A transcriptional switch mediated by cofactor methylation

We describe a molecular switch based on the controlled methylation of nucleosome and the transcriptional cofactors, the CREB-binding proteins (CBP)/p300. The CBP/p300 methylation site is localized to an arginine residue that is essential for stabilizing the structure of the KIX domain, which mediates CREB recruitment. Methylation of KIX by coactivator-associated arginine methyltransferase 1 (CARM1) blocks CREB activation by disabling the interaction between KIX and the kinase inducible domain (KID) of CREB. Thus, CARM1 functions as a corepressor in cyclic adenosine monophosphate signaling pathway via its methyltransferase activity while acting as a coactivator for nuclear hormones. These results provide strong in vivo and in vitro evidence that histone methylation plays a key role in hormone-induced gene activation and define cofactor methylation as a new regulatory mechanism in hormone signaling.

Chromatin-dependent cooperativity between constitutive and inducible activation domains in CREB

The cyclic AMP (cAMP)-responsive factor CREB induces target gene expression via constitutive (Q2) and inducible (KID, for kinase-inducible domain) activation domains that function synergistically in response to cellular signals. KID stimulates transcription via a phospho (Ser133)-dependent interaction with the coactivator paralogs CREB binding protein and p300, whereas Q2 recruits the TFIID complex via a direct association with hTAF(II)130. Here we investigate the mechanism underlying cooperativity between the Q2 domain and KID in CREB by in vitro transcription assay with naked DNA and chromatin templates containing the cAMP-responsive somatostatin promoter. The Q2 domain was highly active on a naked DNA template, and Ser133 phosphorylation had no additional effect on transcriptional initiation in crude extracts. Q2 activity was repressed on a chromatin template, however, and this repression was relieved by the phospho (Ser133) KID-dependent recruitment of p300 histone acetyltransferase activity to the promoter. In chromatin immunoprecipitation assays of NIH 3T3 cells, cAMP-dependent recruitment of p300 to the somatostatin promoter stimulated acetylation of histone H4. Correspondingly, overexpression of hTAFII130 potentiated CREB activity in cells exposed to cAMP, but had no effect on reporter gene expression in unstimulated cells. We propose that cooperativity between the KID and Q2 domains proceeds via a chromatin-dependent mechanism in which recruitment of p300 facilitates subsequent interaction of CREB with TFIID.

CREB regulates hepatic gluconeogenesis through the coactivator PGC-1

When mammals fast, glucose homeostasis is achieved by triggering expression of gluconeogenic genes in response to glucagon and glucocorticoids. The pathways act synergistically to induce gluconeogenesis (glucose synthesis), although the underlying mechanism has not been determined. Here we show that mice carrying a targeted disruption of the cyclic AMP (cAMP) response element binding (CREB) protein gene, or overexpressing a dominant-negative CREB inhibitor, exhibit fasting hypoglycaemia [corrected] and reduced expression of gluconeogenic enzymes. CREB was found to induce expression of the gluconeogenic programme through the nuclear receptor coactivator PGC-1, which is shown here to be a direct target for CREB regulation in vivo. Overexpression of PGC-1 in CREB-deficient mice restored glucose homeostasis and rescued expression of gluconeogenic genes. In transient assays, PGC-1 potentiated glucocorticoid induction of the gene for phosphoenolpyruvate carboxykinase (PEPCK), the rate-limiting enzyme in gluconeogenesis. PGC-1 promotes cooperativity between cyclic AMP and glucocorticoid signalling pathways during hepatic gluconeogenesis. Fasting hyperglycaemia is strongly correlated with type II diabetes, so our results suggest that the activation of PGC-1 by CREB in liver contributes importantly to the pathogenesis of this disease.

Transcriptional regulation by the phosphorylation-dependent factor CREB

The transcription factor CREB -- for 'cyclic AMP response element-binding protein' -- functions in glucose homeostasis, growth-factor-dependent cell survival, and has been implicated in learning and memory. CREB is phosphorylated in response to various signals, but how is specificity achieved in these signalling pathways?

The CREB family of activators is required for endochondral bone development

We have evaluated the importance of the CREB family of transcriptional activators for endochondral bone formation by expressing a potent dominant negative CREB inhibitor (A-CREB) in growth plate chondrocytes of transgenic mice. A-CREB transgenic mice exhibited short-limbed dwarfism and died minutes after birth, apparently due to respiratory failure from a diminished rib cage circumference. Consistent with the robust Ser133 phosphorylation and, hence, activation of CREB in chondrocytes within the proliferative zone of wild-type cartilage during development, chondrocytes in A-CREB mutant cartilage exhibited a profound decrease in proliferative index and a delay in hypertrophy. Correspondingly, the expression of certain signaling molecules in cartilage, most notably the Indian hedgehog (Ihh) receptor patched (Ptch), was lower in A-CREB expressing versus wild-type chondrocytes. CREB appears to promote Ptch expression in proliferating chondrocytes via an Ihh-independent pathway; phospho-CREB levels were comparable in cartilage from Ihh(−/−) and wild-type mice. These results demonstrate the presence of a distinct signaling pathway in developing bone that potentiates Ihh signaling and regulates chondrocyte proliferation, at least in part, via the CREB family of activators.

