IFN signaling and neutrophil degranulation transcriptional signatures are induced during SARS-CoV-2 infection

SARS-CoV-2 virus has infected more than 92 million people worldwide resulting in the Coronavirus disease 2019 (COVID-19). Using a rhesus macaque model of SARS-CoV-2 infection, we have characterized the transcriptional signatures induced in the lungs of juvenile and old macaques following infection. Genes associated with Interferon (IFN) signaling, neutrophil degranulation and innate immune pathways are significantly induced in macaque infected lungs, while pathways associated with collagen formation are downregulated, as also seen in lungs of macaques with tuberculosis. In COVID-19, increasing age is a significant risk factor for poor prognosis and increased mortality. Type I IFN and Notch signaling pathways are significantly upregulated in lungs of juvenile infected macaques when compared with old infected macaques. These results are corroborated with increased peripheral neutrophil counts and neutrophil lymphocyte ratio in older individuals with COVID-19 disease. Together, our transcriptomic studies have delineated disease pathways that improve our understanding of the immunopathogenesis of COVID-19.

Thrombospondin-1 promotes hemostasis through modulation of cAMP signaling in blood platelets

Thrombospondin-1 (TSP-1) is released by platelets upon activation and can increase platelet activation, but its role in hemostasis in vivo is unclear. We show that TSP-1 is a critical mediator of hemostasis that promotes platelet activation by modulating inhibitory cyclic adenosine monophosphate (cAMP) signaling. Genetic deletion of TSP-1 did not affect platelet activation in vitro, but in vivo models of hemostasis and thrombosis showed that TSP-1-deficient mice had prolonged bleeding, defective thrombosis, and increased sensitivity to the prostacyclin mimetic iloprost. Adoptive transfer of wild-type (WT) but not TSP-1-/- platelets ameliorated the thrombotic phenotype, suggesting a key role for platelet-derived TSP-1. In functional assays, TSP-1-deficient platelets showed an increased sensitivity to cAMP signaling, inhibition of platelet aggregation, and arrest under flow by prostacyclin (PGI2). Plasma swap experiments showed that plasma TSP-1 did not correct PGI2 hypersensitivity in TSP-1-/- platelets. By contrast, incubation of TSP-1-/- platelets with releasates from WT platelets or purified TSP-1, but not releasates from TSP-1-/- platelets, reduced the inhibitory effects of PGI2. Activation of WT platelets resulted in diminished cAMP accumulation and downstream signaling, which was associated with increased activity of the cAMP hydrolyzing enzyme phosphodiesterase 3A (PDE3A). PDE3A activity and cAMP accumulation were unaffected in platelets from TSP-1-/- mice. Platelets deficient in CD36, a TSP-1 receptor, showed increased sensitivity to PGI2/cAMP signaling and diminished PDE3A activity, which was unaffected by platelet-derived or purified TSP-1. This scenario suggests that the release of TSP-1 regulates hemostasis in vivo through modulation of platelet cAMP signaling at sites of vascular injury.

Palmitic Acid-Rich High-Fat Diet Exacerbates Experimental Pulmonary Fibrosis by Modulating Endoplasmic Reticulum Stress

The impact of lipotoxicity on the development of lung fibrosis is unclear. Saturated fatty acids, such as palmitic acid (PA), activate endoplasmic reticulum (ER) stress, a cellular stress response associated with the development of idiopathic pulmonary fibrosis (IPF). We tested the hypothesis that PA increases susceptibility to lung epithelial cell death and experimental fibrosis by modulating ER stress. Total liquid chromatography and mass spectrometry were used to measure fatty acid content in IPF lungs. Wild-type mice were fed a high-fat diet (HFD) rich in PA or a standard diet and subjected to bleomycin-induced lung injury. Lung fibrosis was determined by hydroxyproline content. Mouse lung epithelial cells were treated with PA. ER stress and cell death were assessed by Western blotting, TUNEL staining, and cell viability assays. IPF lungs had a higher level of PA compared with controls. Bleomycin-exposed mice fed an HFD had significantly increased pulmonary fibrosis associated with increased cell death and ER stress compared with those fed a standard diet. PA increased apoptosis and activation of the unfolded protein response in lung epithelial cells. This was attenuated by genetic deletion and chemical inhibition of CD36, a fatty acid transporter. In conclusion, consumption of an HFD rich in saturated fat increases susceptibility to lung fibrosis and ER stress, and PA mediates lung epithelial cell death and ER stress via CD36. These findings demonstrate that lipotoxicity may have a significant impact on the development of lung injury and fibrosis by enhancing pro-death ER stress pathways.

A Novel Murine Model Expressing a Chimeric mSCARB2/hSCARB2 Receptor Is Highly Susceptible to Oral Infection with Clinical Isolates of Enterovirus 71

Enterovirus 71 (EV71) infection is generally associated with hand-foot-and-mouth disease (HFMD) and may cause severe neurological disorders and even death. An effective murine oral infection model for studying the pathogenesis of various clinical EV71 isolates is lacking. We developed a transgenic (Tg) mouse that expresses an EV71 receptor, that is, human scavenger receptor class B member 2 (hSCARB2), in a pattern highly similar to that of endogenous murine SCARB2 (mSCARB2) protein. A FLAG-tagged SCARB2 cDNA fragment composed of exons 3 to 12 was inserted into a murine Scarb2 gene-containing bacterial artificial chromosome (BAC) clone, and the resulting transgene was used for establishment of chimeric receptor-expressing Tg mice. Tg mice intragastrically (i.g.) infected with clinical isolates of EV71 showed neurological symptoms, such as ataxia and paralysis, and fatality. There was an age-dependent decrease in susceptibility to viral infection. Pathological characteristics of the infected Tg mice resembled those of encephalomyelitis in human patients. Viral infection was accompanied by microglial activation. Clodronate treatment of the brain slices from Tg mice enhanced viral replication, while lipopolysaccharide treatment significantly inhibited it, suggesting an antiviral role for microglia during EV71 infection. Taken together, this Tg mouse provides a model that closely mimics natural infection for studying EV71 pathogenesis and for evaluating the efficacy of vaccines or other antiviral drugs.IMPORTANCE The availability of a murine model of EV71 infection is beneficial for the understanding of pathogenic mechanisms and the development and assessment of vaccines and antiviral drugs. However, the lack of a murine oral infection model thwarted the study of pathogenesis induced by clinically relevant EV71 strains that are transmitted via the oral-oral or oral-fecal route. Our Tg mice could be intragastrically infected with clinically relevant EV71 strains in an efficient way and developed neurological symptoms and pathological changes strikingly resembling those of human infection. Moreover, these mice showed an age-dependent change in susceptibility that is similar to the human case. This Tg mouse, when combined with the use of other genetically modified mice, potentially contributes to studying the relationship between developmental changes in immunity and susceptibility to virus.

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.

CD36 initiates the secretory phenotype during the establishment of cellular senescence

Cellular senescence is a unique cell fate characterized by stable proliferative arrest and the extensive production and secretion of various inflammatory proteins, a phenomenon known as the senescence-associated secretory phenotype (SASP). The molecular mechanisms responsible for generating a SASP in response to senescent stimuli remain largely obscure. Here, using unbiased gene expression profiling, we discover that the scavenger receptor CD36 is rapidly upregulated in multiple cell types in response to replicative, oncogenic, and chemical senescent stimuli. Moreover, ectopic CD36 expression in dividing mammalian cells is sufficient to initiate the production of a large subset of the known SASP components via activation of canonical Src-p38-NF-κB signaling, resulting in the onset of a full senescent state. The secretome is further shown to be ligand-dependent, as amyloid-beta (Aβ) is sufficient to drive CD36-dependent NF-κB and SASP activation. Finally, loss-of-function experiments revealed a strict requirement for CD36 in secretory molecule production during conventional senescence reprogramming. Taken together, these results uncover the Aβ-CD36-NF-κB signaling axis as an important regulator of the senescent cell fate via induction of the SASP.

CD36 Modulates Fasting and Preabsorptive Hormone and Bile Acid Levels

CONTEXT

Abnormal fatty acid (FA) metabolism contributes to diabetes and cardiovascular disease. The FA receptor CD36 has been linked to risk of metabolic syndrome. In rodents CD36 regulates various aspects of fat metabolism, but whether it has similar actions in humans is unknown. We examined the impact of a coding single-nucleotide polymorphism in CD36 on postprandial hormone and bile acid (BA) responses.

