Rat liver and intestinal mucosa differ in the developmental pattern and hormonal regulation of carbamoyl-phosphate synthetase I and ornithine carbamoyl transferase gene expression

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.

Aestivation induces widespread transcriptional changes in the African lungfish

Aestivation is a special ability possessed by some animals to cope with hot and dry environments utilizing dormancy. At a macroscopic level, dormant animals stop moving and eating. At the microscopic level, the expression of a large number of genes in these animals is strictly controlled. However, little is known about what changes occur during aestivation, especially in fish. In this study, we used transcriptome analysis to examine what changes occur in the gills and lungs of the African lungfish (Protopterus annectens) during the maintenance phase of aestivation and speculated on their causes. We found that aestivating transcriptomes were highly similar between gills and lungs. We also found that some genes showed differential expression or alternative splicing, which may be associated with different organs. In addition, differential expression analysis revealed that the lungs maintained significantly higher bioactivity during aestivation, which suggests that the main respiratory organ in aestivating lungfish can transform. Our study provides a reference point for studying the relationship between aestivation and hibernation and further increases understanding of aestivation.

Simultaneous activation of genes encoding urea cycle enzymes and gluconeogenetic enzymes coincides with a corticosterone surge period before metamorphosis in Xenopus laevis

Amphibian tadpoles are postulated to excrete ammonia as nitrogen metabolites but to shift from ammonotelism to ureotelism during metamorphosis. However, it is unknown whether ureagenesis occurs or plays a functional role before metamorphosis. Here, the mRNA-expression levels of two urea cycle enzymes (carbamoyl phosphate synthetase I [CPSI] and ornithine transcarbamylase [OTC]) were measured beginning with stage-47 Xenopus tadpoles at 5 days post-fertilization (dpf), between the onset of feeding (stage 45, 4 dpf) and metamorphosis (stage 55, 32 dpf). CPSI and OTC expression levels increased significantly from stage 49 (12 dpf). Urea excretion was also detected at stage 47. A transient corticosterone surge peaking at stage 48 was previously reported, supporting the hypothesis that corticosterone can induce CPSI expression in tadpoles, as found in adult frogs and mammals. Stage-46 tadpoles were exposed to a synthetic glucocorticoid, dexamethasone (Dex, 10-500 nM) for 3 days. CPSI mRNA expression was significantly higher in tadpoles exposed to Dex than in tadpoles exposed to the vehicle control. Furthermore, glucocorticoid receptor mRNA expression increased during the pre-metamorphic period. In addition to CPSI and OTC mRNA upregulation, the expression levels of three gluconeogenic enzyme genes (glucose 6-phosphatase, phosphoenolpyruvate carboxykinase, and fructose-1,6-bisphosphatase 1) increased with the onset of urea synthesis and excretion. These results suggest that simultaneous induction of the urea cycle and gluconeogenic enzymes coincided with a corticosterone surge occurring prior to metamorphosis. These metabolic changes preceding metamorphosis may be closely related to the onset of feeding and nutrient accumulation required for metamorphosis.

Global Proteomic Profile Integrated to Quantitative and Morphometric Assessment of Enteric Neurons: Investigation of the Mechanisms Involved in the Toxicity Induced by Acute Fluoride Exposure in the Duodenum

The enteric nervous system is responsible for controlling the gastrointestinal tract (GIT) functions. Enteric neuropathies are highly correlated to the development of several intestinal disturbances. Fluoride (F) is extensively applied for dental health improvement and its ingestion can promote systemic toxicity with mild to severe GIT symptomatology and neurotoxicity. Although F harmful effects have been published, there is no information regarding noxiousness of a high acute F exposure (25 mg F/kg) on enteric neurons and levels of expression of intestinal proteins in the duodenum. Quantitative proteomics of the duodenum wall associated to morphometric and quantitative analysis of enteric neurons displayed F effects of a high acute exposure. F-induced myenteric neuroplasticity was characterized by a decrease in the density of nitrergic neurons and morphometric alterations in the general populations of neurons, nitrergic neurons, and substance P varicosities. Proteomics demonstrated F-induced alterations in levels of expression of 356 proteins correlated to striated muscle cell differentiation; generation of precursor metabolites and energy; NADH and glutathione metabolic process and purine ribonucleoside triphosphate biosynthesis. The neurochemical role of several intestinal proteins was discussed specially related to the modulation of enteric neuroplasticity. The results provide a new perspective on cell signaling pathways of gastrointestinal symptomatology promoted by acute F toxicity.

Developmental gene expression patterns in the brain and liver of Xenopus tropicalis during metamorphosis climax

Thyroid hormones (THs) induce metamorphosis in amphibians, causing dynamic changes, whereas mammalian newborns undergo environmental transition from placenta to open air at birth. The similarity between amphibian metamorphosis and the mammalian perinatal periods has been repeatedly discussed. However, a corresponding developmental gene expression analysis has not yet been reported. In this study, we examined the developmental gene expression profiles in the brain and liver of Xenopus tropicalis during metamorphosis climax and compared them to the respective gene expression profiles of newborn rodents. Many upregulated genes identified in the tadpole brain during metamorphosis are also upregulated in the rodent brain during the first three postnatal weeks when the TH surge occurs. The upregulation of some genes in the brain was inhibited in thyroid hormone receptor α (TRα) knockout tadpoles but not in TRβ-knockout tadpoles, implying that brain metamorphosis is mainly mediated by TRα. The expression of some genes was also increased in the liver during metamorphosis climax. Our data suggest that the rodent brain undergoes TH-dependent remodeling during the first three postnatal weeks as observed in X. tropicalis during the larva-to-adult metamorphosis.

