Metabolism of apoB lipoproteins of intestinal and hepatic origin during constant feeding of small amounts of fat

ApoB100 and Atherosclerosis: What's New in the 21st Century?

ApoB is the main protein of triglyceride-rich lipoproteins and is further divided into ApoB48 in the intestine and ApoB100 in the liver. Very low-density lipoprotein (VLDL) is produced by the liver, contains ApoB100, and is metabolized into its remnants, intermediate-density lipoprotein (IDL) and low-density lipoprotein (LDL). ApoB100 has been suggested to play a crucial role in the formation of the atherogenic plaque. Apart from being a biomarker of atherosclerosis, ApoB100 seems to be implicated in the inflammatory process of atherosclerosis per se. In this review, we will focus on the structure, the metabolism, and the function of ApoB100, as well as its role as a predictor biomarker of cardiovascular risk. Moreover, we will elaborate upon the molecular mechanisms regarding the pathophysiology of atherosclerosis, and we will discuss the disorders associated with the APOB gene mutations, and the potential role of various drugs as therapeutic targets.

Food Intake Suppresses ApoB Secretion and Fractional Catabolic Rates in Humans

BACKGROUND

Humans spend much of the day in the postprandial state. However, most research and clinical guidelines on plasma lipids pertain to blood drawn after a 12-hour fast. We aimed to study the metabolic differences of apoB lipoproteins between the fasting and postprandial states.

METHODS

We investigated plasma apoB metabolism using stable isotope tracers in 12 adult volunteers under fasting and continuous postprandial conditions in a randomized crossover study. We determined the metabolism of apoB in multiple lipoprotein subfractions, including light and dense VLDLs (very-low-density lipoproteins), IDLs (intermediate-density lipoproteins), and light and dense LDLs (low-density lipoproteins) that do or do not contain apoE or apoC3.

RESULTS

A major feature of the postprandial state is 50% lower secretion rate of triglyceride-rich lipoproteins and concurrent slowdown of their catabolism in circulation, as shown by 34% to 55% lower rate constants for the metabolic pathways of conversion by lipolysis from larger to smaller lipoproteins and direct clearance of lipoproteins from the circulation. In addition, the secretion pattern of apoB lipoprotein phenotypes was shifted from particles containing apoE and apoC3 in the fasting state to those without either protein in the postprandial state.

CONCLUSIONS

Overall, during the fasting state, hepatic apoB lipoprotein metabolism is activated, characterized by increased production, transport, and clearance. After food intake, endogenous apoB lipoprotein metabolism is globally reduced as appropriate to balance dietary input to maintain the supply of energy to peripheral tissues.

Bariatric surgery improves postprandial VLDL kinetics and restores insulin-mediated regulation of hepatic VLDL production

Dyslipidemia in obesity results from excessive production and impaired clearance of triglyceride-rich (TG-rich) lipoproteins, which are particularly pronounced in the postprandial state. Here, we investigated the impact of Roux-en-Y gastric bypass (RYGB) surgery on postprandial VLDL1 and VLDL2 apoB and TG kinetics and their relationship with insulin-responsiveness indices. Morbidly obese patients without diabetes who were scheduled for RYGB surgery (n = 24) underwent a lipoprotein kinetics study during a mixed-meal test and a hyperinsulinemic-euglycemic clamp study before the surgery and 1 year later. A physiologically based computational model was developed to investigate the impact of RYGB surgery and plasma insulin on postprandial VLDL kinetics. After the surgery, VLDL1 apoB and TG production rates were significantly decreased, whereas VLDL2 apoB and TG production rates remained unchanged. The TG catabolic rate was increased in both VLDL1 and VLDL2 fractions, but only the VLDL2 apoB catabolic rate tended to increase. Furthermore, postsurgery VLDL1 apoB and TG production rates, but not those of VLDL2, were positively correlated with insulin resistance. Insulin-mediated stimulation of peripheral lipoprotein lipolysis was also improved after the surgery. In summary, RYGB resulted in reduced hepatic VLDL1 production that correlated with reduced insulin resistance, elevated VLDL2 clearance, and improved insulin sensitivity in lipoprotein lipolysis pathways.

