Plasma lipoprotein conversions: the origins of low-density and high-density lipoproteins

Reduced Capacity of High-Density Lipoprotein to Acquire Free Cholesterol From Triglyceride-Rich Lipoproteins Is Associated With Elevated Postprandial Hypertriglyceridemia in Healthy Men

BACKGROUND

The capacity of high-density lipoprotein cholesterol (HDL) to acquire free cholesterol (FC) from triglyceride-rich lipoproteins during lipoprotein lipase-dependent lipolysis in a process of reverse remnant cholesterol transport, has been proposed as a key biological function of HDL particles that underlies the U-shaped relationship between HDLcholesterol and cardiovascular diseases. Although reverse remnant cholesterol transport has been evaluated in a fasting state, it has never been explored under nonfasting conditions.

METHODS AND RESULTS

FC transfer was evaluated in healthy men (n=78) before and throughout the postprandial phase up to 8 hours after consumption of a test meal. Postprandially, the capacity of HDL to acquire FC increased progressively, reaching a maximal mean value of 98.5%±22.5% 6 hours after meal intake (P<0.05). Analysis of the study population according to tertiles of postprandial variation of FC transfer identified subjects exhibiting reduced capacity of HDL to acquire FC (tertile 1), those for whom the capacity of HDL to acquire FC remained unchanged (tertile 2), and subjects characterized by an enhanced FC transfer during the postprandial phase (tertile 3). Across the tertiles, we found an inverse relationship between the maximal postprandial change in FC transfer to HDL and the degree of postprandial triglyceride response.

CONCLUSIONS

Healthy individuals exhibiting exacerbated postprandial triglyceride response and reduced HDL cholesterol levels feature reduced FC transfer to HDL during the postprandial state. These data suggest that to normalize postprandial triglyceride response, 2 conditions need to be fulfilled: notably elevated FC transfer to HDL in the postprandial phase and increased levels of acceptor HDL particles.

High density lipoprotein composition versus heredity for acute myocardial infarction in middle-aged males

The levels of high density lipoprotein (HDL) cholesterol and apolipoprotein AI (Apo AI) were determined in 299 men, 48 years of age. We found a significant negative correlation between HDL cholesterol and serum triglyceride concentrations and a significant correlation between serum triglyceride concentration and the Apo AI/HDL cholesterol ratio. A history of acute myocardial infarction (AMI) was significantly more common than expected among the fathers of men with a low HDL cholesterol level. We also found a relationship between AMI among fathers and high Apo AI level in sons, although this relationship was not as strong as that between AMI among fathers and low HDL cholesterol in sons. No significant relationship was found between AMI among mothers and HDL cholesterol or Apo AI in sons.

LDL composition in E2/2 subjects and LDL distribution by Apo E genotype in type 1 diabetes

Apo E plays an important role in chylomicron and VLDL remnant processing, uptake or conversion to LDL. The type of lipoprotein that isolates in the LDL density of E2/2 subjects was investigated and the effect of the apo E isoforms on LDL mass was determined in all genotypes in a large group of Type 1 diabetics. Analysis of the LDL composition of E2/2 homozygotes (n=6) compared to subjects with the common E3/3 isoform (n=6) demonstrated an enrichment in apo E, unesterified cholesterol, phospholipid and triglyceride relative to apo B in E2/2 subjects, more typical of a dense IDL remnant than of LDL. Although diabetics were studied, these findings are considered to reflect those of the general population. Comparison of the lipoprotein distribution of homozygous and heterozygous subjects revealed that, as genotype changed from E4/4 (n=22) to E3/4 (n=262), E3/3 (n=710)=E2/4 (n=30), E2/3 (n=151), E2/2 (n=6), LDL cholesterol decreased significantly in a stepwise manner. The decrease was not in a specific subgroup of LDL. In conclusion, for E2/2 subjects, lipoproteins isolated in the LDL density range appear to be composed mainly of dense IDL remnants and some Lp(a). The apo E isoform also has a significant effect on LDL concentration in both homozygotes and heterozygotes.

R219K polymorphism of the ABCA1 gene and its modulation of the variations in serum high-density lipoprotein cholesterol and triglycerides related to age and adiposity in white versus black young adults. The Bogalusa heart study

Mutations in adenosine triphosphate (ATP)-binding cassette transporter 1 (ABCA1) gene have been established as the molecular defect in Tangier disease and familial hypoalphalipoproteinemia, uncommon genetic disorders characterized by deficient or depressed high-density lipoprotein (HDL) cholesterol and increased triglycerides. However, information regarding the frequency of common variants, including Arg219Lys (R219K) within the coding region of the ABCA1 gene and their effect on these phenotypes in the general population is limited. This study examined the frequency and phenotypic effect of R219K variant in a community-based sample of 887 white and 390 black young adults aged 20 to 38 years. The frequency of the variant allele (K219) was higher in blacks than in whites (0.595 v 0.262, P<.001), with carriers (KK+RK) representing 83.8% of blacks versus 44.2% of whites. After adjusting for age, body mass index (BMI), and sex, the genotype effect on HDL cholesterol and natural logarithm of triglycerides was not apparent in whites or blacks. However, significant interaction effects of genotype and age on HDL cholesterol (P<.001) and genotype and BMI on triglycerides (P=.029) were found in whites. Carriers (KK+RK), unlike noncarriers (RR) showed a positive relationship between age and HDL cholesterol (regression coefficient beta=0.28, P=.029 for carriers v beta=-0.18, P=.112 for noncarriers). In addition, the variant allele attenuated the adverse positive relationship between BMI and triglycerides (beta=0.032, P<.001 for carriers v beta=0.046, P<.001 for noncarriers). These results indicate that the K219 allele frequency differs markedly between blacks and whites, and that the variant-allele modulates the association between age and HDL cholesterol, as well as body fatness and triglycerides in a beneficial manner only in whites.

Membrane-bound plasma platelet activating factor acetylhydrolase acts on substrate in the aqueous phase

Human plasma platelet activating factor acetylhydrolase (pPAF-AH) is a phospholipase A(2) that specifically hydrolyzes the sn-2 ester of platelet activating factor (PAF) and of phospholipids with oxidatively truncated sn-2 fatty acyl chains. pPAF-AH is bound to lipoproteins in vivo, and it binds essentially irreversibly to anionic and zwitterionic phospholipid vesicles in vitro and hydrolyzes PAF and PAF analogues. Substrate hydrolysis also occurs in the absence of vesicles, with a maximum rate reached at the critical micelle concentration. A novel pre-steady-state kinetic analysis with enzyme tightly bound to vesicles and with a substrate that undergoes slow intervesicle exchange establishes that pPAF-AH accesses its substrate from the aqueous phase and thus is not an interfacial enzyme. Such a mechanism readily explains why this enzyme displays dramatic specificity for phospholipids with short sn-2 chains or with medium-length, oxidatively truncated sn-2 chains since a common feature of these lipids is their relatively high water solubility. It also explains why the enzymatic rate drops as the length of the sn-1 chain is increased. pPAF-AH shows broad specificity toward phospholipids with different polar headgroups. Additional results are that PAF undergoes intervesicle exchange on the subminute time scale and it does not undergo transbilayer movement over tens of minutes.

