Receptor-mediated mechanisms of lipoprotein remnant catabolism

Changes in the Mitochondria in the Aging Process-Can α-Tocopherol Affect Them?

Aerobic organisms use molecular oxygen in several reactions, including those in which the oxidation of substrate molecules is coupled to oxygen reduction to produce large amounts of metabolic energy. The utilization of oxygen is associated with the production of ROS, which can damage biological macromolecules but also act as signaling molecules, regulating numerous cellular processes. Mitochondria are the cellular sites where most of the metabolic energy is produced and perform numerous physiological functions by acting as regulatory hubs of cellular metabolism. They retain the remnants of their bacterial ancestors, including an independent genome that encodes part of their protein equipment; they have an accurate quality control system; and control of cellular functions also depends on communication with the nucleus. During aging, mitochondria can undergo dysfunctions, some of which are mediated by ROS. In this review, after a description of how aging affects the mitochondrial quality and quality control system and the involvement of mitochondria in inflammation, we report information on how vitamin E, the main fat-soluble antioxidant, can protect mitochondria from age-related changes. The information in this regard is scarce and limited to some tissues and some aspects of mitochondrial alterations in aging. Improving knowledge of the effects of vitamin E on aging is essential to defining an optimal strategy for healthy aging.

Genetic Factors Influencing Warfarin Dose in Han Chinese Population: A Systematic Review and Meta-Analysis of Cohort Studies

OBJECTIVE

To investigate the association of single nucleotide polymorphisms (SNPs) of various genes known to influence mean daily warfarin dose (MDWD) in the Han Chinese population.

METHODS

The study is a systematic review and meta-analysis. Selected studies retrieved by searching Pubmed, Embase (Ovid), Medline, CNKI, Wanfang data, and SinoMed (from their inception to 31 August 2022) for the cohort studies assessing genetic variations that may possibly influence MDWD in Chinese patients were included.

RESULT

A total of 46 studies including a total of 10,102 Han Chinese adult patients were finally included in the meta-analysis. The impact of 20 single nucleotide polymorphisms (SNPs) in 8 genes on MDWD was analyzed. The significant impact of some of these SNPs on MDWD requirements was demonstrated. Patients with CYP4F2 rs2108622 TT, EPHX1 rs2260863 GC, or NQO1 rs1800566 TT genotype required more than 10% higher MDWD. Furthermore, patients with ABCB1 rs2032582 GT or GG, or CALU rs2290228 TT genotype required more than 10% lower MDWD. Subgroup analysis showed that patients with EPHX1 rs2260863 GC genotype required 7% lower MDWD after heart valve replacement (HVR).

CONCLUSION

This is the first systematic review and meta-analysis assessing the association between single nucleotide polymorphisms (SNPs) of various genes known to influence MDWD besides CYP2C9 and VKORC1 in the Han Chinese population. CYP4F2 (rs2108622), GGCX (rs12714145), EPHX1 (rs2292566 and rs2260863), ABCB1 (rs2032582), NQO1 (rs1800566), and CALU (rs2290228) SNPs might be moderate factors affecting MDWD requirements.

REGISTERED INFORMATION

PROSPERO International Prospective Register of Systematic Reviews (CRD42022355130).

Purification and Analysis of Circulating Lipid Particles

Lipid particles found in circulating extracellular fluids such as blood or lymph are essential for cellular homeostasis, metabolism and survival. Such particles provide essential lipids and fats which enable cells to synthesize new membranes and regulate different biochemical pathways. Imbalance in lipid particle metabolism can cause pathological states such as atherosclerosis. Here, elevated low-density lipoprotein (LDL) accumulation leads to fat-filled lesions or plaques in arterial walls. In this chapter, we provide a detailed set of protocols for the rapid and safe purification of lipid particles from human blood using high-speed ultracentrifugation. We provide a detailed set of assays for further analysis of the biochemical and cellular properties of these lipid particles. By combining these assays, we can better understand the complex roles of different lipid particles in normal physiology and disease pathology.

Effects of Evolocumab on the ApoA1 Remnant Ratio: A Pooled Analysis of Phase 3 Studies

Introduction

The apolipoprotein A1 (apoA1) remnant ratio has been identified as an independent cardiovascular (CV) risk factor. Higher apoA1 remnant ratios may predict lower CV risk in some patients. This analysis aimed to evaluate the effects of evolocumab on the change from baseline in the apoA1 remnant ratio compared with placebo.

Methods

This pooled post hoc analysis included 2464 patients with mixed dyslipidemia treated with evolocumab 140 mg every 2 weeks (Q2W) or 420 mg once monthly (QM) in three phase 3 evolocumab trials. The apoA1 remnant ratio was calculated by dividing apoA1 by the difference between non-high-density lipoprotein cholesterol (non-HDL-C) and low-density lipoprotein cholesterol (LDL-C). ApoA1 remnant ratio strata were generated using previously published tertile (< 4.7, 4.7–6.8, and > 6.8) and partitioning categories (< 3.6, 3.6–6.0, and > 6.0).

Results

The baseline apoA1 remnant ratio in evolocumab and placebo treatment arms was 7.1 and 7.3, respectively. At week 12, evolocumab 140 mg Q2W and 420 mg QM increased the apoA1 remnant ratio by 25.0% and 33.6%, respectively, versus placebo (p < 0.0001 for both groups). When patients were categorized by week 12 apoA1 remnant ratio thresholds (< 3.6 vs. > 3.6, and < 4.7 vs. > 4.7), those with higher week 12 apoA1 remnant ratios were significantly more likely to have also achieved a target non-HDL-C level of < 100 mg/dl. In the subset of women > 50 years of age, the proportion of patients at apoA1 remnant ratio thresholds < 3.6, 3.6–6.0, and > 6.0 at baseline shifted toward or remained at higher thresholds at week 12.

Conclusions

This post hoc analysis suggests that evolocumab increases the apoA1 remnant ratio.

Funding

Amgen Inc.

Plain Language Summary

Plain language summary available for this article.

Electronic supplementary material

The online version of this article (10.1007/s40119-019-0133-6) contains supplementary material, which is available to authorized users.

Hypothesis II: The majority of VLDL-apoB48 remnants in postprandial plasma are derived from the liver, not from the intestine

We have long thought that remnant lipoproteins (RLP) in the postprandial plasma contain CM remnants (exogenous remnants; RLP-apoB48) and VLDL remnants (endogenous remnants; RLP-apoB100) of different origin, i.e. produced in the intestine and liver, respectively. However, the majority of CM remnants incorporated into liver from the circulation are degraded in liver and may be reused for the remodeling of VLDL. Namely, the most of the apoB48 in CM remnants are smoothly incorporated into the liver after fat intake along with lipids and other apolipoproteins via the LDL receptor and LDL-receptor-related protein (LRP). Subsequently, apoB48 may be reconstituted in VLDL as VLDL apoB48 through an essential physiological pathway similar or the same to that of VLDL apoB100 formation in the liver and secreted into the circulation as VLDL apoB48 to form their remnants. Because those particles are newly reconstituted in liver as a portion of VLDL, we propose that both RLP-apoB100 and RLP-apoB48 are endogenous VLDL remnants produced in liver after fat intake. Also we predict the presence of a new pathway for the formation of VLDL apoB48 along with VLDL apoB100 in liver in humans similar in mice and rats.

