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28 May 2001, Volume 20, Number 24, Pages 3100-3109
Table of contents    Previous  Article  Next   [PDF]
Review
Avian erythroleukemia: a model for corepressor function in cancer
Luc EG Rietvelda, Eric Caldenhovena and Hendrik G Stunnenberg

Department of Molecular Biology, NCMLS, Geert Grooteplein Zuid 26, PO Box 9101 6500 HB Nijmegen, The Netherlands

Correspondence to: H G Stunnenberg, Department of Molecular Biology, NCMLS, Geert Grooteplein Zuid 26, PO Box 9101 6500 HB Nijmegen, The Netherlands

aLEG Rietveld and E Caldenhoven contributed equally

Abstract

Transcriptional regulation at the level of chromatin plays crucial roles during eukaryotic development and differentiation. A plethora of studies revealed that the acetylation status of histones is controlled by multi-protein complexes containing (de)acetylase activities. In the current model, histone deacetylases and acetyltransferases are recruited to chromatin by DNA-bound repressors and activators, respectively. Shifting the balance between deacetylation, i.e. repressive chromatin and acetylation, i.e. active chromatin can lead to aberrant gene transcription and cancer. In human acute promyelocytic leukemia (APL) and avian erythroleukemia (AEL), chromosomal translocations and/or mutations in nuclear hormone receptors, RARalpha [NR1B1] and TRalpha [NR1A1], yielded oncoproteins that deregulate transcription and alter chromatin structure. The oncogenic receptors are locked in their 'off' mode thereby constitutively repressing transcription of genes that are critical for differentiation of hematopoietic cells. AEL involves an oncogenic version of the chicken TRalpha, v-ErbA. Apart from repression by v-ErbA via recruitment of corepressor complexes, other repressors and corepressors appear to be involved in repression of v-ErbA target genes, such as carbonic anhydrase II (CAII). Reactivation of repressed genes in APL and AEL by chromatin modifying agents such as inhibitors of histone deacetylase or of methylation provides new therapeutic strategies in the treatment of acute myeloid leukemia. Oncogene (2001) 20, 3100-3109.

Keywords

Erythroleukemia; corepressor; v-ErbA; histone deacetylase; carbonic anhydrase II; methylation

Introduction

The mechanisms by which oncoproteins deregulate transcription of specific target genes during oncogenic transformation are still poorly understood. Mutations, deletions and fusions resulting from chromosomal translocations in cellular or viral (proto-)oncogenes have altered their cognate protein functions, causing aberrant gene regulation and inducing cancer. Recent studies show that reactivation of genes silenced in cancer can be accomplished by treatment of transformed cells with compounds that affect the structure of chromatin, such as inhibitors of methylation or histone acetylation (Cameron et al., 1999). Thus, altering the state of chromatin may be an important tool in a cure for cancer.

A link between (de)acetylation of histones and transcriptional regulation was proposed several decades ago (Allfrey et al., 1964). The identification of proteins that can modulate the acetylation state of histones provided further evidence for the link between chromatin structure and gene regulation. The extent of histone acetylation is determined by the opposing action of histone acetyltransferases (HATs) and histone deacetylases (HDACs) (reviewed by Kouzarides, 1999). In a simplistic view, these enzymes are recruited to chromatin by DNA-bound transcription factors thereby (locally) modifying the histone tails and consequently the chromatin structure. Transcriptional regulators that recruit histone deacetylases may control tumour development at different stages and can grossly be divided into three different categories. Firstly, transcriptional repressors that control cell-cycle and proliferation such as Mad and Rb. Secondly, methyl-binding proteins that mediate transcriptional silencing of genes which may be critical in the formation of certain cancers. Thirdly, transcriptional regulators such as TR and RAR that control differentiation and development (reviewed in Cress and Seto, 2000).

