Platelet CD36 signaling through ERK5 promotes caspase-dependent procoagulant activity and fibrin deposition in vivo

Voltage-dependent anion channel 1 mediates mitochondrial fission and glucose metabolic reprogramming in response to ionizing radiation

The ionizing radiation (IR) represents a formidable challenge as an environmental factor to mitochondria, leading to disrupt cellular energy metabolism and posing health risks. Although the deleterious impacts of IR on mitochondrial function are recognized, the specific molecular targets remain incompletely elucidated. In this study, HeLa cells subjected to γ-rays exhibited concomitant oxidative stress, mitochondrial structural alterations, and diminished ATP production capacity. The γ-rays induced a dose-dependent induction of mitochondrial fission, simultaneously manifested by an elevated S616/S637 phosphorylation ratio of the dynamin-related protein 1 (DRP1) and a reduction in the expression of the mitochondrial fusion protein mitofusin 2 (MFN2). Knockdown of DRP1 effectively mitigated γ-rays-induced mitochondrial network damage, implying that DRP1 phosphorylation may act as an effector of radiation-induced mitochondrial damage. The mitochondrial outer membrane protein voltage-dependent anion channel 1 (VDAC1) was identified as a crucial player in IR-induced mitochondrial damage. The VDAC1 inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), counteracts the excessive mitochondrial fission induced by γ-rays, consequently rebalancing the glycolytic and oxidative phosphorylation equilibrium. This metabolic shift was uncovered to enhance glycolytic capacity, thus fortifying cellular resilience and elevating the radiosensitivity of cancer cells. These findings elucidate the intricate regulatory mechanisms governing mitochondrial morphology under radiation response. It is anticipated that the development of targeted drugs directed against VDAC1 may hold promise in augmenting the sensitivity of tumor cells to radiotherapy and chemotherapy.

OxLDL enhances procoagulant activity of endothelial cells by TMEM16F-mediated phosphatidylserine exposure

Oxidized low-density lipoprotein (oxLDL), a key component in atherosclerosis and hyperlipidemia, is a risk factor for atherothrombosis in dyslipidemia, yet its mechanism is poorly understood. In this study, we used oxLDL-induced human aortic endothelial cells (HAECs) and high-fat diet (HFD)-fed mice as a hyperlipidemia model. Phosphatidylserine (PS) exposure, cytosolic Ca2+, reactive oxygen species (ROS), and lipid peroxidation were measured by flow cytometer. TMEM16F expression was detected by immunofluorescence, western blot, and reverse transcription polymerase chain reaction. Procoagulant activity (PCA) was measured by coagulation time, intrinsic/extrinsic factor Xase, and thrombin generation. We found that oxLDL-induced PS exposure and the corresponding PCA of HAECs were increased significantly compared with control, which could be inhibited over 90% by lactadherin. Importantly, TMEM16F expression in oxLDL-induced HAECs was upregulated by enhanced intracellular Ca2+ concentration, ROS, and lipid peroxidation, which led to PS exposure. Meanwhile, the knockdown of TMEM16F by short hairpin RNA significantly inhibited PS exposure in oxLDL-induced HAECs. Moreover, we observed that HFD-fed mice dramatically increased the progress of thrombus formation and accompanied upregulated TMEM16F expression by thromboelastography analysis, FeCl3-induced carotid artery thrombosis model, and western blot. Collectively, these results demonstrate that TMEM16F-mediated PS exposure may contribute to prothrombotic status under hyperlipidemic conditions, which may serve as a novel therapeutic target for the prevention of thrombosis in hyperlipidemia.

The Role of CD36/GPIV in Platelet Biology

CD36 (also known as platelet glycoprotein IV) is expressed by a variety of different cell entities, where it possesses functions as a signaling receptor, but additionally acts as a transporter for long-chain fatty acids. This dual function of CD36 has been investigated for its relevance in immune and nonimmune cells. Although CD36 was first identified on platelets, the understanding of the role of CD36 in platelet biology remained scarce for decades. In the past few years, several discoveries have shed a new light on the CD36 signaling activity in platelets. Notably, CD36 has been recognized as a sensor for oxidized low-density lipoproteins in the circulation that mitigates the threshold for platelet activation under conditions of dyslipidemia. Thus, platelet CD36 transduces atherogenic lipid stress into an increased risk for thrombosis, myocardial infarction, and stroke. The underlying pathways that are affected by CD36 are the inhibition of cyclic nucleotide signaling pathways and simultaneously the induction of activatory signaling events. Furthermore, thrombospondin-1 secreted by activated platelets binds to CD36 and furthers paracrine platelet activation. CD36 also serves as a binding hub for different coagulation factors and, thus, contributes to the plasmatic coagulation cascade. This review provides a comprehensive overview of the recent findings on platelet CD36 and presents CD36 as a relevant target for the prevention of thrombotic events for dyslipidemic individuals with an elevated risk for thrombosis.

Targeting Cysteine Oxidation in Thrombotic Disorders

Oxidative stress increases the risk for clinically significant thrombotic events, yet the mechanisms by which oxidants become prothrombotic are unclear. In this review, we provide an overview of cysteine reactivity and oxidation. We then highlight recent findings on cysteine oxidation events in oxidative stress-related thrombosis. Special emphasis is on the signaling pathway induced by a platelet membrane protein, CD36, in dyslipidemia, and by protein disulfide isomerase (PDI), a member of the thiol oxidoreductase family of proteins. Antioxidative and chemical biology approaches to target cysteine are discussed. Lastly, the knowledge gaps in the field are highlighted as they relate to understanding how oxidative cysteine modification might be targeted to limit thrombosis.

Is the suppression of CD36 a promising way for atherosclerosis therapy?

Atherosclerosis is the main underlying pathology of many cardiovascular diseases and is marked by plaque formation in the artery wall. It has posed a serious threat to the health of people all over the world. CD36 acts as a significant regulator of lipid homeostasis, which is closely associated with the onset and progression of atherosclerosis and may be a new therapeutic target. The abnormal overexpression of CD36 facilitates lipid accumulation, foam cell formation, inflammation, endothelial apoptosis, and thrombosis. Numerous natural products and lipid-lowering agents are found to target the suppression of CD36 or inhibit the upregulation of CD36 to prevent and treat atherosclerosis. Here, the structure, expression regulation and function of CD36 in atherosclerosis and its related pharmacological therapies are reviewed. This review highlights the importance of drugs targeting CD36 suppression in the treatment and prevention of atherosclerosis, in order to develop new therapeutic strategies and potential anti-atherosclerotic drugs both preclinically and clinically.

