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

Glutamine Synthetase Drugability beyond Its Active Site: Exploring Oligomerization Interfaces and Pockets

BACKGROUND

Glutamine synthetase (GS) is a crucial enzyme to the nitrogen cycle with great commercial and pharmaceutical value. Current inhibitors target the active site, affecting GS activity indiscriminately in all organisms. As the active site is located at the interface between two monomers, the protein-protein interface (PPI) of GSs gains a new role, by providing new targets for enzyme inhibition. Exploring GSs PPI could allow for the development of inhibitors selective for specific organisms. Here we map the PPI of three GSs-human (hsGS), maize (zmGS) and Mycobacterium tuberculosis (mtGS)-and unravel new drugable pockets.

METHODS

The PPI binding free energy coming from key residues on three GSs from different organisms were mapped by computational alanine scan mutagenesis, applying a multiple dielectric constant MM-PBSA methodology. The most relevant residues for binding are referred as hot-spots. Drugable pockets on GS were detected with the Fpocket software.

RESULTS AND CONCLUSIONS

A total of 23, 19 and 30 hot-spots were identified on hsGS, zmGS and mtGS PPI. Even possessing differences in the hot-spots, hsGS and zmGS PPI are overall very similar. On the other hand, mtGS PPI differs greatly from hsGS and zmGS PPI. A novel drugable pocket was detected on the mtGS PPI. It seems particularly promising for the development of selective anti-tuberculosis drugs given its location on a PPI region that is highly populated with hot-spots and is completely different from the hsGS and zmGS PPIs. Drugs targeting this pockets should be inactive on eukaryotic GS II enzymes.

Insulin resistance in the liver: deficiency or excess of insulin?

In insulin-resistant states (obesity, pre-diabetes, and type 2 diabetes), hepatic production of glucose and lipid synthesis are heightened in concert, implying that insulin deficiency and insulin excess coexists in this setting. The fact that insulin may be inadequate or excessive at any one point in differing organs and tissues has many biologic ramifications. In this context the concept of metabolic compartmentalization in the liver is offered herein as one perspective of this paradox. In particular, we focus on the hypothesis that insulin resistance accentuates differences in periportal and perivenous hepatocytes, namely periportal glucose production and perivenous lipid synthesis. Subsequently, excessive production of glucose and accumulation of lipids could be expected in the livers of patients with obesity and insulin resistance. Overall, in this review, we provide our integrative perspective regarding how excessive production of glucose in periportal hepatocytes and accumulation of lipids in perivenous hepatocytes interact in insulin resistant states.

The effect of portacaval anastomosis on the expression of glutamine synthetase and ornithine aminotransferase in perivenous hepatocytes

There is functional zonation of metabolism across the liver acinus, with glutamine synthetase restricted to a narrow band of cells around the terminal hepatic venules. Portacaval anastomosis, where there is a major rerouting of portal blood flow from the portal vein directly to the vena cava bypassing the liver, has been reported to result in a marked decrease in the activity of glutamine synthetase. It is not known whether this represents a loss of perivenous hepatocytes or whether there is a specific loss of glutamine synthetase. To answer this question, we have determined the activity of glutamine synthetase and another enzyme from the perivenous compartment, ornithine aminotransferase, as well as the immunochemical localization of both glutamine synthetase and ornithine aminotransferase in rats with a portacaval shunt. The portacaval shunt caused a marked decrease in glutamine synthetase activity and an increase in ornithine aminotransferase activity. Immunohistochemical analysis showed that the glutamine synthetase and ornithine aminotransferase proteins maintained their location in the perivenous cells. These results indicate that there is no generalized loss of perivenous hepatocytes, but rather, there is a significant alteration in the expression of these proteins and hence metabolism in this cell population.

Organ patterning in the adult stage: the role of Wnt/beta-catenin signaling in liver zonation and beyond

Wnt/beta-catenin signaling has been found to play key roles in metabolic zonation of adult liver, regeneration, and hepatocellular carcinogenesis. In this review, recent progress in this field is summarized, in particular the rapidly growing knowledge about the various interactions of beta-catenin with many transcription factors involved in controlling metabolism. These interactions may provide the basis for understanding how the wide range of activities of Wnt/beta-catenin signaling is differentially interpreted. Based on these results, a three-level mode for the molecular interpretation of beta-catenin activity gradients in liver is proposed favoring cell differentiation, metabolic zonation, and proliferation. While derangement of the combinatorial interplay of the various transcription factors with beta-catenin at the intermediary activity level may contribute to the development of metabolic diseases, extremely high activation of beta-catenin may eventually lead to initiation and progression of hepatocellular tumors.

