Characterization of agonist stimulation of cAMP-dependent protein kinase and G protein-coupled receptor kinase phosphorylation of the beta2-adrenergic receptor using phosphoserine-specific antibodies

Beta2-agonists and bronchial hyperresponsiveness

Bronchial hyperresponsiveness (BHR) is a characteristic feature of asthma, and individuals with this disease respond to a range of physiological and chemical insults that are otherwise innocuous to healthy subjects, suggesting that the mechanisms underlying this phenomenon are characteristic of the asthma phenotype. BHR can be increased following exposure to environmental allergens in suitably sensitized individuals, pollutants, and certain viruses and can also be exacerbated by exposure to certain drugs, including nonsteroidal anti-inflammatory agents and beta-blockers. Although beta2-agonists administered acutely remain the treatment for the symptoms of asthma, paradoxically, regular treatment with these drugs can result in an increase in BHR, and this has been suggested to contribute to the increase in asthma morbidity and mortality that has been reported by numerous investigators. This article highlights our current understanding of this phenomenon and examines the potential mechanisms responsible for this effect.

Characterization of beta2-adrenergic receptor dephosphorylation: Comparison with the rate of resensitization

Dephosphorylation of the cyclic AMP-dependent protein kinase (PKA) site phosphoserine 262 and the G protein-coupled receptor kinase (GRK) site phosphoserines 355 and 356 of the beta2-adrenergic receptor (beta2AR) were characterized in both intact human embryonic kidney 293 cells and subcellular fractions and were correlated with the rate of resensitization of isoproterenol stimulation of adenylyl cyclase after treatment with isoproterenol and blockade by antagonist. Dephosphorylation of the PKA site after stimulation with 300 pM isoproterenol occurred with a t(1/2) of 9 min (k = 0.08 +/- 0.016/min) in intact cells in the absence of internalization. Dephosphorylation of the GRK sites in intact cells after treatment with 1.0 microM isoproterenol for 5 min exhibited a lag phase of approximately 5 min, after which dephosphorylation proceeded slowly with a t(1/2) of 18 min (k = 0.039 +/- 0.006/min). Consistent with the slow rate of GRK site dephosphorylation, the phosphatase inhibitors calyculin A and okadaic acid failed to augment phosphorylation in intact cells during continuous agonist stimulation indicating that GRK site dephosphorylation was minimal. However, both inhibited dephosphorylation of the GRK sites after the addition of antagonist. Slow GRK site dephosphorylation after antagonist treatment was also demonstrated by the relative stability of internalized phosphorylated beta2AR in cells as observed both by immunofluorescence microscopy using a phospho-site-specific antibody and by studies of the subcellular localization of the GRK-phosphorylated beta2AR on sucrose gradients that revealed nearly equivalent levels of GRK site phosphorylation in the plasma membrane and vesicular fractions. In addition, dephosphorylation of the GRK sites by intrinsic phosphatase activity occurred only in the heavy vesicle fractions. In contrast to the slow rates of dephosphorylation, the rate of resensitization of isoproterenol stimulation of adenylyl cyclase was 5- and 10-fold faster (k = 0.43 +/- 0.009/min; t(1/2) = 1.6 min), than PKA and GRK site dephosphorylation, respectively, clearly dissociating the rapid phase of resensitization (0-5 min) from dephosphorylation.

Functional selectivity and classical concepts of quantitative pharmacology

The concept of intrinsic efficacy has been enshrined in pharmacology for half of a century, yet recent data have revealed that many ligands can differentially activate signaling pathways mediated via a single G protein-coupled receptor in a manner that challenges the traditional definition of intrinsic efficacy. Some terms for this phenomenon include functional selectivity, agonist-directed trafficking, and biased agonism. At the extreme, functionally selective ligands may be both agonists and antagonists at different functions mediated by the same receptor. Data illustrating this phenomenon are presented from serotonin, opioid, dopamine, vasopressin, and adrenergic receptor systems. A variety of mechanisms may influence this apparently ubiquitous phenomenon. It may be initiated by differences in ligand-induced intermediate conformational states, as shown for the beta(2)-adrenergic receptor. Subsequent mechanisms that may play a role include diversity of G proteins, scaffolding and signaling partners, and receptor oligomers. Clearly, expanded research is needed to elucidate the proximal (e.g., how functionally selective ligands cause conformational changes that initiate differential signaling), intermediate (mechanisms that translate conformation changes into differential signaling), and distal mechanisms (differential effects on target tissue or organism). Besides the heuristically interesting nature of functional selectivity, there is a clear impact on drug discovery, because this mechanism raises the possibility of selecting or designing novel ligands that differentially activate only a subset of functions of a single receptor, thereby optimizing therapeutic action. It also may be timely to revise classic concepts in quantitative pharmacology and relevant pharmacological conventions to incorporate these new concepts.

