Agonist-induced Destabilization of β-Adrenergic Receptor mRNA

Dexamethasone attenuates inflammatory-mediated suppression of β2-adrenoceptor expression in rat primary mixed glia

β₂-adrenoceptors are G-protein coupled receptors expressed on both astrocytes and microglia that play a key role in mediating the anti-inflammatory actions of noradrenaline in the CNS. Here the effect of an inflammatory stimulus (LPS + IFN-γ) was examined on glial β₂-adrenoceptor expression and function. Exposure of glia to LPS + IFN-γ decreased β₂-adrenoceptor mRNA and agonist-stimulated production of the intracellular second messenger cAMP. Pre-treatment with the synthetic glucocorticoid and potent anti-inflammatory agent dexamethasone prevented the LPS + IFN-γ-induced suppression of β₂-adrenoceptor mRNA expression. These results raise the possibility that inflammation-mediated β₂-adrenoceptor downregulation in glia may dampen the innate anti-inflammatory properties of noradrenaline in the CNS.

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.

Continuous Inhalation of Ipratropium Bromide for Acute Asthma Refractory to β2-agonist Treatment

To present the case of a patient with persistent bronchospasm, refractory to treatment with β2-agonists, that resolved promptly with continuous inhalation of large dose (1000 mcg/hr) ipratropium bromide, and to discuss the possibility of tolerance to β2-agonists as the cause for his failure to respond to adrenergic medications. The patient had received multiple doses of albuterol, as well as subcutaneous terbutaline (0.3 mg), intravenous magnesium sulfate (1 g) and intravenous dexamethasone (10 mg) prior to his admission to the intensive care unit. He remained symptomatic despite systemic intravenous steroids, continuous intravenous terbutaline (up to 0.6 mcg/kg/min), and continuous nebulized albuterol (up to 20 mg/hr for 57 hr) followed by 49 hours of continuous levalbuterol (7 mg/hr). Due to the lack of response, all β2-agonists were discontinued at 106 hours post-admission, and he was started on large dose ipratropium bromide (1000 mcg/hr) by continuous nebulization. Clinical improvement was evident within 1 hour and complete resolution of his symptoms within 4 hours. Continuous inhalation of large dose ipratropium bromide may be an effective regimen for the treatment of patients hospitalized with acute asthma who are deemed to be nonresponsive and/or tolerant to β2-agonist therapy.

Chronic stress mediators act synergistically on colonic nociceptive mouse dorsal root ganglia neurons to increase excitability

BACKGROUND

Stress hormones can signal to colonic dorsal root ganglia (DRG) neurons and may play a role in sustained hyperexcitability of nociceptors.

METHODS

Mouse DRG neurons were exposed overnight to epinephrine (Epi) 5 nM and/or corticosterone (Cort) 1 μM or prior water-avoidance stress. Patch clamp recordings, visceromotor reflexes (VMRs) and molecular studies were conducted.

KEY RESULTS

Water-avoidance stress induced neuronal hyperexcitability. Incubation of DRG neurons in both Cort and Epi (but neither alone) induced hyperexcitability (rheobase decreased 51%, p < 0.05; action potential discharge increased 95%, p < 0.01); this was blocked by antagonists of the β2 adrenoreceptor (butoxamine, But) and Cort receptor (mifepristone) in combination or alone. Stress hormones enhanced voltage-gated Nav 1.7 currents (p < 0.05) and suppressed IA (p < 0.0001) and IK+ (p < 0.05) currents. Furthermore, stress hormones increased DRG β2 adrenoreceptor mRNA (59%, p = 0.007) and protein (125%, p < 0.05), also Nav 1.7 transcript (45%, p = 0.004) and protein (114%, p = 0.002). In whole-animal studies, the WAS hyperexcitability of DRG neurons was blocked by antagonists of the β2 and glucocorticoid receptors (GCR) but together they paradoxically increased VMRs to colorectal balloon distension.

CONCLUSIONS & INFERENCES

Stress mediators Epi and Cort activate β2 and GCR on DRG neurons which synergistically induce hyperexcitability of nociceptive DRG neurons and cause corresponding changes in voltage-gated Na(+) and K(+) currents. Furthermore, they increase the expression of β2 adrenoreceptors and Nav1.7 channels, suggesting transcriptional changes could contribute to sustained signaling following stress. The paradoxical effects of But and mifepristone in electrophysiological compared to VMR testing may reflect different peripheral and central actions on sensory signaling.

Tolerance & resistance to β₂-agonist bronchodilators

For the past half-century, β₂-agonists have been the mainstay of treatment of the bronchoconstriction associated with asthma. Although their usefulness in reversing acute bronchospasm remains undiminished, there is increasing evidence that chronic use may lead to development of tolerance and thus, potentially increasing morbidity and even mortality. In addition, genetic studies have shown that certain individuals carrying specific mutations may be prone to developing resistance to β₂-agonists regardless of the duration of treatment. This article reviews the current evidence regarding the underlying mechanisms that cause or contribute to the development of the resistance, as well as the strategies for the evaluation and management of patients who are at risk for or have developed tolerance to β₂-agonists.

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.

Effects of budesonide/formoterol combination therapy versus budesonide alone on airway dimensions in asthma

BACKGROUND AND OBJECTIVE

Combination therapy with inhaled corticosteroids and long-acting β(2)-agonists results in improved asthma symptom control compared with the use of inhaled corticosteroids alone. However, the effects of combination therapy on structural changes and inflammation of the airways are still unknown. The aim of this study was to compare the effects of budesonide/formoterol with those of budesonide alone on airway dimensions and inflammation in individuals with asthma.

METHODS

Fifty asthmatic patients were randomized to treatment with budesonide/formoterol (200/6 µg, two inhalations bd) or budesonide (200 µg, two inhalations bd) for 24 weeks. Airway dimensions were assessed using a validated computed tomography technique, and airway wall area (WA) corrected for body surface area (BSA), percentage WA (WA%), wall thickness/Ösquare root BSA, and luminal area (Ai)/BSA at the right apical segmental bronchus, were measured. The percentage of eosinophils in induced sputum, pulmonary function, and Asthma Quality of Life Questionnaires (AQLQ) were also evaluated.

