What is the purpose of triglycerides?
Why are elevated triglycerides significant?
Triglycerides are storage fats.
Elevated triglycerides are important because:
1. Independently predict risk of cardiovascular disease
2. If >1000 mg/dL, increased risk of pancreatitis
Why is Cholesterol important?
1. Essential component of membranes (reduces fluidity)
2. Precursor for bile acids and steroid hormones
3. Enriched in lipid rafts (in golgi and plasma membrane). Lipid rafts are areas enriched in proteins that participate in signal transduction.
4. Involved in covalent modification of embryonic signaling proteins.
What are the four major steps of cholesterol synthesis?
1. 3x Acetate condense to make the 6 carbon Mevalonate
2. Mevalonate converts to phosphorylated 5 carbon Isoprene
3. 6 isoprenes polymerize to form 30 carbon Squalene
4. Squalene cyclizes to form the four rings that are modified to produce Cholesterol
What is the first main step in cholesterol synthesis?
What are the specific steps?
What step is important and why?
Formation of Mevalonate from Acetyl CoA
2 Acetyl-CoAs--> Acetyoacetyl-CoA (catalyzed by acetyl-CoA acyl transferase)
Acetyoacetyl-CoA further combines with another Acetyl-CoA--> HMG-CoA (cat. by HMG-CoA synthase)
HMG-CoA+2 NADPH--> Mevalonate (cat. by HMG-CoA reductase)
THIS LAST STEP IS RATE LIMITING AND POINT OF REGULATION!
What is the second main step of Cholesterol Synthesis?
What are the specific steps?
What are the products?
Conversion of Mevalonate to activated isoprenes
3 phosphates transferred stepwise from ATP to mevalonate.
Decarboxylation and hydrolysis to create diphosphorylated 5 carbon product with a double bond, Isoprene
Isomerization to a second isoprene
Delta-3-isopentyl pyrophosphate and Dimethylallylpyrophosphate
What is the third main step in Cholesterol synthesis?
What are the specific steps?
Six activated isoprene units condense to form squalene
2 isoprenes join to displace one set of diphosphates-->geranyl pyrophosphate
Geranyl pyrophosphate joints to another isopentenyl pyrophosphate--> 15 carbon farnesyl pyrophosphate
2 farnesyl pyrophosphates--> phosphate free 30 carbon squalene
What is the fourth main step of cholesterol synthesis?
Why is this main step interesting?
What are the main products of this main step (in the context of the last question)?
Conversion of squalene to 4 ring Cholesterol
Squalene monoxygenase adds one oxygen to the end of the squalene chain forming squalene 2,3 epoxide
Pathways diverge in animal vs. plant cells
The cyclization product in animals is lanosterol, which converts to cholesterol
The epoxide in plants cyclizes to other sterols like ergosterol
What are the fates of Cholesterol after synthesis?
Most cholesterol synthesized in liver then exported...
As bile acid/Biliary cholesterol/Cholesteryl esters
Other tissues convert cholesterol into steroid hormones
What is the purpose of Cholesteryl Esters (CE)?
What are cholesteryl esters? So what?
What is it involved in?
How is it handled?
STORAGE FORM OF CHOLESTEROL
Fatty acid esterified to the oxygen, making it more hydrophobic and unable to enter membranes
Key player in atherosclerosis
Transported in lipoproteins to other tissues or stored in liver
Where is bile acids stored?
What is it's action?
What is an example of a bile acid?
Stored in gall bladder
Secreted into small intestine after fatty meal to assist in emulsification of fats (surround droplets of fat, increase surface area for attack by lipases)
An example is Taurocholic acid
What are the different classes of cholester-derived steroids?
Adrenal gland-synthesized steroids:
How are Cholesterol and other lipids carried through the plasma?
What is the surface made up of?
What is in the interior?
Lipids are carried through the plasma on spherical particles, lipoproteins.
Surface is made of proteins called apolipoproteins and a phospholipid monolayer
Interior contains: Cholesterol / Triglycerides (TG) / and Cholesteryl esters (CE)
What are the different subclasses of lipoproteins in order of size from smallest to largest?
