G Protein-coupled Receptor Kinase 2 Levels Research Articles

The roles of G protein-coupled receptor kinase 2 in renal diseases.

G protein-coupled receptor (GPCR) kinase 2 (GRK2) is an integrative node in many signalling network cascades. An emerging study indicates that GRK2 can interact with GPCRs and non-GPCR substrates in both kinase-dependent and -independent modes. Alterations in the functional levels of GRK2 have been found in a variety of renal diseases, such as hypertension-related kidney injury, sepsis-associated acute kidney injury (S-AKI), cardiorenal syndrome (CRS), acute kidney injury (AKI), age-related kidney injury or hyperglycemia-related kidney injury. Abnormal GRK2 expression contribute to the development of renal diseases, making them promising molecular targets for treating renal diseases. Blocking the prostaglandin E2 (PGE2)-EP1-Gaq-Ca2+ signal pathway in glomerular mesangial cells (GMCs) by internalizing prostaglandin E2 receptor 1 (EP1) with GRK2 may be a potential treatment for diabetic nephropathy (DN). In addition, GRK2 inhibition may have therapeutic effects in a variety of renal diseases, such as SLE-related kidney injury, DN, age-related kidney injury, hypertension-related kidney injury, and CRS. However, there is still a long way to go for the large-scale application of GRK2 inhibition in the field of renal diseases. In this review, we discuss recent updates in understanding the role of GRK2 in kidney dysfunction. Furthermore, we explore the potential of GRK2 as a possible therapeutic target for renal pathologies. We believe it will shed light on the future development of small-molecule inhibitors of GRK, as well as the clinical applications in renal diseases.

G protein-coupled receptor kinase 2 as a novel therapeutic target for gland fibrosis of Sjögren's syndrome.

Sjogren's syndrome (SS) is a chronic, progressive autoimmune disorder characterized by gland fibrosis. We previously found a close correlation between gland fibrosis and the expression of G protein-coupled receptor kinase 2 (GRK2). In this study we explored the pathological and therapeutic significance of GRK2 in SS. Submandibular gland (SMG) antigen-induced SS mouse model was established in WT and GRK2+/- mice. We showed that the expression levels of GRK2 were significantly up-regulated in glandular tissue and positively correlated with fibrotic morphology in SS patients and mice. Hemizygous knockout of GRK2 significantly inhibited the gland fibrosis. In mouse salivary gland epithelial cells (SGECs), we demonstrated that GRK2 interacted with Smad2/3 to positively regulate the activation of TGF-β-Smad signaling with a TGF-β-GRK2 positive feedback loop contributing to gland fibrosis. Hemizygous knockout of GRK2 attenuated TGF-β-induced collagen I production in SGECs in vitro and hindered gland fibrosis in murine SS though preventing Smad2/3 nuclear translocation. Around 28 days post immunization with SMG antigen, WT SS mice were treated with a specific GRK2 inhibitor paroxetine (Par, 5 mg·kg-1·d-1, i.g. for 19 days). We found that Par administration significantly attenuated gland fibrosis and alleviated the progression of SS in mice. We conclude that genetic knockdown or pharmacological inhibition of GRK2 significantly attenuates gland fibrosis and alleviates the progression of SS. GRK2 binds to Smad2/3 and positively regulates the activation of TGF-β-Smad signaling. A TGF-β-GRK2 positive feedback loop contributes to gland fibrosis. Our research points out that GRK2 could be a promising therapeutic target for treating SS.

GRK2-YAP signaling is implicated in pulmonary arterial hypertension development.

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. 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. 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. 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, an Inhibitor of MALT1-Dependent Oncogenic Signaling, Is Downregulated By microRNA in ABC-DLBCL

