Chapter 25 Lipid Transport

The plasma levels of the major lipids are not only higher than the normal blood glucose level of 100 mg/dl (Table 25.1), but they fluctuate over a wider range, depending on nutrition, lifestyle, and individual constitution. This is possible because none of the major tissues depends on lipids as its only energy source, although some tissues depend on glucose.

Table 25.1 “Normal” Concentrations of Plasma Lipids in the Adult, Determined in the Postabsorptive State 8 to 12 Hours after the Last Meal

Lipid	Normal Range (mg/dl)
Total lipid	400–800
Triglycerides	40–280
Total cholesterol	120–280
LDL cholesterol	65–200
HDL cholesterol	30–90
Phospholipids	125–275
Free fatty acids	8–25

HDL, High-density lipoprotein; LDL, low-density lipoprotein.

Unesterified (“free”) fatty acids are transported in noncovalent binding to serum albumin, but triglycerides, phospholipids, and cholesterol esters form noncovalent aggregates with proteins called lipoproteins. The four pathways of lipid transport in the human body are as follows:

1. Transport of fatty acids from adipose tissue to other tissues

2. Transport of dietary lipids from the intestine to other tissues

3. Transport of endogenously synthesized lipids from the liver to other tissues

4. Reverse transport of cholesterol from extrahepatic tissues to the liver. This pathway is required because cholesterol cannot be degraded locally and must be transported to the liver for biliary excretion.

Most Plasma Lipids Are Components of Lipoproteins

The general structure of a lipoprotein (Fig. 25.1) can be predicted from the solubility properties of the lipids. The hydrophobic triglycerides and cholesterol esters always avoid contact with water. They form the core of the lipoprotein. The amphipathic phospholipids prefer the water-lipid interface. They form a monolayer that covers the surface of the particle. The protein components, or apolipoproteins, are also amphipathic and reside on the surface of the particle. Large lipoprotein particles with a high volume/surface ratio have a high content of nonpolar lipids, and small particles contain mainly polar lipids and protein.

Figure 25.1 General structure of a lipoprotein.

The composition of lipoproteins keeps changing. Most lipids and apolipoproteins can be transferred from one lipoprotein particle to another, and lipids can be acquired from cells, processed by enzymes while in the lipoprotein, and given off to cells. For their final destruction, many lipoproteins are taken up into cells by receptor-mediated endocytosis, followed by lysosomal hydrolysis of their constituents.

The plasma lipoproteins can be separated by electrophoresis, along with the other plasma proteins (see Chapter 15). In fasting serum or plasma, the two most prominent lipoprotein bands are in the α₁ and β fractions. They are designated as α- and β-lipoproteins. A weaker band, the pre–β-lipoproteins, moves slightly ahead of the β-lipoproteins. The chylomicrons, found only after a fatty meal, do not move upon electrophoresis.

Density gradient centrifugation separates the lipoproteins according to their protein/lipid ratio. Nonpolar lipids have densities near 0.9 g/cm³. For lipoprotein particles, the densities increase from 0.95 g/cm³ in the most lipid-rich particles to well above 1.0 g/cm³ in the protein-rich types (Table 25.2). Based on their order of density and protein content, we can distinguish chylomicrons, very-low-density lipoprotein (VLDL), low-density lipoprotein (LDL), and high-density lipoprotein (HDL). The correspondence of these density classes to the electrophoretic separation pattern is shown in Figure 25.2.

Table 25.2 Typical Compositions of Plasma Lipoproteins

Figure 25.2 Electrophoretic mobilities and density classes of plasma lipoproteins. Intermediate-density lipoprotein is included here in low-density lipoprotein (LDL). HDL, High-density lipoprotein; VLDL, very-low-density lipoprotein

Lipoproteins Have Characteristic Lipid and Protein Compositions

Table 25.2 lists the approximate compositions of the lipoprotein classes. In the fasting state, most of the plasma triglyceride is in VLDL, whereas 70% of the total cholesterol is in LDL. Therefore, elevations of plasma triglycerides usually are caused by increased VLDL, and elevations of cholesterol usually are caused by increased LDL.