Density from highest to lowest?
Special class in context of atherosclerosis?
HDL, LDL, IDL, VLDL, Chylomicrons
Same order because the larger the subclass of lipoprotein, hte lower the density
Lp(A) similar to LDL. Not much is known but involved in atherosclerosis
For the various lipoprotein subclasses, what are the important notes including identifying apoLp?
Chylomicron- ApoB-48 and others. Largest lipoprotein that delivers Chol and TG from gut to tissues and liver
VLDL- ApoB-100 and others. Synthesized in liver, delivers TG to tissues as fatty acids via lipoprotein lipases
IDL- ApoB-100. Short lived lipoprotein between VLDL and LDL
LDL- ApoB-100 only. Stays in circulation much longer than other lipoproteins.
Lp(a)- Homologous between LDL and plasminogen
HDL- ApoA-I and ApoA-II. Generally good.
What are apolipoproteins and what are their functions?
Apolipoproteins are the protein part of a lipoprotein particle.
Solubilize the lipoprotein particle in the circulation
Exchangeable and non-exchangeable
Change conformation to adjust to changing lipid compositions and metabolic states of the lipoproteins
Can activate or inhibit plasma enzymes
Serve as ligands for cell surface receptors
What is a key protein that mediates intestinal cholesterol absorption?
What drug targets it?
NPC1L1, or Niemann-Pick C1-like 1 protein
Ezetimibe is a cholesterol absorption inhibitor targeting this
What key proteins export plant sterols back into intestinal lumen?
Where are they located?
What condition arises from mutations in these key proteins?
ATP binding cassette half transporters: ABCG5 and ABCG8
Reside on the apical plasma membrane of enterocytes
Sitosterolemia (autosomal recessive disorder) reuslts from muitations in either of the genes encoding for ABCG5 and ABCG8
What happens in Sitosterolemia?
What what is it associated?
What causes it?
Absorption of unusually large amounts of plant sterols, failure to excrete dietary sterols into the bile, and accumulation of plant sterols in blood and tissues.
Associated with tendon and subcutaenous xanthomas and markedly increased risk of premature CHD
Autosomal recessive disorder with utations in either of the genes encoding ABCG5 and ABCG8
What are the steps of dietary fatty acid absorption?
1. Bile salts emulsify dietary fats iin the small intestine forming mixed micelles
2. Intestinal lipases degrade triacylglycerols
3. Fatty acids and other break down products are taken up by the intestinal mucosa and converted into triacylglycerols
4. Triacylglycerols are incorporated with cholesterol and apolipoproteins, into chylomicrons
5. Chylomicrons move through the lymphatic system and bloodstream to tissues
6. Lipoprotein lipase, activated by apoC-II in the capillary, converts triacylglycerols to fatty acids and glycerol
7. Fatty acids are oxidized as fuel or reesterified for storage
How are lipids transported in the blood?
What is essential about this?
Major characteristics of this?
How long do they remain in plasma?
What are the major apolipoproteins on this?
Essential for absorption of dietary fat and fat soluble vitamins.
Largest and least dense of the lipoproteins. 85% Fatty Acids of dietary triglycerides
Present for 3-6 hrs after a fat containing meal ingested. After a fast of 10-12 hrs, no chylomicrons remain
Some synthesized by intestinal epithelial cells (apoB-48, apoA-I, apoA-IV) and some acquired from HDL after chylomicrons have been secreted into the lymph and enter the plasma (apoE, apoC-I, C-II, C-III)
What are the forms of apoB present in lipoproteins?
What is unique to chylomicrons?
What does ApoC-II do?
ApoB-48 and ApoB-100 are two forms of apoB present in lipoproteins
ApoB-48 synthesized only by intestinal epithelial cells, is unique to chylomicrons
ApoC-II activates lipoprotein lipase to allow FFA release for fuel in adipose tissue, heart, and skeletal muscle
What hydrolyzes triglycerides?
Where is it bound?
Where does lipolysis occur?
How is it activated?
