Adipocytes recognize and degrade oxidized low density lipoprotein through CD36

Obesity, dyslipidemia, and cardiovascular disease: A joint expert review from the Obesity Medicine Association and the National Lipid Association 2024

Background

This joint expert review by the Obesity Medicine Association (OMA) and National Lipid Association (NLA) provides clinicians an overview of the pathophysiologic and clinical considerations regarding obesity, dyslipidemia, and cardiovascular disease (CVD) risk.

Methods

This joint expert review is based upon scientific evidence, clinical perspectives of the authors, and peer review by the OMA and NLA leadership.

Results

Among individuals with obesity, adipose tissue may store over 50% of the total body free cholesterol. Triglycerides may represent up to 99% of lipid species in adipose tissue. The potential for adipose tissue expansion accounts for the greatest weight variance among most individuals, with percent body fat ranging from less than 5% to over 60%. While population studies suggest a modest increase in blood low-density lipoprotein cholesterol (LDL-C) levels with excess adiposity, the adiposopathic dyslipidemia pattern most often described with an increase in adiposity includes elevated triglycerides, reduced high density lipoprotein cholesterol (HDL-C), increased non-HDL-C, elevated apolipoprotein B, increased LDL particle concentration, and increased small, dense LDL particles.

Conclusions

Obesity increases CVD risk, at least partially due to promotion of an adiposopathic, atherogenic lipid profile. Obesity also worsens other cardiometabolic risk factors. Among patients with obesity, interventions that reduce body weight and improve CVD outcomes are generally associated with improved lipid levels. Given the modest improvement in blood LDL-C with weight reduction in patients with overweight or obesity, early interventions to treat both excess adiposity and elevated atherogenic cholesterol (LDL-C and/or non-HDL-C) levels represent priorities in reducing the risk of CVD.

Obesity, dyslipidemia, and cardiovascular disease: A joint expert review from the Obesity Medicine Association and the National Lipid Association 2024

BACKGROUND

This joint expert review by the Obesity Medicine Association (OMA) and National Lipid Association (NLA) provides clinicians an overview of the pathophysiologic and clinical considerations regarding obesity, dyslipidemia, and cardiovascular disease (CVD) risk.

METHODS

This joint expert review is based upon scientific evidence, clinical perspectives of the authors, and peer review by the OMA and NLA leadership.

RESULTS

Among individuals with obesity, adipose tissue may store over 50% of the total body free cholesterol. Triglycerides may represent up to 99% of lipid species in adipose tissue. The potential for adipose tissue expansion accounts for the greatest weight variance among most individuals, with percent body fat ranging from less than 5% to over 60%. While population studies suggest a modest increase in blood low-density lipoprotein cholesterol (LDL-C) levels with excess adiposity, the adiposopathic dyslipidemia pattern most often described with an increase in adiposity includes elevated triglycerides, reduced high-density lipoprotein cholesterol (HDL-C), increased non-HDL-C, elevated apolipoprotein B, increased LDL particle concentration, and increased small, dense LDL particles.

CONCLUSIONS

Obesity increases CVD risk, at least partially due to promotion of an adiposopathic, atherogenic lipid profile. Obesity also worsens other cardiometabolic risk factors. Among patients with obesity, interventions that reduce body weight and improve CVD outcomes are generally associated with improved lipid levels. Given the modest improvement in blood LDL-C with weight reduction in patients with overweight or obesity, early interventions to treat both excess adiposity and elevated atherogenic cholesterol (LDL-C and/or non-HDL-C) levels represent priorities in reducing the risk of CVD.

Feeding-induced hepatokines and crosstalk with multi-organ: A novel therapeutic target for Type 2 diabetes

Hyperglycemia, which can be caused by either an insulin deficit and/or insulin resistance, is the main symptom of Type 2 diabetes, a significant endocrine metabolic illness. Conventional medications, including insulin and oral antidiabetic medicines, can alleviate the signs of diabetes but cannot restore insulin release in a physiologically normal amount. The liver detects and reacts to shifts in the nutritional condition that occur under a wide variety of metabolic situations, making it an essential organ for maintaining energy homeostasis. It also performs a crucial function in glucolipid metabolism through the secretion of hepatokines. Emerging research shows that feeding induces hepatokines release, which regulates glucose and lipid metabolism. Notably, these feeding-induced hepatokines act on multiple organs to regulate glucolipotoxicity and thus influence the development of T2DM. In this review, we focus on describing how feeding-induced cross-talk between hepatokines, including Adropin, Manf, Leap2 and Pcsk9, and metabolic organs (e.g.brain, heart, pancreas, and adipose tissue) affects metabolic disorders, thus revealing a novel approach for both controlling and managing of Type 2 diabetes as a promising medication.

Oxidized LDLs as Signaling Molecules

Levels of oxidized low-density lipoproteins (oxLDLs) are usually low in vivo but can increase whenever the balance between formation and scavenging of free radicals is impaired. Under normal conditions, uptake and degradation represent the physiological cellular response to oxLDL exposure. The uptake of oxLDLs is mediated by cell surface scavenger receptors that may also act as signaling molecules. Under conditions of atherosclerosis, monocytes/macrophages and vascular smooth muscle cells highly exposed to oxLDLs tend to convert to foam cells due to the intracellular accumulation of lipids. Moreover, the atherogenic process is accelerated by the increased expression of the scavenger receptors CD36, SR-BI, LOX-1, and SRA in response to high levels of oxLDL and oxidized lipids. In some respects, the effects of oxLDLs, involving cell proliferation, inflammation, apoptosis, adhesion, migration, senescence, and gene expression, can be seen as an adaptive response to the rise of free radicals in the vascular system. Unlike highly reactive radicals, circulating oxLDLs may signal to cells at more distant sites and possibly trigger a systemic antioxidant defense, thus elevating the role of oxLDLs to that of signaling molecules with physiological relevance.

White Adipose Tissue Surface Expression of LDLR and CD36 is Associated with Risk Factors for Type 2 Diabetes in Adults with Obesity

OBJECTIVE

Human conditions with upregulated receptor uptake of low-density lipoproteins (LDL) are associated with diabetes risk, the reasons for which remain unexplored. LDL induce metabolic dysfunction in murine adipocytes. Thus, it was hypothesized that white adipose tissue (WAT) surface expression of LDL receptor (LDLR) and/or CD36 is associated with WAT and systemic metabolic dysfunction. Whether WAT LDLR and CD36 expression is predicted by plasma lipoprotein-related parameters was also explored.

METHODS

This was a cross-sectional analysis of 31 nondiabetic adults (BMI > 25 kg/m² ) assessed for WAT surface expression of LDLR and CD36 (immunohistochemistry), WAT function, WAT and systemic inflammation, postprandial fat metabolism, and insulin resistance (IR; hyperinsulinemic-euglycemic clamp).

RESULTS

Fasting WAT surface expression of LDLR and CD36 was negatively associated with WAT function (³ H-triglyceride storage, r = -0.45 and -0.66, respectively) and positively associated with plasma IL-1 receptor antagonist (r = 0.64 and 0.43, respectively). Their expression was suppressed 4 hours postprandially, and reduced LDLR was further associated with IR (M/I_clamp , r = 0.61 women, r = 0.80 men). Plasma apolipoprotein B (apoB)-to-PCSK9 ratio predicted WAT surface expression of LDLR and CD36, WAT dysfunction, WAT NLRP3 inflammasome priming and disrupted cholesterol-sensing genes, and systemic IR independent of sex and body composition.

CONCLUSIONS

Higher fasting and lower postprandial WAT surface expression of LDLR and CD36 is associated with WAT dysfunction, systemic inflammation, and IR in adults with overweight/obesity, anomalies that are predicted by higher plasma apoB-to-PCSK9 ratio.

Recent Progress on Lipid Intake and Chronic Kidney Disease

The incidence of chronic kidney disease (CKD) is associated with major abnormalities in circulating lipoproteins and renal lipid metabolism. This article elaborates on the mechanisms of CKD and lipid uptake abnormalities. The viewpoint we supported is that lipid abnormalities directly cause CKD, resulting in forming a vicious cycle. On the theoretical and experiment fronts, this inference has been verified by elaborately elucidating the role of lipid intake and accumulation as well as their influences on CKD. Taken together, these findings suggest that further understanding of lipid metabolism in CKD may lead to novel therapeutic approaches.

