Generic transcription pathway (Homo sapiens)
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Description
Detailed studies of gene transcription regulation in a wide variety of eukaryotic systems has revealed the general principles and mechanisms by which cell- or tissue-specific regulation of differential gene transcription is mediated (reviewed in Naar, 2001. Kadonaga, 2004, Maston, 2006, Barolo, 2002; Roeder, 2005, Rosenfeld, 2006). Of the three major classes of DNA polymerase involved in eukaryotic gene transcription, Polymerase II generally regulates protein-encoding genes. Figure 1 shows a diagram of the various components involved in cell-specific regulation of Pol-II gene transcription.
Core Promoter: Pol II-regulated genes typically have a Core Promoter where Pol II and a variety of general factors bind to specific DNA motifs:
i: the TATA box (TATA DNA sequence), which is bound by the "TATA-binding protein" (TBP).
ii: the Initiator motif (INR), where Pol II and certain other core factors bind, is present in many Pol II-regulated genes.
iii: the Downstream Promoter Element (DPE), which is present in a subset of Pol II genes, and where additional core factors bind.
The core promoter binding factors are generally ubiquitously expressed, although there are exceptions to this.
Proximal Promoter: immediately upstream (5') of the core promoter, Pol II target genes often have a Proximal Promoter region that spans up to 500 base pairs (b.p.), or even to 1000 b.p.. This region contains a number of functional DNA binding sites for a specific set of transcription activator (TA) and transcription repressor (TR) proteins. These TA and TR factors are generally cell- or tissue-specific in expression, rather than ubiquitous, so that the presence of their cognate binding sites in the proximal promoter region programs cell- or tissue-specific expression of the target gene, perhaps in conjunction with TA and TR complexes bound in distal enhancer regions.
Distal Enhancer(s): many or most Pol II regulated genes in higher eukaryotes have one or more distal Enhancer regions which are essential for proper regulation of the gene, often in a cell or tissue-specific pattern. Like the proximal promoter region, each of the distal enhancer regions typically contain a cluster of binding sites for specific TA and/or TR DNA-binding factors, rather than just a single site.
Enhancers generally have three defining characteristics:
i: They can be located very long distances from the promoter of the target gene they regulate, sometimes as far as 100 Kb, or more.
ii: They can be either upstream (5') or downstream (3') of the target gene, including within introns of that gene.
iii: They can function in either orientation in the DNA.
Combinatorial mechanisms of transcription regulation: The specific combination of TA and TR binding sites within the proximal promoter and/or distal enhancer(s) provides a "combinatorial transcription code" that mediates cell- or tissue-specific expression of the associated target gene. Each promoter or enhancer region mediates expression in a specific subset of the overall expression pattern. In at least some cases, each enhancer region functions completely independently of the others, so that the overall expression pattern is a linear combination of the expression patterns of each of the enhancer modules.
Co-Activator and Co-Repressor Complexes: DNA-bound TA and TR proteins typically recruit the assembly of specific Co-Activator (Co-A) and Co-Repressor (Co-R) Complexes, respectively, which are essential for regulating target gene transcription. Both Co-A's and Co-R's are multi-protein complexes that contain several specific protein components.
Co-Activator complexes generally contain at lease one component protein that has Histone Acetyl Transferase (HAT) enzymatic activity. This functions to acetylate Histones and/or other chromatin-associated factors, which typically increases that transcription activation of the target gene. By contrast, Co-Repressor complexes generally contain at lease one component protein that has Histone De-Acetylase (HDAC) enzymatic activity. This functions to de-acetylate Histones and/or other chromatin-associated factors. This typically increases the transcription repression of the target gene.
Adaptor (Mediator) complexes: In addition to the co-activator complexes that assemble on particular cell-specific TA factors, - there are at least two additional transcriptional co-activator complexes common to most cells. One of these is the Mediator complex, which functions as an "adaptor" complex that bridges between the tissue-specific co-activator complexes assembled in the proximal promoter (or distal enhancers). The human Mediator complex has been shown to contain at least 19 protein distinct components. Different combinations of these co-activator proteins are also found to be components of specific transcription Co-Activator complexes, such as the DRIP, TRAP and ARC complexes described below.
TBP/TAF complex: Another large Co-A complex is the "TBP-associated factors" (TAFs) that assemble on TBP (TATA-Binding Protein), which is bound to the TATA box present in many promoters. There are at least 23 human TAF proteins that have been identified. Many of these are ubiquitously expressed, but TAFs can also be expressed in a cell or tissue-specific pattern.
Specific Coactivator Complexes for DNA-binding Transcription Factors.
