TAF4 inactivation in embryonic fibroblasts activates TGF beta signalling and autocrine growth

General transcription factor TAF4 antagonizes epigenetic silencing by Polycomb to maintain intestine stem cell functions

Taf4 (TATA-box binding protein-associated factor 4) is a subunit of the general transcription factor TFIID, a component of the RNA polymerase II pre-initiation complex that interacts with tissue-specific transcription factors to regulate gene expression. Properly regulated gene expression is particularly important in the intestinal epithelium that is constantly renewed from stem cells. Tissue-specific inactivation of Taf4 in murine intestinal epithelium during embryogenesis compromised gut morphogenesis and the emergence of adult-type stem cells. In adults, Taf4 loss impacted the stem cell compartment and associated Paneth cells in the stem cell niche, epithelial turnover and differentiation of mature cells, thus exacerbating the response to inflammatory challenge. Taf4 inactivation ex vivo in enteroids prevented budding formation and maintenance and caused broad chromatin remodeling and a strong reduction in the numbers of stem and progenitor cells with a concomitant increase in an undifferentiated cell population that displayed high activity of the Ezh2 and Suz12 components of Polycomb Repressive Complex 2 (PRC2). Treatment of Taf4-mutant enteroids with a specific Ezh2 inhibitor restored buddings, cell proliferation and the stem/progenitor compartment. Taf4 loss also led to increased PRC2 activity in cells of adult crypts associated with modification of the immune/inflammatory microenvironment that potentiated Apc-driven tumorigenesis. Our results reveal a novel function of Taf4 in antagonizing PRC2-mediated repression of the stem cell gene expression program to assure normal development, homeostasis, and immune-microenvironment of the intestinal epithelium.

Single cell transcriptomics reveal trans-differentiation of pancreatic beta cells following inactivation of the TFIID subunit Taf4

Regulation of gene expression involves a complex and dynamic dialogue between transcription factors, chromatin remodelling and modification complexes and the basal transcription machinery. To address the function of the Taf4 subunit of general transcription factor TFIID in the regulation of insulin signalling, it was inactivated in adult murine pancreatic beta cells. Taf4 inactivation impacted the expression of critical genes involved in beta-cell function leading to increased glycaemia, lowered plasma insulin levels and defective glucose-stimulated insulin secretion. One week after Taf4-loss, single-cell RNA-seq revealed cells with mixed beta cell, alpha and/or delta cell identities as well as a beta cell population trans-differentiating into alpha-like cells. Computational analysis of single-cell RNA-seq defines how known critical beta cell and alpha cell determinants may act in combination with additional transcription factors and the NuRF chromatin remodelling complex to promote beta cell trans-differentiation.

Dynamic Evolution of Clonal Composition and Neoantigen Landscape in Recurrent Metastatic Melanoma with a Rare Combination of Driver Mutations

In melanoma, initiating oncogenic mutations in BRAF or NRAS are detected in premalignant lesions that accumulate additional mutations and genomic instability as the tumor evolves to the metastatic state. Here we investigate evolution of clonal composition and neoantigen landscape in an atypical melanoma displaying recurrent cutaneous lesions over a 6-year period without development of extracutaneous metastases. Whole exome sequencing of four cutaneous lesions taken during the 6-year period identified a collection of single nucleotide variants and small insertions and deletions shared among all tumors, along with progressive selection of subclones displaying fewer single nucleotide variants. Later tumors also displayed lower neoantigen burden compared to early tumors, suggesting that clonal evolution was driven, at least in part, by counter selection of subclones with high neoantigen burdens. Among the selected mutations are a missense mutation in MAP2K1 (F53Y) and an inversion on chromosome 7 generating a AKAP9-BRAF fusion. The mutant proteins cooperatively activate the MAPK signaling pathway confirming they are potential driver mutations of this tumor. We therefore describe the long-term genetic evolution of cutaneous metastatic melanoma characterized by an unexpected phenotypic stability and neoantigen-driven clonal selection.

Targeted alternative splicing of TAF4: a new strategy for cell reprogramming

Reprogramming of somatic cells has become a versatile tool for biomedical research and for regenerative medicine. In the current study, we show that manipulating alternative splicing (AS) is a highly potent strategy to produce cells for therapeutic applications. We demonstrate that silencing of hTAF4-TAFH activity of TAF4 converts human facial dermal fibroblasts to melanocyte-like (iMel) cells. iMel cells produce melanin and express microphthalmia-associated transcription factor (MITF) and its target genes at levels comparable to normal melanocytes. Reprogramming of melanoma cells by manipulation with hTAF4-TAFH activity upon TAFH RNAi enforces cell differentiation towards chondrogenic pathway, whereas ectoptic expression of TAF4 results in enhanced multipotency and neural crest-like features in melanoma cells. In both cell states, iMels and cancer cells, hTAF4-TAFH activity controls migration by supporting E- to N-cadherin switches. From our data, we conclude that targeted splicing of hTAF4-TAFH coordinates AS of other TFIID subunits, underscoring the role of TAF4 in synchronised changes of Pol II complex composition essential for efficient cellular reprogramming. Taken together, targeted AS of TAF4 provides a unique strategy for generation of iMels and recapitulating stages of melanoma progression.

Essential role of the TFIID subunit TAF4 in murine embryogenesis and embryonic stem cell differentiation

TAF4 (TATA-binding protein-associated factor 4) and its paralogue TAF4b are components of the TFIID core module. We inactivated the murine Taf4a gene to address Taf4 function during embryogenesis. Here we show that Taf4a^−/− embryos survive until E9.5 where primary germ layers and many embryonic structures are identified showing Taf4 is dispensable for their specification. In contrast, Taf4 is required for correct patterning of the trunk and anterior structures, ventral morphogenesis and proper heart positioning. Overlapping expression of Taf4a and Taf4b during embryogenesis suggests their redundancy at early stages. In agreement with this, Taf4a^−/− embryonic stem cells (ESCs) are viable and comprise Taf4b-containing TFIID. Nevertheless, Taf4a^−/− ESCs do not complete differentiation into glutamatergic neurons and cardiomyocytes in vitro due to impaired preinitiation complex formation at the promoters of critical differentiation genes. We define an essential role of a core TFIID TAF in differentiation events during mammalian embryogenesis.

