Redox regulation of cGMP-dependent protein kinase Iα in the cardiovascular system

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.

NADPH Oxidases in Pain Processing

Inflammation or injury to the somatosensory nervous system may result in chronic pain conditions, which affect millions of people and often cause major health problems. Emerging lines of evidence indicate that reactive oxygen species (ROS), such as superoxide anion or hydrogen peroxide, are produced in the nociceptive system during chronic inflammatory and neuropathic pain and act as specific signaling molecules in pain processing. Among potential ROS sources in the somatosensory system are NADPH oxidases, a group of electron-transporting transmembrane enzymes whose sole function seems to be the generation of ROS. Interestingly, the expression and relevant function of the Nox family members Nox1, Nox2, and Nox4 in various cells of the nociceptive system have been demonstrated. Studies using knockout mice or specific knockdown of these isoforms indicate that Nox1, Nox2, and Nox4 specifically contribute to distinct signaling pathways in chronic inflammatory and/or neuropathic pain states. As selective Nox inhibitors are currently being developed and investigated in various physiological and pathophysiological settings, targeting Nox1, Nox2, and/or Nox4 could be a novel strategy for the treatment of chronic pain. Here, we summarize the distinct roles of Nox1, Nox2, and Nox4 in inflammatory and neuropathic processing and discuss the effectiveness of currently available Nox inhibitors in the treatment of chronic pain conditions.

The NO signalling pathway in aortic aneurysm and dissection

Recent studies have shown that NO is a central mediator in diseases associated with thoracic aortic aneurysm, such as Marfan syndrome. The progressive dilation of the aorta in thoracic aortic aneurysm ultimately leads to aortic dissection. Unfortunately, current medical treatments have neither halt aortic enlargement nor prevented rupture, leaving surgical repair as the only effective treatment. There is therefore a pressing need for effective therapies to delay or even avoid the need for surgical repair in thoracic aortic aneurysm patients. Here, we summarize the mechanisms through which NO signalling dysregulation causes thoracic aortic aneurysm, particularly in Marfan syndrome. We discuss recent advances based on the identification of new Marfan syndrome mediators related to pathway overactivation that represent potential disease biomarkers. Likewise, we propose iNOS, sGC and PRKG1, whose pharmacological inhibition reverses aortopathy in Marfan syndrome mice, as targets for therapeutic intervention in thoracic aortic aneurysm and are candidates for clinical trials.

Endothelial-to-Mesenchymal Transition in Pulmonary Arterial Hypertension

Endothelial-to-mesenchymal transition (EndMT) is a process that encompasses extensive transcriptional reprogramming of activated endothelial cells leading to a shift toward mesenchymal cellular phenotypes and functional responses. Initially observed in the context of embryonic development, in the last few decades EndMT is increasingly recognized as a process that contributes to a variety of pathologies in the adult organism. Within the settings of cardiovascular biology, EndMT plays a role in various diseases, including atherosclerosis, heart valvular disease, cardiac fibrosis, and myocardial infarction. EndMT is also being progressively implicated in development and progression of pulmonary hypertension (PH) and pulmonary arterial hypertension (PAH). This review covers the current knowledge about EndMT in PH and PAH, and provides comprehensive overview of seminal discoveries. Topics covered include evidence linking EndMT to factors associated with PAH development, including hypoxia responses, inflammation, dysregulation of bone-morphogenetic protein receptor 2 (BMPR2), and redox signaling. This review amalgamates these discoveries into potential insights for the identification of underlying mechanisms driving EndMT in PH and PAH, and discusses future directions for EndMT-based therapeutic strategies in disease management.

Advances in the Development of Phosphodiesterase-4 Inhibitors

Cyclic nucleotide phosphodiesterase 4 (PDE4) specifically hydrolyzes cyclic adenosine monophosphate (cAMP) and plays vital roles in biological processes such as cancer development. To date, PDE4 inhibitors have been widely studied as therapeutics for the treatment of various diseases such as chronic obstructive pulmonary disease, and many of them have progressed to clinical trials or have been approved as drugs. Herein, we review the advances in the development of PDE4 inhibitors in the past decade and will focus on their pharmacophores, PDE4 subfamily selectivity, and therapeutic potential. Hopefully, this analysis will lead to a strategy for development of novel therapeutics targeting PDE4.

