Agonist regulation of gene expression of adrenergic receptors and G proteins

Neuromodulation of inhibitory synaptic transmission in the basolateral amygdala during fear and anxiety

The basolateral amygdala plays pivotal roles in the regulation of fear and anxiety and these processes are profoundly modulated by different neuromodulatory systems that are recruited during emotional arousal. Recent studies suggest activities of BLA interneurons and inhibitory synaptic transmission in BLA principal cells are regulated by neuromodulators to influence the output and oscillatory network states of the BLA, and ultimately the behavioral expression of fear and anxiety. In this review, we first summarize a cellular mechanism of stress-induced anxiogenesis mediated by the interaction of glucocorticoid and endocannabinoid signaling at inhibitory synapses in the BLA. Then we discuss cell type-specific activity patterns induced by neuromodulators converging on the Gq signaling pathway in BLA perisomatic parvalbumin-expressing (PV) and cholecystokinin-expressing (CCK) basket cells and their effects on BLA network oscillations and fear learning.

Desensitization of cAMP Accumulation via Human β3-Adrenoceptors Expressed in Human Embryonic Kidney Cells by Full, Partial, and Biased Agonists

β₃-Adrenoceptors couple not only to cAMP formation but, at least in some cell types, also to alternative signaling pathways such as phosphorylation of extracellular signal-regulated kinase (ERK). β₃-Adrenoceptor agonists are used in long-term symptomatic treatment of the overactive bladder syndrome; it is only poorly understood which signaling pathway mediates the clinical response and whether it undergoes agonist-induced desensitization. Therefore, we used human embryonic kidney cells stably transfected with human β₃-adrenoceptors to compare coupling of ligands with various degrees of efficacy, including biased agonists, to cAMP formation and ERK phosphorylation, particularly regarding desensitization. Ligands stimulated cAMP formation with a numerical rank order of isoprenaline ≥ L 755,507 ≥ CL 316,243 > solabegron > SR 59,230 > L 748,337. Except for the weakest agonist, L 748,337, pretreatment with any ligand reduced cAMP responses to freshly added isoprenaline or forskolin to a similar extent. On the other hand, we were unable to detect ERK phosphorylation despite testing a wide variation of conditions. We conclude that a minor degree of efficacy for cAMP formation may be sufficient to induced full desensitization of that response. Transfected human embryonic kidney cells are not suitable to study desensitization of ERK phosphorylation by β₃-adrenoceptor stimulation.

Beta-adrenergic receptor mechanisms and pain sensitivity in women with menstrually related mood disorders

Somatic symptoms experienced by women with a menstrually related mood disorder (MRMD) during their premenstrual luteal phase contribute to functional impairment. Yet, investigations on pathophysiological mechanisms contributing to heightened pain sensitivity in MRMD are sparse. During the luteal phase, 61 women with an MRMD and 61 non-MRMD controls were evaluated for β-adrenergic receptor (β-AR) responsivity using the isoproterenol sensitivity test. A subset (43 MRMD and 50 non-MRMD) then entered a double-blind, placebo-controlled, crossover protocol to examine the effect of β-AR blockade with intravenous propranolol on sensitivity to experimental (cold pressor and ischemic) and clinical (McGill Pain Questionnaire score) pain. Women with an MRMD exhibited greater β1- and β2-AR responsivity, ischemic pain intensity, and affective clinical pain ratings than controls. Propranolol increased cold pressor pain tolerance in both groups, but it decreased cold pain intensity and ischemic pain unpleasantness ratings only in non-MRMD women. In contrast, propranolol decreased affective ratings of clinical pain in women with MRMD. Exploratory analyses indicated that only in MRMD women did greater β-AR responsivity predict greater sensitivity to cold pressor and ischemic pain. This study provides the first evidence for a role of β-AR mechanisms in the hyperalgesia and clinical pain experienced by women with MRMDs.

PERSPECTIVE

This article describes the effects of β-adrenergic receptor stimulation and blockade on experimental and clinical pain sensitivity in women with an MRMD. The results of this study may have implications for the management of the substantial somatic premenstrual symptomatology experienced by women with an MRMD.

Regulatory mechanism of G protein-coupled receptor trafficking to the plasma membrane: a role for mRNA localization

Trafficking and localization of G protein-coupled receptors (GPCRs) to the plasma membrane and its retention in the agonist-naive state are critically important for signaling by these receptors. Agonist-induced desensitization of activated GPCRs and their removal from the cell surface have been studied and reviewed extensively. However, less attention has been given to the regulatory mechanisms and different steps that control the trafficking of newly synthesized receptors to the plasma membrane. It is generally believed that the mRNAs encoding GPCRs are targeted to the endoplasmic reticulum by a cotranslational, signal-sequence recognition particle-dependent pathway that results in protein translation and translocation to the plasma membrane. In this chapter, we discuss the importance of cis-targeting elements and trans-recognition factors in GPCR mRNA translational silencing, trafficking, and localization within the cell and its importance in receptor trafficking to the plasma membrane. Knockdown of the critical trans-recognition factors (RNA-binding proteins) resulted in translation of GPCR mRNAs in the perinuclear region and the receptors failed to traffic to the plasma membrane. Thus, a new paradigm is emerging in GPCR trafficking that suggests a fundamental role for mRNA partitioning to specific cytoplasmic regions for efficient plasma membrane localization of the receptors.

β-Adrenergic Receptor-Stimulated Cardiac Myocyte Apoptosis: Role of β1 Integrins

Increased sympathetic nerve activity to the myocardium is a central feature in patients with heart failure. Accumulation of catecholamines plays an important role in the pathogenesis of heart disease. Acting via β-adrenergic receptors (β-AR), catecholamines (norepinephrine and isoproterenol) increase cardiac myocyte apoptosis in vitro and in vivo. Specifically, β₁-AR and β₂-AR coupled to Gαs exert a proapoptotic action, while β₂-AR coupled to Gi exerts an antiapoptotic action. β1 integrin signaling protects cardiac myocytes against β-AR-stimulated apoptosis in vitro and in vivo. Interaction of matrix metalloproteinase-2 (MMP-2) with β1 integrins interferes with the survival signals initiated by β1 integrins. This paper will discuss background information on β-AR and integrin signaling and summarize the role of β1 integrins in β-AR-stimulated cardiac myocyte apoptosis.

Both ligand- and cell-specific parameters control ligand agonism in a kinetic model of g protein-coupled receptor signaling

G protein–coupled receptors (GPCRs) exist in multiple dynamic states (e.g., ligand-bound, inactive, G protein–coupled) that influence G protein activation and ultimately response generation. In quantitative models of GPCR signaling that incorporate these varied states, parameter values are often uncharacterized or varied over large ranges, making identification of important parameters and signaling outcomes difficult to intuit. Here we identify the ligand- and cell-specific parameters that are important determinants of cell-response behavior in a dynamic model of GPCR signaling using parameter variation and sensitivity analysis. The character of response (i.e., positive/neutral/inverse agonism) is, not surprisingly, significantly influenced by a ligand's ability to bias the receptor into an active conformation. We also find that several cell-specific parameters, including the ratio of active to inactive receptor species, the rate constant for G protein activation, and expression levels of receptors and G proteins also dramatically influence agonism. Expressing either receptor or G protein in numbers several fold above or below endogenous levels may result in system behavior inconsistent with that measured in endogenous systems. Finally, small variations in cell-specific parameters identified by sensitivity analysis as significant determinants of response behavior are found to change ligand-induced responses from positive to negative, a phenomenon termed protean agonism. Our findings offer an explanation for protean agonism reported in β₂--adrenergic and α_2A-adrenergic receptor systems.

G protein–coupled receptors (GPCRs) are transmembrane proteins involved in physiological functions ranging from vasodilation and immune response to memory. The binding of both endogenous ligands (e.g., hormones, neurotransmitters) and exogenous ligands (e.g., pharmaceuticals) to these receptors initiates intracellular events that ultimately lead to cell responses. We describe a dynamic model for G protein activation, an immediate outcome of GPCR signaling, and use it together with efficient parameter variation and sensitivity analysis techniques to identify the key cell- and ligand-specific parameters that influence G protein activation. Our results show that although ligand-specific parameters do strongly influence cell response (either causing increases or decreases in G protein activation), cellular parameters may also dictate the magnitude and direction of G protein activation. We apply our findings to describe how protean agonism, a phenomenon in which the same ligand may induce both positive and negative responses, may result from changes in cell-specific parameters. These findings may be used to understand the molecular basis of different responses of cell types and tissues to pharmacological treatment. In addition, these methods may be applied generally to models of cellular signaling and will help guide experimental resources toward further characterization of the key parameters in these networks.

