A cluster of phosphorylation sites on the cyclic AMP-regulated nuclear factor CREB predicted by its sequence

Protein kinase C activation selectively increases mRNA levels for one of the regulatory subunits (RI alpha) of cAMP-dependent protein kinases in HT-29 cells

We have examined the effect of the protein kinase C activator, TPA, on mRNA levels for subunits of cAMP-dependent protein kinases in the human colonic cancer cell line HT-29, subline m2. Messenger RNA for the regulatory subunit, RI alpha, of cAMP-dependent protein kinases was shown to be present and regulated by TPA. Other mRNAs for subunits of cAMP-dependent protein kinases (RI beta, RII alpha, RII beta, C alpha, C beta) were also present in these cells, but revealed no or only minor changes upon TPA stimulation. When HT-29 cells were cultured in the presence of 10 nM TPA for various time periods, a biphasic response was observed in RI alpha mRNA levels with a maximal increase (approximately 4 fold) after 24 hours. TPA stimulated RI alpha mRNA increased in a concentration-dependent manner and maximal response (4-8 fold) was seen at 3-10 nM. The TPA-induced increase in RI alpha mRNA was not obtained when cells were incubated with TPA together with the protein kinase C inhibitors, staurosporine or H7. The cAMP-analog 8-CPTcAMP alone induced RI alpha mRNA levels 50% more than TPA. Combined treatment with TPA (10 nM) and 8-CPTcAMP (0.1 mM) gave an increase in RI alpha mRNA similar to TPA. These results demonstrate an interaction between the protein kinase C pathway and mRNA levels for the RI alpha subunit of cAMP-dependent protein kinases in HT-29 cells.

Functional analysis of the murine T-cell receptor beta enhancer and characteristics of its DNA-binding proteins

The minimal T-cell receptor (TCR) beta-chain (TCR beta) enhancer has been identified by transfection into lymphoid cells. The minimal enhancer was active in T cells and in some B-lineage cells. When a larger fragment containing the minimal enhancer was used, its activity was apparent only in T cells. Studies with phytohemagglutinin and 4 beta-phorbol-12,13-dibutyrate revealed that the enhancer activity was increased by these agents. By a combination of DNase I footprinting, gel mobility shift assay, and methylation interference analysis, seven different motifs were identified within the minimal enhancer. Furthermore, competition experiments showed that some of these elements bound identical or similar factors that are known to bind to the TCR V beta promoter decamer or to the immunoglobulin enhancer kappa E2 or muEBP-E motif. These shared motifs may be important in the differential gene activity among the different lymphoid subsets.

E4F and ATF, two transcription factors that recognize the same site, can be distinguished both physically and functionally: a role for E4F in E1A trans activation

Previous experiments have identified an element in the adenovirus E4 promoter that is critical for E1A-dependent trans activation and that can confer inducibility to a heterologous promoter. This DNA element is a recognition site for multiple nuclear factors, including ATF, which is likely a family of DNA-binding factors with similar DNA recognition properties. However, ATF activity was found not to be altered in any demonstrable way as a result of adenovirus infection. In contrast, another factor that recognizes this element, termed E4F, was found at only very low levels in uninfected cells but was increased markedly upon adenovirus infection, as measured in DNA-binding assays. Although both the ATF activity and the E4F activity recognized and bound to the same two sites in the E4 promoter, they differed in their sequence recognition of these sites. Furthermore, E4F bound only to a small subset of the ATF recognition sites; for instance, E4F did not recognize the ATF sites in the E2 or E3 promoters. Various E4F and ATF binding sites were inserted into an expression vector and tested by cotransfection assays for responsiveness to E1A. We found that a sequence capable of binding E4F could confer E1A inducibility. In contrast, a sequence that could bind ATF but not E4F did not confer E1A inducibility. We also found that E4F formed a stable complex with the E4 promoter, whereas the ATF DNA complex was unstable and rapidly dissociated. We conclude that the DNA-binding specificity of E4F as well as the alterations in DNA-binding activity of E4F closely correlates with E1A stimulation of the E4 promoter.

Mutations that define the optimal half-site for binding yeast GCN4 activator protein and identify an ATF/CREB-like repressor that recognizes similar DNA sites

The yeast GCN4 transcriptional activator protein binds as a dimer to a dyad-symmetric sequence, indicative of a protein-DNA complex in which two protein monomers interact with adjacent half-sites. However, the optimal GCN4 recognition site, ATGA(C/G)TCAT, is inherently asymmetric because it contains an odd number of base pairs and because mutation of the central C.G base pair strongly reduces specific DNA binding. From this asymmetry, we suggested previously that GCN4 interacts with nonequivalent and possibly overlapping half-sites (ATGAC and ATGAG) that have different affinities. Here, we examine the nature of GCN4 half-sites by creating symmetrical derivatives of the optimal GCN4 binding sequence that delete or insert a single base pair at the center of the site. In vitro, GCN4 bound efficiently to the sequence ATGACGTCAT, whereas it failed to bind to ATGAGCTCAT or ATGATCAT. These observations strongly suggest that (i) GCN4 specifically recognizes the central base pair, (ii) the optimal half-site for GCN4 binding is ATGAC, not ATGAG, and (iii) GCN4 is a surprisingly flexible protein that can accommodate the insertion of a single base pair in the center of its compact binding site. The ATGACGTCAT sequence strongly resembles sites bound by the yeast and mammalian ATF/CREB family of proteins, suggesting that GCN4 and the ATF/CREB proteins recognize similar half-sites but have different spacing requirements. Unexpectedly, in the context of the his3 promoter, the ATGACGTCAT derivative reduced transcription below the basal level in a GCN4-independent manner, presumably reflecting DNA binding by a distinct ATF/CREB-like repressor protein. In other promoter contexts, however, the same site acted as a weak upstream activating sequence.

