Regulation of plasma lecithin:cholesterol acyltransferase in man. III. Role of high density lipoprotein cholesteryl esters in the activating effect of a high-fat test meal

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.

Postprandial remodeling of high-density lipoprotein following high saturated fat and high carbohydrate meals

BACKGROUND

Humans spend most of the time in the postprandial state, yet most knowledge about high-density lipoproteins (HDL) derives from the fasted state. HDL protein and lipid cargo mediate HDL's antiatherogenic effects, but whether these HDL constituents change in the postprandial state and are affected by dietary macronutrients remains unknown.

OBJECTIVES

This study aimed to assess changes in HDL protein and lipid composition after the consumption of a high-carbohydrate or high saturated fat (HSF) meal.

METHODS

We isolated HDL from plasma collected during a randomized, cross-over study of metabolically healthy subjects. Subjects consumed isocaloric meals consisting predominantly of either carbohydrate or fat. At baseline and at 3 and 6 hours postprandial, we quantified HDL protein and lipid composition by liquid chromatography-mass spectrometry.

RESULTS

A total of 15 subjects were included (60% female, aged 34 ± 15 years, body mass index: 24.1 ± 2.7 kg/m2). Consumption of the HSF meal led to HDL enrichment in total lipid (P = .006), triglyceride (P = .02), and phospholipid (P = .008) content and a corresponding depletion in protein content. After the HSF meal, 16 of the 25 measured phosphatidylcholine species significantly increased in abundance (P values range from .027 to <.001), along with several sphingolipids including ceramides (P < .004), lactosylceramide (P = .023), and sphingomyelin-14 (P = .013). Enrichment in apolipoprotein A-I (P = .001) was the only significant change in HDL protein composition after the HSF meal. The high-carbohydrate meal conferred only minimal changes in HDL composition.

CONCLUSION

Meal macronutrient content acutely affects HDL composition in the postprandial state, with the HSF meal resulting in enrichment of HDL phospholipid content with possible consequences for HDL function.

Plasma cholesteryl ester synthesis, cholesteryl ester transfer protein concentration and activity in hypercholesterolemic women: effects of the degree of saturation of dietary fatty acids in the fasting and postprandial states

Hypercholesterolemic women (n = 19) sequentially maintained on a long-term saturated (SAT) or a polyunsaturated (PUFA) fatty acid-rich diet, respectively, were studied in the fasting state and after a meal rich in SAT or PUFA. When apo B-containing lipoprotein was excluded from plasma the in vitro HDL-14C-cholesterol esterification rate was identical for the saturated (SAT) and polyunsaturated (PUFA) fatty acid diets, and did not increase during the postprandial period. Rates of transfer of 14C-cholesteryl ester to apo B-containing lipoproteins from HDL were also similar for both diets in the fasting state and increased to the same extent in the postprandial period in parallel with the rise in plasma triglycerides. When transfer data were related to the plasma concentration of apo B, the gain of cholesteryl ester by the triglyceride-containing particles (VLDL + LDL) also increased in the postprandial period to a similar extent for both diets. Cholesteryl ester transfer protein (CETP) concentration measured by radioimmunoassay was similar during both experimental diets, although greater in the postprandial period for the PUFA diet. The rate limiting factor for CETP-mediated transfer of HDL-derived cholesteryl ester (CE) was the plasma triglyceride concentration, that is, the content of triglycerides per lipoprotein particle and the quantity of TG-containing particles (VLDL + LDL). In contrast, the fatty acid composition of these particles had less effect on CETP-mediated CE transfer.

