Nucleotide sequence of rat glutamine synthetase mRNA

An intronic silencer element is responsible for specific zonal expression of glutamine synthetase in the rat liver

The most striking phenomenon of glutamine synthetase (GS) expression in the liver is its unique restriction to cells surrounding the terminal hepatic venules. Expression is positively regulated by elements located in the 5'-upstream region and in the first intron of the gene. It was long believed that transcription factors present in GS-positive cells and absent in GS-negative cells are responsible for the phenomenon of zonal expression. However, strong enhancers are equally active in both types of cells. Therefore, the existence of a silencer mechanism in GS-negative hepatocytes was postulated. In the present study, a GS silencer element was investigated that was previously identified within the first intron and was shown to be able to prevent glucocorticoid-induced expression in cells negative for a transacting factor designated GS silencer element-binding protein. Reporter gene assays with the silencer element in combination with the most potent 5'-enhancer of the GS gene demonstrate that the silencer element is able to prevent enhancement mediated by the 5'-enhancer in combination with a heterologous as well as with the homologous promoter. More importantly, the effect of the silencer is shown to be restricted to GS-negative hepatocytes. In conclusion, the phenomenon of zonal expression of GS in the liver is caused by a protein present in GS-negative cells and absent in GS-positive cells that interacts with the silencer element in the first intron and not by a differential expression of enhancer-binding proteins.

Hypoxia regulates glutamate metabolism and membrane transport in rat PC12 cells

We investigated the effect of hypoxia on glutamate metabolism and uptake in rat pheochromocytoma (PC12) cells. Various key enzymes relevant to glutamate production, metabolism and transport were coordinately regulated by hypoxia. PC12 cells express two glutamate-metabolizing enzymes, glutamine synthetase (GS) and glutamate decarboxylase (GAD), as well as the glutamate-producing enzyme, phosphate-activated glutaminase (PAG). Exposure to hypoxia (1% O(2)) for 6 h or longer increased expression of GS mRNA and protein and enhanced GS enzymatic activity. In contrast, hypoxia caused a significant decrease in expression of PAG mRNA and protein, and also decreased PAG activity. In addition, hypoxia led to an increase in GAD65 and GAD67 protein levels and GAD enzymatic activity. PC12 cells express three Na(+)-dependent glutamate transporters; EAAC1, GLT-1 and GLAST. Hypoxia increased EAAC1 and GLT-1 protein levels, but had no effect on GLAST. Chronic hypoxia significantly enhanced the Na(+)-dependent component of glutamate transport. Furthermore, chronic hypoxia decreased cellular content of glutamate, but increased that of glutamine. Taken together, the hypoxia-induced changes in enzymes related to glutamate metabolism and transport are consistent with a decrease in the extracellular concentration of glutamate. This may have a role in protecting PC12 cells from the cytotoxic effects of glutamate during chronic hypoxia.

Glucocorticoid control of glial gene expression

The glucocorticoid signaling pathway is responsive to a considerable number of internal and external signals and can therefore establish diverse patterns of gene expression. A glial-specific pattern, for example, is shown by the glucocorticoid-inducible gene glutamine synthetase. The enzyme is expressed at a particularly high level in glial cells, where it catalyzes the recycling of the neurotransmitter glutamate, and at a low level in most other cells, for housekeeping duties. Glial specificity of glutamine synthetase induction is achieved by the use of positive and negative regulatory elements, a glucocorticoid response element and a neural restrictive silencer element. Though not glial specific by themselves, these elements may establish a glial-specific pattern of expression through their mutual activity and their combined effect. The inductive activity of glucocorticoids is markedly repressed by the c-Jun protein, which is expressed at relatively high levels in proliferating glial cells. The signaling pathway of c-Jun is activated by the disruption of glia-neuron cell contacts, by transformation with v-src, and in proliferating retinal cells of early embryonic ages. The c-Jun protein inhibits the transcriptional activity of the glucocorticoid receptor and thus represses glutamine synthetase expression. This repressive mechanism might also affect the ability of glial cells to cope with glutamate neurotoxicity in injured tissues.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

Heterogeneous (positional) expression of hepatic glutamine synthetase: features, regulation and implications for hepatocarcinogenesis

Glutamine synthetase expression in liver parenchyma is restricted to a small population of pericentral hepatocytes surrounding the central veins. Studies on the development of this heterogeneous (positional) gene expression and of the changes observed in response to experimental alterations of liver physiology or manipulations of hepatocytes in culture have revealed that it is dependent on cell-cell and cell-matrix interactions rather than on the levels of hormones and other modulating factors. The considerable stability of GS expression may point to further events leading to a defined differentiated GS+ phenotype. Observations during experimental hepatocarcinogenesis indicate that strong GS expression may be used for tracing hepatocellular lineages during preneoplastic and early neoplastic stages. Furthermore, these studies suggest a relationship between the GS+ phenotype and enhanced growth of these lesions. Future studies should help to define the diagnostic value of GS and its significance for new chemotherapeutic strategies.

