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

Increased very low density lipoprotein (VLDL) secretion, hepatic steatosis, and insulin resistance

Insulin resistance (IR) affects not only the regulation of carbohydrate metabolism but all aspects of lipid and lipoprotein metabolism. IR is associated with increased secretion of VLDL and increased plasma triglycerides, as well as with hepatic steatosis, despite the increased VLDL secretion. Here we link IR with increased VLDL secretion and hepatic steatosis at both the physiologic and molecular levels. Increased VLDL secretion, together with the downstream effects on high density lipoprotein (HDL) cholesterol and low density lipoprotein (LDL) size, is proatherogenic. Hepatic steatosis is a risk factor for steatohepatitis and cirrhosis. Understanding the complex inter-relationships between IR and these abnormalities of liver lipid homeostasis will provide insights relevant to new therapies for these increasing clinical problems.

Gut-liver interaction in triglyceride-rich lipoprotein metabolism

The liver and intestine have complementary and coordinated roles in lipoprotein metabolism. Despite their highly specialized functions, assembly and secretion of triglyceride-rich lipoproteins (TRL; apoB-100-containing VLDL in the liver and apoB-48-containing chylomicrons in the intestine) are regulated by many of the same hormonal, inflammatory, nutritional, and metabolic factors. Furthermore, lipoprotein metabolism in these two organs may be affected in a similar fashion by certain disorders. In insulin resistance, for example, overproduction of TRL by both liver and intestine is a prominent component of and underlies other features of a complex dyslipidemia and increased risk of atherosclerosis. The intestine is gaining increasing recognition for its importance in affecting whole body lipid homeostasis, in part through its interaction with the liver. This review aims to integrate recent advances in our understanding of these processes and attempts to provide insight into the factors that coordinate lipid homeostasis in these two organs in health and disease.

Acute hypertriglyceridemia promotes intestinal lymphatic lipid and drug transport: a positive feedback mechanism in lipid and drug absorption

Elevated systemic levels of triglyceride-rich lipoproteins (TRL) are a risk factor for the development of atherosclerosis. In patients with metabolic syndrome (MetS), intestinal TRL overproduction contributes to high systemic TRL levels, and recent studies suggest that systemic changes in MetS such as increases in plasma fatty acids and insulin resistance stimulate intestinal TRL production. The current study has examined whether increases in systemic TRL influence intestinal lipid transport and lipoprotein assembly pathways and evaluates the impact of these changes on the absorption and lymphatic transport of lipids and a model lipophilic drug (halofantrine). Mesenteric lymph-duct or bile-duct cannulated rats were administered IV saline or (14)C-labeled chylomicron (CM) (to increase systemic TRL) and intraduodenal (3)H lipids and drug. Changes to biliary lipid output and lymphatic lipid and drug transport were subsequently examined. Increasing systemic TRL concentrations stimulated a significant increase in lymphatic lipid and drug transport. The increased lipids in lymph were not derived from bile or the intestinal blood supply (fatty acid or IV infused (14)C-CM). Rather, an increase in lymphatic transport of duodenally sourced lipids was evident. Increasing plasma levels of TRL therefore stimulated lipid absorption and lymphatic transport via a positive feedback process. The data also suggest that the changes to intestinal TRL formation that result from raised systemic TRL levels may impact on the absorption of highly lipophilic drugs and therefore the reproducibility of drug treatments.

Effects of acute hyperglucagonemia on hepatic and intestinal lipoprotein production and clearance in healthy humans

OBJECTIVE

The metabolism of hepatic- and intestinally derived lipoproteins is regulated in a complex fashion by nutrients, hormones, and neurologic and other factors. Recent studies in animal models suggest an important role for glucagon acting via the glucagon receptor in regulating hepatic triglyceride (TG) secretion. Here we examined the direct effects of glucagon on regulation of hepatic and intestinal lipoprotein metabolism in humans.

RESEARCH DESIGN AND METHODS

Eight healthy men underwent two studies each, in random order, 4-6 weeks apart in which de novo lipogenesis, kinetics of larger VLDL1 TG, and kinetics of VLDL1 and smaller VLDL2 apolipoprotein (apo)B100 and B48 were studied using established stable isotope enrichment methods. Subjects were studied in the constant fed state under conditions of a pancreatic clamp (with infusion of somatostatin, insulin, and growth hormone) at either basal glucagon (BG study, 64.5 ± 2.1 pg/mL) or hyperglucagonemia (high glucagon [HG] study, 183.2 ± 5.1 pg/mL).

RESULTS

There were no significant differences in plasma concentration of VLDL1 or VLDL2 TG, apoB100 or apoB48 between BG and HG studies. There was, however, lower (P < 0.05) VLDL1 apoB100 fractional catabolic rate (-39%) and production rate (-30%) in HG versus BG, but no difference in de novo lipogenesis or TG turnover, and glucagon had no effect on intestinal (B48-containing) lipoprotein metabolism.

CONCLUSIONS

Glucagon acutely regulates hepatic but not intestinal lipoprotein particle metabolism in humans both by decreasing hepatic lipoprotein particle production as well as by inhibiting particle clearance, with no net effect on particle concentration.

Increased risk of cardiovascular disease in Type 1 diabetes: arterial exposure to remnant lipoproteins leads to enhanced deposition of cholesterol and binding to glycated extracellular matrix proteoglycans

AIMS

To determine fasting and postprandial metabolism of apolipoprotein B48 (apoB48) remnant lipoproteins in subjects with Type 1 diabetes and the relationship to progressive cardiovascular disease, and to investigate the impact of remnant lipoprotein cholesterol accumulation associated with arterial wall biglycan using a rodent model of Type 1 diabetes.

