Characterization and regulation of the angiotensin II type-1 receptor (binding and mRNA) in human adrenal fasciculata-reticularis cells

Peptidergic G Protein-Coupled Receptor Regulation of Adrenal Function: Bench to Bedside and Back

An altered secretion of adrenocortical and adrenomedullary hormones plays a role in the clinical syndromes of primary aldosteronism (PA), Cushing, and pheochromocytoma. Moreover, an altered production of adrenocortical hormones and/or an abnormal release of factors by the adrenal medulla are involved in several other diseases, including high blood pressure, congestive heart failure, liver cirrhosis, nephrotic syndrome, primary reninism, renovascular hypertension, Addison disease, Bartter, Gitelman, and virilization syndromes. Understanding the regulation of adrenal function and the interactions between adrenal cortex and medulla is, therefore, the prerequisite for mechanistic understanding of these disorders. Accumulating evidence indicates that the modulation of adrenal hormone biosynthesis is a process far more complex than originally thought, as it involves several factors, each cooperating with the other. Moreover, the tight vascular and neural interconnections between the adrenal cortex and medulla underlie physiologically relevant autocrine/paracrine interactions involving several peptides. Besides playing a pathophysiological role in common adrenal diseases, these complex mechanisms could intervene also in rare diseases, such as pheochromocytoma concomitant with adrenal Cushing or with PA, and PA co-occurring with Cushing, through mechanisms that remain to be fully understood at the molecular levels. Heterodimerization of G protein-coupled receptors (GPCRs) induced by peptide signaling is a further emerging new modulatory mechanism capable of finely tuning adrenal hormones synthesis and release. In this review we will examine current knowledge on the role of peptides that act via GPCRs in the regulation of adrenal hormone secretion with a particular focus on autocrine-paracrine signals.

Angiotensin peptides in the regulation of adrenal cortical function

The adrenal cortex plays a key role in the regulation of metabolism, salt and water homeostasis and sex differentiation by synthesizing glucocorticoid, mineralocorticoid and androgen hormones. Evidence exists that angiotensin II regulates adrenocortical function and it has been contended that angiotensin peptides of the non-canonical branch of the renin angiotensin system (RAS) might also modulate steroidogenesis in adrenals. Thus, the aim of this review is to examine the role of the RAS, and particularly of the angiotensin peptides and their receptors, in the regulation of adrenocortical hormones with particular focus on aldosterone production.

2-Methoxyestradiol causes matrix metalloproteinase 9-mediated transactivation of epidermal growth factor receptor and angiotensin type 1 receptor downregulation in rat aortic smooth muscle cells

Studies have demonstrated the therapeutic potential of estrogen metabolite 2-methoxyestradiol (2ME2) in several cardiovascular disorders, including hypertension. However, the exact mechanism(s) remains unknown. In this study, primary rat aortic smooth muscle cells (RASMCs) were exposed to 2ME2, and angiotensin type 1 receptor (AT1R) expression, function, and associated signaling pathways were evaluated. In RASMCs, 2ME2 downregulated AT1R expression in a concentration- and time-dependent manner, which was correlated with reduced mRNA expression. The 2ME2 effect was through G protein-coupled receptor 30 (GPR30) that inhibits second messenger cAMP. Moreover, 2ME2 exposure phosphorylated ERK1/2 that was sensitive to MEK inhibitor PD98059. Selective epidermal growth factor receptor (EGFR) inhibitor AG1478 blocked 2ME2-induced EGFR transactivation and attenuated subsequent phosphorylation of ERK1/2 preventing AT1R downregulation. The transactivation was dependent on 2ME2-induced release of matrix metalloproteinase 9 (MMP9) and epidermal growth factor demonstrated by ELISA. Furthermore, transfection with small interfering (si) RNA targeting MMP9 impeded ERK1/2 activation and AT1R downregulation in response to 2ME2 and G1 stimulation. Interestingly, under similar conditions, stimulation of GPR30 with the selective agonist G1 elicited similar signaling pathways and downregulated the AT1R expression that was reversed by GPR30 antagonist G15. Furthermore, 2ME2 and G1 inhibited angiotensin II (ANG II) induced Ca2+ release, a response consistent with AT1R downregulation. Collectively, our study demonstrates for the first time that 2ME2 binding to GPR30 induces MMP9 specific transactivation of EGFR that mediates ERK1/2-dependent downregulation of AT1R in RASMCs. The study provides critical insights into the newly discovered role and signaling pathways of 2ME2 in the regulation of AT1R in vascular cells and its potential to be developed as a therapeutic agent that ameliorates hypertension.