The promoter of the human parathyroid hormone gene contains a functional cyclic AMP-response element

We have screened the sequence of the 394 base pairs upstream of the main transcriptional start site of the promoter of the human parathyroid hormone (PTH) gene for well-known protein recognition motifs with the aim to identify potential positive or negative regulatory elements. Within this region we found a potential cAMP-response element (CRE) besides several other putative binding sites for transcription factors. We fused promoter regions that contain this element and extend beyond the transcription start site to an appropriate reporter gene (CAT) and transfected different cell lines with these constructs. Transient expression of the CAT gene from these hybrid genes could be shown to be significantly stimulated by forskolin or isoproterenol thus proving the responsiveness of the whole promoter region towards elevated cAMP levels. DNase I protection studies revealed protein binding around the putative CRE (PTH-CRE) and an adjacent CCAAT element. Gel retardation assays with the PTH-CRE as well as the well-characterized CRE from the rat somatostatin promoter indicated specific binding of the same protein to both elements, although with a slightly reduced affinity of the PTH-CRE. Both of these elements were also able to confer cAMP-responsiveness to a heterologous promoter.

Functional analysis of the murine T-cell receptor beta enhancer and characteristics of its DNA-binding proteins

The minimal T-cell receptor (TCR) beta-chain (TCR beta) enhancer has been identified by transfection into lymphoid cells. The minimal enhancer was active in T cells and in some B-lineage cells. When a larger fragment containing the minimal enhancer was used, its activity was apparent only in T cells. Studies with phytohemagglutinin and 4 beta-phorbol-12,13-dibutyrate revealed that the enhancer activity was increased by these agents. By a combination of DNase I footprinting, gel mobility shift assay, and methylation interference analysis, seven different motifs were identified within the minimal enhancer. Furthermore, competition experiments showed that some of these elements bound identical or similar factors that are known to bind to the TCR V beta promoter decamer or to the immunoglobulin enhancer kappa E2 or muEBP-E motif. These shared motifs may be important in the differential gene activity among the different lymphoid subsets.

E4F and ATF, two transcription factors that recognize the same site, can be distinguished both physically and functionally: a role for E4F in E1A trans activation

Previous experiments have identified an element in the adenovirus E4 promoter that is critical for E1A-dependent trans activation and that can confer inducibility to a heterologous promoter. This DNA element is a recognition site for multiple nuclear factors, including ATF, which is likely a family of DNA-binding factors with similar DNA recognition properties. However, ATF activity was found not to be altered in any demonstrable way as a result of adenovirus infection. In contrast, another factor that recognizes this element, termed E4F, was found at only very low levels in uninfected cells but was increased markedly upon adenovirus infection, as measured in DNA-binding assays. Although both the ATF activity and the E4F activity recognized and bound to the same two sites in the E4 promoter, they differed in their sequence recognition of these sites. Furthermore, E4F bound only to a small subset of the ATF recognition sites; for instance, E4F did not recognize the ATF sites in the E2 or E3 promoters. Various E4F and ATF binding sites were inserted into an expression vector and tested by cotransfection assays for responsiveness to E1A. We found that a sequence capable of binding E4F could confer E1A inducibility. In contrast, a sequence that could bind ATF but not E4F did not confer E1A inducibility. We also found that E4F formed a stable complex with the E4 promoter, whereas the ATF DNA complex was unstable and rapidly dissociated. We conclude that the DNA-binding specificity of E4F as well as the alterations in DNA-binding activity of E4F closely correlates with E1A stimulation of the E4 promoter.

Mutations that define the optimal half-site for binding yeast GCN4 activator protein and identify an ATF/CREB-like repressor that recognizes similar DNA sites

The yeast GCN4 transcriptional activator protein binds as a dimer to a dyad-symmetric sequence, indicative of a protein-DNA complex in which two protein monomers interact with adjacent half-sites. However, the optimal GCN4 recognition site, ATGA(C/G)TCAT, is inherently asymmetric because it contains an odd number of base pairs and because mutation of the central C.G base pair strongly reduces specific DNA binding. From this asymmetry, we suggested previously that GCN4 interacts with nonequivalent and possibly overlapping half-sites (ATGAC and ATGAG) that have different affinities. Here, we examine the nature of GCN4 half-sites by creating symmetrical derivatives of the optimal GCN4 binding sequence that delete or insert a single base pair at the center of the site. In vitro, GCN4 bound efficiently to the sequence ATGACGTCAT, whereas it failed to bind to ATGAGCTCAT or ATGATCAT. These observations strongly suggest that (i) GCN4 specifically recognizes the central base pair, (ii) the optimal half-site for GCN4 binding is ATGAC, not ATGAG, and (iii) GCN4 is a surprisingly flexible protein that can accommodate the insertion of a single base pair in the center of its compact binding site. The ATGACGTCAT sequence strongly resembles sites bound by the yeast and mammalian ATF/CREB family of proteins, suggesting that GCN4 and the ATF/CREB proteins recognize similar half-sites but have different spacing requirements. Unexpectedly, in the context of the his3 promoter, the ATGACGTCAT derivative reduced transcription below the basal level in a GCN4-independent manner, presumably reflecting DNA binding by a distinct ATF/CREB-like repressor protein. In other promoter contexts, however, the same site acted as a weak upstream activating sequence.