Oxidized LDL Modify the Human Adipocyte Phenotype to an Insulin Resistant, Proinflamatory and Proapoptotic Profile

Little information exists in humans on the regulation that oxidized low-density lipoprotein (oxLDL) exerts on adipocyte metabolism, which is associated with obesity and type 2 diabetes. The aim was to analyze the oxLDL effects on adipocytokine secretion and scavenger receptors (SRs) and cell death markers in human visceral adipocytes. Human differentiated adipocytes from visceral adipose tissue from non-obese and morbidly obese subjects were incubated with increasing oxLDL concentrations. mRNA expression of SRs, markers of apoptosis and autophagy, secretion of adipocytokines, and glucose uptake were analyzed. In non-obese and in morbidly obese subjects, oxLDL produced a decrease in insulin-induced glucose uptake, a significant dose-dependent increase in tumor necrosis factor-α (TNF-α), IL-6, and adiponectin secretion, and a decrease in leptin secretion. OxLDL produced a significant increase of Lox-1 and a decrease in Cxcl16 and Cl-p1 expression. The expression of Bnip3 (marker of apoptosis, necrosis and autophagy) was significantly increased and Bcl2 (antiapoptotic marker) was decreased. OxLDL could sensitize adipocytes to a lower insulin-induced glucose uptake, a more proinflammatory phenotype, and could modify the gene expression involved in apoptosis, autophagy, necrosis, and mitophagy. OxLDL can upregulate Lox-1, and this could lead to a possible amplification of proinflammatory and proapoptotic effects of oxLDL.

LDL, LDL receptors, and PCSK9 as modulators of the risk for type 2 diabetes: a focus on white adipose tissue

Type 2 diabetes (T2D) and cardiovascular disease (CVD) share many risk factors such as obesity, unhealthy lifestyle, and metabolic syndrome, whose accumulation over years leads to disease onset. However, while lowering plasma low-density lipoprotein cholesterol (LDLC) is cardio-protective, novel evidence have recognised a role for common LDLC-lowering variants (e.g. in HMGCR, PCSK9, and LDLR) and widely used hypocholesterolemic drugs that mimic the effects of some of these variants (statins) in higher risk for T2D. As these conditions decrease plasma LDLC by increasing tissue-uptake of LDL, a role for LDL receptor (LDLR) pathway was proposed. While underlying mechanisms remain to be fully elucidated, work from our lab reported that native LDL directly provoke the dysfunction of human white adipose tissue (WAT) and the activation of WAT NLRP3 (Nucleotide-binding domain and Leucine-rich repeat Receptor, containing a Pyrin domain 3) inflammasome, which play a major role in the etiology of T2D. However, while elevated plasma numbers of apolipoprotein B (apoB)-containing lipoproteins (measured as apoB, mostly as LDL) is associated with WAT dysfunction and related risk factors for T2D in our cohort, this relation was strengthened in regression analysis by lower plasma proprotein convertase subtilisin/kexin type 9 (PCSK9). This supports a central role for upregulated pathway of LDLR and/or other receptors regulated by PCSK9 such as cluster of differentiation 36 (CD36) in LDL-induced anomalies. Targeting receptor-mediated uptake of LDL into WAT may reduce WAT inflammation, WAT dysfunction, and related risk for T2D without increasing the risk for CVD.

Role of the COP9 Signalosome (CSN) in Cardiovascular Diseases

The constitutive photomorphogenesis 9 (COP9) signalosome (CSN) is an evolutionarily conserved multi-protein complex, consisting of eight subunits termed CSN1-CSN8. The main biochemical function of the CSN is the control of protein degradation via the ubiquitin-proteasome-system through regulation of cullin-RING E3-ligase (CRL) activity by deNEDDylation of cullins, but the CSN also serves as a docking platform for signaling proteins. The catalytic deNEDDylase (isopeptidase) activity of the complex is executed by CSN5, but only efficiently occurs in the three-dimensional architectural context of the complex. Due to its positioning in a central cellular pathway connected to cell responses such as cell-cycle, proliferation, and signaling, the CSN has been implicated in several human diseases, with most evidence available for a role in cancer. However, emerging evidence also suggests that the CSN is involved in inflammation and cardiovascular diseases. This is both due to its role in controlling CRLs, regulating components of key inflammatory pathways such as nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), and complex-independent interactions of subunits such as CSN5 with inflammatory proteins. In this case, we summarize and discuss studies suggesting that the CSN may have a key role in cardiovascular diseases such as atherosclerosis and heart failure. We discuss the implicated molecular mechanisms ranging from inflammatory NF-κB signaling to proteotoxicity and necrosis, covering disease-relevant cell types such as myeloid and endothelial cells or cardiomyocytes. While the CSN is considered to be disease-exacerbating in most cancer entities, the cardiovascular studies suggest potent protective activities in the vasculature and heart. The underlying mechanisms and potential therapeutic avenues will be critically discussed.

Walnuts change lipoprotein composition suppressing TNFα-stimulated cytokine production by diabetic adipocyte

Walnut consumption can provide both vascular and metabolic health benefits, and walnut-induced changes in lipoprotein particle chemical payloads may be responsible for these health benefits. To explore this possibility with a focus on metabolic health, this study investigated the impact of walnut consumption on lipoprotein lipid composition and changes in LDL anti-inflammatory properties, as reported by inflamed adipocyte. Hypercholesterolemic, postmenopausal females were treated with 40 g/day (i.e., 1.6 servings/day; n=15) of walnuts for 4 weeks. Fatty acids and their oxygenated metabolites, i.e., oxylipins, were quantified in isolated lipoproteins. Human primary adipocytes were exposed to LDL and TNFα-stimulated adipokine production was measured. Walnut treatment elevated α-linolenic acid and its epoxides in all lipoproteins and depleted mid-chain alcohols in VLDL and LDL, but not HDL. Walnuts also reduced TNFα-induced diabetic adipocyte production of IL-6 (-48%, P=.0006) and IL-8 (-30%, P=.01), changes inversely correlated with levels of α-linolenic acid-derived epoxides but not α-linolenic acid itself. In conclusion, modest walnut consumption can alter lipoprotein lipid profiles and enhance their ability to inhibit TNFα-dependent pro-inflammatory responses in human diabetic primary adipocytes. Moreover, this study suggests the oxylipins, rather than the parent fatty acids, mediate LDL action of adipocytes.

Overexpression of scavenger receptor and infiltration of macrophage in epicardial adipose tissue of patients with ischemic heart disease and diabetes

Background

Oxidized low-density lipoproteins and scavenger receptors (SRs) play an important role in the formation and development of atherosclerotic plaques. However, little is known about their presence in epicardial adipose tissue (EAT). The objective of the study was to evaluate the mRNA expression of different SRs in EAT of patients with ischemic heart disease (IHD), stratifying by diabetes status and its association with clinical and biochemical variables.

Methods

We analyzed the mRNA expression of SRs (LOX-1, MSR1, CXCL16, CD36 and CL-P1) and macrophage markers (CD68, CD11c and CD206) in EAT from 45 patients with IHD (23 with type 2 diabetes mellitus (T2DM) and 22 without T2DM) and 23 controls without IHD or T2DM.

Results

LOX-1, CL-P1, CD68 and CD11c mRNA expression were significantly higher in diabetic patients with IHD when compared with those without T2DM and control patients. MSR1, CXCL16, CD36 and CD206 showed no significant differences. In IHD patients, LOX-1 (OR 2.9; 95% CI 1.6–6.7; P = 0.019) and CD68 mRNA expression (OR 1.7; 95% CI 0.98–4.5; P = 0.049) were identified as independent risk factors associated with T2DM. Glucose and glycated hemoglobin were also shown to be risk factors.

Conclusions

SRs mRNA expression is found in EAT. LOX-1 and CD68 and were higher in IHD patients with T2DM and were identified as a cardiovascular risk factor of T2DM. This study suggests the importance of EAT in coronary atherosclerosis among patients with T2DM.

The Association of Polyunsaturated Fatty Acid δ-5-Desaturase Activity with Risk Factors for Type 2 Diabetes Is Dependent on Plasma ApoB-Lipoproteins in Overweight and Obese Adults

Background

δ-5 and δ-6 desaturases (D5D and D6D) catalyze the endogenous conversion of n-3 (ω-3) and n-6 (ω-6) polyunsaturated fatty acids (PUFAs). Their activities are negatively and positively associated with type 2 diabetes (T2D), respectively, by unclear mechanisms. Elevated plasma apoB-lipoproteins (measured as plasma apoB), which can be reduced by n-3 PUFA intake, promote T2D risk factors.

Objective

The aim of this study was to test the hypothesis that the association of D5D and D6D activities with T2D risk factors is dependent on plasma apoB.

Methods

This is a pooled analysis of 2 populations recruited for 2 different metabolic studies. It is a post hoc analysis of baseline data of these subjects [n = 98; 60% women (postmenopausal); mean ± SD body mass index (in kg/m2): 32.8 ± 4.7; mean ± SD age: 57.6 ± 6.3 y]. Glucose-induced insulin secretion (GIIS) and insulin sensitivity (IS) were measured using Botnia clamps. Plasma clearance of a high-fat meal (600 kcal/m2, 66% fat) and white adipose tissue (WAT) function (storage of 3H-triolein-labeled substrate) were assessed in a subpopulation (n = 47). Desaturase activities were estimated from plasma phospholipid fatty acids. Associations were examined using Pearson and partial correlations.

