Corepressor CtBP1 interacts with and specifically inhibits CBP activity

A novel CTBP1 variant in a Chinese pediatric patient with a phenotype distinct from hypotonia, ataxia, developmental delay, and tooth enamel defect syndrome

Hypotonia, Ataxia, Developmental Delay, and Tooth Enamel Defect Syndrome (HADDTS) is an exceptionally rare disorder resulting from a heterozygous variant in the C-terminal binding protein 1 (CTBP1) gene. To date, a mere two variants (14 patients) have been documented on a global scale. The aim of this study was to identify a causative CTBP1 variant in a Chinese patient, and to determine the potential pathogenicity of the identified variant. Here, Whole-exome sequencing (WES) was conducted on the proband to pinpoint the candidate variant. Following this, Sanger sequencing was employed to validate the identified candidate variant and examine its co-segregation within the available family members. Employing both in silico prediction and three-dimensional protein modeling, we conducted an analysis to assess the potential functional implications of the variant on the encoded protein. Our investigation led to the identification of a novel heterozygous variant in the CTBP1 gene, namely, c.371 C>T (p.Ser124Phe), in a Chinese patient. This case represents the first confirmed instance of such a variant in a Chinese patient. When comparing the patient's clinical symptoms with those reported in the literature, notable distinctions were observed between her primary symptoms and those associated with HADDTS. She showed other signs such as microcephaly, coarse facial features, single transverse palmar crease, visible beard, myopia, coarse toenail and skeletal anomalies. This study enriching the spectrum of genetic variants observed in different ethnic populations and expanding the phenotypic profile associated with this gene. These findings are expected to contribute to the enhancement of future variant-based screening and genetic diagnosis, while also providing further insights into the pathogenic mechanisms underlying CTBP1-related conditions.

Novel small molecules inhibit proteotoxicity and inflammation: Mechanistic and therapeutic implications for Alzheimer's Disease, healthspan and lifespan- Aging as a consequence of glycolysis

Inflammation drives many age-related, especially neurological, diseases, and likely mediates age-related proteotoxicity. For example, dementia due to Alzheimer's Disease (AD), cerebral vascular disease, many other neurodegenerative conditions is increasingly among the most devastating burdens on the American (and world) health system and threatens to bankrupt the American health system as the population ages unless effective treatments are developed. Dementia due to either AD or cerebral vascular disease, and plausibly many other neurodegenerative and even psychiatric conditions, is driven by increased age-related inflammation, which in turn appears to mediate Abeta and related proteotoxic processes. The functional significance of inflammation during aging is also supported by the fact that Humira, which is simply an antibody to the pro-inflammatory cytokine TNF-a, is the best-selling drug in the world by revenue. These observations led us to develop parallel high-throughput screens to discover small molecules which inhibit age-related Abeta proteotoxicity in a C. elegans model of AD AND LPS-induced microglial TNF-a. In the initial screen of 2560 compounds (Microsource Spectrum library) to delay Abeta proteotoxicity, the most protective compounds were, in order, phenylbutyrate, methicillin, and quetiapine, which belong to drug classes (HDAC inhibitors, beta lactam antibiotics, and tricyclic antipsychotics, respectably) already robustly implicated as promising to protect in neurodegenerative diseases, especially AD. RNAi and chemical screens indicated that the protective effects of HDAC inhibitors to reduce Abeta proteotoxicity are mediated by inhibition of HDAC2, also implicated in human AD, dependent on the HAT Creb binding protein (Cbp), which is also required for the protective effects of both dietary restriction and the daf-2 mutation (inactivation of IGF-1 signaling) during aging. In addition to methicillin, several other beta lactam antibiotics also delayed Abeta proteotoxicity and reduced microglial TNF-a. In addition to quetiapine, several other tricyclic antipsychotic drugs also delayed age-related Abeta proteotoxicity and increased microglial TNF-a, leading to the synthesis of a novel congener, GM310, which delays Abeta as well as Huntingtin proteotoxicity, inhibits LPS-induced mouse and human microglial and monocyte TNF-a, is highly concentrated in brain after oral delivery with no apparent toxicity, increases lifespan, and produces molecular responses highly similar to those produced by dietary restriction, including induction of Cbp inhibition of inhibitors of Cbp, and genes promoting a shift away from glycolysis and toward metabolism of alternate (e.g., lipid) substrates. GM310, as well as FDA-approved tricyclic congeners, prevented functional impairments and associated increase in TNF-a in a mouse model of stroke. Robust reduction of glycolysis by GM310 was functionally corroborated by flux analysis, and the glycolytic inhibitor 2-DG inhibited microglial TNF-a and other markers of inflammation, delayed Abeta proteotoxicity, and increased lifespan. These results support the value of phenotypic screens to discover drugs to treat age-related, especially neurological and even psychiatric diseases, including AD and stroke, and to clarify novel mechanisms driving neurodegeneration (e.g., increased microglial glycolysis drives neuroinflammation and subsequent neurotoxicity) suggesting novel treatments (selective inhibitors of microglial glycolysis).

