CCAAT/enhancer-binding protein beta (C/EBP beta) binds and activates while hepatocyte nuclear factor-4 (HNF-4) does not bind but represses the liver-type arginase promoter

Insulin Resistance in Macrophages Alters Their Metabolism and Promotes an M2-Like Phenotype

Obesity and insulin resistance influences metabolic processes, but whether it affects macrophage metabolism is not known. In this study, we demonstrate that chronic exposure of macrophages to insulin either in culture or in vivo in diet-induced, glucose-intolerant mice rendered them resistant to insulin signals marked by failure to induce Akt2 phosphorylation. Similarly, macrophages lacking Akt2 or IGF1 receptor were also resistant to insulin signals. Insulin-resistant macrophages had increased basal mTORC1 activity, possessed an M2-like phenotype, and reduced LPS responses. Moreover, they exhibited increased glycolysis and increased expression of key glycolytic enzymes. Inhibition of mTORC1 reversed the M2-like phenotype and suppressed glycolysis in insulin-resistant macrophages. In the context of polymicrobial sepsis, mice harboring insulin-resistant macrophages exhibited reduced sepsis-induced lung injury. Thus, macrophages obtain resistance to insulin characterized by increased glycolysis and a unique M2-like phenotype, termed M-insulin resistant, which accounts for obesity-related changes in macrophage responses and a state of trained immunity.

Stable overexpression of arginase I and ornithine transcarbamylase in HepG2 cells improves its ammonia detoxification

HepG2 is an immortalized human hepatoma cell line that has been used for research into bioartificial liver systems. However, a low level of ammonia detoxification is its biggest drawback. In this work, a recombinant HepG2 cell line with stable overexpression of human arginase I (hArgI) and human ornithine transcarbamylase (hOTC), HepG2/(hArgI + hOTC)4, was developed using a eukaryotic dual gene expression vector pBudCE4.1. (1) The hArgI and hOTC enzymatic activity in HepG2/(hArgI + hOTC)4 cells were higher than in the control cells. (2) The ammonia tolerance capacity of HepG2/(hArgI + hOTC)4 cells was three times that of HepG2 cells and 37.5% of that of primary human hepatocytes in cultivation. In the experiment of ammonia detoxification, HepG2/(hArgI + hOTC)4 cells produced 3.1 times more urea (at 180 mM NH(4) Cl) and 3.1 times more glutamine (at 120 mM NH(4) Cl and 15 mM glutamate) than HepG2 cells, reaching 63.1% and 36.0% that of primary human hepatocytes, respectively. (3) The hArgI and hOTC overexpression did not influence the growth of HepG2 cells and also promoted the expression of other ammonia detoxification associated proteins including glutamine synthetase (GS), arginase II (ArgII), arginosuccinate synthase (ASS) and arginosuccinate lyase (ASL) in HepG2 cells. This work illustrates that the modification reported here made significant progress in the improvement of HepG2 cell function and the HepG2/(hArgI + hOTC)4 cells will provide a better selection for the application of bioartificial liver system.

Upregulation of arginase expression and activity in hypertensive rats exposed to chronic intermittent hypobaric hypoxia

The aim of this study was to analyze the activity and expression levels of arginase I and II and to monitor the cardiovascular and hematological responses in tolerant and intolerant rats exposed to chronic intermittent hypobaric hypoxia (CIHH). Male Wistar rats (age: 3.0 +/- 0.4 months, weight: 250 +/- 25 g; n = 30) were randomly divided into two groups: CIHH2 x 2 (2 days hypoxia, 2 days normoxia, n = 20) and NX (normoxia, n = 10). The hypoxia was simulated in a hypobaric chamber at 428 torr. Tolerance was determined according to a previous protocol. Arginase activity was measured in lung and heart tissues, and the expression levels were determined by a (RT-PCR) assay in lung tissue. Results showed that the intolerants rats had lower body weight, higher hematocrit (Hct) (74 +/- 4% vs. 61 +/- 2%, p < 0.05), higher values of systolic blood pressure (SBP) (183 +/- 3.7 mmHg vs. 147 +/- 5.4 mmHg, p < 0.05), and higher arginase activity. In addition, RT-PCR analysis from lung tissue showed an overexpression of arginase II in the intolerant group (p < 0.01). However, tolerants had similar values as the NX group (p = ns). Further, a correlation was found between arginase activity and SBP in the heart (r(2) = 0.596, p < 0.001). An upregulation of arginase type II could be pivotal in understanding the pathogenesis of systolic hypertension and probably other phenomena associated with intermittent hypobaric hypoxia. A schematic explanation of these relations is proposed.