Cysteine and methionine oxidation in thrombotic disorders

Thrombosis is the leading cause of death in many diseased conditions. Oxidative stress is characteristic of these conditions. Yet, the mechanisms through which oxidants become prothrombotic are unclear. Recent evidence suggests protein cysteine and methionine oxidation as prothrombotic regulators. These oxidative post-translational modifications occur on proteins that participate in the thrombotic process, including Src family kinases, protein disulfide isomerase, β2 glycoprotein I, von Willebrand factor, and fibrinogen. New chemical tools to identify oxidized cysteine and methionine proteins in thrombosis and hemostasis, including carbon nucleophiles for cysteine sulfenylation and oxaziridines for methionine, are critical to understanding why clots occur during oxidative stress. These mechanisms will identify alternative or novel therapeutic approaches to treat thrombotic disorders in diseased conditions.

Platelet mitochondria: the mighty few

PURPOSE OF REVIEW

Platelet mitochondrial dysfunction is both caused by, as well as a source of oxidative stress. Oxidative stress is a key hallmark of metabolic disorders such as dyslipidemia and diabetes, which are known to have higher risks for thrombotic complications.

RECENT FINDINGS

Increasing evidence supports a critical role for platelet mitochondria beyond energy production and apoptosis. Mitochondria are key regulators of reactive oxygen species and procoagulant platelets, which both contribute to pathological thrombosis. Studies targeting platelet mitochondrial pathways have reported promising results suggesting antithrombotic effects with limited impact on hemostasis in animal models.

SUMMARY

Targeting platelet mitochondria holds promise for the reduction of thrombotic complications in patients with metabolic disorders. Future studies should aim at validating these preclinical findings and translate them to the clinic.

The significance of ERK5 catalytic-independent functions in disease pathways

Extracellular signal-regulated kinase 5 (ERK5), also known as BMK1 or MAPK7, represents a recent addition to the classical mitogen-activated protein kinase (MAPK) family. This family includes well-known members such as ERK1/2, c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (p38 MAPK), as well as atypical MAPKs such as ERK3, ERK4, ERK7 (ERK8), and Nemo-like kinase (NLK). Comprehensive reviews available elsewhere provide detailed insights into ERK5, which interested readers can refer to for in-depth knowledge (Nithianandarajah-Jones et al., 2012; Monti et al., Cancers (Basel), 2022, 14). The primary aim of this review is to emphasize the essential characteristics of ERK5 and shed light on the intricate nature of its activation, with particular attention to the catalytic-independent functions in disease pathways.

Prognostic Value of the Selected Polymorphisms in the CD36 Gene in the Domain-Encoding Lipid-Binding Region at a 10-Year Follow-Up for Early-Onset CAD Patients

The polymorphism of the CD36 gene may have a decisive impact on the formation and progression of atherosclerotic changes. The aim of the study was to confirm the prognostic values of the previously studied polymorphisms in the CD36 gene within a 10-year follow-up period. This is the first published report confirming the long-term observation of patients with CAD. The study group covered 100 early-onset CAD patients. It included 26 women not older than 55 years and 74 men not older than 50 years, tested in a ten-year study as a long-term follow-up after the first cardiovascular episode. There are no notable differences between the CD36 variants and the number of fatalities during observation, fatalities due to cardiological reasons, cases of myocardial infarction within a ten-year observation period, hospitalizations for cardiovascular issues, all cardiovascular occurrences, and the number of months lived. We have shown that the CD36 variants analyzed in this study do not appear to be related to the risk of early CAD occurrence in the Caucasian population in long-term observation.

Sulforaphane attenuates glycoprotein VI-mediated platelet mitochondrial dysfunction through up-regulating the cAMP/PKA signaling pathway in vitro and in vivo

Platelet mitochondrial dysfunction is crucial for platelet activation, atherosclerosis and thrombosis. Sulforaphane (SFN) is a dietary isothiocyanate enriched in cruciferous vegetables and possesses multiple health benefits including cardiovascular protection. This study aims to investigate whether and how SFN modulates platelet mitochondrial dysfunction and hyperactivity in vitro and in vivo. Using a series of platelet functional assays in human platelets in vitro, we found that SFN at physiological concentrations attenuated oxidative stress-dependent platelet mitochondrial dysfunction (loss of mitochondrial membrane potential), apoptosis (cytochrome c release, caspase 3 activation and phosphatidylserine exposure) and activation induced by glycoprotein VI (GPVI) agonists (e.g., collagen and convulxin). Moreover, 12-week supplementation of SFN-enriched broccoli sprout extract (BSE, 0.06% diet) in C57BL/6J mice also attenuated GPVI-induced platelet mitochondrial dysfunction, apoptosis and hyperreactivity in vivo. Mechanistically, these inhibitory effects of SFN treatment and BSE supplementation were mainly mediated by up-regulating the cAMP/PKA pathway though decreasing phosphodiesterase 3A (PDE3A) activity. Thus, through modulating the PDE3A/cAMP/PKA signaling pathway, and attenuating platelet mitochondrial dysfunction and hyperreactivity, SFN may be a potent cardioprotective agent.

OxLDL sensitizes platelets for increased formation of extracellular vesicles capable of finetuning macrophage gene expression

Platelet extracellular vesicles (PEVs) generated upon platelet activation may play a role in inflammatory pathologies such as atherosclerosis. Oxidized low-density lipoprotein (oxLDL), a well-known contributor to atherogenesis, activates platelets and presensitizes them for activation by other agonists. We studied the effect of oxLDL on the secretion, composition, and inflammatory functions of PEVs using contemporary EV analytics. Platelets were activated by co-stimulation with thrombin (T) and collagen (C) ± oxLDL and characterized by high-resolution flow cytometry, nanoparticle tracking analysis, proximity extension assay, western blot, and electron microscopy. The effect of PEVs on macrophage differentiation and functionality was examined by analyzing macrophage surface markers, cytokine secretion, and transcriptome. OxLDL upregulated TC-induced formation of CD61⁺, P-selectin⁺ and phosphatidylserine⁺ PEVs. Blocking the scavenger receptor CD36 significantly suppressed the oxLDL+TC-induced PEV formation, and HDL caused a slight but detectable suppression. The inflammatory protein cargo differed between the PEVs from stimulated and unstimulated platelets. Both oxLDL+TC- and TC-induced PEVs enhanced macrophage HLA-DR and CD86 expression and decreased CD11c expression as well as secretion of several cytokines. Pathways related to cell cycle and regulation of gene expression, and immune system signaling were overrepresented in the differentially expressed genes between TC PEV -treated vs. control macrophages and oxLDL+TC PEV -treated vs. control macrophages, respectively. In conclusion, we speculate that oxLDL and activated platelets contribute to proatherogenic processes by increasing the number of PEVs that provide an adhesive and procoagulant surface, contain inflammatory mediators, and subtly finetune the macrophage gene expression.

