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

Dioxin increases C/EBPbeta transcription by activating cAMP/protein kinase A

The environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD = dioxin) has been shown to increase the expression of C/EBPbeta. The modulated expression of C/EBPbeta has been suggested to be associated with toxic responses of TCDD such as wasting syndrome, diabetes, and inhibition of adipocyte differentiation. This study focused on the regulatory mechanism of TCDD-mediated transcriptional activation of C/EBPbeta. Elevated C/EBPbeta mRNA and protein levels in mouse embryonic fibroblasts (C3H10T(1/2)) and in mouse hepatoma cells (Hepa1c1c7) were correlated with increased binding affinity of the C/EBPbeta protein. Transfection studies with different deletion constructs of the CCAAT/enhancer-binding protein promoter indicated that a small region located 60-120 bp upstream of the start site of transcription is required for activation of the C/EBPbeta gene by TCDD in both cell lines tested. Further analysis using mutation constructs of the C/EBPbeta promoter demonstrated that activation of the C/EBPbeta promoter is mediated through incomplete cAMP-response element-binding protein (CREB) sites located close to the TATA box of the C/EBPbeta gene. The protein kinase A (PKA) inhibitor H89 completely blocks the TCDD-dependent effect on C/EBPbeta promoter activity, indicating that TCDD activates CREB binding via a cAMP/PKA pathway, which is supported by the increased cAMP level and PKA activity observed after TCDD treatment. Gel shift analyses demonstrated that CREB itself binds to the putative CREB motif that mediates the TCDD-dependent effect on C/EBPbeta gene transcription. Cotransfection experiments with CREB and PKA expression plasmids further supported our conclusions that the TCDD-dependent effect on C/EBPbeta transcription is mediated via PKA-dependent CREB activation.

Induction of CHOP expression by amino acid limitation requires both ATF4 expression and ATF2 phosphorylation

The CHOP gene is transcriptionally induced by amino acid starvation. We have previously identified a genomic cis-acting element (amino acid response element (AARE)) involved in the transcriptional activation of the human CHOP gene by leucine starvation and shown that it binds the activating transcription factor 2 (ATF2). The present study was designed to identify other transcription factors capable of binding to the CHOP AARE and to establish their role with regard to induction of the gene by amino acid deprivation. Electrophoretic mobility shift assay and transient transfection experiments show that several transcription factors that belong to the C/EBP or ATF families bind the AARE sequence and activate transcription. Among all these transcription factors, only ATF4 and ATF2 are involved in the amino acid control of CHOP expression. We show that inhibition of ATF2 or ATF4 expression impairs the transcriptional activation of CHOP by amino acid starvation. The transacting capacity of ATF4 depends on its expression level and that of ATF2 on its phosphorylation state. In response to leucine starvation, ATF4 expression and ATF2 phosphorylation are increased. However, induction of ATF4 expression by the endoplasmic reticulum stress pathway does not fully activate the AARE-dependent transcription. Taken together our results demonstrate that at least two pathways, one leading to ATF4 induction and one leading to ATF2 phosphorylation, are necessary to induce CHOP expression by amino acid starvation. This work was extended to the regulation of other amino acid regulated genes and suggests that ATF4 and ATF2 are key components of the amino acid control of gene expression.

Hypoxia-induced gene expression in human macrophages: implications for ischemic tissues and hypoxia-regulated gene therapy

