The permissive role of glucocorticoids on interleukin-1 stimulation of angiotensinogen gene transcription is mediated by an interaction between inducible enhancers

Potential mechanisms of hypothalamic renin-angiotensin system activation by leptin and DOCA-salt for the control of resting metabolism

The renin-angiotensin system (RAS), originally described as a circulating hormone system, is an enzymatic cascade in which the final vasoactive peptide angiotensin II (ANG) regulates cardiovascular, hydromineral, and metabolic functions. The RAS is also synthesized locally in a number of tissues including the brain, where it can act in a paracrine fashion to regulate blood pressure, thirst, fluid balance, and resting energy expenditure/resting metabolic rate (RMR). Recent studies demonstrate that ANG AT_1A receptors (Agtr1a) specifically in agouti-related peptide (AgRP) neurons of the arcuate nucleus (ARC) coordinate autonomic and energy expenditure responses to various stimuli including deoxycorticosterone acetate (DOCA)-salt, high-fat feeding, and leptin. It remains unclear, however, how these disparate stimuli converge upon and activate this specific population of AT_1A receptors in AgRP neurons. We hypothesize that these stimuli may act to stimulate local expression of the angiotensinogen (AGT) precursor for ANG, or the expression of AT_1A receptors, and thereby local activity of the RAS within the (ARC). Here we review mechanisms that may control AGT and AT_1A expression within the central nervous system, with a particular focus on mechanisms activated by steroids, dietary fat, and leptin.

Regulation of signal transducer and activator of transcription 3 enhanceosome formation by apurinic/apyrimidinic endonuclease 1 in hepatic acute phase response

The signal transducer and activator of transcription-3 (STAT3) is a latent IL-6 inducible transcription factor that mediates hepatic and vascular inflammation. In this study, we make the novel observation that STAT3 forms an inducible complex with the apurinic/apyrimidinic endonuclease 1 (APE1)/redox effector factor-1 (APE1/Ref-1), an essential multifunctional protein in DNA base excision repair, and studied the role of APE1/Ref-1 in STAT3 function. Using a transfection-coimmunoprecipitation assay, we observed that APE1 selectively binds the NH(2)-terminal acetylation domain of STAT3. Ectopic expression of APE1 potentiated inducible STAT3 reporter activity, whereas knockdown of APE1 resulted in reduced IL-6-inducible acute-phase reactant protein expression (C-reactive protein and serum amyloid P) and monocyte chemotactic protein-1 expression. The mechanism for APE1 requirement in IL-6 signaling was indicated by reduced STAT3 DNA binding activity observed in response to small interfering RNA-mediated APE1 silencing. Consistent with these in vitro studies, we also observed that lipopolysaccharide-induced activation of acute-phase reactant protein expression is significantly abrogated in APE1 heterozygous mice compared with wild-type mice. IL-6 induces both STAT3 and APE1 to bind the suppressor of cytokine signaling-3 and gamma-fibrionogen promoters in their native chromatin environment. Moreover, we observed that APE1 knockdown destabilized formation of the STAT3-inducible enhanceosome on the endogenous gamma-fibrionogen promoter. Taken together, our study indicates that IL-6 induces a novel STAT3-APE1 complex, whose interaction is required for stable chromatin association in the IL-6-induced hepatic acute phase response.

Inhibition of angiotensin II action protects rat steatotic livers against ischemia-reperfusion injury

OBJECTIVE

We examined whether pharmacologic strategies blocking angiotensin II actions protect steatotic livers against ischemia-reperfusion (I/R) injury. The effects of ischemic preconditioning (PC) on angiotensin II were also evaluated.

DESIGN

Randomized and controlled animal study.

SETTING

Experimental laboratory.

SUBJECTS

Zucker rats.

INTERVENTIONS

The following experimental groups were studied: I/R, ischemia-reperfusion + angiotensin-converting enzyme inhibitor (I/R+ACE inhibitor), ischemia-reperfusion + angiotensin II type I receptor antagonist (I/R+AT1R antagonist), ischemia-reperfusion + angiotensin II type II receptor antagonist (I/R+AT2R antagonist), and PC (5 mins of ischemia + 10 mins of reperfusion before I/R). In some of these groups, the action of bradykinin (BK) and/or peroxisome-proliferator-activated receptor-gamma (PPARgamma) was altered pharmacologically.

MEASUREMENTS AND MAIN RESULTS

I/R+ACE inhibitor, I/R+AT1R antagonist, and I/R+AT2R antagonist reduced hepatic injury in steatotic livers compared with the I/R group. PC reduced angiotensin II generation and hepatic injury in steatotic livers in comparison to I/R group. Our results revealed that I/R+ACE inhibitor, I/R+AT1R antagonist, I/R+AT2R antagonist, and PC increased BK compared with the I/R group. In addition, the effects of PC on BK and hepatic injury were abolished when angiotensin II was administered. Furthermore, administration of BK receptor antagonists to the I/R+ACE inhibitor, I/R+AT1R antagonist, I/R+AT2R antagonist, and PC groups resulted in hepatic injury similar to the I/R group, indicating that the benefits of ACE inhibitor, AT1R antagonist, AT2R antagonist, and PC were abolished when the action of BK was inhibited. Experiments aimed at investigating why BK was protective in steatotic livers indicated that BK acts as a positive regulator of PPARgamma. If PPARgamma action was inhibited, BK did not protect steatotic livers against hepatic injury.

