A targetable pathway in neutrophils mitigates both arterial and venous thrombosis

Arterial and venous thrombosis constitutes a major source of morbidity and mortality worldwide. Long considered as distinct entities, accumulating evidence indicates that arterial and venous thrombosis can occur in the same populations, suggesting that common mechanisms are likely operative. Although hyperactivation of the immune system is a common forerunner to the genesis of thrombotic events in both vascular systems, the key molecular control points remain poorly understood. Consequently, antithrombotic therapies targeting the immune system for therapeutics gain are lacking. Here, we show that neutrophils are key effectors of both arterial and venous thrombosis and can be targeted through immunoregulatory nanoparticles. Using antiphospholipid antibody syndrome (APS) as a model for arterial and venous thrombosis, we identified the transcription factor Krüppel-like factor 2 (KLF2) as a key regulator of neutrophil activation. Upon activation through genetic loss of KLF2 or administration of antiphospholipid antibodies, neutrophils clustered P-selectin glycoprotein ligand 1 (PSGL-1) by cortical actin remodeling, thereby increasing adhesion potential at sites of thrombosis. Targeting clustered PSGL-1 using nanoparticles attenuated neutrophil-mediated thrombosis in APS and KLF2 knockout models, illustrating the importance and feasibility of targeting activated neutrophils to prevent pathological thrombosis. Together, our results demonstrate a role for activated neutrophils in both arterial and venous thrombosis and identify key molecular events that serve as potential targets for therapeutics against diverse causes of immunothrombosis.

Correction: The thromboprotective effect of traditional Chinese medicine Tongji 2 granules is dependent on anti-inflammatory activity by suppression of NF-κB pathways

[This corrects the article DOI: 10.1371/journal.pone.0241607.].

Myeloid Krüppel-like factor 2 is a critical regulator of metabolic inflammation

Substantial evidence implicates crosstalk between metabolic tissues and the immune system in the inception and progression of obesity. However, molecular regulators that orchestrate metaflammation both centrally and peripherally remains incompletely understood. Here, we identify myeloid Krüppel-like factor 2 (KLF2) as an essential regulator of obesity and its sequelae. In mice and humans, consumption of a fatty diet downregulates myeloid KLF2 levels. Under basal conditions, myeloid-specific KLF2 knockout mice (K2KO) exhibit increased feeding and weight gain. High-fat diet (HFD) feeding further exacerbates the K2KO metabolic disease phenotype. Mechanistically, loss of myeloid KLF2 increases metaflammation in peripheral and central tissues. A combination of pair-feeding, bone marrow-transplant, and microglial ablation implicate central and peripheral contributions to K2KO-induced metabolic dysfunction observed. Finally, overexpression of myeloid KLF2 protects mice from HFD-induced obesity and insulin resistance. Together, these data establish myeloid KLF2 as a nodal regulator of central and peripheral metabolic inflammation in homeostasis and disease.

Inflammation contributes to the development of metabolic disease through incompletely understood mechanisms. Here the authors report that deletion of the transcription factor KLF2 in myeloid cells leads to increased feeding and weight gain in mice with concomitant peripheral and central tissue inflammation, while overexpression protects against diet-induced metabolic disease.

OR04-04 Identification of a Novel Transcriptional Regulator of Metabolic Disease in Circulating and Central Myeloid Cells