Cell and molecular biology of the assembly and secretion of apolipoprotein B-containing lipoproteins by the liver

Triglycerides are one of the most efficient storage forms of free energy. Because of their insolubility in biological fluids, their transport between cells and tissues requires that they be assembled into lipoprotein particles. Genetic disruption of the lipoprotein assembly/secretion pathway leads to several human disorders associated with malnutrition and developmental abnormalities. In contrast, patients displaying inappropriately high rates of lipoprotein production display increased risk for the development of atherosclerotic cardiovascular disease. Insights provided by diverse experimental approaches describe an elegant biological adaptation of basic chemical interactions required to overcome the thermodynamic dilemma of producing a stable emulsion vehicle for the transport and tissue targeting of triglycerides. The mammalian lipoprotein assembly/secretion pathway shows an absolute requirement for: (1) the unique amphipathic protein: apolipoprotein B, in a form that is sufficiently large to assemble a lipoprotein particle containing a neutral lipid core; and, (2) a lipid transfer protein (microsomal triglyceride transfer protein-MTP). In the endoplasmic reticulum apolipoprotein B has two distinct metabolic fates: (1) entrance into the lipoprotein assembly pathway within the lumen of the endoplasmic reticulum; or, (2) degradation in the cytoplasm by the ubiquitin-dependent proteasome. The destiny of apolipoprotein B is determined by the relative availability of individual lipids and level of expression of MTP. The dynamically varied expression of cholesterol-7alpha-hydroxylase indirectly influences the rate of lipid biosynthesis and the assembly and secretion lipoprotein particles by the liver.

Binding steps of apolipoprotein A-I with phospholipid monolayers: adsorption and penetration

In whole HDL particles, the arrangement of apoA-I relative to phospholipids is of crucial interest with respect to the physiological formations of HDL. We report here new data concerning the nature of the interaction of apoA-I with condensed phospholipid (PL) monolayers (phosphatidylcholine and phosphatidylserine). The use of alternative current polarography allowed the detection for the first time of different binding steps which are dependent on apoA-I concentration. At low concentration (below 10 micrograms/mL), apoA-I adsorbs on PL polar headgroups, through electrostatic interactions. Above this threshold concentration, apoA-I penetrates within the monolayer (i.e., part of apoA-I crosses the PL polar headgroup/hydrocarbon chain interface). The process of penetration described here brings experimental evidence supporting Segrest's "snorkel" model. Penetrated helices are lying at the interface, their apolar face in contact with PL hydrocarbon chains and their polar face in contact with PL polar headgroups. In the absence of cholesterol, a second level of penetration was detected at higher apoA-I concentrations. It was facilitated in the presence of phosphatidylserine in comparison to phosphatidylcholine and disappeared in the presence of cholesterol. It is proposed that the C-terminal domain is involved in the first binding steps and that hinged domains may also be implicated. Furthermore, we propose that the apoA-I binding states stabilize the protein/phospholipid layer complex. These different binding states are discussed with respect to their roles in HDL metabolism.

Distribution and correlates of serum high-density lipoprotein subclasses (LpA-I and LpA-I:A-II) in children from a biracial community. The Bogalusa Heart Study

High-density lipoprotein (HDL) subclasses are considered to differ in terms of antiatherogenic potential. Therefore, the distribution and correlates of serum lipoprotein A-I (LpA-I) and LpA-I:A-II were examined in a random community-based subsample of black (n = 1,021) and white (n = 1,087) children aged 5 to 17 years. Black children had significantly higher LpA-I levels than white children. With respect to LpA-I:A-II, prepubertal (age 5 to 10 years) black males and pubertal (age 11 to 17 years) white children showed significantly higher values than their counterparts. With the exception of the LpA-I:A-II difference among prepubertal males, the observed black-white difference was independent of the racial differential in serum triglycerides, a metabolic correlate of HDL. A significant sex differential (males > females) was noted among blacks and whites for both HDL subclasses, with the exception of LpA-I levels at the pubertal age. Among the pubertal age group, a male-female crossover trend (females > males) in LpA-I levels was apparent after age 14. Sexual maturation and age were the major factors (negative) contributing to the variability in the levels of HDL subclasses among race-sex groups; adiposity (negative), insulin (negative), alcohol intake (positive), and oral contraceptive use (positive) emerged as minor but significant predictor variables. In terms of a relation to other lipoprotein variables, LpA-I compared with LpA-I:A-II correlated much more strongly with HDL cholesterol. Unlike LpA-I, LpA-I:A-II was associated significantly (positively) with low-density lipoprotein (LDL) cholesterol. These findings are indicative of intrinsic metabolic differences among the race-sex groups early in life, resulting in variability in the HDL subclass pattern and attendant antiatherogenic potential.

Effect of selenium deficiency on hepatic lipid and lipoprotein metabolism in the rat

Since experimental Se deficiency results in a significant increase in plasma cholesterol concentration the present investigation was undertaken to assess further the influence of this deficiency on the expression of proteins involved in hepatic lipid metabolism. Se deficiency was induced by feeding weanling male Wistar rats on a deficient diet for 6 weeks. Hypercholesterolaemia associated with Se deficiency was related to increased 3-hydroxy-3-methylglutaryl-coA (HMG-CoA) reductase (EC 1.1.1.34) activity in liver microsomes as compared with control animals. Hepatic lipoprotein receptor levels (LDL-receptor and HDL-binding proteins, HB1 and HB2) were not significantly affected by Se deficiency, as assessed by immunoblotting. Plasma triacylglycerol concentrations tended to decrease in Se-deficient rats in concert with their reduced post-Triton secretion. There was no significant effect of Se deficiency on the hepatic synthesis of apolipoproteins. These results point to the need for further investigations into the mechanism related to the increased activity of HMG-CoA reductase and the enhanced cholesterogenesis in the liver of Se-deficient rats likely to result from this.

