative, ITF2357, on AML cells. ITF2357 potently induced histone acetylation. ITF2357 0.1 microM blocked proliferation and induced apoptosis in AML1/ETO-positive Kasumi-1 cells, while AML1/ETO-negative HL60, THP1 and NB4 cell lines were sensitive only to 1 microM ITF2357. Apoptosis was induced by 0.1 microM ITF2357 in AML1/ETO-positive primary blasts and U937-A/E cells induced to express AML1/ETO, but not in U937-A/E cells non-expressing AML1/ETO. In Kasumi-1 cells 0.1 microM ITF2357 induced AML1/ETO degradation through a caspase-dependent mechanism. ITF2357 0.1 microM also determined DNMT1 efflux from, and p300 influx to, the nucleus. Moreover, 0.1 microM ITF2357 determined local H4 acetylation and release of DNMT1, HDAC1 and AML1/ETO, paralleled by recruitment of p300 to the IL-3 gene promoter. ITF2357 treatment, however, did not induce re-expression of IL-3 gene. Accordingly, the methylation level of IL-3 promoter, as well as of several other genes, was unmodified. In conclusion, ITF2357 emerged as an anti-leukaemic agent very potent on AML cells, and on AML1/ETO-positive cells in particular. More relevantly, clearly emerged from our results that ITF2357 could be an ideal agent to treat AML subtypes presenting AML1/ETO fusion protein which determine HDAC involvement in leukaemogenesis.

Butyrates and decitabine cooperate to induce histone acetylation and granulocytic maturation of t(8;21) acute myeloid leukemia blasts

Core histones are proteins organized in octamers, to which DNA is wrapped more or less tightly, depending on their acetylation status. Gene transcription is regulated by a complex series of epigenetic modifications, i.e., histone modification such as methylation and acetylation, events determined by the enzymatic activity of histone methyltransferases, and histone acetyltransferases, respectively, the latter counterbalanced by histone deacetylases (HDAC). Acetylation of histones facilitates destabilization of DNA-nucleosome interaction and renders DNA more accessible to transcription factors. Methylation of different specific lysine residues of histones is differently linked to euchromatin (transcripted DNA) or heterochromatin (silenced DNA). On the other hand, methylation of the promoter regions of some genes by DNA methyltransferases (DNMT) leads to transcriptional silencing and is a common mechanism to regulate gene expression. In normal eukaryotic cells, DNA methylation and histone acetylation are interdependent and maintain equilibrium, allowing temporal expression of genes. In neoplastic cells, this balance is frequently disrupted. In leukemic cells, hypermethylation of CpG islands in the promoter region of genes critical for cell cycle and maturation is frequent, and DNMTs were found to be overexpressed, findings paralleled by evidence of transcriptional repression of downstream genes. Therefore, the combination of HDAC and DNMT inhibitors has been considered to be a possible therapeutic approach to restore normal gene expression in acute myeloid leukemia (AML) and other diseases. Human AML1/ETO Kasumi cells were exposed to the HDAC inhibitor D1 (O-n-butanoil-2,3-O-isopropylidene-alpha-D: -mannofuranoside) and 5-aza-deoxycytidine (decitabine) alone and in combination. Histone acetylation as measured by flow cytometry was increased following treatment with D1 and the combination of D1 and decitabine. Addition of D1 alone or in combination with decitabine also led to inhibition of cell proliferation and induction of apoptosis. Thus, treatment of AML with HDAC inhibitors such as D1 and DNMT inhibitors such as decitabine might have clinical benefit for patients, especially these presenting subtypes of AML, like AML1/ETO, in which the leukemogenic mechanism involves corepressor protein complexes containing HDAC and DNMT.

c-Jun N-terminal kinase mediates AML1-ETO protein-induced connexin-43 expression

AML1-ETO fusion protein, a product of leukemia-related chromosomal translocation t(8;21), was reported to upregulate expression of connexin-43 (Cx43), a member of gap junction-constituted connexin family. However, its mechanism(s) remains unclear. By bioinformatic analysis, here we showed that there are two putative AML1-binding consensus sequences followed by two activated protein (AP)1 sites in the 5'-flanking region upstream to Cx43 gene. AML1-ETO could directly bind to these two AML1-binding sites in electrophoretic mobility shift assay, but luciferase reporter assay revealed that the AML1 binding sites were not indispensable for Cx43 induction by AML1-ETO protein. Conversely, AP1 sites exerted an important role in this event. In agreement, AML1-ETO overexpression in leukemic U937 cells activated c-Jun N-terminal kinase (JNK), while its specific inhibitor SP600125 effectively abrogated AML1-ETO-induced Cx43 expression, indicating that JNK signaling pathway contributes to AML1-ETO induced Cx43 expression. These results would shed new insights for understanding mechanisms of AML1-ETO-associated leukemogenesis.