RESULTS

There were significantly greater decreases in WA/BSA (P < 0.05), WA% (P < 0.001) and wall thickness/square root BSA (P < 0.05), and increases in Ai/BSA (P < 0.05), in subjects treated with budesonide/formoterol compared with those treated with budesonide. The reduction in sputum eosinophils and increase in per cent of predicted forced expiratory volume in 1 s (FEV(1) %) were greater for subjects treated with budesonide/formoterol compared with those treated with budesonide alone. In the budesonide/formoterol group, the changes in WA% were significantly correlated with changes in sputum eosinophils and FEV(1%) (r = 0.84 and r = 0.64, respectively). There were improvements in the AQLQ scores after treatment with budesonide/formoterol.

CONCLUSIONS

Budesonide/formoterol combination therapy is more effective than budesonide alone for reducing airway wall thickness and inflammation in individuals with asthma.

G protein-coupled receptors in the hypothalamic paraventricular and supraoptic nuclei--serpentine gateways to neuroendocrine homeostasis

G protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in the mammalian genome. They are activated by a multitude of different ligands that elicit rapid intracellular responses to regulate cell function. Unsurprisingly, a large proportion of therapeutic agents target these receptors. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important mediators in homeostatic control. Many modulators of PVN/SON activity, including neurotransmitters and hormones act via GPCRs--in fact over 100 non-chemosensory GPCRs have been detected in either the PVN or SON. This review provides a comprehensive summary of the expression of GPCRs within the PVN/SON, including data from recent transcriptomic studies that potentially expand the repertoire of GPCRs that may have functional roles in these hypothalamic nuclei. We also present some aspects of the regulation and known roles of GPCRs in PVN/SON, which are likely complemented by the activity of 'orphan' GPCRs.

β2-Adrenergic receptor signaling in osteoblasts contributes to the catabolic effect of glucocorticoids on bone

The sympathetic nervous system is a physiological regulator of bone homeostasis. Autonomic nerves are indeed present in bone, bone cells express the β2-adrenergic receptors (β2AR), and pharmacological or genetic disruption of sympathetic outflow to bone induces bone gain in rodents. These recent findings implied that conditions that affect β2AR signaling in osteoblasts and/or sympathetic drive to bone may contribute to bone diseases. In this study, we show that dexamethasone stimulates the expression of the β2AR in differentiated primary calvarial osteoblasts, as measured by an increase in Adrβ2 mRNA and β2AR protein level after short-term dexamethasone treatment. Isoproterenol-induced cAMP accumulation and the expression of the β2AR target gene Rankl were also significantly increased after dexamethasone pretreatment, indicating that dexamethasone promotes the responsiveness of differentiated osteoblasts to adrenergic stimulation. These in vitro results led to the hypothesis that glucocorticoid-induced bone loss, provoked by increased endogenous or high-dose exogenous glucocorticoids given for the treatment of inflammatory diseases, might, at least in part, be mediated by increased sensitivity of bone-forming cells to the tonic inhibitory effect of sympathetic nerves on bone formation or their stimulatory effect on bone resorption. Supporting this hypothesis, both pharmacological and genetic β2AR blockade in mice significantly reduced the bone catabolic effect of high-dose prednisolone in vivo. This study emphasizes the importance of sympathetic nerves in the regulation of bone homeostasis and indicates that this neuroskeletal signaling axis can be modulated by hormones or drugs and contribute to enhance pathological bone loss.

Moderate global reduction in maternal nutrition has differential stage of gestation specific effects on {beta}1- and {beta}2-adrenergic receptors in the fetal baboon liver

Hepatic β-adrenergic receptors (β-ARs) play a pivotal role in mobilization of reserves via gluconeogenesis and glycogenolysis to supply the animal with its energy needs during decreased nutrient availability. Using a unique nutrient-deprived baboon model, we have demonstrated for the first time that immunoreactive hepatic β(1)- and β(2)-AR subtypes are regionally distributed and localized on cells around the central lobular vein in 0.5 and 0.9 gestation (G) fetuses of ad libitum fed control (CTR) and maternal nutrient restricted (MNR) mothers. Furthermore, MNR decreased fetal liver immunoreactive β(1)-AR and increased immunoreactive β(2)-AR at 0.5G. However, at 0.9G, immunohistochemistry and Western blot analysis revealed a decrease in β(1)-AR and no change in β(2)-AR levels. Thus, MNR in a nonhuman primate species has effects on hepatic β(1)- and β(2)-ARs that are receptor- and gestation stage-specific and may represent compensatory systems whose effects would increase glucose availability in the presence of nutrient deprivation.

The role of AUF1 in regulated mRNA decay

Messenger ribonucleic acid (mRNA) turnover is a major control point in gene expression. In mammals, many mRNAs encoding inflammatory cytokines, oncoproteins, and G-protein-coupled receptors are destabilized by the presence of AU-rich elements (AREs) in their 3'-untranslated regions. Association of ARE-binding proteins (AUBPs) with these mRNAs promotes rapid mRNA degradation. ARE/poly(U)-binding/degradation factor 1 (AUF1), one of the best-characterized AUBPs, binds to many ARE-mRNAs and assembles other factors necessary to recruit the mRNA degradation machinery. These factors include translation initiation factor eIF4G, chaperones hsp27 and hsp70, heat-shock cognate protein hsc70, lactate dehydrogenase, poly(A)-binding protein, and other unidentified proteins. Numerous signaling pathways alter the composition of this AUF1 complex of proteins to effect changes in ARE-mRNA degradation rates. This review briefly describes the roles of mRNA decay in gene expression in general and ARE-mediated decay (AMD) in particular, with a focus on AUF1 and the different modes of regulation that govern AUF1 involvement in AMD.

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.

Adaptive effects of the beta2-agonist clenbuterol on expression of beta2-adrenoceptor mRNA in rat fast-twitch fiber-rich muscles

Administration of the beta(2)-agonist clenbuterol has been shown to reduce the expression of beta(2)-adrenoceptor (AR) mRNA in fast-twitch fiber-rich (extensor digitorum longus, EDL) muscle without changing that in slow-twitch fiber-rich (soleus, SOL) muscle in rats. However, the regulatory mechanism for muscle fiber type-dependent down-regulation of the expression of beta(2)-AR mRNA induced by clenbuterol is still unclear. Therefore, mRNA expression of transcriptional and post-transcriptional regulatory factors for beta(2)-AR mRNA levels in fast-twitch fiber-rich (EDL and plantaris, PLA) and slow-twitch fiber-rich (SOL) muscles in clenbuterol-administered (1.0 mg/kg body weight/day for 10 days, subcutaneous) rats was studied by real-time reverse transcription-polymerase chain reaction. Administration of clenbuterol significantly reduced expression of beta(2)-AR mRNA in EDL and PLA muscles without changing that in SOL muscle. Administration of clenbuterol also significantly reduced the mRNA expression of transcriptional regulatory factor (glucocorticoid receptor) and mRNA stabilizing factor (Hu antigen R) in EDL and PLA muscles without changing those in SOL muscle. These results suggest that muscle fiber type-dependent effects of clenbuterol on expression of beta(2)-AR mRNA are closely related to the down-regulation of mRNA expression of transcriptional and post-transcriptional regulatory factors for beta(2)-AR mRNA levels.

