Possible role for intracellular cholesteryl ester transfer protein in adipocyte lipid metabolism and storage

The impact of cholesteryl ester transfer protein on the progression of cutaneous leishmaniasis

Introduction

Pathogenesis of cutaneous leishmaniases involves parasite growth, persistent inflammation, and likely participation of lipoproteins (LP). The cholesteryl ester transfer protein (CETP), involved in LP remodeling, has been shown to participate in the inflammatory response and the evolution of infectious conditions.

Methods

We evaluated the impact of the presence of CETP on infection by Leishmania (L.) amazonensis in an experimental model of cutaneous leishmaniasis using C57BL6/J mice transgenic for human CETP (CETP), having as control their littermates that do not express the protein, wild-type (WT) mice. The progression of the lesion after infection in the footpad was monitored for 12 weeks. Two groups of animals were formed to collect the plantar pad in the 4th and 12th week post-infection.

Results

The lesion increased from the 3rd week onwards, in both groups, with a gradual decrease from the 10th week onwards in the CETP group compared to the WT group, showing a reduction in parasitism and an improvement in the healing process, a reduction in CD68+ cells, and an increase in CD163+ and CD206, characterizing a population of M2 macrophages. A reduction in ARG1+ cells and an increase in INOS+ cells were observed. During infection, the LP profile showed an increase in triglycerides in the VLDL fraction in the CETP group at 12 weeks. Gene expression revealed a decrease in the CD36 receptor in the CETP group at 12 weeks, correlating with healing and parasite reduction. In vitro, macrophages derived from bone marrow cells from CETP mice showed lower parasite load at 48 h and, a reduction in arginase activity at 4 h accompanied by increased NO production at 4 and 24 h compared to WT macrophages, corroborating the in vivo findings.

Discussion

The data indicate that the presence of CETP plays an important role in resolving Leishmania (L.) amazonensis infection, reducing parasitism, and modulating the inflammatory response in controlling infection and tissue repair.

Cholesteryl ester transfer protein knock-down in conjunction with a cholesterol-depleting agent decreases tamoxifen resistance in breast cancer cells

The cholesterogenic phenotype, encompassing de novo biosynthesis and accumulation of cholesterol, aids cancer cell proliferation and survival. Previously, the role of cholesteryl ester (CE) transfer protein (CETP) has been implicated in breast cancer aggressiveness, but the molecular basis of this observation is not clearly understood, which this study aims to elucidate. CETP knock-down resulted in a >50% decrease in cell proliferation in both 'estrogen receptor-positive' (ER+; Michigan Cancer Foundation-7 (MCF7) breast cancer cells) and 'triple-negative' breast cancer (TNBC; MDA-MB-231) cell lines. Intriguingly, the abrogation of CETP together with the combination treatment of tamoxifen (5 μM) and acetyl plumbagin (a cholesterol-depleting agent) (5 μM) resulted in twofold to threefold increase in apoptosis in both cell lines. CETP knockdown also showed decreased intracellular CE levels, lipid raft and lipid droplets in both cell lines. In addition, RT2 Profiler PCR array (Qiagen, Germany)-based gene expression analysis revealed an overall downregulation of genes associated in cholesterol biosynthesis, lipid signalling and drug resistance in MCF7 cells post-CETP knock-down. On the contrary, resistance in MDA-MB-231 cells was reduced through increased expression in cholesterol efflux genes and the expression of targetable surface receptors by endocrine therapy. The pilot xenograft mice study substantiated CETP's role as a cancer survival gene as knock-down of CETP stunted the growth of TNBC tumour by 86%. The principal findings of this study potentiate CETP as a driver in breast cancer growth and aggressiveness and thus targeting CETP could limit drug resistance via the reduction in cholesterol accumulation in breast cancer cells, thereby reducing cancer aggressiveness.

CETP inhibition enhances monocyte activation and bacterial clearance and reduces streptococcus pneumonia-associated mortality in mice

Sepsis is a leading cause of mortality worldwide, and pneumonia is the most common cause of sepsis in humans. Low levels of high-density lipoprotein cholesterol (HDL-C) levels are associated with an increased risk of death from sepsis, and increasing levels of HDL-C by inhibition of cholesteryl ester transfer protein (CETP) decreases mortality from intraabdominal polymicrobial sepsis in APOE*3-Leiden.CETP mice. Here, we show that treatment with the CETP inhibitor (CETPi) anacetrapib reduced mortality from Streptococcus pneumoniae-induced sepsis in APOE*3-Leiden.CETP and APOA1.CETP mice. Mechanistically, CETP inhibition reduced the host proinflammatory response via attenuation of proinflammatory cytokine transcription and release. This effect was dependent on the presence of HDL, leading to attenuation of immune-mediated organ damage. In addition, CETP inhibition promoted monocyte activation in the blood prior to the onset of sepsis, resulting in accelerated macrophage recruitment to the lung and liver. In vitro experiments demonstrated that CETP inhibition significantly promoted the activation of proinflammatory signaling in peripheral blood mononuclear cells and THP1 cells in the absence of HDL; this may represent a mechanism responsible for improved bacterial clearance during sepsis. These findings provide evidence that CETP inhibition represents a potential approach to reduce mortality from pneumosepsis.

Dihydromyricetin supplementation improves ethanol-induced lipid accumulation and inflammation

Introduction

Excessive alcohol consumption leads to a myriad of detrimental health effects, including alcohol-associated liver disease (ALD). Unfortunately, no available treatments exist to combat the progression of ALD beyond corticosteroid administration and/or liver transplants. Dihydromyricetin (DHM) is a bioactive polyphenol and flavonoid that has traditionally been used in Chinese herbal medicine for its robust antioxidant and anti-inflammatory properties. It is derived from many plants, including Hovenia dulcis and is found as the active ingredient in a variety of popular hangover remedies. Investigations utilizing DHM have demonstrated its ability to alleviate ethanol-induced disruptions in mitochondrial and lipid metabolism, while demonstrating hepatoprotective activity.

