Aberrant recruitment of the nuclear receptor corepressor-histone deacetylase complex by the acute myeloid leukemia fusion partner ETO

Inhibition of retinoic acid receptor signaling by Ski in acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous disease with multiple different cytogenetic and molecular aberrations contributing to leukemic transformation. We compared gene expression profiles of 4608 genes using cDNA-arrays from 20 AML patients (nine with -7/del7q and 11 with normal karyotype) with 23 CD34+ preparations from healthy bone marrow donors. SKI, a nuclear oncogene, was highly up regulated. In a second set of 183 AML patients analyzed with real-time PCR, the highest expression level of SKI in AML with -7/del7q could be confirmed. As previously described, Ski associates with the retinoic acid receptor (RAR) complex and can repress transcription. We wanted to investigate the interference of Ski with RARalpha signaling in AML. Ski was co-immunoprecipitated and colocalized with RARalpha. We also found that overexpression of wild-type Ski inhibited the prodifferentiating effects of retinoic acid in U937 leukemia cells. Mutant Ski, lacking the N-CoR binding, was no more capable of repressing RARalpha signaling. The inhibition by wild-type Ski could partially be reverted by the histone deacetylase blocking agent valproic acid. In conclusion, Ski seems to be involved in the blocking of differentiation in AML via inhibition of RARalpha signaling.

Dynamic and reversibility of heterochromatic gene silencing in human disease

In eukaryotic organisms cellular fate and tissue specific gene expression are regulated by the activity of proteins known as transcription factors that by interacting with specific DNA sequences direct the activation or repression of target genes. The post genomic era has shown that transcription factors are not the unique key regulators of gene expression. Epigenetic mechanisms such as DNA methylation, post-translational modifications of histone proteins, remodeling of nucleosomes and expression of small regulatory RNAs also contribute to regulation of gene expression, determination of cell and tissue specificity and assurance of inheritance of gene expression levels. The relevant contribution of epigenetic mechanisms to a proper cellular function is highlighted by the effects of their deregulation that cooperate with genetic alterations to the development of various diseases and to the establishment and progression of tumors.

Mtgr1 is a transcriptional corepressor that is required for maintenance of the secretory cell lineage in the small intestine

Two members of the MTG/ETO family of transcriptional corepressors, MTG8 and MTG16, are disrupted by chromosomal translocations in up to 15% of acute myeloid leukemia cases. The third family member, MTGR1, was identified as a factor that associates with the t(8;21) fusion protein RUNX1-MTG8. We demonstrate that Mtgr1 associates with mSin3A, N-CoR, and histone deacetylase 3 and that when tethered to DNA, Mtgr1 represses transcription, suggesting that Mtgr1 also acts as a transcriptional corepressor. To define the biological function of Mtgr1, we created Mtgr1-null mice. These mice are proportionally smaller than their littermates during embryogenesis and throughout their life span but otherwise develop normally. However, these mice display a progressive reduction in the secretory epithelial cell lineage in the small intestine. This is not due to the loss of small intestinal progenitor cells expressing Gfi1, which is required for the formation of goblet and Paneth cells, implying that loss of Mtgr1 impairs the maturation of secretory cells in the small intestine.

Novel RUNX1-PRDM16 fusion transcripts in a patient with acute myeloid leukemia showing t(1;21)(p36;q22)

The t(1;21)(p36;q22) is a recurrent chromosome abnormality associated with therapy-related acute myeloid leukemia (AML). Although involvement of RUNX1 has been detected by fluorescence in situ hybridization analysis, the partner gene has not been reported previously. We identified a novel RUNX1 partner gene, MDS1/EVI1-like-gene 1 (PRDM16), in an AML patient with t(1;21). Alternative splicing of the fusion gene generates five different fusion transcripts. In two of them, the PRDM16 reading frame is maintained in the fusion with RUNX1, suggesting that the RUNX1-PRDM16 gene fusion results in the production of a protein that is highly homologous to the RUNX1-MDS1/EVI1 chimeric protein. It is suggested that PRDM16 and MDS1/EVI1 share a common molecular mechanism for the leukemogenesis of RUNX1-associated leukemia. Characterization of the RUNX1-PRDM16 fusion protein and comparison with the RUNX1-MDS1/EVI1 protein will facilitate the understanding of the mechanisms underlying RUNX1-associated leukemia.

M344 is a novel synthesized histone deacetylase inhibitor that induces growth inhibition, cell cycle arrest, and apoptosis in human endometrial cancer and ovarian cancer cells

OBJECTIVE

Histone deacetylase inhibitors (HDACIs) can inhibit cell proliferation, induce cell cycle arrest, and stimulate apoptosis of cancer cells.

METHODS

We investigated the effects of a novel synthesized HDACI, M344, on Ishikawa endometrial cancer cell line, SK-OV-3 ovarian cancer cell line, and normal human endometrial epithelial cells. Endometrial and ovarian cancer cells were treated with various concentrations of M344, and its effect on cell growth, cell cycle, apoptosis, and related measurements was investigated.

RESULTS

3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays showed that all endometrial and ovarian cancer cell lines were sensitive to the growth inhibitory effect of M344, although normal endometrial epithelial cells were viable after the treatment with the same doses of M344 that induced growth inhibition of endometrial and ovarian cancer cells. Cell cycle analysis indicated that their exposure to M344 decreased the proportion of cells in the S-phase and increased the proportion in the G0/G1 phases of the cell cycle. Induction of apoptosis was confirmed by annexin V staining of externalized phosphatidylserine and loss of the transmembrane potential of mitochondria. This induction occurred in concert with altered expression of genes related to cell growth, malignant phenotype, and apoptosis. Furthermore, M344 treatment of these cell lines increased acetylation of H3 and H4 histone tails.

CONCLUSIONS

These results raise the possibility that M344 may prove particularly effective in the treatment of endometrial cancers and ovarian cancers.

The molecular pathogenesis of acute myeloid leukemia

The description of the molecular pathogenesis of acute myeloid leukemias (AML) has seen dramatic progress over the last years. Two major types of genetic events have been described that are crucial for leukemic transformation: alterations in myeloid transcription factors governing hematopoietic differentiation and activating mutations of signal transduction intermediates. These processes are highly interdependent, since the molecular events changing the transcriptional control in hematopoietic progenitor cells modify the composition of signal transduction molecules available for growth factor receptors, while the activating mutations in signal transduction molecules induce alterations in the activity and expression of several transcription factors that are crucial for normal myeloid differentiation. The purpose of this article is to review the current literature describing these genetic events, their biological consequences and their clinical implications. As the article will show, the recent description of several critical transforming mutations in AML may soon give rise to more efficient and less toxic molecularly targeted therapies of this deadly disease.

Leukemia fusion proteins and co-repressor complexes: changing paradigms

Many cases of acute myelogenous leukemia (AML) are characterized by non-random chromosomal translocations that fuse a DNA-binding protein with a transcriptional regulator, which in turn may aberrantly recruit a co-repressor complex. The similarities in this pattern between different AML chimeric fusions have led to a paradigm that stresses the importance of the co-repressor complex in altering the pattern of expression of genes targeted by the DNA-binding moiety of the fusion. Such findings beg the question of whether the fusion proteins merely serve as anchors to recruit the co-repressor complex or whether they play other significant roles in leukemogenesis. The answers to this question may have therapeutic importance since we now have the ability to target various components of the co-repressor complex, such as the histone deacetylase (HDAC) enzymes. In this Prospect, we wish to highlight some of the complexities and difficulties with the existing molecular paradigm of this challenging group of disorders.