METHODS

Normolipidaemic subjects (n = 9) with long-standing Type 1 diabetes (and advanced cardiovascular disease) and seven healthy control subjects were studied. Fasting and postprandial apoB48 concentration was determined following a sequential meal challenge. A rodent model of streptozotocin-induced diabetes was used to investigate the ex vivo retention of fluorescent-conjugated remnants. Binding of remnant lipoproteins to human recombinant biglycan was assessed in vitro.

RESULTS

A significantly higher concentration of fasting plasma apoB48 remnants was observed in patients with Type 1 diabetes compared with control subjects. Patients with Type 1 diabetes exhibited a greater total plasma apoB48 area under the curve (AUC) and an increased incremental AUC following a second sequential meal compared with control subjects. The arterial retention of remnants ex vivo and associated cholesterol was increased sevenfold in Type 1 diabetes rats relative to controls. Remnants were shown to bind with significant affinity to human biglycan in vitro and a further 2.3-fold increased binding capacity was observed with glycated biglycan. Remnants were shown to colocalize with both arterial biglycan and glycated matrix proteins in the Type 1 diabetes rodent model.

CONCLUSION

Impaired metabolism of remnant lipoproteins associated with enhanced binding to proteoglycans appears to contribute to the arterial cholesterol deposition in Type 1 diabetes. Our findings support the hypothesis that impaired remnant metabolism may contribute to accelerated progression of atherosclerosis in the hyperglycaemic and insulin-deficient state.

Jejunal wall triglyceride concentration of morbidly obese persons is lower in those with type 2 diabetes mellitus

The overproduction of intestinal lipoproteins may contribute to the dyslipidemia found in diabetes. We studied the influence of diabetes on the fasting jejunal lipid content and its association with plasma lipids and the expression of genes involved in the synthesis and secretion of these lipoproteins. The study was undertaken in 27 morbidly obese persons, 12 of whom had type 2 diabetes mellitus (T2DM). The morbidly obese persons with diabetes had higher levels of chylomicron (CM) triglycerides (P < 0.001) and apolipoprotein (apo)B48 (P = 0.012). The jejunum samples obtained from the subjects with diabetes had a lower jejunal triglyceride content (P = 0.012) and angiopoietin-like protein 4 (ANGPTL4) mRNA expression (P = 0.043). However, the apoA-IV mRNA expression was significantly greater (P = 0.036). The jejunal triglyceride content correlated negatively with apoA-IV mRNA expression (r = -0.587, P = 0.027). The variables that explained the jejunal triglyceride content in a multiple linear regression model were the insulin resistance state and the apoA-IV mRNA expression. Our results show that the morbidly obese subjects with diabetes had lower jejunal lipid content and that this correlated negatively with apoA-IV mRNA expression. These findings show that the jejunum appears to play an active role in lipid homeostasis in the fasting state.

The role of fructose in the pathogenesis of NAFLD and the metabolic syndrome

Nonalcoholic fatty liver disease (NAFLD) is the most frequent liver disease worldwide, and is commonly associated with the metabolic syndrome. Secular trends in the prevalence of these diseases may be associated with the increased fructose consumption observed in the Western diet. NAFLD is characterized by two steps of liver injury: intrahepatic lipid accumulation (hepatic steatosis), and inflammatory progression to nonalcoholic steatohepatitis (NASH) (the 'two-hit' theory). In the first 'hit', hepatic metabolism of fructose promotes de novo lipogenesis and intrahepatic lipid, inhibition of mitochondrial beta-oxidation of long-chain fatty acids, triglyceride formation and steatosis, hepatic and skeletal muscle insulin resistance, and hyperglycemia. In the second 'hit', owing to the molecular instability of its five-membered furanose ring, fructose promotes protein fructosylation and formation of reactive oxygen species (ROS), which require quenching by hepatic antioxidants. Many patients with NASH also have micronutrient deficiencies and do not have enough antioxidant capacity to prevent synthesis of ROS, resulting in necroinflammation. We postulate that excessive dietary fructose consumption may underlie the development of NAFLD and the metabolic syndrome. Furthermore, we postulate that NAFLD and alcoholic fatty liver disease share the same pathogenesis.

Insulin acutely inhibits intestinal lipoprotein secretion in humans in part by suppressing plasma free fatty acids

OBJECTIVE

Intestinal lipoprotein production has recently been shown to be increased in insulin resistance, but it is not known whether it is regulated by insulin in humans. Here, we investigated the effect of acute hyperinsulinemia on intestinal (and hepatic) lipoprotein production in six healthy men in the presence and absence of concomitant suppression of plasma free fatty acids (FFAs).

RESEARCH DESIGN AND METHODS

Each subject underwent the following three lipoprotein turnover studies, in random order, 4-6 weeks apart: 1) insulin and glucose infusion (euglycemic-hyperinsulinemic clamp) to induce hyperinsulinemia, 2) insulin and glucose infusion plus Intralipid and heparin infusion to prevent the insulin-induced suppression of plasma FFAs, and 3) saline control.

RESULTS

VLDL1 and VLDL2-apoB48 and -apoB100 production rates were suppressed by 47-62% by insulin, with no change in clearance. When the decline in FFAs was prevented by concomitant infusion of Intralipid and heparin, the production rates of VLDL1 and VLDL2-apoB48 and -apoB100 were intermediate between insulin and glucose infusion and saline control.