Differential phosphorylation and dephosphorylation of beta2-adrenoceptor sites Ser262 and Ser355,356

Activated beta2-adrenoceptors are rapidly desensitized by phosphorylation of Ser262 by protein kinase A (PKA) and of Ser355,356 by G-protein-coupled receptor kinase (GRK). We sought to determine whether the phosphorylation and subsequent dephosphorylation of these sites had similar kinetics and requirements for receptor endocytosis. The phosphorylation of the PKA and GRK sites were measured using antibodies that recognize phosphoserine 262 and phosphoserine 355,356. Endocytosis in stably transfected HEK293 cells was blocked by inducible expression of dominant-negative dynamin-1 K44A or by treatment with hypertonic sucrose. The phosphorylation of the GRK site Ser355,356 during a 10 microM isoprenaline treatment rapidly reached a steady state, and the extent of kinetics of phosphorylation were unaffected by dynamin-1 K44A expression, and minimally by hypertonic sucrose. In contrast, phosphorylation of the PKA site Ser262 during a 10 microM isoprenaline treatment peaked after 2 min and then rapidly declined, while inhibition of endocytosis enhanced and prolonged phosphorylation. Treatment with 300 pM isoprenaline, a concentration too low to provoke endocytosis, also resulted in prolonged PKA site phosphorylation. The dephosphorylation of these sites was measured after removal of agonist. Significant dephosphorylation of phosphoserines 262 and 355,356 was observed under conditions of very low endocytosis, however dephosphorylation of the GRK site was greater if antagonist was present after removal of agonist. The results indicate that the kinetics of beta2-adrenoceptor GRK and PKA site phosphorylation are distinct and differently affected by endocytosis, and that receptor dephosphorylation can occur either at the plasma membrane or in internal compartments.

Bioluminescence resonance energy transfer as a screening assay: Focus on partial and inverse agonism

The reported data for compound screening with the bioluminescence resonance energy transfer (BRET2) assay is very limited, and several questions remain unaddressed, such as the behavior of agonists. Eleven beta2 adrenergic receptor (beta2-AR) agonists were tested for full or partial agonism in an improved version of the receptor/beta-arrestin2 BRET2 assay and in 2 cyclic adenosine monophosphate (cAMP) assays (column cAMP assay and ALPHAscreen cAMP assay). Tested in the highly sensitive ALPHAscreen cAMP assay, all selected agonists behaved as full agonists, using isoproterenol as a reference compound. In the less sensitive column cAMP assay, ephedrine and dopamine had a clear partial response. For the BRET2 assay, a highly graded picture was obtained. Moreover, beta2-AR antagonists were tested for inverse agonism. Pronounced inverse agonism was detected in the ALPHAscreen cAMP assay. Only marginal inverse agonistic responses were seen for alprenolol and pindolol in the column cAMP assay, and no inverse agonism was seen in the BRET2 assay. For the beta2-AR, the BRET2 assay may be superior for secondary screening of agonists where a separation of full and partial agonists is needed and the ALPHAscreen cAMP assay may be preferred for primary screening of agonists where all receptor activating compounds are desired.

The role of intrinsic efficacy in determining response to a beta2-agonist in acute severe asthma

BACKGROUND

Current guidelines recommend repeated doses of albuterol for the emergency treatment of acute asthma. However, approximately one-third of patients show little or no initial response to this partial beta(2)-agonist.