PDX:PBX complexes are required for normal proliferation of pancreatic cells during development

The homeobox factor PDX-1 is a key regulator of pancreatic morphogenesis and glucose homeostasis; targeted disruption of the PDX-1 gene leads to pancreatic agenesis in pdx-1(-/-) homozygotes. Pdx-1 heterozygotes develop normally, but they display glucose intolerance in adulthood. Like certain other homeobox proteins, PDX-1 contains a consensus FPWMK motif that promotes heterodimer formation with the ubiquitous homeodomain protein PBX. To evaluate the importance of PDX-1:PBX complexes in pancreatic morphogenesis and glucose homeostasis, we expressed either wild-type or PBX interaction defective PDX-1 transgenes under control of the PDX-1 promoter. Both wild-type and mutant PDX-1 transgenes corrected glucose intolerance in pdx-1 heterozygotes. The wild-type PDX-1 transgene rescued the development of all pancreatic lineages in pdx-1(-/-) animals, and these mice survived to adulthood. In contrast, pancreata from pdx-1(-/-) mice expressing the mutant PDX-1 transgene were hypoplastic, and these mice died within 3 weeks of birth from pancreatic insufficiency. All pancreatic cell types were observed in pdx-1(-/-) mice expressing the mutant PDX-1 transgene; but the islets were smaller, and increased numbers of islet hormone-positive cells were noted within the ductal epithelium. These results indicate that PDX-1:PBX complexes are dispensable for glucose homeostasis and for differentiation of stem cells into ductal, endocrine, and acinar lineages; but they are essential for expansion of these populations during development.

Characterization of a CREB gain-of-function mutant with constitutive transcriptional activity in vivo

The cyclic AMP (cAMP)-responsive factor CREB promotes cellular gene expression, following its phosphorylation at Ser133, via recruitment of the coactivator paralogs CREB-binding protein (CBP) and p300. CBP and p300, in turn, appear to mediate target gene induction via their association with RNA polymerase II complexes and via intrinsic histone acetyltransferase activities that mobilize promoter-bound nucleosomes. In addition to cAMP, a wide variety of stimuli, including hypoxia, UV irradiation, and growth factor addition, induce Ser133 phosphorylation with stoichiometry and kinetics comparable to those induced by cAMP. Yet a number of these signals are incapable of promoting target gene activation via CREB phosphorylation per se, suggesting the presence of additional regulatory events either at the level of CREB-CBP complex formation or in the subsequent recruitment of the transcriptional apparatus. Here we characterize a Tyr134Phe CREB mutant that behaves as a constitutive activator in vivo. Like protein kinase A (PKA)-stimulated wild-type CREB, the Tyr134Phe polypeptide was found to stimulate target gene expression via the Ser133-dependent recruitment of CBP and p300. Biochemical studies reveal that mutation of Tyr134 to Phe lowers the K(m) for PKA phosphorylation and thereby induces high levels of constitutive Ser133 phosphorylation in vivo. Consistent with its constitutive activity, Tyr134Phe CREB strongly promoted differentiation of PC12 cells in concert with suboptimal doses of nerve growth factor. Taken together, these results demonstrate that Ser133 phosphorylation is sufficient for cellular gene activation and that additional signal-dependent modifications of CBP or p300 are not required for recruitment of the transcriptional apparatus to the promoter.

Stimulus-specific interaction between activator-coactivator cognates revealed with a novel complex-specific antiserum

A number of second messenger pathways propagate inductive signals via protein-protein interactions that are phosphorylation-dependent. The second messenger, cAMP, for example, promotes cellular gene expression via the protein kinase A-mediated phosphorylation of cAMP-response element-binding protein (CREB) at Ser(133), and this modification in turn stimulates the association of CREB with the co-activator, CREB-binding protein (CBP). The solution structure of the CREB.CBP complex, using relevant interaction domains, kinase inducible domain and kinase-induced domain interacting domain, referred to as KID and KIX, respectively, shows that KID undergoes a coil to helix transition, upon binding to KIX, that stabilizes complex formation. Whether such changes occur in the context of the full-length CREB and CBP proteins, however, is unclear. Here we characterize a novel antiserum that specifically binds to the CREB. CBP complex but to neither protein individually. Epitope mapping experiments demonstrate that the CREB.CBP antiserum detects residues in KID that undergo a conformational change upon binding to KIX. The ability of this antiserum to recognize full-length CREB.CBP complexes in a phospho-(Ser(133))-dependent manner demonstrates that the structural transition observed with the isolated KID domain also occurs in the context of the full-length CREB protein. To our knowledge, this is the first report documenting formation of endogenous cellular protein-protein complexes in situ.

