G protein-coupled receptor kinase 2 expression and activity are associated with blood pressure in black Americans

G protein-coupled receptor kinases in hypertension: physiology, pathogenesis, and therapeutic targets

G protein-coupled receptors (GPCRs) mediate cellular responses to a myriad of hormones and neurotransmitters that play vital roles in the regulation of physiological processes such as blood pressure. In organs such as the artery and kidney, hormones or neurotransmitters, such as angiotensin II (Ang II), dopamine, epinephrine, and norepinephrine exert their functions via their receptors, with the ultimate effect of keeping normal vascular reactivity, normal body sodium, and normal blood pressure. GPCR kinases (GRKs) exert their biological functions, by mediating the regulation of agonist-occupied GPCRs, non-GPCRs, or non-receptor substrates. In particular, increasing number of studies show that aberrant expression and activity of GRKs in the cardiovascular system and kidney inhibit or stimulate GPCRs (e.g., dopamine receptors, Ang II receptors, and α- and β-adrenergic receptors), resulting in hypertension. Current studies focus on the effect of selective GRK inhibitors in cardiovascular diseases, including hypertension. Moreover, genetic studies show that GRK gene variants are associated with essential hypertension, blood pressure response to antihypertensive medicines, and adverse cardiovascular outcomes of antihypertensive treatment. In this review, we present a comprehensive overview of GRK-mediated regulation of blood pressure, role of GRKs in the pathogenesis of hypertension, and highlight potential strategies for the treatment of hypertension. Schematic representation of GPCR desensitization process. Activation of GPCRs begins with the binding of an agonist to its corresponding receptor. Then G proteins activate downstream effectors that are mediated by various signaling pathways. GPCR signaling is halted by GRK-mediated receptor phosphorylation, which causes receptor internalization through β-arrestin.

Gender differences in GRK2 in cardiovascular diseases and its interactions with estrogen

G protein-coupled receptor kinase 2 (GRK2) is a multifunctional protein involved in regulating G protein-coupled receptor (GPCR) and non-GPCR signaling in the body. In the cardiovascular system, increased expression of GRK2 has been implicated in the occurrence and development of several cardiovascular diseases (CVDs). Recent studies have found gender differences in GRK2 in the cardiovascular system under physiological and pathological conditions, where GRK2's expression and activity are increased in males than in females. The incidence of CVDs in premenopausal women is lower than in men of the same age, which is related to estrogen levels. Given the shared location of GRK2 and estrogen receptors, estrogen may interact with GRK2 by modulating vital molecules such as calmodulin (CaM), caveolin, RhoA, nitrate oxide (NO), and mouse double minute 2 homolog (Mdm2), via signaling pathways mediated by estrogen's genomic (ERα and ERβ), and non-genomic (GPER) receptors, conferring cardiovascular protection in females. Highlighting the gender differences in GRK2 and understanding its interaction with estrogen in the cardiovascular system is pertinent in treating gender-related CVDs. As a result, this article explores the gender differences of GRK2 in the cardiovascular system and its relationship with estrogen during disease conditions. Estrogen's protective and therapeutic effects and its mechanism on GRK2-related cardiovascular diseases have also been discussed.

Apelin-Induced Relaxation of Coronary Arteries Is Impaired in a Model of Second-Hand Cigarette Smoke Exposure

ABSTRACT

Apelin, an endogenous ligand for APJ receptors, causes nitric oxide (NO)-dependent relaxation of coronary arteries. Little is known about the effects of apelin/APJ receptor signaling in the coronary circulation under pathological conditions. Here, we tested the hypothesis that the vasorelaxing effect of apelin is impaired by cigarette smoke extract (CSE), an established model for second-hand smoke exposure. Isolated rat coronary arteries were treated with 2% CSE for 4 hours. Apelin-induced relaxation of coronary arteries was abolished by CSE exposure, while relaxations to acetylcholine (ACh) (endothelium-dependent relaxation) and to diethyl amine NONOate (NO donor) were similar in control and CSE-treated arteries. Immunoblot analysis demonstrated that apelin increased eNOS ser1177 phosphorylation under control conditions but had no effect after exposure to CSE. Moreover, GRK2 expression was increased in CSE-exposed coronary endothelial cells. Pretreatment with CMPD101, a GRK2 inhibitor, improved the relaxation response to apelin in CSE-exposed coronary arteries. CSE treatment failed to inhibit relaxations evoked by CMF-019, an APJ receptor biased agonist that has little effect on GRK2. In arteries exposed to CSE, apelin impaired the response to ACh but not to diethyl amine NONOate. ACh-induced relaxation was unaffected by CMF-019 in either control or CSE-treated coronary arteries. The results suggest that APJ receptor signaling using the GRK2 pathway contributes to both loss of relaxation to apelin itself and the ability of apelin to inhibit endothelium-dependent relaxation to ACh in CSE-exposed coronary arteries, likely because of impaired production of NO from endothelial cells. These changes in apelin/APJ receptor signaling under pathological conditions (eg, exposure to second-hand smoke) could create an environment that favors increased vasomotor tone in coronary arteries.

