G protein-coupled receptor kinase 2 in multiple sclerosis and experimental autoimmune encephalomyelitis

Neuronal GRK2 regulates microglial activation and contributes to electroacupuncture analgesia on inflammatory pain in mice

Background

G protein coupled receptor kinase 2 (GRK2) has been demonstrated to play a crucial role in the development of chronic pain. Acupuncture is an alternative therapy widely used for pain management. In this study, we investigated the role of spinal neuronal GRK2 in electroacupuncture (EA) analgesia.

Methods

The mice model of inflammatory pain was built by subcutaneous injection of Complete Freund’s Adjuvant (CFA) into the plantar surface of the hind paws. The mechanical allodynia of mice was examined by von Frey test. The mice were subjected to EA treatment (BL60 and ST36 acupuncture points) for 1 week. Overexpression and downregulation of spinal neuronal GRK2 were achieved by intraspinal injection of adeno associated virus (AAV) containing neuron-specific promoters, and microglial activation and neuroinflammation were evaluated by real-time PCR.

Results

Intraplantar injection with CFA in mice induced the decrease of GRK2 and microglial activation along with neuroinflammation in spinal cord. EA treatment increased the spinal GRK2, reduced neuroinflammation, and significantly decreased CFA-induced mechanical allodynia. The effects of EA were markedly weakened by non-cell-specific downregulation of spinal GRK2. Further, intraspinal injection of AAV containing neuron-specific promoters specifically downregulated neuronal GRK2, and weakened the regulatory effect of EA on CFA-induced mechanical allodynia and microglial activation. Meanwhile, overexpression of spinal neuronal GRK2 decreased mechanical allodynia. All these indicated that the neuronal GRK2 mediated microglial activation and neuroinflammation, and subsequently contributed to CFA-induced inflammatory pain.

Conclusion

The restoration of the spinal GRK2 and subsequent suppression of microglial activation and neuroinflammation might be an important mechanism for EA analgesia. Our findings further suggested that the spinal GRK2, especially neuronal GRK2, might be the potential target for EA analgesia and pain management, and we provided a new experimental basis for the EA treatment of pain.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40659-022-00374-6.

Understanding Abnormal c-JNK/p38MAPK Signaling Overactivation Involved in the Progression of Multiple Sclerosis: Possible Therapeutic Targets and Impact on Neurodegenerative Diseases

Demyelination, immune dysregulation, and neuroinflammation are the most common triggers of motor neuron disorders such as multiple sclerosis (MS). MS is a chronic demyelinating neurodegenerative disease of the central nervous system caused by abnormal immune activation, which causes myelin sheath damage. Cell signal transduction pathways are required for a variety of physiological and pathological processes in the brain. When these signaling systems become overactive, they can lead to disease progression. In various physiological conditions, abnormal mitogen-activated protein kinase (MAPK) activation is associated with several physiological dysfunctions that cause neurodegeneration. Previous research indicates that c-JNK and p38MAPK signaling play critical roles in neuronal growth and differentiation. c-JNK/p38MAPK is a member of the MAPK family, which regulates metabolic pathways, cell proliferation, differentiation, and apoptosis that control certain neurological activities. During brain injuries, c-JNK/p38MAPK also affects neuronal elastic properties, nerve growth, and cognitive processing. This review systematically linked abnormal c-JNK/p38MAPK signaling activation to multiple neuropathological pathways in MS and related neurological dysfunctions. MS progression is linked to genetic defects, oligodendrocyte destruction, glial overactivation, and immune dysregulation. We concluded that inhibiting both the c-JNK/p38MAPK signaling pathways can promote neuroprotection and neurotrophic effects against the clinical-pathological presentation of MS and influence other neurological disorders. As a result, the potential benefits of c-JNK/p38MAPK downregulation for the development of disease-modifying treatment interventions in the future could include MS prevention and related neurocomplications.

Potential Regulatory Roles of GRK2 in Endothelial Cell Activity and Pathological Angiogenesis

G protein-coupled receptor (GPCR) kinase 2 (GRK2) is an integrative node in many signaling network cascades. Emerging evidence indicates that GRK2 can interact with a large number of GPCRs and non-GPCR substrates in both kinase-dependent and -independent modes. Some of these pathways are associated with endothelial cell (EC) activity. The active state of ECs is a pivotal factor in angiogenesis. The occurrence and development of some inflammation-related diseases are accompanied by pathological angiogenesis, but there remains a lack of effective targeted treatments. Alterations in the expression and/or localization of GRK2 have been identified in several types of diseases and have been demonstrated to regulate the angiogenesis process in these diseases. GRK2 as a target may be a promising candidate for anti-angiogenesis therapy.

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.

CP-25 inhibits PGE2-induced angiogenesis by down-regulating EP4/AC/cAMP/PKA-mediated GRK2 translocation

G protein-coupled receptor kinase 2 (GRK2), a type of cytosolic enzyme, transiently translocates to the plasma membrane upon G protein-coupled receptors (GPCRs) activation, and it also binds to extracellular signal-regulated kinase (ERK) to inhibit the activation of ERK. GRK2 deficiency in endothelial cells (ECs) leads to increased pro-inflammatory signaling and promotes recruitment of leukocytes to activated ECs. However, the role of GRK2 in regulating angiogenesis remains unclear. Here, we show that GRK2 is a novel regulatory molecule on migration and tube formation of ECs, vessel sprouting ex vivo and angiogenesis in vivo. We identify that EP4/AC/cAMP/protein kinase A (PKA)-mediated GRK2 translocation to cells membrane decreases the binding of GRK2 and ERK1/2 to inhibit ERK1/2 activation, which promotes prostaglandin E2 (PGE2)-induced angiogenesis. GRK2 small interfering RNA (siRNA) inhibits the increase in PGE2-induced HUVECs migration and tube formation. In vivo, PGE2 increases ECs sprouting from normal murine aortic segments and angiogenesis in mice, but not from GRK2-deficient ones, on Matrigel. Further research found that Lys220 and Ser685 of GRK2 play an important role in angiogenesis by regulating GRK2 translocation. Paeoniflorin-6'-O-benzene sulfonate (CP-25), as a novel ester derivative of paeoniflorin (pae), has therapeutic potential for the treatment of adjuvant arthritis (AA) and collagen-induced arthritis (CIA), but the underlying mechanism of CP-25 on angiogenesis has not been elucidated. In our study, CP-25 inhibits the migration and tube formation of HUVECs, and angiogenesis in mice by down-regulating GRK2 translocation activation without affecting GRK2 total expression. Taken together, the present results revealed that CP-25 down-regulates EP4/AC/cAMP/PKA-mediated GRK2 translocation, restoring the inhibition of GRK2 for ERK1/2, thereby inhibiting PGE2-stimulated angiogenesis.

