Lowering of plasma phospholipid transfer protein activity by acute hyperglycaemia-induced hyperinsulinaemia in healthy men

Differential insulin sensitivity of NMR-based metabolomic measures in a two-step hyperinsulinemic euglycemic clamp study

BACKGROUND

Insulin is the key regulator of glucose metabolism, but it is difficult to dissect direct insulin from glucose-induced effects. We aimed to investigate the effects of hyperinsulemia on metabolomic measures under euglycemic conditions in nondiabetic participants.

METHODS

We assessed concentrations of 151 metabolomic measures throughout a two-step hyperinsulinemic euglycemic clamp procedure. We included 24 participants (50% women, mean age = 62 [s.d. = 4.2] years) and metabolomic measures were assessed under baseline, low-dose (10 mU/m²/min) and high-dose (40 mU/m²/min) insulin conditions. The effects of low- and high-dose insulin infusion on metabolomic measures were analyzed using linear mixed-effect models for repeated measures.

RESULTS

After low-dose insulin infusion, 90 metabolomic measures changed in concentration (p < 1.34e^-4), among which glycerol (beta [Confidence Interval] =  - 1.41 [- 1.54, - 1.27] s.d., p = 1.28e^-95) and three-hydroxybutyrate (- 1.22 [- 1.36, - 1.07] s.d., p = 1.44e^-61) showed largest effect sizes. After high-dose insulin infusion, 121 metabolomic measures changed in concentration, among which branched-chain amino acids showed the largest additional decrease compared with low-dose insulin infusion (e.g., Leucine, - 1.78 [- 1.88, - 1.69] s.d., P = 2.7e^-295). More specifically, after low- and high-dose insulin infusion, the distribution of the lipoproteins shifted towards more LDL-sized particles with decreased mean diameters.

CONCLUSION

Metabolomic measures are differentially insulin sensitive and may thus be differentially affected by the development of insulin resistance. Moreover, our data suggests insulin directly affects metabolomic measures previously associated with increased cardiovascular disease risk.

Effects of weight loss, induced by gastric bypass surgery, on HDL remodeling in obese women

Plasma lipoproteins and glucose homeostasis were evaluated after marked weight loss before and over 12 months following Roux-en-Y gastric-bypass (RYGBP) surgery in 19 morbidly obese women. Standard lipids, remnant-lipoprotein cholesterol (RLP-C); HDL-triglyceride (TG); apolipoproteins (apo) A-I, A-II, E, and A-I-containing HDL subpopulations; lecithin-cholesterol acyltransferase (LCAT) and cholesteryl ester transfer protein (CETP) mass and activity; plasma glucose and insulin levels were measured before and at 1, 3, 6, and 12 months after GBP surgery. Baseline concentrations of TG, RLP-C, glucose, and insulin were significantly higher in obese than in normal-weight, age-matched women, whereas HDL cholesterol (HDL-C), apoA-I, apoA-II, alpha-1 and alpha-2 levels were significantly lower. Over 1 year, significant decreases of body mass index, glucose, insulin, TG, RLP-C, HDL-TG, and prebeta-1 levels were observed with significant increases of HDL-C and alpha-1 levels (all P < 0.05). Changes of fat mass were correlated with those of LDL cholesterol (P = 0.018) and LCAT mass (P = 0.011), but not with CETP mass (P = 0.265). Changes of fasting plasma glucose concentrations were inversely correlated with those of CETP mass (P = 0.005) and alpha-1 level (P = 0.004). Changes of fasting plasma insulin concentrations were positively correlated with those of LCAT mass (P = 0.043) and inversely with changes of alpha-1 (P = 0.03) and alpha-2 (P = 0.05) concentrations. These results demonstrate beneficial changes in HDL remodeling following substantial weight loss induced by RYGBP surgery and that these changes are associated with improvement of glucose homeostasis in these patients.

