Sulfo-N-succinimidyl oleate (SSO) inhibits fatty acid uptake and signaling for intracellular calcium via binding CD36 lysine 164: SSO also inhibits oxidized low density lipoprotein uptake by macrophages

The computational analyses, molecular dynamics of fatty-acid transport mechanism to the CD36 receptor

The transmembrane glycoprotein CD36, which is responsible of the metabolic disorders, and the elevated intake of fat induces lipid buildup, is a multifunctional scavenger receptor signaling those functions in high-affinity tissue uptake of long-chain fatty acids. In this study, we used series of molecular dynamics simulations of the wild type and mutants types K164A CD36 protein interacting with one palmitic acid (PLM) besides simulations of the wild type interacting with the three PLM to find out the mechanism of the functioning of the complex CD36/Fatty acids and the unraveling of the role of the mutation. Additionally we determined whether Lys164, mostly exposed to protein surface, played important roles in fatty acid uptake. These simulations revealed, the conformational changes induced by Lys164 residue and the altered interactions induced by the mutagenesis of surface lysine that was badly influencing the folding, utility, solubility, and stability form of the variant. Furthermore, Lys164 residue provided the structural basis of forming an opening at the region of principal portal for the dissociation of palmitic acid. The results of our simulations revealed hole two fatty acids found in CD36 cavity structure and it was the most preferred to CD36 structure stabilization.

Transcription factor SOX2 contributes to nonalcoholic fatty liver disease development by regulating the expression of the fatty acid transporter CD36

Nonalcoholic fatty liver disease (NAFLD) can lead to hepatocellular carcinoma (HCC). The level of the transcription factor SOX2 correlates with HCC progression, but its role in fat accumulation remains unclear. Here, a high-fat diet, with and without fructose, significantly upregulated SOX2 in murine liver tissue. Treatment with free fatty acids (FFAs) and fructose upregulated SOX2 in murine FL83B hepatocytes. SOX2 overexpression or knockdown regulated triglyceride synthesis and lipid accumulation after FFA stimulation. CD36, a fatty acid transporter, and Yes-associated protein (YAP), a downstream molecule of the Hippo signaling pathway, were upregulated by FFA/fructose in vivo and in vitro. Transcriptional regulation of CD36 by SOX2 suggested the involvement of CD36 in SOX2-mediated hepatic steatosis. Thus, SOX2 may be a target to prevent NAFLD development.

Oxidized low-density lipoprotein links hypercholesterolemia and bladder cancer aggressiveness by promoting cancer stemness

Hypercholesterolemia is a prevalent metabolic disorder that has been implicated in the development of steroid-targeted cancers. However, the link between hypercholesterolemia and urinary bladder cancer (UBC), a non-steroid-targeted cancer, remains unresolved. Here we show that diet- and Ldlr deficiency-induced hypercholesterolemia enhances both UBC stemness and progression. Inhibition of intestinal cholesterol absorption by Ezetimibe reversed diet-induced hypercholesterolemia and cancer stemness. As a key component in hypercholesterolemic sera, oxidized low-density lipoprotein (ox-LDL), but not native low-density lipoprotein-cholesterol or metabolite 27-hydroxycholesterol, increased cancer stemness through its receptor CD36. Depletion of CD36, ectopic expression of an ox-LDL binding-disabled mutant form of CD36(K164A), and the neutralization of ox-LDL and CD36 via neutralizing antibodies all reversed ox-LDL-induced cancer stemness. Mechanistically, ox-LDL enhanced the interaction of CD36 and JAK2, inducing phosphorylation of JAK2 and subsequently activating STAT3 signaling, which was not mediated by JAK1 or Src in UBC cells. Finally, ox-LDL levels in serum predicted poor prognosis, and the ox-LDLhigh signature predicted worse survival in UBC patients. These findings indicate that ox-LDL links hypercholesterolemia with UBC progression by enhancing cancer stemness. Lowering serum ox-LDL or targeting the CD36/JAK2/STAT3 axis might serve as a potential therapeutic strategy for UBCs with hypercholesterolemia. Moreover, elevated ox-LDL may serve as a biomarker for UBC.

Qki is an essential regulator of microglial phagocytosis in demyelination

The mechanism underpinning the regulation of microglial phagocytosis in demyelinating diseases is unclear. Here, we showed that the Quaking protein (Qki) in microglia was greatly induced by demyelination in the brains of both mice and humans. Deletion of the Quaking gene (Qk) in microglia severely impaired the clearance of myelin debris. Transcriptomic profiling indicated that depletion of Qki impaired total RNA levels and splicing of the genes involved in phagosome formation and maturation. RNA immunoprecipitation (RIP) confirmed the physical interactions between the Qki protein and the mRNAs of Qki targets that are involved in phagocytosis, indicating that Qki regulates their RNA stability. Both Qki depletion and inhibition of Qki target Cd36 greatly reduced the phagocytic activity of microglia and macrophages. The defective uptake and degradation of myelin debris caused by Qki depletion in microglia resulted in unresolved myelin debris that impaired axon integrity, oligodendrocyte maturation, and subsequent remyelination. Thus, our results demonstrate that Qki is an essential regulator of microglia’s phagocytic activity under demyelinating conditions.

Palmitic Acid Lipotoxicity in Microglia Cells Is Ameliorated by Unsaturated Fatty Acids

Obesity and metabolic syndrome are associated with cognitive decline and dementia. Palmitic acid (PA) is increased in the cerebrospinal fluid of obese patients with cognitive impairment. This study was therefore designed to examine fatty acid (FA) lipotoxicity in BV2 microglia cells. We found that PA induced time- and dose-dependent decrease in cell viability and increase in cell death without affecting the cell cycle profile and that PA lipotoxicity did not depend on cell surface free fatty acid receptors but rather on FA uptake. Treatment with sulfosuccinimidyl oleate (SSO), an irreversible inhibitor of fatty acid translocase CD36, significantly inhibited FA uptake in BSA- and PA-treated cells and blocked PA-induced decrease in cell viability. Inhibition of ER stress or treatment with N-acetylcysteine was not able to rescue PA lipotoxicity. Our study also showed that unsaturated fatty acids (UFAs), such as linoleic acid (LA), oleic acid (OA), α-linolenic acid (ALA), and docosahexaenoic acid (DHA), were not lipotoxic but instead protected microglia against PA-induced decrease in cell viability. Co-treatment of PA with LA, OA, and DHA significantly inhibited FA uptake in PA-treated cells. All UFAs tested induced the incorporation of FAs into and the amount of neutral lipids, while PA did not significantly affect the amount of neutral lipids compared with BSA control.

Fatty acid mobilization from adipose tissue is mediated by CD36 post-translational modifications and intracellular trafficking

The mechanism controlling long-chain fatty acid (LCFA) mobilization from adipose tissue is not well understood. Here, we investigated how the LCFA transporter CD36 regulates this process. By using tissue-specific KO mouse models, we showed that CD36 in adipocytes and endothelial cells mediated both LCFA deposition into and release from adipose tissue. We demonstrated the role of adipocytic and endothelial CD36 in promoting tumor growth and chemoresistance conferred by adipose tissue–derived LCFAs. We showed that dynamic cysteine S-acylation of CD36 in adipocytes, endothelial cells, and cancer cells mediated intercellular LCFA transport. We demonstrated that lipolysis induction in adipocytes triggered CD36 deacylation and deglycosylation, as well as its dissociation from interacting proteins, prohibitin-1 (PHB) and annexin 2 (ANX2). Our data indicate that lipolysis triggers caveolar endocytosis and translocation of CD36 from the cell membrane to lipid droplets. This study suggests a mechanism for both outside-in and inside-out cellular LCFA transport regulated by CD36 S-acylation and its interactions with PHB and ANX2.

CD36 Signal Transduction in Metabolic Diseases: Novel Insights and Therapeutic Targeting

The cluster of differentiation 36 (CD36) is a scavenger receptor present on various types of cells and has multiple biological functions that may be important in inflammation and in the pathogenesis of metabolic diseases, including diabetes. Here, we consider recent insights into how the CD36 response becomes deregulated under metabolic conditions, as well as the therapeutic benefits of CD36 inhibition, which may provide clues for developing strategies aimed at the treatment or prevention of diabetes associated with metabolic diseases. To facilitate this process further, it is important to pinpoint regulatory mechanisms that are relevant under physiological and pathological conditions. In particular, understanding the mechanisms involved in dictating specific CD36 downstream cellular outcomes will aid in the discovery of potent compounds that target specific CD36 downstream signaling cascades.

