The urokinase receptor: involvement in cell surface proteolysis and cancer invasion

The receptor for urokinase plasminogen activator is present in plasma from healthy donors and elevated in patients with paroxysmal nocturnal haemoglobinuria

The urokinase plasminogen activator (uPA) is a proteolytic enzyme which converts the proenzyme plasminogen to the active serine protease plasmin. A cell surface receptor for uPA (uPAR) is attached to the cell membrane by a glycosyl-phosphatidylinositol anchor. Binding of uPA to uPAR leads to an enhanced plasmin formation and thereby an amplification of pericellular proteolysis. We have shown previously that uPAR is expressed on normal blood monocytes and granulocytes, but is deficient on affected blood monocytes and granulocytes in patients with paroxysmal nocturnal haemoglobinuria (PNH), and that uPAR is present in plasma from these patients. In this study a newly established sensitive enzyme-linked immunosorbent assay (ELISA) has been applied for quantitation of uPAR in plasma. Unexpectedly, we found that uPAR is not only present in PNH plasma but also in plasma from healthy individuals. In 39 healthy individuals the mean plasma-uPAR value +/- SD was 31 +/- 15 pM, median 28 (range 11-108), and the corresponding value for six PNH patients was 116 +/- 67 pM, median 90 (range 61-228). The elevated uPAR-level in PNH patients was highly significant (Mann-Whitney test; P < 0.0001), and may possibly contribute to the propensity for thrombosis in PNH by inhibition of the fibrinolytic system. Binding of pro-uPA by uPAR in plasma may interfere with the appropriate binding of pro-uPA to cell-bound uPAR and therefore inhibit cell-associated plasmin generation and fibrinolysis. It is likely that the uPAR in normal plasma reflects the overall level of activity of the uPAR-mediated cell surface proteolysis. The present ELISA may be used for studies of uPAR levels in plasma from patients with conditions in which this activity might be increased, such as cancer and inflammatory disorders. Future studies will determine if uPAR in plasma is a parameter of clinical importance in these diseases.

Urokinase receptor in breast cancer tissue extracts. Enzyme-linked immunosorbent assay with a combination of mono- and polyclonal antibodies

Urokinase plasminogen activator (uPA) is a proteolytic enzyme involved in degradation of the extracellular matrix during cancer invasion. The levels of uPA and its inhibitor PAI-1 in tumor extracts have previously been demonstrated to be of prognostic value in breast cancer as well as other types of cancer. We have previously characterized a specific cell surface receptor for uPA (uPAR) which strongly enhances the catalytic activity of uPA and is expressed during mammary cancer invasion. In order to quantitate uPAR in breast cancer tissue, we have now developed a sensitive enzyme-linked immunosorbent assay (ELISA), with polyclonal catching antibodies and three monoclonal detecting antibodies. The detection limit of the assay is approximately 0.16 fmol of uPAR in a volume of 100 microliters (1.6 pM). There is a linear relationship between signal and uPAR concentration up to at least 6.6 fmol per 100 microliters (66 pM). Both free uPAR and uPAR in complex with uPA is detected. The recovery of an internal uPAR standard in breast cancer tissue extracts is above 87%. The intra-assay and inter-assay variation coefficients are 7% and 13%. In order to find a suitable buffer for extraction of various components of the uPA-system from breast cancer tissue, we tested buffers which previously have been used for optimal extraction of estrogen receptor (A), uPA (B), and uPAR (C). Buffer A and B extracted approximately 30% and 50%, respectively, of the amount of uPAR extracted with buffer C.(ABSTRACT TRUNCATED AT 250 WORDS)

Overexpression of plasminogen activator inhibitor 2 in human melanoma cells inhibits spontaneous metastasis in scid/scid mice

A metastatic human melanoma cell line that produces urokinase-type plasminogen activator was stably transfected with cDNA encoding human plasminogen activator inhibitor 2 (PAI-2). Transfected clones expressed PAI-2 at levels two to nine times higher than both the parental cell line and mock transfectants, as detected by ELISA of cell lysates and conditioned medium. The clone with the highest PAI-2 expression exhibited complete inhibition of soluble and cell-surface-bound plasminogen activator activity. The level of PAI-2 overexpression in these clonal cell lines correlated positively with the inhibition of their ability to degrade extracellular matrix in vitro. Parental, mock-transfected, and PAI-2-transfected cell lines produced rapidly growing tumors when injected s.c. into the skin of mice with severe combined immunodeficiency. The tumors producing the highest levels of PAI-2 were surrounded by a dense tumor capsule. Both parental cells and mock-transfected cells invariably metastasized from s.c. tumors to lymph nodes and lungs of mice. PAI-2-transfected cell lines produced significantly less or no metastases. Taken together, these data indicate a critical role for plasminogen activator activity in melanoma invasion and metastasis.

