WK08L2 - Atherosclerosis (Ben) Flashcards
What class of molecules is increased in areas of disturbed or no flow in the circulation?
What is their function?
How does their expression increase in these areas?
Scavenger Receptors
-
pattern recognition receptors which have broad specificity for abnormal structures
- ex: bacterial LPS, apoptotic debris, oxidated LDL
- bind their ligand and initiate its endocytosis + elimination
- expression increases via PC-1 + its attached nuclear transcription factors which are cleaved from the primary cilium membrane of endothelial cells under low/no flow conditions
(these areas also tend to be sites of atherosclerotic plugs)
What are the classes of scavenger receptors?
Important examples from some classes + facts about them?
A, B, E, F G
-
Class A:
- composed of 3 polypeptide chains
-
Class B:
- SRB-1 - HDL uptake
- CD36 - specific for oxidized LDL
-
Class C:
- LOX-1 - lectin-like receptor induced in low/turb. flow regions near atherosclerotic plugs
Which of the scavenger receptors just mentioned can have a ‘phenotype-changing’ effect on endothelial cells when activated?
What is the general result of its activation?
LOX-1 (Lectin-like Oxidized LDL receptor)
- activation results in intracellular signal transduction which favors processes of thrombosis, atherosclerosis and inflammation
What are some more specific consequences for endothelial cells upon activation of the LOX-1 receptor?
Be specific about the molecules involved.
(which result in the increased thrombosis, inflamm. and atheroscler. mentioned before)
Overexpression of adhesion molecules…
- Immunoglobulin Superfamily:
- ICAM-1 + VCAM-1
- Cytokines:
- MCP-1 (Monocyte Chemotactic Protein)
- Selectins
- overall, this favors leukocyte retention at low/no flow points
What enzyme, normally expressed in leukocytes as an anti-infectious agent, is also expressed in endothelium under low/no flow conditions?
What is the consequence of this?
NADH/NADPH Oxidase
- endothelial cell will produce superoxide which triggers modification of LDL to oxidized LDL (the LOX-1 ligand)
Which enzyme normally present in endothelial cells can have an altered function under oxidative stress?
(resulting in a sort of positive feedback loop on more oxidation)
How is the function altered?
NO synthase
-
Normal conditions:
- uses NADPH, O2, H4Biopterin and Arg to make NO
-
Under oxidative stress:
- if any substrates/cofactors are not present, the enzyme can be “uncoupled”
- H4Biopterin is decreased by oxidation b/c reducing equivalents used to produce H4B will be used up
- electrons from NADPH will go to O2 to produce superoxide, increasing oxidative stress + accelerating atherosclerotic processes
Besides scavenger receptors, what other pattern recognition receptors play a role in atherosclerosis?
Which one in particular has been well studied in mice and what is its role?
Toll-like Receptors
- recognize abnormal molecules such as dsRNA and release cytokines to attract WBCs which eliminate the abnormal factor
-
TLR2 was found to have a pro-atherosclerotic role
- mice w/out LDLRs have increased risk, but mice without LDLRs or TLR2 have no risk increase
- overexpressed at atherosclerotic sites
- provokes inflammation which increases endothelial permeability
How can oxidized LDL be taken up by endothelial cells?
(Specific membrane receptors.)
A complex of TLR4 + TLR6 + CD36 (a B-type scavenger receptor) can initiate endocytotic uptake of oxLDL
After uptake into endothelial cells, what then happens to oxLDL?
Why is this possible, when movement of LDL is normally strictly controlled?
Oxidized LDL can then be taken up via scavenger receptors by smooth muscle cells beneath the endothelium and macrophages.
This is possible because the LDL is oxidized and is therefore a ligand for SRs.
Normal LDL uptake via LDLRs is strictly controlled by negative feedback… too much cholesterol in a cell decreases its LDLR expression.
What are several sources of reactive oxygen species which can oxidize LDL?