Genetic and nongenetic sources of variation in phospholipid transfer protein activity

Phospholipid transfer protein (PLTP) belongs to the lipid transfer/lipopolysaccharide-binding protein gene family. Expression of PLTP has been implicated in the development of atherosclerosis. We evaluated the effects of PLTP region tagging single nucleotide polymorphisms (SNPs) on the prediction of both carotid artery disease (CAAD) and PLTP activity. CAAD effects were evaluated in 442 Caucasian male subjects with severe CAAD and 497 vascular disease-free controls. SNP prediction of PLTP transfer activity was evaluated in both a subsample of 87 subjects enriched for an allele of interest and in a confirmation sample of 210 Caucasian males and females. Hemoglobin A1c or insulin level predicted 11-14% of age- and sex-adjusted PLTP activity. PLTP SNPs that predicted approximately 11-30% of adjusted PLTP activity variance were identified in the two cohorts. For rs6065904, the allele that was associated with CAAD was also associated with elevated PLTP activity in both cohorts. SNPs associated with PLTP activity also predicted variation in LDL-cholesterol and LDL-B level only in the replication cohort. These results demonstrate that PLTP activity is strongly influenced by PLTP region polymorphisms and metabolic factors.

Plasma phospholipid transfer protein activity, a determinant of HDL kinetics in vivo

OBJECTIVE

Phospholipid transfer protein (PLTP) is an important regulator in the transport of surface components of triglyceride-rich lipoprotein (TRL) to high density lipoprotein (HDL) during lipolysis and may therefore play an important role in regulating HDL transport. In this study we investigated the relationship of plasma PLTP activity with HDL metabolism in men.

DESIGN AND METHODS

The kinetics of HDL LpA-I and LpA-I:A-II were measured using intravenous administration of [D3]-leucine, gas chromatography-mass spectrometry (GCMS) and a new multicompartmental model for HDL subpopulation kinetics (SAAM II) in 31 men with wide-ranging body mass index (BMI 18-46 kg/m2). Plasma PLTP activity was determined as the transfer of radiolabelled phosphatidylcholine from small unilamellar phosphatidylcholine vesicles to ultracentrifugally isolated HDL.

RESULTS

PLTP activity was inversely associated with LpA-I concentration and production rate (PR) after adjusting for insulin resistance (P < 0.05). No significant associations were observed between plasma PLTP activity and LpA-I fractional catabolic rate (FCR). In multivariate analysis, including homeostasis model assessment score (HOMA), triglyceride, cholesteryl ester transfer protein (CETP) activity and PLTP activity, PLTP activity was the only significant determinant of LpA-I concentration and PR (P = 0.020 and P = 0.016, respectively).

CONCLUSIONS

Plasma PLTP activity may be a significant, independent determinant of LpA-I kinetics in men, and may contribute to the maintenance of the plasma concentration of these lipoprotein particles in setting of hypercatabolism of HDL.

Plasma phospholipid transfer protein activity and subclinical inflammation in type 2 diabetes mellitus

Phospholipid transfer protein (PLTP) transfers phospholipids between lipoproteins, and plays an essential role in HDL metabolism. The regulation of PLTP is poorly understood and recent evidence suggests that PLTP activity increases during acute-phase response. Since type 2 diabetes is associated with chronic subclinical inflammation, the objective is to determine whether inflammation modulates PLTP in diabetes. Plasma PLTP activity was assayed by measuring the transfer of radiolabeled phosphatidylcholine from liposomes to HDL and high-sensitivity C-reactive protein (CRP) by immunoturbidimetric assay in 280 type 2 diabetic patients and 105 controls. Plasma PLTP activity (2364+/-651 nmol/ml/h versus 1880+/-586 nmol/ml/h in control, mean +/- S.D., P <0.01) and CRP (1.64(0.89-3.23)mg/l versus 0.99(0.53-2.23 mg/l, median (interquartile range), P<0.01) were increased in diabetic subjects. PLTP activity correlated significantly with age, BMI, HbA1c, log(CRP) and apolipoprotein AI and B in diabetic subjects. General linear model analysis showed that only apolipoprotein AI, age, BMI, and log(CRP) were independent determinants of PLTP activity. In conclusion, PLTP activity is increased in diabetes and apolipoprotein AI is a major determinant of PLTP activity. There is also an independent association between CRP and PLTP activity, suggesting that subclinical inflammation may influence PLTP activity in diabetes.