The phosphorylation status of a cyclic AMP-responsive activator is modulated via a chromatin-dependent mechanism

Cyclic AMP (cAMP) stimulates the expression of numerous genes via the protein kinase A (PKA)-mediated phosphorylation of CREB at Ser133. Ser133 phosphorylation, in turn, promotes recruitment of the coactivator CREB binding protein and its paralog p300, histone acetyltransferases (HATs) that have been proposed to mediate target gene activation, in part, by destabilizing promoter bound nucleosomes and thereby allowing assembly of the transcriptional apparatus. Here we show that although histone deacetylase (HDAC) inhibitors potentiate target gene activation via cAMP, they do not stimulate transcription over the early burst phase, during which CREB phosphorylation and CBP/p300 recruitment are maximal. Rather, HDAC inhibitors augment CREB activity during the late attenuation phase by prolonging CREB phosphorylation on chromosomal but, remarkably, not on extrachromosomal templates. In reconstitution studies, assembly of periodic nucleosomal arrays on a cAMP-responsive promoter template potently inhibited CREB phosphorylation by PKA, and acetylation of these template-bound nucleosomes by p300 partially rescued CREB phosphorylation by PKA. Our results suggest a novel regulatory mechanism by which cellular HATs and HDACs modulate the phosphorylation status of nuclear activators in response to cellular signals.

Pbx-Hox heterodimers recruit coactivator-corepressor complexes in an isoform-specific manner

Homeobox (hox) proteins have been shown to regulate cell fate and segment identity by promoting the expression of specific genetic programs. In contrast to their restricted biological action in vivo, however, most homeodomain factors exhibit promiscuous DNA binding properties in vitro, suggesting a requirement for additional cofactors that enhance target site selectivity. In this regard, the pbx family of homeobox genes has been found to heterodimerize with and thereby augment the DNA binding activity of certain hox proteins on a subset of potential target sites. Here we examine the transcriptional properties of a forced hox-pbx heterodimer containing the pancreas-specific orphan homeobox factor pdx fused to pbx-1a. Compared to the pdx monomer, the forced pdx-pbx1a dimer, displayed 10- to 20-fold-higher affinity for a consensus hox-pbx binding site but was completely unable to bind a hox monomer recognition site. The pdx-pbx dimer stimulated target gene expression via an N-terminal trans-activation domain in pdx that interacts with the coactivator CREB binding protein. The pdx-pbx dimer was also found to repress transcription via a C-terminal domain in pbx-1a that associates with the corepressors SMRT and NCoR. The transcriptional properties of the pdx-pbx1 complex appear to be regulated at the level of alternative splicing; a pdx-pbx polypeptide containing the pbx1b isoform, which lacks the C-terminal extension in pbx1a, was unable to repress target gene expression via NCoR-SMRT. Since pbx1a and pbx1b are differentially expressed in endocrine versus exocrine compartments of the adult pancreas, our results illustrate a novel mechanism by which pbx proteins may modulate the expression of specific genetic programs, either positively or negatively, during development.

Mutations in NEUROD1 are associated with the development of type 2 diabetes mellitus

The helix-loop-helix (HLH) protein NEUROD1 (also known as BETA2) functions as a regulatory switch for endocrine pancreatic development. In mice homozygous for a targeted disruption of Neurod, pancreatic islet morphogenesis is abnormal and overt diabetes develops due in part to inadequate expression of the insulin gene (Ins2). NEUROD1, following its heterodimerization with the ubiquitous HLH protein E47, regulates insulin gene (INS) expression by binding to a critical E-box motif on the INS promoter. Here we describe two mutations in NEUROD1, which are associated with the development of type 2 diabetes in the heterozygous state. The first, a missense mutation at Arg 111 in the DNA-binding domain, abolishes E-box binding activity of NEUROD1. The second mutation gives rise to a truncated polypeptide lacking the carboxy-terminal trans-activation domain, a region that associates with the co-activators CBP and p300 (refs 3,4). The clinical profile of patients with the truncated NEUROD1 polypeptide is more severe than that of patients with the Arg 111 mutation. Our findings suggest that deficient binding of NEUROD1 or binding of a transcriptionally inactive NEUROD1 polypeptide to target promoters in pancreatic islets leads to the development of type 2 diabetes in humans.

Role of secondary structure in discrimination between constitutive and inducible activators

We have examined structural differences between the proto-oncogene c-Myb and the cyclic AMP-responsive factor CREB that underlie their constitutive or signal-dependent activation properties. Both proteins stimulate gene expression via activating regions that articulate with a shallow hydrophobic groove in the KIX domain of the coactivator CREB-binding protein (CBP). Three hydrophobic residues in c-Myb that are conserved in CREB function importantly in cellular gene activation and in complex formation with KIX. These hydrophobic residues are assembled on one face of an amphipathic helix in both proteins, and mutations that disrupt c-Myb or CREB helicity in this region block interaction of either factor with KIX. Binding of the helical c-Myb domain to KIX is accompanied by a substantial increase in entropy that compensates for the comparatively low enthalpy of complex formation. By contrast, binding of CREB to KIX entails a large entropy cost due to a random coil-to-helix transition in CREB that accompanies complex formation. These results indicate that the constitutive and inducible activation properties of c-Myb and CREB reflect secondary structural characteristics of their corresponding activating regions that influence the thermodynamics of formation of a complex with CBP.