Derangement in systemic metabolic homeostasis is tightly associated with widespread activation of resident and circulating immune cells, a phenomenon known as ‘metaflammation’. Numerous studies have explored the role of tissue resident and circulating macrophages in contributing to metaflammation, obesity, and their sequelae; however, there is a dearth of information regarding targetable transcriptional regulators of the genesis and persistence of metabolic disease. Here, we identify myeloid Krüppel-like factor 2 (KLF2) as a novel regulator of metabolic disease. Previous reports demonstrate that KLF2 serves as a critical regulator of myeloid cell quiescence and is downregulated in numerous acute and chronic inflammatory states. Specifically in the context of chronic metaflammation, we note that KLF2 expression is decreased in circulating immune cells of obese patients and in adipose tissue macrophages of high fat diet (HFD) fed mice, which is consistent with the hypothesis that KLF2 regulates metaflammation. To explore this further, we utilized mice with myeloid cell-specific deletion of KLF2 (K2KO) which exhibit accelerated obesity and insulin resistance. K2KO mice have widespread central (i.e. CNS) and peripheral metaflammation both in the basal and HFD-stimulated states. To discern whether the effect of myeloid deletion of KLF2 on metabolism is due to deletion in microglia in the feeding centers of the hypothalamus or in peripheral immune cells, bone marrow chimeras with head shielding were created. 50% reconstitution of circulating immune cells with K2KO cells in wildtype (WT) mice was sufficient to maintain the metabolic disease phenotype, while mice with K2KO microglia + WT circulating cells had only slightly improved outcomes compared to K2KO mice. Conversely, ablation of microglia in K2KO mice using PLX5622 formulated in HFD also successfully attenuated the aberrant feeding behavior, weight gain, and glucose dyshomeostasis seen in K2KO mice. Together, these data demonstrate a role for loss of KLF2 in hematopoietic and CNS resident cells in causing metabolic disease. Given that myeloid KLF2 expression decreases under metabolic stress in WT mice and humans, we sought to explore whether maintenance of KLF2 expression in these cells would be protective against diet-induced metabolic disease. Indeed, mice with myeloid-specific overexpression of KLF2 demonstrated a markedly improved metabolic phenotype when challenged with HFD, providing evidence that targeting KLF2 expression in myeloid cells may prove to be a therapeutic option against metaflammation.

GPCRs and Insulin Receptor Signaling in Conversation: Novel Avenues for Drug Discovery

Type 2 diabetes is a major health issue worldwide with complex metabolic and endocrine abnormalities. Hyperglycemia, defects in insulin secretion and insulin resistance are classic features of type 2 diabetes. Insulin signaling regulates metabolic homeostasis by regulating glucose and lipid turnover in the liver, skeletal muscle and adipose tissue. Major treatment modalities for diabetes include the drugs from the class of sulfonyl urea, Insulin, GLP-1 agonists, SGLT2 inhibitors, DPP-IV inhibitors and Thiazolidinediones. Emerging antidiabetic therapeutics also include classes of drugs targeting GPCRs in the liver, adipose tissue and skeletal muscle. Interestingly, recent research highlights several shared intermediates between insulin and GPCR signaling cascades opening potential novel avenues for diabetic drug discovery.

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.

cAMP assays in GPCR drug discovery

GPCRs are a major family of cell surface receptors and the most druggable protein targets in modern medicine. Recent elucidation of crystal structures of a vast number of GPCRs eludes to the finer details of biased agonism or functional selectivity of many of these receptors and warrants a better understanding of their biological effects as well as therapeutic potential. Receptor function is measured in terms desensitization/resensitization, which provides insights on receptor activation and differential coupling to various G proteins. This review article presents thoughts on the potential of GPCR desensitization assay/cAMP assay, a highly sensitive and versatile platform for high-throughput assays for elucidating novel functions of many orphan GPCRs. Besides, these assays also are very sensitive to screen for agonists, partial agonists, or antagonists and provide a simplified system for novel drug discovery for Gs-coupled GPCRs and understanding their physiological implications.

Abstract 189: βAR Resensitization At The Plasma Membrane Provides Beneficial Cardiac Remodeling