Background: The “CBM” complex, composed of the scaffolding protein CARMA1, the adaptor protein BCL10, and the effector protein MALT1, performs multiple pivotal functions as a mediator of antigen receptor-dependent induction of the NF-κB transcription factor and subsequent lymphocyte activation. The effector molecule of the CBM complex, MALT1, regulates downstream signaling via two important functions: first, as a scaffold to recruit and activate components of the canonical NF-κB signaling machinery, and second, as a protease to enzymatically cleave and inactivate multiple substrates including several negative regulators of NF-κB signaling. Deregulated/constitutive activation of MALT1, which can result from gain-of-function mutations in either B cell receptor (BCR) or CARMA1 or from chromosomal translocation involving the MALT1 gene, underlies the pathogenesis of a variety of lymphoid malignancies including activated B-cell type-diffuse large B-cell lymphoma (ABC-DLBCL) and mucosa associated lymphoid tissue (MALT) lymphoma. We recently discovered that G-protein-coupled receptor kinase 2 (GRK2) binds to MALT1 and inhibits MALT1 scaffold and proteolytic activities. Further, we found that knockdown of GRK2 in ABC-DLBCL enhances tumor growth in vivo, suggesting that GRK2 may act as a tumor suppressor in MALT1-dependent lymphomas. Interestingly, we found that GRK2 mRNA levels are markedly lower in a subset of DLBCL cases in comparison to normal B cell controls and that lower GRK2 level is associated with reduced survival in ABC-DLBCL patients. We thus sought to investigate how GRK2 expression is regulated in ABC-DLBCL. Methods and Results: We surveyed 6 published DLBCL sequencing datasets (334 cases total) and identified only one case with a mutation/deletion in the ADRBK1 (GRK2) gene. Thus, we concluded that mutation/deletion could not account for the lower levels of GRK2 in a subset of DLBCL cases. We next investigated other potential mechanisms that could play a role in downregulating GRK2 in ABC-DLBCL, including the impact of microRNA (miRNA). Using DICER-knock-down (KD) and knock-out (KO) HEK 293T cells, we demonstrated that GRK2 protein level significantly increases when miRNA processing is impaired. Next, we performed a bioinformatic analysis which identified two candidate microRNAs, miR-148b and miR-331, whose levels inversely correlate with GRK2 mRNA in ABC-DLBCL patient tumor specimens. These two microRNAs were also identified among the top four candidate hits (miR-125b, miR-148b, miR-125a and miR-331) using mirDIP, a microRNA Data Integration Portal which we used to screen for candidate miRNAs predicted to target the 3'UTR of GRK2. We confirmed direct targeting of GRK2 by three top candidate miRNAs, miR-148b, miR-125a and miR-125b, using a GRK2 3'UTR reporter assay. Notably, we found that expression of these three microRNAs is higher in ABC-DLBCL cells (OCI-Ly3 and TMD8), which demonstrate lower GRK2 mRNA expression in comparison to primary B cells. We showed that overexpression of either miR-125a, miR-125b or miR-148 in OCI-Ly3 and TMD8 cells leads to reduced GRK2 protein level, increased MALT1 scaffolding and proteolytic activities and enhanced ABC-DLBCL cell proliferation. Conversely, anti-miRNA Locked Nucleic Acids (LNA) inhibitors of miR-125a, miR-125b or miR-148 each inhibited ABC-DLBCL cell proliferation and MALT1 activity. Further evaluation of the effect of anti-miRNA LNA inhibitors on tumor progression is under way, using an ABC-DLBCL xenograft animal model. Conclusions: Together, our data show that specific miRNAs (miR-148b, miR-125a and miR-125b) down-regulate GRK2 expression in ABC-DLBCL, which in turn enhances MALT1 scaffolding and proteolytic activities, leading to increased tumor cell proliferation. Future studies investigating miRNA inhibitors which enhance GRK2 expression and thereby suppress MALT1 activity may inform novel strategies for inhibiting MALT1-dependent lymphomagenesis.

Abstract A18: GRK2, an inhibitor of MALT1-dependent oncogenic signaling, is downregulated by microRNA in ABC-DLBCL