Canonical and Non-Canonical Roles of GRK2 in Lymphocytes

G protein-coupled receptor kinase 2 (GRK2) is emerging as a key integrative signaling node in a variety of biological processes ranging from cell growth and proliferation to migration and chemotaxis. As such, GRK2 is now implicated as playing a role in the molecular pathogenesis of a broad group of diseases including heart failure, cancer, depression, neurodegenerative disease, and others. In addition to its long-known canonical role in the phosphorylation and desensitization of G protein-coupled receptors (GPCRs), recent studies have shown that GRK2 also modulates a diverse array of other molecular processes via newly identified GRK2 kinase substrates and via a growing number of protein-protein interaction binding partners. GRK2 belongs to the 7-member GRK family. It is a multidomain protein containing a specific N-terminal region (referred to as αN), followed by a regulator of G protein signaling homology (RH) domain, an AGC (Protein kinase A, G, C serine/threonine kinase family) kinase domain, and a C-terminal pleckstrin homology (PH) domain. GPCRs mediate the activity of many regulators of the immune system such as chemokines and leukotrienes, and thus GRK proteins may play key roles in modulating the lymphocyte response to these factors. As one of the predominant GRK family members expressed in immune cells, GRK2's canonical and noncanonical actions play an especially significant role in normal immune cell function as well as in the development and progression of disorders of the immune system. This review summarizes our current state of knowledge of the roles of GRK2 in lymphocytes. We highlight the diverse functions of GRK2 and discuss how ongoing investigation of GRK2 in lymphocytes may inform the development of new therapies for diseases associated with lymphocyte dysregulation.

β2-Adrenoceptors and GRK2 as Potential Biomarkers in Patients With Chronic Pulmonary Regurgitation

Pulmonary regurgitation (PR) is a frequent complication after repair of congenital heart disease. Three different GRK isoforms (GRK2, GRK5, and GRK3) and two β-adrenoceptors (β1-AR and β2-AR) are present in peripheral blood mononuclear cells (PBMC) and their expression changes as a consequence of the hemodynamic and neurohumoral alterations that occur in some cardiovascular diseases. Therefore, they could be useful as biomarkers in PR. A prospective study was conducted to describe the expression (TaqMan Gene Expression Assays) of β-ARs and GRKs in PBMC isolated (Ficoll^® gradient) from patients with severe PR before and after pulmonary valve replacement and establish if this expression correlates to clinical status. 23 patients with severe PR were included and compared with 22 healthy volunteers (controls). PR patients before the PVR had a significantly lower expression of β2-AR (513.8 ± 261.2 mRNA copies) vs. controls (812.5 ± 497.2 mRNA copies), so as GRK2 expression (503.4 ± 364.9 copies vs. 858.1 ± 380.3 mRNA copies). The expression of β2-AR and GRK2 significantly decreases in symptomatic and asymptomatic patients, as well as in patients under treatment with beta-blockers and non-treated patients. The expression of β2-AR and GRK2 in PR patients recovers the normal values after pulmonary valve replacement (754,8 ± 77,1 and 897,8 ± 87,4 copies, respectively). Therefore, changes in the expression of β2-AR and GRK2 in PBMC of PR patients, could be considered as potential biomarkers to determine clinical decisions.

G Protein-Coupled Receptor Kinase 2 (GRK2) as a Potential Therapeutic Target in Cardiovascular and Metabolic Diseases

G protein-coupled receptor kinase 2 (GRK2) is a central signaling node involved in the modulation of many G protein-coupled receptors (GPCRs) and also displaying regulatory functions in other cell signaling routes. GRK2 levels and activity have been reported to be enhanced in patients or in preclinical models of several relevant pathological situations, such as heart failure, cardiac hypertrophy, hypertension, obesity and insulin resistance conditions, or non-alcoholic fatty liver disease (NAFLD), and to contribute to disease progression by a variety of mechanisms related to its multifunctional roles. Therefore, targeting GRK2 by different strategies emerges as a potentially relevant approach to treat cardiovascular disease, obesity, type 2 diabetes, or NAFLD, pathological conditions which are frequently interconnected and present as co-morbidities.