A number of specific co-activator complexes for DNA-binding transcription factors have been identified, including DRIP, TRAP, and ARC (reviewed in Bourbon, 2004, Blazek, 2005, Conaway, 2005, and Malik, 2005). The DRIP co-activator complex was originally identified and named as a specific complex associated with the Vitamin D Receptor member of the nuclear receptor family of transcription factors (Rachez, 1998). Similarly, the TRAP co-activator complex was originally identified as a complex that associates with the thyroid receptor (Yuan, 1998). It was later determined that all of the components of the DRIP complex are also present in the TRAP complex, and the ARC complex (discussed further below). For example, the DRIP205 and TRAP220 proteins were show to be identical, as were specific pairs of the other components of these complexes (Rachez, 1999).
In addition, these various transcription co-activator proteins identified in mammalian cells were found to be the orthologues or homologues of the Mediator ("adaptor") complex proteins (reviewed in Bourbon, 2004). The Mediator proteins were originally identified in yeast by Kornberg and colleagues, as complexes associated with DNA polymerase (Kelleher, 1990). In higher organisms, Adapter complexes bridge between the basal transcription factors (including Pol II) and tissue-specific transcription factors (TFs) bound to sites within upstream Proximal Promoter regions or distal Enhancer regions (Figure 1). However, many of the Mediator homologues can also be found in complexes associated with specific transcription factors in higher organisms. A unified nomenclature system for these adapter / co-activator proteins now labels them Mediator 1 through Mediator 31 (Bourbon, 2004). For example, the DRIP205 / TRAP220 proteins are now identified as Mediator 1 (Rachez, 1999), based on homology with yeast Mediator 1.
Example Pathway: Specific Regulation of Target Genes During Notch Signaling:
One well-studied example of cell-specific regulation of gene transcription is selective regulation of target genes during Notch signaling. Notch signaling was first identified in Drosophila, where it has been studied in detail at the genetic, molecular, biochemical and cellular levels (reviewed in Justice, 2002; Bray, 2006; Schweisguth, 2004; Louvri, 2006). In Drosophila, Notch signaling to the nucleus is thought always to be mediated by one specific DNA binding transcription factor, Suppressor of Hairless. In mammals, the homologous genes are called CBF1 (or RBPJkappa), while in worms they are called Lag-1, so that the acronym "CSL" has been given to this conserved transcription factor family. There are at least two human CSL homologues, which are now named RBPJ and RBPJL.
In Drosophila, Su(H) is known to be bifunctional, in that it represses target gene transcription in the absence of Notch signaling, but activates target genes during Notch signaling. At least some of the mammalian CSL homologues are believed also to be bifunctional, and to mediate target gene repression in the absence of Notch signaling, and activation in the presence of Notch signaling.
Notch Co-Activator and Co-Repressor complexes: This repression is mediated by at least one specific co-repressor complexes (Co-R) bound to CSL in the absence of Notch signaling. In Drosophila, this co-repressor complex consists of at least three distinct co-repressor proteins: Hairless, Groucho, and dCtBP (Drosophila C-terminal Binding Protein). Hairless has been show to bind directly to Su(H), and Groucho and dCtBP have been shown to bind directly to Hairless (Barolo, 2002). All three of the co-repressor proteins have been shown to be necessary for proper gene regulation during Notch signaling in vivo (Nagel, 2005).
In mammals, the same general pathway and mechanisms are observed, where CSL proteins are bifunctional DNA binding transcription factors (TFs), that bind to Co-Repressor complexes to mediate repression in the absence of Notch signaling, and bind to Co-Activator complexes to mediate activation in the presence of Notch signaling. However, in mammals, there may be multiple co-repressor complexes, rather than the single Hairless co-repressor complex that has been observed in Drosophila.
During Notch signaling in all systems, the Notch transmembrane receptor is cleaved and the Notch intracellular domain (NICD) translocates to the nucleus, where it there functions as a specific transcription co-activator for CSL proteins. In the nucleus, NICD replaces the Co-R complex bound to CSL, thus resulting in de-repression of Notch target genes in the nucleus (Figure 2). Once bound to CSL, NICD and CSL proteins recruit an additional co-activator protein, Mastermind, to form a CSL-NICD-Mam ternary co-activator (Co-A) complex. This Co-R complex was initially thought to be sufficient to mediate activation of at least some Notch target genes. However, there now is evidence that still other co-activators and additional DNA-binding transcription factors are required in at least some contexts (reviewed in Barolo, 2002).
Thus, CSL is a good example of a bifunctional DNA-binding transcription factor that mediates repression of specific targets genes in one context, but activation of the same targets in another context. This bifunctionality is mediated by the association of specific Co-Repressor complexes vs. specific Co-Activator complexes in different contexts, namely in the absence or presence of Notch signaling.
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The DRIP co-activator complex is a subset of 14 proteins from the set of at least 31 Mediator proteins that, in different combinations, form "Adapter" complexes. Adapter complexes bridge between the basal transcription factors (including Pol II) and tissue-specific transcription factors (TFs) bound to sites within upstream Proximal Promoter regions or distal Enhancer regions (reviewed in Maston, 2006 and Naar, 2001).
