c-Fos-induced activation of a TATA-box-containing promoter involves direct contact with TATA-box-binding protein

The role of magnetosomes in cellular homeostasis disorder and development of pathology

Literature data on magnitosomes, the nanocrystals formed during natural biomineralization have been summarized. Special attention is paid to magnitosome effect on physiological and biochemical processes, impairments of cell homeostasis and development of various pathologies. It is suggested that the increase in quantity and sizes of magnetosomes, spatial rearrangement, and modification of their crystalline substance exert substantial effect on development of pathological processes.

A modified version of a Fos-associated cluster in HBZ affects Jun transcriptional potency

Like c-Fos, HBZ (HTLV-I bZIP factor) is able to interact with c-Jun but differs considerably from c-Fos in its ability to activate AP-1-responsive genes since HBZ rather inhibits transcriptional activity of c-Jun. To better understand the molecular mechanisms involved in this down-regulation of c-Jun activity, a large number of HBZ/c-Fos chimeras was constructed and analyzed for their ability to interact with c-Jun, to bind to the AP-1 motif and to stimulate expression of a reporter gene containing the collagenase promoter. By this approach, we demonstrate that the DNA-binding domain of HBZ is responsible for its inhibitory effect on the trans-activation potential of c-Jun. However, unexpectedly, we found that exchange of a cluster of six charged amino acids immediately adjacent to the DNA contact region altered significantly transcriptional activity of chimeras. This particular subdomain could be involved in efficient presentation of the AP-1 complex to the transcriptional machinery. To confirm this role, specific residues present in the cluster of HBZ were substituted for corresponding amino acids in c-Fos. Unlike the JunD-activating potential of wild-type HBZ, this mutant was no longer able to stimulate JunD activity, confirming the key role of this particular cluster in regulation of Jun transcriptional potency.

Regulation of the transcriptional activity of c-Fos by ERK. A novel role for the prolyl isomerase PIN1

The activation of the activating protein-1 (AP-1) family of transcription factors, including c-Fos and c-Jun family members, is one of the earliest nuclear events induced by growth factors that stimulate extracellular signal-regulated kinases (ERKs). In the case of c-Fos, the activation of ERK leads to an increased expression of c-fos mRNA. In turn, we have recently shown that ERK phosphorylates multiple residues within the carboxylterminal transactivation domain (TAD) of c-Fos, thus resulting in its increased transcriptional activity. However, how ERK-dependent phosphorylation regulates c-Fos function is still poorly understood. In this regard, it has been recently observed that the prolyl isomerase Pin1 can interact with proteins phosphorylated on serine or threonine residues that precede prolines (pS/T-P), such as the transcription factors p53 and c-Jun, thereby controlling their activity by promoting the cis-trans isomerization of these pS/T-P bonds. Here, we found that Pin1 binds c-Fos through specific pS/T-P sites within the c-Fos TAD, and that this interaction results in an enhanced transcriptional response of c-Fos to polypeptide growth factors that stimulate ERK. Our findings suggest that c-Fos represents a novel target for the isomerizing activity of Pin1 and support a role for Pin1 in the mechanism by which c-Jun and c-Fos can cooperate to regulate AP-1-dependent gene transcription upon phosphorylation by mitogen-activated kinase (MAPK) family members.

Hepatic stellate cells lack AP-1 responsiveness to electrophiles and phorbol 12-myristate-13-acetate

Stellate cell profibrotic gene induction and transdifferentiation are central events in liver fibrosis. Oxidative stress has been implicated as an activator of the transcription factors Nrf2 and AP-1 through shared kinase signaling pathways that also purportedly contribute to stellate cell activation. The present study examined the role of oxidative stress in ARE- and TRE-regulated gene induction in isolated hepatic stellate cells. Using a portion of the human Nqo1 promoter consisting of an ARE imbedded TRE, it was demonstrated that while the ARE was responsible for mediating inducible gene expression in response to the electrophiles 4-HNE and tBHQ, the TRE was refractory to induction by either electrophiles or PMA. It was demonstrated that stellate cells possess nuclear TRE-binding proteins that were identified as JunB, JunD, Fra1, and Fra2, which were unaffected by either electrophiles or PMA treatment. This report demonstrates that, in contrast to the ARE, the TRE and its binding cognate AP-1 did not mediate independent gene induction in hepatic stellate cells. This observation is significant given the presumed importance attributed to AP-1 in mediating profibrogenic gene expression.