GRK2 and GRK5 as therapeutic targets and their role in maladaptive and pathological cardiac hypertrophy

INTRODUCTION

One in every four deaths in the United States is attributed to cardiovascular disease, hence the development and employment of novel and effective therapeutics are necessary to improve the quality of life and survival of affected patient. Pathological hypertrophy is a maladaptive response by the heart to relieve wall stress that could result from cardiovascular disease. Maladaptive hypertrophy can lead to further disease progression and complications such as heart failure; hence, efforts to target hypertrophy to prevent and treat further morbidity and mortality are necessary. Areas covered: This review summarizes the compelling literature that describes the mechanistic role of GRK2 and GRK5 in maladaptive cardiac hypertrophy; it examines the approaches to inhibit these kinases in hypertrophic animal models and furthermore, it assesses the potential of GRK2 and GRK5 as therapeutic targets for hypertrophy. Expert opinion: GRK2 and GRK5 are novel therapeutic targets for pathological hypertrophy and may have added benefits of ameliorating morbidity and mortality. Despite the lesser researched role of GRK2 in cardiac hypertrophy, it may be the advantageous strategy for treating cardiac hypertrophy because of its role in other maladaptive pathways. Anti-GRK2 therapy optimization and the discovery and development of specific GRK2 and GRK5 small-molecule inhibitors is necessary for the eventual application of successful, effective therapeutics.

GRK2 as negative modulator of NO bioavailability: Implications for cardiovascular disease

Nitric oxide (NO), initially identified as endothelium-derived relaxing factor (EDRF), is a gaso-transmitter with important regulatory roles in the cardiovascular, nervous and immune systems. In the former, this diatomic molecule and free radical gas controls vascular tone and cardiac mechanics, among others. In the cardiovascular system, it is now understood that β-adrenergic receptor (βAR) activation is a key modulator of NO generation. Therefore, it is not surprising that the up-regulation of G protein-coupled receptor kinases (GRKs), in particular GRK2, that restrains βAR activity contributes to impaired cardiovascular functions via alteration of NO bioavailability. This review, will explore the specific interrelation between βARs, GRK2 and NO in the cardiovascular system and their inter-relationship for the pathogenesis of the onset of disease. Last, we will update the readers on the current status of GRK2 inhibitors as a potential therapeutic strategy for heart failure with an emphasis on their ability of rescuing NO bioavailability.

Association Between Polymorphisms of ADRBK1 Gene and Plasma Renin Activity in Hypertensive Patients: A Case-Control Study

Background

Renin is the first step of the RAS cascade, which is a major regulator of salt-volume homeostasis. Adrenergic beta receptor kinase 1 (ADRBK1) plays important roles in regulating blood pressure via the epithelial Na⁺ channel (ENaC), which plays an important role in Na⁺ reabsorption in the renal collecting duct. The present case-control study was designed to investigate the potential relationship between polymorphisms of ADRBK1 and plasma renin activity (PRA) in hypertension.

Material/Methods

We recruited 1831 hypertensive and 422 normotensive Han Chinese subjects. Sitting PRA (ng/mL/h) was measured using radioimmunoassay method. Hypertensive patients were classified into 4 renin categories via PRA quartile. Single-nucleotide polymorphisms (SNPs) of the ADRBK1 gene (rs1894111, rs4930416, rs7127431, rs12286664, and rs3730147) were identified via TaqMan polymerase chain reaction.

Results

Comparison of the hypertensive group and the control group showed significant differences in distribution of genotypes and alleles of rs1894111 (P<0.05). Moreover, distribution of the dominant model (CC vs. CT+TT) in rs1894111 was lower in the hypertensive group than in the control group (P<0.05). Subjects were classified into 4 subgroups based on PRA quartile; the dominant model (CC vs. CT+TT) of rs1894111 was significantly lower in the quartile 1 group (the group with the lowest PRA) than in the control group (P<0.05). Logistic regression analysis demonstrated that the dominant model (CC vs. CT+TT) of rs1894111 was significantly different in the hypertensive group (OR=1.590, 95%CI=1.022–2.474, P<0.05), particularly in the quartile 1 group (OR=1.845, 95%CI=1.119–3.042, P<0.05), but not in the quartile 4 group.

Conclusions

The dominant model (CC vs. CT+TT) of rs1894111 polymorphism in the ADRBK1 gene might be associated with low-renin hypertension in Han Chinese.

G-protein-coupled receptor kinase 2 and endothelial dysfunction: molecular insights and pathophysiological mechanisms

Smooth muscle cells (SMC) and endothelial cells are the major cell types in blood vessels. The principal function of vascular SMC in the body is to regulate blood flow and pressure through contraction and relaxation. The endothelium performs a crucial role in maintaining vascular integrity by achieving whole-organ metabolic homeostasis via the production of factors associated with vasoconstriction or vasorelaxation. In this review, we have focused on the production of nitric oxide (NO), a vasorelaxation factor. The extent of NO production represents a key marker in vascular health. A decrease in NO is capable of inducing pathological conditions associated with endothelial dysfunction, such as obesity, diabetes, cardiovascular disease, and atherosclerosis. Recent studies have strongly implicated the involvement of G-protein-coupled receptor kinase 2 (GRK2) in the progression of cardiovascular disease. Vasculature which is affected by insulin resistance and type 2 diabetes expresses high levels of GRK2, which may induce endothelial dysfunction by reducing intracellular NO. GRK2 activation also induces changes in the subcellular localization of GRK2 itself and also of β-arrestin 2, a downstream protein. In this review, we describe the pathophysiological mechanisms of insulin resistance and diabetes, focusing on the signal transduction for NO production via GRK2 and β-arrestin 2, providing novel insights into the potential field of translational investigation in the treatment of diabetic complications.