Long-term Western diet fed apolipoprotein E-deficient rats exhibit only modest early atherosclerotic characteristics

In the apolipoprotein E–deficient mouse, the gut microbiota has an impact on the development of atherosclerosis, but whether such correlations are also present in rats requires investigation. Therefore, we studied female SD-Apoe^tm1sage (Apoe^−/−) rats fed either a Western diet or a low-fat control diet with or without gluten, which is known to promote gut microbiota changes, until 20 weeks of age. We hypothesized that the manifestation of atherosclerosis would be more severe in Apoe^−/− rats fed the Western high-fat diet, as compared with rats fed the low-fat diet, and that atherosclerosis would be accelerated by gluten. Both Western diet-feeding and gluten resulted in significant changes in gut microbiota, but the microbiota impact of gluten was transient. Compared with Apoe^−/− rats fed a low-fat diet, Western diet-fed Apoe^−/− rats were heavier and became glucose intolerant with increased levels of oxidative stress. They developed early fatty streak lesions in their aortic sinus, while there was no evidence of atherosclerosis in the thoracic aorta. No conclusions could be made on the impact of gluten on atherosclerosis. Although Western diet-fed Apoe^−/− rats exhibited a more human-like LDL dominated blood lipid profile, signs of obesity, type 2 diabetes and cardiovascular disease were modest.

Molecular mechanisms of tributyltin-induced alterations in cholesterol homeostasis and steroidogenesis in hamster testis: In vivo and in vitro studies

Tributyltins (TBT) are ubiquitous and persistent environmental contaminants that disturb normal endocrine function including gonadal function in humans and marine organisms. TBT was administered through oral route to male Syrian hamsters at daily doses of 50, 100, and 150 ppm/kg for 65 days. Changes in testis morphology, immunohistochemistry of iNOS, 3β-HSD, and 17β-HSD, cholesterol transport receptor, nuclear receptors, and transcription factors were analyzed. TBT treatment affected each of these parameters to significant levels in a dose-dependent manner compared to vehicle treated control. Real-time PCR and protein analyses revealed that expression levels of ApoE and LDL-R mRNA were up-regulated in the testis of TBT-treated animals while the expression levels of SR-B1, LXR, PPARs α, β, and γ, SCAP, SREBP 1 and 2, 3β-HSD, 17β-HSD, CYP17A1, and P450_SCC were down-regulated. Leydig cells were isolated and separated adopting percoll gradient centrifugation under aseptic condition. The viability of Leydig cell was affected by TBT treatment in a dose- and time-dependent manner. Further, the mechanism of action of TBT was ascertained by siRNA transfection of ApoE, which was upregulated, and SREBP, which was down-regulated. These observations led us to infer that exposure to TBT hinders intracellular cholesterol transport resulting in abnormal sex steroid biosynthesis and alteration of steroidogenic enzyme activities. Finally, we could recognize ApoE and SREBP as the major factors regulating genes that control cholesterol biosynthesis and steroidogenesis that ultimately inhibit the synthesis of testosterone. Therefore, ApoE is one of the important molecular targets that can be intercepted in context of male infertility/male contraception.

Influence of APOE Gene Polymorphism on Interindividual and Interethnic Warfarin Dosage Requirement: A Systematic Review and Meta-Analysis

BACKGROUND

Warfarin is the most extensively used coumarin anticoagulant. It has been shown that the anticoagulant effect of warfarin is associated with genetic variation. Apolipoprotein E (ApoE) is a possible candidate to influence the maintenance dose of warfarin. ApoE affects the vitamin K cycle by mediating the uptake of vitamin K into the liver. The vitamin K cycle is the drug target of warfarin. However, the association between genetic variants of the APOE gene and warfarin dose requirement is still controversial.

METHODS

Revman 5.3 software was used to analyze the relationship between APOE genotypes and warfarin dose requirements.

RESULTS

In our meta-analysis, the E2/E2 genotype was significantly associated with warfarin dose. E2/E2 patients required 12% (P = 0.0002) lower mean daily warfarin dose than E3/E3 carriers. In addition, subgroup analysis showed that Asians with the E4/E4 genotype tended to need lower warfarin maintenance doses, while the African American E4/E4 carriers needed slightly higher doses than E3/E3 carriers; however, these subgroups were very small.

CONCLUSION

This is the first meta-analysis of the association between APOE genotypes and warfarin dose. APOE E2/E2 might be one of the factors affecting warfarin dose requirements. The effect of APOE may vary between ethnicities.

Lipoprotein-bound LPS induces cytokine tolerance in hepatocytes

Bacterial endotoxin (LPS) elicits dramatic responses in the host including elevated plasma lipid levels due to the increased synthesis and secretion of triglyceride (TG)-rich lipoproteins by the liver. We postulate that this cytokine-induced hyperlipoproteinemia, clinically termed the `lipemia of sepsis', represents an innate, non-adaptive host immune response to infection. Data in support of this hypothesis include the capacity of TG-rich lipoproteins (VLDL and chylomicrons, CM) to bind and neutralize LPS. Herein, we present evidence that CM-bound LPS attenuates the hepatocellular response to pro-inflammatory cytokines. Primary rodent hepatocytes pretreated with CM—LPS complexes for 2 h demonstrated a near 70% reduction in cytokine-induced NO production as compared to non-pretreated control cells ( P ≥ 0.04). Whereas hepatocytes were maximally tolerant to cytokine stimulation 6 h after CM—LPS pretreatment, the cells spontaneously regained cytokine responsiveness within 40 h. The induction of cytokine tolerance in hepatocytes follows the internalization of CM—LPS complexes and is a process regulated by the LDL receptor. CM—LPS complexes failed to induce cytokine tolerance in hepatocytes wherein lipoprotein receptor activity was inhibited with high dose receptor associated protein (30 μg/ml). Similarly, CM-bound LPS did not induce tolerance in hepatocytes from ldlr—/— mice. Thus, the biochemical or genetic inhibition of LDL receptor activity effectively prevented the CM-mediated induction of the cytokine tolerant phenotype. In conclusion, the lipemia of sepsis likely represents a mechanism by which the host combats sporadic, non-life-threatening episodes of endotoxemia. Also, it may indicate a negative regulatory mechanism for the hepatic response to sepsis, serving to effectively down-regulate the acute phase response. A better understanding of how TG-rich lipoproteins modulate the host response to LPS could yield novel biological insights with important clinical implications, including the development of lipid-based therapies for bacterial infections.

Lycopene stabilizes lipoprotein levels during D-galactosamine/lipopolysaccharide induced hepatitis in experimental rats

OBJECTIVE

To investigate the effect of lycopene on lipoprotein metabolism during D-galactosamine/lipopolysaccharide (D-Gal/LPS) induced hepatitis in experimental rats.

METHODS

The efficacy of lycopene was validated during D-Gal/LPS induced hepatitis by analyzing the activity of lipid metabolizing enzymes such as lipoprotein lipase (LPL), lecithin-cholesterol acyl transferase (LCAT) and hepatic triglyceride lipase (HTGL). Lipo protein analyses were done by the estimation of very low density lipoprotein cholesterol (VLDL), low density lipoprotein cholesterol (LDL) and high density lipoprotein cholesterol (HDL).

RESULTS

The toxic insult of D-galactosamine/lipopolysaccharide (D-Gal/LPS) in experimental group of animals reduces the normal values of lipid metabolizing enzymes due to liver injury. The significant drop in the levels of HDL and concomitant increase in the values of VLDL and LDL were observed. The pretreatment of lycopene restore these altered values to near normal level in experimental group of animals.

CONCLUSIONS

In the light of results, it can be concluded that administration lycopene stabilizes the lipoprotein levels by regulating the lipid metabolizing enzymes through its antioxidant defense and helps to maintain the normal lipid metabolism during toxic injury in liver.

