Homologous desensitization of β-adrenergic receptors in lymphoma cells is not altered by the inactivation of Ni (Gi), the inhibitory guanine nucleotide regulatory protein
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Abstract:
We had previously demonstrated that the cyc- mutant of S49 wild-type lymphoma cells both desensitizes and undergoes a sequestration-internalization of the beta-receptor in response to short-term treatment with adrenaline. The cyc- mutant of S49 wild-type lymphoma cells lacks the alpha s subunit of the stimulatory coupling protein Ns, but has fully functional Ni, the inhibitory component of the regulatory complex. This suggested that functional Ns was not required for desensitization. To examine the role of Ni in desensitization, both S49 wild-type and cyc- cells were treated with islet-activating protein under conditions that led to over 85% attenuation of Ni function in S49 wild-type cells and approx. 50% attenuation of Ni function in cyc- cells. This treatment had no effect on the adrenaline-induced desensitization of adenylate cyclase or the sequestration event measured by the apparent movement of beta-adrenergic receptors to a light-vesicle fraction. Further, the desensitization event, which occurs before the sequestration event, observable only in intact cells, was also not altered by islet-activating-protein pretreatment of S49 wild-type cells. The data suggest that a functional Ni is not required for desensitization in the S49 lymphoma cells.Keywords:
Homologous desensitization
Internalization
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This study characterized the rapid desensitization induced by arginine vasopressin (AVP) in vascular smooth muscle cells (VSMC) in culture. The Ca2+ mobilization response and, in some experiments, the intracellular pH changes were used as a probe for the desensitization phenomenon. In VSMC, AVP desensitization was homologous, concentration dependent, and occurred in less than 30 s. The desensitization was complete with 10(-7) M AVP. Receptor occupancy was a critical factor in the maintenance of desensitization, since complete hormone washing by acid glycine buffer produced an earlier (less than 5 min) recovery of the cell response, whereas partial hormone washing with saline (pH 7.4) required 15 min to produce any significant recovery. Protein kinase C activation was a significant mechanism in AVP desensitization, because protein kinase C downregulation inhibited the desensitization phenomenon. Receptor internalization was, however, not important for the desensitization phenomenon, since it still occurred at 4 degrees C. Treatment with pertussis toxin did not affect the Ca2+ mobilization response but decreased the AVP-mediated intracellular alkalinization, therefore suggesting that a Gi or Go protein may be involved in some but not all the aspects of the AVP signal transduction and the desensitization phenomena.
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Abstract G-protein-coupled chemoattractant receptors signal transiently upon ligand binding to effect cell orientation and motility but then are rapidly desensitized. The importance of desensitization has been unclear, because mutated nondesensitizable receptors mediate efficient chemotaxis. We hypothesized that homologous receptor desensitization is required for cellular navigation in fields of competing attractants. Modeling of receptor-mediated orientation shows that desensitization allows integration of attractant signals. Cells expressing normal receptors are predicted to 1) orient preferentially to distant gradients; 2) seek an intermediate position between balanced agonist sources; 3) and can be repositioned between chemoattractant-defined microenvironmental domains by modest changes in receptor number. In contrast, in the absence of desensitization, orientation is dominated by local agonist sources, precluding continued navigation. Furthermore, cell orientation in competing ligand gradients depends on the relative kinetic rates of receptor desensitization and recycling. We propose that homologous receptor desensitization is critical for cellular navigation in complex chemoattractant fields.
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Hepatocytes from hypothyroid rats have a marked beta-adrenergic responsiveness. Preincubation of these hepatocytes with isoprenaline induced a time-dependent and concentration-dependent desensitization of the beta-adrenergic responsiveness without altering that to glucagon (homologous desensitization). The desensitization was evidenced both in the cyclic AMP accumulation and in the stimulation of ureagenesis induced by the beta-adrenergic agonists. Under the same conditions, preincubation with glucagon induced no desensitization. Propranolol was also unable to induce desensitization, but blocked that induced by isoprenaline. Pertussis-toxin treatment did not alter the homologous beta-adrenergic desensitization induced by isoprenaline.
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G Protein‐Coupled Receptor Kinase 4γ Interacts with Inactive Gαs and Gα13 without Competing with Gβγ
G protein‐coupled receptors (GPCRs) are regulated by multiple families of kinases including G protein‐coupled receptor kinases (GRKs). GRK4 has been shown to be constitutively active towards GPCRs, and polymorphisms of GRK4γ are linked to, and experimentally can cause, hypertension. To better understand GRK4γ we examined the interactions between GRK4γ and Gα and Gβ subunits of heterotrimeric G proteins. Because GRK4 has been shown to inhibit Gα s ‐coupled GPCR signaling and does not contain the PH domain found in GRK2, we hypothesized that GRK4γ would interact with active Gα s , but not Gβ. We examined GRK4γ and G protein interactions through co‐immunoprecipitation of GRK4γ with inactive and active Gα proteins and Gβ. Surprisingly, GRK4γ preferentially interacts with inactive Gα s along with Gβ to a greater extent than active Gα s . GRK4γ also interacts with inactive Gα 13 along with Gβ to a lesser extent than Gα s . These results are in contrast to GRK2, which interacts with active Gα q , but not inactive Gα q . Functional studies demonstrate that wild‐type GRK4γ, but not kinase‐dead GRK4γ, ablates isoproterenol‐mediated cAMP production indicating that the kinase domain is responsible for GPCR regulation. This evidence suggests that the binding to inactive Gα s and Gβ may explain the constitutive activity of GRK4γ towards Gα s coupled receptors. Support provided by the American Health Assistance Foundation.
