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

The adrenal cortex after estradiol or daidzein application in a rat model of the andropause: Structural and hormonal study

INTRODUCTION AND AIM

Daidzein application may represent an effective and less harmful alternative to indicated, classical estrogenization of ageing men. The aim of this study was to perform structural and hormonal analysis of the adrenal cortex, after estradiol or daidzein supplementation in a rat model of the andropause.

MATERIAL AND METHODS

Middle-aged Wistar rats were divided into sham operated (SO; n = 8), orchidectomized (Orx; n = 8), estradiol treated orchidectomized (Orx + E; n = 8) and daidzein treated orchidectomized (Orx + D; n = 8) groups. Estradiol (0.625 mg/kg b.m./day) or daidzein (30 mg/kg b.m./day) were administered subcutaneously for three weeks, while the SO and Orx groups received the vehicle alone. Set objectives were achieved using stereology, histochemistry/immunohistochemistry, immunoassays and ultrastructural analysis.

RESULTS

Both estradiol and daidzein treatment significantly increased volumes of the zona glomerulosa cell and nuclei, but decreased circulating aldosterone levels. Estradiol markedly increased volumes of the zona fasciculata cell and nuclei in parallel with significant decrease of the adrenal tissue level of corticosterone, while daidzein significantly decreased both the adrenal and circulating levels of corticosterone. Serum DHEA level and volumes of the zona reticularis cell and nuclei significantly increased upon estradiol treatment, whereas daidzein even stronger increased the circulating level of DHEA. Shunting of the corticosteroidogenesis pathways towards adrenal androgens production, after the treatments, corresponded to the ultrastructural findings and zonal capillary network rearrangements.

CONCLUSIONS

Given the coherence of its effects and relative safety, daidzein could be the remedy of choice for the treatment of ageing-caused androgen deprivation and the hypothalamo-pituitary-adrenal axis hyperfunction/related metabolic issues in males.

Control of aldosterone secretion: a model for convergence in cellular signaling pathways

Aldosterone secretion by glomerulosa cells is stimulated by angiotensin II (ANG II), extracellular K(+), corticotrophin, and several paracrine factors. Electrophysiological, fluorimetric, and molecular biological techniques have significantly clarified the molecular action of these stimuli. The steroidogenic effect of corticotrophin is mediated by adenylyl cyclase, whereas potassium activates voltage-operated Ca(2+) channels. ANG II, bound to AT(1) receptors, acts through the inositol 1,4,5-trisphosphate (IP(3))-Ca(2+)/calmodulin system. All three types of IP(3) receptors are coexpressed, rendering a complex control of Ca(2+) release possible. Ca(2+) release is followed by both capacitative and voltage-activated Ca(2+) influx. ANG II inhibits the background K(+) channel TASK and Na(+)-K(+)-ATPase, and the ensuing depolarization activates T-type (Ca(v)3.2) Ca(2+) channels. Activation of protein kinase C by diacylglycerol (DAG) inhibits aldosterone production, whereas the arachidonate released from DAG in ANG II-stimulated cells is converted by lipoxygenase to 12-hydroxyeicosatetraenoic acid, which may also induce Ca(2+) signaling. Feedback effects and cross-talk of signal-transducing pathways sensitize glomerulosa cells to low-intensity stimuli, such as physiological elevations of [K(+)] (< or =1 mM), ANG II, and ACTH. Ca(2+) signaling is also modified by cell swelling, as well as receptor desensitization, resensitization, and downregulation. Long-term regulation of glomerulosa cells involves cell growth and proliferation and induction of steroidogenic enzymes. Ca(2+), receptor, and nonreceptor tyrosine kinases and mitogen-activated kinases participate in these processes. Ca(2+)- and cAMP-dependent phosphorylation induce the transfer of the steroid precursor cholesterol from the cytoplasm to the inner mitochondrial membrane. Ca(2+) signaling, transferred into the mitochondria, stimulates the reduction of pyridine nucleotides.