Very-low-density lipoprotein subfraction composition and metabolism by adipose tissue

Lipoprotein lipase (LPL) plays a pivotal role in very-low-density lipoprotein (VLDL) metabolism. Within the circulation, the VLDL population is heterogeneous with respect to both size and composition. Several studies have investigated the action of LPL in vitro on different VLDL subfractions, but little is known of the action of LPL in vivo. To investigate this, arterial and adipose tissue venous plasma samples were obtained from 16 normal male healthy volunteers (aged 24.4 +/- 1.8 years; body mass index, 23.5 +/- 0.7 kg.m-2) following an overnight fast. VLDL subfractions were isolated (VLDL1 of Sf 60 to 400 and VLDL2 of Sf 20 to 60) and characterized in terms of triacylglycarol (TAG) and apolipoprotein (apo) B, E, CI, CII, and CIII content. The apolipoprotein content of VLDL1 differed from that of VLDL2: the VLDL2 fraction contained significantly more apo B (0.018 +/- 0.004 v 0.011 +/- 0.003 mumol.L-1, p = .001) but the ratios of TAG:apo B and apo CI:B, and CII:B, and CIII:B were significantly higher in VLDL1 (48,200 +/- 7,980 v 13,860 +/- 2,420, 22.7 +/- 5.5 v 12.5 +/- 2.2, 45.0 +/- 6.3 v 14.9 +/- 2.0, and 0.434 +/- 0.077 v 0.357 +/- 0.054, respectively, molar ratios, all P < .05). The venous blood draining an adipose tissue depot contained less VLDL1-TAG than arterial blood (328 +/- 68 v 381 +/- 83 mumol.L-1, respectively, P < .01), whereas VLDL2-TAG exhibited an opposite tendency (199 +/- 46 v 172 +/- 31 mumol.L-1, NS). Concentrations of VLDL1-apo B, -apo CII, and -apo CIII were significantly less in adipose tissue venous blood compared with arterial blood (0.011 +/- 0.004 v 0.013 +/- 0.004, 0.38 +/- 0.08 v 0.43 +/- 0.10, and 1.33 +/- 0.35 v 1.58 +/- 0.38 mumol.L-1, respectively, all P < .05). These studies demonstrated novel differences in VLDL1 and VLDL2 in terms of composition and metabolism by human adipose tissue LPL in vivo.

Diets deficient in selenium and vitamin E affect plasma lipoprotein and apolipoprotein concentrations in the rat

The present study examined the effects of Se, vitamin E and combined Se and vitamin E deficiencies in rats on plasma lipid, lipoprotein and apolipoprotein (apo) concentrations. Deficiencies were induced by feeding rats the respective diets for 6 weeks. The study shows that Se deficiency results in increased concentrations of plasma cholesterol and apo E. Both could be explained by an increase in the HDL1 fraction. Vitamin E deficiency alone had no significant effect on plasma lipid, lipoprotein and apo concentrations. Se deficiency in combination with vitamin E deficiency leads to an increase in plasma LDL and apo B concentrations. These results point to the need for further investigations on the mechanism by which Se deficiency affects lipoprotein metabolism.

Carotid artery intima-media thickness in elderly patients with NIDDM and in nondiabetic subjects

BACKGROUND AND PURPOSE

The risk of atherosclerotic vascular disease is increased both in subjects with non-insulin-dependent diabetes mellitus (NIDDM) and in those with impaired glucose tolerance compared with nondiabetic subjects. Although classic cardiovascular risk factors are operative in subjects with NIDDM, other factors closely related to insulin resistance syndrome such as diabetic dyslipidemia and hyperglycemia itself may contribute to an excessive cardiovascular disease risk in subjects with NIDDM. The purpose of this study was to investigate the carotid intimal-medial thicknesses (IMTs) and their determinants in elderly patients with NIDDM and in control subjects.

METHODS

We investigated the common carotid and carotid bifurcation IMTs and their determinants in groups of elderly patients (n = 84, age 67.2 +/- 0.6 years) with NIDDM and in 119 control subjects (21 with impaired and 98 with normal glucose tolerance; ages 67.5 +/- 1.0 and 65.1 +/- 0.6 years, respectively).

RESULTS

Common carotid and carotid bifurcation IMTs were greater in the NIDDM group than in control subjects (P < .05 to .01). In NIDDM patients, the mean carotid IMT correlated with postglucose 1-hour plasma insulin (r = .305, P = .01, adjusted for age and sex), serum LDL triglyceride (r = .237, P < .05), and apolipoprotein B concentrations (r = .263, P < .05). Fasting plasma immunoreactive insulin, proinsulin, or specific insulin levels were not significantly associated with carotid IMT. Both diabetic status (P < .05) and the presence of clinical macrovascular disease (P < .01) contributed independently to carotid IMT.

CONCLUSIONS

Carotid IMT was greater in NIDDM patients than in control subjects. The main determinants of IMT in NIDDM patients were related to both postglucose insulin levels and abnormal lipoprotein profiles characteristic of NIDDM and insulin resistance syndrome. Treatment of these factors is likely to reduce the atherosclerotic burden in NIDDM.

Effect of lipid transfer proteins on lipoprotein lipase induced transformation of VLDL and HDL

Lipoprotein lipase-induced lipolysis of human plasma VLDL usually does not yield a complete conversion of VLDL to LDL due to insufficient loss of surface and core lipids and apolipoprotein E. In order to assess the role of lipid transfer proteins in this process human VLDL and apo E free HDL, in approximately physiologic proportions, and with sufficient albumin to bind all released fatty acids, were subjected to 90% lipolysis of triglycerides in 2 h by lipoprotein lipase in the presence or absence of partially purified human cholesteryl ester and phospholipid transfer proteins. Lipoprotein lipase caused a partial transfer of VLDL unesterified cholesterol (16%) and phospholipid (11%), apo E (19%) and almost complete transfer of apo CII and CIII to HDL. VLDL remnants possessed excess apo E and surface and core lipids when compared to plasma LDL, and densities ranging from that of VLDL/IDL to LDL. With addition of the lipid transfer proteins to the lipolysis incubation there was an increased transfer of phospholipid and unesterified cholesterol (2-fold) and apo E (1.6-fold) to HDL over that for lipoprotein lipase incubations. The source of transferred material was primarily from remnants which isolated in the LDL density range in lipoprotein lipase incubations. This transfer resulted in LDL-like particles which had a smaller particle size but lighter density compared to those in lipoprotein lipase incubation. Transfer of cholesteryl esters to VLDL from HDL in exchange for triglyceride was absent or substantially reduced in incubations containing lipoprotein lipase and lipid transfer proteins compared to incubations with only lipid transfer proteins. It is concluded that during rapid lipolysis lipid transfer proteins promote the loss of phospholipid, unesterified cholesterol and apo E from VLDL remnants but do not promote the transfer of cholesteryl ester from HDL to VLDL.

