Cholesterol Absorption,
Synthesis, & Metabolism I
Chapter 34
Nov. 4th 2011
Cholesterol Background
• Atherosclerotic vascular
disease
• Stabilizes cell
membrane
• Precursor to bile salts
and steroid hormones
• Cholesterol precursors
converted to
ubiquinone, dolichol, &
vitamine D
Cholesterol Background
Synthesis
• Obtained through diet
or synthesis
• Synthesized in many
cells, but mostly in the
liver and intestine
• Acetyl coenzyme A
(acetyl CoA) is the
precursor to cholesterol
synthesis
Cholesterol Background
(Transport)
• Chylomicrons & VLDL transport cholesterol to other cells
through the bloodstream
• Chylomicrons package cholesterol in intestine, while VLDL
package in liver
• Triacylglycerols are also transported by Chylomicrons and VLDL
• HDL – reverse cholesterol transport
Student Learning Outcomes
• Describe the rate-limiting step in cholesterol
synthesis and how the HMG-CoA reductase is
regulated
• Briefly describe the fates of cholesterol
• Describe the VLDL to LDL pathway
• The role of HDL
– RCT, apoprotein & lipid exchange
• Explain what occurs during receptor mediated
endocytosis
• Describe the aspects of Atherosclerosis
Cholesterol Synthesis
• Perhydrocyclopentanophenanthrene structure consists
of four fused rings
• Cholesterol contains a hydroxyl group at C3, double
bond between C5 & C6, eight-membered hydrocarbon
chain at C17, & methyl groups at C10 & C13
Fig. 1
Perhydrocyclopentanophenanthrene
Fig.2
Cholesterol
Cholesterol Synthesis
Stage I: Acetyl CoA to Mevalonate
A.
B.
C.
Fig.3
Rate limiting step
Cholesterol Synthesis
Stage I: Transcription Control
Fig. 4A
• Feedback regulatory system
• Rate of HMG-CoA reductase mRNA synthesis controlled
by sterol regulatory element binding protein (SREBP)
• Once in the Golgi, SERBP is cleaved twice by S1p & S2P to
release the transcription factor
Cholesterol Synthesis
Stage I: Proteolytic Degradation of HMG-CoA
Reductase
Fig. 4B
• When sterol present, enzyme undergoes sterol
accelerated ERAD (ER associated degradation)
• HMG-CoA is ubiquitinated and extracted from membrane
where it is then degraded by proteosomes
Cholesterol Synthesis
Stage I: Regulation by Covalent Modification
• Short-term regulation by
phosphorylation &
dephosphorylation
• Adenosine monophosphate
(AMP) activated kinase
phosphorylates HMG-CoA
• Glucagon, sterols,
glucocorticoids & low ATP
levels inactivate HMG-CoA
• Insulin, thyroid hormone,
high ATP levels activate
enzyme
Fig. 4C
Cholesterol Synthesis Stage 2:
Mevalonate to 2 Activated Isoprenes
• Transfer 3 ATP to Mevalonate
in order to activate C5 & OHgroup of C3
• Phosphate group at C3 &
Carboxyl group of C1 leave,
which produces a double
bound
• This allows for two active
isoprenes
Fig.5
Cholesterol Synthesis Stage 3:
Condensation of Isoprenes to for Squalene
• 1) Head to tail attachment of
isoprenes to form Geranyl
pyrophosphate
• 2) Head to tail condensation
of Geranyl pyrophosphate
and
isopentenylpyrophosphate to
form Farnesyl pyrophosphate
• 3) Head to head fusion of two
Farnesyl pyrophosphate to
form squalene
Fig.6
Cholesterol Synthesis Stage 4:
Squalene to Four-Ring Steroid Nucleus
Fig. 7
• Squalene monooxygenase adds oxygen to form an epoxide
• Unsaturated carbons (double bonds) are aligned to allow
cyclization and formation of lanosterol
• After many reaction get cholesterol
Fates of Cholesterol
•
•
•
•
Membranes
Cholesterol Ester
Biliary Cholesterol
Bile Acids
Cholesterol Esters
• Acyl-CoA:cholesterol
acyl transferase (ACAT)
is an ER membrane
protein
• ACAT transfers fatty acid
of CoA to C3 hydroxyl
group of cholesterol
• Excess cholesterol is
stored as cholesterol
esters in cytosolic lipid
droplets
Fig. 8
Bile Salts
•
•
•
•
Bile acids & salts are effective detergents
Synthesized in the liver
Stored & concentrated in the gallbladder
Discharged into gut and aides in absorption of
intraluminal lipids, cholesteral, & fat soluble vitamines
• Bile acid refers to the protonated form while bile salts
refers to the ionized form
– The pH of the intestine is 7 and the pKa of bile salts is 6,
which means that 50% are protonated
• These terms are sometimes used interchangeably
