Current Pharmaceutical Design - Volume 17, Issue 19, 2011

The story that led to the discovery of urokinase receptor (uPAR) system started in 1947 with the report of MacFarlane and Pilling who identified but did not named urokinase (uPA). Today, the uPAR system is recognized as one very important actor in tumourigenesis and is even considered as a valuable tumour marker. Its critical functions justify the important effort of translational research that has produced many inhibitors which unfortunately failed to be transferred in the clinic. However, the role of the uPAR system in cancer development should not shade the vital functions of this system in hematopoietic stem cells mobilization, cognitive functions including language development, inflammation, innate immunity, coagulation and fibrinolysis. All these topics are covered in this special theme issue of the journal Current Pharmaceutical Design that comprises 9 reviews written by leading scientists. Other aspects are also embraced by additional articles including the first attempt to depict the complete atlas of the uPAR interactome, and the regulation of the uPAR system by the LDL receptor-related protein-1 (LRP-1).

The urokinase receptor (uPAR) was originally identified as the membrane receptor of the serine protease urokinase (uPA), thereby implicated in the plasminogen activation cascade and regulation of pericellular proteolysis. Later on, vitronectin was showed to be another major ligand providing uPAR with a role in cell adhesion. Other unrelated ligands have been subsequently reported including for example factor XII and SRPX2 expanding the functions of uPAR to unexpected biological areas such as the initiation of the coagulation cascade or the regulation of language development. Due to its glycosylphosphatidylinositol (GPI) anchor, uPAR has no intracellular domain and thus exerts its signaling capacity through lateral interactions with other components of the plasma membrane that actually mediate uPAR-induced signals. As yet, a total 42 proteins interacting directly with uPAR can be numbered comprising 9 soluble ligands and 33 lateral partners. The fact that uPAR interacts with members of three major families of membrane receptors i.e. G protein-coupled receptors, receptor tyrosine kinases, and integrins implies that the actual number of components constituting the uPAR interacome is extremely high. For example, 156 factors belong to the integrin adhesome. Moreover, in the light of the wide diversity of the components of the uPAR interactome, uPAR appears to be an essential player of major biological systems including the blood coagulation, complement and plasma kallikrein-kinin cascades. This review describes the soluble ligands and lateral partners of the uPAR interactome, the mechanisms regulating uPAR interactions and their proved and/or potential biological functions.

Since decades the urokinase plasminogen activator (uPA) system has been associated with the invasion of malignant cells. The receptor of urokinase (uPAR) is one of the key players in this proteolytic cascade, because it focuses uPA's proteolytic activity to the cell surface and in addition functions as a signaling receptor. uPAR is highly expressed in virtually all human cancers, suggesting possible clinical applications as diagnostic marker, predictive tool of survival or clinical response, and as a target for therapy and imaging. This review summarizes the possibilities of uPAR in clinical applications for cancer patients.

The receptor for urokinase plasminogen activator (uPAR) is required in hematopoietic stem/progenitor cells (HSC, HPC) mobilization in the mouse. Indeed, uPAR Ko mice are deficient both in retention and mobilization of HSC and HPC, because uPAR causes their retention in the BM through its interaction with the α4β1 integrin and at the same time its removal promotes their migration. Normally, the membrane signal is cleaved by plasmin which on one side releases the cells from their osteoblasts interaction and on the other produces a cleaved soluble uPAR product that stimulates mobilization. Available data suggest that a similar mechanism may take place in humans.

