Groucho-dependent and -independent repression activities of Runt domain proteins

All Tcf HMG box transcription factors interact with Groucho-related co-repressors

Tcf/Lef family transcription factors are the downstream effectors of the Wingless/Wnt signal transduction pathway. Upon Wingless/Wnt signalling, beta-catenin translocates to the nucleus, interacts with Tcf (1-3) and thus activates transcription of target genes (4,5). Tcf factors also interact with members of the Groucho (Grg/TLE) family of transcriptional co-repressors (6). We have now tested all known mammalian Groucho family members for their ability to interact specifically with individual Tcf/Lef family members. Transcriptional activation by any Tcf could be repressed by Grg-1, Grg-2/TLE-2, Grg-3 and Grg-4 in a reporter assay. Specific interactions between Tcf and Grg proteins may be achieved in vivo by tissue- or cell type-limited expression. To address this, we determined the expression of all Tcf and Grg/TLE family members in a panel of cell lines. Within any cell line, several Tcfs and TLEs are co-expressed. Thus, redundancy in Tcf/Grg interactions appears to be the rule. The 'long' Groucho family members containing five domains are repressors of Tcf-mediated transactivation, whereas Grg-5, which only contains the first two domains, acts as a de-repressor. As previously shown for Drosophila Groucho, we show that long Grg proteins interact with histone deacetylase-1. Although Grg-5 contains the GP homology domain that mediates HDAC binding in long Grg proteins, Grg-5 fails to bind this co-repressor, explaining how it can de-repress transcription.

The basic helix-loop-helix transcription factor HESR1 regulates endothelial cell tube formation

Human endothelial cells can be induced to form capillary-like tubular networks in collagen gels. We have used this in vitro model and representational difference analysis to identify genes involved in the formation of new blood vessels. HESR1 (HEY-1/HRT-1/CHF-2/gridlock), a basic helix-loop-helix protein related to the hairy/enhancer of split/HES family, is absent in migrating and proliferating cultures of endothelial cells but is rapidly induced during capillary-like network formation. HESR1 is detectable in all adult tissues and at high levels in well vascularized organs such as heart and brain. Its expression is also enriched in aorta and purified capillaries. Overexpression of HESR1 in endothelial cells down-regulates vascular endothelial cell growth factor receptor-2 (VEGFR2) mRNA levels and blocks proliferation, migration, and network formation. Interestingly, reduction of expression of HESR1 by antisense oligonucleotides also blocks endothelial cell network formation in vitro. Finally, HESR1 expression is altered in several breast, lung, and kidney tumors. These data are consistent with a temporal model for HESR1 action where down-regulation at the initiation of new vessel budding is required to allow VEGFR2-mediated migration and proliferation, but re-expression of HESR1 is necessary for induction of tubular network formation and continued maintenance of the mature, quiescent vessel.

The winged-helix protein brain factor 1 interacts with groucho and hes proteins to repress transcription

Brain factor 1 (BF-1) is a winged-helix transcriptional repressor that plays important roles in both progenitor cell differentiation and regional patterning in the mammalian telencephalon. The aim of this study was to elucidate the molecular mechanisms underlying BF-1 functions. It is shown here that BF-1 interacts in vivo with global transcriptional corepressors of the Groucho family and also associates with the histone deacetylase 1 protein. The ability of BF-1 to mediate transcriptional repression is promoted by Groucho and inhibited by the histone deacetylase inhibitor trichostatin A, suggesting that BF-1 recruits Groucho and histone deacetylase activities to repress transcription. Our studies also provide the first demonstration that Groucho mediates a specific interaction between BF-1 and the basic helix-loop-helix protein Hes1 and that BF-1 potentiates transcriptional repression by Hes1 in a Groucho-dependent manner. These findings suggest that Groucho participates in the transcriptional functions of BF-1 by acting as both a corepressor and an adapter between BF-1 and Hes1. Taken together with the demonstration that these proteins are coexpressed in telencephalic neural progenitor cells, these results also suggest that complexes of BF-1, Groucho, and Hes factors may be involved in the regulation of progenitor cell differentiation in the telencephalon.

Contributions of nuclear architecture and chromatin to vitamin D-dependent transcriptional control of the rat osteocalcin gene

The vitamin D response element in the bone tissue-specific osteocalcin gene has served as a prototype for understanding molecular mechanisms regulating physiologic responsiveness of vitamin D-dependent genes in bone cells. We briefly review factors which contribute to vitamin D transcriptional control. The organization of the vitamin D response element (VDRE), the multiple activities of the vitamin D receptor transactivation complex, and the necessity for protein-protein interactions between the VDR-RXR heterodimer activation complex and DNA binding proteins at other regulatory elements, including AP-1 sites and TATA boxes, provide for precise regulation of gene activity in concert with basal levels of transcription. We present evidence for molecular mechanisms regulating vitamin D-dependent mediated transcription of the osteocalcin gene that involve chromatin structure of the gene and nuclear architecture. Modifications in nucleosomal organization, DNase I hypersensitivity and localization of vitamin D receptor interacting proteins in subnuclear domains are regulatory components of vitamin D-dependent gene transcription. A model is proposed to account for the inability of vitamin D induction of the osteocalcin gene in the absence of ongoing basal transcription by competition of the YY1 nuclear matrix-associated transcription factor for TFIIB-VDR interactions. Activation of the VDR-RXR complex at the OC VDRE occurs through modifications in chromatin mediated in part by interaction of OC gene regulatory sequences with the nuclear matrix-associated Cbfa1 (Runx2) transcription factor which is required for osteogenesis.

Brinker is a sequence-specific transcriptional repressor in the Drosophila embryo

A Dpp activity gradient specifies multiple thresholds of gene expression in the dorsal ectoderm of the early embryo. Some of these thresholds depend on a putative repressor, Brinker, which is expressed in the neurogenic ectoderm in response to the maternal Dorsal gradient and Dpp signaling. Here we show that Brinker is a sequence-specific transcriptional repressor. It binds the consensus sequence, TGGCGc/tc/t, and interacts with the Groucho corepressor through a conserved sequence motif, FKPY. An optimal Brinker binding site is contained within an 800-bp enhancer from the tolloid gene, which has been identified as a genetic target of the Brinker repressor. A tolloid-lacZ transgene containing point mutations in this site exhibits an expanded pattern of expression, suggesting that Brinker directly represses tolloid transcription. We discuss other examples of transcriptional repressors constraining the activities of signaling pathways.

