Two distinct pathways for cAMP-mediated down-regulation of the β2-adrenergic receptor

Early life adversity accelerates hypothalamic drive of pubertal timing in female rats with associated enhanced acoustic startle

Early life adversity in the form of childhood maltreatment in humans or as modeled by maternal separation (MS) in rodents is often associated with an earlier emergence of puberty in females. Earlier pubertal initiation is an example of accelerated biological aging and predicts later risk for anxiety in women, especially in populations exposed to early life trauma. Here we investigated external pubertal markers as well as hypothalamic gene expression of pubertal regulators kisspeptin and gonadotropin-releasing hormone, to determine a biological substrate for MS-induced accelerated puberty. We further investigated a mechanism by which developmental stress might regulate pubertal timing. As kisspeptin and gonadotropin-releasing hormone secretion are typically inhibited by corticotropin releasing hormone at its receptor CRH-R1, we hypothesized that MS induces a downregulation of Crhr1 gene transcription in a cell-specific manner. Finally, we explored the association between pubertal timing and anxiety-like behavior in an acoustic startle paradigm, to drive future preclinical research linking accelerated puberty and anxiety. We replicated previous findings that MS leads to earlier puberty in females but not males, and found expression of kisspeptin and gonadotropin-releasing hormone mRNA to be prematurely increased in MS females. RNAscope confirmed increased expression of these genes, and further revealed that kisspeptin-expressing neurons in females were less likely to express Crhr1 after MS. Early puberty was associated with higher acoustic startle magnitude in females. Taken together, these findings indicate precocial maturation of central pubertal timing mechanisms after MS, as well as a potential role of CRH-R1 in these effects and an association with a translational measure of anxiety.

Early-life-trauma triggers interferon-β resistance and neurodegeneration in a multiple sclerosis model via downregulated β1-adrenergic signaling

Environmental triggers have important functions in multiple sclerosis (MS) susceptibility, phenotype, and trajectory. Exposure to early life trauma (ELT) has been associated with higher relapse rates in MS patients; however, the underlying mechanisms are not well-defined. Here we show ELT induces mechanistic and phenotypical alterations during experimental autoimmune encephalitis (EAE). ELT sustains downregulation of immune cell adrenergic receptors, which can be attributed to chronic norepinephrine circulation. ELT-subjected mice exhibit interferon-β resistance and neurodegeneration driven by lymphotoxin and CXCR2 involvement. These phenotypic changes are observed in control EAE mice treated with β1 adrenergic receptor antagonist. Conversely, β1 adrenergic receptor agonist treatment to ELT mice abrogates phenotype changes via restoration of immune cell β1 adrenergic receptor function. Our results indicate that ELT alters EAE phenotype via downregulation of β1 adrenergic signaling in immune cells. These results have implications for the effect of environmental factors in provoking disease heterogeneity and might enable prediction of long-term outcomes in MS.

Interdependence of hypoxia and β-adrenergic receptor signaling in pulmonary arterial hypertension

The β-adrenergic receptor (βAR) exists in an equilibrium of inactive and active conformational states, which shifts in response to different ligands and results in downstream signaling. In addition to cAMP, βAR signals to hypoxia-inducible factor 1 (HIF-1). We hypothesized that a βAR-active conformation (R**) that leads to HIF-1 is separable from the cAMP-activating conformation (R*) and that pulmonary arterial hypertension (PAH) patients with HIF-biased conformations would not respond to a cAMP agonist. We compared two cAMP agonists, isoproterenol and salbutamol, in vitro. Isoproterenol increased cAMP and HIF-1 activity, while salbutamol increased cAMP and reduced HIF-1. Hypoxia blunted agonist-stimulated cAMP, consistent with receptor equilibrium shifting toward HIF-activating conformations. Similarly, isoproterenol increased HIF-1 and erythropoiesis in mice, while salbutamol decreased erythropoiesis. βAR overexpression in cells increased glycolysis, which was blunted by HIF-1 inhibitors, suggesting increased βAR leads to increased hypoxia-metabolic effects. Because PAH is also characterized by HIF-related glycolytic shift, we dichotomized PAH patients in the Pulmonary Arterial Hypertension Treatment with Carvedilol for Heart Failure trial (NCT01586156) based on right ventricular (RV) glucose uptake to evaluate βAR ligands. Patients with high glucose uptake had more severe disease than those with low uptake. cAMP increased in response to isoproterenol in mononuclear cells from low-uptake patients but not in high-uptake patients' cells. When patients were treated with carvedilol for 1 wk, the low-uptake group decreased RV systolic pressures and pulmonary vascular resistance, but high-uptake patients had no physiologic responses. The findings expand the paradigm of βAR activation and uncover a novel PAH subtype that might benefit from β-blockers.

Pathological cardiac hypertrophy: the synergy of adenylyl cyclases inhibition in cardiac and immune cells during chronic catecholamine stress

Response to stressors in our environment and daily lives is an adaptation conserved through evolution as it is beneficial in enhancing the survival and continuity of humans. Although stressors have evolved, the drastic physiological response they elicit still remains unchanged. The chronic secretion and circulation of catecholamines to produce physical responses when they are not required may result in pathological consequences which affect cardiac function drastically. This review seeks to point out the probable implication of chronic stress in inducing an inflammation disorder in the heart. We discussed the likely synergy of a G protein-independent stimuli signaling via β₂-adrenergic receptors in both cardiomyocytes and immune cells during chronic catecholamine stress. To explain this synergy, we hypothesized the possibility of adenylyl cyclases having a regulatory effect on G protein-coupled receptor kinases. This was based on the negative correlations they exhibit during normal cardiac function and heart failures. As such, the downregulation of adenylyl cyclases in cardiomyocytes and immune cells during chronic catecholamine stress enhances the expressions of G protein-coupled receptor kinases. In addition, we explain the maladaptive roles played by G protein-coupled receptor kinase and extracellular signal-regulated kinase in the synergistic cascade that pathologically remodels the heart. Finally, we highlighted the therapeutic potentials of an adenylyl cyclases stimulator to attenuate pathological cardiac hypertrophy (PCH) and improve cardiac function in patients developing cardiac disorders due to chronic catecholamine stress.

Agonist-induced desensitisation of β3 -adrenoceptors: Where, when, and how?

β₃ -Adrenoceptor agonists have proven useful in the treatment of overactive bladder syndrome, but it is not known whether their efficacy during chronic administration may be limited by receptor-induced desensitisation. Whereas the β₂ -adrenoceptor has phosphorylation sites that are important for desensitisation, the β₃ -adrenoceptor lacks these; therefore, it had been assumed that β₃ -adrenoceptors are largely resistant to agonist-induced desensitisation. While all direct comparative studies demonstrate that β₃ -adrenoceptors are less susceptible to desensitisation than β₂ -adrenoceptors, desensitisation of β₃ -adrenoceptors has been observed in many models and treatment settings. Chimeric β₂ - and β₃ -adrenoceptors have demonstrated that the C-terminal tail of the receptor plays an important role in the relative resistance to desensitisation but is not the only relevant factor. While the evidence from some models, such as transfected CHO cells, is inconsistent, it appears that desensitisation is observed more often after long-term (hours to days) than short-term (minutes to hours) agonist exposure. When it occurs, desensitisation of β₃ -adrenoceptors can involve multiple levels including down-regulation of its mRNA and the receptor protein and alterations in post-receptor signalling events. The relative contributions of these mechanistic factors apparently depend on the cell type under investigation. Which if any of these factors is applicable to the human urinary bladder remains to be determined. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Fingolimod Suppresses the Proinflammatory Status of Interferon-γ-Activated Cultured Rat Astrocytes

Astroglia, the primary homeostatic cells of the central nervous system, play an important role in neuroinflammation. They act as facultative immunocompetent antigen-presenting cells (APCs), expressing major histocompatibility complex (MHC) class II antigens upon activation with interferon (IFN)-γ and possibly other proinflammatory cytokines that are upregulated in disease states, including multiple sclerosis (MS). We characterized the anti-inflammatory effects of fingolimod (FTY720), an established drug for MS, and its phosphorylated metabolite (FTY720-P) in IFN-γ-activated cultured rat astrocytes. The expression of MHC class II compartments, β₂ adrenergic receptor (ADR-β₂), and nuclear factor kappa-light-chain enhancer of activated B cells subunit p65 (NF-κB p65) was quantified in immunofluorescence images acquired by laser scanning confocal microscopy. In addition, MHC class II-enriched endocytotic vesicles were labeled by fluorescent dextran and their mobility analyzed in astrocytes subjected to different treatments. FTY720 and FTY720-P treatment significantly reduced the number of IFN-γ-induced MHC class II compartments and substantially increased ADR-β₂ expression, which is otherwise small or absent in astrocytes in MS. These effects could be partially attributed to the observed decrease in NF-κB p65 expression, because the NF-κB signaling cascade is activated in inflammatory processes. We also found attenuated trafficking and secretion from dextran-labeled endo-/lysosomes that may hinder efficient delivery of MHC class II molecules to the plasma membrane. Our data suggest that FTY720 and FTY720-P at submicromolar concentrations mediate anti-inflammatory effects on astrocytes by suppressing their action as APCs, which may further downregulate the inflammatory process in the brain, constituting the therapeutic effect of fingolimod in MS.

