Desensitization of the human beta 1-adrenergic receptor. Involvement of the cyclic AMP-dependent but not a receptor-specific protein kinase

Dopaminergic and adrenergic toxicities on SK-N-MC human neuroblastoma cells are mediated through G protein signaling and oxidative stress

Dopamine and norepinephrine are neurotransmitters which participate in various regulatory functions of the human brain. These functions are lost in neurodegenerative diseases including Parkinson's disease and Alzheimer's disease. In this study, we used SK-N-MC neuroblastoma cells to investigate the cytotoxicities of high concentrations of dopamine and norepinephrine on neuronal cells. Dopamine, norepinephrine, as well as their corresponding synthetic agonists (SKF38393 and isoproterenol, respectively) triggered SK-N-MC cell death when applied at 50-100 muM persistently for 2 days. This catecholamine-induced cell death appears to be neuronal specific, as demonstrated by their inabilities of triggering apoptosis of A549 lung carcinoma cells and Cos-7 kidney fibroblasts. By pretreating SK-N-MC cells with target-specific inhibitors before administration of catecholamine, components of G protein signaling (i.e. G( s )/cAMP/PKA), monoamine oxidases, nitric oxide synthase, c-Jun N-terminal kinase and oxidative stress were found to be involved in this dopamine/norepinephrine-induced cytotoxicity, which subsequently led to caspase-dependent and -independent apoptotic responses as well as DNA degradation. In contrast, agonists of G( i )-coupled dopamine receptors and adrenergic receptors (quinpirole and UK14,304, respectively) were incapable of triggering apoptosis of SK-N-MC cells. Our results suggest that both G protein (G( s ))-mediated signaling cascade and oxidative stress participate in the dopamine/norepinephrine-induced neuronal apoptosis.

Evidence for the existence of an HP1-mediated subcode within the histone code

Currently, the mammalian heterochromatic proteins HP1alpha, HP1beta and the pan-nuclear HP1gamma are considered 'gatekeepers' of methyl-K9-H3-mediated silencing. Understanding how the binding of these proteins to post-translationally modified histones is switched on and off will further our knowledge of how the histone code is modulated. Here, we report that all three HP1 isoforms can be extensively modified, similar to histones, suggesting that the silencing of gene expression may be further regulated beyond the histone code. To assess the potential impact of these modifications, we analysed the phosphorylation of HP1gamma at Ser 83 as a 'model modification'. We demonstrate that P-Ser 83-HP1gamma has an exclusively euchromatic localization, interacts with Ku70 (a regulatory protein involved in multiple nuclear procesess), has impaired silencing activity and serves as a marker for transcription elongation. These observations predict that regulation of silencing by methyl-K9-H3 through modification of mammalian HP1 proteins may be more complex than previously thought and suggests the existence of an HP1-mediated 'silencing subcode' that underlies the instructions of the histone code.

Role of specific protein kinase C isoforms in modulation of β1- and β2-adrenergic receptors

The function of beta-adrenergic receptor (betaAR) is modulated by the activity status of alpha1-adrenergic receptors (alpha1ARs) via molecular crosstalk, and this becomes evident when measuring cardiac contractile responses to adrenergic stimulation. The molecular mechanism underlying this crosstalk is unknown. We have previously demonstrated that overexpression of alpha1B-adrenergic receptor (alpha1BAR) in transgenic mice leads to a marked desensitization of betaAR-mediated adenylyl cyclase stimulation which is correlated with increased levels of activated protein kinase C (PKC) beta, delta and [J. Mol. Cell. Cardiol. 30 (1998) 1827]. Therefore, we wished to determine which PKC isoforms play a role in heterologous betaAR desensitization and also which isoforms of the betaAR were the molecular target(s) for PKC. In experiments using constitutively activated PKC expression constructs transfected into HEK 293 cells also expressing the beta2AR, constitutively active (CA)-PKC overexpression was first confirmed by immunoblots using specific anti-PKC antibodies. We then demonstrated that the different PKC subtypes lead to a decreased maximal cAMP accumulation following isoproterenol stimulation with a rank order of PKCalpha > or = PKCzeta>PKC>PKCbetaII. However, a much more dramatic desensitization of adenylyl cyclase stimulation was observed in cells co-transfected with different PKC isoforms and beta1AR. Further, the modulation of beta1AR by PKC isoforms had a different rank order than for the beta2AR: PKCbetaII>PKCalpha>PKC>PKCzeta. PKC-mediated desensitization was reduced by mutating consensus cAMP-dependent protein kinase (PKA)/PKC sites in the third intracellular loop and/or the carboxy-terminal tail of either receptor. Our results demonstrate therefore that the beta1AR is the most likely molecular target for PKC-mediated heterologous desensitization in the mammalian heart and that modulation of adrenergic receptor activity in any given cell type will depend on the complement of PKC isoforms present.

