Pathophysiological roles of G-protein-coupled receptor kinases

Molecular mechanism for inhibition of g protein-coupled receptor kinase 2 by a selective RNA aptamer

Cardiovascular homeostasis is maintained in part by the rapid desensitization of activated heptahelical receptors that have been phosphorylated by G protein-coupled receptor kinase 2 (GRK2). However, during chronic heart failure GRK2 is upregulated and believed to contribute to disease progression. We have determined crystallographic structures of GRK2 bound to an RNA aptamer that potently and selectively inhibits kinase activity. Key to the mechanism of inhibition is the positioning of an adenine nucleotide into the ATP-binding pocket and interactions with the basic αF-αG loop region of the GRK2 kinase domain. Constraints imposed on the RNA by the terminal stem of the aptamer also play a role. These results highlight how a high-affinity aptamer can be used to selectively trap a novel conformational state of a protein kinase.

Different expression of adrenoceptors and GRKs in the human myocardium depends on heart failure etiology and correlates to clinical variables

Downregulation of β(1)- adrenergic receptors (β(1)-ARs) and increased expression/function of G-protein-coupled receptor kinase 2 (GRK2) have been observed in human heart failure, but changes in expression of other ARs and GRKs have not been established. Another unresolved question is the incidence of these compensatory mechanisms depending on heart failure etiology and treatment. To analyze these questions, we quantified the mRNA/protein expressions of six ARs (α(1A), α(1B), α(1D), β(1), β(2), and β(3)) and three GRKs (GRK2, GRK3, and GRK5) in left (LV) and right ventricle (RV) from four donors, 10 patients with ischemic cardiomyopathy (IC), 14 patients with dilated cardiomyopathy (DC), and 10 patients with nonischemic, nondilated cardiopathies (NINDC). We correlated the changes in the expressions of ARs and GRKs with clinical variables such as left ventricular ejection fraction (LVEF) and left ventricular end-systolic and left ventricular end-diastolic diameter (LVESD and LVEDD, respectively). The main findings were 1) the expression of the α(1A)-AR in the LV positively correlates with LVEF; 2) the expression of GRK3 and GRK5 inversely correlates with LVESD and LVEDD, supporting previous observations about a protective role for both kinases in failing hearts; and 3) β(1)-AR expression is downregulated in the LV and RV of IC, in the LV of DC, and in the RV of NINDC. This difference, better than an increased expression of GRK2 (not observed in IC), determines the lower LVEF in IC and DC vs. NINDC.

Growth inhibition of human hepatocellular carcinoma cells by overexpression of G-protein-coupled receptor kinase 2

Hepatocellular carcinoma (HCC) is one of the deadliest forms of human liver cancer and does not respond well to conventional therapies. Novel effective treatments are urgently in need. G-protein-coupled kinase 2 (GRK2) is unique serine/threonine kinase that involves in many signaling pathways and regulates various essential cellular processes. Altered levels of GRK2 have been linked with several human diseases including cancer. In this study, we investigated a novel approach for HCC treatment by inducing overexpression of GRK2 in human HCC cells. We found that overexpression of GRK2 through recombinant adenovirus transduction inhibits the growth of human HCC cells. BrdU incorporation assay showed that the growth inhibition caused by elevated GRK2 level was due to reduced cell proliferation but not apoptosis. To examine the anti-proliferative function of increased GRK2 level, we performed cell cycle analysis using propidium iodide staining. We found that the proliferation suppression was associated with G2/M phase cell cycle arrest by the wild-type GRK2 but not its kinase-dead K220R mutant. Furthermore, increased levels of wild-type GRK2 induced upregulation of phosphor-Ser(15) p53 and cyclin B1 in a dose-dependent manner. Our data indicate that the anti-proliferative function of elevated GRK2 is associated with delayed cell cycle progression and is GRK2 kinase activity-dependent. Enforced expression of GRK2 in human HCC by molecular delivery may offer a potential therapeutic approach for the treatment of human liver cancer.

GRK5 deficiency decreases diet-induced obesity and adipogenesis

Identification of the protein factors that regulate the adipogenesis and lipid metabolism of adipose tissue is critical for the understanding of the physiology and pathology of obesity and energy homeostasis. In this study, we found that G protein coupled receptor (GPCR) kinase 5 (GRK5) was expressed at a relatively high level in the white adipose tissue. When fed on a high-fat diet, GRK5(-/-) mice gained significantly less weight and had decreased WAT mass than their wild type littermates, which could not be attributed to alterations in food consumption or energy expenditure. However, GRK5(-/-) mice showed a 30-70% decreased expression of lipid metabolism and adipogenic genes in WAT. Moreover, GRK5(-/-) embryonic fibroblasts and preadipocytes exhibited 40-70% decreased expression of adipogenic genes and impaired adipocyte differentiation when induced in vitro. Taken together, these results suggest that GRK5 is an important regulator of adipogenesis and is crucial for the development of diet-induced obesity.

GRK5 promotes F-actin bundling and targets bundles to membrane structures to control neuronal morphogenesis

Neuronal morphogenesis requires extensive membrane remodeling and cytoskeleton dynamics. In this paper, we show that GRK5, a G protein-coupled receptor kinase, is critically involved in neurite outgrowth, dendrite branching, and spine morphogenesis through promotion of filopodial protrusion. Interestingly, GRK5 is not acting as a kinase but rather provides a key link between the plasma membrane and the actin cytoskeleton. GRK5 promoted filamentous actin (F-actin) bundling at the membranes of dynamic neuronal structures by interacting with both F-actin and phosphatidylinositol-4,5-bisphosphate. Moreover, separate domains of GRK5 mediated the coupling of actin cytoskeleton dynamics and membrane remodeling and were required for its effects on neuronal morphogenesis. Accordingly, GRK5 knockout mice exhibited immature spine morphology and deficient learning and memory. Our findings identify GRK5 as a critical mediator of dendritic development and suggest that coordinated actin cytoskeleton and membrane remodeling mediated by bifunctional actin-bundling and membrane-targeting molecules, such as GRK5, is crucial for proper neuronal morphogenesis and the establishment of functional neuronal circuitry.

Suppression of G-protein-coupled receptor kinase 3 expression is a feature of classical GBM that is required for maximal growth

G-protein-coupled receptor kinases (GRK) regulate the function of G-protein-coupled receptors (GPCR). Previously, we found that GPCR (CXCR4)-mediated astrocytoma growth was dependent upon abnormally sustained CXCR4 signaling and was correlated with decreased GRK-mediated receptor phosphorylation. As CXCR4 has also been implicated in the stimulation of high-grade glioma growth, we sought to determine whether dysregulation of GRK expression and/or function might also be present in high-grade gliomas. In an analysis of data from The Cancer Genome Atlas, we found that GRK3 expression is frequently decreased in glioblastoma (GBM) of the classical subtype, which possesses signature amplification or mutational activation of the epidermal growth factor (EGF) receptor. We tested the correlation between GRK3 expression and GBM subtypes, as well as the relationship between the activation of the EGF and other growth factor receptor pathways and GRK expression. In analyses of primary GBM tissue and RNA specimens, we found that GRK3 expression is correlated with established criteria for GBM subtyping including expression of EGF receptor, platelet-derived growth factor receptor (PDGFR)α, NF1, PTEN, CDKN2A, and neurofilament. We also found that established drivers of gliomagenesis, the EGF, PDGF, and TGF-β pathways, all regulate GRK expression. Coculture experiments, designed to mimic critical interactions between tumor and brain microvascular endothelial cells, showed that specifically increasing GRK3 expression reduced the trophic effect of endothelial cells on tumor cells. Together, these experiments show that GRK3 is a negative regulator of cell growth whose expression is preferentially reduced in GBM of the classical subtype as a consequence of activity in primary gliomagenic pathways.

