Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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Spectroscopy: Nuclear Magnetic Resonance; Visible

Spectroscopy and Colorimetry; Tocopherols: Properties

and Determination

Further Reading

Brumley WC, Sheppard AJ, Rudolf TS et al. (1985) Mass

spectrometry and identification of sterols in vegetable

oils as butyryl esters and relative quantitation by gas

chromatography with flame ionization detection. Jour-

nal of the Association of Official Analytical Chemists

68: 701–709.

Christie WW (1987) High-performance Liquid Chroma-

tography and Lipids: A Practical Guide, 1st edn. New

York: Pergamon Press.

Faulkner WR and Meites S (eds) (1982) Selected Methods

for the Small Clinical Chemistry Laboratory, vol. 9, pp.

157–183. Washington, DC: American Association of

Clinical Chemistry.

Fieser LF and Fieser M (1949) Natural Products Related to

Phenanthrene, 3rd edn. New York: Reinhold.

Macrae R (1988) HPLC in Food Analysis, 2nd edn.

London: Academic Press.

Sheppard AJ, Newkirk DR, Hubbard WD and Osgood T

(1977) Gas–liquid chromatographic determination

of cholesterol and other sterols in foods. Journal of

the Association of Official Analytical Chemists 60:

1302–1306.

Windaus A (1932) Uber die Konstitution des Cholesterins

und der Gallensauren. Zeitschrift fu

r Physiologische

Chemie 213: 147–187.

Yasaei P, Sheppard AJ, Brumley WC, Mazzola EP and

Aldridge MH (1989) Structural proof of cholesterol isol-

ated from plants. II. Identification and verification by

C nuclear magnetic resonance spectroscopy and mass

spectrometry. Journal of Micronutrient Analysis 5:

259–267.

Yasaei P, Sheppard AJ, Rudolf TS, Shen C-SJ and Aldridge

MH (1989) Structural proof of cholesterol isolated from

plants. I. Isolation and preliminary identification by

chromatography and infrared spectroscopy. Journal of

Micronutrient Analysis 5: 238–245.

Absorption, Function, and

Metabolism

D R Arnold and P O Kwiterovich Jr, Johns Hopkins

University, Baltimore, MD, USA

Cholesterol Metabolism

0001 Cholesterol metabolism in humans is complex. Chol-

esterol is either supplied from the diet (exogenous)

or synthesized de novo by many cells of the body

(endogenous). The major factors in the diet that may

increase the blood cholesterol level are high intakes of

cholesterol itself, or of saturated fats and excessive

calories. The liver is one of the major sites of en-

dogenous cholesterol synthesis. The pool of choles-

terol in the liver is tightly regulated and reflects the

input of cholesterol from the diet, the biosynthesis of

cholesterol, the secretion and uptake of cholesterol

from plasma lipoproteins, the conversion of choles-

terol into bile, and the reuptake of biliary cholesterol

and bile acids from the intestine to the liver. The

metabolic exogenous and endogenous pathways of

cholesterol metabolism are described in this chapter.

The concentration of cholesterol in the blood in

fasting normal humans is the result of the metabolism

of cholesterol from exogenous and endogenous

sources. Environmental factors such as dietary fatty

acids, and metabolic perturbations such as diabetes

and obesity as well as genetic factors also influence

the level of cholesterol in blood. Consequently, after

several decades, hypercholesterolemia causes athero-

sclerotic vascular disease. Dieting, exercise, and sev-

eral drugs are available as tools to lower blood

cholesterol levels, and the mechanisms by which

these interventions lower elevated cholesterol levels

are discussed here.

Cholesterol – Chemical Structure and

Importance

0002Cholesterol (cholest-5-en-3-ol (3-beta) cholesterol)

and cholesterol metabolites and esters are major com-

ponents of the plasma membrane and of many other

cellular organelles in animals. On average, 1.5 g of

cholesterol is present per kilogram of body weight;

some tissues like fat, brain, and liver contain higher

amounts of cholesterol (e.g., liver contains 4.5 g

1

). Cholesterol is a water-insoluble lipid, made

of a so-called sterol backbone with one hydroxy

group and one double bond, together with a side-

chain made up of eight carbon atoms. Cholesterol is

chemically distinctly different from two other lipids,

triacylglycerols and phopholipids. The steroid back-

bone of cholesterol also constitutes the fundamental

structure of bile acids (with cholic acid and cheno-

deoxycholic acid being the primary bile acids), certain

fat-soluble vitamins, such as vitamin D

, steroid hor-

mones (cortisone and aldosterone), and sex hormones

(estradiol and testosterone, Figure 1).

0003Cholesterol is primarily transported through blood

on low-density lipoproteins (LDL). When present in

elevated amounts, LDL cholesterol causes ‘hardening

of the arteries’ or atherosclerotic cardiovascular dis-

ease (ASCVD). The evidence for a causal relationship

1226 CHOLESTEROL/Absorption, Function, and Metabolism

between elevated blood LDL cholesterol and ASCVD

comes from many research studies in various disci-

plines (feeding studies of animals, epidemiological

surveys, studies of genetic abnormalities of lipid dis-

orders, and large, placebo-controlled, double-blind

clinical trials).

