HC36: Fibrinolysis and atherothrombosis

Fibrinolysis

At some point, the platelet plug needs to be removed to return the normal blood flow.

Fibrinolysis is part of the normal hemostasis. It enables repair of the injured vessel wall and is important in resolving an unwanted clot such as a thrombus.

Fibrinolysis consists of the conversion of earlier formed fibrin, which is insoluble, into soluble fibrin degradation products. This process is important because it prevents venous thrombosis and atherothrombosis by removing unwanted clots. It is part of normal hemostasis.

Plasmin:

Plasmin is a molecule which degrades fibrin → after degradation, only D-dimers of fibrin are left:

1. Cross-linked fibrin is degraded by fibrin degradation products
   • Plasmin cleaves the E-domains
2. D-dimers are formed

A D-dimer is also a biomarker for the fact that thrombosis is present elsewhere.

Plasmin generation

Plasmin generation consists of a sequence of proteolytic reactions on the surface of fibrin. There are 2 pathways, dependent of the activator:

• Pathway 1: tPA dependent plasmin generation
  • Activator: tissue plasminogen activator (tPA)
    • Produced by healthy endothelial cells
  • tPA converts plasminogens to plasmins
    • Fibrin is the co-factor in his reaction → speeds up the reaction
      • tPa is only active when bound to fibrin → advantageous in thrombosis
      • Is the surface where tPA and plasmin react together
• Pathway 2: uPA dependent plasmin generation
  • Activator: sc-uPA or pro-urokinase
    • Produced by endothelial cells, macrophages and epithelial cells
    • Plasmin converts sc-uPA to tc-uPA
      • Tc-uPA is the active form of sc-uPA
  • tc-uPA converts plasminogen to plasmin → amplification
  • The reaction takes place in both presence and absence of fibrin

Regulation:

Plasmin generation is regulated by:

• Presence of tPA and uPA
• Inhibitors of tPA, uPA and plasmin
  • Plasminogen activator inhibitor 1 (PAI-1)
    • Produced by endothelium, platelets and monocytes
    • PAI-1 inhibits tPA and uPA highly efficiently
  • α2-antiplasmin (α2AP)
    • Produced by the liver
    • Limits the diffusion of plasmin into the circulation
    • A fast and highly efficient inhibitor of plasmin
• Fibrin inhibitors
  • Thrombin activatable fibrinolysis inhibitor (TAFI)
    1. TAFI is a plasma pro-carboxypeptidase which circulates in the blood in an inactive form
    2. Thrombin activates TAFI to TAFIa
    3. TAFIa removes lysins from the fibrin surface to which the tPA and fibrinogen bind → inhibition of the conversion of plasminogen to plasmin
    4. Plasmin generation and fibrin degradation are delayed

Thrombosis and fibrinolysis

There is a correlation between thrombosis and fibrinolysis:

• Venous thrombosis and fibrinolysis
  • Cause: thrombi are formed in large veins under low flow (stasis)
  • Therapy: targeting coagulation
    • Heparin antagonists
    • Vitamin K antagonists
    • DOACs
  • Primary aim: preventing occurrence of a pulmonary embolism (PE)
  • The thrombus is removed by endogenous fibrinolysis
• Arterial thrombosis and fibrinolysis
  • A diseased arterial wall triggers thrombosis
    • This may obstruct the arterial flow
    • Atherosclerotic plaques induce the activation of hemostasis
  • The impact is dependent on the rate of thrombus removal such as fibrinolysis → this can be done via:
    • Endogenous fibrinolysis
    • Pharmacologically stimulated fibrinolysis

Atherosclerosis

Atherosclerosis is linked to arterial thrombosis. It is a degenerative disease of the arteries characterized by patchy thickening of the inner lining of the arterial walls, caused by deposits of fatty material (cholesterol). This is a chronic process which can take years and starts early in life. When triggering hemostasis, acute clinical manifestation takes place → atherosclerosis becomes a problem:

1. A thrombus obstructs the arterial blood flow
2. Ischemia of certain organs
3. Infarction of the organs
4. Loss of (vital) organ function