CONCLUSIONS

Pharmacologic blockers of angiotensin II action (ACE inhibitors, AT1R antagonists, and AT2R antagonists) and PC, which reduced angiotensin II generation, increased BK generation in steatotic livers after I/R. This in turn increased PPARgamma and protected this type of liver against I/R injury.

A haplotype of the angiotensinogen gene is associated with hypertension in african americans

1. Hypertension is a serious risk factor for myocardial infarction, heart failure, vascular disease, stroke and renal failure. The incidence of hypertension is 25-30% in the adult Caucasian population and complications due to hypertension are even greater in African Americans. 2. The renin-angiotensin system plays an important role in the regulation of blood pressure and previous studies have suggested that angiotensinogen (AGT) gene locus is linked with human essential hypertension. Earlier studies suggested that a single nucleotide polymorphism (SNP) that converts methionine to threonine at amino acid 235 is associated with hypertension in the Caucasian population. However, this SNP is not associated with hypertension in African American and Chinese populations. 3. We have found an A/G polymorphism at -217 of the human AGT gene promoter and have shown that the frequency of allele A at -217 is significantly increased in the genomic DNA of African American hypertensive patients. 4. We have also shown that: (i) reporter constructs containing the AGT gene promoter with nucleoside A at -217 have increased promoter activity on transient transfection; and (ii) the CCAAT box enhancer binding protein (C/EBP) family of transcription factors and glucocorticoid receptor (GR) bind preferentially to this region of the promoter when nucleoside A is present at -217. In addition, variant -217A is always present with variants -532T, -793A and -1074T in the human AGT gene promoter. 5. These data suggest that the AGT haplotype containing -217A, -532T, -793A and -1074T may be involved in increased transcription of this gene and may play a role in human hypertension.

STAT3 NH2-terminal acetylation is activated by the hepatic acute-phase response and required for IL-6 induction of angiotensinogen

BACKGROUND & AIMS

The signal transducers and activators of transcription (STATs) are cytoplasmic transcription factors mediating acute-phase response (APR) of the human angiotensinogen (hAGT) gene in hepatocytes. The mechanisms of how STAT3 activates target genes are unknown. Here we analyzed the biochemistry of STAT3 activation by interleukin (IL)-6 in hepatocellular carcinoma HepG2 and Balb/C mice.

METHODS

Immunoprecipitation-Western assays and Matrix Assisted Laser Desorption-Time of Flight mass spectrometry determined sites of STAT3 acetylation by the 300-kilodalton target of E1A (p300) co-activator. The subcellular localization of acetylation-deficient STAT3 molecules were studied by microscopic imaging, effects on DNA binding measured by gel shift and chromatin immunoprecipitation (ChIP) assays, and gene transactivation by Northern blot and reporter assays.

RESULTS

Two Lys residues at amino acids 49 and 87 in the STAT3 NH2 terminus are acetylated by p300. Lys-to-Arg point mutations (STAT3 K49R/K87R) had no effect on inducible DNA binding, but blocked p300-mediated acetyl(Ac)-STAT3 formation and abrogated IL-6-induced hAGT activation. Although STAT3 K49R/K87R rapidly translocated into the nucleus, it did not bind p300 and had delayed cytoplasmic redistribution. ChIP assays show IL-6-inducible acetylated STAT3 and p300 binding to the native hAGT promoter. Activation of the APR in mice induces nuclear Tyr phosphorylated and acetylated STAT3 in hepatic nuclei. We also observed that STAT3 interacts with histone deacetylases (HDACs), specifically HDAC 1, that down-regulate IL-6-induced hAGT transactivation.

CONCLUSIONS

IL-6-induced target gene activation requires p300-mediated STAT3 acetylation, and HDACs are involved in the termination of STAT3 action. These studies indicate the acetylation-deacetylation reaction as a novel signaling mechanism controlling the IL-6-STAT3 pathway in the hepatic APR.