Methods

Female c57BL/6J mice (n = 12/group) were treated using the Lieber DeCarli forced-drinking and ethanol (EtOH) containing liquid diet, for 5 weeks. Mice were randomly divided into three groups: (1) No-EtOH, (2) EtOH [5% (v/v)], and (3) EtOH [5% (v/v)] + DHM (6 mg/mL). Mice were exposed to ethanol for 2 weeks to ensure the development of ALD pathology prior to receiving dihydromyricetin supplementation. Statistical analysis included one-way ANOVA along with Bonferroni multiple comparison tests, where p ≤ 0.05 was considered statistically significant.

Results

Dihydromyricetin administration significantly improved aminotransferase levels (AST/ALT) and reduced levels of circulating lipids including LDL/VLDL, total cholesterol (free cholesterol), and triglycerides. DHM demonstrated enhanced lipid clearance by way of increased lipophagy activity, shown as the increased interaction and colocalization of p62/SQSTM-1, LC3B, and PLIN-1 proteins. DHM-fed mice had increased hepatocyte-to-hepatocyte lipid droplet (LD) heterogeneity, suggesting increased neutralization and sequestration of free lipids into LDs. DHM administration significantly reduced prominent pro-inflammatory cytokines commonly associated with ALD pathology such as TNF-α, IL-6, and IL-17.

Discussion

Dihydromyricetin is commercially available as a dietary supplement. The results of this proof-of-concept study demonstrate its potential utility and functionality as a cost-effective and safe candidate to combat inflammation and the progression of ALD pathology.

Study of effect modifiers of genetically predicted CETP reduction

Genetic variants in drug targets can be used to predict the long-term, on-target effect of drugs. Here, we extend this principle to assess how sex and body mass index may modify the effect of genetically predicted lower CETP levels on biomarkers and cardiovascular outcomes. We found sex and body mass index (BMI) to be modifiers of the association between genetically predicted lower CETP and lipid biomarkers in UK Biobank participants. Female sex and lower BMI were associated with higher high-density lipoprotein cholesterol and lower low-density lipoprotein cholesterol for the same genetically predicted reduction in CETP concentration. We found that sex also modulated the effect of genetically lower CETP on cholesterol efflux capacity in samples from the Montreal Heart Institute Biobank. However, these modifying effects did not extend to sex differences in cardiovascular outcomes in our data. Our results provide insight into the clinical effects of CETP inhibitors in the presence of effect modification based on genetic data. The approach can support precision medicine applications and help assess the external validity of clinical trials.

The Cholesteryl Ester Transfer Protein (CETP) raises Cholesterol Levels in the Brain

The cholesteryl ester transfer protein (CETP) is a lipid transfer protein responsible for the exchange of cholesteryl esters and triglycerides between lipoproteins. Decreased CETP activity is associated with longevity, cardiovascular health, and maintenance of good cognitive performance. Interestingly, mice lack the CETP-encoding gene and have very low levels of LDL particles compared with humans. Currently, the molecular mechanisms induced because of CETP activity are not clear. To understand how CETP activity affects the brain, we utilized CETP transgenic (CETPtg) mice that show elevated LDL levels upon induction of CETP expression through a high-cholesterol diet. CETPtg mice on a high-cholesterol diet showed up to 22% higher cholesterol levels in the brain. Using a microarray on mostly astrocyte-derived mRNA, we found that this cholesterol increase is likely not because of elevated de novo synthesis of cholesterol. However, cholesterol efflux is decreased in CETPtg mice along with an upregulation of the complement factor C1Q, which plays a role in neuronal cholesterol clearance. Our data suggest that CETP activity affects brain health through modulating cholesterol distribution and clearance. Therefore, we propose that CETPtg mice constitute a valuable research tool to investigate the impact of cholesterol metabolism on brain function.

Both full length-cholesteryl ester transfer protein and exon 9-deleted cholesteryl ester transfer protein promote triacylglycerol storage in cultured hepatocytes

We previously reported that overexpression of full-length cholesteryl ester transfer protein (FL-CETP), but not its exon 9-deleted variant (∆E9-CETP), in an adipose cell line reduces their triacylglycerol (TAG) content. This provided mechanistic insight into several in vivo studies where FL-CETP levels are inversely correlated with adiposity. However, increased FL-CETP is also associated with elevated hepatic lipids, suggesting that the effect of CETP on cellular lipid metabolism may be tissue-specific. Here, we directly investigated the role of FL-CETP and ∆E9-CETP in hepatic lipid metabolism. FL- or ∆E9-CETP was overexpressed in HepG2-C3A by adenovirus transduction. Overexpression of either FL or ∆E9-CETP in hepatocytes increased cellular TAG mass by 25% but reduced TAG secretion. This cellular TAG was contained in larger and more numerous lipid droplets. Analysis of TAG synthetic and catabolic pathways showed that this elevated TAG content was due to increased incorporation of fatty acid into TAG (24%), and higher de novo synthesis of fatty acid (50%) and TAG from acetate (40%). siRNA knockdown of CETP had the opposite effect on TAG synthesis and lipogenesis, and decreased cellular TAG. This novel increase in cellular TAG by FL-CETP overexpression was reproduced in Caco-2 intestinal epithelial cells. We conclude that, unlike that seen in adipocyte cells, overexpression of either CETP isoform in lipoprotein-secreting cells promotes the accumulation of TAG. These data suggest that the in vivo correlation between CETP levels and hepatic steatosis can be explained, in part, by a direct effect of CETP on hepatocyte cellular metabolism.

Diabetes Mellitus and Its Metabolic Complications: The Role of Adipose Tissues

Many approaches have been used in the effective management of type 2 diabetes mellitus. A recent paradigm shift has focused on the role of adipose tissues in the development and treatment of the disease. Brown adipose tissues (BAT) and white adipose tissues (WAT) are the two main types of adipose tissues with beige subsets more recently identified. They play key roles in communication and insulin sensitivity. However, WAT has been shown to contribute significantly to endocrine function. WAT produces hormones and cytokines, collectively called adipocytokines, such as leptin and adiponectin. These adipocytokines have been proven to vary in conditions, such as metabolic dysfunction, type 2 diabetes, or inflammation. The regulation of fat storage, energy metabolism, satiety, and insulin release are all features of adipose tissues. As such, they are indicators that may provide insights on the development of metabolic dysfunction or type 2 diabetes and can be considered routes for therapeutic considerations. The essential roles of adipocytokines vis-a-vis satiety, appetite, regulation of fat storage and energy, glucose tolerance, and insulin release, solidifies adipose tissue role in the development and pathogenesis of diabetes mellitus and the complications associated with the disease.