Gastrin-releasing peptide links stressor to cancer progression

INTRODUCTION

Gastrin-releasing peptide (GRP) plays an important role in cancer growth and metastasis; however, the mechanisms of how GRP affects cancer progression are not well understood. Recent studies revealed that chronic stress is a major risk factor for cancer progression, and this effect may be mediated by GRP. In this review, we will discuss the mechanisms and implications of GRP linking stressor to cancer progression.

MATERIALS AND METHODS

We retrieved the studies of the relationship between GRP, stress and cancers through PubMed using systematic methods to search, select, and evaluate the findings.

RESULTS

The results suggested that GRP can mediate the effects of stress on cancers at systemic, tissue and cellular levels: Stress elicits the secretion of GRP in the brain and GRP in turn activates the stress response pathways resulting in an elevation of stress hormones and GRP in the plasma and tissues. GRP in synergy with stress hormones stimulates the growth and invasion of cancer cells by suppressing the anti-tumor immune function and directly activating the pro-proliferative and pro-migratory signaling pathways in cancer cells.

CONCLUSION

GRP is a multi-functional peptide, which acts as a stress mediator as well as a growth factor linking stressor to cancer progression. GRP and its high-affinity receptor are useful targets for the diagnosis and treatment of cancers.

Psychological aspect of cancer: From stressor to cancer progression

Substantial evidence indicates that psychological stress can influence the incidence and progression of cancers, and adequate psychotherapies are beneficial to cancer patients. Recently, the mechanisms responsible for the effects of psychological stress on cancer cells have been extensively investigated at the systemic, biochemical and molecular levels. Accumulating data indicate that the effects of psychological stress on cancer cells are mainly mediated by key stress-related mediators and their corresponding receptors in multi-fold pathways: chronic stressors act on the paraventricular nucleus and the suprachiasmatic nuclei. The effects are then transmitted through the sympathetic nervous system and the hypothalamic-pituitary-adrenal axis, amplified by the unchecked release of stress-related mediators and altered behaviors. These mediators act as immunosuppressors or mitogens in the tumor microenvironment. The converging effects of psychological stress on cancer cells finally signal through receptors of the stress mediators and cytokines to activate the intracellular pro-proliferative and pro-migratory signaling pathways, and reset the molecular clock in tumor cells. Understanding these action mechanisms of psychological stress in promoting the growth and invasion of cancer cells is crucial for devising effective interventions.

Effects of the beta2-agonist clenbuterol on beta1- and beta2-adrenoceptor mRNA expressions of rat skeletal and left ventricle muscles

The beta2-agonist clenbuterol [4-amino-alpha(t-butyl-amino)methyl-3,5-dichlorobenzyl alcohol] is used as a non-steroidal anabolic drug for sports doping. The effects of clenbuterol on the transcriptional process and mRNA stability of beta-adrenoceptor (beta-AR) in skeletal and cardiac muscles are still unknown. Therefore, we investigated the effects of clenbuterol on beta1- and beta2-AR mRNA expressions of fast-twitch fiber-rich extensor digitorum longus (EDL), slow-twitch fiber-rich soleus (SOL), and left ventricle (LV) muscles by real-time RT-PCR. Adult male Sprague Dawley rats were divided into the clenbuterol-administered group and control group. The administration (dose = 1.0 mg/kg body weight/day, s.c.) of clenbuterol was maintained for 10 days. The administration of clenbuterol significantly increased the weight, RNA concentration, and total RNA content of EDL muscle. No effects of clenbuterol on those of SOL and LV muscles, however, were observed. The administration of clenbuterol significantly decreased beta1-AR mRNA expression of LV muscle. Furthermore, the administration of clenbuterol significantly decreased beta2-AR mRNA expression of EDL and LV muscles. No effect of clenbuterol on beta2-AR mRNA expression of SOL muscle, however, was observed. These results suggest that the effects of clenbuterol on beta1- and beta2-AR mRNA expressions and muscle hypertrophy depend on muscle fiber types.

Cells, mediators and Toll-like receptors in COPD

Chronic obstructive pulmonary disease (COPD) is a global health problem. Being a progressive disease characterized by inflammation, it deteriorates pulmonary functioning. Research has focused on airway inflammation, oxidative stress, and remodelling of the airways. Macrophages, neutrophils and T cells are thought to be important key players. A number of new research topics received special attention in the last years. The combined use of inhaled corticosteroids and long-acting beta(2)-adrenoceptor agonists produces better control of symptoms and lung function than that of the use of either compound alone. Furthermore, collagen breakdown products might be involved in the recruitment and activation of inflammatory cells by which the process of airway remodelling becomes self-sustaining. Also, TLR (Toll-like receptor)-based signalling pathways seem to be involved in the pathogenesis of COPD. These new findings may lead to new therapeutic strategies to stop the process of inflammation and self-destruction in the airways of COPD patients.

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.

