Defective chylomicron synthesis as a cause of delayed particle clearance in diabetes?

Chylomicron metabolism is abnormal in diabetes and the chylomicron particle may play a very important role in atherosclerosis. The aim of this study was to examine the effect of diabetes on the metabolism of chylomicrons in cholesterol-fed alloxan diabetic and nondiabetic rabbits. Five diabetic rabbits and 5 control rabbits were given [14C]linoleic acid and [3H]cholesterol by gavage. Lymph was collected following cannulation of the lymph duct and radiolabelled chylomicrons were isolated by ultracentrifugation. The chylomicrons from each animal were injected into paired control and diabetic recipients. Lymph apolipoprotein (apo) B48, apo B100, and apo E were measured using sodium dodecyl sulfate-polyacrylamide gradient gel electrophoresis. Mean blood sugar of the diabetic donors and diabetic recipients were 19.7 +/- 2.3 and 17.2 +/- 3.2 mmol/L. Diabetic rabbits had significantly raised plasma triglyceride (10.8 +/- 13.9 versus 0.8 +/- 0.5 mmol/L, P < 0.02). There was a large increase in apo B48 in lymph chylomicrons in the diabetic donor animals (0.19 +/- 0.10 versus 0.04 +/- 0.02 mg/h, P < 0.01) and apo B100 (0.22 +/- 0.15 versus 0.07 +/- 0.07 mg/h, P < 0.05) and a reduction in apo E on the lymph chylomicron particle (0.27 +/- 0.01 versus 0.62 +/- 0.07 mg/mg apo B, P < 0.001). Diabetic recipients cleared both control and diabetic chylomicron triglyceride significantly more slowly than control recipients (P < 0.05). Clearance of control chylomicron cholesterol was delayed when injected into diabetic recipients compared to when these chylomicrons were injected into control recipients (P < 0.005). Clearance of diabetic chylomicron cholesterol was significantly slower when injected into control animals compared to control chylomicron injected into control animals (P < 0.02). In this animal model of atherosclerosis, we have demonstrated that diabetes leads to the production of an increased number of lipid and apo E-deficient chylomicron particles. Chylomicron particles from the control animals were cleared more slowly by the diabetic recipient (both triglyceride and cholesterol). The chylomicron particles obtained from the diabetic animals were cleared even more slowly when injected into the diabetic recipient. Although there was an initial delay in clearance of chylomicron triglyceride from the diabetic particle when injected into the control animals, the clearance over the first 15 minutes was not significantly different when compared to the control chylomicron injected into the control animal. On the other hand, the cholesterol clearance was significantly delayed. Thus, diabetes resulted in the production of an increased number of lipid- and apo E-deficient chylomicron particles. These alterations account, in part, for the delay in clearance of these particles.     

Genes that affect cholesterol synthesis, cholesterol absorption, and chylomicron assembly: the relationship between the liver and intestine in control and streptozotosin diabetic rats

Chylomicrons and very low-density lipoproteins (VLDLs) are abnormal in diabetes. The aim of this study was to compare the expression of Niemann-Pick C1-like1 (NPC1L1), adenosine triphosphate-binding cassette (ABC) proteins G5 and G8, microsomal triglyceride transfer protein (MTP), and 3-hydroxy-3-methylglutaryl coenzyme A (HMGCoA) reductase in the fasting and fed states in nondiabetic Sprague-Dawley rats fed a high-fat/cholesterol diet and to examine the messenger RNA (mRNA) expression of these proteins in the liver and intestine of diabetic and control animals using streptozotosin diabetic cholesterol-fed rats. Chylomicron and VLDL concentrations were significantly lower after a 12-hour fast in fasted compared with fed rats (P < .02). There was no change with fasting in mRNA expression of any of the genes in the intestine, but MTP level was significantly lower in the liver after the 12-hour fast (P < .01). There was a positive correlation between intestinal NPC1L1 mRNA and chylomicron cholesterol (P < .01) and between hepatic NPC1L1 mRNA and VLDL cholesterol (P < .01). The diabetic rats had significantly higher chylomicron and VLDL cholesterol, triglyceride, and apolipoprotein B-48 and B-100 levels compared with control rats (P < .0001). They had significantly increased NPC1L1 and MTP mRNA in both liver and intestine (P < .05 and P < .0005, respectively), and ABCG5 and ABCG8 mRNA were significantly reduced (P < .05). HMGCoA reductase mRNA was increased in diabetic animals (P < .01). In conclusion, fasting intestinal gene expression reflects the fed state. In diabetes, intestinal and hepatic gene expression correlates with abnormalities in chylomicron and VLDL cholesterol.     

Postprandial apolipoprotein B48-and B100-containing lipoproteins in type 2 diabetes: do statins have a specific effect on triglyceride metabolism?