The evolving impact of g protein-coupled receptor kinases in cardiac health and disease

G protein-coupled receptors (GPCRs) are important regulators of various cellular functions via activation of intracellular signaling events. Active GPCR signaling is shut down by GPCR kinases (GRKs) and subsequent β-arrestin-mediated mechanisms including phosphorylation, internalization, and either receptor degradation or resensitization. The seven-member GRK family varies in their structural composition, cellular localization, function, and mechanism of action (see sect. II). Here, we focus our attention on GRKs in particular canonical and novel roles of the GRKs found in the cardiovascular system (see sects. III and IV). Paramount to overall cardiac function is GPCR-mediated signaling provided by the adrenergic system. Overstimulation of the adrenergic system has been highly implicated in various etiologies of cardiovascular disease including hypertension and heart failure. GRKs acting downstream of heightened adrenergic signaling appear to be key players in cardiac homeostasis and disease progression, and herein we review the current data on GRKs related to cardiac disease and discuss their potential in the development of novel therapeutic strategies in cardiac diseases including heart failure.

GRK2 targeted knock-down results in spontaneous hypertension, and altered vascular GPCR signaling

Hypertension, elevated arterial pressure, occurs as the consequence of increased peripheral resistance. G protein-coupled receptors (GPCRs) contribute to the regulation of vasodilator and vasoconstrictor responses, and their activity is regulated by a family of GPCR kinases (GRKs). GRK2 expression is increased in hypertension and this facilitates the development of the hypertensive state by increasing the desensitization of GPCRs important for vasodilation. We demonstrate here, that genetic knockdown of GRK2 using a small hairpin (sh) RNA results in altered vascular reactivity and the development of hypertension between 8-12 weeks of age in shGRK2 mice due to enhanced Gαq/11 signaling. Vascular smooth muscle cells (VSMCs) cultured from shGRK2 knockdown mice show increases in GPCR-mediated Gαs and Gαq/11 signaling, as the consequence of reduced GRK2-mediated desensitization. In addition, agonists and biased agonists exhibited age-dependent alterations in ERK1/2 and Akt signaling, as well as cell proliferation and migration responses in shGRK2 knockdown VSMCs when cultured from mice that are either 3 months or 6 months of age. Changes in angiotensin II-stimulated ERK1/2 phosphorylation are observed in VSMCs derived from 6-week-old shGRK2 mice prior to the development of the hypertensive phenotype. Thus, our findings indicate that the balance between mechanisms regulating vascular tone are shifted to favor vasoconstriction in the absence of GRK2 expression and that this leads to the age-dependent development of hypertension, as a consequence of global alterations in GPCR signaling. Consequently, therapeutic strategies that target GRK2 activity, not expression, may be more effective for the treatment of hypertension.

Increased nitric oxide bioavailability in adult GRK2 hemizygous mice protects against angiotensin II-induced hypertension

G protein-coupled receptor kinase 2 (GRK2) is a ubiquitous serine/threonine protein kinase able to phosphorylate and desensitize the active form of several G protein-coupled receptors. Given the lack of selective inhibitors for GRK2, we investigated the effects elicited by GRK2 inhibition in vascular responses using global adult hemizygous mice (GRK2(+/-)). The vasodilator responses to acetylcholine or isoproterenol were increased in aortas and mesenteric resistance arteries from GRK2(+/-) mice compared with wild-type (WT) littermates. After angiotensin II (AngII) infusion, GRK2(+/-) mice were partially protected against hypertension, vascular remodeling, and mechanical alterations, even when resting basal blood pressures were not significantly different. AngII infusion also (1) increased GRK2 levels in WT but not in GRK2(+/-) vessels; (2) increased vasoconstrictor responses to phenylephrine in WT but not in GRK2(+/-) mice; and (3) decreased vasodilator responses to acetylcholine and vascular pAkt and eNOS levels more in WT than in GRK2(+/-) animals. Vascular NO production and the modulation of vasoconstrictor responses by endothelial-derived NO remained enhanced in GRK2(+/-) mice infused with AngII. Thus, GRK2(+/-) mice are resistant to the development of vascular remodeling and mechanical alterations, endothelial dysfunction, increased vasoconstrictor responses, and hypertension induced by AngII at least partially through the preservation of NO bioavailability. In conclusion, our results describe an important role for GRK2 in systemic hypertension and further establish that an inhibition of GRK2 could be a beneficial treatment for this condition.

