Effect of growth hormone replacement therapy on plasma lecithin:cholesterol acyltransferase and lipid transfer protein activities in growth hormone-deficient adults

The effects of growth hormone (GH) replacement on plasma lecithin:cholesterol acyltransferase (LCAT), cholesteryl ester transfer protein (CETP), and phospholipid transfer protein (PLTP), factors involved in high density lipoprotein (HDL) metabolism, are unknown. We carried out a 6 months study in 24 GH-deficient adults who were randomized to placebo (n = 8), low dose GH (1 U daily, n = 8), and high dose GH (2 U daily, n = 8), followed by a 6 months open extension study with high dose GH (1 drop-out). No significant changes in plasma lipoproteins, LCAT, CETP, and PLTP activities, cholesterol esterification (EST) and cholesteryl ester transfer (CET) were observed after placebo. After 6 months of GH (combined data, n = 24), very low + low density lipoprotein (VLDL + LDL) cholesterol (P < 0.05) and apolipoprotein B (P < 0.05) decreased, whereas HDL cholesterol and HDL cholesteryl ester increased (P < 0. 05). Prolonged treatment showed comparable effects. Plasma apolipoprotein A-I and Lp[a] remained unchanged. Plasma LCAT (P < 0. 01) and CETP activities (P < 0.01), as well as EST (P < 0.01) and CET decreased (P < 0.01) after 12 months of GH (n = 15), but PLTP activity did not significantly change. Changes in EST and CET after 12 months of treatment were independently related to changes in plasma LCAT (P = 0.001 and CETP activity (P = 0.01). In conclusion, GH replacement therapy improves the lipoprotein profile in GH-deficient adults. Chronic GH replacement lowers plasma LCAT and CETP activities, contributing to a decrease in cholesterol esterification and cholesteryl ester transfer. These effects may have consequences for HDL metabolism and reverse cholesterol transport.     

Increased concentration of plasma cholesteryl ester transfer protein in nephrotic syndrome: role in dyslipidemia.

Hyperlipidemia is a prominent feature of the nephrotic syndrome. Lipoprotein abnormalities include increased very low and low density lipoprotein (VLDL and LDL) cholesterol and variable reductions in high density lipoprotein (HDL) cholesterol. We hypothesized that plasma cholesteryl ester transfer protein (CETP), which influences the distribution of cholesteryl esters among the lipoproteins, might contribute to lipoprotein abnormalities in nephrotic syndrome. Plasma CETP, apolipoprotein and lipoprotein concentrations were measured in 14 consecutive untreated and 7 treated nephrotic patients, 5 patients with primary hypertriglyceridemia, and 18 normolipidemic controls. Patients with nephrotic syndrome displayed increased plasma concentrations of apoB, VLDL, and LDL cholesterol. The VLDL was enriched with cholesteryl ester (CE), shown by a CE/triglyceride (TG) ratio approximately twice that in normolipidemic or hypertriglyceridemic controls (P < 0.001). Plasma CETP concentration was increased in patients with untreated nephrotic syndrome compared to controls (3.6 vs. 2.3 mg/l, P < 0.001), and was positively correlated with the CE concentration in VLDL (r = 0.69, P = 0.004) and with plasma apoB concentration (r = 0.68, P = 0.007). Treatment with corticosteroids resulted in normalization of plasma CETP and of the CE/TG ratio in VLDL. An inverse correlation between plasma CETP and HDL cholesterol was observed in hypertriglyceridemic nephrotic syndrome patients (r = -0.67, P = 0.03). The dyslipidemia of nephrotic syndrome includes increased levels of apoB-lipoproteins and VLDL that are unusually enriched in CE and likely to be atherogenic. Increased plasma CETP probably plays a significant role in the enrichment of VLDL with CE, and may also contribute to increased concentrations of apoB-lipoproteins and decreased HDL cholesterol in some patients.     

Acute and chronic effects of a 24-hour intravenous triglyceride emulsion challenge on plasma lecithin: cholesterol acyltransferase, phospholipid transfer protein, and cholesteryl ester transfer protein activities.