Each lipoprotein class has its own characteristic apolipoproteins (Table 25.3). Chylomicrons contain apoB-48; LDL and VLDL contain apoB-100; and HDL contains the A-apolipoproteins. However, with the exception of the B-apolipoproteins, most apolipoproteins can be exchanged among the lipoprotein classes. The apolipoproteins

• regulate lipid-metabolizing enzymes in the blood

• facilitate the transfer of lipids between lipoprotein classes and between lipoproteins and cells

• mediate the endocytosis of lipoproteins by binding to cell surface receptors

Table 25.3 Characteristics of the Apolipoproteins

Dietary Lipids Are Transported by Chylomicrons

Approximately 100 g of dietary triglycerides is transported daily from the small intestine to other tissues. As discussed in Chapter 23, they are transported as constituents of chylomicrons. Chylomicrons are present only after a fatty meal. The fate of chylomicrons is shown in Figure 25.3. They are formed with apoB-48 and the A-apolipoproteins as their only apolipoproteins. ApoE and the C-apolipoproteins are acquired in the blood by transfer from HDL.

Figure 25.3 Metabolism of chylomicrons. C, Free cholesterol; CE, cholesterol ester; HDL, high-density lipoprotein; LPL, lipoprotein lipase; PL, phospholipid; TG, triglyceride.

Lipoprotein lipase (LPL) removes 80% to 90% of the chylomicron triglycerides, most of this in muscle and adipose tissue. LPL is activated by apoC-II on the surface of the chylomicron and is inhibited by apoC-III. During triglyceride hydrolysis, some surface phospholipids and apolipoproteins peel off from the surface of the shrinking particle and are transferred to HDL. Phospholipid transfer requires a specialized phospholipid transfer protein. Thus the large chylomicron, with a diameter of about 1 μm, is reduced to a far smaller chylomicron remnant.

The remnant particles bind to lipoprotein receptors in the liver with the help of apoE, followed by receptor-mediated endocytosis into the hepatocytes. In the cell, lipids and apolipoproteins are hydrolyzed by lysosomal enzymes.

The lifespan of a chylomicron, from its secretion by the intestinal cell to the uptake of the remnant by the liver, is less than 1 hour. Once in the bloodstream, the life expectancy of the chylomicron triglycerides is only 5 to 10 minutes.

VLDL Is a Precursor of LDL

The liver synthesizes 25 to 50 g of triglycerides and smaller amounts of other lipids per day. These lipids are released as VLDL. Like the chylomicrons, VLDL is synthesized in the endoplasmic reticulum (ER) and Golgi apparatus and is released by exocytosis (Fig. 25.4). The liver sinusoids have a fenestrated endothelium that allows the passage of lipoproteins into the sinusoidal blood.

Figure 25.4 Metabolism of very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL). C, Free cholesterol; CE, cholesterol esters; HDL, high-density lipoprotein; HL, hepatic lipase; LPL, Lipoprotein lipase; PL, phospholipid; TG, triglyceride.

VLDL is released with apoB-100, small amounts of apoE and the C-apolipoproteins, and a modest amount of cholesterol esters. Like the chylomicrons, it acquires more C-apolipoproteins and apoE from HDL. Additional cholesterol esters are acquired from circulating HDL. This requires a cholesterol ester transfer protein (CETP).

The major apolipoprotein of VLDL, apoB-100, is a single polypeptide of 4536 amino acids. It is encoded by the same gene as apoB-48, the major apolipoprotein of chylomicrons. Indeed, apoB-48 consists of the first 2152 amino acids of apoB-100, counting from the N-terminus. In the intestine, a CAA codon that codes for glutamine in position 2153 is posttranscriptionally changed into the stop codon UAA. This is an example of tissue-specific editing of an RNA transcript.

Like the chylomicrons, VLDL is metabolized by LPL, although VLDL triglycerides are hydrolyzed a bit more slowly than those in chylomicrons (see Fig. 25.4). Like chylomicrons, VLDL transfers C-apolipoproteins to HDL during triglyceride hydrolysis but retains most of its apoE.

About half of the VLDL remnants, especially larger specimens with multiple copies of apoE, are taken up by the liver. Smaller remnant particles appear initially as intermediate-density lipoprotein (IDL) and eventually are remodeled to LDL. This requires the hydrolysis of excess triglyceride and phospholipid by the hepatic lipase (HL) and the transfer of excess apolipoproteins to HDL (see Fig. 25.4).

