The alpha-isoform of the CCAAT/enhancer-binding protein is required for mediating cAMP responsiveness of the phosphoenolpyruvate carboxykinase promoter in hepatoma cells

Identification of Insulin-Responsive Transcription Factors That Regulate Glucose Production by Hepatocytes

Hepatocyte glucose production is a complex process that integrates cell-autonomous mechanisms with cellular signaling, enzyme activity modulation, and gene transcription. Transcriptional mechanisms controlling glucose production are redundant and involve nuclear hormone receptors and unliganded transcription factors (TFs). Our knowledge of this circuitry is incomplete. Here we used DNA affinity purification followed by mass spectrometry to probe the network of hormone-regulated TFs by using phosphoenolpyruvate carboxykinase (Pck1) and glucose-6-phosphatase (G6pc) in liver and primary hepatocytes as model systems. The repertoire of insulin-regulated TFs is unexpectedly broad and diverse. Whereas in liver the two test promoters are regulated by largely overlapping sets of TFs, in primary hepatocytes Pck1 and G6pc regulation diverges. Insulin treatment preferentially results in increased occupancy by the two promoters, consistent with a model in which the hormone's primary role is to recruit corepressors rather than to clear activators. Nine insulin-responsive TFs are present in both models, but only FoxK1, FoxA2, ZFP91, and ZHX3 require an intact Pck1p insulin response sequence for binding. Knockdown of FoxK1 in primary hepatocytes decreased both glucose production and insulin's ability to suppress it. The findings expand the repertoire of insulin-dependent TFs and identify FoxK1 as a contributor to insulin signaling.

Identification of a Co-repressor That Inhibits the Transcriptional and Growth-Arrest Activities of CCAAT/Enhancer-binding Protein α

We used a yeast two-hybrid screening approach to identify novel interactors of CCAAT/enhancer-binding protein alpha (C/EBPalpha) that may offer insight into its mechanism of action and regulation. One clone obtained was that for CA150, a nuclear protein previously characterized as a transcriptional elongation factor. In this report, we show that CA150 is a widely expressed co-repressor of C/EBP proteins. Two-hybrid and co-immunoprecipitation analyses indicated that CA150 interacts with C/EBPalpha. Overexpression of CA150 inhibited the transactivation produced by C/EBPalpha and was also able to reverse the enhancing effect of the co-activator p300 on C/EBPbeta-mediated transactivation. Analysis of C/EBPalpha mutants indicated that CA150 interacts with C/EBPalpha primarily through a domain spanning amino acids 135-150. Chromatin immunoprecipitation assays showed that CA150 was present on a promoter that is repressed by C/EBPalpha but not present on a promoter that is activated by C/EBPalpha. Finally, we showed that in cells in which growth arrest had been induced by ectopic expression of C/EBPalpha, CA150 was able to release them from growth arrest. Interestingly, CA150 could not reverse the growth arrest produced by the minimal growth-arrest domain of C/EBPalpha (amino acids 175-217), suggesting that the effect of CA150 was directed at a region of C/EBPalpha outside of this minimal domain, consistent with our two-hybrid analysis. Taken together, these data indicate that CA150 is a co-repressor of C/EBP proteins and provides a possible mechanism for how C/EBPalpha can repress transcription of specific genes.

Functional characterisation of the regulation of CAAT enhancer binding protein alpha by GSK-3 phosphorylation of Threonines 222/226

BACKGROUND

Glycogen Synthase Kinase-3 (GSK3) activity is repressed following insulin treatment of cells. Pharmacological inhibition of GSK3 mimics the effect of insulin on Phosphoenolpyruvate Carboxykinase (PEPCK), Glucose-6 Phosphatase (G6Pase) and IGF binding protein-1 (IGFBP1) gene expression. CAAT/enhancer binding protein alpha (C/EBPalpha) regulates these gene promoters in liver and is phosphorylated on two residues (T222/T226) by GSK3, although the functional outcome of the phosphorylation has not been established. We aimed to establish whether CEBPalpha is a link between GSK3 and these gene promoters.

RESULTS

C/EBPalpha represses the IGFBP1 thymine-rich insulin response element (TIRE), but mutation of T222 or T226 of C/EBPalpha to non-phosphorylatable alanines has no effect on C/EBPalpha activity in liver cells (towards the TIRE or a consensus C/EBP binding sequence). Phosphorylation of T222/T226 is decreased by GSK3 inhibition, suggesting GSK3 does phosphorylate T222/226 in intact cells. However, phosphorylation was not altered by treatment of liver cells with insulin. Meanwhile C/EBPalpha activity in 3T3 L1 preadipocytes was enhanced by mutation of T222/T226 and/or S230 to alanine residues. Finally, we demonstrate that C/EBPalpha is a very poor substrate for GSK3 in vitro and in cells.

CONCLUSION

The work demonstrates an important role for this domain in the regulation of C/EBPalpha activity in adipocytes but not hepatocytes, however GSK3 phosphorylation of these residues does not mediate regulation of this C/EBP activity. In short, we find no evidence that C/EBPalpha activity is regulated by direct phosphorylation by GSK3.

Signal-dependent control of gluconeogenic key enzyme genes through coactivator-associated arginine methyltransferase 1

Together with impaired glucose uptake in skeletal muscle, elevated hepatic gluconeogenesis is largely responsible for the hyperglycemic phenotype in type II diabetic patients. Intracellular glucocorticoid and cyclic adenosine monophosphate (cAMP)/protein kinase A-dependent signaling pathways contribute to aberrant hepatic glucose production through the induction of gluconeogenic enzyme gene expression. Here we show that the coactivator-associated arginine methyltransferase 1 (CARM1) is required for cAMP-mediated activation of rate-limiting gluconeogenic phosphoenolpyruvate carboxykinase (PEPCK; EC 4.1.1.32) and glucose-6-phosphatase genes. Mutational analysis showed that CARM1 mediates its effect via the cAMP-responsive element within the PEPCK promoter, which is identified here as a CARM1 target in vivo. In hepatocytes, endogenous CARM1 physically interacts with cAMP-responsive element binding factor CREB and is recruited to the PEPCK and glucose-6-phosphatase promoters in a cAMP-dependent manner associated with increased promoter methylation. CARM1 might, therefore, represent a critical component of cAMP-dependent glucose metabolism in the liver.

Synergistic effect of dexamethasone and beta-adrenergic receptor agonists on the nerve growth factor gene transcription

Activation of beta-adrenergic receptor (betaAR) increases the synthesis of nerve growth factor (NGF) in the brain and in C6-2B glioma cells. However, in the brain, the betaAR-mediated increase in NGF expression appears to require the presence of glucocorticoids, suggesting that NGF promoter may be sensitive to cAMP and glucocorticoid-dependent transcription factors. We tested this hypothesis by exposing C6-2B glioma cells to dexamethasone (DEX) in combination with agents that increase cAMP levels and examining the DNA binding activity of two cAMP-dependent transcription factors that regulate NGF expression: cAMP responsive element binding protein (CREB) and CCAAT/enhancer binding protein delta (C/EBPdelta). Electrophoretic mobility shift assays revealed that the beta(2)AR agonist clenbuterol (CLE) or high levels of cAMP elicited a time-dependent increase in C/EBPdelta binding activity as well as phosphorylated CREB (P-CREB). When DEX, which per se showed little effect on these transcription factors, was combined with CLE, dibutyryl cAMP or isoproterenol, enhanced induction of P-CREB and C/EBP binding activity as well as NGF mRNA was observed. Moreover, the increase in NGF mRNA in the presence of DEX was prolonged compared to that obtained by CLE or other cAMP inducing agents alone. In fact, NGF mRNA levels remained significantly elevated at least for 24 h. These studies suggest that the synergistic effect of DEX on the induction of NGF mRNA may include the ability of this glucocorticoid to potentiate the betaAR-mediated induction of transcription factors.

Transforming growth factor-beta inhibits adipocyte differentiation by Smad3 interacting with CCAAT/enhancer-binding protein (C/EBP) and repressing C/EBP transactivation function

Transforming growth factor (TGF)-beta is a potent inhibitor of adipocyte differentiation. To identify which adipocyte transcription factors might be targeted by TGF-beta, we overexpressed key adipogenic transcription factors, C/EBPbeta, C/EBPdelta, or peroxisome proliferator-activated receptor (PPAR) gamma in NIH3T3 cells and tested the ability of TGF-beta to block adipogenesis. We show that TGF-beta inhibits adipocyte differentiation driven by either C/EBPbeta or C/EBPdelta without affecting C/EBP protein expression levels, suggesting that these C/EBPs are a direct target of TGF-beta action. Because TGF-beta inhibits adipogenesis by signaling through Smad3, we examined physical and functional interactions of Smad3 and Smad4 with C/EBPbeta, C/EBPdelta, and PPARgamma2. C/EBPbeta and C/EBPdelta were found to physically interact with Smad3 and Smad4, and Smad3 cooperated with Smad4 and TGF-beta signaling to repress the transcriptional activity of C/EBPs. Thus, repression of the activity of C/EBPs by Smad3/4 at C/EBP binding sites inhibited transcription from the PPARgamma2 and leptin promoters. In contrast, PPARgamma interacted only very weakly with Smad3 and its transcriptional activity was not repressed by Smad3/4 or in response to TGF-beta. Smad3/4 did not reduce the ability of C/EBP to bind to its cognate DNA sequence, but repressed transcription by inhibiting the transactivation function of C/EBP.

