cAMP-dependent regulation of gene transcription by cAMP response element-binding protein and cAMP response element modulator

Cyclic AMP signaling and gene regulation

Cyclic adenosine monophosphate (cAMP) is a ubiquitous second messenger produced in cells in response to hormones and nutrients. The production of cAMP is dependent upon the actions of many different proteins that affect its synthesis and degradation. An important function of cAMP is to activate the phosphorylating enzyme, protein kinase A. The key roles of cAMP and protein kinase A in the phosphorylation and regulation of enzyme substrates involved in intermediary metabolism are well known. A newly discovered role for protein kinase A is in the phosphorylation and activation of transcription factors that are critical for the control of the transcription of genes in response to elevated levels of cAMP.

Inducible cAMP early repressor ICER down-regulation of CREB gene expression in Sertoli cells

The cAMP response element binding protein (CREB) and the cAMP-responsive element modulator (CREM) are cyclically expressed in the seminiferous tubules during spermatogenesis. In the somatic Sertoli cells, which are the major supporters of germ cell development in the seminiferous tubules, the expression of CREB is cyclical and appears to be regulated by the levels of cAMP produced in response to the pituitary derived follicle-stimulating hormone FSH. Cyclic AMP response elements (CREs) located in the promoter of the CREB gene were shown earlier to be implicated in an autopositive feedback loop that up-regulates the expression of CREB. Here we show that in Sertoli cells FSH-mediated induction of the CREM repressor isoform, ICER (inducible cAMP early repressor) is correlated with the inhibition and delay of CREB gene expression in the seminiferous tubules. ICER binds to the two CREs located in the promoter of the CREB gene and in transient transfection assays of Sertoli cells, ICER expression vectors down-regulate transcription of a reporter gene driven by the CREB gene promoter. In addition, analyses of ICER and CREB gene expression in isolated segments of rat seminiferous tubules reveals stage-specific and cycle-dependent expression of ICER. The periods of enhanced expression of ICER correspond to the stages of spermatogenesis with the lowest levels of CREB expression. We suggest that the expression of ICER in Sertoli cells may contribute to the periodic repression of CREB gene expression during the repeated 12-day cycles of spermatogenesis, and may be required to reset the levels of activator CREB prior to the initiation of each new cycle of spermatogenesis.

Autoregulation of eukaryotic transcription factors

The structures of several promoters regulating the expression of eukaryotic transcription factors have in recent years been examined. In many cases there is good evidence for autoregulation, in which a given factor binds to its own promoter and either activates or represses transcription. Autoregulation occurs in all eukaryotes and is an important component in controlling expression of basal, cell cycle specific, inducible response and cell type-specific factors. The basal factors are autoregulatory, being strictly necessary for their own expression, and as such must be epigenetically inherited. Autoregulation of stimulus response factors typically serves to amplify cellular signals transiently and also to attenuate the response whether or not a given inducer remains. Cell cycle-specific transcription factors are positively and negatively autoregulatory, but this frequently depends on interlocking circuits among family members. Autoregulation of cell type-specific factors results in a form of cellular memory that can contribute, or define, a determined state. Autoregulation of transcription factors provides a simple circuitry, useful in many cellular circumstances, that does not require the involvement of additional factors, which, in turn, would need to be subject to another hierarchy of regulation. Autoregulation additionally can provide a direct means to sense and control the cellular conce]ntration of a given factor. However, autoregulatory loops are often dependent on cellular pathways that create the circumstances under which autoregulation occurs.

The cAMP-response element mediates induction of secretogranin II by CHX and FSK in GH4C1 cells

The effect of cAMP on secretogranin II (SgII) gene transcription in GH4C1 (GH) cells is not observed unless protein synthesis is inhibited. We have defined elements in the SgII promoter that mediate regulation by cycloheximide (CHX) and forskolin (FSK) and characterized the nuclear proteins that interact with them. GH cells were transfected with p2774Luc, p351Luc, p242Luc, and p223Luc containing 2,612, 189, 80, and 61 bp of the SgII promoter upstream of the luciferase gene, respectively. Treatment with CHX and FSK increased promoter activity 8- to 12-fold in cells transfected with p2774Luc, p351Luc, and p242Luc but had not effect in cells transfected with p223Luc. The same 19-bp element (-80 to -62) mediates regulation by CHX alone, as CHX caused a 3.8-fold increase in activity in GH cells transfected with p242Luc but not p223Luc. Gel mobility shifts using sequences -84 to -53 resulted in three complexes, which contained cAMP response element-binding protein heterodimerized with cAMP response element modulator or activating transcription factor-1. No differences were observed in complex formation when cells were treated with either CHX, FSK, or CHX and FSK. Thus CHX affects the response to FSK in GH cells by inhibiting the synthesis of a protein, which does not itself interact with DNA or affect the binding of CRE-binding proteins with the SgII promoter, but likely interferes with the interaction of CRE-binding proteins with the general transcriptional machinery.

