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The Usual Suspects: Cholesterol and Triglyceride
Lipid Structure
Cholesterol:
Membranes
Bile Acids
Steroid Hormones
Protein modification
H
O
Fatty Acids:
Fuel,
Prostanoids
FA for Fuel, Prostanoids
Protein modification
COOH
COO
COOH
COO
COOH
COO
H
O
+
Glycerol
Triglycerides:
Phospholipid: Lecithin
COO
H
O
H
O
COO
+
N
OPOO
Membranes
2nd Messengers
Structure of a
Typical Lipoprotein
Free cholesterol
(surface and core)
Phospholipid
(amphipath at
surface only)
Apolipoprotein
(amphipath at
surface only)
Triglyceride
(core only)
Cholesteryl ester
(core only)
Lipoprotein classes and sub-Classes
0.95
Chylomicron
VLDL
Density (g/ml)
VLDL
Remnants
1.006
IDL
Chylomicron
Remnants
1.02
LDL
1.06
Directly atherogenic
(found in plaque)
HDL2
Lp(a)
1.10
HDL3
pre-β2 HDL
1.20
pre-β1 HDL
5
10
20
40
60
Particle Size (nm)
80
1000
Substrates for Triacylglycerol Synthesis
Plasma
NEFA
Glucose
Triglycerides
Acyl-CoA Synthetase
Glc-6-Pase
CoA
Acyl-CoA
Multiple steps
Glucose-6-P
CPT I
Pyruvate kinase
Mitochondria
Acyl-Carnitine
Acyl-CoA
Fatty Acid Synthase
CPT II
Malonyl-CoA
PEP
Acyl-CoA
Acyl-CoA Carboxylase
Krebs
Cycle
Pyruvate
Citrate
Hepatocyte
PEPCK
Acetyl-CoA
ATP Citrate Lyase
Pyruvate
Acetyl-CoA
HMG-CoA Synthase
Ketone Bodies
PEP = phosphoenolpyruvate
PEPCK = PEP carboxylase
CO2
Citrate
Beta-oxidation
CPT = Carnitine palmitoyl transferase
Structures of Fatty Acids
O
C
HO
O
C
HO
16:0 (palmitic)
cis-18:1 -6 (oleic)
O
C
HO
O
C
HO
trans-18:1 -6 (elaidic
O
C
HO
(alpha
18:3 -3
linolenic)
O
C
HO
18:2 -6 (linoleic)
20:5 -3 (EPA)
Exogenous (dietary) lipid metabolism
Muscle and adipose tissue
Lipoprotein lipase
Fatty acids
Lipoprotein Lipase
Apo C-II enhances
and apo C-III inhibits
LPL activity
Chylomicron
Remnant
Bloodstream
Plasma
chylomicron
I
N
T
E
S
T
I
N
E
Apo B and apo E are
ligands for LDL
receptor
LDL (apo B,E)
receptor clears
Chylomicron
Remnants
LDL
receptor
Lymphatic
chylomicron
Xenical blocks diatary
fat digestion
Hepatocyte
Liver
Endogenous (hepatic) lipid metabolism
Muscle and adipose tissue
Lipoprotein lipase
Fatty acids
Lipoprotein Lipase
And hepatic lipase
LDL
Apo C-II enhances
and apo C-III inhibits
LPL activity
Bloodstream
IDL
Apo B and apo E are
ligands for LDL
receptor
LDL (apo B,E)
receptor clears VLDL,
IDL & LDL
LDL
receptor
VLDL
Hepatocyte
Liver
Clinical Hypertriglyceridaemia
Condition
Features
Secondary
Relatively common (obesity, diabetes,
renal impairment, liver disease, drugs)
Polygenic
Accounts for the majority of cases
Familial HTG
TG predominates. CVD risk varies
Predisposes to massive HTG
Familial
Combined H/L
Overproduction of apo B lipoproteins
TG and TC vary with age and weight
Massive HTG
Lipoprotein Lipase deficiency or
saturation. Risk of pancreatitis
Therapy for Hypertriglyceridaemia
Intervention
Features
Diet, Exercise
Relatively responsive
Alcohol restriction
Often sufficient in heavy intake
Manage 2o causes
Diabetes, renal
Fibrates
Effective in high TG, low HDL
Statins
Mild TG and HDL benefit
Fish oils (eg 6gm/d)
Benefits TG rather than HDL
Niacin
Future
Effective, but increases glu, urate.