Identification and functional characterization of regulatory elements of the glutamine synthetase gene from rat liver

Hepatic glutamine synthetase (GS) shows a unique expression pattern limited to a few hepatocytes surrounding the terminal hepatic veins. Starting from the genomic clone of the rat GS gene, lambda GS1 [Van de Zande, L. P. G. W., Labruyère, W. T., Arnberg, A. C., Wilson, R. H., Van den Bogaert, A. J. W., Das, A. T., Frijters, C., Charles, R., Moorman, A. F. M. & Lamers, W. H. (1990) Gene (Amst.) 87, 225-232] additional genomic clones containing up to 9 kb of 5'flanking region were isolated in order to characterize cis-acting elements involved in the regulation of GS expression. Sequence analysis of the 5'flanking region up to -2520 bp revealed a putative AP2-binding site at -223 bp and a second GC box at -2343 bp in addition to the canonical TATA, CCAAT and GC boxes found proximal to the transcription-start site. A possible negative glucocorticoid-responsive element (GRE) and regions with very weak similarity to a GRE and to a known silencer element were noted at -506 bp, -406 bp and at -798 bp, respectively. Within the sequenced part of the 5'flanking region no known regulatory elements associated with liver-specific gene expression were found except for a putative HNF3-binding site at -896 bp. Functional analysis by transient transfection assays using constructs with the pSSCAT or the pXP1 vector revealed that the elements present within the first 153 bp and particularly the first 368 bp of upstream sequence constitute an active promoter the activity of which is decreased by additional sequences up to -2148 bp. The presence of dexamethasone led to a 2-4-fold increase in the promoter activity of all these constructs. Using the heterologous truncated thymidine-kinase-gene promoter of the plasmid pT81-luc a strong enhancer element was located between -2520 bp and -2148 bp. Its activity was not affected by dexamethasone but was negatively influenced by flanking sequences in both directions. This enhancer was also effective with the homologous GS promoter (-153 to +59 bp) and the heterologous full thymidine-kinase-gene promoter (pT109luc). No further enhancers were found up to -6200 bp. Using the same approach, a second enhancer was found between +259 bp and +950 bp within the first intron. Deoxyribonuclease-I hypersensitivity studies confirmed the presence of a hypersensitive site between +350 bp and +550 bp and suggested a second site between +850 bp and +1200 bp.(ABSTRACT TRUNCATED AT 400 WORDS)

cDNA sequence of the long mRNA for human glutamine synthase

Screening a human liver cDNA library in lambda ZAP revealed several clones for the mRNA of glutamine synthase. The longest clone was completely sequenced and consists of a 109 bp 5' untranslated region, a 1119 bp protein coding region, a 1498 bp 3' untranslated region and a poly(A) tract of 12 bp.

Cloning and functional characterization of the rat glutamine synthetase gene

Glutamine synthetase catalyzes the formation of glutamine from glutamate and ammonia. It plays a central role in both amino acid neurotransmitter metabolism and ammonia detoxification in the central nervous system. Glutamine synthetase expression is regulated in developmental, hormonal, and in tissue- and cell-specific manners. We have cloned a full-length cDNA coding for rat glutamine synthetase, and have found an AT-rich area of conservation in the 3' untranslated regions between rat, mouse, and chicken, which may play a part in the regulation of the stability of the glutamine synthetase message. We have also cloned and mapped the gene coding for rat glutamine synthetase, and identified, by sequence analysis, areas potentially important for the regulation of glutamine synthetase transcription. Transient transfection of a variety of cell lines with deletion constructs of the glutamine synthetase promoter driving a chloramphenicol acetyltransferase reporter gene functionally demonstrates regions of the promoter containing elements important for transcriptional regulation.

Diet- and hormone-induced reversal of the carbamoylphosphate synthetase mRNA gradient in the rat liver lobulus

A hybridocytochemical analysis of adult liver from normal control and from hormonally and dietary-treated rats was carried out, using radioactively-labelled probes for the mRNAs of glutamine synthetase (GS), carbamoylphosphate synthetase (CPS) and phosphoenolpyruvate carboxykinase (PEPCK). In line with previous findings, GS mRNA is exclusively expressed in a small pericentral compartment, CPS mRNA exclusively in a contiguous large periportal compartment and PEPCK mRNA across the entire porto-central distance. The density of labelling in CPS and PEPCK mRNA-positive hepatocytes decreases in a porto-central direction. Starvation resulted in a reversal of the gradient of CPS mRNA within its periportal compartment; glucose refeeding counteracted this effect. Livers of glucocorticosteroid-treated, starved or diabetic rats also revealed a reversal of the normal gradient of CPS mRNA, but now across the entire porto-central distance. The patterns of expression of GS and PEPCK mRNA remained essentially unchanged, notwithstanding substantial changes in the levels of expression. It is concluded that blood-borne factors constitute the major determinants for the expression patterns of CPS mRNA within the context of the architecture of the liver lobulus.

