Agonist-induced endocytosis and signal generation in adrenal glomerulosa cells. A potential mechanism for receptor-operated calcium entry

Angiotensin II receptors and peritoneal dialysis-induced peritoneal fibrosis

The vasoactive hormone angiotensin II initiates its major hemodynamic effects through interaction with AT1 receptors, a member of the class of G protein-coupled receptors. Acting through its AT1R, angiotensin II regulates blood pressure and renal salt and water balance. Recent evidence points to additional pathological influences of activation of AT1R, in particular inflammation, fibrosis and atherosclerosis. The transcription factor nuclear factor κB, a key mediator in inflammation and atherosclerosis, can be activated by angiotensin II through a mechanism that may involve arrestin-dependent AT1 receptor internalization. Peritoneal dialysis is a therapeutic modality for treating patients with end-stage kidney disease. The effectiveness of peritoneal dialysis at removing waste from the circulation is compromised over time as a consequence of peritoneal dialysis-induced peritoneal fibrosis. The non-physiological dialysis solution used in peritoneal dialysis, i.e. highly concentrated, hyperosmotic glucose, acidic pH as well as large volumes infused into the peritoneal cavity, contributes to the development of fibrosis. Numerous trials have been conducted altering certain components of the peritoneal dialysis fluid in hopes of preventing or delaying the fibrotic response with limited success. We hypothesize that structural activation of AT1R by hyperosmotic peritoneal dialysis fluid activates the internalization process and subsequent signaling through the transcription factor nuclear factor κB, resulting in the generation of pro-fibrotic/pro-inflammatory mediators producing peritoneal fibrosis.

Regulation of aldosterone synthesis and secretion

Aldosterone is a steroid hormone synthesized in and secreted from the outer layer of the adrenal cortex, the zona glomerulosa. Aldosterone is responsible for regulating sodium homeostasis, thereby helping to control blood volume and blood pressure. Insufficient aldosterone secretion can lead to hypotension and circulatory shock, particularly in infancy. On the other hand, excessive aldosterone levels, or those too high for sodium status, can cause hypertension and exacerbate the effects of high blood pressure on multiple organs, contributing to renal disease, stroke, visual loss, and congestive heart failure. Aldosterone is also thought to directly induce end-organ damage, including in the kidneys and heart. Because of the significance of aldosterone to the physiology and pathophysiology of the cardiovascular system, it is important to understand the regulation of its biosynthesis and secretion from the adrenal cortex. Herein, the mechanisms regulating aldosterone production in zona glomerulosa cells are discussed, with a particular emphasis on signaling pathways involved in the secretory response to the main controllers of aldosterone production, the renin-angiotensin II system, serum potassium levels and adrenocorticotrophic hormone. The signaling pathways involved include phospholipase C-mediated phosphoinositide hydrolysis, inositol 1,4,5-trisphosphate, cytosolic calcium levels, calcium influx pathways, calcium/calmodulin-dependent protein kinases, diacylglycerol, protein kinases C and D, 12-hydroxyeicostetraenoic acid, phospholipase D, mitogen-activated protein kinase pathways, tyrosine kinases, adenylate cyclase, and cAMP-dependent protein kinase. A complete understanding of the signaling events regulating aldosterone biosynthesis may allow the identification of novel targets for therapeutic interventions in hypertension, primary aldosteronism, congestive heart failure, renal disease, and other cardiovascular disorders.

Regulation of human and pig renal Na(+),K (+)-ATPase activity by tyrosine phosphorylation of their alpha(1)-subunits

Modulation of the physiologically influential Na(+),K(+)-ATPase is a complex process involving a wide variety of factors. To determine the possible effects of the protein tyrosine phosphatase (PTP) inhibitors dephostatin and Et-3,4-dephostatin on human and pig, renal cells and enzymatic extracts, we treated our samples (15 min-24 h) with those PTP inhibitors (0-100 microM). PTP inhibitors were found to possess a concentration-dependent inhibition of Na(+),K(+)-ATPase activity in both human and pig samples. The inhibition was similarly demonstrated on all cellular, microsomal fraction and purified Na(+),K(+)-ATPase levels. Despite rigorous activity recovery attempts, the PTP inhibitors' effects were sustained on Na(+),K(+)-ATPase activity. Western blotting experiments revealed the expression of both alpha(1)- and beta(1)-subunits in both human and pig tissues. alpha(1)-Subunits possessed higher tyrosine phosphorylation levels with higher concentrations of PTP inhibitors. Meanwhile, serine/threonine residues of both alpha(1)- and beta(1)-subunits demonstrated diminished phosphorylation levels upon dephostatin treatment. Accordingly, we provide evidence that Na(+),K(+)-ATPase can be regulated through tyrosine phosphorylation of primarily their alpha(1)-subunits, using PTP inhibitors.

A megalin-like receptor is involved in protein endocytosis in the midgut of an insect (Bombyx mori, Lepidoptera)

The mechanism responsible for fluorescein isothiocyanate (FITC)-albumin internalization by columnar cells in culture obtained from the midgut of Bombyx mori larvae was examined by confocal laser scanning microscopy. Protein uptake changed over time, and it appeared to be energy dependent, since it was strongly reduced by both low temperatures and metabolic inhibitors. Labeled albumin uptake as a function of increasing protein concentration showed a saturation kinetics with a Michaelis constant value of 2.0 +/- 0.6 microM. These data are compatible with the occurrence of receptor-mediated endocytosis. RT-PCR analysis and colocalization experiments with an anti-megalin primary antibody indicated that the receptor involved was a putative homolog of megalin, the multiligand endocytic receptor belonging to the low-density lipoprotein receptor family, responsible for the uptake of various molecules, albumin included, in many epithelial cells of mammals. This insect receptor, like the mammalian counterpart, required Ca(2+) for albumin internalization and was inhibited by gentamicin. FITC-albumin internalization was clathrin mediated, since two inhibitors of this process caused a significant reduction of the uptake, and clathrin and albumin colocalized in the intermicrovillar areas of the apical plasma membrane. The integrity of actin and microtubule organization was essential for the correct functioning of the endocytic machinery.

Two independent forms of endocytosis maintain embryonic cell surface homeostasis during early development

Eukaryotic cells have multiple forms of endocytosis which maintain cell surface homeostasis. One explanation for this apparent redundancy is to allow independent retrieval of surface membranes derived from different types of vesicles. Consistent with this hypothesis we find that sea urchin eggs have at least two types of compensatory endocytosis. One is associated with retrieving cortical vesicle membranes, and formed large endosomes by a mechanism that was inhibited by agatoxin, cadmium, staurosporine and FK506. The second type is thought to compensate for constitutive exocytosis, and formed small endosomes using a mechanism that was insensitive to the above mentioned reagents, but was inhibited by phenylarsine oxide (PAO), and by microinjection of mRNA encoding Src kinase. Both mechanisms could act concurrently, and account for all of the endocytosis occurring during early development. Inhibition of either form did not trigger compensation by the other form, and phorbol ester treatment rescued the endocytotic activity blocked by agatoxin, but not the retrieval blocked by PAO.