There is little information about the effect of an alteration of low-density lipoprotein (LDL) turnover on chylomicron and very-low-density lipoprotein (VLDL) metabolism, yet chylomicron remnant particles are thought to be particularly atherogenic. This study examined the effect of inhibition of cholesterol synthesis on postprandial lipoproteins. Eight type 2 diabetic patients were examined before treatment with the 3-hydroxy-3-methyl glutaryl coenzyme A (HMGCoA) reductase inhibitor cerivastatin, after 4 weeks on active treatment, and 4 weeks after stopping treatment. On each occasion, blood was collected fasting and at 2-hour intervals for up to 8 hours after a high-fat meal. Chylomicrons and VLDLs were isolated by sequential ultracentrifugation. Compositional analysis was performed including the measurement of apolipoprotein B48 (apo B48) and apo B100 using polyacrylamide gradient gel electrophoresis. During statin treatment, there was a significant reduction in the postprandial chylomicron apo B48 area under the curve (AUC) from 23 +/- 16 to 17 +/- 10 (P < .01) and apo B100 in the chylomicron fraction from 166 +/- 148 to 70 +/- 70 (P < .05). Postprandial cholesterol (362 +/- 193 to 74 +/- 39, P < .005), triglyceride (2,222 +/- 1,440 to 746 +/- 329), and phospholipid (518 +/- 267 to 205 +/- 94) also decreased (P < .005). In the VLDL fraction, the postprandial cholesterol and triglyceride AUC were significantly reduced by statin (316 +/- 228 to 171 +/- 78, P < .05, and 1,733 +/- 833 to 857 +/- 468, P < .02, respectively). Four weeks after cessation of treatment, the chylomicron fraction triglyceride AUC had returned to the pretreatment level, but postprandial chylomicron cholesterol and VLDL cholesterol, triglyceride, and phospholipid were significantly lower than baseline (P < .05). Plasma total cholesterol and LDL cholesterol were significantly reduced with treatment (6.2 +/- 0.5 to 4.3 +/- 1.0 mmol/L, P < .001, and 4.5 +/- 0.4 to 2.8 +/- 1.0 mmol/L, P < .01, respectively) and returned to baseline following cessation of treatment. Fasting plasma triglycerides decreased significantly on treatment (2.4 +/- 1.0 to 1.7 +/- 0.2 mmol/L, P < .05) but remained significantly lower than baseline 4 weeks later (1.8 +/- 0.3 mmol/L, P < .05). This study suggests major postprandial lipoprotein changes on statin therapy which may account, in part, for the beneficial effects of statins in the prevention of myocardial infarction.     

Microsomal triglyceride transfer protein: does insulin resistance play a role in the regulation of chylomicron assembly?

We have previously demonstrated that diabetes is associated with an increase in intestinal microsomal triglyceride transfer protein (MTP) mRNA in both the rat and rabbit models. The present study was designed to investigate the relationship between MTP expression and chylomicron assembly in an insulin resistant non-diabetic animal model. Ten insulin resistant Zucker obese fa/fa rats and ten lean fa/− rats were examined at 8–10 weeks of age. The lymph duct was cannulated and lymph collected for 4 h. Lymph chylomicrons were isolated by ultracentrifugation and their composition determined. RNA was extracted from intestinal mucosa and from the liver. MTP mRNA was measured using the RNase protection assay. Blood sugar in the fatty rats was significantly higher (6.3±1.2 vs. 5.4±0.4 P<0.05) and plasma insulin was almost six times that of the lean rats (P<0.001). Plasma cholesterol and phospholipid but not triglyceride were significantly increased in the obese animals (P<0.01). Obese animals secreted significantly more lymph chylomicron apo B48 (0.05±0.02 vs. 0.02±0.01mg/h P<0.005), triglyceride (9.7±5.3 vs. 3.8±1.9 mg/h P<0.005) and phospholipid (1.5±0.7 vs. 0.4±0.3 mg/h P<0.001). The only difference in the chylomicron particle composition between the two groups was a significant increase in phospholipid (P<0.01). Intestinal MTP mRNA expression was significantly higher in the fatty compared to the lean rats (22.1±9.5 vs. 7.8±5.6 amol MTP mRNA/μg total RNA P<0.001) as was hepatic MTP mRNA expression (6.9±3.5 vs. 3.4±1.5 amol MTP mRNA/μg total RNA, P<0.01). Thus in this animal model of insulin resistance, increased MTP, which was associated with increased chylomicron particle number, may play a crucial role in the development of atherosclerosis.     

The relationship between cholesterol absorption and intestinal cholesterol synthesis in the diabetic rat model

