Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:6.3.5.5 (CPS)
 1,262 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Glucocorticosteroids and cyclic AMP induce carbamoylphosphate synthetase (ammonia) (CPS) in rat hepatocytes. Using an enzyme immunoassay applied to hepatocyte cultures fixed in situ, it has been demonstrated that the capacity of hepatocytes to synthesize CPS in the presence of both hormones is present as soon as the cells become recognizable as hepatocytes. Immunochemical staining of the cultures shows that hepatocytes do not acquire or express the capacity to accumulate CPS at high rates synchronously. The average levels of CPS per hepatocyte that are observed upon hormone treatment are approx 50-fold lower in embryonic than in adult hepatocytes, corresponding with an approx 10-fold lower synthetic capacity (per gram hepatocytes) and an approx 5-fold smaller size of embryonic compared to adult hepatocytes. Carbamoylphosphate synthetase levels are therefore a good parameter in studies that aim to establish the mechanisms that underly the ontogenesis of the hepatic phenotype.
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PMID:Inducibility of carbamoylphosphate synthetase (ammonia) in cultures of embryonic hepatocytes: ontogenesis of the responsiveness to hormones. 609 Feb 46

'77orn', a derivative of the Morris rat hepatoma 7777, stably expresses high levels of ornithine transcarbamoylase (OTC) and carbamoylphosphate synthetase I (CPS-I), and is able to grow indefinitely in ornithine-medium (medium with ornithine in place of arginine). Variants that have lost this ability are isolated from 77orn by a 'suicide' selective technique dependent on the cellular incorporation of [3H]ornithine. These variants, which have reduced levels of CPS-I, or of both CPS-I and OTC, are shown to have developed multiple hormonal requirements; their enzyme deficiencies can be reversed by use of an appropriately supplemented medium. In particular, CPS-I is inducible by dexamethasone and dibutyryl-cyclic-AMP in combination. Cholera toxin can be used instead of cyclic-AMP, but then butyrate is additionally required if the induction is to be maintained in the long term. The use of these agents in excess can depress OTC. Several other hepatomas, and alos explanted foetal rat liver cells, have similar requirements for CPS-I expression. It is argued that multiple hormonal requirements for CPS-I production are normal in liver cells in vitro, and that hormone-independent hepatomas should be regarded as abnormal. The implications of this for the somatic cell genetic investigation of differentiation are briefly discussed.
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PMID:Arginine synthesis by hepatomas in vitro. II. Isolation and characterization of Morris hepatoma variants unable to convert ornithine to arginine, and modulation of urea-cycle enzymes by dexamethasone and cyclic-AMP. 609 98

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.
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PMID:The far-upstream enhancer of the carbamoyl-phosphate synthetase I gene is responsible for the tissue specificity and hormone inducibility of its expression. 755 19

We have studied factors regulating the rate of protein degradation in cultured hepatocytes obtained from 17-day-old fetal, 7-day-old suckling, and 20-day-old weanling rats. At all three stages of development 60-70% of protein degradation was sensitive to inhibition by amino acids and 3-methyladenine, an inhibitor of macroautophagy, indicating a major role of the lysosomes in proteolysis under these conditions. A combination of dibutyryl cyclic AMP and dexamethasone strongly stimulated proteolysis in hepatocytes from weanling, but not from fetal and suckling rats. The stimulatory effect of these compounds was eliminated at high amino acid concentrations in the culture medium. Cultured perinatal hepatocytes responded to exposure to dibutyryl cyclic AMP and dexamethasone by de novo synthesis of mRNA for carbamoyl-phosphate synthase and for phosphoenolpyruvate carboxykinase, demonstrating that the developmental change in the effect of dibutyryl cyclic AMP and dexamethasone on proteolysis was due to a developmental change in the regulation of proteolysis. An analysis of the changes in intracellular amino acid concentrations in response to variations in the extracellular amino acid concentrations at all three stages of development showed that of all amino acids that could be identified, only Ile, Leu, Lys, Phe, and Tyr are implicated as possible regulators of hepatic proteolysis. Dibutyryl cyclic AMP and dexamethasone did not affect the intracellular concentrations of these amino acids, showing that hormonal regulation of proteolysis is not mediated by changes in intracellular concentrations of these amino acids. It is concluded that the lack of sensitivity of the proteolytic system to catabolic hormones in the period around birth, combined with higher circulating plasma amino acid concentrations, are mechanisms contributing to the low rate of intrahepatic proteolysis in vivo in the perinatal period and thus to the rapid growth of the liver in this period.
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PMID:Effects of intracellular amino acid concentrations, cyclic AMP, and dexamethasone on lysosomal proteolysis in primary cultures of perinatal rat hepatocytes. 838 May 74

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.
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PMID:The upstream regulatory region of the carbamoyl-phosphate synthetase I gene controls its tissue-specific, developmental, and hormonal regulation in vivo. 894 Jan 27

Amino acids are not only important precursors for the synthesis of proteins and other N-containing compounds, but also participate in the regulation of major metabolic pathways. Glutamate and aspartate, for example, are components of the malate/aspartate shuttle and their concentrations control the rate of mitochondrial oxidation of glycolytic NADH. Glutamate also controls the rate of urea synthesis, not only as the precursor of ammonia and aspartate, but as substrate for synthesis of N-acetylglutamate, the essential activator of carbamoyl-phosphate synthase. This mechanism allows large variations in urea synthesis at relatively constant ammonia concentrations. Increases in intracellular amino acid concentration increase cell volume. Cell swelling per se has anabolic effects on protein, carbohydrate and lipid metabolism: enhanced synthesis of macromolecules compensates for increases in intracellular osmolarity. Mechanisms responsible for cell swelling-induced changes in pathway fluxes include changes in intracellular ion concentrations and in signal transduction. Specific amino acids (e.g., leucine) stimulate protein synthesis and inhibit (autophagic) protein degradation independent of changes in cell volume because they stimulate mTOR (mammalian target of rapamycin), a protein kinase, which is one of the components of a signal transduction pathway used by insulin. When the cellular energy state is low, stimulation of mTOR by amino acids is prevented by activation of AMP-dependent protein kinase. Amino acid-dependent signaling also promotes insulin production by beta-cells. This further adds to the anabolic properties of amino acids. It is concluded that amino acids are important regulators of major metabolic pathways.
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PMID:Amino acids as regulators and components of nonproteinogenic pathways. 1277 65


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