DGAT 2 Inhibitors?
Bile acid resins
Contraindicated. Increase TG
NORMAL CHOLESTEROL ABSORPTION
1,300 mg/day
400 mg/day
Oil phase
NORMAL CHOLESTEROL ABSORPTION
1,300 mg/day
400 mg/day
Oil phase
Plant sterols compete
with cholesterol here
NORMAL CHOLESTEROL ABSORPTION
1,300 mg/day
400 mg/day
17,400 mg/day
Oil phase
850 mg/day
Ezetimibe competes
with cholesterol here
NORMAL CHOLESTEROL ABSORPTION
1,300 mg/day
400 mg/day
Oil phase
17,400 mg/day
850 mg/day
Intracellular cholesterol sensing by SREBPs
(Sterol Regulatory Element Binding Proteins)
SCAP or SREBP activating protein
SCAP
SREBP
WD Reg
ER
bHLH
Cytosol
Nucleus
SRE
bHLH
Lumen
Sterols
Golgi
Apparatus
WD Reg
bHLH
bHLH
ZN++
S1P
Serine protease
S2P
Metalloproteinase
Membrane fluidity
reflects intracellular
cholesterol. Low levels
allow cleavage to active
form which binds
nuclear receptor to
control gene expression.
• SREBP-2 controls
cholesterol synthesis and
sterol metabolism
• SREBP-1c is the major
isoform in liver and is a
key regulator of fatty
acid & triglyceride
synthesis
Other nuclear receptors:
FXR, LXR.
LDL Receptor activity reflects
intracellular cholesterol homeostasis
Cholesterol delivery via LDL-R
alters intracellular membrane
cholesterol and SREBP, which
a) Reduces synthesis via
HMGCoA Reductase
b) Reduces LDL-R synthesis
c) Increases storage as ester
d) Reduces counter-regulatory
PCSK9
*[SREBP] = sterol regulatory element-binding protein.
1. Goldstein JL, et al. Arterioscler Thromb Vasc Biol. 2009;29:431-438..
The Role of HDL in
Reverse Cholesterol Transport
Bile
ABCG1
&SR-B1
Spheroidal HDL
Liver
UC
Pre-β HDL
LCAT
ABCA1
UC
Hepatic lipase,
endothelial lipase
SR-BI
LDL
Receptor
CETP
PL&UC
SR-A
Macrophage
VLDL/LDL
ABCA1, ATP-binding cassette protein A1; CETP, cholesterol ester transfer protein; FC, free cholesterol; LCAT,
lecithin:cholesterol acyltransferase; SR-A, scavenger receptor class A; SR-BI, scavenger receptor class B type I.
Adapted with permission from Cuchel C et al. Arterioscler Thromb Vasc Biol. 2003;23:1710–1712
Clinical Hypercholesterolaemia
Condition
Features
Secondary
Relatively uncommon, but potent
(hypothyroidism, nephrotic syndrome,
primary biliary cirrhosis)
Polygenic
Accounts for the majority of cases
Familial
Prevalent, Accelerates CVD.
Hyperchol’aemia Due to defects in genes related to LDL-R
Familial
Combined H/L
Overproduction of apo B lipoproteins
TG and TC vary with age and weight
Increased HDL
?OK if LDL not raised?
Therapy for Hypercholesterolaemia
Intervention
Features
Diet
Manage 2o causes
Statins
Plant sterols, avoid sat & trans FA
Rarer, but potent: (Thyroid, liver,
renal.)
First line for LDL reduction
Ezetimibe
2nd line. Neutral for TG & HDL
Niacin
Improves LDL, TG, HDL & Lp(a)
Bile acid resins
Colesevalam better tolerated
Future
PCSK9 Inhibitors, MTP inhibitors?
Apo B antisense oligonucleotides
Triglyceride and Cholesterol:
Why are they linked?