Uptake of oxidized lipids by the scavenger receptor CD36 promotes lipid peroxidation and dysfunction in CD8+ T cells in tumors

A common metabolic alteration in the tumor microenvironment (TME) is lipid accumulation, a feature associated with immune dysfunction. Here, we examined how CD8+ tumor infiltrating lymphocytes (TILs) respond to lipids within the TME. We found elevated concentrations of several classes of lipids in the TME and accumulation of these in CD8+ TILs. Lipid accumulation was associated with increased expression of CD36, a scavenger receptor for oxidized lipids, on CD8+ TILs, which also correlated with progressive T cell dysfunction. Cd36-/- T cells retained effector functions in the TME, as compared to WT counterparts. Mechanistically, CD36 promoted uptake of oxidized low-density lipoproteins (OxLDL) into T cells, and this induced lipid peroxidation and downstream activation of p38 kinase. Inhibition of p38 restored effector T cell functions in vitro, and resolution of lipid peroxidation by overexpression of glutathione peroxidase 4 restored functionalities in CD8+ TILs in vivo. Thus, an oxidized lipid-CD36 axis promotes intratumoral CD8+ T cell dysfunction and serves as a therapeutic avenue for immunotherapies.

CD36 as a target for metabolic modulation therapy in cardiac disease

Introduction: Disturbances in myocardial lipid metabolism are increasingly being recognized as drivers of the development and progression of heart disease. Therefore, there is a need for treatments that can directly target lipid metabolic defects in heart failure. The membrane-associated glycoprotein CD36 plays a pivotal role in governing myocardial lipid metabolism by mediating lipid signaling and facilitating the cellular uptake of long-chain fatty acids. Emerging evidence suggests that CD36 is a prominent target in the treatment of heart failure.Areas covered: This article provides an overview of the key role of CD36 for proper contractile functioning of a healthy heart, its implications in the development of cardiac disease (ischemia/reperfusion, cardiac hypertrophy, and diabetic cardiomyopathy), and its application as a target to normalize cardiac metabolism as part of so-called metabolic modulation therapy.Expert opinion: CD36 appears a promising and effective therapeutic target in the treatment of heart failure. Natural compounds and chemical agents known to alter the amount or subcellular distribution of CD36 or inhibit its functioning, should be evaluated for their potency to correct cardiac metabolism and cure heart disease.

Lipolytic enzymes and free fatty acids at the endothelial interface

Lipids released from circulating lipoproteins by intravascular action of lipoprotein lipase (LpL) reach parenchymal cells in tissues with a non-fenestrated endothelium by transfer through or around endothelial cells. The actions of LpL are controlled at multiple sites, its synthesis and release by myocytes and adipocytes, its transit and association with the endothelial cell luminal surface, and finally its activation and inhibition by a number of proteins and by its product non-esterified fatty acids. Multiple pathways mediate endothelial transit of lipids into muscle and adipose tissues. These include movement of fatty acids via the endothelial cell fatty acid transporter CD36 and movement of whole or partially LpL-hydrolyzed lipoproteins via other apical endothelial cell receptors such as SR-B1and Alk1. Lipids also likely change the barrier function of the endothelium and operation of the paracellular pathway around endothelial cells. This review summarizes in vitro and in vivo support for the key role of endothelial cells in delivery of lipids and highlights incompletely understood processes that are the focus of active investigation.

Mitochondrial localization and moderated activity are key to murine erythroid enucleation

Mammalian red blood cells (RBCs), which primarily contain hemoglobin, exemplify an elaborate maturation process, with the terminal steps of RBC generation involving extensive cellular remodeling. This encompasses alterations of cellular content through distinct stages of erythroblast maturation that result in the expulsion of the nucleus (enucleation) followed by the loss of mitochondria and all other organelles and a transition to anaerobic glycolysis. Whether there is any link between erythroid removal of the nucleus and the function of any other organelle, including mitochondria, remains unknown. Here we demonstrate that mitochondria are key to nuclear clearance. Using live and confocal microscopy and high-throughput single-cell imaging, we show that before nuclear polarization, mitochondria progressively move toward one side of maturing erythroblasts and aggregate near the nucleus as it extrudes from the cell, a prerequisite for enucleation to proceed. Although we found active mitochondrial respiration is required for nuclear expulsion, levels of mitochondrial activity identify distinct functional subpopulations, because terminally maturing erythroblasts with low relative to high mitochondrial membrane potential are at a later stage of maturation, contain greatly condensed nuclei with reduced open chromatin-associated acetylation histone marks, and exhibit higher enucleation rates. Lastly, to our surprise, we found that late-stage erythroblasts sustain mitochondrial metabolism and subsequent enucleation, primarily through pyruvate but independent of in situ glycolysis. These findings demonstrate the critical but unanticipated functions of mitochondria during the erythroblast enucleation process. They are also relevant to the in vitro production of RBCs as well as to disorders of the erythroid lineage.

Platelets Fuel the Inflammasome Activation of Innate Immune Cells

The inflammasomes control the bioactivity of pro-inflammatory cytokines of the interleukin (IL)-1 family. The inflammasome assembled by NLRP3 has been predominantly studied in homogeneous cell populations in vitro, neglecting the influence of cellular interactions that occur in vivo. Here, we show that platelets boost the inflammasome capacity of human macrophages and neutrophils and are critical for IL-1 production by monocytes. Platelets license NLRP3 transcription, thereby enhancing ASC oligomerization, caspase-1 activity, and IL-1β secretion. Platelets influence IL-1β production in vivo, and blood platelet counts correlate with plasmatic IL-1β levels in malaria. Furthermore, we reveal an enriched platelet gene signature among the highest-expressed transcripts in IL-1β-driven autoinflammatory diseases. The platelet effect is independent of cell-to-cell contact, platelet-derived lipid mediators, purines, nucleic acids, and a host of platelet cytokines, and it involves the triggering of calcium-sensing receptors on macrophages. Hence, platelets provide an additional layer of regulation of inflammasomes and IL-1-driven inflammation.

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Platelets license NLRP3 for inflammasome activattion in innate immune cells

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Platelets are required for optimal monocyte inflammasome activation

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Platelets shape IL-1β in vivo, and platelet counts correlate with IL-1β in plasma

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A constitutive, heat-sensitive soluble platelet-factor boost IL-1β in macrophages

Effect of Oxidized Low-Density Lipoprotein on Head and Neck Squamous Cell Carcinomas

Cardiovascular disease (CVD) and cancer are two major causes of death worldwide. The question is, "Could there be a link between these two pathologies in addition to their shared, common risk factors?" To find some answers, we studied the effect of oxidized low-density lipoproteins (oxLDL) on head and neck cancer (HNC) cell lines, since oxLDL is a major contributor to atherosclerosis and the principal cause of CVD. In this study, we exposed three HNC cell lines (Detroit 562, UPCI-SCC-131 and FaDu) to oxLDL. We investigated two oxLDL receptors, CD36 and Lox-1, using immunofluorescence. Cancer cell migration was evaluated using Boyden chambers and the Wnt/β-catenin pathway was investigated using Western blotting. We demonstrated that the expression of CD36 and Lox-1 significantly increases after exposure to oxLDL. Moreover, we found that oxLDL reduces the migration of HNC cell lines, an observation that is in line with an increased degradation of β-catenin under oxLDL. Finally, the inhibition of CD36 with sulfosuccinimidyl oleate (SSO) reverses the inhibition of cell migration. In conclusion, we report that oxLDL seems to induce an increase in CD36 expression on HNC cell lines, enhancing the uptake of these lipids in cells to finally decrease cancer cell migration via the CD36/β-catenin pathway.

Shifting the Gears of Metabolic Plasticity to Drive Cell State Transitions in Cancer

Cancer metabolism is a hallmark of cancer. Metabolic plasticity defines the ability of cancer cells to reprogram a plethora of metabolic pathways to meet unique energetic needs during the various steps of disease progression. Cell state transitions are phenotypic adaptations which confer distinct advantages that help cancer cells overcome progression hurdles, that include tumor initiation, expansive growth, resistance to therapy, metastasis, colonization, and relapse. It is increasingly appreciated that cancer cells need to appropriately reprogram their cellular metabolism in a timely manner to support the changes associated with new phenotypic cell states. We discuss metabolic alterations that may be adopted by cancer cells in relation to the maintenance of cancer stemness, activation of the epithelial-mesenchymal transition program for facilitating metastasis, and the acquisition of drug resistance. While such metabolic plasticity is harnessed by cancer cells for survival, their dependence and addiction towards certain metabolic pathways also present therapeutic opportunities that may be exploited.

