A proline-rich TGF-beta-responsive transcriptional activator interacts with histone H3

NFIXing Cancer: The Role of NFIX in Oxidative Stress Response and Cell Fate

NFIX, a member of the nuclear factor I (NFI) family of transcription factors, is known to be involved in muscle and central nervous system embryonic development. However, its expression in adults is limited. Similar to other developmental transcription factors, NFIX has been found to be altered in tumors, often promoting pro-tumorigenic functions, such as leading to proliferation, differentiation, and migration. However, some studies suggest that NFIX can also have a tumor suppressor role, indicating a complex and cancer-type dependent role of NFIX. This complexity may be linked to the multiple processes at play in regulating NFIX, which include transcriptional, post-transcriptional, and post-translational processes. Moreover, other features of NFIX, including its ability to interact with different NFI members to form homodimers or heterodimers, therefore allowing the transcription of different target genes, and its ability to sense oxidative stress, can also modulate its function. In this review, we examine different aspects of NFIX regulation, first in development and then in cancer, highlighting the important role of NFIX in oxidative stress and cell fate regulation in tumors. Moreover, we propose different mechanisms through which oxidative stress regulates NFIX transcription and function, underlining NFIX as a key factor for tumorigenesis.

NFIA differentially controls adipogenic and myogenic gene program through distinct pathways to ensure brown and beige adipocyte differentiation

The transcription factor nuclear factor I-A (NFIA) is a regulator of brown adipocyte differentiation. Here we show that the C-terminal 17 amino acid residues of NFIA (which we call pro#3 domain) are required for the transcriptional activity of NFIA. Full-length NFIA—but not deletion mutant lacking pro#3 domain—rescued impaired expression of PPARγ, the master transcriptional regulator of adipogenesis and impaired adipocyte differentiation in NFIA-knockout cells. Mechanistically, the ability of NFIA to penetrate chromatin and bind to the crucial Pparg enhancer is mediated through pro#3 domain. However, the deletion mutant still binds to Myod1 enhancer to repress expression of MyoD, the master transcriptional regulator of myogenesis as well as proximally transcribed non-coding RNA called ^DRReRNA, via competition with KLF5 in terms of enhancer binding, leading to suppression of myogenic gene program. Therefore, the negative effect of NFIA on the myogenic gene program is, at least partly, independent of the positive effect on PPARγ expression and its downstream adipogenic gene program. These results uncover multiple ways of action of NFIA to ensure optimal regulation of brown and beige adipocyte differentiation.

Obesity and its complications including type 2 diabetes are growing concerns worldwide. While white adipocytes generally store energy in the form of lipid, classical brown and cold- or β-adrenergic stimulation-induced beige adipocytes dissipate chemical energy in the form of heat through uncoupling protein-1 (Ucp1). Since the re-discovery of human brown and beige adipocytes, it has been considered a promising target for the treatment of obesity. During mesenchymal development, not only activation of brown/beige adipocyte gene program but also repression of muscle gene program is required to achieve thermogenic adipocyte differentiation. Previously, we identified the transcription factor nuclear factor I-A (NFIA) as a regulator of brown adipocyte differentiation. Here we show that the C-terminal 17 amino acid residues of NFIA, which we call pro#3 domain, is required for activation of adipocyte differentiation. However, the deletion mutant which lacks this domain is still able to suppress muscle gene program by repressing the expression of Myod1, which encode the master transcriptional regulator of myogenesis, MyoD. We demonstrate that NFIA activates adipogenesis and also “actively” suppresses myogenesis through distinct molecular pathways to ensure brown and beige adipocyte differentiation.

Systematic dissection of genomic features determining transcription factor binding and enhancer function

Enhancers regulate gene expression through the binding of sequence-specific transcription factors (TFs) to cognate motifs. Various features influence TF binding and enhancer function-including the chromatin state of the genomic locus, the affinities of the binding site, the activity of the bound TFs, and interactions among TFs. However, the precise nature and relative contributions of these features remain unclear. Here, we used massively parallel reporter assays (MPRAs) involving 32,115 natural and synthetic enhancers, together with high-throughput in vivo binding assays, to systematically dissect the contribution of each of these features to the binding and activity of genomic regulatory elements that contain motifs for PPARγ, a TF that serves as a key regulator of adipogenesis. We show that distinct sets of features govern PPARγ binding vs. enhancer activity. PPARγ binding is largely governed by the affinity of the specific motif site and higher-order features of the larger genomic locus, such as chromatin accessibility. In contrast, the enhancer activity of PPARγ binding sites depends on varying contributions from dozens of TFs in the immediate vicinity, including interactions between combinations of these TFs. Different pairs of motifs follow different interaction rules, including subadditive, additive, and superadditive interactions among specific classes of TFs, with both spatially constrained and flexible grammars. Our results provide a paradigm for the systematic characterization of the genomic features underlying regulatory elements, applicable to the design of synthetic regulatory elements or the interpretation of human genetic variation.

Nuclear factor one transcription factors as epigenetic regulators in cancer

Tumour heterogeneity poses a distinct obstacle to therapeutic intervention. While the initiation of tumours across various physiological systems is frequently associated with signature mutations in genes that drive proliferation and bypass senescence, increasing evidence suggests that tumour progression and clonal diversity is driven at an epigenetic level. The tumour microenvironment plays a key role in driving diversity as cells adapt to demands imposed during tumour growth, and is thought to drive certain subpopulations back to a stem cell-like state. This stem cell-like phenotype primes tumour cells to react to external cues via the use of developmental pathways that facilitate changes in proliferation, migration and invasion. Because the dynamism of this stem cell-like state requires constant chromatin remodelling and rapid alterations at regulatory elements, it is of great therapeutic interest to identify the cell-intrinsic factors that confer these epigenetic changes that drive tumour progression. The nuclear factor one (NFI) family are transcription factors that play an important role in the development of many mammalian organ systems. While all four family members have been shown to act as either oncogenes or tumour suppressors across various cancer models, evidence has emerged implicating them as key epigenetic regulators during development and within tumours. Notably, NFIs have also been shown to regulate chromatin accessibility at distal regulatory elements that drive tumour cell dissemination and metastasis. Here we summarize the role of the NFIs in cancer, focusing largely on the potential mechanisms associated with chromatin remodelling and epigenetic modulation of gene expression.

Nucleosome distortion as a possible mechanism of transcription activation domain function

After more than three decades since the discovery of transcription activation domains (ADs) in gene-specific activators, the mechanism of their function remains enigmatic. The widely accepted model of direct recruitment by ADs of co-activators and basal transcriptional machinery components, however, is not always compatible with the short size yet very high degree of sequence randomness and intrinsic structural disorder of natural and synthetic ADs. In this review, we formulate the basis for an alternative and complementary model, whereby sequence randomness and intrinsic structural disorder of ADs are necessary for transient distorting interactions with promoter nucleosomes, triggering promoter nucleosome translocation and subsequently gene activation.

Nfib Promotes Metastasis through a Widespread Increase in Chromatin Accessibility

Metastases are the main cause of cancer deaths, but the mechanisms underlying metastatic progression remain poorly understood. We isolated pure populations of cancer cells from primary tumors and metastases from a genetically engineered mouse model of human small cell lung cancer (SCLC) to investigate the mechanisms that drive the metastatic spread of this lethal cancer. Genome-wide characterization of chromatin accessibility revealed the opening of large numbers of distal regulatory elements across the genome during metastatic progression. These changes correlate with copy number amplification of the Nfib locus, and differentially accessible sites were highly enriched for Nfib transcription factor binding sites. Nfib is necessary and sufficient to increase chromatin accessibility at a large subset of the intergenic regions. Nfib promotes pro-metastatic neuronal gene expression programs and drives the metastatic ability of SCLC cells. The identification of widespread chromatin changes during SCLC progression reveals an unexpected global reprogramming during metastatic progression.