Hypoxia-induced proliferation of human pulmonary microvascular endothelial cells depends on epidermal growth factor receptor tyrosine kinase activation

We hypothesized that hypoxia would activate epidermal growth factor receptor (EGFR) tyrosine kinase, leading to increased arginase expression and resulting in proliferation of human pulmonary microvascular endothelial cell (hPMVEC). To test this hypothesis, hPMVEC were incubated in normoxia (20% O(2), 5% CO(2)) or hypoxia (1% O(2), 5% CO(2)). Immunoblotting for EGFR and proliferating cell nuclear antigen was done, and protein levels of both total EGFR and proliferating cell nuclear antigen were greater in hypoxic hPMVEC than in normoxic hPMVEC. Furthermore, hypoxic hPMVEC had greater levels of EGFR activity than did normoxic hPMVEC. Hypoxic hPMVEC had a twofold greater level of proliferation compared with normoxic controls, and this increase in proliferation was prevented by the addition of AG-1478 (a pharmacological inhibitor of EGFR). Immunoblotting for arginase I and arginase II demonstrated a threefold induction in arginase II protein levels in hypoxia, with little change in arginase I protein levels. The hypoxic induction of arginase II protein was prevented by treatment with AG-1478. Proliferation assays were performed in the presence of arginase inhibitors, and hypoxia-induced proliferation was also prevented by arginase inhibition. Finally, treatment with an EGFR small interfering RNA prevented hypoxia-induced proliferation and urea production. These findings demonstrate that hypoxia activates EGFR tyrosine kinase, leading to arginase expression and thereby promoting proliferation in hPMVEC.

Pharmacodynamic Modeling of Acute and Chronic Effects of Methylprednisolone on Hepatic Urea Cycle Genes in Rats

Corticosteroids (CS) regulate many enzymes at both mRNA and protein levels. This study used microarrays to broadly assess regulation of various genes related to the greater urea cycle and employs pharmacokinetic/pharmacodynamic (PK/PD) modeling to quantitatively analyze and compare the temporal profiles of these genes during acute and chronic exposure to methylprednisolone (MPL). One group of adrenalectomized male Wistar rats received an intravenous bolus dose (50 mg/kg) of MPL, whereas a second group received MPL by a subcutaneous infusion (Alzet osmotic pumps) at a rate of 0.3 mg/kg/hr for seven days. The rats were sacrificed at various time points over 72 hours (acute) or 168 hours (chronic) and livers were harvested. Total RNA was extracted and Affymetrix® gene chips (RGU34A for acute and RAE 230A for chronic) were used to identify genes regulated by CS. Besides five primary urea cycle enzymes, many other genes related to the urea cycle showed substantial changes in mRNA expression. Some genes that were simply up- or down-regulated after acute MPL showed complex biphasic patterns upon chronic infusion indicating involvement of secondary regulation. For the simplest patterns, indirect response models were used to describe the nuclear steroid-bound receptor mediated increase or decrease in gene transcription (e.g. tyrosine aminotransferase, glucocorticoid receptor). For the biphasic profiles, involvement of a secondary biosignal was assumed (e.g. ornithine decarboxylase, CCAAT/enhancer binding protein) and more complex models were derived. Microarrays were used successfully to explore CS effects on various urea cycle enzyme genes. PD models presented in this report describe testable hypotheses regarding molecular mechanisms and quantitatively characterize the direct or indirect regulation of various genes by CS.

Expression of the hepatic specific V1 messenger ribonucleic acid of the human growth hormone receptor gene is regulated by hepatic nuclear factor (HNF)-4alpha2 and HNF-4alpha8

Human (h) GH plays an essential role in growth and metabolism, and its effectiveness is modulated by the availability of its specific receptor [hGH receptor (hGHR)] on target cells. The hGHR gene has a complex 5'-regulatory region containing multiple first exons. Seven are clustered within two small regions: V2,V3,V9 (module A) and V1,V4,V7,V8 (module B). Module A-derived mRNAs are ubiquitously expressed whereas those from module B are only found in postnatal liver, suggesting developmental- and liver-specific regulation of module B hGHR gene expression. To characterize the elements regulating module B activity, we studied a 1.8-kb promoter of the highest expressing exon in liver, V1. This promoter was repressed in transfection assays; however, either 5'- or 3'-deletions relieved this, suggesting the presence of multiple negative regulatory elements. Six putative hepatic nuclear factor 4 (HNF-4) response elements were identified. We determined that HNF-4alpha is developmentally regulated in the human liver: HNF-4alpha2 and HNF-4alpha8 are expressed in fetal hepatocytes but only HNF-4alpha2 is expressed in postnatal liver. Transient transfection assays demonstrated that HNF-4alpha2 and HNF-4alpha8 have a similar dual effect on V1 transcription: activation via site 1 in the proximal promoter and repression through site 6, approximately 1.7 kb upstream. EMSA/electrophoretic mobility supershift assays and chromatin immunoprecipitation analyses confirmed these two sites are bound by HNF-4alpha. Based on these data, we speculate there are multiple regions working together to repress the expression of V1 hGHR transcripts in tissues other than the normal postnatal liver, and that HNF-4alpha is a good candidate for regulating V1 hGHR expression in the human hepatocyte.