Sensitivity of Colorectal Cancer to Arginine Deprivation Therapy is Shaped by Differential Expression of Urea Cycle Enzymes

Tumors deficient in the urea cycle enzymes argininosuccinate synthase-1 (ASS1) and ornithine transcarbamylase (OTC) are unable to synthesize arginine and can be targeted using arginine-deprivation therapy. Here, we show that colorectal cancers (CRCs) display negligible expression of OTC and, in subset of cases, ASS1 proteins. CRC cells fail to grow in arginine-free medium and dietary arginine deprivation slows growth of cancer cells implanted into immunocompromised mice. Moreover, we report that clinically-formulated arginine-degrading enzymes are effective anticancer drugs in CRC. Pegylated arginine deiminase (ADI-PEG20), which degrades arginine to citrulline and ammonia, affects growth of ASS1-negative cells, whereas recombinant human arginase-1 (rhArg1peg5000), which degrades arginine into urea and ornithine, is effective against a broad spectrum of OTC-negative CRC cell lines. This reflects the inability of CRC cells to recycle citrulline and ornithine into the urea cycle. Finally, we show that arginase antagonizes chemotherapeutic drugs oxaliplatin and 5-fluorouracil (5-FU), whereas ADI-PEG20 synergizes with oxaliplatin in ASS1-negative cell lines and appears to interact with 5-fluorouracil independently of ASS1 status. Overall, we conclude that CRC is amenable to arginine-deprivation therapy, but we warrant caution when combining arginine deprivation with standard chemotherapy.

The intestinal-renal axis for arginine synthesis is present and functional in the neonatal pig

The intestinal-renal axis for endogenous arginine synthesis is an interorgan process in which citrulline produced in the small intestine is utilized by the kidney for arginine synthesis. The function of this axis in neonates has been questioned because during this period the enzymes needed for arginine synthesis argininosuccinate synthase (ASS1) and lyase (ASL) are present in the gut. However, evidence of high plasma citrulline concentrations in neonates suggests otherwise. We quantified in vivo citrulline production in premature (10 days preterm), neonatal (7 days old), and young pigs (35 days old) using citrulline tracers. Neonatal pigs had higher fluxes (69 µmol·kg^-1·h^-1, P < 0.001) than premature and young pigs (43 and 45 µmol·kg^-1·h^-1, respectively). Plasma citrulline concentration was also greater in neonatal pigs than in the other age groups. We also determined the site of synthesis and utilization of citrulline in neonatal and young pigs by measuring organ balances across the gut and the kidney. Citrulline was released from the gut and utilized by the kidney in both neonatal and young pigs. The abundance and localization of the enzymes involved in the synthesis and utilization were determined in intestinal and kidney tissue. Despite the presence of ASS1 and ASL in the neonatal small intestine, the lack of colocalization with the enzymes that produce citrulline results in the release of citrulline by the PDV and its utilization by the kidney to produce arginine. In conclusion, the intestinal-renal axis for arginine synthesis is present in the neonatal pig.

Carbamoylphosphate synthetase 1 (CPS1) deficiency: clinical, biochemical, and molecular characterization in Malaysian patients

Carbamoyl phosphate synthetase 1 (CPS1) deficiency is a rare autosomal recessive disorder of ureagenesis presenting as life-threatening hyperammonemia. In this study, we present the main clinical features and biochemical and molecular data of six Malaysian patients with CPS1 deficiency. All the patients have neonatal-onset symptoms, initially diagnosed as infections before hyperammonemia was recognized. They have typical biochemical findings of hyperglutaminemia, hypocitrullinemia, and low to normal urinary excretion of orotate. One neonate succumbed to the first hyperammonemic decompensation. Five neonatal survivors received long-term treatment consisting of dietary protein restriction and ammonia-scavenging drugs. They have delayed neurocognitive development of varying severity. Genetic analysis revealed eight mutations in CPS1 gene, five of which were not previously reported. Five mutations were missense changes while another three were predicted to create premature stop codons. In silico analyses showed that these new mutations affected different CPS1 enzyme domains and were predicted to interrupt interactions at enzyme active sites, disturb local enzyme conformation, and destabilize assembly of intact enzyme complex.

CONCLUSION

All mutations are private except one mutation; p.Ile1254Phe was found in three unrelated families. Identification of a recurrent p.Ile1254Phe mutation suggests the presence of a common and unique mutation in our population. Our study also expands the mutational spectrum of the CPS1 gene.

Ammonia metabolism and hyperammonemic disorders

Human adults produce around 1000 mmol of ammonia daily. Some is reutilized in biosynthesis. The remainder is waste and neurotoxic. Eventually most is excreted in urine as urea, together with ammonia used as a buffer. In extrahepatic tissues, ammonia is incorporated into nontoxic glutamine and released into blood. Large amounts are metabolized by the kidneys and small intestine. In the intestine, this yields ammonia, which is sequestered in portal blood and transported to the liver for ureagenesis, and citrulline, which is converted to arginine by the kidneys. The amazing developments in NMR imaging and spectroscopy and molecular biology have confirmed concepts derived from early studies in animals and cell cultures. The processes involved are exquisitely tuned. When they are faulty, ammonia accumulates. Severe acute hyperammonemia causes a rapidly progressive, often fatal, encephalopathy with brain edema. Chronic milder hyperammonemia causes a neuropsychiatric illness. Survivors of severe neonatal hyperammonemia have structural brain damage. Proposed explanations for brain edema are an increase in astrocyte osmolality, generally attributed to glutamine accumulation, and cytotoxic oxidative/nitrosative damage. However, ammonia neurotoxicity is multifactorial, with disturbances also in neurotransmitters, energy production, anaplerosis, cerebral blood flow, potassium, and sodium. Around 90% of hyperammonemic patients have liver disease. Inherited defects are rare. They are being recognized increasingly in adults. Deficiencies of urea cycle enzymes, citrin, and pyruvate carboxylase demonstrate the roles of isolated pathways in ammonia metabolism. Phenylbutyrate is used routinely to treat inherited urea cycle disorders, and its use for hepatic encephalopathy is under investigation.

Ontogeny of ornithine-urea cycle gene expression in zebrafish (Danio rerio)

Although the majority of adult teleosts excrete most of their nitrogenous wastes as ammonia, several fish species are capable of producing urea early in development. In zebrafish, it is unclear whether this results from a functional ornithine-urea cycle (O-UC) and, if so, how it might be regulated. This study examined the spatiotemporal patterns of gene expression of four major O-UC enzymes: carbamoyl phosphate synthase III (CPSIII), ornithine transcarboxylase, arginosuccinate synthetase, and arginosuccinate lyase, using real-time PCR and whole mount in situ hybridization. In addition, we hypothesized that CPSIII gene expression was epigenetically regulated through methylation of its promoter, a widespread mode of differential gene regulation between tissues and life stages in vertebrates. Furthermore, to assess CPSIII functionality, we used morpholinos to silence CPSIII in zebrafish embryos and assessed their nitrogenous waste handling during development, and in response to ammonia injections. Our results suggest that mRNAs of O-UC enzymes are expressed early in zebrafish development and colocalize to the embryonic endoderm. In addition, the methylation status of CPSIII promoter is not consistent with the patterns of expression observed in developing larvae or adult tissues, suggesting other means of transcriptional regulation of this enzyme. Finally, CPSIII morphants exhibited a transient reduction in CPSIII enzyme activity 24 h postfertilization, which was paralleled by reduced urea production during development and in response to an ammonia challenge. Overall, we conclude that the O-UC is functional in zebrafish embryos, providing further evidence that the capacity to produce urea via the O-UC is widespread in developing teleosts.

