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

Artificial cells for the treatment of liver diseases

Liver diseases have become an increasing health burden and account for over 2 million deaths every year globally. Standard therapies including liver transplant and cell therapy offer a promising treatment for liver diseases, but they also suffer limitations such as adverse immune reactions and lack of long-term efficacy. Artificial cells that mimic certain functions of a living cell have emerged as a new strategy to overcome some of the challenges that liver cell therapy faces at present. Artificial cells have demonstrated advantages in long-term storage, targeting capability, and tuneable features. This article provides an overview of the recent progress in developing artificial cells and their potential applications in liver disease treatment. First, the design of artificial cells and their biomimicking functions are summarized. Then, systems that mimic cell surface properties are introduced with two concepts highlighted: cell membrane-coated artificial cells and synthetic lipid-based artificial cells. Next, cell microencapsulation strategy is summarized and discussed. Finally, challenges and future perspectives of artificial cells are outlined. STATEMENT OF SIGNIFICANCE: Liver diseases have become an increasing health burden. Standard therapies including liver transplant and cell therapy offer a promising treatment for liver diseases, but they have limitations such as adverse immune reactions and lack of long-term efficacy. Artificial cells that mimic certain functions of a living cell have emerged as a new strategy to overcome some of the challenges that liver cell therapy faces at present. This article provides an overview of the recent progress in developing artificial cells and their potential applications in liver disease treatment, including the design of artificial cells and their biomimicking functions, two systems that mimic cell surface properties (cell membrane-coated artificial cells and synthetic lipid-based artificial cells), and cell microencapsulation strategy. We also outline the challenges and future perspectives of artificial cells.

Cardiac Fibrosis and Fibroblasts

Cardiac fibrosis is the excess deposition of extracellular matrix (ECM), such as collagen. Myofibroblasts are major players in the production of collagen, and are differentiated primarily from resident fibroblasts. Collagen can compensate for the dead cells produced by injury. The appropriate production of collagen is beneficial for preserving the structural integrity of the heart, and protects the heart from cardiac rupture. However, excessive deposition of collagen causes cardiac dysfunction. Recent studies have demonstrated that myofibroblasts can change their phenotypes. In addition, myofibroblasts are found to have functions other than ECM production. Myofibroblasts have macrophage-like functions, in which they engulf dead cells and secrete anti-inflammatory cytokines. Research into fibroblasts has been delayed due to the lack of selective markers for the identification of fibroblasts. In recent years, it has become possible to genetically label fibroblasts and perform sequencing at single-cell levels. Based on new technologies, the origins of fibroblasts and myofibroblasts, time-dependent changes in fibroblast states after injury, and fibroblast heterogeneity have been demonstrated. In this paper, recent advances in fibroblast and myofibroblast research are reviewed.

Differential Regulation of GPCRs-Are GRK Expression Levels the Key?

G protein-coupled receptors (GPCRs) comprise the largest family of transmembrane receptors and their signal transduction is tightly regulated by GPCR kinases (GRKs) and β-arrestins. In this review, we discuss novel aspects of the regulatory GRK/β-arrestin system. Therefore, we briefly revise the origin of the "barcode" hypothesis for GPCR/β-arrestin interactions, which states that β-arrestins recognize different receptor phosphorylation states to induce specific functions. We emphasize two important parameters which may influence resulting GPCR phosphorylation patterns: (A) direct GPCR-GRK interactions and (B) tissue-specific expression and availability of GRKs and β-arrestins. In most studies that focus on the molecular mechanisms of GPCR regulation, these expression profiles are underappreciated. Hence we analyzed expression data for GRKs and β-arrestins in 61 tissues annotated in the Human Protein Atlas. We present our analysis in the context of pathophysiological dysregulation of the GPCR/GRK/β-arrestin system. This tissue-specific point of view might be the key to unraveling the individual impact of different GRK isoforms on GPCR regulation.