G protein-coupled receptor kinase 2 (GRK2) as a multifunctional signaling hub

Accumulating evidence indicates that G protein-coupled receptor kinase 2 (GRK2) is a versatile protein that acts as a signaling hub by modulating G protein-coupled receptor (GPCR) signaling and also via phosphorylation or scaffolding interactions with an extensive number of non-GPCR cellular partners. GRK2 multifunctionality arises from its multidomain structure and from complex mechanisms of regulation of its expression levels, activity, and localization within the cell, what allows the precise spatio-temporal shaping of GRK2 targets. A better understanding of the GRK2 interactome and its modulation mechanisms is helping to identify the GRK2-interacting proteins and its substrates involved in the participation of this kinase in different cellular processes and pathophysiological contexts.

PBMCs transcriptome profiles identified breed-specific transcriptome signatures for PRRSV vaccination in German Landrace and Pietrain pigs

Porcine reproductive and respiratory syndrome (PRRS) is a devastating viral disease affecting the swine industry worldwide. Genetic variation in host immunity has been considered as one of the potential determinants to improve the immunocompetence, thereby resistance to PRRS. Therefore, the present study aimed to investigate the breed difference in innate immune response to PRRSV vaccination between German Landrace (DL) and Pietrain (Pi) pigs. We analyzed microarray-based transcriptome profiles of peripheral blood mononuclear cells (PBMCs) collected before (0 h) and 24 h after PRRSV vaccination from purebred DL and Pi pigs with three biological replicates. In total 4,269 transcripts were identified to be differentially expressed in PBMCs in at least any of four tested contrast pairs (i.e. DL-24h vs. DL-0h, Pi-24h vs. Pi-0h, DL-0h vs. Pi-0h and DL-24h vs. Pi-24h). The number of vaccine-induced differentially expressed genes (DEGs) was much higher (2,459) in DL pigs than that of Pi pigs (291). After 24 h of PRRSV vaccination, 1,046 genes were differentially expressed in PMBCs of DL pigs compared to that of Pi (DL-24h vs. Pi-24h), indicating the breed differences in vaccine responsiveness. The top biological pathways significantly affected by DEGs of both breeds were linked to immune response functions. The network enrichment analysis identified ADAM17, STAT1, MMS19, RPA2, BAD, UCHL5 and APC as potential regulatory genes for the functional network of PRRSV vaccine response specific for DL; while FOXO3, IRF2, ADRBK1, FHL3, PPP2CB and NCOA6 were found to be the most potential hubs of Pi specific transcriptome network. In conclusion, our data provided insights of breed-specific host transcriptome responses to PRRSV vaccination which might contribute in better understanding of PPRS resistance in pigs.

NFkappaB is a Key Player in the Crosstalk between Inflammation and Cardiovascular Diseases

Inflammation is a key mechanism of cardiovascular diseases. It is an essential component of atherosclerosis and a significant risk factor for the development of cardiovascular events. In the crosstalk between inflammation and cardiovascular diseases, the transcription factor NFκB seems to be a key player since it is involved in the development and progression of both inflammation and cardiac and vascular damage. In this review, we deal with the recent findings of the role of inflammation in cardiac diseases, focusing, in particular, on NFκB as a functional link. We describe strategies for the therapeutic targeting of NFκB as a potential strategy for the failing heart.

Regulatory Role of GRK2 in the TLR Signaling-Mediated iNOS Induction Pathway in Microglial Cells

G protein-coupled receptor kinase 2 (GRK2) is a ubiquitous member of the GRK family that restrains cellular activation by G protein-coupled receptor (GPCR) phosphorylation leading to receptor desensitization and internalization, but has been identified to regulate a variety of signaling molecules, among which may be associated with inflammation. In this study, we attempted to establish the regulatory role of GRK2 in the Toll-like receptor (TLR) signaling pathway for inducible nitric oxide synthase (iNOS) expression in microglial cells. When mouse MG6 cells were stimulated with the TLR4 ligands lipopolysaccharide (LPS) and paclitaxel, we found that interferon regulatory factor 1 (IRF1) protein expression and activation was upregulated, transcription of interferon-β (IFN-β) was accelerated, induction/activation of STAT1 and activation of STAT3 were promoted, and subsequently iNOS expression was upregulated. The ablation of GRK2 by small interfering RNAs (siRNAs) not only eliminated TLR4-mediated upregulation of IRF1 protein expression and nuclear translocation but also suppressed the activation of the STAT pathway, resulting in negating the iNOS upregulation. The TLR3-mediated changes in IRF1 and STAT1/3, leading to iNOS induction, were also abrogated by siRNA knockdown of GRK2. Furthermore, transfection of GRK2 siRNA blocked the exogenous IFN-β supplementation-induced increases in phosphorylation of STAT1 as well as STAT3 and abrogated the augmentation of iNOS expression in the presence of exogenous IFN-β. Taken together, our results show that GRK2 regulates the activation of IRF1 as well as the activation of the STAT pathway, leading to upregulated transcription of iNOS in activated microglial cells. Modulation of the TLR signaling pathway via GRK2 in microglia may be a novel therapeutic target for treatment of neuroinflammatory disorders.

GRK5 - A Functional Bridge Between Cardiovascular and Neurodegenerative Disorders

Complex aging-triggered disorders are multifactorial programs that comprise a myriad of alterations in interconnected protein networks over a broad range of tissues. It is evident that rather than being randomly organized events, pathophysiologies that possess a strong aging component such as cardiovascular diseases (hypertensions, atherosclerosis, and vascular stiffening) and neurodegenerative conditions (dementia, Alzheimer's disease, mild cognitive impairment, Parkinson's disease), in essence represent a subtly modified version of the intricate molecular programs already in place for normal aging. To control such multidimensional activities there are layers of trophic protein control across these networks mediated by so-called "keystone" proteins. We propose that these "keystones" coordinate and interconnect multiple signaling pathways to control whole somatic activities such as aging-related disease etiology. Given its ability to control multiple receptor sensitivities and its broad protein-protein interactomic nature, we propose that G protein coupled receptor kinase 5 (GRK5) represents one of these key network controllers. Considerable data has emerged, suggesting that GRK5 acts as a bridging factor, allowing signaling regulation in pathophysiological settings to control the connectivity between both the cardiovascular and neurophysiological complications of aging.