Immunohistochemical and histoplanimetrical study on the endothelial receptor involved in transportation of minute chylomicrons into subepithelial portal blood in intestinal villi of the rat jejunum

A portion of the minute chylomicrons less than 75 nm in diameter are transcytosed from the extravascular tissue into the subepithelial blood capillaries (sBC) in the villous apices of the rat jejunum. However, the details of the transportation mechanism have not been clarified. In this study, the endothelial receptor involved in the transportation of minute chylomicrons into the sBC’s lumina was immunohistochemically and histoplanimetrically examined in intestinal villi of the rat jejunum. Immunopositivity for very low density lipoprotein (VLDL) receptor was detected on the luminal and basal surfaces of the endothelial cells of sBC in approximately 68% of those apices of jejunal villi that possessed numerous chylomicrons in the lamina propria, while VLDL receptor was detected on the endothelial cells of sBC in only approximately 8% of intestinal villi that possessed few or no chylomicrons in the lamina propria. No immunopositivity for LDL receptor was detected in the sBC of all intestinal villi. These findings suggest that VLDL receptor is expressed by the endothelial cells of the sBC in conjunction with the filling of the lamina propria of jejunal villi with many chylomicrons produced by the villous columnar epithelial cells and that the VLDL receptor mediates the transportation of minute chylomicrons, maybe VLDL, into the subepithelial portal blood from the extravascular tissue of the rat jejunal villi.

Hepatitis C virus stimulates low-density lipoprotein receptor expression to facilitate viral propagation

Lipids play a crucial role in multiple aspects of hepatitis C virus (HCV) life cycle. HCV modulates host lipid metabolism to enrich the intracellular milieu with lipids to facilitate its proliferation. However, very little is known about the influence of HCV on lipid uptake from bloodstream. Low-density lipoprotein receptor (LDLR) is involved in uptake of cholesterol rich low-density lipoprotein (LDL) particles from the bloodstream. The association of HCV particles with lipoproteins implicates their role in HCV entry; however, the precise role of LDLR in HCV entry still remains controversial. Here, we investigate the effect of HCV infection on LDLR expression and the underlying mechanism(s) involved. We demonstrate that HCV stimulates LDLR expression in both HCV-infected Huh7 cells and in liver tissue from chronic hepatitis C patients. Fluorescence activated cell sorting and immunofluorescence analysis revealed enhanced cell surface and total expression of LDLR in HCV-infected cells. Increased LDLR expression resulted in the enhanced uptake of lipoprotein particles by HCV-infected cells. Analysis of LDLR gene promoter identified a pivotal role of sterol-regulatory element binding proteins (SREBPs), in the HCV-mediated stimulation of LDLR transcription. In addition, HCV negatively modulated the expression of proprotein convertase subtilisin/kexin type 9 (PCSK9), a protein that facilitates LDLR degradation. Ectopic expression of wild-type PCSK9 or gain-of-function PCSK9 mutant negatively affected HCV replication. Overall, our results demonstrate that HCV regulates LDLR expression at transcriptional and posttranslational level via SREBPs and PCSK9 to promote lipid uptake and facilitate viral proliferation.

IMPORTANCE

HCV modulates host lipid metabolism to promote enrichment of lipids in intracellular environment, which are essential in multiple aspects of HCV life cycle. However, very little is known about the influence of HCV on lipid uptake from the bloodstream. LDLR is involved in uptake of cholesterol rich lipid particles from bloodstream. In this study, we investigated the effect of HCV on LDLR expression and the underlying mechanism triggered by the virus to modulate LDLR expression. Our observations suggest that HCV upregulates LDLR expression at both the protein and the transcript levels and that this upregulation likely contributes toward the uptake of serum lipids by infected hepatocytes. Abrogation of HCV-mediated upregulation of LDLR inhibits serum lipid uptake and thereby perturbs HCV replication. Overall, our findings highlight the importance of serum lipid uptake by infected hepatocytes in HCV life cycle.

Lipoprotein metabolism in patients with type 1 diabetes under intensive insulin treatment

Background

Type 1 diabetes (T1DM) is frequently accompanied by dyslipidemia related with insulin-dependent steps of the intravascular lipoprotein metabolism. T1DM dyslipidemia may predispose to precocious cardiovascular disease and the lipid status in T1DM under intensive insulin treatment has not been sufficiently explored. The aim was to investigate the plasma lipids and the metabolism of LDL and HDL in insulin-treated T1DM patients with high glycemic levels.

Methods

Sixteen male patients with T1DM (26 ± 7 yrs) with glycated hemoglobin >7%, and 15 control subjects (28 ± 6 yrs) were injected with a lipid nanoemulsion (LDE) resembling LDL and labeled with ¹⁴C-cholesteryl ester and ³H-free-cholesterol for determination of fractional clearance rates (FCR, in h^-1) and cholesterol esterification kinetics. Transfer of labeled lipids from LDE to HDL was assayed in vitro.

Results

LDL-cholesterol (83 ± 15 vs 100 ± 29 mg/dl, p=0.08) tended to be lower in T1DM than in controls; HDL-cholesterol and triglycerides were equal. LDE marker ¹⁴C-cholesteryl ester was removed faster from plasma in T1DM patients than in controls (FCR=0.059 ± 0.022 vs 0.039 ± 0.022h-1, p=0.019), which may account for their lower LDL-cholesterol levels. Cholesterol esterification kinetics and transfer of non-esterified and esterified cholesterol, phospholipids and triglycerides from LDE to HDL were also equal.

Conclusion

T1DM patients under intensive insulin treatment but with poor glycemic control had lower LDL-cholesterol with higher LDE plasma clearance, indicating that LDL plasma removal was even more efficient than in controls. Furthermore, HDL-cholesterol and triglycerides, cholesterol esterification and transfer of lipids to HDL, an important step in reverse cholesterol transport, were all normal. Coexistence of high glycemia levels with normal intravascular lipid metabolism may be related to differences in exogenous insulin bioavailabity and different insulin mechanisms of action on glucose and lipids. Those findings may have important implications for prevention of macrovascular disease by intensive insulin treatment.

Liver X receptors agonists impede hepatitis C virus infection in an Idol-dependent manner

Hepatitis C virus (HCV) is a major human pathogen that causes many serious diseases, including acute and chronic hepatitis, cirrhosis and hepatocellular carcinoma. Treatments for this virus are inadequate, and improved antiviral therapies are necessary. Although the precise mechanisms regulating HCV entry into hepatic cells are still unknown, the low-density lipoprotein receptor (LDLR) has been shown to be essential for entry of infectious HCV particles. Liver X receptors (LXR) were recently reported to control LDLR expression through the regulation of the expression of the Idol (inducible degrader of the LDLR) protein, which could trigger the ubiquitination and degradation of LDLR. In this study, we analyzed the antiviral effect of Idol in vitro. The results demonstrated that Huh7.5.1 cells that exogenously expressed Idol were resistant to HCV infection. Next, the treatment of HCV-infected Huh7.5.1 cells with either synthetic LXR agonists (GW3965 or T0901317) or the natural LXR ligand 24(S),25-epoxycholesterol inhibited HCV infection in a dose-dependent manner. Furthermore, a combination of LXR agonists and HCV RNA replication inhibitors exerted additive effects against HCV, as revealed by isobologram analysis. In conclusion, our data indicate that molecules such as LXR agonists, which could stimulate the expression of Idol, represent a new class of potential anti-HCV compounds, and these compounds could be developed for therapeutic use against HCV infection, either as a monotherapy, or in combination with other anti-HCV drugs.