The role of TFIID core module TAFs (TATA-binding protein-associated factors) in embryogenesis is unknown. Here, the authors show that Taf4 is essential at mid-gestation and for complete neuronal differentiation of embryonic stem cells, but that Taf4a and Taf4b are redundant at early embryonic stages.

A Brn2-Zic1 axis specifies the neuronal fate of retinoic-acid-treated embryonic stem cells

Mouse embryonic stem cells (ESCs) treated with all-trans retinoic acid differentiate into a homogenous population of glutamatergic neurons. Although differentiation is initiated through activation of target genes by the retinoic acid receptors, the downstream transcription factors specifying neuronal fate are less well characterised. Here, we show that the transcription factor Brn2 (also known as Pou3f2) is essential for the neuronal differentiation programme. By integrating results from RNA-seq following Brn2 silencing with results from Brn2 ChIP-seq, we identify a set of Brn2 target genes required for the neurogenic programme. Further integration of Brn2 ChIP-seq data from retinoic-acid-treated ESCs and P19 cells with data from ESCs differentiated into neuronal precursors by Fgf2 treatment and that from fibroblasts trans-differentiated into neurons by ectopic Brn2 expression showed that Brn2 occupied a distinct but overlapping set of genomic loci in these differing conditions. However, a set of common binding sites and target genes defined the core of the Brn2-regulated neuronal programme, among which was that encoding the transcription factor Zic1. Small hairpin RNA (shRNA)-mediated silencing of Zic1 prevented ESCs from differentiating into neuronal precursors, thus defining a hierarchical Brn2-Zic1 axis that is essential to specify neural fate in retinoic-acid-treated ESCs.

Diversity in TAF proteomics: consequences for cellular differentiation and migration

Development is a highly controlled process of cell proliferation and differentiation driven by mechanisms of dynamic gene regulation. Specific DNA binding factors for establishing cell- and tissue-specific transcriptional programs have been characterised in different cell and animal models. However, much less is known about the role of “core transcription machinery” during cell differentiation, given that general transcription factors and their spatiotemporally patterned activity govern different aspects of cell function. In this review, we focus on the role of TATA-box associated factor 4 (TAF4) and its functional isoforms generated by alternative splicing in controlling lineage-specific differentiation of normal mesenchymal stem cells and cancer stem cells. In the light of our recent findings, induction, control and maintenance of cell differentiation status implies diversification of the transcription initiation apparatus orchestrated by alternative splicing.

TAF4, a subunit of transcription factor II D, directs promoter occupancy of nuclear receptor HNF4A during post-natal hepatocyte differentiation

The functions of the TAF subunits of mammalian TFIID in physiological processes remain poorly characterised. In this study, we describe a novel function of TAFs in directing genomic occupancy of a transcriptional activator. Using liver-specific inactivation in mice, we show that the TAF4 subunit of TFIID is required for post-natal hepatocyte maturation. TAF4 promotes pre-initiation complex (PIC) formation at post-natal expressed liver function genes and down-regulates a subset of embryonic expressed genes by increased RNA polymerase II pausing. The TAF4–TAF12 heterodimer interacts directly with HNF4A and in vivo TAF4 is necessary to maintain HNF4A-directed embryonic gene expression at post-natal stages and promotes HNF4A occupancy of functional cis-regulatory elements adjacent to the transcription start sites of post-natal expressed genes. Stable HNF4A occupancy of these regulatory elements requires TAF4-dependent PIC formation highlighting that these are mutually dependent events. Local promoter-proximal HNF4A–TFIID interactions therefore act as instructive signals for post-natal hepatocyte differentiation.

DOI:http://dx.doi.org/10.7554/eLife.03613.001

To decode the information contained within a gene, a number of processes need to occur. For example, the DNA sequence that makes up the gene needs to be copied to make a molecule of RNA, which is then translated to build the corresponding protein. The first steps in the manufacture of RNA involve a structure called a ‘pre-initiation complex’ moving an enzyme called RNA polymerase II to the start of the gene that needs to be copied.

The pre-initiation complex is made up of many types of protein, including a set of proteins called TAFs. However, the way that these proteins work in mammals is not well understood. There are good reasons for this: proteins are often studied by seeing what happens when the protein is removed, but many TAFs are so important that removing them is lethal.

Alpern et al. have now studied the function of TAF4 by removing this protein from mouse liver cells. This causes severe hypoglycemia (that is, a drop in sugar levels in the blood). Moreover, it seems as if these cells start dying before they become fully mature. In liver cells lacking TAF4, some 1408 genes that are normally turned on just after birth are not properly switched on; these genes are necessary for the metabolic functions of the liver. Furthermore, 776 genes that are normally turned off after birth continue to be expressed. It seems that the absence of TAF4 sometimes disrupts the formation of the pre-initiation complex, which would slow down the production of RNA. However, it can also have the opposite effect by increasing the activity of RNA polymerase II, hence making too many copies of RNA from some genes.

Alpern et al. also find that TAF4 is needed to allow a protein called HNF4A, which is important in the development of the liver and in controlling metabolism, to interact with over 7000 important DNA sequences. Mutations in HNF4A are responsible for a syndrome known as Maturity Onset of Diabetes in the Young. The next stage in this work will be to explore if these mutations influence the interaction between HNF4A and TAF4, and if they do, whether these changes contribute to this form of diabetes.

DOI:http://dx.doi.org/10.7554/eLife.03613.002

TAF-4 is required for the life extension of isp-1, clk-1 and tpk-1 Mit mutants

While numerous life-extending manipulations have been discovered in the nematode Caenorhabditis elegans, one that remains most enigmatic is disruption of oxidative phosphorylation. In order to unravel how such an ostensibly deleterious manipulation can extend lifespan, we sought to identify the ensemble of nuclear transcription factors that are activated in response to defective mitochondrial electron transport chain (ETC) function. Using a feeding RNAi approach, we targeted over 400 transcription factors and identified 15 that, when reduced in function, reproducibly and differentially altered the development, stress response, and/or fecundity of isp-1(qm150) Mit mutants relative to wild-type animals. Seven of these transcription factors – AHA-1, CEH-18, HIF-1, JUN-1, NHR-27, NHR-49 and the CREB homolog-1 (CRH-1)-interacting protein TAF-4 – were also essential for isp-1 life extension. When we tested the involvement of these seven transcription factors in the life extension of two other Mit mutants, namely clk-1(qm30) and tpk-1(qm162), TAF-4 and HIF-1 were consistently required. Our findings suggest that the Mit phenotype is under the control of multiple transcriptional responses, and that TAF-4 and HIF-1 may be part of a general signaling axis that specifies Mit mutant life extension.

