Glucocorticoids Induce the Accumulation of Novel Angiotensinogen Gene Transcripts

Dynamic regulation of the angiotensinogen gene by DNA methylation, which is influenced by various stimuli experienced in daily life

Angiotensinogen (AGT) has a central role in maintaining blood pressure and fluid balance. DNA methylation is an epigenomic modification maintaining a steady pattern in somatic cells. Herein we summarize the link between AGT regulation and DNA methylation. DNA methylation negatively regulates AGT expression and dynamically changes in response to continuous AGT promoter stimulation. High-salt intake and excess circulating aldosterone cause DNA demethylation around the CCAAT enhancer-binding protein-binding sites, thereby converting the phenotype of AGT expression from an inactive to an active state in visceral adipose tissue. Salt-dependent hypertension may be partially affected by increased adipose AGT expression. Because angiotensin II is a well-established aldosterone-releasing hormone, stimulation of adipose AGT by aldosterone creates a positive feedback loop. This effect is pathologically associated with obesity-related hypertension, although it would be physiologically favorable for humans to efficiently retain their body fluid. The clear difference in DNA demethylation patterns between aldosterone and cortisol indicates a difference in the respective target DNA-binding sites between mineralocorticoid and glucocorticoid receptors in the AGT promoter. Stimulation-induced interactions between transcription factors and target DNA-binding sites trigger DNA demethylation. Dynamic changes in DNA methylation occur in relaxed chromatin regions both where transcription factors actively interact and where transcription is initiated. In contrast to rapid histone modifications, DNA demethylation and remethylation will progress relatively slowly over days or years. A wide variety of stimuli in daily life will continue to slowly and dynamically change DNA methylation patterns throughout life. Wise choices of beneficial stimuli will improve health.

Differential regulation of angiotensinogen transcripts after renin infusion

To investigate angiotensinogen regulation in high-renin hypertension, we infused porcine renin intravenously at either a low (4 mU/kg per hour, n = 6) or high (20 mU/kg per hour, n = 9) dose into male Sprague-Dawley rats (225 to 250 g) for 5 days using osmotic minipumps. Control rats received 0.9% NaCl. In renin-infused rats, mean arterial pressure and plasma renin activity were significantly elevated. Both low- and high-renin infusions lowered plasma angiotensinogen levels. Plasma angiotension II was elevated in rats given renin but reached statistical significance only at the higher dose. Angiotensinogen mRNA isolated from the liver, adrenal gland, kidney, and brain was measured by slot blot analysis. Both renin doses were associated with significant decreases in the levels of liver and hypothalamic angiotensinogen mRNA. In the medulla oblongata, angiotensinogen mRNA was reduced only by the higher renin dose. The lower dose increased angiotensinogen mRNA in the adrenal gland, and in kidney, angiotensinogen mRNA level was unchanged by renin infusion. Angiotensinogen mRNA visualized on Northern blots showed that the number of mRNA species in liver decreased from three in control rats to a single mRNA species after renin infusion. Tissue differences in the size of the major angiotensinogen mRNA species were also apparent. This, together with changes in the total hybridization signal of angiotensinogen mRNA in tissues, suggests that renin differentially affects the different angiotensinogen mRNA transcripts. Results of this study indicate that angiotensinogen gene expression is regulated not only by alterations in levels of circulating angiotensin II but also by other mechanisms, presently unidentified, that are activated by renin infusions.

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.

Dexamethasone down-regulates the expression of endothelin B receptor mRNA in the rat brain

The present study was designed to examine effects of dexamethasone on the steady state level of endothelin B(ETB) receptor mRNA in in vivo the rat brain. ETB receptor mRNA was very high at the hypothalamus and cerebellum but was comparatively low at the striatum and amygdala. Dexamethasone, 1 and 7 mg/kg, i.p., markedly and dose-relatedly decreased ETB receptor mRNA level with slow onset of 8hr at the hypothalamus and cerebellum, but did not induced a marked decrease at other areas. On the contrary, dexamethasone produced an increase of ET-1 mRNA which preceded to the decrease of ETB receptor mRNA at the same brain areas. Phosphoramidon, a endothelin-converting enzyme inhibitor, did not antagonized but potentiated the effect of dexamethasone. Besides, phosphoramidon per se markedly stimulated the expression of ET-1 mRNA. The results suggested that dexamethasone down-regulates ETB receptor mRNA level at the hypothalamus and cerebellum of rat brain and these effects may be involved in the increase of ET-1 peptide gene transcription.

