G-protein-coupled receptor kinase activity is increased in hypertension

GRK2-YAP signaling is implicated in pulmonary arterial hypertension development

BACKGROUND

Pulmonary arterial hypertension (PAH) is characterized by excessive proliferation of small pulmonary arterial vascular smooth muscle cells (PASMCs), endothelial dysfunction, and extracellular matrix remodeling. G protein-coupled receptor kinase 2 (GRK2) plays an important role in the maintenance of vascular tone and blood flow. However, the role of GRK2 in the pathogenesis of PAH is unknown.

METHODS

GRK2 levels were detected in lung tissues from healthy people and PAH patients. C57BL/6 mice, vascular smooth muscle cell-specific Grk2 -knockout mice ( Grk2ΔSM22 ), and littermate controls ( Grk2flox/flox ) were grouped into control and hypoxia mice ( n  = 8). Pulmonary hypertension (PH) was induced by exposure to chronic hypoxia (10%) combined with injection of the SU5416 (cHx/SU). The expression levels of GRK2 and Yes-associated protein (YAP) in pulmonary arteries and PASMCs were detected by Western blotting and immunofluorescence staining. The mRNA expression levels of Grk2 and Yes-associated protein ( YAP ) in PASMCs were quantified with real-time polymerase chain reaction (RT-PCR). Wound-healing assay, 3-(4,5)-dimethylthiahiazo (-z-y1)-3,5-di-phenytetrazoliumromide (MTT) assay, and 5-Ethynyl-2'-deoxyuridine (EdU) staining were performed to evaluate the proliferation and migration of PASMCs. Meanwhile, the interaction among proteins was detected by immunoprecipitation assays.

RESULTS

The expression levels of GRK2 were upregulated in the pulmonary arteries of patients with PAH and the lungs of PH mice. Moreover, cHx/SU-induced PH was attenuated in Grk2ΔSM22 mice compared with littermate controls. The amelioration of PH in Grk2ΔSM22 mice was accompanied by reduced pulmonary vascular remodeling. In vitro study further confirmed that GRK2 knock-down significantly altered hypoxia-induced PASMCs proliferation and migration, whereas this effect was severely intensified by overexpression of GRK2 . We also identified that GRK2 promoted YAP expression and nuclear translocation in PASMCs, resulting in excessive PASMCs proliferation and migration. Furthermore, GRK2 is stabilized by inhibiting phosphorylating GRK2 on Tyr86 and subsequently activating ubiquitylation under hypoxic conditions.

CONCLUSION

Our findings suggest that GRK2 plays a critical role in the pathogenesis of PAH, via regulating YAP expression and nuclear translocation. Therefore, GRK2 serves as a novel therapeutic target for PAH treatment.

GRK2 expression and catalytic activity are essential for vasoconstrictor/ERK-stimulated arterial smooth muscle proliferation

Prolonged vasoconstrictor signalling found in hypertension, increases arterial contraction, and alters vessel architecture by stimulating arterial smooth muscle cell (ASMC) growth, underpinning the development of re-stenosis lesions and vascular remodelling. Vasoconstrictors interact with their cognate G protein coupled receptors activating a variety of signalling pathways to promote smooth muscle proliferation. Here, angiotensin II (AngII) and endothelin 1 (ET1), but not UTP stimulates ASMC proliferation. Moreover, siRNA-mediated depletion of endogenous GRK2 expression, or GRK2 inhibitors, compound 101 or paroxetine, prevented AngII and ET1-promoted ASMC growth. Depletion of GRK2 expression or inhibition of GRK2 activity ablated the prolonged phase of AngII and ET-stimulated ERK signalling, while enhancing and prolonging UTP-stimulated ERK signalling. Increased GRK2 expression enhanced and prolonged AngII and ET1-stimulated ERK signalling, but suppressed UTP-stimulated ERK signalling. In ASMC prepared from 6-week-old WKY and SHR, AngII and ET1-stimulated proliferation rates were similar, however, in cultures prepared from 12-week-old rats AngII and ET1-stimulated growth was enhanced in SHR-derived ASMC, which was reversed following depletion of GRK2 expression. Furthermore, in ASMC cultures isolated from 6-week-old WKY and SHR rats, AngII and ET1-stimulated ERK signals were similar, while in cultures from 12-week-old rats ERK signals were both enhanced and prolonged in SHR-derived ASMC, and were reversed to those seen in age-matched WKY-derived ASMC following pre-treatment of SHR-derived ASMC with compound 101. These data indicate that the presence of GRK2 and its catalytic activity are essential to enable pro-proliferative vasoconstrictors to promote growth via recruitment and activation of the ERK signalling pathway in ASMC.

G protein-coupled receptor kinase 2 is essential to enable vasoconstrictor-mediated arterial smooth muscle proliferation

Hypertension is associated with increased production and circulation of vasoconstrictors, resulting in enhanced signalling through their cognate G protein-coupled receptors (GPCR). Prolonged vasoconstrictor GPCR signalling increases arterial contraction and stimulates signalling pathways that promote vascular smooth muscle cell (VSMC) proliferation, contributing to the development of atherosclerotic plaques, re-stenosis lesions and vascular remodelling. GPCR signalling through phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt) promotes VSMC proliferation. In VSMC, G protein-coupled receptor kinase 2 (GRK2) is known to regulate numerous vasoconstrictor GPCRs and their downstream signalling pathways. As GRK2 is implicated in controlling various aspects of cellular growth, we examined whether GRK2 could affect VSMC proliferation. Using two indices of cell growth, we show that PI3K inhibition and depletion of GRK2 expression produced a similar ablation of pro-proliferative vasoconstrictor-stimulated VSMC growth. Furthermore, GRK2-knockdown ablated the sustained phase of endothelin-1 and angiotensin-II-stimulated Akt phosphorylation, whilst the peak (5 min) phase was unaffected. Conversely, the GRK2 inhibitor compound 101 did not affect vasoconstrictor-driven Akt phosphorylation. Vasoconstrictor-stimulated phosphorylation of the Akt substrates GSK3α and GSK3β was ablated following RNAi-mediated GRK2 depletion, or after PI3K inhibition. Moreover, GRK2 knockdown prevented endothelin-1 and angiotensin-II from increasing cyclin D1 expression. These data suggest GRK2 expression is essential to facilitate vasoconstrictor-driven VSMC proliferation through its ability to promote efficient prolonged PI3K-Akt signalling, and thus relieve the GSK3-mediated block on cell cycling. Considering VSMC GRK2 expression increases early in the development of hypertension, this highlights the potential for GRK2 to promote VSMC growth and exacerbate hypertensive pathophysiological vascular remodelling.

The GRKs Reactome: Role in Cell Biology and Pathology

G protein-coupled receptor kinases (GRKs) are protein kinases that function in concert with arrestins in the regulation of a diverse class of G protein-coupled receptors (GPCRs) signaling. Although GRKs and arrestins are key participants in the regulation of GPCR cascades, the complex regulatory mechanisms of GRK expression, its alternation, and their function are not thoroughly understood. Several studies together with the work from our lab in recent years have revealed the critical role of these kinases in various physiological and pathophysiological processes, including cardiovascular biology, inflammation and immunity, neurodegeneration, thrombosis, and hemostasis. A comprehensive understanding of the mechanisms underlying functional interactions with multiple receptor proteins and how these interactions take part in the development of various pathobiological processes may give rise to novel diagnostic and therapeutic strategies. In this review, we summarize the current research linking the role of GRKs to various aspects of cell biology, pathology, and therapeutics, with a particular focus on thrombosis and hemostasis.