CONCLUSIONS

This is the first demonstration in humans that intestinal apoB48-containing lipoprotein production is acutely suppressed by insulin, which may involve insulin's direct effects and insulin-mediated suppression of circulating FFAs.

Intestinal absorption of long-chain fatty acids: evidence and uncertainties

Over the two last decades, cloning of proteins responsible for trafficking and metabolic fate of long-chain fatty acids (LCFA) in gut has provided new insights on cellular and molecular mechanisms involved in fat absorption. To this systematic cloning period, functional genomics has succeeded in providing a new set of surprises. Disruption of several genes, thought to play a crucial role in LCFA absorption, did not lead to clear phenotypes. This observation raises the question of the real physiological role of lipid-binding proteins and lipid-metabolizing enzymes expressed in enterocytes. The goal of this review is to analyze present knowledge concerning the main steps of intestinal fat absorption from LCFA uptake to lipoprotein release and to assess their impact on health.

Postprandial lipid and apolipoprotein responses following three consecutive meals associate with liver fat content in type 2 diabetes and the metabolic syndrome

OBJECTIVE

Liver fat is associated with dyslipidemia following a fat load. Previous studies demonstrated that alimentary fat is temporarily retained within enterocytes and mobilized by subsequently ingested nutrients. As this potentially contributes to cumulative postprandial hyperlipidemia, we assessed postprandial lipoprotein changes and their association with liver fat following 3 consecutive meals during a 24 h period in males with type 2 diabetes, and men with the metabolic syndrome (MetS).

METHODS

Plasma lipids were measured in 14 type 2 diabetic, 14 MetS and 14 healthy age-matched males, following a standardized breakfast (t=0 h), lunch (t=4 h) and diner (t=8 h). Blood samples were collected before and at t=2, 4, 6, 8, 12, 16, 20 and 24 h following breakfast. Liver fat was measured by proton magnetic resonance spectroscopy.

RESULTS

Type 2 diabetic (mean age 55 (4.2) years; HbA1c 7.2 (1.1)%) and MetS men had similar BMI, waist, blood pressure and triglycerides. 24 h-AUC triglycerides, ApoB, and cholesterol-rich-remnants, but not ApoB-48, differed significantly among groups (calculated by ANOVA, all P<0.05). Liver fat was independently associated with 24 h-AUC triglycerides, ApoB and cholesterol-rich-remnants (r=0.57, P<0.001, r=0.38, P=0.017; r=0.48, P=0.002, respectively), but not with 24 h-AUC ApoB-48 (r=0.22, P=0.18).

CONCLUSIONS

In type 2 diabetes and the MetS exposure to 3 consecutive meals produced exaggerated 24 h triglyceride, ApoB and cholesterol-rich-remnant concentrations, which were closely associated with liver fat. Instead, ApoB-48 peak was delayed in type 2 diabetes, but not related to liver fat. In addition to liver fat, other mechanisms, including local intestinal processes, determine atherogenic postprandial lipoprotein changes following 3 consecutive meals during 24 h.

APOB-516 T allele homozygous subjects are unresponsive to dietary changes in a three-month primary intervention study targeted to reduce fat intake

Dietary guidelines aim to control fat intake and reduce cardiovascular risk but an important interindividual variability occurs among subjects. The objective was to investigate whether the response of lipid and glucose homeostasis parameters after a three-month diet aimed at reducing cardiovascular risk could be modulated by the -516C/T polymorphism in the apolipoprotein B gene (APOB). Middle-aged men (n = 69) and women (n = 100) with moderate cardiovascular disease risk were advised to reduce total energy and fat intakes and replace saturated dietary fat by monounsaturated and polyunsaturated fat. Subjects were genotyped for APOB-516C/T polymorphism. At the entry and at the end of the three-month period, fasting and postprandial plasma lipid analyses were performed. At entry, subjects homozygous for the APOB-516 T allele exhibited significantly lower fasting plasma concentrations of apolipoprotein B 48, triglycerides and triglyceride-rich lipoproteins-triglycerides compared to C carrier subjects. After the diet period, while C carrier subjects presented a clear improvement of most biological parameters, paradoxically T/T subjects did not modify them. In addition, the apoB 48 postprandial response after a standardized mixed test meal was not improved in T/T subjects after the three-month diet, contrary to C allele carriers. Even though their phenotype at entry does not show any significant increase of risk factors when compared to other groups, subjects homozygous for the APOB-516 T allele are unresponsive to a healthy diet that improves cardiovascular risk status in the whole population.

Methods to assess impaired post-prandial metabolism and the impact for early detection of cardiovascular disease risk

Post-prandial lipaemia has emerged as a key contributor to cardiovascular disease (CVD) risk and progression. Specifically, delayed clearance of chylomicrons (CM) and their remnants increase the delivery of triglyceride and cholesteryl ester to the vessel wall and can accelerate the progression of atherosclerosis, which may be particularly pertinent to individuals with insulin resistance and/or obesity. As the number of studies linking post-prandial metabolism and chronic disease increases, interest has grown in the use of parameters reflecting CM metabolism as a possible indicator of early CVD risk. This, in turn has raised the question of what method might be most appropriate to detect CM and their remnants in plasma accurately. However, the handful of techniques able to measure CM metabolism (triglyceride-rich lipoprotein fractions; remnant-lipoprotein cholesterol; retinyl esters, CM-like emulsion; sodium dodecyl sulphate-polyacrylamide gel electrophoresis; immunoblotting, enzyme-linked immunoabsorbent assays; C(13) breath test; capillary finger prick) differ in their specificity, cost and applicability in research and in the clinical setting. In this review, we explore the scientific and clinical implications of CM methodology to better understand early risk assessment of CVD. We raise ongoing issues of the need to appreciate differential separation of very low-density lipoprotein and CM fractions, as well as to identify the technical basis for imprecision between assays for apolipoprotein B48. We also highlight emerging issues with respect to the practicality of measuring post-prandial metabolism in large clinical studies and offer opinions on the appropriateness of existing techniques in this field.