Alterations in plasma vitamin E distribution in type 2 diabetic patients with elevated plasma phospholipid transfer protein activity

Mouse studies indicated that plasma phospholipid transfer protein (PLTP) determines the plasma distribution of vitamin E, a potent lipophilic antioxidant. Vitamin E distribution, antioxidant status, and titer of anti-oxidized LDLs (oxLDL) autoantibodies were evaluated in plasma from control subjects (n = 31) and type 2 diabetic patients (n = 31) with elevated plasma PLTP concentration. Unlike diabetic and control HDLs, which displayed similar vitamin E contents, diabetic VLDLs and diabetic LDLs contained fewer vitamin E molecules than normal counterparts. Plasma PLTP concentration in diabetic plasmas correlated negatively with vitamin E in VLDL+LDL, but positively with vitamin E in HDL, with an even stronger correlation with the VLDL+LDL-to-HDL vitamin E ratio. Circulating levels of oxLDL were significantly higher in diabetic plasmas than in control plasmas. Whereas the titer of IgG autoantibodies to modified LDL did not differ significantly between diabetic patients and control subjects, diabetic plasmas showed significantly lower levels of potentially protective IgM autoantibodies. The present observations support a pathophysiological role of PLTP in decreasing the vitamin E content of apolipoprotein B-containing lipoproteins, but not of HDL in plasma of type 2 diabetic patients, contributing to a greater potential for LDL oxidation.

PLTP activity decreases with weight loss: changes in PLTP are associated with changes in subcutaneous fat and FFA but not IAF or insulin sensitivity

Phospholipid transfer protein (PLTP) activity is elevated in obese and diabetic subjects. No prospective studies have examined the effect of weight loss on PLTP activity and assessed whether the resultant changes in activity are related to changes in body weight, insulin resistance, or both. PLTP activity was measured at baseline in 46 subjects (body mass index = 19-64 kg/m2) and after diet-induced weight loss in 19 of the obese subjects. Total body fat mass (FM) by dual-energy X-ray absorptiometry, intraabdominal fat (IAF), and abdominal subcutaneous fat (SQF) by CT scan, insulin sensitivity (SI) by frequently sampled intravenous glucose tolerance test, leptin, and lipids were determined. At baseline, PLTP activity correlated with FM (r = 0.36, P = 0.02) and SQF (r = 0.31, P = 0.045), but not with IAF (r = 0.16, P = 0.32) or SI (r = 0.10, P = 0.52). With diet-induced weight loss (16 +/- 7.3 kg), PLTP activity significantly decreased 9.1% (P = 0.002). The change in PLTP activity correlated with the change in SQF (r = 0.55, P = 0.014) (33.6% decrease), but not with IAF (r = 0.09, P = 0.73) or SI (r = 0.18, P = 0.44), and was highly correlated with the change in nonesterified fatty acid (NEFA) (r = 0.71, P < 0.001). In conclusion, elevated PLTP activity in obese subjects is likely a result of increased body fat, reflected by SQF, and is influenced by NEFAs but is not directly related to insulin resistance.

Plasma phospholipid transfer protein activity and small, dense LDL in type 2 diabetes mellitus

BACKGROUND

Phospholipid transfer protein (PLTP) and cholesteryl ester transfer protein (CETP) remodel circulating lipoproteins and play a role in the antiatherogenic reverse cholesterol transport pathway. The present study determined whether abnormalities in the LDL subfraction pattern in type 2 diabetic patients were related to changes in lipid transfer proteins.

METHODS

Low-density lipoprotein (LDL) subfractions were measured by density gradient ultracentrifugation and plasma PLTP and CETP activities by radiometric assays in 240 diabetic patients and 136 controls.

