Prevention of metastasis by inhibition of the urokinase receptor

Co-expression of urokinase, urokinase receptor and PAI-1 is necessary for optimum invasiveness of cultured lung cancer cells

We investigated the importance of the urokinase (uPA)-plasmin system in fostering invasion of human lung cancer cells through artificial basement membranes composed of Matrigel. Eight cell lines (including 1 small cell and 7 non-small cell lines) were examined. One cell line did not express any components of the urokinase system. Four cell lines had substantial levels of endogenous uPA detectable on their surfaces. Three of these cell lines co-expressed the plasminogen activator inhibitor PAI-1 in addition to uPA. Assays for invasiveness revealed 4 cell lines capable of traversing a Matrigel barrier, including the 3 which co-expressed uPA, PAI-1 and uPA receptor. Surprisingly, the cell line expressing only uPA and uPA receptor displayed no invasive capacity despite levels of secreted uPA more than 20-fold higher than the other cell lines studied. Based on these observations, we hypothesized that both uPA and PAI-1 might be important for invasion by lung tumor cells, at least in vitro. We therefore tested polyclonal antibodies which inhibit uPA and PAI-1 activity for their effects on the highly invasive H292 cell line. After 3 days, invasive capacity was inhibited by antibodies to both uPA and PAI-1 in a dose-dependent manner. The plasmin inhibitor aprotinin reduced H292 cell invasion by 70%. Taken together, our data demonstrate that in cultured human lung cancer cells the uPA-plasmin system is important in promoting invasion into basement membranes and suggest that a critical balance between uPA and PAI-1 is necessary for optimal invasiveness. Our data are consistent with results from recent clinical studies showing that PAI-1 expression in tumor tissue is an adverse prognostic feature.

Structure of the human urokinase receptor gene and its similarity to CD59 and the Ly-6 family

Urokinase plasminogen activator receptor (uPAR) gene expression has been implicated in many important biological processes including cell invasiveness and migration. The uPAR gene was cloned from a human genomic library by hybridization with a uPAR cDNA. The complete structure of the human uPAR gene, including a 21.23-kb transcription unit with 204 bp 5' and 239 bp 3' flanking sequences, was determined by comparison with the uPAR cDNA sequence. The uPAR gene is composed of seven exons and six introns. The seven exons of 101, 111, 144, 162, 135, 147 and 563 bp are separated by six introns of approximately 2.04, 2.62, 8.42, 0.906, 3.10 and 2.78 kb. Exons 1-7 encode 19, 37, 48, 54, 45, 49 and 83 amino acid residues, respectively. A CpG-rich island and sequences related to the transcription factors AP-1, AP-2, c-Jun and NF kappa B are present, but no potential TATA or CAAT boxes were found in the proximal 5' region of the uPAR gene. Comparison of the exon organization of the uPAR gene with that of human CD59 and murine Ly-6 reveals similarity to all three domains encoded by the uPAR exons (2 + 3), (4 + 5) and (6 + 7). These data enable elucidation of the mechanisms involved in regulation of the uPAR gene expression and provide further evidence that the uPAR gene belongs to the Ly-6 superfamily.

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.

Augmented urokinase receptor expression in atheroma

Smooth muscle cell proliferation and migration into neointima are hallmarks of atherogenesis. However, mechanisms responsible have not yet been fully elucidated. One potential mediator of both smooth muscle cell proliferation and migration is activation of plasminogen by activators bound to receptors on cells within the vessel wall. To determine whether vascular smooth muscle cells within atheroma express the receptor for urokinase-type plasminogen activator (uPA-R), we characterized atheroma in cholesterol-fed New Zealand White rabbits and human subjects by immunostaining. Intense immunostaining of uPA-R was observed throughout the neointima in both rabbit and human atherosclerotic lesions with the use of a monoclonal antibody to uPA-R. uPA-R was not detectable in normal arterial tissues. uPA-R was localized to macrophages and neointimal smooth muscle cells identified by immunostaining in serial sections. Furthermore, uPA-R protein in extracts from atheroma was present in at least a ninefold greater quantity compared with extracts from normal vessels, as shown by Western blotting. Expression of uPA-R mRNA in migrating vascular smooth muscle cells did not increase significantly. Thus, altered posttranscriptional regulation may be contributing to the increased uPA-R. In vitro, antibodies to uPA-R delayed the migration of cultured vascular smooth muscle cells. Our results suggest that increased cell-surface uPA-R contributes to pericellular proteolysis and consequently increased neointimalization secondary to increased vascular smooth muscle cell migration in atheroma.

Blockage of the urokinase receptor on the cell surface: construction and characterization of a hybrid protein consisting of the N-terminal fragment of human urokinase and human albumin

Receptor-bound urokinase is likely to be a crucial determinant in both tumor invasion and angiogenesis. We report here that a yeast-derived genetic conjugate between human serum albumin and the 1-135 N-terminal residues of urokinase (u-PA) competitively inhibits the binding of exogenous and endogenous u-PA to its cell-anchored receptor (u-PAR). This hybrid molecule (ATF-HSA) also inhibits in vitro pro-urokinase-dependent plasminogen activation in the presence of u-PAR bearing cells. These effects are probably responsible for the observed in vitro inhibition of tumor cell invasion in a reconstituted basement membrane extract (Matrigel).