Signaling Interactions in the Adrenal Cortex

The major physiological stimuli of aldosterone secretion are angiotensin II (AII) and extracellular K(+), whereas cortisol production is primarily regulated by corticotropin (ACTH) in fasciculata cells. AII triggers Ca(2+) release from internal stores that is followed by store-operated and voltage-dependent Ca(2+) entry, whereas K(+)-evoked depolarization activates voltage-dependent Ca(2+) channels. ACTH acts primarily through the formation of cAMP and subsequent protein phosphorylation by protein kinase A. Both Ca(2+) and cAMP facilitate the transfer of cholesterol to mitochondrial inner membrane. The cytosolic Ca(2+) signal is transferred into the mitochondrial matrix and enhances pyridine nucleotide reduction. Increased formation of NADH results in increased ATP production, whereas that of NADPH supports steroid production. In reality, the control of adrenocortical function is a lot more sophisticated with second messengers crosstalking and mutually modifying each other's pathways. Cytosolic Ca(2+) and cGMP are both capable of modifying cAMP metabolism, while cAMP may enhance Ca(2+) release and voltage-activated Ca(2+) channel activity. Besides, mitochondrial Ca(2+) signal brings about cAMP formation within the organelle and this further enhances aldosterone production. Maintained aldosterone and cortisol secretion are optimized by the concurrent actions of Ca(2+) and cAMP, as exemplified by the apparent synergism of Ca(2+) influx (inducing cAMP formation) and Ca(2+) release during response to AII. Thus, cross-actions of parallel signal transducing pathways are not mere intracellular curiosities but rather substantial phenomena, which fine-tune the biological response. Our review focuses on these functionally relevant interactions between the Ca(2+) and the cyclic nucleotide signal transducing pathways hitherto described in the adrenal cortex.

Angiotensin II regulation of adrenocortical gene transcription

Angiotensin II (Ang II) is the key peptide hormone in the renin-angiotensin-aldosterone system (RAAS). Its ability to regulate levels of circulating aldosterone relies on actions on adrenal glomerulosa cells. Many of the Ang II effects on glomerulosa cells involve a precisely coordinated regulation of signaling cascades and gene expression. The development of genome-wide gene arrays has allowed the definition of transcriptome-wide effects of Ang II in adrenocortical cells. Analysis of the Ang II gene targets reveals broad effects on cellular gene expression, particularly the rapid induction of numerous transcription factors that may regulate long-term steroid metabolism and cell growth/proliferation. Herein we discuss the Ang II-induced genes in adrenocortical cells and review the progress in defining the role of these genes in zona glomerulosa function.

Angiotensin II and cAMP regulate AT(1)-mRNA expression in rat cardiomyocytes by transcriptional mechanism

Mechanisms of angiotensin II and cAMP regulating the expression of angiotensin II type 1 (AT(1)) receptor mRNA were studied in neonatal rat cardiomyocytes. Angiotensin II induced a transient decrease of AT(1)-mRNA expression in time- and dose-dependent manner. Maximal decrease (49.2 +/- 9.5% of control) occurred at 6 h of angiotensin II (10 nmol/l) treatment. AT(1) receptor antagonists 4-ethyl-2-n-propyl-1-[2'-(1H-tetrazol-5-yl)biphenyl-4-yl)methyl]imidazole-5-carboxylic acid (DMP811) and losartan as well as 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) reversed the down-regulation of AT(1)-mRNA expression. 6 h of phorbol 12-myristate 13-acetate (PMA) stimulation caused a decrease of AT(1)-mRNA level. Treatment by angiotensin II plus actinomycin D for 6 h produced the same effect as actinomycin D alone. These results suggest that angiotensin II down-regulates AT(1)-mRNA level of rat cardiomyocytes by inhibiting the transcription of AT(1) gene, which is mediated by AT(1) receptor and related to the activation of protein kinase C. Stimulation by forskolin plus 3-isobutyl-1-methyl-xanthine (IBMX) decreased the expression of AT(1)-mRNA to 68.1 +/- 21.5% of control at 6 h treatment; while increased to 207.9 +/- 27.1% of control at 48 h treatment. A series of 5'-upstream deletion mutants of AT(1A) promoter were produced and then were recombined with pGL(3) basic vector utilizing luciferase as reporter gene. Among all the constructors, p(-201/+ 74)Luc was of the highest luciferase activity (5.9 times higher than control) after stimulation by forskolin for 48 h. Further deletion from -201 to -61 resulted in a large decrease of activity. These results indicate that cAMP induces a time-dependent bi-directional regulation of AT(1)-mRNA expression. The cAMP responsible element (CRE) cis-element located in the region -201/-61 of rat AT(1A) promoter is forskolin inducible, which may mediate the up-regulation of AT(1)-mRNA expression induced by cAMP long-lasting stimulation.

Fibronectin, laminin, and collagen IV as modulators of cell behavior during adrenal gland development in the human fetus

The specific development of the human fetal adrenal gland requires cell proliferation, migration, apoptosis, and zone-specific steroidogenic activity. The present work was designed to determine the physiological significance of the previously identified spatial distribution of extracellular matrix components in the fetal gland. Primary cultures of human fetal adrenal cells grown on collagen IV, laminin, or fibronectin revealed that cell morphology was affected by environmental cues. Matrices also modulated the profile of steroid secretion by the fetal cells. Collagen IV favored cortisol secretion after ACTH or angiotensin II stimulation and increased dehydroepiandrosterone production when the AT(2) receptor of angiotensin II was specifically stimulated. These effects were correlated by changes in the mRNA levels of 3beta-hydroxysteroid dehydrogenase and cytochrome P450C17. In contrast, fibronectin and laminin decreased cell responsiveness to ACTH in terms of cortisol secretion, but enhanced ACTH-stimulated androgen secretion. Finally, extracellular matrices were able to orchestrate cell behavior. Collagen IV and laminin enhanced cell proliferation, and fibronectin increased cell death. This study is the first to demonstrate that the nature of extracellular matrix coordinates specific steroidogenic pathways and cell turnover in the developing human fetal adrenal gland.