Regulation of nitric oxide synthesis and apoptosis by arginase and arginine recycling

Nitric oxide (NO) is synthesized from arginine and O2 by NO synthase (NOS). Citrulline formed as a by-product of the NOS reaction can be recycled to arginine by argininosuccinate synthetase (AS) and argininosuccinate lyase (AL). We found that AS and sometimes AL are coinduced with inducible NOS (iNOS) in various cells. In these cells, NO was synthesized from citrulline (via arginine) as well as from arginine, indicating operation of the citrulline-NO cycle. On the other hand, we found that arginase isoforms (types I and II) are coinduced with iNOS by LPS in rodent tissues and cultured macrophages. Km values for arginine of arginase I and II (approximately 10 mmol/L) are much higher than that of iNOS (approximately 5 micromol/L), whereas Vmax of arginase I and II were 10(3)-10(4) times higher than that of iNOS in activated macrophages. Thus, Vmax/Km values of arginases were close to that of iNOS, and these enzymes were expected to compete for arginine in the cells. In fact, NO production by iNOS in activated macrophages was decreased by coinduction of arginase I or arginase II. Low concentrations of NO protect cells from apoptosis, whereas excessive NO causes apoptosis. We found that NO causes endoplasmic reticulum (ER) stress, induces a transcription factor, CAAT/enhancer binding protein (C/EBP) homologous protein (CHOP), and leads to apoptosis. These results suggest that the arginine metabolic enzymes and the ER stress-CHOP pathway can be good targets to regulate NO production and NO-mediated apoptosis in diseases associated with overproduction or impaired production of NO.

Ornithine transcarbamylase and arginase I deficiency are responsible for diminished urea cycle function in the human hepatoblastoma cell line HepG2

A possible cell source for a bio-artificial liver is the human hepatblastoma-derived cell line HepG2 as it confers many hepatocyte functions, however, the urea cycle is not maintained resulting in the lack of ammonia detoxification via this cycle. We investigated urea cycle activity in HepG2 cells at both a molecular and biochemical level to determine the causes for the lack of urea cycle expression, and subsequently addressed reinstatement of the cycle by gene transfer. Metabolic labelling studies showed that urea production from 15N-ammonium chloride was not detectable in HepG2 conditioned medium, nor could 14C-labelled urea cycle intermediates be detected. Gene expression data from HepG2 cells revealed that although expression of three urea cycle genes Carbamoyl Phosphate Synthase I, Arginosuccinate Synthetase and Arginosuccinate Lyase was evident, Ornithine Transcarbamylase and Arginase I expression were completely absent. These results were confirmed by Western blot for arginase I, where no protein was detected. Radiolabelled enzyme assays showed that Ornithine Transcarbamylase functional activity was missing but that Carbamoyl Phosphate Synthase I, Arginosuccinate Synthetase and Arginosuccinate Lyase were functionally expressed at levels comparable to cultured primary human hepatocytes. To restore the urea cycle, HepG2 cells were transfected with full length Ornithine Transcarbamylase and Arginase I cDNA constructs under a CMV promoter. Co-transfected HepG2 cells displayed complete urea cycle activity, producing both labelled urea and urea cycle intermediates. This strategy could provide a cell source capable of urea synthesis, and hence ammonia detoxificatory function, which would be useful in a bio-artificial liver.

Hepatocyte nuclear factor-4alpha mediates redox sensitivity of inducible nitric-oxide synthase gene transcription

The underlying redox-sensitive mechanisms that regulate hepatocyte expression of inducible nitric-oxide synthase (iNOS) and its antioxidant functions are largely unknown. We have demonstrated previously that oxidative stress induced by benzenetriol-mediated superoxide production increases interleukin-1beta-induced iNOS protein synthesis, steady state iNOS mRNA expression, NO production, iNOS gene transcription, and trans-activation of the iNOS promoter in primary cultures of rat hepatocytes. In this study, we extend these studies by establishing the sequence specificity and binding of nuclear protein to the previously described 15-base cis-regulatory element of the rat hepatocyte iNOS promoter, isolating and identifying the cis-regulatory element transcription factor as hepatocyte nuclear factor-4alpha (HNF-4alpha), and confirming the functional role of HNF-4alpha in mediating redox-sensitive iNOS promoter trans-activation. In addition, we demonstrate that binding of HNF-4alpha to the transcriptional coactivator, PC4, in the presence of oxidative stress and interleukin-1beta stimulation is essential for increased iNOS promoter activity in this setting. Our results indicate that HNF-4alpha is the transcription factor that mediates redox regulation of hepatocyte iNOS gene transcription.