This review focuses on the role of nuclear hormone receptors (NR) in cancer. Aberrant nuclear receptor function due to a hormonal imbalance or receptor mutations has been strongly correlated with disease and cancer (reviewed in Forrest et al., 1996; Goldhirsch and Gelber, 1996). Thyroid hormone receptor (TR) and retinoic acid receptor (RAR) are both class II nuclear receptors which are ligand-controlled transcription factors that govern transcription of a multitude of target genes by recruiting multicomponent cofactor complexes. In the absence of ligand, nuclear hormone receptors recruit corepressor complexes containing HDAC activity, whereas in the presence of ligand they recruit coactivator proteins with HAT activity and/or multicomponent coactivator complexes. In human promyelocytic leukemia (APL), chromosomal translocations lead to fusion proteins involving RARalpha and different fusion partners, such as PML, PLZF, NPM and NUMA (reviewed in Redner et al., 1999). APL-patients carrying the PML-RARalpha fusion can be cured by treatment with the RARalpha ligand all-trans retinoic acid (ATRA) which induces terminal differentiation of APL blasts (Warrell et al., 1991, 1993). PLZF-RARalpha patients are not responsive to ATRA, suggesting that different or additional mechanisms are involved in PLZF-RARalpha- compared to PML-RARalpha-induced APL. Treatment with histone deacetylase inhibitors such as trichostatin A (TSA) plus ATRA overcomes the maturation blockade in APL induced by PLZF-RARalpha (reviewed in Redner et al., 1999), suggesting a link between chromatin modifications and cancer development. In chicks, the avian erythroblastosis virus (AEV) causes erythroleukemia (AEL). AEV encodes an oncogenic variant of the TRalpha, v-ErbA, and the mutated EGF-receptor v-ErbB. Due to a number of mutations throughout the ligand-binding domain, v-ErbA is unable to respond to ligand. Restoration of thyroid hormone responsiveness in erythroleukemic cells by overexpression of a ligand-responsive thyroid hormone receptor partially restores the ability of these cells to differentiate (Disela et al., 1991). The inability of oncogenic receptors to effectively respond to physiological concentrations of ligand provides a molecular basis of oncogenesis. Recently, a plethora of molecular studies on the modulation of chromatin by NR have been published that provide new and sometimes contradictory explanations for the mechanisms causing APL and AEL. In this review, we will focus on chromatin modifying complexes and their role in transcriptional repression in AEV-induced leukemia.

AEV and leukemia

AEV is an acute leukemogenic retrovirus that causes fatal erythroleukemia in young chicks. The target cell for AEV is the committed erythroid progenitor (Samarut and Gazzolo, 1982). In culture-transformed erythroblast cells called HD3 are arrested in their ability to terminally differentiate and show an increased rate of proliferation (Graf and Beug, 1983). AEV-transformed HD3 cells provide an excellent model system to investigate the molecular basis of the disease. Besides the v-ErbA oncoprotein, the AEV virus encodes a second oncoprotein, v-ErbB, a mutated constitutively active transmembrane receptor for epidermal growth factor (EGF) (Downward et al., 1984; Sap et al., 1986). v-ErbA has no transformation capacity on its own, unless it is expressed at extremely high levels (Casini and Graf, 1995). Co-expression of v-ErbA with v-ErbB does, however, lead to a much more severe leukemic phenotype compared to expression of v-ErbB alone (Miles and Robinson, 1985; Kahn et al., 1986; Forrest et al., 1990). The current view is that v-ErbA cooperates with ligand-activated tyrosine kinases such as stem cell factor (SCF)-activated c-Kit and TGF-loaded c-ErbB or constitutively active v-ErbB to effectively arrest differentiation and to induce leukemia (reviewed in Beug et al., 1996). Recent studies suggest that v-ErbA mimics unliganded TRalpha in co-operating with ligand-activated c-Kit to cause steroid-independent long-term proliferation and a block in differentiation of primary erythroid progenitor cells (Bauer et al., 1998).

v-ErbA and transcriptional repression

The v-ErbA oncogene is a viral variant of chicken TRalpha and was fused to the viral gag gene during the genesis of AEV. The way in which v-ErbA contributes to the leukemic phenotype has long been elusive. The role of numerous mutations occurring throughout the receptor moiety have been analysed in great detail (Figure 1, reviewed in Beug et al., 1994). Due to the mutations in the Zn-finger, the v-ErbA DNA-binding specificity and affinity have been affected. Furthermore, the ability of v-ErbA to dimerise with RXR is impaired. Several mutations and a small N-terminal deletion also affect the ability of v-ErbA to bind ligand and to activate transcription (Munoz et al., 1988; Zenke et al., 1990; Barettino et al., 1993).