CCAAT/enhancer-binding protein delta activates insulin-like growth factor-I gene transcription in osteoblasts. Identification of a novel cyclic AMP signaling pathway in bone

Insulin-like growth factor-I (IGF-I) plays a key role in skeletal growth by stimulating bone cell replication and differentiation. We previously showed that prostaglandin E2 (PGE2) and other cAMP-activating agents enhanced IGF-I gene transcription in cultured primary rat osteoblasts through promoter 1, the major IGF-I promoter, and identified a short segment of the promoter, termed HS3D, that was essential for hormonal regulation of IGF-I gene expression. We now demonstrate that CCAAT/enhancer-binding protein (C/EBP) delta is a major component of a PGE2-stimulated DNA-protein complex involving HS3D and find that C/EBPdelta transactivates IGF-I promoter 1 through this site. Competition gel shift studies first indicated that a core C/EBP half-site (GCAAT) was required for binding of a labeled HS3D oligomer to osteoblast nuclear proteins. Southwestern blotting and UV-cross-linking studies showed that the HS3D probe recognized a approximately 35-kDa nuclear protein, and antibody supershift assays indicated that C/EBPdelta comprised most of the PGE2-activated gel-shifted complex. C/EBPdelta was detected by Western immunoblotting in osteoblast nuclear extracts after treatment of cells with PGE2. An HS3D oligonucleotide competed effectively with a high affinity C/EBP site from the rat albumin gene for binding to osteoblast nuclear proteins. Co-transfection of osteoblast cell cultures with a C/EBPdelta expression plasmid enhanced basal and PGE2-activated IGF-I promoter 1-luciferase activity but did not stimulate a reporter gene lacking an HS3D site. By contrast, an expression plasmid for the related protein, C/EBPbeta, did not alter basal IGF-I gene activity but did increase the response to PGE2. In osteoblasts and in COS-7 cells, C/EBPdelta, but not C/EBPbeta, transactivated a reporter gene containing four tandem copies of HS3D fused to a minimal promoter; neither transcription factor stimulated a gene with four copies of an HS3D mutant that was unable to bind osteoblast nuclear proteins. These results identify C/EBPdelta as a hormonally activated inducer of IGF-I gene transcription in osteoblasts and show that the HS3D element within IGF-I promoter 1 is a high affinity binding site for this protein.

Insulin-like growth factor I regulates gonadotropin responsiveness in the murine ovary

The present study shows that insulin-like growth factor I (IGF-I) and FSH receptor (FSHR) mRNAs are selectively coexpressed in a subset of healthy-appearing follicles in murine ovaries, irrespective of cycle stage. Aromatase gene expression, a prime marker for FSH effect, is found only in IGF-I/FSHR-positive follicles, showing that these are healthy, gonadotropin-responsive follicles. Given the striking coexpression of FSHR and IGF-I, we hypothesized that FSH was responsible for follicular IGF-I expression. We found, however, that granulosa cell IGF-I mRNA levels are not reduced in hypophysectomized (+/-PMSG) or FSH knockout mice, indicating that FSH does not have a major role in regulation of granulosa cell IGF-I gene expression. To test the alternative hypothesis that IGF-I regulates FSHR gene expression, we studied ovaries from IGF-I knockout mice. FSHR mRNA was significantly reduced in ovaries from homozygous IGF-I knockout compared with wild type mice and was restored to control values by exogenous IGF-I treatment. The functional significance of the reduced FSHR gene expression in IGF-I knockout ovaries is suggested by reduced aromatase expression and by the failure of their follicles to develop normally beyond the early antral stage. In fact, IGF-I knockout and FSH knockout ovaries appear very similar in terms of arrested follicular development. In summary, we have shown that IGF-I and FSHR are selectively coexpressed in healthy, growing murine follicles and that FSH does not affect IGF-I expression but that IGF-I augments granulosa cell FSHR expression. These data suggest that ovarian IGF-I expression serves to enhance granulosa cell FSH responsiveness by augmenting FSHR expression.

