Platelet CD36 surface expression levels affect functional responses to oxidized LDL and are associated with inheritance of specific genetic polymorphisms. Blood. 2011;117:6355-6366. [PMID: 21478428 DOI: 10.1182/blood-2011-02-338582]

Implications of von Willebrand Factor in Inflammatory Bowel Diseases: Beyond Bleeding and Thrombosis

Inflammatory bowel disease (IBD) displays an increased venous and arterial thrombotic risk despite the common occurrence of intestinal bleeding. While some of the mechanisms leading to these thrombotic complications have been studied, other specific changes in the hemostasis profile of IBD patients have been less explored. One such example relates to von Willebrand factor (VWF) whose plasma levels have been reported to be modulated in IBD. Von Willebrand factor is a plasma glycoprotein crucial for hemostatic functions via roles both in platelet function and coagulation. High plasma VWF is a known risk factor for venous thromboembolism. In addition to its canonical roles in hemostasis, VWF is known to be directly or indirectly involved in other vascular processes such as maintenance of endothelial barrier integrity or proliferation of vascular smooth muscle cells. The purpose of this review is to recapitulate and update the existing data about VWF biology in IBD and to highlight its role both in the existing procoagulant phenotype and in vascular alterations that may occur in IBD.

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.

Elevated glycosylation of CD36 in platelets is a risk factor for oxLDL-mediated platelet activation in type 2 diabetes

Platelet activation and related cardiovascular complications are the hallmarks of type 2 diabetes (T2D). We investigated the mechanism of platelet activation in T2D using MS-based identification of differentially expressed platelet proteins with a focus on glycosylated forms. Glycosylation is considered one of the common post-translational modifications in T2D, and N/O-linked glycosylation of glycoproteins (GPs)/integrins is known to play crucial roles in platelet activation. Our platelet proteome data revealed elevated levels of GPs GPIbα, GPIIbIIIa, GPIV (CD36), GPV and integrins in T2D patients. T2D platelets had elevated N-linked glycosylation of CD36 at asparagine (Asn)408,417 . Enrichment analysis revealed a close association of glycosylated CD36 with thrombospondin-1, fibrinogen and SERPINA1 in T2D platelets. The glycosylation of CD36 has previously been reported to increase cellular uptake of long-chain fatty acids. Our in silico molecular docking data also showed a favorable binding of cholesterol with glycosylated Asn417 CD36 compared to the non-glycosylated form. We further investigated the CD36:LDL cholesterol axis in T2D. Elevated levels of oxidized-low density lipoprotein (oxLDL) were found to cause significant platelet activation via CD36-mediated stimulation of Lyn-JNK signaling. Sulfo-N-succinimidyl oleate, an inhibitor of CD36, effectively inhibited oxLDL-mediated platelet activation and adhesion in vitro. Our study suggests increased glycosylation of CD36 in T2D platelets as a potential route for oxLDL-mediated platelet activation. The oxLDL:CD36 axis may thus be exploited as a prospective target to develop therapeutics against thrombosis in T2D.

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.

Genetic Variants in CD36 Involved in Fat Taste Perception: Association with Anthropometric and Clinical Parameters in Overweight and Obese Subjects Affected by Type 2 Diabetes or Dysglycemia-A Pilot Study

Obesity and overweight represent a growing health problem worldwide. Genes regulating the intake and metabolism of different nutrients can positively or negatively influence the efficacy of nutritional interventions against obesity and its complications. The aim of this study was to assess changes in anthropometric and clinical parameters and the adherence to a Mediterranean diet (MedDiet) over time in relation to nutrigenetic variants in overweight or obese subjects affected by Type 2 Diabetes (T2D) or dysglycemia, who were included in a nutritional program. A total of 23 subjects were included in this study. Clinical parameters, physical activity levels, and the adherence to a MedDiet were evaluated at baseline, at 6 (T6), and at 12 months (T12) during and after a diet/lifestyle intervention. In a single blood sample from each subject, rs1984112 (A>G) and rs1761667 (G>A) in CD36; rs7950226 (G>A) in BMAL1; and rs1801260 (A>G), rs4864548 (A>G), and rs3736544 (G>A) in CLOCK were genotyped with Real-Time PCR. Significant associations were observed between CD36 rs1761667 and weight (p = 0.025), hip circumference (p = 0.042), triglycerides (p = 0.047), and HbA1c (p = 0.012) at baseline. Moreover, the genotype AA in CD36 rs1761667 was significantly associated with a lower BMI when compared to G carriers at baseline, at T6, and also at T12. In addition, subjects with the AA genotype at CD36 rs1984112 had significantly lower levels of HbA1c (p = 0.027) than the GG and AG genotypes at baseline. These results show that variants in CD36 can have an impact on anthropometric and clinical parameters in overweight or obese subjects affected by T2D or dysglycemia, and that it might influence the success of the diet/lifestyle intervention.

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.

S100A8/A9 drives the formation of procoagulant platelets through GPIbα

S100A8/A9, also known as "calprotectin" or "MRP8/14," is an alarmin primarily secreted by activated myeloid cells with antimicrobial, proinflammatory, and prothrombotic properties. Increased plasma levels of S100A8/A9 in thrombo-inflammatory diseases are associated with thrombotic complications. We assessed the presence of S100A8/A9 in the plasma and lung autopsies from patients with COVID-19 and investigated the molecular mechanism by which S100A8/A9 affects platelet function and thrombosis. S100A8/A9 plasma levels were increased in patients with COVID-19 and sustained high levels during hospitalization correlated with poor outcomes. Heterodimeric S100A8/A9 was mainly detected in neutrophils and deposited on the vessel wall in COVID-19 lung autopsies. Immobilization of S100A8/A9 with collagen accelerated the formation of a fibrin-rich network after perfusion of recalcified blood at venous shear. In vitro, platelets adhered and partially spread on S100A8/A9, leading to the formation of distinct populations of either P-selectin or phosphatidylserine (PS)-positive platelets. By using washed platelets, soluble S100A8/A9 induced PS exposure but failed to induce platelet aggregation, despite GPIIb/IIIa activation and alpha-granule secretion. We identified GPIbα as the receptor for S100A8/A9 on platelets inducing the formation of procoagulant platelets with a supporting role for CD36. The effect of S100A8/A9 on platelets was abolished by recombinant GPIbα ectodomain, platelets from a patient with Bernard-Soulier syndrome with GPIb-IX-V deficiency, and platelets from mice deficient in the extracellular domain of GPIbα. We identified the S100A8/A9-GPIbα axis as a novel targetable prothrombotic pathway inducing procoagulant platelets and fibrin formation, in particular in diseases associated with high levels of S100A8/A9, such as COVID-19.

