New Therapeutics for Heart Failure: Focusing on cGMP Signaling

Current drugs for treating heart failure (HF), for example, angiotensin II receptor blockers and β-blockers, possess specific target molecules involved in the regulation of the cardiac circulatory system. However, most clinically approved drugs are effective in the treatment of HF with reduced ejection fraction (HFrEF). Novel drug classes, including angiotensin receptor blocker/neprilysin inhibitor (ARNI), sodium-glucose co-transporter-2 (SGLT2) inhibitor, hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blocker, soluble guanylyl cyclase (sGC) stimulator/activator, and cardiac myosin activator, have recently been introduced for HF intervention based on their proposed novel mechanisms. SGLT2 inhibitors have been shown to be effective not only for HFrEF but also for HF with preserved ejection fraction (HFpEF). In the myocardium, excess cyclic adenosine monophosphate (cAMP) stimulation has detrimental effects on HFrEF, whereas cyclic guanosine monophosphate (cGMP) signaling inhibits cAMP-mediated responses. Thus, molecules participating in cGMP signaling are promising targets of novel drugs for HF. In this review, we summarize molecular pathways of cGMP signaling and clinical trials of emerging drug classes targeting cGMP signaling in the treatment of HF.

Knockout of Purinergic P2Y6 Receptor Fails to Improve Liver Injury and Inflammation in Non-Alcoholic Steatohepatitis

Nonalcoholic steatohepatitis (NASH) is a disease that progresses from nonalcoholic fatty liver (NAFL) and which is characterized by inflammation and fibrosis. The purinergic P2Y₆ receptor (P2Y₆R) is a pro-inflammatory G_q/G₁₂ family protein-coupled receptor and reportedly contributes to intestinal inflammation and cardiovascular fibrosis, but its role in liver pathogenesis is unknown. Human genomics data analysis revealed that the liver P2Y₆R mRNA expression level is increased during the progression from NAFL to NASH, which positively correlates with inductions of C-C motif chemokine 2 (CCL2) and collagen type I α1 chain (Col1a1) mRNAs. Therefore, we examined the impact of P2Y₆R functional deficiency in mice crossed with a NASH model using a choline-deficient, L-amino acid-defined, high-fat diet (CDAHFD). Feeding CDAHFD for 6 weeks markedly increased P2Y₆R expression level in mouse liver, which was positively correlated with CCL2 mRNA induction. Unexpectedly, the CDAHFD treatment for 6 weeks increased liver weights with severe steatosis in both wild-type (WT) and P2Y₆R knockout (KO) mice, while the disease marker levels such as serum AST and liver CCL2 mRNA in CDAHFD-treated P2Y₆R KO mice were rather aggravated compared with those of CDAHFD-treated WT mice. Thus, P2Y₆R may not contribute to the progression of liver injury, despite increased expression in NASH liver.

[Biased Signaling through G Protein-coupled Receptors]

It is well-established that G protein-coupled receptors (GPCRs) transduce signals into cells using G proteins as intermediary molecules. β-Arrestins are molecules involved in regulating GPCRs; however, it has recently been reported that β-arrestins can also mediate signaling through GPCRs. Signaling through G proteins or β-arrestins can be activated selectively using specific agonists; of the latter, those that can selectively activate either G proteins or β-arrestins are called biased agonists. The clinical use of biased agonists could potentially induce fewer side effects. However, partial agonists can also explain the mechanism of G protein-biased agonists; thus, appropriate assay systems must be considered. Endogenous agonists are known to bind to orthosteric and allosteric sites in the agonist binding site, and the allosteric site is associated with the activity of biased agonists. This current review presents a detailed discussion of biased agonists.

A TRPC3/6 Channel Inhibitor Promotes Arteriogenesis after Hind-Limb Ischemia

Retarded revascularization after progressive occlusion of large conductance arteries is a major cause of bad prognosis for peripheral artery disease (PAD). However, pharmacological treatment for PAD is still limited. We previously reported that suppression of transient receptor potential canonical (TRPC) 6 channel activity in vascular smooth muscle cells (VSMCs) facilitates VSMC differentiation without affecting proliferation and migration. In this study, we found that 1-benzilpiperadine derivative (1-BP), a selective inhibitor for TRPC3 and TRPC6 channel activities, induced VSMC differentiation. 1-BP-treated mice showed increased capillary arterialization and improvement of peripheral circulation and skeletal muscle mass after hind-limb ischemia (HLI) in mice. 1-BP had no additive effect on the facilitation of blood flow recovery after HLI in TRPC6-deficient mice, suggesting that suppression of TRPC6 underlies facilitation of the blood flow recovery by 1-BP. 1-BP also improved vascular nitric oxide bioavailability and blood flow recovery after HLI in hypercholesterolemic mice with endothelial dysfunction, suggesting the retrograde interaction from VSMCs to endothelium. These results suggest that 1-BP becomes a potential seed for PAD treatments that target vascular TRPC6 channels.

Pharmacology of Antagonism of GPCR

Agonists are defined as the ligands that activate intracellular signaling and evoke cellular responses. Synthetic and endogenous agonists should bind specific amino acids to activate G protein-coupled receptor (GPCR). Agonists that induce maximal responses are full agonists. Partial agonists cannot induce full responses unlike full agonists. In definition, antagonists inhibit agonist-stimulated responses by binding to orthosteric or allosteric sites. Antagonists modulate agonist-induced responses and are often related with inverse agonist activity. However, the relationship between antagonists and partial agonists is complex. An antagonist behaves as a partial agonist when the constitutive activity of the GPCR is high. In contrast, a partial agonist with very weak intrinsic activity may be classified as an antagonist. Thus, antagonisms of the compounds are influenced by constitutive activity of GPCRs, intrinsic activity and differences in the binding sites of GPCRs. Since "antagonism" has been revealed to have multiple aspects and more complex than previously thought, it may be difficult to classify each compound as simply "agonist" or "antagonist" as before. In this review, we discuss the recent findings and perspectives on the pharmacology of GPCR-binding antagonists, inverse agonists, and signaling.