G Protein-Coupled Receptor Kinase 2 (GRK2) Regulates T Cell Response in a Murine Model of House Dust Mite-Induced Asthma

G protein-coupled receptor kinase 2 (GRK2) is an adapter protein that modulates G protein-coupled receptor (GPCR) signaling. It also regulates the functions and activity of other intracellular proteins in many cell types. Accordingly, GRK2 is thought to contribute to disease progression by a variety of mechanisms related to its multifunctional roles. Indeed, GRK2 levels are enhanced in patient samples as well as in preclinical models of several diseases. We have previously shown that GRK2 regulates mast cell functions, and thereby contributes to exacerbated inflammation during allergic reactions. In the current study, we observed that GRK2 levels are enhanced in the lungs of human asthma patients and in mice sensitized to house dust mite extract (HDME) allergen. Consistent with these findings, interleukin (IL)-4 and IL-13 levels were reduced in the lungs of GRK2+/- mice in a HMDE mouse model of asthma. Because Th2 cells are the major source of these cytokines during asthma, we determined the role of GRK2 in regulating T cell-specific responses in our HMDE mouse model. We observed a significant reduction of airway hyperresponsiveness (AHR), lung eosinophil and lymphocyte counts, serum IgE, Th2 cytokines (IL-4 and IL-13), goblet cell hyperplasia and mucus production in mice that had reduced GRK2 expression specifically in T cells. Collectively, our studies reveal an important role for GRK2 in regulating T cell response during asthma pathogenesis and further elucidation of the mechanisms through which GRK2 modulates airway inflammation will lead to the development of new therapeutic strategies for asthma.

Mdm2-Mediated Downmodulation of GRK2 Restricts Centrosome Separation for Proper Chromosome Congression

The timing of centrosome separation and the distance moved apart influence the formation of the bipolar spindle, affecting chromosome stability. Epidermal growth factor receptor (EGFR) signaling induces early centrosome separation through downstream G protein-coupled receptor kinase GRK2, which phosphorylates the Hippo pathway component MST2 (Mammalian STE20-like protein kinase 2), in turn allowing NIMA kinase Nek2A activation for centrosomal linker disassembly. However, the mechanisms that counterbalance centrosome disjunction and separation remain poorly understood. We unveil that timely degradation of GRK2 by the E3 ligase Mdm2 limits centrosome separation in the G2. Both knockout expression and catalytic inhibition of Mdm2 result in GRK2 accumulation and enhanced centrosome separation before mitosis onset. Phosphorylation of GRK2 on residue S670 enables a complex pattern of non-K48-linked polyubiquitin chains assembled by Mdm2, which correlate with kinase protein degradation. Remarkably, GRK2-S670A protein fails to phosphorylate MST2 despite overcoming Mdm2-dependent degradation, which results in defective centrosome separation, shorter spindles, and abnormal chromosome congression. Conversely, extra levels of wild-type kinase in the G2 cause increased inter-centrosome distances with longer spindles, also converging in congression issues. Our findings show that the signals enabling activity of the GRK2/MST2/Nek2A axis for separation also switches on Mdm2 degradation of GRK2 to ensure accurate centrosome dynamics and proper mitotic spindle functionality.

Cardiac GRK2 Protein Levels Show Sexual Dimorphism during Aging and Are Regulated by Ovarian Hormones

Cardiovascular disease (CVD) risk shows a clear sexual dimorphism with age, with a lower incidence in young women compared to age-matched men. However, this protection is lost after menopause. We demonstrate that sex-biased sensitivity to the development of CVD with age runs in parallel with changes in G protein-coupled receptor kinase 2 (GRK2) protein levels in the murine heart and that mitochondrial fusion markers, related to mitochondrial functionality and cardiac health, inversely correlate with GRK2. Young female mice display lower amounts of cardiac GRK2 protein compared to age-matched males, whereas GRK2 is upregulated with age specifically in female hearts. Such an increase in GRK2 seems to be specific to the cardiac muscle since a different pattern is found in the skeletal muscles of aging females. Changes in the cardiac GRK2 protein do not seem to rely on transcriptional modulation since adrbk1 mRNA does not change with age and no differences are found between sexes. Global changes in proteasomal or autophagic machinery (known regulators of GRK2 dosage) do not seem to correlate with the observed GRK2 dynamics. Interestingly, cardiac GRK2 upregulation in aging females is recapitulated by ovariectomy and can be partially reversed by estrogen supplementation, while this does not occur in the skeletal muscle. Our data indicate an unforeseen role for ovarian hormones in the regulation of GRK2 protein levels in the cardiac muscle which correlates with the sex-dependent dynamics of CVD risk, and might have interesting therapeutic applications, particularly for post-menopausal women.