Role of Adenylate Cyclase 9 in the Pharmacogenomic Response to Dalcetrapib: Clinical Paradigm and Molecular Mechanisms in Precision Cardiovascular Medicine

Following the neutral results of the dal-OUTCOMES trial, a genome-wide study identified the rs1967309 variant in the adenylate cyclase type 9 (ADCY9) gene on chromosome 16 as being associated with the risk of future cardiovascular events only in subjects taking dalcetrapib, a CETP (cholesterol ester transfer protein) modulator. Homozygotes for the minor A allele (AA) were protected from recurrent cardiovascular events when treated with dalcetrapib, while homozygotes for the major G allele (GG) had increased risk. Here, we present the current state of knowledge regarding the impact of rs1967309 in ADCY9 on clinical observations and biomarkers in dalcetrapib trials and the effects of mouse ADCY9 gene inactivation on cardiovascular physiology. Finally, we present our current model of the interaction between dalcetrapib and ADCY9 gene variants in the arterial wall macrophage, based on the intracellular role of CETP in the transfer of complex lipids from endoplasmic reticulum membranes to lipid droplets. Briefly, the concept is that dalcetrapib would inhibit CETP-mediated transfer of cholesteryl esters, resulting in a progressive inhibition of cholesteryl ester synthesis and free cholesterol accumulation in the endoplasmic reticulum. Reduced ADCY9 activity, by paradoxically leading to higher cyclic AMP levels and in turn increased cellular cholesterol efflux, could impart cardiovascular protection in rs1967309 AA patients. The ongoing dal-GenE trial recruited 6145 patients with the protective AA genotype and will provide a definitive answer to whether dalcetrapib will be protective in this population.

Revealing the Role of High-Density Lipoprotein in Colorectal Cancer

Colorectal cancer (CRC) is a highly prevalent malignancy with multifactorial etiology, which includes metabolic alterations as contributors to disease development. Studies have shown that lipid status disorders are involved in colorectal carcinogenesis. In line with this, previous studies have also suggested that the serum high-density lipoprotein cholesterol (HDL-C) level decreases in patients with CRC, but more recently, the focus of investigations has shifted toward the exploration of qualitative properties of HDL in this malignancy. Herein, a comprehensive overview of available evidences regarding the putative role of HDL in CRC will be presented. We will analyze existing findings regarding alterations of HDL-C levels but also HDL particle structure and distribution in CRC. In addition, changes in HDL functionality in this malignancy will be discussed. Moreover, we will focus on the genetic regulation of HDL metabolism, as well as the involvement of HDL in disturbances of cholesterol trafficking in CRC. Finally, possible therapeutic implications related to HDL will be presented. Given the available evidence, future studies are needed to resolve all raised issues concerning the suggested protective role of HDL in CRC, its presumed function as a biomarker, and eventual therapeutic approaches based on HDL.

Low-density lipoprotein receptor is required for cholesteryl ester transfer protein to regulate triglyceride metabolism in both male and female mice

Elevated triglycerides (TGs) and impaired TG clearance increase the risk of cardiovascular disease in both men and women, but molecular mechanisms remain poorly understood. Cholesteryl ester transfer protein (CETP) is a lipid shuttling protein known for its effects on high-density lipoprotein cholesterol. Although mice lack CETP, transgenic expression of CETP in mice alters TG metabolism in males and females by sex-specific mechanisms. A unifying mechanism explaining how CETP alters TG metabolism in both males and females remains unknown. Since low-density lipoprotein receptor (LDLR) regulates both TG clearance and very low density lipoprotein (VLDL) production, LDLR may be involved in CETP-mediated alterations in TG metabolism in both males and females. We hypothesize that LDLR is required for CETP to alter TG metabolism in both males and females. We used LDLR null mice with and without CETP to demonstrate that LDLR is required for CETP to raise plasma TGs and to impair TG clearance in males. We also demonstrate that LDLR is required for CETP to increase TG production and to increase the expression and activity of VLDL synthesis targets in response to estrogen. Additionally, we show that LDLR is required for CETP to enhance β-oxidation. These studies support that LDLR is required for CETP to regulate TG metabolism in both males and females.

The Presence of Cholesteryl Ester Transfer Protein (CETP) in Endothelial Cells Generates Vascular Oxidative Stress and Endothelial Dysfunction

Endothelial dysfunction precedes atherosclerosis and is an independent predictor of cardiovascular events. Cholesterol levels and oxidative stress are key contributors to endothelial damage, whereas high levels of plasma high-density lipoproteins (HDL) could prevent it. Cholesteryl ester transfer protein (CETP) is one of the most potent endogenous negative regulators of HDL-cholesterol. However, whether and to what degree CETP expression impacts endothelial function, and the molecular mechanisms underlying the vascular effects of CETP on endothelial cells, have not been addressed. Acetylcholine-induced endothelium-dependent relaxation of aortic rings was impaired in human CETP-expressing transgenic mice, compared to their non-transgenic littermates. However, endothelial nitric oxide synthase (eNOS) activation was enhanced. The generation of superoxide and hydrogen peroxide was increased in aortas from CETP transgenic mice, while silencing CETP in cultured human aortic endothelial cells effectively decreased oxidative stress promoted by all major sources of ROS: mitochondria and NOX2. The endoplasmic reticulum stress markers, known as GADD153, PERK, and ARF6, and unfolded protein response effectors, were also diminished. Silencing CETP reduced endothelial tumor necrosis factor (TNF) α levels, intercellular cell adhesion molecule-1 (ICAM-1), and vascular cell adhesion molecule-1 (VCAM-1) expression, diminishing monocyte adhesion. These results support the notion that CETP expression negatively impacts endothelial cell function, revealing a new mechanism that might contribute to atherosclerosis.

