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

Secretin: structure of the precursor and tissue distribution of the mRNA

Secretin is a 27-amino acid gastrointestinal hormone that stimulates the secretion of bicarbonate-rich pancreatic fluid. The unusually high number of serine, leucine, and arginine residues in secretin has precluded the use of oligonucleotides to screen cDNA libraries to isolate a secretin cDNA. In the present study, a short cDNA encoding porcine secretin was amplified from duodenal mucosal first-strand cDNA template by using 16,384- and 4096-fold degenerate primers in the DNA polymerase chain reaction. From the sequence of the amplified cDNA, an unambiguous oligonucleotide probe was designed to screen a cDNA library. Here we report the sequences of cDNAs encoding the porcine and rat secretin precursors. The predicted amino acid sequences reveal that each precursor consists of a signal peptide, an N-terminal peptide, secretin, and a 72-amino acid C-terminal peptide. Secretin has been highly conserved through evolution. Rat secretin differs from its porcine counterpart by a single glutamine-for-arginine substitution at position 14. In contrast, the amino acid sequences of the C-terminal peptides are only 39% conserved between the two species, suggesting that the C-terminal peptide does not have an essential physiologic function. RNA blot hybridizations reveal that the rat secretin gene is expressed throughout the small intestine. Although secretin immunoreactivity has been localized in the central nervous system by some laboratories, we are unable to detect secretin mRNA in tissues of the central nervous system by Northern blot hybridization.

Characterization of a bipartite activator domain in transcription factor CREB

In this paper, we characterize a trans-activating region in CREB, termed alpha, that interacts cooperatively with the kinase A phosphorylation motif to stimulate transcription. The alpha region appears to be encoded by an alternate exon that is deleted in a CREB-related cDNA named delta CREB. Both proteins are expressed in eukaryotic cells, although the activity of CREB is 10-fold higher than that of delta CREB. Circular dichroism data on a synthetic "alpha peptide" combined with results from in vitro mutagenesis experiments support the hypothesis that the alpha region contains an amphipathic alpha helix whose structure is critical to CREB activity. We propose that phosphorylation by kinase A may stimulate CREB activity in part by modulating the structure of alpha and thus may stimulate its ability to interact with other proteins in the polymerase II complex.

Transcription factor IIIA gene expression in Xenopus oocytes utilizes a transcription factor similar to the major late transcription factor

Xenopus transcription factor IIIA (TFIIIA) gene expression is stringently regulated during development. The steady-state level of TFIIIA mRNA in a somatic cell is approximately 10(6) times less than in an immature oocyte. We have undertaken studies designed to identify differences in how the TFIIIA gene is transcribed in oocytes and somatic cells. In this regard, we have localized an upstream transcriptional control element in the TFIIIA promoter that stimulates transcription from the TFIIIA promoter approximately threefold in microinjected oocytes. The upstream element, in cis. does not stimulate transcription from the TFIIIA promoter in somatic cells. Thus, the element appears to be oocyte specific in the context of the TFIIIA promoter. However, both oocytes and somatic cells contain a protein (or a related protein) that binds the upstream element. We have termed this protein from oocytes the TFIIIA distal element factor. The sequence of the upstream element is similar to the sequence of the upstream element found in the adenovirus major late promoter that is a binding site for the major late transcription factor. By gel shift analysis, chemical footprinting, methylation intereference, and point mutation analysis, we demonstrate that the TFIIIA distal element factor and major late transcription factor have similar DNA-binding properties.

Transcription from a murine T-cell receptor V beta promoter depends on a conserved decamer motif similar to the cyclic AMP response element

We identified a regulatory region of the murine V beta promoter by both in vivo and in vitro analyses. The results of transient transfection assays indicated that the dominant transcription-activating element within the V beta 8.3 promoter is the palindromic motif identified previously as the conserved V beta decamer. Elimination of this element, by linear deletion or specific mutation, reduced transcriptional activity from this promoter by 10-fold. DNase I footprinting, gel mobility shift, and methylation interference assays confirmed that the palindrome acts as the binding site of a specific nuclear factor. In particular, the V beta promoter motif functioned in vitro as a high-affinity site for a previously characterized transcription activator, ATF. A consensus cyclic AMP response element (CRE) but not a consensus AP-1 site, can substitute for the decamer in vivo. These data suggest that cyclic AMP response element-binding protein (ATF/CREB) or related proteins activate V beta transcription.