The homotetramerization of a GPCR transmits the 20-hydroxyecdysone signal and increases its entry into cells for insect metamorphosis

Animal steroid hormones initiate signaling by passive diffusion into cells and binding to their nuclear receptors to regulate gene expression. Animal steroid hormones can initiate signaling via G protein-coupled receptors (GPCRs); however, the underlying mechanisms are unclear. Here, we show that a newly discovered ecdysone-responsive GPCR, ErGPCR-3, transmits the steroid hormone 20-hydroxyecdysone (20E) signal by binding 20E and promoting its entry into cells in the lepidopteran insect Helicoverpa armigera Knockdown of ErGPCR-3 in larvae caused delayed and abnormal pupation, inhibited remodeling of the larval midgut and fat body, and repressed 20E-induced gene expression. Also, 20E induced both the interaction of ErGPCR-3 with G proteins and rapid intracellular increase in calcium, cAMP and protein phosphorylation. ErGPCR-3 was endocytosed by GPCR kinase 2-mediated phosphorylation, and interacted with β-arrestin-1 and clathrin, to terminate 20E signaling under 20E induction. We found that 20E bound to ErGPCR-3 and induced the ErGPCR-3 homodimer to form a homotetramer, which increased 20E entry into cells. Our study revealed that homotetrameric ErGPCR-3 functions as a cell membrane receptor and increases 20E diffusion into cells to transmit the 20E signal and promote metamorphosis.

Involvement of the Catecholamine Pathway in Glioblastoma Development

Glioblastoma (GBM) is the most aggressive tumor of the central nervous system (CNS). The standard of care improves the overall survival of patients only by a few months. Explorations of new therapeutic targets related to molecular properties of the tumor are under way. Even though neurotransmitters and their receptors normally function as mediators of interneuronal communication, growing data suggest that these molecules are also involved in modulating the development and growth of GBM by acting on neuronal and glioblastoma stem cells. In our previous DNA CpG methylation studies, gene ontology analyses revealed the involvement of the monoamine pathway in sequential GBM. In this follow-up study, we quantitated the expression levels of four selected catecholamine pathway markers (alpha 1D adrenergic receptor-ADRA1D; adrenergic beta receptor kinase 1 or G protein-coupled receptor kinase 2-ADRBK1/GRK2; dopamine receptor D2-DRD2; and synaptic vesicle monoamine transporter-SLC18A2) by immunohistochemistry, and compared the histological scores with the methylation levels within the promoters + genes of these markers in 21 pairs of sequential GBM and in controls. Subsequently, we also determined the promoter and gene methylation levels of the same markers in an independent database cohort of sequential GBM pairs. These analyses revealed partial inverse correlations between the catecholamine protein expression and promoter + gene methylation levels, when the tumor and control samples were compared. However, we found no differences in the promoter + gene methylation levels of these markers in either our own or in the database primary-recurrent GBM pairs, despite the higher protein expression of all markers in the primary samples. This observation suggests that regulation of catecholamine expression is only partially related to CpG methylation within the promoter + gene regions, and additional mechanisms may also influence the expression of these markers in progressive GBM. These analyses underscore the involvement of certain catecholamine pathway markers in GBM development and suggest that these molecules mediating or modulating tumor growth merit further exploration.

GRK2 regulates GLP-1R-mediated early phase insulin secretion in vivo

BACKGROUND

Insulin secretion from the pancreatic β-cell is finely modulated by different signals to allow an adequate control of glucose homeostasis. Incretin hormones such as glucagon-like peptide-1 (GLP-1) act as key physiological potentiators of insulin release through binding to the G protein-coupled receptor GLP-1R. Another key regulator of insulin signaling is the Ser/Thr kinase G protein-coupled receptor kinase 2 (GRK2). However, whether GRK2 affects insulin secretion or if GRK2 can control incretin actions in vivo remains to be analyzed.

RESULTS

Using GRK2 hemizygous mice, isolated pancreatic islets, and model β-cell lines, we have uncovered a relevant physiological role for GRK2 as a regulator of incretin-mediated insulin secretion in vivo. Feeding, oral glucose gavage, or administration of GLP-1R agonists in animals with reduced GRK2 levels (GRK2+/- mice) resulted in enhanced early phase insulin release without affecting late phase secretion. In contrast, intraperitoneal glucose-induced insulin release was not affected. This effect was recapitulated in isolated islets and correlated with the increased size or priming efficacy of the readily releasable pool (RRP) of insulin granules that was observed in GRK2+/- mice. Using nanoBRET in β-cell lines, we found that stimulation of GLP-1R promoted GRK2 association to this receptor and that GRK2 protein and kinase activity were required for subsequent β-arrestin recruitment.