Estrogen receptor alpha regulates the expression of adipogenic genes genetically and epigenetically in rat bone marrow-derived mesenchymal stem cells

Regulation of the efficacy of epigenetic modifiers is regarded as an important control mechanism in the determination and differentiation of stem cell fate. Studies are showing that the effect of estrogen is important in the differentiation of mesenchymal stem cells (MSCs) into adipocytes, osteocytes, and chondrocytes. Activation of certain transcription factors and epigenetic modifications in related genes play an active role in the initiation and completion of adipogenic differentiation. Understanding the role of estrogen in diseases such as obesity, which increases with the onset of menopause, will pave the way for more effective use of estrogen as a therapeutic option. Demonstration of the differentiation tendencies of MSCs change in the presence/absence of estrogen, especially the evaluation of reversible epigenetic changes, will provide valuable information for clinical applications. In this study, the effect of estrogen on the expression of genes involved in adipogenic differentiation of MSCs and accompanying epigenetic modifications was investigated. Our results showed that estrogen affects the expression of adipogenesis-related transcription factors such as PPARy, C/EBPα and Adipsin. In addition, after estrogen treatment, increased accumulation of estrogen receptor alpha (ERα) and repressive epigenetic markers such as H3K27me2 and H3K27me3 were observed on the promoter of given transcription factors. By using co-immunoprecipitation experiments we were also able to show that ERα physically interacts with the zeste homolog 2 (EZH2) H3K27 methyltransferase in MSCs. We propose that the increase of H3K27me2 and H3K27me3 markers on adipogenic genes upon estrogen treatment may be mediated by the direct interaction of ERα and EZH2. Taken together, these findings suggest that estrogen has a role as an epigenetic switcher in the regulation of H3K27 methylation leading to suppression of adipogenic differentiation of MSC.

The transrepression and transactivation roles of CtBPs in the pathogenesis of different diseases

Gene transcription is strictly controlled by transcriptional complexes, which are assemblies of transcription factors, transcriptional regulators, and co-regulators. Mammalian genomes encode two C-terminal-binding proteins (CtBPs), CtBP1 and CtBP2, which are both well-known transcriptional corepressors of oncogenic processes. Their overexpression in tumors is associated with malignant behavior, such as uncontrolled cell proliferation, migration, and invasion, as well as with an increase in the epithelial-mesenchymal transition. CtBPs coordinate with other transcriptional regulators, such as histone deacetylases (HDACs) and histone acetyltransferases (p300 and CBP [CREBP-binding protein]) that contain the PXDLS motif, and with transcription factors to assemble transcriptional complexes that dock onto the promoters of genes to initiate gene transcription. Emerging evidence suggests that CtBPs function as both corepressors and coactivators in different biological processes ranging from apoptosis to inflammation and osteogenesis. Therapeutic targeting of CtBPs or the interactions required to form transcriptional complexes has also shown promising effects in preventing disease progression. This review summarizes the most recent progress in the study of CtBP functions and therapeutic inhibitors in different biological processes. This knowledge may enable a better understanding of the complexity of the roles of CtBPs, while providing new insights into therapeutic strategies that target CtBPs.

Downregulation of HERC5 E3 ligase attenuates the ubiquitination of CtBP1 to inhibit apoptosis in colorectal cancer cells

The Homologous to E6AP C-terminus (HECT) domain and RCC1-like domain-containing (HERC) proteins can function as tumour suppressors and as oncogenes, depending on the cancer type. However, the expression patterns of HERCs in colorectal cancer (CRC) cells are unclear. Here, we show that only HERC1 and HERC5 are downregulated in CRC tumours, and we focus our study on revealing HERC5-mediating signalling because the change in downregulation is much more obvious for HERC5 than for HERC1. We demonstrate that HERC5 recruits an adaptor protein, CREB binding protein (CRB), to ubiquitinate C-terminal binding protein 1 (CtBP1) in noncancerous colon cells. The downregulation of HERC5 in CRC cells attenuates the ubiquitination of CtBP1, which then accumulates and assembles into a transcriptional complex with histone deacetylase 1 (HDAC1) and a transcription factor c-MYC. This transcriptional complex binds to the promoters of three proapoptotic genes, Bcl2 associated X (BAX), Bcl2 interacting killer (BIK) and p53upregulated modulator of apoptosis (PUMA), and inhibits their expression, thereby suppressing apoptotic signalling and promoting tumourigenesis. Overexpression of HERC5, downregulation of CtBP1 or blocking of the CtBP1 function with its inhibitors (NSC95397 and 4-methylthio-2-oxobutyric acid [MTOB]) significantly prevents CRC cell proliferation in vitro and tumour growth in vivo. Combining NSC95397 (or MTOB) with chemotherapeutic drugs (oxaliplatin or capecitabine) gives a much stronger inhibition of cell proliferation and tumour growth compared to their single treatments. Collectively, our results reveal that downregulation of HERC5 E3 ligase attenuates the ubiquitination of CtBP1 to inhibit apoptosis. Therefore, CtBP1 may be a promising target in CRC chemotherapy.