Regulation of collagenase gene expression by okadaic acid, an inhibitor of protein phosphatases

Human collagenase gene expression is regulated transcriptionally and is inducible by various mitogens in many cell types. To investigate the molecular mechanisms of this response, we examined the effects on collagenase gene expression of okadaic acid, a non-12-O-tetradecanoyl-phorbol-13-acetate (TPA)-type tumor promoter, which induces apparent "activation" of protein kinases by inhibition of protein phosphatases. Steady state levels of collagenase mRNA were markedly increased by okadaic acid treatment. We show that the AP-1 consensus sequence in the collagenase promoter is required for the induction of collagenase gene expression by okadaic acid, even though sequences upstream of the AP-1 consensus site have an additive effect. We also examined the regulation by okadaic acid of expression of the components of the AP-1 complex, c-fos and c-jun. c-fos expression is dramatically stimulated by okadaic acid, whereas c-jun expression is stimulated to a lesser extent. Induction of c-fos gene mRNA occurs through a region known to contain multiple regulatory elements. These results suggest that phosphorylation regulates collagenase gene expression mediated by an AP-1 binding site.

Lipopolysaccharide-unresponsive mutant pre-B-cell lines blocked in NF-kappa B activation

NF-kappa B activation is a crucial late step in the induction of immunoglobulin kappa light-chain gene expression in pre-B cells by lipopolysaccharide (LPS). We have analyzed NF-kappa B activation in three independent mutant lines of 70Z/3 pre-B cells which are unresponsive to LPS. All three variant cell lines failed to activate NF-kappa B when induced with LPS or the phorbol ester 12-O-tetradecanoylphorbol 13-acetate. However, all three cell lines contained functional NF-kappa B, as revealed by detergent treatment of cytoplasmic extracts. Moreover, cycloheximide induced limited activation of NF-kappa B comparable to that in wild-type 70Z/3 pre-B cells in two of the three variant lines. These results indicate that the mutations blocking kappa gene induction in these variant 70Z/3 pre-B-cell lines affect NF-kappa B activation.

Factor substitution in a human HSP70 gene promoter: TATA-dependent and TATA-independent interactions

To investigate interactions between transcription factors on mammalian promoters, we constructed a set of 24 variations of the human HSP70 gene promoter in which six upstream sequence motifs are paired in every possible combination with four TATA motifs. These promoters were analyzed for in vivo expression, and selected constructs were examined by in vitro template commitment studies. Activation transcription factor (ATF) and CP1 showed dramatically different interactions with the factor(s) bound to the TATA region. CP1 functioned in vivo regardless of the TATA motif that it was paired with and was not capable of sequestering the core promoter complex in a template commitment assay. ATF activity was dramatically altered by changing the TATA motif, and ATF was able to sequester the core promoter complex. These data suggest that CP1 and ATF function by distinct mechanisms that differ with respect to interaction with the factor(s) at the TATA box. Factor Sp1 also appeared to function by a TATA-independent mechanism. These data imply that the ability of a factor to function is determined not only by the intrinsic properties of the factor but also by promoter context.

Coupled and uncoupled induction of fos and jun transcription by different second messengers in cells of hematopoietic origin