PRH represses transcription in hematopoietic cells by at least two independent mechanisms

PRH (proline-rich homeodomain protein) is strongly expressed in the hematopoietic compartment. Here we show that PRH is a repressor of transcription in hematopoietic cells. A fragment of PRH that includes the homeodomain can bind to TATA box sequences in vitro and can also bind to the TATA box-binding protein. PRH represses transcription from TATA box-containing promoters in intact cells but does not repress transcription from a promoter lacking a TATA box. A mutation in the PRH homeodomain that blocks binding to DNA but that has little or no effect on binding to the TATA box-binding protein significantly reduces the ability of the protein to repress transcription and provides the first clear demonstration that a homeodomain can bring about transcriptional repression in vivo by binding to a TATA box. However, we also show that mutation of the PRH homeodomain does not block the ability of PRH to repress transcription when this protein is tethered upstream of the TATA box via a heterologous DNA-binding domain. PRH also contains an N-terminal proline-rich repression domain that is separate from the homeodomain. Deletion mapping suggests that this repression domain contains at least two regions that both independently contribute to transcriptional repression.

Expression and function of a stem cell promoter for the murine CBFalpha2 gene: distinct roles and regulation in natural killer and T cell development

The Runt family transcription factor CBFalpha2 (AML1, PEBP2alphaB, or Runx1) is required by hematopoietic stem cells and expressed at high levels in T-lineage cells. In human T cells CBFalpha2 is usually transcribed from a different promoter (distal promoter) than in myeloid cells (proximal promoter), but the developmental and functional significance of this promoter switch has not been known. Here, we report that both coding and noncoding sequences of the distal 5' end are highly conserved between the human and the murine genes, and the distal promoter is responsible for the overwhelming majority of CBFalpha2 expression in murine hematopoietic stem cells as well as in T cells. Distal promoter activity is maintained throughout T cell development and at lower levels in B cell development, but downregulated in natural killer cell development. The distal N-terminal isoform binds to functionally important regulatory sites from known target genes with two- to threefold higher affinity than the proximal N-terminal isoform. Neither full-length isoform alters growth of a myeloid cell line under nondifferentiating conditions, but the proximal isoform selectively delays mitotic arrest of the cell line under differentiating conditions, resulting in the generation of greater numbers of neutrophils.

Identification of a structural motif that confers specific interaction with the WD40 repeat domain of Arabidopsis COP1

Arabidopsis COP1 is a photomorphogenesis repressor capable of directly interacting with the photomorphogenesis-promoting factor HY5. This interaction between HY5 and COP1 results in targeted deg radation of HY5 by the 26S proteasome. Here we characterized the WD40 repeat domain-mediated interactions of COP1 with HY5 and two new proteins. Mutational analysis of those interactive partners revealed a conserved motif responsible for the interaction with the WD40 domain. This novel motif, with the core sequence V-P-E/D-φ-G (φ = hydrophobic residue) in conjunction with an upstream stretch of 4-5 negatively charged residues, interacts with a defined surface area of the ss-propeller assembly of the COP1 WD40 repeat domain through both hydrophobic and ionic interactions. Several residues in the COP1 WD40 domain that are critical for the interaction with this motif have been revealed. The fact that point mutations either in the COP1 WD40 domain or in the HY5 motif that abolish the interaction between COP1 and HY5 in yeast result in a dramatic reduction of HY5 degradation in transgenic plants validates the biological significance of this defined interaction.

Association with the nuclear matrix and interaction with Groucho and RUNX proteins regulate the transcription repression activity of the basic helix loop helix factor Hes1

Hairy/Enhancer of split 1 (Hes1) is a mammalian transcriptional repressor that plays crucial roles in the regulation of several developmental processes, including neuronal differentiation. The aim of this study was to elucidate the molecular mechanisms that regulate the transcription repression activity of Hes1. It is shown here that Hes1 associates with the nuclear matrix, the ribonucleoprotein network of the nucleus that plays important roles in transcriptional regulation. Nuclear matrix binding is mediated by the same Hes1 C-terminal domain that is also required for transcriptional repression. This domain contains the WRPW motif that acts as a binding site for the transcriptional corepressor Groucho, which also localizes to the nuclear matrix. Both the nuclear matrix association and transcription repression activity of Hes1 are inhibited by deletion of the WRPW motif, indicating that Groucho acts as a transcriptional corepressor for Hes1. This corepressor role is not modulated by the Groucho-related gene product Grg5. In contrast, the Runt-related protein RUNX2, which localizes to the nuclear matrix and interacts with Groucho and Hes1, can inhibit both the Groucho.Hes1 interaction and the transcription repression ability of Hes1. Together, these observations suggest that transcriptional repression by Hes1 requires interactions with Groucho at the nuclear matrix and that RUNX proteins act as negative regulators of the repressive activity of Groucho.Hes1 complexes.

Drosophila C-terminal binding protein functions as a context-dependent transcriptional co-factor and interferes with both mad and groucho transcriptional repression

Drosophila C-terminal binding protein (dCtBP) and Groucho have been identified as Hairy-interacting proteins required for embryonic segmentation and Hairy-mediated transcriptional repression. While both dCtBP and Groucho are required for proper Hairy function, their properties are very different. As would be expected for a co-repressor, reduced Groucho activity enhances the hairy mutant phenotype. In contrast, reduced dCtBP activity suppresses it. We show here that dCtBP can function as either a co-activator or co-repressor of transcription in a context-dependent manner. The regions of dCtBP required for activation and repression are separable. We find that mSin3A-histone deacetylase complexes are altered in the presence of dCtBP and that dCtBP interferes with both Groucho and Mad transcriptional repression. Similar to CtBP's role in attenuating E1A's oncogenicity, we propose that dCtBP can interfere with corepressor-histone deacetylase complexes, thereby attenuating transcriptional repression. Hairy defines a new class of proteins that requires both CtBP and Groucho co-factors for proper function.