Resistant starch is more effective than cholestyramine as a lipid-lowering agent in the rat

Amylase-resistant starch (RS) represents a substrate for the bacterial flora of the colon, and the question arises as whether RS shares with soluble fibers common mechanisms for their lipid-lowering effects. It is uncertain whether a cholesterol-lowering effect depends basically on an enhanced rate of steroid excretion or whether colonic fermentations also play a role in this effect. In the present study, the effect of RS (25% raw potato starch), of a steroid sequestrant (0.8% cholestyramine), or both were compared on bile acid excretion and lipid metabolism in rats fed semipurified diets. RS diets led to a marked rise in cecal size and the cecal pool of short-chain fatty acids (SCFA), as well as SCFA absorption; cholestyramine did not noticeably affect cecal fermentation. Whereas cholestyramine was particularly effective at enhancing bile acid excretion, RS was more effective in lowering plasma cholesterol (-32%) and triglycerides (-29%). The activity of 3-hydroxy-3-methylglutaryl-CoA reductase was increased fivefold by cholestyramine and twofold by RS. This induction in rats fed RS diets was concomittant to a depressed fatty acid synthase activity. In rats fed the RS diet, there was a lower concentration of cholesterol in all lipoprotein fractions, especially the (d = 1.040-1.080) fraction high-density lipoprotein (HDL1), while those fed cholestyramine had only a significant reduction of HDL1 cholesterol. In contrast to cholestyramine, RS also depressed the concentration of triglycerides in the triglyceride-rich lipoprotein fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired postprandial clearance of triacylglycerol-rich lipoproteins in adipose tissue in obese subjects

Adipose tissue is an important site of clearance of circulating triacylglycerol (TAG), especially in the postprandial period. Postprandial lipemia is usually increased in obesity. We studied the extraction of TAG from plasma and TAG-rich lipoproteins (TRLs) in subcutaneous adipose tissue in 11 control and 8 obese subjects before and after a mixed meal. Clearance of plasma TAG and very low-density lipoprotein (VLDL)-TAG was decreased in the obese subjects after an overnight fast. After the meal, chylomicron-TAG extraction increased in the control group whereas VLDL-TAG clearance decreased; these changes were not seen in the obese group, in whom the VLDL particles appeared to be better able to compete with the chylomicrons for clearance by lipoprotein lipase. In the control subjects, removal of TAG from the TRL in the postprandial period was accompanied by a shift toward addition of cholesterol to the high-density lipoprotein (HDL) fraction; this was not observed in the obese subjects. We conclude that disturbed TRL-TAG clearance in adipose tissue is related both to the elevated plasma TAG concentrations and the depressed HDL-cholesterol concentrations typical of obesity.

Characterization of subspecies of apolipoprotein A-I-containing lipoprotein in homozygotes for familial lecithin:cholesterol acyltransferase deficiency

We characterized the two species of lipoproteins containing apolipoprotein A-I (apoA-I), one containing only apoA-I (LpA-I) and the other containing apoA-I and apoA-II (LpA-I/A-II), in four homozygotes for familial lecithin: cholesterol acyltransferase (LCAT) deficiency. Two homozygotes lacked both LCAT mass and activity, whereas the other two had some residual LCAT mass and activity. In these patients, the amount of all apoA-I-containing lipoproteins was one fourth that of normal control subjects, and > 60% was LpA-I. The chemical composition of both LpA-I and LpA-I/A-II is characterized by markedly decreased ratios of neutral to polar lipids compared with those of normals and the sizes of LpA-I and LpA-I/A-II particles are shifted to smaller and larger diameter ranges when compared with those of normal particles. Changes in particle diameter are also reflected in slower electrophoretic mobilities of both LpA-I and LpA-I/A-II particles. All of these abnormalities were more evident in the two homozygotes who lacked LCAT activity. Incubation of LCAT-deficient plasma with LCAT markedly corrected the chemical and physical abnormalities in both LpA-I and LpA-I/A-II particles. These data, taken together, emphasize the importance of LCAT in modifying the chemical composition, size, and shape of LpA-I and LpA-I/A-II particles.

Relation of serum lipoprotein lipids and apolipoproteins to obesity in children: the Bogalusa Heart Study

BACKGROUND

The relationship of serum lipoprotein lipids and apolipoproteins to obesity was studied in a biracial sample of 2,816 children of ages 5-17 in Bogalusa, Louisiana.

METHODS

Two measures of obesity were used: fatness (subscapular skinfold thickness) and fat centrality (the ratio of subscapular to triceps skinfold thickness). Plasma insulin and glucose were included as metabolic markers related to obesity.

RESULTS

The obesity associations were relatively strong with insulin (rs = 0.29, P less than 0.001, skinfold; rs = 0.15, P less than 0.001, skinfold ratio) and triglycerides (rs = 0.25, P less than 0.001, skinfold; rs = 0.19, P less than 0.001, skinfold ratio). The relationships of serum low-density lipoprotein cholesterol (LDL-C) (rs = 0.17, P less than 0.001, skinfold; rs = 0.13, P less than 0.001, skinfold ratio) and apolipoprotein (apo) B (rs = 0.16, P less than 0.001, skinfold; rs = 0.13, P less than 0.001, skinfold ratio) with the obesity measures were of lesser magnitude, but persisted after adjustment for insulin and triglycerides. The inverse association of obesity to serum high-density lipoprotein cholesterol (HDL-C) (rs = -0.13, P less than 0.001, both skinfold and skinfold ratio) and apo A-I (rs = -0.04, P = 0.03, skinfold; rs = -0.05, P = 0.004, skinfold ratio) was significant only before adjustment for insulin and serum triglycerides. Multiple linear regression of obesity measures showed that, like insulin, serum triglycerides had consistently higher standardized coefficients than LDL-C, HDL-C, apo B, and apo A-I. Apo A-I and apo B added only a small amount (less than 2%) of information to the relationship of serum lipoproteins with obesity measures.

CONCLUSION

These results indicate that serum very-low-density lipoprotein (VLDL) levels are directly and independently related to obesity. The well-known inverse association between obesity and serum HDL-C is not independent, but secondary to the elevated VLDL or triglyceride levels associated with obesity. While associations of obesity and lipoprotein cholesterol are found, far fewer occur with apolipoproteins, especially Apo A-I. Interesting race and sex differences in the relationship of obesity to serum lipoproteins and apoproteins are noted, being greater among white children and more consistent in white males.

