Receptor-mediated endocytosis of angiotensin II in rat myometrial cells

Endocytosis and signaling of angiotensin II type 1 receptor

A vasoactive octapeptide angiotensin II (Ang II) hormone is the key regulator of the renin-angiotensin system (RAS). It binds with the two different plasma membrane receptors like angiotensin II type 1 (AT1) and type 2 (AT2) and consequence various biological responses occur. Further, AT1 has two subtypes such as AT1A and AT1B. These angiotensin receptors are classified to be G protein-coupled receptors (GPCRs). The main constituent of RAS is the AT1 receptor (AT1R), and its activation, signal transduction, and regulation have been extensively studied. After Ang II stimulation, the ligand-receptor complexes internalized and trafficked through the early endosome, recycling endosome, and some receptors skipped the recycling endosome and trafficked to the lysosome for metabolic degradation. Moreover, some short sequence motifs located in the carboxyl-terminus (CT) of the receptor play a vital role in the internalization, phosphorylation, subcellular trafficking, signaling, and desensitization. Furthermore, in endocytosis, the various proteins interact with the CT region of the receptor. This chapter highlights the basic mechanism of AT1 receptor internalization, trafficking and signaling in both physiological and pathophysiological conditions.

Live Cell Imaging and 3D Analysis of Angiotensin Receptor Type 1a Trafficking in Transfected Human Embryonic Kidney Cells Using Confocal Microscopy

Live-cell imaging is used to simultaneously capture time-lapse images of angiotensin type 1a receptors (AT1aR) and intracellular compartments in transfected human embryonic kidney-293 (HEK) cells following stimulation with angiotensin II (Ang II). HEK cells are transiently transfected with plasmid DNA containing AT1aR tagged with enhanced green fluorescent protein (EGFP). Lysosomes are identified with a red fluorescent dye. Live-cell images are captured on a laser scanning confocal microscope after Ang II stimulation and analyzed by software in three dimensions (3D, voxels) over time. Live-cell imaging enables investigations into receptor trafficking and avoids confounds associated with fixation, and in particular, the loss or artefactual displacement of EGFP-tagged membrane receptors. Thus, as individual cells are tracked through time, the subcellular localization of receptors can be imaged and measured. Images must be acquired sufficiently rapidly to capture rapid vesicle movement. Yet, at faster imaging speeds, the number of photons collected is reduced. Compromises must also be made in the selection of imaging parameters like voxel size in order to gain imaging speed. Significant applications of live-cell imaging are to study protein trafficking, migration, proliferation, cell cycle, apoptosis, autophagy and protein-protein interaction and dynamics, to name but a few.

Engineering knottins as novel binding agents

Cystine-knot miniproteins, also known as knottins, contain a conserved core of three tightly woven disulfide bonds which impart extraordinary thermal and proteolytic stability. Interspersed between their conserved cysteine residues are constrained loops that possess high levels of sequence diversity among knottin family members. Together these attributes make knottins promising molecular scaffolds for protein engineering and translational applications. While naturally occurring knottins have shown potential as both diagnostic agents and therapeutics, protein engineering is playing an important and increasing role in creating designer molecules that bind to a myriad of biomedical targets. Toward this goal, rational and combinatorial approaches have been used to engineer knottins with novel molecular recognition properties. Here, methods are described for creating and screening knottin libraries using yeast surface display and fluorescence-activated cell sorting. Protocols are also provided for producing knottins by synthetic and recombinant methods, and for measuring the binding affinity of knottins to target proteins expressed on the cell surface.