Domain swapping reveals the modular nature of Fos, Jun, and CREB proteins

The products of the Jun and Fos proto-oncogenes form a heterodimer that binds to and activates transcription from 12-O-tetradecanoylphorbol-13-acetate-responsive promoter elements (TGACTCA) and AP-1-binding sites (TGACATCA). These two proteins belong to a family of related transcription factors which contain similar domains required for protein dimerization and DNA binding but display different protein and DNA binding specificities. The basic region, required for DNA binding, is followed by a leucine zipper structure, a domain that mediates protein-protein interactions. To assess the role of these two domains in three related proteins, Fos, Jun, and CREB, we carried out extensive domain-swapping analysis. We found that (i) dimers formed by two Jun leucine zipper-containing proteins were unable to bind DNA as efficiently as a Fos-Jun combination, regardless of the source of the basic region; (ii) the Fos leucine zipper was unable to form either homo- or heterodimers with a chimeric protein containing a Fos leucine zipper; (iii) the Fos basic region was capable of binding to an AP-1 site; (iv) replacement of the Jun amino terminus with that of CREB had little effect on dimerization, whereas replacement with the amino terminus of Fos disrupted both protein-protein and protein-DNA interactions; (v) changes in relative affinities of the Fos and Jun basic regions for the AP-1 element were dependent on the secondary contributions of amino-terminal residues; and (vi) the Fos-Jun chimeric constructs cooperated in transcriptional transactivation of the Jun promoter in NIH 3T3 cells.

Transcription factor interactions: selectors of positive or negative regulation from a single DNA element

The mechanism by which a single factor evokes opposite regulatory effects from a specific DNA sequence is not well understood. In this study, a 25-base pair element that resides upstream of the mouse proliferin gene was examined; it conferred on linked promoters either positive or negative glucocorticoid regulation, depending upon physiological context. This sequence, denoted a "composite" glucocorticoid response element (GRE), was bound selectively in vitro both by the glucocorticoid receptor and by c-Jun and c-Fos, components of the phorbol ester-activated AP-1 transcription factor. Indeed, c-Jun and c-Fos served as selectors of hormone responsiveness: the composite GRE was inactive in the absence of c-Jun, whereas it conferred a positive glucocorticoid effect in the presence of c-Jun, and a negative glucocorticoid effect in the presence of c-Jun and relatively high levels of c-Fos. The receptor also interacted selectively with c-Jun in vitro. A general model for composite GRE action is proposed that invokes both DNA binding and protein-protein interactions by receptor and nonreceptor factors.

Somatostatin gene regulation: an overview

The somatostatinergic system has proven to be one of the best models of neuropeptide biology. Originally characterized as a hypothalamic regulator of growth hormone secretion, somatostatin also regulates the secretion of several other pituitary, pancreatic, and gastrointestinal (GI) hormones including thyrotropin-stimulating hormone, insulin, glucagon, and gastrin. Disorders in somatostatin metabolism have been proposed to contribute to the pathogenesis of Alzheimer's disease, epilepsy, GI motility disorders, and diabetes. On a more basic level, studies of somatostatin action have integrated divergent concepts of intracellular signal transduction. Advances in the understanding of somatostatin biosynthesis have had an impact on areas outside the field of endocrinology by providing new concepts of eukaryotic gene regulation. This report focuses on the transcriptional regulation of somatostatin gene expression. Two aspects of somatostatin gene transcription will be considered--regulated expression by second messengers and tissue-specific basal expression.

Loss of a phosphorylated form of transcription factor CREB/ATF in poliovirus-infected cells

Host cell RNA synthesis is inhibited by poliovirus infection. We have studied the mechanism of poliovirus-induced inhibition of RNA polymerase II-mediated transcription by using the adenovirus early region 3 (E3) promoter. In vitro transcription from the E3 promoter was severely inhibited in extracts prepared from poliovirus-infected HeLa cells. Four regions in the E3 promoter have been shown to serve as binding sites for cellular transcription factors. These regions contain binding sites for transcription factors NF-1 (site IV), AP-1 (site III), CREB/ATF (site II), and the TATA factor (site I). Binding to these four regions was not significantly altered by poliovirus infection as assayed by DNase I footprinting analysis; furthermore, gel retardation assays failed to reveal dramatic differences in the total amount of CREB/ATF-, AP-1-, and NF-1-binding activity present in mock- or poliovirus-infected cell extracts. Gel retardation assays, however, did reveal significant qualitative differences in the DNA-protein complexes formed with a CREB/ATF-binding site in extracts prepared from poliovirus-infected cells as compared to mock-infected cell extracts. Radioimmunoprecipitation reactions performed with antiserum against CREB/ATF revealed a severe reduction in a phosphorylated form of the protein present in poliovirus-infected cell extracts. However, in vitro kinase reactions demonstrated that mock- and poliovirus-infected cell extracts contained similar levels of CREB/ATF. Expression from the E3 promoter was shown to be activated by CREB/ATF in vivo; this induction was dependent upon the phosphorylation of CREB/ATF. Thus, we propose that poliovirus infection inhibits transcription from the E3 promoter, at least in part, through the dephosphorylation of CREB/ATF.