The DRIP complex was originally identified and named as a co-activator complex associated with the Vitamin D Receptor member of the nuclear receptor family of transcription factors (Rachez, 1998). It was later determined that all of the components of the DRIP complex were also in the TRAP complex, and the ARC complex.
The DRIP complex contains the following 14 proteins, which also are common to the ARC and TRAP complexes: MED1, MED4, MED6, MED7, MED10, MED12, MED13, MED14, MED16, MED17, MED23, MED24, CDK8, CycC.
All of the DRIP adapter complex components are present in the ARC adapter complex, but the ARC complex also has 4 additional components (Rachez, 1999). These ARC-specific components are now called: MED8, MED15, MED25, and MED 26 in the unified nomenclature scheme (Bourbon, 2004).
Similarly, all 14 of the DRIP adapter complex components are present in the TRAP adapter complex, but the TRAP complex also has 4 additional components (Bourbon, 2004), These TRAP-specific components are now called: MED20, MED27, MED30, and MED 31 in the unified nomenclature scheme.
In addition, these various transcription co-activator proteins identified in mammalian cells were found to be the orthologues or homologues of the Mediator complex identified in yeast, first identified by Kornberg and colleagues (Kelleher, 1990).
E2f6 knockout mice are viable and embryonic fibroblasts derived from these mice proliferate normally. Although E2f6 knockout mice appear healthy, they are affected by homeotic transformations of the axial skeleton, involving vertebrae and ribs. Similar skeletal defects have been reported in mice harboring mutations in polycomb genes, suggesting that E2F6 may function in recruitment of polycomb repressor complex(es) to target promoters (Storre et al. 2002).
E2F6 mediates repression of E2F responsive genes. While E2F6 was suggested to maintain G0 state in quiescent cells (Gaubatz et al. 1998, Ogawa et al. 2002), this finding has been challenged (Giangrande et al. 2004, Bertoli et al. 2013, Bertoli et al. 2016). Instead, E2F6-mediated gene repression in proliferating (non-quiescent) cells is thought to repress E2F targets involved in G1/S transition during S phase of the cell cycle. E2F6 does not affect E2F targets involved in G2/M transition (Oberley et al. 2003, Giangrande et al. 2004, Attwooll et al. 2005, Trojer et al. 2011, Bertoli et al. 2013). In the context of the E2F6.com-1 complex, E2F6 was shown to bind to promoters of E2F1, MYC, CDC25A and TK1 genes (Ogawa et al. 2002). E2F6 also binds the promoters of CDC6, RRM1 (RR1), PCNA and TYMS (TS) genes (Giangrande et al. 2004), as well as the promoter of the DHFR gene (Gaubatz et al. 1998). While transcriptional repression by the E2F6.com 1 complex may be associated with histone methyltransferase activity (Ogawa et al. 2002), E2F6 can also repress transcription independently of H3K9 methylation (Oberley et al. 2003).
During S phase, E2F6 is involved in the DNA replication stress checkpoint (Bertoli et al. 2013, Bertoli et al. 2016). Under replication stress, CHEK1-mediated phosphorylation prevents association of E2F6 with its target promoters, allowing transcription of E2F target genes whose expression is needed for resolution of stalled replication forks and restart of DNA synthesis. Inability to induce transcription of E2F target genes (due to CHEK1 inhibition or E2F6 overexpression) leads to replication stress induced DNA damage (Bertoli et al. 2013, Bertoli et al. 2016). E2F6 represses transcription of a number of E2F targets involved in DNA synthesis and repair, such as RRM2, RAD51, BRCA1, and RBBP8 (Oberley et al. 2003, Bertoli et al. 2013).
activity of SMAD2/SMAD3:SMAD4
heterotrimerThe core domain of CBFB binds to the Runt domain of RUNX1, resulting in formation of the RUNX1:CBFB heterodimer. CBFB does not interact with DNA directly. The Runt domain of RUNX1 mediated both DNA binding and heterodimerization with CBFB (Tahirov et al. 2001), while RUNX1 regions that flank the Runt domain are involved in transactivation (reviewed in Zhang et al. 2003) and negative regulation (autoinhibition). CBFB facilitates RUNX1 binding to DNA by stabilizing Runt domain regions that interact with the major and minor grooves of the DNA (Tahirov et al. 2001, Backstrom et al. 2002, Bartfeld et al. 2002). The transactivation domain of RUNX1 is located C-terminally to the Runt domain and is followed by the negative regulatory domain. Autoinhibiton of RUNX1 is relieved by interaction with CBFB (Kanno et al. 1998).
Transcriptional targets of the RUNX1:CBFB complex involve genes that regulate self-renewal of hematopoietic stem cells (HSCs) (Zhao et al. 2014), as well as commitment and differentiation of many hematopoietic progenitors, including myeloid (Friedman 2009) and megakaryocytic progenitors (Goldfarb 2009), regulatory T lymphocytes (Wong et al. 2011) and B lymphocytes (Boller and Grosschedl 2014).