Endothelial Cell Receptors in Tissue Lipid Uptake and Metabolism

Lipid uptake and metabolism are central to the function of organs such as heart, skeletal muscle, and adipose tissue. Although most heart energy derives from fatty acids (FAs), excess lipid accumulation can cause cardiomyopathy. Similarly, high delivery of cholesterol can initiate coronary artery atherosclerosis. Hearts and arteries-unlike liver and adrenals-have nonfenestrated capillaries and lipid accumulation in both health and disease requires lipid movement from the circulation across the endothelial barrier. This review summarizes recent in vitro and in vivo findings on the importance of endothelial cell receptors and uptake pathways in regulating FAs and cholesterol uptake in normal physiology and cardiovascular disease. We highlight clinical and experimental data on the roles of ECs in lipid supply to tissues, heart, and arterial wall in particular, and how this affects organ metabolism and function. Models of FA uptake into ECs suggest that receptor-mediated uptake predominates at low FA concentrations, such as during fasting, whereas FA uptake during lipolysis of chylomicrons may involve paracellular movement. Similarly, in the setting of an intact arterial endothelial layer, recent and historic data support a role for receptor-mediated processes in the movement of lipoproteins into the subarterial space. We conclude with thoughts on the need to better understand endothelial lipid transfer for fuller comprehension of the pathophysiology of hyperlipidemia, and lipotoxic diseases such as some forms of cardiomyopathy and atherosclerosis.

Blockade of fatty acid signalling inhibits lipopolysaccharide-induced macrophage recruitment and progression of apical periodontitis

AIM

To examine the role of palmitic acid in lipopolysaccharide (LPS)-stimulated chemotaxis of macrophages and the potential contribution of saturated fatty acid in signalling during the pathogenesis of apical periodontitis.

METHODOLOGY

J774, a mouse macrophage cell line, was used in the experiments. After treatment with LPS, proteolytic maturation of sterol regulatory element-binding protein-1c (SREBP-1c) and expression of fatty acid synthase (FASN) were examined by Western analysis. Levels of palmitic acid were measured by reverse phase-high performance liquid chromatography-mass spectrometry. Knockdown of SREBP-1c and FASN was accomplished by small interfering RNA technology. Secretion of CC-chemokine ligand 2 (CCL2) and cellular chemotaxis were assessed by enzyme-linked immunosorbent assay and transwell migration assay, respectively. Sulfo-N-succinimidyl oleate (SSO) treatment was used to inhibit fatty acid signalling in vitro and also in a rat model of apical periodontitis. All data were first subjected to Levene's test. In vitro data were then analysed using ANOVA followed by Tukey's multiple comparison test. Data from animal experiments were analysed by independent t-tests. The significant level was set at 0.05.

RESULTS

LPS stimulated proteolytic maturation of SREBP-1c and FASN expression in macrophages and significantly enhanced palmitic acid synthesis (P < 0.05). Knockdown of SREBP-1c attenuated LPS-enhanced FASN expression. Knockdown of FASN significantly suppressed LPS-enhanced palmitic acid synthesis (P < 0.05). LPS and exogenous palmitic acid significantly enhanced CCL2 secretion and macrophage chemotaxis (all P < 0.05). Inhibition of FASN expression significantly alleviated LPS-augmented CCL2 secretion (P < 0.05). SSO significantly suppressed CCL2 secretion and macrophage chemotaxis augmented by LPS and palmitic acid (all P < 0.05). In a rat model of induced apical periodontitis, SSO treatment significantly attenuated progression of apical periodontitis and macrophage recruitment (all P < 0.05).

CONCLUSIONS

LPS/SREBP-1c/FASN/palmitic acid signalling contributed to tissue destruction caused by bacterial infection. Modulation of lipid metabolism and signalling may be helpful for the management of apical periodontitis.

Early Transcriptomic Response to OxLDL in Human Retinal Pigment Epithelial Cells

In the sub-retinal pigment epithelium (sub-RPE) space of the aging macula, deposits of oxidized phospholipids, oxidized derivatives of cholesterol and associated oxidized low-density lipoproteins (OxLDL) are considered contributors to the onset and development of age-related macular degeneration (AMD). We investigated the gene expression response of a human-derived RPE cell line exposed for short periods of time to non-cytotoxic levels of OxLDL or LDL. In our cell model, treatment with OxLDL, but not LDL, generated an early gene expression response which affected more than 400 genes. Gene pathway analysis unveiled gene networks involved in the regulation of various cellular functions, including acute response to oxidative stress via up-regulation of antioxidative gene transcripts controlled by nuclear factor erythroid-2 related factor 2 (NRF2), and up-regulation of aryl hydrocarbon receptor-controlled detoxifying gene transcripts. In contrast, circadian rhythm-controlling genes and genes involved in lipid metabolism were strongly down-regulated. Treatment with low-density lipoprotein (LDL) did not induce the regulation of these pathways. These findings show that RPE cells are able to selectively respond to the oxidized forms of LDL via the up-regulation of gene pathways involved in molecular mechanisms that minimize cellular oxidative damage, and the down-regulation of the expression of genes that regulate the intracellular levels of lipids and lipid derivatives. The effect on genes that control the cellular circadian rhythm suggests that OxLDL might also disrupt the circadian clock-dependent phagocytic activity of the RPE. The data reveal a complex cellular response to a highly heterogeneous oxidative stress-causing agent such as OxLDL commonly present in drusen formations.

Fibrillar amyloid peptides promote platelet aggregation through the coordinated action of ITAM- and ROS-dependent pathways

BACKGROUND

Amyloid peptides Aβ40 and Aβ42, whose deposition in brain correlates with Alzheimer disease, are also present in platelets and have prothrombotic activities.

OBJECTIVE

In this study, we analyze the ability of Aβ peptides to form fibrils and to induce platelet activation and aggregation.

METHODS

Aβ40, Aβ42, and their scrambled peptides were diluted in phosphate buffered saline and fibrillogenesis was investigated by ThioflavinT and Congo Red. Aggregation, protein phosphorylation, and reactive oxygen species (ROS) production were analyzed.

RESULTS

Aβ40 and Aβ42, but not scrambled peptides, were able to form fibrils when diluted in phosphate buffered saline. Fibrillogenesis of Aβ42 was very rapid, whereas fibril formation by Aβ40 was completed only after 48 hours of incubation. Fibrillar Aβ40 and Aβ42 promoted dose-dependent aggregation of washed platelets in the presence of extracellular CaCl2 . Cleavage of GPIbα by mocarhagin or blockade of the ITAM-containing FcγRIIA prevented platelet aggregation induced by fibrillary Aβ40 and Aβ42. Fibrillar Aβ peptides stimulated the phosphorylation of FcγRIIA, resulting in the downstream stimulation of PLC, protein kinase C, and phosphoinositide 3-kinases, whose activity was necessary for full aggregation of platelets. Fibrillar Aβ peptides also induced ROS generation, and NOX inhibitors, as well as ROS scavengers, prevented platelet aggregation. However, Aβ peptide-induced ROS production did not require binding to GPIbα or activation of FcγRIIA, but was initiated by CD36, which provided an important contribution to full platelet aggregation.

CONCLUSION

These results suggest that fibrillar amyloid Aβ40 and Aβ42 induce platelet aggregation through the recruitment of GPIb-IX-V and CD36, which requires the convergence of ITAM- and ROS-dependent pathways.

CD36 facilitates fatty acid uptake by dynamic palmitoylation-regulated endocytosis

Fatty acids (FAs) are essential nutrients, but how they are transported into cells remains unclear. Here, we show that FAs trigger caveolae-dependent CD36 internalization, which in turn delivers FAs into adipocytes. During the process, binding of FAs to CD36 activates its downstream kinase LYN, which phosphorylates DHHC5, the palmitoyl acyltransferase of CD36, at Tyr91 and inactivates it. CD36 then gets depalmitoylated by APT1 and recruits another tyrosine kinase SYK to phosphorylate JNK and VAVs to initiate endocytic uptake of FAs. Blocking CD36 internalization by inhibiting APT1, LYN or SYK abolishes CD36-dependent FA uptake. Restricting CD36 at either palmitoylated or depalmitoylated state eliminates its FA uptake activity, indicating an essential role of dynamic palmitoylation of CD36. Furthermore, blocking endocytosis by targeting LYN or SYK inhibits CD36-dependent lipid droplet growth in adipocytes and high-fat-diet induced weight gain in mice. Our study has uncovered a dynamic palmitoylation-regulated endocytic pathway to take up FAs.

