Aiji Sakamoto

Cloning and functional expression of human cDNA for the ETB endothelin receptor

We report the cloning of human cDNA encoding an ETB (non-isopeptide-selective) subtype of the endothelin receptor. The predicted amino acid sequence of the human ETB endothelin receptor was 87.8% and 62.9% identical with the previously cloned rat ETB and ETA receptors, respectively. COS cells transiently transfected with the cloned cDNA expressed specific, high-affinity binding sites for endothelin isopeptides and responded to the peptides with a transient increase of [Ca2+]i; endothelin-1 and endothelin-3 exhibited approximately equal potencies both in displacing 125I-labeled endothelin-1 binding and in eliciting [Ca2+]i transients. The ETB receptor mRNAs were expressed in various human tissues and also in the intact porcine aortic intimal cells ex vivo.

Contribution of Sustained Ca Elevation for Nitric Oxide Production in Endothelial Cells and Subsequent Modulation of Ca Transient in Vascular Smooth Muscle Cells in Coculture

To elucidate the intracellular Ca (Ca) transient responsible for nitric oxide (NO) production in endothelial cells (ECs) and the subsequent Ca reduction in vascular smooth muscle cells (VSMCs), we administrated four agonists with different Ca-mobilizing mechanisms for both cells in iso- or coculture. We monitored the Ca of both cells by two-dimensional fura-2 imaging, simultaneously measuring NO production as NO. The order of potency of the agonists in terms of the peak Ca in ECs was bradykinin (100 nM) > ATP (10 μM) > ionomycin (50 nM) > thapsigargin (1 μM). In contrast, the order in reference to both the extent of Ca reduction in cocultured VSMCs and the elevation in NO production over the level of basal release in ECs completely matched and was ranked as thapsigargin > ionomycin > ATP > bradykinin. Treatment by N-monomethyl-L-arginine monoacetate but not indomethacin or glybenclamide restored the Ca response in cocultured VSMCs to the isoculture level. In ECs, when the Ca influx was blocked by Ni or by chelating extracellular Ca, all four agonists markedly decreased NO production, the half decay time of the Ca degenerating phase, and the area under the Ca curve. The amount of produced NO hyperbolically correlated to the half decay time and the area under the Ca curve but not to the Ca peak level. Thus, the sustained elevation of Ca in ECs, mainly a result of Ca influx, determines the active NO production and subsequent Ca reduction in adjacent VSMCs. Furthermore, L-arginine but not D-arginine or L-lysine at high dose (5 mM) without agonist enhanced the NO production, weakly reduced the Ca in ECs, and markedly decreased the Ca in VSMCs, demonstrating the autocrine and paracrine effects of NO (Shin, W. S., Sasaki, T., Kato, M., Hara, K., Seko, A., Yang, W. D., Shimamoto, N., Sugimoto, T., and Toyo-oka, T.(1992) J. Biol. Chem. 267, 20377-20382).

Palmitoylation of Human EndothelinB ITS CRITICAL ROLE IN G PROTEIN COUPLING AND A DIFFERENTIAL REQUIREMENT FOR THE CYTOPLASMIC TAIL BY G PROTEIN SUBTYPES

By site-directed mutagenesis, three cysteine residues (amino acids 402, 403, and 405) in the carboxyl terminus of human endothelinB (ETB) were identified as potential palmitoylation sites. Substitutions of all of the three cysteine residues with serine gave an unpalmitoylated mutant, C2S/C3S/C5S. When expressed in Chinese hamster ovary cells, C2S/C3S/C5S was localized on the cell surface, retained high affinities to ET-1 and ET-3, and was rapidly internalized when bound to the ligand. However, unlike the wild-type ETB, C2S/C3S/C5S transmitted neither an inhibitory effect on adenylate cyclase nor a stimulatory effect on phospholipase C, indicating a critical role of palmitoylation in the coupling with G proteins, regardless of the G protein subtypes. Truncation of the carboxyl terminus including Cys403/Cys405 gave a deletion mutant Δ403 that was palmitoylated on Cys402 and lacked the carboxyl terminus downstream to the palmitoylation site. Δ403 did transmit a stimulatory effect on phospholipase C via a pertussis toxin-insensitive G protein but it failed to transmit an inhibitory effect on adenylate cyclase. These results indicated a differential requirement for the carboxyl terminus downstream to the palmitoylation site in the coupling with G protein subtypes, i.e. it is required for the coupling with Gi but not for that with Gq.

