Detection, quantification, and characterization of potentially atherogenic triglyceride-rich remnant lipoproteins

Remnants, LDL, and the Quantification of Lipoprotein-Associated Risk in Atherosclerotic Cardiovascular Disease

Purpose of Review

Implementation of intensive LDL cholesterol (LDL-C) lowering strategies and recognition of the role of triglyceride-rich lipoproteins (TRL) in atherosclerosis has prompted re-evaluation of the suitability of current lipid profile measurements for future clinical practice.

Recent Findings

At low concentrations of LDL-C (< 1.8 mmol/l/70 mg/dl), the Friedewald equation yields estimates with substantial negative bias. New equations provide a more accurate means of calculating LDL-C. Recent reports indicate that the increase in risk per unit increment in TRL/remnant cholesterol may be greater than that of LDL-C. Hence, specific measurement of TRL/remnant cholesterol may be of importance in determining risk. Non-HDL cholesterol and plasma apolipoprotein B have been shown in discordancy analyses to identify individuals at high risk even when LDL-C is low.

Summary

There is a need to adopt updated methods for determining LDL-C and to develop better biomarkers that more accurately reflect the abundance of TRL remnant particles.

Triglyceride-rich lipoproteins and their remnants: metabolic insights, role in atherosclerotic cardiovascular disease, and emerging therapeutic strategies-a consensus statement from the European Atherosclerosis Society

Recent advances in human genetics, together with a large body of epidemiologic, preclinical, and clinical trial results, provide strong support for a causal association between triglycerides (TG), TG-rich lipoproteins (TRL), and TRL remnants, and increased risk of myocardial infarction, ischaemic stroke, and aortic valve stenosis. These data also indicate that TRL and their remnants may contribute significantly to residual cardiovascular risk in patients on optimized low-density lipoprotein (LDL)-lowering therapy. This statement critically appraises current understanding of the structure, function, and metabolism of TRL, and their pathophysiological role in atherosclerotic cardiovascular disease (ASCVD). Key points are (i) a working definition of normo- and hypertriglyceridaemic states and their relation to risk of ASCVD, (ii) a conceptual framework for the generation of remnants due to dysregulation of TRL production, lipolysis, and remodelling, as well as clearance of remnant lipoproteins from the circulation, (iii) the pleiotropic proatherogenic actions of TRL and remnants at the arterial wall, (iv) challenges in defining, quantitating, and assessing the atherogenic properties of remnant particles, and (v) exploration of the relative atherogenicity of TRL and remnants compared to LDL. Assessment of these issues provides a foundation for evaluating approaches to effectively reduce levels of TRL and remnants by targeting either production, lipolysis, or hepatic clearance, or a combination of these mechanisms. This consensus statement updates current understanding in an integrated manner, thereby providing a platform for new therapeutic paradigms targeting TRL and their remnants, with the aim of reducing the risk of ASCVD.

Intestinal and Hepatic Uptake of Dietary Peroxidized Lipids and Their Decomposition Products, and Their Subsequent Effects on Apolipoprotein A1 and Paraoxonase1

Both pro- and antiatherosclerotic effects have been ascribed to dietary peroxidized lipids. Confusion on the role of peroxidized lipids in atherosclerotic cardiovascular disease is punctuated by a lack of understanding regarding the metabolic fate and potential physiological effects of dietary peroxidized lipids and their decomposition products. This study sought to determine the metabolic fate and physiological ramifications of 13-hydroperoxyoctadecadienoic acid (13-HPODE) and 13-HODE (13-hydroxyoctadecadienoic acid) supplementation in intestinal and hepatic cell lines, as well as any effects resulting from 13-HPODE or 13-HODE degradation products. In the presence of Caco-2 cells, 13-HPODE was rapidly reduced to 13-HODE. Upon entering the cell, 13-HODE appears to undergo decomposition, followed by esterification. Moreover, 13-HPODE undergoes autodecomposition to produce aldehydes such as 9-oxononanoic acid (9-ONA). Results indicate that 9-ONA was oxidized to azelaic acid (AzA) rapidly in cell culture media, but AzA was poorly absorbed by intestinal cells and remained detectable in cell culture media for up to 18 h. An increased apolipoprotein A1 (ApoA1) secretion was observed in Caco-2 cells in the presence of 13-HPODE, 9-ONA, and AzA, whereas such induction was not observed in HepG2 cells. However, 13-HPODE treatments suppressed paraoxonase 1 (PON1) activity, suggesting the induction of ApoA1 secretion by 13-HPODE may not represent functional high-density lipoprotein (HDL) capable of reducing oxidative stress. Alternatively, AzA induced both ApoA1 secretion and PON1 activity while suppressing ApoB secretion in differentiated Caco-2 cells but not in HepG2. These results suggest oxidation of 9-ONA to AzA might be an important phenomenon, resulting in the accumulation of potentially beneficial dietary peroxidized lipid-derived aldehydes.

Triglyceride-Rich Lipoprotein Remnants and Cardiovascular Disease

BACKGROUND

Triglycerides, cholesterol, and their metabolism are linked due to shared packaging and transport within circulating lipoprotein particles. While a case for a causal role of cholesterol-carrying low-density lipoproteins (LDLs) in atherosclerosis is well made, the body of scientific evidence for a causal role of triglyceride-rich lipoproteins (TRLs) is rapidly growing, with multiple lines of evidence (old and new) providing robust support.

CONTENT

This review will discuss current perspectives and accumulated evidence that an overabundance of remnant lipoproteins stemming from intravascular remodeling of nascent TRLs-chylomicrons and very low-density lipoproteins (VLDL)-results in a proatherogenic milieu that augments cardiovascular risk. Basic mechanisms of TRL metabolism and clearance will be summarized, assay methods reviewed, and pivotal clinical studies highlighted.

SUMMARY

Remnant lipoproteins are rendered highly atherogenic by their high cholesterol content, altered apolipoprotein composition, and physicochemical properties. The aggregate findings from multiple lines of evidence suggest that TRL remnants play a central role in residual cardiovascular risk.

Lipoprotein Abnormalities in Chronic Kidney Disease and Renal Transplantation

Chronic kidney disease (CKD) is one of the most important risk factors for cardiovascular disease (CVD). Despite the kidney having no direct implications for lipoproteins metabolism, advanced CKD dyslipidemia is usually present in patients with CKD, and the frequent lipid and lipoprotein alterations occurring in these patients play a role of primary importance in the development of CVD. Although hypertriglyceridemia is the main disorder, a number of lipoprotein abnormalities occur in these patients. Different enzymes pathways and proteins involved in lipoprotein metabolism are impaired in CKD. In addition, treatment of uremia may modify the expression of lipoprotein pattern as well as determine acute changes. In renal transplantation recipients, the main lipid alteration is hypercholesterolemia, while hypertriglyceridemia is less pronounced. In this review we have analyzed lipid and lipoprotein disturbances in CKD and also their relationship with progression of renal disease. Hypolipidemic treatments may also change the natural history of CVD in CKD patients and may represent important strategies in the management of CKD patients.