Micellization of coenzyme Q by the fungicide caspofungin allows for safe intravenous administration to reach extreme supraphysiological concentrations

Coenzyme Q₁₀ (CoQ₁₀; also known as ubiquinone) is a vital, redox-active membrane component that functions as obligate electron transporter in the mitochondrial respiratory chain, as cofactor in other enzymatic processes and as antioxidant. CoQ₁₀ supplementation has been widely investigated for treating a variety of acute and chronic conditions in which mitochondrial function or oxidative stress play a role. In addition, it is used as replacement therapy in patients with CoQ deficiency including inborn primary CoQ₁₀ deficiency due to mutations in CoQ₁₀-biosynthetic genes as well as secondary CoQ₁₀ deficiency, which is frequently observed in patients with mitochondrial disease syndrome and in other conditions. However, despite many tests and some promising results, whether CoQ₁₀ treatment is beneficial in any indication has remained inconclusive. Because CoQ₁₀ is highly insoluble, it is only available in oral formulations, despite its very poor oral bioavailability. Using a novel model of CoQ-deficient cells, we screened a library of FDA-approved drugs for an activity that could increase the uptake of exogenous CoQ₁₀ by the cell. We identified the fungicide caspofungin as capable of increasing the aqueous solubility of CoQ₁₀ by several orders of magnitude. Caspofungin is a mild surfactant that solubilizes CoQ₁₀ by forming nano-micelles with unique properties favoring stability and cellular uptake. Intravenous administration of the formulation in mice achieves unprecedented increases in CoQ₁₀ plasma levels and in tissue uptake, with no observable toxicity. As it contains only two safe components (caspofungin and CoQ₁₀), this injectable formulation presents a high potential for clinical safety and efficacy.

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Coenzyme Q₁₀ (CoQ₁₀) can be solubilized by the antifungal drug caspofungin (CF).

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CF is a mild surfactant and solubilizes CoQ₁₀ in water by forming micellar structures with a high CoQ₁₀ content.

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CF/CoQ₁₀ micelles have unique properties favoring rapid and efficient uptake into cells and mitochondria.

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CF/CoQ₁₀ micelles can be intravenously administrated without signs of toxicity.

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Intravenous administration of CF/CoQ₁₀ in mice achieves unprecedented elevation of CoQ₁₀ plasma levels and tissue uptake.

Therapeutic Potentials of Scavenger Receptor CD36 Mediated Innate Immune Responses Against Infectious and Non-Infectious Diseases

CD36 is a multifunctional glycoprotein, expressed in different types of cells and known to play a significant role in the pathophysiology of the host. The structural studies revealed that the scavenger receptor consists of short cytosolic domains, two transmembrane domains, and a large ectodomain. The ectodomain serves as a receptor for a diverse number of endogenous and exogenous ligands. The CD36-specific ligands are involved in regulating the immune response during infectious and non-infectious diseases in the host. The role of CD36 in regulating the innate immune response during Pneumonia, Tuberculosis, Malaria, Leishmaniasis, HIV, and Sepsis in a ligand- mediated fashion. Apart from infectious diseases, it is also considered to be involved in metabolic disorders such as Atherosclerosis, Alzheimer's, cancer, and Diabetes. The ligand binding to scavenger receptor modulates the CD36 down-stream innate immune response, and it can be exploited to design suitable immuno-modulators. Hence, the current review focused on the role of the CD36 in innate immune response and therapeutic potentials of novel heterocyclic compounds as CD36 ligands during infectious and non-infectious diseases.

Nitro-oleic acid, a ligand of CD36, reduces cholesterol accumulation by modulating oxidized-LDL uptake and cholesterol efflux in RAW264.7 macrophages

Macrophages play a pivotal role in the early stages of atherosclerosis development; they excessively accumulate cholesterol in the cytosol in response to modified Low Density Lipoprotein (mLDL). The mLDL are incorporated through scavenger receptors. CD36 is a high-affinity cell surface scavenger receptor that facilitates the binding and uptake of long-chain fatty acids and mLDL into the cell. Numerous structurally diverse ligands can initiate signaling responses through CD36 to regulate cell metabolism, migration, and angiogenesis. Nitro-fatty acids are endogenous electrophilic lipid mediators that react with and modulate the function of multiple enzymes and transcriptional regulatory proteins. These actions induce the expression of several anti-inflammatory and cytoprotective genes and limit pathologic responses in experimental models of atherosclerosis, cardiac ischemia/reperfusion, and inflammatory diseases. Pharmacological and genetic approaches were used to explore the actions of nitro-oleic acid (NO₂-OA) on macrophage lipid metabolism. Pure synthetic NO₂-OA dose-dependently increased CD36 expression in RAW264.7 macrophages and this up-regulation was abrogated in BMDM from Nrf2-KO mice. Ligand binding analysis revealed that NO₂-OA specifically interacts with CD36, thus limiting the binding and uptake of mLDL. Docking analysis shows that NO₂-OA establishes a low binding energy interaction with the alpha helix containing Lys164 in CD36. NO₂-OA also restored autophagy flux in mLDL-loaded macrophages, thus reversing cholesterol deposition within the cell. In aggregate, these results indicate that NO₂-OA reduces cholesterol uptake by binding to CD36 and increases cholesterol efflux by restoring autophagy.

Reprogramming of lipid metabolism in cancer-associated fibroblasts potentiates migration of colorectal cancer cells

Metabolic interaction between cancer-associated fibroblasts (CAFs) and colorectal cancer (CRC) cells plays a major role in CRC progression. However, little is known about lipid alternations in CAFs and how these metabolic reprogramming affect CRC cells metastasis. Here, we uncover CAFs conditioned medium (CM) promote the migration of CRC cells compared with normal fibroblasts CM. CAFs undergo a lipidomic reprogramming, and accumulate more fatty acids and phospholipids. CAFs CM after protein deprivation still increase the CRC cells migration, which suggests small molecular metabolites in CAFs CM are responsible for CRC cells migration. Then, we confirm that CRC cells take up the lipids metabolites that are secreted from CAFs. Fatty acids synthase (FASN), a crucial enzyme in fatty acids synthesis, is significantly increased in CAFs. CAF-induced CRC cell migration is abolished by knockdown of FASN by siRNA or reducing the uptake of fatty acids by CRC cells by sulfo-N-succinimidyloleate sodium in vitro and CD36 monoclonal antibody in vivo. To conclude, our results provide a new insight into the mechanism of CRC metastasis and suggest FASN of CAFs or CD36 of CRC cells may be potential targets for anti-metastasis treatment in the future.

Influence of cellular lipid content on influenza A virus replication

Influenza A virus (IAV) depends on the metabolism of its cellular host to provide energy and essential factors, including lipids, for viral replication. Previous studies have shown that fatty acids (FAs) play an important role in IAV replication and that inhibition of FA biosynthesis can diminish viral replication. However, cellular lipids can either be synthesized intracellularly or be imported from the extracellular environment. Interfering with FA import mechanisms may reduce the cellular lipid content and inhibit IAV replication. To test this hypothesis, MDCK and Detroit 562 cells were infected with IAV followed by exposure to palmitic acid and inhibitors of FA import. Replication of IAV significantly increased when infected cells were supplied with palmitic acid. This enhancement could be reduced by adding an FA import inhibitor. The addition of palmitic acid significantly increased the cellular lipid content, and this increased level was reduced by treatment with an FA import inhibitor. These results show that reducing the cellular lipid level might be an approach for IAV therapy.

