Cellular aspects of intestinal lipoprotein assembly in Psammomys obesus: a model of insulin resistance and type 2 diabetes

Overproduction of intestinal lipoprotein containing apolipoprotein B-48 in Psammomys obesus: impact of dietary n-3 fatty acids

AIMS/HYPOTHESIS

Emerging evidence underscores the important role of the small intestine in the pathogenesis of dyslipidaemia in insulin resistance and type 2 diabetes. We therefore tested the hypothesis that n-3 fatty acids improve the various events governing intra-enterocyte lipid transport in Psammomys obesus gerbils, a model of nutritionally induced metabolic syndrome.

MATERIALS AND METHODS

Experiments were carried out on Psammomys obesus gerbils that were assigned to an isocaloric control diet and a diet rich in fish oil for 6 weeks.

RESULTS

Increased dietary intake of fish oil lowered body weight and improved hyperglycaemia and hyperinsulinaemia. It simultaneously decreased de novo intestinal lipogenesis and lipid esterification of the major lipid classes, e.g. triglycerides, phospholipids and cholesteryl esters, particularly in insulin-resistant and diabetic animals. Accordingly, lessened activity of monoacylglycerol and diacylglycerol acyltransferase was recorded. As assessed in cultured jejunal explants incubated with either [(14)C]-oleic acid or [(35)S]-methionine, fish oil feeding resulted in diminished triglyceride-rich lipoprotein assembly and apolipoprotein (apo) B-48 biogenesis, respectively. The mechanisms did not involve apo B-48 transcription or alter the gene expression and activity of the critical microsomal triglyceride transfer protein. Rather, the suppressed production of apo B-48 by n-3 fatty acids was associated with intracellular proteasome-mediated posttranslational downregulation in insulin-resistant and diabetic animals.

CONCLUSIONS/INTERPRETATION

Our data highlight the beneficial impact of n-3 fatty acids on adverse effects of the metabolic syndrome and emphasise their influence on intestinal lipid transport, an effect which may limit postprandial lipaemia and the risk of atherosclerosis.

Intestinal insulin resistance and aberrant production of apolipoprotein B48 lipoproteins in an animal model of insulin resistance and metabolic dyslipidemia: evidence for activation of protein tyrosine phosphatase-1B, extracellular signal-related kinase, and sterol regulatory element-binding protein-1c in the fructose-fed hamster intestine

Postprandial dyslipidemia is recognized as an important complication of insulin-resistant states, and recent evidence implicates intestinal lipoprotein overproduction as a causative factor. The mechanisms linking intestinal lipoprotein overproduction and aberrant insulin signaling in intestinal enterocytes are currently unknown. Intestinal insulin sensitivity and lipid metabolism were studied in a fructose-fed hamster model of insulin resistance and metabolic dyslipidemia. Intestinal lipoprotein production in chow-fed hamsters was responsive to the inhibitory effects of insulin, and a decrease in circulating levels of triglyceride-rich apolipoprotein (apo)B48-containing lipoproteins occurred 60 min after insulin administration. However, fructose-fed hamster intestine was not responsive to the insulin-induced downregulation of apoB48-lipoprotein production, suggesting insulin insensitivity at the level of the intestine. Enterocytes from the fructose-fed hamster exhibited normal activity of the insulin receptor but reduced levels of insulin receptor substrate-1 phosphorylation and mass and Akt protein mass. Conversely, the protein mass of the p110 subunit of phosphatidylinositol 3-kinase, protein tyrosine phosphatase-1B, and basal levels of phosphorylated extracellular signal-related kinase (ERK) were significantly increased in the fructose-fed hamster intestine. Modulating the ERK pathway through in vivo inhibition of mitogen-activated protein/ERK kinase 1/2, the upstream activator of ERK1/2, we observed a significant decrease in intestinal apoB48 synthesis and secretion. Interestingly, enhanced basal ERK activity in the fructose-fed hamster intestine was accompanied by an increased activation of sterol regulatory element-binding protein. In summary, these data suggest that insulin insensitivity at the level of the intestine and aberrant insulin signaling are important underlying factors in intestinal overproduction of highly atherogenic apoB48-containing lipoproteins in the insulin-resistant state. Basal activation of the ERK pathway may be an important contributor to the aberrant insulin signaling and lipoprotein overproduction in this model.

Hyperinsulinemia is associated with increased production rate of intestinal apolipoprotein B-48-containing lipoproteins in humans

OBJECTIVE

Whereas postprandial hyperlipidemia is a well-described feature of insulin-resistant states and type 2 diabetes, no previous studies have examined intestinal lipoprotein production rates (PRs) in relation to hyperinsulinemia or insulin resistance in humans.

