Endothelial cells, plasminogen activation

There are endogenous and exogenous plasminogen activators. Endothelial cells (andtumor cells) synthesize... 

Tissue-type plasminogen activator (tPA) is a glycoprotein (68 kDa), synthesized by endothelial and tumor-cells. As a serine protease, tPA hydrolyses Arg561-Val562 peptide bond in plasminogen, resulting in plasmin formation. It needs cofactors for efficient plasminogen activation. 

Urokinase-type plasminogen activator (uPA, urokinase) is synthesized by endothelial and tumor cells as a single-chain glycoprotein (scuPA) without catalytic activity. When it is converted to a two-chain protein (tcuPA) by plasmin, an active serine protease center develops, which activates plasminogen. Thus, uPA (55 kDa) results in the amplification of fibrinolysis. 

Mahdi F, Shariat-Madar Z, Todd RF 3rd, Figueroa CD, Schmaier AH Expression and colocahzation of cytokeratin 1 and urokinase plasminogen activator receptor on endothelial cells. Blood 2001 97 2342-2350. 56... 

The antithrombotic factors produced by endothelial cells are thrombomodulin (TM) and protein S (PS), components of the vitamin K-dependent protein C (PC) anticoagulant pathway, inhibiting F-Va-F-Villa (E15) tissue plasminogen activator (tPA), responsible for fibrinolysis (N2, LI8) and the lipoprotein-associated coagulation inhibitor (LACI), which inhibits F-VIIa-TF complex and F-Xa (B51). 

The procoagulant factors produced by endothelial cells are the coagulation factors von Willebrand factor (WF), F-V, F-VIII, tissue factor (TF), and plasminogen activator inhibitor (PAI), which blocks the activators u-PA and t-PA and counteracts fibrinolysis (G21, FI6). It has been shown that under the influence of complement activation (C9), in response to endotoxin in vitro (C24), in experimental E. coli sepsis in baboons (D30), and after stimulation with TNF (Al, N6), endothelial cells up-regulate the expression of TF, down-regulate TM and inhibit the production of t-PA and PAF. Thus, the balance may shift in the procoagulant direction with a large excess of PAI-1. 

F16. De Fouw, N. J., Van Hinsbergh, V. W. M., De Jong, Y. F Haverkate, F., and Bertina, R. M., The interaction of activated protein C and thrombin with the plasminogen activator inhibitor released from human endothelial cells. Thromb. Haemost. 57, 176-182 (1987). 

N2. Nakashima, Y., Sueishi, K., and Tanaka, K Thrombin enhances production and release of tissue plasminogen activator from bovine venous endothelial cells. Fibrinolysis 2, 227-234 (1988). 

Levin E. G., Santell L. Association of a plasminogen activator inhibitor (PAI-1) with the growth substratum and membrane of human endothelial cells. J Cell Biol 1987 105, 2543-9. 

Another process in which Lp(a) can interfere with plasminogen activation to plasmin is on cell surfaces of endothelial and mononucleated cells (G19, Hll, L12, M23) (Fig. 12). 

Fig. 12. Inhibition of l25I-lys-plasminogen binding to human umbilical vein endothelial cells (HUVECs) by Lp(a) and apo(a). Confluent HUVEC monolayers were washed, treated with -aminocaproic acid, rewashed and incubated with 125I-lys-plasminogen (4.95 nM, specific activity 415.000cpm-pmo -1), for 30 min at 4°C in the presence of various excess amounts of unlabeled Lys-PLG (A) Lp(a) (0,0) apo(a) (x) LDL ( , ) or Lp(-) (V). [With permission of Hajjar el at. (HI 1).]...

E10. Etingin, O. R., Hajjar, D. P., Hajjar, K. A., Harpel, P. C., and Nachman, R. L., Lipopro-tein(a) regulates plasminogen activator inhibitor-1 expression in endothelial cells. J. Biol. Chem. 266, 2459-2465 (1991). 

H12. Hajjar, K. A., The endothelial cell tissue plasminogen activator receptor. J. Biol. Chem. 266, 21962-21970 (1991). 

Figure 22.5 (a) Injury to endothelial cells can lead to thrombosis. Healthy endothelial cells secrete two factors that inhibit thrombus formation (i) prostacyclins, which inhibit aggregation of platelets and (ii) a factor that facilitates conversion of plasminogen to plasmin. Injury to endothelial cells can facilitate thrombosis since (i) they do not secrete prostacyclins (ii) they do not secrete plasminogen activator and (iii) they secrete a factor that stimulates thrombosis. (b) Diagram of an atheroscle-rosed artery containing a thrombus. A thrombus is blocking the lumen of the artery that is not totally blocked by plaque. 

Alteplase Alteplase is a drag that activates human tissue plasminogen (t-PA). It is a glycoprotein of molecular mass 68,000 that is synthesized by vascular endothelial cells. t-PA cells were first isolated from cultured human melanoma cells [56-58], but currently a genetically recombined form of rt-PA is genetically engineered. 

Secretion of tissue plasminogen activator by cultured endothelial cells increases within an hour after exposure to arterial levels of shear stress [11], while secretion of the related inhibitor (plasminogen inhibitor-1) by the same cells is unaffected by shear forces over the physiological range. 

S. L. Diamond, S.G. Eskin, L.V. Mclntire, Fluid flow stimulates tissue plasminogen activator secretion by cultured human endothelial cells. Science 243(4897) (1989) 1483. 

While some parenteral injections, such as intravenous administration, provide rapid and predictable access to the circulation and tissues, therapeutic proteins are rapidly cleared from the system, and thus such administrations may result in very short durations of action. Regardless of route of administration, therapeutic proteins may exhibit limited distribution outside of endothelial cells lining blood vessels. This may be advantageous for thrombolytic agents, such as tissue plasminogen activator, which is used for rapid fibrinolytic actions at... 

Fibrinolysis refers to the process of fibrin digestion by the fibrin-specific protease, plasmin. The fibrinolytic system is similar to the coagulation system in that the precursor form of the serine protease plasmin circulates in an inactive form as plasminogen. In response to injury, endothelial cells synthesize and release tissue plasminogen activator (t-PA), which converts plasminogen to plasmin (Figure 34-3). Plasmin remodels the thrombus and limits its extension by proteolytic digestion of fibrin. 

Protein C. This vitamin K-dependent glycoprotein serine protease zymogen is produced in the liver. It is an anticoagulant with species specificity (19—21). Protein C is activated to Protein Ca by thrombomodulin, a protein that resides on the surface of endothelial cells, plus thrombin in the presence of calcium. In its active form, Protein Ca selectively inactivates, by proteolytic degradation, Factors V, Va, VIII, and Villa. In this reaction the efficiency of Protein Ca is enhanced by complex formation with free Protein S. In addition, Protein Ca activates tissue plasminogen activator, which promotes the conversion of plasminogen [9001-91-6] to plasmin [9001-90-5]. 

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