A decade ago, it was demonstrated that v-ErbA constitutively inhibits T3-mediated activation of cognate target genes (Sap et al., 1989; Damm et al., 1989; Pain et al., 1990; Zenke et al., 1990; Baniahmad et al., 1992). From these and many other studies, v-ErbA was initially postulated to merely antagonise TR function through constitutive binding to TR responsive elements (TRE), occluding the endogenous receptor (the 'binding site occlusion' model). Another step was the identification of target genes that were directly regulated by v-ErbA: the erythrocyte anion transport protein gene (Band 3), the delta-aminolevulinate synthase gene (ALA-S), the lysozyme gene and the carbonic anhydrase II gene (CAII) (Zenke et al., 1988; Pain et al., 1990; Baniahmad et al., 1990). Notwithstanding the identification of target genes, progress in elucidating the mechanisms by which v-ErbA silences transcription remained slow. A further piece of the puzzle was provided by Damm and Evans (1993). The authors showed that the transformation-defective AEV-mutant td359 encodes a v-ErbA variant that failed to suppress basal transcription and exhibited an impaired ability to antagonise TR. One of two mutations specific for the td359 variant (Figure 1, Pro144 Arg in the 'hinge' region), abolished repressive functions but not hormone binding and activation properties when introduced in the context of wild type TR. Thus, active repression in addition to TR occlusion appeared to be an important mechanism involved in AEV-induced oncogenesis.

Corepressor complexes

The recent observation that unliganded nuclear receptors can 'actively' repress transcription by recruiting multi-protein corepressor complexes has been a milestone (reviewed in Hu and Lazar, 2000; Burke and Baniahmad, 2000). The repressive complexes are recruited to the DNA by unliganded nuclear hormone receptors and other specific DNA-binding proteins. Yeast-two hybrid screens for proteins interacting with unliganded nuclear receptors led to the identification of the corepressor molecules NCoR (nuclear receptor corepressor) (Horlein et al., 1995; Kurokawa et al., 1995) and the highly related SMRT (Silencing Mediator for Retinoic acid and Thyroid hormone receptors) (Chen and Evans, 1995; Sande and Privalsky, 1996). Subsequent studies suggested that NCoR/SMRT functions as corepressors not only for unliganded nuclear receptors, but for many other unrelated transcription factors including, PLZF (Hong et al., 1997; Grignani et al., 1998), BCL6-POZ (Dhordain et al., 1998) and MyoD (Bailey et al., 1999). While NCoR is a 270 kDa protein, SMRT was initially reported to be a 170 kDa protein but recently a 270 kDa splice variant of SMRT was identified (Ordentlich et al., 1999, Park et al., 1999). The 270 kDa SMRT protein has a high overall homology with NCoR; both possess a bipartite structure with at least three autonomous N-terminal repressor domains and two independent C-terminal nuclear receptor interaction domains (RID) (reviewed by Hu and Lazar, 2000). NCoR/SMRT-mediated repression pathways have been suggested to play a pathophysiological role in APL (Grignani et al., 1998; He et al., 1998), in AML (Gelmetti et al., 1998; Lutterbach et al., 1998) as well as in thyroid hormone resistance syndrome (Safer et al., 1998). In addition to NCoR/SMRT, two other corepressors have been identified that directly interact with unliganded nuclear receptors. SUN-CoR is a highly basic 16 kDa nuclear protein that interacts with RevErb [NR1D1] as well as with TR in vitro (Zamir et al., 1996, 1997). Interestingly, SUN-CoR can also directly bind to NCoR/SMRT suggesting a role for this protein in facilitating NCoR-mediated repression. Recently, a TR-interacting protein called Alien has been identified that is unrelated to NCoR/SMRT (Dressel et al., 1999; Polly et al., 2000). RAR has been reported not to interact with Alien, indicating that Alien displays a receptor specificity distinct from that of NCoR/SMRT.