Mechanisms of transcriptional regulation by Runt domain proteins

Runt domain proteins have vital roles in regulating transcription in developmental pathways extending from sex determination and segmentation in fruit fly embryos to the development of blood and bone in mammals. Many of the insights into the mechanisms by which these proteins act to regulate transcription originate either from studies on the Drosophila runt gene, the founding member of this family, or from work on the mammalian PEBP2/CBF transcription factor. Genetic experiments in the Drosophila system reveal that runt functions both to activate and to repress transcription of different downstream target genes and indicate that different mechanisms are used in the regulation of different specific downstream target genes. These studies have also identified other nuclear factors that work with Runt in some of these pathways. Studies in mammalian systems have provided additional evidence for the complexity of transcriptional regulation by Runt domain proteins and have identified other transcription factors that cooperate with Runt domain proteins to regulate the activity of different specific cis-regulatory enhancers. The emerging view from studies in both systems is that these proteins act as context-dependent regulators of transcription, activating or repressing gene expression dependent upon the constititution of a particular promoter/enhancer in a particular cell type. These results have yielded new insights into the molecular mechanisms that control animal development and provide a framework for investigating fundamental issues in eukaryotic transcriptional regulation.

Alterations of the AML1 transcription factor in human leukemia

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

dCtBP-dependent and -independent repression activities of the Drosophila Knirps protein

Transcriptional repressor proteins play essential roles in controlling the correct temporal and spatial patterns of gene expression in Drosophila melanogaster embryogenesis. Repressors such as Knirps, Krüppel, and Snail mediate short-range repression and interact with the dCtBP corepressor. The mechanism by which short-range repressors block transcription is not well understood; therefore, we have undertaken a detailed structure-function analysis of the Knirps protein. To provide a physiological setting for measurement of repression, the activities of endogenous or chimeric Knirps repressor proteins were assayed on integrated reporter genes in transgenic embryos. Two distinct repression functions were identified in Knirps. One repression activity depends on dCtBP binding, and this function maps to a C-terminal region of Knirps that contains a dCtBP binding motif. In addition, an N-terminal region was identified that represses in a CtBP mutant background and does not bind to the dCtBP protein in vitro. Although the dCtBP protein is important for Knirps activity on some genes, one endogenous target of the Knirps protein, the even-skipped stripe 3 enhancer, is not derepressed in a CtBP mutant. These results indicate that Knirps can utilize two different pathways to mediate transcriptional repression and suggest that the phenomenon of short-range repression may be a combination of independent activities.

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.

Runt and Lozenge function in Drosophila development

Runt and Lozenge (LZ) are members of the Runt domain family of transcriptional regulators and control a large number of developmental processes in Drosophila. Runt is a pair-rule gene, and is part of the network of genes that control pattern formation in the embryo. In the central nervous system, Runt function is necessary for the development of a subset of neurons. Runt is also a key regulator of sex determination, and directly controls Sex-lethal, a master gene that determines sex of the animal and controls dosage compensation. The LZ protein also participates in several key processes. LZ controls pre-patterning and cell-fate choices in the development of the visual system by regulating the expression of several fate-specifying transcription factors, and works in conjunction with general signaling pathways. LZ function is also required in hematopoiesis for the specification of a Drosophila blood cell lineage.

Regulation of collagenase-3 gene expression in osteoblastic and non-osteoblastic cell lines

Collagenase-3 expression in osteoblastic (UMR 106-01, ROS 17/2.8) and non-osteoblastic cell lines (BC1, NIH3T3) was examined. We observed that parathyroid hormone (PTH) induces collagenase-3 expression only in UMR cells but not in BC1 (which express collagenase-3 constitutively) or ROS and NIH3T3 cells. Since we know from UMR cells that the AP-1 factors and Cbfa1 are required for collagenase-3 expression, we analyzed the expression and PTH regulation of these factors by gel shift and Northern blot analysis in all cell lines. Gel mobility shift with a [(32)P]-labeled collagenase-3 AP-1 site probe indicated the induction of c-Fos in osteoblastic cells upon PTH treatment. While c-fos was induced in UMR cells, both c-fos and jun B were induced in ROS cells. Since Jun B is inhibitory of Fos and Jun in the regulation of the rat collagenase-3 gene in UMR cells, it is likely that high levels of Jun B prevent PTH stimulation of collagenase-3 in ROS cells. When we carried out gel shift analysis with a [(32)P]-labeled collagenase-3 RD (runt domain) site probe and Northern blot analysis with a Cbfa1 specific probe, we have observed the presence of Cbfa1 in both osteoblastic and non-osteoblastic cell lines, but there was no change in the levels of Cbfa1 RNA or protein in these cells under either control conditions or PTH treatment. From our studies above, it is evident that the expression of collagenase-3 and its regulation by PTH in osteoblastic and non-osteoblastic cells may be influenced by differential temporal stimulation of the AP-1 family members, especially c-Fos and Jun B along with the potential for posttranslational modification(s) of Cbfa1.

Pair-rule gene runt restricts orthodenticle expression to the presumptive head of the Drosophila embryo

Ectopic expression of the pair-rule gene runt in the anterior end of the Drosophila embryo antagonizes transcriptional activation of the head gap gene orthodenticle (otd) by the anterior morphogen bicoid. Here we investigate the relevance of runt's activity as a repressor of otd in normal Drosophila embryogenesis otd expression is activated in the posterior region of embryos that are mutant for runt. This posterior expression domain of otd depends on the activity of the orphan nuclear receptor protein Tailless. Repression of otd by runt does not require the conserved VVVRPY motif that mediates interaction between Runt and the co-repressor protein Groucho. The observed functional interactions between runt and tailless on otd expression may indicate there are other contexts where members of these two families of transcriptional regulators interact to regulate gene expression during development.

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

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

Characterization of the human and mouse HEY1, HEY2, and HEYL genes: cloning, mapping, and mutation screening of a new bHLH gene family

Many basic helix-loop-helix (bHLH) transcription factors are known as key regulators of embryonic development or differentiation in various species. We have isolated and characterized three new hairy-related bHLH transcription factor genes from mouse and human (hairy and Enhancer-of-split related with YRPW motif; HEY1, HEY2, and HEYL). All three HEY genes have a similar genomic structure with five exons. Together with a highly related Drosophila homologue, they form a new bHLH gene subfamily that is different from both hairy and the known vertebrate Hes and Her genes. While the overall structure with the bHLH domain, Orange domain, and WRPW motif is similar, the last motif is changed to KPYRPWG in Hey1/2 and absent in HeyL. This and other sequence features suggest Hey proteins to have unique functional properties. The genes were mapped by fluorescence in situ hybridization and RH mapping to the following human chromosomes: (HEY1) 8q21, (HEY2) 6q21, and (HEYL) 1p34.3. Based on expression patterns and map location, HEY genes are candidates for several human or mouse disease loci. However, initial screening of DNA from affected individuals for two human disorders and four mouse mutants did not reveal any diagnostic alterations in the coding regions.