The nuclear oncoproteins fos and jun are associated as a heterodimer which binds to TPA (PMA or TPA: phorbol 12-myristate 13-acetate)- responsive promoter elements (TRE), the recognition site for the transcription factor AP-1. The fos/jun heterodimer has a higher affinity to the TRE and stimulates transcription of responsive genes more than the jun homodimer. The association of these two oncoproteins may play a central role in signal transduction and regulation of cell proliferation and differentiation. We further defined the regulation of fos and jun by studying their inducibility by second messengers in cells of hematopoietic origin. In THP-1 monocytic leukemia cells fos and jun mRNA levels are regulated in a coupled manner by second messengers activated after membrane phospholipid turnover. Addition of phospholipase C to cells, as well as stimulation of protein kinase C and release of intracellular Ca2+, caused a rapid induction of fos and jun mRNA levels, but the induction of jun mRNA showed a more persistant and less transient pattern than fos. In contrast to the phosphoinositol system, stimulation of the adenylate cyclase pathway in THP-1 cells induced only fos transcription whereas jun mRNA levels remained unchanged. A similar uncoupling of fos and jun inducibility was found after phorbol ester addition to the human erythroleukemia cell line HEL and the human promyelocytic cell line HL-60. The uncoupling of fos and jun levels might predispose cells to the formation of combinatorial transcription complexes of a different composition and activity than the fos/jun heterodimer. Indeed, nuclear extracts from THP-1 cells before or after activation of the phosphinositol or adenylate cyclase second messenger pathways revealed a correlation in fos and jun expression and specific binding of the heterocomplex to a TRE sequence.

A role for cAMP-dependent protein kinase A in early Dictyostelium development

In Dictyostelium, cAMP functions as an extracellular regulatory molecule that controls aggregation, expression of a number of classes of genes, and cellular differentiation by binding to cell-surface receptors that activate intracellular signal transduction pathways. To investigate possible roles for intracellular cAMP, we have overexpressed the wild-type mouse type-I regulatory subunit (RI) of cAMP-dependent protein C (PKA) in Dictyostelium cells, as well as mutant forms of the subunit that are altered in their ability to bind cAMP. We show that overexpression of a mutated RI, which lacks both cAMP-binding sites and presumably forms a complex with the endogenous Dictyostelium catalytic subunit that cannot be activated by cAMP, results in cells that do not aggregate or express sets of genes that are normally induced in the multicellular stages. Transformations that express the mutant subunit at low levels show no observable phenotype. We show that these cells can respond to pulses of cAMP and activate cAMP receptor/G protein-mediated processes, including the activation of adenylate and guanylate cyclases and the induction of a class of genes known to be regulated through the receptor-mediated pathways; however, the cells do show an altered pattern of expression of other genes normally active during the preaggregation/interphase and aggregation stages. Of interest is a substantial overexpression of the developmentally regulated PDE mRNA. Cell lines carrying constructs encoding the wild-type subunit or mutant subunits lacking one of the two binding sites show no visual phenotype. The results suggest that PKA-mediated functions, presumably controlled by increases in intracellular cAMP, are essential for Dictyostelium aggregation.

Peptide growth factors and cell cycle control

Research on mammalian cell cycle control focuses on the points discussed below. Peptide growth factors are multifunctional regulators of growth and differentiation that act by autocrine and paracrine mechanisms. Gene transcription changes are key steps in the control of the G0 in equilibrium with G1----S transition of the cell cycle. Both peptide growth factors and classical tropic hormones, are capable of rapidly modulating transcription through the induction of genes (fos/jun) that encode nuclear transregulator proteins.

Characterization of a cyclic AMP regulatory element DNA-binding protein

Signal transduction pathways converge ultimately at the level of transcriptional activation to produce specific patterns of gene expression in response to environmental stimuli. The initiation of transcription mediated by these signaling pathways is regulated by the coordinate expression and/or activation of specific transcription factors that bind to the control regions of eukaryotic genes. Specific insights into the mechanisms underlying transcriptional activation have arisen from the studies of the structure and functions of eukaryotic transcription factors. One of these factors, a cyclic (c)AMP response element binding protein (CREB), has only recently been discovered and appears to play a key role in the regulation of gene expression in response to the activation of the cAMP-dependent protein kinase A. The transcription factors related to CREB, c-jun and c-fos, mediate transcriptional responses to activators of protein kinase C.

