ETO, a target of t(8;21) in acute leukemia, interacts with the N-CoR and mSin3 corepressors

Inhibition of the transforming growth factor beta 1 signaling pathway by the AML1/ETO leukemia-associated fusion protein

The t(8;21) translocation, found in adult acute myelogenous leukemia, results in the formation of an AML1/ETO chimeric transcription factor. AML1/ETO expression leads to alterations in hematopoietic progenitor cell differentiation, although its role in leukemic transformation is not clear. The N-terminal portion of AML1, which is retained in AML1/ETO, contains a region of homology to the FAST proteins, which cooperate with Smads to regulate transforming growth factor beta1 (TGF-beta1) target genes. We have demonstrated the physical association of Smad proteins with AML1 and AML1/ETO by immunoprecipitation and have mapped the region of interaction to the runt homology domain in these AML1 proteins. Using confocal microscopy, we demonstrated that AML1, and ETO and/or AML1/ETO, colocalize with Smads in the nucleus of t(8;21)-positive Kasumi-1 cells, in the presence but not the absence of TGF-beta1. Using transient transfection assays and a reporter gene construct that contains both Smad and AML1 consensus binding sequences, we demonstrated that overexpression of AML1B cooperates with TGF-beta1 in stimulating reporter gene activity, whereas AML1/ETO represses basal promoter activity and blocks the response to TGF-beta1. Considering the critical role of TGF-beta1 in the growth and differentiation of hematopoietic cells, interference with TGF-beta1 signaling by AML1/ETO may contribute to leukemogenesis.

P/CAF-mediated acetylation regulates the function of the basic helix-loop-helix transcription factor TAL1/SCL

The basic helix-loop-helix transcription factor TAL1 (or SCL) is a critical regulator of hematopoietic and vascular development and is misexpressed in the majority of patients with T-cell acute lymphoblastic leukemia. We found previously that TAL1 could interact with transcriptional co-activator and co-repressor complexes possessing histone acetyltransferase and deacetylase activities, respectively. Here, we report that TAL1 is subject to acetylation in vivo and can be acetylated by p300 and the p300/CBP-associated factor P/CAF in vitro. P/CAF-mediated acetylation, which mapped to a lysine-rich motif in the loop region, increased TAL1 binding to DNA while selectively inhibiting its interaction with the transcriptional co-repressor mSin3A. Furthermore, P/CAF protein, TAL1-P/CAF interaction and TAL1 acetylation increased significantly in murine erythroleukemia cells induced to differentiate in culture, while enforced expression of an acetylation-defective P/CAF mutant inhibited endogenous TAL1 acetylation, TAL1 DNA-binding activity, TAL1-directed transcription and terminal differentiation of these cells. These results reveal a novel mechanism by which TAL1 activity is regulated and implicate acetylation of this transcription factor in promotion of erythroid differentiation.

AML-1/ETO fusion protein is a dominant negative inhibitor of transcriptional repression by the promyelocytic leukemia zinc finger protein

The AML-1/ETO fusion protein, created by the (8;21) translocation in M2-type acute myelogenous leukemia (AML), is a dominant repressive form of AML-1. This effect is due to the ability of the ETO portion of the protein to recruit co-repressors to promoters of AML-1 target genes. The t(11;17)(q21;q23)-associated acute promyelocytic leukemia creates the promyelocytic leukemia zinc finger PLZFt/RARα fusion protein and, in a similar manner, inhibits RARα target gene expression and myeloid differentiation. PLZF is expressed in hematopoietic progenitors and functions as a growth suppressor by repressing cyclin A2 and other targets. ETO is a corepressor for PLZF and potentiates transcriptional repression by linking PLZF to a histone deacetylase-containing complex. In transiently transfected cells and in a cell line derived from a patient with t(8;21) leukemia, PLZF and AML-1/ETO formed a tight complex. In transient assays, AML-1/ETO blocked transcriptional repression by PLZF, even at substoichiometric levels relative to PLZF. This effect was dependent on the presence of the ETO zinc finger domain, which recruits corepressors, and could not be rescued by overexpression of co-repressors that normally enhance PLZF repression. AML-1/ETO also excluded PLZF from the nuclear matrix and reduced its ability to bind to its cognate DNA-binding site. Finally, ETO interacted with PLZF/RARα and enhanced its ability to repress through the RARE. These data show a link in the transcriptional pathways of M2 and M3 leukemia.

AML-1/ETO fusion protein is a dominant negative inhibitor of transcriptional repression by the promyelocytic leukemia zinc finger protein

The AML-1/ETO fusion protein, created by the (8;21) translocation in M2-type acute myelogenous leukemia (AML), is a dominant repressive form of AML-1. This effect is due to the ability of the ETO portion of the protein to recruit co-repressors to promoters of AML-1 target genes. The t(11;17)(q21;q23)-associated acute promyelocytic leukemia creates the promyelocytic leukemia zinc finger PLZFt/RARα fusion protein and, in a similar manner, inhibits RARα target gene expression and myeloid differentiation. PLZF is expressed in hematopoietic progenitors and functions as a growth suppressor by repressing cyclin A2 and other targets. ETO is a corepressor for PLZF and potentiates transcriptional repression by linking PLZF to a histone deacetylase-containing complex. In transiently transfected cells and in a cell line derived from a patient with t(8;21) leukemia, PLZF and AML-1/ETO formed a tight complex. In transient assays, AML-1/ETO blocked transcriptional repression by PLZF, even at substoichiometric levels relative to PLZF. This effect was dependent on the presence of the ETO zinc finger domain, which recruits corepressors, and could not be rescued by overexpression of co-repressors that normally enhance PLZF repression. AML-1/ETO also excluded PLZF from the nuclear matrix and reduced its ability to bind to its cognate DNA-binding site. Finally, ETO interacted with PLZF/RARα and enhanced its ability to repress through the RARE. These data show a link in the transcriptional pathways of M2 and M3 leukemia.