Characteristics of the cAMP response unit

Cyclic AMP (cAMP) mediates the hormonal stimulation of numerous cellular processes by regulating the phosphorylation of critical target proteins. In this report, we review current work suggesting that cAMP regulates transcription of eukaryotic genes through the reversible phosphorylation of a target nuclear protein called CREB. As CREB is currently the only transcription factor that is regulated by a well-defined cytoplasmic kinase-kinase-A, these studies may help to elucidate the general mechanisms underlying signal transduction.

Characterization of a cAMP-regulated enhancer-binding protein

The transcription regulation of many hormone genes is modulated by intracellular second messengers such as cAMP. The cAMP response element binding protein, CREB, binds to the 8 base pair CRE enhancer, TGACGTCA, that is found in the 5'-flank of certain genes including those for somatostatin and the alpha-subunit of human chorionic gonadotropin. The recent characterization of CREB and CREB-related cDNA clones, combined with Southwesterns and Northern blot analyses, reveals a family of transcription factors that dimerize via a leucine zipper motif and bind to the CRE through positively charged basic regions. The CREB cDNA encoding a 327 residue protein is transcriptionally activated via phosphorylation by protein kinases, including the cAMP-dependent protein kinase-A.

Domain swapping reveals the modular nature of Fos, Jun, and CREB proteins

The products of the Jun and Fos proto-oncogenes form a heterodimer that binds to and activates transcription from 12-O-tetradecanoylphorbol-13-acetate-responsive promoter elements (TGACTCA) and AP-1-binding sites (TGACATCA). These two proteins belong to a family of related transcription factors which contain similar domains required for protein dimerization and DNA binding but display different protein and DNA binding specificities. The basic region, required for DNA binding, is followed by a leucine zipper structure, a domain that mediates protein-protein interactions. To assess the role of these two domains in three related proteins, Fos, Jun, and CREB, we carried out extensive domain-swapping analysis. We found that (i) dimers formed by two Jun leucine zipper-containing proteins were unable to bind DNA as efficiently as a Fos-Jun combination, regardless of the source of the basic region; (ii) the Fos leucine zipper was unable to form either homo- or heterodimers with a chimeric protein containing a Fos leucine zipper; (iii) the Fos basic region was capable of binding to an AP-1 site; (iv) replacement of the Jun amino terminus with that of CREB had little effect on dimerization, whereas replacement with the amino terminus of Fos disrupted both protein-protein and protein-DNA interactions; (v) changes in relative affinities of the Fos and Jun basic regions for the AP-1 element were dependent on the secondary contributions of amino-terminal residues; and (vi) the Fos-Jun chimeric constructs cooperated in transcriptional transactivation of the Jun promoter in NIH 3T3 cells.

Molecular mechanisms of cAMP-regulated gene expression

The ability of many genes to be induced by cAMP is dependent on the presence of enhancers located in the regions of DNA upstream of the start sites to the genes. The two best characterized enhancers are the CRE (5'-TGACGTCA-3') and the AP-2 site (5'-CCCCAGGC-3'). The activity of the CRE is modulated by sequences adjacent to the consensus sequence as well as by promoter context and cell type. The complex control of the CRE is reflected in the large number of cloned CRE binding proteins that arise both from unique genes and from splice variants. These factors are leucine zipper proteins that must dimerize before binding to DNA. Although all of the factors isolated can form active homodimers, many are also able to form heterodimers. The amino termini of these proteins contain consensus phosphorylation sites through which these factors trans-activate their cognate promoters. The diversity of the trans-acting factors and their cis-acting sequences reflects the precise control that cells require in the modulation of gene expression by cAMP.

CREB regulation of cellular cyclic AMP-responsive and adenovirus early promoters

The cyclic AMP (cAMP) response element-binding protein (CREB) has been demonstrated to be a key mediator of cellular promoter response to cAMP. The binding site for this protein in many cellular cAMP inducible promoters (CRE) contains the palindrome sequence TGACGTCA, which contains two half-sites for CREB binding. A related promoter element, with the core sequence TGACG, has significant homology to an AP1-binding site and contains only one half-site for CREB binding. A group of factors known as activating transcription factors (ATF) have been found to bind to the latter and related sequences found upstream of early adenovirus promoters induced by E1A, and these factors are highly homologous to the CREB protein. We wished to characterize CREB, c-jun, and c-fos binding to these sites in the somatostatin gene (CRE) and in the adenovirus early region 3 promoter (E3/ATF). Oligonucleotides complementary to each of these sites were used in gel retardation assays with in vitro-translated CREB protein. These studies indicated that CREB bound primarily as a dimer to both a single and two half-sites, though there was increased affinity to the double compared with the single half-site. The c-jun and c-fos proteins also bound to both the somatostatin CRE- and E3/ATF-binding sites, but CREB did not bind to AP1 recognition sites nor was it capable of forming heterodimers with either c-jun or c-fos. Truncations of the CREB protein, which eliminated regions of the protein containing consensus sites for phosphorylation by protein kinase A, protein kinase C, and casein kinase II, bound to both the CRE and ATF sites, indicating that these consensus sites were not essential for DNA binding or dimer formation. Transfection of CREB and protein kinase A expression constructs into F9 cells with promoters containing either a single or two half-sites for CREB binding indicated that CREB was capable of similar levels of activation of these constructs. However, the fold activation by CREB was higher for constructs containing a single half-site compared with those containing two half-sites. These results demonstrate that multiple mechanisms may regulate CREB binding, including variations in the sequences in the promoter-binding site and the presence of related DNA-binding proteins.