Results

While both desaturase activities were positively associated with percentage of eicosapentaenoic acid, only D5D was negatively associated with plasma apoB (r = -0.30, P = 0.003). Association of D5D activity with second-phase GIIS (r = -0.23, P = 0.029), IS (r = 0.33, P = 0.015, in women) and 6-h area-under-the-curve (AUC6h) of plasma chylomicrons (apoB48, r = -0.47, P = 0.020, in women) was independent of age and adiposity, but was eliminated after adjustment for plasma apoB. D6D activity was associated in the opposite direction with GIIS (r = 0.24, P = 0.049), IS (r = -0.36, P = 0.004) and AUC6h chylomicrons (r = 0.52, P = 0.004), independent of plasma apoB. Both desaturases were associated with plasma interleukin-1-receptor antagonist (D5D: r = -0.45, P < 0.001 in women; D6D: r = -0.33, P = 0.007) and WAT function (trend for D5D: r = 0.30, P = 0.05; D6D: r = 0.39, P = 0.027) independent of any adjustment.

Conclusions

Association of D5D activity with IS, lower GIIS, and plasma chylomicron clearance is dependent on plasma apoB in overweight and obese adults.

Effects of homocysteine on adipocyte differentiation and CD36 gene expression in 3T3-L1 adipocytes

The aim of this study was to investigate the effects of homocysteine (Hcy), a risk factor for cardiovascular diseases, hypertension, stroke and obesity, on expression of CD36 that regulates uptake of oxidized low-density lipoprotein (Ox-LDL) by adipocytes and differentiation of 3T3-L1 cells to adipocytes. Cell viability was determined using MTT assay, and density of triglycerides were measured with Oil Red O staining. The expression levels of CD36 were analyzed using SYBR green assay by quantitative RT-PCR. Our results showed that the addition of Hcy inhibited differentiation of 3T3-L1 preadipocytes in a dose-dependent manner without a significant cell toxicity (p < 0.05). Percentage CD36 gene expression increased in the Hcy treatment groups, but not statistically significantly (p > 0.05) compared to differentiated adipocytes. Hcy reduced adipocyte differentiation, but had no effect on the expression level of CD36 in vitro conditions. The effect of Hcy on uptake and clearance of Ox-LDL by adipose tissue now needs to be investigated in vivo.

ApoA-I Milano stimulates lipolysis in adipose cells independently of cAMP/PKA activation

ApoA-I, the main protein component of HDL, is suggested to be involved in metabolic homeostasis. We examined the effects of Milano, a naturally occurring ApoA-I variant, about which little mechanistic information is available. Remarkably, high-fat-fed mice treated with Milano displayed a rapid weight loss greater than ApoA-I WT treated mice, and a significantly reduced adipose tissue mass, without an inflammatory response. Further, lipolysis in adipose cells isolated from mice treated with either WT or Milano was increased. In primary rat adipose cells, Milano stimulated cholesterol efflux and increased glycerol release, independently of β-adrenergic stimulation and phosphorylation of hormone sensitive lipase (Ser563) and perilipin (Ser522). Stimulation with Milano had a significantly greater effect on glycerol release compared with WT but similar effect on cholesterol efflux. Pharmacological inhibition or siRNA silencing of ABCA1 did not diminish Milano-stimulated lipolysis, although binding to the cell surface was decreased, as analyzed by fluorescence microscopy. Interestingly, methyl-β-cyclodextrin, a well-described cholesterol acceptor, dose-dependently stimulated lipolysis. Together, these results suggest that decreased fat mass and increased lipolysis following Milano treatment in vivo is partly explained by a novel mechanism at the adipose cell level comprising stimulation of lipolysis independently of the canonical cAMP/protein kinase A signaling pathway.

Plasma IL-1Ra: linking hyperapoB to risk factors for type 2 diabetes independent of obesity in humans

Background/Objective:

Plasma apoB predicts the incidence of type 2 diabetes (T2D); however, the link between apoB-linpoproteins and risks for T2D remain unclear. Insulin resistance (IR) and compensatory hyperinsulinemia characterize prediabetes, and the involvement of an activated interleukin-1 (IL-1) family, mainly IL-1β and its receptor antagonist (IL-Ra), is well documented. ApoB-lipoproteins were reported to promote IL-1β secretion in immune cells; however, in vivo evidence is lacking. We hypothesized that obese subjects with hyperapoB have an activated IL-1 system that explains hyperinsulinemia and IR in these subjects.

Subjects/Methods:

We examined 81 well-characterized normoglycemic men and postmenopausal women (⩾27 kg m⁻², 45–74 years, non-smokers, sedentary, free of chronic disease). Insulin secretion and sensitivity were measured by the gold-standard Botnia clamp, which is a combination of a 1-h intravenous glucose tolerance test (IVGTT) followed by 3-h hyperinsulinemic euglycemic clamp.

Results:

Plasma IL-1β was near detection limit (0.071–0.216 pg ml⁻¹), while IL-1Ra accumulated at 1000-folds higher (77–1068 pg ml⁻¹). Plasma apoB (0.34–1.80 g l⁻¹) associated significantly with hypersinsulinemia (total_IVGTT: C-peptide r=0.27, insulin r=0.22), IR (M/I=−0.29) and plasma IL-1Ra (r=0.26) but not with IL-1β. Plasma IL-1Ra associated with plasma IL-1β (r=0.40), and more strongly with hyperinsulinemia and IR than apoB, while the association of plasma IL-1β was limited to second phase and total insulin secretion (r=0.23). Adjusting the association of plasma apoB to hyperinsulinemia and IR for IL-1Ra eliminated these associations. Furthermore, despite equivalent body composition, subjects with hyperapoB (⩾80th percentile, 1.14 g l⁻¹) had higher C-peptide secretion and lower insulin sensitivity than those with low plasma apoB (⩽20th percentile, 0.78 g l⁻¹). Adjustment for plasma IL-1 Ra eliminated all group differences.

Conclusion:

Plasma apoB is associated with hyperinsulinemia and IR in normoglycemic obese subjects, which is eliminated upon adjustment for plasma IL-1Ra. This may implicate the IL-1 family in elevated risks for T2D in obese subjects with hyperapoB.

Pathways commonly dysregulated in mouse and human obese adipose tissue: FAT/CD36 modulates differentiation and lipogenesis

Obesity is linked to adipose tissue hypertrophy (increased adipocyte cell size) and hyperplasia (increased cell number). Comparative analyses of gene datasets allowed us to identify 1426 genes which may represent common adipose phenotype in humans and mice. Among them we identified several adipocyte-specific genes dysregulated in obese adipose tissue, involved in either fatty acid storage (acyl CoA synthase ACSL1, hormone-sensitive lipase LIPE, aquaporin 7 AQP7, perilipin PLIN) or cell adhesion (fibronectin FN1, collagens COL1A1, COL1A3, metalloprotein MMP9, or both (scavenger receptor FAT/CD36). Using real-time analysis of cell surface occupancy on xCELLigence system we developed a new method to study lipid uptake and differentiation of mouse 3T3L1 fibroblasts and human adipose stem cells. Both processes are regulated by insulin and fatty acids such as oleic acid. We showed that fatty acid addition to culture media increased the differentiation rate and was required for full differentiation into unilocular adipocytes. Significant activation of lipogenesis, i.e. lipid accumulation, by either insulin or oleic acid was monitored in times ranging from 1 to 24 h, depending on differentiation state, whereas significant effects on adipogenesis, i.e., surperimposed lipid accumulation and gene transcriptional regulations were measured after 3 to 4 d. Combination of selected times for analysis of lipid contents, cell counts, size fractionations, and gene transcriptional regulations showed that FAT/CD36 specific inhibitor AP5258 significantly increased cell survival of oleic acid-treated mouse and human adipocytes, and partially restored the transcriptional response to oleic acid in the presence of insulin through JNK pathway. Taken together, these data open new perspectives to study the molecular mechanisms commonly dysregulated in mouse and human obesity at the level of lipogenesis linked to hypertrophy and adipogenesis linked to hyperplasia.