Regulation of granulopoiesis by transcription factors and cytokine signals

The development of mature granulocytes from hematopoietic precursor cells is controlled by a myriad of transcription factors which regulate the expression of essential genes, including those encoding growth factors and their receptors, enzymes, adhesion molecules, and transcription factors themselves. In particular, C/EBPalpha, PU.1, CBF, and c-Myb have emerged as critical players during early granulopoiesis. These transcription factors interact with one another as well as other factors to regulate the expression of a variety of genes important in granulocytic lineage commitment. An important goal remains to understand in greater detail how these various factors act in concert with signals emanating from cytokine receptors to influence the various steps of maturation, from the pluripotent hematopoietic stem cell, to a committed myeloid progenitor, to myeloid precursors, and ultimately to mature granulocytes.

Groucho/TLE family proteins and transcriptional repression

The Drosophila Groucho (Gro) protein is the prototype for a large family of corepressors, examples of which are found in most metazoans. This family includes the human transducin-like Enhancer of split (TLE) proteins. As corepressors, Gro/TLE family proteins do not bind to DNA directly, but rather are recruited to the template by DNA-bound repressor proteins. Gro/TLE family proteins are required for many developmental processes, including lateral inhibition, segmentation, sex determination, dorsal/ventral pattern formation, terminal pattern formation, and eye development. These proteins are characterized by a conserved N-terminal glutamine-rich domain and a conserved C-terminal WD-repeat domain. The primary role of the glutamine-rich domain is apparently to mediate tetramerization, while the WD-repeat domain may mediate interactions with DNA-bound repressors. The glutamine rich and WD-repeat domains are separated by a less conserved region containing domains that have been implicated in transcriptional repression and nuclear localization. In addition to encoding full-length Gro/TLE family proteins, most metazoan genomes encode truncated family members that contain the N-terminal oligomerization domain, but lack the C-terminal WD-repeat domain. These truncated proteins may negatively regulate full-length Gro/TLE proteins, perhaps by sequestering them in non-productive complexes. Gro/TLE family proteins probably repress transcription by multiple mechanisms. For example, a glycine/proline-rich domain in the central variable region functions to recruit the histone deacetylase Rpd3 to the template. This histone deacetylase then presumably silences transcription by altering local chromatin structure. Other repression domains in Gro may function in a histone deacetylase-independent manner. Many aspects of Gro/TLE protein function remain to be explored, including the possible post-translational regulation of Gro/TLE activity as well as the mechanisms by which Gro/TLE proteins direct repression at a distance.

Transcriptional repression by Pax5 (BSAP) through interaction with corepressors of the Groucho family

Pax5 (BSAP) functions as both a transcriptional activator and repressor during midbrain patterning, B-cell development and lymphomagenesis. Here we demonstrate that Pax5 exerts its repression function by recruiting members of the Groucho corepressor family. In a yeast two-hybrid screen, the groucho-related gene product Grg4 was identified as a Pax5 partner protein. Both proteins interact cooperatively via two separate domains: the N-terminal Q and central SP regions of Grg4, and the octapeptide motif and C-terminal transactivation domain of Pax5. The phosphorylation state of Grg4 is altered in vivo upon Pax5 binding. Moreover, Grg4 efficiently represses the transcriptional activity of Pax5 in an octapeptide-dependent manner. Similar protein interactions resulting in transcriptional repression were also observed between distantly related members of both the Pax2/5/8 and Groucho protein families. In agreement with this evolutionary conservation, the octapeptide motif of Pax proteins functions as a Groucho-dependent repression domain in Drosophila embryos. These data indicate that Pax proteins can be converted from transcriptional activators to repressors through interaction with corepressors of the Groucho protein family.

Expression patterns of hairy, even-skipped, and runt in the spider Cupiennius salei imply that these genes were segmentation genes in a basal arthropod

There is an ongoing discussion on whether segmentation in different phyla has a common origin sharing a common genetic program. However, before comparing segmentation between phyla, it is necessary to identify the ancestral condition within each phylum. Even within the arthropods it is not clear which parts of the genetic network leading to segmentation are conserved in all groups. In this paper, we analyze the expression of three segmentation genes of the pair-rule class in the spider Cupiennius salei. Spiders are representatives of the Chelicerata, a monophyletic basic arthropod group. We find that in spider embryos, the orthologues for the Drosophila primary pair-rule genes hairy, even-skipped, and runt are expressed in stripes in the growth zone, where the segments are forming, suggesting a role for these genes in chelicerate segmentation. These data imply that the involvement of hairy, even-skipped, and runt in arthropod segmentation is an ancestral character for arthropods and is not restricted to a particular group of insects.

Constitutive expression and regulation of collagenase-3 in human breast cancer cells

Matrix metalloproteinases (MMPs) are a family of secreted or transmembrane proteins that have been implicated in multiple physiological and pathological processes related to extracellular matrix turnover. Recent evidence strongly suggests a role for collagenase-3 (MMP-13) in tumor metastasis and invasion. We report here that collagenase-3 is constitutively expressed in the breast cancer cell line MDA-MB231 (MDA) and outline the molecular mechanism regulating its expression. Functional analysis of the collagenase-3 promoter showed that both the activator protein-1 (AP-1) site and the runt domain (RD) binding site were required for maximal constitutive expression of collagenase-3 in MDA cells. Determination of factors binding to those sites by Northern analysis and transient transfections identified the requirement of Fra-1, c-Jun, and Cbfa1 for basal collagenase-3 promoter activity in MDA cells.