CREB is a regulatory target for the protein kinase Akt/PKB

The nuclear factor CREB stimulates the expression of cellular genes following its protein kinase A-mediated phosphorylation at Ser-133. Ser-133 phosphorylation, in turn, activates target gene expression by promoting recruitment of the co-activator CBP. Recent studies showing that CREB and its paralog CREM are required for survival of certain cell types prompted us to examine whether CREB is a nuclear target for activation via the growth factor-dependent Ser/Thr kinase Akt/PKB. When overexpressed in serum-stimulated cells, Akt/PKB potently induced Ser-133 phosphorylation of CREB and promoted recruitment of CBP. Correspondingly, Akt/PKB stimulated target gene expression via CREB in a phospho(Ser-133)-dependent manner. Akt/PKB induced CREB activity only in response to serum stimulation, and this effect was suppressed by the phosphatidylinositol 3-kinase inhibitor LY 294002. Our results support the notion that Akt/PKB promotes cell survival, at least in part, by stimulating the expression of cellular genes via the CREB/CBP nuclear transduction pathway.

Analysis of an activator:coactivator complex reveals an essential role for secondary structure in transcriptional activation

Ser-133 phosphorylation of CREB within the kinase-inducible domain (KID) promotes target gene activation via complex formation with the KIX domain of the coactivator CBP. Concurrent phosphorylation of CREB at Ser-142 inhibits transcriptional induction via an unknown mechanism. Unstructured in the free state, KID folds into a helical structure upon binding to KIX. Using site-directed mutagenesis based on the NMR structure of the KID:KIX complex, we have examined the mechanisms by which Ser-133 and Ser-142 phosphorylation regulate CREB activity. Our results indicate that phospho-Ser-133 stablizes whereas phospho-Ser-142 disrupts secondary structure-mediated interactions between CREB and CBP. Thus, differential phosphorylation of CREB may form the basis by which upstream signals regulate the specificity of target gene activation.

CREB-binding protein cooperates with transcription factor GATA-1 and is required for erythroid differentiation

The transcription factor GATA-1 coordinates multiple events during terminal erythroid cell maturation. GATA-1 participates in the transcription of virtually all erythroid-specific genes, blocks apoptosis of precursor cells, and controls the balance between proliferation and cell cycle arrest. Prior studies suggest that the function of GATA-1 is mediated in part through association with transcriptional cofactors. CREB-binding protein (CBP) and its close relative p300 serve as coactivators for a variety of transcription factors involved in growth control and differentiation. We report here that CBP markedly stimulates GATA-1's transcriptional activity in transient transfection experiments in nonhematopoietic cells. GATA-1 and CBP also coimmunoprecipitate from nuclear extracts of erythroid cells. Interaction mapping pinpoints contact sites to the zinc finger region of GATA-1 and to the E1A-binding region of CBP. Expression of a conditional form of adenovirus E1A in murine erythroleukemia cells blocks differentiation and expression of endogenous GATA-1 target genes, whereas mutant forms of E1A unable to bind CBP/p300 have no effect. Our findings add GATA-1, and very likely other members of the GATA family, to the growing list of molecules implicated in the complex regulatory network surrounding CBP/p300.

RNA helicase A mediates association of CBP with RNA polymerase II

The coactivator CBP has been proposed to stimulate the expression of certain signal-dependent genes via its association with RNA polymerase II complexes. Here we show that complex formation between CBP and RNA polymerase II requires RNA helicase A (RHA), a nuclear DNA/RNA helicase that is related to the Drosophila male dosage compensation factor mle. In transient transfection assays, RHA was found to cooperate with CBP in mediating target gene activation via the CAMP responsive factor CREB. As a mutation in RHA that compromised its helicase activity correspondingly reduced CREB-dependent transcription, we propose that RHA may induce local changes in chromatin structure that promote engagement of the transcriptional apparatus on signal responsive promoters.

Hormonal regulation of an islet-specific enhancer in the pancreatic homeobox gene STF-1

The homeobox protein STF-1 appears to function as a master control switch for expression of the pancreatic program during development. Here we characterize a composite enhancer which directs STF-1 expression to pancreatic islet cells via two functional elements that recognize the nuclear factors HNF-3beta and BETA-2. In keeping with their inhibitory effects on islet cell maturation, glucocorticoids were found to repress STF-1 gene expression by interfering with HNF-3beta activity on the islet-specific enhancer. Overexpression of HNF-3beta suppressed glucocorticoid receptor-mediated inhibition of the STF-1 gene, and our results suggest that the expansion of pancreatic islet precursor cells during development may be restricted by hormonal cues which regulate STF-1 gene expression.