New insights into the mechanism of transcriptional repression in mammalian cells came from the discovery that the mammalian homologues of yeast Sin3p, the Sin3A and Sin3B proteins, are recruited by the transcriptional repressor Mad (Ayer et al., 1995). In yeast, the Sin3p repressor was reported to act as a global repressor and genetic data indicated that it was a component of a pathway that included the transcriptional repressor, RPD3 (Vidal and Gaber, 1991). The identification of a human histone deacetylase with ~60% identity to RPD3 linked histone deacetylases with Sin3A/Sin3B (Taunton et al., 1996). These and other efforts led to the identification of a multiprotein complex containing Sin3A/B and HDAC1/2, RbAP48 and -46, and two Sin-interacting protein SAP18 and -30 (Table 1, Hassig et al., 1997; Kadosh and Struhl, 1997; Laherty et al., 1997; Zhang et al., 1997). RbAp46/48, originally identified to associate with Rb, are of particular interest as they can bind to helix 1 of histone H4 in vitro (Verreault et al., 1998). Concomitant with the identification of the Sin3/HDAC complex, other studies reported on protein-protein interactions between the Sin3/HDAC complex and the nuclear corepresssor NCoR/SMRT (Heinzel et al., 1997; Nagy et al., 1997; Alland et al., 1997). Finally, the Sin3/HDAC complex also participates in mediating repression by the methyl-CpG binding protein, MeCP2 (Nan et al., 1998; Jones et al., 1998), suggesting a direct link between methylation and deacetylation.

Recently, it was shown that the class II histone deacetylases HDAC4, HDAC5 and HDAC7 directly interacted with NCoR/SMRT devoid of Sin3 or HDAC1/2 proteins (Kao et al., 2000; Huang et al., 2000). HDAC4, -5 and -7 share homology with the yeast HDA1 protein (Fischle et al., 1999; Grozinger et al., 1999; Verdel and Khochbin, 1999) and contain a non-catalytic N-terminal domain which may execute additional functions. HDAC6 is unique because it contains two deacetylase domains that are less conserved compared to other class II HDACs (Grozinger et al., 1999; Zhou et al., 2000; Kao et al., 2000). The finding that class II HDACs are differentially expressed suggests that they are not redundant but rather have distinct physiological functions (Grozinger et al., 1999; Verdel and Khochbin, 1999; Kao et al., 2000). Although it has been shown that several HDACs can directly or indirectly interact with NCoR/SMRT in vitro, immunopurified NCoR/SMRT complexes contained predominantly HDAC3 (Table 1, Li et al., 2000; Guenther et al., 2000; Urnov et al., 2000; Wen et al., 2000). The NCoR/HDAC3 complex further contained TBL1 (Transducin beta-like 1), a novel WD40-repeat containing protein with analogy to the TUP1 and Groucho corepressors. TBL-1 has been reported to bind histone H3 suggesting that TBL-1 forms a bridge between NCoR/SMRT and chromatin (Guenther et al., 2000). Independent evidence was obtained from Xenopus oocyte injections: unliganded v-ErbA/TRalpha receptors were shown to interact with the NCoR/HDAC3 but not with the Sin3/HDAC complex (Urnov et al., 2000). Finally, two chromatographically distinct NCoR complexes, NCoR-1 and NCoR-2 have been identified (Underhill et al., 2000). Whereas the NCoR-1 complex contained HDAC3 and the SWI-SNF related proteins BRG1, BAF170, BAF155, BAF47 and KAP, the NCoR-2 complex predominantly contained HDAC1 and HDAC2 and other components typical of the Sin3/HDAC complex (Table 1).

The first compelling in vivo evidence for the role of NCoR as a corepressor protein was obtained by disrupting the NCoR gene in mice (Jepsen et al., 2000). NCoR-/- embryos exhibited defects in erythrocytes, thymocytes and neuronal development. NCoR-/- erythrocyte progenitors were impaired in definitive erythropoiesis resulting in severe anemia in the mutant embryos. Furthermore, transient transfections in mouse embryonic fibroblasts showed that NCoR is involved in repression mediated by specific nuclear receptors as well as other unrelated transcription factors.

HDAC1 and HDAC2 have also been detected in the Mi-2/NuRD multi-protein complex (Table 1), which contain two cancer related proteins Mi-2beta and MTA2 (reviewed in Ayer, 1999; Knoepfler and Eisenmann, 1999). Mi-2beta, originally identified as a dermatomyositis-specific auto-antigen, is a member of the SNF-2 family of proteins and harbors ATP-dependent chromatin-remodeling activity (Boyer et al., 2000). Thus, Mi-2/NuRD complexes may not only be involved in histone deacetylation but also in nucleosome remodeling, providing a link between both chromatin remodeling and histone acetylation. MTA2 is related to the metastasis-associated protein MTA1. The NuRD complex also contains a 32 kDa subunit corresponding to MBD3a/b, a member of a family of proteins containing methyl-CpG binding domains (Wade et al., 1999; Zhang et al., 1999). The fact that multiple distinct corepressor complexes co-exist indicates that they may perform tissue- or gene-specific functions depending on the DNA-bound transcriptional repressor.