OBJECTIVE

To examine whether the minor allele (G) of coding CD36 variant rs3211938 (G/T), which reduces CD36 level by ∼50%, influences hormonal responses to a high-fat meal (HFM).

DESIGN

Obese African American (AA) women carriers of the G allele of rs3211938 (G/T) and weight-matched noncarriers (T/T) were studied before and after a HFM.

SETTING

Two-center study.

PARTICIPANTS

Obese AA women.

INTERVENTION

HFM.

MAIN OUTCOME MEASURES

Early preabsorptive responses (10 minutes) and extended excursions in plasma hormones [C-peptide, insulin, incretins, ghrelin fibroblast growth factor (FGF)19, FGF21], BAs, and serum lipoproteins (chylomicrons, very-low-density lipoprotein) were determined.

RESULTS

At fasting, G-allele carriers had significantly reduced cholesterol and glycodeoxycholic acid and consistent but nonsignificant reductions of serum lipoproteins. Levels of GLP-1 and pancreatic polypeptide (PP) were reduced 60% to 70% and those of total BAs were 1.8-fold higher. After the meal, G-allele carriers displayed attenuated early (-10 to 10 minute) responses in insulin, C-peptide, GLP-1, gastric inhibitory peptide, and PP. BAs exhibited divergent trends in G allele carriers vs noncarriers concomitant with differential FGF19 responses.

CONCLUSIONS

CD36 plays an important role in the preabsorptive hormone and BA responses that coordinate brain and gut regulation of energy metabolism.

Intestinal CD36 and Other Key Proteins of Lipid Utilization: Role in Absorption and Gut Homeostasis

Several proteins have been implicated in fatty acid (FA) transport by enterocytes including the scavenger receptor CD36 (SR-B2), the scavenger receptor B1 (SR-B1) a member of the CD36 family and the FA transport protein 4 (FATP4). Here, we review the regulation of enterocyte FA uptake and its function in lipid absorption including prechylomicron formation, assembly and transport. Emphasis is given to CD36, which is abundantly expressed along the digestive tract of rodents and humans and has been the most studied. We also address the pleiotropic functions of CD36 that go beyond lipid absorption and metabolism to include recent evidence of its impact on intestinal homeostasis and barrier maintenance. Areas of progress involving contribution of membrane phospholipid remodeling and of cytosolic FA-binding proteins, FABP1 and FABP2 to fat absorption will be covered. © 2018 American Physiological Society. Compr Physiol 8:493-507, 2018.

[Effect of microRNA-155 on inflammatory response and lipid uptake of macrophages and its mechanism]

Objective To investigate the effect of microRNA-155 on inflammatory response and lipid uptake of macrophages after the cells are stimulated by ox-LDL and its potential mechanism. Methods Macrophage RAW264.7 cells were treated with 0, 25, 50 and 100 μg/mL ox-LDL for 24 hours or with 50 μg/mL ox-LDL for 0, 6, 12, 24 hours. The level of miR-155 was evaluated in all above samples through real-time quantitative PCR. In our research, RAW264.7 cells were divided into six groups: control group, ox-LDL group, ox-LDL/negative control group, ox-LDL/anti-miR-155 group, ox-LDL/shRNA negative control group and ox-LDL/PPARγ-shRNA group. Oil red O staining was used to observe lipid uptake in the cells. Filipin staining was used to evaluate the cellular uptake of ox-LDL. Cholesterol testing was performed to examine the levels of total cholesterol (TC) and free cholesterol (FC). Real-time quantitative PCR was done to detect the expressions of tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and IL-6 mRNAs. According to study purpose, we explored the potential mechanisms of miR-155 inhibitor (including control group, ox-LDL group, ox-LDL/negative control group and ox-LDL/miR-155 inhibitor group), miR-155 mimic (including negative control group and miR-155 mimic group), and PPARγ shRNA (including control group, ox-LDL group, ox-LDL/shRNA negative control group and ox-LDL/PPARγ shRNA group) in ox-LDL-treated RAW264.7 cells through evaluating the expressions of p-STAT3, PPARγ, CD36 and NF-κBp65 using Western blotting. Results Ox-LDL stimulation increased the relative expression of miR-155 in a dose- and time-dependent manner. Through oil red O staining, Filipin staining, cholesterol testing and real-time PCR experiment, we found the relative absorbance, levels of TC and FC, filipin fluorescence intensity, and levels of TNF-α, IL-1β and IL-6 mRNAs were significantly lower in ox-LDL/anti-miR-155 group than in ox-LDL and ox-LDL/negative control group. Similarly, the relative absorbance, levels of TC and FC, filipin fluorescence intensity and levels of TNF-α, IL-1β and IL-6 mRNAs were significantly lower in ox-LDL/ PPARγ shRNA group than in ox-LDL group and ox-LDL/shRNA negative control group. The expressions of p-STAT3, PPARγ, CD36 and NF-κBp65 proteins were suppressed in ox-LDL/anti-miR-155 group as compared with ox-LDL group and ox-LDL/negative control group. Similarly, p-STAT3, PPARγ, CD36 and NF-κBp65 protein levels decreased in ox-LDL/PPARγ shRNA as compared with ox-LDL/vector group. Moreover, p-STAT3, PPARγ, CD36 and NF-κBp65 protein levels were higher in miR-155 mimic group than in negative control group. Conclusion Mediated by PPARγ, miR-155 induced inflammation response and lipid uptake of macrophages via STAT3/NF-κB signal pathway and CD36.

The role of CD36 receptor in the pathogenesis of atherosclerosis

Atherosclerosis is a progressive, chronic inflammation in artery walls. Oxidized low density lipoproteins (ox-LDL) play an important role in atherosclerotic plaque formation. ox-LDL are taken up by macrophages mainly through scavenger receptors, among which CD36 is considered to be the most important. Animal studies have shown that crossing atherogenic mice with a strain lacking the expression of CD36 prevented the development of atherosclerosis despite a diet rich in saturated LCFA. In humans, autopsy studies performed in obese patients have demonstrated increased expression of CD36 receptor on macrophages, comprised within atherosclerotic plaques. Until recently it had been believed that CD36 is a major player in atherosclerosis progression in humans. However, recent studies challenge this conviction, showing increased incidence of coronary heart disease in the subgroup of patients with decreased expression of CD36. This article reviews the role of CD36 receptor in the development of atherosclerosis. The authors also discuss current possibilities to interfere with CD36, their potential benefits and hazards.

Myocardial energy provision is preserved by increased utilization of glucose and ketone bodies in CD36 knockout mice

AIMS

CD36 is an important transporter of long-chain fatty acids (LCFAs) in the myocardium. As we have reported previously, CD36-deficient patients demonstrate a marked reduction in myocardial uptake of (123)I-15-(p-iodophenyl)-(R, S)-methyl pentadecanoic acid (BMIPP), which is an analog of LCFAs, while myocardial (18)F-fluorodeoxy-glucose (FDG) uptake is increased. However, it has not been clarified whether energy provision is preserved in patients with CD36 deficiency. The aims of the current study were to investigate the myocardial uptake of glucose and alterations in myocardial metabolites in wild-type (WT) and CD36 knockout (KO) mice.

METHODS AND RESULTS

High-resolution positron emission tomography (PET) demonstrated markedly enhanced glucose uptake in KO mouse hearts compared with those of WT mice in real-time. The myocardial protein expression of glucose transporter protein 1 (GLUT1) was significantly enhanced in KO mice compared to WT mice, whereas that of GLUT4 was not altered. While the myocardial expression of genes involved in fatty acid metabolism did not increase in KO mice, that of genes related to glucose utilization compensatorily increased in KO mice. The metabolomic analysis of cardiac tissues revealed that the myocardial concentrations of ATP and phosphocreatine were maintained, even in KO mice. The concentration of 3-hydroxybutyric acid and mRNA expression of hydroxybutyrate dehydrogenase in the heart were significantly higher in KO than in WT mice.

CONCLUSION

These data suggest that high-energy phosphate might be preserved by the increased utilization of glucose and ketone bodies in CD36KO mouse hearts under conditions of deficient LCFA uptake.