Swimming improves platelet dysfunction in mice fed with a high-fat diet

BACKGROUND

We investigated the effects and mechanism of swimming on platelet function in mice fed with a high-fat diet.

MATERIAL AND METHODS

Mice were randomly divided into the control group (NC), high-fat group (HF), and high-fat diet combined with swimming group (FE). The FE group swam for 60 min a day, 5 days a week, for 8 weeks.

RESULTS

Compared with the NC group, the HF group had significant weight gain, dyslipidemia, abbreviated bleeding time after tail breakage, increased clot retraction, increased platelet aggregation rate, increased spread of platelets on fibrinogen, and increased pAKT level in platelets. Compared with the HF group, the FE group had lower body weight, improved dyslipidemia, prolonged bleeding time, reduced clot retraction, reduced platelet aggregation rate, decreased spread of platelets on fibrinogen, and decreased pAKT level in platelets.

CONCLUSIONS

By inhibiting the level of pAKT in platelets, swimming improves platelet dysfunction in mice fed with a high-fat diet.

Targeting macrophages in atherosclerosis using nanocarriers loaded with liver X receptor agonists: A narrow review

Macrophages are involved in the whole process of atherosclerosis, which is characterized by accumulation of lipid and inflammation. Presently, clinically used lipid-lowering drugs cannot completely retard the progress of atherosclerosis. Liver X receptor (LXR) plays a key role in regulation of lipid metabolism and inflammation. Accumulating evidence have demonstrated that synthetic LXR agonists can significantly retard the development of atherosclerosis. However, these agonists induce sever hypertriglyceridemia and liver steatosis. These side effects have greatly limited their potential application for therapy of atherosclerosis. The rapid development of drug delivery system makes it possible to delivery interested drugs to special organs or cells using nanocarriers. Macrophages express various receptors which can recognize and ingest specially modified nanocarriers loaded with LXR agonists. In the past decades, a great progress has been made in this field. These macrophage-targeted nanocarriers loaded with LXR agonists are found to decrease atherosclerosis by reducing cholesterol accumulation and inflammatory reactions. Of important, these nanocarriers can alleviate side effects of LXR agonists. In this article, we briefly review the roles of macrophages in atherosclerosis, mechanisms of action of LXR agonists, and focus on the advances of macrophage-targeted nanocarriers loaded with LXR agonists. This work may promote the potential clinical application of these nanocarriers.

Procoagulant platelets: novel players in thromboinflammation

Platelets play a key role in maintaining hemostasis. However, dysregulated platelet activation can lead to pathological thrombosis or bleeding. Once a platelet gets activated, it will either become an aggregatory platelet or eventually a procoagulant platelet with both types playing distinct roles in thrombosis and hemostasis. Although aggregatory platelets have been extensively studied, procoagulant platelets have only recently come into the spotlight. Procoagulant platelets are a subpopulation of highly activated platelets that express phosphatidylserine and P-selectin on their surface, allowing for coagulation factors to bind and thrombin to be generated. In recent years, novel roles for procoagulant platelets have been identified and they have increasingly been implicated in thromboinflammatory diseases. Here, we provide an up-to-date review on the mechanisms resulting in the formation of procoagulant platelets and how they contribute to hemostasis, thrombosis, and thromboinflammation.

Platelet Redox Imbalance in Hypercholesterolemia: A Big Problem for a Small Cell

The imbalance between reactive oxygen species (ROS) synthesis and their scavenging by anti-oxidant defences is the common soil of many disorders, including hypercholesterolemia. Platelets, the smallest blood cells, are deeply involved in the pathophysiology of occlusive arterial thrombi associated with myocardial infarction and stroke. A great deal of evidence shows that both increased intraplatelet ROS synthesis and impaired ROS neutralization are implicated in the thrombotic process. Hypercholesterolemia is recognized as cause of atherosclerosis, cerebro- and cardiovascular disease, and, closely related to this, is the widespread acceptance that it strongly contributes to platelet hyperreactivity via direct oxidized LDL (oxLDL)-platelet membrane interaction via scavenger receptors such as CD36 and signaling pathways including Src family kinases (SFK), mitogen-activated protein kinases (MAPK), and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. In turn, activated platelets contribute to oxLDL generation, which ends up propagating platelet activation and thrombus formation through a mechanism mediated by oxidative stress. When evaluating the effect of lipid-lowering therapies on thrombogenesis, a large body of evidence shows that the effects of statins and proprotein convertase subtilisin/kexin type 9 inhibitors are not limited to the reduction of LDL-C but also to the down-regulation of platelet reactivity mainly by mechanisms sensitive to intracellular redox balance. In this review, we will focus on the role of oxidative stress-related mechanisms as a cause of platelet hyperreactivity and the pathophysiological link of the pleiotropism of lipid-lowering agents to the beneficial effects on platelet function.

Oxidised Low-Density Lipoprotein-Induced Platelet Hyperactivity—Receptors and Signalling Mechanisms

Dyslipidaemia leads to proatherogenic oxidative lipid stress that promotes vascular inflammation and thrombosis, the pathologies that underpin myocardial infarction, stroke, and deep vein thrombosis. These prothrombotic states are driven, at least in part, by platelet hyperactivity, and they are concurrent with the appearancxe of oxidatively modified low-density lipoproteins (LDL) in the circulation. Modified LDL are heterogenous in nature but, in a general sense, constitute a prototype circulating transporter for a plethora of oxidised lipid epitopes that act as danger-associated molecular patterns. It is well-established that oxidatively modified LDL promote platelet activation and arterial thrombosis through a number of constitutively expressed scavenger receptors, which transduce atherogenic lipid stress to a complex array of proactivatory signalling pathways in the platelets. Stimulation of these signalling events underlie the ability of modified LDL to induce platelet activation and blunt platelet inhibitory pathways, as well as promote platelet-mediated coagulation. Accumulating evidence from patients at risk of arterial thrombosis and experimental animal models of disease suggest that oxidised LDL represents a tangible link between the dyslipidaemic environment and increased platelet activation. The aim of this review is to summarise recent advances in our understanding of the pro-thrombotic signalling events induced in platelets by modified LDL ligation, describe the contribution of individual platelet scavenger receptors, and highlight potential future challenges of targeting these pathways.

