SMRT corepressor interacts with PLZF and with the PML-retinoic acid receptor alpha (RARalpha) and PLZF-RARalpha oncoproteins associated with acute promyelocytic leukemia

Thebric à braclocus consists of two paralogous genes encoding BTB/POZ domain proteins and acts as a homeotic and morphogenetic regulator of imaginal development inDrosophila

The bric à brac (bab) locus acts as a homeotic and morphogenetic regulator in the development of ovaries, appendages and the abdomen. It consists of two structurally and functionally related genes, bab1 and bab2, each of which encodes a single nuclear protein. Bab1 and Bab2 have two conserved domains in common, a BTB/POZ domain and a Psq domain, a motif that characterizes a subfamily of BTB/POZ domain proteins in Drosophila. The tissue distribution of Bab1 and Bab2 overlaps, with Bab1 being expressed in a subpattern of Bab2. Analysis of a series of mutations indicates that the two bab genes have synergistic, distinct and redundant functions during imaginal development. Interestingly, several reproduction-related traits that are sexually dimorphic or show diversity among Drosophila species are highly sensitive to changes in the bab gene dose, suggesting that alterations in bab activity may contribute to evolutionary modification of sex-related morphology.

Interactions of GATA-2 with the promyelocytic leukemia zinc finger (PLZF) protein, its homologue FAZF, and the t(11;17)-generated PLZF-retinoic acid receptor alpha oncoprotein

Transcription factor GATA-2 is implicated in the survival and growth of multipotential progenitors. Here we report that the promyelocytic leukemia zinc finger (PLZF) protein can interact with GATA-2 and can modify its transactivation capacity. Fanconi anemia zinc finger (FAZF), a PLZF-homologous protein that has been variously described as ROG (repressor of GATA), and TZFP (testis zinc finger protein) also interact with GATA-2. The zinc finger region of GATA-2 is required for binding to PLZF and FAZF, but distinct interfaces on the PLZF and FAZF molecules mediate the interaction, suggesting that GATA-2 activity is controlled by these 2 homologous proteins through distinct mechanisms. GATA-2 can also physically associate with the PLZF-RARalpha fusion protein generated by the t(11;17) chromosomal translocation associated with acute promyelocytic leukemia (APL). Functional experiments showed that this interaction has the capacity to render GATA-dependent transcription responsive to treatment with a combination of all-trans retinoic acid and the histone deacetylase inhibitor trichostatin A (TSA). This combination of drugs has been shown to stimulate the terminal differentiation of leukemic t(11;17)-associated APL blasts, raising the possibility that GATA target genes may be involved in the molecular pathogenesis of APL.

Nuclear receptor coregulators: multiple modes of modification

Many proteins have been characterized as coregulators that can be recruited by DNA-binding nuclear receptors to influence transcriptional regulation. Recent genetic and biochemical studies have shown that cellular levels of coregulators are crucial for nuclear receptor-mediated transcription, and many coregulators have been shown to be targets for diverse intracellular signaling pathways and post-translational modifications. This review focuses on the different modes of regulation of nuclear receptor coregulators and the implications for tissue- and context-specific transcriptional responses to hormone and membrane receptor signaling.

Critical residues within the BTB domain of PLZF and Bcl-6 modulate interaction with corepressors

The PLZF (promyelocytic leukemia zinc finger) transcriptional repressor, when fused to retinoic acid receptor alpha (RARalpha), causes a refractory form of acute promyelocytic leukemia. The highly conserved N-terminal BTB (bric a brac, tramtrack, broad complex)/POZ domain of PLZF plays a critical role in this disease, since it is required for transcriptional repression by the PLZF-RARalpha fusion protein. The crystal structure of the PLZF BTB domain revealed an obligate homodimer with a highly conserved charged pocket formed by apposition of the two monomers. An extensive structure-function analysis showed that the charged pocket motif plays a major role in transcriptional repression by PLZF. We found that mutations of the BTB domain that neutralize key charged pocket residues did not disrupt dimerization, yet abrogated the ability of PLZF to repress transcription and led to the loss of interaction with N-CoR, SMRT, and histone deacetylases (HDACs). We extended these studies to the Bcl-6 protein, which is linked to the pathogenesis of non-Hodgkin's lymphomas. In this case, neutralizing the charged pocket also resulted in loss of repression and corepressor binding. Experiments with purified protein showed that corepressor-BTB interactions were direct. A comparison of the PLZF, Bcl-6, and the FAZF (Fanconi anemia zinc finger)/ROG protein shows that variations in the BTB pocket result in differential affinity for corepressors, which predicts the potency of transcriptional repression. Thus, the BTB pocket represents a molecular structure involved in recruitment of transcriptional repression complexes to target promoters.

FBI-1 can stimulate HIV-1 Tat activity and is targeted to a novel subnuclear domain that includes the Tat-P-TEFb-containing nuclear speckles

FBI-1 is a cellular POZ-domain-containing protein that binds to the HIV-1 LTR and associates with the HIV-1 transactivator protein Tat. Here we show that elevated levels of FBI-1 specifically stimulate Tat activity and that this effect is dependent on the same domain of FBI-1 that mediates Tat-FBI-1 association in vivo. FBI-1 also partially colocalizes with Tat and Tat's cellular cofactor, P-TEFb (Cdk9 and cyclin T1), at the splicing-factor-rich nuclear speckle domain. Further, a less-soluble population of FBI-1 distributes in a novel peripheral-speckle pattern of localization as well as in other nuclear regions. This distribution pattern is dependent on the FBI-1 DNA binding domain, on the presence of cellular DNA, and on active transcription. Taken together, these results suggest that FBI-1 is a cellular factor that preferentially associates with active chromatin and that can specifically stimulate Tat-activated HIV-1 transcription.

Lola regulates midline crossing of CNS axons in Drosophila

The pattern and level of expression of axon guidance proteins must be choreographed with exquisite precision for the nervous system to develop its proper connectivity. Previous work has shown that the transcription factor Lola is required for central nervous system (CNS) axons of Drosophila to extend longitudinally. We show here that Lola is simultaneously required to repel these same longitudinal axons away from the midline, and that it acts, in part, by augmenting the expression both of the midline repellant, Slit, and of its axonal receptor, Robo. Lola is thus the examplar of a class of axon guidance molecules that control axon patterning by coordinating the regulation of multiple, independent guidance genes, ensuring that they are co-expressed at the correct time, place and relative level.

