Cholesteryl ester transfer protein protects against insulin resistance in obese female mice

Cholesteryl Ester Transfer Protein (CETP) Variations in Relation to Lipid Profiles and Cardiovascular Diseases: An Update

Lipid metabolism plays an essential role in the pathogenesis of cardiovascular and metabolic diseases. Cholesteryl ester transfer protein (CETP) is a crucial glycoprotein involved in lipid metabolism by transferring cholesteryl esters (CE) and triglycerides (TG) between plasma lipoproteins. CETP activity results in reduced HDL-C and increased VLDL- and LDL-C concentrations, thus increasing the risk of cardiovascular and metabolic diseases. In this review, we discuss the structure of CETP and its mechanism of action. Furthermore, we focus on recent experiments on animal CETP-expressing models, deciphering the regulation and functions of CETP in various genetic backgrounds and interaction with different external factors. Finally, we discuss recent publications revealing the association of CETP single nucleotide polymorphisms (SNPs) with the risk of cardiovascular and metabolic diseases, lifestyle factors, diet and therapeutic interventions. While CETP SNPs can be used as effective diagnostic markers, diet, lifestyle, gender and ethnic specificity should also be considered for effective treatment.

CETP expression ameliorates endothelial function in female mice through estrogen receptor-α and endothelial nitric oxide synthase pathway

Endothelial dysfunction is an early manifestation of atherosclerosis. The cholesteryl ester transfer protein (CETP) has been considered proatherogenic by reducing plasma HDL levels. However, CETP may exhibit cell- or tissue-specific effects. We have previously reported that male mice expressing the human CETP gene show impaired endothelium-mediated vascular relaxation associated with oxidative stress. Although sexual dimorphisms on the metabolic role of CETP have been proposed, possible sex differences in the vascular effects of CETP were not previously studied. Thus, here we investigated the endothelial function of female CETP transgenic mice as compared with nontransgenic controls (NTg). Aortas from CETP females presented preserved endothelium-dependent relaxation to acetylcholine and an endothelium-dependent reduction of phenylephrine-induced contraction. eNOS phosphorylation (Ser1177) and calcium-induced NO levels were enhanced, whereas reactive oxygen species (ROS) production and NOX2 and SOD2 expression were reduced in the CETP female aortas. Furthermore, CETP females exhibited increased aortic relaxation to 17β-estradiol (E2) and upregulation of heat shock protein 90 (HSP90) and caveolin-1, proteins that stabilize estrogen receptor (ER) in the caveolae. Indeed, CETP females showed an increased E2-induced relaxation in a manner sensitive to estrogen receptor-α (ERα) and HSP90 inhibitors methylpiperidinopyrazole (MPP) and geldanamycin, respectively. MPP also impaired the relaxation response to acetylcholine in CETP but not in NTg females. Altogether, the study indicates that CETP expression ameliorates the anticontractile endothelial effect and relaxation to E2 in females. This was associated with less ROS production, and increased eNOS-NO and E2-ERα pathways. These results highlight the need for considering the sex-specific effects of CETP on cardiovascular risk.NEW & NOTEWORTHY Here we demonstrated that CETP expression has a sex-specific impact on the endothelium function. Contrary to what was described for males, CETP-expressing females present preserved endothelium-dependent relaxation to acetylcholine and improved relaxation response to 17β-estradiol. This was associated with less ROS production, increased eNOS-derived NO, and increased expression of proteins that stabilize estrogen receptor-α (ERα), thus increasing E2-ERα signaling sensitivity. These results highlight the need for considering the sex-specific effects of CETP on cardiovascular risk.