Factor substitution in a human HSP70 gene promoter: TATA-dependent and TATA-independent interactions

To investigate interactions between transcription factors on mammalian promoters, we constructed a set of 24 variations of the human HSP70 gene promoter in which six upstream sequence motifs are paired in every possible combination with four TATA motifs. These promoters were analyzed for in vivo expression, and selected constructs were examined by in vitro template commitment studies. Activation transcription factor (ATF) and CP1 showed dramatically different interactions with the factor(s) bound to the TATA region. CP1 functioned in vivo regardless of the TATA motif that it was paired with and was not capable of sequestering the core promoter complex in a template commitment assay. ATF activity was dramatically altered by changing the TATA motif, and ATF was able to sequester the core promoter complex. These data suggest that CP1 and ATF function by distinct mechanisms that differ with respect to interaction with the factor(s) at the TATA box. Factor Sp1 also appeared to function by a TATA-independent mechanism. These data imply that the ability of a factor to function is determined not only by the intrinsic properties of the factor but also by promoter context.

The palindromic series I repeats in the simian cytomegalovirus major immediate-early promoter behave as both strong basal enhancers and cyclic AMP response elements

A 600-base-pair (bp) enhancer region upstream from the major IE94 gene of simian cytomegalovirus (SCMV) produces very strong basal expression of associated gene products. This domain consists of multiple sets of interspersed repetitive elements, including 11 copies of a conserved 16-bp palindromic sequence with the consensus CCATTGACGTCAATGG. These series I repeats contain an 8-bp core TGACGTCA that resembles the cyclic AMP (cAMP) response element (CRE) of cellular genes. In transient chloramphenicol acetyltransferase assays in K562 human erythroleukemia cells, a set of deleted variants of the IE94 promoter all responded up to 15-fold to induction by cAMP. However, successive removal of most of the SCMV 16-bp motifs reduced basal expression over 20-fold. The cAMP stimulation was also manifested at the steady-state RNA level after SCMV infection of K562 cells and was detectable within 1.5 h after treatment of DNA-transfected cells. Addition of a single 30-bp oligonucleotide encompassing the 16-bp palindrome conveyed up to 10-fold cAMP responsiveness onto a heterologous weak promoter but had no effect on basal expression. In contrast, two or more adjacent copies produced 20- to 40-fold increases in basal expression and provided greater than 200-fold activation in the presence of cAMP. Similar effects were obtained when the oligonucleotides were placed in a downstream location relative to the reporter gene. Studies with mutant oligonucleotides revealed that both the core CRE and the flanking sequence portions of the 16-bp elements were essential for enhancer function. Both components were also important for maximum cAMP responsiveness. Band shift assays with fractionated nuclear extracts from Raji lymphocytes revealed multiple competable complexes with cellular DNA-binding factors that recognized the series I elements. Three distinct CREB-like factors were detected that required only the core 8-bp elements for binding. We conclude that the 16-bp series I repeats provide a major contribution to the constitutive enhancer properties of the IE94 promoter and also act as functional CREs. The cAMP response properties appear likely to play a key role in reactivation of the virus from a latent state in appropriately differentiating cell types.

Lipopolysaccharide-unresponsive mutant pre-B-cell lines blocked in NF-kappa B activation

NF-kappa B activation is a crucial late step in the induction of immunoglobulin kappa light-chain gene expression in pre-B cells by lipopolysaccharide (LPS). We have analyzed NF-kappa B activation in three independent mutant lines of 70Z/3 pre-B cells which are unresponsive to LPS. All three variant cell lines failed to activate NF-kappa B when induced with LPS or the phorbol ester 12-O-tetradecanoylphorbol 13-acetate. However, all three cell lines contained functional NF-kappa B, as revealed by detergent treatment of cytoplasmic extracts. Moreover, cycloheximide induced limited activation of NF-kappa B comparable to that in wild-type 70Z/3 pre-B cells in two of the three variant lines. These results indicate that the mutations blocking kappa gene induction in these variant 70Z/3 pre-B-cell lines affect NF-kappa B activation.

Maize regulatory gene opaque-2 encodes a protein with a "leucine-zipper" motif that binds to zein DNA

The opaque-2 locus (o2) in maize regulates the expression of many members of the zein multigene family of storage proteins. cDNA clones for a wild-type allele of the (o2) locus (O2) were isolated from a maize endosperm cDNA library and sequenced. We found a 258-nucleotide 5' leader sequence containing three short open reading frames followed by a sequence specifying a protein of 437 amino acids. The presumptive amino acid sequence of the protein (O2) specified by the O2 cDNA contains a "leucine-zipper" domain characteristic of some mammalian and fungal transcription activation factors. lacZ-O2 fusion constructs, using nearly the entire coding region of O2 or only a fragment specifying the leucine-zipper domain, were expressed in Escherichia coli. In an in vitro binding assay, the beta-galactosidase-O2 fusion proteins bound to two specific regions on the 5' side of the coding sequence in a zein genomic clone. This suggests that the O2 protein affects zein transcription through direct interaction with one or more zein promoter elements.