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.

The Metabolic Role of GRK2 in Insulin Resistance and Associated Conditions

Insulin resistance (IRES) is a pathophysiological condition characterized by the reduced response to insulin of several tissues, including myocardial and skeletal muscle. IRES is associated with obesity, glucose intolerance, dyslipidemia, and hypertension, evolves toward type 2 diabetes, and increases the risk of developing cardiovascular diseases. Several studies designed to explore the mechanisms involved in IRES allowed the identification of a multitude of potential molecular targets. Among the most promising, G Protein Coupled Receptor Kinase type 2 (GRK2) appears to be a suitable one given its functional implications in many cellular processes. In this review, we will discuss the metabolic role of GRK2 in those conditions that are characterized by insulin resistance (diabetes, hypertension, heart failure), and the potentiality of its inhibition as a therapeutic strategy to revert both insulin resistance and its associated phenotypes.

Role of adrenergic receptor signalling in neuroimmune communication

Neuroimmune communication plays a crucial role in maintaining homeostasis and promptly responding to any foreign insults. Sympathetic nerve fibres are innervated into all the lymphoid organs (bone marrow, thymus, spleen, and lymph nodes) and provide a communication link between the central nervous system (CNS) and ongoing immune response in the tissue microenvironment. Neurotransmitters such as catecholamines (epinephrine and norepinephrine) bind to adrenergic receptors present on most immune and non-immune cells, establish a local neuroimmune-communication system, and help regulate the ongoing immune response. The activation of these receptors varies with the type of receptor-activated, target cell, the activation status of the cells, and timing of activation. Activating adrenergic receptors, specifically β-adrenergic signalling in immune cells leads to activation of the cAMP-PKA pathway or other non-canonical pathways. It predominantly leads to immune suppression such as inhibition of IL-2 secretion and a decrease in macrophages phagocytosis. This review discusses the expression of different adrenergic receptors in various immune cells, signalling, and how it modulates immune cell function and contributes to health and diseases. Understanding the neuroimmune communication through adrenergic receptor signalling in immune cells could help to design better strategies to control inflammation and autoimmunity.

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Primary and secondary lymphoid organs are innervated with sympathetic nerve fibres.

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Adrenergic receptor expression on immune and non-immune cells establishes a local neuroimmune communication system.

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Adrenergic receptor signalling in immune cells controls the differentiation and function of various immune cells.

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Modulating adrenergic receptor signalling with a specific agonist or antagonist also affect the immune response.

Antidiabetic and Cardioprotective Effects of Pharmacological Inhibition of GRK2 in db/db Mice

Despite the availability of several therapies for the management of blood glucose in diabetic patients, most of the treatments do not show benefits on diabetic cardiomyopathy, while others even favor the progression of the disease. New pharmacological targets are needed that might help the management of diabetes and its cardiovascular complications at the same time. GRK2 appears a promising target, given its established role in insulin resistance and in systolic heart failure. Using a custom peptide inhibitor of GRK2, we assessed in vitro in L6 myoblasts the effects of GRK2 inhibition on glucose extraction and insulin signaling. Afterwards, we treated diabetic male mice (db/db) for 2 weeks. Glucose tolerance (IGTT) and insulin sensitivity (ITT) were ameliorated, as was skeletal muscle glucose uptake and insulin signaling. In the heart, at the same time, the GRK2 inhibitor ameliorated inflammatory and cytokine responses, reduced oxidative stress, and corrected patterns of fetal gene expression, typical of diabetic cardiomyopathy. GRK2 inhibition represents a promising therapeutic target for diabetes and its cardiovascular complications.

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.

Therapeutic Targets for Treatment of Heart Failure: Focus on GRKs and β-Arrestins Affecting βAR Signaling

Heart failure (HF) is a heart disease that is classified into two main types: HF with reduced ejection fraction (HFrEF) and HF with preserved ejection fraction (HFpEF). Both types of HF lead to significant risk of mortality and morbidity. Pharmacological treatment with β-adrenergic receptor (βAR) antagonists (also called β-blockers) has been shown to reduce the overall hospitalization and mortality rates and improve the clinical outcomes in HF patients with HFrEF but not HFpEF. Although, the survival rate of patients suffering from HF continues to drop, the management of HF still faces several limitations and discrepancies highlighting the need to develop new treatment strategies. Overstimulation of the sympathetic nervous system is an adaptive neurohormonal response to acute myocardial injury and heart damage, whereas prolonged exposure to catecholamines causes defects in βAR regulation, including a reduction in the amount of βARs and an increase in βAR desensitization due to the upregulation of G protein-coupled receptor kinases (GRKs) in the heart, contributing in turn to the progression of HF. Several studies show that myocardial GRK2 activity and expression are raised in the failing heart. Furthermore, β-arrestins play a pivotal role in βAR desensitization and, interestingly, can mediate their own signal transduction without any G protein-dependent pathway involved. In this review, we provide new insight into the role of GRKs and β-arrestins on how they affect βAR signaling regarding the molecular and cellular pathophysiology of HF. Additionally, we discuss the therapeutic potential of targeting GRKs and β-arrestins for the treatment of HF.

G-protein-coupled receptor kinase-5 promotes glioblastoma progression by targeting the nuclear factor kappa B pathway

G-protein-coupled receptor kinase-5 (GRK5) plays essential roles in multiple celluar events. However, its role in the development and progression of glioma is poorly understood. In this research, we found that GRK5 is significantly upregulated in human gliomas. For the first time, a close relationship was noted between GRK5 expression and blood vessel development in human glioma. Specifically co-expression of GRK5 and the tumor stem cell marker CD133 was observed in the cytoplasm of high grade glioma cells. The depletion of GRK5 suppressed the proliferation, migration and invasion in glioma cells, and promoted apoptosis. We next discovered that GRK5 knockdown inhibits the nuclear factor kappa B (NF-κB) pathway, thus resulting in downregulation of key downstream secretory products CCL2, IL-6 and IL-8 in glioma cell conditioned medium (CM). In addition, treatment of cells with the NF-κB stimulator PMA reversed this effect and increased the GRK5 level. Our results demonstrate an oncogenic role for GRK5 and reveal an activation of the GRK5-NF-κB pathway during the malignant progression of glioma.