In vivo monitoring of a cAMP-stimulated DNA-binding activity

The transcriptional activity of the tyrosine aminotransferase (TAT) gene is influenced by two major signal transduction pathways, by glucocorticoids and by glucagon acting via cAMP. We analyzed the effect of cAMP on protein-DNA interactions in vivo and on the transcription rate of the TAT gene. We demonstrate that a cAMP-responsive element (CRE) is located in a tissue-specific DNase I-hypersensitive region, 3.6 kb upstream of the start site of transcription. By using the genomic footprinting technique, we show that this sequence is occupied by protein in uninduced cells and that the in vivo footprint is transiently increased upon cAMP induction. Protein binding at the TAT-CRE correlates with the rate of transcription of the TAT gene. Cycloheximide treatment reveals that the genomic footprint is subject to rapid turnover; however, subsequent cAMP induction in the continued presence of cycloheximide restores the footprint partially. We conclude that as a part of the signal transduction pathway, a cAMP-dependent, post-translational modification increases the DNA-binding activity of a protein to the TAT-CRE and thereby stimulates the transcription rate of the TAT gene.

Assignment of the human gene for CREB1 to chromosome 2q32.3-q34

The nuclear phosphoprotein cAMP-response-element-binding-protein 1 (CREB1) is involved in the transcriptional regulation of cAMP-inducible genes. We have determined the chromosomal location of the human CREB1 gene by using a cDNA probe. Southern blot analysis of genomic DNA from a panel of mouse-human somatic cell hybrids indicated that the CREB1 gene resides on human chromosome 2. Analysis of a somatic cell hybrid containing a chromosome 2 translocation narrowed assignment of the CREB1 locus to the terminal third of chromosome 2. Finally, in situ hybridization allowed localization of the gene to the 2q32.3-q34 region. This knowledge may prove valuable in molecular analysis of translocations occurring in this region and in exploring the possibility of a role for the CREB1 locus in malignancy.

Two distinct forms of active transcription factor CREB (cAMP response element binding protein)

Mammalian cells express two distinct forms of transcription factor CREB (cAMP response element binding protein) that are apparently the products of alternative splicing of the CREB gene transcript. The two proteins differ by a 14-amino acid serine-rich insertion present in one of the CREB isoforms. We show that both CREB isoforms are expressed in many cell types and mammalian species. Both encode proteins that bind specifically to a cAMP response element in vitro. As expected for proteins of this class, the CREB proteins bind DNA as dimers. Both proteins impart cAMP-regulated transcriptional activity to a heterologous DNA-binding domain, showing that cAMP directly modulates the transcriptional stimulatory activity of CREB. The presence of multiple CREB isoforms with identical DNA-binding specificities but differences in the presumed regulatory domain raises the possibility that CREB proteins may be able to integrate distinct regulatory signals at the level of gene transcription.

Cyclic AMP induces activity increase of kinase FA (a transmembrane signal of insulin) during NG108-15 hybrid cell differentiation

The ATP.Mg-dependent protein phosphatase activating factor (protein kinase FA) has been identified to exist in neuroblastoma x glioma hybrid 108-15 cells (NG108-15 cells). More importantly, when NG cells were induced to differentiate with N6, O2'-dibutyryl adenosine 3',5'-cyclic monophosphate (dibutyryl cAMP), the cellular activity of kinase FA was found to increase dramatically. Time course study further revealed that induction of differentiation in NG cells by dibutyryl cAMP treatment increased the FA activity to over 3 times the levels found in undifferentiated cells and in a linear day-dependent manner, indicating that the FA activity level is correlated with the state of differentiation of NG108-15 cells. This is the first report providing initial evidence that protein kinase FA (a transmembrane signal of insulin) is involved in the induction of neuronal cell differentiation.

A specific member of the ATF transcription factor family can mediate transcription activation by the adenovirus E1a protein

The adenovirus E1a protein stimulates transcription of viral early genes. Recent experiments indicate that E1a contains a transcriptional activating region, which functions when directed to a promoter. Because E1a is not a sequence-specific DNA binding protein, how it targets to viral promoters has been a question. Several of the viral early promoters contain one or more binding sites for ATFs, a family of cellular transcription factors. Here we show that E1a can function through a specific ATF protein, designated ATF-2. We provide evidence that E1a interacts with a discrete region of promoter-bound ATF-2, thereby positioning the E1a activating region at a viral promoter.

Regulation of the pituitary-specific homeobox gene GHF1 by cell-autonomous and environmental cues

Homeodomain proteins function in determination of mating type in yeast, segmentation in fruit flies and cell-type specific gene expression in mammals. In Drosophila, expression of homeobox genes is controlled by cell-autonomous interactions between regulatory proteins and environmental clues. Similar controls may operate during mammalian limb development and frog embryogenesis. But, the exact way in which expression of homeodomain proteins is regulated in these systems is not clear and requires biochemical analysis of homeobox gene transcription. We now describe such an analysis of the GHF1 gene, which encodes a mammalian homeodomain protein specifying expression of the growth hormone (GH) gene in anterior pituitary somatotrophs. GHF1 is transcribed in a highly restricted manner and the presence of GHF1 protein is correlated both temporally and spatially with activation of the GH gene during pituitary development. Analysis of the GHF1 promoter indicates that transcription is also controlled by cell-autonomous interactions involving positive autoregulation by GHF1, and environmental cues that modulate the intracellular level of cyclic AMP and thereby the activity of cAMP response element binding protein (CREB), a ubiquitous transactivator that binds to the GHF1 promoter.

