Cholesterol and Steroid Metabolism

I. Overview

Cholesterol characteristic steroid alcohol of animal

tissues

- Structural component of all cell membranes

(modulate its fluidity)

- Precursor of bile acids, steroid hormones, and

vitamin D (specialized tissues)

Liver regulate bodys cholesterol homeostasis

Cholesterol sources:

Dietary cholesterol

Cholesterol synthesized de novo by

extrahepatic tissues and by the liver itself

Fates of cholesterol:

Eliminated from the liver as unmodified

cholesterol in the bile

Converted to bile salts that are secreted into

the intestinal lumen

Component of plasma lipoproteins sent to the

peripheral tissues

Atherosclerosis lipid deposition leads to plaque

formation causing narrowing of blood vessels

II. Structure of Cholesterol

Cholesterol very hydrophobic

- Consists of 4 fused hydrocarbon rings (steroid

nucleus)

- Has an eight-carbon, branched hydrocarbon chain

attached to carbon 17 of the D ring

- Ring A has OH at carbon 3

- Ring B has a double bond between carbon 5 and

carbon 6

A. Sterols

- Steroids with 8 to 10 carbon atoms in the side

chain at carbon 17 and OH at carbon 3

Cholesterol major sterol in animal tissues

Plant sterols (e.g. -sitosterol) poorly absorbed by

humans

- After entering enterocytes, they are actively

transported back into the intestinal lumen

- Reduce absorption of dietary cholesterol used in

dietary treatment of hypercholesteremia

- Commercially available trans fatty acid-free

margarine

B. Cholesteryl esters

- Not bound in membranes

- Normally present in low levels in most cells

- Must be transported in association with protein as

a component of a lipoprotein particle or be

solubilized by phospholipids and bile salts in the

bile

III. Synthesis of Cholesterol

- Endergonic

- Driven by hydrolysis of the high-energy thioester

bond of acetyl coenzyme A (CoA) and the terminal

phosphate bond of ATP

- Requires enzymes in both the cytosol and the

membrane of the smooth ER

- Responsive to changes in cholesterol

concentration

Cholesterol synthesized by virtually all tissues in

humans

Make the largest contributions to the bodys

cholesterol pool:

Liver

Intestine

Adrenal cortex

Ovaries

Testes

Placenta

Acetate provides all the carbon atoms in cholesterol

NADPH provides the reducing equivalents

Imbalance in regulation can lead to elevation in

circulating levels of plasma cholesterol with the

potential for vascular disease

A. Synthesis of 3-hydroxy-3-methylglutaryl (HMG) CoA

- First 2 reactions are similar in the ketone bodies

pathway

- Result in the production of HMG CoA

HMG CoA six-carbon compound

First 2 acetyl CoA molecules condense to form acetoacetyl CoA

Third molecule of acetyl CoA is added producing

HMG CoA

Liver parenchymal cells

- contain 2 isozymes of HMG CoA synthase

- Cytosolic enzyme: participates in cholesterol

synthesis

- Mitochondrial enzyme: functions in the pathway for

ketone body synthesis

B. Synthesis of mevalonate

- Reduction of HMG CoA to mevalonate

- Catalyzed by HMG CoA reductase

- Rate-limiting and key regulated step in cholesterol

synthesis

- Occurs in the cytosol

- Uses 2 molecules of NADPH as reducing agent;

releases CoA

- Irreversible

HMG CoA reductase intrinsic membrane protein of

the ER with the enzymes catalytic domain projecting

into the cytosol

C. Synthesis of cholesterol

IPP precursor of a family of molecules with diverse

functions, the isoprenoids

Cholesterol sterol isoprenoid

Nonsterol isoprenoids

e.g. dolichol and ubiquinone

Prenylation covalent attachment of farnesyl to

proteins

- One mechanism for anchoring proteins to plasma

membranes

Squalene formed from 6 isoprenoid units

- 3 ATP are hydrolyzed per mevalonate residue

converted to IPP

Total: 18 ATP required to make the

polyisoprenoid squalene

Final step ER-associated pathway

- Includes several different enzymatic reactions

Smith-Lemli-Opitz syndrome (SLOS)

- Relatively common autosomal recessive order of

cholesterol biosynthesis

- Caused by partial deficiency in 7-

dehydrocholesterol-7-reductase

- One of several multisystem, embryonic

malformation syndromes associated with impaired

cholesterol synthesis

7-dehydrocholesterol-7-reductase enzyme involved in

the migration of the double bond

Mevalonate is converted to 5-pyrophosphomevalonate in 2 steps each of

which transfers a phosphate group from ATP

Isopentenyl pyrophosphate (IPP) is formed by the decarboxylation of 5-

pyrophosphomevalonate. The reaction requires ATP.

