Phosphorylation of cellular proteins regulates their binding to the cAMP response element

Evidence for association of two variants of the nociceptin/orphanin FQ receptor gene OPRL1 with vulnerability to develop opiate addiction in Caucasians

OBJECTIVES

The OPRL1 gene encodes the nociceptin/orphanin FQ receptor, which plays a role in regulating tolerance and behavioral responses to morphine. However, there is limited information on whether variants of OPRL1 are associated with vulnerability to develop opiate addiction. In this study, we examined five variants of OPRL1 and their role in determining vulnerability to develop opiate addiction.

METHODS

We recruited 447 individuals: 271 former severe heroin addicts and 176 healthy controls. Using a 5'-fluorogenic exonuclease assay, we genotyped individuals at five variants in OPRL1. It was then determined whether there was a significant association of allele, genotype, or haplotype frequency with vulnerability to develop opiate addiction.

RESULTS

When the cohort was stratified by ethnicity, we found that, in Caucasians but not in African-Americans or Hispanics, the allele frequency of rs6090041 and rs6090043 were significantly associated point-wise with opiate addiction (P=0.03 and 0.04, respectively). Of the haplotypes formed by these two variants, one haplotype was found to be associated with protection from developing opiate addiction in both African-Americans (point-wise P=0.04) and Caucasians (point-wise P=0.04), and another haplotype with vulnerability to develop opiate addiction in Caucasians only (P=0.020).

CONCLUSION

This study provides evidence for an association of two variants of the OPRL1 gene, rs6090041 and rs6090043, with vulnerability to develop opiate addiction, suggesting a role for nociceptin/orphanin FQ receptor in the development of opiate addiction.

Profiling the thermodynamic softness of adenoviral promoters

We showed previously that anharmonic DNA dynamical features correlate with transcriptional activity in selected viral promoters, and hypothesized that areas of DNA softness may represent loci of functional significance. The nine known promoters from human adenovirus type 5 were analyzed for inherent DNA softness using the Peyrard-Bishop-Dauxois model and a statistical mechanics approach, using a transfer integral operator. We found a loosely defined pattern of softness peaks distributed both upstream and downstream of the transcriptional start sites, and that early transcriptional regions tended to be softer than late promoter regions. When reported transcription factor binding sites were superimposed on our calculated softness profiles, we observed a close correspondence in many cases, which suggests that DNA duplex breathing dynamics may play a role in protein recognition of specific nucleotide sequences and protein-DNA binding. These results suggest that genetic information is stored not only in explicit codon sequences, but also may be encoded into local dynamic and structural features, and that it may be possible to access this obscured information using DNA dynamics calculations.

DNA Binding and Phosphorylation Induce Conformational Alterations in the Kinase-inducible Domain of CREB

CREB-mediated activation of target gene transcription is stimulated by protein kinase A (PKA) phosphorylation at serine 133. This is followed by recruitment of the coactivators CREB-binding protein (CBP) or p300. Conversely, the decline in expression during the attenuation phase is linked to CREB dephosphorylation by nuclear phosphatases. The CREB bZIP domain, which promotes dimerization and promoter binding, as well as the kinase-inducible domain (KID), which interacts with the KIX domain of CBP/p300, are both largely unstructured in solution and become more structured once bound to their respective ligands. In this study, we biochemically characterize DNA- and phosphorylation-induced conformational alterations in CREB that may play a role in its transcriptionally poised, activated state. We find that sequence-specific DNA binding of pCREB renders the protein resistant to serine 133 dephosphorylation by protein phosphatase 1. Paradoxically, CREB bound to DNA and chromatin is efficiently phosphorylated by PKA, indicating that the KID region exists in a different conformation depending on its phosphorylation state. Consistent with this observation, we find that phosphorylation of DNA-bound CREB promotes an alternate conformation characterized by an apparent increase in the size or asymmetry of the complex and a qualitative change in proteolytic sensitivity. Together, our data indicate that DNA binding promotes a global conformational change in CREB that alters the structure of KID. PKA phosphorylation of KID in the DNA-bound state induces a phosphatase-resistant conformation that may prolong transcriptional activity.

CREB, memory enhancement and the treatment of memory disorders: promises, pitfalls and prospects

The treatment of memory disorders, such as the gradual weakening of memory with age, the ravages of Alzheimer's disease and the cognitive deficits in various forms of mental retardation, may greatly benefit from a better understanding of the molecular and cellular mechanisms of memory formation. There is increasing interest in the possibility of pharmacologically enhancing learning and memory even in the absence of specific anatomically evident pathology. Substantial evidence in experimental systems ranging from molluscs to humans indicates that the cAMP response element binding protein (CREB) is a core component of the molecular switch that converts short- to long-term memory. Recent studies have greatly strengthened and refined our understanding of the role of CREB in learning and memory in mammals, in addition to providing greater insight into the molecular mechanisms of CREB regulation and function. This involvement of CREB and the upstream signalling pathways leading to its activation in learning-associated plasticity makes them attractive targets for drugs aimed at improving memory function, in both diseased and healthy individuals. However, CREB and its close relatives cAMP response element modulator and activating transcription factor-1 are ubiquitous proteins with several critical functions. This creates hurdles that the authors believe may limit the usefulness of CREB per se as a target for the development of memory-enhancing drugs, and focus on components of the upstream signalling pathways or on specific downstream targets will be required.