The chylomicron remnant particle is thought to be particularly atherogenic and we have previously shown alterations in post-prandial lipoproteins which could contribute to their atherogenicity. Cholesterol metabolism is disturbed in diabetes, yet the effect of diabetes on intestinal cholesterol synthesis and absorption has rarely been investigated. The aim of this study was to examine cholesterol absorption and intestinal synthesis of cholesterol in the streptozotocin diabetic rat. Twelve diabetic rats were paired with 12 control rats. [14C]-Cholesterol emulsion was administered and the lymph duct was canulated. Lymph was collected for 4h. At sacrifice blood was taken for plasma lipoprotein measurements. Chylomicrons were prepared from the lymph by ultracentrifugation and [14C]-cholesterol content was determined by liquid scintillation counting. Lymph apolipoprotein B48 was isolated by gradient gel electrophoresis, and quantified by densitometric scanning. Serum triglyceride and cholesterol were greatly elevated in diabetic compared to control animals (260+/-90 and 9.8+/-8.0 mg/ml vs. 1.0+/-0.4 and 0.6+/-0.3 mg/ml, p < 0.0001 respectively). Lymph chylomicron apo B48 was similar in the two groups. Cholesterol absorption was not significantly different in diabetic compared to control rats but cholesterol synthesis was significantly higher in the diabetic animals (550+/-352 vs. 322+/-113 microg/h p < 0.03). There was a positive correlation between apo B48 and cholesterol absorption (r = 0.70, p < 0.01) in the diabetic rats and control rats (r = 0.71, p < 0.01) but no correlation between apo B48 and cholesterol synthesis in either group. This study demonstrates that cholesterol synthesis was increased in diabetes whereas cholesterol absorption was unaffected suggesting that intestinal cholesterol synthesis made an important contribution to the hypercholesterolaemia seen in the diabetic animals.     

The Role of microsomal triglyceride transfer protein and dietary cholesterol in chylomicron production in diabetes

Aims/hypothesis. The aim of this study was to examine factors involved in chylomicron production in the streptozotocin diabetic rat, our hypothesis being that the synthesis of the chylomicron is abnormal in diabetes. Methods. Diabetic rats (n = 20) were paired with control rats (n = 20). Cholesterol emulsion was given by gavage and the lymph duct was cannulated. Lymph was collected for 4 h. Chylomicrons were prepared from the lymph by ultracentrifugation. Lymph apolipoprotein B48 was isolated by gradient gel electrophoresis and quantified by densitometric scanning. Intestinal microsomal triglycerol transfer protein mRNA was measured by solution hybridisation nuclease protection, using a rat specific [³²P]-labelled cRNA probe. Results. Serum triglyceride and cholesterol were greatly increased in diabetic compared with control animals (258 ± 77 and 8.9 ± 6.4 mg/ml vs 1.04 ± 0.37 and 0.54 ± 0.03 mg/ml, p < 0.0001). Lymph chylomicron triglyceride and cholesterol were also higher in diabetic rats (29.4 ± 27.3 and 0.28 ± 0.3 mg/h vs 16.8 ± 10.6 and 0.18 ± 0.09 mg/h, p < 0.05). Lymph chylomicron apo B48 was similar in the two groups. Intestinal microsomal triglycerol transfer protein mRNA was higher in the diabetic rats (12.6 ± 3.2 vs 3.8 ± 3.0 amol/μg RNA, p < 0.0001) and there was a positive correlation between lymph triglyceride and microsomal triglycerol transfer protein mRNA in the whole group (r = 0.65, p < 0.01). Conclusion/interpretation. The study shows that microsomal triglycerol transfer protein mRNA is raised in diabetes without an increase in apolipoprotein B48 in the lymph suggesting that microsomal triglycerol transfer protein regulates chylomicron triglyceride content but not particle number. [Diabetologia (1999) 42: 944–948] Received: 9 December 1998 and in revised form: 6 April 1999     

Does pravastatin increase chylomicron remnant catabolism in postmenopausal women with type 2 diabetes mellitus?

OBJECTIVE: We investigated the effects of pravastatin on chylomicron remnant catabolism measured with a 13C stable isotope breath test and plasma apolipoprotein (apo) B-48 and remnant-like particle (RLP)-cholesterol in postmenopausal women with type 2 diabetes mellitus. PATIENTS AND MEASUREMENTS: Nineteen postmenopausal women with type 2 diabetes were randomized to receive 40 mg/day pravastatin or no treatment for 6 weeks followed by a 2-week washout period, and crossed over for a further 6 weeks. Fractional catabolic rate (FCR) of a chylomicron remnant-like emulsion was determined from 13CO2 enrichment in the breath and plasma using isotope-ratio mass spectrometry and multicompartmental modelling. Plasma apo B-48 and RLP-cholesterol concentrations were also measured as static markers of chylomicron remnant metabolism. RESULTS: Pravastatin significantly reduced plasma concentrations of cholesterol (5.9 +/- 0.3 vs. 4.8 +/- 0.2 mmol/l; P < 0.001), low density lipoprotein (LDL)-cholesterol (3.5 +/- 0.2 vs. 2.6 +/- 0.2 mmol/l; P < 0.001), triglyceride (2.1 +/- 0.3 vs. 1.7 +/- 0.2 mmol/l; P = 0.017), non-high density lipoprotein (HDL)-cholesterol (4.4 +/- 0.3 vs. 3.3 +/- 0.2 mmol/l; P < 0.001), lathosterol/total cholesterol ratio (2.6 +/- 0.2 vs. 2.0 +/- 0.3, P = 0.035), apo B-100 (1.1 +/- 0.1 vs. 0.8 +/- 0.1 g/l; P = 0.001), apo B-48 (4.8 +/- 0.9 vs. 3.3 +/- 0.6 mg/l; P = 0.016), and RLP-cholesterol (31.4 +/- 8.2 vs. 18.6 +/- 4.6 mg/dl; P = 0.024). Pravastatin was also associated with an increase in sitosterol/total cholesterol ratio (2.8 +/- 0.3 vs. 3.1 +/- 0.3, P = 0.029). Chylomicron remnant-like emulsion catabolism was not, however, significantly altered by pravastatin estimated by either breath or plasma clearance measurements. CONCLUSIONS: In postmenopausal women, pravastatin decreases plasma concentrations of remnant lipoproteins by a mechanism that may relate chiefly to inhibition of remnant production, but this requires further evaluation.     