Angiotensin II type 1 receptor and ACTH receptor expression in human adrenocortical neoplasms

OBJECTIVE

Type 1 angiotensin II (Ang II) receptors transduce most of the known actions of Ang II, including steroidogenesis and trophic actions on the adrenal cortex. We investigated the type 1 Ang II receptor expression in adrenocortical tissues to define its regulation in adrenocortical neoplasms and to compare its expression with that of the ACTH receptor (ACTH-R).

PATIENTS AND MEASUREMENTS

Poly A RNA was extracted from tumour tissue and electrophoresed through a 1.0% agarose gel, blotted and hybridized with alpha32P-CTP labelled PCR generated type 1 Ang II receptor cDNA probe. Receptor autoradiography was performed on slices from normal adrenals and tumour tissue by incubation with 125I-Sar1, Ile8-Ang II with and without pretreatment with cold Ang II or with the selective type 1 receptor antagonist losartan.

RESULTS

Ang II type 1 receptor mRNA was high in cortisol producing (CPA; n = 5) and aldosterone producing (APA; n = 4) adenomas (normal adrenals 100 +/- 12% vs. 180 +/- 16% in CPA and 154 +/- 26% in APA, mean +/- SEM), but was low in nonfunctioning adenomas (NFA; n = 2; 2 +/- 1%). ACTH receptor mRNA followed a similar pattern (CPA 178 +/- 17, APA 196 +/- 30, NFA 0%, carcinomas 56 +/- 11%) with a good correlation between Ang II type 1 receptor and ACTH-R mRNA of r = 0.692, P = 0.0019. Receptor autoradiography in normal adrenals demonstrated Ang II type 1 receptors predominantly in the zona glomerulosa. In tumour tissue, mainly type 1 receptor expression was found confirming the Northern blot data.

CONCLUSIONS

Angiotensin II type 1 receptor and ACTH receptor expression seems to be correlated with the functional status of adrenocortical tumours, suggesting regulation by similar factors. The predominant receptor expressed in adrenocortical tumours is the Angiotensin II type 1 receptor whereas type 2 receptor expression is minimal.

Ectopic and abnormal hormone receptors in adrenal Cushing's syndrome

The mechanism by which cortisol is produced in adrenal Cushing's syndrome, when ACTH is suppressed, was previously unknown and was referred to as being "autonomous." More recently, several investigators have shown that some cortisol and other steroid-producing adrenal tumors or hyperplasias are under the control of ectopic (or aberrant, illicit, inappropriate) membrane hormone receptors. These include ectopic receptors for gastric inhibitory polypeptide (GIP), beta-adrenergic agonists, or LH/hCG; a similar outcome can result from altered activity of eutopic receptors, such as those for vasopressin (V1-AVPR), serotonin (5-HT4), or possibly leptin. The presence of aberrant receptors places adrenal cells under stimulation by a trophic factor not negatively regulated by glucocorticoids, leading to increased steroidogenesis and possibly to the proliferative phenotype. The molecular mechanisms responsible for the abnormal expression and function of membrane hormone receptors are still largely unknown. Identification of the presence of these illicit receptors can eventually lead to new pharmacological therapies as alternatives to adrenalectomy, now demonstrated by the long-term control of ectopic P-AR- and LH/hCGR-dependent Cushing's syndrome by propanolol and leuprolide acetate. Further studies will potentially identify a larger diversity of hormone receptors capable of coupling to G proteins, adenylyl cyclase, and steroidogenesis in functional adrenal tumors and probably in other endocrine and nonendocrine tumors.

Local renin-angiotensin system is involved in K+-induced aldosterone secretion from human adrenocortical NCI-H295 cells

NCI-H295, a human adrenocarcinoma cell line, has been proposed as a model system to define the role of the renin-angiotensin system in the regulation of aldosterone production in humans. Because the precise cellular localization of the components of the renin-angiotensin system in human adrenal cortical cells remains unclear, we investigated their localization in this defined cell system. NCI-H295 cells expressed both angiotensinogen and renin as shown by reverse transcriptase polymerase chain reaction and immunohistochemistry. Human angiotensin-converting enzyme (ACE) was not detectable by immunocytochemistry, ACE binding, or reverse transcriptase polymerase chain reaction. However, 3.5 mmol/L K+ stimulated the formation of both angiotensin I and angiotensin II 1. 9- and 2.5-fold, respectively, and increased aldosterone release 3. 0-fold. The K+-induced stimulation of aldosterone release was decreased by captopril and enalaprilat (24% and 26%, respectively) and by the angiotensin type 1 (AT1)-receptor antagonist losartan (28%). Angiotensin II-induced stimulation of aldosterone release was abolished by losartan treatment. Specific [125I]Sar1-angiotensin II binding was detected by receptor autoradiography. The binding of [125I]Sar1-angiotensin II was completely displaced by the AT1 antagonist losartan but not by the AT2 receptor ligand PD 123319, confirming the expression of angiotensin II AT1 receptors in NCI-H295 cells. Our results demonstrate that NCI-H295 cells express most of the components of the renin-angiotensin system. Our failure to detect ACE, however, suggests that the production of angiotensin II in NCI-H295 cells may be ACE independent. NCI-H295 cells are able to produce angiotensin II, and K+ increases aldosterone secretion in part through an angiotensin-mediated pathway. The production of angiotensin II in NCI-H295 cells demonstrates that this human cell line can be useful to characterize the role of locally produced angiotensin II in the regulation of aldosterone release.

