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

The Roles of Cardiovascular H2-Histamine Receptors Under Normal and Pathophysiological Conditions

This review addresses pharmacological, structural and functional relationships among H₂-histamine receptors and H₁-histamine receptors in the mammalian heart. The role of both receptors in the regulation of force and rhythm, including their electrophysiological effects on the mammalian heart, will then be discussed in context. The potential clinical role of cardiac H₂-histamine-receptors in cardiac diseases will be examined. The use of H₂-histamine receptor agonists to acutely increase the force of contraction will be discussed. Special attention will be paid to the potential role of cardiac H₂-histamine receptors in the genesis of cardiac arrhythmias. Moreover, novel findings on the putative role of H₂-histamine receptor antagonists in treating chronic heart failure in animal models and patients will be reviewed. Some limitations in our biochemical understanding of the cardiac role of H₂-histamine receptors will be discussed. Recommendations for further basic and translational research on cardiac H₂-histamine receptors will be offered. We will speculate whether new knowledge might lead to novel roles of H₂-histamine receptors in cardiac disease and whether cardiomyocyte specific H₂-histamine receptor agonists and antagonists should be developed.

Histamine H2 Receptor in Blood Cells: A Suitable Target for the Treatment of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) consists in a cancer of early hematopoietic cells arising in the bone marrow, most often of those cells that would turn into white blood cells (except lymphocytes). Chemotherapy is the treatment of choice for AML but one of the major complications is that current drugs are highly toxic and poorly tolerated. In general, treatment for AML consists of induction chemotherapy and post-remission therapy. If no further post-remission is given, almost all patients will eventually relapse. Histamine, acting at histamine type-2 (H₂) receptors on phagocytes and AML blast cells, helps prevent the production and release of oxygen-free radicals, thereby protecting NK and cytotoxic T cells. This protection allows immune-stimulating agents, such as interleukin-2 (IL-2), to activate cytotoxic cells more effectively, enhancing the killing of tumor cells. Based on this mechanism, post-remission therapy with histamine and IL-2 was found to significantly prevent relapse of AML. Alternatively, another potentially less toxic approach to treat AML employs drugs to induce differentiation of malignant cells. It is based on the assumption that many neoplastic cell types exhibit reversible defects in differentiation, which upon appropriate treatment results in tumor reprogramming and the induction of terminal differentiation. There are promissory results showing that an elevated and sustained signaling through H₂ receptors is able to differentiate leukemia-derived cell lines, opening the door for the use of H₂ agonists for specific differentiation therapies. In both situations, histamine acting through H₂ receptors constitutes an eligible treatment to induce leukemic cell differentiation, improving combined therapies.

The Pharmacological and Physiological Role of Multidrug-Resistant Protein 4

Multidrug-resistant protein 4 (MRP4), a member of the C subfamily of ATP-binding cassette transporters, is distributed in a variety of tissues and a number of cancers. As a drug transporter, MRP4 is responsible for the pharmacokinetics and pharmacodynamics of numerous drugs, especially antiviral drugs, antitumor drugs, and diuretics. In this regard, the functional role of MRP4 is affected by a number of factors, such as genetic mutations; tissue-specific transcriptional regulations; post-transcriptional regulations, including miRNAs and membrane internalization; and substrate competition. Unlike other C family members, MRP4 is in a pivotal position to transport cellular signaling molecules, through which it is tightly connected to the living activity and physiologic processes of cells and bodies. In the context of several cancers in which MRP4 is overexpressed, MRP4 inhibition shows striking effects against cancer progression and drug resistance. In this review, we describe the role of MRP4 more specifically in both healthy conditions and disease states, with an emphasis on its potential as a drug target.

Homologous desensitization of human histamine H₃ receptors expressed in CHO-K1 cells