Redox Mechanisms of Platelet Activation in Aging

Aging is intrinsically linked with physiologic decline and is a major risk factor for a broad range of diseases. The deleterious effects of advancing age on the vascular system are evidenced by the high incidence and prevalence of cardiovascular disease in the elderly. Reactive oxygen species are critical mediators of normal vascular physiology and have been shown to gradually increase in the vasculature with age. There is a growing appreciation for the complexity of oxidant and antioxidant systems at the cellular and molecular levels, and accumulating evidence indicates a causal association between oxidative stress and age-related vascular disease. Herein, we review the current understanding of mechanistic links between oxidative stress and thrombotic vascular disease and the changes that occur with aging. While several vascular cells are key contributors, we focus on oxidative changes that occur in platelets and their mediation in disease progression. Additionally, we discuss the impact of comorbid conditions (i.e., diabetes, atherosclerosis, obesity, cancer, etc.) that have been associated with platelet redox dysregulation and vascular disease pathogenesis. As we continue to unravel the fundamental redox mechanisms of the vascular system, we will be able to develop more targeted therapeutic strategies for the prevention and management of age-associated vascular disease.

Is Arsenic Exposure a Risk Factor for Metabolic Syndrome? A Review of the Potential Mechanisms

Exposure to arsenic in drinking water is a worldwide health problem. This pollutant is associated with increased risk of developing chronic diseases, including metabolic diseases. Metabolic syndrome (MS) is a complex pathology that results from the interaction between environmental and genetic factors. This condition increases the risk of developing type 2 diabetes, cardiovascular diseases, and cancer. The MS includes at least three of the following signs, central obesity, impaired fasting glucose, insulin resistance, dyslipidemias, and hypertension. Here, we summarize the existing evidence of the multiple mechanisms triggered by arsenic to developing the cardinal signs of MS, showing that this pollutant could contribute to the multifactorial origin of this pathology.

Rhubarb Enema Decreases Circulating Trimethylamine N-Oxide Level and Improves Renal Fibrosis Accompanied With Gut Microbiota Change in Chronic Kidney Disease Rats

Objectives: Trimethylamine N-oxide (TMAO), a metabolic product of gut flora, is increased in chronic kidney disease (CKD) subjects and is recognized as one type of uremic toxins which is associated with poor cardiovascular outcomes and kidney function loss. Previous studies have suggested that rhubarb enema could reduce circulating uremic toxins such as urea, creatinine, and indoxyl sulfate and also regulate the intestinal microbiota. However, whether rhubarb enema retards kidney dysfunction by reducing circulating TMAO and its underlying mechanism, are still unclear. The present study aims to investigate the impact of rhubarb enema on TMAO and its precursors, as well as on the intestinal microbiota in 5/6 nephrectomized (5/6Nx) CKD rats.

Design: Rats in the treatment groups were given rhubarb enema after modeling. At the end of the study, blood, feces, and kidney tissues were collected and processed for biochemical analyses, histological and western blot analyses, 16S rRNA sequence and untargeted metabolomic analyses.

Results: Rhubarb enema reduced serum TMAO and trimethylamine (TMA) levels, inhibited the expression of inflammatory markers (interleukin-6, tumor necrosis factor α and Interferon-γ) and alleviated tubular atrophy, monocyte infiltration and interstitial fibrosis in 5/6Nx CKD rats. Moreover, rhubarb enema significantly increased the abundance of some symbiotic bacteria and probiotics, while reduced the abundance of some potential pathogens at the genus level. In addition, Spearman’s correlation analysis revealed that lachnospiraceae and romboutsia were positively correlated with TMAO.

Conclusion: Rhubarb enema decreases circulating TMAO level and improves renal fibrosis in 5/6Nx CKD rats, which may be related to the regulation of intestinal microbial community.

The role of P-glycoprotein (P-gp) and inwardly rectifying potassium (Kir) channels in sudden unexpected death in epilepsy (SUDEP)

Sudden unexpected death in epilepsy (SUDEP) is the major cause of death that affects patients with epilepsy. The risk of SUDEP increases according to the frequency and severity of uncontrolled seizures; therefore, SUDEP risk is higher in patients with refractory epilepsy (RE), in whom most antiepileptic drugs (AEDs) are ineffective for both seizure control and SUDEP prevention. Consequently, RE and SUDEP share a multidrug resistance (MDR) phenotype, which is mainly associated with brain overexpression of ABC-transporters such as P-glycoprotein (P-gp). The activity of P-gp can also contribute to membrane depolarization and affect the normal function of neurons and cardiomyocytes. Other molecular regulators of membrane potential are the inwardly rectifying potassium channels (Kir), whose genetic variants have been related to both epilepsy and heart dysfunctions. Although it has been suggested that dysfunctions of the cardiac, respiratory, and brainstem arousal systems are the causes of SUDEP, the molecular basis for explaining its dysfunctions remain unknown. In rats, repetitive seizures or status epilepticus induced high expression of P-gp and loss Kir expression in the brain and heart, and promoted membrane depolarization, malignant bradycardia, and the high rate of mortality. Here we reviewed clinical and experimental evidences suggesting that abnormal expression of depolarizing/repolarizing factors as P-gp and Kir could favor persistent depolarization of membranes without any rapid functional recovery capacity. This condition induced by convulsive stress could be the molecular mechanism leading to acquired severe bradycardia, as an ineffective heart response generating the appropriate scenario for SUDEP development. This article is part of the Special Issue "NEWroscience 2018".

Fatty acids and evolving roles of their proteins in neurological, cardiovascular disorders and cancers

The dysregulation of fat metabolism is involved in various disorders, including neurodegenerative, cardiovascular, and cancers. The uptake of long-chain fatty acids (LCFAs) with 14 or more carbons plays a pivotal role in cellular metabolic homeostasis. Therefore, the uptake and metabolism of LCFAs must constantly be in tune with the cellular, metabolic, and structural requirements of cells. Many metabolic diseases are thought to be driven by the abnormal flow of fatty acids either from the dietary origin and/or released from adipose stores. Cellular uptake and intracellular trafficking of fatty acids are facilitated ubiquitously with unique combinations of fatty acid transport proteins and cytoplasmic fatty acid-binding proteins in every tissue. Extensive data are emerging on the defective transporters and metabolism of LCFAs and their clinical implications. Uptake and metabolism of LCFAs are crucial for the brain's functional development and cardiovascular health and maintenance. In addition, data suggest fatty acid metabolic transporter can normalize activated inflammatory response by reprogramming lipid metabolism in cancers. Here we review the current understanding of how LCFAs and their proteins contribute to the pathophysiology of three crucial diseases and the mechanisms involved in the processes.