METHODS

We conducted a randomized, double-blind, proof-of-concept study to investigate whether a full beta(2)-agonist, isoproterenol, offers a therapeutic advantage in adults presenting with acute severe asthma (FEV(1)<50%) who fail to respond to an initial treatment of the partial beta(2)-agonist, albuterol. Study subjects were randomized to receive a 2-h continuous nebulization of either albuterol (7.5mg/h) (n=10, mean FEV(1)=37% predicted) or isoproterenol (7.5mg/h) (n=9, mean FEV(1)=33% predicted). Respiratory symptoms, vital signs and pulmonary function measures were collected.

RESULTS

Subjects from both treatment groups had similar baseline characteristics. The percent improvements from baseline FEV(1) at 60 and 120min were significantly higher in subjects receiving isoproterenol than those receiving albuterol (44 vs. 17% and 63 vs. 24%, respectively, P<0.05). The change in symptoms measured by the modified Borg score was also significantly greater in subjects receiving isoproterenol (P<0.01). Both treatments were well tolerated, though the mean increase in pulse rate at 60 and 120min (21 vs. 1 and 23 vs. 6beats/min, respectively, P<0.05) and the mean change in serum potassium at 120min (-0.52 vs. -0.07meq/L, P<0.05) from baseline were significantly greater in the isoproterenol group.

CONCLUSIONS

Our data suggest that in subjects presenting with acute severe asthma who fail to show an initial response to albuterol, the use of a beta(2)-agonist of higher intrinsic efficacy can be more effective in improving lung function and symptoms.

Effect of age on vascular beta2-adrenergic receptor desensitization is not mediated by the receptor coupling to Galphai proteins

Beta-adrenergic receptor (beta-AR)-mediated vasorelaxation declines with age. In the vasculature, beta2-AR undergoes protein kinase A-mediated desensitization that causes a switch in the G protein coupled to beta2-AR; Galphai links instead of Galphas. We exposed Fischer 344 rat aortae of increasing age to a desensitizing dose of isoproterenol, and determined its effect on beta2-AR-mediated vasorelaxation. Desensitization decreased beta2-AR-mediated vasorelaxation in young aortae only. Subsequently, we used pertussis toxin to block Galphai to determine whether changes in beta2-AR/G protein coupling occurred. Galphai inhibition did not reverse desensitization or the age-related change, but there appears to be a population of beta2-AR linked to Galphai, as pertussis toxin treatment improved beta2-AR-mediated vasorelaxation in aortae from animals of all ages. These findings suggest aortic beta2-AR in older animals may be maximally desensitized, which would explain impaired vasorelaxation. Our results also imply that protein kinase A-mediated beta2-AR desensitization may not be responsible for the age-related decline.

Salmeterol stimulation dissociates beta2-adrenergic receptor phosphorylation and internalization

Salmeterol is a long-acting beta(2)-adrenergic receptor (beta(2)AR) agonist commonly used in the treatment of asthma and chronic obstructive pulmonary disease. It differs from other beta-agonists in that it has a very low intrinisic efficacy, especially when compared with the other available long-acting beta-agonist, formoterol. Receptor desensitization and down-regulation has been described with the chronic use of beta-agonists. This effect may not be the same with all beta-agonists and may be related to their stabilization of altered receptor states. The extreme hydrophobicity and high-affinity quasi-irreversible binding of salmeterol have rendered studies examining the mechanisms by which it mediates receptor desensitization, down-regulation, and internalization difficult. We determined the capacity of salmeterol to induce beta(2)AR endocytosis, G protein-coupled receptor kinase (GRK)-site phosphorylation, degradation, and beta-arrestin2 translocation in HEK293 cells as compared with other agonists of varying intrinsic efficacies. Despite stimulating GRK-mediated phosphorylation of Ser355,356 after 30 min and 18 h to an extent similar to that observed with agonists of high intrinsic efficacy, such as epinephrine and formoterol, salmeterol did not induce significant beta(2)AR internalization or degradation and was incapable of stimulating the translocation of enhanced green fluorescent protein-beta-arrestin2 chimera (EGFP-beta-arrestin2) to the cell surface. Salmeterol-induced receptor endocytosis was rescued, at least in part, by the overexpression of EGFP-beta-arrestin2. Our data indicate that salmeterol binding induces an active receptor state that is unable to recruit beta-arrestin or undergo significant endocytosis or degradation despite stimulating considerable GRK-site phosphorylation. Defects in these components of salmeterol-induced receptor desensitization may be important determinants of its sustained bronchodilation with chronic use.