RESULTS

The diabetic patients had lower LDL-I (P < 0.001) and higher LDL-III concentrations than the controls (P < 0.001). Plasma PLTP activity was increased (P < 0.001) whereas no significant differences were seen in CETP activity. In the diabetic patients, small, dense LDL-III correlated with plasma triglyceride (r = 0.18, P < 0.01), HDL (r = -0.14, P < 0.05), PLTP (r = 0.29, P < 0.001) and CETP activity (r = 0.15, P < 0.05). Linear regression analysis showed that plasma PLTP activity, triglyceride and age were the major determinants of LDL-III concentration (r2 = 28%, P < 0.001). The univariate relationship between CETP and LDL-III was no longer significant after adjusting for PLTP activity.

CONCLUSIONS

The increase in plasma PLTP activity was independently associated with small, dense LDL concentrations in type 2 diabetes. Hence, elevated PLTP activity might have both antiatherogenic and pro-atherogenic potential in these patients.

Relationship of phospholipid transfer protein activity to HDL and apolipoprotein B-containing lipoproteins in subjects with and without type 1 diabetes

Patients with type 1 diabetes have greatly increased phospholipid transfer protein (PLTP) activity and have an altered HDL subclass distribution. In 195 patients with type 1 diabetes and in 194 men and women aged 30-55 years, we examined the relationship of PLTP activity to HDL and examined whether PLTP activity contributes to differences in HDL found in type 1 diabetes. PLTP activity was measured using an exogenous substrate assay. Average HDL particle size and HDL subclasses were measured using nuclear magnetic resonance spectroscopy. Apolipoprotein AI (apoAI) and apoAII were measured by immunoturbidimetry. The amount of apoAI present in LpAI was measured using a differential electroimmunoassay, and the amount of apoAI in LpAIAII was inferred from the apoAI and LpAI data. Higher PLTP activity was associated with more large HDL (P < 0.001) and less small HDL (P < 0.01), more apoAI and apoAII (both at P < 0.001), and more apoAI in both LpAI and LpAIAII (P = 0.02 and P < 0.001, respectively). These associations were independent of other lipids and enzyme activities. Adjusting for PLTP activity halved the difference between subjects with and without diabetes in apoA1 (from 10.1 mg/dl higher in subjects with diabetes to 4.6 mg/dl higher) and large HDL (2.4 micro mol/l higher to 1.2 micro mol/l higher) and reduced the difference in HDL size (from 0.31 nm higher to 0.26 nm higher). PLTP activity was also positively associated with apoB, total VLDL and LDL particle number, and IDL level in subjects with diabetes. These data support the idea that PLTP is a major factor in HDL conversion and remodeling in humans and that higher PLTP activity makes an important contribution to the higher apoAI levels and altered HDL subclass distribution in type 1 diabetes. They also support a role for PLTP in the metabolism of apoB-containing lipoproteins.

Influence of leptin and insulin on lipid transfer proteins in human hepatoma cell line, HepG2

AIM

Phospholipid transfer protein (PLTP) and cholesteryl ester transfer protein (CETP) are key enzymes in lipoprotein metabolism facilitating the transfer and exchange of cholesteryl esters, triglycerides and phospholipids between lipoproteins. In the study presented here, we investigated the influence of two hormones-the adipocyte-derived hormone leptin as well as insulin on the hepatic secretion of both, PLTP and CETP.

METHODS

PLTP activity and CETP concentration-measured by exogenous substrate assay and enzyme-linked immunosorbent assay-were determined in supernatant of human hepatoma cell line HepG2 after single or combined exposure to leptin and insulin at physiological and supraphysiological concentrations, respectively. Messenger-RNA of PLTP and CETP was quantified by Northern blot analysis.

RESULTS

Leptin suppressed PLTP activity and CETP-concentration by up to 33% and 23%, respectively. Insulin also suppressed PLTP activity by up to 11% and CETP-concentration by up to 16%. In combination, the two hormones had additive suppressive effects for both, PLTP activity and CETP-concentration. Northern blot analysis showed no difference in m-RNA levels after exposure to leptin or insulin.