The Emerging Roles of Nicotinamide Adenine Dinucleotide Phosphate Oxidase 2 in Skeletal Muscle Redox Signaling and Metabolism

Significance: Skeletal muscle is a crucial tissue to whole-body locomotion and metabolic health. Reactive oxygen species (ROS) have emerged as intracellular messengers participating in both physiological and pathological adaptations in skeletal muscle. A complex interplay between ROS-producing enzymes and antioxidant networks exists in different subcellular compartments of mature skeletal muscle. Recent evidence suggests that nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (NOXs) are a major source of contraction- and insulin-stimulated oxidants production, but they may paradoxically also contribute to muscle insulin resistance and atrophy. Recent Advances: Pharmacological and molecular biological tools, including redox-sensitive probes and transgenic mouse models, have generated novel insights into compartmentalized redox signaling and suggested that NOX2 contributes to redox control of skeletal muscle metabolism. Critical Issues: Major outstanding questions in skeletal muscle include where NOX2 activation occurs under different conditions in health and disease, how NOX2 activation is regulated, how superoxide/hydrogen peroxide generated by NOX2 reaches the cytosol, what the signaling mediators are downstream of NOX2, and the role of NOX2 for different physiological and pathophysiological processes. Future Directions: Future research should utilize and expand the current redox-signaling toolbox to clarify the NOX2-dependent mechanisms in skeletal muscle and determine whether the proposed functions of NOX2 in cells and animal models are conserved into humans.

Metabolism and Redox in Pulmonary Vascular Physiology and Pathophysiology

Significance: This review considers how some systems controlling pulmonary vascular function are potentially regulated by redox processes to examine how and why conditions such as prolonged hypoxia, pathological mediators, and other factors promoting vascular remodeling contribute to the development of pulmonary hypertension (PH). Recent Advances and Critical Issues: Aspects of vascular remodeling induction mechanisms described are associated with shifts in glucose metabolism through the pentose phosphate pathway and increased cytosolic NADPH generation by glucose-6-phosphate dehydrogenase, increased glycolysis generation of cytosolic NADH and lactate, mitochondrial dysfunction associated with superoxide dismutase-2 depletion, changes in reactive oxygen species and iron metabolism, and redox signaling. Future Directions: The regulation and impact of hypoxia-inducible factor and the function of cGMP-dependent and redox regulation of protein kinase G are considered for their potential roles as key sensors and coordinators of redox and metabolic processes controlling the progression of vascular pathophysiology in PH, and how modulating aspects of metabolic and redox regulatory systems potentially function in beneficial therapeutic approaches.

Oxidation of PKGIα mediates an endogenous adaptation to pulmonary hypertension

Chronic hypoxia causes pulmonary hypertension (PH), vascular remodeling, right ventricular (RV) hypertrophy, and cardiac failure. Protein kinase G Iα (PKGIα) is susceptible to oxidation, forming an interprotein disulfide homodimer associated with kinase targeting involved in vasodilation. Here we report increased disulfide PKGIα in pulmonary arteries from mice with hypoxic PH or lungs from patients with pulmonary arterial hypertension. This oxidation is likely caused by oxidants derived from NADPH oxidase-4, superoxide dismutase 3, and cystathionine γ-lyase, enzymes that were concomitantly increased in these samples. Indeed, products that may arise from these enzymes, including hydrogen peroxide, glutathione disulfide, and protein-bound persulfides, were increased in the plasma of hypoxic mice. Furthermore, low-molecular-weight hydropersulfides, which can serve as "superreductants" were attenuated in hypoxic tissues, consistent with systemic oxidative stress and the oxidation of PKGIα observed. Inhibiting cystathionine γ-lyase resulted in decreased hypoxia-induced disulfide PKGIα and more severe PH phenotype in wild-type mice, but not in Cys42Ser PKGIα knock-in (KI) mice that are resistant to oxidation. In addition, KI mice also developed potentiated PH during hypoxia alone. Thus, oxidation of PKGIα is an adaptive mechanism that limits PH, a concept further supported by polysulfide treatment abrogating hypoxia-induced RV hypertrophy in wild-type, but not in the KI, mice. Unbiased transcriptomic analysis of hypoxic lungs before structural remodeling identified up-regulation of endothelial-to-mesenchymal transition pathways in the KI compared with wild-type mice. Thus, disulfide PKGIα is an intrinsic adaptive mechanism that attenuates PH progression not only by promoting vasodilation but also by limiting maladaptive growth and fibrosis signaling.