Dissection of a novel autocrine signaling pathway via quantitative secretome and interactome mapping

Epidermal homeostasis is a balancing act governed by a multitude of underlying regulatory events, and several growth factors and signaling pathways have been implicated in regulation of the balance between proliferation and differentiation in keratinocytes. We show here that the signal transducer/transcription factor FIZ1 (Flt3 interacting zinc finger protein-1) is a previously unknown player in this regulatory axis, promoting an increase in proliferation of HaCaT human immortalized keratinocytes that is driven by more rapid G1/S progression and mediated by activation of the MAP/ERK kinase pathway. Utilizing quantitative SILAC-based secretome analysis, we identified the insulin growth factor binding protein IGFBP3 as the key mediating factor, demonstrating that elevated FIZ1 levels promote increased IGFBP3 expression and secretion and a concurrent increased sensitivity to IGF1 signaling, while antibody-based neutralization of IGFBP3 abrogates the FIZ1-induced growth advantage. To identify underlying protein-protein interactions likely to govern these events, we mapped the interactome of FIZ1 and found eight novel binding partners that form complexes with the protein in the cytoplasm and nucleus. These include signal transduction and transcription factors and the cell cycle regulatory NDR (Nuclear Dbf2-related) kinases. Our results provide further insight into the complex balance of epidermal homeostasis and identify FIZ1 as a novel therapeutic target.

Targeting TBP-Associated Factors in Ovarian Cancer

As ovarian tumors progress, they undergo a process of dedifferentiation, allowing adaptive changes in growth and morphology that promote metastasis and chemoresistance. Herein, we outline a hypothesis that TATA-box binding protein associated factors (TAFs), which compose the RNA Polymerase II initiation factor, TFIID, contribute to regulation of dedifferentiation states in ovarian cancer. Numerous studies demonstrate that TAFs regulate differentiation and proliferation states; their expression is typically high in pluripotent cells and reduced upon differentiation. Strikingly, TAF2 exhibits copy number increases or mRNA overexpression in 73% of high-grade serous ovarian cancers (HGSC). At the biochemical level, TAF2 directs TFIID to TATA-less promoters by contact with an Initiator element, which may lead to the deregulation of the transcriptional output of these tumor cells. TAF4, which is altered in 66% of HGSC, is crucial for the stability of the TFIID complex and helps drive dedifferentiation of mouse embryonic fibroblasts to induced pluripotent stem cells. Its ovary-enriched paralog, TAF4B, is altered in 26% of HGSC. Here, we show that TAF4B mRNA correlates with Cyclin D2 mRNA expression in human granulosa cell tumors. TAF4B may also contribute to regulation of tumor microenvironment due to its estrogen-responsiveness and ability to act as a cofactor for NFκB. Conversely, TAF9, a cofactor for p53 in regulating apoptosis, may act as a tumor suppressor in ovarian cancer, since it is downregulated or deleted in 98% of HGSC. We conclude that a greater understanding of mechanisms of transcriptional regulation that execute signals from oncogenic signaling cascades is needed in order to expand our understanding of the etiology and progression of ovarian cancer, and most importantly to identify novel targets for therapeutic intervention.

TAF4 inactivation reveals the 3 dimensional growth promoting activities of collagen 6A3

Collagen 6A3 (Col6a3), a component of extracellular matrix, is often up-regulated in tumours and is believed to play a pro-oncogenic role. However the mechanisms of its tumorigenic activity are poorly understood. We show here that Col6a3 is highly expressed in densely growing mouse embryonic fibroblasts (MEFs). In MEFs where the TAF4 subunit of general transcription factor IID (TFIID) has been inactivated, elevated Col6a3 expression prevents contact inhibition promoting their 3 dimensional growth as foci and fibrospheres. Analyses of gene expression in densely growing Taf4^−/− MEFs revealed repression of the Hippo pathway and activation of Wnt signalling. The Hippo activator Kibra/Wwc1 is repressed under dense conditions in Taf4^−/− MEFs, leading to nuclear accumulation of the proliferation factor YAP1 in the cells forming 3D foci. At the same time, Wnt9a is activated and the Sfrp2 antagonist of Wnt signalling is repressed. Surprisingly, treatment of Taf4^−/− MEFs with all-trans retinoic acid (ATRA) restores contact inhibition suppressing 3D growth. ATRA represses Col6a3 expression independently of TAF4 expression and Col6a3 silencing is sufficient to restore contact inhibition in Taf4^−/− MEFs and to suppress 3D growth by reactivating Kibra expression to induce Hippo signalling and by inducing Sfrp2 expression to antagonize Wnt signalling. All together, these results reveal a critical role for Col6a3 in regulating both Hippo and Wnt signalling to promote 3D growth, and show that the TFIID subunit TAF4 is essential to restrain the growth promoting properties of Col6a3. Our data provide new insight into the role of extra cellular matrix components in regulating cell growth.

A TAF4 coactivator function for E proteins that involves enhanced TFIID binding

The multisubunit TFIID plays a direct role in transcription initiation by binding to core promoter elements and directing preinitiation complex assembly. Although TFIID may also function as a coactivator through direct interactions with promoter-bound activators, mechanistic aspects of this poorly defined function remain unclear. Here, biochemical studies show a direct TFIID-E-protein interaction that (1) is mediated through interaction of a novel E-protein activation domain (activation domain 3 [AD3]) with the TAF homology (TAFH) domain of TAF4, (2) is critical for activation of a natural target gene by an E protein, and (3) mechanistically acts by enhancing TFIID binding to the core promoter. Complementary assays establish a gene-specific role for the TAFH domain in TFIID recruitment and activation of a large subset of genes in vivo. These results firmly establish TAF4 as a bona fide E-protein coactivator as well as a mechanism involving facilitated TFIID binding through direct interaction with an E-protein activation domain.