Different transcription factor binding arrays modulate the cAMP responsivity of the phosphoenolpyruvate carboxykinase gene promoter

The cAMP responsiveness of the phosphoenolpyruvate carboxykinase (PEPCK) gene promoter is mediated by a cAMP response unit, which includes three CCAAT/enhancer-binding protein (C/EBPs) sites, and a cAMP response element (CRE). Because both the CRE-binding protein and several C/EBP isoforms can to bind to the CRE with similar affinity, a variety of transcription factor bindings arrays in the cAMP response unit are possible that may affect the protein kinase A (PKA) responsivity of the promoter. To explore this issue, we have designed PEPCK promoter variants that have the native cis-elements within the cAMP response unit replaced with one or more LexA- and/or GAL4-binding sites. We also engineered the corresponding C/EBP and CRE-binding protein chimeras, which have their basic region leucine zipper domains replaced with LexA or GAL4 DNA-binding domains. Using this approach, we have reconstituted the PKA responsiveness of permissive PEPCK promoters in hepatoma cells and have characterized the PKA responsivity of the promoter under defined transcription factor occupancy patterns. Furthermore, analysis of deletion mutants of C/EBPalpha indicated that the domains that mediate its constitutive and PKA-inducible activities vary depending on which cis-element it occupies on the PEPCK promoter. These results suggest that promoter context may influence which domains within a transcription factor are employed to mediate transactivation.

Conserved amino acids within CCAAT enhancer-binding proteins (C/EBP(alpha) and beta) regulate phosphoenolpyruvate carboxykinase (PEPCK) gene expression

Thyroid hormone and cAMP stimulate transcription of the gene for phosphoenolpyruvate carboxykinase (PEPCK). CCAAT enhancer-binding proteins (C/EBP(alpha) and beta) are involved in multiple aspects of the nutritional, developmental and hormonal regulation of PEPCK gene expression. Previously, we have identified a thyroid hormone response element in the PEPCK promoter and demonstrated that C/EBP proteins bound to the P3(I) site are participants in the induction of PEPCK gene expression by thyroid hormone and cAMP. Here, we identify several peptide regions within the transactivation domain of C/EBP(alpha) that enhance the ability of T(3) to stimulate gene transcription. We also demonstrate that several conserved amino acids in the transactivation domain of C/EBP(alpha) and C/EBPbeta are required for the stimulation of basal gene expression and identify amino acids within C/EBPbeta that participate in the cAMP induction of the PEPCK gene. Finally, we show that the CREB-binding protein (CBP) enhanced the induction of PEPCK gene transcription by thyroid hormone and that CBP is associated with the PEPCK gene in vivo. Our results indicate that both C/EBP proteins and CBP participate in the regulation of PEPCK gene transcription by thyroid hormone.

CCAAT/enhancer binding proteins: do they possess intrinsic cAMP-inducible activity?

CCAAT/enhancer binding proteins (C/EBPs) are transcription factors that are enriched in tissues which play a central role in energy metabolism, such as adipose and liver. Structure/function analyses of these proteins have identified several transactivation domains, some of which can physically interact with general transcription factors present in the preinitiation complex. C/EBPs are generally considered to be constitutively-acting factors, unlike other transcription factors whose activities can be regulated by covalent modification, binding of a specific ligand, etc. However, studies of the regulatory property of the phosphoenolpyruvate carboxykinase gene promoter have uncovered a role for C/EBPs in mediating cAMP responsiveness, and identified specific domains within the proteins, which mediate this effect. Interestingly, a number of other gene promoters that are activated in response to cAMP also contain binding sites for C/EBP, and these binding sites are often located within the region of the promoter that is responsible for mediating the acute responsiveness to cAMP. The evidence presented in this review provides compelling support for the hypothesis that C/EBPs have both constitutive and cAMP-inducible activities, and should be considered as a cAMP-responsive nuclear regulator.

The role of C/EBP in nutrient and hormonal regulation of gene expression

C/EBPs are a family of transcription factors that play important roles in energy metabolism. Although initially thought to be constitutive regulators of transcription, an increasing amount of evidence indicates that their transactivating capacity within the cell can be modulated by nutrients and hormones. There are several mechanisms whereby this occurs. First, hormones/nutrients are known to directly alter the expression of C/EBPs. Second, hormones/nutrients may cause an alteration in the phosphorylation state of C/EBPs, which can affect their DNA-binding activity or transactivating capacity. Third, C/EBPs can function as accessory factors on gene promoters within a hormone response unit, interacting with other transcription factors to enhance the degree of responsiveness to specific hormones. Given their role in regulating genes involved in a wide variety of metabolic events, advancing our understanding of the molecular mechanism of action of C/EBPs will undoubtedly further our appreciation for the role these transcription factors play in both health and disease.

C/EBPbeta contributes to cAMP-activated transcription of phosphoenolpyruvate carboxykinase in LLC-PK(1)-F+ cells

Phosphoenolpyruvate carboxykinase (PEPCK) is a key regulatory enzyme in renal gluconeogenesis. Activation of various PEPCK(-2300)Luc reporter constructs in LLC-PK(1)-F+ cells, a gluconeogenic line of porcine renal proximal tubule-like cells, by protein kinase A (PKA) is mediated, in part, through the cAMP-response element (CRE)-1 of the PEPCK promoter. Incubation of a CRE-1 containing oligonucleotide with nuclear extracts from LLC-PK(1)-F+ cells produced multiple bands, all of which were blocked by antibodies that are specific for C/EBPbeta but not for C/EBPalpha or C/EBPdelta. Treatment of cells with cAMP did not affect the expression of C/EBPbeta, but the observed binding activity was increased nearly threefold. Mutation of CRE-1 to a Gal-4 binding site reduced the PKA-dependent activation of PEPCK(-2300)Luc to 40% of that observed with the wild-type construct. Coexpression of a chimeric protein containing a Gal-4 binding domain and the transactivation domain of C/EBPbeta, but not of C/EBPalpha or CRE binding protein (CREB), restored full activation by PKA. A deletion construct that lacks the activation domain of C/EBPbeta functions as a dominant negative inhibitor. Thus the binding of C/EBPbeta to the CRE-1 may contribute to the cAMP-dependent activation of the PEPCK promoter in kidney cells.

Gene- and activation-specific mechanisms for insulin inhibition of basal and glucocorticoid-induced insulin-like growth factor binding protein-1 and phosphoenolpyruvate carboxykinase transcription. Roles of forkhead and insulin response sequences

The insulin response sequence (IRS) of the phosphoenolpyruvate carboxykinase (PEPCK) promoter, located within the glucocorticoid response unit, was first characterized by its ability to mediate insulin inhibition when inserted into a thymidine kinase promoter. The IRSs of the PEPCK and insulin-like growth factor binding protein-1 (IGFBP-1) promoters have been proposed to contribute to regulation by glucocorticoids and insulin. Forkhead (FKHR) recognizes IRS sequences, is phosphorylated in response to insulin, and mediates insulin inhibition of basal IGFBP-1 transcription in an IRS-dependent manner. Here, we investigate the contributions of FKHR and IRSs to insulin inhibition of basal and glucocorticoid-induced transcription of PEPCK and IGFBP-1. Expression of T/S/S, in which three putative protein kinase B (PKB) sites in FKHR are mutated, reduced insulin inhibition of basal expression of IGFBP-1 but not PEPCK. Mutation of the IGFBP-1 IRSs abolished insulin inhibition in the presence of T/S/S. Mutation of the PEPCK IRS had no effect on insulin inhibition in the presence of T/S/S, indicating that insulin inhibits PEPCK transcription independently of the IRS or of the putative PKB phosphorylation sites in FKHR. Mutations in the IRS or FKHR had no effect on insulin inhibition of glucocorticoid-induced transcription of either the PEPCK or IGFBP-1 gene. Thus, insulin uses gene- and activation-specific mechanisms to regulate the basal and glucocorticoid-induced activity of these genes.

Characterization of domains in C/EBPalpha that mediate its constitutive and cAMP-inducible activities

Structure/function analysis of CCAAT/enhancer binding proteins (C/EBP) alpha and beta have shown that they possess both constitutive and cAMP inducible activities. Three regions conserved between C/EBPalpha and beta were identified which lie within the cAMP inducible domains of each protein. Deletion analysis of these conserved regions within C/EBPalpha show that conserved region 2 plays a particularly critical role in mediating the PKA inducible activity of the protein, however, the constitutive activity of conserved region 2 depends on promoter context. This data supports previous findings that constitutive and cAMP responsiveness are mediated by domains of the protein that do not directly overlap, suggesting that they occur through distinct mechanisms.