Abstract Introduction and Background: The “CBM” complex is composed of the scaffolding protein CARMA1, the adaptor protein BCL10, and the effector protein MALT1. This complex performs multiple pivotal functions as a mediator of antigen receptor-dependent induction of the NF-κB transcription factor and subsequent lymphocyte activation. The key downstream effector of the CBM complex, MALT1, regulates downstream signaling via both scaffolding and proteolytic functions. Inappropriate activation of MALT1, which can result from somatic gain-of-function mutation in upstream regulators of MALT1 or chromosomal translocation involving the MALT1 gene, underlies the pathogenesis of a variety of lymphoid malignancies including activated B cell type-diffuse large B cell lymphoma (ABC-DLBCL) and mucosa associated lymphoid tissue (MALT) lymphoma. We recently discovered that G-protein-coupled receptor kinase 2 (GRK2) binds to MALT1 and inhibits MALT1 scaffold and proteolytic activities. Further, we found that knockdown of GRK2 in ABC-DLBCL enhances tumor growth in vivo, suggesting that GRK2 may act as a tumor suppressor in MALT1-dependent lymphomas. Interestingly, we found that GRK2 mRNA levels are markedly lower in a subset of DLBCL cases in comparison to normal B-cell controls and that lower GRK2 level is associated with reduced survival in ABC-DLBCL patients. We thus sought to investigate how GRK2 expression is regulated in ABC-DLBCL. Methods and Results: We first investigated whether mutations/deletions in the ADRBK1 gene (GRK2) account for the lower levels of GRK2 expression in a subset of DLBCL cases. Only one mutation among 6 published DLBCL sequencing datasets (334 cases total) was identified. We next considered that other mechanisms, such as regulation by microRNA (miRNA), could play a role in downregulating GRK2. Using DICER-deficient HEK 293T cells, we demonstrated that GRK2 protein level significantly increases when miRNA processing is impaired. We then used mirDIP, a microRNA Data Integration Portal, to screen for candidate miRNAs predicted to target the 3’UTR of GRK2. We identified 4 top candidate hits, 2 of which were also identified in bioinformatic analysis as miRNA whose level inversely correlates with GRK2 mRNA level in ABC-DLBCL patient tumor specimens. Using a GRK2 3’ UTR reporter system, we confirmed direct targeting of GRK2 by these miRNAs. Next, we generated stable ABC-DLBCL cell lines (TMD8, OCI-Ly3) overexpressing candidate miRNAs. Our results demonstrate that candidate miRNA overexpression results in reduced GRK2 expression and increased MALT1 scaffolding and proteolytic activities, NF-κB transcriptional activity and ABC-DLBCL cell proliferation. Conclusions: Together, our data suggest that miRNAs down-regulate expression of GRK2 in ABC-DLBCL which in turn enhances MALT1 scaffolding and proteolytic activities, leading to increased tumor cell proliferation. Future studies are aimed at identifying potential miRNA inhibitors which enhance GRK2 expression and thereby suppress MALT1 pro-tumorigenic activity in ABC-DLBCL. Citation Format: Jing Cheng, Mei Smyers, Matt Trotta, Neil M Carleton, Lisa M Maurer, Peter C Lucas, Linda M McAllister-Lucas. GRK2, an inhibitor of MALT1-dependent oncogenic signaling, is downregulated by microRNA in ABC-DLBCL [abstract]. In: Proceedings of the Third AACR International Meeting: Advances in Malignant Lymphoma: Maximizing the Basic-Translational Interface for Clinical Application; 2022 Jun 23-26; Boston, MA. Philadelphia (PA): AACR; Blood Cancer Discov 2022;3(5_Suppl):Abstract nr A18.