Signal transduction pathways and the modification of chromatin structure

Mechanical and chemical signaling pathways are involved in transmitting information from the exterior of a cell to its chromatin. The mechanical signaling pathway consists of a tissue matrix system that links together the three-dimensional skeletal networks, the extracellular matrix, cytoskeleton, and karyoskeleton. The tissue matrix system governs cell and nuclear shape and forms a structural and functional connection between the cell periphery and chromatin. Further, this mechanical signaling pathway has a role in controlling cell cycle progression and gene expression. Chemical signaling pathways such as the Ras/mitogen-activated protein kinase (MAPK) pathway can stimulate the activity of kinases that modify transcription factors, nonhistone chromosomal proteins, and histones. Activation of the Ras/MAPK pathway results in the alteration of chromatin structure and gene expression. The tissue matrix and chemical signaling pathways are not independent and one signaling pathway can affect the other. In this chapter, we will review chromatin organization, histone variants and modifications, and the impact that signaling pathways have on chromatin structure and function.

The Pit-1beta domain dictates active repression and alteration of histone acetylation of the proximal prolactin promoter

A critical problem in current molecular biology is to gain a detailed understanding of the molecular mechanisms by which related transcription factor isoforms with identical DNA sequence specificity mediate distinct transcription responses. Pit-1 and Pit-1beta constitute such a pair of transcription factor isoforms. Pit-1 enhances the Ras signaling pathway to the prolactin promoter, and Pit-1beta represses basal prolactin promoter activity as well as Ras signaling to the prolactin promoter in pituitary cells. We have previously demonstrated that the beta-domain amino acid sequence dictates the transcriptional properties of Pit-1beta. Here, we show that five hydrophobic beta-domain residues are required for Pit-1 isoform-specific repression of Ras signaling, and we demonstrate that sodium butyrate and trichostatin A, pharmacological inhibitors of histone deacetylation, as well as viral Ski protein, a dominant-negative inhibitor of recruitment of N-CoR/mSin3 histone deacetylase complexes, specifically reverse beta isoform-specific repression of Ras signaling. Moreover, we directly demonstrate, with a chromatin immunoprecipitation assay, that the Pit-1beta isoform alters the histone acetylation state of the proximal prolactin promoter. This differential analysis of Pit-1/Pit-1beta isoform function provides significant insights into the structural determinants that govern how different transcription factors with identical DNA sequence specificity can display opposite effects on target gene activity.

Recruitment of the nuclear receptor corepressor N-CoR by the TEL moiety of the childhood leukemia–associated TEL-AML1 oncoprotein

The t(12;21)(p13;q22) chromosomal translocation is the most frequent illegitimate gene recombination in a pediatric cancer and occurs in approximately 25% of common acute lymphoblastic leukemia (cALL) cases. This rearrangement results in the in frame fusion of the 5′-region of the ETS-related gene, TEL(ETV6), to almost the entire acute myeloid leukemia 1 (AML1) (also called CBFA2 orPEBP2AB1) locus and expression of the TEL-AML1 chimeric protein. Although AML1 stimulates transcription, TEL-AML1 functions as a repressor of some AML1 target genes. In contrast to the wild type AML1 protein, both TEL and TEL-AML1 interact with N-CoR, a component of the nuclear receptor corepressor complex with histone deacetylase activity. The interaction between TEL and N-CoR requires the central region of TEL, which is retained in TEL-AML1, and TEL lacking this domain is impaired in transcriptional repression. Taken together, our results suggest that TEL-AML1 may contribute to leukemogenesis by recruiting N-CoR to AML1 target genes and thus imposing an altered pattern of their expression.

Alterations of the AML1 transcription factor in human leukemia

The identification of clonal chromosomal translocations in human leukemias provided one of the first insights into the underlying pathogenesis of this clinically heterogeneous disease. Over the last decade a large number of these chromosomal rearrangements have been molecularly cloned and the involved genes identified. A surprising finding that has emerged from this work is that many of these chromosomal alterations target the genes encoding the AML1/CBFbeta transcription factor complex, a critical regulator of normal hematopoiesis. In this review, we summarize our present understanding of the mechanisms through which alterations of AML1/CBFbeta contribute to leukemogenesis.

Co-repressors 2000

In the last 5 years, many co-repressors have been identified in eukaryotes that function in a wide range of species, from yeast to Drosophila and humans. Co-repressors are coregulators that are recruited by DNA-bound transcriptional silencers and play essential roles in many pathways including differentiation, proliferation, programmed cell death, and cell cycle. Accordingly, it has been shown that aberrant interactions of co-repressors with transcriptional silencers provide the molecular basis of a variety of human diseases. Co-repressors mediate transcriptional silencing by mechanisms that include direct inhibition of the basal transcription machinery and recruitment of chromatin-modifying enzymes. Chromatin modification includes histone deacetylation, which is thought to lead to a compact chromatin structure to which the accessibility of transcriptional activators is impaired. In a general mechanistic view, the overall picture suggests that transcriptional silencers and co-repressors act in analogy to transcriptional activators and coactivators, but with the opposite effect leading to gene silencing. We provide a comprehensive overview of the currently known higher eukaryotic co-repressors, their mechanism of action, and their involvement in biological and pathophysiological pathways. We also show the different pathways that lead to the regulation of co-repressor-silencer complex formation.

Location and function of critical genes in leukemogenesis inferred from cytogenetic abnormalities in hematologic malignancies

Dramatic advances in the cytogenetic analysis of chromosomal rearrangements of hematopoietic malignancies have occurred over the past years. These are due to considerable improvement in the techniques of molecular cytogenetics. Various applications of fluorescence in situ hybridization (FISH), used in conjunction with conventional cytogenetics, make the recognition of some abnormalities easier, and the localization of chromosomal breakpoints in structural rearrangements more precise. Under many circumstances, accurate breakpoint localization is the first step toward the identification of genes involved in translocations and inversions. Some of the genes recently discovered may be rearranged with several partner genes. These promiscuous genes are natural experiments that generate mutants which help to identify the function of genes rearranged in hematopoietic malignancies as well as that of their normal counterparts. The diversity of the genes implicated in leukemogenesis makes their functional study a challenge, but, as recently shown by their role in chromatin remodeling, increasing recognition of cross-talk between many of these genes justifies the development of analyses of leukemia-associated chromosome abnormalities and of their functional consequences.