G-protein-coupled receptor kinase specificity for beta-arrestin recruitment to the beta2-adrenergic receptor revealed by fluorescence resonance energy transfer

The small family of G-protein-coupled receptor kinases (GRKs) regulate cell signaling by phosphorylating heptahelical receptors, thereby promoting receptor interaction with beta-arrestins. This switches a receptor from G-protein activation to G-protein desensitization, receptor internalization, and beta-arrestin-dependent signal activation. However, the specificity of GRKs for recruiting beta-arrestins to specific receptors has not been elucidated. Here we use the beta(2)-adrenergic receptor (beta(2)AR), the archetypal nonvisual heptahelical receptor, as a model to test functional GRK specificity. We monitor endogenous GRK activity with a fluorescence resonance energy transfer assay in live cells by measuring kinetics of the interaction between the beta(2)AR and beta-arrestins. We show that beta(2)AR phosphorylation is required for high affinity beta-arrestin binding, and we use small interfering RNA silencing to show that HEK-293 and U2-OS cells use different subsets of their expressed GRKs to promote beta-arrestin recruitment, with significant GRK redundancy evident in both cell types. Surprisingly, the GRK specificity for beta-arrestin recruitment does not correlate with that for bulk receptor phosphorylation, indicating that beta-arrestin recruitment is specific for a subset of receptor phosphorylations on specific sites. Moreover, multiple members of the GRK family are able to phosphorylate the beta(2)AR and induce beta-arrestin recruitment, with their relative contributions largely determined by their relative expression levels. Because GRK isoforms vary in their regulation, this partially redundant system ensures beta-arrestin recruitment while providing the opportunity for tissue-specific regulation of the rate of beta-arrestin recruitment.

Adrenergic receptor agonists prevent bile duct injury induced by adrenergic denervation by increased cAMP levels and activation of Akt

Loss of parasympathetic innervation after vagotomy impairs cholangiocyte proliferation, which is associated with depressed cAMP levels, impaired ductal secretion, and enhanced apoptosis. Agonists that elevate cAMP levels prevent cholangiocyte apoptosis and restore cholangiocyte proliferation and ductal secretion. No information exists regarding the role of adrenergic innervation in the regulation of cholangiocyte function. In the present studies, we investigated the role of adrenergic innervation on cholangiocyte proliferative and secretory responses to bile duct ligation (BDL). Adrenergic denervation by treatment with 6-hydroxydopamine (6-OHDA) during BDL decreased cholangiocyte proliferation and secretin-stimulated ductal secretion with concomitant increased apoptosis, which was associated with depressed cholangiocyte cAMP levels. Chronic administration of forskolin (an adenylyl cyclase activator) or beta(1)- and beta(2)-adrenergic receptor agonists (clenbuterol or dobutamine) prevented the decrease in cholangiocyte cAMP levels, maintained cholangiocyte secretory and proliferative activities, and decreased cholangiocyte apoptosis resulting from adrenergic denervation. This was associated with enhanced phosphorylation of Akt. The protective effects of clenbuterol, dobutamine, and forskolin on 6-OHDA-induced changes in cholangiocyte apoptosis and proliferation were partially blocked by chronic in vivo administration of wortmannin. In conclusion, we propose that adrenergic innervation plays a role in the regulation of biliary mass and cholangiocyte functions during BDL by modulating intracellular cAMP levels.

