Reply to: “Methodological issues regarding: “A third of nonfasting plasma cholesterol is in remnant lipoproteins: Lipoprotein subclass profiling in 9293 individuals””

Small Remnants versus Large Triglyceride-Rich Lipoproteins in Risk of Atherosclerotic Cardiovascular Disease

BACKGROUND

Small remnants may penetrate the arterial intima more efficiently compared to large triglyceride-rich lipoproteins (TGRL). We tested the hypothesis that the importance of remnant cholesterol for the risk of atherosclerotic cardiovascular disease (ASCVD) may depend on the size of the remnants and TGRL carrying cholesterol.

METHODS

The cholesterol content of small remnants and large TGRL were measured in 25 572 individuals from the Copenhagen General Population Study (2003-2015) and in 222 721 individuals from the UK Biobank (2006-2010) using nuclear magnetic resonance spectroscopy. In the Copenhagen cohort during up to 15 years of follow-up and in the UK Biobank cohort during up to 16 years of follow-up, the numbers of individuals diagnosed with ASCVD (=myocardial infarction, ischemic stroke, and peripheral artery disease) in national health registries were 3869 and 11 424, respectively.

RESULTS

Compared to individuals with low cholesterol content in both small remnants and large TGRL (cutpoints were median cholesterol content), multivariable-adjusted hazard ratios for risk of ASCVD were 1.21 (95% confidence interval: 1.07-1.37) for individuals with high cholesterol content in small remnants only and 0.94 (0.83-1.07) for individuals with high cholesterol content in large TGRL only; the multivariable-adjusted hazard ratio for risk of ASCVD per 10 percentile-units higher cholesterol content in small remnants vs that in large TGRL was 1.04 (1.01-1.07). In the UK Biobank cohort, corresponding hazard ratios were 1.11 (1.03-1.20), 1.01 (0.93-1.09), and 1.05 (1.04-1.07), respectively.

CONCLUSION

The importance of remnant cholesterol for the risk of ASCVD may depend on the size of the TGRL and remnants carrying cholesterol.

Remnant Cholesterol: Quantification, Concentrations by Sex and Age, and Risk of Ischemic Heart Disease

BACKGROUND

Observational and genetic causal studies have shown an association between high concentrations of remnant cholesterol and increased risk of ischemic heart disease. However, findings from randomized intervention trials that reduced plasma triglycerides, a surrogate marker of remnant cholesterol, have been conflicting. The exact mechanisms by which remnant cholesterol contributes to atherosclerosis and, ultimately, ischemic heart disease remain incompletely understood. Additionally, insight on sex and age differences and the importance of measurement differences of remnant cholesterol in plasma concentrations and risk of ischemic heart disease are sparse.

CONTENT

This review covers current knowledge regarding remnant cholesterol and its role in ischemic heart disease, with particular attention to measurement and sex- and age-specific differences.

SUMMARY

Findings from observational, genetic, and mechanistic studies support the notion that higher remnant cholesterol may be an important cause of ischemic heart disease in both women and men. Concentrations of remnant cholesterol vary by age, with a sharp increase at early adulthood for men and around menopause for women. Remnant cholesterol can be calculated from a standard lipid profile and in addition measured directly using manual ultracentrifugation, automated assays, and nuclear magnetic resonance spectroscopy. Irrespective of the method used to assess plasma concentrations, high concentrations of remnant cholesterol are consistently associated with increased risk of myocardial infarction and ischemic heart disease in observational and genetic causal studies; cholesterol rather than triglycerides in remnants drive this risk. Importantly, results from ongoing randomized clinical trials aiming specifically at lowering remnant cholesterol and ischemic heart disease are eagerly awaited.

Remnant Cholesterol, Not LDL Cholesterol, Explains Peripheral Artery Disease Risk Conferred by apoB: A Cohort Study

BACKGROUND

Elevated apoB-containing lipoproteins (=remnants+LDLs [low-density lipoproteins]) are a major risk factor for atherosclerotic cardiovascular disease, including peripheral artery disease (PAD) and myocardial infarction. We tested the hypothesis that remnants and LDL both explain part of the increased risk of PAD conferred by elevated apoB-containing lipoproteins. For comparison, we also studied the risk of chronic limb-threatening ischemia and myocardial infarction.

