Control of glomerulosa cell function by angiotensin II: transduction by G-proteins and inositol polyphosphates

Losartan attenuates aortic endothelial apoptosis induced by chronic intermittent hypoxia partly via the phospholipase C pathway

PURPOSE

Endoplasmic reticulum (ER) stress is known to play key roles in the development of endothelial cell apoptosis induced by chronic intermittent hypoxia (CIH), and the angiotensin II-phospholipase C-inositol-1,4,5-triphosphate (AngII-PLC-IP3) pathway has been demonstrated to induce ER stress. To explore whether the AngII-PLC-IP3 pathway is involved in the vascular damage induced by CIH, we examined whether the AngII-PLC-IP3 pathway is involved in ER stress induced by CIH and whether losartan, a selective angiotensin II type 1 receptor (AT1R) blocker, could suppress endothelial cell apoptosis induced by CIH.

METHODS

Adult male Sprague Dawley rats were subjected to 8 h/day of intermittent hypoxia/normoxia, with or without losartan, a selective AT1R blocker, and/or U73122, a selective PLC inhibitor, for 8 weeks. Endothelial cell apoptosis, ER stress markers, and levels of PLC-γ1 and IP3R expression were determined.

RESULTS

Losartan prevented increases in PLC-γ1 and IP3R protein levels and inhibited ER stress markers induced by CIH. Addition of U73122 reproduced all the protective effects of losartan. Losartan administration before CIH significantly ameliorated CIH-induced endothelial cell apoptosis.

CONCLUSIONS

This study showed that the AngII-PLC-IP3 pathway is involved in ER stress induced by CIH and that pre-losartan administration ameliorates endothelial cell apoptosis following CIH partly via inhibition of the AngII-PLC-IP3 pathway and ER stress.

NADPH oxidase-derived H(2)O(2) contributes to angiotensin II-induced aldosterone synthesis in human and rat adrenal cortical cells

BACKGROUND

The Renin-Angiotensin-Aldosterone-System plays a pivotal role in hypertension. Angiotensin II (Ang II) is a major regulator of aldosterone synthesis and secretion, and it is known to facilitate reactive oxygen species (ROS) generation in many cell types.

AIMS

Here, we assessed the role of ROS signaling in Ang II-induced aldosterone synthesis by focusing on the regulation of aldosterone synthase (CYP11B2), a cytochrome P450 oxidase that catalyzes the final step in aldosterone biosynthetic pathway.

RESULTS

Ang II increased CYP11B2 activity, mRNA and protein with a concomitant elevation of 6-Carboxy- 2',7'-dichlorodihydrofluorescein diacetate fluorescence, malondialdehyde and protein carbonyl levels (indices of ROS), NADPH oxidase (Nox) activity, and H(2)O(2) levels in human and rat adrenal cortical cells. The expression of nuclear receptor related 1 protein, a transcription factor known to regulate CYP11B2 expression, was also augmented by Ang II. These Ang II-evoked effects were either abolished or attenuated by pretreatment of cells with either Ang II type I receptor (AT(1)R) antagonist, or antioxidants or Nox inhibitor or siRNA silencing of Nox1, 2 and 4, or inhibitors of phospholipase C and protein kinase C. Exogenous H(2)O(2) mimicked the facilitatory effects of Ang II on CYP11B2 activity, mRNA, and protein expression, and these changes were significantly reduced by PEG-catalase.

INNOVATION

ROS, particularly H(2)O(2), is identified as a key regulator of aldosterone production.

CONCLUSION

Our results suggest that Ang II facilitates CYP11B2 activity and the ensuing aldosterone production via activation of AT(1)R-Nox-H(2)O(2) signaling pathway.

Inhibitory effects of digoxin and ouabain on aldosterone synthesis in human adrenocortical NCI-H295 cells