Cyclic AMP stimulates somatostatin gene transcription by phosphorylation of CREB at serine 133

In this paper, we demonstrate that phosphorylation of CREB at Ser-133 is induced 6-fold in vivo, following treatment of PC12 cells with forskolin. By contrast, no such induction was observed in the kinase A-deficient PC12 line A126-1B2 (A126). Using F9 teratocarcinoma cells, which are unresponsive to cAMP, we initiated a series of transient expression experiments to establish a causal link between phosphorylation of CREB and trans-activation of cAMP-responsive genes. Inactivating the kinase A phosphorylation site by in vitro mutagenesis of the cloned CREB cDNA at Ser-133 completely abolished CREB transcriptional activity. As CREB mutants containing acidic residues in place of the Ser-133 phosphoacceptor were also transcriptionally inactive, these results suggest that phosphorylation of CREB may stimulate transcription by a mechanism other than by simply providing negative charge.

Transcription from a murine T-cell receptor V beta promoter depends on a conserved decamer motif similar to the cyclic AMP response element

We identified a regulatory region of the murine V beta promoter by both in vivo and in vitro analyses. The results of transient transfection assays indicated that the dominant transcription-activating element within the V beta 8.3 promoter is the palindromic motif identified previously as the conserved V beta decamer. Elimination of this element, by linear deletion or specific mutation, reduced transcriptional activity from this promoter by 10-fold. DNase I footprinting, gel mobility shift, and methylation interference assays confirmed that the palindrome acts as the binding site of a specific nuclear factor. In particular, the V beta promoter motif functioned in vitro as a high-affinity site for a previously characterized transcription activator, ATF. A consensus cyclic AMP response element (CRE) but not a consensus AP-1 site, can substitute for the decamer in vivo. These data suggest that cyclic AMP response element-binding protein (ATF/CREB) or related proteins activate V beta transcription.

Transcription factor IIIA gene expression in Xenopus oocytes utilizes a transcription factor similar to the major late transcription factor

Xenopus transcription factor IIIA (TFIIIA) gene expression is stringently regulated during development. The steady-state level of TFIIIA mRNA in a somatic cell is approximately 10(6) times less than in an immature oocyte. We have undertaken studies designed to identify differences in how the TFIIIA gene is transcribed in oocytes and somatic cells. In this regard, we have localized an upstream transcriptional control element in the TFIIIA promoter that stimulates transcription from the TFIIIA promoter approximately threefold in microinjected oocytes. The upstream element, in cis. does not stimulate transcription from the TFIIIA promoter in somatic cells. Thus, the element appears to be oocyte specific in the context of the TFIIIA promoter. However, both oocytes and somatic cells contain a protein (or a related protein) that binds the upstream element. We have termed this protein from oocytes the TFIIIA distal element factor. The sequence of the upstream element is similar to the sequence of the upstream element found in the adenovirus major late promoter that is a binding site for the major late transcription factor. By gel shift analysis, chemical footprinting, methylation intereference, and point mutation analysis, we demonstrate that the TFIIIA distal element factor and major late transcription factor have similar DNA-binding properties.

Structure and associated DNA-helicase activity of a general transcription initiation factor that binds to RNA polymerase II

RAP30/74 is a heteromeric general transcription initiation factor which binds to RNA polymerase II. Here we report that preparations of RAP30/74 contain an ATP-dependent DNA helicase whose probable function is to melt the DNA at transcriptional start sites. The sequence of the RAP30 subunit of RAP30/74 indicates that RAP30 may be distantly related to bacterial sigma factors.