Tumor Necrosis Factor-α in Heart Failure: an Updated Review

PURPOSE OF THE REVIEW

Proinflammatory cytokines are consistently elevated in congestive heart failure. In the current review, we provide an overview on the current understanding of how tumor necrosis factor-α (TNFα), a key proinflammatory cytokine, potentiates heart failure by overwhelming the anti-inflammatory responses disrupting the homeostasis.

RECENT FINDINGS

Studies have shown co-relationship between severity of heart failure and levels of the proinflammatory cytokine TNFα and one of its secondary mediators interleukin-6 (IL-6), suggesting their potential as biomarkers. Recent efforts have focused on understanding the mechanisms of how proinflammatory cytokines contribute towards cardiac dysfunction and failure. In addition, how unchecked proinflammatory cytokines and their cross-talk with sympathetic system overrides the anti-inflammatory response underlying failure. The review offers insights on how TNFα and IL-6 contribute to cardiac dysfunction and failure. Furthermore, this provides a forum to begin the discussion on the cross-talk between sympathetic drive and proinflammatory cytokines and its determinant role in deleterious outcomes.

Low GRK2 Underlies Hyperalgesic Priming by Glial Cell-Derived Neurotrophic Factor

Background: We recently identified the balance between the level of G protein coupled receptor kinase 2 (GRK2) and Epac1 in nociceptors as a key factor in the transition from acute to chronic pain that occurs in mice 'primed' by an inflammatory stimulus. Here, we examined the contribution of GRK2 and Epac-signaling to growth factor-induced hyperalgesic priming. Methods: Mice were primed by intraplantar injection with glial cell-derived neurotrophic factor (GDNF). Mechanical allodynia in response to PGE₂ was followed over time in primed and non-primed animals. GRK2 protein levels in dorsal root ganglion (DRG) neurons were quantified by immunohistochemistry. The effect of herpes simplex virus (HSV)-GRK2 amplicons to restore GRK2 levels or of an Epac inhibitor on PGE₂ allodynia in primed mice was examined. Results: Glial cell-derived neurotrophic factor-induced hyperalgesia disappeared within 12 days. The hyperalgesic response to a subsequent intraplantar injection of PGE₂ was prolonged from <24 h in control mice to more than 72 h in GDNF-primed mice. In male and female primed mice, PGE₂ hyperalgesia was inhibited by oral administration of the Epac inhibitor ESI-09, while the drug had no effect in control mice. Mice primed with GDNF had reduced levels of GRK2 in IB4(+) small DRG neurons, but normal GRK2 levels in IB4(-) DRG neurons. Intraplantar administration of HSV-GRK2 amplicons to increase GRK2 protein levels prevented the prolongation of PGE₂-induced hyperalgesia in GDNF-primed mice. Conclusion: Low GRK2 in nociceptors is critical to develop a primed state in response to GDNF and leads to engagement of Epac signaling and transition to chronic PGE₂-induced hyperalgesia. Increasing GRK2 protein or inhibiting Epac signaling may represent new avenues for preventing transition to a chronic pain state.

Role of G protein-coupled receptor kinase 2 in oxidative and nitrosative stress-related neurohistopathological changes in a mouse model of sepsis-associated encephalopathy

Sepsis-associated encephalopathy (SAE), characterized as diffuse brain dysfunction and neurological manifestations secondary to sepsis, is a common complication in critically ill patients and can give rise to poor outcome, but understanding the molecular basis of this disorder remains a major challenge. Given the emerging role of G protein-coupled receptor 2 (GRK2), first identified as a G protein-coupled receptor (GPCR) regulator, in the regulation of non-G protein-coupled receptor-related molecules contributing to diverse cellular functions and pathology, including inflammation, we tested the hypothesis that GRK2 may be linked to the neuropathogenesis of SAE. When mouse MG6 microglial cells were challenged with lipopolysaccharide (LPS), GRK2 cytosolic expression was highly up-regulated. The ablation of GRK2 by small interfering RNAs (siRNAs) prevented an increase in intracellular reactive oxygen species generation in LPS-stimulated MG6 cells. Furthermore, the LPS-induced up-regulation of inducible nitric-oxide synthase expression and increase in nitric oxide production were negated by GRK2 inhibitor or siRNAs. However, GRK2 inhibition was without effect on overproduction of tumor necrosis factor-α, interleukin (IL)-6, and IL-1β in LPS-stimulated MG cells. In mice with cecal ligation and puncture-induced sepsis, treatment with GRK2 inhibitor reduced high levels of oxidative and nitrosative stress in the mice brains, where GRK2 expression was up-regulated, alleviated neurohistological damage observed in cerebral cortex sections, and conferred a significant survival advantage to CLP mice. Altogether, these results uncover the novel role for GRK2 in regulating cellular oxidative and nitrosative stress during inflammation and suggest that GRK2 may have a potential as an intriguing therapeutic target to prevent or treat SAE.

Proinflammatory Cytokines Mediate GPCR Dysfunction

Proinflammatory reaction by the body occurs acutely in response to injury that is considered primarily beneficial. However, sustained proinflammatory cytokines observed with chronic pathologies such as metabolic syndrome, cancer, and arthritis are detrimental and in many cases is a major cardiovascular risk factor. Proinflammatory cytokines such as interleukin-1, interleukin-6, and tumor necrosis factor α (TNFα) have long been implicated in cardiovascular risk and considered to be a major underlying cause for heart failure (HF). The failure of the anti-TNFα therapy for HF indicates our elusive understanding on the dichotomous role of proinflammatory cytokines on acutely beneficial effects versus long-term deleterious effects. Despite these well-described observations, less is known about the mechanistic underpinnings of proinflammatory cytokines especially TNFα in pathogenesis of HF. Increasing evidence suggests the existence of an active cross-talk between the TNFα receptor signaling and G-protein-coupled receptors such as β-adrenergic receptor (βAR). Given that βARs are the key regulators of cardiac function, the review will discuss the current state of understanding on the role of proinflammatory cytokine TNFα in regulating βAR function.