High-throughput LC-MS method to investigate postprandial lipemia: considerations for future precision nutrition research

Elevated postprandial lipemia is an independent risk factor for cardiovascular disease, yet methods to quantitate postmeal handling of dietary lipids in humans are limited. This study tested a new method to track dietary lipid appearance using a stable isotope tracer (²H₁₁-oleate) in liquid meals containing three levels of fat [low fat (LF), 15 g; moderate fat (MF), 30 g; high fat (HF), 60 g]. Meals were fed to 12 healthy men [means ± SD, age 31.3 ± 9.2 yr, body mass index (BMI) 24.5 ± 1.9 kg/m²] during four randomized study visits; the HF meal was administered twice for reproducibility. Blood was collected over 8 h postprandially, triglyceride (TG)-rich lipoproteins (TRL), and particles with a Svedberg flotation rate >400 (S_f > 400, n = 8) were isolated by ultracentrifugation, and labeling of two TG species (54:3 and 52:2) was quantified by LC-MS. Total plasma TRL-TG concentrations were threefold greater than S_f > 400-TG. Both S_f > 400- and TRL-TG 54:3 were present at higher concentrations than 52:2, and singly labeled TG concentrations were higher than doubly labeled. Furthermore, TG 54:3 and the singly labeled molecules demonstrated higher plasma absolute entry rates differing significantly across fat levels within a single TG species (P < 0.01). Calculation of fractional entry showed no significant differences in label handling supporting the utility of either TG species for appearance rate calculations. These data demonstrate the utility of labeling research meals with stable isotopes to investigate human postprandial lipemia while simultaneously highlighting the importance of examining individual responses. Meal type and timing, control of prestudy activities, and effects of sex on outcomes should match the research goals. The method, optimized here, will be beneficial to conduct basic science research in precision nutrition and clinical drug development.NEW & NOTEWORTHY A novel method to test human intestinal lipid handling using stable isotope labeling is presented and, for the first time, plasma appearance and lipid turnover were quantified in 12 healthy men following meals with varying amounts of fat. The method can be applied to studies in precision nutrition characterizing individual response to support basic science research or drug development. This report discusses key questions for consideration in precision nutrition that were highlighted by the data.

Apolipoprotein B48 metabolism in chylomicrons and very low-density lipoproteins and its role in triglyceride transport in normo- and hypertriglyceridemic human subjects

BACKGROUND

Renewed interest in triglyceride-rich lipoproteins as causative agents in cardiovascular disease mandates further exploration of the integrated metabolism of chylomicrons and very low-density lipoproteins (VLDL).

METHODS

Novel tracer techniques and an integrated multi-compartmental model were used to determine the kinetics of apoB48- and apoB100-containing particles in the chylomicron and VLDL density intervals in 15 subjects with a wide range of plasma triglyceride levels.

RESULTS

Following a fat-rich meal, apoB48 appeared in the chylomicron, VLDL₁ and VLDL₂ fractions in all subjects. Chylomicrons cleared rapidly from the circulation but apoB48-containing VLDL accumulated, and over the day were 3-fold higher in those with high versus low plasma triglyceride. ApoB48-containing particles were secreted directly into both the chylomicron and VLDL fractions at rates that were similar across the plasma triglyceride range studied. During fat absorption, whilst most triglyceride entered the circulation in chylomicrons, the majority of apoB48 particles were secreted into the VLDL density range.

CONCLUSION

The intestine secretes apoB48-containing particles not only as chylomicrons but also directly into the VLDL₁ and VLDL₂ density ranges both in the basal state and during dietary lipid absorption. Over the day, apoB48-containing particles appear to comprise about 20-25% of circulating VLDL and, especially in those with elevated triglycerides, form part of a slowly cleared 'remnant' particle population, thereby potentially increasing CHD risk. These findings provide a metabolic understanding of the potential consequences for increased CHD risk when slowed lipolysis leads to the accumulation of remnants, especially in individuals with hypertriglyceridemia.

