Atherosclerosis - Basics and Roles of Inflammatory Cells Flashcards
Describe the difference in the function of macrophages and smooth muscle cells in an atherosclerotic plaque.
Macrophages remove arterial tissue
Smooth muscle cells deposit arterial tissue
Macrophages
- remove arterial tissue
VSMC
- make more arterial tissue (angiogenesis)
- Protect plaque integrity by making more tissue to strengthen artery
Describe the effects of macrophages on smooth muscle cells.
Macrophages stimulate smooth muscle cell proliferation and makes the smooth muscle cells produce more collagen to strengthen the fibrous cap.
Smooth muscle cells protect plaque integrity, they make more tissue which strengthens the artery.
What are modifiable and non-modifiable risk factors of atherosclerosis?
Modifiable:
- Smoking
- Fats/lipids
- Blood pressure
- Diabetes
- Obesity
- Lack of exercise
Non-modifiable:
- Age, sex
- Genetic background
Where does atherosclerosis form?
At places of turbulent flow such as bifurcations (branch points).
It forms as the bifurcation of the common carotid artery.
What is the reason for the formation of atherosclerosis at places of turbulent flow?
- Blood flow in arteries is LAMINAR so the blood flows fastest in the middle of the vessel and slower around the outside.
- when the blood goes around a corner too quickly it sets up EDDYs (turbulent flow).
- Therefore atherosclerotic lesions tend to appear on the outside of a bend rather than on the inside.
- Laminar flow maintains the shear stress of a vessel so promotes endothelial cell survival (with secretion of NO etc.)
- In places of turbulent flow, shear stress decreases which is bad (eddy currents can form)
where do LDLs deposit and what does it bind to
-LDLs deposit in the SUBINTIMAL SPACE and binds to matrix proteoglycans
How does atherosclerosis progress?
Low Density Lipoproteins (LDLs) circulate in blood
Carry cholesterol synthesised in liver to body
Phospholipid MONOLAYER
Have APOPROTEINS on their surface
Improve solubility and receptor recognition
LDLs “leak” through endothelium
Mechanism under research
Bind to matrix proteoglycans (carbohydrates in the ECM)
Held in place, so are prone to oxidation
Monocytes circulate in blood Are called MACROPHAGES when in tissue
Normally have receptors to detect pathogenic antigens SCAVENGER RECEPTORS A and B
These “accidently” fit to Oxidised LDL ligands
Macrophages naturally residing in tissue take in oxidised LDL via PHAGOCYTOSIS
Macrophages that have taken up significant LDLs are called FOAM CELLS
Macrophages have enzymes that further oxidise the LDLs
NADPH Oxidase
Myeloperoxide
Internal accumulation of highly oxidised LDLs triggers upregulations of NUCLEAR FACTOR KAPPA B (NFkB)
A transcription factor, called the “master regulator” of INFLAMMATION
NFkB activates genes that code for CYTOKINES and CHEMOKINES
CYTOKINES that activate strengthen endothelium adhesion to monocytes
Interleukin-1 : Upregulates production of VCAM-1
VCAM-1 : Mediates tight monocyte binding
CHEMOKINES that drawn in monocytes via chemotaxis
Monocyte chemotactic protein-1 (MCP-1) : Binds to CCR2 receptors (G-protein) of the monocytes
Monocytes are brought into atheroma and become foam cells
Leads to POSITIVE FEEDBACK LOOP of atheroma proliferation
What are the types of lesions?
Type II lesion
- > Type III (preatheroma)
- > Type IV (atheroma)
- > Type V (fibroatheroma)
- > Type VI (complicated lesion)
Type I: Tunica Media Hypertrophy and Isolated Foam Cells Type II: Foam Cell Layers Type III: Extracellular lipid Type IV: Lipid Core Type V: Fibrosis Type VI: Complications
What are the main cell types and their roles?
- Vascular endothelial cells
- Barrier function e.g. to lipoproteins
- Leukocyte recruitment - Platelets
- Thrombus generation
- Cytokine and growth factor release - Monocyte-macrophages
- Foam cell formation
- Cytokine and growth factor release
- A major source of free radicals
- Metalloproteinases (degrade thickened areas and promote plaque rupture) - VSMCs
- Migration and proliferation
- Collagen synthesis
- Remodeling and fibrous cap formation - T-lymphocytes
- Macrophage activation
what are the 2 systems responsible for homeostasis?
- Clotting cascade
2. Platelet aggregation
What are macrophages derived from?
- In atherosclerosis, the main inflammatory cells are macrophages, which are derived from blood monocytes
what do macrophages produce?
-Macrophages secrete cytokines and growth factors which are a key source of free radicals which the immune system makes as part of the natural immune function to kill microbes.
what happens inside vascular smooth muscle cells in an abnormal artery
In an abnormal artery with an atherosclerotic plaque, can synthesise and secrete collagen which contributes to the stabilisation of the plaque and the fibrous cap.
How are T cells involved in macrophages recruitment
T cells: the feedback of macrophages activated T cell and the activated T cells, in turn, activate macrophages.
what are the main inflammatory cells in atherosclerosis?