βAR downregulation and desensitization are hallmarks of heart failure. Agonist occupied βARs undergo desensitization through phosphorylation by G-protein coupled receptor kinases leading to βAR internalization. Phosphorylated βAR becomes resensitized following dephosphorylation by PP2A in the endosomes. In contrast to this paradigm our recent studies have shown that resensitization of βARs can occur at the plasma membrane through the inhibition of PP2A activity by I2PP2A. We generated a stable HEK cell line expressing short hairpin RNA targeting I2PP2A (shRNA-I2PP2A). Confocal microscopy studies show cells expressing shRNA-I2PP2A resulted in significant loss of receptor phosphorylation despite the presence of the agonist. Radioligand binding and confocal imaging also showed marked inhibition of receptor internalization upon depletion of I2PP2A with significant PP2A activation. We also observed preservation of βAR function despite the presence of agonist measured by cAMP generation and adenylyl cyclase activity. To further dissect the interaction of I2PP2A-PP2A we generated mutants of PP2A from amino acids 263-309. Over expression of these PP2A mutant peptides significantly reversed receptor phosphorylation upon isoproterenol (ISO) stimulation compared to full length PP2A. Also, expression of PP2A mutant showed marked increase in adenylyl cyclase activity in response ISO stimulation suggesting that this mutant I2PP2A competes out inhibitory I2PP2A interaction with PP2A. To test whether alteration in βAR resensitization contributes to cardiac dysfunction with stress we generated transgenic (Tg) mice with cardiac specific over expression of wt I2PP2A and mutant I2PP2A (phopspho- and dephospho-). ISO treatment of wt I2PP2A Tg and littermate controls showed that wt I2PP2A Tg mice had significant βAR dysfunction and cardiac hypertrophy. Assessment of age dependent cardiac function of these mice showed that wt I2PP2A Tg and phospho-I2PP2A Tg mice have cardiac hypertrophic response followed by dilated cardiomyopathy; expression of dephospho-I2PP2A mutant reversed this phenotype. These studies suggest targeting I2PP2A alters receptor function and may have implications in cardiac remodeling and hypertrophy with cardiac stress.

Gβγ-independent recruitment of G-protein coupled receptor kinase 2 drives tumor necrosis factor α-induced cardiac β-adrenergic receptor dysfunction

BACKGROUND

Proinflammatory cytokine tumor necrosis factor-α (TNFα) induces β-adrenergic receptor (βAR) desensitization, but mechanisms proximal to the receptor in contributing to cardiac dysfunction are not known.

METHODS AND RESULTS

Two different proinflammatory transgenic mouse models with cardiac overexpression of myotrophin (a prohypertrophic molecule) or TNFα showed that TNFα alone is sufficient to mediate βAR desensitization as measured by cardiac adenylyl cyclase activity. M-mode echocardiography in these mouse models showed cardiac dysfunction paralleling βAR desensitization independent of sympathetic overdrive. TNFα-mediated βAR desensitization that precedes cardiac dysfunction is associated with selective upregulation of G-protein coupled receptor kinase 2 (GRK2) in both mouse models. In vitro studies in β2AR-overexpressing human embryonic kidney 293 cells showed significant βAR desensitization, GRK2 upregulation, and recruitment to the βAR complex following TNFα. Interestingly, inhibition of phosphoinositide 3-kinase abolished GRK2-mediated βAR phosphorylation and GRK2 recruitment on TNFα. Furthermore, TNFα-mediated βAR phosphorylation was not blocked with βAR antagonist propranolol. Additionally, TNFα administration in transgenic mice with cardiac overexpression of Gβγ-sequestering peptide βARK-ct could not prevent βAR desensitization or cardiac dysfunction showing that GRK2 recruitment to the βAR is Gβγ independent. Small interfering RNA knockdown of GRK2 resulted in the loss of TNFα-mediated βAR phosphorylation. Consistently, cardiomyocytes from mice with cardiac-specific GRK2 ablation normalized the TNFα-mediated loss in contractility, showing that TNFα-induced βAR desensitization is GRK2 dependent.

CONCLUSIONS

TNFα-induced βAR desensitization is mediated by GRK2 and is independent of Gβγ, uncovering a hitherto unknown cross-talk between TNFα and βAR function, providing the underpinnings of inflammation-mediated cardiac dysfunction.

miRNA-548c: a specific signature in circulating PBMCs from dilated cardiomyopathy patients

