Agonist-induced down-regulation of the angiotensin II receptor in primary cultures of rat hepatocytes

Structural perspectives on the mechanism of signal activation, ligand selectivity and allosteric modulation in angiotensin receptors: IUPHAR Review 34

Functional advances have guided our knowledge of physiological and fatal pathological mechanisms of the hormone angiotensin II (AngII) and its antagonists. Such studies revealed that tissue response to a given dose of the hormone or its antagonist depends on receptors that engage the ligand. Thus, we need to know much more about the structures of receptor-ligand complexes at high resolution. Recently, X-ray structures of both AngII receptors (AT1 and AT2 receptors) bound to peptide and non-peptide ligands have been elucidated, providing new opportunities to examine the dynamic fluxes in the 3D architecture of the receptors, as the basis of ligand selectivity, efficacy, and regulation of the molecular functions of the receptors. Constituent structural motifs cooperatively transform ligand selectivity into specific functions, thus conceptualizing the primacy of the 3D structure over individual motifs of receptors. This review covers the new data elucidating the structural dynamics of AngII receptors and how structural knowledge can be transformative in understanding the mechanisms underlying the physiology of AngII.

Deficiency of peroxisome proliferator-activated receptor α attenuates apoptosis and promotes migration of vascular smooth muscle cells

Peroxisome proliferator-activated receptor (PPAR) α is widely expressed in the vasculature and has pleiotropic and lipid-lowering independent effects, but its role in the growth and function of vascular smooth muscle cells (VSMCs) during vascular pathophysiology is still unclear. Herein, VSMC-specific PPARα-deficient mice (Ppara ^ΔSMC) were generated by Cre-LoxP site-specific recombinase technology and VSMCs were isolated from mice aorta. PPARα deficiency attenuated VSMC apoptosis induced by angiotensin (Ang) II and hydrogen peroxide, and increased the migration of Ang II-challenged cells.

A single conserved leucine residue on the first intracellular loop regulates ER export of G protein-coupled receptors

The intrinsic structural determinants for export trafficking of G protein-coupled receptors (GPCRs) have been mainly identified in the termini of the receptors. In this report, we determined the role of the first intracellular loop (ICL1) in the transport from the endoplasmic reticulum (ER) to the cell surface of GPCRs. The alpha(2B)-adrenergic receptor (AR) mutant lacking the ICL1 is unable to traffic to the cell surface and to initiate signaling measured as ERK1/2 activation. Mutagenesis studies identify a single Leu48 residue in the ICL1 modulates alpha(2B)-AR export from the ER. The ER export function of the Leu48 residue can be substituted by Phe, but not Ile, Val, Tyr and Trp, and is unlikely involved in correct folding or dimerization of alpha(2B)-AR in the ER. Importantly, the isolated Leu residue is remarkably conserved in the center of the ICL1s among the family A GPCRs and is also required for the export to the cell surface of beta(2)-AR, alpha(1B)-AR and angiotensin II type 1 receptor. These data indicate a crucial role for a single Leu residue within the ICL1 in ER export of GPCRs.

Expression and regulation of AT1 receptor in rat lung microvascular endothelial cell

BACKGROUND

The renin-angiotensin system is thought to be involved in the development and progression of vascular endothelium inflammation, thereby contributing to vascular endothelium injury. To clarify the role of angiotensin II (Ang II) in rat pulmonary microvascular endothelial cells (RPMVECs), we examined the expression and functional significance of angiotensin II (Ang II) receptors in normal and lipopolysacchride (LPS) treated RPMVECs.

METHODS

The expressions of Ang II type 1(AT(1)) and Ang II type 2 (AT(2)) receptors in cultured RPMVECs were identified by the reverse transcription-polymerase chain reaction (RT-PCR) technique, Western blot and (125)I-labeled [Sar(1),Ile(8)] Ang II binding assays. The RPMVECs were treated with LPS (0.1-100 microg/ml) and Ang II (10(-8)-10(-5) M) for 24 h, respectively. Next, RPMVECs were treated with 10 microg/ml LPS or 10(-7) M Ang II for various times (3, 6, 12, and 24 h). The mRNA and protein levels of, AT(1) and AT(2) receptors, were evaluated at 3, 6, 12, and 24 h, respectively.