The effect of inhaled budesonide and formoterol on bronchial remodeling and HRCT features in young asthmatics

Asthma is a chronic disease that may cause remodeling of the airways. We aimed to observe the effects of the combined use of inhaled budesonide and formoterol on both the reversibility of remodeling and structural changes in the airways. Thirty-six male patients (age range, 20-31) with mild-to-moderate persistent asthma were given inhaled formoterol and budesonide treatment for three months. Bronchial diameter (BD) and bronchial wall thickness (BWT), as measured by high-resolution computerized tomography, and reticular basement membrane thickness (RBMT), assessed in bronchoscopic biopsy specimens, were compared with pretreatment findings. Twenty-two age-matched male controls were also enrolled. BDs of the patients were significantly smaller than in the controls, whereas BWT and RBMT were greater. After three months BWT and RBMT of the subsegmental airways significantly decreased and BD increased. There was a prominent eosinophilic and lymphocytic infiltration in the bronchial mucosa of the asthmatics, and the eosinophilic infiltration significantly improved with treatment. Both serum total IgE and eosinophil counts were related to eosinophilic infiltration in the biopsy samples (r = 0.494 and r = 0.463, respectively). FEV(1) was positively correlated with the diameters of the segmental and subsegmental airways (r = 0.491 and r = 0.265, respectively) and negatively correlated with BWT of the subsegmental airways (r = -0.293) and with the RBMT of both the segmental and subsegmental airways (r = -0.597 and r = -0.590, respectively). We suggest that treatment with inhaled formoterol and budesonide may reverse increased RBMT and BWT as part of remodeling in patients with asthma.

Selective upregulation of β1-adrenergic receptors and dephosphorylation of troponin I in end-stage heart failure patients supported by ventricular assist devices

In terminal failing hearts, adrenergic receptors are downregulated and intracellular adrenergic signal transduction is inhibited. Mechanical circulatory support by ventricular assist devices (VAD) is used to bridge patients to heart transplantation. Mechanical unloading by VAD may induce reverse remodeling in heart transplantation (HTx) candidates. However, little is known on beta-adrenergic receptor subtype regulation and adrenergic signal transduction under VAD-support. We investigated paired myocardial samples from 16 VAD-supported patients and 9 non-failing donor hearts. We analyzed beta-adrenergic receptor subtype regulation by real-time PCR and radioligand binding and cardiac troponin I phosphorylation (by phospho-cTnI-specific antibodies). We found that the beta1-adrenergic receptor (beta1AR) is downregulated at VAD-implantation on mRNA and protein levels whereas the beta2-adrenergic receptor (beta2AR) was not. After VAD-support, beta1AR protein but not its mRNA was upregulated, whereas the degree of cTnI-phosphorylation was reduced. Upregulation of beta1AR was enhanced by beta blocking medication during VAD-support. However, in 9 out of 15 patients, beta1AR-density remained below the 0.25 percentile of donor hearts. VAD-support is associated with partial normalization of the betaAR-signal transduction pathways. This beneficial effect is related to a posttranscriptional increase in beta1AR-density.

Changes in mGlu5 receptor expression in the basal ganglia of reserpinised rats

Dopamine depletion in Parkinson's disease results in a series of pathophysiological changes in the basal ganglia circuitry. Increased release of glutamate plays an important role in this motor disorder, therefore, agents interacting with glutamatergic transmission may have therapeutic potential. In this study we investigated changes in both mRNA expression and the number of binding sites of the mGlu5 receptor in a reserpinised rat model of Parkinson's disease. The in situ hybridisation demonstrated that acute reserpine treatment caused a significant decrease in the expression of mGlu5 receptor mRNA in the rostral and caudal parts of the rat striatum. At the same time, tritium-labelled 2-ethyl-6-(phenylethynyl)-pyridine ([(3)H]MPEP) ligand binding experiments detected a significant increase in the total number of mGlu5 receptors in the same region of the motor loop. These apparently contradictory data can be explained by mGlu5 receptor turnover being down-regulated in reserpinised rats, due possibly to an imbalance in the rates of synthesis/insertion and internalisation/degradation of the receptor. These findings suggest that changes such as these affecting mGlu5 receptors may be involved in the pathophysiological consequences of dopamine depletion in the brain.

CRM 1-mediated degradation and agonist-induced down-regulation of beta-adrenergic receptor mRNAs

The beta1-adrenergic receptor (beta1-AR) mRNAs are post-transcriptionally regulated at the level of mRNA stability and undergo accelerated agonist-mediated degradation via interaction of its 3' untranslated region (UTR) with RNA binding proteins, including the HuR nuclear protein. In a previous report [Kirigiti et al. (2001). Mol. Pharmacol. 60:1308-1324], we examined the agonist-mediated down-regulation of the rat beta1-AR mRNAs, endogenously expressed in the rat C6 cell line and ectopically expressed in transfectant hamster DDT1MF2 and rat L6 cells. In this report, we determined that isoproterenol treatment of neonatal rat cortical neurons, an important cell type expressing beta1-ARs in the brain, results in significant decreases in beta1-AR mRNA stability, while treatment with leptomycin B, an inhibitor of the nuclear export receptor CRM 1, results in significant increases in beta1-AR mRNA stability and nuclear retention. UV-crosslinking/immunoprecipitation and glycerol gradient fractionation analyses indicate that the beta1-AR 3' UTR recognize complexes composed of HuR and multiple proteins, including CRM 1. Cell-permeable peptides containing the leucine-rich nuclear export signal (NES) were used as inhibitors of CRM 1-mediated nuclear export. When DDT1MF2 transfectants were treated with isoproterenol and peptide inhibitors, only the co-addition of the NES inhibitor reversed the isoproterenol-induced reduction of beta1-AR mRNA levels. Our results suggest that CRM 1-dependent NES-mediated mechanisms influence the degradation and agonist-mediated down-regulation of the beta1-AR mRNAs.

Tolerance to bronchodilation during treatment with long-acting beta-agonists, a randomised controlled trial

BACKGROUND

Regular use of beta-agonists leads to tolerance to their bronchodilator effects. This can be demonstrated by measuring the response to beta-agonist following bronchoconstriction using methacholine. However most studies have demonstrated tolerance after a period of beta-agonist withdrawal, which is not typical of their use in clinical practice. This study assessed tolerance to the bronchodilator action of salbutamol during ongoing treatment with long-acting beta-agonist.

METHODS

Random-order, double-blind, placebo-controlled, crossover trial. After 1 week without beta-agonists, 13 asthmatic subjects inhaled formoterol 12 microg twice daily or matching placebo for 1 week. Eight hours after the first and last doses subjects inhaled methacholine to produce a 20% fall in FEV1. Salbutamol 100, 200 and 400 microg (cumulative dose) was then given at 5-minute intervals and FEV1 was measured 5 minutes after each dose. After a 1 week washout subjects crossed over to the other treatment. Unscheduled use of beta-agonists was not allowed during the study. The main outcome variable was the area under the salbutamol response curve.