ABT1-associated protein (ABTAP), a novel nuclear protein conserved from yeast to mammals, represses transcriptional activation by ABT1

Various TATA-binding protein (TBP)-associated proteins are involved in the regulation of gene expression through control of basal transcription directed by RNA polymerase (Pol) II. We recently identified a novel nuclear protein, activator of basal transcription 1 (ABT1), which binds TBP and DNA, and enhances Pol II-directed basal transcription. To better understand regulatory mechanisms for ABT1, we searched for ABT1-binding proteins using a yeast two-hybrid screening and isolated a cDNA clone encoding a novel protein termed ABT1-associated protein (ABTAP). ABTAP formed a complex with ABT1 and suppressed the ABT1-induced activation of Pol II-directed transcription in mammalian cells. Furthermore, ABTAP directly bound to ABT1, disrupted the interaction between ABT1 and TBP, and suppressed the ABT1-induced activation of Pol II-directed basal transcription in vitro. These two proteins colocalized in the nucleolus and nucleoplasm and were concomitantly relocalized into discrete nuclear bodies at higher expression of ABTAP. Taken together, these results suggest that ABTAP binds and negatively regulates ABT1. The ABT1/ABTAP complex is evolutionarily conserved and may constitute a novel regulatory system for basal transcription.

Temporal and regional expression of Fos-related proteins in response to ischemic injury

The AP-1 transcription factor family has been widely studied in the response to ischemic brain injury. The data to date have demonstrated a complex involvement that depends on stimulus, subunit composition and brain region. One member in particular, the Fos-related antigen FRA-2, has demonstrated a potential for controlling neuroprotective gene expression. This study characterized the temporal and regional expression of a variety of proteins following ischemic injury induced by occlusion of the middle cerebral artery in rats. The results demonstrated upregulation of both c-Fos and FRA-2 in penumbral regions that preceded upregulation of the classic injury-associated proteins expressed by astrocytes and microglia and, in the case of FRA-2, appeared to correlate with neuronal survival. A further, previously undescribed, expression of FRA-2 in endothelial cells of the core ischemic region was also demonstrated.

The RNA polymerase II core promoter

The events leading to transcription of eukaryotic protein-coding genes culminate in the positioning of RNA polymerase II at the correct initiation site. The core promoter, which can extend ~35 bp upstream and/or downstream of this site, plays a central role in regulating initiation. Specific DNA elements within the core promoter bind the factors that nucleate the assembly of a functional preinitiation complex and integrate stimulatory and repressive signals from factors bound at distal sites. Although core promoter structure was originally thought to be invariant, a remarkable degree of diversity has become apparent. This article reviews the structural and functional diversity of the RNA polymerase II core promoter.

AP1 genes in Fugu indicate a divergent transcriptional control to that of mammals

The draft genomic sequence of the Japanese puffer fish, Fugu rubripes, has now been announced. This is the first complete sequence of a teleost fish and the second available vertebrate sequence, the first being that of human. For the first time, whole-genome comparisons between two vertebrates can be undertaken. Early analysis has suggested that there may be surprising differences in gene regulation between human and fish. In mammals, a gene commonly has several functions, and this may not always be the case in fish. Many gene families comprise more members in fish than they do in mammals, possibly because each fish gene has evolved an individual function. Complexities of gene regulation in mammals has hampered studies of all biological processes from cell proliferation to cell death. Determining the activities of the AP1 transcription factor proteins has been non-trivial. The AP1 complex typically comprises two proteins, a Jun (c-Jun, JunB, and JunD) and a Fos (c-Fos, FosB, Fra1, and Fra2). These proteins can form both homodimers and heterodimers among-themselves and can interact with additional proteins; thus, dissecting their individual roles has been difficult. We have determined that Fugu has more Jun and Fos genes than mammals, and if each proves to have a separate function, then addressing the roles of the individual AP1 proteins in Fugu may be simpler than in human.