Lipoproteins:

In case of high LDL cholesterol levels, LDL can travel out of the bloodstream into the tissue → becomes trapped in the arterial vessel wall. This can also happen at young age and can be enhanced by conditions that alter the barrier function of the endothelium:

• Smoking
• High glucose
• High blood pressure
• Chemotherapy
• Infections
• Radiation

Formation of atherosclerosis:

The forming of atherosclerosis consists of 6 steps:

1. Step 1: endothelial dysfunction
   1. Retention of LDL in the intimal space
      • Locations of LDL trapping depend on hemodynamic flow patterns
      • Increased at locations with high oscillating shear stress
        • E.g. branches and bifurcations
   2. The prolonged half-life of LDL results in LDL oxidation → OxLDL is formed
   3. Oxidated LDL is toxic and affects cells of the arterial wall → induces:
      • Activation of endothelium by cytokines and expression of cell adhesion molecules
      • Adhesion, transmigration and retention of cells involved in inflammation → monocytes are recruited
2. Step 2: macrophage-foam cell formation
   1. Recruited monocytes differentiate to macrophages
   2. Macrophages clear the oxLDL
   3. OxLDL-loaded macrophages are unable to leave the vessel wall → foamy macrophages remain present and form a “fatty-streak”
      • Reside under the epithelium and on top of the smooth muscle cells (SMC)
3. Step 3: SMC proliferation and migration
   1. As a “healing” response, SMCs proliferate and migrate → cover the foam cells
   2. SMCs start producing collagen and elastin → a protective SMC-rich fibrous cap is formed
4. Step 4: atheroma formation
   1. The death of foam cells results in extracellular cholesterol deposition → a necrotic core
5. Step 5: calcification
   1. The plaque cell ages and dies → calcium deposition
   2. Calcium deposition results in loss of elasticity → the vessel wall hardens (sclerosis)
   3. An atherosclerotic vessel wall leads to 2 types of responses
      • Outward remodeling
        • Normal flow
        • No clinical symptoms
      • Inward remodeling
        • Stenosis and reduced flow, often in the arteries of the leg
          • Intermittent claudication
          • A gradual process
        • No acute clinical symptoms
6. Step 6: plaque rupture and thrombosis
   1. Inflammatory cells produce degrading enzymes, which degrade the fibrous capsule around the thrombus
   2. Collagen degradation exceeds synthesis → the capsule thins and ruptures
   3. Thrombogenic materials are exposed to the bloodstream
   4. Within hours, the vessel lumen and flow are obstructed
   5. Acute clinical manifestation takes place

Myocardial infarction:

A myocardial infarction can occur during plaque rupture and thrombosis (stage 6 of atherosclerosis). The following happens:

1. The coronary artery flow is obstructed by a thrombus → atherothrombosis
2. Ischemia of the heart muscle tissue leads infarction
3. Infarction leads to loss of function

The impact this has on the heart depends on the location of the thrombus and the rate of endogenous fibrinolysis.

Ischemic stroke:

A cerebrovascular accident or ischemic stroke can occur during plaque rupture and thrombosis (stage 6 of atherosclerosis). The following happens:

1. Atherothrombosis obstructs carotid artery flow and/or embolizes to the middle cerebral artery → ischemia
2. Ischemia leads to infarction
3. Infarction leads to loss of function
4. A transient ischemic attack (TIA) or ischemic stroke occurs
   • Aphasia and paralysis are examples of ischemic strokes

The impact of this depends on the location and success or failure of endogenous fibrinolysis:

• A TIA occurs when fibrinolysis has succeeded
• An ischemic stroke occurs when fibrinolysis has failed

Therapeutic stimulation:

After for example a myocardial infarct or stroke, fibrinolysis needs to be therapeutically stimulated. This can be done by giving recombinant tPA. This is only beneficial shortly after the event (up to 3 hours after a stroke). The aim is to restore the blood flow in the affected part of the heart or brain. A side effect which frequently occurs are bleedings everywhere throughout the body.