Chylomicron Remnants Research Articles

Biological sex-related differences in the postprandial triglyceride response to intermittent hypoxaemia in young adults: a randomized crossover trial.

Obstructive sleep apnoea is characterized by chronic intermittent hypoxaemia and is independently associated with an increased risk of metabolic comorbidities (e.g. type II diabetes and ischaemic heart disease). These comorbidities could be attributable to hypoxaemia-induced alterations in blood lipid profiles. However, it remains unclear whether intermittent hypoxaemia alters triglyceridaemia differently between biological sexes. Therefore, we used a randomized crossover design to examine whether 6h of moderate intermittent hypoxaemia (15hypoxaemic cycles/h, 85% oxyhaemoglobin saturation) alters plasma triglyceride levels differently between men and women after a high-fat meal. Relative to men, women displayed lower levels of total triglycerides, in addition to denser triglyceride-rich lipoprotein triglycerides (TRL-TG; mainly very low-density lipoprotein triglycerides and chylomicron remnant triglycerides) and buoyant TRL-TG (mainly chylomicron triglycerides) during normoxia (ambient air) and intermittent hypoxaemia (sex × time: all P ≤ 0.008). Intermittent hypoxaemia led to higher triglyceride levels (condition: all P ≤ 0.016); however, this effect was observed only in men (sex × condition: all P ≤ 0.002). Compared with normoxia, glucose levels were higher in men and lower in women during intermittent hypoxaemia (sex × condition: P<0.001). The different postprandial responses between biological sexes occurred despite similar reductions in mean oxyhaemoglobin saturation and similar elevations in insulin levels, non-esterified fatty acid levels and mean heart rate (sex × condition: all P ≥ 0.185). These results support growing evidence showing that intermittent hypoxaemia impacts men and women differently, and they might help to explain biological sex-related discrepancies in the rate of certain comorbidities associated with intermittent hypoxaemia. KEY POINTS: Intermittent hypoxaemia is a key characteristic of obstructive sleep apnoea and alters lipid metabolism in multiple tissues, resulting in increased circulating triglyceride levels, an important risk factor for cardiometabolic diseases. Circulating triglyceride levels are regulated differently between biological sexes, with women typically displaying much lower fasting and postprandial triglyceride levels than men, partly explaining why women of all ages experience lower mortality rates from cardiometabolic diseases. In this study, healthy young men and women consumed a high-fat meal and were then exposed to 6h of intermittent hypoxaemia or ambient air. We show that postprandial triglyceride levels are significantly lower in women compared with men and that intermittent hypoxaemia leads to higher postprandial triglyceride levels in men only. These results might help us to understand better why women living with obstructive sleep apnoea experience lower rates of cardiometabolic diseases (e.g. type II diabetes and ischaemic heart disease) than men living with obstructive sleep apnoea.

Proteomic Analysis of Human Chylomicron Remnants Isolated by Apolipoprotein B-48 Immunoprecipitation.

Postprandial hypertriglyceridemia (PHTG) is an independent risk factor for coronary heart diseases. PHTG exhibits accumulation of apoB-48 containing chylomicron remnants (CM-Rs) and apoB-100 containing VLDL remnants (VLDL-Rs), which are both known to be atherogenic. However, unlike VLDL-Rs, structural and functional characterization of CM-Rs remains to be elucidated due to challenges in separating CM-Rs from VLDL-Rs. Recently, we successfully isolated CM-Rs and VLDL-Rs utilizing anti-apoB-48 or apoB-100 specific antibodies. This study aimed to characterize the proteome of CM-Rs along with that of VLDL-Rs. Eight healthy subjects were enrolled. Venous blood was drawn 3 hours after high-fat-containing meals. We isolated CM-Rs and VLDL-Rs from sera through combination of ultracentrifugation and immunoprecipitation using apoB-48 or apoB-100 specific antibodies, followed by shotgun proteomic analysis. We identified 42 CM-Rs or VLDL-Rs-associated proteins, including 11 potential newly identified proteins such as platelet basic protein (PPBP) and platelet factor 4, which are chemokines secreted from platelets. ApoA-I, apoA-IV, and clusterin, which are also known as HDL-associated proteins, were significantly more abundant in CM-Rs. Interestingly, apoC-I, which reduces the activity of lipoprotein lipase and eventually inhibits catabolism of remnant proteins, was also more abundant in CM-Rs. Moreover, we identified proteins involved in complement regulation such as complement C3 and vitronectin, and those involved in acute-phase response such as PPBP, serum amyloid A protein 2, and protein S100-A8, in both CM-Rs and VLDL-Rs. We have firstly characterized the proteome of CM-Rs. These findings may provide an explanation for the atherogenic properties of CM-Rs.