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.

Carbamoyl phosphate synthetase 1 deficiency in Italy: clinical and genetic findings in a heterogeneous cohort

Carbamoyl Phosphate Synthetase 1 deficiency (CPS1D) is a rare autosomal recessive urea cycle disorder, potentially leading to lethal hyperammonemia. Based on the age of onset, there are two distinct phenotypes: neonatal and late form. The CPS1 enzyme, located in the mitochondrial matrix of hepatocytes and epithelial cells of intestinal mucosa, is encoded by the CPS1 gene. At present more than 220 clear-cut genetic lesions leading to CPS1D have been reported. As most of them are private mutations diagnosis is complicated. Here we report an overview of the main clinical findings and biochemical and molecular data of 13 CPS1D Italian patients. In two of them, one with the neonatal form and one with the late form, cadaveric auxiliary liver transplant was performed. Mutation analysis in these patients identified 17 genetic lesions, 9 of which were new confirming their "private" nature. Seven of the newly identified mutations were missense/nonsense changes. In order to study their protein level effects, we performed an in silico analysis whose results indicate that the amino acid substitutions occur at evolutionary conserved positions and affect residues necessary for enzyme stability or function.

Loss of carbamoyl phosphate synthetase I in small-intestinal adenocarcinoma

Carbamoyl phosphate synthetase I (CPS1), normally found in hepatocytes and small-intestine (SI) enterocytes, is the antigen of Hep Par 1 antibody. Expression of CPS1 in invasive SI adenocarcinoma seems to be lost. We retrospectively collected 36 total specimens, which included 31 SI adenomas and 21 adenocarcinomas. We used 34 cases of duodenitis as a control group. Immunohistochemical and Western blot analyses were performed to determine CPS1 expression. The normal SI mucosa, all 34 cases of duodenitis, and all 29 adenomas with low-grade dysplasia demonstrated diffuse Hep Par 1 expression. Of the 21 invasive adenocarcinomas, 15 lost antigen expression (71%). These data are statistically significant (P < .05). Western blot analysis confirmed the immunohistochemical findings, with strong CPS1 expression within the normal mucosa and adenoma and complete loss in the invasive tumor. The differential expression of Hep Par 1 in dysplastic vs malignant tumors of the SI may be diagnostically useful in difficult cases.

Therapeutic use of citrulline in cardiovascular disease

L-citrulline is the natural precursor of L-arginine, substrate for nitric oxide synthase (NOS) in the production of NO. Supplemental administration L-arginine has been shown to be effective in improving NO production and cardiovascular function in cardiovascular diseases associated with endothelial dysfunction, such as hypertension, heart failure, atherosclerosis, diabetic vascular disease and ischemia-reperfusion injury, but the beneficial actions do not endure with chronic therapy. Substantial intestinal and hepatic metabolism of L-arginine to ornithine and urea by arginase makes oral delivery very ineffective. Additionally, all of these disease states as well as supplemental L-arginine enhance arginase expression and activity, thus reducing the effectiveness of L-arginine therapy. In contrast, L-citrulline is not metabolized in the intestine or liver and does not induce tissue arginase, but rather inhibits its activity. L-citrulline entering the kidney, vascular endothelium and other tissues can be readily converted to L-arginine, thus raising plasma and tissue levels of L-arginine and enhancing NO production. Supplemental L-citrulline has promise as a therapeutic adjunct in disease states associated with L-arginine deficiencies.

Somatotropin-induced amino acid conservation in pigs involves differential regulation of liver and gut urea cycle enzyme activity

Somatotropin (ST) treatment promotes animal growth and allows for the conservation of amino acids by increasing nitrogen retention and reducing ureagenesis and amino acid oxidation. To determine whether the improvement in amino acid conservation with ST treatment involves regulation of urea cycle enzyme activities in both liver and intestine, growing swine were treated with either ST (150 microg x kg(-1) x d(-1)) or saline for 7 d. Fully fed pigs (n = 20) were infused intravenously for 2 h with NaH(13)CO(3) followed by a 4-h intraduodenal infusion of [1-(13)C]phenylalanine. Arterial and portal venous blood and breath samples were obtained at baseline and steady-state conditions for measurement of amino acid and blood urea nitrogen (BUN) concentrations and whole-body phenylalanine oxidation. Urea cycle enzyme activities were determined in liver and jejunum. ST decreased BUN (-46%), arterial (-34%) and portal venous (-43%) amino acid concentrations and whole-body phenylalanine oxidation (-30%). The activities of carbamoylphosphate synthase-I (-45%), argininosuccinate synthase (-38%), argininosuccinate lyase (-23%), arginase (-27%), and glutaminase (-18%), but not of ornithine carbamoyltransferase, ornithine aminotransferase, or glutamate dehydrogenase were reduced in liver of ST-treated pigs. ST slightly increased intestinal activity of glutaminase (+9%) but did not affect that of any other enzymes. ST decreased hepatic, but increased jejunal, N-acetylglutamate (an essential allosteric activator of carbamoylphosphate synthase-I; -26% and +32%, respectively) and carbamoylphosphate (a substrate for ornithine carbamoyltransferase; -20% and +28%, respectively) content. These results demonstrate that the reduced amino acid catabolism with ST treatment in growing pigs involves a reduction in hepatic urea cycle enzyme activities. The effect of ST treatment on porcine urea cycle enzymes is tissue-specific and is associated with a reduction in substrate availability for hepatic ureagenesis.