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.

Impact of the interaction between the polymorphisms and hypermethylation of the CD36 gene on a new biomarker of type 2 diabetes mellitus: circulating soluble CD36 (sCD36) in Senegalese females

Background

Several predisposing factors for diabetes mellitus have been identified, including cluster determinant 36 (CD36) receptor expression. We aimed to determine the effects of CD36 gene polymorphisms and hypermethylation on the plasma CD36 protein levels in type 2 diabetes.

Materials and methods

We conducted a cross-sectional study involving 100 females (lean healthy control subjects and subjects with type 2 diabetes). This study was conducted at the Human Physiology Laboratory at the Dakar Faculty of Medicine in Senegal. Circulating sCD36 levels and DNA methyltransferase 3a levels were determined by enzyme-linked immunosorbent assay. The other biological parameters were evaluated in a biochemical laboratory. CD36 gene polymorphisms and methylation were explored by real-time polymerase chain reaction and methylation-specific polymerase chain reaction, respectively.

Results

sCD36 was negatively correlated with HDL-cholesterol levels (r =  − 0.52 p = 0.0001) and triglyceride levels (r =  − 0.36 p = 0.01) in control subjects. However, in the type 2 diabetes group, sCD36 levels were positively correlated with total cholesterol levels (r = 0.28 p = 0.04). For rs3211867, control subjects harboring the CC genotypes had significantly higher sCD36 levels than control subjects harboring the AA/AC genotype (p = 0.02); in the type 2 diabetes group, the sCD36 level was not significantly lower in subjects harboring the AA/AC genotype than in subjects harboring the CC genotype (p = 0.27). CD36 gene methylation reduced the sCD36 level in the control subjects compared to control subjects without CD36 gene methylation (p = 0.03). This difference was not significant in the type 2 diabetes group comparing subjects with diabetes with CD36 gene methylation to subjects with diabetes without CD36 gene methylation (p = 0.09). We noted a nonsignificant increase in sCD36 levels in subjects with diabetes with CD36 gene methylation compared to control subjects with CD36 gene methylation (p = 0.27). A combination of the CD36 polymorphism effect and the CD36 methylation effect did not significantly reduce sCD36 levels in subjects with type 2 diabetes.

Conclusion

CD36 gene polymorphisms and CD36 gene methylation separately reduce sCD36 levels. Their impacts are compensated for in subjects with type 2 diabetes by an increase in sCD36 levels, the mechanism of which needs to be elucidated.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12920-022-01337-2.

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.

CD36, a signaling receptor and fatty acid transporter that regulates immune cell metabolism and fate

CD36 is a type 2 cell surface scavenger receptor widely expressed in many immune and non-immune cells. It functions as both a signaling receptor responding to DAMPs and PAMPs, as well as a long chain free fatty acid transporter. Recent studies have indicated that CD36 can integrate cell signaling and metabolic pathways through its dual functions and thereby influence immune cell differentiation and activation, and ultimately help determine cell fate. Its expression along with its dual functions in both innate and adaptive immune cells contribute to pathogenesis of common diseases, including atherosclerosis and tumor progression, which makes CD36 and its downstream effectors potential therapeutic targets. This review comprehensively examines the dual functions of CD36 in a variety of immune cells, especially macrophages and T cells. We also briefly discuss CD36 function in non-immune cells, such as adipocytes and platelets, which impact the immune system via intercellular communication. Finally, outstanding questions in this field are provided for potential directions of future studies.

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.

Platelets, a Key Cell in Inflammation and Atherosclerosis Progression

Platelets play important roles in thrombosis-dependent obstructive cardiovascular diseases. In addition, it has now become evident that platelets also participate in the earliest stages of atherosclerosis, including the genesis of the atherosclerotic lesion. Moreover, while the link between platelet activity and hemostasis has been well established, the role of platelets as modulators of inflammation has only recently been recognized. Thus, through their secretory activities, platelets can chemically attract a diverse repertoire of cells to inflammatory foci. Although monocytes and lymphocytes act as key cells in the progression of an inflammatory event and play a central role in plaque formation and progression, there is also evidence that platelets can traverse the endothelium, and therefore be a direct mediator in the progression of atherosclerotic plaque. This review provides an overview of platelet interactions and regulation in atherosclerosis.

Scavenger receptor CD36 governs recruitment of myeloid cells to the blood-CSF barrier after stroke in neonatal mice

Background

Ischemic stroke induces the activation and recruitment of peripheral leukocytes to the injured brain. These cells can infiltrate the brain through multiple routes, either by penetrating blood–brain barrier or via blood–CSF barriers at the meninges or the choroid plexus (CP). We previously showed that myeloid cell trafficking via the CP occurs early after neonatal arterial stroke and modulates injury. CD36 is a receptor that mediates function of endothelial cells and cells of the monocyte lineage under various neurodegenerative conditions and can influence brain injury after neonatal stroke. Here we asked whether CD36 impacts injury by altering leukocyte trafficking through the CP in neonatal mice subjected to transient middle cerebral artery occlusion (tMCAO).

Methods

In neonatal mice with intact or globally disrupted CD36 signalling (CD36 KO), we characterized the phenotypes of myeloid cells by flow cytometry and the underlying gene expression signatures in the CPs contralateral and ipsilateral to tMCAO by RNA sequencing analyses, focussing on early post-reperfusion time window.

Results

Flow cytometry in the isolated CPs revealed that CD36 mediates stepwise recruitment of myeloid cells to the CP ipsilateral to tMCAO early after reperfusion, with a predominant increase first in inflammatory monocyte subsets and neutrophils followed by patrolling monocytes. RNA sequencing analyses demonstrated marked changes in gene expression in the CP ipsilateral compared to the CP contralateral to tMCAO in wild type mice. Changes were further modified by lack of CD36, including distinction in several clusters of genes involved in inflammatory, metabolic and extracellular matrix signalling in the CP ipsilateral to tMCAO.

Conclusion

Altogether, our data suggest cooperation between blood–CSF–brain interface via the CP through CD36-mediated signalling following neonatal stroke with a key role for inflammatory monocytes and neutrophils.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02388-z.