Keratinocytes as depository of ammonium-inducible glutamine synthetase: age- and anatomy-dependent distribution in human and rat skin

In inner organs, glutamine contributes to proliferation, detoxification and establishment of a mechanical barrier, i.e., functions essential for skin, as well. However, the age-dependent and regional peculiarities of distribution of glutamine synthetase (GS), an enzyme responsible for generation of glutamine, and factors regulating its enzymatic activity in mammalian skin remain undisclosed. To explore this, GS localization was investigated using immunohistochemistry and double-labeling of young and adult human and rat skin sections as well as skin cells in culture. In human and rat skin GS was almost completely co-localized with astrocyte-specific proteins (e.g. GFAP). While GS staining was pronounced in all layers of the epidermis of young human skin, staining was reduced and more differentiated among different layers with age. In stratum basale and in stratum spinosum GS was co-localized with the adherens junction component beta-catenin. Inhibition of, glycogen synthase kinase 3beta in cultured keratinocytes and HaCaT cells, however, did not support a direct role of beta-catenin in regulation of GS. Enzymatic and reverse transcriptase polymerase chain reaction studies revealed an unusual mode of regulation of this enzyme in keratinocytes, i.e., GS activity, but not expression, was enhanced about 8-10 fold when the cells were exposed to ammonium ions. Prominent posttranscriptional up-regulation of GS activity in keratinocytes by ammonium ions in conjunction with widespread distribution of GS immunoreactivity throughout the epidermis allows considering the skin as a large reservoir of latent GS. Such a depository of glutamine-generating enzyme seems essential for continuous renewal of epidermal permeability barrier and during pathological processes accompanied by hyperammonemia.

Hepatocellular expression of glutamine synthetase: an indicator of morphogen actions as master regulators of zonation in adult liver

Glutamine synthetase (GS) has long been known to be expressed exclusively in pericentral hepatocytes most proximal to the central veins of liver lobuli. This enzyme as well as its peculiar distribution complementary to the periportal compartment for ureogenesis plays an important role in nitrogen metabolism, particularly in homeostasis of blood levels of ammonium ions and glutamine. Despite this fact and intensive studies in vivo and in vitro, many aspects of the regulation of its activity on the protein and on the genetic level remained enigmatic. Recent experimental advances using transgenic mice and new analytic tools have revealed the fundamental role of morphogens such as wingless-type MMTV integration site family member signals (Wnt), beta-catenin, and adenomatous polyposis coli in the regulation of this particular enzyme. In addition, novel information concerning the structure of transcription factor binding sites within regulatory regions of the GS gene and their interactions with signalling pathways could be collected. In this review we focus on all aspects of the regulation of GS in the liver and demonstrate how the new findings have changed our view of the determinants of liver zonation. What appeared as a simple response of hepatocytes to blood-derived factors and local cellular interactions must now be perceived as a fundamental mechanism of adult tissue patterning by morphogens that were considered mainly as regulators of developmental processes. Though GS may be the most obvious indicator of morphogen action among many other targets, elucidation of the complex regulation of the expression of the GS gene could pave the road for a better understanding of the mechanisms involved in patterning of liver parenchyma. Based on current knowledge we propose a new concept of how morphogens, hormones and other factors may act in concert, in order to restrict gene expression to small subpopulations of one differentiated cell type, the hepatocyte, in different anatomical locations. Although many details of this regulatory network are still missing, and an era of exciting new discoveries is still about to come, it can already be envisioned that similar mechanisms may well be active in other organs contributing to the fine-tuning of organ-specific functions.

Reduction and expansion of the glutamine synthetase expressing zone in livers from tetracycline controlled TGF-beta1 transgenic mice and multiple starved mice

BACKGROUND/AIMS

To learn more about tissue remodelling in fibrotic livers of tetracycline-controlled TGF-beta1 transgenic mice (TGF-beta1-on-mice) and during regeneration after removal of the fibrotic stimulus (off-mice), we investigated the expression of glutamine synthetase (GS), an exclusive pericentrally expressed enzyme.

METHODS

GS was localised immunohistochemically and quantified by real-time RT-PCR and enzymatic activity measurement. Apoptosis in livers of TGF-beta1-on-mice was demonstrated by in situ apoptosis detection kit (TUNEL reaction).