A cAMP response element within the latency-associated transcript promoter of HSV-1 facilitates induced ocular reactivation in a mouse hyperthermia model

Herpes simplex virus type 1 (HSV-1) recombinant strain 17CRE contains a site-directed mutation in the 7-bp CRE consensus sequence located 38 nucleotides upstream of the transcription start site. Scarified mouse corneas received inoculations of 17syn+ (parent), 17CRE, and rescue 17CREr. Slit lamp examination of herpetic lesions and tear film swabs containing infectious virus showed that 17CRE had the same acute phenotype as 17syn+ and 17CREr. At 4 weeks, when the corneas had healed and latency was established, mice received hyperthermic shock. Eye swabs taken 24 h after hyperthermia showed that 17CRE reactivated significantly less than 17syn+ and 17CREr, while no significant differences were found in HSV-1 DNA genome copy numbers and latent virus in the trigeminal ganglia. These results are evidence that this CRE site in the LAT promoter facilitates ocular HSV-1 reactivation in mice.

Characterization of three new manganese peroxidase genes from the ligninolytic basidiomycete Ceriporiopsis subvermispora

Three new genes (Cs-mnp2A, Cs-mnp2B and Cs-mnp3) coding for manganese-dependent peroxidase (MnP) have been identified in the white-rot basidiomycete Ceriporiopsis subvermispora. The mature proteins contain 366 (MnP2A and MnP2B) and 364 (MnP3) amino acids, which are preceded by leader sequences of 21 and 24 amino acids, respectively. Cs-mnp2A and Cs-mnp2B appear to be alleles, since the corresponding protein sequences differ in only five residues. The upstream region of Cs-mnp2B contains a TATA box, AP-1 and AP-2 sites, as well as sites for transcription regulation by metals (two), cAMP (two) and xenobiotics (one). Some of these elements are also found in the regulatory region of Cs-MnP3. Transcription of Cs-mnp2A and Cs-mnp2B, but not that of Cs-mnp3, is activated by manganese.

Regulation of the Mts1-Mts2-dependent ade6-M26 meiotic recombination hot spot and developmental decisions by the Spc1 mitogen-activated protein kinase of fission yeast

The M26 meiotic recombination hot spot in the ade6 gene of Schizosaccharomyces pombe is activated by the heterodimeric M26 binding protein Mts1-Mts2. The individual Mts1 (Atf1, Gad7) and Mts2 (Pcr1) proteins are also transcription factors involved in developmental decisions. We report that the Mts proteins are key effectors of at least two distinct classes of developmental decisions regulated by the mitogen-activated protein (MAP) kinase cascade. The first class (osmoregulation, spore viability, and spore quiescence) requires the Spc1 MAP kinase and the Mts1 protein but does not require the Mts2 protein. The second class (mating, meiosis, and recombination hot spot activation) requires the Spc1 kinase and the Mts1-Mts2 heterodimer. Northern and Western blotting eliminated any significant role for the Spc1 kinase in regulating the expression levels of the Mts proteins. Gel mobility shift experiments indicated that the Mts1-Mts2 heterodimer does not need to be phosphorylated to bind to ade6-M26 DNA in vitro. However, in vivo dimethyl sulfate footprinting demonstrated that protein-DNA interaction within cells is dependent upon the Spc1 MAP kinase, which phosphorylates the Mts1 protein. Thus, the Spc1 kinase helps regulate the effector activities of the Mts1-Mts2 heterodimer in part by modulating its ability to occupy the M26 DNA site in vivo. Meiotic recombination hot spot function is likely the result of DNA conformational changes imparted by binding of the Mts1-Mts2 meiotic transcription factor.

Transcriptional activator-coactivator recognition: nascent folding of a kinase-inducible transactivation domain predicts its structure on coactivator binding

A model of transcriptional activator-coactivator recognition is provided by the mammalian CREB activation domain and the KIX domain of coactivator CBP. The CREB kinase-inducible activation domain (pKID, 60 residues) is disordered in solution and undergoes an alpha-helical folding transition on binding to CBP [Radhakrishan, I., Perez-Alvarado, G. C., Parker, D., Dyson, H. J., Montminy, M. R., and Wright, P. E. (1997) Cell 91, 741-752]. Binding requires phosphorylation of a conserved serine (RPpSYR) in pKID associated in vivo with the biological activation of CREB signaling pathways. The CBP-bound structure of CREB contains two alpha-helices (designated alphaA and alphaB) flanking the phosphoserine; the bound structure is stabilized by specific interactions with CBP. Here, the nascent structure of an unbound pKID domain is characterized by multidimensional NMR spectroscopy. The solubility of the phosphopeptide (46 residues) was enhanced by truncation of N- and C-terminal residues not involved in pKID-CBP interactions. Although disordered under physiologic conditions, the pKID fragment and its unphosphorylated parent peptide exhibit partial folding at low temperatures. One recognition helix (alphaA) is well-defined at 4 degreesC, whereas the other (alphaB) is disordered but inducible in 40% trifluoroethanol (TFE). Such nascent structure is independent of serine phosphorylation and correlates with the relative extent of engagement of the two alpha-helices in the pKID-KIX complex; whereas alphaA occupies a peripheral binding site with few intermolecular contacts, the TFE-inducible alphaB motif is deeply engaged in a hydrophobic groove. Our results support the use of TFE as an empirical probe of hidden structural propensities and define a correspondence between induced fit and the nascent structure of peptide fragments.