Intermedin inhibits uptake of oxidized LDL via CD36 pathway in RAW264.7 cells

Intermedin (IMD) exerts a potent function in preventing atherosclerosis, while the mechanism remains unclear. Here we investigated the potential molecular mechanism responsible for the protective function of IMD in preventing foam cell formation in RAW264.7 cells. In our present study, IMD significantly inhibited intracellular cholesterol accumulation. Additionally, IMD dose-dependently down-regulated CD36 expression, which was confirmed by real-time quantitative reverse transcription-PCR and Western blot analysis. Our data suggest that IMD could inhibit the formation of foam cells through, at least partly, a CD36-dependent mechanism. This study suggests that IMD may be a therapeutic candidate for treating atherosclerosis.

The thrombospondin-1 receptor CD36 is an important mediator of ovarian angiogenesis and folliculogenesis

BACKGROUND

Ovarian angiogenesis is a complex process that is regulated by a balance between pro- and anti-angiogenic factors. Physiological processes within the ovary, such as folliculogenesis, ovulation, and luteal formation are dependent upon adequate vascularization and anything that disrupts normal angiogenic processes may result in ovarian dysfunction, and possibly infertility. The objective of this study was to evaluate the role of the thrombospondin-1 (TSP-1) receptor CD36 in mediating ovarian angiogenesis and regulating ovarian function.

METHODS

The role of CD36 was evaluated in granulosa cells in vitro and ovarian morphology and protein expression were determined in wild type and CD36 null mice.

RESULTS

In vitro, CD36 inhibition increased granulosa cell proliferation and decreased apoptosis. Granulosa cells in which CD36 was knocked down also exhibited an increase in expression of survival and angiogenic proteins. Ovaries from CD36 null mice were hypervascularized, with increased expression of pro-angiogenic vascular endothelial growth factor (VEGF) and its receptor VEGFR-2. Ovaries from CD36 null mice contained an increase in the numbers of pre-ovulatory follicles and decreased numbers of corpora lutea. CD36 null mice also had fewer number of offspring compared to wild type controls.

CONCLUSIONS

The results from this study demonstrate that CD36 is integral to the regulation of ovarian angiogenesis by TSP-1 and the expression of these family members may be useful in the control of ovarian vascular disorders.

What are lipoproteins doing in the brain?

Lipoproteins in plasma transport lipids between tissues, however, only high-density lipoproteins (HDL) appear to traverse the blood-brain barrier (BBB); thus, lipoproteins found in the brain must be produced within the central nervous system. Apolipoproteins E (ApoE) and ApoJ are the most abundant apolipoproteins in the brain, are mostly synthesized by astrocytes, and are found on HDL. In the hippocampus and other brain regions, lipoproteins help to regulate neurobehavioral functions by processes that are lipoprotein receptor-mediated. Moreover, lipoproteins and their receptors also have roles in the regulation of body weight and energy balance, acting through lipoprotein lipase (LPL) and the low-density lipoprotein (LDL) receptor-related protein (LRP). Thus, understanding lipoproteins and their metabolism in the brain provides a new opportunity with potential therapeutic relevance.

Thrombospondin-1/CD36 pathway contributes to bone marrow-derived angiogenic cell dysfunction in type 1 diabetes via Sonic hedgehog pathway suppression

Refractory wounds in diabetic patients present a significant clinical problem. Sonic hedgehog (SHH), a morphogenic protein central to wound repair, is deficient in diabetes. Regulation of SHH in wound healing is poorly understood. We hypothesize that thrombospondin-1 (TSP-1), through its receptor CD36, contributes to the SHH signaling defect in bone marrow-derived angiogenic cells (BMACs) in type 1 diabetic mice. Isolated BMACs from TSP-1-knockout mice demonstrated improved tube formation, migration, and adhesion in parallel with active SHH signaling. BMACs from STZ-induced type 1 diabetic mice showed significantly impaired Matrigel tube formation (n = 5; P < 0.05 vs. control), which was rescued by TSP-1 depletion (n = 5; P < 0.05 STZ-TSP-1(-/-) vs. STZ-WT) or exogenous SHH (20 mg/l, 24 h, n = 4; P < 0.05 vs. STZ-control). The expression of CD36 was elevated in BMACs from STZ mice (n = 4; P < 0.05). SHH signaling was significantly higher in BMACs from TSP-1(-/-) mice and TSP-1 receptor CD36-knockout mice (n = 6; P < 0.05 vs. WT) but not CD47-knockout mice (n = 3; P > 0.05 vs. WT). The impairment of recombinant human TSP-1 (2.2 nM, 24 h) on BMAC Matrigel tube formation was delayed significantly by CD36 deletion (n = 5; P < 0.05). CD36(-/-) BMACs demonstrated better tube formation under both normal and diabetic conditions with active SHH signaling (n = 4; P < 0.05 vs. WT BMACs). In conclusion, The TSP-1/CD36 pathway contributes to the SHH signaling defect, resulting in BMAC dysfunction in type 1 diabetic mice.

CD36 is important for adipocyte recruitment and affects lipolysis

OBJECTIVE

The scavenger receptor CD36 facilitates the cellular uptake of long-chain fatty acids. As CD36-deficiency attenuates the development of high fat diet (HFD)-induced obesity, the role of CD36-deficiency in preadipocyte recruitment and adipocyte function was set out to characterize.

DESIGN AND METHODS

Fat cell size and number were determined in gonadal, visceral, and subcutaneous adipose tissue of CD36(-/-) and WT mice after 6 weeks on HFD. Basal lipolysis and insulin-inhibited lipolysis were investigated in gonadal adipose tissue.

RESULTS

CD36(-/-) mice showed a reduction in adipocyte size in all fat pads. Gonadal adipose tissue also showed a lower total number of adipocytes because of a lower number of very small adipocytes (diameter <50 μm). This was accompanied by an increased pool of preadipocytes, which suggests that CD36-deficiency reduces the capacity of preadipocytes to become adipocytes. Regarding lipolysis, in adipose tissue from CD36(-/-) mice, cAMP levels were increased and both basal and 8-bromo-cAMP stimulated lipolysis were higher. However, insulin-mediated inhibition of lipolysis was more potent in CD36(-/-) mice.

CONCLUSIONS

These results indicate that during fat depot expansion, CD36-deficiency negatively affects preadipocyte recruitment and that in mature adipocytes, CD36-deficiency is associated with increased basal lipolysis and insulin responsiveness.

Role of scavenger receptors in the pathophysiology of chronic liver diseases

Scavenger receptors comprise a large family of structurally diverse proteins that are involved in many homeostatic functions. They recognize a wide range of ligands, from pathogen-associated molecular patterns (PAMPs) to endogenous, as well as modified host-derived molecules (DAMPs). The liver deals with blood micro-organisms and DAMPs released from injured organs, thus performing vital metabolic and clearance functions that require the uptake of nutrients and toxins. Many liver cell types, including hepatocytes and Kupffer cells, express scavenger receptors that play key roles in hepatitis C virus entry, lipid uptake, and macrophage activation, among others. Chronic liver disease causes high morbidity and mortality worldwide. Hepatitis virus infection, alcohol abuse, and non-alcoholic fatty liver are the main etiologies associated with this disease. In this context, continuous inflammation as a result of liver damage leads to hepatic fibrosis, which frequently brings about cirrhosis and ultimately hepatocellular carcinoma. In this review, we will summarize the role of scavenger receptors in the pathophysiology of chronic liver diseases. We will also emphasize their potential as biomarkers of advanced liver disease, including cirrhosis and cancer.

Amyloid-β-induced astrocytic phagocytosis is mediated by CD36, CD47 and RAGE

Astrocytes, the most numerous glial cell in the brain, have multiple functions and are key to maintenance of homeostasis in the central nervous system. Microglia are the resident immunocompetent cells in the brain and share several functions with macrophages, including their phagocytic ability. Indeed microglia are the resident phagocytes in the brain and express numerous cell surface proteins which act to enable receptor-mediated phagocytosis. However recent evidence suggests that astrocytes express some genes which permit phagocytosis of phosphatidylserine-decorated cells and this probably explains sporadic reports in the literature which suggest that astrocytes become phagocytic following brain trauma. Here we examined the potential of astrocytes to phagocytose fluorescently-labelled latex beads and amyloid-β (Aβ) and report that they competently engulf both in a manner that relies on actin polymerization since it was inhibited by cytochalasin D. The data indicate that incubation of cultured astrocytes or microglia with Aβ increased phagocytosis and markers of activation of both cell types. Aβ was found to markedly increase expression of the putative Aβ-binding receptors CD36 and CD47 in astrocytes, while it decreased expression of the receptor for advanced glycation endproducts (RAGE). It is demonstrated that blocking these receptors using a neutralizing antibody attenuated Aβ-induced phagocytosis of latex beads by astrocytes. Interestingly blocking these receptors also decreased uptake of beads even in the absence of Aβ. Here we demonstrate that astrocytes are competent phagocytes and are capable of engulfing Aβ.