Histamine H₃ receptors (H₃Rs) modulate the function of the nervous system at the pre- and post-synaptic levels. In this work we aimed to determine whether, as other G protein-coupled receptors (GPCRs), H₃Rs desensitize in response to agonist exposure. By using CHO-K1 cells stably transfected with the human H₃R (hH3R) we show that functional responses (inhibition of forskolin-induced cAMP accumulation in intact cells and stimulation of [(35)S]-GTPγS binding to cell membranes) were markedly reduced after agonist exposure. For cAMP accumulation assays the effect was significant at 60 min with a maximum at 90 min. Agonist exposure resulted in decreased binding sites for the radioligand [(3)H]-N-methyl-histamine ([(3)H]-NMHA) to intact cells and modified the sub-cellular distribution of H₃Rs, as detected by sucrose density gradients and [(3)H]-NMHA binding to cell membranes, suggesting receptor internalization. The reduction in the inhibition of forskolin-stimulated cAMP formation observed after agonist pre-incubation was prevented by incubation in hypertonic medium or in ice-cold medium. Agonist-induced loss in binding sites was also prevented by hypertonic medium or incubation at 4 °C, but not by filipin III, indicating clathrin-dependent endocytosis. Immunodetection showed that CHO-K1 cells express GPCR kinases (GRKs) 2/3, and both the GRK general inhibitor ZnCl₂ and a small interfering RNA against GRK-2 reduced receptor desensitization. Taken together these results indicate that hH₃Rs experience homologous desensitization upon prolonged exposure to agonists, and that this process involves the action of GRK-2 and internalization via clathrin-coated vesicles.

Molecular and cellular analysis of human histamine receptor subtypes

The human histamine receptors hH(1)R and hH(2)R constitute important drug targets, and hH(3)R and hH(4)R have substantial potential in this area. Considering the species-specificity of pharmacology of H(x)R orthologs, it is important to analyze hH(x)Rs. Here, we summarize current knowledge of hH(x)Rs endogenously expressed in human cells and hH(x)Rs recombinantly expressed in mammalian and insect cells. We present the advantages and disadvantages of the various systems. We also discuss problems associated with the use of hH(x)R antibodies, an issue of general relevance for G-protein-coupled receptors (GPCRs). There is much greater overlap in activity of 'selective' ligands for other hH(x)Rs than the cognate receptor subtype than generally appreciated. Studies with native and recombinant systems support the concept of ligand-specific receptor conformations, encompassing agonists and antagonists. It is emerging that for characterization of hH(x)R ligands, one cannot rely on a single test system and a single parameter. Rather, multiple systems and parameters have to be studied. Although such studies are time-consuming and expensive, ultimately, they will increase drug safety and efficacy.

Cyclic nucleotides and phosphodiesterases in monocytic differentiation

Monocytes are immune cells that can differentiate into a number of cell types including macrophages, dendritic cells, and osteoclasts upon exposure to various cytokines. The phenotypes of these differentiated cells are highly heterogeneous and their differentiation can be affected by the cyclic nucleotides, 3'-5'-cyclic adenosine monophosphate (cAMP) and 3'-5'-cyclic guanosine monophosphate (cGMP). The intracellular levels of cAMP and cGMP are controlled through regulation of production by adenylyl and guanylyl cyclases and through degradation by cyclic nucleotide phosphodiesterases (PDEs). PDE inhibition and subsequent changes in cyclic nucleotide levels can alter the final phenotype of a differentiating monocyte with regards to surface marker expression, gene expression, or changes in secreted chemokine and cytokine levels. The differentiation process itself can also be either inhibited or augmented by changes in cyclic nucleotide levels, depending on the system being studied and the timing of cyclic nucleotide elevation. This chapter explores the effects of PDE inhibition and increases in cGMP and cAMP on monocytic differentiation into osteoclasts, dendritic cells, and macrophages.

Roles of phosphorylation-dependent and -independent mechanisms in the regulation of histamine H2 receptor by G protein-coupled receptor kinase 2

It is widely assumed that G protein-coupled receptor kinase 2 (GRK2)-mediated specific inhibition of G protein-coupled receptors (GPCRs) response involves GRK-mediated receptor phosphorylation followed by β-arrestin binding and subsequent uncoupling from the heterotrimeric G protein. It has recently become evident that GRK2-mediated GPCRs regulation also involves phosphorylation-independent mechanisms. In the present study we investigated whether the histamine H2 receptor (H2R), a Gα(s)-coupled GPCR known to be desensitized by GRK2, needs to be phosphorylated for its desensitization and/or internalization and resensitization. For this purpose we evaluated the effect of the phosphorylating-deficient GRK2K220R mutant on H2R signaling in U937, COS7, and HEK293T cells. We found that although this mutant functioned as dominant negative concerning receptor internalization and resensitization, it desensitized H2R signaling in the same degree as the GRK2 wild type. To identify the domains responsible for the kinase-independent receptor desensitization, we co-transfected the receptor with constructions encoding the GRK2 RGS-homology domain (RH) and the RH or the kinase domain fused to the pleckstrin-homology domain. Results demonstrated that the RH domain of GRK2 was sufficient to desensitize the H2R. Moreover, disruption of RGS functions by the use of GRK2D110A/K220R double mutant, although coimmunoprecipitating with the H2R, reversed GRK2K220R-mediated H2R desensitization. Overall, these results indicate that GRK2 induces desensitization of H2R through a phosphorylation-independent and RGS-dependent mechanism and extends the GRK2 RH domain-mediated regulation of GPCRs beyond Gα(q)-coupled receptors. On the other hand, GRK2 kinase activity proved to be necessary for receptor internalization and the resulting resensitization.