Regulation of transcription factor AP-2 by the morphogen retinoic acid and by second messengers

The expression of the transcription factor AP-2 recently has been shown to be enhanced during retinoic acid (RA)-induced differentiation of NT2 cells, a human teratocarcinoma cell line. Here we show that this induction of AP-2 mRNA is at the level of transcription and is transient, reaching a peak 48-72 hr after the addition of RA and declining thereafter, even in the continuous presence of RA. Increased levels of AP-2 mRNA are reflected in a similar elevation of AP-2 protein and accompanied by an increase in the AP-2-binding site-dependent transcriptional activity of a reporter gene. AP-2 also has been proposed to confer TPA and cAMP inducibility on promoters/enhancers containing AP-2-binding sites. We investigated the effect of these agents on the expression of AP-2 protein and mRNA. Our experiments demonstrate that expression of the AP-2 gene in HeLa cells is not elevated significantly by TPA or by a calcium ionophore and is not enhanced at all by agents that increase intracellular cAMP concentration. In fact, AP-2 mRNA is repressed by both TPA and the calcium ionophore A23187 through a delayed response. These data suggest that the AP-2-binding site-mediated cAMP and TPA responses are not regulated at the level of AP-2 expression but, rather, achieved either by post-translational changes in AP-2 or in conjunction with another protein.

Two tobacco DNA-binding proteins with homology to the nuclear factor CREB

The 35S promoter of the cauliflower mosaic virus (CaMV) contains a tandem repeat of the sequence TGACG in the region -83 to -63. This 21-base pair (bp) sequence, called as-1, is involved in root expression of the 35S promoter. When inserted in a promoter of a gene expressed specifically in photosynthetic tissues, as-1 confers high level expression in roots. We have described a factor, ASF-1, that binds specifically to as-1 in vitro. There is a good correlation between ASF-1 binding affinity to as-1 related sequences in vitro and the function of these sequences in vivo. These results strongly suggest that ASF-1 is responsible for the function of as-1. Here we report the isolation of tobacco complementary DNA clones encoding two TGACG-sequence-specific binding-proteins (TGA1a and TGA1b). Sequence analysis of the cDNA clones shows that both proteins contain a basic region that shows high homology to a stretch of basic amino acids in the nuclear factors CREB, GCN4, and c-Jun to a 'leucine-zipper' region. On the basis of binding specificity we propose TGA1a to be a good candidate for ASF-1.

Transcriptional regulation in mammalian cells by sequence-specific DNA binding proteins

The cloning of genes encoding mammalian DNA binding transcription factors for RNA polymerase II has provided the opportunity to analyze the structure and function of these proteins. This review summarizes recent studies that define structural domains for DNA binding and transcriptional activation functions in sequence-specific transcription factors. The mechanisms by which these factors may activate transcriptional initiation and by which they may be regulated to achieve differential gene expression are also discussed.

Homology of nuclear factor I with the protein kinase family

It is observed that a putative DNA binding domain in nuclear factor I (NF I) which is a eukaryotic sequence-specific DNA binding protein participating in both regulation of DNA replication and transcription displays sequence homology with catalytic domains in various protein kinases. In addition, hydropathy analysis reveals that the NF I polypeptide chain is likely to fold into similar secondary and tertiary structures to those of the protein kinases. Although it is not known whether NF I functions as a protein kinase in addition to recognizing a specific sequence on DNA, it is concluded that some of the eukaryotic DNA binding proteins and protein kinases may belong to a gene family, various members of which are evolutionarily related and responsible for the regulation of metabolism in diverse compartments of the cell. The present finding also suggests that a number of kinases may to varying extents be capable of direct interaction with DNA.

Differential activation of the 21-base-pair enhancer element of human T-cell leukemia virus type I by its own trans-activator and cyclic AMP

