Atherosclerosis

Definition/Overview

Nature. 2002 Dec 19-26;420(6917):868-74.
J Am Coll Cardiol. 2006 Apr 18;47(8 Suppl):C7-12.
N Engl J Med. 2005 Apr 21;352(16):1685-95.

Atherosclerosis: disease of large and medium-sized arteries characterized by endothelial dysfunction, vascular inflammation and accumulation of lipids, cholesterol, calcium and cellular debris within the intima of the vessel wall.

Inflammatory activation of vascular cells

In contrast to the homeostasis that exists between the endothelium and smooth muscle cells (SMCs) of healthy vessels, inflammatory activation of vascular cells in diseased vessels corrupts their normal functions and favours processes that contribute to the atherosclerotic plaque development.

Table 1. Consequences of inflammatory activation in vascular cells

Stages of plaque development

Stroke. 2006 Jul;37(7):1923-32.
Stroke. 2004 Nov;35(11 Suppl 1):2712-9 

1. Fatty streak

• Earliest visible lesions that appear as areas of yellow discoloration on artery’s inner surface; blood flow is not yet impeded at this stage
• Central to this process is endothelial dysfunction, which allows entry and modification of lipids in the vessel subintima; these lipids then serve as pro-inflammatory mediators that initiate leukocyte recruitment and foam cell formation

Endothelial dysfunction

• Triggered by injury to the arterial endothelium
  • Exposure to physical forces, e.g. shear stress
  • Chemical irritants and toxins
    • Increased production of reactive oxygen species (ROS) in cigarette smoking, elevated circulating low density lipoprotein (LDL) levels and diabetes
• Resulting activated state (Table 1) sets the stage for subsequent development of atherosclerosis

Lipoprotein entry and modification

• Accumulation of lipoprotein particles in the intima
  • Increased endothelial permeability allows for entry of LDL into the vessel intima
  • LDL binds to proteoglycans in the extracellular matrix and becomes trapped
• Chemical modification of lipoproteins
  • Oxidation – by local ROS derived from endothelial cells or macrophages that penetrate the vessel wall
  • Glycation – in diabetic patients with sustained hyperglycemia
• Modified LDL has antigenic and pro-inflammatory properties and contributes to leukocyte recruitment and foam cell formation

Leukocyte recruitment

• Modified LDL induces local pro-inflammatory cytokine elaboration (e.g. IL-1, TNF-α) by endothelial and SMCs
• Cytokines promote increased expression of:
  • Adhesion molecules (e.g. VCAM-1, ICAM-1, E-selectin, P-selectin) – bind leukocytes
  • Chemoattractant molecules (e.g. MCP-1, IL-8) – direct leukocyte migration into the vessel intima
• Monocytes, and to a lesser extent T lymphocytes, are attracted to the vessel wall

Foam cell formation

• Upon entering the intima, monocytes differentiate into phagocytic macrophages and upregulate their expression of scavenger receptors in response to local macrophage colony-stimulating factor (M-CSF)
• Scavenger receptors mediate the uptake of modified LDL into macrophages
• Macrophages develop into foam cells which produce additional cytokines that perpetuate the process of atherosclerotic plaque formation

2. Plaque progression

• Thickening of the intima due to migration of SMCs from the media to the intima, proliferation of SMCs and extracellular matrix production
  • Foam cells, activated platelets and endothelial cells release cytokines and growth factors (e.g. TNF-α, IL-1, FGF, TGF-β) that stimulate SMC migration and proliferation
  • SMC activation and cytokine release reinforces and maintains inflammation in the lesion
• Fatty streak evolves into a fibrofatty lesion
• Calcification can occur at later stages and fibrosis continues
• Apoptosis of SMCs yields a relatively acellular fibrous capsule that surrounds a lipid-rich core

• Late plaque growth can significantly restrict the vessel lumen and impede perfusion
  • Flow-limiting plaques can lead to tissue ischemia and cause symptoms such as angina pectoris or claudication

3. Plaque disruption

• Fibrous cap integrity depends on net extracellular matrix metabolism
  • SMCs synthesize constituents of the fibrous cap such as collagen and elastin
  • Foam cells synthesize proteolytic enzymes including collagen-degrading matrix metalloproteinases
• Over time hemodynamic stresses and degradation of extracellular matrix increase the risk of fibrous cap rupture
• While plaques with thicker fibrous caps tend to cause more pronounced arterial narrowing, they have less propensity to rupture (stable plaques); conversely, thinner less obstructive plaques tend to be more fragile and rupture (vulnerable plaques)
• When the fibrous cap ruptures, pro-thrombotic molecules within the lipid core are exposed and can sometimes, but not necessarily, precipitate formation of an acute thrombus which occludes the arterial lumen
  • This is the underlying mechanism in most acute coronary syndrome

Risk factors for atherosclerosis

Modifiable risk factors

• Dyslipidemia (elevated LDL, decreased HDL)
  • Excess LDL accumulates in the intima and undergoes modifications that initiate and perpetuate the development of atherosclerotic lesions
• Tobacco smoking
  • Enhances oxidative modification of LDL, contributes to endothelial dysfunction via oxidant stress and increases expression of leukocyte adhesion molecules, among other factors
• Hypertension 
  • Increases permeability of vessel wall to lipoproteins and promotes retention of LDL in the vessel intima by accentuating production of LDL-binding proteoglycans by SMCs
• Diabetes mellitus
  • Enhances glycation of LDL and is associated with endothelial dysfunction
• Obesity and lack of physical activity
  • Contribute to dyslipidemia, hypertension and insulin resistance

Non-modifiable risk factors

• Advanced age
• Male gender
  • Lack of atheroprotective properties of estrogen which raises HDL and lowers LDL
• Heredity – history of coronary artery disease (CAD) among first-degree relatives at a young age (before 55 for males and before 65 for females)

 Biological markers

• Homocysteine 
  • High levels may promote oxidative stress, vascular inflammation and platelet adhesiveness
• Lipoprotein particle Lp(a) 
  • Detrimental effect may be attributed to competition with normal plasminogen activity
• C-reactive protein and other markers of inflammation
  • Activates complement and contributes to a sustained inflammatory state

Complications of atherosclerotic plaques

• Calcification – makes vessel more rigid and increases its fragility
  • Myocardial ischemia, peripheral artery disease, renal artery stenosis
• Rupture – plaque rupture exposes pro-coagulant molecules within the plaque to circulating blood and can potentially lead to formation of a thrombus and infarction of the supplied tissue
  • Unstable angina, myocardial infarction, thrombotic stroke
• Hemorrhage – an intramural hematoma can form secondary to plaque rupture
  • Myocardial infarction, thrombotic stroke
• Embolization – disrupted fragments from the atherosclerotic plaque can embolize to distal vascular sites
  • Embolic stroke
• Aneurysm – the fibrous plaque increases pressure in the vessel media and can provoke weakening of elastic tissue, with subsequent dilatation of the artery
  • Abdominal aortic aneurysm