RUNX1 binds to promoters of many genes involved in ribosomal biogenesis (Ribi) and is thought to stimulate their transcription. RUNX1 loss-of-function decreases ribosome biogenesis and translation in hematopoietic stem and progenitor cells (HSPCs). RUNX1 loss-of-function is therefore associated with a slow growth, but at the same time it results in reduced apoptosis and increases resistance of cells to genotoxic and endoplasmic reticulum stress, conferring an overall selective advantage to RUNX1 deficient HSPCs (Cai et al. 2015).
RUNX1 is implicated as a tumor suppressor in breast cancer. RUNX1 forms a complex with the activated estrogen receptor alpha (ESR1) and regulates expression of estrogen-responsive genes (Chimge and Frenkel 2013).
RUNX1 is overexpressed in epithelial ovarian carcinoma where it may contribute to cell proliferation, migration and invasion (Keita et al. 2013).
RUNX1 may cooperate with TP53 in transcriptional activation of TP53 target genes upon DNA damage (Wu et al. 2013).
RUNX1 is needed for the maintenance of skeletal musculature (Wang et al. 2005).
During mouse embryonic development, Runx1 is expressed in most nociceptive sensory neurons, which are involved in the perception of pain. In adult mice, Runx1 is expressed only in nociceptive sensory neurons that express the Ret receptor and is involved in regulation of expression of genes encoding ion channels (sodium-gated, ATP-gated and hydrogen ion-gated) and receptors (thermal receptors, opioid receptor MOR and the Mrgpr class of G protein coupled receptors). Mice lacking Runx1 show defective perception of thermal and neuropathic pain (Chen CL et al. 2006). Runx1 is thought to activate the neuronal differentiation of nociceptive dorsal root ganglion cells during embryonal development possibly through repression of Hes1 expression (Kobayashi et al. 2012). In chick and mouse embryos, Runx1 expression is restricted to the dorso-medial domain of the dorsal root ganglion, to TrkA-positive cutaneous sensory neurons. Runx3 expression in chick and mouse embryos is restricted to ventro-lateral domain of the dorsal root ganglion, to TrkC-positive proprioceptive neurons (Chen AI et al. 2006, Kramer et al. 2006). RUNX1 mediated regulation of neuronally expressed genes will be annotated when mechanistic details become available.
RUNX2 expression is regulated by estrogen signaling, and RUNX2 is implicated in breast cancer development and metastasis (reviewed in Wysokinski et al. 2014). Besides estrogen receptor alpha (ESR1) and estrogen-related receptor alpha (ERRA) (Kammerer et al. 2013), RUNX2 transcription is also regulated by TWIST1 (Yang, Yang et al. 2011), glucocorticoid receptor (NR3C1) (Zhang et al. 2012), NKX3-2 (BAPX1) (Tribioli and Lufkin 1999, Lengner et al. 2005), DLX5 (Robledo et al. 2002, Lee et al. 2005) and MSX2 (Lee et al. 2005). RUNX2 can autoregulate, by directly inhibiting its own transcription (Drissi et al. 2000). Several E3 ubiquitin ligases target RUNX2 for proteasome-mediated degradation: FBXW7a (Kumar et al. 2015), STUB1 (CHIP) (Li et al. 2008), SMURF1 (Zhao et al. 2003, Yang et al. 2014), WWP1 (Jones et al. 2006), and SKP2 (Thacker et al. 2016). Besides formation of RUNX2:CBFB heterodimers, transcriptional activity of RUNX2 is regulated by binding to a number of other transcription factors, for example SOX9 (Zhou et al. 2006, TWIST1 (Bialek et al. 2004) and RB1 (Thomas et al. 2001).
RUNX2 regulates expression of several genes implicated in cell migration during normal development and bone metastasis of breast cancer cells. RUNX2 stimulates transcription of the ITGA5 gene, encoding Integrin alpha 5 (Li et al. 2016) and the ITGBL1 gene, encoding Integrin beta like protein 1 (Li et al. 2015). RUNX2 mediated transcription of the MMP13 gene, encoding Colagenase 3 (Matrix metalloproteinase 13), is stimulated by AKT mediated phosphorylation of RUNX2 (Pande et al. 2013). RUNX2 is implicated in positive regulation of AKT signaling by stimulating expression of AKT-activating TORC2 complex components MTOR and RICTOR, which may contribute to survival of breast cancer cells (Tandon et al. 2014).
RUNX2 inhibits CDKN1A transcription, thus preventing CDKN1A-induced cell cycle arrest. Phosphorylation of RUNX2 by CDK4 in response to high glucose enhances RUNX2-mediated repression of the CDKN1A gene in endothelial cells (Pierce et al. 2012). In mice, Runx2-mediated repression of Cdkn1a may contribute to the development of acute myeloid leukemia (AML) (Kuo et al. 2009). RUNX2 can stimulate transcription of the LGALS3 gene, encoding Galectin-3 (Vladimirova et al. 2008, Zhang et al. 2009). Galectin 3 is expressed in myeloid progenitors and its levels increase during the maturation process (Le Marer 2000).