Insulin-like growth factor-I provokes functional antagonism and internalization of beta1-adrenergic receptors

Hormones that activate receptor tyrosine kinases have been shown to regulate G protein-coupled receptors, and herein we investigate the ability of IGF-I to regulate the beta(1)-adrenergic receptor. Treating Chinese hamster ovary cells in culture with IGF-I is shown to functionally antagonize the ability of expressed beta(1)-adrenergic receptors to accumulate intracellular cAMP in response to stimulation by the beta-adrenergic agonist Iso. The attenuation of beta(1)-adrenergic action was accompanied by internalization of beta(1)-adrenergic receptors in response to IGF-I. Inhibiting either phosphatidylinositol 3-kinase or the serine/threonine protein kinase Akt blocks the ability of IGF-I to antagonize and to internalize beta(1)-adrenergic receptors. Mutation of one potential Akt substrate site Ser412Ala, but not another Ser312Ala, of the beta(1)-adrenergic receptor abolishes the ability of IGF-I to functionally antagonize and to sequester the beta(1)-adrenergic receptor. We also tested the ability of IGF-I to regulate beta(1)-adrenergic receptors and their signaling in adult canine cardiac myocytes. IGF-I attenuates the ability of beta(1)-adrenergic receptors to accumulate intracellular cAMP in response to Iso and promotes internalization of beta(1)-adrenergic receptors in these cardiac myocytes.

Lysophosphatidic acid regulates trafficking of beta2-adrenergic receptors: the Galpha13/p115RhoGEF/JNK pathway stimulates receptor internalization

Lysophosphatidic acid is an important lipid ligand regulating many aspects of cell function, including proliferation and migration. Operating via heterotrimeric G proteins to downstream effectors, lysophosphatidic acid was shown to regulate the function and trafficking of the G protein-coupled beta(2)-adrenergic receptor. C3 exotoxin, expression of dominant negative RhoA, and inhibition of c-Jun N-terminal kinase blocked the ability of lysophosphatidic acid to sequester the beta(2)-adrenergic receptor, whereas expression of constitutively active Galpha(13), p115RhoGEF, or RhoA mimicked lysophosphatidic acid (LPA) action, stimulating the internalization of the Galpha(s)-coupled beta(2)-adrenergic receptor. This study revealed a novel cross-talk exerted from the LPA/Galpha(13)/p115RhoGEF/RhoA pathway to the beta(2)-adrenergic receptor/Galpha(s)/adenylyl cyclase pathway, attenuating the ability of beta-adrenergic agonists to act following stimulation of cells by LPA as may occur during beta-adrenergic therapy of an inflammatory response.

Sensitization of adenylate cyclase by Galpha i/o-coupled receptors

Activation of receptors coupled to inhibitory G proteins (Galpha i/o) has opposing consequences for cyclic AMP accumulation and the activity of cyclic AMP-dependent protein kinase, depending on the duration of stimulation. Acute activation inhibits the activity of adenylate cyclase, thereby attenuating cyclic AMP accumulation; in contrast, persistent activation of Galpha i/o-coupled receptors produces a paradoxical enhancement of adenylate cyclase activity, thus increasing cyclic AMP accumulation when the action of the inhibitory receptor is terminated. This heterologous sensitization of cyclic AMP signaling, also called superactivation or supersensitization, likely represents a cellular adaptive response, a mechanism by which the cell compensates for chronic inhibitory input. Recent advances in our knowledge of G protein-mediated signaling, regulation of adenylate cyclase, and other cellular signaling mechanisms have extensively increased our insight into the mechanisms and significance of this phenomenon. In particular, recent evidence points to the Galpha(s)-adenylate cyclase interface as a locus for the expression of the sensitized adenylate cyclase response, and to isoform-specific phosphorylation of adenylate cyclase as one mechanism that can produce sensitization. Galpha i/o-coupled receptor-induced heterologous sensitization may contribute to enhanced Galpha(s)-coupled receptor signaling following neurotransmitter elevations induced by the administration of drugs of abuse and during other types of neuronal function or dysfunction. This review will focus on recent advances in our understanding of signaling pathways that are involved in sensitization and describe the potential role of sensitization in neuronal function.

Intracellular signaling pathways for norepinephrine- and endothelin-1-mediated regulation of myocardial cell apoptosis

Accumulating data support the idea that apoptosis in cardiac myocytes, in part, contributes to the development of heart failure. Since a number of neurohormonal factors are activated in this state, these factors may be involved in the positive and negative regulation of apoptosis in cardiac myocytes. Norepinephrine is one such factor and induces apoptosis in cardiac myocytes via a beta-adrenergic receptor pathway. beta-adrenergic agonist-induced apoptosis in cardiac myocytes is dependent on the activation of the cAMP/protein kinase A pathway. Interestingly, the activation of this pathway protects PC12 cells from apoptosis, suggesting that cAMP/protein kinase A regulates apoptosis in a cell type-specific manner. Another neurohormonal factor activated in heart failure is endothelin-1, which acts as a potent survival factor against myocardial cell apoptosis. Intracellular signaling pathways for endothelin-1-mediated protection include activation of MEK-1 /ERK1/2 and PI3 kinase. In addition to these protective pathways common among cell types, endothelin- activates the calcium-activated phosphatase calcineurin, which is necessary for the nuclear import of NFAT transcription factors. These factors interact with the cardiac-restricted zinc finger protein GATA-4 and induce transcription and expression of anti-apoptotic molecule bcl-2. Thus, myocardial cell apoptosis is regulated by pathways unique to cardiac myocytes as well as by those common among cell types. It should be further determined whether agents that specifically block myocardial cell apoptosis will attenuate the progression of heart failure.

Sensitization of adenylate cyclase by short-term activation of 5-HT1A receptors

Long-term (18 h) activation of 5-HT1A receptors alters 5-HT1A receptor-G protein coupling and leads to heterologous sensitization of adenylate cyclase. In contrast, the effects of short-term (2 h) 5-HT1A receptor activation on subsequent adenylate cyclase activity have not been determined. The present study examined and characterized 5-HT1A receptor-induced heterologous sensitization following short-term activation in CHO-5-HT1A cells. Short-term activation of 5-HT1A receptors with full agonists, as well as the partial agonist, buspirone, markedly enhanced subsequent forskolin-stimulated cyclic AMP accumulation. This heterologous sensitization was evident after 30 min treatment with 5HT and appeared to be near maximal following 2 h agonist treatment. Sensitization was characterized by a dose-dependent increase in forskolin-stimulated cyclic AMP accumulation and was prevented by WAY 100635 or by pertussis toxin treatment. The ability of the 5-HT1A agonists to induce heterologous sensitization was not significantly altered by agents shown previously to modulate 5-HT1A-mediated inhibition of cyclic AMP accumulation.

Effects of chronic hypoxia on maternal vasodilation and vascular reactivity in guinea pig and ovine pregnancy

During pregnancy, exposure to chronic hypoxia is thought to be associated with an increased risk of preeclampsia and fetal intrauterine growth restriction (IUGR). While some studies suggest that this process may be mediated through effects of chronic hypoxia on uterine artery vasodilation and growth, these observations are likely to be species specific and may represent genetic variability in maternal adaptation to hypoxia. This review is a comparative analysis of the effects of chronic hypoxia on vascular reactivity in pregnant and nonpregnant guinea pig and sheep. Data suggest that exposure to chronic hypoxia is associated with enhanced uterine artery blood flow in the sheep, whereas, in the guinea pig, blood flow is decreased.

Sensitization of adenylate cyclase: a general mechanism of neuroadaptation to persistent activation of Galpha(i/o)-coupled receptors?

Acute activation of Galphas-coupled receptors stimulates cyclic AMP accumulation leading to the activation of downstream signaling cascades. These Galphas-mediated events can be countered by acute activation of inhibitory G proteins (Galpha(i/o)), which inhibit the activity of adenylate cyclase, thereby attenuating cyclic AMP accumulation. Furthermore, an additional, less direct mechanism for Galpha(i/o) proteins modulation of cyclic AMP signaling also has been described. Persistent activation of several Galpha(i/o)-coupled receptors has been shown to result in a subsequent paradoxical enhancement of adenylate cyclase activity in response to drug-stimulated cyclic AMP accumulation. This sensitization of adenylate cyclase likely represents a cellular adaptive response following prolonged activation of inhibitory receptors. Recent advances in our knowledge of G protein signaling, adenylate cyclase regulation, and other cellular signaling mechanisms have extensively increased our insight into this phenomenon. It is now thought that sensitization occurs as part of a compensatory mechanism by which the cell adapts to chronic inhibitory input. Such a mechanism may be involved in modulating Galphas-coupled receptor signaling following neurotransmitter elevations that occur in psychiatric disease states or following the administration of many drugs of abuse. This review will focus on recent advances in the understanding of molecular signaling pathways that are involved in sensitization and describe the potential role of sensitization in neuronal cell function.