Cystathionine γ-Lyase-Produced Hydrogen Sulfide Controls Endothelial NO Bioavailability and Blood Pressure

Hydrogen sulfide (H₂S) and NO are important gasotransmitters, but how endogenous H₂S affects the circulatory system has remained incompletely understood. Here, we show that CTH or CSE (cystathionine γ-lyase)-produced H₂S scavenges vascular NO and controls its endogenous levels in peripheral arteries, which contribute to blood pressure regulation. Furthermore, eNOS (endothelial NO synthase) and phospho-eNOS protein levels were unaffected, but levels of nitroxyl were low in CTH-deficient arteries, demonstrating reduced direct chemical interaction between H₂S and NO. Pretreatment of arterial rings from CTH-deficient mice with exogenous H₂S donor rescued the endothelial vasorelaxant response and decreased tissue NO levels. Our discovery that CTH-produced H₂S inhibits endogenous endothelial NO bioavailability and vascular tone is novel and fundamentally important for understanding how regulation of vascular tone is tailored for endogenous H₂S to contribute to systemic blood pressure function.

Oxidant sensor in the cGMP-binding pocket of PKGIα regulates nitroxyl-mediated kinase activity

Despite the mechanisms for endogenous nitroxyl (HNO) production and action being incompletely understood, pharmacological donors show broad therapeutic promise and are in clinical trials. Mass spectrometry and site-directed mutagenesis showed that chemically distinct HNO donors 1-nitrosocyclohexyl acetate or Angeli’s salt induced disulfides within cGMP-dependent protein kinase I-alpha (PKGIα), an interdisulfide between Cys42 of the two identical subunits of the kinase and a previously unobserved intradisulfide between Cys117 and Cys195 in the high affinity cGMP-binding site. Kinase activity was monitored in cells transfected with wildtype (WT), Cys42Ser or Cys117/195Ser PKGIα that cannot form the inter- or intradisulfide, respectively. HNO enhanced WT kinase activity, an effect significantly attenuated in inter- or intradisulfide-deficient PKGIα. To investigate whether the intradisulfide modulates cGMP binding, real-time imaging was performed in vascular smooth muscle cells expressing a FRET-biosensor comprising the cGMP-binding sites of PKGIα. HNO induced FRET changes similar to those elicited by an increase of cGMP, suggesting that intradisulfide formation is associated with activation of PKGIα. Intradisulfide formation in PKGIα correlated with enhanced HNO-mediated vasorelaxation in mesenteric arteries in vitro and arteriolar dilation in vivo in mice. HNO induces intradisulfide formation in PKGIα, inducing the same effect as cGMP binding, namely kinase activation and thus vasorelaxation.

Divergent roles of endothelial nitric oxide synthases system in maintaining cardiovascular homeostasis

Accumulating evidence has demonstrated the importance of reactive oxygen species (ROS) as an essential second messenger in health and disease. Endothelial dysfunction is the hallmark of atherosclerotic cardiovascular diseases, in which pathological levels of ROS are substantially involved. The endothelium plays a crucial role in modulating tone of underlying vascular smooth muscle by synthesizing and releasing nitric oxide (NO) and endothelium-dependent hyperpolarization (EDH) factors in a distinct vessel size-dependent manner through the diverse roles of the endothelial NO synthases (NOSs) system. Endothelium-derived hydrogen peroxide (H₂O₂) is a physiological signaling molecule serving as one of the major EDH factors especially in microcirculations and has gained increasing attention in view of its emerging relevance for cardiovascular homeostasis. In the clinical settings, it has been reported that antioxidant supplements are unexpectedly ineffective to prevent cardiovascular events. These lines of evidence indicate the potential importance of the physiological balance between NO and H₂O₂/EDH through the diverse functions of endothelial NOSs system in maintaining cardiovascular homeostasis. A better understanding of cardiovascular redox signaling is certainly needed to develop novel therapeutic strategies in cardiovascular medicine. In this review, we will briefly summarize the current knowledge on the emerging regulatory roles of redox signaling pathways in cardiovascular homeostasis, with particular focus on the two endothelial NOSs-derived mediators, NO and H₂O₂/EDH.