Recruitment of the nuclear receptor corepressor N-CoR by the TEL moiety of the childhood leukemia–associated TEL-AML1 oncoprotein

The t(12;21)(p13;q22) chromosomal translocation is the most frequent illegitimate gene recombination in a pediatric cancer and occurs in approximately 25% of common acute lymphoblastic leukemia (cALL) cases. This rearrangement results in the in frame fusion of the 5′-region of the ETS-related gene, TEL(ETV6), to almost the entire acute myeloid leukemia 1 (AML1) (also called CBFA2 orPEBP2AB1) locus and expression of the TEL-AML1 chimeric protein. Although AML1 stimulates transcription, TEL-AML1 functions as a repressor of some AML1 target genes. In contrast to the wild type AML1 protein, both TEL and TEL-AML1 interact with N-CoR, a component of the nuclear receptor corepressor complex with histone deacetylase activity. The interaction between TEL and N-CoR requires the central region of TEL, which is retained in TEL-AML1, and TEL lacking this domain is impaired in transcriptional repression. Taken together, our results suggest that TEL-AML1 may contribute to leukemogenesis by recruiting N-CoR to AML1 target genes and thus imposing an altered pattern of their expression.

The mechanism of action of thyroid hormones

Thyroid hormone is essential for normal development, differentiation, and metabolic balance. Thyroid hormone action is mediated by multiple thyroid hormone receptor isoforms derived from two distinct genes. The thyroid hormone receptors belong to a nuclear receptor superfamily that also includes receptors for other small lipophilic hormones. Thyroid hormone receptors function by binding to specific thyroid hormone-responsive sequences in promoters of target genes and by regulating transcription. Thyroid hormone receptors often form heterodimers with retinoid X receptors. Heterodimerization is regulated through distinct mechanisms that together determine the specificity and flexibility of the sequence recognition. Amino-terminal regions appear to modulate thyroid hormone receptor function in an isoform-dependent manner. Unliganded thyroid hormone receptor represses transcription through recruitment of a corepressor complex, which also includes Sin3A and histone deacetylase. Ligand binding alters the conformation of the thyroid hormone receptor in such a way as to release the corepressor complex and recruit a coactivator complex that includes multiple histone acetyltransferases, including a steroid receptor family coactivator, p300/CREB-binding protein-associated factor (PCAF), and CREB binding protein (CBP). The existence of histone-modifying activities in the transcriptional regulatory complexes indicates an important role of chromatin structure. Stoichiometric, structural, and sequence-specific rules for coregulator interaction are beginning to be understood, as are aspects of the tissue specificity of hormone action. Moreover, knockout studies suggest that the products of two thyroid hormone receptor genes mediate distinct functions in vivo. The increased understanding of the structure and function of thyroid hormone receptors and their interacting proteins has markedly clarified the molecular mechanisms of thyroid hormone action.

Atrophin-1, the dentato-rubral and pallido-luysian atrophy gene product, interacts with ETO/MTG8 in the nuclear matrix and represses transcription

Dentato-rubral and pallido-luysian atrophy (DRPLA) is one of the family of neurodegenerative diseases caused by expansion of a polyglutamine tract. The drpla gene product, atrophin-1, is widely expressed, has no known function or activity, and is found in both the nuclear and cytoplasmic compartments of neurons. Truncated fragments of atrophin-1 accumulate in neuronal nuclei in a transgenic mouse model of DRPLA, and may underlie the disease phenotype. Using the yeast two-hybrid system, we identified ETO/MTG8, a component of nuclear receptor corepressor complexes, as an atrophin-1-interacting protein. When cotransfected into Neuro-2a cells, atrophin-1 and ETO/MTG8 colocalize in discrete nuclear structures that contain endogenous mSin3A and histone deacetylases. These structures are sodium dodecyl sulfate-soluble and associated with the nuclear matrix. Cotransfection of ETO/MTG8 with atrophin-1 recruits atrophin-1 to the nuclear matrix, while atrophin-1 and ETO/MTG8 cofractionate in nuclear matrix preparations from brains of DRPLA transgenic mice. Furthermore, in a cell transfection-based assay, atrophin-1 represses transcription. Together, these results suggest that atrophin-1 associates with nuclear receptor corepressor complexes and is involved in transcriptional regulation. Emerging links between disease-associated polyglutamine proteins, nuclear receptors, translocation-leukemia proteins, and the nuclear matrix may have important repercussions for the pathobiology of this family of neurodegenerative disorders.

TEL, a putative tumor suppressor, modulates cell growth and cell morphology of ras-transformed cells while repressing the transcription of stromelysin-1

TEL is a member of the ETS family of transcription factors that interacts with the mSin3 and SMRT corepressors to regulate transcription. TEL is biallelically disrupted in acute leukemia, and loss of heterozygosity at the TEL locus has been observed in various cancers. Here we show that expression of TEL in Ras-transformed NIH 3T3 cells inhibits cell growth in soft agar and in normal cultures. Unexpectedly, cells expressing both Ras and TEL grew as aggregates. To begin to explain the morphology of Ras-plus TEL-expressing cells, we demonstrated that the endogenous matrix metalloproteinase stromelysin-1 was repressed by TEL. TEL bound sequences in the stromelysin-1 promoter and repressed the promoter in transient-expression assays, suggesting that it is a direct target for TEL-mediated regulation. Mutants of TEL that removed a binding site for the mSin3A corepressor but retained the ETS domain failed to repress stromelysin-1. When BB-94, a matrix metalloproteinase inhibitor, was added to the culture medium of Ras-expressing cells, it caused a cell aggregation phenotype similar to that caused by TEL expression. In addition, TEL inhibited the invasiveness of Ras-transformed cells in vitro and in vivo. Our results suggest that TEL acts as a tumor suppressor, in part, by transcriptional repression of stromelysin-1.