Analysis of a cAMP-responsive activator reveals a two-component mechanism for transcriptional induction via signal-dependent factors

We have examined the mechanism by which the cAMP-responsive factor CREB stimulates target gene expression following its phosphorylation at Ser-133. Using an in vitro transcription assay, we found that two signals were required for target gene activation: a phospho(Ser-133)-dependent interaction of CREB with RNA polymerase II via the coactivator CBP and a glutamine-rich domain interaction with TFIID via hTAF(II)130. The adenovirus E1A oncoprotein was found to inhibit phospho(Ser-133) CREB activity by binding to CBP and specifically blocking recruitment of RNA Pol II to the promoter. Our results suggest that the recruitment of CBP-RNA Pol II complexes per se is not sufficient for transcriptional activation and that activator-mediated recruitment of TFIID is additionally required for induction of signal-dependent genes.

Transcriptional regulation by cyclic AMP

A number of hormones and growth factors have been shown to stimulate target cells via second messenger pathways that in turn regulate the phosphorylation of specific nuclear factors. The second messenger cyclic AMP, for example, regulates a striking number of physiologic processes, including intermediary metabolism, cellular proliferation, and neuronal signaling, by altering basic patterns of gene expression. Our understanding of cyclic AMP signaling in the nucleus has expanded considerably over the past decade, owing in large part to the characterization of cyclic AMP-responsive promoter elements, transcription factors that bind them, and signal-dependent coactivators that mediate target gene induction. More importantly, these studies have revealed new insights into biological problems as diverse as biological clocks and long-term memory. The purpose of this review is to describe the components of the cyclic AMP response unit and to analyze how these components cooperate to induce target gene expression in response to hormonal stimulation.

Role of CBP/P300 in nuclear receptor signalling

The nuclear receptor superfamily includes receptors for steroids, retinoids, thyroid hormone and vitamin D, as well as many related proteins. An important feature of the action of the lipophilic hormones and vitamins is that the maintenance of homeostatic function requires both intrinsic positive and negative regulation. Here we provide in vitro and in vivo evidence that identifies the CREB-binding protein (CBP) and its homologue P300 (refs 6,7) as cofactors mediating nuclear-receptor-activated gene transcription. The role of CBP/P300 in the transcriptional response to cyclic AMP, phorbol esters, serum, the lipophilic hormones and as the target of the E1A oncoprotein suggests they may serve as integrators of extracellular and intracellular signalling pathways.

Regulation of somatostatin gene transcription by cyclic adenosine monophosphate

Cyclic adenosine monophosphate (cAMP) stimulates transcription of somatostatin and other target genes with burst-attenuation kinetics. The kinetics of protein kinase (PK-A)-dependent cAMP response element binding protein (CREB) phosphorylation closely parallel the changes in transcription of cAMP-responsive genes by run-on assay. Nuclear translocation of PK-A, visualized by microinjection of fluorescently labeled PK-A holoenzyme, appears to represent the rate-limiting step in CREB phosphorylation and transcriptional activation. We and others have recently characterized a CREB-binding protein (CBP), which specifically recognizes sequences within the Ser133 phosphorylated form of CREB. CBP does not regulate the DNA binding, dimerization, or nuclear targeting properties of CREB, but binds selectively to the kinase-inducible 60 amino acid trans-activation domain (KID) of CREB, critical for PK-A-inducible transcription. We developed an antiserum directed against amino acid 634-648 within the CREB-binding domain of CBP. We detected a 265-kd polypeptide by Western blot as predicted from the cDNA, which coincided with the predominant phospho-CREB-binding activity in Hela nuclear extracts by "Far Western" blot assay. An identical phospho-CREB-binding activity was also found in NIH-3T3 cells. This phospho-CREB-binding protein appeared to be specific for Ser133-phosphorylated CREB, because no such band was detected with CREB labeled to the same specific activity at a nonregulatory phosphoacceptor site (Ser156) by casein kinase II (CKII). Following microinjection into nuclei of NIH-3T3 cells, a cAMP response element (CRE)-lacZ reporter was markedly induced by treatment with 8-Br cAMP plus isobutyl methyl xanthine (IBMX). Coinjection of CBP antiserum with the CRE-lacZ plasmid inhibited cAMP-dependent activity in a dose-dependent manner, but control immunoglobulin G (lgG) had no effect on this response. We can now begin reconstituting PK-A-dependent transcription in vitro, using well-characterized proteins such as CREB, TAF 110, and CBP. The assembly of such factors on cAMP-regulated promoters like somatostain may enable responsiveness to a variety of hormonal stimuli that employ cAMP as their second messenger.

