Are we ready for a prospective study to investigate the role of chylomicrons in cardiovascular disease?

The chylomicron saga: time to focus on postprandial metabolism

Since statins have had such tremendous therapeutic success over the last three decades, the field of atherosclerosis has become somewhat LDL-centric, dismissing the relevance of triglycerides (TG), particularly chylomicrons, in atherogenesis. Nonetheless, 50% of patients who take statins are at risk of developing atherosclerotic cardiovascular disease (ASCVD) and are unable to achieve their goal LDL-C levels. This residual risk is mediated, in part by triglyceride rich lipoproteins (TRL) and their remnants. Following his seminal investigation on the subject, Zilversmit proposed that atherosclerosis is a postprandial event in 1979 (1-4). In essence, the concept suggests that remnant cholesterol-rich chylomicron (CM) and very-low density lipoprotein (VLDL) particles play a role in atherogenesis. Given the foregoing, this narrative review addresses the most recent improvements in our understanding of postprandial dyslipidemia. The primary metabolic pathways of chylomicrons are discussed, emphasizing the critical physiological role of lipoprotein lipase and apoCIII, the importance of these particles' fluxes in the postprandial period, their catabolic rate, the complexities of testing postprandial metabolism, and the role of angiopoietin-like proteins in the partition of CM during the fed cycle. The narrative is rounded out by the dysregulation of postprandial lipid metabolism in insulin resistance states and consequent CVD risk, the clinical evaluation of postprandial dyslipidemia, current research limits, and potential future study directions.

Triglyceride-Rich Lipoprotein Metabolism: Key Regulators of Their Flux

The residual risk for arteriosclerotic cardiovascular disease after optimal statin treatment may amount to 50% and is the consequence of both immunological and lipid disturbances. Regarding the lipid disturbances, the role of triglyceride-rich lipoproteins (TRLs) and their remnants has come to the forefront in the past decade. Triglycerides (TGs) stand as markers of the remnants of the catabolism of TRLs that tend to contain twice as much cholesterol as compared to LDL. The accumulation of circulating TRLs and their partially lipolyzed derivatives, known as "remnants", is caused mainly by ineffective triglyceride catabolism. These cholesterol-enriched remnant particles are hypothesized to contribute to atherogenesis. The aim of the present narrative review is to briefly summarize the main pathways of TRL metabolism, bringing to the forefront the newly discovered role of apolipoproteins, the key physiological function of lipoprotein lipase and its main regulators, the importance of the fluxes of these particles in the post-prandial period, their catabolic rates and the role of apo CIII and angiopoietin-like proteins in the partition of TRLs during the fast-fed cycle. Finally, we provide a succinct summary of the new and old therapeutic armamentarium and the outcomes of key current trials with a final outlook on the different methodological approaches to measuring TRL remnants, still in search of the gold standard.

The Postprandial Appearance of Features of Cardiometabolic Risk: Acute Induction and Prevention by Nutrients and Other Dietary Substances

The purpose of this review is to provide an overview of diets, food, and food components that affect postprandial inflammation, endothelial function, and oxidative stress, which are related to cardiometabolic risk. A high-energy meal, rich in saturated fat and sugars, induces the transient appearance of a series of metabolic, signaling and physiological dysregulations or dysfunctions, including oxidative stress, low-grade inflammation, and endothelial dysfunction, which are directly related to the amplitude of postprandial plasma triglycerides and glucose. Low-grade inflammation and endothelial dysfunction are also known to cluster together with insulin resistance, a third risk factor for cardiovascular diseases (CVD) and type-II diabetes, thus making a considerable contribution to cardiometabolic risk. Because of the marked relevance of the postprandial model to nutritional pathophysiology, many studies have investigated whether adding various nutrients and other substances to such a challenge meal might mitigate the onset of these adverse effects. Some foods (e.g., nuts, berries, and citrus), nutrients (e.g., l-arginine), and other substances (various polyphenols) have been widely studied. Reports of favorable effects in the postprandial state have concerned plasma markers for systemic or vascular pro-inflammatory conditions, the activation of inflammatory pathways in plasma monocytes, vascular endothelial function (mostly assessed using physiological criteria), and postprandial oxidative stress. Although the literature is fragmented, this topic warrants further study using multiple endpoints and markers to investigate whether the interesting candidates identified might prevent or limit the postprandial appearance of critical features of cardiometabolic risk.