Follicle stimulating hormone-regulated expression of serum/glucocorticoid-inducible kinase in rat ovarian granulosa cells: a functional role for the Sp1 family in promoter activity

Recently, a family of novel, serine/threonine protein kinases has been identified. One of these transcriptionally inducible, immediate-early genes encodes serum/glucocorticoid inducible-protein kinase, sgk. By in situ hybridization, we show that sgk expression in the rat ovary is selectively localized to granulosa cells. In culture, FSH or forskolin, activators of the protein kinase A (PKA) pathway, rapidly (2 h) and transiently increased sgk mRNA levels in undifferentiated granulosa cells. Sgk mRNA exhibited a biphasic expression pattern, with maximal levels observed at 48 h of FSH/forskolin as granulosa cells differentiate to the preovulatory phenotype. Deletion analyses using sgk promoter-reporter constructs (-4.0 kb to -35 bp) identified a region between -63 and -43 bp that mediated FSH and forskolin-responsive transcription in undifferentiated and differentiated granulosa cells. This G/C-rich region 1) conferred both basal and inducible transcription to the minimal -35 sgk promoter chloramphenicol acetyltransferase reporter construct, 2) specifically bound Sp1 and Sp3 present in granulosa cell extracts, and 3) bound recombinant Sp1. Mutation of 2 bp in this region not only prevented Sp1 and Sp3 binding, but also abolished the PKA-mediated transactivation observed when using the wild type construct. Sp1 and Sp3 DNA-binding activity and protein levels did not change significantly during sgk induction. Collectively, these data indicate that Sp1/Sp3 transactivation of the sgk promoter likely involves regulated, phosphorylation-dependent interaction with other factors. Thus the novel, biphasic induction of sgk that correlates with granulosa cell progression from proliferation to differentiation appears to involve sequential, coordinated actions of FSH, PKA, and transcription factors, including Sp1 and Sp3.

Dog CREM transcription factors: cloning, tissue distribution, and identification of new isoforms

CREM (cAMP Response Element Modulator) transcription factors are involved in the cAMP-dependent transcriptional regulation of CRE-containing genes. Multiple CREM transactivators and repressors are generated from a single gene by alternative splicings and use of an alternative intronic promoter. Here we report the cloning and sequencing of the full-length dog CREM cDNA, corresponding to the CREMtau alpha splice variant. Amino acid sequence identity with mouse and human orthologs reached 94.5% and 91.0% respectively. Using the RNAse Protection Assay (RPA) method with three distinct probes, we analyzed the expression of the various CREM transcripts in several dog tissues. We showed that CREM transcription factors have a restricted tissue distribution and that the ratio between activators and repressors varies considerably from one tissue to another. Moreover, we amplified, by RT-PCR, a cDNA that corresponds to two new CREM isoforms and confirmed, by RPA experiments, the presence of these mRNAs in dog thyroid and in other tissues. These transcripts result from splicing of the gamma domain and encode potential CREM transactivators (CREMtau alphagamma and CREMtau2 alphagamma).

Quantification of the cAMP response element binding protein in ventricular nuclear protein from failing and nonfailing human hearts

Alterations in the expression of myocardial regulatory proteins (e.g. beta-adrenoceptor, inhibitory G-proteins) in human heart failure are associated with excessive stimulation of the cAMP signalling pathway by endogenous catecholamines. The transcription factor cAMP response element binding protein (CREB) mediates cAMP-dependent transcriptional activation and is expressed in the human heart. Here, CREB protein was immunologically quantified in ventricular nuclear protein preparations from nonfailing donor hearts (n = 8) and from failing hearts transplanted due to dilative (n = 10) or ischemic cardiomyopathy (n = 6). CREB expression was unchanged in ventricular nuclei from failing hearts compared to the nonfailing controls suggesting that expressional alterations in human heart failure cannot be explained by altered expression of CREB.

Neuropeptide Y treatment and food deprivation increase cyclic AMP response element-binding in rat hypothalamus

Intrahypothalamic (IHT) administration of neuropeptide Y (NPY) induces a robust feeding response in rats. We have shown previously that NPY-induced feeding is mediated by a pertussis-toxin-sensitive G protein in rats. NPY receptors are coupled to cAMP and Ca2+. Because these second messengers are known to activate cAMP response element binding proteins, (CREB), cAMP response element modulators, or activating transcription factor 1, we investigated the involvement of these transcription factors in NPY-induced feeding in rats. Compared with control injections of cerebrospinal fluid (1 microl), IHT administration of NPY increased cAMP response element (CRE) binding to rat hypothalamic nuclear extracts in a time-dependent manner, as detected by an electrophoretic mobility shift assay. In contrast, IHT administration of the anorectic neuropeptide, pituitary adenylate cyclase activating polypeptide, strongly inhibited the CRE binding. Food deprivation for 48 hr also increased CRE binding, whereas 8 hr of refeeding normalized CRE activity. Preincubation of the hypothalamic nuclear extracts of NPY-treated and unfed rats with antibody specific to CREB blocked CRE binding, whereas preincubation with phosphoCREB antibody retarded the migration of CRE-protein complex, indicating that phosphoCREB is involved in this process. Consistently, immunohistochemical studies with food-deprived rats showed an intense phosphoCREB signal in the paraventricular nuclei and ventromedial hypothalamus in comparison to rats fed ad libitum. Hypothalamic calcium/calmodulin-dependent protein kinase II activity was also increased by IHT-NPY. These results suggest that calcium/calmodulin-dependent protein kinase II induced phosphorylation of CREB may be involved in regulating feeding behavior induced by NPY.