Redox-dependent internalization of the purinergic P2Y6 receptor limits colitis progression

[Figure: see text].

Alteration of circadian machinery in monocytes underlies chronic kidney disease-associated cardiac inflammation and fibrosis

Dysfunction of the circadian clock has been implicated in the pathogenesis of cardiovascular disease. The CLOCK protein is a core molecular component of the circadian oscillator, so that mice with a mutated Clock gene (Clk/Clk) exhibit abnormal rhythms in numerous physiological processes. However, here we report that chronic kidney disease (CKD)-induced cardiac inflammation and fibrosis are attenuated in Clk/Clk mice even though they have high blood pressure and increased serum angiotensin II levels. A search for the underlying cause of the attenuation of heart disorder in Clk/Clk mice with 5/6 nephrectomy (5/6Nx) led to identification of the monocytic expression of G protein-coupled receptor 68 (GPR68) as a risk factor of CKD-induced inflammation and fibrosis of heart. 5/6Nx induces the expression of GPR68 in circulating monocytes via altered CLOCK activation by increasing serum levels of retinol and its binding protein (RBP4). The high-GPR68-expressing monocytes have increased potential for producing inflammatory cytokines, and their cardiac infiltration under CKD conditions exacerbates inflammation and fibrosis of heart. Serum retinol and RBP4 levels in CKD patients are also sufficient to induce the expression of GPR68 in human monocytes. Our present study reveals an uncovered role of monocytic clock genes in CKD-induced heart failure.

Pressure Overload-Mediated Sustained PKR2 (Prokineticin-2 Receptor) Signaling in Cardiomyocytes Contributes to Cardiac Hypertrophy and Endotheliopathies

[Figure: see text].

Efferocytosis during myocardial infarction

Myocardial infarction is one of the major causes of death worldwide. Many heart cells die during myocardial infarction through various processes such as necrosis, apoptosis, necroptosis, autophagy-related cell death, pyroptosis and ferroptosis. These dead cells in infarcted hearts expose the so-called 'eat-me' signals, such as phosphatidylserine, on their surfaces, enhancing their removal by professional and non-professional phagocytes. Clearance of dead cells by phagocytes in the diseased hearts plays a crucial role in the pathology of myocardial infarction by inhibiting the inflammatory responses caused by the leakage of contents from dead cells. This review focuses on the rapidly growing understanding of the molecular mechanisms of dead cell phagocytosis, termed efferocytosis, during myocardial infarction, which contributes to the pathophysiology of myocardial infarction.

Gα12/13 signaling in metabolic diseases

As the key governors of diverse physiological processes, G protein-coupled receptors (GPCRs) have drawn attention as primary targets for several diseases, including diabetes and cardiovascular disease. Heterotrimeric G proteins converge signals from ~800 members of the GPCR family. Among the members of the G protein α family, the Gα₁₂ family members comprising Gα₁₂ and Gα₁₃ have been referred to as gep oncogenes. Gα_12/13 levels are altered in metabolic organs, including the liver and muscles, in metabolic diseases. The roles of Gα_12/13 in metabolic diseases have been investigated. In this review, we highlight findings demonstrating Gα_12/13 amplifying or dampening regulators of phenotype changes. We discuss the molecular basis of G protein biology in the context of posttranslational modifications to heterotrimeric G proteins and the cell signaling axis. We also highlight findings providing insights into the organ-specific, metabolic and pathological roles of G proteins in changes associated with specific cells, energy homeostasis, glucose metabolism, liver fibrosis and the immune and cardiovascular systems. This review summarizes the currently available knowledge on the importance of Gα_12/13 in the physiology and pathogenesis of metabolic diseases, which is presented according to the basic understanding of their metabolic actions and underlying cellular and molecular bases.

Understanding the activities of two members of a vital category of proteins called G proteins, which initiate metabolic changes when signaling molecules bind to cells, could lead to new therapies for many diseases. Researchers in South Korea and Japan, led by Sang Geon Kim at Seoul National University, review the significance of the Gα12 and Gα13 proteins in diseases characterised by significant changes in metabolism, including liver conditions and disorders of the cardiovascular and immune systems. Specific roles for the proteins have been identified by a variety of methods, including studying the effect of disabling the genes that code for them in mice. Recent insights suggest that drugs interfering with the activity of these Gα proteins might help treat many conditions in which the molecular signalling networks involving the proteins are disrupted.

Drebrin is induced during myofibroblast differentiation and enhances the production of fibrosis-related genes

Fibrosis is attributed to excess deposition of extracellular matrix (ECM) proteins including collagen and is associated with various organ dysfunction. This excessive ECM is produced by myofibroblasts, which are differentiated from various cells by a variety of stimuli, represented by TGF-β. However, molecular mechanisms for the regulation of ECM production in myofibroblasts remain obscure. In this study, we demonstrate that the expression of drebrin, which binds to and increases the stability of actin filament in neurons, is increased in mouse hearts and lungs upon fibrosis. Drebrin is mainly expressed in myofibroblasts in the fibrotic hearts and lungs and promotes the expression of fibrosis-related genes, such as Acta2 and Col1a1. Taken together, our study identifies drebrin as a molecule that promotes the production of fibrosis-related genes in myofibroblasts.