History

0004Poulletier de la Salle discovered cholesterol in gall-

stones in 1769, but Chevreul gave it the name ‘cho-

lesterine’ in 1815. Ignatowsky first demonstrated in

1908 the connection between intake of cholesterol in

Progesterone

Cholesterol

Aldosterone

Estradiol

Cortisol

Testosterone

Vitamin D

fig0001 Figure 1 Biochemical structure of cholesterol in comparison with other hormones derived from cholesterol. Cholesterol is

homologous to several steroid hormones: cortisol, a stress hormone, progesterone, testosterone and estradiol (sex hormones),

aldosterone (important for blood pressure control and salt–fluid balance) and vitamin D

(important for calcium absorption in the gut

and homeostasis).

CHOLESTEROL/Absorption, Function, and Metabolism 1227

the diet of rabbits and the development of arterio-

sclerosis. Anitschkow demonstrated in 1913 that it

was cholesterol feeding only that caused athero-

sclerotic changes in the blood vessels of rabbits.

Cholesterol De-novo Production

0005 Cholesterol can be either absorbed from food or

made de novo in multiple human tissues in varying

amounts. The biosynthesis of cholesterol starts with

acetyl coenzyme A (acetyl CoA). Three molecules of

acetyl CoA are condensed to form 3-hydroxy-3-

methylglutaryl CoA (HMG CoA). The conversion of

HMG CoA to mevalonate by HMG CoA reductase is

the rate-limiting step of cholesterol synthesis. A

schematic diagram summarizing the de-novo biosyn-

thesis of cholesterol is shown in Figure 2. While most

of the sterol pool of the body is made de novo in

extrahepatic tissues, the liver is a major organ

involved both in the synthesis of cholesterol and

COOH

3-Hydroxy-3-methylglutaryl CoA

(HMG CoA)

Acetoacetyl CoA + acetyl CoA

Farnesyl pyrophosphate (FP)

OPP

Squalene

Cholesterol

Mevalonic acid

COOH

HMG CoA

reductase

3 - Isopentenyl pyrophosphate (IPP)

OPP

Lanosterol

c. 20

Enzymatic

steps

CoA

fig0002 Figure 2 De-novo biosynthesis of cholesterol. The biosynthesis of cholesterol begins with the condensation of three acetyl

molecules to form 3-hydroxy-3-methylglutaryl CoA (HMG CoA). The acetyl-CoA can be derived from glucose, fatty acids, or amino

acids. Reduction of HMG CoA produces mevalonate, which gets further processed to isopentenyl pyrophosphate (IPP). Three

molecules of IPP are condensed to make up farnesyl pyrophosphate (FP). Two molecules of FP are used to make up squalene,

which gets converted by three enzymatic steps to lanosterol. Approximately 20 additional chemical steps are needed to make up

cholesterol.

1228 CHOLESTEROL/Absorption, Function, and Metabolism

in the processing and repackaging of cholesterol and

other lipids absorbed from the diet. By using H

labeled water, the average production of cholesterol

in man has been estimated to about 9 mg per kilo-

gram per day. Using this technique, the most active

cholesterol processing tissue is the liver followed by

the gastrointestinal tract, skin, striated muscle and

bone marrow while minor amounts are made in the

adrenal glands, and other endocrine organs like ovar-

ies and testis.

Cholesterol from Diet and Factors

Influencing Cholesterol Absorption/

de novo

Synthesis

0006 The amount of synthesis of cholesterol in mammalian

liver is tightly controlled by multiple factors. Modu-

lating dietary factors that affect plasma cholesterol

levels include energy restriction in the form of de-

creased lipid and carbohydrate intake (diet), meal

frequency, dietary fat type (saturated, unsaturated or

trans), and cholesterol content of the food. Plants

make sitosterol but not cholesterol, and an average

American diet contains about 100–200 mg of this

plant sterol per day. In the intestine, inhibition of

the uptake of cholesterol occurs with intake of these

so-called phytosterols, and this can lower the blood

cholesterol level. Diseases of the liver, or exposure to

drugs and toxins such as alcohol, can easily disturb

the ability of the liver to handle cholesterol and other

fats, and may result in the initiation of ‘fatty degener-

ation,’ thus demonstrating further the importance of

the liver in the metabolic processing of lipids.

0007 Cholesterol in the intestinal lumen comes either

from the diet (250–500 mg) or the bile (c. 600–

1000 mg). While bile acids, such as cholic acid and

chendeoxycholic acid, are nearly completely recircu-

lated, only about 50% of the dietary cholesterol is

absorbed at an average intake of 300–600 mg per day.