Causes and Consequences of Hypertriglyceridemia

Elevations in plasma triglyceride are the result of overproduction and impaired clearance of triglyceride-rich lipoproteins-very low-density lipoproteins (VLDL) and chylomicrons. Hypertriglyceridemia is characterized by an accumulation in the circulation of large VLDL-VLDL₁-and its lipolytic products, and throughout the VLDL-LDL delipidation cascade perturbations occur that give rise to increased concentrations of remnant lipoproteins and small, dense low-density lipoprotein (LDL). The elevated risk of atherosclerotic cardiovascular disease in hypertriglyceridemia is believed to result from the exposure of the artery wall to these aberrant lipoprotein species. Key regulators of the metabolism of triglyceride-rich lipoproteins have been identified and a number of these are targets for pharmacological intervention. However, a clear picture is yet to emerge as to how to relate triglyceride lowering to reduced risk of atherosclerosis.

Apolipoprotein B, Triglyceride-Rich Lipoproteins, and Risk of Cardiovascular Events in Persons with CKD

BACKGROUND AND OBJECTIVES

Triglyceride-rich lipoproteins may contribute to the high cardiovascular risk of patients with CKD. This study evaluated associations of apo-B and markers of triglyceride-rich lipoproteins with cardiovascular events in people with CKD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Analyses were conducted in 9270 participants with CKD in the Study of Heart and Renal Protection (SHARP): 6245 not on dialysis (mean eGFR 26.5 ml/min per 1.73 m²), and 3025 on dialysis when recruited. Cox regression methods were used to evaluate associations of lipids with incident atherosclerotic and nonatherosclerotic vascular events, adjusting for demographics and clinical characteristics. Hazard ratios (HRs) were calculated per 1 SD higher level for apo-B, HDL cholesterol, LDL cholesterol, triglyceride-rich lipoprotein cholesterol (i.e., total cholesterol minus LDL cholesterol minus HDL cholesterol), non-HDL cholesterol, log triglyceride, and log ratio of triglyceride to HDL cholesterol.

RESULTS

During a median follow-up of 4.9 years (interquartile range, 4.0-5.5 years), 1406 participants experienced at least one atherosclerotic vascular event. In multivariable adjusted models, positive associations with atherosclerotic vascular events were observed for apo-B (HR per 1 SD, 1.19; 95% confidence interval, 1.12 to 1.27), triglycerides (1.06; 1.00 to 1.13), the ratio of triglyceride to HDL cholesterol (1.10; 1.03 to 1.18), and triglyceride-rich lipoprotein cholesterol (1.14; 1.05 to 1.25). By contrast, inverse associations with nonatherosclerotic vascular events were observed for each of these lipid markers: apo-B (HR per 1 SD, 0.92; 0.85 to 0.98), triglycerides (0.86; 0.81 to 0.92), the ratio of triglyceride to HDL cholesterol (0.88; 0.82 to 0.94), and triglyceride-rich lipoprotein cholesterol (0.85; 0.77 to 0.94).

CONCLUSIONS

Higher apo-B, triglycerides, ratio of triglyceride to HDL cholesterol, and triglyceride-rich lipoprotein cholesterol concentrations were associated with increased risk of atherosclerotic vascular events in CKD. Reducing triglyceride-rich lipoproteins using novel therapeutic agents could potentially lower the risk of atherosclerotic cardiovascular disease risk in the CKD population.

Pediatric Metabolic Syndrome: Pathophysiology and Laboratory Assessment

Pediatric overweight and obesity is an emerging public health priority as rates have rapidly increased worldwide. Obesity is often clustered with other metabolic abnormalities including hypertension, dyslipidemia, and insulin resistance, leading to increased risk of cardiovascular disease. This cluster of risk factors, termed the metabolic syndrome, has traditionally been reported in adults. However, with the increased prevalence of pediatric obesity, the metabolic syndrome is now evident in children and adolescents. This complex cluster of risk factors is the result of the pathological interplay between several organs including adipose tissue, muscle, liver, and intestine with a common antecedent - insulin resistance. The association of the metabolic syndrome with several systemic alterations that involve numerous organs and tissues adds to the complexity and challenge of diagnosing the metabolic syndrome and identifying useful clinical indicators of the disease. The complex physiology of growing and developing children and adolescents further adds to the difficulties in standardizing laboratory assessment, diagnosis, and prognosis for the diverse pediatric population. However, establishing a consensus definition is critical to identifying and managing children and adolescents at high risk of developing the metabolic syndrome. As a result, the examination of novel metabolic syndrome biomarkers which can detect these metabolic abnormalities early with high specificity and sensitivity in the pediatric population has been of interest. Understanding this complex cluster of risk factors in the pediatric population is critical to ensure that this is not the first generation where children have a shorter life expectancy than their parents. This review will discuss the pathophysiology, consensus definitions and laboratory assessment of pediatric metabolic syndrome as well as potential novel biomarkers.