Signal transduction and neurosurvival in experimental models of brain injury

Brain injury and neurodegenerative disease are linked by their primary pathological consequence-death of neurons. Current approaches for the treatment of neurodegeneration are limited. In this review, we discuss animal models of human brain injury and molecular biological data that have been obtained from their analysis. In particular, signal transduction pathways that are associated with neurosurvival following injury to the brain are presented and discussed.

Intrinsic disorder in cell-signaling and cancer-associated proteins

The number of intrinsically disordered proteins known to be involved in cell-signaling and regulation is growing rapidly. To test for a generalized involvement of intrinsic disorder in signaling and cancer, we applied a neural network predictor of natural disordered regions (PONDR VL-XT) to four protein datasets: human cancer-associated proteins (HCAP), signaling proteins (AfCS), eukaryotic proteins from SWISS-PROT (EU_SW) and non-homologous protein segments with well-defined (ordered) 3D structure (O_PDB_S25). PONDR VL-XT predicts >or=30 consecutive disordered residues for 79(+/-5)%, 66(+/-6)%, 47(+/-4)% and 13(+/-4)% of the proteins from HCAP, AfCS, EU_SW, and O_PDB_S25, respectively, indicating significantly more intrinsic disorder in cancer-associated and signaling proteins as compared to the two control sets. The disorder analysis was extended to 11 additional functionally diverse categories of human proteins from SWISS-PROT. The proteins involved in metabolism, biosynthesis, and degradation together with kinases, inhibitors, transport, G-protein coupled receptors, and membrane proteins are predicted to have at least twofold less disorder than regulatory, cancer-associated and cytoskeletal proteins. In contrast to 44.5% of the proteins from representative non-membrane categories, just 17.3% of the cancer-associated proteins had sequence alignments with structures in the Protein Data Bank covering at least 75% of their lengths. This relative lack of structural information correlated with the greater amount of predicted disorder in the HCAP dataset. A comparison of disorder predictions with the experimental structural data for a subset of the HCAP proteins indicated good agreement between prediction and observation. Our data suggest that intrinsically unstructured proteins play key roles in cell-signaling, regulation and cancer, where coupled folding and binding is a common mechanism.

The absence of activator protein 1-dependent gene expression in THP-1 macrophages stimulated with phorbol esters is due to lack of p38 mitogen-activated protein kinase activation

Activator protein 1 (AP-1) binds to the promoters of many genes involved in immune and inflammatory responses. We have previously shown that the p38 mitogen-activated protein (MAP) kinase regulates NF-kappa B-dependent gene expression by modulating the phosphorylation and subsequent activation of TATA-binding protein (TBP). In this study, we asked whether the p38 MAP kinase regulated the transcriptional activity of AP-1. We found that phorbol 12-myristate 13-acetate (PMA) was unable to drive the AP-1-dependent reporter gene in THP-1 cells. PMA activated both the extracellular signal-regulated kinase and c-Jun NH(2)-terminal kinase MAP kinases, but it did not activate the p38 MAP kinase. We found that cells expressing MAP kinase kinase 6(Glu), which is the upstream kinase that activates the p38 MAP kinase, had significantly increased AP-1-dependent gene expression alone and when stimulated with PMA. These cells also had increased phosphorylation of native c-Jun, suggesting that both c-Jun NH(2)-terminal kinase and p38 MAP kinases phosphorylate c-Jun. More importantly, expression of a constitutive active MAP kinase kinase 6(Glu) resulted in the phosphorylation of a His-TBP fusion protein and increased direct interaction of TBP with c-Jun. These findings suggest that in macrophages, the p38 MAP kinase regulates AP-1-driven transcription by modulating the activation of TBP.