Acetyl-CoA Metabolism and Histone Acetylation in the Regulation of Aging and Lifespan

Acetyl-CoA is a metabolite at the crossroads of central metabolism and the substrate of histone acetyltransferases regulating gene expression. In many tissues fasting or lifespan extending calorie restriction (CR) decreases glucose-derived metabolic flux through ATP-citrate lyase (ACLY) to reduce cytoplasmic acetyl-CoA levels to decrease activity of the p300 histone acetyltransferase (HAT) stimulating pro-longevity autophagy. Because of this, compounds that decrease cytoplasmic acetyl-CoA have been described as CR mimetics. But few authors have highlighted the potential longevity promoting roles of nuclear acetyl-CoA. For example, increasing nuclear acetyl-CoA levels increases histone acetylation and administration of class I histone deacetylase (HDAC) inhibitors increases longevity through increased histone acetylation. Therefore, increased nuclear acetyl-CoA likely plays an important role in promoting longevity. Although cytoplasmic acetyl-CoA synthetase 2 (ACSS2) promotes aging by decreasing autophagy in some peripheral tissues, increased glial AMPK activity or neuronal differentiation can stimulate ACSS2 nuclear translocation and chromatin association. ACSS2 nuclear translocation can result in increased activity of CREB binding protein (CBP), p300/CBP-associated factor (PCAF), and other HATs to increase histone acetylation on the promoter of neuroprotective genes including transcription factor EB (TFEB) target genes resulting in increased lysosomal biogenesis and autophagy. Much of what is known regarding acetyl-CoA metabolism and aging has come from pioneering studies with yeast, fruit flies, and nematodes. These studies have identified evolutionary conserved roles for histone acetylation in promoting longevity. Future studies should focus on the role of nuclear acetyl-CoA and histone acetylation in the control of hypothalamic inflammation, an important driver of organismal aging.

SWI/SNF complexes act through CBP-1 histone acetyltransferase to regulate acute functional tolerance to alcohol

Background

SWI/SNF chromatin remodeling genes are required for normal acute responses to alcohol in C. elegans and are associated with alcohol use disorder in two human populations. In an effort to discover the downstream genes that are mediating this effect, we identified SWI/SNF-regulated genes in C. elegans.

Results

To identify SWI/SNF-regulated genes in adults, we compared mRNA expression in wild type and swsn-1(os22ts) worms under conditions that produce inactive swsn-1 in mature cells. To identify SWI/SNF-regulated genes in neurons, we compared gene expression in swsn-9(ok1354) null mutant worms that harbor a neuronal rescue or a control construct. RNA sequencing was performed to an average depth of 25 million reads per sample using 50-base, paired-end reads. We found that 6813 transcripts were significantly differentially expressed between swsn-1(os22ts) mutants and wild-type worms and 2412 transcripts were significantly differentially expressed between swsn-9(ok1354) mutants and swsn-9(ok1354) mutants with neuronal rescue. We examined the intersection between these two datasets and identified 603 genes that were differentially expressed in the same direction in both comparisons; we defined these as SWI/SNF-regulated genes in neurons and in adults. Among the differentially expressed genes was cbp-1, a C. elegans homolog of the mammalian CBP/p300 family of histone acetyltransferases. CBP has been implicated in the epigenetic regulation in response to alcohol in animal models and a polymorphism in the human CBP gene, CREBBP, has been associated with alcohol-related phenotypes. We found that cbp-1 is required for the development of acute functional tolerance to alcohol in C. elegans.

Conclusions

We identified 603 transcripts that were regulated by two different SWI/SNF complex subunits in adults and in neurons. The SWI/SNF-regulated genes were highly enriched for genes involved in membrane rafts, suggesting an important role for this membrane microdomain in the acute alcohol response. Among the differentially expressed genes was cbp-1; CBP-1 homologs have been implicated in alcohol responses across phyla and we found that C. elegans cbp-1 was required for the acute alcohol response in worms.

