Objective-We sought to compare the synthesis and metabolism of VLDL 1 and VLDL 2 in patients with type 2 diabetes mellitus (DM2) and nondiabetic subjects. Methods and Results-We used a novel multicompartmental model to simultaneously determine the kinetics of apolipoprotein (apo) B and triglyceride (TG) in VLDL 1 and VLDL 2 after a bolus injection of [ 2 H 3 ]leucine and [ 2 H 5 ]glycerol and to follow the catabolism and transfer of the lipoprotein particles. Our results show that the overproduction of VLDL particles in DM2 is explained by enhanced secretion of VLDL 1 apoB and TG. Direct production of VLDL 2 apoB and TG was not influenced by diabetes per se. The production rates of VLDL 1 apoB and TG were closely related, as were the corresponding pool sizes. VLDL 1 and VLDL 2 compositions did not differ in subjects with DM2 and controls, and the TG to apoB ratio of newly synthesized particles was very similar in the 2 groups. Plasma glucose, insulin, and free fatty acids together explained 55% of the variation in VLDL 1 TG production rate. Conclusion-Insulin resistance and DM2 are associated with excess hepatic production of VLDL 1 particles similar in size and composition to those in nondiabetic subjects. We propose that hyperglycemia is the driving force that aggravates overproduction of VLDL 1 in DM2. Key Words: diabetes Ⅲ dyslipidemia Ⅲ VLDL Ⅲ apolipoprotein B Ⅲ triglycerides Ⅲ compartmental modeling Ⅲ kinetics Ⅲ stable isotope B y 2025, Ͼ300 million people worldwide will have type 2 diabetes mellitus (DM2). Because atherosclerosis is an important complication of DM2, this will contribute significantly to an expected increase in cardiovascular disease worldwide. 1 One important cardiovascular risk factor associated with DM2 is a dyslipidemia characterized by high levels of triglyceride (TG)-rich VLDL, low levels of HDL cholesterol, small, dense LDL, and impaired and prolonged postprandial hyperlipidemia. 2 These abnormalities are present for years before DM2 is diagnosed clinically.The discovery of heterogeneity within the major lipoprotein classes (VLDL, LDL, and HDL) has opened new avenues to identify specific perturbations of diabetic dyslipidemia. 3 VLDL particles secreted from the liver vary in size and composition and can be classified by their density (0.94 to 1.06 g/mL), diameter (20 to 75 nm), and flotation [Svedberg flotation rate (Sf) 20 to 400]. VLDL can be separated into 2 main classes: large, buoyant VLDL 1 particles (Sf 60 to 400) and small, dense VLDL 2 particles (Sf 20 to 60). VLDL 1 particles contain more TG than VLDL 2 particles and are rich in apolipoprotein (apo) CIII and apoE. 4 Large VLDL 1 particles are the major subclass of endogenous TG-rich lipoproteins and seem to be the major determinant of the plasma TG concentration in normolipidemic subjects. 5 Although elevation of plasma TG is a consistent feature of diabetic dyslipidemia, little attention has focused on the VLDL subclass distribution in DM2. However, emerging data indicate a higher increase of VLDL 1 particles than of VL...