Inhibition of the expression of the gene for the angiotensin AT1 receptor by angiotensin II in the rat adrenal gland

The expression of angiotensin AT1A and AT1B receptor mRNA after continuous angiotensin II administration was investigated in the rat adrenal gland. Angiotensin AT1 receptor mRNA detected by Northern blot analysis decreased to 52.7+/-16.1% of control after the administration of angiotensin II (20 microg/h) for 24 h, and to 70.8+/-8.0% after 1 week. A low dose of angiotensin II (0.2 microg/h) also decreased angiotensin AT1 receptor mRNA to 73.0+/-5.5% after 1 week. Competitive reverse transcription and polymerase chain reaction (RT-PCR) experiments revealed that both angiotensin AT1A and AT1B receptor mRNAs decreased after administration of angiotensin II (20 or 0.2 microg/h) for 1 week. Analysis of the angiotensin AT1A promoter by using luciferase-reporter system showed that angiotensin II (up to 1 microM) did not have any effects on the promoter activity (106+/-5.7% after 0.1 microM angiotensin II stimulation) in Y1 cells and cultured vascular smooth muscle cells, although phorbol myristate acetate (PMA) decreased the promoter activity by about 40% compared with control. These results suggest that angiotensin AT1 receptor gene expression in the rat adrenal gland is inhibited by angiotensin II and it may not be due to suppression of promoter activity. Other mechanisms such as destabilization of angiotensin AT1 receptor mRNA or angiotensin II-induced increased blood pressure may be involved in the inhibition.

Distribution and functional significance of angiotensin-II AT1- and AT2-receptor subtypes in the rat adrenal gland

The distribution and the functional significance of angiotensin-II (ANG-II) receptor subtypes, AT1 and AT2, in the rat adrenal gland has been investigated in vitro. Autoradiographic assessment of the selective displacement of [125I]ANG-II binding by selective ligands of the two receptor subtypes indicated that zona glomerulosa (ZG) was provided with both AT1 and AT2, and adrenal medulla (AM) almost exclusively with AT2 receptors. ANG-II (10(-9) M) evoked a marked rise in the secretion of aldosterone by dispersed ZG cells and catecholamines by AM fragments. The selective AT1-receptor antagonist DuP753 blocked aldosterone response to ANG-II, while the selective AT2-receptor antagonist PD123319 was ineffective. Catecholamine response to ANG-II was inhibited by PD123319 and only moderately affected by high concentrations of DuP753. The selective AT2-receptor agonist CGP42112 did not change basal aldosterone release of ZG cells, but concentration-dependently enhanced basal catecholamine release by AM fragments. In light of these findings the conclusion is drawn that in the rat the aldosterone secretagogue effect of ANG-II is exclusively mediated by the AT1 receptors present in the ZG, while the catecholamine secretagogue action preminently involves the activation of AT2 receptor located on medullary chromaffin cells.

Expression of angiotensin type-1 (AT1) and type-2 (AT2) receptor mRNAs in the adult rat brain: a functional neuroanatomical review

The discovery that all components of the renin-angiotensin system (RAS) are present in the central nervous system led investigators to postulate the existence of a local brain RAS. Supporting this, angiotensin immunoreactive neurons have been visualized in the brain. Two major pathways were described: a forebrain pathway which connects circumventricular organs to the median preoptic nucleus, paraventricular nucleus, and supraoptic nucleus, and a second pathway connecting the hypothalamus to the medulla oblongata. Blood-brain barrier deficient circumventricular organs are rich in angiotensin II receptors. By activating these receptors, circulating angiotensin II may act on central cardiovascular centers via angiotensinergic neurons, providing a link between peripheral and central angiotensin II systems. Among the effector peptides of the brain RAS, angiotensin II and angiotensin III have the same affinity for the two pharmacologically well-defined receptors: type 1 (AT1) and type 2 (AT2). When injected in the brain, these peptides increase blood pressure, water intake, and anterior and posterior pituitary hormone release and may modify memory and learning. The cloning of AT1 and AT2 receptor cDNAs has revealed that these receptors belong to the seven transmembrane domain receptor family. In rodents, two AT1 receptor subtypes, AT1A and AT1B, have been isolated. Using specific riboprobes for in situ hybridization histochemistry, recent studies mapped the distribution of AT1A, AT1B, and AT2 receptor mRNAs in the adult rat and found a predominant expression of AT1A and AT2 mRNA in the brain and of AT1B in the pituitary. Very limited overlap was found between the brain expression of AT1A and AT2 mRNAs. In several functional entities of the brain, such as the preoptic region, the hypothalamus, the olivocerebellary system, and the brainstem baroreflex arc, the colocalization of receptor mRNA, binding sites, and angiotensin immunoreactive nerve terminals suggests local synthesis and expression of angiotensin II receptors. In other areas, such as the bed nucleus of the stria terminalis, the median eminence, or certain parts of the nucleus of the solitary tract, angiotensin II receptors are likely of extrinsic origin. The neuronal expression of AT1A and AT2 receptors was demonstrated in the subfornical organ, the hypothalamus, and the lateral septum. By using double label in situ hybridization, AT1A receptor expression was localized in corticotropin releasing hormone but not in vasopressin containing neurons in the hypothalamus. The information is discussed together with functional data concerning the role of brain angiotensins, in an attempt to provide a better understanding of the physiological and functional roles of each receptor subtype.