[Impact of anti-OX-LDL antibodies on CD36 mRNA expression in monocytes]

AIM

To investigate the impact of antibodies to oxidized low-density lipoprotein (OX-LDL) on CD36 mRNA expression in monocytes and explore the mechanism underlying the impact on the formation of foam cells.

METHODS

U937 cells and the monocytes of New Zealand rabbit were respectively cultured in vitro and divided into 4 groups: the control group (cultured in nutrient medium of RPMI1640), the OX-LDL group (with additional OX-LDL of 50 μg/L in nutrient medium), the OX-LDL+Ab-OX-LDL group (with additional OX-LDL of 50 μg/L and Ab-OX-LDL of 100 μg/L in nutrient medium) and the Ab-OX-LDL group (with additional Ab-OX-LDL of 100 μg/L in nutrient medium). After 24-hour culture, the expression of CD36 mRNA was detected by semi-quantitative RT-PCR.

RESULTS

The expression of CD36 mRNA, either in the OX-LDL group or in the OX-LDL+Ab-OX-LDL group, was higher than that in the control group. After intervened by Ab-OX-LDL, the expression was respectively down-regulated by 64.80% in U937 cells and 35.18% in the monocytes of rabbit, which was statistically significant between the two species.

CONCLUSION

Antibodies to OX-LDL could negatively regulate the expression of CD36 mRNA in monocytes, and prevent monocyte in taking OX-LDL through the pathway of antigen CD36.

Advanced glycation end products induce a prothrombotic phenotype in mice via interaction with platelet CD36

Diabetes mellitus has been associated with platelet hyperreactivity, which plays a central role in the hyperglycemia-related prothrombotic phenotype. The mechanisms responsible for this phenomenon are not established. In the present study, we investigated the role of CD36, a class-B scavenger receptor, in this process. Using both in vitro and in vivo mouse models, we demonstrated direct and specific interactions of platelet CD36 with advanced glycation end products (AGEs) generated under hyperglycemic conditions. AGEs bound to platelet CD36 in a specific and dose-dependent manner, and binding was inhibited by the high-affinity CD36 ligand NO(2)LDL. Cd36-null platelets did not bind AGE. Using diet- and drug-induced mouse models of diabetes, we have shown that cd36-null mice had a delayed time to the formation of occlusive thrombi compared with wild-type (WT) in a FeCl(3)-induced carotid artery injury model. Cd36-null mice had a similar level of hyperglycemia and a similar level of plasma AGEs compared with WT mice under this condition, but WT mice had more AGEs incorporated into thrombi. Mechanistic studies revealed that CD36-dependent JNK2 activation is involved in this prothrombotic pathway. Therefore, the results of the present study couple vascular complications in diabetes mellitus with AGE-CD36-mediated platelet signaling and hyperreactivity.

[Oxidized low density lipoprotein induces macrophage endoplasmic reticulum stress via CD36.]

The purpose of the present study is to explore the effect of oxidized low density lipoprotein (ox-LDL) on the induction of endoplasmic reticulum stress (ERS) and the underlying mechanisms in ox-LDL-induced macrophage foam-forming process. RAW264.7 macrophages were cultured in DMEM medium containing 10% fetal bovine serum, and then treated with ox-LDL (25, 50 and 100 mg/L), anti-CD36 monoclonal antibody+ox-LDL and tunicamycin (TM), respectively. After incubation for 24 h, the cells were collected. The cellular lipid accumulation was showed by oil red O staining and the content of cellular total cholesterol was quantified by enzymatic colorimetry. The expression of glucose-regulated protein 94 (GRP94), a molecular marker of ERS, was determined by immunocytochemistry assay. The levels of GRP94 protein, phosphorylated inositol-requiring enzyme 1 (p-IRE1) and X box binding protein 1 (XBP1) in RAW264.7 cells were detected by Western blotting. The results indicated that after incubation with ox-LDL (25, 50 and 100 mg/L) for 24 h, a large amount of lipid droplets were found in the cytoplasm, and the contents of cellular total cholesterol were increased by 2.1, 2.8 and 3.1 folds compared with the control, respectively. Anti-CD36 antibody decreased markedly the cellular lipid accumulation induced by ox-LDL at 100 mg/L. Both ox-LDL and TM, a specific ERS inducer, could up-regulate the protein expression of GRP94 in a dose-dependent manner. Furthermore, p-IRE1 and XBP1, two key components of the unfolded protein response, were also significantly induced by the treatment with ox-LDL. The up-regulations of the three proteins induced by ox-LDL were inhibited significantly when the macrophages were pre-incubated with anti-CD36 antibody. These results suggest that ox-LDL may induce ERS in a dose-dependent way and subsequently activate the unfolded protein response signaling pathway in RAW264.7 macrophages, which is potentially mediated by scavenger receptor CD36.

Alterations in skeletal muscle fatty acid handling predisposes middle-aged mice to diet-induced insulin resistance

OBJECTIVE

Although advanced age is a risk factor for type 2 diabetes, a clear understanding of the changes that occur during middle age that contribute to the development of skeletal muscle insulin resistance is currently lacking. Therefore, we sought to investigate how middle age impacts skeletal muscle fatty acid handling and to determine how this contributes to the development of diet-induced insulin resistance.

RESEARCH DESIGN AND METHODS

Whole-body and skeletal muscle insulin resistance were studied in young and middle-aged wild-type and CD36 knockout (KO) mice fed either a standard or a high-fat diet for 12 weeks. Molecular signaling pathways, intramuscular triglycerides accumulation, and targeted metabolomics of in vivo mitochondrial substrate flux were also analyzed in the skeletal muscle of mice of all ages.

RESULTS

Middle-aged mice fed a standard diet demonstrated an increase in intramuscular triglycerides without a concomitant increase in insulin resistance. However, middle-aged mice fed a high-fat diet were more susceptible to the development of insulin resistance-a condition that could be prevented by limiting skeletal muscle fatty acid transport and excessive lipid accumulation in middle-aged CD36 KO mice.

CONCLUSION

Our data provide insight into the mechanisms by which aging becomes a risk factor for the development of insulin resistance. Our data also demonstrate that limiting skeletal muscle fatty acid transport is an effective approach for delaying the development of age-associated insulin resistance and metabolic disease during exposure to a high-fat diet.

Fatty acid transport across the cell membrane: regulation by fatty acid transporters

Transport of long-chain fatty acids across the cell membrane has long been thought to occur by passive diffusion. However, in recent years there has been a fundamental shift in understanding, and it is now generally recognized that fatty acids cross the cell membrane via a protein-mediated mechanism. Membrane-associated fatty acid-binding proteins ('fatty acid transporters') not only facilitate but also regulate cellular fatty acid uptake, for instance through their inducible rapid (and reversible) translocation from intracellular storage pools to the cell membrane. A number of fatty acid transporters have been identified, including CD36, plasma membrane-associated fatty acid-binding protein (FABP(pm)), and a family of fatty acid transport proteins (FATP1-6). Fatty acid transporters are also implicated in metabolic disease, such as insulin resistance and type-2 diabetes. In this report we briefly review current understanding of the mechanism of transmembrane fatty acid transport, and the function of fatty acid transporters in healthy cardiac and skeletal muscle, and in insulin resistance/type-2 diabetes. Fatty acid transporters hold promise as a future target to rectify lipid fluxes in the body and regain metabolic homeostasis.