CONCLUSIONS

Leptin and insulin, both known to increase with body fat mass, suppress production of PLTP and CETP in HepG2 cells. When extrapolated to the in vivo situation, this suppressive effect may constitute a mechanism counteracting the potentially harmful action of lipid transfer proteins, particularly reduction of HDL-cholesterol, in conditions frequently associated with increased plasma triglyceride levels such as obesity and insulin resistance.

Glucose regulates the transcription of human genes relevant to HDL metabolism: responsive elements for peroxisome proliferator-activated receptor are involved in the regulation of phospholipid transfer protein

Phospholipid transfer protein (PLTP) plays an important role in human plasma HDL metabolism. Clinical data have recently indicated that plasma PLTP activity and mass were both higher in diabetic patients concomitant with hyperglycemia. The present study shows that high glucose increases both PLTP mRNA and functional activity in HepG2 cells, due to a significant increase in the promoter activity of human PLTP gene. The glucose-responsive elements are located between -759 and -230 of the PLTP 5'-flanking region, within which two binding motifs (-537 to -524 and -339 to -327) for either peroxisome proliferator-activated receptor or farnesoid X-activated receptor are involved in this glucose-mediated transcriptional regulation. This finding suggests that high glucose upregulates the transcription of human PLTP gene via nuclear hormone receptors. In addition, high glucose increases mRNA levels for several genes that are functionally important in HDL metabolism, including human ATP-binding cassette transporter A1, apolipoprotein A-I, scavenger receptor BI, and hepatic lipase. The functional promoter activities of these genes are enhanced by high glucose in three cell lines tested, indicating that glucose may also regulate these genes at the transcriptional level. Our findings provide a molecular basis for a role of hyperglycemia in altered HDL metabolism.

The impact of phospholipid transfer protein (PLTP) on HDL metabolism

High-density lipoproteins (HDL) play a major protective role against the development of coronary artery disease. Phospholipid transfer protein (PLTP) is a main factor regulating the size and composition of HDL in the circulation and plays an important role in controlling plasma HDL levels. This is achieved via both the phospholipid transfer activity of PLTP and its capability to cause HDL conversion. The present review focuses on the impact of PLTP on HDL metabolism. The basic characteristics and structure of the PLTP protein are described. The two main functions of PLTP, PLTP-mediated phospholipid transfer and HDL conversion are reviewed, and the mechanisms and control, as well as the physiological significance of these processes are discussed. The relationship between PLTP and the related cholesteryl ester transfer protein (CETP) is reviewed. Thereafter other functions of PLTP are recapitulated: the ability of PLTP to transfer cholesterol, alpha-tocopherol and lipopolysaccharide (LPS), and the suggested involvement of PLTP in cellular cholesterol traffic. The discussion on PLTP activity and mass in (patho)physiological settings includes new data on the presence of two forms of PLTP in the circulation, one catalytically active and the other inactive. Finally, future directions for PLTP research are outlined.

Plasma lipid transfer proteins, high-density lipoproteins, and reverse cholesterol transport

Cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP) are members of the lipid transfer/lipopolysaccharide binding protein gene family. Recently, the crystal structure of one of the members of the gene family, bactericidal permeability increasing protein, was solved, providing potential insights into the mechanisms of action of CETP and PLTP. These molecules contain intrinsic lipid binding sites and appear to act as carrier proteins that shuttle between lipoproteins to redistribute lipids. The phenotype of human CETP genetic deficiency states and CETP transgenic mice indicates that CETP plays a major role in the catabolism of high-density lipoprotein (HDL) cholesteryl esters and thereby influences the concentration, apolipoprotein content, and size of HDL particles in plasma. PLTP also appears to have an important role in determining HDL levels and speciation. Recent data indicate that genetic CETP deficiency is associates with an excess of coronary heart disease in humans, despite increased HDL levels. Also, CETP expression is anti-atherogenic in many mouse models, even while lowering HDL. These data tend to support the reverse cholesterol transport hypothesis, i.e., that anti-atherogenic properties of HDL are related to its role in reverse cholesterol transport. Recently, another key molecule involved in this pathway was identified, scavenger receptor BI; this mediates the selective uptake of HDL cholesteryl esters in the liver and thus constitutes a pathway of reverse cholesterol transport parallel to that mediated by CETP. Reflecting its role in reverse cholesterol transport, the CETP gene is up-regulated in peripheral tissues and liver in responses to dietary or endogenous hypercholesterolemia. An analysis of the CETP proximal promoter indicates that it contains sterol regulatory elements highly homologous to those present in 3-hydroxy-3-methylglutaryl-coenzyme A reductase; the CETP gene is transactivated by the binding of SREBP-1 to these elements. A challenge for the future will be the manipulation of components of the reverse cholesterol transport pathway, such as CETP, PLTP, or scavenger receptor BI for therapeutic benefit.