HL is anchored to the surface of hepatocytes by heparan sulfate proteoglycans. Like LPL, it is released by heparin; however, unlike LPL, it is not activated by apoC-II and does not attack triglycerides in chylomicrons and VLDL. HL hydrolyzes triglycerides and, to some extent, phosphoglycerides in IDL and HDL. It also facilitates the uptake of remnant particles into hepatocytes.

CLINICAL EXAMPLE 25.1: ApoC-III Mutations and Plasma Lipids

Elevated levels of VLDL as well as LDL increase the risk of atherosclerosis. Therefore genetic traits and pharmacological manipulations that enhance the activity of LPL are expected to reduce the incidence and progression of atherosclerosis.

Ordinarily, apoC-II activates and apoC-III inhibits LPL. Among the Old Order Amish of Pennsylvania, approximately 5% of the population is heterozygous for a null mutation in the gene for apoC-III, which reduces the level of this apolipoprotein by 50%.

As a result, the fasting and postprandial plasma triglyceride levels are 45% lower in carriers than in noncarriers. In addition, the mutation raises HDL cholesterol and lowers LDL cholesterol by approximately 20% each. As expected, carriers have a reduced risk of atherosclerotic lesions in their coronary arteries.

Whereas the Amish mutation is rare, a common polymorphism has been observed in Ashkenazi Jews. In this population, homozygosity for an SNP variant 641 base pairs upstream of the transcriptional start site leads to reduced transcription of the APOC3 gene and reduced levels of circulating apoC-III. Homozygosity for this promoter variant was found in 25% of centenarians but in only 10% of controls.

LDL Is Removed by Receptor-Mediated Endocytosis

LDL has a well-defined structure. Its only apolipoprotein is a solitary apo B-100 molecule, and its lipid component includes a high proportion of cholesterol and cholesterol esters (see Table 25.2).

Unlike VLDL and chylomicrons, which are metabolized within minutes to hours, LDL circulates in the plasma for an average of 3 days. Eventually, LDL is removed by receptor-mediated endocytosis. This requires the binding of apoB-100 to the LDL receptor (apoB-100/apoE receptor). The endocytosed LDL is directed to the lysosomes, and its apolipoproteins, cholesterol esters, and other lipids are hydrolyzed by lysosomal enzymes.

Approximately two thirds of the LDL ends up in the liver. However, for the extrahepatic tissues, LDL acquired through the LDL receptor is the major external source of cholesterol.

Not all LDL is cleared by the LDL receptor. Macrophages and some endothelial cells possess alternative lipoprotein receptors, collectively known as scavenger receptors. They have a four to seven times higher Michaelis constant (K_m), or lower affinity, for LDL than does the regular LDL receptor. Therefore their contribution to LDL metabolism is greatest when the plasma LDL concentration is high.

LDL that has been chemically modified by acetylating or oxidizing agents or by exposure to the cross-linking agent malondialdehyde (formed during lipid peroxidation; see Chapter 23) has a higher affinity for scavenger receptors than does virgin LDL. Therefore one likely function of these receptors is the removal of aberrant or aged lipoproteins that are no longer good ligands for the other lipoprotein receptors. The scavenger receptors bind not only lipoproteins but also other particles with negative surface charges, including some bacteria. Therefore they can participate in the defense against infections.

More than 90% of LDL uptake is through the LDL receptor in liver, ovaries, adrenal glands, lungs, and kidneys. However, 44% of LDL uptake is independent of the LDL receptor in the intestine and 72% in the spleen. These organs contain many macrophages, which remove LDL with their scavenger receptors.

CLINICAL EXAMPLE 25.2: Abetalipoproteinemia

Abetalipoproteinemia is a near-complete absence of the apoB-containing lipoproteins LDL, VLDL, and chylomicrons, caused by the inherited deficiency of a triglyceride transfer protein in the ER. Plasma cholesterol and triglyceride levels are reduced to 20% or 25% of normal.

This rare recessively inherited disease (incidence at birth less than 1:100,000) leads to severe fat malabsorption and steatorrhea and to the accumulation of triglycerides in intestinal mucosa and liver. In the absence of LDL, cholesterol is transported to the tissues by apoE-rich HDL particles that are endocytosed through apoB/apoE receptors.

Clinical findings include acanthocytosis (star-shaped erythrocytes), neuropathy, myopathy, and atypical retinitis pigmentosa. If treated with high doses of fat-soluble vitamins, patients survive with little disability and without developing cardiovascular disease as they get older.