Antidepressant-induced switch of beta 1-adrenoceptor trafficking as a mechanism for drug action

Reduction in surface beta(1)-adrenoceptor (beta1AR) density is thought to play a critical role in mediating the therapeutic long term effects of antidepressants. Since antidepressants are neither agonists nor antagonists for G protein-coupled receptors, receptor density must be regulated through processes independent of direct receptor activation. Endocytosis and recycling of the beta1AR fused to green fluorescent protein at its carboxyl-terminus (beta1AR-GFP) were analyzed by confocal fluorescence microscopy of live cells and complementary ligand binding studies. In stably transfected C6 glioblastoma cells, beta1AR-GFP displayed identical ligand-binding isotherms and adenylyl cyclase activation as native beta1AR. Upon exposure to isoproterenol, a fraction of beta1AR-GFP (10-15%) internalized rapidly and colocalized with endocytosed transferrin receptors in an early endosomal compartment in the perinuclear region. Chronic treatment with the tricyclic antidepressant desipramine (DMI) did not affect internalization characteristics of beta1AR-GFP when challenged with isoproterenol. However, internalized receptors were not able to recycle back to the cell surface in DMI-treated cells, whereas recycling of transferrin receptors was not affected. Endocytosed receptors were absent from structures that stained with fluorescently labeled dextran, and inhibition of lysosomal protease activity did not restore receptor recycling, indicating that beta1AR-GFP did not immediately enter the lysosomal compartment. The data suggest a new mode of drug action causing a "switch" of receptor fate from a fast recycling pathway to a slowly exchanging perinuclear compartment. Antidepressant-induced reduction of receptor surface expression may thus be caused by modulation of receptor trafficking routes.

G protein-coupled receptors: dominant players in cell-cell communication

The G protein-coupled receptors (GPCRs) are the most numerous and the most diverse type of receptors (1-5% of the complete invertebrate and vertebrate genomes). They transduce messages as different as odorants, nucleotides, nucleosides, peptides, lipids, and proteins. There are at least eight families of GPCRs that show no sequence similarities and that use different domains to bind ligands and activate a similar set of G proteins. Homo- and heterodimerization of GPCRs seem to be the rule, and in some cases an absolute requirement, for activation. There are about 100 orphan GPCRs in the human genome which will be used to find new message molecules. Mutations of GPCRs are responsible for a wide range of genetic diseases. The importance of GPCRs in physiological processes is illustrated by the fact that they are the target of the majority of therapeutical drugs and drugs of abuse.

Regulation of human beta(1)-adrenergic receptors and their mRNA in neuroepithelioma SK-N-MC cells: effects of agonist, forskolin, and protein kinase A

We determined the effect of long-term exposure to beta-agonists on beta(1)-adrenergic receptors (beta(1)-AR) in human neuroepithelioma SK-N-MC cells because earlier studies have indicated that beta(1)-AR in this cell line are resistant to agonist-induced down-regulation. Exposing SK-N-MC cells to isoproterenol for 24 hr reduced the density of beta(1)-AR by 72%, whereas forskolin, an activator of all the isoforms of adenylyl cyclase, failed to affect the density of beta(1)-AR. Measurement of beta(1)-AR mRNA levels by the ribonuclease protection assay revealed that isoproterenol-induced down-regulation of beta(1)-AR was associated with a sharp decline in beta(1)-AR mRNA, while forskolin also failed to affect this parameter. The differences between the effects of isoproterenol and forskolin on beta(1)-AR were unrelated to cyclic AMP levels, since both agents increased cyclic AMP equally. Next, we determined the role of cyclic AMP-dependent protein kinase A (PKA) in this phenomenon. Inhibition of PKA by its specific inhibitor, H-89 [N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide, 2HCl], markedly reduced the magnitude of the isoproterenol-mediated down-regulation of the beta(1)-AR and its mRNA. Transient expression of the catalytic subunit of PKA in SK-N-MC cells down-regulated beta(1)-AR independently of isoproterenol. Therefore, PKA is central to the effect of beta-agonists in down-regulating beta(1)-AR, and its spatial compartmentalization and access to the receptor appear to be essential components of its action.