Inhibition of CD36-dependent phagocytosis by prostaglandin E2 contributes to the development of endometriosis

Dysfunction in macrophage-mediated phagocytosis of aberrant cells that undergo retrograde transport to the peritoneal cavity is considered an important factor in the development of endometriosis. However, the mechanisms responsible for the loss of function of macrophages remain largely unknown. Herein, we report that prostaglandin (PG) E(2), via the EP2 receptor-dependent signaling pathway, inhibits the expression of CD36 in peritoneal macrophages, resulting in reduced phagocytic ability. PGE(2)-mediated inhibition of macrophage phagocytic capability was restored by ectopic expression of CD36. Treatment with PGE(2) inhibited CD36-dependent phagocytosis of peritoneal macrophages and increased the number and size of endometriotic lesions in mice. In contrast, blockade of PGE(2) production by cyclooxygenase inhibitors enhanced the phagocytic ability of peritoneal macrophages and reduced endometriotic lesion formation. Taken together, our findings reveal a potential mechanism of immune dysfunction during endometriosis development and may contribute to the design of an effective prevention/treatment regimen.

Mechanisms of cell signaling by the scavenger receptor CD36: implications in atherosclerosis and thrombosis

CD36 is a multifunctional membrane receptor present on mononuclear phagocytes, platelets, and other cells that serves as a scavenger receptor for oxidized phospholipids, apoptotic cells and certain microbial pathogens. On macrophages, CD36 interaction with oxidized LDL (oxLDL) triggers a signaling response that is pro-inflammatory and pro-atherogenic. The signaling pathway involves activation of src-family kinases, MAP kinases, and Vav family guanine nucleotide exchange factors and results in ligand internalization, foam cell formation and inhibition of migration. On platelets, CD36 interaction with oxLDL and cell-derived microparticles transduces intracellular signals that render them more reactive to low concentrations of classical agonists. In vitro studies and in vivo experiments in CD36 null mice have revealed an important mechanistic role for CD36 in atherosclerosis and thrombosis. Identification of the precise CD36 signaling pathways in specific cells elicited in response to specific ligands may yield novel targets for drug development in athero-thrombotic disorders.

Regulation of nitric oxide signalling by thrombospondin 1: implications for anti-angiogenic therapies

In addition to long-term regulation of angiogenesis, angiogenic growth factor signalling through nitric oxide (NO) acutely controls blood flow and haemostasis. Inhibition of this pathway may account for the hypertensive and pro-thrombotic side effects of the vascular endothelial growth factor antagonists that are currently used for cancer treatment. The first identified endogenous angiogenesis inhibitor, thrombospondin 1, also controls tissue perfusion, haemostasis and radiosensitivity by antagonizing NO signalling. We examine the role of these and other emerging activities of thrombospondin 1 in cancer. Clarifying how endogenous and therapeutic angiogenesis inhibitors regulate vascular NO signalling could facilitate development of more selective inhibitors.

Glucose-oxidized low-density lipoproteins enhance insulin-like growth factor I-stimulated smooth muscle cell proliferation by inhibiting integrin-associated protein cleavage

Prior published reports have demonstrated that glucose-oxidized low-density lipoproteins (g-OxLDL) enhance the proliferative response of vascular smooth muscle cells (SMC) to IGF-I. Our previous studies have determined that the regulation of cleavage of integrin-associated protein (IAP) by matrix-metalloprotease-2 (MMP-2) in diabetic mice in response to hyperglycemia is a key regulator of the response of SMC to IGF-I. Because chronic hyperglycemia enhances glucose-induced LDL oxidation, these studies were conducted to determine whether g-OxLDL modulates the response of SMC to IGF-I by regulating MMP-2-mediated cleavage of IAP. We determined that exposure of SMC to g-OxLDL, but not native LDL, was sufficient to facilitate an increase in cell proliferation in response to IGF-I. Exposure to an anti-CD36 antibody, which has been shown to inhibit g-OxLDL-mediated signaling, inhibited the effects of g-OxLDL on IGF-I-stimulated SMC proliferation. The effect of g-OxLDL could be attributed, in part, to an associated decrease in proteolytic cleavage of IAP leading to increase in the basal association between IAP and Src homology 2 domain-containing protein tyrosine phosphatase substrate-1, which is required for IGF-I-stimulated proliferation. The inhibitory effect of g-OxLDL on IAP cleavage appeared to be due to its ability to decrease the amount of activated MMP-2, the protease responsible for IAP cleavage. In conclusion, these data provide a molecular mechanism to explain previous studies that have reported an enhancing effect of g-OxLDL on IGF-I-stimulated SMC proliferation.

Cellular fatty acid uptake: a pathway under construction

Membrane uptake of long-chain fatty acids (FAs) is the first step in cellular FA utilization and a point of metabolic regulation. CD36 facilitates a major fraction of FA uptake by key tissues. This review highlights the contribution of CD36 to pathophysiology in rodents and humans. Novel concepts regarding regulation of CD36-facilitated uptake are discussed (i.e. the role of membrane rafts and caveolae, CD36 recycling between intracellular depots and the membrane, and chemical modifications of the protein that impact its turnover and recruitment). Importantly, CD36 membrane levels and turnover are abnormal in diabetes, resulting in dysfunctional FA utilization. In addition, variants in the CD36 gene were shown recently to influence susceptibility for the metabolic syndrome, which greatly increases the risk of diabetes and heart disease.

Contribution of FAT/CD36 to the regulation of skeletal muscle fatty acid oxidation: an overview

Long chain fatty acids (LCFAs) are an important substrate for ATP production within the skeletal muscle. The process of LCFA delivery from adipose tissue to muscle mitochondria involves many regulatory steps. Recently, it has been recognized that LCFA oxidation is not only dependent on LCFA delivery to the muscle, but also on regulatory steps within the muscle. Increasing selected fatty acid binding proteins/transporters on the plasma membrane facilitates a very rapid LCFA increase into the muscle, independent of any changes in LCFA delivery to the muscle. Such a mechanism of LCFA transporter translocation is activated by muscle contraction. Intramuscular triacylglycerols may also be hydrolysed to provide fatty acids for mitochondrial oxidation, particularly during exercise, when hormone-sensitive lipase and other enzymes are activated. Mitochondrial LCFA entry is also highly regulated. This however does not involve only the malonyl CoA carnitine palmitoyltransferase-I (CPTI) axis. Exercise-induced fatty acid entry into mitochondria is also regulated by at least one of the proteins (FAT/CD36) that also regulates plasma membrane fatty acid transport. Among individuals, differences in mitochondrial fatty acid oxidation appear to be correlated with the content of mitochondrial CPTI and FAT/CD36. This paper provides a brief overview of mechanisms that regulate LCFA uptake and oxidation in skeletal muscle during exercise and in obesity. We focus largely on our own work on FAT/CD36, which contributes to regulating, in a coordinated fashion, LCFA uptake across the plasma membrane and the mitochondrial membrane. Very little is known about the roles of FATP1-6 on fatty acid transport in skeletal muscle.

CD36 deficiency leads to choroidal involution via COX2 down-regulation in rodents

BACKGROUND

In the Western world, a major cause of blindness is age-related macular degeneration (AMD). Recent research in angiogenesis has furthered the understanding of choroidal neovascularization, which occurs in the "wet" form of AMD. In contrast, very little is known about the mechanisms of the predominant, "dry" form of AMD, which is characterized by retinal atrophy and choroidal involution. The aim of this study is to elucidate the possible implication of the scavenger receptor CD36 in retinal degeneration and choroidal involution, the cardinal features of the dry form of AMD.

METHODS AND FINDINGS

We here show that deficiency of CD36, which participates in outer segment (OS) phagocytosis by the retinal pigment epithelium (RPE) in vitro, leads to significant progressive age-related photoreceptor degeneration evaluated histologically at different ages in two rodent models of CD36 invalidation in vivo (Spontaneous hypertensive rats (SHR) and CD36-/- mice). Furthermore, these animals developed significant age related choroidal involution reflected in a 100%-300% increase in the avascular area of the choriocapillaries measured on vascular corrosion casts of aged animals. We also show that proangiogenic COX2 expression in RPE is stimulated by CD36 activating antibody and that CD36-deficient RPE cells from SHR rats fail to induce COX2 and subsequent vascular endothelial growth factor (VEGF) expression upon OS or antibody stimulation in vitro. CD36-/- mice express reduced levels of COX2 and VEGF in vivo, and COX2-/- mice develop progressive choroidal degeneration similar to what is seen in CD36 deficiency.

CONCLUSIONS

CD36 deficiency leads to choroidal involution via COX2 down-regulation in the RPE. These results show a novel molecular mechanism of choroidal degeneration, a key feature of dry AMD. These findings unveil a pathogenic process, to our knowledge previously undescribed, with important implications for the development of new therapies.