G protein coupled receptor kinases as therapeutic targets in cardiovascular disease

G protein-coupled receptors (GPCRs) represent the largest family of membrane receptors and are responsible for regulating a wide variety of physiological processes. This is accomplished via ligand binding to GPCRs, activating associated heterotrimeric G proteins and intracellular signaling pathways. G protein-coupled receptor kinases (GRKs), in concert with β-arrestins, classically desensitize receptor signal transduction, thus preventing hyperactivation of GPCR second-messenger cascades. As changes in GRK expression have featured prominently in many cardiovascular pathologies, including heart failure, myocardial infarction, hypertension, and cardiac hypertrophy, GRKs have been intensively studied as potential diagnostic or therapeutic targets. Herein, we review our evolving understanding of the role of GRKs in cardiovascular pathophysiology.

Prolyl isomerase Pin1 as a molecular switch to determine the fate of phosphoproteins

Pin1 is a highly conserved enzyme that only isomerizes specific phosphorylated Ser/Thr-Pro bonds in certain proteins, thereby inducing conformational changes. Such conformational changes represent a novel and tightly controlled signaling mechanism regulating a spectrum of protein activities in physiology and disease; often through phosphorylation-dependent, ubiquitin-mediated proteasomal degradation. In this review, we summarize recent advances in elucidating the role and regulation of Pin1 in controlling protein stability. We also propose a mechanism by which Pin1 functions as a molecular switch to control the fates of phosphoproteins. We finally stress the need to develop tools to visualize directly Pin1-catalyzed protein conformational changes as a way to determine their roles in the development and treatment of human diseases.

Modulating micro-opioid receptor phosphorylation switches agonist-dependent signaling as reflected in PKCepsilon activation and dendritic spine stability

A new role of G protein-coupled receptor (GPCR) phosphorylation was demonstrated in the current studies by using the μ-opioid receptor (OPRM1) as a model. Morphine induces a low level of receptor phosphorylation and uses the PKCε pathway to induce ERK phosphorylation and receptor desensitization, whereas etorphine, fentanyl, and [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) induce extensive receptor phosphorylation and use the β-arrestin2 pathway. Blocking OPRM1 phosphorylation (by mutating Ser363, Thr370 and Ser375 to Ala) enabled etorphine, fentanyl, and DAMGO to use the PKCε pathway. This was not due to the decreased recruitment of β-arrestin2 to the receptor signaling complex, because these agonists were unable to use the PKCε pathway when β-arrestin2 was absent. In addition, overexpressing G protein-coupled receptor kinase 2 (GRK2) decreased the ability of morphine to activate PKCε, whereas overexpressing dominant-negative GRK2 enabled etorphine, fentanyl, and DAMGO to activate PKCε. Furthermore, by overexpressing wild-type OPRM1 and a phosphorylation-deficient mutant in primary cultures of hippocampal neurons, we demonstrated that receptor phosphorylation contributes to the differential effects of agonists on dendritic spine stability. Phosphorylation blockage made etorphine, fentanyl, and DAMGO function as morphine in the primary cultures. Therefore, agonist-dependent phosphorylation of GPCR regulates the activation of the PKC pathway and the subsequent responses.

Multiple scaffolding functions of {beta}-arrestins in the degradation of G protein-coupled receptor kinase 2

G protein-coupled receptor kinase 2 (GRK2) plays a fundamental role in the regulation of G protein-coupled receptors (GPCRs), and changes in GRK2 expression levels can have an important impact on cell functions. GRK2 is known to be degraded by the proteasome pathway. We have shown previously that β-arrestins participate in enhanced kinase turnover upon GPCR stimulation by facilitating GRK2 phosphorylation by c-Src or by MAPK or by recruiting the Mdm2 E3 ubiquitin ligase to the receptor complex. In this report, we have investigated how such diverse β-arrestin scaffold functions are integrated to modulate GRK2 degradation. Interestingly, we found that in the absence of GPCR activation, β-arrestins do not perform an adaptor role for GRK2/Mdm2 association, but rather compete with GRK2 for direct Mdm2 binding to regulate basal kinase turnover. Upon agonist stimulation, β-arrestins-mediated phosphorylation of GRK2 at serine 670 by MAPK facilitates Mdm2-mediated GRK2 degradation, whereas c-Src-dependent phosphorylation would support the action of an undetermined β-arrestin-recruited ligase in the absence of GPCR activation. The ability of β-arrestins to play different scaffold functions would allow coordination of both Mdm2-dependent and -independent processes aimed at the specific modulation of GRK2 turnover in different signaling contexts.

The complex G protein-coupled receptor kinase 2 (GRK2) interactome unveils new physiopathological targets

GRK2 is a ubiquitous member of the G protein-coupled receptor kinase (GRK) family that appears to play a central, integrative role in signal transduction cascades. GRKs participate together with arrestins in the regulation of G protein-coupled receptors (GPCR), a family of hundreds of membrane proteins of key physiological and pharmacological importance, by triggering receptor desensitization from G proteins and GPCR internalization, and also by helping assemble macromolecular signalosomes in the receptor environment acting as agonist-regulated adaptor scaffolds, thus contributing to signal propagation. In addition, emerging evidence indicates that GRK2 can phosphorylate a growing number of non-GPCR substrates and associate with a variety of proteins related to signal transduction, thus suggesting that this kinase could also have diverse 'effector' functions. We discuss herein the increasing complexity of such GRK2 'interactome', with emphasis on the recently reported roles of this kinase in cell migration and cell cycle progression and on the functional impact of the altered GRK2 levels observed in several relevant cardiovascular, inflammatory or tumour pathologies. Deciphering how the different networks of potential GRK2 functional interactions are orchestrated in a stimulus, cell type or context-specific way is critical to unveil the contribution of GRK2 to basic cellular processes, to understand how alterations in GRK2 levels or functionality may participate in the onset or development of several cardiovascular, tumour or inflammatory diseases, and to assess the feasibility of new therapeutic strategies based on the modulation of the activity, levels or specific interactions of GRK2.

Molecular basis for activation of G protein-coupled receptor kinases

G protein-coupled receptor (GPCR) kinases (GRKs) selectively recognize and are allosterically regulated by activated GPCRs, but the molecular basis for this interaction is not understood. Herein, we report crystal structures of GRK6 in which regions known to be critical for receptor phosphorylation have coalesced to stabilize the kinase domain in a closed state and to form a likely receptor docking site. The crux of this docking site is an extended N-terminal helix that bridges the large and small lobes of the kinase domain and lies adjacent to a basic surface of the protein proposed to bind anionic phospholipids. Mutation of exposed, hydrophobic residues in the N-terminal helix selectively inhibits receptor, but not peptide phosphorylation, suggesting that these residues interact directly with GPCRs. Our structural and biochemical results thus provide an explanation for how receptor recognition, phospholipid binding, and kinase activation are intimately coupled in GRKs.