Role of low-density lipoprotein receptor in the hepatitis C virus life cycle

Hepatitis C virus (HCV) particles are known to be in complex with lipoproteins. As a result of this interaction, the low-density lipoprotein (LDL) receptor (LDLR) has been proposed as a potential entry factor for HCV; however, its implication in virus entry remains unclear. Here, we reinvestigated the role of the LDLR in the HCV life cycle by comparing virus entry to the mechanism of lipoprotein uptake. A small interfering RNA targeting the LDLR in Huh-7 cells reduced HCV infectivity, confirming that this receptor plays a role in the life cycle of HCV generated in cell culture. However, kinetics of internalization were much faster for lipoproteins than for infectious HCV particles. Furthermore, a decrease in HCV RNA replication was observed by blocking the LDLR with a specific antibody, and this was associated with an increase in the ratio of phosphatidylethanolamine to phosphatidylcholine in host cells. Nevertheless, a soluble form of the LDLR inhibited both HCV entry into the hepatocytes and its binding to the LDLR expressed on Chinese hamster ovary cells, suggesting a direct interaction between the HCV particle and the LDLR. Finally, we showed that modification of HCV particles by lipoprotein lipase (LPL) reduces HCV infectivity and increases HCV binding to LDLR. Importantly, LPL treatment also induced an increase in RNA internalization, suggesting that LDLR, at least in some conditions, leads to nonproductive internalization of HCV.

CONCLUSION

The LDLR is not essential for infectious HCV particle entry, whereas the physiological function of this receptor is important for optimal replication of the HCV genome.

Very Low Density Lipoprotein Metabolism in Patients with Chronic Kidney Disease

BACKGROUND: Hypertriglyceridemia is a common metabolic complication of chronic kidney disease (CKD) and an important risk factor for coronary heart disease in this patient population. The mechanisms responsible for the development of hypertriglyceridemia in subjects with CKD are not clear. METHODS: We studied very low density lipoprotein triglyceride (VLDL-TG) and VLDL-apolipoprotein B-100 (VLDL-apoB-100) kinetics in vivo in 6 subjects with non-dialysis-dependent CKD (CKD-ND), 6 subjects with CKD treated with peritoneal dialysis (CKD-PD) and 24 sex-, age- and body mass index-matched control subjects with normal renal function (12 control subjects each matched with the CKD-ND and CKD-PD group, respectively). RESULTS: The secretion rates of VLDL-TG and VLDL-apoB-100 into plasma were not different between CKD-ND or CKD-PD and their respective control groups. The mean residence times of VLDL-TG and VLDL-apoB-100 in plasma, which represents the time VLDL-TG and VLDL-apoB-100 spend in the circulation after secretion by the liver, tended to be greater in subjects with CKD-ND than in control subjects (222 ± 38 vs. 143 ± 21 min, p = 0.07, and 352 ± 102 vs. 200 ± 20 min, p = 0.06, respectively) and were about two-fold greater in subjects with CKD-PD compared with their control group (248 ± 51 vs. 143 ± 21 min and 526 ± 116 vs. 182 ± 16 min, respectively; both p ≤ 0.01). CONCLUSION: Impaired plasma clearance of VLDL-TG and VLDL-apoB-100 is the major abnormality associated with hypertriglyceridemia in patients with either CKD-ND or CKD-PD.

Very-low-density lipoprotein: complex particles in cardiac energy metabolism

The heart is a major consumer of energy and is able to utilise a wide range of substrates including lipids. Nonesterified fatty acids (NEFA) were thought to be a favoured carbon source, but their quantitative contribution is limited because of their relative histotoxicity. Circulating triacylglycerols (TAGs) in the form of chylomicrons (CMs) and very-low-density lipoprotein (VLDL) are an alternative source of fatty acids and are now believed to be important in cardiac metabolism. However, few studies on cardiac utilisation of VLDL have been performed and the role of VLDL in cardiac energy metabolism remains unclear. Hearts utilise VLDL to generate ATP, but the oxidation rate of VLDL-TAG is relatively low under physiological conditions; however, in certain pathological states switching of energy substrates occurs and VLDL may become a major energy source for hearts. We review research regarding myocardial utilisation of VLDL and suggest possible roles of VLDL in cardiac energy metabolism: metabolic regulator and extracardiac energy storage for hearts.

Apolipoprotein E on hepatitis C virion facilitates infection through interaction with low-density lipoprotein receptor

Hepatitis C virus (HCV) infection is a major cause of liver disease. HCV associates with host apolipoproteins and enters hepatocytes through complex processes involving some combination of CD81, claudin-1, occludin, and scavenger receptor BI. Here we show that infectious HCV resembles very low density lipoprotein (VLDL) and that entry involves co-receptor function of the low-density lipoprotein receptor (LDL-R). Blocking experiments demonstrate that beta-VLDL itself or anti-apolipoprotein E (apoE) antibody can block HCV entry. Knockdown of the LDL-R by treatment with 25-hydroxycholesterol or siRNA ablated ligand uptake and reduced HCV infection of cells, whereas infection was rescued upon cell ectopic LDL-R expression. Analyses of gradient-fractionated HCV demonstrate that apoE is associated with HCV virions exhibiting peak infectivity and dependence upon the LDL-R for cell entry. Our results define the LDL-R as a cooperative HCV co-receptor that supports viral entry and infectivity through interaction with apoE ligand present in an infectious HCV/lipoprotein complex comprising the virion. Disruption of HCV/LDL-R interactions by altering lipoprotein metabolism may therefore represent a focus for future therapy.

Apolipoprotein E on hepatitis C virion facilitates infection through interaction with low-density lipoprotein receptor

Hepatitis C virus (HCV) infection is a major cause of liver disease. HCV associates with host apolipoproteins and enters hepatocytes through complex processes involving some combination of CD81, claudin-1, occludin, and scavenger receptor BI. Here we show that infectious HCV resembles very low density lipoprotein (VLDL) and that entry involves co-receptor function of the low-density lipoprotein receptor (LDL-R). Blocking experiments demonstrate that beta-VLDL itself or anti-apolipoprotein E (apoE) antibody can block HCV entry. Knockdown of the LDL-R by treatment with 25-hydroxycholesterol or siRNA ablated ligand uptake and reduced HCV infection of cells, whereas infection was rescued upon cell ectopic LDL-R expression. Analyses of gradient-fractionated HCV demonstrate that apoE is associated with HCV virions exhibiting peak infectivity and dependence upon the LDL-R for cell entry. Our results define the LDL-R as a cooperative HCV co-receptor that supports viral entry and infectivity through interaction with apoE ligand present in an infectious HCV/lipoprotein complex comprising the virion. Disruption of HCV/LDL-R interactions by altering lipoprotein metabolism may therefore represent a focus for future therapy.

Apolipoprotein B-associated cholesterol is a determinant of treatment outcome in patients with chronic hepatitis C virus infection receiving anti-viral agents interferon-alpha and ribavirin

BACKGROUND

Hepatitis C virus (HCV) co-opts very-low-density lipoprotein (VLDL) pathways for replication, secretion and entry into hepatocytes and associates with apolipoprotein B (apoB) in plasma. Each VLDL contains apoB-100 and variable amounts of apolipoproteins E and C, cholesterol and triglycerides.

AIM

To determine whether baseline lipid levels predicted treatment outcome.

METHODS

Retrospective analysis was performed of 250 chronic hepatitis C (CHC) patients who had received anti-viral agents interferon-alpha and ribavirin; 165 had a sustained virological response (SVR). Pre- and post-treatment nonfasting lipid profiles were measured and non-high-density lipoprotein (non-HDL) cholesterol (i.e. apoB-associated) was calculated. Binary logistic regression analysis assessed factors independently associated with treatment outcome.