Beta-adrenergic receptors and angiotensinogen gene expression in mouse hepatoma cells in vitro

We have previously reported that addition of 8-bromocyclic AMP enhances the stimulatory effect of dexamethasone on the expression of the angiotensinogen gene in mouse hepatoma cells in vitro. Isoproterenol is known to stimulate the synthesis of hepatic intracellular cyclic AMP via beta-adrenergic receptors. To study the possible effect of beta-adrenergic receptors on the expression of the angiotensinogen gene in mouse hepatoma cells, we transiently transfected them with a fusion gene with the 5'-flanking region of the angiotensinogen gene linked to a bacterial chloramphenicol acetyltransferase coding sequence as a reporter, pOCAT (ANG N-1498/+18). The addition of isoproterenol (10(-9) to 10(-5) mol/L) alone had no stimulatory effect on the expression of pOCAT (ANG N-1498/+18). In the presence of dexamethasone (10(-6) mol/L), however, isoproterenol enhanced the stimulatory effect on the dexamethasone on the expression of pOCAT (ANG N-1498/+18). The enhancing effect of isoproterenol was inhibited by the presence of propranolol (beta 1- and beta 2-adrenergic receptor antagonist) and ICI 118,551 (beta 2-adrenergic receptor antagonist) but not by the presence of atenolol (beta 1-adrenergic receptor antagonist). Furthermore, the addition of Rp-cAMP (an inhibitor of protein kinase A I and II) blocked the enhancing effect of isoproterenol. These studies demonstrated that isoproterenol enhances the stimulatory effect of dexamethasone on the expression of the angiotensinogen gene in mouse hepatoma cells via beta 2-adrenergic receptor and cyclic AMP-dependent protein kinase pathways. Our data may be important in understanding the molecular mechanism(s) of the stimulatory effect of catecholamines/glucocorticoid-induced expression of the angiotensinogen gene in the liver.

The brain renin-angiotensin system: molecular mechanisms of cell to cell interactions

The components of the Renin-Angiotensin System (RAS) have been found to be expressed in the brain. Angiotensinogen, the high molecular weight precursor of the system, is widely distributed and expressed in areas not related to control of blood pressure and body fluid homeostasis as well. It has been shown that it is regulated by steroid hormones independently from the liver and that it is also regulated in a different manner in several brain areas. Angiotensin II, the effector peptide of the system, may be generated in the brain via the classical pathway, using renin and angiotensin converting enzyme or directly from angiotensinogen by cathepsin G or tonin. N-terminal peptides of angiotensin II have been found in several brain areas with ANG (1-7) involved in vasopressin release however without influence on blood pressure and with ANG III acting as potent as ANG II. Transgenic animals may be used to study the pathophysiology of an activated brain RAS.

Estrogen action on hepatic synthesis of angiotensinogen and IGF-I: direct and indirect estrogen effects

In the present study effects of estrogens (natural estradiol and synthetic ethinyl estradiol) on liver derived proteins (angiotensinogen, IGF-I) were investigated in vivo in ovariectomized rats and in vitro in a rat hepatoma cell line (Fe33). The aim of this study was to establish both an animal and an in vitro model for quantification of the hepatic activity of given estrogenic compounds, and to study underlying mechanisms as regards the question of direct or indirect mode of estrogen action. In ovariectomized rats subcutaneous (s.c.)-treatment for 11 days with either estradiol (E2) or ethinyl estradiol (EE) (dose range 0.1-3 micrograms/animal/day) induced a comparable dose-dependent increase in uterine weight indicating a similar estrogenic potency of the two estrogens. Equipotency was also found as regards the effects on IGF-I plasma levels which dose-dependently decreased by about 50% at the highest dose tested (3 micrograms/animal/day). The decrease in IGF-I serum levels was accompanied by a significant 40% decrease in liver IGF-I mRNA. In contrast angiotensinogen plasma levels were affected only by EE (60% increase for the 3 micrograms/animal/day dose) but not by E2. When rats, in addition to ovariectomy, were also hypophysectomized (substituted with human growth hormone and dexamethasone) angiotensinogen again increased by 80% upon administration of 3 micrograms/animal/day EE, whereas IGF-I remained unaffected by EE. In a rat hepatoma cell line (Fe33) which is stably transfected with an estrogen receptor expression plasmid, 10 nmol/l EE for 24 h caused a 2.4-fold increase in angiotensinogen mRNA level. We conclude from our studies that estrogen effects on angiotensinogen serum levels in the rat are direct effects via the hepatic estrogen receptor, whereas estrogen effects on IGF-I serum levels are indirect effects, the primary target of estrogen action being probably the pituitary. The changes in angiotensinogen serum levels in the rat model are comparable to the situation in humans indicating the rat model and the Fe33 model to be useful tools to study the hepatic activity of estrogenic compounds.