Dependence of brown adipose tissue function on CD36-mediated coenzyme Q uptake

Brown adipose tissue (BAT) possesses the inherent ability to dissipate metabolic energy as heat through uncoupled mitochondrial respiration. An essential component of the mitochondrial electron transport chain is coenzyme Q (CoQ). While cells synthesize CoQ mostly endogenously, exogenous supplementation with CoQ has been successful as a therapy for patients with CoQ deficiency. However, which tissues depend on exogenous CoQ uptake as well as the mechanism by which CoQ is taken up by cells and the role of this process in BAT function are not well understood. Here, we report that the scavenger receptor CD36 drives the uptake of CoQ by BAT and is required for normal BAT function. BAT from mice lacking CD36 displays CoQ deficiency, impaired CoQ uptake, hypertrophy, altered lipid metabolism, mitochondrial dysfunction, and defective nonshivering thermogenesis. Together, these data reveal an important new role for the systemic transport of CoQ to BAT and its function in thermogenesis.

High-density lipoprotein increases the uptake of oxidized low density lipoprotein via PPARγ/CD36 pathway in inflammatory adipocytes

Aim: Previous studies have demonstrated that the dysregulated-secretion of adipokines by adipocytes may contribute to obesity-associated atherosclerosis (As) and high density lipoprotein (HDL) may protect against atherogenesis through multiple pathways. This study was to explore the effect of HDL on the oxLDL uptake in inflammatory adipocytes stimulated by endotoxin lipopolysaccharide (LPS) and the possible mechanism.

Methods and Results: 3T3-L1 adipocytes were cultured and induced to differentiation and maturation. Acute inflammation in adipocytes was induced by LPS (100 ng/ml) for 6 hours. The adipocytes were pretreated with HDL in various concentrations (10, 50, 100 μg/ml) for 16 hours or with specific PPARγ antagonist (GW9662, 10 μM) or agonist (Rosiglitazone, 10 μM) for 30 min before administration of LPS. The results showed that LPS significantly increased the release of inflammation-related adipokines, such as monocyte chemoattractant protein-1 (MCP-1), plasminogen activator inhibitor 1 (PAI-1), tumor necrosis factor-alpha (TNF-α), interleukin (IL)-8 and IL-6, while decreasing the release of leptin and adiponectin. Meanwhile, LPS reduced the uptake and degradation of ¹²⁵I-oxLDL, and down-regulated the expression of PPARγ and CD36. Pretreatment with HDL dose-dependently affected the release of IL-8 and IL-6 and the reduced uptake and degradation of oxLDL of adipocytes stimulated by LPS, accompanied with marked upregulation of PPARγ and CD36 expression. Pretreatment with GW9662 markedly inhibited the upregulation of CD36 expression mediated by HDL (100 μg/ml), while the effects of Rosiglitazone were opposite to GW9662.

Conclusions: HDL may increase oxLDL uptake of inflammatory adipocytes stimulated by LPS via upregulation of PPARγ/CD36 pathway, which may be a new mechanism of anti-atherosclerosis mediated by HDL.

Hypercholesterolemia induces adipose dysfunction in conditions of obesity and nonobesity

It is well known that hypercholesterolemia can lead to atherosclerosis and coronary heart disease. Adipose tissue represents an active endocrine and metabolic site, which might be involved in the development of chronic disease. Because adipose tissue is a key site for cholesterol metabolism and the presence of hypercholesterolemia has been shown to induce adipocyte cholesterol overload, it is critical to investigate the role of hypercholesterolemia on normal adipose function. Studies in preadipocytes revealed that cholesterol accumulation can impair adipocyte differentiation and maturation by affecting multiple transcription factors. Hypercholesterolemia has been observed to cause adipocyte hypertrophy, adipose tissue inflammation, and disruption of endocrine function in animal studies. Moreover, these effects can also be observed in obesity-independent conditions as confirmed by clinical trials. In humans, hypercholesterolemia disrupts adipose hormone secretion of visfatin, leptin, and adiponectin, adipokines that play a central role in numerous metabolic pathways and regulate basic physiologic responses such as appetite and satiety. Remarkably, treatment with cholesterol-lowering drugs has been shown to restore adipose tissue endocrine function. In this review the role of hypercholesterolemia on adipose tissue differentiation and maturation, as well as on hormone secretion and physiologic outcomes, in obesity and non–obesity conditions is presented.

A potent soluble epoxide hydrolase inhibitor, t-AUCB, modulates cholesterol balance and oxidized low density lipoprotein metabolism in adipocytes in vitro

The cholesterol metabolism in adipose tissue is dependent on the balance between cholesterol uptake and efflux. Adipocytes dysfunction and its cholesterol imbalance are associated with obesity. Adipocytes are the site for clearance of oxidized low density lipoprotein (oxLDL) in blood. Soluble epoxide hydrolase (sEH) is highly expressed in adipocytes. sEH converts epoxyeicosatrienoic acids (EETs) into less bioactive dihydroxyeicosatrienoic acids, which regulate cholesterol metabolism in adipocytes and block the development of atherosclerosis. In vitro, 3T3-L1 differentiated adipocytes were incubated with the sEH inhibitor t-AUCB (0, 1, 10, 50 or 100 mmol/l) for 24 h with or without the PPARγ inhibitor GW9662. To determine the effect of t-AUCB on oxLDL endocytosis, degradation and cholesterol efflux from adipocytes, we demonstrated that t-AUCB enhances the CD36-mediated recognition and degradation of oxLDL and improves cholesterol efflux via the upregulation of ABCA1 expression. Furthermore, t-AUCB blocked TNF-α secretion and increased adiponectin levels found in adipocytes culture medium. We provide evidence that these effects are PPARγ-dependent. These results suggest that an increase in EETs because of sEH inhibition could maintain cellular cholesterol homeostasis by the regulation of oxLDL clearance and cholesterol efflux via the EETs–PPARγ pathway.

Ox-LDL induces ER stress and promotes the adipokines secretion in 3T3-L1 adipocytes

Adipocytes behave as a rich source of adipokines, which may be the link between obesity and its complications. Endoplasmic reticulum (ER) stress in adipocytes can modulate adipokines secretion. The aim of this study is to evaluate the effect of oxidized low density lipoprotein（ox-LDL）treatment on ER stress and adipokines secretion in differentiated adipocytes. 3T3-L1 pre-adipocytes were cultured and differentiated into mature adipocytes in vitro. Differentiated adipocytes were incubated with various concentrations of ox-LDL (0-100 µg/ml) for 48 hours; 50µg/ml ox-LDL for various times (0-48 hours) with or without tauroursodeoxycholic acid (TUDCA) (0-400µM) pre-treatment. The protein expressions of ER stress markers, glucose regulated protein 78(GRP78) and CCAAT/enhancer binding protein [C/EBP] homologous protein (CHOP) in adipocytes were detected by Western blot. The mRNA expressions of visfatin and resistin were measured by real-time PCR and the protein release of visfatin and resistin in supernatant were determined by ELISA. Treatment with ox-LDL could increase the cholesterol concentration in adipocytes. Ox-LDL induced the expressions of GRP78 and CHOP protein in adipocytes and promoted visfatin and resistin secretion in culture medium in dose and time-dependent manner. TUDCA could attenuate the effect of ox-LDL on GRP78 and CHOP expressions and reduce visfatin and resistin at mRNA and protein level in dose-dependent manner. In conclusion, ox-LDL promoted the expression and secretion of visfatin and resistin through its activation of ER stress, which may be related to the increase of cholesterol load in adipocytes.

Paraoxonase 1 activities and genetic variation in childhood obesity

Changes in paraoxonase 1 (PON1) activities have been observed in a variety of diseases involving oxidative stress, such as CVD. However, its role in obesity has not been fully established. In the present study, we aimed (1) to genotype sixteen PON1 SNP, (2) to measure serum PON1 activities and (3) to correlate these findings with the incidence of childhood obesity and related traits. We conducted a case–control study of 189 normal-weight and 179 obese prepubertal children, and we measured four different PON1 activities: lactonase; paraoxonase; arylesterase; diazoxonase. Although none of these activities was significantly different between the obese and normal-weight children, lactonase activity was found to be positively correlated with HDL-cholesterol and ApoA1 levels and negatively correlated with myeloperoxidase and fatty acid-binding protein 4 levels. Among the sixteen genotyped PON1 SNP, only the intronic SNP rs854566 exhibited a significant association with obesity (OR 0·61, 95 % CI 0·41, 0·91; P= 0·016). This genetic variant was also associated with increased diazoxonase, lactonase and arylesterase activities and decreased paraoxonase activity. Other genetic variants exhibited different association patterns with serum activities based on their location within the PON1 gene, and SNP that were located within the promoter were strongly associated with lactonase, arylesterase and diazoxonase activities. The functional variant Q192R exhibited the greatest effect on paraoxonase activity (P= 5·88 × 10− 42). In conclusion, SNP rs854566 was negatively associated with childhood obesity and with increased serum PON1 activities in prepubertal children. We determined that lactonase is a reliable indicator of PON1 activities and should be included in future studies of PON1 function.