Conventional and Novel Lipid Measures and Risk of Peripheral Artery Disease

OBJECTIVE

The aim of this study was to comprehensively assess the association of multiple lipid measures with incident peripheral artery disease (PAD). Approach and Results: We used Cox proportional hazards models to characterize the associations of each of the fasting lipid measures (total cholesterol, LDL-C [low-density lipoprotein cholesterol], HDL-C [high-density lipoprotein cholesterol], triglycerides, RLP-C [remnant lipoprotein cholesterol], LDL-TG [LDL-triglycerides], sdLDL-C [small dense LDL-C], and Apo-E-HDL [Apo-E-containing HDL-C]) with incident PAD identified by pertinent International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) hospital discharge codes (eg, 440.2) among 8330 Black and White ARIC (Atherosclerosis Risk in Communities) participants (mean age 62.8 [SD 5.6] years) free of PAD at baseline (1996-1998) through 2015. Since lipid traits are biologically correlated to each other, we also conducted principal component analysis to identify underlying components for PAD risk. There were 246 incident PAD cases with a median follow-up of 17 years. After accounting for potential confounders, the following lipid measures were significantly associated with PAD (hazard ratio per 1-SD increment [decrement for HDL-C and Apo-E-HDL]): triglycerides, 1.21 (95% CI, 1.08-1.36); RLP-C, 1.18 (1.08-1.29); LDL-TG, 1.18 (1.05-1.33); HDL-C, 1.39 (1.16-1.67); and Apo-E-HDL, 1.27 (1.07-1.51). The principal component analysis identified 3 components (1: mainly loaded by triglycerides, RLP-C, LDL-TG, and sdLDL-C; 2: by HDL-C and Apo-E-HDL; and 3: by LDL-C and RLP-C). Components 1 and 2 showed independent associations with incident PAD.

CONCLUSIONS

Triglyceride-related and HDL-related lipids were independently associated with incident PAD, which has implications on preventive strategies for PAD. However, none of the novel lipid measures outperformed conventional ones. Graphic Abstract: A graphic abstract is available for this article.

Evaluation of atherogenic lipoprotein-cholesterol to HDL cholesterol ratio as a prognostic test for ST-segment elevation myocardial infarction

Background: The detectable component of triglyceride-rich lipoproteins (TGRLs), remnant lipoprotein cholesterol (RLP-c), has been proven being correlated with the progression of atherosclerosis and myocardial infarction. However, when taken as a risk predictor, the prognostic and diagnostic potential of RLP-c remains controversial in studies. In this study, we evaluated the hypothesis that atherogenic lipoprotein-cholesterol (AL-c), representing the sum of RLP-c and the sd-LDL-c, to the HDL-c ratio, could represent a better predictive indicator than RLP-c alone in ST-segment elevation myocardial infarction (STEMI). Methods: The 316 consecutive patients suffering from persistent chest discomfort admitted to the Shanghai General Hospital between January 2018 and June 2018 were enrolled. 149 STEMI patients (62% men, mean age 69.6 ± 13.3 years) were included as the study cohort. The AL-c/HDL-c ratio was calculated on admission in a cohort of electrocardiogram-confirmed STEMI patients and compared to other lipid profiles as a predictive indicator. Results: The AL-c/HDL-c ratio was significantly increased in STEMI patients compared with apparently healthy adults (0.93; IQR [0.71-1.18] vs 0.70; IQR [0.45-1.04]; p < 0.001). Gender dependency existed, and the male and female patients had median AL-c/HDL-c ratios of 1.01 and 0.79, respectively (p < 0.001). Compared to RLP-c, the AL-c/HDL-c ratio had a better prognostic value to predict STEMI risk in both sexes (AUC of 0.672 with a sensitivity of 0.794 in males and 0.613 with a sensitivity of 0.684 in females). Conclusions: The AL-c/HDL-c ratio could represent a convenient and sensitive biomarker for screening and predicting STEMI risk.

Pathophysiology of Type 2 Diabetes Mellitus

Type 2 Diabetes Mellitus (T2DM), one of the most common metabolic disorders, is caused by a combination of two primary factors: defective insulin secretion by pancreatic β-cells and the inability of insulin-sensitive tissues to respond appropriately to insulin. Because insulin release and activity are essential processes for glucose homeostasis, the molecular mechanisms involved in the synthesis and release of insulin, as well as in its detection are tightly regulated. Defects in any of the mechanisms involved in these processes can lead to a metabolic imbalance responsible for the development of the disease. This review analyzes the key aspects of T2DM, as well as the molecular mechanisms and pathways implicated in insulin metabolism leading to T2DM and insulin resistance. For that purpose, we summarize the data gathered up until now, focusing especially on insulin synthesis, insulin release, insulin sensing and on the downstream effects on individual insulin-sensitive organs. The review also covers the pathological conditions perpetuating T2DM such as nutritional factors, physical activity, gut dysbiosis and metabolic memory. Additionally, because T2DM is associated with accelerated atherosclerosis development, we review here some of the molecular mechanisms that link T2DM and insulin resistance (IR) as well as cardiovascular risk as one of the most important complications in T2DM.

Prognostic utility of triglyceride-rich lipoprotein-related markers in patients with coronary artery disease

TG-rich lipoprotein (TRL)-related biomarkers, including TRL-cholesterol (TRL-C), remnant-like lipoprotein particle-cholesterol (RLP-C), and apoC-III have been associated with atherosclerosis. However, their prognostic values have not been fully determined, especially in patients with previous CAD. This study aimed to examine the associations of TRL-C, RLP-C, and apoC-III with incident cardiovascular events (CVEs) in the setting of secondary prevention of CAD. Plasma TRL-C, RLP-C, and total apoC-III were directly measured. A total of 4,355 participants with angiographically confirmed CAD were followed up for the occurrence of CVEs. During a median follow-up period of 5.1 years (interquartile range: 3.9-6.4 years), 543 (12.5%) events occurred. Patients with incident CVEs had significantly higher levels of TRL-C, RLP-C, and apoC-III than those without events. Multivariable Cox analysis indicated that a log unit increase in TRL-C, RLP-C, and apoC-III increased the risk of CVEs by 49% (95% CI: 1.16-1.93), 21% (95% CI: 1.09-1.35), and 40% (95% CI: 1.11-1.77), respectively. High TRL-C, RLP-C, and apoC-III were also independent predictors of CVEs in individuals with LDL-C levels ≤1.8 mmol/l (n = 1,068). The addition of RLP-C level to a prediction model resulted in a significant increase in discrimination, and all three TRL biomarkers improved risk reclassification. Thus, TRL-C, RLP-C, and apoC-III levels were independently associated with incident CVEs in Chinese CAD patients undergoing statin therapy.

Comparing remnant lipoprotein cholesterol measurement methods to evaluate efficacy of ezetimibe/statin vs statin therapy

BACKGROUND

Elevated remnant lipoprotein cholesterol (RLP-C) levels increase cardiovascular disease risk. However, RLP-C measurement methods are not standardized, leading to variations across studies.

OBJECTIVE

To evaluate the effect of ezetimibe (Eze) + statins vs statin monotherapy on RLP-C using immunoseparation (IM), vertical auto profile (VAP) ultracentrifugation, and calculated RLP-C measurement methods.