RESULTS

There was an independent association between higher apoB-associated cholesterol (non-HDL-C) and increased odds of SVR (odds ratio 2.09, P = 0.042). In multivariate analysis, non-HDL-C was significantly lower in HCV genotype 3 (g3) than genotype 1 (P = 0.007); this was reversible upon eradication of HCVg3 (pre-treatment non-HDL-C = 2.8 mmol/L, SVR = 3.6 mmol/L, P < 0.001).

CONCLUSIONS

Higher apoB-associated cholesterol is positively associated with treatment outcome in CHC patients receiving anti-viral therapy, possibly due to competition between apoB-containing lipoproteins and infectious low-density HCV lipo-viral particles for hepatocyte entry via shared lipoprotein receptors.

The differential hepatic uptake of chylomicron remnants of different fatty acid composition is not mediated by hepatic lipase

The hypothesis that hepatic lipase mediates the differential hepatic uptake of chylomicron remnants of different fatty acid composition, demonstrated in previous work from our laboratory, was tested by investigating the effect of antibodies to the enzyme on the uptake of remnants enriched with saturated orn-3 polyunsaturated fatty acids by the perfused rat liver. After perfusion of rat livers with polyclonal antibodies to rat hepatic lipase raised in rabbits or with rabbit non-immune serum for 15 min, [3H]oleate-labelled chylomicron remnants, derived from chylomicrons of rats given a bolus of either palm (rich in saturated fatty acids) oil or fish (rich inn-3 polyunsaturated fatty acids) oil, were added. The disappearance of radioactivity from the perfusate during 120 min and its recovery in the liver at the end of the experiments were then measured. Although the rabbit anti-rat hepatic lipase antiserum was shown to inhibit hepatic lipase activity by up to 90 %, and to bind extensively to hepatic sinusoidal surfaces when added to the perfusate, radioactivity from remnants of chylomicrons from rats given a bolus of fish oil as compared with palm oil disappeared from the perfusate and appeared in the liver more rapidly in the presence both the antiserum and the non-immune serum, and the differences between the uptake of the two types of remnants were similar. We conclude, therefore, that differential interaction with hepatic lipase is not responsible for the differences in the rate of removal of chylomicron remnants of different fatty acid composition from the blood.

Reduced atherosclerosis in chow-fed mice expressing high levels of a catalytically inactive human hepatic lipase

Increased expression of catalytically inactive hepatic lipase (ciHL) lowers remnants and low-density lipoproteins (LDL) and may reduce atherosclerosis in mice lacking both LDLreceptors (LDLR) and murine (m) HL. However, in a previous study, ciHL expression failed to reduce atherosclerosis but increased liver fat accumulation after a 3-month high-fat diet, suggesting that diet-induced metabolic changes compromised the antiatherogenic effects of ciHL. Therefore, we hypothesized that reduced dietary fat would reduce atherosclerosis in ciHL expressing mice. Mice lacking both LDLR and mHL, alone, or expressing ciHL were fed a low-fat (chow) diet for 9 months to match the cumulative cholesterol exposure resulting from a 3-month high-fat diet. Plasma lipids and lipoproteins as well as atherosclerosis were determined at sacrifice. Also, liver expression of receptors and proteins contributing to cholesterol delivery including the LDLreceptor related protein (LRP), scavenger receptor (SR)-B1 and apoE were determined. At 9 months, ciHL expression reduced plasma cholesterol by approximately 20% and atherosclerosis by 79% (from 2.67+/-0.61% of aortic surface, Ldlr-/-hl-/-, n=9, to 0.55+/-0.32% of aortic surface, Ldlr-/-hl-/-ciHL, n=7, P=0.01). Also, LRP-expression increased approximately 4-fold, whereas SR-B1 and apoE remained unchanged. These results demonstrate that ciHL expression reduces atherosclerosis. Also, these results demonstrate that ciHL increases LRP expression and suggest increased LRP-mediated lipoprotein clearance as a pathway for ciHL-mediated atherosclerosis reduction.

ATP-Binding cassette cholesterol transporters and cardiovascular disease

A hallmark of atherosclerotic cardiovascular disease (CVD) is the accumulation of cholesterol in arterial macrophages. Factors that modulate circulating and tissue cholesterol levels have major impacts on initiation, progression, and regression of CVD. Four members of the ATP-binding cassette (ABC) transporter family play important roles in this modulation. ABCA1 and ABCG1 export excess cellular cholesterol into the HDL pathway and reduce cholesterol accumulation in macrophages. ABCG5 and ABCG8 form heterodimers that limit absorption of dietary sterols in the intestine and promote cholesterol elimination from the body through hepatobiliary secretion. All 4 transporters are induced by the same sterol-sensing nuclear receptor system. ABCA1 expression and activity are also highly regulated posttranscriptionally by diverse processes. ABCA1 mutations can cause a severe HDL-deficiency syndrome characterized by cholesterol deposition in tissue macrophages and prevalent atherosclerosis. ABCG5 or ABCG8 mutations can cause sitosterolemia, in which patients accumulate cholesterol and plant sterols in the circulation and develop premature CVD. Disrupting Abca1 or Abcg1 in mice promotes accumulation of excess cholesterol in macrophages, and manipulating mouse macrophage ABCA1 expression affects atherogenesis. Overexpressing ABCG5 and ABCG8 in mice attenuates diet-induced atherosclerosis in association with reduced circulating and liver cholesterol. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and inhibit transcription of all 4 transporters. Thus, impaired ABC cholesterol transporters might contribute to the enhanced atherogenesis associated with common inflammatory and metabolic disorders. Their beneficial effects on cholesterol homeostasis have made these transporters important new therapeutic targets for preventing and reversing CVD.

Involvement of apolipoprotein E in excess fat accumulation and insulin resistance

Although apolipoprotein E (apoE) is well known to play a major role in lipid metabolism, its role in glucose and energy homeostasis remains unclear. Herein, we established apoE-deficient genetically obese Ay (apoE(-/-);Ay/+) mice. ApoE deficiency in Ay mice prevented the development of obesity, with decreased fat accumulation in the liver and adipose tissues. ApoE(-/-);Ay/+ mice exhibited better glucose tolerance than apoE(+/+);Ay/+ mice. Insulin tolerance testing and hyperinsulinemic-euglycemic clamp study revealed marked improvement of insulin sensitivity, despite increased plasma free fatty acid levels. These metabolic phenotypes were reversed by adenoviral replenishment of apoE protein, indicating circulating apoE to be involved in increased adiposity and obesity-related metabolic disorders. Uptake of apoE-lacking VLDL into the liver and adipocytes was markedly inhibited, but adipocytes in apoE(-/-);Ay/+ mice exhibited normal differentiation, suggesting that apoE-dependent VLDL transport is involved in the development of obesity, i.e., surplus fat accumulation. Interestingly, apoE(-/-);Ay/+ mice exhibited decreased food intake and increased energy expenditure. Pair-feeding experiments indicate these phenomena to both contribute to the obesity-resistant phenotypes associated with apoE deficiency. Thus, apoE is involved in maintaining energy homeostasis. ApoE-dependent excess fat accumulation is a promising therapeutic target for the metabolic syndrome.

Warfarin dose related to apolipoprotein E (APOE) genotype

OBJECTIVE

Warfarin is an anticoagulant which acts through interference with the recycling of vitamin K in the liver, leading to reduced activation of several clotting factors. Apolipoprotein E plays a central role in the uptake of the lipid-soluble vitamin K. The apolipoprotein E (APOE) alleles E2, E3 and E4 encode the three major isoforms of apolipoprotein E. The aim of this project was to evaluate whether variation in the APOE gene influences warfarin dose.