Leukotriene B4 receptor 1 exacerbates inflammation following myocardial infarction

Leukotriene B₄ receptor 1 (BLT1), a high-affinity G-protein-coupled receptor for leukotriene B4 (LTB₄ ), is expressed on various inflammatory cells and plays critical roles in several inflammatory diseases. In myocardial infarction (MI), various inflammatory cells are known to be recruited to the infarcted area, but the function of BLT1 in MI is poorly understood. Here, we investigated the role of BLT1 in MI and the therapeutic effect of a BLT1 antagonist, ONO-4057, on MI. Mice with infarcted hearts showed increased BLT1 expression and LTB₄ levels. BLT1-knockout mice with infarcted hearts exhibited attenuated leukocyte infiltration, proinflammatory cytokine production, and cell death, which led to reduced mortality and improved cardiac function after MI. Bone-marrow transplantation studies showed that BLT1 expressed on bone marrow-derived cells was responsible for the exacerbation of inflammation in infarcted hearts. Furthermore, ONO-4057 administration attenuated the inflammatory responses in hearts surgically treated for MI, which resulted in reduced mortality and improved cardiac function after MI. Our study demonstrated that BLT1 contributes to excessive inflammation after MI and could represent a new therapeutic target for MI.

Therapeutic Targets for the Treatment of Cardiac Fibrosis and Cancer: Focusing on TGF-β Signaling

Transforming growth factor-β (TGF-β) is a common mediator of cancer progression and fibrosis. Fibrosis can be a significant pathology in multiple organs, including the heart. In this review, we explain how inhibitors of TGF-β signaling can work as antifibrotic therapy. After cardiac injury, profibrotic mediators such as TGF-β, angiotensin II, and endothelin-1 simultaneously activate cardiac fibroblasts, resulting in fibroblast proliferation and migration, deposition of extracellular matrix proteins, and myofibroblast differentiation, which ultimately lead to the development of cardiac fibrosis. The consequences of fibrosis include a wide range of cardiac disorders, including contractile dysfunction, distortion of the cardiac structure, cardiac remodeling, and heart failure. Among various molecular contributors, TGF-β and its signaling pathways which play a major role in carcinogenesis are considered master fibrotic mediators. In fact, recently the inhibition of TGF-β signaling pathways using small molecule inhibitors, antibodies, and gene deletion has shown that the progression of several cancer types was suppressed. Therefore, inhibitors of TGF-β signaling are promising targets for the treatment of tissue fibrosis and cancers. In this review, we discuss the molecular mechanisms of TGF-β in the pathogenesis of cardiac fibrosis and cancer. We will review recent in vitro and in vivo evidence regarding antifibrotic and anticancer actions of TGF-β inhibitors. In addition, we also present available clinical data on therapy based on inhibiting TGF-β signaling for the treatment of cancers and cardiac fibrosis.

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

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

Gα12 ablation exacerbates liver steatosis and obesity by suppressing USP22/SIRT1-regulated mitochondrial respiration

Nonalcoholic fatty liver disease (NAFLD) arises from mitochondrial dysfunction under sustained imbalance between energy intake and expenditure, but the underlying mechanisms controlling mitochondrial respiration have not been entirely understood. Heterotrimeric G proteins converge with activated GPCRs to modulate cell-signaling pathways to maintain metabolic homeostasis. Here, we investigated the regulatory role of G protein α12 (Gα12) on hepatic lipid metabolism and whole-body energy expenditure in mice. Fasting increased Gα12 levels in mouse liver. Gα12 ablation markedly augmented fasting-induced hepatic fat accumulation. cDNA microarray analysis from Gna12-KO liver revealed that the Gα12-signaling pathway regulated sirtuin 1 (SIRT1) and PPARα, which are responsible for mitochondrial respiration. Defective induction of SIRT1 upon fasting was observed in the liver of Gna12-KO mice, which was reversed by lentivirus-mediated Gα12 overexpression in hepatocytes. Mechanistically, Gα12 stabilized SIRT1 protein through transcriptional induction of ubiquitin-specific peptidase 22 (USP22) via HIF-1α increase. Gα12 levels were markedly diminished in liver biopsies from NAFLD patients. Consistently, Gna12-KO mice fed a high-fat diet displayed greater susceptibility to diet-induced liver steatosis and obesity due to decrease in energy expenditure. Our results demonstrate that Gα12 regulates SIRT1-dependent mitochondrial respiration through HIF-1α-dependent USP22 induction, identifying Gα12 as an upstream molecule that contributes to the regulation of mitochondrial energy expenditure.

The MAPK Erk5 is necessary for proper skeletogenesis involving a Smurf-Smad-Sox9 molecular axis

Erk5 belongs to the mitogen-activated protein kinase (MAPK) family. Following its phosphorylation by Mek5, Erk5 modulates several signaling pathways in a number of cell types. In this study, we demonstrated that Erk5 inactivation in mesenchymal cells causes abnormalities in skeletal development by inducing Sox9, an important transcription factor of skeletogenesis. We further demonstrate that Erk5 directly phosphorylates and activates Smurf2 (a ubiquitin E3 ligase) at Thr²⁴⁹, which promotes the proteasomal degradation of Smad proteins and phosphorylates Smad1 at Ser²⁰⁶ in the linker region known to trigger its proteasomal degradation by Smurf1. Smads transcriptionally activated the expression of Sox9 in mesenchymal cells. Accordingly, removal of one Sox9 allele in mesenchymal cells from Erk5-deficient mice rescued some abnormalities of skeletogenesis. These findings highlight the importance of the Mek5-Erk5-Smurf-Smad-Sox9 axis in mammalian skeletogenesis.

An Assay to Determine Phagocytosis of Apoptotic Cells by Cardiac Macrophages and Cardiac Myofibroblasts

In myocardial infarction (MI), a number of cardiomyocytes undergo apoptosis. These apoptotic cardiomyocytes are promptly engulfed by phagocytes. If the dead cells are not engulfed, their noxious contents are released outside, resulting in induction of inflammation. Therefore, the removal of these dead cells is necessary. However, the contribution of each phagocyte type to the removal of apoptotic cells in infarcted hearts remains unresolved. Here, we describe an in vitro protocol for a phagocytosis assay to compare the engulfment ability of cardiac macrophages and cardiac myofibroblasts.