Multidrug resistance protein 4 (MRP4/ABCC4) regulates cAMP cellular levels and controls human leukemia cell proliferation and differentiation

Increased intracellular cAMP concentration plays a well established role in leukemic cell maturation. We previously reported that U937 cells stimulated by H2 receptor agonists, despite a robust increase in cAMP, fail to mature because of rapid H2 receptor desensitization and phosphodiesterase (PDE) activation. Here we show that intracellular cAMP levels not only in U937 cells but also in other acute myeloid leukemia cell lines are also regulated by multidrug resistance-associated proteins (MRPs), particularly MRP4. U937, HL-60, and KG-1a cells, exposed to amthamine (H2-receptor agonist), augmented intracellular cAMP concentration with a concomitant increase in the efflux. Extrusion of cAMP was ATP-dependent and probenecid-sensitive, supporting that the transport was MRP-mediated. Cells exposed to amthamine and the PDE4 inhibitor showed enhanced cAMP extrusion, but this response was inhibited by MRP blockade. Amthamine stimulation, combined with PDE4 and MRP inhibition, induced maximal cell arrest proliferation. Knockdown strategy by shRNA revealed that this process was mediated by MRP4. Furthermore, blockade by probenecid or MRP4 knockdown showed that increased intracellular cAMP levels induce maturation in U937 cells. These findings confirm the key role of intracellular cAMP levels in leukemic cell maturation and provide the first evidence that MRP4 may represent a new potential target for leukemia differentiation therapy.

Regulatory mechanisms underlying GKR2 levels in U937 cells: evidence for GRK3 involvement

G protein-coupled receptors represent the most diverse group of proteins involved in transmembrane signalling, that participate in the regulation of a wide range of physicochemical messengers through the interaction with heterotrimeric G proteins. In addition, GPCRs stimulation also triggers a negative feedback mechanism, known as desensitization that prevents the potentially harmful effects caused by persistent receptor stimulation. In this adaptative response, G protein-coupled receptor kinases (GRKs) play a key role and alterations in their function are related to diverse pathophysiological situations. Based on the scarce knowledge about the regulation of GRK2 by other kinases of the same family, the aim of the present work was to investigate the regulation of GRK2 levels in systems where other GRKs are diminished by antisense technique. Present findings show that in U937 cells GRK2 levels are regulated by GRK3 and not by GRK6 through a mechanism involving InsP upregulation. This work reports a novel GRK3-mediated GRK2 regulatory mechanism and further suggests that GRK2 may also act as a compensatory kinase tending to counterbalance the reduction in GRK3 levels. This study provides the first evidence for the existence of GRKs cross-regulation.

Increasing synaptic noradrenaline, serotonin and histamine enhances in vivo binding of phosphodiesterase-4 inhibitor (R)-[11C]rolipram in rat brain, lung and heart

Phosphodiesterase-4 (PDE4) is one of the main enzymes that specifically terminate the action of cAMP, thereby contributing to intracellular signaling following stimulation of various G protein-coupled receptors. PDE4 expression and activity are modulated by agents affecting cAMP levels. The selective PDE4 inhibitor (R)-rolipram labeled with C-11 was tested in vivo in rats to analyze changes in PDE4 levels following drug treatments that increase synaptic noradrenaline (NA), serotonin (5HT), histamine (HA) and dopamine (DA) levels. We hypothesized that increasing synaptic neurotransmitter levels and subsequent cAMP-mediated signaling would significantly enhance (R)-[(11)C]rolipram retention and specific binding to PDE4 in vivo. Pre-treatments were performed 3 h prior to tracer injection, and rats were sacrificed 45 min later. Biodistribution studies revealed a dose-dependent increase in (R)-[(11)C]rolipram uptake following administration of the monoamine oxidase (MAO) inhibitor tranylcypromine, NA and 5HT reuptake inhibitors (desipramine [DMI], maprotiline; and fluoxetine, sertraline, respectively), and the HA H(3) receptor antagonist (thioperamide), but not with DA transporter blockers GBR 12909, cocaine or DA D(1) agonist SKF81297. Significant increases in rat brain and heart reflect changes in PDE4 specific binding (total-non-specific binding [coinjection with saturating dose of (R)-rolipram]). These results demonstrate that acute treatments elevating synaptic NA, 5HT and HA, but not DA levels, significantly enhance (R)-[(11)C]rolipram binding. Use of (R)-[(11)C]rolipram and positron emission tomography as an index of PDE4 activity could provide insight into understanding disease states with altered NA, 5HT and HA concentrations.