Cellular and Molecular Mechanisms of Fat Taste Perception

During the last couples of years, a number of studies have increasingly accumulated on the gustatory perception of dietary fatty acids in rodent models and human beings in health and disease. There is still a debate to coin a specific term for the gustatory perception of dietary fatty acids either as the sixth basic taste quality or as an alimentary taste. Indeed, the psycho-physical cues of orosensory detection of dietary lipids are not as distinctly perceived as other taste qualities like sweet or bitter. The cellular and molecular pharmacological mechanisms, triggered by the binding of dietary long-chain fatty acids (LCFAs) to tongue taste bud lipid receptors like CD36 and GPR120, involve Ca²⁺ signaling as other five basic taste qualities. We have not only elucidated the role of Ca²⁺ signaling but also identified different components of the second messenger cascade like STIM1 and MAP kinases, implicated in fat taste perception. We have also demonstrated the implication of Calhm1 voltage-gated channels and store-operated Ca²⁺ (SOC) channels like Orai1, Orai1/3, and TRPC3 in gustatory perception of dietary fatty acids. We have not only employed siRNA technology in vitro and ex vivo on tissues but also used animal models of genetic invalidation of STIM1, ERK1, Orai1, Calhm1 genes to explore their implications in fat taste signal transduction. Moreover, our laboratory has also demonstrated the importance of LCFAs detection dysfunction in obesity in animal models and human beings.

Worldwide diversity, association potential, and natural selection in the superimposed taste genes, CD36 and GNAT3

CD36 and GNAT3 mediate taste responses, with CD36 acting as a lipid detector and GNAT3 acting as the α subunit of gustducin, a G protein governing sweet, savory, and bitter transduction. Strikingly, the genes encoding CD36 and GNAT3 are genomically superimposed, with CD36 completely encompassing GNAT3. To characterize genetic variation across the CD36-GNAT3 region, its implications for phenotypic diversity, and its recent evolution, we analyzed from ~2,500 worldwide subjects sequenced by the 1000 Genomes Project (1000GP). CD36-GNAT3 harbored extensive diversity including 8,688 single-nucleotide polymorphisms (SNPs), 414 indels, and other complex variants. Sliding window analyses revealed that nucleotide diversity and population differentiation across CD36-GNAT3 were consistent with genome-wide trends in the 1000GP (π = 0.10%, P = 0.64; FST = 9.0%, P = 0.57). In addition, functional predictions using SIFT and PolyPhen-2 identified 60 variants likely to alter protein function, and they were in weak linkage disequilibrium (r2 < 0.17), suggesting their effects are largely independent. However, the frequencies of predicted functional variants were low (P¯ = 0.0013), indicating their contributions to phenotypic variance on population scales are limited. Tests using Tajima's D statistic revealed that pressures from natural selection have been relaxed across most of CD36-GNAT3 during its recent history (0.39 < P < 0.67). However, CD36 exons showed signs of local adaptation consistent with prior reports (P < 0.035). Thus, CD36 and GNAT3 harbor numerous variants predicted to affect taste sensitivity, but most are rare and phenotypic variance on a population level is likely mediated by a small number of sites.

Platelet proteome dynamics in hibernating 13-lined ground squirrels

Hibernating mammals undergo a dramatic drop in temperature and blood flow during torpor, yet avoid stasis blood clotting through mechanisms that remain unspecified. The effects of hibernation on hemostasis are especially complex, as cold temperatures generally activate platelets, resulting in platelet clearance and cold storage lesions in the context of blood transfusion. With a hibernating body temperature of 4°C-8°C, 13-lined ground squirrels (Ictidomys tridecemlineatus) provide a model to study hemostasis as well as platelet cold storage lesion resistance during hibernation. Here, we quantified and systematically compared proteomes of platelets collected from ground squirrels at summer (active), fall (entrance), and winter (topor) to elucidate how molecular-level changes in platelets may support hemostatic adaptations in torpor. Platelets were isolated from a total of 11 squirrels in June, October, and January. Platelet lysates from each animal were digested with trypsin prior to 11-plex tandem mass tag (TMT) labeling, followed by LC-MS/MS analysis for relative protein quantification. We measured >700 proteins with significant variations in abundance in platelets over the course of entrance, torpor, and activity-including systems of proteins regulating translation, secretion, metabolism, complement, and coagulation cascades. We also noted species-specific differences in levels of hemostatic, secretory, and inflammatory regulators in ground squirrel platelets relative to human platelets. Altogether, we provide the first ever proteomic characterization of platelets from hibernating animals, where systematic changes in metabolic, hemostatic, and other proteins may account for physiological adaptations in torpor and also inform translational effort to improve cold storage of human platelets for transfusion.

Association of FTO rs9939609 and CD36 rs1761667 with Visceral Obesity

In 2016, more than 1.9 billion adults were overweight; of which, over 650 million of adults were obese. Genetics and lifestyle play important roles in the development of obesity. Studies have shown that genetic variants contribute in developing obesity; such as FTO and CD36, which regulate metabolism and food preferences. Many researches have also emphasized the importance of lifestyle in obesity prevention. However, the interactions of both factors were still underexplored. Therefore, this study was aimed to assess the interaction between FTO-CD36 variants and fat consumption on the metabolic status of healthy Indonesians. Twenty-one females and seventeen males were involved in this cross-sectional study. CD36 rs1761667 and FTO rs9939609 genotypes were identified from blood samples using PCR-RFLP. Data were compared with dietary patterns (24-h food recall), physical activities (IPAQ), medical records, and body compositions (InBody720). Results: CD36 rs1761667 AA and AG group showed higher -but not significant- fat consumption, WHR, and VFA compared to GG. The trend persisted after gender and physical activity adjustment. Meanwhile, FTO rs9939609 AT group showed significant higher WC, WHR and VFA in male subjects after gender and energy balance adjustment: WC (TT: 74.40±3.85, AT: 85.50±5.92, p=0.011), WHR (TT: 0.85±0.02, AT: 0.92±0.04, p=0.010), and VFA (TT: 48.65±10.61, AT: 78.48±15.18, p=0.010). CD36 rs1761667 might be correlated with higher fat consumption and visceral obesity; while FTO rs9939609 showed a significant association with male visceral obesity. These results indicates that both genetic variants were potential as visceral obesity markers.

The Associations Between Bitter and Fat Taste Sensitivity, and Dietary Fat Intake: Are They Impacted by Genetic Predisposition?