Postprandial lipoprotein responses in hypertriglyceridemic subjects with and without cardiovascular disease

Three groups of age- and weight-matched men (aged 40 to 70 years) without diabetes were studied: controls (n = 10), plasma triglycerides (TG) less than 180 mg/dL and no cardiovascular disease (CVD); HTG-CVD (n = 11), hypertriglyceridemic (HTG) (TG > 240 mg/dL) without CVD; and HTG+CVD (n = 10), HTG (TG > 240 mg/dL) with documented CVD. HTG+CVD subjects had higher fasting and post-oral glucose tolerance test insulin levels than the other two groups, respectively. Very-low-density lipoprotein (VLDL)+chylomicrons (CMs), intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL), and three high-density lipoprotein (HDL) subfractions (HDL-L, HDL-M, and HDL-D, from least to most dense) were isolated by gradient ultracentrifugation. Fasting lipoproteins were similar in HTG groups, except for higher VLDL lipid to apolipoprotein (apo) B ratios (P < .04) in the HTG+CVD group. Subjects were fed a high-fat mixed meal, and lipoprotein composition was determined at 3, 6, 9, and 12 hours postprandially. Postprandial responses of the core lipids (TG and cholesterol esters [CE]) in all of the lipoprotein subfractions were similar in the two HTG groups at each time point. However, both controls and HTG-CVD subjects had increases in HDL-M phospholipid (PL) at 9 and 12 hours with no change in HDL-D PL. The HTG+CVD group, on the other hand, had no increase in HDL-M PL and had a substantial reduction in HDL-D PL. These changes resulted in significant increases in HDL-M and HDL-D PL to apo A-I ratios in both controls and HTG-CVD subjects between 6 and 12 hours, whereas there was no increase seen in the HTG+CVD group. The HTG-CVD group also had a significantly greater increase in the VLDL+CM PL to apo B ratio (P = .038) at 3 hours than the HTG+CVD group. This diminished amount of surface lipid per VLDL particle may account for the late decrease in the HDL-D PL to apo A-I ratio seen in HTG+CVD patients. There were no other postprandial lipid or apolipoprotein differences between the two HTG groups. We conclude therefore that the major postprandial lipoprotein abnormality in these HTG+CVD patients was a failure to increase the PL content per particle in VLDL+CM, HDL-M, and HDL-D. This abnormality could prevent the usual increase in reverse cholesterol transport seen in postprandial plasma and therefore contribute to their increased incidence of CVD. The greater insulin resistance seen in these patients also appears to contribute significantly to their CVD.

Corn oil and beef tallow elicit different postprandial responses in triglycerides and cholesterol, but similar changes in constituents of high-density lipoprotein

OBJECTIVE

The study was designed to compare, in a homogeneous, normolipidemic population, the postprandial responses of plasma lipids, in particular, high-density lipoprotein (HDL) constituents, after administration of a polyunsaturated fat and a more saturated fat.

METHODOLOGY

Emulsions of 100 g corn oil (CO) and 100 g beef tallow (BT) were given in a crossover protocol to 12 male subjects (21-24 years). Plasma cholesterol (TC), triglycerides (TG), and HDL lipid and protein constituents were measured at 0, 2, 4, 7 and 10 hours.

RESULTS

A postprandial increase in TG at 2 hours after CO ingestion (96%) was twice that with BT (48%); TG returned to near fasting levels at 10 hours after ingestion of either fat. Areas under the TG response curves for CO and BT were 6.29 +/- 1.67 and 1.75 +/- 0.60 mmol x hour/L (mean +/- SE), respectively. TC and low-density lipoprotein cholesterol (LDL-C) levels were unchanged at 10 hours after CO ingestion, but they were increased 8.1% and 9.3%, respectively, with BT. Both fats increased HDL TG at 2-4 hours, and both similarly increased HDL free cholesterol, cholesterol ester, phospholipid, apolipoproteins A-I and A-II, and lipoprotein (A-I) levels at 7-10 hours. Changes in HDL were predominantly in HDL3.

CONCLUSIONS

The increase in LDL-C with BT at 10 hours suggests that levels may be abnormally elevated in "fasting" samples, dependent on the amount and type of fat in a prior meal. The increase in LDL-C is consistent with short-term regulation of hepatic LDL-receptor activity and/or LDL synthesis. Similar increases in HDL constituents at 7-10 hours after CO or BT, despite the difference in TG responses, suggests differences in the metabolism of chylomicrons and/or HDL due to the type of fat ingested.