METHODS

apoB, remnant cholesterol, and LDL cholesterol were measured in 93 461 individuals without statin use at baseline from the Copenhagen General Population Study (2003-2015). During up to 15 years of follow-up, 1207 had PAD, 552 had chronic limb-threatening ischemia, and 2022 had myocardial infarction in the Danish National Patient Registry. Remnant and LDL cholesterol were calculated from a standard lipid profile. Remnant and LDL particle counts were additionally measured with nuclear magnetic resonance spectroscopy in 25 347 of the individuals. Results were replicated in 302 167 individuals without statin use from the UK Biobank (2004-2010).

RESULTS

In the Copenhagen General Population Study, multivariable adjusted hazard ratios for risk of PAD per 1 mmol/L (39 mg/dL) increment in remnant and LDL cholesterol were 1.9 (95% CI, 1.5-2.4) and 1.1 (95% CI, 1.0-1.2), respectively; corresponding results in the UK Biobank were 1.7 (95% CI, 1.4-2.1) and 0.9 (95% CI, 0.9-1.0), respectively. In the association from elevated apoB to increased risk of PAD, remnant and LDL cholesterol explained 73% (32%-100%) and 8% (0%-46%), respectively; corresponding results were 63% (30%-100%) and 0% (0%-33%) for risk of chronic limb-threatening ischemia and 41% (27%-55%) and 54% (38%-70%) for risk of myocardial infarction; results for remnant and LDL particle counts corroborated these findings.

CONCLUSIONS

PAD risk conferred by elevated apoB-containing lipoproteins was explained mainly by elevated remnants, while myocardial infarction risk was explained by both elevated remnants and LDL.

Triglyceride content increases while cholesterol content decreases in HDL and LDL+IDL fractions following normal meals: The Copenhagen General Population Study of 25,656 individuals

BACKGROUND AND AIMS

During fat tolerance tests, plasma triglycerides increase while high-density lipoprotein (HDL) cholesterol, low-density lipoprotein (LDL) cholesterol, and intermediate-density lipoprotein (IDL) cholesterol decrease. However, it is unknown whether triglyceride content increases and cholesterol content decreases in HDL and LDL + IDL fractions following normal meals in the general population. Therefore, we tested the hypothesis that triglyceride content increases while cholesterol content decreases in HDL and LDL + IDL fractions following normal meals.

METHODS

In this cross-sectional study, we included 25,656 individuals aged 20-100 years, all without lipid-lowering therapy at examination and selected for metabolomic profiling from the Copenhagen General Population Study. Triglyceride and cholesterol content of 14 lipoprotein fractions weas measured using nuclear magnetic resonance (NMR) spectroscopy. Time since last meal was recorded by the examiner immediately before blood sampling.

RESULTS

Following normal meals in age and sex-adjusted analyses and when compared with fasting levels, plasma triglycerides were higher for up to 5-6 h, and triglyceride content was higher for up to 6-7 h in HDL fractions, for up to 6-7 h in LDL + IDL fractions, and for up to 5-6 h in very-low-density lipoprotein (VLDL) fractions. Further, plasma cholesterol was lower for up to 2-3 h, and cholesterol content was lower for up to 0-1 h in HDL fractions and for up to 4-5 h in LDL + IDL fractions, while cholesterol content was higher for up to 4-5 h in VLDL fractions.

CONCLUSIONS

Following normal meals, triglyceride content increases while cholesterol content decreases in HDL and LDL + IDL fractions.

Insulin Resistance Is Associated with an Unfavorable Serum Lipoprotein Lipid Profile in Women with Newly Diagnosed Gestational Diabetes