Problems of Pathogenesis and Pathogenetic Therapy of COVID-19 from the Perspective of the General Theory of Pathological Systems (General Pathological Processes)

The COVID-19 pandemic examines not only the state of actual health care but also the state of fundamental medicine in various countries. Pro-inflammatory processes extend far beyond the classical concepts of inflammation. They manifest themselves in a variety of ways, beginning with extreme physiology, then allostasis at low-grade inflammation, and finally the shockogenic phenomenon of "inflammatory systemic microcirculation". The pathogenetic core of critical situations, including COVID-19, is this phenomenon. Microcirculatory abnormalities, on the other hand, lie at the heart of a specific type of general pathological process known as systemic inflammation (SI). Systemic inflammatory response, cytokine release, cytokine storm, and thrombo-inflammatory syndrome are all terms that refer to different aspects of SI. As a result, the metabolic syndrome model does not adequately reflect the pathophysiology of persistent low-grade systemic inflammation (ChSLGI). Diseases associated with ChSLGI, on the other hand, are risk factors for a severe COVID-19 course. The review examines the role of hypoxia, metabolic dysfunction, scavenger receptors, and pattern-recognition receptors, as well as the processes of the hemophagocytic syndrome, in the systemic alteration and development of SI in COVID-19.

Fine-Tuning of Platelet Responses by Serine/Threonine Protein Kinases and Phosphatases-Just the Beginning

Comprehensive proteomic analyses of human and murine platelets established an extraordinary intracellular repertoire of signaling components, which control crucial functions. The spectrum of platelet serine/threonine protein kinases (more than 100) includes the AGC family (protein kinase A, G, C [PKA, PKG, PKC]), the mitogen-activated protein kinases (MAPKs), and others. PKA and PKG have multiple significantly overlapping substrates in human platelets, which possibly affect functions with clear "signaling nodes" of regulation by multiple protein kinases/phosphatases. Signaling nodes are intracellular Ca²⁺ stores, the contractile system (myosin light chains), and other signaling components such as G-proteins, protein kinases, and protein phosphatases. An example for this fine-tuning is the tyrosine kinase Syk, a crucial component of platelet activation, which is controlled by several serine/threonine and tyrosine protein kinases as well as phosphatases. Other protein kinases including PKA/PKG modulate protein phosphatase 2A, which may be a master regulator of MAPK signaling in human platelets. Protein kinases and in particular MAPKs are targeted by an increasing number of clinically used inhibitors. However, the precise regulation and fine-tuning of these protein kinases and their effects on other signaling components in platelets are only superficially understood-just the beginning. However, promising future approaches are in sight.

Inhibitors of Protein Convertase Subtilisin/Kexin 9 (PCSK9) and Acute Coronary Syndrome (ACS): The State-of-the-Art

Acute Coronary Syndrome (ACS) remains one of the most frequent causes of morbidity and mortality in the world. Although the age- and gender-adjusted incidence of ACS is decreasing, the mortality associated with this condition remains high, especially 1-year after the acute event. Several studies demonstrated that PCSK9 inhibitors therapy determine a significant reduction of major adverse cardiovascular events (MACE) in post-ACS patients, through a process of plaque modification, by intervening in lipid metabolism and platelet aggregation and finally determining an improvement in endothelial function. In the EVACS (Evolocumab in Acute Coronary Syndrome) study, evolocumab allows >90% of patients to achieve LDL-C < 55 mg/dL according to ESC/EAS guidelines compared to 11% of patients who only receive statins. In the EVOPACS (EVOlocumab for Early Reduction of low-density lipoprotein (LDL)-cholesterol Levels in Patients With Acute Coronary Syndromes) study, evolocumab determined LDL levels reduction of 40.7% (95% CI: 45.2 to 36.2; p < 0.001) and allowed 95.7% of patients to achieve LDL levels <55 mg/dL. In ODYSSEY Outcome trial, alirocumab reduced the overall risk of MACE by 15% (HR = 0.85; CI: 0.78-0.93; p = 0.0003), with a reduced risk of all-cause mortality (HR = 0.85; CI: 0.73-0.98: nominal p = 0026), and fewer deaths for coronary heart disease (CHD) compared to the control group (HR = 0.92; CI: 0.76-1.11; p = 0.38). The present review aimed at describing the beneficial effect of PCSK9 inhibitors therapy early after ACS in reducing LDL circulating levels (LDL-C) and the risk of major adverse cardiovascular events, which was very high in the first year and persists higher later after the acute event.

Platelet Proteomes, Pathways, and Phenotypes as Informants of Vascular Wellness and Disease

Platelets rapidly undergo responsive transitions in form and function to repair vascular endothelium and mediate hemostasis. In contrast, heterogeneous platelet subpopulations with a range of primed or refractory phenotypes gradually arise in chronic inflammatory and other conditions in a manner that may indicate or support disease. Qualitatively distinguishable platelet phenotypes are increasingly associated with a variety of physiological and pathological circumstances; however, the origins and significance of platelet phenotypic variation remain unclear and conceptually vague. As changes in platelet function in disease exhibit many similarities to platelets following the activation of platelet agonist receptors, the intracellular responses of platelets common to hemostasis and inflammation may provide insights to the molecular basis of platelet phenotype. Here, we review concepts around how protein-level relations-from platelet receptors through intracellular signaling events-may help to define platelet phenotypes in inflammation, immune responses, aging, and other conditions. We further discuss how representing systems-wide platelet proteomics data profiles as circuit-like networks of causally related intracellular events, or, pathway maps, may inform molecular definitions of platelet phenotype. In addition to offering insights into platelets as druggable targets, maps of causally arranged intracellular relations underlying platelet function can also advance precision and interceptive medicine efforts by leveraging platelets as accessible, dynamic, endogenous, circulating biomarkers of vascular wellness and disease. Graphic Abstract: A graphic abstract is available for this article.

Lipoprotein receptor signalling in atherosclerosis

The founding member of the lipoprotein receptor family, low-density lipoprotein receptor (LDLR) plays a major role in the atherogenesis through the receptor-mediated endocytosis of LDL particles and regulation of cholesterol homeostasis. Since the discovery of the LDLR, many other structurally and functionally related receptors have been identified, which include low-density lipoprotein receptor-related protein (LRP)1, LRP5, LRP6, very low-density lipoprotein receptor, and apolipoprotein E receptor 2. The scavenger receptor family members, on the other hand, constitute a family of pattern recognition proteins that are structurally diverse and recognize a wide array of ligands, including oxidized LDL. Among these are cluster of differentiation 36, scavenger receptor class B type I and lectin-like oxidized low-density lipoprotein receptor-1. In addition to the initially assigned role as a mediator of the uptake of macromolecules into the cell, a large number of studies in cultured cells and in in vivo animal models have revealed that these lipoprotein receptors participate in signal transduction to modulate cellular functions. This review highlights the signalling pathways by which these receptors influence the process of atherosclerosis development, focusing on their roles in the vascular cells, such as macrophages, endothelial cells, smooth muscle cells, and platelets. Human genetics of the receptors is also discussed to further provide the relevance to cardiovascular disease risks in humans. Further knowledge of the vascular biology of the lipoprotein receptors and their ligands will potentially enhance our ability to harness the mechanism to develop novel prophylactic and therapeutic strategies against cardiovascular diseases.

