Transducin-like Enhancer Of Split Research Articles

Differentiating aortic fibrosarcoma from acute intramural hematoma

FIGURE 1. CTwith contrast shows the tumor (asterisk) in both long-axis and transverse views. The pericardial effusion is also evident in the transverse view (arrow). An 86-year-old woman with a medical history of hypertension was transferred to the Hennepin County Medical Center for severe substernal chest pain radiating to her back, associated with coughing. Physical examination revealed normal heart sounds with irregular heart rate, symmetric pulses, and intact neurologic examination. Electrocardiogram on admission showed atrial fibrillation. Chest computed tomography (CT) with contrast obtained before her transfer demonstrated soft tissue surrounding her ascending aorta, which was diagnosed as an aortic intramural hematoma (Figure 1) and a small pericardial effusion. An aortic dissection flap was not observed. Transthoracic echocardiography before emergency surgical exploration showed a soft tissue collection posterior to the ascending aorta without a dissection flap and a small pericardial effusion. On surgical exploration, a large white mass completely surrounding the distal ascending aorta and compressing the superior vena cava was observed. Biopsies from this mass were obtained showing a monomorphic spindle cell sarcoma (Figure 2). Immunostains for a variety of markers were negative, including wide spectrum cytokeratin, cytokeratin 5/6, Wilms’ tumor protein 1, Transducinlike enhancer of split 1, CD34, actin, desmin, and S-100. The morphology and immunohistochemistry were consistent with a high-grade adult-type fibrosarcoma. No other surgical intervention was performed. The patient elected to have her care at a different institution, and therefore no follow-up data are available. On the basis of the above stains, the tumor could be aortic or cardiac in origin.

TLE1 Modifies the Effects of NOD2 in the Pathogenesis of Crohn's Disease

The mechanisms by which specific mutations in NOD2/CARD15 increase the risk for Crohn's disease (CD) are unclear. We identified proteins that interact with NOD2 and investigated them by expression, genetic, and functional analyses. By using a yeast 2-hybrid screen of an intestinal epithelial library, we identified proteins that interact with NOD2 and confirmed the interactions in mammalian cells using co-immunoprecipitation. We used microarray analysis to analyze gene expression patterns in 302 intestinal biopsy samples (129 from patients with ulcerative colitis [UC], 106 with CD, and 67 controls). Eighty single-nucleotide polymorphisms within the genes that encoded 6 interacting proteins were genotyped in a discovery cohort (869 cases of inflammatory bowel disease [IBD], 885 controls) and a replication cohort (504 patients with IBD, 713 controls). We investigated interaction between transducin-like enhancer of split 1 (TLE1) and NOD2 in HEK293 cells. We identified 6 NOD2-interacting proteins (TLE1, UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 2 [GALNT2], HIV-1 Tat interactive protein [HTATIP], Vimentin, fission 1 (mitochondrial outer membrane) homolog [FIS1], and protein phosphatase 2, regulatory subunit B', epsilon isoform [PPP2R5E]). Of these, expression of GALNT2 (CD, P = .004) and vimentin (CD, P = .006; UC, P = .0025) was altered in patients with IBD compared with controls. Single-nucleotide polymorphisms within TLE1 were associated with susceptibility to CD, specifically with ileal disease (rs6559629, P = 3.1 × 10⁻⁵; odds ratio, 1.45). The TLE1 risk allele is required for susceptibility to CD in carriers of NOD2 mutations. In cells, TLE1 and NOD2 co-localized around the nuclear membrane and TLE1 inhibited activation of nuclear factor-κB by NOD2. Epistatic and biological interactions between TLE1 and NOD2 are involved in IBD pathogenesis. NOD2 might be involved in a series of pathways such as epigenetic regulation of expression (via TLE1 and HTATIP), biosynthesis of mucin (via GALNT2), apoptosis (via PPP2R5E and FIS1), and integrity of the intracellular cytoskeleton (vimentin).