Exon 9-deleted CETP inhibits full length-CETP synthesis and promotes cellular triglyceride storage

Cholesteryl ester transfer protein (CETP) exists as full-length (FL) and exon 9 (E9)-deleted isoforms. The function of E9-deleted CETP is poorly understood. Here, we investigated the role of E9-deleted CETP in regulating the secretion of FL-CETP by cells and explored its possible role in intracellular lipid metabolism. CETP overexpression in cells that naturally express CETP confirmed that E9-deleted CETP is not secreted, and showed that cellular FL- and E9-deleted CETP form an isolatable complex. Coexpression of CETP isoforms lowered cellular levels of both proteins and impaired FL-CETP secretion. These effects were due to reduced synthesis of both isoforms; however, the predominate consequence of FL- and E9-deleted CETP coexpression is impaired FL-CETP synthesis. We reported previously that reducing both CETP isoforms or overexpressing FL-CETP impairs cellular triglyceride (TG) storage. To investigate this further, E9-deleted CETP was expressed in SW872 cells that naturally synthesize CETP and in mouse 3T3-L1 cells that do not. E9-deleted CETP overexpression stimulated SW872 triglyceride synthesis and increased stored TG 2-fold. Expression of E9-deleted CETP in mouse 3T3-L1 cells produced a similar lipid phenotype. In vitro, FL-CETP promotes the transfer of TG from ER-enriched membranes to lipid droplets. E9-deleted CETP also promoted this transfer, although less effectively, and it inhibited the transfer driven by FL-CETP. We conclude that FL- and E9-deleted CETP isoforms interact to mutually decrease their intracellular levels and impair FL-CETP secretion by reducing CETP biosynthesis. E9-deleted CETP, like FL-CETP, alters cellular TG metabolism and storage but in a contrary manner.

Cholesteryl Ester Transfer Protein and Lipid Metabolism and Cardiovascular Diseases

In this chapter, we present the major advances in CETP research since the detection, isolation, and characterization of its activity in the plasma of humans and several species. Since CETP is a major modulator of HDL plasma levels, the clinical importance of CETP activity was recognized very early. We describe the participation of CETP in reverse cholesterol transport, conflicting results in animal and human genetic studies, possible new functions of CETP, and the results of the main clinical trials on CETP inhibition. Despite major setbacks in clinical trials, the hypothesis that CETP inhibitors are anti-atherogenic in humans is still being tested.

Epigenetic modifications in adipose tissue - relation to obesity and diabetes

The growing number of people suffering from obesity and type 2 diabetes mellitus (T2DM) is a global health problem that results in increased mortality from their complications, mainly cardiovascular diseases. Although the relationship between obesity and T2DM is well established, the common molecular pathomechanisms are still under investigation. Recently, it has been suggested that epigenetic modifications may be involved in both obesity and T2DM development. Epigenetics plays a pivotal role in the regulation of gene expression by the reversible modifications of chromatin structure without any changes in DNA sequence. Epigenetic modifications include DNA methylation, posttranslational histone modifications and miRNA interference. Therefore, the aim of this article is to discuss the current knowledge on epigenetic modifications in adipose tissue and their association with obesity and T2DM.

Identification of CETP as a molecular target for estrogen positive breast cancer cell death by cholesterol depleting agents

Cholesterol and its metabolites act as steroid hormone precursors, which promote estrogen receptor positive (ER+) breast cancer (BC) progression. Development of cholesterol targeting anticancer drugs has been hindered due to the lack of knowledge of viable molecular targets. Till now, Cholesteryl ester transfer protein (CETP) has been envisaged as a feasible molecular target in atherosclerosis, but for the first time, we show that CETP contributes to BC cell survival when challenged with cholesterol depleting agents. We show that MCF-7 CETP knockout BC cells pose less resistance towards cytotoxic compounds (Tamoxifen and Acetyl Plumbagin (AP)), and were more susceptible to intrinsic apoptosis. Analysis of differentially expressed genes using Ingenuity Pathway Analysis (IPA), in vivo tumor inhibition, and in vitro phenotypic responses to AP revealed a unique CETP-centric cholesterol pathway involved in sensitizing ER+ BC cells to intrinsic mitochondrial apoptosis. Furthermore, analysis of cell line, tissue and patient data available in publicly available databases linked elevated CETP expression to cancer, cancer relapse and overall poor survival. Overall, our findings highlight CETP as a pharmacologically relevant and unexploited cellular target in BC. The work also highlights AP as a promising chemical entity for preclinical investigations as a cholesterol depleting anticancer therapeutic agent.

Lipid-based transfection reagents can interfere with cholesterol biosynthesis

Lipid-based transfection reagents are widely used for delivery of small interfering RNA into cells. We examined whether the commonly used commercial transfection reagents DharmaFECT-4 and Lipofectamine 2000 can interfere with lipid metabolism by studying cholesterogenesis. Cholesterol de novo synthesis from [(14)C]acetate was assessed in human hepatocyte-derived Huh-7 cells. The results revealed that DharmaFECT, but not Lipofectamine, markedly inhibited cholesterol biosynthesis by approximately 70%. Cell viability was not significantly altered. These findings suggest that caution is required in the choice of certain lipid-based transfection reagents for gene silencing experiments, particularly when assessing cholesterol metabolism.

Impacts of Alternative Splicing Events on the Differentiation of Adipocytes

Alternative splicing was found to be a common phenomenon after the advent of whole transcriptome analyses or next generation sequencing. Over 90% of human genes were demonstrated to undergo at least one alternative splicing event. Alternative splicing is an effective mechanism to spatiotemporally expand protein diversity, which influences the cell fate and tissue development. The first focus of this review is to highlight recent studies, which demonstrated effects of alternative splicing on the differentiation of adipocytes. Moreover, use of evolving high-throughput approaches, such as transcriptome analyses (RNA sequencing), to profile adipogenic transcriptomes, is also addressed.