Competitive cofactor recruitment by orphan receptor hepatocyte nuclear factor 4alpha1: modulation by the F domain

For most ligand-dependent nuclear receptors, the status of endogenous ligand modulates the relative affinities for corepressor and coactivator complexes. It is less clear what parameters modulate the switch between corepressor and coactivator for the orphan receptors. Our previous work demonstrated that hepatocyte nuclear factor 4alpha1 (HNF4alpha1, NR2A1) interacts with the p160 coactivator GRIP1 and the cointegrators CBP and p300 in the absence of exogenously added ligand and that removal of the F domain enhances these interactions. Here, we utilized transient-transfection analysis to demonstrate repression of HNF4alpha1 activity by the corepressor silencing mediator of retinoid and thyroid receptors (SMRT) in several cell lines and on several HNF4alpha-responsive promoter elements. Glutathione S-transferase pulldown assays confirmed a direct interaction between HNF4alpha1 and receptor interaction domain 2 of SMRT. Loss of the F domain resulted in marked reduction of the ability of SMRT to interact with HNF4alpha1 in vitro and repress HNF4alpha1 activity in vivo, although the isolated F domain itself failed to interact with SMRT. Surprisingly, loss of both the A/B and F domains restored full repression by SMRT, suggesting involvement of both domains in the SMRT interaction. Finally, we show that when coexpressed along with HNF4alpha1 and GRIP1, CBP, or p300, SMRT can titer out HNF4alpha1-mediated transactivation in a dose-dependent manner and that this competition derives from mutually exclusive binding. Collectively, these results suggest that HNF4alpha can functionally interact with both a coactivator and a corepressor without altering the status of any putative ligand and that the presence of the F domain may play a role in discriminating between the different coregulators.

Genetic lesions and perturbation of chromatin architecture: a road to cell transformation

Differential gene expression is a rigorously precise procedure that defines the developmental program of cells, tissues, organs, and of the entire organism. The correct execution of this program requires the participation of multiple and complex groups of regulators. In addition to transcription factors, which are key tools in ontogenesis by providing sequential switch of different genes, the structure of the chromatin is a dominant determinant leading to gene expression. Through the novel and insightful work of several investigators, it appears that the architecture of the chromatin spanning the genes can and does influence the efficiency of RNA transcription, and therefore of gene expression. Several new enzymatic complexes have been identified that reversibly modify the chromatin architecture by methylation, phosphorylation, and acetylation of the nucleosomal core proteins. These enzymes are crucial for the proper balance and maintenance of gene expression, and are often the target of mutations and alterations in human cancer. Here, we review briefly the current models proposing how some of these enzymes normally modify the chromatin structure and how their functional disruption leads to inappropriate gene expression and cell transformation.

Biological roles and mechanistic actions of co-repressor complexes

Transcriptional repression, which plays a crucial role in diverse biological processes, is mediated in part by non-DNA-binding co-repressors. The closely related co-repressor proteins N-CoR and SMRT, although originally identified on the basis of their ability to associate with and confer transcriptional repression through nuclear receptors, have been shown to be recruited to many classes of transcription factor and are in fact components of multiple protein complexes containing histone deacetylase proteins. This association with histone deacetylase activity provides an important component of the mechanism that allows DNA-binding proteins interacting with N-CoR or SMRT to repress transcription of specific target genes. Both N-CoR and SMRT are important targets for cell signaling pathways, which influence their expression levels, subcellular localization and association with other proteins. Recently, the biological importance of these proteins has been revealed by studies of genetically engineered mice and human diseases such as acute promyelocytic leukemia (APL) and resistance to thyroid hormone(RTH).

Structural basis for antagonist-mediated recruitment of nuclear co-repressors by PPARalpha

Repression of gene transcription by nuclear receptors is mediated by interactions with co-repressor proteins such as SMRT and N-CoR, which in turn recruit histone deacetylases to the chromatin. Aberrant interactions between nuclear receptors and co-repressors contribute towards acute promyelocytic leukaemia and thyroid hormone resistance syndrome. The binding of co-repressors to nuclear receptors occurs in the unliganded state, and can be stabilized by antagonists. Here we report the crystal structure of a ternary complex containing the peroxisome proliferator-activated receptor-alpha ligand-binding domain bound to the antagonist GW6471 and a SMRT co-repressor motif. In this structure, the co-repressor motif adopts a three-turn alpha-helix that prevents the carboxy-terminal activation helix (AF-2) of the receptor from assuming the active conformation. Binding of the co-repressor motif is further reinforced by the antagonist, which blocks the AF-2 helix from adopting the active position. Biochemical analyses and structure-based mutagenesis indicate that this mode of co-repressor binding is highly conserved across nuclear receptors.

The acute promyelocytic leukemia-associated protein, promyelocytic leukemia zinc finger, regulates 1,25-dihydroxyvitamin D(3)-induced monocytic differentiation of U937 cells through a physical interaction with vitamin D(3) receptor

Monocyte differentiation induced by 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) is interrupted during the course of acute promyelocytic leukemia (APL). One form of APL is associated with the translocation t(11;17), which joins the promyelocytic leukemia zinc finger (PLZF) and retinoic acid receptor alpha (RARalpha) genes. Because PLZF is coexpressed in the myeloid lineage with the vitamin D(3) receptor (VDR), the interplay between PLZF and VDR was examined. It was found that PLZF interacts directly with VDR. This occurred at least partly through contacts in the DNA-binding domain of VDR and the broad complex, tram-trak, bric-a-brac/pox virus zinc finger (BTB/POZ) domain of PLZF. Moreover, PLZF altered the mobility of VDR derived from nuclear extracts when bound to its cognate binding site, forming a slowly migrating DNA-protein complex. Overexpression of PLZF in a monocytic cell line abrogated 1,25(OH)(2)D(3) activation from both a minimal VDR responsive reporter and the promoter of p21(WAF1/CIP1), a target gene of VDR. Deletion of the BTB/POZ domain significantly relieved PLZF-mediated repression of 1,25(OH)(2)D(3)-dependent activation. In addition, stable, inducible expression of PLZF in U937 cells inhibited the ability of 1,25(OH)(2)D(3) to induce surface expression of the monocytic marker CD14 and morphologic changes associated with differentiation. These results suggest that PLZF may play an important role in regulating the process by which 1,25(OH)(2)D(3) induces monocytic differentiation in hematopoietic cells.