RESULTS

The analysis showed a significant time by treatment interaction indicating that the response to salbutamol fell during formoterol therapy compared to placebo. After 1 week of formoterol the area under the salbutamol response curve was 48% (95% confidence interval 28 to 68%) lower than placebo. This reduction in response remained significant when the analyses were adjusted for changes in the pre-challenge FEV1 and dose of methacholine given (p = 0.001).

CONCLUSION

The bronchodilator response to salbutamol is significantly reduced in patients taking formoterol. Clinically relevant tolerance to rescue beta-agonist treatment is likely to occur in patients treated with long-acting beta-agonists.

Translational control of beta2-adrenergic receptor mRNA by T-cell-restricted intracellular antigen-related protein

Cellular expression of the beta(2)-adrenergic receptor (beta(2)-AR) is suppressed at the translational level by 3'-untranslated region (UTR) sequences. To test the possible role of 3'-UTR-binding proteins in translational suppression of beta(2)-AR mRNA, we expressed the full-length 3'-UTR or the adenylate/uridylate-rich (A+U-rich element (ARE)) RNA from the 3'-UTR sequences of beta(2)-AR in cell lines that endogenously express this receptor. Reversal of beta(2)-adrenergic receptor translational repression by retroviral expression of 3'-UTR sequences suggested that ARE RNA-binding proteins are involved in translational suppression of beta(2)-adrenergic receptor expression. Using a 20-nucleotide ARE RNA from the receptor 3'-UTR as an affinity ligand, we purified the proteins that bind to these sequences. T-cell-restricted intracellular antigen-related protein (TIAR) was one of the strongly bound proteins identified by this method. UV-catalyzed cross-linking experiments using in vitro transcribed 3'-UTR RNA and glutathione S-transferase-TIAR demonstrated multiple binding sites for this protein on beta(2)-AR 3'-UTR sequences. The distal 340-nucleotide region of the 3'-UTR was identified as a target RNA motif for TIAR binding by both RNA gel shift analysis and immunoprecipitation experiments. Overexpression of TIAR resulted in suppression of receptor protein synthesis and a significant shift in endogenously expressed beta(2)-AR mRNA toward low molecular weight fractions in sucrose gradient polysome fractionation. Taken together, our results provide the first evidence for translational control of beta(2)-AR mRNA by TIAR.

Regulation of the rainbow trout (Oncorhynchus mykiss) hepatic beta2-adrenoceptor by adrenergic agonists

The characteristics of hepatic beta(2)-adrenoceptors (AR) were examined in rainbow trout (Oncorhynchus mykiss) chased once per day to exhaustion for up to 7 days or fed the repartitioning agents clenbuterol (CLEN) or ractopamine (RACT) that function in mammals as beta-agonists. A one-day chase and feeding the CLEN for 37 days resulted in a significant 27% and 33% decrease, respectively, in the number of CGP-binding sites (B(max)) with no significant change in affinity (Kd) of hepatic beta(2)-ARs. Despite the significant decrease in beta(2)-AR numbers with CLEN feeding, no significant differences were found for either beta(2)-AR mRNA levels or adenylyl cyclase (ACase) activities. In addition, CLEN displayed only partial agonist activities as it was found to be more effective at blocking isoproterenol-stimulated cAMP production in isolated hepatocytes than stimulating cAMP production. The small affects of RACT may be related to its low active stereoisomer content and low affinity for the trout beta(2)-AR. Agonist regulation of the trout hepatic beta(2)-ARs may involve down-regulation of the receptors without affecting responsiveness.

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.

Direct effects of testosterone, 17 beta-estradiol, and progesterone on adrenergic regulation in cultured brown adipocytes: potential mechanism for gender-dependent thermogenesis

Previous studies suggest that sex hormones could be responsible, at least in part, for the gender-dependent thermogenesis found in the adrenergic control of brown adipose tissue (BAT) under control conditions and in response to diet and cold. Catecholamines, as well as several hormones, including sex hormones, may alter the function or expression of different adrenoceptor subtypes in brown adipocytes in vivo, and a confirmation could be provided by in vitro experiments. Therefore, the effect of testosterone, 17 beta-estradiol, progesterone, and norepinephrine (NE) on adrenergic receptor (AR) gene expression (alpha 2A-, beta 1-, -, and beta 3-AR) and lipolytic activity was investigated in differentiated brown adipocytes in culture. We report that the expression of each AR subtype gene was distinctively regulated by NE and sex hormones in brown adipocytes. Testosterone-treated cells had lower lipolytic activity and increased expression of antilipolytic receptors alpha 2A-AR. Both 17 beta-estradiol and progesterone decreased alpha 2A-AR expression and alpha 2A/beta 3-AR protein ratio, but progesterone had higher potency than 17 beta-estradiol, increasing beta-AR levels, mainly beta 3-AR expression, and enhancing lipolysis stimulated by NE. In conclusion, our results support the idea that male and female sex hormones, as a part of the hormonal environment of BAT, have direct and opposite effects on the AR balance and lipolytic activity, and they might play a role in the gender dimorphism for the recruitment process in BAT.

Regulation of the cAMP-elevating effects of isoproterenol and forskolin in cardiac myocytes by treatments that cause increases in cAMP

Regulation of the cAMP-elevating effects of isoproterenol and forskolin in cardiac myocytes by treatments that cause increases in cAMP. We have found that elevations in cyclic AMP (cAMP) have long-term effects on both the beta-adrenergic receptor and adenylyl cyclase in cultured chick ventricular myocytes. Pretreatment with isoproterenol for 15min markedly reduced the cAMP-elevating effect of a subsequent treatment with isoproterenol 18h later. Responses to isoproterenol were similarly reduced after overnight treatments with forskolin or phosphodiesterase inhibitors. Furthermore, these same treatments also markedly blunted the cAMP-elevating effect of forskolin, a direct activator of adenylyl cyclase. The blunting of the isoproterenol effect was greater than that of the forskolin effect, at least partially because the pretreatments caused a decrease in the number of beta-adrenergic receptors as well as a net decrease in adenylyl cyclase activity. Experiments with a recombinant adenovirus to express luciferase under the control of cAMP responsive elements (CREs) showed that the same treatments elevated cAMP sufficiently to drive the transcription of a gene with CREs in its promoter. The blunting of both the isoproterenol and forskolin responses was blocked by the inhibition of protein synthesis or by infecting cells with a recombinant adenovirus that expresses rabbit muscle cAMP-dependent protein kinase inhibitor (PKI). It is hypothesized that one or more adenylyl cyclase isozymes responsible for the generation of cAMP in the myocytes, along with other proteins previously reported to regulate beta-adrenergic receptors and perhaps adenylyl cyclase, are negatively regulated by cAMP, most likely at the level of gene expression, and that this regulation may have therapeutic consequences in the treatment of cardiac diseases.