Lecithin:cholesterol acyltransferase (LCAT), phospholipid transfer protein (PLTP), and cholesteryl ester transfer protein (CETP) are key factors in remodeling of high density lipoproteins (HDL) and triglyceride-rich lipoproteins. We examined the effect of a large, 24 h intravenous fat load on plasma lipids and free fatty acids (FFA) as well as on plasma LCAT, PLTP, and CETP activity levels in 8 healthy men. The effect of concomitant insulin infusion was also studied, with 1 week between the study days. During Lipofundin(R) infusion, plasma triglycerides and FFA strongly increased after 8 and 24 h (P < 0.001), whereas HDL cholesterol decreased (P < 0.01). The increase in triglycerides was mitigated with concomitant insulin infusion (P < 0.05 from without insulin). Plasma LCAT activity increased by 17.7 +/- 7.7% after 8 h (P < 0.001) and by 26.1 +/- 11. 1% after 24 h (P < 0.001), PLTP activity increased by 19.7 +/- 15.6% after 24 h (P < 0.001), but CETP activity remained unchanged. Concomitant insulin infusion blunted the increase in plasma LCAT activity (P < 0.05 from without insulin), but not that in PLTP activity. One week after the first fat load, plasma non-HDL cholesterol (P < 0.02), and triglycerides (P = 0.05) were increased, whereas HDL cholesterol was decreased (P < 0.02). Plasma CETP and PLTP activity levels were increased by 34.8 +/- 30.4% (P < 0.02) and by 15.9 +/- 6.4% (P < 0.02), respectively, but LCAT activity was then unaltered. In summary, plasma LCAT, PLTP, and CETP activity levels are stimulated by a large intravenous fat load, but the time course of their responses and the effects of insulin coadministration are different. Changes in plasma LCAT and PLTP activities may be implicated in HDL and triglyceride-rich lipoprotein remodeling under the present experimental conditions.     

Enhanced cholesteryl ester transfer protein activities and abnormalities of high density lipoproteins in familial hypercholesterolemia.

Cholesteryl ester transfer protein may play a role in the cholesteryl ester metabolism between high density lipoproteins (HDL) and apolipoprotein B-containing lipoproteins. To investigate relationship between HDL and cholesteryl ester transfer protein (CETP) activity in the development of atherosclerosis, the present study has focused on CETP activity in the patients with familial hypercholesterolemia (GH). HDL-C and HDL-C/apo A-I mass ratio in heterozygous FH were lower than those in normolipidemic controls. There was a 2-fold increase in total CETP activity in incubated FH serum compared with normolipidemic controls. Assays for CETP activity in the lipoprotein deficient serum (d greater than 1.215 g/ml) were carried out by measuring the transfer of radioactive cholesteryl ester from HDL (1.125 less than d less than 1.21 g/ml) to LDL (1.019 less than d less than 1.060 g/ml). CETP activities in heterozygous FH (79 +/- 4 nmol/ml/h) was significantly higher than those in normolipidemic controls (54 +/- 6 nmol/ml/h). The increased total cholesteryl ester transfer mainly results from increased CETP activity in the d greater than 1.215 g/ml, possibly reflecting an increase in CETP mass in serum. Increased CETP activity in the d greater than 1.215 g/ml was correlated positively with IDL-cholesterol/triglyceride mass ratio (r = 0.496, p less than 0.01), and negatively with HDL-cholesterol/apo A-I mass ratio (r = -0.334, p less than 0.05). These results indicate that the enhanced CETP activities may contribute to increase risk for developing atherosclerosis in FH by changing the distribution of cholesteryl ester in serum lipoproteins.     

Plasma cholesteryl ester transfer protein mass and phospholipid transfer protein activity are associated with leptin in type 2 diabetes mellitus

Adipose tissue contributes to plasma levels of lipid transfer proteins and is also the major source of plasma adipokines. We hypothesized that plasma cholesteryl ester transfer protein (CETP) mass, phospholipid transfer protein (PLTP) activity and cholesteryl ester transfer (CET, a measure of CETP action) are determined by adipokine levels. In this study, relationships of plasma CETP mass, PLTP activity and CET with leptin, resistin and adiponectin were analyzed in type 2 diabetic patients and control subjects. Plasma PLTP activity (P<0.001), CET (P<0.001), leptin (P=0.003), resistin (P<0.001), high sensitive C-reactive protein (P=0.005), and insulin resistance (HOMA(ir)) (P<0.001) were higher, whereas HDL cholesterol (P<0.001) and plasma adiponectin (P<0.001) were lower in 83 type 2 diabetic patients (32 females) than in 83 sex-matched control subjects. Multiple linear regression analysis demonstrated that in diabetic patients plasma leptin levels were related to plasma CETP mass (P=0.018) and PLTP activity (P<0.001), but not to the other adipokines measured. Plasma CET was inversely correlated with adiponectin in univariate analysis, but this association disappeared in multivariate models that included plasma lipids and CETP. In conclusion, both plasma CETP mass and PLTP activity are associated with plasma leptin in type 2 diabetes. The elevated CET in these patients is not independently related to any of the measured plasma adipokines.     