Phosphodiesterase-4 influences the PKA phosphorylation status and membrane translocation of G-protein receptor kinase 2 (GRK2) in HEK-293beta2 cells and cardiac myocytes

Membrane-recruitment of GRK2 (G-protein receptor kinase 2) provides a fundamental step in the desensitization process controlling GPCRs (G-protein-coupled receptors), such as the beta2AR (beta2-adrenergic receptor). In the present paper, we show that challenge of HEK-293beta2 [human embryonic kidney cells stably overexpressing the FLAG-tagged beta2AR-GFP (green fluorescent protein)] cells with the beta-adrenoceptor agonist, isoprenaline, causes GRK2 to become phosphorylated by PKA (cAMP-dependent protein kinase). This action is facilitated when cAMP-specific PDE4 (phosphodiesterase-4) activity is selectively inactivated, either chemically with rolipram or by siRNA (small interfering RNA)-mediated knockdown of PDE4B and PDE4D. PDE4-selective inhibition by rolipram facilitates the isoprenaline-induced membrane translocation of GRK2, phosphorylation of the beta2AR by GRK2, membrane translocation of beta-arrestin and internalization of beta2ARs. PDE4-selective inhibition also enhances the ability of isoprenaline to trigger the PKA phosphorylation of GRK2 in cardiac myocytes. In the absence of isoprenaline, rolipram-induced inhibition of PDE4 activity in HEK-293beta2 cells acts to stimulate PKA phosphorylation of GRK2, with consequential effects on GRK2 membrane recruitment and GRK2-mediated phosphorylation of the beta2AR. We propose that a key role for PDE4 enzymes is: (i) to gate the action of PKA on GRK2, influencing the rate of GRK2 phosphorylation of the beta2AR and consequential recruitment of beta-arrestin subsequent to beta-adrenoceptor agonist challenge, and (ii) to protect GRK2 from inappropriate membrane recruitment in unstimulated cells through its phosphorylation by PKA in response to fluctuations in basal levels of cAMP.

Role of the G protein-coupled receptor kinase site serine cluster in beta2-adrenergic receptor internalization, desensitization, and beta-arrestin translocation

There is considerable evidence for the role of carboxyl-terminal serines 355, 356, and 364 in G protein-coupled receptor kinase (GRK)-mediated phosphorylation and desensitization of beta(2)-adrenergic receptors (beta(2)ARs). In this study we used receptors in which these serines were changed to alanines (SA3) or to aspartic acids (SD3) to determine the role of these sites in beta-arrestin-dependent beta(2)AR internalization and desensitization. Coupling efficiencies for epinephrine activation of adenylyl cyclase were similar in wild-type and mutant receptors, demonstrating that the SD3 mutant did not drive constitutive GRK desensitization. Treatment of wild-type and mutant receptors with 0.3 nm isoproterenol for 5 min induced approximately 2-fold increases in the EC(50) for agonist activation of adenylyl cyclase, consistent with protein kinase A (PKA) site-mediated desensitization. When exposed to 1 mum isoproterenol to trigger GRK site-mediated desensitization, only wild-type receptors showed significant further desensitization. Using a phospho site-specific antibody, we determined that there is no requirement for these GRK sites in PKA-mediated phosphorylation at high agonist concentration. The rates of agonist-induced internalization of the SD3 and SA3 mutants were 44 and 13%, respectively, relative to that of wild-type receptors, but the SD3 mutant recruited enhanced green fluorescent protein (EGFP)-beta-arrestin 2 to the plasma membrane, whereas the SA3 mutant did not. EGFP-beta-Arrestin2 overexpression triggered a significant increase in the extent of SD3 mutant desensitization but had no effect on the desensitization of wild-type receptors or the SA3 mutant. Expression of a phosphorylation-independent beta-arrestin 1 mutant (R169E) significantly rescued the internalization defect of the SA3 mutant but inhibited the phosphorylation of serines 355 and 356 in wild-type receptors. Our data demonstrate that (i) the lack of GRK sites does not impair PKA site phosphorylation, (ii) the SD3 mutation inhibits GRK-mediated desensitization although it supports some agonist-induced beta-arrestin binding and receptor internalization, and (iii) serines 355, 356, and 364 play a pivotal role in the GRK-mediated desensitization, beta-arrestin binding, and internalization of beta(2)ARs.