Ectopic and abnormal hormone receptors in adrenal Cushing's syndrome

The mechanism by which cortisol is produced in adrenal Cushing's syndrome, when ACTH is suppressed, was previously unknown and was referred to as being "autonomous." More recently, several investigators have shown that some cortisol and other steroid-producing adrenal tumors or hyperplasias are under the control of ectopic (or aberrant, illicit, inappropriate) membrane hormone receptors. These include ectopic receptors for gastric inhibitory polypeptide (GIP), beta-adrenergic agonists, or LH/hCG; a similar outcome can result from altered activity of eutopic receptors, such as those for vasopressin (V1-AVPR), serotonin (5-HT4), or possibly leptin. The presence of aberrant receptors places adrenal cells under stimulation by a trophic factor not negatively regulated by glucocorticoids, leading to increased steroidogenesis and possibly to the proliferative phenotype. The molecular mechanisms responsible for the abnormal expression and function of membrane hormone receptors are still largely unknown. Identification of the presence of these illicit receptors can eventually lead to new pharmacological therapies as alternatives to adrenalectomy, now demonstrated by the long-term control of ectopic P-AR- and LH/hCGR-dependent Cushing's syndrome by propanolol and leuprolide acetate. Further studies will potentially identify a larger diversity of hormone receptors capable of coupling to G proteins, adenylyl cyclase, and steroidogenesis in functional adrenal tumors and probably in other endocrine and nonendocrine tumors.

Interaction with beta-arrestin determines the difference in internalization behavor between beta1- and beta2-adrenergic receptors

The beta(1)-adrenergic receptor (beta(1)AR) shows the resistance to agonist-induced internalization. As beta-arrestin is important for internalization, we examine the interaction of beta-arrestin with beta(1)AR with three different methods: intracellular trafficking of beta-arrestin, binding of in vitro translated beta-arrestin to intracellular domains of beta(1)- and beta(2)ARs, and inhibition of betaAR-stimulated adenylyl cyclase activities by beta-arrestin. The green fluorescent protein-tagged beta-arrestin 2 translocates to and stays at the plasma membrane by beta(2)AR stimulation. Although green fluorescent protein-tagged beta-arrestin 2 also translocates to the plasma membrane, it returns to the cytoplasm 10-30 min after beta(1)AR stimulation. The binding of in vitro translated beta-arrestin 1 and beta-arrestin 2 to the third intracellular loop and the carboxyl tail of beta(1)AR is lower than that of beta(2)AR. The fusion protein of beta-arrestin 1 with glutathione S-transferase inhibits the beta(1)- and beta(2)AR-stimulated adenylyl cyclase activities, although inhibition of the beta(1)AR-stimulated activity requires a higher concentration of the fusion protein than that of the beta(2)AR-stimulated activity. These results suggest that weak interaction of beta(1)AR with beta-arrestins explains the resistance to agonist-induced internalization. This is further supported by the finding that beta-arrestin can induce internalization of beta(1)AR when beta-arrestin 1 does not dissociate from beta(1)AR by fusing to the carboxyl tail of beta(1)AR.

Analysis of domain responsible for desensitization of beta1-adrenergic receptor

When the wild type beta1-adrenergic receptor (WT-beta1AR) was expressed in Sf9 cells, the beta1AR-stimulated adenylyl cyclase activities were desensitized by prior treatment with isoproterenol. The extent of beta1AR desensitization was not modified, and the onset was not promoted by the overexpression of G protein-coupled receptor kinase 2 (GRK2), GRK5 or GRK6. However, overexpression of the dominant negative mutant of GRK2 appeared to inhibit desensitization of the beta1AR. The change of the potential protein kinase A phosphorylation site located at the intracellular third loop did not affect beta1AR desensitization. Desensitization of the truncated mutant, in which nearly all of the serine and threonine residues from the carboxyl terminus were eliminated, was the same as that of the WT-beta1AR. A deletion mutant that lacked serine and threonine residues of the intracellular third loop was also desensitized by isoproterenol stimulation. Furthermore, the deletion of serine and threonine residues from both the intracellular third loop and carboxyl terminus did not affect desensitization of the beta1AR. These results suggested that phosphorylation by endogenous GRKs in Sf9 cells contributed to desensitization of the beta1AR and that the regions other than third intracellular loop and carboxyl terminus may be responsible for beta1AR desensitization.