A CREB-mediated increase in miRNA let-7f during prolonged β-agonist exposure: a novel mechanism of β2-adrenergic receptor down-regulation in airway smooth muscle

β₂-Adrenergic receptors (β₂ARs) desensitize during continuous agonist activation, which manifests clinically as tachyphylaxis. β-Agonist desensitization of β₂ARs in human airway smooth muscle (HASM) cells is recognized in the treatment of asthma and may be related to poor outcomes. Rapid events in desensitization include receptor phosphorylation and internalization, but mechanisms responsible for the decrease in receptor protein after prolonged agonist exposure (down-regulation) are ill defined. The microRNA (miRNA) let-7f regulates β₂AR expression by translational repression. In cultured HASM cells from nonasthmatic and asthmatic lungs, 18 h of β-agonist exposure increased let-7f by 2-3-fold, concomitant with a ∼90% decrease in β₂ARs. Inhibition of let-7f attenuated this down-regulation response by ∼50%. The let-7f increase was found to be cAMP/PKA-dependent. The mechanism of the let-7f increase was found by chromatin immunoprecipitation to be from activated cAMP response element-binding protein (CREB) binding to the let-7f promoter, thereby increasing let-7f expression. Knockdown of CREB attenuated agonist-promoted β₂AR down-regulation by ∼50%. Thus, β₂AR down-regulation occurs as a result of not only internalized receptor degradation but also a novel cAMP/PKA/CREB-mediated increase in let-7f, which causes enhanced repression of the β₂AR gene, adrenoreceptor β₂ ( ADRB2) translation and represents ∼50% of the net loss of receptors observed after prolonged agonist exposure. This mechanism is apparent in asthmatic HASM cells, indicating relevance in a disease model.-Kim, D., Cho, S., Woo, J. A., Liggett, S. B. A CREB-mediated increase in miRNA let-7f during prolonged β-agonist exposure: a novel mechanism of β₂-adrenergic receptor down-regulation in airway smooth muscle.

Benzo(a)pyrene triggers desensitization of β2-adrenergic pathway

Exposure to environmental polycyclic aromatic hydrocarbons (PAHs), such as benzo(a)pyrene (B(a)P), has been linked to several health-threatening risks. PAHs were also shown to hinder adrenergic receptor (ADR) responses. As we previously demonstrated that B(a)P can directly interact with the β2ADR, we investigated here whether B(a)P could decrease β2ADR responsiveness by triggering receptor desensitization phenomena. We firstly showed that exposure to B(a)P reduced β2ADR-mediated epinephrine-induced induction of NR4A gene mRNAs and of intracellular cAMP. Analysis of β2ADR protein expression demonstrated that B(a)P rapidly decreased membrane expression of β2ADR with a subsequent degradation of receptor protein. B(a)P exposure concomitantly rapidly increased the β2ADR mRNA levels. The use of the β-blockers, propranolol and ICI 118.551, demonstrated the involvement of β2ADR itself in this increase. However, sustained exposure to B(a)P induced a diminution of β2ADR mRNA steady-state as a result of the acceleration of its degradation. Together, these results show that, beside the well-known activation of the aryl hydrocarbon receptor, PAH deleterious effects may involve the dysfunction of adrenergic responses through, in part, the desensitization of β2ADR. This may be taken in consideration when β2-agonists/antagonists are administered in patients exposed to important concentrations of PAHs, e.g. in cigarette smokers.

GPCR desensitization: Acute and prolonged phases

G protein-coupled receptors (GPCRs) transduce a wide array of extracellular signals and regulate virtually every aspect of physiology. While GPCR signaling is essential, overstimulation can be deleterious, resulting in cellular toxicity or uncontrolled cellular growth. Accordingly, nature has developed a number of mechanisms for limiting GPCR signaling, which are broadly referred to as desensitization, and refer to a decrease in response to repeated or continuous stimulation. Short-term desensitization occurs over minutes, and is primarily associated with β-arrestins preventing G protein interaction with a GPCR. Longer-term desensitization, referred to as downregulation, occurs over hours to days, and involves receptor internalization into vesicles, degradation in lysosomes and decreased receptor mRNA levels through unclear mechanisms. Phosphorylation of the receptor by GPCR kinases (GRKs) and the recruitment of β-arrestins is critical to both these short- and long-term desensitization mechanisms. In addition to phosphorylation, both the GPCR and β-arrestins are modified post-translationally in several ways, including by ubiquitination. For many GPCRs, receptor ubiquitination promotes degradation of agonist-activated receptors in the lysosomes. Other proteins also play important roles in desensitization, including phosphodiesterases, RGS family proteins and A-kinase-anchoring proteins. Together, this intricate network of kinases, ubiquitin ligases, and adaptor proteins orchestrate the acute and prolonged desensitization of GPCRs.

A dynamic interface between ubiquitylation and cAMP signaling

Phosphorylation waves drive the propagation of signals generated in response to hormones and growth factors in target cells. cAMP is an ancient second messenger implicated in key biological functions. In mammals, most of the effects elicited by cAMP are mediated by protein kinase A (PKA). Activation of the kinase by cAMP results in the phosphorylation of a variety of cellular substrates, leading to differentiation, proliferation, survival, metabolism. The identification of scaffold proteins, namely A-Kinase Anchor proteins (AKAPs), that localize PKA in specific cellular districts, provided critical cues for our understanding of the role played by cAMP in cell biology. Multivalent complexes are assembled by AKAPs and include signaling enzymes, mRNAs, adapter molecules, receptors and ion channels. A novel development derived from the molecular analysis of these complexes nucleated by AKAPs is represented by the presence of components of the ubiquitin-proteasome system (UPS). More to it, the AKAP complex can be regulated by the UPS, eliciting relevant effects on downstream cAMP signals. This represents a novel, yet previously unpredicted interface between compartmentalized signaling and the UPS. We anticipate that impairment of these regulatory mechanisms could promote cell dysfunction and disease. Here, we will focus on the reciprocal regulation between cAMP signaling and UPS, and its relevance to human degenerative and proliferative disorders.

Membrane-permeable tastants amplify β2-adrenergic receptor signaling and delay receptor desensitization via intracellular inhibition of GRK2's kinase activity

BACKGROUND

Amphipathic sweet and bitter tastants inhibit purified forms of the protein kinases GRK2, GRK5 and PKA activities. Here we tested whether membrane-permeable tastants may intracellularly interfere with GPCR desensitization at the whole cell context.

METHODS

β2AR-transfected cells and cells containing endogenous β2AR were preincubated with membrane-permeable or impermeable tastants and then stimulated with isoproterenol (ISO). cAMP formation, β2AR phosphorylation and β2AR internalization were monitored in response to ISO stimulation. IBMX and H89 inhibitors and GRK2 silencing were used to explore possible roles of PDE, PKA, and GRK2 in the tastants-mediated amplification of cAMP formation and the tastant delay of β2AR phosphorylation and internalization.

RESULTS

Membrane-permeable but not impermeable tastants amplified the ISO-stimulated cAMP formation in a concentration- and time-dependent manner. Without ISO stimulation, amphipathic tastants, except caffeine, had no effect on cAMP formation. The amplification of ISO-stimulated cAMP formation by the amphipathic tastants was not affected by PDE and PKA activities, but was completely abolished by GRK2 silencing. Amphipathic tastants delayed the ISO-induced GRK-mediated phosphorylation of β2ARs and GRK2 silencing abolished it. Further, tastants also delayed the ISO-stimulated β2AR internalization.

CONCLUSION

Amphipathic tastants significantly amplify β2AR signaling and delay its desensitization via their intracellular inhibition of GRK2.

GENERAL SIGNIFICANCE

Commonly used amphipathic tastants may potentially affect similar GPCR pathways whose desensitization depends on GRK2's kinase activity. Because GRK2 also modulates phosphorylation of non-receptor components in multiple cellular pathways, these gut-absorbable tastants may permeate into various cells, and potentially affect GRK2-dependent phosphorylation processes in these cells as well.

Non-monotonic dose-response relationships and endocrine disruptors: a qualitative method of assessment

Experimental studies investigating the effects of endocrine disruptors frequently identify potential unconventional dose-response relationships called non-monotonic dose-response (NMDR) relationships. Standardized approaches for investigating NMDR relationships in a risk assessment context are missing. The aim of this work was to develop criteria for assessing the strength of NMDR relationships. A literature search was conducted to identify published studies that report NMDR relationships with endocrine disruptors. Fifty-one experimental studies that investigated various effects associated with endocrine disruption elicited by many substances were selected. Scoring criteria were applied by adaptation of an approach previously used for identification of hormesis-type dose-response relationships. Out of the 148 NMDR relationships analyzed, 82 were categorized with this method as having a "moderate" to "high" level of plausibility for various effects. Numerous modes of action described in the literature can explain such phenomena. NMDR can arise from numerous molecular mechanisms such as opposing effects induced by multiple receptors differing by their affinity, receptor desensitization, negative feedback with increasing dose, or dose-dependent metabolism modulation. A stepwise decision tree was developed as a tool to standardize the analysis of NMDR relationships observed in the literature with the final aim to use these results in a Risk Assessment purpose. This decision tree was finally applied to studies focused on the effects of bisphenol A.