Molecular and cellular basis of cancer invasion and metastasis: implications for treatment

In the past decade significant advances in establishing the underlying biological mechanisms of tumour invasion and metastasis have been made. Some of the triggering factors and genes relevant to metastatic spread have been identified. Advances have also been made in understanding the signal transduction pathways involved in invasion and metastasis. This increased comprehension of the malignant metastatic process has enabled new antimetastatic strategies to be devised. This review summarizes progress in these areas and discusses the implications for the treatment of metastasis.

Human urokinase receptor expression is inhibited by amiloride and induced by tumor necrosis factor and phorbol ester in colon cancer cells

The modulation of urokinase plasminogen activator receptor (uPAR) gene expression by tumor necrosis factor alpha (TNF alpha), phorbol ester (PMA) and amiloride was studied in three colon cancer cell lines. uPAR mRNA and protein were induced by TNF alpha and by PMA but were inhibited by amiloride at concentrations of 0.1 to 1 mM in the presence or absence of TNF alpha and PMA. Nuclear run-on transcription assay indicated that the effects of amiloride and TNF alpha were mediated at least in part at the transcriptional level, whereas PMA may act in part via a posttranscriptional mechanism. These results suggested that uPAR gene expression is modulated by multiple signal transduction pathways.

The alpha v beta 6 integrin promotes proliferation of colon carcinoma cells through a unique region of the beta 6 cytoplasmic domain

Cell-matrix interactions are assumed to be important in regulating differentiation and tumor cell growth; however, the precise roles of individual matrix receptors in producing cellular responses are still unclear. We have previously described the alpha v beta 6 integrin, an epithelial cell fibronectin receptor expressed in many carcinoma cell lines. Here we show that heterologous expression of alpha v beta 6 in a human colon carcinoma cell line (SW480) enhances the proliferative capacity of these cells, both in vitro and in vivo in nude mice. This property of alpha v beta 6 correlates with the presence of an 11-amino acid region at the COOH terminus of the beta 6 cytoplasmic domain. This 11-amino acid sequence is required for the growth stimulatory effect, but not for other functions of the beta 6 cytoplasmic domain, such as promoting cell adhesion and focal contact localization.

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.

Chromosomal localization of the human urokinase plasminogen activator receptor and plasminogen activator inhibitor type-2 genes: implications in colorectal cancer

Activation of the proenzyme of urokinase (uPA) on the surface of cancer cells has been implicated in the initiation of focal proteolytic mechanisms that permit invasion and metastasis by colon cancers. The activity of uPA on the cell surface appears to be a function of the number of uPA-specific receptors (uPAR) and the extent of inhibition of uPA by plasminogen activator inhibitors (PAI). The mapping of the genes coding for uPAR, and for PAI-2, was performed to determine whether their chromosomal localization suggested their involvement in the genetic alterations associated with cancer cell DNA. This study confirms the localization of the human urokinase plasminogen activator receptor gene to chromosome 19q and, using in situ hybridization, provides a precise localization to chromosome 19q13.2. In addition, our results confirm the previous allocation of the human plasminogen activator inhibitor-2 gene to a location 18q21.3-->18q22.1, a location that corresponds to the commonest (> 70%) somatic deletions found in colorectal carcinomas. The mapping of the uPAR and PAI-2 genes enables the elucidation of their possible involvement in the genetic alterations that determine the invasive and metastatic phenotypes in colorectal cancer.

Expression of uPA and its receptor by both neoplastic and stromal cells during xenograft invasion

Recent studies have shown that molecules involved in generation and regulation of extracellular proteolytic activity are often expressed by non-malignant stromal cells during human cancer invasion. We have studied the expression of the urokinase-type plasminogen activator and the urokinase-type plasminogen activator cell-surface receptor in xenografts of human MDA-MB-231 mammary carcinoma cells growing invasively in nude mice. Northern analysis showed the presence of both human and mouse urokinase-type plasminogen activator and urokinase-type plasminogen activator receptor mRNA in tumor extracts. By in situ hybridization, mRNA for human urokinase-type plasminogen activator and its receptor was detected in virtually all the cancer cells, while mouse urokinase-type plasminogen activator and urokinase-type plasminogen activator receptor mRNA was expressed by tumor-infiltrating fibroblast-like and macrophage-like cells. In invasive areas the cells expressing the 2 murine mRNAs were either the same or located immediately adjacent to each other. This model system has several advantages for studies of the mechanism by which cancer cells induce or recruit stromal cells to produce molecules involved in proteolysis.