RESULTS

Livers of TGF-beta1-on-mice harbour a reduced number of GS-positive hepatocytes and expression of GS is downregulated, while multiple starved mice serving as controls for malnutrition during TGF-beta1 exposure surprisingly showed an impressive amplification of GS-positive hepatocytes. Apoptotic events were frequent around central veins in livers of TGF-beta1-on-mice, while in multiple induced mice apoptosis was dominant around all vessels and weak in midzonal areas. During regeneration from fibrosis, control levels were regained within 21 days. Beta-catenin was dislocated from plasma membrane to cytoplasm exclusively in pericentral hepatocytes during a short time slot after a unique expression of TGF-beta1.

CONCLUSIONS

Reduction of GS in TGF-beta1-on-mice results from apoptosis of GS-positive hepatocytes rather than downregulation of GS expression. Beta-catenin seems involved in the recovery of GS-positive hepatocytes.

A silencer element in the first intron of the glutamine synthetase gene represses induction by glucocorticoids

The enzyme glutamine synthetase (GS) ranks as one of the most remarkable glucocorticoid-inducible mammalian genes. In many tissues and cell lines, the synthetic glucocorticoid dexamethasone alone increases GS expression several fold. The direct response is mainly mediated by a cellular glucocorticoid receptor that, upon binding of the hormone, interacts with glucocorticoid responsive elements (GREs) of the gene. In cells of hepatocellular origin the response is mediated by a GRE located in the first intron of the gene. Surprisingly, hepatocytes do not respond to glucocorticoids with enhanced GS expression, despite the presence of an intact glucocorticoid receptor, which, in the same cells, stimulates expression of other genes such as tyrosine amino transferase. Reporter gene assays identified a sequence element downstream from the intronic GRE that inhibits the enhancement of expression by glucocorticoids. This silencer was designated GS silencer element of the rat. Gel mobility shift assays demonstrate the binding of a factor in hepatocyte nuclear extract. This yet unknown factor was designated GS silencer-binding protein. It is absent in FAO cells that respond to glucocorticoids with enhanced expression of GS and present in HepG2 cells that do not respond.

Effect of oxygen concentration on the expression of glutathione S-transferase activity in periportal and perivenous rat hepatocyte cultures

Cultures of perivenous (PV) and periportal (PP) hepatocytes could provide suitable in vitro models for studying the zone-specific hepatotoxic potential of xenobiotics. However, it is not known whether cultured PP and PV hepatocytes keep their phenotypes when the microcirculation of the liver changes. This question has been studied by culturing rat hepatocytes at 13 and 4% (v/v) O(2), respectively, mimicking the acinar oxygen gradient. PP and PV adult rat hepatocytes were isolated by digitonin-collagenase in situ perfusion and cultured on plastic Falcon and gas-permeable Petriperm dishes in Williams' E medium and kept at 13 and 4% (v/v) O(2), respectively. Cultures at 20% (v/v) O(2) on plastic dishes served as a control. Two types of cultures were studied, namely conventional cultures either unsupplemented or supplemented with 30 mM pyruvate. The activities of glutamine synthetase (GS) and glutathione S-transferase (GST) were measured in freshly isolated PP and PV hepatocytes and all cultures. The heterogeneous expression of GS (PV>PP), observed in freshly isolated hepatocytes, was kept for at least 4 days in culture. Total, Mu and Alpha class GST activities were predominantly expressed in PV freshly isolated cells. However, no beneficial effect could be observed in culture by exposing the cells to their specific in vivo oxygen concentration. The best maintenance of GST PV predominance in culture was observed in Petriperm dishes at 20% (v/v) O(2), as well in pyruvate-supplemented as unsupplemented cultures. PV GST predominance was thus kept in particular when the highest oxygen concentration was used and made available to the cells through the gas-permeable membranes. The results on GS PV predominance support these findings.

Identification of a cis-acting element and a novel trans-acting factor of the glutamine synthetase gene in liver cells

In the mammalian liver the expression of the enzyme glutamine synthetase (GS) is restricted to a small population of hepatocytes. In cells expressing the enzyme up to 3.5% of total cellular protein is GS. In order to identify enhancer elements contributing to this extraordinarily high level of expression we focused on a region roughly 2.5 kbp upstream of the GS promoter. Gel mobility shift assays revealed binding of an unknown protein within the most distal part of this region and reportergene assays demonstrated that roughly 60 bp downstream from position -2503 are indispensable for protein binding and the full effect of the enhancer. In UV cross-link analysis a 38 kDa nuclear protein that binds to the sequence was identified in rat hepatocytes. This nuclear protein, designated as upstream binding factor of the GS gene (UFGS) seems to play an important role in high-level expression of GS in liver.