AML1B transcriptional repressor function is impaired by the Flt3-internal tandem duplication

Fms-like tyrosine kinase 3 (Flt3) is a type III receptor tyrosine kinase. The internal tandem duplication (ITD) of the juxtamembrane region of this receptor is the most prevalent mutation in acute myeloid leukaemia (AML). The silencing mediator of retinoic and thyroid hormone receptors (SMRT) co-repressor recruits histone deacetylases (HDAC) and mediates transcriptional repression by interacting with various transcription factors. We recently reported that Flt3-ITD interferes with the transcriptional and biological action of promyelocytic leukaemia zinc finger transcriptional repressor by dissociating it from SMRT. In this study, we aimed to clarify whether the repressional activity of other well-known oncoproteins, such as AML1/Runx1 (AML1), is also affected by Flt3-ITD. We verified that the repression activity of AML1B, the isoform of AML1, is dependent on HDAC activity by using HDAC inbitor trichostatin A in GAL4 reporter assays. Mammalian two-hybrid assays demonstrated that this protein interacts with SMRT. Furthermore, this AML1B-SMRT interaction was disrupted by the overexpression of Flt3-ITD, leading to the reduction of AML1B repression activity. Additionally, we showed AML1B repression target, p21 (WAF1/CIP1), was aberrantly expressed in Flt3-ITD stably expressed BaF3 cells. Taken together, Flt3-ITD disrupts transcriptional repressor functions resulting in aberrant gene regulation in leukaemic cells.

The clinical relevance of steroid hormone receptor corepressors

Steroid hormone receptors are ligand-dependent transcription factors that control a variety of essential physiologic and developmental processes in humans. The functional activity of a steroid receptor is regulated not only by hormones but also by an array of regulatory proteins such as coactivators, corepressors, and chromatin modifiers. Contrary to an earlier notion that corepressors and coactivators exist in separate complexes, these molecules, which have apparently opposite functions, are increasingly being found in the same complex, which allows for efficient transcriptional control mechanisms. These control mechanisms are in turn regulated by an array of post-translational modifications under the influence of upstream and local signaling networks. Because the outcome of steroidal hormone receptor transcriptional complexes is measured in terms of the expression of target genes, any dysregulation of coregulator complexes perturbs normal homeostasis and could contribute to the development and maintenance of malignant phenotypes. Increasing evidence implicating steroid hormone receptors and their coregulators in various pathophysiologic conditions has elicited interest in their structure and biology. Further advances in this field of study should open up a unique window for novel targeted therapies for diseases such as cancer. Here we briefly review the clinical relevance of corepressors, with a particular focus on their role in the development of cancerous phenotypes.

Drug therapy for acute myeloid leukemia

Although improvement in outcomes has occurred in younger adults with acute myeloid leukemia (AML) during the past 4 decades, progress in older adults has been much less conspicuous, if at all. Approximately 50% to 75% of adults with AML achieve complete remission (CR) with cytarabine and an anthracycline such as daunorubicin or idarubicin or the anthracenedione mitoxantrone. However, only approximately 20% to 30% of the patients enjoy long-term disease survival. Various postremission strategies have been explored to eliminate minimal residual disease. The optimal dose, schedule, and number of cycles of postremission chemotherapy for most patients are not known. A variety of prognostic factors can predict outcome and include the karyotype of the leukemic cells and the presence of transmembrane transporter proteins, which extrude certain chemotherapy agents from the cell and confer multidrug resistance and mutations in or over expressions of specific genes such as WT1, CEBPA, BAX and the ratio of BCL2 to BAX, BAALC, EVI1, KIT, and FLT3. Most recently, insights into the molecular pathogenesis of AML have led to the development of more specific targeted agents and have ushered in an exciting new era of antileukemia therapy. Such agents include the immunoconjugate gemtuzumab ozogamicin, multidrug resistance inhibitors, farnesyl transferase inhibitors, histone deacetylase and proteosome inhibitors, antiangiogenesis agents, Fms-like tyrosine kinase 3 (FLT3) inhibitors, and apoptosis inhibitors.

The Kasumi-1 cell line: a t(8;21)-kit mutant model for acute myeloid leukemia

The Kasumi-1 cell line is an intensively investigated model system of Acute Myeloid Leukemia with t(8;21) translocation, that represents 1 of the 2 main subtypes of Core Binding Factor Leukemia (CBFL). Since establishment in 1991 the Kasumi-1 cell line has provided the tool to study the peculiar molecular, morphologic, immunophenotypic findings of AML with t(8;21) and the functional consequences of the AML1-ETO fusion oncogene on myeloid differentiation. Leukemogenesis involves multiple genetic changes and, as suggested by murine experiments and other findings in humans, AML1-ETO expression may not be sufficient for full blown leukemia. In agreement with the "two hits" model of leukemogenesis, based on the cooperation between 1 class of mutations that impair hematopoietic differentiation and a second class of mutations that confer a proliferative and/or survival advantage to hematopoietic progenitors an activating mutation in the tyrosine kinase domain of the c-kit gene was identified in the AML1/ETO expressing Kasumi-1 cell line. The dosage of the Asn822Lys mutated allele was shown to be about 5-fold compared to the normal allele and c-kit amplification was found to map to minute 4cen-q11 marker chromosomes, likely derived from the extra chromosome 4 recorded in the newly established cell line. The combination of t(8;21) and trisomy 4 leading to enhanced dosage of a mutated kit allele is a feature of a few CBFL patients reproduced by the Kasumi-1 cell model. The Kasumi-1 cell line, paralleling the commitment stage of CBF leukemia also provides a valuable resource to investigate the effect of tyrosine kinase kit mutant on the main KIT-regulated signal transduction pathways, i.e. MAPK, PI3K/AKT and STAT3 and the diverse inhibitory effect exerted by STI 571 on these KIT mutant activated pathways. PI3K-dependent activation of AKT and STAT activation was observed in Kasumi-1 cells. Contrary to the expectations for an amplified tyrosine kinase kit mutant, we found that STI 571 inhibited KIT Asn822Lys tyrosine phosphorylation and downstream JNK and STAT3 effectors in Kasumi-1 cells, but had no effect on constitutive activation of AKT, suggesting that signaling by tyrosine kinases other than KIT may be responsible for its activation in Kasumi-1 cells. Independent findings on the same model system provide complementary insights into designing strategies for treatment of CBF leukemia associated with mutations in the KIT catalytic domain.

Functional interactions with Pit-1 reorganize co-repressor complexes in the living cell nucleus

The co-repressor proteins SMRT and NCoR concentrate in specific subnuclear compartments and function with DNA-binding factors to inhibit transcription. To provide detailed mechanistic understanding of these activities, this study tested the hypothesis that functional interactions with transcription factors, such as the pituitary-gland-specific Pit-1 homeodomain protein, direct the subnuclear organization and activity of co-repressor complexes. Both SMRT and NCoR repressed Pit-1-dependent transcription, and NCoR was co-immunoprecipitated with Pit-1. Immunofluorescence experiments confirmed that endogenous NCoR is concentrated in small focal bodies and that incremental increases in fluorescent-protein-tagged NCoR expression lead to progressive increases in the size of these structures. In pituitary cells, the endogenous NCoR localized with endogenous Pit-1 and the co-expression of a fluorescent-protein-labeled Pit-1 redistributed both NCoR and SMRT into diffuse nucleoplasmic compartments that also contained histone deacetylase and chromatin. Automated image-analysis methods were applied to cell populations to characterize the reorganization of co-repressor proteins by Pit-1 and mutation analysis showed that Pit-1 DNA-binding activity was necessary for the reorganization of co-repressor proteins. These data support the hypothesis that spherical foci serve as co-repressor storage compartments, whereas Pit-1/co-repressor complexes interact with target genes in more widely dispersed subnuclear domains. The redistribution of co-repressor complexes by Pit-1 might represent an important mechanism by which transcription factors direct changes in cell-specific gene expression.

New method to detect histone acetylation levels by flow cytometry

BACKGROUND

Reversible histone acetylation affects chromatin structural organization, thus regulating gene expression and other nuclear events. Levels of histone acetylation are tightly modulated in normal cells, and alterations of their regulating mechanisms have been shown to be involved in tumorigenesis.