Phosphorylation of the deubiquitinase USP20 by protein kinase A regulates post-endocytic trafficking of β2 adrenergic receptors to autophagosomes during physiological stress

Ubiquitination by the E3 ligase Nedd4 and deubiquitination by the deubiquitinases USP20 and USP33 have been shown to regulate the lysosomal trafficking and recycling of agonist-activated β2 adrenergic receptors (β2ARs). In this work, we demonstrate that, in cells subjected to physiological stress by nutrient starvation, agonist-activated ubiquitinated β2ARs traffic to autophagosomes to colocalize with the autophagy marker protein LC3-II. Furthermore, this trafficking is synchronized by dynamic posttranslational modifications of USP20 that, in turn, are induced in a β2AR-dependent manner. Upon β2AR activation, a specific isoform of the second messenger cAMP-dependent protein kinase A (PKAα) rapidly phosphorylates USP20 on serine 333 located in its unique insertion domain. This phosphorylation of USP20 correlates with a characteristic SDS-PAGE mobility shift of the protein, blocks its deubiquitinase activity, promotes its dissociation from the activated β2AR complex, and facilitates trafficking of the ubiquitinated β2AR to autophagosomes, which fuse with lysosomes to form autolysosomes where receptors are degraded. Dephosphorylation of USP20 has reciprocal effects and blocks trafficking of the β2AR to autophagosomes while promoting plasma membrane recycling of internalized β2ARs. Our findings reveal a dynamic regulation of USP20 by site-specific phosphorylation as well as the interdependence of signal transduction and trafficking pathways in balancing adrenergic stimulation and maintaining cellular homeostasis.

Dual regulation of β2-adrenoceptor messenger RNA expression in human lung fibroblasts by β2-cAMP signaling; delayed upregulated inhibitors oppose a rapid in onset, direct stimulation of gene expression

Based on their bronchodilatory effect, β2-adrenoceptor agonists constitute essential elements in the treatment of bronchial asthma and COPD. As treatment with β2-adrenoceptor agonists has been associated with worsening of airway hyper-reactivity, possibly because of loss of β-adrenoceptor function, molecular mechanism of the regulation of β2-adrenoceptor expression were studied. MRC-5 human lung fibroblasts were cultured in absence or presence of test substances followed by β2-adrenoceptor messenger RNA (mRNA) determination by qPCR. After inhibition of mRNA synthesis by actinomycin D, β2-adrenoceptor mRNA decreased with a half-life of 23 min, whereas inhibition of protein synthesis by cycloheximide caused an about 5- and 6-fold increase within 1.5 and 4 h, respectively. β2-Adrenoceptor mRNA was increased by about 100 % after 1 h exposure to formoterol or olodaterol but decreased by about 60 % after 4 h agonist exposure. Both effects of β2-adrenoceptor agonists were mimicked by forskolin, a direct activator of adenylyl cyclase and cholera toxin, which stimulates adenylyl cyclase by permanent activation of Gs. β2-Adrenoceptor agonist-induced upregulation of β2-adrenoceptor mRNA was blocked by the β2-adrenoceptor antagonist ICI 118551 and prevented by actinomycin D, but not by cycloheximide. Moreover, in presence of cycloheximide, β2-adrenoceptor agonist-induced reduction in β2-adrenoceptor mRNA was converted into stimulation, resulting in a more than 10-fold increase. In conclusion, expression of β2-adrenoceptors in human lung fibroblasts is highly regulated at transcriptional level. The β2-adrenoceptor gene is under strong inhibitory control of short-living suppressor proteins. β2-Adrenoceptor activation induces via adenylyl cyclase - cyclic adenosine monophosphate (cAMP) signaling a rapid in onset direct stimulation of the β2-adrenoceptor gene transcription, an effect opposed by a delayed upregulation of inhibitory factors.

Assessment of myometrial transcriptome changes associated with spontaneous human labour by high-throughput RNA-seq

The transition of the human uterus from a quiescent to a contractile state takes place over a number of weeks. On such biological time scales, cellular phenotype is modified by changes in the transcriptome, which in turn is under the control of the underlying endocrine, paracrine, and biophysical processes resulting from the ongoing pregnancy. In this study, we characterize the transition of the human myometrial transcriptome at term from not in labour (NIL) to in labour (LAB) using high throughput RNA sequencing (RNA-seq). RNA was isolated from the myometrium of uterine biopsies from patients at term who were not in labour (n = 5) and at term in spontaneous labour (n = 5) without augmentation. A total of 143.6 million separate reads were sequenced, achieving, on average, ∼13 times coverage of the expressed human transcriptome per sample. Principal component analysis indicated that the NIL and LAB transcriptomes could be distinguished as two distinct clusters. A comparison of the NIL and LAB groups, using three different statistical approaches (baySeq, edgeR, and DESeq), demonstrated an overlap of 764 differentially expressed genes. A comparison with currently available microarray data revealed only a partial overlap in differentially expressed genes. We conclude that the described RNA-seq data sets represent the first fully annotated catalogue of expressed mRNAs in human myometrium. When considered together, the full expression repertoire and the differentially expressed gene sets should provide an excellent resource for formulating new hypotheses of physiological function, as well as the discovery of novel therapeutic targets.

Agonist-induced desensitization of human β3-adrenoceptors expressed in human embryonic kidney cells

β3-Adrenoceptors are resistant to agonist-induced desensitization in some cell types but susceptible in others including transfected human embryonic kidney (HEK) cells. Therefore, we have studied cellular and molecular changes involved in agonist-induced β3-adrenoceptor desensitization in HEK cells. Cells were treated with isoprenaline or forskolin, and following wash-out, cyclic adenosine monophosphate (cAMP) accumulation in response to freshly added agonist was quantified. Receptor and G protein expression were quantified by radioligand binding and immunoblot experiments, respectively. Treatment with isoprenaline induced a concentration- and time-dependent desensitization of cAMP accumulation in response to freshly added isoprenaline. This functional desensitization primarily consisted of reduced maximum responses with little change of agonist potency. Maximum desensitization was achieved by pre-treatment with 10 μM isoprenaline for 24 h. It was not accompanied by changes in β3-adrenoceptor density as assessed in saturation radioligand-binding studies. The desensitization was associated with a small reduction in immunoreactivity for α-subunits for Gs and Gi1, whereas that for Gi2, Gi3, and Gq/11 was not significantly altered. In cells treated with pertussis toxin, isoprenaline-induced cAMP accumulation as well as desensitization by isoprenaline pre-treatment remained unchanged. Isoprenaline pre-treatment also reduced forskolin-induced cAMP accumulation; conversely, pre-treatment with forskolin caused a similar desensitization of isoprenaline-induced cAMP accumulation. We conclude that agonist-induced β3-adrenoceptor desensitization in HEK cells does not involve reduced receptor numbers and small, if any, reduction of Gs expression; changes at the level of adenylyl cyclase function can fully explain this desensitization.

Micronucleus induction in the bone marrow of rats by pharmacological mechanisms. II: long-acting beta-2 agonism

AZD9708 is a new chemical entity with selective and long-acting β2-agonistic properties currently being evaluated by AstraZeneca for potential use in treatment of respiratory diseases by the inhaled route. As part of the toxicological characterisation of this compound, an increased incidence of micronucleated immature erythrocytes (MIEs) was seen in the bone marrow of rats following single intravenous doses near the maximum tolerated. This effect was seen in the absence of in vitro genotoxicity in bacterial and mammalian cells and no consistent evidence of in vivo DNA damage in the the bone marrow or liver using the comet assay was observed. Because of the lack of signals for mutagenic potential, combined with the observation that MIE frequencies appeared to be increased in only some of the rats and the clearest response was seen at the intermediate dose, it was hypothesised that the effect was secondary to β2-adrenergic receptor overstimulation. Because it appears that this has not been previously described for β2-agonists and because pharmacodynamic/pharmacokinetic factors may influence the response, studies using repeated dosing were performed to investigate whether this would lead to compound-induced tachyphylaxis with tolerance induction and decreased responses indicated by β2-effect biomarkers. A series of experiments confirmed that a sequence of five escalating daily doses leading to systemic exposure corresponding to that after a single dose led to symptomatic tolerance, declining or diminished effects on plasma biomarkers of β2-effects (plasma glucose and potassium) and elimination of the micronucleus response. This suggests that the increased MIE frequencies after single doses of AZD9708 are secondary to physiological overstimulation of β2-adrenergic receptors, not a consequence of genotoxicity.