Distribution of human plasma PLTP mass and activity in hypo- and hyperalphalipoproteinemia

Plasma phospholipid transfer protein (PLTP) plays an important role in lipoprotein metabolism and reverse cholesterol transport. We have recently reported that plasma PLTP concentration correlates positively with plasma HDL cholesterol (HDL-C) but not with PLTP activity in healthy subjects. We have also shown that PLTP exists as active and inactive forms in healthy human plasma. In the present study, we measured plasma PLTP concentration and PLTP activity, and analyzed the distribution of PLTP in normolipidemic subjects (controls), cholesteryl ester transfer protein (CETP) deficiency, and hypo-alphalipoproteinemia (hypo-ALP). Plasma PLTP concentration was significantly lower (0.7 +/- 0.4 mg/l, mean +/- SD, n = 9, P < 0.001) in the hypo-ALP subjects, and significantly higher (19.5 +/- 4.3 mg/l, n = 17, P < 0.001) in CETP deficiency than in the controls (12.4 +/- 2.3 mg/l, n = 63). In contrast, we observed no significant differences in plasma PLTP activity between controls, hypo-ALP subjects, and CETP deficiency (6.2 +/- 1.3, 6.1 +/- 1.8, and 6.8 +/- 1.2 micro mol/ml/h, respectively). There was a positive correlation between PLTP concentration and plasma HDL-C (r = 0.81, n = 89, P < 0.001). By size exclusion chromatography analysis, we found that the larger PLTP containing particles without PLTP activity (inactive form of PLTP) were almost absent in the plasma of hypo-ALP subjects, and accumulated in the plasma of CETP deficiency compared with those of controls. These results indicate that the differences in plasma PLTP concentrations between hypo-ALP subjects, CETP deficiency, and controls are mainly due to the differences in the amount of the inactive form of PLTP.     

Molecular biology and pathophysiological aspects of plasma cholesteryl ester transfer protein

Plasma cholesteryl ester transfer protein (CETP) facilitates the transfer of cholesteryl ester (CE) from high density lipoprotein (HDL) to apolipoprotein B-containing lipoproteins. Since CETP regulates the plasma levels of HDL cholesterol and the size of HDL particles, CETP is considered to be a key protein in reverse cholesterol transport, a protective system against atherosclerosis. CETP, as well as plasma phospholipid transfer protein, belongs to members of the lipid transfer/lipopolysaccharide-binding protein (LBP) gene family, which also includes the lipopolysaccharide-binding protein (LBP) and bactericidal/permeability-increasing protein. Although these four proteins possess different physiological functions, they share marked biochemical and structural similarities. The importance of plasma CETP in lipoprotein metabolism was demonstrated by the discovery of CETP-deficient subjects with a marked hyperalphalipoproteinemia (HALP). Two common mutations in the CETP gene, intron 14 splicing defect and exon 15 missense mutation (D442G), have been identified in Japanese HALP patients with CETP deficiency. The deficiency of CETP causes various abnormalities in the concentration, composition, and functions of both HDL and low density lipoprotein. Although the pathophysiological significance of CETP in terms of atherosclerosis has been controversial, the in vitro experiments showed that large CE-rich HDL particles in CETP deficiency are defective in cholesterol efflux. Epidemiological studies in Japanese-Americans and in the Omagari area where HALP subjects with the intron 14 splicing defect of CETP gene are markedly frequent, have shown an increased incidence of coronary atherosclerosis in CETP-deficient patients. The current review will focus on the recent findings on the molecular biology and pathophysiological aspects of plasma CETP, a key protein in reverse cholesterol transport.     