Isolation and characterization of two novel, closely related ATF cDNA clones from HeLa cells

ATF or CRE binding proteins are cellular transcription factors involved in the regulation of adenovirus Ela-responsive and cellular cAMP-inducible promoters. We report the isolation from a HeLa cell cDNA library of two clones that encode proteins with specific ATF/CRE DNA binding activity. The two clones differ by a 63 bp element which is retained in one (ATF-a) and deleted from the other (ATF-a delta) and which may correspond to an alternative exon. The peptide sequences (483 and 462 amino acids, respectively) derived from each of these cDNAs are identical, except for the additional 21 amino acids in ATF-a, but clearly differ from the other ATF/CREB proteins reported. All of them, however, share a conserved leucine zipper domain also found in other transcription factors. ATF-a and ATF-a delta therefore represent two closely related members of a larger multigene family of proteins that interact with conserved promoter elements.

Injection of the cAMP-responsive element into the nucleus of Aplysia sensory neurons blocks long-term facilitation

In both vertebrates and invertebrates, long-term memory differs from short-term in requiring protein synthesis during training. Studies of the gill and siphon withdrawal reflex in Aplysia indicate that similar requirements can be demonstrated at the level of sensory and motor neurons which may participate in memory storage. A single application of serotonin, a transmitter that mediates sensitization, to individual sensory and motor cells in dissociated cell cultures leads to enhanced transmitter release from the sensory neurons that is independent of new macromolecular synthesis. Five applications of serotonin cause a long-term enhancement, lasting one or more days, which requires translation and transcription. Prolonged application or intracellular injection into the sensory neuron of cyclic AMP, a second messenger for the action of serotonin, also produce long-term increases in synaptic strength, suggesting that some of the gene products important for long-term facilitation are cAMP-inducible. In eukaryotic cells, most cAMP-inducible genes so far studied are activated by the cAMP-dependent protein kinase (A kinase), which phosphorylates transcription factors that bind the cAMP-responsive element TGACGTCA. The cAMP-responsive element (CRE) binds a protein dimer of relative molecular mass 43,000, the CRE-binding protein (CREBP), which has been purified and shown to increase transcription when phosphorylated by the A kinase. Here we show that extracts of the Aplysia central nervous system and extracts of sensory neurons contain a set of proteins, including one with properties similar to mammalian CREBPs, that specifically bind the mammalian CRE sequence. Microinjection of the CRE sequence into the nucleus of a sensory neuron selectively blocks the serotonin-induced long-term increase in synaptic strength, without affecting short-term facilitation. Taken together, these observations suggest that one or more CREB-like transcriptional activators are required for long-term facilitation.

A cyclic AMP response element mediates repression of tyrosine aminotransferase gene transcription by the tissue-specific extinguisher locus Tse-1

Tyrosine aminotransferase (TAT) gene expression is liver specific and inducible by glucocorticoids and via the cAMP signaling pathway. In fibroblasts and other nonliver cells the gene is subject to negative control by the trans-dominant tissue-specific extinguisher locus Tse-1. We identified a hepatocyte-specific enhancer that is repressed by Tse-1. Two distinct sequence motifs are absolutely essential for function of this enhancer: a cAMP response element (CRE), which is the target for repression by Tse-1, and a hepatocyte-specific element. The specificity of the enhancer is generated by the combination of these two essential elements, which are fully interdependent. In vivo footprinting indicates that Tse-1 acts by affecting protein binding at the CRE. A direct antagonism between Tse-1 and the cAMP signaling pathway suggests that Tse-1 plays a role in control of developmental activation of the TAT gene.

mXBP/CRE-BP2 and c-Jun form a complex which binds to the cyclic AMP, but not to the 12-O-tetradecanoylphorbol-13-acetate, response element

Proto-oncogene products c-Fos and c-Jun form a complex which binds with high affinity to the 12-O-tetradecanoylphorbol-13-acetate (TPA) response DNA element and which stimulates transcription of phorbol ester- inducible genes. We have previously identified, by screening a lambda gt11 expression library, murine protein mXBP, which binds to a sequence which overlaps the 3' end of the murine class II major histocompatibility complex A alpha gene X box, a conserved transcription element found upstream of all class II genes. Here, we demonstrate that the target sequence for mXBP is a consensus cyclic AMP response element (CRE). mXBP is a member of the leucine zipper family of DNA-binding proteins and has significant homology to oncoproteins c-Fos and c-Jun. The inferred amino acid sequence of mXBP shows near identity to human CRE-BP1, except it does not contain an internal proline-rich domain. Immunoprecipitation and glutaraldehyde cross-linking studies show that mXBP/CRE-BP2 can form a complex with c-Jun. Complex formation is dependent on intact leucine zipper domains in both proteins. mXBP-c-Jun complexes can coexist with c-Fos-c-Jun complexes and can bind with high affinity to CRE, but not to TPA response DNA element, sequences. These results suggest that changes in the expression of mXBP/CRE-BP2, c-Fos, and c-Jun, which alter the ratio of mXBP-c-Jun to c-Fos-c-Jun complexes, would affect the relative expression of cyclic AMP and phorbol ester-responsive genes. This provides support for a combinatorial model of gene regulation, whereby protein-protein interactions which alter the DNA binding specificity of protein complexes can expand the flexibility of cellular transcriptional responses.

