The gene for human urokinase-type plasminogen activator receptor (uPAR) is located on chromosome 19q13 within a 2 Mb cluster harbouring all presently known glycosylphosphatidylinositol (GPI)-anchored, multi-domain proteins of the Ly6/uPAR/alpha-neurotoxin (LU) domain family (Kjaergaard et al., 2008).

24254 bp; 7 exons (Figure 1).

Transcription of the uPAR gene is regulated by a TATA-less proximal promoter, partly through binding to SP1 (Soravia et al., 1995).

uPAR was originally identified on the monocyte-like human cell line U937 as the membrane receptor for the serine protease urokinase-type plasminogen activator (uPA) (Vassalli et al., 1985). It has since been implicated in a large number of physiological and pathological conditions, including cancer invasion and metastasis.

uPAR is a multi-domain member of the Ly6/uPAR/alpha-neurotoxin (LU) protein family (Ploug, 2003), containing three of these LU domains (DI, DII and DIII; Figure 2), each of ~ 90 amino acids, adopting a "three-fingered" folding topology, and encompassing 4 consensus disulfide bonds and an invariant C-terminal asparagine (Figure 2B). Intriguingly, one of the consensus disulfide bonds, which is crucial to the proper folding of the single domain LU proteins, is missing in the N-terminal LU domain of uPAR. As evident from the crystal structures solved for human uPAR, the three LU domains cooperate in creating a deep and hydrophobic ligand-binding cavity (Figure 2C), in which the growth factor-like domain (GFD) of the cognate protease ligand uPA (Huai et al., 2006; Barinka et al., 2006) or synthetic peptide antagonists (Llinas et al., 2005) are buried during formation of the corresponding high-affinity receptor complexes.
The 335 amino acid residue long single polypeptide chain of human uPAR is processed to a mature protein of only 283 residues after post-translational excision of signal peptides at the N- and C-termini, the latter event being responsible for tethering uPAR to the cell membrane via a GPI moiety (Figure 2A+C; Ploug et al 1991). Human uPAR contains 5 potential N-glycosylation sites, of which only four are utilized (Ploug et al., 1998; Gårdsvoll et al., 2004).

Under normal homeostatic conditions, uPAR is expressed by the following bone marrow-derived blood cells: monocytes, neutrophils, eosinophils and macrophages. In the bone marrow itself, uPAR has been demonstrated in myelocytes, mature myeloid elements and monocytes. Expression of the receptor is significantly upregulated upon cytokine stimulation of various monocyte-derived cell lines in vitro (Plesner et al., 1994a). Consistent with these observations, uPAR is classified as a differentiation antigen and is also denoted CD87. The amount of uPAR in homeostatic organs is in general low, and when present, usually associated to quiescent endothelial cells, as demonstrated in e.g. the lung, kidney, thymus, liver and heart of normal mice (Solberg et al., 2001). Expression of uPAR in kidney and thymus has also been recapitulated in human samples (Wei et al., 2008; Plesner et al., 1994a).
Whereas uPAR is scarce under normal conditions, pronounced receptor expression has been observed in various non-homeostatic tissue remodeling processes. First, during wound healing, strong uPAR immunoreactivity is found in migrating keratinocytes at the leading re-epithelialization edge of the wound, while non-migrating keratinocytes are negative (Rømer et al., 1994). In addition, uPAR is located in infiltrating granulocytes located underneath the wound crush, and in endothelial cells in the wound area (Solberg et al., 2001). In squamous cell carcinoma of the skin, uPAR mRNA and protein is seen in well-differentiated cancer cells at the invasive front of the tumour lesion (Rømer et al., 2001; Ferrier et al., 2002). In view of the localization of uPAR in the leading-edge keratinocytes in regenerating epidermis during mouse skin wound healing, it has been proposed that similarities exist between the mechanisms of generation and regulation of extracellular proteolysis during skin re-epithelialization and squamous cell carcinoma invasion (Rømer et al., 2001). As a second example of tissue remodeling, late pregnancy in mouse shows uPAR expression in spongiotrophoblasts and endothelial cells in the placenta (Solberg et al., 2001). In the human counterpart, uPAR was encountered in endothelial cells and macrophages in association with fibrinoid deposits, suggesting a participation of uPAR in placental development and fibrin surveillance (Pierleoni et al., 1998 and 2003). Third, uPAR is present in the regression of mammary glands in post-lactational involution in mice (Solberg et al., 2001). These three processes mimic some of the characteristics of cancer invasion and can accordingly be used as model systems for the elucidation of the role of uPAR in malignant transformation. Indeed, the receptor is upregulated in several cancer types, expression often being confined to stromal cells associated with the tumour, as detailed later for gastro-intestinal, breast and squamous cell cancers (Figure 3). Other examples conforming to this pattern include hepatocellular carcinoma, where uPAR has been found in macrophages and fibroblasts, as well as in a subpopulation of rare CK7-positive tumoural hepatocytes (Akahane et al., 1998; Dubuisson et al., 2000). In invasive lesions of human prostate adenocarcinoma, uPAR mRNA and protein is also expressed by macrophages, located throughout the interstitial tissue of tumours (Figure 3D), whereas in benign lesions, it is confined to intraluminal macrophages (Usher et al., 2005). A summary of uPAR expression patterns in various cancer forms can be found in Table 1.