Close encounters of many kinds: Fos-Jun interactions that mediate transcription regulatory specificity

Fos and Jun family proteins regulate the expression of a myriad of genes in a variety of tissues and cell types. This functional versatility emerges from their interactions with related bZIP proteins and with structurally unrelated transcription factors. These interactions at composite regulatory elements produce nucleoprotein complexes with high sequence-specificity and regulatory selectivity. Several general principles including binding cooperativity and conformational adaptability have emerged from studies of regulatory complexes containing Fos-Jun family proteins. The structural properties of Fos-Jun family proteins including opposite orientations of heterodimer binding and the ability to bend DNA can contribute to the assembly and functions of such complexes. The cooperative recruitment of transcription factors, coactivators and chromatin remodeling factors to promoter and enhancer regions generates multiprotein transcription regulatory complexes with cell- and stimulus-specific transcriptional activities. The gene-specific architecture of these complexes can mediate the selective control of transcriptional activity.

Mechanisms underlying hypoxia-induced neuronal apoptosis

In vivo models of cerebral hypoxia-ischemia have shown that neuronal death may occur via necrosis or apoptosis. Necrosis is, in general, a rapidly occurring form of cell death that has been attributed, in part, to alterations in ionic homeostasis. In contrast, apoptosis is a delayed form of cell death that occurs as the result of activation of a genetic program. In the past decade, we have learned considerably about the mechanisms underlying apoptotic neuronal death following cerebral hypoxia-ischemia. With this growth in knowledge, we are coming to the realization that apoptosis and necrosis, although morphologically distinct, are likely part of a continuum of cell death with similar operative mechanisms. For example, following hypoxia-ischemia, excitatory amino acid release and alterations in ionic homeostasis contribute to both necrotic and apoptotic neuronal death. However, apoptosis is distinguished from necrosis in that gene activation is the predominant mechanism regulating cell survival. Following hypoxic-ischemic episodes in the brain, genes that promote as well as inhibit apoptosis are activated. It is the balance in the expression of pro- and anti-apoptotic genes that likely determines the fate of neurons exposed to hypoxia. The balance in expression of pro- and anti-apoptotic genes may also account for the regional differences in vulnerability to hypoxic insults. In this review, we will examine the known mechanisms underlying apoptosis in neurons exposed to hypoxia and hypoxia-ischemia.

The retinoblastoma gene family members pRB and p107 coactivate the AP-1-dependent mouse tissue factor promoter in fibroblasts

Serum-stimulation of quiescent mouse fibroblasts results in transcriptional activation of tissue factor (TF), the cellular initiator of blood coagulation. This requires the rapid entry of c-Fos into specific AP-1 DNA-binding complexes and can be strongly inhibited by the adenovirus EIA 12S gene product. In this study, we utilized a panel of E1A mutants deficient in cellular protein binding to analyse the molecular basis for EIA inhibition of a minimal, c-Fos-dependent TF promoter/ reporter construct in mouse AKR-2B fibroblasts. Mutations which impaired binding of the retinoblastoma tumor suppressor protein family members pRB, p107, and p130 relieved E1A-mediated inhibition of transcription in response to serum-stimulation or c-Fos overexpression. Inhibition was restricted to the G0 to G1 transition, consistent with the specificity of E1A for hypophosphorylated forms of RB proteins. Although E1A mutants deficient in CBP/p300 binding retained the ability to inhibit TF transcription, deletion of the amino-terminal portion of the CBP/p300 interaction domain was required to permit rescue of TF promoter activity by coexpression of pRB. Moreover, ectopic p107 could effectively substitute for pRB in relieving E1A-mediated repression. In primary mouse embryo fibroblasts, activity of the minimal AP-1-dependent TF promoter was suppressed in Rb(-/-) cells compared to parallel Rb(+/-) and Rb(+/+) transfectants. Ectopic expression of either pRB or p107 markedly enhanced TF promoter activity in Rb(-/-) fibroblasts. Collectively, these data imply that pRB and p107 can cooperate with c-Fos to activate TF gene transcription in fibroblasts and suggest a requirement for another, as yet unidentified, E1A-binding protein.