METHODS

We developed a new flow cytometric technique for detection of histone acetylation, based on a specific monoclonal antibody that recognizes acetylated histone tails. Bivariate analysis for histone acetylation levels and DNA were performed to study modulation of chromatin organization during the cell cycle and after induction of histone hyperacetylation by the histone deacetylase (HDAC) inhibitor trichostatin A (TSA). Histone acetylation and transcription levels were monitored during differentiation induced by retinoic acid alone or in combination with TSA. Blood samples from patients were analyzed with the described protocol to monitor the effects of HDAC inhibitors in vivo and validate the developed protocol for clinical usage.

RESULTS

Flow cytometric detection of acetylation status can successfully detect modifications induced by HDAC inhibitor treatment in vivo as demonstrated by analysis of various blood samples from patients treated with valproic acid. Changes in acetylation levels during the cell cycle demonstrated a reproducible increase in histone acetylation during the replication phase that was subsequently decreased at the G2M entrance, thus paralleling the behavior of DNA replication and transcriptional activity.

CONCLUSIONS

Multiparameter analysis of histone acetylation and expression of molecular markers, DNA ploidy, and/or cell cycle kinetics can provide a quick and statistically reliable tool for the diagnosis and evaluation of treatment efficacy in clinical trials using HDAC inhibitors.

Significant growth suppression of synovial sarcomas by the histone deacetylase inhibitor FK228 in vitro and in vivo

About 97% of synovial sarcomas harbor the SYT-SSX fusion gene by chromosomal translocation. We found that the histone deacetylase (HDAC) inhibitor FK228 significantly suppressed the growth of synovial sarcoma cells as compared with that of osteosarcoma. The 50% growth inhibition IC50 value we obtained for FK228 was 0.02-0.2 nM, and it indicates that its suppression effect on synovial sarcoma cells is the highest of any of the HDAC inhibitors yet reported. It was not likely that the growth suppression of FK228 depends on the doubling time of these cells. Introduction of SYT-SSX cDNA into HEK293 cells enhanced the sensitivity of the cells for FK228. Immunostaining of the FK228-treated cells using an anti-acetyl-histone H3 antibody showed that FK228 inhibits deacetylation of histone. In a mice assay, the growth of synovial sarcoma cells was markedly inhibited by FK228 treatment, and the invasion of tumors into surrounding tissues was suppressed. These results suggest that FK228 may be useful in developing therapeutic strategies to treat synovial sarcoma.

MYND-less splice variants of AML1-MTG8 (RUNX1-CBFA2T1) are expressed in leukemia with t(8;21)

The AML1-MTG8 fusion gene is generated by chromosome translocation t(8;21), which is frequently observed in acute myeloid leukemia. The fusion gene produces a chimeric transcription factor that suppresses the expression of AML1-target genes via the MTG8 part of the chimeric protein, which is thought to be the primary cause of leukemia. The C-terminal region of MTG8 contains the MYND domain, represented by highly conserved zinc-finger-like protein motifs, and is known to interact with corepressor proteins. We found that, instead of the MYND domain, an alternative last exon of MTG8 encoding 27 amino acids in-frame is expressed naturally in human adult testis and in several leukemia cell lines. This type of alternative splicing also occurred in the AML1-MTG8 fusion gene at high levels in leukemia cell lines with t(8;21), as well as in blast cells of leukemia patients with t(8;21). The variant proteins of both MTG8 and AML1-MTG8 reduced transcriptional repressor activity in a mammalian two-hybrid assay. However, mixed expression of these variants with wild-type MTG8 recovered their repressor activity, suggesting that these variants also act as repressors in vivo where wild-type MTG8 and other family members exist in abundance. On the other hand, the MYND-less variants acquired a higher affinity for binding to MTG8 and formed a multimer, whereas the wild-type protein forms a dimer. Thus, expression of the MYND-less variants by the dysregulation of splicing machinery, which stimulates the oligomerization of fusion proteins in leukemia cells, may enhance malignant conversion of hematopoietic cells.

The histone deacetylase inhibitor valproic acid alters sensitivity towards all trans retinoic acid in acute myeloblastic leukemia cells

Acute myeloblastic leukemia (AML) may be classified in a number of ways. Using the French American British classification, the M3 form of the disease or acute promyelocytic leukemia (APL) has been found to be sensitive in vitro and in vivo to the retinoid all trans retinoic acid (ATRA). The mechanism for this is by restoration of normal gene expression through the release of histone deacetylase complexes (HDACs). In contrast to APL, other forms of AML are either nonresponsive or show blunted responses to ATRA. We evaluated if the inhibitor of HDAC activity, valproic acid (VPA), could mimic or enhance retinoid sensitivity in the AML cell line, OCI/AML-2, and clinical samples derived from patients with AML. An Affymetrix GeneChip experiment demonstrated that VPA modulated the expression of numerous genes in OCI/AML-2 cells that were not affected by ATRA including p21, a retinoid responsive gene in APL. VPA induced p21 expression in OCI/AML-2 cells and the majority of the AML samples tested; this was associated with cell cycle arrest and apoptosis not seen with ATRA alone. The addition of ATRA to VPA accentuated many of these responses, supporting the potential beneficial combination of these drugs in the treatment of AML.

The RUNX genes: gain or loss of function in cancer

The RUNX genes have come to prominence recently because of their roles as essential regulators of cell fate in development and their paradoxical effects in cancer, in which they can function either as tumour-suppressor genes or dominant oncogenes according to context. How can this family of transcription factors have such an ambiguous role in cancer? How and where do these genes impinge on the pathways that regulate growth control and differentiation? And what is the evidence for a wider role for the RUNX genes in non-haematopoietic cancers?

The t(8;21) translocation converts AML1 into a constitutive transcriptional repressor

The human translocation (t8;21) is associated with approximately 12% of the cases of acute myelogenous leukemia. Two genes, AML1 and ETO, are fused together at the translocation breakpoint, resulting in the expression of a chimeric protein called AML1-ETO. AML1-ETO is thought to interfere with normal AML1 function, although the mechanism by which it does so is unclear. Here, we have used Drosophila genetics to investigate two models of AML1-ETO function. In the first model, AML1-ETO is a constitutive transcriptional repressor of AML1 target genes, regardless of whether they are normally activated or repressed by AML1. In the second model, AML1-ETO dominantly interferes with AML1 activity by, for example, competing for a common co-factor. To discriminate between these models, the effects of expressing AML1-ETO were characterized and compared with loss-of-function phenotypes of lozenge (lz), an AML1 homolog expressed during Drosophila eye development. We also present results of genetic interaction experiments with AML1 co-factors that are not consistent with AML1-ETO behaving as a dominant-negative factor. Instead, our data suggest that AML1-ETO acts as a constitutive transcriptional repressor.

The Leukemia-associated ETO homologues are differently expressed during hematopoietic differentiation

The Eight twenty-one (ETO) homologues are nuclear repressor proteins including ETO, myeloid-transforming gene-related protein 1 (MTGR1), and myeloid-transforming gene chromosome 16 (MTG16). ETO and MTG16 are both part of fusion proteins resulting from chromosomal translocations associated with acute myeloid leukemia. Expression of these chimeras results in a differentiation block that contributes to the onset of leukemia. In order to elucidate the relation between the ETO homologues and hematopoietic differentiation, we determined the expression of the homologues during differentiation of leukemic and normal hematopoietic cells. Our results showed MTGR1 and MTG16 to be ubiquitously expressed in leukemic cell lines, whereas expression of ETO was observed only in an erythroleukemic cell line. The MTGR1 and MTG16 proteins decreased during all trans-retinoic acid-, but not vitamin D(3)-induced differentiation of leukemic cells. The reduction seemed to reflect a decrease in transcript levels as well as in protein stability. MTGR1 transcripts were ubiquitously expressed in human bone marrow cells. The MTG16 transcripts of CD34(+) progenitor cells were rapidly downregulated by cytokine-induced differentiation into myeloid or erythroid lineages. ETO transcripts, present at very low abundance in CD34(+) progenitor cells, were transiently upregulated during erythroid differentiation. In conclusion, the differential expression of the ETO homologues suggests that they may have a potential role in hematopoietic differentiation.