Lipid transfer protein activities in type 1 diabetic patients without renal failure and nondiabetic control subjects and their association with coronary artery calcification

This study examined the role of cholesteryl ester transfer (CET), cholesteryl ester transfer protein (CETP) activity, and phospholipid transfer protein (PLTP) activity in the increased prevalence of coronary artery calcification (CAC) in diabetic subjects compared with nondiabetic subjects and in the loss of the sex difference in CAC in diabetes. CETP activity, PLTP activity, and CET were measured in 195 type 1 diabetic subjects without renal failure and 194 nondiabetic control subjects of similar age (30-55 years) and sex distribution (50% female). CAC was quantified with electron beam computed tomography. CETP activity was higher in diabetic subjects (mean 84 arbitrary units [AU]) than in nondiabetic subjects (80 AU, P = 0.028). PLTP activity was also higher in diabetic subjects (96 AU) than in nondiabetic subjects (81 AU, P < 0.001). However, CET was lower in diabetic men (geometric mean 32 nmol. ml(-1).h(-1)) than nondiabetic men (37 nmol.ml(-1).h(-1), P = 0.004) and did not differ between diabetic (30 nmol. ml(-1).h(-1)) and nondiabetic (32 nmol.ml(-1).h(-1), P = 0.3) women. CETP and PLTP activities were not associated with CAC. CET was positively associated with CAC in both diabetic and nondiabetic subjects (odds ratio per 10 nmol.ml(-1).h(-1) increase in CET in all subjects = 1.4, P = 0.001). The prevalence of CAC was similar in diabetic (51%) and nondiabetic (54%, P = 0.7) men but was much higher in diabetic (47%) than nondiabetic (21%, odds ratio 3.6, P < 0.001) women so that there was no sex difference in CAC in diabetic subjects. The odds of CAC in diabetic women compared with nondiabetic women was altered little by adjustment for CETP activity, PLTP activity, or CET (odds ratio on adjustment 3.7, P < 0.001). The greater effect of diabetes on CAC in women than in men, i.e., the loss of the sex difference in CAC, was independent of CETP and PLTP activity and CET. In conclusion, among both diabetic and nondiabetic subjects, higher cholesteryl ester transfer is a risk factor for CAC. However, abnormalities in cholesteryl ester transfer or lipid transfer protein activities do not underlie the increased CAC risk in diabetic women compared with nondiabetic women or the loss of the sex difference in CAC in diabetes.

Quantification of human plasma phospholipid transfer protein (PLTP): relationship between PLTP mass and phospholipid transfer activity

