Expression of plasminogen activator inhibitor type 1 by human prostate carcinoma cells inhibits primary tumor growth, tumor-associated angiogenesis, and metastasis to lung and liver in an athymic mouse model

Human Kallikrein 2 (hK2) and prostate-specific antigen (PSA): two closely related, but distinct, kallikreins in the prostate

Recent studies on human kallikrein 2 (hK2) have revealed striking similarities and significant differences with the closely related kallikrein PSA. Both PSA and hK2 are primarily localized to the prostate and share close structural similarities. Although both kallikreins are produced by the same secretory epithelial cells in the prostate, hK2 is associated more with prostate tumors than PSA and is highly expressed in poorly differentiated cancer cells. The potent trypsin-like activity of hK2 contrasts with the weak chymotrypsin-like activity of PSA. The inactive precursor form of PSA, proPSA, is converted rapidly to active PSA by hK2, suggesting an important in vivo regulatory function by hK2 on PSA activity. The high homology between hK2 and PSA results in significant cross-reactivity to hK2 by polyclonal and some monoclonal antibodies to PSA. Future studies on both PSA and hK2 need to take into account this potential for cross-reactivity. Specific monoclonal antibodies to hK2 have now demonstrated that serum levels of hK2, like PSA, are correlated with prostate cancer. The production of hK2 protein in active protease form and specific monoclonal antibodies to the hK2 antigen will allow extensive future studies delineating the physiological and clinical utility of this new prostate antigen.

Inhibition of Metastasis of Intraocular Melanomas by Adenovirus-Mediated Gene Transfer of Plasminogen Activator Inhibitor Type 1 (PAI-1) in an Athymic Mouse Model

Uveal melanoma is the most common intraocular malignancy in adults and results in the death of 50% of the patients. Plasminogen activators (PA) are believed to facilitate tumor metastasis by promoting invasion of tissue barriers. The present study explored the possibility of preventing the metastasis of intraocular melanomas by disrupting plasminogen activator function through gene transfer. A replication-deficient adenovirus vector was used for the in vivo transfer of plasminogen activator inhibitor type 1 (PAI-1) cDNA. Intraocular injection of an adenovirus vector (AdCMV-PAI-1) expressing plasminogen activator inhibitor-1 resulted in: (1) the transduction of more than 95% of human and murine uveal melanoma cells in the eyes of nude mice; (2) a 50% reduction in the number of animals developing liver metastases; and (3) a 78% reduction in the metastatic tumor burden in animals that eventually developed metastases. In other experiments intravenous injections of AdCMV-PAI-1 resulted in transduction of normal liver cells and culminated in a sharp reduction in the incidence of metastases and a significant prolongation of host survival. The results support the feasibility of disruption of PA function through gene transfer as a therapeutic strategy for preventing metastases and prolonging host survival.

Plasminogen activators, integrins, and the coordinated regulation of cell adhesion and migration

Cellular migration is critically dependent on an interplay between forces of attachment and detachment. Recent studies show that the serine protease urokinase and its major inhibitor and receptor regulate the adhesive properties of integrins, at least in part through initiation of cellular signals. These new functions for an old protease system imply intricate connections between proteolysis and adhesion that operate at the cell surface to regulate migration.

Inhibition of Metastasis of Intraocular Melanomas by Adenovirus-Mediated Gene Transfer of Plasminogen Activator Inhibitor Type 1 (PAI-1) in an Athymic Mouse Model

Uveal melanoma is the most common intraocular malignancy in adults and results in the death of 50% of the patients. Plasminogen activators (PA) are believed to facilitate tumor metastasis by promoting invasion of tissue barriers. The present study explored the possibility of preventing the metastasis of intraocular melanomas by disrupting plasminogen activator function through gene transfer. A replication-deficient adenovirus vector was used for the in vivo transfer of plasminogen activator inhibitor type 1 (PAI-1) cDNA. Intraocular injection of an adenovirus vector (AdCMV-PAI-1) expressing plasminogen activator inhibitor-1 resulted in: (1) the transduction of more than 95% of human and murine uveal melanoma cells in the eyes of nude mice; (2) a 50% reduction in the number of animals developing liver metastases; and (3) a 78% reduction in the metastatic tumor burden in animals that eventually developed metastases. In other experiments intravenous injections of AdCMV-PAI-1 resulted in transduction of normal liver cells and culminated in a sharp reduction in the incidence of metastases and a significant prolongation of host survival. The results support the feasibility of disruption of PA function through gene transfer as a therapeutic strategy for preventing metastases and prolonging host survival.