Phagocytosis Assay of Necroptotic Cells by Cardiac Myofibroblasts

In myocardial infarction (MI), a plenty of cardiomyocytes undergo necrosis and necroptosis due to the lack of oxygen and nutrients. The dead cardiomyocytes are promptly engulfed by phagocytes. When the dead cells are not engulfed, the noxious contents of the cells are released outside, and thus, induce inflammation, and obstruct the function of organs. Therefore, phagocytosis is crucial for maintaining homeostasis of organs. Herein, we describe a protocol of an in vitro phagocytosis assay of necroptotic cells.

The proton-sensing G protein-coupled receptor T-cell death-associated gene 8 (TDAG8) shows cardioprotective effects against myocardial infarction

Myocardial infarction (MI) is an ischaemic heart condition caused by the occlusion of coronary arteries. Following MI, lactic acid from anaerobic glycolysis increases and infiltrating immune cells produce severe inflammation, which leads to acidosis in the ischaemic heart. However, the physiological implication of this pH reduction remains largely unknown. T-cell death-associated gene 8 (TDAG8) is a proton-sensing G protein-coupled receptor found on cardiac macrophages that recognise increases in extracellular protons. We demonstrated that TDAG8 negatively regulates the transcription of the chemokine Ccl20. The infarcted hearts of TDAG8 KO mice showed an increase in CCL20 expression and the number of infiltrating IL-17A-producing γδT cells that express CCR6, a receptor for CCL20. Accordingly, excessive IL-17A production, which is linked to the functional deterioration after MI, was observed in MI-operated TDAG8 KO mice. The survival rate and cardiac function significantly decreased in TDAG8 KO mice compared with those in wild-type mice after MI. Thus, our results suggest that TDAG8 is a key regulator of MI and a potential therapeutic target.

Dioxin-induced increase in leukotriene B4 biosynthesis through the aryl hydrocarbon receptor and its relevance to hepatotoxicity owing to neutrophil infiltration

Dioxin and related chemicals alter the expression of a number of genes by activating the aryl hydrocarbon receptors (AHR) to produce a variety of disorders including hepatotoxicity. However, it remains largely unknown how these changes in gene expression are linked to toxicity. To address this issue, we initially examined the effect of 2,3,7,8-tetrachrolodibenzo-p-dioxin (TCDD), a most toxic dioxin, on the hepatic and serum metabolome in male pubertal rats and found that TCDD causes many changes in the level of fatty acids, bile acids, amino acids, and their metabolites. Among these findings was the discovery that TCDD increases the content of leukotriene B4 (LTB4), an inducer of inflammation due to the activation of leukocytes, in the liver of rats and mice. Further analyses suggested that an increase in LTB4 comes from a dual mechanism consisting of an induction of arachidonate lipoxygenase-5, a rate-limiting enzyme in LTB4 synthesis, and the down-regulation of LTC4 synthase, an enzyme that converts LTA4 to LTC4. The above changes required AHR activation, because the same was not observed in AHR knock-out rats. In agreement with LTB4 accumulation, TCDD caused the marked infiltration of neutrophils into the liver. However, deleting LTB4 receptors (BLT1) blocked this effect. A TCDD-produced increase in the mRNA expression of inflammatory markers, including tumor-necrosis factor and hepatic damage, was also suppressed in BLT1-null mice. The above observations focusing on metabolomic changes provide novel evidence that TCDD accumulates LTB4 in the liver by an AHR-dependent induction of LTB4 biosynthesis to cause hepatotoxicity through neutrophil activation.

β-Adrenergic Receptor and Insulin Resistance in the Heart

Insulin resistance is characterized by the reduced ability of insulin to stimulate tissue uptake and disposal of glucose including cardiac muscle. These conditions accelerate the progression of heart failure and increase cardiovascular morbidity and mortality in patients with cardiovascular diseases. It is noteworthy that some conditions of insulin resistance are characterized by up-regulation of the sympathetic nervous system, resulting in enhanced stimulation of β-adrenergic receptor (βAR). Overstimulation of βARs leads to the development of heart failure and is associated with the pathogenesis of insulin resistance in the heart. However, pathological consequences of the cross-talk between the βAR and the insulin sensitivity and the mechanism by which βAR overstimulation promotes insulin resistance remain unclear. This review article examines the hypothesis that βARs overstimulation leads to induction of insulin resistance in the heart.

Cardiac myofibroblast engulfment of dead cells facilitates recovery after myocardial infarction

Myocardial infarction (MI) results in the generation of dead cells in the infarcted area. These cells are swiftly removed by phagocytes to minimize inflammation and limit expansion of the damaged area. However, the types of cells and molecules responsible for the engulfment of dead cells in the infarcted area remain largely unknown. In this study, we demonstrated that cardiac myofibroblasts, which execute tissue fibrosis by producing extracellular matrix proteins, efficiently engulf dead cells. Furthermore, we identified a population of cardiac myofibroblasts that appears in the heart after MI in humans and mice. We found that these cardiac myofibroblasts secrete milk fat globule-epidermal growth factor 8 (MFG-E8), which promotes apoptotic engulfment, and determined that serum response factor is important for MFG-E8 production in myofibroblasts. Following MFG-E8-mediated engulfment of apoptotic cells, myofibroblasts acquired antiinflammatory properties. MFG-E8 deficiency in mice led to the accumulation of unengulfed dead cells after MI, resulting in exacerbated inflammatory responses and a substantial decrease in survival. Moreover, MFG-E8 administration into infarcted hearts restored cardiac function and morphology. MFG-E8-producing myofibroblasts mainly originated from resident cardiac fibroblasts and cells that underwent endothelial-mesenchymal transition in the heart. Together, our results reveal previously unrecognized roles of myofibroblasts in regulating apoptotic engulfment and a fundamental importance of these cells in recovery from MI.