The architecture of human general transcription factor TFIID core complex

The initiation of gene transcription by RNA polymerase II is regulated by a plethora of proteins in human cells. The first general transcription factor to bind gene promoters is transcription factor IID (TFIID). TFIID triggers pre-initiation complex formation, functions as a coactivator by interacting with transcriptional activators and reads epigenetic marks. TFIID is a megadalton-sized multiprotein complex composed of TATA-box-binding protein (TBP) and 13 TBP-associated factors (TAFs). Despite its crucial role, the detailed architecture and assembly mechanism of TFIID remain elusive. Histone fold domains are prevalent in TAFs, and histone-like tetramer and octamer structures have been proposed in TFIID. A functional core-TFIID subcomplex was revealed in Drosophila nuclei, consisting of a subset of TAFs (TAF4, TAF5, TAF6, TAF9 and TAF12). These core subunits are thought to be present in two copies in holo-TFIID, in contrast to TBP and other TAFs that are present in a single copy, conveying a transition from symmetry to asymmetry in the TFIID assembly pathway. Here we present the structure of human core-TFIID determined by cryo-electron microscopy at 11.6 Å resolution. Our structure reveals a two-fold symmetric, interlaced architecture, with pronounced protrusions, that accommodates all conserved structural features of the TAFs including the histone folds. We further demonstrate that binding of one TAF8-TAF10 complex breaks the original symmetry of core-TFIID. We propose that the resulting asymmetric structure serves as a functional scaffold to nucleate holo-TFIID assembly, by accreting one copy each of the remaining TAFs and TBP.

Retinoic acid receptors recognize the mouse genome through binding elements with diverse spacing and topology

Retinoic acid receptors (RARs) heterodimerize with retinoid X receptors (RXRs) and bind to RA response elements (RAREs) in the regulatory regions of their target genes. Although previous studies on limited sets of RA-regulated genes have defined canonical RAREs as direct repeats of the consensus RGKTCA separated by 1, 2, or 5 nucleotides (DR1, DR2, DR5), we show that in mouse embryoid bodies or F9 embryonal carcinoma cells, RARs occupy a large repertoire of sites with DR0, DR8, and IR0 (inverted repeat 0) elements. Recombinant RAR-RXR binds these non-canonical spacings in vitro with comparable affinities to DR2 and DR5. Most DR8 elements comprise three half-sites with DR2 and DR0 spacings. This specific half-site organization constitutes a previously unrecognized but frequent signature of RAR binding elements. In functional assays, DR8 and IR0 elements act as independent RAREs, whereas DR0 does not. Our results reveal an unexpected diversity in the spacing and topology of binding elements for the RAR-RXR heterodimer. The differential ability of RAR-RXR bound to DR0 compared to DR2, DR5, and DR8 to mediate RA-dependent transcriptional activation indicates that half-site spacing allosterically regulates RAR function.

The general transcription factor TAF7 is essential for embryonic development but not essential for the survival or differentiation of mature T cells

TAF7, a component of the TFIID complex that nucleates the assembly of transcription preinitiation complexes, also independently interacts with and regulates the enzymatic activities of other transcription factors, including P-TEFb, TFIIH, and CIITA, ensuring an orderly progression in transcription initiation. Since not all TAFs are required in terminally differentiated cells, we examined the essentiality of TAF7 in cells at different developmental stages in vivo. Germ line disruption of the TAF7 gene is embryonic lethal between 3.5 and 5.5 days postcoitus. Mouse embryonic fibroblasts with TAF7 deleted cease transcription globally and stop proliferating. In contrast, whereas TAF7 is essential for the differentiation and proliferation of immature thymocytes, it is not required for subsequent, proliferation-independent differentiation of lineage committed thymocytes or for their egress into the periphery. TAF7 deletion in peripheral CD4 T cells affects only a small number of transcripts. However, T cells with TAF7 deleted are not able to undergo activation and expansion in response to antigenic stimuli. These findings suggest that TAF7 is essential for proliferation but not for proliferation-independent differentiation.

Interconversion between active and inactive TATA-binding protein transcription complexes in the mouse genome

The TATA binding protein (TBP) plays a pivotal role in RNA polymerase II (Pol II) transcription through incorporation into the TFIID and B-TFIID complexes. The role of mammalian B-TFIID composed of TBP and B-TAF1 is poorly understood. Using a complementation system in genetically modified mouse cells where endogenous TBP can be conditionally inactivated and replaced by exogenous mutant TBP coupled to tandem affinity purification and mass spectrometry, we identify two TBP mutations, R188E and K243E, that disrupt the TBP–BTAF1 interaction and B-TFIID complex formation. Transcriptome and ChIP-seq analyses show that loss of B-TFIID does not generally alter gene expression or genomic distribution of TBP, but positively or negatively affects TBP and/or Pol II recruitment to a subset of promoters. We identify promoters where wild-type TBP assembles a partial inactive preinitiation complex comprising B-TFIID, TFIIB and Mediator complex, but lacking TFIID, TFIIE and Pol II. Exchange of B-TFIID in wild-type cells for TFIID in R188E and K243E mutant cells at these primed promoters completes preinitiation complex formation and recruits Pol II to activate their expression. We propose a novel regulatory mechanism involving formation of a partial preinitiation complex comprising B-TFIID that primes the promoter for productive preinitiation complex formation in mammalian cells.

BzpF is a CREB-like transcription factor that regulates spore maturation and stability in Dictyostelium

The cAMP response element-binding protein (CREB) is a highly conserved transcription factor that integrates signaling through the cAMP-dependent protein kinase A (PKA) in many eukaryotes. PKA plays a critical role in Dictyostelium development but no CREB homologue has been identified in this system. Here we show that Dictyostelium utilizes a CREB-like protein, BzpF, to integrate PKA signaling during late development. bzpF(-) mutants produce compromised spores, which are extremely unstable and germination defective. Previously, we have found that BzpF binds the canonical CRE motif in vitro. In this paper, we determined the DNA binding specificity of BzpF using protein binding microarray (PBM) and showed that the motif with the highest specificity is a CRE-like sequence. BzpF is necessary to activate the transcription of at least 15 PKA-regulated, late-developmental target genes whose promoters contain BzpF binding motifs. BzpF is sufficient to activate two of these genes. The comparison of RNA sequencing data between wild type and bzpF(-) mutant revealed that the mutant fails to express 205 genes, many of which encode cellulose-binding and sugar-binding proteins. We propose that BzpF is a CREB-like transcription factor that regulates spore maturation and stability in a PKA-related manner.