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

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

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

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

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

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

The mammalian basic helix loop helix protein HES-1 binds to and modulates the transactivating function of the runt-related factor Cbfa1

Drosophila Runt is the founding member of a family of related transcription factors involved in the regulation of a variety of cell-differentiation events in invertebrates and vertebrates. Runt-related proteins act as both transactivators and transcriptional repressors, suggesting that context-dependent mechanisms modulate their transcriptional properties. The aim of this study was to elucidate the molecular mechanisms that contribute to the regulation of the functions of the mammalian Runt-related protein, Cbfa1. Here we provide the first demonstration that Cbfa1 (as well as the related protein, Cbfa2/AML1) physically interacts with the basic helix loop helix transcription factor, HES-1, a mammalian counterpart of the Drosophila Hairy and Enhancer of split proteins. This interaction is mediated by the carboxyl-terminal domains of Cbfa1 and HES-1, but does not require their respective tetrapeptide motifs, WRPY and WRPW. Our studies also show that HES-1 can antagonize the binding of Cbfa1 to mammalian transcriptional corepressors of the Groucho family. Moreover, HES-1 can potentiate Cbfa1-mediated transactivation in transfected cells. Taken together, these findings implicate HES-1 in the transcriptional functions of Cbfa1 and suggest that the concerted activities of Groucho and HES proteins modulate the functions of mammalian Runt-related proteins.

Quantitative analysis of gene function in the Drosophila embryo

The specific functions of gene products frequently depend on the developmental context in which they are expressed. Thus, studies on gene function will benefit from systems that allow for manipulation of gene expression within model systems where the developmental context is well defined. Here we describe a system that allows for genetically controlled overexpression of any gene of interest under normal physiological conditions in the early Drosophila embryo. This regulated expression is achieved through the use of Drosophila lines that express a maternal mRNA for the yeast transcription factor GAL4. Embryos derived from females that express GAL4 maternally activate GAL4-dependent UAS transgenes at uniform levels throughout the embryo during the blastoderm stage of embryogenesis. The expression levels can be quantitatively manipulated through the use of lines that have different levels of maternal GAL4 activity. Specific phenotypes are produced by expression of a number of different developmental regulators with this system, including genes that normally do not function during Drosophila embryogenesis. Analysis of the response to overexpression of runt provides evidence that this pair-rule segmentation gene has a direct role in repressing transcription of the segment-polarity gene engrailed. The maternal GAL4 system will have applications both for the measurement of gene activity in reverse genetic experiments as well as for the identification of genetic factors that have quantitative effects on gene function in vivo.

Identification of CtBP1 and CtBP2 as corepressors of zinc finger-homeodomain factor deltaEF1

deltaEF1, a representative of the zinc finger-homeodomain protein family, is a transcriptional repressor which binds E2-box (CACCTG) and related sequences and counteracts the activators through transrepression mechanisms. It has been shown that the N-proximal region of the protein is involved in the transrepression. Here we demonstrate that deltaEF1 has a second mechanism of transrepression recruiting CtBP1 or CtBP2 as its corepressor. A two-hybrid screen of mouse cDNAs with various portions of deltaEF1 identified these proteins, which bind to deltaEF1 in a manner dependent on the PLDLSL sequence located in the short medial (MS) portion of deltaEF1. CtBP1 is the mouse orthologue of human CtBP, known as the C-terminal binding protein of adenovirus E1A, while CtBP2 is the second homologue. Fusion of mouse CtBP1 or CtBP2 to Gal4DBD (Gal4 DNA binding domain) made them Gal4 binding site-dependent transcriptional repressors in transfected 10T1/2 cells, indicating their involvement in a transcriptional repression mechanism. When the MS portion of deltaEF1 was used to Gal4DBD and used to transfect cells, a strong transrepression activity was generated, but this activity was totally dependent on the PLDLSL sequence which served as the site for interaction with endogenous CtBP proteins, indicating that CtBP1 and -2 can act as corepressors. Exogenous CtBP1/2 significantly enhanced transcriptional repression by deltaEF1, and this enhancement was lost if the PLDLSL sequence was altered, demonstrating that CtBP1 and -2 act as corepressors of deltaEF1. In the mouse, CtBP1 is expressed from embryo to adult, but CtBP2 is mainly expressed during embryogenesis. In developing embryos, CtBP1 and CtBP2 are expressed broadly with different tissue preferences. Remarkably, their high expression occurs in subsets of deltaEF1-expressing tissues, e.g., cephalic and dorsal root ganglia, spinal cord, posterior-distal halves of the limb bud mesenchyme, and perichondrium of forming digits, supporting the conclusion that CtBP1 and -2 play crucial roles in the repressor action of deltaEF1 in these tissues.

The homeodomain protein NK-3 recruits Groucho and a histone deacetylase complex to repress transcription

Transcriptional repression by sequence-specific DNA binding factors is mediated by the recruitment of a corepressor complex to the promoter region. The NK-3 homeodomain protein is a transcriptional repressor that recruits the nuclear protein kinase, homeodomain interacting protein kinase 2 (HIPK2). Here we show that HIPK2 is a component of a corepressor complex containing Groucho and a histone deacetylase complex. Groucho, like HIPK2, acts as a corepressor for NK-3 and binds to NK-3 and HIPK2. Moreover, HIPK2 appears to regulate the corepressor activity of Groucho. Transcriptional repression by NK-3 and Groucho is relieved by the histone deacetylase inhibitor trichostatin A, and both NK-3 and Groucho directly interact with the histone deacetylase HDAC1 that is associated with mSin3A in vivo. Recruitment of the histone deacetylase complex by NK-3 decreases the acetylated histones that are associated with the target gene promoter. These results indicate that NK-3 represses transcription by recruiting a complex containing Groucho and a histone deacetylase complex that leads to histone modification on chromatin and suggest that HIPK2 may play a regulatory role in the corepressor complex formation.

UNC-4/UNC-37-dependent repression of motor neuron-specific genes controls synaptic choice in Caenorhabditis elegans

The UNC-4 homeoprotein and the Groucho-like corepressor UNC-37 specify synaptic choice in the Caenorhabditis elegans motor neuron circuit. In unc-4 mutants, VA motor neurons are miswired with inputs from interneurons normally reserved for their lineal sisters, the VB motor neurons. Here we show that UNC-4 and UNC-37 function together in VA motor neurons to repress VB-specific genes and that this activity depends on physical contact between UNC-37 and a conserved Engrailed-like repressor domain (eh1) in UNC-4. Missense mutations in the UNC-4 eh1 domain disrupt interactions between UNC-4 and UNC-37 and result in the loss of UNC-4-dependent repressor activity in vivo. A compensatory amino acid substitution in UNC-37 suppresses specific unc-4 alleles by restoring physical interactions with UNC-4 as well as UNC-4-dependent repression of VB-specific genes. We propose that repression of VB-specific genes by UNC-4 and UNC-37 is necessary for the creation of wild-type inputs to VA motor neurons. The existence of mammalian homologs of UNC-4 and UNC-37 indicates that a similar mechanism could regulate synaptic choice in the vertebrate spinal cord.