Nuclear import and subnuclear localization of the proto-oncoprotein ETO (MTG8)

ETO (MTG8) was first described due to its involvement in the (8;21) translocation frequently observed in acute myeloid leukemias. In the t(8;21) the AML1 gene on chromosome 21 is fused to ETO on chromosome 8. The resultant hybrid protein is comprised of the DNA binding domain of AML-1 and the majority of ETO. This study examines the subnuclear distributions of ETO, AML-1B and AML-1/ETO proteins fused to green fluorescence protein in living cells using fluorescence microscopy. Further, we identified a 40 amino acid portion of ETO (amino acids 241-280) that was sufficient to cause nuclear import of green fluorescent protein. Mutational analysis demonstrated that lysine 265 and/or arginine 266 were required for nuclear import of ETO, but that the surrounding basic residues were not critical. ETO interacted with the nuclear import proteins importin-alpha and beta in vitro, and mutations in ETO that abolish nuclear localization also abolished the in vitro interaction with importin-alpha and beta. These data suggest that ETO enters the nucleus via an importin-mediated pathway. Additionally, ETO and AML-1/ETO co-localized to punctate nuclear bodies distinct from those containing promyelocytic leukemia protein. Nuclear body formation was dependent upon a region of ETO N-terminal to the nuclear localization signal. Thus, ETO and AML-1/ETO reside in potentially novel subnuclear compartments.

Analysis of genes under the downstream control of the t(8;21) fusion protein AML1-MTG8: overexpression of the TIS11b(ERF-1, cMG1) gene induces myeloid cell proliferation in response to G-CSF

The AML1-MTG8 fusion transcription factor generated by t(8;21) translocation is thought to dysregulate genes that are crucial for normal differentiation and proliferation of hematopoietic progenitors to cause acute myelogenous leukemia (AML). Although AML1-MTG8 has been shown to repress the transcription of AML1 targets, none of the known targets of AML1 are probably responsible for AML1-MTG8-mediated leukemogenesis. In this study, 24 genes under the downstream control of AML1-MTG8 were isolated by using a differential display technique. Analysis with deletion mutants of AML1-MTG8 demonstrated that the regulation of the majority of these genes requires the region of 51 residues (488-538) containing the Nervy homology region 2 (NHR2), through which AML1-MTG8 interacts with MTGR1. Among the 24 genes identified, 10 were considered to be genes under the control of AML1, because their expression was altered by AML1b or AML1a or both. However, the other 14 genes were not affected by either AML1b or AML1a, suggesting the possibility that AML1-MTG8 regulates a number of specific target genes that are not normally regulated by AML1. Furthermore, an up-regulated gene, TIS11b (ERF-1,cMG1), was highly expressed in t(8;21) leukemic cells, and the overexpression of TIS11b induced myeloid cell proliferation in response to granulocyte colony-stimulating factor. These results suggest that the high-level expression of TIS11b contributes to AML1-MTG8-mediated leukemogenesis.

Analysis of genes under the downstream control of the t(8;21) fusion protein AML1-MTG8: overexpression of the TIS11b(ERF-1, cMG1) gene induces myeloid cell proliferation in response to G-CSF

The AML1-MTG8 fusion transcription factor generated by t(8;21) translocation is thought to dysregulate genes that are crucial for normal differentiation and proliferation of hematopoietic progenitors to cause acute myelogenous leukemia (AML). Although AML1-MTG8 has been shown to repress the transcription of AML1 targets, none of the known targets of AML1 are probably responsible for AML1-MTG8-mediated leukemogenesis. In this study, 24 genes under the downstream control of AML1-MTG8 were isolated by using a differential display technique. Analysis with deletion mutants of AML1-MTG8 demonstrated that the regulation of the majority of these genes requires the region of 51 residues (488-538) containing the Nervy homology region 2 (NHR2), through which AML1-MTG8 interacts with MTGR1. Among the 24 genes identified, 10 were considered to be genes under the control of AML1, because their expression was altered by AML1b or AML1a or both. However, the other 14 genes were not affected by either AML1b or AML1a, suggesting the possibility that AML1-MTG8 regulates a number of specific target genes that are not normally regulated by AML1. Furthermore, an up-regulated gene, TIS11b (ERF-1,cMG1), was highly expressed in t(8;21) leukemic cells, and the overexpression of TIS11b induced myeloid cell proliferation in response to granulocyte colony-stimulating factor. These results suggest that the high-level expression of TIS11b contributes to AML1-MTG8-mediated leukemogenesis.

Histone deacetylases, transcriptional control, and cancer

A key event in the regulation of eukaryotic gene expression is the posttranslational modification of nucleosomal histones, which converts regions of chromosomes into transcriptionally active or inactive chromatin. The most well studied posttranslational modification of histones is the acetylation of epsilon-amino groups on conserved lysine residues in the histones' amino-terminal tail domains. Significant advances have been made in the past few years toward the identification of histone acetyltransferases and histone deacetylases. Currently, there are over a dozen cloned histone acetyltransferases and at least eight cloned human histone deacetylases. Interestingly, many histone deacetylases can function as transcriptional corepressors and, often, they are present in multi-subunit complexes. More intriguing, at least some histone deacetylases are associated with chromatin-remodeling machines. In addition, several studies have pointed to the possible involvement of histone deacetylases in human cancer. The availability of the cloned histone deacetylase genes has provided swift progress in the understanding of the mechanisms of deacetylases, their role in transcription, and their possible role in health and disease.