RESULTS

The presence of specific Ang II binding sites in RPMVECs was found by Ang II saturated assays. RT-PCR revealed that only the AT(1) receptor mRNA is presented in RPMVECs. Western blot analysis of the RPMVECs protein extracts showed only one prominent band of the protein at approximately 41 KDa when probed with anti-AT(1) antibody and anti-AT(2) antibody. No AT(2) receptor mRNA and protein was detected. LPS treated cells resulted in an increase in the mRNA and protein levels of AT(1) receptor, whereas, Ang II treated cells showed a decrease in the mRNA and protein levels of AT(1) receptor.

CONCLUSIONS

We found that primary cultured RPMVECs expressed only AT(1) receptor, but not AT(2) receptor. LPS up-regulated the transcriptional and post-transcriptional expression of AT(1) receptor in RPMVECS; in contrast, Ang II treatment caused a reduction in the mRNA and protein of AT(1) receptor in a time-dependent manner.

Regulation of the Cell Surface Expression and Function of Angiotensin II Type 1 Receptor by Rab1-mediated Endoplasmic Reticulum-to-Golgi Transport in Cardiac Myocytes

Rab1 GTPase coordinates vesicle-mediated protein transport specifically from the endoplasmic reticulum (ER) to the Golgi apparatus. We recently demonstrated that Rab1 is involved in the export of angiotensin II (Ang II) type 1 receptor (AT1R) to the cell surface in HEK293 cells and that transgenic mice overexpressing Rab1 in the myocardium develop cardiac hypertrophy. To expand these studies, we determined in this report whether the modification of Rab1-mediated ER-to-Golgi transport can alter the cell surface expression and function of endogenous AT1R and AT1R-mediated hypertrophic growth in primary cultures of neonatal rat ventricular myocytes. Adenovirus-mediated gene transfer of wild-type Rab1 (Rab1WT) significantly increased cell surface expression of endogenous AT1R in neonatal cardiomyocytes, whereas the dominant-negative mutant Rab1N124I had the opposite effect. Brefeldin A treatment blocked the Rab1WT-induced increase in AT1R cell surface expression. Fluorescence analysis of the subcellular localization of AT1R revealed that Rab1 regulated AT1R transport specifically from the ER to the Golgi in HL-1 cardiomyocytes. Consistent with their effects on AT1R export, Rab1WT and Rab1N124I differentially modified the AT1R-mediated activation of ERK1/2 and its upstream kinase MEK1. More importantly, adenovirus-mediated expression of Rab1N124I markedly attenuated the Ang II-stimulated hypertrophic growth as measured by protein synthesis, cell size, and sarcomeric organization in neonatal cardiomyocytes. In contrast, Rab1WT expression augmented the Ang II-mediated hypertrophic response. These data strongly indicate that AT1R function in cardiomyocytes can be modulated through manipulating AT1R traffic from the ER to the Golgi and provide the first evidence implicating the ER-to-Golgi transport as a regulatory site for control of cardiomyocyte growth.

The relationship between activation of phosphoinositide-specific phospholipase C and growth stimulation by Ca2+-mobilizing hormones in hepatocytes

Previous studies have shown that while vasopressin and angiotensin II are markedly more effective than norepinephrine and prostaglandin F2alpha in eliciting an acute elevation of inositol 1,4,5-trisphosphate (IP3), norepinephrine and prostaglandin F2alpha produce larger enhancement of DNA synthesis. This suggests that the initial activation of phosphoinositide-specific phospholipase C is not a common factor for the growth response to these agonists, but does not exclude a role of the integral of phospholipase C activity over a prolonged part of the prereplicative period, during which agonist-specific changes in responsiveness might occur. We show that vasopressin and angiotensin II also cause a prolonged elevation of cellular IP3 levels. which remain elevated for at least 60 min., while norepinephrine and prostaglandin F2alpha elevate IP3 levels slightly and transiently For vasopressin the dose-effect curves for IP3 accumulation and stimulation of DNA synthesis were closely parallel, while this was not the case for angiotensin II, norepinephrine, or prostaglandin F2alpha. After cultivation of the hepatocytes, hormone-stimulated IP3 accumulation rapidly declined, particularly in response to norepinephrine and prostaglandin F2alpha. When the IP3 response to norepinephrine and prostaglandin F2alpha was completely down-regulated, these agonists still enhanced the DNA synthesis. These results suggest that other mechanisms in addition to IP3 accumulation and Ca2+ release are likely to be involved in the growth stimulatory effects of the Ca2+-mobilizing agonists studied here, in particular for angiotensin II, norepinephrine, and prostaglandin F2alpha.