Methylation and oncogenesis

Gene silencing by means of methylation has received a lot of attention in recent years. Methylation of CpG residues has been tightly correlated with transcriptional repression and histone hypoacetylation (Siegfried et al., 1999). Several studies have suggested that methylation-dependent transcriptional silencing may be a critical event in the etiology of cancer. A global-genome approach to analyse methylation of CpG islands in normal tissues and tumours suggested that methylation of CpG islands specifically occurs in tumours and bears consequences for tumour formation (Costello et al., 2000). It was estimated that in average 600 of the roughly 45 000 CpG islands in the human genome were aberrantly methylated in tumours. Other studies suggested that methylation of specific tumour suppressor genes, such as Rb and the cdk-inhibitors p15 and p16, may be a critical event in tumourigenesis (reviewed by Jones and Laird, 1999). Although various mechanisms may underlie methylation-dependent repression, recent work has demonstrated the involvement of methyl-CpG binding proteins. MeCP2 was the first characterised methyl-CpG binding protein (Lewis et al., 1992; Meehan et al., 1992; Nan et al., 1998). MeCP2 can bind a single methylated CpG residue and was shown to recruit the Sin3/HDAC complex (Nan et al., 1998; Jones et al., 1998). MeCP2 is widely expressed and is thought to associate mainly with dispersed methylated CpGs to prevent transcription, providing what has been referred to as a 'transcriptional noise reduction system' to the genome (Bird, 1995). Mutations of MeCP2 lead to childhood neurodevelopmental disorders, the Rett-syndrome (Amir et al., 1999; Willard and Hendrich, 1999). Rett syndrome mutations appearing in the methyl-binding domain of MeCP2 affect DNA-binding by these proteins and probably result in activation of normally silenced genes (Ballestar et al., 2000).

MeCP2 is the founding member of a family of methyl-binding proteins (MBD1-4) that are present in different repressor complexes and share a domain homologous to MeCP2. MBD3 is a component of the Mi-2/NuRD complex, involved in chromatin remodelling and histone deacetylation. The methyl-binding complex MeCP1 contains MBD2 along with HDAC1 and -2 as well as the histone binding proteins RbAP46/48 (reviewed by Bird and Wolffe, 1999). The tissue-specific distribution of these proteins suggests cell-type specific functions.

CpG methylation patterns within the genome are very stable and appear to be important for maintenance of transcriptionally silenced chromatin by methyl-binding proteins such as MeCP2. In contrast, DNA (de)methylation is remarkably dynamic during early mammalian development and tumourigenesis (Wolffe et al., 1999). Alterations in the methylation status of specific genes can promote tumourigenesis. Little is still known about the enzymatic activities that are involved in these alterations, but recent data have shed some light on these processes. A DNA-demethylase has been identified that is identical to the previously identified methyl-binding protein MBD2b (Hendrich and Bird, 1998, Battacharya et al., 1999). This protein is widely expressed and may catalyse the demethylation of a single methyl-CpG residue. Demethylases antagonise the activity of another group of proteins that catalyse the addition of methyl-groups to CpG residues, the methylases. Until recently, only one DNA-methyltransferase, DNMT1, had been cloned (Leonhardt et al., 1992). Disruption of the DNMT1 gene in mice had severe effects on DNA methylation and resulted in embryonic lethality (Li et al., 1992). Dnmt-/- ES cells are, however, viable and still possess the ability to methylate DNA de novo, suggesting the presence of additional DNMT activities (Lei et al., 1996). A second potential DNMT, DNMT2, was isolated by two different groups but hitherto has not been shown to possess DNA-methyltransferase activity (Yoder and Bestor, 1998; Okano et al., 1998a). Recently, DNMT3a and DNMT3b were found in a database search (Okano et al., 1998b) and shown to methylate hemimethylated and unmethylated DNA. DNMT3a/b are expressed at high levels in undifferentiated ES cells and are downregulated during ES cell differentiation. Overexpression of DNMT1 and DNMT3 has been reported in human tumours which might contribute to methylation abnormalities in cancer cells. The mechanism by which DNMTs act is still unclear but recent data show that DNMT1 can participate in silencing of transcription in association with HDAC1 (Rountree et al., 2000; Fuks et al., 2000). This suggests that DNMT1/HDAC1 proteins may be required to assure that deacetylated histones are assembled on newly replicated DNA. DNMT1 would then generate methylated and hypo-acetylated chromatin, which might be maintained by recruiting methyl-binding proteins such as MeCP2. Mutations in DNMTs, might disrupt its methyltransferase function thereby creating demethylated chromatin resulting in histone hyperacetylation, aberrant gene activation and tumourigenesis. A direct correlation between oncogenic transformation, histone acetylation and DNMT1 activity was found in fos-transformed fibroblasts that have a threefold higher DNMT1 expression than normal fibroblasts (Bakin and Curran, 1999). Inhibition of DNMT1 activity as well as inhibition of histone deacetylation resulted in reversion of fos-induced transformation, suggesting that fos-mediated transformation requires alterations in histone deacetylation and DNA methylation. This in turn could suggest a mechanism in which oncogenes can transform cells by affecting the methylation- and/or acetylation machinery in the cell.