Cross-talk between the signals hypoxia and glucose at the glucose response element of the L-type pyruvate kinase gene

The signals oxygen and glucose play an important role in metabolism, angiogenesis, tumorigenesis, and embryonic development. Little is known about an interaction of these two signals. We demonstrate here the cross-talk between oxygen and glucose in the regulation of L-type pyruvate kinase (L-PK) gene expression in the liver. In the liver the periportal to perivenous drop in O(2) tension was proposed to be an endocrine key regulator for the zonated gene expression. In primary rat hepatocyte cultures the expression of the L-PK gene on mRNA and on protein level was induced by venous pO(2), whereas its glucose-dependent induction occurred predominantly under arterial pO(2). It was shown by transient transfection of L-PK promoter luciferase and glucose response element (Glc(PK)RE) SV40 promoter luciferase gene constructs that the modulation by O(2) of the glucose-dependent induction occurred at the Glc(PK)RE in the L-PK gene promoter. The reduction of the glucose-dependent induction of the L-PK gene expression under venous pO(2) appeared to be mediated via an interference between hypoxia inducible factor-1 (HIF-1) and upstream stimulating factor at the Glc(PK)RE. The glucose response element also functioned as an hypoxia response element which was confirmed in cotransfection assays with Glc(PK)RE luciferase gene constructs and HIF-1alpha expression vectors. Furthermore, it was found by gel shift and supershift assay that HIF-1alpha and USF-1 or USF-2 could bind to the Glc(PK)RE. Our findings implicate that the cross-talk between oxygen and glucose might have a fundamental role in the regulation of several physiological and pathophysiological processes.

Triiodothyronine downregulates the periportal expression of alpha class glutathione S-transferase in rat liver

Most drug-metabolizing phase I and phase II enzymes, including the glutathione S-transferases (GST), exhibit a zonated expression in the liver, with lower expression in the upstream, periportal region. To elucidate the involvement of pituitary-dependent hormones in this zonation, the effect of hypophysectomy and 3,3',5-triiodo-L-thyronine (T3) on the distribution of GST was studied in rats. Hypophysectomy increased total GST activity both in the periportal and perivenous liver region. Subsequent T3 treatment counteracted this effect in the perivenous zone. However, analysis for either mu class M1/M2-specific (1,2-dichloro-4-nitrobenzene) or alpha class A1/A2-specific (7-chloro-4-nitrobenzo-2-oxa-1,3-diazole) GST activity revealed that T3 treatment did not significantly affect the perivenous activity of these GST classes. In contrast, T3 was found to significantly counteract the increase of alpha class GST activity caused by hypophysectomy in the periportal zone. To establish whether this effect was T3-specific, hepatocytes were isolated from either the periportal and perivenous zone by digitonin/collagenase perfusion and cultured either as pyruvate-supplemented monolayer or as co-culture with rat liver epithelial cells. Only in the latter it was found that T3 suppressed the A1/A2-specific GST activity and alpha class proteins predominantly in periportal cells. The data demonstrate that T3 is an important factor responsible for the low expression of alpha GST in the periportal region. T3 may be involved in the periportal downregulation of other phase I and II enzymes as well.

Prognostic significance of glutamine synthetase expression in unifocal advanced hepatocellular carcinoma

BACKGROUND/AIMS

Glutamine synthetase (GS) catalyzes the synthesis of glutamine, a major energy source of cells, and is upregulated in a subset of human hepatocellular carcinomas (HCCs). GS expression may be related to tumor recurrence, since GS-expressing tumors have a growth advantage in that they are independent of the extracellular glutamine supply. However, there are no studies concerning the prognostic value of GS expression in patients with HCC.

METHODS

Seventy-three patients with a single advanced HCC nodule who underwent curative hepatectomy were included in the study. GS expression in the HCC nodules was analyzed immunohistochemically and was compared with clinicopathologic features and the behavior of the tumors. Survival curves were assessed according to the Kaplan-Meier product-limit method and multivariate analysis based on the Cox regression model was performed.