Cardiac progenitor cells engineered with βARKct have enhanced β-adrenergic tolerance

Stem cell survival and retention in myocardium after injury following adoptive transfer is low. Elevated catecholamine levels coinciding with myocardial injury adversely affect cardiac progenitor cell (CPC) survival. The G protein-coupled receptor kinase 2 (GRK2)-derived inhibitory peptide, βARKct, enhance myocyte contractility, survival, and normalize the neurohormonal axis in failing heart, however salutary effects of βARKct on CPC survival and proliferation are unknown. Herein, we investigated whether the protective effects of βARKct expression seen in the failing heart relate to CPCs. Modified CPCs expressing βARKct enhanced AKT/eNOS signaling through protective β2-adrenergic receptors (β2-ARs). In addition, to the actions of βARKct expression on β2- AR signaling, pharmacologic inhibition of GRK2 also increased β2-AR signaling in nonengineered CPCs (lacking βARKct) but had limited effects in βARKct engineered CPCs providing evidence for the strength of the βARKct in inhibiting GRK2 in these cells. Increased proliferation and metabolic activity were observed in βARKct-engineered CPCs following catecholamine stimulation indicating improved adrenergic tolerance. βARKct modification of CPCs increased survival and proliferation following adoptive transfer in an acute myocardial infarction model concomitant with increased expression of β-AR. Thus, βARKct engineering of CPCs promotes survival and proliferation of injected cells following myocardial infarction, which includes improved β-adrenergic tolerance essential for stem cell survival.

Improvement of vascular insulin sensitivity by downregulation of GRK2 mediates exercise-induced alleviation of hypertension in spontaneously hypertensive rats

Exercise training lowers blood pressure and is a recommended nonpharmacological strategy and useful adjunctive therapy for hypertensive patients. Studies demonstrate that physical activity attenuates progression of hypertension. However, underlying mechanisms remain elusive. Vascular insulin resistance and endothelial dysfunction plays a critical role in the development of hypertension. The present study investigated whether long-term physical exercise starting during the prehypertensive period prevents the development of hypertension via improving vascular insulin sensitivity. Young (4 wk old) prehypertensive spontaneously hypertensive rats (SHRs) and their normotensive Wistar-Kyoto (WKY) control rats were subjected to a 10-wk free-of-loading swim training session (60 min/day, 5 days/wk). Blood pressure, mesenteric arteriolar vasorelaxation, G protein-coupled receptor kinase-2 (GRK2) expression and activity, and insulin-stimulated Akt/endothelial nitric oxide synthase (eNOS) activation were determined. SHRs had higher systolic blood pressure, systemic insulin resistance, and impaired vasodilator actions of insulin in resistance vessels when compared with WKY rats. Systolic blood pressure in SHRs postexercise was significantly lower than that in sedentary rats. Vascular insulin sensitivity in mesenteric arteries was improved after exercise training as evidenced by an increased vasodilator response to insulin. In addition, exercise downregulated vascular GRK2 expression and activity, which further increased insulin-stimulated vascular Akt/eNOS activation in exercised SHRs. Specific small interfering RNA knockdown of GRK2 in endothelium mimicked the effect of exercise-enhanced vascular insulin sensitivity. Likewise, upregulation of GRK2 by Chariot-mediated delivery opposed exercise-induced vascular insulin sensitization. Taken together, our results suggest that long-term exercise beginning at the prehypertensive stage improves vascular insulin sensitivity via downregulation of vascular GRK2 that may help to limit the progression of hypertension.

Increased circulating inflammatory endothelial cells in blacks with essential hypertension

Morbidity and mortality attributable to hypertension are higher in black essential hypertensive (EH) compared with white EH patients, possibly related to differential effects on vascular injury and repair. Although circulating endothelial progenitor cells (EPCs) preserve endothelial integrity, inflammatory endothelial cells (IECs) detach from sites of injury and represent markers of vascular damage. We hypothesized that blood levels of IECs and inflammatory markers would be higher in black EH compared with white EH patients. Inferior vena cava and renal vein levels of CD34+/KDR+ (EPC) and VAP-1+ (IEC) cells were measured by fluorescence-activated cell sorting in white EH and black EH patients under fixed sodium intake and blockade of the renin-angiotensin system, and compared with systemic levels in normotensive control subjects (n=19 each). Renal vein and inferior vena cava levels of inflammatory cytokines and EPC homing factors were measured by Luminex. Blood pressure, serum creatinine, lipids, and antihypertensive medications did not differ between white and black EH patients, and EPC levels were decreased in both. Circulating IEC levels were elevated in black EH patients, and inversely correlated with EPC levels (R(2)=0.58; P=0.0001). Systemic levels of inflammatory cytokines and EPC homing factors were higher in black EH compared with white EH patients, and correlated directly with IECs. Renal vein inflammatory cytokines, EPCs, and IECs did not differ from their circulating levels. Most IECs expressed endothelial markers, fewer expressed progenitor cell markers, but none showed lymphocyte or phagocytic cell markers. Thus, increased release of cytokines and IECs in black EH patients may impair EPC reparative capacity and aggravate vascular damage, and accelerate hypertension-related complications.