G protein-coupled receptor kinase 2 involvement in desensitization of corticotropin-releasing factor (CRF) receptor type 1 by CRF in murine corticotrophs

Hypothalamic CRF stimulates synthesis and secretion of ACTH via CRF receptor type 1 (CRFR1) in the anterior pituitary gland. After agonist-activated stimulation of receptor signaling, CRFR1 is down-regulated and desensitized. Generally, it is thought that G protein-coupled receptors may be desensitized by G protein-coupled receptor kinases (GRKs). However, the role of GRKs in corticotropic cells has not been determined. In this study we focused on involvement of GRKs in desensitization of CRFR1 by CRF in corticotropic cells. We found that GRK2 (but not GRK3) mRNA and protein were expressed in rat anterior pituitary cells and AtT-20 cells (a line of mouse corticotroph tumor cells). To determine the role of GRK2 in CRF-induced desensitization of CRFR1 in mouse corticotrophs, AtT-20 cells were transfected with a dominant-negative mutant GRK2 construct. CRF desensitized the cAMP-dependent response by CRFR1. Desensitization of CRFR1 by CRF was significantly less in AtT-20 cells transfected with the dominant-negative mutant GRK2 construct compared with desensitization in control (an empty vector-transfected) AtT-20 cells. Furthermore, pretreatment with a protein kinase A inhibitor also partially blocked desensitization of CRFR1 by CRF. These results suggest that GRK2 is involved in CRF-induced desensitization of CRFR1 in AtT-20 cells, and the protein kinase A pathway may also have an important role in desensitization of CRFR1 by CRF seen in corticotropic cells.

Protein kinase C potentiates homologous desensitization of the beta2-adrenoceptor in bovine tracheal smooth muscle

Preincubation (30 min) of bovine tracheal smooth muscle with various concentrations (0.1, 1 and 10 microM) of fenoterol decreased isoprenaline-induced maximal relaxation (E(max)) of methacholine-contracted preparations in a concentration dependent fashion, indicating desensitization of the beta(2)-adrenoceptor. Preincubation with 1 microM of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) caused a small but significant decrease in isoprenaline-induced E(max), indicating activated PKC-mediated heterologous beta(2)-adrenoceptor desensitization. To investigate the capacity of activated PKC to regulate homologous desensitization, we incubated the smooth muscle strips with the combination of both 1 microM PMA and 1 microM fenoterol. This combined treatment synergistically decreased the isoprenaline-induced maximal relaxation, as compared to the individual effects of PMA and fenoterol alone, indicating a common pathway for heterologous and homologous desensitization. Moreover, the specific PKC-inhibitor 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl) maleimide (GF 109203X) markedly increased the potency and E(max) of isoprenaline for all conditions used, including control conditions, and the synergistic effects of PMA and fenoterol were completely prevented. In conclusion, the present study demonstrates that homologous desensitization of the beta(2)-adrenergic receptor can be enhanced by PKC activation. For the first time we have provided evidence that this concept is functionally operative in airway smooth muscle, and it may explain the reduced bronchodilator response to beta(2)-adrenoceptor agonists in patients with asthma during a severe exacerbation.

beta-arrestin-dependent, G protein-independent ERK1/2 activation by the beta2 adrenergic receptor

Physiological effects of beta adrenergic receptor (beta2AR) stimulation have been classically shown to result from G(s)-dependent adenylyl cyclase activation. Here we demonstrate a novel signaling mechanism wherein beta-arrestins mediate beta2AR signaling to extracellular-signal regulated kinases 1/2 (ERK 1/2) independent of G protein activation. Activation of ERK1/2 by the beta2AR expressed in HEK-293 cells was resolved into two components dependent, respectively, on G(s)-G(i)/protein kinase A (PKA) or beta-arrestins. G protein-dependent activity was rapid, peaking within 2-5 min, was quite transient, was blocked by pertussis toxin (G(i) inhibitor) and H-89 (PKA inhibitor), and was insensitive to depletion of endogenous beta-arrestins by siRNA. beta-Arrestin-dependent activation was slower in onset (peak 5-10 min), less robust, but more sustained and showed little decrement over 30 min. It was insensitive to pertussis toxin and H-89 and sensitive to depletion of either beta-arrestin1 or -2 by small interfering RNA. In G(s) knock-out mouse embryonic fibroblasts, wild-type beta2AR recruited beta-arrestin2-green fluorescent protein and activated pertussis toxin-insensitive ERK1/2. Furthermore, a novel beta2AR mutant (beta2AR(T68F,Y132G,Y219A) or beta2AR(TYY)), rationally designed based on Evolutionary Trace analysis, was incapable of G protein activation but could recruit beta-arrestins, undergo beta-arrestin-dependent internalization, and activate beta-arrestin-dependent ERK. Interestingly, overexpression of GRK5 or -6 increased mutant receptor phosphorylation and beta-arrestin recruitment, led to the formation of stable receptor-beta-arrestin complexes on endosomes, and increased agonist-stimulated phospho-ERK1/2. In contrast, GRK2, membrane translocation of which requires Gbetagamma release upon G protein activation, was ineffective unless it was constitutively targeted to the plasma membrane by a prenylation signal (CAAX). These findings demonstrate that the beta2AR can signal to ERK via a GRK5/6-beta-arrestin-dependent pathway, which is independent of G protein coupling.