Multiple Cbfa/AML sites in the rat osteocalcin promoter are required for basal and vitamin D-responsive transcription and contribute to chromatin organization

Three Cbfa motifs are strategically positioned in the bone-specific rat osteocalcin (rOC) promoter. Sites A and B flank the vitamin D response element in the distal promoter and sites B and C flank a positioned nucleosome in the proximal promoter. The functional significance of each Cbfa element was addressed by mutating individual or multiple Cbfa sites within the context of the -1.1-kb rOC promoter fused to a chloramphenicol acetyltransferase reporter gene. Promoter activity was assayed following transient transfection and after stable genomic integration in ROS 17/2.8 osteoblastic cell lines. We show that all three Cbfa sites are required for maximal basal expression of the rOC promoter. However, the distal sites A and B each contribute significantly more (P < 0.001) to promoter activity than site C. In a genomic context, sites A and B can largely compensate for a mutation at the proximal site C, and paired mutations involving site A (mAB or mAC) result in a far greater loss of activity than the mBC mutation. Strikingly, mutation of the three Cbfa sites leads to abrogation of responsiveness to vitamin D. Vitamin D-enhanced activity is also not observed when sites A and B are mutated. Significantly, related to these losses in transcriptional activity, mutation of the three Cbfa sites results in altered chromatin structure as reflected by loss of DNase I-hypersensitive sites at the vitamin D response element and over the proximal tissue-specific basal promoter. These findings strongly support a multifunctional role for Cbfa factors in regulating gene expression, not only as simple transcriptional transactivators but also by facilitating modifications in promoter architecture and chromatin organization.

TCF transcription factors: molecular switches in carcinogenesis

Although originally cloned as lymphoid transcription factors, members of the T-cell factor (Tcf) family are now well recognized as key activators/repressors in many developmental processes. Transcriptionally inert Tcf factors become potent transactivators upon interaction with the Wnt signaling product beta-catenin or its Drosophila counterpart Armadillo. In contrast, Tcf proteins mediate repression when bound to members of the Groucho family of transcriptional repressors, CBP and CtBP. Recently, Tcf factors have been reported as tumor inducers, aberrantly activating their target genes as a result of elevated beta-catenin levels in many types of cancer. These abnormal beta-catenin levels are usually caused by stabilizing mutations in beta-catenin itself or truncating mutations in the adenomatous polyposis coli (APC) tumor suppressor gene. In this review, we will give a chronological overview of the Tcf factors and the phenotypes of Tcf mutant mice, as well as Tcf-binding partners. We will discuss Tcf signaling upon interaction with different partners, resulting in activator and repressor roles of Tcf factors in the light of carcinogenic events.

The inv(16) encodes an acute myeloid leukemia 1 transcriptional corepressor

The inv(16) is one of the most frequent chromosomal translocations associated with acute myeloid leukemia (AML). The inv(16) fusion protein acts by dominantly interfering with AML-1/core binding factor beta-dependent transcriptional regulation. Here we demonstrate that the inv(16) fusion protein cooperates with AML-1B to repress transcription. This cooperativity requires the ability of the translocation fusion protein to bind to AML-1B. Mutational analysis and cell fractionation experiments indicated that the inv(16) fusion protein acts in the nucleus and that repression occurs when the complex is bound to DNA. We also found that the inv(16) fusion protein binds to AML-1B when it is associated with the mSin3A corepressor. An AML-1B mutant that fails to bind mSin3A was impaired in cooperative repression, suggesting that the inv(16) fusion protein acts through mSin3 and possibly other corepressors. Finally, we demonstrate that the C-terminal portion of the inv(16) fusion protein contains a repression domain, suggesting a molecular mechanism for AML-1-mediated repression.

AML3/CBFalpha1 is required for androgen-specific activation of the enhancer of the mouse sex-limited protein (Slp) gene

A complex 120-base pair enhancer, derived from the mouse sex-limited protein (Slp) gene, is activated solely by the androgen receptor (AR) in specific tissues, although it contains a hormone response element recognized by several steroid receptors. The generation of this transcriptional specificity has been ascribed to the interactions of the receptor with tissue-specific nonreceptor factors bound to accessory sites within the enhancer. Protein-DNA interaction assays revealed two factors binding the 5' part of the enhancer that differ widely in abundance between cells showing AR-specific activation of the Slp element compared with those that also permit activation by glucocorticoid receptor (GR). The factor designated B formed a complex centered on the sequence TGTGGT, a core motif recognized by members of the AML/CBFalpha transcription factor family. This complex was competed by a high affinity binding site specific for AML/CBFalpha and was specifically supershifted by an antibody to AML3/CBFalpha1, placing factor B within the AML3/CBFalpha1 subclass. Interestingly, this factor was shown to bind to a second site in the 3' part of the enhancer, positioned between the two critical AR binding sites. Transfection studies revealed that AML1-ETO, a dominant-negative AML/CBFalpha construct, abrogated AR induction of the enhancer, but not of simple hormone response elements. Furthermore, overexpression of AML3/CBFalpha1 could rescue the AML1-ETO repression. Finally, glutathione S-transferase-AML/CBFalpha fusion proteins demonstrated direct interaction between AML/CBFalpha and steroid receptors. Although this interaction was equivalent between AML1/CBFalpha2 and AR or GR, AML3/CBFalpha1 showed stronger interaction with AR than with GR. These data demonstrate that AML3/CBFalpha1 is functionally required for hormonal induction of the Slp enhancer and that direct, preferential protein-protein interactions may contribute to AR-specific activation. These results demonstrate an intriguing role of AML3/CBFalpha1 in steroid- as well as tissue-specific activation of target genes.