IPP is isomerized to 3,3-dimethylallyl pyrophosphate (DPP)

IPP and DPP condense to form 10-carbon geranyl pyrophosphate (GPP)

Second molecule of IPP condenses with GPP to form 15-carbon farnesyl pyrophosphate

2 molecules of FPP combine, releasing pyrophosphate, and are reduced forming the

30-carbon compound squalene

Squalene is converted to the sterol lanosterol by a sequence of reactions catalyzed by ER-

associated enzymes that use O2 and NADPH. Hydroxylation of squalene triggers the cyclization of the strucure of lanosterol

The conversion of lanosterol to cholesterol results to shortening of the carbon chain from 30 to 27 carbons, removal of 2 methyl groups

at carbon 4, migration of double bond from carbon 8 to carbon 5, and reduction of double

bond between carbon 24 and carbon 25

D. Regulation of cholesterol synthesis

HMG CoA reductase rate-limiting enzyme

- Major control point for cholesterol biosynthesis

- Subject to different kinds of metabolic control

1. Sterol-dependent regulation of gene expression

Sterol regulatory element-binding protein-2 (SREBP-2)

- Transcription factor that controls the gene

expression for HMG CoA reductase

- Binds DNA at the cis-acting sterol regulatory

element (SRE) of the reductase gene

SREBP an integral protein of the ER membrane

- Associates with a second ER membrane protein,

SCAP (SREBP cleavage-acting protein)

2. Sterol-accelerated enzyme degradation

Reductase a sterol-sensing integral protein of

the ER membrane

sterol levels in the cell reductase binds to insig

proteins ubiquitination and proteasomal

degradation of the reductase

3. Sterol-independent

phosphorylation/dephosphorylation

AMP-activated protein kinase (AMPK) +

phosphoprotein phosphatase controls covalently

the activity of CoA reductase

Phosphorylated inactive enzyme

Dephosphorylated active enzyme

AMPK activated by AMP

ATP availability, cholesterol synthesis

4. Hormonal regulation

insulin and thyroxine, upregulation of expression

of the gene for HMG CoA reductase

glucagon and glucocorticoids, downregulation of

expression of the gene for HMG CoA reductase

When the sterol level in the cell is low, the SREBP-SCAP complex is sent out

of the ER to the Golgi

In the Golgi SREBP is sequentially acted upon by 2 proteases, which generate a soluble fragment that

enters the nucleus, binds the SRE and functions as a transcription factor

Increased synthesis of HMG CoA reductase

Increased cholesterol synthesis

If sterols are abundant, they bind to SCAP at its sterol-sensing domain

Binding of SCAP to other ER membrane proteins

(insigs) is induced

Retention of SCAP-SREBP complex in the

ER

Prevent the activation of SREBP

Down-regulation of cholesterol synthesis

5. Inhibition by drugs

Statin drugs structural analogs of HMG CoA

- Are (or are metabolized to) reversible, competitive

inhibitors of HMG CoA reductase

- Used to decrease plasma cholesterol levels in

patients with hypercholesterolemia

IV. Degradation of Cholesterol

Coprostanol, cholestanol, and cholesterol make up

the bulk of neutral fecal sterols

V. Bile Acids and Bile Salts

Bile watery mixture of organic and inorganic

compounds

- Can either:

Pass directly from the liver where it is

synthesized into the duodenum through

the common bile duct

Stored in the gallbladder when not

immediately needed for digestion

Phosphatidylcholine (lecithin) and bile salts

(conjugated bile acids) quantitatively the most

important organic compounds of bile

A. Structure of bile acids

Bile acids contain 24 carbons with 2 or 3 OH groups

and a side chain that terminates in a carboxyl group

- Amphipathic (-OH groups are in orientation- lie

below the plane of the rings) can act as

emulsifying agents in the intestine; help prepare

dietary TAG and other complex lipids for

degradation by pancreatic digestive enzymes

- Methyl groups are (lie above the plane of the

rings)

Carboxyl group pKa 6

- Not fully ionized at physiologic pH

B. Synthesis of bile acids

Rate-limiting step: introduction of OH at carbon 7 of

the steroid nucleus by cholesterol-7--hydroxylase

Cholesterol-7--hydroxylase an ER-associated

cytochrome P450 (CYP) enzyme found only in liver

- Down-regulated by cholic acid

Intact sterol nucleus is converted to bile acids and bile salts

Excreted in the feces and by secretion of cholesterol into the bile

Transported to the intestine for elimination

Some of the choleterol in the intestine is modified by bacteria

before excretion

Primary compounds made are isomers coprostanol and cholestanol - reduced derivatives of cholesterool