GRK2 knockdown in mice exacerbates kidney injury and alters renal mechanisms of blood pressure regulation

The renin-angiotensin system regulates blood pressure and fluid balance in the body primarily via angiotensin receptor 1 (AT1R). Renal AT1R was found to be primarily responsible for Ang II-mediated hypertension. G protein-coupled receptor kinase 2 (GRK2) modulates AT1R desensitization and increased GRK2 protein expression is reported in hypertensive patients. However, the consequences of GRK2 inhibition on kidney functions remain unknown. We employed shGRK2 knockdown mice (shGRK2 mice) to test the role of GRK2 in kidney development and function that can be ultimately linked to the hypertensive phenotype detected in shGRK2 mice. GRK2 knockdown reduced kidney size, nephrogenesis and glomerular count, and impaired glomerular filtration. Glomerular damage in adult shGRK2 mice was associated with increased renin- and AT1R-mediated production of reactive oxygen species. The AT1R blocker, Losartan, normalized elevated blood pressure and markedly improved glomerular filtration in the shGRK2 knockdown mice. Our findings provide evidence for the crucial role of GRK2 in renal regulation of blood pressure. It also suggests that the detrimental outcomes of GRK2 inhibitors on the kidney should be carefully examined when used as antihypertensive.

Inactivation of MAPK in epididymal fat and amelioration of triglyceride secretion by injection of GRK2 siRNA in ob/ob mice

Abnormal G protein-coupled receptor kinase 2 (GRK2) accumulation has a crucial role in the development of insulin resistance and diabetes. Although GRK2 siRNA transfection in the liver improves insulin resistance-related vascular complications, the effects of GRK2 siRNA in lipid metabolism and obesity remain unknown. To investigate how GRK2 siRNA affects obesity, ob/ob mice were transfected with GRK2 siRNA, mainly in the liver, by using a hydrodynamic-based procedure. Epididymal fat, glucose, triglyceride, non-esterified fatty acid (NEFA), and alanine transaminase activity were higher in the control siRNA-transfected ob/ob mice than in the control siRNA-transfected Lean mice, but these parameters were reduced by GRK2 siRNA transfection into the ob/ob mice. GRK2 expression in epididymal fat was not altered among the 3 groups, although hepatic GRK2 expression was higher in the control siRNA-transfected ob/ob mice than in the control siRNA-transfected Lean mice. Additionally, we found that Akt interacted with GRK2 in the liver. Furthermore, phosphorylation levels of ERK1/2 and JNK were higher in the epididymal fats from the control siRNA-transfected ob/ob mice than in those from the control siRNA-transfected Lean mice, but they were lowered by transfection with GRK2 siRNA. The study results showed that GRK2 siRNA improved blood triglyceride levels and abnormal or excessive activity of mitogen-activated protein kinases in epididymal fat. This effect may be promoted by inhibition of the NEFA production pathway in the liver. Therefore, the interaction of organs (hepatic GRK2-epididymal fat) may help improve insulin resistance and diabetes-associated pathophysiology.

GRK5 functions as an oncogenic factor in non-small-cell lung cancer

Lung cancer is the leading cause of cancer-related deaths worldwide, and non-small-cell lung cancer (NSCLC) accounts for about 80% of all cases, which is the major subgroup of lung cancer. G protein-coupled receptor kinase 5 (GRK5) has been demonstrated to play pivotal roles in both development and progression of several pathological conditions including cancer. Here, we found that GRK5 expression was significantly increased in 539 NSCLC cancerous tissues than that in 99 normal non-cancerous tissues by immunohistochemistry analysis; we also showed intensive higher positive staining percentage in female and adenocarcinoma (ADC) NSCLC patients than that in male and squamous cell carcinoma (SCC) patients, respectively. In addition, GRK5 high expression NSCLC patients had a worse overall survival rate than the low expression patients. We provided evidence showing that both the mRNA and protein expression levels of GRK5 were increased in NSCLC cancerous cell lines (GLC-82, SPC-A-1, H520, H838, H358, A549, and H1299) comparing with that in normal human bronchial epithelium cell line (BEAS-2B), and identified many GRK5 mutations in NSCLC cancerous tissues. In addition, we found that depletion of GRK5 inhibited NSCLC cancerous cell proliferation, migration in vitro, and xenograft tumor formation in vivo. Furthermore, GRK5 knockdown promoted cell cycle arrest at G2/M phase and induced cellular apoptosis. In summary, our data reveal an oncogenic role of GRK5 in NSCLC progression, indicating that GRK5 could be used as a new therapeutic target in future.

Opsin 3 and 4 mediate light-induced pulmonary vasorelaxation that is potentiated by G protein-coupled receptor kinase 2 inhibition

We recently demonstrated that blue light induces vasorelaxation in the systemic mouse circulation, a phenomenon mediated by the nonvisual G protein-coupled receptor melanopsin (Opsin 4; Opn4). Here we tested the hypothesis that nonvisual opsins mediate photorelaxation in the pulmonary circulation. We discovered Opsin 3 (Opn3), Opn4, and G protein-coupled receptor kinase 2 (GRK2) in rat pulmonary arteries (PAs) and in pulmonary arterial smooth muscle cells (PASMCs), where the opsins interact directly with GRK2, as demonstrated with a proximity ligation assay. Light elicited an intensity-dependent relaxation of PAs preconstricted with phenylephrine (PE), with a maximum response between 400 and 460 nm (blue light). Wavelength-specific photorelaxation was attenuated in PAs from Opn4^-/- mice and further reduced following shRNA-mediated knockdown of Opn3. Inhibition of GRK2 amplified the response and prevented physiological desensitization to repeated light exposure. Blue light also prevented PE-induced constriction in isolated PAs, decreased basal tone, ablated PE-induced single-cell contraction of PASMCs, and reversed PE-induced depolarization in PASMCs when GRK2 was inhibited. The photorelaxation response was modulated by soluble guanylyl cyclase but not by protein kinase G or nitric oxide. Most importantly, blue light induced significant vasorelaxation of PAs from rats with chronic pulmonary hypertension and effectively lowered pulmonary arterial pressure in isolated intact perfused rat lungs subjected to acute hypoxia. These findings show that functional Opn3 and Opn4 in PAs represent an endogenous "optogenetic system" that mediates photorelaxation in the pulmonary vasculature. Phototherapy in conjunction with GRK2 inhibition could therefore provide an alternative treatment strategy for pulmonary vasoconstrictive disorders.

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.

Molecular Mechanisms Linking Autonomic Dysfunction and Impaired Cardiac Contractility in Critical Illness

OBJECTIVES

Molecular mechanisms linking autonomic dysfunction with poorer clinical outcomes in critical illness remain unclear. We hypothesized that baroreflex dysfunction alone is sufficient to cause cardiac impairment through neurohormonal activation of (nicotinamide adenine dinucleotide phosphate oxidase dependent) oxidative stress resulting in increased expression of G-protein-coupled receptor kinase 2, a key negative regulator of cardiac function.

DESIGN

Laboratory/clinical investigations.

SETTING

University laboratory/medical centers.

SUBJECTS

Adult rats; wild-type/nicotinamide adenine dinucleotide phosphate oxidase subunit-2-deficient mice; elective surgical patients.

INTERVENTIONS

Cardiac performance was assessed by transthoracic echocardiography following experimental baroreflex dysfunction (sino-aortic denervation) in rats and mice. Immunoblots assessed G-protein-coupled receptor recycling proteins expression in rodent cardiomyocytes and patient mononuclear leukocytes. In surgical patients, heart rate recovery after cardiopulmonary exercise testing, time/frequency measures of parasympathetic variables were related to the presence/absence of baroreflex dysfunction (defined by spontaneous baroreflex sensitivity of <6 ms mm Hg). The associations of baroreflex dysfunction with intraoperative cardiac function and outcomes were assessed.