Hyperglycemia exacerbates ischemic stroke outcome independent of platelet glucose uptake

OBJECTIVE

Hyperglycemia is a common comorbidity for ischemic stroke and is associated with worsened neurological outcomes. Platelets are central mediators of ischemic stroke and hyperglycemia mediates platelet hyperactivity. In this study, we investigated the contribution of platelet glucose metabolism to ischemic stroke.

METHODS

Mice lacking both Glut1 and Glut3 specifically in platelets (DKO) and their littermate controls (WT) were subjected to 1-hour transient middle cerebral artery occlusion under normoglycemic and streptozotocin-induced hyperglycemic conditions after which stroke outcomes, platelet activation, and platelet-neutrophil aggregate (PNA) formation were examined.

RESULTS

Under normoglycemic conditions, DKO mice were protected from ischemic stroke with smaller brain infarct volumes and improved cerebral blood flow. In addition, DKO mice had reduced platelet activation, PNA, and cerebral neutrophil recruitment after stroke. Hyperglycemia significantly increased infarct size and cerebral Evans blue extravasation and worsened neurological outcomes and cerebral blood flow in both WT and DKO mice, abolishing the protective effect witnessed under normoglycemic conditions. Flow cytometric analysis after stroke demonstrated increased platelet activation and neutrophil trafficking to the brain, independent of platelet glucose metabolism. Finally, platelets from healthy DKO mice were unable to become procoagulant upon dual agonist stimulation. Conversely, hyperglycemia increased platelet mitochondrial reactive oxygen species production which potentiated procoagulant platelet formation in WT mice and restored procoagulant platelet formation in DKO mice.

CONCLUSION

Hyperglycemia aggravates ischemic stroke outcome independent of platelet glucose uptake. Furthermore, we demonstrated that hyperglycemia primes procoagulant platelet formation. This underlines the therapeutic potential for strategies targeting procoagulant platelet formation for the treatment of acute ischemic stroke.

Mild hyperlipidemia in mice aggravates platelet responsiveness in thrombus formation and exploration of platelet proteome and lipidome

Hyperlipidemia is a well-established risk factor for cardiovascular diseases. Millions of people worldwide display mildly elevated levels of plasma lipids and cholesterol linked to diet and life-style. While the prothrombotic risk of severe hyperlipidemia has been established, the effects of moderate hyperlipidemia are less clear. Here, we studied platelet activation and arterial thrombus formation in Apoe^-/- and Ldlr^-/- mice fed a normal chow diet, resulting in mildly increased plasma cholesterol. In blood from both knockout mice, collagen-dependent thrombus and fibrin formation under flow were enhanced. These effects did not increase in severe hyperlipidemic blood from aged mice and upon feeding a high-fat diet (Apoe^-/- mice). Bone marrow from wild-type or Ldlr^-/- mice was transplanted into irradiated Ldlr^-/- recipients. Markedly, thrombus formation was enhanced in blood from chimeric mice, suggesting that the hyperlipidemic environment altered the wild-type platelets, rather than the genetic modification. The platelet proteome revealed high similarity between the three genotypes, without clear indication for a common protein-based gain-of-function. The platelet lipidome revealed an altered lipid profile in mildly hyperlipidemic mice. In conclusion, in Apoe^-/- and Ldlr^-/- mice, modest elevation in plasma and platelet cholesterol increased platelet responsiveness in thrombus formation and ensuing fibrin formation, resulting in a prothrombotic phenotype.

The role of CD36 in cardiovascular disease

CD36, also known as the scavenger receptor B2, is a multifunctional receptor widely expressed in various organs. CD36 plays a crucial role in the uptake of long-chain fatty acids, the main metabolic substrate in myocardial tissue. The maturation and transportation of CD36 is regulated by post-translational modifications, including phosphorylation, ubiquitination, glycosylation, and palmitoylation. CD36 is decreased in pathological cardiac hypertrophy caused by ischaemia-reperfusion and pressure overload, and increased in diabetic cardiomyopathy and atherosclerosis. Deficiency of CD36 alleviates diabetic cardiomyopathy and atherosclerosis, while overexpression of CD36 eliminates ischaemia-reperfusion damage, together suggesting that CD36 is closely associated with the progression of cardiovascular diseases and may be a new therapeutic target. This review summarizes the regulation and post-translational modifications of CD36 and evaluates its role in cardiovascular diseases and its potential as a therapeutic target.

Cysteine sulfenylation by CD36 signaling promotes arterial thrombosis in dyslipidemia

Arterial thrombosis in the setting of dyslipidemia promotes clinically significant events, including myocardial infarction and stroke. Oxidized lipids in low-density lipoproteins (oxLDL) are a risk factor for athero-thrombosis and are recognized by platelet scavenger receptor CD36. oxLDL binding to CD36 promotes platelet activation and thrombosis by promoting generation of reactive oxygen species. The downstream signaling events initiated by reactive oxygen species in this setting are poorly understood. In this study, we report that CD36 signaling promotes hydrogen peroxide flux in platelets. Using carbon nucleophiles that selectively and covalently modify cysteine sulfenic acids, we found that hydrogen peroxide generated through CD36 signaling promotes cysteine sulfenylation of platelet proteins. Specifically, cysteines were sulfenylated on Src family kinases, which are signaling transducers that are recruited to CD36 upon recognition of its ligands. Cysteine sulfenylation promoted activation of Src family kinases and was prevented by using a blocking antibody to CD36 or by enzymatic degradation of hydrogen peroxide. CD36-mediated platelet aggregation and procoagulant phosphatidylserine externalization were inhibited in a concentration-dependent manner by a panel of sulfenic acid-selective carbon nucleophiles. At the same concentrations, these probes did not inhibit platelet aggregation induced by the purinergic receptor agonist adenosine diphosphate or the collagen receptor glycoprotein VI agonist collagen-related peptide. Selective modification of cysteine sulfenylation in vivo with a benzothiazine-based nucleophile rescued the enhanced arterial thrombosis seen in dyslipidemic mice back to control levels. These findings suggest that CD36 signaling generates hydrogen peroxide to oxidize cysteines within platelet proteins, including Src family kinases, and lowers the threshold for platelet activation in dyslipidemia.

Platelet MAPKs-a 20+ year history: What do we really know?