-OH groups are inserted at specific positions on the steroid

structure

Double bond of cholesterol B rings is reduced

Hydrocarbon chain is shortened by 3 carbons, introducing a

carboxyl group at the end of the chain

Product: "Primary" bile acids:

cholic acid (triol)

and chenodeoxycholic acid (diol)

C. Synthesis of bile salts

- Bile acids are conjugated to a molecule of either

glycine or taurine by an amide bond between the

carboxyl group of the bile acid and the amino

group of the added compound before they leave

the liver

New structures:

Glycocholic acid

Glycochenodeoxycholic acids

Taurocholic

Taurochenodeoxycholic acids

Taurine endproduct of cysteine metabolism

3:1 ratio of glycine to taurine forming in the bile

Addition of glycine or taurine

- Results in the presence of carboxyl group with a

lower pKa (from glycine) or a sulfonate group (from

taurine) both of which are fully ionized (negatively

charged) at physiologic pH

Bile salts conjugated forms

- More effective detergents than bile acids because

of their enhanced amphipathic nature; thus, only

the conjugated forms are found in the bile

- Provide the only significant mechanism for

cholesterol excretion, both as a metabolic product

of cholesterol and as a solubilizer of cholesterol in

bile

Exogenously supplied chenodeoxycholic acid

- Treatment for individuals with genetic deficiencies

in the conversion of cholesterol to bile acids

D. Action of intestinal flora on bile salts

Bacteria in the intestine

- Can remove glycine and taurine from bile salts

regenerating bile acids

- Convert some of the primary bile acids into

secondary bile acids by removing a OH group,

producing:

Deoxycholic acid from cholic acid

Lithocholic acid from chenodeoxycholic

acid

E. Enterohepatic circulation

- Bile salts secreted into the intestine are efficiently

reabsorbed (> 95%) and reused

- Liver converts both primary and secondary bile

acids into bile salts by conjugation with glycine or

taurine secreted into the bile

Ileum via a Na+-bile salt cotransporter where bile

acids + bile salts is primarily absorbed

Bile acids + bile salts actively transported out of the

ileal mucosal cells into the portal blood and are

efficiently taken up by the hepatocytes via an isoform

of the cotransporter

Bile acids hydrophobic

- Require a carrier in the portal blood

Albumin carries bile acids in a noncovalent complex

Enterohepatic circulation

- Continuous process of secretion of bile salts into

the bile passage through the duodenum where

some are converted to bile acids uptake in the

ileum subsequent return to the liver as a

mixture of bile acids and salts

Bile acid sequesterants (e.g. cholestyramine) bind

bile acids in the gut

- Prevent reabsorption of bile acids promote

excretion

- Used in the treatment of hypercholesterolemia

because the removal of bile acids relieves the

inhibition on bile acid synthesis in the liver

divert additional cholesterol into that pathway

Dietary fiber also binds bile acids and increases their

excretion

F. Bile salt deficiency: cholelithiasis

- disruption of the simultaneous movement of

cholesterol from the liver into the bile and

secretion of phospholipid and bile salts more

cholesterol enters the bile than can be solubilized

by the bile salts and phosphatidylcholine present

precipitation of cholesterol in the gallbladder

leading to cholesterol gallstone disease

- typically caused by bile acids in the bile which

may result from:

gross malabsorption of bile acids from the

intestine (seen in patients with severe ileal

disease)

obstruction of biliary tract interrupted

enterohepatic circulation

severe hepatic dysfunction decreased

synthesis of bile salts or other

abnormalities in bile production

excessive feedback suppression of bile

acid synthesis due to accelerated rate

of recycling of bile acids

- may also result from increased biliary cholesterol

excretion (seen with use of fibrates)

Fibrates (e.g. gemfibrozil) derivatives of fibric acid

- used to reduce TAG levels in blood through up-

regulation of fatty acid -oxidation

Laparoscopic cholecystectomy surgical removal of

gallbladder through a small incision

- treatment of choice

Oral administration of chenodeoxycholic acid

- for patients who are unable to undergo surgery

- supplement bodys supply of bile acids gradual

(months to years) dissolution of gallstones

VI. Plasma Lipoproteins

- Spherical macromolecular complexes of lipids and

specific proteins (apolipoproteins or apoproteins)

Lipoprotein particles

- Include:

Chylomicrons (CM)