Macrophage subtypes are regulated by combinations of transcription factors binding to regulatory sequences on DNA
- Inflammatory: adapted to kill microorganisms
- Resident : normally homeostatic (suppress inflammatory activity)
- e.g alveolar resident macrophages (surfactant lipid homeostasis), osteoclasts (calcium and phosphate homeostasis), spleen (iron homeostasis)
What are the 3 subforms of lipoproteins?
LDL
- ‘Bad’ cholesterol - Synthesised in liver.
- Carries cholesterol from liver to rest of the body including arteries
HDL
- ‘Good’ cholesterol
- Carries cholesterol from ‘peripheral tissues’ including arteries back to liver (=”reverse cholesterol transport”)
Oxidised LDL, modified LDL
- Due to action of free radicals on LDL
- Multiple substances act on LDLs to create inflammatory LDLs known as oxidised LDLs
- Families of highly inflammatory and toxic forms of LDL found in vessel walls
Describe the structure of a lipoprotein?
- LDL consists of a monolayer
- It has a protein on the outside (apoproteins) which tells it where to go
What causes the oxidation of LDLs?
Free radicals
Describe what happens to LDLs after endothelial activation.
Endothelial activation increases the membrane permeability
LDLs enter and are oxidised by free radicals and become stuck in the subendothelial layer
OxLDLs are phagocytosed by macrophages to form foam cells
The foam cells accumulate and cause chronic inflammation
What is Familial Hypercholesterolemia and its clinical signs?
- Autosomal dominant genetic disease.
- Massively elevated cholesterol (20mmol/L).
- Failure to clear LDL from blood
- Xanthomas form (fat deposits in the skin) and early atherosclerosis; if untreated fatal myocardial infarction before age 20
How was the LDL receptor discovered?
- Brown and Goldstein discovered the LDL receptor (LDLR) which is negatively regulated by intracellular cholesterol and that cholesterol synthesis is negatively regulated by cellular cholesterol –> led to statins
- In LDLR negative patients, foam cells still form so must be another receptor for LDL not under feedback control from atherosclerotic lesions –> Scavenger receptor (cleans up oxidized LDL)
What are scavenger receptors?
Scavenger receptors are pathogen receptors that accidentally bind to oxidised LDLs
What are the 2 types of macrophage scavenger receptors?
- Macrophage scavenger receptor A
- Known as CD204
- Binds to oxidised LDL
- Binds to Gram-positive bacteria
- Binds to dead cells - Macrophage scavenger receptor B
- Known as CD36
- Binds to oxidised LDL
- Binds to malaria parasites
- Binds to dead cells
What are the 2 pathways of action of arterial ox-LDL deposits?
LARGE deposit –> inflammation.
SMALL deposit –> safe clearance
What is the LDLR regulated by?
It is negatively regulated by intracellular cholesterol
What determines the function of the second LDL receptor?
The level of oxidised LDLs present
At relatively low levels of OxLDLs, the abnormal materials are taken up by macrophages and cleared safely by reverse cholesterol transport.
At high levels of OxLDLs, it triggers an inflammatory reaction
What enzyme do macrophages have that affects oxygen?
NADPH Oxidase - reduces oxygen to make superoxide (O2-)
What are the processes macrophages carry out within plaques?
- Generate free radicals that further oxidise lipoproteins
- Phagocytose modified lipoproteins, & become foam cells
3a. Become activated by OxLDLs to express cytokine
3b. Become activated by OxLDLs to express chemoattractants & growth factors for VSMC
3c. Express Proteinases that degrade tissue - Macrophage Apoptosis - contributing to the lipid-rich core
What enzyme do macrophages have that affects oxygen?
NADPH Oxidase - reduces oxygen to make superoxide (O2-)
What does the superoxide end up producing?
Superoxide ends up producing hypochlorous acid (by the action of myeloperoxidase from hydrogen peroxide and chlorine)
What enzyme is involved in the production of HOCl and what other product can it produce?
Myeloperoxidase - also produces a more unstable form called peroxynitrite
What cytokines are released by macrophages?
IL-1 = upregulates VCAM-1 thus increasing leukocyte migration
explain how macrophages accumulate oxidative enzymes that can modify native LDL?
- Macrophages enter the sub-endothelial layer and as they go further in, they get larger as they absorb OxLDLs.
- As they get larger they bloat and eventually they apoptose causing contents to spill out (fat)
Overall, macrophages accumulate modified LDLs to become enlarged foam cells
What two growth factors are produced by macrophages and how do they affect smooth muscle cells?
Platelet derived growth factor (PDGF) = stimulates smooth muscle chemotaxis, survival and division
Transforming growth factor - beta = stimulates collagen synthesis
Describe how the normal function of a smooth muscle cell is different to the synthetic smooth muscle cell in atherosclerosis.
Normal - more contractile filaments and less collagen deposition
Synthetic - fewer contractile filaments and more collagen deposition
PDGF and TGF-beta can make the VSMC become the synthetic phenotype
What do the matrix metalloproteinases expressed by macrophages do?
They break down the collagen in the ECM and hence weakens the plaque
What are the characteristics of unstable plaques?
Thin fibrous cap
Reduced VSMC and collagen content
Infiltration of activated macrophages expressing MMPs
Large, soft, eccentric lipid-rich necrotic core
What is a master transcription factor involved in atherosclerosis and what does it stimulate?
Nuclear Factor Kappa B - stimulates matrix metalloproteinases and stimulates inducible nitric oxide synthase