Leukemia-targeting ligands isolated from phage-display peptide libraries

Ligands specifically binding to leukemia cells may be used for drug targeting, resulting in more effective treatment with less side effects. Little is known about receptors specifically expressed on acute myeloid leukemia (AML) cells or ligands thereof. We selected random phage display peptide libraries on Kasumi-1 AML cells. A peptide with the sequence CPLDIDFYC was enriched. Phage displaying this peptide strongly bound to Kasumi-1 and SKNO-1 cells and binding could be inhibited by the cognate peptide. Both, Kasumi-1 and SKNO-1 cells carry the chromosomal translocation t(8;21), leading to aberrant expression of the fusion protein AML1/ETO. CPLDIDFYC also strongly and specifically bound primary AML1/ETO-positive AML blasts as well as U-937 cells with forced AML1/ETO expression, suggesting that the CPLDIDFYC receptor may be upregulated upon AML1/ETO expression. Gene expression profiling comparing a panel of CPLDIDFYC-binding and CPLDIDFYC-nonbinding cell lines identified a set of potential receptors for the CPLDIDFYC peptide. Further analysis suggested that alpha4beta1 integrin (VLA-4) is the CPLDIDFYC receptor. Finally, we showed that the CPLDIDFYC-phage is internalized upon receptor binding, suggesting that the CPLDIDFYC-receptor-ligand interaction may be exploitable for targeting drugs or gene therapy vectors to leukemia cells carrying the suitable receptor.

Inducible expression of AML1-ETO fusion protein endows leukemic cells with susceptibility to extrinsic and intrinsic apoptosis

AML1-ETO, a leukemia-associated fusion protein generated by the frequently occurred chromosome translocation t(8;21) in acute myeloid leukemia, was shown to exert dichotomous functions in leukemic cells, that is, growth arrest versus differentiation block. By the analysis of oligonucleotide microarray, AML1-ETO was shown to modulate the expressions of an impressive array of pro- and anti-apoptotic genes. Here, we investigate potential effects of the ecdysone inducible AML1-ETO expression on apoptosis of leukemic U937 cell line. We show that AML1-ETO significantly stabilizes death receptor Fas protein and increases proapoptotic Bak in addition to reducing Bcl-2 expression. Accordingly, inducible AML1-ETO expression is followed by apoptosis to a lower degree. Especially, AML1-ETO endows leukemic cells with the susceptibility to anti-Fas agonist antibody, ultraviolet light and camptothecin analog NSC606985-induced apoptosis with increased activation of caspase-3/8. Considering that apoptosis-enhancing effect of AML1-ETO would not be favorable to the leukemogenesis harboring the t(8;21) translocation, it must be overcome to fulfill their leukemogenic potential. Complementary to this prediction is that two AML1-ETO-carrying leukemic cells, Kasumi-1 and SKNO-1, present similar sensitivity to apoptosis induction with AML1-ETO-negative leukemic cells. Therefore, genetic and/or epigenetic screenings of apoptosis-related genes modulated by AML1-ETO deserve to be explored for understanding the mechanisms of AML1-ETO-induced leukemogenesis.

Ntal/Lab/Lat2

Non-T cell activation linker (NTAL)/linker for activation of B cells (LAB), now officially termed LAT2 (linker for activation of T cells 2) is a 25-30kDa transmembrane adaptor protein (TRAP) associated with glycolipid-enriched membrane fractions (GEMs; lipid rafts) in specific cell types of hematopoietic lineage. Tyrosine phosphorylation of NTAL/LAB/LAT2 is induced by FcvarepsilonRI aggregation and Kit dimerization in mast cells, FcgammaRI aggregation in monocytes, and BCR aggregation in B cells. NTAL/LAB/LAT2 is also expressed in resting NK cells but, unlike the related TRAP, LAT, not in resting T cells. As demonstrated in monocytes and B cells, phosphorylated NTAL/LAB/LAT2 recruits signaling molecules such as Grb2, Gab1 and c-Cbl into receptor-signaling complexes. Although gene knock out and knock down studies have indicated that NTAL/LAB/LAT2 may function as both a positive and negative regulator of mast cell activation, its precise role in the activation of these and other hematopoietic cells remains enigmatic.