Macrophages accumulate in ischemic areas of such pathological tissues as solid tumors, atherosclerotic plaques and arthritic joints. Studies have suggested that hypoxia alters the phenotype of macrophages in a way that promotes these lesions. However, the genes up-regulated by macrophages in such hypoxic tissues are poorly characterized. Here, we have used cDNA array hybridization to investigate the effects of hypoxia on the mRNAs of 1185 genes in primary human monocyte-derived macrophages. As shown previously in other cell types, mRNA levels for vascular endothelial growth factor (VEGF) and glucose transporter 1 (GLUT-1) were up-regulated by hypoxia. However, the mRNAs of other genes were also up-regulated including matrix metalloproteinase-7 (MMP-7), neuromedin B receptor, and the DNA-binding protein inhibitor, Id2. The promoters of GLUT-1 and MMP-7 confer hypoxic inducibility on a reporter gene in RAW 264.7 macrophages, indicating that the hypoxic up-regulation of these mRNAs may occur, at least in part, at the transcriptional level. GLUT-1 and MMP-7 mRNA were also shown to be up-regulated in hypoxic macrophages in vitro by real-time RT-PCR, and these proteins were elevated in hypoxic macrophages in vitro and in hypoxic areas of human breast tumors. The hypoxia up-regulated genes identified could be important for the survival and functioning of macrophages in hypoxic diseased tissues, and their promoters could prove useful in macrophage-delivered gene therapy.

CCAAT/enhancer-binding protein family members recruit the coactivator CREB-binding protein and trigger its phosphorylation

CCAAT/enhancer-binding protein (C/EBP) family members are transcription factors involved in important physiological processes, such as cellular proliferation and differentiation, regulation of energy homeostasis, inflammation, and hematopoiesis. Transcriptional activation by C/EBPalpha and C/EBPbeta involves the coactivators CREB-binding protein (CBP) and p300, which promote transcription by acetylating histones and recruiting basal transcription factors. In this study, we show that C/EBPdelta is also using CBP as a coactivator. Based on sequence homology with C/EBPalpha and -beta, we identify in C/EBPdelta two conserved amino acid segments that are necessary for the physical interaction with CBP. Using reporter gene assays, we demonstrate that mutation of these residues prevents CBP recruitment and diminishes the transactivating potential of C/EBPdelta. In addition, our results indicate that C/EBP family members not only recruit CBP but specifically induce its phosphorylation. We provide evidence that CBP phosphorylation depends on its interaction with C/EBPdelta and define point mutations within one of the two conserved amino acid segments of C/EBPdelta that abolish CBP phosphorylation as well as transcriptional activation, suggesting that this new mechanism could be important for C/EBP-mediated transcription.

Common gene polymorphisms and nutrition: emerging links with pathogenesis of multifactorial chronic diseases (review)

Rapid progress in human genome decoding has accelerated search for the role of gene polymorphisms in the pathogenesis of complex multifactorial diseases. This review summarizes the results of recent studies on the associations of common gene variants with multifactorial chronic conditions strongly affected by nutritional factors. Three main individual sections discuss genes related to energy homeostasis regulation and obesity, cardiovascular disease (CVD), and cancer. It is evident that several major chronic diseases are closely related (often through obesity) to deregulation of energy homeostasis. Multiple polymorphic genes encoding central and peripheral determinants of energy intake and expenditure have been revealed over the past decade. Food intake control may be affected by polymorphisms in the genes encoding taste receptors and a number of peripheral signaling peptides such as insulin, leptin, ghrelin, cholecystokinin, and corresponding receptors. Polymorphic central regulators of energy intake include hypothalamic neuropeptide Y, agouti-related protein, melanocortin pathway factors, CART (cocaine- and amphetamine-regulated transcript), some other neuropeptides, and receptors for these molecules. Potentially important polymorphisms in the genes encoding energy expenditure modulators (alpha- and beta- adrenoceptors, uncoupling proteins, and regulators of adipocyte growth and differentiation) are also discussed. CVD-related gene polymorphisms comprising those involved in the pathogenesis of atherosclerosis, blood pressure regulation, hemostasis control, and homocysteine metabolism are considered in a separate section with emphasis on multiple polymorphisms affecting lipid transport and metabolism and their interactions with diet. Cancer-associated polymorphisms are discussed for groups of genes encoding enzymes of xenobiotic metabolism, DNA repair enzymes, factors involved in the cell cycle control, hormonal regulation-associated proteins, enzymes related to DNA methylation through folate metabolism, and angiogenesis-related factors. There is an apparent progress in the field with hundreds of new gene polymorphisms discovered and characterized, however firm evidence consistently linking them with pathogenesis of complex chronic diseases is still limited. Ways of improving the efficiency of candidate gene approach-based studies are discussed in a short separate section. Successful unraveling of interaction between dietary factors, polymorphisms, and pathogenesis of several multifactorial diseases is exemplified by studies of folate metabolism in relation to CVD and cancer. It appears that several new directions emerge as targets of research on the role of genetic variation in relation to diet and complex chronic diseases. Regulation of energy homeostasis is a fundamental problem insufficiently investigated in this context so far. Impacts of genetic variation on systems controlling angiogenesis, inflammatory reactions, and cell growth and differentiation (comprising regulation of the cell cycle, DNA repair, and DNA methylation) are also largely unknown and need thorough analysis. These goals can be achieved by complex simultaneous analysis of multiple polymorphic genes controlling carefully defined and selected elements of relevant metabolic and regulatory pathways in meticulously designed large-scale studies.

