Arterial entrance and metabolism of free and esterified plasma cholesterol measured in vivo in experimental animals by a dual isotope method

Induction of Atherosclerosis by Human Chylomicron Remnants: A Hypothesis

Epidemiologic studies have provided support for the association between delayed remnant removal and premature atherosclerosis. Triglyceride-rich particles such as chylomicrons and chylomicron remnants that carry dietary derived fats, may play a role in the early stages of developing arteriosclerosis. Currently research focuses on these lipoprotein classes seeking distinguishing factors that causes some lipoproteins to be atherogenic while others are not. Such lipoproteins could be involved in atherogenesis directly or indirectly. Direct involvement occurs by interaction of triglyceride-rich particles with the arterial wall, possibly affecting the artery wall by oxidative stress, direct endothelial toxicity by constituents such as lysophosphatidylcholine or oxysterols, induction of prothrombotic changes, stimulation of endothelial expression of cell adhesion molecules and direct interaction with circulating blood cells. Indirect involvement refers to the influence of triglyceride-rich lipoproteins on other lipoproteins on the composition of low density lipoprotein (LDL) and high density lipoprotein (HDL) particles. We propose that in individuals with delayed removal of chylomicron remnants, the prolonged exposure of areas of endothelium that have been partially activated by turbulent flow, to specific components of the remnants, results in the endothelial cells becoming further activated and able to bind monocytes. During or shortly after the transcytosis to the intima and transformation of monocytes to macrophages, the macrophages become engorged with remnant derived lipids and form the nidus of a fatty streak.

In vivo influx of free and esterified plasma cholesterol into human aortic tissue without atherosclerotic lesions

In order to determine the in vivo influx of plasma cholesterol into human aortic intimamedia tissue, specimens of the ascending aortic wall without visible atherosclerosis were obtained from patients undergoing aortic valve replacement. Before the operation the patients were intravenously injected with autologous plasma in which the lipoproteins were labeled with radioactive cholesterol. The influence of the duration of the exposure time (0.3-114 h) and of the distribution of radioactivity between free and esterified cholesterol in plasma on the amount of radioactivity found in the arterial wall was studied by the simultaneous use of 3H- and 14C-cholesterol. It was shown that the influx of free and esterified cholesterol into the intima-media layer of the tissue could be calculated from a set of linear equations that relate the labeled sterols in the tissue to the average specific activities in plasma. In nine patients between 50 and 70 yr of age with 4.2-5.9 mM total cholesterol in plasma, the influx of free cholesterol and of esterified cholesterol was 1.2-8.8 and 1.0-12.5 nmol X cm-2 X d-1, respectively. Both hydrolysis and esterification of the sterol fractions in the aortic tissue and exchange of free cholesterol between the plasma lipoproteins and the tissue were demonstrated. The cholesterol content of the intima-media layer was 0.6-2.3 mumol X cm-2. This corresponds to the influx of esterified cholesterol during a period of only 0.1-3.5 yr, which is short compared with the lifespan of the patient. Our data thus suggest that removal of esterified cholesterol from aortic tissue without visible atherosclerosis represents a major importance for the cholesterol concentration in the tissue.

Arterial influx of esterified cholesterol from two plasma lipoprotein fractions and its hydrolysis in vivo in hypercholesterolemic rabbits