Mammalian adipose tissue and muscle are major sources of lipid transfer protein mRNA

The plasma cholesteryl ester transfer protein (CETP) catalyzes the transfer of cholesteryl esters from high density lipoproteins (HDL) to triglyceride-rich lipoproteins and plays a major role in the catabolism of HDL. Lipoprotein lipase (LPL) is the rate-limiting enzyme for hydrolysis of circulating triglyceride and is involved in HDL formation. We show that tissues containing LPL are major sources of CETP mRNA in several mammalian species, including some with low cholesteryl ester transfer activity in plasma. In hamsters, adipose tissue and heart were found to be the richest sources of both CETP and LPL mRNA; in situ hybridization studies showed that the same cell types (i.e. adipocytes or myocytes) contained CETP and LPL mRNA in these tissues. Isolated adipocytes synthesized active CETP. Dietary studies revealed a complex pattern of response of CETP mRNA levels in different tissues, which showed partial similarity to the changes in LPL mRNA abundance. However, high cholesterol diets resulted in increased CETP mRNA abundance in adipose tissue, heart, and skeletal muscle, without equivalent changes in LPL mRNA. Plasma HDL cholesteryl ester levels showed strong inverse correlations with CETP mRNA abundance in adipose tissue. The results suggest a conserved function of CETP in adipose tissue and heart, such as a co-ordinate action with LPL to enhance HDL turnover. Although there is considerable overlap in the tissue- and cell-specific pattern of CETP and LPL gene expression, dietary studies revealed only limited parallelism in response at the mRNA level. The increase in CETP mRNA in peripheral tissues in response to increased dietary cholesterol suggests that local induction of CETP synthesis may help to recycle cholesterol deposited in these tissues during lipolysis of dietary lipoproteins.     

Familial cholesteryl ester transfer protein deficiency is associated with triglyceride-rich low density lipoproteins containing cholesteryl esters of probable intracellular origin

The net transfer of core lipids between lipoproteins is facilitated by cholesteryl ester transfer protein (CETP). We have recently documented CETP deficiency in a family with hyperalphalipoproteinemia, due to a CETP gene splicing defect. The purpose of the present study was to characterize the plasma lipoproteins within the low density lipoprotein (LDL) density range and also the cholesteryl ester fatty acid distribution amongst lipoproteins in CETP-deficient subjects. In CETP deficiency, the conventional LDL density range contained both an apoE-rich enlarged high density lipoprotein (HDL) (resembling HDLc), and also apoB-containing lipoproteins. Native gradient gel electrophoresis revealed clear speciation of LDL subclasses, including a distinct population larger in size than normal LDL. Anti-apoB affinity-purified LDL from the CETP-deficient subjects were shown to contain an elevated triglyceride to cholesteryl ester ratio, and also a high ratio of cholesteryl oleate to cholesteryl linoleate, compared to their own HDL or to LDL from normal subjects. Addition of purified CETP to CETP-deficient plasma results in equilibration of very low density lipoprotein (VLDL) cholesteryl esters with those of HDL. These data suggest that, in CETP-deficient humans, the cholesteryl esters of VLDL and its catabolic product, LDL, originate predominantly from intracellular acyl-CoA:cholesterol acyltransferase (ACAT). The CETP plays a role in the normal formation of LDL, removing triglyceride and transferring LCAT-derived cholesteryl esters into LDL precursors.     

Constitutive inhibition of plasma CETP by apolipoprotein C1 is blunted in dyslipidemic patients with coronary artery disease

Plasma cholesteryl ester transfer protein (CETP) promotes the cholesterol enrichment of apoB-containing lipoproteins (VLDL and LDL) at the expense of HDL. Recent studies demonstrated that apoC1 is a potent CETP inhibitor in plasma of healthy, normolipidemic subjects. Our goal was to establish whether the modulation of CETP activity by apoC1 is influenced by dyslipidemia in patients with documented coronary artery disease (CAD). In the total CAD population studied (n = 240), apoC1 levels correlated negatively with CETP activity, independently of apoE-epsilon, CETP-Taq1B, and apoC1-Hpa1 genotypes. In multivariate analysis, the negative relationship was observed only in normolipidemic patients, not in those with hypercholesterolemia, hypertriglyceridemia, or combined hyperlipidemia. In the normolipidemic subjects, apoC1 levels were positively associated with higher HDL- to LDL-cholesterol ratio (r = 0.359, P < 0.001). It is concluded that apoC1 as a CETP inhibitor no longer operates on cholesterol redistribution in high-risk patients with dyslipidemia, probably due to increasing amounts of VLDL-bound apoC1, which is inactive as a CETP inhibitor. Patients with dyslipidemia could experience major benefits from treatment with pharmacological CETP inhibitors, which might compensate for blunted endogenous inhibition.     