A cDNA for a human cyclic AMP response element-binding protein which is distinct from CREB and expressed preferentially in brain

The cyclic AMP response element (CRE) is found in many cellular genes regulated by cyclic AMP, and similar elements are present in the early genes of adenovirus that are activated by E1A. The transcription factor CREB has previously been shown to bind this site, and cDNAs for CREB have recently been characterized. We report here the isolation of a cDNA encoding a human DNA-binding protein that also recognizes this motif in cellular and viral promoters. This protein, HB16, displays structural similarity to CREB and to c-Jun and c-Fos, which bind the related 12-O-tetradecanoylphorbol-13-acetate response element (TRE). HB16 contains a highly basic, putative DNA-binding domain and a leucine zipper structure thought to be involved in dimerization. Deletional analysis of HB16 demonstrated that the leucine zipper is required for its interaction with DNA. In addition, HB16 could form a complex with c-Jun but not with c-Fos. Despite its structural similarity to c-Jun and c-Fos and its interaction with c-Jun, HB16 had approximately a 10-fold-lower affinity for the TRE sequence than for the CRE sequence. Although HB16 and CREB both recognized the CRE motif, an extensive binding analysis of HB16 revealed differences in the fine specificity of binding of the two proteins. HB16 mRNA was found at various levels in many human tissues but was most abundant in brain, where its expression was widespread. The existence of more than one CRE-binding protein suggests that the CRE motif could serve multiple regulatory functions.

Regulation of cAMP-inducible genes by CREB

A large number of neuropeptides and neurotransmitters stimulate neuronal cells through the second messenger cAMP. These synaptic signals often cause profound changes in neuronal function by altering basic patterns of gene expression. Cyclic AMP, in turn, regulates a number of these genes through a conserved cAMP response element (CRE). Recently, a nuclear CRE-binding protein, CREB, has been shown to bind to the CRE and stimulate the transcription of cAMP-responsive genes. This article reviews recent progress towards understanding the mechanism by which cAMP modulates the activity of CREB to stimulate gene transcription.

Phytohemagglutinin-induced activity of cyclic AMP (cAMP) response elements from cytomegalovirus is reduced by cyclosporine and synergistically enhanced by cAMP

The 19-base-pair enhancer repeat of the human cytomegalovirus immediate-early 1 gene mediates cyclic AMP- and phytohemagglutinin-induced expression in Jurkat T cells. Synergistic activity was observed in the presence of both drugs, suggesting a convergence of the protein kinase A and C pathways on this transcription element. In addition, the immunosuppressive drug cyclosporine strongly reduced the ability of the 19-base-pair repeat to activate gene expression in phytohemagglutinin-stimulated T cells.

Activin is a nerve cell survival molecule

The structures of five neurotrophic molecules have so far been published. Nerve growth factor, fibroblast growth factor and purpurin, have been identified as nerve-cell survival molecules. More recently, brain-derived neurotrophic factor (BDNF) and ciliary neurotrophic factor have been cloned and sequenced. As all these proteins stimulate the survival of ciliary or sensory neurons, a new cell survival assay is required if novel neurotrophic molecules are to be discovered. P19 teratoma cells differentiate to nerve-like cells in the presence of 5 x 10(-7) M retinoic acid (RA). But when P19 cells are plated in N2 synthetic medium without being exposed to RA, they die within 48 h. In an attempt to identify a molecule(s) that can substitute for RA in promoting P19 survival, we assayed serum-free growth-conditioned media for their ability to promote P19 survival. One cell line from the rat eye secreted a molecule that promoted the survival of P19 cells and some types of nerve cell. We identified this molecule as activin, better known for its role in hormone secretion.

Macromolecular interaction on a cAMP responsive region in the urokinase-type plasminogen activator gene: a role of protein phosphorylation

We have studied the regulation of urokinase-type plasminogen activator gene expression by cAMP in LLC-PK1 cells. We found a cAMP responsive region 3.4 kb upstream of the transcription initiation site, which comprised three protein-binding domains designated A, B, and C. Domains A and B both contain a sequence, TGACG, homologous to a consensus cAMP response element (CRE; TGACGTCA). Effective cAMP-mediated induction was achieved when these two domains were linked with domain C, which by itself did not confer cAMP responsiveness to a heterologous promoter nor contained CRE-like sequence, suggesting a functional cooperation among these domains. Results of competition studies using gel retardation and DNase I footprinting assays suggest that there is a protein-protein interaction between a CRE binding protein and a domain C binding protein. In gel retardation assays, binding of a nuclear protein to domains A and B was strongly augmented by addition of the catalytic subunit of cAMP-dependent protein kinase, whereas the protein binding to domain C was slightly inhibited, suggesting that protein phosphorylation is involved in the regulation of protein-DNA interaction.