Transcription cooperation by NFAT.C/EBP composite enhancer complex

The nuclear factor of activated T cells (NFAT) group of transcription factors regulates gene expression in immune and non-immune cells. NFAT-mediated gene transcription is orchestrated, in part, by formation of a composite regulatory element. Here we demonstrate that NFAT interacts with transcription factor CCAAT/enhancer-binding protein (C/EBP) to form a composite enhancer complex, to potentiate expression of the peroxisome proliferator-activated receptor-gamma2 gene. Formation of a ternary NFAT.C/EBP.DNA complex is required for the transcriptional cooperation. A similar NFAT.C/EBP composite element is found in the regulatory region of the insulin-like growth factor 2, angiotensin-converting enzyme homolog, and transcription factor POU4F3 genes. Thus, the NFAT.C/EBP composite element represents a novel regulatory enhancer to direct NFAT-mediated gene transcription.

The role of C/EBPbeta in mammary gland development and breast cancer

The CCAAT/enhancer binding protein (C/EBP) family of bZIP transcription factors control the proliferation and differentiation of a variety of tissues. While C/EBPalpha and -delta are also expressed in the mammary gland, the multiple protein isoforms of C/EBPbeta appear to play a critical role in mammary gland development and breast cancer. Targeted deletion of all the C/EBPbeta isoforms results in a severe inhibition of lobuloalveolar development and a block to functional differentiation, as well as more subtle changes in ductal morphogenesis. The altered expression of a number of molecular markers, including the progesterone, estrogen, and prolactin receptors, the transporter proteins (NKCC1 and aquaporin 5), and several markers of skin differentiation (Sprr2A and keratin 6), suggests that germline deletion of C/EBPbeta results in an altered cell fate. Thus, C/EBPbeta appears to play a role in the specification of progenitor cell fate not only in the mammary gland, but also in a number of other tissues.

Mitochondrial threshold effects

The study of mitochondrial diseases has revealed dramatic variability in the phenotypic presentation of mitochondrial genetic defects. To attempt to understand this variability, different authors have studied energy metabolism in transmitochondrial cell lines carrying different proportions of various pathogenic mutations in their mitochondrial DNA. The same kinds of experiments have been performed on isolated mitochondria and on tissue biopsies taken from patients with mitochondrial diseases. The results have shown that, in most cases, phenotypic manifestation of the genetic defect occurs only when a threshold level is exceeded, and this phenomenon has been named the 'phenotypic threshold effect'. Subsequently, several authors showed that it was possible to inhibit considerably the activity of a respiratory chain complex, up to a critical value, without affecting the rate of mitochondrial respiration or ATP synthesis. This phenomenon was called the 'biochemical threshold effect'. More recently, quantitative analysis of the effects of various mutations in mitochondrial DNA on the rate of mitochondrial protein synthesis has revealed the existence of a 'translational threshold effect'. In this review these different mitochondrial threshold effects are discussed, along with their molecular bases and the roles that they play in the presentation of mitochondrial diseases.