Expression patterns of mRNAs for ammonia-metabolizing enzymes in the developing rat: the ontogenesis of hepatocyte heterogeneity

The expression patterns of the mRNAs for the ammonia-metabolizing enzymes carbamoylphosphate synthetase (CPS), glutamine synthetase (GS) and glutamate dehydrogenase (GDH) were studied in developing pre- and neonatal rat liver by in situ hybridization. In the period of 11 to 14 embryonic days (ED) the concentrations of GS and GDH mRNA increases rapidly in the liver, whereas a substantial rise of CPS mRNA in the liver does not occur until ED 18. Hepatocyte heterogeneity related to the vascular architecture can first be observed at ED 18 for GS mRNA, at ED 20 for GDH mRNA and three days after birth for CPS mRNA. The adult phenotype is gradually established during the second neonatal week, i.e. GS mRNA becomes confined to a pericentral compartment of one to two hepatocytes thickness, CPS mRNA to a large periportal compartment being no longer expressed in the pericentral compartment and GDH mRNA is expressed over the entire porto-central distance, decreasing in concentration going from central to portal. Comparison of the observed mRNA distribution patterns in the perinatal liver, with published data on the distribution of the respective proteins, points to the occurrence of posttranslational, in addition to pretranslational control mechanisms in the period of ontogenesis of hepatocyte heterogeneity. Interestingly, during development all three mRNAS are expressed outside the liver to a considerable extent and in a highly specific way, indicating that several organs are involved in the developmentally regulated expression of the mRNAs for the ammonia-metabolizing enzymes, that were hitherto not recognized as such.

Isolation and characterization of the rat glutamine synthetase-encoding gene

From a rat genomic library in phage lambda Charon4A, a complete glutamine synthetase-encoding gene was isolated. The gene is 9.5-10 kb long, consists of seven exons, and codes for two mRNA species of 1375 nucleotides (nt) and 2787 nt, respectively. For both mRNAs, full-length cDNAs containing a short poly(A) tract were identified. The sequences of the entire mRNA and of the exon-intron transitions were determined. The smaller mRNA is identical to the 5' 1375 nt of the long mRNA and contains the entire protein-coding region. The position of the transcription start point was mapped. Within the first 118 bp of promoter sequence, a (T)ATAA-box, a CCAAT-box and an SP1-binding site were identified.

The structure of the chicken glutamine synthetase-encoding gene

Complementary DNA (cDNA) and genomic clones encoding chicken glutamine synthetase (Glns) have been isolated. The nucleotide (nt) sequence of the 2728-bp cDNA specifies a 91-nt 5' untranslated sequence, a 1119-nt open reading frame, and a 1518-nt 3' untranslated sequence that contains several A + T-rich regions but lacks a canonical endonucleolytic-cleavage/polyadenylation signal. Based on sequence analysis of the cloned gene, the Glns transcription unit spans 7.0 kb and contains seven exons.

Mouse glutamine synthetase is encoded by a single gene that can be expressed in a localized fashion

Two mouse glutamine synthetase (GSase) cDNAs were cloned that correspond to the 2.8 kb and 1.4 kb mRNA species found in many mouse tissues (1 kb = 10(3) base-pairs). There is a sequence homology of about 90% to other mammalian GSase cDNAs in the coding region. A 2.1 kb mRNA can be discerned in fat tissue, the most abundant source of GSase mRNA. Three genomic clones G4, G21 and G2 contain GSase sequences. By several criteria G21 and G2 are pseudogenes, while G4 is a functional gene composed of seven exons and six introns. Primer extension, RNase protection and Northern analysis provide evidence that all tissues use the same major RNA start site and the different-sized mRNAs are due to the usage of two different poly(A) sites, neither of which has the consensus AAUAAA sequence. When tested by transfection into Hep G2 human hepatoma cells the G4 promoter can produce correctly initiated mRNA with only 350 base-pairs of 5' regulatory sequences. A major interest in GSase expression is its restriction to pericentral hepatocytes in adult liver. In this paper we show by in situ hybridization that GSase mRNA is only found in glial cells in the adult brain and in proximal tubular epithelium of the kidney. Coupled with the earlier demonstration of expression of GSase only in pericentral hepatocytes, it is clear that this gene is regulated by position-specific signals in many cell types.