The use of stable isotopes in conjunction with compartmental modeling analysis has greatly facilitated studies of the metabolism of the apolipoprotein B (apoB)-containing lipoproteins in humans. The aim of this study was to develop a multicompartment model that allows us to simultaneously determine the kinetics of apoB and triglyceride (TG) in VLDL 1 and VLDL 2 after a bolus injection of [ 2 H 3 ]leucine and [ 2 H 5 ]glycerol and to follow the catabolism and transfer of the lipoprotein particles. Here, we describe the model and present the results of its application in a fasting steadystate situation in 17 subjects with lipid values representative of a Western population. Analysis of the correlations showed that plasma TG was determined by the VLDL 1 and VLDL 2 apoB and TG fractional catabolic rate. Furthermore, the model showed a linear correlation between VLDL 1 TG and apoB production. A novel observation was that VLDL TG entered the circulation within 21 min after its synthesis, whereas VLDL apoB entered the circulation after 33 min. These observations are consistent with a sequential assembly model of VLDL and suggest that the TG is added to a primordial apoB-containing particle in the liver. Regulation of the metabolism of VLDL subfractions has been an area of active interest that received fresh impetus from the introduction of stable isotope-based techniques in the late 1980s (1, 2). The use of tracer models has generated direct information on lipoprotein synthetic rates, which previously could only be inferred from the turnover of radiolabeled lipoproteins. One common approach is to inject a bolus of radioactive tracer, such as [ 3 H, 14 C]glycerol, and determine the subsequent monoexponential slope of the decline in plasma VLDL-specific radioactivity. A disadvantage of this approach is that it can underestimate the true VLDL turnover rate because it does not account for recycling of the injected bolus of tracer (3). Multicompartmental modeling improves the accuracy by attempting to account for tracer recycling (3-8). Such studies have revealed that VLDL 1 apolipoprotein B-100 (apoB-100) production and VLDL 2 apoB-100 production are independently regulated (9-11), indicating that regulatory steps in the assembly of VLDL govern the lipid content of the secreted particles. However, it is still unclear how the liver regulates the triglyceride (TG) content of VLDL particles to produce large VLDL 1 or small VLDL 2 . VLDL assembly is thought to involve at least two steps in which nascent VLDL particles are formed and then TG is added, resulting in larger particles (12,13).Several studies have analyzed VLDL TG turnover kinetics using stable isotopically labeled glycerol or palmitate tracers and mathematical modeling. However, VLDL subclasses were not analyzed in those studies, and VLDL apoB was not included in the models (3,14,15). To enhance our understanding of the pathways leading to VLDL 1 and VLDL 2 and of the metabolic fate of these particles, we developed for the first time a multicompartmental m...

We performed a genomewide scan for genes that predispose to low serum HDL cholesterol (HDL-C) in 25 well-defined Finnish families that were ascertained for familial low HDL-C and premature coronary heart disease. The potential loci for low HDL-C that were identified initially were tested in an independent sample group of 29 Finnish families that were ascertained for familial combined hyperlipidemia (FCHL), expressing low HDL-C as one component trait. The data from the previous genome scan were also reanalyzed for this trait. We found evidence for linkage between the low-HDL-C trait and three loci, in a pooled data analysis of families with low HDL-C and FCHL. The strongest statistical evidence was obtained at a locus on chromosome 8q23, with a two-point LOD score of 4.7 under a recessive mode of inheritance and a multipoint LOD score of 3.3. Evidence for linkage also emerged for loci on chromosomes 16q24.1-24.2 and 20q13.11, the latter representing a recently characterized region for type 2 diabetes. Besides these three loci, loci on chromosomes 2p and 3p showed linkage in the families with low HDL-C and a locus on 2ptel in the families with FCHL.

Several genomewide screens have been performed to identify novel loci predisposing to unfavorable serum lipid levels and coronary heart disease (CHD). We hypothesized that the accumulating data of these screens in different study populations could be combined to verify which of the identified loci truly harbor susceptibility genes. The power of this strategy has recently been demonstrated with other complex diseases, such as inflammatory bowel disease and asthma. We assessed the largely unknown genetic background of CHD by investigating the most common dyslipidemia predisposing to CHD, familial combined hyperlipidemia (FCHL), affecting 1%-2% of Western populations and 10%-20% of families with premature CHD. To be able to perform a combined data analysis, we unified the diagnostic criteria for FCHL and its component traits and combined the data from two genomewide scans performed in two populations, the Finns and the Dutch. As a result of our pooled data analysis, we identified three chromosomal regions, on chromosomes 2p25.1, 9p23, and 16q24.1, exceeding the statistical significance level of a LOD score>2.0. The 2p25.1 region was detected for the FCHL trait, and the 9p23 and 16q24.1 regions were detected for the low high-density lipoprotein cholesterol (HDL-C) trait. In addition, the previously recognized 1q21 region also obtained additional support in the other study sample, when the triglyceride trait was used. Analysis of the 16q24.1 region resulted in a statistically significant LOD score of 3.6 when the data from Finnish families with low HDL-C were included in the analysis. To search for the underlying gene in the 16q24.1 region, we investigated a novel functional and positional candidate gene, helix/forkhead transcription factor (FOXC2), by sequencing and by genotyping of two single-nucleotide polymorphisms in the families.