Notch- and Transducin-like Enhancer of Split (TLE)-dependent Histone Deacetylation Explain Interleukin 12 (IL-12) p70 Inhibition by Zymosan

The fungal analog zymosan induces IL-23 and low amounts of IL-12 p70. This study addresses the molecular mechanisms underlying this cytokine pattern in human monocyte-derived dendritic cells. The transcriptional regulation of il23a, one of the chains of IL-23, depended on the activation of c-Rel and histone H3 phosphorylation, as judged from the association of c-Rel with the il23a promoter and the correlation between IL-23 production and Ser-10-histone H3 phosphorylation. Consistent with its reduced ability to produce IL-12 p70, zymosan induced a transient occupancy of the il12a promoter by c-Rel, blocked the production of IL-12 p70 and the transcription of il12a induced by other stimuli, and triggered the expression and nuclear translocation of the transcriptional repressors of the Notch family hairy and enhancer of split (Hes)-1, Hes5, hairy/enhancer-of-split related with YRPW motif protein (Hey)-1, and transducin-like enhancer of split (TLE). Zymosan also induced the interaction of Hes1 and TLE with histone H3 phosphorylated on Ser-10 and deacetylated on Lys-14. Inhibition of class III histone deacetylases increased the production of IL-12 p70 and partially blunted the inhibitory effect of zymosan on the production of IL-12 p70 elicited by LPS and IFN-γ. These results indicate that the selective induction of IL-23 by β-glucans is explained by the activation of c-Rel associated with Ser-10-histone H3 phosphorylation in the il23a promoter mediated by mitogen- and stress-activated kinase and/or protein kinase A and inhibition of il12a transcription by a mechanism involving activation of several corepressors with the ability to bind TLE and to promote histone deacetylation.

The role of TLE1 in synovial sarcoma

The treatment outcome of synovial sarcoma is poor owing to its resistance to radiation and chemotherapy. The transducin-like enhancer of split 1 (TLE1) is a co-repressor that involves many signaling pathways like cell survival, hematopoiesis and differentiation. Although TLE1 is uniquely expressed in synovial sarcomas, the biological role of TLE1 is not completely understood. This study evaluated the function of TLE1 in synovial sarcomas using knock-down of TLE1, and examined whether the inhibition of TLE1 suppresses the proliferation of synovial sarcomas and enhances the cytotoxicity caused by doxorubicin (doxo). The over-expression of TLE1 was first confirmed in synovial sarcoma cells (HS-SYII). When the HS-SYII cells and normal fibroblast were transiently transfected with TLE1 siRNA, the MTT assay revealed growth inhibition in the HS-SY-11 cells but not in the normal fibroblast. TLE1 silencing also potentiated the cytotoxic effects of doxo against HS-SYII cells. This effect of TLE1 silencing was attributed mainly to the induction of apoptosis. Subsequent analysis revealed that Bcl-2 is a possible downstream target of TLE1 signaling. This study demonstrated that TLE1 is a critical factor for the survival of synovial sarcomas. Overall, the inhibition of TLE1 affects cell proliferation and the apoptosis pathway by suppressing the expression of Bcl-2.

Expression, purification, crystallization, and preliminary X-ray diffraction analysis of the human TLE1 Q domain

Human transducin-like enhancer of split 1 (TLE1) plays crucial roles in a number of developmental processes and is involved in pathogenesis of malignancy tumors. The N-terminal glutamine-rich domain (Q domain) of TLE1 mediates its tetramerization and interactions with different DNA-binding transcription factors to regulate Notch and Wnt signaling pathways. To better understand the molecular mechanism of TLE1's functions in these pathways, we cloned, purified, and crystallized the TLE1 Q domain (TLE1-Q). The crystals belong to space group C222(1), with the complete diffraction data of the native and Se-Met TLE1-Q collected to 3.5 and 4.1 Å resolutions, respectively. The phasing-solving and model building are in progress.

Molecular characterization of a novel, de novo, cryptic interstitial deletion on 19p13.3 in a child with a cutis aplasia and multiple congenital anomalies