The pancreatic islet factor STF-1 binds cooperatively with Pbx to a regulatory element in the somatostatin promoter: importance of the FPWMK motif and of the homeodomain

A number of homeodomain proteins have been shown to regulate cellular development by stimulating the transcription of specific target genes. In contrast to their distinct activities in vivo, however, most homeodomain proteins bind indiscriminately to potential target sites in vitro, suggesting the involvement of cofactors which specify target site selection. One such cofactor, termed extradenticle, has been shown to influence segmental morphogenesis in Drosophila melanogaster by binding cooperatively with certain homeodomain proteins to target regulatory elements. Here we demonstrate that STF-1, an orphan homeodomain protein required for pancreatic development in mammals, binds cooperatively to DNA with Pbx, the mammalian homolog of extradenticle. Cooperative binding with Pbx requires a pentapeptide motif (FPWMK) which is well conserved among a large subset of homeodomain proteins. The FPMWK motif is not sufficient to confer Pbx cooperativity on other homeodomain proteins, however; the N-terminal arm of the STF-1 homeodomain is also essential. As cooperative binding with Pbx occurs on only a subset of potential STF-1 target sites, our results suggest that Pbx may specify target gene selection in the developing pancreas by forming heterodimeric complexes with STF-1.

Multiple protein kinase A-regulated events are required for transcriptional induction by cAMP

The second messenger cAMP stimulates the expression of numerous genes via the protein kinase A-mediated phosphorylation of the cAMP response element-binding protein (CREB) at Ser-133. Ser-133 phosphorylation, in turn, appears to induce target gene expression by promoting interaction between CREB and CBP, a 265-kDa nuclear phospho-CREB-binding protein. It is unclear, however, whether Ser-133 phosphorylation per se is sufficient for CREB-CBP complex formation and for target gene induction in vivo. Here we examine CREB activity in Jurkat T cells after stimulation of the T-cell receptor (TCR), an event that leads to calcium entry and diacylglycerol production. Triggering of the TCR stimulated Ser-133 phosphorylation of CREB with high stoichiometry, but TCR activation did not promote CREB-CBP complex formation or target gene induction unless suboptimal doses of cAMP agonist were provided as a costimulus. Our results demonstrate that, in addition to mediating Ser-133 phosphorylation of CREB, protein kinase A regulates additional proteins that are required for recruitment of the transcriptional apparatus to cAMP-responsive genes.

The cyclic adenosine 3',5'-monophosphate-responsive factor CREB is constitutively activated in human somatotroph adenomas

Oncogenic gsp proteins appear to stimulate the transformation of pituitary somatotrophs by inducing the constitutive activation of adenyl cyclase. Previous work implicating the cAMP-responsive transcription factor CREB as a biochemical intermediate in the proliferative response to cAMP led us to examine whether CREB activity was correspondingly elevated in human somatotroph adenomas. In a series of 15 human GH-secreting tumors, we found that each of these contained elevated levels of Ser133-phosphorylated and, hence, activated CREB compared with nonfunctioning pituitary tumors. Four of the GH-secreting adenomas contained an oncogenic gsp gene by polymerase chain reaction analysis, and two additional adenomas expressed wild-type G alpha s protein at 5- to 10-fold higher levels than nonfunctioning pituitary tumors. As both oncogenic gsp and overexpressed G alpha s proteins can induce Ser133 phosphorylation and cAMP-responsive gene expression in transfected GC somatotroph cells, our studies indicate that these proteins may promote somatotroph transformation in part by inducing the transcription of specific CREB-dependent target genes.

A refractory phase in cyclic AMP-responsive transcription requires down regulation of protein kinase A

Cyclic AMP (cAMP) stimulates the expression of numerous genes through the protein kinase A (PK-A)-mediated phosphorylation of the nuclear factor CREB at Ser-133 (G. A. Gonzalez and M. R. Montminy, Cell 59:675-680, 1989). Like other signal transduction pathways, cAMP induces gene expression with burst-attenuation kinetics; cAMP-dependent transcription and CREB phosphorylation peak within 30 min and decline steadily over the next 4 to 6 h via the protein phosphatase 1-mediated dephosphorylation of CREB (M. Hagiwara, A. Alberts, P. Brindle, J. Meinkoth, J. Feramisco, T. Deng, M. Karin, S. Shenolikar, and M. Montminy, Cell 70:105-113, 1992). Here we characterize a third phase in cAMP-responsive transcription--a refractory period during which hormone-treated cells become transcriptionally unresponsive to subsequent stimulation by cAMP. This refractory period begins 6 to 8 h after stimulation and lasts 3 to 5 days after the removal of hormone. In contrast to the earlier attenuation phase, transcription of cAMP-responsive genes during the refractory period is not restored by inhibitors of protein phosphatase 1 activity. Rather, the establishment and maintenance of this phase rely on a marked reduction in PK-A catalytic subunit expression at the translational level. As overexpression of C-subunit protein can reactive transcription of cAMP-responsive genes during the refractory period, our results suggest that hormone-responsive cells may stimulate, attenuate, and then silence signal-dependent genes through distinct regulatory mechanisms.