Postprandial Metabolism of Macronutrients and Cardiometabolic Risk: Recent Developments, Emerging Concepts, and Future Directions

Cardiovascular disease (CVD) is the leading cause of death in the United States. Although the role of habitual lifestyle factors such as physical activity and dietary patterns in increasing CVD risk has long been appreciated, less is known about how acute daily activities may cumulatively contribute to long-term disease risk. Here, the term acute refers to metabolic responses occurring in a short period of time after eating, and the goal of this article is to review recently identified stressors that can occur after meals and during the sleep-wake cycle to affect macronutrient metabolism. It is hypothesized that these events, when repeated on a regular basis, contribute to the observed long-term behavioral risks identified in population studies. In this regard, developments in research methods have supported key advancements in 3 fields of macronutrient metabolism. The first of these research areas is the focus on the immediate postmeal metabolism, spanning from early intestinal adsorptive events to the impact of incretin hormones on these events. The second topic is a focus on the importance of meal components on postprandial vasculature function. Finally, some of the most exciting advances are being made in understanding dysregulation in metabolism early in the day, due to insufficient sleep, that may affect subsequent processing of nutrients throughout the day. Key future research questions are highlighted which will lead to a better understanding of the relations between nocturnal, basal (fasting), and early postmeal events, and aid in the development of optimal sleep and targeted dietary patterns to reduce cardiometabolic risk.

Multiple apolipoprotein kinetics measured in human HDL by high-resolution/accurate mass parallel reaction monitoring

Endogenous labeling with stable isotopes is used to study the metabolism of proteins in vivo. However, traditional detection methods such as GC/MS cannot measure tracer enrichment in multiple proteins simultaneously, and multiple reaction monitoring MS cannot measure precisely the low tracer enrichment in slowly turning-over proteins as in HDL. We exploited the versatility of the high-resolution/accurate mass (HR/AM) quadrupole Orbitrap for proteomic analysis of five HDL sizes. We identified 58 proteins in HDL that were shared among three humans and that were organized into five subproteomes according to HDL size. For seven of these proteins, apoA-I, apoA-II, apoA-IV, apoC-III, apoD, apoE, and apoM, we performed parallel reaction monitoring (PRM) to measure trideuterated leucine tracer enrichment between 0.03 to 1.0% in vivo, as required to study their metabolism. The results were suitable for multicompartmental modeling in all except apoD. These apolipoproteins in each HDL size mainly originated directly from the source compartment, presumably the liver and intestine. Flux of apolipoproteins from smaller to larger HDL or the reverse contributed only slightly to apolipoprotein metabolism. These novel findings on HDL apolipoprotein metabolism demonstrate the analytical breadth and scope of the HR/AM-PRM technology to perform metabolic research.

Metabolism of apolipoprotein A-II containing triglyceride rich ApoB lipoproteins in humans

OBJECTIVE

To characterize human triglyceride-rich lipoproteins (TRL) with and without apoA-II and to study their metabolism in vivo.

METHODS

Plasma from 11 participants on a controlled diet given a bolus infusion of [D5]l-phenylalanine to label apoB was combined into four pools and applied to anti-apoA-II immunoaffinity columns. Fractions with and without apoA-II were separated into VLDL and IDL by ultracentrifugation; lipids and apolipoproteins were measured. For kinetic measurements, apoB was isolated and hydrolyzed to the constituent amino acids. Tracer enrichment was measured by GCMS. Metabolic rates were determined by SAAM-II.