Elevated Serum Low-Density Lipoproteins-Cholesterol Levels and B1B2/B2B2 CETP Genotype Are Positively Associated with Nonalcoholic Fatty Liver Disease in Women with Gallstone Disease

Background: Nonalcoholic fatty liver disease (NAFLD) is generated by the interaction between environmental and genetic factors, and the presence of metabolic alterations. Since Taq1B cholesteryl ester transfer protein (CETP) polymorphism is associated with abnormal serum lipid values, it could be related to NAFLD. The aim of this study was to determine the role of the Taq1B CETP polymorphism with serum lipids, anthropometric variables, and the extent of steatosis in Mexican-mestizo women with gallstone disease (GD). Methods: Sixty-two women were enrolled in this cross-sectional study. Serum lipids were determined by dry chemistry. The Taq1B CETP polymorphism was determined by allelic discrimination. CETP serum levels were measured by enzyme-linked immunosorbent assay, and the extent of steatosis with a biopsy staining with Oil-Red-O. Results: Subjects with the B1B2/B2B2 genotype had higher percentage of degree of steatosis than those with B1B1 (11.95% vs. 2.19%, P = 0.008). The B1B2/B2B2 genotype (odds ratio [OR] 3.90 [confidence interval {CI} 95% 1.891-8.536], P = 0.04) and an elevated low-density lipoproteins (LDL)-cholesterol (OR 3.54 [CI 95% 1.042-2.058, P = 0.039) significantly increase the risk for NAFLD. Conclusions: This study provides evidence that the B1B2/B2B2 genotype of CETP and the elevated LDL-cholesterol serum levels increase the risk of NAFLD in women with GD.

Immune and metabolic cross-links in the pathogenesis of comorbid non-alcoholic fatty liver disease

In recent years, there has been a steady growth of interest in non-alcoholic fatty liver disease (NAFLD), which is associated with negative epidemiological data on the prevalence of the disease and its clinical significance. NAFLD is closely related to the metabolic syndrome and these relationships are the subject of active research. A growing body of evidence shows cross-linkages between metabolic abnormalities and the innate immune system in the development and progression of NAFLD. These links are bidirectional and largely still unclear, but a better understanding of them will improve the quality of diagnosis and management of patients. In addition, lipid metabolic disorders and the innate immune system link NAFLD with other diseases, such as atherosclerosis, which is of great clinical importance.

Effects of Montmorency Tart Cherry and Blueberry Juice on Cardiometabolic and Other Health-Related Outcomes: A Three-Arm Placebo Randomized Controlled Trial

The current study aimed to investigate the influence of tart cherry and blueberry juices on cardiometabolic and other health indices following a 20-day supplementation period. Forty-five adults were randomly assigned to receive tart cherry, blueberry, or a placebo, of which they drank 60 mL per day for 20 days. The primary outcome, which was systolic blood pressure, and secondary measures, including anthropometric, energy expenditure, substrate oxidation, hematological, diastolic blood pressure/resting heart rate, psychological wellbeing, and sleep efficacy, were measured before and after the intervention. There were no statistically significant differences (p > 0.05) for systolic blood pressure; however, total and LDL cholesterol were significantly improved with blueberry intake (pre: total cholesterol = 4.36 mmol/L and LDL cholesterol = 2.71 mmol/L; post: total cholesterol = 3.79 mmol/L and LDL cholesterol = 2.23 mmol/L) compared to placebo (pre: total cholesterol = 4.01 mmol/L and LDL cholesterol = 2.45 mmol/L; post: total cholesterol = 4.34 mmol/L and LDL cholesterol = 2.67 mmol/L). Furthermore, psychological wellbeing indices measured using the Beck Depression Inventory, State Trait Anxiety Inventory, and COOP WONCA improved statistically in the blueberry arm compared to placebo. Given the clear association between lipid concentrations and the risk of cardiovascular disease as well as the importance of psychological wellbeing to health-related quality of life, this investigation indicates that it could be an effective approach to assist in managing cardiometabolic disease.

Expressing the Human Cholesteryl Ester Transfer Protein Minigene Improves Diet-Induced Fatty Liver and Insulin Resistance in Female Mice