Molecular mechanisms for heterologous sensitization of adenylate cyclase

The nine membrane-bound isoforms of the enzyme adenylate cyclase (EC 4.6.1.1) are highly regulated by neurotransmitters and drugs acting through G protein-coupled receptors to modulate intracellular cAMP levels. In general, acute activation of Galpha(s)-coupled receptors stimulates cAMP accumulation, whereas acute activation of Galpha(i/o)-coupled receptors typically inhibits cAMP accumulation. It is also well established that persistent activation of G-protein coupled receptors will alter subsequent drug-modulated cAMP accumulation. These alterations are thought to represent cellular adaptive responses following prolonged receptor activation. One phenomenon commonly observed, heterologous sensitization of adenylate cyclase, is characterized by an enhanced responsiveness to drug-stimulated cAMP accumulation following persistent activation of Galpha(i/o)-coupled receptors. Heterologous sensitization of adenylate cyclase was originally proposed to explain tolerance and withdrawal following chronic opiate administration and may be a mechanism by which cells adapt to prolonged activation of inhibitory receptors. Such an adaptive mechanism has been suggested to play a role in the processes of addiction to and withdrawal from many drugs of abuse and in psychiatric disorders including schizophrenia and depression. Although the precise mechanisms remain unknown, research over the last decade has led to advances toward understanding the molecular events associated with heterologous sensitization of recombinant and endogenous adenylate cyclases in cellular models. These events include the pertussis toxin-sensitive events that are associated with the development of heterologous sensitization and the more recently identified Galpha(s)-dependent events that are involved in the expression of heterologous sensitization.

Dopamine D2 receptor-induced heterologous sensitization of adenylyl cyclase requires Galphas: characterization of Galphas-insensitive mutants of adenylyl cyclase V

Whereas acute stimulation of Galphai/o-coupled receptors inhibits the activity of adenylyl cyclase, a delayed consequence of persistent activation of the receptors is heterologous sensitization, an enhanced responsiveness of adenylyl cyclase to activators such as forskolin or agonists of Galphas-coupled receptors. Galphas-insensitive mutants of adenylyl cyclase type V were used to test the hypothesis that heterologous sensitization requires Galphas-dependent activation of adenylyl cyclase. When adenylyl cyclase was stably expressed in human embryonic kidney (HEK) 293 cells with the D2L dopamine receptor, basal, forskolin-stimulated, and isoproterenol-stimulated cyclic AMP accumulation were all enhanced by 2-h pretreatment with the D2 receptor agonist quinpirole. Transient expression of wild-type adenylyl cyclase and three Galphas-insensitive mutants (F379L, R1021Q, and F1093S) in HEK293 cells stably expressing the D2L receptor demonstrated that all three mutants had little or no responsiveness to beta-adrenergic receptor-mediated activation of Galphas but that the mutants retained sensitivity to forskolin and to D2L receptor-mediated inhibition. Transiently expressed adenylyl cyclase V was robustly sensitized by 2-h pretreatment with quinpirole. In contrast, the Galphas-insensitive mutants displayed no sensitization of forskolin-stimulated cyclic AMP accumulation, indicating that responsiveness to Galphas is required for the expression of heterologous sensitization.

Adrenergic regulation of cardiac myocyte apoptosis

The direct effects of catecholamines on cardiac myocytes may contribute to both normal physiologic adaptation and pathologic remodeling, and may be associated with cellular hypertrophy, apoptosis, and alterations in contractile function. Norepinephrine (NE) signals via alpha- and beta-adrenergic receptors (AR) that are coupled to G-proteins. Pharmacologic studies of cardiac myocytes in vitro demonstrate that stimulation of beta1-AR induces apoptosis which is cAMP-dependent and involves the voltage-dependent calcium influx channel. In contrast, stimulation of beta2-AR exerts an anti-apoptotic effect which appears to be mediated by a pertussis toxin-sensitive G protein. Stimulation of alpha1-AR causes myocyte hypertrophy and may exert an anti-apoptotic action. In transgenic mice, myocardial overexpression of either beta1-AR or G(alpha)s is associated with myocyte apoptosis and the development of dilated cardiomyopathy. Myocardial overexpression of beta2-AR at low levels results in improved cardiac function, whereas expression at high levels leads to dilated cardiomyopathy. Overexpression of wildtype alpha1B-AR does not result in apoptosis, whereas overexpression of G(alpha)q results in myocyte hypertrophy and/or apoptosis depending on the level of expression. Differential activation of the members of the mitogen-activated protein kinase (MAPK) superfamily and production of reactive oxygen species appear to play a key role in mediating the actions of adrenergic pathways on myocyte apoptosis and hypertrophy. This review summarizes current knowledge about the molecular and cellular mechanisms involved in the regulation of cardiac myocyte apoptosis via stimulation of adrenergic receptors and their coupled effector pathways.

Protein kinase C-mediated down-regulation of beta(2)-adrenergic receptor and gene expression in rat C6 glioma cells

We investigated the regulation of beta(2)-adrenergic receptors (beta(2)AR) by protein kinase C (PKC) in rat C6 glioma cells at the levels of receptor activity, protein expression and gene expression. Cells exposed to 4beta-phorbol-12-myristate-13-acetate (PMA), a potent activator of PKC, exhibited a time- and concentration-dependent decrease in beta(2)AR binding activity. Maximum down-regulation was approximately 50% by 24 h and western blot analysis revealed a parallel decrease in beta(2)AR protein. In addition, PMA treatment resulted in an acute desensitization of beta(2)AR-stimulated cyclic AMP response prior to any reduction in receptor levels. PMA exposure also affected steady-state beta(2)AR mRNA levels in a time-dependent, biphasic manner. During the first 4 h, levels decreased by approximately 60% and then slowly recovered to approximately 75% of control by 24 h. As the reduction in receptor mRNA was not due to a decrease in its stability, we examined beta(2)AR gene transcription by nuclear run-on assays. Transcriptional activity in nuclei from C6 cells treated with PMA for 2 h was reduced by 70% compared to controls. Thus PKC can regulate beta(2)AR at least two levels: the first being an acute desensitization of receptor function, and the second being a more prolonged repression of receptor gene transcription that in turn results in decreased receptor expression.

Activation of phosphatidylinositol-3 kinase (PI-3K) and extracellular regulated kinases (Erk1/2) is involved in muscarinic receptor-mediated DNA synthesis in neural progenitor cells

Muscarinic acetylcholine receptor (mAChR), a member of the G-protein-coupled receptors (GPCRs) gene superfamily, has been shown to mediate the effects of acetylcholine on differentiation and proliferation in the CNS. However, the mechanism or mechanisms whereby mAChRs regulate cell proliferation remain poorly understood. Here we show that in vitro bFGF-expanded neural progenitor cells dissociated from rat cortical neuroepithelium express muscarinic acetylcholine receptor subtype mRNAs. We demonstrate that stimulation of these mAChRs with carbachol, a muscarinic agonist, activated extracellular-regulated kinases (Erk1/2) and phosphatidylinositol-3 kinase (PI-3K). This, in turn, stimulated DNA synthesis in neural progenitor cells. MEK inhibitor PD98059 and PI-3K inhibitors wortmannin and LY294002 inhibited a carbachol-induced increase in DNA synthesis. These findings indicate that the activation of both PI-3 kinase and MEK signaling pathways via muscarinic receptors is involved in stimulating DNA synthesis in the neural progenitor cells during early neurogenesis.

Cirrhosis of the liver and receptor-mediated function in vascular smooth muscle

Altered regulation of receptors on the vascular smooth muscle has been proposed as one of the mechanisms that may account for the vascular abnormalities in patients with cirrhosis of the liver. Impaired contractility and down-regulation of contractile receptors have been demonstrated in cirrhotic patients and animal models, although interpretation of the literature is hampered by methodological variation and conflicting results. There is little evidence, however, that receptor down-regulation is the cause of contractile dysfunction in either patients or animal models. Receptor desensitisation may contribute to impaired contraction in human arteries, but further investigation is required to confirm this possibility.