Dopamine receptor subtypes in the human coronary vessels of healthy subjects

OBJECTIVE

Dopamine D(1)-D(5) receptors subtypes were studied in human coronary vessels of healthy subjects to assess their localization and their expression.

METHODS

Samples of intraparenchymal and extraparenchymal branches of human coronary arteries and veins were harvested from four normal native hearts explanted from four young brain dead heart donors in case of orthoptic transplant, not carried out for technical reasons. In all the samples morphological, biochemical, immunochemical, and morphometrical studies were performed including quantitative analysis of images and evaluation of data.

RESULTS

Microanatomical section showed healthy coronary vessels, which expressed all dopamine receptors (from D(1) to D(5)) with a different pattern of distribution between the different layers, in the intra and in the extraparenchymal branches.D(1) and D(5) (with a prevalence D(1) over D(5)) were distributed in the adventitia and to a lesser extent in the outer media but they were absent in arterioles, capillaries and venules. Endothelial and the middle layer showed D(2), D(3) and D(4) receptors, with a greater expression of D(2). Immunoblot analysis of dopamine monoclonal antibodies and dopamine receptors showed a different migration band for each receptor: D(1) (45 KDa); D(2) (43 KDa); D(3) (42 kDa); D(4) (40-42 KDa); D(5) (38-40 KDa)

CONCLUSION

These findings demonstrate the presence of all dopamine receptor subtypes in the wall of human coronary vessels of healthy subjects. Dopamine D(1) and D(2) receptor subtypes are the most expressed, suggesting their prominent role in the coronary vasoactivity.

Human 5-HT(4) and 5-HT(7) receptor splice variants: are they important?

G-protein-coupled receptors (GPCRs), which are encoded by >300 genes in the human genome, are by far the largest class of targets for modern drugs. These macromolecules display inherent adaptability of function, which is partly due to the production of different forms of the receptor protein. These are commonly called 'isoforms' or 'splice variants' denoting the molecular process of their production/assembly. Not all GPCRs are expressed as splice variants, but certain subclasses of 5-HT receptors are for example, the 5-HT(4) and 5-HT(7) receptors. There are at least 11 human 5-HT(4) and three h5-HT(7) receptor splice variants. This review describestheir discoveries, nomenclature and structures. The discovery that particular splice variants are tissue specific (or prominent) has highlighted their potential as future drug targets. In particular, this review examines the functional relevance of different 5-HT(4) and 5-HT(7) receptor splice variants. Examples are given to illustrate that splice variants have differential modulatory influences on signalling processes. Differences in agonist potency and efficacies and also differences in desensitisation rates to 5-HT occur with both 5-HT(4) and 5-HT(7) receptor splice variants. The known and candidate signalling systems that allow for splice variant specific responses include GPCR interacting proteins (GIPs) and GPCR receptor kinases (GRKs) which are examined.Finally, the relevance of 5-HT receptor splice variants to clinical medicine and to the pharmaceutical industry is discussed.

Q344ter mutation causes mislocalization of rhodopsin molecules that are catalytically active: a mouse model of Q344ter-induced retinal degeneration

Q344ter is a naturally occurring rhodopsin mutation in humans that causes autosomal dominant retinal degeneration through mechanisms that are not fully understood, but are thought to involve an early termination that removed the trafficking signal, QVAPA, leading to its mislocalization in the rod photoreceptor cell. To better understand the disease mechanism(s), transgenic mice that express Q344ter were generated and crossed with rhodopsin knockout mice. Dark-reared Q344ter(rho+/-) mice exhibited retinal degeneration, demonstrating that rhodopsin mislocalization caused photoreceptor cell death. This degeneration is exacerbated by light-exposure and is correlated with the activation of transducin as well as other G-protein signaling pathways. We observed numerous sub-micrometer sized vesicles in the inter-photoreceptor space of Q344ter(rho+/-) and Q344ter(rho-/-) retinas, similar to that seen in another rhodopsin mutant, P347S. Whereas light microscopy failed to reveal outer segment structures in Q344ter(rho-/-) rods, shortened and disorganized rod outer segment structures were visible using electron microscopy. Thus, some Q344ter molecules trafficked to the outer segment and formed disc structures, albeit inefficiently, in the absence of full length wildtype rhodopsin. These findings helped to establish the in vivo role of the QVAPA domain as well as the pathways leading to Q344ter-induced retinal degeneration.

Further evidence supporting a role for gs signal transduction in severe malaria pathogenesis

With the functional demonstration of a role in erythrocyte invasion by Plasmodium falciparum parasites, implications in the aetiology of common conditions that prevail in individuals of African origin, and a wealth of pharmacological knowledge, the stimulatory G protein (Gs) signal transduction pathway presents an exciting target for anti-malarial drug intervention. Having previously demonstrated a role for the G-alpha-s gene, GNAS, in severe malaria disease, we sought to identify other important components of the Gs pathway. Using meta-analysis across case-control and family trio (affected child and parental controls) studies of severe malaria from The Gambia and Malawi, we sought evidence of association in six Gs pathway candidate genes: adenosine receptor 2A (ADORA2A) and 2B (ADORA2B), beta-adrenergic receptor kinase 1 (ADRBK1), adenylyl cyclase 9 (ADCY9), G protein beta subunit 3 (GNB3), and regulator of G protein signalling 2 (RGS2). Our study amassed a total of 2278 cases and 2364 controls. Allele-based models of association were investigated in all genes, and genotype and haplotype-based models were investigated where significant allelic associations were identified. Although no significant associations were observed in the other genes, several were identified in ADORA2A. The most significant association was observed at the rs9624472 locus, where the G allele (∼20% frequency) appeared to confer enhanced risk to severe malaria [OR = 1.22 (1.09–1.37); P = 0.001]. Further investigation of the ADORA2A gene region is required to validate the associations identified here, and to identify and functionally characterize the responsible causal variant(s). Our results provide further evidence supporting a role of the Gs signal transduction pathway in the regulation of severe malaria, and request further exploration of this pathway in future studies.

Caenorhabditis elegans TRPV channels function in a modality-specific pathway to regulate response to aberrant sensory signaling

Olfaction and some forms of taste (including bitter) are mediated by G protein-coupled signal transduction pathways. Olfactory and gustatory ligands bind to chemosensory G protein-coupled receptors (GPCRs) in specialized sensory cells to activate intracellular signal transduction cascades. G protein-coupled receptor kinases (GRKs) are negative regulators of signaling that specifically phosphorylate activated GPCRs to terminate signaling. Although loss of GRK function usually results in enhanced cellular signaling, Caenorhabditis elegans lacking GRK-2 function are not hypersensitive to chemosensory stimuli. Instead, grk-2 mutant animals do not chemotax toward attractive olfactory stimuli or avoid aversive tastes and smells. We show here that loss-of-function mutations in the transient receptor potential vanilloid (TRPV) channels OSM-9 and OCR-2 selectively restore grk-2 behavioral avoidance of bitter tastants, revealing modality-specific mechanisms for TRPV channel function in the regulation of C. elegans chemosensation. Additionally, a single amino acid point mutation in OCR-2 that disrupts TRPV channel-mediated gene expression, but does not decrease channel function in chemosensory primary signal transduction, also restores grk-2 bitter taste avoidance. Thus, loss of GRK-2 function may lead to changes in gene expression, via OSM-9/OCR-2, to selectively alter the levels of signaling components that transduce or regulate bitter taste responses. Our results suggest a novel mechanism and multiple modality-specific pathways that sensory cells employ in response to aberrant signal transduction.