Molecular and cellular biology of adrenergic receptors

Adrenergic receptors form the interface between the sympathetic nervous system and the cardiovascular system. Genomic or cDNA clones for 8 types of mammalian adrenergic receptors have been obtained. Much has been learned about the structure and functional properties of the β(2)-adrenergic receptor. Less is known about the functional properties and the physiologic role of the other adrenergic receptors. Further progress in this field may lead to the development of more selective drugs to modify the physiologic processes controlled by these receptors.

Protective effects of glycyrrhizin against β₂-adrenergic receptor agonist-induced receptor internalization and cell apoptosis

It has been reported that treatment with β₂ adrenergic receptor (β₂AR) agonist bronchodilators may result in airway β₂ARs internalization and cardiac muscle cells apoptosis. This could lead to the loss of pharmacological effect of β₂AR agonists and increase adverse cardiovascular events in asthma patients receiving β₂AR agonist therapy. Glycyrrhizin, the major bioactive component of licorice root extract, has been reported to exhibit protective effect on respiratory system. Here, we investigate the effects of glycyrrhizin against β₂AR agonist salbutamol-induced receptor internalization and cell apoptosis. In our study, the live cell confocal imaging and fixed-cell enzyme-linked immunosorbent assay (ELISA) assay revealed that glycyrrhizin significantly inhibited salbutamol-induced surface β₂AR internalization. The underlying mechanisms were then identified to be that glycyrrhizin could reduce the association of β₂ARs with β-arrestins and clathrin heavy chain as well as the level of G protein-coupled receptor kinase (GRK) mediated phosphorylation of β₂ARs. The inhibition of receptor internalization by glycyrrhizin further lead to stabilization of the β₂AR mRNA and protein expression, thus amplified the transmembrane signaling via the β₂ARs. We also proved that glycyrrhizin could profoundly attenuate salbutamol-induced early cellular apoptosis by regulating the expressions of B-cell lymphoma 2 (Bcl-2) family genes. Taken together, our results suggest that glycyrrhizin exhibits protective effects against β₂AR agonist-induced receptor internalization and cell apoptosis. These findings might have practical implications for future strategies of combined application of glycyrrhizin with β₂AR receptor agonists to improve the efficacy of bronchodilators in patients with asthma and chronic obstructive pulmonary disease (COPD).

How can 1 + 1 = 3? β2-adrenergic and glucocorticoid receptor agonist synergism in obstructive airway diseases

For a long time it was believed that β(2)-adrenergic receptor agonists used in the treatment of obstructive airway diseases worked primarily on airway smooth muscle cells, causing relaxation, whereas glucocorticoids primarily improved airway function via their anti-inflammatory action, indicating that their clinical synergism occurred at the organism rather than the cellular level. However, it is now becoming clear that both drug classes can affect airway function at multiple levels, including an integrated effect on several cell types. This article summarizes data on the molecular interaction between the two receptor systems, particularly with relevance to phenomena of β(2)-adrenergic receptor desensitization and glucocorticoid insensitivity in the airways. These molecular interactions may contribute to the observed clinical synergism between both drug classes in the treatment of obstructive airway diseases.

MicroRNA let-7 establishes expression of beta2-adrenergic receptors and dynamically down-regulates agonist-promoted down-regulation

Although β(2)-adrenergic receptors (β(2)AR) are expressed on most cell types, mechanisms that establish expression levels and regulate expression by chronic agonist remain unclear. The 3' UTR of ADRB2 has a conserved 8-nucleotide seed region that we hypothesized is targeted by the let-7 family of miRNAs leading to translational repression. In luciferase assays with transfected cells, luc-β(2)WT3'UTR had decreased expression when cotransfected with let-7f, but a mutated luc-β(2)3'UTR lacking the seed was unaffected by let-7f; a mutated let-7f also had no effect on luc-β(2)WT3'UTR expression. ADRB2 mRNA was in greater abundance in immunoprecipitates of Ago2, a core component of the miRNA-induced silencing complex, when cells were transfected with let-7f, but not with a mutated let-7f, indicating a direct interaction with the silencing mechanism. H292 cells transfected with let-7f caused ∼60% decrease in native β(2)AR expression, but transfection with let-7f-specific locked nucleic acid anti-miRNA increased β(2)AR expression by ∼twofold. We considered that an increase in let-7f leading to greater repression of translation contributes to agonist-promoted down-regulation. Paradoxically, in cells and in lungs from mice treated in vivo, an ∼50% decrease in let-7f occurs during long-term agonist exposure, indicating a counterregulatory event. Consistent with this notion, let-7f locked nucleic acid transfection caused depressed agonist-promoted down-regulation. Thus, let-7f miRNA regulates baseline β(2)AR expression and decreases in let-7f evoked by agonist attenuate down-regulation. This positive feedback loop has not previously been described for a G protein-coupled receptor and its miRNA. Methods to decrease let-7f expression in targeted cells may increase therapeutic responses to β-agonist by increasing β(2)AR expression or minimizing tachyphylaxis.

Paradoxical attenuation of β2-AR function in airway smooth muscle by Gi-mediated counterregulation in transgenic mice overexpressing type 5 adenylyl cyclase

The limiting component within the receptor-G protein-effector complex in airway smooth muscle (ASM) for β(2)-adrenergic receptor (β(2)-AR)-mediated relaxation is unknown. In cardiomyocytes, adenylyl cyclase (AC) is considered the "bottleneck" for β-AR signaling, and gene therapy trials are underway to increase inotropy by increasing cardiac AC expression. We hypothesized that increasing AC in ASM would increase relaxation from β-agonists, thereby providing a strategy for asthma therapy. Transgenic (TG) mice were generated with approximately two- to threefold overexpression of type 5 AC (AC5) in ASM. cAMP and airway relaxation in response to direct activation of AC by forskolin were increased in AC5-TG. Counter to our hypothesis, isoproterenol-mediated airway relaxation was significantly attenuated (∼50%) in AC5-TG, as was cAMP production, suggesting compensatory regulatory events limiting β(2)-AR signaling when AC expression is increased. In contrast, acetylcholine-mediated contraction was preserved. G(αi) expression and ERK1/2 activation were markedly increased in AC5-TG (5- and 8-fold, respectively), and β-AR expression was decreased by ∼40%. Other G proteins, G protein-coupled receptor kinases, and β-arrestins were unaffected. β-agonist-mediated airway relaxation of AC5-TG was normalized to that of nontransgenic mice by pertussis toxin, implicating β(2)-AR coupling to the increased G(i) as a mechanism of depressed agonist-promoted relaxation in these mice. The decrease in β(2)-AR may account for additional relaxation impairment, given that there is no enhancement over nontransgenic after pertussis toxin, despite AC5 overexpression. ERK1/2 inhibition had no effect on the phenotype. Thus perturbing the ratio of β(2)-AR to AC in ASM by increasing AC fails to improve (and actually decreases) β-agonist efficacy due to counterregulatory events.

β2-Adrenergic receptor desensitization in perirenal adipose tissue in fetuses and lambs with placental insufficiency-induced intrauterine growth restriction

Placental insufficiency-induced intrauterine growth restriction (IUGR) fetuses have chronic hypoxaemia and elevated plasma catecholamine concentrations. In this study, we determined whether adrenergic responsiveness becomes desensitized in the perirenal adipose tissue of IUGR fetuses and lambs by measuring adrenergic receptor (AR) mRNA and protein levels. We also tested the ability of adrenaline to mobilize non-esterified fatty acids (NEFAs) in young lambs. Perirenal adipose tissue was collected from IUGR and control fetuses at 133 days of gestational age (dGA) and lambs at 18 days of age (dA). β(2)-AR mRNA concentrations were 59% and 74% lower (P < 0.05) in IUGR fetuses and lambs compared to controls, respectively, which also resulted in lower protein levels (P < 0.05). No treatment differences were detected for α(1A)-, α(1B)-, α(1D)-, α(2A)-, α(2B)-, α(2C)-, β(1)- and β(3)-AR expression. mRNA concentrations were also determined for hormone sensitive lipase (HSL), perilipin (lipid droplet-associated protein), and two adipokines, leptin and adiponectin. Adiponectin and HSL were not different between treatments at either age. Compared to controls, perilipin and leptin mRNA concentrations were lower (P < 0.05) in IUGR fetuses but not in lambs. Because of the β(2)-AR results, we challenged a second cohort of lambs with exogenous adrenaline at 21 dA. The ability of adrenaline to mobilize NEFA was 55 ± 15% lower (P < 0.05) in IUGRs than controls. Collectively, our findings indicate that elevated catecholamine exposure in utero causes desensitization of adipose tissue by down-regulation of β(2)-AR, and this persists in lambs. This impairment in adrenergic stimulated lipolysis might partially explain early onset obesity in IUGR offspring.

Beta-Adrenergic Agonists

Inhaled β₂-adrenoceptor (β₂-AR) agonists are considered essential bronchodilator drugs in the treatment of bronchial asthma, both as symptoms-relievers and, in combination with inhaled corticosteroids, as disease-controllers. In this article, we first review the basic mechanisms by which the β₂-adrenergic system contributes to the control of airway smooth muscle tone. Then, we go on describing the structural characteristics of β₂-AR and the molecular basis of G-protein-coupled receptor signaling and mechanisms of its desensitization/ dysfunction. In particular, phosphorylation mediated by protein kinase A and β-adrenergic receptor kinase are examined in detail. Finally, we discuss the pivotal role of inhaled β₂-AR agonists in the treatment of asthma and the concerns about their safety that have been recently raised.