High-affinity urokinase receptor antagonists identified with bacteriophage peptide display

Affinity selection of a 15-mer random peptide library displayed on bacteriophage M13 has been used to identify potent ligands for the human urokinase receptor, a key mediator of tumor cell invasion. A family of receptor binding bacteriophage ligands was obtained by sequentially and alternately selecting the peptide library on COS-7 monkey kidney cells and baculovirus-infected Sf9 insect cells overexpressing the human urokinase receptor. Nineteen peptides encoded by the random DNA regions of the selected bacteriophage were synthesized and tested in a urokinase receptor binding assay, where they competed with the labeled N-terminal fragment of urokinase with IC50 values ranging from 10 nM to 10 microM. All of the isolated peptides were linear and showed two relatively short conserved subsequences: LWXXAr (Ar = Y, W, F, or H) and XFXXYLW, neither of which is found in urokinase or its receptor. Competition experiments demonstrated that the most potent peptide, clone 20, prevented binding of bacteriophage displaying the urokinase receptor binding sequence (urokinase residues 13-32). In addition, this peptide blocked other apparently unrelated receptor binding bacteriophage, suggesting overlapping receptor interaction sites for all of these sequences. These results provide a demonstration of bacteriophage display identifying peptide ligands for a receptor expressed on cells and yield leads for the development of urokinase receptor antagonists.

Immunohistochemical detection of the receptor for urokinase plasminogen activator in human colon cancer

Paraffin-wax embedded specimens from 30 cases of colonic adenocarcinoma were investigated for immunoreactivity for the receptor of urokinase-type plasminogen activator (uPAR). In all cases there was a strong signal, predominantly at the invasive foci. The positive cells were mainly tumour-infiltrating macrophages but neutrophils and eosinophils were also strongly stained. The neoplastic cells were positive in 19 of the samples with staining of occasional or a moderate number of cells. In uninvolved, normal-appearing mucosa adjacent to the malignant infiltrates, immunostaining of both macrophages and neutrophils was seen, but the labelling was less intense than that seen in the malignant lesions. Weak to moderate staining of normal intestinal epithelium was also seen at the luminal surface. Comparison between immunoreactivity and in situ hybridization showed a similar distribution of protein and mRNA with two exceptions: first, neutrophils (strongly immunoreactive for uPAR) were negative or only weakly positive for uPAR/mRNA; and second, many cancer cells at invasive foci showed prominent hybridization signals but no detectable uPAR immunoreactivity. Together with previous findings of urokinase plasminogen activator (uPA) protein and mRNA being expressed in tumour-infiltrating fibroblast-like cells at the invasive foci, these results support the view that the uPA pathway of plasminogen activation is involved in tissue degradation in colon cancer. The results also extend and consolidate an emerging picture of non-neoplastic tumour stromal cells producing molecules involved in the generation and regulation of extracellular proteolysis in cancer.

Proteolysis and invasiveness of brain tumors: role of urokinase-type plasminogen activator receptor

The cellular receptor for urokinase-type plasminogen activator (uPAR) in glioblastoma cell lines has been identified and found to be similar to the uPAR expressed by other tumor cell lines. Increased levels of uPAR have been found in primary malignant brain tumor tissues, especially highly malignant glioblastoma, and, to a lesser degree, in malignant astrocytomas, suggesting that this receptor might be involved in efficient activation of pro-uPA and confinement of uPA activity on the cell surface of invading brain tumors. The cell surface uPARs in gliomas could constitute an optimum environment for the generation and activity of plasmin, which is known to play a crucial role in the dissolution of the extracellular matrix during tumor cell invasion. In situ hybridization studies have shown that uPAR mRNA is expressed abundantly in tumor cells and is consistently present at the invasive edges of malignant gliomas. These results imply that uPAR is involved in plasmin-catalyzed proteolysis during glioma invasion and that interference with the uPA:uPAR interactions could constitute a novel approach for developing therapeutic strategies to counteract invasion of brain tumors.

