A noninternalized nondesensitized truncated AT1A receptor transduces an amplified ANG II signal

Sex Differences in the Behavioral Desensitization of Water Intake Observed After Repeated Central Injections of Angiotensin II

Previous in vivo and in vitro studies demonstrate that the angiotensin type 1 receptor rapidly desensitizes after exposure to angiotensin II (AngII). Behaviorally, this likely underlies the reduced drinking observed after acute repeated central injections of AngII. To date, this phenomenon has been studied exclusively in male subjects. Because there are sex differences in the dipsogenic potency of AngII, we hypothesized that sex differences also exist in desensitization caused by AngII. As expected, when male rats were pretreated with AngII, they drank less water after a test injection of AngII than did rats pretreated with vehicle. Intact cycling female rats, however, drank similar amounts of water after AngII regardless of the pretreatment. To probe the mechanism underlying this sex difference, we tested the role of gonadal hormones in adult and developing rats. Gonadectomy in adults did not produce a male-like propensity for desensitization of water intake in female rats, nor did it produce a female-like response in male rats. To test if neonatal brain masculinization generated a male-like responsiveness, female pups were treated at birth with vehicle, testosterone propionate (TP), or dihydrotestosterone (DHT). When tested as adults, TP-treated female rats showed a male-like desensitization after repeated AngII that was not found in vehicle- or DHT-treated rats. Together, these data reveal a striking sex difference in the behavioral response to elevated AngII that is mediated by organizational effects of gonadal hormones and provide an example of one of the many ways that sex influences the renin-angiotensin-aldosterone system.

Structural determinants for binding, activation, and functional selectivity of the angiotensin AT1 receptor

The renin-angiotensin system (RAS) plays an important role in the pathophysiology of cardiovascular disorders. Pharmacologic interventions targeting the RAS cascade have led to the discovery of renin inhibitors, angiotensin-converting enzyme inhibitors, and AT(1) receptor blockers (ARBs) to treat hypertension and some cardiovascular and renal disorders. Mutagenesis and modeling studies have revealed that differential functional outcomes are the results of multiple active states conformed by the AT(1) receptor upon interaction with angiotensin II (Ang II). The binding of agonist is dependent on both extracellular and intramembrane regions of the receptor molecule, and as a consequence occupies more extensive area of the receptor than a non-peptide antagonist. Both agonist and antagonist bind to the same intramembrane regions to interfere with each other's binding to exhibit competitive, surmountable interaction. The nature of interactions with the amino acids in the receptor is different for each of the ARBs given the small differences in the molecular structure between drugs. AT(1) receptors attain different conformation states after binding various Ang II analogues, resulting in variable responses through activation of multiple signaling pathways. These include both classical and non-classical pathways mediated through growth factor receptor transactivations, and provide cross-communication between downstream signaling molecules. The structural requirements for AT(1) receptors to activate extracellular signal-regulated kinases 1 and 2 through G proteins, or G protein-independently through β-arrestin, are different. We review the structural and functional characteristics of Ang II and its analogs and antagonists, and their interaction with amino acid residues in the AT(1) receptor.

Shear stress activates extracellular signal-regulated kinase 1/2 via the angiotensin II type 1 receptor

Mechanical factors such as strain, pressure, and shear stress are key regulators of cell function, but the molecular mechanisms underlying the detection and responses to such stimuli are poorly understood. Whether the angiotensin II (AngII) AT1 receptor (AT1R) transduces shear stress in endothelial cells (ECs) is unknown. We exposed human umbilical cord endothelial cells (HUVECs) to a shear stress of 0 (control) or 15 dyn/cm(2) for 5 or 10 min. The colocalization of AT1R with caveolin-1 (Cav1), endosomal markers Rab5, EEA1, and Rab7, and lysosomal marker Lamp-1 increased in shear stimulated cells, detected by immunocytochemistry. Shear stress reduced labeling of wild-type mouse ECs (18±3% of unsheared control, P<0.01) but not Cav1(-/-) ECs (90±10%) with fluorescent AngII, confirming that internalization of AT1R requires Cav1. Shear stress activated ERK1/2 2-fold (P<0.01), which was prevented by the AT1R blocker losartan. NADPH oxidase inhibition with apocynin prevented both the colocalization of AT1R with Cav1 and the induction of ERK1/2 by shear stress. Moreover, shear-dependent ERK1/2 activation was minimal in CHO cells expressing an AT1Ra mutant that does not internalize, compared with cells expressing wild-type AT1Ra (P<0.05). Hence, AT1R may be an important transducer of shear stress-dependent activation of ERK1/2.