For a review of RUNX2 function, please refer to Long 2012 and Ito et al. 2015.
During nervous system formation, the Cbfb:Runx3 complex is involved in development of mouse proprioceptive dorsal root ganglion neurons by regulating expression of Ntrk3 (Neurotrophic tyrosine kinase receptor type 3) and possibly other genes (Inoue et al. 2002, Kramer et al. 2006, Nakamura et al. 2008, Dykes et al. 2011, Ogihara et al. 2016). It is not yet known whether RUNX3 is involved in human neuronal development and neuronal disorders.
RUNX3 plays a major role in immune response. RUNX3 regulates development of T lymphocytes. In mouse hematopoietic stem cells, expression of Runx3 is regulated by the transcription factor TAL1 (Landry et al. 2008). RUNX3 promotes the CD8+ lineage fate in developing thymocytes. In the CD4+ thymocyte lineage in mice, the transcription factor ThPOK induces transcription of SOCS family members, which repress Runx3 expression (Luckey et al. 2014). RUNX3, along with RUNX1 and ETS1, is implicated in regulation of transcription of the CD6 gene, encoding a lymphocyte surface receptor expressed on developing and mature T cells (Arman et al. 2009). RUNX3 and ThPOK regulate intestinal CD4+ T cell immunity in a TGF-beta and retinoic acid-dependent manner, which is important for cellular defense against intestinal pathogens (Reis et al. 2013). Besides T lymphocytes, RUNX3 is a key transcription factor in the commitment of innate lymphoid cells ILC1 and ILC3 (Ebihara et al. 2015). RUNX3 regulates expression of CD11A and CD49D integrin genes, involved in immune and inflammatory responses (Dominguez-Soto et al. 2005). RUNX3 is involved in mouse TGF-beta-mediated dendritic cell function and its deficiency is linked to airway inflammation (Fainaru et al. 2004).
In addition to its developmental role, RUNX3 is implicated as a tumor suppressor. The loss of RUNX3 expression and function was first causally linked to the genesis and progression of human gastric cancer (Li et al. 2002). Expression of RUNX3 increases in human pancreatic islet of Langerhans cells but not in pancreatic adenocarcinoma cells in response to differentiation stimulus (serum withdrawal) (Levkovitz et al. 2010). Hypermethylation of the RUNX3 gene is associated with an increased risk for progression of Barrett's esophagus to esophageal adenocarcinoma (Schulmann et al. 2005). Hypermethylation-mediated silencing of the RUNX3 gene expression is also frequent in granulosa cell tumors (Dhillon et al. 2004) and has also been reported in colon cancer (Weisenberger et al. 2006), breast cancer (Lau et al. 2006, Huang et al. 2012), bladder cancer (Wolff et al. 2008) and gastric cancer (Li et al. 2002). In colorectal cancer, RUNX3 is one of the five markers in a gene panel used to classify CpG island methylator phenotype (CIMP+) (Weisenberger et al. 2006).
RUNX3 and CBFB are frequently downregulated in gastric cancer. RUNX3 cooperates with TGF-beta to maintain homeostasis in the stomach and is involved in TGF-beta-induced cell cycle arrest of stomach epithelial cells. Runx3 knockout mice exhibit decreased sensitivity to TGF-beta and develop gastric epithelial hyperplasia (Li et al. 2002, Chi et al. 2005). RUNX3-mediated inhibition of binding of TEADs:YAP1 complexes to target promoters is also implicated in gastric cancer suppression (Qiao et al. 2016).
RUNX3 is a negative regulator of NOTCH signaling and RUNX3-mediated inhibition of NOTCH activity may play a tumor suppressor role in hepatocellular carcinoma (Gao et al. 2010, Nishina et al. 2011).
In addition to RUNX3 silencing through promoter hypermethylation in breast cancer (Lau et al. 2006), Runx3+/- mice are predisposed to breast cancer development. RUNX3 downregulates estrogen receptor alpha (ESR1) protein levels in a proteasome-dependent manner (Huang et al. 2012).
Besides its tumor suppressor role, mainly manifested through its negative effect on cell proliferation, RUNX3 can promote cancer cell invasion by stimulating expression of genes involved in metastasis, such as osteopontin (SPP1) (Whittle et al. 2015).