Upregulation of renin-angiotensin system during differentiation of monocytes to macrophages

BACKGROUND

We have demonstrated that accumulated macrophages in human coronary arteries strongly express angiotensin converting enzyme in accordance with the development of atheromatous plaques. However, there are few reports on the regulation of the renin-angiotensin system in macrophages and in monocytes as their source.

OBJECTIVE

To examine whether the renin-angiotensin system is upregulated during the differentiation of monocytes to macrophages, and whether it is further regulated by angiotensin II and cytokines.

MATERIALS AND METHODS

We used a human leukemia cell line, THP-1, for monocytes. Differentiated THP-1, induced by adding phorbol 12-myristate 13-acetate for 24 h, were used as macrophages. Expression of messenger RNA of the renin-angiotensin system components was measured by quantitative reverse-transcriptase polymerase chain reaction. Angiotensin converting enzyme activity and subtype-specific angiotensin-binding sites of cultured cells, and angiotensin II production in the culture medium were measured.

RESULTS

Macrophages expressed all components of the renin-angiotensin system except chymase. Cellular angiotensin converting enzyme activity and angiotensin II in the medium were increased 3.2- and 4.5-fold during differentiation, respectively. Expression of angiotensin II type 1 (AT1) and type 2 (AT2) receptors was increased 6.2-and 6.4-fold during differentiation, and was sustained for 7 days. Incubation with angiotensin II for 24 h caused downregulation of both AT1 and AT2 receptor messenger RNA, but the expression levels were still more than threefold higher compared with monocytes. The density of binding sites of AT1 and AT2 receptors in macrophages was 0.26 +/- 0.02 and 0.15 +/- 0.01 fmol/10(6) cells, respectively.

CONCLUSION

The renin-angiotensin system is markedly activated during monocyte/macrophage differentiation, and may participate in the development of atherosclerosis.

Regulation of angiotensin II type 1 (AT1) receptor function

The type 1 angiotensin receptor (AT1) mediates the important biological actions of the peptide hormone, angiotensin II (AngII), by activating an array of intracellular signaling pathways. The unique temporal arrangement and duration of AngII-stimulated signals suggests a hierarchy of post-AT1 receptor binding events that permits activation of selective effector pathways. Moreover, it predicts that the coupling of AT1 receptors is tightly regulated, allowing cells to differentiate acute responses from those requiring longer periods of stimulation. Recent studies have concentrated on delineating the molecular processes involved in modulating AT1 receptor activity. In addition to AT1 receptor modification (phosphorylation), trafficking (internalization and degradation) and interaction with regulatory intracellular proteins, other processes may include receptor dimerization, cross-regulation by other receptor systems, and receptor isomerization between activated and non-activated forms. This review focuses on recent advances in this area of research, highlighting directions for future investigation.

Evidence of an important and direct role for protein kinase C in agonist-induced phosphorylation leading to desensitization of the angiotensin AT1A receptor

1. The role of protein kinase C (PKC) in the mechanism underlying rapid agonist-induced desensitization of angiotensin AT1 receptors remains unresolved. A major problem has been to isolate these receptors in a sufficiently purified form to allow study of their phosphorylation state. 2. A cleavable (His)6 affinity tag was introduced into the N-terminus of the recombinant AT1A receptor and stably expressed in human embryonic kidney cells. This affinity tag allowed rapid isolation, purification and determination of the phosphorylation state of the AT1A receptor. Using these cells, we determined the role of PKC in both agonist-induced receptor phosphorylation and desensitization under identical conditions. 3. Agonist-induced phosphorylation of the AT1A receptor was observed at both low and high concentrations of angiotensin II (AII). Preincubation of cells with Ro-31-8220 (a PKC specific inhibitor) revealed that at low concentrations of AII (1 nM), PKC appeared to be the main kinase involved in receptor phosphorylation. In contrast, at high concentrations of AII (100 nM), although PKC-mediated phosphorylation of the receptor was observed, this was overshadowed by a second kinase. 4. In preliminary desensitization studies we observed that at a low concentration of AII, preincubation with Ro-31-8220 attenuated desensitization, whilst at high concentrations of AII (100 nM) it had little or no effect on the level of desensitization observed. 5. These data directly demonstrate an association between PKC-induced receptor phosphorylation and desensitization at low concentrations of AII. Since circulating concentrations of AII are in the picomolar range, we propose that PKC is the physiologically relevant mediator of AT1 receptor desensitization.

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.

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.