High fidelity genome-wide expression analysis has strengthened the idea that microRNA (miRNA) signatures in peripheral blood mononuclear cells (PBMCs) can be potentially used to predict the pathology when anatomical samples are inaccessible like the heart. PBMCs from 48 non-failing controls and 44 patients with relatively stable chronic heart failure (ejection fraction of ≤ 40%) associated with dilated cardiomyopathy (DCM) were used for miRNA analysis. Genome-wide miRNA-microarray on PBMCs from chronic heart failure patients identified miRNA signature uniquely characterized by the downregulation of miRNA-548 family members. We have also independently validated downregulation of miRNA-548 family members (miRNA-548c & 548i) using real time-PCR in a large cohort of independent patient samples. Independent in silico Ingenuity Pathway Analysis (IPA) of miRNA-548 targets shows unique enrichment of signaling molecules and pathways associated with cardiovascular disease and hypertrophy. Consistent with specificity of miRNA changes with pathology, PBMCs from breast cancer patients showed no alterations in miRNA-548c expression compared to healthy controls. These studies suggest that miRNA-548 family signature in PBMCs can therefore be used to detect early heart failure. Our studies show that cognate networking of predicted miRNA-548 targets in heart failure can be used as a powerful ancillary tool to predict the ongoing pathology.

Phosphoinositide 3-kinase γ inhibits cardiac GSK-3 independently of Akt

Activation of cardiac phosphoinositide 3-kinase α (PI3Kα) by growth factors, such as insulin, or activation of PI3Kγ downstream of heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptors stimulates the activity of the kinase Akt, which phosphorylates and inhibits glycogen synthase kinase-3 (GSK-3). We found that PI3Kγ inhibited GSK-3 independently of the insulin-PI3Kα-Akt axis. Although insulin treatment activated Akt in PI3Kγ knockout mice, phosphorylation of GSK-3 was decreased compared to control mice. GSK-3 is activated when dephosphorylated by the protein phosphatase 2A (PP2A), which is activated when methylated by the PP2A methyltransferase PPMT-1. PI3Kγ knockout mice showed increased activity of PPMT-1 and PP2A and enhanced nuclear export of the GSK-3 substrate NFATc3. GSK-3 inhibits cardiac hypertrophy, and the hearts of PI3Kγ knockout mice were smaller compared to those of wild-type mice. Cardiac overexpression of a catalytically inactive PI3Kγ (PI3Kγ(inact)) transgene in PI3Kγ knockout mice reduced the activities of PPMT-1 and PP2A and increased phosphorylation of GSK-3. Furthermore, PI3Kγ knockout mice expressing the PI3Kγ(inact) transgene had larger hearts than wild-type or PI3Kγ knockout mice. Our studies show that a kinase-independent function of PI3Kγ could directly inhibit GSK-3 function by preventing the PP2A-PPMT-1 interaction and that this inhibition of GSK-3 was independent of Akt.

Abstract 218: PI3Kγ Regulates Age-Dependent Cardiac Hypertrophy Through Kinase-Independent GSK-3-PP2A Axis

Activation of phosphoinositide 3-kinase α (PI3Kα) by Receptor Tyrosine Kinase (RTK) or PI3Kγ by G-protein coupled receptor (GPCR) inhibits glycogen synthase kinase-3 (GSK-3) via protein kinase B (Akt). We show that in addition to promoting GSK-3 phosphorylation through Akt, PI3Kγ in parallel suppresses PP2A dependent GSK-3 dephosphorylation. This is evidenced by accelerated GSK-3 dephosphorylation in PI3Kγ knock out (PI3Kγ-KO) mice downstream of RTK-PI3Kα-Akt axis despite robust Akt activation by insulin. Confocal microscopy and immunoblotting show marked reduction of steady state GSK-3 phosphorylation in PI3Kγ-KO compared to littermate controls. Assessment of GSK-3 dephosphorylating enzyme protein phosphatase 2A (PP2A) showed significant elevation in PP2A and GSK-associated phosphatase activity in PI3Kγ-KO mice compared to controls. Mechanistically, we found that elevated PP2A activity in PI3Kγ-KO was due to PP2A methylation mediated by elevated PP2A methyl transferase (PPMT-1) activity. Consistent with the elevated anti-hypertrophic GSK-3 activity, we observed reduced heart size in PI3Kγ-KO mice at 6, 12, and 18 months compared to age matched littermate controls. To test in vivo whether PI3Kγ activity regulates cardiac GSK-3 function through PP2A, we bred transgenic mice with cardiac overexpression of inactive PI3Kγ (PI3Kγ inact ) with PI3Kγ-KO mice. Surprisingly, cardiac overexpression of PI3Kγ inact transgene in PI3Kγ-KO background completely normalized cardiac PPMT-1 activity resulting in reduced PP2A activity and increased GSK-3 phosphorylation. Expression of PI3Kγ inact transgene in PI3Kγ-KO resulted in normalization of heart size compared to PI3Kγ-KO littermates consistent with the increased GSK-3 phosphorylation and consequent inhibition of GSK-3 activity suggesting a novel kinase independent role of PI3Kγ downstream of growth factor receptor in regulating cardiac growth with age.