Desensitization of AT1 receptor-mediated cellular responses requires long term receptor down-regulation in bovine adrenal glomerulosa cells

Angiotensin II (Ang II) regulates aldosterone production in bovine adrenal glomerulosa cells by interacting with the AT1 receptor. This receptor is coupled to a G protein that controls the activity of phospholipase C. With a primary culture of bovine adrenal glomerulosa cells, we evaluated the desensitization of cellular responses after pretreatment with Ang II. When cells were pretreated for 30 min with 1 microM Ang II at 37 C, we observed a 48% loss of [125I]Ang II-binding activity. Scatchard analysis revealed that this decreased binding activity corresponded to a 53% loss of the total number of binding sites. This phenomenon was time dependent, with a t(1/2) of 20 min, and a maximal loss of 76% of the total binding sites was observed after 14 h. A time-dependent decrease in AT1 receptor messenger RNA levels was also observed after pretreatment with 1 microM Ang II for 12-24 h. Taken together, these results are interpreted as a down-regulation of the AT1 receptor. Desensitization of phospholipase C activity under similar conditions was, however, a slower process, with a t(1/2) of 9 h and a maximal response reduction of 83% observed after 24 h. Dose-response experiments indicated that maximal phospholipase C desensitization was obtained in the presence of 1 microM Ang II, with an EC50 of 90 nM. The desensitization was of a homologous nature, as a 24-h pretreatment with Ang II did not affect bradykinin-induced inositol phosphate production. A 24-h pretreatment with 1 microM Ang II also significantly desensitized the steroidogenic effect of Ang II and the potentiating effect of Ang II on ACTH-induced cAMP production. Lower concentrations of Ang II (10 nM) did not produce any desensitizing effect on these two parameters. This study provides evidence that glomerulosa cells are functionally resistant to short term desensitization of the AT1 receptor and that long term down-regulation with high concentrations of Ang II is needed to desensitize AT1-mediated cellular responses.

Stimulation of the angiotensin II type I receptor on bovine adrenal glomerulosa cells activates a temperature-sensitive internalization-recycling pathway

Angiotensin II (Ang II) is an important regulator of aldosterone production by bovine adrenal glomerulosa cells. On these cells Ang II interacts with the AT1 receptor that is coupled to a G protein controlling the activity of phospholipase C. A primary culture of bovine adrenal glomerulosa cells was used to study the internalization-recycling mechanism of the AT1 receptor after stimulation with Ang II. When cells were pretreated with 10 nM Ang II for 30 min at 37 degrees C and binding studies were performed at 12 degrees C we observed a 48% loss in [125I]Ang II binding. Scatchard analysis revealed that this loss in binding translated into a decreased affinity of the AT1 receptor without any loss in the total amount of binding sites. Under the same conditions an important internalization of [125I]Ang II was invariably observed. These observations suggest that a mechanism was at work to recycle the internalized receptors to the cell surface during the binding studies. Following internalization we indeed observed an externalization of [125I]Ang II. This phenomenon relatively rapid at 37 degrees C was much slower at 12 degrees C and completely inhibited at 4 degrees C. When cells were pretreated with 10 nM Ang II for 30 min at 37 degrees C binding assays at 4 degrees C no longer revealed a loss of binding affinity but rather a 54% reduction in the total amount of binding sites. The maximal binding capacity could be recovered during incubations at 12 degrees C. These results reveal the existence of a dynamic recycling process for the AT1 receptor. In accordance with this interpretation the phenomenon was blocked by monensin, a known inhibitor of receptor recycling. These studies suggest that the stimulation of the AT1 receptor sets in motion an internalization-recycling process that seems to be a fundamental aspect of the AT1 receptor transduction mechanism.

Angiotensin II receptors in cortical and medullary adrenal tumors

Several pieces of evidences suggest that angiotensin II (Ang II) has mitogenic effects, and a link between Ang II receptors and adrenal tumors can be suggested. In various adrenal tumors, aldosterone-producing adenoma (APA), Cushing's adrenal adenomas (Cush), pheochromocytomas (Pheo), and adrenal carcinomas, we studied the density, affinity, and subtype of Ang II receptors. Ang II binding was tested in cell membrane homogenates. [125I]Ang II was used as ligand, and Losartan and CGP 42112 were used as selective Ang II type 1 and type 2 antagonists, respectively. In APA, Ang II receptor density was 178.5 +/- 82.7 fmol/mg: however, due to the high degree of variability, the receptor density was not significantly higher than that in nontumorous adrenal cortex (59.3 +/- 8.4 fmol/mg). In Cush, the receptor density (27.6 +/- 8.2 fmol/mg; P < 0.05) was significantly lower than that in controls, whereas in Pheo and cortical carcinoma, Ang II binding was very low and in several cases almost undetectable. There was no remarkable difference in the Ang II receptor affinity among all tissues tested. The ratio between type 1 and type 2 Ang II receptors showed a large prevalence of type 1 in controls, APA, and three cases of Cush; in two cases of Cush, this ratio was reversed. In conclusion, our data indicate that Ang II receptors are normally expressed in APA and can also be detected in Cush, whereas they have a very low density in Pheo and adrenal carcinoma. Therefore, Ang II receptors are not involved in the lack of response to Ang II that is characteristic of APA; additionally, a reduction of Ang II receptors can be associated with dedifferentiation or malignancy of adrenal tumors. Further investigation of the expression and functional characterization of Ang II receptors is required to better clarify their possible role in adrenal tumorigenesis.