Cell lineage analysis during liver development using the spf(ash)-heterozygous mouse

Biliary epithelial cells differentiate from periportal hepatoblasts during fetal mouse liver development. It remains to be determined whether each hepatoblast is equivalent for differentiation into hepatocytes and biliary epithelial cells in normal liver development. To resolve this question, the mosaic pattern of ornithine transcarbamylase (OTC) expression was analyzed in the hepatoblast population of spf(ash) (sparse-fur with abnormal skin and hair)-heterozygous fetal mouse livers, in which random inactivation of either the X chromosome carrying the spf(ash) gene (causing OTC deficiency) or its wild-type gene occurs. Aggregates (patches) of OTC-positive hepatoblasts showed very complex patterns, and their shapes and size distributions were similar in sections from periportal regions and nonperiportal regions of the fetal liver in which bile duct differentiation by periportal hepatoblasts occurred. Average sizes of periportal patches were larger than those of nonperiportal patches because of the presence of more hemopoietic cells in the latter region. The OTC mosaicism in periportal bile duct progenitors and hepatoblast islands of other liver parenchyma was also similar. These results suggest that the growth patterns of hepatoblasts are similar in both periportal and nonperiportal regions. Isolated three-dimensional patches comprising hepatoblasts giving rise to only biliary epithelial cells or hepatoblasts giving rise to both hepatocytes and biliary epithelial cells were observed in periportal regions. In nonperiportal regions, patches consisting of hepatoblasts differentiating into hepatocytes were also seen. Thus, it is likely that there are three lineages for the developmental fates of hepatoblasts: hepatoblasts giving rise to only biliary epithelial cells, hepatoblasts giving rise to only hepatocytes, and hepatoblasts giving rise to both of them.

A cortisol surge mediates the enhanced expression of pig intestinal pyrroline-5-carboxylate synthase during weaning

Citrulline synthesis from glutamine is enhanced remarkably in enterocytes of weanling pigs, but the molecular mechanism(s) involved are not known. The objective of this study was to determine whether a cortisol surge mediates the enhanced expression of intestinal citrulline-synthetic enzymes during weaning. Jejunal enterocytes were prepared from 29-d-old weanling pigs treated with or without metyrapone (an inhibitor of cortisol synthesis), or from age-matched unweaned pigs. The mRNA levels and activities of phosphate-dependent glutaminase (PDG), pyrroline-5-carboxylate synthase (P5CS), ornithine aminotransferase (OAT), carbamoyl-phosphate synthase I (CPS-I) and ornithine carbamoyltransferase (OCT) were determined. The mRNA levels for PDG, P5CS, OAT and OCT were 139, 157, 102 and 55% higher, respectively, in weanling pigs compared with suckling pigs. The activities of PDG and P5CS were 38 and 692% higher, respectively, in weanling pigs compared with unweaned pigs, but the activities of OAT, CPS-I and OCT did not differ between these two groups of pigs. The effects of metyrapone administration to weanling pigs were as follows: 1) prevention of a cortisol surge, 2) abolition of the increases in both mRNA levels and activity of P5CS, 3) no alteration in the mRNA levels and activities of PDG and CPS-I, 4) increases in the mRNA levels for OAT (216%) and OCT (39%) and in OAT activity (30%), and 5) prevention of the increase in intestinal synthesis of citrulline from glutamine. These results suggest that increased P5CS activity reflects in large part the increased levels of P5CS mRNA and is responsible for the increased synthesis of citrulline from glutamine in enterocytes of weanling pigs; these increases may be mediated by a cortisol surge during weaning that can be blocked by metyrapone administration.

Quantitative analysis of cell allocation during liver development, using the spf(ash)-heterozygous female mouse

Mosaicism of ornithine transcarbamylase (OTC) expression in hepatocytes was quantitatively analyzed during liver development of the spf(ash)-heterozygous female mouse. Because the mosaic patterns depend on cell migration and cell mixing, such analysis could give insights on the growth pattern or allocation pattern of hepatocytes during liver development. Complex mosaic patterns of OTC-positive and -negative hepatocytes were observed in sections of fetal and postnatal livers. Sizes of patches, which were aggregates of OTC-positive or -negative hepatocytes, increased during development. Patches were slender and comparatively simple in 15.5- and 17.5-day fetal and neonatal livers. Quantitative analysis of patch shapes demonstrated that undulation of patches was maximal at 7 postnatal days. Patches with nodular shapes also started to increase in number at this stage. Isolated patches in sections of fetal livers and postnatal livers three-dimensionally connected with one another. However, especially in fetal livers, in which OTC-positive patches were minor, due to the presence of abundant hemopoietic cells, isolated three-dimensional patches consisting of approximately 5 to 70 cells were often found. They were shaped like slender branching or zigzag-shaped cords, but no definite orientation such as portal-central was observed in them at any stage. These results suggest that hepatocytes contiguously allocate their daughter cells as zigzag-shaped or branching cords at younger stages. Some hepatocytes grow with nodular formation after 7 postnatal days. Migration and mixing of hepatocytes appear to be more extensive at fetal stages than in the adult liver. Immunohistochemical analysis of intercellular junction proteins (E-cadherin, connexins 26 and 32, occludin, and ZO-1) also revealed that their expression and distribution changed in hepatocytes during development, which may be correlated with the OTC mosaic patterns.

Cell-specific expression of mitochondrial carbonic anhydrase in the human and rat gastrointestinal tract

Mitochondrial carbonic anhydrase V (CA V) in liver provides HCO3- to pyruvate carboxylase for the first step in gluconeogenesis and HCO3- to carbamyl phosphate synthetase I for the first step in ureagenesis. Because carbamyl phosphate synthetase I and ornithine transcarbamylase are also expressed in enterocytes, we tested the hypothesis that CA V is expressed in the gastrointestinal tract in addition to liver. Polyclonal rabbit antisera were raised against a polypeptide of 17 C-terminal amino acids of human CA V and against purified recombinant mouse isozyme and were used in Western blotting and immunoperoxidase staining of human and rat tissues. Immunohistochemistry showed that CA V is expressed cell-specifically in the alimentary canal mucosa from stomach to rectum. Immunoreactions for CA V were detected in the parietal cells and gastrin-producing G-cells of the stomach and in intestinal enterocytes. Western blotting of human and rat gastrointestinal tissues with isozyme-specific antibodies showed positive signals for CA V with the expected molecular mass. The findings in human tissues paralleled those in rat. The cell-specific pattern of CA V expression suggests a role for CA V in alimentary canal physiology. We propose that mitochondrial CA V participates in the detoxification of ammonia produced in the gastrointestinal tract by providing bicarbonate to carbamyl phosphate synthetase I. (J Histochem Cytochem 47:517-524, 1999)