Histone deacetylase 3 (HDAC3) activity is regulated by interaction with protein serine/threonine phosphatase 4

Histone deacetylase 3 (HDAC3) is one of four members of the human class I HDACs that regulates gene expression by deacetylation of histones and nonhistone proteins. Early studies have suggested that HDAC3 activity is regulated by association with the corepressors N-CoR and SMRT. Here we demonstrate that, in addition to protein-protein interactions with NCoR/SMRT, the activity of HDAC3 is regulated by both phosphorylation and dephosphorylation. A protein kinase CK2 phosphoacceptor site in the HDAC3 protein was identified at position Ser424, which is a nonconserved residue among the class I HDACs. Mutation of this residue was found to reduce deacetylase activity. Interestingly, unlike other class I HDACs, HDAC3 uniquely copurifies with the catalytic and regulatory subunits of the protein serine/threonine phosphatase 4 complex (PP4c/PP4R1). Furthermore, HDAC3 complexes displayed protein phosphatase activity and a series of subsequent mutational analyses revealed that the N terminus of HDAC3 (residues 1-122) was both necessary and sufficient for HDAC3-PP4c interactions. Significantly, both overexpression and siRNA knock-down approaches, and analysis of cells devoid of PP4c, unequivocally show that HDAC3 activity is inversely proportional to the cellular abundance of PP4(c). These findings therefore further highlight the importance of protein-protein interactions and extend the significance of dephosphorylation in the regulation of HDAC activity, as well as present a novel alternative pathway by which HDAC3 activity is regulated.

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

The chromosomal translocation t(8;21) fuses the AML1 (RUNX1) gene on chromosome 21 and the ETO gene on chromosome 8 in human acute myeloid leukemias (AMLs), resulting in expression of the chimeric transcription factor AML1/ETO. AML1/ETO-mediated dysregulation of target genes critical for hematopoietic differentiation and proliferation is thought to contribute to the leukemic phenotype. Several mechanisms, including recruitment of histone deacetylases (HDACs) to AML1 target genes, may be responsible for altered gene expression. We used an ecdysone-inducible expression system in the human monoblastic U-937 cell line to isolate genes that were differentially expressed upon induction of AML1/ETO expression. By representational difference analysis (cDNA-RDA), we identified 26 genes whose expression levels were significantly modulated following AML1/ETO induction for 48 h. None of these genes has previously been described as a target of AML1, ETO or AML1/ETO. One gene downregulated by AML1/ETO in vitro, Williams Beuren syndrome critical region 5 (WBSCR5), was expressed in primary t(8;21)-negative AML blasts but not in primary t(8;21)-positive AML blasts, strongly implying a role of this gene in the phenotype of t(8;21)-positive AML. Four upregulated and four downregulated genes were further studied with all-trans-retinoic acid (ATRA), an inducer of differentiation of U-937 cells, and Trichostatin A (TSA), an HDAC inhibitor. Three out of eight genes including WBSCR5 were regulated during ATRA-induced monocytic differentiation of U-937 cells, however, none of them antagonistically, upon both ATRA treatment and AML1/ETO induction. AML1/ETO-associated dysregulation of gene expression was not mediated by a TSA-sensitive mechanism. The identified genes provide a useful model to study the mechanism by which the AML1/ETO fusion protein exerts its function in transcriptional dysregulation in AML. The possible role of WBSCR5 in normal and malignant hematopoiesis warrants further study.

The expression and function of MTG/ETO family proteins during neurogenesis

The proneural basic helix-loop-helix (bHLH) proteins promote neurogenesis by inducing changes in gene expression required for neuronal differentiation. Here we characterize one aspect of this differentiation program by analyzing a small family of putative corepressors encoded by MTG genes. We show that MTG genes are expressed sequentially during neurogenesis as cells undergo neuronal differentiation in both the chick spinal cord and in the Xenopus primary nervous system. Using in ovo electroporation, we show that misexpressing wild-type forms of MTG proteins in the developing chick spinal cord does not detectably alter neuronal differentiation. By contrast, the number of differentiated neurons is markedly reduced when a putative dominant-negative mutant of the MTG proteins is expressed in neural precursors in a manner that can be rescued by wild-type MTGR1. Together, these results suggest that MTG family members act downstream of proneural proteins, presumably as corepressors, to promote neuronal differentiation.

Interplay of RUNX1/MTG8 and DNA Methyltransferase 1 in Acute Myeloid Leukemia

The translocation t(8;21)(q22;q22) in acute myeloid leukemia (AML) results in the expression of the fusion protein RUNX1/MTG8, which in turn recruits histone deacetylases (HDAC) to silence RUNX1 target genes [e.g., interleukin-3 (IL-3)]. We previously reported that expression of the RUNX1/MTG8 target gene IL-3 is synergistically restored by the combination of inhibitors of HDACs (i.e., depsipeptide) and DNA methyltransferases (DNMT; i.e., decitabine) in RUNX1/MTG8-positive Kasumi-1 cells. Thus, we hypothesized that DNMT1 is also part of the transcriptional repressor complex recruited by RUNX1/MTG8. By a chromatin immunoprecipitation assay, we identified a RUNX1/MTG8-DNMT1 complex on the IL-3 promoter in Kasumi-1 cells and in primary RUNX1/MTG8-positive AML blasts. The physical association of RUNX1/MTG8 with DNMT1 was shown by coimmunoprecipitation experiments. Furthermore, RUNX1/MTG8 and DNMT1 were concurrently released from the IL-3 promoter by exposure to depsipeptide or stabilized on the promoter by decitabine treatment. Finally, we proved that RUNX1/MTG8 and DNMT1 were functionally interrelated by showing an enhanced repression of IL-3 after coexpression in 293T cells. These results suggest a novel mechanism for gene silencing mediated by RUNX1/MTG8 and support the combination of HDAC and DNMT inhibitors as a novel therapeutic approach for t(8;21) AML.

Targeting fusion protein/corepressor contact restores differentiation response in leukemia cells

The AML1/ETO and PML/RARalpha leukemia fusion proteins induce acute myeloid leukemia by acting as transcriptional repressors. They interact with corepressors, such as N-CoR and SMRT, that recruit a multiprotein complex containing histone deacetylases on crucial myeloid differentiation genes. This leads to gene repression contributing to generate a differentiation block. We expressed in leukemia cells containing PML/RARalpha and AML1/ETO N-CoR protein fragments derived from fusion protein/corepressor interaction surfaces. This blocks N-CoR/SMRT binding by these fusion proteins, and disrupts the repressor protein complex. In consequence, the expression of genes repressed by these fusion proteins increases and differentiation response to vitamin D3 and retinoic acid is restored in previously resistant cells. The alteration of PML/RARalpha-N-CoR/SMRT connections triggers proteasomal degradation of the fusion protein. The N-CoR fragments are biologically effective also when directly transduced by virtue of a protein transduction domain. Our data indicate that fusion protein activity is permanently required to maintain the leukemia phenotype and show the route to developing a novel therapeutic approach for leukemia, based on its molecular pathogenesis.