Potentiation of purinergic transmission by angiotensin in prostatic rat vas deferens

1. Angiotensin II (AII) elicited only a minute, if any, direct contractile response in smooth muscle cells of prostatic rat vas deferens, but it potentiated contractile responses to field stimulation. 2. Angiotensin-potentiated contractile response to field stimulation was concentration-dependent, and the order of potency was AII > AIII approximately AI. The EC50 of AII was 8.11 +/- 2.79 nM. 3. AII did not modify the contractile response of exogenous noradrenaline (NA) on non-stimulated prostatic vas deferens. Furthermore, the concentration-response curve for AII-potentiated contractile responses to field stimulation in reserpine-treated rats did not significantly differ from the control group. 4. Desensitization of purinoceptors with 30 microM alpha, beta-methylene-ATP almost completely abolished the potentiation of the contractile response to field stimulation by AII. 5. The response to AII in the prostatic rat vas deferens was blocked by the AT1 selective antagonist losartan, but not by the AT2 selective antagonist CGP 42112. Losartan acted as a competitive antagonist with a pA2 value of 8.75. 6. In conclusion, AII potentiated purinergic transmission in the prostatic rat vas deferens via the AT1 receptor.

Inhibition of protein kinase C prevents rapid desensitization of type 1B angiotensin II receptor

The type 1B angiotensin II (AT1B) receptor cloned from rat kidney was stably expressed in Chinese hamster ovary cells. The stably expressed receptor was characterized by radioligand binding studies and functional coupling to inositol 1,4,5-triphosphate (IP3) formation. Exposure of cells expressing the AT1B receptor to angiotensin II (Ang II) resulted in a rapid and dose-dependent homologous desensitization of receptor-mediated production of IP3, with an essentially complete desensitization at an agonist concentration > 10 nmol/L. Binding studies revealed no significant change in the number of AT1B receptors in transfected cells exposed to 1 nmol/L Ang II, whereas exposure to 100 nmol/L Ang II caused a rapid decrease of cell surface receptors, with a 75% loss of receptor number seen at 1 hour. Rapid desensitization occurred in the absence of receptor internalization. Blockade of receptor internalization with concanavalin A had at most only a slight effect on the agonist-induced desensitization. This indicates that factors other than internalization are chiefly responsible for the rapid agonist-induced desensitization. Phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, caused rapid desensitization of the receptor-mediated IP3 response. Neither tyrosine kinase inhibitors nor a protein kinase A activator affected the receptor-mediated IP3 response. The specific PKC inhibitor GF109203X or PKC depletion by prolonged treatment with 1 mumol/L PMA completely blocked the PMA-dependent desensitization. Desensitization evoked by a low Ang II agonist concentration (1 nmol/L) was reversed by the PKC-specific inhibitor GF109203X or PKC depletion, whereas the desensitizing effect at a high agonist concentration (100 nmol/L) is only partially prevented by PKC inhibitory treatment. These results demonstrate that PKC plays a crucial role in the desensitization of the AT1B receptor. They also suggest that receptor internalization and an additional PKC-independent pathway also contribute to desensitization of the AT1B receptor in transfected cells.

Differential regulation of angiotensin II receptor subtypes in rat kidney by low dietary sodium

This study was designed to determine whether expression of renal messenger RNA (mRNA) encoding the two known angiotensin II type 1 (AT1) receptor subtypes (AT1A and AT1B) can be regulated by dietary sodium. Seven-week-old male Wistar rats were fed a low-sodium diet (0.07%, n = 9) or a normal-sodium diet (0.5%, n = 9 [control]) for 14 days. A rat AT1 complementary DNA (cDNA) probe, which hybridizes to mRNA encoding both the AT1A and AT1B receptor subtypes, and cDNA probes, which are selective for AT1A or AT1B mRNA, were used in Northern blot or in situ hybridization analysis. By use of Northern blot analysis, renal mRNA levels for the AT1 and AT1A receptors in rats fed a low-sodium diet were found to be increased twofold (P < .05) compared with control. Because renal AT1B mRNA content was not detected by Northern blot analysis, quantitative image analysis of in situ hybridization with a digoxigenin-labeled cRNA probe made from AT1B cDNA was used. In situ hybridization analysis indicated that AT1B mRNA was expressed in the proximal and collecting tubules of the kidney in rats fed a normal-sodium diet. The low-sodium diet significantly decreased the percent positive staining area of AT1B mRNA in the renal cortex (5.51 +/- 0.77% versus 2.73 +/- 0.35%, P < .05) and medulla (4.76 +/- 0.70% versus 2.01 +/- 0.43%, P < .05) compared with the control diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin converting enzyme inhibition does not affect the response to exogenous angiotensin II in the human forearm