We report on a de novo constitutional deletion within G-band region 19p13.3 in a girl with cutis aplasia of the scalp, facial anomalies, structural heart abnormalities, hypotonia, mild mental retardation and conductive hearing loss which we characterized with chromosomal microarray, fluorescence in situ hybridization (FISH), and SNP analyses. Initial microarray analysis revealed a 6-BAC-clone deletion covering an approximately 1.612 Mb region within 19p13.3. Subsequent BAC FISH studies delineated the proximal deletion breakpoint to within BAC clone RP11-125C3 and the distal deletion breakpoint to within BAC clone RP11-648B14. SNP analysis showed the deletion to be of paternal origin and further refined its distal breakpoint to within a 20 kb region between rs11666694 and novel SNP2 that we identified at g.2,924,845, and its proximal deletion breakpoint to within a 22 kb region between rs35280644 and rs262562. Accordingly, the size of the deletion was revised to 1.89-1.932 Mb in length. We identified many Alu, L1, and L2 repeats, as well as SINE and LINE sequences at both deletion breakpoints. We found the deletion to encompass 71 genes, two of which appear to be good candidates for the patient's observed craniofacial and cardiac anomalies: guanine nucleotide binding protein (G protein), alpha 11 (Gq class)(GNA11), and Transducin-like Enhancer of Split 2 (E(sp1) homolog, Drosophila)(TLE2).

TLEs Regulate Myeloid Proliferation and Differentiation In Association with Modulation of NF-κB and Wnt Signaling

AbstractAbstract 4176We recently identified two members of the Groucho family of co-repressors, TLE (transducin-like enhancer of split) 1 and TLE4, as candidate tumor suppressor genes from the commonly deleted del(9q) region in acute myeloid leukemia (AML). This deletion is frequently associated with presence of the t(8;21) fusion gene AML1-ETO (RUNX1-RUNX1T1) and we have previously shown loss of these TLEs cooperates with AML1-ETO to increase survival, block differentiation, and induce myeloid cell proliferation. We have also shown that the proliferation and differentiation of several myeloid cell lines is markedly affected by modulation of TLE levels. Over-expression of TLEs induced THP-1 or HL-60 myeloid cell differentiation and blocks proliferation, while knockdown of TLEs triggers cell proliferation.We previously showed that knockdown of TLE1 and TLE4 inhibited all-trans retinoic acid (ATRA) induced differentiation of HL-60 cells. In this study we show knockdown of the TLEs also inhibits 12-O-tetradecanoyl-phorbol-13-acetate (TPA) induced differentiation of THP-1 cells, further pointing to the importance of these proteins in myeloid differentiation. Myeloid differentiation induced by TPA in THP-1 cells was associated with a transient translocation of TLE1 and TLE4 into the nucleus. We show that lowest levels of TLE1 and TLE4 are seen in the hematopoietic stem cell containing compartment of cord blood cells. We found a peak in TLE1/4 expression in early myeloid cells with lower levels in mature granulocytes. When we simultaneously knocked down both TLE1 and TLE4 we saw an expansion of the cord blood primitive progenitor population as measured by an increase of CD34+CD38- cells, 2.9 fold increase of colony forming units and 3.5 fold increase of Long-Term Culture-Initiating Cells. This data indicates low levels of TLE expression may maintain or expand undifferentiated hematopoietic progenitor cells and that a transient increase in their expression with translocation into the nucleus may trigger myeloid differentiation.We sought to further evaluate the mechanisms by which the TLEs regulate myeloid differentiation and proliferation. Using ImageStream analysis we found that TLE expression is able to inhibit activation of NFκB as evidenced by inhibition of nuclear NFκB translocation induced by lipopolysaccharide. Expression of the TLEs also reduces both of the nuclear translocation of β-catenin and total β-catenin in THP-1 cells. TLE1 inhibits LiCl-induced activation of Wnt signaling as measured by TOPFLASH. Knockdown of TLE1 and/or TLE4 in Kasumi1 cells induced expression of plakoglobin, a mediator of Wnt signaling. Blocking ATRA-induced myeloid differentiation by knocking down both TLE1 and TLE4 is associated with down-regulation of Nab2 and C/EBPe. In addition to the deletion of TLE1 and TLE4 in a subset of AMLs, epigenetic inactivation of TLE1 has also been reported in several myeloid malignancies, including AML. NF-κB and Wnt/β-catenin pathways in particular appear to be constitutively activated in AML and are believed to contribute to hematopoietic stem cell (HSC) expansion and increased survival. The loss of TLEs may contribute to the constitutive activation of these pathways in AML. Disclosures:No relevant conflicts of interest to declare.