Low density lipoprotein delays clearance of triglyceride-rich lipoprotein by human subcutaneous adipose tissue

Delayed clearance of triglyceride-rich lipoprotein (TRL) by white adipose tissue (WAT) promotes hypertriglyceridemia and elevated apoB-lipoproteins, which are primarily in the form of LDL. This study examines whether LDL promotes delayed clearance of TRL by WAT. Following the ingestion of a (13)C-triolein-labeled high-fat meal, obese women with high plasma apoB (> median 0.93 g/l, N = 11, > 98% as IDL/LDL) had delayed clearance of postprandial (13)C-triglyceride and (13)C-NEFA over 6 h compared with controls. AUC6 h of plasma (13)C-triglyceride and (13)C-NEFA correlated with plasma apoB but not with LDL diameter or adipocyte area. There was no group difference in (13)C-triolein oxidation rate, which suggests lower (13)C-NEFA storage in peripheral tissue in women with high apoB. Ex vivo/in vitro plasma apoB correlated negatively with WAT (3)H-lipid following a 4 h incubation of women's WAT with synthetic (3)H-triolein-TRL. LDL-differentiated 3T3-L1 adipocytes had lower (3)H-TRL hydrolysis and (3)H-NEFA storage. Treatment of women's WAT with their own LDL decreased (3)H-TRL hydrolysis and (3)H-NEFA uptake. Finally, LDL, although not an LPL substrate, reduced LPL-mediated (3)H-TRL hydrolysis as did VLDL and HDL. Exposure to LDL decreases TRL clearance by human WAT ex vivo. This may promote production of apoB-lipoproteins and hypertriglyceridemia through a positive-feedback mechanism in vivo.

Simvastatin inhibits ox-LDL-induced inflammatory adipokines secretion via amelioration of ER stress in 3T3-L1 adipocyte

Adipocytes behave as a rich source of pro-inflammatory cytokines including tumor necrosis factor-α (TNF-α) and monocyte chemoattractant protein 1 (MCP-1). Endoplasmic reticulum (ER) stress in adipocytes can alter adipokines secretion and induce inflammation. The aim of this study is to evaluate the effect of simvastatin on the ox-LDL-induced ER stress and expression and secretion of TNF-α and MCP-1 in 3T3-L1 adipocytes. Differentiated adipocytes were treated with various concentrations of ox-LDL (0-100 μg/ml) for 24h with or without simvastatin pre-treatment. The protein expressions of ER stress markers, glucose-regulated protein 78 (GRP78) and C/EBP homology protein (CHOP), were determined by Western blot analysis. The mRNA expressions of TNF-α and MCP-1 were measured by real-time PCR. The protein release of TNF-α and MCP-1 in culture medium were evaluated by ELISA. Ox-LDL treatment led to significant up-regulation of GRP78 and CHOP in dose-dependent manner. The expressions of TNF-α and MCP-1 were dose-dependently increased at mRNA and protein levels after ox-LDL intervention. The effects of ox-LDL on adipocytes were abolished by pre-treatment with 4-phenylbutyrate (4-PBA), a chemical chaperone known to ameliorate ER stress. Simvastatin could inhibit ox-LDL-induced ER stress and reduce the expression of TNF-α and MCP-1 at mRNA and protien level in dose dependent manner. In conclusion, ox-LDL can stimulate the expression and secretion of TNF-α and MCP-1 through its activation of ER stress in adipocytes. Simvastatin might exert direct anti-inflammatory effects in adipocytes through amelioration of ER stress.

Insulin and chromium picolinate induce translocation of CD36 to the plasma membrane through different signaling pathways in 3T3-L1 adipocytes, and with a differential functionality of the CD36

Chromium picolinate (CrPic) has been indicated to activate glucose transporter 4 (GLUT4) trafficking to the plasma membrane (PM) to enhance glucose uptake in 3T3-L1 adipocytes. In skeletal and heart muscle cells, insulin directs the intracellular trafficking of the fatty acid translocase/CD36 to induce the uptake of cellular long-chain fatty acid (LCFA). The current study describes the effects of CrPic and insulin on the translocation of CD36 from intracellular storage pools to the PM in 3T3-L1 adipocytes in comparison with that of GLUT4. Immunofluorescence microscopy and immunoblotting revealed that both CD36 and GLUT4 were expressed and primarily located intracellularly in 3T3-L1 adipocytes. Upon insulin or CrPic stimulation, PM expression of CD36 increased in a similar manner as that for GLUT4; the CrPic-stimulated PM expression was less strong than that of insulin. The increase in PM localization for these two proteins by insulin paralleled LCFA ([1-(14)C]palmitate) or [(3)H]deoxyglucose uptake in 3T3-L1 adipocytes. The induction of the PM expression of GLUT4, but not CD36, or substrate uptake by insulin and CrPic appears to be additive in adipocytes. Furthermore, wortmannin completely inhibited the insulin-stimulated translocation of GLUT4 or CD36 and prevented the increased uptake of glucose or LCFA in these cells. Taken together, for the first time, these findings suggest that both insulin and CrPic induce CD36 translocation to the PM in 3T3-L1 adipocytes and that their translocation-inducing effects are not additive. The signaling pathway inducing the translocations is different, apparently resulting in a differential activity of CD36.

CCAAT/enhancer-binding protein-β participates in oxidized LDL-enhanced proliferation in 3T3-L1 cells

Increased circulating oxidized LDL (oxLDL) have been found in obese subjects. Obesity is characterized by an excess of fat mass resulting from an increase in adipocyte number and size. The generation of new adipocytes is a tightly controlled process where multiple factors acting in a signaling cascade follow a precise temporal expression pattern; oxLDL appear to have a role in the impairment of this process. The purpose of this study was to examine the effects of oxLDL on the mechanisms involved in the proliferative stage of the differentiation process in 3T3-L1 cells. After hormonal induction, 3T3-L1 cells undergo approximately two rounds of mitotic clonal expansion (MCE), a process required for adipogenesis. CCAAT/enhancer-binding protein β (C/EBPβ) is immediately expressed after induction, and plays a crucial role in MCE, but its expression must decrease to allow preadipocytes to mature into adipocytes. We found that, in the presence of stimuli to differentiate, oxLDL induced a higher proliferation rate in this cell line, associated with a sustained up-regulation of C/EBPβ, which remained activated inside the nucleus for several days. RNAi-mediated knockdown of C/EBPβ 24 h after oxLDL treatment counteracted the increase in proliferation rate. Both C/EBPβ expression and proliferation processes appear to be influenced by cAMP/protein kinase A (PKA) and extracellular signal-regulated kinases1/2 (ERK1/2) pathways. OxLDL treatment led to increased levels of cAMP, and to a strong, prolonged phosphorylation of ERK1/2 and C/EBPβ. The addition of cAMP and PKA inhibitors, SQ22536 and H-89, respectively, reduced proliferation only in oxLDL-treated cells, whereas the addition of ERK1/2 inhibitor U0126 blocked proliferation in both control and oxLDL-treated cells. C/EBPβ nuclear expression and DNA-binding activity were reduced by U0126, under all tested conditions. These findings show that the altered expression pattern of C/EBPβ is involved in the increase in the number of proliferating cells induced by oxLDL, in hormone-stimulated 3T3-L1 cells.

Variants in the CD36 gene locus determine whole-body adiposity, but have no independent effect on insulin sensitivity

CD36 variants have been associated with type 2 diabetes, features of the metabolic syndrome, and alterations in lipid metabolism. In contrast, the effect of single-nucleotide polymorphisms (SNPs) in CD36 on insulin resistance is controversial in literature. Therefore, we investigated whether genetic variation within the CD36 gene locus affects insulin resistance in a well-phenotyped cohort of white European subjects at increased risk for type 2 diabetes. We genotyped 1,790 subjects (1,174 women, 616 men) for six SNPs tagging 100% of common variants (minor allele frequency ≥0.05) within the CD36 gene locus with an r² ≥ 0.8. All subjects underwent an oral glucose tolerance test (OGTT) and a subset additionally a hyperinsulinemic-euglycemic clamp (n = 523). Ectopic hepatic lipids (n = 346) were assessed by magnetic resonance spectroscopy. After appropriate adjustment and Bonferroni correction for multiple comparisons, the four CD36 SNPs rs9784998, rs3211883, rs3211908, and rs3211956 significantly associated with BMI and rs3211883 and rs3211908 significantly associated with waist circumference (all P < 0.0042). In contrast, CD36 SNPs rs3211816 and rs3211960 were not associated with measures of adiposity (all P ≥ 0.11). No reliable association was detected between the six CD36 SNPs and insulin sensitivity or ectopic hepatic lipid accumulation after adjustment for age, gender, and BMI. In the long run, genetic variation within the CD36 locus may contribute to metabolic disease via its effect on body adiposity, but not via an independent effect on insulin sensitivity.