Mechanisms of differential regulation of interleukin-6 mRNA accumulation by tumor necrosis factor alpha and lymphotoxin during monocytic differentiation

In the present report we compare the capacity of two related cytokines, tumor necrosis factor (TNF) alpha and lymphotoxin (LT), to modulate mRNA levels of interleukin-6 (IL-6) in cells representing different stages of monocytic differentiation including the human leukemia cell lines HL 60, U 937, THP-1, MonoMac 1 and peripheral blood monocytes. We show that the capacity of TNF alpha and LT to induce IL-6 mRNA accumulation increases as monocytic differentiation proceeds with TNF alpha being more potent than LT, suggesting that alternate pathways may be used by differentiating cells to control expression of IL-6. In contrast, in monocytes which constitutively synthesize IL-6 transcripts, TNF alpha and LT treatment had opposite effects on levels of IL-6 mRNA accumulation. In these cells TNF alpha enhanced steady state levels of IL-6 transcripts due to mRNA stabilization, whereas LT shortened IL-6 mRNA half-life, most likely due to induction of a RNA destabilizer since LT-mediated downregulation of levels of IL-6 mRNA in monocytes could be prevented by inhibition of protein synthesis. Neither TNF alpha nor LT altered IL-6 mRNA accumulation by interfering with preexisting transcription factors since both TNF alpha and LT required de novo protein synthesis to exert their effects.

A cDNA for a human cyclic AMP response element-binding protein which is distinct from CREB and expressed preferentially in brain

The cyclic AMP response element (CRE) is found in many cellular genes regulated by cyclic AMP, and similar elements are present in the early genes of adenovirus that are activated by E1A. The transcription factor CREB has previously been shown to bind this site, and cDNAs for CREB have recently been characterized. We report here the isolation of a cDNA encoding a human DNA-binding protein that also recognizes this motif in cellular and viral promoters. This protein, HB16, displays structural similarity to CREB and to c-Jun and c-Fos, which bind the related 12-O-tetradecanoylphorbol-13-acetate response element (TRE). HB16 contains a highly basic, putative DNA-binding domain and a leucine zipper structure thought to be involved in dimerization. Deletional analysis of HB16 demonstrated that the leucine zipper is required for its interaction with DNA. In addition, HB16 could form a complex with c-Jun but not with c-Fos. Despite its structural similarity to c-Jun and c-Fos and its interaction with c-Jun, HB16 had approximately a 10-fold-lower affinity for the TRE sequence than for the CRE sequence. Although HB16 and CREB both recognized the CRE motif, an extensive binding analysis of HB16 revealed differences in the fine specificity of binding of the two proteins. HB16 mRNA was found at various levels in many human tissues but was most abundant in brain, where its expression was widespread. The existence of more than one CRE-binding protein suggests that the CRE motif could serve multiple regulatory functions.

Membrane depolarization and calcium induce c-fos transcription via phosphorylation of transcription factor CREB

The mechanism by which the calcium influx signal, triggered by membrane depolarization, is transduced to the nucleus to activate c-fos proto-oncogene transcription has been characterized. A calcium response element (CaRE) that is indistinguishable from a cAMP response element (CRE) mediates transcriptional inducibility by depolarization. Its cognate transcription factor CREB is the target for both calcium and cAMP signals. CREB is rapidly phosphorylated in response to depolarization or cAMP, at a site known to be important for the transcriptional activating function of this protein. The convergent effects of calcium and cAMP on CREB activation are mediated by distinct protein kinase signaling pathways. CREB and its binding site, the Ca/CRE, can thus function as a regulatory element that integrates both calcium and cAMP signals in the control of gene expression.

mXBP/CRE-BP2 and c-Jun form a complex which binds to the cyclic AMP, but not to the 12-O-tetradecanoylphorbol-13-acetate, response element

Proto-oncogene products c-Fos and c-Jun form a complex which binds with high affinity to the 12-O-tetradecanoylphorbol-13-acetate (TPA) response DNA element and which stimulates transcription of phorbol ester- inducible genes. We have previously identified, by screening a lambda gt11 expression library, murine protein mXBP, which binds to a sequence which overlaps the 3' end of the murine class II major histocompatibility complex A alpha gene X box, a conserved transcription element found upstream of all class II genes. Here, we demonstrate that the target sequence for mXBP is a consensus cyclic AMP response element (CRE). mXBP is a member of the leucine zipper family of DNA-binding proteins and has significant homology to oncoproteins c-Fos and c-Jun. The inferred amino acid sequence of mXBP shows near identity to human CRE-BP1, except it does not contain an internal proline-rich domain. Immunoprecipitation and glutaraldehyde cross-linking studies show that mXBP/CRE-BP2 can form a complex with c-Jun. Complex formation is dependent on intact leucine zipper domains in both proteins. mXBP-c-Jun complexes can coexist with c-Fos-c-Jun complexes and can bind with high affinity to CRE, but not to TPA response DNA element, sequences. These results suggest that changes in the expression of mXBP/CRE-BP2, c-Fos, and c-Jun, which alter the ratio of mXBP-c-Jun to c-Fos-c-Jun complexes, would affect the relative expression of cyclic AMP and phorbol ester-responsive genes. This provides support for a combinatorial model of gene regulation, whereby protein-protein interactions which alter the DNA binding specificity of protein complexes can expand the flexibility of cellular transcriptional responses.

Growth-related protein kinases

A protein kinase cascade is involved in the action of some mitogens. The cascade begins with receptor tyrosine kinase activation by growth factors. The resulting signal is transmitted into cells via phospholipid metabolism which produces a variety of second messengers and by intracellular protein kinase activation. The signal is then propagated and disseminated via a network of other protein kinases and protein phosphatases. Recent research suggests that ribosomal protein S6 kinase and casein kinase II are two important elements in the kinase cascade that leads to the initiation of growth. The nature and some properties of these hitherto lesser known enzymes is considered.