Functional interactions, phosphorylation, and levels of 3',5'-cyclic adenosine monophosphate-regulatory element binding protein and steroidogenic factor-1 mediate hormone-regulated and constitutive expression of aromatase in gonadal cells

The proximal promoter of the rat aromatase CYP19 gene contains two functional regions that, by 5'-deletion analyses, have been shown to confer hormone/ cAMP inducibility to chimeric genes in primary cultures of rat granulosa cells and constitutive expression in R2C Leydig cells. Promoter region A binds Steroidogenic Factor-1 (SF-1); region B binds cAMP-regulatory element binding protein (CREB) and two other factors (designated X and Y). Mutations were generated within the context of the intact promoter to selectively eliminate the binding of either SF-1, CREB, CREB plus factors X and Y, or all of the above. When expression vectors that failed to bind either CREB alone or CREB plus factors X and Y were transfected into granulosa cells, cAMP-dependent chloramphenicol acetyltransferase (CAT) activity was reduced 65% indicating that CREB alone, and not factors X and Y, mediates the cAMP response of this cAMP response element-like domain. Similarly, cAMP-dependent CAT activity was reduced 50% in constructs that failed to bind SF-1 and was abolished with vectors that were unable to bind either factor. In R2C Leydig cells, the absence of either CREB or SF-1 binding resulted in an almost complete loss in CAT activity. Both immunoreactive CREB and phosphorylated CREB (phospho-CREB) were present in extracts and nuclei of R2C cells. Immunoreactive phosphoCREB was low in granulosa cell extracts and nuclei but increased rapidly (90 min) in response to FSH/cAMP and was highest at 48 h, at a time when SF-1 was also phosphorylated and expression of the endogenous gene was elevated. Although the amount of CREB and SF-1 remained unchanged in response to FSH, LH mediated a rapid decrease in the amount of SF-1 (but not CREB) that is coincident with decreased aromatase mRNA in luteinizing granulosa cells. Taken together, the data indicate that expression of the aromatase gene is dependent on the additive interactions of regions A and B of the aromatase promoter in granulosa cells and the synergistic interactions of these same regions in R2C cells and that these interactions are dependent, in turn, on the phosphorylation of CREB and SF-1 and the content of these factors, as well as the presence of putative coregulatory molecules.

TGF-beta signaling in murine embryonic palate cells involves phosphorylation of the CREB transcription factor

A number of studies over the last several years have demonstrated a crucial role for TGF-beta in epithelial and mesenchymal differentiation during development of the embryonic palate. Molecular mechanism(s) of signal transduction responsible for eliciting these responses remain unresolved. Since cAMP signaling also modulates the same tissue differentiation in the developing palate and palate-derived cells, we hypothesized that TGF-beta activity may be mediated through cAMP-inducible pathways. We thus examined the effects of TGF-beta on activation of the cAMP regulatory element binding protein CREB, a nuclear transcription factor which mediates transcription of genes containing CRE recognition sequences in their promoters. We examined the ability of TGF-beta-treated murine embryonic palate mesenchymal (MEPM) cells to phosphorylate CREB on the amino acid residue serine 133, phosphorylation of which is indispensable for transcriptional activation. TGF-beta treatment led to increased phosphorylation of CREB ser-133 in a time- and dose-dependent manner. Inhibition of serine-threonine phosphatases by okadaic acid enhanced but did not prolong this response. TGF-beta failed to induce the activity of protein kinase A (PKA), a known CREB kinase. Inhibition of either PKA or calcium/calmodulin kinase II (CaMK II) did not abrogate phosphorylation of CREB by TGF-beta. TGF-beta treatment also did not induce phosphorylation of mitogen-activated protein kinases, erk-1 and erk-2, on tyrosine 185, suggesting that these kinases do not mediate CREB phosphorylation by TGF-beta. Additionally, TGF-beta had no effect on CREB binding to known CREB DNA consensus recognition sequences, CRE and TRE. Together, these data suggest an alternative or novel CREB kinase in MEPM cells through which TGF-beta acts to induce CREB ser-133 phosphorylation and subsequent activation of CRE-containing genes.