Postprandial Hyperlipidemia: Its Pathophysiology, Diagnosis, Atherogenesis, and Treatments.

Postprandial hyperlipidemia showing postprandial increases in serum triglyceride (TG) is associated with the development of atherosclerotic cardiovascular disease (ASCVD). To diagnose postprandial hyperlipidemia, the oral fat loading test (OFLT) should be performed; however, this test is very time-consuming and is difficult to perform. Elevated serum TG levels reflect an increase in TG-rich lipoproteins (TRLs), such as chylomicrons (CM), very low-density lipoproteins (VLDL), and their remnants (CM remnants [CMRs] and VLDL remnants [VLDLRs]). Understanding of elevation in CMR and/or VLDLR can lead us to understand the existence of postprandial hyperlipidemia. The measurement of apo B48, which is a constituent of CM and CMR; non-fasting TG, which includes TG content in all lipoproteins including CM and CMR; non-high-density lipoprotein cholesterol (non-HDL-C), which includes TRLs and low-density lipoprotein; and remnant cholesterol are useful to reveal the existence of postprandial hyperlipidemia. Postprandial hyperlipidemia is observed in patients with familial type III hyperlipoproteinemia, familial combined hyperlipidemia, chronic kidney disease, metabolic syndrome and type 2 diabetes. Postprandial hyperlipidemia is closely related to postprandial hyperglycemia, and insulin resistance may be an inducing and enhancing factor for both postprandial hyperlipidemia and postprandial hyperglycemia. Remnant lipoproteins and metabolic disorders associated with postprandial hyperlipidemia have various atherogenic properties such as induction of inflammation and endothelial dysfunction. A healthy diet, calorie restriction, weight loss, and exercise positively impact postprandial hyperlipidemia. Anti-hyperlipidemic drugs such pemafibrate, fenofibrate, bezafibrate, ezetimibe, and eicosapentaenoic acid have been shown to improve postprandial hyperlipidemia. Anti-diabetic drugs including metformin, alpha-glucosidase inhibitors, pioglitazone, dipeptidyl-peptidase-4 inhibitors and glucagon-like peptide 1 analogues have been shown to ameliorate postprandial hyperlipidemia. Although sodium glucose cotransporter-2 inhibitors have not been proven to reduce postprandial hyperlipidemia, they reduced fasting apo B48 and remnant lipoprotein cholesterol. In conclusion, it is important to appropriately understand the existence of postprandial hyperlipidemia and to connect it to optimal treatments. However, there are some problems with the diagnosis for postprandial hyperlipidemia. Postprandial hyperlipidemia cannot be specifically defined by measures such as TG levels 2 h after a meal. To study interventions for postprandial hyperlipidemia with the outcome of preventing the onset of ASCVD, it is necessary to define postprandial hyperlipidemia using reference values such as IGT.

Role of glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 in hypertriglyceridemia and diabetes.

In diabetes, the impairment of insulin secretion and insulin resistance contribute to hypertriglyceridemia, as the enzymatic activity of lipoprotein lipase (LPL) depends on insulin action. The transport of LPL to endothelial cells and its enzymatic activity are maintained by the formation of lipolytic complex depending on the multiple positive (glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 [GPIHBP1], apolipoprotein C-II [APOC2], APOA5, heparan sulfate proteoglycan [HSPG], lipase maturation factor 1 [LFM1] and sel-1 suppressor of lin-12-like [SEL1L]) and negative regulators (APOC1, APOC3, angiopoietin-like proteins [ANGPTL]3, ANGPTL4 and ANGPTL8). Among the regulators, GPIHBP1 is a crucial molecule for the translocation of LPL from parenchymal cells to the luminal surface of capillary endothelial cells, and maintenance of lipolytic activity; that is, hydrolyzation of triglyceride into free fatty acids and monoglyceride, and conversion from chylomicron to chylomicron remnant in the exogenous pathway and from very low-density lipoprotein to low-density lipoprotein in the endogenous pathway. The null mutation of GPIHBP1 causes severe hypertriglyceridemia and pancreatitis, and GPIGBP1 autoantibody syndrome also causes severe hypertriglyceridemia and recurrent episodes of acute pancreatitis. In patients with type 2 diabetes, the elevated serum triglyceride levels negatively correlate with circulating LPL levels, and positively with circulating APOC1, APOC3, ANGPTL3, ANGPTL4 and ANGPTL8 levels. In contrast, circulating GPIHBP1 levels are not altered in type 2 diabetes patients with higher serum triglyceride levels, whereas they are elevated in type 2 diabetes patients with diabetic retinopathy and nephropathy. The circulating regulators of lipolytic complex might be new biomarkers for lipid and glucose metabolism, and diabetic vascular complications.