LDL concentration is correlated with the removal from the plasma of a chylomicron-like emulsion in subjects with coronary artery disease

In animal model studies, the uptake of chylomicron remnants after entering in the space of Disse occurs mainly by low-density lipoprotein (LDL) receptor and LDL receptor-related protein (LRP). In subjects, the relative importance of each one of these receptors for the clearance of chylomicron remnants is not fully understood. In our study, LDL cholesterol and apolipoprotein (apo) B were correlated to the plasma kinetics of a chylomicron-like emulsion in 77 subjects (11 women, mean age 58 +/- 12 years) with coronary artery disease (CAD). Their total cholesterol was 227 +/- 25 mg/dl, triglyceride 159 +/- 25 mg/dl, LDL cholesterol 148 +/- 27 mg/dl, HDL cholesterol 40 +/- 9 mg/dl, apo A1 1.80 +/- 0.53 g/l and apo B 1.65 +/- 0.48 g/l. The emulsion was double-labeled with 3H-triolein and 14C-cholesteryl oleate and injected intravenously after 12-h fasting. The decay curves of the radioisotopes were determined from blood samples collected at predetermined intervals during 60 min. A negative correlation between FCR of the emulsion cholesterol esters and LDL cholesterol and apo B plasma concentrations was found (r=-0.4, P=0.005 and r=-0.3, P=0.01, respectively) whereas FCR of the emulsion triglycerides did not correlate with any of the plasma lipids or apolipoprotein parameters. Concluding, in patients with CAD, LDL catabolic pathway significantly influences the removal from plasma of chylomicron remnants.     

Lipoprotein alterations in hemodialysis: differences between diabetic and nondiabetic patients

Both renal failure and type 2 diabetes may contribute synergistically to the dyslipemia of diabetic renal failure with the development of atherosclerosis as the possible consequence. It has not yet been conclusively evaluated whether diabetic patients with end-stage renal failure under maintenance hemodialysis (HD) show accentuated alterations in plasma lipids and lipoproteins in comparison to nondiabetics under HD. These abnormalities would involve hepatic lipase activity and the regulation of triglyceride-rich lipoprotein metabolism. The purpose of the present study was to evaluate whether type 2 diabetic patients undergoing HD exhibited a lipid-lipoprotein profile different from that of nondiabetic hemodialyzed patients. We compared plasma lipids, apoprotein (apo) A-I and B, and lipoprotein parameters among 3 groups: 25 type 2 diabetics, 25 nondiabetics, both undergoing HD, and 20 healthy control subjects. Intermediate-density lipoprotein (IDL) and low-density lipoprotein (LDL) were isolated by sequential ultracentrifugation. Hepatic lipase activity was measured in postheparin plasma. Both groups of HD patients showed higher triglyceride and IDL cholesterol (P <.001), and lower high-density lipoprotein (HDL) cholesterol (P <.01) and apo A-I (P <.001) levels compared to the control group, even after adjustment for age and body mass index (BMI). However, no differences were found in lipid, lipoprotein, and apoprotein concentrations between diabetic and nondiabetic HD patients, except for high LDL triglyceride content of diabetic HD patients (P <.01). Nondiabetics undergoing HD also presented higher LDL triglyceride levels than controls (P <.05). LDL triglyceride correlated with plasma triglycerides (r = 0.51, P <.001). A lower LDL cholesterol/apo B ratio was found in each group of HD patients in comparison to controls (P <.02). Comparing the diabetic and nondiabetic patients, hepatic lipase activity remained unchanged, but significantly lower than control subjects (P <.001). Hepatic lipase correlated with log-triglyceride (r = -0.31, P <.01), IDL cholesterol (r = -0.41, P <.001), and LDL triglyceride (r = -0.32, P <.01). In conclusion, both diabetic and nondiabetic HD patients shared unfavorable alterations in lipid-lipoprotein profile not different between them but different from a healthy control group. The only difference between the groups of HD patients was a significant LDL triglyceride enrichment, which correlated negatively with hepatic lipase activity. Lipoprotein abnormalities in HD patients would enhance their risk for the development of atherosclerosis.     