β-Arrestin Mediates Desensitization and Internalization but Does Not Affect Dephosphorylation of the Thyrotropin-releasing Hormone Receptor

The G protein-coupled thyrotropin-releasing hormone (TRH) receptor is phosphorylated and binds to beta-arrestin after agonist exposure. To define the importance of receptor phosphorylation and beta-arrestin binding in desensitization, and to determine whether beta-arrestin binding and receptor endocytosis are required for receptor dephosphorylation, we expressed TRH receptors in fibroblasts from mice lacking beta-arrestin-1 and/or beta-arrestin-2. Apparent affinity for [(3)H]MeTRH was increased 8-fold in cells expressing beta-arrestins, including a beta-arrestin mutant that did not permit receptor internalization. TRH caused extensive receptor endocytosis in the presence of beta-arrestins, but receptors remained primarily on the plasma membrane without beta-arrestin. beta-Arrestins strongly inhibited inositol 1,4,5-trisphosphate production within 10 s. At 30 min, endogenous beta-arrestins reduced TRH-stimulated inositol phosphate production by 48% (beta-arrestin-1), 71% (beta-arrestin-2), and 84% (beta-arrestins-1 and -2). In contrast, receptor phosphorylation, detected by the mobility shift of deglycosylated receptor, was unaffected by beta-arrestins. Receptors were fully phosphorylated within 15 s of TRH addition. Receptor dephosphorylation was identical with or without beta-arrestins and almost complete 20 min after TRH withdrawal. Blocking endocytosis with hypertonic sucrose did not alter the rate of receptor phosphorylation or dephosphorylation. Expressing receptors in cells lacking Galpha(q) and Galpha(11) or inhibiting protein kinase C pharmacologically did not prevent receptor phosphorylation or dephosphorylation. Overexpression of dominant negative G protein-coupled receptor kinase-2 (GRK2), however, retarded receptor phosphorylation. Receptor activation caused translocation of endogenous GRK2 to the plasma membrane. The results show conclusively that receptor dephosphorylation can take place on the plasma membrane and that beta-arrestin binding is critical for desensitization and internalization.

NNK-induced hamster lung adenocarcinomas over-express beta2-adrenergic and EGFR signaling pathways

Pulmonary adenocarcinoma (PAC) is the most common type of human lung cancer. A diagnosis of PAC, history of non-smoking and presence of mutations in the EGFR are predictive factors for responsiveness of lung cancer to EGFR-specific tyrosine kinase inhibitors. Unfortunately, less than 50% of PAC cases demonstrate this mutation-based responsiveness. Our immunohistochemical analysis of NNK-induced PAC in hamsters demonstrates the simultaneous over-expression of a beta2-adrenergic receptor pathway, including PKA, cAMP, CREB and phosphorylated CREB and of an EGFR pathway, including over-expression of EGFR-specific phosphorylated tyrosine kinase, Raf-1 and ERK1/2 and their phosphorylated forms. These findings implicate, for the first time, PKA/CREB-mediated signaling in the development and regulation of any type of lung cancer. In light of reports that NNK acts as a beta-adrenergic agonist and that beta-blockers inhibit the growth of PAC of Clara cell lineage in the NNK hamster model and in human cancer cell lines from smokers, our current data suggest transactivation of the EGFR pathway via beta-adrenergic signaling as a novel regulatory mechanism in a subpopulation of PACs in smokers. Taken together, these data point to PKA/CREB as novel targets for the development of cancer therapeutics for PAC patients non-responsive to EGFR-specific tyrosine kinase inhibitors.