Mitochondrial GRK2 is a Novel Regulator of Cardiac Energetics

G protein‐coupled receptor (GPCR) kinase 2 (GRK2) is highly expressed in the heart, where during injury or heart failure (HF), both its levels and activity increase. GRK2 is canonically studied in the context of GPCR phosphorylation; however, noncanonical activities of GRK2 have emerged and it is now appreciated that GRK2 has a large non‐GPCR interactome. For example, in cardiac myocytes, GRK2 translocates from the cytosol to mitochondria (mtGRK2) following oxidative stress or ischemia injury, and this pool of mtGRK2 is associated with negative effects on metabolism and also induces myocyte cell death. However, the mechanisms by which mtGRK2 contributes to cardiac dysfunction and HF are not fully understood. We hypothesized that mtGRK2 could have novel substrates and phosphorylate proteins involved in mitochondrial bioenergetics, thus contributing to our previously established post‐injury phenotype. Stress‐induced mitochondrial translocation of cytosolic GRK2 was validated in cell and animal models and the mtGRK2 interactome was identified using liquid chromatography‐mass spectroscopy (LCMS). Proteomics analysis identified mtGRK2 interacting proteins which were involved in mitochondrial dysfunction, bioenergetics, and OXPHOS, particularly complexes I, II, IV and V of the electron transport chain (ETC). Specifically, mtGRK2 interactions with Complex V (ATP synthase) subunits were particularly increased following stress. We established that mtGRK2 phosphorylates ATP synthase on the F1 catalytic barrel, which is critical for oxidative phosphorylation and ATP production. We have also determined that alterations in either the levels or activity of GRK2 appear to alter ATP synthase enzymatic activity in vitro. Excitingly, in vivo data suggest that reducing levels of GRK2 in a mouse model of myocardial infarction prevents the post‐injury reduction in ATP synthesis. We are currently assessing the ability of the SSRI drug paroxetine, a GRK2 inhibitor, to preserve mitochondrial bioenergetics in a transgenic GRK2 mouse model. Thus, phosphorylation of the ATP synthesis machinery by mtGRK2 may contribute to the impaired mitochondrial phenotype observed in injured or failing heartssuch as reduced fatty acid metabolism and substrate utilization. These data uncover a druggable, novel target for rescuing cardiac function in patients with injured and/or failing hearts.

Myocardial GRK2 Reduces Fatty Acid Metabolism and β-Adrenergic Receptor-Mediated Mitochondrial Responses.

G-protein coupled receptor (GPCR) kinase 2 (GRK2) is upregulated in heart failure (HF) patients and mouse models of cardiac disease. GRK2 is a regulator of β-adrenergic receptors (βARs), a GPCR involved in ionotropic and chronotropic responses. We and others have recently reported GRK2 to be localized in the mitochondria, although its function in the mitochondria and/or metabolism remain not clearly defined. We hypothesized that upregulation of GRK2 reduced mitochondrial respiratory function and responses to βAR activation. Utilizing isolated mouse primary adult cardiomyocytes (ACMs), we investigated the role of glucose, palmitate, ketone bodies, and BCAAs in mediating cell survival. Our results showed that myocyte upregulation of GRK2 promotes palmitate-induced cell death. Isotopologue labeling and mass spectrometry showed that the upregulation of GRK2 reduces β-hydroxybutyryl CoA generation. Next, using isoproterenol (ISO), a non-selective βAR-agonist, we determined mitochondrial function in mouse and human primary ACMs. Upregulation of GRK2 impaired ISO-mediated mitochondrial functional responses, which we propose is important for metabolic adaptations in pathological conditions. Increased cardiac levels of GRK2 reduced fatty acid-specific catabolic pathways and impaired ISO-stimulated mitochondrial function. Our data support the notion that GRK2 participates in bioenergetic remodeling and may be an important avenue for the development of novel pharmacological strategies in HF.

The ROS/GRK2/HIF-1α/NLRP3 Pathway Mediates Pyroptosis of Fibroblast-Like Synoviocytes and the Regulation of Monomer Derivatives of Paeoniflorin.

Hypoxia is an important factor in the development of synovitis in rheumatoid arthritis (RA). The previous study of the research group found that monomeric derivatives of paeoniflorin (MDP) can alleviate joint inflammation in adjuvant-induced arthritis (AA) rats by inhibiting macrophage pyroptosis. This study revealed increased levels of hypoxia-inducible factor- (HIF-) 1α and N-terminal p30 fragment of GSDMD (GSDMD-N) in fibroblast-like synoviocytes (FLS) of RA patients and AA rats, while MDP significantly inhibited their expression. Subsequently, FLS were exposed to a hypoxic environment or treated with cobalt ion in vitro. Western blot and immunofluorescence analysis showed increased expression of G protein-coupled receptor kinase 2 (GRK2), HIF-1α, nucleotide-binding oligomerization segment-like receptor family 3 (NLRP3), ASC, caspase-1, cleaved-caspase-1, and GSDMD-N. Electron microscopy revealed FLS pyroptosis after exposure in hypoxia. Next, corresponding shRNAs were transferred into FLS to knock down hypoxia-inducible factor- (HIF-) 1α, and in turn, NLRP3 and western blot results confirmed the same. The enhanced level of GSDMD was reversed under hypoxia by inhibiting NLRP3 expression. Knockdown and overexpression of GRK2 in FLS revealed GRK2 to be a positive regulator of HIF-1α. Levels of GRK2 and HIF-1α were inhibited by eliminating excess reactive oxygen species (ROS). Furthermore, MDP reduced FLS pyroptosis through targeted inhibition of GRK2 phosphorylation. According to these findings, hypoxia induces FLS pyroptosis through the ROS/GRK2/HIF-1α/NLRP3 pathway, while MDP regulates this pathway to reduce FLS pyroptosis.