Parallel increase of plasma apoproteins C-II and C-III in Type 2 diabetic patients

AIMS

To determine plasma levels of apoprotein (apo) C-II and apoprotein C-III in Type 2 diabetic patients and to examine the clinical and biological factors that are associated with elevated apoC concentrations.

METHODS

We measured apoC-II and apoC-III in total plasma and in non-high-density lipoprotein fractions by an immunoturbidimetric assay in 88 Caucasian Type 2 diabetic patients and in 138 healthy control subjects.

RESULTS

Plasma levels of both apoC-II and apoC-III were increased in Type 2 diabetic patients. The clinical conditions associated with an increase of plasma apoC-II and apoC-III were abdominal obesity, body mass index, poor glycaemic control and lack of insulin treatment. However, when multivariate analysis was used, plasma apoCs levels correlated with triglyceride levels only. The apoC-III/apoC-II ratio was similar in the Type 2 diabetic and control subjects.

CONCLUSIONS

Our study shows the parallel increase of apoC-II and C-III in Type 2 diabetic patients. This parallel increase is related to hypertriglyceridaemia only.

Overexpression of apolipoprotein A5 in mice is not protective against body weight gain and aberrant glucose homeostasis

Apolipoprotein A5 (APOA5) is expressed primarily in the liver and modulates plasma triglyceride levels in mice and humans. Mice overexpressing APOA5 exhibit reduced plasma triglyceride levels. Because there is a tight association between plasma triglyceride concentration and traits of the metabolic syndrome, we used transgenic mice overexpressing human APOA5 to test the concept that these mice would be protected from diet-induced obesity and insulin resistance. Male and female transgenic and wild-type mice on the FVB/N genetic background were fed standard rodent chow or a diet rich in fat and sucrose for 18 weeks, during which time clinical phenotypes associated with obesity and glucose homeostasis were measured. We found that APOA5 transgenic (A5tg) mice were resistant to diet-induced changes in plasma triglyceride but not total cholesterol levels. Body weights were similar between the genotypes for females and males, although male A5tg mice showed a modest but significant increase in the relative size of inguinal fat pads. Although male A5tg mice showed a significantly increased ratio of plasma glucose to insulin, profiles of glucose clearance as evaluated after injections of glucose or insulin failed to reveal any differences between genotypes. Overall, our data showed that there was no advantage to responses to diet-induced obesity with chronic reduction of plasma triglyceride levels as mediated by overexpression of APOA5.

Short-term adaptation of postprandial lipoprotein secretion and intestinal gene expression to a high-fat diet

Western diet is characterized by a hypercaloric and hyperlipidic intake, enriched in saturated fats, that is associated with the increased occurrence of metabolic diseases. To cope with this overload of dietary lipids, the intestine, which delivers dietary lipids to the body, has to adapt its capacity in lipid absorption and lipoprotein synthesis. We have studied the early effects of a high-fat diet (HFD) on intestinal lipid metabolism in mice. After 7 days of HFD, mice displayed normal fasting triglyceridemia but postprandial hypertriglyceridemia. HFD induced a decreased number of secreted chylomicrons with increased associated triglycerides. Secretion of larger chylomicrons was correlated with increased intestinal microsomal triglyceride transfer protein (MTP) content and activity. Seven days of HFD induced a repression of genes involved in fatty acid synthesis (FAS, ACC) and an increased expression of genes involved in lipoprotein assembly (apoB, MTP, and apoA-IV), suggesting a coordinated control of intestinal lipid metabolism to manage a high-fat loading. Of note, the mature form of the transcription factor SREBP-1c was increased and translocated to the nucleus, suggesting that it could be involved in the coordinated control of gene transcription. Activation of SREBP-1c was partly independent of LXR. Moreover, HFD induced hepatic insulin resistance whereas intestine remained insulin sensitive. Altogether, these results demonstrate that a short-term HFD is sufficient to impact intestinal lipid metabolism, which might participate in the development of dyslipidemia and metabolic diseases.

Omega 3 fatty acids induce a marked reduction of apolipoprotein B48 when added to fluvastatin in patients with type 2 diabetes and mixed hyperlipidemia: a preliminary report

Backgorund

Mixed hyperlipidemia is common in patients with diabetes. Statins, the choice drugs, are effective at reducing lipoproteins that contain apolipoprotein B100, but they fail to exert good control over intestinal lipoproteins, which have an atherogenic potential. We describe the effect of prescription omega 3 fatty acids on the intestinal lipoproteins in patients with type 2 diabetes who were already receiving fluvastatin 80 mg per day.

Methods

Patients with type 2 diabetes and mixed hyperlipidemia were recruited. Fasting lipid profile was taken when patients were treated with diet, diet plus 80 mg of fluvastatin and diet plus fluvastatin 80 mg and 4 g of prescription omega 3 fatty acids. The intestinal lipoproteins were quantified by the fasting concentration of apolipoprotein B48 using a commercial ELISA.

Results

The addition of 4 g of prescription omega 3 was followed by significant reductions in the levels of triglycerides, VLDL triglycerides and the triglyceride/HDL cholesterol ratio, and an increase in HDL cholesterol (P < 0.05). Fluvastatin induced a reduction of 26% in B100 (P < 0.05) and 14% in B48 (NS). However, the addition of omega 3 fatty acids enhanced this reduction to 32% in B100 (NS) and up to 36% in B48 (P < 0.05).

