The carbamyl phosphate synthetase promoter contains multiple binding sites for C/EBP-related proteins

Datamining approaches for examining the low prevalence of N-acetylglutamate synthase deficiency and understanding transcriptional regulation of urea cycle genes

Ammonia, which is toxic to the brain, is converted into non-toxic urea, through a pathway of six enzymatically catalyzed steps known as the urea cycle. In this pathway, N-acetylglutamate synthase (NAGS, EC 2.3.1.1) catalyzes the formation of N-acetylglutamate (NAG) from glutamate and acetyl coenzyme A. NAGS deficiency (NAGSD) is the rarest of the urea cycle disorders, yet is unique in that ureagenesis can be restored with the drug N-carbamylglutamate (NCG). We investigated whether the rarity of NAGSD could be due to low sequence variation in the NAGS genomic region, high NAGS tolerance for amino acid replacements, and alternative sources of NAG and NCG in the body. We also evaluated whether the small genomic footprint of the NAGS catalytic domain might play a role. The small number of patients diagnosed with NAGSD could result from the absence of specific disease biomarkers and/or short NAGS catalytic domain. We screened for sequence variants in NAGS regulatory regions in patients suspected of having NAGSD and found a novel NAGS regulatory element in the first intron of the NAGS gene. We applied the same datamining approach to identify regulatory elements in the remaining urea cycle genes. In addition to the known promoters and enhancers of each gene, we identified several novel regulatory elements in their upstream regions and first introns. The identification of cis-regulatory elements of urea cycle genes and their associated transcription factors holds promise for uncovering shared mechanisms governing urea cycle gene expression and potentially leading to new treatments for urea cycle disorders.

CPS1: Looking at an ancient enzyme in a modern light

The mammalian urea cycle (UC) is responsible for siphoning catabolic waste nitrogen into urea for excretion. Disruptions of the functions of any of the enzymes or transporters lead to elevated ammonia and neurological injury. Carbamoyl phosphate synthetase 1 (CPS1) is the first and rate-limiting UC enzyme responsible for the direct incorporation of ammonia into UC intermediates. Symptoms in CPS1 deficiency are typically the most severe of all UC disorders, and current clinical management is insufficient to prevent the associated morbidities and high mortality. With recent advances in basic and translational studies of CPS1, appreciation for this enzyme's essential role in the UC has been broadened to include systemic metabolic regulation during homeostasis and disease. Here, we review recent advances in CPS1 biology and contextualize them around the role of CPS1 in health and disease.

Role of CPS1 in Cell Growth, Metabolism and Prognosis in LKB1-Inactivated Lung Adenocarcinoma

Background

Liver kinase B1 ( LKB1 ) is a tumor suppressor in lung adenocarcinoma (LADC). We investigated the proteomic profiles of 45 LADC cell lines with and without LKB1 inactivation. Carbamoyl phosphate synthetase 1 (CPS1), the first rate-limiting mitochondrial enzyme in the urea cycle, was distinctively overexpressed in LKB1-inactivated LADC cell lines. We therefore assessed the role of CPS1 and its clinical relevance in LKB1-inactivated LADC.

Methods

Mass spectrometric profiling of proteome and metabolome and function of CPS1 were analyzed in LADC cell lines. CPS1 and LKB1 expression in tumors from 305 LADC and 160 lung squamous cell carcinoma patients was evaluated by immunohistochemistry. Kaplan-Meier and Cox regression analyses were applied to assess the association between overall survival and CPS1 and LKB1 expression. All statistical tests were two-sided.

Results

CPS1 knockdown reduced cell growth, decreased metabolite levels associated with nucleic acid biosynthesis pathway, and contributed an additive effect when combined with gemcitabine, pemetrexed, or CHK1 inhibitor AZD7762. Tissue microarray analysis revealed that CPS1 was expressed in 65.7% of LKB1-negative LADC, and only 5.0% of LKB1-positive LADC. CPS1 expression showed statistically significant association with poor overall survival in LADC (hazard ratio = 3.03, 95% confidence interval = 1.74 to 5.25, P < .001).