Very-low-density lipoproteins

Low-density lipoproteins

High-density lipoproteins

- Differ in lipid and protein composition, size, density

and site of origin

- Function both:

to keep their component lipids soluble as

they transport them in the plasma

to provide an efficient mechanism for

transporting their lipid contents to (and

from) the tissues

- Humans experience a gradual deposition of lipid

(especially cholesterol) in tissues potentially life-

threatening occurrence when the lipid deposition

contributes to plaque formation atherosclerosis

A. Composition of plasma lipoproteins

Lipoproteins neutral lipid core (TAG + cholesteryl

esters) surrounded by a shell of amphipathic

apolipoproteins, phospholipid and nonesterified (free)

cholesterol

- Constantly interchange lipids and apolipoproteins

with each other

Shell of amphipathic apolipoproteins, phospholipid,

and nonesterified cholesterol (free)

- Oriented so that their polar portions are exposed

on the surface of the lipoprotein, thus making the

particle soluble in aqueous solution

TAG and chlolesterol carried by lipoproteins are

obtained from:

Diet (exogenous source)

De novo synthesis (endogenous source)

1. Size and density of lipoprotein particles

Chylomicrons lipoprotein particles lowest in

density and largest in size

- Contain the highest percentage of lipid

- Lowest percentage of protein

VLDLs and LDLs

- Successively denser

- Higher ratios of protein to lipid

HDL particles

- Densest

Plasma lipoproteins

- Can be separated on the basis of their

electrophoretic mobility or on the basis of their

density by ultracentrifugation

2. Apolipoproteins

- Associated with lipoprotein particles

Functions:

Provide recognition sites for cell-surface

receptors

Serve as activators or coenzymes for enzymes

involved in lipoprotein metabolism

- Required as essential structural components of the

particles and cannot be removed (particles cannot

be produced without them), whereas others are

transferred freely between lipoproteins

5 Major Classes

A through E

Subclasses

Apolipoprotein (or apo) A-I

Apo C-II

B. Metabolism of chylomicrons

Intestinal mucosal cells where chylomicrons are

assembled

Chylomicrons carry:

Dietary TAG

Cholesterol

Fat-soluble vitamins

Cholesteryl esters (plus additional lipids made

in these cells)

to the peripheral tissues

TAG account for close to 90% of lipids in a

chylomicron

1. Synthesis of apolipoproteins

Apolipoprotein B-48 unique to chylomicrons

- Constitutes the N-terminal, 48% of the protein

coded for by the gene for apo B

Rough ER where synthesis of apolipoprotein B-48

begins glycosylated as it moves through the RER

and Golgi

Apo B-100 synthesized by the liver

- Found in VLDL and LDL

- Represents the entire protein coded for by the apo

B gene

Nonsense codon created by posttranscriptional

editing of a cytosine to a uracil in intestinal apo B-100

mRNA

- Allow translation of only 48% of mRNA

2. Assembly of chylomicrons

- Occurs before transition from the ER to the Golgi,

where the particles are packaged in secretory

vesicles fuse with the plasma membrane

releasing lipoproteins enter the lymphatic

system enter the blood

Smooth ER where enzymes involved in TAG,

cholesterol, and phospholipid synthesis are located

Microsomal TAG transfer protein required in

assembly of apolipoproteins and lipid into chylomicrons

- Loads apo B-48 with lipid

3. Modification of nascent chylomicron particles

Nascent chylomicron particle released by the

intestinal mucosal cell

- Receives apolipoprotein E and C when it reaches

the plasma

Apolipoprotein E recognized by hepatic receptors

Apolipoprotein C includes apo C-II necessary for

activation of lipoprotein lipase

Lipoprotein lipase degrades the TAG contained in the

chylomicron

HDL source of these apolipoproteins

4. Degradation of TAG by lipoprotein lipase

Lipoprotein lipase extracellular enzyme that is

anchored by heparin sulfate to the capillary walls of

most tissues, but predominantly those of:

adipose tissue

cardiac muscle

skeletal muscle

- activated by apo C-II on circulating lipoprotein

particles

- hydrolyzes TAG contained in lipoprotein particles to

yield fatty acids and glycerol

Adult liver does not have lipoprotein lipase

Hepatic lipase found on the surface of endothelial

cells of the liver

- plays some role in TAG degradation in CM and

VLDL

- particularly important in HDL metabolism

Fatty acids stored by the adipose or used for energy

by the muscle

- if not immediately taken up by a cell, LCFA are

transported by serum albumin until their uptake

does occur

Glycerol used by the liver in:

lipid synthesis

glycolysis

gluconeogenesis

Lipoprotein lipase deficiency or apo C-II (Type 1

hyperlipoproteinemia or familial lipoprotein lipase

deficiency)