As a key component of the plasminogen activation system, uPAR, the receptor for the plasminogen activator of the urokinase type, is involved in many physiological and pathological processes. Besides its classical roles, there has been increased evidence that uPAR or uPAR-associated pathways, participate in the development, in the functioning and in the pathology of the central nervous system. Qualitative and quantitative changes in the expressions of uPAR and of its canonical ligand uPA have been observed in a large variety of epileptic disorders, either in human or in animal models, as well as in other brain diseases (stroke and brain trauma, multiple sclerosis, Alzheimer's disease, cerebral malaria, HIV-associated leukoencephalopathy and encephalitis). The variety of such pathological conditions and the different brain areas and cell types involved, likely reflects the wide range and the complexity of the multiple and somehow intertwined pathophysiological mechanisms related with uPAR. In the mouse, the knock-out of the Upar-encoding gene (Plaur) leads to significant and nearly complete loss in parvalbumin-containing interneurons during brain development. This is associated with increased susceptibility to spontaneous and chemically-induced seizures and with increased anxiety and impaired social interactions. The recent identification of the novel uPAR ligand SRPX2 (Sushi repeat protein, X-linked 2) and the regulation of both the SRPX2 and PLAUR genes by transcription factor FOXP2 has shed novel and exciting insights into the role of uPAR-related molecular networks in rolandic epilepsy, in developmental verbal dyspraxia, in perisylvian polymicrogyria, and generally in disorders of the speech areas and circuits. uPAR, its regulators and partners, as well as other proteins containing Ly-6/uPAR/alpha-neurotoxin domains, represent key entry points for present and future studies not only on speech-related disorders but also on epilepsy and autism spectrum disorders.

The urokinase plasminogen activator (uPA) and its receptor (uPAR) provide a cell surface integrated multimolecular complex that exerts pleiotropic functions influencing the development of inflammatory, immune, coagulation and fibrinolytic responses. Here we review the evidences indicating a role of the uPA/uPAR system in the regulation of the innate immune system in the inflammation process, of the adaptive immune response, as well as the role of fibrin and fibrin degradation products at the cross-road between coagulation and inflammation. Comparative studies have clearly highlighted the notion that coagulation and immunity are co-regulated and intertwined. The implication is that the vertebrate blood clotting system is evolutionarily by product of the innate immune system, where the blood clotting proteases have diverged from those comprising the complement system. Differences have emerged gradually, as shown by the acquisition of unique protein structures, such as kringle domains and gla (glutammic acid) domains, in order to comply with the increasingly complex vertebrate systems and to defend higher organisms against a range of infections and injuries. Plasminogen activation also controls the formation of complement anaphylotoxins (responsibe for vasodilatation, increase of venular permeability and leukocyte chemotaxis) and of bradykinin (which accounts for vasodilatation, increase of venular permeability and pain) by regulating the plasma contact system. The urokinase plasminogen activator and its cellular receptor, expressed on the surface of human leukocytes, provide a functional unit that, by regulating interaction of leukocytes with extracellular matrix, as well as its degradation, is critical for the migration of leukocytes and for their movement in the damaged tissues.

Urokinase (uPA) was originally identified in human urine for its ability to catalyse the transformation of plasminogen into its active form, plasmin which degrades fibrin and extracellular matrix components. Two major, functionally independent regions have been identified in the uPA molecule: a non-catalytic N-terminal region (residues 1-135) and a large catalytic region (residues 159-411) spaced by the “connecting peptide” (residues 136-158). Binding of uPA to its specific surface receptor (uPAR) amplifies cell surface plasminogen activation, thus enhancing pericellular proteolysis. The uPAR, linked to the lipid bilayer via a glycosylphosphatidylinositol anchor, mediates signaling through the assembly of a multiprotein complex with transmembrane receptors, like integrins, EGFR, GPCRs. Receptor engagement with uPA results in a variety of cell responses, including increased proliferation, survival, migration and invasion. These responses may be enhanced by the concomitant binding of the uPA “connecting peptide” region to αvβ5 integrin, thus favoring uPARintegrin association. Receptors engaged with uPA exhibit a high affinity binding for vitronectin, stimulating cell adhesion. The uPA/uPAR system is regarded as one of the key systems driving tumour invasion and metastases. Different strategies to prevent the activity of the protease, as well as the interactions of uPAR with integrins and GPCRs have been designed. Many preclinical studies are ongoing and, at least, two uPA-related compounds have reached Phase II clinical trials. The aim of this review is to provide a comprehensive picture of the functionally relevant interactions, together with a description of the promising compounds and strategies to control uPA activity and signaling in human pathologies.