Molecular interactions between glucocorticoids and long-acting beta2-agonists

beta(2)-Adrenergic receptor agonists and glucocorticoids are the two most effective treatments for asthma, and used in combination they are more effective than either alone. Glucocorticoids mediate their anti-inflammatory effects through the action of activated glucocorticoid receptors (GRs), with the level of activity being related to the number of nuclear receptors. Glucocorticoids can upregulate the synthesis of several genes in human lung cells through interaction with specific DNA binding regions (glucocorticoid response elements) within the promoter region of glucocorticoid-responsive genes. Many of the down-regulating effects of GRs on the synthesis of cytokines and other inflammatory mediators are due to repression of other transcription factors, such as activator protein-1 and nuclear factor kappaB. GR functions such as nuclear localization and gene activation can be regulated by phosphorylation status. Long-acting beta(2)-agonists may affect GR nuclear localization through modulation of GR phosphorylation and furthermore through priming of GR functions within the nucleus by modifying GR or GR-associated protein phosphorylation. Glucocorticoids in turn may regulate beta(2)-adrenergic receptor function by increasing its expression, acting through glucocorticoid response elements, and, importantly, by restoring G-protein-beta(2)-receptor coupling and inhibiting beta(2)-receptor downregulation, thereby preventing desensitization.

Molecular interactions between glucocorticoid and catecholamine signaling pathways

To study the mechanism underlying glucocorticoid regulation of the beta(1)-adrenergic receptor (beta(1)AR), we identified a 43-bp region (-1274 to -1232 from the translation start site) that contains a novel glucocorticoid regulatory unit (GRU) that confers glucocorticoid responsiveness. The sequence encompassing the GRU is (5')TAATTA(3'), which is a core-binding motif for the homeodomain proteins; an E-box ((5')CACGTG(3')) binding site for the Myc/Max family proteins, and an overlapping glucocorticoid response element half-site ((5')TGTTCT(3')). We showed that the half-site is critical for GRU-protein interactions, which also require binding of proteins to the E-box and the homeodomain region. Expression of proteins binding to the GRU was shown to be developmentally regulated, being high in embryonic hearts, reduced in newborn hearts, and undetectable in adult hearts. Overexpression of c-myc antisense significantly reduced glucocorticoid responsiveness of the beta(1)AR gene. We further demonstrated that transcriptional regulation of the beta(1)AR gene is closely related to that of the c-myc gene and that the beta(1)AR may be a potential target of c-myc. We conclude that the ovine beta(1)AR gene contains a novel, functional GRU and that the nuclear factors that transactivate through this element may have important developmental implications.

Selected contribution: long-term effects of beta(2)-adrenergic receptor stimulation on alveolar fluid clearance in mice

Stimulation of active fluid transport with beta-adrenergic receptor (betaAR) agonists can accelerate the resolution of alveolar edema. However, chronic betaAR-agonist administration may cause betaAR desensitization and downregulation that may impair physiological responsiveness to betaAR-agonist stimulation. Therefore, we measured baseline and terbutaline- (10(-3) M) stimulated alveolar fluid clearance in mice that received subcutaneously (miniosmotic pumps) either saline or albuterol (2 mg. kg(-1). day(-1)) for 1, 3, or 6 days. Continuous albuterol administration increased plasma albuterol levels (10(-5) M), an effect that was associated with 1) a significant decrease in betaAR density and 2) attenuation, but not ablation, of maximal terbutaline-induced cAMP production. Forskolin-mediated cAMP-release was unaffected. Continuous albuterol infusion stimulated alveolar fluid clearance on day 1 but did not increase alveolar fluid clearance on days 3 and 6. However, terbutaline-stimulated alveolar fluid clearance in albuterol-treated mice was not reduced compared with saline-treated mice. Despite significant reductions in betaAR density and agonist-mediated cAMP production by long-term betaAR-agonist exposure, maximal betaAR-agonist-mediated increase in alveolar fluid clearance is not diminished in mice.

Prolonged isoproterenol infusion impairs the ability of beta(2)-agonists to increase alveolar liquid clearance

We determined if prolonged isoproterenol (Iso) infusion in rats impaired the ability of the beta(2)-adrenergic agonist terbutaline to increase alveolar liquid clearance (ALC). We infused rats with Iso (at rates of 4, 40, or 400 microg.kg(-1).h(-1)) or vehicle (0.001 N HCl) for 48 h using subcutaneously implanted miniosmotic pumps. After this time, the rats were anesthetized, and ALC was determined (by mass-balance after instillation of Ringer lactate containing albumin into the lungs) under baseline conditions and after terbutaline administration. Baseline and terbutaline-stimulated ALC in vehicle-infused rats averaged, respectively, 19.6 +/- 1.2% (SE) and 44.7 +/- 1.5%/h. The ability of terbutaline to increase ALC was eliminated at 400 microg.kg(-1).h(-1)Iso, inhibited by 26% at 40 microg.kg(-1).h(-1) Iso, and was not affected by 4 microg.kg(-1).h(-1) Iso. beta-adrenergic receptor (betaAR) density of freshly isolated alveolar epithelial type II (ATII) cells from Iso-infused rats was reduced by the 40 and 400 microg.kg(-1).h(-1) infusion rates. These data demonstrate that prolonged exposure to beta-agonists can impair the ability of beta(2)-agonists to stimulate ALC and produce ATII cell betaAR downregulation.