CONCLUSIONS

Overall, our data suggest that GRK2 is an important negative modulator of GLP-1R-mediated insulin secretion and that GRK2-interfering strategies may favor β-cell insulin secretion specifically during the early phase, an effect that may carry interesting therapeutic applications.

Why Do We Not Assess Sympathetic Nervous System Activity in Heart Failure Management: Might GRK2 Serve as a New Biomarker?

Heart failure (HF) represents the end-stage condition of several structural and functional cardiovascular diseases, characterized by reduced myocardial pump function and increased pressure load. The dysregulation of neurohormonal systems, especially the hyperactivity of the cardiac adrenergic nervous system (ANS), constitutes a hallmark of HF and exerts a pivotal role in its progression. Indeed, it negatively affects patients’ prognosis, being associated with high morbidity and mortality rates, with a tremendous burden on global healthcare systems. To date, all the techniques proposed to assess the cardiac sympathetic nervous system are burdened by intrinsic limits that hinder their implementation in clinical practice. Several biomarkers related to ANS activity, which may potentially support the clinical management of such a complex syndrome, are slow to be implemented in the routine practice for several limitations due to their assessment and clinical impact. Lymphocyte G-protein-coupled Receptor Kinase 2 (GRK2) levels reflect myocardial β-adrenergic receptor function in HF and have been shown to add independent prognostic information related to ANS overdrive. In the present manuscript, we provide an overview of the techniques currently available to evaluate cardiac ANS in HF and future perspectives in this field of relevant scientific and clinical interest.

Paroxetine-mediated GRK2 inhibition is a disease-modifying treatment for osteoarthritis

Osteoarthritis (OA) is a debilitating joint disease characterized by progressive cartilage degeneration, with no available disease-modifying therapy. OA is driven by pathological chondrocyte hypertrophy (CH), the cellular regulators of which are unknown. We have recently reported the therapeutic efficacy of G protein-coupled receptor kinase 2 (GRK2) inhibition in other diseases by recovering protective G protein-coupled receptor (GPCR) signaling. However, the role of GPCR-GRK2 pathway in OA is unknown. Thus, in a surgical OA mouse model, we performed genetic GRK2 deletion in chondrocytes or pharmacological inhibition with the repurposed U.S. Food and Drug Administration (FDA)-approved antidepressant paroxetine. Both GRK2 deletion and inhibition prevented CH, abated OA progression, and promoted cartilage regeneration. Supporting experiments with cultured human OA cartilage confirmed the ability of paroxetine to mitigate CH and cartilage degradation. Our findings present elevated GRK2 signaling in chondrocytes as a driver of CH in OA and identify paroxetine as a disease-modifying drug for OA treatment.

Paroxetine-Overview of the Molecular Mechanisms of Action

In the 21st century and especially during a pandemic, the diagnosis and treatment of depression is an essential part of the daily practice of many family doctors. It mainly affects patients in the age category 15–44 years, regardless of gender. Anxiety disorders are often diagnosed in children and adolescents. Social phobias can account for up to 13% of these diagnoses. Social anxiety manifests itself in fear of negative social assessment and humiliation, which disrupts the quality of social functioning. Treatment of the above-mentioned disorders is based on psychotherapy and pharmacotherapy. Serious side effects or mortality from antidepressant drug overdose are currently rare. Recent studies indicate that paroxetine (ATC code: N06AB), belonging to the selective serotonin reuptake inhibitors, has promising therapeutic effects and is used off-label in children and adolescents. The purpose of this review is to describe the interaction of paroxetine with several molecular targets in various points of view including the basic chemical and pharmaceutical properties. The central point of the review is focused on the pharmacodynamic analysis based on the molecular mechanism of binding paroxetine to various therapeutic targets.

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.