- dramatic accumulation of chylomicron-TAG in the

plasma (hypertriacylglycerolemia) even in the

fasted state

5. Regulation of lipoprotein lipase activity

Insulin stimulate lipoprotein lipase synthesis and

transfer to the luminal surface of the capillary (fed

state)

Adipose enzyme has a large Km allows removal of

fatty acids from circulating lipoprotein particles and

their storage as TAG only when plasma lipoprotein

concentrations are elevated

Heart muscle lipoprotein lipase has a small Km

- Allows the heart continuing access to the

circulating fuel, even when plasma lipoprotein

concentrations are low

Cardiac muscle has the highest concentration of

lipoprotein lipase reflect the use of fatty acids to

provide much energy needed for cardiac function

6. Formation of chylomicron remnants

- As the chylomicron circulates and more than 90%

of TAG in its core is degraded by lipoprotein lipase,

the particle size and density

- C apoproteins but not apo E are returned to HDL

Remnant rapidly removed from the circulation by the

liver cell membranes contain lipoprotein receptors

that recognize apo E

C. Metabolism of VLDL

VLDL produced in the liver

- Composed predominantly of endogenous TAG

(approximately 60%)

- Function: carry endogenous TAG from the liver (site

of synthesis) to the peripheral tissues

Peripheral tissues where TAG is degraded by

lipoprotein lipase

Fatty liver (hepatic steatosis) occurs in conditions in

which there is an imbalance between hepatic TAG

synthesis and the secretion of VLDL

- Characterized by:

Obesity

Uncontrolled diabetes mellitus

Chronic ethanol digestion

1. Release of VLDL

- VLDL are secreted directly into the blood by the

liver as nascent VLDL particles containing apo B-

100 must obtain apo C-II and apo E from

circulating HDL

Chylomicrons

- Apo C-II is required for activation of lipoprotein

lipase

Abetalipoproteinemia rare hypolipoproteinemia

- Caused by a defect in microsomal TAG transfer

protein (MTP) inability to load apo B with lipid

no VLDL or chylomicrons are formed and TAG

accumulate in the liver and intestine

Chylomicron remnants bind to lipoprotein receptors

Taken into the hepatocytes by endocytosis

Endocytosed vesicle fuses with a lysosome

Apolipoproteins, cholesteryl esters, and other components of the

remnant are hydrolytically degraded

Amino acids, free cholesterol, and fatty acids are released

Receptor is recycled

2. Modification of circulating VLDL

3. Production of LDL from VLDL in the plasma

- VLDL is converted in the plasma to LDL

Intermediate-density lipoproteins (IDL) or VLDL

remnants observed during this transition

IDL can also be taken up by cells through receptor-

mediated endocytosis that uses apo E as the ligand

Apo E normally present in 3 isoforms:

E-2 binds poorly to receptors

E-3

E-4

Patients homozygotic for apo E-2 are deficient in the

clearance of chylomicron remnants and IDL have

Type III hyperlipoproteinemia (familial

dysbetalipoproteinemia or broad beta disease) with

hypercholesterolemia and premature atherosclerosis

- E-4 isoform confers increased susceptibility to and

decreased age of onset of late-onset Alzheimer

disease, doubling lifetime risk

D. Metabolism of LDL

LDL contain much less TAG than their VLDL

predecessors

- Have a high concentration of cholesterol and

cholesteryl esters

1. Receptor-mediated endocytosis

Primary function of LDL particles: provide cholesterol to

the peripheral tissues (or return it to the liver) by

binding to cell surface membrane LDL receptors that

recognize apo B-100 (but not apo B-48) these

receptors can also bind apo E; they are also known as

apo B-100/apo E receptors

VLDL pass through the circulation

TAG is degraded by lipoprotein lipase

VLDL decrease in size and become denser

Surface components, including the C and E apoproteins, are returned to HDL, but the particles retain apo B-

100

Some TAG are transferred from VLDL to HDL in an exchange reaction that

concomitantly transfers some cholesteryl esters from HDL to VLDL (accomplished by cholesteryl ester

transfer protein or CETP)

