Tumor cell metastasis is believed to occur due to the serine protease urokinase. Metastasis is required for a cancer cell to leave its primary tumor site to migrate through the blood or lymph system to a new tissue or organ, where it will grow into a second tumor. The increased synthesis of urokinase is related to the ability of increased metastasis.
Urokinase activates plasmin from plasminogen, which is located everywhere in extracellular space. The activation of plasmin causes degradation o proteins in the extracellular matrix through which metstasizing tumor cells invade. Plasmin can activate procollagen which promotes the degradation of the collagen in the capillaries and lymph system. Promotion of this proteolytic degradation activity is activated by urokinase that is secreted by tumor cells allowing for the invasion into target tissue and the formation of secondary tumor sites 1.
Metastasis the act of cancer cells leaving a primary tumor site and moving through the blood or lymph systems to a new tissue or organ, where it starts the growth of a secondary tumor. There are a few steps involved for metastastis to occur:
1) There must be the ability to loosen adhesion to original neighboring tissue.
2) The tumor cells must escape the tissue.
3) Burrow through other tissue until it reaches a blood vessel or lymphatic vessel.
4) Cross basal lamina and endothelial lining.
5) Enter circulation.
6) Exit circulation, invade secondary tissue, survive and proliferate 2.
Role of Urokinase
Why does synthesis increase?
Researchers have concluded that a high level of urokinase is a consequence of an abundant mRNA encoding the plasminogen activator. Therefore elevated level of transcription of urokinase results in the amplification, increased message stability, or transcriptional activation 3.
Expression of urokinase-plasminogen activator
As stated earlier, u-PA appears to play a role in the aggressive invasion of adjacent normal tissue. u-PA has been correlated with invasive phenotype of breast, lung, and colon carcinoma and invasive carcinoma. u-PA is bound to a cell membrane -specific receptor, urokinase receptor (u-PAR) which localizes u-PA protease activity to specific sites on the cell membrane surface. The substrate of urokinase is plasminogen, once bound activation of plasmin occurs. The plasmin is then able to degrade extracellular matrix, including fibrin, fibronectin laminicin, brain myelin basic protein, and type IV collagen. Inhibition of the u-PAR by antibodies has been shown to prevent metastasis, which concludes that urokinase does definitely have a role in metastasis 4.
Kinetics of Urokinase
Pro-urokinase (U) and plasminogen (P) are converted to urokinase (u) and plasmin (p). Three reactions are involved and they follow the Michaelis-Menten Kinetics.
(I) U + P <---> UP-----> U + p
(II) p + U<---> pU-----> p + u
(III) u + P <--->uP-----> u + p
In reaction (I) pro-urokinase behaves as an enzyme, activating plasminogen to plasmin. In the second reaction pro-urokinase behaves as a substrate and is converted to urokinase by plasmin generated in the first reaction. In reaction (III) formed urokinase then activates plasminogen to plasmin. Reaction three is the step to keep in mind because activation of plasmin from plasminogen causes the degradation of proteins in the extracellular matrix allowing for invasion of tumor cells.
Km(microM) k2(1/s) k2/Km (micro1/M1/s)
(I) .4 .02 .05
(II) 3.3 1.7 .5
(III) 50 1.0 .02
Looking at the data Km for reaction III is much greater then the Km value for reaction I and the k2 is also much greater, this shows that there is an increased rate of plasmin formation. Reaction (I) and reaction (III) work differently because reaction (I) has better binding ability, with a low Km value, whereas reaction (III) has a higher k2 value, with better releasing ability. Both reactions form plasmin, which can then help in tumor metastasis. The data also shows that pro-urokinase is an equally potent activator of plasminogen with a k2/Km ratio of .05 where urokinase has a ratio of .02 5.
1)Advance Cancer Research, 44: 139, 1985.
2)Alberts, Bray, Lewis, Kaff, Roberts, Watson. Molecular Biology of the Cell, 1989.
3)International Journal of Cancer, 54: 73-80, 1993.
4)American Journal of Pathology 146: 1150-60, 1995.
5)Journal of Biological Chemistry 261: 1259-66, 1986.