Tissue-specific regulation of avian glutamine synthetase expression during development and in response to glucocorticoid hormones

Weak mitochondrial targeting sequence determines tissue-specific subcellular localization of glutamine synthetase in liver and brain cells

Evolution of the uricotelic system for ammonia detoxification required a mechanism for tissue-specific subcellular localization of glutamine synthetase (GS). In uricotelic vertebrates, GS is mitochondrial in liver cells and cytoplasmic in brain. Because these species contain a single copy of the GS gene, it is not clear how tissue-specific subcellular localization is achieved. Here we show that in chicken, which utilizes the uricotelic system, the GS transcripts of liver and brain cells are identical and, consistently, there is no difference in the amino acid sequence of the protein. The N-terminus of GS, which constitutes a 'weak' mitochondrial targeting signal (MTS), is sufficient to direct a chimeric protein to the mitochondria in hepatocytes and to the cytoplasm in astrocytes. Considering that a weak MTS is dependent on a highly negative mitochondrial membrane potential (DeltaPsi) for import, we examined the magnitude of DeltaPsi in hepatocytes and astrocytes. Our results unexpectedly revealed that DeltaPsi in hepatocytes is considerably more negative than that of astrocytes and that converting the targeting signal into 'strong' MTS abolished the capability to confer tissue-specific subcellular localization. We suggest that evolutional selection of weak MTS provided a tool for differential targeting of an identical protein by taking advantage of tissue-specific differences in DeltaPsi.

Cytoskeletal and cell contact control of the glucocorticoid pathway

The cytoskeleton is a dynamic network that undergoes restructuring during a variety of cellular events including cell contact formation, cell invasion and the mitotic phase of the cell cycle. Here, we review the contribution of the cytoskeletal network to the inductive activity of glucocorticoids by focusing on the hormonal control of glutamine synthetase in the chick neural retina. Depolymerization of the cytoskeleton in cells of the intact retinal tissue inhibits the hormonal induction of glutamine synthetase, but does not alter the cellular amount of the glucocorticoid-receptor protein or the ability of the receptor molecules to translocate into the nucleus. Inhibition of glutamine synthetase induction occurs via a mechanism that involves elevation of c-Jun protein accumulation and repression of glucocorticoid-receptor transcriptional activity. Unlike growth factors and other c-Jun inducing stimuli that control the transcription of the c-Jun gene, depolymerization of the cytoskeleton elevates c-Jun accumulation by upregulating the translation of the c-Jun transcript. We postulate that the cytoskeletal-dependent increase in c-Jun accumulation is involved in cell contact control of both cell proliferation and transcriptional activity of the glucocorticoid-receptor protein.

A single glutamine synthetase gene produces tissue-specific subcellular localization by alternative splicing

Glutamine synthetase (GS) plays a key role in two major biochemical pathways: In liver GS catalyzes ammonia detoxification, whereas in neural tissues it also functions in recycling of the neurotransmitter glutamate. In most species the GS gene gives rise to a cytoplasmic protein in both liver and neural tissues. However, in species that utilize the ureosmotic or uricotelic system for ammonia detoxification, the enzyme is cytoplasmic in neural tissues, but mitochondrial in liver cells. Since most vertebrates have a single copy of the GS gene, it is not clear how tissue-specific subcellular localization is achieved. Here we show that in the ureosmotic elasmobranch, Squalus acanthias (spiny dogfish), two different GS transcripts are generated by tissue-specific alternative splicing. The liver transcript contains an alternative exon that is not present in the neural one. This exon leads to acquisition of an upstream in-frame start codon and formation of a mitochondrial targeting signal (MTS). Therefore, the liver product is targeted to the mitochondria while the neural one is retained in the cytoplasm. These findings present a mechanism in which alternative splicing of an MTS-encoding exon is used to generate tissue-specific subcellular localization.