Angiotensin II receptor types 1A, 1B, and 2 in murine neuroblastoma Neuro-2a cells

In this study, the mouse neuroblastoma cell line Neuro-2a was analyzed for expression of angiotensin II receptors. Reverse-transcriptase polymerase chain reaction (RT-PCR) showed that Neuro-2a cells express mRNA of angiotensin II (AngII) receptor subtypes AT1A, AT1B, and AT2. Analysis of Neuro-2a cells by Western blotting revealed AT1 and AT2 receptor protein expression. The predominant molecular weights were determined to be 50.4 kDa for the AT1 receptor and 62.4 kDa for the AT2 receptor. Observation of AT1 and AT2 receptor localization within Neuro-2a cells using immunocytochemistry showed distribution similar to other G-protein coupled receptors with diffuse distribution in the cytosol, perinuclear enrichment and accumulation of receptors on the outer cellular periphery with extension into the neurites. Furthermore, we observed InsP3 formation following AngII induction that could be abolished in presence of the AT1A receptor antagonist losartan. The results clearly show expression of the AngII receptor types AT1A and AT2 in the Neuro-2a cell line. We conclude that Neuro-2a cells represent an interesting model cell line for study of mechanisms that control the interplay between these receptors.

Mechanisms and functions of AT(1) angiotensin receptor internalization

The type 1 (AT(1)) angiotensin receptor, which mediates the known physiological and pharmacological actions of angiotensin II, activates numerous intracellular signaling pathways and undergoes rapid internalization upon agonist binding. Morphological and biochemical studies have shown that agonist-induced endocytosis of the AT(1) receptor occurs via clathrin-coated pits, and is dependent on two regions in the cytoplasmic tail of the receptor. However, it is independent of G protein activation and signaling, and does not require the conserved NPXXY motif in the seventh transmembrane helix. The dependence of internalization of the AT(1) receptor on a cytoplasmic serine-threonine-rich region that is phosphorylated during agonist stimulation suggests that endocytosis is regulated by phosphorylation of the AT(1) receptor tail. beta-Arrestins have been implicated in the desensitization and endocytosis of several G protein-coupled receptors, but the exact nature of the adaptor protein required for association of the AT(1) receptor with clathrin-coated pits, and the role of dynamin in the internalization process, are still controversial. There is increasing evidence for a role of internalization in sustained signal generation from the AT(1) receptor. Several aspects of the mechanisms and specific function of AT(1) receptor internalization, including its precise mode and route of endocytosis, and the potential roles of cytoplasmic and nuclear receptors, remain to be elucidated.

Angiotensin II-induced nuclear targeting of the angiotensin type 1 (AT1) receptor in brain neurons

Angiotensin II (Ang II) interaction with the neuronal AT1 receptor results in a chronic stimulation of neuromodulation that involves the expression of norepinephrine transporter (NET) and tyrosine hydroxylase (TH). In view of this unique property and the presence of putative nuclear localization signal (NLS) consensus sequence in the AT1 receptor, this study was conducted to investigate the hypothesis that Ang II would induce nuclear sequestration of this G protein-coupled receptor and that the sequestration may have implications on Ang II-induced expression of NET and TH genes. Incubation of neuronal cultures with Ang II caused a time- and dose-dependent increase in the levels of AT1 receptor immunoreactivity in the nucleus. A 6.7-fold increase was observed with 100 nM Ang II, in 15 min, that was blocked by losartan, an AT1 receptor-specific antagonist. Ang II-induced nuclear sequestration was specific for AT1 receptor, because Ang II failed to produce a similar effect on neuronal AT2 receptors. The presence of the putative NLS sequence in the cytoplasmic tail of the AT1 receptor seems to be the key in nuclear targeting because: 1) nuclear targeting was attenuated by a peptide of the AT1 receptor that contained the putative NLS sequence; and 2) Ang II failed to cause nuclear translocation of the AT2 receptor, which does not contain the putative NLS. Ang II also caused a time- and dose-dependent stimulation of P62 phosphorylation, a glycoprotein of the nuclear pore complex. A 6-fold stimulation of phosphorylation was observed with 100 nM Ang II, in 15 min, that was completely blocked by losartan and not by PD123,319, an AT2 receptor specific antagonist. Preloading of neurons with p62-pep (a peptide containing consenses of mitogen-activated protein kinase in p62) resulted in a loss of Ang II-induced p62 phosphorylation and stimulation of NET and TH messenger RNA levels. In conclusion, these data demonstrate that Ang II induces nuclear sequestration of AT1 receptor involving NLS in the AT1 receptor and p62 of the nuclear pore complex in brain neurons. A possible role of such a nuclear targeting of the AT1 receptor on chronic neuromodulatory actions of Ang II has been discussed.

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.