Repression of an erythroid enhancer

Unravelling repression of endogenous target genes has become of utmost importance to understand the function of corepressor complexes in transcriptional repression in normal and transformed cells. Inhibitors that can affect corepressor function such as AZAdC and TSA have been successfully used in clinical trials to treat patients with leukemia and myelodysplastic syndromes (Lubbert, 2000; Limonta et al., 1993; Richel et al., 1991). However, details on the molecular mechanisms underlying gene repression and the effects of AZAdC and TSA on APL blasts are currently scars or missing. A severe drawback is the lack of well characterised cell systems and target genes to study the mechanisms of repression. AEL provides a good model system to study corepressor function in vivo because the v-ErbA protein as well as v-ErbA target genes have been characterised in great detail.

More than a decade ago, v-ErbA was postulated to act as a transcriptional repressor of erythroid-specific genes involved in differentiation and to contribute to leukemogenisis by efficiently arresting terminal differentiation of erythroid precursor cells. Ensuing studies identified potential v-ErbA target genes such as Band3, ALA-S, lysozyme and the carbonic anhydrase II gene (CAII) (Zenke et al., 1988; Baniahmad et al., 1990; Disela et al., 1991; Fuerstenberg et al., 1992). Repression of two of these potential target genes, band3 and CAII, was shown to be important for the v-ErbA-induced malignant phenotype.

Initial attempts to identify v-ErbA responsive elements in the CAII locus using transient transfection approaches yielded ambiguous or even conflicting results (Disela et al., 1991; Herman et al., 1993; Rascle et al., 1994). In an unbiased approach to map the regulatory regions of CAII and possibly a v-ErbA responsive element (VRE), two DNaseI hypersensitive sites (HS1 and HS2) were identified. Subsequent studies concentrated on HS2, that is located in the second intron and harbours a VRE, a direct repeat with a spacer of four nucleotides, as well as three putative GATA-binding sites. In vivo footprint studies showed that the VRE as well as two of the adjacent GATA sites were protected in proliferating HD3 cells. Transfection assays showed that the HS2 region acts as a silencer that can be turned into a potent enhancer in v-ErbA expressing cells by mutation of the VRE, indicating that binding of v-ErbA to the VRE nullifies its intrinsic enhancer activity (Ciana et al., 1998; Braliou et al., 2000). The fact that the HS2 is transcriptionally silent notwithstanding the binding of GATA-transactivators, suggested that v-ErbA interfered with the positive activity of the erythroid GATA-1 factors bound to the enhancer. Thus, v-ErbA converted an enhancer into a silencer.