The existence of mitogen activated protein kinases (MAPKs) in platelets has been known for more than 20 years. Since that time hundreds of reports have been published describing the conditions that cause MAPK activation in platelets and their role in regulating diverse platelet functions from the molecular to physiological level. However, this cacophony of reports, with inconsistent and sometimes contradictory findings, has muddied the waters leading to great confusion. Since the last review of platelet MAPKs was published more than a decade ago, there have been more than 50 reports, including the description of novel knockout mouse models, that have furthered our knowledge. Therefore, we undertook an extensive literature review to delineate what is known about platelet MAPKs. We specifically discuss what is currently known about how MAPKs are activated and what signaling cascades they regulate in platelets incorporating recent findings from knockout mouse models. In addition, we will discuss the role each MAPK plays in regulating distinct platelet functions. In doing so, we hope to clarify the role for MAPKs and identify knowledge gaps in this field that await future researchers. In addition, we discuss the limitations of current studies with a particular focus on the off-target effects of commonly used MAPK inhibitors. We conclude with a look at the clinical utility of MAPK inhibitors as potential antithrombotic therapies with an analysis of current clinical trial data.

CD36 in Atherosclerosis: Pathophysiological Mechanisms and Therapeutic Implications

PURPOSE OF REVIEW

Atherosclerosis is a chronic disease characterized by lipid retention and inflammation in the artery wall. The retention and oxidation of low-density lipoprotein (LDL) in sub-endothelial space play a critical role in atherosclerotic plaque formation and destabilization. Oxidized LDL (ox-LDL) and other modified LDL particles are avidly taken up by endothelial cells, smooth muscle cells, and macrophages mainly through several scavenger receptors, including CD36 which is a class B scavenger receptor and membrane glycoprotein.

RECENT FINDINGS

Animal studies performed on CD36-deficient mice suggest that deficiency of CD36 prevents the development of atherosclerosis, though with some debate. CD36 serves as a signaling hub protein at the crossroad of inflammation, lipid metabolism, and fatty acid metabolism. In addition, the level of soluble CD36 (unattached to cells) in the circulating blood was elevated in patients with atherosclerosis and other metabolic disorders. We performed a state-of-the-art review on the structure, ligands, functions, and regulation of CD36 in the context of atherosclerosis by focusing on the pathological role of CD36 in the dysfunction of endothelial cells, smooth muscle cells, monocytes/macrophages, and platelets. Finally, we highlight therapeutic possibilities to target CD36 expression/activity in atherosclerosis.

CD36-fibrin interaction propagates FXI-dependent thrombin generation of human platelets

Thrombin converts fibrinogen to fibrin and activates blood and vascular cells in thrombo-inflammatory diseases. Platelets are amplifiers of thrombin formation when activated by leukocyte- and vascular cell-derived thrombin. CD36 on platelets acts as sensitizer for molecules with damage-associated molecular patterns, thereby increasing platelet reactivity. Here, we investigated the role of CD36 in thrombin-generation on human platelets, including selected patients with advanced chronic kidney disease (CKD). Platelets deficient in CD36 or blocked by anti-CD36 antibody FA6.152 showed impaired thrombin generation triggered by thrombin in calibrated automated thrombography. Using platelets with congenital function defects, blocking antibodies, pharmacological inhibitors, and factor-depleted plasma, CD36-sensitive thrombin generation was dependent on FXI, fibrin, and platelet signaling via GPIbα and SFKs. CD36-deficiency or blocking suppressed thrombin-induced platelet αIIbβ3 activation, granule exocytosis, binding of adhesion proteins and FV, FVIII, FIX, FX, but not anionic phospholipid exposure determined by flow cytometry. CD36 ligated specifically soluble fibrin, which recruited distinct coagulation factors via thiols. Selected patients with CKD showed elevated soluble fibrin plasma levels and enhanced thrombin-induced thrombin generation, which was normalized by CD36 blocking. Thus, CD36 is an important amplifier of platelet-dependent thrombin generation when exposure of anionic phospholipids is limited. This pathway might contribute to hypercoagulability in CKD.

Optimal concentration of necrostatin-1 for protecting against hippocampal neuronal damage in mice with status epilepticus

Hippocampal neurons undergo various forms of cell death after status epilepticus. Necrostatin-1 specifically inhibits necroptosis mediated by receptor interacting protein kinase 1 (RIP1) and RIP3 receptors. However, there are no reports of necroptosis in mouse models of status epilepticus. Therefore, in this study, we investigated the effects of necrostatin-1 on hippocampal neurons in mice with status epilepticus, and, furthermore, we tested different amounts of the compound to identify the optimal concentration for inhibiting necroptosis and apoptosis. A mouse model of status epilepticus was produced by intraperitoneal injection of kainic acid, 12 mg/kg. Different concentrations of necrostatin-1 (10, 20, 40, and 80 μM) were administered into the lateral ventricle 15 minutes before kainic acid injection. Hippocampal damage was assessed by hematoxylin-eosin staining 24 hours after the model was successfully produced. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling staining, western blot assay and immunohistochemistry were used to evaluate the expression of apoptosis-related and necroptosis-related proteins. Necrostatin-1 alleviated damage to hippocampal tissue in the mouse model of epilepsy. The 40 μM concentration of necrostatin-1 significantly decreased the number of apoptotic cells in the hippocampal CA1 region. Furthermore, necrostatin-1 significantly downregulated necroptosis-related proteins (MLKL, RIP1, and RIP3) and apoptosis-related proteins (cleaved-Caspase-3, Bax), and it upregulated the expression of anti-apoptotic protein Bcl-2. Taken together, our findings show that necrostatin-1 effectively inhibits necroptosis and apoptosis in mice with status epilepticus, with the 40 μM concentration of the compound having an optimal effect. The experiments were approved by the Animal Ethics Committee of Fujian Medical University, China (approval No. 2016-032) on November 9, 2016.

Procoagulant Phosphatidylserine-Exposing Platelets in vitro and in vivo

The physiological heterogeneity of platelets leads to diverse responses and the formation of discrete subpopulations upon platelet stimulation. Procoagulant platelets are an example of such subpopulations, a key characteristic of which is exposure either of the anionic aminophospholipid phosphatidylserine (PS) or of tissue factor on the activated platelet surface. This review focuses on the former, in which PS exposure on a subpopulation of platelets facilitates assembly of the intrinsic tenase and prothrombinase complexes, thereby accelerating thrombin generation on the activated platelet surface, contributing importantly to the hemostatic process. Mechanisms involved in platelet PS exposure, and accompanying events, induced by physiologically relevant agonists are considered then contrasted with PS exposure resulting from intrinsic pathway-mediated apoptosis in platelets. Pathologies of PS exposure, both inherited and acquired, are described. A consideration of platelet PS exposure as an antithrombotic target concludes the review.