• Most lipoproteins have TG and/or CE in their core
• Hepatic Triglyceride rich lipoproteins are precursors of
cholesterol-rich LDL
• Cholesterol-ester transfer protein allows all
triglyceride-rich lipoproteins to modify the composition
of cholesterol-rich HDL and LDL. As a result,
hypertriglyceridaemia is associated with reduced HDL
cholersterol as well as “small dense LDL”.
• The major gene regulators for lipid metabolism affect
both TG and Chol
•
Key Regulators of Genes in
Fatty Acid and Triglyceride Metabolism
Bile Acids
SHP = Short Heterodimer Partner
SHP
SREBP-1c
LXR
Acetyl CoA Carboxylase Fatty
Acid Synthase
Spot 14
NEFA
PPAR
FXR
Acyl CoA
Unsaturated
Saturated
HNF-4α
L-FABP
RXR
Fatty acid metabolism
Apolipoproteins
Pyruvate kinase
Glucose-6-phosphalase
Transferin
Bile Acids
Transport
Oxidation
Fatty Acid Binding Protein
Ketogenesis
Adapted from Pegorier JP et al. J Nutr 2004;134:2444S-9S
Key Regulators of Genes in Lipid Metabolism
Sterol Regulatory Element Binding Protein (SREBP2)
Peroxisome proliferated
Synthesis activator receptors PPARs Delivery
Acquisition
Cellular
Cholesterol
Homeostasis
Excretion
Hepatobiliary
Farnesoid X Receptor (FXR)
Intestine
Liver X Receptor (LXR)
Link to mixed HL cases
Case MC
Patient is a 43 year-old male with a strong family history of
premature CVD who presents for initial evaluation.
He has a 10 year history of dyslipidaemia and hypertension, for
which he has received beta blockers in the past. More recently
he has been on an ARB/diuretic combination. Three months
prior to this visit a fasting lipid profile showed:
Total Cholesterol: 5.7 mmol/L
HDL-C:
0.7 mmol/L
Triglyceride: 2.8 mmol/L
LDL-C:
3.6 mmol/L
He has managed to lose 3 kg and today results include:
Total Cholesterol:
HDL-C:
6.9 mmol/L
0.8 mmol/L
Triglyceride: 1.8 mmol/L
LDL-C:
5.2 mmol/L
Questions concerning Mr M.C.
• 1)
Is ethnicity an independent risk factor for CVD? Yes / No?
• 2)
In the absence of any symptoms or signs of hypothyroidism,
would you perform thyroid function tests? Yes / No?
• 3)
His brother’s lipids include
LDL = 5.4 mmol/l, TG = 1.9 mmol/l,
HDL = 0.9 mmol/l .
What is the most likely cause of MC’s lipid abnormality?
•
•
•
•
•
A)
B)
C)
D)
E)
Dyslipidaemia secondary to Insulin resistance and the Metabolic Syndrome
Polygenc dyslipidamia
Familial Combined Hyperlipidaemia
Familial Hypercholesterolaemia
Lipids aren’t really an issue in this patient
Patient is a 43 year-old male with a strong family history of
premature CVD who presents for initial evaluation.
He has a 10 year history of dyslipidaemia. Hypertension, for
which he has received beta blockers in the past. More recently
he has been on an ARB/diuretic combination. Three months
prior to this visit a fasting lipid profile showed:
TC: 5.7 Triglyceride: 2.8 mmol/L
HDL-C: 0.7 mmol/L LDL-C: 3.6 mmol/L
He has managed to lose 3 kg and today results include:
TC: 6.9 Triglyceride:1.8 mmol/L HDL-C: 0.8 mmol/L LDL-C: 5.2 mmol/L
• 1)
• 2)
Is ethnicity an independent risk factor for CVD? Yes / No?
In the absence of any symptoms or signs of hypothyroidism,
would you perform thyroid function tests? Yes / No?
His brother’s lipids include
LDL = 5.4 mmol/l, TG = 1.9 mmol/l,
HDL = 0.9 mmol/l .
What is the most likely cause of MC’s lipid abnormality?