The hcKrox gene family regulates multiple extracellular matrix genes

The transcription factor cKrox was originally identified as a protein that bound to a negative transcription regulatory element in the murine alpha1(I) collagen promoter. We recently reported the cloning and characterization of human cKrox (hcKrox). Overexpression of hcKrox in NIH3T3 fibroblasts efficiently repressed the promoters of the fibronectin and alpha1(I) collagen genes (70-90%) in transient transfection assays and suppressed the endogenous genes in hcKrox expressing permanent cell lines. We have now isolated genomic clones and cDNAs encoding two novel transcription factors related to hcKrox termed hcKrox-beta and hcKrox-gamma (the original clone is now referred to as hcKrox-alpha). Both contain three kruppel-like zinc-finger DNA binding motifs that are 71-78% identical to those of hcKrox-alpha. The NH(2)-terminus of all three proteins contains a POZ domain, a conserved 120 amino acid motif involved in transcriptional repression and protein dimerization. RT-PCR experiments demonstrate that all three hcKrox family members are expressed in foreskin and dermal fibroblasts. Transient transfection studies in NIH3T3 fibroblasts demonstrate that hcKrox-alpha -beta and -gamma, as well as the murine cKrox-beta homologue, LRF, suppress transcription driven by promoters for the alpha1(I) and alpha2(I) collagen, fibronectin and elastin genes. Electrophoretic mobility shift assays and coimmunoprecipitation studies suggest that homo- and heterodimerization occurs between cKrox family members. Dimer formation is influenced by amino acids in the NH(2)-terminal POZ domain and the Zn(+2)-finger region. Immunoprecipitation studies indicate that cKrox can form heterodimers in solution in the absence of DNA. Thus, a multi-gene family exists that can coordinately regulate several extracellular matrix genes and has the potential to form many heterodimeric transcription factors.

Arsenic trioxide is a potent inhibitor of the interaction of SMRT corepressor with Its transcription factor partners, including the PML-retinoic acid receptor alpha oncoprotein found in human acute promyelocytic leukemia

The SMRT corepressor complex participates in transcriptional repression by a diverse array of vertebrate transcription factors. The ability to recruit SMRT appears to play a crucial role in leukemogenesis by the PML-retinoic acid receptor alpha (RARalpha) oncoprotein, an aberrant nuclear hormone receptor implicated in human acute promyelocytic leukemia (APL). Arsenite induces clinical remission of APL through a incompletely understood mechanism. We report here that arsenite is a potent inhibitor of the interaction of SMRT with its transcription factor partners, including PML-RARalpha. Arsenite operates, in part, through a mitogen-activated protein (MAP) kinase cascade culminating in phosphorylation of the SMRT protein, dissociation of SMRT from its nuclear receptor partners, and a relocalization of SMRT out of the nucleus into the cytoplasm of the cell. Conversely, inhibition of this MAP kinase cascade attenuates the effects of arsenite on APL cells. Our results implicate SMRT as an important biological target for the actions of arsenite in both normal and neoplastic cells.

Function of RARalpha during the maturation of neutrophils

The retinoic acid receptor alpha gene is the target of chromosomal rearrangements in all cases of acute promyelocytic leukemia (APL). This recurrent involvement of RARalpha in the pathogenesis of APL is likely to reflect an important role played by this receptor during the differentiation of immature myeloid cells to neutrophils. RARalpha is a negative regulator of promyelocyte differentiation when not complexed with RA, and stimulates this differentiation when bound to RA. Since RARs are dispensable for the generation of mature neutrophils, their role thus appears to be to modulatory, rather than obligatory, for the control of neutrophil differentiation. In vitro, retinoic acid is also a potent inducer of neutrophil cell fate, suggesting that it might play a role in the commitment of pluripotent hematopoietic progenitors to the neutrophil lineage. Thus, the APL translocations target an important regulator of myeloid cell differentiation.

Transcriptional regulation in acute promyelocytic leukemia

It has been 10 years since the seminal discovery that a mutant form of a retinoid acid receptor (RARalpha) is associated with acute promyelocytic leukemia (APL). This finding, coupled with the remarkable success of retinoic acid (RA), the natural ligand of RARalpha, in the treatment of APL, has made APL a unique model system in the study of oncogenic conversion of transcription factors in hematological malignancies. Indeed, subsequent basic and clinical studies showed that chromosomal translocation involving the RARalpha gene is the cytogenetic hallmark of APL and that these mutant forms of RARs are the oncogenes in APL that interfere with the proliferation and differentiation pathways controlled by both RAR and their fusion partners. However, it was not until recently that the role of aberrant transcriptional regulation in the pathogenesis of APL was revealed. In this review, we summarize the biochemical and biological mechanisms of transcriptional regulation by mutant RARs and their corresponding wild-type fusion partner PML and PLZF. These studies have been instrumental in our understanding of the process of leukemogenesis in general and have laid the scientific foundation for the novel concept of transcription therapy in the treatment of human cancer.

Translocations of the RARalpha gene in acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) has been recognized as a distinct clinical entity for over 40 years. Although relatively rare among hematopoietic malignancies (approximately 10% of AML cases), this disease has attracted a particularly good share of attention by becoming the first human cancer in which all-trans-retinoic acid (ATRA), a physiologically active derivative of vitamin A, was able to induce complete remission (CR). ATRA induced remission is not associated with rapid cell death, as in the case of conventional chemotherapy, but with a restoration of the 'normal' granulocytic differentiation pathway. With this remarkable medical success story APL has overnight become a paradigm for the differentiation therapy of cancer. A few years later, excitement with APL was further enhanced by the discovery that a cytogenetic marker for this disease, the t(15:17) reciprocal chromosomal translocation, involves a fusion between the retinoic acid receptor alpha (RARalpha) gene and a previously unknown locus named promyelocytic leukemia (PML). Consequence of this gene rearrangement is expression of the PML-RARalpha chimeric oncoprotein, which is responsible for the cellular transformation as well as ATRA response that is observed in APL. Since this initial discovery, a number of different translocation partner genes of RARalpha have been reported in rarer cases of APL, strongly suggesting that disruption of RARalpha underlies its pathogenesis. This article reviews various rearrangements of the RARalpha gene that have so far been described in literature, functions of the proteins encoded by the different RARalpha partner loci, and implications that these may have for the molecular pathogenesis of APL.

A new functional domain of Bcl6 family that recruits histone deacetylases

The proto-oncogene Bcl6 and its family gene, BAZF, encode a sequence-specific transcriptional repressor which contains the BTB/POZ domain in NH(2)-terminal region and zinc finger motifs in COOH-terminal region. The BTB/POZ domain and the middle portion of Bcl6 and BAZF are known to display transrepressor activity. Since we have identified the identical 17-amino acid (aa) sequence in the middle portion of Bcl6 and BAZF, the 17aa region may be another repressive domain of the middle portion. The reporter gene assay indicates that the 27aa sequence including the 17aa region recruits histone deacetylases to express transrepressor activity. Furthermore, overexpression of Bcl6 or Bcl6(POZ-) (Bcl6 deleted with the BTB/POZ domain) induced apoptosis in NIH3T3 cells, and the apoptosis was inhibited by the addition of histone deacetylase inhibitor in the culture. However, apoptosis was not induced in NIH3T3 cells by overexpression of Bcl6(POZ-) deleted with the 17aa region. These results indicate that the 17aa region in the middle portion of Bcl6 is a functional domain of transrepressor activity and is responsible for inducibility of apoptosis in NIH3T3 cells.