Formulas developed by Keys and by Hegstedt and

coworkers predict the average increase in the blood

cholesterol level that will result from a given change

in the amounts of dietary cholesterol, saturated and

unsaturated fatty acids. The Hegstedt formula is as

follows:

< Blood cholesterol ðmg dl

1

¼ 2:16 < S  1:65 < P þ 0:068 < C,

where < symbolizes a change in; S ¼ dietary saturated

fatty acids (% of total calories), P ¼ dietary poly-

unsaturated fatty acids (% total calories) and C ¼

dietary cholesterol (mg per day). Examples of

polyunsaturated fats include linoleic acid (C18:2),

linolenic acid (C18:3), and arachidonic acid

(C20:4). Linoleic acid is the essential fatty acid,

which can be elongated and desaturated to form

other polyunsaturated fats. Fish oils include eicosa-

pentaenoic acid (EPA: C20:5) and docosahexaenoic

acid (DHA: C22:6).

0008Without drug treatment, dietary cholesterol and

saturated fats, and the de novo production and re-

moval of LDL, determine the blood LDL-cholesterol

level. Saturated fatty acids, particularly with 14 and

16 carbons, suppress the receptor-mediated uptake of

LDL and increase the LDL cholesterol level, whereas

unsaturated fatty acids like 9-cis 18:1 increase

receptor-mediated LDL uptake and lower the LDL

cholesterol level.

Excretion of Cholesterol

0009The human does not have an enzyme that can degrade

the sterol ring of cholesterol; the backbone/sterol ring

of cholesterol thus has to be excreted in the bile and

stool. All cholesterol metabolites, including hor-

mones and vitamin metabolites, are excreted as bile

acids or metabolites in the intestine, but they are

partially reabsorbed in the intestine via the entero-

hepatic pathway. The sterol ring of hormones and

vitamins are metabolized (hydroxylated/sulfonu-

rated) by multiple enzymes in the liver to make the

sterol more water-soluble, which promotes excretion

in bile and the urine as well. The delicate balance of

intake and excretion of cholesterol and cholesterol

products is tightly regulated. Evolutionarily, excess

cholesterol must be avoided inside cells as it can be

toxic. Excess cholesterol in the bloodstream is also

undesirable, as it causes hardening of the arteries

(atherosclerosis) and consequently, ASCVD.

Lipoproteins

0010To make lipids soluble in blood, they usually are

packaged inside a lipid-rich core, around which

are wrapped amphipathic proteins, whose nonpolar

amino acid groups are exposed to the lipid core,

whereas their polar or charged amino acids are

oriented to the outer surface, making the entire lipo-

protein particle water-soluble. The proteins associ-

ated with lipoproteins are called apolipoproteins

and act as surface components of the particle, as

enzymes key to lipid metabolism in blood, or as

ligands to cell-surface receptors promoting the bind-

ing and uptake of lipoproteins. A list of these apo-

lipoproteins explaining their functions are listed in

Table 1. Multiple apolipoproteins can be present on

the surface of the lipoprotein particle and provide

information about the tissue of origin and the func-

tion of the lipoprotein. For example, apoB-48, a

CHOLESTEROL/Absorption, Function, and Metabolism 1229

truncated version of apolipoprotein B, is only made in

the intestine and is present only on chylomicrons and

chylomicron remnants, whereas apoB-100 is made in

the liver and is the major apolipoprotein on very-low-

density lipoproteins(VLDL) andtheproducts ofVLDL

catabolism, VLDL remnants and intermediate density

lipoproteins (IDL), and LDL. Multiple enzymes are

present on the capillary walls and on the lipoprotein

particles that metabolize/transfer the lipids present in

the particles (see Table 2 and also below).

0011 There are principally five major classes of lipopro-

tein particles in the blood: chylomicrons (CM), VLDL,

IDL, LDL, and high-density lipoproteins (HDL),

based on their physicochemical properties. This clas-

sification is based on a separation via a salt density

gradient (chylomicrons are the lightest particles,

whereas HDL particles are the heaviest; see Figure 3

for further explanations).

Chylomicrons and Transport of Dietary (Exogenous)

Fat

0012 Dietary cholesterol and triacylglycerol are emulsified

in the intestinal lumen by bile salts; the cholesteryl

esters are hydrolyzed into unesterified cholesterol

and free fatty acids (FFA), and the triacylglycerol

into 2-monoglycerol and FFA. These constituents

are absorbed into the intestinal epithelial cells where

they are repackaged into chylomicrons. Chylo-

microns are large (c.1mm) and their lipid-containing

core is surrounded on the surface by apoproteins

which are amphipathic molecules that allow the

lipid core to be soluble in aqueous lymph fluid and

blood. These particles contain the apolipoproteins B-

48, A-1, A-2, and A-4. The secretion of the chylomi-

crons from the intestinal cells into the lymph requires

the presence of apoB-48. The lymph from the gut

collects in the thoracic duct and enters the blood

stream via the right subclavian vein. Even before

the chylomicrons reach the systemic circulation, the

apoA-1 content is decreased, and apoE, apoC-1,

apoC-2, and apoC-3 are acquired as a result of

HDL particles interacting with chylomicrons in the

intestinal lymph capillaries.