A thyroid hormone response unit formed between the promoter and first intron of the carnitine palmitoyltransferase-Ialpha gene mediates the liver-specific induction by thyroid hormone

Carnitine palmitoyltransferase-I (CPT-I) catalyzes the rate-controlling step of fatty acid oxidation. CPT-I converts long-chain fatty acyl-CoAs to acylcarnitines for translocation across the mitochondrial membrane. The mRNA levels and enzyme activity of the liver isoform, CPT-Ialpha, are greatly increased in the liver of hyperthyroid animals. Thyroid hormone (T3) stimulates CPT-Ialpha transcription far more robustly in the liver than in non-hepatic tissues. We have shown that the thyroid hormone receptor (TR) binds to a thyroid hormone response element (TRE) located in the CPT-Ialpha promoter. In addition, elements in the first intron participate in the T3 induction of CPT-Ialpha gene expression, but the CPT-Ialpha intron alone cannot confer a T3 response. We found that deletion of sequences in the first intron between +653 and +744 decreased the T3 induction of CPT-Ialpha. Upstream stimulatory factor (USF) and CCAAT enhancer binding proteins (C/EBPs) bind to elements within this region, and these factors are required for the T3 response. The binding of TR and C/EBP to the CPT-Ialpha gene in vivo was shown by the chromatin immunoprecipitation assay. We determined that TR can physically interact with USF-1, USF-2, and C/EBPalpha. Transgenic mice were created that carry CPT-Ialpha-luciferase transgenes with or without the first intron of the CPT-Ialpha gene. In these mouse lines, the first intron is required for T3 induction as well as high levels of hepatic expression. Our data indicate that the T3 stimulates CPT-Ialpha gene expression in the liver through a T3 response unit consisting of the TRE in the promoter and additional factors, C/EBP and USF, bound in the first intron.

cAMP-induced Interleukin-10 promoter activation depends on CCAAT/enhancer-binding protein expression and monocytic differentiation

The molecular mechanisms underlying the regulation of interleukin (IL)-10 transcription in monocytic cells by various stimuli during inflammation and the stress reaction are not fully understood. Recently, we provided evidence that stress-induced IL-10 promoter activation in monocytic cells is mediated by catecholamines via a cAMP-dependent signaling pathway including CREB/ATF (cAMP-responsive element binding protein/activating transcription factor) binding to two CRE motifs. However, the mutation of these sites diminished cAMP responsiveness by only 50%, suggesting a role for additional transcription factors and elements in the cAMP-dependent regulation of the human IL-10 promoter. Here, we analyze the functional role of one such factor, C/EBP, in two cell lines of myelomonocytic origin, THP-1 and HL-60, which are known to differ in their differentiation status and C/EBP protein content. We show that the level of basal as well as cAMP-stimulated IL-10 transcription depends on the expression of C/EBP alpha and beta and their binding to three motifs in the promoter/enhancer region. The C/EBP5 motif, which is located between the TATA-box and the translation start point, is essential for the C/EBP-mediated constitutive and most of the cAMP-stimulated expression as its mutation nearly abolished IL-10 promoter activity. Our results suggest a dominant role of C/EBP transcription factors relative to CREB/ATF in tissue-specific and differentiation-dependent IL-10 transcription.

Differences in the molecular mechanisms involved in the transcriptional activation of the CHOP and asparagine synthetase genes in response to amino acid deprivation or activation of the unfolded protein response