Activation of cAMP and mitogen responsive genes relies on a common nuclear factor

A number of signalling pathways stimulate transcription of target genes through nuclear factors whose activities are primarily regulated by phosphorylation. Cyclic AMP regulates the expression of numerous genes, for example, through the protein kinase-A (PKA)-mediated phosphorylation of transcription factor CREB at Ser 133. Although phosphorylation may stimulate transcriptional activators by modulating their nuclear transport or DNA-binding affinity, CREB belongs to a class of proteins whose phosphorylation appears specifically to enhance their trans-activation potential. Recent work describing a phospho-CREB binding protein (CBP) which interacts specifically with the CREB trans-activation domain prompted us to examine whether CBP is necessary for cAMP regulated transcription. We report here that microinjection of an anti-CBP antiserum into fibroblasts can inhibit transcription from a cAMP responsive promoter. Surprisingly, CBP also cooperates with upstream activators such as c-Jun, which are involved in mitogen responsive transcription. We propose that CBP is recruited to the promoter through interaction with certain phosphorylated factors, and that CBP may thus play a critical role in the transmission of inductive signals from cell surface receptor to the transcriptional apparatus.

Expression of a peptide inhibitor of protein phosphatase 1 increases phosphorylation and activity of CREB in NIH 3T3 fibroblasts

We have examined the activity and phosphorylation state of the cyclic AMP (cAMP) response element binding factor (CREB) in intact NIH 3T3 cells following microinjection of expression plasmids encoding regulatory proteins of type 1 (PP1) and 2A (PP2A) serine/threonine-specific protein phosphatases. Changes in CREB phosphorylation in the injected cells were monitored by indirect immunofluorescence using an affinity-purified antiserum (Ab5322) which specifically recognizes CREB phosphorylated at Ser-133, and changes in transcriptional activity of CREB were monitored by expression of a reporter gene regulated by cAMP. cAMP-stimulated phosphorylation in NIH 3T3 cells is normally transient, and as expected, after stimulation of cells with cell-permeable cAMP analogs, the level of phosphorylated CREB was found to initially increase and then return to a basal level within 4 h. Microinjection of an expression vector encoding a constitutively active form of inhibitor 1 (I-1), a PP1-specific inhibitor, by itself resulted in an apparent increase in phosphorylated CREB in unstimulated cells. Moreover, injection of the I-1 vector resulted in the prolonged appearance of phosphorylated CREB in cells after cAMP stimulation. In contrast, injection of a plasmid encoding simian virus 40 small t antigen, which interacts with PP2A to inhibit its activity towards several phosphoprotein substrates, had no effect on the phosphorylation state of CREB in stimulated or unstimulated NIH 3T3 cells. Consistent with these results, injection of the I-1 expression vector activated expression from a coinjected CRE-lacZ reporter plasmid, indicating that the increased phosphorylation of CREB also activated its transcriptional activity. These results provide further evidence for a role of a PP1 as the primary protein (Ser/Thr) phosphatase regulating the dephosphorylation of Ser-133 and thereby limiting the transcriptional activity of CREB.

The cAMP-regulated transcription factor CREB interacts with a component of the TFIID complex

cAMP regulates the expression of a number of genes through the protein kinase A-mediated phosphorylation of CREB at Ser-133. The effects of Ser-133 phosphorylation appear to be primarily transmitted through a modulatory kinase-inducible domain in CREB that functions cooperatively with a 120-amino acid glutamine-rich region (Q2) to stimulate transcription. Indeed, the kinase-inducible domain activity alone is not sufficient to sustain a transcriptional response as illustrated by the CREM family of repressors, which contain the kinase-inducible domain but lack the Q2 region. Here we demonstrate that Q2 functions as a potent constitutive activator in vitro. The transcription factor TFIID fraction supports transcriptional activation by Q2, although the "TATA" binding protein alone does not, suggesting that other components of the TFIID complex mediate the response to CREB Q2. In fact, Q2 associates with the TATA binding protein-associated factor dTAFII110. As the transcriptionally inactive CREM alpha and beta proteins lack sequences in Q2 that are necessary for binding dTAFII110, our results suggest that these proteins may repress transcription because they are unable to interact with the basal transcription complex.

Signal transduction through the cAMP-dependent protein kinase

Temporal cellular events responsible for hormonal activation of responses mediated by the cAMP-dependent protein kinase (PKA) have been studied in living cells. By selectively perturbing molecular function of Gs, the catalytic subunit of PKA (C), or the nuclear factor CREB, in cells through microinjection of inhibitory agents specific for these molecules or activated forms of these molecules, we have obtained evidence for a requirement for the function of each of these molecules in the hormonal stimulation of cAMP-regulated genes. Moreover, by introducing fluorescently labeled PKA subunits into these cells as molecular tracers, or by immunofluorescence of C subunit, we have observed biological translocation of C subunit from the cytoplasm to the nucleus during transcriptional activation and a quenching of this by the inhibitor molecule, PKI. The implications of these cellular and molecular events in the signal transduction of hormonal responses are discussed.