Table 1

Cancer type	Cancer cells	Macrophages	Fibroblast-like cells	Neutrophils	Endothelial cells	Nerves	References
Breast	+	+	+	+			Pyke et al., 1993; Nielsen et al., 2007
Colon	+	+		+		+	Pyke et al., 1994; Illemann et al., 2009
Gastric	+	+	+			+	Migita et al., 1999; Zhang et al., 2006; Alpízar-Alpízar et al., 2009
Glioblastoma	+				+		Yamamoto et al., 1994
Liver		+	+				Dubuisson et al., 2000
Lung	+	+	+				Pappot et al., 1997; Morita et al., 1998
Oral	+	+	+	+			Lindberg et al., 2006
Prostate		+		+			Usher et al., 2005
Skin	+	+					Rømer et al., 2001; Ferrier et al., 2002

The cell membrane attachment of uPAR via GPI implicates a differential partitioning of the receptor into detergent-resistant microdomain lipid rafts that are enriched in cholesterol and sphingolipids. In complex with uPA and the plasminogen activator inhibitor type 1, uPAR can also be internalized (Cubellis et al., 1990), and later recycled back to the membrane (Nykjær et al., 1997). Interestingly, upon stimulation of resting neutrophils, uPAR is rapidly translocated from secretory vesicles to the cell surface (Plesner et al., 1994b). Proteolytic cleavage of the receptor either in the linker region between DI and DII, e.g. by its own ligand uPA (Høyer-Hansen et al., 1992), or between domain III and the GPI-anchor, yield various soluble uPAR fragments that are detectable in body fluids such as plasma and urine, the levels of which have been shown to correlate to overall survival in several human cancers (Høyer-Hansen and Lund, 2007).

Through its high-affinity interaction with the serine protease urokinase-type plasminogen activator (uPA), uPAR plays a central role in cell surface-associated plasminogen activation, entailing proteolytic degradation of the extracellular matrix surrounding the cells (Ellis et al., 1989; Stephens et al., 1989). This cascade, mediated by the broad-spectrum serine protease plasmin, is involved in several tissue remodeling processes such as wound healing and mammary gland involution (Ploug, 2003; Danø et al., 2005). The proteolytic role of uPAR in tumour invasion and metastasis is reflected by the frequently encountered expression of uPAR at the invasive front of cancer tissue, and by the impact of uPAR status in malignant cells disseminated to the bone marrow on the prognosis of gastric cancer patients (Heiss et al., 2002).
Other ligands with which uPAR allegedly also interacts include the matrix component vitronectin (Waltz and Chapman, 1994; Wei et al., 1994; Gårdsvoll and Ploug, 2007; Huai et al., 2008), various integrins (Wei et al., 1996; Aguirre Ghiso et al., 1999) and the G-protein coupled receptor FPRL1 (Resnati et al., 1996), affecting cell adhesion and migration, as well as signal transduction (Figure 4; for reviews, see Ossowski and Aguirre-Ghiso, 2000; Blasi and Carmeliet, 2002; Kjøller, 2002; Kugler et al., 2003; Ragno, 2006; Tang and Wei, 2008).

uPAR is homologous to the other multi-domain proteins of the Ly6/uPAR/alpha-neurotoxin protein domain family (C4.4A, PRV-1/CD177, TEX101, PRO4356, GPQH2552), of which C4.4A is the best-studied until now (Jacobsen and Ploug, 2008), and to a vast number of single LU-domain proteins such as the Ly-6 antigens, CD59, SLURP 1 and SLURP 2, the extracellular ligand-binding domains of the TGF-receptor family and the snake venom alpha-neurotoxins (Ploug and Ellis, 1994; Ploug, 2003).

An mRNA splice variant of uPAR, lacking exons 4 and 5, which encode domain II of the receptor, has recently been shown to have prognostic relevance in breast cancer (Kotzsch et al., 2008). Restriction fragment length polymorphisms of the uPAR gene have been identified by EcoRI and PstI (Børglum et al., 1991 and 1992), and a highly polymorphic CA/GT repeat is present in intron 3 (Kohonen-Corish et al., 1996). In a comparison of the latter in colon cancer patients and controls, however, there were no significant differences in the frequencies of alleles (Przybylowska et al., 2008; Kohonen-Corish et al., 1998). Genetic linkage and association analyses on 587 families with high incidences of asthma have revealed a correlation between asthma/lung function decline and certain SNP in the uPAR gene (Barton et al., 2009).