Abstract 229: Endothelial Cell Handling Of Atherogenic ApoB-carrying Lipoproteins

Atherosclerosis is triggered by the subendothelial retention of cholesterol-rich, apoB -containing LDL (which carries apoB100) and chylomicron remnants (which carry apoB48, the 48% N terminal sequence of apoB). In addition, multiple studies have established elevated postprandial TRLs as an independent risk factor for CVD. However, the mechanisms whereby postprandial TRLs (which circulate mostly within chylomicrons) exert their atherogenic effects remain unknown. Two receptors - SR-BI and ALK1 - have been shown to bind apoB and mediate transcytosis of LDL across the endothelium. We have shown that SR-BI also mediates EC uptake of nascent chylomicrons. RNAseq analysis revealed that ECs exposed to chylomicrons express a host of inflammatory and immune response genes, which does not occur with knockdown of SR-BI. In addition, following chylomicron uptake ECs release small extracellular vesicles that induce lipid accumulation in macrophages and lead to reduced eNOS phosphorylation in naïve ECs. Conversely, although binding of ALK1 to apoB is believed to occur through the N-terminus of apoB, the receptor is not involved in chylomicron uptake. Using a series of N terminal apoB peptides (apoB8-18, with sequences comprising the 8-18% N terminus of apoB), we found that chylomicron uptake was inhibited by apoB15 and apoB18, and LDL uptake was additionally inhibited by apoB12. These observations suggest that a) the binding sequence to SR-BI is between residues 547-684 of apoB, and b) an additional sequence between residues 365-547 participates in the binding to ALK1. Molecular modeling of apoB interactions identified an additional sequence in the C terminus of apoB potentially involved in the binding to ALK1. This sequence is absent in chylomicrons, which may explain why ALK1 inhibition and competition with apoB12 fail to significantly inhibit chylomicron uptake. Consistently, uptake of apoB100-carrying chylomicrons from Apobec -/- mice was inhibited by both ALK1 siRNA and competition with apoB12. Our observations suggest that a) the N terminus of apoB harbors distinct binding sequences to SR-BI and ALK1, and b) chylomicron uptake by ECs contributes to atherogenesis by inducing endothelial dysfunction, inflammation, and macrophage foam cell formation.

Primary Chylomicronemia in Patients with Severe Familial Hypertriglyceridemia Responds to Long-Term Treatment with (n-3) Fatty Acids

Because it disturbs microcirculation, chylomicronemia can cause severe clinical complications such as acute pancreatitis. Dietary measures are very important in the prevention and treatment of this condition. Dietary supplementation with (n-3) fatty acids has been shown to be of benefit in short-term treatment of chylomicronemia. To evaluate the long-term efficacy of (n-3) fatty acids we added 2.16 or 4.32 g/d (for 3 mo) and 3.24 g/d (for a further 8 mo after 1 mo of no treatment (n-3) fatty acids to the diet of eight patients with primary chylomicronemia. Serum triglycerides decreased significantly from 15.0 ± 2.5 to 9.9 ± 2.8 mmol/L (P < 0.01) after 11 mo of treatment, and VLDL triglycerides decreased from 12.3 ± 2.8 to 9.0 ± 2.4 mmol/L (P < 0.01), resulting in a significant decrease of plasma viscosity from 1.56 ± 0.17 mPa·s to 1.47 ± 0.12 mPa·s (P < 0.01). Total cholesterol, HDL and LDL cholesterol as well as lipoprotein (a) and fibrinogen concentrations were not affected by fish oil treatment. The presence of chylomicrons and chylomicron remnants (apolipoprotein B 48 concentration in the lipoprotein fraction with a density lower than 1.006 kg/L) was significantly reduced from 4.82 ± 1.08 to 1.06 ± 0.38 mg/L (P < 0.01). Body weight increased significantly from 71.0 ± 4.6 kg to 75.2 ± 5.3 kg after 11 mo of treatment with (n-3) fatty acids. These data indicate that despite an increase in body weight, long-term supplementation of a diet with (n-3) fatty acids results in a persistent decrease of serum triglyceride concentrations, a pronounced reduction in the number of chylomicrons present in the fasting state, and an improvement in plasma viscosity.