Protein-kinase-A-dependent activator in transcription factor CREB reveals new role for CREM repressors

Hormonally induced increases in cyclic AMP levels induce phosphorylation of the transcription factor CREB at a serine residue at position 133 by protein kinase A (ref. 1), enhancing its ability to activate transcription without affecting its intracellular location or DNA-binding activity. This effect is dependent on a 60-amino-acid region of CREB that contains Ser133 and is termed the kinase-inducible domain (KID)2, which also occurs in the CREB-related CREM-alpha and -beta proteins, although these are transcriptional repressors. Here we show that the KID domain confers a cAMP-inducible increase on the activity of the Q2 activation domain from CREB and the acidic activation domains from the yeast proteins GAL4 and GCN4. Remarkably, it retains this ability even when attached to a separate polypeptide bound to an adjacent site in the promoter. KID may therefore be the first of a new class of conditional activators that work through other promoter-bound factors to stimulate gene expression in response to hormonal stimuli.

The LIM family transcription factor Isl-1 requires cAMP response element binding protein to promote somatostatin expression in pancreatic islet cells

Many eukaryotic genes are regulated by cAMP through a conserved cAMP response element (CRE). Here we show that, in the pancreatic islet cell line Tu6, a well-characterized CRE in the somatostatin gene does not provide cAMP responsiveness but functions as an essential element for its basal activity. DNA-binding and functional analyses indicate that the cAMP-responsive factor CREB regulates somatostatin expression in these cells without requirement for phosphorylation at the protein kinase A-regulated Ser-133 phosphorylation site. In addition to the CRE site, cell-specific expression of the somatostatin gene requires a second promoter element, which binds the recently characterized LIM family protein Isl-1. Thus, Isl-1 and CREB appear to synergize on the somatostatin promoter to stimulate high-level expression in Tu6 cells. The ability of CREB to function in a phosphorylation-independent manner suggests a mechanism by which this protein can regulate gene transcription.

Transcriptional attenuation following cAMP induction requires PP-1-mediated dephosphorylation of CREB

We have examined the mechanism by which the transcriptional activity of the cAMP-responsive factor CREB is attenuated following induction with forskolin. Metabolic labeling studies reveal that, after an initial burst of phosphorylation in response to cAMP, CREB is dephosphorylated and transcription of the cAMP-responsive somatostatin gene is correspondingly reduced. The phosphatase inhibitor 1 protein and okadaic acid both prevented the dephosphorylation of CREB at Ser-133 in PC12 cells and also augmented the transcriptional response to cAMP. Of the four Ser/Thr phosphatases described to date, only PP-1 appears to be similarly inhibited by these agents. As PP-1 specifically dephosphorylates CREB at Ser-133 and inhibits cAMP-dependent transcription, we propose that this phosphatase is the major regulator of CREB activity in cAMP-responsive cells.

Negative and positive regulation by transcription factor cAMP response element-binding protein is modulated by phosphorylation

We have shown that the transcriptional activity of the protooncogene jun (c-jun) promoter is repressed by a transcription factor, the cAMP response element-binding protein (CREB). This repression can be alleviated when CREB is phosphorylated by the catalytic subunit of protein kinase A. Repression cannot be alleviated by a mutant CREB deficient in the protein kinase A phosphorylation site (M1 CREB Ser-133----Ala), suggesting that phosphorylation of CREB at this site is essential for the relief of repression. Repression by CREB requires its binding to the c-jun promoter. In NIH 3T3 cells stably expressing CREB, c-jun is no longer induced by serum, but this repression can be relieved by treatment of the cells with forskolin, an agonist of the adenylate cyclase pathway. Thus, CREB has a dual function, that of a repressor in the absence of phosphorylation and an activator when phosphorylated by protein kinase A.

Both the basic region and the ‘leucine zipper’ domain of the cyclic AMP response element binding (CREB) protein are essential for transcriptional activation

Second messengers like cAMP can activate the transcription of genes containing consesus cAMP response element (CRE). A 43 kd nuclear phosphoprotein previously identified as the cAMP response element binding (CREB) protein has been shown to bind as a dimer to CRE and activate gene transcription. The rat and human CREB protein contain the 'leucine zipper' motif. We have analyzed the role of both leucine zipper domain and the amino-terminal basic region by making site-specific mutations. Our results show that the first three leucines int he leucine zipper domain are essential for efficient dimer formation. Mutations of two consecutive leucines in the leucine zipper domain completely abolish the ability to form dimers. Mutant CREB protein unable to form homodimers is also unable to bind to DNA. In contrast, however, mutations, in the DNA binding region had no effect on dimer formation but were unable to bind to CRE sites or activate transcription. We propose that CREB protein functions by forming homodimers which bind to CRE and activate transcription. Furthermore, the CREB protein needs to be phosphorylated before activating transcription. Finally, we show that the CREB basic region mutant acts as a trans-dominant transcriptional suppressor of wild-type CREB function.

Shock after phenothiazine administration in a pregnant patient with a pheochromocytoma. A case report and literature review

A pregnant patient with undiagnosed bilateral pheochromocytoma developed shock and pulmonary edema subsequent to the administration of a phenothiazine. The patient had developed a paradoxic hypertensive response to hydralazine. A high index of suspicion is required for diagnosing a pheochromocytoma that mimics toxemia of pregnancy. There is an urgent need for prompt plasma expansion for the pregnant patient in shock, even when pulmonary edema is present.