Nuclear Factor I-C acts as a regulator of hepatocyte proliferation at the onset of liver regeneration

BACKGROUND & AIMS

Knockout studies of the murine Nuclear Factor I-C (NFI-C) transcription factor revealed abnormal skin wound healing and growth of its appendages, suggesting a role in controlling cell proliferation in adult regenerative processes. Liver regeneration following partial hepatectomy (PH) is a well-established regenerative model whereby changes elicited in hepatocytes lead to their rapid and phased proliferation. Although NFI-C is highly expressed in the liver, no hepatic function was yet established for this transcription factor. This study aimed to determine whether NFI-C may play a role in hepatocyte proliferation and liver regeneration.

METHODS

Liver regeneration and cell proliferation pathways following two-thirds PH were investigated in NFI-C knockout (ko) and wild-type (wt) mice.

RESULTS

We show that the absence of NFI-C impaired hepatocyte proliferation because of plasminogen activator I (PAI-1) overexpression and the subsequent suppression of urokinase plasminogen activator (uPA) activity and hepatocyte growth factor (HGF) signalling, a potent hepatocyte mitogen. This indicated that NFI-C first acts to promote hepatocyte proliferation at the onset of liver regeneration in wt mice. The subsequent transient down regulation of NFI-C, as can be explained by a self-regulatory feedback loop with transforming growth factor beta 1 (TGF-ß1), may limit the number of hepatocytes entering the first wave of cell division and/or prevent late initiations of mitosis.

CONCLUSION

NFI-C acts as a regulator of the phased hepatocyte proliferation during liver regeneration.

Extracellular deposition of matrilin-2 controls the timing of the myogenic program during muscle regeneration

Here, we identify a role for the matrilin-2 (Matn2) extracellular matrix protein in controlling the early stages of myogenic differentiation. We observed Matn2 deposition around proliferating, differentiating and fusing myoblasts in culture and during muscle regeneration in vivo. Silencing of Matn2 delayed the expression of the Cdk inhibitor p21 and of the myogenic genes Nfix, MyoD and Myog, explaining the retarded cell cycle exit and myoblast differentiation. Rescue of Matn2 expression restored differentiation and the expression of p21 and of the myogenic genes. TGF-β1 inhibited myogenic differentiation at least in part by repressing Matn2 expression, which inhibited the onset of a positive-feedback loop whereby Matn2 and Nfix activate the expression of one another and activate myoblast differentiation. In vivo, myoblast cell cycle arrest and muscle regeneration was delayed in Matn2(-/-) relative to wild-type mice. The expression levels of Trf3 and myogenic genes were robustly reduced in Matn2(-/-) fetal limbs and in differentiating primary myoblast cultures, establishing Matn2 as a key modulator of the regulatory cascade that initiates terminal myogenic differentiation. Our data thus identify Matn2 as a crucial component of a genetic switch that modulates the onset of tissue repair.

Nfix is a novel regulator of murine hematopoietic stem and progenitor cell survival

Hematopoietic stem cells are both necessary and sufficient to sustain the complete blood system of vertebrates. Here we show that Nfix, a member of the nuclear factor I (Nfi) family of transcription factors, is highly expressed by hematopoietic stem and progenitor cells (HSPCs) of murine adult bone marrow. Although short hairpin RNA-mediated knockdown of Nfix expression in Lineage(-)Sca-1(+)c-Kit(+) HSPCs had no effect on in vitro cell growth or viability, Nfix-depleted HSPCs displayed a significant loss of colony-forming potential, as well as short- and long-term in vivo hematopoietic repopulating activity. Analysis of recipient mice at 4 to 20 days posttransplant revealed that Nfix-depleted HSPCs are established in the bone marrow, but fail to persist due to increased apoptotic cell death. Gene expression profiling of Nfix-depleted HSPCs reveals that loss of Nfix expression in HSPCs is concomitant with a decrease in the expression of multiple genes known to be important for HSPCs survival, such as Erg, Mecom, and Mpl. These data reveal that Nfix is a novel regulator of HSPCs survival posttransplantation and establish a role for Nfi genes in the regulation of this cellular compartment.

Epigenetic regulatory elements associate with specific histone modifications to prevent silencing of telomeric genes

In eukaryotic cells, transgene expression levels may be limited by an unfavourable chromatin structure at the integration site. Epigenetic regulators are DNA sequences which may protect transgenes from such position effect. We evaluated different epigenetic regulators for their ability to protect transgene expression at telomeres, which are commonly associated to low or inconsistent expression because of their repressive chromatin environment. Although to variable extents, matrix attachment regions (MARs), ubiquitous chromatin opening element (UCOE) and the chicken cHS4 insulator acted as barrier elements, protecting a telomeric-distal transgene from silencing. MARs also increased the probability of silent gene reactivation in time-course experiments. Additionally, all MARs improved the level of expression in non-silenced cells, unlike other elements. MARs were associated to histone marks usually linked to actively expressed genes, especially acetylation of histone H3 and H4, suggesting that they may prevent the spread of silencing chromatin by imposing acetylation marks on nearby nucleosomes. Alternatively, an UCOE was found to act by preventing deposition of repressive chromatin marks. We conclude that epigenetic DNA elements used to enhance and stabilize transgene expression all have specific epigenetic signature that might be at the basis of their mode of action.

Nuclear Factor I genomic binding associates with chromatin boundaries

Background

The Nuclear Factor I (NFI) family of DNA binding proteins (also called CCAAT box transcription factors or CTF) is involved in both DNA replication and gene expression regulation. Using chromatin immuno-precipitation and high throughput sequencing (ChIP-Seq), we performed a genome-wide mapping of NFI DNA binding sites in primary mouse embryonic fibroblasts.

Results

We found that in vivo and in vitro NFI DNA binding specificities are indistinguishable, as in vivo ChIP-Seq NFI binding sites matched predictions based on previously established position weight matrix models of its in vitro binding specificity. Combining ChIP-Seq with mRNA profiling data, we found that NFI preferentially associates with highly expressed genes that it up-regulates, while binding sites were under-represented at expressed but unregulated genes. Genomic binding also correlated with markers of transcribed genes such as histone modifications H3K4me3 and H3K36me3, even outside of annotated transcribed loci, implying NFI in the control of the deposition of these modifications. Positional correlation between + and - strand ChIP-Seq tags revealed that, in contrast to other transcription factors, NFI associates with a nucleosomal length of cleavage-resistant DNA, suggesting an interaction with positioned nucleosomes. In addition, NFI binding prominently occurred at boundaries displaying discontinuities in histone modifications specific of expressed and silent chromatin, such as loci submitted to parental allele-specific imprinted expression.

Conclusions

Our data thus suggest that NFI nucleosomal interaction may contribute to the partitioning of distinct chromatin domains and to epigenetic gene expression regulation.

NFI ChIP-Seq and input control DNA data were deposited at Gene Expression Omnibus (GEO) repository under accession number GSE15844. Gene expression microarray data for mouse embryonic fibroblasts are on GEO accession number GSE15871.