Composition of very low density lipoproteins and in vitro effect of lipoprotein lipase

In order to clarify the relationship between composition and lipolytic responses to lipoprotein lipase (LPL), very low density lipoproteins (VLDL) from rats or humans were incubated with a commercially available LPL or with a partially purified LPL from postheparin human plasma and fatty acids released from VLDL were determined in vitro. VLDL from rats fed a diet containing 0.25% cholesterol for 6 months were rich in cholesterol and poor in triglycerides, and released less fatty acids from incubation with LPL than those from control rats. VLDL from normo-and hypertriglyceridemic human subjects were incubated with LPL. The fatty acid release poorly correlated with the apoprotein ratios of VLDL, apo C-III/C-II, B/E, and C/E with the exception of apo B/C, but it correlated well with the ratio of triglyceride/either one of the surface components including total apoproteins, free cholesterol and phospholipids in VLDL or the ratio of the triglyceride/total sum of the surface components. The correlation coefficients between fatty acid release and a ratio of triglyceride/total surface components were 0.774 (using the commercially available LPL) and 0.786 (using the partially purified human LPL). The fatty acid release increased after pretreatment of VLDL with phospholipase A2. The phospholipid content of VLDL was reduced without significant changes in other VLDL components. Thus, the responses of VLDL to LPL treatment may depend mainly upon the surface: core relationship of VLDL rather than its apoprotein composition except in rare clinical cases such as apo C-II deficiency.

Fish plasma lipoproteins--comparative observations in serranides and sparides

1. Diet, time from last feeding, temperature, season and sexual stage are some of the factors influencing the lipoprotein pattern. 2. Keeping these factors constant species-specific differences observed among lipoprotein patterns of Sparus aurata, Puntazzo puntazzo, Diplodus sargus, Diplodus vulgaris and Dicentrarchus labrax are discussed. 3. Feeding habits and therefore lipid absorption and the rate of lipoprotein maturation process are the factors determining the observed differences.

The effect of high-fibre diet on plasma lipoproteins and hormones in genetically obese Zucker rats

The effects of prolonged substitution of readily digested carbohydrates (wheat starch) for poorly digested carbohydrates (mixture of various fibres and crude potato starch) on plasma lipids, lipoproteins and hormones were investigated in Zucker, genetically obese rats. Lean rats were also studied in parallel. Usually, plasma lipid and insulin levels and insulin/glucose ratio were higher in obese rats vs. their lean littermates. High-fibre diet intake led to weight loss in both obese and lean rats. With a high-fibre diet, plasma lipid, insulin and glucose postprandial levels in both groups were reduced when compared with their respective control animals. Dietary fibre modified the lipoprotein profile. Triglycerides and cholesterol were reduced in all studied lipoprotein fractions. The study of high density lipoprotein (HDL) fraction in rats fed a high-fibre diet demonstrated a decrease in the HDL1 subpopulation and in the apolipoprotein (apo)E proportion. The findings show that a high-fibre diet modulates plasma lipid, insulin and glucose levels and modifies the plasma lipoprotein distribution and composition in normolipaemic, lean rats as well as in hyperlipaemic, genetically obese rats.

Abnormal composition of hypertriglyceridemic very low density lipoprotein determines abnormal cell metabolism

The metabolism of very low density lipoprotein (VLDL) from normolipemic (NTG) subjects, hypertriglyceridemic (HTG) subjects, and hypertriglyceridemic subjects treated with bezafibrate (BZ) was studied in cultured human skin fibroblasts. The binding, cell association, and proteolytic degradation of 125I-labeled lipoproteins and the capacity to regulate cellular sterol synthesis was determined with and without maximal stimulation of the lipoprotein by exogenous recombinant or plasmatic apolipoprotein (apo) E-3. The VLDL was separated into three density subfractions: I, II, and III. Multiple differences between HTG and NTG lipoproteins were found, which all reverted toward normal with therapy. Even in the presence of an optimal concentration of apo E-3, HTG-VLDL demonstrated 100% to 200% higher metabolic activities, indicating a better association or a better biological expression of apo E-3 at the surface of the lipoprotein. There was a strong and linear relationship between the cholesterol ester/protein ratios of the different VLDLs and their proteolytic degradations by the cells (r = 0.95). Thus, the composition/structure alterations of VLDL appear to determine their apo E-3-dependent cellular catabolism. In addition, HTG-VLDLs not enriched with apo E-3 exhibited a capacity to down-regulate cellular sterol synthesis independently of their uptake and degradation by the cells. This abnormality appeared to reflect the ability of the VLDL to donate cholesterol to the cells and was not observed in receptor-negative cells. Thus, HTG-VLDL is much more capable than NTG-VLDL of introducing cholesterol to cells by at least two mechanisms: 1) accelerated uptake and degradation and 2) direct transfer of cholesterol to the cells. Both processes are potentially atherogenic and are reversible when triglyceride-lowering therapy is instituted.

Plasma lipoproteins in dairy cows with naturally occurring severe fatty liver: evidence of alteration in the distribution of apo A-I-containing lipoproteins

The relationships between fatty liver in dairy cows and reduced levels of plasma lipoproteins, and particularly of low density lipoproteins (LDL), has been previously described. Since electrophoretic heterogeneity of ultracentrifugally isolated LDL (d, 1.006-1.063 g/ml) has been found, the exact nature of this reduction in cows with fatty liver was investigated. Lipoproteins from control and severely afflicted animals were isolated by ultracentrifugation and affinity chromatography on heparin-Sepharose CL 6 B. Gradient gel electrophoresis of lipoproteins on 4-30% gels and an immunolocalization study of apoprotein A-I (apo A-I) showed that control animals have two subpopulations of apo A-I-containing particles with a mean radius of 6.52 and 5.05 nm. In the fatty liver cows, the former was clearly shifted toward smaller particles. We concluded that the depressed level and compositional modifications of LDL in severe fatty liver cows result from a decrease in the oversized apo A-I-containing lipoproteins which can be isolated in the LDL density range. This could stem from the decreased supply of triglyceride-rich lipoprotein surface components for the production of these lipoproteins. The modifications can be plausibly explained by a reduced synthesis or secretion of very low density lipoproteins (VLDL) by the liver.