Protease inhibitors: role and potential therapeutic use in human cancer

Proteases and protease inhibitors have been increasingly recognised as important factors in the physiopathology of human diseases, and our understanding of their role in cancer has dramatically increased over the last decade. We have obtained causal evidence linking proteases to tumour invasion and metastasis, and have become aware of genuine mechanisms used by tumour cells to optimise the use of proteases in the pericellular matrix. Many synthetic and natural inhibitors of these proteases have also been characterised, and their mechanisms of interaction with their corresponding enzymes are progressively unveiled as the X-ray crystal structures of these enzymes and their inhibitors are now reported. It has also become evident that many of these inhibitors, in addition to preventing the dissemination of cancer cells, have an inhibitory effect on tumour growth. Thus protease inhibitors are emerging as potentially therapeutic tools to treat cancer. In this article, recent studies on the role of proteases and their inhibitors in cancer are reviewed, and current ideas on their potential use as therapeutic agents are discussed.

A novel, specific pro-urokinase complex on monocyte-like cells, detected by transglutaminase-catalyzed cross-linking

Radiolabeled pro-urokinase plasminogen activator (pro-uPA) was cross-linked to a specific protein on the surface of human monocyte-like U937 cells in a reaction catalyzed by tissue transglutaminase. The conjugate formed with this unknown component had a much higher molecular weight (apparent M(r) 250,000-300,000) than the complex of pro-uPA and the urokinase plasminogen activator receptor (uPAR). There was a strong preference for the pro-form of uPA. The conjugate was recognized by antibodies against uPA but not by anti-uPAR antibodies. Nevertheless, the blocking of uPAR with a monoclonal antibody abolished the formation of the conjugate, thus showing a role of uPAR in this process.

Specific inhibition of the activity of the urokinase receptor-mediated cell-surface plasminogen activation system by suramin

Urokinase-type plasminogen activator (uPA) is involved in generating the proteolytic activity necessary for invasive processes, and is dependent on binding to its specific cellular receptor (uPAR) for efficient function. We report here that the polysulphonated napthylurea compound suramin inhibits the activity of this cell-associated proteolytic system, in a manner independent of its antagonism of the uPA-uPAR interaction [Behrendt, Rønne and Danø (1993) J. Biol. Chem. 268, 5985-5989], occurring at a 25-100-fold-lower suramin concentration. This inhibition was found to be due to effects on the activation of both pro-uPA and plasminogen. Suramin inhibited plasmin activation of pro-uPA by a non-competitive mechanism (Ki approx. 2 micrograms/ml), which did not involve a direct effect on plasmin catalytic activity. Similarly, its effect on plasminogen activation was not due to a direct inhibition of uPA. The inhibition of plasminogen activation, which occurred exclusively with receptor-bound uPA, appeared to be due to a reversal of the favourable kinetics which result from the activation of cell-associated plasminogen, although suramin did not inhibit the cellular binding of 125I-labelled plasminogen. This suggests that this effect is due to interference with interactions between components of this system on the cell surface, and that suramin may be useful in gaining further insight into the molecular mechanisms involved in the functional assembly of this proteolytic system. Furthermore the effective inhibition of this system by suramin indicates an anti-invasive potential that may contribute to the anti-tumour effect of suramin in vivo.

An alternatively spliced variant of mRNA for the human receptor for urokinase plasminogen activator

Using 3' RACE (rapid amplification of cDNA ends), we have isolated a cDNA variant for the receptor for human urokinase plasminogen activator (uPAR). The deduced protein includes the amino-terminal ligand binding domain in uPAR, but lacks the carboxy-terminal membrane attachment by a glycolipid anchor. Genomic DNA analysis showed that the uPAR mRNA variant is generated by alternative splicing. The new variant mRNA is expressed in various human cell lines and tissues and both variants are up-regulated by phorbol ester in A549 cells. We propose that the alternatively spliced uPAR mRNA encodes a soluble uPA binding protein, the possible function of which is discussed.