METHODS

This post hoc analysis evaluated data pooled from 3 first-line (all-statin [simvastatin 10/20/40/80 mg] vs Eze + statin [Eze 10 mg + simvastatin]) and 2 second-line (statin [atorvastatin uptitrated to 40/80 mg] vs statin + Eze [atorvastatin 20/40 mg + Eze 10 mg]) studies. Similarity of RLP-C methods was evaluated using Pearson correlation coefficients and Bland-Altman plots. RLP-C changes and percent changes from baseline were measured by all 3 methods in first-line and VAP and calculated methods in second-line studies.

RESULTS

Correlations between methods were generally moderate to strong for RLP-C levels, changes, and percent changes across treatment groups (r = 0.29-0.79) but with little evidence of agreement by Bland-Altman plots. Baseline RLP-C levels for Eze + statin vs all-statin groups were lower by IM (14.0 vs 14.0) compared with VAP (36.9 vs 35.9) and calculated (32.8 vs 33.3) methods. RLP-C changes (mg/dL) and percent changes from baseline were significantly greater (P < .01) with Eze + statins vs statins by VAP, calculated, and IM methods (between-treatment differences: -5.0 and -12.0, -2.0 and -5.4, and -1.5 and -12.1, respectively) in first-line, and VAP and calculated methods (between-treatment differences: -5.0 and -19.9 and -2.0 and -7.3) in second-line studies.

CONCLUSION

Although the 3 methods showed little agreement, each supported Eze + statins for achieving greater RLP-C reductions vs statin monotherapy; variability of results reinforces urgent need to standardize RLP-C measurements.

Development of a Novel Homogeneous Assay for Remnant Lipoprotein Particle Cholesterol

Background

Quantification of remnant lipoprotein particle cholesterol (RLP-C) by automated assay is useful in routine clinical laboratories to assess coronary artery disease risk and diagnose type III hyperlipoproteinemia.

Methods

Enzymes and surfactants were screened to establish a homogeneous RLP-C assay using the chylomicron-VLDL, LDL, and HDL fractions isolated by ultracentrifugation, along with the RLP fraction isolated by immunoaffinity gel. All data were generated using a Hitachi analyzer.

Results

A specific cholesterol esterase with a polyoxyethelene styrenated phenyl ether derivative (surfactant) was used for the establishment of a homogeneous RLP-C assay. This cholesterol esterase with subunits of &gt;40 kDa (H-CE) was found to react with lipoproteins other than RLP, whereas this enzyme with subunits of &lt;40 kDa (L-CE) reacted with RLP. H-CE was applied for the first reaction step with the specific surfactant to decompose non-RLP lipoproteins, degrading non-RLP cholesterol into water and oxygen in the presence of cholesterol oxidase and catalase. For the second step, L-CE was applied to release cholesterol from RLP, and then the released RLP-C was determined in a standard cholesterol oxidase and peroxidase system. This new homogeneous assay exhibited good correlation with the RLP-C immunoseparation method.

Conclusions

We established a simple, rapid, automated homogeneous assay for RLP-C. The assay can determine RLP-C levels in 10 min in a fully automated manner, processing a large number of samples in routine clinical laboratories.

Update on the laboratory investigation of dyslipidemias

The role of the clinical laboratory is evolving to provide more information to clinicians to assess cardiovascular disease (CVD) risk and target therapy more effectively. Current routine methods to measure LDL-cholesterol (LDL-C), the Friedewald calculation, ultracentrifugation, electrophoresis and homogeneous direct methods have established limitations. Studies suggest that LDL and HDL size or particle concentration are alternative methods to predict future CVD risk. At this time there is no consensus role for lipoprotein particle or subclasses in CVD risk assessment. LDL and HDL particle concentration are measured by several methods, namely gradient gel electrophoresis, ultracentrifugation-vertical auto profile, nuclear magnetic resonance and ion mobility. It has been suggested that HDL functional assays may be better predictors of CVD risk. To assess the issue of lipoprotein subclasses/particles and HDL function as potential CVD risk markers robust, simple, validated analytical methods are required. In patients with small dense LDL particles, even a perfect measure of LDL-C will not reflect LDL particle concentration. Non-HDL-C is an alternative measurement and includes VLDL and CM remnant cholesterol and LDL-C. However, apolipoprotein B measurement may more accurately reflect LDL particle numbers. Non-fasting lipid measurements have many practical advantages. Defining thresholds for treatment with new measurements of CVD risk remain a challenge. In families with genetic variants, ApoCIII and lipoprotein (a) may be additional risk factors. Recognition of familial causes of dyslipidemias and diagnosis in childhood will result in early treatment. This review discusses the limitations in current laboratory technologies to predict CVD risk and reviews the evidence for emergent approaches using newer biomarkers in clinical practice.

Atherogenic postprandial remnant lipoproteins; VLDL remnants as a causal factor in atherosclerosis

Oxidized LDL (Ox-LDL) and chylomicron (CM) remnants have been suggested to be the most atherogenic lipoproteins that initiate and exacerbate coronary atherosclerosis. In this review, we propose a hypothesis of the causal lipoproteins in atherosclerosis based on our recent findings on postprandial remnant lipoproteins (RLP). Plasma RLP-C and RLP-TG increased significantly after food intake, especially a fat load. More than 80% of the TG increase after the fat load consisted of the TG in RLP, which contained significantly greater apoB100 than apoB48 particles as VLDL remnants. The majority of the LPL in non-heparin plasma was found in RLP as an RLP-LPL complex and released into the circulation after hydrolysis. Plasma LPL did not increase after food intake, which may have caused the partial hydrolysis of CM and VLDL as well as the significant increase of RLP-TG in the postprandial plasma. LPL was inversely correlated with the RLP particle size after food intake. We showed that VLDL remnants are the major atherogenic lipoproteins in the postprandial plasma associated with insufficient LPL activity and a causal factor in the initiation and progression of atherosclerosis. We also propose "LPL bound TG-rich lipoproteins" as a new definition of remnant lipoproteins based on the findings of the RLP-LPL complex in the non-heparin plasma.

Remnant cholesterol predicts periprocedural myocardial injury following percutaneous coronary intervention in poorly-controlled type 2 diabetes

BACKGROUND

Remnant cholesterol (RC) is receiving increasing attention regarding its relation to cardiovascular risk. Whether RC is associated with periprocedural myocardial injury (PMI) following percutaneous coronary intervention (PCI) in type 2 diabetes (T2D) is currently unknown.

METHODS

We prospectively enrolled 1182 consecutive T2D patients who were scheduled for PCI but with baseline normal preprocedural cardiac troponin I (cTnI). Patients were divided according to their glycemic control status: group A [glycated hemoglobin (HbA1c)<7%, n=563] and group B (HbA1c≥7%, n=619). PMI was evaluated by cTnI analysis within 24h. The associations of preprocedural RC and the RC to high-density lipoprotein cholesterol ratio (RC/HDL-C) with PMI were investigated.