Normalization of GRK2 protein and mRNA measures in patients with depression predict response to antidepressants

G-protein-coupled receptor kinases (GRKs) interfere in receptor-G-protein coupling leading to desensitization of G-protein-mediated receptor signalling. G-protein-coupled receptor signalling and its desensitization were previously implicated in the pathophysiology, diagnosis and treatment of mood disorders. The present study aimed to evaluate alterations in GRK2 protein and mRNA levels in mononuclear leukocytes (MNL) of untreated patients with major depression and the effects and time-course of antidepressant treatments on these alterations. Repeated GRK2 protein and mRNA measurements were carried in MNL of 24 patients with major depression. Each patient was examined while untreated and after 1, 2, 3 and 4 wk of antidepressant treatment; 24 healthy subjects were also studied. GRK2 protein and mRNA levels were evaluated through immunoblot analyses using monoclonal antibodies against GRK2 and reverse transcriptase-polymerase chain reaction, respectively. GRK2 protein and mRNA levels in MNL of untreated patients with major depression were significantly lower than the measures characterizing healthy subjects. The decreased GRK2 protein and mRNA levels were alleviated by antidepressant treatment. Normalization of GRK2 measures preceded, and, thus, could predict clinical improvement by 1-2 wk. These findings support the implication of GRK2 in the pathophysiology of major depression and in the mechanism underlying antidepressant-induced receptor down-regulation and therapeutic effects. GRK2 measurements in patients with depression may potentially serve for biochemical diagnostic purposes and for monitoring and predicting response to antidepressants.

G protein-coupled receptor kinase 2 (GRK2) modulation and cell cycle progression

Cell cycle progression requires changes in the activity or levels of a variety of key signaling proteins. G protein-coupled receptor kinase 2 (GRK2) plays a central role in G protein-coupled receptor regulation. Recent research is uncovering its involvement in additional cellular functions, but the potential role of GRK2 in the cell cycle has not been addressed. We report that GRK2 protein levels are transiently down-regulated during the G2/M transition by a mechanism involving CDK2-mediated phosphorylation of GRK2 at Serine670, which triggers binding to the prolyl-isomerase Pin1 and subsequent degradation. Prevention of GRK2 phosphorylation at S670 impedes normal GRK2 down-regulation and markedly delays cell cycle progression. Interestingly, we find that endogenous GRK2 down-regulation is prevented on activation of the G2/M checkpoint by doxorubicin and that stabilized GRK2 levels in such conditions inversely correlate with the p53 response and the induction of apoptosis, suggesting that GRK2 participates in the regulatory network controlling cell cycle arrest and survival in such conditions.

G-protein-coupled receptor kinases in cardiovascular conditions: focus on G-protein-coupled receptor kinase 2, a gain in translational medicine

With increasing knowledge of the regulatory mechanisms of G-protein-coupled receptor signaling in heart physiology, many studies have focused on the role of this system in cardiovascular disease. In recent years, scientists have moved their attention from the receptors to their regulatory proteins: the G-protein-coupled receptor kinases. This class of protein is indispensable for terminating signaling of G-protein-coupled receptors through receptor desensitization and downregulation. This article attempts to assemble the currently available information regarding G-protein-coupled receptor kinases and their role in cardiovascular disease and, in particular, the potential employment of G-protein-coupled receptor kinase 2 as biomarker of cardiac dysfunction.

A surface of the kinase domain critical for the allosteric activation of G protein-coupled receptor kinases

G protein-coupled receptor (GPCR) kinases (GRKs) phosphorylate activated GPCRs and initiate their desensitization. Many prior studies suggest that activated GPCRs dock to an allosteric site on the GRKs and thereby stimulate kinase activity. The extreme N-terminal region of GRKs is clearly involved in this process, but its role is not understood. Using our recent structure of bovine GRK1 as a guide, we generated mutants of solvent-exposed residues in the GRK1 kinase domain that are conserved among GRKs but not in the extended protein kinase A, G, and C family and evaluated their catalytic activity. Mutation of select residues in strands beta1 and beta3 of the kinase small lobe, alphaD of the kinase large lobe, and the protein kinase A, G, and C kinase C-tail greatly impaired receptor phosphorylation. The most dramatic effect was observed for mutation of an invariant arginine on the beta1-strand (approximately 1000-fold decrease in k(cat)/K(m)). These residues form a continuous surface that is uniquely available in GRKs for protein-protein interactions. Surprisingly, these mutants, as well as a 19-amino acid N-terminal truncation of GRK1, also show decreased catalytic efficiency for peptide substrates, although to a lesser extent than for receptor phosphorylation. Our data suggest that the N-terminal region and the newly identified surface interact and stabilize the closed, active conformation of the kinase domain. Receptor binding is proposed to promote this interaction, thereby enhancing GRK activity.

Characterization of oxidized guanosine 5'-triphosphate as a viable inhibitor of soluble guanylyl cyclase

The guanine base is prone to oxidation by free radicals regardless of the cellular moiety it is bound to. However, under conditions of oxidative stress, 8-oxoguanosine triphosphate (oxo(8)GTP) formation has been shown to occur without oxidation of the guanine base in DNA. In vitro studies have suggested that oxo(8)GTP could impact G-protein signaling and RNA synthesis. Whether increased levels of oxo(8)GTP translate into cellular malfunction is unknown. Data presented herein show that oxo(8)GTP is formed in cell-free preparations as well as in PC12 cells after exposure to physiologically relevant oxidative conditions generated with 10 microM copper sulfate and 1 mM L-ascorbic acid (Cu/Asc). We also determined that oxo(8)GTP has biological activity as a potent inhibitor of nitric oxide-stimulated soluble guanylyl cyclase (sGC). The increase in oxo(8)GTP formation in purified GTP and PC12 cells exposed to Cu/Asc caused a significant reduction in the product of sGC activity, cGMP. This oxidation of GTP was attenuated by the addition of reduced glutathione under these same Cu/Asc conditions, thus preventing the decrease in sGC activity. This suggests that oxo(8)GTP is produced by free radicals in vivo and could have significant impact on cell functions regulated by sGC activity such as synaptic plasticity in the central nervous system.