Effect of cholesterol feeding on lipolytic activities in high- and low-responding rhesus monkeys

We compared the activities of lipoprotein (LPL) and hepatic lipase (HL) in postheparin plasma in groups of high- and low-responding rhesus monkeys fed a low cholesterol diet followed by a high cholesterol diet. Cholesterol feeding resulted in a two-phase response in lipolytic activities: an initial phase lasting about 21 days that was similar in the two groups, followed by the second phase in which major differences became apparent between the groups. In the initial phase, LPL and HL activities increased along with plasma cholesterol and high density lipoprotein (HDL) cholesterol, but there was no change in plasma triglycerides or apolipoprotein (apo) A-I levels. These changes suggest that both high and low responders respond in a similar manner to metabolize an increased lipid load and that both groups continue to remove triglyceride efficiently while cholesterol accumulation begins. The second phase (between 21 and 42 days and thereafter) in high responders was characterized by a decrease in LPL and HL activities along with plasma HDL cholesterol and apo A-I levels, continued increase in plasma cholesterol, and a slow increase in plasma triglycerides. In low responders, LPL activity was maintained at a high level, HL activity decreased to the basal level, and plasma lipids were unchanged. All changes occurred simultaneously, suggesting metabolic relationships between plasma lipolytic activities, plasma lipids, and apolipoprotein.

Characteristics associated with apoprotein and lipoprotein lipid levels in middle-aged women

Recent evidence indicates that measurement of apoproteins may enhance evaluation of coronary heart disease risk. The purpose of the present study was to identify factors associated with interindividual variation in apoproteins (apo) A-I, A-II, and B and lipoprotein lipid levels in 541 healthy premenopausal women, a random sample ages 42 to 50 taken from driver's license lists. The results of multivariate analyses that included alcohol intake, obesity, smoking, exercise, and age as predictor variables showed alcohol consumption to be strongly, positively related to apo A-I and A-II and smoking and obesity to have modest lowering effects on apo A-I. Concentration of the high density lipoprotein subfraction, HDL2c, however, was highly negatively related to body mass index, with alcohol intake and smoking each contributing about 5% to the variation. HDL3c had a similar relationship to obesity, alcohol, and smoking, but the magnitude of effect was much smaller than that for HDL2c. Thus, the concentration of cholesterol relative to protein found in HDL, particularly HDL2, was lower in overweight women and higher in women who reported alcohol intake. About 10% of variation in low density lipoprotein cholesterol (LDLc) was explained jointly by smoking, obesity, and alcohol intake compared with 15% of variation in apo B associated primarily with obesity (8%) and, to a lesser extent, with age and smoking. Physical activity was not independently associated with any of the lipoprotein lipid or apoprotein measures. In sum, results show that obese women exhibited reduced HDLc per mole of protein and that alcohol intake was linked to increased HDL particle number.(ABSTRACT TRUNCATED AT 250 WORDS)

Black-white differences in cholesterol levels of serum high-density lipoprotein subclasses among children: the Bogalusa Heart Study

Cholesterol levels of serum high-density lipoprotein (HDL) subclasses, HDL2 and HDL3, were examined in a random subsample (n = 561) of children (7 to 17 years of age) from a total biracial community. Overall, black children in younger (7 to 10 years) and older (11 to 17 years) age groups alike had significantly higher HDL2 cholesterol (HDL2-C) and HDL3-C than their white counterparts. In addition, black children had a relatively higher frequency of joint occurrence of high levels of both HDL2-C and HDL3-C. A significant sex-related difference, with girls showing higher values than boys, was noted among younger age groups for HDL2-C. A male-female crossover trend in HDL2-C levels was apparent only among white children, with girls showing higher values after age 11. Both age and sexual maturation were inversely associated with HDL3-C levels in white children, irrespective of sex (p less than .001). Serum triglycerides were inversely related to both HDL2-C and HDL3-C only in white children (p less than .001). A black-white difference in HDL2-C persisted only among boys and girls in the older age group after adjusting for the covariates (sexual maturation, age, adiposity, oral contraceptive use, cigarette smoking, alcohol use, and serum triglycerides). With respect to HDL3-C, the covariate-adjusted difference remained significant only among boys in the older age group. Metabolic variations between the races in response to both physiologic and environmental factors likely account for the divergence in antiatherogenic HDL pattern.

Effects of Triton WR 1339 and orotic acid on lipid metabolism in rats

In order to investigate the effect of hepatic cholesterol flux on biliary bile acids, Triton WR 1339 and orotic acid were administered to rats, and the biliary cholesterol, phospholipids and bile acids were analyzed together with serum lipoproteins and hepatic lipids. Triton, which raised serum very low density lipoprotein and lipid levels and decreased serum high density lipoprotein liver lipid levels, increase the biliary cholic acid group/chenodeoxycholic acid group ratio (CA/CDCA) in the bile without affecting the total amount of bile acids and the other biliary lipids. Orotic acid, which decreased serum lipid and lipoprotein concentrations and increased liver lipid levels, increased the biliary excretion of cholesterol and phospholipids, but produced no significant change in the total amount of bile acids and in the CA/CDCA ratio in bile.

Platelet activation by plasma lipoproteins

Enhanced platelet activity in patients with hypercholesterolemia and moderate hypertriglyceridemia can be attributed to increased LDL and VLDL and/or decreased HDL concentrations. In marked hypertriglyceridemia, where there is an accumulation of chylomicrons, platelet function is reduced. Treatment, whether by diet, medication, or plasmapheresis, that will result in a change in lipoprotein pattern is accompanied by a parallel change in platelet responsiveness. Incubation of lipoproteins with isolated platelets results in enhancement of platelet activation by LDL and VLDL and suppression of activity by HDL and chylomicrons. These findings have in vivo confirmation. They are even more pronounced and sometimes altered when the lipoproteins are derived from hyperlipidemic subjects. The effects of the lipoproteins on platelet activity appear to be dependent on lipoprotein composition and on factors such as cholesterol:protein ratio, apo C-III0:apo C-III2 ratio, apo B concentration, and triglyceride:protein ratio. The lipoproteins interact with platelets at specific receptor sites. Rapid change in platelet composition, particularly with regard to cholesterol, phospholipid, and fatty acid content, might ensue, with consequent alterations in membrane fluidity and enzyme activities and either suppression or activation of platelet function. This review has indicated that lipoproteins have a clear influence on platelet function. This interaction could be well of paramount importance in determining atherogenic risk.