AP-1 and Oct-1 transcription factors down-regulate the expression of the human PIT1/GHF1 gene

The pituitary-specific transcription factor Pit-1/GHF-1 is a member of the POU domain family of regulatory proteins. It is involved in the commitment and expansion of the somatotropic cell lineage and activates the transcription of a set of anterior pituitary genes. We have cloned the human PIT1/GHF1 gene and characterized the regulatory mechanisms controlling its promoter activation and regulation. A minimal promoter region (-102 to +15) contains the cis-acting elements that confer to the human PIT1/GHF1 gene a high basal transcriptional activity, the tissue-specific expression, and the autoregulation by Pit-1/GHF-1 protein. The upstream promoter region contains a multiplicity of Pit-1/GHF-1 binding sites that do not show any synergistic interaction with the minimal promoter. The transcriptional activity is negatively regulated by Oct-1 and mediated by an octamer-binding site (OTF). In addition, we have also identified a 12-O-tetradecanoylphorbol-13-acetate-responsive element, which overlaps with a Pit-1/GHF-1 binding site. A mutually exclusive binding of the activator protein-1 (AP-1) and Pit-1/GHF-1 has been observed on this composite site, and AP-1 was shown to down-regulate PIT1/GHF1 transcription.

Identification of the cAMP response element that controls transcriptional activation of the insulin-like growth factor-I gene by prostaglandin E2 in osteoblasts

Insulin-like growth factor-I (IGF-I), a multifunctional growth factor, plays a key role in skeletal growth and can enhance bone cell replication and differentiation. We previously showed that prostaglandin E2 (PGE2) and other agents that increase cAMP activated IGF-I gene transcription in primary rat osteoblast cultures through promoter 1 (P1), the major IGF-I promoter, and found that transcriptional induction was mediated by protein kinase A. We now have identified a short segment of P1 that is essential for full hormonal regulation and have characterized inducible DNA-protein interactions involving this site. Transient transfections of IGF-I P1 reporter genes into primary rat osteoblasts showed that the 328-base pair untranslated region of exon 1 was required for a full 5.3-fold response to PGE2; mutation in a previously footprinted site, HS3D (base pairs +193 to +215), reduced induction by 65%. PGE2 stimulated nuclear protein binding to HS3D. Binding, as determined by gel mobility shift assay, was not seen in nuclear extracts from untreated osteoblast cultures, was detected within 2 h of PGE2 treatment, and was maximal by 4 h. This DNA-protein interaction was not observed in cytoplasmic extracts from PGE2-treated cultures, indicating nuclear localization of the protein kinase A-activated factor(s). Activation of this factor was not blocked by cycloheximide (Chx), and Chx did not impair stimulation of IGF-I gene expression by PGE2. In contrast, binding to a consensus cAMP response element (CRE; 5'-TGACGTCA-3') from the rat somatostatin gene was not modulated by PGE2 or Chx. Competition gel mobility shift analysis using mutated DNA probes identified 5'-CGCAATCG-3' as the minimal sequence needed for inducible binding. All modified IGF-I P1 promoterreporter genes with mutations within this CRE sequence also showed a diminished functional response to PGE2. These results identify the CRE within the 5'-untranslated region of IGF-I exon 1 that is required for hormonal activation of IGF-I gene transcription by cAMP in osteoblasts.

Role of transcription factors CREB and CREM in cAMP-regulated transcription during spermatogenesis

The cAMP response element binding protein (CREB) and the cAMP-responsive element modulator (CREM) are cyclically expressed at high levels during spermatogenesis. Cyclical expression of CREB and CREM in germ and somatic Sertoli cells correlates with the fluctuations in cAMP signaling induced by the pituitary gonadotropic hormones FSH and LH both during sexual maturation of the testis and during the 12-day cycles of spermatogenesis that occur in the adult testis. CREB and CREM are expressed at different times during the spermatogenic cycle, undergo programmed sequential switches from activator to repressor isoforms by mechanisms of alternative exon splicing and promoter usage, and are autoregulated by cAMP signaling in opposing directions. cAMP response elements located in the promoter of the CREB gene upregulate the expression of activator CREBs, whereas cAMP autoregulatory response elements in the internal promoter of the CREM gene induce expression of repressor CREM isoforms. The complex mechanisms for the regulation of the expression of CREB and CREM in the testis appear to reflect critical adaptations for regulating key target genes essential for the development of germ cells.