RESULTS

GS expression was strong in 26 cases (35.6%, high-GS group) and weak or absent in 47 cases (64.4%, low-GS group). Univariate analysis showed that the high-GS group had a significantly shorter disease-free survival time than the low-GS group (p=0.042). Multivariate analysis revealed that GS expression (p=0.021), as well as Child's classification (p=0.005) and portal invasion (p=0.039), was a significant and independent prognostic parameter that affected tumor recurrence.

CONCLUSION

The results of this study indicate that GS expression may enhance the metastatic potential in HCC, and GS immunostaining may be helpful in identifying HCC patients at high risk for disease recurrence.

A mechanistic model for the development and maintenance of portocentral gradients in gene expression in the liver

In the liver, genes are expressed along a portocentral gradient. Based on their adaptive behavior, a gradient versus compartment type, and a dynamic versus stable type of gradient have been recognized. To understand at least in principle the development and maintenance of these gradients in gene expression in relation to the limited number of signal gradients, we propose a simple and testable model. The model uses portocentral gradients of signal molecules as input, while the output depends on two gene-specific variables, viz., the affinity of the gene for its regulatory factors and the degree of cooperativity that determines the response in the signal-transduction pathways. As a preliminary validity test for its performance, the model was tested on control and hormonally induced expression patterns of phosphoenolpyruvate carboxykinase (PCK), carbamoylphosphate synthetase I (CPS), and glutamine synthetase (GS). Affinity was found to determine the overall steepness of the gradient, whereas cooperativity causes these gradients to steepen locally, as is necessary for a compartment-like expression pattern. Interaction between two or more different signal gradients is necessary to ensure a stable expression pattern under different conditions. The diversity in sequence and arrangement of related DNA-response elements of genes appears to account for the gene-specific shape of the portocentral gradients in expression. The feasibility of testing the function of hepatocyte-specific DNA-response units in vivo is demonstrated by integrating such units into a ubiquitously active promoter/enhancer and analyzing the pattern of expression of these constructs in transgenic mice.

Acquisition of glutamine synthetase expression in human hepatocarcinogenesis: relation to disease recurrence and possible regulation by ubiquitin-dependent proteolysis

BACKGROUND

The authors previously reported increased ubiquitin (Ub) immunoreactivity in hepatocellular carcinomas (HCCs) and suggested a possible correlation between changes in ubiquitinated protein levels and multistep hepatocarcinogenesis. The current study was performed to identify one of these ubiquitinated proteins (42 kDa) and to analyze the clinical significance of its accumulation.

METHODS

The protein was purified using two-dimensional gel electrophoresis and identified by amino acid sequence analysis. The authors studied the expression of this protein in 101 HCCs and 23 precancerous lesions by immunohistochemical methods and in 26 HCCs by immunoblot analysis. A survival analysis was performed on patients with advanced HCC using the Kaplan-Meier method with approximate chi-square statistics for the log rank test.

RESULTS

The target protein for ubiquitination was identified as glutamine synthetase (GS). Accumulation of GS was found in 19 of 49 advanced HCCs (38.8%) by immunohistochemical methods and in 9 of 16 (56.3%) by immunoblot analysis, whereas the frequency was much lower in early HCCs (12.9% and 33.3%, respectively) and precancerous lesions (4.3% by immunostaining). In the Ub immunoblot analysis of strongly GS positive specimens, an intense 42-kDa ubiquitinated band was observed. Nine of 21 (42.9%) nodule-in-nodule type HCCs showed a GS positive, high-grade component within a GS negative, low-grade component, indicating the acquisition of GS expression during progression. Among 23 patients with a single advanced HCC nodule, the relapse free survival time was significantly shorter in the GS positive group than in the GS negative group.

CONCLUSIONS

The results of this study demonstrate the acquisition of GS expression during hepatocarcinogenesis and the possible regulation of GS enzyme activity by a Ub-dependent proteolytic system. Moreover, GS might play a significant role in promoting the metastatic potential of HCC.

Hepatic localisation of rat cysteine dioxygenase

BACKGROUND/AIM

Cysteine dioxygenase (CDO, E.C. 1.13.11.20) is the main catabolic enzyme of cysteine, metabolising cysteine to cysteinesulphinic acid. CDO abnormality has been implicated in a number of neurological and non-neurological diseases, with CDO deficiency possibly leading to excitotoxic damage to the brain and impaired Phase II metabolism in the liver.