Theobromine Does Not Affect Fasting and Postprandial HDL Cholesterol Efflux Capacity, While It Decreases Fasting miR-92a Levels in Humans

SCOPE

Chocolate consumption lowers cardiovascular disease risk, which might be attributed to the methylxanthine theobromine. These effects may be mediated through effects on HDL-mediated cholesterol efflux, which may be affected by microRNA (miRNA) levels in the HDL particles. Therefore, the aim of this study is to investigate effects of theobromine consumption on fasting and postprandial cholesterol efflux and miRNAs levels.

METHODS AND RESULTS

Thirty overweight and 14 obese healthy men and women participated in this randomized, double-blind crossover study. Participants consumed 500 mg d-1 of theobromine or placebo for 4 weeks. ABCA1-mediated cholesterol efflux was measured using J774 macrophages. MiRNAs levels (miR-92a, miR-223, miR-135a*) were quantified in apolipoprotein B-depleted serum. Theobromine consumption did not affect fasting and postprandial cholesterol efflux. Fasting miR-223 and miR-135a levels were unchanged, while miR-92a levels were decreased (-0.21; p < 0.05). The high-fat meal increased postprandial cholesterol efflux capacity (+4.3 percentage points; p ≤ 0.001), miR-92a (+1.21; p < 0.001), and miR-223 (+1.79; p < 0.001) levels, while a trend was found for miR-135a (+1.08; p = 0.06).

CONCLUSION

Theobromine did not improve fasting and postprandial ABCA1-mediated cholesterol efflux capacity, but decreased fasting miR-92a levels. High-fat meal intake increased postprandial cholesterol efflux and the three selected miRNAs levels.

An acute intake of theobromine does not change postprandial lipid metabolism, whereas a high-fat meal lowers chylomicron particle number

Postprandial responses predict cardiovascular disease risk. However, only a few studies have compared acute postprandial effects of a low-fat, high-carbohydrate (LF) meal with a high-fat, low-carbohydrate (HF) meal. Furthermore, theobromine has favorably affected fasting lipids, but postprandial effects are unknown. Because both fat and theobromine have been reported to increase fasting apolipoprotein A-I (apoA-I) concentrations, the main hypothesis of this randomized, double-blind crossover study was that acute consumption of an HF meal and a theobromine meal increased postprandial apoA-I concentrations, when compared with an LF meal. Theobromine was added to the LF meal. Nine healthy men completed the study. After meal intake, blood was sampled frequently for 4hours. Postprandial apoA-I concentrations were comparable after intake of the 3 meals. Apolipoprotein B48 curves, however, were significantly lower and those of triacylglycerol were significantly higher after HF as compared with LF consumption. Postprandial free fatty acid concentrations decreased less, and glucose and insulin concentrations increased less after HF meal consumption. Except for an increase in the incremental area under the curve for insulin, theobromine did not modify responses of the LF meal. These data show that acute HF and theobromine consumption does not change postprandial apoA-I concentrations. Furthermore, acute HF consumption had divergent effects on postprandial apolipoprotein B48 and triacylglycerol responses, suggesting the formation of less, but larger chylomicrons after HF intake. Finally, except for an increase in the incremental area under the curve for insulin, acute theobromine consumption did not modify the postprandial responses of the LF meal.

Identification of a botanical inhibitor of intestinal diacylglyceride acyltransferase 1 activity via in vitro screening and a parallel, randomized, blinded, placebo-controlled clinical trial

Background

Diacylglyceride acyltransferase 1 (DGAT1) is the enzyme that adds the final fatty acid on to a diacylglyceride during triglyceride (TG) synthesis. DGAT1 plays a key role in the repackaging of dietary TG into circulating TG rich chylomicrons. A growing amount of research has indicated that an exaggerated postprandial circulating TG level is a risk indicator for cardiovascular and metabolic disorders. The aim of this research was to identify a botanical extract that inhibits intestinal DGAT1 activity and attenuates postprandial hypertriglyceridemia in overweight and obese humans.