MEASUREMENTS AND MAIN RESULTS

Experimental baroreflex dysfunction in rats and mice resulted in impaired cardiac contractility and upregulation of G-protein-coupled receptor kinase 2 expression. In mice, genetic deficiency of gp91 nicotinamide adenine dinucleotide phosphate oxidase subunit-2 prevented upregulation of G-protein-coupled receptor kinase 2 expression in conditions of baroreflex dysfunction and preserved cardiac function. Baroreflex dysfunction was present in 81 of 249 patients (32.5%) and was characterized by lower parasympathetic tone and increased G-protein-coupled receptor kinase 2 expression in mononuclear leukocytes. Baroreflex dysfunction in patients was also associated with impaired intraoperative cardiac contractility. Critical illness and mortality were more frequent in surgical patients with baroreflex dysfunction (relative risk, 1.66 [95% CI, 1.16-2.39]; p = 0.006).

CONCLUSIONS

Reduced baroreflex sensitivity is associated with nicotinamide adenine dinucleotide phosphate oxidase subunit-2-mediated upregulation of G-protein-coupled receptor kinase 2 expression in cardiomyocytes and impaired cardiac contractility. Autonomic dysfunction predisposes patients to the development of critical illness and increases mortality.

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.

Dual role of GRK5 in cancer development and progression

GRK5 is a multifunctional protein that is able to move within the cell in response to various stimuli to regulate key intracellular signaling from receptor activation, on plasmamembrane, to gene transcription, in the nucleus. Thus, GRK5 is involved in the development and progression of several pathological conditions including cancer. Several reports underline the involvement of GRK5 in the regulation of tumor growth even if they appear controversial. Indeed, depending on its subcellular localization and on the type of cancer, GRK5 is able to both inhibit cancer progression, through the desensitization of GPCR and non GPCR-receptors (TSH, PGE2R, PDGFR), and induce tumor growth, acting on non-receptor substrates (p53, AUKA and NPM1). All these findings suggest that targeting GRK5 could be an useful anti-cancer strategy, for specific tumor types. In this review, we will discuss the different effects of this kinase in the induction and progression of tumorigenesis, the molecular mechanisms by which GRK5 exerts its effects, and the potential therapeutic strategies to modulate them.

SAR study and conformational analysis of a series of novel peptide G protein-coupled receptor kinase 2 inhibitors

G protein-coupled receptor kinase 2 (GRK2) plays a central role in the cellular transduction network. In particular, during chronic heart failure GRK2 is upregulated and believed to contribute to disease progression. Thereby, its inhibition offers a potential therapeutic solution to several pathological conditions. In the present study, we performed a SAR study and a NMR conformational analysis of peptides derived from HJ loop of GRK2 and able to selectively inhibit GRK2. From Ala-scan and D-Ala point replacement, we found that Arg residues don't affect the inhibitory properties, while a D-amino acid at position 5 is key to the activity. Conformational analysis identified two β-turns that involve N-terminal residues, followed by a short extended region. These information can help the design of peptides and peptido-mimetics with enhanced GRK2 inhibition properties.

Atomic Structure of GRK5 Reveals Distinct Structural Features Novel for G Protein-coupled Receptor Kinases

G protein-coupled receptor kinases (GRKs) are members of the protein kinase A, G, and C families (AGC) and play a central role in mediating G protein-coupled receptor phosphorylation and desensitization. One member of the family, GRK5, has been implicated in several human pathologies, including heart failure, hypertension, cancer, diabetes, and Alzheimer disease. To gain mechanistic insight into GRK5 function, we determined a crystal structure of full-length human GRK5 at 1.8 Å resolution. GRK5 in complex with the ATP analog 5'-adenylyl β,γ-imidodiphosphate or the nucleoside sangivamycin crystallized as a monomer. The C-terminal tail (C-tail) of AGC kinase domains is a highly conserved feature that is divided into three segments as follows: the C-lobe tether, the active-site tether (AST), and the N-lobe tether (NLT). This domain is fully resolved in GRK5 and reveals novel interactions with the nucleotide and N-lobe. Similar to other AGC kinases, the GRK5 AST is an integral part of the nucleotide-binding pocket, a feature not observed in other GRKs. The AST also mediates contact between the kinase N- and C-lobes facilitating closure of the kinase domain. The GRK5 NLT is largely displaced from its previously observed position in other GRKs. Moreover, although the autophosphorylation sites in the NLT are >20 Å away from the catalytic cleft, they are capable of rapid cis-autophosphorylation suggesting high mobility of this region. In summary, we provide a snapshot of GRK5 in a partially closed state, where structural elements of the kinase domain C-tail are aligned to form novel interactions to the nucleotide and N-lobe not previously observed in other GRKs.

Defining the roles of arrestin2 and arrestin3 in vasoconstrictor receptor desensitization in hypertension

Prolonged vasoconstrictor-stimulated phospholipase C activity can induce arterial constriction, hypertension, and smooth muscle hypertrophy/hyperplasia. Arrestin proteins are recruited by agonist-occupied G protein-coupled receptors to terminate signaling and counteract changes in vascular tone. Here we determine whether the development of hypertension affects arrestin expression in resistance arteries and how such changes alter arterial contractile signaling and function. Arrestin2/3 expression was increased in mesenteric arteries of 12-wk-old spontaneously hypertensive rats (SHR) compared with normotensive Wistar-Kyoto (WKY) controls, while no differences in arrestin expression were observed between 6-wk-old SHR and WKY animals. In mesenteric artery myography experiments, high extracellular K(+)-stimulated contractions were increased in both 6- and 12-wk-old SHR animals. Concentration-response experiments for uridine 5'-triphosphate (UTP) acting through P2Y receptors displayed a leftward shift in 12-wk, but not 6-wk-old animals. Desensitization of UTP-stimulated vessel contractions was increased in 12-wk-old (but not 6-wk-old) SHR animals. Dual IP3/Ca(2+) imaging in mesenteric arterial cells showed that desensitization of UTP and endothelin-1 (ET1) responses was enhanced in 12-wk-old (but not 6-wk-old) SHR compared with WKY rats. siRNA-mediated depletion of arrestin2 for UTP and arrestin3 for ET1, reversed the desensitization of PLC signaling. In conclusion, arrestin2 and 3 expression is elevated in resistance arteries during the emergence of the early hypertensive phenotype, which underlies an enhanced ability to desensitize vasoconstrictor signaling and vessel contraction. Such regulatory changes may act to compensate for increased vasoconstrictor-induced vessel contraction.

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.