Groucho/TLE/R-esp proteins associate with the nuclear matrix and repress RUNX (CBF(alpha)/AML/PEBP2(alpha)) dependent activation of tissue-specific gene transcription

The Runt related transcription factors RUNX (AML/CBF(alpha)/PEBP2(alpha)) are key regulators of hematopoiesis and osteogenesis. Using co-transfection experiments with four natural promoters, including those of the osteocalcin (OC), multi drug resistance (MDR), Rous Sarcoma Virus long terminal repeat (LTR), and bone sialoprotein (BSP) genes, we show that each of these promoters responds differently to the forced expression of RUNX proteins. However, the three RUNX subtypes (i.e. AML1, AML2, and AML3) regulate each promoter in a similar manner. Although the OC promoter is activated in a C terminus dependent manner, the MDR, LTR and BSP promoters are repressed by three distinct mechanisms, either independent of or involving the AML C terminus, or requiring only the conserved C-terminal pentapeptide VWRPY. Using yeast two hybrid assays we find that the C terminus of AML1 interacts with a Groucho/TLE/R-esp repressor protein. Co-expression assays reveal that TLE proteins repress AML dependent activation of OC gene transcription. Western and northern blot analyses suggest that TLE expression is regulated reciprocally with the levels of OC gene expression during osteoblast differentiation. Digital immunofluorescence microscopy results show that TLE1 and TLE2 are both associated with the nuclear matrix, and that a significant subset of each colocalizes with AML transcription factors. This co-localization of TLE and AML proteins is lost upon removing the C terminus of AML family members. Our findings indicate that suppression of AML-dependent gene activation by TLE proteins involves functional interactions with the C terminus of AML at the nuclear matrix in situ. Our data are consistent with the concept that the C termini of AML proteins support activation or repression of cell-type specific genes depending on the regulatory organization of the target promoter and subnuclear localization.

Oligomerization of RAR and AML1 transcription factors as a novel mechanism of oncogenic activation

RAR and AML1 transcription factors are found in leukemias as fusion proteins with PML and ETO, respectively. Association of PML-RAR and AML1-ETO with the nuclear corepressor (N-CoR)/histone deacetylase (HDAC) complex is required to block hematopoietic differentiation. We show that PML-RAR and AML1-ETO exist in vivo within high molecular weight (HMW) nuclear complexes, reflecting their oligomeric state. Oligomerization requires PML or ETO coiled-coil regions and is responsible for abnormal recruitment of N-CoR, transcriptional repression, and impaired differentiation of primary hematopoietic precursors. Fusion of RAR to a heterologous oligomerization domain recapitulated the properties of PML-RAR, indicating that oligomerization per se is sufficient to achieve transforming potential. These results show that oligomerization of a transcription factor, imposing an altered interaction with transcriptional coregulators, represents a novel mechanism of oncogenic activation.

Role of the transcription factor AML-1 in acute leukemia and hematopoietic differentiation

Chromosomal translocations affecting the AML-1 gene are among the most frequent aberrations found in acute leukemia. Because the AML-1 transcription factor is a critical regulator of hematopoeitic cell development, normal homeostasis is disrupted in cells containing these translocations. In this review we describe the mechanisms of transcriptional activation and repression by AML-1 and how this transcriptional control is disrupted by the chromosomal translocations that affect AML-1. Finally, we discuss how the mechanism of transcriptional repression by these chromosomal translocation fusion proteins is a possible target of therapeutic intervention in acute leukemia.

The adenovirus E1A binding protein BS69 is a corepressor of transcription through recruitment of N-CoR

BS69 was first identified as a protein that interacts directly with the transactivation domain (conserved region 3) of the 289R adenovirus type 5 E1A protein. We show here that BS69 is a potent repressor of transcription. BS69 mediates repression, at least in part, through interaction with the co-repressor N-CoR. BS69 interacts with N-CoR through a MYND domain in its carboxyl terminus. A recently cloned splice variant of BS69, designated BRAM1, is also capable of interacting with N-CoR and E1A, but unlike BS69, is not able to repress transcription, indicating that N-CoR interaction is necessary but not sufficient for BS69 repression. Expression of E1A inhibits repression mediated by BS69. Our data suggest that BS69 participates in transcriptional repressor complexes and that E1A can modulate these complexes through interaction with BS69.

mSin3A regulates murine erythroleukemia cell differentiation through association with the TAL1 (or SCL) transcription factor

Activation of the TAL1 (or SCL) gene is the most frequent gain-of-function mutation in T-cell acute lymphoblastic leukemia (T-ALL). TAL1 belongs to the basic helix-loop-helix (HLH) family of transcription factors that bind as heterodimers with the E2A and HEB/HTF4 gene products to a nucleotide sequence motif termed the E-box. Reported to act both as an activator and as a repressor of transcription, the mechanisms underlying TAL1-regulated gene expression are poorly understood. We report here that the corepressor mSin3A is associated with TAL1 in murine erythroleukemia (MEL) and human T-ALL cells. Interaction mapping showed that the basic-HLH domain of TAL1 was both necessary and sufficient for TAL1-mSin3A interaction. TAL1 was found, in addition, to interact with the histone deacetylase HDAC1 in vitro and in vivo, and a specific histone deacetylase inhibitor, trichostatin A (TSA), relieved TAL1-mediated repression of an E-box-containing promoter and a GAL4 reporter linked to a thymidine kinase minimal promoter. Further, TAL1 association with mSin3A and HDAC1 declined during dimethyl sulfoxide-induced differentiation of MEL cells in parallel with a decrease in mSin3A abundance. Finally, TSA had a synergistic effect with enforced TAL1 expression in stimulating MEL cells to differentiate, while constitutive expression of mSin3A inhibited MEL cell differentiation. These results demonstrate that a corepressor complex containing mSin3A and HDAC1 interacts with TAL1 and restricts its function in erythroid differentiation. This also has implications for this transcription factor's actions in leukemogenesis.