A single-nucleotide polymorphism in human angiotensinogen gene is associated with essential hypertension and affects glucocorticoid induced promoter activity

Hypertension is a serious health problem particularly for African-Americans. Previous studies have suggested that angiotensinogen (AGT) gene locus is involved in human essential hypertension. We have recently shown that an A/G polymorphism at -217 in the promoter of the AGT gene is associated with essential hypertension especially in African-Americans. We report here that A/G polymorphism at -217 affects the glucocorticoid-induced promoter activity of the human AGT gene. We show that recombinant glucocorticoid receptor (GR) binds strongly to the AGT gene promoter when nucleoside A is present at -217, and dexamethasone treatment increases the interleukin 6 induced promoter activity of reporter constructs containing nucleoside A at -217. Similarly cotransfection of GR and C/EBP beta or C/EBP delta increases the promoter activity of reporter construct containing nucleoside A at -217. Since AGT is an acute phase protein, we propose that increased expression of -217A allele of the AGT gene by glucocorticoids and C/EBP family of transcription factors may be involved in essential hypertension.

Characterization of cis-elements mediating the stimulation of glucose-6-phosphate transporter promoter activity by glucocorticoids

The endoplasmatic glucose-6-phosphate transporter is involved in the control of hepatic glucose production and blood glucose homeostasis. In this study, the expression of a luciferase reporter gene under the control of the glucose-6-phosphate transporter gene promoter was examined in transiently transfected hepatoma cells. The promoter activity was stimulated approximately 2.5-fold by dexamethasone. Mutational analyses demonstrated that the regions nucleotide (nt) -215/-209 and nt -197/-183 relative to the translation start site were critical for this regulation. In gel electrophoretic mobility shift assays the transcription factor Fox O1, also called forkhead in rhabdomyosarcoma (FKHR), overexpressed in 293 cells, bound to a probe with the sequence nt -215/-209. The overexpression of Fox O1 stimulated the induction of glucose-6-phosphate transporter promoter activity by dexamethasone via nt -215/-209 in hepatoma cells. Recombinant glucocorticoid receptor DNA binding domain protein bound to a probe with the sequence of nt -197/-183 in gel electrophoretic mobility shift assays and an oligonucleotide with this sequence transferred glucocorticoid responsiveness to a heterologous promoter. The data indicate that the glucose-6-phosphate transporter promoter contains a glucocorticoid response unit consisting of binding sites for Fox O1 and the glucocorticoid receptor.

The interplay between the glucocorticoid receptor and nuclear factor-kappaB or activator protein-1: molecular mechanisms for gene repression

The inflammatory response is a highly regulated physiological process that is critically important for homeostasis. A precise physiological control of inflammation allows a timely reaction to invading pathogens or to other insults without causing overreaction liable to damage the host. The cellular signaling pathways identified as important regulators of inflammation are the signal transduction cascades mediated by the nuclear factor-kappaB and the activator protein-1, which can both be modulated by glucocorticoids. Their use in the clinic includes treatment of rheumatoid arthritis, asthma, allograft rejection, and allergic skin diseases. Although glucocorticoids have been widely used since the late 1940s, the molecular mechanisms responsible for their antiinflammatory activity are still under investigation. The various molecular pathways proposed so far are discussed in more detail.

Mutational analysis of the kinetics and thermodynamics of transcription factor NF-kappaB homodimerisation

Dimeric transcription factors of the NF-kappaB/Rel family are sequence-specific DNA-binding proteins that mediate the inducible expression of immunologically important eukaryotic genes by competing for kappaB sites. The kinetic and thermodynamic components of these interactions were probed by mutation of the subunit interface of the p50 homodimer, a paradigm for other family members. Guided by the crystal structure, we selected the side chains of five key residues (R255, Y270, L272, A311 and V313) for individual and combinatorial truncation, with the aim of generating a mutant panel. Homodimerisation was assessed indirectly by measurement of DNA binding with an optical biosensor in order to unmask the relative contributions of each residue. Surface plasmon resonance revealed that a unanimous bias for a palindromic kappaB site over an asymmetric one was mainly the result of a slower dissociation rate for the DNA/homodimer complex in the case of the palindromic kappaB site. Y270 and L272 were individually the most critical residues in homodimerisation. DNA binding was abolished when all five residues were substituted, which reinforces the notion that only a subset of residues contributes crucial dimer-forming contacts. The role of Y270 was unique, since its mutation to glycine dramatically slowed both the association and dissociation rates for DNA binding. Surprisingly, R255 was shown to be of little importance in the stability of the p50 homodimer, despite its apparent participation in a salt bridge at the dimer interface. Our results suggest that binding modes inferred from structural data should be treated cautiously.