METHODS

We genotyped APOE in 183 warfarin-treated patients. Information about warfarin dose, prothrombin time, age, gender, body weight, treatment indication and duration, other diseases and concurrent medication was taken from the patients' medical records. Cytochrome P450 2C9 genotyping had been performed previously, and patients were stratified according to CYP2C9 genotype.

RESULTS

Patients homozygous for APOE*E4 tended to receive higher warfarin doses than others. Among CYP2C9 extensive metabolisers, APOE*E4 homozygous patients received significantly higher warfarin doses than patients with one or no E4 alleles; 56.9 compared with 34.3 and 34.6 mg/week, (Bonferroni corrected P=0.008 and 0.007, respectively). APOE genotype explains 6% of warfarin dose variance among CYP2C9 extensive metabolisers (analysis of variance, P=0.009).

CONCLUSION

Previous studies have shown that individuals carrying the APOE*E4 allele have a faster uptake of lipoproteins into the liver and lower levels of circulating vitamin K than others. It is therefore plausible that patients carrying E4 alleles have an enhanced uptake of vitamin K into the liver and require higher doses of warfarin to compensate for this.

The composition, structural properties and binding of very-low-density and low-density lipoproteins to the LDL receptor in normo- and hypertriglyceridemia: relation to the apolipoprotein E phenotype

The composition, apolipoprotein structure and lipoprotein binding to the LDL receptor were studied for very-low-density (VLDL) and low-density lipoprotein (LDL) particles isolated from subjects with apoE phenotype E3/3 (E3), E2/2 or E2/3 (E2+) and E3/4 or E4/4 (E4+) and a wide range of plasma triglyceride (TG) contents. The data combined for all three phenotype groups can be summarized as follows. (i) A decrease in accessibility of VLDL tryptophan residues to I- anions with a decrease in tryptophan surface density, concomitant with an increase in VLDL dimensions, reflects the increased efficiency of protein-protein interactions. (ii) A gradual increase in the quenching constant for LDL apoB fluorescence with an increase in TG/cholesterol (Chol) ratio reflects the 'freezing' effect of Chol molecules on apoB dynamics. (iii) Different mechanisms specific for a particular lipoprotein from E3/3 or E2/3 subjects are responsible for apoE-mediated VLDL binding and apoB-mediated LDL binding to the LDL receptor in a solid-phase binding assay. (iv) The 'spacing' effect of apoC-III molecules on apoE-mediated VLDL binding results in a decrease in the number of binding sites. (v) The maximum of the dependence of the LDL binding affinity constant on relative tryptophan density corresponds to LDL intermediate size. VLDL particles from hypertriglyceridemic E2/3 heterozygotic individuals had remnant-like properties (increased cholesterol, apoE and decreased apoC-III content) while their binding efficiency was unchanged. Based on the affinity constant value and LDL-Chol content, increased competition between VLDL and LDL for the binding to the LDL receptor upon increase in plasma TG is suggested, and LDL from hypertriglyceridemic E3/3 homozygotic individuals is the most efficient competitor.

Lipoprotein transport in the metabolic syndrome: methodological aspects of stable isotope kinetic studies

The metabolic syndrome encapsulates visceral obesity, insulin resistance, diabetes, hypertension and dyslipidaemia. Dyslipidaemia is a cardinal feature of the metabolic syndrome that accelerates the risk of cardiovascular disease. It is usually characterized by high plasma concentrations of triacylglycerol (triglyceride)-rich and apoB (apolipoprotein B)-containing lipoproteins, with depressed concentrations of HDL (high-density lipoprotein). However, lipoprotein metabolism is complex and abnormal plasma concentrations can result from alterations in the rates of production and/or catabolism of these lipoprotein particles. Our in vivo understanding of kinetic defects in lipoprotein metabolism in the metabolic syndrome has been achieved chiefly by ongoing developments in the use of stable isotope tracers and mathematical modelling. This review deals with the methodological aspects of stable isotope kinetic studies. The design of in vivo turnover studies requires considerations related to stable isotope tracer administration, duration of sampling protocol and interpretation of tracer data, all of which are critically dependent on the kinetic properties of the lipoproteins under investigation. Such models provide novel insight that further understanding of metabolic disorders and effects of treatments. Future investigations of the pathophysiology and therapy of the dyslipoproteinaemia of the metabolic syndrome will require the development of novel kinetic methodologies. Specifically, new stable isotope techniques are required for investigating in vivo the turnover of the HDL subpopulation of particles, as well as the cellular efflux of cholesterol into the extracellular space and its subsequent transport in plasma and metabolic fate in the liver.

Chylomicron margination, lipolysis, and vitamin a uptake in the lactating rat mammary gland: implications for milk retinoid content

We have reported previously that the concentration of vitamin A (VA) in the milk of lactating rats varies with dietary VA intake, even when plasma retinol concentration is unaffected. In the current study, we investigated the role of lipolysis in the uptake of chylomicron (CM) VA into mammary tissue of lactating rats and estimated the proportion of CM-VA that is associated with the mammary gland during CM clearance. Chylomicrons containing [(3)H]VA, mainly as retinyl esters, were prepared in donor rats and administered intravenously to lactating recipient rats. Chylomicron VA rapidly disappeared from plasma and appeared in mammary tissue (maximum within 2-3 mins), followed by a decline. Concomitantly, uptake by liver increased continuously, reaching a plateau within 20-30 mins. Active lipolysis in mammary tissue was necessary for rapid VA uptake, as significantly less CM-VA was recovered in mammary tissue of postlactating rats than of lactating rats, after heparin treatment in lactating rats, or after injection of preformed CM remnants in lactating rats. [(3)H]Vitamin A uptake by mammary tissue increased linearly with CM-VA dose over a 150-fold dose range (R(2) = 0.972, P = 0.0001), suggesting a high capacity for uptake and apparent first-order assimilation of CM-VA during CM remnant formation in situ. Model-based compartmental analysis using WinSAAM predicted that approximately 42% of CM-VA marginated, that is, were temporarily removed, from plasma to the mammary glands during lipolysis and that a total of 3.8% of CM-VA was transferred to mammary tissue. The model-predicted t(1/2) for CM remnants was 3.04 mins. The metabolism of CM-VA in the lactating mammary gland, in proportion to VA absorption and CM-VA contents, may explain how milk VA concentration varies even when plasma retinol levels are unchanged. The mechanism of CM margination and mammary gland uptake described here for VA may be similar for other lipophilic substances.

Endocytosis of hepatic lipase and lipoprotein lipase into rat liver hepatocytes in vivo is mediated by the low density lipoprotein receptor-related protein

In isolated cell studies, the internalization and degradation of hepatic lipase (HL) has been linked to its binding to the low density lipoprotein receptor-related protein (LRP). We have utilized the receptor-associated protein (RAP), a universal inhibitor of high affinity ligand binding to LRP, to evaluate the participation of LRP in the endocytosis of HL and lipoprotein lipase (LPL). We isolated a total endosome fraction from rat livers after a 30-min infusion of recombinant RAP, administered as a glutathione S-transferase conjugate (GST-RAP). GST-RAP infusion had no effect on the concentration of HL in liver homogenates, but its concentration in blood plasma increased progressively by 20%, and enrichment over homogenate of HL in endosomes was reduced by 50% as compared with infusion of GST alone. The concentrations of LPL in liver and plasma were 1.4 and 0.5%, respectively, those of HL, but endosomal enrichment of the two enzymes was similar ( approximately 10-fold). GST-RAP infusion had no effect on the concentration of LPL in liver but increased its concentration in blood plasma by 250% and reduced its endosomal enrichment by 95% or greater. GST-RAP infusion also reduced endosomal enrichment of LRP by 40%, but enrichment of several other endocytic receptors was unaffected. Endosomal enrichment of several membrane trafficking proteins associated with the endocytic pathway in hepatocytes was unaffected by GST-RAP with the exception of early endosome endosome antigen 1, which was reduced by 85%. We conclude that HL is partially and LPL almost exclusively taken up into rat hepatocytes after binding to the endocytic receptor LRP.