The present study was to investigate the effects and action mechanisms of digoxin and ouabain on steroidogenesis in human adrenocortical NCI-H295 cells. Administration of digoxin or ouabain for 24 h decreased the basal and angiotensin II (Ang II)-stimulated release of aldosterone by NCI-H295 cells. The conversions of corticosterone (substrate of cytochrome P450 aldosterone synthase, P450c11AS) to aldosterone or deoxycortisol (substrate of cytochrome P450 11beta-hydroxylase, P450c11beta) to cortisol were reduced by digoxin or ouabain. The basal and 22-hydroxy-cholesterol (a membrane-permeable cholesterol, substrate of cytochrome P450 side-chain cleavage enzyme, P450scc)-stimulated pregnenolone release in mitochondria was inhibited by digoxin or ouabain. Digoxin or ouabain suppressed the basal and Ang II-stimulated protein expression of steroidogenic acute regulatory (StAR) protein and P450scc. Incubation of digoxin or ouabain for 24 h reduced P450c11AS mRNA expression in NCI-H295 cells. Digoxin or ouabain (10(-6) M, 24 h)-treated cells showed a lower resting intracellular Ca2+ concentration ([Ca2+]i) and an attenuated response of [Ca2+]i to Ang II. Since no significant cytotoxicity was observed at 10(-6) M digoxin or ouabain, the digoxin- or ouabain-induced decrease of aldosterone or cortisol release was not associated with cytotoxicity. These results demonstrate that digoxin or ouabain inhibits the aldosterone or cortisol release via reduction of P450c11AS or P450c11beta and P450scc activities, inhibition of StAR and P450scc protein expression, suppression of P450c11AS mRNA expression, and attenuation of Ca2+ mobilization in NCI-H295 cells.

Structural determinants for the activation mechanism of the angiotensin II type 1 receptor differ for phosphoinositide hydrolysis and mitogen-activated protein kinase pathways

While the mechanism whereby the angiotensin II type 1 receptor (AT(1) receptor) activates its classical effector phospholipase C-beta (PLC-beta) has largely been elucidated, there is little consensus on how this receptor activates a more recently identified effector, the p42/44 mitogen-activated protein kinases (p42/44(MAPK)). Using transfected COS-1 cells, we investigated the activation of this signaling pathway at the receptor level itself. Previous mutational studies that relied on phosphoinositide turnover as an index of receptor activation have indicated that key residues in the second and seventh transmembrane domains participate in AT(1) receptor activation mechanisms. Thus, we introduced a variety of mutations-AT(1)[D74N], AT(1)[Y292F], AT(1)[N295S], and AT(1)[AT(2) TM7], which is composed of a chimeric substitution of the AT(1) seventh transmembrane domain with its AT(2) counterpart. These mutations that strongly diminished the receptor's ability to activate PLC-beta had little to no effect on its ability to activate p42/44(MAPK), which not only suggests that p42/44(MAPK) does not exclusively lie downstream of the G-protein G(q)/PLC-beta pathway but also indicates that more than one activation state may exist for the AT(1) receptor. The failure of a protein kinase C inhibitor to block AT(1) receptor activation of p42/44(MAPK) further corroborated evidence that the receptor's activation of p42/44(MAPK) is largely independent of the G(q)/PLC-beta/PKC pathway. Taken together, the experimental evidence strongly suggests that the mechanism whereby the AT(1) receptor activates p42/44(MAPK) is fundamentally different from that for PLC-beta, even at the level of the receptor itself.

Adrenocortical zonation and ACTH

The clear morphological distinction between the cells of the different adrenocortical zones has attracted speculation and experiment to interpret their functions and the ways in which they are regulated. Considerable data have been produced in recent years that has benefited a fuller understanding of the processes of steroidogenesis and of cell proliferation at the molecular level. This now enables the reexamination of earlier concepts. It is evident that there is considerable species variation, and this article, dealing mainly with the rat, reaches conclusions that do not necessarily apply to other mammals. In the rat adrenal, however, the evidence suggests that the greatest differences between the functions of the zones are between the glomerulosa and the fasciculata. Here the sometimes all-or-nothing demarcation in their complement of components associated with steroidogenesis or with cell proliferation suggests a stark division of labor. In this model the fasciculata is the main engine of steroid hormone output and the glomerulosa is the site of cell proliferation, recruitment, and differentiation. Regulating these functions are angiotensin II and other paracrine components that modulate and maintain the glomerulosa, and ACTH, that maintains the fasciculata, and recruits new fasciculata cells by transformation of proliferating glomerulosa cells. Grafted onto this mostly vegetative function of the glomerulosa is CYP11B2, limited to just a fraction of the outer glomerulosa in rats on a normal laboratory diet and generating aldosterone (and 18-hydroxycorticosterone) from precursors whose origin is not, from the evidence summarized here, very clear, but may include the fasciculata, directly or indirectly. The biosynthesis of aldosterone in the rat certainly requires reinterpretation.