The BTB/POZ domain of the regulatory proteins Bric à brac 1 (BAB1) and Bric à brac 2 (BAB2) interacts with the novel Drosophila TAF(II) factor BIP2/dTAF(II)155

The BTB/POZ domain is an evolutionarily conserved protein-protein interaction domain present in the N-terminal region of numerous transcription factors involved in development, chromatin remodeling, and human cancers. This domain is involved in homomeric and heteromeric associations with other BTB/POZ domains. The Drosophila BTB/POZ proteins Bric à brac 1 (BAB1) and Bric à brac 2 (BAB2) are developmentally regulated transcription factors which are involved in pattern formation along the proximo-distal axis of the leg and antenna, in the morphogenesis of the adult ovaries, and in the control of sexually dimorphic characters. We have identified partners of the BAB1 protein by using the two-hybrid system. The characterization of one of these proteins, called BIP2 for BAB Interacting Protein 2, is presented. BIP2 is a novel Drosophila TATA-box Protein Associated Factor (TAF(II)), also named dTAF(II)155. We show that the BTB/POZ domains of BAB1 and BAB2 are sufficient to mediate a direct interaction with BIP2/dTAF(II)155. This provides a direct link between these BTB/POZ transcription factors and the basal transcriptional machinery. We discuss the implications of the interaction between a BTB/POZ domain and a TAF(II) for the molecular mechanisms of transcriptional control mediated by BTB/POZ transcription factors.

Genomic organization and refined mapping of the human nuclear corepressor 2 (NCOR2)/ silencing mediator of retinoid and thyroid hormone receptor (SMRT) gene on chromosome 12q24.3

The human nuclear co-repressor 2 (N-CoR2) gene (NCOR2, previously called silencing mediator for retinoid and thyroid hormone receptor SMRT) is recruited to nuclear and non-nuclear receptors in a large repressing complex containing also N-CoR1, mSin3 and HDACs. This large complex represses transcription in absence of ligand. Herein we report the high- resolution and refined mapping of NCOR2 at the boundary of sub-bands 12q24.23 and 12q24.31, and its intron/exon structure. The gene contains 45 exons. This information should allow further study of potential NCOR2 genomic alteration in some subsets of malignancies.

SMRTE inhibits MEF2C transcriptional activation by targeting HDAC4 and 5 to nuclear domains

The silencing mediator for retinoic acid and thyroid hormone receptors (SMRT) mediates transcriptional repression by recruiting histone deacetylases (HDACs) to the DNA-bound nuclear receptor complex. The full-length SMRT (SMRTe) contains an N-terminal sequence that is highly conserved to the nuclear receptor corepressor N-CoR. To date, little is known about the activity and function of the full-length SMRTe protein, despite extensive studies on separated receptor interaction and transcriptional repression domains. Here we show that SMRTe inhibits MEF2C transcriptional activation by targeting selective HDACs to unique subnuclear domains. Indirect immunofluorescence studies with anti-SMRTe antibody reveal discrete cytoplasmic and nuclear speckles, which contain RARalpha in an RA-sensitive manner. Formation of the SMRTe nuclear speckles results in recruitment of several class I and class II HDACs to these subnuclear domains in a process depending on HDAC enzymatic activity. Intriguingly, although HDAC4 is located primarily in the cytoplasm, coexpression of SMRTe dramatically translocates HDAC4 from the cytoplasm into the nucleus, where HDAC4 prevents MEF2C from activating muscle differentiation. SMRTe also translocates HDAC5 from diffusive nucleoplasm into discrete nuclear domains. Accordingly, SMRTe synergizes with HDAC4 and 5 to inhibit MEF2C transactivation of target promoter, suggesting that nuclear domain targeting of HDAC4/5 may be important in preventing muscle cell differentiation. These results highlight an unexpected new function of the nuclear receptor corepressor SMRTe for its role in regulating cellular trafficking of nuclear receptor and selective HDACs that may play an important role in regulation of cell growth and differentiation.

The zinc finger domain of Tzfp binds to the tbs motif located at the upstream flanking region of the Aie1 (aurora-C) kinase gene

Our previous studies showed that Aie1 (aurora-C), is a novel testis kinase belonging to the aurora kinase family (). In this report, we describe a testis zinc finger protein (Tzfp) that binds to the upstream flanking sequence of the Aie1 gene. The mouse Tzfp gene, mapped to chromosome 7 B2-B3, encodes a 465-amino acid transcription factor containing a conserved N-terminal BTB/POZ domain and three C-terminal PLZF-like C(2)H(2) zinc fingers. The zinc finger domain of Tzfp binds to the TGTACAGTGT motif (Tzfp binding site, termed tbs) located at the upstream flanking sequence of the Aie1 gene by gel mobility shift, DNase I footprinting, and competition analyses. When the C-terminal zinc fingers of Tzfp were fused to the transactivation domain of VP16, the chimera activated transcription of a reporter construct containing multiple copies of the tbs. In contrast, the same chimera did not activate the reporter gene when an essential nucleotide fifth C was mutated to A at the tbs. Furthermore, we showed that the N-terminal BTB/POZ domain of TZFP has a repressor activity. Taken together, our results indicate that Tzfp recognizes a sequence-specific motif (tbs) and may play a role in the regulation of the genes carrying the tbs.

Avian erythroleukemia: a model for corepressor function in cancer

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.

The SMRT corepressor is a target of phosphorylation by protein kinase CK2 (casein kinase II)

The Silencing-Mediator for Retinoid/Thyroid hormone receptors (SMRT) interacts with, and mediates transcriptional repression by, a variety of eukaryotic transcription factors, including the nuclear hormone receptors. The ability of SMRT to function as a transcriptional 'corepressor' is regulated by a variety of signal transduction pathways. We report here that SMRT is a phosphoprotein in vivo, and is also phosphorylated in vitro by unfractionated cell extracts. A major site of phosphorylation of SMRT is a protein kinase CK2 motif centered on serine 1492, and located within a C-terminal SMRT domain that mediates interaction of the corepressor with the nuclear hormone receptors. Phosphorylation of SMRT by CK2 stabilizes the ability of the SMRT protein to interact with nuclear hormone receptors. Our results indicate that SMRT is a member of an expanding family of transcriptional regulators that are modified, and potentially regulated, in response to protein kinase CK2.