Molecular weight of different angiotensin II polymers directly determines: density of endothelial membrane AT1 receptors and coronary vasoconstriction

We have shown that angiotensin II (Ang II) does not diffuse across the vessel wall, remaining intravascularly confined and acting solely on the coronary endothelial luminal membrane (CELM) receptors. A sustained intracoronary infusion of Ang II causes transient coronary vasoconstriction (desensitization) due to membrane internalization of CELM Ang II type 1 receptors (CELM-AT1R). In contrast, sustained intracoronary infusion of a non-diffusible polymer of Ang II (Ang II-Pol, 15,000 kDa) causes a sustained vasoconstriction by preventing CELM-AT1R internalization. In addition, a sustained intracoronary infusion of Ang II leads to a depressed response following a secondary Ang II administration (tachyphylaxis) that is reversed by Ang II-Pol. These findings led us to hypothesize that the rate of desensitization, tachyphylaxis, and AT1R internalization were dependent on Ang II-Pol molecular weight. To test this hypothesis, we synthesized Ang II-Pols of the following molecular weights (in kDa): 1.3, 2.7, 11, 47, 527, 3270 and 15,000. Vasoconstriction was measured following intracoronary infusion of Ang II-Pols in Langendorff-perfused guinea pig hearts at constant flow. The CELM protein fraction was extracted using the silica pellicle technique at different time points in order to determine the rate of AT1R internalization following each Ang II-Pol infusion. CELM-AT1R density was quantified by Western blot. We found that the rate of desensitization and the tachyphylaxis effect varied inversely with the molecular weight of the Ang II-Pols. Inversely proportional to the molecular weight of Ang II-Pol the CELM-AT1R density decreases over time. These results indicate that the mechanism responsible for the decreased rate of desensitization and tachyphylaxis by higher molecular weight Ang II polymers is due to reduction in the rate of CELM-AT1R internalization. These Ang II polymers would be valuable tools for studying the relationship between AT1R internalization and physiological effects.

Biased signaling of the angiotensin II type 1 receptor can be mediated through distinct mechanisms

BACKGROUND

Seven transmembrane receptors (7TMRs) can adopt different active conformations facilitating a selective activation of either G protein or β-arrestin-dependent signaling pathways. This represents an opportunity for development of novel therapeutics targeting selective biological effects of a given receptor. Several studies on pathway separation have been performed, many of these on the Angiotensin II type 1 receptor (AT1R). It has been shown that certain ligands or mutations facilitate internalization and/or recruitment of β-arrestins without activation of G proteins. However, the underlying molecular mechanisms remain largely unresolved. For instance, it is unclear whether such selective G protein-uncoupling is caused by a lack of ability to interact with G proteins or rather by an increased ability of the receptor to recruit β-arrestins. Since uncoupling of G proteins by increased ability to recruit β-arrestins could lead to different cellular or in vivo outcomes than lack of ability to interact with G proteins, it is essential to distinguish between these two mechanisms.

METHODOLOGY/PRINCIPAL FINDINGS

We studied five AT1R mutants previously published to display pathway separation: D74N, DRY/AAY, Y292F, N298A, and Y302F (Ballesteros-Weinstein numbering: 2.50, 3.49-3.51, 7.43, 7.49, and 7.53). We find that D74N, DRY/AAY, and N298A mutants are more prone to β-arrestin recruitment than WT. In contrast, receptor mutants Y292F and Y302F showed impaired ability to recruit β-arrestin in response to Sar1-Ile4-Ile8 (SII) Ang II, a ligand solely activating the β-arrestin pathway.

CONCLUSIONS/SIGNIFICANCE

Our analysis reveals that the underlying conformations induced by these AT1R mutants most likely represent principally different mechanisms of uncoupling the G protein, which for some mutants may be due to their increased ability to recruit β-arrestin2. Hereby, these findings have important implications for drug discovery and 7TMR biology and illustrate the necessity of uncovering the exact molecular determinants for G protein-coupling and β-arrestin recruitment, respectively.