Desensitization of G-protein-coupled receptors in the cardiovascular system

Multiple mechanisms exist to control the signaling and density of G-protein-coupled receptors (GPRs). Upon agonist binding and receptor activation, a series of reactions participate in the turn off or desensitization of GPRs. Many GPRs are phosphorylated by protein kinases and consequently uncoupled from G proteins. In addition, many GPRs are sequestered from the cell surface and become inaccessible to their activating ligands. Both receptor:G protein uncoupling and receptor sequestration may involve the participation of arrestins or other proteins. A model for receptor regulation has been developed from studies of the beta-adrenergic receptor. However, recent studies suggest that other GPRs important in the cardiovascular system, such as the muscarinic cholinergic receptors that regulate heart rate, might be regulated by mechanisms other than those that regulate the beta-adrenergic receptors. This review summarizes our current understanding of the processes involved in the desensitization of GPRs.

Endogenous beta 3- but not beta 1-adrenergic receptors are resistant to agonist-mediated regulation in human SK-N-MC neurotumor cells

Although there is considerable interest in the regulation of the different beta-adrenergic receptor (AR) subtypes, most previous studies have utilized stably transfected cells expressing recombinant receptors under the control of viral promoters. Human SK-N-MC neurotumor cells appear to be novel, since they express both endogenous beta 1AR and beta 3AR based on radioligand binding and on functional response. Saturation binding of either the hydrophilic ligand (-)-[3H]CGP-12177 or the more hydrophobic (-)-[125I]iodocyanopindolol indicated the presence of two populations of binding sites with high and low affinities. With either ligand, the beta 1AR antagonist CGP-20712A preferentially inhibited binding to the high-affinity sites. This is consistent with the latter representing beta 1AR whereas the low-affinity sites represent beta 3AR. Both subtypes appeared to be functional on the basis of isoproterenol stimulation of cyclic adenosine monophosphate (cAMP) in intact cells and adenylyl cyclase activity in cell membranes in the absence and presence of CGP-20712A. SK-N-MC-IXC cells, derived by twice subcloning the parental cells, also expressed both beta AR subtypes, indicating that they co-exist in the same cell. SK-N-MC cells exposed to isoproterenol exhibited a rapid sequestration and a slower downregulation of beta 1AR. The latter subtype also underwent desensitization, as indicated by a rightward shift to less sensitivity in the EC50 for isoproterenol stimulation of adenylyl cyclase activity. In contrast, the beta 3AR subtype was resistant to agonist-mediated sequestration, downregulation, and desensitization. Thus, when endogenously expressed in the same cell line, human beta 1AR and beta 3AR display differences in their ability to be regulated by agonist.

Beta-adrenergic receptors in failing human myocardium

In the human heart the beta-adrenergic receptor-G-protein-adenylyl cyclase system is the most powerful physiologic mechanism to acutely increase contractility and/or heart rate. In the failing human myocardium beta 1-adrenergic receptor number is decreased, and this is accompanied by a reduced beta 1-adrenergic receptor mediated positive inotropic effect. Cardiac beta 2-adrenergic receptor number may or may not decrease; however, beta 2- adrenergic receptor mediated positive inotropic effects are also reduced, possibly because the functional activity of myocardial Gi is increased, thereby inhibiting cyclic AMP formation. The aging human heart shows some similarities with the failing human heart: in both settings, of chronic heart failure and age, beta-adrenergic receptor mediated effects and all other cyclic AMP dependent effects are depressed and Gi-protein is increased.