Defective triglyceride biosynthesis in CETP-deficient SW872 cells

We previously reported that reducing the expression of cholesteryl ester transfer protein (CETP) disrupts cholesterol homeostasis in SW872 cells and causes an ∼50% reduction in TG. The causes of this reduced TG content, investigated here, could not be attributed to changes in the differentiation status of CETP-deficient cells, nor was there evidence of endoplasmic reticulum (ER) stress. In short-term studies, the total flux of oleate through the TG biosynthetic pathway was not altered in CETP-deficient cells, although mRNA levels of some pathway enzymes were different. However, the conversion of diglyceride (DG) to TG was impaired. In longer-term studies, newly synthesized TG was not effectively transported to lipid droplets, yet this lipid did not accumulate in the ER, apparently due to elevated lipase activity in this organelle. DG, shown to be a novel CETP substrate, was also inefficiently transferred to lipid droplets. This may reduce TG synthesis on droplets by resident diacylglycerol acyltransferase. Overall, these data suggest that the decreased TG content of CETP-deficient cells arises from the reduced conversion of DG to TG in the ER and/or on the lipid droplet surface, and enhanced TG degradation in the ER due to its ineffective transport from this organelle.

Oxidative stress and adipocyte biology: focus on the role of AGEs

Diabetes is a major health problem that is usually associated with obesity, together with hyperglycemia and increased advanced glycation endproducts (AGEs) formation. Elevated AGEs elicit severe downstream consequences via their binding to receptors of AGEs (RAGE). This includes oxidative stress and oxidative modifications of biological compounds together with heightened inflammation. For example, albumin (major circulating protein) undergoes increased glycoxidation with diabetes and may represent an important biomarker for monitoring diabetic pathophysiology. Despite the central role of adipose tissue in many physiologic/pathologic processes, recognition of the effects of greater AGEs formation in this tissue is quite recent within the obesity/diabetes context. This review provides a brief background of AGEs formation and adipose tissue biology and thereafter discusses the impact of AGEs-adipocyte interactions in pathology progression. Novel data are included showing how AGEs (especially glycated albumin) may be involved in hyperglycemia-induced oxidative damage in adipocytes and its potential links to diabetes progression.

Curcumin triggers apoptosis via upregulation of Bax/Bcl-2 ratio and caspase activation in SW872 human adipocytes

Induction of adipocyte apoptosis has been postulated as a novel strategy with which to treat obesity. The effects of curcumin, a polyphenol compound, on the apoptotic signaling pathway in SW872 adipocytes were investigated in the present study. The results showed that cell viability decreased following curcumin treatment in a time- and dose-dependent manner. The results from a single-stranded DNA ELISA assay indicated that curcumin causes the number of apoptotic cells to increase in a concentration-dependent manner. In addition, curcumin treatment resulted in an increased expression of Bax, and a decrease in that of of Bcl-2, with a concomitant upregulation of the Bax/Bcl-2 ratio. Curcumin treatment also led to the release of cytochrome c from mitochondria into the cytosol. Similarly, caspase-dependent poly (ADP) ribose polymerase (PARP) cleavage by curcumin was observed in the current study. In conclusion the results indicate that curcumin is an effective therapeutic agent with which to induce apoptosis in adipocytes. This effect is, in part, mediated through the mitochondrial pathway, which involves upregulation of the Bax/Bcl-2 ratio, cytochrome c release, activation of caspase-3 and the cleavage of PARP.

Overexpression of full-length cholesteryl ester transfer protein in SW872 cells reduces lipid accumulation

Cells produce two cholesteryl ester transfer protein (CETP) isoforms, full-length and a shorter variant produced by alternative splicing. Blocking synthesis of both isoforms disrupts lipid metabolism and storage. To further define the role of CETP in cellular lipid metabolism, we stably overexpressed full-length CETP in SW872 cells. These CETP(+) cells had several-fold higher intracellular CETP and accumulated 50% less TG due to a 26% decrease in TG synthesis and 2.5-fold higher TG turnover rate. Reduced TG synthesis was due to decreased fatty acid uptake and impaired conversion of diglyceride to TG even though diacylglycerol acyltransferase activity was normal. Sterol-regulatory element binding protein 1 mRNA levels were normal, and although PPARγ expression was reduced, the expression of several of its target genes including adipocyte triglyceride lipase, FASN, and APOE was normal. CETP(+) cells contained smaller lipid droplets, consistent with their higher levels of perilipin protein family (PLIN) 3 compared with PLIN1 and PLIN2. Intracellular CETP was mostly associated with the endoplasmic reticulum, although CETP near lipid droplets poorly colocalized with this membrane. A small pool of CETP resided in the cytoplasm, and a subfraction coisolated with lipid droplets. These data show that overexpression of full-length CETP disrupts lipid homeostasis resulting in the formation of smaller, more metabolically active lipid droplets.

Cholesteryl ester transfer proteins from different species do not have equivalent activities

Site-specific changes in the amino acid composition of human cholesteryl ester transfer protein (CETP) modify its preference for triglyceride (TG) versus cholesteryl ester (CE) as substrate. CETP homologs are found in many species but little is known about their activity. Here, we examined the lipid transfer properties of CETP species with 80-96% amino acid identity to human CETP. TG/CE transfer ratios for recombinant rabbit, monkey, and hamster CETPs were 1.40-, 1.44-, and 6.08-fold higher than human CETP, respectively. In transfer assays between VLDL and HDL, net transfers of CE into VLDL by human and monkey CETPs were offset by equimolar net transfers of TG toward HDL. For hamster CETP this process was not equimolar but resulted in a net flow of lipid (TG) into HDL. When assayed for the ability to transfer lipid to an acceptor particle lacking CE and TG, monkey and hamster CETPs were most effective, although all CETP species were able to promote this one-way movement of neutral lipid. We conclude that CETPs from human, monkey, rabbit, and hamster are not functionally equivalent. Most unique was hamster CETP, which strongly prefers TG as a substrate and promotes the net flow of lipid from VLDL to HDL.