AML1-ETO fusion protein up-regulates TRKA mRNA expression in human CD34+ cells, allowing nerve growth factor-induced expansion

The AML1-ETO fusion protein, generated by the t(8;21) in acute myeloid leukemia (AML), exerts dominant-negative functions and a variety of gains of function, including a positive effect on the growth of primary human CD34+ hematopoietic stem/progenitor cells. We now show that AML1-ETO expression up-regulates the level of TRKA mRNA and protein in these cells and that AML1-ETO-expressing CD34+ hematopoietic cells grown in the presence of five early-acting hematopoietic cytokines further proliferate in response to nerve growth factor (NGF). These cells also show a unique response to NGF and IL-3; namely, they expand in liquid culture. To determine the biological relevance of our findings, we analyzed 262 primary AML patient samples using real-time RT-PCR and found that t(8;21)-positive AML samples express significantly higher levels of TRKA mRNA than other subtypes of AML. NGF, which is normally expressed by bone marrow stromal cells, could provide important proliferative or survival signals to AML1-ETO-expressing leukemic or preleukemic cells, and the NGF/TRKA signaling pathway may be a suitable target for therapeutic approaches to AML.

Somatic point mutations in RUNX1/CBFA2/AML1 are common in high-risk myelodysplastic syndrome, but not in myelofibrosis with myeloid metaplasia

OBJECTIVE

Acquired somatic point mutations in RUNX1/CBFA2/AML1 have recently been described in a subset of patients with myelodysplastic syndrome (MDS) and acute myeloid leukaemia (AML). Given the importance of core-binding factor in megakaryocytic differentiation and platelet production, as well as the central role of megakaryocytes in the pathophysiology of myelofibrosis with myeloid metaplasia (MMM), we hypothesised that RUNX1 gene mutations might be common in MMM. In addition, it is unclear whether patients with MDS-associated acquired alpha thalassaemia (ATMDS), a special subgroup with a very high incidence of point mutations in the ATRX gene, have an especially high incidence of RUNX1 mutations.

METHODS

We analysed samples from 78 patients for RUNX1 point mutations by denaturing high-performance liquid chromatography (DHPLC): 26 with MMM and 52 with MDS, including 18 with ATMDS.

RESULTS

We found five RUNX1 mutations in MDS patients (9.6%), all of whom had RAEB-2 or a history of treated AML, but none in MMM patients. ATMDS patients did not have an increased risk of RUNX1 point mutations (2/18, 11.1%) when compared with MDS without thalassaemia (3/34, 8.8%; P = 0.58).

CONCLUSION

RUNX1 point mutations are common in high-risk MDS, but not in MMM. DHPLC is a useful technique for high-throughput analysis of RUNX1 mutation status in myeloid disorders, and may be complementary to screening via other methods.

The MTG proteins: chromatin repression players with a passion for networking

The human myeloid translocation genes (MTGs) encode a family of proteins with a modular structure that can be traced to the Drosophila protein nervy. The nuclear MTGs can mediate the formation of complex protein networks among nuclear corepressors (Sin3a, N-CoR, SMRT), chromatin-modifying enzymes (histone deacetylases), and DNA-binding transcription factors. Hierarchical modulation of repression at target genes by MTG protein complexes is likely required for fine spatial and temporal gene regulation during development and differentiation. Genomic changes can disrupt these sophisticated protein networks and underlie novel pathogenic causes of cancer and neurodegeneration.

Human ovarian carcinoma cells: histone deacetylase inhibitors exhibit antiproliferative activity and potently induce apoptosis

BACKGROUND

Histone deacetylase inhibitors (HDACIs) can inhibit proliferation, stimulate apoptosis, and induce cell cycle arrest in malignant cells.

METHODS

The authors investigated the effects of four HDACIs on nine ovarian carcinoma cell lines in vitro and in vivo. Ovarian carcinoma cells were treated with a variety of HDACIs, and their effects on cell growth, the cell cycle, apoptosis, and related events were investigated. The ability of valproic acid (VPA) to inhibit the growth of ovarian tumors in immunodeficient mice was also assessed.

RESULTS

Clonogenic assays showed that all ovarian carcinoma cell lines were sensitive to the growth-inhibitory effects of the HDACIs. Cell cycle analysis indicated that their exposure to HDACIs decreased the proportion of cells in S phase and increased the proportion of cells in the G0/G1 and/or G2/M phases of the cell cycle. Terminal deoxynucleotidyltransferase-mediated uridine triphosphate end-labeling assays demonstrated that HDACIs induced apoptosis, which occurred in concert with alterations in the expression of genes related to apoptosis, cell growth, and malignant phenotype, including the activation of caspase-9 and caspase-3. Chromatin immunoprecipitation analysis revealed a notable increase in levels of acetylated histones associated with the p21 promoter after treatment with suberoylanilide bishydroxamine. In addition, in experiments involving nude mice, VPA significantly inhibited human ovarian tumor growth without toxic side effects.

CONCLUSIONS

The results of the current study suggest that HDACIs may be particularly effective in the treatment of ovarian tumors.

Deletion of an AML1-ETO C-terminal NcoR/SMRT-interacting region strongly induces leukemia development

Normal blood-cell differentiation is controlled by regulated gene expression and signal transduction. Transcription deregulation due to chromosomal translocation is a common theme in hematopoietic neoplasms. AML1-ETO, which is a fusion protein generated by the 8;21 translocation that is commonly associated with the development of acute myeloid leukemia, fuses the AML1 runx family DNA-binding transcription factor to the ETO corepressor that associates with histone deacetylase complexes. Analyses have demonstrated that AML1-ETO blocks AML1 function and requires additional mutagenic events to promote leukemia. Here, we report that the loss of the molecular events of AML1-ETO C-terminal NCoR/SMRT-interacting domain transforms AML1-ETO into a potent leukemogenic protein. Contrary to full-length AML1-ETO, the truncated form promotes in vitro growth and does not obstruct the cell-cycle machinery. These observations suggest a previously uncharacterized mechanism of tumorigenesis, in which secondary mutation(s) in molecular events disrupting the function of a domain of the oncogene promote the development of malignancy.

Concepts of human leukemic development

Two fundamental problems in cancer research are identification of the normal cell within which cancer initiates and identification of the cell type capable of sustaining the growth of the neoplastic clone. There is overwhelming evidence that virtually all cancers are clonal and represent the progeny of a single cell. What is less clear for most cancers is which cells within the tumor clone possess tumorigenic or 'cancer stem cell' (CSC) properties and are capable of maintaining tumor growth. The concept that only a subpopulation of rare CSC is responsible for maintenance of the neoplasm emerged nearly 50 years ago. Testing of this hypothesis is most advanced for the hematopoietic system due to the establishment of functional in vitro and in vivo assays for stem and progenitor cells at all stages of development. This body of work led to conclusive proof for CSC with the identification and purification of leukemic stem cells capable of repopulating NOD/SCID mice. This review will focus on the historical development of the CSC hypothesis, the mechanisms necessary to subvert normal developmental programs, and the identification of the cell in which these leukemogenic events first occur.

Histone deacetylase inhibitors have a profound antigrowth activity in endometrial cancer cells

PURPOSE

HDAC inhibitors (HDACIs) have been shown to inhibit cancer cell proliferation, stimulate apoptosis, and induce cell cycle arrest. Our purpose was to investigate the antiproliferative effects of the HDACIs [suberoyl anilide bishydroxamine, valproic acid (VPA), trichostatin A, and sodium butyrate] against six endometrial cancer cell lines.

EXPERIMENTAL DESIGN

Endometrial cancer cells were treated with a variety of HDACIs, and the effect on cell growth, cell cycle, and apoptosis was measured. The ability of VPA to inhibit the growth of endometrial tumors growing in immunodeficient mice was also assessed.