GRKs and Epac1 Interaction in Cardiac Remodeling and Heart Failure

β-adrenergic receptors (β-ARs) play a major role in the physiological regulation of cardiac function through signaling routes tightly controlled by G protein-coupled receptor kinases (GRKs). Although the acute stimulation of β-ARs and the subsequent production of cyclic AMP (cAMP) have beneficial effects on cardiac function, chronic stimulation of β-ARs as observed under sympathetic overdrive promotes the development of pathological cardiac remodeling and heart failure (HF), a leading cause of mortality worldwide. This is accompanied by an alteration in cAMP compartmentalization and the activation of the exchange protein directly activated by cAMP 1 (Epac1) signaling. Among downstream signals of β-ARs, compelling evidence indicates that GRK2, GRK5, and Epac1 represent attractive therapeutic targets for cardiac disease. Here, we summarize the pathophysiological roles of GRK2, GRK5, and Epac1 in the heart. We focus on their signalosome and describe how under pathological settings, these proteins can cross-talk and are part of scaffolded nodal signaling systems that contribute to a decreased cardiac function and HF development.

GRK Inhibition Potentiates Glucagon-Like Peptide-1 Action

The glucagon-like peptide-1 receptor (GLP-1R) is a G-protein-coupled receptor (GPCR) whose activation results in suppression of food intake and improvement of glucose metabolism. Several receptor interacting proteins regulate the signaling of GLP-1R such as G protein-coupled receptor kinases (GRK) and β-arrestins. Here we evaluated the physiological and pharmacological impact of GRK inhibition on GLP-1R activity leveraging small molecule inhibitors of GRK2 and GRK3. We demonstrated that inhibition of GRK: i) inhibited GLP-1-mediated β-arrestin recruitment, ii) enhanced GLP-1-induced insulin secretion in isolated islets and iii) has additive effect with dipeptidyl peptidase 4 in mediating suppression of glucose excursion in mice. These findings highlight the importance of GRK to modulate GLP-1R function in vitro and in vivo. GRK inhibition is a potential therapeutic approach to enhance endogenous and pharmacologically stimulated GLP-1R signaling.

Hit-to-lead optimization and discovery of a potent, and orally bioavailable G protein coupled receptor kinase 2 (GRK2) inhibitor

G-protein coupled receptor kinase 2 (GRK2), which is upregulated in the failing heart, appears to play a critical role in heart failure (HF) progression in part because enhanced GRK2 activity promotes dysfunction of β-adrenergic signaling and myocyte death. An orally bioavailable GRK2 inhibitor could offer unique therapeutic outcomes that cannot be attained by current heart failure treatments that directly target GPCRs or angiotensin-converting enzyme. Herein, we describe the discovery of a potent, selective, and orally bioavailable GRK2 inhibitor, 8h, through high-throughput screening, hit-to-lead optimization, structure-based design, molecular modelling, synthesis, and biological evaluation. In the cellular target engagement assays, 8h enhances isoproterenol-mediated cyclic adenosine 3',5'-monophosphate (cAMP) production in HEK293 cells overexpressing GRK2. Compound 8h was further evaluated in a human stem cell-derived cardiomyocyte (HSC-CM) contractility assay and potentiated isoproterenol-induced beating rate in HSC-CMs.

Myeloid GRK2 Regulates Obesity-Induced Endothelial Dysfunction by Modulating Inflammatory Responses in Perivascular Adipose Tissue

Perivascular adipose tissue (PVAT) is increasingly being regarded as an important endocrine organ that directly impacts vessel function, structure, and contractility in obesity-associated diseases. We uncover here a role for myeloid G protein-coupled receptor kinase 2 (GRK2) in the modulation of PVAT-dependent vasodilation responses. GRK2 expression positively correlates with myeloid- (CD68) and lymphoid-specific (CD3, CD4, and CD8) markers and with leptin in PVAT from patients with abdominal aortic aneurysms. Using mice hemizygous for GRK2 in the myeloid lineage (LysM-GRK2^+/−), we found that GRK2 deficiency in myeloid cells allows animals to preserve the endothelium-dependent acetylcholine or insulin-induced relaxation, which is otherwise impaired by PVAT, in arteries of animals fed a high fat diet (HFD). Downregulation of GRK2 in myeloid cells attenuates HFD-dependent infiltration of macrophages and T lymphocytes in PVAT, as well as the induction of tumor necrosis factor-α (TNFα) and NADPH oxidase (Nox)1 expression, whereas blocking TNFα or Nox pathways by pharmacological means can rescue the impaired vasodilator responses to insulin in arteries with PVAT from HFD-fed animals. Our results suggest that myeloid GRK2 could be a potential therapeutic target in the development of endothelial dysfunction induced by PVAT in the context of obesity.