CCAAT/enhancer-binding protein beta is required for activation of genes for ornithine cycle enzymes by glucocorticoids and glucagon in primary-cultured hepatocytes

Transcription of genes for enzymes of the ornithine cycle is activated by hormones such as glucocorticoids and glucagon. Promoters and enhancers of several genes for the enzymes interact with the CCAAT/enhancer-binding protein (C/EBP) family of transcription factors, and C/EBPbeta has been suggested to mediate glucocorticoid response of the gene for arginase, the last enzyme of the cycle. To determine the contribution of C/EBPbeta to hormonal regulation of genes for ornithine cycle enzymes, we examined mice with targeted disruption of the C/EBPbeta gene. Induction of genes for the enzymes by intraperitoneal injection of dexamethasone and glucagon was almost intact in the liver of C/EBPbeta-deficient mice. On the other hand, in primary-cultured hepatocytes derived from C/EBPbeta-deficient mice, induction of genes for the first enzyme carbamylphosphate synthetase, as well as for arginase, in response to dexamethasone and/or glucagon was severely impaired. Therefore, C/EBPbeta is required for hormonal induction of the genes for ornithine cycle enzymes in primary-cultured hepatocytes, while the deficiency of C/EBPbeta is compensated for in vivo.

The gene for hepatocyte nuclear factor (HNF)-4alpha is activated by glucocorticoids and glucagon, and repressed by insulin in rat liver

The gene for a transcription factor hepatocyte nuclear factor-4alpha (HNF-4alpha) is responsible for maturity-onset diabetes of the young, type 1. We examined hormonal regulation of the HNF-4alpha gene in the liver. Stimulation of primary-cultured rat hepatocytes with dexamethasone or glucagon led to induction of HNF-4alpha mRNA, being antagonized by insulin. In the liver of streptozotocin-induced diabetic rat, mRNA and protein levels for HNF-4alpha were elevated, and were normalized by insulin treatment. Therefore, HNF-4alpha in the liver is likely to be involved in the regulation of glucose metabolism in response to these hormones.

IL-4 and IL-13 upregulate arginase I expression by cAMP and JAK/STAT6 pathways in vascular smooth muscle cells

The objectives of this study were to determine whether rat aortic smooth muscle cells (RASMC) express arginase and to elucidate the possible mechanisms involved in the regulation of arginase expression. The results show that RASMC contain basal arginase I (AI) activity, which is significantly enhanced by stimulating the cells with either interleukin (IL)-4 or IL-13, but arginase II (AII) expression was not detected under any condition studied here. We further investigated the signal transduction pathways responsible for AI induction. AI mRNA and protein levels were enhanced by addition of forskolin (1 microM) and inhibited by H-89 (30 microM), suggesting positive regulation of AI by a protein kinase A pathway. Genistein (10 microgramg/ml) and sodium orthovanadate (Na(3)VO(4); 10 microM) were used to investigate the role of tyrosine phosphorylation in the control of AI expression. Genistein inhibited, whereas Na(3)VO(4) enhanced the induction of AI by IL-4 or IL-13. Along with immunoprecipitation and immunoblot analyses, these data implicate the JAK/STAT6 pathway in AI regulation. Dexamethasone (Dex) and interferon (IFN)-gamma were investigated for their effects on AI induction. Dex (1 microM) and IFN-gamma (100 U/ml) alone had no effect on basal AI expression in RASMC, but both reduced AI induction by IL-4 and IL-13. In combination, Dex and IFN-gamma abolished AI induction by IL-4 and IL-13. Finally, both IL-4 and IL-13 significantly increased RASMC DNA synthesis as monitored by [(3)H]thymidine incorporation, demonstrating that upregulation of AI is correlated with an increase in cell proliferation. Blockade of AI induction by IFN-gamma, H-89, or genistein also blocked the increase in cell proliferation. These observations are consistent with the possibility that upregulation of AI might play an important role in the pathophysiology of vascular disorders characterized by excessive smooth muscle growth.

C/EBPβ and GATA-1 Synergistically Regulate Activity of the Eosinophil Granule Major Basic Protein Promoter: Implication for C/EBPβ Activity in Eosinophil Gene Expression