Divergent levels of high density lipoprotein cholesterol and apolipoprotein A-I in children. The Bogalusa Heart Study

Clinical studies indicate that levels of apolipoprotein A-I (apo A-I), the major protein moiety of the high density lipoprotein (HDL) particle, may provide more information concerning the risk of future cardiovascular disease than do levels of HDL cholesterol (HDL-C). Therefore, the relationship of HDL-C to apo A-I levels was examined in a biracial sample of 2849 5- to 17-year-olds. The mean HDL-C to apo A-I ratio, a measure of HDL composition, was 0.42. However, marked interindividual variation was found: HDL-C/apo A-I levels varied from 0.27 (10th percentile) to 0.57 (90th percentile). Furthermore, only 26% of the variation in HDL-C levels was explained by concomitant variation in apo A-I. Increasing levels of triglyceride (and very low density lipoprotein cholesterol) were related to decreases in both the HDL-C/apo A-I ratio and the magnitude of the correlation between HDL-C and apo A-I. Low density lipoprotein cholesterol, race, and age were also related to the HDL-C/apo A-I ratio, but influenced HDL composition less strongly than did triglyceride levels. These observations may be explained by the bidirectional transfer of cholesteryl esters and triglycerides between HDL and triglyceride-rich lipoproteins. The current study documents the influence of triglyceride levels on HDL composition in a general population of children and adolescents, and emphasizes the interrelationships between the various lipid and lipoprotein fractions.

Dietary carbohydrate type and cholesterol-induced hypercholesterolemia in cynomolgus monkeys: influence of oral antibiotic

The relationship of carbohydrate type to cholesterol-induced hypercholesterolemia and the potential role of intestinal flora in the above process were examined in 12 male cynomolgus monkeys (M. fascicularis). Semipurified diets provided two types of carbohydrates (starch or sucrose, 49% by calorie) with 0.4 mg cholesterol/kcal. Six weeks of the starch diet resulted in significantly enhanced hypercholesterolemia when compared to sucrose diet. Starch in relation to sucrose produced cholesterol enrichment of intermediate density lipoproteins and increase in low density lipoprotein particles, whereas sucrose increased high density lipoprotein constituents (phospholipids, cholesterol, and apoA-I) and triglyceride content of very low density lipoproteins. Fecal Escherichia counts were high during the starch diet as contrasted with sucrose diet, but the Escherichia, Streptococcus, and Bacteroides groups did not show differences by diet following each consecutive 4-week period of oral neomycin (107 mg/kg body wt) treatment and withdrawal. The magnitude of hypercholesterolemia during these periods also remained similar between starch and sucrose, suggesting formation of germfree metabolic characteristics. Thus, the magnitude of cholesterol-induced hypercholesterolemia can be affected by the type of carbohydrate, which may be in part determined by intestinal flora metabolism.

Alterations of secretory pattern of intestinal lipoproteins by the benzoyl ester derivative of poloxalene surfactant (BEP)

Hydrophobic surfactant BEP was administered intraduodenally as part of lipid emulsion to rats with cannulated mesenteric lymphatic duct. The effect on the size and composition of intestinal triglyceride-rich lipoproteins (TRLp) was assessed by comparing the results with those obtained during infusion of the lipid emulsion alone. Administration of BEP decreased intestinal capacity to transport triglyceride and cholesterol in large TRLp, SF greater than 2000, and resulted in a significant reduction of total triglyceride in lymph. Non-apoB apolipoproteins decreased significantly in large and increased in small TRLp without appreciable change in total content. Contrary to these findings total apoB protein content increased significantly, primarily due to an increase in small TRLp. Changes in lipid and protein content of apolipoproteins produced by BEP resulted in increased ratios of apolipoproteins to lipids in TRLp. It was therefore concluded that inhibition of lipid transport by BEP was not a result of apolipoprotein deficiency. Discontinuation of BEP administration resulted in a prompt recovery of the intestinal lipid transport system.

Racial (black-white) comparisons of the relationship of levels of endogenous sex hormones to serum lipoproteins during male adolescence: the Bogalusa Heart Study

The cross-sectional relationship of endogenous androgens (testosterone, androstenedione, and dehydroepiandrosterone sulfate [DHEA-S]), estrogen (estradiol) and progestin (progesterone) to serum levels of lipoprotein cholesterol (very low-density [VLDL], low-density [LDL], and high-density lipoprotein [HDL]) and apolipoproteins (apo A-I and apo B) were studied in white (n = 251) and black (n = 258) adolescent boys, ages 11 to 17 years, as part of the Bogalusa Heart Study. Black boys had significantly higher levels of estradiol, HDL cholesterol, and apo A-I, and lower levels of androstenedione and VLDL cholesterol than white boys, independent of age and adiposity. Age was correlated strongly with testosterone and androstenedione, and moderately with DHEA-S and estradiol levels in both races. However, only in white boys was age consistently related to VLDL cholesterol (positively), HDL cholesterol (negatively), and apo A-I (negatively). Overall, testosterone was associated inversely with HDL cholesterol and apo A-I in white boys, while progesterone was related positively to apo A-I in both races after adjusting for age and adiposity. However, these relationships were found to differ with age. Partial correlations between levels of sex hormones and lipoproteins adjusted for age and adiposity showed no associations in the 11 to 12 year age group in boys of either race. A significant positive relation of testosterone to VLDL cholesterol, and inverse relations of testosterone to HDL cholesterol and apo A-I and DHEA-S to HDL cholesterol were apparent only in white boys in the 13 to 14 year age group.(ABSTRACT TRUNCATED AT 250 WORDS)

Proteolytic processing and compartmentalization of the primary translation products of mammalian apolipoprotein mRNAs

The steps involved in the initial assembly of apolipoproteins and lipids into supramolecular arrays (nascent lipoprotein particles) are largely unknown. Examination of the proteolytic processing and compartmentalization of the primary translation products of apolipoprotein mRNAs represents one approach to deciphering the molecular details of lipoprotein assembly. The structures of the primary translation products of seven mammalian apolipoprotein mRNAs has been determined in the past several years. The organization of apolipoprotein signal peptides is typical of eukaryotic prepeptides, although an unusual degree of sequence conservation is present among the signal segments of apo AI, AIV, and E. For those apolipoprotein sequences studied in detail, SRP-dependent cotranslational translocation and proteolytic processing appears to be highly efficient and results in sequestration of the processed protein within the lumen of the endoplasmic reticulum (ER). However the mechanism by which these lipid-binding proteins avoid arrest during their translocation through the lipid bilayer of the ER membrane remains obscure. The two principal human HDL apolipoproteins undergo novel extracellular post-translational proteolytic processing, which results in removal of nonhomologous propeptides. The proteases responsible for proapo AI and AII processing appear to be different. The processing of these proapolipoproteins provides a potential series of steps for regulating the ordered assembly of HDL constituents.