Conclusions

Our findings suggest functional relevance of CPS1 in LKB1-inactivated LADC and association with worse outcome of LADC. CPS1 is a promising therapeutic target in combination with other chemotherapy agents, as well as a prognostic biomarker, enabling a personalized approach to treatment of LADC.

The activity of the carbamoyl phosphate synthase 1 promoter in human liver-derived cells is dependent on hepatocyte nuclear factor 3-beta

Carbamoyl phosphate synthase 1 (CPS1) is the rate‐limiting enzyme in the first step of the urea cycle and an indispensable enzyme in the metabolism of human liver. However, CPS1 epigenetic regulation involves promoter analysis and the role of liver‐enriched transcription factors (LETFs), which is not fully elucidated. In this work, the promoter region of hCPS1 gene was cloned, and its activity was investigated. An LETF, hepatocyte nuclear factor 3‐beta (HNF3β), was found to promote the transcriptional expression of CPS1 in liver‐derived cell lines. In addition, dual‐luciferase reporter assay shows that the essential binding sites of the HNF3β may exist in the oligonucleotide −70 nt to +73 nt. Two putative binding sites are available for HNF3β. Mutation analysis results show that the binding site 2 of HNF3β was effective, and the transcriptional activity of CPS1 promoter significantly decreased after mutation. Electrophoretic mobile shift assay (EMSA) and ChIP assay confirmed that HNF3β can interact with the binding site in the CPS1 promoter region of −70 nt to +73 nt promoter region in vivo and in vitro to regulate the transcription of CPS1. Moreover, HNF3β overexpression enhanced the transcription of CPS1 and consequently improved the mRNA and protein levels of CPS1, whereas the knockdown of HNF3β showed the opposite effects. Finally, urea production in cells was measured, and ammonia detoxification improved significantly in cells after transfection with HNF3β. HNF3β plays a vital role in regulation of CPS1 gene and could promote the metabolism of ammonia by regulating CPS1 expression.

Ammonia-lowering activities and carbamoyl phosphate synthetase 1 (Cps1) induction mechanism of a natural flavonoid

OBJECTIVE

Ammonia detoxification is essential for physiological well-being, and the urea cycle in liver plays a predominant role in ammonia disposal. Nobiletin (NOB), a natural dietary flavonoid, is known to exhibit various physiological efficacies. In the current study, we investigated a potential role of NOB in ammonia control and the underlying cellular mechanism.

MATERIALS/METHODS

C57BL/6 mice were fed with regular chow (RC), high-fat (HFD) or high-protein diet (HPD) and treated with either vehicle or NOB. Serum and/or urine levels of ammonia and urea were measured. Liver expression of genes encoding urea cycle enzymes and C/EBP transcription factors was determined over the circadian cycle. Luciferase reporter assays were carried out to investigate function of CCAAT consensus elements on the carbamoyl phosphate synthetase (Cps1) gene promoter. A circadian clock-deficient mouse mutant, Clock (Δ19/Δ19) , was utilized to examine a requisite role of the circadian clock in mediating NOB induction of Cps1.

RESULTS

NOB was able to lower serum ammonia levels in mice fed with RC, HFD or HPD. Compared with RC, HFD repressed the mRNA and protein expression of Cps1, encoding the rate-limiting enzyme of the urea cycle. Interestingly, NOB rescued CPS1 protein levels under the HFD condition via induction of the transcription factors C/EBPα and C/EBPβ. Expression of other urea cycle genes was also decreased by HFD relative to RC and again restored by NOB to varying degrees, which, in conjunction with Cps1 promoter reporter analysis, suggested a C/EBP-dependent mechanism for the co-induction of urea cycle genes by NOB. In comparison, HPD markedly increased CPS1 levels relative to RC, yet NOB did not further enrich CPS1 to a significant extent. Using the circadian mouse mutant Clock (Δ19/Δ19) , we also showed that a functional circadian clock, known to modulate C/EBP and CPS1 expression, was required for NOB induction of CPS1 under the HFD condition.