How in molecular terms v-ErbA quenches the activity of GATA-factors is yet unclear but several not necessarily mutually exclusive models can be envisioned. The close proximity of the GATA-sites to the VRE could imply that binding of a multi-protein corepressor complex to v-ErbA might sterically interfere with binding of coactivator components to GATA-1, such as p300/CBP or FOG thereby preventing signal transduction to or recruitment of the basal machinery. A second mechanism could involve deacetylation of GATA-1. It was shown by Boyes and coworkers that acetylation of GATA1 potentiated its transcriptional activity and was essential for erythroid differentiation (Boyes et al., 1998; Hung et al., 1999). The association of an HDAC-containing corepressor complex with v-ErbA could cause deacetylation of GATA-1 and render it inactive in proliferating, undifferentiated HD3 cells. Thirdly, the corepressor complex recruited by v-ErbA may inhibit GATA-1 or GATA-1-associated coactivators through direct physical contact. A similar mechanism has been postulated for inhibition of GATA-1 function by the glucocorticoid receptor (Chang et al., 1993).

The emerging model for the role of v-ErbA in CAII silencing is that of a nuclear receptor locked in its repressive 'off' mode by constitutive recruitment of an HDAC-containing corepressor complex. Supporting evidence came from experiments with the HDAC-inhibitor Trichostatin A (TSA), which caused an activation of CAII transcription (Ciana et al., 1998). Unpublished observations from our lab showed that v-ErbA indeed recruits HDAC-activity to the silencer in proliferating HD3 cells (manuscript in preparation). Similarly, Xenopus oocyte injection studies showed recruitment of corepressor proteins by v-ErbA. Injected v-ErbA and unliganded TR recruited NCoR/HDAC3 to an artificial v-ErbA responsive promoter resulting in repression (Urnov et al., 2000). Interestingly, neither in the oocyte system nor in HD3 cells the initially reported interaction between NCoR and the Sin3/HDAC complex (Nagy et al., 1997; Heinzel et al., 1997) could not be detected (Urnov et al., 2000, manuscript in preparation). The use of different cell systems and experimental set-up might explain the differences in corepressor components recruited by v-ErbA and NCoR/SMRT. The observation that the association of corepressor components with unliganded nuclear receptors may be under control of specific signalling pathways shed new light on this ambiguous issue. Hong and Privalsky (2000) showed that the ability of TR to mediate repression via NCoR/SMRT was strongly inhibited by activation of tyrosine kinase signaling pathways, such as that originating from v-ErbB. The authors reached the intriguing conclusion that SMRT function was potently inhibited by a mitogen-activated protein kinase (MAPK) cascade that operates downstream of the constitutively active growth factor receptor v-ErbB. SMRT itself appeared to be a substrate for phosphorylation by protein kinases operating in this MAPK pathway. Phosphorylation of SMRT by MEKK-1 and, to a lesser extent, MEK-1 inhibited the ability of SMRT to bind unliganded nuclear receptors in transient transfection experiments. The finding that phosphorylation triggered by v-ErbB abolishes the interaction of SMRT interaction with unliganded TR/v-ErbA is in apparent contradiction with the postulated role of v-ErbA in leukemia. In AEV-transformed HD3 cells, where v-ErbB is constitutively active, v-ErbA is able to repress CAII transcription and to interact with the corepressor protein SMRT/NCoR (EC and LR, manuscript in preparation). The apparent discrepancies may be explained if v-ErbB activates distinct signalling pathways in different cell systems.

In our lab, we have obtained compelling data showing that an additional pathway is in place to repress CAII transcription in HD3 cells. Treatment of HD3 cells with the methylation inhibitor AZAdC caused an activation of CAII transcription (manuscript in preparation). Inspection of the CAII locus revealed that the CAII promoter region is G/C-rich and contains a hypermethylated CpG-island (Figure 2). Preliminary data suggest that the prototypical member of the methyl-CpG binding protein family, MeCP2, may bind to the hypermethylated promoter and recruit a multi-protein corepressor complex that includes the corepressor Sin3A and the histone deacetylases HDAC1 and HDAC2 (Table 1) (Nan et al., 1998; Jones et al., 1998). The observation that both MeCP2 and nuclear hormone receptors recruit corepressor complexes is suggestive of cross-talk and synergy between the repressive complexes bound to the CAII locus (Figure 3). Protein-protein interactions between subunits in the distinct corepressor complexes-bound at the enhancer and at the promoter-might result in the formation of a higher-order complex, the 'repressosome'. The repressosome might be stabilised by interaction between unliganded TR or v-ErbA with components of the general transcriptional machinery, such as TFIIB and TBP. Although this may apply to unliganded TR, an interaction between v-ErbA and TFIIB seems less likely because of its reported lower affinity for the basal transcription factor (Urnov et al., 2000). Interactions between the corepressor proteins SMRT/NCoR, Sin3 and MeCP2 with TFIIB have also been observed (Wong and Privalsky, 1998; Kaludov and Wolffe, 2000; Muscat et al., 1998). Interactions of repressosome components with basal transcription factors may inhibit the activity or assembly of a transcription competent preinitiation complex, to 'actively' repress transcription (Baniahmad et al., 1993; Fondell et al., 1993, 1996; Urnov et al., 2000). The multitude of possible interactions suggests that transcriptional silencing by nuclear hormone receptors involves a higher-order network of concordant physical interactions between the nuclear receptor, the corepressor complex and the general transcriptional machinery.

