News and Publications

Protease Receptors as Multifunctional Molecules

B.M. Mueller, W. Ruf, L. Goretzki, M. Yebra

Production of molecules with proteolytic activities and regulation of these molecules by cellular receptors are crucial for both physiologic and pathophysiologic processes of tissue remodeling and cell invasion. Binding of ligands to protease receptors, however, can also influence intracellular processes and thereby alter the activation state of the cells. Our research focuses on the contribution of several receptors for serine proteases to tumor cell invasion and metastasis.

The receptor for urokinase-type plasminogen activator (u-PA) enables invading cells to direct and enhance generation of pericellular plasmin. However, binding of catalytically inactive u-PA to the receptor also initiates signaling events that lead to enhanced cell migration, for example, in a model of prostate carcinoma cells migrating on a fibronectin gradient. Because the u-PA receptor is anchored with a glycolipid moiety in the plasma membrane and lacks an intracellular domain, it probably requires a transmembrane adaptor molecule to transduce signals into the cytoplasm. We suggest that an integrin-type adhesion receptor functions in our prostate carcinoma model to couple the u-PA receptor to the cytoplasm, because binding of u-PA induces interaction of the u-PA receptor with the integrin ₅ß₁ and results in enhanced tyrosine phosphorylation of intracellular protein kinases and adaptor proteins downstream of the integrin.

The low density lipoprotein receptor--related protein (LRP) is a multiligand endocytotic receptor that mediates the internalization of protease-inhibitor complexes and thereby regulates the proteolytic balance on the surface of invasive cells. Internalization of LRP was thought to be a constitutive process that happens independent of ligand binding. We found that binding of ligand to LRP activates a stimulatory heterotrimeric G protein and a cAMP-dependent protein kinase. We also showed that the internalization of LRP ligands depends on the activity of the protein kinase. We will further investigate whether binding of ligand to LRP also induces gene transcription, particularly of proteins implicated in the regulation of the invasive phenotype.

Another protease receptor involved in tumor cell invasion and metastasis is tissue factor (TF), the cellular receptor and cofactor for the coagulation protease factor VIIa. We recently showed that TF function in metastasis involves the formation of a catalytically active complex of TF and VIIa that leads to generation of thrombin and specific functions of the cytoplasmic domain of TF. We are investigating what intracellular proteins interact with the cytoplasmic domain of TF in the context of tumor cell metastasis.

PUBLICATIONS

Goretzki, L., Mueller, B.M. Receptor-mediated endocytosis of urokinase-type plasminogen activator is regulated by cAMP-dependent protein kinase. J. Cell Sci. 110:1395, 1997.

Mueller, B.M., Ruf, W. Requirement for binding of catalytically active factor VIIa in tissue factor dependent experimental metastasis. J. Clin. Invest. 101:1372, 1998.

Wong, N.C., Mueller, B.M., Barbas, C.F. III, Ruminski, P., Quaranta, V., Lin, E.C.K., Smith, J.W. _v integrins mediate adhesion and migration of breast carcinoma cell lines. Clin. Exp. Metastasis 16:50, 1998.