CONCLUSION

NOB, a dietary flavonoid, exhibits a broad activity in ammonia control across varying diets, and regulates urea cycle function via C/EBP-and clock-dependent regulatory mechanisms.

Transcriptional regulation of N-acetylglutamate synthase

The urea cycle converts toxic ammonia to urea within the liver of mammals. At least 6 enzymes are required for ureagenesis, which correlates with dietary protein intake. The transcription of urea cycle genes is, at least in part, regulated by glucocorticoid and glucagon hormone signaling pathways. N-acetylglutamate synthase (NAGS) produces a unique cofactor, N-acetylglutamate (NAG), that is essential for the catalytic function of the first and rate-limiting enzyme of ureagenesis, carbamyl phosphate synthetase 1 (CPS1). However, despite the important role of NAGS in ammonia removal, little is known about the mechanisms of its regulation. We identified two regions of high conservation upstream of the translation start of the NAGS gene. Reporter assays confirmed that these regions represent promoter and enhancer and that the enhancer is tissue specific. Within the promoter, we identified multiple transcription start sites that differed between liver and small intestine. Several transcription factor binding motifs were conserved within the promoter and enhancer regions while a TATA-box motif was absent. DNA-protein pull-down assays and chromatin immunoprecipitation confirmed binding of Sp1 and CREB, but not C/EBP in the promoter and HNF-1 and NF-Y, but not SMAD3 or AP-2 in the enhancer. The functional importance of these motifs was demonstrated by decreased transcription of reporter constructs following mutagenesis of each motif. The presented data strongly suggest that Sp1, CREB, HNF-1, and NF-Y, that are known to be responsive to hormones and diet, regulate NAGS transcription. This provides molecular mechanism of regulation of ureagenesis in response to hormonal and dietary changes.

Y-box binding protein-1 down-regulates expression of carbamoyl phosphate synthetase-I by suppressing CCAAT enhancer-binding protein-alpha function in mice

BACKGROUND & AIMS

Carbamoyl phosphate synthetase-I (CPS1) is a key enzyme in the urea cycle and patients with defects in the function or expression of CPS1 suffer from hyperammonemia. CPS1 is expressed in the liver at neonatal and adult stages in a CCAAT enhancer-binding protein-alpha (C/EBPalpha)-dependent manner. Despite expression of C/EBPalpha, CPS1 is not expressed in fetal liver, indicating an additional factor is involved in the regulation of CPS1 expression. The aim of this study was to elucidate the mechanism of CPS1 expression.

METHODS

Microarray was performed to find Y-box binding protein-1 (YB-1) that was expressed in mouse fetal liver. The role of YB-1 in CPS1 expression was investigated by overexpression of YB-1 in mouse fetal liver culture and luciferase reporter assays using the CPS1 promoter. Chromatin immunoprecipitation assay was used to examine recruitment of YB-1 to the CPS1 promoter in vivo.

RESULTS

Expression of YB-1 and CPS1 was inversely correlated in vivo, and YB-1 inhibited CPS1 expression and ammonia clearance in fetal liver culture. Although YB-1 was not expressed in adult liver, acute liver injury up-regulated YB-1 and down-regulated CPS1, accompanying an increase of the serum ammonia level. YB-1 inhibited C/EBPalpha-induced transcription from the CPS1 promoter via the Y-box near the C/EBPalpha-binding site. Chromatin immunoprecipitation assays demonstrated that YB-1 was recruited to the CPS1 promoter in fetal and injured adult liver, but not in normal adult liver.

CONCLUSIONS

YB-1 is a key regulator of ammonia detoxification by negatively regulating CPS1 expression via suppression of C/EBPalpha function.