Soluble HMGB1 is a novel adipokine stimulating IL-6 secretion through RAGE receptor in SW872 preadipocyte cell line: contribution to chronic inflammation in fat tissue

Low-grade inflammation (LGI) is a central phenomenon in the genesis of obesity and insulin-resistance characterized by IL-6 in human serum. Whereas this LGI was initially thought to be mainly attributed to macrophage activation, it is now known that pre-adipocytes and adipocytes secrete several adipokines including IL-6 and participate to LGI and associated pathologies. In macrophages, HMGB1 is a nuclear yet secreted protein and acts as a cytokine to drive the production of inflammatory molecules through RAGE and TLR2/4. In this paper we tested the secretion of HMGB1 and the auto- and paracrine contribution to fat inflammation using the human preadipocyte cell line SW872 as a model. We showed that 1) human SW872 secreted actively HMGB1, 2) IL-6 production was positively linked to high levels of secreted HMGB1, 3) recombinant HMGB1 boosted IL-6 expression and this effect was mediated by the receptor RAGE and did not involve TLR2 or TLR4. These results suggest that HMGB1 is a major adipokine contributing to LGI implementation and maintenance, and can be considered as a target to develop news therapeutics in LGI associated pathologies such as obesity and type II diabetes.

Direct interaction of Plin2 with lipids on the surface of lipid droplets: a live cell FRET analysis

Despite increasing awareness of the health risks associated with excess lipid storage in cells and tissues, knowledge of events governing lipid exchange at the surface of lipid droplets remains unclear. To address this issue, fluorescence resonance energy transfer (FRET) was performed to examine live cell interactions of Plin2 with lipids involved in maintaining lipid droplet structure and function. FRET efficiencies (E) between CFP-labeled Plin2 and fluorescently labeled phosphatidylcholine, sphingomyelin, stearic acid, and cholesterol were quantitated on a pixel-by-pixel basis to generate FRET image maps that specified areas with high E (>60%) in lipid droplets. The mean E and the distance R between the probes indicated a high yield of energy transfer and demonstrated molecular distances on the order of 44-57 Å, in keeping with direct molecular contact. In contrast, FRET between CFP-Plin2 and Nile red was not detected, indicating that the CFP-Plin2/Nile red interaction was beyond FRET proximity (>100 Å). An examination of the effect of Plin2 on cellular metabolism revealed that triacylglycerol, fatty acid, and cholesteryl ester content increased while diacylglycerol remained constant in CFP-Plin2-overexpressing cells. Total phospholipids also increased, reflecting increased phosphatidylcholine and sphingomyelin. Consistent with these results, expression levels of enzymes involved in triacylglycerol, cholesteryl ester, and phospholipid synthesis were significantly upregulated in CFP-Plin2-expressing cells while those associated with lipolysis either decreased or were unaffected. Taken together, these data show for the first time that Plin2 interacts directly with lipids on the surface of lipid droplets and influences levels of key enzymes and lipids involved in maintaining lipid droplet structure and function.

Human cholesteryl ester transfer protein enhances insulin-mediated glucose uptake in adipocytes

AIMS

Adipose tissue plays an important role in the pathogenesis of insulin resistance, obesity, and Type-2 diabetes. Human adipocytes express abundant cholesteryl ester transfer protein (CETP). However, the function and role of CETP in regulating lipoprotein metabolism are mostly unknown. In this study, we examined whether CETP affected the insulin-mediated responses in adipocytes.

MAIN METHODS

Because mouse 3T3-L1 preadipocytes do not express CETP, we established a stable cell line expressing human CETP by transfecting the cells with pcDNA3.1/human CETP. We used a standard approach to differentiate the cells into mature adipocytes, and we examined the cholesterol balance and insulin responses.

KEY FINDINGS

The human CETP stable cell line expressed stable levels of CETP without affecting the expression of either peroxisome proliferator-activated receptor-gamma (PPARγ) or glucose transporter-4 (GLUT4) throughout cell differentiation. CETP expression significantly increased the level of both total and free cholesterol in the mature adipocytes. Upon insulin stimulation, CETP expressing cells had significantly higher protein kinase B (Akt) phosphorylation and 2-(3)H-deoxyglucose uptake, as compared with 3T3-L1 cells and cells transfected with control vector.

SIGNIFICANCE

Human CETP expression increased cellular cholesterol levels and enhanced insulin-stimulated Akt phosphorylation and glucose uptake in adipocytes. Thus, CETP may modulate glucose metabolism and insulin action in addition to its effects on lipoprotein metabolism.

Quantitative monitoring of lipid accumulation over time in cultured adipocytes as function of culture conditions: toward controlled adipose tissue engineering

Adipose tissue engineering is investigated for native fat substitutes and wound healing model systems. Research and clinical applications of bioartificial fat require a quantitative and objective method to continuously measure adipogenesis in living cultures as opposed to currently used culture-destructive techniques that stain lipid droplet (LD) accumulation. To allow standardization, automatic quantification of LD size is further needed, but currently LD size is measured mostly manually. We developed an image processing-based method that does not require staining to monitor adipose cell maturation in vitro nondestructively using optical micrographs taken consecutively during culturing. We employed our method to monitor LD accumulation in 3T3-L1 and mesenchymal stem cells over 37 days. For each cell type, percentage of lipid area, number of droplets per cell, and droplet diameter were obtained every 2-3 days. In 3T3-L1 cultures, high insulin concentration (10 microg/mL) yielded a significantly different (p < 0.01) time course of all three outcome measures. In mesenchymal stem cell cultures, high fetal bovine serum concentration (12.5%) produced significantly more lipid area (p < 0.01). Our method was able to successfully characterize time courses and extents of adipogenesis and is useful for a wide range of applications testing the effects of biochemical, mechanical, and thermal stimulations in tissue engineering of bioartificial fat constructs.

Citrus fruit extracts reduce advanced glycation end products (AGEs)- and H₂O₂-induced oxidative stress in human adipocytes

Diabetes is a reactive oxygen species (ROS)-mediated pathology, with a worldwide prevalence estimated to double by 2030. A major effort has been launched to find therapeutic means to improve health conditions of diabetic patients. Recent data show that supplemental natural antioxidants represent a potential strategy as adjunct therapy. Despite the major role of adipocytes in the etiology of diabetes, little is known about the effect of natural antioxidants on adipocyte response to oxidative stress. Using a diabetes-like oxidative stress model, the potential protective effect of antioxidative flavedo, albedo, and pulp extracts of (1) tangor Elendale (Citrus reticulata × Citrus sinensis) and (2) tangelo Minneola (C. reticulata × Citrus paradisis) was investigated on human adipocytes. Besides the retardation of free-radical-induced hemolysis of human erythrocytes, non-cytotoxic concentrations of tangelo and tangor flavedo extracts significantly reduced the levels of protein carbonyls in response to advanced glycation end products (AGEs) generated by albumin glycation in SW872 cells. Flavedo extracts lowered carbonyl accumulation in H2O2-treated adipocytes, while tangelo and tangor flavedo, albedo, and pulp extracts suppressed ROS production in SW872 cells with or without the addition of H2O2. Our results clearly show that Mauritian Citrus fruit extracts represent an important source of antioxidants, with a novel antioxidative role at the adipose tissue level.