Eosinophil granule major basic protein (MBP) is expressed exclusively in eosinophils and basophils in hematopoietic cells. In our previous study, we demonstrated a major positive regulatory role for GATA-1 and a negative regulatory role for GATA-2 in MBP gene transcription. Further analysis of the MBP promoter region identified a C/EBP (CCAAT/enhancer-binding protein) consensus binding site 6 bp upstream of the functional GATA-binding site in the MBP gene. In the cell line HT93A, which is capable of differentiating towards both the eosinophil and neutrophil lineages in response to retinoic acid (RA), C/EBP mRNA expression decreased significantly concomitant with eosinophilic and neutrophilic differentiation, whereas C/EBPβ expression was markedly increased. Electrophoretic mobility shift assays (EMSAs) showed that recombinant C/EBPβ protein could bind to the potential C/EBP-binding site (bp −90 to −82) in the MBP promoter. Furthermore, we have demonstrated that both C/EBPβ and GATA-1 can bind simultaneously to the C/EBP- and GATA-binding sites in the MBP promoter. To determine the functionality of both the C/EBP- and GATA-binding sites, we analyzed whether C/EBPβ and GATA-1 can stimulate the MBP promoter in the C/EBPβ and GATA-1 negative Jurkat T-cell line. Cotransfection with C/EBPβ and GATA-1 expression vectors produced a 5-fold increase compared with cotransfection with the C/EBPβ or GATA-1 expression vectors individually. In addition, GST pull-down experiments demonstrated a physical interaction between human GATA-1 and C/EBPβ. Expression of FOG (F̲riendo̲fG̲ATA), which binds to GATA-1 and acts as a cofactor for GATA-binding proteins, decreased transactivation activity of GATA-1 for the MBP promoter in a dose-dependent manner. Our results provide the first evidence that both GATA-1 and C/EBPβ synergistically transactivate the promoter of an eosinophil-specific granule protein gene and that FOG may act as a negative cofactor for the eosinophil lineage, unlike its positively regulatory function for the erythroid and megakaryocyte lineages.

C/EBPβ and GATA-1 Synergistically Regulate Activity of the Eosinophil Granule Major Basic Protein Promoter: Implication for C/EBPβ Activity in Eosinophil Gene Expression

Eosinophil granule major basic protein (MBP) is expressed exclusively in eosinophils and basophils in hematopoietic cells. In our previous study, we demonstrated a major positive regulatory role for GATA-1 and a negative regulatory role for GATA-2 in MBP gene transcription. Further analysis of the MBP promoter region identified a C/EBP (CCAAT/enhancer-binding protein) consensus binding site 6 bp upstream of the functional GATA-binding site in the MBP gene. In the cell line HT93A, which is capable of differentiating towards both the eosinophil and neutrophil lineages in response to retinoic acid (RA), C/EBP mRNA expression decreased significantly concomitant with eosinophilic and neutrophilic differentiation, whereas C/EBPβ expression was markedly increased. Electrophoretic mobility shift assays (EMSAs) showed that recombinant C/EBPβ protein could bind to the potential C/EBP-binding site (bp −90 to −82) in the MBP promoter. Furthermore, we have demonstrated that both C/EBPβ and GATA-1 can bind simultaneously to the C/EBP- and GATA-binding sites in the MBP promoter. To determine the functionality of both the C/EBP- and GATA-binding sites, we analyzed whether C/EBPβ and GATA-1 can stimulate the MBP promoter in the C/EBPβ and GATA-1 negative Jurkat T-cell line. Cotransfection with C/EBPβ and GATA-1 expression vectors produced a 5-fold increase compared with cotransfection with the C/EBPβ or GATA-1 expression vectors individually. In addition, GST pull-down experiments demonstrated a physical interaction between human GATA-1 and C/EBPβ. Expression of FOG (F̲riendo̲fG̲ATA), which binds to GATA-1 and acts as a cofactor for GATA-binding proteins, decreased transactivation activity of GATA-1 for the MBP promoter in a dose-dependent manner. Our results provide the first evidence that both GATA-1 and C/EBPβ synergistically transactivate the promoter of an eosinophil-specific granule protein gene and that FOG may act as a negative cofactor for the eosinophil lineage, unlike its positively regulatory function for the erythroid and megakaryocyte lineages.

CCAAT/enhancer-binding proteins. A role in regulation of human involucrin promoter response to phorbol ester

The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) is a potent inducer of keratinocyte differentiation and of involucrin gene expression. In the present study we show that a CCAAT/enhancer-binding protein (C/EBP) site in the proximal regulatory region is required for the phorbol ester response. Mutation of the C/EBP site results in the loss of basal and TPA-responsive activity. Gel mobility supershift analysis shows that C/EBPalpha binding to this site is increased by TPA treatment. Moreover, cotransfection of the human involucrin reporter plasmid with C/EBPalpha increases promoter activity to an extent comparable with TPA treatment. Mutation of the C/EBP-binding site eliminates these responses. Transfection experiments using GADD153 to create C/EBP-null conditions confirm that C/EBP factors are absolutely required for promoter activity and TPA responsiveness. C/EBPbeta and C/EBPdelta inhibit both TPA- and C/EBPalpha-dependent promoter activation, indicating functional differences among C/EBP family members. These results suggest that C/EBP transcription factor activity is necessary for basal promoter activity and TPA response of the involucrin gene.

The C/EBP family of transcription factors in the liver and other organs

Members of the CCAAT/enhancer-binding protein (C/EBP) family of transcription factors are pivotal regulators of liver functions such as nutrient metabolism and its control by hormones, acute-phase response and liver regeneration. Recent progress in clarification of regulatory mechanisms for the C/EBP family members gives insight into understanding the liver functions at the molecular level.