METHODS

Two novel anti-CDO antibodies raised against linear synthetic peptides corresponding to two distinct epitopes on the 22 kDa gene product of the CDO-I gene were used for immunohistochemistry and Western blotting. These antibodies were characterised by their ability to both block and precipitate CDO enzyme activity as well as the ability of the respective antigenic peptides to absorb the antibodies and prevent the immunodetection of CDO.

RESULTS

The antibodies were found to detect the presence of a 68 kDa protein, which was subsequently shown to be CDO. Distribution was found to be centrilobular and did not alter when CDO was induced with cysteine or methionine; however, the intensity of staining increased, indicating an increase in the levels of CDO in that region.

CONCLUSIONS

These results suggest that the 68 kDa Type II is the predominant isoform in vitro and in vivo and that its centrilobular localisation may allow CDO to initiate the production of sulphate and taurine for Phase II conjugation in the liver.

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.

The spontaneous induction of glutamine synthetase in pig hepatocytes cocultured with RL-ET-14 cells is completely inhibited by trijodothyronine and okadaic acid

Cocultivation of primary pig hepatocytes with RL-ET-14 cells, an endothelial-like cell line resulted in spontaneous induction of glutamine synthetase (GS)-activity in the hepatocytes by more than 10-fold within 120 h to 144 h. Hepatocyte-specific induction was confirmed by immunocytochemistry. Addition of trijodothyronine (T3) to the culture medium inhibited the induction in a concentration dependent manner. No comparable influence of T3 was seen with pure cultures of either cell type suggesting that only the spontaneous induction was affected. Other hormones such as glucagon, insulin, growth hormone, epinephrine and testosterone did not interfere with the induction. Addition of several protein kinase-inhibitors such as staurosporine and genistein were without influence. However, a strong inhibition was found after addition of okadaic acid in nanomolar concentrations indicating an involvement of protein-phosphatase 2A in the induction process.

Hepatic glutamine transport and metabolism

Although the liver was long known to play a major role in the uptake, synthesis, and disposition of glutamine, metabolite balance studies across the whole liver yielded apparently contradictory findings suggesting that little or no net turnover of glutamine occurred in this organ. Efforts to understand the unique regulatory properties of hepatic glutaminase culminated in the conceptual reformulation of the pathway for glutamine synthesis and turnover, especially as regards the role of sub-acinar distribution of glutamine synthetase and glutaminase. This chapter describes these processes as well as the role of glutamine in hepatocellular hydration, a process that is the consequence of cumulative, osmotically active uptake of glutamine into cells. This topic is also examined in terms of the effects of cell swelling on the selective stimulation or inhibition of other far-ranging cellular processes. The pathophysiology of the intercellular glutamine cycle in cirrhosis is also considered.

Heterogeneous carbamoylphosphate synthetase I expression in testicular transplants of fetal mouse liver

Expression of carbamoylphosphate synthetase I (CPSI; EC 6.3.4.16) was examined immunohistochemically in normal development of the mouse liver, and in testicular transplants of fetal liver fragments. CPSI started to be expressed in all hepatocytes around 15 days of gestation, and became heterogeneous (i.e. absent from pericentral hepatocytes) around 2 weeks after birth. Most hepatocytes in fetal liver fragments placed for 2 months under the testicular capsule expressed this enzyme except for the pericentral ones, most of which were positively stained with anti-glutamine synthetase (GS; EC 6.3.1.2) antiserum. This distribution resembled that in the adult liver. The steep change in CPSI immunostaining in liver lobules suggests that the microenvironment tightly connected to the central veins plays an important role in the suppression of CPSI expression in the pericentral hepatocytes. Some pericentral hepatocytes were also negative for both enzymes. Thus, control mechanisms of CPSI expression may be different from those of GS expression in pericentral hepatocytes.

Periportal localization of glucagon receptor mRNA in rat liver and regulation of its expression by glucose and oxygen in hepatocyte cultures

Glucagon is the major hormone activating glycogenolysis and gluconeogenesis both localized in the periportal, more aerobic zone of the liver. Accordingly, the glucagon receptor (GcgR) mRNA was found to be predominantly expressed in this area. In hepatocyte cultures high glucose concentrations as reached after a meal induced GcgR mRNA under arterial but not venous pO2. The induction by glucose was partially antagonized by insulin and unaffected by glucagon. The modulation by 02 of the glucose-dependent induction would contribute to the zonated expression of GcgR mRNA.