Myofibroblasts and inflammatory cells as players of cardiac fibrosis

On myocardial infarction, many cells are injured or died owing to arterial occlusion. Intracellular molecules released from injured or dead cells initiate inflammatory responses that play important roles in cardiac remodeling including fibrosis. Fibrosis is an excess accumulation of extracellular collagen. Currently, drugs used to treat cardiac fibrosis are not commercially available. Myofibroblasts are responsible for the production and secretion of collagen. Infiltrating inflammatory cells interact with fibroblasts or other cells and promote myofibroblast formation. Inflammatory cells also modulate the activities of myofibroblasts. Regulation of collagen production is critical for modulating the progression of fibrosis. Hence, the manipulation of activities of inflammatory cells and myofibroblasts will provide promising therapeutic targets for treatment of cardiac fibrosis.

Identification of a Potent and Selective GPR4 Antagonist as a Drug Lead for the Treatment of Myocardial Infarction

GPR4, a pH-sensing G protein-coupled receptor, is highly expressed in endothelial cells and may be activated in myocardial infarction due the decreased tissue pH. We are interested in GPR4 antagonists as potential effective pharmacologic tools and/or drug leads for the treatment of myocardial infarction. We investigated the structure-activity relationship of a known GPR4 antagonist 1 as a lead compound to identify 3b as the first potent and selective GPR4 antagonist, whose effectiveness was demonstrated in a mouse myocardial infarction model.

Purinergic P2Y6 receptors heterodimerize with angiotensin AT1 receptors to promote angiotensin II–induced hypertension

The angiotensin (Ang) type 1 receptor (AT1R) promotes functional and structural integrity of the arterial wall to contribute to vascular homeostasis, but this receptor also promotes hypertension. In our investigation of how Ang II signals are converted by the AT1R from physiological to pathological outputs, we found that the purinergic P2Y6 receptor (P2Y6R), an inflammation-inducible G protein (heterotrimeric guanine nucleotide–binding protein)–coupled receptor (GPCR), promoted Ang II–induced hypertension in mice. In mice, deletion of P2Y6R attenuated Ang II–induced increase in blood pressure, vascular remodeling, oxidative stress, and endothelial dysfunction. AT1R and P2Y6R formed stable heterodimers, which enhanced G protein–dependent vascular hypertrophy but reduced β-arrestin–dependent AT1R internalization. Pharmacological disruption of AT1R-P2Y6R heterodimers by the P2Y6R antagonist MRS2578 suppressed Ang II–induced hypertension in mice. Furthermore, P2Y6R abundance increased with age in vascular smooth muscle cells. The increased abundance of P2Y6R converted AT1R-stimulated signaling in vascular smooth muscle cells from β-arrestin–dependent proliferation to G protein–dependent hypertrophy. These results suggest that increased formation of AT1R-P2Y6R heterodimers with age may increase the likelihood of hypertension induced by Ang II.

A new approach of indirect enzyme-linked immunosorbent assay for determination of D-glutamic acid through in situ conjugation

We propose a new approach of an indirect enzyme-linked immunosorbent assay (ELISA) for determination of D-glutamic acid (D-Glu) using a monoclonal antibody against D-glutamic acid (D-Glu-MAb), which recognizes D-Glu-glutaraldehyde (GA) molecule but not D-Glu molecule. Human serum albumin (HSA) was coated on an immunoplate and reacted with D-Glu via GA to produce D-Glu-GA-HSA conjugates in situ in the well to be recognized by D-Glu-MAb, which enabled the development of an indirect ELISA for the determination of free D-Glu. In this indirect ELISA, D-Glu can be specifically detected with limit of detection of 7.81 μ g/mL. Since anti-conjugate antibodies are often produced, even though anti-hapten antibodies are desired, this new approach could be very useful as an application of anti-conjugate antibodies to the development of quantitative analysis for detecting hapten.

Abstract 411: Discovery and Cardioprotective Effects of the First Non-peptide Agonists of Prokineticin Receptor-1

Prokineticins are angiogenic hormones that activate two G protein-coupled receptors: PKR1 and PKR2. PKR1 has emerged as a critical mediator of cardiovascular homeostasis and cardioprotection. Identification of non-peptide PKR1 agonists that contribute to myocardial repair and collateral vessel growth hold promises for treatment of heart diseases. Through a combination of in silico studies, medicinal chemistry, and pharmacological profiling approaches, we designed, synthesized, and characterized the first PKR1 agonists, demonstrating their cardioprotective activity against myocardial infarction (MI) in mice. Based on high throughput docking protocol, 250,000 compounds were computationally screened for putative PKR1 agonistic activity, using a homology model, and 10 virtual hits were pharmacologically evaluated. One hit internalizes PKR1, increases calcium release and activates ERK and Akt kinases. Among the 30 derivatives of the hit compound, the most potent derivative, IS20, was confirmed for its selectivity and specificity through genetic gain- and loss-of-function of PKR1. Importantly, IS20 prevented cardiac lesion formation and improved cardiac function after MI in mice, promoting proliferation of cardiac progenitor cells and neovasculogenesis. The preclinical investigation of the first PKR1 agonists provides a novel approach to promote cardiac neovasculogenesis after MI.