CREB and the CRTC co-activators: sensors for hormonal and metabolic signals

The cyclic AMP-responsive element-binding protein (CREB) is phosphorylated in response to a wide variety of signals, yet target gene transcription is only increased in a subset of cases. Recent studies indicate that CREB functions in concert with a family of latent cytoplasmic co-activators called cAMP-regulated transcriptional co-activators (CRTCs), which are activated through dephosphorylation. A dual requirement for CREB phosphorylation and CRTC dephosphorylation is likely to explain how these activator-co-activator cognates discriminate between different stimuli. Following their activation, CREB and CRTCs mediate the effects of fasting and feeding signals on the expression of metabolic programmes in insulin-sensitive tissues.

TAF4b and Jun/activating protein-1 collaborate to regulate the expression of integrin alpha6 and cancer cell migration properties

The TAF4b subunit of the transcription factor IID, which has a central role in transcription by polymerase II, is involved in promoter recognition by selective recruitment of activators. The activating protein-1 (AP-1) family members participate in oncogenic transformation via gene regulation. Utilizing immunoprecipitation of endogenous protein complexes, we documented specific interactions between Jun family members and TATA box binding protein-associated factors (TAF) in colon HT29 adenocarcinoma cells. Particularly, TAF4b and c-Jun were found to colocalize and interact in the nucleus of advanced carcinoma cells and in cells with epithelial-to-mesenchymal transition (EMT) characteristics. TAF4b was found to specifically regulate the AP-1 target gene involved in EMT integrin alpha6, thus altering related cellular properties such as migration potential. Using a chromatin immunoprecipitation approach in colon adenocarcinoma cell lines, we further identified a synergistic role for TAF4b and c-Jun and other AP-1 family members on the promoter of integrin alpha6, underlining the existence of a specific mechanism related to gene expression control. We show evidence for the first time of an interdependence of TAF4b and AP-1 family members in cell type-specific promoter recognition and initiation of transcription in the context of cancer progression and EMT.

Genome-wide analysis of POU3F2/BRN2 promoter occupancy in human melanoma cells reveals Kitl as a novel regulated target gene

POU3F2 is a POU-Homeodomain transcription factor expressed in neurons and melanoma cells. In melanoma lesions, cells expressing high levels of POU3F2 show enhanced invasive and metastatic capacity that can in part be explained by repression of Micropthalmia-associated Transcription Factor (MITF) expression via POU3F2 binding to its promoter. To identify other POU3F2 target genes that may be involved in modulating the properties of melanoma cells, we performed ChIP-chip experiments in 501Mel melanoma cells. 2108 binding loci located in the regulatory regions of 1700 potential target genes were identified. Bioinformatic and experimental assays showed the presence of known POU3F2-binding motifs, but also many AT-rich sequences with only partial similarity to the known motifs at the occupied loci. Functional analysis indicates that POU3F2 regulates the stem cell factor (Kit ligand, Kitl) promoter via a cluster of four closely spaced binding sites located in the proximal promoter. Our results suggest that POU3F2 may regulate the properties of melanoma cells via autocrine KIT ligand signalling.

Genes influencing spinal bone mineral density in inbred F344, LEW, COP, and DA rats

Previously, we identified the regions of chromosomes 10q12-q31 and 15p16-q21 harbor quantitative trait loci (QTLs) for lumbar volumetric bone mineral density (vBMD) in female F2 rats derived from Fischer 344 (F344) x Lewis (LEW) and Copenhagen 2331 (COP) x Dark Agouti (DA) crosses. The purpose of this study is to identify the candidate genes within these QTL regions contributing to the variation in lumbar vBMD. RNA was extracted from bone tissue of F344, LEW, COP, and DA rats. Microarray analysis was performed using Affymetrix Rat Genome 230 2.0 Arrays. Genes differentially expressed among the rat strains were then ranked based on the strength of the correlation with lumbar vBMD in F2 animals derived from these rats. Quantitative PCR (qPCR) analysis was performed to confirm the prioritized candidate genes. A total of 285 genes were differentially expressed among all strains of rats with a false discovery rate less than 10%. Among these genes, 18 candidate genes were prioritized based on their strong correlation (r (2) > 0.90) with lumbar vBMD. Of these, 14 genes (Akap1, Asgr2, Esd, Fam101b, Irf1, Lcp1, Ltc4s, Mdp-1, Pdhb, Plxdc1, Rabep1, Rhot1, Slc2a4, Xpo4) were confirmed by qPCR. We identified several novel candidate genes influencing spinal vBMD in rats.

Direct transactivator-transcription factor IID (TFIID) contacts drive yeast ribosomal protein gene transcription

Transcription factor IID (TFIID) plays a key role in regulating eukaryotic gene expression by directly binding promoters and enhancer-bound transactivator proteins. However, the precise mechanisms and outcomes of transactivator-TFIID interaction remain unclear. Transcription of yeast ribosomal protein genes requires TFIID and the DNA-binding transactivator Rap1. We have previously shown that Rap1 directly binds to the TFIID complex through interaction with its TATA-binding protein-associated factor (Taf) subunits Taf4, -5, and -12. Here, we identify and characterize the Rap1 binding domains (RBDs) of Taf4 and Taf5. These RBDs are essential for viability but dispensable for Taf-Taf interactions and TFIID stability. Cells expressing altered Rap1 binding domains exhibit conditional growth, synthetic phenotypes when expressed in combination or with altered Rap1, and are selectively defective in ribosomal protein gene transcription. Taf4 and Taf5 proteins with altered RBDs bind Rap1 with reduced affinity. We propose that collectively the Taf4, Taf5, and Taf12 subunits of TFIID represent the physical and functional targets for Rap1 interaction and, furthermore, that these interactions drive ribosomal protein gene transcription.

Promoting developmental transcription

Animal growth and development depend on the precise control of gene expression at the level of transcription. A central role in the regulation of developmental transcription is attributed to transcription factors that bind DNA enhancer elements, which are often located far from gene transcription start sites. Here, we review recent studies that have uncovered significant regulatory functions in developmental transcription for the TFIID basal transcription factors and for the DNA core promoter elements that are located close to transcription start sites.