regulation by the AP-2 (TFAP2) family of transcription
factorsAP-2 dimers bind palindromic GC-rich DNA response elements that match the consensus sequence 5'-GCCNNNGGC-3' (Williams and Tjian 1991a, Williams and Tjian 1991b). Transcriptional co-factors from the CITED family interact with the helix-span-helix (HSH) domain of TFAP2 (AP-2) family of transcription factors and recruit transcription co-activators EP300 (p300) and CREBBP (CBP) to TFAP2-bound DNA elements. CITED2 shows the highest affinity for TFAP2 proteins, followed by CITED4, while CITED1 interacts with TFAP2s with a very low affinity. Mouse embryos defective for CITED2 exhibit neural crest defects, cardiac malformations and adrenal agenesis, which can at least in part be attributed to a defective Tfap2 transactivation (Bamforth et al. 2001, Braganca et al. 2002, Braganca et al. 2003). Transcriptional activity of AP-2 dimers in inhibited by binding of KCTD1 or KCTD15 to the AP-2 transactivation domain (Ding et al. 2009, Zarelli and Dawid 2013). Transcriptional activity of TFAP2A, TFAP2B and TFAP2C is negatively regulated by SUMOylation mediated by UBE2I (UBC9) (Eloranta and Hurst 2002, Berlato et al. 2011, Impens et al. 2014, Bogachek et al. 2014).
During embryonic development, AP-2 transcription factors stimulate proliferation and suppress terminal differentiation in a cell-type specific manner (Eckert et al. 2005).
TFAP2A and TFAP2C directly stimulate transcription of the estrogen receptor ESR1 gene (McPherson and Weigel 1999). TFAP2A expression correlates with ESR1 expression in breast cancer, and TFAP2C is frequently overexpressed in estrogen-positive breast cancer and endometrial cancer (deConinck et al. 1995, Turner et al. 1998). TFAP2A, TFAP2C, as well as TFAP2B can directly stimulate the expression of ERBB2, another important breast cancer gene (Bosher et al. 1996). Association of TFAP2A with the YY1 transcription factor significantly increases the ERBB2 transcription rate (Begon et al. 2005). In addition to ERBB2, the expression of another receptor tyrosine kinase, KIT, is also stimulated by TFAP2A and TFAP2B (Huang et al. 1998), while the expression of the VEGF receptor tyrosine kinase ligand VEGFA is repressed by TFAP2A (Ruiz et al. 2004, Li et al. 2012). TFAP2A stimulates transcription of the transforming growth factor alpha (TGFA) gene (Wang et al. 1997). TFAP2C regulates EGFR in luminal breast cancer (De Andrade et al. 2016).
TFAP2C plays a critical role in maintaining the luminal phenotype in human breast cancer and in influencing the luminal cell phenotype during normal mammary development (Cyr et al. 2015).
In placenta, TFAP2A and TFAP2C directly stimulate transcription of both subunits of the human chorionic gonadotropin, CGA and CGB (Johnson et al. 1997, LiCalsi et al. 2000).
TFAP2A and/or TFAP2C, in complex with CITED2, stimulate transcription of the PITX2 gene, involved in left-right patterning and heart development (Bamforth et al. 2004, Li et al. 2012).
TFAP2A and TFAP2C play opposing roles in transcriptional regulation of the CDKN1A (p21) gene locus. While TFAP2A stimulates transcription of the CDKN1A cyclin-dependent kinase inhibitor (Zeng et al. 1997, Williams et al. 2009, Scibetta et al. 2010), TFAP2C represses CDKN1A transcription (Williams et al. 2009, Scibetta et al. 2010, Wong et al. 2012). Transcription of the TFAP2A gene may be inhibited by CREB and E2F1 (Melnikova et al. 2010).
For review of the AP-2 family of transcription factors, please refer to Eckert et al. 2005.
(TAZ)-stimulated
gene expressionAnnotated Interactions
The ARC co-activator complex is a subset of 18 proteins from the set of at least 31 Mediator proteins that, in different combinations, form "Adapter" complexes in human cells. Adapter complexes bridge between the basal transcription factors (including Pol II) and tissue-specific transcription factors (TFs) bound to sites within upstream Proximal Promoter regions or distal Enhancer regions (reviewed in Maston, 2006 and Naar, 2001).
The ARC complex was originally identified and named as a co-activator complex associated with transcription activator proteins (reviewed in Malik, 2005 and references therein). It was subsequently determined that many of the components of the ARC complex are also in the DRIP complex, and in the TRAP complex..
The ARC complex contains the following 14 proteins, which also are common to the DRIP and TRAP complexes: MED1, MED4, MED6, MED7, MED10, MED12, MED13, MED14, MED16, MED17, MED23, MED24, CDK8, CycC.
The ARC complex also contains 4 additional, ARC-specific components, which are now called: MED8, MED15, MED25, and MED 26 in the unified nomenclature scheme (Bourbon, 2004).
In addition, these various transcription co-activator proteins identified in mammalian cells were found to be the orthologues or homologues of the Mediator complex proteins in yeast, first identified by Kornberg and colleagues (Kelleher, 1990). The unified nomenclature system for these adapter / co-activator proteins now labels them Mediator 1 through Mediator 31 (Bourbon, 2004).
The order of addition of the ARC proteins during complex assembly is not fully determined, and may vary in different cell contexts. Therefore, ARC complex assembly is represented as a single reaction event, in which all 19 components assemble simultaneously into the ARC co-activator complex.