Cellular and synaptic adaptations mediating opioid dependence

Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

Alpha 1-adrenergic receptor regulation: basic science and clinical implications

Adrenergic receptors (ARs) are members of the G-protein-coupled receptor family, which includes alpha 1ARs, alpha 2ARs, beta 1ARs, beta 2ARs, beta 3ARs, adenosine, muscarinic, angiotensin, endothelin receptors, and many others that are responsible for a large variety of physiologic effects through G-protein coupling. This review focuses on alpha 1ARs and their regulation at both the mRNA and protein levels. Currently, three alpha 1AR subtypes have been characterized both pharmacologically and at the gene level: alpha 1aAR, alpha 1bAR, and alpha 1dAR. These are expressed in a species- and tissue-dependent manner. Mutagenesis approaches have been extremely valuable in the identification of key residues that govern alpha 1AR ligand binding and signaling. These studies reveal that alpha 1ARs have evolved an exquisitely sensitive regulation of their activity in which any disruption of the native structure has profound effects on subsequent function and effector coupling. Significant advances have also been made in the elucidation of signaling pathway components, resulting in the identification of novel pathways that can lead to pathologic conditions. Specific topics include mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and G-protein-coupled receptor cross-talk pathways. Within this context, recent studies identifying underlying transcriptional mechanisms involved in the regulation of the alpha 1AR subtypes are also discussed. Finally, given the potentially important role of alpha 1ARs in the vasculature, as well as in the pathology of many diseases, such as myocardial hypertrophy and benign prostatic hyperplasia, the clinical relevance of alpha 1AR distribution, pharmacology, and therapeutic intervention is reviewed.

Rapid desensitization and slow recovery of the cyclic AMP response mediated by histamine H(2) receptors in the U937 cell line

The present study focused on the desensitization process of the H(2) receptor in U937 cells and the recovery of the cyclic AMP (cAMP) response. Treatment of U937 leukemic cells with the H(2) histamine receptor agonists (+/-)-N(1)-[3-(3, 4-difluorophenyl)-3-(pyridin-2-yl)propyl]-N(2)-[3-(1H-imidazol-4-yl)p ropyl]guanidine (BU-E-75) and amthamine produced a rapid desensitization characterized by decreased cAMP production (T(1/2) = 20 min). Pretreatment with 10 microM BU-E-75 did not induce modifications in the responses to prostaglandin E(2), isoproterenol, or forskolin. H(2) receptor desensitization was not affected by protein kinase A and C inhibitors, but was reduced drastically by Zn(2+) and heparin, known to act as inhibitors of G protein-coupled receptor kinases. Recovery studies of the cAMP response showed that cAMP levels reached 50% of the initial values within 5 hr. Furthermore, desensitization produced an important decrease in the basal level of this cyclic nucleotide. The minimal value was observed 12 hr later, and corresponded to approximately 1.3% of the initial basal level (7.5 vs 0.1 pmol/10(6) cells). This result could be explained by an increase in phosphodiesterase activity following 10 microM BU-E-75 treatment. When cells were exposed for 2 hr to an H(2) agonist, binding assays showed no modification in the number of H(2) receptors; internalization began just after 8 hr. Although the initial desensitization seems to involve G protein-coupled receptor kinases, results indicate that additional mechanisms of regulation were triggered by the H(2) agonists.

Type-specific sorting of G protein-coupled receptors after endocytosis

The beta(2)-adrenergic receptor (B2AR) and delta-opioid receptor (DOR) are structurally distinct G protein-coupled receptors (GPCRs) that undergo rapid, agonist-induced internalization by clathrin-coated pits. We have observed that these receptors differ substantially in their membrane trafficking after endocytosis. B2AR expressed in stably transfected HEK293 cells exhibits negligible (<10%) down-regulation after continuous incubation of cells with agonist for 3 h, as assessed both by radioligand binding (to detect functional receptors) and immunoblotting (to detect total receptor protein). In contrast, DOR exhibits substantial (>/=50%) agonist-induced down-regulation when examined by similar means. Degradation of internalized DOR is sensitive to inhibitors of lysosomal proteolysis. Flow cytometric and surface biotinylation assays indicate that differential sorting of B2AR and DOR between distinct recycling and non-recycling pathways (respectively) can be detected within approximately 10 min after endocytosis, significantly before the onset of detectable proteolytic degradation of receptors ( approximately 60 min after endocytosis). Studies using pulsatile application of agonist suggest that after this sorting event occurs, later steps of membrane transport leading to lysosomal degradation of receptors do not require the continued presence of agonist in the culture medium. These observations establish that distinct GPCRs differ significantly in endocytic membrane trafficking after internalization by the same membrane mechanism, and they suggest a mechanism by which brief application of agonist can induce substantial down-regulation of receptors.

Functional significance of stimulatory GTP-binding protein in hippocampus is associated with kindling-elicited epileptogenesis

In order to evaluate the involvement of the stimulatory G-protein (Gs)-related transduction system in the basic mechanisms of epilepsy, we examine the expression levels of Gsalpha mRNA and specific GTP-binding ability in the hippocampus of amygdaloid-kindled rats at various seizure stages. Northern blot analysis showed a significant increase in the Gsalpha mRNA expression level in the bilateral hippocampus at 24h after the last generalized seizure. The [3H]-GTP-binding assay with isoproterenol (IPN), a beta-receptor agonist, revealed a remarkable increase of Bmax values in the sham-operated control and partially kindled groups. However, the IPN-induced increase of Bmax values was abolished on both sides of the hippocampus at 24 h after and at 4 weeks after the last generalized seizure in fully kindled rats. These data suggest that alteration in the Gs function and beta-adrenergic receptor-Gs coupling might be implicated in the neurobiological basis of the induction mechanisms of the generalization of seizures and the mechanisms of the maintenance of enduring epileptogenesis. Conversely, the Gs-related transduction system might have a lesser impact on the acquisition process of epileptogenesis.

Platelet alpha2A-adrenoceptor function in major depression: Gi coupling, effects of imipramine and relationship to treatment outcome

Studies suggest alpha2A-adrenoceptors (alpha(2A)AR) dysregulation in major depressive disorder (MDD). Platelet alpha(2A)ARs exist in high- and low-conformational states that are regulated by Gi protein. Although alpha(2A)AR coupling to Gi protein plays an important role in signal transduction and is modulated by antidepressants, it has not been previously investigated. Alpha2AR density in the high- and low-conformational states, agonist affinity and coupling efficiency were investigated in 27 healthy control subjects, 23 drug-free MDD patients and 16 patients after imipramine treatment using [3H]yohimbine saturation and norepinephrine displacement of [3H]yohimbine binding experiments. Coupling measures were derived from NE-displacement experiments. Patients had significantly higher alpha(2A)AR density, particularly in the high-conformational state, than control subjects. Coupling indices were normal in patients. High pre-treatment agonist affinity to the receptor in the high-conformational state and normal coupling predicted positive treatment outcome. Decreased coupling to Gi predicted a negative treatment outcome. Imipramine induced uncoupling (-11%) and redistribution of receptor density in treatment responders only, but had no effect on alpha(2A)AR coupling or density in treatment non-responders. Increased alpha(2A)AR density may represent a trait marker in MDD. The results provide indirect evidence for abnormal protein kinase A (PKA) and protein kinase C (PKC) in MDD which may be pursued in future investigations.

Neutrophil beta(2)-adrenoceptor function in major depression: G(s) coupling, effects of imipramine and relationship to treatment outcome

Abnormal beta(2)-adrenoceptor density and beta(2)-adrenoceptor-mediated cyclic adenosine monophosphate (cAMP) responses were inconsistently reported in major depressive disorder. Tricyclic antidepressants downregulate beta-adrenoceptor density and decrease coupling to G(s) protein. Abnormal beta-adrenoceptor coupling may exist in major depressive disorder and may relate to treatment response. We investigated beta(2)-adrenoceptor coupling to G(s) protein in 25 controls, 23 major depressive disorder drug-free patients and 16 major depressive disorder patients after chronic imipramine treatment using agonist displacement experiments. Pretreatment beta(2)-adrenoceptor coupling and density were normal in patients as a whole. Chronic imipramine induced beta(2)-adrenoceptor uncoupling. This effect was observed in treatment responders who had increased beta(2)-adrenoceptor density in the high-conformational state and supercoupling prior to treatment. Beta(2)-adrenoceptor density decreased after imipramine treatment. Treatment non-responders had seemingly normal pretreatment beta(2)-adrenoceptor function, which was not changed by imipramine. Differences in beta(2)-adrenoceptor regulation in major depressive disorder may underlie treatment response. The results indirectly implicate abnormal agonist-mediated beta(2)-adrenoceptor gene expression, protein kinase A, and protein kinase C in major depressive disorder.