Lipoprotein Lipase (LPL)
Bound to capillary endothelium in heart, skeletal muscle, adipose tissue, mammary gland, and other tissues
Lipolysis occurs at the vascular endothelial surface
LPL activated by presence of apoC-II on CMs and VLDL
Hydrolyzes TG to DG to MAG, resulting in a shrunken TG-rich particle (CM remnants, IDL, LDL) where cholesterol phospholipids and apolipoproteins are transferred to HDL
How is LPL regulated?
When glucose levels in plasma elevated, release of insulin is stimulated resulting in LPL transcriptionally upregulated.
In fasting or diabetic ketoacidosis, LPL activity of adipose tissue falls preventing storage of fatty acids
Apo C-II is an obligatory cofactor
To measure LPL activity, you need heparin to displace LPL (since heparin is a cofactor for LPL) into plasma
What happens once Chylomicrons are depleted of their dietary TG via LPL?
1. Chylomicron remnants go to liver to release their dietary cholesterol.
2. ApoB-48 is degraded.
3. Chylomicron remnants are rapidly removed from the circulation by endocytosis at the liver thorugh a process requiring apoE as ligand for hepatic receptors such as the LDL receptor or the LDL receptor-related protein (LRP)
4. Surface lipids and C proteins transferred to HDL
5. Few chylomicrons or remnants present after 12 hr fast (except in those w disorders of chylomicron metabolism)
6. Dietary cholesterol delivered to liver by remnants exerts feedback control of cholesterol biosynthesis
What is the LRP, or LDL receptor-related protein?
What can a deficiency in this cause?
Backup receptor responsible for uptake of apoE-enriched remnants of chylomicrons and VLDL
Inherited absence of a functional apoE impedes remnant clearance by the LDL receptor and the LRP, increasing TG and cholesterol rich remnant lipoproteins in the plasma. Type III hyperlipoproteinemia
What is the function of VLDL? (Very low density lipoproteins)
When is it produced?
What apolipoproteins does it contain?
Transport endogenous lipids. Cholesteryl esters and TG from excess FA and cholesterol are packed into the core of VLDL and exported to peripheral tissues. Excess carbohydrate in the diet can also be made into TG in the liver and packed into VLDL
Produced in the liver when TG production stimulated by increased flux of free fatty acids or by increased de novo synthesis of fatty acids by the liver.
Contains apoB-100, apoC-I, apoC-II, apoC-III, and apoE (all constitutively synthesized in the liver)
What protein transfer TG to the VLDL core?
How does it work?
If it's dysfunctional, what is failed to be made?
What is the genetic disorder for mutations in this protein?
MTP, Microsomal triglyceride Transfer Protein.
After their synthesis in the ER, TG is transferred by MTP to the site where newly synthesized apoB-100 is available to form VLDL (or sites of apoB-48 synthesis for chylomicron formation)
Dysfunctional MTP fail to make any of the apoB-containing lipoproteins (chylomicrons, VLDL, or LDL)
Abetalipoproteinemia is caused by mutations of MTP (vitamin deficiency, fat in stool, developmental delays)
What is ACAT?
When does it function?
Where is it found?
What variant is involved in foam cell formation and cholesterol homeostasis in extra-hepatic tissues?
Aceyl-CoA:cholesterol Acyltransferase, esterifies free cholesterol to form cholesteryl esters (CE)
When dietary or endogenous cholesterol is INXS of need of membrane synthesis, it is metabolized to CE by ACAT for storage
ACAT-2 is found in the intestine and in the liver where cellular free cholesterol is esterified before triglyceride-rich lipoproteins (chylomicrons and VLDL) are assembled.
ACAT-1 is expressed in macrophages, including foam cells, adrenocortical cells, and skin sebaceous glands. ACAT-1 plays a critical role in foam cell formation and cholesterol homeostasis in extra-hepatic tissues
What happens to VLDL after hydrolysis by LPL?
Depletion of core TG, producing particles of smaller diameter (VLDL remnants or IDL).