- Metabolism - mitochondrial/microsomal by-products
- Exogenous sources - smoking, radiation
- Aging - age-related disease
- produce superoxide, hydrogen peroxide, and OH-
What is a cell called when it has take up too much oxLDL via un-regulated scavenger receptors and stored them in cholesteryl esters?
Foam Cell
- due to its foamy appearance in microscopy
Describe the response cascade in a foam cell due to cholesterol overload.
- Excess cholesterol forms crystals
- Crystals are recognized by intracellular TLR analogs such as NLRP-3 (nucleotide-binding leucine-rich protein)
- NLRP-3 complexes with ASC (apoptotic speck-like protein)
- Complex activates procaspase-1 to caspase-1
- Caspase-1 activates IL-1 which initiates inflammation around foam cells.
Besides cytokine production, what process occurs in foam cells due to activation of caspase-1?
How?
Pyroptosis - a type of cell death accompanied by fever
- caspase-1 targets membrane ion channels, causing influx of ions + water which leads to swelling, lysis and death
What happens at pyroptotic sites after cells rupture?
- Leukocytes are attracted to the rupture site and release matrix metalloproteases (MMPs)
- MMPs target collagen and other extracellular proteins for degradation
- Fibroblasts initiate regeneration of EC matrix, but new tissue is abnormal
- Can either have stable fibrous cap formation or fibrous cap rupture resulting in platelet-EC matrix contact and thrombus formation
Which drug targets uptake of cholesterol by enterocytes?
How?
Ezetimibe
inhibits the NPC1L1 protein on enterocyte membranes which plays a role in cholesterol uptake
How can de novo synthesis of cholesterol be targeted pharmacologically?
Aside from direct inhibition of synthesis, what downstream effect do these drugs have which is beneficial for atherosclerosis prevention?
Statins
- inhibit HMG-CoA reductase in the liver to reduce cholesterol synthesis
- resulting decrease in intracellular cholesterol concentration increases LDLR expression
Which plasma protein alters lipoproteins in ways that favor atherosclerosis?
Generally, what effect does it have?
CETP
Cholesteryl Ester Transfer Protein
- exchanges TAGs and cholesteryl esters between lipoproteins
How does CETP affect HDL and what is the consequence?
- exchanges TAGs from LDL for cholesteryl esters (CE) from HDL
- new TAG-rich HDL is acted on by hepatic lipase forming a smaller, denser HDL3
- HDL3 loses some ApoA-1 from its surface
- free ApoA-1 can then be filtered by the kidney and its lower levels lead to lower levels of HDL
- all of this is pro-atherosclerotic
How does CETP affect LDL and what is the consequence?
- Exchanges CEs from HDL for TAGs from VLDL
- CE-rich LDL is acted on by hepatic lipase resulting in denser LDL
- smaller, denser LDL can circulate longer (less LDLR uptake) and therefore be oxidized more
High levels of what lipid results in initiation of these CETP processes?
high triglycerides (TAGs) initiate CETP transfer processes
How was the importance of CETP in atherosclerosis (and the potential for its inhibition as a therapeutic target) discovered?
A CETP variant known as I405V (isoleucine 405 switched with valine) was discovered in 25% of people 95-107 yrs old in a study.
The variant was found to be less active, conferring an anti-atherosclerotic benefit.
What is an important inhibitor of CETP?
Torcetrapib
What effect can red wine have on atherosclerosis and how?
its polyphenol constituents neutralize ROSs
in this way, it decreases oxidization of LDL and decrease atherosclerotic risk
How do unsaturated fatty acids affect cholesterol levels?
- ACAT (acyl-CoA cholesteryl acyltransferase) is the enyzme which combines fatty acids and cholesterol to form cholesteryl esters
- it has a low Km (high affinity) for unsaturated fatty acids
- in this way, UFAs increase production of cholesteryl esters
- less free cholesterol leads to SREBP cleavage –> higher levels of the LDL-R –> decreased plasma cholesterol
What new therapies are being used to increase levels of the LDL receptor?
Antibodies or antisense nucleotides for PCSK9, a protein which binds LDLRs and induces their degeneration.