Regulation of chemokine receptor by Toll-like receptor 2 is critical to neutrophil migration and resistance to polymicrobial sepsis

Patients with sepsis have a marked defect in neutrophil migration. Here we identify a key role of Toll-like receptor 2 (TLR2) in the regulation of neutrophil migration and resistance during polymicrobial sepsis. We found that the expression of the chemokine receptor CXCR2 was dramatically down-regulated in circulating neutrophils from WT mice with severe sepsis, which correlates with reduced chemotaxis to CXCL2 in vitro and impaired migration into an infectious focus in vivo. TLR2 deficiency prevented the down-regulation of CXCR2 and failure of neutrophil migration. Moreover, TLR2(-/-) mice exhibited higher bacterial clearance, lower serum inflammatory cytokines, and improved survival rate during severe sepsis compared with WT mice. In vitro, the TLR2 agonist lipoteichoic acid (LTA) down-regulated CXCR2 expression and markedly inhibited the neutrophil chemotaxis and actin polymerization induced by CXCL2. Moreover, neutrophils activated ex vivo by LTA and adoptively transferred into naïve WT recipient mice displayed a significantly reduced competence to migrate toward thioglycolate-induced peritonitis. Finally, LTA enhanced the expression of G protein-coupled receptor kinases 2 (GRK2) in neutrophils; increased expression of GRK2 was seen in blood neutrophils from WT mice, but not TLR2(-/-) mice, with severe sepsis. Our findings identify an unexpected detrimental role of TLR2 in polymicrobial sepsis and suggest that inhibition of TLR2 signaling may improve survival from sepsis.

New roles of G protein-coupled receptor kinase 2 (GRK2) in cell migration

G protein-coupled receptor kinase 2 (GRK2) was initially identified as a key player, together with beta-arrestins, in the regulation of multiple G protein-coupled receptors (GPCR). Further research has revealed a complex GRK2 interactome, that includes a variety of proteins related to cell motility, and a role for GRK2 kinase activity in inhibiting chemokine-induced immune cell migration. In addition, we have recently reported that GRK2 positively regulates integrin and sphingosine-1-phosphate-dependent motility in epithelial cell types and fibroblasts, acting as a scaffold molecule. We suggest that the positive or negative correlation of GRK2 levels with cell migration would depend on the cell type, specific stimuli acting through plasma membrane receptors, or on the signalling context, leading to differential networks of interaction of GRK2 with cell migration-related signalosomes.

Pharmacogenomics of beta-adrenergic receptors and their accessory signaling proteins in heart failure

beta-Adrenergic receptors (betaAR) are widely expressed on cardiovascular cells. Pharmacological stimulation or blockade of betaAR signaling is the therapeutic mainstay in cardiogenic shock, hypertension, ischemia, arrhythmias, and heart failure. Interindividual variability in the response to betaAR agonists and antagonists has prompted examination of variability in the genes encoding betaAR signaling pathway members. Prominent among the genes that have been examined so far in heart failure are the beta(1)AR, beta(2)AR, and G-protein-coupled receptor kinase 5 (GRK5). Each has nonsynonymous polymorphisms that alter amino acid sequence and protein function and regulation in cell-based systems, genetically altered mouse models, or human hearts. Here, we review these phenotypes and results from published clinical studies, with a focus on heart failure pharmacogenomics. Thus far, very few studies have utilized analogous protocols or drugs, and discrepancies in the clinical studies are apparent. A compelling approach is the use of multiple methods to understand the molecular, cellular, and organ phenotypes of a variant and couple these with clinical studies designed to specifically address the relevance of those phenotypes in humans. Undoubtedly, additional loci will be identified, and together, will provide for genetically driven, individualized treatments for heart failure.

Homocysteine effects classical pathway of GPCR down regulation: Galpha(q/11), Galpha(12/13), G(i/o)

G protein-coupled receptors (GPCRs) are known to modulate intracellular effectors involved in cardiac function. We recently reported homocysteine (Hcy)-induced ERK-phosphorylation was suppressed by pertussis toxin (PTX), which suggested the involvement of GPCRs in initiating signal transduction. An activated GPCR undergoes down regulation via a known mechanism involving ERK, GRK2, beta-arrestin1: ERK activity increases; GRK2 activity increases; beta-arrestin1 is degraded. We hypothesized that Hcy treatment leads to GPCR activation and down regulation. Microvascular endothelial cells were treated with Hcy. Expression of phospho-ERK1 and phospho-GRK2 was determined using Western blot, standardized to ERK1, GRK2, and beta-actin. Hcy was shown to dephosphorylate GRK2, thereby enhancing the activity. The results provided further evidence that Hcy acts as an agonist to activate GPCRs, followed by their down regulation. Hcy was also shown to decrease the content of the following G proteins and other proteins: beta-arrestin1, Galpha(q/11), Galpha(12/13), G(i/o).

Leukocyte analysis from WHIM syndrome patients reveals a pivotal role for GRK3 in CXCR4 signaling

Leukocytes from individuals with warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome, a rare immunodeficiency, and bearing a wild-type CXCR4 ORF (WHIM(WT)) display impaired CXCR4 internalization and desensitization upon exposure to CXCL12. The resulting enhanced CXCR4-dependent responses, including chemotaxis, probably impair leukocyte trafficking and account for the immunohematologic clinical manifestations of WHIM syndrome. We provided here evidence that GPCR kinase-3 (GRK3) specifically regulates CXCL12-promoted internalization and desensitization of CXCR4. GRK3-silenced control cells displayed altered CXCR4 attenuation and enhanced chemotaxis, as did WHIM(WT) cells. These findings identified GRK3 as a negative regulator of CXCL12-induced chemotaxis and as a candidate responsible for CXCR4 dysfunction in WHIM(WT) leukocytes. Consistent with this, we showed that GRK3 overexpression in both leukocytes and skin fibroblasts from 2 unrelated WHIM(WT) patients restored CXCL12-induced internalization and desensitization of CXCR4 and normalized chemotaxis. Moreover, we found in cells derived from one patient a profound and selective decrease in GRK3 products that probably resulted from defective mRNA synthesis. Taken together, these results have revealed a pivotal role for GRK3 in regulating CXCR4 attenuation and have provided a mechanistic link between the GRK3 pathway and the CXCR4-related WHIM(WT) disorder.

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.

Different kinases desensitize the human delta-opioid receptor (hDOP-R) in the neuroblastoma cell line SK-N-BE upon peptidic and alkaloid agonists

In a previous work, we described a differential desensitization of the human delta-opioid receptor (hDOP-R) by etorphine (a non-selective and alkaloid agonist) and delta-selective and peptidic agonists (DPDPE ([D-Pen(2,5)]enkephalin) and deltorphin I (Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH(2))) in the neuroblastoma cell line SK-N-BE (Allouche et al., Eur. J. Pharmacol., 371, 235, 1999). In the present study, we explored the putative role of different kinases in this differential regulation. First, selective chemical inhibitors of PKA, PKC and tyrosine kinases were used and we showed a significant reduction of etorphine-induced opioid receptor desensitization by the bisindolylmaleimide I (PKC inhibitor) while genistein (tyrosine kinase inhibitor) was potent to impair desensitization induced by the different agonists. When the PKA was inhibited by H89 pretreatment, no modification of opioid receptor desensitization was observed whatever the agonist used. Second, we further studied the role of G protein-coupled receptor kinases (GRKs) and by using western-blot experiments we observed that only the GRK2 isoform was expressed in the SK-N-BE cells. Next, the neuroblastoma cells were transfected with the wild type GRK2 or its dominant negative mutant GRK2-K220R and the inhibition on cAMP level was determined in naïve and agonist-pretreated cells. We showed that over-expression of GRK2-K220R totally abolished etorphine-induced receptor desensitization while no effect was observed with peptidic agonists and over-expression of GRK2 selectively impaired cAMP inhibition promoted by etorphine suggesting that this kinase was involved in the regulation of hDOP-R activated only by etorphine. Third, correlation between functional experiments and phosphorylation of the hDOP-R after agonist activation was assessed by western-blot using the specific anti-phospho-DOP-R Ser(363) antibody. While all agonists were potent to increase phosphorylation of opioid receptor, we showed no impairment of receptor phosphorylation level after PKC inhibitor pretreatment. Upon agonist activation, no enhancement of receptor phosphorylation was observed when the GRK2 was over-expressed while the GRK2-K220R partially reduced the hDOP-R Ser(363) phosphorylation only after peptidic agonists pretreatment. In conclusion, hDOP-R desensitization upon etorphine exposure relies on the GRK2, PKC and tyrosine kinases while DPDPE and deltorphin I mediate desensitization at least via tyrosine kinases. Although the Ser(363) was described as the primary phosphorylation site of the mouse DOP-R, we observed no correlation between desensitization and phosphorylation of this amino acid.