Role of beta-arrestin1/ERK MAP kinase pathway in regulating adenosine A1 receptor desensitization and recovery

Exposure of cells to adenosine receptor (AR) agonists leads to receptor uncoupling from G proteins and downregulation of the A(1)AR. The receptor levels on the cell surface generally recover on withdrawal of the agonist, because of either translocation of the sequestered A(1)AR back to plasma membrane or de novo synthesis of A(1)AR. To examine the mechanism(s) underlying A(1)AR downregulation and recovery, we treated ductus deferens tumor (DDT(1) MF-2) cells with the agonist R-phenylisopropyladenosine (R-PIA) and showed a decrease in membrane A(1)AR levels by 24 h, which was associated with an unexpected 11-fold increase in A(1)AR mRNA. Acute exposure of these cells to R-PIA resulted in a rapid translocation of beta-arrestin1 to the plasma membrane. Knockdown of beta-arrestin1 by short interfering RNA (siRNA) blocked R-PIA-mediated downregulation of the A(1)AR, suppressed R-PIA-dependent ERK1/2 and activator protein-1 (AP-1) activity, and reduced the induction of A(1)AR mRNA. Withdrawal of the agonist after a 24-h exposure resulted in rapid recovery of plasma membrane A(1)AR. This was dependent on the de novo protein synthesis and on the activity of ERK1/2 but independent of beta-arrestin1 and nuclear factor-kappaB. Together, these data suggest that exposure to A(1)AR agonist stimulates ERK1/2 activity via beta-arrestin1, which subserves receptor uncoupling and downregulation, in addition to the induction of A(1)AR expression. We propose that such a pathway ensures both the termination of the agonist signal and recovery by priming the cell for rapid de novo synthesis of A(1)AR once the drug is terminated.

Targeted transgenesis reveals discrete attenuator functions of GRK and PKA in airway beta2-adrenergic receptor physiologic signaling

Phosphorylation by protein kinase A (PKA) and G protein-coupled receptor kinases (GRKs) desensitize beta2-adrenergic receptor (beta2AR) signaling, and these are thought to be mechanisms involved with cell and organ homeostasis and tolerance to agonists. However, there is little direct evidence that these events are relevant to beta2AR physiological function, such as airway smooth muscle (ASM) relaxation leading to bronchodilation. To maintain cell- and receptor-specificity without altering the natural complement of kinases/arrestins, transgenic mice were generated expressing the human WT and mutated beta2ARs lacking PKA and/or GRK phosphorylation sites on ASM at approximately 4-fold over background. Functional gains in response to beta-agonist from the selective loss of these mechanisms were determined in mouse airways. Relaxation kinetics were altered in all mutant airways compared with beta2WT. At low receptor occupancy, beta2PKA(-) had enhanced agonist-promoted relaxation, while beta2GRK(-) airways were unaffected. In contrast, at saturating agonist concentrations, the greatest relaxation enhancement was with beta2GRK(-), with no evidence for additivity when PKA sites were also removed. For the full range of responses, the beta2PKA(-)/GRK(-) airways had the greatest relaxation efficiency, indicating a graded effect of GRKs as agonist concentration increased. ASM cAMP levels paralleled relaxation phenotypes. No interaction between PKA phosphorylation of beta2AR and GRK-promoted events was identified by beta-arrestin-2 recruitment. Thus, these two mechanisms indeed impact a relevant beta2AR physiologic function, acting as attenuators of the acute response, and represent specific interfaces where adjunct therapy or biased ligands may improve beta-agonist treatment of obstructive lung disease.

Cargo-mediated regulation of a rapid Rab4-dependent recycling pathway

Membrane trafficking is well known to regulate receptor-mediated signaling processes, but less is known about whether signaling receptors conversely regulate the membrane trafficking machinery. We investigated this question by focusing on the beta-2 adrenergic receptor (B2AR), a G protein-coupled receptor whose cellular signaling activity is controlled by ligand-induced endocytosis followed by recycling. We used total internal reflection fluorescence microscopy (TIR-FM) and tagging with a pH-sensitive GFP variant to image discrete membrane trafficking events mediating B2AR endo- and exocytosis. Within several minutes after initiating rapid endocytosis of B2ARs by the adrenergic agonist isoproterenol, we observed bright "puffs" of locally increased surface fluorescence intensity representing discrete Rab4-dependent recycling events. These events reached a constant frequency in the continuous presence of isoproterenol, and agonist removal produced a rapid (observed within 1 min) and pronounced (approximately twofold) increase in recycling event frequency. This regulation required receptor signaling via the cAMP-dependent protein kinase (PKA) and a specific PKA consensus site located in the carboxyl-terminal cytoplasmic tail of the B2AR itself. B2AR-mediated regulation was not restricted to this membrane cargo, however, as transferrin receptors packaged in the same population of recycling vesicles were similarly affected. In contrast, net recycling measured over a longer time interval (10 to 30 min) was not detectably regulated by B2AR signaling. These results identify rapid regulation of a specific recycling pathway by a signaling receptor cargo.

In vitro hypoxia impairs beta2-adrenergic receptor signaling in primary rat alveolar epithelial cells

Hypoxia inhibits beta(2)-adrenergic receptor (beta(2)-AR) signaling in a variety of tissues, but effects in alveolar epithelium are unclear. We therefore examined the effect of 24 h of hypoxia on beta(2)-AR function in primary rat alveolar epithelial [alveolar type II (ATII)] cells. ATII cells were isolated, cultured to confluence, and incubated in normoxia or hypoxia (3% O(2)) for 24 h. Hypoxia decreased maximal terbutaline-stimulated cAMP production by 37%; potency of terbutaline was not affected. Reoxygenation (3 h) reversed this effect. Density of beta(2)-AR assessed by (-)-[(125)I]iodocyanopindolol binding was decreased in hypoxia (-22%). Hypoxia did not affect terbutaline binding affinity to beta(2)-AR. Hypoxia decreased G(s) protein levels by 27%, whereas no change was observed in G(i1/2), G(i3), and Gbeta subunits. Forskolin-stimulated cAMP production was not inhibited by hypoxia. Pertussis toxin (PTX; 0.5 microg/ml, 2 h), an inhibitor of G(i/o) proteins, restored terbutaline-stimulated cAMP production of hypoxic ATII cells to normoxic control values. Cholera toxin (CTX)-stimulated G(s) protein activity did not change in hypoxia. Hypoxia increased the sensitivity of beta(2)-AR to desensitization. These results indicate that despite the decrease in G(s) protein level G(s) protein was still functional and that hypoxia impairs beta(2)-AR signaling due to an increased activity of G(i/o) proteins.

Nedd4 mediates agonist-dependent ubiquitination, lysosomal targeting, and degradation of the beta2-adrenergic receptor

Agonist-stimulated beta(2)-adrenergic receptor (beta(2)AR) ubiquitination is a major factor that governs both lysosomal trafficking and degradation of internalized receptors, but the identity of the E3 ubiquitin ligase regulating this process was unknown. Among the various catalytically inactive E3 ubiquitin ligase mutants that we tested, a dominant negative Nedd4 specifically inhibited isoproterenol-induced ubiquitination and degradation of the beta(2)AR in HEK-293 cells. Moreover, siRNA that down-regulates Nedd4 expression inhibited beta(2)AR ubiquitination and lysosomal degradation, whereas siRNA targeting the closely related E3 ligases Nedd4-2 or AIP4 did not. Interestingly, beta(2)AR as well as beta-arrestin2, the endocytic and signaling adaptor for the beta(2)AR, interact robustly with Nedd4 upon agonist stimulation. However, beta(2)AR-Nedd4 interaction is ablated when beta-arrestin2 expression is knocked down by siRNA transfection, implicating an essential E3 ubiquitin ligase adaptor role for beta-arrestin2 in mediating beta(2)AR ubiquitination. Notably, beta-arrestin2 interacts with two different E3 ubiquitin ligases, namely, Mdm2 and Nedd4 to regulate distinct steps in beta(2)AR trafficking. Collectively, our findings indicate that the degradative fate of the beta(2)AR in the lysosomal compartments is dependent upon beta-arrestin2-mediated recruitment of Nedd4 to the activated receptor and Nedd4-catalyzed ubiquitination.

Regulation of G protein-coupled receptor signalling: focus on the cardiovascular system and regulator of G protein signalling proteins

G protein-coupled receptors (GPCRs) are involved in many biological processes. Therefore, GPCR function is tightly controlled both at receptor level and at the level of signalling components. Well-known mechanisms by which GPCR function can be regulated comprise desensitization/resensitization processes and GPCR up- and downregulation. GPCR function can also be regulated by several proteins that directly interact with the receptor and thereby modulate receptor activity. An additional mechanism by which receptor signalling is regulated involves an emerging class of proteins, the so-called regulators of G protein signalling (RGS). In this review we will describe some of these control mechanisms in more detail with some specific examples in the cardiovascular system. In addition, we will provide an overview on RGS proteins and the involvement of RGS proteins in cardiovascular function.