Variability in the response of different male subjects to the effect of marathon running on the increase in plasma high density lipoprotein

The acute effects of running a 42.2 km marathon race on the concentration and composition of the plasma lipoproteins were studied in 56 men of varying fitness, training experience, age and physical characteristics. There was no change in the mean concentration of total serum cholesterol, but a 10.9% increase (P less than 0.001) in the mean concentrations of high-density lipoprotein cholesterol (HDL-TC), representing an 11.1% increase (P less than 0.001) in the cholesteryl ester (CE) and 9.9% increase (P less than 0.001) in the unesterified cholesterol (UC) moieties of HDL. The ratio of total serum cholesterol to HDL-TC decreased significantly (P less than 0.001) during the exercise. Changes in lipoprotein concentrations during the marathon varied considerably between individual subjects, with a small proportion of subjects exhibiting relatively large increases or decreases in HDL-TC, HDL-CE and HDL-UC. Small sub-populations of runners were identified who showed abnormally large decreases in HDL-UC and abnormally small increases in HDL-CE relative to HDL-UC. A correlation (P less than 0.05) was found between the average weekly mileage of training and the increase in HDL-TC, whilst faster runners (finishing time less than 3 h; n = 13) had a significantly greater (P less than 0.02) increase in HDL-TC than slower runners (greater than 4 h; n = 14).(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of phospholipid transfer between liposomes (neutral or negatively charged) and high-density lipoproteins: a spin-label study of early events

The kinetics of spin-labeled phosphatidylcholine transfer between vesicles and HDL particles exhibited a two-phase process, as seen by ESR spectroscopy. The results were analyzed by considering several possible steps in the overall transfer, whose aspects were also studied: (i) micellar complex formation after HDL apolipoprotein-vesicle mixture, (ii) the rate of PC transfer from the micellar complex to HDL, (iii) the rate of the reverse reaction between overloaded HDL particles and other particles such as HDLs, LDLs, and lipid vesicles. The results agree most convincingly with a mechanism in which the diffusion of phospholipids into the HDL-endogenous lipids is the limiting step, occurring as a two-step process. In addition, we observed a negative charge effect on the lipid transfer rates and yields.

Immunoaffinity isolation of apolipoprotein E-containing lipoproteins

Discrete apolipoprotein E-containing lipoproteins can be identified when EDTA plasma is fractionated on columns of 4% agarose. The present study has demonstrated, by physical and metabolic criteria, that these apolipoprotein E-containing lipoprotein subclasses may be further isolated by immunoaffinity chromatography. Whole plasma was first bound to an anti-apolipoprotein E immunoadsorbent prior to gel filtration on 4% agarose. After elution from the affinity column and dialysis, the bound fraction was chromatographed on 4% agarose. Discrete subfractions of apolipoprotein E could be demonstrated within elution volumes similar to those observed in the original plasma. When whole plasma was first submitted to gel filtration and the apolipoprotein E-containing lipoproteins of either intermediate- or of high-density lipoprotein (HDL) size were subsequently bound to anti-apolipoprotein E columns, the bound eluted fractions maintained their size and physical properties as shown by electron microscopy and by rechromatography on columns of 4% agarose. The metabolic integrity of apolipoprotein E-containing very-low-density lipoproteins (VLDL) was examined by coinjection into a cynomolgus monkey of 125I-labeled apolipoprotein E-rich and 131I-labeled apolipoprotein E-deficient human VLDL which had been separated by immunoaffinity chromatography. The plasma specific activity time curves of the apolipoprotein B in VLDL, intermediate-density (IDL) and low-density (LDL) lipoproteins demonstrated rates of decay and precursor-product relationships similar to those obtained after injection of whole labeled VLDL, supporting the metabolic integrity of VLDL isolated by immunoaffinity chromatography.

Variations in HDL and VLDL levels chronic alcoholics. Influence of the degree of liver damage and of withdrawal of alcohol

Changes in plasma HDL and VLDL levels were investigated in 284 chronic alcoholics staying in a Detoxification Centre where they initiated or continued abstinence. The data show that the variations in plasma HDL are modulated by the degree of liver injury. In severe hepatic damage HDL levels are sharply decreased. An alcohol-induced rise in HDL can occur in the only subjects with no signs of hepatic insufficiency. This elevation is rapidly reversible after withdrawal of alcohol. Such a rise might reflect enhanced synthesis and release by liver but might also be due to an accelerated turnover of the VLDL.

Alterations of lecithin-cholesterol acyltransferase activity during Plasmodium chabaudi rodent malaria

In non fatal and synchronous P. chabaudi rodent malaria, we observed at the stage of parasitaemia peak, an alteration (50 % decrease) in LCAT activity. This decrease could be related partly to hepatic dysfunction, and mainly to circulating inhibitors released into blood from parasitized red blood cells at each end of a schizogonic cycle. This decrease in LCAT activity, at this step of the infection, accounts for part of the dyslipoproteinemia previously observed (i.e., increase in cholesterol and phospholipids into VLDL-LDL and decrease in the EC series and delayed conversion of Tg-rich lipoproteins into LDL-HDL. At a prepatent step of infection and after the parasitaemia peak, the alterations observed in LCAT activity, (respectively, increase and then decrease), would be related to similar changes in levels of cholesterol of HDL associated to complex changes in triacylglyceride transport and metabolism.

Effects of hormones and pyruvate on the rates of secretion of very-low-density lipoprotein triacylglycerol and cholesterol by rat hepatocytes

The secretion of very-low-density lipoprotein (VLDL) triacylglycerol and cholesterol was determined under various conditions in hepatocytes prepared from rats maintained on a controlled lighting and feeding schedule. The rate of lipogenesis in hepatocytes prepared from rats during the feeding period was 2-3-fold higher than that in cells prepared immediately before the animals had access to food. However, there were no corresponding changes in the rates of secretion of triacylglycerol and cholesterol. Pyruvate alone stimulated triacylglycerol secretion but had no effect on the secretion of cholesterol. Despite its stimulation of lipogenesis, insulin suppressed the secretion of both triacylglycerol and cholesterol. This effect on triacylglycerol secretion was more pronounced when lipogenesis was enhanced in the presence of pyruvate. Thus, insulin may act to alleviate hypertriglyceridaemia, which may arise during periods of increased hepatic lipogenesis. The inhibitory effect of glucagon on cholesterol secretion was much less pronounced than that on the secretion of triacylglycerol. The inhibitory effects of glucagon were reversed by pyruvate on cholesterol secretion differed according to whether glucagon was present or absent. These results suggest that the rate of hepatic VLDL triacylglycerol secretion is not necessarily coupled to the rate of lipogenesis in the liver; nor is there any obligatory coupling between the output of triacylglycerol and cholesterol associated with VLDL.