A nucleoprotein complex containing CCAAT/enhancer-binding protein beta interacts with an insulin response sequence in the insulin-like growth factor-binding protein-1 gene and contributes to insulin-regulated gene expression

Highly related insulin response sequences (IRSs) mediate effects of insulin on the expression of multiple genes in the liver, including insulin-like growth factor binding protein-1 (IGFBP-1) and phosphoenolpyruvate carboxykinase (PEPCK). Gel shift studies reveal that oligonucleotide probes containing an IRS from the IGFBP-1 or PEPCK gene form a similar complex with hepatic nuclear proteins. Unlabeled competitors containing the IGFBP-1 or PEPCK IRS or a binding site for C/EBP proteins inhibit the formation of this complex. Antibody against C/EBPbeta (but not other C/EBP proteins) supershifts this complex, and Western blotting of affinity purified proteins confirms that C/EBPbeta is present in this complex. Studies with affinity purified and recombinant protein indicate that C/EBPbeta does not interact directly with the IRS, but that other factors are required. Gel shift assays and reporter gene studies with constructs containing point mutations within the IRS reveal that the ability to interact with factors required for the formation of this complex correlates well with the ability of insulin to regulate promoter activity via this IRS (r = 0.849, p < 0.01). Replacing the IRS in reporter gene constructs with a C/EBP-binding site (but not an HNF-3/forkhead site or cAMP response element) maintains the effect of insulin on promoter activity. Together, these findings indicate that a nucleoprotein complex containing C/EBPbeta interacts with IRSs from the IGFBP-1 and PEPCK genes in a sequence-specific fashion and may contribute to the ability of insulin to regulate gene expression.

Insulin suppresses transactivation by CAAT/enhancer-binding proteins beta (C/EBPbeta). Signaling to p300/CREB-binding protein by protein kinase B disrupts interaction with the major activation domain of C/EBPbeta

CAAT/enhancer-binding proteins (C/EBPs) play an important role in the regulation of gene expression in insulin-responsive tissues. We have found that a complex containing C/EBPbeta interacts with an insulin response sequence in the insulin-like growth factor-binding protein-1 (IGFBP-1) gene and that a C/EBP-binding site can mediate effects of insulin on promoter activity. Here, we examined mechanisms mediating this effect of insulin. The ability of insulin to suppress promoter activity via a C/EBP-binding site is blocked by LY294002, a phosphatidylinositol 3-kinase inhibitor, but not by rapamycin, which blocks activation of p70(S6 kinase). Dominant negative phosphatidylinositol 3-kinase and protein kinase B (PKB) block the effect of insulin, while activated PKB suppresses promoter function via a C/EBP-binding site, mimicking the effect of insulin. Coexpression studies indicate that insulin and PKB suppress transactivation by C/EBPbeta, but not C/EBPalpha, and that N-terminal transactivation domains in C/EBPbeta are required. Studies with Gal4 fusion proteins reveal that insulin and PKB suppress transactivation by the major activation domain in C/EBPbeta (AD II), located between amino acids 31 and 83. Studies with E1A protein indicate that interaction with p300/CBP is required for transactivation by AD II and the effect of insulin and PKB. Based on a consensus sequence, we identified a PKB phosphorylation site (Ser(1834)) within the region of p300/CBP known to bind C/EBPbeta. Mammalian two-hybrid studies indicate that insulin and PKB disrupt interactions between this region of p300 and AD II and that Ser(1834) is critical for this effect. Signaling by PKB and phosphorylation of Ser(1834) may play an important role in modulating interactions between p300/CBP and transcription factors and mediate effects of insulin and related growth factors on gene expression.

HIV-1-associated central nervous system dysfunction

Despite more than 15 years of extensive investigative efforts, a complete understanding of the neurological consequences of HIV-1 CNS infection remains elusive. Although the resources of numerous investigators have been focused on studies of HIV-1-associated CNS disease, the complex nature of the disease processes that underlie the clinical, pathological, and cellular manifestations of HIV-1 CNS infection have required a larger volume of studies than was initially envisioned. Several major areas remain as the focus of current research efforts. One of the more pressing issues facing researchers and clinicians alike is the search for correlates to the development of HIV-1-associated CNS neuropathology and the onset of HIVD. Although numerous parameters have been studied, none have been shown to be absolute predictors or markers of HIV-1-related CNS dysfunction. The identification of solid correlates of HIVD is an important goal that would permit clinical identification of individuals at risk for developing potentially crippling, life-threatening CNS abnormalities and would facilitate early treatment of nascent neurological problems. A more complete comprehension of the cellular foundations of CNS dysfunction and HIVD is also a fundamental part of strategies designed to treat or prevent HIV-1-associated CNS disease. Future investigations will strive to expand the body of knowledge concerning the complex interactions between infected and uninfected neuroglial cells and the roles of numerous cytokines, chemokines, and other soluble agents that are deregulated during HIV-1 CNS infection. In particular, a thorough understanding of the mechanisms of neurotoxicity may facilitate the development of new therapies that alleviate or eliminate the clinical consequences of CNS infection. Finally, investigators will continue to study HIVD within the context of single and combination drug therapies used in the treatment of HIV-1 infection and AIDS. As newer and more effective systemic treatments for HIV-1 infection and AIDS are introduced, the effects of these treatments on the onset, incidence, and severity of HIVD will also require intensive study. The impact of drug therapies on the ability of the CNS to act as an HIV-1 reservoir will also need to be addressed. Introduction of each new drug or drug combination will necessitate studies of drug penetration into the CNS and efficacy against the development of CNS abnormalities. Furthermore, as more effective treatments prolong the lifespan of individuals infected with HIV-1, the impact of extended survival on the occurrence and severity of HIVD will also require further investigations. The quest for answers to these and other questions will be complicated by the diversity of experimental systems used to study different aspects of HIV-1 CNS infection and HIVD. Each system has its own unique strengths and weaknesses. Clinical observations provide a continuous spectrum of symptomatic findings but reveal little about the underlying mechanisms of disease. In vivo imaging techniques, such as CT and MRI, also provide a continuum of observations, but the images are limited in their resolution. Neuropathological examinations of postmortem HIV-1-infected brains offer gross, cellular, and molecular views (including phenotypic and genotypic analyses of CNS viral isolates) of the diseased brain, but only provide a snapshot of the end-stage neurologic dysfunction. Studies that rely on animal surrogates for HIV-1, including SIV, simian-HIV (SHIV), feline immunodeficiency virus (FIV), visna virus, and HIV-1 SCID-hu models, permit experimental protocols that cannot be carried out in humans, but are limited by the fidelity with which each virus and animal model emulates the conditions and events observed in the human host. Finally, in vitro techniques, which include the use of primary cells and cell lines, adult or fetal human cell cultures, and BBB barrier model systems, are also convenient means by which aspe

Cooperative mechanism of transcriptional activation by a cyclic AMP-response element modulator alpha mutant containing a motif for constitutive binding to CREB-binding protein

Cyclic AMP-response element modulator alpha (CREMalpha) is a transcription factor that is highly related to cAMP-response element-binding protein (CREB) but represses cAMP-induced gene expression from simple artificial promoters containing a cAMP-response element (CRE). CREMalpha lacks two glutamine-rich Q regions that, in CREB, are thought to be necessary for transcriptional activation. Nevertheless, protein kinase A stimulation induces CREMalpha to activate the complex native promoter in the phosphoenolpyruvate carboxykinase (PEPCK) gene. To study this phenomenon in the absence of protein kinase A stimulation, we introduced a mutation into CREMalpha to allow constitutive binding to the coactivator CREB-binding protein. This mutant, CREMalpha(DIEDML), constitutively activated the PEPCK promoter. By engineering the leucine zipper regions of CREMalpha(DIEDML) and CREB(DIEDML) to direct their patterns of dimerization, we found that only CREMalpha(DIEDML) homodimers fully activated the PEPCK promoter. By using a series of deletion and block mutants of the PEPCK promoter, we found that activation by CREMalpha(DIEDML) depended on the CRE and two CCAAT/enhancer-binding protein (C/EBP) sites. A dominant negative inhibitor of C/EBP, A-C/EBP, suppressed activation by CREMalpha(DIEDML). Furthermore, a GAL4-C/EBPalpha fusion protein and CREMalpha(DIEDML) cooperatively activated a promoter containing three GAL4 sites and the PEPCK CRE. Thus, we propose that the C/EBP sites in the PEPCK promoter allow CREMalpha to activate transcription despite its lack of Q regions.