G protein-coupled receptor kinase 4gamma interacts with inactive Galpha(s) and Galpha13

G protein-coupled receptors (GPCRs) are regulated by multiple families of kinases including GPCR kinases (GRKs). GRK4 is constitutively active towards GPCRs, and polymorphisms of GRK4gamma are linked to hypertension. We examined, through co-immunoprecipitation, the interactions between GRK4gamma and the Galpha and Gbeta subunits of heterotrimeric G proteins. Because GRK4 has been shown to inhibit Galpha(s)-coupled GPCR signaling and lacks a PH domain, we hypothesized that GRK4gamma would interact with active Galpha(s), but not Gbeta. Surprisingly, GRK4gamma preferentially interacts with inactive Galpha(s) and Gbeta to a greater extent than active Galpha(s). GRK4gamma also interacts with inactive Galpha(13) and Gbeta. Functional studies demonstrate that wild-type GRK4gamma, but not kinase-dead GRK4gamma, ablates isoproterenol-mediated cAMP production indicating that the kinase domain is responsible for GPCR regulation. This evidence suggests that binding to inactive Galpha(s) and Gbeta may explain the constitutive activity of GRK4gamma towards Galpha(s)-coupled receptors.

Differential D1 and D5 receptor regulation and degradation of the angiotensin type 1 receptor

Renal sodium transport is increased by the angiotensin type 1 receptor (AT(1)R), which is counterregulated by dopamine via unknown mechanisms involving either the dopamine type 1 (D(1)R) or dopamine type 5 receptor (D(5)R) that belong to the D(1)-like receptor family of dopamine receptors. We hypothesize that the D(1)R and D(5)R differentially regulate AT(1)R protein expression and signaling, which may have important implications in the pathogenesis of essential hypertension. D(1)R and D(5)R share the same agonists and antagonists; therefore, the selective effects of either D(1)R or D(5)R stimulation on AT(1)R expression in human renal proximal tubule cells were determined using antisense oligonucleotides selective to either D(1)R or D(5)R. We also determined the role of receptor tyrosine kinase and the proteosome on the D(1)R/D(5)R-mediated effects on AT(1)R expression and internalization. In renal proximal tubule cells, D(5)R (not D(1)R) decreased AT(1)R expression (half-life: 0.47+/-0.18 hours) and AT(1)R-mediated extracellular signal-regulated kinase 1/2 phosphorylation (232+/-18.9 U with angiotensin II [10(-7) mol/L] versus 81+/-8.9 U with angiotensin II [10(-7) mol/L] and fenoldopam [D(1)R/D(5)R agonist; 10(-6) mol/L; P<0.05; n=6). The fenoldopam-induced decrease in AT(1)R expression was reversed by 4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo (3,4-d) pyrimidine (c-Src tyrosine-kinase inhibitor) and clasto-lactacystin beta-lactone (proteasome inhibitor), demonstrating that the fenoldopam-mediated decrease in total cell AT(1)R expression is a result of a c-Src- and proteasome-dependent process. D(5)R stimulation decreases AT(1)R expression and is c-Src and proteasome dependent. The discovery of differential regulation by D(1)R and D(5)R opens new avenues for the development of agonists selective to either receptor subtype as targeted antihypertensive agents that can decrease AT(1)R-mediated antinatriuresis.

Human G(salpha) mutant causes pseudohypoparathyroidism type Ia/neonatal diarrhea, a potential cell-specific role of the palmitoylation cycle

Pseudohypoparathyroidism type Ia (PHP-Ia) results from the loss of one allele of G(salpha), causing resistance to parathyroid hormone and other hormones that transduce signals via G(s). Most G(salpha)mutations cause the complete loss of protein expression, but some cause loss of function only, and these have provided valuable insights into the normal function of G proteins. Here we have analyzed a mutant G(salpha) (alphas-AVDT) harboring AVDT amino acid repeats within its GDP/GTP binding site, which was identified in unique patients with PHP-Ia accompanied by neonatal diarrhea. Biochemical and intact cell analyses showed that alphas-AVDT is unstable but constitutively active as a result of rapid GDP release and reduced GTP hydrolysis. This instability underlies the PHP-Ia phenotype. alphas-AVDT is predominantly localized in the cytosol, but in rat and mouse small intestine epithelial cells (IEC-6 and DIF-12 cells) alphas-AVDT was found to be localized predominantly in the membrane where adenylyl cyclase is present and constitutive increases in cAMP accumulation occur in parallel. The likely cause of this membrane localization is the inhibition of an activation-dependent decrease in alphas palmitoylation. Upon the overexpression of acyl-protein thioesterase 1, however, alphas-AVDT translocates from the membrane to the cytosol, and the constitutive accumulation of cAMP becomes attenuated. These results suggest that PHP-Ia results from the instability of alphas-AVDT and that the accompanying neonatal diarrhea may result from its enhanced constitutive activity in the intestine. Hence, palmitoylation may control the activity and localization of G(salpha) in a cell-specific manner.

Gene expression analysis in LLC-PK1 renal tubular cells by atrial natriuretic peptide (ANP): correlation of homologous human genes with renal response

We used human DNA microarray to explore the differential gene expression profiling of atrial natriuretic peptide (ANP)-stimulated renal tubular epithelial kidney cells (LLC-PK1) in order to understand the biological effect of ANP on renal kidney cell's response. Gene expression profiling revealed 807 differentially expressed genes, consisting of 483 up-regulated and 324 down-regulated genes. The bioinformatics tool was used to gain a better understanding of differentially expressed genes in porcine genome homologous with human genome and to search the gene ontology and category classification, such as cellular component, molecular function and biological process. Four up-regulated genes of ATP1B1, H3F3A, ITGB1 and RHO that were typically validated by real-time quantitative PCR (RT-qPCR) analysis serve important roles in the alleviation of renal hypertrophy as well as other related effects. Therefore, the human array can be used for gene expression analysis in pig kidney cells and we believe that our findings of differentially expressed genes served as genetic markers and biological functions can lead to a better understanding of ANP action on the renal protective system and may be used for further therapeutic application.