The role of glucocorticoid activity in the inheritance of hypertension: studies in the rat

Young (3-week-old) spontaneously hypertensive rats (SHR) had significantly higher basal plasma corticosterone levels than WKY rats and maximum responses to ACTH were also higher. In isolated adrenocortical cells from these rats, corticosterone production was also more responsive to ACTH in SHR. There was no significant difference in aldosterone production. Mononuclear leucocytes from older (10-week-old) SHR had a higher affinity for dexamethasone but a smaller number of binding sites per cell. The SHR therefore has higher circulating glucocorticoid levels and the target cells have a higher apparent affinity for this agonist. However, the target cells also have a smaller binding capacity. The precise resultant effect of these changes on glucocorticoid activity will require additional studies on specific glucocorticoid-dependent variables.

Intracardiac detection of angiotensinogen and renin: a localized renin-angiotensin system in neonatal rat heart

There is increasing evidence that the renin-angiotensin system (RAS) modulates cardiovascular function through both blood-borne and tissue-derived components. The existence of a local RAS has been proposed in the heart based on biochemical and molecular biological studies that identify angiotensinogen and renin. We conducted the present study to determine the chamber localization of angiotensinogen and renin mRNA in neonatal rat heart and whether these components could be identified in cultured cardiomyocytes and fibroblasts obtained from neonatal rat heart. Experiments using polymerase chain reaction (PCR) indicated that whole hearts obtained from neonatal rats contained both angiotensinogen and renin mRNA. With the use of radiolabeled cDNA probes and in situ hybridization, angiotensinogen and renin transcripts were localized both in the atria and ventricles of neonatal rat hearts. Relative signal strengths for angiotensinogen were highest in the left and right ventricles. In contrast, renin signal strength was overall much lower and preferentially localized in the left ventricle. To investigate the cellular source of angiotensinogen and renin, cultured neonatal heart cardiomyocytes and ventricular fibroblasts were screened for angiotensinogen and renin messenger RNA and protein using PCR and indirect immunofluorescent staining, respectively. These experiments demonstrated that both cell types produce transcripts and the respective translation products for angiotensinogen and renin. These data suggest that the site of angiotensin II synthesis can occur at the level of the individual cardiomyocyte and fibroblast, where it may serve to directly and/or indirectly regulate cardiac rate, force, growth, and development in the neonate.

Changes in angiotensinogen messenger RNA in differentiating 3T3-F442A adipocytes

Angiotensinogen messenger RNA (mRNA) has been identified in both brown and white adipose tissue. Recently we have shown that when 3T3-L1 cells were treated with isobutylmethylxanthine (IBMX) to accelerate differentiation, angiotensinogen mRNA increased markedly in adipocytes as compared with preadipocytes. To determine if a correlation existed between the regulatory events associated with the differentiation process, we compared the change in angiotensinogen mRNA in spontaneously differentiating 3T3-F442A cells with two established parameters of differentiation in adipocyte cell lines. Differentiation was assessed by visual examination of cells for lipid droplets, fluorescent staining of the F-actin fibers, and increases in glycerol phosphate dehydrogenase mRNA. F-actin fibers were highly structured in preadipocytes, becoming disassembled and very disorganized as cells differentiated into adipocytes. The quantity of angiotensinogen mRNA increased as the number of lipid-containing cells increased within a culture. Glycerol phosphate dehydrogenase mRNA accumulated in differentiated adipocytes to about the same extent as angiotensinogen mRNA. Thus, increases in angiotensinogen mRNA were associated with the morphological and biochemical changes that occur during the phenotypic modulation of 3T3-F442A cells.