Mounting evidence has shown that CETP has important physiological roles in adapting to chronic nutrient excess, specifically, to protect against diet-induced insulin resistance. However, the underlying mechanisms for the protective roles of CETP in metabolism are not yet clear. Mice naturally lack CETP expression. We used transgenic mice with a human CETP minigene (huCETP) controlled by its natural flanking region to further understand CETP-related physiology in response to obesity. Female huCETP mice and their wild-type littermates were fed a high-fat diet for 6 months. Blood lipid profile and liver lipid metabolism were studied. Insulin sensitivity was analyzed with euglycemic-hyperinsulinemic clamp studies combined with ³H-glucose tracer techniques. While high-fat diet feeding induced obesity for huCETP mice and their wild-type littermates lacking CETP expression, insulin sensitivity was higher for female huCETP mice than for their wild-type littermates. There was no difference in insulin sensitivity for male huCETP mice vs. littermates. The increased insulin sensitivity in females was largely caused by the better insulin-mediated suppression of hepatic glucose production. In huCETP females, CETP in the circulation decreased HDL-cholesterol content and increased liver cholesterol uptake and liver cholesterol and oxysterol contents, which was associated with the upregulation of LXR target genes in long-chain polyunsaturated fatty acid biosynthesis and PPARα target genes in fatty acid β-oxidation in the liver. The upregulated fatty acid β-oxidation may account for the improved fatty liver and liver insulin action in female huCETP mice. This study provides further evidence that CETP has beneficial physiological roles in the metabolic adaptation to nutrient excess by promoting liver fatty acid oxidation and hepatic insulin sensitivity, particularly for females.

Novel role of cholesteryl ester transfer protein (CETP): attenuation of adiposity by enhancing lipolysis and brown adipose tissue activity

OBJECTIVE

The systemic function of CETP has been well characterized. CETP plasma activity reduces HDL cholesterol and thus increases the risk of atherosclerosis. Here, we investigated whether CETP expression modulate adiposity.

METHODS

Body adiposity and energy metabolism related assays and gene/protein expression were compared in CETP transgenic and non-transgenic mice and in hamsters treated with CETP neutralizing antibody.

RESULTS

We found that transgenic mice expressing human CETP present less white adipose tissue mass and lower leptinemia than nontransgenic (NTg) littermates. No differences were found in physical activity, food intake, fat fecal excretion, lipogenesis or exogenous lipid accumulation in adipose depots. Nonetheless, adipose lipolysis rates and whole-body energy expenditure were elevated in CETP mice. In accordance, lipolysis-related gene expression and protein content were increased in visceral and brown adipose tissue (BAT). In addition, we verified increased BAT temperature and oxygen consumption. These results were confirmed in two other animal models: 1) hamsters treated with CETP neutralizing antibody and 2) an independent line of transgenic mice expressing simian CETP.

CONCLUSIONS

These findings reveal a novel anti-adipogenic role for CETP.

Cholesteryl Ester Transfer Protein Impairs Triglyceride Clearance via Androgen Receptor in Male Mice

Elevated postprandial triacylglycerols (TAG) are an important risk factor for cardiovascular disease. Men have higher plasma TAG and impaired TAG clearance compared to women, which may contribute to sex differences in risk of cardiovascular disease. Understanding mechanisms of sex differences in TAG metabolism may yield novel therapeutic targets to prevent cardiovascular disease. Cholesteryl ester transfer protein (CETP) is a lipid shuttling protein known for its effects on high‐density lipoprotein (HDL) cholesterol levels. Although mice lack CETP, we previously demonstrated that transgenic CETP expression in female mice alters TAG metabolism. The impact of CETP on TAG metabolism in males, however, is not well understood. Here, we demonstrate that CETP expression increases plasma TAG in males, especially in very‐low density lipoprotein (VLDL), by impairing postprandial plasma TAG clearance compared to wild‐type (WT) males. Gonadal hormones were required for CETP to impair TAG clearance, suggesting a role for sex hormones for this effect. Testosterone replacement in the setting of gonadectomy was sufficient to restore the effect of CETP on TAG. Lastly, liver androgen receptor (AR) was required for CETP to increase plasma TAG. Thus, expression of CETP in males raises plasma TAG by impairing TAG clearance via testosterone signaling to AR. Further understanding of how CETP and androgen signaling impair TAG clearance may lead to novel approaches to reduce TAG and mitigate risk of cardiovascular disease.

A lipidome-wide association study of the lipoprotein insulin resistance index

Background

The lipoprotein insulin resistance (LPIR) score was shown to predict insulin resistance (IR) and type 2 diabetes (T2D) in healthy adults. However, the molecular basis underlying the LPIR utility for classification remains unclear.