Angiotensin II receptor subtypes AT1 and AT2 are down-regulated by angiotensin II through AT1 receptor by different mechanisms

The regulatory effects of angiotensin II (AngII) on its receptor subtypes, AT1 and AT2, were studied using cultured bovine adrenal cells (BAC), which express both receptor subtypes, and PC12W and R3T3 cells, which express only AT2 receptors. In BAC, AngII caused a decrease in AT1- and AT2-binding sites and their corresponding messenger RNAs (mRNAs), but with different kinetics. AT1-binding sites decreased by more than 50% within the first 3 h, whereas AT1 mRNA started to decline after a lag period of 3 h. Both AT2-binding sites and mRNA remained stable within the first 6 h of AngII treatment. Then, AT2 mRNA decreased rapidly with an apparent half-life of 2-3 h, whereas AT2-binding sites declined with an apparent half-life of about 16 h. Measurement of transcription rate and mRNA half-life by the [3H]uridine-thiouridine method revealed that AngII reduced by 90% the rate of AT1 transcription, but had no effect on AT1 mRNA half-life, whereas it slightly reduced AT2 transcription, but markedly reduced AT2 mRNA stability. All of the effects of AngII on both AT1 and AT2 receptors were blocked by losartan, indicating that they were mediated exclusively through the AT1 receptor. In PC12W cells, AngII was unable to modify AT2-binding sites or mRNA. Moreover, in BAC, [125I]AngII was internalized through the AT1 receptor, whereas occupancy of AT2 receptors in either BAC or PC12W did not produce internalization of the hormone. These results indicate that AngII, through the AT1 receptor, down-regulates both AT1 and AT2, but by different mechanisms; AT1 receptor is regulated through internalization-degradation of the occupied receptor and inhibition of transcription, whereas AT2 receptor is regulated mainly by decreasing the stability of its mRNA. Moreover, the phorbol ester phorbol 12-myristate 13-acetate mimicked most of the effects of AngII in BAC and decreased both AT2-binding sites and mRNA on PC12W cells, indicating that the hormonal regulation of both AT1 and AT2 receptors is mediated through protein kinase C activation.

Angiotensin II stimulates secretion of endogenous ouabain from bovine adrenocortical cells via angiotensin type 2 receptors

Angiotensin II stimulates secretion of corticosteroids and ouabain-like activity from adrenocortical cells. Distinct adrenocortical angiotensin II receptor subtypes (AT1, AT2) have been described, and the present studies investigated their roles in steroid secretion. Using primary bovine adrenocortical cell cultures under serum free conditions, angiotensin II stimulated the secretions of aldosterone, cortisol, and endogenous ouabain as verified by high-performance chromatography. The dose-response curves for stimulated steroid secretion were parallel with unitary slopes while the half-maximally effective concentrations of angiotensin II were 0.31 to 0.38 nmol/L for secretions of aldosterone and cortisol and 2.3 nmol/L for endogenous ouabain. The nonselective mammalian antagonist (Sar1-Ile8) angiotensin II blocked stimulated secretion of all three steroids without affecting basal output. In the presence of the AT1 antagonist DuP753, angiotensin II-stimulated secretions of aldosterone and cortisol were blocked while secretion of endogenous ouabain was unaffected. In the presence of the AT2 antagonist PD123319, both basal and angiotensin II-stimulated secretions of aldosterone and cortisol were normal while stimulated secretion of endogenous ouabain was inhibited. The secretion of endogenous ouabain was activated maximally by the AT2 agonist CGP42112 under conditions in which aldosterone secretion was unaffected. These results demonstrate that AT2 receptors stimulate secretion of endogenous ouabain from bovine adrenocortical cells. The specificity of AT1 and AT2 receptor stimulation indicates that separate signaling mechanisms having minimal cross talk control the adrenocortical secretions of corticosteroids and cardiac-active steroids. Adrenocortical AT2 receptors may be important in the adaptation to low salt diets and other conditions in which angiotensin II is increased.

Regulation of primary response and specific genes in adrenal cells by peptide hormones and growth factors

Using cultured bovine adrenal fasciculata cells (BAC), we investigated the effects of two hormones, corticotropin (ACTH) and angiotensin II (Ang-II) and two growth factors, insulin-like growth factors I (IGF-I) and transforming growth factor beta 1 (TGF beta 1), on the mRNA levels of nuclear proto-oncogenes of the Fos and Jun families and on the mRNA levels of genes expressed in BAC coding for ACTH and AT1 receptors, cytochrome P450scc and P450 17 alpha and 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD). ACTH and IGF-1 increased c-fos and jun-B mRNA levels early with later increases in the levels of mRNA for the ACTH receptor and the three steroidogenic enzymes, and enhanced steroidogenic responses to both ACTH and Ang-II. In contrast, Ang-II increased mRNA coding for the three proto-oncogenes (cfos, c-jun, and jun-B), decreased those for P450 17 alpha and 3 beta-HSD, and caused marked homologous and heterologous steroidogenic desensitization. TGF beta 1 increased only jun-B mRNA and markedly reduced BAC-differentiated functions and steroidogenic responsiveness to both ACTH and Ang-II. The long-term effects of ACTH on human adrenal fasciculata cells were comparable with those observed in BAC, whereas the long term effects of Ang-II and TGF beta 1 were different from those observed in BAC. Whether these species-specific differences are related to a different effect of these factors on proto-oncogene expression is not yet known.