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G beta-gamma complex
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Homologous (agonist-specific) desensitization of beta-adrenergic receptors (beta ARs) is accompanied by and appears to require phosphorylation of the receptors. We have recently described a novel protein kinase, beta AR kinase, which phosphorylates beta ARs in vitro in an agonist-dependent manner. This kinase is inhibited by two classes of compounds, polyanions and synthetic peptides derived from the beta 2-adrenergic receptor (beta 2AR). In this report we describe the effects of these inhibitors on the process of homologous desensitization induced by the beta-adrenergic agonist isoproterenol. Permeabilization of human epidermoid carcinoma A431 cells with digitonin was used to permit access of the charged inhibitors to the cytosol; this procedure did not interfere with the pattern of isoproterenol-induced homologous desensitization of beta 2AR-stimulated adenylyl cyclase. Inhibitors of beta AR kinase markedly inhibited homologous desensitization of beta 2ARs in the permeabilized cells. Inhibition of desensitization by heparin, the most potent of the polyanion inhibitors of beta AR kinase, occurred over the same concentration range (5-50 nM) as inhibition of purified beta AR kinase assessed in a reconstituted system. Inhibition of desensitization by heparin was accompanied by a marked reduction of receptor phosphorylation in the permeabilized cells. Whereas inhibitors of beta AR kinase inhibited homologous desensitization, inhibitors of protein kinase C and of cyclic-nucleotide-dependent protein kinases were ineffective. These data establish that phosphorylation of beta ARs by beta AR kinase is an essential step in homologous desensitization of the receptors. They further suggest a potential therapeutic value of inhibitors of beta AR kinase in inhibiting agonist-induced desensitization.
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G protein-coupled receptors (GPCRs) are involved in a multitude of signaling processes and respond to a wide range of ligands. The activity of GPCRs is subject to three principal modes of regulation: desensitization, trafficking, and down-regulation. Desensitization is defined as a loss in the responsiveness of a signaling system. The generally established paradigm for GPCR desensitization involves receptor phosphorylation by GPCR kinases (GRKs), initiated by agonist-induced conformational changes in the receptor or by kinases activated by specific signaling pathways. GRKs have several interaction domains and may be able to contribute to receptor desensitization through mechanisms that do not involve the kinase activity of GRK. Pao and Benovic discuss some of these interactions and their relevance for the regulation of GPCR signaling.
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We previously reported that G protein-coupled receptor kinase (GRK) may contribute to beta-adrenergic receptor (beta-AR) uncoupling occurring just before parturition in rat uterine muscle (myometrium). To identify the GRK involved, we set up in this study a primary cell culture retaining the morphological and functional characteristics of myometrial tissue as well as the in vivo pattern of GRK expression (GRK2, GRK5, and GRK6). In this model, homologous beta-AR desensitization was assessed by an approximately 60% decrease in cAMP production to a subsequent challenge with the beta-agonist, isoproterenol. Desensitization was reduced by 36% with a GRK inhibitor, heparin, and by 31% with a protein kinase A in-hibitor, H89. Using antibodies known to specifically inhibit either GRK2/3 or GRK4-6 families, we demonstrated that only the GRK4-6 family mediated beta-AR desensitization. To discriminate between endogenous GRK5 and GRK6, we attempted to inhibit their action by introducing, into myometrial cells, kinase-dead dominant-negative mutants ((K215R)GRK5 and (K215R)GRK6). Expression of (K215R)GRK6 increased by approximately 70% the cAMP response to isoproterenol without effect on forskolin stimulation. Conversely, expression of (K215R)GRK5 or (K220R)GRK2 had no effect on beta-adrenergic signaling. These results strongly suggest that endogenous GRK6 mediate homologous beta-AR desensitization in myometrial cells.
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Rhodopsin-like receptors
Arrestin
Homologous desensitization
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The patterns and mechanism of vascular desensitization to agonists were studied in isolated arteries of Wistar and SHR rats. The results showed that: (1) Arteries were much easier to desensitize peptidergic agonists (A II, AVP) than catecholaminer gic agonists (NE, PE); (2) the desensitization induced by peptidergic agonists and catecholaminer gic agonists were homologous within 30 minutes; (3) the endothelium-EDRF-cGMP pathway was not involved in agonist-induced desensitization; (4) inhibition of surface receptor internalization by phenylarsine oxide (PAO, 10(-5)mol/L) could transiently reverse the desensitization and the blocking of intracellular receptor re-insertion into the membrane by chloroauine (10(-5)mol/L) could accelerate the desensitization in varying degrees with different arteries, which demonstrated the involvement of receptor cycling in homologous desensitization, but was not the unique mechanism; (5) there was no change in the function of voltage-dependent Ca2+ channel during homologous desensitization; (6) the concentration-contraction curve of GTP gamma S shifted down-ward during homologous desensitization, suggesting the functional change of G protein (s); (7) the desensitization to agonists didn't decrease in arteries of SHR compared with the arteries of Wistar rats.
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Leucocytes from allergic donors were preincubated with suboptimal concentrations of ragweed or anti-IgE and then challenged with increasing concentrations of the homologous or heterologous agonist. The initial incubation resulted in desensitization, as judged by a reduced reactivity relative to controls preincubated without agonist but challenged similarly. Both homologous and heterologous desensitization were observed and were dose dependent. Evidence was obtained for both a reversible and irreversible component of desensitization, which was also agonist-concentration related. Reversibility occurred to a similar degree either by incubation of suboptimally desensitized cells with optimal concentrations of agonist or by removal of IgE and resensitization. This could implicate IgE-agonist aggregation on the basophil surface as a mechanism of desensitization. Histamine release from desensitized cells was highly correlated with degranulation, suggesting that individual cells were desensitized in an all-or-none manner.
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