Type II hyperlipidemia (familial hypercholesterolemia, FH)

and premature atherosclerosis

- deficiency of functional LDL receptors

- plasma LDL and plasma cholesterol

FH can also be caused by:

protease activity that degrades the receptor

Defects in apo B-100 that reduce its binding to the

receptor

CURL compartment for uncoupling of receptor and ligand

- Where receptors migrate to

Wolman disease

- Storage disease caused by rare autosomal

recessive deficiencies in the ability to hydrolyze

lysosomal cholesteryl esters

Niemann-Pick disease, Type C

- Inability to transport unesterified cholesterol out of

the lysosome

2. Effect of endocytosed cholesterol on cellular

cholesterol homeostasis

a. HMG CoA reductase is inhibited by cholesterol;

de novo cholesterol synthesis

b. synthesis of new LDL receptor protein by

expression of LDL receptor gene limited entry of

LDL cholesterol into cells

c. If the cholesterol is not required immediately for

some structural or synthetic purpose, it is

esterified by acyl CoA: cholesterol acyltransferase

(ACAT)

SRE and SREBP (SREBP-2) involved in the regulation

of LDL receptor gene

ACAT transfers fatty acid from fatty acyl CoA

derivative to cholesterol; Product: cholesteryl ester that

can be stored in the cell

- Activity is enhanced in the presence of increased

intracellular cholesterol

3. Uptake of chemically modified LDL by macrophage

scavenger

Macrophages possess high levels of scavenger

receptor activity

Scavenger receptor class A (SR-A)

- Can bind a broad range of ligands

- Mediate endocytosis of chemically modified LDL in

which the lipid components of apo B have been

oxidized

LDL receptors are negatively charged glycoproteins that are clustered in pits on cell

membranes. The cytosolic side of the pit is coated with the protein clathrin, which stabilizes the

shape of the pit

After binding, the LDL-receptor complex is internalized by endocytosis

The vesicle containing LDL loses its clathrin coat and fuses with other similar vesicles, forming

larger vesicles (endosomes)

THe pH of the endosome falls due to the proton-pumping activity of endosomal ATPase - allows

separation of LDL from its receptor

Receptor migrate to one side of the endosome, whereas the LDLs stay free within the lumen of

the vesicle

The receptors can be recycled, whereas the lipoprotein remnants in the vesicle are transferred

to lysosomes and degraded by lysosomal acid hydrolases, releasing free cholesterol, amino acids, fatty acids, and phospholipids. These

compounds are reutralized by the cell.

- Not down-regulated in response to intracellular

cholesterol

Cholesteryl esters accumulate in macrophages

- Cause transformation of macrophages into foam

cells participate in the formation of

atherosclerotic plaque

E. Metabolism of HDL

HDL comprise of heterogeneous family of

lipoproteins with a complex metabolism

HDL particles formed in blood by the addition of lipid

to apo A-1

Apo A-1 apolipoprotein made by the liver and

intestine and secreted into blood

- Accounts for about 70% of the apoproteins in HDL

Functions of HDL

1. HDL is a reservoir of apolipoproteins

HDL particles serve as circulating reservoir of apo C-II

Apo C-II apolipoprotein that is transferred to VLDL

and chylomicrons

- Activator of lipoprotein lipase

Apo E apolipoprotein required for the receptor-

mediated endocytosis of IDLs and chylomicron

remnants

2. HDL uptake of unesterified cholesterol

Nascent HDL disk-shaped particles containing

primarily phospholipid (largely phosphatidylcholine)

and apolipoproteins A, C, and E

- Take up cholesterol from non-hepatic (peripheral)

tissues and return it to the liver as cholesteryl

esters

HDL particles excellent acceptors of unesterified

cholesterol as a result of their high concentration of

phospholipids, which are important solubilizers of

cholesterol

3. Esterification of cholesterol

Cholesterol when taken up by HDL, it is immediately

esterified by the plasma enzyme lecithin:cholesterol

acyltransferase (LCAT or PCAT; P =

Phosphatidylcholine)

LCAT synthesized by the liver

- Binds to nascent HDL

- Activated by: Apo A-I

- Transfers fatty acid from carbon 2 of

phosphatidylcholine to cholesterol

Product: hydrophobic cholesteryl ester

(sequestered in the core of HDL) and

lysophosphatidylcholine (binds to albumin)

Esterification maintains cholesterol concentration

gradient allow continued efflux of cholesterol to HDL

Discoidal nascent HDL accumulates cholesteryl

esters

- First becomes a spherical, relatively cholesteryl

ester-poor HDL3 cholesteryl ester-rich HDL2

particle that carries these esters to the liver

Cholesterol ester transfer protein (CETP) moves

some of the cholesteryl esters from HDL to VLDL in

exchange for TAG relieve product inhibition of LCAT

Because VLDL are catabolized to LDL, cholesteryl

esters are ultimately taken up by the liver

4. Reverse cholesterol transport

- Involves

efflux of cholesterol from peripheral cells

to HDL mediated, at least in part, by the

transport protein ABCA1

esterification of cholesterol by LCAT

binding of cholesteryl ester-rich HDL

(HDL2) to liver and steroidogenic cells

selective transfer of the cholesteryl esters

into these cells

release of lipid-depleted HDL (HDL3)