Arginine metabolism: nitric oxide and beyond

Arginine is one of the most versatile amino acids in animal cells, serving as a precursor for the synthesis not only of proteins but also of nitric oxide, urea, polyamines, proline, glutamate, creatine and agmatine. Of the enzymes that catalyse rate-controlling steps in arginine synthesis and catabolism, argininosuccinate synthase, the two arginase isoenzymes, the three nitric oxide synthase isoenzymes and arginine decarboxylase have been recognized in recent years as key factors in regulating newly identified aspects of arginine metabolism. In particular, changes in the activities of argininosuccinate synthase, the arginases, the inducible isoenzyme of nitric oxide synthase and also cationic amino acid transporters play major roles in determining the metabolic fates of arginine in health and disease, and recent studies have identified complex patterns of interaction among these enzymes. There is growing interest in the potential roles of the arginase isoenzymes as regulators of the synthesis of nitric oxide, polyamines, proline and glutamate. Physiological roles and relationships between the pathways of arginine synthesis and catabolism in vivo are complex and difficult to analyse, owing to compartmentalized expression of various enzymes at both organ (e.g. liver, small intestine and kidney) and subcellular (cytosol and mitochondria) levels, as well as to changes in expression during development and in response to diet, hormones and cytokines. The ongoing development of new cell lines and animal models using cDNA clones and genes for key arginine metabolic enzymes will provide new approaches more clearly elucidating the physiological roles of these enzymes.

Arginine-metabolizing enzymes in the developing rat small intestine

Before weaning, arginine biosynthesis from citrulline most likely takes place in the small intestine rather than in the kidney. We studied the expression of ornithine cycle enzymes in the rat small intestine during perinatal development. The spatiotemporal patterns of expression of ornithine aminotransferase, carbamoylphosphate synthetase, ornithine transcarbamoylase, argininosuccinate synthetase, argininosuccinate lyase, and arginase mRNAs were studied by Northern blot analysis and in situ hybridization. In addition, the expression of carbamoylphosphate synthetase and argininosuccinate synthetase protein was studied by immunohistochemistry. Before birth, the developmentally more mature proximal loops of the intestine expressed the mRNAs at higher concentrations than the more distal loops. After birth, this difference was no longer obvious. The mRNAs of argininosuccinate synthetase and argininosuccinate lyase, the enzymes that metabolize citrulline to arginine, were detectable only in the upper part of the villi, whereas the other mRNAs were concentrated in the crypts. The distribution of argininosuccinate synthetase protein corresponded with that of the mRNA, whereas carbamoylphosphate synthetase protein was present in all enterocytes of the crypts and villi. Hepatic arginase mRNA could not be detected in the enterocytes. The spatial distribution of the respective mRNAs and proteins along the villus axis of the suckling small intestine indicates that the basal enterocytes synthesize citrulline, whereas the enterocytes in the upper half of the villus synthesize arginine.

Compartmentation and kinetics of urea cycle enzymes in porcine enterocytes

We have recently reported the synthesis of urea from ammonia, glutamine and arginine in enterocytes of postweaning pigs. The present study was conducted to determine the compartmentation and kinetics of urea cycle enzymes in these cells. Carbamoyl phosphate synthase I (CPS I) and ornithine carbamoyltransferase (OCT) were located exclusively in mitochondria, whereas argininosuccinate synthase (ASS) and argininosuccinate lyase (ASL) were found in the cytosol. Arginase isozymes were present in both the cytosol and mitochondria of enterocytes, and differed in their sensitivity to heat inactivation. Except for OCT, Vmax values of urea cycle enzymes were much lower in enterocytes than in the liver of pigs, and vice versa for their Km values. Because of a low rate of ureagenesis in enterocytes compared with the liver, intestinal urea cycle enzymes may function primarily to synthesize citrulline. The co-localization of CPS I and OCT and a high activity of OCT in enterocyte mitochondria favors the intestinal synthesis of citrulline from ammonia, HCO3- and ornithine. Low activities of cytosolic ASS and ASL minimize the conversion of citrulline into arginine and therefore, the recycling of citrulline into ornithine via arginase in postweaning-pig enterocytes. These kinetic properties of intestinal urea cycle enzymes maximize the net synthesis of citrulline from glutamine and explain the release of large amounts of citrulline by the pig small intestine. The two compartmentally separated arginase isozymes in enterocytes may play an important role in regulating the intestinal metabolism of proline, nitric oxide and polyamines.

Intestinal carbamoyl phosphate synthase I in human and rat. Expression during development shows species differences and mosaic expression in duodenum of both species

The clinical importance of carbamoyl phosphate synthase I (CPSI) relates to its capacity to metabolize ammonia, because CPSI deficiencies cause lethal serum ammonia levels. Although some metabolic parameters concerning liver and intestinal CPSI have been reported, the extent to which enterocytes contribute to ammonia conversion remains unclear without a detailed description of its developmental and spatial expression patterns. Therefore, we determined the patterns of enterocytic CPSI mRNA and protein expression in human and rat intestine during embryonic and postnatal development, using in situ hybridization and immunohistochemistry. CPSI protein appeared during human embryogenesis in liver at 31-35 e. d. (embryonic days) before intestine (59 e.d.), whereas in rat CPSI detection in intestine (at 16 e.d.) preceded liver (20 e.d.). During all stages of development there was a good correlation between the expression of CPSI protein and mRNA in the intestinal epithelium. Strikingly, duodenal enterocytes in both species exhibited mosaic CPSI protein expression despite uniform CPSI mRNA expression in the epithelium and the presence of functional mitochondria in all epithelial cells. Unlike rat, CPSI in human embryos was expressed in liver before intestine. Although CPSI was primarily regulated at the transcriptional level, CPSI protein appeared mosaic in the duodenum of both species, possibly due to post-transcriptional regulation.

Effect of an elemental vs a complex diet on L-citrulline production from L-arginine in rat isolated enterocytes

BACKGROUND

L-Arginine and L-glutamine are highly metabolized by intestinal cells, leading to various metabolites, including L-citrulline, which is required for optimal growth. Elemental diets, used in clinical practice to treat growth failure and malnutrition, are very different from complex diets normally consumed. The aim of the present study was to assess the effects of an elemental diet compared with a complex diet on L-arginine metabolism in rat isolated enterocytes and its modulation by L-glutamine.