Repeated administration of angiotensin II reduces its dipsogenic effect without affecting saline intake

Angiotensin II (Ang II) acts at central type 1 (AT(1)) receptors to increase intake of water and saline. In vitro studies demonstrated rapid desensitization of the AT(1) receptor after Ang II exposure, and behavioural studies in rats suggest that exposure to Ang II decreases the dipsogenic potency of subsequent Ang II. Nevertheless, the effect of repeated Ang II injections on saline intake remains untested, and a reliable protocol for examining this purported behavioural desensitization has not emerged from the literature. To address these issues, we established a reliable approach to study Ang II-induced dipsetic desensitization and used this approach to test the requirement of central AT(1) receptors and the specificity of the effect for water intake. Rats given a treatment regimen of three injections of Ang II (300 ng, intracerebroventricular), each separated by 20 min, drank less water than control rats after a subsequent test injection of Ang II. The effect was relatively short lasting, dependent on the dose and timing of Ang II, and was almost completely blocked by the AT(1) receptor antagonist losartan. In further testing, when rats were given access to both water and 1.5% saline, animals that received an Ang II treatment regimen drank less water than control animals, but saline intake was unaffected. These data support previous suggestions that Ang II-induced water and saline intakes are separable. Given the role of G protein uncoupling in desensitization of the AT(1) receptor, these data are consistent with the emerging hypothesis that AT(1) receptor G protein-dependent intracellular signalling pathways are more relevant for water, but not saline, intake.

TRPV1 gene deficiency attenuates miniature EPSC potentiation induced by mannitol and angiotensin II in supraoptic magnocellular neurons

The release of arginine vasopressin (AVP) from the magnocellular neurosecretory cells (MNCs) in the supraoptic nucleus (SON) is crucial for body fluid homeostasis. The MNC activity is modulated by synaptic inputs and humoral factors. A recent study demonstrated that an N-terminal splice variant of the transient receptor potential vanilloid type 1 (TRPV1) is essential for osmosensory transduction in the SON. In the present study, we examined the effects of mannitol and angiotensin II on miniature EPSCs (mEPSCs) in the supraoptic MNCs using whole-cell patch-clamp recording in in vitro slice preparation. Mannitol (60 mm) and angiotensin II (0.1 microm) increased the frequency of mEPSCs without affecting the amplitude. These effects were attenuated by pre-exposure to a nonspecific TRPV channel blocker, ruthenium red (10 microm) and enhanced by pre-exposure to cannabinoid type1 receptor antagonist, AM251 (2 microm). Mannitol-induced potentiation of mEPSCs was not attenuated by angiotensin II receptor antagonist, losartan (10 microm), indicating independent pathways of mannitol and angiotensin II to the TRPV channels. The potentiation of mEPSCs by mannitol was not mimicked by a TRPV1 agonist, capsaicin, and also not attenuated by TRPV1 blockers, capsazepine (10 microm). PKC was involved in angiotensin II-induced potentiation of mEPSCs. The effects of mannitol and angiotensin II on the supraoptic MNCs in trpv1 knock-out mice were significantly attenuated compared with those in wild-type mice counterparts. The results suggest that hyperosmotic stimulation and angiotensin II independently modulate mEPSCs through capsaicin-insensitive TRPV1 channel in the presynaptic terminals of the SON.

Atrap deficiency increases arterial blood pressure and plasma volume

The angiotensin receptor-associated protein (Atrap) interacts with angiotensin II (AngII) type 1 (AT1) receptors and facilitates their internalization in vitro, but little is known about the function of Atrap in vivo. Here, we detected Atrap expression in several organs of wild-type mice; the highest expression was in the kidney where it localized to the proximal tubule, particularly the brush border. There was no Atrap expression in the renal vasculature or juxtaglomerular cells. We generated Atrap-deficient (Atrap-/-) mice, which were viable and seemed grossly normal. Mean systolic BP was significantly higher in Atrap-/- mice compared with wild-type mice. Dose-response relationships of arterial BP after acute AngII infusion were similar in both genotypes. Plasma volume was significantly higher and plasma renin concentration was markedly lower in Atrap-/- mice compared with wild-type mice. (125)I-AngII binding showed enhanced surface expression of AT1 receptors in the renal cortex of Atrap-/- mice, accompanied by increased carboanhydrase-sensitive proximal tubular function. In summary, Atrap-/- mice have increased arterial pressure and plasma volume. Atrap seems to modulate volume status by acting as a negative regulator of AT1 receptors in the renal tubules.