Cell-specific interaction of retinoic acid receptors with target genes in mouse embryonic fibroblasts and embryonic stem cells

All-trans retinoic acid (RA) induces transforming growth factor beta (TGF-beta)-dependent autocrine growth of mouse embryonic fibroblasts (MEFs). We have used chromatin immunoprecipitation to map 354 RA receptor (RAR) binding loci in MEFs, most of which were similarly occupied by the RAR alpha and RAR gamma receptors. Only a subset of the genes associated with these loci are regulated by RA, among which are several critical components of the TGF-beta pathway. We also show RAR binding to a novel series of target genes involved in cell cycle regulation, transformation, and metastasis, suggesting new pathways by which RA may regulate proliferation and cancer. Few of the RAR binding loci contained consensus direct-repeat (DR)-type elements. The majority comprised either degenerate DRs or no identifiable DRs but anomalously spaced half sites. Furthermore, we identify 462 RAR target loci in embryonic stem (ES) cells and show that their occupancy is cell type specific. Our results also show that differences in the chromatin landscape regulate the accessibility of a subset of more than 700 identified loci to RARs, thus modulating the repertoire of target genes that can be regulated and the biological effects of RA.

Shifting players and paradigms in cell-specific transcription

Historically, developmental-stage- and tissue-specific patterns of gene expression were assumed to be determined primarily by DNA regulatory sequences and their associated activators, while the general transcription machinery including core promoter recognition complexes, coactivators, and chromatin modifiers was held to be invariant. New evidence suggests that significant changes in these general transcription factors including TFIID, BAF, and Mediator may facilitate global changes in cell-type-specific transcription.

Transforming growth factor-beta regulates basal transcriptional regulatory machinery to control cell proliferation and differentiation in cranial neural crest-derived osteoprogenitor cells

Transforming growth factor-beta (Tgf-beta) signaling is crucial for regulating craniofacial development. Loss of Tgf-beta signaling results in defects in cranial neural crest cells (CNCC), but the mechanism by which Tgf-beta signaling regulates bone formation in CNCC-derived osteogenic cells remains largely unknown. In this study, we discovered that Tgf-beta regulates the basal transcriptional regulatory machinery to control intramembranous bone development. Specifically, basal transcription factor Taf4b is down-regulated in the CNCC-derived intramembranous bone in Tgfbr2(fl/fl);Wnt1-Cre mice. Tgf-beta specifically induces Taf4b expression. Moreover, small interfering RNA knockdown of Taf4b results in decreased cell proliferation and altered osteogenic differentiation in primary mouse embryonic maxillary mesenchymal cells, as seen in Tgfbr2 mutant cells. In addition, we show that Taf1 is decreased at the osteogenic initiation stage in the maxilla of Tgfbr2 mutant mice. Furthermore, small interfering RNA knockdown of Taf4b and Taf1 together in primary mouse embryonic maxillary mesenchymal cells results in up-regulated osteogenic initiator Runx2 expression, with decreased cell proliferation and altered osteogenic differentiation. Our results indicate a critical function of Tgf-beta-mediated basal transcriptional factors in regulating osteogenic cell proliferation and differentiation in CNCC-derived osteoprogenitor cells during intramembranous bone formation.

TAF4/4b x TAF12 displays a unique mode of DNA binding and is required for core promoter function of a subset of genes

The major core promoter-binding factor in polymerase II transcription machinery is TFIID, a complex consisting of TBP, the TATA box-binding protein, and 13 to 14 TBP-associated factors (TAFs). Previously we found that the histone H2A-like TAF paralogs TAF4 and TAF4b possess DNA-binding activity. Whether TAF4/TAF4b DNA binding directs TFIID to a specific core promoter element or facilitates TFIID binding to established core promoter elements is not known. Here we analyzed the mode of TAF4b.TAF12 DNA binding and show that this complex binds DNA with high affinity. The DNA length required for optimal binding is approximately 70 bp. Although the complex displays a weak sequence preference, the nucleotide composition is less important than the length of the DNA for high affinity binding. Comparative expression profiling of wild-type and a DNA-binding mutant of TAF4 revealed common core promoter features in the down-regulated genes that include a TATA-box and an Initiator. Further examination of the PEL98 gene from this group showed diminished Initiator activity and TFIID occupancy in TAF4 DNA-binding mutant cells. These findings suggest that DNA binding by TAF4/4b-TAF12 facilitates the association of TFIID with the core promoter of a subset of genes.

Recent advances in understanding the structure and function of general transcription factor TFIID

The general transcription factor TFIID is a macromolecular complex comprising the TATA-binding protein (TBP) and a set of 13-14 TBP associated factors (TAFs). This review discusses biochemical, genetic and electron microscopic data acquired over the past years that provide a model for the composition, organisation and assembly of TFIID. We also revisit ideas on how TFIID is recruited to the promoters of active and possibly repressed genes. Recent observations show that recognition of acetylated and methylated histone residues by structural domains in several TAFs plays an important role. Finally, we highlight several genetic studies suggesting that TFIID is required for initiation of transcription, but not for maintaining transcription once a promoter is in an active state.

TATA box-binding protein-associated factor 12 is important for RAS-induced transformation properties of colorectal cancer cells

Activating mutations in the RAS proto-oncogene result in constant stimulation of its downstream pathways, further leading to tumorigenesis. Transcription factor IID (TFIID) can be regulated by cellular signals to specifically alter transcription of particular subsets of genes. To investigate potential links between the regulation of TFIID function and the RAS-induced carcinogenesis, we monitored the expression of the TATA box-binding protein and its associated factors (TAF) in human colon carcinoma cells. We primarily identified TAF12 levels as being up-regulated in cell lines bearing natural RAS mutations or stably overexpressing a mutated RAS isoform via a mitogen-activated protein kinase/extracellular signal-regulated kinase kinase-dependent pathway. We further showed by electrophoretic mobility shift assays and chromatin immunoprecipitation that the ETS1 protein was interacting with an ETS-binding site on the TAF12 promoter and was regulating TAF12 expression. The binding was enhanced in extracts from oncogenic RAS-transformed cells, pointing to a role in the RAS-mediated regulation of TAF12 expression. Reduction of TAF12 levels by small interfering RNA treatment induced a destabilization of the TFIID complex, enhanced E-cadherin mRNA and protein levels, and reduced migration and adhesion properties of RAS-transformed cells with epithelial to mesenchymal transition. Overall, our study indicates the importance of TAF12 in the process of RAS-induced transformation properties of human colon cells and epithelial to mesenchymal transition, most notably those related to increased motility, by regulating specifically expression of genes such as E-cadherin.