Proof of Principle for a Novel Class of Antihypertensives That Target the Oxidative Activation of PKG Iα (Protein Kinase G Iα)

Arterial hypertension continues to be a major health burden. Development of new antihypertensive drugs that engage vasodilatory mechanisms not harnessed by available therapies offer therapeutic potential. Oxidants induce an interprotein disulfide in PKG Iα (protein kinase G Iα) at C42, which is associated with its targeting and activation, resulting in vasodilation and blood pressure lowering. Consequently, we developed an assay and screened for electrophilic drugs that activate PKG Iα by selectively targeting C42, as such compounds have potential as novel antihypertensives with a mechanism of action that differs from current therapies. In this way, a drug that we termed G1 was identified, which targets C42 of PKG Iα to induce vasodilation of isolated resistance blood vessels and blood pressure lowering in a mouse model of angiotensin II-induced hypertension. In contrast, these antihypertensive effects were deficient in angiotensin II-induced hypertensive C42S PKG Iα knockin mice. These transgenic mice were engineered to have the reactive cysteinyl thiol replaced with a hydroxyl so that it cannot react with endogenous vasodilatory oxidants or electrophiles such as drug G1. These studies, therefore, provide validation of PKG Iα C42 as the target of G1, as well as proof-of-principle for a new class of antihypertensive drugs that have potential for further development for clinical use in humans.

Relaxing Responses to Hydrogen Peroxide and Nitric Oxide in Human Pericardial Resistance Arteries Stimulated with Endothelin-1

In human pericardial resistance arteries, effects of the endothelium-dependent vasodilator bradykinin are mediated by NO during contraction induced by K⁺ or the TxA₂ analogue U46619 and by H₂ O₂ during contraction by endothelin-1 (ET-1), respectively. We tested the hypotheses that ET-1 reduces relaxing effects of NO and increases those of H₂ O₂ in resistance artery smooth muscle of patients with cardiovascular disease. Arterial segments, dissected from the parietal pericardium of 39 cardiothoracic surgery patients, were studied by myography during amplitude-matched contractions induced by K⁺ , the TXA₂ analogue U46619 or ET-1. Effects of the NO donor Na-nitroprusside (SNP) and of exogenous H₂ O₂ were recorded in the absence and presence of inhibitors of cyclooxygenases, NO synthases and small and intermediate conductance calcium-activated K⁺ channels. During contractions induced by either of the three stimuli, the potency of SNP did not differ and was not modified by the inhibitors. In vessels contracted with ET-1, the potency of H₂ O₂ was on average and in terms of interindividual variability considerably larger than in K⁺ -contracted vessels. Both differences were not statistically significant in the presence of inhibitors of mechanisms of endothelium-dependent vasodilatation. In resistance arteries from patients with cardiovascular disease, ET-1 does not selectively modify smooth muscle relaxing responses to NO or H₂ O₂ . Furthermore, the candidate endothelium-derived relaxing factor H₂ O₂ also acts as an endothelium-dependent vasodilator.

Oxidant signaling underlies PKGIα modulation of Ca2+ sparks and BKCa in myogenically active arterioles

Local blood flow autoregulation in response to intraluminal pressure requires small artery myogenic vasoconstriction, the extent of which is thought to be governed by a feedback process that depends on Ca²⁺ signaling. In this issue of Science Signaling, Khavandi et al suggest a role for cyclic guanosine monophosphate (cGMP)-dependent protein kinase G Iα (PKGIα) activated by oxidants in a cGMP-independent manner.

Role of Protein Kinase G and Reactive Oxygen Species in the Regulation of Podocyte Function in Health and Disease

Podocytes and their foot processes form an important cellular layer of the glomerular barrier involved in the regulation of glomerular permeability. Disturbing the function of podocytes plays a central role in the development of proteinuria in diabetic nephropathy. Retraction of the podocyte foot processes that form slit diaphragms is a common feature of proteinuria; although, the correlation between these events in not well understood. Notably, it is unclear whether podocyte foot processes are able to regulate slit diaphragm permeability and glomerular ultrafiltration. The occurrence of reactive oxygen species generation, insulin resistance, and hyperglycemia characterizes early stages of type 2 diabetes. Protein kinase G type I alpha (PKGIα) is an intracellular target for vasorelaxant factors. It is activated in both cGMP-dependent and cGMP-independent manners. Recently, we demonstrated a relationship between oxidative stress, PKGIα activation, actin reorganization, and changes in the permeability of the filtration barrier. This review discusses how redox imbalance affects both the activity of PKGIα and PKGI-dependent signaling pathways in podocytes. J. Cell. Physiol. 232: 691-697, 2017. © 2016 Wiley Periodicals, Inc.