AT1 angiotensin II receptor mediates intracellular calcium mobilization in normal and cancerous breast cells in primary culture

Angiotensin II (Ang II) increases intracellular calcium concentration ([Ca2+]i) in both normal and cancerous human breast cells in primary culture. Maximal [Ca2+]i increase is obtained using 100nM Ang II in both cell types; in cancerous breast cells, [Ca2+]i increase (delta[Ca2+]i) is 135+/-10nM, while in normal breast cells it reaches 65+/-5 nM (P<0.0001). In both cell types, Ang II evokes a Ca2+ transient peak mediated by thapsigargin (TG) sensitive stores; neither Ca2+ entry through L-type membrane channels or capacitative Ca2+ entry are involved. Type I Ang II receptor subtype (AT1) mediates Ang II-dependent [Ca2+]i increase, since losartan, an AT1 inhibitor, blunted [Ca2+]i increase induced by Ang II in a dose-dependent manner, while CGP 4221A, an AT2 inhibitor, does not. Phospholipase C (PLC) is involved in this signaling mechanism, as U73122, a PLC inhibitor, decreases Ang II-dependent [Ca2+]i transient peak in a dose-dependent mode.Thus, the present study provides new information about Ca2+ signaling pathways mediated through AT1 in breast cells in which no data were yet available.

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.

Functional interactions of L-162,313 with angiotensin II receptor subtypes and mutants

A nonpeptide ligand, L-162,313 (5,7-dimethyl-2-ethyl-3-[[4-[2(n-butyloxycarbonylsulfonamido)-5-is obutyl-3-thienyl]phenyl]methyl]imidazo[4,5,6]pyridine) was characterized on the angiotensin II receptors. This compound displaced [125I][Sar1]angiotensin II from rat angiotensin AT1A, AT1B or AT2 receptor individually expressed in COS-7 cells (Ki = 207 nM, 226 nM and 276 nM, respectively). In monkey kidney cells expressing angiotensin AT1A or AT1B receptors, it stimulated inositol phosphate accumulation, but the maximal response was 34.9 and 23.3%, respectively, of those of angiotensin II. Furthermore, an antagonist effect of L-162.313 was observed in response to angiotensin II. Single-point substitutions in the second and third transmembrane domains of the rat angiotensin AT1A receptor, which impaired the binding of losartan (2-n-butyl-4-chloro-5-hydroxymethyl-1[(1H-tetrazol-5-yl)biphenyl-4 -yl)methyl]imidazole), also affected the binding of L-162,313. Losartan and L-162,313 require some common structural determinants for non-peptide recognition on the angiotensin AT1 receptor. Furthermore, some of these substitutions, which impaired the inositol phosphate accumulation in response to angiotensin II, also impaired the response to L-162,313. Angiotensin II and L-162,313 require common critical residues for angiotensin AT1 receptor activation.

Angiotensin II activation of Ca(2+)-permeant nonselective cation channels in rat adrenal glomerulosa cells

A Ca(2+)-permeant, nonselective cation channel was observed in cell-attached and inside-out membrane patches from rat adrenal glomerulosa cells maintained in primary cell culture. In cell-attached patches under near physiological ionic conditions, single-channel currents exhibited a reversal potential near -10 mV, inward rectification, a nearly linear slope conductance between 0 and -80 mV of 17.4 pS, and voltage-dependent block at potentials more negative than -80 mV. Channels exhibiting identical conductance and gating properties were observed in inside-out patches; however, channel gating ran down within minutes in this configuation. In the inside-out configuration, channel gating did not require cytosolic Ca2+ (Ca2+ < 10(-9) M), and inward rectification was relieved by removal of intracellular Mg2+. Relative ionic permeability was calculated using reversal potential measurements from inside-out patches under bi-ionic conditions. The channel discriminated poorly among monovalent cations (PLi > PK > PCs > PNa) and was not significantly permeable to anions. The channel was permeable to Ca2+, exhibiting a relative permeability ratio of 0.29 PCa/PNa) when measured with 110 mM Ca2+ on the intracellular face and a permeability ratio of 4.38 (PCa/PNa) with 110 mM Ca2+ on the extracellular face. Channel gating behavior was episodic with open times ranging from milliseconds to tens of seconds and closed times lasting up to several minutes or longer. Channel gating appeared to be relatively voltage independent except that mean channel open time and open probability were reduced by membrane hyperpolarization. In cell-attached patches, bath application of 1 nM angiotensin II (ANG II) increased the channel open probability, primarily affecting channels exhibiting a low open probability, primarily affecting channels exhibiting a low open probability before stimulation. With the use of nystatin perforated-patch current clamp to measure membrane potential, ANG II was observed to induce large transient membrane depolarizations, consistent with activation of an inward current. We hypothesize that this channel is an important component of ANG II-induced membrane depolarization and Ca2+ influx during stimulation of aldosterone secretion.