The role of G protein coupled receptor kinases in neurocardiovascular pathophysiology

In coronary artery disease the G protein related kinases (GRKs) play a role in desensitization of β-adrenoreceptors (AR) after coronary occlusion. Targeted deletion and lowering of cardiac myocyte GRK-2 decreases the risk of post-ischemic heart failure (HF). Studies carried out in humans confirm the role of GRK-2 as a marker for the progression of HF after myocardial infarction (MI). The level of GRK-2 could be an indicator of β-AR blocker efficacy in patients with acute coronary syndrome. Elevated levels of GRK-2 are an early ubiquitous consequence of myocardial injury. In hypertension an increased level of GRK-2 was reported in both animal models and human studies. The role of GRKs in vagally mediated disorders such as vasovagal syncope and atrial fibrillation remains controversial. The role of GRKs in the pathogenesis of neurocardiological diseases provides an insight into the molecular pathogenesis process, opens potential therapeutic options and suggests new directins for scientific research.

G protein-coupled receptor kinases in cardiovascular disease: why "where" matters

Cardiac function is mainly controlled by β-adrenergic receptors (β-ARs), members of the G protein-coupled receptor (GPCR) family. GPCR signaling and expression are tightly controlled by G protein-coupled receptor kinases (GRKs), which induce GPCR internalization and signal termination through phosphorylation. Reduced β-AR density and activity associated with elevated cardiac GRK expression and activity have been described in various cardiovascular diseases. Moreover, alterations in extracardiac GRKs have been observed in blood vessels, adrenal glands, kidneys, and fat cells. The broad tissue distribution of GPCRs and GRKs suggests that a keen appreciation of integrative physiology may drive future therapeutic development. In this review, we provide a brief summary of GRK isoforms, subcellular localization, and interacting partners that impinge directly or indirectly on the cardiovascular system. We also discuss GRK/GPCR interactions and their implications in cardiovascular pathophysiology.

G protein receptor kinase 4 polymorphisms: β-blocker pharmacogenetics and treatment-related outcomes in hypertension

G protein-coupled receptor kinases (GRKs) are important regulatory proteins for many G protein-coupled receptors, but little is known about GRK4 pharmacogenetics. We hypothesized that 3 nonsynonymous GRK4 single-nucleotide polymorphisms, R65L (rs2960306), A142V (rs1024323), and A486V (rs1801058), would be associated with blood pressure response to atenolol, but not hydrochlorothiazide, and would be associated with long-term cardiovascular outcomes (all-cause death, nonfatal myocardial infarction, nonfatal stroke) in participants treated with an atenolol-based versus verapamil-SR-based antihypertensive strategy. GRK4 single-nucleotide polymorphisms were genotyped in 768 hypertensive participants from the Pharmacogenomic Evaluation of Antihypertensive Responses (PEAR) trial. In whites and blacks, increasing copies of the variant 65L-142V haplotype were associated with significantly reduced atenolol-induced diastolic blood pressure lowering (-9.1±6.8 versus -6.8±7.1 versus -5.3±6.4 mm Hg in participants with 0, 1, and 2 copies of 65L-142V, respectively; P=0.0088). One thousand four hundred sixty participants with hypertension and coronary artery disease from the INternational VErapamil SR/Trandolapril STudy (INVEST) were genotyped, and variant alleles of all 3 GRK4 single-nucleotide polymorphisms were associated with increased risk for adverse cardiovascular outcomes in an additive fashion, with 486V homozygotes reaching statistical significance (odds ratio, 2.29 [1.48-3.55]; P=0.0002). These effects on adverse cardiovascular outcomes were independent of antihypertensive treatment. These results suggest that the presence of GRK4 variant alleles may be important determinants of blood pressure response to atenolol and risk for adverse cardiovascular events. The associations with GRK4 variant alleles were stronger in patients who were also ADRB1 389R homozygotes, suggesting a potential interaction between these 2 genes.

Abnormalities in renal dopamine signaling and hypertension: the role of GRK4

PURPOSE OF REVIEW

This review will highlight the recent findings concerning the role of the intrarenal dopaminergic system in hypertension, especially the role of alterations in G-protein receptor kinase 4 (GRK4) activity.

RECENT FINDINGS

Recent studies highlight the importance of the intrarenal dopaminergic system in blood pressure regulation and how defects in dopamine signaling are involved in the development of hypertension. There are recent experimental models that definitively demonstrate that abnormalities in intrarenal dopamine production or receptor signaling can predispose to salt-sensitive hypertension and a dysregulated renin-angiotensin system. Furthermore, studies in experimental animal models and in humans with salt-sensitive hypertension implicate abnormalities in dopamine receptor regulation because of receptor desensitization resulting from increased GRK4 activity. Functional polymorphisms that predispose to increased basal GRK4 activity both decrease dopamine receptor activity and increase angiotensin II AT1 receptor activity and are associated with essential hypertension in a number of different human cohorts.