Reversal of p15/INK4b hypermethylation in AML1/ETO-positive and -negative myeloid leukemia cell lines

In vitro and in vivo, myeloid leukemic and preleukemic cells exhibit variable sensitivity to the antiproliferative and proapoptotic effects induced already at low concentrations of DNA methyltransferase (DNMT) inhibitors. The molecular mechanisms underlying this variable sensitivity of leukemic blasts to azanucleosides such as 5-azacytidine and 5-aza-2'-deoxycytidine (DAC) may involve modifier effects of specific fusion proteins such as AML1/ETO. The cyclin-dependent kinase inhibitor p15/INK4b is one potential target of DNA demethylating activity in AML and MDS where it is frequently silenced by hypermethylation. To study sensitivity to DAC in myeloid leukemia cells, we chose the myeloid cell lines Kasumi-1 (expressing AML1/ETO), KG-1 and KG-1a (both AML1/ETO-negative) all of which a highly methylated p15/INK4b gene. Treatment with DAC resulted in dose-dependent regional demethylation of p15/INK4b in Kasumi-1 and KG-1, but only to a modest degree in KG-1a cells. Demethylation was associated with induction of p15/INK4b protein expression. Growth-inhibitory and proapoptotic activity of DAC was significantly higher in Kasumi-1 than in KG-1a cells, and sensitization of cells to a cooperating effect of All-trans retinoic acid and of the histone deacetylase (HDAC) inhibitor Trichostatin A was observed. DAC-induced growth inhibition and apoptosis were enhanced when AML1/ETO was conditionally expressed in AML1/ETO-negative U-937 cells. In conclusion, hypomethylation and reactivation of p15/INK4b in myeloid cell lines are among the molecular events associated with DAC-induced growth arrest and apoptosis. Further studies of AML1/ETO as a modifier of the epigenotype and sensitivity of myeloid cells to inhibitors of DNMTs and HDACs appear warranted.

Leukemogenic AML1-ETO fusion protein upregulates expression of connexin 43: the role in AML 1-ETO-induced growth arrest in leukemic cells

AML1-ETO, a fusion protein generated by the chromosomal translocation t(8;21), is frequently associated with acute myeloid leukemia (AML). In addition to blocking differentiation, AML1-ETO is also shown to induce growth arrest in AML cells, which is unfavorable for leukemogenesis harboring the t(8;21) translocation. However, its precise mechanism is still unclear. Here we provide the first demonstration that the conditional expression of AML1-ETO by the ecdysone-inducible system dramatically increases the expression of connexin 43 (CX43), together with growth arrest at G1 phase in leukemic U937 cells. We also show that the CX43 induction inhibits the proliferation of U937 cells at G1 phase, while the suppression of CX43 expression by small interfering RNA (siRNA) effectively overcomes the growth-inhibitory effect of AML1 -ETO in leukemic cells. Furthermore, either AML1-ETO or CX43 induction elevates cell-cycle negative regulator P27(kip1) protein by inhibiting its degradation, which is antagonized by siRNA against CX43. Taken together, our data indicate that CX43 plays a role in AML1-ETO-induced growth arrest possibly through the accumulation of P27(kip1) protein. The potential mutation or/and epigenetic alterations of CX43 and its related gene(s) deserve to be explored in AML1-ETO-positive AML patients.

Inhibitors of DNA methylation and histone deacetylation independently relieve AML1/ETO-mediated lysozyme repression