A promoter element called the amino acid response element (AARE), which is essential for the induction of CHOP (a CCAAT/enhancer-binding protein-related gene) transcription by amino acid depletion, has been previously characterized. Conversely, the human asparagine synthetase (AS) promoter contains two cis-acting elements termed nutrient-sensing response elements (NSRE-1 and NSRE-2) that are required to activate the gene by either amino acid deprivation or the endoplasmic reticulum stress response. The results reported here document the comparison between CHOP and AS transcriptional control elements used by the amino acid pathway. We first establish that the AS NSRE-1 sequence shares nucleotide sequence and functional similarities with the CHOP AARE. However, we demonstrate that the CHOP AARE can function independently, whereas AS NSRE-1 is functionally weak by itself and instead requires the presence of NSRE-2. Furthermore, AS NSRE-2 can confer endoplasmic reticulum stress responsiveness to the CHOP AARE. Using activating transcription factor-2-deficient mouse embryonic fibroblasts, we also show that lack of this transcription factor does not abolish the amino acid inducibility of AS transcription, but this transcription factor is necessary to obtain the full AS response to amino acid starvation. Collectively, these results document that there are significant differences in the molecular mechanisms involved in the transcriptional activation of CHOP and AS by amino acid limitation.

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.

Genistein inhibits CCAAT/enhancer-binding protein beta (C/EBPbeta) activity and 3T3-L1 adipogenesis by increasing C/EBP homologous protein expression

The tyrosine kinase inhibitor genistein inhibits 3T3-L1 adipogenesis when present during the first 72 h of differentiation. In this report, we investigated the underlying mechanisms involved in the anti-adipogenic effects of genistein. We found that genistein blocked the DNA binding and transcriptional activity of CCAAT/enhancer-binding protein beta (C/EBPbeta) during differentiation by promoting the expression of C/EBP homologous protein, a dominant-negative member of the C/EBP family. Loss of C/EBPbeta activity was manifested as a loss of differentiation-induced C/EBPalpha and peroxisome-proliferator-activated receptor gamma protein expression and a dramatic reduction in lipid accumulation. Further, we documented for the first time that C/EBPbeta was tyrosine-phosphorylated in vivo during differentiation and in vitro by activated epidermal growth factor receptor. Genistein inhibited both of these events. Collectively, these results indicate that genistein blocks adipogenesis and C/EBPbeta activity by increasing the level of C/EBP homologous protein and possibly by inhibiting the tyrosine phosphorylation of C/EBPbeta.

In vivo functional characterization of the aldolase B gene enhancer

A 400-bp intronic enhancer fragment in conjunction with the proximal promoter of the aldolase B gene provided correct tissue-specific expression in transgenic mice together with hormonal regulation in the liver. We investigated in vivo and in cultured cells the contribution of the intronic regulatory sequences and their interaction with the promoter elements in controlling aldolase B gene expression. Transgene activity was completely abolished by disruption of the two hepatocyte nuclear factor 1 (HNF1) binding sites in the enhancer, whereas mutation of one HNF1 site had no effect in the liver but strongly decreased activity in the kidney. Our data show that the HNF1 binding site(s) in the enhancer were key regulators of aldolase B transgene expression both in the liver and kidney. Deletion of the CCAAT/enhancer-binding protein site in the promoter completely abolished the enhancer function in HepG2 cells. These results suggest that expression of the aldolase B gene in the liver requires cooperative interactions between CCAAT/enhancer-binding protein and HNF1. Deletion of the HNF4 binding site in the enhancer suppressed expression in both liver and kidney in half of the transgenic lines, suggesting that this element might play a role in chromatin opening at the insertion site. We firmly establish that the endogenous aldolase B gene's first response to glucagon or cyclic AMP exposure was a transient increase in the expression in the liver, followed by a secondary decline in the transcription, as previously reported. This response was reproduced by all transgenes studied, indicating that neither HNF1 nor HNF4 binding sites in the enhancer were involved in this biphasic cyclic AMP response.