Differential roles of CD36, ICAM-1, and P-selectin in Plasmodium falciparum cytoadherence in vivo

Cytoadherence of Plasmodium falciparum-infected red blood cells (IRBCs) on human microvascular endothelium is mediated by synergistic adhesive interactions with different adhesion molecules in vitro. Here, the authors used a unique human/severe combined immunodeficient (SCID) mouse chimeric model to directly visualize IRBC-endothelial interactions in an intact human microvasculature in vivo. Stimulation of human skin grafts with 100 ng TNF-alpha for 4 h led to a dramatic reduction in the distance rolled by IRBCs before arrest, so that the majority of IRBCs adhered directly to the endothelium with a 1.8-fold increase in the number of adherent cells. The decrease in rolling distance and increase in adhesion could be reversed by anti-ICAM-1. More importantly, the effect of TNF-alpha could be seen only in the presence of CD36. A further increase in adhesion by 4.9-fold was observed after 24 h of TNF-alpha stimulation. The increase could be reversed by anti-ICAM-1, but not anti-VCAM-1. In histamine-stimulated grafts, the rolling flux fraction and adhesion increased by 2.8- and 1.6-fold, respectively. The increases were attributable to P-selectin as an inhibitory anti-P-selectin antibody abrogated both the increased rolling flux fraction and firm adhesion. These findings indicate that in addition to CD36, ICAM-1, and P-selectin are major contributors to the dynamic process of IRBC adhesion by different mechanisms in vivo.

Interleukin-10 expression in macrophages during phagocytosis of apoptotic cells is mediated by homeodomain proteins Pbx1 and Prep-1

Production of interleukin (IL)-10, a major immunoregulatory cytokine, by phagocytes during clearance of apoptotic cells is critical to ensuring cellular homeostasis and suppression of autoimmunity. Little is known about the regulatory mechanisms in this fundamental process. We report that IL-10 production stimulated by apoptotic cells was regulated at the point of transcription in a manner dependent on p38 mitogen-activated protein kinase, partially on the scavenger receptor CD36, and required cell-cell contact but not phagocytosis. By using a reporter assay, we mapped the apoptotic-cell-response element (ACRE) in the human IL10 promoter and provide biochemical and physiological evidence that ACRE mediates the transcriptional activation of IL10 by pre-B cell leukemia transcription factor-1b and another Hox cofactor Pbx-regulating protein 1 in response to apoptotic cells. This study establishes a role of two developmentally critical factors (Pbx1 and Prep-1) in the regulation of homeostasis in the immune system.

The inhibition of endocytosis affects HDL-lipid uptake mediated by the human scavenger receptor class B type I

The scavenger receptor SR-BI plays an important role in the hepatic clearance of HDL cholesterol and other lipids, driving reverse cholesterol transport and contributing to protection against atherosclerosis in mouse models. We characterized the role of endocytosis in lipid uptake from HDL, mediated by the human SR-BI, using a variety of approaches to inhibit endocytosis, including hypertonic shock, potassium or energy depletion and disassembly of the actin cytoskeleton. Our studies revealed that unlike mouse SR-BI, human SR-BI-mediated HDL-lipid uptake was reduced by inhibition of endocytosis. This was not dependent on the cytoplasmic C-terminus of SR-BI. Monitoring the uptake of both the protein and lipid components of HDL revealed that although overall lipid uptake was decreased, the degree of selective lipid uptake was increased. These data suggest that that endocytosis is a dynamic regulator of SR-BI's selective lipid uptake activity.

Evidence for concerted action of FAT/CD36 and FABPpm to increase fatty acid transport across the plasma membrane

There is substantial molecular, biochemical and physiologic evidence that long-chain fatty acid transport involves a protein-mediated process. A number of fatty acid transport proteins have been identified, and for unknown reasons, some of these are coexpressed in the same tissues. In muscle and heart FAT/CD36 and FABPpm appear to be key transporters. Both proteins are regulated acutely (within minutes) and chronically (hours to days) by selected physiologic stimuli (insulin, AMP kinase activation). Acute regulation involves the translocation of FAT/CD36 by insulin, muscle contraction and AMP kinase activation, while FABPpm is induced to translocate by muscle contraction and AMP kinase activation, but not by insulin. Protein expression ofFAT/CD36 and FABPpm is regulated by prolonged AMP kinase activation (heart) or increased muscle contraction. Prolonged insulin exposure increases the expression of FAT/CD36 but not FABPpm. Trafficking of fatty acid transporters between an intracellular compartment(s) and the plasma membrane is altered in insulin-resistant skeletal muscle, as some FAT/CD36 is permanently relocated to plasma membrane, thereby contributing to insulin resistance due to the increased influx of fatty acids into muscle cells. Studies in FAT/CD36 null mice have revealed that this transporter is key to regulating the increase in the rate of fatty acid metabolism in heart and skeletal muscle. It appears based on a number of experiments that FAT/CD36 and FABPpm may collaborate to increase the rates of fatty acid transport, as these proteins co-immunoprecipitate.

CD36 gene deletion reduces fat preference and intake but not post-oral fat conditioning in mice

Several findings suggest the existence of a “fatty” taste, and the CD36 fatty acid translocase is a candidate taste receptor. The present study compared fat preference and acceptance in CD36 knockout (KO) and wild-type (WT) mice using nutritive (triglyceride and fatty acid) and nonnutritive (Sefa Soyate oil) emulsions. In two-bottle tests (24 h/day) naive KO mice, unlike WT mice, displayed little or no preference for dilute soybean oil, linoleic acid, or Sefa Soyate emulsions. At high concentrations (2.5–20%), KO mice developed significant soybean oil preferences, although they consumed less oil than WT mice. The postoral actions of fat likely conditioned these preferences. KO mice, like WT mice, learned to prefer a flavored solution paired with intragastric soybean oil infusions. These findings support CD36 mediation of a gustatory component to fat preference but demonstrate that it is not essential for fat-conditioned flavor preferences. The finding that oil-naive KO mice failed to prefer a nonnutritive oil, assumed to provide texture rather than taste cues, requires explanation. Finally, CD36 deletion decreased fat consumption and enhanced the ability of the mice to compensate for the calories provided by their optional fat intake.

Cardiac contractile dysfunction in insulin-resistant rats fed a high-fat diet is associated with elevated CD36-mediated fatty acid uptake and esterification

AIMS/HYPOTHESIS

Changes in cardiac substrate utilisation leading to altered energy metabolism may underlie the development of diabetic cardiomyopathy. We studied cardiomyocyte substrate uptake and utilisation and the role of the fatty acid translocase CD36 in relation to in vivo cardiac function in rats fed a high-fat diet (HFD).

METHODS

Rats were exposed to an HFD or a low-fat diet (LFD). In vivo cardiac function was monitored by echocardiography. Substrate uptake and utilisation were determined in isolated cardiomyocytes.

RESULTS

Feeding an HFD for 8 weeks induced left ventricular dilation in the systolic phase and decreased fractional shortening and the ejection fraction. Insulin-stimulated glucose uptake and proline-rich Akt substrate 40 phosphorylation were 41% (p < 0.001) and 45% (p < 0.05) lower, respectively, in cardiomyocytes from rats on the HFD. However, long-chain fatty acid (LCFA) uptake was 1.4-fold increased (p < 0.001) and LCFA esterification into triacylglycerols and phospholipids was increased 1.4- and 1.5-fold, respectively (both p < 0.05), in cardiomyocytes from HFD compared with LFD hearts. In the presence of the CD36 inhibitor sulfo-N-succinimidyloleate, LCFA uptake and esterification were similar in LFD and HFD cardiomyocytes. In HFD hearts CD36 was relocated to the sarcolemma, and basal phosphorylation of a mediator of CD36-trafficking, i.e. protein kinase B (PKB/Akt), was increased.

CONCLUSIONS/INTERPRETATION

Feeding rats an HFD induced cardiac contractile dysfunction, which was accompanied by the relocation of CD36 to the sarcolemma, and elevated basal levels of phosphorylated PKB/Akt. The permanent presence of CD36 at the sarcolemma resulted in enhanced rates of LCFA uptake and myocardial triacylglycerol accumulation, and may contribute to the development of insulin resistance and diabetic cardiomyopathy.