G protein-coupled receptor kinase-2-deficient mice are protected from dextran sodium sulfate-induced acute colitis

G protein-coupled receptor kinase 2 (GRK2) is a serine/threonine kinase and plays a key role in different disease processes. Previously, we showed that GRK2 knockdown enhances wound healing in colonic epithelial cells. Therefore, we hypothesized that ablation of GRK2 would protect mice from dextran sodium sulfate (DSS)-induced acute colitis. To test this, we administered DSS to wild-type (GRK2^+/+) and GRK2 heterozygous (GRK^+/-) mice in their drinking water for 7 days. As predicted, GRK2^+/- mice were protected from colitis as demonstrated by decreased weight loss (20% loss in GRK2^+/+ vs. 11% loss in GRK2^+/-). lower disease activity index (GRK2^+/+ 9.1 vs GRK2^+/- 4.1), and increased colon lengths (GRK2^+/+ 4.7 cm vs GRK2^+/- 5.3 cm). To examine the mechanisms by which GRK2^+/- mice are protected from colitis, we investigated expression of inflammatory genes in the colon as well as immune cell profiles in colonic lamina propria, mesenteric lymph node, and in bone marrow. Our results did not reveal differences in immune cell profiles between the two genotypes. However, expression of inflammatory genes was significantly decreased in DSS-treated GRK2^+/- mice compared with GRK2^+/+. To understand the mechanisms, we generated myeloid-specific GRK2 knockout mice and subjected them to DSS-induced colitis. Similar to whole body GRK2 heterozygous knockout mice, myeloid-specific knockout of GRK2 was sufficient for the protection from DSS-induced colitis. Together our results indicate that deficiency of GRK2 protects mice from DSS-induced colitis and further suggests that the mechanism of this effect is likely via GRK2 regulation of inflammatory genes in the myeloid cells.

G-protein-coupled receptor kinase-2 is a critical regulator of TNFα signaling in colon epithelial cells

G-protein-coupled receptor kinase-2 (GRK2) belongs to the GRK family of serine/threonine protein kinases critical in the regulation of G-protein-coupled receptors. Apart from this canonical role, GRK2 is also involved in several signaling pathways via distinct intracellular interactomes. In the present study, we examined the role of GRK2 in TNFα signaling in colon epithelial cell-biological processes including wound healing, proliferation, apoptosis, and gene expression. Knockdown of GRK2 in the SW480 human colonic cells significantly enhanced TNFα-induced epithelial cell wound healing without any effect on apoptosis/proliferation. Consistent with wound-healing effects, GRK2 knockdown augmented TNFα-induced matrix metalloproteinases (MMPs) 7 and 9, as well as urokinase plasminogen activator (uPA; factors involved in cell migration and wound healing). To assess the mechanism by which GRK2 affects these physiological processes, we examined the role of GRK2 in TNFα-induced MAPK and NF-κB pathways. Our results demonstrate that while GRK2 knockdown inhibited TNFα-induced IκBα phosphorylation, activation of ERK was significantly enhanced in GRK2 knockdown cells. Our results further demonstrate that GRK2 inhibits TNFα-induced ERK activation by inhibiting generation of reactive oxygen species (ROS). Together, these data suggest that GRK2 plays a critical role in TNFα-induced wound healing by modulating MMP7 and 9 and uPA levels via the ROS-ERK pathway. Consistent with in vitro findings, GRK2 heterozygous mice exhibited enhanced intestinal wound healing. Together, our results identify a novel role for GRK2 in TNFα signaling in intestinal epithelial cells.

G Protein-Coupled Receptor Kinases in the Inflammatory Response and Signaling

G protein-coupled receptor kinases (GRKs) are serine/threonine kinases that regulate a large and diverse class of G protein-coupled receptors (GPCRs). Through GRK phosphorylation and β-arrestin recruitment, GPCRs are desensitized and their signal terminated. Recent work on these kinases has expanded their role from canonical GPCR regulation to include noncanonical regulation of non-GPCR and nonreceptor substrates through phosphorylation as well as via scaffolding functions. Owing to these and other regulatory roles, GRKs have been shown to play a critical role in the outcome of a variety of physiological and pathophysiological processes including chemotaxis, signaling, migration, inflammatory gene expression, etc. This diverse set of functions for these proteins makes them popular targets for therapeutics. Role for these kinases in inflammation and inflammatory disease is an evolving area of research currently pursued in many laboratories. In this review, we describe the current state of knowledge on various GRKs pertaining to their role in inflammation and inflammatory diseases.

Adrenergic signaling in heart failure and cardiovascular aging

Both cardiovascular disease and aging are associated with changes in the sympathetic nervous system. Indeed, mounting evidence indicates that adrenergic receptors are functionally involved in numerous processes underlying both aging and cardiovascular disorders, in particular heart failure. This article will review the pathophysiological role of the sympathetic nervous system in heart failure and cardiovascular aging.

Cardiac Fibroblast GRK2 Deletion Enhances Contractility and Remodeling Following Ischemia/Reperfusion Injury

RATIONALE

G protein-coupled receptor kinase 2 (GRK2) is an important molecule upregulated after myocardial injury and during heart failure. Myocyte-specific GRK2 loss before and after myocardial ischemic injury improves cardiac function and remodeling. The cardiac fibroblast plays an important role in the repair and remodeling events after cardiac ischemia; the importance of GRK2 in these events has not been investigated.

OBJECTIVE

The aim of this study is to elucidate the in vivo implications of deleting GRK2 in the cardiac fibroblast after ischemia/reperfusion injury.

METHODS AND RESULTS

We demonstrate, using Tamoxifen inducible, fibroblast-specific GRK2 knockout mice, that GRK2 loss confers a protective advantage over control mice after myocardial ischemia/reperfusion injury. Fibroblast GRK2 knockout mice presented with decreased infarct size and preserved cardiac function 24 hours post ischemia/reperfusion as demonstrated by increased ejection fraction (59.1±1.8% versus 48.7±1.2% in controls; P<0.01). GRK2 fibroblast knockout mice also had decreased fibrosis and fibrotic gene expression. Importantly, these protective effects correlated with decreased infiltration of neutrophils to the ischemia site and decreased levels of tumor necrosis factor-α expression and secretion in GRK2 fibroblast knockout mice.

CONCLUSIONS

These novel data showing the benefits of inhibiting GRK2 in the cardiac fibroblast adds to previously published data showing the advantage of GRK2 ablation and reinforces the therapeutic potential of GRK2 inhibition in the heart after myocardial ischemia.