Conclusion

Our preliminary findings therefore suggest an additional benefit on postprandial atherogenic particles when omega 3 fatty acids are added to standard treatment with fluvastatin.

Intestinal lipid transport and chylomicron production: possible links to exacerbated atherogenesis in a rodent model of the metabolic syndrome

Post-prandial lipaemia is prevalent during conditions of obesity and insulin-resistance (IR), and has been associated with mediating the accelerated progression of cardiovascular disease (CVD). Our group has contributed to the concept that intestinally derived chylomicron lipoproteins are atherogenic and are associated with increased cholesterol accumulation in arterial vessels. More recently we have established the JCR:LA-cp rodent model of post-prandial dyslipidemia during conditions of the metabolic syndrome (MetS): including obesity, insulin-resistance and intimal atherogenesis. We have used this model as a novel physiological approach to investigate intestinal lipid transport and metabolism in the 'absorption-to-chylomicron secretion' axis, in the context of IR. The purpose of this review is to highlight recent preliminary data that has been collected using a range of different methodologies in this unique model of MetS. For the first time we report that the JCR:LA-cp rodent has over-production of intestinal chylomicrons and that this is associated with intestinal villus hypertrophy. We have also observed that vascular re-modelling associated with increased arterial accumulation of atherogenic lipoproteins is evident in this model. We discuss our findings in the context of a void of knowledge in the understanding of intestinal lipid metabolism, and the potential significance of these pathways in contributing to dyslipidemia in MetS.

Triglyceride-rich lipoprotein-associated apolipoprotein C-III production is stimulated by plasma free fatty acids in humans

OBJECTIVE

Insulin resistant states are associated with increased fatty acid flux to liver and intestine, which stimulates the production of triglyceride-rich lipoproteins (TRL). ApoC-III production and plasma and TRL concentrations are increased in insulin resistance and may contribute to the hypertriglyceridemia of these conditions. The mechanism underlying that increase is not known, but because apoC-III and VLDL production are closely linked we hypothesized that FFAs may stimulate TRL apoC-III production.

METHODS AND RESULTS

We used Intralipid/heparin (IH) to raise plasma FFA in 12 healthy men in the fed state, and stable isotopes to examine apoC-III metabolism. TRL apoC-III concentration was significantly higher in the IH study, and this increase was associated with higher production (PR) and fractional catabolic rate (FCR). The increase in production was greater than in FCR (90% versus 30%, respectively), accounting for the elevated concentration. Glycerol infusion had no effect on apoC-III concentration, PR, or FCR compared to saline, indicating that the effect was not attributable to glycerol released from intralipid.

CONCLUSIONS

These findings confirm that TRL apoC-III production is stimulated by an acute elevation of plasma FFAs, suggesting a novel regulatory pathway that may play a role in the overproduction of TRL apoC-III in insulin resistant states.

Intestinal lipoprotein overproduction in insulin-resistant states

PURPOSE OF REVIEW

Excessive postprandial lipemia is highly prevalent in obese and insulin-resistant/type 2 diabetic individuals and substantially increases the risk of atherosclerosis and cardiovascular disease. This article will review our current understanding of the link between insulin resistance and intestinal lipoprotein overproduction and highlight some of the key recent findings in the field.

RECENT FINDINGS

Emerging evidence from several animal models of insulin resistance as well as insulin-resistant humans clearly supports the link between insulin resistance and aberrant intestinal lipoprotein metabolism. In insulin-resistant states, elevated free fatty acid flux into the intestine, downregulation of intestinal insulin signaling and upregulation of microsomal triglyceride transfer protein all appear to stimulate intestinal lipoprotein production. Gut peptides, GLP-1 and GLP-2, may be important regulators of intestinal lipid absorption and lipoprotein production.

SUMMARY

Available evidence in humans and animal models strongly favors the concept that the small intestine is not merely an absorptive organ but rather plays an active role in regulating the rate of production of triglyceride-rich lipoproteins. Metabolic signals in insulin resistance and type 2 diabetes and in some cases an aberrant intestinal response to these factors all contribute to the enhanced formation and secretion of triglyceride-rich lipoproteins.

Dissociation between the insulin-sensitizing effect of rosiglitazone and its effect on hepatic and intestinal lipoprotein production

CONTEXT

Despite its potent, well-documented insulin-sensitizing effects, rosiglitazone (RSG) does not effectively ameliorate the hypertriglyceridemia of insulin-resistant or diabetic individuals and has even been shown to slightly but significantly increase triglyceride-rich lipoproteins (TRL) in some studies. The mechanism of this effect is currently not known.

OBJECTIVE

We investigated the effect of RSG treatment on TRL metabolism.

DESIGN

This was a 12-wk, single-sequence, cross-over study of rosiglitazone vs. placebo for 6 wk.

PARTICIPANTS

Participants included 17 nondiabetic men with a broad range of insulin sensitivity.

INTERVENTION

INTERVENTION included rosiglitazone 8 mg/d vs. placebo for 6 wk.

MAIN OUTCOME MEASURE

TRL metabolism (concentration, production and catabolic rates) was assessed in a constant fed state with a 12-h primed constant infusion of [D3]l-leucine and multicompartmental modeling.