• 3)
•
•
•
•
•
A)
B)
C)
D)
E)
Dyslipidaemia secondary to Insulin resistance and the Metabolic Syndrome
Polygenc dyslipidamia
Familial Combined Hyperlipidaemia
Familial Hypercholesterolaemia
Lipids aren’t really an issue in this patient
Is ethnicity an independent risk factor for
CVD? Yes / No?
• YES:
• NO:
Is ethnicity an independent risk factor for CVD?
Case for a qualified “Yes”.
Same risk factors, different pattern
INTERHEART, Karthikeyan et al 2009,
INTERHEART, Joshi et al 2007
2) In the absence of any symptoms or signs
of hypothyroidism, would you perform thyroid
function tests? Yes / No?
• Yes:
• No:
2) In the absence of any symptoms or signs
of hypothyroidism, would you perform thyroid
function tests?
The case for “Yes”
What is the most likely cause of MC’s lipid
abnormality?
• A) Dyslipidaemia secondary
to Insulin resistance and
the Metabolic Syndrome
• B) Polygenc dyslipidamia
• C) Familial Combined
Hyperlipidaemia
• D) Familial
Hypercholesterolaemia
• E) Lipids aren’t really an
issue in this patient
What is the most likely cause of MC’s lipid
abnormality? The case for “C”, maybe “A” or “B”
Condition
Features
Secondary
Relatively uncommon, but potent
(hypothyroidism, nephrotic syndrome,
primary biliary cirrhosis)
Polygenic
Accounts for the majority of cases
Familial
Prevalent, Accelerates CVD.
Hyperchol’aemia Due to defects in genes related to LDL-R
Familial
Combined H/L
Overproduction of apo B lipoproteins
TG and TC vary with age and weight
Increased HDL
?OK if LDL not raised?
Case MC (continued)
Mr MC started statin, therapy, Atorvastatin 20 mg/ day,
but unfortunately he had and inferior AMI still 4 months
later. His discuharge medication include:
Atorvastatin 20mg, Metoprolol 20 mg, Aspirin 100mg,
and his previous ARB/diuretic. Follow-up 2 months later
reveals: 2 kg weight loss, BP 118 / 78, Fasting tests:
• Glu 5.3 mmol/l, TC 4.4 mmol/l TG 4.2 mmol/l,
• HDL 0.7 mmol/l, LDL 1.8 mmol/l
Case MC: Further questions:
• Should you stop his beta blocker? Yes / No?
• Do you trust the LDL-C result? Yes / No?
• Is it practical to try to manage Mr M.C’s lipid profile to
target levels?
Yes / No?
• What is the next lipid-lowering drug that you would add to
his therapy?
a) Ezetimibe
b) Niacin
c) I would increase Atorvastatin to 80 mg but I wouldn’t
give anything other than a statin
d) Fenofibrate
e) Fish Oil
Mr MC started statin, therapy, Atorvastatin 20 mg/ day, but
unfortunately he had and inferior AMI still 4 months later. His
discuharge medication include:
Atorvastatin 20mg, Metoprolol 20 mg, Aspirin 100mg, and his
previous ARB/diuretic. Follow-up 2 months later reveals:
2 kg weight loss, BP 118 / 78, Fasting tests:
• Glu 5.3 mmol/l, TC 4.4 mmol/l TG 4.2 mmol/l,
• HDL 0.7 mmol/l, LDL 1.8 mmol/l
• Should you stop his beta blocker? Yes / No?
• Do you trust the LDL-C result? Yes / No?
• Is it practical to try to manage Mr M.C’s lipid profile to target
levels? Yes / No?
• What is the next lipid-lowering drug that you would add to his
therapy?
a) Ezetimibe b) Niacin c) Increase Atorvastatin to 80 mg but
don’t give anything other than a statin
d) Fenofibrate e) Fish Oil
Should you stop his beta blocker?
• Yes
• No
Should you stop his beta blocker?
The case for “no”
Some β-blockers decrease HDL and increase triglycerides.25 In
spite of this, BHAT data showed that propranolol improves
survival after MI.26 Low-dose metoprolol CR/XL alone or in
combination with a statin resulted in significant slowing of the
progression of carotid artery’s intima-media thickness over a 3year period.27
M Gheorghiade et al Circulation.2002; 106: 394-398
Do you trust the LDL-C result?