Zonation of hepatic cytochrome P-450 expression and regulation

The CYP genes encode enzymes of the cytochrome P-450 superfamily. Cytochrome P-450 (CYP) enzymes are expressed mainly in the liver and are active in mono-oxygenation and hydroxylation of various xenobiotics, including drugs and alcohols, as well as that of endogenous compounds such as steroids, bile acids, prostaglandins, leukotrienes and biogenic amines. In the liver the CYP enzymes are constitutively expressed and commonly also induced by chemicals in a characteristic zonated pattern with high expression prevailing in the downstream perivenous region. In the present review we summarize recent studies, mainly based on rat liver, on the factors regulating this position-dependent expression and induction. Pituitary-dependent signals mediated by growth hormone and thyroid hormone seem to selectively down-regulate the upstream periportal expression of certain CYP forms. It is at present unknown to what extent other hormones that also affect total hepatic CYP activities, i.e. insulin, glucagon, glucocorticoids and gonadal hormones, act zone-specifically. The expression and induction of CYP enzymes in the perivenous region probably have important toxicological implications, since many CYP-activated chemicals cause cell injury primarily in this region of the liver.

Strain- and sex-specific variations in hepatic glutamine synthetase activity and distribution in rats and mice

The distribution of glutamine synthetase (GS) in a mammalian liver is restricted to a small zone of hepatocytes surrounding the central veins. The determination of the size of the GS+ zone in rats by immunohistochemistry revealed that it differed between rat strains and was larger in males than in females of each strain. Accordingly, the means of the relative mean width (RMW) values that characterize the size of the GS+ zone were 19%, 26%, and 39% lower in females than in males of Sprague-Dawley, Wistar, and Fischer rats, respectively. Upon orchidectomy of male rats, the size of the GS+ zone diminished towards the value found in females, while ovariectomy was without effect. This orchidectomy-induced reduction was reflected in corresponding changes of the RMW values as well as in the number of GS+ cells per pericentral field and was not due to the slightly smaller size of the GS+ hepatocytes in the orchidectomized males. No such sex difference was found in M775 mice. Biochemical GS activity was higher in the male rats than in the female rats and changed correspondingly to the distribution after gonadectomy. In the mice, only the specific activity of GS dropped after orchidectomy. In primary cultures of rat hepatocytes, no influence of testosterone or estrogen on GS activity and cellular distribution was observed, even after stimulation of GS activity with dexamethasone and growth hormone. Both sex hormones, however, were able to affect the activity of glucose-6-phosphate dehydrogenase (G6PD). The observed sex differences in the activity and distribution of GS in rat livers suggest that sex hormones not only modulate the level of this enzyme but are at least partially involved in the determination of the size of the compartment of GS expression. According to the results in the cell cultures, the effects of the sex hormones appear indirect rather than direct.

Changes in distribution and activity of glutamine synthetase in carbon tetrachloride-induced cirrhosis in the rat: potential role in hyperammonemia

Cirrhosis induced in rats by carbon tetrachloride was used to study alterations in the activities and lobular distribution of carbamoylphosphate synthetase and glutamine synthetase. Specific activity of carbamoylphosphate synthetase in cirrhotic subjects was decreased to 70% of controls. Staining was homogeneous within micronodular areas, but varied from area to area and generally showed a decreased intensity. Specific activity of glutamine synthetase and the size of the glutamine synthetase-positive area were decreased to 20% and less of controls. Glutamine synthetase-positive hepatocytes were rare and scattered at the periphery of nodular areas and within fibrous septa, the normal association with the central veins being widely lost. Rarely, complete micronodules showed a slight homogeneous staining for glutamine synthetase. Arginase activity was not affected, whereas glutaminase activity was enhanced by 50%. Serum levels of ammonia were elevated more than 2-fold and those of glutamine by 30%. In contrast, urea levels tended to be slightly diminished. Serum ammonia levels showed a clear negative correlation with the specific activity of glutamine synthetase and the size of the glutamine synthetase-positive area. Furthermore, blood urea levels correlated with the sum of ammonia and glutamine concentrations, but not with each of these substrate concentrations alone. These data suggest that the changes in activity and distribution of glutamine synthetase contribute to hyperammonemia in cirrhosis. Despite a reduced activity of the initial enzyme of the urea cycle, urea synthesis is not diminished accordingly. This may be due to an enhanced flux caused by the elevated blood level of ammonia and an increased hydrolysis of glutamine, because of higher levels of glutaminase.