METHODS

Rats were fed the elemental diet (group ED) or the control diet (group C) for 14 days. Villus enterocytes then were isolated, and metabolic capacities or enzyme activities were assessed.

RESULTS

The incubation of enterocytes isolated from group C with 0.1 mmol/L L-[U-14C]-arginine led to the production of 125 +/- 25 pmol L-citrulline/10(6) cells per 30 minutes. This production showed a twofold increase in the presence of 2 mmol/L L-glutamine. In group ED, L-citrulline synthesis from L-arginine was markedly lower in the absence or in the presence of L-glutamine. This coincided with lower carbamoylphosphate synthase I activity and carbamoylphosphate (CP) content of enterocytes. Other L-arginine and L-glutamine metabolic pathways were not affected. Similar results were obtained when the elemental diet was administered continuously through a gastric catheter or fed by mouth.

CONCLUSIONS

L-Glutamine favors the synthesis of L-citrulline from L-arginine in isolated enterocytes, probably via an increase in CP production. Changing the diet composition, from a complex to an elemental diet, results in an alteration of the enterocyte capacity to synthesize L-citrulline from L-arginine, irrespective of the rhythm of delivery.

Regulation of argininosuccinate synthetase mRNA level in rat foetal hepatocytes

Expression of the hepatic gene for argininosuccinate synthase (ASS), one of the key enzymes of the urea cycle, was analysed during the perinatal period in the rat. To this end, the amount of specific mRNA was measured in the liver at various stages of development and in cultured foetal hepatocytes maintained in different hormonal conditions. The ASS mRNA was first detected in 15.5-day foetuses and its level increased concomitantly with a rise in the enzyme activity, suggesting that the appearance of the ASS activity reflects the turning on of specific gene transcription. This was demonstrated by run-on assay which showed an enhanced rate of transcription of the ASS gene during the perinatal period. When foetal hepatocytes were cultured with dexamethasone, a dose-dependent increase in ASS mRNA was measured, which was completely abolished by actinomycin D addition. The transcription rate of the gene was increased about twofold in the presence of the steroid, as measured by nuclear run-on assay. This transcriptional action could additionally require a protein factor since it could be inhibited by the simultaneous addition of puromycin. Insulin or glucagon respectively repressed or enhanced the dexamethasone-induced accumulation of ASS mRNA when added simultaneously with the steroid for 24 h. This developmental regulation of the ASS mRNA by glucocorticoids, insulin and glucagon could account for the modulation of the enzyme activity previously observed in vivo and in vitro in the foetal liver.

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.

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.

Abnormal gene expression and regulation in the liver of jvs mice with systemic carnitine deficiency

Carnitine-deficient jvs mice expressed reduced levels of a group of genes which are preferentially expressed in the liver, including urea cycle enzyme genes (Biochim. Biophys. Acta 1138, 167-171, 1992). The expression of alpha-fetoprotein and aldolase A was elevated, indicating that the liver of jvs mice is undifferentiated or dedifferentiated (FEBS Lett. 311, 63-66, 1992). Studies of the hormone signal transduction pathway showed that serum cortisol and plasma glucagon levels of jvs mice were 2 and 3 times higher, respectively, than those of normal mice, and that the hormone binding activity of glucocorticoid receptor (GR) in the cytosol of jvs liver was 50% of normal mice, which reflected the amount of receptor protein in the cytosol. On the other hand, GR protein accumulated in the nuclear fraction in jvs mice. Exogenously administrated dexamethasone induced carbamoyl phosphate synthetase (CPS) and tyrosine aminotransferase (TAT) mRNAs in jvs mice, indicating that CPS and TAT genes in jvs mice are responsive to induction by glucocorticoid and cAMP. Analysis of transacting factors by gel retardation assay revealed that HNF-1, COUP-TF and SP-1 were detected at almost the same level in the hepatic nuclear fraction of jvs mice as in normal littermates, and C/EBP and CREB were a little higher in jvs mice, suggesting that these factors are probably not targets of jvs mutation causing abnormal gene expression in the liver. On the other hand, AP-1 binding activity was much higher in jvs mice from an early age, preceding the abnormal expression of urea cycle enzyme, and carnitine administration normalized AP-1 binding activity. We suggest that elevated AP-1 binding induced by carnitine deficiency is closely connected with the abnormal gene expression in the liver.

Expression patterns of ammonia-metabolizing enzymes in the liver, mesonephros, and gut of human embryos and their possible implications

Human and ungulate embryos can catabolize amino acids for energy production, whereas rodent embryos cannot, raising the question whether studies of rodent model systems are suitable for extrapolation to the human situation. Therefore, we investigated the expression of the amino acid- and ammonia-metabolizing enzymes glutaminase, glutamate dehydrogenase, glutamine synthase, carbamoylphosphate synthase, and arginase immunohistochemically in a graded series of human embryos and fetuses. During human development the expression of these enzymes is first seen in the liver, then in the mesonephric kidney, and finally in the small intestine. Such a simultaneous expression of nitrogen-metabolizing enzymes was not seen in any other organ. The early appearance of the enzymes involved in amino acid and ammonia metabolism in the human liver, compared to, for example, the rat liver, suggests that catabolism of amino acids may provide an important supply of metabolic energy for the human embryo. The coexpression of glutaminase, glutamate dehydrogenase, and carbamoylphosphate synthase, but not of arginase, in the mesonephros and the small intestine suggests that these organs are involved in the biosynthesis of intermediates of the ornithine cycle, e.g., arginine or citrulline. From a comparison of the developmental appearance of ornithine cycle enzymes in different mammalian species we postulate that an early appearance of these enzymes is generally associated with a relatively slow prenatal growth rate and the use of amino acids as metabolic fuel.