Fosl1 is a transcriptional target of c-Fos during osteoclast differentiation

Osteoclasts are bone-resorbing cells derived from haematopoietic precursors of the monocyte-macrophage lineage. Mice lacking Fos (encoding c-Fos) develop osteopetrosis due to an early differentiation block in the osteoclast lineage. c-Fos is a component of the dimeric transcription factor activator protein-1 (Ap-1), which is composed mainly of Fos (c-Fos, FosB, Fra-1 and Fra-2) and Jun proteins (c-Jun, JunB and JunD). Unlike Fra-1 (encoded by Fosl1), c-Fos contains transactivation domains required for oncogenesis and cellular transformation. The mechanism by which c-Fos exerts its specific function in osteoclast differentiation is not understood. Here we show by retroviral-gene transfer that all four Fos proteins, but not the Jun proteins, rescue the differentiation block in vitro. Structure-function analysis demonstrated that the major carboxy-terminal transactivation domains of c-Fos and FosB are dispensable and that Fra-1 (which lacks transactivation domains) has the highest rescue activity. Moreover, a transgene expressing Fra-1 rescues the osteopetrosis of c-Fos-mutant mice in vivo. The osteoclast differentiation factor Rankl (also known as TRANCE, ODF and OPGL; refs 8-11) induces transcription of Fosl1 in a c-Fos-dependent manner, thereby establishing a link between Rank signalling and the expression of Ap-1 proteins in osteoclast differentiation.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

The MDM2 C-terminal region binds to TAFII250 and is required for MDM2 regulation of the cyclin A promoter

MDM2 proto-oncogene expression is aberrant in many human tumors. Its normal role is to modulate the functions of p53. The N terminus of MDM2 interacts with p53, whereas the properties of the rest of the molecule are poorly understood. We show that MDM2 binds to the general transcription factor TFIID in vivo. The C-terminal Ring finger interacts with TAFII250/CCG1, and the central acidic domain interacts with TBP. Expression of MDM2 activates the cyclin A gene promoter but not c-fos, showing that the effects of MDM2 are specific. Deletion of the C-terminal region of MDM2 abolishes activation, showing that the C-terminal domain of MDM2 is functionally important. We found that increasing MDM2 expression to higher levels inhibits the cyclin A promoter. Inhibition appears to result from titration of general transcription factors because MDM2 overexpression inhibits c-fos as well as other promoters in vivo and basal transcription in vitro. The mechanisms of repression of the cyclin A and fos promoters appear to be different. Cyclin A repression is lost by deleting the C terminus, whereas that of c-fos is lost by removal of the acidic domain. These results reinforce the conclusion that the C terminus of MDM2 mediates effects on the cyclin A promoter. MDM2 transformed cells contain elevated levels of cyclin A mRNA, showing that activation occurs under physiological conditions. There is a positive correlation between MDM2 binding to TAFII250 and MDM2 activation of the cyclin A promoter. The C-terminal region of MDM2, which contains the Ring finger, interacts with TAFII250 and is required for regulation of the cyclin A promoter by MDM2. Our results link the activity of MDM2, a transforming protein implicated in many human tumors, with cyclin A, a regulator of the cell cycle.

Effects of Ca++ mobilization on expression of androgen-regulated genes: interference with androgen receptor-mediated transactivation by AP-I proteins

BACKGROUND

The adult prostate is maintained through an equilibrium between cell growth and death rates. Androgen deprivation induces an increase in intracellular Ca++, AP-1 gene expression of androgen-inducible genes.

METHODS

Northern blot analysis, band-shift assays, and transient cotransfection assays were used to study the effects of Ca++ mobilizer A23187 on gene expression in human prostate cancer cells.

RESULTS

A23187 repressed androgen-upregulated mRNAs for prostate-specific antigen (PSA) and hKLK2, and rapidly induced mRNA levels for c-fos and c-jun. AP-1 protein-DNA binding activities were elevated after A23187 treatments. Androgen receptor (AR)-mediated induction of chloramphenicol acetyltransferase (CAT) reporter was repressed by AP-1 proteins.

CONCLUSIONS

The repression of AR-mediated induction of PSA and hKLK2 genes by Ca++ mobilizers is due to the interference of AR transactivation activity by AP-1 proteins.