TAF6delta controls apoptosis and gene expression in the absence of p53

BACKGROUND

Life and death decisions of metazoan cells hinge on the balance between the expression of pro- versus anti-apoptotic gene products. The general RNA polymerase II transcription factor, TFIID, plays a central role in the regulation of gene expression through its core promoter recognition and co-activator functions. The core TFIID subunit TAF6 acts in vitro as an essential co-activator of transcription for the p53 tumor suppressor protein. We previously identified a splice variant of TAF6, termed TAF6delta that can be induced during apoptosis.

METHODOLOGY/PRINCIPAL FINDINGS

To elucidate the impact of TAF6delta on cell death and gene expression, we have employed modified antisense oligonucleotides to enforce expression of endogenous TAF6delta. The induction of endogenous TAF6delta triggered apoptosis in tumor cell lines, including cells devoid of p53. Microarray experiments revealed that TAF6delta activates gene expression independently of cellular p53 status.

CONCLUSIONS

Our data define TAF6delta as a pivotal node in a signaling pathway that controls gene expression programs and apoptosis in the absence of p53.

Structural changes in TAF4b-TFIID correlate with promoter selectivity

Proper ovarian development requires the cell type-specific transcription factor TAF4b, a subunit of the core promoter recognition complex TFIID. We present the 35 A structure of a cell type-specific core promoter recognition complex containing TAF4b and TAF4 (4b/4-IID), which is responsible for directing transcriptional synergy between c-Jun and Sp1 at a TAF4b target promoter. As a first step toward correlating potential structure/function relationships of the prototypic TFIID versus 4b/4-IID, we have compared their 3D structures by electron microscopy and single-particle reconstruction. These studies reveal that TAF4b incorporation into TFIID induces an open conformation at the lobe involved in TFIIA and putative activator interactions. Importantly, this open conformation correlates with differential activator-dependent transcription and promoter recognition by 4b/4-IID. By combining functional and structural analysis, we find that distinct localized structural changes in a megadalton macromolecular assembly can significantly alter its activity and lead to a TAF4b-induced reprogramming of promoter specificity.

Retinoic acid induces TGFbeta-dependent autocrine fibroblast growth

To evaluate the role of murine TFIID subunit TAF4 in activation of cellular genes by all-trans retinoic acid (T-RA), we have characterized the T-RA response of taf4(lox/-) and taf4(-/-) embryonic fibroblasts. T-RA regulates almost 1000 genes in taf4(lox/-) cells, but less than 300 in taf4(-/-) cells showing that TAF4 is required for T-RA regulation of most, but not all cellular genes. We further show that T-RA-treated taf4(lox/-) cells exhibit transforming growth factor (TGF)beta-dependent autocrine growth and identify a set of genes regulated by loss of TAF4 and by T-RA corresponding to key mediators of the TGFbeta signalling pathway. T-RA rapidly and potently induces expression of connective tissue growth factor (CTGF) via a conserved DR2 type response element in its proximal promoter leading to serum-free autocrine growth. These results highlight the role of TAF4 as a cofactor in the cellular response to T-RA and identify the genetic programme of a novel cross talk between the T-RA and TGFbeta pathways that leads to deregulated cell growth.

Quantitative proteomics reveals regulation of dynamic components within TATA-binding protein (TBP) transcription complexes

Affinity purification in combination with isotope labeling of proteins has proven to be a powerful method to discriminate specific from nonspecific interactors. However, in the standard SILAC (stable isotope labeling by amino acids in cell culture) approach dynamic components may easily be assigned as nonspecific. We compared two affinity purification protocols, which in combination revealed information on the dynamics of protein complexes. We focused on the central component in eukaryotic transcription, the human TATA-binding protein, which is involved in different complexes. All known TATA-binding protein-associated factors (TAFs) were detected as specific interactors. Interestingly one of them, BTAF1, exchanged significantly in cell extracts during the affinity purification. The other TAFs did not display this behavior. Cell cycle synchronization showed that BTAF1 exchange was regulated during mitosis. The combination of the two affinity purification protocols allows a quantitative approach to identify transient components in any protein complex.

The TFIID subunit TAF4 regulates keratinocyte proliferation and has cell-autonomous and non-cell-autonomous tumour suppressor activity in mouse epidermis

The TAF4 subunit of transcription factor TFIID was inactivated in the basal keratinocytes of foetal and adult mouse epidermis. Loss of TAF4 in the foetal epidermis results in reduced expression of the genes required for skin barrier function, leading to early neonatal death. By contrast, TAF4 inactivation in adult epidermis leads to extensive fur loss and an aberrant hair cycle characterised by a defective anagen phase. Although the mutant epidermis contains few normal anagen-phase hair follicles, many genes expressed at this stage are strongly upregulated indicating desynchronised and inappropriate gene expression. The TAF4 mutant adult epidermis also displays interfollicular hyperplasia associated with a potent upregulation of several members of the EGF family of mitogens. Moreover, loss of TAF4 leads to malignant transformation of chemically induced papillomas and the appearance of invasive melanocytic tumours. Together, our results show that TAF4 is an important regulator of keratinocyte proliferation and has cell-autonomous and non-cell-autonomous tumour suppressor activity.

Individual CREB-target genes dictate usage of distinct cAMP-responsive coactivation mechanisms

CREB is a key mediator of cAMP- and calcium-inducible transcription, where phosphorylation of serine 133 in its Kinase-Inducible Domain (KID) is often equated with transactivation. Phospho-Ser133 is required for CREB to bind the KIX domain of the coactivators CBP and p300 (CBP/p300) in vitro, although the importance of this archetype coactivator interaction for endogenous gene expression is unclear. Here, we show that the CREB interaction with KIX is necessary for only a part of cAMP-inducible transcription and CBP/p300 recruitment. Surprisingly, individual cAMP-inducible genes with CREB bound at their promoters differed in their reliance on KIX and none examined showed complete dependence. Alternatively, we found that arginine 314 (Arg314) in the CREB basic-leucine zipper (bZIP) domain contributed to CBP/p300 recruitment and KIX-independent CREB transactivation function. This implicates Transducer Of Regulated CREB (TORC), an unrelated cAMP-responsive coactivator that binds via Arg314, and which can bind CBP/p300, in these functions. Interestingly, KIX was also required for the full cAMP induction of a gene that did not require CREB. Thus, individual CREB-target gene context dictates the relative contribution of at least two different cAMP-responsive coactivation mechanisms.