Delayed effects of prolonged exercise on serum lipoproteins

The delayed lipoprotein changes after a 3-hour running test were examined in 14 moderately trained young male subjects. Fasting blood samples were obtained one day before, immediately before, and one, two, and four days after the race. Nonfasting samples were collected immediately after, one, and three hours after exercise. Three hours after the race, the ratio of unesterified cholesterol to cholesteryl esters was significantly increased, and one and two days after the race it was significantly decreased compared to the preexercise value. The HDL2/HDL3 ratio, measured by density-gradient ultracentrifugation was one and three hours after the running significantly elevated. However, no redistribution of the HDL2/HDL3 cholesterol ratio determined by a precipitation method with polyanions was found at this time. One day postexercise HDL cholesterol rose significantly above the preexercise value, and this was associated with an elevation of the HDL3 subfraction. On the following day a significant increase of HDL2 cholesterol and the HDL2/HDL3 cholesterol ratio was found. The apolipoproteins A-I, A-II, and B, measured by radial immunodiffusion, did not change during the first hours and the first two days after the race. On the second postexercise day the Lp(a) lipoprotein rose significantly above the preexercise value. Compared with the preexercise level the LCAT activity was significantly elevated three hours after the race and significantly decreased two days later. The present study suggests that during the first few days after prolonged exercise a number of plasma lipoprotein changes take place that are similar to those observed after a period of physical training.

Cholesteryl ester and apolipoprotein E transfer between human high density lipoproteins and chylomicrons

The transfer of cholesteryl esters and apolipoprotein E has been studied between plasma HDL and chylomicrons isolated either from ascitic fluid or from the plasma of a patient with type V hyperlipoproteinemia. Whereas apolipoprotein E transfer was rapid and occurred at low temperature, cholesteryl ester transfer was suppressed at 4 degrees C. Apolipoprotein E transfer did not depend upon the presence of cholesteryl ester transfer protein and was in fact inhibited by the partially purified preparation of this protein. Apolipoprotein E transfer was not increased by reduction with dithiothreitol. The transfer of cholesteryl esters increased sharply at a chylomicron to HDL ratio of cholesteryl ester above 1/10, a value which may be of physiological significance at the peak of postprandial lipemia. At this ratio, the transfer of apolipoprotein E was minimal and increased only at ratios above 2/1. From these results, it is concluded that there is no connection between apolipoprotein E and cholesteryl ester transfer from HDL to chylomicrons. It is, therefore, proposed that whereas chylomicron apolipoprotein E is acquired rapidly and mostly in the lymphatic system, the concentration of chylomicron cholesteryl esters increases significantly and independently in the circulation.

Lipoprotein products of lecithin: cholesterol acyltransferase and cholesteryl ester transfer

High-density lipoprotein substrates and products of human plasma lecithin: cholesterol acyltransferase have been labelled with radioisotopic cholesteryl esters in order to facilitate identification. [3H]Cholesteryl esters were formed by endogenous HDL3/VHDL enzyme (d greater than 1.125 g/ml) following incubation with mixed vesicles of phosphatidylcholine, unesterified cholesterol and 3H-labelled unesterified cholesterol. Transfer of labelled esters to acceptor lipoproteins (VLDL+LDL, d less than 1.063 g/ml) was employed to distinguish a hypothetical transfer complex. Separation of labelled HDL3/VHDL was by gel-permeation chromatography. The results indicate that a subpopulation of labelled HDL3/VHDL cholesteryl esters (43-61% of total) were removed by VLDL/LDL during a 3 h transfer period and these derive from the smaller lipoproteins of the spectrum. HDL carrying non-transferable [3H]cholesteryl esters localize to the larger HDL3. Transfer rates were proportional to ratios of acceptor to donor lipoproteins. Net transfer of cholesteryl esters from the smaller HDL3 also occurred, but was smaller in magnitude (about 10.5% of total). Acyltransferase assays indicated that enzyme distribution is skewed to larger-sized HDL3, suggesting that the non-transferable components might be lecithin: cholesterol acyltransferase-containing parent complexes, while the smaller transfer products contain little acyltransferase. The results fit the hypothesis that a parent HDL3-lecithin: cholesterol acyltransferase complex generates a smaller-sized lipoprotein product which is active in cholesteryl ester transport.