C-terminal of E1A binding protein 1 enhances the migration of gastric epithelial cells and has a clinicopathologic significance in human gastric carcinoma

Background

Recent studies have claimed that the C-terminal of E1A binding proteins (CtBPs) influence tumorigenesis through participating in cell signal transduction in various human tumors. However, the detailed expression profiles of CtBP isoforms in human gastric cancer (GC) and the molecular mechanisms of CtBP involvement in tumor cell phenotypes warrant further investigation.

Materials and methods

The expression of CtBPs in GC cell lines and a human gastric epithelial cell line were explored via RT-qPCR and Western blotting assays. Moreover, the expression profiles of CtBPs in GC and histologically noncancerous tissues were explored by immunohistochemistry. To explore the effects of CtBP1 on the metastatic phenotype in GC, gastric epithelial cells were transfected with a eukaryotic expression plasmid to overexpress CTBP1, and the endogenous CtBP1 or JAK1 in GC cells was silenced through an RNA interference (RNAi) method. These transfections were validated via Western blotting, and the activation state of the JAK1/Stat3 signaling pathway was also explored via Western blotting. Furthermore, the malignant phenotype of GC cells was evaluated via a Cell Counting Kit-8 (CCK8) assay, colony formation assay, transwell assay, and wound-healing experiment.

Results

Our data revealed that the expression of CtBP1, but not CTBP2, was upregulated in 102 GC tissue samples compared with 98 noncancerous tissue samples, and the elevated expression level of CtBP1 was notably associated with distant metastasis. CTBP1 modulated cell migration and invasion through the JAK1/Stat3 signaling pathway in gastric epithelial cells. In addition, genetic silence of CtBP1 expression in GC cells notably constrained cell proliferation, invasion and migration abilities through inhibiting the activation of the JAK1/Stat3 pathway in GC cells.

Conclusion

Our data reveal that the knockout of CtBP1 notably constrains distant metastasis in GC through the JAK1/Stat3 pathway, suggesting that targeting CtBP1 is a practical anti-tumor approach to restrain tumor progression in GC.

C-terminal of E1A binding protein 2 promotes the malignancy of osteosarcoma cells via JAK1/Stat3 signaling

Previous studies have demonstrated that the C-terminal of E1A binding proteins (CtBPs) influences tumorigenesis by participating in cell signal transduction in various human malignancies. However, the detailed expression patterns of CtBP isoforms in human osteosarcoma (OS) and the molecular mechanisms of CtBP involvement in tumor cell phenotypes requires further investigation. In the present study, the expression patterns of CtBP2 in OS cells and tissues were explored by immunohistochemistry. Fetal osteoblast cells were transfected with a eukaryotic expression plasmid to overexpress CtBP2, and the endogenous CtBP2 in OS cells was silenced via a short hairpin RNA. These transfections were validated and the phosphorylation levels of the JAK1/Stat3 signaling pathway were explored via western blotting. Furthermore, the malignant phenotype of OS cells was evaluated via a Cell Counting Kit-8 assay, cell colony formation assay, cell migration assay and scratch wound healing assay. The results revealed that the expression of CtBP2, but not CtBP1, was upregulated in OS tissue samples and the elevated expression level of CtBP2 was notably associated with distant metastasis. CtBP2 was demonstrated to modulate cell migration and invasion via JAK1/Stat3 signaling pathway in fetal osteoblast cells. In addition, genetic silencing of CtBP2 expression in OS cells notably reduced cell migration abilities and the phosphorylation of the JAK1/Stat3 pathway. In summary, the present studies revealed that the loss of CtBP2 constrained distant metastasis through the JAK1/Stat3 pathway in OS, suggesting that targeting CtBP2 may be a practical anti-tumor approach to prevent OS tumor progression.

The Role of CtBP1 in Oncogenic Processes and Its Potential as a Therapeutic Target

Transcriptional corepressor proteins have emerged as an important facet of cancer etiology. These corepressor proteins are often altered by loss- or gain-of-function mutations, leading to transcriptional imbalance. Thus, research directed at expanding our current understanding of transcriptional corepressors could impact the future development of new cancer diagnostics, prognostics, and therapies. In this review, our current understanding of the CtBP corepressors, and their role in both development and disease, is discussed in detail. Importantly, the role of CtBP1 overexpression in adult tissues in promoting the progression of multiple cancer types through their ability to modulate the transcription of developmental genes ectopically is explored. CtBP1 overexpression is known to be protumorigenic and affects the regulation of gene networks associated with "cancer hallmarks" and malignant behavior, including increased cell survival, proliferation, migration, invasion, and the epithelial-mesenchymal transition. As a transcriptional regulator of broad developmental processes capable of promoting malignant growth in adult tissues, therapeutically targeting the CtBP1 corepressor has the potential to be an effective method for the treatment of diverse tumor types. Although efforts to develop CtBP1 inhibitors are still in the early stages, the current progress and the future perspectives of therapeutically targeting this transcriptional corepressor are also discussed. Mol Cancer Ther; 16(6); 981-90. ©2017 AACR.