T-cell proto-oncogene rhombotin-2 is a complex transcription regulator containing multiple activation and repression domains

The LIM domain protein rhombotin-2 (RBTN-2/TTG-2/LMO2) is involved in many processes, including leukemogenesis and erythropoiesis. It is thought that the principle role of RBTN-2 in these processes is to regulate transcription. To examine the potential for RBTN-2 to modulate transcription, we constructed RBTN-2/GAL4 DNA-binding domain fusion proteins and measured their ability to activate transcription of a reporter gene construct. From these studies we identified a transcription activation domain within the NH2 terminus of RBTN-2. This activation domain was further localized within a proline-rich 19-amino acid region. A second activation domain of 11 amino acids was also identified. This domain was located within the COOH terminus of RBTN-2, and functioned in mammalian cells but not in yeast. Furthermore, the two LIM domains of RBTN-2 were shown to function as transcription repression domains. Each individual LIM domain acted as an independent transcription repression domain on a heterologous activation domain. However, in context of full-length RBTN-2, the LIM domains selectively repressed the NH2-terminal activation domain, but had no effect on the COOH-terminal domain. Overall, these results demonstrate that the T-cell oncogene RBTN-2 is a complex transcription factor possessing multiple transcription regulatory modules, including two activation domains and two repression domains.

Effect of c-Fos overexpression on the murine macrophage cell line P388DI

In order to study the role of Fos on the regulation of proliferation in the monocyte-macrophage lineage we realized a stable transfection of the murine P388D1 cell line by the murine c-fos gene under the control of the human metallothionein IIA promoter. Several clones have been selected by geneticin: they show a variable number of integrated transgene (two to ten copies). Their expression has been analyzed in the presence or absence of cadmium chloride as inducer (5 x 10(-6) M). In one clone especially, the c-fos mRNA and Fos protein levels were respectively 6- and 10-fold increased. The study of cell growth by tritiated thymidine incorporation indicates a negative effect of the overexpressed Fos protein in the absence of any other stimulus.

Functional analysis of the Schistosoma mansoni 28 kDa glutathione S-transferase gene promoter: involvement of SMNF-Y transcription factor in multimeric complexes

The ability of the 5' flanking region of the gene encoding the 28 kDa glutathione S-transferase of Schistosoma mansoni gene to promote transcription, was studied in different mammalian cell lines. Results of transient transfection assays showed a strong activity of the -277 to +1 nt region of the Sm28GST gene, comparable to that of well-studied promoters. Deletion analysis indicated that an AP-1 site and two closely located CCAAT (Y1 and Y2) boxes were the principal motifs responsible for the promoter activity. Binding of the NF-Y complex to Y1 and Y2, as well as to a third CCAAT box (Y3) close to the promoter TATA box, was compared in gel shift and super-shift experiments. All of the three Y boxes bound protein complexes from S. mansoni nuclear extracts that were shown to contain the A subunit of the schistosome NF-Y complex (SMNF-YA). Competition assays revealed a differential affinity of the Y1, Y2 and Y3 sequences for NF-Y. The Y1, Y2 and Y3 regions were also shown to activate transcription when included in an heterologous promoter and data obtained strongly suggested the involvement of SMNF-Y in multimeric complexes during this process.

Wavelength-specific induction of immediate early genes by ultraviolet radiation

Exposure of skin in vivo to ultraviolet B (UVB) or ultraviolet A (UVA) radiation produces a variety of distinct clinical manifestations. In the present study, we characterized the immediate early genes that are activated in an epidermoid carcinoma cell line (A431) when exposed to UVB (FS20 sunlamp) or UVA radiation (window glass-filtered black light). We observed that: (a) c-jun mRNA expression is upregulated predominantly by UVB; (b) fra-1 and c-myc are downregulated by UVB, whereas both are upregulated by UVA; (c) fra-2 and AP-2 are downregulated modestly by UVB, (d) c-fos is unaffected, and (e) optimal regulation of each gene is achieved at environmentally relevant fluences (25-100 J/m2 for UVB and 2500-10 000 J/m2 for UVA). Thus, distinct sets of genes are activated (or repressed) by UVB and UVA irradiation. Treatment with organic hydrogen peroxides mimicked UVB radiation in upregulating c-jun expression, suggesting the participation of reactive oxygen intermediates in the UVB-signaling pathway. We propose that wavelength-specific regulation of nuclear mediator genes accounts for the development of at least some of the wavelength-specific cutaneous manifestations of ultraviolet radiation.