A transcriptional trans-acting factor p40tax of human T-cell leukemia virus type I (HTLV-I) functions as an inducer for expression of HTLV-I provirus via activation of the enhancer in the long terminal repeat of HTLV-I. In addition to p40tax and a tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), an activator of protein kinase C, we report here that forskolin, an activator of adenyl cyclase, also induces function of the HTLV-I enhancer. Experiments with mutants of the HTLV-I enhancer revealed that TPA-induced activation was not mediated by solely a 21-base-pair (bp) sequence that is repeated three times in the enhancer, whereas the 21-bp enhancer element can act as a sufficient cis-acting sequence for activation by both p40tax and forskolin. In addition, we found that nuclear factor(s) like the cyclic AMP-responsive element (CRE) binding factor could bind to the HTLV-I 21-bp enhancer element. However, a difference was found in sequences required for activation by p40tax and forskolin. A CRE related sequence present in the 21-bp enhancer element was enough for forskolin-induced activation. On the other hand, p40tax required a much longer sequence that is overlapping but not identical to the CRE related sequence, suggesting that the forskolin-induced cyclic AMP pathway may be partly involved in, but not sufficient for p40tax-mediating trans-activation of the HTLV-I enhancer.

Regulation of transcription by cyclic AMP-dependent protein kinase

cAMP-dependent protein kinase (PKA; ATP: protein phosphotransferase; EC 2.7.1.37) appears to be the major mediator of cAMP responses in mammalian cells. We have investigated the role of PKA subunits in the regulation of specific genes in response to cAMP by cotransfection of wild-type or mutant subunits of PKA together with cAMP-inducible reporter genes. Overexpression of catalytic subunit induced expression from three cAMP-regulated promoters (alpha-subunit, c-fos, E1A) in the absence of elevated levels of cAMP but did not affect expression from two unregulated promoters (Rous sarcoma virus, simian virus 40). Cotransfection of a regulatory subunit gene containing mutations in both cAMP binding sites strongly repressed both basal and induced expression from the cAMP-responsive alpha-subunit promoter without affecting expression from the Rous sarcoma virus promoter. These experiments indicate that cAMP induces gene expression through phosphorylation by the catalytic subunit and that the ambient degree of phosphorylation dictates the level of basal as well as induced expression of the cAMP-regulated alpha-subunit gene.

Leucine zipper structure of the protein CRE-BP1 binding to the cyclic AMP response element in brain

By screening a lambda gt11 library with the multimerized sequence of the cAMP response element (CRE), we isolated human clones encoding the CRE binding protein, CRE-BP1, from a human brain cDNA library. CRE-BP1 expressed in Escherichia coli bound not only to the CRE element of the somatostatin and fibronectin genes, but also to the CRE element of the adenovirus E4 gene, suggesting that the protein was not distinguishable from the adenovirus transcription factor, ATF. The human CRE-BP1 clone encoded a 54.5 kd protein similar at its carboxy terminus to the leucine zipper motifs found in other enhancer binding proteins such as C/EBP and c-jun/AP-1. CRE-BP1 mRNA was expressed in all of the cells examined and was abundant in brain. The structure of CRE-BP1 and its recognition elements suggest that cellular response to extracellular stimuli is controlled by a family of transcription factors that bind to related cis-active elements and that contain several highly conserved domains.

cAMP stimulates transcription of the beta 2-adrenergic receptor gene in response to short-term agonist exposure

In addition to conveying cellular responses to an effector molecule, receptors are often themselves regulated by their effectors. We have demonstrated that epinephrine modulates both the rate of transcription of the beta 2-adrenergic receptor (beta 2AR) gene and the steady-state level of beta 2AR mRNA in DDT1MF-2 cells. Short-term (30 min) exposure to epinephrine (100 nM) stimulates the rate of beta 2AR gene transcription, resulting in a 3- to 4-fold increase in steady-state beta 2AR mRNA levels. These effects are mimicked by 1 mM N6,O2'-dibutyryladenosine 3',5'-cyclic monophosphate (Bt2cAMP) or foskolin but not by phorbol esters. The half-life of the beta 2AR mRNA after addition of actinomycin D (46.7 +/- 10.2 min; mean +/- SEM; n = 5) remained unchanged after 30 min of epinephrine treatment (46.8 +/- 10.6 min; mean +/- SEM; n = 4), indicating that a change in transcription rate is the predominant factor responsible for the increase of beta 2AR mRNA. Whereas brief exposure to epinephrine or Bt2cAMP does not significantly affect the total number of cellular beta 2ARs (assessed by ligand binding), continued exposure results in a gradual decline in beta 2AR number to approximately 20% (epinephrine) or approximately 45% (Bt2cAMP) of the levels in control cells by 24 hr. Similar decreases in agonist-stimulated adenylyl cyclase activity are observed. This loss of receptors with prolonged agonist exposure is accompanied by a 50% reduction in beta 2AR mRNA. Transfection of the beta 2AR promoter region cloned onto a reporter gene (bacterial chloramphenicol acetyltransferase) allowed demonstration of a 2- to 4-fold induction of transcription by agents that elevate cAMP levels, such as forskolin or phosphodiesterase inhibitors. These results establish the presence of elements within the proximal promoter region of the beta 2AR gene responsible for the transcriptional enhancing activity of cAMP and demonstrate that beta 2AR gene expression is regulated by a type of feedback mechanism involving the second messenger cAMP.