Epigenetic regulation by Z-DNA silencer function controls cancer-associated ADAM-12 expression in breast cancer: cross-talk between MeCP2 and NF1 transcription factor family

A disintegrin and metalloprotease domain-containing protein 12 (ADAM-12) is upregulated in many human cancers and promotes cancer metastasis. Increased urinary level of ADAM-12 in breast and bladder cancers correlates with disease progression. However, the mechanism of its induction in cancer remains less understood. Previously, we reported a Z-DNA-forming negative regulatory element (NRE) in ADAM-12 that functions as a transcriptional suppressor to maintain a low-level expression of ADAM-12 in most normal cells. We now report here that overexpression of ADAM-12 in triple-negative MDA-MB-231 breast cancer cells and breast cancer tumors is likely due to a marked loss of this Z-DNA-mediated transcriptional suppression function. We show that Z-DNA suppressor operates by interaction with methyl-CpG-binding protein, MeCP2, a prominent epigenetic regulator, and two members of the nuclear factor 1 family of transcription factors, NF1C and NF1X. While this tripartite interaction is highly prevalent in normal breast epithelial cells, both in vitro and in vivo, it is significantly lower in breast cancer cells. Western blot analysis has revealed significant differences in the levels of these 3 proteins between normal mammary epithelial and breast cancer cells. Furthermore, we show, by NRE mutation analysis, that interaction of these proteins with the NRE is necessary for effective suppressor function. Our findings unveil a new epigenetic regulatory process in which Z-DNA/MeCP2/NF1 interaction leads to transcriptional suppression, loss of which results in ADAM-12 overexpression in breast cancer cells.

Stimulation of BK virus DNA replication by NFI family transcription factors

BK polyomavirus (BKV) establishes persistent, low-level, and asymptomatic infections in most humans and causes polyomavirus-associated nephropathy (PVAN) and other pathologies in some individuals. The activation of BKV replication following kidney transplantation, leading to viruria, viremia, and, ultimately, PVAN, is associated with immune suppression as well as inflammation and stress from ischemia-reperfusion injury of the allograft, but the stimuli and molecular mechanisms leading to these pathologies are not well defined. The replication of BKV DNA in cell cultures is regulated by the viral noncoding control region (NCCR) comprising the core origin and flanking sequences, to which BKV T antigen (Tag), cellular proteins, and small regulatory RNAs bind. Six nuclear factor I (NFI) binding sites occur in sequences flanking the late side of the core origin (the enhancer) of the archetype virus, and their mutation, either individually or in toto, reduces BKV DNA replication when placed in competition with templates containing intact BKV NCCRs. NFI family members interacted with the helicase domain of BKV Tag in pulldown assays, suggesting that NFI helps recruit Tag to the viral core origin and may modulate its function. However, Tag may not be the sole target of the replication-modulatory activities of NFI: the NFIC/CTF1 isotype stimulates BKV template replication in vitro at low concentrations of DNA polymerase-α primase (Pol-primase), and the p58 subunit of Pol-primase associates with NFIC/CTF1, suggesting that NFI also recruits Pol-primase to the NCCR. These results suggest that NFI proteins (and the signaling pathways that target them) activate BKV replication and contribute to the consequent pathologies caused by acute infection.

Evolutionarily conserved, growth plate zone-specific regulation of the matrilin-1 promoter: L-Sox5/Sox6 and Nfi factors bound near TATA finely tune activation by Sox9

To help uncover the mechanisms underlying the staggered expression of cartilage-specific genes in the growth plate, we dissected the transcriptional mechanisms driving expression of the matrilin-1 gene (Matn1). We show that a unique assembly of evolutionarily conserved cis-acting elements in the Matn1 proximal promoter restricts expression to the proliferative and prehypertrophic zones of the growth plate. These elements functionally interact with distal elements and likewise are capable of restricting the domain of activity of a pancartilaginous Col2a1 enhancer. The proximal elements include a Pe1 element binding the chondrogenic L-Sox5, Sox6, and Sox9 proteins, a SI element binding Nfi proteins, and an initiator Ine element binding the Sox trio and other factors. Sox9 binding to Pe1 is indispensable for functional interaction with the distal promoter. Binding of L-Sox5/Sox6 to Ine and Nfib to SI modulates Sox9 transactivation in a protein dose-dependent manner, possibly to enhance Sox9 activity in early stages of chondrogenesis and repress it at later stages. Hence, our data suggest a novel model whereby Sox and Nfi proteins bind to conserved Matn1 proximal elements and functionally interact with each other to finely tune gene expression in specific zones of the cartilage growth plate.

Nuclear factor I-C links platelet-derived growth factor and transforming growth factor beta1 signaling to skin wound healing progression

Transforming growth factor beta (TGF-beta) and platelet-derived growth factor A (PDGFAlpha) play a central role in tissue morphogenesis and repair, but their interplay remain poorly understood. The nuclear factor I C (NFI-C) transcription factor has been implicated in TGF-beta signaling, extracellular matrix deposition, and skin appendage pathologies, but a potential role in skin morphogenesis or healing had not been assessed. To evaluate this possibility, we performed a global gene expression analysis in NFI-C(-/-) and wild-type embryonic primary murine fibroblasts. This indicated that NFI-C acts mostly to repress gene expression in response to TGF-beta1. Misregulated genes were prominently overrepresented by regulators of connective tissue inflammation and repair. In vivo skin healing revealed a faster inflammatory stage and wound closure in NFI-C(-/-) mice. Expression of PDGFA and PDGF-receptor alpha were increased in wounds of NFI-C(-/-) mice, explaining the early recruitment of macrophages and fibroblasts. Differentiation of fibroblasts to contractile myofibroblasts was also elevated, providing a rationale for faster wound closure. Taken together with the role of TGF-beta in myofibroblast differentiation, our results imply a central role of NFI-C in the interplay of the two signaling pathways and in regulation of the progression of tissue regeneration.

Nuclear factor I regulates brain fatty acid-binding protein and glial fibrillary acidic protein gene expression in malignant glioma cell lines

Glial fibrillary acidic protein (GFAP), an intermediate filament protein normally found in astrocytes, and the radial glial marker brain fatty acid-binding protein (B-FABP; also known as FABP7) are co-expressed in malignant glioma cell lines and tumors. Nuclear factor I (NFI) recognition sites have been identified in the B-FABP and GFAP promoters, and transcription of both genes is believed to be regulated by NFI. Here, we study the role of the different members of the NFI family in regulating endogenous and ectopic B-FABP and GFAP gene transcription in human malignant glioma cells. We show by gel shifts that all four members of the NFI family (NFIA, NFIB, NFIC, and NFIX) bind to B-FABP and GFAP NFI consensus sites. Over-expression of NFIs, in conjunction with mutation analysis of NFI consensus sites using a reporter gene assay, supports a role for all four NFIs in the regulation of the GFAP and B-FABP genes. Knock-down of single or combined NFIs reveals promoter-dependent and promoter-context-dependent interaction patterns and suggests cross talk between the different members of the NFI family. Our data indicate that the NFI family of transcription factors plays a key role in the regulation of both the B-FABP and GFAP genes in malignant glioma cells.

A DNA sequence directed mutual transcription regulation of HSF1 and NFIX involves novel heat sensitive protein interactions

BACKGROUND

Though the Nuclear factor 1 family member NFIX has been strongly implicated in PDGFB-induced glioblastoma, its molecular mechanisms of action remain unknown. HSF1, a heat shock-related transcription factor is also a powerful modifier of carcinogenesis by several factors, including PDGFB. How HSF1 transcription is controlled has remained largely elusive.