Glutamine synthetase enhances the clearance of extracellular glutamate by the neural retina

Clearance of synaptic glutamate by glial cells is required for the normal function of excitatory synapses and for prevention of neurotoxicity. Although the regulatory role of glial glutamate transporters in glutamate clearance is well established, little is known about the influence of glial glutamate metabolism on this process. This study examines whether glutamine synthetase (GS), a glial-specific enzyme that amidates glutamate to glutamine, affects the uptake of glutamate. Retinal explants were incubated in the presence of [(14)C]glutamate and glutamate uptake was assessed by measurement of the amount of radioactively labeled molecules within the cells and the amount of [(14)C]glutamine released to the medium. An increase in GS expression in Müller glial cells, caused by induction of the endogenous gene, did not affect the amount of glutamate accumulated within the cells, but led to a dramatic increase in the amount of glutamine released. This increase, which was directly correlated with the level of GS expression, was dependent on the presence of external sodium ions, and could be completely abolished by methionine sulfoximine, a specific inhibitor of GS activity. Our results demonstrate that GS activity significantly influences the uptake of glutamate by the neural retina and suggest that this enzyme may represent an important target for neuroprotective strategies.

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.

A silencer element in the regulatory region of glutamine synthetase controls cell type-specific repression of gene induction by glucocorticoids

Glutamine synthetase is a key enzyme in the recycling of the neurotransmitter glutamate. Expression of this enzyme is regulated by glucocorticoids, which induce a high level of glutamine synthetase in neural but not in various non-neural tissues. This is despite the fact that non-neural cells express functional glucocorticoid receptor molecules capable of inducing other target genes. Sequencing and functional analysis of the upstream region of the glutamine synthetase gene identified, 5' to the glucocorticoid response element (GRE), a 21-base pair glutamine synthetase silencer element (GSSE), which showed considerable homology with the neural restrictive silencer element NRSE. The GSSE was able to markedly repress the induction of gene transcription by glucocorticoids in non-neural cells and in embryonic neural retina. The repressive activity of the GSSE could be conferred on a heterologous GRE promoter and was orientation- and position-independent with respect to the transcriptional start site, but appeared to depend on a location proximal to the GRE. Gel-shift assays revealed that non-neural cells and cells of early embryonic retina contain a high level of GSSE binding activity and that this level declines progressively with age. Our results suggest that the GSSE might be involved in the restriction of glutamine synthetase induction by glucocorticoids to differentiated neural tissues.

Cyclic adenosine-3',5'-monophosphate-mediated activation of a glutamine synthetase composite glucocorticoid response element

The glutamate synthetase gene (GS) contains a composite glucocorticoid response element (cGRE) comprised of a GRE and an adjacent element with features of both a cAMP-response element (CRE) and a 12-O-tetradecanoylphorbol 13-acetate (TPA) response element (TRE). The CRE/TRE element of the cGRE contributed to two modes of transcriptional activation: 1) enhancement of the response to cortisol and 2) a synergistic response to cortisol and increased cAMP. COS-7 cells transfected with a cGRE-luciferase construct show minimal expression under basal conditions or forskolin treatment. After cortisol treatment, luciferase activity from the cGRE is enhanced 4- to 8-fold greater than the GRE portion of the cGRE or a GRE from the tyrosine aminotransferase gene. Treatment with both forskolin and cortisol produced a 2- to 4-fold synergistic response over cortisol alone. Synergy is also seen with 8-bromo-cAMP, is specific for the cGRE, and occurs in a number of established cell lines. Elimination of the GRE or CRE/TRE reduces the synergy by 70-100%. Altering the CRE/TRE to GRE spacing changed both enhancement and synergy. Moving the elements 3 bp closer or extending 15 bp reduced enhancement. Synergy was markedly reduced when elements were one half of a helical turn out of phase. Western blots verified that CREB (cAMP-responsive binding protein) and ATF-1 (activating transcription factor-1) binds to the cGRE sequence. A specific dominant negative inhibitor of the CREB family, A-CREB, reduced synergy by 50%. These results suggest that the GS cGRE can potentially integrate signaling from both the cAMP and glucocorticoid receptor transduction pathways and that CREB/ATF-1 may play an important role in this process.

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.

Organ-specific activity of the 5' regulatory region of the glutamine synthetase gene in developing mice

Glutamine synthetase (GS) converts ammonia and glutamate into glutamine. We assessed the activity of the 5' regulatory region of the GS gene in developing transgenic mice carrying the chloramphenicol acetyltransferase (CAT) gene under the control of 3150 bp of the upstream sequence of the rat GS gene to obtain insight into the spatiotemporal regulation of its pattern of expression. To determine the organ-specific activity of the 5' regulatory region CAT and GS mRNA expression were compared by ribonuclease-protection and semi-quantitative in situ hybridization analyses. Three patterns were observed: the 5' region is active and involved in the regulation of GS expression throughout development (pericentral hepatocytes, intestines and epididymis); the 5' region shows no activity at any of the ages investigated (periportal hepatocytes and white adipose tissue); and the activity of the 5' region becomes repressed during development (stomach, muscle, brown adipose tissue, kidney, lung and testis). In the second group, an additional element must be responsible for the activation of GS expression. The last group included organs in which the 5' regulatory region is active, but not in the cells that express GS. In these organs, the activity of the 5' regulatory region must be repressed by other regulatory regions of the GS gene that are missing from the transgenic construct. These findings indicate that in addition to the 5' regulatory region, at least two unidentified elements are involved in the spatiotemporal pattern of expression of GS.