0013Chylomicrons then interact with endothelial cells

in the arterioles lining a number of tissues. The tri-

acylglycerols in the core of the chylomicrons, through

the interaction of apoC-2 with the enzyme lipopro-

tein lipase (LPL) bound to the surface of endothelial

tbl0001 Table 1 Characteristics of apolipoproteins: their origins, functions and distribution in plasma lipoprotein classes

Apolipoprotein Origin Function Presentin lipoprotein

classes

Molecular

weight (kDa)

A-1 Liver, intestine Activator of LCAT CM, HDL 29 016

A-2 Liver, intestine Inhibits HL and VLDL hydrolysis CM, HDL 17 414

A-4 Liver, intestine Activates LCAT CM, HDL 44 465

B B100 liver Ligand for LDL receptor B100!VLDL, LDL 512 723

B48 intestine Necessary for secretion of intestinal and hepatic fat B48!CM 241 000

C-1 Liver Cofactor for LCAT CM, VLDL, HDL 6 630

C-2 Liver Activates LPL, cofactor for LCAT CM, VLDL, HDL 8 900

C-3 Liver, intestine Inhibits LPL/inhibits binding of ApoE to the LDL

receptor

CM, VLDL, HDL 8 800

D Many sources Reverse cholesterol transport HDL 19 000

E (2,3, and 4) Liver, peripheral tissues Ligand for LDL receptor and LRP receptor CM, VLDL, HDL 34 145

CM, chylomicron; LDL, low-density lipoprotein; HDL, high-density lipoprotein; VLDL, very-low-density lipoprotein; LPL, lipoprotein lipase; HL, hepatic

lipase; LCAT, lecithin cholesterol acyltransferase; LRP, low-density lipoprotein receptor-like protein.

tbl0002 Table 2 Characteristics of key enzymes and transfer proteins of lipoprotein particles in the blood

Enzyme Origin Function Lipoproteinclasses

involved

Molecular

weight (kDa)

LPL Capillaries of muscle and

adipose tissue

Hydrolyzes triglycerides and phospholipids CM, large VLDL 50 394

HL Liver Hydrolyzes triglycerides and phospholipids Small VLDL, IDL, HDL 53 222

LCAT Liver Converts free cholesterol from cell membranes to

cholesterol esters

HDL 47 090

CETP Liver, spleen, adipose tissue Transfers cholesteryl esters from HDL to ApoB

containing lipoproteins

HDL, LDL, VLDL 74 000

PTP Pancreas, adipose tissue, lung Transfers the majority of phospholipids in plasma HDL 81 000

CM , chylomicron; LDL , low-density lipoprotein; HDL , high-density lipoprotein; VLDL , very-low-density lipoprotein; LPL , lipoprotein lipase; HL , hepatic

lipase; LCAT, lecithin cholesterol acyltransferase; CETP, cholesterol transfer protein; PTP, phospholipid transfer protein.

1230 CHOLESTEROL/Absorption, Function, and Metabolism

cells, are hydrolyzed into FFA and glycerol, which are

then absorbed into adipocytes and other cells. During

hydrolysis, phospholipids and apolipoproteins are

removed from the surface of the chylomicrons and

transferred to HDL. By removing apoC-2, an activa-

tor of LPL, the process starts to slow down. The

resultant chylomicron remnant, which contains less

triacylglycerols and relatively more esterified choles-

terol, is removed rapidly from the circulation by its

interaction with the chylomicron receptor (low-

density, lipoprotein receptor-like protein (LRP)) on

the surface of the liver cells. The LDL receptor and

proteoglycans can also facilitate the uptake of chylo-

micron remnants into the liver. ApoE is the ligand

that permits the binding and uptake of the chylomi-

cron remnant.

Very-low-density Lipoproteins and Transport of

Hepatic (Endogenous) Fat

0014 In the liver, cholesterol, triacylglycerols, and other

fats are packaged into the VLDL particle that is

surrounded by apoB-100. Triacylglycerol and choles-

teryl ester production is necessary for VLDL synthesis

by the liver. The availability of apoB-100 affects both

the assembly and secretion of the VLDL. The apoB-

100 gene is constitutively expressed.

0015The amount of apoB-100 available is regulated by

the rate of proteolysis of apoB-100. When excess lipid

is available, proteolysis is decreased, and more apoB-

100 is available for VLDL production. When lipid is

depleted, apoB-100 proteolysis increases. The triacyl-

glycerols and apoB-100 interaction requires the

presence of the microsomal triacylglycerol transfer

protein (MTP). MTP is a heteromer that consists of

two subunits, a protein widely distributed in many

cells, called disulfide isomerase, and a unique 97-kDa

large subunit. A mutation in the 97-kDa protein pre-

vents the association of lipid with apoB-100, leading

to little, if any, secretion of VLDL and no production

of IDL and LDL. At least a dozen mutations in the

97-kDa protein have been described in patients with

the recessive disorder, abetalipoproteinemia.