Characterization of Imidazopyridine Compounds as Negative Allosteric Modulators of Proton-Sensing GPR4 in Extracellular Acidification-Induced Responses

G protein-coupled receptor 4 (GPR4), previously proposed as the receptor for sphingosylphosphorylcholine, has recently been identified as the proton-sensing G protein-coupled receptor (GPCR) coupling to multiple intracellular signaling pathways, including the G_s protein/cAMP and G₁₃ protein/Rho. In the present study, we characterized some imidazopyridine compounds as GPR4 modulators that modify GPR4 receptor function. In the cells that express proton-sensing GPCRs, including GPR4, OGR1, TDAG8, and G2A, extracellular acidification stimulates serum responsive element (SRE)-driven transcriptional activity, which has been shown to reflect Rho activity, with different proton sensitivities. Imidazopyridine compounds inhibited the moderately acidic pH-induced SRE activity only in GPR4-expressing cells. Acidic pH-stimulated cAMP accumulation, mRNA expression of inflammatory genes, and GPR4 internalization within GPR4-expressing cells were all inhibited by the GPR4 modulator. We further compared the inhibition property of the imidazopyridine compound with psychosine, which has been shown to selectively inhibit actions induced by proton-sensing GPCRs, including GPR4. In the GPR4 mutant, in which certain histidine residues were mutated to phenylalanine, proton sensitivity was significantly shifted to the right, and psychosine failed to further inhibit acidic pH-induced SRE activation. On the other hand, the imidazopyridine compound almost completely inhibited acidic pH-induced action in mutant GPR4. We conclude that some imidazopyridine compounds show specificity to GPR4 as negative allosteric modulators with a different action mode from psychosine, an antagonist susceptible to histidine residues, and are useful for characterizing GPR4-mediated acidic pH-induced biological actions.

Discovery and cardioprotective effects of the first non-Peptide agonists of the G protein-coupled prokineticin receptor-1

Prokineticins are angiogenic hormones that activate two G protein-coupled receptors: PKR1 and PKR2. PKR1 has emerged as a critical mediator of cardiovascular homeostasis and cardioprotection. Identification of non-peptide PKR1 agonists that contribute to myocardial repair and collateral vessel growth hold promises for treatment of heart diseases. Through a combination of in silico studies, medicinal chemistry, and pharmacological profiling approaches, we designed, synthesized, and characterized the first PKR1 agonists, demonstrating their cardioprotective activity against myocardial infarction (MI) in mice. Based on high throughput docking protocol, 250,000 compounds were computationally screened for putative PKR1 agonistic activity, using a homology model, and 10 virtual hits were pharmacologically evaluated. One hit internalizes PKR1, increases calcium release and activates ERK and Akt kinases. Among the 30 derivatives of the hit compound, the most potent derivative, IS20, was confirmed for its selectivity and specificity through genetic gain- and loss-of-function of PKR1. Importantly, IS20 prevented cardiac lesion formation and improved cardiac function after MI in mice, promoting proliferation of cardiac progenitor cells and neovasculogenesis. The preclinical investigation of the first PKR1 agonists provides a novel approach to promote cardiac neovasculogenesis after MI.

Production and characterization of highly specific monoclonal antibodies to D-glutamic acid

Most of the functions of D-amino acids (D-AA) remain unclear because of little analytic methods for specific detection/determination. In this study, a highly specific monoclonal antibody to D-glutamic acid (D-Glu-MAb) was produced using a hybridoma method. Characterization of D-Glu-MAb by indirect enzyme-linked immunosorbent assay (ELISA) revealed that it has high selectivity against D-Glu-glutaraldehyde (GA) conjugates, while no cross-reaction was observed when 38 other kinds of AA-GA conjugates were used. Moreover, subsequent indirect competitive ELISA disclosed that an epitope of D-Glu-MAb is a D-Glu-GA molecule in the conjugates, suggesting that D-Glu-MAb could be a useful tool to investigate the functional analysis of D-Glu in immunostaining.

Multiple functions of G protein-coupled receptor kinases

Desensitization is a physiological feedback mechanism that blocks detrimental effects of persistent stimulation. G protein-coupled receptor kinase 2 (GRK2) was originally identified as the kinase that mediates G protein-coupled receptor (GPCR) desensitization. Subsequent studies revealed that GRK is a family composed of seven isoforms (GRK1–GRK7). Each GRK shows a differential expression pattern. GRK1, GRK4, and GRK7 are expressed in limited tissues. In contrast, GRK2, GRK3, GRK5, and GRK6 are ubiquitously expressed throughout the body. The roles of GRKs in GPCR desensitization are well established. When GPCRs are activated by their agonists, GRKs phosphorylate serine/threonine residues in the intracellular loops and the carboxyl-termini of GPCRs. Phosphorylation promotes translocation of β-arrestins to the receptors and inhibits further G protein activation by interrupting receptor-G protein coupling. The binding of β-arrestins to the receptors also helps to promote receptor internalization by clathrin-coated pits. Thus, the GRK-catalyzed phosphorylation and subsequent binding of β-arrestin to GPCRs are believed to be the common mechanism of GPCR desensitization and internalization. Recent studies have revealed that GRKs are also involved in the β-arrestin-mediated signaling pathway. The GRK-mediated phosphorylation of the receptors plays opposite roles in conventional G protein- and β-arrestin-mediated signaling. The GRK-catalyzed phosphorylation of the receptors results in decreased G protein-mediated signaling, but it is necessary for β-arrestin-mediated signaling. Agonists that selectively activate GRK/β-arrestin-dependent signaling without affecting G protein signaling are known as β-arrestin-biased agonists. Biased agonists are expected to have potential therapeutic benefits for various diseases due to their selective activation of favorable physiological responses or avoidance of the side effects of drugs. Furthermore, GRKs are recognized as signaling mediators that are independent of either G protein- or β-arrestin-mediated pathways. GRKs can phosphorylate non-GPCR substrates, and this is found to be involved in various physiological responses, such as cell motility, development, and inflammation. In addition to these effects, our group revealed that GRK6 expressed in macrophages mediates the removal of apoptotic cells (engulfment) in a kinase activity-dependent manner. These studies revealed that GRKs block excess stimulus and also induce cellular responses. Here, we summarized the involvement of GRKs in β-arrestin-mediated and G protein-independent signaling pathways.