SSO and other putative inhibitors of FA transport across membranes by CD36 disrupt intracellular metabolism, but do not affect FA translocation

Membrane-bound proteins have been proposed to mediate the transport of long-chain FA (LCFA) transport through the plasma membrane (PM). These proposals are based largely on reports that PM transport of LCFAs can be blocked by a number of enzymes and purported inhibitors of LCFA transport. Here, using the ratiometric pH indicator (2',7'-bis-(2-carboxyethyl)-5-(and-6-)-carboxyfluorescein and acrylodated intestinal FA-binding protein-based dual fluorescence assays, we investigated the effects of nine inhibitors of the putative FA transporter protein CD36 on the binding and transmembrane movement of LCFAs. We particularly focused on sulfosuccinimidyl oleate (SSO), reported to be a competitive inhibitor of CD36-mediated LCFA transport. Using these assays in adipocytes and inhibitor-treated protein-free lipid vesicles, we demonstrate that rapid LCFA transport across model and biological membranes remains unchanged in the presence of these purported inhibitors. We have previously shown in live cells that CD36 does not accelerate the transport of unesterified LCFAs across the PM. Our present experiments indicated disruption of LCFA metabolism inside the cell within minutes upon treatment with many of the "inhibitors" previously assumed to inhibit LCFA transport across the PM. Furthermore, using confocal microscopy and a specific anti-SSO antibody, we found that numerous intracellular and PM-bound proteins are SSO-modified in addition to CD36. Our results support the hypothesis that LCFAs diffuse rapidly across biological membranes and do not require an active protein transporter for their transmembrane movement.

Positioning Metabolism as a Central Player in the Diabetic Heart

In type 2 diabetes (T2D), the leading cause of death is cardiovascular complications. One mechanism contributing to cardiac pathogenesis is alterations in metabolism, with the diabetic heart exhibiting increased fatty acid oxidation and reduced glucose utilisation. The processes classically thought to underlie this metabolic shift include the Randle cycle and changes to gene expression. More recently, alternative mechanisms have been proposed, most notably, changes in post-translational modification of mitochondrial proteins in the heart. This increased understanding of how metabolism is altered in the diabetic heart has highlighted new therapeutic targets, with an aim to improve cardiac function in T2D. This review focuses on metabolism in the healthy heart and how this is modified in T2D, providing evidence for the mechanisms underlying this shift. There will be emphasis on the current treatments for the heart in diabetes, alongside efforts for metabocentric pharmacological therapies.

Lipid oversupply induces CD36 sarcolemmal translocation via dual modulation of PKCζ and TBC1D1: an early event prior to insulin resistance

Lipid oversupply may induce CD36 sarcolemmal translocation to facilitate fatty acid transport, which in turn causes dyslipidemia and type 2 diabetes. However, the underlying mechanisms of CD36 redistribution are still yet to be unraveled.

Methods: High fat diet fed mice and palmitate/oleic acid-treated L6 cells were used to investigate the initial events of subcellular CD36 recycling prior to insulin resistance. The regulation of CD36 sarcolemmal translocation by lipid oversupply was assessed by insulin tolerance test (ITT), oral glucose tolerance test (OGTT), glucose/fatty acid uptake assay, surface CD36 and GLUT4 detection, and ELISA assays. To elucidate the underlying mechanisms, specific gene knockout, gene overexpression and/or gene inhibition were employed, followed by Western blot, co-immunoprecipitation, immunostaining, and kinase activity assay.

Results: Upon lipid/fatty acid overload, PKCζ activity and TBC1D1 phosphorylation were enhanced along with increased sarcolemmal CD36. The inhibition of PKCζ or TBC1D1 was shown to block fatty acid-induced CD36 translocation and was synergistic in impairing CD36 redistribution. Mechanically, we revealed that AMPK was located upstream of PKCζ to control its activity whereas Rac1 facilitated PKCζ translocation to the dorsal surface of the cell to cause actin remodeling. Furthermore, AMPK phosphorylated TBC1D1 to release retained cytosolic CD36. The activated PKCζ and phosphorylated TBC1D1 resulted in a positive feedback regulation of CD36 sarcolemmal translocation.

Conclusion: Collectively, our study demonstrated exclusively that lipid oversupply induced CD36 sarcolemmal translocation via dual modulation of PKCζ and TBC1D1, which was as an early event prior to insulin resistance. The acquired data may provide potential therapy targets to prevent lipid oversupply-induced insulin resistance.

Pcsk9 knockout exacerbates diet-induced non-alcoholic steatohepatitis, fibrosis and liver injury in mice

The fatty acid translocase, also known as CD36, is a well-established scavenger receptor for fatty acid (FA) uptake and is abundantly expressed in many metabolically active tissues. In the liver, CD36 is known to contribute to the progression of non-alcoholic fatty liver disease and to the more severe non-alcoholic steatohepatitis, by promoting triglyceride accumulation and subsequent lipid-induced endoplasmic reticulum (ER) stress. Given the recent discovery that the hepatocyte-secreted proprotein convertase subtilisin/kexin type 9 (PCSK9) blocks CD36 expression, we sought to investigate the role of PCSK9 in liver fat accumulation and injury in response to saturated FAs and in a mouse model of diet-induced hepatic steatosis.

Methods

In this study, we investigated the role of PCSK9 on the uptake and accumulation of FAs, as well as FA-induced toxicity, in a variety of cultured hepatocytes. Diet-induced hepatic steatosis and liver injury were also assessed in Pcsk9^-/- mice.

Results

Our results indicate that PCSK9 deficiency in cultured hepatocytes increased the uptake and accumulation of saturated and unsaturated FAs. In the presence of saturated FAs, PCSK9 also protected cultured hepatocytes from ER stress and cytotoxicity. In line with these findings, a metabolic challenge using a high-fat diet caused severe hepatic steatosis, ER stress inflammation and fibrosis in the livers of Pcsk9^-/- mice compared to controls. Given that inhibition of CD36 ablated the observed accumulation of lipid in vitro and in vivo, our findings also highlight CD36 as a strong contributor to steatosis and liver injury in the context of PCSK9 deficiency.

Conclusions

Collectively, our findings demonstrate that PCSK9 regulates hepatic triglyceride content in a manner dependent on CD36. In the presence of excess dietary fats, PCSK9 can also protect against hepatic steatosis and liver injury.

Lay summary

The proprotein convertase subtilisin/kexin type 9 (PCSK9) is a circulating protein known to reduce the abundance of receptors on the surface of liver cells charged with the task of lipid uptake from the circulation. Although PCSK9 deficiency is known to cause lipid accumulation in mice and in cultured cells, the toxicological implications of this observation have not yet been reported. In this study, we demonstrate that PCSK9 can protect against cytotoxicity in cultured liver cells treated with a saturated fatty acid and we also show that Pcsk9 knockout mice develop increased liver injury in response to a high-fat diet.

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PCSK9 reduces the expression of hepatic CD36 in mice.

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CD36 is a known driver of liver steatosis and injury.

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PCSK9 protects from palmitate-induced ER stress and ROS in cultured hepatocytes.

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High-fat diet causes severe hepatic steatosis, ER stress, inflammation and insulin resistance in Pcsk9^-/- mice.

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PCSK9 expression protects mice from diet-induced liver injury.

Meningeal Foam Cells and Ependymal Cells in Axolotl Spinal Cord Regeneration

A previously unreported population of foam cells (foamy macrophages) accumulates in the invasive fibrotic meninges during gap regeneration of transected adult Axolotl spinal cord (salamander Ambystoma mexicanum) and may act beneficially. Multinucleated giant cells (MNGCs) also occurred in the fibrotic meninges. Actin-label localization and transmission electron microscopy showed characteristic foam cell and MNGC podosome and ruffled border-containing sealing ring structures involved in substratum attachment, with characteristic intermediate filament accumulations surrounding nuclei. These cells co-localized with regenerating cord ependymal cell (ependymoglial) outgrowth. Phase contrast-bright droplets labeled with Oil Red O, DiI, and DyRect polar lipid live cell label showed accumulated foamy macrophages to be heavily lipid-laden, while reactive ependymoglia contained smaller lipid droplets. Both cell types contained both neutral and polar lipids in lipid droplets. Foamy macrophages and ependymoglia expressed the lipid scavenger receptor CD36 (fatty acid translocase) and the co-transporter toll-like receptor-4 (TLR4). Competitive inhibitor treatment using the modified fatty acid Sulfo-N-succinimidyl Oleate verified the role of the lipid scavenger receptor CD36 in lipid uptake studies in vitro. Fluoromyelin staining showed both cell types took up myelin fragments in situ during the regeneration process. Foam cells took up DiI-Ox-LDL and DiI-myelin fragments in vitro while ependymoglia took up only DiI-myelin in vitro. Both cell types expressed the cysteine proteinase cathepsin K, with foam cells sequestering cathepsin K within the sealing ring adjacent to the culture substratum. The two cell types act as sinks for Ox-LDL and myelin fragments within the lesion site, with foamy macrophages showing more Ox-LDL uptake activity. Cathepsin K activity and cellular localization suggested that foamy macrophages digest ECM within reactive meninges, while ependymal cells act from within the spinal cord tissue during outgrowth into the lesion site, acting in complementary fashion. Small MNGCs also expressed lipid transporters and showed cathepsin K activity. Comparison of ³H-glucosamine uptake in ependymal cells and foam cells showed that only ependymal cells produce glycosaminoglycan and proteoglycan-containing ECM, while the cathepsin studies showed both cell types remove ECM. Interaction of foam cells and ependymoglia in vitro supported the dispersion of ependymal outgrowth associated with tissue reconstruction in Axolotl spinal cord regeneration.