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: structure, function and regulation

CCAAT/enhancer binding proteins (C/EBPs) are a family of transcription factors that all contain a highly conserved, basic-leucine zipper domain at the C-terminus that is involved in dimerization and DNA binding. At least six members of the family have been isolated and characterized to date (C/EBP alpha[bond]C/EBP zeta), with further diversity produced by the generation of different sized polypeptides, predominantly by differential use of translation initiation sites, and extensive protein-protein interactions both within the family and with other transcription factors. The function of the C/EBPs has recently been investigated by a number of approaches, including studies on mice that lack specific members, and has identified pivotal roles of the family in the control of cellular proliferation and differentiation, metabolism, inflammation and numerous other responses, particularly in hepatocytes, adipocytes and haematopoietic cells. The expression of the C/EBPs is regulated at multiple levels during several physiological and pathophysiological conditions through the action of a range of factors, including hormones, mitogens, cytokines, nutrients and certain toxins. The mechanisms through which the C/EBP members are regulated during such conditions have also been the focus of several recent studies and have revealed an immense complexity with the potential existence of cell/tissue- and species-specific differences. This review deals with the structure, biological function and the regulation of the C/EBP family.

Long-term modulation of glucose utilization by IL-1 alpha and TNF-alpha in astrocytes: Na+ pump activity as a potential target via distinct signaling mechanisms

Interleukin-1alpha (IL-1alpha) and tumor necrosis factor-alpha (TNF-alpha) markedly stimulate glucose utilization in primary cultures of mouse cortical astrocytes. The mechanism that gives rise to this effect, which takes place several hours after application of cytokine, has remained unclear. Experiments were conducted to identify the major signaling cascades involved in the metabolic action of cytokine. First, the selective IL-1 receptor antagonist (IL-1ra) prevents the effect of IL-1alpha on glucose utilization in a concentration-dependent manner, whereas it has no effect on the action of TNF-alpha. Then, using inhibitors of three classical signaling cascades known to be activated by cytokines, it appears that the PI3 kinase is essential for the effect of both IL-1alpha and TNF-alpha, whereas the action of IL-1alpha also requires activation of the MAP kinase pathway. Participation of a phospholipase C-dependent pathway does not appear critical for both IL-1alpha and TNF-alpha. Inhibition of NO synthase by L-NAME did not prevent the metabolic response to both IL-1alpha and TNF-alpha, indicating that nitric oxide is probably not involved. In contrast, the Na(+)/K(+) ATPase inhibitor ouabain prevents the IL-1alpha- and TNF-alpha-stimulated 2-deoxyglucose (2DG) uptake. When treatment of astrocytes with a cytokine was followed 24 h later by an acute application of glutamate, a synergistic enhancement in glucose utilization was observed. This effect was greatly reduced by ouabain. These data suggest that Na(+) pump activity is a common target for both the long-term metabolic action of cytokines promoted by the activation of distinct signaling pathways and the enhanced metabolic response to glutamate.

USF2 inhibits C/EBP-mediated transcriptional regulation of the RIIbeta subunit of cAMP-dependent protein kinase

BACKGROUND

Cyclic AMP-dependent protein kinase (PKA) plays a central role in regulation of energy metabolism. Upon stimulation of testicular Sertoli cells by follicle stimulating hormone (FSH), glycolysis is activated to increase the production of nutrients for the germ cells, and a new regulatory subunit of cAMP-dependent protein kinase, RIIbeta, is induced. We have previously shown that production of the transcription factor C/EBPbeta is rapidly increased by FSH and cAMP in primary Sertoli cell cultures, and that C/EBPbeta induces the RIIbeta promoter.

RESULTS

In this work we show that USF1, USF2 and truncated USF isoforms bind to a conserved E-box in the RIIbeta gene. Interestingly, overexpression of USF2, but not USF1, led to inhibition of both cAMP- and C/EBPbeta-mediated induction of RIIbeta. Furthermore, Western blots show that a novel USF1 isoform is induced by cAMP in Sertoli cells.

CONCLUSIONS

These results indicate that the expression of various USF isoforms may be regulated by cAMP, and that the interplay between USF and C/EBPbeta is important for cAMP-mediated regulation of RIIbeta expression. The counteracting effects of USF2 and C/EBPbeta observed on the RIIbeta promoter is in accordance with the hypothesis that C/EBP and USF play opposite roles in regulation of glucose metabolism.

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.

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.