• Yes
• No
Do you trust the LDL-C result?
The case for “No”
Discussion of the effect of Cholesterol ester transfer protein will
Explain why LDL-C underestimates risk when TG is elevated
Is it practical to try to manage Mr M.C’s lipid
profile to target levels?
•
Yes
• No
Is it practical to try to manage Mr M.C’s lipid
profile to target levels? The case for “Yes”
Combination therapy is safe and effective, but yet to
be supported by clinical endpoint data.
What is the next lipid-lowering drug that you
would add to his therapy?
a) Ezetimibe
b) Niacin
c) I would increase Atorvastatin to 80 mg but I wouldn’t
give anything other than a statin
d) Fenofibrate
e) Fish Oil
What is the next lipid-lowering drug that you
would add to his therapy?
The case for “d” or “b”, possibly “c” or “e”
anticipate “residual risk” module
Case GS
Patient is a 47 year-old female who has been gaining weight
for several years.
She has a 10 year history of mildly elevated triglyceride. She
received therapeutic lifestyle counseling but she has been
largely non-compliant. Three months prior to this visit, a
fasting lipid profile showed: Total Cholesterol: 5.5mmol/L
Triglyceride: 2.4mmol/L HDL-C: 1.0mmol/L
LDL-C:3.6 mmol/L
Now she has symptoms of hyperglycaemia and repeat fasting
glucose confirms Type 2 diabetes
Case GS
Review of Symptoms: Thirst, polyuria, folliculitis,
Weight unchanged (increased 2kg, then lost when
polyuria commenced
BP 118/72 Pulse 72 Wt 85kg Ht 175cm Waist 93 cm
BMI 27.8: Physical examination unremarkable
Current fasting lipid results surprise you:
Total Cholesterol: 8.5mmol/L
Triglyceride:
7.4mmol/L
HDL-C:
1.0mmol/L
LDL-C:
unable to be calculated
Questions:
How could you obtain an LDL-C result?
a) Friedewald equation
b) Abusive phonecall to lab
c) “Direct method” involving detergents
d) Ultracentifugation
e) Subtract HDL-C from Total cholesterol
Which class or classes of lipoproteins would you expect to be
increased?
a) Chylomicrons and LDL
b) VLDL and LDL
c) VLDL and HDL
d) IDL and chylmicron “remnants”
e) Why bother? It doesn’t matter
Which combination of extra tests would be most useful?
a) LDL size+HDL subfractions b) Lipid EPG+ApoE phenotype
c) LDL subfractions HDL size d) Lp(a)+ homocysteine
e) Routine fasting lipids are the only lipid tests that are ever
required
She has a 10 year history of mildly elevated triglyceride. She
received therapeutic lifestyle counseling but she has been
largely non-compliant. Three months prior to this visit, a fasting
lipid profile showed: Total Cholesterol: 5.5mmol/L Triglyceride:
2.4mmol/L HDL-C: 1.0mmol/L LDL-C:3.6 mmol/L Now she
has symptoms of hyperglycaemia and repeat fasting glucose
confirms Type 2 diabetes. Symptoms: Thirst, polyuria, folliculitis,
Weight increased 2kg, then lost when polyuria commenced
BP 118/72 Pulse 72 Wt 85kg Ht 175cm Waist 93 cm
BMI 27.8: Physical examination unremarkable
Current fasting lipid results surprise you: TC: 8.5mmol/L
TG: 7.4mmol/L HDL-C: 1.0mmol/L
LDL-C: unable to be calculated
• How could you obtain an LDL-C result?
• Which class or classes of lipoproteins would you expect
to be increased?
• Which combination of extra tests would be most useful?
How could you obtain an LDL-C result?
a) Friedewald equation
b) Abusive phonecall to lab
c) “Direct method” involving detergents
d) Ultracentifugation
e) Subtract HDL-C from Total cholesterol
How could you obtain an LDL-C result?