A relationship between bitter and fat taste sensitivity, CD36 rs1761667 and TAS2R38 has been demonstrated. However, research is scarce and does not take diet into account. This study aimed to explore associations between genetics, fat and bitter taste sensitivity and dietary fat intake in healthy UK adults. A cross-sectional study was carried out on 88 Caucasian participants (49 females and 39 males aged 35 ± 1 years; body mass index 24.9 ± 0.5 kg/m2). Bitter taste sensitivity was assessed using phenylthiocarbamide (PTC) impregnated strips and the general Labeled Magnitude Scale. Fat taste sensitivity was assessed by the Ascending Forced Choice Triangle Procedure and dietary intake with a semi-quantitative food frequency questionnaire. Genotyping for rs713598, rs1726866, rs10246939, and rs1761667 was performed. Participants with TAS2R38 PAV/PAV diplotype perceived PTC strips as more bitter than groups carrying AVI haplotypes (AVI/AVI, P = 1 × 10-6; AVI/AAV, P = 0.029). CD36 rs1761667 was associated with fat taste sensitivity (P = 0.008). A negative correlation between bitter taste sensitivity and saturated fat intake was observed (rs = -0.256, P = 0.016). When combining the CD36 genotypes and TAS2R38 diplotypes into one variable, participants carrying both TAS2R38 AVI haplotype and CD36 A allele had a higher intake of saturated fat compared to carriers of CD36 GG genotype or TAS2R38 PAV/PAV and PAV/AAV diplotypes (13.8 ± 0.3 vs. 12.6 ± 0.5%TEI, P = 0.047) warranting further exploration in a larger cohort.

Feedback regulation of coronary artery disease susceptibility gene ADTRP and LDL receptors LDLR/CD36/LOX-1 in endothelia cell functions involved in atherosclerosis

A high level of low-density lipoprotein cholesterol (LDL) is one of the most important risk factors for coronary artery disease (CAD), the leading cause of death worldwide. However, a low concentration of LDL may be protective. Genome-wide association studies revealed that variation in ADTRP gene increased the risk of CAD. In this study, we found that a low concentration of oxidized-LDL induced the expression of ADTRP. Further analyses showed that knockdown of the expression of LDL receptor genes LDLR, CD36, or LOX-1 significantly downregulated ADTRP expression, whereas overexpression of LDLR/CD36/LOX-1 markedly increased ADTRP expression through the NF-κB pathway. Like ADTRP, LDLR, CD36 and LOX-1 were all involved in endothelial cell (EC) functions relevant to the initiation of atherosclerosis. Downregulation of LDLR/CD36/LOX-1 promoted monocyte adhesion to ECs and transendothelial migration of monocytes by increasing expression of ICAM-1, VCAM-1, E-selectin and P-selectin, decreased EC proliferation and migration, and increased EC apoptosis, thereby promoting the initiation of atherosclerosis. Opposite effects were observed with the overexpression of ADTRP and LDLR/CD36/LOX-1 in ECs. Interestingly, through the NF-κB and AKT pathways, overexpression of ADTRP significantly upregulated the expression of LDLR, CD36, and LOX-1, and knockdown of ADTRP expression significantly downregulated the expression of LDLR, CD36, and LOX-1. These data suggest that ADTRP and LDL receptors LDLR/CD36/LOX-1 positively regulate each other, and form a positive regulatory loop that regulates endothelial cell functions, thereby providing a potential protective mechanism against atherosclerosis. Our findings provide a new molecular mechanism by which deregulation of ADTRP and LDLR/CD36/LOX-1 promote the development of atherosclerosis and CAD.

Single nucleotide polymorphism in CD36: Correlation to peptide YY levels in obese and non-obese adults

BACKGROUND & AIMS

Human beings are often driven to exhibit dietary preference according to their hedonic characteristics. Though previous studies proposed that the fat taste preference of an obese individual was associated with BMI, the perception of fat taste differs for every individual. The genetic variation among populations in taste receptor genes such as CD36 may be a contributing factor for this difference. Satiety peptides can also play a role in the regulation of fat taste perception. Generally, this hormone helps us to feel the sense of satiety.

METHODS

We have analysed the relationship among oro-gustatory perception of dietary lipids, salivary peptide-YY and genetic polymorphism in CD36. Oral fatty acid sensitivity analysis was performed by alternative forced choice method. Salivary peptide-YY concentration was analysed by ELISA and single nucleotide polymorphism (SNP) in CD36 gene was determined by Real-Time PCR experiments.

RESULTS

We observed that the SNP at rs1761667 of CD36 and oral detection threshold for linoleic acid (LA) are associated with choice of food, lipid profiles, peptide-YY as well as adiposity parameters in obese population. Obese peoples had significantly low levels of peptide YY than people with BMI less than 25. These factors possibly play a role in preference for energy rich diets, development of obesity and associated complications.

CONCLUSION

This study provides a solid foundation for understanding the alterations in the dietary fat intake and levels of peptide-YY, which are associated with polymorphism in fat taste receptor. This is the first report that shows a significant relationship between the satiety hormone level, SNP in CD36 gene and oral fat detection threshold in human subjects.

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.

Changes of Taste, Smell and Eating Behavior in Patients Undergoing Bariatric Surgery: Associations with PROP Phenotypes and Polymorphisms in the Odorant-Binding Protein OBPIIa and CD36 Receptor Genes

Bariatric surgery is the most effective long-term treatment for severe obesity and related comorbidities. Although patients who underwent bariatric surgery report changes of taste and smell perception, results from sensory studies are discrepant and limited. Here, we assessed taste and smell functions in 51 patients before, one month, and six months after undergoing bariatric surgery. We used taste strip tests to assess gustatory function (including sweetness, saltiness, sourness, umaminess, bitterness and oleic acid, a fatty stimulus), the “Sniffin’ Sticks” test to assess olfactory identification and the 3-Factor Eating Questionnaire to assess eating behavior. We also explored associations between these phenotypes and flavor-related genes. Results showed an overall improvement in taste function (including increased sensitivity to oleic acid and the bitterness of 6-n-propylthiouracil (PROP)) and in olfactory function (which could be related to the increase in PROP and oleic acid sensitivity), an increase in cognitive restraint, and a decrease in disinhibition and hunger after bariatric surgery. These findings indicate that bariatric surgery can have a positive impact on olfactory and gustatory functions and eating behavior (with an important role of genetic factors, such PROP tasting), which in turn might contribute to the success of the intervention.

LDL-Cholesterol and Platelets: Insights into Their Interactions in Atherosclerosis

Atherosclerosis and its complications, including acute coronary syndromes, are the major cause of death worldwide. The two most important pathophysiological mechanisms underlying atherosclerosis include increased platelet activation and increased low-density lipoproteins (LDL) concentration. In contrast to LDL, oxidized (ox)-LDL have direct pro-thrombotic properties by functional interactions with platelets, leading to platelet activation and favoring thrombus formation. In this review, we summarize the currently available evidence on the interactions between LDL-cholesterol and platelets, which are based on (i) the presence of ox-LDL-binding sites on platelets, (ii) generation of ox-LDL by platelets and (iii) the role of activated platelets and ox-LDL in atherosclerosis. In addition, we elaborate on the clinical implications of these interactions, including development of the new therapeutic possibilities. The ability to understand and modulate mechanisms governing interactions between LDL-cholesterol and platelets may offer new treatment strategies for atherosclerosis prevention.