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

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

A functional interaction between the histone deacetylase Rpd3 and the corepressor groucho in Drosophila development

The Drosophila gene groucho (gro) encodes a transcriptional corepressor that has critical roles in many development processes. In an effort to illuminate the mechanism of Gro-mediated repression, we have employed Gro as an affinity reagent to purify Gro-binding proteins from embryonic nuclear extracts. One of these proteins was found to be the histone deacetylase Rpd3. Protein-protein interaction assays suggest that Gro and Rpd3 form a complex in vivo and that they interact directly via the glycine/proline rich (GP) domain in Gro. Cell culture assays demonstrate that Rpd3 potentiates repression by the GP domain. Furthermore, experiments employing a histone deacetylase inhibitor, as well as a catalytically inactive form of Rpd3, imply that histone deacetylase activity is required for efficient Gro-mediated repression. Finally, mutations in gro and rpd3 have synergistic effects on embryonic lethality and pattern formation. These findings support the view that Gro mediates repression, at least in part, by the direct recruitment of the histone deacetylase Rpd3 to the template, where it can modulate local chromatin structure. They also provide evidence for a specific role of Rpd3 in early development.

Huckebein repressor activity in Drosophila terminal patterning is mediated by Groucho

The Groucho corepressor mediates negative transcriptional regulation in association with various DNA-binding proteins in diverse developmental contexts. We have previously implicated Groucho in Drosophila embryonic terminal patterning, showing that it is required to confine tailless and huckebein terminal gap gene expression to the pole regions of the embryo. Here we reveal an additional requirement for Groucho in this developmental process by establishing that Groucho mediates repressor activity of the Huckebein protein. Putative Huckebein target genes are derepressed in embryos lacking maternal groucho activity and biochemical experiments demonstrate that Huckebein physically interacts with Groucho. Using an in vivo repression assay, we identify a functional repressor domain in Huckebein that contains an FRPW tetrapeptide, similar to the WRPW Groucho-recruitment domain found in Hairy-related repressor proteins. Mutations in Huckebein's FRPW motif abolish Groucho binding and in vivo repression activity, indicating that binding of Groucho through the FRPW motif is required for the repressor function of Huckebein. Taken together with our earlier results, these findings show that Groucho-repression regulates sequential aspects of terminal patterning in Drosophila.

Differential interactions between Brother proteins and Runt domain proteins in the Drosophila embryo and eye

Brother and Big brother were isolated as Runt-interacting proteins and are homologous to CBF(beta), which interacts with the mammalian CBF(alpha) Runt-domain proteins. In vitro experiments indicate that Brother family proteins regulate the DNA binding activity of Runt-domain proteins without contacting DNA. In both mouse and human there is genetic evidence that the CBF(alpha) and CBF(beta) proteins function together in hematopoiesis and leukemogenesis. Here we demonstrate functional interactions between Brother proteins and Runt domain proteins in Drosophila. First, we show that a specific point mutation in Runt that disrupts interaction with Brother proteins but does not affect DNA binding activity is dysfunctional in several in vivo assays. Interestingly, this mutant protein acts dominantly to interfere with the Runt-dependent activation of Sxl-lethal transcription. To investigate further the requirements for Brother proteins in Drosophila development, we examine the effects of expression of a Brother fusion protein homologous to the dominant negative CBF(beta)::SMMHC fusion protein that is associated with leukemia in humans. This Bro::SMMHC fusion protein interferes with the activity of Runt and a second Runt domain protein, Lozenge. Moreover, we find that the effects of lozenge mutations on eye development are suppressed by expression of wild-type Brother proteins, suggesting that Brother/Big brother dosage is limiting in this developmental context. Results obtained when Runt is expressed in developing eye discs further support this hypothesis. Our results firmly establish the importance of the Brother and Big brother proteins for the biological activities of Runt and Lozenge, and further suggest that Brother protein function is not restricted to enhancing DNA-binding.

Collagenase 3 is a target of Cbfa1, a transcription factor of the runt gene family involved in bone formation

Collagenase 3 (MMP-13) is a recently identified member of the matrix metalloproteinase (MMP) gene family that is expressed at high levels in diverse human carcinomas and in articular cartilage from arthritic patients. In addition to its expression in pathological conditions, collagenase 3 has been detected in osteoblasts and hypertrophic chondrocytes during fetal ossification. In this work, we have evaluated the possibility that Cbfa1 (core binding factor 1), a transcription factor playing a major role in the expression of osteoblastic specific genes, is involved in the expression of collagenase 3 during bone formation. We have functionally characterized a Cbfa motif present in the promoter region of collagenase 3 gene and demonstrated, by cotransfection experiments and gel mobility shift assays, that this element is involved in the inducibility of the collagenase 3 promoter by Cbfa1 in osteoblastic and chondrocytic cells. Furthermore, overexpression of Cbfa1 in osteoblastic cells unable to produce collagenase 3 leads to the expression of this gene after stimulation with transforming growth factor beta. Finally, we show that mutant mice deficient in Cbfa1, lacking mature osteoblasts but containing hypertrophic chondrocytes which are also a major source of collagenase 3, do not express this protease during fetal development. These results provide in vivo evidence that collagenase 3 is a target of the transcriptional activator Cbfa1 in these cells. On the basis of these transcriptional regulation studies, together with the potent proteolytic activity of collagenase 3 on diverse collagenous and noncollagenous bone and cartilage components, we proposed that this enzyme may play a key role in the process of bone formation and remodeling.

XCtBP is a XTcf-3 co-repressor with roles throughout Xenopus development

XTcf-3 is an HMG box transcription factor that mediates Xenopus dorsal-ventral axis formation. As a Wnt pathway effector, XTcf-3 interacts with beta-catenin and activates the expression of the dorsal organizing gene siamois, while in the absence of beta-catenin, XTcf-3 functions as a transcriptional repressor. We show that XTcf-3 contains amino- and carboxy-terminal repressor domains and have identified a Xenopus member of the C-terminal Binding Protein family of transcriptional co-repressors (XCtBP) as the C-terminal co-repressor. We show that two XCtBP binding sites near the XTcf-3 carboxy-terminus are required for the interaction of XTcf-3 and XCtBP and for the transcriptional repression mediated by the XTcf-3 carboxy-terminal domain. By fusing the GAL4 activation domain to XCtBP we have generated an antimorphic protein, XCtBP/G4A, that activates siamois transcription through an interaction with endogenous XTcf-3. Ectopic expression of XCtBP/G4A demonstrates that XCtBP functions in the regulation of head and notochord development. Our data support a role for XCtBP as a co-repressor throughout Xenopus development and indicate that XCtBP/G4A will be a useful tool in determining how XCtBP functions in various developmental processes.