The role of proximal-enhancer elements in the glucocorticoid regulation of carbamoylphosphate synthetase gene transcription from the upstream response unit

As part of the urea cycle, carbamoylphosphate synthetase (CPS) converts toxic ammonia resulting from amino-acid catabolism into urea. Liver-specific and glucocorticoid-dependent expression of the gene involves a distal enhancer, a promoter-proximal enhancer, and the minimal promoter itself. When challenged with glucocorticoids, the glucocorticoid-responsive unit (GRU) in the distal enhancer of the carbamoylphosphate-synthetase gene can only activate gene expression if, in addition to the minimal promoter, the proximal enhancer is present. Here, we identify and characterise two elements in the proximal CPS enhancer that are involved in glucocorticoid-dependent gene activation mediated by the GRU. A purine-rich stretch forming a so-called GAGA-box and a glucocorticoid-response element (GRE) are both crucial for the efficacy of the GRU and appear to constitute a promoter-proximal response unit that activates the promoter. The glucocorticoid response of the CPS gene is, therefore, dependent on the combined action of a distal and a promoter-proximal response unit.

A single regulatory module of the carbamoylphosphate synthetase I gene executes its hepatic program of expression

A 469-base pair (bp) upstream regulatory fragment (URF) and the proximal promoter of the carbamoylphosphate synthetase I (CPS) gene were analyzed for their role in the regulation of spatial, developmental, and hormone-induced expression in vivo. The URF is essential and sufficient for hepatocyte-specific expression, periportal localization, perinatal activation and induction by glucocorticoids, and cAMP in transgenic mice. Before birth, the transgene is silent but can be induced by cAMP and glucocorticoids, indicating that these compounds are responsible for the activation of expression at birth. A 102-bp glucocorticoid response unit within the URF, containing binding sites for HNF3, C/EBP, and the glucocorticoid receptor, is the main determinant of the hepatocyte-specific and hormone-controlled activity. Additional sequences are required for a productive interaction between this minimal response unit and the core CPS promoter. These results show that the 469-bp URF, and probably only the 102-bp glucocorticoid response unit, functions as a regulatory module, in that it autonomously executes a correct spatial, developmental and hormonal program of CPS expression in the liver.

Role for the Rana catesbeiana homologue of C/EBP alpha in the reprogramming of gene expression in the liver of metamorphosing tadpoles

During the spontaneous or thyroid hormone (TH)-induced metamorphosis of Rana catesbeiana, developmental changes occur in its liver that are necessary for the transition of this organism from an ammonotelic larva to a ureotelic adult. These changes include the coordinated expression of genes encoding the urea cycle enzymes carbamyl phosphate synthetase (CPS-I) and arnithine transcarbamylase (OTC). Although the expression of these genes is dependent on TH, the mechanisms(s) by which TH initiates this tissue-specific response is thought to be indirect and to involve early TH-induced upregulation of a gene(s), which, in turn, upregulates the coordinated expression of these urea-cycle enzyme genes. Herein, we demonstrate that mRNAs encoding the Rana homologue of the mammalian transcription factor C/EBP alpha (designated RcC/EBP-1) accumulate early in response to TH and that the product of these mRNAs can bind to and transactivate the promoters of both the Rana CPS-1 and OTC genes. These results support the contention that the reprogramming of gene expression in the liver of metamorphosing tadpoles involves a TH-induced cascade of gene activity in which RcC/EBP-1 and, perhaps, other transcription factors coordinate the expression of genes, such as those encoding CPS-I and OTC, whose products are characteristic of the adult liver phenotype.

Involvement of a cis-acting element in the suppression of carbamoyl phosphate synthetase I gene expression in the liver of carnitine-deficient mice