RESULTS

VLDL and IDL with apoA-II comprised 7% and 9% of total VLDL and IDL apoB respectively. VLDL with apoA-II was enriched in apoC-III, apoE, and cholesterol compared to VLDL without apoA-II. Mean apoB FCR of VLDL with apoA-II was significantly lower than for VLDL without apoA-II (2.80 ± 0.96 pools/day v.s. 5.09 ± 1.69 pools/day, P = 0.009). A higher percentage of VLDL with apoA-II was converted to IDL than was cleared from circulation, compared to VLDL without apoA-II (96 ± 8% vs. 45 ± 22%; P = 0.007). The rate constants for conversion of VLDL to IDL were similar for VLDL with and without apoA-II. Thus, a very low rate constant for clearance accounted for the lower FCR of VLDL with apoA-II.

CONCLUSION

VLDL with apoA-II represents a small pool of VLDL particles that has a slow FCR and is predominantly converted to IDL rather than cleared from the circulation.

ApoC-III and visceral adipose tissue contribute to paradoxically normal triglyceride levels in insulin-resistant African-American women

Background

African-Americans are more insulin-resistant than whites but have lower triglyceride (TG) concentrations. The metabolic basis for this is unknown. Our goal was to determine in a cross-sectional study the effect of insulin resistance, visceral adipose tissue (VAT) and the apolipoproteins, B, C-III and E, on race differences in TG content of very low density lipoproteins (VLDL).

Methods

The participants were 31 women (16 African-American, 15 white) of similar age (37 ± 9 vs. 38 ± 11y (mean ± SD), P = 0.72) and BMI (32.4 ± 7.2 vs. 29.3 ± 6.0 kg/m², P = 0.21). A standard diet (33% fat, 52% carbohydrate, 15% protein) was given for 7 days followed by a test meal (40% fat, 40% carbohydrate, 20% protein) on Day 8. Insulin sensitivity index (S_I) was calculated from the minimal model. VAT was measured at L2-3. The influence of race, S_I, VAT and apolipoproteins on the TG content of VLDL was determined by random effects models (REM).

Results

African-Americans were more insulin-resistant (S_I: 3.6 ± 1.3 vs. 5.6 ± 2.6 mU/L^-1.min^-1, P < 0.01) with less VAT (75 ± 59 vs. 102 ± 71 cm², P < 0.01). TG, apoB and apoC-III content of light and dense VLDL were lower in African-Americans (all P < 0.05 except for apoC-III in light VLDL, P = 0.11). ApoE content did not vary by race. In REM, VAT but not S_I influenced the TG concentration of VLDL. In models with race, S_I, VAT and all apolipoproteins entered, race was not significant but apoC-III and VAT remained significant determinants of TG concentration in light and dense VLDL.

Conclusions

Low concentrations of apoC-III and VAT in African-Americans contribute to race differences in TG concentrations.

Trial registration

ClinicalTrials.gov Identifier: NCT00484861

A novel method for measuring intestinal and hepatic triacylglycerol kinetics

This study aimed to 1) develop a method that completely separated hepatic (VLDL1, VLDL2) and intestinal [chylomicron (CM)] lipoproteins and 2) use the method to measure triacylglycerol (TAG) kinetics in these lipoproteins in the fed and fasting state in healthy subjects, using intravenous [²H₅]glycerol as the tracer. An immunoaffinity method that completely separated hepatic and intestinal particles using sequential binding to three antibodies to apolipoprotein B-100 (apoB-100) was established and validated. Six healthy volunteers were studied in a fasted and continuous feeding study (study 1). Five additional healthy volunteers were studied in a continuous feeding study that included an oral [¹³C₃]glycerol tripalmitin tracer (study 2). In both studies, an intravenous bolus of [²H₅]glycerol was administered to label TAG in hepatic and intestinal lipoproteins. In both feeding studies there was sufficient incorporation of the [²H₅]glycerol tracer into the exogenous lipoproteins to enable isotopic enrichment to be measured. In study 2, the oral tracer enrichment in VLDL1 was <5% of CM enrichment 150 min after tracer administration, demonstrating negligible contamination of VLDL1 with apoB-48. Western blotting showed no detectable apoB-100 in CMs. VLDL1 and VLDL2 TAG fractional catabolic rate (FCR) did not differ between feeding and fasting (study 1). There was no difference between CM and VLDL1 TAG FCR in both fed studies. In fed study 2, 47% of the total TAG production rate (CM + VLDL1) was from CM. This methodology may be a useful tool for understanding the abnormalities in postprandial TAG kinetics in metabolic syndrome and type 2 diabetes.