Suppression of endogenous levels of angiotensin II by angiotensin converting enzyme inhibition, may result in up-regulation of vascular AT1 receptors. We have evaluated the effects of orally administered enalapril on angiotensin II induced vasoconstriction in the human forearm. Subjects received in random order, enalapril (20 mg) or matched placebo daily for 2 weeks. Forearm blood flow response to increasing doses of angiotensin II was measured using venous occlusion plethysmography at the beginning of the study and at the end of each 2 week treatment period. Treatment with enalapril significantly reduced plasma angiotensin II levels and supine blood pressure compared with placebo. The percentage reductions in forearm blood flow in the infused arm, in response to the maximum dose of angiotensin II (50,000 fmol min-1) were 48.1 +/- 3.6% at baseline, 57.5 +/- 3.6% on placebo and 54.5 +/- 4.2% on enalapril. The differences were not significantly different. This demonstrates that suppression of plasma angiotensin II for a 14 day period does not enhance the response to exogenous intra-arterial angiotensin II in the human forearm of healthy salt replete subjects.

Activation of glycogenolysis by methotrexate. Influence of calcium and inhibitors of hormone action

The influence of Ca2+ and the possible action of hormone blockers on the activation of glycogenolysis by methotrexate were investigated. Methotrexate was inactive on glycogenolysis and oxygen uptake when the liver, depleted of intracellular Ca2+, was perfused with Ca(2+)-free medium. The action of methotrexate in calcium-depleted hepatocytes could be restored by the addition of extracellular Ca2+. When Ca2+ was absent in the extracellular medium, but the intracellular stores were not depleted, methotrexate produced transient and progressively attenuated increases in glycogenolysis and oxygen uptake. Like many agonists, methotrexate produced transient increases in Ca2+ efflux. The action of methotrexate was not blocked by the antagonists of norepinephrine, phenylephrine, isoproterenol, vasopressin and angiotensin II. It was concluded that methotrexate acts through a Ca(2+)-dependent mechanism, which is similar to that of the Ca(2+)-dependent agonists. This action, however, seems not to be receptor mediated.

Two distinct pathways in the down-regulation of type-1 angiotension II receptor gene in rat glomerular mesangial cells

The mRNA level of the type-1 angiotensin II receptor (AT1) was down-regulated by angiotensin II in cultured rat glomerular mesangial cells. The effect was maximum with 1 microM AII at 6 h, sensitive to cycloheximide, and specific to AT1 since this phenomenon was blocked by DuP753, an AT1 antagonist, but not by type-2 antagonist PD123319. Dibutyryl cAMP, forskolin, and cholera toxin also caused AT1 down-regulation. These effects were not altered by either the protein kinase A inhibitor H-8 or cycloheximide. Calcium ionophore A23187, pertussis toxin, protein kinase C inhibitor staurosporine, or prolonged incubation with phorbol ester were without effect. These results suggest that there are at least two pathways to down-regulate AT1 mRNA; one way is an angiotensin II-induced, protein kinase C-independent, and cycloheximide-sensitive pathway and the other is an angiotensin II-independent, cAMP-induced, and cycloheximide-insensitive pathway.

Identification and regulation of angiotensin II receptor subtypes on NG108-15 cells

NG108-15 cells, a neurally derived clonal cell line, express various components of the renin-angiotensin system and thus serve as a model of the cellular action of angiotensin (Ang) II. NG108-15 cells contain a high-affinity binding site for Ang II, with a Kd of 1.1 nM and a Bmax of 6.5 fmol/mg protein. Ang peptides competed for 125I-Ang II binding with an order of potency of Ang II greater than Ang-(2-8) much greater than Ang-(1-7). The subtype 1 (or B)-selective Ang II receptor antagonist DuP 753 as well as [Sar1,Ile8]Ang II and [Sar1,Thr8]Ang II competed for Ang II binding with high affinity, whereas the subtype 2 (or A)-selective Ang receptor antagonist CGP 42112A was partially effective only at a 300-fold higher concentration. When NG108-15 cells were induced to differentiate by treatment with dibutyryl cyclic adenosine 3',5'-monophosphate, the density of Ang II receptors increased dramatically, with little change in affinity (1.1 versus 4.2 nM) or competition by Ang peptides. In marked contrast to undifferentiated cells, CGP 42112A became a potent competitor (IC50, 1 nM) for the majority (90-95%) of Ang II binding, whereas DuP 753 competed for only 5-10% of the binding sites. Ang II caused a dose-dependent mobilization of cytosolic Ca2+ in undifferentiated NG108-15 cells through activation of phospholipase C and the production of inositol 1,4,5-trisphosphate. In these cells, Ca2+ mobilization was blocked by either DuP 753 or the sarcosine Ang II analogues, whereas CGP 42112A was ineffective. Ang II also mobilized intracellular Ca2+ in differentiated NG108-15 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in vasopressin and angiotensin II receptors and responses during culture of rat liver cells