Abstract 14556: Deep Phenotyping APOC3 Knockouts in a Population With High Consanguinity

Apolipoprotein C-III (APOC3) plays an integral role in the regulation of triglyceride-rich lipoproteins, by inhibiting the clearance of triglycerides carried by VLDL and chylomicron remnants in the blood. Prior work has shown that heterozygous loss of function (LOF) carriers of APOC3 have lower triglyceride levels and a lower risk of coronary artery disease (CAD). The quantitative impact of disease risk remains unknown in complete knockouts (KOs). Additionally, the effects and safety implications of complete APOC3 LOF have not been characterized. APOC3 inhibition is an active therapeutic strategy to lower CAD risk; hence these questions have therapeutic relevance. Among 37,244 unrelated sequenced individuals, including 19,681 cases of myocardial infarction (MI), in the Pakistan Genomic Resource - a biobank with high levels of consanguinity - we identified 207 heterozygous LOF carriers and 14 KOs. As expected, the KOs had undetectable APOC3 levels. We also observed a decrease in plasma triglycerides (P = 9E-85), VLDL-C levels (P = 2E-73), APOE levels (P = 5.4E-9) and an increase in HDL-C levels (P = 1E-34) and APOA1 levels (P = 6E-5) consistent with a gene-dosage effect. We observed a significant decrease in the risk of MI among heterozygous carriers (P = 0.01); however, we did not observe any protection from MI risk in complete KOs. Conversely, we observed a non-significant increase in the risk of MI in complete KOs compared to non-carriers; of the 14 knockouts identified, 9 were found to have MI. The loss of protection against MI could not be explained by the genetic background or by the increase in levels of homozygosity of the KOs. By recalling complete KOs and their family members, we were able to identify and phenotype an additional 33 complete KOs and 152 heterozygotes and assess other safety concerns related to complete APOC3 inhibition (i.e., glucose intolerance, fat content in the liver, etc.). In conclusion, by leveraging a highly consanguineous cohort, we have identified and phenotyped APOC3 KOs that have, hitherto, not been identified elsewhere. We did not observe APOC3 LOF to confer protection in complete KOs and observed other biomarker and phenotypic associations; these findings should inform existing therapeutic programs targeting APOC3.

Remnant cholesterol and atherosclerotic cardiovascular disease: Metabolism, mechanism, evidence, and treatment.

This review aimed to summarize the evidence of elevated remnant cholesterol and the risks of atherosclerotic cardiovascular disease (ASCVD) and to search for further guidance in clinical therapy. The lipids-lowering treatments such as statins and ezetimibe targeted on low-density lipoprotein cholesterol (LDL-C) have always been the first-line therapy for ASCVD. However, even after statins or new lipid-lowering drugs lowered LDL-C to recommended concentrations, and with other risk factors well-controlled, such as high blood pressure, the risks of developing ASCVD remained. Remnant cholesterol (RC) referred to the cholesterol contained in all remnant lipoprotein particles, which was the cholesterol in the hydrolyzed very-low-density lipoprotein and intermediate-density lipoprotein in the fasting state, and the cholesterol in the chylomicron remnants in the postprandial state. Evidence from in vitro and animal pathogenic mechanisms studies, epidemiology, and genetic studies all indicated that RC played an important role in predicting the incidence of ASCVD. As a new indicator to reflect atherosclerosis, especially when LDL-C has been controlled to a recommended level, RC was considered as a priority treatment target for people at high risk of ASCVD. The use of statins, fibrates, APOC3 inhibitors, PCSK9 inhibitors, and omega-3 fatty acids to reduce RC levels in the plasma may provide long-term benefits. However, the standardized detection of RC was still controversial, and more studies on appropriate treatments of elevated RC are urgently needed. These positive trials may benefit more patients at high ASCVD risks worldwide in the future.

Physiological Bases for the Superiority of Apolipoprotein B Over Low-Density Lipoprotein Cholesterol and Non-High-Density Lipoprotein Cholesterol as a Marker of Cardiovascular Risk.