VLDL IDL LDL HDL

UC residue

400 100 20 12 0

510 20

Flotation rate

)

Chylomicra

500

700 400 300 200 100

Molecular diameter

(Assumed spherical)

0−50

(0−2)

120

(20)

300

(135)

250

(45)

6000

(0)

1.007

g ml

−1

1.063

g ml

−1

1.200

g ml

−1

(pre-β) (pre-β) (β) (α)

S rate

Average concentration (Chol, mg dl

−1

)

fig0003 Figure 3 Characteristics of the major lipoproteins present in normal human plasma. Flotation rates are measured in S

units. The

flotation of VLDL, IDL and LDL is performed at a salt gradient of 1.063 g ml

1

, whereas a density of 1.200 g ml

1

is used for HDL. During

ultracentrifugation at 200 000 g for 1 h, a light beam is directed through an aperture in the cell and the changes in diffraction recorded to

outline the boundaries caused by the floating lipoproteins. The class of lipoprotein is indicated at the top, corresponding to

abbreviations and a description given in the text. ‘UC-residue’ describes nonlipid plasma proteins that sediment at a density of

1.200 g ml

1

. The amount of each lipoprotein class is proportional to the area under each curve shown numerically at the bottom. There

are three subclasses of HDL (1, 2, and 3), and these are indicated by numbers and colored black. HDL1 occurs only in hyperlipemic

plasma. The approximate diameter of each lipoprotein class particle is shown in A

ngstro

ms (1 A

¼ 0.1 nm), and the average amount of

plasma cholesterol present in each class of lipoprotein is shown in parentheses (for example, 45 mg dl

1

for HDL lipoproteins). From

Olson RE (1998) Discovery of the lipoproteins: their role in fat transport and their significance as risk factors. Journal of Nutrition

128(Supplement 2): 439S–443S, with permission.

CHOLESTEROL/Absorption, Function, and Metabolism 1231

0016 VLDL is secreted from the liver into the blood-

stream surrounded by apoB-100, apoE, and the

apoCs. Once in the bloodstream, the apoC-2 on

VLDL binds to the same LPL site as chylomicrons in

the capillaries surrounding various tissues.

Intermediate-density lipoproteins

0017 As the core triacylglycerol in VLDL is hydrolyzed, a

remnant VLDL is formed, and the released FFA are

taken up by tissues. Upon further hydrolysis of tri-

acylglycerol, the final remnant, IDL, is formed. The

liver takes up some IDL through the interaction of

apoE with the LDL (B, E) receptor. The rest of the

IDL is modified at the surface of the liver by hepatic

lipase (HL), producing the cholesteryl ester-enriched

LDL.

Low-density Lipoproteins – Production, Uptake,

and Catabolism

0018 The majority of LDL is formed from VLDL. The

major apolipoprotein of LDL is apoB-100, which

serves as the ligand for the binding of LDL to the

LDL (B, E) receptor in clustered pits on the surface

of liver and peripheral cells. The bound LDL and its

receptor are internalized into the cell. LDL are taken

up and degraded in the lysosomes, while the LDL

receptor is released and recycled back to the cell

surface. Cholesteryl esters in the core of LDL are

hydrolyzed into unesterified (free) cholesterol and

FFA. The enzyme acyl cholesterol acyl transferase

(ACAT) controls the cellular levels of unesterified

(free) cholesterol versus cholesteryl esters. Monoun-

saturated fat such as oleic acid is a preferred substrate

for ACAT, whereas saturated FFA is not. In the pres-

ence of excess dietary saturated FFA, less cholesterol

is esterified, and the excess cholesterol inhibits the

release of the transcription factor from the nuclear

membrane. Thus, less of this factor is available to go

into the nucleus where it ordinarily upregulates the

transcription of both the LDL receptor gene and the

gene for HMG CoA reductase, the rate-limiting

enzyme of cholesterol production (see below).

High-density Lipoproteins and Reverse Cholesterol

Transport

0019 HDL play a paramount role in the reverse cholesterol

transport pathway, which mediates the removal of

cholesterol from peripheral cells for transport back

to the liver. HDL are secreted from the liver and

intestine as nascent, disc-shaped particles that consist

primarily of phospholipids and apoA-1, with very

little cholesterol. The nascent HDL interacts with

the ABCA1 transport protein on the surface of

peripheral cells such as macrophages and removes

unesterified cholesterol. In the bloodstream, the free

cholesterol is esterified by lecithin cholesterol acyl

transferase (LCAT) and its cofactor, apoA-1, produ-

cing a spherical HDL particle with a cholesteryl ester

in its core. The core cholesteryl ester in HDL can be

selectively taken up in liver and other steroidogenic

tissues such as the adrenal gland, ovaries, and testes

through the interaction of HDL with the SR-B1 recep-

tor. Alternatively, the cholesteryl esters in HDL can be

exchanged for the triacylglycerols in VLDL and IDL

through the action of cholesteryl ester transfer protein

(CETP). Such a cholesteryl ester is eventually re-

moved by the interaction of IDL or LDL with the

LDL receptor on the surface of liver or peripheral

cells.