Background: Gestational diabetes (GDM) is associated with various degrees of insulin resistance—a feature related to increased risk of adverse perinatal outcomes. We aimed to determine the previously poorly investigated associations between maternal insulin resistance and serum fasting metabolome at the time of GDM diagnosis. Methods: Serum lipoprotein and amino acid profile was analyzed in 300 subjects with newly diagnosed GDM using a validated nuclear magnetic resonance spectroscopy protocol. Associations between insulin resistance (homeostasis model assessment of insulin resistance, HOMA2-IR) and serum metabolites were examined with linear regression. Results: We found insulin resistance to be associated with a distinct lipid pattern: increased concentration of VLDL triglycerides and phospholipids and total triglycerides. VLDL size was positively related and LDL and HDL sizes were inversely related to insulin resistance. Of fatty acids, increased total fatty acids, relative increase in saturated and monounsaturated fatty acids, and relative decrease in polyunsaturated and omega fatty acids were related to maternal insulin resistance. Conclusions: In newly diagnosed GDM, the association between maternal insulin resistance and serum lipoprotein profile was largely as described in type 2 diabetes. Lifestyle interventions aiming to decrease insulin resistance from early pregnancy could benefit pregnancy outcomes via more advantageous lipid metabolism.

Elevated LDL Triglycerides and Atherosclerotic Risk

BACKGROUND

It is unclear whether elevated low-density lipoprotein (LDL) triglycerides are associated with an increased risk of atherosclerotic cardiovascular disease (ASCVD).

OBJECTIVES

This study tested the hypothesis that elevated LDL triglycerides are associated with an increased risk of ASCVD and of each ASCVD component individually.

METHODS

The study investigators used the Copenhagen General Population Study, which measured LDL triglycerides in 38,081 individuals with a direct automated assay (direct LDL triglycerides) and in another 30,208 individuals with nuclear magnetic resonance (NMR) spectroscopy (NMR LDL triglycerides). Meta-analyses aggregated the present findings with previously reported results.

RESULTS

During a median follow-up of 3.0 and 9.2 years, respectively, 872 and 5,766 individuals in the 2 cohorts received a diagnosis of ASCVD. Per 0.1 mmol/L (9 mg/dL) higher direct LDL triglycerides, HRs were 1.26 (95% CI: 1.17-1.35) for ASCVD, 1.27 (95% CI: 1.16-1.39) for ischemic heart disease, 1.28 (95% CI: 1.11-1.48) for myocardial infarction, 1.22 (95% CI: 1.08-1.38) for ischemic stroke, and 1.38 (95% CI: 1.21-1.58) for peripheral artery disease. Corresponding HRs for NMR LDL triglycerides were 1.26 (95% CI: 1.20-1.33), 1.33 (95% CI: 1.25-1.41), 1.41 (95% CI: 1.31-1.52), 1.13 (95% CI: 1.05-1.23), and 1.26 (95% CI: 1.10-1.43), respectively. The foregoing results were not entirely statistically explained by apolipoprotein B levels. In meta-analyses for the highest quartile vs the lowest quartile of LDL triglycerides, random-effects risk ratios were 1.50 (95% CI: 1.35-1.66) for ASCVD (4 studies; 71,526 individuals; 8,576 events), 1.62 (95% CI: 1.37-1.93) for ischemic heart disease (6 studies; 107,538 individuals; 9,734 events), 1.30 (95% CI: 1.13-1.49) for ischemic stroke (4 studies; 78,026 individuals; 4,273 events), and 1.53 (95% CI: 1.29-1.81) for peripheral artery disease (4 studies; 107,511 individuals; 1,848 events).

CONCLUSIONS

Elevated LDL triglycerides were robustly associated with an increased risk of ASCVD and of each ASCVD component individually in 2 prospective cohort studies and in meta-analyses of previous and present studies combined.

Apt interpretation of comprehensive lipoprotein data in large-scale epidemiology: disclosure of fundamental structural and metabolic relationships

BACKGROUND

Quantitative lipoprotein analytics using nuclear magnetic resonance (NMR) spectroscopy is currently commonplace in large-scale studies. One methodology has become widespread and is currently being utilized also in large biobanks. It allows the comprehensive characterization of 14 lipoprotein subclasses, clinical lipids, apolipoprotein A-I and B. The details of these data are conceptualized here in relation to lipoprotein metabolism with particular attention on the fundamental characteristics of subclass particle numbers, lipid concentrations and compositional measures.

