Cholesterol and Steroid Metabolism
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Transcript of Cholesterol and Steroid Metabolism
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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