The intestine as a regulator of cholesterol homeostasis in diabetes

The chylomicron influences very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) composition but itself is atherogenic. Thus abnormalities of chylomicron production are of interest particularly in conditions such as diabetes which confer major cardiovascular risk. Intestinal function is abnormal in diabetes and is a major cause of the dyslipidaemia found in this condition. Studies have suggested that cholesterol absorption is decreased in diabetes and cholesterol synthesis increased. Molecular mechanisms involved in insulin resistance in the intestine and its effect on cholesterol homeostasis in diabetes are described. Abnormalities in triglyceride synthesis and alterations genes regulating cholesterol absorption and intestinal synthesis are discussed. In particular, increase in apolipoprotein B48 synthesis has been demonstrated in animal models of diabetes and insulin resistance. Intestinal mRNA expression of Niemann Pick C1-like 1, protein is increased in both experimental and human diabetes suggesting that an increase in cholesterol transportation does occur. mRNA expression of the ATP binding cassette proteins (ABC) G5 and G8, two proteins working in tandem to excrete cholesterol have been shown to be decreased suggesting increased delivery of cholesterol for absorption. Expression of microsomal triglyceride transfer protein, which assembles the chylomicron particle, is increased in diabetes leading to increase in both number and cholesterol content. In conclusion, diabetes is associated with considerable dysfunction of the intestine leading to abnormal chylomicron composition which may play a major part in the premature development of atherosclerosis.     

Metabolism of chylomicrons in patients with congenital lipoatrophic diabetes: a study with emulsion models of chylomicrons

BACKGROUND: Lipoatrophic diabetes is characterized by the near absence of adipose tissue and the presence of insulin-resistant diabetes. Fasting hypertriglyceridaemia and increased postprandial lipidaemia are also present, but the metabolism of chylomicrons, the triglyceride-rich lipoproteins in the circulation that carry the dietary fats absorbed by the intestine, was not specifically investigated. Because both the activity of insulin-dependent lipoprotein lipase that catalyses the chylomicron lipolysis and the storage of the lipolysis products are affected in the disease, it is important to evaluate how those changes may ultimately affect the chylomicron lipolysis and removal of chylomicron remnants from the circulation. OBJECTIVE: The aim of the study was to evaluate the chylomicron intravascular metabolism in patients with lipoatrophic diabetes. PATIENTS: Six patients with lipoatrophic diabetes (four females, two males) aged 22.2 +/- 4.4 years, with body mass index (BMI) 21.6 +/- 3.6 kg/m(2), were compared with 12 healthy control subjects (seven females, five males) aged 24.3 +/- 2.1 years with BMI 22.5 +/- 2.7 kg/m(2). MEASUREMENTS: The plasma kinetics of intravenously injected chylomicron-like emulsions labelled with (3)H-triglycerides ((3)H-TG) and with (14)C-cholesteryl esters ((14)C-CE) were determined, the former tracing the chylomicron lipolysis by lipoprotein lipase and the latter the removal of chylomicron remnants from the plasma. RESULTS: Triglyceride values (8.3 +/- 9.2 mmol/l) in the patients were higher (P < 0.005) and high density lipoprotein (HDL) cholesterol values (0.8 +/- 0.2 mmol/l) lower (P < 0.0005) than in controls (0.7 +/- 0.2 and 1.3 +/- 0.4 mmol/l, respectively) whereas total cholesterol, apoprotein B (apo B) and apo A1 were similar. The fractional clearance rate (FCR, in min(-1)) of (3)H-TG was 0.014 +/- 0.016 and the FCR of (14)C-CE was 0.008 +/- 0.012 in the patients and 0.046 +/- 0.024 and 0.024 +/- 0.012 in the controls, respectively (P < 0.05). Thus FCRs of both emulsion labels were markedly reduced in the patients, indicating that lipolysis and remnant removal were diminished. Diminished remnant removal may be due to either deficient lipolysis or deficient removal mechanisms. CONCLUSION: The metabolism of chylomicrons tested by the emulsion method is impaired in lipoatrophic diabetes.     

Chylomicron remnant metabolism studied with a new breath test in postmenopausal women with and without type 2 diabetes mellitus

OBJECTIVES: The kinetic basis for the effect of type 2 diabetes mellitus (DM) on postprandial lipoproteins has not been fully established. We investigated chylomicron remnant metabolism using a stable isotope breath test and fasting measurements of plasma apolipoprotein (apo) B-48 and apoC-III concentrations in postmenopausal women with and without type 2 DM. PATIENTS: Twenty-four postmenopausal women without DM and 14 postmenopausal women with diet-controlled DM of similar age and body mass index (BMI) were studied in the postabsorptive state. METHODS: The fractional catabolic rate (FCR) of an intravenously injected chylomicron remnant-like emulsion was determined from the appearance of 13CO2 in the breath using isotope-ratio mass spectrometry and multicompartmental modelling. apoB-48, a marker of particle number of intestinal lipoproteins, was determined immunoelectrophoretically. apoC-III was measured by immunoturbidimetric assay. RESULTS: Compared with the nondiabetic women, the women with DM had significantly higher plasma apoB-48 concentration (16.40 +/- 1.18 mg/l vs. 13.0 +/- 0.9 mg/l; mean +/- standard error mean; P = 0.021), higher plasma apoC-III concentration (204.24 +/- 15.18 mg/l vs. 170.74 +/- 10.75 mg/l; P = 0.042) and lower FCR of the chylomicron remnant-like emulsion (0.06 +/- 0.05 pools/h vs. 0.12 +/- 0.02 pools/h; P < 0.001). In the diabetic patients, the FCR of the emulsion was correlated significantly with plasma apoB-48 levels (r = -0.641, P = 0.007) but not with apoC-III levels. CONCLUSIONS: In postmenopausal women, diabetes mellitus appears to decrease the catabolism of chylomicron remnants and result in an accumulation of these particles in plasma. This may chiefly be due to decreased clearance by hepatic receptors related to an effect of insulin resistance. Impairment in the catabolism of chylomicron remnants may contribute to increased risk of atherosclerosis in postmenopausal women with type 2 diabetes mellitus.     