Molecular biology of leukemia

Identification and characterization of leukemia-related chromosomal translocations have had significant impact on all aspects of the management of acute leukemia, including its diagnosis, assignment of prognosis, and development of an appropriate treatment plan. Several genes are recurrent targets of chromosomal abnormalities, suggesting that they play a key role in leukemogenesis. Significant progress has been made to define potentially unifying molecular mechanisms of leukemic transformation. Hopefully, these findings will provide the basis for molecularly targeted therapies for leukemia.

New agents for acute myelogenous leukemia

New agents for the treatment of acute myelogenous leukemia are discussed that reflect different treatment mechanisms. These include histone acetylation, angiogenesis inhibition, protein kinase inhibitors, and a novel retinoid. Efficacy and safety in phase I and phase II trials reviewed, as well as the problems involved in crossing over from treatment of solid tumors to blood disorders.

The ETO protein disrupted in t(8;21)-associated acute myeloid leukemia is a corepressor for the promyelocytic leukemia zinc finger protein

The ETO protein was originally identified by its fusion to the AML-1 transcription factor in translocation (8;21) associated with the M2 form of acute myeloid leukemia (AML). The resulting AML-1-ETO fusion is an aberrant transcriptional regulator due to the ability of ETO, which does not bind DNA itself, to recruit the transcriptional corepressors N-CoR, SMRT, and Sin3A and histone deacetylases. The promyelocytic leukemia zinc finger (PLZF) protein is a sequence-specific DNA-binding transcriptional factor fused to retinoic acid receptor alpha in acute promyelocytic leukemia associated with the (11;17)(q23;q21) translocation. PLZF also mediates transcriptional repression through the actions of corepressors and histone deacetylases. We found that ETO is one of the corepressors recruited by PLZF. The PLZF and ETO proteins associate in vivo and in vitro, and ETO can potentiate transcriptional repression by PLZF. The N-terminal portion of ETO forms complexes with PLZF, while the C-terminal region, which was shown to bind to N-CoR and SMRT, is required for the ability of ETO to augment transcriptional repression by PLZF. The second repression domain (RD2) of PLZF, not the POZ/BTB domain, is necessary to bind to ETO. Corepression by ETO was completely abrogated by histone deacetylase inhibitors. This identifies ETO as a cofactor for a sequence-specific transcription factor and indicates that, like other corepressors, it functions through the action of histone deactylase.

Expression of the AML-1 oncogene shortens the G(1) phase of the cell cycle

The AML-1-encoded transcription factor, AML-1B, regulates numerous hematopoietic-specific genes. Inappropriate expression of AML-1-family proteins is oncogenic in cell culture systems and in mice. To understand the oncogenic functions of AML-1, we established cell lines expressing AML-1B to examine the role of AML-1 in the cell cycle. DNA content analysis and bromodeoxyuridine pulse-chase studies indicated that entry into the S phase of the cell cycle was accelerated by up to 4 h in AML-1B-expressing 32D.3 myeloid progenitor cells as compared with control cells or cells expressing E2F-1. However, AML-1B was not able to induce continued cell cycle progression in the absence of growth factors. The DNA binding and transactivation domains of AML-1B were required for altering the cell cycle. Thus, AML-1B is the first transcription factor that affects the timing of the mammalian cell cycle.

Structure and expression pattern of a human MTG8/ETO family gene, MTGR1

AML1-MTG8 fusion protein, which is produced from the rearranged gene formed between AML1 and MTG8 in myeloid leukemia with t(8;21) chromosomal translocation, plays an important role in the pathogenesis of leukemia. We previously showed that ectopically expressed AML1-MTG8 fusion protein is associated with an MTG8-like protein in the mouse myeloid precursor cell line L-G, and this association seemed to be required for AML1-MTG8 to stimulate proliferation. As a candidate cDNA for this MTG8-like protein, a 6.4 kb MTGR1 cDNA encoding human MTGR1b protein of 604 amino acids was isolated. Since this cDNA was shorter than the main mRNA (about 7.5 kb), the 5'-end of the MTGR1 cDNA was extended using Marathon Ready cDNA. When the newly obtained 5'-sequence was combined with the previous cDNA, the resultant MTGR1 cDNA (6995 bp), including exon 3 that the previous cDNA lacked, could encode MTGR1a protein of 575 amino acids. Transcripts of the MTGR1 gene were expressed ubiquitously in the human tissues and cell lines examined. PCR analyses of the cDNAs from human tissues showed the presence of various splicing variants with regard to the 5'-region including exons 1, 2 and 3. The MTGR1 gene consists of 14 exons and spans about 68 kb. The genomic structure of MTGR1 is highly similar to those of other MTG 8-family genes, MTG8 and MTG16. MTG16 was recently cloned from the translocation breakpoint of myeloid malignancies with t(16;21) chromosomal translocation.

A mechanism of repression by acute myeloid leukemia-1, the target of multiple chromosomal translocations in acute leukemia

AML1 is one of the most frequently translocated genes in human leukemia. Here we demonstrate that acute myeloid leukemia-1 (AML-1) (Runx-1) represses transcription from a native promoter, p21(Waf1/Cip1). Unexpectedly, this repression did not require interactions with the Groucho co-repressor. To define the mechanism of repression, we asked whether other co-repressors could interact with AML-1. We demonstrate that AML-1 interacts with the mSin3 co-repressors. Moreover, endogenous AML-1 associated with endogenous mSin3A in mammalian cells. A deletion mutant of AML-1 that did not interact with mSin3A failed to repress transcription. The AML-1/mSin3 association suggests a mechanism of repression for the chromosomal translocation fusion proteins that disrupt AML-1.

Transcriptional repression by nuclear hormone receptors

Repression by nuclear receptors plays important roles in acute promyelocytic leukemia and other diseases. Nuclear receptor corepressor (N-CoR) and SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) are corepressor proteins whose modular structure facilitates receptor interaction as well as transduction of repression signals involving histone deacetylation, alterations in chromatin structure and direct interactions with the basal transcription machinery. Interactions between nuclear receptors and corepressor complexes have multiple determinants. This allows regulation, and potentially therapeutic manipulation, of receptor, corepressor, cell-type and target-gene specificity.

