EXPRESSION OF UROKINASE-TYPE PLASMINOGEN-ACTIVATOR IN VARIOUS HUMAN THYROID TISSUES

High levels of urokinase-type plasminogen activator (u-PA) have been d etected in malignant tissues of various organs and these are thought t o be related to the local invasion and distant metastasis of carcinoma s. In the present study, the expression of u-PA was immunohistochemica lly investigated in various thyroid tissues by using formaline-fixed, paraffin-embedded tissue sections. Normal thyroid expressed hardly any u-PA. In some hyperplasias, weak u-PA immunoreactivity was observed i n small follicles and in clusters of follicular cells in the process o f regenerating hyperplastic follicles. However very intense u-PA stain ing was observed in all cases of chronic thyroiditis in the small foll icules surrounded by numerous lymphocytes. This indicates that u-PA mu st be strongly implicated in this disease together with various cytoki nes. As much as 90% of the carcinomas in our study expressed u-PA in t he cytoplasm of carcinoma cells and the expression in carcinomas was m uch more extensive than that in benign adenomas. Although this carcino ma is known to have a generally good prognosis, our findings suggest t hat u-PA reflects malignancy in tumoral growth of the thyroid and modu lates this growth from its early stage.