Intracellular angiotensin II activates rat myometrium

Angiotensin II is a modulator of myometrial activity; both AT(1) and AT(2) receptors are expressed in myometrium. Since in other tissues angiotensin II has been reported to activate intracellular receptors, we assessed the effects of intracellular administration of angiotensin II via microinjection on myometrium, using calcium imaging. Intracellular injection of angiotensin II increased cytosolic Ca(2+) concentration ([Ca(2+)](i)) in myometrial cells in a dose-dependent manner. The effect was abolished by the AT(1) receptor antagonist losartan but not by the AT(2) receptor antagonist PD-123319. Disruption of the endo-lysosomal system, but not that of Golgi apparatus, prevented the angiotensin II-induced increase in [Ca(2+)](i). Blockade of AT(1) receptor internalization had no effect, whereas blockade of microautophagy abolished the increase in [Ca(2+)](i) produced by intracellular injection of angiotensin II; this indicates that microautophagy is a critical step in transporting the peptide into the endo-lysosomes lumenum. The response to angiotensin II was slightly reduced in Ca(2+)-free saline, indicating a major involvement of Ca(2+) release from internal stores. Blockade of inositol 1,4,5-trisphosphate (IP(3)) receptors with heparin and xestospongin C or inhibition of phospholipase C (PLC) with U-73122 abolished the response to angiotensin II, supporting the involvement of PLC-IP(3) pathway. Angiotensin II-induced increase in [Ca(2+)](i) was slightly reduced by antagonism of ryanodine receptors. Taken together, our results indicate for the first time that in myometrial cells, intracellular angiotensin II activates AT(1)-like receptors on lysosomes and activates PLC-IP(3)-dependent Ca(2+) release from endoplasmic reticulum; the response is further augmented by a Ca(2+)-induced Ca(2+) release mechanism via ryanodine receptors activation.

Methyl-β-cyclodextrin Prevents Angiotensin II-Induced Tachyphylactic Contractile Responses in Rat Aorta

Tachyphylaxis or desensitization is frequently observed following angiotensin II type I (AT1) receptor activation by angiotensin II. One of the possible mechanisms contributing to receptor desensitization involves receptor internalization. In addition to clathrin-coated pits/vesicles, caveolae, small invaginations in the plasma membrane rich in cholesterol, may also be involved in receptor internalization. After activation, AT1 receptor partially redistributes to lipid-enriched domains. We hypothesize that AT1 receptor internalization via caveolae contributes to the tachyphylactic response observed to angiotensin II. Endothelium-denuded rat aortic rings were exposed to increasing concentrations of angiotensin II or phenylephrine, generating two cumulative concentration-effect curves (CCEC) with a 90-min interval separating each curve (CCEC-I and CCEC-II). CCEC-II was performed in the presence of either vehicle or methyl-beta-cyclodextrin (CD), a drug that depletes cholesterol from the membrane and disassembles caveolae. CCEC-II to angiotensin II, but not to phenylephrine, was blunted in aortic rings treated with vehicle. In the presence of CD, CCEC-II did not differ significantly from CCEC-I for both agonists. CCEC-I to angiotensin II was abolished when in the presence of the AT1 receptor antagonist. The presence of AT1 receptors at the aortic smooth muscle cells' membrane treated with angiotensin II was observed by immunofluorescence only in the presence of CD. In addition, caveolin-1 coimmunoprecipitated with AT1 receptor after agonist stimulation, and this interaction was inhibited by CD. Our data suggest that caveolae are involved in the tachyphylactic contractile response induced by angiotensin II in rat aorta, and this effect is related to receptor internalization.

Estrogen regulates adrenal angiotensin type 1 receptors by modulating adrenal angiotensin levels

Estrogen inhibits adrenal angiotensin type 1 receptor (AT(1)R) binding sites and attenuates the adrenal responsivity to angiotensin II (Ang II). Ang II modulates AT(1)R expression. Here, we determined if estrogen-induced down-regulation of adrenal AT(1)Rs involves modulation of adrenal Ang II. Female rats were ovariectomized (OVX) and injected with 17beta-estradiol benzoate (E(2); 40 micro g/kg) or vehicle for 7 d. Adrenal Ang II was separated from other angiotensin peptides by HPLC and measured by RIA. Scatchard analysis of radioligand binding curves showed that E(2) or captopril (Cap; 0.5 g/liter water) significantly reduced adrenal AT(1)R binding (maximum binding capacity) by 22% and 19%, respectively, compared with OVX (276 +/- 2.09 fmol/mg protein). E(2) and Cap lowered adrenal Ang II levels by 39% and 21%, respectively, compared with OVX (4.10 +/- 0.44 pmol/g). E(2) caused no further reductions in adrenal AT(1)R binding or in Ang II levels in Cap-treated OVX rats. High-dose Ang II infusion (1000 ng/kg.min) increased adrenal Ang II levels by 71% and lowered AT(1)R binding by 18%. Under these infusion conditions, E(2) did not reduce adrenal Ang II or AT(1)R binding. No differences in AT(1)R affinity (dissociation constant) were observed among groups. These data suggest that E(2) regulates the number of adrenal AT(1)R binding sites indirectly by modulating adrenal Ang II.