METHODOLOGY/PRINCIPAL FINDINGS

By combining microarray expression profiling and a yeast-two-hybrid screen, we identified that NFIX and its interactions with CGGBP1 and HMGN1 regulate expression of HSF1. We found that CGGBP1 organizes a bifunctional transcriptional complex at small CGG repeats in the HSF1 promoter. Under chronic heat shock, NFIX uses CGGBP1 and HMGN1 to get recruited to this promoter and in turn affects their binding to DNA. Results show that the interactions of NFIX with CGGBP1 and HMGN1 in the soluble fraction are heat shock sensitive due to preferential localization of CGGBP1 to heterochromatin after heat shock. HSF1 in turn was found to bind to the NFIX promoter and repress its expression in a heat shock sensitive manner.

CONCLUSIONS/SIGNIFICANCE

NFIX and HSF1 exert a mutual transcriptional repressive effect on each other which requires CGG repeat in HSF1 promoter and HSF1 binding site in NFIX promoter. We unravel a unique mechanism of heat shock sensitive DNA sequence-directed reciprocal transcriptional regulation between NFIX and HSF1. Our findings provide new insights into mechanisms of transcription regulation under stress.

Transcription factor CTF1 acts as a chromatin domain boundary that shields human telomeric genes from silencing

Telomeres are associated with chromatin-mediated silencing of genes in their vicinity. However, how epigenetic markers mediate mammalian telomeric silencing and whether specific proteins may counteract this effect are not known. We evaluated the ability of CTF1, a DNA- and histone-binding transcription factor, to prevent transgene silencing at human telomeres. CTF1 was found to protect a gene from silencing when its DNA-binding sites were interposed between the gene and the telomeric extremity, while it did not affect a gene adjacent to the telomere. Protein fusions containing the CTF1 histone-binding domain displayed similar activities, while mutants impaired in their ability to interact with the histone did not. Chromatin immunoprecipitation indicated the propagation of a hypoacetylated histone structure to various extents depending on the telomere. The CTF1 fusion protein was found to recruit the H2A.Z histone variant at the telomeric locus and to restore high histone acetylation levels to the insulated telomeric transgene. Histone lysine trimethylations were also increased on the insulated transgene, indicating that these modifications may mediate expression rather than silencing at human telomeres. Overall, these results indicate that transcription factors can act to delimit chromatin domain boundaries at mammalian telomeres, thereby blocking the propagation of a silent chromatin structure.

Acetylation of EKLF is essential for epigenetic modification and transcriptional activation of the beta-globin locus

Posttranslational modifications of transcription factors provide alternate protein interaction platforms that lead to varied downstream effects. We have investigated how the acetylation of EKLF plays a role in its ability to alter the beta-like globin locus chromatin structure and activate transcription of the adult beta-globin gene. By establishing an EKLF-null erythroid line whose closed beta-locus chromatin structure and silent beta-globin gene status can be rescued by retroviral infection of EKLF, we demonstrate the importance of EKLF acetylation at lysine 288 in the recruitment of CBP to the locus, modification of histone H3, occupancy by EKLF, opening of the chromatin structure, and transcription of adult beta-globin. We also find that EKLF helps to coordinate this process by the specific association of its zinc finger domain with the histone H3 amino terminus. Although EKLF interacts equally well with H3.1 and H3.3, we find that only H3.3 is enriched at the adult beta-globin promoter. These data emphasize the critical nature of lysine acetylation in transcription factor activity and enable us to propose a model of how modified EKLF integrates coactivators, chromatin remodelers, and nucleosomal components to alter epigenetic chromatin structure and stimulate transcription.

Chromatin-dependent cooperativity between site-specific transcription factors in vivo

Accessing binding sites in DNA wrapped around histones in condensed chromatin is an obstacle that transcription factors must overcome to regulate gene expression. Here we demonstrate cooperativity between two transcription factors, the glucocorticoid receptor (GR) and nuclear factor 1 (NF1) to bind the mouse mammary tumor virus promoter organized as regular chromatin in vivo. This cooperativity is not observed when the promoter is introduced transiently into cells. Using RNA interference to deplete NF1 protein levels in the cells, we confirmed that NF1 promotes binding of GR to the promoter. Furthermore, we observed a similar synergism between GR and NF1 binding on the endogenous 11beta-hydroxysteroid dehydrogenase promoter, also regulated by GR and NF1. Our results suggest that the chromatin architecture of the promoters does not permit strong association of GR in the absence of NF1. Therefore we propose that cooperativity among DNA binding factors in binding to their cognate recognition sites in chromatin may be an important feature in the regulation of gene expression.

Chromatin remodeling complexes: ATP-dependent machines in action

Since the initial characterization of chromatin remodeling as an ATP-dependent process, many studies have given us insight into how nucleosome-remodeling complexes can affect various nuclear functions. However, the multistep DNA-histone remodeling process has not been completely elucidated. Although new studies are published on a nearly weekly basis, the nature and roles of interactions of the individual SWI/SNF- and ISWI-based remodeling complexes and DNA, core histones, and other chromatin-associated proteins are not fully understood. In addition, the potential changes associated with ATP recruitment and its subsequent hydrolysis have not been fully characterized. This review explores possible mechanisms by which chromatin-remodeling complexes are recruited to specific loci, use ATP hydrolysis to achieve actual remodeling through disruption of DNA-histone interactions, and are released from their chromatin template. We propose possible roles for ATP hydrolysis in a chromatin-release/target-scanning process that offer an alternative to or complement the often overlooked function of delivering the energy required for sliding or dislodging specific subsets of core histones.

nfi-I affects behavior and life-span in C. elegans but is not essential for DNA replication or survival

BACKGROUND

The Nuclear Factor I (one) (NFI) family of transcription/replication factors plays essential roles in mammalian gene expression and development and in adenovirus DNA replication. Because of its role in viral DNA replication NFI has long been suspected to function in host DNA synthesis. Determining the requirement for NFI proteins in mammalian DNA replication is complicated by the presence of 4 NFI genes in mice and humans. Loss of individual NFI genes in mice cause defects in brain, lung and tooth development, but the presence of 4 homologous NFI genes raises the issue of redundant roles for NFI genes in DNA replication. No NFI genes are present in bacteria, fungi or plants. However single NFI genes are present in several simple animals including Drosophila and C. elegans, making it possible to test for a requirement for NFI in multicellular eukaryotic DNA replication and development. Here we assess the functions of the single nfi-1 gene in C. elegans.

RESULTS

C. elegans NFI protein (CeNFI) binds specifically to the same NFI-binding site recognized by vertebrate NFIs. nfi-1 encodes alternatively-spliced, maternally-inherited transcripts that are expressed at the single cell stage, during embryogenesis, and in adult muscles, neurons and gut cells. Worms lacking nfi-1 survive but have defects in movement, pharyngeal pumping and egg-laying and have a reduced life-span. Expression of the muscle gene Ce titin is decreased in nfi-1 mutant worms.

CONCLUSION

NFI gene function is not needed for survival in C. elegans and thus NFI is likely not essential for DNA replication in multi-cellular eukaryotes. The multiple defects in motility, egg-laying, pharyngeal pumping, and reduced lifespan indicate that NFI is important for these processes. Reduction in Ce titin expression could affect muscle function in multiple tissues. The phenotype of nfi-1 null worms indicates that NFI functions in multiple developmental and behavioral systems in C. elegans, likely regulating genes that function in motility, egg-laying, pharyngeal pumping and lifespan maintenance.