Anomalous behavior of CGP 12177A on beta 1-adrenergic receptors

CGP 12177A originally was developed as a hydrophilic antagonist to detect cell surface beta 1- and beta 2-adrenergic receptors, and subsequently was found to be a partial agonist for the atypical or beta 3-adrenergic receptor. Using hamster cells stably expressing either the human beta 1-, human beta 2- or rat beta 1-adrenergic receptor, we found that CGP 12177A behaved as an agonist of beta 1-adrenergic receptors. Whereas at low concentrations, CGP 12177a behaved as an antagonist and inhibited isoproterenol stimulation of adenylyl cyclase activity, at higher concentrations, it stimulated a response even in the absence of isoproterenol. The agonistic properties of CGP 12177A were positively correlated with the level of beta 1-adrenergic receptor expression. Thus, at low receptor of densities, CGP 12177A behaved as a weak, partial agonist whereas as high receptor densities, the drug was a full agonist. At similar high densities of the beta 2-adrenergic receptor, CGP 12177A acted only as a partial agonist. Competition binding studies to membranes from cells expressing beta 1-adrenergic receptors indicated that approximately 90% of the receptors were in a high affinity, guanine nucleotide-insensitive state for CGP 12177A whereas approximately 10% of the receptors were in a lower affinity, guanine nucleotide-sensitive state for CGP 12177A. We propose that the latter receptors are precoupled to stimulatory G proteins and recognize CGP 12177A as an agonist whereas the high affinity, uncoupled receptors recognize CGP 12177A as an antagonist.

Adrenergic receptors and fat cells: differential recruitment by physiological amines and homologous regulation

The control of fat cell lipolysis by the catecholamines involves at least four different adrenoceptor subtypes; three beta (beta 1-, beta 2-, and beta 3-ARs) and one alpha 2-adrenoceptor (alpha 2-AR). The physiological importance of the beta- and alpha 2A-ARs varies according to the species, the sex, the age, the anatomical location of fat deposits and the degree of obesity in humans and animals. The physiological amines operate through differential recruitment of these sites on the basis of their relative affinities. This point has been assessed by in vitro studies and has partly been confirmed in in vivo experiments using selected alpha/beta-AR antagonists and in situ microdialysis. The affinity of the beta 3-AR for catecholamines is less than that of the classical beta 1- and beta 2-ARs in the various species investigated. Conversely, it is the alpha 2-AR which exhibit the highest affinity for the physiological amines in all fat cells. The relative order of affinity of the various fat cell ARs for the physiological amines defined in binding studies and in vitro assays is alpha 2 > beta 1 > or = beta 2 > beta 3 for norepinephrine and alpha 2 > beta 2 > beta 1 > beta 3 for epinephrine. When considering differential beta-AR recruitment by catecholamines, it is the beta 1-AR which is always activated at the lowest norepinephrine levels, whatever the species, while the activation of the beta 3-AR requires higher norepinephrine levels. In addition to the differential recruitment, differential regulation by hormones could also occur for each fat cell AR subtype. The alpha 2-and beta 3-ARs are less prone to desensitization and down-regulation by comparison with the beta 1- and beta 2-AR.

Phosphorylation and desensitization of the human beta 1-adrenergic receptor. Involvement of G protein-coupled receptor kinases and cAMP-dependent protein kinase

Persistent stimulation of the beta 1-adrenergic receptor (beta 1AR) engenders, within minutes, diminished responsiveness of the beta 1 AR/adenylyl cyclase signal transduction system. This desensitization remains incompletely defined mechanistically, however. We therefore tested the hypothesis that agonist-induced desensitization of the beta 1AR (like that of the related beta 2AR) involves phosphorylation of the receptor itself, by cAMP-dependent protein kinase (PKA) and the beta-adrenergic receptor kinase (beta ARK1) or other G protein-coupled receptor kinases (GRKs). Both Chinese hamster fibroblast and 293 cells demonstrate receptor-specific desensitization of the beta 1 AR within 3-5 min. Both cell types also express beta ARK1 and the associated inhibitory proteins beta-arrestin-1 and beta-arrestin-2, as assessed by immunoblotting. Agonist-induced beta 1AR desensitization in 293 cells correlates with a 2 +/- 0.3-fold increase in phosphorylation of the beta 1AR, determined by immunoprecipitation of the beta 1AR from cells metabolically labeled with 32P(i). This agonist-induced beta 1AR phosphorylation derives approximately equally from PKA and GRK activity, as judged by intact cell studies with kinase inhibitors or dominant negative beta ARK1 (K220R) mutant overexpression. Desensitization, likewise, is reduced by only approximately 50% when PKA is inhibited in the intact cells. Overexpression of rhodopsin kinase, beta ARK1, beta ARK2, or GRK5 significantly increases agonist-induced beta 1AR phosphorylation and concomitantly decreases agonist-stimulated cellular cAMP production (p < 0.05). Furthermore, purified beta ARK1, beta ARK2, and GRK5 all demonstrate agonist-dependent phosphorylation of the beta 1AR. Consistent with a GRK mechanism, receptor-specific desensitization of the beta 1AR was enhanced by overexpression of beta-arrestin-1 and -2 in transfected 293 cells. We conclude that rapid agonist-induced desensitization of the beta 1AR involves phosphorylation of the receptor by both PKA and at least beta ARK1 in intact cells. Like the beta 2AR, the beta 1AR appears to bind either beta-arrestin-1 or beta-arrestin-2 and to react with rhodopsin kinase, beta ARK1, beta ARK2, and GRK5.