In 2019, the European Society of Cardiology/European Atherosclerosis Society stated that apolipoprotein B (apoB) was a more accurate marker of cardiovascular risk than low-density lipoprotein cholesterol (LDL-C) and non-high-density lipoprotein cholesterol. Since then, the evidence has continued to mount in favor of apoB. This review explicates the physiological mechanisms responsible for the superiority of apoB as a marker of the cardiovascular risk attributable to the atherogenic apoB lipoprotein particles chylomicron remnants, very low-density lipoprotein, and low-density lipoprotein particles. First, the nature and relative numbers of these different apoB particles will be outlined. This will make clear why low-density lipoprotein particles are almost always the major determinants of cardiovascular risk and why the concentrations of triglycerides and LDL-C may obscure this relation. Next, the mechanisms that govern the number of very low-density lipoprotein and low-density lipoprotein particles will be outlined because, except for dysbetalipoproteinemia, the total number of apoB particles determines cardiovascular risk, Then, the mechanisms that govern the cholesterol mass within very low-density lipoprotein and low-density lipoprotein particles will be reviewed because these are responsible for the discordance between the mass of cholesterol within apoB particles, measured either as LDL-C or non-high-density lipoprotein cholesterol, and the number of apoB particles measured as apoB, which creates the superior predictive power of apoB over LDL-C and non-high-density lipoprotein cholesterol. Finally, the major apoB dyslipoproteinemias will be briefly outlined. Our objective is to provide a physiological framework for health care givers to understand why apoB is a more accurate marker of cardiovascular risk than LDL-C or non-high-density lipoprotein cholesterol.

INTESTINAL ADAPTATION TO SHORT-TERM, EXTREME FAT CONSUMPTION ALTERS TRIGLCYERIDE-RICH LIPOPROTEIN SECRETION AND INTESTINAL LIPID HANDLING IN MALE AND FEMALE MICE

Dysregulated postprandial metabolism including elevated chylomicron remnants, contributes to atherogenesis. The ketogenic diet is a patient-accessible metabolic health intervention whereby nutritional ketosis is achieved by consuming predominantly fat (>70% of kcal) with restricted carbohydrates and protein. It has been established that intestinal enterocytes package dietary triglycerides (TG) into chylomicrons for transport into circulation and store excess TGs as cytosolic lipid droplets (CLDs) such that nutrient absorption is maximized.

Intestinal lipid absorption and transport in type 2 diabetes.

Postprandial hyperlipidaemia is an important feature of diabetic dyslipidaemia and plays an important role in the development of cardiovascular disease in individuals with type 2 diabetes. Postprandial hyperlipidaemia in type 2 diabetes is secondary to increased chylomicron production by the enterocytes and delayed catabolism of chylomicrons and chylomicron remnants. Insulin and some intestinal hormones (e.g. glucagon-like peptide-1 [GLP-1]) influence intestinal lipid metabolism. In individuals with type 2 diabetes, insulin resistance and possibly reduced GLP-1 secretion are involved in the pathophysiology of postprandial hyperlipidaemia. Several factors are involved in the overproduction of chylomicrons: (1) increased expression of microsomal triglyceride transfer protein, which is a key enzyme in chylomicron synthesis; (2) higher stability and availability of apolipoprotein B-48; and (3) increased de novo lipogenesis. Individuals with type 2 diabetes present with disorders of cholesterol metabolism in the enterocytes with reduced absorption and increased synthesis. The increased production of chylomicrons in type 2 diabetes is also associated with a reduction in their catabolism, mostly because of a reduction in activity of lipoprotein lipase. Modification of the microbiota, which is observed in type 2 diabetes, may also generate disorders of intestinal lipid metabolism, but human data remain limited. Some glucose-lowering treatments significantly influence intestinal lipid absorption and transport. Postprandial hyperlipidaemia is reduced by metformin, pioglitazone, alpha-glucosidase inhibitors, dipeptidyl peptidase 4 inhibitors and GLP-1 agonists. The most pronounced effect is observed with GLP-1 agonists, which reduce chylomicron production significantly in individuals with type 2 diabetes and have a direct effect on the intestine by reducing the expression of genes involved in intestinal lipoprotein metabolism. The effect of sodium-glucose cotransporter 2 inhibitors on intestinal lipid metabolism needs to be clarified.

Common Dietary Modifications in Preclinical Models to Study Skeletal Health.