Hypoglycemia-associated hyperammonemia caused by impaired expression of ornithine cycle enzyme genes in C/EBPalpha knockout mice

Ammonia produced by amino acid metabolism is detoxified through conversion into urea by the ornithine cycle in the liver, whereas carbon skeletons of amino acids are converted to glucose by gluconeogenic enzymes. Promoter and enhancer sequences of several genes for ornithine cycle enzymes interact with members of the CCAAT/enhancer-binding protein (C/EBP) transcription factor family. Disruption of the C/EBPalpha gene in mice causes hypoglycemia associated with the impaired expression of gluconeogenic enzymes. Here we examined the expression of ornithine cycle enzyme genes in the livers of C/EBPalpha-deficient mice. mRNA levels for the first, third, fourth, and fifth enzymes of five enzymes in the cycle were decreased in C/EBPalpha-deficient mice. Protein levels for the first, second, fourth, and fifth enzymes were also decreased. In situ hybridization analysis revealed that the enzyme mRNAs were distributed normally in the periportal region but were disordered in C/EBPalpha-deficient mice with relatively higher mRNA levels in the midlobular region. Blood ammonia concentrations in the mutant mice were severalfold higher than in wild-type mice. Thus, C/EBPalpha is crucial for ammonia detoxification by ornithine cycle enzymes and for coordination of gluconeogenesis and urea synthesis.

An array of binding sites for hepatocyte nuclear factor 4 of high and low affinities modulates the liver-specific enhancer for the human alpha1-microglobulin/bikunin precursor

Alpha1-Microglobulin and bikunin are two plasma glycoproteins encoded by a gene for alpha1-microglobulin/bikunin precursor (AMBP). The strict liver-specific transcription of the AMBP gene is controlled by an elaborate and remote enhancer made of six clustered boxes numbered 1 to 6 (core enhancer) that are binding sites for the hepatocyte-enriched nuclear factors HNF-1, HNF-4, HNF-3, HNF-1, HNF-3 and HNF-4 respectively. Three further boxes, 7 to 9, have now been found in the enhancer area in a position 5' of box 2, 5' of box 1 and 3' of box 6, respectively. Electrophoretic mobility-shift assays with nuclear extracts from the HepG2 hepatoma cell line demonstrated that boxes 7 and 8 are both functional HNF-4-binding sites of high and low affinity respectively, whereas no binding capacity of box 9 was detected by this method. Transfection of HepG2 and Chinese hamster ovary cells with chloramphenicol acetyltransferase constructs harbouring the core or extended AMBP enhancer with wild-type or mutated boxes and co-transfection with expression plasmids for a wild-type or defective HNF-4 identified box 7 as an essential element for the basal activity of this enhancer. The response of boxes 7 and 8 varies with the level of HNF-4 in cells. Box 9 exhibits a repressor activity that can be detected when box 8 is ablated. In vivo this corresponds to conditions of low box 8 occupancy when the intracellular level of HNF-4 is limited. These results reinforce the view that the AMBP enhancer is a quite elaborate and unusual example of a modular enhancer whose activity is fine-tuned by the level of cognate nuclear factors in the cell.

Mechanisms of Transcription in Eosinophils: GATA-1, but not GATA-2, Transactivates the Promoter of the Eosinophil Granule Major Basic Protein Gene

Granule major basic protein (MBP) is expressed exclusively in eosinophils, basophils, and placental trophoblasts. To identify thecis-elements and transcription factors involved in regulating MBP expression, we subcloned 3.2 kb of sequence upstream of the exon 9 transcriptional start site (P2 promoter) and serial 5′ deletions into the pXP2 luciferase reporter vector. An 80% decrement in promoter activity was obtained when MBP sequences between bp −117 to −67 were deleted. To identify transcription factors that bind to and transactivate through the bp −117 to −67 region, we first compared the upstream genomic sequences of human and murine MBP; a potential GATA binding consensus site was conserved in the 50-bp region between the two genes. To determine which GATA proteins bind this consensus site, we performed electrophoretic mobility shift assays (EMSAs), which showed that both GATA-1 and GATA-2 can bind to this consensus site. To determine the functionality of this site, we tested whether GATA-1 and GATA-2, either individually or in combination, can transactivate the MBP promoter in the Jurkat T cell line. Cotransfection with a GATA-1 expression vector produced 20-fold augmentation of MBP promoter activity, whereas GATA-2 had no activity. In contrast, combined cotransfection of GATA-1 and GATA-2 decreased the ability of GATA-1 to transactivate the MBP promoter by approximately 50%. Our results provide the first evidence for a GATA-1 target gene in eosinophils, a negative regulatory role for GATA-2 in MBP expression, and possibly eosinophil gene transcription in general during myelopoiesis.