Hormonal and non-hormonal regulation of glutamine synthetase in the developing neural retina

Two isoforms of the glucocorticoid receptor, with apparent molecular mass of 90 and 95 kDa, are expressed in embryonic chicken neural retina. The 95-kDa receptor represents a hyperphosphorylated form of the 90-kDa receptor. Activation of the glucocorticoid receptor by cortisol results in a dose-dependent increase in receptor phosphorylation, translocation of receptor molecules into the nucleus and a decline in the total amount of the receptor. Activation of the glucocorticoid receptor can also be observed in the developing retinal tissue in ovo. At late embryonic ages, when the systemic level of glucocorticoids increases, a substantial quantity of receptor molecules becomes translocated into the nucleus, the relative level of the 95-kDa isoform increases, and the total amount of receptor declines. Activation of the receptor molecules in ovo correlates directly with an increase in transcription of the glucocorticoid-inducible gene, glutamine synthetase. The close correlation between the increase in systemic glucocorticoids, activation of glucocorticoid receptor molecules and induction of glutamine synthetase gene transcription suggests that glucocorticoids are directly involved in the developmental control of glutamine synthetase expression. Long-term organ culturing of embryonic retinal tissue in the absence of hormone results in an increase in glutamine synthetase expression. This increase, which is only 5 to 10% of that observed in ovo, is not mediated by activated receptor molecules and represents a mechanism for non-hormonal regulation of glutamine synthetase.

Glutamine synthetase induction by glucocorticoids is preserved in skeletal muscle of aged rats

Glutamine synthetase (GS) is a glucocorticoid-inducible enzyme that has a key role for glutamine synthesis in muscle. We hypothesized that the glucocorticoid induction of GS could be altered in aged rats, because alterations in the responsiveness of some genes to glucocorticoids were reported in aging. We compared the glucocorticoid-induced GS in fast-twitch and slow-twitch skeletal muscles (tibialis anterior and soleus, respectively) and heart from adult (age 6-8 mo) and aged (age 22 mo) female rats. All animals received dexamethasone (Dex) in their drinking water (0.77 +/- 0.10 and 0.80 +/- 0.08 mg/day per adult and aged rat, respectively) for 5 days. Dex caused an increase in both GS activity and GS mRNA in fast-twitch and slow-twitch skeletal muscles from adult and aged rats. In contrast, Dex increased GS activity in heart of adult rats, without any concomitant change in GS mRNA levels. Furthermore, Dex did not affect GS activity in aged heart. Thus the responsiveness of GS to an excess of glucocorticoids is preserved in skeletal muscle but not in heart from aged animals.

Identification of a Xenopus glutamine synthetase gene abundantly expressed in the embryonic nervous system but not in adult brain

We used a PCR-based subtraction cloning procedure with concanavalin A-treated and -untreated animal caps from stage 9 Xenopus embryos to search for genes up-regulated during early neural development. One such gene was found to encode a protein homologous to several known glutamine synthetases, and we named it xGS. Molecular hybridization studies revealed that xGS mRNA is maternally transmitted and abundantly expressed in neuroectoderm-derived tissues during the gastrula and neurula stages. The expression of xGS mRNA in the nervous system continues until the larval stages, but declines thereafter and becomes undetectable in adult brain. Considering its metabolic activity and potential neuroprotective effect against the neurotoxic substances such as glutamate and ammonia, the glutamine synthetase may play an important role in the early stages of vertebrate neural development.