PSF is a novel corepressor that mediates its effect through Sin3A and the DNA binding domain of nuclear hormone receptors

Members of the type II nuclear hormone receptor subfamily (e.g., thyroid hormone receptors [TRs], retinoic acid receptors, retinoid X receptors [RXRs], vitamin D receptor, and the peroxisome proliferator-activated receptors) bind to their response sequences with or without ligand. In the absence of ligand, these DNA-bound receptors mediate different degrees of repression or silencing of gene expression which is thought to result from the association of their ligand binding domains (LBDs) with corepressors. Two related corepressors, N-CoR and SMRT, interact to various degrees with the LBDs of these type II receptors in the absence of their cognate ligands. N-CoR and SMRT have been proposed to act by recruiting class I histone deacetylases (HDAC I) through an association with Sin3, although they have also been shown to recruit class II HDACs through a Sin3-independent mechanism. In this study, we used a biochemical approach to identify novel nuclear factors that interact with unliganded full-length TR and RXR. We found that the DNA binding domains (DBDs) of TR and RXR associate with two proteins which we identified as PSF (polypyrimidine tract-binding protein-associated splicing factor) and NonO/p54(nrb). Our studies indicate that PSF is a novel repressor which interacts with Sin3A and mediates silencing through the recruitment of HDACs to the receptor DBD. In vivo studies with TR showed that although N-CoR fully dissociates in the presence of ligand, the levels of TR-bound PSF and Sin3A appear to remain unchanged, indicating that Sin3A can be recruited to the receptor independent of N-CoR or SMRT. RXR was not detected to bind N-CoR although it bound PSF and Sin3A as effectively as TR, and this association with RXR did not change with ligand. Our studies point to a novel PSF/Sin3-mediated pathway for nuclear hormone receptors, and possibly other transcription factors, which may fine-tune the transcriptional response as well as play an important role in mediating the repressive effects of those type II receptors which only weakly interact with N-CoR and SMRT.

Silencing mediator for retinoid and thyroid hormone receptors interacts with octamer transcription factor-1 and acts as a transcriptional repressor

Octamer transcription factor-1 (Oct-1) is a member of the POU (Pit-1, Oct-1, unc-86) family of transcription factors and is involved in the transcriptional regulation of a variety of gene expressions related to cell cycle regulation, development, and hormonal signals. It has been shown that Oct-1 acts not only as a transcriptional activator but also as a transcriptional repressor for certain genes. The mechanism of the repressive function of Oct-1 has not been well understood. Here we demonstrate by using the glutathione S-transferase pull-down assays and coimmunoprecipitation assays that the POU domain of Oct-1 directly interacts with a silencing mediator for retinoid and thyroid hormone receptors (SMRT). The interaction surfaces are located in the C-terminal region of SMRT, which are different from previously described silencing domains I and II or receptor interacting domains I and II. In transient transfection assays in COS1 cells, overexpression of SMRT attenuated the augmentation of Oct-1 transcriptional activity by OBF-1/OCA-B, activator for Oct-1. In pull-down assays, increasing amounts of SMRT could compete the binding of OCA-B to Oct-1 POU domain. The activity of Oct-1 could be determined by a regulated balance between SMRT and OCA-B. Furthermore, cotransfected unliganded thyroid hormone receptor enhanced the transactivation by Oct-1, and addition of 3,3',5-tri-iodo-l-thyronine obliterated the stimulatory effects. Consequently, in the presence of cotransfected thyroid hormone receptor, the octamer response element acts as an element negatively regulated by 3,3',5-tri-iodo-l-thyronine. The results suggest that the transcriptional activity of Oct-1 can be modulated by interaction through its POU domain by a silencing mediator SMRT resulting in the cross-talk between Oct-1 and nuclear receptors.

Positive and negative regulation of granulopoiesis by endogenous RARalpha

Acute promyelocytic leukemia (APL) is always associated with chromosomal translocations that disrupt the retinoic acid receptor alpha (RARalpha) gene. Whether these translocations relate to a role for endogenous RARalpha in normal granulopoiesis remains uncertain because most studies addressing this question have used non-physiological overexpression systems. Granulocyte differentiation in cells derived from RARalpha-deficient (RARalpha(-/-)) mice was studied and evaluated in the context of agonist-bound and ligand-free RARalpha. Our results demonstrate that RARalpha is dispensable for granulopoiesis, as RARalpha(-/-) mice have a normal granulocyte population despite an impaired ability to respond to retinoids. However, although it is not absolutely required, RARalpha can bidirectionally modulate granulopoiesis. RARalpha stimulates differentiation in response to exogenous retinoic acid. Furthermore, endogenous retinoids control granulopoiesis in vivo, as either vitamin A-deficient mice or animals treated with an RAR antagonist accumulate more immature granulocytes in their bone marrow. Conversely, RARalpha acts to limit differentiation in the absence of ligand because granulocyte precursors from RARalpha(-/-) mice differentiate earlier in culture. Thus, the block in granulopoiesis exerted by RARalpha fusion proteins expressed in APL cells may correspond to an amplification of a normal function of unliganded RARalpha.

Transcription factors and translocations in lymphoid and myeloid leukemia

Chromosomal translocations involving transcription factors and aberrant expression of transcription factors are frequently associated with leukemogenesis. Transcription factors are essential in maintaining the regulation of cell growth, development, and differentiation in the hematopoietic system. Alterations in the mechanisms that normally control these functions can lead to hematological malignancies. Further characterization of the molecular biology of leukemia will enhance our ability to develop disease-specific treatment strategies, and to develop effective methods of diagnosis and prognosis.

DNA recognition by the aberrant retinoic acid receptors implicated in human acute promyelocytic leukemia

Human acute promyelocytic leukemias (APLs) are associated with chromosomal translocations that replace the NH2 terminus of wild-type retinoic acid receptor (RAR) alpha with portions of the promyelocytic leukemia protein (PML) or promyelocytic leukemia zinc-finger protein (PLZF). The wild-type RARalpha readily forms heterodimers with the retinoid X receptors (RXRs), and these RAR/RXR heterodimers appear to be the principal mediators of retinoid signaling in normal cells. In contrast, PML-RARalpha and PLZF-RARa display an enhanced ability to form homodimers, and this enhanced homodimer formation is believed to contribute to the neoplastic properties of these chimeric oncoproteins. We report here that the DNA recognition specificity of the RXRalpha/RARa heterodimer, which is presumed to be the dominant receptor species in normal cells, differs from that of the PML-RARalpha and PLZF-RARalpha homodimers, which are thought to prevail in the oncogenic cell. We suggest that differences in target gene recognition by the normal and oncogenic RARalpha proteins may contribute to the leukemogenic phenotype.