Interaction between CCAAT/enhancer binding protein and cyclic AMP response element binding protein 1 regulates human immunodeficiency virus type 1 transcription in cells of the monocyte/macrophage lineage

Recent observations have shown two CCAAT/enhancer binding protein (C/EBP) binding sites to be critically important for efficient human immunodeficiency virus type 1 (HIV-1) replication within cells of the monocyte/macrophage lineage, a cell type likely involved in transport of the virus to the brain. Additionally, sequence variation at C/EBP site I, which lies immediately upstream of the distal nuclear factor kappa B site and immediately downstream of a binding site for activating transcription factor (ATF)/cyclic AMP response element binding protein (CREB), has been shown to affect HIV-1 long terminal repeat (LTR) activity. Given that C/EBP proteins have been shown to interact with many other transcription factors including members of the ATF/CREB family, we proceeded to determine whether an adjacent ATF/CREB binding site could affect C/EBP protein binding to C/EBP site I. Electrophoretic mobility shift analyses indicated that selected ATF/CREB site variants assisted in the recruitment of C/EBP proteins to an adjacent, naturally occurring, low-affinity C/EBP site. This biophysical interaction appears to occur via at least two mechanisms. First, low amounts of CREB-1 and C/EBP appear to heterodimerize and bind to a site consisting of a half site from both the ATF/CREB and C/EBP binding sites. In addition, CREB-1 homodimers bind to the ATF/CREB site and recruit C/EBP dimers to their cognate weak binding sites. This interaction is reciprocal, since C/EBP dimer binding to a strong C/EBP site leads to enhanced CREB-1 recruitment to ATF/CREB sites that are weakly bound by CREB. Sequence variation at both C/EBP and ATF/CREB sites affects the molecular interactions involved in mediating both of these mechanisms. Most importantly, sequence variation at the ATF/CREB binding site affected basal LTR activity as well as LTR function following interleukin-6 stimulation, a treatment that leads to increases in C/EBP activation. Thus, HIV-1 LTR ATF/CREB binding site sequence variation may modulate cellular signaling at the viral promoter through the C/EBP pathway.

Characterization of elements mediating regulation of phosphoenolpyruvate carboxykinase gene transcription by protein kinase A and insulin. Identification of a distinct complex formed in cells that mediate insulin inhibition

The in vivo pattern of induction of phosphoenolpyruvate carboxykinase (PEPCK) gene transcription by cAMP and its inhibition by insulin is reproduced in H4IIe cells and is mediated by a bipartite cAMP/insulin response unit (C/IRU) consisting of a cAMP response element (-95/-87) and an upstream enhancer, AC (-271/-225). Studies in HepG2 cells showed that binding of AP-1 and CAAT/enhancer-binding protein (C/EBP) to AC is required for induction by cAMP, but insulin did not inhibit cAMP-induced PEPCK expression in HepG2 cells. Binding of H4IIe nuclear proteins to an AC element probe was inhibited by antibodies or a consensus site for C/EBP, but not AP-1. Transfection with dominant negative bZIP factors, which prevent endogenous factors from binding to DNA, showed that elimination of cAMP regulatory element-binding protein CREB or C/EBP activity blocked induction by protein kinase A (PKA), whereas elimination of AP-1 activity had no effect. In addition, promoters with multiple CREB sites, or a single CREB site and multiple C/EBP sites, mediated PKA induction, but this was inhibited to no greater extent than basal activity was by insulin. These results indicate that an AC factor other than C/EBP must mediate insulin inhibition. An A-site probe (-265/-247) or a probe across the middle of the AC element (-256/-237) competed for complexes formed by factors other than AP-1 or C/EBP. However, analysis of competitor oligonucleotides and antibodies for candidate factors failed to identify other factors. Scanning mutations throughout the AC element interfered with induction but allowed us to define five overlapping sites for regulatory factors in AC and to design probes binding just one or two factors. Comparison of the protein-DNA complexes formed on these smaller probes revealed that a specific complex present in rat liver and H4IIe cell nuclear extracts differed from those formed by HepG2 cell nuclear extracts. Our results suggest that multiple factors binding the AC element of the C/IRU interact with each other and CREB to regulate PEPCK induction by cAMP and inhibition by insulin and that the unique factor expressed in H4IIe cells is a candidate for involvement in insulin regulation of PKA-induced PEPCK gene transcription.

What is a cAMP response unit?

Phosphoenolpyruvate carboxykinase (PEPCK) is the rate-limiting enzyme of gluconeogenesis, and most, if not all, of the regulation of its activity is exerted at the level of gene expression, with transcriptional regulation being the most predominant. A number of hormones regulate transcription of this gene, but in a defined, tissue-specific fashion. For example, cAMP strongly induces PEPCK gene transcription in liver, but provides only a weak response in kidney. Results from a number of different studies indicate that cAMP responsiveness of this gene is mediated by a 'cAMP response unit' (CRU), consisting of five cis-elements. All five sequences are required for maximal responsiveness and, potentially, four of these are binding sites for a CCAAT/enhancer binding protein (C/EBP). Since alpha- and beta-isoforms of C/EBP are liver-enriched, this may provide the molecular basis for the liver-specific responsiveness to cAMP. A curiosity of this promoter is that one of the cis-elements present in the CRU is a cAMP response element (CRE), which typically acts as a binding site for CRE binding protein (CREB). However, the non-consensus CRE in the PEPCK promoter also binds C/EBP proteins with high affinity, and C/EBPalpha can functionally substitute for CREB in this cAMP response unit while C/EBPbeta cannot. The available data suggest that the PEPCK promoter can exist in altered states of cAMP responsivity, depending on which transcription factors occupy specific cis-elements in the CRU.

Transcriptional induction of heme oxygenase-1 gene expression by okadaic acid in primary rat hepatocyte cultures

Heme oxygenase (HO) catalyzes the rate-limiting enzymatic step of heme degradation and regulates the cellular heme content. The gene expression of the inducible isoform of HO, HO-1, is up-regulated in response to various agents causing oxidative stress. To investigate the regulatory role of protein phosphatases in the hepatic regulation of HO-1 gene expression, primary cultures of rat hepatocytes were treated with okadaic acid (OA), which specifically inhibits the serine threonine protein phosphatases 1 and 2A. Both protein synthesis and mRNA expression of HO-1 were induced by OA in cultured hepatocytes, but not in cultured tissue macrophages of rat liver. The HO-1 mRNA induction by OA occurred in a time- and concentration-dependent manner. Simultaneous treatment with OA plus dibutyryl cAMP caused a synergistic up-regulation of steady-state levels of HO-1 mRNA, and the specific protein kinase A inhibitor KT5720 markedly reduced the OA-dependent HO-1 mRNA induction. In contrast, the dibutyryl cAMP-dependent induction of the phosphoenolpyruvate carboxykinase mRNA expression and enzyme activity was inhibited by simultaneous treatment with OA in hepatocytes. The induction of the HO-1 gene expression by OA was transcriptional as determined by studies with actinomycin D, nuclear run-off assay, and measurement of the half-life of HO-1 mRNA. Luciferase reporter constructs containing DNA sequences of the rat HO-1 promoter 5'-flanking region were up-regulated by OA in transiently transfected hepatocytes. Mutation of the cAMP response element/activator protein-1 (-665/-654) site obliterated the OA-dependent induction, suggesting that this element is involved in the transcriptional induction of the rat HO-1 gene by OA.

Hormonal regulation of the phosphoenolpyruvate carboxykinase gene. Role of specific CCAAT/enhancer-binding protein isoforms

The CCAAT/enhancer-binding protein alpha (C/EBP) is a transcription factor that trans-activates a number of metabolically important genes. Previous work has demonstrated that C/EBPalpha and C/EBPbeta have the potential to mediate the cAMP responsiveness of phosphoenolpyruvate carboxykinase (PEPCK) in liver cells. However, these studies used GAL4 fusion proteins and artificial promoter-reporter gene vectors in transfection experiments; as a result, these studies only indicated that both isoforms had the potential to mediate the hormonal response and not which isoform actually participated in vivo. To address this issue, we produced hepatoma cell lines that stably expressed either a dominant negative inhibitor or antisense RNA for these two main liver C/EBP isoforms. Inhibition of all C/EBP isoforms via expression of the dominant negative protein eliminated cAMP responsiveness, and reduced glucocorticoid responsiveness, of the endogenous PEPCK gene in hepatoma cells. Antisense directed against C/EBPalpha mRNA, which reduced C/EBPalpha protein levels by nearly 80%, also significantly reduced the cAMP responsiveness of the endogenous PEPCK promoter, whereas antisense directed against C/EBPbeta was without effect. There was no major alteration in cAMP signaling in the C/EBPalpha antisense cells, as cAMP induction of the C/EBPbeta gene was similar to that in wild-type H4IIE cells. These data suggest that the alpha-isoform of C/EBP is specifically utilized for mediating the cAMP responsiveness of the PEPCK gene.

Negative cyclic AMP response elements in the promoter of the L-type pyruvate kinase gene

L-type pyruvate kinase gene expression is modulated by hormonal and nutritional conditions. Here, we show by transient transfections in hepatocytes in primary culture that both the glucose response element and the contiguous hepatocyte nuclear factor 4 (HNF4) binding site (L3) of the promoter were negative cyclic AMP (cAMP) response elements and that cAMP-dependent inhibition through L3 requires HNF4 binding. Another HNF4 binding site-dependent construct was also inhibited by cAMP. However, HNF4 mutants whose putative PKA-dependent phosphorylation sites have been mutated still conferred cAMP-sensitive transactivation of a L3-dependent reporter gene. Overexpression of the CREB binding protein (CBP) increased the HNF4-dependent transactivation but this effect remained sensitive to cAMP inhibition.