Important Enzymes Responsible for the

Processing of Lipoprotein Particles in the

Bloodstream and Peripheral Tissues

0020Apolipoproteins present on the surface of lipid

particles serve as ligands for receptors as well as

activators or inhibitors of enzymes important in the

lipoprotein-processing pathway. The names and func-

tions of enzymes playing a major role in the process-

ing of lipid particles are summarized in Table 2.

Functions of some of the important apolipoproteins

and enzymes will be discussed briefly.

0021LPL catabolizes the triacylglyceride in the core of

many lipoprotein particles. LPL converts chylo-

microns into chylomicron remnants, and VLDL par-

ticles into VLDL remnants and IDL. LPL is bound to

heparan sulfate proteoglycans on the surface of capil-

laries, and production of this enzyme is upregulated

by insulin. Heparinase-induced proteoglycan defi-

ciency (heparinase cuts off proteoglycans from the

cells) impairs the function of LPL. Both chylomicrons

and chylomicron remnants are cleared from the

bloodstream by the liver through the binding of the

ligand, apoE, either to the LRP receptor or to the LDL

(B,E) receptor. This initiates internalization via

coated pits and lysosomal degradation of the apoE-

coated lipoprotein particles.

0022LCAT is made in the liver and is present on HDL

particles. Initially, HDL has very little cholesteryl

ester but is loaded up with cholesteryl ester by the

action of LCAT. This HDL particle then undergoes

exchange of cholesteryl esters for triacylglycerol on

the triacylglycerol-rich lipoproteins through the

action of the CETP. The triacylglycerol-rich lipopro-

teins are then processed by HL, present on the surface

of liver cells. HL hydrolyzes triacylglycerols and

phospholipids. HL is similar to LPL and belongs

to the lipase superfamily. Both enzymes, CETP and

HL, are needed for reverse cholesterol transport as

1232 CHOLESTEROL/Absorption, Function, and Metabolism

demonstrated by transgenic experiments/substitution

experiments. Estrogen is an important negative regu-

lator of HL. Phospholipid transfer proteins (PTP) are

made in multiple tissues. PTPs play an important role

in the maintenance of plasma HDL content and

remodeling of HDL in the circulation.

Role of Transcription Factors in the

Regulation of Cholesterol Metabolism

0023 Recently, transcription factors (proteins that bind to

DNA and control transcription of proteins important

in lipid metabolism) have been described that tightly

control the amount of cholesterol present inside the

cell.

0024 Uptake of cholesterol in liver cells is mediated by

receptors such as the LRP receptor and the LDL

receptor. Brown, Goldstein, and coworkers have de-

scribed transcription factors called sterol regulatory

element-binding proteins (SREBPs). SREBPs control

the transcription of enzymes such as HMG CoA

reductase, essential in cholesterol biosynthesis, and

of the LDL receptor, which mediates the uptake

of LDL by the liver. It has been known for years

that cholesterol in the diet shuts down the synthesis

of new cholesterol in the liver. It does so by down-

regulating the rate-limiting enzyme in the biosyn-

thesis of cholesterol, HMG CoA reductase. It is now

known that an increase in hepatic cholesterol levels

inhibits the sequential release of an active form of

SREBP by two proteases from the nuclear membrane.

Conversely, a decrease in hepatic cholesterol levels

promotes the proteolysis and release of SREBP,

allowing its translocation into the nucleus where

SREBP binds to the promoter of the LDL receptor

gene and activates its transcription (Figure 4). The

increased production of the LDL receptor molecule

results in an increased uptake of LDL cholesterol by

the cells. Also, SREBP interacts with the gene for

HMG CoA reductase, enhancing its transcription

and increasing the number of enzyme molecules.

This increases the biosynthesis of cholesterol. Thus,

both an increased uptake of cholesterol on LDL from

the blood and enhanced production of cholesterol

synthesis occur when the level of cholesterol in the

liver is decreased. Conversely, as the cholesterol pool

in the liver is increased, the release of SREBP is in-

hibited, the LDL receptor gene and HMG CoA reduc-

tase genes are downregulated, and an equilibrium is

achieved (see Figure 4).

LDL receptor LDL receptor

Nucleus

SREBP response element

Nuclear membrane

Decreased proteolysis of SREBP Increased proteolysis of SREBP

Cell membrane

Cholesterol

Cholesterol Cholesterol esters

Fatty acids

ACAT

fig0004 Figure 4 LDL-receptor/SREBP pathway. Liver cells adsorb cholesterol and fatty acids, and the amount of cholesterol versus

cholesterol ester in the cell cytoplasm is controlled by the enzyme acyl cholesterol acyl transferase (ACAT). If the level of free

cholesterol is decreased (on the right), a proteolytic process is activated to free the steroid response element-binding protein (SREBP)

which then translocates to the cell nucleus and activates the transcription of the LDL-receptor gene. This leads to an increased

expression of LDL-receptors on the surface of the liver cell and to enhanced clearance of LDL from the bloodstream. The reverse is

true (on the left) if the intracellular cholesterol level is increased.