Type 1 angiotensin receptor (AT1-R)-mediated decrease in type 2 angiotensin receptor mRNA level is dependent on Gq and extracellular signal-regulated kinase 1//2 in AT1-R-transfected PC12 cells

Angiotensin II (Ang II) functions through two major Ang II receptor subtypes, type 1 (AT1-R) and type 2 (AT2-R), both of which are classified to be G protein-coupled receptors. AT2-R is highly expressed at the fetal stage, and in heart remodelling and brain ischaemia; therefore, it is important to clarify the regulatory mechanisms of AT2-R expression. Although AT1-R is generally believed to modulate AT2-R expression in some tissues or cells, the exact mechanism remains to be clarified. In the present study, we examined the effect of AT1-R stimulation on expression of endogenous rat AT2-R (rAT2-R) in AT1-R-transfected PC12 cells. rAT2-R mRNA and protein expression were decreased by Ang II in PC12 cells transfected with rAT1A-R in a time-dependent manner, mediated through a decline in mRNA stability. The C-terminus of G protein-coupled receptor (GPCR) is important for GPCR-mediated signal transduction. Therefore, we used C-terminus-deleted human AT1-R (hAT1-327STOP), which is thought to be a nondesensitised mutant of hAT1-R. As a result, Ang II decreased rAT2-R mRNA expression to a greater extent in cells transfected with hAT1-327STOP than with wild-type hAT1-R. The decrease was completely reversed by AT1-R antagonist candesartan, G(q) inhibitor YM254980, and mitogen-activated protein kinase (MAPK) kinase 1/2 inhibitor U0126, but not by pertussis toxin, which uncouples the receptor with G(i), or p38 MAPK inhibitor SB203580. We suggest, possibly for the first time, that the hAT1-R/G(q)/extracellular signal-regulated kinase 1/2 pathway is involved in the down-regulation of AT2-R using PC12 cells transfected with AT1-R.

Distinct motifs of neuropeptide Y receptors differentially regulate trafficking and desensitization

Activated human neuropeptide Y Y(1) receptors rapidly desensitize and internalize through clathrin-coated pits and recycle from early and recycling endosomes, unlike Y(2) receptors that neither internalize nor desensitize. To identify motifs implicated in Y(1) receptor desensitization and trafficking, mutants with varying C-terminal truncations or a substituted Y(2) C-terminus were constructed. Point mutations of key putative residues were made in a C-terminal conserved motif [phi-H-(S/T)-(E/D)-V-(S/T)-X-T] that we have identified and in the second intracellular i2 loop. Receptors were analyzed by functional assays, spectrofluorimetric measurements on living cells, flow cytometry, confocal imaging and bioluminescence resonance energy transfer assays for beta-arrestin activation and adaptor protein (AP-2) complex recruitment. Inhibitory GTP-binding protein-dependent signaling of Y(1) receptors to adenylyl cyclase and desensitization was unaffected by C-terminal truncations or mutations, while C-terminal deletion mutants of 42 and 61 amino acids no longer internalized. Substitutions of Thr357, Asp358, Ser360 and Thr362 by Ala in the C-terminus abolished both internalization and beta-arrestin activation but not desensitization. A Pro145 substitution by His in an i2 consensus motif reported to mediate phosphorylation-independent recruitment of beta-arrestins affected neither desensitization, internalization or recycling kinetics of activated Y(1) receptors nor beta-arrestin activation. Interestingly, combining Pro145 substitution by His and C-terminal substitutions significantly attenuates Y(1) desensitization. In the Y(2) receptor, replacement of His155 with Pro at this position in the i2 loop motif promotes agonist-mediated desensitization, beta-arrestin activation, internalization and recycling. Overall, our results indicate that beta-arrestin-mediated desensitization and internalization of Y(1) and Y(2) receptors are differentially regulated by the C-terminal motif and the i2 loop consensus motif.

Gain-of-function mutant of angiotensin II receptor, type 1A, causes hypertension and cardiovascular fibrosis in mice

The role of the renin-angiotensin system has been investigated by overexpression or inactivation of its different genes in animals. However, there is no data concerning the effect of the constitutive activation of any component of the system. A knockin mouse model has been constructed with a gain-of-function mutant of the Ang II receptor, type 1A (AT(1A)), associating a constitutively activating mutation (N111S) with a C-terminal deletion, which impairs receptor internalization and desensitization. In vivo consequences of this mutant receptor expression in homozygous mice recapitulate its in vitro characteristics: the pressor response is more sensitive to Ang II and longer lasting. These mice present with a moderate (~20 mmHg) and stable increase in BP. They also develop early and progressive renal fibrosis and cardiac fibrosis and diastolic dysfunction. However, there was no overt cardiac hypertrophy. The hormonal parameters (low-renin and inappropriately normal aldosterone productions) mimic those of low-renin human hypertension. This new model reveals that a constitutive activation of AT(1A) leads to cardiac and renal fibrosis in spite of a modest effect on BP and will be useful for investigating the role of Ang II in target organs in a model similar to some forms of human hypertension.