Analysis of the AU-rich elements in the 3'-untranslated region of beta 2-adrenergic receptor mRNA by mutagenesis and identification of the homologous AU-rich region from different species

The 35000-Mr beta-adrenergic receptor mRNA binding protein (beta ARB) is induced by beta-adrenergic agonists and binds to G-protein-linked receptor mRNAs that exhibit agonist-induced destabilization. Recently, we identified a 20-nucleotide, AU-rich region in the 3'-untranslated region of the hamster beta 2-adrenergic receptor mRNA consisting of an AUUUUA hexamer flanked by U-rich regions, which constitutes the binding domain for beta ARB. U to G substitution in the hexamer region attenuates the binding of beta ARB, whereas U to G substitution of hexamer and flanking U-rich domains abolishes binding of beta ARB and stabilizes beta 2-adrenergic receptor mRNA levels in transfectant clones challenged with either isoproterenol or cyclic AMP. In the study presented here, we mutated the 20-nucleotide ARE region to establish the minimal AU-rich sequence required for beta ARB binding. U to G substitutions of flanking poly(U) regions and of the hexamer established the nature of the binding properties. Using various mutants, we demonstrated also that binding of beta ARB correlates with the extent of destabilization of beta 2-adrenergic receptor mRNA in response to agonist stimulation. High-affinity binding of hamster, rat, mouse, porcine, and human ARE sequences to beta ARB was revealed by SDS-polyacrylamide gel electrophoresis following UV-catalyzed cross-linking and by gel mobility shift assays. Further, beta ARB was shown to bind more avidly to the 20-nucleotide ARE region than to well-established mRNA destablization sequences of tandem repeats of five pentamers. Thus, for beta 2-adrenergic receptor, mRNA destabilization likely occurs via conserved AU-rich elements present in the 3'-untranslated regions of receptor mRNAs.

Restoration of ocular dominance plasticity mediated by adenosine 3',5'-monophosphate in adult visual cortex

Noradrenaline (NA)-stimulated beta-adrenoreceptors activate adenylate cyclase via excitatory G-proteins (Gs). Activated adenylate cyclase in turn promotes the production of cAMP. Critical roles of cAMP-dependent protein kinase A (PKA) in divergent cellular functions have been shown, including memory, learning and neural plasticity. Ocular dominance plasticity (ODP) is strongly expressed in early postnatal life and usually absent in the mature visual cortex. Here, we asked whether the activation of cAMP-dependent PKA could restore ODP to the aplastic visual cortex of adult cats. Concurrent with brief monocular deprivation, each of the following cAMP-related drugs was directly and continuously infused in the adult visual cortex: cholera toxin (a Gs-protein stimulant), forskolin (a Gs-protein-independent activator of adenylate cyclase) and dibutyryl cAMP (a cAMP analogue). We found that the ocular dominance distribution became W-shaped, the proportion of binocular cells being significantly lower than that in respective controls. We concluded that the activation of cAMP cascades rapidly restores ODP to the adult visual cortex, though moderately. The finding further extends the original hypothesis that the NA-beta-adrenoreceptors system is a neurochemical mechanism of cortical plasticity.

Characterization of alpha(s)-immunoreactive ADP-ribosylated proteins in postmortem human brain

ADP-ribosylation of the stimulatory G protein alpha subunit, alpha(s), has been demonstrated in a number of different mammalian tissues. However, little is known about the occurrence and role of this process in modifying alpha(s) levels/function in human brain. In the present study, endogenous and cholera toxin (CTX)-catalyzed [32P]ADP-ribosylated products were characterized in postmortem human temporal cortex by (1) immunoprecipitation with alpha(s) antisera (RM/1), (2) comparisons of immunoblots and autoradiograms of the [32P]ADP-ribosylated products, and (3) limited protease digestion. Of the three major endogenous [32P]ADP-ribosylated products (48, 45, and 39 kDa) in postmortem brain, the 48-kDa and 45-kDa bands were clearly identified as alpha(s-L) (long isoform) and alpha(s-S) (short isoform), respectively. RM/1 immunoprecipitated the 39-kDa [32P]ADP-ribosylated protein, and overlays of immunoblots and autoradiograms showed that this product corresponded to an alpha(s)-like-immunoreactive protein. Furthermore, limited protease digestion of the 39-kDa endogenous [32P]ADP-ribosylated band generated peptide fragments similar to both endogenous and CTX-catalyzed [32P]ADP-ribosylated alpha(s-S). Two major CTX-catalyzed [32P]ADP-ribosylated products were also identified as alpha(s-L) (52 kDa) and alpha(s-S) (45 kDa). These findings clearly demonstrate that alpha(s) is a substrate for endogenous and CTX-catalyzed [32P]ADP-ribosylation in postmortem human brain. Furthermore, a lower molecular weight alpha(s)-like immunoreactive protein is also expressed in human brain and is a substrate for endogenous but not CTX-catalyzed [32P]ADP-ribosylation.

Beta-adrenergic receptors and receptor signaling in heart failure

Cardiac beta-adrenergic receptors, which respond to neuronally released and circulating catecholamines, are important regulators of cardiac function. Congestive heart failure, a common clinical condition, is associated with a number of alterations in the activation and deactivation of beta-adrenergic receptor pathways. Studies with failing hearts from humans and animals indicate that such alterations include changes in the expression or function of beta-adrenergic receptors, G-proteins, adenylyl cyclases, and G-protein receptor kinases. The net effect of these alterations is the substantial blunting of beta-adrenergic receptor-mediated cardiac response. An important unanswered question is whether the loss of cardiac beta-adrenergic receptor responsiveness is a contributing cause, or a result, of ventricular dysfunction. Even though this question remains unanswered, the concept of targeting the beta-adrenergic pathway in the failing heart is becoming increasing popular and several new therapeutic strategies are in development.

Differential spatiotemporal alterations in adrenoceptor mRNAs and binding sites in cerebral cortex following spreading depression: selective and prolonged up-regulation of alpha1B-adrenoceptors

Noradrenaline, an important transmitter in the CNS, is involved in cerebral plasticity and functional recovery after injury. Experimental brain injury, including KCl application onto the brain surface, induces a slow-moving cortical depolarization/depression wave called cortical spreading depression (CSD). Interestingly, CSD does not produce neuronal damage but can protect cortical neurons against subsequent neurotoxic insults, although the mechanisms involved are unknown. This study examined the status of alpha- and beta-adrenoceptors (ARs) in cerebral cortex following CSD. Anesthetized rats had unilateral CSD induced by a 10-min topical application of KCl to the frontoparietal cortex and were killed at various times thereafter. Levels of alpha1-, alpha2-, beta1-, and beta2-AR mRNA and binding were examined using in situ hybridization histochemistry and radioligand autoradiography. Levels of alpha1b-AR mRNA in the affected neocortex were significantly increased by 20-40% at 1, 2, and 7 days (P </= 0.01) compared with contralateral levels, but were not significantly above control values at 2 and 4 weeks after CSD induction. Cortical alpha1B-AR binding sites were also increased by 45-65% 1 and 2 weeks (P </= 0.01) after CSD in a similar, but delayed, profile to alpha1b-AR mRNA. CSD rapidly increased beta1-AR mRNA by 45% at 1 h (P </= 0.01) and produced a delayed decrease of 25% in alpha2a-AR mRNA at 2 days and 1 week (P </= 0.05), but had no effect on corresponding levels of binding sites. In contrast, CSD had no effect on the remaining AR-subtype mRNAs or binding levels in neocortex under identical conditions. These results reveal a long-term up-regulation of alpha1B-ARs induced by an acute cortical stimulation/depression. Subtype-selective responses of ARs to CSD reflect an important differential regulation of expression of each receptor in vivo and suggest that alpha1B-ARs are particularly likely to be involved in cortical adaptive responses to physical injury at both local and distant locations.

Oxidative damage to DNA in lymphocytes from AD patients

OBJECTIVE

Several studies show structural and functional alterations in peripheral cells in AD. The purpose of this study was to evaluate oxidative stress in AD lymphocytes.

BACKGROUND

The literature supports the role of reactive oxygen species in the pathogenesis of AD because several markers of oxidative damage have been detected in AD brain.