RESULTS

Clonogenic assays showed that all cancer cell lines were sensitive to the growth inhibitory effect of HDACIs. Cell cycle analysis indicated that treatment with HDACIs decreased the proportion of cells in S phase and increased the proportion of cells in the G(0)-G(1) and/or G(2)-M phases of the cell cycle. Terminal deoxynucleotidyl transferase-mediated nick end labeling assays showed that HDACIs induced apoptosis. This was concomitant with altered expression of genes related to malignant phenotype, including an increase in p21(Waf1), p27(Kip7), and E-cadherin and a decrease in Bcl-2 and cyclin-D1 and -D2. Chromatin immunoprecipitation analysis revealed a remarkable increase in levels of acetylated histones associated with the p21 promoter after suberoyl anilide bishydroxamine treatment. In nude mice experiments, VPA inhibited significantly human uterine tumor growth without toxic side effects.

CONCLUSIONS

These results suggest that HDACIs are effective in inhibiting growth of endometrial cancer cells in vitro and in nude mice, without toxic side effects. The findings raise the possibility that HDACIs may prove particularly effective in treatment of endometrial cancers.

The nuclear receptor co-repressor (N-CoR) utilizes repression domains I and III for interaction and co-repression with ETO

The acute human leukemias are associated with the presence of chimeric gene products that arise from spontaneous chromosomal translocations. The t(8;21) translocation gene product led to the discovery of the Eight Twenty-One (ETO) gene. When fused to RUNX1, ETO is thought to mediate the formation of a repressive complex at RUNX1-dependent genes. ETO has also been found to act as a co-repressor of the promyelocytic zinc finger and Bcl-6 oncoproteins, suggesting that it may play a common role as a transcriptional co-repressor leading to human disease. An analysis of ETO-mediated repression revealed that one of the key binding partners of ETO is the nuclear receptor co-repressor (N-CoR). It is shown that two highly conserved domains of ETO interact with repression domains I and III of N-CoR. One of the ETO domains displays significant homology to Drosophila TAF(II)110, whereas the other is a predicted zinc binding motif that engages a conserved PPLXP motif in repression domain III of N-CoR. Together, these domains of ETO cooperate in repression with N-CoR and the binding sites in N-CoR overlap with those for other repressive factors. Thus, ETO has the potential to participate in a number of repressive complexes, which can be distinguished by their binding partners and target genes.

Histone deacetylase inhibition improves dendritic cell differentiation of leukemic blasts with AML1-containing fusion proteins

Recurrent cytogenetic abnormalities in leukemic blasts make these an attractive source for dendritic cells (DC) to induce a leukemia-specific immune response. In this study, three leukemic cell lines were investigated: Kasumi-1 and SKNO-1 (two acute myeloid leukemia (AML) cell lines carrying the (8;21)-chromosomal translocation, resulting in the expression of the leukemia-specific fusion protein AML1-eight-twenty-one) and REH, an acute lymphoblastic leukemia cell line with the (12;21)-chromosomal translocation and expression of translocation ETS-like leukemia-AML1. These fusion proteins are implicated in the pathogenesis of the leukemic state by recruiting corepressors and histone deacetylases (HDAC), which interfere with normal cell differentiation. In vitro generation of DC was achieved using a cytokine cocktail containing tumor necrosis factor alpha, granulocyte macrophage-colony stimulating factor, c-kit ligand, and soluble CD40 ligand; yet, addition of the HDAC inhibitor (Hdi) trichostatin A enhanced DC differentiation with retention of the fusion transcripts. These leukemic DC showed high-level CD83 and human leukocyte antigen (HLA)-DR expression and had a high allostimulatory potential. Only DC generated from these cell lines after Hdi induced blast-specific cytotoxic T cell responses in HLA-A-matched T cells with a cytotoxicity of 42% in parental Kasumi-1 and 83% in parental REH cells, respectively. This model system suggests that the Hdi supports the in vitro differentiation of DC from leukemic blasts with AML1-containing fusion proteins.

Aberrant CBFA2T3B gene promoter methylation in breast tumors

Background

The CBFA2T3 locus located on the human chromosome region 16q24.3 is frequently deleted in breast tumors. CBFA2T3 gene expression levels are aberrant in breast tumor cell lines and the CBFA2T3B isoform is a potential tumor suppressor gene. In the absence of identified mutations to further support a role for this gene in tumorigenesis, we explored whether the CBFA2T3B promoter region is aberrantly methylated and whether this correlates with expression.

Results

Aberrant hypo and hypermethylation of the CBFA2T3B promoter was detected in breast tumor cell lines and primary breast tumor samples relative to methylation index interquartile ranges in normal breast counterpart and normal whole blood samples. A statistically significant inverse correlation between aberrant CBFA2T3B promoter methylation and gene expression was established.

Conclusion

CBFA2T3B is a potential breast tumor suppressor gene affected by aberrant promoter methylation and gene expression. The methylation levels were quantitated using a second-round real-time methylation-specific PCR assay. The detection of both hypo and hypermethylation is a technicality regarding the methylation methodology.

The oncogenic fusion protein RUNX1-CBFA2T1 supports proliferation and inhibits senescence in t(8;21)-positive leukaemic cells

BACKGROUND

The fusion protein RUNX1-CBFA2T1 associated with t(8;21)-positive acute myeloid leukaemia is a potent inhibitor of haematopoetic differentiation. The role of RUNX1-CBFA2T1 in leukaemic cell proliferation is less clear. We examined the consequences of siRNA-mediated RUNX1-CBFA2T1 depletion regarding proliferation and clonogenicity of t(8;21)-positive cell lines.

METHODS

The t(8;21)-positive cell line Kasumi-1 was electroporated with RUNX1-CBFA2T1 or control siRNAs followed by analysis of proliferation, colony formation, cell cycle distribution, apoptosis and senescence.

RESULTS

Electroporation of Kasumi-1 cells with RUNX1-CBFA2T1 siRNAs, but not with control siRNAs, resulted in RUNX1-CBFA2T1 suppression which lasted for at least 5 days. A single electroporation with RUNX1-CBFA2T1 siRNA severely diminished the clonogenicity of Kasumi-1 cells. Prolonged RUNX1-CBFA2T1 depletion inhibited proliferation in suspension culture and G1-S transition during the cell cycle, diminished the number of apoptotic cells, but induced cellular senescence. The addition of haematopoetic growth factors could not rescue RUNX1-CBFA2T1-depleted cells from senescence, and could only partially restore their clonogenicity.

CONCLUSIONS

RUNX1-CBFA2T1 supports the proliferation and expansion of t(8;21)-positive leukaemic cells by preventing cellular senescence. These findings suggest a central role of RUNX1-CBFA2T1 in the maintenance of the leukaemia. Therefore, RUNX1-CBFA2T1 is a promising and leukaemia-specific target for molecularly defined therapeutic approaches.

AML1-ETO Decreases ETO-2 (MTG16) Interactions with Nuclear Receptor Corepressor, an Effect That Impairs Granulocyte Differentiation

The t(8;21) chromosome abnormality in acute myeloid leukemia targets the AML1 and ETO genes to produce the leukemia fusion protein AML1-ETO. Another member of the ETO family, ETO-2/MTG16, is highly expressed in murine and human hematopoietic cells, bears >75% homology to ETO, and like ETO, contains a conserved MYND domain that interacts with the nuclear receptor corepressor (N-CoR). AML1-ETO prevents granulocyte but not macrophage differentiation of murine 32Dcl3 granulocyte/macrophage progenitors. One possible mechanism is recruitment of N-CoR to aberrantly repress AML1 target genes. We wished to examine another mechanism by which AML1-ETO might impair granulocyte differentiation. We demonstrate that AML1-ETO decreases interactions between ETO-2 and N-CoR. Furthermore, overexpression of ETO-2 relieves AML1-ETO-induced granulocyte differentiation arrest. This suggests that decreased interactions between ETO-2 and N-CoR may contribute to granulocyte differentiation impairment. The MYND domain coimmunoprecipitates with N-CoR and inhibits interactions between ETO-2 and N-CoR, presumably by occupying the ETO-2 binding site on N-CoR. This inhibition of ETO-2 interactions with N-CoR is specific because the MYND domain does not inhibit retinoic acid receptor interactions with N-CoR. To examine the effect of decreasing interactions between ETO-2 and N-CoR in hematopoietic cells, without effects of AML1-ETO such as direct repression of AML1 target genes, the MYND domain was expressed in 32Dcl3 and human CD34+ cells. The MYND domain prevented granulocyte but not macrophage differentiation of both 32Dcl3 and human CD34+ cells, recapitulating this effect of AML1-ETO. In conclusion, decreasing interactions between ETO-2 and N-CoR, an effect of AML1-ETO, inhibits granulocyte differentiation.