NF-kappa B-inducible BCL-3 expression is an autoregulatory loop controlling nuclear p50/NF-kappa B1 residence

NF-kappa B is a transcription factor whose nuclear residence is controlled by I kappa B family members. In the NF-kappa B-I kappa B autoregulatory loop, activated (nuclear) Rel A.NF-kappa B1 induces the resynthesis of I kappa B alpha recapturing nuclear Rel A back into the cytoplasm within 1 h of stimulation. In contrast, NF-kappa B1 subunits redistribute more slowly into the cytoplasm (from 6 to 12 h). Here we examine the role of inducible cytoplasmic BCL-3 expression in terminating nuclear NF-kappa B1. Although BCL-3 is a nuclear protein in B lymphocytes, surprisingly, BCL-3 is primarily a cytoplasmic protein in HepG2 cells. Cytoplasmic BCL-3 abundance is induced 6-12 h after tumor necrosis factor-alpha stimulation where it complexes with NF-kappa B1 homodimers. Moreover, BCL-3 mRNA and protein expression are induced by NF-kappa B-activating agents. Two observations are interpreted to indicate that bcl-3 is transactivated by NF-kappa B/Rel A: 1) expression of a dominant negative NF-kappa B inhibitor blocks tumor necrosis factor-alpha-induced BCL-3 expression and 2) expression of constitutively active Rel A is sufficient to induce BCL-3 expression. In gene transfer studies, we identify two high affinity NF-kappa B-binding sites, kappa B1 (located at -872 to -861 nucleotides) and kappa B2 (-106 to -96 nucleotides), and although both bind with high affinity to Rel A, only kappa B2 is required for NF-kappa B-dependent induction of the native BCL-3 promoter. Down-regulation of BCL-3 induction results in prolonged, enhanced NF-kappa B1 binding and increased NF-kappa B-dependent transcription. Together, these data suggest the presence of an NF-kappa B-BCL-3 autoregulatory loop important in terminating NF-kappa B1 action and that individual NF-kappa B isoforms are actively terminated through coordinate induction of inhibitory I kappa B molecules to restore cellular homeostasis.

Transcriptional regulation of angiotensinogen gene expression

The renin--angiotensin system (RAS) is an extracellular hormonal system implicated in acute, homeostatic control of peripheral vascular resistance and electrolyte homeostasis. In this tightly regulated system, physiological regulators of blood pressure and fluid balance induce the production of the potent vasoactive angiotensin peptides by sequential proteolysis of the angiotensinogen (AGT) prohormone. AGT is the only known precursor of the angiotensin peptides, whose circulating concentrations influence the tonic activity of the RAS. AGT abundance is regulated at the transcriptional level through hormonal and cell-type specific regulators. In this review, we will discuss the identified mechanisms controlling AGT expression separately for the rodent and human genes. The most intensively investigated gene (rodent AGT) is regulated constitutively by multiple positive- and negative-acting cis factors that function in a cell-type dependent fashion. Inducible rodent AGT expression is mediated through a multihormone-inducible enhancer that integrates signals from steroid and cytokine hormones into AGT transcription. We review recent advances in understanding the mechanism of the nuclear factor-kappa B (NF-kappa B) family in mediating cytokine-induced AGT expression and our recent discoveries on the existence of differentially inducible pools of cytoplasmic NF-kappa B. Constitutive control of the human AGT gene will be discussed; there is surprisingly little information on the cis- and trans-acting regulators controlling inducible expression of human AGT. Finally, we will explore some of the recent developments in gene linkage studies where human AGT alleles have been associated with hypertensive phenotypes through a mechanism that may involve enhanced transcription. These studies have provided a molecular explanation for a subset of heritable hypertensive disorders in humans.

Association of 3 gene polymorphisms with atopic diseases

BACKGROUND

Various peptidases, including angiotensin-converting enzyme (ACE), inactivate some inflammatory peptides that are considered to influence the pathogenesis of atopic diseases. This enzyme is also involved in the conversion or activation of 2 bronchoconstriction mediators: angiotensin II from angiotensinogen and endothelin (ET), respectively.

OBJECTIVE

We tested a hypothesis that asthma or other atopic diseases are associated with insertion/deletion ACE, M235T angiotensinogen, and TaqI ET-1 gene polymorphisms.

METHODS

A case-control approach was used in the study. Healthy subjects (141 persons) were used as control subjects, and 231 patients with histories of atopic asthma, allergic rhinitis, atopic dermatitis, or a combination thereof were studied. ACE genotype was determined by PCR, angiotensinogen M235T and ET-1 by PCR, and restriction analysis by AspI and TaqI, respectively.

RESULTS

We found the significant association of the insertion/deletion polymorphism of the ACE, as well as that of M235T polymorphism of the angiotensinogen genes, with the group of patients with atopic diseases ( P =.0025 and P =.0204, respectively). No difference was proved for the intron 4 (position 8000) polymorphism in the ET-1 gene when comparing the atopic patients with the control group (P =.1774). A significant difference was found between groups of patients with both asthma and rhinitis and patients without both respiratory atopic diseases (P =.0033).

CONCLUSION

It follows that the examined polymorphisms in the genes for ACE, angiotensinogen, and ET-1 could participate in the etiopathogenesis of atopic diseases.

