Plasminogen activator content of human tumor and adjacent normal tissue measured with fibrin and non-fibrin assays

Abstract

Plasminogen activators are enzymes which convert the zymogen to plasmin, the physiological enzyme for dissolving fibrin. There are two different physiological activator enzymes, urokinase (UK) and tissue plasminogen activator (t-PA, also known as vascular activator). The most striking difference in the behavior of the two activators is the ability of fibrin to augment the activity of t-PA but not of UK. Since tumor and normal tissues have been shown to contain different ratios of UK:t-PA, it can be anticipated that their comparative activator activities measured with fibrinolytic assays would yield different results from those measured with non-fibrin tests. This study was designed to test the validity of earlier conclusions that: (a) the measured activity of t-PA is augmented in fibrinolytic assays when compared with a non-fibrin assay based on azocaseinolysis; and (b) this difference could explain the failure of some laboratories using fibrinolytic assays to detect a difference in activator activity between tumor and normal tissues or to find more activity in the normal tissue. Azocaseinolytic and fibrinolytic (fibrin plate) assays were used to measure activator activity in a series of 14 normal-tumor tissue pairs. Using azocasein tests, cancer tissues were found to contain significantly higher median activities than normal tissues [Wilcoxon test, P less than 0.05; 13.8 versus 3.7 Committee on Thrombolytic Agents (CTA) units/g tissue, respectively], whereas no significant difference was found with fibrin assays (43.5 versus 69.0 CTA units/g tissue, respectively). Of total activator activity, the median percentage of UK was significantly higher in tumor (95%) than in normal tissue (58%). In addition, using azocaseinolysis it was found that the median UK activity was significantly higher in tumor (12.1 CTA units/g) relative to normal (3.51 CTA units/g) tissues, whereas no difference was found for t-PA. To explain these results in tumor and normal tissues, a mathematical model was derived to describe the difference between azocasein and fibrin assays for both purified plasminogen activator enzymes and activator enzymes in tissue extracts. The model fits the data well, confirming in a quantitative manner the hypotheses of the study. In addition, the study revealed that the azocaseinolytic assay was able to measure the full potential activator activity of purified pro-urokinase enzyme. Pro-urokinase activity could not be measured with standard fibrinolytic assays. These results show the importance of selection and interpretation of plasminogen activator assays in studies dealing with malignant transformation.