Objective

To identify small molecule lipids associated with variation in the LPIR score, a weighted index of lipoproteins measured by nuclear magnetic resonance, in the Genetics of Lipid Lowering Drugs and Diet Network (GOLDN) study (n = 980).

Methods

Linear mixed effects models were used to test the association between the LPIR score and 413 lipid species and their principal component analysis-derived groups. Significant associations were tested for replication with homeostatic model assessment-IR (HOMA-IR), a phenotype correlated with the LPIR score (r = 0.48, p <  0.001), in the Heredity and Phenotype Intervention (HAPI) Heart Study (n = 590).

Results

In GOLDN, 319 lipids were associated with the LPIR score (false discovery rate-adjusted p-values ranging from 4.59 × 10^− 161 to 49.50 × 10^− 3). Factors 1 (triglycerides and diglycerides/storage lipids) and 3 (mixed lipids) were positively (β = 0.025, p = 4.52 × 10^− 71 and β = 0.021, p = 5.84 × 10^− 41, respectively) and factor 2 (phospholipids/non-storage lipids) was inversely (β = − 0.013, p = 2.28 × 10^− 18) associated with the LPIR score. These findings were replicated for HOMA-IR in the HAPI Heart Study (β = 0.10, p = 1.21 × 10^− 02 for storage, β = − 0.13, p = 3.14 × 10^− 04 for non-storage, and β = 0.19, p = 8.40 × 10^− 07 for mixed lipids).

Conclusions

Non-storage lipidomics species show a significant inverse association with the LPIR metabolic dysfunction score and present a promising focus for future therapeutic and prevention studies.

Proteomics study of the effect of high-fat diet on rat liver

Excessive intake of high-energy diets is an important cause of most obesity. The intervention of rats with high-fat diet can replicate the ideal animal model for studying the occurrence of human nutritional obesity. Proteomics and bioinformatics analyses can help us to systematically and comprehensively study the effect of high-fat diet on rat liver. In the present study, 4056 proteins were identified in rat liver by using tandem mass tag. A total of 198 proteins were significantly changed, of which 103 were significantly up-regulated and ninety-five were significantly down-regulated. These significant differentially expressed proteins are primarily involved in lipid metabolism and glucose metabolism processes. The intake of a high-fat diet forces the body to maintain physiological balance by regulating these key protein spots to inhibit fatty acid synthesis, promote fatty acid oxidation and accelerate fatty acid degradation. The present study enriches our understanding of metabolic disorders induced by high-fat diets at the protein level.

CETP Inhibition Improves HDL Function but Leads to Fatty Liver and Insulin Resistance in CETP-Expressing Transgenic Mice on a High-Fat Diet

In clinical trials, inhibition of cholesteryl ester transfer protein (CETP) raises HDL cholesterol levels but does not robustly improve cardiovascular outcomes. Approximately two-thirds of trial participants are obese. Lower plasma CETP activity is associated with increased cardiovascular risk in human studies, and protective aspects of CETP have been observed in mice fed a high-fat diet (HFD) with regard to metabolic outcomes. To define whether CETP inhibition has different effects depending on the presence of obesity, we performed short-term anacetrapib treatment in chow- and HFD-fed CETP transgenic mice. Anacetrapib raised HDL cholesterol and improved aspects of HDL functionality, including reverse cholesterol transport, and HDL's antioxidative capacity in HFD-fed mice was better than in chow-fed mice. Anacetrapib worsened the anti-inflammatory capacity of HDL in HFD-fed mice. The HDL proteome was markedly different with anacetrapib treatment in HFD- versus chow-fed mice. Despite benefits on HDL, anacetrapib led to liver triglyceride accumulation and insulin resistance in HFD-fed mice. Overall, our results support a physiologic importance of CETP in protecting from fatty liver and demonstrate context selectivity of CETP inhibition that might be important in obese subjects.