Mechanisms of enhancement of cholesteryl ester transfer protein activity by lipolysis

Lipoprotein lipase enhances the cholesteryl ester transfer protein (CETP)-mediated transfer of cholesteryl esters from plasma high density lipoproteins (HDL) to very low density lipoproteins (VLDL). In time course studies the stimulation of cholesteryl ester transfer by bovine milk lipase was correlated with accumulation of fatty acids in VLDL remnants. As the amount of fatty acid-poor albumin in the incubations was increased, there was decreased accumulation of fatty acids in VLDL remnants and a parallel decrease in the stimulation of cholesteryl ester transfer by lipolysis. Addition of sodium oleate to VLDL and albumin resulted in stimulation of the CETP-mediated transfer of cholesteryl esters from HDL to VLDL. The stimulation of transfer of cholesteryl esters into previously lipolyzed VLDL was abolished by lowering the pH from 7.5 to 6.0, consistent with a role of lipoprotein ionized fatty acids. CETP-mediated cholesteryl ester transfer from HDL to VLDL was also augmented by phosholipase A2 and by a bacterial lipase which lacked phospholipase activity. When VLDL and HDL were re-isolated after a lipolysis experiment, both lipoproteins stimulated CETP activity. Postlipolysis VLDL and HDL bound much more CETP than native VLDL or HDL. Lipolysis of apoprotein-free phospholipid/triglyceride emulsions also resulted in enhanced binding of CETP to the emulsion particles. Incubation conditions which abolished the enhanced cholesteryl ester transfer into VLDL remnants reduced binding of CETP to remnants, emulsions, and HDL. In conclusion, the enhanced CETP-mediated transfer of cholesteryl esters from HDL to VLDL during lipolysis is related to the accumulation of products of lipolysis, especially fatty acids, in the lipoproteins. Lipids accumulating in VLDL remnants and HDL as a result of lipolysis may augment binding of CETP to these lipoproteins, leading to more efficient transfer of cholesteryl esters from HDL to VLDL.     

Elevation of plasma phospholipid transfer protein in transgenic mice increases VLDL secretion

Two lipid transfer proteins are active in human plasma, cholesteryl ester transfer protein (CETP), and phospholipid transfer protein (PLTP). Mice by nature do not express CETP. Additional inactivation of the PLTP gene resulted in reduced secretion of VLDL and subsequently in decreased susceptibility to diet-induced atherosclerosis. The aim of this study is to assess possible effects of differences in PLTP expression on VLDL secretion in mice that are proficient in CETP and PLTP. We compared human CETP transgenic (huCETPtg) mice with mice expressing both human lipid transfer proteins (huCETPtg/huPLTPtg). Plasma cholesterol in huCETPtg mice was 1.5-fold higher compared with huCETPtg/huPLTPtg mice (P < 0.001). This difference was mostly due to a lower HDL level in the huCETPtg/huPLTPtg mice, which subsequently could lead to the somewhat decreased CETP activity and concentration that was found in huCETPtg/huPLTPtg mice (P < 0.05). PLTP activity was 2.8-fold increased in these animals (P < 0.001). The human PLTP concentration was 5 microg/ml. Moderate overexpression of PLTP resulted in a 1.5-fold higher VLDL secretion compared with huCETPtg mice (P < 0.05). The composition of nascent VLDL was similar in both strains. These results indicate that elevated PLTP activity in huCETPtg mice results in an increase in VLDL secretion. In addition, PLTP overexpression decreases plasma HDL cholesterol as well as CETP.     