Identification of a cAMP response element within the glucose- 6-phosphatase hydrolytic subunit gene promoter which is involved in the transcriptional regulation by cAMP and glucocorticoids in H4IIE hepatoma cells

The expression of a luciferase reporter gene under the control of the human glucose 6-phosphatase gene promoter was stimulated by both dexamethasone and dibutyryl cAMP in H4IIE hepatoma cells. A cis-active element located between nucleotides -161 and -152 in the glucose 6-phosphatase gene promoter was identified and found to be necessary for both basal reporter-gene expression and induction of expression by both dibutyryl cAMP and dexamethasone. Nucleotides -161 to -152 were functionally replaced by the consensus sequence for a cAMP response element. An antibody against the cAMP response element-binding protein caused a supershift in gel-electrophoretic-mobility-shift assays using an oligonucleotide probe representing the glucose 6-phosphatase gene promoter from nucleotides -161 to -152. These results strongly indicate that in H4IIE cells the glucose 6-phosphatase gene-promoter sequence from -161 to -152 is a cAMP response element which is important for the regulation of transcription of the glucose 6-phosphatase gene by both cAMP and glucocorticoids.

Identification of a glucocorticoid response element in the rat beta2-adrenergic receptor gene

Regulation of beta2-adrenergic receptor (beta2AR) levels by glucocorticoids is a physiologically important mechanism for altering beta2AR responsiveness. Glucocorticoids increase beta2AR density by increasing the rate of beta2AR gene transcription, but the cis-elements involved have not been well characterized. We now show that one of six potential glucocorticoid response elements (GREs) in the 5'-flanking region of the rat beta2AR gene is necessary for glucocorticoid-dependent stimulation of receptor gene expression. Using a nested set of deletion fragments of the rat beta2AR gene 5'-flanking region fused to a luciferase reporter gene, glucocorticoid-dependent induction of reporter gene expression in HepG2 cells was localized to a region between positions -643 and -152, relative to the transcription initiation site. In electrophoretic mobility shift assays, a double-stranded oligonucleotide incorporating a near-consensus GRE from this region (positions -379 to -365) formed complexes with the human recombinant glucocorticoid receptor, as well as with nuclear protein from dexamethasone-treated HepG2 cells. Mutation of a single base within this GRE sequence greatly diminished interaction of the mutated oligonucleotide with the human recombinant glucocorticoid receptor. The functional activity of the GRE was characterized using a luciferase reporter construct driven by a minimal thymidine kinase promoter. In HepG2 cells transfected with constructs containing the GRE, dexamethasone increased reporter gene expression approximately 3-fold, whereas a dexamethasone effect was not observed with constructs lacking the GRE. Taken together, these findings show that a GRE located at positions -379 to -365 in the 5'-flanking region of the rat beta2AR gene mediates glucocorticoid stimulation of beta2AR gene transcription.

Tissue angiotensinogen gene expression induced by lipopolysaccharide in hypertensive rats

There is now convincing evidence that various tissues express their own tissue renin-angiotensin system, which may be regulated independently of the systemic renin-angiotensin system. However, little information is available on the regulation of the tissue renin-angiotensin system. We investigated the regulation of tissue angiotensinogen gene expression with respect to the development of hypertension. We measured basal and lipopolysaccharide-stimulated plasma angiotensinogen concentrations by radioimmunoassay and examined the expression of tissue angiotensinogen by Northern blot analysis in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) at 4 and 13 weeks of age. Basal plasma angiotensinogen concentration in SHR was comparable to that in WKY at 4 weeks of age and was significantly higher than that in WKY at 13 weeks of age. Lipopolysaccharide induced a significant increase in plasma angiotensinogen concentration in both WKY and SHR at 4 and 13 weeks of age. At 4 weeks of age, the basal levels of angiotensinogen mRNA in the liver, fat, adrenal, and aorta were higher in WKY than in SHR. At 13 weeks of age, the basal levels of angiotensinogen mRNA in the fat, adrenal, aorta, spleen, and kidney were higher in WKY than in SHR, while that in the liver did not differ significantly between the two strains. At 4 weeks of age, pretreatment with lipopolysaccharide increased the angiotensinogen mRNA levels in the liver, fat, adrenal, and aorta in both WKY and SHR. At 13 weeks of age, pretreatment with lipopolysaccharide increased the angiotensinogen mRNA levels in the liver, aorta, and adrenal; decreased those in the spleen; and had no effect in the kidney in both WKY and SHR. Interestingly, lipopolysaccharide increased the angiotensinogen mRNA level in fat only in SHR, with no effect in WKY, at 13 weeks of age. Lipopolysaccharide stimulated tumor necrosis factor-a mRNA expression in fat of WKY and SHR, and the increase in tumor necrosis factor-alpha mRNA level in SHR was significantly greater than that in WKY. Therefore, the increased tumor necrosis factor-alpha mRNA expression may be involved in the increased lipopolysaccharide-induced expression of angiotensinogen gene in fat of SHR at 13 weeks of age. These data suggest that the transcriptional and probably posttranscriptional regulation of angiotensinogen mRNA differs between SHR and WKY, that the regulation of angiotensinogen gene expression is tissue-specific, and that the altered expression of the angiotensinogen gene may be involved in the development of hypertension.