The expression of carbamoyl phosphate synthetase I (CPS) gene is suppressed in the liver of carnitine-deficient juvenile visceral steatosis (JVS) mice at weaning and under starvation at adult age. To clarify the suppression mechanism, we produced CPSL transgenic JVS mice carrying a transgene composed of the chloramphenicol acetyltransferase (CAT) gene with the upstream region (-12 kb to +138) of the rat CPS gene and CPSE transgenic JVS mice carrying a transgene composed of the luciferase gene with minimal promoter (299 bp from -161 to +138) and enhancer (469 bp around -6.3 kb) fragments of the rat gene. The expression of the CAT gene as well as the endogenous CPS was suppressed in CPSL transgenic JVS mice, but luciferase gene expression was not suppressed in CPSE transgenic JVS mice. We isolated the 5'-upstream region of the mouse CPS gene and identified an activator protein-1 (AP-1) site downstream of the minimum enhancer region of both rat and mouse CPS genes. In conjunction with the 313-bp mouse promoter region, the 714-bp mouse enhancer fragment conferred a cell-type-dependent hormone responsiveness. In rat primary cultured hepatocytes, the addition of oleic acid suppressed reporter gene expression induced by dexamethasone in the construct containing the enhancer fragment of 714 bp with the AP-1 site, but not in its AP-1 site mutants or in 519 bp without the AP-1 site. These results strongly suggest that direct protein-protein interaction between AP-1 and glucocorticoid receptor is not involved in the suppression of the CPS gene in JVS mice and that the AP-1 element is the cis-element which is responsible for the suppression.

D-site binding protein transactivation requires the proline- and acid-rich domain and involves the coactivator p300

The D-site binding protein (DBP) is a member of the proline- and acid-rich (PAR) domain subfamily of basic/leucine zipper proteins and is involved in transcriptional regulation in the liver. Deletion analysis of the DBP protein was carried out in an effort to define the function of the conserved PAR domain. Internal deletions of the protein, i.e. removing portions of the PAR domain, resulted in a substantial loss in transactivation of a high affinity DBP reporter construct when assayed in Hep G2 cells. These same sequences conferred significant transactivation to GAL4 DNA binding domain fusion proteins, indicating that this region acts as part of an independent activation domain comprised of sequences in both the amino terminus and in the PAR domain of DBP. The coexpression of full-length expression constructs for both DBP and hepatic leukemia factor resulted in a dramatic increase in activation mediated by the GAL4-DBP fusion proteins, suggesting the involvement of a regulated coactivator in this process. DBP transactivation appears to be a p300-dependent process, as a 12 S E1A expression construct disrupted DBP-mediated transactivation, and a p300 expression vector, but not a CREB binding protein vector, was able to restore DBP transactivation. These results suggest that the PAR domain is required for DBP activation, which occurs through a regulated, p300-dependent process.

Regulation of the DBP promoter by PAR proteins and in leukemic cells bearing an E2A/HLF translocation

The D-site binding protein (DBP) is a member of the PAR domain subfamily of b/ZIP proteins, whose expression in the liver is highly sensitive to the growth state of that organ. This paper examines the regulation of the DBP promoter by C/EBP alpha and examines the role of autoregulation in its expression. Of four previously characterized proximal promoter sites, sites I and III are shown to bind C/EBP alpha, but cotransfection in Hep G2 cells of a C/EBP alpha expression vector is unable to transactivate the promoter. In contrast, the expression of DBP, particularly in conjunction with the related protein HLF, is able to dramatically upregulate expression directed by the proximal promoter. Deletion analysis and the use of single site reporter constructs demonstrate that sites II and IV are highly responsive to transactivation by DBP and HLF. The DBP promoter is active in the UOC-B1 cell line, which bears a 17:19 translocation resulting in the creation of an E2A:HLF fusion protein. The proteins binding to site IV are elevated in this line, suggesting that upregulation of DBP expression in response to inappropriate HLF activity may be mediated through this site.

Regions of human chromosome 2 (2q32-q35) and mouse chromosome 1 show synteny with the pufferfish genome (Fugu rubripes)

We have isolated and sequenced a cosmid clone from the compact genome of the Japanese pufferfish (Fugu rubripes) containing portions of three genes that have the same order as in human. The gene order is microtubule-associated protein (MAP-2), myosin light chain (MYL-1), and carbamoyl phosphate synthetase (CPS III). The intron-exon organization of Fugu CPS III is identical with that of rat CPS I, although the equivalent genomic fragments of rat and Fugu CPS span 87.9 and 21 kb, respectively. This is the first report of a piscine CPS III genomic structure and predicts a close evolutionary link between CPS III and CPS I. The 8-kb intergenic region between MYL-1 and CPS gave no clear areas of transcription factor-binding sites by pairwise comparison with shark or rat CPS promoter regions. However, there was a match with the rat myosin light chain 2 (MLC-2) gene promoter and a MyoD transcription factor-binding site 874 bp upstream of the MYL-1 gene.

