PLAUR (plasminogen activator, urokinase receptor)

Note	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).
	Figure 1: Location of the uPAR gene in the uPAR-like gene cluster on chromosome 19q13. Each of the three LU domains of uPAR is encoded by separate exon sets, flanked by phase-1 introns (Casey et al., 1994). The introns dividing these 3 exon sets are also phase-1 and are located at a position corresponding to the surface-exposed tip of loop 2 in the three-finger fold of the LU domains (Ploug, 2003). The other known multi LU-domain members of this protein family, which are all present within this gene cluster, are: PRV1/CD177, TEX101, C4.4A, PRO4356 and GPQH2552.
Description	24254 bp; 7 exons (Figure 1).
Transcription	Transcription of the uPAR gene is regulated by a TATA-less proximal promoter, partly through binding to SP1 (Soravia et al., 1995).

Note	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.
	Figure 2: Structure of the uPAR protein. Panel A - Schematic representation of the amino acid sequence of human uPAR showing its three homologous LU domains. Consensus disulfide bonds defining the LU domains are coloured black. The position of the C-terminal glycolipid anchor (GPI) is shown (modified from Ploug and Ellis, 1994, with permission). Insert: The archetypical three-finger fold is illustrated by a ribbon diagram for a single secreted LU-domain protein (snake venom toxin-a) using the PDB coordinates 1NEA and PyMOLTM (DeLano Scientific). Panel B - LU domain signatures in the primary sequence of human uPAR. The three LU domains (DI, DII and DIII) of uPAR are aligned with the consensus structures being highlighted (disulfide bonds in yellow and the invariant asparagines in red). The number of residues between the individual cysteines is represented by dots or numbers in brackets (modified from Kjaergaard et al., 2008). Panel C - The crystal structure solved for uPAR in complex with a peptide antagonist is shown as a ribbon diagram (Llinas et al., 2005). The individual LU domains are colour-coded (DI in yellow, DII in blue and DIII in red), and N-linked carbohydrates are shown as white sticks. The attachment to the cell surface by a glycolipid anchor is modelled in this cartoon. The insert shows uPAR in a surface representation, with the hydrophobic ligand-binding cavity marked with hatched lines; carbon, nitrogen and oxygen atoms are coloured white, blue and red, respectively. These structures are visualized by PyMOLTM (DeLano Scientific), using the PDB coordinates 1YWH (reproduced from Kjaergaard et al., 2008).
Description	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).
Expression	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
	Figure 3: Expression of uPAR in cancer tissue. Peroxidase stainings with the same batch of a polyclonal antibody against uPAR produced at the Finsen Laboratory (Copenhagen, Denmark), showing reactivity in stromal cells associated with the tumour. Panel A - Primary colorectal adenocarcinoma (1) and a corresponding liver metastasis (2) (reproduced from Illemann et al., 2009). Panel B - Ductal carcinoma in situ lesion of the breast with microinvasion (1) and normal breast (2) (reproduced from Nielsen et al., 2007). Panel C - Gastric cancer, intestinal subtype (courtesy of W. Alpízar-Alpízar, Finsen Laboratory, Copenhagen, Denmark). Panel D - Prostate carcinoma (reproduced from Usher et al., 2005, Copyright (2004, Wiley-Liss, Inc.), with permission of John Wiley & Sons, Inc.).
Localisation	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).
Function	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).
	Figure 4: uPAR ligands. Schematic representation of the functions of uPAR via its interaction with uPA, integrins, and vitronectin (modified from Kjaergaard et al., 2008).
Homology	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).

	As a result of the well-established correlation of uPAR to tumour invasion and metastasis, several targeting strategies with therapeutic potential have been devised to interfere with the receptor (Mazar, 2008): A series of small molecule antagonists of the uPA-uPAR interaction have been identified by screening in chemical libraries (Tyndall et al., 2008). Synthetic peptide antagonists have also been developed, either on the basis of the receptor-binding region of human uPA (Figure 5A), or by combinatorial chemistry of lead compounds selected by random phage-display technology (Figure 5B) (Reuning et al., 2003; Rømer et al., 2004; Jacobsen and Ploug, 2008). The most potent of the latter group (the AE105 peptide) has already shown its applicability in vivo. Conjugation of modified versions of this peptide with the metal chelator DOTA enabled its use in the non-invasive molecular bioimaging of uPAR expression in xenotransplanted human glioblastomas in nude mice by PET scanning using the positron emitter 64Cu (Figure 5C; Li et al., 2008). The biodistributions of 111In- and 213Bi-labelled peptide derivatives of AE105 were also explored (Liu et al., 2009; Knör et al., 2008), the latter with a view to subsequent studies on uPAR-targeted radiotherapy. Monoclonal antibodies are being developed aiming at inhibiting the uPA-uPAR interaction and thereby providing a means of cancer patient treatment (Bauer et al., 2005; Jögi et al 2007; Pass et al., 2007). Anti-tumour toxins targeting uPAR include engineered anthrax toxins that rely on activation by receptor-bound uPA to unleash their cytotoxicity (Liu et al., 2001), and fusion proteins encompassing the uPAR-binding part of uPA linked to e.g. diphtheria toxins (Rustamzadeh et al., 2003). In the field of gene therapy, downregulation of uPAR gene expression has been done by anti-sense or siRNA technology (Pillay et al., 2007). New strategies are constantly unfolding regarding the intervention of the uPA-uPAR interaction, as illustrated by the recent reports of synthetic self-assembly nanoparticles taken up by uPAR-expressing cells via receptor-mediated endocytosis (Wang et al., 2009), molecular imaging of pancreatic cancer using dual-modality nanoparticles (Yang et al., 2009), and an oncolytic measles virus retargeted against the receptor with potent anti-tumour effect in a breast cancer xenograft model (Jing et al., 2009).

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