Dexamethasone enhances the osteogenic effects of fluoride in human TE85 osteosarcoma cells in vitro

The in vitro osteogenic effects of fluoride have not always been consistently observed in human bone cells. The present study sought to test if dexamethasone (Dex) could potentiate the action of fluoride to increase the detectability of the stimulatory effects of fluoride on [3H]thymidine incorporation, alkaline phosphatase (ALP) specific activity, collagen synthesis, and osteocalcin secretion in human TE85 osteosarcoma cells. Neither Dex at 10(-10)-10(-6) M or fluoride at a mitogenic dose (100 microM) had any consistent stimulatory effects on thymidine incorporation. When the cells were treated with both agents simultaneously, significant and highly reproducible stimulations were observed. The mitogenic effects of the two agents were confirmed with cell number counting. Analysis of variance (ANOVA) revealed a significant interaction (P < 0.001) between fluoride and Dex on cell proliferation. The enhancing effect of Dex on [3H]thymidine incorporation was not due to a shift of the optimal dose response of fluoride. Though fluoride alone or Dex alone also had no consistent effect on ALP specific activity, the co-treatment with fluoride and Dex for 24 hours produced significant (P < 0.001, ANOVA) stimulation in ALP specific activity. Fluoride alone had no consistent effect on collagen synthesis and on 1, 25(OH)2D3-dependent osteocalcin secretion, whereas Dex treatment consistently inhibited these two osteoblastic parameters in a dose-dependent manner. However, both the collagen synthesis and osteocalcin secretion rates were significantly higher (P < 0.001 ANOVA for each) when the cells were co-treated with Dex and fluoride (100 microM) than when they were treated with Dex alone. Thus, these data indicate that the response in collagen synthesis and osteocalcin secretion to fluoride stimulation was more readily observed in the presence of Dex than in its absence. ANOVA analysis revealed that the interaction between fluoride and Dex on collagen synthesis, but not the 1,25(OH)2D3-dependent osteocalcin secretion, was significant (P < 0.02). In summary, we have demonstrated for the first time that in TE85 cells (1) Dex potentiated the effects of fluoride on cell proliferation, ALP specific activity, and collagen synthesis; (2) while Dex at 10(-7)-10(-6) M alone inhibited the collagen synthesis and at 10(-9)-10(-6) M reduced osteocalcin secretion, Dex at 10(-8)-10(-6) M significantly stimulated the proliferation of TE85 cells; and (3) Dex interacted with fluoride to increase the percentage of experiments showing an osteogenic action of fluoride. In conclusion, the in vitro osteogenic actions of fluoride in human TE85 cells are more consistently observed in the presence than in the absence of Dex.

Tumor necrosis factor activates angiotensinogen gene expression by the Rel A transactivator

Angiotensinogen encodes the only known precursor of angiotensin II, a critical regulator of the cardiovascular system. Transcriptional control of angiotensinogen in hepatocytes is an important regulator of circulating angiotensinogen concentrations. Angiotensinogen transcription is increased by the inflammatory cytokine tumor necrosis factor (TNF)-alpha by a nuclear factor-kappaB-like protein binding to an inducible enhancer called the acute-phase response element. By gel mobility shift assays, we observe two specific acute-phase response element-binding complexes, C1 and C2. The abundance of C2 is not changed by TNF treatment. In contrast, C1 is faintly detected in untreated cells, and its abundance increases by fivefold after stimulation. We identify the nuclear factor-kappaB subunits in these complexes using subunit-specific antibodies in the gel mobility "supershift" assay. The transcriptionally inert nuclear factor-kappaB DNA-binding subunit NF-kappaB1 is present in both control and stimulated hepatocyte nuclei. Its abundance changes weakly upon TNF stimulation. In contrast, the potent transactivating protein Rel A is not found in unstimulated hepatocyte nuclei and is recruited by TNF-alpha into the C1 DNA-binding complex. Overexpression of Rel A results in acute-phase response element transcription. Cotransfection of a chimeric GAL4-Rel A protein with GAL4 DNA-binding sites is a strategy that allows for selective study of Rel A. The GAL4:Rel A chimera is a TNF-alpha-inducible transactivator. Deletion of the amino-terminal 254 amino acids of Rel A produces a constitutive activator (that is no longer TNF-alpha inducible). The cytokine induction of Rel A, then, is mediated through its amino-terminal 254 amino acids. We conclude that Rel A:NF-kappaB1 is a crucial cytokine-inducible transcription factor complex regulating angiotensinogen gene synthesis in hepatocytes and may be involved in controlling the activity of the renin-angiotensin system.