Histamine H2 receptor overexpression induces U937 cell differentiation despite triggered mechanisms to attenuate cAMP signalling

Knowing that cell-surface receptors that recognize and respond to extracellular stimuli are key components for the regular communication between individual cells required for the survival of any living organism, the aim of the present work was to investigate the effect of H2R overexpression on the U937 signal transduction pathway and its consequences on cell proliferation and differentiation. The overexpression of H2R led to an increase in cAMP basal levels, a leftward shift of agonist concentration-response curves, and similar maximal response to agonist treatment, suggesting that overexpressed H2Rs act as functional spare receptors. In this system cells triggered several mechanisms tending to restore cAMP basal levels to those of the naïve cells. H2R overexpression induced PDE activity stimulation and GRK2 overexpression. In spite of the onset of these regulatory mechanisms, H2 agonist and rolipram treatments induced the terminal differentiation of the H2R overexpressed clone, conversely to the naïve cells. Present findings show that stably H2R overexpression alters cAMP signalling as the result of not only the amounts of second messenger generated but also the activation or upregulation of various components of signalling cascade, leading to an adapted biologically unique system. This adaptation may represent an advantage or a disadvantage, depending on the biological system, but in any case, the existence of compensatory mechanisms should be considered when a clinical treatment is designed.

Differences among desensitization of histamine H2 receptor induced by three H2 receptor antagonists on rat gastric parietal cells

AIM: To investigate the differences of the desensitization of the histamine H₂ receptor induced by three H₂ receptor antagonists on the gastric parietal cells in rats.

METHODS: The gastric parietal cells were isolated by pronase digestion and then divided into three groups: cimetidine, ranitidine and famotidine treatment group. The activity of the H⁺-K⁺-ATPase was detected by H⁺-K⁺-ATPase kit after the cells were treated with different concentrations of the antagonists for different times.

RESULTS: Significant changes of the H⁺-K⁺-ATPase activity were observed after the cells were treated with different concentrations of the antagonists at different times. The activities of H⁺-K⁺-ATPase were significantly higher in famotidine group at 1, 2, and 4 h (589.34±2.7, 812.82±8.35, 637.15±4.59) than those in ranitidine (169.38±93.64, 343.46±44.88, 234.07±4.72) and cimetidine (118.42±5.91, 110.62±1.28, 102.43±3.44) group (P <0.01). The activity in ranitidine group was markedly higner than that in cimetidine group at 2 and 4 h. Famotidine increased the activities of H⁺-K⁺-ATPase significantly at the concentrations of 10 and 100 mg/L(178.21±20.38, 225.65±16.41) as compared with ranitidine and cimetidine did (70.88±21.44, 128.03±8.22; 123.62±4.32, 125.40±7.45)(P <0.01). At the concentration of 1000 mg/L, both famotidine and ranitidine increased the activity of H⁺-K⁺-ATPase obviously as compared with cimetidine did (233.44±6.24, 131.58±11.50 vs 109.88±0.69, P <0.01, P <0.05).

CONCLUSION: Three H₂ receptor antagonists can induce different desensitization of the H₂ receptor, among which famotidine induces the strongest and cimetidine does the weakest.

The time-course of cyclic AMP signaling is critical for leukemia U-937 cell differentiation

The regulation of the cAMP signaling is intimately involved in several cellular processes, including cell differentiation. Here, we provide strong evidence supporting that the time-course of cAMP signal is critical for leukemia U-937 cell differentiation. Three stimulating-cAMP agents were used to analyze the correlation between cAMP time-course and cell differentiation. All three agents denoted similar cAMP maximal responses in dose-response experiments. The kinetic of desensitization showed differential characteristics, while H2 receptor desensitized homologously without affecting PGE2 or forskolin effect, PGE2 response showed mixed desensitization characterized by a homologous initial phase followed by a heterologous phase. Regarding forskolin, long-term stimuli attenuated PGE2 and H2 agonist response without affecting adenylyl cyclase activity. In the absence of phosphodiesterase inhibitors, the three agents induced similar maximal cAMP levels after 5 min, but only that induced by the H2 agonist returned to basal levels. Consistent with this observation, H2 agonist was not able to induce U-937 cell maturation in contrast to PGE2 and forskolin, supporting the importance of time-course signaling in the determination of cell behavior.