Destabilization of AT(1) receptor mRNA by calreticulin

AT(1) receptor activation leads to vasoconstriction, blood pressure increase, free radical release, and cell growth. AT(1) receptor regulation contributes to the adaptation of the renin-angiotensin system to long-term stimulation and serves as explanation for the involvement of the AT(1) receptor in the pathogenesis of cardiovascular disease. The molecular mechanisms involved in AT(1) receptor regulation are poorly understood. Here, we report that angiotensin II accelerates AT(1) receptor mRNA decay in vascular smooth muscle cells. A cognate mRNA region within the 3' untranslated region at bases 2175 to 2195 governs the inducible decay of the AT(1) receptor mRNA. Sequential protein purifications led to the discovery of a novel mRNA binding protein, calreticulin, which mediates destabilization of the AT(1) receptor mRNA. Angiotensin II-caused phosphorylation of calreticulin enables binding of calreticulin to the AT(1) receptor mRNA at bases 2175 to 2195 and propagates calreticulin-induced acceleration of AT(1) receptor mRNA decay. Thus, a novel mRNA binding protein, calreticulin, is discovered, which causes AT(1) receptor mRNA degradation via binding to a distinct mRNA region in the 3' untranslated region. These findings display a novel mechanism of posttranscriptional mRNA processing.

Agonist-mediated down-regulation of rat beta1-adrenergic receptor transcripts: role of potential post-transcriptional degradation factors

The human beta1-adrenergic receptor (AR) and hamster beta2-AR transcripts can be post-transcriptionally regulated at the level of mRNA stability and undergo accelerated agonist-mediated degradation via interaction of their 3' untranslated regions (UTR) with RNA binding proteins. Using RNase protection assays, we have determined that chronic isoproterenol exposure of rat C6 glioma cells results in the accelerated reduction of beta1-AR mRNAs. To determine the role of cellular environment on the agonist-independent and agonist-mediated degradation of beta1-AR mRNAs, we transfected rat beta1-AR expression recombinants into both hamster DDT1MF2 cells and rat L6 cells. The rat beta1-AR mRNAs in the two transfectant cell pools retain longer agonist-independent half-lives than in the C6 environment and undergo accelerated degradation upon chronic agonist exposure. Using UV-cross-linking/immunoblot and immunoprecipitation analyses, we have determined that the rat beta1-AR 3' UTR recognizes a predominant M(r) 39,000 component, identified as the mammalian elav-like protein HuR, and several other minor components, including the heteronuclear protein hnRNP A1. HuR levels are more highly expressed in C6 cells than in DDT1MF2 and L6 cells and are induced after chronic isoproterenol treatment. Furthermore, C6 transfectants containing an HuR expression recombinant exhibit reduced beta1-AR mRNA half-lives that were statistically comparable with half-lives identified in isoproterenol-treated C6 cells. These results imply that HuR plays a potential role in the agonist-independent and agonist-mediated down-regulation of beta1-AR mRNAs.

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

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

Altered beta-adrenergic receptor gene regulation and signaling in chronic heart failure

J. D. Port and M. R. Bristow. Altered Beta-adrenergic Receptor Gene Regulation and Signaling in Chronic Heart Failure. Journal of Molecular and Cellular Cardiology (2001) 33, 887-905. Beta adrenergic receptors (beta -ARs) are critical regulators of cardiac function in both normal and pathophysiological states. Under normal conditions, beta -ARs and their signaling pathways modulate both the rate and force of myocardial contraction and relaxation, allowing individuals to respond appropriately to physiological stress or exercise. However, in chronic heart failure, sustained activation of the beta -AR signaling pathways can have overtly negative biological consequences. This notion is reinforced by the positive outcomes of a number of clinical trials demonstrating the usefulness of beta-blocker therapy in chronic congestive heart failure. During the last few years, significant progress has been made in understanding the molecular biological basis of beta -AR function, both at the biochemical and genetic levels. In this review, the biological basis of adrenergic signaling and how this changes in heart failure is discussed. Aspects of adrenergic receptor pharmacology relevant to heart failure are reviewed, including the recently emerging differences described for beta(1)- v beta(2)-AR signaling pathways. Highlighting these differences is recent evidence that over-stimulation of the beta(1)-AR pathway in cardiac myocytes appears to be pro-apoptotic, whereas stimulation of the beta(2)-AR pathway may be anti-apoptotic. Overview of beta -AR gene regulation, transgenic models of beta -AR overexpression, and beta -AR polymorphisms as they relate to heart failure progression are also discussed.

The role of protein kinase C in the regulation of serotonin-2A receptor expression

We have investigated in C6 glioma cells the involvement of protein kinase C (PKC) in the regulation of serotonin-(2A) receptor (5-HT(2A) receptor) expression by agonist treatment. Comparison of the time-courses of agonist-induced downregulation of receptor number and mRNA indicate that a decrease in the number of 5-HT(2A) receptor binding sites in response to serotonin (5-HT) treatment is preceded by a decrease in 5-HT(2A) receptor mRNA. This decrease in 5-HT(2A) receptor mRNA as a result of agonist exposure was not due to a change in the stability or half-life of the transcript. Pretreatment of cells with the PKC inhibitor bisindolylmaleimide blocked the decrease in 5-HT(2A) receptor mRNA levels, and attenuated the down-regulation of 5-HT(2A) receptor binding sites induced by treatment with 5-HT. Experiments performed with the PKC inhibitors calphostin C and Gö 6976 confirmed that PKC was involved in the regulation of 5-HT(2A) receptor mRNA by agonist and implicate the conventional subgroup of PKC isoforms. Western blot analysis, using isoform-specific anti-PKC antibodies showed that under our culture conditions C6 glioma cells express the conventional isoforms PKC alpha, PKC gamma, as well as the novel isoforms PKC delta, PKC epsilon, and the atypical isoforms PKC lambda and PKC iota. Upon treatment with 5-HT for 10 min levels of the conventional isoforms PKC alpha and PKC gamma increased in the nuclear fraction. Taken together, our results implicate PKC alpha and/or PKC gamma in the regulation of 5-HT(2A) mRNA receptor and binding sites in response to agonist treatment.

Reactive oxygen species-mediated homologous downregulation of angiotensin II type 1 receptor mRNA by angiotensin II

Recent studies suggest a crucial role of reactive oxygen species (ROS) for the signaling of angiotensin (Ang) II through Ang II type 1 receptor (AT(1)-R). However, the role of ROS in the regulation of AT(1)-R expression has not been explored. In this study, we examined the effect of an antioxidant on the homologous downregulation of AT(1)-R by Ang II. Ang II (10(-6) mol/L) decreased AT(1)-R mRNA with a peak suppression at 6 hours of stimulation in rat aortic vascular smooth muscle cells. Preincubation of vascular smooth muscle cells with N:-acetylcysteine (NAC), a potent antioxidant, almost completely inhibited the Ang II-induced downregulation of AT(1)-R mRNA. The effect of NAC was due to stabilization of the AT(1)-R mRNA that was destabilized by Ang II. The Ang II-induced AT(1)-R mRNA downregulation was also blocked by PD98059, an extracellular signal-regulated protein kinase (ERK) kinase inhibitor. Ang II-induced ERK activation was inhibited by NAC as well as by PD98059. Exogenous H(2)O(2) also suppressed AT(1)-R mRNA. These results suggest that the production of ROS and the activation of ERK are critical for the downregulation of AT(1)-R mRNA. The generation of ROS through stimulation of AT(1)-R not only mediates signaling of Ang II but also may play a crucial role in the adaptation process of AT(1)-R to the sustained stimulation of Ang II.