METHODS

8-hydroxy-2'-deoxyguanosine (8OHdG), a marker of oxidative stress in DNA, was measured in lymphocytes of AD patients and healthy aged controls with high-pressure liquid chromatography with electrochemical detection, both at basal condition and after acute oxidative stress with hydrogen peroxide.

RESULTS

A significantly higher concentration of 8OHdG in lymphocytes occurred in AD patients compared with controls. In this latter group, 8OHdG increased progressively with age. After acute oxidative stress, levels of formed 8OHdG did not differ between AD patients and controls.

CONCLUSIONS

Our results support that AD is affected by oxidative stress, detectable not only in the brain but also in peripheral cells; oxidative mechanisms may contribute to the pathogenesis of AD. Additional studies in other neurodegenerative diseases are needed to evaluate these findings.

Regulation of G proteins and adenylyl cyclase in brain regions of caffeine-tolerant and -dependent mice

Regulation of post-receptor signaling provides a mechanism of adaptation to chronic psychotropic drug treatment. In this study, the regulation of guanine nucleotide binding proteins (G proteins) and G protein-stimulated adenylyl cyclase activity was examined in brain regions of caffeine-tolerant and -dependent mice. Chronic caffeine doses were administered via mini-osmotic pumps over 7 days at 0, 42, 85 and 125 mg kg-1 day-1. These chronic caffeine doses were linearly correlated with plasma caffeine concentrations. In behavioral studies, the stimulant effects of acute caffeine on motor activity were significantly diminished in a dose-dependent manner after chronic caffeine, suggesting the development of tolerance. Abrupt discontinuation of chronic caffeine treatment (at 85 and 125 mg kg-1 day-1) produced a dose-dependent and reversible reduction in motor activity 24 h later, suggestive of a caffeine withdrawal syndrome. Utilizing quantitative immunoblotting methods, we found that hippocampal Gialpha1,2 and Gialpha3 subunits were significantly reduced by 20.2% and 11.1%, respectively, in caffeine tolerant/dependent mice (caffeine 125 mg kg-1 day-1 vs. vehicle controls). Decreases in inhibitory G protein subunit concentrations in hippocampus were accompanied by a significant increase (by 21%) in hippocampal G protein function, as measured by guanine nucleotide-stimulated adenylyl cyclase activity, in caffeine-treated mice. This same caffeine treatment also produced significant decreases in cortical Gsalpha subunits of 14.0%. Since short-term caffeine treatment has been shown to reduce adenylyl cyclase activity, chronic caffeine treatment could produce adaptive increases in G protein-stimulated adenylyl cyclase to oppose this effect via G protein regulation.

Promoter-independent regulation of cell-specific dopamine receptor expression

Here we describe the construction of recombinant adenoviruses expressing dopamine D2 and D4 receptors, and their ability to mediate high levels of heterologous expression in a variety of cell types in vitro and in vivo for at least 7 days post infection. These experiments demonstrated that maximum receptor expression is achieved generally within 24 h and remains constant thereafter. Maximum expression levels were highly variable between cell lines and dependent on infection efficiency and promoter strength. Correction for these two variables revealed differences in relative expression levels between cell lines varying by two orders of magnitude. Our results indicate that in addition to gene transcription, post-transcriptional mechanisms play a dominant role in determining dopamine receptor levels in this system.

Nerve growth factor regulation of m4 muscarinic receptor mRNA stability but not gene transcription requires mitogen-activated protein kinase activity

Nerve growth factor (NGF) up-regulated steady-state levels of m4 muscarinic acetylcholine receptor (mAChR) mRNA in PC12 cells. Up-regulation of mRNA levels was associated with a corresponding increase in mAChR binding sites. Two other growth factors, basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF), up-regulated m4 mRNA and mAChR binding sites. Treatment of PC12 cells with NGF and bFGF, but not EGF, has previously been demonstrated to result in sustained activation of mitogen-activated protein kinase (MAPK). Analogously, NGF and bFGF, but not EGF, increased the stability of m4 mRNA in PC12 cells. In HER-PC12 cells, a clonal PC12 cell transfectant overexpressing EGF receptors and displaying sustained MAPK activation upon receptor stimulation, EGF treatment stabilized the m4 transcript. A synthetic inhibitor of MAPK kinase, PD98059, inhibited growth factor-induced stabilization of the m4 transcript in both PC12 and HER-PC12 cells. These findings demonstrate that the MAPK pathway is involved in transcript stabilization. Cycloheximide pretreatment abolished the post-transcriptional effect of NGF, indicating that de novo protein synthesis was required for the observed increase in m4 mRNA stability. By contrast, cycloheximide had no discernible post-transcriptional effect if added after NGF treatment, suggesting that an inducible yet stable protein factor was involved in m4 mRNA decay. An unusually well conserved 137 nucleotides of m4 3'-untranslated region has been identified by sequence comparison with other mRNAs that are post-transcriptionally regulated by NGF. In PC12 cells that heterologously overexpress this region, we demonstrate that NGF no longer stabilizes endogenous m4 mRNA. This conserved region probably represents an NGF-responsive element involved in mRNA stability regulation. Finally, transcription of the m4 gene can be induced by all three growth factors but is not dependent on MAPK activity, unlike growth factor-induced m4 mRNA stabilization.

Adrenergic receptors in premenstrual dysphoric disorder. II. Neutrophil beta2-adrenergic receptors: Gs protein coupling, phase of menstrual cycle and prediction of luteal phase symptom severity

Abnormal beta2-adrenergic receptor coupling to Gs protein is implicated in depressive disorders. Steroid hormones and antidepressants modulate beta-adrenergic receptor coupling, which may relate to the therapeutic efficacy of antidepressants. We examined beta2-adrenergic receptors in 18 patients with premenstrual dysphoric disorder (PMDD), in 15 control subjects during the follicular phase and in 12 patients during late luteal phase. Antagonist-measured receptor density, agonist-measured receptor density in the high- and low-conformational states and agonist affinity to both states were measured. Coupling indices to Gs protein were determined from agonist-displacement experiments. Follicular beta2-adrenergic receptor density was higher in patients than in control subjects, with a trend for higher receptor density in the high-conformational state. The phase of menstrual cycle had no effect on beta2-adrenergic receptor regulation in PMDD. Exploratory correlations showed that the K(L)/K(H) ratio was related to anxiety ratings in control subjects and %R(H) was correlated with symptom severity in patients. In patients, follicular beta2-adrenergic receptor binding measures were correlated with luteal symptom severity. These findings suggest abnormal beta2-adrenergic receptor regulation in PMDD. Further exploration of the role of beta-adrenergic receptor kinase, sex steroid hormones and antidepressants on beta-adrenergic receptor regulation in PMDD is warranted.

Agonist regulation of human beta2-adrenergic receptor mRNA stability occurs via a specific AU-rich element

Prolonged agonist stimulation of beta2-adrenergic receptors results in receptor down-regulation, which is closely associated with a reduction of the corresponding mRNA, an effect mediated in part by changes in mRNA stability. Transfection experiments with human beta2-adrenergic receptor cDNAs bearing or lacking the untranslated regions suggested that the essential agonist sensitivity of the mRNA resides within the 3'-untranslated region. The importance of this region was further confirmed in gel shift experiments; cytosolic preparations from agonist-stimulated DDT1-MF2 smooth muscle cells caused a shift of beta2-adrenergic receptor mRNAs containing the 3'-untranslated region. Progressive 3'-terminal truncations of the receptor cDNA led to the identification of an AU-rich element at positions 329-337 of the 3'-untranslated region as the responsible cis-acting element. Substitution of this motif by cytosine residues almost completely abolished mRNA down-regulation and inhibited the formation of the RNA-protein complex. Even though the beta2-adrenergic receptor AU-rich element showed two U --> A transitions compared with the recently proposed AU-rich element consensus sequence, it revealed an almost identical destabilizing potency. Fusion of the beta2-adrenergic receptor 3'-untranslated region to the beta-globin coding sequence dramatically reduced the half-life of the chimeric transcript in an agonist- and cAMP-dependent manner. This suggests that the agonist-induced beta2-adrenergic receptor mRNA destabilization is regulated by cAMP-dependent RNA-binding protein(s) via a specific AU-rich element.