Molecular mechanisms underlying β-adrenergic receptor-mediated cross-talk between sympathetic neurons and immune cells

Cross-talk between the sympathetic nervous system (SNS) and immune system is vital for health and well-being. Infection, tissue injury and inflammation raise firing rates of sympathetic nerves, increasing their release of norepinephrine (NE) in lymphoid organs and tissues. NE stimulation of β2-adrenergic receptors (ARs) in immune cells activates the cAMP-protein kinase A (PKA) intracellular signaling pathway, a pathway that interfaces with other signaling pathways that regulate proliferation, differentiation, maturation and effector functions in immune cells. Immune-SNS cross-talk is required to maintain homeostasis under normal conditions, to develop an immune response of appropriate magnitude after injury or immune challenge, and subsequently restore homeostasis. Typically, β2-AR-induced cAMP is immunosuppressive. However, many studies report actions of β2-AR stimulation in immune cells that are inconsistent with typical cAMP-PKA signal transduction. Research during the last decade in non-immune organs, has unveiled novel alternative signaling mechanisms induced by β2-AR activation, such as a signaling switch from cAMP-PKA to mitogen-activated protein kinase (MAPK) pathways. If alternative signaling occurs in immune cells, it may explain inconsistent findings of sympathetic regulation of immune function. Here, we review β2-AR signaling, assess the available evidence for alternative signaling in immune cells, and provide insight into the circumstances necessary for "signal switching" in immune cells.

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.

Biased β2-adrenoceptor signalling in heart failure: pathophysiology and drug discovery

The body is constantly faced with a dynamic requirement for blood flow. The heart is able to respond to these changing needs by adjusting cardiac output based on cues emitted by circulating catecholamine levels. Cardiac β-adrenoceptors transduce the signal produced by catecholamine stimulation via Gs proteins to their downstream effectors to increase heart contractility. During heart failure, cardiac output is insufficient to meet the needs of the body; catecholamine levels are high and β-adrenoceptors become hyperstimulated. The hyperstimulated β1-adrenoceptors induce a cardiotoxic effect, which could be counteracted by the cardioprotective effect of β2-adrenoceptor-mediated Gi signalling. However, β2-adrenoceptor-Gi signalling negates the stimulatory effect of the Gs signalling on cardiomyocyte contraction and further exacerbates cardiodepression. Here, further to the localization of β1- and β2-adrenoceptors and β2-adrenoceptor-mediated β-arrestin signalling in cardiomyocytes, we discuss features of the dysregulation of β-adrenoceptor subtype signalling in the failing heart, and conclude that Gi-biased β2-adrenoceptor signalling is a pathogenic pathway in heart failure that plays a crucial role in cardiac remodelling. In contrast, β2-adrenoceptor-Gs signalling increases cardiomyocyte contractility without causing cardiotoxicity. Finally, we discuss a novel therapeutic approach for heart failure using a Gs-biased β2-adrenoceptor agonist and a β1-adrenoceptor antagonist in combination. This combination treatment normalizes the β-adrenoceptor subtype signalling in the failing heart and produces therapeutic effects that outperform traditional heart failure therapies in animal models. The present review illustrates how the concept of biased signalling can be applied to increase our understanding of the pathophysiology of diseases and in the development of novel therapies.

Prenatal lipopolysaccharide exposure results in dysfunction of the renal dopamine D1 receptor in offspring

Adverse environment in early life can modulate the adult phenotype, including blood pressure. Lipopolysaccharide (LPS) exposure in utero results in increased blood pressure in the offspring, but the exact mechanisms are not clear. Studies have shown that the renal dopamine D1 receptor (D1R) plays an important role in maintaining sodium homeostasis and normal blood pressure; dysfunction of D1R is associated with oxidative stress and hypertension. In this study, we determined if dysfunction of the renal D1R is involved in fetal-programmed hypertension, and if oxidative stress contributes to this process. Pregnant Sprague-Dawley (SD) rats were intraperitoneally injected with LPS (0.79 mg/kg) or saline at gestation days 8, 10, and 12. As compared with saline-injected (control) dams, offspring of LPS-treated dams had increased blood pressure, decreased renal sodium excretion, and increased markers of oxidative stress. In addition, offspring of LPS-treated dams had decreased renal D1R expression, increased D1R phosphorylation, and G protein-coupled receptor kinase type 2 (GRK2) and type 4 (GRK4) protein expression, and impaired D1R-mediated natriuresis and diuresis. All of the findings in the offspring of LPS-treated dams were normalized after treatment with TEMPOL, an oxygen free radical scavenger. In conclusion, prenatal LPS exposure, via an increase in oxidative stress, impairs renal D1R function and leads to hypertension in the offspring. Normalization of renal D1R function by amelioration of oxidative stress may be a therapeutic target of fetal programming of hypertension.

β-adrenoceptors as molecular targets in the treatment of hypertension

Regulation of sympathoadrenal activity has been a long-time target in the management of hypertension. Regulation of β-adrenoceptor (βAR) function has been the most therapeutically important of these targets. The development of effective antihypertensive treatments based on βAR antagonism paralleled the elucidation of the molecular basis of β-adrenergic effects by the family of βARs, which are members of the G-protein-coupled receptor (GPCR) superfamily. βARs serve as the extracellular face of the transmembrane signalling pathway that results in the consequent activation of heterotrimeric G-proteins and the activation of several other newly appreciated signalling molecules that include β-arrestins and GPCR kinases (GRKs). The aggregate effect of the activation of these signalling pathways mediates the response to βAR activation. Paradoxically, the hypertensive state is characterized by impaired βAR responsiveness. This defect is common to many other receptor systems linked to the stimulator G protein (Gs) and adenylyl cyclase activation. This impairment is principally mediated by receptor-G-protein uncoupling, which has been linked to increased expression and activity of GRK2.

Cardiovascular gene therapy for myocardial infarction

INTRODUCTION

Cardiovascular gene therapy is the third most popular application for gene therapy, representing 8.4% of all gene therapy trials as reported in 2012 estimates. Gene therapy in cardiovascular disease is aiming to treat heart failure from ischemic and non-ischemic causes, peripheral artery disease, venous ulcer, pulmonary hypertension, atherosclerosis and monogenic diseases, such as Fabry disease.

AREAS COVERED

In this review, we will focus on elucidating current molecular targets for the treatment of ventricular dysfunction following myocardial infarction (MI). In particular, we will focus on the treatment of i) the clinical consequences of it, such as heart failure and residual myocardial ischemia and ii) etiological causes of MI (coronary vessels atherosclerosis, bypass venous graft disease, in-stent restenosis).

EXPERT OPINION

We summarise the scheme of the review and the molecular targets either already at the gene therapy clinical trial phase or in the pipeline. These targets will be discussed below. Following this, we will focus on what we believe are the 4 prerequisites of success of any gene target therapy: safety, expression, specificity and efficacy (SESE).

Constitutive activities and inverse agonism in dopamine receptors

The concept of activation in the absence of agonists has been demonstrated for many GPCRs and is now solidified as one of the principal aspects of GPCR signaling. In this chapter, we review how dopamine receptors demonstrate this ability. Although difficult to prove in vivo due to the presence of endogenous dopamine and lack of subtype-selective inverse agonists and "pure" antagonists (neutral ligands), in vitro assays such as measuring intracellular cAMP, [(35)S]GTPγS binding, and [(3)H]thymidine incorporation have uncovered the constitutive activation of D1- and D2-class receptors. Nevertheless, because of limited and inconsistent findings, the existence of constitutive activity for D2-class receptors is currently not well established. Mutagenesis studies have shown that basal signaling, notably by D1-class receptors, is governed by the collective contributions of transmembrane domains and extracellular/intracellular loops, such as the third extracellular loop, the third intracellular loop, and C-terminal tail. Furthermore, constitutive activities of D1-class receptors are subjected to regulation by kinases. Among the dopamine receptor family, the D5 receptor subtype exhibits a higher basal signaling and bears resemblance to constitutively active mutant forms of GPCRs. The presence of its constitutive activity in vivo and its pathophysiological relevance, with a brief mention of other subtypes, are also discussed.