Oligomerization of ETO is obligatory for corepressor interaction

Nearly 40% of cases of acute myelogenous leukemia (AML) of the M2 subtype are due to a chromosomal translocation that combines a sequence-specific DNA binding protein, AML1, with a potent transcriptional repressor, ETO. ETO interacts with nuclear receptor corepressors SMRT and N-CoR, which recruit histone deacetylase to the AML1-ETO oncoprotein. SMRT-N-CoR interaction requires each of two zinc fingers contained in C-terminal Nervy homology region 4 (NHR4) of ETO. However, here we show that polypeptides containing NHR4 are insufficient for interaction with SMRT. NHR2 is also required for SMRT interaction and repression by ETO, as well as for inhibition of hematopoietic differentiation by AML1-ETO. NHR2 mediates oligomerization of ETO as well as AML1-ETO. Fusion of NHR4 polypeptide to a heterologous dimerization domain allows strong interaction with SMRT in vitro. These data support a model in which NHR2 and NHR4 have complementary functions in repression by ETO. NHR2 functions as an oligomerization domain bringing together NHR4 polypeptides that together form the surface required for high-affinity interaction with corepressors. As nuclear receptors also interact with corepressors as dimers, oligomerization may be a common mechanism regulating corepressor interactions.

Colocalization and heteromerization between the two human oncogene POZ/zinc finger proteins, LAZ3 (BCL6) and PLZF

Most acute promyelocytic leukemia (APL) cases are associated with recurrent translocations between the gene of retinoic receptor alpha and that of PML (t(15;17)) or PLZF (t(11;17)). PML localizes onto discrete intranuclear domains, the PML-nuclear bodies, and displays anti-oncogenic and pro-apoptotic properties. PLZF encodes a transcription factor belonging to the POZ/domain and Krüppel zinc finger (POK) family which interacts directly with PML. PLZF is related to another POK protein, LAZ3(BCL6), which is structurally altered, and presumably misexpressed, in many non-Hodgkin lymphoma (NHL) cases. PLZF and LAZ3 share many functional properties: both inhibit cell growth, concentrate into punctated nuclear subdomains and are sequence-specific transcriptional repressors recruiting a histone deacetylase-repressing complex. Given these similarities, we tested whether both proteins could be targeted by each other. Here, LAZ3 and PLZF are shown to colocalize onto nuclear dots. Moreover, truncated derivatives of one protein, which display a diffuse nuclear localization, are recruited onto nuclear dots by the full-length other. The colocalization and the reciprocal 'rescue' is the result of a direct interaction between LAZ3 and PLZF, as indicated by yeast two hybrid assays, in vitro immunoprecipitations, and GST pull down experiments. In contrast to LAZ3 homomerization, LAZ3/PLZF heteromerization in yeast does not solely depend on POZ/POZ contacts but rather also relies on interactions between the two zinc finger regions and 'cross' contacts between the zinc finger region and the POZ domain of each partner. Likewise, LAZ3 shows some colocalization with the PLZF partner PML upon stable overexpression of both proteins in CHO cells and interacts with PML in yeast. Finally, endogenous LAZ3 and PLZF are co-induced and partially colocalized in myeloid MDS cells. These data indicate that a physical interaction between LAZ3 and PLZF underlies their simultaneous recruitment onto multiproteic nuclear complexes, presumably involved in transcriptional silencing and whose integrity (for APL) and/or function (for APL and NHL) may be altered in oncogenesis.

Identification and characterization of SKAT-2, a novel Th2-specific zinc finger gene

We have identified a novel Kruppel-type zinc finger (ZF) gene, SKAT-2, which is selectively expressed by murine Th2 cells. The protein encoded by this gene has 14 C2H2-type ZF tandemly arrayed at its C terminus and N-terminal SCAN box and KRAB domains. SKAT-2 is tissue restricted in expression at the RNA level, detectable only in brain and at low levels in kidney and spleen and few hematopoietic cell lines. By in situ hybridization, SKAT-2 expression was found to peak in antigen-stimulated CD4(+) T cells after 2-3 days of culture under Th2 but not Th1 biasing conditions. This pattern of expression closely mirrored that of GATA-3 in the same cells. In transient transfection experiments in phorbol 12-myristate 13-acetate/ionomycin-stimulated EL4 cells, SKAT-2 was found to up-regulate the activity of the IL-4 but not the IL-5 promoter, contrasting with the ability of GATA-3 to activate both promoters. This result was confirmed using clones of EL4 cells stably expressing an inducible form of SKAT-2, thus SKAT-2 is a novel Th2-specific gene that may play a role in selective regulation of cytokine genes in T cells.

Functional studies of the BTB domain in the Drosophila GAGA and Mod(mdg4) proteins

The BTB/POZ (BTB) domain is an approximately 120 residue sequence that is conserved at the N-terminus of many proteins in both vertebrates and invertebrates. We found that the protein encoded by a lethal allele of the Drosophila modifier of mdg4 [mod(mdg4)] gene has two mutated residues in its BTB domain. The identities of the residues at the positions of these mutations are highly conserved in the BTB domain family of proteins, and when the corresponding mutations were engineered into the BTB domain-containing GAGA protein, the activity of GAGA as a transcription activator in a transient transfection assay was severely reduced. The functional equivalence of the BTB domains was established by showing that the BTB domain of the mod(mdg4) protein can effectively substitute for that of GAGA.

Retinoic acid receptor alpha (RAralpha) Mutations in Human Leukemia

The retinoic acid receptor alpha (RARalpha) plays a central role in the biology of the myeloid cellular compartment. Chromosomal translocations involving the RARalpha locus probably represent the malignant initiating events in acute promyelocytic leukemia (APL). Recent studies that identify novel interactions between RARalpha and the nuclear receptor co-activators and co-repressors, new functions of the oncogenic RARalpha fusion proteins and their catabolism in retinoic acid-induced differentiation, and the availability of new transgenic mice models have provided important insights into our understanding of the mechanisms by which mutant forms of RARalpha can be implicated in the development of leukemia. Novel alterations of the RARalpha gene identified in hematopoietic malignant disorders other than APL, such as myelodysplastic syndromes, non-APL acute myeloid leukemias and B-chronic lymphocytic leukemias, suggest that disruption of the RARalpha gene might predispose to myeloid and lymphoid disorders.

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.

Overlapping activators and repressors delimit transcriptional response to receptor tyrosine kinase signals in the Drosophila eye

Regulated transcription of the prospero gene in the Drosophila eye provides a model for how gene expression is specifically controlled by signals from receptor tyrosine kinases. We show that prospero is controlled by signals from the EGF receptor DER and the Sevenless receptor. A direct link is established between DER activation of a transcription enhancer in prospero and binding of two transcription factors that are targets of DER signaling. Binding of the cell-specific Lozenge protein is also required for activation, and overlapping Lozenge protein distribution and DER signaling establishes expression in a subset of equivalent cells competent to respond to Sevenless. We show that Sevenless activates prospero independent of the enhancer and involves targeted degradation of Tramtrack, a transcription repressor.

