Plasminogen activator inhibitor-1 (PAI-1) also known as endothelial plasminogen activator inhibitor (serpin E1) is a protein that in humans is encoded by the SERPINE1gene. Elevated PAI-1 is a risk factor for thrombosis and atherosclerosis.[5]
The other PAI, plasminogen activator inhibitor-2 (PAI-2) is secreted by the placenta and only present in significant amounts during pregnancy. In addition, proteasenexin acts as an inhibitor of tPA and urokinase. PAI-1, however, is the main inhibitor of the plasminogen activators.
Genetics
The PAI-1 gene is SERPINE1, located on chromosome 7 (7q21.3-q22). There is a common polymorphism known as 4G/5G in the promoter region. The 5G allele is slightly less transcriptionally active than the 4G.
Function
PAI-1's main function entails the inhibition of urokinase plasminogen activator (uPA), an enzyme responsible for the cleavage of plasminogen to form plasmin. Plasmin mediates the degradation of the extracellular matrix either by itself or in conjunction with matrix metalloproteinases. In this scenario, PAI-1 inhibits uPA via active site binding, preventing the formation of plasmin. Additional inhibition is mediated by PAI-1 binding to the uPA/uPA receptor complex, resulting in the latter's degradation.[6] Thus, PAI can be said to inhibit the serine proteases tPA and uPA/urokinase, and hence is an inhibitor of fibrinolysis, the physiological process that degrades blood clots. In addition, PAI-1 inhibits the activity of matrix metalloproteinases, which play a crucial role in invasion of malignant cells through the basal lamina.
Fibrinolysis (simplified). Blue arrows denote stimulation, and red arrows inhibition.
Role in disease
Congenital deficiency of PAI-1 has been reported; as fibrinolysis is not suppressed adequately, it leads to a hemorrhagic diathesis (a tendency to hemorrhage).
PAI-1 is present in increased levels in various disease states (such as a number of forms of cancer), as well as in obesity and the metabolic syndrome. It has been linked to the increased occurrence of thrombosis in patients with these conditions.
In inflammatory conditions in which fibrin is deposited in tissues, PAI-1 appears to play a significant role in the progression to fibrosis (pathological formation of connective tissue). Presumably, lower PAI levels would lead to less suppression of fibrinolysis and conversely a more rapid degradation of the fibrin.
Angiotensin II increases the synthesis of plasminogen activator inhibitor-1, so it accelerates the development of atherosclerosis.
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