Exercise Training Favorably Modulates Gene and Protein Expression That Regulate Arterial Cholesterol Content in CETP Transgenic Mice

Aerobic exercise training (AET) improves the reverse cholesterol transport (RCT) in cholesteryl ester transfer protein-transgenic (CETP-tg) mice. We aimed at investigating the role of AET in the expression of genes and proteins involved in lipid flux in the aorta and macrophages of CETP-tg mice. Three-month-old male mice were randomly divided into trained (T; treadmill 15 m/min; 30 min/day) and sedentary (S) groups. After 6 weeks, peritoneal macrophages and the aortic arch were obtained immediately (0 h) or 48 h after the last exercise session. mRNA was determined by RT-qPCR, protein levels by immunoblot and ¹⁴C-cholesterol efflux determined in macrophages. AET did not change body weight, plasma cholesterol, triglycerides, glucose and CETP activity. In macrophages, at time 0 h, a higher expression of genes that encode PPAR gamma, ABCA-1 and a lower expression of MCP-1 and IL-10, was observed in T as compared to S. After 48 h, lower expressions of MCP-1 and PPAR gamma genes were observed in T mice. Increase in ABCA-1, SR-BI and IL-6 and decrease of LOX-1, MCP-1, TNF and IL-10 gene expression was observed in the aorta of T compared to S mice (0 h) and LOX-1 and MCP-1 remained diminished after 48 h. The protein level of MCP-1 and SR-BI in the aortic arch was unchanged in T animals after 48 h as compared to S, but LOX-1 was reduced confirming data of gene expression. The apo A-I and the HDL₂ mediated-cholesterol efflux (8 and 24 h) were not different between T and S animals. In the presence of CETP, AET positively influences gene expression in the arterial wall and macrophages of CETP-tg mice contributing to the RCT and prevention of atherosclerosis. These changes were perceptible immediately after the exercise session and were influenced by the presence of CETP although independent of changes in its activity. Reductions in gene and protein expression of LOX-1 were parallel and reflect the ability of exercise training in reducing the uptake of modified LDL by the arterial wall macrophages.

Involvement of CETP (Cholesteryl Ester Transfer Protein) in the Shift of Sphingosine-1-Phosphate Among Lipoproteins and in the Modulation of its Functions

OBJECTIVE

Sphingosine-1-phosphate (S1P) is a vasoprotective lipid mediator. About two thirds of plasma S1P rides on high-density lipoprotein (HDL), and several pleiotropic properties of HDL have been ascribed to S1P. In human subjects, CETP (cholesteryl ester transfer protein) greatly influences HDL quantities. In this study, we attempted to elucidate the roles of CETP in the metabolism of S1P.

APPROACH AND RESULTS

We overexpressed CETP in mice that lacked CETP and found that CETP overexpression decreased the HDL level but failed to modulate the levels of S1P and apolipoprotein M (apoM), a carrier of S1P, in the total plasma. We observed, however, that the distribution of S1P and apoM shifted from HDL to apoB-containing lipoproteins. When we administered C₁₇S1P bound to apoM-containing lipoprotein, C₁₇S1P and apoM were rapidly transferred to apoB-containing lipoproteins in CETP-overexpressing mice. When HDL containing C₁₇S1P was mixed with low-density lipoprotein ex vivo, C₁₇S1P shifted to the low-density lipoprotein fraction independent of the presence of CETP. Concordant with these results, apoM was distributed mainly to the same fraction as apo AI in a CETP-deficient subject, although apoM was also detected in apo AI-poor fractions in a corresponding hypercholesterolemia subject. About the bioactivities of S1P carried on each lipoprotein, S1P riding on apoB-containing lipoproteins induced the phosphorylation of Akt (AKT8 virus oncogene cellular homolog) and eNOS (endothelial nitric oxide synthase) in human umbilical vein endothelial cells, and CETP overexpression increased insulin secretion and sensitivity, which was inhibited by an S1P receptor 1 or 3 antagonist.

CONCLUSIONS

CETP modulates the distribution of S1P among lipoproteins, which affects the bioactivities of S1P.

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.