Methods

Twenty individual phytochemicals and an internal proprietary botanical extract library were screened with a primary cell-free DGAT1 enzyme assay that contained dioleoyl glycerol and palmitoleoyl Coenzyme A as substrates plus human intestinal microsomes as the DGAT1 enzyme source. Botanical extracts with IC50 values < 100 μg/mL were evaluated in a cellular DGAT1 assay. The cellular DGAT1 assay comprised the analysis of ¹⁴C labeled TG synthesis in cells incubated with ¹⁴C-glycerol and 0.3 mM oleic acid. Lead botanical extracts were then evaluated in a parallel, double-blind, placebo-controlled clinical trial. Ninety healthy, overweight and obese participants were randomized to receive 2 g daily of placebo or individual botanical extracts (the investigational product) for seven days. Serum TG levels were measured before and after consuming a high fat meal (HFM) challenge (0.354 L drink/shake; 77 g fat, 25 g carbohydrate and 9 g protein) as a marker of intestinal DGAT1 enzyme activity.

Results

Phenolic acids (i.e., gallic acid) and polyphenols (i.e., cyanidin) abundantly found in nature appeared to inhibit DGAT1 enzyme activity in vitro. Four polyphenolic rich botanical extracts were identified from in vitro evaluation in both cell-free and cellular model systems: apple peel extract (APE), grape extract (GE), red raspberry leaf extract (RLE) and apricot/nectarine extract (ANE) (IC50 = 1.4, 5.6, and 10.4 and 3.4 μg/mL, respectively). In the seven day clinical trial, compared to placebo, only GE significantly reduced the baseline subtracted change in serum TG AUC following consumption of the HFM (AUC = 281 ± 37 vs. 181 ± 30 mg/dL*h, respectively; P = 0.021). Chromatographic characterization of the GE revealed a large number of closely eluting components containing proanthocyanidins, catechins, anthocyanins and their secondary metabolites that corresponded with the observed DGAT1 enzyme inhibition in the cell-free model.

Conclusion

These data suggest that a dietary GE has the potential to attenuate postprandial hypertriglyceridemia in part by the inhibition of intestinal DGAT1 enzyme activity without intolerable side effects.

Trial registration

This trial was registered with ClinicalTrials.gov NCT02333461

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-015-0025-2) contains supplementary material, which is available to authorized users.

Cideb facilitates the lipidation of chylomicrons in the small intestine

Cell death-inducing DFF45-like effector b (Cideb), an endoplasmic reticulum (ER)- and lipid droplet (LD)-associated protein, has been shown to play a critical role in maintaining hepatic lipid homeostasis by promoting the lipidation and maturation of VLDL particles. Here, we observed that Cideb is expressed in the jejunum and ileum sections of the small intestine, and its expression was induced by high-fat diet. Intragastric gavage with lipids resulted in the retention of lipids in the intestine in Cideb-deficient mice. In addition, we observed that mice with Cideb deficiency exhibited reduced intestinal chylomicron-TG secretion and increased lipid accumulation in the enterocytes. The sizes of chylomicrons secreted from the small intestine of Cideb-deficient mice were also smaller than those from wild-type mice. Furthermore, the overexpression of Cideb increased TG secretion and reduced lipid accumulation in the enterocyte-like Caco-2 cells. In addition, we proved that Cideb was localized to the ER and LDs and could interact with ApoB48 in Caco-2 cells. Overall, these data revealed that Cideb plays an important role in controlling intestinal chylomicron lipidation.

The lipid-lowering effect of dietary proanthocyanidins in rats involves both chylomicron-rich and VLDL-rich fractions