In vitro evidence for an intracellular site of angiotensin action

To differentiate the relative effects of nuclear and cell surface angiotensin II (Ang II) receptors, we mutated the angiotensinogen cDNA by removing the signal sequence-encoding region to produce a nonsecreted form of angiotensinogen [Ang(-S)Exp]. Rat hepatoma cells (which produce renin and angiotensin-converting enzyme mRNAs) were stably transfected with Ang(-S)Exp/pSVL (or a corresponding control) expression plasmid, and mitotic indices were measured for stably transfected cell lines. Experimental clonal cell lines demonstrate an average of 33+/-4.4% (P<0.001) increase in percentage-labeled nuclei compared with control cell lines. The mitogenic effect is blocked by 10(-6) mol/L losartan and by 1 micromol/L renin antisense phosphorothioate oligomers but not by 10(-6) mol/L candesartan. In addition, phenylarsine oxide, which blocks angiotensin receptor internalization, abolishes the losartan inhibitory effect, suggesting that after cell-surface receptor-mediated endocytosis, losartan blocks Ang II nuclear receptors. PDGF mRNA levels are elevated 2.2-fold in Ang(-S)Exp transfected cell lines; addition of anti-PDGF antibodies to the culture medium partially blocks the mitogenic effect of Ang(-S)Exp, while anti-Ang II antibodies have no effect. These results suggest that the Ang(-S)Exp growth effect is due, in part, to autocrine/paracrine stimulation by secreted PDGF after Ang II/Ang II receptor intracellular interactions. We further demonstrate that these cells produce the alternative renin transcript, renin 1A, which apparently lacks a signal sequence and is maintained intracellularly. Collectively, these studies of cultured cells suggest that some cell types may possess components of the renin-angiotensin system that permit intracellular processing of angiotensinogen to Ang II and that Ang II generated intracellularly may be mitogenic.

The role of G protein activation in the toxicity of amyloidogenic Abeta-(1-40), Abeta-(25-35), and bovine calcitonin

More than 16 different proteins have been identified as amyloid in clinical diseases; among these, beta-amyloid (Abeta) of Alzheimer's disease is the best characterized. In the present study, we performed experiments with Abeta and calcitonin, another amyloid-forming peptide, to examine the role of G protein activation in amyloid toxicity. We demonstrated that the peptides, when prepared under conditions that promoted beta-sheet and amyloid fibril (or protofibril) formation, increased high affinity GTPase activity, but the nonamyloidogenic peptides had no discernible effects on GTP hydrolysis. These increases in GTPase activity were correlated to toxicity. In addition, G protein inhibitors significantly reduced the toxic effects of the amyloidogenic Abeta and calcitonin peptides. Our results further indicated that the amyloidogenic peptides significantly increased GTPase activity of purified Galpha(o) and Galpha(i) subunits and that the effect was not receptor-mediated. Collectively, these results imply that the amyloidogenic structure, regardless of the actual peptide or protein sequence, may be sufficient to cause toxicity and that toxicity is mediated, at least partially, through G protein activation. Our abilities to manipulate G protein activity may lead to novel treatments for Alzheimer's disease and the other amyloidoses.

Cutting edge: detection of antigen-specific CD4+ T cells by HLA-DR1 oligomers is dependent on the T cell activation state

Class I MHC tetramers have proven to be invaluable tools for following and deciphering the CD8(+) T cell response, but the development of similar reagents for detection of CD4(+) T cells based on class II MHC proteins has been more difficult. We evaluated fluorescent streptavidin-based oligomers of HLA-DR1 for use as reagents to analyze Ag-specific human CD4(+) T cells. Staining was blocked at low temperatures and by drugs that disrupt microfilament formation and endocytosis. Cell-associated MHC oligomers were resistant to a surface stripping protocol and were observed by microscopy in intracellular compartments. This behavior indicates that detection of CD4(+) T cells using class II MHC oligomers can depend on an active cellular process in which T cells cluster and/or endocytose their Ag receptors. T cells of identical specificity but in different activation states varied greatly in their ability to be detected by class II MHC oligomers.