The TRAP co-activator complex is a subset of 18 proteins from the set of at least 31 Mediator proteins that, in different combinations and in different contexts, form specific co-activator or "Adapter" complexes in human cells. These complexes bridge between the basal transcription factors (including Pol II) and tissue-specific transcription factors (TFs) bound to sites within upstream Proximal Promoter regions or distal Enhancer regions (reviewed in Maston, 2006 and Naar, 2001).
The TRAP complex was originally identified and named as a co-activator complex associated with the Thyroid Hormone Receptor member of the nuclear receptor family of transcription factors (Yuan, 1998). It was later determined that many of the components of the TRAP complex are also in the DRIP complex, and in the ARC complex.
The TRAP complex contains the following 14 proteins, which also are common to the DRIP and ARC complexes: MED1, MED4, MED6, MED7, MED10, MED12, MED13, MED14, MED16, MED17, MED23, MED24, CDK8, CycC.
The TRAP complex also contains 4 additional components, which are now called: MED20, MED27, MED30, and MED 31 in the unified nomenclature scheme (Bourbon, 2004).
In addition, these various transcription co-activator proteins identified in mammalian cells were found to be the orthologues or homologues of the Mediator complex proteins in yeast, first identified by Kornberg and colleagues (Kelleher, 1990). The unified nomenclature system for these adapter / co-activator proteins now labels them Mediator 1 through Mediator 31 (Bourbon, 2004).
The order of addition of the TRAP proteins during complex assembly is not fully determined, and may vary in different cell contexts. Therefore, TRAP co-activator complex assembly is represented as a single reaction event, in which all 18 components assemble simultaneously into the TRAP co-activator complex.
The DRIP co-activator complex is a subset of 14 proteins from the set of at least 31 Mediator proteins that, in different combinations, form "Adapter" complexes. Adapter complexes bridge between the basal transcription factors (including Pol II) and tissue-specific transcription factors (TFs) bound to sites within upstream Proximal Promoter regions or distal Enhancer regions (reviewed in Maston, 2006 and Naar, 2001).
The DRIP complex was originally identified and named as a co-activator complex associated with the Vitamin D Receptor member of the nuclear receptor family of transcription factors (Rachez, 1998). It was later determined that all of the components of the DRIP complex were also in the TRAP complex, and the ARC complex.
The DRIP complex contains the following 14 proteins, which also are common to the ARC and TRAP complexes: MED1, MED4, MED6, MED7, MED10, MED12, MED13, MED14, MED16, MED17, MED23, MED24, CDK8, CycC.
All of the DRIP adapter complex components are present in the ARC adapter complex, but the ARC complex also has 4 additional components (Rachez, 1999). These ARC-specific components are now called: MED8, MED15, MED25, and MED 26 in the unified nomenclature scheme (Bourbon, 2004).
Similarly, all 14 of the DRIP adapter complex components are present in the TRAP adapter complex, but the TRAP complex also has 4 additional components (Bourbon, 2004), These TRAP-specific components are now called: MED20, MED27, MED30, and MED 31 in the unified nomenclature scheme.
In addition, these various transcription co-activator proteins identified in mammalian cells were found to be the orthologues or homologues of the Mediator complex identified in yeast, first identified by Kornberg and colleagues (Kelleher, 1990).
A general feature of the NR proteins is that they each contain a specific protein interaction domain (PID), or domains, that mediates the specific binding interactions with the MED1 proteins. In the ligand-bound state, NRs each take part in an NR-MED1 binding reaction to form an NR-MED1 complex. The bound MED1 then functions to nucleate the assembly of additional specific coactivator proteins, depending on the cell and DNA context, such as what specific target gene promoter or enhancer they are bound to, and in what cell type.
The formation of specific MED1-containing coactivator complexes on specific NR proteins has been well-characterized for a number of the human NR proteins. For example, binding of Vitamin D to the human Vitamin D3 Receptor was found to result in the recruitment of a specific complex of D Receptor Interacting Proteins - the DRIP coactivator complex (Rachez, 1998). Within the DRIP complex, the DRIP205 subunit was later renamed human "MED1", based on sequence similarities with yeast MED1 (reviewed in Bourbon, 2004).
Similarly, binding of thyroid hormone (TH) to the human TH Receptor (THRA or THRB) was found to result in the recruitment of a specific complex of Thyroid Receptor Associated Proteins - the TRAP coactivator complex (Yuan, 1998). The TRAP220 subunit was later identified to be the Mediator 1 (MED1) homologue (summarized in Bourbon, et al., 2004; Table 1).