Amino acid limitation induces expression of CHOP, a CCAAT/enhancer binding protein-related gene, at both transcriptional and post-transcriptional levels

In mammals, plasma concentrations of amino acids are affected by nutritional or pathological conditions. Here we examined the role of amino acid limitation in regulating the expression of CHOP, a CCAAT/enhancer binding protein (C/EBP)-related gene. CHOP protein is capable of interacting with other C/EBPs to modify their DNA binding activities and may function as a negative regulator of these transcription factors. Our data show that leucine limitation in human cell lines leads to induction of CHOP mRNA and protein in a dose-dependent manner. CHOP mRNA induction is rapidly reversed by leucine replenishment. Elevated mRNA levels result from both an increase in the rate of CHOP transcription and an increase in the CHOP mRNA stability. Using a transient expression assay, we show that a promoter fragment, when linked to a reporter gene, is sufficient to mediate the regulation of CHOP expression by leucine starvation in HeLa cells. In addition, we found that decreasing amino acid concentration by itself can induce CHOP expression independently of a cellular stress due to protein synthesis inhibition. Moreover, CHOP expression is induced at leucine concentrations in the range of those observed in blood of protein-restricted animals suggesting that amino acids can participate, in concert with hormones, in the regulation of gene expression.

The upstream regulatory region of the carbamoyl-phosphate synthetase I gene controls its tissue-specific, developmental, and hormonal regulation in vivo

The carbamoyl-phosphate synthetase I gene is expressed in the periportal region of the liver, where it is activated by glucocorticosteroids and glucagon (via cyclic AMP), and in the crypts of the intestinal mucosa. The enhancer of the gene is located 6.3 kilobase pairs upstream of the transcription start site and has been shown to direct the hormone-dependent hepatocyte-specific expression in vitro. To analyze the function of the upstream region in vivo, three groups of transgenic mice were generated. In the first group the promoter drives expression of the reporter gene, whereas the promoter and upstream region including the far upstream enhancer drive expression of the reporter gene in the second group. In the third group the far upstream enhancer was directly coupled to a minimized promoter fragment. Reporter-gene expression was virtually undetectable in the first group. In the second group spatial, temporal, and hormonal regulation of expression of the reporter gene and the endogenous carbamoyl-phosphate synthetase gene were identical. The third group showed liver-specific periportal reporter gene expression, but failed to activate expression in the intestine. These results show that the upstream region of the carbamoyl-phosphate synthetase gene controls four characteristics of its expression: tissue specificity, spatial pattern of expression within the liver and intestine, hormone sensitivity, and developmental regulation. Within the upstream region, the far upstream enhancer at -6.3 kilobase pairs is the determinant of the characteristic hepatocyte-specific periportal expression pattern of carbamoyl-phosphate synthetase.

The promoter region of the carbamoyl-phosphate synthetase III gene of Squalus acanthias