TLE1 is a robust diagnostic biomarker for synovial sarcomas and correlates with t(X;18): Analysis of 319 cases

Introduction Genomewide expression profiling has identified a number of genes expressed at higher levels in synovial sarcoma than in other sarcomas. Our objectives in this study were (1) to test whether the differentially expressed gene, Transducin-Like Enhancer of split (TLE1) belonging to the groucho/TLE family, is also distinct on the protein level; (2) to evaluate this biomarker in a series of well-characterised synovial sarcomas on standard, full-sized tissue sections and (3) to correlate the expression of TLE1 with t(X;18) and other established biomarkers. Methods Three-hundred and eighty four spindle cell sarcomas from the German consultation and reference centre of soft tissue tumours initially suspected for synovial sarcoma were revisited. Three-hundred and nineteen of these specimens were analysed immunohistochemically using a monoclonal antibody TLE1 and standard, full-sized tissue sections. The nuclear staining was scored semiquantitatively as −, negative; +, weak; ++, moderate and +++, strong positive. Furthermore, 118 specimens among these were further analysed using FISH and/or PCR to detect t(X;18). We correlated the TLE1 expression with the t(X;18) translocation and other established biomarkers (EMA, PanCK, CK7, CD34 and BCL2). Results TLE1 expression was observed in 96% of the synovial sarcomas (score ⩾ +, 249/259) and discriminates them from other soft tissue tumours ( p < 0.001). Multivariate analysis showed that positive TLE1 staining was a statistically independent diagnostic marker. Furthermore molecular analysis showed that t(X;18) was clearly correlated with TLE1 protein expression ( p < 0.001). Conclusions Expression of TLE1 is significantly correlated with t(X;18) and may serve as a new robust diagnostic biomarker in synovial sarcomas and potential therapeutic target.

Formation and patterning of the forebrain and olfactory system by zinc‐finger genes Fezf1 and Fezf2

The zinc finger genes Fezf1 (Fez) and Fezf2 (Fez-like, Fezl, Zfp312) were initially identified as anterior neuroectoderm-specific genes in Xenopus and zebrafish. They encode transcriptional regulators containing an Engrailed homology 1 (Eh1) repressor motif, which is known to interact with Groucho/TLE (Transducin-Like Enhancer of Split)-type transcriptional co-repressors. Both Fezf1 and Fezf2 are expressed in the prospective forebrain region during early embryogenesis, and they subsequently show both overlapping and distinct expression domains in the olfactory epithelium and forebrain. Loss-of-function studies in mouse and zebrafish revealed roles for Fezf1 and Fezf2 in the development of the olfactory system and forebrain. In mice, Fezf1, expressed in olfactory sensory neurons, is required for the axonal projection of olfactory sensory neurons, and controls the layer formation of the olfactory bulb in a non-cell autonomous manner. Fezf2 is involved in the differentiation of subplate neurons and the formation of the fimbria and fornix. Fezf2 is also essential for specification of the subcerebral projection neurons in the neocortex. Fezf1 and Fezf2 control the rostro-caudal patterning of the diencephalon by repressing the caudal diencephalon fate in the rostral diencephalon in mice and zebrafish. In zebrafish, fezf2 is also required for the development of monoaminergic (dopaminergic and serotonergic) neurons in the basal forebrain.

Epigenetic Inactivation of the Groucho Homologue Gene TLE1 in Hematologic Malignancies

An undifferentiated status and the epigenetic inactivation of tumor-suppressor genes are hallmarks of transformed cells. Promoter CpG island hypermethylation of differentiating genes, however, has rarely been reported. The Groucho homologue Transducin-like Enhancer of Split 1 (TLE1) is a multitasked transcriptional corepressor that acts through the acute myelogenous leukemia 1, Wnt, and Notch signaling pathways. We have found that TLE1 undergoes promoter CpG island hypermethylation-associated inactivation in hematologic malignancies, such as diffuse large B-cell lymphoma and AML. We also observed a mutual exclusivity of the epigenetic alteration of TLE1 and the cytogenetic alteration of AML1. TLE1 reintroduction in hypermethylated leukemia/lymphoma cells causes growth inhibition in colony assays and nude mice, whereas TLE1-short hairpin RNA depletion in unmethylated cells enhances tumor growth. We also show that these effects are mediated by TLE1 transcriptional repressor activity on its target genes, such as Cyclin D1, Colony-Stimulating Factor 1 receptor, and Hairy/Enhancer of Split 1. These data suggest that TLE1 epigenetic inactivation contributes to the development of hematologic malignancies by disrupting critical differentiation and growth-suppressing pathways.