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.

Human endogenous retrovirus protein cORF supports cell transformation and associates with the promyelocytic leukemia zinc finger protein

Human endogenous retrovirus sequences (HERVs) reside in the genomes of primates and humans for several million years. The majority of HERVs is non-coding but a limited set is intact and can express proteins. We have recently identified an almost intact HERV-K(HML-2) provirus on chromosome 7 and have documented that most patients with germ cell tumors (GCTs) display antibodies directed against proteins of HERV-K(HML-2). To address whether these proteins merely represent tumor markers or contribute to neoplastic transformation, we examined the transforming potential of various HERV sequences and studied physical interactions between HERV and cellular proteins by yeast two-hybrid and biochemical assays. cORF, a protein encoded by the C-terminal open reading frame within the env gene, supports tumor growth in nude mice and associates with the promyelocytic leukemia zinc finger protein (PLZF). The interaction domains map between amino acid residues 21 and 87 of cORF, and between residues 245 and 543 of PLZF. PLZF is critical for spermatogenesis in mice. Abnormal spermatogenesis or maturation of gonocytes is thought to predispose humans to the development of germ cell tumors. Thus, cORF of human endogenous retroviruses may contribute to tumor development by interfering with processes during spermatogenesis that involve PLZF.

In-depth mutational analysis of the promyelocytic leukemia zinc finger BTB/POZ domain reveals motifs and residues required for biological and transcriptional functions

The promyelocytic leukemia zinc finger (PLZF) protein is a transcription factor disrupted in patients with t(11;17)(q23;q21)-associated acute promyelocytic leukemia. PLZF contains an N-terminal BTB/POZ domain which is required for dimerization, transcriptional repression, formation of high-molecular-weight DNA-protein complexes, nuclear sublocalization, and growth suppression. X-ray crystallographic data show that the PLZF BTB/POZ domain forms an obligate homodimer via an extensive interface. In addition, the dimer possesses several highly conserved features, including a charged pocket, a hydrophobic monomer core, an exposed hydrophobic surface on the floor of the dimer, and two negatively charged surface patches. To determine the role of these structures, mutational analysis of the BTB/POZ domain was performed. We found that point mutations in conserved residues that disrupt the dimer interface or the monomer core result in a misfolded nonfunctional protein. Mutation of key residues from the exposed hydrophobic surface suggests that these are also important for the stability of PLZF complexes. The integrity of the charged-pocket region was crucial for proper folding of the BTB/POZ domain. In addition, the pocket was critical for the ability of the BTB/POZ domain to repress transcription. Alteration of charged-pocket residue arginine 49 to a glutamine (mutant R49Q) yields a domain that can still dimerize but activates rather than represses transcription. In the context of full-length PLZF, a properly folded BTB/POZ domain was required for all PLZF functions. However, PLZF with the single pocket mutation R49Q repressed transcription, while the double mutant D35N/R49Q could not, despite its ability to dimerize. These results indicate that PLZF requires the BTB/POZ domain for dimerization and the charged pocket for transcriptional repression.

The SMRT corepressor is regulated by a MEK-1 kinase pathway: inhibition of corepressor function is associated with SMRT phosphorylation and nuclear export

The SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) corepressor participates in the repression of target gene expression by a variety of transcription factors, including the nuclear hormone receptors, promyelocytic leukemia zinc finger protein, and B-cell leukemia protein 6. The ability of SMRT to associate with these transcription factors and thereby to mediate repression is strongly inhibited by activation of tyrosine kinase signaling pathways, such as that represented by the epidermal growth factor receptor. We report here that SMRT function is potently inhibited by a mitogen-activated protein kinase (MAPK) kinase kinase (MAPKKK) cascade that operates downstream of this growth factor receptor. Intriguingly, the SMRT protein is a substrate for phosphorylation by protein kinases operating at multiple levels in this MAPKKK pathway, including the MAPKs, MAPK-extracellular signal-regulated kinase 1 (MEK-1), and MEK-1 kinase (MEKK-1). Phosphorylation of SMRT by MEKK-1 and, to a lesser extent, MEK-1 inhibits the ability of SMRT to physically tether to its transcription factor partners. Notably, activation of MEKK-1 or MEK-1 signaling in transfected cells also leads to a redistribution of the SMRT protein from a nuclear compartment to a more perinuclear or cytoplasmic compartment. We suggest that SMRT-mediated repression is regulated by the MAPKKK cascade and that changes both in the affinity of SMRT for its transcription factors and in the subcellular distribution of SMRT contribute to the loss of SMRT function that is observed in response to kinase signal transduction.

The N-terminal BTB/POZ domain and C-terminal sequences are essential for Tramtrack69 to specify cell fate in the developing Drosophila eye

The BTB/POZ (broad complex Tramtrack bric-a-brac/Pox virus and zinc finger) domain is an evolutionarily conserved protein-protein interaction motif. Many BTB-containing proteins are transcriptional regulators involved in a wide range of developmental processes. However, the significance of the BTB domain in development has not been evaluated. Here we present evidence that overexpression of the Tramtrack69 (Ttk69) protein not only blocks neuronal photoreceptor differentiation but also promotes nonneuronal cone cell specification in early Drosophila eye development. We show that the BTB domain is essential for Ttk69 function and single amino acid changes in highly conserved residues in this domain abolish Ttk69 activity. Interestingly, the Ttk69 BTB can be substituted by the BTB of the human Bcl-6 protein, suggesting that BTB function has been conserved between Drosophila and humans. We found that the Ttk69 BTB domain is critical for mediating interaction with the Drosophila homolog of C-terminal-binding protein (dCtBP) in vitro, and dCtBP(-) mutations genetically interact with ttk69. Furthermore, the C-terminal region downstream of the DNA-binding zinc fingers is shown to be essential for Ttk69 function. A dCtBP consensus binding motif in the C terminus appears to contribute to Ttk69 activity, but it cannot be fully responsible for the function of the C terminus.