Identification of a novel transcriptional regulatory element within the promoter region of the keratinocyte growth factor gene that mediates inducibility to cyclic AMP

Keratinocyte growth factor (KGF) plays a critical role for the normal development and morphogenesis of many different tissues and organs. Furthermore, its expression is induced during wound healing and in various chronic inflammatory diseases. To determine the molecular mechanisms which regulate KGF gene induction at the transcriptional level, we carried out in vitro studies using the human KGF promoter. We have identified a novel regulatory element, TGAGGTCAG, located between -39 and -46 bp (relative to the transcription start site) in the KGF basal promoter region, which binds to inducible transcription factors as determined by electrophoretic mobility shift assay. When cloned in front of a heterologous SV40 promoter this region conferred inducibility to forskolin, a stimulator of adenylate cyclase. In contrast, various mutated forms of this region were either partially or completely impaired in their ability to mediate induction to forskolin. The TGAGGTCAG sequence shared homology to both the cAMP responsive element (CRE) and CCAAT/enhancer binding protein (C/EBP) consensus binding sites. An oligonucleotide comprising a consensus CRE binding site partially competed for the nuclear protein binding to the TGAGGTCAG site. Gel mobility supershift assays indicated that two members of the activating transcription factor (ATF) family of CRE binding proteins, ATF1 and ATF2, were part of the nuclear protein complex bound to this regulatory region. Furthermore, purified recombinant ATF2 was able to directly recognize and bind the TGAGGTCAG sequence. In contrast, no evidence was obtained for C/EBP transcription factors being part of the complex. These results suggest that members of the ATF family are involved in mediating the transcriptional regulation of the KGF gene in response to extracellular stimuli via a novel CRE regulatory element.

CCAAT/enhancer-binding protein beta is an accessory factor for the glucocorticoid response from the cAMP response element in the rat phosphoenolpyruvate carboxykinase gene promoter

The cyclic AMP response element (CRE) of the rat phosphoenolpyruvate carboxykinase (PEPCK) gene promoter is required for a complete glucocorticoid response. Proteins known to bind the PEPCK CRE include the CRE-binding protein (CREB) and members of the CCAAT/enhancer-binding protein (C/EBP) family. We took two different approaches to determine which of these proteins provides the accessory factor activity for the glucocorticoid response from the PEPCK CRE. The first strategy involved replacing the CRE of the PEPCK promoter/chloramphenicol acetyltransferase reporter plasmid (pPL32) with a consensus C/EBP-binding sequence. This construct, termed pDeltaCREC/EBP, binds C/EBPalpha and beta but not CREB, yet it confers a nearly complete glucocorticoid response when transiently transfected into H4IIE rat hepatoma cells. These results suggest that one of the C/EBP family members may be the accessory factor. The second strategy involved co-transfecting H4IIE cells with a pPL32 mutant, in which the CRE was replaced with a GAL4-binding sequence (pDeltaCREGAL4), and various GAL4 DNA-binding domain (DBD) fusion protein expression vectors. Although chimeric proteins consisting of the GAL4 DBD fused to either CREB or C/EBPalpha are able to confer an increase in basal transcription, they do not facilitate the glucocorticoid response. In contrast, a fusion protein consisting of the GAL4 DBD and amino acids 1-118 of C/EBPbeta provides a significant glucocorticoid response. Additional GAL4 fusion studies were done to map the minimal domain of C/EBPbeta needed for accessory factor activity to the glucocorticoid response. Chimeric proteins containing amino acid regions 1-84, 52-118, or 85-118 of C/EBPbeta fused to the GAL4 DBD do not mediate a glucocorticoid response. We conclude that the amino terminus of C/EBPbeta contains a multicomponent domain necessary to confer accessory factor activity to the glucocorticoid response from the CRE of the PEPCK gene promoter.

Role of CCAAT enhancer-binding protein beta in the thyroid hormone and cAMP induction of phosphoenolpyruvate carboxykinase gene transcription

Transcription of the gene for phosphoenolpyruvate carboxykinase (PEPCK) is stimulated by thyroid hormone (T3) and cAMP. Two DNA elements in the PEPCK promoter are required for T3 responsiveness including a thyroid hormone response element and a binding site called P3(I) for the CCAAT enhancer-binding protein (C/EBP). Both the alpha and beta isoforms of C/EBP are highly expressed in the liver. C/EBPalpha contributes to the liver-specific expression and cAMP responsiveness of the PEPCK gene. In this study, we examined the ability of C/EBPbeta when bound to the P3(I) site to regulate PEPCK gene expression. We report that C/EBPbeta can stimulate basal expression and participate in the induction of PEPCK gene transcription by T3 and cAMP. The cAMP-responsive element-binding protein and AP1 proteins that contribute to the induction by cAMP are not involved in the stimulation by T3. A small region of the transactivation domain of C/EBPbeta is sufficient for the stimulation of basal expression and cAMP responsiveness. Our results suggest that C/EBPalpha and C/EBPbeta are functionally interchangeable when bound to the P3(I) site of the PEPCK promoter.

The C/EBP family of transcription factors in the liver and other organs

Members of the CCAAT/enhancer-binding protein (C/EBP) family of transcription factors are pivotal regulators of liver functions such as nutrient metabolism and its control by hormones, acute-phase response and liver regeneration. Recent progress in clarification of regulatory mechanisms for the C/EBP family members gives insight into understanding the liver functions at the molecular level.

Activation of haptoglobin gene expression by cAMP involves CCAAT/enhancer-binding protein isoforms in intestinal epithelial cells

CCAAT/enhancer-binding protein (C/EBP) isoforms are expressed in rodent intestine and in the rat intestinal epithelial cell line IEC-6 but their role remains to be determined. Treatment of IEC-6 cells with the adenylate cyclase activator forskolin led to coordinate induction of C/EBP isoforms alpha, beta and delta at the mRNA and protein levels. Transient transfection assays showed that their expression is controlled at the transcriptional level. Forskolin treatment induced haptoglobin mRNA levels. Electrophoretic mobility shift and supershift assays demonstrated an increase in DNA-binding activities of the three C/EBP isoforms to the haptoA and haptoC C/EBP DNA-binding sites of the proximal haptoglobin promoter. Site-specific mutations of both sites led to a decrease in transcriptional induction by forskolin, suggesting that C/EBP isoforms are involved in the cAMP-dependent regulation of the acute-phase protein gene haptoglobin in intestinal epithelial cells.

Glucocorticoid receptor, C/EBP, HNF3, and protein kinase A coordinately activate the glucocorticoid response unit of the carbamoylphosphate synthetase I gene

A single far-upstream enhancer is sufficient to confer hepatocyte-specific, glucocorticoid- and cyclic AMP-inducible periportal expression to the carbamoylphosphate synthetase I (CPS) gene. To identify the mechanism of hormone-dependent activation, the composition and function of the enhancer have been analyzed. DNase I protection and gel mobility shift assays revealed the presence of a cyclic AMP response element, a glucocorticoid response element (GRE), and several sites for the liver-enriched transcription factor families HNF3 and C/EBP. The in vivo relevance of the transcription factors interacting with the enhancer in the regulation of CPS expression in the liver was assessed by the analysis of knockout mice. A strong reduction of CPS mRNA levels was observed in glucocorticoid receptor- and C/EBPalpha-deficient mice, whereas the CPS mRNA was normally expressed in C/EBPbeta knockout mice and in HNF3alpha and -gamma double-knockout mice. (The role of HNFbeta could not be assessed, because the corresponding knockout mice die at embryonic day 10). In hepatoma cells, most of the activity of the enhancer is contained within a 103-bp fragment, which depends for its activity on the simultaneous occupation of the GRE, HNF3, and C/EBP sites, thus meeting the requirement of a glucocorticoid response unit. In fibroblast-like CHO cells, on the other hand, the GRE in the CPS enhancer does not cooperate with the C/EBP and HNF3 elements in transactivation of the CPS promoter. In both hepatoma and CHO cells, stimulation of expression by cyclic AMP depends mainly on the integrity of the glucocorticoid pathway, demonstrating cross talk between this pathway and the cyclic AMP (protein kinase A) pathway.

PMA and staurosporine affect expression of the PCK gene in LLC-PK1-F+ cells

The addition of phorbol 12-myristate 13-acetate (PMA) to renal LLC-PK1-F+ cells caused a rapid decrease in the level of phosphoenolpyruvate carboxykinase (PCK) mRNA and reversed the stimulatory effects of exposure to acidic medium (pH 6.9, 10 mM HCO-3) or cAMP. In contrast, prolonged treatment with PMA increased the levels of PCK mRNA. The two effects correlated with the membrane translocation and downregulation of the alpha-isozyme of protein kinase C and were blocked by pretreatment with specific inhibitors of protein kinase C. The rapid decrease in PCK mRNA caused by PMA occurred with a half-life (t1/2 = 1 h) that is significantly faster than that measured during recovery from acid medium or following inhibition of transcription (t1/2 = 4 h). The effect of PMA was reversed by staurosporine, which apparently acts by inhibiting a signaling pathway other than protein kinase C. Staurosporine had no effect on the half-life of the PCK mRNA, but it stimulated the activity of a chloramphenicol acetyltransferase gene that was driven by the initial 490 base pairs of the PCK promoter and transiently transfected into LLC-PK1-F+ cells. This effect was additive to that of cAMP, and neither stimulation was reversed by PMA. The stimulatory effect of staurosporine was mapped to the cAMP response element (CRE-1) and P3(II) element of the PCK promoter. The data indicate that, in LLC-PK1-F+ cells, activation of protein kinase C decreases the stability of the PCK mRNA, whereas transcription of the PCK gene may be suppressed by a kinase that is inhibited by staurosporine.