Histone deacetylase‐related protein inhibits AES‐mediated neuronal cell death by direct interaction

Histone deacetylase-related protein (HDRP), an alternatively spliced and truncated form of histone deacetylase-9 that lacks a C-terminal catalytic domain, protects neurons from death. In an effort to understand the mechanism by which HDRP mediates its neuroprotective effect, we screened for proteins in the brain that interact with HDRP by using a yeast two-hybrid assay. One of the HDRP-interacting proteins identified in this screen was amino enhancer of split (AES), a 197-amino acid protein belonging to the Groucho family. Interaction between HDRP and AES was verified by in vitro binding assays, coimmunoprecipitation, and colocalization studies. To investigate the significance of the HDRP-AES association to the regulation of neuronal survival, we used cultured cerebellar granule neurons, which undergo apoptosis when treated with low potassium (LK) medium. We found that in contrast to HDRP, whose expression is markedly reduced by LK treatment, AES expression was not appreciably altered. Forced expression of AES in healthy neurons results in cell death, an action that is blocked by the coexpression of HDRP. AES is a truncated version of larger Groucho-related proteins, one of which is transducin-like enhancer of split (TLE)-1. We found that the expression of TLE1 is reduced in LK-treated neurons and the forced expression of TLE1 blocks LK-induced neuronal death as well as death induced by AES. Our results show that AES has apoptotic activity in neurons and suggest that neuroprotection by HDRP is mediated by the inhibition of this activity through direct interaction.

Sirt1 interacts with transducin-like enhancer of split-1 to inhibit nuclear factor κB-mediated transcription

Sirt1 is an NAD+-dependent deacetylase that plays a role in cellular processes such as transcriptional regulation, stress response, longevity and apoptosis. Sirt1 deacetylates histone proteins and certain transcription factors such as p53, CTIP2 (chicken ovalbumin upstream promoter-transcription factor-interacting protein 2), FOXO (forkhead box O) and NF-kappaB (nuclear factor kappaB). To identify potential Sirt1-interacting factors, we performed a yeast two-hybrid screen. The screen identified TLE1 (transducin-like enhancer of split-1) as a possible Sirt1-interacting factor, which was then confirmed by co-immunoprecipitation. TLE1 is a non-DNA binding co-repressor for several transcriptional factors including NF-kappaB. We have demonstrated using co-transfection assays that Sirt1 and TLE1 repress NF-kappaB activity. The catalytic mutant of Sirt1, Sirt1-H363Y, and the N-terminal Sirt1 fragment (amino acids 1-270) also show similar repression activity, suggesting that the deacetylase activity of Sirt1 may not be critical for its effect on NF-kappaB activity. Furthermore, analysis in Sirt1-null MEFs (murine embryonic fibroblasts) and HeLa cells stably expressing siRNA (small interfering RNA) specific to Sirt1 or TLE1 demonstrate that both Sirt1 and TLE1 are required for negative regulation of NF-kappaB activity. Taken together, these results suggest that the interaction between Sirt1 and TLE1 is important for mediating repression of NF-kappaB activity.

The Groucho Corepressors, TLE1 and TLE4, Are Candidate Del(9q) AML Tumor Suppressor Genes That Can Complement AML1-ETO.