Lack of evidence for reduced plasma apo B48 catabolism in patients with heterozygous familial hypercholesterolemia carrying the same null LDL receptor gene mutation

Increasing evidence suggests that remnants of chylomicrons and very low density lipoprotein (VLDL) also known as triglyceride-rich lipoproteins (TRL) are directly related to the pathogenesis of atherosclerosis. While studies in animals suggest that low density lipoprotein (LDL) receptor deficiency delays clearance of chylomicron remnants, human data supporting this hypothesis are conflicting. The objective of this study was to compare the fractional catabolic rate (FCR) and production rate (PR) of TRL apolipoprotein B48, the principal structural protein of intestinally derived chylomicron remnants, between familial hypercholesterolemic (FH) heterozygotes and non-FH controls. This was achieved by examining the kinetics of TRL apo B48 labelled with a stable isotope (L-(5,5,5-D3)leucine) in five normolipidemic males (age: 24.7 +/- 1.3 years; body mass index (BMI): 23.9 +/- 1.4 kg/m2) and six genetically defined FH heterozygous males (age: 29.7 +/- 9.9 years; (BMI): 22.0 +/- 4.3 kg/m2) carrying the same null LDL receptor gene mutation. All participants were apo E3 homozygotes. During the kinetic study, the subjects consumed 1/30 of their daily food intake every 30 min over a 15 h period. No significant difference was observed between FH heterozygotes and controls for FCR of TRL apo B48 (7.9 +/- 2.1 versus 7.9 +/- 2.6 pools per day, P = 0.99) while the TRL apo B48 pool size (10.5 +/- 5.4 versus 5.7 +/- 2.4 mg, P = 0.03) and PR (1.1 +/- 0.3 versus 0.6 +/- 0.3 mg kg(-1) per day, P = 0.02) were significantly higher among FH than in controls. In conclusion, this study shows no evidence for reduced plasma apo B48 catabolism in patients with heterozygous FH carrying the same null LDL receptor gene mutation and suggests that the plasma levels of intestinally derived TRL are elevated in FH due to an increased production rate.     

A functional polymorphism in the promoter region of the microsomal triglyceride transfer protein (MTP -493G/T) influences lipoprotein phenotype in familial hypercholesterolemia

The microsomal triglyceride transfer protein (MTP) has a key function in intracellular apolipoprotein (apo) B lipidation and secretion of very low density lipoprotein (VLDL). A recently discovered functional polymorphism in the promoter of the MTP gene (-493G/T) affects the plasma concentration of low density lipoprotein (LDL) cholesterol and the VLDL distribution between large and small particle species in healthy men. This phenotype is likely to be explained by an effect on VLDL synthesis. Against this background, we studied the effect of the MTP-493G/T polymorphism in a large cohort (217 men and 211 women) with heterozygous familial hypercholesterolemia (FH). A 40% to 50% lower serum triglyceride level was observed in homozygous carriers of the MTP-493 T allele (T/T, 0.93+/-0.34; G/T, 1.54+/-1.40; and G/G, 1.56+/-1.24 mmol/L; T/T vs G/T P=0.04, T/T vs G/G P=0.02). In contrast to the situation in healthy subjects, the MTP promoter polymorphism did not have a significant effect on the LDL cholesterol levels in FH subjects, although the same trend was observed (T/T, 7.31+/-1.87; G/T, 7. 80+/-2.12; and G/G, 7.91+/-2.31 mmol/L, NS). Adjustment for the apo E gene polymorphism by inclusion of subjects homozygous for the apo E3 allele only revealed a reciprocal high density lipoprotein cholesterol-elevating effect (T/T, 1.41+/-0.73; G/T, 1.18+/-0.27; and G/G, 1.16+/-0.29 mmol/L; T/T vs G/T P=0.06, T/T vs G/G P=0.04). This effect seemed to be sex-specific because it was accounted for by the female patients. In conclusion, the LDL cholesterol-lowering effect of the rare MTP gene promoter variant (MTP-493T) present in healthy subjects is shifted to a triglyceride-lowering effect in FH. These data suggest that the MTP gene has a role in modulating the clinical phenotype of FH.     