Nuclear receptor corepressors partner with class II histone deacetylases in a Sin3-independent repression pathway

Transcriptional repression mediated by corepressors N-CoR and SMRT is a critical function of nuclear hormone receptors, and is dysregulated in human myeloid leukemias. At the present time, these corepressors are thought to act exclusively through an mSin3/HDAC1 complex. Surprisingly, however, numerous biochemical studies have not detected N-CoR or SMRT in mSin3- and HDAC1-containing complexes. Each corepressor contains multiple repression domains (RDs), the significance of which is unknown. Here we show that these RDs are nonredundant, and that one RD, which is conserved in N-CoR and SMRT, represses transcription by interacting directly with class II HDAC4 and HDAC5. Endogenous N-CoR and SMRT each associate with HDAC4 in a complex that does not contain mSin3A or HDAC1. This is the first example of a single corepressor utilizing distinct domains to engage multiple HDAC complexes. The alternative HDAC complexes may mediate specific repression pathways in normal as well as leukemic cells.

Nuclear receptor corepressors partner with class II histone deacetylases in a Sin3-independent repression pathway

Transcriptional repression mediated by corepressors N-CoR and SMRT is a critical function of nuclear hormone receptors, and is dysregulated in human myeloid leukemias. At the present time, these corepressors are thought to act exclusively through an mSin3/HDAC1 complex. Surprisingly, however, numerous biochemical studies have not detected N-CoR or SMRT in mSin3- and HDAC1-containing complexes. Each corepressor contains multiple repression domains (RDs), the significance of which is unknown. Here we show that these RDs are nonredundant, and that one RD, which is conserved in N-CoR and SMRT, represses transcription by interacting directly with class II HDAC4 and HDAC5. Endogenous N-CoR and SMRT each associate with HDAC4 in a complex that does not contain mSin3A or HDAC1. This is the first example of a single corepressor utilizing distinct domains to engage multiple HDAC complexes. The alternative HDAC complexes may mediate specific repression pathways in normal as well as leukemic cells.

The t(8;21) chromosomal translocation in acute myelogenous leukemia modifies intranuclear targeting of the AML1/CBFalpha2 transcription factor

Targeting of gene regulatory factors to specific intranuclear sites may be critical for the accurate control of gene expression. The acute myelogenous leukemia 8;21 (AML1/ETO) fusion protein is encoded by a rearranged gene created by the ETO chromosomal translocation. This protein lacks the nuclear matrix-targeting signal that directs the AML1 protein to appropriate gene regulatory sites within the nucleus. Here we report that substitution of the chromosome 8-derived ETO protein for the multifunctional C terminus of AML1 precludes targeting of the factor to AML1 subnuclear domains. Instead, the AML1/ETO fusion protein is redirected by the ETO component to alternate nuclear matrix-associated foci. Our results link the ETO chromosomal translocation in AML with modifications in the intranuclear trafficking of the key hematopoietic regulatory factor, AML1. We conclude that misrouting of gene regulatory factors as a consequence of chromosomal translocations is an important characteristic of acute leukemias.

Recruitment of SMRT/N-CoR-mSin3A-HDAC-repressing complexes is not a general mechanism for BTB/POZ transcriptional repressors: the case of HIC-1 and gammaFBP-B

Hypermethylated in cancer (HIC-1), a new candidate tumor suppressor gene located in 17p13.3, encodes a protein with five C(2)H(2) zinc fingers and an N-terminal broad complex, tramtrack, and bric à brac/poxviruses and zinc-finger (BTB/POZ) domain found in actin binding proteins or transcriptional regulators involved in chromatin modeling. In the human B cell lymphoma (BCL-6) and promyelocityc leukemia (PLZF) oncoproteins, this domain mediates transcriptional repression through its ability to recruit a silencing mediator of retinoid and thyroid hormone receptor (SMRT)/nuclear receptor corepressor (N-CoR)-mSin3A-histone deacetylase (HDAC) complex, a mechanism shared with numerous transcription factors. HIC-1 appears unique because it contains a 13-aa insertion acquired late in evolution, because it is not found in its avian homologue, gammaF1-binding protein isoform B (gammaFBP-B), a transcriptional repressor of the gammaF-crystallin gene. This insertion, located in a conserved region involved in the dimerization and scaffolding of the BTB/POZ domain, mainly affects slightly the ability of the HIC-1 and gammaFBP-B BTB/POZ domains to homo- and heterodimerize in vivo, as shown by mammalian two-hybrid experiments. Both the HIC-1 and gammaFBP-B BTB/POZ domains behave as autonomous transcriptional repression domains. However, in striking contrast with BCL-6 and PLZF, both HIC-1 and gammaFBP-B similarly fail to interact with members of the HDAC complexes (SMRT/N-CoR, mSin3A or HDAC-1) in vivo and in vitro. In addition, a general and specific inhibitor of HDACs, trichostatin A, did not alleviate the HIC-1- and gammaFBP-B-mediated transcriptional repression, as previously shown for BCL-6. Taken together, our studies show that the recruitment onto target promoters of an HDAC complex is not a general property of transcriptional repressors containing a conserved BTB/POZ domain.

Nuclear DEAF-1-related (NUDR) protein contains a novel DNA binding domain and represses transcription of the heterogeneous nuclear ribonucleoprotein A2/B1 promoter

Nuclear DEAF-1-related (NUDR) protein is a novel transcriptional regulator with sequence similarity to developmental and oncogenic proteins. NUDR protein deletions were used to localize the DNA binding domain between amino acids 167 and 368, and site-specific DNA photocross-linking indicated at least two sites of protein-DNA contact within this domain. The DNA binding domain contains a proline-rich region and a region with similarity to a Myc-type helix-loop-helix domain but does not include the zinc finger motif at the C terminus. Deoxyribonuclease I protection assays confirmed the presence of multiple NUDR binding motifs (TTC(C/G)G) in the heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNP A2/B1) promoter and also in the 5'-untranslated region (UTR) of hNUDR cDNA. NUDR produced a 65-70% repression of the hnRNP A2/B1 promoter activity, and NUDR binding motifs in the 5'-UTR were found to mediate this repression. NUDR-dependent repression was also observed when the 5'-UTR of NUDR was placed onto a heterologous thymidine kinase promoter in an analogous 5'-UTR position but not when placed upstream of transcription initiation. These results suggest that NUDR may regulate the in vivo expression of hnRNP A2/B1 and NUDR genes and imply that inactivation of NUDR could contribute to the overexpression of hnRNP A2/B1 observed in some human cancers.