Mechanisms for inducible control of angiotensinogen gene transcription

The intravascular renin-angiotensin system is an endocrine system designed to maintain cardiovascular homeostasis in response to hypotension. Under normal conditions, angiotensinogen concentrations circulating in the plasma are rate limiting for the maximum velocity of angiotensin I formation. In the liver, the major site of circulating angiotensinogen synthesis, angiotensinogen expression is under exquisite hormonal control. We review the mechanisms by which hormones effect transcriptional control of angiotensinogen expression. Adrenal-derived glucocorticoids produce the translocation of the glucocorticoid receptor into the nucleus. It in turn binds to two glucocorticoid response elements and stimulates angiotensinogen gene transcription. Inflammation activates angiotensinogen transcription as a result of the macrophage-derived cytokines interleukin-1 and tumor necrosis factor-alpha. These cytokines change the abundance of two transcription factor families that bind a single regulatory site in the angiotensinogen promoter, the acute-phase response element. These proteins include the nuclear factor-kappaB complex and the CCAAT/enhancer binding protein family. Activation of the renin-angiotensin system, through production of angiotensin II, results in feedback stimulation of angiotensinogen synthesis (the "positive feedback loop"). We have discovered that the nuclear factor-kappaB transcription factor is regulated by angiotensin II, a finding that provides a mechanism for the transcriptional component of angiotensinogen gene synthesis in the positive feedback loop. These studies underscore the concept that induction of the angiotensinogen gene by diverse physiological stimuli is mediated through changes in the nuclear abundance of sequence-specific transcription factors. The intracellular convergence of cytokine- and angiotensin II-induced signaling pathways on the nuclear factor-kappaB transcription factor provides a point for "cross talk" between angiotensin- and cytokine-activated second messenger pathways.

Transcription factors modulating angiotensinogen gene expression in hepatocytes

The gene encoding angiotensinogen is regulated at the transcriptional level in hepatocytes in response to glucocorticoids and inflammatory cytokines (IL-1 and TNF). These hormones activate transcription of the angiotensinogen gene by changing the abundance of DNA binding proteins that interact with a multihormone-inducible enhancer located between nucleotides -615 to -440 upstream of the major transcription start site. Activation of this enhancer in hepatocytes is effected by glucocorticoid- and cytokine-inducible DNA binding proteins. Cytokine induction is mediated through the interaction of two classes of transcription factors that bind to the acute-phase response element (APRE): nuclear factor-kappa B (NF-kappa B), and CCAAT-Box/Enhancer Binding Protein (C/EBP). NF-kappa B is a multiprotein DNA binding complex sequestered in the cytoplasm that is induced in the nucleus by cytokines, whereas C/EBP is a nuclear transcription factor family implicated in the expression of differentiated hepatic proteins. During the acute-phase response, individual C/EBP family members are discordinately regulated: C/EBP alpha levels fall, whereas another C/EBP family member termed nuclear factor IL6 (NF-IL6), is induced. We investigated the interaction between the two acute-phase induced APRE-binding proteins: NF-kappa B and NF-IL6. Both proteins bind to overlapping nucleotides in a mutually exclusive fashion with similar affinities for the APRE. NF-IL6, a less potent transactivator, attenuates NF-kappa B mediated transcription late in the evolution of the acute-phase response. These observations argue for a temporal model of sequentially-expressed transcription factors occupying the APRE during the evolution of the inflammatory process.

Control of activator protein-1 and nuclear factor kappa B activity by interleukin-1, interleukin-6 and metals in HEPG2 cells

The intracellular signals induced by IL-1 and IL-6 have been described but there are few details of the signals they induce in liver-derived cells during initiation of acute phase protein synthesis. We therefore used an in vitro system to investigate signalling by IL-1 and IL-6 in the human liver cell line, HepG2. Chloramphenicol acetyl transferase (CAT) expression vectors, under the control of activator protein-1 (pTRE-CAT), nuclear factor kappa B (pNF-CAT) or no enhancer region (pBLCAT2), were transfected into HepG2 cells and the effects of the cytokines on their activity was studied. Profound changes in liver processing of heavy metals and the induction of metal-dependent acute proteins are also seen during the acute phase response. To determine if the supply of metal ions could itself influence signalling we also investigated the effects of cadmium and zinc on the activity of the transfected vectors. Both alpha and beta forms of interleukin-1 increased the expression of pTRE-CAT and pNF-CAT, but not pBLCAT2, while interleukin-6 had no effect, suggesting that activator protein-1 and nuclear factor kappa B activity was induced by interleukin-1, but not interleukin-6. Specificity of the effect of interleukin-1 alpha was confirmed using an anti-interleukin-1 alpha monoclonal antibody. Zinc and cadmium also increased pTRE-CAT expression, but not pNF-CAT or pBLCAT2. Removal of heavy metal ions from the culture medium resulted in decreased pTRE-CAT expression, while pNF-CAT and pBLCAT2 were relatively unaffected, confirming the stimulatory effect of metals on activator protein-1, but not nuclear protein kappa B activity. Therefore, metal and interleukin-1-mediated signal transduction may involve overlapping pathways, whereas interleukin-1 and interleukin-6 act via different pathways in liver cells.