Cooperative interactions between CBP and TORC2 confer selectivity to CREB target gene expression

A number of hormones and growth factors stimulate gene expression by promoting the phosphorylation of CREB (P-CREB), thereby enhancing its association with the histone acetylase paralogs p300 and CBP (CBP/p300). Relative to cAMP, stress signals trigger comparable amounts of CREB phosphorylation, but have minimal effects on CRE-dependent transcription. Here, we show that the latent cytoplasmic coactivator TORC2 mediates target gene activation in response to cAMP signaling by associating with CBP/p300 and increasing its recruitment to a subset of CREB target genes. TORC2 is not activated in response to stress signals, however; and in its absence, P-CREB is unable to stimulate CRE-dependent transcription, due to a block in CBP recruitment. The effect of TORC2 on CBP/p300 promoter occupancy appears pivotal because a gain of function mutant CREB polypeptide with increased affinity for CBP restored CRE-mediated transcription in cells exposed to stress signals. Taken together, these results indicate that TORC2 is one of the long sought after cofactors that mediates the differential effects of cAMP and stress pathways on CREB target gene expression.

A gene expression profile for vascular invasion can predict the recurrence after resection of hepatocellular carcinoma: a microarray approach

BACKGROUND

Recurrence after hepatocellular carcinoma (HCC) resection is the major obstacle to improved survival. The presence of vascular invasion (VI) in pathology specimens is a well-known unfavorable prognostic factor for HCC recurrence. Though some VI-related genes have been reported, their association with recurrence-free survival is not known. We hypothesized that a gene expression profile for VI can predict the recurrence of HCC after liver resection.

METHODS

Eighteen patients receiving complete HCC resection were included as a "training group". Genome-wide gene expression profile was obtained for each tumor using a microarray technique. Datasets were subjected to clustering analysis supervised by the presence or absence of VI to obtain 14 discriminative genes. We then applied those genes to execute pattern recognition using the k-Nearest Neighbor (KNN) classification method, and the best model for this VI gene signature to predict recurrence-free survival in the training group was obtained. The resulting model was then tested in an independent "test group" of 35 patients.

RESULTS

A 14-gene profile was extracted which could accurately separate ten patients with VI and eight patients without VI in the "training group". In the "test group", significant difference in disease-free survival was found between patients predicted to have and not to have recurrence (P = .02823). In patients with stage_I disease, this model can also predict outcomes (P = .000205).

CONCLUSIONS

Using the 14-gene expression profile extracted from microarrays based on the presence of VI can effectively predict recurrence after HCC resection. This approach might facilitate "personalized medicine" for HCC patients after surgical resection.

Sumoylation delays the ATF7 transcription factor subcellular localization and inhibits its transcriptional activity

Over the past few years, small ubiquitin-like modifier (SUMO) modification has emerged as an important regulator of diverse pathways and activities including protein localization and transcriptional regulation. We identified a consensus sumoylation motif (IKEE), located within the N-terminal activation domain of the ATF7 transcription factor and thus investigated the role of this modification. ATF7 is a ubiquitously expressed transcription factor, homologous to ATF2, that binds to CRE elements within specific promoters. This protein is able to heterodimerize with Jun or Fos proteins and its transcriptional activity is mediated by interaction with TAF12, a subunit of the general transcription factor TFIID. In the present article, we demonstrate that ATF7 is sumoylated in vitro (using RanBP2 as a E3-specific ligase) and in vivo. Moreover, we show that ATF7 sumoylation affects its intranuclear localization by delaying its entry into the nucleus. Furthermore, SUMO conjugation inhibits ATF7 transactivation activity by (i) impairing its association with TAF12 and (ii) blocking its binding-to-specific sequences within target promoters.

Ovarian granulosa cell survival and proliferation requires the gonad-selective TFIID subunit TAF4b

Oocyte development in the mammalian ovary requires productive interactions with somatic granulosa cells of the ovarian follicle. Proliferating granulosa cells support the progression of follicular growth and maturation, multiplying dramatically as it unfolds. The cell cycle recruitment of granulosa cells is regulated at least in part by hormones such as follicle-stimulating hormone (FSH) and estrogen. Follicles recruited into the growth phase following formation of multiple layers of granulosa cells have two major fates: either to continue proliferation followed by differentiation, or to die by programmed cell death, or atresia. While many of the signaling pathways orchestrating ovarian follicle development are known, the downstream transcriptional regulators that integrate such signals in the mammalian ovary remain to be defined. Recent experiments in diverse organisms have revealed multiple instances of gonad-selective components of the basal transcriptional machinery. One such protein, TAF4b, is a gonadal-enriched coactivator subunit of the TFIID complex required for normal female fertility in the mouse. To determine the etiology of female infertility of the TAF4b-deficient mice, we have determined multiple functions of TAF4b during postnatal ovarian follicle development. Here we demonstrate that the TAF4b protein is expressed in the granulosa cell compartment of the mammalian ovarian follicle. Furthermore, TAF4b-deficient mouse ovaries contain reduced numbers of primordial as well as growing follicles and a concomitant increased proportion of apoptotic follicles in comparison to wild type counterparts. Importantly, TAF4b-null follicles are largely resistant to induction of proliferation in response to multiple hormonal stimuli including estrogen and FSH and demonstrate compromised granulosa cell survival. Together, these data suggest that TAF4b integrates a program of granulosa cell gene expression required for normal ovarian follicle survival and proliferation in response to diverse ovarian signaling events.

Cell-type-selective induction of c-jun by TAF4b directs ovarian-specific transcription networks

Cell-type-selective expression of the TFIID subunit TAF(II)105 (renamed TAF4b) in the ovary is essential for proper follicle development. Although a multitude of signaling pathways required for folliculogenesis have been identified, downstream transcriptional integrators of these signals remain largely unknown. Here, we show that TAF4b controls the granulosa-cell-specific expression of the proto-oncogene c-jun, and together they regulate transcription of ovary-selective promoters. Instead of using cell-type-specific activators, our findings suggest that the coactivator TAF4b regulates the expression of tissue-specific genes, at least in part, through the cell-type-specific induction of c-jun, a ubiquitous activator. Importantly, the loss of TAF4b in ovarian granulosa cells disrupts cellular morphologies and interactions during follicle growth that likely contribute to the infertility observed in TAF4b-null female mice. These data highlight a mechanism for potentiating tissue-selective functions of the basal transcription machinery and reveal intricate networks of gene expression that orchestrate ovarian-specific functions and cell morphology.