Both TEL and AML-1 contribute repression domains to the t(12;21) fusion protein

t(12;21) is the most frequent translocation found in pediatric B-cell acute lymphoblastic leukemias. This translocation fuses a putative repressor domain from the TEL DNA-binding protein to nearly all of the AML-1B transcription factor. Here, we demonstrate that fusion of the TEL pointed domain to the GAL4 DNA-binding domain resulted in sequence-specific transcriptional repression, indicating that the pointed domain is a portable repression motif. The TEL pointed domain functioned equally well when the GAL4 DNA-binding sites were moved 600 bp from the promoter, suggesting an active mechanism of repression. This lead us to demonstrate that wild-type TEL and the t(12;21) fusion protein bind the mSin3A corepressor. In the fusion protein, both TEL and AML-1B contribute mSin3 interaction domains. Deletion mutagenesis indicated that both the TEL and AML-1B mSin3-binding domains contribute to repression by the fusion protein. While both TEL and AML-1B associate with mSin3A, TEL/AML-1B appears to bind this corepressor much more stably than either wild-type protein, suggesting a mode of action for the t(12;21) fusion protein.

Complex Complexes: Report of an NIDDK Workshop on Coactivators and Corepressors, Bethesda, USA, 15-16 December 1998

Exciting new observations about receptor-interacting and nuclear accessory proteins and their ability to regulate gene expression have provided insight into hormonal regulation of gene expression. The goal of a recent NIDDK workshop was to discuss how diverse signaling pathways could converge on target genes and provide regulatory input to either activate or repress transcription.

A novel role for helix 12 of retinoid X receptor in regulating repression

Nutrients, drugs, and hormones influence transcription during differentiation and metabolism by binding to high-affinity nuclear receptors. In the absence of ligand, some but not all nuclear receptors repress transcription as a heterodimer with retinoid X receptor (RXR). Here we define a novel role for helix 12 (H12) in sterically masking the corepressor (CoR) binding site in apo-RXR. Removing H12 converts RXR to a potent transcriptional repressor. The length but not the specific sequence of H12 is critical for masking RXR's intrinsic repression function. This contrasts with the amphipathic character required for mediating ligand-dependent activation and coactivator recruitment. Physiologically, we show that heterodimerization of RXR with apo-thyroid hormone receptor (TR) unmasks the CoR binding site in RXR and allows the TR-RXR heterodimer to repress. A molecular mechanism that involves sequence-specific interaction between RXR H12 and the coactivator-binding surface of the nuclear receptor is proposed for this heterodimerization-mediated unmasking. Peroxisome proliferator-activated receptor gamma does not interact as well with RXR H12, thus explaining its inability to repress transcription as an RXR heterodimer. The requirement to unmask RXR's latent repression function explains why only certain RXR partners repress transcription.

Organization of chromatin in cancer cells: role of signalling pathways

The role of mechanical and chemical signalling pathways in the organization and function of chromatin is the subject of this review. The mechanical signalling pathway consists of the tissue matrix system that links together the three-dimensional skeletal networks, the extracellular matrix, cytoskeleton, and nuclear matrix. Intermediate filament proteins are associated with nuclear DNA, suggesting that intermediate filaments may have a role in the organization of chromatin. In human hormone-dependent breast cancer cells, the interaction between cytokeratins and chromatin is regulated by estrogens. Transcription factors, histone acetyltransferases, and histone deacetylases, which are associated with the nuclear matrix, are components of the mechanical signalling pathway. Recently, we reported that nuclear matrix-bound human and chicken histone deacetylase 1 is associated with nuclear DNA in situ, suggesting that histone deacetylase has a role in the organization of nuclear DNA. Chemical signalling pathways such as the Ras/mitogen-activated protein kinase (Ras/MAPK) pathway stimulate the activity of kinases that modify transcription factors, nonhistone chromosomal proteins, and histones. The levels of phosphorylated histones are increased in mouse fibroblasts transformed with oncogenes, the products of which stimulate the Ras/MAPK pathway. Histone phosphorylation may lead to decondensation of chromatin, resulting in aberrant gene expression.Key words: histone acetylation, histone phosphorylation, nuclear matrix, cytoskeleton, histone deacetylase, cancer.

SMRTe, a silencing mediator for retinoid and thyroid hormone receptors-extended isoform that is more related to the nuclear receptor corepressor

SMRT (silencing mediator for retinoid and thyroid hormone receptors) and N-CoR (nuclear receptor copressor) mediate transcriptional repression of important regulators that are involved in many signaling pathways. SMRT and N-CoR are related proteins that form complexes with mSin3A/B and histone deacetylases to induce local chromatin condensation and transcriptional repression. However, SMRT is substantially smaller than N-CoR, lacking an N-terminal domain of approximately 1,000 aa that are present in N-CoR. Here, we report the identification of SMRT-extended (SMRTe), which contains an N-terminal sequence that shows striking similarity with N-CoR. As in N-CoR, this SMRTe-N-terminal domain also represses basal transcription. We find that SMRTe expression is regulated during cell cycle progression and SMRTe transcripts are present in many embryonic tissues. These data redefine a structurally and functionally more related nuclear receptor corepressor family and suggest an additional role for SMRTe in the regulation of cycle-specific gene expression in diverse signaling pathways.