Current Pharmaceutical Design - Volume 9, Issue 19, 2003

Urokinase type plasminogen activator (uPA) activates plasminogen to plasmin and is often associated with diseases where tissue remodeling is essential (e.g. cancer, macular degeneration, atherosclerosis). We discuss some of the mechanisms of uPA action in diseases, and evidence that some of the early uPA inhibitors can modulate the progression of these diseases. Recently, a number of research groups have discovered, with the aid of structure-based design, a new generation of uPA inhibitors. These inhibitors are much more potent and selective than their predecessors. We will review this progress here, and give particular attention to the structural rationale associated with these observed increases in potency and selectivity.

Degradation of the extracellular matrix plays an important role in a number of normal and pathological conditions involving active tissue remodeling such as postlactational mammary gland involution, wound healing and tumor invasion and metastasis. The expression of a high-affinity, glycolipid-anchored receptor for the urokinase-type plasminogen activator (uPAR) is often up-regulated during such tissue remodeling events. uPAR may, therefore, in cooperation with various matrix metalloproteases, serve to facilitate the proteolytic breakdown of the extracellular matrix via uPA catalyzed plasminogen activation at the foci where cellular invasion occurs. Consistent with such a role for uPAR in pericellular proteolysis is the observation that the membrane assembly of both plasminogen, via its lysine binding-sites, and of pro-uPA, via its tight binding to uPAR, is required to favor and confine plasminogen activation potential in proximity of the cell surface.This review will focus on molecular properties of uPAR including its membrane attachment by glycosylphosphatidylinositol, its multidomain structure and its relationship to the Ly-6 / uPAR / α-neurotoxin protein domain family. Furthermore a mapping of the functional epitopes for uPA binding as well as a competitive peptide antagonist of the uPA-uPAR interaction will be discussed.

In cancer, increased levels of the tumor-associated serine protease uPA (urokinase-type plasminogen activator) and its receptor uPAR (CD87) are linked to tumor progression, metastasis, and shortened survival in patients afflicted with this disease. Strong clinical and experimental evidence has accumulated that the cell surface interaction of uPA with uPAR facilitates extravasation and intravasation of cancer cells by regulating local proteolysis and attachment of the cells to components of the extracellular matrix. Moreover, the uPA / uPAR system is also implicated in proliferation of some tumor cells and migration of tumor and endothelial cells. Thus, metastasis formation is facilitated via tumor cell spread through the blood circulation system and neovascularization at the metastatic site. This multifunctional potential has rendered the uPA / uPAR system an attractive novel target for anti-metastatic therapy. Consequently, inhibitors of the uPA / uPAR interaction have been and are currently developed for suppression of tumor growth and angiogenesis. In addition to antibodies and recombinant uPA- or uPAR-derived proteins, various linear and cyclic peptides as well as small molecules have been designed and synthesized which potently interfere with the uPA / uPAR interaction, leading to reduced tumor progression in experimental animals. Such compounds affecting the uPA / uPAR system represent novel tumor biology-based therapeutic agents, thereby opening new ways for patient optimized and individualized cancer therapy.

Plasminogen activator inhibitor-1 (PAI-1) is the principal inhibitor of urokinase type plasminogen activator (uPA) and tissue-type plasminogen activator (tPA), and as such is thought to play an important role in the regulation of extracellular matrix remodeling. In blood, PAI-1 is bound to the adhesion protein vitronectin and is associated with vitronectin in fibrin clots and the provisional matrix. Elevated levels of PAI-1 are associated with atherosclerosis and an increased thrombotic tendency, while PAI-1 deficiency leads to increased fibrinolysis and bleeding. PAI-1 is also elevated in many solid tumors and is associated with a poor prognosis in cancer. PAI-1 has been shown to be a potent regulator of both vascular cell migration in vitro and of angiogenesis and tumor growth in vivo. PAI-1 can both promote and inhibit tumor growth and angiogenesis. Low concentrations of PAI-1 can stimulate tumor angiogenesis while treatment of animals with high doses of PAI-1 inhibits angiogenesis and tumor growth. Hence, PAI-1 appears to have a multifunctional role in regulating the migratory and fibrinolytic activity of vascular cells, and this, in turn, may help to explain the many varied actions of PAI-1.

Urokinase receptors (uPAR) were initially thought to function simply as a mechanism to concentrate the urokinase / plasmin system toward the cell surface. However, extensive evidence has accumulated that this glycolipidanchored receptor also functions in both the adhesive and signaling pathways of many migratory cells. Mechanisms by which uPAR exercises these functions involve complexing with other membrane proteins for signal transduction. One set of functional partners for uPAR on the cell surface are integrins. Recent studies point to important structural features of uPAR:integrin interactions, indicating uPAR to be a cis-acting integrin ligand. In vivo data reveal altered integrin function and cell migration when uPAR:integrin interactions are impaired. Together these observations support the idea that uPAR:integrin interactions may be a focal point of intervention in pathobiology where integrin function is crucial, such as tumor metastasis.