RESULTS

RSG treatment resulted in significant insulin sensitization with no change in body weight. Fasting plasma triglyceride (TG) concentration, however, was higher with RSG vs. placebo (P = 0.0006), as were fasting and fed TRL-TG, TRL-apoB-48, and TRL-apoB-100 (fed TRL-apoB-48: 0.93 +/- 0.08 vs. 0.76 +/- 0.07 mg/dl, P =0.017, and fed TRL-apoB-100: 15.57 +/- 0.90 vs. 13.71 +/- 1.27 mg/dl, P = 0.029). This small but significant increase in plasma TRL concentration was explained by a tendency for RSG to increase TRL production and reduce particle clearance, as indicated by the significantly increased production to clearance ratios for both apoB-48-containing (0.43 +/- 0.03 vs. 0.34 +/- 0.03, P = 0.048) and apoB-100-containing (7.0 +/- 0.4 vs. 6.2 +/- 0.6, P = 0.029) TRL.

CONCLUSION

These data indicate dissociation between the insulin-sensitizing effects of RSG and absence of anticipated reductions in production rates of apoB-100- and apoB-48-containing-TRL particles, which may explain the absence of TG lowering seen in humans treated with this agent.

Both intestinal and hepatic lipoprotein production are stimulated by an acute elevation of plasma free fatty acids in humans

BACKGROUND

Hepatic lipoprotein production has been shown previously to be regulated by free fatty acid (FFA) flux to the liver, whereas intestinal lipoprotein production is stimulated mainly by ingested fat absorbed from the intestinal lumen. Emerging evidence indicates that intestinal lipoprotein production is increased in insulin resistance and type 2 diabetes mellitus, conditions that are associated with increased levels of circulating FFAs. Here we investigated whether short-term elevation of plasma FFAs stimulates intestinal apolipoprotein (apo) B-48- and hepatic apoB-100-containing triglyceride-rich lipoprotein (TRL) production in humans in the fed state.

METHODS AND RESULTS

TRL apoB-48 and apoB-100 metabolism were examined in 12 healthy men during a constant fed state. The studies were as follows, respectively: (1) Intralipid/heparin was infused intravenously immediately before and during the kinetics study to induce an approximately 3-fold difference in plasma FFA compared with the saline study; (2) saline was infused intravenously as a control. ApoB-48- and apoB-100-containing TRL production and clearance were determined with a 12-hour primed constant infusion of [D3]L-leucine and multicompartmental kinetic modeling. TRL apoB-48 production rate was 69% higher in the Intralipid/heparin study than in the saline control (5.95+/-1.13 versus 3.53+/-0.58 mg/kg per day; P=0.027), and there was no significant difference in TRL apoB-48 clearance. TRL apoB-100 concentrations were also increased (P<0.001) and TRL apoB-100 production rate was 35% higher in the Intralipid/heparin study compared with saline (28+/-4 versus 21+/-3 mg/kg per day; P=0.020).

CONCLUSIONS

This is the first study to demonstrate that intestinal TRL apoB-48 production is increased after short-term elevation of plasma FFAs in humans in the fed state, similar to the well-described stimulation of hepatic TRL apoB100-containing particles by FFAs.

Both intestinal and hepatic lipoprotein production are stimulated by an acute elevation of plasma free fatty acids in humans

BACKGROUND

Hepatic lipoprotein production has been shown previously to be regulated by free fatty acid (FFA) flux to the liver, whereas intestinal lipoprotein production is stimulated mainly by ingested fat absorbed from the intestinal lumen. Emerging evidence indicates that intestinal lipoprotein production is increased in insulin resistance and type 2 diabetes mellitus, conditions that are associated with increased levels of circulating FFAs. Here we investigated whether short-term elevation of plasma FFAs stimulates intestinal apolipoprotein (apo) B-48- and hepatic apoB-100-containing triglyceride-rich lipoprotein (TRL) production in humans in the fed state.

METHODS AND RESULTS

TRL apoB-48 and apoB-100 metabolism were examined in 12 healthy men during a constant fed state. The studies were as follows, respectively: (1) Intralipid/heparin was infused intravenously immediately before and during the kinetics study to induce an approximately 3-fold difference in plasma FFA compared with the saline study; (2) saline was infused intravenously as a control. ApoB-48- and apoB-100-containing TRL production and clearance were determined with a 12-hour primed constant infusion of [D3]L-leucine and multicompartmental kinetic modeling. TRL apoB-48 production rate was 69% higher in the Intralipid/heparin study than in the saline control (5.95+/-1.13 versus 3.53+/-0.58 mg/kg per day; P=0.027), and there was no significant difference in TRL apoB-48 clearance. TRL apoB-100 concentrations were also increased (P<0.001) and TRL apoB-100 production rate was 35% higher in the Intralipid/heparin study compared with saline (28+/-4 versus 21+/-3 mg/kg per day; P=0.020).

CONCLUSIONS

This is the first study to demonstrate that intestinal TRL apoB-48 production is increased after short-term elevation of plasma FFAs in humans in the fed state, similar to the well-described stimulation of hepatic TRL apoB100-containing particles by FFAs.