G-Protein-Coupled Receptor Kinase 2 (GRK2) Inhibitors: Current Trends and Future Perspectives

G-protein-coupled receptor kinase 2 (GRK2) is a G-protein-coupled receptor kinase that is ubiquitously expressed in many tissues and regulates various intracellular mechanisms. The up- or down-regulation of GRK2 correlates with several pathological disorders. GRK2 plays an important role in the maintenance of heart structure and function; thus, this kinase is involved in many cardiovascular diseases. GRK2 up-regulation can worsen cardiac ischemia; furthermore, increased kinase levels occur during the early stages of heart failure and in hypertensive subjects. GRK2 up-regulation can lead to changes in the insulin signaling cascade, which can translate to insulin resistance. Increased GRK2 levels also correlate with the degree of cognitive impairment that is typically observed in Alzheimer's disease. This article reviews the most potent and selective GRK2 inhibitors that have been developed. We focus on their mechanism of action, inhibition profile, and structure-activity relationships to provide insight into the further development of GRK2 inhibitors as drug candidates.

Chapter Three - Ubiquitination and Protein Turnover of G-Protein-Coupled Receptor Kinases in GPCR Signaling and Cellular Regulation

G-protein-coupled receptors (GPCRs) are responsible for regulating a wide variety of physiological processes, and distinct mechanisms for GPCR inactivation exist to guarantee correct receptor functionality. One of the widely used mechanisms is receptor phosphorylation by specific G-protein-coupled receptor kinases (GRKs), leading to uncoupling from G proteins (desensitization) and receptor internalization. GRKs and β-arrestins also participate in the assembly of receptor-associated multimolecular complexes, thus initiating alternative G-protein-independent signaling events. In addition, the abundant GRK2 kinase has diverse "effector" functions in cellular migration, proliferation, and metabolism homeostasis by means of the phosphorylation or interaction with non-GPCR partners. Altered expression of GRKs (particularly of GRK2 and GRK5) occurs during pathological conditions characterized by impaired GPCR signaling including inflammatory syndromes, cardiovascular disease, and tumor contexts. It is increasingly appreciated that different pathways governing GRK protein stability play a role in the modulation of kinase levels in normal and pathological conditions. Thus, enhanced GRK2 degradation by the proteasome pathway occurs upon GPCR stimulation, what allows cellular adaptation to chronic stimulation in a physiological setting. β-arrestins participate in this process by facilitating GRK2 phosphorylation by different kinases and by recruiting diverse E3 ubiquitin ligase to the receptor complex. Different proteolytic systems (ubiquitin-proteasome, calpains), chaperone activities and signaling pathways influence the stability of GRKs in different ways, thus endowing specificity to GPCR regulation as protein turnover of GRKs can be differentially affected. Therefore, modulation of protein stability of GRKs emerges as a versatile mechanism for feedback regulation of GPCR signaling and basic cellular processes.

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.

The ubiquitin ligase Mdm2 controls oligodendrocyte maturation by intertwining mTOR with G protein-coupled receptor kinase 2 in the regulation of GPR17 receptor desensitization

During oligodendrocyte precursor cell (OPC) differentiation, defective control of the membrane receptor GPR17 has been suggested to block cell maturation and impair remyelination under demyelinating conditions. After the immature oligodendrocyte stage, to enable cells to complete maturation, GPR17 is physiologically down-regulated via phosphorylation/desensitization by G protein-coupled receptor kinases (GRKs); conversely, GRKs are regulated by the "mammalian target of rapamycin" mTOR. However, how GRKs and mTOR are connected to each other in modulating GPR17 function and oligodendrogenesis has remained elusive. Here we show, for the first time, a role for Murine double minute 2 (Mdm2), a ligase previously involved in ubiquitination/degradation of the onco-suppressor p53 protein. In maturing OPCs, both rapamycin and Nutlin-3, a small molecule inhibitor of Mdm2-p53 interactions, increased GRK2 sequestration by Mdm2, leading to impaired GPR17 down-regulation and OPC maturation block. Thus, Mdm2 intertwines mTOR with GRK2 in regulating GPR17 and oligodendrogenesis and represents a novel actor in myelination.

G-protein-coupled receptor kinases in inflammation and disease

G-protein-coupled receptor kinases (GRKs) are serine/threonine protein kinases originally discovered for their role in G-protein-coupled receptor (GPCR) phosphorylation. Recent studies have demonstrated a much broader function for this kinase family including phosphorylation of cytosolic substrates involved in cell signaling pathways stimulated by GPCRs, as well as by non-GPCRs. In addition, GRKs modulate signaling via phosphorylation-independent functions. Because of these various biochemical functions, GRKs have been shown to affect critical physiological and pathophysiological processes, and thus are considered as drug targets in diseases such as heart failure. Role of GRKs in inflammation and inflammatory diseases is an evolving area of research and several studies including work from our lab in the recent years have demonstrated critical role of GRKs in the immune system. In this review, we discuss the classical and the newly emerging functions of GRKs in the immune system and their role in inflammation and disease processes.

Inhibition of G-Protein βγ Signaling Decreases Levels of Messenger RNAs Encoding Proinflammatory Cytokines in T Cell Receptor-Stimulated CD4(+) T Helper Cells

Background: Inhibition of G-protein βγ (Gβγ) signaling was found previously to enhance T cell receptor (TCR)-stimulated increases in interleukin 2 (IL-2) mRNA in CD4⁺ T helper cells, suggesting that Gβγ might be a useful drug target for treating autoimmune diseases, as low dose IL-2 therapy can suppress autoimmune responses. Because IL-2 may counteract autoimmunity in part by shifting CD4⁺ T helper cells away from the Type 1 T helper cell (TH1) and TH17 subtypes towards the TH2 subtype, the purpose of this study was to determine if blocking Gβγ signaling affected the balance of TH1, TH17, and TH2 cytokine mRNAs produced by CD4⁺ T helper cells.

Methods: Gallein, a small molecule inhibitor of Gβγ, and siRNA-mediated silencing of the G-protein β₁ subunit (Gβ₁) were used to test the effect of blocking Gβγ on mRNA levels of cytokines in primary human TCR-stimulated CD4⁺ T helper cells.

Results: Gallein and Gβ₁ siRNA decreased interferon-γ (IFN-γ) and IL-17A mRNA levels in TCR-stimulated CD4⁺ T cells grown under TH1-promoting conditions. Inhibiting Gβγ also decreased mRNA levels of STAT4, which plays a positive role in TH1 differentiation and IL-17A production. Moreover, mRNA levels of the STAT4-regulated TH1-associated proteins, IL-18 receptor β chain (IL-18Rβ), mitogen-activated protein kinase kinase kinase 8 (MAP3K8), lymphocyte activation gene 3 (LAG-3), natural killer cell group 7 sequence (NKG7), and oncostatin M (OSM) were also decreased upon Gβγ inhibition. Gallein also increased IL-4, IL-5, IL-9, and IL-13 mRNA levels in TCR-stimulated memory CD4⁺ T cells grown in TH2-promoting conditions.