NAC-1 is a brain POZ/BTB protein that can prevent cocaine-induced sensitization in the rat

Levels of the mRNA NAC-1 are increased in the rat forebrain weeks after cocaine exposure. This long-term neuroadaptation occurs during the expression of behavioral sensitization, a model of psychostimulant-induced paranoia. NAC-1, the protein encoded by this cocaine-regulated mRNA, contains a Pox virus and zinc finger/bric-a-brac tramtrack broad complex (POZ/BTB) motif, which mediates interactions among several transcriptional regulators. The present studies demonstrate that NAC-1 acts as a transcription factor. NAC-1 was localized to the nucleus of neurons in the brain. Transfection of NAC-1 in cell culture repressed transcription of a reporter gene. NAC-1 was also able to affect the actions of other POZ/BTB proteins in mammalian two-hybrid studies; these interactions required the presence of the POZ/BTB domain. However, NAC-1 appears to be a unique POZ/BTB transcriptional regulator because it does not contain any zinc finger regions found in these other DNA-binding proteins. Adenoviral-mediated overexpression of NAC-1 protein in the rat nucleus accumbens prevented the development but not the expression of behavioral sensitization produced by repeated administration of cocaine. Thus, NAC-1 may modify the long-term behaviors of psychostimulant abuse by regulating gene transcription in the mammalian brain.

Gene expression networks underlying retinoic acid–induced differentiation of acute promyelocytic leukemia cells

To elucidate the molecular mechanism of all-trans-retinoic acid (ATRA)–induced differentiation of acute promyelocytic leukemia (APL) cells, the gene expression patterns in the APL cell line NB4 before and after ATRA treatment were analyzed using complementary DNA array, suppression-subtractive hybridization, and differential-display–polymerase chain reaction. A total of 169 genes, including 8 novel ones, were modulated by ATRA. The ATRA-induced gene expression profiles were in high accord with the differentiation and proliferation status of the NB4 cells. The time courses of their modulation were interesting. Among the 100 up-regulated genes, the induction of expression occurred most frequently 12-48 hours after ATRA treatment, while 59 of 69 down-regulated genes found their expression suppressed within 8 hours. The transcriptional regulation of 8 induced and 24 repressed genes was not blocked by cycloheximide, which suggests that these genes may be direct targets of the ATRA signaling pathway. A balanced functional network seemed to emerge, and it formed the foundation of decreased cellular proliferation, maintenance of cell viability, increased protein modulation, and promotion of granulocytic maturation. Several cytosolic signaling pathways, including JAKs/STAT and MAPK, may also be implicated in the symphony of differentiation.

Both corepressor proteins SMRT and N-CoR exist in large protein complexes containing HDAC3

We present evidence that both corepressors SMRT and N-CoR exist in large protein complexes with estimated sizes of 1.5-2 MDa in HeLa nuclear extracts. Using a combination of conventional and immunoaffinity chromatography, we have successfully isolated a SMRT complex and identified histone deacetylase 3 (HDAC3) and transducin (beta)-like I (TBL1), a WD-40 repeat-containing protein, as the subunits of the purified SMRT complex. We show that the HDAC3-containing SMRT and N-CoR complexes can bind to unliganded thyroid hormone receptors (TRs) in vitro. We demonstrate further that in Xenopus oocytes, both SMRT and N-CoR also associate with HDAC3 in large protein complexes and that injection of antibodies against HDAC3 or SMRT/N-CoR led to a partial relief of repression by unliganded TR/RXR. These findings thus establish both SMRT and N-CoR complexes as bona fide HDAC-containing complexes and shed new light on the molecular pathways by which N-CoR and SMRT function in transcriptional repression.

Gene expression networks underlying retinoic acid–induced differentiation of acute promyelocytic leukemia cells

To elucidate the molecular mechanism of all-trans-retinoic acid (ATRA)–induced differentiation of acute promyelocytic leukemia (APL) cells, the gene expression patterns in the APL cell line NB4 before and after ATRA treatment were analyzed using complementary DNA array, suppression-subtractive hybridization, and differential-display–polymerase chain reaction. A total of 169 genes, including 8 novel ones, were modulated by ATRA. The ATRA-induced gene expression profiles were in high accord with the differentiation and proliferation status of the NB4 cells. The time courses of their modulation were interesting. Among the 100 up-regulated genes, the induction of expression occurred most frequently 12-48 hours after ATRA treatment, while 59 of 69 down-regulated genes found their expression suppressed within 8 hours. The transcriptional regulation of 8 induced and 24 repressed genes was not blocked by cycloheximide, which suggests that these genes may be direct targets of the ATRA signaling pathway. A balanced functional network seemed to emerge, and it formed the foundation of decreased cellular proliferation, maintenance of cell viability, increased protein modulation, and promotion of granulocytic maturation. Several cytosolic signaling pathways, including JAKs/STAT and MAPK, may also be implicated in the symphony of differentiation.

NAC-1 is a brain POZ/BTB protein that can prevent cocaine-induced sensitization in the rat

Levels of the mRNA NAC-1 are increased in the rat forebrain weeks after cocaine exposure. This long-term neuroadaptation occurs during the expression of behavioral sensitization, a model of psychostimulant-induced paranoia. NAC-1, the protein encoded by this cocaine-regulated mRNA, contains a Pox virus and zinc finger/bric-a-brac tramtrack broad complex (POZ/BTB) motif, which mediates interactions among several transcriptional regulators. The present studies demonstrate that NAC-1 acts as a transcription factor. NAC-1 was localized to the nucleus of neurons in the brain. Transfection of NAC-1 in cell culture repressed transcription of a reporter gene. NAC-1 was also able to affect the actions of other POZ/BTB proteins in mammalian two-hybrid studies; these interactions required the presence of the POZ/BTB domain. However, NAC-1 appears to be a unique POZ/BTB transcriptional regulator because it does not contain any zinc finger regions found in these other DNA-binding proteins. Adenoviral-mediated overexpression of NAC-1 protein in the rat nucleus accumbens prevented the development but not the expression of behavioral sensitization produced by repeated administration of cocaine. Thus, NAC-1 may modify the long-term behaviors of psychostimulant abuse by regulating gene transcription in the mammalian brain.

BCoR, a novel corepressor involved in BCL-6 repression

BCL-6 encodes a POZ/zinc finger transcriptional repressor that is required for germinal center formation and may influence apoptosis. Aberrant expression ofBCL-6 due to chromosomal translocations is implicated in certain subtypes of non-Hodgkin's lymphoma. The POZ domains of BCL-6 and several other POZ proteins interact with corepressors N-CoR and SMRT. Here we identify and characterize a novel corepressor BCoR (BCL-6 interacting corepressor), which is expressed ubiquitously in human tissues. BCoR can function as a corepressor when tethered to DNA and, when overexpressed, can potentiate BCL-6 repression. Specific class I and II histone deacetylases (HDACs) interact in vivo with BCoR, suggesting that BCoR may functionally link these two classes of HDACs. Strikingly, BCoR interacts selectively with the POZ domain of BCL-6 but not with eight other POZ proteins tested, including PLZF. Additionally, interactions between the BCL-6 POZ domain and SMRT, N-CoR, and BCoR are mutually exclusive. The specificity of the BCL-6/BCoR interaction suggests that BCoR may have a role in BCL-6-associated lymphomas.