Effects of quinapril, losartan and hydralazine on cardiac hypertrophy and beta-adrenergic neuroeffector mechanisms in transgenic (mREN2)27 rats

1. Desensitization of the myocardial beta-adrenergic signal transduction pathway is an important mechanism which is involved in the progression of hypertensive heart disease. The aim of the present study was to evaluate the differential effects of chronic pharmacotherapy with an angiotensin converting enzyme (ACE)-inhibitor, an AT1-receptor antagonist and a direct vasodilator on blood pressure, cardiac hypertrophy and the beta-adrenergic signal transduction. Therefore, transgenic TG(mREN2)27 (TG) rats overexpressing the mouse renin gene were used. This strain is characterized by the development of fulminant hypertension with cardiac hypertrophy. 2. Seven week old heterozygous TG(mREN2)27 rats were treated for 11 weeks with the AT1-receptor antagonist losartan (10 mg kg[-1]), the ACE-inhibitor quinapril (15 mg kg[-1]) and the direct vasodilator hydralazine (30 mg kg[-1]). Untreated TG and normotensive Sprague-Dawley rats (SD) served as controls. 3. TG(mREN2)27-rats were characterized by arterial hypertension (TG 194+/-3.2 mmHg vs SD 136+/-2.9 mmHg systolic blood pressure), increased left ventricular weights (TG 4.3+/-0.3 vs SD 3.0+/-0.1 mg g(-1) body weight), decreased myocardial neuropeptide Y (NPY) concentrations (TG 1143+/-108 vs SD 1953+/-134 pg g(-1) wet weight), reduced beta-adrenoceptor densities (TG 51.1+/-1.9 vs SD 63.4+/-3.7 fmol mg[-1]) as assessed by [125I]-cyanopindolol binding studies, and increased Gi(alpha)-activities (TG 4151+/-181 vs SD 3169+/-130 densitometric units) as assessed by pertussis toxin catalyzed [32P]-ADP-ribosylation. Downregulation of beta-adrenoceptors and increased Gi(alpha) were accompanied by significantly reduced isoprenaline-, Gpp(NH)p- and forskolin-stimulated adenylyl cyclase activity. Catalyst activity as determined by forskolin plus Mn2+ co-stimulation of adenylyl cyclase did not differ between TG(mREN2)27- and SD control-rats. 4. Losartan and quinapril significantly restored systolic blood pressures, left ventricular weights, beta-adrenoceptor densities, myocardial neuropeptide Y-concentrations, adenylyl cyclase activities and Gi(alpha)-activities towards the values in Sprague-Dawley-controls. No differences were observed between the effects of quinapril- and losartan-treatment. In contrast, hydralazine had only minor effects on blood pressure reduction, regression of left ventricular hypertrophy and neuroeffector defects in TG(mREN2)27. 5. In conclusion, direct vasodilatation is not able to overcome the pathophysiological alterations in TG caused by transgene overexpression. In contrast, ACE-inhibitors and AT1-receptor antagonists, which inhibit the renin angiotensin system, equally exert beneficial effects on blood pressure, myocardial hypertrophy and neuroeffector mechanisms. Modulation of the sympathetic tone and resensitization of the beta-adrenergic signal transduction system may contribute to the special effectiveness of these drugs in the treatment of the hypertensive cardiomyopathy.

Desensitization of adenosine A1 receptor-mediated inhibition of adenylyl cyclase in cerebellar granule cells

Agonist-induced desensitization of adenosine A1 receptor-mediated inhibition of adenylyl cyclase has been studied in cerebellar granule cells. Exposure of cells to the adenosine A1 receptor agonist R-phenylisopropyl adenosine (R-PIA) from 2 to 48 h brings about desensitization of this signal transduction pathway. Associated with the desensitization process, a decrease in radioligand binding performed in intact cells with the adenosine A1 receptor agonist [3H]cyclohexyladenosine (CHA) has been detected. Simultaneously, an increase of adenosine A1 radioligand binding has also been detected in microsomes. A decrease in the steady-state level of alpha-Gi in both, plasma membrane and microsomes also has been detected during the desensitization process. These data may account for the desensitization of the inhibitory pathway of the adenylyl cyclase in cerebellar granule cells described herein. After a transient increase in adenosine A1 receptor mRNA, no changes were observed in this parameter after 12 hr of treatment with the adenosine A1 agonist R-PIA, suggesting a post-transcriptional regulation of this receptor during long-term desensitization.

Modulation of alpha1B-adrenoceptor expression by agonist and protein kinase inhibitors

The agonist-induced up-regulation of alpha1B-adrenoceptors in clone H99 of transfected Chinese hamster ovary cells that we reported previously (Zhu et al., 1996) was further investigated. Studies with a larger number of clones revealed that the up-regulation observed in H99 cells is atypical and that most other clones exhibit down-regulation under the same conditions. The role of protein kinases in the up-regulation of alpha1B-adrenoceptors in clone H99 was further investigated. Surprisingly, the protein kinase inhibitor staurosporine induced a similar up-regulation. Neither the selective protein kinase C inhibitor GF109203X nor the activator phorbol 12-myristate, 13-acetate altered receptor expression. The tyrosine kinase inhibitors genistein and its weaker analog daidzein did not induce up-regulation but blocked the up-regulation induced by epinephrine and by staurosporine. Up-regulation was blocked by the protein synthesis inhibitor cycloheximide. These studies suggest multiple mechanisms by which different protein kinases can modulate the expression of transfected alpha1B-adrenoceptors.

In vivo homologous regulation of mu-opioid receptor gene expression in the mouse

Regulation of the mu-opioid receptor gene by opioid analgesic drugs has not been observed in rats and mice following in vivo treatments that produce tolerance. Although in vivo heterologous regulation of mu-opioid receptor mRNA by non-opioid compounds has been reported, the failure to observe changes in mu-opioid receptor mRNA levels in vivo after treatment with opioid agonists raised the possibility that in vivo homologous regulation by agonists may not occur. Therefore, in the present study, the effect of a high intrinsic efficacy opioid receptor agonist on opioid receptor density, gene expression and tolerance was determined. Mice were infused with etorphine for 7 days using an osmotic minipump, then the pump was removed and studies conducted 16-168 h later. Etorphine (50-250 microg/kg/day) infusion produced significant dose-dependent tolerance to the analgesic (tailflick) effects of etorphine, as well as dose-dependent mu-opioid receptor downregulation in brain at 16 h following the end of the infusion. Mu-opioid receptor density returned to control levels over a 168 h period following the end of etorphine (250 microg/kg/day) infusion. Similarly, the magnitude of tolerance decreased over the same period. Evaluation of changes in brain mu-opioid receptor mRNA 16 h following etorphine infusion indicated that there was dose-dependent increase in steady-state levels, with no significant change in GAPDH mRNA. The increase in mu-opioid receptor mRNA was approximately 55-65% over control at the highest etorphine infusion dose. Mu-opioid receptor mRNA returned to control levels over a 168 h period following the end of etorphine (250 microg/kg/day) infusion. These data suggest that the increase in mu-opioid receptor mRNA following the termination of etorphine treatment may drive the recovery of mu-opioid receptors. These data are the first demonstration of in vivo homologous regulation of mu-opioid receptor gene expression in the mouse by an opioid receptor agonist that produces tolerance and receptor downregulation.

Cross talk between angiotensin AT1 and alpha 1-adrenergic receptors: angiotensin II downregulates alpha 1a-adrenergic receptor subtype mRNA and density in neonatal rat cardiac myocytes

Signaling mediated by the angiotensin (Ang) II and alpha1-adrenergic receptor (alpha1-AR) pathways is important for cardiovascular homeostasis. However, it is unknown whether Ang II has any direct effect on alpha1-AR expression and signaling in cardiac myocytes. In the present study, we determined alpha1-AR subtype mRNA levels by RNase protection; receptor density by competition binding with 5-methylurapidil; and alpha1-AR-mediated c-fos expression by Northern blot analysis. We found that Ang II had no effect on alpha1b- and alpha1d-AR mRNA levels but decreased the alpha1a-AR mRNA level in a time- and dose-dependent manner. The maximal effect occurred at 6 hours with 100 nmol/L Ang II (40.0+/-8.2% reduction, n=4, P<.01). The decrease in alpha1a-AR mRNA level induced by Ang II is mediated by the Ang II AT1 receptor subtype and is associated with decreased stability of alpha1a-AR mRNA. Corresponding to the changes in the alpha1a-AR mRNA level, Ang II (100 nmol/L, 24 hours) reduced the density of high-affinity sites for 5-methylurapidil (alpha1A-AR) by 29% (56.5+/-6.4 versus 79.0+/-11.6 fmol/mg protein, n=4, P<.05). Alpha1-AR-stimulated c-fos induction, which could be blocked by 5-methylurapidil but not by chloroethylclonidine, was attenuated by Ang II preincubation (100 nmol/L, 24 hours). We conclude that there is previously undescribed cross talk between AT1 receptors and alpha1-ARs. Ang II selectively downregulates alpha1a-AR subtype mRNA and its corresponding receptor as well as alpha1a-AR-mediated expression of the immediate-early gene c-fos in cardiac myocytes.