Histamine inhibits atrial myocytic ANP release via H2 receptor-cAMP-protein kinase signaling

Changes in cyclic nucleotide production and atrial dynamics have been known to modulate atrial natriuretic peptide (ANP) release. Although cardiac atrium expresses histamine receptors and contains histamine, the role of histamine in the regulation of ANP release has to be defined. The purpose of the present study was to define the effect of histamine on the regulation of ANP release in perfused beating rabbit atria. Histamine decreased ANP release concomitantly with increases in cAMP efflux and atrial dynamics in a concentration-dependent manner. Histamine-induced decrease in ANP release was a function of an increase in cAMP production. Blockade of histamine H2 receptor with cimetidine but not of H1 receptor with triprolidine abolished the responses of histamine. Cell-permeable cAMP analog, 8-Br-cAMP, mimicked the effects of histamine, and the responses were dose-dependent and blocked by a protein kinase A (PKA)-selective inhibitor, KT5720. Nifedipine failed to modulate histamine-induced decrease in ANP release. Protein kinase nonselective inhibitor staurosporine blocked histamine-induced changes in a concentration-dependent manner. KT5720 and RP-adenosine 3',5'-cyclic monophosphorothioate, another PKA-selective inhibitor, attenuated histamine-induced changes. These results suggest that histamine decreases atrial ANP release by H2 receptor-cAMP signaling via PKA-dependent and -independent pathways.

Tiotidine, a histamine H2 receptor inverse agonist that binds with high affinity to an inactive G-protein-coupled form of the receptor. Experimental support for the cubic ternary complex model

Knowing the importance for research and pharmacological uses of proper ligand classification into agonists, inverse agonists, and antagonists, the aim of this work was to study the behavior of tiotidine, a controversial histamine H2 receptor ligand. We found that tiotidine, described previously as an H2 antagonist, actually behaves as an inverse agonist in U-937 cells, diminishing basal cAMP levels. [3H]Tiotidine showed two binding sites, one with high affinity and low capacity and the other with low affinity and high capacity. The former site disappeared in the presence of guanosine 5'-O-(3-thio)triphosphate, indicating that it belongs to a subset of receptors coupled to G-protein, showing the classic binding profile for an agonist. Considering the occupancy models developed up to now, the only one that explains tiotidine dual behavior is the cubic ternary complex (CTC) model. This model allows G-protein to interact with the receptor even in the inactive state. We showed by theoretical simulations based on the CTC model of dose-response and binding experiments that tiotidine biases the system to a G-protein-coupled form of the receptor that is unable to evoke a response. This theoretical approach was supported by experimental results in which an unrelated G-protein-coupled receptor that also signals through Galphas-protein (beta2-adrenoreceptor) was impeded by tiotidine. This interference clearly implies that tiotidine biases the system to Galphas-coupled form of the H2 receptor and turns Galphas-protein less available to interact with beta2-adrenoreceptor. These findings not only show that tiotidine is an H2 inverse agonist in U-937 cells but also provide experimental support for the CTC model.

Reduction of G protein-coupled receptor kinase 2 expression in U-937 cells attenuates H2 histamine receptor desensitization and induces cell maturation

Histamine and H2 agonists transiently induce an important cAMP response in promonocytic U-937 cells but fail to induce monocytic differentiation because of a rapid receptor desensitization mediated by G protein-coupled receptor kinases (GRKs). The aims of the present study were to investigate the participation of GRK2 in the desensitization mechanism of the H2 receptor in U-937 cells by reducing GRK2 levels through antisense technology and to evaluate the differentiating capacity of cells expressing lower GRK2 level, stimulated by H2 agonists. By stable U-937 cell transfection with a GRK2-antisense cDNA, we obtained D5 and A2 cell clones exhibiting a reduction in GRK2 expression and an H3 clone with no significant difference in GRK2 expression from control cells. The cAMP response induced by the H2 agonist in D5 and A2 but not in H3 cells was higher than in U-937 and persisted for a longer period of time, although the number of H2 receptors in D5 and A2 cells was lower than in U-937. Furthermore, D5 and A2 cells treated with H2 agonist showed patterns of c-Fos and CD88 expression consistent with monocytic differentiated cells. Overall, these results indicate a direct correlation between the expression of GRK2 and the desensitization of natively expressed H2 receptors in U-937 cells, suggesting that GRK2 plays a major role in the regulation of these receptors' response. In turn, desensitization process is a key component of H2 receptor signaling, determining the differentiation capability of promonocytic cells.