METHODS AND RESULTS

Apolipoprotein B-48 (apoB-48)-containing lipoprotein metabolism was examined in the steady-state fed condition with a 15-hour primed constant infusion of [D3]-l-leucine in 14 nondiabetic men with a broad range of body mass index (BMI) and insulin sensitivity. To examine the relationship between indices of insulin resistance and intestinal lipoprotein PR data were analyzed in 2 ways: by correlation and by comparing apoB-48 PRs in those whose fasting plasma insulin concentrations were above or below the median for the 14 subjects studied (60 pmol/L). ApoB-48 PR was significantly higher in hyperinsulinemic, insulin-resistant subjects (1.73+/-0.39 versus 0.88+/-0.13 mg/kg per day; P<0.05) and correlated with fasting plasma insulin concentrations (r=0.558; P=0.038), despite great heterogeneity in apoB-48 kinetic parameters, particularly among the obese subjects. There was no significant difference in clearance of apoB-48 between the 2 groups, nor was there a significant correlation between apoB-48 fractional clearance rate and fasting insulin or homeostasis model assessment-insulin resistance.

CONCLUSIONS

These are the first human data to conclusively demonstrate that intestinal apoB-48-containing triglyceride-rich lipoprotein PR is increased in hyperinsulinemic, insulin-resistant humans. Intestinal lipoprotein particle overproduction is a newly described feature of insulin resistance in humans.

Fasting apoprotein B-48 level and coronary artery disease in a population without frank fasting hypertriglyceridemia

The aim of this study was to test the hypothesis that fasting apoprotein B-48 level might be a surrogate marker of postprandial lipemia in evaluating the risk of coronary artery disease (CAD) in a population without frank abnormality in fasting lipid profile. One hundred twenty-three patients tested by coronary angiography were selected on the criteria of absence of treatment with hypolipidemic drugs, obvious hypertriglyceridemia (>2.85 mmol/L), or other conditions that may interfere with lipoprotein metabolism except diabetes. CAD was defined by more than 50% narrowing of vessel lumen, and its severity is determined by the number of arteries involved. Fasting apoprotein B-48 was measured by a competitive enzyme-linked immunosorbent assay method. There was no difference in fasting apoprotein B-48 levels between the groups with and without CAD (0.123+/-0.096 vs 0.136+/-0.125 microg/mL, respectively), whatever the sex or whether with or without diabetes. The apoprotein B-48 level was not related to the presence or the severity of CAD. There was also no correlation between fasting apoprotein B-48 levels and age, sex, body mass index, and usual fasting lipid parameters in both patients with and without angiographically proven CAD. Finally, among the features of metabolic syndrome, apoprotein B-48 was correlated with fasting triglyceride levels (r=0.357, P<.01) only. In conclusion, the present study shows that in the absence of any major fasting abnormality in plasma lipid parameters, fasting apoprotein B-48 level, which has been associated with postprandial hyperlipidemia, does not predict the risk of CAD.

Intestinal lipoprotein assembly

PURPOSE OF REVIEW

The assembly of intestinal lipoproteins is critical for the transport of fat and fat-soluble vitamins. In this review we propose a nomenclature for these lipoproteins and have summarized recent data about their intracellular assembly and factors that modulate their secretion.

RECENT FINDINGS

The assembly and secretion of intestinal lipoproteins increases with the augmented synthesis of apoB, apoAIV and lipids. Chylomicron assembly begins with the formation of primordial, phospholipid-rich particles in the membrane, and their conversion to large chylomicrons occurs in the lumen of the smooth endoplasmic reticulum. Chylomicrons are transported from the endoplasmic reticulum via specialized vesicles to the Golgi for secretion. The identification of genetic mutations in chylomicron retention disease indicates that Sar1b may play a critical role in this process. In addition to chylomicron assembly, intestinal cells have been shown to transport dietary cholesterol via apoB-independent pathways, such as efflux.

SUMMARY

Understanding the mechanisms involved in the intracellular transport of chylomicrons and chylomicron-independent secretion pathways are expected to be the next frontiers in the field of intestinal lipoprotein assembly and secretion.