A transcriptional enhancer of the glutamine synthetase gene that is selective for retinal Müller glial cells

This article demonstrates that the chicken glutamine synthetase (GS) promoter contains cis-acting elements that direct transcription to retinal Müller glial cells. The transient assay system developed to identify these elements involved electroporation of intact retinal tissue with GS-beta-galactosidase fusion genes followed by preparation of primary cultures and histochemical assay of cells expressing beta-galactosidase. Plasmids containing beta-galactosidase under transcriptional control by two different viral promoters are expressed primarily in neuronal cells after transfection of intact embryonic d 12 retina. In sharp contrast, expression is primarily in Müller glia after transfection with a GS-beta-galactosidase fusion gene. Although GS is glucocorticoid inducible, steroid hormone is not required to achieve Müller cell-selective expression of the GS-beta-galactosidase fusion gene. Deletion studies indicate that multiple cis-acting elements located between nucleotides-436 and -61 relative to the GS transcription start site contribute to produce Müller cell selectivity. Moreover, these upstream elements enhance expression of a heterologous promoter in Müller cells not neurons. These results indicate that an enhancer located between 61 and 436 nucleotides upstream of the transcription start site contributes to Müller cell-selective expression of the GS gene in the retina.

Genetic basis for tissue isozymes of glutamine synthetase in elasmobranchs

Tissue-specific isozymes of glutamine synthetase are present in elasmobranchs. A larger isozyme occurs in tissues in which the enzyme is localized in mitochondria (liver, kidney) whereas a smaller form occurs in tissues in which it is cytosolic (brain, spleen, etc.). The nucleotide sequence of spiny dogfish shark (Squalus acanthias) liver glutamine synthetase mRNA, derived from its cDNA, shows there are two in-frame initiation codons (AUG) at the N-terminus which will account for the size differences between the two isozymes. Initiation at the up-stream and down-stream sites would yield peptides of 45,406 and 41,869 mol. wts. representing the precursor of the mitochondrial isozyme and the cytosolic isozyme, respectively. The additional N-terminal 29 amino acids present in the mitochondrial isozyme precursor contains two putative cleavage sites based on the Arg-X-(Phe,Ile,Leu) motif. The predicted two-step processing would remove 14 of the 29 N-terminal amino acids. These 14 amino acids can be predicted to form a very strong amphipathic mitochondrial targeting signal. Their removal would yield a mature peptide of 43,680 mol. wt. The calculated mol. wts. based on the derived amino acid sequence are therefore in good agreement with previous estimates of an approximately 1.5-2-kDa difference between the M(r)s of the mitochondrial and cytosolic isozymes. A model for the evolution of the mitochondrial targeting of glutamine synthetase in vertebrates is proposed.

Gene expression in astrocytes is affected by subculture

We have investigated the effects of cell passaging and time in culture on astrocyte morphology, transferrin expression and the expression of two main astrocyte markers, glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS: EC 6.3.1.2). When primary astrocytes were subcultured, giving rise to secondary and tertiary cultures, their morphology changed, regardless of the split ratio used to passage the cells. Correlating with this morphological change, a dramatic increase in the accumulation of GFAP and GS mRNAs was observed after cells had been passaged. This effect was in marked contrast to the moderate increase in the levels of GFAP and GS mRNAs observed over several weeks in primary culture. Hydrocortisone induction of GS gene expression was not affected by cell passage. Transferrin mRNA, which is not normally found in astrocytes in vivo, was expressed at a high level in primary cultures of astrocytes. However, transferring mRNA almost completely disappeared after the second passage. Astrocyte-conditioned media, or co-cultures with oligodendrocytes, modified transferrin gene expression. Taken together, these results show that subculturing of primary rat astrocytes leads to a dramatic change in the genetic expression of several proteins and provides a new approach to modify astrocyte differentiation in vitro.

Molecular basis for differential expression of glutamine synthetase in retina glia and neurons

Glutamine synthetase (GS) is a differentiation marker of retina glial cell. It is expressed in the chicken neural retina at a particularly high level, is inducible by glucocorticoids and is always confined to Müller glia. This study investigated the molecular basis for tissue and cell-type specific expression of the GS gene. A high level of GS expression in the retina was found to coincide with the accumulation of a relatively high level of GS mRNA in this tissue. The gliatoxic agent alpha-aminoadipic acid, which can selectively destroy glia cells, was used to demonstrate that restriction of GS induction to Müller glia is controlled at a transcriptional level. Cortisol could induce accumulation of GS mRNA and transcription of the GS gene in Müller glia but not in retina neurons. Glia and neurons were also found to differ in their ability to express the glucocorticoid inducible CAT construct, p delta G46TCO, which is controlled by a 'simple GRE' promoter. When introduced into cells of retina tissue, this construct was cortisol-inducible in glia whereas in neurons it was only slightly inducible or not at all. Introduction of a glucocorticoid receptor expression vector into the cells facilitated induction of the CAT construct in neurons. Analysis by immunoblotting revealed that expression of the glucocorticoid receptor protein is predominantly restricted to Müller glia. These results suggest that differential levels of glucocorticoid receptor expression in glia and neurons might be the basis for cell-type specific induction of GS.