The role of endocytosis in renal dopamine D1 receptor signaling

Desensitization of G-protein-coupled receptors (GPCR) includes receptor endocytosis. This phenomenon is suggested, at least for some receptors, to be associated with receptor resensitization. Here, we examined the role of receptor endocytosis for two different GPCR, the dopamine-1 (D1) receptor and the beta1-adrenoceptor (beta(1)-AR) in renal tissue. The functional role of receptor endocytosis was examined on Na+, K+ -ATPase activity in microdissected proximal tubules from rat kidney. The spatial regulation of endogenous D1 receptors and beta(1)-AR was examined by confocal microscopy techniques in LLCPK cells. Phenylarsine oxide (PAO) an endocytosis inhibitor, attenuated isoproterenol-induced decrease in Na+, K+ -ATPase activity but had no such effect on dopamine-induced decrease in Na+, K+ -ATPase activity. We have previously shown that isoproterenol sensitizes the renal dopamine system, by recruiting silent D1 receptors from the interior of the cell towards the plasma membrane. This effect was attenuated by PAO as well as by cytochalasin D while these substances had no effect on dopamine-induced D1 receptor recruitment. The beta(1)-AR was localized to the plasma membrane in control cells. Isoproterenol induced a rapid internalization of the beta(1)-AR; which was prevented by PAO. The results suggest that endocytosis of beta(1)-AR in renal proximal tubular cells is an important step in signal generation, while endocytosis of proximal tubular D1 receptor is not.

Activation of second messenger-dependent protein kinases induces muscarinic acetylcholine receptor desensitization in rat thyroid epithelial cells

Internalization and phosphorylation of G protein-coupled receptors (GPCR) are considered two important regulatory events of receptor signal transduction. In Fischer rat thyroid (FRT) epithelial cells, we have shown that muscarinic acetylcholine receptor (mAChR) stimulation induces intracellular Ca2+ mobilization via Ca2+ store release, capacitative Ca2+ entry and voltage-dependent Ca2+ channels activation. In the present study, the role of mAChR internalization and phosphorylation on receptor signalling pathway was examined by means of intracellular Ca2+ measurement in these cells. Exposure of FRT cells to carbachol (Cch), a mAChR agonist, resulted in a desensitization of receptor-mediated intracellular Ca2+ mobilization and induced the internalization of constitutively expressed mAChR in this cell type. Treatment of FRT cells with hypertonic sucrose, which markedly reduced agonist-receptor complex internalization, or phenylarsine oxide (PAO) diminished the Cch-induced intracellular Ca2+ response. Moreover, pretreatment of cells with phorbol-12-myristate-13-acetate (PMA), an activator of protein kinase C (PKC), completely abolished Cch-evoked Ca2+ mobilization, whereas it was significantly increased by the preincubation of cells with GF109203X, a selective inhibitor of PKC. We also found a marked decrease on Cch-stimulated Ca2+ mobilization in pretreated FRT cells with forskolin, an activator of protein kinase A (PKA), but the preincubation of cells with genistein, an inhibitor of protein tyrosine kinases, had no effect on Ca2+ mobilization induced by Cch. These findings seem to indicate that mAChR in FRT cells exhibit a desensitization, which may be mediated, at least in part, through activation of second messenger-dependent protein kinases and that receptor internalization could be necessary for signalling.

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.

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.

Independence of angiotensin II-induced MAP kinase activation from angiotensin type 1 receptor internalization in clone 9 hepatocytes

The agonist-induced internalization of several G protein-coupled receptors is an obligatory requirement for their activation of MAPKs. Studies on the relationship between endocytosis of the angiotensin II (Ang II) type 1 receptor (AT1-R) and Ang II-induced ERK1/2 activation were performed in clone 9 (C9) rat hepatic cells treated with inhibitors of endocytosis [sucrose, phenylarsine oxide (PAO), and concanavalin A]. Although Ang II-induced endocytosis of the AT1-R was prevented by sucrose and PAO, and was partially inhibited by concanavalin A, there was no impairment of Ang II-induced ERK activation. However, the specific epidermal growth factor receptor (EGF-R) kinase inhibitor, AG1478, abolished Ang II-induced activation of ERK1/2. Sucrose and PAO also inhibited EGFinduced internalization of the EGF-R in C9 cells, and the inability of these agents to impair EGF-induced ERK activation suggested that the latter is also independent of receptor endocytosis. In COS-7 cells transiently expressing the rat AT1A-R, Ang II also caused ERK activation through EGF-R transactivation. Furthermore, a mutant AT1A-R with truncated carboxyl terminus and impaired internalization retained full ability to activate ERK1/2 in response to Ang II stimulation. These findings demonstrate that Ang II-induced ERK1/2 activation in C9 hepatocytes is independent of both AT1-R and EGF-R endocytosis and is mediated by transactivation of the EGF-R.

Calcium-independent activation of extracellularly regulated kinases 1 and 2 by angiotensin II in hepatic C9 cells: roles of protein kinase Cdelta, Src/proline-rich tyrosine kinase 2, and epidermal growth receptor trans-activation

Agonist activation of endogenous angiotensin II (Ang II) AT(1) receptors expressed in hepatic C9 cells markedly stimulated inositol phosphate production, phosphorylation of the proline-rich tyrosine kinase PyK-2, and ERK activation. Ang II caused activation of protein kinase C delta (PKCdelta) in C9 cells, and its stimulatory actions on Pyk2 and extracellularly regulated kinase (ERK) phosphorylation were abolished by PKC depletion and selective inhibition of PKCdelta by rottlerin, but not by Ca(2+)-chelators. These effects, and the similar actions of the Src kinase inhibitor PP2 indicate the involvement of PKCdelta and Src kinase in ERK activation. In C9 cells, phorbol-12-myristate-13-acetate (PMA) caused much greater phosphorylation of Pyk2 and ERK than the Ca(2+) ionophore ionomycin, and the effects of PMA and Ang II were abolished in PKC-depleted cells. Ang II increased the association of Pyk2 with Src and with the epidermal growth factor receptor (EGF-R). EGF caused much greater tyrosine phosphorylation of the EGF-R than Ang II and PMA. Ang II-induced activation of ERK, but not Pyk2, was prevented by inhibition of EGF receptor phosphorylation by AG 1478 and of Src kinase by PP1. Ang II also increased the association of the adaptor protein Grb2 with the EGF-R. These findings indicate that Src and Pyk2 act upstream of the EGF-R and that the majority of Ang II-induced ERK phosphorylation is dependent on trans-activation of the EGF-R. Ang II-induced ERK activation in C9 cells is initiated by a PKCdelta-dependent but Ca(2+)-independent mechanism and is mediated by the Src/Pyk2 complex through trans-activation of the EGF-R.