Phosphorylation of GRK2 by PKA augments GRK2-mediated phosphorylation, internalization, and desensitization of VPAC2 receptors in smooth muscle

The smooth muscle of the gut expresses mainly G(s) protein-coupled vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activating peptide receptors (VPAC(2) receptors), which belong to the secretin family of G protein-coupled receptors. The extent to which PKA and G protein-coupled receptor kinases (GRKs) participate in homologous desensitization varies greatly among the secretin family of receptors. The present study identified the novel role of PKA in homologous desensitization of VPAC(2) receptors via the phosphorylation of GRK2 at Ser(685). VIP induced phosphorylation of GRK2 in a concentration-dependent fashion, and the phosphorylation was abolished by blockade of PKA with cell-permeable myristoylated protein kinase inhibitor (PKI) or in cells expressing PKA phosphorylation-site deficient GRK2(S685A). Phosphorylation of GRK2 increased its activity and binding to G betagamma. VIP-induced phosphorylation of VPAC(2) receptors was abolished in muscle cells expressing kinase-deficient GRK2(K220R) and attenuated in cells expressing GRK2(S685A) or by PKI. VPAC(2) receptor internalization (determined from residual (125)I-labeled VIP binding and receptor biotinylation after a 30-min exposure to VIP) was blocked in cells expressing GRK2(K220R) and attenuated in cells expressing GRK2(S685A) or by PKI. Finally, VPAC(2) receptor degradation (determined from residual (125)I-labeled VIP binding and receptor expression after a prolonged exposure to VIP) and functional VPAC(2) receptor desensitization (determined from the decrease in adenylyl cyclase activity and cAMP formation after a 30-min exposure to VIP) were abolished in cells expressing GRK2(K220R) and attenuated in cells expressing GRK2(S685A). These results demonstrate that in gastric smooth muscle VPAC(2) receptor phosphorylation is mediated by GRK2. Phosphorylation of GRK2 by PKA enhances GRK2 activity and its ability to induce VPAC(2) receptor phosphorylation, internalization, desensitization, and degradation.

Inverse agonist exposure enhances ligand binding and G protein activation of the human MT1 melatonin receptor, but leads to receptor down-regulation

Melatonin binds and activates G protein-coupled melatonin receptors. The density and affinity of the endogenous melatonin receptors change throughout the 24-hr day, and the exposure of recombinant melatonin receptors to melatonin often results in desensitization of the receptors. Receptor density, G protein activation and expression level were analyzed in CHO cell lines stably expressing the human MT1 receptors after 1 or 72 hr of exposure to melatonin (agonist, 10 nm) and luzindole (antagonist/inverse agonist, 10 microm). The 72-hr exposure to luzindole significantly increased the apparent receptor density in cell lines with both high and low MT1 receptor expression levels (MT1(high) and MT1(low) cells, respectively). In the constitutively active MT1(high) cells, luzindole pretreatment also stimulated the functional response to melatonin in [(35)S]GTPgammaS binding assays, whereas melatonin pretreatment attenuated the functional response at both time points. Receptor ELISA was used to analyze the cell membrane and total expression level of the MT1 receptor in intact and permeabilized cells, respectively. Luzindole pretreatment decreased the total cellular level of MT1 receptor in the MT1(high) cells at both time points but increased the cell surface expression of MT1 receptor at 72 hr. Melatonin significantly decreased MT1 receptor cell surface expression only in MT1(high) cells after a 1-hr treatment. These results indicate that melatonin treatment desensitizes MT1 receptors, whereas luzindole increases ligand binding and G-protein activation. Luzindole also stimulates downregulation of the MT1 receptor protein, interfering with the synthesis and/or degradation of the receptor.

Chronic direct stimulation of adenylyl cyclase induces cardiac desensitization to catecholamine and beta-adrenergic receptor downregulation in rabbits

Chronic stimulation of beta-adrenergic receptors (betaARs) induces betaAR downregulation. However, it is not known whether continuous activation of adenylyl cyclase without direct stimulation of betaARs leads to receptor downregulation. This study investigated the effects of chronic stimulation of adenylyl cyclase with colforsin, on hemodynamic variables, and on myocardial betaAR density. In all, 55 rabbits received intravenous colforsin (1.6 microg/kg/min, n = 20), isoproterenol (ISO; 0.4 microg/kg/min, n = 16), or saline (n = 19) for two weeks. After chronic drug administration, responses of systolic (Delta% peak LV +dP/dt) and diastolic function (Delta% peak LV -dP/dt), and heart rate (Delta% heart rate), to acute administration of ISO (0.05 to 0.2 microg/kg/min) or colforsin (5 to 20 nmol/kg/min) were decreased compared to those before chronic administration. betaAR density in the colforsin group (69.8 +/- 13.8 fmol/ml protein) was less than that in the saline group (79.8 +/- 15.0 fmol/ml protein, P < 0.05), but was greater than that in the ISO group (56.3 +/- 8.4 fmol/ml protein, P < 0.05). Thus, chronic direct stimulation of adenylyl cyclase elicited systolic and diastolic functional desensitization to betaAR stimulation or adenylyl cyclase stimulation, and myocardial betaAR downregulation.

Cytokines regulate beta-2-adrenergic receptor responsiveness in airway smooth muscle via multiple PKA- and EP2 receptor-dependent mechanisms

Beta2AR desensitization in airway smooth muscle (ASM) mediated by airway inflammation has been proposed to contribute to asthma pathogenesis and diminished efficacy of beta-agonist therapy. Mechanistic insight into this phenomenon is largely conceptual and lacks direct empirical evidence. Here, we employ molecular and genetic strategies to reveal mechanisms mediating cytokine effects on ASM beta2AR responsiveness. Ectopic expression of inhibitory peptide (PKI-GFP) or a mutant regulatory subunit of PKA (RevAB-GFP) effectively inhibited intracellular PKA activity in cultured human ASM cells and enhanced beta2AR responsiveness by mitigating both agonist-specific (beta-agonist-mediated) desensitization and cytokine (IL-1beta and TNF-alpha)-induced heterologous desensitization via actions on multiple targets. In the absence of cytokine treatment, PKA inhibition increased beta2AR-mediated signaling by increasing both beta2AR-G protein coupling and intrinsic adenylyl cyclase activity. PKI-GFP and RevAB-GFP expression also conferred resistance to cytokine-promoted beta2AR-G protein uncoupling and disrupted feed-forward mechanisms of PKA activation by attenuating the induction of COX-2 and PGE2. Cytokine treatment of tracheal ring preparations from wild-type mice resulted in a profound loss of beta-agonist-mediated relaxation of methacholine-contracted rings, whereas rings from EP2 receptor knockout mice were largely resistant to cytokine-mediated beta2AR desensitization. These findings identify EP2 receptor- and PKA-dependent mechanisms as the principal effectors of cytokine-mediated beta2AR desensitization in ASM.

Chronic beta-adrenergic receptor stimulation enhances the expression of G-Protein coupled receptor kinases, GRK2 and GRK5, in both the heart and peripheral lymphocytes

BACKGROUND

Enhanced expression of G protein-coupled receptor kinase (GRK) has been reported in failing hearts and in the present study the stability of enhanced GRK mRNA expression, and the correlation between the expression level of GRK mRNA in peripheral lymphocytes and in the heart were both evaluated.

METHODS AND RESULTS

Isoproterenol was injected into rats for 2 weeks, and then GRK5 mRNA was assessed by quantitative reverse transcriptase-palymerase chain reaction. An enhanced expression of cardiac GRK5 mRNA was observed even after 4 weeks of recovery. The isoproterenol-induced increased expression of GRK2 and GRK5 mRNA was equally observed in the heart and lymphocytes, and there was a close correlation between the heart and lymphocytes in the level of expression of each GRK mRNA.

CONCLUSIONS

The GRK mRNA level is maintained at a high level for a long period without continuous beta-adrenergic receptor stimulation. The level in circulating lymphocytes could be used as a surrogate marker to estimate the level of cardiac GRK expression and, presumably, the beta-adrenergic receptor function of cardiomyocytes.