LDL receptor-related protein (LRP1) is an endocytic receptor for multiple ligands, including proteases, growth factors, apolipoproteins, and extracellular matrix proteins. In some cell types, including neurons, neuron-like cells, and Schwann cells, ligand-binding to LRP1 triggers robust cell-signaling. This “direct” pathway by which LRP1 regulates cell-signaling promotes cell survival and cell migration. LRP1 also regulates the composition of the plasma membrane proteome. Although multiple mechanisms are involved, LRP1 and receptors in the same gene family facilitate the endocytosis of other plasma membrane proteins. When LRP1 regulates the abundance or trafficking of another cell-signaling receptor in the plasma membrane, activation of important cell-signaling pathways may be controlled “indirectly” by LRP1. The urokinase receptor (uPAR) was the first cell-signaling receptor identified as a member of the LRP1-regulated plasma membrane proteome. Because LRP1 down-regulates cell-surface uPAR by facilitating its endocytosis, under some conditions, uPAR-initiated cell-signaling may be inhibited by LRP1. However, the relationship between LRP1 and uPAR is complicated because uPAR endocytosis may be necessary for sustained uPAR-initiated cell-signaling. Certain cell-signaling factors, including ERK, phosphatidylinositol 3-kinase, and Rac1 are regulated by LRP1, directly, and indirectly through uPAR. Thus, the predominant effect of LRP1 on cell-signaling, in different cell types, may depend on the abundance of LRP1 and uPAR and on the availability of ligands for LRP1 and uPAR. Opportunities for targeting the uPAR-LRP1 system through drug discovery are discussed.

The urokinase plasminogen activator receptor (uPAR) mediates cell motility and tissue remodeling. Although uPAR may be expressed transiently in many tissues during development and wound healing, its constitutive expression appears to be associated with several pathological conditions, including cancer. uPAR expression has been demonstrated in most solid tumors and several hematologic malignancies including multiple myeloma and acute leukemias.Unlike many tumor antigens, uPAR is present not only in tumor cells but also in a number of tumor-associated cells including angiogenic endothelial cells and macrophages. The expression of uPAR has been shown to be fairly high in tumor compared to normal, quiescent tissues, which has led to uPAR being proposed as a therapeutic target, as well as a targeting agent, for the treatment of cancer. The majority of therapeutic approaches that have been investigated to date have focused on inhibiting the urokinase plasminogen activator (uPA)-uPAR interaction but these have not led to the development of a viable uPAR targeted clinical candidate. Genetic knockdown approaches e.g. siRNA, shRNA focused on decreasing uPAR expression have demonstrated robust antitumor activity in pre-clinical studies but have been hampered by the obstacles of stability and drug delivery that have limited the field of RNA nucleic acid based therapeutics. More recently, novel approaches that target interactions of uPAR that are downstream of uPA binding e.g. with integrins or that exploit observations describing the biology of uPAR such as mediating uPA internalization and signaling have generated novel uPAR targeted candidates that are now advancing towards clinic evaluation. This review will discuss some of the pitfalls that have delayed progress on uPAR-targeted interventions and will summarize recent progress in the development of uPAR-targeted therapeutics.

The urokinase receptor (uPAR) exerts essential functions in the pathophysiology of cancers and therefore constitutes an important drug target. In order to generate efficient drugs against uPAR, a new approach includes chimeric proteins associating one molecular address to specifically target uPAR and one bacterial or plant toxin that will eventually kill the tumoral cell. Using this frame, several recombinant toxins have been designed namely DTAT, DTAT13, EGFATFKDEL 7 mut, and ATF-SAP. As molecular address, all of these fusion proteins use the amino-terminal fragment of urokinase that binds with high affinity to uPAR through its growth factor domain (GFD). The various toxin moieties were derived from either diphtheria toxin, Pseudomonas exotoxin A (PE38), or saporin. In this review, we describe the rational, design, production and therapeutic anti-cancer potential of these chimeric toxins.