Getting the label in: practical research strategies for tracing dietary fat

The observation that events occurring after consumption of a meal can directly affect metabolic risk has been gaining interest over the past 40 years. As a result, the desire for investigators to conduct postprandial studies has also increased. Study design decisions pertaining to the choice of meal quantity and composition are more difficult than may be readily apparent, and there is now ample evidence available in the literature to suggest that what is fed on the test day significantly affects postprandial metabolism and can therefore influence interpretation of results. In addition, events occurring before the testing day (food intake and activities) can also have an impact on the observed postprandial response. The goal of this review is to present aspects of study design critical to the investigation of postprandial metabolism. These details include subject preparation, meal quantity, form and composition, as well as sampling protocols for measuring metabolites. Key factors and practical examples are provided to minimize the impact of nonresearch variables on subject variability. Finally, aspects related to using stable isotope tracers to measure metabolism of meal fat are discussed, including choice of tracer form, dose and delivery in food. Given that fed-state events contribute significantly to chronic disease risk, improved methods to study the absorption and disposal of food energy will support the development of strategies designed to prevent and treat diseases associated with overconsumption of nutrients.

Intravascular transfer contributes to postprandial increase in numbers of very-low-density hepatitis C virus particles

BACKGROUND & AIMS

The physical association of hepatitis C virus (HCV) particles with lipoproteins in plasma results in distribution of HCV in a broad range of buoyant densities. This association is thought to increase virion infectivity by mediating cell entry via lipoprotein receptors. We sought to determine if factors that affect triglyceride-rich lipoprotein (TRL) metabolism alter the density and dynamics of HCV particles in the plasma of patients with chronic HCV infection.

METHODS

Fasting patients (n = 10) consumed a high-fat milkshake; plasma was collected and fractionated by density gradients. HCV- RNA was measured in the very-low-density fraction (VLDF, d < 1.025 g/mL) before and at 7 serial time points postprandially.

RESULTS

The amount of HCV RNA in the VLDF (HCV(VLDF)) increased a mean of 26-fold, peaking 180 minutes after the meal (P < .01). Quantification of HCV RNA throughout the density gradient fractions revealed that HCV(VLDF) rapidly disappeared, rather than migrating into the adjacent density fraction. Immuno-affinity separation of the VLDF, using antibodies that recognize apolipoprotein B-100 and not apolipoprotein B-48, showed that HCV(VLDF) is composed of chylomicron- and VLDL-associated HCV particles; peaking 120 and 180 minutes after the meal, respectively. Plasma from fasting HCV-infected patients mixed with uninfected plasma increased the quantity of HCV(VLDF), compared with that mixed with phosphate-buffered saline, showing extracellular assembly of HCV(VLDF).

CONCLUSIONS

Dietary triglyceride alters the density and dynamics of HCV in plasma. The rapid clearance rate of HCV(VLDF) indicates that association with TRL is important for HCV infectivity. HCV particles, such as exchangeable apolipoproteins, appear to reassociate with TRLs in the vascular compartment.

Vitamin A equivalence of the ß-carotene in ß-carotene-biofortified maize porridge consumed by women

BACKGROUND

β-Carotene-biofortified maize is being developed through plant breeding as a sustainable agronomic approach to alleviate vitamin A deficiency.

OBJECTIVE

Our objective was to quantify the vitamin A equivalence of the β-carotene in β-carotene-biofortified maize based on consumption of a single serving of maize porridge.

DESIGN

Six healthy women each consumed three 250-g portions of maize porridge as follows: 1) β-carotene-biofortified maize porridge containing 527 μg (0.98 μmol) total β-carotene, 2) white maize porridge with a β-carotene reference dose containing 595 μg (1.11 μmol) added β-carotene, and 3) white maize porridge with a vitamin A reference dose containing 286 μg retinol activity equivelent (1.00 μmol) added retinyl palmitate. Each portion contained 8.0 g added sunflower oil. The porridges were consumed in random order separated by ≥2 wk. Blood samples were collected over 9 h. Retinyl palmitate was analyzed in plasma triacylglycerol-rich lipoprotein (TRL) fractions by HPLC with coulometric array electrochemical detection.