RESULTS

The associations of RC and RC/HDL-C with PMI were observed in group B (both p<0.05) but not in group A (both p>0.05). Patients in group B, a 1-SD increase of RC produced 30% and 32% increased risk for postprocedural cTnI>3× upper limit of normal (ULN) and >5×ULN, respectively. The odds ratios for RC/HDL-C were the highest compared with any cholesterol fractions including total cholesterol (TC)/HDL-C, low density lipoprotein cholesterol (LDL-C)/HDL-C, nonHDL-C/HDL-C, and triglyceride/HDL-C with 1.43 [95% confidence interval (CI): 1.10-1.88] for >3× ULN and 1.49 (95% CI: 1.13-1.97) for >5× ULN. However, no such associations were found in group A. Furthermore, patients with RC >27.46mg/dL (third tertile) [RC≤14.15mg/dL (first tertile) as reference] were associated with a 1.57-fold and 2-fold increased risk for >3× ULN and >5× ULN in group B, respectively.

CONCLUSIONS

RC and RC/HDL-C might be valuable, independent predictors for PMI in poorly-controlled diabetic patients undergoing PCI.

Postprandial Hyperlipidemia and Remnant Lipoproteins

Fasting hypertriglyceridemia is positively associated with the morbidity of coronary heart disease (CHD), and postprandial (non-fasting) hypertriglyceridemia is also correlated with the risk status for CHD, which is related to the increase in chylomicron (CM) remnant lipoproteins produced from the intestine. CM remnant particles, as well as oxidized low density lipoprotein (LDL) or very low density lipoprotein (VLDL) remnants, are highly atherogenic and act by enhancing systemic inflammation, platelet activation, coagulation, thrombus formation, and macrophage foam cell formation. The cholesterol levels of remnant lipoproteins significantly correlate with small, dense LDL; impaired glucose tolerance (IGT) and CHD prevalence. We have developed an assay of apolipoprotein (apo)B-48 levels to evaluate the accumulation of CM remnants. Fasting apoB-48 levels correlate with the morbidity of postprandial hypertriglyceridemia, obesity, type III hyperlipoproteinemia, the metabolic syndrome, hypothyroidism, chronic kidney disease, and IGT. Fasting apoB-48 levels also correlate with carotid intima-media thickening and CHD prevalence, and a high apoB-48 level is a significant predictor of CHD risk, independent of the fasting TG level. Diet interventions, such as dietary fibers, polyphenols, medium-chain fatty acids, diacylglycerol, and long-chain n-3 polyunsaturated fatty acids (PUFA), ameliorate postprandial hypertriglyceridemia, moreover, drugs for dyslipidemia (n-3 PUFA, statins, fibrates or ezetimibe) and diabetes concerning incretins (dipeptidyl-peptidase IV inhibitor or glucagon like peptide-1 analogue) may improve postprandial hypertriglyceridemia. Since the accumulation of CM remnants correlates to impaired lipid and glucose metabolism and atherosclerotic cardiovascular events, further studies are required to investigate the characteristics, physiological activities, and functions of CM remnants for the development of new interventions to reduce atherogenicity.

Polyhedral 3D structure of human plasma very low density lipoproteins by individual particle cryo-electron tomography1

Human VLDLs assembled in the liver and secreted into the circulation supply energy to peripheral tissues. VLDL lipolysis yields atherogenic LDLs and VLDL remnants that strongly correlate with CVD. Although the composition of VLDL particles has been well-characterized, their 3D structure is elusive because of their variations in size, heterogeneity in composition, structural flexibility, and mobility in solution. Here, we employed cryo-electron microscopy and individual-particle electron tomography to study the 3D structure of individual VLDL particles (without averaging) at both below and above their lipid phase transition temperatures. The 3D reconstructions of VLDL and VLDL bound to antibodies revealed an unexpected polyhedral shape, in contrast to the generally accepted model of a spherical emulsion-like particle. The smaller curvature of surface lipids compared with HDL may also reduce surface hydrophobicity, resulting in lower binding affinity to the hydrophobic distal end of the N-terminal β-barrel domain of cholesteryl ester transfer protein (CETP) compared with HDL. The directional binding of CETP to HDL and VLDL may explain the function of CETP in transferring TGs and cholesteryl esters between these particles. This first visualization of the 3D structure of VLDL could improve our understanding of the role of VLDL in atherogenesis.

Atherogenic Lipoproteins in Subclinical Hypothyroidism and Their Relationship with Hepatic Lipase Activity: Response to Replacement Treatment with Levothyroxine

BACKGROUND

Qualitative lipoprotein changes, such as an increase in fasting remnants, are reported in subclinical hypothyroidism (SCH). It was hypothesized that such changes are due to reduced hepatic lipase (HL) activity in SCH: HL is an enzyme regulated by thyroid hormones, and is involved in the degradation of triglyceride (TG)-rich remnants. This study aimed to quantify remnant-like lipoproteins (RLP), small dense LDL (sdLDL), and HL activity in women with SCH, and to assess these parameters after levothyroxine replacement therapy.

METHODS

This was an observational cross-sectional study with a subsequent longitudinal follow-up. Findings in women with thyrotropin levels >4.5 mIU/L (SH group) were compared with age- and body mass index (BMI)-matched euthyroid women (control group). In addition, a subgroup analysis was undertaken in SCH women who chose to receive levothyroxine treatment (0.9 μg/kg/day) for 6 months. RLP was quantified by measuring cholesterol (RLP-C) and triglycerides (RLP-TG) after immunoaffinity chromatography, and sdLDL by automated standardized methods; HL activity was measured in post-heparin plasma.

RESULTS

The SCH group included 37 women; 29 women were included in the control group. In addition, 22 women with SCH were included in the subgroup analysis (levothyroxine treatment). Significantly higher RLP values were observed in the SCH group than in the control group: RLP-C (median [range], mg/dL): 20.3 (5.8-66.8) versus 10.2 (2.7-36.3), p = 0.005; RLP-TG (mg/dL): 26.3 (3.2-123.3) versus 12.1 (2.5-61.6), p = 0.033. HL activity (mean ± standard deviation [SD], μmol free fatty acid/mL post-heparin plasma.h)-9.83 ± 4.25 versus 9.92 ± 5.20, p = 0.707-and sdLDL levels (mg/dL)-23.1 ± 10.7 versus 22.6 ± 8.4, p = 0.83-were similar. After levothyroxine, RLP-C decreased-21.5 (5.8-66.8) versus 17.2 (4.1-45.6), p = 0.023-and HL increased-9.75 ± 4.04 versus 11.86 ± 4.58, p = 0.012-in the subgroup of SCH women. No changes in sdLDL were observed.

CONCLUSIONS

Women with SCH have higher RLP levels than matched controls do, but their RLP-C levels decrease significantly following levothyroxine therapy. Furthermore, HL activity also increases after levothyroxine therapy and can be interpreted as a possible explanation for the decrease in RLP-C.