A sensitive sandwich-type enzyme-linked immunosorbent assay (ELISA) for human plasma phospholipid transfer protein (PLTP) has been developed using a monoclonal capture antibody and a polyclonal detection antibody. The ELISA allows for the accurate quantification of PLTP in the range of 25-250 ng PLTP/assay. Using the ELISA, the mean plasma PLTP concentration in a Finnish population sample (n = 159) was determined to be 15.6 +/- 5.1 mg/l, the values ranging from 2.30 to 33.4 mg/l. PLTP mass correlated positively with HDL-cholesterol (r = 0.36, P < 0.001), apoA-I (r = 0.37, P < 0.001), apoA-II (r = 0.20, P < 0.05), Lp(A-I) (r=0.26, P=0.001) and Lp(A-I/A-II) particles (r=0.34, P<0.001), and negatively with body mass index (BMI) (r = -0.28, P < 0.001) and serum triacylglycerol (TG) concentration (r = -0.34, P < 0.001). PLTP mass did not correlate with phospholipid transfer activity as measured with a radiometric assay. The specific activity of PLTP, i.e. phospholipid transfer activity divided by PLTP mass, correlated positively with plasma TG concentration (r=0.568, P<0.001), BMI (r=0.45, P<0.001), apoB (r = 0.45, P < 0.001). total cholesterol (r=0.42, P < 0.001), LDL-cholesterol (r = 0.34, P < 0.001) and age (r = 0.36, P < 0.001), and negatively with HDL-cholesterol (r= -0.33, P < 0.001), Lp(A-I) (r= -0.21, P < 0.01) as well as Lp(A-I/A-II) particles (r = -0.32, P < 0.001). When both PLTP mass and phospholipid transfer activity were adjusted for plasma TG concentration, a significant positive correlation was revealed (partial correlation, r = 0.31, P < 0.001). The results suggest that PLTP mass and phospholipid transfer activity are strongly modulated by plasma lipoprotein composition: PLTP mass correlates positively with parameters reflecting plasma high density lipoprotein (HDL) levels, but the protein appears to be most active in subjects displaying high TG concentration.

Insulin decreases plasma cholesteryl ester transfer but not cholesterol esterification in healthy subjects as well as in normotriglyceridaemic patients with type 2 diabetes

BACKGROUND

Plasma cholesterol esterification (EST) and subsequent cholesteryl ester transfer (CET) from high-density lipoproteins (HDLs) towards apolipoprotein (apo) B-containing lipoproteins are key steps in HDL metabolism.

MATERIALS AND METHODS

The effects of exogenous hyperinsulinaemia on plasma CET and EST, measured with isotope methods, were evaluated in 10 male normotriglyceridaemic (plasma triglycerides <2.0 mmol L-1) patients with type 2 diabetes and 10 individually matched healthy subjects during a two-step hyperinsulinaemic euglycaemic clamp over 6-7 h.

RESULTS

No between-group differences in baseline plasma lipid parameters were observed, but the HDL cholesteryl ester content was lower (P < 0.02) and the HDL triglyceride content was higher (P < 0.05) in diabetic patients. Baseline CET and EST were similar in the groups. In both groups, hyperinsulinaemia decreased plasma triglycerides (P < 0.01) and the HDL triglyceride content (P < 0.01) compared with saline infusion in healthy subjects, whereas the HDL cholesteryl ester content increased (P < 0.05 vs. saline infusion) in diabetic patients. CET was similarly decreased by hyperinsulinaemia in both groups (P < 0.01 vs. saline infusion). In contrast, the change in EST in either group was not different from that during saline administration. In the combined group, baseline CET was positively correlated with plasma triglycerides (Rs = 0.68, P < 0.01). The HDL cholesteryl ester content was negatively (Rs = -0.48, P < 0.05) and the HDL triglyceride content was positively (Rs = 0.64, P < 0.01) correlated with CET.

CONCLUSION

Insulin infusion decreases plasma CET in conjunction with a fall in triglycerides but does not decrease cholesterol esterification in healthy and type 2 diabetic subjects, indicating that acute hyperinsulinaemia has a different effect on these processes involved in HDL metabolism. Despite unaltered fasting plasma CET, HDL core lipid composition was abnormal in diabetic patients, suggesting that additional mechanisms may contribute to changes in HDL metabolism in diabetes mellitus.

High density associated enzymes: their role in vascular biology

Enzymes associated with circulating HDL include lecithin: cholesterol acyl transferase, phospholipid transfer protein, cholesterol ester transfer protein, paraoxonase 1 and platelet activating factor acetylhydrolase. Together with lipoprotein lipase and hepatic lipase these enzymes produce important lipoprotein remodeling and modulate their structure and function and therefore their role in artery wall metabolism.