RESULTS

Mean (± SD) areas under the curve for retinyl palmitate in the TRL fractions (nmol ⋅ h) were 24.0 ± 9.4, 89.7 ± 34.7, and 80.1 ± 24.8 after ingestion of the β-carotene-biofortified maize porridge, the white maize porridge with the β-carotene reference dose, and the white maize porridge with the vitamin A reference dose, respectively. On average, 6.48 ± 3.51 μg (mean ± SD) of the β-carotene in β-carotene-biofortified maize porridge and 2.34 ± 1.61 μg of the β-carotene in the reference dose were each equivalent to 1 μg retinol.

CONCLUSION

β-Carotene in biofortified maize has good bioavailability as a plant source of vitamin A.

Elevation of tumor necrosis factor-alpha induces the overproduction of postprandial intestinal apolipoprotein B48-containing very low-density lipoprotein particles: evidence for related gene expression of inflammatory, insulin and lipoprotein signaling in enterocytes

The aim of this study was to determine whether systemic elevation of tumor necrosis factor (TNF)-alpha induces intestinal-derived apolipoprotein B (apoB)48-containing very low-density lipoprotein (VLDL) production in hamsters after fat loading and whether TNF-alpha disturbs the related mRNA expression in inflammatory, insulin and lipoprotein signaling pathways in primary enterocytes. In vivo TNF-alpha and Triton-WR1339 infusion, Western blotting and reverse transcriptase-polymerase chain reaction were combined to explore the mechanisms underlying intestinal overproduction of apoB48-containing chylomicrons and VLDL(1) particles by TNF-alpha. TNF-alpha infusion increased intestinal production of chylomicron and VLDL(1)-apoB48 in postprandial (fat load) states. Following TNF-alpha-treatment in enterocytes, there was enhanced gene expression of Il1alpha and beta, Il6 and Tnf and decreased mRNA levels of components of the insulin signaling pathway including the insulin receptor (Ir), Ir substrate-1 and 2, PI3 k, and Akt, but increased phosphatase and tensin homolog deleted on chromosome ten (Pten) protein and mRNA expression. TNF-alpha also induced Cd36 and peroxisome proliferators-activated receptor (Ppar)gamma expression, as well as microsomal triglyceride transfer protein (Mtp) protein and mRNA, but suppressed the sterol regulatory element binding protein (Srebp)1c protein and mRNA level. Systemic elevation of TNF-alpha stimulates the postprandial overproduction of apoB48-containing chylomicrons and VLDL(1) particles by disturbing intestinal gene expression of the inflammatory, insulin and lipoprotein pathways. These findings provide mechanistic links among the inflammatory factor, TNF-alpha, intestinal inflammatory/insulin insensitivity and the overproduction of intestinal apoB48-containing lipoproteins.

Dietary monounsaturated fat activates metabolic pathways for triglyceride-rich lipoproteins that involve apolipoproteins E and C-III

BACKGROUND

Dietary monounsaturated fat (MUFA) and complex carbohydrates have different effects on triglyceride-rich lipoprotein (TRL) metabolism.

OBJECTIVE

We hypothesized that apolipoprotein (apo) E and apo C-III might be involved in these dietary effects because of their crucial role in TRL metabolism.

DESIGN

Twelve adults consumed, for 3 wk each, 2 isocaloric diets: first a carbohydrate-rich diet (48% complex carbohydrate, 8% MUFAs) and then a MUFA-rich diet (31% complex carbohydrate, 24% MUFAs) 12 mo later. The dietary composition of other macronutrients in the 2 diets was similar. Body weight was kept constant. Postprandial apo B kinetic studies using stable-isotope tracers were performed after each dietary intervention. Multiple VLDL, intermediate-density lipoprotein (IDL), and LDL fractions were prepared on the basis of apo E and apo C-III contents.