Localization of the iron transport proteins Mobilferrin and DMT-1 in the duodenum: the surprising role of mucin

There are two pathways for inorganic iron uptake in the intestine, the ferric pathway, mediated by the key protein mobilferrin, and the ferrous pathway, mediated by DMT-1. Previous studies reported that the amount of DMT-1 increased in the intestinal mucosa in iron deficiency and the increase was seen in the apical portion of the villus of the duodenal mucosa. Mobilferrin did not quantitatively increase but became localized at the cell membrane. However, studies on fresh tissue have not previously been performed and the localization to the microvillae has not been demonstrated. In order to more definitively localize these proteins immunofluorescent and electron microscopic studies were undertaken. Samples were also subjected to biochemical analysis and Western analysis. In iron-deficient animals both DMT-1 and Mobilferrin were concentrated in the apical surface of the villae. Electron microscopy revealed that the majority of this increase in the amount of these proteins near the luminal surface was due to increased binding of the proteins to mucin in vesicles near the surface. A significant portion of the iron transport proteins was localized in the goblet cells and outside the cell in the luminal mucin, as demonstrated by immunofluorescence, electron microscopy, and isolation of the mucin by cesium chloride gradient centrifugation and Western analysis. A new model for the transport of metal ions was suggested. The metal transport proteins travel from vesicles inside the cell out to the lumen mucin. This increases the surface area and allows a greater portion of the lumen contents to be exposed to the binding proteins. Once the metal is bound to the externalized protein it is internalized into the cell. This explains many of the unique properties of the iron-binding proteins and suggests that it may be a more general model for the absorption of other nutrients.

Lower plasma levels of lipoprotein lipase after infusion of low molecular weight heparin than after administration of conventional heparin indicate more rapid catabolism of the enzyme

The functional pool of lipoprotein lipase (LPL) is anchored to heparan sulfate at the vascular endothelium. Injection of heparin releases the enzyme into the circulating blood. Animal experiments have shown that the enzyme is then extracted and degraded by the liver. Low molecular weight (LMW) heparin preparations are widely used in the clinic and are supposed to release less LPL. In this study, we infused a LMW heparin into healthy volunteers for 8 hours. The peak of LPL activity was only about 30% and the subsequent plateau of LPL activity only about 40% compared with those seen with conventional heparin. When a bolus of heparin was given after 4 hours' infusion of LMW or conventional heparin, only relatively small, and similar, amounts of LPL entered plasma. This suggests that the difference between LMW and conventional heparin lay in the ability to retain LPL in the circulating blood, not in the ability to release the lipase. Triglycerides (TGs) decreased when the heparin infusion was started, as expected from the high circulating LPL activities. After 1 to 2 hours, TG levels increased again, and after 8 hours they were about twice as high as before the heparin infusion. This indicates that the amount of LPL available for lipoprotein metabolism had become critically low in relation to TG transport rates. This study indicates that LMW heparin compared with conventional heparin causes as much or more depletion of LPL and subsequent impairment of TG clearing.

The low density lipoprotein receptor-related protein complexes with cell surface heparan sulfate proteoglycans to regulate proteoglycan-mediated lipoprotein catabolism

It has been proposed that clearance of cholesterol-enriched very low density lipoprotein (VLDL) particles occurs through a multistep process beginning with their initial binding to cell-surface heparan sulfate proteoglycans (HSPG), followed by their uptake into cells by a receptor-mediated process that utilizes members of the low density lipoprotein receptor (LDLR) family, including the low density lipoprotein receptor-related protein (LRP). We have further explored the relationship between HSPG binding of VLDL and its subsequent internalization by focusing on the LRP pathway using a cell line deficient in LDLR. In this study, we show that LRP and HSPG are part of a co-immunoprecipitable complex at the cell surface demonstrating a novel association for these two cell surface receptors. Cell surface binding assays show that this complex can be disrupted by an LRP-specific ligand binding antagonist, which in turn leads to increased VLDL binding and degradation. The increase in VLDL binding results from an increase in the availability of HSPG sites as treatment with heparinase or competitors of glycosaminoglycan chain addition eliminated the augmented binding. From these results we propose a model whereby LRP regulates the availability of VLDL binding sites at the cell surface by complexing with HSPG. Once HSPG dissociates from LRP, it is then able to bind and internalize VLDL independent of LRP endocytic activity. We conclude that HSPG and LRP together participate in VLDL clearance by means of a synergistic relationship.

Charge-based heterogeneity of human plasma lipoproteins at hypertriglyceridemia: capillary isotachophoresis study

To reveal the metabolic links between and within pools of pro-atherogenic triglyceride(TG)-rich lipoproteins and anti-atherogenic high density lipoproteins (HDL), the changes in lipoprotein profile at hypertriglyceridemia were analyzed by capillary isotachophoresis. Plasma samples from patients with apoE3/3 phenotype were stained with a fluorescent probe NBD-C6-ceramide and lipoproteins resolved into six H-, one (V+I) and four L-components which belong to HDL, very low and intermediate density (VLDL+IDL) and low density lipoproteins (LDL), respectively. The expected correlation between the relative size of the combined fractions and lipid and apolipoprotein values was obtained confirming the validity of the approach. The new findings were obtained as follows. (1) The fast L-component correlated inversely with HDL-cholesterol (Chol), while intermediate and slow H-components correlated inversely with plasma and LDL-Chol and apoB. (2) The content of intermediate and slow H-components increased within H-pool and decreased relative TG-rich lipoproteins as hypertriglyceridemia rose due to the impairment of triglyceride hydrolysis by lipoprotein lipase within TG-rich particles. (3) A predictive value of the ratios of fast to slow H-components as an indicator of lecithin:cholesterol acyltransferase activity was demonstrated which tended to decrease at hypertriglyceridemia. (4) The L1/L2 ratio may be considered as an indicator of the accumulation of small dense LDL, which is a feature of clinically manifested atherogenic B-pattern. The competition between H(DL) and L(DL) particles for hepatic lipase and significant contribution of apoE to functional deficiency of H(DL) particles at hypertriglyceridemia are suggested.

Induction of cytokine tolerance in rodent hepatocytes by chylomicron-bound LPS is low-density lipoprotein receptor dependent

We examined the role of lipoprotein receptors in mediating chylomicron-bound lipopolysaccharide (CM-LPS)-induced cytokine tolerance in rodent hepatocytes. We found that 2 h of pretreatment with CM-LPS (5 mg TG/mL) was sufficient to induce cytokine tolerance, as measured by decreased nitric oxide (NO) production by hepatocytes (20% of the Control group, P < 0.03). Tolerance was evident as early as 2 h after pretreatment and disappeared after 40 h. Furthermore, we evaluated the roles of the low-density lipoprotein (LDL) receptor (LDLR) and LDL receptor-related protein (LRP) in the induction of cytokine tolerance in hepatocytes. Biochemical inhibition of the receptors or use of hepatocytes from LDLR-deficient mice revealed that functional LDLR was necessary for the induction of tolerance. However, by increasing the pretreatment time, the LRP compensated for the absence of the LDLR in induction of cytokine tolerance. In conclusion, CM-LPS-mediated induction of cytokine tolerance to proinflammatory cytokines is a time- and dose-dependent process that requires functional lipoprotein receptors. These findings underscore an interrelationship between TG-rich lipoprotein metabolism and innate immunity.