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.

Identification and characterization of the human adipocyte apM-1 promoter

The human adipocyte-specific apM-1 gene encodes a secretory protein of the adipose tissue and seems to play a role in the pathogenesis of obesity. A 1.3 kb amount of the proximal promoter region has been cloned and analyzed for the presence of putative transcription factor binding sites. Several binding sites known to be involved in adipogenesis and regulation of adipocyte-specific genes (C/EBP, SREBP) are present. No TATA box, but a classical CCAAT box could be identified. To confirm functionality and cell specificity of the 1.3 kb promoter, a series of 5'-deleted fragments were ligated in front of the luciferase gene and the constructs were transfected into 3T3-L1 adipocytes. The reporter gene was effectively transcribed, as demonstrated by the expression of enzyme activity. The 5'-end of the human cDNA was completed by 5'-RACE-PCR. Several alternative transcription start sites were detected by RNase protection assay and primer extension analysis. In addition, an exon/intron boundary was mapped at the extreme 5'-end of the cDNA sequence. Genomic Southern blotting suggests that the human apM-1 gene is a single copy gene.

A tripartite array of transcription factor binding sites mediates cAMP induction of phosphoenolpyruvate carboxykinase gene transcription and its inhibition by insulin

Transcription of the phosphoenolpyruvate carboxykinase (PEPCK) gene is induced upon activation of protein kinase A by cAMP and phosphorylation of Ser-133 in the transcription factor, cAMP-response element binding protein (CREB), and this induction is inhibited by insulin. We show here that insulin does not act by dephosphorylating CREB or by affecting heterologous kinases that phosphorylate Ser-129 or Ser-142 in CREB. In addition, insulin inhibition of minimal PEPCK promoter activity induced by CREB-GAL4 + protein kinase A was equivalent to inhibition of basal transcription, and thus cAMP-independent. On the other hand, nearly complete insulin inhibition is observed with the full PEPCK promoter (-600/+69), indicating that other factors are involved. The additional promoter elements required for induction by protein kinase A lie within -271 nucleotides of the start site and correspond to putative binding sites for activator protein-1 and CAAT/enhancer-binding protein (C/EBP), first identified by Roesler et al. (Roesler, W. J., McFie, P. J., and Puttick, D. M., (1993) J. Biol. Chem. 268, 3791-3796). This tripartite array of binding sites for CREB, C/EBP, and activator protein-1 (AP-1) factors forms a cAMP response unit that, together with the minimal promoter, can mediate both induction by cAMP and inhibition by insulin. Thus, for the PEPCK gene with a single CREB site, the CREB.CBP.RNA polymerase II complex cannot mediate either induction by cAMP or inhibition by insulin.

Assessment of the roles of mitogen-activated protein kinase, phosphatidylinositol 3-kinase, protein kinase B, and protein kinase C in insulin inhibition of cAMP-induced phosphoenolpyruvate carboxykinase gene transcription

Transcription of the phosphoenolpyruvate carboxykinase (PEPCK) gene is induced by glucagon, acting through cAMP and protein kinase A, and this induction is inhibited by insulin. Conflicting reports have suggested that insulin inhibits induction by cAMP by activating the Ras/mitogen-activated protein kinase (MAPK) pathway or by activating the phosphatidylinositol 3-kinase (PI3-kinase), but not MAPK, pathway. Insulin activated PI3-kinase phosphorylates lipids that activate protein kinase B (PKB) and Ca2+/diacylglycerol-insensitive forms of protein kinase C (PKC). We have assessed the roles of these pathways in insulin inhibition of cAMP/PKA-induced transcription of PEPCK by using dominant negative and dominant active forms of regulatory enzymes in the Ras/MAPK and PKB pathways and chemical inhibitors of PKC isoforms. Three independently acting inhibitory enzymes of the Ras/MAPK pathway, blocking SOS, Ras, and MAPK, had no effect upon insulin inhibition. However, dominant active Ras prevented induction of PEPCK and also stimulated transcription mediated by Elk, a MAPK target. Insulin did not stimulate Elk-mediated transcription, indicating that insulin did not functionally activate the Ras/MAPK pathway. Inhibitors of PI3-kinase, LY294002 and wortmannin, abolished insulin inhibition of PEPCK gene transcription. However, inhibitors of PKC and mutated forms of PKB, both of which are known downstream targets of PI3-kinase, had no effect upon insulin inhibition. Dominant negative forms of PKB did not interfere with insulin inhibition and a dominant active form of PKB did not prevent induction by PKA. Phorbol ester-mediated inhibition of PEPCK transcription was blocked by bisindole maleimide and by staurosporine, but insulin-mediated inhibition was unaffected. Thus, insulin inhibition of PKA-induced PEPCK expression does not require MAPK activation but does require activation of PI3-kinase, although this signal is not transmitted through the PKB or PKC pathways.

Characterization of CCAAT/enhancer-binding protein alpha as a cyclic AMP-responsive nuclear regulator

The alpha isoform of CCAAT/enhancer-binding protein (C/EBPalpha) is a transcription factor that regulates expression of genes linked to adipose differentiation and hepatic nutrient metabolism. Recently, our laboratory has characterized a role for C/EBPalpha in mediating hormonal responsiveness. For example, the cAMP responsiveness of the phosphoenolpyruvate carboxykinase gene promoter in liver requires synergism among the cAMP response element-binding protein (CREB), C/EBPalpha, and activator protein-1. In the present study, we show that C/EBPalpha can functionally substitute for CREB in this cAMP response unit, i.e. cAMP responsiveness can occur in the absence of CREB. This observation is physiologically relevant since both CREB and C/EBPalpha have been shown to bind with high affinity to the cAMP response element in this particular promoter. Structure/function analysis of C/EBPalpha identified specific mutations that differentially affected its constitutive and protein kinase A-inducible activities. This finding suggests that the mechanism whereby C/EBPalpha mediates constitutive transactivation is distinct from that whereby it mediates cAMP responsiveness. These data support the hypothesis that C/EBPalpha plays a critical role in metabolism, in part by participating in the hormonal regulation of expression of metabolically important genes.

Glucocorticoids are insufficient for neonatal gene induction in the liver

Glucocorticoids and their receptor (GR) play a key role in perinatal gene induction. In the liver, the GR is essential for the neonatal induction of a number of genes, including that coding for tyrosine aminotransferase (TAT). To assess the function of the GR in the perinatal period, we have compared the activity of two types of glucocorticoid responsive elements in transgenic mice; one is the Tat gene glucocorticoid-responsive unit (GRU), an assembly of numerous binding sites for transcription factors, including the GR; the other is a simple dimer of high-affinity GR binding sites (GREs). Both elements confer strong glucocorticoid response in the adult liver. However, only the Tat GRUs are able to promote neonatal induction; the GRE dimer is unresponsive. Because this dimer is responsive to glucocorticoid administration in the neonate, the absence of neonatal induction is not due to the inactivity of the GR at this stage. At birth, the neonate has to withstand a brief period of starvation and hypoglycemia, a nutritional and hormonal situation that resembles fasting in the adult. In transgenic mice, the responses at birth and after fasting in the adult are similar: the Tat GRUs but not the dimeric GREs are activated. Our results show that, in rodents, glucocorticoids are not sufficient for neonatal gene induction in the liver and support the conclusion that the hypoglycemia at birth is the main trigger for expression.

Regulation of CCAAT/enhancer binding protein alpha (C/EBP alpha) gene expression by thiazolidinediones in 3T3-L1 adipocytes

Thiazolidinediones are a class of antidiabetic drugs that induce preadipocyte differentiation by binding and activating peroxisome proliferator-activated receptor gamma 2. Although thiazolidinediones are commonly thought of as insulin-sensitizing agents, these drugs have opposing and antagonistic effects to that of insulin on CCAAT/enhancer binding protein alpha (C/EBP alpha) gene expression in fully differentiated 3T3-L1 adipocytes. Thiazolidinediones induce expression of C/EBP alpha mRNA and protein, while insulin stimulates a rapid decline in C/EBP alpha mRNA and protein. When added in combination, thiazolidinediones block the suppression of C/EBP alpha mRNA by insulin; however, thiazolidinediones do not block the insulin-induced decline in GLUT4 mRNA, indicating that repression of C/EBP alpha mRNA is not required for insulin to suppress expression of a C/EBP alpha-responsive gene such as GLUT4. Instead, insulin may regulate GLUT4 mRNA by inactivating C/EBP alpha through dephosphorylation as well as by inducing the expression of the dominant-negative form of C/EBP beta (liver inhibitory protein), since both of these processes occur in the presence of thiazolidinediones.

