Novel Pathways of Apolipoprotein A-I Metabolism in High-Density Lipoprotein of Different Sizes in Humans

Abstract

Objective: A prevailing concept is that high-density lipoprotein (HDL) is secreted into the systemic circulation as a small mainly discoidal particle, which expands progressively and becomes spherical by uptake and esterification of cellular cholesterol and then contracts by cholesterol ester delivery to the liver, a process known as reverse cholesterol transport, thought to be impaired in people with low HDL cholesterol (HDLc). This metabolic framework has not been established in humans.

Approach and results: We studied the metabolism of apolipoprotein A-I in 4 standard HDL sizes by endogenous isotopic labeling in 6 overweight adults with low HDLc and in 6 adults with normal body weight with high plasma HDLc. Contrary to expectation, HDL was secreted into the circulation in its entire size distribution from very small to very large similarly in both groups. Very small (prebeta) HDL comprised only 8% of total apolipoprotein A-I secretion. Each HDL subfraction circulated mostly within its secreted size range for 1 to 4 days and then was cleared. Enlargement of very small and medium to large and very large HDL and generation of very small from medium HDL were minor metabolic pathways. Prebeta HDL was cleared slower, whereas medium, large, and very large HDL were cleared faster in the low HDLc group.

Conclusions: A new model is proposed from these results in which HDL is metabolized in plasma mainly within several discrete, stable sizes across the common range of HDLc concentrations.

Keywords: apolipoprotein; cholesterol; high-density lipoprotein; liver; metabolism.

Figure 1. Optimized model structure for apoA-I metabolism in plasma

White circles represent pools of apoA-I in HDL size subfractions. All 4 HDL sizes appear directly in plasma and may undergo holoparticle clearance. Very small-discoidal HDL may be enlarged to large and very large HDL; and medium HDL may be enlarged to large. Very small HDL may be regenerated from medium HDL through release of lipidated apoA-I (gray circle). All HDL size fractions can appear in plasma with or without an extravascular processing compartment, called a delay pool (gray rectangles).

Figure 2. Apo A-I secretion rate into plasma (Panel A), and apoA-I fractional catabolic rate (FCR) (Panel B), and apoA-I mass (Panel C) of the 4 HDL size subfractions

White bars represent the low HDLc group, gray bars represent the high HDLc group. P-values are from 2-way ANOVAs in which HDLc group and HDL mass, secretion rate or FCR were fixed factors.

Figure 3. Sources of apoA-I in HDL of each size by group

P-values are from 2-way ANOVA in which HDLc group and HDL source were fixed factors.

Figure 4. Model-derived rates and pool sizes for the two study groups

Numbers inside pools represent mean pool sizes in mg. Numbers above lines or after arrowheads represent mean rates in pools/24h. Percentages in parentheses after rates are the percentages of apoA-I metabolized by that pathway (percent of that pool´s total FCR). Numbers above the arrows out of the box labeled “ApoA-I production” represent the mean percent of apoA-I production going into each HDL subfraction. White circles represent pools that were actually separated and measured, and the number inside them represents mean measured pool size in mg apoA-I. Gray circles represent nonsampled pools whose size was estimated by the model. Gray rectangles represent nonsampled extravascular delay pools in which processing of HDL may occur. *p<0.05 for between-group difference.

Figure 5. Metabolism of very small-discoidal and medium HDL

Numbers are percentage apoA-I flux. P-values pertain to the difference between the low and high HDLc groups.

Figure 6. Integrated proposal for the in vivo metabolism of plasma HDL in humans

ApoA-I is secreted by hepatocytes variably lipidated with phospholipid and unesterified cholesterol, and interacts with ABCA1 to take up more unesterified cholesterol and form nascent HDL in a range of sizes. Exposure to LCAT in the hepatic lymphatics and sinusoids esterifies the cholesterol to form an HDL core of cholesterol ester while the size of most of the HDL does not enlarge enough to become a larger subfraction. HDL in these discrete sizes enters the systemic circulation, and then escapes to the interstitial space or lymphatic system. In these extravascular-extrahepatic spaces, very small, medium, large and very large HDL take up and esterify free cholesterol from cells. Simultaneously or right after HDL goes back to the systemic circulation, CETP catalyzes the exchange of recently formed cholesterol esters for triglycerides present in apoB lipoproteins. Circulating HDL may come into contact with SRB1 on hepatocytes, which selectively takes up cholesterol ester from HDL. HDL particles do not experience major changes in size because the influx of cholesterol from cells is balanced by the efflux of cholesterol esters from each HDL particle by the CETP and SRB1 pathways. Hepatic lipase, acting on the triglyceride from the CETP reaction, also may maintain stability of size. This cycle of HDL circulation between plasma and the extravascular space can be repeated many times over several days. In the vascular or extravascular space, small-discoidal HDL can, in a minor pathway, take up and esterify cholesterol from cells, becoming large or very large HDL. In a process facilitated by plasma PLTP, HL, CETP and LCAT, medium size HDL particles experience fusion to generate new small-discoidal HDL and perhaps new medium or large HDL. All HDL size subspecies can be cleared from circulation via holoparticle uptake. Reverse cholesterol transport can be accomplished by coupling cholesterol uptake from cells with CETP-mediated cholesterol ester transfer to apoB lipoproteins and SRB1-mediated selective CE uptake by the liver, without transforming the size of the individual HDL. ABCA1: ATP-binding cassette transporter A1, FC: Free cholesterol, CE: Cholesterol esters, TG: Triglycerides, CETP: Cholesterol-ester transfer protein, PLTP: Phospholipid transfer protein, HL: Hepatic lipase, LCAT: Lecithin-cholesterol acyltransferase, SRB1: scavenger receptor B1. VS: very small discoidal; M: medium; L: large; VL: very large size HDL subfractions.