Intestinal assembly and secretion of highly dense/lipid-poor apolipoprotein B48-containing lipoprotein particles in the fasting state: evidence for induction by insulin resistance and exogenous fatty acids

Emerging evidence suggests that overproduction of intestinally derived apolipoprotein (apo) B48-containing lipoprotein particles may be an important contributor to both fasting and postprandial dyslipidemia in insulin-resistant states. Mechanisms regulating the assembly and secretion of apoB48-containing lipoproteins are not fully understood particularly in the diabetic/insulin-resistant intestine. In the present study, we have investigated the density profile of apoB48 lipoproteins assembled in primary hamster enterocytes. Both intracellular and secreted apoB48 particles were examined in intestinal enterocytes isolated from normal or insulin-resistant fructose-fed hamsters, as well as in enterocytes treated with exogenous oleic acid. Microsomal luminal contents and culture media were analyzed by discontinuous and sequential ultracentrifugation on sucrose and KBr gradients, respectively. ApoB48 was mostly secreted on VLDL-, LDL-, and denser HDL-sized particles in the fasting state. In pulse-chase labeling experiments, nascent apoB48-containing particles initially accumulated in the microsomal lumen as HDL-sized particles, with subsequent formation of apoB48-VLDL particles, with only a minute amount of chylomicrons observed. Treatment with 720 mu mol/L of oleic acid, increased microsomal apoB48 HDL synthesis, and induced a marked shift toward lighter more buoyant particles. A marked enhancement in assembly of apoB48-containing lipoproteins was also observed in the microsomal lumen of fructose-fed hamster enterocytes, suggesting facilitated assembly and secretion of dense intestinal lipoprotein particles in insulin-resistant states. Overall, these observations suggest that a major proportion of apoB48-containing lipoprotein particles is assembled and secreted as highly dense, HDL-sized particles. The production of these small, dense, and potentially atherogenic apoB48 particles can be stimulated by increased free fatty acid flux as well as in insulin-resistant diabetes.

Intestinal fatty acid binding protein and microsomal triglyceride transfer protein polymorphisms in French-Canadian youth

Growing evidence suggests an association between lipid abnormalities and fatty acid binding protein (FABP) and microsomal triglyceride transfer protein (MTP) gene variants. Our objectives were to determine whether Ala54Thr FABP2 and G-493T MTP polymorphisms are associated with increased risks of insulin resistance syndrome (IRS) in youth and/or modify the expression of accompanying dyslipidemia. Our study of 1,742 French-Canadians aged 9, 13, and 16 years did not provide evidence of a potential predisposition to IRS related to either FABP2 or MTP genotypes. However, we observed a heterogeneity of the FABP2 effect by IRS status on total cholesterol (TC), low density lipoprotein-cholesterol (LDL-C), and apolipoprotein B (apoB) concentrations (P for interaction=0.045, 0.018, and 0.017, respectively). Among the metabolic components of IRS, only triglyceride (TG) displayed an interaction with FABP2 polymorphism: compared with Thr/Ala and Ala/Ala, the Thr/Thr genotype was associated with a steeper increase in TC, LDL-C, and apoB parallel to TG concentrations (P <0.001). IRS did not modify the associations between the MTP polymorphism and any of the biochemical parameters. Our study suggests that the effects of FABP2 allelic variations on lipid traits are context dependent, indicating that this variant may play an important role in cardiovascular pathogenesis in the presence of IRS or hypertriglyceridemia.

Intestinal lipoprotein production is stimulated by an acute elevation of plasma free fatty acids in the fasting state: studies in insulin-resistant and insulin-sensitized Syrian golden hamsters

It is not known whether intestinal lipoprotein production is stimulated by an acute elevation of plasma free fatty acids (FFA). We examined the effect of an intralipid and heparin infusion on the intestinal lipoprotein production rate (PR) in insulin-sensitive [chow-fed (CHOW)], insulin-resistant [60% fructose (FRUC) or 60% fat-fed (FAT)], and insulin-sensitized [FRUC or FAT plus rosiglitazone (RSG)-treated] Syrian Golden hamsters. After 5 wk of treatment, overnight-fasted hamsters underwent in vivo Triton WR-1339 studies for measurement of apolipoprotein B48 (apoB48) PR in large (Svedberg unit, >400) and small (Svedberg unit, 100-400) lipoprotein fractions, with an antecedent 90-min infusion of 20% intralipid and heparin (IH) to raise plasma FFA levels approximately 5- to 8-fold vs. those in the saline control study. IH markedly increased apoB48 PR in CHOW by 3- to 5-fold, which was confirmed ex vivo in pulse-chase experiments in primary cultured hamster enterocytes. Oleate, but not glycerol, infusion was associated with a similar elevation of apoB48 PR as IH. In FRUC and FAT, basal (saline control) apoB48 PR was approximately 4-fold greater than that in CHOW; there was no additional stimulation with IH in vivo and only minimal additional stimulation ex vivo. RSG partially normalized basal apoB48 PR in FAT and FRUC, and PR was markedly stimulated with IH. We conclude that intestinal lipoprotein production is markedly stimulated by an acute elevation of plasma FFAs in insulin-sensitive hamsters, in which basal production is low, but minimally in insulin-resistant hamsters, in which basal production is already elevated. With RSG treatment, basal PR is partially normalized, and they become more susceptible to the acute FFA stimulatory effect.