Ligand-induced endocytosis and nuclear localization of angiotensin II receptors expressed in CHO cells

A construct (AT1R-NF) containing a "Flag" sequence added to the N-terminus of the rat AT1 receptor was stably expressed in Chinese hamster ovary cells and quantified in the cell membrane by confocal microscopy after reaction with a fluorescein-labeled anti-Flag monoclonal antibody. Angiotensin II bound to AT1R-NF and induced endocytosis with a half-time of 2 min. After 60-90 min, fluorescence accumulated around the cell nucleus, suggesting migration of the ligand-receptor complex to the nuclear membrane. Angiotensin antagonists also induced endocytosis, suggesting that a common step in the transduction signal mechanism occurring after ligand binding may be responsible for the ligand-receptor complex internalization.

Monitoring agonist-induced phospholipase C activation in live cells by fluorescence resonance energy transfer

Agonist-induced intracellular Ca(2+) signals following phospholipase C (PLC) activation display a variety of patterns, including transient, sustained, and oscillatory behavior. These Ca(2+) changes have been well characterized, but detailed kinetic analyses of PLC activation in single living cells is lacking, due to the absence of suitable indicators for use in vivo. Recently, green fluorescent protein-tagged pleckstrin homology domains have been employed to monitor PLC activation in single cells, based on (confocal) imaging of their fluorescence translocation from the membrane to the cytosol that occurs upon hydrolysis of phosphatidylinositol bisphosphate. Here we describe fluorescence resonance energy transfer between pleckstrin homology domains of PLCdelta1 tagged with cyan and yellow fluorescent proteins as a sensitive readout of phosphatidylinositol bisphosphate metabolism for use both in cell populations and in single cells. Fluorescence resonance energy transfer requires significantly less excitation intensity, enabling prolonged and fast data acquisition without the cell damage that limits confocal experiments. It also allows experiments on motile or extremely flat cells, and can be scaled to record from cell populations as well as single neurites. Characterization of responses to various agonists by this method reveals that stimuli that elicit very similar Ca(2+) mobilization responses can exhibit widely different kinetics of PLC activation, and that the latter appears to follow receptor activation more faithfully than the cytosolic Ca(2+) transient.

Angiotensin-(1-7) downregulates the angiotensin II type 1 receptor in vascular smooth muscle cells

Angiotensin (Ang)-(1-7) is a biologically active peptide of the renin-angiotensin system that has both vasodilatory and antiproliferative activities that are opposite the constrictive and proliferative effects of angiotensin II (Ang II). We studied the actions of Ang-(1-7) on the Ang II type 1 (AT(1)) receptor in cultured rat aortic vascular smooth muscle cells to determine whether the effects of Ang-(1-7) are due to its regulation of the AT(1) receptor. Ang-(1-7) competed poorly for [(125)I]Ang II binding to the AT(1) receptor on vascular smooth muscle cells, with an IC(50) of 2.0 micromol/L compared with 1.9 nmol/L for Ang II. The pretreatment of vascular smooth muscle cells with Ang-(1-7) followed by treatment with acidic glycine to remove surface-bound peptide resulted in a significant decrease in [(125)I]Ang II binding; however, reduced Ang II binding was observed only at micromolar concentrations of Ang-(1-7). Scatchard analysis of vascular smooth muscle cells pretreated with 1 micromol/L Ang-(1-7) showed that the reduction in Ang II binding resulted from a loss of the total number of binding sites [B(max) 437.7+/-261.5 fmol/mg protein in Ang-(1-7)-pretreated cells compared with 607.5+/-301.2 fmol/mg protein in untreated cells, n=5, P<0.05] with no significant effect on the affinity of Ang II for the AT(1) receptor. Pretreatment with the AT(1) receptor antagonist L-158,809 blocked the reduction in [(125)I]Ang II binding by Ang-(1-7) or Ang II. Pretreatment of vascular smooth muscle cells with increasing concentrations of Ang-(1-7) reduced Ang II-stimulated phospholipase C activity; however, the decrease was significant (81.2+/-6.4%, P<0.01, n=5) only at 1 micromol/L Ang-(1-7). These results demonstrate that pharmacological concentrations of Ang-(1-7) in the micromolar range cause a modest downregulation of the AT(1) receptor on vascular cells and a reduction in Ang II-stimulated phospholipase C activity. Because the antiproliferative and vasodilatory effects of Ang-(1-7) are observed at nanomolar concentrations of the heptapeptide, these responses to Ang-(1-7) cannot be explained by competition of Ang-(1-7) at the AT(1) receptor or Ang-(1-7)-mediated downregulation of the vascular AT(1) receptor.

Downregulation of the AT1A receptor by pharmacologic concentrations of Angiotensin-(1-7)

Angiotensin (Ang)-(1-7), the amino terminal heptapeptide fragment of Ang II, is an endogenous Ang peptide with vasodilatory and antiproliferative actions. Because Ang II causes vasoconstriction and promotes growth through activation of Ang type 1 (AT1) receptors, we investigated whether the actions of Ang-(1-7) are due to its regulation of these receptors. Studies were performed in CHO cells stably transfected with the AT1A receptor. Ang-(1-7) competed poorly with [125I]-Ang II for the AT1A binding site and was ineffective at shifting the IC50 for Ang II competition with [125I]-Ang II for binding to the AT1A receptor. However, if CHO-AT1A cells were pretreated with Ang-(1-7) and then treated with acidic glycine to remove surface-bound ligand, the heptapeptide caused a concentration-dependent reduction in Ang II binding, with a maximal inhibition to 67.8 +/- 4.6% of total (p < 0.05) at 1 microM Ang-(1-7) compared with a reduction to 24% of total by 10 nM Ang II. Ang-(1-7) pretreatment caused a small but significant decrease in the affinity of [125I]-Ang II for the AT1A receptor and a significant reduction in the total number of binding sites. The Ang-(1-7)-induced reduction in binding was rapid (occurring as early as 5 min after exposure to the peptide), was maintained for 30 min during continued exposure of the cells to Ang-(1-7), and rapidly recovered after removal of the heptapeptide. The AT1 receptor antagonist L-158,809 reduced the Ang-(1-7)-induced downregulation of the AT1A receptor, suggesting that interactions with AT1A receptors mediate the regulatory events. Pretreatment with 1 microM or 10 microM Ang-(1-7) significantly reduced inositol phosphate production in response to 10 nM Ang II. The decrease in binding and responsiveness of the AT1A receptor after exposure to micromolar concentrations of Ang-(1-7) suggests that the heptapeptide downregulates the AT1A receptor to reduce responses to Ang II. Because downregulation of the receptor only occurred at micromolar concentrations of the heptapeptide, our findings suggest that Ang-(1-7) is not a potent antagonist at the AT1A receptor. However, when the balance between Ang II and Ang-(1-7) is shifted in favor of Ang-(1-7), such as during inhibition of Ang-converting enzyme, some contribution of this mechanism may come into play.

