Cholesterol synthesis and breakdown
Dr. Carolyn K. Suzuki
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OBJECTIVES
To compare and contrast the different mechanisms by which
cholesterol biosynthesis are regulated.
To predict whether intracellular cholesterol synthesis will be
up- or down-regulated in response to energy availability as
influenced by diet, hormones and exercise.
To distinguish the different mechanisms by which plasma
cholesterol levels are controlled by clinically adminstrered
pharmacological agents.
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Cholesterol in cellular membranes
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4
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Cholesterol
•27 carbons all derived from acetate
•C-3 hydroxyl group
•C-17 side chain with 8 carbons
cholesterol
Sources in the body
• synthesized primarily in liver and intestine
• not required in diet
• intestinal uptake from diet
hydrocarbon tail
C
A
D
steroid nucleus
B
Elimination
• converted into bile acids and bile salts in liver
• stored in gall bladder, secreted into intestine
• small % excreted in feces
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Cholesterol esters
• esterification at C-3 with fatty acid
• primary form transported in plasma
• packaged in lipoprotein particles
(e.g. LDL, HDL)
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cholesterol ester
C
3
A
D
B
fatty acid
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A. Synthesis of HMG CoA
2 acetyl CoA (2C)
thiolase
cytosol
mitochondria
peroxisomes
CoA
2 acetoacetyl CoA (4C)
HMG CoA synthase
acetyl CoA
CoA
3-hydroxy-3-methylglutaryl CoA (6C)
(HMG CoA)
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B. Synthesis of mevalonic acid
HMG CoA (6C)
2 NADPH
RATE LIMITING STEP
HMG CoA reductase
CoA
2 NADP+
endoplasmic
reticulum
Mevalonic acid (6C)
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C. Cholesterol synthesis
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C. Cholesterol synthesis
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The transcription factor regulating cholesterol synthesis genes is
SREBP- sterol responsive element binding protein
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Sterol-dependent regulation of cholesterol synthesis genes
SREBP- Sterol Regulatory Element Binding Protein
a transmembrane protein
has a DNA binding domain
has a SCAP interacting domain
SCAP- SREBP Cleavage Activating Protein
a transmembrane protein
has a sterol sensing domain
binds to SREBP in the ER
when ER sterols are low, SCAP-SREBP move to the Golgi
Protease 1 and Protease 2localized to the Golgi
responsible for the two step cleavage of SREBP resulting
in soluble, cytosolic SREBP
Mature, proteolytically-processed SREBP
translocates from the Golgi to the nucleus
activates the expression of cholesterol synthesis genes
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nucleus
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step #1
when sterol levels
are low
SCAP and SREBP
are transported to
the Golgi
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step #2
SREBP
release
step #3
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step #4
SREBP
translocates
to nucleus
nucleus
transcriptional activation of
sterol responsive element (SRE)
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controlled genes
When intracellular cholesterol is low
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Cholesterol-dependent degradation
of HMG CoA reductase
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(AMP kinase)
AMP kinase is different from cAMP-dependent kinase (PKA)
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Statins competitively inhibit HMGRthey mimic the transient intermediate mevadyl CoA
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Zetia (ezetimibe)
Mechanism of action• acts at small intestine brush border
• does not enter the bloodstream, no side effects
• inhibits absorption of cholesterol
• does not block absorption of triglycerides or
fat-soluble vitamins
Vytorin (ezetimibe + simvastatin)
• ezetimibe administered in combination with a
simavastatin (i.e. a statin)
• further reduces total cholesterol levels as compared
to statin alone
• blocks cholesterol absorption in the intestine and
cholesterol synthesis in the liver
• permits reduced doses of statins, which have
side effects
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Reduction of LDL-cholesterol across all tested statins
Mean %
change in LDL-C
from
baseline
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Bile acids and bile salts
• cholesterol is the precursor of bile acids and bile salts
• synthesized in the liver
• stored in the gall bladder
• secreted into intestine
• aids digestion by emulsifying dietary lipids making them
accessible to pancreatic lipases
• aids intestinal absorption of fat-soluble vitamins (A, D, E, K)
• ~95% are reabsorbed in ileum and returned to liver
• ~5% of bile salts are excreted in feces
Enterohepatic circuit
•synthesis in the liver
•storage in the gall bladder
•secretion into intestine
•re-circulation to liver
Excretion of bile salts is the principal mechanism
for eliminating cholesterol from the body
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Primary bile acids
are formed from
cholesterol
7-a-hydroxylase
rate limiting step
+ cholesterol
- cholic acid
cholic acid
cholesterol
hydroxylation of C7
addition of OH group
7-a-hydroxycholesterol
chenodeoxycholic acid
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7-a-hydroxylase
Primary
bile acids
-
cholic acid
+
cholesterol
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7-a-hydroxylase
Primary
bile acids
+
cholic acid
cholesterol
Secondary
bile acids
• Hypercholesterolemia is often treated with
“sequestrants” that bind bile acids in the intestine.
These compounds:
prevent reabsorbtion of bile acids
increase conversion of cholesterol to bile acids
increase bile salt elimination in feces
• Dietary fiber also sequesters bile acids
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Increased elimination of cholesterol from the body
Primary
bile acids
Secondary
bile acids
Bile saltsglycine or taurine
conjugated to
bile acids in liver
5% lost in feces Recent R & D efforts focusing on bile acid receptors
as drug targets for treating liver disease, liver cancer,
metabolic disease.
Review- you tell me !!!!
•How many carbons are there in cholesterol?
•Which carbons are the business ends of the
cholesterol molecule?
•When cholesterol levels are high, HMG CoA reductase is
regulated by which of the following mechanisms?
•When cholesterol levels are high, HMG CoA reductase is
regulated by which of the following mechanisms?
•What organ STORES bile acids and bile salts?