Epigenetic mechanisms underlying lifespan and age-related effects of dietary restriction and the ketogenic diet

Aging constitutes the central risk factor for major diseases including many forms of cancer, neurodegeneration, and cardiovascular diseases. The aging process is characterized by both global and tissue-specific changes in gene expression across taxonomically diverse species. While aging has historically been thought to entail cell-autonomous, even stochastic changes, recent evidence suggests that modulation of this process can be hierarchal, wherein manipulations of nutrient-sensing neurons (e.g., in the hypothalamus) produce peripheral effects that may modulate the aging process itself. The most robust intervention extending lifespan, plausibly impinging on the aging process, involves different modalities of dietary restriction (DR). Lifespan extension by DR is associated with broad protection against diseases (natural and engineered). Here we review potential epigenetic processes that may link lifespan to age-related diseases, particularly in the context of DR and (other) ketogenic diets, focusing on brain and hypothalamic mechanisms.

Role of the hypothalamus in mediating protective effects of dietary restriction during aging

Dietary restriction (DR) can extend lifespan and reduce disease burden across a wide range of animals and yeast but the mechanisms mediating these remarkably protective effects remain to be elucidated despite extensive efforts. Although it has generally been assumed that protective effects of DR are cell-autonomous, there is considerable evidence that many whole-body responses to nutritional state, including DR, are regulated by nutrient-sensing neurons. In this review, we explore the hypothesis that nutrient sensing neurons in the ventromedial hypothalamus hierarchically regulate the protective responses of dietary restriction. We describe multiple peripheral responses that are hierarchically regulated by the hypothalamus and we present evidence for non-cell autonomous signaling of dietary restriction gathered from a diverse range of models including invertebrates, mammalian cell culture, and rodent studies.

CBP histone acetyltransferase activity regulates embryonic neural differentiation in the normal and Rubinstein-Taybi syndrome brain

Increasing evidence indicates that epigenetic changes regulate cell genesis. Here, we ask about neural precursors, focusing on CREB binding protein (CBP), a histone acetyltransferase that, when haploinsufficient, causes Rubinstein-Taybi syndrome (RTS), a genetic disorder with cognitive dysfunction. We show that neonatal cbp(+/-) mice are behaviorally impaired, displaying perturbed vocalization behavior. cbp haploinsufficiency or genetic knockdown with siRNAs inhibited differentiation of embryonic cortical precursors into all three neural lineages, coincident with decreased CBP binding and histone acetylation at promoters of neuronal and glial genes. Inhibition of histone deacetylation rescued these deficits. Moreover, CBP phosphorylation by atypical protein kinase C zeta was necessary for histone acetylation at neural gene promoters and appropriate differentiation. These data support a model in which environmental cues regulate CBP activity and histone acetylation to control neural precursor competency to differentiate, and indicate that cbp haploinsufficiency disrupts this mechanism, thereby likely causing cognitive dysfunction in RTS.

Estrogen receptor alpha represses transcription of early target genes via p300 and CtBP1

The regulation of gene expression by nuclear receptors controls the phenotypic properties and diverse biologies of target cells. In breast cancer cells, estrogen receptor alpha (ERalpha) is a master regulator of transcriptional stimulation and repression, yet the mechanisms by which agonist-bound ERalpha elicits repression are poorly understood. We analyzed early estrogen-repressed genes and found that ERalpha is recruited to ERalpha binding sites of these genes, albeit more transiently and less efficiently than for estrogen-stimulated genes. Of multiple cofactors studied, only p300 was recruited to ERalpha binding sites of repressed genes, and its knockdown prevented estrogen-mediated gene repression. Because p300 is involved in transcription initiation, we tested whether ERalpha might be trying to stimulate transcription at repressed genes, with ultimately failure and a shift to a repressive program. We found that estrogen increases transcription in a rapid but transient manner at early estrogen-repressed genes but that this is followed by recruitment of the corepressor CtBP1, a p300-interacting partner that plays an essential role in the repressive process. Thus, at early estrogen-repressed genes, ERalpha initiates transient stimulation of transcription but fails to maintain the transcriptional process observed at estrogen-stimulated genes; rather, it uses p300 to recruit CtBP1-containing complexes, eliciting chromatin modifications that lead to transcriptional repression.