Differences in desensitization between human beta 1- and beta 2-adrenergic receptors stably expressed in transfected hamster cells

It is well-established that agonist-mediated desensitization of the beta 2-adrenergic receptor (beta 2AR) involves its phosphorylation by protein kinase A (PKA) and the beta AR kinase (beta ARK). The phosphorylated receptor is less efficient at mediating agonist stimulation of adenylyl cyclase activity. The result is an increase in the concentration of agonist required for half-maximal stimulation (EC50) and a reduction in maximal stimulation (Vmax). As less is known about desentization of the human beta 1 AR, we compared the desensitization pattern of human beta 1 AR and beta 2AR stably expressed in two different hamster cell lines: Chinese hamster ovary (CHO), and Chinese hamster fibroblast (CHW). Following agonist treatment, all of the cell lines exhibited an increase in EC50, and a reduction in Vmax was observed in CHO-beta 2 but not beta 1 cells. CHW-beta 1 cells were resistant to acute agonist-mediated reduction in Vmax compared to CHW-beta 2 cells. More prolonged agonist exposure produced a modest reduction in Vmax and this effect was more noticeable when the CHW cells expressed lower levels of beta 1AR. To explore the role of protein kinases in these effects, digitonin-permeabilized CHW cells were loaded either with heparin (a beta ARK inhibitor) or a peptide inhibitor of PKA and exposed to agonist. In both beta 2AR- and beta 1AR-expressing cells, heparin inhibited the reduction in Vmax and the PKA inhibitor blocked the increase in EC50. Finally, exposing CHW cells expressing either subtype to a permeable cyclic AMP derivative caused an increase in EC50 similar to that observed in agonist-treated cells, but without any reduction in maximal activity. Our data suggest that whereas PKA-mediated desensitization is not subtype-specific, human beta 1AR is more resistant to beta ARK-mediated desensitization compared to the human beta 2AR.

The heat-labile enterotoxin of Escherichia coli binds to polylactosaminoglycan-containing receptors in CaCo-2 human intestinal epithelial cells

The E. coli type I heat-labile enterotoxin (LT-I) shares considerable functional, structural, and immunological homology with cholera toxin (CT). Although the ganglioside GM1 is the sole receptor for CT, LT-I also appears to utilize additional, unique receptors on intestinal cells not recognized by CT. We characterized this second class of LT-I receptors using the human intestinal epithelial cell line, CaCo-2. CaCo-2 cells bound 8-fold more LT-I than CT, and some of these additional LT-I receptors appeared to be functional, as CT-B only partially inhibited LT-I activity at concentrations that completely inhibited CT activity. Membranes from unlabeled or [3H]galactose-labeled cells were incubated with toxin B subunits and extracted with Triton X-100, and the solubilized toxin B-receptor complexes were immunoabsorbed with anti-B bound to protein A-Sepharose. When organic extracts of the complexes were separated by thin-layer chromatography and overlayed with [125I]toxin, both toxins were found to bind only GM1. Separation of the complexes from [3H]galactose-labeled membranes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a series of galactoproteins specifically recognized by LT-I but not by CT. Similar proteins were detected on Western blots probed with [125I]toxin. LT-I activity on intact cells and binding to membranes and the above galactoproteins were enhanced by neuraminidase treatment even in the presence of CT-B. beta-1,4-Galactosidase and endo-beta-1,4-galactosidase, but not beta-1,3-galactosidase, significantly reduced LT-I binding. LT-I binding to fetuin and transferrin exhibited a similar glycosidase sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Independent and coordinate regulation of beta 1- and beta 2-adrenergic receptors in rat C6 glioma cells