The human lysozyme (LZM) gene is highly methylated in LZM-nonexpressor immature myeloid and in nonmyeloid cells and unmethylated only in LZM-expressing cells. Extended methylation analyses of the CpG-poor 5' flanking region and of the exon 4 CpG island (both containing Alu elements) of the LZM gene were now performed. Marked demethylation was noted after treatment of AML1/ETO-positive Kasumi-1 cells with the DNA methyltransferase (DNMT) inhibitor 5-aza-2'-deoxycytidine (5-azaCdR), not associated with cellular differentiation. LZM mRNA in Kasumi-1, but not in several AML1/ETO-negative myeloid cell lines, was specifically and independently up-regulated upon treatment with 5-azaCdR and, to a lesser extent, with the histone deacetylase (HDAC) inhibitor trichostatin A (TSA). Increased chromatin accessibility within the 5' LZM gene was observed concomitantly with 5-azaCdR-induced demethylation. In contrast, TSA treatment had no effect on chromatin accessibility, but, as shown by chromatin immunoprecipitation, resulted in increased acetylation of histones H3 and H4. Repression of LZM transcription is mediated by conditional AML1/ETO expression in an inducible cell line model (U-937), and is reversed by siRNA "knock-down" of AML1/ETO in Kasumi-1 cells (Dunne et al., Oncogene 25: 2006). Antagonization of LZM repression following conditional expression of AML1/ETO was achieved by TSA. In conclusion, we demonstrate complex interactions between DNA methylation and histone modifications in mediating LZM repression, which implicate AML1/ETO as one component involved in local chromatin remodeling. Interestingly, inhibitors of DNMTs and HDACs independently relieve repression of this CpG-poor gene in AML1/ETO-positive cells.

Aberrant transcriptional regulation of the MLL fusion partner EEN by AML1-ETO and its implication in leukemogenesis

The EEN (extra eleven nineteen) gene, located on chromosome 19p13, was cloned as a fusion with MLL from a patient with acute myeloid leukemia (AML) with translocation t(11;19)(q23;p13). In this study, we characterized the genomic structure of the EEN gene, including its 5′ regulatory region and transcription start site (TSS). We found that Sp1 could bind to the guanine-cytosine (GC)–stretch of the EEN promoter and was critical for the normal EEN expression, whereas the leukemia-associated fusion protein AML1-ETO could aberrantly transactivate the EEN gene through an AML1 binding site. Of note, overexpressed EEN showed oncogenic properties, such as transforming potential in NIH3T3 cells, stimulating cell proliferation, and increasing the activity of transcriptional factor AP-1. Retroviral transduction of EEN increased self-renewal and proliferation of murine hematopoietic progenitor cells. Moreover, Kasumi-1 and HL60-cell growth was inhibited with down-regulation of EEN by RNAi. These findings demonstrate that EEN might be a common target in 2 major types of AML associated with MLL or AML1 translocations, and overexpression of EEN may play an essential role in leukemogenesis.

siRNA-mediated AML1/MTG8 depletion affects differentiation and proliferation-associated gene expression in t(8;21)-positive cell lines and primary AML blasts

The chromosomal translocation t(8;21) is associated with 10-15% of all cases of acute myeloid leukaemia (AML). The resultant fusion protein AML1/MTG8 interferes with haematopoietic gene expression and is an important regulator of leukaemogenesis. We studied the effects of small interfering RNA (siRNA)-mediated AML1/MTG8 depletion on global gene expression in t(8;21)-positive leukaemic cell lines and in primary AML blasts using cDNA arrays, oligonucleotide arrays and real-time reverse transcription-polymerase chain reaction (RT-PCR). Suppression of AML1/MTG8 results in the increased expression of genes associated with myeloid differentiation, such as AZU1, BPI, CTSG, LYZ and RNASE2 as well as of antiproliferative genes such as IGFBP7, MS4A3 and SLA both in blasts and in cell lines. Furthermore, expression levels of several genes affiliated with drug resistance or indicative of poor prognosis AML (BAALC, CD34, PRG2, TSPAN7) are affected by AML1/MTG8 depletion. In conclusion, siRNA-mediated suppression of AML1/MTG8 cause very similar changes in gene expression pattern in t(8;21)-positive cell lines and in primary AML blasts. Furthermore, the results suggest that the specific targeting of AML1/MTG8 function may be a promising approach for complementing existing treatment strategies.

ELA2 is regulated by hematopoietic transcription factors, but not repressed by AML1-ETO