Phosphorylation of the cAMP response element binding protein CREB by cAMP-dependent protein kinase A and glycogen synthase kinase-3 alters DNA-binding affinity, conformation, and increases net charge

The cAMP response element binding protein CREB activates the transcription of genes in response to phosphorylation by cAMP-dependent protein kinase A (PKA) and other protein kinases. Phosphorylated CREB activates transcription by recruiting transcriptional co-activators such as the CREB binding protein. Here, we describe experiments that analyze the effects of phosphorylation on the DNA binding affinity of CREB and the structural characteristics of the CREB/DNA complex in solution. Analysis of deletion mutants of CREB indicate that amino acid sequences within the transactivation domain promote high-affinity binding of CREB to fluorescently labeled oligonucleotides containing cAMP response elements. In vitro experiments indicate that phosphorylation is processive between PKA as the initial kinase and glycogen synthase kinase-3 (GSK-3) but not casein kinase II as the secondary kinase. Fluorescent electrophoretic mobility shift assays show that phosphorylation by PKA results in a 3-5-fold increase in the binding affinity of CREB to both the symmetrical somatostatin CRE (SMS-CRE) and the asymmetric somatostatin upstream element (SMS-UE). Processive phosphorylation of CREB by GSK-3 attenuates the enhanced DNA binding in response to PKA thus acts as an inhibitor of PKA-induced binding. Ferguson plot analyses demonstrate that phosphorylation of CREB by PKA and GSK-3 result in an increase in the spherical size and the net positive surface charge of the CREB/DNA complex. Moreover, these analyses uncovered the unexpected finding that CREB associates as a tetramer both in the presence and absence of DNA. These findings suggest a model by which phosphorylation of CREB alters the secondary structure and charge characteristics of the CREB/DNA complex resulting in an alteration in binding affinity.

Modulation of transcription factor Sp1 by cAMP-dependent protein kinase

Transcription factor Sp1 is a phosphoprotein whose level and DNA binding activity are markedly increased in doxorubicin-resistant HL-60 (HL-60/AR) leukemia cells. The trans-activating and DNA binding properties of Sp1 in HL-60/AR cells are stimulated by cAMP-dependent protein kinase (PKA) and PKA agonists and inhibited by PKA antagonists as well as by the PKA regulatory subunit. Reporter gene activity under the control of the Sp1-dependent SV40 promoter is stimulated in insect cells transiently expressing Sp1 and PKA, and the DNA binding activity of recombinant Sp1 is activated by exogenous PKA in vitro. These results indicate that Sp1 is a cAMP-responsive transcription factor and that Sp1-dependent genes may be modulated through a cAMP-dependent signaling pathway.

Phosphorylation of ATF-1 enhances its DNA binding and transcription of the Na,K-ATPase alpha 1 subunit gene promoter

Transcriptional activity of both ATF-1 and CREB is enhanced by protein phosphorylation. While enhancement has been attributed to an increase in binding affinity for a co-activator (CBP), induction of the DNA binding activity by phosphorylation is an open question. Using the Na,K-ATPase alpha1 subunit gene promoter, which has an asymmetrical ATF/CRE site, we analyzed the effect of phosphorylation on DNA binding activity of the ATF-1-CREB heterodimer. Dephosphorylation of the heterodimer in nuclear extracts reduced binding to the ATF/CRE site. Phosphorylation of ATF-1 at Ser63 enhanced its binding to the ATF/CRE site in both the homodimeric and heterodimeric forms. Transcription of the Na,K-ATPase alpha 1 subunit gene promoter was also stimulated by phosphorylated ATF-1 in vitro.

Analysis of the structural properties of cAMP-responsive element-binding protein (CREB) and phosphorylated CREB

The transcription factor CREB (cAMP responsive element binding protein) is activated by protein kinase A (PKA) phosphorylation of a single serine residue. To investigate possible mechanisms of CREB regulation by phosphorylation, we initiated a structural and biophysical characterization of the full-length, wild-type CREB protein, an altered CREB protein (CREB/SER) in which the three cysteine residues in the DNA-binding domain were replaced with serine residues and a truncated protein (ACT265) which encompasses the entire activation domain of CREB. Circular dichroism (CD) reveals that CREB and CREB/SER have identical secondary structures and contain approximately 20% alpha-helix, 9% beta-strand, 34% beta-turn, and 37% random coil structures. PKA phosphorylation does not alter the CD spectra, and therefore the secondary structure, of CREB or of CREB bound to DNA. Protease cleavage patterns indicate that PKA phosphorylation does not induce a global conformational change in CREB. Furthermore, PKA phosphorylation does not change the DNA binding affinity of CREB for either canonical or non-canonical CRE sequences as measured by a fluorescence anisotropy DNA binding assay. Since PKA phosphorylation of CREB results in its specific binding to the transcriptional co-activators CREB-binding protein and p300, we suggest that the PKA activation of CREB occurs by the production of specific, complementary interactions with these proteins, rather than through the previously proposed mechanisms of a phosphorylation-dependent conformational change or increased DNA binding affinity.