Deletion of the long arm of chromosome 9, del(9q), is one of the most common cytogenetic abnormalities seen with t(8;21) AML. This close association suggests that that the loss of a critical gene(s) on chr9q cooperates with the AML1-ETO (RUNX1-MTG8) fusion gene produced by t(8;21) in leukemogenesis. We recently mapped the commonly deleted region of del(9q) AML to less than 2.4 Mb and identified all the genes mapping to this region. Extensive sequence analysis of these genes failed to reveal obvious mutations in del(9q) AML samples although the expression of a number of these genes appeared low specifically in del(9q) samples (Genes Chr and Ca 44, 279–291). To help determine the critical affected genes in the del(9q) CDR we used an siRNA expression library directed against all del(9q) CDR gene transcripts in an in vitro complementation assay with AML1-ETO. Inducible expression of AML1-ETO in U937 cells (U937T-A/E, Mol Cell Biol 21, 5577–5590) leads to cell cycle arrest and apoptosis. We introduced the del(9q) CDR siRNA library into U937T-A/E cells with the goal of finding siRNAs that rescued these cells after induction of AML1-ETO expression. The most commonly recovered siRNA using flanking PCR primers was directed against TLE1. Transducin-like enhancer of split (TLE) 1 and TLE4, are two members of the Groucho family of co-repressors that map adjacent to the del(9q) CDR. We subsequently demonstrated that knockdown of either TLE1 or TLE4 with specific siRNAs were able to overcome the induced cell cycle arrest in U937T-A/E cells. We further demonstrate that knockdown of either TLE in the AML1-ETO+ Kasumi-1 cell line increased cell division, increased expression of cyclin D1 and Ki67 and reduced the G0/G1 fraction, while expression of TLE1 or TLE4 in Kasumi-1 cells induced cell cycle arrest and apoptosis. Understanding how various mutations work cooperatively to produce a malignant phenotype is one of the great challenges in oncology. The TLEs are known to inhibit NFKB as well as Wnt signaling, two pathways implicated leukemic stem cell expansion. TLE inhibition may represent an important cooperating mutation with AML11-ETO in AML that links Wnt signaling and core binding protein transcription factors. Studies are currently underway to demonstrate this cooperativity and more directly evaluate the role of TLE inhibition in leukemogenesis.

Transcriptional co-repressors of Runx2

Runx2 is an essential transcription factor for skeletal mineralization because it stimulates osteoblast differentiation of mesenchymal stem cells, promotes chondrocyte hypertrophy, and contributes to endothelial cell migration and vascular invasion of developing bones. Runx2 is also expressed during mouse embryo development in nascent mammary gland epithelium. Recent evidence implicates deregulation of Runx2 as a contributing factor in breast cancer-induced osteolysis and invasion, as well as in ectopic vascular calcification. Like other Runt domain proteins, Runx2 is a context-dependent transcriptional activator and repressor of genes that regulate cellular proliferation and differentiation. Proteins that temporally and spatially associate with Runx2 dictate these opposing transcriptional activities. Recent studies have identified several co-repressor proteins that bind to Runx2 to regulate gene expression. These co-factors include histone deacetylases (HDACs), transducin-like enhancer of split (TLE) proteins, mSin3a, and yes-associated protein (YAP). These proteins do not bind DNA themselves and appear to act by preventing Runx2 from binding DNA, altering chromatin structure, and/or by possibly blocking co-activator complexes. The nuclear localization of several of these factors is regulated by extracellular signaling events. Understanding the mechanisms whereby co-repressor proteins affect Runx2 activity during normal cellular development and tumor progression will identify new therapeutic targets for skeletal disorders such as osteoporosis and for bone metastatic cancers.

TLE Corepressors as Novel Candidate AML Tumor Suppressor Genes That Regulate Wnt Signaling, Cell Cycle Progression, and Myeloid Differentiation.