Molecular Aspects of Pathophysiology of Platelet Receptors

Receptor is a dynamic instrumental surface protein that helps to interact with specific molecules to respond accordingly. Platelet is the smallest in size among the blood components, but it plays many pivotal roles to maintain hemostasis involving its surface receptors. It (platelet) has cell adhesion receptors (e.g., integrins and glycoproteins), leucine-rich repeats receptors (e.g., TLRs, glycoprotein complex, and MMPs), selectins (e.g., CLEC, P-selectin, and CD), tetraspanins (e.g., CD and LAMP), transmembrane receptors (e.g., purinergic—P2Y and P2X1), prostaglandin receptors (e.g., TxA2, PGH2, and PGI2), immunoglobulin superfamily receptors (e.g., FcRγ and FcεR), etc. on its surface. The platelet receptors (e.g., glycoproteins, protease-activated receptors, and GPCRs) during platelet activation are over expressed and their granule contents are secreted (including neurotransmitters, cytokines, and chemokines) into circulation, which are found to be correlated with different physiological conditions. Interestingly, platelets promote metastasis through circulation protecting from cytolysis and endogenous immune surveillance involving several platelets receptors. The updated knowledge about different types of platelet receptors in all probable aspects, including their inter- and intra-signaling mechanisms, are discussed with respect to not only its (platelets) receptor type but also under different pathophysiological conditions.

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.

PCSK9 (Proprotein Convertase Subtilisin/Kexin 9) Enhances Platelet Activation, Thrombosis, and Myocardial Infarct Expansion by Binding to Platelet CD36

BACKGROUND

PCSK9 (proprotein convertase subtilisin/kexin 9), mainly secreted by the liver and released into the blood, elevates plasma low-density lipoprotein cholesterol by degrading low-density lipoprotein receptor. Pleiotropic effects of PCSK9 beyond lipid metabolism have been shown. However, the direct effects of PCSK9 on platelet activation and thrombosis, and the underlying mechanisms, as well, still remain unclear.

METHODS

We detected the direct effects of PCSK9 on agonist-induced platelet aggregation, dense granule ATP release, integrin αIIbβ3 activation, α-granule release, spreading, and clot retraction. These studies were complemented by in vivo analysis of FeCl₃-injured mouse mesenteric arteriole thrombosis. We also investigated the underlying mechanisms. Using the myocardial infarction (MI) model, we explored the effects of PCSK9 on microvascular obstruction and infarct expansion post-MI.

RESULTS

PCSK9 directly enhances agonist-induced platelet aggregation, dense granule ATP release, integrin αIIbβ3 activation, P-selectin release from α-granules, spreading, and clot retraction. In line, PCSK9 enhances in vivo thrombosis in a FeCl₃-injured mesenteric arteriole thrombosis mouse model, whereas PCSK9 inhibitor evolocumab ameliorates its enhancing effects. Mechanism studies revealed that PCSK9 binds to platelet CD36 and thus activates Src kinase and MAPK (mitogen-activated protein kinase)-extracellular signal-regulated kinase 5 and c-Jun N-terminal kinase, increases the generation of reactive oxygen species, and activates the p38MAPK/cytosolic phospholipase A2/cyclooxygenase-1/thromboxane A₂ signaling pathways downstream of CD36 to enhance platelet activation, as well. Using CD36 knockout mice, we showed that the enhancing effects of PCSK9 on platelet activation are CD36 dependent. It is important to note that aspirin consistently abolishes the enhancing effects of PCSK9 on platelet activation and in vivo thrombosis. Last, we showed that PCSK9 activating platelet CD36 aggravates microvascular obstruction and promotes MI expansion post-MI.

CONCLUSIONS

PCSK9 in plasma directly enhances platelet activation and in vivo thrombosis, and MI expansion post-MI, as well, by binding to platelet CD36 and thus activating the downstream signaling pathways. PCSK9 inhibitors or aspirin abolish the enhancing effects of PCSK9, supporting the use of aspirin in patients with high plasma PCSK9 levels in addition to PCSK9 inhibitors to prevent thrombotic complications.

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.

PCSK9 Regulates Nox2-Mediated Platelet Activation via CD36 Receptor in Patients with Atrial Fibrillation

Background: High levels of proprotein convertase subtilisin/kexin 9 (PCSK9) is predictive of cardiovascular events (CVEs) in atrial fibrillation (AF). We hypothesized that PCSK9 may directly induce platelet activation (PA). Methods: We measured platelet aggregation, recruitment, Thromboxane B2 (TxB2) formation and soluble P-selectin levels as markers of PA and soluble Nox2-derived peptide (sNox2-dp), H₂O₂, isoprostanes and oxidized Low-Density-Lipoprotein (oxLDL) to analyze oxidative stress (OS) in 88 patients having PCSK9 values < (n = 44) or > (n = 44) 1.2 ng/mL, balanced for age, sex and cardiovascular risk factors. Furthermore, we investigated if normal (n = 5) platelets incubated with PCSK9 (1.0–2.0 ng/mL) alone or with LDL (50 µg/mL) displayed changes of PA, OS and down-stream signaling. Results: PA and OS markers were significantly higher in patients with PCSK9 levels > 1.2 ng/mL compared to those with values < 1.2 ng/mL (p < 0.001). Levels of PCSK9 significantly correlated with markers of PA and OS. Platelets incubation with PCSK9 increased PA, OS and p38, p47 and Phospholipase A2 (PLA2) phosphorylation. These changes were amplified by adding LDL and blunted by CD36 or Nox2 inhibitors. Co-immunoprecipitation analysis revealed an immune complex of PCSK9 with CD36. Conclusions: We provide the first evidence that PCSK9, at concentration found in the circulation of AF patients, directly interacts with platelets via CD36 receptor and activating Nox2: this effect is amplified in presence of LDL.

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.

Taste Changes in Patients with Inflammatory Bowel Disease: Associations with PROP Phenotypes and polymorphisms in the salivary protein, Gustin and CD36 Receptor Genes

Inflammatory bowel disease (IBD) is a chronic inflammatory condition of the gastrointestinal tract resulting from interactions among various factors with diet being one of the most significant. IBD-related dietary behaviors are not clearly related to taste dysfunctions. We analyzed body mass index (BMI) and perception of six taste qualities and assessed effects of specific taste genes in IBD patients and healthy subjects (HC). BMI in IBD patients was higher than in HC subjects. Taste sensitivity to taste qualities was reduced in IBD patients, except for sour taste, which was higher than in HC subjects. Genetic variations were related to some taste responses in HC subjects, but not in IBD patients. Frequencies of genotype AA and allele A in CD36 polymorphism (rs1761667) were significantly higher in IBD patients than in HC subjects. The taste changes observed could be explained by the oral pathologies and microbiome variations known for IBD patients and can justify their typical dietary behaviors. The lack of genetic effects on taste in IBD patients indicates that IBD might compromise taste so severely that gene effects cannot be observed. However, the high frequency of the non-tasting form of CD36 substantiates the fact that IBD-associated fat taste impairment may represent a risk factor for IBD.