GRK4-mediated adiponectin receptor-1 phosphorylative desensitization as a novel mechanism of reduced renal sodium excretion in hypertension

Hypertensive patients have impaired sodium excretion. However, the mechanisms are incompletely understood. Despite the established association between obesity/excess adiposity and hypertension, whether and how adiponectin, one of the adipokines, contributes to impaired sodium excretion in hypertension has not been previously investigated. The current study tested the hypothesis that adiponectin promotes natriuresis and diuresis in the normotensive state. However, impaired adiponectin-mediated natriuresis and diuresis are involved in pathogenesis of hypertension. We found that sodium excretion was reduced in adiponectin knockout (Adipo-/-) mice; intrarenal arterial infusion of adiponectin-induced natriuresis and diuresis in Wistar-Kyoto (WKY) rats. However, the natriuretic and diuretic effects of adiponectin were impaired in spontaneously hypertensive rats (SHRs), which were ascribed to the hyperphosphorylation of adiponectin receptor and subsequent uncoupling from Gαi. Inhibition of adiponectin receptor phosphorylation by a specific point mutation restored its coupling with Gαi and the adiponectin-mediated inhibition of Na+-K+-ATPase activity in renal proximal tubule (RPT) cells from SHRs. Finally, we identified G protein-coupled receptor kinase 4 (GRK4) as a mediator of adiponectin receptor hyperphosphorylation; mice transgenic for a hyperphosphorylating variant of GRK4 replicated the abnormal adiponectin function observed in SHRs, whereas down-regulation of GRK4 by renal ultrasound-directed small interfering RNA (siRNA) restored the adiponectin-mediated sodium excretion and reduced the blood pressure in SHRs. We conclude that the stimulatory effect of adiponectin on sodium excretion is impaired in hypertension, which is ascribed to the increased renal GRK4 expression and activity. Targeting GRK4 restores impaired adiponectin-mediated sodium excretion in hypertension, thus representing a novel strategy against hypertension.

Identification of common and divergent gene expression signatures in patients with venous and arterial thrombosis using data from public repositories

STRENGTHS AND LIMITATIONS OF THIS STUDY

Our results represent the first comparison of venous and arterial thrombosis at the transcriptomic level.Our main result was the demonstration that immunothrombosis pathways are important to the pathophysiology of these conditions, also at the transcriptomic level.A specific signature for venous and arterial thrombosis was described, and validated in independent cohorts.The limited number of public repositories with gene expression data from patients with venous thromboembolism limits the representation of these patients in our analyses.In order to gather a meaningful number of studies with gene expression data we had to include patients in different time-points since the index thrombotic event, which might have increased the heterogeneity of our population.

Design of substrates and inhibitors of G protein-coupled receptor kinase 2 (GRK2) based on its phosphorylation reaction

The G protein-coupled receptor kinase (GRK) family consists of seven cytosolic serine/threonine (Ser/Thr) protein kinases, and among them, GRK2 is involved in the regulation of an enormous range of both G protein-coupled receptors (GPCRs) and non-GPCR substrates that participate in or regulate many critical cellular processes. GRK2 dysfunction is associated with multiple diseases, including cancers, brain diseases, cardiovascular and metabolic diseases, and therefore GRK2-specific substrates/inhibitors are needed not only for studies of GRK2-mediated cellular functions but also for GRK2-targeted drug development. Here, we first review the structure, regulation and functions of GRK2, and its synthetic substrates and inhibitors. We then highlight recent work on synthetic peptide substrates/inhibitors as promising tools for fundamental studies of the physiological functions of GRK2, and as candidates for applications in clinical diagnostics.