Smads and chromatin modulation

Smad proteins are critical intracellular effector proteins and regulators of transforming growth factor type beta (TGFbeta) modulated gene transcription. They directly convey signals that initiate at ligand-bound receptor complexes and end in the nucleus with changes in programs of gene expression. Activated Smad proteins seem to recruit chromatin modifying proteins to target genes besides cooperating with DNA-bound transcription factors. We survey here the current and still emerging knowledge on Smad-binding factors, and their different mechanisms of chromatin modification in particular, in Smad-dependent TGFbeta signaling.

Alpha2(I) collagen gene regulation by protein kinase C signaling in human dermal fibroblasts

We investigated the mechanisms by which protein kinase C (PKC) regulates the expression of the α2(I) collagen gene in normal dermal fibroblasts. Reduction of PKC-α activity by treatment with Gö697-6 or by overexpression of a dominant negative (DN) mutant form decreased α2(I) collagen gene expression. This decrease required a sequence element in the collagen promoter that contains Sp1/Sp3 binding sites. Reduction of PKC-δ activity by rottlerin or overexpression of DN PKC-δ also decreased α2(I) collagen gene expression. This effect required a separate sequence element containing Sp1/Sp3-binding sites and an Ets-binding site. In both cases, point mutations within the response elements abrogated the response to PKC inhibition. Forced overexpression of Sp1 rescued the PKC inhibitor-mediated reduction in collagen protein expression. A DNA affinity precipitation assay revealed that inhibition of PKC-δ by rottlerin increased the binding activity of endogenous Fli1 and decreased that of Ets1. On the other hand, TGF-β1, which increased the expression of PKC-δ, had the opposite effect, increasing the binding activity of Ets1 and decreasing that of Fli1. Our results suggest that PKC-δ is involved in the regulation of the α2(I) collagen gene in the presence or absence of TGF-β. Alteration of the balance of Ets1 and Fli1 may be a novel mechanism regulating α2(I) collagen expression.

Reversal of the silencing of tetracycline-controlled genes requires the coordinate action of distinctly acting transcription factors

BACKGROUND

Regulation of genes transferred to eukaryotic organisms is often limited by the lack of consistent expression levels in all transduced cells, which may result in part from epigenetic gene silencing effects. This reduces the efficacy of ligand-controlled gene switches designed for somatic gene transfers such as gene therapy.

METHODS

A doxycycline-controlled transgene was stably introduced in human cells, and clones were screened for epigenetic silencing of the transgene. Various regulatory proteins were targeted to the silent transgene, to identify those that would mediate regulation by doxycycline.

RESULTS

A doxycycline-controlled minimal promoter was found to be prone to gene silencing, which prevents activation by a fusion of the bacterial TetR DNA-binding domain with the VP16 activator. DNA modification studies indicated that the silenced transgene adopts a poorly accessible chromatin structure. Several cellular transcriptional activators were found to restore an accessible DNA structure when targeted to the silent transgene, and they cooperated with Tet-VP16 to mediate regulation by doxycycline.

CONCLUSIONS

Reversal of the silencing of a tetracycline-regulated minimal promoter requires a chromatin-remodeling activity for subsequent promoter activation by the Tet-VP16 fusion protein. Thus, distinct regulatory elements may be combined to obtain long-term regulation and persistent expression of exogenous genes in eukaryotic cells.

Chromatin domain boundaries delimited by a histone-binding protein in yeast

When located next to chromosomal elements such as telomeres, genes can be subjected to epigenetic silencing. In yeast, this is mediated by the propagation of the SIR proteins from telomeres toward more centromeric regions. Particular transcription factors can protect downstream genes from silencing when tethered between the gene and the telomere, and they may thus act as chromatin domain boundaries. Here we have studied one such transcription factor, CTF-1, that binds directly histone H3. A deletion mutagenesis localized the barrier activity to the CTF-1 histone-binding domain. A saturating point mutagenesis of this domain identified several amino acid substitutions that similarly inhibited the boundary and histone binding activities. Chromatin immunoprecipitation experiments indicated that the barrier protein efficiently prevents the spreading of SIR proteins, and that it separates domains of hypoacetylated and hyperacetylated histones. Together, these results suggest a mechanism by which proteins such as CTF-1 may interact directly with histone H3 to prevent the propagation of a silent chromatin structure, thereby defining boundaries of permissive and silent chromatin domains.

Activation domains of gene-specific transcription factors: are histones among their targets?

Activation domains of promoter-specific transcription factors are critical entities involved in recruitment of multiple protein complexes to gene promoters. The activation domains often retain functionality when transferred between very diverse eukaryotic phyla, yet the amino acid sequences of activation domains do not bear any specific consensus or secondary structure. Activation domains function in the context of chromatin structure and are critical for chromatin remodeling, which is associated with transcription initiation. The mechanisms of direct and indirect recruitment of chromatin-remodeling and histone-modifying complexes, including mechanisms involving direct interactions between activation domains and histones, are discussed.Key words: activation domain, transcription, chromatin, nucleosome.

Hypoxia modulates the effects of transforming growth factor-beta isoforms on matrix-formation by primary human lung fibroblasts

Chronic hypoxia is implicated in lung fibrosis, which is characterized by enhanced deposition of extracellular matrix (ECM) molecules. Transforming growth factor-beta (TGF-beta) plays a key role in fibroblast homeostasis and is involved in disease states characterized by excessive fibrosis, such as pulmonary fibrosis. In this study, we investigated if hypoxia modulates the effects of TGF-beta on the expression of gelatinases: matrix metalloproteinase (MMP)-2 and MMP-9, interstitial collagenases: MMP-1 and MMP-13, tissue inhibitors of MMP (TIMP), collagen type I and interleukin-6 (IL-6). Primary human lung fibroblasts, established from tissue biopsies, were cultivated under normoxia or hypoxia in the presence of TGF-beta1, TGF-beta2 or TGF-beta3. Gelatinases were assessed by gelatin zymography and collagenases, TIMP, collagen type I and IL-6 by ELISA. Under normoxia fibroblasts secreted MMP-2, collagenases, TIMP, collagen type I and IL-6. TGF-betas significantly decreased MMP-1 and increased TIMP-1, IL-6 and collagen type I. Hypoxia significantly enhanced MMP-2, and collagenases. Compared to normoxia, the combination of TGF-beta and hypoxia reduced MMP-1, and further amplified the level of TIMP, IL-6, and collagen type I. Thus, in human lung fibroblasts hypoxia significantly increases the TGF-betas-induced secretion of collagen type I and may be associated to the accumulation of ECM observed in lung fibrosis.