The 48 human NR proteins each contain the PID(s) known to mediate interaction with the human MED1 protein. Direct NR-MED1 protein-protein interactions have been shown for a number of the NR proteins. The MED1-interacting PIDs are conserved in all of the human NRs. Therefore, each of the human NRs is known or expected to interact with MED1 in the appropriate cell context, depending on the cell type, the cell state, and the target gene regulatory region involved.Formation of the KRAB ZNF / KAP1 Corepressor Complex:
Transcription factors which contain tandem copies of the C2H2 zinc finger DNA binding motif (ZNFs) are the most abundant class of TFs in the human proteome, comprising more than 1000 members. The KRAB ZNF proteins are the largest subset of these (with 423 members) and are defined by having an additional conserved domain, the KRAB domain (Bellefroid,1991, Margolin, 1994, Urrutia, 2003, Huntley, 2006). The Kruppel Associated Box (KRAB) domain is a transcription repression domain (Margolin, 1994) which mediates the recruitment of a specific and dedicated co repressor protein for the KRAB-ZNF family - KAP1 - which is required for transcriptional repression and gene silencing (Friedman, 1996).
The larger family of ZNF transcription factors are present in almost all metazoans and generally their DNA binding specificities and transcription regulation functions are conserved from Drosophila to humans. Although the biological functions of most ZNF TFs is not known, they often function biochemically as sequence specific DNA binding proteins and can be activators, or more oftenly observed, repressors of transcription, depending on cellular context. Transcriptional repression is mediated via specific protein protein interaction surfaces in the ZNF that function as repression domains, by recruiting specific co repressors, such as KAP1 in humans (Friedman, 1996), and dCTBP in Drosophila (Nibu, 1998).
In contrast to the larger ZNF family, the KRAB-ZNFs only appear much later in vertebrate evolution: genes encoding the primordial KRAB ZNF subfamily first arose in tetrapods and the family has been greatly expanded in numbers and complexity in mammals. Interestingly,a large fraction of KRAB-ZNFs are found only in primates. In addition to their rapid and dynamic evolutionary history, comparative genomics and expression studies of primate KRAB-ZNFs suggest that these genes have played a significant role in shaping primate specific traits (Huntley, 2006, Nowick, 2009).
The biochemical pathway utilized by KRAB-ZNFs is well defined and probably nearly identical for each member: All KRAB-ZNF proteins which have been studied in detail are repressors and utilize the KRAB domain to bind the KAP1 co-repressor. This interaction is direct, of high affinity, and is obligate for the KRAB-ZNF to function as a repressor when bound to DNA in vivo (Peng, 2000a,b).. The KAP1co-repressor appears to function as a scaffold protein to assemble and coordinate multiple enzymes (histone de-acetylases, histone methyltransferases and heterochromatin proteins) which target and modify chromatin structure thus leading to a compacted, silent state (Lechner, 2000; Schultz, 2001 Schultz, 2002 , Ayyanathan, 2003). The post-translational modification of KAP1 by SUMO controls its ability to assemble the enzymatic apparatus in chromatin (Ivanov, 2007; Zeng, 2008). It is formally possible that some KRAB ZNF proteins may have additional functional domains that recruit coactivators in specific contexts, given that such bifunctionality is common for many classes of DNA binding transcription factors,. However, there is no experimental evidence for this yet.
There also is good evidence that the KRAB ZNF-KAP1 complex proteins can have long range gene silencing functions, by nucleating chromatin complexes that inactivate transcription of large numbers of genes over large distances by assembling silent heterochromatin (Ayyanathan, 2003). Although KAP1 was originally identified as a mediator of specific gene transcription repression, subsequent studies have shown that KAP1 also is involved in the recruitment of homologues of the HP1 protein family (Ryan, 1999, Ayyanathan, 2003; Lechner, 2000). These nonhistone heterochromatin associated proteins were first shown to have an epigenetic gene silencing function in Drosophila and more recently in mammalian cells . These studies suggest that KRAB ZNF proteins and KAP1 may also be involved in large scale chromatin regulation and gene silencing, not just in gene specific transcriptional repression. Whether this is a general property of most or all KRAB ZNF proteins will require additional studies.
Finally, several KRAB containing ZNFs in mammals also contain a conserved SCAN domain which, like the KRAB domain also functions as a protein protein interaction domain. (Edelstein, 2005, Peng, 2000a,b). The SCAN domain does not participate in KAP1 binding but rather functions to mediate homodimerization, or selective heterodimerization with other SCAN containing proteins. However, the biochemical and biological functions of the SCAN domain in KRAB-ZNF mediated repression are not known.
Remaining Questions: The single most important unanswered question for KRAB-ZNFDs is to determine their biological functions. While the mechanism utilized by the KRAB ZNF / KAP1 protein complex to mediate gene specific transcription repression is well understood , much less known about the specific biological pathways they control. Preliminary evidence from recent whole genome analysis of the target genes for the KRAB- ZNF263 protein suggest that it can have both positive and negative effects on transcriptional regulation of its target genes (Frietze, 2010). Presumably, each KRAB-ZNF, via its array of zinc fingers can bind to specific DNA recognition sequences in target promoters. This, combined with highly tissue specific expression of each gene, makes the potential transcriptome controlled by the 423 KRAB-ZNFs extremely large.