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.

Obesity, cholesterol, and clear-cell renal cell carcinoma (RCC)

Multiple epidemiologic studies have linked the development of renal cancer to obesity. In this chapter, we begin with a review of selected population studies, followed by recent mechanistic discoveries that further link lipid deregulation to the RCC development. The upregulation of leptin and downregulation of adiponectin pathways in obesity fit well with our molecular understanding of RCC pathogenesis. In addition, two forms of hereditary RCC involve proteins, Folliculin and TRC8, that are positioned to coordinately regulate lipid and protein biosynthesis. Both of these biosynthetic pathways have important downstream consequences on HIF-1/2alpha levels and angiogenesis, key aspects in the disease pathogenesis. The role of lipid biology and its interface with protein translation regulation represents a new dimension in RCC research with potential therapeutic implications.

Adipocyte modulation of high-density lipoprotein cholesterol

BACKGROUND

Adipose harbors a large depot of free cholesterol. However, a role for adipose in cholesterol lipidation of high-density lipoprotein (HDL) in vivo is not established. We present the first evidence that adipocytes support transfer of cholesterol to HDL in vivo as well as in vitro and implicate ATP-binding cassette subfamily A member 1 (ABCA1) and scavenger receptor class B type I (SR-BI), but not ATP-binding cassette subfamily G member 1 (ABCG1), cholesterol transporters in this process.

METHODS AND RESULTS

Cholesterol efflux from wild-type, ABCA1(-/-), SR-BI(-/-), and ABCG1(-/-) adipocytes to apolipoprotein A-I (apoA-I) and HDL3 were measured in vitro. 3T3L1 adipocytes, labeled with (3)H-cholesterol, were injected intraperitoneally into wild-type, apoA-I transgenic, and apoA-I(-/-) mice, and tracer movement onto plasma HDL was monitored. Identical studies were performed with labeled wild-type, ABCA1(-/-), or SR-BI(-/-) mouse embryonic fibroblast adipocytes. The effect of tumor necrosis factor-alpha on transporter expression and cholesterol efflux was monitored during adipocyte differentiation. Cholesterol efflux to apoA-I and HDL3 was impaired in ABCA1(-/-) and SR-BI(-/-) adipocytes, respectively, with no effect observed in ABCG1(-/-) adipocytes. Intraperitoneal injection of labeled 3T3L1 adipocytes resulted in increased HDL-associated (3)H-cholesterol in apoA-I transgenic mice but reduced levels in apoA-I(-/-) animals. Intraperitoneal injection of labeled ABCA1(-/-) or SR-BI(-/-) adipocytes reduced plasma counts relative to their respective controls. Tumor necrosis factor-alpha reduced both ABCA1 and SR-BI expression and impaired cholesterol efflux from partially differentiated adipocytes.

CONCLUSIONS

These data suggest a novel metabolic function of adipocytes in promoting cholesterol transfer to HDL in vivo and implicate adipocyte SR-BI and ABCA1, but not ABCG1, in this process. Furthermore, adipocyte modulation of HDL may be impaired in adipose inflammatory disease states such as type 2 diabetes mellitus.

Differential epigenomic and transcriptomic responses in subcutaneous adipose tissue between low and high responders to caloric restriction

BACKGROUND

Caloric restriction is recommended for the treatment of obesity, but it is generally characterized by large interindividual variability in responses. The factors affecting the magnitude of weight loss remain poorly understood. Epigenetic factors (ie, heritable but reversible changes to genomic function that regulate gene expression independently of DNA sequence) may explain some of the interindividual variability seen in weight-loss responses.

OBJECTIVE

The objective was to determine whether epigenetics and gene expression changes may play a role in weight-loss responsiveness.

DESIGN

Overweight/obese postmenopausal women were recruited for a standard 6-mo caloric restriction intervention. Abdominal subcutaneous adipose tissue biopsy samples were collected before (n = 14) and after (n = 14) intervention, and the epigenomic and transcriptomic profiles of the high and low responders to dieting, on the basis of changes in percentage body fat, were compared by using microarray analysis.

RESULTS

Significant DNA methylation differences at 35 loci were found between the high and low responders before dieting, with 3 regions showing differential methylation after intervention. Some of these regions contained genes known to be involved in weight control and insulin secretion, whereas others were localized in known imprinted genomic regions. Differences in gene expression profiles were observed only after dieting, with 644 genes being differentially expressed between the 2 groups. These included genes likely to be involved in metabolic pathways related to angiogenesis and cerebellar long-term depression.

CONCLUSIONS

These data show that both DNA methylation and gene expression are responsive to caloric restriction and provide new insights about the molecular pathways involved in body weight loss as well as methylation regulation during adulthood.

High expression of apolipoprotein E impairs lipid storage and promotes cell proliferation in human adipocytes

Apolipoprotein E (apoE), a key regulator of lipid metabolism, is highly produced by adipose tissue and adipocytes. However, there is little information about its role on adipocyte functions. Because apoE-deficiency in adipocytes was shown to impair adipocyte differentiation, we investigated the consequences of apoE high expression on differentiation and proliferation of a human adipocytic cell line (SW872). SW872 cells were transfected with human apoE to induce a fivefold increase in apoE production and secretion. Adipocyte differentiation and proliferation were assayed by measuring lipid content, adipogenic gene expression, cell number, cell resistance to serum deprivation, and cell division kinetics. Cultured apoE-transfected cells accumulated less triglycerides and less cholesterol than control cells. This decrease in lipid accumulation was associated with a strong downregulation of peroxisome proliferator-activated receptors gamma1 and gamma2 and stearoyl-CoA desaturase 1. The decrease in lipid accumulation was not dependent on the presence of lipids, lipoproteins, or PPAR-gamma agonists in the culture medium, nor was it observed with exogenously added apoE. Moreover, we observed that apoE-transfected cells were more resistant to death induced by serum deprivation, and that these cells underwent more cell divisions than control cells. These results bring new evidence of apoE-involvement in metabolic disorders at the adipocyte level.