Platelet lipidome: Dismantling the "Trojan horse" in the bloodstream

The platelet-lipid chapter in the story of atherothrombosis is an old one, recapitulated and revised in many contexts. For decades several stimulating facets have been added to it, both unraveling and increasing the perplexity of platelet-lipid interplay and its pathophysiological consequences. The recent paradigm shift in our perspective has evolved with lipidomic analysis of the intraplatelet compartment and platelet releasate. These investigations have disclosed that platelets are in constant interaction with circulatory lipids, often reflected in their lipid repertoire. In addition, they offer a shielded intracellular space for oxidative lipid metabolism generating "toxic" metabolites that escape degradation by plasma lipases and antioxidant defense, circulate undetected by conventional plasma lipid profile, and deposited at atherosclerotic lesions or thrombus. Lipidomics divulges this silent invader in platelet vehicles, thereby providing potential biomarkers of pathologic manifestations and therapeutic targets to be exploited, which is surmised in this review.

Gut Microbiota-Dependent Marker TMAO in Promoting Cardiovascular Disease: Inflammation Mechanism, Clinical Prognostic, and Potential as a Therapeutic Target

Cardiovascular disease (CVD) is the leading cause of death worldwide, especially in developed countries, and atherosclerosis (AS) is the common pathological basis of many cardiovascular diseases (CVDs) such as coronary heart disease (CHD). The role of the gut microbiota in AS has begun to be appreciated in recent years. Trimethylamine N-oxide (TMAO), an important gut microbe-dependent metabolite, is generated from dietary choline, betaine, and L-carnitine. Multiple studies have suggested a correlation between plasma TMAO levels and the risk of AS. However, the mechanism underlying this relationship is still unclear. In this review, we discuss the TMAO-involved mechanisms of atherosclerotic CVD from the perspective of inflammation, inflammation-related immunity, cholesterol metabolism, and atherothrombosis. We also summarize available clinical studies on the role of TMAO in predicting prognostic outcomes, including major adverse cardiovascular events (MACE), in patients presenting with AS. Finally, since TMAO may be a novel therapeutic target for AS, several therapeutic strategies including drugs, dietary, etc. to lower TMAO levels that are currently being explored are also discussed.

CD36 and ERK5 link dyslipidemia to apoptotic-like platelet procoagulant function

PURPOSE OF REVIEW

Metabolic diseases, including dyslipidemia, diabetes mellitus, and chronic inflammation are risk factors for clinically significant thrombotic events. Thrombosis in these settings is multifaceted with coordinated mechanisms between platelet activation and the hemostatic pathways. This review focuses on recent advances in platelet procoagulant and apoptotic signaling with emphasis on the pathophysiologic mechanisms induced by platelet CD36 in dyslipidemia, and the key unaddressed questions relating to the field.

RECENT FINDINGS

CD36 promotes platelet activation and increases the risk for thrombosis through signaling events. These include generation of reactive oxygen species, activation of redox-sensitive MAP kinase ERK5, and promotion of a pro-thrombotic phenotype. CD36 promotes phosphatidylserine externalization leading to a procoagulant function downstream from MAP kinase ERK5 that is separate from a pro-aggregatory function. Phosphatidylserine externalization requires maladaptive caspase activation, promotes assembly of the factor tenase and prothrombinase complex, and promotes fibrin formation. It is distinct from the canonical pathways mediating platelet procoagulant function by strong physiologic stimuli or by the platelet apoptotic-like Bak/Bax-mediated pathway for cellular clearance.

SUMMARY

Understanding CD36 signaling in the context of dyslipidemia, or other metabolic diseases will identify important and novel signaling hubs that could be potential therapeutic targets for intervention without impacting hemostasis.

Atherogenic lipid stress induces platelet hyperactivity through CD36-mediated hyposensitivity to prostacyclin: the role of phosphodiesterase 3A

Prostacyclin (PGI₂) controls platelet activation and thrombosis through a cyclic adenosine monophosphate (cAMP) signaling cascade. However, in patients with cardiovascular diseases this protective mechanism fails for reasons that are unclear. Using both pharmacological and genetic approaches we describe a mechanism by which oxidized low density lipoproteins (oxLDL) associated with dyslipidemia promote platelet activation through impaired PGI₂ sensitivity and diminished cAMP signaling. In functional assays using human platelets, oxLDL modulated the inhibitory effects of PGI₂, but not a phosphodiesterase (PDE)-insensitive cAMP analog, on platelet aggregation, granule secretion and in vitro thrombosis. Examination of the mechanism revealed that oxLDL promoted the hydrolysis of cAMP through the phosphorylation and activation of PDE3A, leading to diminished cAMP signaling. PDE3A activation by oxLDL required Src family kinases, Syk and protein kinase C. The effects of oxLDL on platelet function and cAMP signaling were blocked by pharmacological inhibition of CD36, mimicked by CD36-specific oxidized phospholipids and ablated in CD36^−/− murine platelets. The injection of oxLDL into wild-type mice strongly promoted FeCl3-induced carotid thrombosis in vivo, which was prevented by pharmacological inhibition of PDE3A. Furthermore, blood from dyslipidemic mice was associated with increased oxidative lipid stress, reduced platelet sensitivity to PGI₂ex vivo and diminished PKA signaling. In contrast, platelet sensitivity to a PDE-resistant cAMP analog remained normal. Genetic deletion of CD36 protected dyslipidemic animals from PGI₂ hyposensitivity and restored PKA signaling. These data suggest that CD36 can translate atherogenic lipid stress into platelet hyperactivity through modulation of inhibitory cAMP signaling.

CD36 signaling in vascular redox stress

Scavenger receptor CD36 is a multifunctional membrane protein that promotes thrombosis in conditions of oxidative stress such as metabolic disorders including dyslipidemia, diabetes mellitus, and chronic inflammation. In these conditions, specific reactive oxidant species are generated that are context and cell dependent. In the vasculature, CD36 signaling in smooth muscle cells and endothelial cells promotes generation of reactive oxygen species, genetic downregulation of antioxidant genes, and impaired smooth muscle and endothelial function. In hematopoietic cells, CD36 signaling enhances platelet dysfunction thus decreasing the threshold for platelet activation and accelerating arterial thrombosis, whereas in macrophages, CD36 promotes lipid-laden foam cell formation and atherosclerosis. These clinically significant processes are mediated through complex redox regulated signaling mechanisms that include Src-family kinases, MAP kinases and other downstream effectors. We provide an overview of CD36 signaling in vascular redox stress highlighting the role in oxidant generation in vascular and hematopoietic cells, but with special emphasis on platelets and dyslipidemia.