CHOLESTEROL/Absorption, Function, and Metabolism 1233

0025 Other enzymes, such as the intracellular ACAT,

also play a role in this regulatory pathway; for

example, as ACAT converts free cholesterol into

cholesteryl esters, less unesterified cholesterol is avail-

able to inhibit the proteolysis of SREBP, leading to

increased release of SREBP and the upregulation of

both the LDL receptor and HMG CoA reductase. If

ACAT is less active, as might occur in the presence of

saturated fats (which are not preferred substrates for

the enzyme), more free cholesterol will be available to

inhibit the release of SREBP, leading to a downregu-

lation of LDL receptors and HMG CoA reductase. If

the inhibition of the LDL receptors is greater than the

inhibition of HMG CoA reductase, as occurs in about

one in four humans, an increase in the blood choles-

terol and LDL cholesterol occurs.

0026 Another important protein involved in the control

of cholesterol synthesis is the SREBP cleavage acti-

vation protein (SCAP). The NH

-terminal protein

domain of SCAP resembles the NH

-terminal pro-

tein domain of HMG CoA reductase, and it is

thought that both function as sterol-sensing domains.

SCAP is a required activator of SREBP cleavage, as

cleavage is usually abolished in the presence of

sterols. The family of SREBP transcription factors

also appears to be important in the regulation of

fatty acid synthesis. acetyl CoA carboxylase, fatty

acid synthase, stearyl CoA desaturase-1 (a necessary

enzyme for the generation of unsaturated fatty acids),

and even the enzyme LPL appear to be modulated by

SREBP.

Inherited Mechanisms of Disorders of

Lipid and Lipoprotein Disorders

0027 Apolipoproteins are very important in cholesterol

metabolism as they act as enzymes and ligands for

receptors mediating lipid modification and absorp-

tion (see also above). Most apolipoproteins are

made in the liver and intestine, but a small amount

of apoB can be made in heart tissue and apoE is made

by macrophages. All tissues can degrade apolipo-

proteins, but most of them are degraded in the liver.

0028 One of the most serious inherited abnormalities

of the cholesterol metabolism is a condition called

familial hypercholesterolemia (FH). In FH, the LDL

receptor is defective, and liver cells have a reduced

capacity to bind and take up LDL cholesterol from

blood. FH can occur in a severe homozygous form

where patients have few, if any, functional LDL recep-

tors. Homozygotes for FH are rare, about one in a

million, whereas patients with one mutant allele and

one normal allele for the LDL receptor, FH heterozy-

gotes, occur in about one in 500 persons. The ligand

for the LDL receptor, apoB-100, on the surface of

LDL particle can also be defective, and this may also

lead to inherited high total cholesterol and LDL

cholesterol levels similar to those with FH.

0029More than 150 mutations in the LDL receptor gene

have been described, and grouped into five different

classes: the class I mutations do not permit the LDL

receptor to be synthesized; class II mutations interfere

with the transport of the receptor to the surface of the

cell; class III mutations produce a receptor that does

not bind normally to its ligand, apoB-100; in class IV

mutations, the LDL receptor binds to LDL but it can

not be internalized; and in class V mutations, the

receptor cannot be reused by recycling.

0030Treatment of the homozygous FH individuals is

problematic, since it is difficult in many patients to

induce functional LDL receptors, either with a statin

or with a bile acid sequestrant. Niacin and a high dose

of statins may decrease the production of LDL by

inhibiting to a degree the formation and secretion of

VLDL in liver. FH homozygotes often die from

ASCVD before the age of 20 years. The atheroscler-

osis also affects the aortic valve and can lead to life-

threatening aortic stenosis. Transplantation of a liver

with functional LDL receptors was once used but has

fallen out of favor because of the significant morbid-

ity and mortality associated with this procedure. LDL

phoresis can often decrease the LDL levels by half;

however, the procedure must be repeated every 2

weeks, and sufficient blood flow can be a problem

in very young FH homozygoyes. There has been a

strong interest in gene therapy as a potential success-

ful treatment, but it is still experimental. Heterozy-

gous FH individuals can be successfully treated by a

combination of diet, exercise, and drugs.

Treatment of Lipid Disorders

0031Lipid disorders have traditionally been classified

according to the increased amount of lipoprotein

particles present in blood (Fredrickson’s class I–V,

Table 3).

0032With increasing knowledge, mutations in recep-

tors, apolipoproteins, and enzymes have been identi-

fied as the cause of these abnormalities, but many

defects are still not known, and often, their expres-

sion is modulated by environmental and genetic

factors as well. Many lipid disorders are secondary

to other diseases. For example, the LPL is upregulated

by insulin, and diabetic patients who do not produce

enough insulin often manifest a hypertriacylglycerol-

emia secondary to their diabetes and relative LPL

deficiency. Other medical conditions such as hypo-

thyroidism can cause an elevated LDL cholesterol

level, and in such cases, the other medical condition

must be treated first. Certain classes of drugs can also

1234 CHOLESTEROL/Absorption, Function, and Metabolism

contribute to lipid abnormalities and, if so, can be

discontinued if medically feasible.