Wild-type and transactivation-defective mutants of human immunodeficiency virus type 1 Tat protein bind human TATA-binding protein in vitro

SUMMARY

Tat regulates human immunodeficiency virus type 1 (HIV-1) gene expression by increasing both the rate of transcription initiation and the efficiency of transcription elongation. The ability of Tat to facilitate HIV-1 transcription preinitiation complex formation suggests that components of the basal transcriptional machinery may be targeted by Tat. Previous studies have demonstrated that Tat interacts directly with the human TATA-binding protein (TBP) and specific TBP-associated factors (TAFS) that comprise the TFIID complex. Here, in vitro glutathione S-transferase protein binding assays containing fully functional or transactivation-defective mutant Tat proteins have been used to investigate the functional significance of the direct interaction between Tat and TBP relative to Tat transactivation. Results demonstrate that full-length Tat, as well as the activation domain of Tat alone, binds human TBP in vitro. Site-directed mutations within the activation domain of Tat (C22G and P18IS) that abrogate transactivation by Tat in vivo fail to inhibit Tat-TBP binding. Full-length Tat, the activation domain of Tat alone, and a transactivation-defective mutant of Tat that lacks N-terminal amino acid residues 2-36 bind with equal efficiencies to TBP provided that the H1 alpha helical domain that maps to amino acids 167-220 within the highly conserved carboxyl terminus of TBP is maintained. These data indicate that an activity mapped within the activation domain of Tat, which is distinct from Tat-TBP binding. is required for transactivation by Tat.

TEF-1 transrepression in BeWo cells is mediated through interactions with the TATA-binding protein, TBP

Transcription enhancer factor-1 (TEF-1) has been implicated in transactivating a placental enhancer (CSEn) that regulates human chorionic somatomammotropin (hCS) gene activity. We demonstrated that TEF-1 represses hCS promoter activity in choriocarcinoma (BeWo) cells (Jiang, S.W., and Eberhardt, N.L. (1995) J. Biol. Chem. 270, 13609-13915), suggesting that TEF-1 interacts with basal transcription factors. Here we demonstrate that hTEF-1 overexpression inhibits minimal hCS promoters containing TATA and/or initiator elements, Rous sarcoma virus and thymidine kinase promoters in BeWo cells. Cotransfection of TEF-1 antisense oligonucleotides alleviated exogenous TEF-1-mediated repression and increased basal hCS promoter activity, indicating that endogenous TEF-1 exerts repressor activity. GST-TEF-1 fusion peptides fixed to glutathione-Sepharose beads retained in vitro-generated human TATA-binding protein, hTBP. The TEF-1 proline-rich domain was essential for TBP binding, but polypeptides also containing the zinc finger domain bound TBP with higher apparent affinity. TBP supershifted hTEF-GT-IIC DNA complexes, but TEF-1 inhibited in vitro binding of TBP to the TATA motif. Coexpression of TBP and TEF-1 in BeWo cells alleviated TEF-1-mediated transrepression, indicating that the TBP-TEF-1 interaction is functional in vivo. The data indicate that TEF-1 transrepression is mediated by direct interactions with TBP, possibly by inhibiting preinitiation complex formation.

Transcription factor AP-1, and the role of Fra-2

Transcription factors function to regulate gene transcription. They may be constitutively expressed or may only be activated during specific situations. Activator protein-1 (AP-1) is an inducible transcription factor, and is comprised of multiple protein complexes that include the gene products of the fos and jun gene families. Numerous cellular and viral genes contain AP-1 binding sites within their promoters and, accordingly, AP-1 has been shown to play a role in the regulation of both basal and inducible transcription of these genes. fos-related antigen-2 (fra-2) has been found to have both similar and unique properties to that of other fos gene members in terms of its regulation and expression. The analysis and determination of the function of Fra-2 will provide further information on the role of AP-1.