Intestinal apolipoprotein B-48 synthesis and lymphatic cholesterol transport are lower in swine fed high fat, high cholesterol diet with soy protein than with casein

Effects of dietary proteins on intestinal lipoproteins were studied in 8 Yorkshire swine fed a high fat, high cholesterol diet with either casein or soy protein. After 5 weeks of feeding, the casein group exhibited moderately elevated levels of serum cholesterol (334 +/- 46 mg/dl). The soy protein group showed significantly less hypercholesterolemia as compared to the casein group (122 +/- 8 mg/dl). Swine were subjected to cannulation of mesenteric lymph duct under halothane anesthesia. A single dose of 250 microCi [14C]cholesterol and 10 mCi [3H]leucine was infused into the upper jejunum 2 h after one-fifth of daily food was given. The 3-h lymphatic transport of cholesterol in casein-fed swine was significantly higher than in those fed soy protein. Triglyceride transport values were similar in the 2 groups. The [3H]leucine incorporation study revealed that transport of apo B-48 bore a significant positive relationship to transport of cholesterol in both chylomicron and VLDL fractions of mesenteric lymph. A greater apo B-48 secretion with higher specific activity was probably responsible for the greater transport of cholesterol in chylomicrons in casein-fed than in soy protein-fed swine. Similarly, the transport of lymph VLDL cholesterol in swine fed casein or soy protein paralleled the amount of accompanying apo B-48. Dietary proteins probably influence the intestinal synthesis of apo B-48 which in turn affects cholesterol transport into the lymphatics.     

Atorvastatin improves postprandial lipoprotein metabolism in normolipidemlic subjects

Atorvastatin is a potent HMG-CoA reductase inhibitor that decreases low-density lipoprotein (LDL) cholesterol and fasting triglyceride concentrations. Because of the positive association between elevated postprandial lipoproteins and atherosclerosis, we investigated the effect of atorvastatin on postprandial lipoprotein metabolism. The effect of 4 weeks of atorvastatin therapy (10 mg/day) was evaluated in 10 normolipidemic men (30+/-2 yr; body mass index, 22+/-3 kg/m2; cholesterol, 4.84+/-0.54 mmol/L; triglyceride, 1.47+/-0.50 mmol/L; high-density lipoprotein cholesterol, 1.17+/-0.18 mmol/L; LDL-cholesterol, 3.00+/-0.49 mmol/L). Postprandial lipoprotein metabolism was evaluated with a standardized fat load (1300 kcal, 87% fat, 7% carbohydrates, 6% protein, 80,000 IU vitamin A) given after 12 h fast. Plasma was obtained every 2 h for 14 h. A chylomicron (CM) and a chylomicron-remnant (CR) fraction was isolated by ultracentrifugation, and triglycerides, cholesterol, apolipoprotein B, apoB-48, and retinyl-palmitate were determined in plasma and in each lipoprotein fraction. Atorvastatin therapy significantly (P < 0.001) decreased fasting cholesterol (-28%), triglycerides (-30%), LDL-cholesterol (-41%), and apolipoprotein B (-39%), whereas high-density lipoprotein cholesterol increased (4%, not significant). The area under the curve for plasma triglycerides (-27%) and CR triglycerides (-40%), cholesterol (-49%), and apoB-48 (-43%) decreased significantly (P < 0.05), whereas CR retinyl-palmitate decreased (-34%) with borderline significance (P = 0.08). However, none of the CM parameters changed with atorvastatin therapy. This indicates that, in addition to improving fasting lipoprotein concentrations, atorvastatin improves postprandial lipoprotein metabolism presumably by increasing CR clearance or by decreasing the conversion of CMs to CRs, thus increasing the direct removal of CMs from plasma.     

Effects of atorvastatin on triglyceride-rich lipoproteins, low-density lipoprotein subclass, and C-reactive protein in hemodialysis patients

Dyslipidemia is an important risk factor for cardiovascular disease in patients with chronic renal failure (CRF). We evaluated the safety and efficacy of atorvastatin in patients with dyslipidemia associated with CRF who were undergoing hemodialysis (HD). Thirty-five patients who were receiving HD were given atorvastatin (10 mg/d) for 3 months. Chylomicron (CM), light and dense very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and light and dense low-density lipoprotein (LDL) were separated by ultracentrifugation. Apolipoprotein (apo) B was measured by electroimmunoassay. Mean LDL particle diameter was measured by gradient gel electrophoresis. Atorvastatin therapy reduced LDL-cholesterol (C) by 36% and remnant-like particle (RLP)-C by 58%. Atorvastatin significantly reduced apo B, apo CIII, and apo E in VLDL by 40% to 46% and IDL-apo B by 66%. Atorvastatin also significantly reduced cholesterol in CM, light VLDL, and dense VLDL without consistently affecting triglyceride (TG) in these lipoproteins. Atorvastatin similarly reduced both light and dense LDL-apo B by 38%. LDL particle size in the HD patients significantly increased during atorvastatin treatment from 25.7 +/- 0.4 to 26.2 +/- 0.6 nm. High sensitive C-reactive protein (HS-CRP) was halved by atorvastatin decreasing from 0.08 +/- 0.05 to 0.04 +/- 0.03 mg/dL. Atorvastatin treatment did not affect the creatinine kinase level, and no classical adverse effects were observed during the study. These results suggest that atorvastatin is safe and effective for the management of dyslipidemia in patients with CFR who are receiving HD, which may help to suppress the development of atherosclerosis.     