Cloned neuropeptide Y (NPY) Y1 and pancreatic polypeptide Y4 receptors expressed in Chinese hamster ovary cells show considerable agonist-driven internalization, in contrast to the NPY Y2 receptor

Guinea-pig neuropeptide Y1 and rat pancreatic polypeptide Y4 receptors expressed in Chinese hamster ovary cells were internalized rapidly upon attachment of selective peptide agonists. The Y1 and Y2, but not the Y4, receptor also internalized the nonselective neuropeptide Y receptor agonist, human/rat neuropeptide Y. The internalization of guinea-pig neuropeptide Y2 receptor expressed in Chinese hamster ovary cells was small at 37 degrees C, and essentially absent at or below 15 degrees C, possibly in connection to the large molecular size of the receptor-ligand complexes (up to 400 kDa for the internalized fraction). The rate of intake was strongly temperature dependent, with essentially no internalization at 6 degrees C for any receptor. Internalized receptors were largely associated with light, endosome-like particulates. Sucrose dose-dependently decreased the internalization rate for all receptors, while affecting ligand attachment to cell membrane sites much less. Internalization of the Y1 and the Y4 receptors could be blocked, and that of the Y2 receptor significantly inhibited, by phenylarsine oxide, which also unmasked spare cell-surface receptors especially abundant for the Y2 subtype. The restoration of Y1 and Y4 receptors after agonist peptide pretreatment was decreased significantly by cycloheximide and monensin. Thus, in Chinese hamster ovary cells the Y1 and Y4 receptors have much larger subcellular dynamics than the Y2 receptor. This differential could also hold in organismic systems, and is comparable with the known differences in internalization of angiotensin, bradykinin, somatostatin and opioid receptor subtypes.

ATRAP, novel AT1 receptor associated protein, enhances internalization of AT1 receptor and inhibits vascular smooth muscle cell growth

We have identified a novel, membrane-located protein that interacts specifically with the carboxyl-terminal cytoplasmic domain of the AT1a receptor, which we named ATRAP (for AT1 receptor-associated protein). To further investigate the role of ATRAP in AT1 receptor function, we examined the effect of overexpression of ATRAP on angiotensin II (Ang II)-induced AT1 receptor desensitization and/or internalization, and cell proliferation in adult vascular smooth muscle cells (VSMCs). Transfection of ATRAP potentiated AT1 receptor internalization upon Ang II stimulation in these VSMCs. Moreover, we observed that AT1 receptor-induced DNA synthesis was markedly inhibited in ATRAP transfected VSMCs associated with the inhibition of the phosphorylation of signal transducers and activators of transcription (STAT) 3 and Akt. Our results suggest that ATRAP functions as a negative regulator in AT1 receptor-mediated cell proliferation in VSMCs.

Inhibitors of tyrosine phosphatases block angiotensin II inhibition of Na(+) pump

To determine how angiotensin II inhibits the Na(+) pump (Na(+), K(+)-ATPase) in rat zona glomerulosa, we selectively blocked signaling proteins that could be activated by the angiotensin AT(1) receptor and known to affect Na(+) pump activity. Inhibitors of protein kinase C [calphostin C (1 microM); staurosporine (1 microM)], phospholipase A(2) [arachidonyl triflouromethyl ketone (25 microM); quinacrine (75 microM)], diacylgycerol lipase [RHC-80267 (5 microM)], and tyrosine phosphorylation [tyrphostin 47 (100 microM)] had no effect on angiotensin II inhibition of the Na(+) pump. On the other hand, inhibitors of tyrosine phosphatases [phenylarsine oxide (5 microM) and 4-bromotetramisole oxalate (100 microM)] blocked angiotensin II inhibition, where as inhibitors of serine/threonine phosphatases [okadaic acid (1 microM) and microcystin (1.5 microM)] did not. Thus, angiotensin II inhibition of the Na(+) pump may in part be mediated by a tyrosine phosphatase.

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.

Lamellar body membrane turnover is stimulated by secretagogues

Lamellar bodies are specialized cellular organelles used for storage of surfactant by alveolar type II cells of the lung. We utilized monoclonal antibody (MAb) 3C9, which recognizes an integral lamellar body-limiting membrane protein of 180 kDa, to follow lamellar body trafficking. (125)I-labeled MAb 3C9 bound to the surface of type II cells and was internalized by the cells in a time- and concentration-dependent manner that was inhibitable by excess unlabeled antibody. The internalized antibody remained undegraded over a 4-h time period. The L2 rat lung cell line that does not have lamellar bodies did not bind iodinated 3C9. Exposure of type II cells to the secretagogues ATP, phorbol 12-myristate 13-acetate, and cAMP resulted in a 1.5- to 2-fold enhancement of binding and uptake of MAb 3C9. Calphostin C inhibited phorbol 12-myristate 13-acetate-stimulated phospholipid secretion and also reduced binding and uptake of MAb 3C9 by type II cells. Treatment of type II cells with phenylarsine oxide to obstruct clathrin-mediated endocytosis had no effect on the internalization of MAb 3C9 while markedly blocking the uptake of surfactant protein A and transferrin. An actin-mediated process was important for lamellar body membrane uptake because incubation with cytochalasin D partially inhibited MAb 3C9 incorporation by type II cells. These studies are compatible with enhanced lamellar body membrane turnover associated with surfactant secretion and indicate that this process can be monitored by the trafficking of the antigen reporter MAb 3C9.

Intracellular dynamics of sst5 receptors in transfected COS-7 cells: maintenance of cell surface receptors during ligand-induced endocytosis

Internalization of G protein-coupled receptors is crucial for resensitization of phosphorylation-desensitized receptors, but also for their long term desensitization through sequestration. To elucidate the mechanisms regulating cell surface availability of the somatostatin (SRIF) receptor subtype sst5, we characterized its internalization properties in transfected COS-7 cells using biochemical, confocal microscopic, and electron microscopic techniques. Our results demonstrated rapid and efficient sequestration of specifically bound [125I]Tyr0-D-Trp8-SRIF (up to 45% of bound radioactivity). Combined immunocytochemical detection of sst5 and visualization of a fluorescent SRIF analog by confocal microscopy revealed that whereas the internalized ligand progressively clustered toward the cell center with time, immunoreactive receptors remained predominantly associated with the plasma membrane. The preservation of cell surface receptors was confirmed by binding experiments on whole cells revealing a lack of saturability of [125I]Tyr0-D-Trp8-SRIF binding at 37 C. Binding was rendered saturable by the drug monensin, showing that receptor recycling played a key role in the preservation of cell surface receptors. Electron microscopy demonstrated that in addition to receptor recycling, internalization of receptor-ligand complexes triggered a massive recruitment of sst5 receptor molecules from intracellular stores to the membrane. This combination of recycling and recruitment of spare receptors may protect sst5 from long term down-regulation through sequestration and, therefore, facilitate extended SRIF signaling.