GRK2 promotes activation of lung fibroblast cells and contributes to pathogenesis of pulmonary fibrosis through increasing Smad3 expression.

Pulmonary fibrosis is a chronic, progressive, and irreversible interstitial lung disease. Transforming growth factor-β1 (TGF-β1) plays a major role in lung fibroblast cell differentiation to myofibroblast cells and production of extracellular matrix, which are hallmarks of pulmonary fibrosis. G protein-coupled receptor kinase-2 (GRK2) has been shown to play controversial roles in TGF-β1-induced signal transduction in different cell types; however, the role of GRK2 in TGF-β1-induced activation of lung fibroblast cells and development of pulmonary fibrosis has not been revealed. In this study, we found that GRK2 levels were increased in lungs and isolated fibroblast cells in a murine model of pulmonary fibrosis, as well as TGF-β1-treated lung fibroblasts. GRK2 levels were not changed in lungs in the injury phase of pulmonary fibrosis. Posttreatment with GRK2 inhibitor reduced extracellular matrix (ECM) accumulation in lungs in bleomycin-challenged mice, suggesting that GRK2 activation contributes to the progressive phase of pulmonary fibrosis. Inhibition or downregulation of GRK2 attenuates fibronectin, collagen, and α-smooth muscle actin expression in TGF-β1-induced lung fibroblast cells or myofibroblast cells isolated from patients with pulmonary fibrosis. Furthermore, we showed that GRK2 regulates Smad3 expression, indicating that inhibition of GRK2 attenuates ECM accumulation through downregulation of Smad3 expression. This study reveals that GRK2 is a therapeutic target in treating pulmonary fibrosis and inhibition of GRK2 dampens pulmonary fibrosis by suppression of Smad3 expression, eventually attenuating TGF-β1 signal pathway and ECM accumulation.

Dynamics of GRK2 in the kidney: a putative mechanism for sepsis-associated kidney injury.

Renal vascular reactivity to vasoconstrictors is preserved in sepsis in opposition to what happens in the systemic circulation. We studied whether this distinct behavior was related to α1 adrenergic receptor density, G protein-coupled receptor kinase 2 (GRK2) and the putative role of nitric oxide (NO). Sepsis was induced in female mice by cecal ligation and puncture (CLP). Wildtype mice were treated with prazosin 12 h after CLP or nitric oxide synthase 2 (NOS-2) inhibitor, 30 min before and 6 and 12 h after CLP. In vivo experiments and biochemistry assays were performed 24 h after CLP. Sepsis decreased the systemic mean arterial pressure (MAP) and the vascular reactivity to phenylephrine. Sepsis also reduced basal renal blood flow which was normalized by treatment with prazosin. Sepsis led to a substantial decrease in GRK2 level associated with an increase in α1 adrenergic receptor density in the kidney. The disappearance of renal GRK2 was prevented in NOS-2-KO mice or mice treated with 1400 W. Treatment of non-septic mice with an NO donor reduced GRK2 content in the kidney. Therefore, our results show that an NO-dependent reduction in GRK2 level in the kidney leads to the maintenance of a normal α1 adrenergic receptor density. The preservation of the density and/or functionality of this receptor in the kidney together with a higher vasoconstrictor tonus in sepsis lead to vasoconstriction. Thus, the increased concentration of vasoconstrictor mediators together with the preservation (and even increase) of the response to them may help to explain sepsis-induced acute kidney injury.