Essentiality of fatty acid synthase in the 2D to anchorage-independent growth transition in transforming cells

Upregulation of fatty acid synthase (FASN) is a common event in cancer, although its mechanistic and potential therapeutic roles are not completely understood. In this study, we establish a key role of FASN during transformation. FASN is required for eliciting the anaplerotic shift of the Krebs cycle observed in cancer cells. However, its main role is to consume acetyl-CoA, which unlocks isocitrate dehydrogenase (IDH)-dependent reductive carboxylation, producing the reductive power necessary to quench reactive oxygen species (ROS) originated during the switch from two-dimensional (2D) to three-dimensional (3D) growth (a necessary hallmark of cancer). Upregulation of FASN elicits the 2D-to-3D switch; however, FASN's synthetic product palmitate is dispensable for this process since cells satisfy their fatty acid requirements from the media. In vivo, genetic deletion or pharmacologic inhibition of FASN before oncogenic activation prevents tumor development and invasive growth. These results render FASN as a potential target for cancer prevention studies.

Scavenger receptor class B, type 1 facilitates cellular fatty acid uptake

SR-B1 belongs to the class B scavenger receptor, or CD36 super family. SR-B1 and CD36 share an affinity for a wide array of ligands. Although they exhibit similar ligand binding specificity, SR-B1 and CD36 have some very specific lipid transport functions. Whereas SR-B1 primarily facilitates the selective delivery of cholesteryl esters (CEs) and cholesterol from HDL particles to the liver and non-placental steroidogenic tissues, as well as participating in cholesterol efflux from cells, CD36 primarily mediates the uptake of long-chain fatty acids in high fatty acid-requiring organs such as the heart, skeletal muscle and adipose tissue. However, CD36 also mediates cholesterol efflux and facilitates selective lipoprotein-CE delivery, although less efficiently than SR-B1. Interestingly, the ability or efficiency of SR-B1 to mediate fatty acid uptake has not been reported. In this paper, using overexpression and siRNA-mediated knockdown of SR-B1, we show that SR-B1 possesses the ability to facilitate fatty acid uptake. Moreover, this function is not blocked by BLT-1, a specific chemical inhibitor of HDL-CE uptake activity of SR-B1, nor by sulfo-N-succinimidyl oleate, which inhibits fatty acid uptake by CD36. Attenuated fatty acid uptake was also observed in primary adipocytes isolated from SR-B1 knockout mice. In conclusion, facilitation of fatty acid uptake is an additional function that is mediated by SR-B1.

Fatty-acid receptor CD36 functions as a hydrogen sulfide-targeted receptor with its Cys333-Cys272 disulfide bond serving as a specific molecular switch to accelerate gastric cancer metastasis

BACKGROUND

Hydrogen Sulfide (H2S), a third member of gasotransmitter family along with nitric oxide (NO) and carbon monoxide (CO), exerts a wide range of cellular and molecular actions in our body. There is a large body of evidence suggesting that H2S plays an important role in cancer metastasis; however, the molecular mechanisms of H2S-mediated acceleration of cancer metastasis remain unknown.

METHODS

We examined the promote effects of H2S on phenotype of gastric cancer (GC) cells (including those of express wild type CD36 and mutant CD36) in vitro and in vivo. GC patients' samples were used for clinical translational significance evaluation.

FINDINGS

H2S triggered lipid metabolism reprogramming by significantly up-regulating the expression of the fatty-acid receptor CD36 (CD36) and directly activating CD36 in GC cells. Mechanistically, a disulfide bond located between cysteine (Cys)333 and Cys272 within the CD36 protein structure that was labile to H2S-mediated modification. The long chain-fatty acid (LC-FA) binding pocket was capped by a turn in the CD36 protein, located between helical and sheet structures that were stabilized by the Cys333-Cys272. This limited the secondary binding between LC-FAs and lysine (Lys)334. Breaking the Cys333-Cys272 disulfide bond restored the second LC-FA binding conformation of CD36. Targeting CD36 in vivo blocked H2S-promoted metastasis and improved animal survival.

INTERPRETATION

These findings identify that the Cys333-Cys272 disulfide bond disrupted the integrity of the second LC-FA binding conformation of CD36. Therefore, CD36 can directly activate LC-FA access to the cytoplasm by acting as a direct target molecule for H2S.

Increased uptake of oxLDL does not exert lipotoxic effects in insulin-secreting cells

Modified lipoproteins can negatively affect beta cell function and survival. However, the mechanisms behind interactions of modified lipoproteins with beta cells - and in particular, relationships to increased uptake - are only partly clarified. By over-expressing the scavenger receptor CD36 (Tet-on), we increased the uptake of fluorescent low-density modified lipoprotein (oxLDL) into insulin-secreting INS-1 cells. The magnitude of uptake followed the degree of CD36 over-expression. CD36 over-expression increased concomitant efflux of 3H-cholesterol in proportion to the cellular contents of 3H-cholesterol. Exposure to concentrations of oxLDL from 20 to 100 µg/mL dose-dependently increased toxicity (evaluated by MTT) as well as apoptosis. However, the increased uptake of oxLDL due to CD36 over-expression did not exert additive effects on oxLDL toxicity - neither on viability, nor on glucose-induced insulin release and cellular content. Reciprocally, blocking CD36 receptors by Sulfo-N-Succinimidyl Oleate decreased the uptake of oxLDL but did not diminish the toxicity. Pancreatic islets of CD36-/- mice displayed reduced uptake of 3H-cholesterol-labeled oxLDL vs wild type but similar toxicity to oxLDL. OxLDL was found to increase the expression of CD36 in islets and INS-1 cells. In summary, given the experimental conditions, our results indicate that (1) increased uptake of oxLDL is not responsible for toxicity of oxLDL, (2) increased efflux of the cholesterol moiety of oxLDL counterbalances, at least in part, increased uptake and (3) oxLDL participates in the regulation of CD36 in pancreatic islets and in INS-1 cells.

HIF-1α Is a Metabolic Switch between Glycolytic-Driven Migration and Oxidative Phosphorylation-Driven Immunosuppression of Tregs in Glioblastoma

The mechanisms by which regulatory T cells (Tregs) migrate to and function within the hypoxic tumor microenvironment are unclear. Our studies indicate that specific ablation of hypoxia-inducible factor 1α (HIF-1α) in Tregs results in enhanced CD8+ T cell suppression versus wild-type Tregs under hypoxia, due to increased pyruvate import into the mitochondria. Importantly, HIF-1α-deficient Tregs are minimally affected by the inhibition of lipid oxidation, a fuel that is critical for Treg metabolism in tumors. Under hypoxia, HIF-1α directs glucose away from mitochondria, leaving Tregs dependent on fatty acids for mitochondrial metabolism within the hypoxic tumor. Indeed, inhibition of lipid oxidation enhances the survival of mice with glioma. Interestingly, HIF-1α-deficient-Treg mice exhibit significantly enhanced animal survival in a murine model of glioma, due to their stymied migratory capacity, explaining their reduced abundance in tumor-bearing mice. Thus HIF-1α acts as a metabolic switch for Tregs between glycolytic-driven migration and oxidative phosphorylation-driven immunosuppression.

oxLDL inhibits differentiation of mesenchymal stem cells into osteoblasts via the CD36 mediated suppression of Wnt signaling pathway

Bone abnormalities as a consequence of osteoblast deregulation are associated with several diseases such as diabetes and chronic kidney disease. Important role for oxidized low density lipoproteins (oxLDL) in the pathophysiology of bone disorders has been reported. However, little is known about the effects and mechanisms of oxLDL on the process of osteoblastogenesis in human mesenchymal stem cells (MSCs). We show that oxLDL concentrations of ~ 10-25 µg protein (0.43-1.0 µM MDA/mg protein) inhibited the differentiation of MSCs to osteoblasts. We demonstrate that the underlying mechanism entails the suppression of the Wnt signaling through the down-regulation of β-catenin. Further, we show the association of scavenger receptor CD36 with the receptors LRP5/6 and Frizzled in mediating the oxLDL effects on the differentiation of MSCs to pre-osteoblasts. Inhibiting CD36 restored osteoblasts differentiation in the presence of oxLDL. Our findings suggest that oxLDL interferes with the canonical Wnt signaling pathway in a CD36 dependent manner leading to an inhibition of osteoblastogenesis.