Cholesteryl ester transfer protein inhibition, high-density lipoprotein metabolism and heart disease risk reduction

PURPOSE OF REVIEW: Cholesteryl ester transfer protein (CETP) inhibitors (JTT-705 and torcetrapib) are currently in clinical testing, and significantly raise high-density lipoprotein (HDL) cholesterol levels. Low HDL cholesterol is a significant independent predictor of coronary heart disease (CHD) and HDL raising has been associated with coronary heart disease risk reduction, but there is debate about whether CETP inhibition will reduce coronary heart disease risk. RECENT FINDINGS: It has been documented in transgenic mouse models that apolipoprotein (apo) C-I inhibits CETP, and that high mono-unsaturated fat diets prevent the normal stimulation of CETP activity by dietary cholesterol. In rabbits, torcetrapib markedly decreases clearance of HDL cholesteryl ester via an indirect pathway, but has no effect on total plasma cholesteryl ester clearance. In humans, torcetrapib raises HDL apoA-I by modestly decreasing its fractional catabolic rate, while having a very profound effect on raising HDL cholesterol and large alpha-1 migrating HDL particles by more than 50%, with no effect on fecal cholesterol excretion. When JTT-705 at 600 mg/day was given to hypercholesterolemic patients already on pravastatin 40 mg/day, the combination was well tolerated and increases in HDL cholesterol of 28% were noted. SUMMARY: In our view, CETP inhibitors in combination with statins will be profoundly beneficial in reducing human atherosclerosis, primarily because they normalize HDL particles and prevent the transfer of cholesteryl ester from HDL to atherogenic lipoproteins.     

Human apoA-I expression in CETP transgenic rats leads to lower levels of apoC-I in HDL and to magnification of CETP-mediated lipoprotein changes

Plasma cholesteryl ester transfer protein (CETP) has a profound effect on neutral lipid transfers between HDLs and apolipoprotein B (apoB)-containing lipoproteins when it is expressed in combination with human apoA-I in HuAI/CETP transgenic (Tg) rodents. In the present study, human apoA-I-mediated lipoprotein changes in HuAI/CETPTg rats are characterized by 3- to 5-fold increments in the apoB-containing lipoprotein-to-HDL cholesterol ratio, and in the cholesteryl ester-to-triglyceride ratio in apoB-containing lipoproteins. These changes occur despite no change in plasma CETP concentration in HuAI/CETPTg rats, as compared with CETPTg rats. A number of HDL apolipoproteins, including rat apoA-I and rat apoC-I are removed from the HDL surface as a result of human apoA-I overexpression. Rat apoC-I, which is known to constitute a potent inhibitor of CETP, accounts for approximately two-thirds of CETP inhibitory activity in HDL from wild-type rats, and the remainder is carried by other HDL-bound apolipoprotein inhibitors. It is concluded that human apoA-I overexpression modifies HDL particles in a way that suppresses their ability to inhibit CETP. An apoC-I decrease in HDL of HuAI/CETPTg rats contributes chiefly to the loss of the CETP-inhibitory potential that is normally associated with wild-type HDL.     

Effects of alcohol on the major steps of reverse cholesterol transport

The effects of moderate alcohol consumption on the capacity of blood sera to promote acceptance of cholesterol (C) from Fu5AH hepatoma cells, esterification of delivered free C, and transfer of produced cholesteryl esters to apolipoprotein (apo) B-containing lipoproteins have been studied. Twenty male subjects with relatively high (>50 mg/dl, n = 10) and low (<50 mg/dl, n = 10) high density lipoprotein (HDL) C levels consumed for eight weeks red grape wine (0.3 g ethanol/kg body mass per day). Alcohol consumption reduced total C and low density lipoprotein C levels in both groups of subjects. Low HDL C subjects showed an increase in HDL C, apo AI, apo AII, and lipoprotein (Lp) AI particle levels after alcohol consumption. Alcohol did not affect free C efflux from the cells. However, after the following period of substitution of alcohol with an isocaloric amount of red grape juice, cellular C efflux markedly reduced. While lecithin:cholesterol acyltransferase (LCAT) activity increased during alcohol consumption only in subjects with low HDL C, high HDL C subjects showed a significant decrease in cholesteryl ester transfer protein (CETP) activity. At the same time, alcohol consumption reduced the endogenous C esterification rate and increased the transfer of endogenous cholesteryl esters to apo B-containing lipoproteins in both groups. Thus, alcohol consumption in moderate doses enhanced the anti-atherogenicity of the serum lipoprotein spectrum, supporting more effective C efflux from peripheral cells and transport of accepted C to apo B-containing lipoproteins. The effects of alcohol on the reverse cholesterol transport depend on the initial HDL C level.     