Bone is a highly dynamic tissue that undergoes continuous remodeling by bone resorbing osteoclasts and bone forming osteoblasts, a process regulated in large part by osteocytes. Dysregulation of these coupled catabolic and anabolic processes as in the case of menopause, type 2 diabetes mellitus, anorexia nervosa, and chronic kidney disease is known to increase fracture risk. Recent advances in the field of bone cell metabolism and bioenergetics have revealed that maintenance of the skeleton places a high energy demand on these cells involved in bone remodeling. These new insights highlight the reason that bone tissue is the beneficiary of a substantial proportion of cardiac output and post-prandial chylomicron remnants and requires a rich supply of nutrients. Studies designed for the specific purpose of investigating the impact of dietary modifications on bone homeostasis or that alter diet composition and food intake to produce the model can be found throughout the literature; however, confounding dietary factors are often overlooked in some of the preclinical models. This review will examine some of the common pre-clinical models used to study skeletal biology and its pathologies and the subsequent impact of various dietary factors on these model systems. Furthermore, the review will include how inadvertent effects of some of these dietary components can influence bone cell function and study outcomes.

Sleep and Late-Onset Alzheimer's Disease: Shared Genetic Risk Factors, Drug Targets, Molecular Mechanisms, and Causal Effects.

Late-onset Alzheimer’s disease (AD) is associated with sleep-related phenotypes (SRPs). The fact that whether they share a common genetic etiology remains largely unknown. We explored the shared genetics and causality between AD and SRPs by using high-definition likelihood (HDL), cross-phenotype association study (CPASSOC), transcriptome-wide association study (TWAS), and bidirectional Mendelian randomization (MR) in summary-level data for AD (N = 455,258) and summary-level data for seven SRPs (sample size ranges from 359,916 to 1,331,010). AD shared a strong genetic basis with insomnia (r g = 0.20; p = 9.70 × 10–5), snoring (r g = 0.13; p = 2.45 × 10–3), and sleep duration (r g = −0.11; p = 1.18 × 10–3). The CPASSOC identifies 31 independent loci shared between AD and SRPs, including four novel shared loci. Functional analysis and the TWAS showed shared genes were enriched in liver, brain, breast, and heart tissues and highlighted the regulatory roles of immunological disorders, very-low-density lipoprotein particle clearance, triglyceride-rich lipoprotein particle clearance, chylomicron remnant clearance, and positive regulation of T-cell–mediated cytotoxicity pathways. Protein–protein interaction analysis identified three potential drug target genes (APOE, MARK4, and HLA-DRA) that interacted with known FDA-approved drug target genes. The CPASSOC and TWAS demonstrated three regions 11p11.2, 6p22.3, and 16p11.2 may account for the shared basis between AD and sleep duration or snoring. MR showed insomnia had a causal effect on AD (ORIVW = 1.02, P IVW = 6.7 × 10–6), and multivariate MR suggested a potential role of sleep duration and major depression in this association. Our findings provide strong evidence of shared genetics and causation between AD and sleep abnormalities and advance our understanding of the genetic overlap between them. Identifying shared drug targets and molecular pathways can be beneficial for treating AD and sleep disorders more efficiently.

Abstract 115: N-terminus Of ApoB Mediates Endothelial Cell Chylomicron Uptake

The development of atherosclerosis is triggered by the endothelial crossing and subendothelial retention of apolipoprotein B (ApoB)-containing LDL and chylomicron remnants. LDL has been reported to cross the endothelial cell (EC) barrier via two receptors: scavenger receptor-B1 (SR-B1) and activin receptor-like kinase 1 (ALK1). We have shown that SR-B1 also mediates EC uptake of nascent chylomicrons, leading to the accumulation of lipid droplets (LD) within the aortic endothelium and skin macrophages of hyperchylomicronemic LpL-deficient mice. In vitro, chylomicron-exposed ECs induced LD accumulation in co-cultured macrophages in the absence of any other lipid source. Surprisingly, conditioned media from chylomicron-exposed ECs primarily contained phospholipids and diglycerides, rather than triglycerides and fatty acids. Electron microscopy analysis revealed the presence of small extracellular vesicles (sEVs), and filtration of the media to remove sEVs prevented LD accumulation in macrophages. To determine the molecular interaction of chylomicrons and SR-B1 we performed a series of competition experiments. At physiologic concentrations found in postprandial blood, chylomicrons blocked LDL uptake into cultured ECs. Moreover, chylomicron uptake was blocked by co-incubation with ApoB18, a lipid-free peptide comprising the 18% N-terminus of ApoB. To assess this in vivo, we used an adeno-associated virus to overexpress ApoB18 in LpL-deficient mice. ApoB18-expressing mice exhibited a ≈ 50% reduction in aortic EC LD content, suggesting that the peptide effectively inhibits chylomicron uptake in vivo. Further, ApoB18 blocked LDL uptake by ECs in culture. These results suggest that a) the N-terminus of ApoB harbors a binding sequence for SR-B1 and b) endothelial uptake of both chylomicrons and LDL can be blocked by competition with non-lipoprotein ApoB peptides.