Mechanisms of Transcription in Eosinophils: GATA-1, but not GATA-2, Transactivates the Promoter of the Eosinophil Granule Major Basic Protein Gene

Granule major basic protein (MBP) is expressed exclusively in eosinophils, basophils, and placental trophoblasts. To identify thecis-elements and transcription factors involved in regulating MBP expression, we subcloned 3.2 kb of sequence upstream of the exon 9 transcriptional start site (P2 promoter) and serial 5′ deletions into the pXP2 luciferase reporter vector. An 80% decrement in promoter activity was obtained when MBP sequences between bp −117 to −67 were deleted. To identify transcription factors that bind to and transactivate through the bp −117 to −67 region, we first compared the upstream genomic sequences of human and murine MBP; a potential GATA binding consensus site was conserved in the 50-bp region between the two genes. To determine which GATA proteins bind this consensus site, we performed electrophoretic mobility shift assays (EMSAs), which showed that both GATA-1 and GATA-2 can bind to this consensus site. To determine the functionality of this site, we tested whether GATA-1 and GATA-2, either individually or in combination, can transactivate the MBP promoter in the Jurkat T cell line. Cotransfection with a GATA-1 expression vector produced 20-fold augmentation of MBP promoter activity, whereas GATA-2 had no activity. In contrast, combined cotransfection of GATA-1 and GATA-2 decreased the ability of GATA-1 to transactivate the MBP promoter by approximately 50%. Our results provide the first evidence for a GATA-1 target gene in eosinophils, a negative regulatory role for GATA-2 in MBP expression, and possibly eosinophil gene transcription in general during myelopoiesis.

Biphasic regulation of transcription factor nuclear factor-kappaB activity in human endothelial cells by lysophosphatidylcholine through protein kinase C-mediated pathway

Lysophosphatidylcholine (lysoPC), which is generated in oxidized LDL (Ox-LDL) and abundantly exists in atherosclerotic arterial walls, has been shown to alter various endothelial functions and induces several endothelial genes expressed in atherosclerotic arterial walls. Nuclear factor-kappa B (NF-kappaB), a pleiotropic transcription factor, plays an important role in regulation of expression of various genes implicated in atherosclerosis. We have previously reported that lysoPC transferred from Ox-LDL to endothelial surface membrane activates endothelial protein kinase C (PKC), leading to modulated endothelial functions. This study was aimed at determining whether lysoPC could modulate activity of transcription factors in cultured human umbilical vein endothelial cells (HUVECs) by using electrophoretic mobility shift assay. LysoPC was found to increase DNA-binding activity of NF-kappaB in HUVECs within 15 minutes, which peaked at 1 to 2 hours and subsequently declined to the baseline level at 6 hours. Lower concentrations (5 to 15 micromol/L) of lysoPC markedly increased NF-kappaB activity, but higher concentration (50 micromol/L) of lysoPC inhibited the activity. Phorbol 12-myristate 13-acetate, a potent activator of PKC, also augmented NF-kappaB activity in HUVECs, mimicking the effects of lysoPC; furthermore, calphostin C and chelerythrine chloride, specific PKC inhibitors, and alpha-tocopherol, a clinically potent PKC inhibitor, suppressed the lysoPC-induced NF-kappaB activation. These results indicate that lysoPC regulates NF-kappaB activity in a biphasic manner dependent on its concentrations and incubation time in human endothelial cells and the endothelial PKC activation may in part be involved in the lysoPC-induced NF-kappaB activation. Thus, the time course and the positive and negative biphasic regulatory actions of lysoPC on NF-kappaB activity in endothelial cells might exhibit a unique effect of lysoPC in arterial walls on the different stages of atherosclerosis.

Distinct arginase isoforms expressed in primary and transformed macrophages: regulation by oxygen tension

Experiments were performed to identify arginase isoforms expressed in primary and transformed rodent macrophages and to determine the molecular mechanisms for the previously observed increase in arginase activity in macrophages cultured in hypoxia or anoxia. Results demonstrate the following: 1) mRNA and protein for hepatic-type AI arginase are expressed in primary cultures of rat and mouse peritoneal macrophages and are enhanced seven- and nine-fold, respectively, by lipopolysaccharide (LPS). 2) mRNA for extrahepatic-type AII arginase is constitutively expressed in mouse, but not rat, peritoneal macrophages and is detected in RAW264.7 cells after LPS treatment; neither J774A.1 nor P388D1 cells contain arginase mRNA. 3) AI arginase mRNA, arginase activity in cell lysates, and L-arginine flux through arginase in intact cells are all increased in rat wound-derived and mouse peritoneal macrophages by hypoxic or anoxic culture; AII arginase mRNA is, in contrast, suppressed > 50% by O2 deprivation. 4) Expression of the L-arginine transporter mCAT-2 is increased greater than twofold by reduced O2 culture. These results demonstrate substantial variability in arginase isoform expression among primary and transformed rodent macrophages. They also identify AI and AII arginase and the mCAT-2 L-arginine transporter as O2-regulated genes.