Rat C6 glioma cells have both beta 1- and beta 2-adrenergic receptors in approximately 7:3 ratio. When the cells were exposed to the beta-adrenergic agonist isoproterenol, there was a rapid sequestration of up to 50% of the surface receptor population over a 30-min period as measured by the loss of binding of the hydrophilic ligand [3H] CGP-12177 to intact cells. Using the beta 1-selective antagonist CGP 20712A to quantify the proportion of the two subtypes, it was found that although both beta 1 and beta 2 receptors were sequestered, the latter were sequestered initially twice as fast as the former. More prolonged agonist exposure led to a down-regulation of approximately 90% of the total receptor population by 6 h as measured by the loss of binding of the more hydrophobic ligand [125I]iodocyanopindolol to cell lysates. The two subtypes, however, underwent down-regulation with similar kinetics. Treatment of the cells with agents that raise cyclic AMP levels such as cholera toxin and forskolin resulted in a slower, but still coordinated down-regulation of both subtypes. Thus, there appears to be both independent and coordinate regulation of endogenous beta 1-and beta 2-adrenergic receptors in the same cell line.

Autoimmunity in idiopathic dilated cardiomyopathy. Characterization of antibodies against the beta 1-adrenoceptor with positive chronotropic effect

BACKGROUND

Autoantibodies against the beta 1-adrenoceptor have been detected in the sera of patients with idiopathic dilated cardiomyopathy (DCM). The mechanisms by which these autoantibodies can alter normal receptor function are investigated, and the results are interpreted in the light of the beneficial effects of beta 1-blockade in some of these patients.

METHODS AND RESULTS

Autoantibodies against the beta 1-adrenoceptor, affinity purified from sera of patients with idiopathic DCM, were analyzed in a functional test system of spontaneously beating neonatal rat heart myocytes. Antibodies from rabbits immunized with peptides derived from the amino acid sequence of this receptor were also analyzed. Autoantibodies, against the second extracellular loop increased the beating frequency of isolated myocytes in a concentration-dependent manner, to approximately 80% of maximal isoproterenol stimulation. Rabbit anti-peptide antibodies against the second extracellular loop increased the beating frequency correspondingly. Autoantibodies and rabbit anti-peptide antibodies against the second extracellular loop were able to immunoprecipitate the unliganded receptor but not the antagonist-occupied receptor. In contrast, rabbit antibodies against the extracellular N-terminal sequence 34-57 of the beta 1-adrenoceptor were able to immunoprecipitate both the unliganded and the antagonist-occupied receptor although with no effect on the beating frequency of myocytes. The positive chronotropic effect of the antibodies was completely neutralized both by the addition of increasing concentrations of the beta 1-selective antagonist bisoprolol and by preincubation with the peptide corresponding to the second extracellular loop. The antibody-induced increase in beating frequency remained unchanged for more than 6 hours. This should be compared with the isoproterenol-stimulated beating frequency, which undergoes desensitization within 60 minutes. Addition of isoproterenol to autoantibody-stimulated myocytes resulted in only a small increase in beating frequency and did not cause desensitization. Antibodies had only a marginal effect on cyclic AMP production of stimulated cardiomyocytes compared with the 10-fold increase obtained after stimulation with isoproterenol.

CONCLUSIONS

The second extracellular loop of the beta 1-adrenoceptor is a specific target for antibodies with stimulatory activity detected in patients with idiopathic DCM. The antibodies have a positive chronotropic effect on isolated rat heart myocytes. Autoantibody stimulation does not cause the normal agonist-induced desensitization phenomena of the effector system. These findings could contribute to our understanding of the pathophysiological mechanisms of the autoantibodies and of the beneficial effect of beta 1-blocking agents in the treatment of patients with idiopathic DCM.

Primary structure of the mouse beta 1-adrenergic receptor gene

The mouse beta 1-adrenergic receptor was isolated from a genomic library and cloned into pBluescript SK-. Characterization of the clone revealed an open reading frame which encodes a predicted protein of 466 amino acids. The mouse beta 1 receptor is 92.7% identical to the human sequence, 98.5% identical to the rat sequence, and contains a consensus site for N-linked glycosylation at Asn-15 and a cAMP-dependent protein kinase phosphorylation site at Ser-301.