0033 Elevated LDL-cholesterol levels are strongly correl-

ated with development of ASCVD, whereas a high

HDL-cholesterol level is usually protective against

ASCVD. The National Cholesterol Educational Pro-

gram (NCEP) uses measurements of LDL-cholesterol

as guidelines for treatment with diet and drug

therapy in addition to multiple risk factors (Tables 4

and 5).

0034The first step in treatment of a primary disorder of

lipid and lipoprotein metabolism is a diet reduced in

total fat, saturated fat, and cholesterol (NCEP step I

and step II diets; see Table 6). Weight loss is particu-

larly important, especially in those with hyper-

triacylglycerolemia, low HDL cholesterol, glucose

intolerance, and insulin resistance. Drug treatment is

usually based on the particular lipoprotein that is

elevated starting with the atherogenic LDL. For

every individual, positive and negative risk factors

as outlined by the NCEP III guidelines determine

whether the aim of treatment with diet, exercise, and

drug treatment has been reached (Tables 4 and 5).

Guidelines have also been proposed for hypertriacyl-

glycerolemia and low HDL cholesterol. A plasma

triacylglycerol level in adults above 400 mg dl

1

elevated, 200–400 mg dl

1

is borderline, and below

200 mg dl

1

is normal. An HDL cholesterol less than

40 mg dl

1

is too low.

0035The drugs of choice to treat an elevated LDL chol-

esterol level are the HMG CoA reductase inhibitors,

or the so-called ‘statins.’ Inhibition of this enzyme

causes a reduction in the hepatic cholesterol pool,

leading to an induction of LDL receptors and a fall

in blood LDL cholesterol levels (see above). In 1976,

a fungal metabolite, called mevinolin or mevastatin,

was isolated, which inhibited HMG CoA reductase.

Most of the currently available cholesterol-lowering

drugs, which target these enzymes, are chemically

modified fungal metabolites or completely synthetic

analogs of this compound. These include: atorvasta-

tin, fluvastatin, lovastatin, pravastatin, and simvasta-

tin. When given in sufficiently high doses, these drugs

can also produce a significant fall in triacylglycerols,

presumably by decreasing hepatic VLDL production

and by removing the IDL molecules. HDL levels often

increase with statin therapy, but the mechanism is not

currently known.

0036Other drugs that are given when the LDL choles-

terol level is elevated are the bile acid sequestrants or

resins (cholestyramine or colestipol) and niacin.

tbl0003 Table 3 Classification of lipid disorders by phenotype (after Fredrickson)

Group Lipoproteinin excess Plasma cholesterol Plasma triacylglycerol Plasma appearance

I Chylomicrons Elevated or very elevated Very elevated Clear plasma with creamy layer on top

IIa LDL Elevated Normal Clear

IIb LDLþVLDL Elevated Moderately elevated Clear, turbid

III b-VLDL, IDL Elevated Elevated Turbid

IV VLDL Normal or moderately elevated Elevated Turbid

V ChylomicronsþVLDL Elevated or very elevated Very elevated Turbid plasma, creamy layer on top

tbl0004 Table 4 Intervention National Cholesterol Education Program

(NCEP) guidelines

LDL-cholesterollevelwith Initiation of diet Addeddrug therapy

Less than two risk factors >160 mg dl

1

>190 mg dl

1

More than two risk factors >130 mg dl

1

>160 mg dl

1

ASCD >100 mg dl

1

Diabetes mellitus >100 mg dl

1

Patients with diabetes mellitus or ASVD have a treatment goal for LDL

cholesterol of equal or less than 100 mg/dl independent of the presence or

absence of risk factors.

From National Cholesterol Education Panel (NCEP) (1993) Adult treatment

panel II. Journal of the American Medical Association 269: 3015–3023, with

permission.

tbl0005 Table 5 Atheroclerotic vascular disease (ASVD) risk factors

after the National Cholesterol Education Program (NCEP)

guidelines

Positive

risk factor

Negative

risk factor

HDL cholesterol value <40 mg dl

1

>60 mg dl

1

Age Men >45 years of age

or older

Women >55 years of age

or older

Family history of

premature ASVD

Present

Current cigarette

smoking

Present

Hypertension Present

Each person’s risk factors are to be determined and used as a guideline

for treatment. Patients with diabetes mellitus or ASVD have a treatment

goal for LDL cholesterol of equal or less than 100 mg dl

1

independent of

the presence or absence of risk factors.

From National Cholesterol Education Panel (NCEP) (1993) Adult treatment

panel II. Journal of the American Medical Association 269: 3015–3023, with

permission.

CHOLESTEROL/Absorption, Function, and Metabolism 1235