METHODS AND RESULTS

The NMR methodology was applied to fasting serum samples from Northern Finland Birth Cohorts 1966 and 1986 with 5651 and 5605 participants, respectively. All results were highly consistent between the cohorts. Circulating lipid concentrations in a particular lipoprotein subclass arise predominantly as the result of the circulating number of those subclass particles. The spherical lipoprotein particle shape, with a radially oriented surface monolayer, imposes size-dependent biophysical constraints for the lipid composition of individual subclass particles and inherently restricts the accommodation of metabolic changes via compositional modifications. The new finding that the relationship between lipoprotein subclass particle concentrations and the particle size is log-linear reveals that circulating lipoprotein particles are also under rather strict metabolic constraints for both their absolute and relative concentrations.

CONCLUSIONS

The fundamental structural and metabolic relationships between lipoprotein subclasses elucidated in this study empower detailed interpretation of lipoprotein metabolism. Understanding the intricate details of these extensive data is important for the precise interpretation of novel therapeutic opportunities and for fully utilizing the potential of forthcoming analyses of genetic and metabolic data in large biobanks.

Activation of pregnane X receptor induces atherogenic lipids and PCSK9 by a SREBP2-mediated mechanism

BACKGROUND AND PURPOSE

Many drugs and environmental contaminants induce hypercholesterolemia and promote the risk of atherosclerotic cardiovascular disease. We tested the hypothesis that pregnane X receptor (PXR), a xenobiotic-sensing nuclear receptor, regulates the level of circulating atherogenic lipids in humans and utilized mouse experiments to identify the mechanisms involved.

EXPERIMENTAL APPROACH

We performed serum NMR metabolomics in healthy volunteers administered rifampicin, a prototypical human PXR ligand or placebo in a crossover setting. We used high-fat diet fed wild-type and PXR knockout mice to investigate the mechanisms mediating the PXR-induced alterations in cholesterol homeostasis.

KEY RESULTS

Activation of PXR induced cholesterogenesis both in pre-clinical and clinical settings. In human volunteers, rifampicin increased intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL) and total cholesterol and lathosterol-cholesterol ratio, a marker of cholesterol synthesis, suggesting increased cholesterol synthesis. Experiments in mice indicated that PXR activation causes widespread induction of the cholesterol synthesis genes including the rate-limiting Hmgcr and upregulates the intermediates in the Kandutsch-Russell cholesterol synthesis pathway in the liver. Additionally, PXR activation induced plasma proprotein convertase subtilisin/kexin type 9 (PCSK9), a negative regulator of hepatic LDL uptake, in both mice and humans. We propose that these effects were mediated through increased proteolytic activation of sterol regulatory element-binding protein 2 (SREBP2) in response to PXR activation.

CONCLUSION AND IMPLICATIONS

PXR activation induces cholesterol synthesis, elevating LDL and total cholesterol in humans. The PXR-SREBP2 pathway is a novel regulator of the cholesterol and PCSK9 synthesis and a molecular mechanism for drug- and chemical-induced hypercholesterolemia.

Per-Particle Triglyceride-Rich Lipoproteins Imply Higher Myocardial Infarction Risk Than Low-Density Lipoproteins: Copenhagen General Population Study

[Figure: see text].

Leptin alters energy intake and fat mass but not energy expenditure in lean subjects

Based on studies in mice, leptin was expected to decrease body weight in obese individuals. However, the majority of the obese are hyperleptinemic and do not respond to leptin treatment, suggesting the presence of leptin tolerance and questioning the role of leptin as regulator of energy balance in humans. We thus performed detailed novel measurements and analyses of samples and data from our clinical trials biobank to investigate leptin effects on mechanisms of weight regulation in lean normo- and mildly hypo-leptinemic individuals without genetic disorders. We demonstrate that short-term leptin administration alters food intake during refeeding after fasting, whereas long-term leptin treatment reduces fat mass and body weight, and transiently alters circulating free fatty acids in lean mildly hypoleptinemic individuals. Leptin levels before treatment initiation and leptin dose do not predict the observed weight loss in lean individuals suggesting a saturable effect of leptin. In contrast to data from animal studies, leptin treatment does not affect energy expenditure, lipid utilization, SNS activity, heart rate, blood pressure or lean body mass.

Leptin treatment is effective to reduce body weight in animal models, but patients with obesity and associated hyperleptinemia do not respond well to leptin therapy. Here the authors report a retrospective analysis of four clinical trials in normo- and mildly hypoleptinemic individuals and show that leptin therapy alters food intake in the short term and reduces weight and fat mass in the long term without effects on energy expenditure.