Deletion of the long arm of chromosome 9, del(9q), is one of the most common mutations associated with t(8;21) AML. Up to 50% of del(9q) AML is seen in association with t(8;21). The Runx1-ETO (AML1-ETO) fusion gene produced by t(8;21) is insufficient for leukemogenesis and loss of a critical gene(s) on chr9q apparently cooperates in leukemogenesis. We recently identified two members of the Groucho family of co-repressors, Transducin-like enhancer of split (TLE) 1 and TLE4, as candidate tumor suppressor genes, based on our mapping of the commonly deleted region in del(9q) AML. These proteins are known to inhibit Wnt signaling which has been implicated in hematopoietic stem cell renewal and they interact with hematopoietic transcription factors such as Runx1 and Pu.1. We have demonstrated that the expression of these two genes is specifically repressed in both del(9q) and t(8;21) AML samples. Knockdown of TLE1 or TLE4 mRNA using specific small interfering RNAs (siRNA) activates Wnt signaling in 293T cells as measured by TOPFLASH activity, while forced expression of TLE1 or 4 inhibits TOPFLASH. We show that siRNA against TLE4 using a specific siRNA increased cell cycle progression and cell division, while over-expression of TLE1 or TLE4 slowed cell cycle progression and lead to a pronounced growth disadvantage in THP-1 and HL60 myeloid cell lines. In addition to these effects on cell proliferation, these genes also affected myeloid cell differentiation. Over-expression of either TLE1 or TLE4 in monocytic THP-1 and promyelocytic HL-60 cell lines initiated myeloid differentiation as monitored by CD11b expression. These two genes have different effects on more terminal myeloid differentiation as TLE4, but not TLE1, was able to induce more terminal differentiation of THP-1 cells into monocytes as measured by CD14 expression, while TLE1 was able to induce the granulocytic marker CD15 in HL-60 cells. Furthermore, lentiviral delivery of siRNAs for either TLE1 or TLE4 to HL-60 cells inhibited induction of granulocytic and monocytic differentiation with all-trans retinoic acid (ATRA) or 1,25-dihydroxycholechalciferol (Vit.D3). TLE1 siRNA inhibited ATRA and Vit.D3 induced CD11b expression by more than 75%. While TLE4 siRNA decreased the induction of monocytic CD14 after Vit.D3 by 40%, TLE1 siRNA almost completely abrogated induction of CD14 with Vit.D3 or granulocytic CD15 with ATRA in HL-60 cells. Ex vivo culture of siRNA infected CD34 sorted human cord blood (CB) cells in semi-liquid media with differentiation inducing cytokines for seven days exhibited a decrease in the percentage of early CD33+ myeloid cells by 15% and 70% relative to controls in TLE1 and TLE4 siRNA infected CB cells, respectively. These results indicate that inhibition of TLE activity can promote cell cycle progression and inhibit myeloid differentiation. This study is the first to demonstrate a potential role for the TLEs in leukemogenesis and also indicates the TLEs may have non-redundant functions in myeloid differentiation. Understanding how various mutations work cooperatively to produce a malignant phenotype is one of the great challenges in oncology. The TLEs may represent an important cooperating mutation with Runx1-ETO in AML that links Wnt signaling and core binding protein transcription factors. Studies are currently underway to demonstrate this cooperativity.

The Groucho-related Gene Family Regulates the Gonadotropin-releasing Hormone Gene through Interaction with the Homeodomain Proteins MSX1 and OCT1

Gonadotropin-releasing hormone (GnRH) is exclusively expressed in a unique population of hypothalamic neurons that controls reproductive function. GnRH gene expression is highly dynamic. Its transcriptional activity is regulated in a complex spatiotemporal manner during embryonic development and postnatal life. Although a variety of transcription factors have been identified as regulators of GnRH transcription, most are promiscuous in their DNA-binding requirements, and none are solely expressed in GnRH neurons. Their specific activity is probably determined by interactions with distinct cofactors. Here we find that the Groucho-related gene (GRG) family of co-repressors is expressed in a model cell line for the GnRH neuron and co-expresses with GnRH during prenatal development. GRG proteins associate in vivo with the GnRH promoter. Furthermore, GRG proteins interact with two regulators of GnRH transcription, the homeodomain proteins MSX1 and OCT1. Co-transfection experiments indicate that GRG proteins regulate GnRH promoter activity. The long GRG forms enhance MSX1 repression and counteract OCT1 activation of the GnRH gene. In contrast, the short form, GRG5, has a dominant-negative effect on MSX1-dependent repression. Taken together, these data suggest that the dynamic switch between activation and repression of GnRH transcription is mediated by recruitment of the GRG co-regulators.

Notch Signals Inhibit the Development of Erythroid/Megakaryocytic Cells by Suppressing GATA-1 Activity through the Induction of HES1