The role of G‐protein‐coupled receptor kinase 2 in diabetic mechanical hyperalgesia in rats

Previous studies have indicated a negative correlation between GRK2 expression and pain development and transmission. Here, we investigated whether G-protein-coupled receptor kinase 2 (GRK2) was involved in regulating diabetic mechanical hyperalgesia (DMH). The adeno-associated viral vectors containing the GRK2 gene (AAV-GRK2) were used to up-regulate GRK2 protein expression. The expression of GRK2 and exchange protein directly activated by cyclic adenosine monophosphate 1 (Epac1) in the dorsal root ganglion (DRG) of lumbar 4-6 was detected via immunoblotting and immunohistochemistry, and the transfection of the GRK2 gene was detected by immunofluorescence. Low levels of GRK2 were able to sustain STZ-induced pain in DMH rats. Intrathecal injection of AAV-GRK2 vector up-regulated GRK2 expression, providing pain rain to rats with DMH. With an increase in DMH duration, there was a decrease in paw withdrawal threshold (PWT) value, aggravating the pain, resulting in a decreasing pattern in GRK2 protein expression over time, whereas Epac1 protein expression showed an opposite trend. GRK2 expression regulated DMH progression and is expected to play a role in the development of targeted therapy for DMH. GRK2 and Epac1 expressions play a vital role in maintaining pain in DMH rats.

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.

G-protein coupled receptor kinase 2 (GRK2) regulates T cell response in a murine model of house dust mite-induced asthma

Abstract G protein-coupled receptor kinase 2 (GRK2) is an adapter protein that modulates G protein-coupled receptor (GPCRs) 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 count, 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.

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

BackgroundInsulin 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.ResultsUsing 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.ConclusionsOverall, 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.

A novel GPCR target in correlation with androgen deprivation therapy for prostate cancer drug discovery.

Most prostate carcinomas require androgen stimulation to grow, and for nearly 70years, androgen ablation therapy has been one of the central therapeutic strategies against advanced prostate cancer. Although most tumours initially respond to this therapy, some will be acquired resistant and progress to metastatic castration-resistant (mCRPC) disease which clinically tends to progress more rapidly than earlier disease manifestations. The underlying molecular biology of mCRPC is highly complex, and numerous mechanisms have been proposed that promote and retain androgen independence. In various clinical and preclinical data explored, the nature of intracellular signalling pathways mediating mitogenic acquired resistant effects of GPCRs in prostate cancer is poorly defined. G-protein-coupled receptor kinase 2 (GRK2) contributes to the modulation of basic cellular functions-such as cell proliferation, survival or motility-and is involved in metabolic homeostasis, inflammation or angiogenic processes. Moreover, altered GRK2 levels are starting to be reported in different tumoural contexts and shown to promote breast tumourigenesis or to trigger the tumoural angiogenic switch.Thus, we are exploring recent findings that present unexpected opportunities to interfere with major tumourigenic signals by manipulating GPCR-mediated pathways.

Role of Endothelial G Protein-Coupled Receptor Kinase 2 in Angioedema.

Excessive BK (bradykinin) stimulation is responsible for the exaggerated permeabilization of the endothelium in angioedema. However, the molecular mechanisms underlying these responses have not been investigated. BK receptors are Gq-protein-coupled receptors phosphorylated by GRK2 (G protein-coupled receptor kinase 2) with a hitherto unknown biological and pathophysiological significance. In the present study, we sought to identify the functional role of GRK2 in angioedema through the regulation of BK signaling. We found that the accumulation of cytosolic Ca2+ in endothelial cells induced by BK was sensitive to GRK2 activity, as it was significantly augmented by inhibiting the kinase. Accordingly, permeabilization and NO production induced by BK were enhanced, as well. In vivo, mice with reduced GRK2 levels in the endothelium (Tie2-CRE/GRK2fl+/fl-) exhibited an increased response to BK in terms of vascular permeability and extravasation. Finally, patients with reduced GRK2 levels displayed a severe phenotype of angioedema. Taken together, these findings establish GRK2 as a novel pivotal regulator of BK signaling with an essential role in the pathophysiology of vascular permeability and angioedema.

GRK2 suppresses lymphomagenesis by inhibiting the MALT1 proto-oncoprotein.