The AT1A receptor "gain-of-function" mutant N111S/delta329 is both constitutively active and hyperreactive to angiotensin II

The renin-angiotensin-aldosterone system (RAAS) is central to cardiovascular and renal physiology. However, there is no animal model in which the activation of the RAAS only reflects the activation of the angiotensin II (ANG II) AT1 receptor. As a first step to developing such a model, we characterized a gain-of-function mutant of the mouse AT1A receptor. This mutant carries two mutations: N111S predicted to activate the receptor constitutively and a COOH-terminal deletion, delta329, expected to reduce receptor internalization and desensitization. We expressed this double mutant (AT1A-N111S/delta329) in heterologous cells. It showed a pharmacological profile consistent with that of other constitutively active mutants. Furthermore, it increased basal production of inositol phosphates, as well as basal cytosolic and nuclear ERK activities. Basal proliferation of cells expressing the mutant was also greater than that of the wild type. The double mutant was poorly internalized and failed to recruit beta-arrestin 2 in the presence of ANG II. It also showed hypersensitive and hyperreactive responses to ANG II for both inositol phosphate production and ERK activation. The additivity of the phenotypes of the two mutations makes this mutant an appropriate candidate to test the physiological consequences of the AT1A receptor activation itself in transgenic animal models.

Structural determinants of agonist-induced signaling and regulation of the angiotensin AT1 receptor

Angiotensin II (Ang II) regulates aldosterone secretion by stimulating inositol phosphate production and Ca(2+) signaling in adrenal glomerulosa cells via the G(q)-coupled AT(1) receptor, which is rapidly internalized upon agonist binding. Ang II also binds to the heptahelical AT(2) receptor, which neither activates inositol phosphate signaling nor undergoes receptor internalization. The differential behaviors of the AT(1) and AT(2) receptors were analyzed in chimeric angiotensin receptors created by swapping the second (IL2), the third (IL3) intracellular loops and/or the cytoplasmic tail (CT) between these receptors. When transiently expressed in COS-7 cells, the chimeric receptors showed only minor alterations in their ligand binding properties. Measurements of the internalization kinetics and inositol phosphate responses of chimeric AT(1A) receptors indicated that the CT is required for normal receptor internalization, and IL2 is a determinant of G protein activation. In addition, the amino-terminal portion of IL3 is required for both receptor functions. However, only substitution of IL2 impaired Ang II-induced ERK activation, suggesting that alternative mechanisms are responsible for ERK activation in signaling-deficient mutant AT(1) receptors. Substitution of IL2, IL3, or CT of the AT(1A) receptor into the AT(2) receptor sequence did not endow the latter with the ability to internalize or to mediate inositol phosphate signaling responses. These data suggest that the lack of receptor internalization and inositol phosphate signal generation by the AT(2) receptor is a consequence of its different activation mechanism, rather than the inability of its cytoplasmic domains to couple to intracellular effectors.