EVI1 recruits the histone methyltransferase SUV39H1 for transcription repression

EVI1 is an oncoprotein inappropriately expressed in acute myeloid leukemia and myelodysplastic syndrome cells. In vitro studies indicate that diverse biological properties can be attributed to this protein. Its role in leukemogenesis is still unclear but it is thought that overall EVI1 can act mostly as a transcription repressor through its interaction with a subset of histone deacetylases. Studies with histone deacetylase inhibitors have however indicated that EVI1-mediated repression can be only partially rescued by deacetylase inhibitor drugs, suggesting that additional chromosomal modifications might occur to induce gene repression by EVI1. To investigate whether histone methylation contributes to the repressive potential of EVI1, we examined a potential association between EVI1, the histone methyltransferase (HMT) SUV39H1, and methyltransferase activity in vitro. We find that EVI1 directly interacts with SUV39H1 and that the proteins form an active complex with methyltransferase activity in vitro. Our data indicate that SUV39H1 enhances the transcription repressive potential of EVI1 in vivo. We suggest that EVI1 affects promoters' activity in two different pathways, by association with histone deacetylases and by recruiting chromatin-modifying enzymes to impose a heterochromatin-like structure establishing a lasting transcription repression.

Subversion of CtBP1-controlled macropinocytosis by human adenovirus serotype 3

Endocytosis supports cell communication, growth, and pathogen infection. The species B human adenovirus serotype 3 (Ad3) is associated with epidemic conjunctivitis, and fatal respiratory and systemic disease. Here we show that Ad3 uses dynamin-independent endocytosis for rapid infectious entry into epithelial and haematopoietic cells. Unlike Ad5, which uses dynamin-dependent endocytosis, Ad3 endocytosis spatially and temporally coincided with enhanced fluid-phase uptake. It was sensitive to macropinocytosis inhibitors targeting F-actin, protein kinase C, the sodium-proton exchanger, and Rac1 but not Cdc42. Infectious Ad3 macropinocytosis required viral activation of p21-activated kinase 1 (PAK1) and the C-terminal binding protein 1 of E1A (CtBP1), recruited to macropinosomes. These macropinosomes also contained the Ad3 receptors CD46 and alpha v integrins. CtBP1 is a phosphorylation target of PAK1, and is bifunctionally involved in membrane traffic and transcriptional repression of cell cycle, cancer, and innate immunity pathways. Phosphorylation-defective S147A-CtBP1 blocked Ad3 but not Ad5 infection, providing a direct link between PAK1 and CtBP1. The data show that viruses induce macropinocytosis for infectious entry, a pathway used in antigen presentation and cell migration.

Inhibition of agouti-related peptide expression by glucose in a clonal hypothalamic neuronal cell line is mediated by glycolysis, not oxidative phosphorylation

The regulation of neuroendocrine electrical activity and gene expression by glucose is mediated through several distinct metabolic pathways. Many studies have implicated AMP and ATP as key metabolites mediating neuroendocrine responses to glucose, especially through their effects on AMP-activated protein kinase (AMPK), but other studies have suggested that glycolysis, and in particular the cytoplasmic conversion of nicotinamide adenine dinucleotide (NAD+) to reduced NAD (NADH), may play a more important role than oxidative phosphorylation for some effects of glucose. To address these molecular mechanisms further, we have examined the regulation of agouti-related peptide (AgRP) in a clonal hypothalamic cell line, N-38. AgRP expression was induced monotonically as glucose concentrations decreased from 10 to 0.5 mm glucose and with increasing concentrations of glycolytic inhibitors. However, neither pyruvate nor 3-beta-hydroxybutyrate mimicked the effect of glucose to reduce AgRP mRNA, but on the contrary, produced the opposite effect of glucose and actually increased AgRP mRNA. Nevertheless, 3beta-hydroxybutyrate mimicked the effect of glucose to increase ATP and to decrease AMPK phosphorylation. Similarly, inhibition of AMPK by RNA interference increased, and activation of AMPK decreased, AgRP mRNA. Additional studies demonstrated that neither the hexosamine nor the pentose/carbohydrate response element-binding protein pathways mediate the effects of glucose on AgRP expression. These studies do not support that either ATP or AMPK mediate effects of glucose on AgRP in this hypothalamic cell line but support a role for glycolysis and, in particular, NADH. These studies support that cytoplasmic or nuclear NADH, uniquely produced by glucose metabolism, mediates effects of glucose on AgRP expression.