In Vivo Gene Transfection of Human Endothelial Cell Nitric Oxide Synthase in Cardiomyocytes Causes Apoptosis-Like Cell Death Identification Using Sendai Virus–Coated Liposomes

BACKGROUND Nitric oxide (NO) has various actions on the cardiovascular system, although its pathophysiological significance in myocardial cells remains obscure. The aim of the present study was to identify direct NO actions on cardiomyocytes by gene transfection in vivo using a newly developed vector under physiological conditions. METHODS AND RESULTS Liposomes containing the beta-galactosidase (beta-gal) gene alone or with the human endothelial cell nitric oxide synthase (ecNOS) gene were coated with UV-inactivated Sendai virus and injected into the left ventricular wall of rat heart in vivo. Histological examination confirmed that the transfection efficiency was comparable to adenovirus-mediated transfection and that the new vector per se caused no inflammation. beta-Gal expression was confined to cardiomyocytes between two intercalated discs, suggesting that the transfected gene did not permeate the discs. An immunohistochemical study showed that cotransfection of the ecNOS gene induced massive myocardial cell shrinkage in both transfected cells and the adjacent myocytes in a time- and dose-dependent manner. Histochemical findings in shrunk cells coincided with apoptosis as identified by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labeling. Electron microscopy of the lesion revealed myofibrillar degradation and accumulation of mitochondria but no apoptotic bodies. Pre-treatment with the NOS inhibitor N omega-nitro-L-arginine methyl ester abolished these morphological alterations. CONCLUSIONS The efficient expression of the human ecNOS gene in vivo suggests that NO or its toxic metabolite caused myocardial degradation, a part of which was compatible with apoptosis of the transfected cardiomyocytes themselves and the adjacent cells as a paracrine effect. These morphological features mimicked acute myocarditis or ischemic injury.

Long-lasting activation of cation current by low concentration of endothelin-1 in mouse fibroblasts and smooth muscle cells of rabbit aorta.

1 Recombinant human ETA receptors were expressed in a mouse fibroblast cell line (Ltk−cell) and functional coupling of the receptors with Ca2+ permeable channels at low concentrations of endothelin‐l (ET‐1) was investigated using whole‐cell recordings and monitoring the changes in intracellular free Ca2+ concentrations ([Ca2+]i) with a Ca2+ indicator, fluo‐3. A similar type of coupling was investigated in freshly dispersed vascular smooth muscle cells (VSMCs) of rabbit thoracic aorta by use of whole‐cell recordings. 2 In Ltk−cells expressing recombinant human ETA receptors, concentrations of ET‐1 (10−8 M, 10−9 M) evoked an initial transient peak and a subsequent sustained elevation in [Ca2+]i whereas a lower concentration of ET‐1 (10−10 M) evoked only a sustained elevation of [Ca2+]i. After removal of extracellular Ca2+, ET‐1 evoked only an initial peak without a sustained elevation of [Ca2+]i. The sustained elevation induced by 10−10 M ET‐1 was blocked by 300 μM mefenamic acid (a cation channel blocker) but not by 10 μM nifedipine (a blocker of voltage‐operated Ca2+ channel). 3 In whole‐cell recordings with Ltk− cells, a brief (3–5 min) application of ET‐1 (10−10 M) induced a sustained inward current at a holding potential of −60 mV. The current‐voltage relationship revealed that the reversal potential of the ET‐1‐induced current was close to 0 mV (1.9 mV) and was not altered by reducing the concentration of C1− in the bath solution, indicating that the current is carried by cations. The current was reversibly blocked by 300 μM mefenamic acid, and it persisted after all cations in the bath solution had been replaced by Ca2+ (5 or 30 mM) and nonpermeant cation N‐methyl‐D‐glucamine, indicating that the ET‐1‐activated channel is permeable to Ca2+. Activation of the current was independent of membrane potential and the current was induced even after addition of a high concentration (10 mM) of a Ca2+ chelator, EGTA, to the pipette solution. 4 In whole‐cell recordings from rabbit aortic VSMCs, ET‐1 (10−10 M) induced a sustained inward current at a holding potential of −60 mV. The reversal potential was −12 mV and was not altered when the concentration of Cl− in the pipette solution was decreased, indicating that the current is carried by cations. Again activation of the current was independent of membrane potential and was observed even after addition of a high concentration (10 mM) of a Ca2+ chelator, EGTA to the pipette solution. The current was reversibly blocked by 300 μM mefenamic acid and was permeable to Ca2+, showing marked similarities to ET‐1‐induced cationic current in Ltk− cells. 5 These results indicate that in Ltk− cells transfected with cDNA for recombinant ETA receptors and VSMCs, ETA receptors can functionally couple with a nonselective cation channel permeable to Ca2+. Thus the present data suggest that the cation channel plays an essential role in the sustained elevation of [Ca2+]i at low concentrations of ET‐1 by causing Ca2+ entry through the channel.

STRUCTURAL BASIS OF THE FUNCTION OF ENDOTHELIN RECEPTOR

Endothelin receptor is a good model for analysis of the function of heptahelical G-protein coupled receptor. In ligand binding to the heptahelical receptor, the receptor has two functions, i.e. ‘message’ and ‘address’ functions. Each function has been assigned to different domain of the receptor. A different part of the ligand structure also corresponds to each domain of the receptor. Classically, classification of receptor has been done according to the difference of address domain, i.e. affinity difference of the receptor. However, present results predict that the classification of receptor is also possible according to the message domain.After stimulation of ET receptor by a ligand, the receptor transmits a signal to G-protein. Several kinds of G-proteins can possibly be activated. Different structural domains of the receptor are assigned to the coupling of the different Gα-protein. Activated G-protein transmits the message to effector. Each Gα-protein acts on different target molecules, resulting in different responses. However, the activation of each Gα-protein presumably depends on its intracellular level. Even if the same receptor is activated with the same ligand, resulting final response is different from cell to cell. Therefore, classification of receptor according to the function of the receptor is difficult.