Effects of cocaine administration on receptor binding and subunits mRNA of GABA(A)-benzodiazepine receptor complexes

The effects of intermittent intraperitoneal (i.p.) administration of cocaine (20 mg/kg) on GABA(A)-benzodiazepine (BZD) receptors labeled by t-[(35)S]butylbicyclophosphorothionate (TBPS), and on several types of mRNA subunits were investigated in rat brain by in vitro quantitative receptor autoradiography and in situ hybridization. Phosphor screen imaging with high sensitivity and a wide linear range of response was utilized for imaging analysis. There was a significant decrease in the level of alpha 1, alpha 6, beta 2, beta 3, and gamma 2 subunits mRNA, with no alteration of [(35)S]TBPS binding in any regions in the brain of rats at 1 h following a single injection of cocaine. In chronically treated animals, the mean scores of stereotyped behavior were increased with the number of injections. The level of beta 3 subunit mRNA was decreased in the cortices and caudate putamen, at 24 h after a final injection of chronic administrations for 14 days. In the withdrawal from cocaine, the frontal cortex and hippocampal complexes showed a significant increase in [(35)S]TBPS binding and alpha1 and beta 3 subunit mRNA in the rats 1 week after a cessation of chronic administration of cocaine. These findings suggest that the disruption of GABA(A)-BZD receptor formation is closely involved in the development of cocaine-related behavioral disturbances. Further studies on the physiological functions on GABA(A)-BZD receptor complex will be necessary for an explanation of the precise mechanisms underlying the acute effects, development of hypersensitization, and withdrawal state of cocaine.

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.

Reconstitution of angiotensin receptor mRNA down-regulation in vascular smooth muscle. Post-transcriptional control by protein kinase a but not mitogenic signaling directed by the 5'-untranslated region

Cell surface receptor activation generally leads to changes in mRNA abundance, which may involve regulatory targets in processes working at the post-transcriptional level. Many types of agonists down-regulate vascular smooth muscle angiotensin receptor (AT(1)-R) gene expression, but it is unclear which of these activate post-transcriptional mechanisms. To reconstitute faithfully the normal AT(1)-R mRNA regulatory environment, tetracycline-suppressible promoters drive highly accurate recombinant AT(1)-R mRNA mimics in vascular smooth muscle cells that co-express an endogenous AT(1)-R mRNA. Down-regulation of the latter occurs shortly after stimulating mitogenic receptors or by using forskolin, but only cAMP signaling reduces expression of the recombinant AT(1)-R mRNA. Transcription of the recombinant mRNA is unaffected by cAMP signaling. Deletions of the AT(1)-R mRNA 3'-untranslated region do not impair cAMP-mediated down-regulation. Both loss of function and gain of function mutants show the response is mediated by the 5'-untranslated region. These observations provide the first direct functional evidence for modulation of vascular AT(1)-R gene expression by a mechanism involving a protein kinase A-regulated post-transcriptional process.

Purification and characterization of beta-adrenergic receptor mRNA-binding proteins

Beta-adrenergic receptors (beta-ARs), like other G-protein-coupled receptors, can undergo post-transciptional regulation at the level of mRNA stability. In particular, the human beta(1)- and beta(2)-ARs and the hamster beta(2)-AR mRNA undergo beta-agonist-mediated destabilization. By UV cross-linking, we have previously described an approximately M(r) 36,000 mRNA-binding protein, betaARB, that binds to A/C+U-rich nucleotide regions within 3'-untranslated regions. Further, we have demonstrated previously that betaARB is immunologically distinct from AUF1/heterogeneous nuclear ribonucleoprotein (hnRNP) D, another mRNA-binding protein associated with destabilization of A+U-rich mRNAs (Pende, A., Tremmel, K. D., DeMaria, C. T., Blaxall, B. C., Minobe, W., Sherman, J. A., Bisognano, J., Bristow, M. R., Brewer, G., and Port, J. D. (1996) J. Biol. Chem. 271, 8493-8501). In this report, we describe the peptide composition of betaARB. Mass spectrometric analysis of an approximately M(r) 36,000 band isolated from ribosomal salt wash proteins revealed the presence of two mRNA-binding proteins, hnRNP A1, and the elav-like protein, HuR, both of which are known to bind to A+U-rich nucleotide regions. By immunoprecipitation, HuR appears to be the biologically dominant RNA binding component of betaARB. Although hnRNP A1 and HuR can both be immunoprecipitated from ribosomal salt wash proteins, the composition of betaARB (HuR alone versus HuR and hnRNP A1) appears to be dependent on the mRNA probe used. The exact role of HuR and hnRNP A1 in the regulation of beta-AR mRNA stability remains to be determined.

How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions

The secretion of glucocorticoids (GCs) is a classic endocrine response to stress. Despite that, it remains controversial as to what purpose GCs serve at such times. One view, stretching back to the time of Hans Selye, posits that GCs help mediate the ongoing or pending stress response, either via basal levels of GCs permitting other facets of the stress response to emerge efficaciously, and/or by stress levels of GCs actively stimulating the stress response. In contrast, a revisionist viewpoint posits that GCs suppress the stress response, preventing it from being pathologically overactivated. In this review, we consider recent findings regarding GC action and, based on them, generate criteria for determining whether a particular GC action permits, stimulates, or suppresses an ongoing stress-response or, as an additional category, is preparative for a subsequent stressor. We apply these GC actions to the realms of cardiovascular function, fluid volume and hemorrhage, immunity and inflammation, metabolism, neurobiology, and reproductive physiology. We find that GC actions fall into markedly different categories, depending on the physiological endpoint in question, with evidence for mediating effects in some cases, and suppressive or preparative in others. We then attempt to assimilate these heterogeneous GC actions into a physiological whole.