Platelet activation by oxidized low density lipoprotein is mediated by CD36 and scavenger receptor-A

OBJECTIVE

The interaction of platelets with low density lipoprotein (LDL) contributes to the development of cardiovascular disease. Platelets are activated by native LDL (nLDL) through apoE Receptor 2' (apoER2')-mediated signaling to p38(MAPK) and by oxidized LDL (oxLDL) through lysophosphatidic acid (LPA) signaling to Rho A and Ca2+. Here we report a new mechanism for platelet activation by oxLDL.

METHODS AND RESULTS

Oxidation of nLDL increases p38(MAPK) activation through a mechanism that is (1) independent of LPA, and (2) unlike nLDL-signaling not desensitized by prolonged platelet-LDL contact or inhibited by receptor-associated protein or chondroitinase ABC. Antibodies against scavenger receptors CD36 and SR-A alone fail to block p38(MAPK) activation by oxLDL but combined blockade inhibits p38(MAPK) by >40% and platelet adhesion to fibrinogen under flow by >60%. Mouse platelets deficient in either CD36 or SR-A show normal p38(MAPK) activation by oxLDL but combined deficiency of CD36 and SR-A disrupts oxLDL-induced activation of p38(MAPK) by >70%.

CONCLUSION

These findings reveal a novel platelet-activating pathway stimulated by oxLDL that is initiated by the combined action of CD36 and SR-A.

Platelet CD36 links hyperlipidemia, oxidant stress and a prothrombotic phenotype

Dyslipidemia is associated with a prothrombotic phenotype; however, the mechanisms responsible for enhanced platelet reactivity remain unclear. Proatherosclerotic lipid abnormalities are associated with both enhanced oxidant stress and the generation of biologically active oxidized lipids, including potential ligands for the scavenger receptor CD36, a major platelet glycoprotein. Using multiple mouse in vivo thrombosis models, we now demonstrate that genetic deletion of Cd36 protects mice from hyperlipidemia-associated enhanced platelet reactivity and the accompanying prothrombotic phenotype. Structurally defined oxidized choline glycerophospholipids that serve as high-affinity ligands for CD36 were at markedly increased levels in the plasma of hyperlipidemic mice and in the plasma of humans with low HDL levels, were able to bind platelets via CD36 and, at pathophysiological levels, promoted platelet activation via CD36. Thus, interactions of platelet CD36 with specific endogenous oxidized lipids play a crucial role in the well-known clinical associations between dyslipidemia, oxidant stress and a prothrombotic phenotype.

Thiamine attenuates the hypertension and metabolic abnormalities in CD36-defective SHR: uncoupling of glucose oxidation from cellular entry accompanied with enhanced protein O-GlcNAcylation in CD36 deficiency

BACKGROUND AND OBJECTIVES

The spontaneous hypertensive rat (SHR) is a widely studied model of hypertension that exhibits metabolic abnormalities, which share features with the human metabolic syndrome. Genetic linkage studies have revealed a defective CD36 gene, encoding a membrane fatty acid (FA) transporter, in hyperinsulinemia of the SHR. However, there is no unifying mechanism that can explain these phenotypes as a consequence of a defective CD36 gene. Impaired fatty acid uptake is compensated by increased glucose uptake. We hypothesized that (1) the abundant intracellular glucose is not oxidized proportionally and (2) the correction of the uncoupling of glucose oxidation to its cellular entry might be effective against the pathophysiology of CD36-defective SHR. Therefore, we attempted to activate glucose oxidation with the repletion of thiamine, a coenzyme for multiple steps of glucose metabolism.

METHODS AND RESULTS

In one series of experiments, intracellular glucose fate was assessed by the ratio of [(14)C]glucose/[(3)H]deoxyglucose radioactivity, which suggested that glucose oxidation was uncoupled from its cellular entry in SHR. Protein O-GlcNAcylation was intense in the hearts of CD36-defective SHR compared with that of wild-type CD36 rats [Wister Kyoto rats (WKY)], indicating the shunt of glucose through the hexosamine biosynthetic pathway (HBP). In another series of studies, 4-week-old SHR were maintained with water containing 0.2% thiamine for 10 weeks. Systolic blood pressure, plasma insulin and norepinephrine levels were significantly lower in the thiamine-group, as compared with the untreated-group. In epididymal adipose tissue, thiamine repletion down-regulated the expression levels of mRNA transcripts for UDP-N-acetylglucosamine:peptide glycosyltransferase, angiotensinogen, angiotensin type 1 receptor, transforming growth factor-beta1 and plasminogen activator inhibitor-1.

CONCLUSIONS

The hearts of CD36-defective SHR exhibited uncoupling of glucose oxidation from its cellular entry, accompanied with the enhanced protein O-GlcNAcylation, suggesting increased glucose shunt through the HBP. Thiamine repletion in CD36-defective SHR resulted in (1) the correction of the uncoupling of glucose oxidation to its cellular entry, concomitant with reduced protein O-GlcNAcylation, (2) the down-regulation of the expression of mRNAs involved in HBP, the renin-angiotensin system and adipokines in epididymal adipose tissue, and (3) the attenuation of the hypertension and hyperinsulinemia. We propose that interventions targeting glucose oxidation with thiamine repletion may provide a novel adjunctive approach to attenuate metabolic abnormalities and related hypertension.

CD36 signals to the actin cytoskeleton and regulates microglial migration via a p130Cas complex

The pattern recognition receptor CD36 initiates a signaling cascade that promotes microglial activation and recruitment to beta-amyloid deposits in the brain. In the present study we identify the focal adhesion-associated proteins p130Cas, Pyk2, and paxillin as novel members of the tyrosine kinase signaling pathway downstream of CD36 and show that assembly of this complex is essential for microglial migration. In primary microglia and macrophages exposed to beta-amyloid, the scaffolding protein p130Cas is rapidly tyrosine-phosphorylated and co-localizes with CD36 to membrane ruffles contemporaneous with F-actin polymerization. These beta-amyloid-stimulated events are not detected in CD36 null cells and are dependent on CD36 activation of Src family tyrosine kinases. Fyn, a Src kinase known to interact with CD36, co-precipitates with p130Cas and is an essential upstream intermediate in the signaling pathways leading to phosphorylation of the p130Cas substrate domain. Furthermore, the p130Cas-interacting kinase Pyk2 and the cytoskeletal adapter protein paxillin also demonstrate CD36-dependent phosphorylation, identifying these focal adhesion molecules as additional members of this beta-amyloid signaling cascade. Disruption of this p130Cas complex by small interfering RNA silencing inhibits p44/42 mitogen-activated protein kinase phosphorylation and microglial migration, illustrating the importance of this pathway in microglial activation and recruitment. Together, these data are the first to identify the signaling cascade that directly links CD36 to the actin cytoskeleton and, thus, implicates it in diverse processes such as cellular migration, adhesion, and phagocytosis.

Lipids in the heart: a source of fuel and a source of toxins

PURPOSE OF REVIEW

How do lipids arrive in the heart and other tissues? This review focuses on new information on pathways of lipid uptake into the heart.

RECENT FINDINGS

Fatty acids, the major cardiac fuel, are obtained from either lipoproteins or free fatty acids associated with albumin. The heart is the tissue with the most robust expression of lipoprotein lipase, and recent data attest to the importance of this enzyme in supplying optimal amounts of fatty acids for the heart. Genetic deletion of CD36 also shows that this transporter is important for cardiac uptake of lipids. Retinoid acquisition by the heart involves pathways parallel to those used for fatty acid uptake: a pathway for acquisition of core lipoprotein retinyl ester and another for nonlipoprotein retinol. Dilated lipotoxic cardiomyopathy is the consequence of excess lipid uptake.

SUMMARY

Genetic modifications that affect lipid uptake, oxidation, and storage are being exploited to elucidate the pathophysiology of cardiomyopathies and to discover how lipids relate to heart failure in humans with obesity and diabetes mellitus. This information is likely to lead to new diagnostic categories of cardiomyopathy and more pathophysiologically appropriate treatments.