Effects of dapagliflozin on postprandial lipid metabolism in type 2 diabetes mellitus.

Sodium-glucose cotransporter 2 inhibitors (SGLT2i) modulate lipid metabolism and improve cardiovascular morbidity and mortality in patients with type 2 diabetes mellitus (T2DM). The exact cardioprotective mechanism of SGLT2i is unclear. We evaluated the effects of SGLT2i on postprandial lipids, lipoprotein concentrations, glucose and fatty acids. A placebo-controlled randomized, proof-of-concept study. Fourteen male patients with T2DM on intensive insulin regimen were randomly and double-blind allocated to 12 weeks dapagliflozin (10 mg) or placebo. Postprandial effects were assessed with an 8-h standardized oral fat loading test. Mean glycated A1c did not change by dapagliflozin, but the mean daily insulin dose was significantly reduced. Although dapagliflozin did not affect fasting or postprandial levels of glucose and insulin, it increased the postprandial levels of glucagon. While fasting levels of free fatty acids and beta-hydroxybutyrate (bHBA) were unchanged, dapagliflozin significantly increased the postprandial bHBA response. This was seen in the context of increased postprandial glucagon levels by dapagliflozin, without influencing postprandial insulin or glucose levels. Dapagliflozin did not affect fasting or postprandial plasma cholesterol and triglycerides nor postprandial inflammatory markers. Fasting apolipoprotein B48 was decreased without affecting the postprandial response. Markers of inflammation and vascular function did not change. Treatment with dapagliflozin of patients with T2DM led to a reduction of fasting chylomicron remnants and increased postprandial ketone bodies compared to placebo suggesting enhanced hepatic fatty acid oxidation. The latter may have been caused by decreasing the insulin-glucagon ratio. The beneficial clinical effects seen in the trials using dapagliflozin most likely are not due to effects on postprandial inflammation nor postprandial lipemia.

GLP-2 Regulation of Dietary Fat Absorption and Intestinal Chylomicron Production via Neuronal Nitric Oxide Synthase (nNOS) Signaling.

Postprandial dyslipidemia is a metabolic condition commonly associated with insulin-resistant states, such as obesity and type 2 diabetes. It is characterized by the overproduction of intestinal chylomicron particles and excess atherogenic chylomicron remnants in circulation. We have previously shown that glucagon-like peptide 2 (GLP-2) augments dietary fat uptake and chylomicron production in insulin-resistant states; however, the underlying mechanisms remain unclear. Previous studies have implicated nitric oxide (NO) in the absorptive actions of GLP-2. In this study, we report a novel role for neuronal NO synthase (nNOS)-mediated NO generation in lipid uptake and chylomicron formation based on studies in C57BL/6J mice, nNOS-/- mice, and Syrian golden hamsters after intraduodenal and oral fat administration. GLP-2 treatment in wild-type (WT) mice significantly increased postprandial lipid accumulation and circulating apolipoprotein B48 protein levels, while these effects were abolished in nNOS-/- mice. nNOS inhibition in Syrian golden hamsters and protein kinase G (PKG) inhibition in WT mice also abrogated the effect of GLP-2 on postprandial lipid accumulation. These studies demonstrate a novel mechanism in which nNOS-generated NO is crucial for GLP-2-mediated lipid absorption and chylomicron production in both mouse and hamster models. Overall, our data implicate an nNOS-PKG-mediated pathway in GLP-2-mediated stimulation of dietary fat absorption and intestinal chylomicron production.

From Dietary Cholesterol to Blood Cholesterol, Physiological Lipid Fluxes, and Cholesterol Homeostasis.

Dietary cholesterol (C) is a major contributor to the endogenous C pool, and it affects the serum concentration of total C, particularly the low-density lipoprotein cholesterol (LDL-C). A high serum concentration of LDL-C is associated with an increased risk for atherosclerosis and cardiovascular diseases. This concentration is dependent on hepatic C metabolism creating a balance between C input (absorption and synthesis) and C elimination (conversion to bile acids and fecal excretion). The daily C absorption rate is determined by dietary C intake, biliary C secretion, direct trans-intestinal C excretion (TICE), and the fractional C absorption rate. Hepatic C metabolism coordinates C fluxes entering the liver via chylomicron remnants (CMR), LDL, high-density lipoproteins (HDL), hepatic C synthesis, and those leaving the liver via very low-density lipoproteins (VLDL), biliary secretion, and bile acid synthesis. The knowns and the unknowns of this C homeostasis are discussed.