Role of the PLDLS-binding cleft region of CtBP1 in recruitment of core and auxiliary components of the corepressor complex

C-terminal binding protein (CtBP) family proteins CtBP1 and CtBP2 are highly homologous transcriptional corepressors and are recruited by a large number of transcription factors to mediate sequence-specific transcriptional repression. In addition to DNA-binding repressors, the nuclear protein complex of CtBP1 consists of enzymatic constituents such as histone deacetylases (HDAC1/2), histone methyl transferases (HMTases; G9a and GLP), and the lysine-specific demethylase (LSD1). Additionally, CtBPs also recruit the components of the sumoylation machinery. The CtBPs contain two different unique structural elements, a hydrophobic cleft, with which factors that contain motifs related to the E1A PLDLS motif bind, and a surface groove that binds with factors containing motifs related to the sequence RRTGXPPXL (RRT motif). By structure-based functional dissection of CtBP1, we show that the PLDLS-binding cleft region functions as the primary recruitment center for DNA-binding factors and for the core and auxiliary enzymatic constituents of the CtBP1 corepressor complex. We identify HDAC1/2, CoREST/LSD1, and Ubc9 (E2) as the core constituents of the CtBP1 complex, and these components interact with the PLDLS cleft region through non-PLDLS interactions. Among the CtBP core constituents, HDACs contribute predominantly to the repression activity of CtBP1. The auxiliary components include an HMTase complex (G9a/Wiz/CDYL) and two SUMO E3 ligases, HPC2 and PIAS1. The interaction of auxiliary components with CtBP1 is excluded by PLDLS (E1A)-mediated interactions. Although monomeric CtBP1 is proficient in the recruiting of both core and auxiliary components, NAD(H)-dependent dimerization is required for transcriptional repression. We also provide evidence that CtBP1 functions as a platform for sumoylation of cofactors.

Transcriptional regulation by C-terminal binding proteins

C-terminal binding protein family members function predominantly as transcriptional corepressors in association with sequence specific DNA-binding transcriptional repressors. The vertebrates have two CtBP genes while the invertebrates contain a single gene. Genetic studies indicate that the CtBP genes play pivotal roles in animal development. The vertebrate C-terminal binding proteins (CtBP1 and CtBP2) are highly related and are functionally redundant for certain developmental processes and non-redundant for others. The animal C-terminal binding proteins exhibit structural and functional similarity to d-isomer-specific 2-hydroxy acid dehydrogenases (D2-HDH). They function as dimers, recruiting transcriptional regulators through two protein-binding interfaces in each monomer. The corepressor complex of CtBP1 contains enzymatic constituents that mediate coordinated histone modification by deacetylation and methylation of histone H3-Lysine 9 and demethylation of histone H3-Lysine 4. CtBP also recruits the small ubiquitin-related modifier (SUMO) conjugating E2 enzyme UBC9 and a SUMO E3 ligase (HPC2), suggesting that CtBP-mediated transcriptional regulation may also involve SUMOylation of transcription factors. In addition to gene-specific transcriptional repression, CtBP1 appears to antagonize the activity of the global transcriptional coactivators, p300/CBP. Genetic evidence also suggests that the fly CtBP (dCtBP) and the vertebrate CtBP2 might activate transcription in a context-dependent manner. The transcriptional regulatory activity of CtBP is modulated by the nuclear NADH/NAD+ ratio and hence appears to be influenced by the metabolic status of the cell. The nuclear dinucleotide ratio may differentially influence the repression activities of factors that recruit CtBP through PLDLS-like motifs and those through non-PLDLS-motifs.

Coregulator exchange and sphingosine-sensitive cooperativity of steroidogenic factor-1, general control nonderepressed 5, p54, and p160 coactivators regulate cyclic adenosine 3',5'-monophosphate-dependent cytochrome P450c17 transcription rate

Transcription of the cytochrome P450 17 (CYP17) gene is regulated by cAMP-dependent binding of steroidogenic factor-1 (SF-1) to its promoter in the adrenal cortex. Using temporal chromatin immunoprecipitation and mammalian two-hybrid experiments, we establish the reciprocal presence of coactivators [general control nonderepressed (GCN5), cAMP response element-binding protein-binding protein, p300, p300/cAMP response element-binding protein-binding protein CBP associated factor, p160s, polypyrimidine tract associated splicing factor, and p54(nrb)], corepressors (class I histone deacetylases, receptor interacting protein, nuclear receptor corepressor, and Sin3A), and SWI/SNF (human homolog of yeast mating type switching/sucrose nonfermenting) and imitation SWI chromatin remodeling ATPases on the CYP17 promoter during transcription cycles in the H295R adrenocortical cell line. A ternary GCN5/SRC-1/SF-1 complex forms on the CYP17 promoter with cAMP-dependence within 30 min of cAMP stimulation, and corresponds with SWI/SNF chromatin remodeling. This complex is sensitive to the SF-1 antagonist sphingosine and results in decreased transcription of CYP17. GCN5 acetyltransferase activity and carboxy terminus binding proteins alternatively mediate disassembly of the complex. This work establishes the temporal order of cAMP-induced events on the promoter of a key steroidogenic gene during SF-1-mediated transcription.