Betahistine dihydrochloride interaction with the histaminergic system in the cat: neurochemical and molecular mechanisms

Drugs interfering with the histaminergic system facilitate behavioral recovery after vestibular lesion, likely by increasing histamine turnover and release. The effects of betahistine (structural analogue of histamine) on the histaminergic system were tested by quantifying messenger RNA for histidine decarboxylase (enzyme synthesizing histamine) by in situ hybridization and binding to histamine H(3) receptors (mediating, namely, histamine autoinhibition) using a histamine H(3) receptor agonist ([(3)H]N-alpha-methylhistamine) and radioautography methods. Experiments were done in brain sections of control cats (N=6) and cats treated with betahistine for 1 (N=6) or 3 (N=6) weeks. Betahistine treatment induced symmetrical changes with up-regulation of histidine decarboxylase mRNA in the tuberomammillary nucleus and reduction of [(3)H]N-alpha-methylhistamine labeling in both the tuberomammillary nucleus, the vestibular nuclei complex and nuclei of the inferior olive. These findings suggest that betahistine upregulates histamine turnover and release, very likely by blocking presynaptic histamine H(3) receptors, and induces histamine H(3) receptor downregulation. This action on the histaminergic system could explain the effectiveness of betahistine in the treatment of vertigo and vestibular disease.

Histamine H2 receptor desensitization: involvement of a select array of G protein-coupled receptor kinases

The histamine H2 receptor (H2r) belongs to the heptahelical receptor family; upon agonist binding, members of this family activate a G protein and the downstream effector adenylyl cyclase. Like other G protein-coupled receptors, exposure of H2r to agonists produces a desensitization of the response. The present study focused on the desensitization mechanism of this receptor. Using transiently transfected COS-7 cells expressing tagged-H2r, the desensitization induced by amthamine, characterized by decreased cAMP production, was studied. Results show that the receptor was rapidly desensitized with a t(1/2) = 0.49 +/- 0.01 min. Because of the rapid nature of H2r desensitization, receptor phosphorylation was examined as a likely mechanism for signal attenuation. H2r desensitization was not affected by protein kinases A and C (PKA and PKC) inhibitors but was remarkably reduced by Zn(2+), an inhibitor of G protein-coupled receptor kinases (GRKs). Cotransfection experiments using tagged H2r and different GRKs (2, 3, 5, or 6), demonstrated that GRK2 and GRK3 were the most potent in augmenting desensitization, causing a reduction in the maximal response to amthamine and a decrease of the t(1/2) for desensitization, whereas GRK5 and GRK6 did not affect the signaling. Receptor phosphorylation correlates with desensitization for each GRK studied, whereas phosphorylation that is dependent on protein kinases A and C seemed irrelevant in receptor signal termination. These results indicate that in H2r-transfected COS-7 cells, exposure to an agonist caused desensitization controlled by H2r phosphorylation via GRK2 and GRK3.

GRK3 mediates desensitization of CRF1 receptors: a potential mechanism regulating stress adaptation

Potential G protein-coupled receptor kinase (GRK) and protein kinase A (PKA) mediation of homologous desensitization of corticotropin-releasing factor type 1 (CRF1) receptors was investigated in human retinoblastoma Y-79 cells. Inhibition of PKA activity by PKI(5-22) or H-89 failed to attenuate homologous desensitization of CRF1 receptors, and direct activation of PKA by forskolin or dibutyryl cAMP failed to desensitize CRF-induced cAMP accumulation. However, treatment of permeabilized Y-79 cells with heparin, a nonselective GRK inhibitor, reduced homologous desensitization of CRF1 receptors by approximately 35%. Furthermore, Y-79 cell uptake of a GRK3 antisense oligonucleotide (ODN), but not of a random or mismatched ODN, reduced GRK3 mRNA expression by approximately 50% without altering GRK2 mRNA expression and inhibited homologous desensitization of CRF1 receptors by approximately 55%. Finally, Y-79 cells transfected with a GRK3 antisense cDNA construct exhibited an approximately 50% reduction in GRK3 protein expression and an ~65% reduction in homologous desensitization of CRF1 receptors. We conclude that GRK3 contributes importantly to the homologous desensitization of CRF1 receptors in Y-79 cells, a brain-derived cell line.