Measurement of Hepatic Lipids.

Liver plays a central role in lipid metabolism, uptake of lipoproteins and lipids from the circulation (e.g., chylomicron remnant), and secretions of very low-density lipoproteins (VLDL). Therefore, measurements of lipid levels in the liver have been broadly used to check hepatic function, especially in subjects who have chronic liver diseases, such as nonalcoholic steatohepatitis (NASH), in which there is accumulation of fat, inflammation, and damage to liver cells. In this chapter, we describe the processes of extracting hepatic lipids by the method of Folch et al., and measuring the levels of cholesterol, triglycerides, phospholipids, and non-esterified fatty acids using enzymatic assays.

Lean adolescents with insulin resistance display higher angiopoietin like protein 3, ApoC-III and chylomicron remnant dyslipidemia

BackgroundTriglyceride-rich lipoproteins (TRL: chylomicrons and VLDL) are a key component of diabetes dyslipoproteinemia and cardiovascular risk. We have shown that it is already prevalent in obese adolescents in association with lipoprotein lipase (LPL) dysregulation. Insulin resistance (IR) suffices to produce TRL dyslipoproteinemia and LPL dysfunction even in the absence of obesity. MethodsThis cross-sectional study included euglycemic adolescents between 15 and 19 y, classified in 4 groups according to BMI, HOMA-IR and fasting lipid as: metabolically healthy lean (MHL, n = 30), metabolically unhealthy lean (MUL, n = 25), metabolically healthy obese (MHO, = 30), and metabolically unhealthy obese (MUO, n = 42). ResultsAs compared to MHL, MUL participants showed 73% higher concentrations of ApoB-48; 84% of ApoC-III; 24% ANGPTL-3; 200% of TG; 218% of VLDL-C and 238% of TG/HDL-C c, No changes were found in LPL mass. Interestingly, the differences in these parameters between MUL and MHO were not significant. ConclusionEuglycemic lean adolescents with IR display TRL dyslipoproteinemia with increased inhibition of LPL as highlighted by higher concentrations of ANGPTL-3, ApoC-III and fasting chylomicron remnants (ApoB-48).

Alcohol-associated capillarization of sinusoids: A critique since the discovery by Schaffner and Popper in 1963.

This article reviews the literature on capillarization of hepatic sinusoids since its discovery in 1963. Liver sinusoidal endothelial cells are uniquely fenestrated and lack an underlying basement membrane. In chronic liver disease, the sinusoids capillarize and transform into systemic capillaries, a process termed capillarization of sinusoids. The histopathology is marked by defenestration, basement membrane formation, and space of Disse fibrogenesis. Capillarized sinusoids compromise the bidirectional exchange of materials between sinusoids and hepatocytes, leading to hepatocellular dysfunction. Sinusoidal capillarization was first described in active cirrhosis of alcoholics in 1963. Since then, it has been found in early and progressive stages of alcoholic hepatic fibrosis before the onset of cirrhosis. The sinusoidal structure is not altered in alcoholic steatosis without fibrosis. Defenestration impairs the ability of the endothelium to filter chylomicron remnants from sinusoids into the Disse's space, contributing to alcohol-induced postprandial hyperlipidemia and possibly atherosclerosis. Ethanol also modulates the fenestration dynamics in animals. In baboons, chronic alcohol consumption diminishes endothelial porosity in concomitance with hepatic fibrogenesis and in rats defenestrates the endothelium in the absence of fibrosis, and sometimes capillarizes the sinusoids. Acute ethanol ingestion enlarges fenestrations in rats and contracts fenestrations in rabbits. In sinusoidal endothelial cell culture, ethanol elicits fenestration dilation, which is likely related to its interaction with fenestration-associated cytoskeleton. Ethanol potentiates sinusoidal injury caused by cocaine, acetaminophen or lipopolysaccharide in mice and rats. Understanding ethanol's mechanisms on pathogenesis of sinusoidal capillarization and fenestration dynamics will lead to development of methods to prevent risks for atherosclerosis in alcoholism.