Aggregated ursolic acid, a natural triterpenoid, induces IL-1beta release from murine peritoneal macrophages: role of CD36

IL-1beta has been shown to play a pivotal role in the development of inflammatory disorders. We recently found that a natural triterpene, ursolic acid (UA), enhanced MIF release from nonstimulated macrophages. In this study, we examined the effects of UA on the production of several cytokines in resident murine peritoneal macrophages (pMphi). UA increased the protein release of IL-1beta, IL-6, and MIF, but not of TNF-alpha, in dose- and time-dependent manners. This triterpene also strikingly induced the activation of p38 MAPK and ERK1/2 together with that of upstream kinases. The release of UA-induced IL-1beta was significantly inhibited by the inhibitors of p38 MAPK, MEK1/2, ATP-binding cassette transporter, and caspase-1. Furthermore, UA induced intracellular ROS generation for IL-1beta production, which was suppressed by an antioxidant. Pretreatment with an anti-CD36 Ab significantly suppressed IL-1beta release, and surface plasmon resonance assay results showed that UA bound to CD36 on macrophages. In addition, the amount of IL-1beta released from UA-treated pMphi of CD36-deficient mice was markedly lower than that from those of wild-type mice. Interestingly, UA was found to aggregate in culture medium, and the aggregates were suggested to be responsible for IL-1beta production. In addition, i.p. administration of UA increased the levels of IL-1beta secretion and MPO activity in colonic mucosa of ICR mice. Taken together, our results indicate that aggregated UA is recognized, in part, by CD36 on macrophages for generating ROS, thereby activating p38 MAPK, ERK1/2, and caspase-1, as well as releasing IL-1beta protein via the ATP-binding cassette transporter.

CD36 deficiency rescues lipotoxic cardiomyopathy

Obesity-related diabetes mellitus leads to increased myocardial uptake of fatty acids (FAs), resulting in a form of cardiac dysfunction referred to as lipotoxic cardiomyopathy. We have shown previously that chronic activation of the FA-activated nuclear receptor, peroxisome proliferator-activated receptor alpha (PPARalpha), is sufficient to drive the metabolic and functional abnormalities of the diabetic heart. Mice with cardiac-restricted overexpression of PPARalpha (myosin heavy chain [MHC]-PPARalpha) exhibit myocyte lipid accumulation and cardiac dysfunction. We sought to define the role of the long-chain FA transporter CD36 in the pathophysiology of lipotoxic forms of cardiomyopathy. MHC-PPARalpha mice were crossed with CD36-deficient mice (MHC-PPARalpha/CD36-/- mice). The absence of CD36 prevented myocyte triacylglyceride accumulation and cardiac dysfunction in the MHC-PPARalpha mice under basal conditions and following administration of high-fat diet. Surprisingly, the rescue of the MHC-PPARalpha phenotype by CD36 deficiency was associated with increased glucose uptake and oxidation rather than changes in FA utilization. As predicted by the metabolic changes, the activation of PPARalpha target genes involved in myocardial FA-oxidation pathways in the hearts of the MHC-PPARalpha mice was unchanged in the CD36-deficient background. However, PPARalpha-mediated suppression of genes involved in glucose uptake and oxidation was reversed in the MHC-PPARalpha/ CD36-/- mice. We conclude that CD36 is necessary for the development of lipotoxic cardiomyopathy in MHC-PPARalpha mice and that novel therapeutic strategies aimed at reducing CD36-mediated FA uptake show promise for the prevention or treatment of cardiac dysfunction related to obesity and diabetes.

Regulation of fatty acid transport by fatty acid translocase/CD36

Fatty acid (FA) translocase (FAT)/CD36 is a key protein involved in regulating the uptake of FA across the plasma membrane in heart and skeletal muscle. A null mutation of FAT/CD36 reduces FA uptake rates and metabolism, while its overexpression increases FA uptake rates and metabolism. FA uptake into the myocyte may be regulated (a) by altering the expression of FAT/CD36, thereby increasing the plasmalemmal content of this protein (i.e. streptozotocin-induced diabetes, chronic muscle stimulation), or (b) by relocating this protein to the plasma membrane, without altering its expression (i.e. obese Zucker rats). By repressing FAT/CD36 expression, and thereby lowering the plasmalemmal FAT/CD36 (i.e. leptin-treated animals), the rate of FA transport is reduced. Within minutes of beginning muscle contraction or being exposed to insulin FA transport is increased. This increase is a result of the contraction- and insulin-induced translocation of FAT/CD36 from an intracellular depot to the cell surface. Neither PPARα nor PPARγ activation alter FAT/CD36 expression in muscle, despite the fact that PPARα activation increases FAT/CD36 by 80% in liver. A novel observation is that FAT/CD36 also appears to be involved in mitochondrial FA oxidation, as this protein is located on the mitochondrial membrane and seems to be required to participate in moving FA across the mitochondrial membrane. Clearly, FAT/CD36 has an important role in FA homeostasis in skeletal muscle and the heart.

Importance of the carboxyl terminus of FAT/CD36 for plasma membrane localization and function in long-chain fatty acid uptake

This study investigates the role of the cytoplasmic C terminus of fatty acid translocase (FAT/CD36) in localization of the molecule to the plasma membrane, its insertion into lipid rafts, and its ability to enhance long-chain fatty acid uptake in transfected H4IIE rat hepatoma cells. In these cells, wild-type FAT/CD36 is localized to both lipid raft and nonraft domains of the plasma membrane. Interestingly, a FAT/CD36 truncation mutant lacking the final 10 amino acids of the cytoplasmic C terminus was retained within the cell in detergent-resistant membranes, and unlike wild-type FAT/CD36, it did not enhance oleate uptake. Furthermore, expression of FAT/CD36 in these cells increased the incorporation of oleate into diacylglycerol, a property that was not shared by truncated FAT/CD36. To examine whether the C terminus itself has an intrinsic ability to dictate the plasma membrane localization of FAT/CD36, this region was fused in-frame to enhanced green fluorescent protein (EGFP). This domain was sufficient to attach EGFP to cellular membranes, suggesting an involvement in the intracellular traffic of the molecule. We conclude that the C terminus of FAT/CD36 is required for localization of the receptor to the cell surface and its ability to enhance cellular oleate uptake.

A growth hormone-releasing peptide promotes mitochondrial biogenesis and a fat burning-like phenotype through scavenger receptor CD36 in white adipocytes

Whereas the uptake of oxidized lipoproteins by scavenger receptor CD36 in macrophages has been associated with foam cell formation and atherogenesis, little is known about the role of CD36 in regulating lipid metabolism in adipocytes. Here we report that treatment of 3T3-L1 adipocytes with hexarelin, a GH-releasing peptide that interacts with CD36, resulted in a depletion of intracellular lipid content with no significant change in CD36 expression. Microarray analysis revealed an increased pattern in several genes involved in fatty acid mobilization toward the mitochondrial oxidative phosphorylation process in response to hexarelin. Interestingly, many of these up-regulated genes are known targets of peroxisomal proliferator-activated receptor (PPAR)-gamma, such as FATP, CPT-1, and F(1)-ATPase, suggesting that adipocyte response to hexarelin may involve PPARgamma activation. Expression studies also indicate an increase in thermogenic markers PPARgamma coactivator 1alpha and uncoupling protein-1, which are normally expressed in brown adipocytes. Electron microscopy of hexarelin-treated 3T3-L1 adipocytes showed an intense and highly organized cristae formation that spans the entire width of mitochondria, compared with untreated cells, and cytochrome c oxidase activity was enhanced by hexarelin, two features characteristic of highly oxidative tissues. A similar mitochondrial phenotype was detected in epididymal white fat of mice treated with hexarelin, along with an increased expression of thermogenic markers that was lost in treated CD36-null mice, suggesting that the ability of hexarelin to promote a brown fat-like phenotype also occurs in vivo and is dependent on CD36. These results provide a potential role for CD36 to impact the overall metabolic activity of fat usage and mitochondrial biogenesis in adipocytes.

CD36 is a receptor for oxidized high density lipoprotein: Implications for the development of atherosclerosis

Atherosclerotic plaques result from the excessive deposition of cholesterol esters derived from lipoproteins and lipoprotein fragments. Tissue macrophage within the intimal space of major arterial vessels have been shown to play an important role in this process. We demonstrate in a transfection system using two human cell lines that the macrophage scavenger receptor CD36 selectively elicited lipid uptake from Cu(2+)-oxidized high density lipoprotein (HDL) but not from native HDL or low density lipoprotein (LDL). The uptake of oxHDL displayed morphological and biochemical similarities with the CD36-dependent uptake of oxidized LDL. CD36-mediated uptake of oxidized HDL by macrophage may therefore contribute to atheroma formation.