Adrenergic signaling and oxidative stress: a role for sirtuins?

The adrenergic system plays a central role in stress signaling and stress is often associated with increased production of ROS. However, ROS overproduction generates oxidative stress, that occurs in response to several stressors. β-adrenergic signaling is markedly attenuated in conditions such as heart failure, with downregulation and desensitization of the receptors and their uncoupling from adenylyl cyclase. Transgenic activation of β2-adrenoceptor leads to elevation of NADPH oxidase activity, with greater ROS production and p38MAPK phosphorylation. Inhibition of NADPH oxidase or ROS significantly reduced the p38MAPK signaling cascade. Chronic β2-adrenoceptor activation is associated with greater cardiac dilatation and dysfunction, augmented pro-inflammatory and profibrotic signaling, while antioxidant treatment protected hearts against these abnormalities, indicating ROS production to be central to the detrimental signaling of β2-adrenoceptors. It has been demonstrated that sirtuins are involved in modulating the cellular stress response directly by deacetylation of some factors. Sirt1 increases cellular stress resistance, by an increased insulin sensitivity, a decreased circulating free fatty acids and insulin-like growth factor (IGF-1), an increased activity of AMPK, increased activity of PGC-1a, and increased mitochondrial number. Sirt1 acts by involving signaling molecules such P-I-3-kinase-Akt, MAPK and p38-MAPK-β. βAR stimulation antagonizes the protective effect of the AKT pathway through inhibiting induction of Hif-1α and Sirt1 genes, key elements in cell survival. More studies are needed to better clarify the involvement of sirtuins in the β-adrenergic response and, overall, to better define the mechanisms by which tools such as exercise training are able to counteract the oxidative stress, by both activation of sirtuins and inhibition of GRK2 in many cardiovascular conditions and can be used to prevent or treat diseases such as heart failure.

Adrenergic signaling and oxidative stress: a role for sirtuins?

The adrenergic system plays a central role in stress signaling and stress is often associated with increased production of ROS. However, ROS overproduction generates oxidative stress, that occurs in response to several stressors. β-adrenergic signaling is markedly attenuated in conditions such as heart failure, with downregulation and desensitization of the receptors and their uncoupling from adenylyl cyclase. Transgenic activation of β2-adrenoceptor leads to elevation of NADPH oxidase activity, with greater ROS production and p38MAPK phosphorylation. Inhibition of NADPH oxidase or ROS significantly reduced the p38MAPK signaling cascade. Chronic β2-adrenoceptor activation is associated with greater cardiac dilatation and dysfunction, augmented pro-inflammatory and profibrotic signaling, while antioxidant treatment protected hearts against these abnormalities, indicating ROS production to be central to the detrimental signaling of β2-adrenoceptors. It has been demonstrated that sirtuins are involved in modulating the cellular stress response directly by deacetylation of some factors. Sirt1 increases cellular stress resistance, by an increased insulin sensitivity, a decreased circulating free fatty acids and insulin-like growth factor (IGF-1), an increased activity of AMPK, increased activity of PGC-1a, and increased mitochondrial number. Sirt1 acts by involving signaling molecules such P-I-3-kinase-Akt, MAPK and p38-MAPK-β. βAR stimulation antagonizes the protective effect of the AKT pathway through inhibiting induction of Hif-1α and Sirt1 genes, key elements in cell survival. More studies are needed to better clarify the involvement of sirtuins in the β-adrenergic response and, overall, to better define the mechanisms by which tools such as exercise training are able to counteract the oxidative stress, by both activation of sirtuins and inhibition of GRK2 in many cardiovascular conditions and can be used to prevent or treat diseases such as heart failure.

Design, synthesis and efficacy of novel G protein-coupled receptor kinase 2 inhibitors

G protein-coupled receptor kinase 2 (GRK2) is a relevant signaling node of the cellular transduction network, playing major roles in the physiology of various organs/tissues including the heart and blood vessels. Emerging evidence suggests that GRK2 is up regulated in pathological situations such as heart failure, hypertrophy and hypertension, and its inhibition offers a potential therapeutic solution to these diseases. We explored the GRK2 inhibitory activity of a library of cyclic peptides derived from the HJ loop of G protein-coupled receptor kinases 2 (GRK2). The design of these cyclic compounds was based on the conformation of the HJ loop within the X-ray structure of GRK2. One of these compounds, the cyclic peptide 7, inhibited potently and selectively the GRK2 activity, being more active than its linear precursor. In a cellular system, this peptide confirms the beneficial signaling properties of a potent GRK2 inhibitor. Preferred conformations of the most potent analog were investigated by NMR spectroscopy.

GRK2 negatively regulates IGF-1R signaling pathway and cyclins' expression in HepG2 cells

G protein coupled receptor kinase 2 (GRK2) plays a central role in the regulation of a variety of important signaling pathways. Alternation of GRK2 protein level and activity casts profound effects on cell physiological functions and causes diseases such as heart failure, rheumatoid arthritis, and obesity. We have previously reported that overexpression of GRK2 has an inhibitory role in cancer cell growth. To further examine the role of GRK2 in cancer, in this study, we investigated the effects of reduced protein level of GRK2 on insulin-like growth factor 1 receptor (IGF-1R) signaling pathway in human hepatocellular carcinoma (HCC) HepG2 cells. We created a GRK2 knockdown cell line using a lentiviral vector mediated expression of GRK2 specific short hairpin RNA (shRNA). Under IGF-1 stimulation, HepG2 cells with reduced level of GRK2 showed elevated total IGF-1R protein expression as well as tyrosine phosphorylation of receptor. In addition, HepG2 cells with reduced level of GRK2 also demonstrated increased tyrosine phosphorylation of IRS1 at the residue 612 and increased phosphorylation of Akt, indicating a stronger activation of IGF-1R signaling pathway. However, HepG2 cells with reduced level of GRK2 did not display any growth advantage in culture as compared with the scramble control cells. We further detected that reduced level of GRK2 induced a small cell cycle arrest at G2/M phase by enhancing the expression of cyclin A, B1, and E. Our results indicate that GRK2 has contrasting roles on HepG2 cell growth by negatively regulating the IGF-1R signaling pathway and cyclins' expression.

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.

G protein-coupled receptor kinase 2 (GRK2) is localized to centrosomes and mediates epidermal growth factor-promoted centrosomal separation

G protein-coupled receptor kinases (GRKs) play a central role in regulating receptor signaling, but recent studies suggest a broader role in modulating normal cellular functions. For example, GRK5 has been shown to localize to centrosomes and regulate microtubule nucleation and cell cycle progression. Here we demonstrate that GRK2 is also localized to centrosomes, although it has no role in centrosome duplication or microtubule nucleation. Of interest, knockdown of GRK2 inhibits epidermal growth factor receptor (EGFR)-mediated separation of duplicated centrosomes. This EGFR/GRK2-mediated process depends on the protein kinases mammalian STE20-like kinase 2 (Mst2) and Nek2A but does not involve polo-like kinase 1. In vitro analysis and dominant-negative approaches reveal that GRK2 directly phosphorylates and activates Mst2. Collectively these findings demonstrate that GRK2 is localized to centrosomes and plays a central role in mitogen-promoted centrosome separation most likely via its ability to phosphorylate Mst2.

