Lars Kjøller

The urokinase‐type plasminogen activator system in cancer metastasis: A review

The urokinase‐type plasminogen activator (u‐PA) system consists of the serine proteinases plasmin and u‐PA; the serpin inhibitors α2‐anti‐plasmin, PAI‐1 and PAI‐2; and the u‐PA receptor (u‐PAR). Two lines of evidence have strongly suggested an important and apparently causal role for the u‐PA system in cancer metastasis: results from experimental model systems with animal tumor metastasis and the finding that high levels of u‐PA, PAI‐1 and u‐PAR in many tumor types predict poor patient prognosis. We discuss here recent observations related to the molecular and cellular mechanisms underlying this role of the u‐PA system. Many findings suggest that the system does not support tumor metastasis by the unrestricted enzyme activity of u‐PA and plasmin. Rather, pericellular molecular and functional interactions between u‐PA, u‐PAR, PAI‐1, extracellular matrix proteins, integrins, endocytosis receptors and growth factors appear to allow temporal and spatial re‐organizations of the system during cell migration and a selective degradation of extracellular matrix proteins during invasion. Differential expression of components of the system by cancer and non‐cancer cells, regulated by paracrine mechanisms, appear to determine the involvement of the system in cancer cell–directed tissue remodeling. A detailed knowledge of these processes is necessary for utilization of the therapeutic potential of interfering with the action of the system in cancers. Int. J. Cancer 72:1–22, 1997.

Receptor‐mediated endocytosis of plasminogen activators and activator/inhibitor complexes

Recent findings have elucidated the mechanism for clearance from the extracellular space of the two types of plasminogen activators, urokinase‐type plasminogen activator (u‐PA) and tissue‐type plasminogen activator (t‐PA), and their type‐1 inhibitor (PAI‐1). Activator/PAI‐1 complexes and uncomplexed t‐PA bind to the multiligand receptors α2 macroglubulin receptor/low density lipoprotein receptor‐related protein (α2MR) and epithelial glycoprotein 330 (gp330). These receptors mediate endocytosis and degradation of u‐PA/PAI‐1 complex bound to the glycosyl phosphatidyl inositol‐anchored urokinase receptor (u‐PAR) on cell surfaces, and participate, in cooperation with other receptors, in hepatic clearance of activator/PAI‐1 complexes and uncomplexed t‐PA from blood plasma. The α2MR‐ and gp330‐mediated endocytosis of a ligand (u‐PA/PAI‐1 complex) initially bound to another receptor (u‐PAR) is a novel kind of interaction between membrane receptors. Binding to α2MR and gp330 is a novel kind of molecular recognition of serine proteinases and serpins.

Very low density lipoprotein receptor from mammary gland and mammary epithelial cell lines binds and mediates endocytosis of M(r) 40,000 receptor associated protein.

We here report that the M r 40,000 receptor associated protein (RAP), previously found to bind to α2‐macroglobulin receptor/low density lipoprotein receptor related protein (α2MR/LRP) and glycoprotein 330 (gp330), binds to an M r, 105,000 membrane protein from bovine mammary gland, human mamma tumors and mammary epithelial cell lines. We have purified this protein from bovine and human sources. N‐terminal amino acid sequencing and immunoblotting analyses showed that the protein was identical or closely related to very low density lipoprotein receptor (VLDL‐R). Experiments with the human mamma carcinoma cell line MCF‐7 showed that this receptor was able to mediate an efficient endocytosis of RAP. These novel findings strongly suggest that RAP functions as a modulator of ligand binding to VLDL‐R, similarly to α2MR/LRP and gp330.

Differential regulation of urokinase-type-1 inhibitor complex endocytosis by phorbol esters in different cell lines is associated with differential regulation of α2-macroglobulin receptor and urokinase receptor expression

Complex between urokinase and its type-1 inhibitor (uPA-PAI-1) may, when bound to the urokinase receptor (uPAR), be endocytosed by an ensuing binding of the complex to the multiligand receptors alpha (2)-macroglobulin receptor/low density lipoprotein receptor-related protein (alpha 2MR/LRP) and glycoprotein 330 (gp330). We have found that phorbol esters regulate endocytosis of uPA-PAI-1 differently in different cell lines. In COS-1 cells, expressing uPAR and high levels of alpha 2MR/LRP under basal conditions, phorbol esters cause a time-dependent decrease in endocytosis concomitantly with a parallel down-regulation of alpha 2MR/LRP expression. An up-regulation of uPAR expression was also observed. General endocytosis via the clathrin-coated pit pathway was not affected by PMA treatment, as judged from measurements of transferrin endocytosis. In LLC-PK1 cells, expressing alpha 2MR/LRP but not uPAR under basal conditions, phorbol esters transiently increase endocytosis in parallel with a transient induction of uPAR expression, while there was virtually no change in alpha 2MR/LRP expression. Differential regulation of endocytosis therefore seems to be caused by differential regulation of the receptors, with either the alpha 2MR/LRP-level (in COS)-1 cells) or the uPAR-level (in LLC-PK1 cells) being rate-limiting.