SUMMARY

The ongoing elucidation of this important regulatory pathway further emphasizes the importance of the kidney in maintenance of blood pressure control and may help to delineate the underlying mechanisms predisposing individuals or populations to increased risk for development of hypertension.

GRK2 levels in umbilical arteries of pregnancies complicated by gestational hypertension and preeclampsia

BACKGROUND

G-Protein coupled receptor kinase 2 (GRK2) represents a regulator of cell function in different cardiovascular conditions, including high blood pressure. The relationship between elevated GRK2 levels and impaired vasorelaxant responses is causative of hypertension through the increase in vascular resistances. The aim of this study is to ascertain if this feature is present in the fetal placental vasculature of pregnancies complicated by hypertensive disorders.

METHODS

We have assessed GRK2 levels in the umbilical arteries (UA) of 21 preeclamptic or gestational hypertensive and 23 normotensive women at time of delivery.

RESULTS

GRK2 levels were increased in the hypertensive group (0.83 ± 0.14 vs. 0.48 ± 0.06 densitometry units; P < 0.05). GRK2 levels were in correlation and in linear regression with systolic, diastolic, and mean arterial pressure (P < 0.05, r(2) = 0.12, r(2) = 0.11, r(2) = 0.12). Correlations did not reach a significant value for other clinical parameters such as gestational age at birth, umbilical artery pulsatility index, maternal proteinuria, and neonatal birth weight. Out of the 21 hypertensive women, 7 who developed a preeclampsia associated with early preterm delivery (before 34 weeks) had a significantly lower GRK2 levels compared to the remaining 14 (0.51 ± 0.12 vs. 1.08 ± 0.20 densitometry units, P < 0.05).

CONCLUSIONS

We conclude that elevated GRK2 levels in the umbilical vasculature is correlated to elevated blood pressure levels, with a likely compensatory rather than causative role since the lack of protective effect of elevated GRK2 levels may negatively affect the outcome of the hypertensive state.

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.

G protein-coupled receptor kinases: more than just kinases and not only for GPCRs

G protein-coupled receptor (GPCR) kinases (GRKs) are best known for their role in homologous desensitization of GPCRs. GRKs phosphorylate activated receptors and promote high affinity binding of arrestins, which precludes G protein coupling. GRKs have a multidomain structure, with the kinase domain inserted into a loop of a regulator of G protein signaling homology domain. Unlike many other kinases, GRKs do not need to be phosphorylated in their activation loop to achieve an activated state. Instead, they are directly activated by docking with active GPCRs. In this manner they are able to selectively phosphorylate Ser/Thr residues on only the activated form of the receptor, unlike related kinases such as protein kinase A. GRKs also phosphorylate a variety of non-GPCR substrates and regulate several signaling pathways via direct interactions with other proteins in a phosphorylation-independent manner. Multiple GRK subtypes are present in virtually every animal cell, with the highest expression levels found in neurons, with their extensive and complex signal regulation. Insufficient or excessive GRK activity was implicated in a variety of human disorders, ranging from heart failure to depression to Parkinson's disease. As key regulators of GPCR-dependent and -independent signaling pathways, GRKs are emerging drug targets and promising molecular tools for therapy. Targeted modulation of expression and/or of activity of several GRK isoforms for therapeutic purposes was recently validated in cardiac disorders and Parkinson's disease.

A role for GRK2 in myocardial ischemic injury: indicators of a potential future therapy and diagnostic

Morbidity and mortality of myocardial infarction remains significant with resulting left ventricular function presenting as a major determinant of clinical outcome. Protecting the myocardium against ischemia reperfusion injury has become a major therapeutic goal and the identification of key signaling pathways has paved the way for various interventions, but until now with disappointing results. This article describes the recently discovered new role of G-protein-coupled receptor kinase-2 (GRK2), which is known to critically influence the development and progression of heart failure, in acute myocardial injury. This article focuses on potential applications of the GRK2 peptide inhibitor βARKct in ischemic myocardial injury, the use of GRK2 as a biomarker in acute myocardial infarction and discusses the challenges of translating GRK2 inhibition as a cardioprotective strategy to a possible future clinical application.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Taking the heart failure battle inside the cell: small molecule targeting of Gβγ subunits

Heart failure (HF) is devastating disease with poor prognosis. Elevated sympathetic nervous system activity and outflow, leading to pathologic attenuation and desensitization of β-adrenergic receptors (β-ARs) signaling and responsiveness, are salient characteristic of HF progression. These pathologic effects on β-AR signaling and HF progression occur in part due to Gβγ-mediated signaling, including recruitment of receptor desensitizing kinases such as G-protein coupled receptor (GPCR) kinase 2 (GRK2) and phosphoinositide 3-kinase (PI3K), which subsequently phosphorylate agonist occupied GPCRs. Additionally, chronic GPCR signaling signals chronically dissociated Gβγ subunits to interact with multiple effector molecules that activate various signaling cascades involved in HF pathophysiology. Importantly, targeting Gβγ signaling with large peptide inhibitors has proven a promising therapeutic paradigm in the treatment of HF. We recently described an approach to identify small molecule Gβγ inhibitors that selectively block particular Gβγ functions by specifically targeting a Gβγ protein-protein interaction "hot spot." Here we describe their effects on Gβγ downstream signaling pathways, including their role in HF pathophysiology. We suggest a promising therapeutic role for small molecule inhibition of pathologic Gβγ signaling in the treatment of HF. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