Neointimal hyperplasia: are fatty acid transport proteins a new therapeutic target?

PURPOSE OF REVIEW

High-fat diets contribute to hyperlipidemia and dysregulated metabolism underlying insulin resistant states and cardiovascular diseases. Neointimal hyperplasia is a significant resulting morbidity. Increased fatty acid (FA) levels lead to dysfunctional endothelium, defined as activated, proinflammatory and prothrombotic. The purpose of this review is to assess the recent literature on the emerging concept that uptake of FA into many tissues is regulated at the endothelial level, and this in turn contributes to endothelial dysfunction, an initiating factor in insulin resistant states, atherosclerosis and neointimal hyperplasia.

RECENT FINDINGS

Studies support the role of endothelial FA uptake proteins as an additional level of regulation in tissue FA uptake. These proteins include CD36, FA transport proteins, FA-binding proteins and caveolin-1. In many cases, inappropriate expression of these proteins can result in a change in FA and glucose uptake, storage and utilization. Accumulation of plasma FA is one mechanism by which alterations in expression of FA uptake proteins can lead to endothelial dysfunction; changes in tissue substrate metabolism leading to inflammation are also implicated.

SUMMARY

Identification of the critical players and regulators can lead to therapeutic targeting to reduce endothelial dysfunction and sequela such as insulin resistance and neointimal hyperplasia.

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

Dyslipidemia is a risk factor for clinically significant thrombotic events. In this condition, scavenger receptor CD36 potentiates platelet reactivity through recognition of circulating oxidized lipids. CD36 promotes thrombosis by activating redox-sensitive signaling molecules, such as the MAPK extracellular signal-regulated kinase 5 (ERK5). However, the events downstream of platelet ERK5 are not clear. In this study, we report that oxidized low-density lipoprotein (oxLDL) promotes exposure of procoagulant phosphatidylserine (PSer) on platelet surfaces. Studies using pharmacologic inhibitors indicate that oxLDL-CD36 interaction-induced PSer exposure requires apoptotic caspases in addition to the downstream CD36-signaling molecules Src kinases, hydrogen peroxide, and ERK5. Caspases promote PSer exposure and, subsequently, recruitment of the prothrombinase complex, resulting in the generation of fibrin from the activation of thrombin. Caspase activity was observed when platelets were stimulated with oxLDL. This was prevented by inhibiting CD36 and ERK5. Furthermore, oxLDL potentiates convulxin/glycoprotein VI-mediated fibrin formation by platelets, which was prevented when CD36, ERK5, and caspases were inhibited. Using 2 in vivo arterial thrombosis models in apoE-null hyperlipidemic mice demonstrated enhanced arterial fibrin accumulation upon vessel injury. Importantly, absence of ERK5 in platelets or mice lacking CD36 displayed decreased fibrin accumulation in high-fat diet-fed conditions comparable to that seen in chow diet-fed animals. These findings suggest that platelet signaling through CD36 and ERK5 induces a procoagulant phenotype in the hyperlipidemic environment by enhancing caspase-mediated PSer exposure.

Platelet Acetyl-CoA Carboxylase Phosphorylation: A Risk Stratification Marker That Reveals Platelet-Lipid Interplay in Coronary Artery Disease Patients

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Platelet phosphoACC is a marker for risk stratification in suspected CAD patients. It identifies high-risk CAD patients and correlates with severity of coronary artery calcification.

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The triglycerides/high-density lipoprotein cholesterol ratio is strongly associated with increased phosphoACC in circulating platelets. PhosphoACC is a metabolic signature of the platelet-proatherogenic lipid interplay in CAD patients.

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Phosphorylation and inhibition of acetyl-CoA carboxylase impacts platelet lipid content by down-regulating triglycerides lipid species.

Adenosine monophosphate–activated protein kinase (AMPK) acetyl-CoA carboxylase (ACC) signaling is activated in platelets by atherogenic lipids, particularly by oxidized low-density lipoproteins, through a CD36-dependent pathway. More interestingly, increased platelet AMPK–induced ACC phosphorylation is associated with the severity of coronary artery calcification as well as acute coronary events in coronary artery disease patients. Therefore, AMPK–induced ACC phosphorylation is a potential marker for risk stratification in suspected coronary artery disease patients. The inhibition of ACC resulting from its phosphorylation impacts platelet lipid content by down-regulating triglycerides, which in turn may affect platelet function.

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.

Functionalization of CD36 cardiovascular disease and expression associated variants by interdisciplinary high throughput analysis

CD36 is a platelet membrane glycoprotein whose engagement with oxidized low-density lipoprotein (oxLDL) results in platelet activation. The CD36 gene has been associated with platelet count, platelet volume, as well as lipid levels and CVD risk by genome-wide association studies. Platelet CD36 expression levels have been shown to be associated with both the platelet oxLDL response and an elevated risk of thrombo-embolism. Several genomic variants have been identified as associated with platelet CD36 levels, however none have been conclusively demonstrated to be causative. We screened 81 expression quantitative trait loci (eQTL) single nucleotide polymorphisms (SNPs) associated with platelet CD36 expression by a Massively Parallel Reporter Assay (MPRA) and analyzed the results with a novel Bayesian statistical method. Ten eQTLs located 13kb to 55kb upstream of the CD36 transcriptional start site of transcript ENST00000309881 and 49kb to 92kb upstream of transcript ENST00000447544, demonstrated significant transcription shifts between their minor and major allele in the MPRA assay. Of these, rs2366739 and rs1194196, separated by only 20bp, were confirmed by luciferase assay to alter transcriptional regulation. In addition, electromobility shift assays demonstrated differential DNA:protein complex formation between the two alleles of this locus. Furthermore, deletion of the genomic locus by CRISPR/Cas9 in K562 and Meg-01 cells results in upregulation of CD36 transcription. These data indicate that we have identified a variant that regulates expression of CD36, which in turn affects platelet function. To assess the clinical relevance of our findings we used the PhenoScanner tool, which aggregates large scale GWAS findings; the results reinforce the clinical relevance of our variants and the utility of the MPRA assay. The study demonstrates a generalizable paradigm for functional testing of genetic variants to inform mechanistic studies, support patient management and develop precision therapies.