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 receptor internalization and signaling in isolated rat hepatocytes

Since angiotensin II (Ang II)-induced receptor internalization is required to maintain the production of certain intracellular signals in some target cells, we investigated the relationships between Ang II receptor endocytosis and the generation of second messengers in rat hepatocytes. The results of the present study demonstrate that in response to exposure of hepatocytes to Ang II, a decrease in surface Ang II receptors occurred, consistent with a rapid endocytosis of the receptor-bound hormone complex. Pretreatment of cells with okadaic acid (OA) did not have any effect on receptor-mediated internalization. In contrast, a marked reduction of the Ang II receptor endocytosis process occurred after treatment of hepatocytes with phenylarsine oxide (PAO), indicating that cysteine residues could be involved in receptor-mediated endocytosis. Stimulation of cells with Ang II blocked the generation of cyclic adenosine monophosphate (cAMP), which follows the stimulation of hepatocytes with forskolin. Moreover, Ang II increased both inositol 4,5-bisphosphate (IP2) and inositol 1,4,5-trisphosphate (IP3) generation, and enhanced intracellular calcium concentration ([Ca2+]i). Exposure of cells to PAO did not alter the effect of Ang II on the accumulation of cAMP after forskolin stimulation, indicating that endocytosis of the agonist-receptor complex is not involved in adenylate cyclase inhibition. Conversely, PAO and OA markedly reduced IP2 and IP3 synthesis, and the plateau phase of Ang II-induced Ca2+ mobilization. The relationship between Ang II-induced endocytosis and the generation of phosphoinositols and increment in [Ca2+]i indicates that sequestration of the Ang II receptor is necessary to maintain the production of these intracellular signals in rat hepatocytes.

Transport of the delta-opioid receptor agonist [D-penicillamine2,5] enkephalin across the blood-brain barrier involves transcytosis1

The delta opioid receptor antagonist [D-penicillamine2,5]enkephalin (DPDPE) is an enzymatically stable peptide analogue of Met-enkephalin. DPDPE uses a saturable transport mechanism to cross the blood-brain barrier (BBB), though the exact mechanism is not fully understood. The aim of the present study was to identify the mechanism by which DPDPE enters the brain. The effect of phenylarsine oxide (PAO), an endocytosis inhibitor, on the transport of [3H]DPDPE was investigated using both in vitro and in situ transport studies. Two in vitro models of the BBB utilizing primary bovine brain microvascular endothelial cells (BBMEC) were studied. [3H]DPDPE permeability across monolayers of BBMEC grown on polycarbonate filters was studied. PAO significantly reduced the permeability of [3H]DPDPE across the monolayer. PAO also reduced the uptake of [3H]DPDPE into BBMEC cells, without affecting binding to the cells. The in situ perfusion model of the BBB was also studied, PAO reduced DPDPE uptake by the brain in a dose-dependent manner. These studies indicate that DPDPE enters the brain via an energy-dependent transcytotic mechanism.

The human placental renin-angiotensin system

The human placenta and related tissues are considered to be examples of the recently accepted local renin-angiotensin systems (RAS). The brain is another example of a system that is thought to be regulated independently of the kidney and the role of angiotensin within the CNS as a neural mediator has drawn considerable attention. It has been known for a long time that many of the neuroendocrine mediators and receptors are expressed in the placenta and it has been suggested that there are many parallels between the classical neuroendocrine system and the placental one. The present review summarizes information that components of the RAS are expressed in uteroplacental tissues, are regulated by endogenous substances, and have important biological functions within this reproductive system. A comparison of similarities and differences between the classical and the placental RAS may provide clues to functions in other endocrine and neuroendocrine systems. The major components of the placental RAS that are considered are renin, prorenin, angiotensin I, angiotensin II, angiotensin converting enzyme (ACE), angiotensin receptors, and angiotensinogen (renin substrate). The factors that regulate these components at the cellular and the nuclear level are described. It is concluded that prorenin via angiotensin-dependent and angiotensin-independent mechanisms influences functions within uteroplacental tissues. Some of these actions are direct and others are mediated by the release of different signalling molecules. These features are similar to many neuroendocrine systems and utilize some of the same messengers.

Receptor-mediated angiotensin II transcytosis by brain microvessel endothelial cells

Angiotensin II (Ang II) uptake and transport across monolayers of bovine brain microvessel endothelial cells (BMECs) was demonstrated. Ang II transport was linear up to 2 h, saturable with a K(m) of 1.7 nM, and tended to be polarized with the apical-to-basolateral transport being greater. [3H]Ang II transport was found to be inhibited by excess unlabeled Ang II, by the Ang II analog sarathrin, and by the endocytic inhibitor phenylarsine oxide. Ang II-(2-8) and-(3-8) were shown to significantly increase the transport of Ang II. These results demonstrate for the first time the receptor-mediated transcytosis of Ang II across brain microvessel endothelium.