Vasopressin and angiotensin II binding and responses were studied in hepatocytes in primary culture for 4 h and 24 h. After 24 h of culture, angiotensin II was completely ineffective in elevating cytosolic [Ca2+], whereas the maximum [Ca2+] response to vasopressin was decreased by 66% and the sensitivity to the hormone was decreased approx. 20-fold compared with values after 4 h of culture. The dissociation constant (KD) for vasopressin binding to the cells was not significantly changed during 24 h of culture, but the Bmax was decreased by 63% compared with 4 h of culture. There was also no change in the KD for angiotensin II binding from 4 h to 24 h, but the Bmax was decreased by 90%. After 24 h of culture, there was no change in the plasma membrane concentration of phosphatidylinositol 4,5-bisphosphate or in the basal cell concentration of inositol trisphosphate. However, the trisphosphate did not increase with 100 nM angiotensin II and the response to 100 nM vasopressin was reduced by 66% compared with that at 4 h. The effect of guanosine 5'-(3-O-thiol) triphosphate on the polyphosphoinositide phospholipase C activity of liver cell plasma membranes was also measured. There was no decrease in the degree of stimulation of the phospholipase by this nucleotide after 24 h of culture. It is concluded that the loss of vasopressin and angiotensin II responses in cultured liver cells is due in part to changes in receptors and also in their coupling to a guanine nucleotide binding protein.

Discrimination of two angiotensin II receptor subtypes with a selective agonist analogue of angiotensin II, p-aminophenylalanine6 angiotensin II

Angiotensin II receptor binding sites in rat liver and PC12 cells differ in their affinities for a nonpeptidic antagonist, DuP 753, and p-aminophenylalanine6 angiotensin II. In liver, which primarily contains the sulfhydryl reducing agent-inhibited type of angiotensin II receptor, which we refer to as the AII alpha subtype, DuP 753 displays an IC50 of 55 nM, while p-aminophenylalanine6 angiotensin II displays an IC50 of 8-9 microM. In PC12 cells, which primarily contain the angiotensin II receptor type whose binding affinity is enhanced by sulfhydryl reducing agents (AII beta), DuP 753 displays an IC50 in excess of 100 microM, while p-aminophenylalanine6 angiotensin II displays an IC50 of 12 nM. p-Aminophenylalanine6 angiotensin II binding affinity in liver is decreased in the presence of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) suggesting that this analogue is an agonist.

Inhibition of angiotensinogen production by angiotensin II analogues in human hepatoma cell line

The aim of this study was to examine in Hep G2, a human hepatoma-derived cell line, the presence of angiotensin II (ANG II) receptors and the effect of ANG II and its analogues on angiotensinogen production. The presence of ANG II receptors was demonstrated using a long-acting ANG II analogue, 125I-labeled [Sar1]ANG II. A single class of specific binding sites was identified in these cells with a dissociation constant (Kd) of 2 nM. The number and affinity of these binding sites were not changed by [Sar1]ANG II treatment over 24 h. ANG II showed an inhibitory effect on angiotensinogen production. [Sar1]ANG II also exhibited a similar inhibitory effect as that of ANG II but to a greater extent and therefore was used throughout these studies. [Sar1]ANG II inhibited angiotensinogen production in a dose-dependent manner, exhibiting a half-maximal inhibitory concentration (IC50) of 2 nM. Other ANG II analogues showed similar effects on angiotensinogen production. In order of decreasing ability, they were [Sar1]ANG II greater than [Sar1-Ala8]ANG II greater than [Sar1-Val8]ANG II greater than [Sar1-Val5-(Br5)-Phe8]ANG II greater than [Sar1-Val5-DPhe8]ANG II. Results of these studies show that the Hep G2 cell possesses specific ANG II receptors and that [Sar1]ANG II induces a dose-dependent inhibition of angiotensinogen production in this system.