Pathophysiologic and treatment strategies for cardiovascular disease in end-stage renal disease and kidney transplantations

The inextricable link between the heart and the kidneys predestines that significant cardiovascular disease ensues in the face of end-stage renal disease (ESRD). As a point of fact, the leading cause of mortality of patients on dialysis is still from cardiovascular etiologies, albeit differing in particular types of disease from the general population. For example, sudden cardiac death outnumbers coronary artery disease in patients with ESRD, which is the reverse for the general population. In this review, we will focus on the pathophysiology and treatment options of important traditional and nontraditional risk factors for cardiovascular disease in ESRD patients such as hypertension, anemia, vascular calcification, hyperparathyroidism, uremia, and oxidative stress. The evidence of erythropoietin-stimulating agents, phosphate binders, calcimimetics, and dialysis modalities will be presented. We will then discuss how these risk factors may be changed and perhaps exacerbated after renal transplantation. This is largely due to the immunosuppressive agents that are both crucial yet potentially detrimental in the posttransplant state. Calcineurin inhibitors, corticosteroids, and mammalian target of rapamycin inhibitors, the mainstay of transplant immunosuppression, are all known to increase the risks of developing new onset diabetes as well as the metabolic syndrome. Thus, we need to carefully negotiate between patients' cardiovascular profile and their risks of rejection. Finally, we end by considering strategies by which we may minimize cardiovascular disease in the transplant population, as this modality still confers the highest chance of survival in patients with ESRD.

Premature myocardial infarction is strongly associated with increased levels of remnant cholesterol

BACKGROUND

Remnant cholesterol has been defined as the cholesterol present in triglyceride-rich remnant lipoproteins. Elevated levels of remnant cholesterol have been associated with increased cardiovascular risk. Acute myocardial infarction (AMI) in very young individuals (≤40 years) represents a rare disease with a typical risk factor profile and a lipid phenotype that is characterized by a predominance of elevated triglyceride-rich lipoproteins.

OBJECTIVE

The aim of this study was to investigate the role of remnant cholesterol in premature AMI.

METHODS

We prospectively enrolled 302 patients into our multicenter case-control study comprising 102 consecutive myocardial infarction survivors (≤40 years) and 200 hospital controls. Myocardial infarction patients were frequency matched for age, gender, and center. Remnant cholesterol was calculated from standard lipid parameters.

RESULTS

Remnant cholesterol was 1.7-fold higher in premature AMI patients compared with controls (61.1 ± 36.8 vs 35.8 ± 16.8 mg/dL; P < .001). Remnant cholesterol was the lipid fraction most strongly associated with premature myocardial infarction (odds ratio 3.87; 95% confidence interval 2.26-6.64; P < .001) for an increase of 1-standard deviation. This observation was independent from clinical risk factors and plasma lipid levels.

CONCLUSIONS

Remnant cholesterol is strongly associated with premature myocardial infarction, can be easily calculated, and might serve as a new potent risk marker in this young patient population.

Hyperlipoproteinemia type 3: the forgotten phenotype

Hyperlipoproteinemia type 3 (HLP3) is caused by impaired removal of triglyceride-rich lipoproteins (TGRL) leading to accumulation of TGRL remnants with abnormal composition. High levels of these remnants, called β-VLDL, promote lipid deposition in tuberous xanthomas, atherosclerosis, premature coronary artery disease, and early myocardial infarction. Recent genetic and molecular studies suggest more genes than previously appreciated may contribute to the expression of HLP3, both through impaired hepatic TGRL processing or removal and increased TGRL production. HLP3 is often highly amenable to appropriate treatment. Nevertheless, most HLP3 probably goes undiagnosed, in part because of lack of awareness of the relatively high prevalence (about 0.2% in women and 0.4-0.5% in men older than 20 years) and largely because of infrequent use of definitive diagnostic methods.

New Insights into the Regulation of Chylomicron Production

Dietary lipids are efficiently absorbed by the small intestine, incorporated into triglyceride-rich lipoproteins (chylomicrons), and transported in the circulation to various tissues. Intestinal lipid absorption and mobilization and chylomicron synthesis and secretion are highly regulated processes. Elevated chylomicron production rate contributes to the dyslipidemia seen in common metabolic disorders such as insulin-resistant states and type 2 diabetes and likely increases the risk for atherosclerosis seen in these conditions. An in-depth understanding of the regulation of chylomicron production may provide leads for the development of drugs that could be of therapeutic utility in the prevention of dyslipidemia and atherosclerosis. Chylomicron secretion is subject to regulation by various factors, including diet, body weight, genetic variants, hormones, nutraceuticals, medications, and emerging interventions such as bariatric surgical procedures. In this review we discuss the regulation of chylomicron production, mechanisms that underlie chylomicron dysregulation, and potential avenues for future research.

Identification of molecular species of oxidized triglyceride in plasma and its distribution in lipoproteins

The role of triglycerides carried in the triglyceride-rich lipoproteins (TRL) in the progression of atherosclerosis is uncertain. Identification of oxidized triglycerides and its possible association with atherosclerosis were largely ignored. Here we applied mass spectrometric approach to detect and identify triglyceride hydroperoxides (TGOOH) in human plasma and lipoproteins.

EDTA plasma was collected from healthy human volunteers (n=9) after 14–16 h of fasting. Very low-density lipoprotein (VLDL)

We identified 11 molecular species of TGOOH in either plasma or VLDL and IDL, of which TGOOH-18:1/18:2/16:0, TGOOH-18:1/18:1/16:0, TGOOH-16:0/18:2/16:0, TGOOH-18:1/18:1/18:1, and TGOOH-16:0/20:4/16:0 were most dominant. These TGOOH molecules are carried by TRL but not by LDL and HDL. Mean concentration of TGOOH in plasma, VLDL and IDL were, respectively, 56.1±25.6, 349.8±253.6 and 512.5±173.2 μmol/mol of triglycerides.

This is the first report to identify several molecular species of oxidized triglycerides in TRL. Presence of oxidized triglyceride may contribute to the atherogenicity of TRL. Further work is needed to elucidate the association of the oxidized triglyceride in atherosclerosis.

Lipoprotein lipase: from gene to atherosclerosis

Lipoprotein lipase (LPL) is a key enzyme in lipid metabolism and responsible for catalyzing lipolysis of triglycerides in lipoproteins. LPL is produced mainly in adipose tissue, skeletal and heart muscle, as well as in macrophage and other tissues. After synthesized, it is secreted and translocated to the vascular lumen. LPL expression and activity are regulated by a variety of factors, such as transcription factors, interactive proteins and nutritional state through complicated mechanisms. LPL with different distributions may exert distinct functions and have diverse roles in human health and disease with close association with atherosclerosis. It may pose a pro-atherogenic or an anti-atherogenic effect depending on its locations. In this review, we will discuss its gene, protein, synthesis, transportation and biological functions, and then focus on its regulation and relationship with atherosclerosis and potential underlying mechanisms. The goal of this review is to provide basic information and novel insight for further studies and therapeutic targets.

Novel unbiased equations to calculate triglyceride-rich lipoprotein cholesterol from routine non-fasting lipids

Background

Non-fasting triglyceride-rich lipoproteins cholesterol (TRL-C) contributes to cardiovascular risk, in that it includes remnant cholesterol (RC). TRL-C is computed as total C - [LDL-C + HDL-C]. Such calculation applies only if LDL-C is directly measured, or obtained from a non-Friedewald’s formula, a method as yet never benchmarked against independent markers of TRL burden.