CD36 mediates albumin transcytosis by dermal but not lung microvascular endothelial cells: role in fatty acid delivery

In healthy blood vessels, albumin crosses the endothelium to leave the circulation by transcytosis. However, little is known about the regulation of albumin transcytosis or how it differs in different tissues; its physiological purpose is also unclear. Using total internal reflection fluorescence microscopy, we quantified transcytosis of albumin across primary human microvascular endothelial cells from both lung and skin. We then validated our in vitro findings using a tissue-specific knockout mouse model. We observed that albumin transcytosis was saturable in the skin but not the lung microvascular endothelial cells, implicating a receptor-mediated process. We identified the scavenger receptor CD36 as being both necessary and sufficient for albumin transcytosis across dermal microvascular endothelium, in contrast to the lung where macropinocytosis dominated. Mutations in the apical helical bundle of CD36 prevented albumin internalization by cells. Mice deficient in CD36 specifically in endothelial cells exhibited lower basal permeability to albumin and less basal tissue edema in the skin but not in the lung. Finally, these mice also exhibited a smaller subcutaneous fat layer despite having identical total body weights and circulating fatty acid levels as wild-type animals. In conclusion, CD36 mediates albumin transcytosis in the skin but not the lung. Albumin transcytosis may serve to regulate fatty acid delivery from the circulation to tissues.

Impairment of Angiogenesis by Fatty Acid Synthase Inhibition Involves mTOR Malonylation

The role of fatty acid synthesis in endothelial cells (ECs) remains incompletely characterized. We report that fatty acid synthase knockdown (FASN^KD) in ECs impedes vessel sprouting by reducing proliferation. Endothelial loss of FASN impaired angiogenesis in vivo, while FASN blockade reduced pathological ocular neovascularization, at >10-fold lower doses than used for anti-cancer treatment. Impaired angiogenesis was not due to energy stress, redox imbalance, or palmitate depletion. Rather, FASN^KD elevated malonyl-CoA levels, causing malonylation (a post-translational modification) of mTOR at lysine 1218 (K1218). mTOR K-1218 malonylation impaired mTOR complex 1 (mTORC1) kinase activity, thereby reducing phosphorylation of downstream targets (p70S6K/4EBP1). Silencing acetyl-CoA carboxylase 1 (an enzyme producing malonyl-CoA) normalized malonyl-CoA levels and reactivated mTOR in FASN^KD ECs. Mutagenesis unveiled the importance of mTOR K1218 malonylation for angiogenesis. This study unveils a novel role of FASN in metabolite signaling that contributes to explaining the anti-angiogenic effect of FASN blockade.

Intestinal Saturated Long-Chain Fatty Acid, Glucose and Fructose Transporters and Their Inhibition by Natural Plant Extracts in Caco-2 Cells

The intestinal absorption of fatty acids, glucose and fructose is part of the basic requirements for the provision of energy in the body. High access of saturated long-chain fatty acids (LCFA), glucose and fructose can facilitate the development of metabolic diseases, particularly the metabolic syndrome and type-2 diabetes mellitus (T2DM). Research has been done to find substances which decelerate or inhibit intestinal resorption of these specific food components. Promising targets are the inhibition of intestinal long-chain fatty acid (FATP2, FATP4), glucose (SGLT1, GLUT2) and fructose (GLUT2, GLUT5) transporters by plant extracts and by pure substances. The largest part of active components in plant extracts belongs to the group of polyphenols. This review summarizes the knowledge about binding sites of named transporters and lists the plant extracts which were tested in Caco-2 cells regarding uptake inhibition.

Epigenetic-Transcriptional Regulation of Fatty Acid Metabolism and Its Alterations in Leukaemia

In recent years fatty acid metabolism has gained greater attention in haematologic cancers such as acute myeloid leukaemia. The oxidation of fatty acids provides fuel in the form of ATP and NADH, while fatty acid synthesis provides building blocks for cellular structures. Here, we will discuss how leukaemic cells differ from healthy cells in their increased reliance on fatty acid metabolism. In order to understand how these changes are achieved, we describe the main pathways regulating fatty acid metabolism at the transcriptional level and highlight the limited knowledge about related epigenetic mechanisms. We explore these mechanisms in the context of leukaemia and consider the relevance of the bone marrow microenvironment in disease management. Finally, we discuss efforts to interfere with fatty acid metabolism as a therapeutic strategy along with the use of metabolic parameters as biomarkers.

Palmitate aggravates proteinuria-induced cell death and inflammation via CD36-inflammasome axis in the proximal tubular cells of obese mice

High levels of serum free fatty acids (FFAs) and proteinuria have been implicated in the pathogenesis of obesity-related nephropathy. CD36, a class B scavenger receptor, is highly expressed in the renal proximal tubules and mediates FFA uptake. It is not clear whether FFA- and proteinuria-mediated CD36 activation coordinates NLRP3 inflammasomes to induce renal tubular injury and inflammation. In this study, we investigated the roles of CD36 and NLRP3 inflammasomes in FFA-induced renal injury in high-fat diet (HFD)-induced obesity. HFD-fed C57BL/6 mice and palmitate-treated HK2 renal tubular cells were used as in vivo and in vitro models. Immunohistochemical staining showed that CD36, IL-1β, and IL-18 levels increased progressively in the kidneys of HFD-fed mice. Sulfo- N-succinimidyl oleate (SSO), a CD36 inhibitor, attenuated the HFD-induced upregulation of NLRP3, IL-1β, and IL-18 and suppressed the colocalization of NLRP3 and ASC in renal tubular cells. In vitro, SSO abolished the palmitate-induced activation of IL-1β, IL-18, and caspase-1 in HK2 proximal tubular cells. Furthermore, treatment with SSO and the knockdown of caspase-1 expression by siRNA both inhibited palmitate-induced cell death and apoptosis in HK2 cells. Collectively, palmitate causes renal tubular inflammation, cell death, and apoptosis via the CD36/NLRP3/caspase-1 axis, which may explain, at least in part, the mechanism underlying FFA-related renal tubular injury. The blockade of CD36-induced cellular processes is therefore a promising strategy for treating obesity-related nephropathy.

Comparative analysis of endothelial cell and sub-endothelial cell elastic moduli in young and aged mice: Role of CD36

OBJECTIVE

To perform comparative analysis of the role of scavenger receptor CD36 on endothelial vs. sub-endothelial elastic modulus (stiffness) in the aortas of young and aged mice.

APPROACHES AND RESULTS

Elastic moduli of endothelial and sub-endothelial layers of freshly isolated mouse aortas were quantified using atomic force microscopy. In young mice (4-6 months old), we found that while endothelial stiffness is markedly reduced in aortas of CD36^-/-mice, as compared to WT controls, no difference between CD36^-/- and WT aortas is observed in the stiffness of the sub-endothelial layer in denuded arteries. Additionally, inhibition of myosin phosphorylation also decreases the elastic modulus in the EC, but not the sub-EC layer in WT mice. Moreover, inhibiting CD36 mediated uptake of oxLDL in intact WT aortas abrogated oxLDL-induced endothelial stiffening. Further analysis of aged mice (22-25 months) revealed that aging resulted not only in significant stiffening of the denuded arteries, as was previously known, but also a comparable increase in the elastic modulus of the endothelial layer. Most significantly, this stiffening in the EC layer is dependent on CD36, whereas the denuded layer is not affected.

CONCLUSIONS

Our results show that the role CD36 in stiffening of cellular components of intact aortas is endothelial-specific and that genetic deficiency of CD36 protects against endothelial stiffening in aged mice. Moreover, these data suggest that endothelial stiffness in intact mouse aortas depends more on the expression of CD36 than on the stiffness of the sub-endothelial layer.