Angiotensin and calcium signaling in the pituitary and hypothalamus

1) In the rat pituitary, angiotensin type 1B receptors (AT1B) are located in lactotrophs and corticotrophs. 2) Activation of AT1B receptors are coupled to Gq/11 (Guanine protein coupled receptor, or GPCR); they increase phospholipase beta C (PLC) activity resulting in inositol 1,4,5 triphosphate (InsP3) and diacylglycerol (DAG) formation. A biphasic increase in [Ca2+]i triggered by InsP3 and DAG ensues. 3) As many GPCRs, AT1B pituitary receptors rapidly desensitize. 4) This was observed in the generation of InsP3, the mobilization of intracellular Ca(2+), and in prolactin release. Both homologous and heterologous desensitization was evidenced. 5) Desensitization of the angiotensin II type 1 (AT1) receptor in the pituitary shares similarities and differences with endogenously expressed or transfected AT1 receptors in different cell types. 6) In the pituitary hyperplasia generated by chronic estrogen treatment there was desensitization or alteration in angiotensin II (Ang II) evoked intracellular Ca2+ increase, InsP3 generation, and prolactin release. This correlates with a downregulation of AT1 receptors. 7) In particular, in hyperplastic cells Ang II failed to evoke a transient acute peak in [Ca2+]i, which was replaced by a persistent plateau phase of [Ca2+]i increase. 8) Different calcium channels participate in Ang II induced [Ca2+]i increase in control and hyperplastic cells. While spike phase in control cells is dependent on intracellular stores sensitive to thapsigargin, in hyperplastic cells plateau increase is dependent on extracellular calcium influx. 9) Signal transduction of the AT1 pituitary receptor is greatly modified by hyperplasia, and it may be an important mechanism in the control of the hyperplastic process. 10) In the hypothalamus and brain stem there is a predominant expression of AT1A and AT2 mRNA. 11) Ang II acts at specific receptors located on neurons in the hypothalamus and brain stem to elicit alterations in blood pressure, fluid intake, and hormone secretion. 12) Calcium channels play important roles in the Ang II induced behavioral and endocrine responses. 13) Ang II, in physiological concentrations, can activate AT1 receptors to stimulate both Ca2+ release from intracellular stores and Ca2+ influx from the extracellular space to increase [Ca2+]i in polygonal and stellate astroglia of the hypothalamus and brain stem. 14) In primary cell culture of neurons from newborn rat hypothalamus and brain stem, it has also been determined that Ang II elicits an AT1 receptor mediated inhibition of delayed rectifier K(+) current and a stimulation of Ca2+ current. 15) In primary cell cultures derived from the subfornical organ or the organum vasculosum laminae terminalis of newborn rat pups, Ang II produced a pronounced desensitization of the [Ca2+]i response. 16) Hypothalamic and pituitary Ang II systems are involved in different functions, some of which are related. At both levels Ang II signals through [Ca2+]i in a characteristic way.

Desensitization of angiotensin II: effect on [Ca2+]i, inositol triphosphate, and prolactin in pituitary cells

Activation of pituitary angiotensin (ANG II) type 1 receptors (AT1) mobilizes intracellular Ca2+, resulting in increased prolactin secretion. We first assessed desensitization of AT1 receptors by testing ANG II-induced intracellular Ca2+ concentration ([Ca2+](i)) response in rat anterior pituitary cells. A period as short as 1 min with 10(-7) M ANG II was effective in producing desensitization (remaining response was 66.8 +/- 2.1% of nondesensitized cells). Desensitization was a concentration-related event (EC(50): 1.1 nM). Although partial recovery was obtained 15 min after removal of ANG II, full response could not be achieved even after 4 h (77.6 +/- 2.4%). Experiments with 5 x 10(-7) M ionomycin indicated that intracellular Ca2+ stores of desensitized cells had already recovered when desensitization was still significant. The thyrotropin-releasing hormone (TRH)-induced intracellular Ca2+ peak was attenuated in the ANG II-pretreated group. ANG II pretreatment also desensitized ANG II- and TRH-induced inositol phosphate generation (72.8 +/- 3.5 and 69.6 +/- 6.1%, respectively, for inositol triphosphate) and prolactin secretion (53.4 +/- 2.3 and 65.1 +/- 7.2%), effects independent of PKC activation. We conclude that, in pituitary cells, inositol triphosphate formation, [Ca2+](i) mobilization, and prolactin release in response to ANG II undergo rapid, long-lasting, homologous and heterologous desensitization.

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 signal transduction pathways

It has been 100 years since the discovery of renin by Tigerstedt and Bergman. Since that time, numerous discoveries have advanced our understanding of the renin-angiotensin system, including the observation that angiotensin II is the effector molecule of this system. A remarkable aspect of angiotensin II is the many different physiological responses this simple peptide induces in different cell types. Here, we focus on the signal transduction pathways that are activated as a consequence of angiotensin II binding to the AT1 receptor. Classical signaling pathways such as the activation of heterotrimeric G proteins by the AT1 receptor are discussed. In addition, recent work examining the role of tyrosine phosphorylation in angiotensin II-mediated signal transduction is also examined. Understanding how these distinct signaling pathways transduce signals from the cell surface will advance our understanding of how such a simple molecule elicits such a wide variety of specific cellular responses.