The Neurospora clock gene frequency shares a sequence element with the Drosophila clock gene period

The isolation and characterization of single gene mutations affecting the circadian biological clocks of several organisms has left little doubt that circadian rhythms can be subjected to classical genetical analysis. Many of these mutations occur at the same few genetic loci (frequency (frq) in the fungus Neurospora, and period (per) in fruit fly Drosophila); these loci represent the best studied clock-affecting genes known. Mutant strains are usually affected in more than one basic clock property, suggesting an inter-relatedness at the molecular level among these basic properties that would not have been predicted a priori. The extensive background information available concerning the frq locus provides a basis for the molecular dissection of the Neurospora circadian clock--the most minimal circadian system thus far described. We report here the cloning and analysis of the frq locus and show it to be larger and more complex than would have been predicted from the available genetic data. Complete rescue of all of the pleiotropic mutant phenotypes of the recessive frq allele requires transformation with a 7.7-kilobase (kb) region of DNA encoding at least two transcripts. Sequence analysis of this region has allowed the identification of a common element between frq and per which, given the background similarities in their classical genetic characteristics, suggests the possibility of a common element in the clock mechanisms of these two organisms.

Molecular biology and regulation of the hypothalamic hormones

Over the past twenty years, each of the five major hypothalamic releasing or release-inhibiting hormones has been sequenced and its gene structure determined. With the use of molecular biological techniques, such as in situ hybridization, Northern blot analysis or gene constructs for in vitro or in vivo transfection studies--together with 'traditional' neuroendocrinological techniques, such as immunocytochemistry, radio-immunoassay and portal vessel cannulation--investigators have been able to address major issues in neuroendocrine regulation. Several common themes have emerged: messenger RNA expression is uniformly present in neurons that are immunopositive for the specific hypothalamic hormone. Steady state RNA levels within the hypophysiotropic neuron groups are either increased or reduced by changes in specific target hormones that conform to predictions based on previous physiological data. Regulation by the requisite peripheral hormone is exquisitely anatomically specific and is not evident in extrahypophysiotropic regions. Determining the receptor or genetic basis of this specificity is a major focus of current research. Clarifying the apparently lesser role of afferent neural pathways to the hypothalamus in regulating releasing hormone mRNA levels is also an important challenge. Clinically, the measurement of levels of releasing hormones in the peripheral circulation appears to be of limited usefulness, except in rare cases of ectopic GRH or CRH secretion. For diagnostic purposes, each of the releasing hormones has specific utility in amplifying the release and measurement of pituitary hormones, both to clarify the overall physiological activity of the hypothalamic-pituitary-target hormone axis and to further define the anatomic locus of any underlying disturbance. The usefulness of somatostatin as a diagnostic tool is presently limited, but the development of SS receptor antagonists might have significant impact in future clinical investigation. The molecular mechanisms of action of the hypothalamic hormones have been separated into those whose receptor-effector function is mediated by the cAMP-adenylate cyclase pathway(s), GRH and CRH, and those working through the phosphoinositide-protein kinase C cascade, GnRH and TRH. Each of the hormone receptors is coupled to intermediary G proteins, somatostatin uniquely to the inhibitory subclass. The mechanisms responsible for sensitization (priming) or desensitization are not fully understood but are presumably related to receptor down regulation and protein phosphorylation.(ABSTRACT TRUNCATED AT 400 WORDS)