Protein kinase C mediates phosphorylation, desensitization, and trafficking of the D2 dopamine receptor

Previously, D2 dopamine receptors (D2 DARs) have been shown to undergo G-protein-coupled receptor kinase phosphorylation in an agonist-specific fashion. We have now investigated the ability of the second messenger-activated protein kinases, protein kinase A (PKA) and protein kinase C (PKC), to mediate phosphorylation and desensitization of the D2 DAR. HEK293T cells were transiently transfected with the D2 DAR and then treated with intracellular activators and inhibitors of PKA or PKC. Treatment with agents that increase cAMP, and activate PKA, had no effect on the phosphorylation state of the D2 DAR, suggesting that PKA does not phosphorylate the D2 DAR in HEK293T cells. In contrast, cellular treatment with phorbol 12-myristate 13-acetate (PMA), a PKC activator, resulted in an approximately 3-fold increase in D2 DAR phosphorylation. The phosphorylation was specific for PKC as the PMA effect was mimicked by phorbol 12,13-dibutyrate, but not by 4alpha-phorbol 12,13-didecanoate, active and inactive, phorbol diesters, respectively. The PMA-mediated D2 DAR phosphorylation was completely blocked by co-treatment with the PKC inhibitor, bisindolylmaleimide II, and augmented by co-transfection with PKCbetaI. In contrast, PKC inhibition had no effect on agonist-promoted phosphorylation, suggesting that PKC is not involved in this response. PKC phosphorylation of the D2 DAR was found to promote receptor desensitization as reflected by a decrease in agonist potency for inhibiting cAMP accumulation. Most interestingly, PKC phosphorylation also promoted internalization of the D2 DAR through a beta-arrestin- and dynamin-dependent pathway, a response not usually associated with PKC phosphorylation of G-protein-coupled receptors. Site-directed mutagenesis experiments resulted in the identification of two domains of PKC phosphorylation sites within the third intracellular loop of the receptor. Both of these domains are involved in regulating sequestration of the D2 DAR, whereas only one domain is involved in receptor desensitization. These results indicate that PKC can mediate phosphorylation of the D2 DAR, resulting in both functional desensitization and receptor internalization.

Influence of agonist intrinsic activity on the desensitisation of beta2-adrenoceptor-mediated responses in mast cells

1. The aim of the present study was to determine whether the intrinsic activity of an agonist influences the extent of desensitisation of beta(2)-adrenoceptor-mediated responses in human lung mast cells. 2. The effects of a wide range of beta-adrenoceptor agonists (10(-10)-10(-5) m) on the IgE-mediated release of histamine from mast cells were determined. The intrinsic activity of agonists was established by comparing the maximal inhibitory response (E(max)) of an agonist relative to the maximal response obtained with the full agonist, isoprenaline. The intrinsic activity order for the inhibition of histamine release was isoprenaline (1.0)>formoterol (0.94)>fenoterol (0.89)>terbutaline (0.84)>salbutamol (0.69)>clenbuterol (0.65)>salmeterol (0.30)>dobutamine (0.20). 3. There was a significant (P<0.05) positive correlation (r=0.81) between the extent to which beta-adrenoceptor agonists inhibited histamine release and the degree to which the agonists caused elevations in cAMP in mast cells. 4. Further studies investigated the effects of long-term (24 h) incubation of mast cells with beta-adrenoceptor agonists on the subsequent ability of isoprenaline to inhibit histamine release. At concentrations of agonists selected to occupy a large percentage (88%) of beta(2)-adrenoceptors, there was a significant (P<0.05) correlation (r=0.73) between the relative intrinsic activity of agonists as inhibitors of histamine release and the extent of functional desensitisation induced by the agonists. At lower receptor occupancies, however, there was no correlation between the relative intrinsic activity of agonists and the extent of agonist-induced desensitisation. 5. These data indicate that, under experimental conditions where high receptor occupancies prevail, agonist intrinsic activity influences the extent of desensitisation of beta(2)-adrenoceptor-mediated responses in mast cells.