Role of RUNX family members in transcriptional repression and gene silencing

RUNX family members are DNA-binding transcription factors that regulate the expression of genes involved in cellular differentiation and cell cycle progression. The RUNX family includes three mammalian RUNX proteins (RUNX1, -2, -3) and two homologues in Drosophila. Experiments in Drosophila and mouse indicate that the RUNX proteins are required for gene silencing of engrailed and CD4, respectively. RUNX-mediated repression involves recruitment of corepressors such as mSin3A and Groucho as well as histone deacetylases. Furthermore, RUNX1 and RUNX3 associate with SUV39H1, a histone methyltransferase involved in gene silencing. RUNX1 is frequently targeted in human leukemia by chromosomal translocations that fuse the DNA-binding domain of RUNX1 to other transcription factors and corepressor molecules. The resulting leukemogenic fusion proteins are transcriptional repressors that form stable complexes with corepressors, histone deacetylases and histone methyltransferases. Thus, transcriptional repression and gene silencing through RUNX1 contribute to the mechanisms of leukemogenesis of the fusion proteins. Therapies directed at the associated cofactors may be beneficial for treatment of these leukemias.

ETO interacting proteins

The 8;21 translocation produces a fusion between the ETO gene and that encoding the myeloid transcription factor AML1. The AML1-ETO fusion substitutes the majority of the ETO protein for the coregulator recruitment domains of AML1. Biochemical analyses of ETO have led to the identification of numerous interacting proteins including many corepressors. Importantly, the proteins interacting with ETO are different from those of wild-type AML1, suggesting that altered coregulator recruitment underlies the oncogenic properties of AML1-ETO. The list of corepressors capable of binding ETO includes histone deacetylases (HDACs) and components of distinct HDAC core complexes. These investigations have provided mechanistic insight into corepressor recruitment by ETO and clues to the leukemogenic activity of AML1-ETO.

Nuclear accumulation of histone deacetylase 4 (HDAC4) coincides with the loss of androgen sensitivity in hormone refractory cancer of the prostate

OBJECTIVES

To examine the effect of androgen treatment upon histone deacetylase 4 (HDAC4) localisation and, thus, enzymatic function in androgen sensitive prostate cancer (CaP) models. To study HDAC4 expression in benign prostatic hyperplasia, primary and hormone refractory (HR) CaP and to investigate the involvement of histone deacetylase activity in the development of the androgen insensitive phenotype.

METHODS

Immunohistochemical staining of prostate sections of both benign tissue and primary and hormone relapsed prostate cancer, as well as of the CWR22 mouse xenograft model, and indirect quantitative immunofluorescence staining of endogenous HDAC4 in LNCaP cells.

RESULTS

HDAC4 is recruited to the nuclei of HR cancer cells, where it may exert an inhibitory effect on differentiation and contribute to the development of the aggressive phenotype of late stage CaP. The above may result from the loss of androgen responsiveness characterising HR CaP, since HDAC4 nuclear localisation is regulated by androgens in androgen responsive systems (i.e. LNCaP, CWR22) reflecting earlier phase disease.

CONCLUSIONS

HDAC4 may contribute to the development of HR CaP and, therefore, constitute a potential therapeutic target, particularly in the most lethal phase of androgen independence.

The 8;21 translocation in leukemogenesis

A common chromosomal translocation in acute myeloid leukemia (AML) involves the AML1 (acute myeloid leukemia 1, also called RUNX1, core binding factor protein (CBF alpha), and PEBP2 alpha B) gene on chromosome 21 and the ETO (eight-twenty one, also called MTG8) gene on chromosome 8. This translocation generates an AML1-ETO fusion protein. t(8;21) is associated with 12% of de novo AML cases and up to 40% in the AML subtype M2 of the French-American-British classification. Furthermore, it is also reported in a small portion of M0, M1, and M4 AML samples. Despite numerous studies on the function of AML1-ETO, the precise mechanism by which the fusion protein is involved in leukemia development is still not fully understood. In this review, we will discuss structural aspects of the fusion protein and the accumulated knowledge from in vitro analyses on AML1-ETO functions, and outline putative mechanisms of its leukemogenic potential.

Translocation products in acute myeloid leukemia activate the Wnt signaling pathway in hematopoietic cells

The acute myeloid leukemia (AML)-associated translocation products AML1-ETO, PML-retinoic acid receptor alpha (RARalpha), and PLZF-RARalpha encode aberrant transcription factors. Several lines of evidence suggest similar pathogenetic mechanisms for these fusion proteins. We used high-density oligonucleotide arrays to identify shared target genes in inducibly transfected U937 cells expressing AML1-ETO, PML-RARalpha, or PLZF-RARalpha. All three fusion proteins significantly repressed the expression of 38 genes and induced the expression of 14 genes. Several of the regulated genes were associated with Wnt signaling. One of these, plakoglobin (gamma-catenin), was induced on the mRNA and protein level by all three fusion proteins. In addition, primary AML blasts carrying one of the fusion proteins significantly overexpressed plakoglobin. The plakoglobin promoter was cloned and shown to be induced by AML1-ETO, with promoter activation depending on the corepressor and histone deacetylase binding domains. The induction of plakoglobin by AML fusion proteins led to downstream signaling and transactivation of TCF- and LEF-dependent promoters, including the c-myc promoter, which was found to be bound by plakoglobin in vivo after AML1-ETO expression. beta-Catenin protein levels and TCF and LEF target genes such as c-myc and cyclin D1 were found to be induced by the fusion proteins. On the functional level, a dominant negative TCF inhibited colony growth of AML1-ETO-positive Kasumi cells, whereas plakoglobin transfection into myeloid 32D cells enhanced proliferation and clonal growth. Injection of plakoglobin-expressing 32D cells into syngeneic mice accelerated the development of leukemia. Transduction of plakoglobin into primitive murine hematopoietic progenitor cells preserved the immature phenotype during colony growth, suggesting enhanced self-renewal. These data provide evidence that activation of Wnt signaling is a common feature of several balanced translocations in AML.

The role of corepressors in transcriptional regulation by nuclear hormone receptors

Nuclear receptors (also known as nuclear hormone receptors) are hormone-regulated transcription factors that control many important physiological and developmental processes in animals and humans. Defects in receptor function result in disease. The diverse biological roles of these receptors reflect their surprisingly versatile transcriptional properties, with many receptors possessing the ability to both repress and activate target gene expression. These bipolar transcriptional properties are mediated through the interactions of the receptors with two distinct classes of auxiliary proteins: corepressors and coactivators. This review focuses on how corepressors work together with nuclear receptors to repress gene transcription in the normal organism and on the aberrations in this process that lead to neoplasia and endocrine disorders. The actions of coactivators and the contributions of the same corepressors to the functions of nonreceptor transcription factors are also touched on.

Upregulation and nuclear recruitment of HDAC1 in hormone refractory prostate cancer

BACKGROUND

Histone deacetylase 1 (HDAC1) is a co-repressor involved in differentiation and proliferation control. It is upregulated in malignant compared to benign tissue, and targets a number of transcription factors including p53.

METHODS

By immunohistochemistry, HDAC1 protein expression was investigated in human prostate specimens and the CWR22 mouse xenograft model. Flow cytometry and deconvolution immunofluorescence were also performed.