Tyrosine kinase inhibition affects type 1 angiotensin II receptor internalization

Growth factor receptors activate tyrosine kinases and undergo endocytosis. Recent data suggest that tyrosine kinase inhibition can affect growth factor receptor internalization. The type 1 angiotensin II receptor (AT1R) which is a G-protein-coupled receptor, also activates tyrosine kinases and undergoes endocytosis. Thus, we examined whether tyrosine kinase inhibition affected AT1R internalization. To verify protein tyrosine phosphorylation, both LLCPKCl4 cells expressing rabbit AT1R (LLCPKAT1R) and cultured rat mesangial cells (MSC) were treated with angiotensin II (Ang II) [1-100 nM] then solubilized and immunoprecipitated with antiphosphotyrosine antisera. Immunoblots of these samples demonstrated that Ang II stimulated protein tyrosine phosphorylation in both cell types. Losartan [1 microM], an AT1R antagonist, inhibited Ang II-stimulated protein tyrosine phosphorylation. LLCPKAT1R cells displayed specific 125I-Ang II binding at apical (AP) and basolateral (BL) membranes, and both AP and BL AT1R activated tyrosine phosphorylation. LLCPKAT1R cells, incubated with genistein (Gen) [200 microM] or tyrphostin B-48 (TB-48) [50 microM], were assayed for acid-resistant specific 125I-Ang II binding, a measure of Ang II internalization. Both Gen (n = 7) and TB-48 (n = 3) inhibited AP 125I-Ang II internalization (80+/-7% inhibition; p<0.025 vs. control). Neither compound affected BL internalization. TB-1, a non-tyrosine kinase-inhibiting tyrphostin, did not affect AP 125I-Ang II endocytosis (n = 3), suggesting that the TB-48 effect was specific for tyrosine kinase inhibition. Incubating MSC with Gen (n = 5) or herbimycin A [150 ng/ml] (n = 4) also inhibited MSC 125I-Ang II internalization (82+/-11% inhibition; p<0.005 vs. control). Thus, tyrosine kinase inhibition prevented Ang II internalization in MSC and selectively decreased AP Ang II internalization in LLCPKAT1R cells suggesting that AP AT1R in LLCPKAT1R cells and MSC AT1R have similar endocytic phenotypes, and tyrosine kinase activity may play a role in AT1R internalization.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Receptor-mediated internalization is critical for the inhibition of the expression of growth hormone by somatostatin in the pituitary cell line AtT-20

The inhibitory effect of the neuropeptide somatostatin on the expression of growth hormone was measured by quantitative polymerase chain reaction in the pituitary cell line AtT-20. We demonstrate that this effect is dependent on the internalization of somatostatin-receptor complexes and that it is totally independent from the peptide-induced inhibition of adenylate cyclase. Indeed, the inhibitory effect of the peptide on growth hormone mRNA levels was totally insensitive to pertussis toxin treatment but was totally abolished under conditions which block somatostatin receptor internalization. Comparative confocal microscopic imaging of fluorescent somatostatin sequestration and fluorescence immunolabeling of sst1, sst2A, and sst5 receptors suggests that sst2A is most probably responsible of the inhibitory effect of somatostatin on growth hormone expression.

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.

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

The structural determinants of the rat angiotensin (ANG) II AT1A receptor involved in receptor internalization, desensitization, and activation are investigated by producing six mutants that had progressively larger deletions of the cytoplasmic tail (-13, -19, -24, -31, -46, and -56 residues, respectively). After stable transfection of the cDNAs into Chinese hamster ovary cells, all mutants, except the most truncated, exhibit normal [Sar1]ANG II affinities [dissociation constant (Kd) = 0.19-0.70 nM] compared with the wild-type (WT) receptor (Kd = 0.62 nM) and are able to activate a Gq/11 protein and a phospholipase C as measured by the ANG II-induced inositol phosphate (IP) turnover in the different clones. However, one of these mutants, delta 329 (deletion of 31 residues), exhibits a peculiar phenotype. This mutant shows a reduced ligand-induced internalization as measured by the acid-washing procedure (only 32% of receptors are internalized vs. 83% for WT). Moreover, the delta 329 mutant is less desensitized by a pretreatment with either ANG II (15% desensitization of ANG II-stimulated IP turnover vs. 60% for WT receptor) or the phorbol ester phorbol 12-myristate 13-acetate (no desensitization vs. 29% for WT receptor). These functional modifications of the delta 329 mutant are associated with the transduction of an amplified signal as demonstrated on both IP turnover and an integrated physiological effect of ANG II. Taken together, these data indicate that the sequence 329SLSTKMS335 of the rat AT1A receptor is involved in both receptor internalization and desensitization. This is the first demonstration that a desensitization- and internalization-defective AT1A receptor mutant is also hyperreactive and mediates augmented cellular responses.

Apical ANG II-stimulated PLA2 activity and Na+ flux: a potential role for Ca2+-independent PLA2

Type 1 angiotensin II (ANG II) receptors (AT1R), which mediate proximal tubule (PT) salt and water reabsorption, undergo endocytosis and recycling. Prior studies in a PT-like model (LLC-PKcl4 cells expressing rabbit AT1R) (LLC-PK-AT1R cells) determined that quinacrine, a nonspecific phospholipase A2 (PLA2) inhibitor, and the haloenol lactone suicide substrate (HELSS), a Ca2+-independent PLA2 inhibitor, attenuated apical (AP) AT1R recycling. Further studies were undertaken to examine the association between AT1R endocytotic movement and PLA2 activity in this model. AP ANG II (100 nM) increased [3H]arachidonic acid ([3H]AA) release 4.4 +/- 0.38-fold in LLC-PK-AT1R cells cultured on permeable supports. Basolateral (BL) ANG II had no significant effect. Reversed-phase high-performance liquid chromatography confirmed that AP ANG II stimulated free [3H]AA release. Quinacrine, HELSS, and palmitoyl trifluoromethyl ketone, another Ca2+-independent PLA2 inhibitor, inhibited AP ANG II-stimulated [3H]AA release, as did inhibiting AP AT1R internalization with phenylarsine oxide. The role of HELSS-inhibitable AA release in ANG II-mediated 22Na flux was examined, given the effects of AT1R-mediated PLA2 activity on salt and water reabsorption. AP ANG II (100 nM) stimulated 22Na flux (AP--> BL), a response inhibited by HELSS. Thus, in this model, AP AT1R activated PLA2 with concomitant 22Na flux (AP --> BL), suggesting a link between AP AT1R endocytotic movement, AT1R-stimulated PLA2 activity, and 22Na flux in this model. The effects of HELSS suggest that Ca2+-independent PLA2 activity may be involved in this AP ANG II response.

Endocytosis and recycling of G protein-coupled receptors

Agonist stimulation of G protein-coupled receptors causes a dramatic reorganization of their intracellular distribution. Activation of receptors triggers receptor endocytosis and, since receptors recycle back to the surface continuously, a new steady state is reached where a significant proportion of receptors is located internally. Although this movement of receptors is remarkable, its role has been enigmatic. Recent developments have provided insight into the compartments through which the receptors move, the nature of the signals that trigger receptor translocation, and the significance of receptor cycling for cell function. In this article, Jennifer Koenig and Michael Edwardson review recent progress in this field and place receptor cycling into a mathematical framework that reveals the extent and rate of intracellular receptor movement.