Bioamine receptors: evolutionary and functional variations of a structural leitmotiv

Bioamines act as neurohormonal messengers through their binding to receptors which belong to the largest membrane protein family known so far: the seven spanning membrane receptors. This class of receptors transmits the effect of agonist binding to intracellular effectors by interacting with an intermediary G-protein. The diversity of receptor subtypes inside the protein family, observed in many animal species, is the result of a long evolutionary process. The tendency to protein diversification depends upon gene duplications and upon the continuous accumulation of mutations. The maintenance of vital functions in organisms, however, strictly requires enough structural conservation to ensure the functionality of the corresponding proteins. Both forces cooperate to ensure the adaptation of organisms to a changing environment. We have reviewed here the main conformational and functional constraints exerted on the structure of the bioamine receptors. They are mainly the transmembrane conformation of the receptors, their ability to bind ligands, to interact with G-proteins and to desensitize. The molecular basis of the biochemical and pharmacological differences used to classify the members of the receptor family have also been examined. Interestingly, this classification is very close to that obtained by the molecular phylogeny methods, used to elucidate the evolutionary relationships between bioamine receptors. However, this latter classification allows to accurately distinguish between different receptor subtypes (paralogous genes) and species homologous (orthologous genes). In addition, the calculation of phylogenetical distances reveals two main periods of diversification: the first one occurred before the separation of arthropods from vertebrates, in the Precambrian, and corresponds to the appearance of the main subtypes of the bioamine receptors. The second one, which occurred about 400 million years ago, might accompany the cephalization of the CNS in vertebrates.

α- and β-Adrenoceptors in Hypertension: Molecular Biology and Pharmacological Studies

Recent years have witnessed astonishing progress in our understanding of the molecular basis of adrenoceptor structure, function and regulation and revealed an unexpected heterogeneity of adrenoceptors demonstrating the existence of at least 11 subtypes. This paper discusses the implications of these advances on studies regarding a specific role of adrenoceptors in the development of genetic hypertension. The available data indicate that among the alpha-adrenoceptor subtypes the alpha 2A-adrenoceptor is the most likely candidate for an alteration specifically linked to genetic hypertension in the animal model of the spontaneously hypertensive rat and possibly in some patients. Alterations of other alpha-adrenoceptor subtypes may be specific for some forms of genetic hypertension but are unlikely to play an important role for blood pressure regulation. Most beta-adrenoceptor alterations appear to occur secondary to blood pressure elevation independently of whether hypertension has occurred on a genetic basis or not. Moreover, the mechanisms regulating alpha- and beta-adrenoceptor responsiveness upon prolonged agonist exposure may be altered in hypertension and thereby contribute to the pathophysiology of this disease.

Rapid agonist-induced beta-adrenergic receptor kinase translocation in C6 glioma cells

Exposure of C6 glioma cells to 1 microM isoproterenol leads to fast desensitization of the beta-adrenergic receptor/adenylyl cyclase system and transient receptor sequestration. It also triggers a very rapid and transient translocation to the plasma membrane of beta-adrenergic receptor kinase (beta ARK), a specific cytoplasmic kinase that phosphorylates only the agonist-occupied form of several G protein-coupled receptors. beta ARK-mediated receptor phosphorylation appears to be a suitable mechanism for the rapid regulation of adrenergic receptor function in the nervous tissue.

Insulin desensitized beta 1-adrenergic receptor-mediated stimulation of adenylyl cyclase in SK-N-MC cells

Receptor cross-talk is an emerging field which investigates cross-regulation between distinct classes of receptors. In the present work, we investigated the influence of activating the insulin receptor, a tyrosine kinase receptor, on beta-agonist activation of adenylyl cyclase, which is mediated by a G protein-linked receptor. Treatment of SK-N-MC neuroepithelioma cells with insulin generated a marked attenuation of beta 1-adrenergic receptor-mediated stimulation of adenylyl cyclase. This effect required nanomolar concentrations of insulin, occurred within minutes of exposure of these cells to insulin, and did not result from down-regulation of beta-adrenergic receptors. Insulin alone reduced the maximal isoproterenol-mediated stimulation of adenylyl cyclase by 50%, while the co-addition of the phosphatase inhibitor sodium vanadate increased the magnitude of insulin inhibition to 90%. Insulin provides an additional avenue for heterologous desensitization of beta-adrenergic receptors and their transmembranal signalling pathway.