Antagonistic Roles of GRK2 and GRK5 in Cardiac Aldosterone Signaling Reveal GRK5-Mediated Cardioprotection via Mineralocorticoid Receptor Inhibition

Aldosterone (Aldo), when overproduced, is a cardiotoxic hormone underlying heart failure and hypertension. Aldo exerts damaging effects via the mineralocorticoid receptor (MR) but also activates the antiapoptotic G protein-coupled estrogen receptor (GPER) in the heart. G protein-coupled receptor (GPCR)-kinase (GRK)-2 and -5 are the most abundant cardiac GRKs and phosphorylate GPCRs as well as non-GPCR substrates. Herein, we investigated whether they phosphorylate and regulate cardiac MR and GPER. To this end, we used the cardiomyocyte cell line H9c2 and adult rat ventricular myocytes (ARVMs), in which we manipulated GRK5 protein levels via clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 and GRK2 activity via pharmacological inhibition. We report that GRK5 phosphorylates and inhibits the cardiac MR whereas GRK2 phosphorylates and desensitizes GPER. In H9c2 cardiomyocytes, GRK5 interacts with and phosphorylates the MR upon β2-adrenergic receptor (AR) activation. In contrast, GRK2 opposes agonist-activated GPER signaling. Importantly, GRK5-dependent MR phosphorylation of the MR inhibits transcriptional activity, since aldosterone-induced gene transcription is markedly suppressed in GRK5-overexpressing cardiomyocytes. Conversely, GRK5 gene deletion augments cardiac MR transcriptional activity. β2AR-stimulated GRK5 phosphorylates and inhibits the MR also in ARVMs. Additionally, GRK5 is necessary for the protective effects of the MR antagonist drug eplerenone against Aldo-induced apoptosis and oxidative stress in ARVMs. In conclusion, GRK5 blocks the cardiotoxic MR-dependent effects of Aldo in the heart, whereas GRK2 may hinder beneficial effects of Aldo through GPER. Thus, cardiac GRK5 stimulation (e.g., via β2AR activation) might be of therapeutic value for heart disease treatment via boosting the efficacy of MR antagonists against Aldo-mediated cardiac injury.

A high-content, in vitro cardiac fibrosis assay for high-throughput, phenotypic identification of compounds with anti-fibrotic activity

A key feature in the pathogenesis of heart failure is cardiac fibrosis, but effective treatments that specifically target cardiac fibrosis are currently not available. A major impediment to progress has been the lack of reliable in vitro models with sufficient throughput to screen for activity against cardiac fibrosis. Here, we established cell culture conditions in micro-well format that support extracellular deposition of mature collagen from primary human cardiac fibroblasts - a hallmark of cardiac fibrosis. Based on robust biochemical characterization we developed a high-content phenotypic screening platform, that allows for high-throughput identification of compounds with activity against cardiac fibrosis. Our platform correctly identifies compounds acting on known cardiac fibrosis pathways. Moreover, it can detect anti-fibrotic activity for compounds acting on targets that have not previously been reported in in vitro cardiac fibrosis assays. Taken together, our experimental approach provides a powerful platform for high-throughput screening of anti-fibrotic compounds as well as discovery of novel targets to develop new therapeutic strategies for heart failure.

Autophagy mediates hepatic GRK2 degradation to facilitate glucagon-induced metabolic adaptation to fasting

The liver plays a key role during fasting to maintain energy homeostasis and euglycemia via metabolic processes mainly orchestrated by the insulin/glucagon ratio. We report here that fasting or calorie restriction protocols in C57BL6 mice promote a marked decrease in the hepatic protein levels of G protein-coupled receptor kinase 2 (GRK2), an important negative modulator of both G protein-coupled receptors (GPCRs) and insulin signaling. Such downregulation of GRK2 levels is liver-specific and can be rapidly reversed by refeeding. We find that autophagy, and not the proteasome, represents the main mechanism implicated in fasting-induced GRK2 degradation in the liver in vivo. Reducing GRK2 levels in murine primary hepatocytes facilitates glucagon-induced glucose production and enhances the expression of the key gluconeogenic enzyme Pck1. Conversely, preventing full downregulation of hepatic GRK2 during fasting using adenovirus-driven overexpression of this kinase in the liver leads to glycogen accumulation, decreased glycemia, and hampered glucagon-induced gluconeogenesis, thus preventing a proper and complete adaptation to nutrient deprivation. Overall, our data indicate that physiological fasting-induced downregulation of GRK2 in the liver is key for allowing complete glucagon-mediated responses and efficient metabolic adaptation to fasting in vivo.