Induction of c-fos and zif/268 gene expression in rat striatal neurons, following stimulation of D1-like dopamine receptors, involves protein kinase A and protein kinase C

Changes in the level of dopaminergic activity in the rat striatum lead to the induction of a number of immediate-early genes, including c-fos and zif/268. These immediate-early genes are thought in turn to alter the rate of transcription of downstream genes. There is evidence that the dopaminergic activation of the c-fos and zif/268 genes in the striatum in vivo is linked to stimulation of D1-like dopamine receptors. We have used primary cultures of embryonic rat striatal neurons to identify the intracellular pathways involved in this response. Dopamine (10 nM-5 microM) caused a marked increase in the levels of c-fos mRNA and zif/268 mRNA in cultured striatal neurons, an effect that was reproduced by the D1-like dopamine receptor agonist SKF38393 (10 nM-5 microM). These actions were attenuated by the D1-like antagonist SCH23390 (1 microM) but not by the D2-like antagonist eticlopride (1 microM). The D2-like agonist quinpirole did not increase zif/268 mRNA above basal levels at concentrations up to 5 microM, but caused a slight increase in the levels of c-fos mRNA. The stimulation of c-fos mRNA levels caused by 1 microM SKF38393 was reduced by 45% following pretreatment with the selective protein kinase A inhibitor KT5720, and by 87% following pretreatment with the selective protein kinase C inhibitor calphostin C. The stimulation of zif/268 mRNA levels caused by 1 microM SKF38393 was reduced by 90% following pretreatment with KT5720, but was not significantly affected by pretreatment with calphostin C. In addition, the actions of SKF38393 to stimulate the expression of both immediate-early genes were attenuated by coadministration of quinpirole. These results suggest that SKF38393 acts on striatal neurons to stimulate c-fos expression predominantly through protein kinase C, but also partially through protein kinase A. Conversely, SKF38393 induces zif/268 expression through protein kinase A. The ability of quinpirole to antagonize the actions of SKF38393 on cultured neurons is consistent with the presence of both D1-like receptors on the same neuronal population.

Regulation of multidrug resistance through the cAMP and EGF signalling pathways

The development of cross-resistance to many natural product anticancer drugs, termed multidrug resistance (MDR), is a serious limitation to cancer chemotherapy. MDR is often associated with overexpression of the MDR1 gene product, P-glycoprotein, a multifunctional drug transporter. Understanding the mechanisms that regulate the transcriptional activation of MDR1 may afford a means of reducing or eliminating MDR. We have found that MDR1 expression can be modulated by type I cAMP-dependent protein kinase (PKA). This suggests that MDR may be modulated by selectively downregulating PKA activity to effect inhibition of PKA-dependent trans-activating factors which may be involved in MDR1 transcription. High levels of type I PKA occur in primary breast carcinomas and patients exhibiting this phenotype show decreased survival. The selective type I PKA inhibitors, 8-Cl-cAMP and Rp8-Cl-cAMP[S], may be particularly useful for downregulating PKA, and inhibit transient expression of a reporter gene under the control of MDR1 promoter elements. Thus, investigations of the signalling pathways involved in transcriptional regulation of MDR1 may lead to a greater understanding of the mechanisms governing the expression of MDR and provide a focus for pharmacological intervention.

Regulation of the transforming growth factor-beta 2 gene promoter in embryonal carcinoma cells and their differentiated cells: differential utilization of transcription factors

Previous studies demonstrated that differentiation of embryonal carcinoma (EC) cells increases the expression of the TGF-beta 2 gene and identified a CRE/ATF-like motif in the TGF-beta 2 promoter that is necessary for its activity. This suggested that differentiation may increase the transcription of this gene by differential binding of transcription factors to the CRE/ATF-like motif. To test this possibility, we performed gel mobility shift analysis using double-stranded oligodeoxynucleotides containing the TGF-beta 2 CRE/ATF-like motif and nuclear extracts prepared from F9 EC cells and F9-differentiated cells. We determined that the DNA/protein complexes formed by the EC nuclear extracts, but not the complexes formed by differentiated cell nuclear extracts, are recognized and supershifted by an ATF-1 specific antibody. This observation is consistent with our Western immunoblot analysis that detects AFT-1 in the EC cells, but not in their differentiated counterparts. In addition, we provide evidence that protein phosphorylation influences the formation of complexes between F9 nuclear proteins and the CRE/ATF-like motif. Together, our studies identify a likely role for the CRE/ATF-like motif in the regulation of TGF-beta 2 and suggest that this site binds one set of nuclear proteins in EC cells, where the gene is not expressed, and a different set of nuclear proteins in the differentiated cells, where the gene is expressed.