A functional initiator element in the human beta-globin promoter

Core promoters are defined by the presence of either a TATA box at approximately 30 base pairs upstream of the transcriptional start site (+1) and/or an initiator element centered around the +1 site. The prevalence, function, and significance of the various combinations of core promoter elements are as yet unclear. We describe here the identification and characterization of an initiator element in the TATA-containing human beta-globin promoter. Mutagenesis of the beta-globin initiator element at positions +2/+3 and +4/+5 abrogates transcription in a heterologous construct. Interestingly, we have found a beta-globin initiator binding activity in nuclear extracts whose presence or absence correlates with function of the beta-globin initiator. Accordingly, this binding activity may be part of the machinery required for beta-globin initiator-dependent transcription. Our analysis further describes a previously uncharacterized beta-thalassemia mutation at the +1 site as a mutation that decreases beta-globin initiator activity. Finally, consistent with other initiator elements, the beta-globin initiator requires a TFIID-containing fraction for in vitro activity. Thus, the human beta-globin promoter contains an initiator element whose function, as revealed by a beta-thalassemia mutation, is of physiological relevance.

Bovine papillomavirus type 1 E2 transcriptional regulators directly bind two cellular transcription factors, TFIID and TFIIB

The bovine papillomavirus type 1 (BPV-1) E2 translational open reading frame encodes three proteins that regulate viral transcription and DNA replication: the E2 transcriptional activator (E2TA), the E2 transcriptional repressor (E2TR) and the E8/E2 transcriptional repressor (E8/E2TR). E2TA is a strong activator of papillomaviral promoters and is required for viral DNA replication. E2TR and E8/E2TR inhibit the activities of E2TA but also possess weak transactivational properties of their own. Two components of the cellular transcription apparatus, TFIID and TFIIB, have previously been shown to associate with other viral and cellular transcriptional activators. We present evidence here that E2TA, the full-length E2 open reading frame gene product, directly binds both of these transcription factors in vitro. Glutathione S-transferase E2TA (GST-E2TA) fusion protein bound in vitro-synthesized TATA-box-binding protein (TBP), a component of TFIID, and in vitro-synthesized TFIIB. Likewise, GST-E2TA bound TFIID and TFIIB present in a nuclear extract from the human cervical cancer-derived cell line, HeLa. The binding of GST-E2TA to TBP and TFIIB required no additional mammalian factors, as shown by direct binding of GST-E2TA to bacterially synthesized recombinant TBP and recombinant TFIIB. The domain of E2TA required for its interaction with both TBP and TFIIB was localized to the C terminus of E2TA, a region also present in E2TR and E8/E2TR. This domain lies within the region of E2TA previously shown to confer cooperative DNA binding by E2TA and TBP and overlaps with the region of E2TA required for DNA binding and dimerization. Our findings, taken in context with previous studies, lead us to conclude that (i) cooperative DNA binding by E2 proteins and TBP is likely mediated by the direct binding of E2 proteins to TBP, (ii) the weak transcriptional transactivation by E2TR and E8/E2TR may result as a consequence of direct TBP and TFIIB binding by these proteins, and (iii) TBP and/or TFIIB binding may be required but is not sufficient for E2TA's strong transactivational activity.

The c-jun proto-oncogene down-regulates the rat alpha-fetoprotein promoter in HepG2 hepatoma cells without binding to DNA

The effects of a phorbol ester (TPA) and of members of the Jun and Fos oncoprotein family on the activity of the rat alpha-fetoprotein (AFP) promoter were checked by using transient expression experiments in HepG2 hepatoma cells. TPA blocked the activity of the rat AFP promoter in a dose-dependent manner. Overexpression of c-Jun specifically repressed the rat AFP promoter but not the albumin promoter. JunB and JunD were poorer inhibitors. c-Fos expression did not potentiate the negative effect of Jun. The Jun-induced repression does not require binding of c-Jun to the AFP promoter. DNase 1 footprinting experiments did not display any high affinity binding site for Jun on the AFP promoter. Integrity of the c-Jun DNA binding domain is not required for the c-Jun protein to block the AFP promoter. The N-terminal part of Jun, which contains the activating domain, is responsible for the repression as shown by using Jun-Gal4 chimera. Jun likely exerts its negative control on the AFP promoter via protein-protein interactions with a not yet identified trans-activating factor within the -134 to +6 region or with a component of the general machinery of transcription. Jun proteins can thus be key intermediates in regulatory cascades which result in the differential modulation of the AFP and albumin gene expression in the course of liver development and carcinogenesis.