Sex Differences in High Fat Diet-Induced Metabolic Alterations Correlate with Changes in the Modulation of GRK2 Levels

A differential sex-related sensitivity has been reported in obesity and insulin resistance-related cardio-metabolic diseases, with a lower incidence of these pathologies being observed in young females when compared to age-matched males. However, such relative protection is lost with age. The mechanisms underlying such sex and age-related changes in the susceptibility to diabetes and obesity are not fully understood. Herein, we report that the relative protection that is displayed by young female mice, as compared to male littermates, against some of the metabolic alterations that are induced by feeding a high fat diet (HFD), correlates with a lower upregulation of the protein levels of G protein-coupled receptor kinase (GRK2), which is a key regulator of both insulin and G protein-coupled receptor signaling, in the liver and adipose tissue. Interestingly, when the HFD is initiated in middle-aged (32 weeks) female mice, these animals are no longer protected and display a more overt obese and insulin-resistant phenotype, along with a more evident increase in the GRK2 protein levels in metabolically relevant tissues in such conditions. Our data suggest that GRK2 dosage might be involved in the sex and age-biased sensitivity to insulin resistance-related pathologies.

Degradation of GRK2 and AKT is an early and detrimental event in myocardial ischemia/reperfusion

BACKGROUND

Identification of signaling pathways altered at early stages after cardiac ischemia/reperfusion (I/R) is crucial to develop timely therapies aimed at reducing I/R injury. The expression of G protein-coupled receptor kinase 2 (GRK2), a key signaling hub, is up-regulated in the long-term in patients and in experimental models of heart failure. However, whether GRK2 levels change at early time points following myocardial I/R and its functional impact during this period remain to be established.

METHODS

We have investigated the temporal changes of GRK2 expression and their potential relationships with the cardioprotective AKT pathway in isolated rat hearts and porcine preclinical models of I/R.

FINDINGS

Contrary to the maladaptive up-regulation of GRK2 reported at later times after myocardial infarction, successive GRK2 phosphorylation at specific sites during ischemia and early reperfusion elicits GRK2 degradation by the proteasome and calpains, respectively, thus keeping GRK2 levels low during early I/R in rat hearts. Concurrently, I/R promotes decay of the prolyl-isomerase Pin1, a positive regulator of AKT stability, and a marked loss of total AKT protein, resulting in an overall decreased activity of this pro-survival pathway. A similar pattern of concomitant down-modulation of GRK2/AKT/Pin1 protein levels in early I/R was observed in pig hearts. Calpain and proteasome inhibition prevents GRK2/Pin1/AKT degradation, restores bulk AKT pathway activity and attenuates myocardial I/R injury in isolated rat hearts.

INTERPRETATION

Preventing transient degradation of GRK2 and AKT during early I/R might improve the potential of endogenous cardioprotection mechanisms and of conditioning strategies.

A Review of Endothelium-Dependent and -Independent Vasodilation Induced by Phytochemicals in Isolated Rat Aorta

Simple Summary

Cardiovascular diseases are the leading cause of death worldwide, so the investigation of new therapeutic tools is a priority for their prevention and treatment. This review shows the relevant contribution of the isolated rat aorta as an in vitro experimental model to validate the therapeutic potential of phytochemicals, mainly those present in plants traditionally used in folk medicine to relieve hypertension. The results of the assays carried out in this model show that a variety of plant extracts and their isolated compounds produce vasodilation, which may explain their use, especially to treat hypertension.

Abstract

This review discusses the contribution of the use of the isolated rat aorta (IRA) as a model for the evaluation of extracts and metabolites produced by plants with a vasodilator effect in animals. This model continues to be a valuable approach for the search and development of new phytochemicals consumed as medicinal plants or foods. In most cases, the sources of phytochemicals have been used in folk medicine to treat ailments that include hypertension. In this model, the endothelium is emphasized as a key component that modulates the vessel contractility, and therefore the basal tone and blood pressure. Based on the functional nature of the model, we focused on studies that determined the endothelium-dependent and -independent vasodilatory activity of phytochemicals. We describe the mechanisms that account for aorta contraction and relaxation, and subsequently show the vasoactive effect of a series of phytochemicals acting as vasodilators and its endothelium dependence. We highlight information regarding the cardiovascular benefits of phytochemicals, especially their potential antihypertensive effect. On this basis, we discuss the advantages of the IRA as a predictive model to support the research and development of new drugs that may be of help in the prevention and treatment of cardiovascular diseases, the number one cause of death worldwide.