Development of pyrroline-5-carboxylate synthase and N-acetylglutamate synthase and their changes in lactation and aging

Using newly developed assay procedures, we studied the development of pyrroline-5-carboxylate synthase (PCS) and N-acetylglutamate synthase (AGAS) activity in rat tissues. PCS in the small intestine of fetuses was 1/5 that of adults and reached an adult level as early as postnatal Day 1. The highest peak was observed at Day 14, and then activity decreased to the adult level. However, PCS in the brain was highest at birth and quickly inactivated in a few days. AGAS in the fetus small intestine was 1/3 that of adults and became higher than the adult level by 40% at Day 1 but was reduced to 1/2 that of adults at Day 3. Subsequently activity increased gradually to the adult level at Day 24. On the contrary, AGAS in the fetus liver was only 1/20 that of adults, and activity increased slowly up to 10 weeks and more. Pregnancy and lactation reduced liver AGAS markedly up to Day 8 and intestinal PCS considerably up to Day 14 after parturition. PCS in the small intestine of senescent rats was almost halved compared to young controls on a whole tissue basis. AGAS in the small intestine was also halved on a gram wet weight basis. Nonetheless the liver AGAS of 430-day-old rats was higher than that of the controls, although that of 630-day rats was lower. The results indicate that the arginine synthesizing enzymes in the small intestine are highly activated in suckling and weaning, and raise a question whether arginine remains fully dispensable in pregnancy, lactation, and senescence.

The interaction of cardiolipin with rat liver carbamoyl phosphate synthetase I

A selective interaction of rat liver carbamoyl phosphate synthetase I with cardiolipin, and other anionic phospholipids, has been demonstrated. The enzymatic activity of the synthetase is inhibited by cardiolipin and, to a lesser extent, by phosphatidylglycerol, phosphatidylinositol, and phosphatidylserine. This group of anionic phospholipids also induced a conformational change in the synthetase, yielding a species with increased exposure of the linkages between independently folded domains of the enzyme, as determined by limited proteolysis under nondenaturing conditions. The interaction of cardiolipin with carbamoyl phosphate synthetase I was a fairly slow process, with complex kinetics, and was apparently irreversible. The inclusion of Mg2+ or of MgATP in the incubation mixture prevented the cardiolipin effects. The zwitterionic phospholipids phosphatidylcholine and phosphatidylethanolamine had negligible effects on the structure and activity of the synthetase. This interaction between cardiolipin and carbamoyl phosphate synthetase I potentially constitutes one of the mechanisms by which the synthetase forms its loose association with the inner mitochondrial membrane. Multiple mechanisms, including synthetase conformational changes, cardiolipin phase changes, and ATP/ADP binding site involvement, are possibly involved in the phospholipid/synthetase interaction and the resulting potential regulatory mechanism(s) for urea cycle activity.

Expression of argininosuccinate lyase mRNA in foetal hepatocytes. Regulation by glucocorticoids and insulin

Argininosuccinate lyase (ASL), the fourth enzyme of the urea cycle, belongs to a group of liver enzymes appearing in the late foetal period in the rat. Several hormones, including glucocorticosteroids and insulin have been implicated in the control of the development of this enzyme activity. In this study, the cloned cDNA was used to measure the relative abundance of ASL mRNA in the livers of rats at various stages of perinatal development and in cultured foetal hepatocytes during hormonal manipulations. The ASL mRNA was first detectable on day 15.5 of gestation and increased in amount concomitantly with the rise in the enzyme activity, suggesting that the appearance of enzyme activity reflects the turning on of specific gene transcription. When foetal hepatocytes were exposed to dexamethasone, an increase in ASL mRNA was detected, which was completely abolished by addition of actinomycin D, suggesting a transcriptional effect of the steroid. In contrast, administration of cortisol to foetuses in utero had no effect on the mRNA level, suggesting that the steroid action is inhibited in the intra-uterine environment. Insulin might be the inhibiting factor since it completely repressed the dexamethasone-induced accumulation of ASL mRNA in foetal hepatocytes. These data were confirmed in vivo by experiments using streptozotocin, which produces insulin-depleted foetuses and causes the accumulation of ASL mRNA. This regulation of ASL mRNA by glucocorticoids and insulin could account for the modulation of the enzyme activity observed in vivo and in vitro.

Promoter and 11-kilobase upstream enhancer elements responsible for hepatoma cell-specific expression of the rat ornithine transcarbamylase gene

The gene for ornithine transcarbamylase (OTC; EC 2.1.3.3), a urea cycle enzyme, is expressed almost exclusively in the liver and small intestine. To identify DNA elements regulating transcription of the OTC gene in the liver, transient expression analysis was carried out by using hepatoma (HepG2) and nonhepatic (CHO) cell lines. The 1.3-kilobase 5'-flanking region of the rat OTC gene directed expression of the fused chloramphenicol acetyltransferase gene in HepG2 cells much more efficiently than in CHO cells. Analysis of deletion mutants of the 5'-flanking region in HepG2 cells revealed that there are at least one negative and two positive regulatory elements within the about 220-base-pair immediate 5'-flanking region. DNase I footprint analysis showed the presence of factors binding to these regulatory elements in nuclear extracts of rat liver and brain, and footprint profiles at the two positive elements exhibited liver-specific features. Transient expression analysis also revealed the existence of an enhancer region located 11 kilobases upstream of the transcription start site. The OTC enhancer was able to activate both its own and heterologous promoters in HepG2 but not in CHO cells. The enhancer was delimited to an about 230-base-pair region, and footprint analysis of this region revealed four protected areas. Footprint profiles at two of the four areas exhibited liver-specific features, and gel shift competition analysis showed that a factor(s) binding to the two liver-specific sites is related to C/EBP. These results suggest that both liver-specific promoter and enhancer elements regulate expression of the OTC gene through interaction with liver-specific factors binding to these elements.

Promoter and 11-kilobase upstream enhancer elements responsible for hepatoma cell-specific expression of the rat ornithine transcarbamylase gene

The gene for ornithine transcarbamylase (OTC; EC 2.1.3.3), a urea cycle enzyme, is expressed almost exclusively in the liver and small intestine. To identify DNA elements regulating transcription of the OTC gene in the liver, transient expression analysis was carried out by using hepatoma (HepG2) and nonhepatic (CHO) cell lines. The 1.3-kilobase 5'-flanking region of the rat OTC gene directed expression of the fused chloramphenicol acetyltransferase gene in HepG2 cells much more efficiently than in CHO cells. Analysis of deletion mutants of the 5'-flanking region in HepG2 cells revealed that there are at least one negative and two positive regulatory elements within the about 220-base-pair immediate 5'-flanking region. DNase I footprint analysis showed the presence of factors binding to these regulatory elements in nuclear extracts of rat liver and brain, and footprint profiles at the two positive elements exhibited liver-specific features. Transient expression analysis also revealed the existence of an enhancer region located 11 kilobases upstream of the transcription start site. The OTC enhancer was able to activate both its own and heterologous promoters in HepG2 but not in CHO cells. The enhancer was delimited to an about 230-base-pair region, and footprint analysis of this region revealed four protected areas. Footprint profiles at two of the four areas exhibited liver-specific features, and gel shift competition analysis showed that a factor(s) binding to the two liver-specific sites is related to C/EBP. These results suggest that both liver-specific promoter and enhancer elements regulate expression of the OTC gene through interaction with liver-specific factors binding to these elements.