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.

betaARKct: a therapeutic approach for improved adrenergic signaling and function in heart disease

One of the most powerful regulators of cardiovascular function is catecholamine-stimulated adrenergic receptor (AR) signaling. The failing heart is characterized by desensitization and impaired beta-AR responsiveness as a result of upregulated G protein-coupled receptor kinase-2 (GRK2) present in injured myocardium. Deterioration of cardiac function is progressively enhanced by chronic adrenergic over-stimulation due to increased levels of circulating catecholamines. Increased GRK2 activity contributes to this pathological cycle of over-stimulation but lowered responsiveness. Over the past two decades the GRK2 inhibitory peptide betaARKct has been identified as a potential therapy that is able to break this vicious cycle of self-perpetuating deregulation of the beta-AR system and subsequent myocardial malfunction, thus halting development of cardiac failure. The betaARKct has been shown to interfere with GRK2 binding to the betagamma subunits of the heterotrimeric G protein, therefore inhibiting its recruitment to the plasma membrane that normally leads to phosphorylation and internalization of the receptor. In this article we summarize the current data on the therapeutic effects of betaARKct in cardiovascular disease and report on recent and ongoing studies that may pave the way for this peptide towards therapeutic application in heart failure and other states of cardiovascular disease.

Dopamine and G protein-coupled receptor kinase 4 in the kidney: role in blood pressure regulation

Complex interactions between genes and environment result in a sodium-induced elevation in blood pressure (salt sensitivity) and/or hypertension that lead to significant morbidity and mortality affecting up to 25% of the middle-aged adult population worldwide. Determining the etiology of genetic and/or environmentally-induced high blood pressure has been difficult because of the many interacting systems involved. Two main pathways have been implicated as principal determinants of blood pressure since they are located in the kidney (the key organ responsible for blood pressure regulation), and have profound effects on sodium balance: the dopaminergic and renin-angiotensin systems. These systems counteract or modulate each other, in concert with a host of intracellular second messenger pathways to regulate sodium and water balance. In particular, the G protein-coupled receptor kinase type 4 (GRK4) appears to play a key role in regulating dopaminergic-mediated natriuresis. Constitutively activated GRK4 gene variants (R65L, A142V, and A486V), by themselves or by their interaction with other genes involved in blood pressure regulation, are associated with essential hypertension and/or salt-sensitive hypertension in several ethnic groups. GRK4γ 142Vtransgenic mice are hypertensive on normal salt intake while GRK4γ 486V transgenic mice develop hypertension only with an increase in salt intake. GRK4 gene variants have been shown to hyperphosphorylate, desensitize, and internalize two members of the dopamine receptor family, the D(1) (D(1)R) and D(3) (D(3)R) dopamine receptors, but also increase the expression of a key receptor of the renin-angiotensin system, the angiotensin type 1 receptor (AT(1)R). Knowledge of the numerous blood pressure regulatory pathways involving angiotensin and dopamine may provide new therapeutic approaches to the pharmacological regulation of sodium excretion and ultimately blood pressure control.

Regulation of GPCR signaling in hypertension

Hypertension represents a complex, multifactorial disease and contributes to the major causes of morbidity and mortality in industrialized countries: ischemic and hypertensive heart disease, stroke, peripheral atherosclerosis and renal failure. Current pharmacological therapy of essential hypertension focuses on the regulation of vascular resistance by inhibition of hormones such as catecholamines and angiotensin II, blocking them from receptor activation. Interaction of G-protein coupled receptor kinases (GRKs) and regulator of G-protein signaling (RGS) proteins with activated G-protein coupled receptors (GPCRs) effect the phosphorylation state of the receptor leading to desensitization and can profoundly impair signaling. Defects in GPCR regulation via these modulators have severe consequences affecting GPCR-stimulated biological responses in pathological situations such as hypertension, since they fine-tune and balance the major transmitters of vessel constriction versus dilatation, thus representing valuable new targets for anti-hypertensive therapeutic strategies. Elevated levels of GRKs are associated with human hypertensive disease and are relevant modulators of blood pressure in animal models of hypertension. This implies therapeutic perspective in a disease that has a prevalence of 65million in the United States while being directly correlated with occurrence of major adverse cardiac and vascular events. Therefore, therapeutic approaches using the inhibition of GRKs to regulate GPCRs are intriguing novel targets for treatment of hypertension and heart failure.