Identification of NF1 as a silencer protein of the human adenine nucleotide translocase-2 gene

The human adenine nucleotide translocase-2 (ANT2) promoter contains a silencer region that confers partial repression on the heterologous herpes simplex virus thymidine kinase (HSVtk) promoter [Barath, P., Albert-Fournier, B., Luciakova, K., Nelson, B.D. (1999) J. Biol. Chem.274, 3378-3384]. Two sequences in the silencer (Site-2 and Site-3) are protected in the DNase I assay in vitro, and one of these is a repeated GTCCTG element previously shown to act as the active repressor element. We have now purified the DNA binding protein, and identified it using MALDI-TOF MS as a 33-kDa member of the nuclear factor 1 (NF1) family of transcription factors. NF1 purified from rat liver and HeLa cell nuclei bind to both silencer Site-2 and Site-3, resulting in a DNase I footprint identical to that obtained with purified recombinant NF1. Furthermore, transient transfection experiments with reporter constructs containing mutated silencer Site-2 and/or Site-3 show that both sites contribute to repression of the HSVtk promoter. Finally, chromatin immunoprecipitation analysis reveals that NF1 is bound to both elements on the endogenous HeLa cell ANT2 promoter. Our data support the belief that NF1 acts as a repressor when bound to silencing Site-2 and Site-3 of the ANT2 gene.

Relationship between the DNA binding domains of SMAD and NFI/CTF transcription factors defines a new superfamily of genes

Transcription factors of the SMAD family relay signals from cell surface receptors to the nucleus in response to TGF-beta related soluble factors. Members of the nuclear factor I/CAAT box binding family (NFI/CTF) have been implicated as regulators of diverse biological processes such as adenovirus replication and transcription of TGF-responsive genes. There are highly conserved DNA binding domains in SMAD and NFI/CTF transcription factors that allow sequence specific DNA binding for members of each family. However, no homology relationship has been established for the DNA binding domains present in these families. For a better understanding of the structure and evolution of SMAD genes, we carried out a sensitive PSI-BLAST database search. This revealed significant similarities between the DNA binding domains of SMADs and NFI/CTF transcription factors. Enhanced graphic matrix analysis and multiple sequence alignment of the amino acid sequences of the SMAD and NFI/CTF DNA binding domains also show that these two classes of domains share considerable structural similarity. These results strongly suggest that these two classes of factors share a homologous DNA binding domain presumably resulting from a common ancestry. In contrast, the C-terminal transcription modulation domains of both SMAD and NFI/CTF families do not show any sequence similarity. Based on the structural relationship of their DNA binding domains, we propose that the SMAD and NFI/CTF transcription factors belong to new superfamily of genes.

Glom is a novel mitochondrial DNA packaging protein in Physarum polycephalum and causes intense chromatin condensation without suppressing DNA functions

Mitochondrial DNA (mtDNA) is packed into highly organized structures called mitochondrial nucleoids (mt-nucleoids). To understand the organization of mtDNA and the overall regulation of its genetic activity within the mt-nucleoids, we identified and characterized a novel mtDNA packaging protein, termed Glom (a protein inducing agglomeration of mitochondrial chromosome), from highly condensed mt-nucleoids of the true slime mold, Physarum polycephalum. This protein could bind to the entire mtDNA and package mtDNA into a highly condensed state in vitro. Immunostaining analysis showed that Glom specifically localized throughout the mt-nucleoid. Deduced amino acid sequence revealed that Glom has a lysine-rich region with proline-rich domain in the N-terminal half and two HMG boxes in C-terminal half. Deletion analysis of Glom revealed that the lysine-rich region was sufficient for the intense mtDNA condensation in vitro. When the recombinant Glom proteins containing the lysine-rich region were expressed in Escherichia coli, the condensed nucleoid structures were observed in E. coli. Such in vivo condensation did not interfere with transcription or replication of E. coli chromosome and the proline-rich domain was essential to keep those genetic activities. The expression of Glom also complemented the E. coli mutant lacking the bacterial histone-like protein HU and the HMG-boxes region of Glom was important for the complementation. Our results suggest that Glom is a new mitochondrial histone-like protein having a property to cause intense DNA condensation without suppressing DNA functions.

Repression of the human adenine nucleotide translocase-2 gene in growth-arrested human diploid cells: the role of nuclear factor-1

Adenine nucleotide translocase-2 (ANT2) catalyzes the exchange of ATP for ADP across the mitochondrial membrane, thus playing an important role in maintaining the cytosolic phosphorylation potential required for cell growth. Expression of ANT2 is activated by growth stimulation of quiescent cells and is down-regulated when cells become growth-arrested. In this study, we address the mechanism of growth arrest repression. Using a combination of transfection, in vivo dimethyl sulfate mapping, and in vitro DNase I mapping experiments, we identified two protein-binding elements (Go-1 and Go-2) that are responsible for growth arrest of ANT2 expression in human diploid fibroblasts. Proteins that bound the Go elements were purified and identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry as members of the NF1 family of transcription factors. Chromatin immunoprecipitation analysis showed that NF1 was bound to both Go-1 and Go-2 in quiescent human diploid cells in vivo, but not in the same cells stimulated to growth by serum. NF1 binding correlated with the disappearance of ANT2 transcripts in quiescent cells. Furthermore, overexpression of NF1-A, -C, and -X in NIH3T3 cells repressed expression of an ANT2-driven reporter gene construct. Two additional putative repressor elements in the ANT2 promoter, an Sp1 element juxtaposed to the transcription start site and a silencer centered at nucleotide -332, did not appear to contribute to growth arrest repression. Thus, enhanced binding of NF1 is a key step in the growth arrest repression of ANT2 transcription. To our knowledge, this is the first report showing a role for NF1 in growth arrest.

Developmentally regulated recruitment of transcription factors and chromatin modification activities to chicken lysozyme cis-regulatory elements in vivo

Expression of the chicken lysozyme gene is upregulated during macrophage differentiation and reaches its highest level in bacterial lipopolysaccharide (LPS)-stimulated macrophages. This is accompanied by complex alterations in chromatin structure. We have previously shown that chromatin fine-structure alterations precede the onset of gene expression in macrophage precursor cells and mark the lysozyme chromatin domain for expression later in development. To further examine this phenomenon and to investigate the basis for the differentiation-dependent alterations of lysozyme chromatin, we studied the recruitment of transcription factors to the lysozyme locus in vivo at different stages of myeloid differentiation. Factor recruitment occurred in several steps. First, early-acting transcription factors such as NF1 and Fli-1 bound to a subset of enhancer elements and recruited CREB-binding protein. LPS stimulation led to an additional recruitment of C/EBPbeta and a significant change in enhancer and promoter structure. Transcription factor recruitment was accompanied by specific changes in histone modification within the lysozyme chromatin domain. Interestingly, we present evidence for a transient interaction of transcription factors with lysozyme chromatin in lysozyme-nonexpressing macrophage precursors, which was accompanied by a partial demethylation of CpG sites. This indicates that a partially accessible chromatin structure of lineage-specific genes is a hallmark of hematopoietic progenitor cells.

Getting into chromatin: how do transcription factors get past the histones?

Transcriptional activators and the general transcription machinery must gain access to DNA that in eukaryotes may be packaged into nucleosomes. In this review, I discuss this problem from the standpoint of the types of chromatin structures that these DNA-binding proteins may encounter, and the mechanisms by which they may contend with various chromatin structures. The discussion includes consideration of experiments in which chromatin structure is manipulated in vivo to confront activators with nucleosomal binding sites, and the roles of nucleosome dynamics and activation domains in facilitating access to such sites. Finally, the role of activators in facilitating access of the general transcriptional machinery to sites in chromatin is discussed.