Increased ubiquitination and the crosstalk of G protein signaling in cardiac myocytes: involvement of Ric-8B in Gs suppression by Gq signal

Hyperactivation of Gq signaling causes cardiac hypertrophy, and β-adrenergic receptor-mediated Gs signaling is attenuated in hypertrophic cardiomyocytes. Here, we found the increase in a global ubiquitination in hypertrophic mouse heart. The activation of Gq signaling resulted in the ubiquitination of Gαs in neonatal rat cardiomyocytes, reduced Gαs expression, and suppressed cAMP response to β-adrenergic receptor stimulation. Ectopic expression of Gαq induced a similar suppression, which is due to the degradation of Gαs by a ubiquitin-proteasome pathway. Co-expression of Ric-8B, a positive regulator of Gαs, effectively canceled the Gαq-induced ubiquitination of Gαs and recovered the cAMP accumulation. In vitro, Gαq competes for the binding of Gαs to Ric-8B. These data show a new role of Ric-8B in the crosstalk of two distinct G protein signaling pathways, which are possibly involved in a part of mechanisms of chronic heart failure.

β-arrestin2 in infiltrated macrophages inhibits excessive inflammation after myocardial infarction

Beta-arrestins (β-arrestin1 and β-arrestin2) are known as cytosolic proteins that mediate desensitization and internalization of activated G protein-coupled receptors. In addition to these functions, β-arrestins have been found to work as adaptor proteins for intracellular signaling pathways. β-arrestin1 and β-arrestin2 are expressed in the heart and are reported to participate in normal cardiac function. However, the physiological and pathological roles of β-arrestin1/2 in myocardial infarction (MI) have not been examined. Here, we demonstrate that β-arrestin2 negatively regulates inflammatory responses of macrophages recruited to the infarct area. β-arrestin2 knockout (KO) mice have higher mortality than wild-type (WT) mice after MI. In infarcted hearts, β-arrestin2 was strongly expressed in infiltrated macrophages. The production of inflammatory cytokines was enhanced in β-arrestin2 KO mice. In addition, p65 phosphorylation in the macrophages from the infarcted hearts of β-arrestin2 KO mice was increased in comparison to that of WT mice. These results suggest that the infiltrated macrophages of β-arrestin2 KO mice induce excessive inflammation at the infarct area. Furthermore, the inflammation in WT mice transplanted with bone marrow cells of β-arrestin2 KO mice is enhanced by MI, which is similar to that in β-arrestin2 KO mice. In contrast, the inflammation after MI in β-arrestin2 KO mice transplanted with bone marrow cells of WT mice is comparable to that in WT mice transplanted with bone marrow cells of WT mice. In summary, our present study demonstrates that β-arrestin2 of infiltrated macrophages negatively regulates inflammation in infarcted hearts, thereby enhancing inflammation when the β-arrestin2 gene is knocked out. β-arrestin2 plays a protective role in MI-induced inflammation.

Attenuated desensitization of β-adrenergic receptor by water-soluble N-nitrosamines that induce S-nitrosylation without NO release

RATIONALE

The clinical problem of loss of β-adrenergic receptor (β-AR) response, both in the pathogenesis of heart failure and during therapeutic application of β-agonists, is attributable, at least in part, to desensitization, internalization, and downregulation of the receptors. In the regulation of β-AR signaling, G protein-coupled receptor kinase 2 (GRK2) primarily phosphorylates agonist-occupied β-ARs, and this modification promotes desensitization, internalization, and downregulation of β-ARs. It has been demonstrated that GRK2 is inhibited by its S-nitrosylation. However, compounds that induce S-nitrosylation, such as S-nitrosoglutathione, simultaneously generate NO, which has been demonstrated to operate for cardiovascular protection.

OBJECTIVE

We examine whether S-nitrosylation without NO generation inhibits desensitization of β(2)-AR by GRK2. We thus aim to synthesize compounds that specifically induce S-nitrosylation.

METHODS AND RESULTS

We have developed water-soluble N-nitrosamines that have S-nitrosylating activity but lack NO-generating activity. These compounds, at least partly, rescue β-AR from desensitization in HEK 293 cells expressing FLAG-tagged human β(2)-AR and in rat cardiac myocytes. They inhibit isoproterenol-dependent phosphorylation and internalization of β(2)-AR. Indeed, they nitrosylate GRK2 in vitro and in cells, and their S-nitrosylation of GRK2 likely underlies their inhibition of β(2)-AR desensitization.

CONCLUSIONS

Compounds that induce S-nitrosylation without NO release inhibit GRK2 and attenuate β(2)-AR desensitization. Developing water-soluble drugs that specifically induce S-nitrosylation may be a promising therapeutic strategy for heart failure.

Induction of cardiac fibrosis by β-blocker in G protein-independent and G protein-coupled receptor kinase 5/β-arrestin2-dependent Signaling pathways

G-protein coupled receptors (GPCRs) have long been known as receptors that activate G protein-dependent cellular signaling pathways. In addition to the G protein-dependent pathways, recent reports have revealed that several ligands called "biased ligands" elicit G protein-independent and β-arrestin-dependent signaling through GPCRs (biased agonism). Several β-blockers are known as biased ligands. All β-blockers inhibit the binding of agonists to the β-adrenergic receptors. In addition to β-blocking action, some β-blockers are reported to induce cellular responses through G protein-independent and β-arrestin-dependent signaling pathways. However, the physiological significance induced by the β-arrestin-dependent pathway remains much to be clarified in vivo. Here, we demonstrate that metoprolol, a β(1)-adrenergic receptor-selective blocker, could induce cardiac fibrosis through a G protein-independent and β-arrestin2-dependent pathway. Metoprolol, a β-blocker, increased the expression of fibrotic genes responsible for cardiac fibrosis in cardiomyocytes. Furthermore, metoprolol induced the interaction between β(1)-adrenergic receptor and β-arrestin2, but not β-arrestin1. The interaction between β(1)-adrenergic receptor and β-arrestin2 by metoprolol was impaired in the G protein-coupled receptor kinase 5 (GRK5)-knockdown cells. Metoprolol-induced cardiac fibrosis led to cardiac dysfunction. However, the metoprolol-induced fibrosis and cardiac dysfunction were not evoked in β-arrestin2- or GRK5-knock-out mice. Thus, metoprolol is a biased ligand that selectively activates a G protein-independent and GRK5/β-arrestin2-dependent pathway, and induces cardiac fibrosis. This study demonstrates the physiological importance of biased agonism, and suggests that G protein-independent and β-arrestin-dependent signaling is a reason for the diversity of the effectiveness of β-blockers.