Differential effect of fenofibrate and atorvastatin on in vivo kinetics of apolipoproteins B-100 and B-48 in subjects with type 2 diabetes mellitus with marked hypertriglyceridemia

The specific impact of 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitors and fibrates on the in vivo metabolism of apolipoprotein (apo) B has not been systematically investigated in patients with type 2 diabetes mellitus with high plasma triglyceride (TG) levels. Therefore, the objective of this 2-group parallel study was to examine the differential effects of a 6-week treatment with atorvastatin or fenofibrate on in vivo kinetics of apo B-48 and B-100 in men with type 2 diabetes mellitus with marked hypertriglyceridemia. Apolipoprotein B kinetics were assessed at baseline and at the end of the intervention using a primed constant infusion of [5,5,5-D(3)]-l-leucine for 12 hours in the fed state. Fenofibrate significantly decreased plasma TG levels with no significant change in plasma low-density lipoprotein cholesterol (LDL-C) and apo B levels. On the other hand, atorvastatin significantly reduced plasma levels of TG, LDL-C, and apo B. After treatment with fenofibrate, very low-density lipoprotein (VLDL) apo B-100 pool size (PS) was decreased because of an increase in the fractional catabolic rate (FCR) of VLDL apo B-100. No significant change was observed in the kinetics of LDL apo B-100. Moreover, fenofibrate significantly decreased TG-rich lipoprotein (TRL) apo B-48 PS because of a significant increase in TRL apo B-48 FCR. After treatment with atorvastatin, VLDL and IDL apo B-100 PSs were significantly decreased because of significant elevations in the FCR of these subfractions. Low-density lipoprotein apo B-100 PS was significantly lowered because of a tendency toward decreased LDL apo B-100 production rate (PR). Finally, atorvastatin reduced TRL apo B-48 PS because of a significant decrease in the PR of this subfraction. These results indicate that fenofibrate increases TRL apo B-48 as well as VLDL apo B-100 clearance in men with type 2 diabetes mellitus with marked hypertriglyceridemia, whereas atorvastatin increases both VLDL and IDL apo B-100 clearance and decreases TRL apo B-48 and LDL apo B-100 PR.

Association of liver disease with postprandial large intestinal triglyceride-rich lipoprotein accumulation and pro/antioxidant imbalance in normolipidemic non-alcoholic steatohepatitis

BACKGROUND

Dietary fat excess and antioxidant deficiency, altered lipid metabolism, and increased lipoperoxidation have been associated with non-alcoholic steatohepatitis (NASH), but the relative importance of each of these factors is unclear.

AIMS

To assess acute intestinal and hepatic very-low-density lipoprotein (VLDL) subfraction metabolism, lipid peroxidation, and pro/antioxidant imbalance after a fat load in NASH.

METHODS

Dietary habits, circulating adipokines, fasting and postprandial lipids, intestinal and hepatic VLDL, oxidized low-density lipoproteins (oxLDL), and total antioxidant status (TAS) were correlated to postprandial liver enzymes and to liver histology in 28 non-obese non-diabetic normolipidemic patients with NASH and 28 healthy controls.

RESULTS

Despite similar fasting profiles, NASH had more pronounced intestinal and hepatic VLDL1 accumulation, LDL lipid peroxidation and TAS fall postprandially. Postprandial intestinal VLDL1 independently predicted oxLDL and TAS responses in NASH. In NASH, hepatic steatosis was independently associated with postprandial intestinal VLDL1 and TAS; necroinflammation with postprandial serum gamma-glutamyltransferase, oxLDL and TAS responses; and fibrosis with adiponectin and postprandial TAS and oxLDL responses.

CONCLUSIONS

Postprandial intestinal VLDL1 accumulation is associated with a pro-oxidant imbalance in normolipidemic non-diabetic NASH, and both correlate with the severity of liver disease. Modulating postprandial lipoprotein metabolism may be beneficial in NASH, even if normolipidemic.

Effects of a 1,3-diacylglycerol oil-enriched diet on postprandial lipemia in people with insulin resistance

Postprandial hypertriglyceridemia is common in individuals with insulin resistance, and diets enriched in 1,3-diacylglycerol (DAG) may reduce postprandial plasma triglycerides (PPTGs). We enrolled 25 insulin-resistant, nondiabetic individuals in a double-blind, randomized crossover trial to test the acute and chronic effects of a DAG-enriched diet on PPTG. Participants received either DAG or triacylglycerol (TAG) oil, in food products, for 5 weeks. Fasting lipids, and two separate postprandial tests, one with DAG oil and one with TAG oil, were performed at the end of each 5 week diet period. We found no acute or chronic effects of DAG oil on PPTG. Thus, neither the DAG oil PPTG (h/mg/dl) on a chronic TAG diet [area under the curve (AUC) = 503 +/- 439] nor the TAG oil PPTG on a chronic DAG diet (AUC = 517 +/- 638) was different from the TAG oil PPTG on a chronic TAG diet (AUC = 565 +/- 362). Five weeks of a DAG-enriched diet had no acute or chronic effects on PPTG in insulin-resistant individuals. We suggest further studies to evaluate the effects of DAG on individuals with low and high TG levels.

Remarkable increase of apolipoprotein B48 level in diabetic patients with end-stage renal disease

Apolipoprotein (apo) B48 is a structural protein of chylomicrons. Fasting serum levels of apoB48 suggest the presence of small number of remnant chylomicron particles which are thought to be an atherogenic lipoprotein. In view of the high incidence of coronary heart disease (CHD) in patients with diabetic nephropathy, we decided to measure the plasma apoB48 level in type 2 diabetics with diabetic nephropathy at various stages to ascertain how apoB48 relates to the progression of diabetic nephropathy. Patients with type 2 diabetes (n=105) were stratified into four groups: normo-albuminuria, micro-albuminuria, overt-proteinuria, and patients with end-stage renal disease (ESRD) receiving hemodialysis. Age-matched-diabetic hypertensive patients (n=24) and non-diabetic ESRD patients on hemodialysis (n=47) were also enrolled. Plasma triglyceride (TG) levels rose as diabetic nephropathy progressed to overt-proteinuria. No further elevation in TG was observed in diabetic ESRD, however, and the TG levels were normal in non-diabetic ESRD. A similar pattern was observed for remnant-like particle-cholesterol (RLP-C). In contrast to the changes observed for TG and RLP-C, the levels of apoB48 increased steadily as the diabetic nephropathy progressed (control, 3.7; normo, 5.7; micro, 6.9; overt, 10.6 mg/l, respectively). ApoB48 peaked in the diabetic ESRD (19 mg/l) and was also markedly elevated in non-diabetic ESRD (10.1mg/l). The apoB48/TG and apoB48/total-apoB ratios were substantially elevated in both diabetic and non-diabetic ESRD. These results are the first to demonstrate remarkable elevations of plasma apoB48 in patients with both diabetic and non-diabetic ESRD. The remarkably high level of apoB48 in diabetic ESRD seems to be attributable to dyslipidemia induced by both diabetic nephropathy and ESRD.