Cholesteryl Ester Transfer Protein Inhibitors

The cholesteryl ester transfer protein (CETP) is a plasma protein that plays an important role in the transfer of lipids between plasma lipoproteins. The CETP inhibitors have been widely studied as a pharmacologic therapy to target plasma cholesterol in order to reduce the risk of atherosclerotic cardiovascular disease . Using CETP inhibitors as cholesterol modifiers was based on the genetic research that found correlations between CETP activity and cholesterol levels. Although CETP inhibitors are successful at altering targeted cholesterol markers, recent phase 3 outcome trials have shown limited benefit on cardiovascular outcomes when combined with the current standard of care. We discuss the science of CETP inhibition, compare the CETP inhibitors developed (torcetrapib, evacetrapib, dalcetrapib, and anacetrapib), the findings from the CETP inhibitor trials, and the future outlook for CETP inhibitors in cholesterol modification.

Cholesteryl ester transfer protein alters liver and plasma triglyceride metabolism through two liver networks in female mice

Elevated plasma TGs increase risk of cardiovascular disease in women. Estrogen treatment raises plasma TGs in women, but molecular mechanisms remain poorly understood. Here we explore the role of cholesteryl ester transfer protein (CETP) in the regulation of TG metabolism in female mice, which naturally lack CETP. In transgenic CETP females, acute estrogen treatment raised plasma TGs 50%, increased TG production, and increased expression of genes involved in VLDL synthesis, but not in nontransgenic littermate females. In CETP females, estrogen enhanced expression of small heterodimer partner (SHP), a nuclear receptor regulating VLDL production. Deletion of liver SHP prevented increases in TG production and expression of genes involved in VLDL synthesis in CETP mice with estrogen treatment. We also examined whether CETP expression had effects on TG metabolism independent of estrogen treatment. CETP increased liver β-oxidation and reduced liver TG content by 60%. Liver estrogen receptor α (ERα) was required for CETP expression to enhance β-oxidation and reduce liver TG content. Thus, CETP alters at least two networks governing TG metabolism, one involving SHP to increase VLDL-TG production in response to estrogen, and another involving ERα to enhance β-oxidation and lower liver TG content. These findings demonstrate a novel role for CETP in estrogen-mediated increases in TG production and a broader role for CETP in TG metabolism.

Stress-impaired transcription factor expression and insulin secretion in transplanted human islets

Type 2 diabetes is characterized by insulin resistance, hyperglycemia, and progressive β cell dysfunction. Excess glucose and lipid impair β cell function in islet cell lines, cultured rodent and human islets, and in vivo rodent models. Here, we examined the mechanistic consequences of glucotoxic and lipotoxic conditions on human islets in vivo and developed and/or used 3 complementary models that allowed comparison of the effects of hyperglycemic and/or insulin-resistant metabolic stress conditions on human and mouse islets, which responded quite differently to these challenges. Hyperglycemia and/or insulin resistance impaired insulin secretion only from human islets in vivo. In human grafts, chronic insulin resistance decreased antioxidant enzyme expression and increased superoxide and amyloid formation. In human islet grafts, expression of transcription factors NKX6.1 and MAFB was decreased by chronic insulin resistance, but only MAFB decreased under chronic hyperglycemia. Knockdown of NKX6.1 or MAFB expression in a human β cell line recapitulated the insulin secretion defect seen in vivo. Contrary to rodent islet studies, neither insulin resistance nor hyperglycemia led to human β cell proliferation or apoptosis. These results demonstrate profound differences in how excess glucose or lipid influence mouse and human insulin secretion and β cell activity and show that reduced expression of key islet-enriched transcription factors is an important mediator of glucotoxicity and lipotoxicity.

Cholesteryl Ester Transfer Protein (CETP) expression does not affect glucose homeostasis and insulin secretion: studies in human CETP transgenic mice

Background

Cholesteryl ester transfer protein (CETP) is a plasma protein that mediates the exchange of triglycerides for esterified cholesterol between HDL and apoB-lipoproteins. Previous studies suggest that CETP may modify glucose metabolism in patients or cultured cells. In this study, we tested if stable CETP expression would impair glucose metabolism.

Methods

We used human CETP transgenic mice and non-transgenic littermate controls (NTg), fed with control or high fat diet, as well as in dyslipidemic background and aging conditions. Assays included glucose and insulin tolerance tests, isolated islets insulin secretion, tissue glucose uptake and adipose tissue GLUT mRNA expression.