Relationships between insulin resistance and lipoproteins in nondiabetic African Americans, Hispanics, and non-Hispanic whites: the Insulin Resistance Atherosclerosis Study

The study purpose was to explore the association between dyslipidemia and insulin resistance in three ethnic groups. The Insulin Resistance Atherosclerosis Study (IRAS) is a multicenter epidemiologic study conducted at four clinical centers in California, Texas, and Colorado. The study population for this analysis consisted of 931 non-Hispanic white, African American, and Hispanic men and women (aged 45 to 64 years) without diabetes. The IRAS clinical examinations included lipoprotein measures, a 75-g glucose tolerance test, and the frequently sampled intravenous glucose tolerance (FSIGT) test. The results show a consistent relationship between insulin-mediated glucose disposal and dyslipidemia in African American, Hispanic, and non-Hispanic white men and women. Further, LDL size was inversely associated with insulin resistance in all three ethnic groups. These findings indicate that dyslipidemia is a fundamental part of the insulin resistance syndrome in all of the ethnic groups studied.

Elevated plasma cholesteryl ester transfer in NIDDM: relationships with apolipoprotein B-containing lipoproteins and phospholipid transfer protein

Lecithin:cholesteryl acyl transferase (LCAT) and cholesteryl ester transfer protein (CETP) are key factors in the esterification of cholesterol and the subsequent transfer of cholesteryl ester from high density lipoproteins (HDL) towards very low and low density lipoproteins (VLDL + LDL). Phospholipid transfer protein (PLTP), lipoprotein lipase (LPL) and hepatic lipase (HL) are involved in plasma phospholipid and triglyceride metabolism and also affect HDL. Equivocal changes in plasma cholesteryl ester transfer have been reported in non-insulin-dependent diabetes mellitus (NIDDM). In 16 NIDDM men with plasma triglycerides < or = 4.5 mmol/l and cholesterol < or = 8.0 mmol/l. plasma cholesteryl ester transfer (CET), cholesterol esterification rate, LCAT and PLTP activity levels were higher (P < 0.05 to P < 0.02) in conjunction with higher plasma triglycerides (P < 0.01) and lower HDL cholesterol and cholesteryl ester levels (P < 0.05) compared to 16 matched healthy men. Multiple stepwise regression analysis demonstrated that CET was positively related to VLDL + LDL cholesterol (P < 0.001), triglycerides (P = 0.001), PLTP activity (P = 0.007) and CETP activity (P = 0.008, multiple r = 0.94). NIDDM had no effect on CET, independently from these parameters. HDL cholesteryl ester was negatively related to CET (P= 0.017), HL activity (P = 0.033) and NIDDM (P = 0.047) and positively to LCAT activity levels (P = 0.034, multiple r = 0.68). It is concluded that the elevated CET in plasma from NIDDM patients is associated with higher plasma triglycerides and PLTP activity levels. Furthermore, our data suggest that in normo- and moderately dyslipidaemic subjects PLTP and CETP activity levels per se may influence the rate of cholesteryl ester transfer in plasma. Plasma cholesteryl ester transfer appears to be a determinant of HDL cholesteryl ester, but other factors are likely to contribute to lower HDL cholesteryl ester levels in NIDDM.

Structure and function of the plasma phospholipid transfer protein

Recent cloning and sequencing of plasma phospholipid transfer protein complementary DNA revealed that phospholipid transfer protein belongs to the lipid transfer/lipopolysaccharide binding protein family that includes the cholesteryl ester transfer protein, the bactericidal permeability increasing protein and the lipopolysaccharide-binding protein. In addition to structural similarities, members of the lipid transfer/lipopolysaccharide-binding protein family might share some common functional properties, and recent studies demonstrated that phospholipid transfer protein can act in several distinct metabolic processes. In particular, the molecular transfer of phospholipids, unesterified cholesterol, alpha-tocopherol and lipopolysaccharides by phospholipid transfer protein suggests that it might be involved both in lipoprotein metabolism and in antimicrobial defence, resulting in a growing interest in this protein.