Influence of insulin sensitivity and the TaqIB cholesteryl ester transfer protein gene polymorphism on plasma lecithin:cholesterol acyltransferase and lipid transfer protein activities and their response to hyperinsulinemia in non-diabetic men.

Lecithin:cholesteryl acyl transferase (LCAT), cholesteryl ester transfer protein (CETP), phospholipid transfer protein (PLTP), and lipoprotein lipases are involved in high density lipoprotein (HDL) metabolism. We evaluated the influence of insulin sensitivity and of the TaqIB CETP gene polymorphism (B1B2) on plasma LCAT, CETP, and PLTP activities (measured with exogenous substrates) and their responses to hyperinsulinemia. Thirty-two non-diabetic men without hyperlipidemia were divided in quartiles of high (Q(1)) to low (Q(4)) insulin sensitivity. Plasma total cholesterol, very low + low density lipoprotein cholesterol, triglycerides, and apolipoprotein (apo) B were higher in Q(4) compared to Q(1) (P < 0.05 for all), whereas HDL cholesterol and apoA-I were lowest in Q(4) (P < 0.05 for both). Plasma LCAT activity was higher in Q(4) than in Q(1) (P < 0. 05) and PLTP activity was higher in Q(4) than in Q(2) (P < 0.05). Insulin sensitivity did not influence plasma CETP activity. Postheparin plasma lipoprotein lipase activity was highest and hepatic lipase activity was lowest in Q(1). Insulin infusion decreased PLTP activity (P < 0.05), irrespective of the degree of insulin sensitivity. The CETP genotype exerted no consistent effects on baseline plasma lipoproteins and LCAT, CETP, and PLTP activities. The decrease in plasma PLTP activity after insulin was larger in B1B1 than in B2B2 homozygotes (P < 0.05). These data suggest that insulin sensitivity influences plasma LCAT, PLTP, lipoprotein lipase, and hepatic lipase activities in men. As PLTP, LCAT, and hepatic lipase may enhance reverse cholesterol transport, it is tempting to speculate that high levels of these factors in association with insulin resistance could be involved in an antiatherogenic mechanism. A possible relationship between the CETP genotype and PLTP lowering by insulin warrants further study.     

Human plasma cholesteryl ester transfer protein enhances the transfer of cholesteryl ester from high density lipoproteins into cultured HepG2 cells

The role of human plasma cholesteryl ester transfer protein (CETP) in the cellular uptake of high density lipoprotein (HDL) cholesteryl ester (CE) was studied in a liver tumor cell line (HepG2). When HepG2 cells were incubated with [3H]cholesteryl ester-labeled HDL3 in the presence of increasing concentrations of CETP there was a progressive increase in cell-associated radioactivity to levels that were 2.8 times control. The CETP-dependent uptake of HDL-CE was found to be saturated by increasing concentrations of both CETP and HDL. The CETP-dependent uptake of CE radioactivity increased continuously during an 18-h incubation. In contrast to the effect on cholesteryl ester, CETP failed to enhance HDL protein cell association or degradation. Enhanced uptake of HDL cholesteryl ester was shown for the d greater than 1.21 g/ml fraction of human plasma, partially purified CETP, and CETP purified to homogeneity, but not for the d greater than 1.21 g/ml fraction of rat plasma which lacks cholesteryl ester transfer activity. HDL cholesteryl ester entering the cell under the influence of CETP was largely degraded to free cholesterol by a process inhibitable by chloroquine. CETP enhanced uptake of HDL [3H]CE in cultured smooth muscle cells and to a lesser extent in fibroblasts but did not significantly influence uptake in endothelial cells or J774 macrophages. These experiments show that, in addition to its known role in enhancing the exchange of CE between lipoproteins, plasma CETP can facilitate the in vitro selective transfer of CE from HDL into certain cells.     