The case for “d”, but “c” is misleading
Lab Tests Online:
“Direct LDL-C is ordered whenever calculation of LDL cholesterol will
not be accurate because the person's triglyceridesare significantly
elevated. It may be ordered by a doctor when prior test results have
indicated high triglycerides. In some laboratories, this direct LDL test
will automatically be performed when the triglyceride levels are too high
to calculate LDL-C. This saves the doctor time by not needing to order
another test, saves the patient time by not needing to have a second
blood sample drawn, and speeds up the time to provide the test result.”
Ultracentrifuge gives absolute result. Detergent methods assume LDL
Which class or classes of lipoproteins would
you expect to be increased?
a) Chylomicrons and LDL
b) VLDL and LDL
c) VLDL and HDL
d) IDL and chylmicron “remnants”
e) Why bother? It doesn’t matter
Which class or classes of lipoproteins would
you expect to be increased?
The case for “b”(orange) or “d” (green)
Which combination of extra
tests would be most useful?
• a) LDL size+HDL subfractions
• b) Lipid EPG+ApoE genotype/phenotype
• c) LDL subfractions HDL size
• d) Lp(a)+ homocysteine
• e) Routine fasting lipids are the only lipid tests that are
ever required
Which combination of extra
tests would be most useful?
The case for “b”
CM
β preβ α
ApoE isoforms
Case GS (continued)
The patient subsequently complied with diet and started on
Simvastatin 40 mg daily and other treatment, which she
tolerates without difficulty. Current Medications:
1. Metformin 850 mg bid
2. Enalapril 10 mg q day
3. ASA 81 mg q day
4. Simvastatin 40 mg q day
Subsequent results include:
LEPG – Broad beta pattern present
Apo E Genotype: Apo E2:E2
Questions
This implies the accumulation of which lipoprotein class(es)?
a) VLDL + LDL
b) IDL and Chylomicron “remnants”
Which lipid-lowering drug is the ideal treatment for this situation?
a) Simvastatin b) Nicotinic Acid c) Questran d) Fibrate
e) Fish oil f) Ezetimibe
Which lipid-lowering therapy is strongly CONTRAindicated?
a) Simvastatin b) Nicotinic Acid c) Questran d) Fibrate
e) Fish oil f) Ezetimibe
Would you stop his statin therapy? Yes / No
Do you agree with the use of low-dose aspirin in this patient? Yes / No
The patient subsequently complied with diet and started on
Simvastatin 40 mg daily and other treatment, which she
tolerates without difficulty. Current Medications:
1. Metformin 850 mg bid
2. Enalapril 10 mg q day
3. ASA 81 mg q day
4. Simvastatin 40 mg q day
Subsequent results include:
LEPG – Broad beta pattern present
Apo E Genotype: Apo E2:E2
This implies the accumulation of which lipoprotein class(es)?
Which lipid-lowering drug is the ideal treatment for this situation?
Which lipid-lowering therapy is strongly CONTRAindicated?
Would you stop his statin therapy?
Do you agree with the use of low-dose aspirin in this patient?
The patient asks you about his risk of Alzheimers’ Disease. Is it increased?
This implies the accumulation of which
lipoprotein class(es)?
a) VLDL + LDL
b) IDL and Chylomicron “remnants”
This implies the accumulation of which
lipoprotein class(es)? The case for “b”
Which lipid-lowering drug is the ideal
treatment for this situation?
a) Simvastatin
b) Nicotinic Acid
c) Questran
d) Fibrate
e) Fish oil
f) Ezetimibe
Which lipid-lowering drug is the ideal
treatment for this situation? The case for “d”
Which lipid-lowering therapy is strongly
CONTRAindicated?
a) Simvastatin
b) Nicotinic Acid
c) Questran
d) Fibrate
e) Fish oil
f) Ezetimibe
Which lipid-lowering therapy is strongly
CONTRAindicated? The case for “c”
Hepatocyte
Heterodimerizatio
n with RXR
Acetyl CoA
SREPB-1c
Bile Duct
FA, TG
MDRP2/
3
Phospholipid
s
FXR
VLDL
(TG levels)
Do you agree with the aspirin dose?
Comment on the role of aspirin in this patient.
• Yes
• No
Do you agree with the aspirin dose?
Comment on the role of aspirin in this patient.
Evidence and opinion tending towards “no”?