Key component of cholesterol homeostasis:

- Selective transfer of cholesterol from peripheral

cells to HDL and from HDL to the liver for bile acid

synthesis or disposal via the bile and to

steroidogenic cells for hormone synthesis

- Basis for

inverse relationship seen between plasma

HDL concentration and atherosclerosis

HDLs designation as the good

cholesterol carrier

Tangier disease very rare deficiency of ABCA1

- Characterized by virtual absence of HDL particles

due to degradation of lipid-poor apo A-1

SR-B1 (scavenger receptor class B type 1)

- Cell-surface receptor

- - mediates the uptake of cholesteryl esters by the

liver

- Binds HDL

Hepatic lipase can degrade both TAG and

phospholipids

- Participates in the conversion of HDL2 to HDL3

ABCA1 an ATP-binding cassette (ABC) protein

ABC proteins use energy from ATP hydrolysis to

transport materials, including lipids, in and out of cells

and across intracellular compartments

Defects in specific ABC proteins result in:

X-linked adrenoleukodystrophy

Respiratory distress syndrome due to

decreased surfactant secretion

Cystic fibrosis

F. Role of lipoprotein (a) in heart disease

Lipoprotein (a) or Lp (a)

- Particle, when present in large quantities in the

plasma, is associated with an increased risk of

coronary heart disease

- Nearly identical in structure to an LDL particle

- Distinguishing feature: presence of additional

apolipoprotein molecule (apo (a)) that is covalently

linked at a single site to apo B-100

Circulating levels of Lp(a) are determined primarily by

genetics

Diet may play some role as trans fatty acids have

been shown to Lp(a)

Estrogen - both LDL and Lp(a)

Apo(a) structurally homologous to plasminogen

Plasminogen precursor of blood protease whose

target is fibrin

Fibrin main protein component of blood clots

Lp(a) slows down the breakdown of blood clots that

trigger heart attacks because it competes with

plasminogen for binding to fibrin

Niacin reduces Lp(a) and raises HDL

VII. Steroid Hormones

Cholesterol precursor of all classes of steroid

hormones:

Glucocorticoids e.g. cortisol

Mineralocorticoids e.g. aldosterone

Sex hormones e.g. androgens, estrogens, and

progestins

Corticosteroids collective term for glucocorticoids

and mineralocorticoids

Adrenal cortex where synthesis and secretion of

cortisol, aldosterone, and androgens occur

Ovaries and placenta where synthesis and secretion

of estrogens and progestins occur

Testes where synthesis and secretion of testosterone

occurs

Steroid hormones transported by the blood from their

sites of synthesis to their target organs

- Must be complexed with a plasma protein because

of their hydrophobicity

Plasma albumin can act as a nonspecific carrier

- carry aldosterone

Specific steroid-carrier plasma proteins

- bind steroid hormones more tightly than does

albumin

E.g. corticosteroid-binding globulin (transcortin)

- responsible for transporting cortisol

A. Synthesis of steroid hormones

- Involves shortening hydrocarbon chain of

cholesterol and hydroxylation of the steroid

nucleus

Initial and rate-limiting reaction converts cholesterol

to the 21-carbon pregnenolone

Cholesterol side-chain cleavage enzyme complex

(desomolase, P450scc) catalyzes the conversion of

cholesterol to 21-carbon pregnenolone

- A cytochrome P450 (CYP) mixed function oxidase

of the inner mitochondrial membrane

NADPH and O2 required for reaction

Cholesterol substrate can be newly synthesized,

taken up from lipoproteins, or released from

cholesteryl esters stored in the cytosol of steroidogenic

tissues

Important control point:

- Movement of cholesterol into mitochondria

mediated by StAR (steroidogenic acute regulatory

protein)

Pregnenolone parent compound for all steroid

hormones

- Oxidized and then isomerized to progesterone

which is further modified to the other steroid

hormones by hydroxylation reactions that occur in

the ER and mitochondria

Enzymes primarily are CYP proteins

- A defect in the activity or amount of an enzyme in

this pathway can lead to a deficiency in the

synthesis of hormones beyond the affected step

and to an excess in the hormones or metabolites

before that step

Congenital adrenal hyperplasias collective term for

enzyme deficiencies

Addison disease due to autoimmune destruction of

the adrenal cortex

- Characterized by adrenocortical insufficiency

B. Secretion of adrenal cortical steroid hormones

Steroid hormones secreted on demand from their

tissues of origin in response to hormonal signals

Corticosteroids and androgens made in different

regions of the adrenal cortex

- Secreted into blood in response to different signals

1. Cortisol

Middle layer (zona fasciculate) of the adrenal cortex

where cortisol is produced controlled by the

hypothalamus to which pituitary gland is attached

Corticotropin-releasing hormone (CRH)