A 117 bp fragment of the human ELA2 promoter has been characterized that can act as a minimal promoter for the expression of neutrophil elastase. Chromatin immunoprecipitation and siRNAs revealed that expression of ELA2 is regulated by the acute myeloid human leukemia 1 protein (AML1), C/EBPalpha, PU.1 and c-Myb transcription factors. ELA2 has also been investigated as a possible target of the leukemic fusion protein AML1-ETO resulting from the t(8;21) chromosomal translocation. AML1-ETO, like AML1, binds the ELA2 promoter in the myeloid cell lines Kasumi-1 and U937, but unexpectedly fails to significantly alter expression of ELA2. Although AML1-ETO downregulates the expression of C/EBPalpha, changes in C/EBPalpha expression do not correlate with changes in the expression of ELA2. Our observations indicate that AML1-ETO may not be a constitutive repressor of gene expression in every case in which it can associate with DNA, either on its own or in conjunction with C/EBPalpha. Since neither ETO nor AML1-ETO are typically expressed in hematopoietic progenitors, we hypothesize that it is the interactions between AML1-ETO and regulatory cofactors in disease-state cells that alter gene expression programs during hematopoiesis. These protein-protein interactions may not require simultaneous DNA binding by AML1-ETO for the deleterious effects of the fusion protein to be realized.

Genomic organization, promoter activity, and expression of the human choline transporter-like protein 1

Choline transporter-like (CTL) proteins of the CTL1 family are novel transmembrane proteins implicated in choline transport for phospholipid synthesis. In this study, we characterized the 5'-flanking region of the human (h)CTL1 gene and examined some of the possible mechanisms of its regulation, including promoter activity, splicing, and expression. The transcription start site of the hCTL1 gene was mapped by 5'-rapid amplification of cDNA ends (RACE), and the presence of two splice variants, hCTL1a and hCTL1b, was investigated using isoform-specific PCR and 3'-RACE. The hCTL1 promoter region of approximately 900 bp was isolated from MCF-7 human breast cancer cells. The promoter was TATA-less and driven by a long stretch of GC-rich sequence in accordance with widespread expression of hCTL1 at both mRNA and protein levels. Deletion analyses demonstrated that a very strong promoter is contained within 500 bp of the transcription start site, and more upstream regions did not increase its activity. The core promoter that conferred the minimal transcription is within the -188/+27-bp region, and its activity varied in human breast cancer and mouse skeletal muscle cells. Multiple motifs within the promoter regulatory region bound nuclear factors from both cultured cells and normal human skeletal muscle. The motifs within the three regions [S1 (-92/-61 bp), S2 (-174/-145 bp), and S3 (-289/-260 bp)] contained overlapping binding sites for hematopoietic transcription factors and ubiquitous transcription factors, in line with the expected gene function. Genomic analyses demonstrated a high conservation of hCTL1 and mouse CTL1 proximal promoters. Accordingly, mRNA profiles demonstrated that human splice variants were expressed ubiquitously, as demonstrated for the mouse transcripts; however, they differed from the profiles of rat CTL1 transcripts, which were more restricted to neurons and intestinal tissues. The shorter hCTL1b variant contained the cytosolic COOH-terminal motif L651KKR654 for endoplasmic reticulum retrieval/retention. This retention signal was conserved in hCTL1b and rat and mouse CTL1b and is typical for transmembrane proteins of type 1 topology.

Desferrioxamine induces leukemic cell differentiation potentially by hypoxia-inducible factor-1 alpha that augments transcriptional activity of CCAAT/enhancer-binding protein-alpha

We reported recently that cobalt chloride-simulated hypoxia and mild hypoxia modified the differentiation of human acute myeloid leukemic (AML) cells, probably acting via a hypoxia-inducible factor-1 alpha (HIF-1 alpha)-dependent mechanism. In this study, we investigated the effect of desferrioxamine (DFO), an iron chelator with 'hypoxia-mimetic' activity, on the differentiation of AML cells. The results showed that DFO at nontoxic concentrations induced the differentiation of AML cell lines NB4 and U937, as assessed by morphological criteria and differentiation-associated antigens. DFO-induced differentiation parallel to the rapid accumulation of HIF-1 alpha protein in these two cell lines. Of importance, the transient transfection of HIF-1 alpha cDNA induced U937 cells to develop the differentiation-related alterations such as growth arrest and increased CD11b expression. Furthermore, the inducible expression of chromosome translocation t(8;21)-generated leukemogenic AML1-ETO fusion gene attenuated DFO-induced differentiation of U937 cells with the decrease of CCAAT/enhancer-binding protein alpha (C/EBP alpha), a critical factor for granulocytic differentiation. Using immunoprecipitation and luciferase reporter assay, HIF-1 alpha was also shown to interact physically with and to increase the transcriptional activity of C/EBP alpha. Taken together, these results provided novel evidence for a role of HIF-1 alpha in AML cell differentiation, and suggested that C/EBP alpha might be a downstream effector for HIF-1 alpha-mediated differentiation.