Mammalian formin Fhod3 plays an essential role in cardiogenesis by organizing myofibrillogenesis

Heart development requires organized integration of actin filaments into the sarcomere, the contractile unit of myofibrils, although it remains largely unknown how actin filaments are assembled during myofibrillogenesis. Here we show that Fhod3, a member of the formin family of proteins that play pivotal roles in actin filament assembly, is essential for myofibrillogenesis at an early stage of heart development. Fhod3^−/− mice appear normal up to embryonic day (E) 8.5, when the developing heart, composed of premyofibrils, initiates spontaneous contraction. However, these premyofibrils fail to mature and myocardial development does not continue, leading to embryonic lethality by E11.5. Transgenic expression of wild-type Fhod3 in the heart restores myofibril maturation and cardiomyogenesis, which allow Fhod3^−/− embryos to develop further. Moreover, cardiomyopathic changes with immature myofibrils are caused in mice overexpressing a mutant Fhod3, defective in binding to actin. These findings indicate that actin dynamics, regulated by Fhod3, participate in sarcomere organization during myofibrillogenesis and thus play a crucial role in heart development.

β-arrestin-mediated signaling improves the efficacy of therapeutics

β-Arrestins (β-arrestin-1 and β-arrestin-2) were first identified as proteins that have the ability to desensitize G protein-coupled receptors (GPCRs). However, it has recently been found that β-arrestins can activate signaling pathways independent of G protein activation. The diversity of these signaling pathways has also been recognized. This leads to an appreciation of β-arrestin-biased agonists, which is a new class of drugs that selectively activate β-arrestin-mediated signaling without G protein activation. In this review, we will discuss the recent advance of β-arrestin-mediated signaling pathways, including a brief account of different biased agonists, their pharmacological applications, and novel β-arrestin research.

β-arrestin-mediated signaling improves the efficacy of therapeutics

β-Arrestins (β-arrestin-1 and β-arrestin-2) were first identified as proteins that have the ability to desensitize G protein-coupled receptors (GPCRs). However, it has recently been found that β-arrestins can activate signaling pathways independent of G protein activation. The diversity of these signaling pathways has also been recognized. This leads to an appreciation of β-arrestin-biased agonists, which is a new class of drugs that selectively activate β-arrestin-mediated signaling without G protein activation. In this review, we will discuss the recent advance of β-arrestin-mediated signaling pathways, including a brief account of different biased agonists, their pharmacological applications, and novel β-arrestin research.

Cilostazol suppresses angiotensin II-induced vasoconstriction via protein kinase A-mediated phosphorylation of the transient receptor potential canonical 6 channel

OBJECTIVE

The goal of this study was to determine whether inhibition of transient receptor potential canonical (TRPC) channels underlies attenuation of angiotensin II (Ang II)-induced vasoconstriction by phosphodiesterase (PDE) 3 inhibition.

METHODS AND RESULTS

Pretreatment of rat thoracic aorta with cilostazol, a selective PDE3 inhibitor, suppressed vasoconstriction induced by Ang II but not that induced by KCl. The Ang II-induced contraction was largely dependent on Ca(2+) influx via receptor-operated cation channels. Cilostazol specifically suppressed diacylglycerol-activated TRPC channels (TRPC3/TRPC6/TRPC7) through protein kinase A (PKA)-dependent phosphorylation of TRPC channels in HEK293 cells. In contrast, we found that phosphorylation of TRPC6 at Thr69 was essential for the suppression of Ang II-induced Ca(2+) influx by PDE3 inhibition in rat aortic smooth muscle cells (RAoSMCs). Cilostazol specifically induced phosphorylation of endogenous TRPC6 at Thr69. The endogenous TRPC6, but not TRPC3, formed a ternary complex with PDE3 and PKA in RAoSMCs, suggesting the specificity of TRPC6 phosphorylation by PDE3 inhibition. Furthermore, inhibition of PDE3 suppressed the Ang II-induced contraction of reconstituted ring with RAoSMCs, which were abolished by the expression of a phosphorylation-deficient mutant of TRPC6.

CONCLUSIONS

PKA-mediated phosphorylation of TRPC6 at Thr69 is essential for the vasorelaxant effects of PDE3 inhibition against the vasoconstrictive actions of Ang II.

G(i/o) protein-dependent and -independent actions of Pertussis Toxin (PTX)

Pertussis toxin (PTX) is a typical A-B toxin. The A-protomer (S1 subunit) exhibits ADP-ribosyltransferase activity. The B-oligomer consists of four subunits (S2 to S5) and binds extracellular molecules that allow the toxin to enter the cells. The A-protomer ADP-ribosylates the α subunits of heterotrimeric G_i/o proteins, resulting in the receptors being uncoupled from the G_i/o proteins. The B-oligomer binds proteins expressed on the cell surface, such as Toll-like receptor 4, and activates an intracellular signal transduction cascade. Thus, PTX modifies cellular responses by at least two different signaling pathways; ADP-ribosylation of the Gα_i/o proteins by the A-protomer (G_i/o protein-dependent action) and the interaction of the B-oligomer with cell surface proteins (G_i/o protein-independent action).