Dopamine D1-like receptor-mediated inhibition of Cl/HCO3- exchanger activity in rat intestinal epithelial IEC-6 cells is regulated by G protein-coupled receptor kinase 6 (GRK 6)

The present study investigated the effect of dopamine D1-like receptor stimulation on the Cl-/HCO3- exchange activity in rat intestinal epithelial IEC-6 cells. The Cl-/HCO3- exchange activity was found to be a chloride-dependent, DIDS-sensitive and niflumate-insensitive process. The presence of the SLC26A6 anion exchanger was detected by both RT-PCR and immunoblotting analysis in IEC-6 cells, in which three different small interfering RNAs (siRNAs) targeting SLC26A6 markedly inhibited Cl-/HCO3- exchange. Activation of dopamine D1-like receptors with SKF 38393 inhibited Cl-/HCO3- exchanger activity, this being antagonized by the D1 selective antagonist SKF 83566. However, effects of SKF 38393 were maximal at 5 min of exposure to the agonist and rapidly diminished with no effect at 15 min, suggestive of agonist-induced desensitization of D1-like receptors. Pretreatment of cells with heparin, a non-selective inhibitor of G protein-coupled receptor kinases (GRKs), prevented the observed attenuation of SKF 38393-induced inhibition of Cl-/HCO3- exchange. Overnight pretreatment with anti-GRK6A and anti-GRK6B, but not with anti-GRK4 antibodies, prevented the loss of SKF 38393-mediated effects. Both PKA and PKC signaling pathways participate in SKF 38393-mediated inhibition of Cl-/HCO3- exchange. These findings suggest that SLC26A6 is at least one of the anion exchanger's family members responsible for Cl-/HCO3- exchange in IEC-6 cells. Dopamine D1 receptors in IEC-6 rapidly desensitize to D1-like agonist stimulation and GRK 6, but not GRK 4, appear to be involved in agonist-mediated responsiveness and desensitization.

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.

Regulators of G-protein-coupled receptor-G-protein coupling: antidepressants mechanism of action

There is a significant gap between advances in medication for mental disorders and the present static situation of biological diagnosis and monitoring treatment. The system of neural transmission and signal transduction is a complicated, highly regulated cascade of biochemical events. Growing evidence suggests that receptor-G-protein coupling may be involved in both the pathogenesis and treatment of mood disorders. Our knowledge concerning the basic mechanisms underlying the phenomenon of desensitization, internalization, downregulation and resensitization of the G-protein-coupled receptor has been advanced during the last decade. The present review discusses the possible involvement of regulators of G-protein-coupled receptor-G-protein coupling: beta-arrestins, G-protein-coupled receptor kinases and phosducin-like proteins, as well as beta-arrestins alternative signaling events, in the pathophysiology, diagnosis and treatment monitoring of mood disorders and in the mechanism of action of antidepressant medications.

Mechanisms of regulation and function of G-protein-coupled receptor kinases

G-protein-coupled receptor kinases (GRKs) interact with the agonist-activated form of G-protein-coupled receptor (GPCR) to affect receptor phosphorylation and to initiate profound impairment of receptor signaling, or desensitization. GPCR forms the largest family of cell surface receptors, and defects in GRK function have the potential consequence to affect GPCR-stimulated biological responses in many pathological situations.

Phosphorylation of p38 by GRK2 at the docking groove unveils a novel mechanism for inactivating p38MAPK

p38 Mitogen-activated protein kinases (MAPK) are a family of Ser/Thr kinases that regulate important cellular processes such as stress responses, differentiation, and cell-cycle control . Activation of MAPK is achieved through a linear signaling cascade in which upstream kinases (MAPKKs) dually phosphorylate MAPKs at a conserved 3-amino-acid motif (Thr-X-Tyr) . G-protein-coupled receptor kinases (GRKs) are known to selectively phosphorylate G-protein-coupled receptors (GPCRs) and thus trigger desensitization . We report that GRK2 is a novel inactivating kinase of p38MAPK. p38 associates with GRK2 endogenously and is phosphorylated by GRK2 at Thr-123, a residue located at its docking groove. Mimicking phosphorylation at this site impairs the binding and activation of p38 by MKK6 and diminishes the capacity of p38 to bind and phosphorylate its substrates. Accordingly, p38 activation is decreased or increased when cellular GRK2 levels are enhanced or reduced, respectively. Changes in GRK2 levels and activity can modify p38-dependent processes such as differentiation of preadipocytic cells and LPS-induced cytokine release, enhanced in macrophages from GRK2(+/-) mice. Phosphorylation of p38 at a region key for its interaction with different partners uncovers a new mechanism for the regulation of this important family of kinases.

Mechanisms of disease: the role of GRK4 in the etiology of essential hypertension and salt sensitivity

Hypertension and salt sensitivity of blood pressure are two conditions the etiologies of which are still elusive because of the complex influences of genes, environment, and behavior. Recent understanding of the molecular mechanisms that govern sodium homeostasis is shedding new light on how genes, their protein products, and interacting metabolic pathways contribute to disease. Sodium transport is increased in the proximal tubule and thick ascending limb of Henle of the kidney in human essential hypertension. This Review focuses on the counter-regulation between the dopaminergic and renin-angiotensin systems in the renal proximal tubule, which is the site of about 70% of total renal sodium reabsorption. The inhibitory effect of dopamine is most evident under conditions of moderate sodium excess, whereas the stimulatory effect of angiotensin II is most evident under conditions of sodium deficit. Dopamine and angiotensin II exert their actions via G protein-coupled receptors, which are in turn regulated by G protein-coupled receptor kinases (GRKs). Polymorphisms that lead to aberrant action of GRKs cause a number of conditions, including hypertension and salt sensitivity. Polymorphisms in one particular member of this family-GRK4-have been shown to cause hyperphosphorylation, desensitization and internalization of a member of the dopamine receptor family, the dopamine 1 receptor, while increasing the expression of a key receptor of the renin-angiotensin system, the angiotensin II type 1 receptor. Novel diagnostic and therapeutic approaches for identifying at-risk subjects, followed by selective treatment of hypertension and salt sensitivity, might center on restoring normal receptor function through blocking the effects of GRK4 polymorphisms.

Type 1 angiotensin receptor pharmacology: signaling beyond G proteins

Drugs that inhibit the production of angiotensin II (AngII) or its access to the type 1 angiotensin receptor (AT₁R) are prescribed to alleviate high blood pressure and its cardiovascular complications. Accordingly, much research has focused on the molecular pharmacology of AT₁R activation and signaling. An emerging theme is that the AT₁R generates G protein dependent as well as independent signals and that these transduction systems separately contribute to AT₁R biology in health and disease. Regulatory molecules termed arrestins are central to this process as is the capacity of AT₁R to crosstalk with other receptor systems, such as the widely studied transactivation of growth factor receptors. AT₁R function can also be modulated by polymorphisms in the AGTR gene, which may significantly alter receptor expression and function; a capacity of the receptor to dimerize/oligomerize with altered pharmacology; and by the cellular environment in which the receptor resides. Together, these aspects of the AT₁R “flavour” the response to angiotensin; they may also contribute to disease, determine the efficacy of current drugs and offer a unique opportunity to develop new therapeutics that antagonize only selective facets of AT₁R function.