Changes in C-terminal binding protein 2 (CtBP2) corepressor complex induced by E1A and modulation of E1A transcriptional activity by CtBP2

The N-terminal region of adenovirus E1A interacts with histone acetyl transferases (HATs) such as p300, P/CAF, and GCN5. The C-terminal region interacts with the transcriptional corepressors CtBP1 and CtBP2. The functional significance of co-recruitment of HATs and CtBPs by E1A is not well understood. In this study, we have shown that E1A enhanced acetylation of CtBP2 by recruitment of p300 to the CtBP2 complex. Additionally, E1A also displaced the histone methyltransferase G9a and the E-box repressor ZEB from the CtBP2 complex through the C-terminal CtBP-binding domain. A transcriptional activation function encoded by the E1A N-terminal region was efficiently inhibited by CtBP2 but not by a mutant with an N-terminal deletion or by a mutant deficient in interaction with E1A. Two isoforms of CtBP1 (CtBP1-L and CtBP1-S) poorly inhibited transcriptional activity of the E1A N-terminal region. Thus, the N-terminal domain of CtBP2 may contribute a unique transcriptional regulatory activity of CtBP2. Our results provide new insights by which CtBP might modulate the biochemical activities of E1A.

Role of the C-terminal binding protein PXDLS motif binding cleft in protein interactions and transcriptional repression

C-terminal binding proteins (CtBPs) are multifunctional proteins that can mediate gene repression. CtBPs contain a cleft that binds Pro-X-Asp-Leu-Ser (PXDLS) motifs. PXDLS motifs occur in numerous transcription factors and in effectors of gene repression, such as certain histone deacetylases. CtBPs have been depicted as bridging proteins that self-associate and link PXDLS-containing transcription factors to PXDLS-containing chromatin-modifying enzymes. CtBPs also recruit effectors that do not contain recognizable PXDLS motifs. We have investigated the importance of the PXDLS binding cleft to CtBP's interactions with various partner proteins and to its ability to repress transcription. We used CtBP cleft mutant and cleft-filled fusion derivatives to distinguish between partner proteins that bind in the cleft and elsewhere on the CtBP surface. Functional assays demonstrate that CtBP mutants that carry defective clefts retain repression activity when fused to heterologous DNA-binding domains. This result suggests that the cleft is not essential for recruiting effectors. In contrast, when tested in the absence of a fused DNA-binding domain, disruption of the cleft abrogates repression activity. These results demonstrate that the PXDLS binding cleft is functionally important but suggest that it is primarily required for localization of the CtBP complex to promoter-bound transcription factors.

Acetylation by p300 Regulates Nuclear Localization and Function of the Transcriptional Corepressor CtBP2

CtBP family members, CtBP1 and CtBP2, are unique transcriptional regulators that adapt a metabolic enzyme fold, and their activities are regulated by NAD(H)-binding. CtBP1 is both cytoplasmic and nuclear, and its subcellular localization is regulated by sumoylation, phosphorylation, and binding to a PDZ protein. In contrast, we showed that CtBP2 is exclusively nuclear. CtBP1 and CtBP2 are highly similar, but differ at the N-terminal 20 amino acid region. Substitution of the N-terminal domain of CtBP1 with the corresponding CtBP2 domain confers a dominant nuclear localization pattern to CtBP1. The N-terminal domain of CtBP2 contains three Lys residues. Our results show that these Lys residues are acetylated by the nuclear acetylase p300. Although all three Lys residues of CtBP2 (Lys-6, Lys-8, and Lys-10) appear to be acetylated, acetylation of Lys-10 is critical for nuclear localization. CtBP2 with a single amino acid substitution at Lys-10 (K10R) is predominantly localized in the cytoplasm. The cytoplasmic localization of the K10R mutant is correlated with enhanced nuclear export that is inhibited by leptomycin B. Furthermore, lack of acetylation at Lys-10 renders CtBP2 to be more efficient in repression of the E-cadherin promoter. Our studies have revealed the important roles of acetylation in regulating subcellular localization and transcriptional activity of CtBP2.