RESULTS

The MUFA diet increased by approximately 4-6-fold, the secretion of VLDLs and IDLs containing both apo E and apo C-III (E+CIII+) (P < 0.05). These are TRLs that mostly cleared from the circulation and are minor precursors of LDL. The MUFA diet also decreased by 60% (P < 0.05) the secretion of the TRLs without apo E or apo C-III (major precursors of LDL in plasma) and decreased their flux to LDLs. Total LDL flux did not change because the MUFA diet increased the flux to LDL from E-CIII+ TRLs, a process that requires the removal of apo C-III. In addition, the MUFA diet significantly increased the TRL fractional catabolic rate by 50% and doubled the percentage of TRLs that were cleared rather than being converted to LDLs.

CONCLUSION

MUFA intake activates synthetic and rapid catabolic pathways for TRL metabolism that involve apo E and apo C-III and suppresses the metabolism of more slowly metabolized VLDLs and IDLs, which do not contain these apolipoproteins.

Tumor necrosis factor-alpha directly stimulates the overproduction of hepatic apolipoprotein B100-containing VLDL via impairment of hepatic insulin signaling

Insulin-resistant states are commonly associated with both increased circulating levels of tumor necrosis factor (TNF)-alpha and hepatic overproduction of very low density lipoproteins (VLDL). Here, we provide evidence that increased TNF-alpha can directly stimulate the hepatic assembly and secretion of apolipoprotein B (apoB) 100-containing VLDL(1), using the Syrian golden hamster, an animal model that closely resembles humans in hepatic VLDL-apoB100 metabolism. In vivo TNF-alpha infusion for 4 h in chow-fed hamsters induced whole-body insulin resistance on the basis of euglycemic hyperinsulinemic clamp studies. Immunoprecipitation and immunoblotting analysis of livers from TNF-alpha-treated hamsters indicated decreased tyrosine phosphorylation of insulin receptor (IR)-beta, IR substrate-1 (Tyr), Akt (Ser(473)), p38, ERK1/2, and JNK but increased serine phosphorylation of IRS-1 (Ser(307)) and Shc. TNF-alpha infusion also significantly increased hepatic production of total circulating apoB100 and VLDL-apoB100 in both fasting and postprandial (fat load) states. Ex vivo experiments, using cultured primary hepatocytes from hamsters, also showed TNF-alpha-induced VLDL-apoB100 oversecretion, an effect that was blocked by TNF receptor 2 antibody. Unexpectedly, TNF-alpha decreased the sterol regulatory element-binding protein-1c mass and mRNA levels but significantly increased microsomal triglyceride transfer protein mass and mRNA levels in primary hepatocytes. In summary, these data provide direct evidence that TNF-alpha induces whole-body insulin resistance and impairs hepatic insulin signaling accompanied by overproduction of apoB100-containing VLDL particles, an effect likely mediated via TNF receptor 2.

Development of a novel method to determine very low density lipoprotein kinetics

Isotopic tracer methods of determining triglyceride-rich lipoprotein (TRL) kinetics are costly, time-consuming, and labor-intensive. This study aimed to develop a simpler and cost-effective method of obtaining TRL kinetic data, based on the fact that chylomicrons compete with large VLDL (VLDL(1); S(f) = 60-400) for the same catalytic pathway. Ten healthy subjects [seven men; fasting triglyceride (TG), 44.3-407.6 mg/dl; body mass index, 21-35 kg/m(2)] were given an intravenous infusion of a chylomicron-like TG emulsion (Intralipid; 0.1 g/kg bolus followed by 0.1 g/kg/h infusion) for 75-120 min to prevent the clearance of VLDL(1) by lipoprotein lipase. Multiple blood samples were taken during and after infusion for separation of Intralipid, VLDL(1), and VLDL(2) by ultracentrifugation. VLDL(1)-apolipoprotein B (apoB) and TG production rates were calculated from their linear increases in the VLDL(1) fraction during the infusion. Intralipid-TG clearance rate was determined from its exponential decay after infusion. The production rates of VLDL(1)-apoB and VLDL(1)-TG were (mean +/- SEM) 25.4 +/- 3.9 and 1,076.7 +/- 224.7 mg/h, respectively, and the Intralipid-TG clearance rate was 66.9 +/- 11.7 pools/day. Kinetic data obtained from this method agree with values obtained from stable isotope methods and show the expected relationships with indices of body fatness and insulin resistance (all P < 0.05). The protocol is relatively quick, inexpensive, and transferable to nonspecialist laboratories.