Relationship between the paraoxonase (PON1) L55M and Q192R polymorphisms and obesity in a Mexican population: a pilot study

The aim of this study was to examine the relationship between the L55M and Q192R paraoxonase (PON1) polymorphisms and obesity in a population of adult Mexican workers. The study population included 127 adult individuals from the Universidad Autónoma del Estado de Morelos, ranging in age from 20 to 56 years and representing both sexes. Based on body mass index, 63 individuals were classified as obese and 64 as normal weight. The PON1-Q192R and PON1-L55M polymorphisms were determined by restriction fragment length polymorphism PCR analysis. Both arylesterase and paraoxonase activity levels were similar in both groups, whereas systolic pressure, triglyceride, total cholesterol, low-density lipoprotein cholesterol, very-low-density lipoprotein cholesterol, glucose, and insulin levels were higher in the obese group than in the normal-weight group (P < 0.05). An exception was the high-density lipoprotein cholesterol (HDL-C) levels, which were lower in the obese group (P < 0.05). Although the PON1-Q192R polymorphism was not associated with either group, the frequency of the homozygous L genotype for the PON1-L55M polymorphism was higher in the obese group than in the normal-weight group (P < 0.05). In conclusion, this study established a positive association between the PON1-L55M homozygous L genotype and obesity.

Pathogenic role of scavenger receptor CD36 in the metabolic syndrome and diabetes

Obesity is increasing at epidemic proportions in the United States and is a major contributor to the development of both metabolic syndrome (glucose intolerance, dyslipidemia, hypertension) and atherosclerotic cardiovascular disease. A wide body of evidence has linked systemic low-grade inflammation as underlying obesity and insulin-resistant states via monocyte/macrophage activation. Transgenic deletion of scavenger receptor type B CD36 in rodents has suggested a pivotal role for CD36 in mediating inflammation, insulin resistance, and atherogenesis through transport of fatty acids and uptake of oxidized lipids, respectively. CD36 signaling pathways involving c-Jun N-terminal kinase (JNK) activation and Toll-like receptors have been implicated in the induction of insulin resistance. This review will focus on the pathogenic role of CD36 receptors in metabolic syndrome and type 2 diabetes.

Oxidized LDL and lysophosphatidylcholine stimulate plasminogen activator inhibitor-1 expression through reactive oxygen species generation and ERK1/2 activation in 3T3-L1 adipocytes

Plasminogen activator inhibitor-1 (PAI-1) is secreted from adipose tissue and is considered to be a risk factor for both atherosclerosis and insulin resistance. Here we report for the first time that PAI-1 expression is enhanced by oxidized low-density lipoprotein (OxLDL) and its lipid component lysophosphatidylcholine (LPC) in mouse 3T3-L1 adipocytes. In fully differentiated 3T3-L1 cells, OxLDL treatment increased the mRNA expression and protein secretion of PAI-1 in a dose- and time-dependent manner, whereas native LDL had no effect. The addition of an anti-CD36 antibody suppressed OxLDL-stimulated PAI-1 expression by 50%, suggesting that adipose-derived CD36 contributes to roughly half of the PAI-1 expression stimulated by OxLDL. In addition, pharmacological experiments showed that the OxLDL-stimulated enhancement in PAI-1 expression was mediated through the generation of reactive oxygen species (ROS) and phosphorylation of extracellular signal-regulated kinase 1/2. Furthermore, LPC, a major lipid component of OxLDL, was responsible for the enhanced expression of PAI-1 as phospholipase A(2)-treated acetyl LDL, which generates LPC, strongly stimulated PAI-1 expression, whereas acetyl LDL itself had no such activity. These data demonstrate that the uptake of OxLDL and, in particular, its lipid component LPC into adipocytes triggers aberrant ROS-mediated PAI-1 expression, which may be involved in the pathogenesis of metabolic syndrome.

Cholesterol imbalance in adipocytes: a possible mechanism of adipocytes dysfunction in obesity

Studies of the past decade have increased our understanding of the role of adipose tissue dysfunction in obesity and obesity-related insulin resistance and type 2 diabetes. Although adipose tissue is the body's largest pool of free cholesterol, adipocytes have limited activity in cholesterol synthetic pathway. Thus, the majority of adipocyte cholesterol originates from circulating lipoproteins. To maintain cholesterol homeostasis, adipocytes have developed multiple pathways for cholesterol efflux. Several transcriptional factors, such as sterol regulatory element-binding proteins and liver X receptors may be responsible for the regulation of cholesterol homeostasis in adipocytes. Most notably, because altering cholesterol balance profoundly modifies adipocyte metabolism in a way resembling that seen in hypertrophied adipocytes, cholesterol imbalance is recognized as a characteristic for enlarged adipocytes per se in the obese state. In addition, plasma membrane cholesterol normalization by chromium picolinate can fully restore insulin-stimulated glucose transport, further supporting the role of the adipocyte cholesterol imbalance in obesity and insulin resistance.

Oxidized LDL impair adipocyte response to insulin by activating serine/threonine kinases

Oxidized LDL (oxLDL) increase in patients affected by type-2 diabetes, obesity, and metabolic syndrome. Likewise, insulin resistance, an impaired responsiveness of target tissues to insulin, is associated with those pathological conditions. To investigate a possible causal relationship between oxLDL and the onset of insulin resistance, we evaluated the response to insulin of 3T3-L1 adipocytes treated with oxLDL. We observed that oxLDL inhibited glucose uptake (-40%) through reduced glucose transporter 4 (GLUT4) recruitment to the plasma membrane (-70%), without affecting GLUT4 gene expression. These findings were associated to the impairment of insulin signaling. Specifically, in oxLDL-treated cells insulin receptor (IR) substrate-1 (IRS-1) was highly degraded likely because of the enhanced Ser(307)phosphorylation. This process was largely mediated by the activation of the inhibitor of kappaB-kinase beta (IKKbeta) and the c-Jun NH(2)-terminal kinase (JNK). Moreover, the activation of IKKbeta positively regulated the nuclear content of nuclear factor kappaB (NF-kappaB), by inactivating the inhibitor of NF-kappaB (IkappaBalpha). The activated NF-kappaB further impaired per se GLUT4 functionality. Specific inhibitors of IKKbeta, JNK, and NF-kappaB restored insulin sensitivity in adipocytes treated with oxLDL. These data provide the first evidence that oxLDL, by activating serine/threonine kinases, impaired adipocyte response to insulin affecting pathways involved in the recruitment of GLUT4 to plasma membranes (PM). This suggests that oxLDL might participate in the development of insulin resistance.

[Seeking "Etwas Neues"--from bioorganic chemistry to Alzheimer's disease]

Introduction to bioorganic chemistry by Prof. Kanaoka at the entrance of my research works affects greatly throughout the life afterward. Chemical modification studies of enzyme proteins taught me quality of chemical reactions. For example, triethyloxonium fluoroborate (Et3O+ BF4(-)), a Meerwein reagent, selectively reacted with a particular carboxyl group (Asp-177) in the substrate binding site of trypsin, even though the reaction was performed in aqueous solution. A series of ion channel studies intoxicate me how exciting the science works are. Purification of sodium channel protein from electric eels initiated the collaboration work to reveal total primary structure of the molecule, as an inaugurating work of ion channel molecules. Photoaffinity labeling proved to be an efficient method to elucidate ligand binding sites, such as TTX binding site within the sodium channel and the sites for calcium anatagonists in L-type calcium channels. Encounter with CD36 molecule expands our works to more pathobiochemical field. We revealed CD36, a class B scavenger receptor, is related to development of atherosclerosis by phagocytosis of ox-LDL in macrophages and even matured adipocytes. In microglia, however, CD36 plays clearance role of oligomeric beta-amyloid peptides in IL-4 activated type-2 microglia, suggesting the activation of type-2 microglia may be useful for developing a new method to treat or prevent from Alzheimer's disease.

Oxidised LDL up-regulate CD36 expression by the Nrf2 pathway in 3T3-L1 preadipocytes

The effect of oxLDL on CD36 expression has been assessed in preadipocytes induced to differentiate. Novel evidence is provided that oxLDL induce a peroxisome proliferator-activated receptor gamma-independent CD36 overexpression, by up-regulating nuclear factor erythroid 2 (NF-E2)-related factor 2 (Nrf2). The nuclear translocation of Nrf2 appeared to depend on PKC pathway activation. In adipocytes, the CD36 up-regulation may indicate a compensation mechanism to meet the demand of excess oxLDL and oxidised lipids in blood, reducing the risk of atherogenesis. Besides strengthening the hypothesis that oxLDL can contribute to the onset of insulin-resistance, data herein presented highlight the significance of oxLDL-induced CD36 overexpression within the cellular defence response.