Are so many adrenergic receptor subtypes really present in domestic animal tissues? A pharmacological perspective

Adrenergic receptors (ARs) are the cellular membrane binding sites through which natural catecholamines and sympathomimetic drugs exert their physiological and pharmacological effects. In recent decades, studies to clarify the distribution and function of ARs have been performed mostly on cultured cells, laboratory animals and human target tissues, but little is known about these aspects in domestic animals. This review focuses on AR structure, classification and signalling pathways and on AR subtype distribution in target tissues of some domestic animals, namely dogs, horses and bovines. In these species, different alpha- and beta-AR subtypes have been characterized and the functions controlled by the adrenergic systems have been studied. In the dog, the role played by the adrenergic system in the pathogenesis of cardiovascular disorders and in the modulation of canine aggression has roused particular interest. In dogs affected by dilated cardiomyopathy a significant down-regulation of beta-ARs has been observed both in the heart and circulating lymphocytes. This finding confirms the involvement of the adrenergic system in the pathogenesis and progression of the disorder and suggests new therapeutic strategies. In the horse, AR distribution has been studied in the cardiac, respiratory and gastrointestinal systems as well as in digital veins and arteries. The cardiac beta-ARs in healthy horses seem to be predominantly represented by the beta(1) subtype. In this species, heart failure may increase the expression of the beta(2) subtype, rather than causing AR down-regulation. Different beta- and alpha-AR subtypes have been characterized in the smooth muscle of equine ileum. The sympathetic relaxation of equine ileum smooth muscle seems to depend mainly on beta(3)-AR subtype activation, with minor involvement of the beta(2) subtype. In the respiratory tract, regional differences have been evidenced in the functionality of beta-AR subtype. The beta(2) subtype predominates in all segments but the beta(2) subtype-mediated adenyl cyclase response is tissue-dependent, with higher activity in tracheal membranes than bronchial or pulmonary ones. Both alpha- and beta-AR subtypes are present in the genital tract of cows. Bovine ovarian and myometrial cell membranes express higher concentrations of beta(2)-ARs than the beta(1) subtype, whereas as far as alpha-ARs are concerned, a single class of alpha(1)-ARs and two distinct classes of alpha(2)-AR binding sites have been discriminated. Interestingly, it has been observed that the activation of the sympathetic system could play an important role in the pathogenesis of bovine ovarian cysts as suggested by the modifications in beta-AR levels in the hypophysis and ovary of cows affected by ovarian cysts. In this species, the phenomenon of down-regulation has been well studied in different organs of veal calves treated with clenbuterol as a "partitioning agent". Since differences exist in AR distribution among species, data obtained in laboratory animals or in human beings cannot be extrapolated to domestic animals and further investigation on AR subtypes in domestic animal tissues is necessary.

Molecular mechanisms of ligand binding, signaling, and regulation within the superfamily of G-protein-coupled receptors: molecular modeling and mutagenesis approaches to receptor structure and function

The superfamily of G-protein-coupled receptors (GPCRs) could be subclassified into 7 families (A, B, large N-terminal family B-7 transmembrane helix, C, Frizzled/Smoothened, taste 2, and vomeronasal 1 receptors) among mammalian species. Cloning and functional studies of GPCRs have revealed that the superfamily of GPCRs comprises receptors for chemically diverse native ligands including (1) endogenous compounds like amines, peptides, and Wnt proteins (i.e., secreted proteins activating Frizzled receptors); (2) endogenous cell surface adhesion molecules; and (3) photons and exogenous compounds like odorants. The combined use of site-directed mutagenesis and molecular modeling approaches have provided detailed insight into molecular mechanisms of ligand binding, receptor folding, receptor activation, G-protein coupling, and regulation of GPCRs. The vast majority of family A, B, C, vomeronasal 1, and taste 2 receptors are able to transduce signals into cells through G-protein coupling. However, G-protein-independent signaling mechanisms have also been reported for many GPCRs. Specific interaction motifs in the intracellular parts of these receptors allow them to interact with scaffold proteins. Protein engineering techniques have provided information on molecular mechanisms of GPCR-accessory protein, GPCR-GPCR, and GPCR-scaffold protein interactions. Site-directed mutagenesis and molecular dynamics simulations have revealed that the inactive state conformations are stabilized by specific interhelical and intrahelical salt bridge interactions and hydrophobic-type interactions. Constitutively activating mutations or agonist binding disrupts such constraining interactions leading to receptor conformations that associates with and activate G-proteins.

A primate-dominant third glycosylation site of the beta2-adrenergic receptor routes receptors to degradation during agonist regulation

beta(2)-adrenergic receptors (beta(2)AR) of all species are N-linked glycosylated at amino terminus residues approximately 6 and approximately 15. However, the human beta(2)AR has a potential third N-glycosylation site at ECL2 residue 187. To determine whether this residue is glycosylated and to ascertain function, all possible single/multiple Asn --> Gln mutations were made in the human beta(2) AR at positions 6, 15, and 187 and were expressed in Chinese hamster fibroblast cells. Substitution of Asn-187 alone or with Asn-6 or Asn-15 decreased the apparent molecular mass of the receptor on SDS-PAGE in a manner consistent with Asn-187 glycosylation. All receptors bound the agonist isoproterenol and functionally coupled to adenylyl cyclase. However, receptors without 187 glycosylation failed to display long term agonist-promoted down-regulation. In contrast, loss of Asn-6/Asn-15 glycosylation did not alter down-regulation. Cell surface distribution and agonist-promoted internalization of receptors and recruitment of beta-arrestin 2 were unaffected by the loss of 187 glycosylation. Furthermore, acutely internalized wild-type and Gln-187 receptors were both localized by confocal microscopy to early endosomes. During prolonged agonist exposure, wild-type beta(2)AR co-localized with lysosomes, consistent with trafficking to a degradation compartment. However, Gln-187 beta(2)AR failed to co-localize with lysosomes despite agonist treatments up to 18 h. Phylogenetic analysis revealed that this third glycosylation site is found in humans and other higher order primates but not in lower order primates such as the monkey. Nor is this third site found in rodents, which are frequently utilized as animal models. These data thus reveal a previously unrecognized beta(2)AR regulatory motif that appeared late in primate evolution and serves to direct internalized receptors to lysosomal degradation during long term agonist exposure.

Differential functional expression of human myocardial G protein receptor kinases in left ventricular cardiac diseases

The relationship between myocardial G protein receptor kinase (GRK) expression and beta-adrenoceptor signalling in human left heart diseases has not been fully elucidated yet. In this study, we characterized and compared the GRK2-7 expression in patients with left ventricular volume overload disorders and dilated cardiomyopathic hearts, and evaluated the relationship of this expression with alterations in myocardial beta-adrenoceptor signalling in volume overload, in order to test the notion that GRK functional expression is influenced in a disease-specific and selective fashion. We established that GRK2, GRK3, and GRK5 are well expressed, while GRK4, GRK6, and GRK7 are only scarcely detectable in the healthy human heart. Compared to control hearts (n=8), GRK2 mRNA expression was elevated by 71% (P<0.005) in the left ventricle, 110% (P<0.05) in the right ventricle, 130% (P<0.05) in the left atrium, and 1300% (P<0.005) in the right atrium (RA) of the dilated cardiomyopathy hearts (n=6). In the volume overload group (n=10), it was increased by approximately 40% (P<0.05) in the left ventricle, 38% in the right ventricle, 81% (P<0.05) in the left atrium, and 850% (P<0.005) in the right atrium. On the other hand, GRK5 was significantly elevated only in the left ventricle by 68% (P<0.05) in the dilated cardiomyopathy hearts and by 48% (P<0.01) in volume overload patients, while in contrast, GRK3 remained unchanged in dilated cardiomyopathy, but was slightly elevated by 36% (P=0.05) in the right ventricle of the volume overload patients. The alterations in GRK expression were accompanied with a decrease in myocardial beta(1)-adrenoceptor mRNA in all four chambers, and these trends in gene expression were paralleled with those of their immunodetectable protein levels. Furthermore, these changes were in association with a decrease in downstream receptor-stimulated, adenylyl cyclase-mediated functional expression and an increase in ventricular protein kinase A activity. The results point to differences in which myocardial GRKs are regulated in cardiac disease, whereby changes in GRK2 expression may be related to the global effects of the disease on myocardial adrenoceptor function and those in GRK5 may be localized to the ventricles, depending on the nature of the myocardial load.

The role of catecholamines in memory impairment in chicks following reduced gas exchange in ovo

We have shown previously that reducing gas exchange to chick embryos by half wrapping eggs with an impermeable membrane from either days 14-18 (W14-18) or days 10-18 (W10-18) of the 21 day incubation results in post-hatch memory deficits. In the W10-18 chicks, short-term memory following training is impaired, whereas in the W14-18 chicks, memory is intact for 30 min but does not consolidate into long-term storage. The reduction in gas exchange caused by half wrapping eggs resulted in alterations in hematocrit, O2 and CO2 tensions suggesting that the embryos are hypoxic and hypercapnic. Our aim was to test the hypothesis that increases in circulating levels of catecholamines in ovo, as a result of hypoxia, lead to a disturbance of the central noradrenergic pathways resulting in cognitive impairment. Noradrenaline is critical for memory consolidation and a disturbance during development could compromise cognitive ability. In the present study, plasma noradrenaline levels were significantly elevated compared with control levels 2 days after hatch in W14-18 chicks. There was also a decrease in tissue noradrenaline concentration in the anterior forebrain in both W14-18 and W10-18 chicks. The differential ability of centrally administered beta2- and beta3-adrenoceptor agonists to overcome the memory deficit post-training, suggests altered responsiveness of central beta2-adrenoceptors to noradrenaline in W14-18 chicks. By comparing the W10-18 and W14-18 chicks with those from eggs wrapped from W10-14 we show that it is the timing of the prenatal hypoxia, rather than its duration, that determines the nature of cognitive dysfunction. We conclude that prenatal hypoxia induced by restriction of gas exchange can disrupt or alter central noradrenergic transmission causing cognitive impairment.