Methods

The Discriminant Ratio (DR) methodology was used in 120 type 2 diabetic patients in order: (i) to compute TRL-C from non-fasting lipids; (ii) to establish the performance of TRL-C and TRL-C/apoA-I (vs. TG-based markers) to grade TRLs and atherogenic dyslipidemia (AD); and (iii) to relate TRL-C with non-fasting TG.

Results

Depending on apoB₁₀₀ availability, TRL-C (mg/dL) can be derived from non-fasting lipids in two ways: (a) total cholesterol (TC) - [(0.0106 * TC - 0.0036 * TG + 0.017 * apoB₁₀₀ - 0.27) * 38.6] - HDL-C; and (b) TC - [(0.0106 * TC - 0.0036 * TG + 0.017 * [0.65 * (TC - HDL-C) + 6.3] - 0.27) * 38.6] - HDL-C. Discrimination between log[TG] and TRL-C was similar (DR 0.94 and 0.84, respectively), whereas that of log[TG]/HDL-C was better than TRL-C/apoA-I (DR 1.01 vs. 0.65; p 0.0482). All Pearson’s correlations between pairs reached unity, allowing formulation of two unbiased equivalence equations: (a) TRL-C = 97.8 * log[TG] - 181.9; and (b) TRL-C/apoA-I = 8.15 * (log[TG]/HDL-C) - 0.18.

Conclusions

TRL-C and log[TG] are as effective and interchangeable for assessing remnant atherogenic particles. For grading TRL-AD, it is best to use log[TG]/HDL-C, inherently superior to TRL-C/apoA-I, while measuring the same underlying variable.

Rare PPARA variants and extreme response to fenofibrate in the Genetics of Lipid-Lowering Drugs and Diet Network Study

OBJECTIVE

Fenofibrate, a peroxisome proliferator-activated receptor-α (PPARα) agonist, reduces triglyceride (TG) concentrations by 25-60%. Given significant interindividual variations in the TG response, we investigated the association of PPARA rare variants with treatment response in the Genetics of Lipid-Lowering Drugs and Diet Network study.

METHODS

We calculated the change in the TG concentration (ΔTG) among 861 GOLDN participants treated with fenofibrate (160 mg/day) for 3 weeks. From the distribution of ΔTG adjusted for age and sex, the 150 highest and 150 lowest fenofibrate responders were selected from the tails of the distribution for PPARA resequencing. The resequencing strategy was based on VariantSEQr technology for the amplification of exons and regulatory regions.

RESULTS

We identified 73 variants with an average minor allele frequency of 4.8% (range: 0.2-16%). We tested the association of rare variants located in a coding or a regulatory region (minor allele frequency<1%, 13 variants) with treatment response group by an indicator variable (presence/absence of ≥1 rare variant) using general linear mixed models to allow for adjustment for family relationship. After adjusting for baseline, fasting TG concentration carrying at least one rare variant was associated with a low fenofibrate response (odds ratio=6.46; 95% confidence interval: 1.4-30.8). Carrier status was also associated with a relative change in the total cholesterol concentration (P=0.02), but not high-density lipoprotein or low-density lipoprotein concentration.

CONCLUSION

Rare, potentially functional variants in PPARA may play a role in the TG response to fenofibrate, but future experimental studies will be necessary to replicate the findings and confirm functional effects.

Consumption of low doses of fat prevents the postprandial rise in chylomicron particle concentration and remnant accumulation in healthy normolipidaemic males

Chylomicron particles are continually synthesised and secreted from the intestine even in the absence of ingested fat. It is possible that following consumption of low doses of fat the basal level of chylomicron secretion and subsequent metabolism are sufficient to metabolise this fat without an increase in postprandial chylomicron concentrations. To test this hypothesis, healthy male subjects were randomised to receive, on three separate occasions, meals containing a range of doses of fat (average 8·1–19 g) and effects on postprandial lipaemia and chylomicron concentration were determined. Furthermore, to delineate the effect on lipid-rich v. lipid-poor (remnant) forms lipid levels were also determined in a density <1·006 g/ml fraction. Following consumption of the very low dose of fat the postprandial concentration of chylomicrons was unaltered, whereas following the medium dose postprandial chylomicron concentrations were significantly increased. Interestingly, this increase was only detected in the lipid-rich chylomicron fraction, with postprandial levels of chylomicron remnants remaining unchanged. In conclusion, it appears that consumption of what would be considered low to medium doses of fat are not associated with transient postprandial increases in chylomicron remnants in healthy male subjects.

High levels of remnant lipoprotein cholesterol is a risk factor for large artery atherosclerotic stroke

BACKGROUND AND PURPOSE

Remnant lipoproteins (RLPs) are products of partially catabolized chylomicrons and very-low-density lipoprotein, from which some triglycerides have been removed. These particles are smaller and denser than the parent particles and are believed to be strongly atherogenic. We explored the association between RLP cholesterol (RLP-C) and ischemic stroke, including stroke subtypes.

METHODS

A cohort of 142 ischemic stroke patients (90 men and 52 women; age, 65.2±12.8 years, mean±SD) was enrolled; all had acute infarcts confirmed by diffusion-weighted MRI, and had fasting lipograms. A full stroke-related evaluation was conducted on each patient. An outpatient population of 88 subjects without a history of cerebrovascular or cardiovascular disease served as a control group. Serum RLP fractions were isolated using an immunoaffinity gel containing specific antiapolipoprotein (anti-apo)B-100 and anti-apoA-I antibodies. RLP-C values were considered to be high when they were in the highest quartile of all values in the study.

RESULTS

High RLP-C values were more common in stroke patients than in control patients (31.0% vs. 14.8%, p=0.01), when 5.6 mg/dL (>75th percentile) was used as the cutoff value. Multivariable analyses indicated that RLP-C was a risk factor for stroke, with an odds ratio of 2.54 (p=0.045). The RLP-C level was higher in the large artery atherosclerosis subgroup (5.7±3.9 mg/dL) than in any other stroke subgroup (small vessel occlusion, 4.9±5.9 mg/dL; cardioembolism, 1.8±2.3 mg/dL; stroke of undetermined etiology, 3.1±2.9 mg/dL).

CONCLUSIONS

We have found an association between high RLP-C levels and ischemic stroke, and in particular large artery atherosclerotic stroke.