Parallels between the regulation of transcription by v-ErbA/TR at the chicken lysozyme as well as CAII gene are striking. A silencer element 2.4 kb upstream of the lysozyme transcription start site has been identified. It is composed of two elements F1 and F2; F2 is a TR/v-ErbA responsive element, whereas the F1 element harbours a binding site for the CTCF protein (reviewed in Lutz et al., 2000a). v-ErbA or unliganded TR cooperate with CTCF in repressing lysozyme transcription. The molecular mechanism underlying this synergism is unclear, but cooperation between transcription corepressor complexes recruited by v-ErbA and CTCF might be involved, because also CTCF can recruit the Sin3/HDAC complex (Lutz et al., 2000b). Collectively the data suggest that cooperative recruitment of multiple corepressor complexes by v-ErbA and CTCF or v-ErbA and MeCP2 at the lysozyme and CAII loci, respectively, may lead to efficient repression.

The cooperation of multiple corepressor complexes in mediating repression of genes in cancer might be a recurring theme and may have important implications for the treatment of cancer. Combinatorial treatments of cancer cells with agents that target different corepressor complexes might proof to have important therapeutic value. Combinatorial treatment has already been successfully used in APL-patients carrying the PLZF-RARalpha fusion. The patients are not responsive to ATRA alone, but the addition of histone deacetylase inhibitors such as trichostatin A (TSA) plus ATRA can cure patients (reviewed in Redner et al., 1999). Thus, a 'chromatin therapy' focussing on the inhibition of corepressor function might provide new and powerful tools in a cure for cancer.

Acknowledgements

The authors wish to thank C Logie for critical reading of the manuscript. E Caldenhoven was supported by a KWF-fellowship. This work was supported by a EU Biomed II Network grant.

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Figures

Figure 1 Comparison of the domain structure of the thyroid hormone receptor c-ErbA/TRa, with the oncogenic, mutated receptor v-ErbA and the transformation deficient td359 variant. Highlighted are the DNA-binding and ligand binding domains and the activation function AF-2. v-ErbA contains an virus derived gag sequence, N- and C-terminal deletions and 13 point mutations (solid dots). The transformation-defective AEV-mutant td359 harbours a v-ErbA variant with two additional mutations, including the mutation Pro144 Arg in the `hinge' region

Figure 2 Schematic representation of the CAII genomic locus. Only exons 1-4 are indicated in black boxes. DNaseI hypersensitive sites HS1, HS2 and promoter HS (prHS) are indicated with arrow heads, the size is indicative of relative DNaseI hypersensitivity. GATA sites I and II and the VRE that are protected in vivo footprinting are indicated, as well as the distribution of CpG residues in the promoter region between -800 and +200

Figure 3 Models for CAII gene repression (a) Binding of two similar corepressor complexes, a MeCP2/Sin3/HDAC complex binding to a CpG island in the promoter and a v-ErbA/NCoR/Sin3/HDAC complex binding to the silencer, (b) Two distinct corepressor complexes, a MeCP2/Sin3/HDAC complex and a v-ErbA/NCoR/HDAC3/TBL-1 complex, (c) Formation of a higher order repressosome through physical interactions between subunits in the corepressor complexes bound to the promoter as well as the silencer thereby facilitating promoter-silencer communication

Tables

Table 1 Corepressor complexes

28 May 2001, Volume 20, Number 24, Pages 3100-3109
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