Functional and physical interactions between AML1 proteins and an ETS protein, MEF: implications for the pathogenesis of t(8;21)-positive leukemias

The AML1 and ETS families of transcription factors play critical roles in hematopoiesis; AML1, and its non-DNA-binding heterodimer partner CBFbeta, are essential for the development of definitive hematopoiesis in mice, whereas the absence of certain ETS proteins creates specific defects in lymphopoiesis or myelopoiesis. The promoter activities of numerous genes expressed in hematopoietic cells are regulated by AML1 proteins or ETS proteins. MEF (for myeloid ELF-1-like factor) is a recently cloned ETS family member that, like AML1B, can strongly transactivate several of these promoters, which led us to examine whether MEF functionally or physically interacts with AML1 proteins. In this study, we demonstrate direct interactions between MEF and AML1 proteins, including the AML1/ETO fusion protein, in t(8;21)-positive acute myeloid leukemia (AML) cells. Using mutational analysis, we identified a novel ETS-interacting subdomain (EID) in the C-terminal portion of the Runt homology domain (RHD) in AML1 proteins and determined that the N-terminal region of MEF was responsible for its interaction with AML1. MEF and AML1B synergistically transactivated an interleukin 3 promoter reporter gene construct, yet the activating activity of MEF was abolished when MEF was coexpressed with AML1/ETO. The repression by AML1/ETO was independent of DNA binding but depended on its ability to interact with MEF, suggesting that AML1/ETO can repress genes not normally regulated by AML1 via protein-protein interactions. Interference with MEF function by AML1/ETO may lead to dysregulation of genes important for myeloid differentiation, thereby contributing to the pathogenesis of t(8;21) AML.

Transcriptional coregulators in development

Small differences in the levels of an extracellular signaling molecule can specify cell fate during development. Threshold responses are often determined at the level of transcription. Cell-specific and spatially localized patterns of gene expression depend on combinations of sequence-specific activators and repressors that bind to extensive cis-regulatory regions. Different mechanisms for integrating this complex regulatory information are discussed, particularly the role of coregulatory proteins, which are recruited to the DNA template by sequence-specific transcription factors. Recent studies suggest that a growing set of coactivators and corepressors mediate communication between diverse upstream regulatory proteins and the core RNA polymerase II transcription complex.

A Cbfa1-dependent genetic pathway controls bone formation beyond embryonic development

The molecular mechanisms controlling bone extracellular matrix (ECM) deposition by differentiated osteoblasts in postnatal life, called hereafter bone formation, are unknown. This contrasts with the growing knowledge about the genetic control of osteoblast differentiation during embryonic development. Cbfa1, a transcriptional activator of osteoblast differentiation during embryonic development, is also expressed in differentiated osteoblasts postnatally. The perinatal lethality occurring in Cbfa1-deficient mice has prevented so far the study of its function after birth. To determine if Cbfa1 plays a role during bone formation we generated transgenic mice overexpressing Cbfa1 DNA-binding domain (DeltaCbfa1) in differentiated osteoblasts only postnatally. DeltaCbfa1 has a higher affinity for DNA than Cbfa1 itself, has no transcriptional activity on its own, and can act in a dominant-negative manner in DNA cotransfection assays. DeltaCbfa1-expressing mice have a normal skeleton at birth but develop an osteopenic phenotype thereafter. Dynamic histomorphometric studies show that this phenotype is caused by a major decrease in the bone formation rate in the face of a normal number of osteoblasts thus indicating that once osteoblasts are differentiated Cbfa1 regulates their function. Molecular analyses reveal that the expression of the genes expressed in osteoblasts and encoding bone ECM proteins is nearly abolished in transgenic mice, and ex vivo assays demonstrated that DeltaCbfa1-expressing osteoblasts were less active than wild-type osteoblasts. We also show that Cbfa1 regulates positively the activity of its own promoter, which has the highest affinity Cbfa1-binding sites characterized. This study demonstrates that beyond its differentiation function Cbfa1 is the first transcriptional activator of bone formation identified to date and illustrates that developmentally important genes control physiological processes postnatally.

Cbfa1 isoforms exert functional differences in osteoblast differentiation

Cbfa1 is an essential transcription factor for osteoblast differentiation and bone formation. We investigated functional differences among three isoforms of Cbfa1: Type I (originally reported as Pebp2alphaA by Ogawa et al. (Ogawa, E., Maruyama, M., Kagoshima, H., Inuzuka, M., Lu, J., Satake, M., Shigesada, K., and Ito, Y. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 6859-6863), Type II (originally reported as til-1 by Stewart et al. (Stewart, M., Terry, A., Hu, M., O'Hara, M., Blyth, K., Baxter, E., Cameron, E., Onions, D. E., and Neil, J. C. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 8646-8651), and Type III (originally reported as Osf2/Cbfa1 by Ducy et al. (Ducy, P., Zhang, R., Geoffroy, V., Ridall, A. L., and Karsenty, G. (1997) Cell 89, 747-754). A reverse transcriptase-polymerase chain reaction analysis demonstrated that these isoforms were expressed in adult mouse bones. The transient transfection of Type I or Type II Cbfa1 in a mouse fibroblastic cell line, C3H10T1/2, induced the expression of alkaline phosphatase (ALP) activity. This induction was synergistically enhanced by the co-introduction of Xenopus BMP-4 cDNA. In contrast, the transient transfection of Type III cDNA induced no ALP activity. In C3H10T1/2 cells stably transfected with each isoform of Cbfa1, the gene expression of ALP was also strongly induced in cells transfected with Type I and Type II Cbfa1 but not in cells with Type III Cbfa1. Osteocalcin, osteopontin,and type I collagen gene expressions were induced or up-regulated in all of the cells stably transfected with each isoform of Cbfa1, and Type II transfected cells exhibited the highest expression level of osteocalcin gene. A luciferase reporter gene assay using a 6XOSE2-SV40 promoter (6 tandem binding elements for Cbfa1 ligated in front of the SV40 promoter sequence), a mouse osteocalcin promoter, and a mouse osteopontin promoter revealed the differences in the transcriptional induction of target genes by each Cbfa1 isoform with or without its beta-subunit. These results suggest that all three of the Cbfa1 isoforms used in the present study are involved in the stimulatory action of osteoblast differentiation, but they exert different functions in the process of osteoblast differentiation.