Effect of YM-53601, a novel squalene synthase inhibitor, on the clearance rate of plasma LDL and VLDL in hamsters

1. To better understand how it decreases plasma cholesterol and triglyceride, we evaluated the effect of YM-53601 ((E-2-[2-fluoro-2-(quinuclidin-3-ylidene) ethoxy]-9H-carbozole monohydrochloride) on the clearance rate of low density lipoprotein (LDL) and very low density lipoprotein (VLDL) in hamsters. 2. Treatment with YM-53601 at 50 mg kg(-1) for 5 days in hamsters fed a normal diet enhanced the disappearance of 1,1'-Dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-VLDL and DiI-LDL. This effect on DiI-LDL was lost in the early phase after DiI-methyl(met)-LDL, chemically modified to block LDL receptor binding, was injected in hamsters, but was retained in the late phase. Pre-treatment with protamine sulphate, which inhibits the activity of LPL, also failed to enhance DiI-VLDL clearance rate by YM-53601. 3. Even on single oral administration at 30 mg kg(-1), YM-53601 enhanced the disappearance of the high concentration of plasma triglyceride after injection of intrafat, an emulsion of fat. Plasma triglyceride was significantly decreased as soon as 1 h after single administration of YM-53601 in hamsters fed a normal diet. 4. These results indicate that the decrease in plasma total cholesterol and triglyceride after the treatment with YM-53601 is due to its enhancement of the clearance rate of LDL and VLDL, respectively. Moreover, YM-53601 may be effective in decreasing plasma triglyceride levels early in the course of treatment of hypertriglyceridaemia in humans.

ApoB-48 and apoB-100 differentially influence the expression of type-III hyperlipoproteinemia in APOE*2 mice

Apolipoprotein E (apoE) is essential for the clearance of plasma chylomicron and VLDL remnants. The human APOE locus is polymorphic and 5-10% of APOE*2 homozygotes exhibit type-III hyperlipoproteinemia (THL), while the remaining homozygotes have less than normal plasma cholesterol. In contrast, mice expressing APOE*2 in place of the mouse Apoe (Apoe(2/2) mice) are markedly hyperlipoproteinemic, suggesting a species difference in lipid metabolism (e.g., editing of apolipoprotein B) enhances THL development. Since apoB-100 has an LDLR binding site absent in apoB-48, we hypothesized that the Apoe(2/2) THL phenotype would improve if all Apoe(2/2) VLDL contained apoB-100. To test this, we crossed Apoe(2/2) mice with mice lacking the editing enzyme for apoB (Apobec(-/-)). Consistent with an increase in remnant clearance, Apoe(2/2). Apobec(-/-) mice have a significant reduction in IDL/LDL cholesterol (IDL/LDL-C) compared with Apoe(2/2) mice. However, Apoe(2/2).Apobec(-/-) mice have twice as much VLDL triglyceride as Apoe(2/2) mice. In vitro tests show the apoB-100-containing VLDL are poorer substrates for lipoprotein lipase than apoB-48-containing VLDL. Thus, despite a lowering in IDL/LDL-C, substituting apoB-48 lipoproteins with apoB-100 lipoproteins did not improve the THL phenotype in the Apoe(2/2).Apobec(-/-) mice, because apoB-48 and apoB-100 differentially influence the catabolism of lipoproteins.

Molecular modeling of the dimeric structure of human lipoprotein lipase and functional studies of the carboxyl-terminal domain

Lipoprotein lipase (LPL) plays a key role in lipid metabolism. Molecular modeling of dimeric LPL was carried out using insight ii based upon the crystal structures of human, porcine, and horse pancreatic lipase. The dimeric model reveals a saddle-shaped structure and the key heparin-binding residues in the amino-terminal domain located on the top of this saddle. The models of two dimeric conformations - a closed, inactive form and an open, active form - differ with respect to how surface-loop positions affect substrate access to the catalytic site. In the closed form, the surface loop covers the catalytic site, which becomes inaccessible to solvent. Large conformational changes in the open form, especially in the loop and carboxyl-terminal domain, allow substrate access to the active site. To dissect the structure-function relationships of the LPL carboxyl-terminal domain, several residues predicted by the model structure to be essential for the functions of heparin binding and substrate recognition were mutagenized. Arg405 plays an important role in heparin binding in the active dimer. Lys413/Lys414 or Lys414 regulates heparin affinity in both monomeric and dimeric forms. To evaluate the prediction that LPL forms a homodimer in a 'head-to-tail' orientation, two inactive LPL mutants - a catalytic site mutant (S132T) and a substrate-recognition mutant (W390A/W393A/W394A) - were cotransfected into COS7 cells. Lipase activity could be recovered only when heterodimerization occurred in a head-to-tail orientation. After cotransfection, 50% of the wild-type lipase activity was recovered, indicating that lipase activity is determined by the interaction between the catalytic site on one subunit and the substrate-recognition site on the other.

LDL receptor but not apolipoprotein E deficiency increases diet-induced obesity and diabetes in mice

The aim of this study was to determine whether phenotypes associated with type 2 diabetes are altered in dyslipidemic obese mice. C57BL/6 wild-type, low-density lipoprotein (LDL) receptor-deficient (LDLR-/-), and apolipoprotein E-deficient (apoE-/-) mice were fed a high-fat, high-carbohydrate diet (diabetogenic diet), and the development of obesity, diabetes, and hypertriglyceridemia was examined. Wild-type mice became obese and developed hyperglycemia, but not hypertriglyceridemia, in response to this diet. LDLR-/- mice fed the diabetogenic diet became more obese than wild-type mice and developed severe hypertriglyceridemia and hyperleptinemia. Surprisingly, glucose levels were only modestly higher and insulin levels and insulin-to-glucose ratios were not strikingly different from those of wild-type mice. In contrast, diabetogenic diet-fed apoE-/- mice were resistant to changes in glucose and lipid homeostasis despite becoming obese. These data suggest that modifications in lipoprotein profiles associated with loss of the LDL receptor or apoE function have profound and unique consequences on susceptibility to diet-induced obesity and type 2 diabetic phenotypes.

Macrophage-specific expression of human lipoprotein lipase accelerates atherosclerosis in transgenic apolipoprotein e knockout mice but not in C57BL/6 mice

Transgenic mice with macrophage-specific expression of human (hu) lipoprotein lipase (LPL) were generated to determine the contribution of macrophage LPL to atherogenesis. Macrophage specificity was accomplished with the scavenger receptor A promoter. Complete characterization demonstrated that macrophages from these mice expressed huLPL mRNA and secreted enzymatically active huLPL protein. Expression of huLPL was macrophage specific, because total RNA isolated from heart, thymus, lung, liver, muscle, and adipose tissues was devoid of huLPL mRNA. Macrophage-specific expression of huLPL did not exacerbate lesions in aortas of C57BL/6 mice even after 32 weeks on an atherosclerotic diet. However, when expressed in apolipoprotein E knockout background, the extent of occlusion in the aortic sinus region of male huLPL+ mice increased 51% (n=9 to 11, P<0.002) compared with huLPL- mice after they had been fed a Western diet for 8 weeks. The proatherogenic effect of macrophage LPL was confirmed in serial sections of the aorta obtained after mice had been fed a Western diet for 3 weeks. By immunohistochemical analysis, huLPL protein was detected in the lesions of huLPL+ mice but not in huLPL- mice. Our results establish that macrophage LPL accelerates atherosclerosis in male apolipoprotein E knockout mice.