Melatonin regulates the phosphorylation of CREB in ovine pars tuberalis

This study investigated whether melatonin could modulate the phosphorylation of the calcium/cyclic AMP response-element binding-protein (CREB) within primary cell cultures of ovine pars tuberalis (oPT) and pars distalis (oPD). Gel shift assays confirmed the presence of nuclear factors able to alter the electrophoretic mobility of a 32P-labelled CRE oligonucleotide. Two shifted bands were observed probably due to monomer and dimer binding to the CRE. Each band was supershifted by antisera directed against both CREB and the phosphorylated form of CREB (P-CREB), consistent with a specific role of CREB proteins in transcriptional regulation. To study the physiological role of CREB, the nuclear immunoreactivity for P-CREB was followed in primary cultures of oPT given different pharmacological treatments. Cells stimulated with forskolin responded with a robust time- and dose-dependent increase in nuclear phospho-CREB immunoreactivity (P-CREB-ir), confirming that activation of this transcription factor occurred through the cyclic AMP-PKA pathway. Maximal stimulation was achieved within 15 min and persisted for up to 1 h. Treatment with melatonin alone did not alter basal P-CREB-ir levels, yet melatonin inhibited the forskolin-induced increase in P-CREB-ir in a dose-dependent manner (IC50 of between 10(-10) M and 10(-8) M melatonin when tested against 1 microM forskolin). In contrast, in primary cultures of oPD, melatonin failed to block forskolin-stimulated increases in either the content of cyclic AMP or the intensity of nuclear P-CREB-ir, confirming that the action of melatonin upon P-CREB-ir is tissue specific. These results demonstrate that, consistent with its inhibitory effect on the activation of PKA within oPT, melatonin prevents or reverses the phosphorylation of CREB induced by activation of the cyclic AMP signal transduction pathway. Therefore melatonin has the potential to regulate gene expression in the oPT by acting upon the CREB transcription factor. However, this paper also shows that 12-O-tetradecanoylphorbol-13-acetate (TPA) which activates PKC also leads to the phosphorylation of CREB in oPT cells, suggesting the potential involvement of other signal transduction pathways in the transcriptional regulation of these cells.

UV-responsive element (TGACAACA) from rat fibroblasts to human melanomas

When taken together, several lines of evidence suggest that the URE-bound proteins are associated with DNA replication. (1) A trans-acting factor of 60 kDa (which may include the 68-kDa URE-binding proteins) was found to be induced by UV and to mediate polyoma DNA replication; (2) the URE was able to compete for the binding of factors that promote polyoma replication in rodent cells; (3) URE-bound proteins are expressed to a higher extent at the S phase of the cell cycle; and (4) they are induced following treatment with aphidicholin. These observations may suggest that the URE may play a role in growth "release" (as opposed to growth "arrest") which would assist in restoring normal growth following DNA damage. It is clear that URE-bound proteins consist of multiple transcription factors, some of which are well characterized (i.e., jun, CREB, and fos families); however, it is likely that the growth release phenomenon we relate to is also mediated by (1) other members of these transcription factor families which have not been identified as yet and (2) a specific combination of known transcription factors which bind to this response element under certain circumstances. This hypothesis is outlined in the enclosed model (Fig. 3).

Transcriptional regulation of multidrug resistance in breast cancer

The development of cross-resistance to many natural product anticancer drugs, termed multidrug resistance (MDR), is one of the major reasons why cancer chemotherapy ultimately fails. This type of MDR is often associated with over-expression of the MDR1 gene product, P-glycoprotein (Pgp), a multifunctional drug transporter. The expression of MDR in breast tumors is related to their origination from a tissue that constitutively expresses Pgp as well as to the development of resistance during successive courses of chemotherapy. Therefore, understanding the mechanisms that regulate the transcriptional activation of MDR1 may afford a means of reducing or eliminating MDR. We have found that MDR1 expression can be modulated by type I cAMP-dependent protein kinase (PKA), opening up the possibility of modulating MDR by selectively down-regulating the activity of PKA-dependent transcription factors which upregulate MDR1 expression. High levels of type I PKA occurs in primary breast carcinomas and patients exhibiting this phenotype show decreased survival. The selective type I cAMP-dependent protein kinase (PKA) inhibitors, 8-Cl-cAMP and Rp8-Cl-cAMP[S] may be particularly useful for downregulating PKA-dependent MDR-associated transcription factors, and we have found these compounds to downregulate transient expression of a reporter gene under the control of several MDR1 promoter elements. Thus, investigations of this nature should not only lead to a greater understanding of the mechanisms governing the expression of MDR, but also provide a focus for pharmacologic intervention by a new class of inhibitors.