Dopamine receptor downregulation: an alternative strategy for schizophrenia treatment

Schizophrenia is a common and devastating illness. The cause of schizophrenia is still unknown and the simplest formulation of the "Dopamine hypothesis" posits that schizophrenia results from dopaminergic hyperactivity. Under the hypothesis of dopaminergic hyperactivity in schizophrenia, antipsychotics blocking the dopamine D2 receptor (DRD2) and other approaches to reduce dopamine (DA) transmission have been used to treat schizophrenia. I propose that dopamine receptor (DR) downregulation could be an alternative strategy to compromise dopaminergic overactivity implicated in the pathogenesis of schizophrenia. Agonist-induced receptor downregulation includes receptor proteolysis, modulation of receptor gene transcription and affecting of RNA stability. These processes cause a decrease of existing receptors and reduction of receptor synthesis. This hypothesis could explain the antipsychotic mechanisms of DA agonists or partial agonists, like aripiprazole. It is suggested that the development of agents that increase DR downregulation could be an alternative strategy for schizophrenia treatment.

Regulation of beta-adrenergic responses in airway smooth muscle

Decreased responsiveness to beta-adrenergic receptor agonists is a characteristic feature of human asthma. This review summarizes data regarding the impact of chronic beta agonist stimulation, cytokines, prostanoids and other factors on beta-adrenergic responses in human airway smooth muscle, as well as the impact of polymorphisms of the beta(2)-adrenergic receptor on these responses. Effects of beta-agonists on both airway smooth muscle relaxation and gene expression are considered. Understanding the regulation of beta-adrenergic responses in airway smooth muscle cells may prove to be an important step in improving the efficacy of beta-agonists for the treatment of asthma.

Resistance of the human beta 1-adrenergic receptor to agonist-mediated down-regulation. Role of the C terminus in determining beta-subtype degradation

Prolonged agonist stimulation results in down-regulation of most G protein-coupled receptors. When we exposed baby hamster kidney cells stably expressing the human beta1-adrenergic receptor (beta 1AR) to agonist over a 24-h period, we instead observed an increase of approximately 30% in both beta 1AR binding activity and immune-detected receptors. In contrast, beta 2AR expressed in these cells exhibited a decrease of > or =50%. We determined that the basal turnover rates of the two subtypes were similar (t(1/2) approximately 7 h) and that agonist stimulation increased beta 2AR but not beta 1AR turnover. Blocking receptor trafficking to lysosomes with bafilomycin A1 had no effect on basal turnover of either subtype but blocked agonist-stimulated beta 2AR turnover. As beta 1AR mRNA levels increased in agonist-stimulated cells, beta 1AR up-regulation appeared to result from increased synthesis with no change in degradation. To explore the basis for the subtype differences, we expressed chimeras in which the C termini had been exchanged. Each chimera responded to persistent agonist stimulation based on the source of its C-tail; beta 1AR with a beta 2AR C-tail underwent down-regulation, and beta 2AR with a beta 1AR C-tail underwent up-regulation. The C-tails had a corresponding effect on agonist-stimulated receptor phosphorylation and internalization with the order being beta 2AR > beta 1AR with beta 2AR C-tail > beta 2AR with a beta 1AR C-tail > beta 1AR. As internalization may be a prerequisite for down-regulation, we addressed this possibility by co-expressing each subtype with arrestin-2. Although beta 1AR internalization was increased to that of beta 2AR, down-regulation still did not occur. Instead, beta 1AR accumulated inside the cells. We conclude that in unstimulated cells, both subtypes appear to be turned over by the same mechanism. Upon agonist stimulation, both subtypes are internalized, and beta 2AR but not beta 1AR undergoes lysosomal degradation, the fate of each subtype being regulated by determinants in its C-tail.

Trafficking of the muscarinic m2 autoreceptor in cholinergic basalocortical neurons in vivo: differential regulation of plasma membrane receptor availability and intraneuronal localization in acetylcholinesterase-deficient and -inhibited mice

In vivo, the abundance of receptors at the neuronal plasma membrane may be critical in the mediation of pre- and postsynaptic responses. Thus, we have studied the membrane availability and intraneuronal distribution of the m2 muscarinic autoreceptor (m2R) in cholinergic neurons of the nucleus basalis magnocellularis (NBM) projecting to the frontal cortex (FC). We have studied the subcellular compartmentalization of m2R at somatodendritic postsynaptic and axonal presynaptic sites in control animals (AChE +/+) and in two animal models: mice displaying acute acetylcholinesterase (AChE) inhibition by treatment with metrifonate, and AChE-deficient mice (AChE -/-). In control animals, m2R was mainly located at the plasma membrane in the somatodendritic field of NBM and in cortical varicosities. Acute AChE inhibition and chronic AChE deficiency induced a dramatic decrease of cell surface m2R in the somatodendritic compartment. This finding was associated with two different intracytoplasmic events: (1). internalization of m2R in endosomes after acute AChE inhibition, (2). exaggerated storage of m2R in the endoplasmic reticulum and Golgi complex in AChE -/- mice. In contrast, the m2R density was higher at the membrane of cortical varicosities in AChE -/- mice but unchanged in acutely AChE-inhibited mice. Our data demonstrate that acute and chronic stimulation provoke, in vivo, depletion of the membrane store of somatodendritic m2R through different intracellular mechanisms: endocytosis of receptors from the plasma membrane to the cytoplasm (acute) or regulation of their delivery from intracytoplasmic stores to the plasma membrane (chronic). The increase of m2R at the membrane of axonal varicosities after chronic stimulation suggest modulation of presynaptic cholinergic activity, including neurotransmitter release.

Downregulation of bradykinin B2 receptor in human fibroblasts during prolonged agonist exposure

Sustained activation of G protein-coupled receptors results in an attenuation of cellular responses, a phenomenon termed desensitization. Whereas mechanisms for rapid desensitization of ligand-receptor-G protein-effector systems are relatively well characterized, much less is known about long-term adaptation processes that occur in the continuous presence of an agonist. Here we have studied the fate of endogenously expressed bradykinin B(2) receptors on human fibroblasts during prolonged agonist treatment. Stimulation with bradykinin for up to 24 h resulted in a 50% reduction of surface binding sites that was paralleled by a similar decrease of total B(2) receptor protein followed by Western blotting using monoclonal antibodies to the B(2) receptor. Whereas B(2) receptor mRNA levels did not change during 24 h of agonist treatment, B(2) receptor de novo synthesis was attenuated by 35-50%, indicating translational control of B(2) receptor levels. Furthermore, the half-life of B(2) receptor protein was shortened by 20-40% as shown by (35)S-labeled pulse-chase and immunoprecipitation experiments. This study demonstrates that bradykinin B(2) receptor expression during long-term agonist treatment is primarily regulated on the (post)translational level, i.e., by attenuation of de novo synthesis and by reduction of receptor stability.

Mechanisms of metabotropic glutamate receptor desensitization: role in the patterning of effector enzyme activation

Metabotropic glutamate receptors (mGluRs) constitute an unique subclass of G protein-coupled receptors (GPCRs). These receptors are activated by the excitatory amino acid glutamate and play an essential role in regulating neural development and plasticity. In the present review, we overview the current understanding regarding the molecular mechanisms involved in the desensitization and endocytosis of Group 1 mGluRs as well as the relative contribution of desensitization to the spatial-temporal patterning of glutamate receptor signaling. Similar to what has been reported previously for prototypic GPCRs, mGluRs desensitization is mediated by second messenger-dependent protein kinases and GPCR kinases (GRKs). However, it remains to be determined whether mGluRs phosphorylation by GRKs and beta-arrestin binding are absolutely required for desensitization. Group 1 mGluRs endocytosis is both agonist-dependent and -independent. Agonist-dependent mGluRs internalization is mediated by a beta-arrestin- and dynamin-dependent clathrin-coated vesicle dependent endocytic pathway. The activation of Group 1 mGluRs also results in oscillatory Gq protein-coupling leading to the cyclical activation of phospholipase Cbeta thereby stimulating oscillations in both inositol 1,4,5-triphosphate formation and Ca(2+) release from intracellular stores. These glutamate receptor-stimulated Ca(2+) oscillations are translated into the synchronous activation of protein kinase C (PKC), which has led to the hypothesis that oscillatory mGluRs signaling involves the repetitive phosphorylation of mGluRs by PKC. However, recent experimental evidence suggests that oscillatory signaling is an intrinsic glutamate receptor property that is independent of feedback receptor phosphorylation by PKC. The challenge in the future will be to determine the structural determinants underlying mGluRs-mediated spatial-temporal signaling as well as to understand how complex signaling patterns can be interpreted by cells in both the developing and adult nervous systems.