Adrenal androgen biosynthesis with special attention to P450c17

In vitro studies of human adrenal androgen synthesis are limited because of the difficulties in obtaining adrenals. We describe the use of the human adrenocortical tumor H295 cell line as a model to evaluate mechanisms controlling C19-steroid production. The cells were characterized with regard to responsiveness to a variety of agents as measured by steroid secretion and induction of 17 alpha-hydroxylase cytochrome P450 (P450c17) expression, a key enzyme in C19-steroid production. Forskolin and dibutyryl cAMP, which were more effective than ACTH, enhanced the production of DHEA and androstenedione over a 48-hour treatment period. Agents that act by increasing intracellular calcium (angiotensin II and K+ ions) as well as protein kinase A pathway activators (ACTH, forskolin, and dibutyryl cAMP) individually increased the mRNA levels and activity of P450c17. In addition, angiotensin II but not K+ ions attenuated the increased expression promoted by the kinase A agonists. Thus, the complexity of human adrenal P450c17 expression through multiple signaling pathways may contribute importantly to the diverse patterns of human adrenocortical steroidogenesis.

Angiotensin receptors in the brain

Angiotensin receptors have recently become a focus of scientific interest due to the recent development of specific receptor ligands which allow to distinguish between various angiotensin II receptor subtypes, notably the angiotensin II type 1 receptor (AT1) and angiotensin II type 2 receptor (AT2). Although both receptors belong to the seven transmembrane domain receptor family they feature less than 35% homology and differ in their signal transduction mechanisms and in the effects mediated. In the brain, both angiotensin receptor types and probably some further subtypes are present and have been localized in distinct regions. In the adult brain, the AT1 receptor dominates by far and is responsible for most of the known central actions of angiotensin peptides, for example blood pressure increase, release of vasopressin from the pituitary gland, natriuresis, drinking and induction of immediate early genes in distinct brain areas. Some of the AT1 receptor-mediated effects have been shown to be enhanced by blockade of AT2 receptors in the brain suggesting that the central AT2 receptor can exert an inhibitory control on AT1 receptor-mediated actions in the brain.

The relationship between the adrenal tissue renin-angiotensin system, internalization of the type I angiotensin II receptor (AT1) and angiotensin II function in the rat adrenal zona glomerulosa cell

Many data suggest that the elements of the tissue renin-angiotensin system (RAS) in the adrenal cortex are mostly located in the zona glomerulosa. The relationship of this paracrine/autocrine system with the cellular localization of the angiotensin II (AII) receptor has not bee clarified. Using a specific monoclonal antibody (6313/G2) to the first extracellular domain of the type 1 receptor (AT1), we show here that most of the receptor is internalized in the rat glomerulosa cell. This may result from tonic stimulation by the tissue RAS, and consequent permanent receptor occupancy. When viable glomerulosa cells are incubated with 6313/G2, the receptor is transiently concentrated on the cell surface, and aldosterone output is stimulated. This stimulated output is enhanced by neither threshold nor maximal stimulatory concentrations of AII amide, although the antibody does not inhibit AII binding to the receptor. The antibody directly stimulates inositol trisphosphate (IP3) generation, but, while having no intrinsic action on protein kinase C (PKC) activation, significantly inhibits the PKC response to angiotensin II. The data suggest that although the receptor is mostly internalized, recycling to the plasma membrane is constitutive, or regulated by unknown factors. Retention of the AT1 receptor in the membrane is alone enough to allow sufficient G protein interaction to generate maximal steroidogenic effects, through IP3 generation. PKC activation induced by angiotensin II has no bearing on steroidogenesis in the dispersed glomerulosa cell system.