Receptor-mediated endocytosis of angiotensin II in rat myometrial cells

The events involved in the processing of the angiotensin II (Ang II)-receptor complex were studied in primary cultures of rat myometrial cells. Ang II bound to rat myometrial cells in a specific, time- and temperature-dependent fashion. Pretreatment with cycloheximide did not interfere with binding up to 3 hr, but inhibited increases in binding observed over longer periods. The [3H]Ang II binding to intact cells was inhibited by dithiothreitol (DTT), and the rank order of potency of Ang II and nonpeptide antagonists to inhibit the [3H]Ang II binding was Ang II > Losartan >> PD 123319 or CGP 42112B, indicating the presence of the AT1 receptor type. Whereas most of the [3H]Ang II binding at 4 degrees was susceptible to acid or pronase treatment, binding at 35 degrees was resistant to both treatments, suggesting an internalization of the Ang II-receptor complex. Phenylarsine oxide (PAO) and N-ethylmaleimide (NEM) caused a concentration-dependent inhibition when the binding assay was performed at 35 degrees, but no effect was observed at 4 degrees, indicating that these agents did not alter cell-surface binding but actually prevented the internalization process. Simultaneous treatment with 1 mM DTT or beta-mercaptoethanol prevented the inhibitory effect of NEM, but only DTT could prevent the inhibition caused by PAO, indicating that two closely located sulfhydryl groups must be involved in the internalization process. Chloroquine (100 microM) inhibited the [3H]Ang II dissociation from cells, and monensin (25 microM) induced a 30% inhibition of [3H]Ang II binding (35 degrees, 3 hr), suggesting endosomal processing of the Ang II-receptor complex with receptor recycling to the cell surface. These results indicate that Ang II binding to AT1 receptors in rat myometrial cells is followed by internalization of the Ang II-receptor complex and recycling of the receptor to the cell surface.

Angiotensin II type 1 (AT1) receptor-mediated accumulation of angiotensin II in tissues and its intracellular half-life in vivo

Angiotensin II (Ang II) is internalized by various cell types via receptor-mediated endocytosis. Little is known about the kinetics of this process in the whole animal and about the half-life of intact Ang II after its internalization. We measured the levels of 125I-Ang II and 125I-Ang I that were reached in various tissues and blood plasma during infusions of these peptides into the left cardiac ventricle of pigs. Steady-state concentrations of 125I-Ang II in skeletal muscle, heart, kidney, and adrenal were 8% to 41%, 64% to 150%, 340% to 550%, and 680% to 2100%, respectively, of the 125I-Ang II concentration in arterial blood plasma (ranges of six experiments). The tissue concentrations of 125I-Ang I were less than 5% of the arterial plasma concentrations. 125I-Ang II accumulation seen in heart, kidney, and adrenal was almost completely blocked by a specific Ang II type 1 (AT1) receptor antagonist. Steady-state concentrations of 125I-Ang II were reached within 30 to 60 minutes in the tissues and within 5 minutes in blood plasma. The in vivo half-life of intact 125I-Ang II in heart, kidney, and adrenal was approximately 15 minutes, compared with 0.5 minute in the circulation. Thus, Ang II, but not Ang I, from the circulation is accumulated by some tissues, and this is mediated by AT1 receptors. The time course of this process and the long half-life of the accumulated Ang II support the contention that this Ang II has been internalized after its binding to the AT1 receptor, so that it is protected from rapid degradation by endothelial peptidases. The results of this study are in agreement with growing evidence of an important physiological role for internalized Ang II.

Chloroquine, quinine and quinidine inhibit calcium release from macrophage intracellular stores by blocking inositol 1,4,5-trisphosphate binding to its receptor

The binding of many ligands to cellular receptors induces a signaling cascade which generates inositol 1,4,5-trisphosphate (IP3). IP3 binding to its receptors in various internal compartments causes a rapid Ca2+ efflux into the cytosol. We now demonstrate that chloroquine blocks ligand-induced Ca2+ mobilization without affecting IP3 synthesis. The effect is independent of the ligand employed and occurred with five unrelated ligands; namely, alpha 2-macroglobulin-methylamine, angiotensin II, bradykinin, carbachol, and epidermal growth factor. Chloroquine, quinidine, and quinine, however, block binding of [3H]IP3 to its receptors by 90%, 88%, and 71%, respectively. These observations suggest a previously undetected mechanism by which these agents may in part function as antimalarials.

Somatostatin receptors in Neuro2A neuroblastoma cells: ligand internalization

1. Receptor-dependent internalization of somatostatin (SRIF) agonists has been a matter of controversy probably because [125I]Tyr11-SRIF-14 is rapidly degraded. We have studied the internalization of a stable somatostatin analogue, [125I]-BIM-23027, in a neuronal cell line, Neuro2A, which natively expresses somatostatin sst2 receptors. 2. Incubation of Neuro2A cells with [125I]-BIM-23027 at 37 degrees C resulted in a time-dependent internalization of the ligand, which reached a maximum at 30 min. Acid-washing showed that cell-surface binding of the ligand accounted for only 34% of total binding at this time. Internalization was dramatically reduced at 15 degrees C. 3. Internalization of [125I]-BIM-23027 was prevented by inclusion of unlabelled somatostatin receptor agonists in a concentration-dependent manner. The IC50 values for inhibition of [125I]-BIM-23027 internalization were approximately 100 fold lower than for inhibition of [125I]-BIM-23027 binding to membrane homogenates but followed the same rank order of potencies. 4. Disruption of G-protein coupling by treatment with pertussis toxin caused a 60% reduction in internalization of ligand. A combination of antimycin (50 nM) and deoxyglucose (50 mM) pretreatment, which leads to a depletion of cellular ATP, decreased internalization of [125I]-BIM-23027 by 66% of control and increased the proportion of surface-bound ligand. Hypertonic sucrose, which prevents clathrin-mediated endocytosis, reversibly abolished the internalization of ligand without increasing the proportion bound at the cell surface. 5. After internalization of [125I]-BIM-23027, approximately half of the ligand was recycled back to the extracellular medium within 20 min at 37 degrees C. This finding suggests that the intracellular content of [125I]-BIM-23027 reaches a steady state which is determined by the rates of both internalization and recycling of the ligand. In contrast to studies in which the internalization of [125I]-Tyr11-SRIF-14 was examined, neither internalized nor recycled [125I]-BIM-23027 was degraded to its component amino acids. 6. These findings indicate that the somatostatin agonist, [125I]-BIM-23027, is internalized in a receptor-dependent manner which involves clathrin-coated pits in Neuro2A cells. Furthermore, much of the internalized ligand is rapidly recycled back to the extracellular medium without undergoing significant degradation.