Results

CETP expression did not modify glucose or insulin tolerance in all tested conditions such as chow and high fat diet, adult and aged mice, normo and dyslipidemic backgrounds. Fasting and fed state plasma levels of insulin were not differ in CETP and NTg mice. Direct measurements of isolated pancreatic islet insulin secretion rates induced by glucose (11, 16.7 or 22 mM), KCl (40 mM), and leucine (10 mM) were similar in NTg and CETP mice, indicating that CETP expression did not affect β-cell function in vivo and ex vivo. Glucose uptake by insulin target tissues, measured in vivo using ³H-2-deoxyglucose, showed that CETP expression had no effect on the glucose uptake in liver, muscle, perigonadal, perirenal, subcutaneous and brown adipose tissues. Accordingly, GLUT1 and GLUT4 mRNA in adipose tissue were not affected by CETP.

Conclusions

In summary, by comparing the in vivo all-or-nothing CETP expressing mouse models, we demonstrated that CETP per se has no impact on the glucose tolerance and tissue uptake, global insulin sensitivity and beta cell insulin secretion rates.

CETP Expression Protects Female Mice from Obesity-Induced Decline in Exercise Capacity

Pharmacological approaches to reduce obesity have not resulted in dramatic reductions in the risk of coronary heart disease (CHD). Exercise, in contrast, reduces CHD risk even in the setting of obesity. Cholesteryl Ester Transfer Protein (CETP) is a lipid transfer protein that shuttles lipids between serum lipoproteins and tissues. There are sexual-dimorphisms in the effects of CETP in humans. Mice naturally lack CETP, but we previously reported that transgenic expression of CETP increases muscle glycolysis in fasting and protects against insulin resistance with high-fat diet (HFD) feeding in female but not male mice. Since glycolysis provides an important energy source for working muscle, we aimed to define if CETP expression protects against the decline in exercise capacity associated with obesity. We measured exercise capacity in female mice that were fed a chow diet and then switched to a HFD. There was no difference in exercise capacity between lean, chow-fed CETP female mice and their non-transgenic littermates. Female CETP transgenic mice were relatively protected against the decline in exercise capacity caused by obesity compared to WT. Despite gaining similar fat mass after 6 weeks of HFD-feeding, female CETP mice showed a nearly two-fold increase in run distance compared to WT. After an additional 6 weeks of HFD-feeding, mice were subjected to a final exercise bout and muscle mitochondria were isolated. We found that improved exercise capacity in CETP mice corresponded with increased muscle mitochondrial oxidative capacity, and increased expression of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α). These results suggest that CETP can protect against the obesity-induced impairment in exercise capacity and may be a target to improve exercise capacity in the context of obesity.

Macrophage-independent regulation of reverse cholesterol transport by liver X receptors

OBJECTIVE

The ability of high-density lipoprotein (HDL) particles to accept cholesterol from peripheral cells, such as lipid-laden macrophages, and to transport cholesterol to the liver for catabolism and excretion in a process termed reverse cholesterol transport (RCT) is thought to underlie the beneficial cardiovascular effects of elevated HDL. The liver X receptors (LXRs; LXRα and LXRβ) regulate RCT by controlling the efflux of cholesterol from macrophages to HDL and the excretion, catabolism, and absorption of cholesterol in the liver and intestine. Importantly, treatment with LXR agonists increases RCT and decreases atherosclerosis in animal models. Nevertheless, LXRs are expressed in multiple tissues involved in RCT, and their tissue-specific contributions to RCT are still not well defined.

APPROACH AND RESULTS

Using tissue-specific LXR deletions together with in vitro and in vivo assays of cholesterol efflux and fecal cholesterol excretion, we demonstrate that macrophage LXR activity is neither necessary nor sufficient for LXR agonist-stimulated RCT. In contrast, the ability of LXR agonists primarily acting in the intestine to increase HDL mass and HDL function seems to underlie the ability of LXR agonists to stimulate RCT in vivo.

CONCLUSIONS

We demonstrate that activation of LXR in macrophages makes little or no contribution to LXR agonist-stimulated RCT. Unexpectedly, our studies suggest that the ability of macrophages to efflux cholesterol to HDL in vivo is not regulated by macrophage activity but is primarily determined by the quantity and functional activity of HDL.