Angiotensin II and dexamethasone regulate angiotensinogen mRNA by different mechanisms

Angiotensinogen synthesis and secretion in the liver is regulated by glucocorticoids and angiotensin II. In isolated hepatocytes in suspension culture, both dexamethasone and angiotensin II induced an increase in angiotensinogen mRNA (2.5- and 4-fold, respectively) with half maximal stimulation at 20 and 200 nM, respectively. In a nuclear run on assay, transcription of the angiotensinogen gene in nuclei from hepatocytes exposed to angiotensin II was not significantly different from controls, whereas dexamethasone-pretreatment dramatically stimulated angiotensinogen mRNA synthesis. By inhibition of transcription in hepatocytes, as well as in [32P]uridine pulse and chase experiments, angiotensin II was shown to stabilize angiotensinogen mRNA, prolonging the intracellular half-life from 83 to 191 min. In polysomal extracts from hepatocytes, a 12 kDa protein could be identified, that binds to a probe of the 3'-untranslated region (UTR) angiotensinogen mRNA. The binding activity of this protein appears to be higher in hepatocytes exposed to angiotensin II, and to have a stabilizing effect on angiotensinogen mRNA. It is proposed that angiotensin II enhances the binding activity of a 12 kDa protein the 3'-UTR of angiotensinogen mRNA, which results in increased stability and transcription of angiotensinogen mRNA.

Tissue renin-angiotensin systems in renal hypertension

Angiotensinogen messenger RNA (mRNA) levels were measured in the brain (hypothalamus, lower brain stem, cerebellum), liver, kidneys, and adrenal glands of rats made hypertensive by ligation of the aorta between the renal arteries. We also measured renin mRNA in the kidneys of these renal hypertensive rats. The early phase of hypertension (day 6) was associated with significant increases in plasma renin activity and levels of circulating angiotensin II. The circulating renin-angiotensin system was not activated in the later phase of hypertension (day 24). Angiotensinogen mRNA levels were elevated in the lower brain stem of hypertensive rats at both stages of hypertension. In contrast, angiotensinogen mRNA levels in the hypothalamus were increased only at day 6 after aortic ligation. Decreased levels of angiotensinogen mRNA were observed in the cerebellum in both the early and later phases of the hypertension. Angiotensinogen mRNA levels in the adrenal gland below the ligature fell in the early phases but rose in the later phases of hypertension. Renin mRNA levels of the ischemic kidney remained elevated at both the early and later phases, whereas in both ischemic and nonischemic kidneys, levels of angiotensinogen mRNA remained below sham values throughout the period of study. These results indicate differential expression of renin-angiotensin system mRNAs in tissues of renal hypertensive rats. The differential changes in the expression of angiotensinogen mRNA over the course of development and maintenance of renal hypertension suggest that factors in addition to angiotensin II are important in modulating the expression of renin-angiotensin system genes.

Effects of cytokines on the liver

Cytokines are essential for the communication not only between the liver and extrahepatic sites but also within the liver itself. Cytokines regulate the intermediary metabolism of amino acids, proteins, carbohydrates, lipids and minerals. Cytokines partially interact with classical hormones such as glucocorticoids, resulting in a complex network of mutual control. Since many cytokines exert growth factor-like activities in addition to their specific proinflammatory effects, the distinction between cytokines and growth factors is somewhat artificial. The liver is an important site of synthesis and the major clearance organ for several cytokines. In liver disease, cytokines are involved in the onset of intrahepatic immune responses (e.g., during viral hepatitis), in liver regeneration (e.g., after partial hepatectomy) and in the fibrotic and cirrhotic transformation of the liver such as chronic chemical injury or viral infection. Further studies of cytokine actions may lead to a better understanding of liver diseases and to the development of new immunomodulating therapeutic options.

Transcriptional regulation of hepatic angiotensinogen gene expression by the acute-phase response

The acute-phase response is a protective physiological reaction to tissue injury manifested by the immediate increase in production and secretion of liver proteins the function of which is to re-establish the homeostasis altered by injury. Such proteins include blood coagulation factors, opsonins, protease-inhibitors and angiotensinogen, a precursor of the potent vasopressor peptide angiotensin II. The angiotensinogen gene is typical of genes regulated during the acute-phase response inasmuch as the promoter regulating its transcription rate is acutely responsive to three known mediators of the acute-phase response: glucocorticoids, and the cytokines interleukin-1 and tumor necrosis factor. We present a model, based on experimental evidence, for the mechanism by which angiotensinogen gene transcription is regulated in a graded fashion by the interplay of several hormonally-inducible transcription factors that bind a hormonally-inducible enhancer unit of the angiotensinogen promoter. These factors include the glucocorticoid receptor, nuclear factor kappa B and members of the CAAT/viral enhancer (C/EBP) family of DNA-binding proteins.