The G protein-coupled receptor kinase (GRK) interactome: role of GRKs in GPCR regulation and signaling

G protein-coupled receptor kinases (GRKs) and arrestins are key participants in the canonical pathways leading to phosphorylation-dependent GPCR desensitization, endocytosis, intracellular trafficking and resensitization as well as in the modulation of important intracellular signaling cascades by GPCR. Novel studies have revealed a phosphorylation-independent desensitization mechanism operating through their RGS-homology (RH) domain and the recent determination of the crystal structures of GRK2 and GRK6 has uncovered interesting details on the structure-function relationships of these kinases. Emerging evidence indicates that the activity of GRKs is tightly modulated by mechanisms including phosphorylation by different kinases and interaction with several cellular proteins such as calmodulin, caveolin or RKIP. In addition, GRKs are involved in multiple interactions with non-receptor proteins (PI3K, Akt, GIT or MEK) that point to novel GRK cellular roles. In this article, our purpose is to describe the ever increasing map of functional interactions for GRK proteins as a basis to better understand its contribution to cellular processes.

Mdm2 is involved in the ubiquitination and degradation of G-protein-coupled receptor kinase 2

G-protein-coupled receptor kinase 2 (GRK2) is a central regulator of G-protein-coupled receptor signaling. We report that Mdm2, an E3-ubiquitin ligase involved in the control of cell growth and apoptosis, plays a key role in GRK2 degradation. Mdm2 and GRK2 association is enhanced by beta(2)-adrenergic receptor stimulation and beta-arrestin. Increased Mdm2 expression accelerates GRK2 proteolysis and promotes kinase ubiquitination at defined residues, whereas GRK2 turnover is markedly impaired in Mdm2-deficient cells. Moreover, we find that activation of the PI3K/Akt pathway by insulin-like growth factor-1 alters Mdm2-mediated GRK2 degradation, leading to enhanced GRK2 stability and increased kinase levels. These data put forward a novel mechanism for controlling GRK2 expression in physiological and pathological conditions.

Hydrogen peroxide impairs GRK2 translation via a calpain-dependent and cdk1-mediated pathway

Oxidative mechanisms of injury are involved in many neurodegenerative diseases such as stroke, ischemia-reperfusion injury and multiple sclerosis. G protein-coupled receptor kinase 2 (GRK2) plays a key role in G protein-coupled receptor (GPCR) signaling modulation, and its expression levels are decreased after brain hypoxia/ischemia and reperfusion as well as in several inflammatory conditions. We report here that hydrogen peroxide downregulates GRK2 expression in C6 rat glioma cells. The hydrogen peroxide-induced decrease in GRK2 is prevented by a calpain protease inhibitor, but does not involve increased GRK2 degradation or changes in GRK2 mRNA level. Instead we show that hydrogen peroxide treatment impairs GRK2 translation in a process that requires Cdk1 activation and involves the mTOR pathway. This novel mechanism for the control of GRK2 expression in glial cells upon oxidative stress challenge may contribute to the modulation of GPCR signaling in different pathological conditions.

Defective vasodilatory mechanisms in hypertension: a G-protein-coupled receptor perspective

PURPOSE OF REVIEW

The purpose of this article is to review recent evidence relating to the regulation of vasodilatation and alterations in these mechanisms in the hypertensive state. In particular, we will focus on signaling systems regulating nitric oxide synthase and intracellular cyclic AMP - the two principal mechanisms mediating vasodilatation.

RECENT FINDINGS

G-protein-coupled-receptor-mediated, endothelial-dependent processes are increasingly being seen as critical vasodilatory mechanisms. Impairment of endothelial responses to G-protein-coupled receptor activation is a key component of the decrease in G-protein-coupled-receptor-mediated vasodilatation in hypertension. In addition, an 'uncoupling' of the G-protein-coupled receptor/G-protein complex is the principal mechanism underlying impaired G-protein-coupled-receptor-mediated vasodilatation in hypertension. Alterations in G-protein-coupled receptor kinase function have a central role underlying this defect. Finally, the importance of the expression of genetic variants of G-protein-coupled receptors and G-proteins underlying the defect in vasodilatation in hypertension remains contentious.

SUMMARY

G-protein signaling pathways in the vasculature play an important role in both the development and the maintenance of the hypertensive state. It is unlikely, however, that any single defect in the G-protein-linked vasodilatory pathway will ever be shown to be the sole cause of hypertension.

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Tyrosine phosphorylation of G-protein-coupled-receptor kinase 2 (GRK2) by c-Src modulates its interaction with Galphaq

G-protein-coupled-receptor kinase 2 (GRK2) plays a key role in the modulation of G-protein-coupled-receptor (GPCR) signaling by both phosphorylating agonist-occupied GPCRs and by directly binding to activated Galphaq subunits, inhibiting downstream effectors activation. The GRK2/Galphaq interaction involves the N-terminal region of the kinase that displays homology to regulators of G-protein signaling (RGS) proteins. We have previously reported that upon GPCR stimulation, GRK2 can be phosphorylated by c-Src on tyrosine residues that are present in the RGS-homology (RH) region of this kinase. Here, we demonstrate that c-Src kinase activity increases the interaction between GRK2 and Galphaq. Tyrosine phosphorylation of GRK2 appears to be critically involved in the modulation of this interaction since the stimulatory effect of c-Src is not observed with a GRK2 mutant with impaired tyrosine phosphorylation (GRK2 Y13,86,92F), whereas a mutant that mimics GRK2 tyrosine phosphorylation in these residues displays an increased interaction with Galphaq. As evidence for a physiological role of this modulatory mechanism, activation of the muscarinic receptor M1, a Galphaq-coupled receptor, promotes an increase in GRK2/Galphaq co-immunoprecipitation that parallels the enhanced GRK2 phosphorylation on tyrosine residues. Moreover, c-Src activation enhances inhibition of the Galphaq/phospholipase Cbeta signaling pathway in intact cells, in a GRK2-tyrosine-phosphorylation-dependent manner. Our results suggest a feedback mechanism by which phosphorylation of GRK2 by c-Src increases both GRK2 kinase activity towards GPCRs and its specific interaction with Galphaq subunits, leading to a more rapid switch off of Galphaq-mediated signaling.

GRK et arrestines : la piste thérapeutique ?

Phosphorylation of the agonist-activated form of G-protein-coupled receptors (GPCRs) by a protein kinase from the G-protein-coupled receptor kinase (GRK) family initiates, with arrestin proteins, a negative feedback process known as desensitization. Because these receptors are involved in so many vital functions, it seems likely that disorders affecting GRK- or arrestin-mediated regulation of GPCRs would contribute to, if not engender, disease. Traditionally, it is believed that the desensitization process protects the cell against an overstimulation; however, in certain situations, this process is maladjusted and participes in disease progression. For example, in Oguchi disease, excessive rhodopsin stimulation due to a functional loss of GRK1 or arrestin 1 leads to light sensitization and stationary night blindness. Also, transgenic mice with vascular smooth muscle-targeted overexpression of GRK2 showed an elevated resting blood pressure, suggesting that increase in GRK2 level in humans is involved in hypertension associated with a decreased effect of beta-adrenergic receptor-mediated vasorelaxation. The restoration of normal GPCR function in modulating the desensitization process has been successfully demonstrated in animal models of heart failure, which indicates that targeting GRKs or arrestins may open a novel therapeutic strategy in human diseases with GPCR dysregulation. However, the few effective pharmacological compounds in this domain currently preclude human clinical tests.