Modulation of LDL particle size after an oral glucose load is associated with insulin levels

The differential effects of thiazolidindiones on atherogenic dyslipidemia in type 2 diabetes: what is the clinical significance?

BACKGROUND

Diabetic dyslipidemia is typically characterized by an increase in plasma triglycerides, a decrease in high-density lipoprotein cholesterol and a concomitant increase in atherogenic small dense low-density lipoproteins. Thiazolidindiones are able to lower the levels of fasting glucose and glycated hemoglobin significantly by improving insulin sensitivity, as well as improving some aspects of diabetic dyslipidemia: total cholesterol, low-density lipoprotein cholesterol and high-density lipoprotein cholesterol tend to increase while triglycerides are generally decreased.

OBJECTIVE

This paper reviewed the effects of pioglitazone and rosiglitazone on atherogenic diabetic dyslipidemia, in particular on small dense low-density lipoprotein particles.

METHODS

A literature search (by Medline and Scopus) was performed up to 15 March 2008. The authors also manually reviewed the references of selected articles for any pertinent material.

RESULTS

Pioglitazone showed an additional beneficial effect on triglycerides, high-density lipoprotein cholesterol and the levels of small dense low-density lipoprotein compared to rosiglitazone.

CONCLUSIONS

Since recent studies have suggested that these agents may also have a differential effect on long-term cardiovascular end-points despite similar improvements in glycated hemoglobin and insulin sensitivity, the different impact on atherogenic diabetic dyslipidemia may help to explain these findings.

Post-prandial alterations in LDL size and subclasses in patients with growth hormone deficiency

OBJECTIVE

Several studies have suggested that lipoproteins generated during the post-prandial phase are highly atherogenic, with modifications in low-density lipoproteins (LDL) size and density. In the present study we assessed post-prandial variations in LDL size and subclasses in patients with growth hormone deficiency (GHD).

DESIGN

We studied in 12 hypopituitary patients with GHD and 10 healthy control subjects matched for gender, age and body mass index (BMI) post-prandial variations after a standardized meal consisting of 35% fat, 45% carbohydrate and 20% of protein (Clinutren Mix, Nestlé) and containing calories corresponding to 1/3 of estimated basal metabolic rate. Blood samples were collected at baseline and after 2 and 4h to measure plasma lipids and LDL size and subclasses by nondenaturing polyacrylamide gradient gel electrophoresis.

RESULTS

At baseline patients had similar plasma lipids than controls, with the exception of higher triglycerides (1.2+/-0.8 vs. 0.7+/-0.4mmol/L, p=.0024). Baseline LDL size was similar between the two groups and LDL subclass analysis revealed a small increase in LDL-IIIA (p=.0046). During post-prandial phase no significant differences were found in LDL size and subclasses in patients vs. controls with the sole exception of increased levels of LDL-IVB after 2h (p=.0295) and LDL-IIIB after 4h (p=.0478).

CONCLUSIONS

It is, therefore, unlikely that a post-prandial variation in levels of small, dense LDL may significantly contribute to the atherogenic potential in hypopituitary patients with GHD.

Factors influencing the formation of small dense low-density lipoprotein particles in dependence on the presence of the metabolic syndrome and on the degree of glucose intolerance

BACKGROUND

Small dense low-density lipoprotein (LDL) particles are known to be especially atherogenic. Several mechanisms are involved in this atherogenicity.

AIMS

We wanted to look for the presence of small dense LDL particles depending on gender, metabolic syndrome (MS) and different degrees of glucose intolerance. Moreover, we looked for anthropometric factors and factors of lipid and carbohydrate metabolism that are associated with changes in the LDL size.

RESULTS

We studied 752 persons (330 males, 422 females; age 40 +/- 17 years). LDL particle size was estimated with polyacrylamide gel electrophoresis. Males had smaller LDL particles than females. Probands with the MS had smaller LDL particles than those without this syndrome. With rising plasma triglyceride (TG) levels more small dense LDL particles were seen. The highest proportion of these small dense LDL particles was observed in the subgroup of type 2 diabetic patients. In the whole material, the mean LDL diameter was correlated negatively with plasma TG and very low-density lipoprotein components (TG, cholesterol and proteins) and positively with high-density lipoprotein cholesterol. In a linear stepwise regression analysis different significant factors influencing the LDL size were found in the whole population, in normoglycaemic probands, in persons with impaired glucose tolerance, in type 2 diabetic patients and in type 2 diabetic patients injecting insulin.