Carbamoyl-phosphate synthetase III (CPSase III) of Squalus acanthias (spiny dogfish) is a nuclear-encoded mitochondrial enzyme that catalyzes glutamine-dependent formation of carbamoyl phosphate for urea synthesis. In this paper we report the results of cloning a 10-kb segment of genomic DNA which includes the region flanking the 5' end of the spiny dogfish CPSase III gene. A total of 1,295 base pairs of sequence straddling the start codon was obtained. Primer extension experiments revealed that the transcription start site is the G located 114 residues upstream of the translation start codon ATG. The first exon has 240 base pairs, including the 5' untranslated region, the coding sequence for the signal peptide (38 amino acids), and the four N-terminal amino acids of the mature enzyme. The boundary of the first exon and the first intron of the CPSase III gene is concordant with that of rat and frog (Rana catesbeiana) CPSase I, which have been suggested to have evolved from CPSase III. The putative TATA box sequence, TACAAA, is located at position -31 with an uncommonly found C at the third position. Two C/EBP binding site sequences, ATTCTGCAAG (-405 to -397) and GTGCAGTAAG (-168 to -160), were identified in the promoter region, which suggests that spiny dogfish CPSase III might be subjected to transactivation of transcription by C/EBP-related proteins, as has been reported for rat CPSase I. The preparation and binding of a recombinant RcC/EBP-1 protein (the R. catesbeiana homolog of the mammalian C/EBP alpha) to the two spiny dogfish C/EBP binding sequences are described. Two putative heat-shock binding elements were also identified in the promoter region.

CAAT/enhancer-binding proteins are involved in beta-globin gene expression and are differentially expressed in murine erythroleukemia and K562 cells

Acting in cis with the beta-globin locus control region, the CAAT box of the beta-globin gene promoter stimulates transcription 10-fold in murine erythroleukemia (MEL) cells but is without effect in K562 cells. Our previous studies suggested that of four proteins from MEL cells that bind to this CAAT box region (CP1, GATA-1, and two factors that were denoted DSFr and DSF1) DSFr is involved in the up-regulation of transcription. In the present report, the DSFr protein in MEL cells was identified as C/EBPgamma through expression cloning and antibody studies. C/EBPgamma DNA binding activity could not be detected in K562 cells. However, K562 cells, but not MEL cells, were found to express LIP, which is a truncated form of C/EBPbeta and is an inhibitor of transcription. Thus, the differential expression of C/EBP members could account for the ability of the beta-globin CAAT box to stimulate transcription in MEL cells, but not function in K562 cells. Juxtaposing a specific C/EBP binding sequence next to the beta-globin promoter, in constructs in which the CAAT box had been rendered inactive by mutation or deletion, restored full promoter activity in MEL cells only if CP1 still bound to the promoter. In conjunction with previous mutation analyses, these results suggest that C/EBPgamma may collaborate with CP1 to enhance transcription through the beta-globin CAAT box.

Dietary calorie restriction in mice induces carbamyl phosphate synthetase I gene transcription tissue specifically

Dietary calorie restriction (CR) delays age-related physiologic changes, increases maximum life span, and reduces cancer incidence. Here, we present the novel finding that chronic reduction of dietary calories by 50% without changing the intake of dietary protein induced the activity of mouse hepatic carbamyl phosphate synthetase I (CpsI) 5-fold. In liver, CpsI protein, mRNA, and gene transcription were each stimulated by approximately 3-fold. Thus, CR increased both the rate of gene transcription and the specific activity of the enzyme. Short-term feeding studies demonstrated that higher cpsI expression was due to CR and not consumption of more dietary protein. Intestinal CpsI activity was stimulated 2-fold, while its mRNA level did not change, suggesting enzyme activity or translation efficiency was stimulated. CpsI catalyzes the conversion of metabolic ammonia to carbamyl phosphate, the rate-limiting step in urea biosynthesis. cpsI induction suggests there is a shift in the metabolism of calorie-restricted animals toward protein catabolism. CpsI induction likely facilitates metabolic detoxification of ammonia, a strong neurotoxin. Enhanced protein turnover and metabolic detoxification may extend life span. Physiologic similarities between calorie-restricted and hibernating animals suggest the effects of CR may be part of a spectrum of adaptive responses that include hibernation.