Angiotensin II regulates both adrenocortical and adrenomedullary function in isolated perfused pig adrenals

The effect of angiotensin II (ANG II) on all four zones of the adrenal gland was studied in preparations of isolated perfused porcine adrenals. The experimental design offered the possibility to analyze directly the actions of ANG II while preserving the structure of the gland. ANG II stimulated aldosterone, cortisol, and androstenedione release in a dose-dependent manner. At a final ANG II concentration of 10(-8) M aldosterone increased from 0.7 +/- 0.05 to 3.4 +/- 0.9 ng/ml, cortisol from 50 +/- 5 to 430 +/- 60 micrograms/l, and androstenedione from 1.4 +/- 0.2 to 4.4 +/- 0.8 ng/ml. In addition, ANG II provoked a release of adrenaline from 4.1 +/- 0.6 to 27.5 +/- 0.5 micrograms/ml and of noradrenaline from 5.5 +/- 1.1 to 36.0 +/- 8.7 micrograms/ml. Our results show that secretion of both adrenocortical steroids and adrenomedullary catecholamines can be evoked by ANG II. ANG II seems to influence not only the function of the zona glomerulosa but the function of the entire adrenal gland.

Identification and characterization of functional angiotensin II receptors in human neuroblastoma cells

The presence of specific AII receptors in 6 different human neuroblastoma cell lines was investigated using binding, cAMP and [Ca2+]i studies. We found high affinity (0.1 nM), low capacity ((1-2).10(3) sites/cell) binding sites for [125I](Sar-1,Ile-8)AII in one half of the cell lines studied. In the positive cell lines mathematical modeling of multiple competition curves among AII and analogs strongly indicated the presence of a homogenous class of sites, i.e., AT1 receptors. The presence of AT1 receptors was further substantiated by AII-induced inhibition of VIP-stimulated cAMP levels and by AII-evoked [Ca2+]i transient. The density of AT1 receptors in neuroblastoma cells was not affected by treatment with pertussis toxin and retinoic acid but was significantly increased by subacute treatment with VIP. In neuroblastoma cells, AII does not stimulate DNA synthesis, suggesting other roles rather than mitogenesis. Neuroblastoma cells represents an interesting model to investigate the function of AII in neural crest derived tissues.

Hormonal regulation of angiotensin II type 1 receptor expression and AT1-R mRNA levels in human adrenocortical cells

Human adrenocortical H295R cells express AII receptors which are predominantly of the AT1 but not AT2 subclass. These receptors are functionally coupled to phosphoinositidase C in a manner similar to that seen in fetal human, sheep and bovine adrenocortical cells. Treatment of H295R cells with forskolin or dbcAMP to activate the protein kinase A pathway caused a rapid (maximal by 3 h) and sustained decrease in AT1-R mRNA levels which in turn preceded a time-dependent (maximal by 12 h) and dose-dependent loss of [125I]AII binding and phosphoinositidase C activation on subsequent AII challenge. Thus, both decreased AT1-R mRNA levels and functional receptor expression appear to parallel each other in response to activation of protein kinase A. Activation of the Ca2+/protein kinase C pathways by treatment with AII also caused a rapid (maximal by 3 h) and dose-dependent loss in AT1-R mRNA, but mRNA levels subsequently rose again, approaching control levels by 36 h. Treatment with AII for 48 h had little effect on either [125I]AII binding or the subsequent phosphoinositidase C response. The effect of AII, but not forskolin, was blocked by the presence of cycloheximide. The action of AII on AT1-R mRNA was probably mediated through both protein kinase C and Ca(2+)-sensitive protein kinases as the effect at 4 h was not completely reproduced by phorbol ester alone, but was fully reproduced by a combination of phorbol ester and Ca2+ ionophore. However, increased Ca2+ influx alone, due to treatment with BAYK8644 or elevated extracellular K+, also resulted in a decrease in AT1-R mRNA levels. Thus in the H295R cell, control of AT1-R expression appears to be complex, being achieved at least in part through control of the level of AT1-R mRNA by multiple independent signaling pathways including protein kinase A, protein kinase C and Ca2+.

Regulation of ACTH receptor mRNA and binding sites by ACTH and angiotensin II in cultured human and bovine adrenal fasciculata cells

Human (HAC) and bovine (BAC) adrenal fasciculata cells express ACTH and angiotensin-II (A-II) receptors. In the present work, we have studied the effects of both hormones on ACTH receptor (ACTH-R) mRNA and binding sites. Both HAC and BAC expressed several ACTH-R transcripts. Although in both cell types, ACTH and A-II increased ACTH-R transcripts in a time- and dose-dependent manner, the maximal effects were different. Thus, ACTH at 10(-9) M enhanced 21- and 5-fold the level of ACTH-R mRNA and binding sites in HAC, whereas in BAC both parameters were enhanced only 3-fold. A-II at 10(-7) M increased 17- and 3.5-fold ACTH-R mRNA and binding sites in HAC, whereas in BAC, it caused only a 2-fold increase in ACTH-R mRNA and a small decrease in receptor number. In HAC, the stimulatory effects of both hormones on ACTH-R mRNA are mainly transcriptional, whereas in BAC they are mainly post-transcriptional, by decreasing the rate of degradation of ACTH-R mRNA. The stimulatory effects of ACTH on ACTH-R in both HAC and BAC were associated with an enhanced steroidogenic response to further hormonal stimulation. In contrast, specific species differences were observed with A-II. Thus, in HAC A-II increased ACTH-R mRNA and binding sites and the ACTH-induced cortisol production, whereas in BAC, A-II caused a slight decrease of ACTH binding sites and steroidogenic desensitization.