CONCLUSIONS

Our data point to different mechanisms of the formation of small dense LDL particles in dependence on the degree of glucose intolerance. Moreover, the target values for plasma TG should be set lower.

Low-density lipoproteins generated during an oral fat load in mild hypertriglyceridemic and healthy subjects are smaller, denser, and have an increased low-density lipoprotein receptor binding affinity

Triglyceride-rich lipoproteins generated during the postprandial phase are atherogenic. Large very low-density lipoproteins (LDLs) or chylomicrons (CMs) are not as atherogenic as their remnants (Rem). Small and dense LDLs are associated with cardiovascular disease. Low-density lipoprotein size is partly under genetic control and is considered as a relatively stable LDL feature. In this article, we present data on retinyl palmitate kinetics correlated with the modification of LDL features in terms of size, density, and in vitro receptor binding affinity after an oral fat load. Six nondiabetic, hypertriglyceridemic (HTG) patients and 6 healthy controls were examined. Low-density lipoprotein size was assessed by gradient gel electrophoresis, and LDL density by density gradient ultracentrifugation. Low-density lipoprotein binding affinity was tested by in vitro competition binding assay on normal human skin fibroblasts (HSFs) and hepatoma cells (HepG2). Kinetic parameters were estimated in CM and Rem fractions by compartmental modeling. Hypertriglyceridemic patients showed significantly higher triglyceride area and a slower CM fractional catabolic rate. Postprandial LDL density increased both in HTG patients and in the control group with a significant difference between groups at 6 hours. Fasting LDL size was lower in HTG patients vs controls but decreased similarly in the postprandial phase. Low-density lipoprotein size and density postprandial modifications were not correlated with any investigated parameter. Postprandial LDLs were internalized more efficiently by HSF than baseline LDL only in the HTG group. In conclusion, postprandial LDLs are smaller and denser compared with fasting LDLs after an oral fat load. Postprandial LDLs also slightly increased their affinity to the HSF cell receptors.

Evidence that acute insulin administration enhances LDL cholesterol susceptibility to oxidation in healthy humans

Increased free radical production and hyperinsulinemia are thought to play a role in experimental and human atherosclerosis, but the relation between the 2 abnormalities has not been studied. In 23 healthy volunteers, we measured the susceptibility of circulating low-density lipoprotein (LDL) cholesterol particles to in vitro copper sulfate oxidation (measured as the lag phase) and cell-mediated oxidative modification (measured as malondialdehyde generation in LDL during incubation with human umbilical vein endothelial cells), as well as the vitamin E content of LDL cholesterol at baseline and after 2 hours of physiological hyperinsulinemia (euglycemic insulin clamp). The lag time of LDL oxidation decreased from control values of 108+/-3 and 107+/-3 minutes (at baseline and after 2 hours of saline infusion) to 101+/-3 minutes after 2 hours of clamping (P<0.0001). At corresponding times, cell-mediated malondialdehyde generation in LDL rose from 4.96+/-0.11 and 4.98+/-0.10 to 5.28+/-0.10 nmol/L (P=0. 0006), whereas the LDL vitamin E content decreased from 6.78+/-0.06 and 6.77+/-0.06 to 6.64+/-0.06 microg/mg (P<0.04). The insulin-induced shortening of the lag phase was directly related to the decrement of vitamin E in LDL; furthermore, in subjects with higher baseline serum triglyceride levels, insulin induced a greater shortening of the lag phase than in subjects with low baseline triglycerides. We conclude that in healthy humans acute physiological hyperinsulinemia enhances the oxidative susceptibility of LDL cholesterol particles. This effect may have pathogenic significance for atherogenesis in insulin resistant states.