Conclusions: Inhibiting Gβγ to produce these shifts in cytokine mRNA production might be beneficial for patients with autoimmune diseases such as rheumatoid arthritis (RA), Crohn’s disease (CD), psoriasis, multiple sclerosis (MS), and Hashimoto’s thyroiditis (HT), in which both IFN-γ and IL-17A are elevated.

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.

Development of cerebral gray and white matter injury and cerebral inflammation over time after inflammatory perinatal asphyxia

Antenatal inflammation is associated with increased severity of hypoxic-ischemic (HI) encephalopathy and adverse outcome in human neonates and experimental rodents. We investigated the effect of lipopolysaccharide (LPS) on the timing of HI-induced cerebral tissue loss and gray matter injury, white matter injury and integrity, and the cerebral inflammatory response. On postnatal day 9, mice underwent HI by unilateral carotid artery occlusion followed by systemic hypoxia which resulted in early neuronal damage (MAP2 loss) at 3 h that did not increase up to day 15. LPS injection 14 h before HI (LPS+HI) significantly and gradually aggravated MAP2 loss from 3 h up to day 15, resulting in an acellular cystic lesion. LPS+HI increased white matter damage, reduced myelination in the corpus callosum and increased white matter fiber coherency in the cingulum. The number of oligodendrocytes throughout the lineage (Olig2-positive) was increased whereas more mature myelinating (CNPase-positive) oligodendrocytes were strongly decreased after LPS+HI. LPS+HI induced an increased and prolonged expression of cerebral cytokines/chemokines compared to HI. Additionally, LPS+HI increased macrophage/microglia activation and influx of neutrophils in the brain compared to HI. This study demonstrates the sensitizing effect of LPS on neonatal HI brain injury for an extended time-frame up to 15 days postinsult. LPS before HI induced a gradual increase in gray and white matter deficits, including reduced numbers of more mature myelinating oligodendrocytes and a decrease in white matter integrity. Moreover, LPS+HI prolonged and intensified the cerebral inflammatory response, including cellular infiltration. In conclusion, as the timing of damage and/or involved pathways are changed when HI is preceded by inflammation, experimental therapies might require modifications in the time window, dosage or combinations of therapies for efficacious neuroprotection.

Monocytes/Macrophages control resolution of transient inflammatory pain

Insights into mechanisms governing resolution of inflammatory pain are of great importance for many chronic pain-associated diseases. Here we investigate the role of macrophages/monocytes and the anti-inflammatory cytokine interleukin-10 (IL-10) in the resolution of transient inflammatory pain. Depletion of mice from peripheral monocytes/macrophages delayed resolution of intraplantar IL-1β- and carrageenan-induced inflammatory hyperalgesia from 1 to 3 days to >1 week. Intrathecal administration of a neutralizing IL-10 antibody also markedly delayed resolution of IL-1β- and carrageenan-induced inflammatory hyperalgesia. Recently, we showed that IL-1β- and carrageenan-induced hyperalgesia is significantly prolonged in LysM-GRK2(+/-) mice, which have reduced levels of G-protein-coupled receptor kinase 2 (GRK2) in LysM(+) myeloid cells. Here we show that adoptive transfer of wild-type, but not of GRK2(+/-), bone marrow-derived monocytes normalizes the resolution of IL-1β-induced hyperalgesia in LysM-GRK2(+/-) mice. Adoptive transfer of IL-10(-/-) bone marrow-derived monocytes failed to normalize the duration of IL-1β-induced hyperalgesia in LysM-GRK2(+/-) mice. Mechanistically, we show that GRK2(+/-) macrophages produce less IL-10 in vitro. In addition, intrathecal IL-10 administration attenuated IL-1β-induced hyperalgesia in LysM-GRK2(+/-) mice, whereas it had no effect in wild-type mice. Our data uncover a key role for monocytes/macrophages in promoting resolution of inflammatory hyperalgesia via a mechanism dependent on IL-10 signaling in dorsal root ganglia.

PERSPECTIVE

We show that IL-10-producing monocytes/macrophages promote resolution of transient inflammatory hyperalgesia. Additionally, we show that reduced monocyte/macrophage GRK2 impairs resolution of hyperalgesia and reduces IL-10 production. We propose that low GRK2 expression and/or impaired IL-10 production by monocytes/macrophages represent peripheral biomarkers for the risk of developing chronic pain after inflammation.

Dexmedetomidine alleviates rat post-ischemia induced allodynia through GRK2 upregulation in superior cervical ganglia

A transient decrease in G protein-coupled receptor kinase 2 (GRK2) in nociceptors can produce long-lasting neuroplastic changes in nociceptor function, eventually enhancing and prolonging inflammatory hyperalgesia. Here, we investigated the effects of selective α2-adrenoceptor agonist dexmedetomidine (DMED) on GRK2 expression in superior cervical ganglion (SCG) in a rat model of complex regional pain syndrome type I (CRPS-I). The ipsilateral 50% paw withdrawal thresholds (PWTs) to mechanical stimuli decreased significantly starting from 24 h after ischemia-reperfusion (I/R) injury, and lasted for over 3 weeks; the ipsilateral cold allodynia scores, GRK2 protein and mRNA levels in SCGs all increased significantly. No significant differences were found in the contralateral side except GRK2 mRNA reduced significantly after 48 h I/R injury, but still higher than those in the ipsilateral side. Following daily injection of 10 μg/kg of DMED for a maximum of 7 days, the ipsilateral PWTs on days 1, 2, 7, 14, and 21 after DMED administration were significantly higher than those in control group; the GRK2 protein and mRNA expressions in the ipsilateral SCGs were also significantly upregulated; the ipsilateral cold allodynia scores were significantly reduced. No significant differences were found in the contralateral 50%PWTs, cold allodynia scores, and GRK2 protein level except GRK2 mRNA levels increased significantly on days 1 to 7 after DMED administration. Therefore, a transient decrease of GRK2 expression in SCG neurons might be involved in the development and maintenance of allodynia in CRPS-I and DMED might alleviate this allodynia through GRK2 upregulation in SCG neurons.