Triacylglycerol increase in plasma very low density lipoproteins in cyclophosphamide-treated rabbit: relationship with cholesteryl ester transfer activity

We have studied the cholesteryl ester transfer between HDL and VLDL in cyclophosphamide-treated rabbits, in order to explain the abnormal cholesteryl ester partition between these two lipoprotein classes. The hypertriglyceridemia caused by treatment with the drug was associated with cholesteryl ester- and triacylglycerol-rich VLDL and with HDL poor in esterified cholesterol but relatively enriched in triacylglycerol. These two lipoprotein classes were characterized by their chemical composition and by gel filtration chromatography. VLDL particles were slightly larger in size, compared with controls. Different transfer combinations were envisaged between these abnormal lipoproteins and control ones. The transfer study involved the plasma fraction of d greater than 1.21 g/ml containing the cholesteryl ester transfer protein (CETP). It appeared that the chemical composition of lipoproteins was responsible for the level of cholesteryl ester transfer between lipoproteins. Actually, when the cholesteryl ester acceptor lipoproteins (VLDL) were enriched in triacylglycerol, the transfer was enhanced. Therefore, the effect of lipolysis on the transfer has also been explored. Lipoprotein lipase seemed to enhance the transfer of cholesteryl ester from HDL to VLDL when these lipoproteins were normal, but an important decline was obtained when triacylglycerol-rich VLDL were lipolyzed. This study defines the relationship between lipoprotein chemical composition and transfer activity of cholesteryl ester from HDL to VLDL.     

Influence of dietary fatty acid composition on the relationship between CETP activity and plasma lipoproteins in monkeys.

CETP activity, measured as transfer of cholesteryl ester from exogenous HDL to exogenous VLDL and LDL, reflecting CETP mass as determined by ELISA, was documented in three groups of St. Kitts vervet monkeys fed diets enriched in saturated (Sat), monounsaturated (Mono), or n-6 polyunsaturated (Poly) fatty acids. CETP activity was not different when comparing the three dietary fats. However, CETP activity was significantly higher when cholesterol was added to each of the diets. Significant positive associations between CETP activity and VLDL and LDL cholesterol concentrations were found whereas significant negative associations were seen between CETP activity and HDL cholesterol in each of the diet groups. The strength of these associations was highest in the Sat group. Cholesteryl ester (CE) fatty acid composition of lipoproteins varied widely among diet groups, with the more polyunsaturated CE of the Poly group being associated with a higher rate of CE transfer to endogenous acceptor apolipoprotein B-containing lipoproteins. Finally, only the Sat diet group showed significant positive correlations of CETP activity with LDL particle diameter (r = 0.76), cholesteryl ester percentage (r = 0.67), and a strong negative correlation (r = -0.86) with LDL receptor function, estimated as the difference between native and methylated LDL turnover rates. We speculate that strong associations between CETP and LDL metabolism may explain, at least in part, the increased atherogenicity of dietary saturated fat.     

Effect of a neutralizing monoclonal antibody to cholesteryl ester transfer protein on the redistribution of apolipoproteins A-IV and E among human lipoproteins

The effect of inhibiting cholesteryl ester transfer protein (CETP) on the in vitro redistribution of apolipoproteins(apo) A-IV and apoE among lipoproteins in whole plasma was studied in seven normal male subjects. Plasmas were incubated in the presence of a purified monoclonal antibody TP2 (Mab TP2) that neutralizes the activity of CETP. Mab TP2 had no effect on lecithin:cholesterol acyltransferase (LCAT) activity. Prior to and following a 6-h incubation at 37 degrees C in the presence of Mab TP2 or a control mouse myeloma immunoglobulin (IgG), plasmas were gel-filtered on Sephacryl S-300 and the distribution of apoA-IV and apoE among lipoproteins was determined by radioimmunoassay. Incubation (i.e., with active LCAT and CETP) increased the amount of apoA-IV associated with lipoproteins by 240%. When CETP activity was inhibited during incubation, the amount of apoA-IV that became lipoprotein-associated was significantly increased (315% of basal). Plasma incubation also caused a redistribution of apoE from high density lipoproteins (HDL) to larger lipoproteins (131% of basal); however, when CETP was inhibited, significantly greater amounts of apoE became associated with the larger particles (155% of basal). These effects were observed in all seven subjects. Increased movement of apoE from HDL to triglyceride-rich particles was not due to displacement by apoA-IV since loss of apoE from HDL was still observed when no movement of apoA-IV onto HDL occurred, such as during LCAT or combined LCAT and CETP inhibition. We speculate that low CETP activity (e.g., in species such as rats) may lead to an increased content of HDL apoA-IV and also to apoE enrichment of triglyceride-rich lipoproteins, augmenting their clearance.