Proanthocyanidins have been shown to improve postprandial hypertriacylglycerolaemia. The present study aims to determine the actual contribution of chylomicrons (CM) and VLDL in the hypotriacylglycerolaemic action of grape seed proanthocyanidin extract (GSPE) in the postprandial state and to characterise the mechanisms by which the GSPE treatment reduces TAG-rich lipoproteins in vivo. A plasma lipid tolerance test was performed on rats fasted for 14 h and orally loaded with lard containing either GSPE or not. GSPE (250 mg/kg body weight) markedly blocked the increase in plasma TAG induced by lard, with a statistically significant reduction of 22 % in the area under the curve. The VLDL-rich fraction was the major contributor (72 %) after 1 h, whereas the CM-rich fraction was the major contributor (85 %) after 3 h. At 5 and 7 h after treatment, CM-rich and VLDL-rich fractions showed a similar influence. Plasma post-heparin lipoprotein lipase (LPL) activity and LPL mRNA levels in white adipose tissue and muscle were not affected by GSPE. On the contrary, GSPE treatment significantly repressed (30 %) the secretion of VLDL-TAG. In the liver, GSPE treatment induced different effects on the expression of acyl-coenzyme A synthetase long-chain family member 1, Apoc3 and 3-hydroxy-3-methylglutaryl-coenzyme A reductase at 1 h and Cd36 at 5 h, compared to those induced by lard. Furthermore, GSPE treatment significantly increased the activity of carnitine palmitoyltransferase 1a at 1 h. In conclusion, both CM-rich and VLDL-rich fractions contributed to the hypotriacylglycerolaemic action of GSPE, but their influence depended on time. GSPE induces hypotriacylglycerolaemic actions by repressing lipoprotein secretion and not by increasing LPL activity.

The chylomicron: relationship to atherosclerosis

The B-containing lipoproteins are the transporters of cholesterol, and the evidence suggests that the apo B48-containing postprandial chylomicron particles and the triglyceride-rich very low density lipoprotein (VLDL) particles play an important part in the development of the plaque both directly and indirectly by their impact on LDL composition. The ratio of dietary to synthesised cholesterol is variable but tightly regulated: hence intervention with diet at best reduces serum cholesterol by <20% andusually <10%. Statins are the mainstay of cholesterol reduction therapy, but they increase cholesterol absorption, an example of the relationship between synthesis and absorption. Inhibition of cholesterol absorption with Ezetimibe, an inhibitor of Niemann Pick C1-like 1 (NPC1-L1), the major regulator of cholesterol absorption, increases cholesterol synthesis and hence the value of adding an inhibitor of cholesterol absorption to an inhibitor of cholesterol synthesis. Apo B48, the structural protein of the chylomicron particle, is synthesised in abundance so that the release of these particles is dependent on the amount of cholesterol and triglyceride available in the intestine. This paper will discuss cholesterol absorption and synthesis, chylomicron formation, and the effect of postprandial lipoproteins on factors involved in atherosclerosis.

Use of a meal challenge test to estimate peak postprandial triglyceride concentrations in dogs

OBJECTIVE

To develop a standardized meal challenge test by assessing associations between food-withheld preprandial (ie, fasting) and postprandial triglyceride concentrations, determining the most appropriate sampling time to detect the peak concentration (highest postprandial concentration), and estimating reference intervals for fasting and postprandial concentrations in healthy dogs.

ANIMALS

12 lean healthy mixed-breed dogs.

PROCEDURES

Dogs were fed a dry commercially available diet (fat, 31% metabolizable energy) for 3 weeks. After food was withheld for 23 to 24 hours, plasma triglyceride concentrations were measured 1 and 0.083 hours before and 1, 2, 3, 4, 5, 6, 9, and 12 hours after feeding of a standardized challenge meal (median amount eaten, 63 kcal/kg [127 kcal/kg⁰.⁷⁵]). Correlation and agreement between concentrations at peak and other time points were assessed by use of correlation coefficients and Bland-Altman limits of agreement. Reference intervals were calculated by use of a robust method.

RESULTS

Fasting and peak triglyceride concentrations were not closely associated. The highest concentration among samples obtained 2, 5, and 6 hours after meal consumption had closest agreement with peak concentration. In 5 of 12 dogs, concentrations 12 hours after eating were still significantly above baseline concentration (mean of each dog's fasting concentrations).

CONCLUSIONS AND CLINICAL RELEVANCE

Fasting triglyceride concentration could not be used to accurately predict peak concentration. When estimating peak concentration, multiple samples should be collected 2, 5, and 6 hours after consumption of a standardized meal. Food may need to be withheld for > 12 hours when assessing fasting concentrations in healthy dogs.