STAT3-activated CD36 facilitates fatty acid uptake in chronic lymphocytic leukemia cells

Although several studies established that unlike normal B cells chronic lymphocytic leukemia (CLL) cells metabolize fatty acids (FA), how CLL cells internalize FA is poorly understood. Because in various cell types CD36 facilitates FA uptake, we wondered whether a similar mechanism is operative CLL. We found that CD36 levels are higher in CLL cells than in normal B cells, and that small interfering RNA, CD36 neutralizing antibodies or sulfosuccinimidyl oleate (SSO) that inhibits CD36 significantly reduced the oxygen consumption of CLL cells incubated with FA. Because CD36 is oeverexpressed and STAT3 is constitutively activated in CLL cells, we wondered whether STAT3 induces CD36 expression. Sequence analysis identified putative STAT3 binding sites in the CD36 gene promoter. Chromatin immunoprecipitation and an electrophoretic mobility shift assay revealed that STAT3 binds to the CD36 gene promoter. A luciferase assay and STAT3-small hairpin RNA, that significantly decreased the levels of CD36 in CLL cells, established that STAT3 activates the transcription of the CD36 gene. Furthermore, SSO induced a dose-dependent apoptosis of CLL cells. Taken together, our data suggest that STAT3 activates CD36 and that CD36 facilitates FA uptake in CLL cells. Whether CD36 inhibition would provide clinical benefits in CLL remains to be determined.

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.

The rs1527483, but not rs3212018, CD36 polymorphism associates with linoleic acid detection and obesity in Czech young adults

Recent evidence has raised the possibility of the existence of a sixth taste modality – that is, taste for fat – which is mediated by lingual CD36 and plays a role in obesity. Consequently, the genetic polymorphism of CD36 has been shown to be associated with altered oro-sensory detection of dietary lipids. In the present study, we investigated the relationship between oro-sensory perception of linoleic acid (LA), two CD36 polymorphisms (rs1527483 and rs3212018), obesity parameters and craving habits for dietary lipids in young Czech adults. We also sequenced 5 and 6 exons of CD36 to trace out any new mutation that might be responsible for the difference in taste perception. We observed that craving for dietary lipids was correlated with anthropometric parameters (P<0·05) and LA detection threshold (P=0·033). The participants with the CC genotype of the rs1527483 polymorphism had lower BMI (P=0·011), waist circumference (P=0·005), waist:height ratio (P=0·010) and higher sensitivity for LA (P=0·037) than the participants with the CT and TT genotypes. Interestingly, we did not observe any association between the rs3212018 polymorphism and the studied parameters. Moreover, we did not observe any mutation in exons 5 and 6 of the CD36 gene in these subjects. Finally, we can state that rs1527483, but not rs3212018, is associated with high body weight in young Czech subjects.

CD36 mediates lipid accumulation in pancreatic beta cells under the duress of glucolipotoxic conditions: Novel roles of lysine deacetylases

The cluster of differentiation 36 (CD36) is implicated in the intake of long-chain fatty acids and fat storage in various cell types including the pancreatic beta cell, thus contributing to the pathogenesis of metabolic stress and diabetes. Recent evidence indicates that CD36 undergoes post-translational modifications such as acetylation-deacetylation. However, putative roles of such modifications in its functional activation and onset of beta cell dysregulation under the duress of glucolipotoxicity (GLT) remain largely unknown. Using pharmacological approaches, we validated, herein, the hypothesis that acetylation-deacetylation signaling steps are involved in CD36-mediated lipid accumulation and downstream apoptotic signaling in pancreatic beta (INS-1832/13) cells under GLT. Exposure of these cells to GLT resulted in significant lipid accumulation without affecting the CD36 expression. Sulfo-n-succinimidyl oleate (SSO), an irreversible inhibitor of CD36, significantly attenuated lipid accumulation under GLT conditions, thus implicating CD36 in this metabolic step. Furthermore, trichostatin A (TSA) or valproic acid (VPA), known inhibitors of lysine deacetylases, markedly suppressed GLT-associated lipid accumulation with no discernible effects on CD36 expression. Lastly, SSO or TSA prevented caspase 3 activation in INS-1832/13 cells exposed to GLT conditions. Based on these findings, we conclude that an acetylation-deacetylation signaling step might regulate CD36 functional activity and subsequent lipid accumulation and caspase 3 activation in pancreatic beta cells exposed to GLT conditions. Identification of specific lysine deacetylases that control CD36 function should provide novel clues for the prevention of beta-cell dysfunction under GLT.

Microfluidic systems for studying dynamic function of adipocytes and adipose tissue

Recent breakthroughs in organ-on-a-chip and related technologies have highlighted the extraordinary potential for microfluidics to not only make lasting impacts in the understanding of biological systems but also to create new and important in vitro culture platforms. Adipose tissue (fat), in particular, is one that should be amenable to microfluidic mimics of its microenvironment. While the tissue was traditionally considered important only for energy storage, it is now understood to be an integral part of the endocrine system that secretes hormones and responds to various stimuli. As such, adipocyte function is central to the understanding of pathological conditions such as obesity, diabetes, and metabolic syndrome. Despite the importance of the tissue, only recently have significant strides been made in studying dynamic function of adipocytes or adipose tissues on microfluidic devices. In this critical review, we highlight new developments in the special class of microfluidic systems aimed at culture and interrogation of adipose tissue, a sub-field of microfluidics that we contend is only in its infancy. We close by reflecting on these studies as we forecast a promising future, where microfluidic technologies should be capable of mimicking the adipose tissue microenvironment and provide novel insights into its physiological roles in the normal and diseased states. Graphical abstract This critical review focuses on recent developments and challenges in applying microfluidic systems to the culture and analysis of adipocytes and adipose tissue.

Lysosomal integral membrane protein-2 as a phospholipid receptor revealed by biophysical and cellular studies

Lysosomal integral membrane protein-2 (LIMP-2/SCARB2) contributes to endosomal and lysosomal function. LIMP-2 deficiency is associated with neurological abnormalities and kidney failure and, as an acid glucocerebrosidase receptor, impacts Gaucher and Parkinson's diseases. Here we report a crystal structure of a LIMP-2 luminal domain dimer with bound cholesterol and phosphatidylcholine. Binding of these lipids alters LIMP-2 from functioning as a glucocerebrosidase-binding monomer toward a dimeric state that preferentially binds anionic phosphatidylserine over neutral phosphatidylcholine. In cellular uptake experiments, LIMP-2 facilitates transport of phospholipids into murine fibroblasts, with a strong substrate preference for phosphatidylserine. Taken together, these biophysical and cellular studies define the structural basis and functional importance of a form of LIMP-2 for lipid trafficking. We propose a model whereby switching between monomeric and dimeric forms allows LIMP-2 to engage distinct binding partners, a mechanism that may be shared by SR-BI and CD36, scavenger receptor proteins highly homologous to LIMP-2.

Sulfosuccinimidyl oleate sodium is neuroprotective and alleviates stroke-induced neuroinflammation

BACKGROUND

Ischemic stroke is one of the main causes of death and disability worldwide. It is caused by the cessation of cerebral blood flow resulting in the insufficient delivery of glucose and oxygen to the neural tissue. The inflammatory response initiated by ischemic stroke in order to restore tissue homeostasis in the acute phase of stroke contributes to delayed brain damage.

METHODS

By using in vitro models of neuroinflammation and in vivo model of permanent middle cerebral artery occlusion, we demonstrate the neuroprotective and anti-inflammatory effects of sulfosuccinimidyl oleate sodium (SSO).

RESULTS

SSO significantly reduced the lipopolysaccharide/interferon-γ-induced production of nitric oxide, interleukin-6 and tumor necrosis factor-α, and the protein levels of inflammatory enzymes including nitric oxide synthase 2, cyclooxygenase-2 (COX-2), and p38 mitogen-activated protein kinase (MAPK) in microglia, without causing cell toxicity. Although SSO failed to directly alleviate glutamate-induced excitotoxicity in murine cortical neurons, it prevented inflammation-induced neuronal death in microglia-neuron co-cultures. Importantly, oral administration of SSO in Balb/c mice subjected to permanent occlusion of the middle cerebral artery reduced microglial activation in the peri-ischemic area and attenuated brain damage. This in vivo neuroprotective effect of SSO was associated with a reduction in the COX-2 and heme oxygenase-1 immunoreactivities.

CONCLUSIONS

Our results suggest that SSO is an anti-inflammatory and a possible therapeutic candidate in diseases such as stroke where inflammation is a central hallmark.