Acute effects of the pattern of fat ingestion on plasma high density lipoprotein components in man

Since Apoprotein A-I (apo A-I) is partially synthesized in the intestine and is secreted into plasma via chylomicrons, we have tested the effects of the daily distribution of fat intake on the concentration and composition of plasma high density lipoprotein (HDL). Ten normal subjects ingested 136 g fat either as a single load (SL) at 0 h or in 6 divided doses from 0 to 10 h (DL). Serial blood samples were obtained over a 24-h period. Studies were performed 7 days apart using a double crossover design and paired comparisons. HDL apo A-I increased during DL (+11% at 9 h, P < 0.01) but was not significantly altered after SL. The HDL Apo A-II concentration did not change. HDL cholesterol decreased significantly (-4 to -7%) during postprandial lipaemia in both phases of the study. Apo A-I and A-II were detected in lipoproteins of d < 1.006 during alimentary lipaemia but not in fasting plasma. These results indicate that HDL apo A-I increases when fat intake is distributed throughout the day (DL), perhaps due to intestinal production of particles with a higher protein/lipid ratio during DL than after a large bolus of oral lipid (SL). Alimentary lipaemia is associated with acute alterations in HDL components including a transient reduction in HDL cholesterol and an increase in the apo A-I/A-II ratio during DL.

Effect of lipoprotein concentration and lecithin: cholesterol acyltransferase activity on cholesterol esterification in human plasma after plasma exchange

The rate of cholesterol esterification in plasma, plasma lecithin cholesterol acyltransferase (LCAT) activity and plasma lipoprotein levels have been measured in five subjects who underwent therapeutic plasma exchange to reduce their plasma cholesterol concentration. In the week following the exchange the cholesterol esterification rate and the plasma triglyceride concentration returned rapidly in parallel to pre-exchange levels, while high density lipoprotein (HDL) cholesterol and LCAT activity returned to normal more slowly but also in parallel. The data suggest that the rate-limiting factor for cholesterol esterification in plasma is unlikely to be solely the enzyme levels, but is probably a combination of factors, including the enzyme level and either substrate availabiltiy or product removal. Plasma very low density lipoprotein (VLDL) may either provide substrates for the reaction or provide a means of removing one of the products from the site of reaction.

Transfer of cholesterol esters between human high density lipoproteins and triglyceride-rich lipoproteins controlled by a plasma protein factor

A protein factor from the d greater than 1.25 g/ml plasma fraction controls the transfer of cholesterol esters between high density lipoproteins and very low density lipoproteins. This transfer is time-dependent, and follows saturation kinetics relative to the concentration ratio of acceptor to donor lipoproteins. Although the process is reversible, the transfer rates are faster from high density to very low density lipoproteins and result in a net increase of cholesterol esters in the very low density lipoproteins. Under the same conditions, there is also a net mass transfer of cholesterol esters from high density lipoproteins to chylomicrons. This constitutes the first demonstration of cholesterol ester mass transfer between isolated lipoproteins and contrasts with the equilibrium of cholesterol esters between HDL and LDL which we previously demonstrated [4]. The apparent maximum transfer rate of cholesterol esters from high density to very low density lipoproteins was calculated to be about 80 nmoles cholesterol esters/h/ml plasma, which is very similar to the initial rate of reaction of lecithin:cholesterol acyltransferase in plasma. It is concluded that cholesterol ester formation in high density lipoproteins and their transfer to triglyceride-rich lipoproteins may be closely coupled.