When TG hydrolysis is nearly complete, the VLDL remnants/IDL are released from the capillary endothelium and re-enter the circulation
2 potential fates
-40-60% cleared from plasma by liver via LDL receptors and LRP which recognize ligands (apoB-100 and apoE) on the remnants
- LPL and the hepatic lipase (HL) convert the remainder of the remnants/IDL to LDL by removal of additional
The C apolipoproteins and apoE, as well as surface lipids, redistribute to HDL. They have lost about 70% of their TG and are enriched in CE
Where does the majority of LDL particles int he plasma derive from?
What are LDL? What does it do?
What is in it?
What is the major apolipoprotein?
How is it cleared from plasma?
What does LDL enable?
Low density lipoproteins, deposit cholesterol in peripheral tissues.
Enriched in cholesterol/cholesteryl esters
ApoB-100 is the major apolipoprotien
Cleared from plasma by LDL receptors. The liver expresses a large complemeent of LDL receptors and removes 75% of all LDL from the plasma. Manipulation of hepatic LDL receptor gene expression is the most effective way to modulate plasma LDL-C levels
LDL enables myocytes and adipocytes to take up cholesterol via receptor-mediated endocytosis
What are the half lifes of chylomicron/VLDL/LDL
5-20 min (Chylomicrons)
30 min to 1 hr (VLDL)
2.5 days (LDL)
What does mutations of the LDL receptor (LDLR) cause?
Hypercholesterolemia, autosomal dominant. Causes high levels of plasma LDL and familial hypercholesterolemia
Mutations in the ligand binding region of the LDLR disrupt binding and also are a cause of autosomal dominant hypercholesterolemia
How is LDL receptor gene expression regulated?
NOTE: manipulatoin of hepatic ldl receptor gene expression is the most effective way to modulate plasma LDL-C levels
Thyroxine and estrogen enhance LDL receptor gene expression (explaining their LDL-C-Lowering effects
What is PCSK9?
Why is this a good target?
A serine protease that decreases the steady-state level of expression of the LDL receptor on the hepatocyte cell membrane (binds to the EGF-A domain of LDLR)
LDLr/PCSK9 complex gets internalized and targeted to the lysosomal compartment for degradation
Inhibition of the recycling of the LDLr back to the cell surface results in increased plasma LDL levels. So if you can target this... inhibit the inhibition of the receptors, you would get higher levels of the LDL receptor, lower LDL cholesterol levels, and thus protection from coronary heart disease
What is lipoprotein(a) or Lp(a)
An LDL-like particle where apoB-100 is covalently bound to apolipoprotein. It is a cardiovascular risk factor
What is the good cholesterol?
What is the bad cholesterol?
HDL-takes cholesterol from peripheral tissues and take it back to the liver for excretion.
LDL-will eventually deposit cholesterol into peripheral tissues.
What does HDL do in all?
HDL is an athero-protective lipoprotein that...
- reduces vascular adhesion molecules on endothelium
- stimulates endothelial repair
- promotes endothelial function
- lowers inflammation
- stabilizes (atherosclerotic) plaques
What is reverse cholesterol transport?
Transport of cholesterol from peripheral cells and returns it to the liver for excretion via bile
What are the main proteins of HDL?
ApoA-I, most abundant (70% of total protein)
ApoA-II 2nd most abundant (20% of total protein)
C apolipoproteins carried on HDL and transferred to nascent VLDL and chylomicrons
Lecithin-cholesterol acyl transferase (LCAT)
What is the role of apoA-I
The primary protein component of HDL that is synthesized in liver and intestine and is required for normal production of HDL
What is ABCA1?
What is the disease caused by mutations in this?
ATP binding cassette transporter that helps release free cholesterol to apoA-I to make discoidal HDL
Tangier disease results form mutations in ABCA1. Characterized by extremely low levels of HDL and has cholesterol (CE) accumulation in the liver, spleen, tonsils, and neurons of peripheral nerves
What is the other name for discoidal HDL?
Where does the majority of the formation of discoidal HDL occur?
The majority of preBETA-HDL formation occurs at the liver and intestine (also sites of apoA-I synthesis and ABCA1 expression).