Identification of nuclear factor 1 (NF1) as a transcriptional modulator of rat A(2A) adenosine receptor

By a combination of PCR and DNA walking technique, we isolated a 4.8-kb DNA fragment containing a 4.3 kb 5'-flanking region and a 0.5-kb 5'-untranslated region of the rat A(2A) adenosine receptor (A(2A)-R) gene. Various lengths of the 5'-flanking region of the A(2A)-R gene were inserted into an expression vector and transfected into several different cell lines for promoter analysis. Our results reveal that a consensus NF1 element (designated as A(2A)-R/NF1), located between bases -2846 and -2827 of the A(2A)-R gene, functions as a repressor for A(2A)-R promoters in the rat brain-derived type-2 astrocyte cell line (RBA2), which expresses no A(2A)-R. Electrophoretic gel mobility shift assay (EMSA) revealed that two A(2A)-R/NF1-protein complexes of RBA2 nuclear extract were formed. Supershift experiments using an anti-NF1 antibody suggest that NF1 proteins exist in both A(2A)-R/NF1-protein complexes. Furthermore, mutations in the conserved NF1 binding site of this A(2A)-R/NF1 element disturbed DNA-protein formation. Thus, NF1 proteins appear to mediate this cell line-specific suppression of A(2A)-R promoters in RBA2 cells. The importance of NF1 proteins in regulating A(2A)-R promoters was further confirmed in another cell line (Siha) which expresses no endogenous A(2A)-R. Moreover, addition of the A(2A)-R/NF1element upstream of an irrelevant thymidine kinase (TK) promoter suppressed its promoter activity in Siha cells, but not in RBA2 cells. Thus, the NF1-mediated inhibition of the A(2A)-R promoter was promoter- and cell line-specific. In summary, we have defined a distal negative element (A(2A)-R/NF1) that plays a functional role in modulating the expression of A(2A)-R.

Dynamic chromatin alterations triggered by natural and synthetic activation domains

The activation domains (ADs) of transcription activators recruit a multiplicity of enzymatic activities to gene promoters. The mechanisms by which such recruitment takes place are not well understood. Using chromatin immunoprecipitation, we demonstrate dynamic alterations in the abundance of histones H2A, H3, and H4 at promoters of genes regulated by the HSF and Gal4 activators of Saccharomyces cerevisiae. Transcriptional activation of these genes, particularly those regulated by HSF, is accompanied by a significant reduction in both acetylated and unacetylated histones at promoters and may involve the transient displacement of histone octamers. To gain insight into the function of ADs, we conducted a genetic screen to identify polypeptides that could substitute for the 340-residue C-terminal activator of HSF and rescue the temperature sensitivity caused by its deletion. We found that the ts(-) phenotype of HSF(1-493) could be complemented by peptides as short as 11 amino acids. Such peptides are enriched in acidic and hydrophobic residues, and exhibit both trans-activating and chromatin-modifying activities when fused to the Gal4 DNA-binding domain. We also demonstrate that a previously identified 14-amino acid histone H3-binding module of human CTF1/NF1, which is similar to synthetic ADs, can substitute for the HSF C-terminal activator in conferring temperature resistance and can mediate the modification of promoter chromatin structure. Possible mechanisms of AD function, including one involving direct interactions with histones, are discussed.

Nuclear factor 1 is required for both hormone-dependent chromatin remodeling and transcriptional activation of the mouse mammary tumor virus promoter

The mouse mammary tumor virus (MMTV) promoter has been used as a model to study how the glucocorticoid receptor (GR) remodels chromatin to allow other transcription factors to bind and activate transcription. To dissect the precise role of nuclear factor 1 (NF1) in chromatin remodeling and transcriptional activation, we used linker-scanning mutants of transcription factor binding sites on the MMTV promoter. We compared the NF1 mutant MMTV promoter in the context of transiently transfected templates (transient transfection) and templates organized as chromatin (stable transfection) to understand the effect of chromatin on factor binding and transcription. We show that on a transiently transfected template, mutation in the NF1 binding site reduces both basal and hormone-dependent transcription. This suggests that NF1 is required for transcription in the absence of organized chromatin. We also found that binding of NF1 on a transiently transfected template is independent of mutation in hormone response elements or the octamer transcription factor (OTF) binding site. In contrast, the binding of OTF proteins to a transiently transfected template was found to be dependent on the binding of NF1, which may imply that NF1 has a stabilizing effect on OTF binding. On a chromatin template, mutation in the NF1 binding site does not affect the positioning of nucleosomes on the promoter. We also show that in the absence of NF1 binding, GR-mediated chromatin remodeling of nucleosome B is reduced and hormone-dependent activation of transcription is abolished. Further, we demonstrate that NF1 is required for both the association of BRG1 chromatin remodeling complex and the GR on the promoter in vivo. These results suggest the novel possibility that NF1 may participate in chromatin remodeling activities in addition to directly enhancing transcription and that in the absence of its binding site the GR is unable to effectively bind the promoter and recruit the remodeling complex.

Nuclear factor 1-C2 contributes to the tissue-specific activation of a milk protein gene in the differentiating mammary gland

Members of the nuclear factor 1 (NF1) transcription factor family have been postulated to be involved in the regulation of milk genes. In this work we have been able to identify the splice variant NF1-C2 as an important member of a tissue-specific activating complex that regulates the milk gene encoding carboxyl ester lipase (CEL). Mutation of the NF1-binding site in the CEL gene promoter results in a drastic reduction of the gene expression to about 15% in mammary epithelial cells. Furthermore, we demonstrate that the NF1-C2 protein interacts with a higher affinity to the NF1-binding site in the CEL gene promoter than other NF1 family members do and that NF1-C2 in the mouse mammary gland is a phosphorylated protein. During development of the mouse mammary gland, binding of NF1-C2 to the CEL gene promoter is induced at midpregnancy, in correlation with the induction of CEL gene expression. The fact that the NF1-C2 involving complex remains throughout the lactation period and decreases during the weaning period, when the CEL gene is down-regulated, supports its importance in the regulation of CEL gene expression. To our knowledge, this is the first report identifying a specific, endogenously expressed NF1 isoform to be involved in the tissue-specific activation of a gene.

Differential role of the proline-rich domain of nuclear factor 1-C splice variants in DNA binding and transactivation

We have addressed the functional significance of the existence of several natural splice variants of NF1-C* differing in their COOH-terminal proline-rich transactivation domain (PRD) by studying their specific DNA binding and transactivation in the yeast Saccharomyces cerevisiae. These parameters yielded the intrinsic transactivation potential (ITP), defined as the activation observed with equal amounts of DNA bound protein. Exchange of 83 amino acids at the COOH-terminal end of the PRD by 16 unrelated amino acids, as found in NF1-C2, and splicing out the central region of the PRD, as found in NF1-C7, enhanced DNA binding in vivo and in vitro. However, the ITP of the splice variants NF1-C2 and NF1-C7 was found to be similar to that of the intact NF1-C1. Additional mutations showed that the ITP of NF1-C requires the synergistic action of the PRD and a novel domain encoded in exons 5 and 6. Intriguingly the carboxyl-terminal domain-like motif encoded in exons 9/10 is not essential for transactivation of a reporter with a single NF1 site but is required for activation of a reporter with six NF1 sites in tandem. Our results imply that differential splicing is used to regulate transcription by generating variants with different DNA binding affinities but similar ITPs.