Angiotensin II receptor subtype antagonists can both stimulate and inhibit salt appetite in rats

In urethane-anaesthetised male Wistar rats iontophoretic application of the angiotensin II (Ang II) type 1 (AT-1) receptor specific nonpeptide antagonist losartan in the septo-preoptic continuum can produce neuronal excitation of short- and long-term duration which has been interpreted as removal of tonic Ang II-induced inhibition. Mineralocorticoid pretreatment, which increases neuronal sensitivity to Ang II to enhance salt appetite, also removes this losartan-induced long-term excitation. These results suggested steroid modulation of the AT-1 receptor and a complex involvement of Ang II in salt appetite. To investigate the role of the inhibitory action of central Ang II on salt appetite, we gave intracerebroventicular injections of a single, low dose (10 ng) of losartan in normal euhydrated rats and this produced, paradoxically, a progressive increase in salt intake (1.6 +/- 0.3 ml/day to 3.5 +/- 0.9 ml/day, n = 15, P < 0.05). Treatment of these salt enhanced rats with DOCA (0.5 mg/day, s.c., for 3 days) further increased the salt appetite, but then a second i.c.v. injection of the same dose of losartan significantly inhibited the enhanced salt appetite (4.7 +/- 0.7 to 1.3 +/- 0.4, n = 9, P < 0.05). These results provide evidence for a complex action of Ang II on the At-1 receptor mediating the generation of salt appetite that appears to involve either at least two functional subtypes of this AT-1 receptor, as already suggested by previous electrophysiological experiments, or one AT-1 receptor with several activation states.

Intracellular signal transduction: The role of endosomes

Polypeptide hormones, growth factors, and other biologically significant molecules are specifically internalized by target cells. Exposure of cells to these ligands results in the formation of ligand-receptor complexes on the cell surface and subsequent internalization of these complexes into the endosomal apparatus (endosomes, or ENs). The study of ENs has identified several important functions for this unique cellular organelle. These include the dissociation of ligand from receptor and receptor recycling to the cell surface and the degradation of some internalized ligands, as well as the delivery of others to lysosomes. More recently, it has become apparent that ENs fulfill another critical role, that of signal transduction. In this article, we review the evidence substantiating this role for ENs and propose three models by which ENs participate in cell signaling.

Characteristics of the binding of [3H]-mepyramine to intact human U373 MG astrocytoma cells: evidence for histamine-induced H1-receptor internalisation

1. The kinetics of the binding of 5 nM [3H]-mepyramine to sites on intact human U373 MG astrocytoma cells, sensitive to inhibition by 2 microM pirdonium, were temperature-dependent. At 37 degrees C the half-time for association was 0.9 +/- 0.4 min and at 4 degrees C 19 +/- 3 min. Dissociation of bound [3H]-mepyramine was fast at 37 degrees C, t0.5 1.5 +/- 0.3 min, but at 6 degrees C dissociation initiated by dilution or addition of unlabelled mepyramine was negligible over 120 min. The very slow dissociation at 6 degrees C made it possible to reduce the level of pirdonium-insensitive binding from 56 +/- 5% to 39 +/- 5% by washing the cells in ice-cold medium before filtration. 2. The binding of [3H]-mepyramine sensitive to 2 microM temelastine, measured after 10 min equilibration at 37 degrees C, failed to saturate and was resolved into an hyperbola and an apparently linear component, whereas the fit to the binding of [3H]-mepyramine sensitive to 2 microM pirdonium was not significantly improved over that to an hyperbola. The mean Kd for the binding of [3H]-mepyramine to the saturable component, 2.5 +/- 0.4 nM, was in close agreement with the value of 3.5 nM for mepyramine derived from inhibition of histamine H1-receptor-mediated inositol phosphate formation in U373 MG cells. 3. Curves for the inhibition of the binding of 5 nM [3H]-mepyramine to U373 MG cells by histamine H1-receptor antagonists were biphasic and were fitted to a two site-model. Affinities calculated from the best-fit IC50 values for the high-affinity site correlated well with those expected for binding to H1-receptors. 4. The percentages of the high-affinity site in curves of the inhibition of [3H]-mepyramine binding to intact U373 MG cells by two tertiary amine antagonists, norpirdonium and 4-methyldiphenhydramine, 68 +/- 3 and 63 +/- 4%, were significantly greater than the percentages of the high-affinity site in the inhibition curves of their quaternary derivatives, 50 +/- 1 and 45 +/- 3%, respectively. Similarly, the percentage of the high-affinity site for unlabelled mepyramine, 65 +/- 7%, was greater than for the non-cell penetrant H1-antagonist temelastine, 42 +/- 5%. 5. Incubation of U373 MG cells with 100 microM histamine at 37 degrees C, followed by washing twice in ice-cold medium and then incubation with 1-15 nM [3H]-mepyramine for 120 min at 4 degrees C, resulted in a decrease in the binding of [3H]-mepyramine sensitive to 2 microM pirdonium, compared to control cells not exposed to histamine. The binding of [3H]-mepyramine in the absence of pirdonium was not altered by histamine pretreatment, whereas the level of the pirdonium-insensitive binding was significantly increased, except after 1 min exposure to histamine. The decreases in the pirdonium-sensitive binding after 5, 10 and 60 min incubation with 100 microM histamine were 41 +/- 6, 56 +/- 6 and 67 +/- 8%, respectively, but the decrease after 1 min incubation with histamine, 16 +/- 8%, was not statistically significant. 6. The results are consistent with the binding of [3H]-mepyramine to intact U373 MG cells being to both plasma membrane and intracellular histamine H1-receptors. The high-affinity binding sensitive to the non-cell penetrant quaternary compounds and to temelastine is thus to plasma membrane H1-receptors. On exposure to 100 microM histamine receptors are translocated to the intracellular pool, since the change in the high-affinity binding of [3H]-mepyramine is primarily in the level of the pirdonium-insensitive binding, rather than in the total binding.

Polarized rabbit type 1 angiotensin II receptors manifest differential rates of endocytosis and recycling

Receptor-mediated endocytosis and recycling have been described for extrarenal angiotensin II (ANG II) receptors. In proximal tubule (PT) epithelia expressing polarized ANG II receptors, these processes have not been examined as thoroughly. We utilized a PT cell model, LLC-PKCl4 cells stably transfected with rabbit type 1 ANG II receptor (AT1R) cDNA, to investigate these properties. LLC-PK-AT1R cells expressed the rabbit AT1R transcript and displayed losartan-inhibitable specific 125I-labeled ANG II binding at apical (AP) and basolateral (BL) membranes when grown on permeable supports. AP AT1R internalized 125I-ANG II more rapidly than BL AT1R, and phenylarsine oxide treatment inhibited AP AT1R internalization without significantly affecting BL AT1R endocytosis. Pertussis toxin had no effect on AP or BL AT1R endocytosis. In addition, AP AT1R recovered specific 125I-ANG II binding after ANG II treatment (a measure of recycling). BL AT1R displayed minimal recovery of 125I-ANG II binding after ANG II pretreatment. These data suggested that AP AT1R enter endocytic/endosomal pathways. Phospholipase A2 (PLA2) activity has been linked to endosomal fusion in other systems, and PT brush-border membrane AT1R also have been associated with PLA2 activity. LLC-PK-AT1R cells were therefore treated with quinacrine, a nonspecific PLA2 inhibitor, or Compound I (CI), a selective Ca(2+)-independent PLA2 inhibitor, to determine if PLA2 activity was linked to AT1R recycling. Both quinacrine and CI decreased AP AT1R recycling without affecting BL AT1R recycling. Polarized AT1R in LLC-PKCl4 cells thus display differential rates of endocytosis and recycling.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II receptor endocytosis involves two distinct regions of the cytoplasmic tail. A role for residues on the hydrophobic face of a putative amphipathic helix