Transcription of the yeast mitochondrial genome requires cyclic AMP

Using various mutant strains and nutritional manipulations, we investigated a potential role for cyclic AMP (cAMP) in the regulation of mitochondrial (mt) gene expression in the yeast Saccharomyces cerevisiae. In RAS mutants known to have either abnormally low or high cellular levels of this nucleotide, we show that both mt transcription rate and overall mt transcript levels vary directly with cellular cAMP levels. We further show that nutritional downshift of actively growing cells causes a severe, rapid fall in cAMP levels, and that this fall is concomitant with the stringent mt transcriptional curtailment that we and others have previously shown to follow this nutritional manipulation. In in vitro mt transcription assays using intact organelles from downshifted and actively growing cells, stringently curtailed mt gene expression can be restored to 75% of control levels by addition of cAMP to the assay mix. Consistent with these observations a RAS2vall9 mutant strain, which cannot adjust cAMP levels in response to external stimuli, shows no mt stringent response following nutritional downshift. We also demonstrate a significant but transient increase in both mt transcript levels and mt transcription rate following shift of actively respiring wild-type cells to glucose-based medium, a manipulation known to cause a short-lived pulse of cAMP in yeast; similar manipulation of the RAS2vall9 mutant strain generates no such response. Taken together all these observations indicate that cellular cAMP levels are involved in the regulation of mt transcription in yeast. Moreover, the lack of a mt stringent transcriptional response following downshift in a strain in which the BCY1 gene had been insertionally inactivated suggests that cAMP may influence mt transcription via a mt cAMP-dependent protein kinase. These results link mt gene expression with mechanisms governing growth control and nutrient adaptation in yeast, and they provide a means by which mt gene expression might be coordinated with that of related nuclear genes.

DNA topoisomerase I is involved in both repression and activation of transcription

Reconstituted transcription reactions containing the seven general transcription factors, in addition to RNA polymerase II, respond poorly to transcriptional activators. Two factors, Dr2 and ACF, necessary for high levels of transcription in response to an activator have been identified. ACF can enhance basal and activated transcription. Dr2 represses basal transcription, but this can be overcome by transcriptional activators or TFIIA. Dr2 is human DNA topoisomerase I. The DNA relaxation activity of topoisomerase I is dispensable for transcriptional repression. The effect of Dr2 is specific for TATA-box-containing promoters and is mediated by the TATA-binding protein.

v-ras and protein kinase C dedifferentiate thyroid cells by down-regulating nuclear cAMP-dependent protein kinase A

Ras proteins are membrane-associated transducers of eternal stimuli to unknown intracellular targets. The constitutively activated v-ras oncogene induces dedifferentiation in thyroid cells. v-Ras appears to act by stimulating protein kinase C (PKC), which inhibits the nuclear migration of the catalytic subunit of the cAMP-dependent protein kinase A (PKA). Nuclear tissue-specific and housekeeping trans-acting factors that are dependent on phosphorylation by PKA are thus inactivated. Exclusion of the PKA subunit from the nucleus could represent a general mechanism for the pleiotropic effects of Ras and PKC on cellular growth and differentiation.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates cyclic AMP formation as well as peptide output of cultured pituitary melanotrophs and AtT-20 corticotrophs

The present study was aimed at investigating whether PACAP stimulates accumulation of cAMP, as well as hormonal secretion of homogeneous populations of pituitary proopiomelanocortin (POMC) cells, namely melanotrophs and AtT-20 corticotrophs. PACAP was shown to enhance cAMP accumulation in a dose-dependent fashion in both cell types (with EC50 values of approx. 10(-10) M) and elicited additive increases of cAMP production with CRF in melanotrophs, but not in corticotrophs. PACAP also stimulated dose-dependently the secretion of alpha-MSH and ACTH, with EC50 concentrations of about 10(-9) M. In melanotrophs, bromocriptine significantly depressed PACAP-induced cAMP formation and blunted by more than 90% stimulated alpha-MSH release. This study shows that (1) pituitary POMC cells did respond to PACAP by enhancing cAMP accumulation and elevating hormone secretion as well; (2) the effect of PACAP was additive with CRF on cAMP production in melanotrophs, but not in corticotrophs, while there was no additivity on peptide output from both cell types; (3) activation of dopamine receptors in melanotrophs dampened both cAMP formation and peptide secretion. These findings are consistent with PACAP playing a possible hypophysiotropic role in the regulation of pituitary POMC cell activity.

Antibodies against the cystic fibrosis transmembrane regulator

Rabbit polyclonal antibodies have been raised against high-performance liquid chromatography purified synthetic peptides corresponding to two discrete regions of the cystic fibrosis transmembrane regulator (CFTR) protein: the R-domain (residues 785-796) and the extreme COOH-terminus (residues 1467-1480). Antibodies (Ab) to each of these peptides were affinity purified either by passage over a peptide-derivatized agarose matrix (Ab 785) to produce monospecific polyclonal antibodies or by protein A affinity chromatography to purify the immunoglobulin G1 fraction free of other serum proteins (Ab 1467). These antibodies recognize a candidate CFTR protein in the colonic cell line T84, as determined by Western blot analysis and by immunoprecipitation and labeling of the precipitate with [gamma-32P]ATP in the presence of protein kinase A. Both antibodies precipitated CFTR-related polypeptides from four mammalian cell types (HeLa, Bsc-40, HEp-2, and Chinese hamster ovary cells) transfected with the full-length CFTR cDNA clone using a vaccinia T7 protein expression system. Similar results were observed using a yeast CFTR expression system. In each case the Mr values of the bands observed were consistent with that expected for the CFTR protein. These antibodies should be useful probes for the immunocytochemical localization, immunoaffinity purification, and ultimately the functional characterization of the CFTR protein.