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

Numerous factors partake in the fine-tuning of arterial blood pressure. The heptahelical G-protein-coupled receptors (GPCRs) represent one of the largest classes of cell-surface receptors. Further, ligands directed at GPCRs account for nearly 30 % of current clinical pharmaceutical agents available. Given the wide variety of GPCRs involved in blood pressure control, it is reasonable to speculate for a potential role of established intermediaries involved in the GPCR desensitization process, like the G-protein-coupled receptor kinases (GRKs), in the regulation of vascular tone. Of the seven mammalian GRKs, GRK2 seems to be the most relevant isoform at the cardiovascular level. This review attempts to assemble the currently available information concerning GRK2 and hypertension, opening new potential fields of translational investigation to treat this vexing disease.

Pinpointing beta adrenergic receptor in ageing pathophysiology: victim or executioner? Evidence from crime scenes

G protein-coupled receptors (GPCRs) play a key role in cellular communication, allowing human cells to sense external cues or to talk each other through hormones or neurotransmitters. Research in this field has been recently awarded with the Nobel Prize in chemistry to Robert J. Lefkowitz and Brian K. Kobilka, for their pioneering work on beta adrenergic receptors (βARs), a prototype GPCR. Such receptors, and β2AR in particular, which is extensively distributed throughout the body, are involved in a number of pathophysiological processes. Moreover, a large amount of studies has demonstrated their participation in ageing process. Reciprocally, age-related changes in regulation of receptor responses have been observed in numerous tissues and include modifications of βAR responses. Impaired sympathetic nervous system function has been indeed evoked as at least a partial explanation for several modifications that occur with ageing. This article represents an updated presentation of the current knowledge in the field, summarizing in a systematic way the major findings of research on ageing in several organs and tissues (crime scenes) expressing βARs: heart, vessels, skeletal muscle, respiratory system, brain, immune system, pancreatic islets, liver, kidney and bone.

Inhibition of G-protein-coupled receptor kinase 2 (GRK2) triggers the growth-promoting mitogen-activated protein kinase (MAPK) pathway

Inhibition of G-protein-coupled receptor kinase 2 (GRK2) is an emerging treatment option for heart failure. Because GRK2 is also indispensable for growth and development, we analyzed the impact of GRK2 inhibition on cell growth and proliferation. Inhibition of GRK2 by the dominant-negative GRK2-K220R did not affect the proliferation of cultured cells. In contrast, upon xenograft transplantation of cells into immunodeficient mice, the dominant-negative GRK2-K220R or a GRK2-specific peptide inhibitor increased tumor mass. The enhanced tumor growth upon GRK2 inhibition was attributed to the growth-promoting MAPK pathway because dual inhibition of the GRK2 and RAF-MAPK axis by the Raf kinase inhibitor protein (RKIP) did not increase tumor mass. The MAPK cascade contributed to the cardioprotective profile of GRK2 inhibition by preventing cardiomyocyte death, whereas dual inhibition of RAF/MAPK and GRK2 by RKIP induced cardiomyocyte apoptosis, cardiac dysfunction, and signs of heart failure. Thus, cardioprotective signaling induced by GRK2 inhibition is overlapping with tumor growth promotion.

Arrestins in the cardiovascular system

Of the four mammalian arrestins, only the β-arrestins (βarrs; Arrestin2 and -3) are expressed throughout the cardiovascular system, where they regulate, as either desensitizers/internalizers or signal transducers, several G-protein-coupled receptors (GPCRs) critical for cardiovascular homeostasis. The cardiovascular roles of βarrs have been delineated at an accelerated pace via a variety of techniques and tools, such as knockout mice, siRNA knockdown, artificial or naturally occurring polymorphic GPCRs, and availability of new βarr "biased" GPCR ligands. This chapter summarizes the current knowledge of cardiovascular arrestin physiology and pharmacology, addressing the individual cardiovascular receptors affected by βarrs in vivo, as well as the individual cell types, tissues, and organs of the cardiovascular system in which βarr effects are exerted; for example, cardiac myocyte or fibroblast, vascular smooth muscle, adrenal gland and platelet. In the broader scope of cardiovascular βarr pharmacology, a discussion of the βarr "bias" of certain cardiovascular GPCR ligands is also included.

Suppressed G-protein-coupled receptor kinase 2 activity protects female diabetic-mouse aorta against endothelial dysfunction

AIM

Pre-menopausal women have less cardiovascular disease and lower cardiovascular morbidity and mortality than men the same age. Previously, we noted in mice that G-protein-coupled receptor kinase 2 (GRK2) negatively regulates the Akt/eNOS pathway in male diabetic aortas and that endothelial function via the Akt/eNOS pathway is less affected in female diabetic aortas. The cellular mechanisms underlying these sex differences remain unclear. We aimed to investigate the ways in which GRK2 might modulate vascular functions in male and female diabetic mice (DM).

METHODS

Vascular functions were examined in aortic rings. GRK2, β-arrestin 2 and Akt/eNOS-signalling-pathway protein levels and activities were assayed by Western blotting.

RESULTS

Phenylephrine-induced contraction was greater, while both clonidine-induced and insulin-induced relaxations were weaker (vs. male controls), in aortas from male type 2 DM, suggesting impairments of the Akt/eNOS pathway and α-adrenoceptor function. GRK2-inhibitor reversed only the impairment in Akt/eNOS-pathway-mediated relaxation in male DM. Increases in GRK2 activity, GRK2 expression in the membrane, plasma Ang II and systolic blood pressure were seen in male DM (vs. male controls) but not in female DM; these increases were attenuated by GRK2-inhibitor treatment. Repeatedly obtaining clonidine concentration-response curves led to reduced relaxation in male and in female DM aortas, indicating similar desensitization between female DM and male DM. This effect was reversed by GRK2-inhibitor in both sexes.

CONCLUSION

GRK2 plays a key role in modulating the aortic vasodilator effect of clonidine by selectively affecting the Akt/eNOS pathway. This action of GRK2 is more powerful in male than in female DM.