Transcription factor FoxA (HNF3) on a nucleosome at an enhancer complex in liver chromatin

Nucleosome-like particles and acetylated histones occur near active promoters and enhancers, and certain transcription factors can recognize their target sites on the surface of a nucleosome in vitro; yet it has been unclear whether transcription factors can occupy target sites on nucleosomes in native chromatin. We developed a method for sequential chromatin immunoprecipitation of distinct nuclear proteins that are simultaneously cross-linked to nucleosome-sized genomic DNA segments. We find that core histone H2A co-occupies, along with the FoxA (hepatocyte nuclear factor-3) transcription factor, DNA for the albumin transcriptional enhancer in native liver chromatin, where the enhancer is active. Because histone H2A on nuclear DNA is only known to exist in nucleosomes, we conclude that transcription factors can form a stable complex on nucleosomes at an active enhancer element in vivo.

Solution structures of the N-terminal domain of histone H4

Histones, nuclear proteins that interact with DNA to form nucleosomes, are essential for both the regulation of transcription and the packaging of DNA within chromosomes. The N-terminal domain of histone H4 contains four acetylation sites at lysine residues and may play a separate role in chromatin structure from the remainder of the H4 chain. We performed circular dichroism and NMR characterization of both native (H4NTP) and acetylated (Ace-H4NTP) peptides containing N-terminal acetylation domain of histone H4 for various pH environments. Data from CD and NMR suggested that H4NTP exhibited a pH-dependent conformational change, whereas the Ace-H4NTP is insensitive to pH change. However, both peptides showed a defined structural form at acidic pH environments. The solution structure for Ace-H4NTP shows two structurally independent regions comprising residues of Leu10-Gly13 and Arg19-Leu22, demonstrating relatively well-defined turn-type structures. Our results suggest that N-terminal acetylated region of H4 prefers an extended backbone conformation at neutral pH, however, upon acetylation, the regions containing lysine residues induce structural transition, having defined structural form for its optimum function.

Spl phosphorylation induced by serum stimulates the human alpha2(I) collagen gene expression

Serum has been known to stimulate collagen production by dermal fibroblasts. As part of an ongoing study of the molecular mechanisms of collagen production, we have investigated transcriptional regulation of the human alpha2(I) collagen gene by serum in human dermal fibroblasts. Serum responsive elements were mapped by deletion analysis between bp -353 and -264, and between -148 and -108 in the alpha2(I) collagen promoter. Further functional analysis of the alpha2(I) collagen promoter containing various substitution mutations revealed that serum stimulation of this promoter is mediated equally by a GC-rich region located between bp -303 and -271 and by the TCCTCC motif located between bp -123 and -128, both of which constitute binding sites for transcription factor Spl and Sp3. No differences were observed in electrophoretic mobility shift assays between unstimulated and serum stimulated fibroblasts. The Spl inhibitor mithramycin blocked stimulation of the alpha2(I) collagen promoter activity by serum. Furthermore, immunoprecipitation analysis showed that serum stimulation increased Spl phosphorylation. In conclusion, this study characterized response elements that mediate serum stimulation of the human alpha2(I) collagen promoter and suggests that serum stimulation was mediated via Sp1/Sp3 binding sites in this promoter.

Glucocorticoid hormone-induced receptor localization to the chromatin fibers formed on injected DNA in Xenopus oocytes

Oocytes from Xenopus laevis have provided a model system for studying the dynamic changes that occur in chromatin during gene activation. We have reconstituted glucocorticoid receptor (GR) induced transcription from the mouse mammary tumor virus (MMTV) promoter by intranuclear injection of an MMTV-driven reporter and cytoplasmic injection of synthetic mRNA(GR) into Xenopus oocytes. Here we investigate the intranuclear distribution of injected DNA, which is assembled into chromatin. We show that this chromatin is organized as an intranuclear fibrous network. Unliganded GR is located in the cytosol and hormone triggers its nuclear translocation and association with the chromatin fibers. Furthermore, we analyze the intranuclear distribution of other factors involved in transcription from the MMTV promoter. Indirect immunofluorescence microscopy on cryostat-sectioned oocytes revealed that BRG1, which is a subunit of the SWI/SNF chromatin remodeling complex, as well as RNA polymerase II and recombinantly expressed Xenopus nuclear factor 1-B, are all associated with the endogenous chromosomes and the chromatin fibers formed on injected DNA. This association does not depend on specific DNA binding sites and appears to be nonspecific.

Heterochromatin formation in mammalian cells: interaction between histones and HP1 proteins

Members of the heterochromatin protein 1 (HP1) family are silencing nonhistone proteins. Here, we show that in P19 embryonal carcinoma (EC) nuclei, HP1 alpha, beta, and gamma form homo- and heteromers associated with nucleosomal core histones. In vitro, all three HP1s bind to tailed and tailless nucleosomes and specifically interact with the histone-fold of histone H3. Furthermore, HP1alpha interacts with the linker histone H1. HP1alpha binds to H3 and H1 through its chromodomain (CD) and hinge region, respectively. Interestingly, the Polycomb (Pc1/M33) CD also interacts with H3, and HP1alpha and Pc1/M33 binding to H3 is severely impaired by CD mutations known to abrogate HP1 and Polycomb silencing in Drosophila. These results define a novel function for the conserved CD and suggest that HP1 self-association and histone binding may play a crucial role in HP1-mediated heterochromatin assembly.

Tumor suppression activity of adenovirus E1a protein: anoikis and the epithelial phenotype

Adenovirus E1a proteins reverse-transform diverse human tumor cells in culture. This has stimulated interest in the arenas of clinical and basic cancer research. Clinically, cancer gene therapy trials on E1a are in progress, and drug discovery strategies based on E1a are being considered. Biologically, the effect of E1a is unique in that it overrides most or all oncogenic signaling pathways to yield nontumorigenic cells. Apparently, this is a consequence of the ability of E1a to reprogram transcription in tumor cells so as to produce an epithelial phenotype that is refractory to oncogenic growth stimulation. The molecular basis for this effect is emerging.

Repression of cytochrome P450 1A1 gene expression by oxidative stress: mechanisms and biological implications

Cytochrome P450 1A1 (CYP1A1) is a member of a multigenic family of xenobiotic-metabolizing enzymes. Beyond its usual role in the detoxification of polycyclic aromatic compounds, the activity of this enzyme can be deleterious since it can generate mutagenic metabolites and oxidative stress. The CYP1A1 gene is highly inducible by the environmental contaminants dioxin and benzo[a]pyrene. We discuss here the regulatory mechanisms that limit this induction. Several feedback loops control the activation of this gene and the subsequent potential toxicity. The oxidative repression of the CYP1A1 gene seems to play a central role in these regulations. The transcription factor Nuclear Factor I/CCAAT Transcription Factor (NFI/CTF), which is important for the transactivation of the CYP1A1 gene promoter, is particularly sensitive to oxidative stress. A critical cysteine within the transactivating domain of NFI/CTF appears to be the target of H(2)O(2). The DNA-binding domains of several transcription factors have been described as targets of oxidative stress. However, recent studies described here suggest that more attention should be given to transactivating domains that may represent biologically relevant redox targets of cellular signaling.

An activation-independent role of transcription factors in insulator function

Chromatin insulators are defined as transcriptionally neutral elements that prevent negative or positive influence from extending across chromatin to a promoter. Here we show that yeast subtelomeric anti-silencing regions behave as boundaries to telomere-driven silencing and also allow discontinuous propagation of silent chromatin. These two facets of insulator activity, boundary and silencing discontinuity, can be recapitulated by tethering various transcription activation domains to tandem sites on DNA. Importantly, we show that these insulator activities do not involve direct transcriptional activation of the reporter promoter. These findings predict that certain promoters behave as insulators and partition genomes in functionally independent domains.