Cell type-specific regulation of beta2-adrenoceptor mRNA by agonists

Prolonged agonist stimulation of beta2-adrenoceptors results in receptor down-regulation which is often paralleled by a reduction of the corresponding mRNA. In this study, we investigated the agonist-dependent regulation of beta2-adrenoceptor mRNA in DDT1-MF2 smooth muscle cells and C6 glioma cells. In DDT1-MF2 cells the half-life of the mRNA was 12 h in monolayer compared to 2 h in suspension cultures. Under both conditions, the agonist isoproterenol reduced this half-life by a factor of 2. In contrast, in C6 glioma cells isoproterenol had no effect on the mRNA stability, even though it reduced mRNA levels by approximately 50%. Isoproterenol-induced downregulation of beta2-adrenoceptor mRNA was completely blocked in C6 cells by the presence of a protein synthesis inhibitor, while this was not so in DDT1-MF2-cells. These data show that beta2-adrenoceptor downregulation occurs via cell-type specific mechanisms.

Mechanical load opposes angiotensin-mediated decrease in vascular alpha 1-adrenoceptors

alpha 1-Adrenergic receptor contraction of vascular smooth muscle is augmented by increases in angiotensin II and also in several forms of hypertension. Whether angiotensin directly modulates alpha 1-adrenoceptor subtype expression to contribute to this effect is unknown. In a previous study, we demonstrated that increased mechanical load (pressure) per se does not alter expression of alpha 1B- and alpha 1D-adrenoceptors in rat aortic smooth muscle in cell culture, in vitro or in vivo. However, findings in aortic coarctation hypertension suggested that a humoral factor, possibly angiotensin, selectively reduces alpha 1B-adrenoceptors and that increased mechanical load opposes this decrease. The present study examined this hypothesis by determining the effect of angiotensin alone and in the presence of mechanical loading on the expression of alpha 1D- and alpha 1B-adrenergic receptor mRNAs and alpha 1-receptor density in cultured aortic smooth muscle cells. alpha 1D mRNA content, per smooth muscle cell, concentration-dependently decreased after 3 hours of exposure to 0.3 nmol/L to 1 mumol/L angiotensin but by 24 hours had returned to control levels. In contrast, alpha 1B mRNA concentration-dependently declined at a later time (24 hours) and remained decreased at 48 hours to 27 +/- 6% of control with 1 mumol/L angiotensin. Angiotensin also decreased alpha 1-adrenoceptor density in a dose-dependent manner. Angiotensin had no effect on cell number in these confluent, quiescent cells but did increase cell protein and total RNA. This cellular hypertrophy and the decreases in alpha 1-adrenoceptor mRNAs were blocked by the angiotensin type 1 receptor antagonist losartan. Cyclic mechanical loading of smooth muscle cells opposed the angiotensin-mediated hypertrophy and decrease in alpha 1B mRNA expression and alpha 1-adrenergic receptor density. These data suggest that angiotensin and intravascular pressure interact to affect cell growth and expression of alpha 1B-adrenergic receptors by vascular smooth muscle.

A 20-nucleotide (A + U)-rich element of beta2-adrenergic receptor (beta2AR) mRNA mediates binding to beta2AR-binding protein and is obligate for agonist-induced destabilization of receptor mRNA

The Mr 35,000 beta-adrenergic receptor mRNA-binding protein, termed betaARB protein, is induced by beta-adrenergic agonists and binds to beta2-receptor mRNAs that display agonist-induced destabilization. A cognate sequence in the mRNA was identified previously that provides for betaARB protein binding in vitro. In the present work, the sequence established in vitro for binding of betaARB protein to hamster beta2-adrenergic receptor mRNA was probed in vivo by site-directed mutagenesis of the 3'-untranslated region and expression in Chinese hamster ovary cells. A 20-nucleotide, (A + U)-rich region in the 3'-untranslated region consisting of an AUUUUA hexamer flanked by defined U-rich regions constitutes the binding domain for betaARB protein. U to G substitution in the hexamer region attenuates the binding of betaARB protein, whereas U to G substitution of hexamer and flanking U-rich domains abolishes binding of betaARB protein and stabilizes beta2-adrenergic receptor mRNA levels in transfectant clones challenged with either isoproterenol or cyclic AMP. These results demonstrate that binding of betaARB protein to the 20-nucleotide, (A + U)-rich domain mediates the agonist and cyclic AMP-induced mRNA decay of G protein-linked receptors, such as the beta2-adrenergic receptor.

Regulation of pineal alpha1B-adrenergic receptor mRNA: day/night rhythm and beta-adrenergic receptor/cyclic AMP control

Mammalian pineal function is regulated by norepinephrine acting through alpha1beta- and beta1-adrenergic receptors (ARs). Noradrenergic stimulation of alpha1beta-ARs potentiates the beta1-AR-driven increase in cAMP, serotonin N-acetyltransferase, and melatonin production. In the present study, we describe a 3-fold daily rhythm in mRNA-encoding alpha1beta-ARs in the pineal gland, with a peak at midnight. Pharmacological studies indicate that this increase in alpha1beta-AR mRNA is due to activation of beta-ARs. Second messenger studies indicate that alpha1beta-AR mRNA is increased by agents that increase cAMP, including dibutyryl cAMP, cholera toxin, forskolin, or vasoactive intestinal peptide. These observations indicate that alpha1beta-AR mRNA can be physiologically regulated by a beta-AR-dependent enhancement of cAMP. It also was observed that in vivo and in vitro changes in alpha1beta-AR mRNA are not accompanied by similar changes in alpha1beta-AR binding, indicating that turnover of alpha1beta-AR protein is significantly slower than that of alpha1beta-AR mRNA and that post-transcriptional mechanisms play an important role in regulating alpha1beta-AR binding.

Hyperexpression of mitogen-activated protein kinase in human breast cancer

Mitogen-activated protein (MAP) kinases act as transducers of extracellular signaling via tyrosine kinase-growth factor receptors and G-protein-linked receptors to elements regulating transcription. The activity, abundance, and localization of MAP kinase was investigated in normal and malignant neoplasia of the breast. In carcinoma of the breast, MAP kinase was heavily phosphorylated on tyrosyl residues and its activity elevated 5-10-fold over benign conditions, such as fibroadenoma and fibrocystic disease. By in situ reverse transcription-polymerase chain reaction, hyperexpression of MAP kinase mRNA can be localized to malignant, epithelial cells. Metastatic cells within involved lymph nodes of patients with breast cancer also display hyperexpression of MAP kinase. In spite of persistent activation via phosphorylation, MAP kinase expression is upregulated 5-20-fold and this hyperexpression may be a critical element to initiation as well as the metastatic potential of various forms of human breast cancer.

Regulatory mechanisms that modulate signalling by G-protein-coupled receptors

The large and functionally diverse group of G-protein-coupled receptors includes receptors for many different signalling molecules, including peptide and non-peptide hormones and neuro-transmitters, chemokines, prostanoids and proteinases. Their principal function is to transmit information about the extracellular environment to the interior of the cell by interacting with the heterotrimeric G-proteins, and they thereby participate in many aspects of regulation. Cellular responses to agonists of these receptors are usually rapidly attenuated. Mechanisms of signal attenuation include removal of agonists from the extracellular fluid, receptor desensitization, endocytosis and down-regulation. Agonists are removed by dilution, uptake by transporters and enzymic degradation. Receptor desensitization is mediated by receptor phosphorylation by G-protein receptor kinases and second-messenger kinases, interaction of phosphorylated receptors with arrestins and receptor uncoupling from G-proteins. Agonist-induced receptor endocytosis also contributes to desensitization by depleting the cell surface of high-affinity receptors, and recycling of internalized receptors contributes to resensitization of cellular responses. Receptor down-regulation is a form of desensitization that occurs during continuous, long-term exposure of cells to receptor agonists. Down-regulation, which may occur during the development of drug tolerance, is characterized by depletion of the cellular receptor content, and is probably mediated by alterations in the rates of receptor degradation and synthesis. These regulatory mechanisms are important, as they govern the ability of cells to respond to agonists. A greater understanding of the mechanisms that modulate signalling may lead to the development of new therapies and may help to explain the mechanism of drug tolerance.