Characterization of the transcription start site of the ACTH receptor gene: presence of an intronic sequence in the 5'-flanking region

Corticotropin (ACTH) regulates glucocorticoid production through specific receptors on the adrenal cortex. Analysis of the ACTH receptor mRNA in human adrenal has revealed the presence of five transcripts ranging from 1.8 to 11 kilobases (kb). Characterization of the 5'-untranslated regions (UTRs) of the ACTH receptor mRNA demonstrated the presence of one major initiation site of transcription 177 bp away from the ATG codon. Analysis of this 5' sequence showed a perfect alignment with the previously described genomic sequence until position -128 bp from the ATG. The upstream 49-bp sequence was divergent, suggesting the occurrence of a splicing and indicating the presence of an intronic sequence in the UTRs, as well as the presence of an upstream exon containing this 49-bp sequence and located at least 1.8 kb away from the exon encoding the protein.

Regulation of corticotropin and steroidogenic enzyme mRNAs in human fetal adrenal cells by corticotropin, angiotensin-II and transforming growth factor beta 1

Using cultured human fetal adrenal cells, we have investigated the basal secretion of cortisol and dehydroepiandrosterone sulfate (DHAS) and the effect of corticotropin (ACTH), angiotensin-II (A-II) and transforming growth factor beta 1 (TGF beta 1) on the secretion of these steroids and on the mRNA levels of ACTH receptor (ACTHR), cytochrome P-450scc (cholesterol side-chain cleavage), P450 17 alpha (17 alpha-hydroxylase/17-20 lyase) and 3 beta-HSD (3 beta-hydroxysteroid dehydrogenase). The basal DHAS/cortisol ratio declined progressively between 12.5 and 21 weeks. ACTH treatment enhanced the secretion of cortisol and to a lesser extent that of DHAS, and increased the steroidogenic response to an acute stimulation with ACTH. These changes were associated with increased mRNA levels of ACTHR and of the steroidogenic enzymes. A-II treatment also increased the secretion of both DHAS and cortisol, but less than ACTH, enhanced the responsiveness to ACTH and increased ACTHR, P450scc and P450 17 alpha mRNA levels. In contrast, TGF beta 1 alone or together with ACTH decreased DHAS secretion, but not cortisol secretion. Moreover, TGF beta 1 had no effect on ACTHR and P450scc mRNA levels, decreased by about 50% the mRNA levels of P450 17 alpha both in the absence or presence of ACTH, but enhanced the stimulatory effects of ACTH on 3 beta-HSD mRNA. These results, along with those previously reported, suggest that both A-II and TGF beta may play a role in fetal adrenal function. In addition, they show that the effects of both peptides are qualitatively different from, even sometimes opposite to, those previously reported in bovine and ovine adrenal cells.

Regulation of corticotropin receptor number and messenger RNA in cultured human adrenocortical cells by corticotropin and angiotensin II

The regulation of ACTH receptor binding sites and mRNA by ACTH and angiotensin II (A-II) was studied using cultured human adrenal fasciculata reticularis cells (HAC). These cells expressed two major ACTH receptor transcripts of 1.8 and 3.4 kb and three minor ones of 4, 7, and 11 kb. ACTH increased the levels of all these transcripts in a time- and dose-dependent manner. At a maximal concentration of 10(-8) M, ACTH enhanced 21- and 4-fold the level of ACTH receptor mRNA and the number of receptors per cell, respectively. Pretreatment of HAC with A-II produced a dose-dependent enhancement of ACTH receptor mRNA that was associated with an increase of both ACTH receptor number and responsiveness to this hormone. The effects of A-II were completely blocked by an AT1 receptor subtype antagonist but not by an AT2 antagonist. The effects of ACTH together with A-II on ACTH receptor mRNA were greater than those induced by each hormone alone. These results show that ACTH receptor number and mRNA are positively regulated by the two main hormones (ACTH and A-II) which, in vivo, regulate adrenocortical functions. In addition, they also show that HAC are a target for A-II. Thus, regulation of ACTH receptors may be one mechanism by which ACTH and A-II regulate adrenocortical functions under both normal and pathological conditions.

Structural analysis and regulation of angiotensin II receptors

Angiotensin (Ang II) is an octapeptide hormone that plays a crucial role in the maintenance of electrolyte homeostasis and cardiovascular function. The hemodynamic and cardiovascular effects o f Ang II are mediated by high-affinity cell-surface receptors of the AT(1) pharmacologic class. The mammalian AT(1) receptor has recently been cloned and found to encode a 359-amino-acid protein of 41,000 molecular weight. The AT, receptor belongs to the guanine nucleotide regulatory-proteincoupled receptor family and is coupled to the phospholipase C signal transduction pathway as evidenced by intracellular calcium mobilization and inositol trisphosphate production upon receptor activation. Cloning of the AT(1) receptor has facilitated the study of structure-function correlates and molecular mechanisms of receptor regulation, and will lead to substantial progress in elucidating the mechanisms governing Ang II actions.