Apolipoproteins: metabolic role and clinical biochemistry applications

Lipoprotein metabolism is dependent on apolipoproteins, multifunctional proteins that serve as templates for the assembly of lipoprotein particles, maintain their structure and direct their metabolism through binding to membrane receptors and regulation of enzyme activity. The three principal functions of lipoproteins are contribution to interorgan fuel (triglyceride) distribution (by means of the fuel transport pathway), to the maintenance of the extracellular cholesterol pool (by means of the overflow pathway) and reverse cholesterol transport. The most important clinical application of apolipoprotein measurements in the plasma is in the assessment of cardiovascular risk. Concentrations of apolipoprotein B and apolipoprotein AI (and their ratio) seem to be better markers of cardiovascular risk than conventional markers such as total cholesterol and LDL-cholesterol. Apolipoprotein measurements are also better standardized than the conventional tests. We suggest that measurements of apolipoprotein AI and apolipoprotein B are included as a part of the specialist lipid profile. We also suggest that lipoprotein (a) should be measured as part of the initial assessment of dyslipidaemias because of its consistent association with cardiovascular risk. Genotyping of apolipoprotein E isoforms remains useful in the investigation of mixed dyslipidaemias. Lastly, the role of postprandial metabolism is increasingly recognized in the context of atherogenesis, obesity and diabetes. This requires better markers of chylomicrons, very-low-density lipoproteins and remnant particles. Measurements of apolipoprotein B48 and remnant lipoprotein cholesterol are currently the key tests in this emerging field.

Understanding postprandial inflammation and its relationship to lifestyle behaviour and metabolic diseases

Postprandial hyperlipidemia with accumulation of remnant lipoproteins is a common metabolic disturbance associated with atherosclerosis and vascular dysfunction, particularly during chronic disease states such as obesity, the metabolic syndrome and, diabetes. Remnant lipoproteins become attached to the vascular wall, where they can penetrate intact endothelium causing foam cell formation. Postprandial remnant lipoproteins can activate circulating leukocytes, upregulate the expression of endothelial adhesion molecules, facilitate adhesion and migration of inflammatory cells into the subendothelial space, and activate the complement system. Since humans are postprandial most of the day, the continuous generation of remnants after each meal may be one of the triggers for the development of atherosclerosis. Modulation of postprandial lipemia by lifestyle changes and pharmacological interventions could result in a further decrease of cardiovascular mortality and morbidity. This paper will provide an update on current concepts concerning the relationship between postprandial lipemia, inflammation, vascular function, and therapeutic options.

New insights into how the intestine can regulate lipid homeostasis and impact vascular disease: frontiers for new pharmaceutical therapies to lower cardiovascular disease risk

In recent years, evidence has emerged that the intestine is a significant regulator of systemic cholesterol homeostasis and can contribute to raised plasma cholesterol concentration. In this review we provide a context for the role the intestine may have in cardiovascular disease during conditions of chronic disease (insulin resistance, obesity). In particular, we highlight the physiological role of the intestine in lipid absorption, identify novel elements in enterocyte molecular biology, review the concept that chylomicrons and their remnants contribute to atherogenesis during chronic disease, and address new principles of chylomicron overproduction during conditions of insulin resistance including the associated hormonal control of the intestine during these conditions. Finally, we raise the issue of a growing need for novel lipid-lowering pharmaceutical therapies that target intestinal lipid metabolism.

Ezetimibe improves postprandial hyperlipemia and its induced endothelial dysfunction

OBJECTIVE

Postprandial hyperlipemia has been shown to impair endothelial function and contribute to the development of atherosclerosis. We investigated the association between postprandial lipid profiles and endothelial function, and we examined the effects of ezetimibe on postprandial hyperlipemia and lipemia-induced endothelial dysfunction.

METHODS

A randomized prospective trial in which 10 mg/day of ezetimibe was administered to 10 subjects for 4 weeks and not administered to 10 subjects (control group) was performed. Lipid profiles and endothelial function, assessed by brachial artery flow-mediated dilation (FMD) during a fasting state and at 2, 4, 6 and 8 h after an oral cookie loading test, were determined before and after treatment for 4 weeks.

RESULTS

In all subjects before treatment, the maximum reduction in postprandial %FMD was significantly correlated with the maximum increases in postprandial triglyceride (TG) (r=-0.499, P<0.05) and apolipoprotein B-48 (apoB-48) concentrations (r=-0.551, P<0.05). Ezetimibe treatment for 4 weeks significantly suppressed postprandial elevation in TG (area under the incremental curve, from 1419±594 to 968±32 1 mg h/dl, P<0.05), remnant lipoprotein cholesterol (from 66.9±27.6 to 38.9±15.4 mg h/dl, P<0.01) and apoB-48 (from 58.8±27.5 to 36.2±17.0 μg h/ml, P<0.05) concentrations, and postprandial endothelial dysfunction assessed by %FMD (maximum reduction in %FMD, from -2.6±1.1% to -1.2±0.8%, P<0.05), whereas no significant changes were observed in the control group.

CONCLUSION

Postprandial hyperlipemia is closely correlated with transient endothelial dysfunction. Ezetimibe improves postprandial hyperlipemia and its induced endothelial dysfunction.

Recent insights into factors affecting remnant lipoprotein uptake

PURPOSE OF REVIEW

Remnant lipoproteins that persist in the bloodstream after each meal have become increasingly important contributors to atherosclerotic vascular disease, owing to the spread of overnutrition, underexertion, obesity, insulin resistance, and type 2 diabetes. Here, we review recent work that clarified long-standing controversies over the molecular mediators of remnant clearance by the liver, as well as their dysregulation - but possible correction - during alterations in caloric balance.

RECENT FINDINGS

Two endocytic receptors, the syndecan-1 heparan sulfate proteoglycan (HSPG) and the LDL receptor, plus one docking receptor, SR-BI, significantly contribute to normal hepatic remnant catabolism. Compelling evidence exists for dysfunction of the syndecan-1 HSPG in diabetic states. The major molecular defect identified so far in poorly controlled type 1 diabetes is impaired hepatic HSPG assembly. In contrast, the primary defect in hepatic HSPGs in type 2 diabetes appears to arise from accelerated de-sulfation, owing to the induction of a sulfatase. Moreover, short-term caloric restriction restores hepatic expression of this sulfatase towards normal.

SUMMARY

Correct identification of hepatic remnant receptors has finally allowed investigations of their molecular dysregulation in diabetes and related conditions. New work points to novel therapeutic targets to correct postprandial dyslipoproteinemia and its consequent arterial damage.

Plant stanol supplementation decreases serum triacylglycerols in subjects with overt hypertriglyceridemia

Evidence is accumulating that high serum concentrations of triacylglycerols (TAG) are, like LDL cholesterol, causally related to cardiovascular disease. A recent meta-analysis has indicated that plant stanol ester (PSE) intake not only lowered LDL cholesterol, but also serum TAG concentrations, especially in subjects with high baseline TAG concentrations. We therefore evaluated the effects of PSE supplementation on lipid metabolism in a population with elevated fasting TAG concentrations. In a randomized, placebo-controlled, parallel study, 28 subjects with elevated TAG concentrations (>1.7 mmol/L) were studied. After a 1-week run-in period during which a control margarine was used, subjects consumed for 3 weeks either control or PSE-enriched margarine (2.5 g/day of plant stanols). Serum plant stanol concentrations increased in all subjects receiving the PSE-enriched margarines, demonstrating good compliance. PSE supplementation significantly decreased serum total (6.7%, P = 0.015) and LDL cholesterol (9.5%, P = 0.041). A significant interaction between baseline TAG concentrations and PSE intake was found; PSE intake lowered TAG concentrations, particularly in subjects with high baseline TAG concentrations (>2.3 mmol/L; P = 0.009). Additionally, a significant interaction between baseline total number of LDL particles (LDL-P) and PSE intake was found (P = 0.020). PSE consumption lowered LDL-P, primarily in subjects with elevated baseline values; this was mainly due to a non-significant decrease in the number of atherogenic small LDL-P. Circulating levels of hs-CRP, glucose, and insulin were not changed after PSE intake. Taken together, PSE supplementation not only lowered LDL cholesterol, but also serum TAG concentrations, especially in subjects with overt hypertriglyceridemia.