Synergistic effects of cyclic AMP and Ca2+ ionophore A23187 on de novo synthesis of histidine decarboxylase in mastocytoma P-815 cells

In the preceding paper (Kawai, H. et al. (1992) Biochim. Biophys. Acta 1133, 172-178), we reported that in mastocytoma P-815 cells dexamethasone and 12-O-tetradecanoylphorbol-13-acetate (TPA) synergistically enhanced the de novo synthesis of L-histidine decarboxylase (HDC). Here we found that Ca2+ acted synergistically with cAMP in the induction of HDC mRNA and HDC activity in mastocytoma P-815 cells, and that the mechanism underlying the enzyme induction by Ca2+ plus cAMP was distinguishable from that by dexamethasone plus TPA. Ca2+ ionophore A23187, itself having no significant activity, markedly enhanced the induction of HDC activity by N6,O2'-dibutyryl cAMP (db cAMP) or cAMP-inducible prostaglandins such as PGE1, PGE2 and PGI2 in the presence of the phosphodiesterase inhibitor, Ro201724. However, A23187 had little effect on increases in HDC activity induced by other known stimulants, such as TPA, dexamethasone and sodium butyrate. These results suggest that A23187 has a specific effect on the induction of HDC activity due to an increased level of cAMP. The finding that both A23187 and cAMP enhanced HDC activity suggests that both Ca2+/calmodulin and cyclic nucleotide dependent protein kinase play essential roles in the process of enhancement of HDC activity. To examine this possibility, we studied the effects of W-7, an inhibitor of calmodulin, removal of extracellular Ca2+, and H-8, an inhibitor of cAMP-dependent protein kinase, on the enhancing activity of A23187 plus db cAMP. The enhancement of HDC activity by A23187 plus db cAMP was inhibited by W-7, removal of extracellular Ca2+, and H-8. The increase in HDC activity was due to the de novo synthesis of the enzyme, since it was suppressed by the addition of cycloheximide or actinomycin D, and was well correlated with the marked accumulation of a 2.7 kilobase HDC mRNA. Furthermore, the mechanism underlying the induction of HDC by db cAMP plus A23187 is distinguishable from that in the case of dexamethasone plus TPA, since preexposure to dexamethasone plus TPA for 12 h, for a plateau level to be reached, did not affect the subsequent increase in HDC activity due to db cAMP plus A23187.

A cAMP-binding ectoprotein in the yeast Saccharomyces cerevisiae

Purified plasma membranes from the yeast Saccharomyces cerevisiae bind about 1.2 pmol of cAMP/mg of protein with high affinity (Kd = 6 nM). By using photoaffinity labeling with 8-N3-[32P]cAMP, we have identified in plasma membrane vesicles a cAMP-binding protein (Mr = 54,000) that is present also in bcy1 disruption mutants, lacking the cytoplasmic R subunit of protein kinase A (PKA). This argues that it is genetically unrelated to PKA. Neither high salt, nor alkaline carbonate, nor cAMP extract the protein from the membrane, suggesting that it is not peripherally bound. The observation that (glycosyl)phosphatidylinositol-specific phospholipases (or nitrous acid) release the amphiphilic protein from the membrane, thereby converting it to a hydrophilic form, indicates anchorage by a glycolipidic membrane anchor. Treatment with N-glycanase reduces the Mr to 44,000-46,000 indicative of a modification by N-linked carbohydrate side chain(s). In addition to the action of a phospholipase, the efficient release from the membrane requires the removal of the carbohydrate side chain(s) or the presence of high salt or methyl alpha-mannopyranoside, suggesting complex interactions with the membrane involving not only the glycolipidic anchor but also the glycan side chain(s). Topological studies show that the protein is exposed to the periplasmic space, raising intriguing questions for the function of this protein.

Redundant elements in the adenovirus type 5 inverted terminal repeat promote bidirectional transcription in vitro and are important for virus growth in vivo

The adenovirus inverted terminal repeat (ITR) contains a number of cis-acting elements that are involved in the initiation of viral DNA replication, as well as multiple binding motifs for the cellular transcription factors SP1 and ATF. In this study, we utilized a Hela cell transcription extract to demonstrate that the adenovirus type 5 ITR promotes bidirectional transcription in vitro. Primer extension analyses demonstrated that the ITR directed transcription at initiation sites both within the terminal repeat and at fixed distances outside of the ITR. The ITR also strongly stimulated transcription at the early region 1A (E1A) initiation site when it was situated immediately upstream of the E1A TATA box region. Deletion and point mutational analyses demonstrated that two distinct cis-acting elements were involved in these ITR-dependent transcriptional activities in vitro. Cellular transcription factors SP1 and ATF were previously shown to bind to these two regions. Analysis of viral mutants in vivo demonstrated that the NFIII/OCT-1 binding site and a conserved ATF motif were important for efficient viral growth. Regulatory elements in the ITR flanking region were found to functionally substitute for these sites.