Dual effects of oxidized low-density lipoprotein on LXR-ABCA1-apoA-I pathway in 3T3-L1 cells

BACKGROUND

The adipocyte has been proven to recognize and degrade oxidized low-density lipoprotein (oxLDL), while cholesterol efflux from adipocytes to clear excess cholesterol loaded by oxLDL is essential to maintain its normal function. Thus, it is intriguing to explore the effects of oxLDL on cholesterol efflux in adipocytes.

METHODS

Fully differentiated 3T3-L1 cells were incubated in the medium containing various concentrations of oxLDL (0 to 50 microg/mL) for 8 or 24 h. 10 micromol/L 22(R)-hydroxycholesterol was exposed to preconditioned adipocytes with 25 microg/mL oxLDL for 24 h. Reverse transcription polymerase chain reaction (RT-PCR) was used to evaluate adipocytes mRNA expression. Cholesterol efflux rate was determined through measuring release of radioactivity from (3)H-cholesterol prelabeled cells into medium containing apolipoprotein A-I (apoA-I).

RESULTS

Low concentrations of oxLDL caused a significant increase in apoA-I-mediated cholesterol efflux via enhancement of ATP binding cassette transporter A1 (ABCA1) pathway, whereas higher concentrations were incapable. In adipocytes preincubated with 25 microg/mL oxLDL for 24 h, 22(R)-hydroxycholesterol could increase ABCA1 and LXR* mRNA levels and apoA-I-mediated cholesterol efflux.

CONCLUSION

OxLDL has dual effects on ABCA1 pathway in adipocytes. It depends on the concentration and exposure time. The new action of low levels of oxLDL may provide further understanding to its atheroprotective effects.

Intra- and interindividual variation in gene expression in human adipose tissue

Adipose tissue is a highly plastic tissue with an important endocrine and metabolic function. To understand its role in human health and disease, it is necessary to understand the extent of variation and the specific differences within and between different depots and subjects. We employed cDNA microarray analysis to investigate this in human subjects ranging from lean to mildly obese. We observe (1) high similarity between different samples of one adipose depot, (2) only small differences between the subcutaneous and visceral adipose tissue depot and (3) larger differences in gene expression between different individuals (per depot). The major variation within adipose depots can be attributed to differences in the non-adipocyte component of adipose tissue. Using only non-obese subjects, we identified genes that were consistently differentially expressed between subcutaneous and omental adipose tissue, despite the variation in gene expression between these subjects. Using quantitative real time polymerase chain reaction (PCR), fatty acid binding protein 4 (FABP4), vimentin (Vim), four and a half LIMs domains (FHL1), CD36 (all higher in subcutaneous adipose tissue) and Matrix Gla protein (MGP; lower in subcutaneous adipose tissue) were confirmed to be significantly differentially expressed between depots.

Is adipose tissue a place for Mycobacterium tuberculosis persistence?

Background

Mycobacterium tuberculosis, the etiological agent of tuberculosis (TB), has the ability to persist in its human host for exceptionally long periods of time. However, little is known about the location of the bacilli in latently infected individuals. Long-term mycobacterial persistence in the lungs has been reported, but this may not sufficiently account for strictly extra-pulmonary TB, which represents 10–15% of the reactivation cases.

Methodology/Principal Findings

We applied in situ and conventional PCR to sections of adipose tissue samples of various anatomical origins from 19 individuals from Mexico and 20 from France who had died from causes other than TB. M. tuberculosis DNA could be detected by either or both techniques in fat tissue surrounding the kidneys, the stomach, the lymph nodes, the heart and the skin in 9/57 Mexican samples (6/19 individuals), and in 8/26 French samples (6/20 individuals). In addition, mycobacteria could be immuno-detected in perinodal adipose tissue of 1 out of 3 biopsy samples from individuals with active TB. In vitro, using a combination of adipose cell models, including the widely used murine adipose cell line 3T3-L1, as well as primary human adipocytes, we show that after binding to scavenger receptors, M. tuberculosis can enter within adipocytes, where it accumulates intracytoplasmic lipid inclusions and survives in a non-replicating state that is insensitive to the major anti-mycobacterial drug isoniazid.

Conclusions/Significance

Given the abundance and the wide distribution of the adipose tissue throughout the body, our results suggest that this tissue, among others, might constitute a vast reservoir where the tubercle bacillus could persist for long periods of time, and avoid both killing by antimicrobials and recognition by the host immune system. In addition, M. tuberculosis-infected adipocytes might provide a new model to investigate dormancy and to evaluate new drugs for the treatment of persistent infection.

Adipose fatty acid composition and rate of incorporation of alpha-linolenic acid differ between normal and lipoprotein lipase-deficient cats

Normal adiposity occurs in humans and mice deficient of adipose lipoprotein lipase (LPL) activity. Subnormal adiposity found in LPL-deficient cats is indicative of limited de novo synthesis of fatty acids (FAs). In 14 LPL-deficient (3.0 +/- 0.1 kg) and 8 normal (3.7 +/- 0.1 kg) queens, FAs in triacylglycerol (TAG), phospholipid (PL), and nonesterified FAs (NEFAs) of plasma and inguinal subcutaneous adipose were determined before and after (d 38, 61, 110, 117, and 251) dietary linseed oil supplementation (30 g/kg). By d 60, LPL-deficient queens gained body weight (+0.4 +/- 0.1 kg), developed normal body fat mass (25 +/- 2%), and were enriched in 18:3(n-3) in their plasma and adipose lipids. Adipose TAG 18:3(n-3) enrichment in LPL-deficient queens was subnormal at all sampling times and, as observed in normal queens, apparently not equilibrated by d 251. Adipose FA profiles in TAG but not PL were substantially different (P < 0.05) between LPL-deficient and normal queens; the 16:0 to 18:2(n-6) ratio was high in LPL-deficient (2.4-4.4) relative to normal queens (1.0-1.4). In LPL-deficient queens, fed-state plasma NEFA (n-6) and (n-3) enrichments were similar to those in adipose TAG, and plasma NEFA concentration was high (0.62 +/- 0.05 mmol/L) and similar to that in normal queens after withholding diet for 16 h. These data indicate that LPL deficiency in cats reduces dietary FA storage efficiency, favors storage of saturated over unsaturated FAs, and stimulates de novo FA synthesis substantive enough to support normal adiposity.

Oxidised LDL modulate adipogenesis in 3T3-L1 preadipocytes by affecting the balance between cell proliferation and differentiation

The effects of oxidised LDL (oxLDL) on cell proliferation, apoptosis and hormone-induced differentiation have been evaluated for the first time in 3T3-L1 preadipocytes. Unlike control cells, oxLDL-treated preadipocytes showed a high proliferation rate, a low apoptosis level, and an impaired differentiation process with an increased preadipocyte factor-1 (Pref-1) mRNA expression at late times. By silencing Pref-1 mRNA or inhibiting its expression with an increased dexamethasone concentration, differentiation occurred as usual, which demonstrates the key role of Pref-1 overexpression. The results suggest a specific action of oxLDL on the adipogenesis inhibitor Pref-1, as indicated also by its reappearance in mature adipocytes treated with oxLDL. The inhibitory effects of oxLDL on differentiation required oxLDL uptake by CD36, and were associated with lipoprotein lipids. These results point to oxLDL as a modulator of adipose tissue mass and as possible link between obesity and its clinical complications.

Pathological Roles of Advanced Glycation End Product Receptors SR-A and CD36

The pathological significance of advanced glycation end product (AGE)-modified proteins deposited in several lesions is generally accounted for by their cellular interaction via the AGE receptors and subsequent acceleration of the inflammatory process. In this study, we focused on two AGE receptors-specifically, the role of SR-A in pathogenesis of diabetic nephropathy and the role of CD36 in AGE-induced downregulation of leptin by adipocytes. In terms of SR-A, diabetic wild-type mice exhibited increased urinary albumin excretion, glomerular hypertrophy, and mesangial matrix expansion, whereas SR-A-knockout mice showed reduced glomerular size and mesangial matrix area. In these diabetic SR-A-knockout mice, the number of macrophages that infiltrated into glomeruli was remarkably reduced (P < 0.05), suggesting that SR-A-dependent glomerular migration of macrophages plays an important role in the pathogenesis of diabetic nephropathy. In terms of CD36, incubation of glycolaldehyde-modified bovine serum albumin (GA-BSA) with 3T3-L1 adipocytes reduced leptin secretion by these cells. The binding of GA-BSA to these cells and subsequent endocytic degradation were effectively inhibited by a neutralizing anti-CD36 antibody. AGE-induced downregulation of leptin was protected by N-acetyl-cysteine, an antioxidant. These results indicate that the interaction of AGE ligands with 3T3-L1 adipocytes via CD36 induces oxidative stress and leads to inhibition of leptin expression by these cells, suggesting a potential link of this phenomenon to exacerbation of the insulin sensitivity in metabolic syndrome.

