Urokinase receptor cleavage and shedding: occurrence and consequences.

The urokinase-type plasminogen activator receptor (uPAR), is a multifunctional protein with an impressive range of distinct, but overlapping functions in the process of tissue remodelling and cell migration: 1) uPAR regulates extracellular proteolysis by promoting plasminogen activation; 2) uPAR regulates cell adhesion as an adhesion receptor for vitronectin and by its capacity to modulate integrin function; 3) uPAR regulates cell migration as a signal transduction molecule and by its intrinsic chemotactic activity. In this thesis I have analysed the consequences, and occurrence, of uPAR cleavage and shedding. In chapter 3 I analyse the structural requirements for uPAR to promote cellular adhesion to vitronectin. I demonstrate that cell surface expression of intact uPAR is necessary and sufficient to promote binding of the myeloid cell line 32D to vitronectin. In this cell system the uPAR mediated cell binding does not lead to cell spreading and does not require integrin activation. In chapter 4 I demonstrate that the chemotactic activity of uPAR maps to the linker region which connects the first and second domain of uPAR. This chemotactic epitope (the SRSRY motif) is required and sufficient for the chemotactic activity of soluble uPAR fragments and appears to induce signalling similar to that induced by uPAR ligands such as uPA. In chapter 5 I show that uPAR and the uPAR fragments Dl and D2D3 are indeed generated on the cell surface and released to the surroundings by several different cell types. In chapter 6 I describe and characterise the presence of soluble uPAR and uPAR fragments in vivo. I demonstrate that suPAR as well as the suPAR fragments Dl and D2D3 are present in human urine. Using mice xenografted with human model tumours I demonstrate that the tumour tissue is a source of urinary suPAR antigen and that the suPAR fragment pattern in urine correlates with uPAR cleavage in the tumour tissue

Efferocytosis: another function of uPAR

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uPAR-induced cell adhesion and migration: vitronectin provides the key

Expression of the membrane receptor uPAR induces profound changes in cell morphology and migration, and its expression correlates with the malignant phenotype of cancers. To identify the molecular interactions essential for uPAR function in these processes, we carried out a complete functional alanine scan of uPAR in HEK293 cells. Of the 255 mutant receptors characterized, 34 failed to induce changes in cell morphology. Remarkably, the molecular defect of all of these mutants was a specific reduction in integrin-independent cell binding to vitronectin. A membrane-tethered plasminogen activator inhibitor-1, which has the same binding site in vitronectin as uPAR, replicated uPAR-induced changes. A direct uPAR–vitronectin interaction is thus both required and sufficient to initiate downstream changes in cell morphology, migration, and signal transduction. Collectively these data demonstrate a novel mechanism by which a cell adhesion molecule lacking inherent signaling capability evokes complex cellular responses by modulating the contact between the cell and the matrix without the requirement for direct lateral protein–protein interactions

Negative regulation of urokinase receptor activity by a GPI-specific phospholipase C in breast cancer cells.

The urokinase receptor (uPAR) is a glycosylphosphatidylinositol (GPI)-anchored protein that promotes tissue remodeling, tumor cell adhesion, migration and invasion. uPAR mediates degradation of the extracellular matrix through protease recruitment and enhances cell adhesion, migration and signaling through vitronectin binding and interactions with integrins. Full-length uPAR is released from the cell surface, but the mechanism and significance of uPAR shedding remain obscure. Here we identify transmembrane glycerophosphodiesterase GDE3 as a GPI-specific phospholipase C that cleaves and releases uPAR with consequent loss of function, whereas its homologue GDE2 fails to attack uPAR. GDE3 overexpression depletes uPAR from distinct basolateral membrane domains in breast cancer cells, resulting in a less transformed phenotype, it slows tumor growth in a xenograft model and correlates with prolonged survival in patients. Our results establish GDE3 as a negative regulator of the uPAR signaling network and, furthermore, highlight GPI-anchor hydrolysis as a cell-intrinsic mechanism to alter cell behavior

Monomer–dimer dynamics and distribution of GPI-anchored uPAR are determined by cell surface protein assemblies

To search for functional links between glycosylphosphatidylinositol (GPI) protein monomer–oligomer exchange and membrane dynamics and confinement, we studied urokinase plasminogen activator (uPA) receptor (uPAR), a GPI receptor involved in the regulation of cell adhesion, migration, and proliferation. Using a functionally active fluorescent protein–uPAR in live cells, we analyzed the effect that extracellular matrix proteins and uPAR ligands have on uPAR dynamics and dimerization at the cell membrane. Vitronectin directs the recruitment of dimers and slows down the diffusion of the receptors at the basal membrane. The commitment to uPA–plasminogen activator inhibitor type 1–mediated endocytosis and recycling modifies uPAR diffusion and induces an exchange between uPAR monomers and dimers. This exchange is fully reversible. The data demonstrate that cell surface protein assemblies are important in regulating the dynamics and localization of uPAR at the cell membrane and the exchange of monomers and dimers. These results also provide a strong rationale for dynamic studies of GPI-anchored molecules in live cells at steady state and in the absence of cross-linker/clustering agents

Stroma-regulated HMGA2 is an independent prognostic marker in PDAC and AAC

Background: The HMGA2 protein has experimentally been linked to EMT and cancer stemness. Recent studies imply that tumour-stroma interactions regulate these features and thereby contribute to tumour aggressiveness. Methods: We analysed 253 cases of pancreatic ductal adenocarcinoma (PDAC) and 155 cases of ampullary adenocarcinoma (AAC) for HMGA2 expression by IHC. The data were correlated with stroma abundance and supplemented by experimental studies. Results: HMGA2 acts as an independent prognostic marker associated with a significantly shorter overall survival in both tumour types. Overall, HMGA2-positivity was more frequent in patients with PDAC than with AAC. The HMGA2 status in tumour cells significantly correlated with the abundance of PDGFRβ-defined stroma cells. In vivo co-injection of Panc-1 cancer cells with pancreatic stellate cells increased tumour growth in a manner associated with increased HMGA2 expression. Furthermore, in vitro treatment of Panc-1 with conditioned media from PDGF-BB-activated stellate cells increased their ability to form tumour spheroids. Conclusions: This study identifies HMGA2 expression in tumour cells as an independent prognostic marker in PDAC and AAC. Correlative data analysis gives novel tissue-based evidence for a heterotypic cross-talk with stroma cells as a possible mechanism for HMGA2 induction, which is further supported by experimental models