RESULTS

HDAC1 was upregulated in pre-malignant and malignant lesions, with the highest increase in expression in hormone refractory (HR) cancer. Using the CWR22 xenograft model we showed androgen dependent regulation of HDAC1. HDAC1 overexpression led to a significant increase in proliferation and a shift towards the undifferentiated cytokeratin (CK) profile in a PC3M derivative clone constitutively expressing HDAC1.

CONCLUSION

This study underlines the importance of HDAC1 in cell proliferation and the development of prostate cancer (CaP) and proposes a mechanism for HDAC1 nuclear recruitment. HDAC1 may constitute a crucial therapeutic target particularly in the most lethal phase of androgen independence.

Mechanisms involved in the induced differentiation of leukemia cells

Despite the remarkable progress achieved in the treatment of leukemias over the last several years, many problems (multidrug resistance [MDR], cellular heterogeneity, heterogeneous molecular abnormalities, karyotypic instability, and lack of selective action of antineoplastic agents) still remain. The recent progress in tumor molecular biology has revealed that leukemias are likely to arise from disruption of differentiation of early hematopoietic progenitors that fail to give birth to cell lineage restricted phenotypes. Evidence supporting such mechanisms has been derived from studying bone marrow leukemiogenesis and analyzing differentiation of leukemic cell lines in culture that serve as models of erythroleukemic (murine erythroleukemia [MEL] and human leukemia [K562] cells) and myeloid (human promyelocytic leukemia [HL-60] cells) cell maturation. This paper reviews the current concepts of differentiation, the chemical/pharmacological inducing agents developed thus far, and the mechanisms involved in initiation of leukemic cell differentiation. Emphasis was given on commitment and the cell lineage transcriptional factors as key regulators of terminal differentiation as well as on membrane-mediated events and signaling pathways involved in hematopoietic cell differentiation. The developmental program of MEL cells was presented in considerable depth. It is quite remarkable that the erythrocytic maturation of these cells is orchestrated into specific subprograms and gene expression patterns, suggesting that leukemic cell differentiation represents a highly coordinated set of events that lead to irreversible growth arrest and expression of cell lineage restricted phenotypes. In MEL and other leukemic cells, differentiation appears to be accompanied by differentiation-dependent apoptosis (DDA), an event that can be exploited chemotherapeutically. The mechanisms by which the chemical inducers promote differentiation of leukemic cells have been discussed.

Selective growth inhibition of tumor cells by a novel histone deacetylase inhibitor, NVP-LAQ824

We have synthesized a histone deacetylase inhibitor, NVP-LAQ824, a cinnamic hydroxamic acid, that inhibited in vitro enzymatic activities and transcriptionally activated the p21 promoter in reporter gene assays. NVP-LAQ824 selectively inhibited growth of cancer cell lines at submicromolar levels after 48-72 h of exposure, whereas higher concentrations and longer exposure times were required to retard the growth of normal dermal human fibroblasts. Flow cytometry studies revealed that both tumor and normal cells arrested in the G(2)-M phase of the cell cycle after compound treatment. However, an increased sub-G(1) population at 48 h (reminiscent of apoptotic cells) was observed only in the cancer cell line. Annexin V staining data supported our hypothesis that NVP-LAQ824 induced apoptosis in tumor and transformed cells but not in normal cells. Western blotting experiments showed an increased histone H3 and H4 acetylation level in NVP-LAQ824-treated cancer cells, suggesting that the likely in vivo target of NVP-LAQ824 was histone deacetylase(s). Finally, NVP-LAQ824 exhibited antitumor effects in a xenograft animal model. Together, our data indicated that the activity of NVP-LAQ824 was consistent with its intended mechanism of action. This novel histone deacetylase inhibitor is currently in clinical trials as an anticancer agent.

Domains involved in ETO and human N-CoR interaction and ETO transcription repression

The (8;21) translocation between the AML1 and ETO genes is seen in approximately 12-15% of all acute myeloid leukemia (AML) and is a frequently observed nonrandom genetic alteration associated with AML. The ETO moiety was shown to interact with the nuclear receptor co-repressor (N-CoR) complex, which includes mSin3A and the histone deacetylase, HDAC1. Repression of AML1-responsive hematopoietic genes by AML1-ETO and the N-CoR complex may play a mechanistic role in t(8;21) leukemogenesis. In order to characterize the interaction between ETO and N-CoR, mutants of either protein were constructed and tested for binding in both yeast two-hybrid and immunoprecipitation assays. We found that two domains of human N-CoR, amino acid residues 988-1126 and 1551-1803, were necessary for interaction with ETO. Previously, we and other investigators had reported that two unusual zinc finger motifs at the C-terminus of ETO mediated binding to N-CoR. Here, using mammalian two-hybrid assays, we found that transcription repression by ETO was substantially decreased when either zinc finger motif of ETO is deleted or mutated. In addition, we identified a second transcription repression domain located between residues 275 and 487. Characterization of the ETO interaction domains within human N-CoR and of the transcription domains of ETO is a first step in designing targeted molecular therapy for t(8;21) AML.

The Flt3 internal tandem duplication mutant inhibits the function of transcriptional repressors by blocking interactions with SMRT

Fms-like tyrosine kinase 3 (Flt3) is a type III receptor tyrosine kinase (RTK). Between 20% and 30% of acute myeloid leukemia (AML) patients have either an internal tandem duplication (ITD) of the juxtamembrane region or a point mutation of the Flt3 receptor leading to the constitutive activation of downstream signaling pathways and aberrant cell growth. The silencing mediator of retinoic and thyroid hormone receptors (SMRT) corepressor mediates transcriptional repression by interacting with transcription factors such as the promyelocytic leukemia zinc finger (PLZF) protein. Previous reports indicate that SMRT interaction with transcription factors can be disrupted by phosphorylation through activation of RTK pathways. We report here that the Flt3-ITD interferes with the transcriptional and biologic action of the PLZF transcriptional repressor. In the presence of Flt3-ITD, PLZF-SMRT interaction was reduced, transcriptional repression by PLZF was inhibited, and PLZF-mediated growth suppression of leukemia cells was partially blocked. Furthermore, overexpression of Flt3-ITD led to a partial relocalization of SMRT protein from the nucleus to the cytoplasm. Nuclear export was dependent on the SMRT receptor interaction domain (RID), and Flt3-ITD enhances the binding of nuclear-cytoplasm shuttling protein nuclear factor-kappaB-p65 (NFkappaB-p65) to this region. These data suggest that activating mutations of Flt3 may disrupt transcriptional repressor function resulting in aberrant gene regulation and abnormal leukemia cell growth.

Acute myeloid leukemia fusion proteins deregulate genes involved in stem cell maintenance and DNA repair

Acute myelogenous leukemias (AMLs) are genetically heterogeneous and characterized by chromosomal rearrangements that produce fusion proteins with aberrant transcriptional regulatory activities. Expression of AML fusion proteins in transgenic mice increases the risk of myeloid leukemias, suggesting that they induce a preleukemic state. The underlying molecular and biological mechanisms are, however, unknown. To address this issue, we performed a systematic analysis of fusion protein transcriptional targets. We expressed AML1/ETO, PML/RAR, and PLZF/RAR in U937 hemopoietic precursor cells and measured global gene expression using oligonucleotide chips. We identified 1,555 genes regulated concordantly by at least two fusion proteins that were further validated in patient samples and finally classified according to available functional information. Strikingly, we found that AML fusion proteins induce genes involved in the maintenance of the stem cell phenotype and repress DNA repair genes, mainly of the base excision repair pathway. Functional studies confirmed that ectopic expression of fusion proteins constitutively activates pathways leading to increased stem cell renewal (e.g., the Jagged1/Notch pathway) and provokes accumulation of DNA damage. We propose that expansion of the stem cell compartment and induction of a mutator phenotype are relevant features underlying the leukemic potential of AML-associated fusion proteins.