Cloning and characterization of the promoter for the liver isoform of the rat carnitine palmitoyltransferase I (L-CPT I) gene

Carnitine palmitoyltransferase I (CPTI) catalyses the transfer of long chain fatty acids to carnitine for translocation across the mitochondrial inner membrane. The cDNAs of two isoforms of CPT I, termed the hepatic and muscle isoforms, have been cloned. Expression of the hepatic CPT I gene (L-CPT I) is subject to developmental, hormonal and tissue specific regulation. We have cloned the promoter of the L-CPTI gene from a rat genomic library. In the L-CPTI gene, there are two exons 5' to the exon containing the ATG that initiates translation. Exon 1 and the 5' end of exon 2 contain sequences that were not previously described in the rat L-CPTI cDNA. There is an alternatively spliced form of the L-CPTI mRNA in which exon 2 is skipped. The proximal promoter of the L-CPTI gene is extremely GC rich and does not contain a TATA box. There are several putative Sp1 binding sites near the transcriptional start site. A 190 base pair fragment of the promoter can efficiently drive transcription of luciferase and CAT (chloramphenicol acetyltransferase) reporter genes transiently transfected into HepG2 cells. Sequences in both the first intron and the promoter contribute to basal expression. Our results provide the foundation for further studies into the regulation of L-CPTI gene expression.

Hormone response units: one plus one equals more than two

The transcription rate of many genes, and particularly those which code for metabolically important proteins, is regulated by various hormones. Detailed analysis of the promoters of these genes has shown that, while functional 'Hormone response elements' exist, the hormonal responsiveness of many promoters is often synergistically mediated by several cis-elements, collectively referred to as a hormone response unit. The utilization of a hormone response unit to mediate a response offers several regulatory advantages, including an expansion of the range of transcriptional responses and modulation of the response by tissue- and developmental-specific cues. Furthermore, the presence of Hormone Response Units may provide a mechanism for the coordination of information from two or more signaling pathways into a single, integrated and exquisitely controlled transcriptional response. The protein-protein interactions that likely mediate many of the synergistic functional characteristics of Hormone Response Units may provide unique targets for therapeutic intervention.

Cell-specific regulation of transcription of the malic enzyme gene: characterization of cis-acting elements that modulate nuclear T3 receptor activity

Stimulation of malic enzyme transcription by triiodothyronine (T3) is robust (> 60-fold) in chick embryo hepatocytes, weak (5-fold) in chick embryo fibroblasts that stably overexpress the nuclear T3 receptor-alpha, and still weaker (1-fold) in chick embryo fibroblasts which contain nuclear T3 receptor levels that are similar to those of chick embryo hepatocytes. Using DNase I hypersensitivity, functional transfection, and in vitro DNA-binding analyses, four cis-acting elements were identified in the malic enzyme 5'-flanking DNA that conferred differences in nuclear T3 receptor activity between chick embryo hepatocytes and chick embryo fibroblasts. These cell-specific regulatory elements are located at -3895/-3890, -3761/-3744, -3703/-3686, and -3474/-2715 bp and overlap with DNase I hypersensitive sites that are observed in chromatin of chick embryo hepatocytes. Each element enhances T3 responsiveness of the malic enzyme promoter in chick embryo hepatocytes but has no effect on T3 responsiveness in chick embryo fibroblasts. Three of the cell-specific regulatory elements flank a previously identified DNA fragment (-3889 to -3769 bp; Hodnett et al., Arch. Biochem. Biophys. 334, 309-324, 1996) that contains one major and four minor T3 response elements. The cell-specific regulatory element at -3703/-3686 bp binds to the liver-enriched factor, CCAAT/enhancer-binding protein-alpha, whereas cell-specific regulatory elements at -3895/-3890 and -3761/-3744 bp bind proteins of unknown identity. While the cell-specific regulatory element at -3761/-3744 bp contains sequences that resemble binding sites for CCAAT/enhancer-binding protein, activator protein-1, cyclic AMP response element binding protein, and NF-1, none of these proteins appear to bind to this DNA fragment. These data suggest that cell-specific differences in T3 responsiveness of the malic enzyme gene are mediated in large part by nonreceptor proteins that augment the transcriptional activity of the nuclear T3 receptor in hepatocytes.

Signaling pathways through which insulin regulates CCAAT/enhancer binding protein alpha (C/EBPalpha) phosphorylation and gene expression in 3T3-L1 adipocytes. Correlation with GLUT4 gene expression

Treatment of 3T3-L1 adipocytes with insulin (IC50 approximately 200 pM insulin) or insulin-like growth factor-1 (IC50 approximately 200 pM IGF-1) stimulates dephosphorylation of CCAAT/enhancer binding protein alpha (C/EBPalpha), a transcription factor involved in preadipocyte differentiation. As assessed by immunoblot analysis of one- and two-dimensional PAGE, insulin appears to dephosphorylate one site within p30C/EBPalpha and an additional site within p42C/EBPalpha. Consistent with insulin causing dephosphorylation of C/EBPalpha through activation of phosphatidylinositol 3-kinase, addition of phosphatidylinositol 3-kinase inhibitors (e.g. wortmannin) blocks insulin-stimulated dephosphorylation of C/EBPalpha. In the absence of insulin, wortmannin or LY294002 enhance C/EBPalpha phosphorylation. Similarly, blocking the activity of FKBP-rapamycin-associated protein with rapamycin increases phosphorylation of C/EBPalpha in the absence of insulin. Dephosphorylation of C/EBPalpha by insulin is partially blocked by rapamycin, consistent with a model in which activation of FKBP-rapamycin-associated protein by phosphatidylinositol 3-kinase results in dephosphorylation of C/EBPalpha. The dephosphorylation of C/EBPalpha by insulin, in conjunction with the insulin-dependent decline in C/EBPalpha mRNA and protein, has been hypothesized to play a role in repression of GLUT4 transcription by insulin. Consistent with this hypothesis, the decline of GLUT4 mRNA following exposure of adipocytes to insulin correlates with dephosphorylation of C/EBPalpha. However, the repression of C/EBPalpha mRNA and protein levels by insulin is blocked with an inhibitor of the mitogen-activated protein kinase pathway without blocking the repression of GLUT4 mRNA, thus dissociating the regulation of C/EBPalpha and GLUT4 mRNAs by insulin. A decline in C/EBPalpha mRNA and protein may not be required to suppress GLUT4 transcription because insulin also induces expression of the dominant-negative form of C/EBPbeta (liver inhibitory protein), which blocks transactivation by C/EBP transcription factors.

Luman, a new member of the CREB/ATF family, binds to herpes simplex virus VP16-associated host cellular factor

The human host cell factor (HCF) is expressed in a variety of adult and fetal tissues, and its gene is conserved in animals as diverse as mammals and insects. However, its only known function is to stabilize the herpes simplex virus virion transactivator VP16 in a complex with the cellular POU domain protein Oct-1 and cis-acting regulatory elements in promoters of immediate-early viral genes. To identify a cellular function for HCF, we used the yeast two-hybrid system to identify a cellular ligand for HCF. This protein, Luman, appears to be a cyclic AMP response element (CRE)-binding protein/activating transcription factor 1 protein of the basic leucine zipper superfamily. It binds CREs in vitro and activates CRE-containing promoters when transfected into COS7 cells. This activation of transcription was synergistically enhanced by the presence of CCAAT/enhancer-binding protein elements and inhibited by AP-1 elements in the promoter. In addition to a basic DNA binding domain, Luman possesses an unusually long leucine zipper and an acidic amino-terminal activation domain. These features in Luman are also present in what appear to be homologs in the mouse, Drosophila melanogaster, and Caenorhabditis elegans. Luman and VP16 appear to have similar mechanisms for binding HCF, as in vitro each competitively inhibited the binding of the other to HCF. In transfected cells, however, while VP16 strongly inhibited the ability of GAL-Luman to activate transcription from a GAL4 upstream activation sequence-containing promoter, Luman was unable to inhibit the activity of GAL-VP16. Luman appears to be a ubiquitous transcription factor, and its mRNA was detected in all human adult and fetal tissues examined. The possible role of HCF in regulating the function of this ubiquitous transcription factor is discussed.

C/EBP factor suppression of inhibition of type II secreted phospholipase A2 promoter in HepG2 cells: possible role of single-strand binding proteins

We previously reported that the type II secreted phospholipase A2 (sPLA2) promoter from positions (-326 to +20) ([-326;+20] promoter) is negatively regulated by two adjacent regulatory elements, C (-210 to -176) and D (-247 to -210). This study examines in greater detail the way in which this negative regulation operates. Successive 5' deletions of the [-326;+20] type II sPLA2 promoter indicated that the region upstream of position -195 inhibits the transcription activity sixfold in HepG2 cells but not in HeLa cells. Although the whole [-326;-176] region decreased the activity of a heterologous thymidine kinase promoter, this effect was orientation and position sensitive. C/EBP beta, C/EBP alpha, and C/EBP delta, which bind to element C, prevented the inhibition of promoter activity. Electrophoretic mobility shift experiments identified the binding of NF1-like proteins to the [-225;-218] site, which overlaps an insulin response-like sequence, 5'-TGTTTTG-3'. This sequence bound a factor which also recognized the promoters of the apolipoproteins C-III and A-II. Substitutions preventing the binding of this factor or the NF1-like proteins did not increase the transcription activity, but substitution in the [-217;-204] sequence blocked the transcription inhibition. This sequence did not bind any double-strand binding factor, but its antisense strand is critical for the binding of single-strand binding proteins to the [-232;-191] region. We therefore suggest that these single-strand binding proteins are involved in the inhibitory mechanism.