Effects of olanzapine and risperidone on lipid metabolism in chronic schizophrenic patients with long-term antipsychotic treatment: a randomized five month study

OBJECTIVE

Metabolic syndrome and elevated lipids, related to cardiovascular risk factors, are more prevalent in schizophrenia and there has been much debate about the extent to which specific antipsychotics contribute more to the increased risk of developing hyperlipidemia and metabolic syndrome. Most studies have concentrated on fasting levels in patients recently started on medication. Randomized prospective studies of metabolic effects of 2nd generation antipsychotics using both fasting measures and provocative tests may provide results that are more informative. We present results of a randomized prospective study of lipid metabolism and metabolic syndrome in chronic schizophrenic patients using both fasting and post-prandial lipid measures.

METHOD

Hospitalized patients with chronic schizophrenia, most of whom had been treated with multiple antipsychotics in the past, were randomly assigned to treatment with a single antipsychotic, olanzapine or risperidone, for a period of 5 months. At baseline and every treatment month thereafter, fasting levels of lipids, free fatty acid (FFA) and leptin were assessed. At baseline and end of month 2 of treatment patients had a fatty meal test (FMT) in which postprandial lipids were measured at several time points before and after meal ingestion. Weight was assessed monthly and waist measures were taken at baseline and month 5. Data was analyzed on 23 patients randomized to risperidone and 23 patients randomized to olanzapine.

RESULTS

Overall, there were no differential drug effects on any fasting lipid measure and fasting triglycerides did not increase in olanzapine treated patients after 5 months of treatment. However, at 2 months of drug treatment the FMT revealed a significantly greater increase in triglycerides, and very low density (VLDL) cholesterol and triglycerides, in olanzapine compared to risperidone patients (Ps=.05-.01). There was no difference between treatments with olanzapine vs. risperidone on development of metabolic syndrome during the 5 month treatment period.

CONCLUSIONS

Chronic schizophrenic patients treated for years with first and second generation antipsychotics may have developed tolerance to the effects of olanzapine on increasing fasting triglycerides and other lipids, but some underlying metabolic abnormalities may be revealed in postprandial tests of lipid metabolism. These findings suggest that the development of standardized tests and criteria for measurement of postprandial triglycerides and related lipid levels, in addition to fasting levels, may be helpful in identifying metabolic effects of olanzapine and other second generation antipsychotics in chronically treated schizophrenics. In some reports postprandial increases in triglycerides have been identified as important risk factors for cardiovascular disease, but the actual differential consequences of these lipid metabolic differences for development of atherosclerosis and cardiovascular disease in patients treated with different antipsychotics need more objective outcome measures to determine and quantify cardiovascular risk outcomes.

In vivo efficacy of acyl CoA: diacylglycerol acyltransferase (DGAT) 1 inhibition in rodent models of postprandial hyperlipidemia

Postprandial serum triglyceride concentrations have recently been identified as a major, independent risk factor for future cardiovascular events. As a result, postprandial hyperlipidemia has emerged as a potential therapeutic target. The purpose of this study was two-fold. Firstly, to describe and characterize a standardized model of postprandial hyperlipidemia in multiple rodent species; and secondly, apply these rodent models to the evaluation of a novel class of pharmacologic agent; acyl CoA:diacylglycerol acyltransferase (DGAT) 1 inhibitors. Serum triglycerides were measured before and for 4h after oral administration of a standardized volume of corn oil, to fasted C57BL/6, ob/ob, apoE(-/-) and CD-1 mice; Sprague-Dawley and JCR/LA-cp rats; and normolipidemic and hyperlipidemic hamsters. Intragastric administration of corn oil increased serum triglycerides in all animals evaluated, however the magnitude and time-course of the postprandial triglyceride excursion varied. The potent and selective DGAT-1 inhibitor A-922500 (0.03, 0.3 and 3 mg/kg, p.o.), dose-dependently attenuated the maximal postprandial rise in serum triglyceride concentrations in all species tested. At the highest dose of DGAT-1 inhibitor, the postprandial triglyceride response was abolished. This study provides a comprehensive characterization of the time-course of postprandial hyperlipidemia in rodents. In addition, the ability of DGAT-1 inhibitors to attenuate postprandial hyperlipidemia in multiple rodent models, including those that feature insulin resistance, is documented. Exaggerated postprandial hyperlipidemia is inherent to insulin-resistant states in humans and contributes to the substantially elevated cardiovascular risk observed in these patients. Therefore, by attenuating postprandial hyperlipidemia, DGAT-1 inhibition may represent a novel therapeutic approach to reduce cardiovascular risk.