G protein-coupled receptor kinase 2 moderates recruitment of THP-1 cells to the endothelium by limiting histamine-invoked Weibel-Palade body exocytosis

BACKGROUND

G protein-coupled receptors (GPCRs) are a major family of signaling molecules, central to the regulation of inflammatory responses. Their activation upon agonist binding is attenuated by GPCR kinases (GRKs), which desensitize the receptors through phosphorylation. G protein-coupled receptor kinase 2(GRK2) down-regulation in leukocytes has been closely linked to the progression of chronic inflammatory disorders such as rheumatoid arthritis and multiple sclerosis. Because leukocytes must interact with the endothelium to infiltrate inflamed tissues, we hypothesized that GRK2 down-regulation in endothelial cells would also be pro-inflammatory.

OBJECTIVES

To determine whether GRK2 down-regulation in endothelial cells is pro-inflammatory.

METHODS

siRNA-mediated ablation of GRK2 in human umbilical vein endothelial cells (HUVECs) was used in analyses of the role of this kinase. Microscopic and biochemical analyses of Weibel-Palade body (WPB) formation and functioning, live cell imaging of calcium concentrations and video analyses of adhesion of monocyte-like THP-1 cells provide clear evidence of GRK2 function in histamine activation of endothelial cells.

RESULTS

G protein-coupled receptor kinase 2 depletion in HUVECs increases WPB exocytosis and P-selectin-dependent adhesion of THP-1 cells to the endothelial surface upon histamine stimulation, relative to controls. Further, live imaging of intracellular calcium concentrations reveals amplified histamine receptor signaling in GRK2-depleted cells, suggesting GRK2 moderates WPB exocytosis through receptor desensitization.

CONCLUSIONS

G protein-coupled receptor kinase 2 deficiency in endothelial cells results in increased pro-inflammatory signaling and enhanced leukocyte recruitment to activated endothelial cells. The ability of GRK2 to modulate initiation of inflammatory responses in endothelial cells as well as leukocytes now places GRK2 at the apex of control of this finely balanced process.

Neuroinflammation and comorbidity of pain and depression

Comorbid depression and chronic pain are highly prevalent in individuals suffering from physical illness. Here, we critically examine the possibility that inflammation is the common mediator of this comorbidity, and we explore the implications of this hypothesis. Inflammation signals the brain to induce sickness responses that include increased pain and negative affect. This is a typical and adaptive response to acute inflammation. However, chronic inflammation induces a transition from these typical sickness behaviors into depression and chronic pain. Several mechanisms can account for the high comorbidity of pain and depression that stem from the precipitating inflammation in physically ill patients. These mechanisms include direct effects of cytokines on the neuronal environment or indirect effects via downregulation of G protein-coupled receptor kinase 2, activation of the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase that generates neurotropic kynurenine metabolites, increased brain extracellular glutamate, and the switch of GABAergic neurotransmission from inhibition to excitation. Despite the existence of many neuroimmune candidate mechanisms for the co-occurrence of depression and chronic pain, little work has been devoted so far to critically assess their mediating role in these comorbid symptoms. Understanding neuroimmune mechanisms that underlie depression and pain comorbidity may yield effective pharmaceutical targets that can treat both conditions simultaneously beyond traditional antidepressants and analgesics.

Balancing GRK2 and EPAC1 levels prevents and relieves chronic pain

Chronic pain is a major clinical problem, yet the mechanisms underlying the transition from acute to chronic pain remain poorly understood. In mice, reduced expression of GPCR kinase 2 (GRK2) in nociceptors promotes cAMP signaling to the guanine nucleotide exchange factor EPAC1 and prolongs the PGE2-induced increase in pain sensitivity (hyperalgesia). Here we hypothesized that reduction of GRK2 or increased EPAC1 in dorsal root ganglion (DRG) neurons would promote the transition to chronic pain. We used 2 mouse models of hyperalgesic priming in which the transition from acute to chronic PGE2-induced hyperalgesia occurs. Hyperalgesic priming with carrageenan induced a sustained decrease in nociceptor GRK2, whereas priming with the PKCε agonist ΨεRACK increased DRG EPAC1. When either GRK2 was increased in vivo by viral-based gene transfer or EPAC1 was decreased in vivo, as was the case for mice heterozygous for Epac1 or mice treated with Epac1 antisense oligodeoxynucleotides, chronic PGE2-induced hyperalgesia development was prevented in the 2 priming models. Using the CFA model of chronic inflammatory pain, we found that increasing GRK2 or decreasing EPAC1 inhibited chronic hyperalgesia. Our data suggest that therapies targeted at balancing nociceptor GRK2 and EPAC1 levels have promise for the prevention and treatment of chronic pain.

Endothelial G protein-coupled receptor kinase 2 regulates vascular homeostasis through the control of free radical oxygen species

OBJECTIVE

The role of endothelial G protein-coupled receptor kinase 2 (GRK2) was investigated in mice with selective deletion of the kinase in the endothelium (Tie2-CRE/GRK2(fl/fl)).

APPROACH AND RESULTS

Aortas from Tie2-CRE/GRK2(fl/fl) presented functional and structural alterations as compared with control GRK2(fl/fl) mice. In particular, vasoconstriction was blunted to different agonists, and collagen and elastic rearrangement and macrophage infiltration were observed. In primary cultured endothelial cells deficient for GRK2, mitochondrial reactive oxygen species was increased, leading to expression of cytokines. Chronic treatment with a reactive oxygen species scavenger in mice corrected the vascular phenotype by recovering vasoconstriction, structural abnormalities, and reducing macrophage infiltration.

CONCLUSIONS

These results demonstrate that GRK2 removal compromises vascular phenotype and integrity by increasing endothelial reactive oxygen species production.

Mitochondrial G protein coupled receptor kinase 2 regulates proinflammatory responses in macrophages

G-protein-coupled receptor kinase 2 (GRK2) levels are elevated in inflammation but its role is not clear yet. Here we show that GRK2 expression is dependent on NFκB transcriptional activity. In macrophages, LPS induces GRK2 accumulation in mitochondria increasing biogenesis. The overexpression of the carboxy-terminal domain of GRK2 (βARK-ct), known to displace GRK2 from plasma membranes, induces earlier localization of GRK2 to mitochondria in response to LPS leading to increased mt-DNA transcription and reduced ROS production and cytokine expression. Our study shows the relevance of GRK2 subcellular localization in macrophage biology and its potential therapeutic properties in inflammation.

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.

