Centro di Endocrinologia ed Oncologia Sperimentale, Consiglio Nazionale delle Ricerche, Naples, Italy.
Five rat thyroid cell lines were tested for the expression of the cell surface receptor for urokinase type plasminogen activator (uPA). All tested lines were found to bind uPA, but transformed 1-5G and Ki-Mol cells, which are also high uPA producers, bound at least ten times more uPA, as compared to non-producers, 'normal' TL5 cells. Moreover, it was possible to remove membrane-bound uPA by treating the cells with phosphatidylinositol-specific phospholipase C, suggesting that rat uPAR, like its human counterpart, is linked to the membrane by a glucosyl-phosphatidylinositol anchor. The specificity of the binding was tested by competition with three different synthetic peptides corresponding to amino acids 14-37 of human, rat and mouse uPA. The results indicate also that the receptor binding region of rat uPA is located within the growth factor domain of the molecule and that its expression may be dependent on the transformed state of the cells.