Negative Chronotropic Effect of Endothelin 1 Mediated Through ETA Receptors in Guinea Pig Atria

Endothelins exert potent excitatory cardiac effects by acting on specific receptors on myocytes. In this study, we have examined the signal transduction mechanism for the chronotropic effect of endothelins in guinea pig atria. A competition binding of [125I]endothelin 1 ([125I]ET-1) using the recently developed ETA receptor-selective antagonist BQ123 showed the presence of almost equal populations of ETA (44%) and ETB (56%) receptors in the guinea pig right atria. In a concentration-response study, endothelin 3 (ET-3), an agonist with higher affinity to ETB receptors than to ETA receptors, and sarafotoxin S6c (STXS6c), an ETB receptor-selective agonist, increased the rate of spontaneous beating at all concentrations tested (10 pmol/L to 100 nmol/L). In contrast, ET-1, a nonselective agonist, increased the heart rate at lower concentrations (10 pmol/L to 10 nmol/L) but decreased it at higher concentrations (30 to 100 nmol/L). When ET-1 (100 nmol/L) was applied in a single amount, heart rate was strongly increased; however, this increase was followed by a rapid decline in the response. ET-1 (100 nmol/L) but not ET-3 or STXS6c significantly reduced the heart rate when it was raised by isoproterenol (ISO, 300 nmol/L) either in the absence or presence of a phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX). Correspondingly, ET-1 significantly reduced the ISO-induced elevation of cAMP accumulation (19.1 +/- 1.7 pmol/mg protein [n = 8] and 12.6 +/- 1.2 pmol/mg protein [n = 7] in the absence and presence of ET-1, respectively; P < .01), which was also observed even in the presence of IBMX.(ABSTRACT TRUNCATED AT 250 WORDS)

DELINEATION OF GENOMIC DELETION IN CARDIOMYOPATHIC HAMSTER

Cardiomyopathic hamster is a representative animal model for autosomal recessive cardiomyopathy. We have previously shown that the transcript of δ‐sarcoglycan is missing in the heart of cardiomyopathic hamster due to genomic deletion. Here we define the normal genomic region deleted in cardiomyopathic hamster, which spans about 30 kb interval and includes the two first exons of the δ‐sarcoglycan gene. RNA blot analysis using genomic DNA fragments covering the entire deletion as probes failed to detect any transcript other than δ‐sarcoglycan in normal hamster heart, suggesting that δ‐sarcoglycan is the only transcript defective in the heart of cardiomyopathic hamster.

Concomitant expression of receptor subtype and isopeptide of endothelin by human adrenal gland.

We studied whether specific receptors for endothelin (ET) isopeptide exist in human aldosterone-producing adenoma and normal adrenal cortex, and whether ET isopeptides are produced by human adrenal gland. Competitive binding studies using [125I]ET-1 as a radioligand revealed the presence of a single class of high-affinity binding sites for ET-1 with the apparent KD of 70 +/- 31 pM and Bmax of 226 +/- 139 fmol/mg protein in adenoma membranes almost comparable to those in adjacent normal cortex. The apparent Ki for ET-2 and ET-3 were 89 +/- 33 pM and 82 +/- 16 pM, respectively. Northern blot analysis of poly(A)+ RNA of adenoma and adjacent normal cortex using cDNAs for ET receptor subtype (ETA, ETB) and ET isopeptide (ET-1, ET-3) as probes revealed that ETA and ETB receptors as well as ET isopeptides (preproET-1, preproET-3) are concomitantly expressed in both tissues. Our data demonstrate for the first time that ET receptor subtype (ETA and ETB) and ET isopeptide (ET-1 and ET-3) are concomitantly expressed by human adrenal cortex, suggesting the potential role of ETs as a local mediator in human adrenal gland.

Endothelin receptors in human parathyroid gland

We studied whether specific receptors for endothelins (ETs) exist in human parathyroid tissues and whether ETs may have any effect on secretion of PTH from parathyroid cells. Binding studies using [125I]ET-1 to the parathyroid membranes obtained from patients with hyperparathyroidism (2 adenomas, 2 hyperplasias) revealed that ET-1 competitively inhibited the binding of [125I]ET-1 to the membranes (the apparent Kd: 62 +/- 18 pM), whereas ET-3 showed biphasic and less steep inhibition curve than ET-1 in all tissue membranes examined. Northern blot analysis of poly(A)+ RNA from the parathyroid adenoma clearly demonstrated gene expression of both ETA and ETB receptors as well as preproET-1. ET-1 inhibited basal PTH secretion from dispersed adenoma cells more potently than ET-3. The present study clearly demonstrates the presence of both ETA and ETB receptor subtypes in human parathyroid tissues through which ETs may modulate PTH secretion in an autocrine and/or paracrine manner.