Targeting G protein coupled receptor-related pathways as emerging molecular therapies

G protein coupled receptors (GPCRs) represent the most important targets in modern pharmacology because of the different functions they mediate, especially within brain and peripheral nervous system, and also because of their functional and stereochemical properties. In this paper, we illustrate, via a variety of examples, novel advances about the GPCR-related molecules that have been shown to play diverse roles in GPCR pathways and in pathophysiological phenomena. We have exemplified how those GPCRs’ pathways are, or might constitute, potential targets for different drugs either to stimulate, modify, regulate or inhibit the cellular mechanisms that are hypothesized to govern some pathologic, physiologic, biologic and cellular or molecular aspects both in vivo and in vitro. Therefore, influencing such pathways will, undoubtedly, lead to different therapeutical applications based on the related pharmacological implications. Furthermore, such new properties can be applied in different fields. In addition to offering fruitful directions for future researches, we hope the reviewed data, together with the elements found within the cited references, will inspire clinicians and researchers devoted to the studies on GPCR’s properties.

The downstream regulation of chemokine receptor signalling: implications for atherosclerosis

Heterotrimeric G-protein-coupled receptors (GPCRs) are key mediators of intracellular signalling, control numerous physiological processes, and are one of the largest class of proteins to be pharmacologically targeted. Chemokine-induced macrophage recruitment into the vascular wall is an early pathological event in the progression of atherosclerosis. Leukocyte activation and chemotaxis during cell recruitment are mediated by chemokine ligation of multiple GPCRs. Regulation of GPCR signalling is critical in limiting vascular inflammation and involves interaction with downstream proteins such as GPCR kinases (GRKs), arrestin proteins and regulator of G-protein signalling (RGS) proteins. These have emerged as new mediators of atherogenesis by functioning in internalisation, desensitisation, and signal termination of chemokine receptors. Targeting chemokine signalling through these proteins may provide new strategies to alter atherosclerotic plaque formation and plaque biology.

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.

Hematopoietic G-protein-coupled receptor kinase 2 deficiency decreases atherosclerotic lesion formation in LDL receptor-knockout mice

Leukocyte chemotaxis is deemed instrumental in initiation and progression of atherosclerosis. It is mediated by G-protein-coupled receptors (e.g., CCR2 and CCR5), the activity of which is controlled by G-protein-coupled receptor kinases (GRKs). In this study, we analyzed the effect of hematopoietic deficiency of a potent regulator kinase of chemotaxis (GRK2) on atherogenesis. LDL receptor-deficient (LDLr(-/-)) mice with heterozygous hematopoietic GRK2 deficiency, generated by bone marrow transplantation (n=15), displayed a dramatic attenuation of plaque development, with 79% reduction in necrotic core and increased macrophage content. Circulating monocytes decreased and granulocytes increased in GRK2(+/-) chimeras, which could be attributed to diminished granulocyte colony-forming units in bone marrow. Collectively, these data pointed to myeloid cells as major mediators of the impaired atherogenic response in GRK2(+/-) chimeras. LDLr(-/-) mice with macrophage/granulocyte-specific GRK2 deficiency (LysM-Cre GRK2(flox/flox); n=8) failed to mimic the aforementioned phenotype, acquitting these cells as major responsible subsets for GRK2 deficiency-associated atheroprotection. To conclude, even partial hematopoietic GRK2 deficiency prevents atherosclerotic lesion progression beyond the fatty streak stage, identifying hematopoietic GRK2 as a potential target for intervention in atherosclerosis.

End-point effector stress mediators in neuroimmune interactions: their role in immune system homeostasis and autoimmune pathology

Much evidence has identified a direct anatomical and functional link between the brain and the immune system, with glucocorticoids (GCs), catecholamines (CAs), and neuropeptide Y (NPY) as its end-point mediators. This suggests the important role of these mediators in immune system homeostasis and the pathogenesis of inflammatory autoimmune diseases. However, although it is clear that these mediators can modulate lymphocyte maturation and the activity of distinct immune cell types, their putative role in the pathogenesis of autoimmune disease is not yet completely understood. We have contributed to this field by discovering the influence of CAs and GCs on fine-tuning thymocyte negative selection and, in particular, by pointing to the putative CA-mediated mechanisms underlying this influence. Furthermore, we have shown that CAs are implicated in the regulation of regulatory T-cell development in the thymus. Moreover, our investigations related to macrophage biology emphasize the complex interaction between GCs, CAs and NPY in the modulation of macrophage functions and their putative significance for the pathogenesis of autoimmune inflammatory diseases.

Prostaglandin E2 affects T cell responses through modulation of CD46 expression

The ubiquitous protein CD46, a regulator of complement activity, promotes T cell activation and differentiation toward a regulatory Tr1-like phenotype. The CD46-mediated differentiation pathway is defective in several chronic inflammatory diseases, underlying the importance of CD46 in controlling T cell function and the need to understand its regulatory mechanisms. Using an RNA interference-based screening approach in primary T cells, we have identified that two members of the G protein-coupled receptor kinases were involved in regulating CD46 expression at the surface of activated cells. We have investigated the role of PGE(2), which binds to the E-prostanoid family of G protein-coupled receptors through four subtypes of receptors called EP 1-4, in the regulation of CD46 expression and function. Conflicting roles of PGE(2) in T cell functions have been reported, and the reasons for these apparent discrepancies are not well understood. We show that addition of PGE(2) strongly downregulates CD46 expression in activated T cells. Moreover, PGE(2) differentially affects T cell activation, cytokine production, and phenotype depending on the activation signals received by the T cells. This was correlated with a distinct pattern of the PGE(2) receptors expressed, with EP4 being preferentially induced by CD46 activation. Indeed, addition of an EP4 antagonist could reverse the effects observed on cytokine production after CD46 costimulation. These data demonstrate a novel role of the PGE(2)-EP4 axis in CD46 functions, which might at least partly explain the diverse roles of PGE(2) in T cell functions.

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.

GRK2-dependent S1PR1 desensitization is required for lymphocytes to overcome their attraction to blood

Lymphocytes egress from lymphoid organs in response to sphingosine-1-phosphate (S1P); minutes later they migrate from blood into tissue against the S1P gradient. The mechanisms facilitating cell movement against the gradient have not been defined. Here, we show that heterotrimeric guanine nucleotide-binding protein-coupled receptor kinase-2 (GRK2) functions in down-regulation of S1P receptor-1 (S1PR1) on blood-exposed lymphocytes. T and B cell movement from blood into lymph nodes is reduced in the absence of GRK2 but is restored in S1P-deficient mice. In the spleen, B cell movement between the blood-rich marginal zone and follicles is disrupted by GRK2 deficiency and by mutation of an S1PR1 desensitization motif. Moreover, delivery of systemic antigen into follicles is impaired. Thus, GRK2-dependent S1PR1 desensitization allows lymphocytes to escape circulatory fluids and migrate into lymphoid tissues.

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

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