Recombinant soluble urokinase receptor as a scavenger for urokinase-type plasminogen activator (uPA). Inhibition of proliferation and invasion of human ovarian cancer cells

A recombinant soluble human urokinase receptor comprising amino acids 1-277 was cloned and transfected into CHO cells. The mutant protein (rec-uPAR277), purified from the CHO cell supernatant by affinity chromatography on immobilized urokinase (uPA), in a four-fold excess, completely abolished the binding of FITC-labeled pro-uPA to the human ovarian cancer cell line, OV-MZ-6. This invasive and tumorigenic cancer cell line expresses uPA, its inhibitor PAI-1, and the high-affinity receptor for uPA, uPAR. Rec-uPAR277 significantly reduced the proliferation of OV-MZ-6 cells in a concentration-dependent manner without altering the viability of the cells. Invasion of OV-MZ-6 cells tested in an in vitro Matrigel invasion assay was inhibited by rec-uPAR277 up to 75%. In conclusion, these results demonstrate that rec-uPAR277 can function as a scavenger for uPA in vitro by inhibiting proliferation and invasion of human cancer cells.

Osteolytic bone metastasis in breast cancer

Metastasis of breast cancer cells to bone consists of multiple sequential steps. To accomplish the process of metastasis to bone, breast cancer cells are required to intrinsically possess or acquire the capacities that are necessary for them to proliferate, invade, migrate, survive, and ultimately arrest in bone. These capacities are essential for any cancer cells to develop distant metastases in organs such as lungs and liver as well as bone. Once breast cancer cells arrest in bone, bone is a storehouse of a variety of cytokines and growth factors and thus provides an extremely fertile environment for the cells to grow. However, breast cancer cells are unable to progress in bone unless they destroy bone with the assistance of bone-resorbing osteoclasts. Thus, the capacity of breast cancer cells to collaborate with osteoclasts is likely to be specific and is likely critical for them to cause osteolytic bone metastases. Evidence to support the concept that there is an intimate relationship between breast cancer cells and osteoclasts is described using an in vivo bone metastasis model in which human breast cancer cells are inoculated into the left ventricle of nude mice. The roles of cell adhesion molecules including cadherins and laminin and matrix metalloproteinases in the development of osteolytic bone metastases by breast cancer are also discussed.

Matrix-degrading proteases in hormone-dependent breast cancer

Proteases have emerged as important modulators of the metastatic capacity of cancer. However, metastasis is regulated by multiple other characteristics of the tumor cell and evidence suggests the participation of multiple classes of proteases. In the present article we review the literature concerning the potential biological roles of multiple proteases in breast cancer. In particular, we focus on the gelatin-degrading metallo proteinase and on a novel 80 KDa matrix-degrading protease that appears to be commonly expressed in hormone dependent breast cancer cell lines.

Activities, localizations, and roles of serine proteases and their inhibitors in human brain tumor progression

The plasminogen activation system consists of plasminogen activators and their inhibitors, serine proteases, and serpins. The proteases and inhibitors regulate a variety of processes in tissue morphogenesis, differentiation, cell migration, and cancer cell invasiveness and metastasis. One of the plasminogen activators, urokinase-type plasminogen activator (uPA), binds to a specific surface and provides a localized cell surface proteolytic activity required for the destruction of extracellular matrix, which is a vital step in tumor cell invasion. The proteolytic activity of uPA is modulated by its cell surface receptor, as well as by plasminogen activator inhibitor type-1 (PAI-1) and, to a lesser degree, by other inhibitors. The role of plasminogen activators and their inhibitors in cancer invasion can be demonstrated in the development and progression of malignant brain tumors. Our findings indicate that uPA and PAI-1 expression are dramatically upregulated in malignant brain tumors in parallel with the histological progression of the tumors. The results suggest that these molecules may contribute to tumor invasion in addition to their significant role in angiogenesis. An evaluation of the plasminogen activation system could add diagnostic and prognostic significance to the evaluation of individual patients.

Comparative aspects of neoplastic invasion of the brain

This review of the invasion of the human brain by tumors is based upon a comparative approach. The taxonomic distribution of a structure of the CNS to be considered a brain is present in the following taxonomic groups: Mollusca (Cephalopoda, Pulmonata) Sipunculida, Echiurida, Annelida, Arthropoda and Vertebrata (fishes, amphibians, reptiles, birds and mammals). The comparative approach (inter-, and intraspecies specific) provides a more informative, indicative, understanding of invasion of the human brain; especially in view of the morphological and functional heterogeneity of structures playing a role in the neoplastic invasion to and from the brain. It can be distinguished among primary invasiveness (as cancer in situ) and local recurrence on the one hand and remote spreading, such as metastasis, on the other. A review of the recent literature arranged according to tumor types is provided and comparative conclusions elucidated, especially to underline the tumor specificity of the invasion of brain tumors, especially the second or intra-species specific approach is therapeutically important. This is set forth against a background of epidemiology and species specificity. Due to the lack of bony skull capsule an extensive knowledge of the brain of cephalopod molluscs with its functional and morphological similarity to the human eye, and also that of the pulmonate Helix pomatia, the vineyard snail, could well serve as excellent oncologic models. Some conclusions can be drawn dealing with the theoretical possibility of first appearance of the different and heterogeneous components of the complicated structure known as brain including its supporting accessory organs during phylogeny.