Methods to assess impaired post-prandial metabolism and the impact for early detection of cardiovascular disease risk

Post-prandial lipaemia has emerged as a key contributor to cardiovascular disease (CVD) risk and progression. Specifically, delayed clearance of chylomicrons (CM) and their remnants increase the delivery of triglyceride and cholesteryl ester to the vessel wall and can accelerate the progression of atherosclerosis, which may be particularly pertinent to individuals with insulin resistance and/or obesity. As the number of studies linking post-prandial metabolism and chronic disease increases, interest has grown in the use of parameters reflecting CM metabolism as a possible indicator of early CVD risk. This, in turn has raised the question of what method might be most appropriate to detect CM and their remnants in plasma accurately. However, the handful of techniques able to measure CM metabolism (triglyceride-rich lipoprotein fractions; remnant-lipoprotein cholesterol; retinyl esters, CM-like emulsion; sodium dodecyl sulphate-polyacrylamide gel electrophoresis; immunoblotting, enzyme-linked immunoabsorbent assays; C(13) breath test; capillary finger prick) differ in their specificity, cost and applicability in research and in the clinical setting. In this review, we explore the scientific and clinical implications of CM methodology to better understand early risk assessment of CVD. We raise ongoing issues of the need to appreciate differential separation of very low-density lipoprotein and CM fractions, as well as to identify the technical basis for imprecision between assays for apolipoprotein B48. We also highlight emerging issues with respect to the practicality of measuring post-prandial metabolism in large clinical studies and offer opinions on the appropriateness of existing techniques in this field.

Oral supplementation with L-aspartate and L-glutamate inhibits atherogenesis and fatty liver disease in cholesterol-fed rabbit

Previous studies have shown that dietary supplementation with L-aspartate and L-glutamate inhibits fatty streak initiation in cholesterol-fed rabbit. The present study investigates the role of dicarboxylic amino acids on the progression of fatty streaks and the development of fatty liver disease, which were caused in New Zealand White rabbits after a 0.5% w/w cholesterol diet for 7 weeks. A group of animals additionally received a combination of 12.5 mM L-aspartate and 12.5 mM L-glutamate per day through drinking water. Total cholesterol (TC), high-density lipoproteins cholesterol (HDLC), non-HDLC and triacylglycerol (TAG) concentrations were measured in plasma. Serum gamma-glutamyl transferase (gamma-GT), aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities were also determined. At the end of dietary intervention, animals were sacrificed. Aortic, hepatic and brain lesions were evaluated after staining with hematoxylin and eosin. Supplementation with dicarboxylic amino acids inhibited the progression of aortic intima thickness (P < 0.05) and the development of liver lesions (P < 0.05). TC, non-HDLC and TAG were similarly increased in both cholesterol-fed groups. Serum gamma-GT and AST activities elevated during the study in all cholesterol-fed animals but the elevation of gamma-GT was milder and significantly lower in rabbits treated with L-aspartate and L-glutamate (P < 0.05). ALT activity was not affected by cholesterol feeding. In conclusion, oral supplementation with L-aspartate and L-glutamate inhibits the progression of atherogenesis and the development of fatty liver disease in the animal model of cholesterol-fed rabbit. The beneficial effects of dicarboxylic amino acids reflect the limited elevation of serum gamma-GT activity.

Remnant lipoproteins as strong key particles to atherogenesis

Recent epidemiologic studies have revealed that hypertriglyceridemia is associated with atherosclerosis independent of other coronary risk factors. However, it is difficult to select patients at high risk for coronary artery disease using only serum triglyceride levels compared with low-density lipoprotein cholesterol levels since multiple factors are associated with elevating triglycerides. Atherosclerotic diseases with high triglyceride levels can be found in patients with familial combined hyperlipidemia, diabetes mellitus, and metabolic syndrome, in which remnant lipoproteins accumulate in the circulating blood. Recent researches have paid attention to remnant lipoproteins as atherogenic particles with the development of methods for measuring remnant cholesterol levels and apolipoprotein B-48 levels directly from human serum. Measurement of these parameters in addition to serum triglycerides may help to distinguish high-risk patients and enable us to prevent or suppress the progression of atherosclerotic diseases in those patients. However, questions remain to be answered to evaluate the significance of remnant lipoproteins. Here, we focus on three issues: the underlying problems in measuring remnant lipoprotein cholesterol, the assessment of postprandial hyperlipidemia as an atherogenic condition, and finally a review of our experimental and clinical findings about the mechanisms by which remnant lipoproteins induce atherosclerosis.

Altered composition of triglyceride-rich lipoproteins and coronary artery disease in a large case-control study

BACKGROUND

Traditional beta-quantification of plasma lipoproteins by ultracentrifugation separates triglyceride-rich lipoproteins (TGRL) from higher density lipoproteins. The cholesterol in the TGRL fraction is referred to as measured very low-density lipoprotein cholesterol (VLDL-C) recognizing that other TGRL may be present. The measured VLDL-C to total plasma triglyceride (VLDL-C/TG) has long been considered an index of average TGRL composition with abnormally high VLDL-C/TG ratios (>or=0.30 with TG>150mg/dL) indicative of atherogenic remnant accumulation (type III hyperlipidemia). However, virtually no reports are available which examine potential associations between CAD and VLDL-C/TG at the lower end of the spectrum.

METHODS AND RESULTS

We performed ultracentrifugation in 1170 cases with premature-onset, familial CAD and 1759 population-based controls and examined the VLDL-C/TG ratio as an index of TGRL composition. As expected, we found very high CAD risk associated with severe type III hyperlipidemia (OR 10.5, p=0.02). Unexpectedly, however, we found a robust, graded, and independent association between CAD risk and lower than average VLDL-C/TG ratios (p<0.0001 as ordered categories or as a continuous variable). Among those in the lowest VLDL-C/TG category (a ratio <0.12), CAD risk was clearly increased (OR 4.5, 95% CI 2.9-6.9) and remained significantly elevated in various subgroups including those with triglycerides below 200mg/dl, in males and females separately, as well as among those with no traditional CAD risk factors (OR 5.8, 95% CI 1.5-22). Significant compositional differences by case status were confirmed in a subset whose samples were re-spun with measurement of lipids and apolipoprotein B (apo B) in each subfraction.

CONCLUSIONS

We found a strong, graded, independent, and robust association between CAD and lower VLDL-C/TG ratios. We consider this a novel, hypothesis-generating observation which will hopefully generate additional future studies to provide confirmation and further insight into potential mechanisms.