Transcriptional regulation of genes for ornithine cycle enzymes

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The far-upstream enhancer of the carbamoyl-phosphate synthetase I gene is responsible for the tissue specificity and hormone inducibility of its expression

The role of the proximal promoter and the far-upstream enhancer in the hepatocyte-specific and hormonal regulation of the carbamoyl-phosphate synthetase I (CPS) gene was investigated in transient transfection assays using primary rat hepatocytes, hepatoma cells, and fibroblasts. These experiments revealed that the activity of the promoter is comparable in all cells tested and is, therefore, not responsible for tissue-specific expression. The 5'-untranslated region of the mRNA is a major, non-tissue specific stimulator of expression in FTO-2B hepatoma cells, acting at the post-transcriptional level. A 469-base pair DNA fragment, 6 kilobase pairs upstream of the transcription start-site in the CPS gene, confers strong hormone-dependent tissue specific expression, both in combination with the CPS promoter and a minimized viral thymidine kinase promoter. Sequences similar to a cyclic AMP-responsive element and a glucocorticosteroid-responsive element were found in the isolated enhancer. Substitutional mutations in these sites strongly affected hormone-induced expression. Analysis of the interaction between the enhancer and parts of the CPS promoter revealed that, in addition to the TATA box, the GAG box, a motif similar to the GC box near the TATA motif, is instrumental in conferring the enhancer activity.

C/EBP alpha: a critical regulator of genes governing integrative metabolic processes

The role of C/EBPalpha in the developmental expression of a subset of genes governing essential metabolic processes has recently been elucidated using a mutant mouse model that lacks this transcription factor. The mutation results in a failure of the liver and white and brown adipose tissue to develop normal metabolic functions in the immediate perinatal period, including hepatic glycogen synthesis and gluconeogenesis and the synthesis and deposition of triglyceride in adipose tissue. The metabolic alterations are very similar to those of human infants born prior to the third trimester and suggest that many of the medical complications of prematurity are a result of the lack of activation of C/EBPalpha in development.

A gene-type-specific enhancer regulates the carbamyl phosphate synthetase I promoter by cooperating with the proximal GAG activating element

The rat carbamyl phosphate synthetase I gene is expressed in two cell types: hepatocytes and epithelial cells of the intestinal mucosa. The proximal promoter contains a single activating element, GAG, two repressor elements (sites I and III) and an anti-repressor element (site II). Although these elements together exhibit the potential for complex regulation, they are unable to confer tissue-specific promoter activity. Here we have identified a cell-type-specific enhancer that lies 10 kilobases upstream of the promoter. Unexpectedly, the enhancer also functioned in a gene-type-specific manner. The enhancer stimulated promoter activity exclusively through the proximal GAG element. Abrogation of GAG, either directly by mutation of GAG or indirectly by sites I and III repressors, abolished enhancer activation. Conversely, activation of the heterologous thymidine kinase promoter by the enhancer required the introduction of GAG. The requirement for GAG, therefore, functions to constrain the enhancer to a specific target promoter.

Factors interacting with the rat carbamyl phosphate synthetase promoter in expressing and nonexpressing tissues

The proximal promoter of the rat carbamyl phosphate synthetase-encoding gene (CPS) contains at least six potential cis-acting regulatory elements (sites I-VI), as judged by DNase I footprint analysis using rat liver nuclear extracts; all six regions bind proteins with DNA recognition properties similar to those of CAAT and enhancer-binding protein alpha (C/EBP alpha) [Lagacé et al., Gene 118 (1992) 231-238]. In contrast, nuclear extracts from kidney, brain and spleen contain proteins that recognize CPS promoter sites II, V and VI, but not sites I, III and IV. Mutation of the octameric sequence (5'-GTTGCAAC) within site II, which is a recognition element for C/EBP alpha, abolished binding of nuclear proteins to site II oligodeoxyribonucleotides (oligos) in all tissues. As well, the site II mutation reduced the level of in vitro transcription from the CPS promoter by about 50% in liver and spleen nuclear extracts, but had a negligible effect in brain and kidney extracts. The fact that promoter activity was observed in extracts of tissues that do not express the endogenous CPS gene (i.e., brain, kidney and spleen) indicates that these tissues, nevertheless, contain factors with the potential to activate transcription through a limited number of CPS promoter elements. Tissue-specific regulation, therefore, must involve steps to prevent these factors from acting on the endogenous CPS promoter in situ.