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

The classical concept of human adrenal physiology indicates that only glomerulosa cells are the target of A-II. Herein, we demonstrated that cultured human adrenal fasciculata-reticularis cells were also responsive to this hormone. Indeed, these cells contained high affinity (Kd = 0.9-1.1 nM) and low capacity (8,000-13,000 sites/cell) A-II receptors, and more than 95% of them were of the type-1. These AT1 receptors are functional since A-II was able to increase cortisol production after 48 h of treatment. These effects were inhibited by losartan, an AT1 antagonist, but not by CGP42112A, an AT2 antagonist. The expression of the type-1 A-II receptor mRNA was detected in the whole adrenal in both adult and fetus, and in cultured human adrenal fasciculata-reticularis cells. In these cells A-II negatively regulated AT1 receptor mRNA, and this effect was also mediated through the AT1 receptor subtype.

Factors controlling steroid biosynthesis in the zona glomerulosa of the adrenal

The biosynthesis of aldosterone in the adrenal zona glomerulosa is influenced by a number of factors of which the main physiological regulator is the octapeptide, angiotensin II (AII). Sodium restriction increases plasma aldosterone, adrenal glomerulosa AII receptors and the activity of enzymes of the early and late aldosterone biosynthetic pathway. The effects of sodium restriction are mimicked by prolonged administration of low doses of AII, and prevented by blockade of AII formation using converting enzyme inhibitors, indicating that the effects of sodium restriction are mediated by AII. However, the adrenal glomerulotrophic actions of AII are impaired in rats on high sodium diet indicating that other factors are modulating the effects of AII in these conditions. A number of factors are known to influence aldosterone secretion, several of which have been shown to preferentially modulate the effect of AII. While the stimulatory effect of AII is potentiated by serotonin or increases in extracellular potassium, it is inhibited by dopamine, somatostatin and atrial natriuretic peptide. Future investigations will be important to understand the relative role of the individual regulators in the physiological control of adrenal sensitivity to AII, and how activation of various intracellular messenger systems results in changes in activity of the enzymes of the aldosterone biosynthetic pathway.

Guanabenz-induced inhibition of aldosterone secretion from isolated rat adrenal glomerulosa cells

The authors examined the effects of the alpha 2-adrenergic agonist guanabenz and other alpha-adrenergic ligands on aldosterone secretion and cyclic nucleotide content in isolated rat adrenal glomerulosa cells. Guanabenz inhibited aldosterone secretion stimulated by potassium, angiotensin II (AII), and adrenocorticotropic hormone (ACTH), exhibiting IC50 values of 35 microM, 43 microM, and 58 microM for stimulation by 10 mM K+, 1 nM AII, and 10 pM ACTH, respectively. Guanabenz did not affect the cGMP content of purified adrenal glomerulosa cells but inhibited ACTH stimulation of cAMP accumulation. Guanabenz inhibition of ACTH-induced cAMP may represent a mechanism for inhibition of aldosterone secretion, however, guanabenz also inhibited aldosterone secretion stimulated by the cAMP analog dibutyryl cAMP. The effect of guanabenz on the early and late pathways of steroidogenesis was tested in the isolated rat glomerulosa cells using 25-OH cholesterol and steroid precursors to aldosterone. Guanabenz inhibited the steroidogenic response to 25-OH cholesterol stimulation of aldosterone secretion but induced a much smaller inhibition of the steroidogenic response to exogenous pregnenolone, progesterone, and 11-deoxycorticosterone. These results suggested that guanabenz inhibited aldosterone secretion primarily through inhibition of the early component of the steroidogenic pathway prior to pregnenolone formation. The effects of guanabenz were not mimicked by other alpha-adrenergic ligands suggesting that these effects of guanabenz were not mediated through activation of alpha-adrenergic receptors.

Temperature dependence of angiotensin II-mediated depolarisation of the rat isolated nodose ganglion

The ability of angiotensin II (A II) and 5-hydroxytryptamine (5-HT) to depolarise the rat isolated nodose ganglion preparation was examined. 5-HT depolarised the nodose ganglion, both at room temperature (20-24 degrees C) and at 35-37 degrees C. However, A II depolarised the nodose ganglion only under the latter condition, and these responses were blocked by the A II receptor antagonist saralasin. This study extends previous findings which have demonstrated A II binding sites on the nodose ganglion and axon, and identifies the rat nodose ganglion as a sensitive preparation in which to study the interactions between neuronal A II receptor activation and its blockade by A II receptor antagonists.