G-protein coupled receptor resensitization-appreciating the balancing act of receptor function

G-protein coupled receptors (GPCRs) are seven transmembrane receptors that are pivotal regulators of cellular responses including vision, cardiac contractility, olfaction, and platelet activation. GPCRs have been a major target for drug discovery due to their role in regulating a broad range of physiological and pathological responses. GPCRs mediate these responses through a cyclical process of receptor activation (initiation of downstream signals), desensitization (inactivation that results in diminution of downstream signals), and resensitization (receptor reactivation for next wave of activation). Although these steps may be of equal importance in regulating receptor function, significant advances have been made in understanding activation and desensitization with limited effort towards resensitization. Inadequate importance has been given to resensitization due to the understanding that resensitization is a homeostasis maintaining process and is not acutely regulated. Evidence indicates that resensitization is a critical step in regulating GPCR function and may contribute towards receptor signaling and cellular responses. In light of these observations, it is imperative to discuss resensitization as a dynamic and mechanistic regulator of GPCR function. In this review we discuss components regulating GPCR function like activation, desensitization, and internalization with special emphasis on resensitization. Although we have used β-adrenergic receptor as a proto-type GPCR to discuss mechanisms regulating receptor function, other GPCRs are also described to put forth a view point on the universality of such mechanisms.

Regulation of β-adrenergic receptor function: an emphasis on receptor resensitization

G protein-coupled receptors are the largest family of cell surface receptors regulating multiple cellular processes. β-adrenergic receptor (βAR) is a prototypical member of GPCR family and has been one of the most well studied receptors in determining regulation of receptor function. Agonist activation of βAR leads to conformational change resulting in coupling to G protein generating cAMP as secondary messenger. The activated βAR is phosphorylated resulting in binding of β-arrestin that physically interdicts further G protein coupling leading to receptor desensitization. The phosphorylated βAR is internalized and undergoes resensitization by dephosphorylation mediated by protein phosphatase 2A in the early endosomes. Although desensitization and resensitization are two sides of the same coin maintaining the homeostatic functioning of the receptor, significant interest has revolved around understanding mechanisms of receptor desensitization while little is known about resensitization. In our current review we provide an overview on regulation of βAR function with a special emphasis on receptor resensitization and its functional relevance in the context of fine tuning receptor signaling.

Inhibition of protein phosphatase 2A activity by PI3Kγ regulates β-adrenergic receptor function

Phosphoinositide 3-kinase γ (PI3Kγ) is activated by G protein-coupled receptors (GPCRs). We show here that PI3Kγ inhibits protein phosphatase 2A (PP2A) at the β-adrenergic receptor (βAR, a GPCR) complex altering G protein coupling. PI3Kγ inhibition results in significant increase of βAR-associated phosphatase activity leading to receptor dephosphorylation and resensitization preserving cardiac function. Mechanistically, PI3Kγ inhibits PP2A activity at the βAR complex by phosphorylating an intracellular inhibitor of PP2A (I2PP2A) on serine residues 9 and 93, resulting in enhanced binding to PP2A. Indeed, enhanced phosphorylation of β2ARs is observed with a phosphomimetic I2PP2A mutant that was completely reversed with a mutant mimicking dephosphorylated state. siRNA depletion of endogenous I2PP2A augments PP2A activity despite active PI3K resulting in β2AR dephosphorylation and sustained signaling. Our study provides the underpinnings of a PI3Kγ-mediated regulation of PP2A activity that has significant consequences on receptor function with broad implications in cellular signaling.