- Produced by the hypothalamus

- Travels through capillaries to the anterior lobe of

the pituitary in response to severe stress (e.g.

infection)

- Induces production and secretion of

adrenocorticotropic hormone (ACTH) in the

pituitary

Polypeptide ACTH stress hormone

- Stimulates adrenal cortex to synthesize and

secrete the glucocorticoid cortisol

Cortisol allows the body to respond to stress through

its effects on intermediary metabolism (e.g. increased

gluconeogenesis) and inflammatory and immune

responses - cortisol, release of CRH and ACTH is

inhibited

ACTH binds to a membrane G-protein coupled

receptor results in cAMP production and activation of

protein kinase A

PKA phosphorylates the esterase that converts

cholesteryl ester to cholesterol and stimulates

synthesis of StAR protein

2. Aldosterone

- Primary effect on kidney tubules: stimulates

sodium uptake and potassium excretion

- BP

Outer layer (zona glomerulosa) of the adrenal cortex

where aldosterone is produced induced by plasma

Na+/K+ ratio and by angiotensin II

Angiotensin II an octapeptide

- Produced from angiotensin I (decapeptide) by

angiotensin-converting enzyme (ACE) found

predominantly in the lungs, but which is also

distributed widely in the body

- Binds to cell-surface receptors

- Effects are mediated through the

phosphatidylinositol 4,5-bisphosphate (PIP2)

pathway and not by cAMP

Inhibitors of ACE are used to treat renin-dependent

hypertension

Angiotensin I produced in the blood by cleavage of an

inactive precursor (angiotensin) secreted by the liver

- Cleavage is accomplished by the enzyme renin,

made and secreted by the kidney

3. Androgens

Inner (zona reticularis) and middle layers of the adrenal

cortex produce androgens, primarily

dehydroepiandrosterone and androstenedione

Adrenal androgens weak

- Converted in peripheral tissues to testosterone

and to estrogens

Testosterone stronger androgen

Estrogens derived from androstenedione and

testosterone by aromatase (CYP19)

Aromatase inhibitors used in the treatment of

estrogen-responsive breast cancer in post-menopausal

women

C. Secretion of steroid hormones from gonads

Testes and ovaries synthesize hormones necessary

for sexual differentiation and reproduction

Gonadotropin-releasing hormone single

hypothalamic-releasing factor

- Stimulates the anterior pituitary to release the:

Glycoproteins

Luteinizing hormone (LH) stimulates the

testes to produce testosterone and the

ovaries to produce estrogens and

progesterone

Follicle-stimulating hormone (FSH)

regulates the growth of ovarian follicles

and stimulates testicular spermatogenesis

LH and FSH bind to surface receptors

- Cause an increase in cAMP

D. Mechanism of steroid hormone action

HRE found in the promoter (or an enhancer element)

for genes that respond to a specific steroid hormone

ensure coordinated regulation of these genes

Hormone-receptor complexes can also inhibit

transcription in association with corepressors

Binding of a hormone to its receptor causes a

conformational change in the receptor that uncovers its

DNA-binding domain allow complex to interact

through a Zn-finger motif with the appropriate

sequence on the DNA

Receptors for steroid hormones, thyroid hormone,

retinoic acid, and 1,25-dihydroxycholecalciferol

(vitamin D) members of a superfamily of

structurally related gene regulators that function in a

similar way

E. Further metabolism of steroid hormones

Steroid hormones generally converted into inactive

metabolic excretion products in the liver

Reactions include:

Reduction of unsaturated bonds

Introduction of additional hydroxyl groups

Resulting structure: made more soluble by conjugation

with glucuronic acid or sulfate (from 3-

phosphoadenosyl-5phosphosulfate)

Approximately 20-30% of these metabolites are

secreted into the bile and then excreted in the feces

The remainder are released into the blood and filtered

from the plasma in the kidney, passing into the urine

Conjugated metabolites are fairly water-soluble and do

not need protein carriers Each steroid hormone diffuses

across the plasma membrane of its target cell and binds to a specific

cytosolic or nuclear receptor

Receptor-ligand complexes accumulate in the nucleus

Receptor-ligand complexes dimerize

Bind to specific regulatory DNA sequences (hromone-response

elements, HRE) in association with coactivator proteins

Promoter activation and increased transcription of targeted genes