Molecular control of glutamine synthetase expression in the developing retina tissue

Glutamine synthetase is a differentiation marker of the neural retina, whose expression is restricted to Müller glia cells, is inducible by glucocorticoids and is dependent on tissue development. The retina tissue acquires the competence to express GS in response to glucocorticoids with development, although the level of hormone binding activity in the cells does not alter with age. Using CAT constructs that are controlled by "simple GRE" promoters we demonstrated that glucocorticoid receptor transcription activity in retina cells increases with development. The increase in receptor activity correlates directly with the increase in inducibility of the glutamine synthetase gene and inversely with the rate of retina cell proliferation. At early developmental ages, when retina cells are still proliferating, the glucocorticoid receptor is transcriptionally inactive and glutamine synthetase expression cannot be induced. Receptor activity increases progressively with development and by day 12, when cell proliferation ceases, competence for glutamine synthetase induction is high. This competence for glutamine synthetase induction can be repressed by overexpressing the oncogene v-src, which stimulates retina cell proliferation. We discuss possible mechanisms for developmental-dependent modulation of glucocorticoid receptor transcriptional activity.

Inhibition of T lymphocyte proliferation by retinal glial Müller cells: reversal of inhibition by glucocorticoids

Study of interactions between retinal glial Müller cells and T lymphocytes have revealed a wide array of reciprocal influences on the functions of these cells. In the present study we show that these interactions can be further modified by corticosteroid hormones. The primary effect of Müller cells on T lymphocytes is an inhibition of the T-cell proliferative response, and it is exerted via a membrane-bound factor. In this report we show that glucocorticoids can reverse the inhibition by suppressing the expression of the Müller cell inhibitory factor. This effect was independent of the action of glucocorticoids on arachidonic acid metabolism.

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.

Positive and negative regulation of a transfected chimeric tyrosine aminotransferase gene: effect of copy number

To define a selective system for the study of rat tyrosine aminotransferase (TAT; EC 2.6.1.5) gene expression, we have introduced into cultured cells the selectable bacterial gene gpt linked to TAT gene flanking sequences. After integration in host cell DNA, the chimeric gene exhibits the same pattern of regulation as the TAT gene. In hepatoma cells, its expression is induced after glucocorticoid hormone treatment and repressed after fusion with fibroblasts. In fibroblasts, the chimeric gene is not expressed. The correct pattern of regulation is lost when the number of integrated copies is high. At copy number above 10, the transfected gene responds poorly to glucocorticoids in hepatoma cells. At copy number above 50, the gene is expressed in fibroblasts. Another gene present in the same construction and controlled by the SV40 early promoter and enhancer is positively regulated by glucocorticoids in hepatoma cells but not after fusion with fibroblasts. These data indicate that in hybrid cells, both TAT promoter and glucocorticoid-responsive elements are negatively regulated.

Murine glutamine synthetase: cloning, developmental regulation, and glucocorticoid inducibility

We have cloned the murine glutamine synthetase (GS) gene and measured GS enzyme activity and mRNA in five tissues (retina, brain, liver, kidney, and skeletal muscle) during perinatal development. Retinal GS enzyme activity increases 200-fold between Day 1 and Day 21 and is accompanied by an increase in the level of GS mRNA; developmental regulation in other tissues is much less dramatic. Based on Southern blotting analysis, a single GS gene gives rise to the tissue-specific patterns of GS mRNA expression. The increase in murine retinal GS observed during perinatal development is similar in magnitude to that observed in the chicken retina just prior to hatching. In the embryonic chicken retina, glucocorticoid hormones mediate a large increase in the level of GS mRNA. However, although glucocorticoids induce a 12-fold increase in GS mRNA in murine skeletal muscle, expression of the retinal enzyme and mRNA is only modestly glucocorticoid-inducible in the mouse. Therefore, despite the hormonal responsiveness of the murine GS gene, it is not likely that glucocorticoids are important physiological modulators of the developmental rise in murine retinal GS.