Chromosomal localization of the human arginase II gene and tissue distribution of its mRNA

Liver-type arginase (arginase I) is expressed almost exclusively in the liver and catalyzes the last step of urea synthesis, whereas the nonhepatic type (arginase II) is expressed in extrahepatic tissues and is probably involved in down-regulation of nitric oxide synthesis. We isolated cDNA for human arginase II (T. Gotoh et al., 1996, FEBS Lett. 395, 119-122). Fluorescence in situ hybridization mapping and PCR mapping studies with somatic cell hybrid panels and a radiation hybrid panel localized the arginase II gene to chromosome 14q24.1-24.3. Dot-blot analysis showed that arginase II mRNA is expressed strongly in the adult human kidney and weakly in the prostate, pituitary gland, lung, liver, thyroid gland, and small intestine. The mRNA was either at very low levels or not detectable in the fetal kidney, lung, and liver. Thus, expression of the human arginase II gene is regulated both tissue-specifically and developmentally.

Coinduction of nitric-oxide synthase and arginase I in cultured rat peritoneal macrophages and rat tissues in vivo by lipopolysaccharide

Nitric oxide is synthesized by nitric-oxide synthase from arginine, a common substrate of arginase. Rat peritoneal macrophages were cultured in the presence of bacterial lipopolysaccharide (LPS), and expression of the inducible isoform of nitric-oxide synthase (iNOS) and liver-type arginase (arginase I) was analyzed. mRNAs for iNOS and arginase I were induced by LPS in a dose-dependent manner. iNOS mRNA appeared 2 h after LPS treatment and increased to a near maximum at 8-12 h. On the other hand, arginase I mRNA that was undetectable prior to the treatment began to increase after 4 h with a lag time and reached a maximum at 12 h. Immunoblot analysis showed that iNOS and arginase I proteins were also induced. mRNA for arginase II, an arginase isozyme, was not detected in the LPS-activated peritoneal cells. mRNA for CCAAT/enhancer-binding protein beta (C/EBPbeta), a transactivator of the arginase I gene, was also induced, and the induction was more rapid than that of arginase I mRNA. Changes in iNOS and arginase I mRNAs were also examined in LPS-injected rats in vivo. iNOS mRNA increased rapidly in the lung and spleen, reached a maximum 2-6 h after the LPS treatment, and decreased thereafter. Arginase I mRNA was induced markedly and more slowly in both tissues, reaching a maximum in 12 h. Thus, arginase I appears to have an important role in down-regulating nitric oxide synthesis in murine macrophages by decreasing the availability of arginine, and the induction of arginase I is mediated by C/EBPbeta.

The glucocorticoid-responsive gene cascade. Activation of the rat arginase gene through induction of C/EBPbeta

The gene for liver-type arginase, an ornithine cycle enzyme, is induced by glucocorticoids in a delayed secondary manner. An enhancer element located around intron 7 of the rat arginase gene shows delayed glucocorticoid responsiveness, and it harbors two sites binding with members of the CCAAT/enhancer binding protein (C/EBP) family. Here, we investigate the role of these C/EBP binding sites in glucocorticoid response of the arginase gene. When inserted in front of the herpes simplex virus thymidine kinase promoter, these C/EBP sites exhibited glucocorticoid responsiveness in reporter transfection assay using rat hepatoma H4IIE cells. In footprint analysis using nuclear extracts of H4IIE cells, profiles of the protected areas of the two C/EBP sites changed when cells were treated with dexamethasone. In gel shift analysis, the complex formation for the two C/EBP sites was augmented in response to dexamethasone. Antibody supershift/inhibition analysis demonstrated that a major portion of the binding proteins induced by dexamethasone is C/EBPbeta. Induction of arginase mRNA by dexamethasone was preceded by augmentation of the C/EBP site-binding activities, which followed increase in C/EBPbeta mRNA. These results were consistent with the notion that the glucocorticoid response of the arginase gene is mediated by C/EBPbeta.

Molecular cloning of cDNA for nonhepatic mitochondrial arginase (arginase II) and comparison of its induction with nitric oxide synthase in a murine macrophage-like cell line

Arginase exists in two isoforms. Liver-type arginase (arginase I) is expressed almost exclusively in the liver and catalyzes the last step of urea synthesis, whereas the nonhepatic type (arginase II) is expressed in extrahepatic tissues. Arginase II has been proposed to play a role in down-regulation of nitric oxide synthesis. A cDNA for human arginase II was isolated. A polypeptide of 354 amino acid residues including the putative NH2-terminal presequence for mitochondrial import was predicted. It was 59% identical with arginase I. The arginase II precursor synthesized in vitro was imported into isolated mitochondria and proteolytically processed. mRNA for human arginase II was present in the kidney and other tissues, but was not detected in the liver. Arginase II mRNA was coinduced with nitric oxide synthase mRNA in murine macrophage-like RAW 264.7 cells by lipopolysaccharide. This induction was enhanced by dexamethasone and dibutyryl cAMP, and was prevented by interferon-gamma. Possible roles of arginase II in NO synthesis are discussed.