The effects of Notch signals on the erythroid/megakaryocytic differentiation of hematopoietic cells were examined. Activation of Notch signals by the intracellular Notch1 or an estradiol-inducible form of Notch1/ER suppressed the expression of the erythroid marker glycophorin A in an erythroid/megakaryocytic cell line K562. Although Mock-transfected K562 cells underwent megakaryocytic differentiation in response to 12-O-tetradecanoylphorbol-13-acetate (TPA), estradiol-activated Notch1/ER induced apoptosis during TPA treatment in the transfectant, which was accompanied by the reduced expression of an antiapoptotic molecule Bcl-XL. Even when apoptosis was prevented by the overexpression of Bcl-XL, activated Notch signals still inhibited TPA-induced megakaryocytic differentiation. As for this mechanism, Notch1/recombination signal binding protein J-kappa-induced HES1 but not HES5 was found to inhibit the function of an erythroid/megakaryocytic lineage-specific transcription factor GATA-1. Although HES1 did not affect the DNA binding activity of GATA-1 in gel shift and chromatin immunoprecipitation assays, it directly bound to GATA-1 and dissociated a critical transcriptional cofactor, p300, from GATA-1. Furthermore, overexpressed HES1 inhibited the development of erythroid and megakaryocytic cells in colony assays. Also, the Notch ligand Jagged1 expressed on NIH3T3 cells suppressed the development of erythroid and megakaryocytic cells from cocultured Lin-Sca-1+ hematopoietic stem/progenitor cells. These results suggest that Notch1 inhibits the development of erythroid/megakaryocytic cells by suppressing GATA-1 activity through HES1.

EGFR signaling attenuates Groucho-dependent repression to antagonize Notch transcriptional output

Crosstalk between signaling pathways is crucial for the generation of complex and varied transcriptional networks. Antagonism between the EGF-receptor (EGFR) and Notch pathways in particular is well documented, although the underlying mechanism is poorly understood. The global corepressor Groucho (Gro) and its transducin-like Enhancer-of-split (TLE) mammalian homologs mediate repression by a myriad of repressors, including effectors of the Notch, Wnt (Wg) and TGF-beta (Dpp) signaling cascades. Given that there are genetic interactions between gro and components of the EGFR pathway (ref. 9 and P.H. et al., unpublished results), we tested whether Gro is at a crossroad between this and other pathways. Here we show that phosphorylation of Gro in response to MAPK activation weakens its repressor capacity, attenuating Gro-dependent transcriptional silencing by the Enhancer-of-split proteins, effectors of the Notch cascade. Thus, Gro is a new junction between signaling pathways, enabling EGFR signaling to antagonize transcriptional output by Notch and potentially other Gro-dependent pathways.

Histone Modifications in Corepressor Functions

Publisher Summary This chapter emphasizes histone modifications in corepressor functions. Histone modifications play a regulatory role in gene expression by altering higher order chromatin structures and by providing docking sites for non-histone regulatory proteins. Many corepressor complexes contain or recruit histone-modifying activities. A few examples of such interactions are described to illustrate the fundamental connections between transcriptional repression and the organization of chromatin––namely, (1) Tup1–Ssn6 and Groucho/ transducin-like enhancer of split (TLE) proteins, (2) silencing mediator of retinoic acid and thyroid hormone receptors (SMRT)/nuclear receptor corepressor (N-CoR), (3) Sin3, (4) C-terminal binding protein (CtBP), (5) retinoblastoma (Rb) protein, and (6) nucleosome remodeling and deacetylase (NuRD). Tup1–Ssn6 mediates repression of a large and diverse set of genes in Saccharomyces cerevisiae. Examples of gene classes regulated by this corepressor complex are genes that are repressed by glucose, genes that respond to hypoxia, genes induced by DNA damage, and cell type-specific genes.

The transcriptional repressor protein PRH interacts with the proteasome.

PRH (proline-rich homeodomain protein)/Hex is important in the control of cell proliferation and differentiation. We have shown previously that PRH contains two domains that can bring about transcriptional repression independently; the PRH homeodomain represses transcription by binding to TATA box sequences, whereas the proline-rich N-terminal domain can repress transcription by interacting with members of the Groucho/TLE (transducin-like enhancer of split) family of co-repressor proteins. The proteasome is a multi-subunit protein complex involved in the processing and degradation of proteins. Some proteasome subunits have been suggested to play a role in the regulation of transcription. In the present study, we show that PRH interacts with the HC8 subunit of the proteasome in the context of both 20 and 26 S proteasomes. Moreover, we show that PRH is associated with the proteasome in haematopoietic cells and that the proline-rich PRH N-terminal domain is responsible for this interaction. Whereas PRH can be cleaved by the proteasome, it does not appear to be degraded rapidly in vitro or in vivo, and the proteolytic activity of the proteasome is not required for transcriptional repression by PRH. However, proteasomal digestion of PRH can liberate truncated PRH proteins that retain the ability to bind to DNA. We discuss these findings in terms of the biological role of PRH in gene regulation and the control of cell proliferation.