Atorvastatin improves diabetic dyslipidemia and increases lipoprotein lipase activity in vivo

A major factor contributing to cardiovascular mortality in type 2 diabetes is dyslipidemia, characterized by low HDL cholesterol and high triglycerides, rather than elevated LDL cholesterol. Lipoprotein lipase (LPL) is the rate-limiting enzyme of triglyceride removal from plasma and has been implicated in atherosclerosis. Since treatment with statins significantly reduces cardiovascular morbidity in diabetes, we analyzed the lipid profile and LPL activities in 61 patients with type 2 diabetes before and 8 weeks after initiation of atorvastatin (40 mg) or placebo treatment. Lipid parameters and LPL activity were unchanged under treatment with placebo. Atorvastatin treatment resulted in a 30% reduction of total and a 45% reduction of LDL cholesterol (6.06 +/- 1.39 mmol/L versus 4.14 +/- 1.27 mmol/L and 4.11 +/- 1.13 mmol/L versus 2.27 +/- 0.89 mmol/L, both P < 0.0001). Triglycerides and VLDL cholesterol were also significantly reduced by statin therapy (2.24 +/- 2.11 mmol/L versus 1.82 +/- 1.46 mmol/L and 1.08 +/- 1.56 mmol/L versus 0.67 +/- 0.66 mmol/L, both P < 0.05). HDL cholesterol was not different between the atorvastatin and the placebo group. Compared to baseline, LPL activity was increased by 25% after atorvastatin treatment (213.0 +/- 28.1 nmol/mL/min versus 171.9 +/- 17.7 nmol/mL/min, P < 0.01). Our data demonstrate that atorvastatin induces a significant improvement of diabetic dyslipidemia and a significant increase of LPL activity. Since low LPL activity indicates an increased cardiovascular risk, the statin-mediated increase in LPL activity may help to explain the reduction of CAD in diabetic patients treated with statins.     

Differences in postprandial concentrations of very-low-density lipoprotein and chylomicron remnants between normotriglyceridemic and hypertriglyceridemic men with and without coronary heart disease

It has been suggested that the postprandial elevation of plasma triglycerides is more closely linked to coronary heart disease (CHD) than the fasting triglyceride level. However, the postprandial situation is complex, as hepatogenous triglyceride-rich lipoprotein (TRL) particles (apolipoprotein [apo]B-100 and very-low-density lipoprotein [VLDL]) are mixed in the blood with apoB-48-containing lipoproteins secreted from the intestine. To analyze the relative proportion of liver-derived and intestinal apoB-containing TRL in subjects with and without CHD, we performed standardized oral fat-loading tests in young survivors of myocardial infarction, a large proportion of whom are hypertriglyceridemic (HTG), as well as sex- and population-matched healthy control subjects. A special effort was made to recruit healthy HTG subjects as controls for the HTG patients. Fasting plasma triglycerides (3.74+/-1.35 v3.01+/-0.83, NS), low-density lipoprotein (LDL) cholesterol, and VLDL lipids, and apoB-100 and apoB-48 content at Svedberg flotation rate (Sf) 60-400, Sf 20-60, and Sf 12-20 did not differ between HTG patients (n = 10) and HTG controls (n = 14). Normotriglyceridemic (NTG) patients (n = 15) had higher fasting plasma triglycerides (1.44+/-0.39 v 0.98+/-0.33 mmol/L, P < .05) and LDL cholesterol (4.07+/-0.71 v 3.43+/-0.64, P < .05) than NTG controls (n = 34). The triglyceride elevation was accounted for by a higher level of small VLDL (apoB-100 in the Sf 20-60 fraction, 52+/-17 v29+/-20 mg/L, P < .05). HTG patients responded with clearly elevated plasma triglycerides in the late postprandial phase, ie, 7, 8, and 9 hours after fat intake. Essentially, this was explained by a retention of large VLDL particles, since HTG patients exhibited no major differences in apoB-48 concentrations in the Sf > 400, Sf 60-400, and Sf 20-60 fractions but showed marked differences in the level of apoB-100 at Sf 60-400 (large VLDL) 9 hours after fat intake when compared with HTG controls (101+/-13 v 57+/-5 mg/L, P < .01). NTG patients were characterized by a more rapid increase of large VLDL in the early postprandial state, ie, 3 hours after fat intake, with a mean increase from baseline to 3 hours of 24.1+/-6.7 mg/L for NTG patients and 11.8+/-2.0 mg/L for controls (P < .05). ApoB-48 levels were also slightly higher, but all TRL parameters returned to baseline within 9 hours after fat intake. In conclusion, elevated triglyceride levels in the postprandial state in CHD patients are explained to a large extent by the accumulation of endogenous TRL. This suggests that the postprandial dyslipidemia encountered in CHD is more dependent on a failure of regulation of endogenous TRL versus the exogenous TRL species.