Renoprotective effects of berberine and its possible molecular mechanisms in combination of high-fat diet and low-dose streptozotocin-induced diabetic rats

Berberine (BBR), an effective compound of Chinese traditional herbal medicine, has preventive effects on diabetes and its complications. In this study, we investigated the therapeutic effects and underlying molecular mechanisms of BBR in rats with high-fat diet and streptozotocin (STZ)-induced diabetic nephropathy model. BBR (50, 100, 200 mg/kg/d) were orally administered to male Sprague-Dawley rats after STZ injection and conducted for 8 weeks. Renal damage was evaluated by kidney weight to body weight ratio (KW/BW), urine microalbumin (UMAlb), urine protein for 24 h (UP24 h), urine creatinine (UCr), and histological examination. Type IV collagen and transforming growth factor-beta1 (TGF-β1) were detected by immunohistochemistry and ultrastructure of glomeruli was observed. Fasting blood glucose (FBG),serum creatinine (SCr), blood urea nitrogen (BUN), total cholesterol (TC), triglyceride (TG), high-density lipoprotein-cholesterol (HDL-c), low-density lipoprotein-cholesterol (LDL-c) in serum and G protein-coupled receptor kinases (GRKs), cAMP in kidney were measured. Remarkable renal damage, hyperglycemia and hyperlipidemia were observed in DN rats. BBR could restore renal functional parameters, suppress alterations in histological and ultrastructural changes in the kidney tissues, improve glucose and lipid metabolism disorders, and increase cAMP levels compared with those of DN model group. Furthermore, BBR down-regulated total protein expression of GRK2, GRK3 and up-regulated expression of GRK6 of renal cortex in DN rats, but had a slight effects on GRK4 and GRK5. These studies demonstrate, for the first time, that BBR exerts renoprotection in high-fat diet and STZ-induced DN rats by modulating the proteins expression of GRKs in G protein- AC-cAMP signaling pathway.

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-coupled receptor kinase 2, with β-arrestin 2, impairs insulin-induced Akt/endothelial nitric oxide synthase signaling in ob/ob mouse aorta

In type 2 diabetes, impaired insulin-induced Akt/endothelial nitric oxide synthase (eNOS) signaling may decrease the vascular relaxation response. Previously, we reported that this response was negatively regulated by G protein-coupled receptor kinase 2 (GRK2). In this study, we investigated whether/how in aortas from ob/ob mice (a model of type 2 diabetes) GRK2 and β-arrestin 2 might regulate insulin-induced signaling. Endothelium-dependent relaxation was measured in aortic strips. GRK2, β-arrestin 2, and Akt/eNOS signaling pathway proteins and activities were mainly assayed by Western blotting. In ob/ob (vs. control [Lean]) aortas: 1) insulin-induced relaxation was reduced, and this deficit was prevented by GRK2 inhibitor, anti-GRK2 antibody, and an siRNA specifically targeting GRK2. The Lean aorta relaxation response was reduced to the ob/ob level by pretreatment with an siRNA targeting β-arrestin 2. 2) Insulin-stimulated Akt and eNOS phosphorylations were decreased. 3) GRK2 expression in membranes was elevated, and, upon insulin stimulation, this expression was further increased, but β-arrestin 2 was decreased. In ob/ob aortic membranes under insulin stimulation, the phosphorylations of Akt and eNOS were augmented by GRK2 inhibitor. In mouse aorta, GRK2 may be, upon translocation, a key negative regulator of insulin responsiveness and an important regulator of the β-arrestin 2/Akt/eNOS signaling, which is implicated in diabetic endothelial dysfunction.

Pharmacogenomics of the heptahelical receptor regulators G-protein-coupled receptor kinases and arrestins: the known and the unknown

Heptahelical G-protein-coupled receptors are the most diverse and therapeutically important family of receptors, playing major roles in the physiology of various organs and tissues. They couple their ligand binding to G-protein activation, which then transmits intracellular signals. G-protein signaling is terminated by phosphorylation of the receptor by the family of G-protein-coupled receptor kinases (GRKs), followed by arrestin (Arr) binding, which uncouples the phosphorylated receptor from the G-protein and subsequently targets the receptor for internalization. Moreover, Arrs can transmit signals in their own right during receptor internalization. Genetic polymorphisms in receptors, as well as in GRK and Arr family members per se, which affect regulation of receptor signaling and function, have just started being identified and characterized. The present review will discuss what is known so far in this evolving field of GRK/Arr pharmacogenomics, as well as highlight important areas likely to produce invaluable information in the future.

Inhibitor of G protein-coupled receptor kinase 2 normalizes vascular endothelial function in type 2 diabetic mice by improving β-arrestin 2 translocation and ameliorating Akt/eNOS signal dysfunction

In type 2 diabetes, although Akt/endothelial NO synthase (eNOS) activation is known to be negatively regulated by G protein-coupled receptor kinase 2 (GRK2), it is unclear whether the GRK2 inhibitor would have therapeutic effects. Here we examined the hypotensive effect of the GRK2 inhibitor and its efficacy agonist both vascular (aortic) endothelial dysfunction (focusing especially on the Akt/eNOS pathway) and glucose intolerance in two type 2 diabetic models (ob/ob mice and nicotinamide+streptozotocin-induced diabetic mice). Mice were treated with a single injection of the GRK2 inhibitor or vehicle, and the therapeutic effects were compared by examining vascular function and by Western blotting. The GRK2 inhibitor lowered blood pressure in both diabetic models but not in their age-matched controls. The GRK2 inhibitor significantly improved clonidine-induced relaxation only in diabetic (ob/ob and DM) mice, with accompanying attenuations of GRK2 activity and translocation to the plasma membrane. These protective effects of the GRK2 inhibitor may be attributable to the augmented Akt/eNOS pathway activation (as evidenced by increases in Akt phosphorylation at Ser(473) and at Thr(308), and eNOS phosphorylation at Ser(1177)) and to the prevention of the GRK2 translocation and promotion of β-arrestin 2 translocation to the membrane under clonidine stimulation. Moreover, the GRK2 inhibitor significantly improved the glucose intolerance seen in the ob/ob mice. Our work provides the first evidence that in diabetes, the GRK2 inhibitor ameliorates vascular endothelial dysfunction via the Akt/eNOS pathway by inhibiting GRK2 activity and enhancing β-arrestin 2 translocation under clonidine stimulation, thereby contributing to a blood pressure-lowering effect. We propose that the GRK2 inhibitor may be a promising therapeutic agent for cardiovascular complications in type 2 diabetes.

G protein-coupled receptor kinase 5 phosphorylates nucleophosmin and regulates cell sensitivity to polo-like kinase 1 inhibition

G protein-coupled receptor kinases (GRKs) phosphorylate activated G protein-coupled receptors, leading to their desensitization and endocytosis. GRKs have also been implicated in phosphorylating other classes of proteins and can localize in a variety of cellular compartments, including the nucleus. Here, we attempted to identify potential nuclear substrates for GRK5. Our studies reveal that GRK5 is able to interact with and phosphorylate nucleophosmin (NPM1) both in vitro and in intact cells. NPM1 is a nuclear protein that regulates a variety of cell functions including centrosomal duplication, cell cycle control, and apoptosis. GRK5 interaction with NPM1 is mediated by the N-terminal domain of each protein, and GRK5 primarily phosphorylates NPM1 at Ser-4, a site shared with polo-like kinase 1 (PLK1). NPM1 phosphorylation by GRK5 and PLK1 correlates with the sensitivity of cells to undergo apoptosis with cells having higher GRK5 levels being less sensitive and cells with lower GRK5 being more sensitive to PLK1 inhibitor-induced apoptosis. Taken together, our results demonstrate that GRK5 phosphorylates Ser-4 in nucleophosmin and regulates the sensitivity of cells to PLK1 inhibition.

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.