Multiple mechanisms by which glutamine synthetase levels are controlled in murine tissue culture cells

We report the isolation of a complimentary DNA (cDNA) clone encoding glutamine synthetase, derived from a population of methionine sulfoxime-resistant mouse GF1 fibroblasts. When GF1 cells are incubated for 48 h in the presence of the glucocorticoid hormone dexamethasone, the specific activity of glutamine synthetase (GS), assayed as glutamyltransferase activity, increases by threefold. Based on dot hybridization analysis, hormonal treatment also produces a similar increase in the level of GS mRNA. When GF1 cells or mouse Neuro 2A neuroblastoma cells are transferred from medium containing 4 mM glutamine to glutamine-free medium, glutamyltransferase activity increases by at least fivefold. However, the presence or absence or glutamine in the medium does not affect the relative level of glutamine synthetase mRNA in either cell line. With both GF1 and Neuro 2A cells, the half-time for the decline in glutamine synthetase enzyme activity on addition of glutamine to the medium is approximately 1.5 h. This rapid decline, coupled with the lack of effect of glutamine on the level of GS messenger RNA in Neuro 2A cells, renders it unlikely that neural cells alter glutamine synthetase levels in response to glutamine by a biosynthetic mechanism, as suggested by previous authors [L. Lacoste, K.D. Chaudhary, and J. Lapointe (1982) J. Neurochem. 39, 78-85].

Mechanisms of glucocorticoid function in human leukemic cells: analysis of receptor gene mutants of the activation-labile type using the covalent affinity ligand dexamethasone mesylate

In the cultured acute lymphoblastic leukemic (ALL) cell line, clones of sensitive cells are killed by receptor-occupying concentrations of glucocorticoids. In addition, several types of resistance have been identified. The types of resistance are r- (glucocorticoid binding site loss), ract/l (activation labile receptors) and r+ly- (defective lysis mechanism). The two types of receptor mutants have been examined for the presence and expression of the glucocorticoid receptor (GR) gene. Southern blot analysis, using a full-length cDNA probe for human GR, shows that the gene in both is grossly intact. Examination of the expression of the gene by Northern blots reveals the presence of normal, 7-kb message in both types of receptor mutants, though in amounts somewhat reduced from wild-type. This report focuses on the activation labile mutants. Since characterization of these mutants suggests that they can bind ligand but not retain it during activation, we hypothesized that they would respond normally to a ligand that could not be lost during activation. This seems to be the case. When the covalent affinity ligand dexamethasone mesylate, itself a partial glucocorticoid agonist/antagonist, is used, the ract/l cells are killed to an extent corresponding to that evoked by a sub-optimal concentration of the full agonist dexamethasone. We conclude: (1) that the ract/l receptors can function to kill cells if provided a ligand that they do not lose during activation; (2) that the partial agonist activity of dexamethasone mesylate for cell killing is not due to release of a small amount of free dexamethasone; (3) that the poor agonist activity of dexamethasone mesylate receptor complexes suggests that the role of steroid is strictly to participate in conversion of the receptor to its DNA binding form, after which presence of the steroid actually interferes with proper receptor action.

Developmentally regulated primary glucocorticoid hormone induction of chick retinal glutamine synthetase mRNA

We have characterized the glucocorticoid hormone induction of glutamine synthetase mRNA in embryonic chick retinal organ cultures by quantitative dot hybridization using a cDNA clone derived from chick retinal RNA. Hydrocortisone (Kapp = 3-4 nM) and dexamethasone (Kapp = 1-2 nM) produce an approximate 30-fold increase in glutamine synthetase mRNA after incubation of organ cultures derived from embryonic day 12 retinae with either hormone for 3 hr. Progesterone is a poor inducer. The glucocorticoid-mediated rise is rapid (t1/2 = 2-3 hr) and occurs in the presence of either of the protein synthesis inhibitors cycloheximide or puromycin, indicating that the induction is a primary or direct response to the hormone. However, the magnitude of the hormonal response observed in culture increases markedly during retinal development. These observations, coupled with the previously reported absence of a hormonal induction in embryonic liver, raise the possibility of a synergistic mechanism, involving tissue-specific regulatory molecules in addition to the glucocorticoid hormone receptor, to explain the retinal-specific primary glucocorticoid hormone induction of glutamine synthetase mRNA.