Platelets are anucleate cells that are best known as regulators of vascular hemostasis and thrombosis but also play important roles in cancer, angiogenesis, and inflammation. CD36 is a platelet surface marker that can activate platelet in response to oxidized low density lipoprotein (oxLDL). CD36 has been associated with numerous cardiovascular traits in human including blood lipid levels, platelet count, and cardiovascular disease prevalence in human genetic studies. Human variability in platelet CD36 levels are associated with the platelet response to oxLDL. However, the genetic mechanisms responsible for the variability of CD36 levels are unknown. We examined 81 genetic variants associated with CD36 levels for functionality using a high-throughput assay. Of the ten variants that were identified in that assay, one doublet, rs2366739 and rs1194196, were confirmed using additional molecular and cellular assays. Deletion of the genomic region containing rs2366739 and rs1194196 resulted in overexpression of CD36 in a cell culture system. This finding indicates a control locus which can serve as a potential target in modulating CD36 expression and altering platelet function in cardiovascular disease.

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.

Bariatric surgery reduces CD36-bearing microvesicles of endothelial and monocyte origin

Background

Bariatric surgery is a widely adopted treatment for obesity and its secondary complications. In the past decade, microvesicles (MVs) and CD36 have increasingly been considered as possible biomarkers for obesity, the metabolic syndrome (MetSy), type 2 diabetes mellitus (T2DM). Thus, the purpose of this study was to investigate how weight loss resulting from bariatric surgery affects levels of specific MV phenotypes and their expression of CD36 scavenger receptor. Additionally, we hypothesised that subjects with MetSy had higher baseline concentrations of investigated MV phenotypes.

Methods

Twenty individuals undergoing Roux-en-Y gastric bypass surgery were evaluated before and 3 months after surgery. MVs were characterised by flow cytometry at both time points and defined as lactadherin-binding particles within a 100-1000 nm size gate. MVs of monocyte (CD14) and endothelial (CD62E) origin were defined by cell-specific markers, and their expression of CD36 was investigated.

Results

Following bariatric surgery, subjects incurred an average BMI reduction (delta) of − 8.4 ± 1.4 (p < 0.0001). Significant reductions were observed for the total MVs (− 66.55%, p = 0.0017) and MVs of monocyte (− 36.11%, p = 0.0056) and endothelial (− 40.10%, p = 0.0007) origins. Although the bulk of CD36-bearing MVs were unaltered, significant reductions were observed for CD36-bearing MVs of monocyte (− 60.04%, p = 0.0192) and endothelial (− 54.93%, p = 0.04) origin. No differences in levels of MVs were identified between subjects who presented with MetSy at baseline (n = 13) and those that did not (n = 7).

Conclusion

Bariatric surgery resulted in significantly altered levels of CD36-bearing MVs of monocyte and endothelial origin. This likely reflects improvements in ectopic fat distribution, plasma lipid profile, low-grade inflammation, and oxidative stress following weight loss. Conversely, however, the presence of MetSy at baseline had no impact on MV phenotypes.

Electronic supplementary material

The online version of this article (10.1186/s12986-018-0309-4) contains supplementary material, which is available to authorized users.

Regulation of Insulin Receptor Pathway and Glucose Metabolism by CD36 Signaling

During reduced energy intake, skeletal muscle maintains homeostasis by rapidly suppressing insulin-stimulated glucose utilization. Loss of this adaptation is observed with deficiency of the fatty acid transporter CD36. A similar loss is also characteristic of the insulin-resistant state where CD36 is dysfunctional. To elucidate what links CD36 to muscle glucose utilization, we examined whether CD36 signaling might influence insulin action. First, we show that CD36 deletion specific to skeletal muscle reduces expression of insulin signaling and glucose metabolism genes. It decreases muscle ceramides but impairs glucose disposal during a meal. Second, depletion of CD36 suppresses insulin signaling in primary-derived human myotubes, and the mechanism is shown to involve functional CD36 interaction with the insulin receptor (IR). CD36 promotes tyrosine phosphorylation of IR by the Fyn kinase and enhances IR recruitment of P85 and downstream signaling. Third, pretreatment for 15 min with saturated fatty acids suppresses CD36-Fyn enhancement of IR phosphorylation, whereas unsaturated fatty acids are neutral or stimulatory. These findings define mechanisms important for muscle glucose metabolism and optimal insulin responsiveness. Potential human relevance is suggested by genome-wide analysis and RNA sequencing data that associate genetically determined low muscle CD36 expression to incidence of type 2 diabetes.

Effect of chemical interaction between oleic acid and L-Arginine on oral perception, as a function of polymorphisms of CD36 and OBPIIa and genetic ability to taste 6-n-propylthiouracil

Oral sensitivity to fats varies in individuals influencing nutritional status and health. Variations in oleic acid perception are associated with CD36 and odorant binding protein (OBPIIa) polymorphisms, and 6-n-propylthiouracil (PROP) sensitivity, which is mediated by TAS2R38 receptor. L-Arginine (L-Arg) supplementation was shown to modify the perception of the five taste qualities. Here we analyzed the effect of three concentrations (5, 10, 15 mmol/L) of L-Arg on oral perception of oleic acid in forty-six subjects classified for PROP taster status and genotyped for TAS2R38, CD36 and OBPIIa polymorphisms. L-Arg supplementation was effective in increasing the perceived intensity of oleic acid in most subjects. The lowest concentration was the most effective, especially in PROP non-tasters or medium tasters, and in subjects with at least an allele A in CD36 and OBPIIa loci. Density Functional Theory (DFT) calculations were exploited to characterize the chemical interaction between L-Arg and oleic acid, showing that a stable 1:1 oleate·ArgH⁺ adduct can be formed, stabilized by a pair of hydrogen bonds. Results indicate that L-Arg, acting as a ‘carrier’ of fatty acids in saliva, can selectively modify taste response, and suggest that it may to be used in personalized dietetic strategies to optimize eating behaviors and health.

Polymorphism rs1761667 in the CD36 Gene Is Associated to Changes in Fatty Acid Metabolism and Circulating Endocannabinoid Levels Distinctively in Normal Weight and Obese Subjects

The multifunctional CD36 scavenger receptor facilitates fatty acid (FA) uptake and oxidation and it has been involved in the pathophysiology related to dysfunctional FA metabolism. The common variant in the CD36 gene, rs1761667 (A/G), whose allele A is characterized by a reduced protein expression, has been associated with taste sensitivity to and preference for fat. We therefore aimed at evaluating whether the CD36 polymorphism may influence fatty acid metabolism and endocannabinoid biosynthesis in normal weight (NW) and obese (OB) subjects. Red blood cell (RBC) fatty acid composition, and plasma endocannabinoid levels were determined. In NW subjects with AA genotype was found a marked reduction of RBC saturated fatty acids and palmitic/linoleic ratio (PA/LA), considered as de novo lipogenesis (DNL) biomarkers. Remarkably, to the reduction of DNL biomarkers corresponded an increase of omega-6 index, an indirect marker of the impact on fatty acid metabolism of dietary omega-6 fatty acids, endocannabinoid levels and a higher waist/hip ratio. The presence of the G allele was instead associated with increased endocannabinoid plasma levels and a trend for increased waist/hip ratio in obese subjects, even though exhibited decreased BMI with respect to those with AA genotype. These data indicate that the CD36 polymorphism, rs1761667, leads to a distinct metabolic pattern in NW and in OB subjects. Therefore, their determination may be crucial in developing personalized therapeutic strategies for ameliorating dyslipidemia and other metabolic disorders.