Activity of the carbamyl phosphate synthetase I promoter in liver nuclear extracts is dependent on a cis-acting C/EBP recognition element

We have identified an essential cis element in the proximal promoter region of the rat carbamyl phosphate synthetase I (CPSI) gene that is requisite for promoter activity in liver nuclear extracts. Excess synthetic oligonucleotides specifying this region abolished promoter-dependent in vitro transcription. We show that C/EBP, a nuclear factor enriched in liver but found as well in other tissues, such as gut, fat, and lung, interacts with an inverted repeat, GTTGCAAC, at the core of the essential cis element. In brain, a tissue that did not express CPSI or contain significant levels of C/EBP, a different factor was capable of binding at or near the C/EBP recognition element. Activity of the CPSI promoter in liver nuclear extracts was also dependent on sequences 5' to the C/EBP motif; presumably, factors binding to elements within this upstream region are instrumental in restricting CPSI gene expression to liver and intestinal mucosa.

Activity of the carbamyl phosphate synthetase I promoter in liver nuclear extracts is dependent on a cis-acting C/EBP recognition element

We have identified an essential cis element in the proximal promoter region of the rat carbamyl phosphate synthetase I (CPSI) gene that is requisite for promoter activity in liver nuclear extracts. Excess synthetic oligonucleotides specifying this region abolished promoter-dependent in vitro transcription. We show that C/EBP, a nuclear factor enriched in liver but found as well in other tissues, such as gut, fat, and lung, interacts with an inverted repeat, GTTGCAAC, at the core of the essential cis element. In brain, a tissue that did not express CPSI or contain significant levels of C/EBP, a different factor was capable of binding at or near the C/EBP recognition element. Activity of the CPSI promoter in liver nuclear extracts was also dependent on sequences 5' to the C/EBP motif; presumably, factors binding to elements within this upstream region are instrumental in restricting CPSI gene expression to liver and intestinal mucosa.

Abundance of mRNAs encoding urea cycle enzymes in fetal and neonatal mouse liver

The relative abundances of mRNAs encoding the five urea cycle enzymes during development of mouse liver have been determined and compared with those of mRNAs encoding four other liver-specific proteins (phosphoenolpyruvate carboxykinase, tyrosine aminotransferase, alpha-fetoprotein, and albumin). Urea cycle enzyme mRNAs in fetal liver are expressed at 2-14% of the abundance in adult liver as early as 6 days before birth. Expression of the urea cycle enzyme mRNAs is not coordinate during the fetal and neonatal period. However, profiles of three urea cycle enzyme mRNAs are quite similar to that of alpha-fetoprotein mRNA, suggesting the possibility of a common response to regulatory signals during fetal development. With the exception of ornithine transcarbamylase mRNA, the urea cycle enzyme mRNAs have been shown previously to be inducible by cAMP and glucocorticoids. However, only argininosuccinate lyase mRNA exhibits any significant change in abundance at birth, resembling postnatal expression of tyrosine aminotransferase mRNA. The results indicate that the urea cycle enzyme mRNAs are potentially useful markers for elucidating various features of hepatocyte differentiation in mammals.

Tissue- and developmental stage-specific expression of the rat ornithine carbamoyltransferase gene in transgenic mice

Rat ornithine carbamoyltransferase (OCT; EC 2.1.3.3) is encoded by a large gene of 75 kilobases. Expression of this gene is restricted to the liver and small intestine, and there is an increase in expression late in gestation. The recombinant gene carrying 1.3 kilobases of the 5' flanking region of the gene fused to the rat OCT cDNA was microinjected into fertilized eggs, and 17 transgenic mice were produced. Expression in the liver of the transgene was detected in three mice. In these mice, the transgene expression was observed exclusively in the liver and small intestine. Expression of the transgene in the intestine was comparable to that of the endogenous mouse OCT gene, whereas expression in the liver was much lower than that of the endogenous gene. The developmental pattern of expression of the transgene was similar to that of the endogenous gene. Therefore, the 5' flanking sequence of the rat OCT gene seems to be sufficient for the developmental and tissue-specific expression of the gene. An explanation for low expression in the liver remains the subject of ongoing study.

Long-term ingestion of ammonium increases acetylglutamate and urea levels without affecting the amount of carbamoyl-phosphate synthase

Rats were fed the following diets: standard (20% protein), high-protein (80%), protein-free, standard plus ammonium and protein-free plus ammonium for six weeks. The standard plus ammonium diet was prepared to contain ammonia equivalent to that supplied by the high-protein diet. Addition of ammonium acetate (20% by mass) to the 20% protein or protein-free diets results in 2.3- and 10-fold increases of urea excretion respectively, without increase of carbamoyl-phosphate synthase. Supplementation of the standard diet with ammonium increases the mitochondrial content of acetylglutamate from 830 to 1590 pmol/mg protein, and of the protein-free diet from 130 to 1040 pmol/mg. However, ingestion of ammonium did not increase the activity of acetylglutamate synthase. Therefore the efflux of acetylglutamate from mitochondria was determined. After 30 min at 37 degrees C liver mitochondria from rats on standard diet released 61% of the initial acetylglutamate while mitochondria from animals on standard plus ammonium diet released only 20%. These results indicate that ingestion of ammonium increases the content of acetylglutamate in rat liver by decreasing its efflux from mitochondria. This effect is similar to that produced in mice by a high protein diet [Morita et al. (1982) J. Biochem. (Tokyo) 91, 563-569]. However, while the high-protein diet increases carbamoylphosphate synthase content, the ammonium diet does not.