How do you get spherical HDL?
What helps with the process?
When you esterify cholesterol.
Lecithin-Cholesterol Acyl Transferase (LCAT) helps form the CE core of HDL.
It is secreted by liver and circulates in blood, physically assoc with HDL at times. It esterifies free cholesterol acquired by the preBETA-HDL. The newly esterified and nonpolar cholesterol moves into the core of the discoidal HDL. Becomes spherical
apoA-I activates LCAT
What is ABCG1 and how does it compare to ABCA1?
Expressed in spleen, thymus, lung and brain, liver and mcarophages.
Promotes cholesterol efflux to HDL and NOT lipid free apoA-I.
Just like ABCA1, but sends to spherical HDL.
What is CETP?
What does it do?
Cholesteryl ester transfer protein
Helps exchange lipids between HDL and LDL. CE from HDL, TG in exchange.
CETP accounts for removal of about 2/3 of the CE associated with HDL
What is PLTP?
Phospholipid transfer protein
Catalyzes transfer of phospholipids between lipoprotein classes, transfers excess free cholesterol and phospholipids liberated from the surface monolayers of chylomicrons and VLDL following hydrolysis of TG to HDL
What is hepatic lipase?
AFter CETP and PLTP mediated lipid exchange, the TG enriched HDL becomes a better substrate for HL
Hydrolyzes TG and Pl to generate smaller spherical HDL particles that recirculate and acquire additional free cholesterol from tissues
Androgens increase HL activity, Estrogens reduce HL activity
What is endothelial lipase?
Hydrolyzes HDL phospholipids, generating smaller HDL particles that are catabolized faster.
Modifies HDL to such an extent that it displays reduced affinity to SR-BI
OVerexpression reduced apoA-I levels and HDL-C levels to amost zero.
ApoA-II may inhibit EL activity (apoA-II-containing HDL are catabolized at a slower rate)
What is the Scavenger receptor BI
What happens if its disrupted?
Is the HDL receptor that increases HDL-CE uptake and decreases HDL-cholesterol levels. Also increases biliary cholesterol excretion.
Disruption of SR-BI decreases neutral lipid stores in adrenal glands and ovaries (steroidogenic tissues)
How does HDL recirculation/catabolism work?
CE core is transferred to cells via SR-BI by selective uptake. ONLY lipid is transferred, the ENTIRE HDL particle is NOT internalized.
The lipid depleted particle can circulate back to peripheral tissues to pick up more cholesterol.
What is the enterohepatic circulation?
In the liver, cholesterol is secreted into the bile.
Cholesterol converted to bile acids by a pathway initiated by enzyme cholesterol 7alpha-hydroxylase
Conjugated bile acids secreted from liver and stored in gall bladder.
Once through bile duct into intestine, emulsify lipids in food to enable fat digestion and absorption through intestinal wall
Most bile acids reabsorbed, returning to liver via portal vein and resecreted.
The small amount lost in stool provides means of elimination of cholesterol.
What are the regulations of cholesterol synthesis and transport?
1. Covalent modfiication of HMG-CoA reductase
2. Transcriptional regulation of HMG-CoA gene
3. Activation of ACAT, increasing esterification for storage
4. Transcriptional regulation of the LDL receptor
Covalent modification of HMG-CoA reductase?
1. AMP-dependent protein kinase. When AMP rises, kinase phorphorylates HMG-CoA reductase, leading to decreased activity and decreased cholesterol synthesis
2. Glucagon, epinephrine cascades lead to phosphorylation, decreasing activity.
3. Insulin cascades lead to dephosphorylation, increasing activity.
*COVALENT MODIFICATION PROVIDES SHORT TERM REGULATION
What is a longer term regulation of HMG-CoA reductase through transcriptional control?
Sterol Regulatory Element-Binding Proteins (SREBPs)
When cellular sterol levels are high, SREBP is in ER membrane with other proteins
When cellular sterol levels decrease, complex is cleaved and moves to the nucleus
-Activates transcription of HMG-CoA reductase and LDL receptor as well as other genes