CREB controls LAP/C/EBP beta transcription

LAP/C/EBP beta is a member of the C/EBP family of transcription factors and is involved in hepatocyte-specific gene expression. Recently we showed that, besides its posttranscriptional regulation, LAP/C/EBP beta mRNA is modulated during liver regeneration. Therefore, in this study we investigated mechanisms which control LAP/C/EBP beta gene transcription. Deletion analysis of the 5'-flanking region, located upstream of the start site of transcription in the LAP/C/EBP beta gene, demonstrated that a small region in close proximity to the TATA box is important in maintaining a high level of transcription of the luciferase reporter gene constructs. In gel shift experiments two sites were identified which are important for specific complex formation within this region. Further analysis by cross-linking, super shift, and competition experiments was performed with liver cell nuclear extracts, hepatoma cell nuclear extracts, or recombinant CREB protein. These experiments conclusively demonstrated that CREB binds to both sites in the LAP/C/EBP beta promoter with an affinity similar to that with the CREB consensus sequence. Transfection experiments with promoter constructs where the CREB sites were mutated showed that these sites are important to maintain both basal promoter activity and LAP/C/EBP beta inducibility through CREB. Northern blot analysis and runoff transcription assays demonstrated that the protein kinase A pathway not only stimulated the activity of the luciferase reporter construct but also the transcription of the endogenous LAP/C/EBP beta gene in different cell types. Western blot analysis of rat liver cell nuclear extracts and runoff transcription assays of rat liver cell nuclei after two-thirds hepatectomy showed a functional link between the induction of CREB phosphorylation and LAP/C/EBP beta mRNA transcription during liver regeneration. These results demonstrate that the two CREB sites are important to control LAP/C/EBP beta transcription in vivo. As several pathways control CREB phosphorylation, our results provide evidence for the transcriptional regulation of LAP/C/EBP beta via CREB under different physiological conditions.

Differential regulation of the mitogen-activated protein and stress-activated protein kinase cascades by adrenergic agonists in quiescent and regenerating adult rat hepatocytes

To study the mechanisms by which catecholamines regulate hepatocyte proliferation after partial hepatectomy (PHX), hepatocytes were isolated from adult male rats 24 h after sham operation or two-thirds PHX and treated with catecholamines and other agonists. In freshly isolated sham cells, p42 mitogen-activated protein (MAP) kinase activity was stimulated by the alpha1-adrenergic agonist phenylephrine (PHE). Activation of p42 MAP kinase by growth factors was blunted by pretreatment of sham hepatocytes with glucagon but not by that with the beta2-adrenergic agonist isoproterenol (ISO). In PHX cells, the ability of PHE to activate p42 MAP kinase was dramatically reduced, whereas ISO became competent to inhibit p42 MAP kinase activation. PHE treatment of sham but not PHX and ISO treatment of PHX but not sham hepatocytes also activated the stress-activated protein (SAP) kinases p46/54 SAP kinase and p38 SAP kinase. These data demonstrate that an alpha1- to beta2-adrenergic receptor switch occurs upon PHX and results in an increase in SAP kinase versus MAP kinase signaling by catecholamines. In primary cultures of hepatocytes, ISO treatment of PHX but not sham cells inhibited [3H]thymidine incorporation. In contrast, PHE treatment of sham but not PHX cells stimulated [3H]thymidine incorporation, which was reduced by approximately 25 and approximately 95% with specific inhibitors of p42 MAP kinase and p38 SAP kinase function, respectively. Inhibition of the p38 SAP kinase also dramatically reduced basal [3H]thymidine incorporation. These data suggest that p38 SAP kinase plays a permissive role in liver regeneration. Alterations in the abilities of catecholamines to modulate the activities of protein kinase A and the MAP and SAP kinase pathways may represent one physiological mechanism by which these agonists can regulate hepatocyte proliferation after PHX.

CCAAT-enhancer-binding protein alpha (C/EBP alpha) is required for the thyroid hormone but not the retinoic acid induction of phosphoenolpyruvate carboxykinase (PEPCK) gene transcription

Transcription of the gene for phosphoenolpyruvate carboxykinase (PEPCK) is stimulated by cAMP, the thyroid hormone tri-iodothyronine (T3) and retinoic acid (RA). Regulation of PEPCK transcription by T3 involves two sites in the promoter including a thyroid-hormone-response element (TRE) and a CCAAT-enhancer-binding protein (C/EBP) binding site called P3I. Mutation of either the TRE or P3I eliminates the T3 response. In this study, we examined the role of C/EBPs in the induction of PEPCK transcription by T3 and RA. PEPCK-CAT vectors were transfected into HepG2 cells. Co-transfection of a dominant negative C/EBP eliminated the T3 stimulation indicating that a member of the C/EBP family is required. To determine which C/EBP isoform was required, Gal4 fusion proteins were created that contained the Gal4 DNA-binding domain ligated to the transcriptional activation domain of C/EBP alpha, C/EBP beta or the cAMP-responsive-element-binding protein. A Gal4 DNA-binding site was introduced into the P3(I) site of the PEPCK-CAT vector. Only co-transfection of the Gal4-C/EBP alpha vector was able to restore T3 responsiveness to the PEPCK-CAT vector. The T3 and RA receptors are members of the nuclear receptor superfamily and bind to repeats of the AGGTCA motif. We found that the RA receptor can bind to sequences within the PEPCK-TRE and contribute to RA responsiveness of the PEPCK gene. However, the RA induction of PEPCK transcription was found to be independent of C/EBPs, further demonstrating the specificity of the involvement of C/EBP alpha in the T3 effect.

The promoter regulatory regions of the genes for the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) from the chicken and the rat have different species-specific roles in gluconeogenesis

Hepatic expression of the gene for phosphoenolpyruvate carboxykinase (GTP) (PEPCK-C) (EC 4.1.1.32) in birds occurs prior to birth and decreases to negligible levels before hatching, whereas in mammals the gene for PEPCK-C in the liver is expressed at birth and is active throughout the life of the animal. The administration of cyclic AMP to adult chickens results in the induction of transcription of the gene for PEPCK-C and the transient accumulation of PEPCK-C mRNA in the liver. DNase I footprint analysis of 330 bp of the avian PEPCK-C promoter immediately 5' of the start-site of transcription indicated the presence of several protein binding domains, purified CAAT/enhancer binding protein alpha, cAMP regulatory element binding protein and nuclear factor-1 bound to these regions of the promoter. Sequences corresponding to an hepatic nuclear factor-1 binding domain and to the insulin response sequence, previously identified in the rat PEPCK-C promoter, were also found in the chicken PEPCK-C promoter. Co-transfection of an expression vector for CAAT/enhancer binding protein alpha or CAAT/enhancer binding protein beta markedly stimulated transcription from both the chicken and rat PEPCK-C promoters in human hepatoma cells. Sequences involved in the regulation of gene transcription by cyclic AMP and insulin were found to reside between -210 and +1 of the avian PEPCK-C promoter. In general, transcription from the avian promoter was more sensitive to inhibition by insulin than was noted for the rat PEPCK-C promoter, which may explain in part the lack of expression of the gene for PEPCK-C in the livers of adult birds.

Regulation of phosphoenolpyruvate carboxykinase (GTP) gene expression

Phosphoenolpyruvate carboxykinase (GTP) (EC 4.1.1.32) (PEPCK) is a key enzyme in the synthesis of glucose in the liver and kidney and of glyceride-glycerol in white adipose tissue and the small intestine. The gene for the cytosolic form of PEPCK (PEPCK-C) is acutely regulated by a variety of dietary and hormonal signals, which result in alteration of synthesis of the enzyme. Major factors that increase PEPCK-C gene expression include cyclic AMP, glucocorticoids, and thyroid hormone, whereas insulin inhibits this process. PEPCK-C is absent in fetal liver but appears at birth, concomitant with the capacity for gluconeogenesis. Regulatory elements that control transcription of the PEPCK-C gene in liver, kidney, and adipose tissue have been delineated, and many of the transcription factors that bind to these elements have been identified. Transgenic mice have been especially useful in elucidating the physiological roles of specific sequence elements in the PEPCK-C gene promoter and in demonstrating the key role played at these sites by the isoforms of CAAT/enhancer binding protein in patterning of PEPCK-C gene expression during the perinatal period. The PEPCK-C gene provides a model for the metabolic control of gene transcription.