Antigen receptor-dependent (AgR-dependent) stimulation of the NF-κB transcription factor in lymphocytes is a required event during adaptive immune response, but dysregulated activation of this signaling pathway can lead to lymphoma. AgR stimulation promotes assembly of the CARMA1-BCL10-MALT1 complex, wherein MALT1 acts as (a) a scaffold to recruit components of the canonical NF-κB machinery and (b) a protease to cleave and inactivate specific substrates, including negative regulators of NF-κB. In multiple lymphoma subtypes, malignant B cells hijack AgR signaling pathways to promote their own growth and survival, and inhibiting MALT1 reduces the viability and growth of these tumors. As such, MALT1 has emerged as a potential pharmaceutical target. Here, we identified G protein-coupled receptor kinase 2 (GRK2) as a new MALT1-interacting protein. We demonstrated that GRK2 binds the death domain of MALT1 and inhibits MALT1 scaffolding and proteolytic activities. We found that lower GRK2 levels in activated B cell-type diffuse large B cell lymphoma (ABC-DLBCL) are associated with reduced survival, and that GRK2 knockdown enhances ABC-DLBCL tumor growth in vitro and in vivo. Together, our findings suggest that GRK2 can function as a tumor suppressor by inhibiting MALT1 and provide a roadmap for developing new strategies to inhibit MALT1-dependent lymphomagenesis.

GRK2 levels in myeloid cells modulate adipose-liver crosstalk in high fat diet-induced obesity.

Macrophages are key effector cells in obesity-associated inflammation. G protein-coupled receptor kinase 2 (GRK2) is highly expressed in different immune cell types. Using LysM-GRK2+/- mice, we uncover that a reduction of GRK2 levels in myeloid cells prevents the development of glucose intolerance and hyperglycemia after a high fat diet (HFD) through modulation of the macrophage pro-inflammatory profile. Low levels of myeloid GRK2 confer protection against hepatic insulin resistance, steatosis and inflammation. In adipose tissue, pro-inflammatory cytokines are reduced and insulin signaling is preserved. Macrophages from LysM-GRK2+/- mice secrete less pro-inflammatory cytokines when stimulated with lipopolysaccharide (LPS) and their conditioned media has a reduced pathological influence in cultured adipocytes or naïve bone marrow-derived macrophages. Our data indicate that reducing GRK2 levels in myeloid cells, by attenuating pro-inflammatory features of macrophages, has a relevant impact in adipose-liver crosstalk, thus preventing high fat diet-induced metabolic alterations.

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.

G protein-coupled receptor kinase-2: A potential biomarker for early diabetic cardiomyopathy.

BackgroundG protein‐coupled receptor kinase‐2 (GRK2) has been shown as a key regulator of cardiac function, and the myocardial GRK2 levels are mirrored by the levels in peripheral blood mononuclear cells (PBMCs). In this study, we evaluated the myocardial and PBMCs GRK2 levels in early diabetic cardiomyopathy (DCM).MethodsC57BL/KS‐db/db male diabetic mice at 12 weeks of age, as the type 2 diabetes (T2DM) animal model of early DCM were evaluated. Forty‐four T2DM patients with left ventricular diastolic dysfunction (LVDD), without evidence of hypertension, coronary artery diseases, congestive heart failure, and diabetic complications and without evidence of ischemia in a maximal treadmill exercise test, were recruited as the DM + LVDD group; 30 age‐matched T2DM patients without LVDD were recruited as the DM control group. Left ventricular diastolic function was evaluated by cardiac tissue Doppler. The pseudonormal pattern of ventricular filling and E′/A′ < 1 were regarded as LVDD.ResultsCompared to 8‐week‐old diabetic mice and 12‐week‐old control mice, GRK2‐mRNA level and expression in myocardial tissues of 12‐week‐old diabetic mice were significantly increased, as well as the left ventricular wall thickness and systolic function. And the collagen volume fraction (CVF), collagen‐3 expression, P53 expression, and cell apoptotic rate in the myocardium of 12‐week‐old diabetic mice elevated as well. The GRK2‐mRNA level in PBMCs of DM with LVDD was significantly higher than in DM control without LVDD.ConclusionsGRK2 expression increased in the myocardial tissue and the PBMCs at the early stage of DCM. These data support further research on the role of GRK2 as the clinical biomarker for early DCM.