Synthesis of Bile Salts
Fig. 9
Fig. 10
• Rate-limiting step performed by the 7α-hydroxylase (CYP7A1) and is
regulated by bile salt concentration
• End product: Cholic acid series & Chenocholic acid series
• Bile salts can be conjugated & become better detergents
Fate of Bile Salts
Fig. 12
Cholesterol Transport by Blood
Lipoproteins
• Cholesterol, cholesterol esters, triacylglycerols, &
phospholipids are insoluble and must travel via lipoproteins
VLDL to LDL
Fig. 14
•
•
•
•
•
The TG, free & esterified cholesterol, FA, & apoB-100 are packaged into nascent VLDL
Nascent VLDL are secreted to bloodstream and acquire apoCII & apoE from HDL to
form a mature VLDL
Hepatic triglyceride lipase (HTGL) hydrolyzes additional triglycerides to produce LDL
40% of LDL transported to extrahepatic tissues
Excess LDL is taken up by macrophages
Reverse Cholesterol Transport (RCT)
Oram, JF & Vaughan, AM. (2000) ABCA1-mediated transport of cellular cholesterol &
phospholipids to HDL apolipoproteins. Curr Opin Lipidol. June;11(3):253-60
• HDL removes cholesterol from cells and returns it to the liver
• ABC1 transport protein uses ATP hydrolysis to move cholesterol from
inner leaflet to outer leaflet of membrane
• HDL receives cholesterol and uses the LCAT enzyme to modify & trap the
cholesterol
Fate of HDL
HDL binds SR-B1 receptor
Transfers cholesterol &
cholesterol ester to cell
Depleted HDL dissociates
& re-enters circulation
• HDL can bind to specific hepatic receptors, but primary HDL clearance
occurs through uptake by scavenger receptor SR-B1
• Present on many cells
• SR-B1 can be upregulated in cells that require more cholesterol
• SR-B1 is not downregulated when cholesterol levels are high
HDL Interactions with Other Particles
Fig. 16
Fig. 17
• HDL transfers apoE & apoCII to Chylomicrons & VLDL
• HDL either transfers cholesterol & cholesterol esters directly to liver or by
means of CETP to VLDL (or other TG-rich lipoproteins)
• In exchange, HDL receives triacylglyceroles
• Prior to CETP mature HDL particles are HDL3, post CETP they become larger
and are called HDL2
Receptor-Mediated Endocytosis
of Lipoproteins
• LDL receptor are located at
coated pits, which also
contain clathrin
• Vesicles fuse with lysosome
where cholesterol esters are
hydrolyzed into cholesterol &
re-esterified by ACAT
• This avoids damaging effects
of high concentrations of free
cholesterol on membrane
• Unlike cholesterol esters of
LDL, these cholesterol esters
are monosaturated
Fig. 18
Feedback Regulation of
Receptors
• Regulation by SREBP or its cofactor
• Low levels of cholesterol leads to up regulation
of receptor genes
– Increase amount of cholesterol in cells
• High levels suppress expression of receptor
genes
– Reduces amount of cholesterol that enters cells
Lipoprotein Receptors
• LDL receptor most well
characterized & contains
6 different regions
• LDL receptor-related
proteins are structurally
related but recognize
more ligands
• Macrophage scavenger
receptor : SR-AI & SR-A2
– Take up oxidatively
modified LDL
– When engorged with lipids
macrophages become
foam cells
Anatomical & Biochemical
Aspects of Atherosclerosis
Fig 21. Layers of arterial wall
• Initial step is formation of fatty streak (foam cells) in subintimal
space
• Foam cells separate endothelial cells exposing them to blood,
which leads to plaques & thrombin at these sites
• When plaque content exposed to procoagulant elements in
circulation, acute thrombus formation occurs
• Further thrombus formation leads to complete occlusion of lumen
& eventually AMI or CVA
Key Concepts
• HMG-CoA conversion to mevalonate is the rate limiting step
of cholesterol synthesis
– HMG-CoA reductase regulated by feedback, degradation,
modification
• Cholesterol fate: membranes, esters, biliary cholesterol, bile
salts
– Bile salts aide in absorption of lipids
• Hydrolysis of VLDL leads to LDL, which transport TG & CE to
peripheral cells & macrophages
• HDL involved in RCT & apoprotein/lipid exchange
• LDL enters cells via receptor-mediated endocytosis
• Excess LDL taken up by macrophage leads to the formation
of foam cells, which is the beginning of atherosclerosis