Arterial influx of esterified cholesterol from 2 plasma lipoprotein fractions, d less than 1.019 and d greater than 1.019, and influx of plasma free cholesterol were determined in each of 15 hypercholesterolemic rabbits with approximately the same plasma cholesterol concentrations but with different extents of arterial lesions. The procedure consisted of injecting intravenously into recipient rabbits [14C]- or [3H]cholesterol-labeled lipoproteins prepared from donor rabbits. The esterified cholesterol of one lipoprotein fraction was labeled primarily with one isotope and that of the other lipoprotein fraction was labeled with the other isotope. Thoracic aortas were removed 4-6 h after lipoprotein injections. The arterial influx of esterified cholesterol was up to 50 times higher in rabbits with maximal lesions than in those with minimal cholesterol deposits. the arterial influx of cholesteryl ester derived from d less than 1.019 lipoproteins was about equal to that derived from the d greater than 1.019 fraction. The amount of cholesteryl ester in plasma d less than 1.019 was approximately 3 times that in d greater than 1.019. Thus, per unit of cholesteryl ester concentration the d less than 1.019 lipoproteins delivered about 1/3 as much cholesteryl ester to the artery as the lipoproteins in the higher density fractions. some 5-40% of plasma esterified cholesterol which had entered the artery was hydrolyzed in the artery during the experimental period. The influx of free cholesterol that could not be accounted for by the influx of intact plasma lipoproteins was 5-80% of the free cholesterol influx. This excess probably represents free cholesterol influx by an exchange between the plasma lipoproteins and the intimal surface of the artery.

In vivo influx, tissue esterification and hydrolysis of free and esterified plasma cholesterol in the cholesterol-fed rabbit

The influx of free and esterified cholesterol into various tissues of cholesterol-fed rabbits is calculated from the tissue [3H] cholesterol and [14C] cholesterol content - corrected for radioactivity in contaminating plasma - after a 3--6 h exposure to in vivo-labeled plasma. The plasma free cholesterol was labeled primarily with 3H and the esterified cholesterol with 14C or vice versa. The influx calculation is based on total 3H and 14C in tissues and two linear equations that take into account esterification and hydrolysis of sterol fractions by the tissues. The influx of esterified cholesterol into tissue samples from aorta, heart, small intestine and lung was 10--80 nmol-g--1-h--1, whereas the influx into adrenal, spleen and liver was from 400--2500 nmol--g-1--h-1. The influx of free cholesterol was considerably higher than expected if free and esterified cholesterol had entered the tissues together as part of plasma lipoproteins. This excess of free cholesterol influx can be ascribed to cholesterol exchange between plasma lipoproteins and tissues, which in several tissues amounted to more than 80% of the total free cholesterol influx. From tissue free and esterified cholesterol radioactivity, one can calculate that 20--70% of the newly entered esterified cholesterol was hydrolyzed by various tissues and that most tissues esterified less than 10% of newly entered cholesterol during the experimental period. However, esterification of plasma cholesterol by adrenals averaged 50% of that taken up during a 3-6 h period.

Transfer of plasma lipoprotein components and of plasma proteins into aortas of cholesterol-fed rabbits. Molecular size as a determinant of plasma lipoprotein influx

The arterial influx of esterified and free cholesterol from low density lipoproteins and very low density lipoproteins in 20 hypercholesterolemic rabbits was measured simultaneously by the use of lipoproteins labeled in vivo with [3H]- and [14C]-cholesterol. The simultaneous arterial influx of either [3H]-leucine-labeled very low density lipoproteins, low density lipoproteins, high density lipoproteins, or plasma proteins was also measured in each rabbit. The arterial influx was calculated as intimal clearance, i.e., the influx of a given fraction divided by its plasma concentration. The intimal clearance of low density lipoprotein esterified cholesterol was equal to that for the apolipoproteins of that fraction, which is compatible with an arterial influx of intact low density lipoprotein molecules. The intimal clearance of very low density apolipoprotein or cholesteryl ester was less than that for low density lipoprotein, whereas high density lipoprotein and albumin clearances exceeded low density lipoprotein clearance by 1.5- to 3-fold. The intimal clearances of plasma proteins, high density, low density, and very low density lipoproteins decreased linearly with the logarithm of the macromolecular diameter. This indicates that the arterial influx of three plasma lipoprotein fractions and of plasma proteins proceeds by similar mechanisms. Apparently the relative intimal clearances of lipoproteins are more dependent on their size relative to pores or vesicular diameters at the plasma-artery interface than on specific interactions between lipoproteins and the arterial intimal surface.