Following agonist stimulation, many receptors are rapidly internalized from the plasma membrane via a mechanism which presumably involves recognition motifs within the cytoplasmic domains of the receptor. We have previously demonstrated (Thomas, W. G., Thekkumkara, T. J., Motel, T. J., and Baker, K. M. (1995) J. Biol. Chem. 270, 207-213) that truncation of the angiotensin II (AT1A) receptor, to remove 45 amino acids from the cytoplasmic tail, markedly reduced agonist stimulated receptor endocytosis. In the present study, we have stably and transiently expressed wild type and carboxyl terminus mutated AT1A receptors in Chinese hamster ovary cells to identify regions and specific amino acids important for this process. Wild type AT1A receptors rapidly internalized (t1/2 = 2.5 min; Ymax = 76.4%) after AII stimulation. Using AT1A receptor mutants, truncated and deleted at the carboxyl terminus, two distinct regions important for internalization were identified: one membrane proximal site between residues 315-329 and another distal to Lys333, within the terminal 26 amino acids. Point mutations (Y302A, Y312A, L316F, Y319A, and K325A) were performed to identify residues contributing to the membrane proximal site. Mutation of Y302A, Y312A, and K325A had little effect on the rate (t1/2 = 4.3, 2.8, and 2.8 min) and maximal amount (Ymax = 81.7, 67.8, and 73.5%) of AII induced internalization. In contrast, L316F and Y319A mutations displayed an approximately 2.5-fold reduction in rate (t1/2 = 6.1 and 6.2 min) and L316F a decreased maximal level (Ymax = 38.1 and 71.4%, respectively) compared to wild type. Interestingly, Leu316 and Tyr319 are closely aligned within the hydrophobic aspect of a putative amphipathic helix, possibly representing an internalization motif for the AT1A receptor. We conclude that the AT1A receptor does not use NPXXY (NPLFY302) motif, first described for the beta 2-adrenergic receptor, to mediate agonist stimulated endocytosis. Rather, two distinct regions of the carboxyl terminus are utilized: one involving hydrophobic and aromatic residues on a putative alpha-helix and another serine/threonine-rich domain.

A conserved NPLFY sequence contributes to agonist binding and signal transduction but is not an internalization signal for the type 1 angiotensin II receptor

A conserved NPX2-3Y sequence that is located in the seventh transmembrane helix of many G protein-coupled receptors has been predicted to participate in receptor signaling and endocytosis. The role of this sequence (NPLFY) in angiotensin II receptor function was studied in mutant and wild-type rat type 1a angiotensin II receptors transiently expressed in COS-7 cells. The ability of the receptor to interact with G proteins and to stimulate inositol phosphate responses was markedly impaired by alanine replacement of Asn298 and was reduced by replacement of Pro299 or Tyr302. The F301A mutant receptor exhibited normal G protein coupling and inositol phosphate responses, and the binding of the peptide antagonist, [Sar1,Ile8]angiotensin II, was only slightly affected. However, its affinity for angiotensin II and the nonpeptide antagonist losartan was reduced by an order of a magnitude, suggesting that angiotensin II and losartan share an intramembrane binding site, possibly through their aromatic moieties. None of the agonist-occupied mutant receptors, including Y302A and triple alanine replacements of Phe301, Tyr302, and Phe304, showed substantial changes in their internalization kinetics. These findings demonstrate that the NPLFY sequence of the type 1a angiotensin II receptor is not an important determinant of agonist-induced internalization. However, the Phe301 residue contributes significantly to agonist binding, and Asn298 is required for normal receptor activation and signal transduction.

Critical role of a conserved intramembrane tyrosine residue in angiotensin II receptor activation

The rat type 1a (AT1a) angiotensin II (Ang II) receptor contains a highly conserved tyrosine residue in the fifth transmembrane region that is present in most G protein-coupled receptors. The role of this amino acid in AT1 receptor activation was analyzed in a mutant receptor (Y215F) created by replacing Tyr215 with phenylalanine. The mutant receptor was highly expressed in transfected COS-7 cells, and its binding affinity for the peptide antagonist [Sar1,Ile8]Ang II was similar to that of the wild type receptor. Although the structural integrity of the peptide ligand binding domain was preserved in the Y215F mutant receptor, its affinity for the native agonist, Ang II, was significantly reduced. Also, whereas guanosine 5'-3-O-(thio)triphosphate markedly reduced Ang II binding to the wild type receptor, it had little effect on agonist binding to the mutant receptor. Agonist-induced internalization of the mutant receptor was also impaired, and its ability to mediate inositol phosphate responses to Ang II stimulation was abolished. The concomitant decreases in receptor internalization and G protein-mediated signaling of the Y215F mutant receptor indicate that Tyr215 has a critical role in AT1 receptor activation. In view of its conservation among members of the seven transmembrane domain receptor superfamily, this residue is likely to be of general importance in signal transduction from G protein-coupled receptors.

Internalisation of the type I angiotensin II receptor (AT1) and angiotensin II function in the rat adrenal zona glomerulosa cell

Using a specific monoclonal antibody (6313/G2) to the first extracellular domain of the type 1 receptor (AT1), we showed that most of the receptor is internalised in the rat glomerulosa cell. When viable glomerulosa cells are incubated with 6313/G2, the receptor is transiently concentrated on the cell surface, and aldosterone output is stimulated. This stimulated output is enhanced by neither threshold nor maximal stimulatory concentrations of AII amide, although the antibody does not inhibit AII binding to the receptor. The antibody directly stimulates inositol trisphosphate (IP3) generation, but, while having no intrinsic action on protein kinase C (PKC) activation, it significantly inhibits the PKC response to angiotensin II. The data suggest that although the receptor is mostly internalized, recycling to the plasma membrane is constitutive, or regulated by unknown factors. Retention of the AT1 receptor in the membrane is alone enough to allow sufficient G protein interaction to generate maximal steroidogenic effects, through IP3 generation. PKC activation induced by angiotensin II has no bearing on steroidogenesis in the dispersed glomerulosa cell system.

Role of carboxyl tail of the rat angiotensin II type 1A receptor in agonist-induced internalization of the receptor

Binding of angiotensin II (Ang II) to its receptor type 1A (AT1A) is known to trigger its internalization. We studied the role of cytosolic segments of AT1A in the internalization, and obtained results indicating a functional role of the cytosolic carboxyl terminal tail of AT1A in the internalization. Deletion of 50 amino acids from the carboxyl terminus abolished the receptor internalization. Deletion mutants lacking 13 and 32 amino acid residues in the carboxyl terminal cytosolic region were internalized to the same extent as wild type AT1A; however, internalization of a mutant lacking the last 42 residues was partially suppressed. Thus, residues 310 through 327 were shown to be essential for the internalization. We propose that a short domain in the cytoplasmic tail (residues 310 to 327) may play a dominant role in the agonist-induced receptor internalization of AT1A. Our results also suggest that the molecular determinants of the AT1A receptor involved in receptor internalization are distinct from those participating in the desensitization process.