Evidence of increased secretion of apolipoprotein B-48-containing lipoproteins in subjects with type 2 diabetes

Patients with type 2 diabetes have high levels of triglyceride-rich lipoproteins (TRLs), including apolipoprotein B-48 (apoB-48)-containing TRLs of intestinal origin, but the mechanism leading to overaccumulation of these lipoproteins remains to be fully elucidated. Therefore, the objective of this study was to examine the in vivo kinetics of TRL apoB-48 and VLDL, intermediate density lipoprotein (IDL), and LDL apoB-100 in type 2 diabetic subjects (n = 11) and nondiabetic controls (n = 13) using a primed-constant infusion of l-[5,5,5-D(3)]leucine for 12 h in the fed state. Diabetic subjects had significantly higher fasting glycemia, higher fasting insulinemia, higher plasma triglyceride, and lower HDL-cholesterol levels than controls. Compared with controls, diabetic subjects had increased TRL apoB-48, VLDL apoB-100, and IDL apoB-100 pool sizes as a result of increased production rates (PRs) and reduced fractional catabolic rates of these lipoprotein subfractions. Furthermore, multiple linear regression analyses revealed that the diabetic/control status was an independent predictor of TRL apoB-48 PR and represented nearly 35% of its variance. These results suggest that the overaccumulation of TRLs seen in patients with type 2 diabetes is attributable to increased PRs of both intestinally derived apoB-48-containing lipoproteins and TRL apoB-100 of hepatic origin and to decreased catabolism of these subfractions.

Tumor necrosis factor-alpha induces intestinal insulin resistance and stimulates the overproduction of intestinal apolipoprotein B48-containing lipoproteins

There is growing evidence suggesting intestinal insulin resistance and overproduction of apolipoprotein (apo) B48-containing chylomicrons in insulin-resistant states. In the current study, we investigated the potential role of the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) in the development of insulin resistance and aberrant lipoprotein metabolism in the small intestine in a Syrian golden hamster model. TNF-alpha infusion decreased whole-body insulin sensitivity, based on in vivo euglycemic clamp studies in chow-fed hamsters. Analysis of intestinal tissue in TNF-alpha-treated hamsters indicated impaired phosphorylation of insulin receptor-beta, insulin receptor substrate-1, Akt, and Shc and increased phosphorylation of p38, extracellular signal-related kinase-1/2, and Jun NH(2)-terminal kinase. TNF-alpha infusion also increased intestinal production of total apoB48, triglyceride-rich lipoprotein apoB48, and serum triglyceride levels in both fasting and postprandial (fat load) states. The effects of TNF-alpha on plasma apoB48 levels could be blocked by the p38 inhibitor SB203580. Ex vivo experiments using freshly isolated enterocytes also showed TNF-alpha-induced p38 phosphorylation and intestinal apoB48 overproduction, effects that could be blocked by SB203580. Interestingly, TNF-alpha increased the mRNA and protein mass of intestinal microsomal triglyceride transfer protein without altering apoB mRNA levels. Enterocytes were found to have detectable levels of both TNF-alpha receptor types (p55 and p75), and antibodies against either of the two TNF-alpha receptors partially blocked the stimulatory effect of TNF-alpha on apoB48 production and p38 phosphorylation. In summary, these data suggest that intestinal insulin resistance can be induced in hamsters by TNF-alpha infusion, and it is accompanied by intestinal overproduction of apoB48-containing lipoproteins. TNF-alpha-induced stimulation of intestinal lipoprotein production appears to be mediated via TNF-alpha receptors and the p38 mitogen-activated protein kinase pathway.

A moderate-fat diet for combined hyperlipidemia and metabolic syndrome

A low-fat diet is recommended for hyperlipidemia. However, low-density lipoprotein (LDL) responses depend on the type of hyperlipidemia (ie, simple hypercholesterolemia or combined hyperlipidemia). In combined hyperlipidemia, which is typical of patients with metabolic syndrome, LDL levels are only one third as responsive to fat and cholesterol as simple hypercholesterolemia. The diminished dietary sensitivity of combined hyperlipidemia is explained by diminished intestinal absorption of cholesterol, a feature of metabolic syndrome. In turn, combined hyperlipidemia is caused by heightened lipid secretion by the liver. A moderate-fat, moderate-carbohydrate diet employing allowable fats has the promise of reducing endogenous lipoprotein production in combined hyperlipidemia. Triglyceride, LDL, and small-dense LDL should be lower, and high-density lipoprotein, apoprotein A-I, and buoyant LDL should be higher. A test of this dietary strategy on lipoproteins and downstream benefits on inflammatory mediators, oxidative stress, and vascular reactivity is now underway.