Platelet CD36 promotes thrombosis by activating redox sensor ERK5 in hyperlipidemic conditions

Atherothrombosis is a process mediated by dysregulated platelet activation that can cause life-threatening complications and is the leading cause of death by cardiovascular disease. Platelet reactivity in hyperlipidemic conditions is enhanced when platelet scavenger receptor CD36 recognizes oxidized lipids in oxidized low-density lipoprotein (oxLDL) particles, a process that induces an overt prothrombotic phenotype. The mechanisms by which CD36 promotes platelet activation and thrombosis remain incompletely defined. In this study, we identify a mechanism for CD36 to promote thrombosis by increasing activation of MAPK extracellular signal-regulated kinase 5 (ERK5), a protein kinase known to be exquisitely sensitive to redox stress, through a signaling pathway requiring Src kinases, NADPH oxidase, superoxide radical anion, and hydrogen peroxide. Pharmacologic inhibitors of ERK5 blunted platelet activation and aggregation in response to oxLDL and targeted genetic deletion of ERK5 in murine platelets prevented oxLDL-induced platelet deposition on immobilized collagen in response to arterial shear. Importantly, in vivo thrombosis experiments after bone marrow transplantation from platelet-specific ERK5 null mice into hyperlipidemic apolipoprotein E null mice showed decreased platelet accumulation and increased thrombosis times compared with mice transplanted with ERK5 expressing control bone marrows. These findings suggest that atherogenic conditions critically regulate platelet CD36 signaling by increasing superoxide radical anion and hydrogen peroxide through a mechanism that promotes activation of MAPK ERK5.

Microvesicles Correlated with Components of Metabolic Syndrome in Men with Type 2 Diabetes Mellitus and Lowered Testosterone Levels But Were Unaltered by Testosterone Therapy

Aims. To investigate how circulating microvesicle phenotypes correlate with insulin sensitivity, body composition, plasma lipids, and hepatic fat accumulation. We hypothesized that changes elicited by testosterone replacement therapy are reflected in levels of microvesicles. Methods. Thirty-nine type 2 diabetic males with lowered testosterone levels were assigned to either testosterone replacement therapy or placebo and evaluated at baseline and after 24 weeks. Microvesicles were analysed by flow cytometry and defined as lactadherin-binding particles within the 0.1-1.0 μm gate. Microvesicles of platelet, monocyte, and endothelial cell origin were identified by cell-specific markers and their expression of CD36 was investigated. Results. Triglycerides correlated positively with all investigated microvesicle phenotypes in this study (p < 0.05), and indicators of hepatic fat accumulation, alanine aminotransferase, and gamma glutamyltransferase correlated with platelet and endothelial microvesicles and CD36-expressing microvesicles from platelets and monocytes (p < 0.05). BMI, waist circumference, and fat percentage correlated with CD36-expressing monocyte microvesicles (p < 0.05), while insulin sensitivity did not correlate with any microvesicle phenotypes. Microvesicle levels were unaffected by testosterone therapy. Conclusions. Metabolic syndrome components and hepatic fat accumulation correlated with microvesicle phenotypes, supporting the involvement of especially CD36 on monocytes in metabolic syndrome pathogenesis. Although testosterone therapy improved body composition measures, microvesicle phenotype levels were unaffected. This trial was registered at ClinicalTrials.gov (NCT01560546).

Higher chylomicron remnants and LDL particle numbers associate with CD36 SNPs and DNA methylation sites that reduce CD36

Cluster of differentiation 36 (CD36) variants influence fasting lipids and risk of metabolic syndrome, but their impact on postprandial lipids, an independent risk factor for cardiovascular disease, is unclear. We determined the effects of SNPs within a ∼410 kb region encompassing CD36 and its proximal and distal promoters on chylomicron (CM) remnants and LDL particles at fasting and at 3.5 and 6 h following a high-fat meal (Genetics of Lipid Lowering Drugs and Diet Network study, n = 1,117). Five promoter variants associated with CMs, four with delayed TG clearance and five with LDL particle number. To assess mechanisms underlying the associations, we queried expression quantitative trait loci, DNA methylation, and ChIP-seq datasets for adipose and heart tissues that function in postprandial lipid clearance. Several SNPs that associated with higher serum lipids correlated with lower adipose and heart CD36 mRNA and aligned to active motifs for PPARγ, a major CD36 regulator. The SNPs also associated with DNA methylation sites that related to reduced CD36 mRNA and higher serum lipids, but mixed-model analyses indicated that the SNPs and methylation independently influence CD36 mRNA. The findings support contributions of CD36 SNPs that reduce adipose and heart CD36 RNA expression to inter-individual variability of postprandial lipid metabolism and document changes in CD36 DNA methylation that influence both CD36 expression and lipids.

CD36 Provides Host Protection Against Klebsiella pneumoniae Intrapulmonary Infection by Enhancing Lipopolysaccharide Responsiveness and Macrophage Phagocytosis

Klebsiella pneumoniae remains an important cause of intrapulmonary infection and invasive disease worldwide. K. pneumoniae can evade serum killing and phagocytosis primarily through the expression of a polysaccharide capsule, but its pathogenicity is also influenced by host factors. We examined whether CD36, a scavenger receptor that recognizes pathogen and modified self ligands, is a host determinant of K. pneumoniae pathogenicity. Despite differences in serum sensitivity and virulence of 3 distinct K. pneumoniae (hypermucoviscous K1, research K2, and carbapenemase-producing ST258) strains, the absence of CD36 significantly increased host susceptibility to acute intrapulmonary infection by K. pneumoniae, regardless of strain. We demonstrate that CD36 enhances LPS responsiveness to K. pneumoniae to increase downstream cytokine production and macrophage phagocytosis that is independent of polysaccharide capsular antigen. Our study provides new insights into host determinants of K. pneumoniae pathogenicity and raises the possibility that functional mutations in CD36 may predispose individuals to K. pneumoniae syndromes.