Apolipoprotein E limits oxidative stress-induced cell dysfunctions in human adipocytes

Oxidative stress in adipose tissue constitutes a pathological process involved in obesity-linked metabolic disorders. Apolipoprotein E (apoE), which exhibits antioxidant properties in plasma and brain, is highly produced by adipose tissue and adipocytes. In this study, we investigated the role of apoE in the human adipocyte response to oxidative stress. We first demonstrated that apoE secretion by adipocytes was stimulated by oxidative stress. We also observed that apoE overexpression protected adipocytes from hydrogen peroxide-induced damages, by mitigating intracellular oxidation and exerting extracellular antioxidant properties. Our findings clearly show a novel antioxidant role for apoE in adipose tissue.

Effects of nutritional antioxidants on AAPH- or AGEs-induced oxidative stress in human SW872 liposarcoma cells

High levels of oxidative stress were reported in obesity-linked type 2 diabetes and were associated with elevated formation of advanced glycation end products (AGEs). Many studies have focused on the effect of antioxidants on vascular and circulating cells such as macrophages. However, despite the major role of adipocytes in the etiology of diabetes, little is known about the effect of natural antioxidants on adipocyte response to oxidative stress. The present study reports the differential protective effects of plant nutrients toward adipose cells subjected to oxidative stress. Caffeic acid, quercetin, L: -ascorbic acid, and alpha-tocopherol were tested on SW872 liposarcoma cells subjected to a free radical generator or to AGEs. Proliferation, viability, free radical formation, and superoxide dismutase expression were assessed in treated cells. Caffeic acid and quercetin appeared as the most potent antioxidant nutrients. Our findings clearly show a novel antioxidant role for caffeic acid and quercetin at the adipose tissue level. These new data confirm the beneficial role of phytotherapy as an interesting alternative mean for the development of novel therapeutical and nutritional strategy to prevent metabolic disorders inherent to obesity-linked diabetes.

Molecular cloning of hamster lipid transfer inhibitor protein (apolipoprotein F) and regulation of its expression by hyperlipidemia

Lipid transfer inhibitor protein (LTIP) is a regulator of cholesteryl ester transfer protein (CETP) function. Factors affecting plasma LTIP levels are poorly understood. In humans, plasma LTIP is elevated in hypercholesterolemia. To define possible mechanisms by which hyperlipidemia modifies LTIP, we investigated the effects of hypercholesterolemic diets on plasma LTIP and mRNA levels in experimental animals. The hamster, which naturally expresses CETP, was shown to express LTIP. Hamster LTIP mRNA, exclusively detected in the liver, defined a predicted LTIP protein that is 69% homologous to human, with an isoelectric point of 4.15 and Mr = approximately 16.4 kDa. Hyperlipidemia induced by feeding hydrogenated coconut oil, cholesterol, or both lipids increased plasma LTIP mass up to 2.5-fold, with LTIP mass correlating strongly with plasma cholesterol levels. CETP mass was similarly affected by these diets. In contrast, these diets reduced LTIP hepatic mRNA levels by >50%, whereas CETP mRNA was increased. Similar results for both CETP and LTIP were also observed in cholesterol-fed rabbits. In conclusion, we report in hamster and rabbit that dietary lipids regulate LTIP. Diet-induced hypercholesterolemia markedly increased plasma LTIP mass while concomitantly depressing LTIP gene expression. CETP and LTIP have distinct responses to dietary lipids.

The Taq1B-variant in the cholesteryl ester-transfer protein gene and the risk of metabolic syndrome

The metabolic syndrome is associated with low high-density lipoprotein-cholesterol (HDL-C) and decreased low-density lipoprotein (LDL) particle size. The Taq1B-polymorphism in the cholesteryl ester-transfer protein (CETP)-gene influences HDL-C, CETP concentration, and LDL-size. We investigated the effect of the Taq1B-polymorphism on the risk of the metabolic syndrome in 1,503 participants (973 men, 530 women) of the Salzburg Atherosclerosis Prevention program in subjects at High Individual Risk study. CETP concentration was determined in a subgroup (n = 486) by an enzyme-linked immunosorbent assay. Prevalence of the metabolic syndrome was 16.7% (18.5% in men, 13.5% in women). The Taq1B-polymorphism influenced significantly CETP concentrations, HDL-C levels, and LDL-size (P < 0.001 for all). The relative risk of the metabolic syndrome was reduced by 32% (odds ratio (OR) 0.68 (95% CI: 0.51-0.89), P = 0.005) in carriers of the B2 variant. This risk reduction persisted after adjustment for age and sex (OR 0.69 (0.53-0.92), P = 0.01) and after further adjustment for body mass index, waist-to-hip ratio, blood pressure, insulin resistance (IR), HDL-C, and triglycerides (TGs) (OR 0.43 (0.26-0.72), P = 0.001). Furthermore, the risk reduction was more pronounced in men than in women. We conclude that CETP plays an important role in the metabolic syndrome, possibly involving novel functions of CETP.

Is raising HDL a futile strategy for atheroprotection?

The dramatic failure of clinical trials evaluating the cholesterol ester transfer protein inhibitor torcetrapib has led to considerable doubt about the value of raising high-density lipoprotein cholesterol (HDL-C) as a treatment for cardiovascular disease. These results have underscored the intricacy of HDL metabolism, with functional quality perhaps being a more important consideration than the circulating quantity of HDL. As a result, HDL-based therapeutics that maintain or enhance HDL functionality warrant closer investigation. In this article, we review the complexity of HDL metabolism, discuss clinical-trial data for HDL-raising agents, including possible reasons for the failure of torcetrapib, and consider the potential for future HDL-based therapies.