Differences in induction of c-fos transcription by cholera toxin-derived cyclic AMP and Ca2+ signals in astrocytes and 3T3 fibroblasts

The B subunit of cholera toxin, a protein which binds specifically to membrane ganglioside GM1, is known to affect cell growth and differentiation. To investigate the mechanism of these cellular responses at the nuclear level, we used the induction of c-fos in astrocytes and 3T3 fibroblasts as a model. Northern blot analysis showed that treatment with B subunit provokes a rapid and transient expression of c-fos mRNA, independent of a measurable increase in cyclic AMP. The B subunit signal, which is mediated by Ca2+, was compared to cholera toxin and other agents which increase intracellular cyclic AMP levels. In transient transfection assays of astrocytes and fibroblasts, functional analysis of c-fos promoter deletions was used to identify the elements involved in transcriptional activation by B subunit. In astrocytes, the DNA region including the serum response element and the cyclic AMP response element (CRE) are equally required, whereas 3T3 cells require only the CRE for maximal induction. A synergistic effect of signal transduction was mediated by calcium and cyclic AMP on the CRE, being positive in 3T3 cells and negative in astrocytes. Diverse regulatory elements may be thus involved in responses of different cell types to the same extracellular signal. Furthermore, a single regulatory element (CRE) can integrate both calcium and cyclic AMP signals in the control of gene expression.

CREM gene: use of alternative DNA-binding domains generates multiple antagonists of cAMP-induced transcription

We isolated a gene from a mouse pituitary cDNA library that encodes a protein highly homologous to nuclear factor CREB, an activator of cAMP-responsive promoter elements (CREs). We demonstrate that while CREB is expressed uniformly in several cell types, this gene, termed CREM, shows cell-specific expression. CREM has a remarkable organization, since down-stream of the stop codon there is a second, out-of-frame DNA-binding domain. Using PCR and RNAase protection analysis, we have identified three mRNA isoforms that appear to be obtained by differential cell-specific splicing. Sequencing of the isoforms demonstrated alternative usage of the two DNA-binding domains. CREM proteins reveal the same efficiency and specificity of binding to CRE sequences as CREB, but in contrast to CREB, CREM acts as a down-regulator of cAMP-induced transcription.

Reversible inhibition of a thyroid-specific trans-acting factor by Ras

Exposure of rat thyroid cells for 1 week to a temperature-sensitive variant of Kirsten murine sarcoma virus (KiMSV) Ras inactivated the thyroglobulin promoter (pTg). Cellular dedifferentiation was paralleled by the loss of the thyroid-specific trans-acting factor, TgTF1, which binds to pTg. When Ras was denatured by shifting cells to 39 degrees C, TgTF1 binding and pTg function recovered rapidly without the synthesis of new protein. TgTF1 could be reactivated in vitro by treating nuclear extracts with protein kinase A. After 4 weeks of exposure to the oncogene, denaturation of Ras no longer restored TgTF1 binding or reactivated pTg. Incubation of nuclear extracts with protein kinase A likewise did not reactivate TgTF1. Cells chronically exposed to Ras did, however, yield differentiated clones after treatment with 5-azacytidine. We suggest that Ras induces dedifferentiation in two sequential steps: (1) Ras reduces PKA activity; TgTF1 (or an auxiliary protein) becomes dephosphorylated, and binding to pTg is abolished. (2) The effects of Ras become imprinted by methylation, possibly of the TgTF1 gene.

Activation of protein kinase C differentially regulates corticotropin-releasing factor-stimulated peptide secretion and cyclic AMP formation of intermediate and anterior pituitary cells in culture

The present study was aimed at investigating the effect of protein kinase C (PKC) activation on CRF receptor function of proopiomelanocortin (POMC) cells in culture. Incubation of tissues with the phorbol ester PMA selectively potentiated corticotropin-releasing factor (CRF)-stimulated ACTH secretion and cyclic AMP formation of anterior pituitary (AP) cells, while, in sharp contrast, it failed to similarly affect intermediate pituitary (IP) cells and AtT-20 corticotrophs exposed to CRF. Unexpectedly, however, long-term treatment of cultures with PMA, which depletes cell stores of PKC, resulted in a similar dramatic attenuation of stimulated peptide release from both corticotrophs and melanotrophs, while being without significant effect on cyclic AMP production. Exposure of cells to PMA did not change either basal or CRF-enhanced levels of POMC mRNA. We conclude that activation of PKC fails to synergize with CRF-mediated signalling in IP and AtT-20 cells, although optimal CRF receptor expression requires the presence of a functional kinase C pathway, thus suggesting cross-talks between both messenger systems.