Glycolaldehyde-modified bovine serum albumin downregulates leptin expression in mouse adipocytes via a CD36-mediated pathway

Previous observations by us have clarified that proteins modified by advanced glycation end products (AGEs) are recognized as effective ligands by CD36-overexpressed CHO cells and undergo receptor-mediated endocytosis. CD36, a member of the class B scavenger receptor family, also acts as a fatty acid transporter in adipocytes. Oxidized low-density lipoprotein (Ox-LDL), a ligand for CD36, is known to upregulate CD36 by activating peroxisome proliferator-activated receptor gamma (PPAR-gamma) in macrophages, whereas PPAR-gamma ligands such as troglitazone and 15-deoxy-delta12,14-prostaglandin J2 decrease leptin secretion from adipocytes. The purpose of this study was to examine effects of AGE ligands on leptin expression in adipocytes. Glycolaldehyde-modified bovine serum albumin (GA-BSA) decreased leptin expression at both the protein and mRNA levels in 3T3-L1 adipocytes and mouse epididymal adipocytes. The binding to and subsequent endocytic degradation of GA-BSA by 3T3-L1 adipocytes were effectively inhibited by a neutralizing anti-CD36 antibody. These results indicate that the ligand interaction of GA-BSA with CD36 leads to downregulation of leptin expression in 3T3-L1 adipocytes, suggesting that AGE-induced leptin downregulation is linked to reduction of the insulin sensitivity in metabolic syndrome.

Surface protein expression between human adipose tissue-derived stromal cells and mature adipocytes

Adipose tissue contains a stroma that can be easily isolated. Thus, human adipose tissue presents an source of multipotent stromal cells. In order to determine the implication of hematopoietic markers in adipocyte biology, we have defined part of the phenotype of the human adipose tissue-derived stromal cells, and compared this to fully differentiated adipocytes. Flow cytometry demonstrates that the protein expression phenotype of both cell types are similar and includes the expression of CD10, CD13, CD34, CD36, CD55, CD59 and CD65. No significant difference between subcutaneous and omental adipose tissue could be demonstrated concerning the expression of these markers. However, the expression of CD34, CD36 and CD65 is cell-dependent. While the expression of CD36 and CD65 doubled between stromal cells and mature adipocytes, the expression of CD34 decreased, despite this protein being present on the mature adipocyte. As CD34 is described as a stem cell marker and it being unlikely to be expressed on differentiated cells, this result was confirmed by immunostaining and western blot. The clear function of this protein on the adipocyte membrane remains to be determined. The characterization of new proteins on mature adipocytes could have broad implications for the comprehension of the biology of this tissue.

PPARgamma regulates adipocyte cholesterol metabolism via oxidized LDL receptor 1

In addition to its role in energy storage, adipose tissue also accumulates cholesterol. Concentrations of cholesterol and triglycerides are strongly correlated in the adipocyte, but little is known about mechanisms regulating cholesterol metabolism in fat cells. Here we report that antidiabetic thiazolidinediones (TZDs) and other ligands for the nuclear receptor PPARgamma dramatically upregulate oxidized LDL receptor 1 (OLR1) in adipocytes by facilitating the exchange of coactivators for corepressors on the OLR1 gene in cultured mouse adipocytes. TZDs markedly stimulate the uptake of oxidized LDL (oxLDL) into adipocytes, and this requires OLR1. Increased OLR1 expression, resulting either from TZD treatment or adenoviral gene delivery, significantly augments adipocyte cholesterol content and enhances fatty acid uptake. OLR1 expression in white adipose tissue is increased in obesity and is further induced by PPARgamma ligand treatment in vivo. Serum oxLDL levels are decreased in both lean and obese diabetic animals treated with TZDs. These data identify OLR1 as a novel PPARgamma target gene in adipocytes. While the physiological role of adipose tissue in cholesterol and oxLDL metabolism remains to be established, the induction of OLR1 is a potential means by which PPARgamma ligands regulate lipid metabolism and insulin sensitivity in adipocytes.

CD36 Is Not Involved in Scavenger Receptor–Mediated Endocytic Uptake of Glycolaldehyde- and Methylglyoxal-Modified Proteins by Liver Endothelial Cells

Circulating proteins modified by advanced glycation end-products (AGE) are mainly taken up by liver endothelial cells (LECs) via scavenger receptor-mediated endocytosis. Endocytic uptake of chemically modified proteins by macrophages and macrophage-derived cells is mediated by class A scavenger receptor (SR-A) and CD36. In a previous study using SR-A knockout mice, we demonstrated that SR-A is not involved in endocytic uptake of AGE proteins by LECs [Matsumoto et al. (2000) Biochem. J. 352, 233-240]. The present study was conducted to determine the contribution of CD36 to this process. Glycolaldehyde-modified BSA (GA-BSA) and methylglyoxal-modified BSA (MG-BSA) were used as AGE proteins. 125I-GA-BSA and 125I-MG-BSA underwent endocytic degradation by these cells at 37 degrees C, and this process was inhibited by several ligands for the scavenger receptors. However, this endocytic uptake of 125I-GA-BSA by LECs was not inhibited by a neutralizing anti-CD36 antibody. Similarly, hepatic uptake of (111)In-GA-BSA after its intravenous injection was not significantly attenuated by co-administration of the anti-CD36 antibody. These results clarify that CD36 does not play a significant role in elimination of GA-BSA and MG-BSA from the circulation, suggesting that the receptor involved in endocytic uptake of circulating AGE proteins by LEC is not SR-A or CD36.

Fenofibrate enhances CD36 mediated endocytic uptake and degradation of oxidized low density lipoprotein in adipocytes from hypercholesterolemia rabbit

BACKGROUND

CD36 as a fatty acid transporter is predominantly expressed in adipocytes. We studied whether adipocytes could uptake and degrade OxLDL through CD36 and explored the effect of fenofibrate on OxLDL uptake in adipocytes from hypercholesterolemia rabbits.

METHODS

Subcutaneous adipose tissues were collected from normal, high-cholesterol and high-cholesterol plus fenofibrate treatment rabbits for adipocytes culture. CD36 and peroxisome proliferator-activated receptor gamma (PPARgamma) mRNA expression were evaluated by RT-PCR.

RESULTS

Cellular expression of CD36 was confirmed during differentiation of adipose cell by RT-PCR. Upon incubation at 37 degrees C, (125)I-OxLDL was endocytosed in a dose-dependent fashion and underwent lysosomal degradation by adipocytes. In binding experiments at 4 degrees C, (125)I-OxLDL exhibited specific and saturable binding to adipocytes (K(D) = 4.2 microg/mL). The endocytic uptake and degradation of (125)I-OxLDL by adipocytes were inhibited by 56 and 54% with anti-CD36 antibody. Fenofibrate treatment enhanced the (125)I-OxLDL uptake and degradation and up-regulated CD36 mRNA expression in adipocytes and suppressed PPARgamma mRNA expression in adipose tissue from hypercholesterolemia rabbits.

CONCLUSION

CD36 plays a novel role in adipose tissues and adipocytes possibly involve in clearance of OxLDL in blood. Fenofibrate treatment improved the OxLDL uptake and degradation in adipocytes from hypercholesterolemia rabbits.

Advanced glycation end products-modified proteins and oxidized LDL mediate down-regulation of leptin in mouse adipocytes via CD36

Advanced glycation end products (AGE)-modified proteins as well as oxidized-LDL (Ox-LDL) undergo receptor-mediated endocytosis by CHO cells overexpressing CD36, a member of class B scavenger receptor family. The purpose of the present study was to examine the effects of glycolaldehyde-modified BSA (GA-BSA) as an AGE-ligand and Ox-LDL on leptin expression in adipocytes. GA-BSA decreased leptin expression at both protein and mRNA levels in 3T3-L1 adipocytes and mouse epididymal adipocytes. Ox-LDL showed a similar inhibitory effect on leptin expression in 3T3-L1 adipocytes, which effect was protected by N-acetylcysteine, a reactive oxygen species (ROS) inhibitor. Binding of (125)I-GA-BSA or (125)I-Ox-LDL to 3T3-L1 adipocytes and subsequent endocytic degradation were inhibited by a neutralizing anti-CD36 antibody. Furthermore, this antibody also suppressed Ox-LDL-induced leptin down-regulation. These results clarify that the interaction of GA-BSA and Ox-LDL with CD36 leads to down-regulation of leptin expression via ROS system(s) in 3T3-L1 adipocytes, suggesting that a potential link of AGE- and/or Ox-LDL-induced leptin down-regulation might be linked to insulin-sensitivity in metabolic syndrome.