Characterization of the precursor of prostate-specific antigen. Activation by trypsin and by human glandular kallikrein

The precursor or zymogen form of prostate-specific antigen (pro-PSA) is composed of 244 amino acid residues including an amino-terminal propiece of 7 amino acids. Recombinant pro-PSA was expressed in Escherichia coli, isolated from inclusion bodies, refolded, and purified. The zymogen was readily activated by trypsin at a weight ratio of 50:1 to generate PSA, a serine protease that cleaves the chromogenic chymotrypsin substrate 3-carbomethoxypropionyl-L-arginyl-L-prolyl-L-tyrosine-p-nitroanili ne- HCl (S-2586). In this activation, the amino-terminal propiece Ala-Pro-Leu-Ile-Leu-Ser-Arg was released by cleavage at the Arg-Ile peptide bond. The recombinant pro-PSA was also activated by recombinant human glandular kallikrein, another prostate-specific serine protease, as well as by a partially purified protease(s) from seminal plasma. The recombinant PSA was inhibited by alpha1-antichymotrypsin, forming an equimolar complex with a molecular mass of approximately 100 kDa. The recombinant PSA failed to activate single chain urokinase-type plasminogen activator, in contrast to the recombinant hK2, which readily activated single chain urokinase-type plasminogen activator. These results indicate that pro-PSA is converted to an active serine protease by minor proteolysis analogous to the activation of many of the proteases present in blood, pancreas, and other tissues. Furthermore, PSA is probably generated by a cascade system involving a series of precursor proteins. These proteins may interact in a stepwise manner similar to the generation of plasmin during fibrinolysis or thrombin during blood coagulation.

The urokinase-type plasminogen activator system in cancer metastasis: a review

The urokinase-type plasminogen activator (u-PA) system consists of the serine proteinases plasmin and u-PA; the serpin inhibitors alpha2-anti-plasmin, PAI-1 and PAI-2; and the u-PA receptor (u-PAR). Two lines of evidence have strongly suggested an important and apparently causal role for the u-PA system in cancer metastasis: results from experimental model systems with animal tumor metastasis and the finding that high levels of u-PA, PAI-1 and u-PAR in many tumor types predict poor patient prognosis. We discuss here recent observations related to the molecular and cellular mechanisms underlying this role of the u-PA system. Many findings suggest that the system does not support tumor metastasis by the unrestricted enzyme activity of u-PA and plasmin. Rather, pericellular molecular and functional interactions between u-PA, u-PAR, PAI-1, extracellular matrix proteins, integrins, endocytosis receptors and growth factors appear to allow temporal and spatial re-organizations of the system during cell migration and a selective degradation of extracellular matrix proteins during invasion. Differential expression of components of the system by cancer and non-cancer cells, regulated by paracrine mechanisms, appear to determine the involvement of the system in cancer cell-directed tissue remodeling. A detailed knowledge of these processes is necessary for utilization of the therapeutic potential of interfering with the action of the system in cancers.

Plasmin and plasminogen activator inhibitor type 1 promote cellular motility by regulating the interaction between the urokinase receptor and vitronectin

The urokinase receptor (uPAR) coordinates plasmin-mediated cell-surface proteolysis and promotes cellular adhesion via a binding site for vitronectin on uPAR. Because vitronectin also binds plasminogen activator inhibitor type 1 (PAI-1), and plasmin cleavage of vitronectin reduces PAI-1 binding, we explored the effects of plasmin and PAI-1 on the interaction between uPAR and vitronectin. PAI-1 blocked cellular binding of and adhesion to vitronectin by over 80% (IC50 approximately 5 nM), promoted detachment of uPAR-bearing cells from vitronectin, and increased cellular migration on vitronectin. Limited cleavage of vitronectin by plasmin also abolished cellular binding and adhesion and induced cellular detachment. A series of peptides surrounding a plasmin cleavage site (arginine 361) near the carboxy-terminal end of vitronectin were synthesized. Two peptides spanning res 364-380 blocked binding of uPAR to vitronectin (IC50 approximately 8-25 microM) identifying this region as an important site of uPAR-vitronectin interaction. These data illuminate a complex regulatory scheme for uPAR-dependent cellular adhesion to vitronectin: Active urokinase promotes adhesion and also subsequent detachment through activation of plasmin or complex formation with PAI-1. Excess PAI-1 may also promote migration by blocking cellular adhesion and/or promoting detachment, possibly accounting in part for the strong correlation between PAI-1 expression and tumor cell metastasis.

Manipulating angiogenesis. From basic science to the bedside

Considerable progress has been made recently in understanding the molecular mechanisms of angiogenesis, which like most other biological processes is the result of subtle and often complex interactions between molecules that have regulatory (eg, cytokines and their receptors) and effector (eg, extracellular matrix, integrins, and proteases) functions. The title of this review was chosen to reflect a recent trend in which knowledge acquired through a molecular/cell biological approach is being rapidly transferred to the clinical setting. As a result, by manipulating angiogenesis either positively or negatively, considerable therapeutic benefit can now be envisaged in physiological and pathological settings in which neovascularization is a prominent component.

Melanoma cell migration on vitronectin: regulation by components of the plasminogen activation system

Tumor cell migration and invasion require complex interactions between tumor cells and the surrounding extracellular matrix. These interactions are modified by cell adhesion receptors, as well as by proteolytic enzymes and their receptors. Here, we study the influence of the protease urokinasetype plasminogen activator (uPA) and its receptor (uPAR) on melanoma cell adhesion to, and migration on, the extracellular matrix protein vitronectin (VN). Cell adhesion to VN, but not to type I collagen, is significantly enhanced in the presence of either uPA or its amino-terminal fragment (ATF). Soluble uPAR can inhibit this effect, indicating that uPA/uPAR on melanoma cells can function as a VN receptor. In the absence of bivalent cations, uPA/uPAR can promote cell attachment on VN, but not cell spreading, suggesting that the glycosylphosphatidylinositol (GPI)-anchored uPAR alone is unable to organize the cytoskeleton. Chemotactic melanoma cell migration on a uniform VN matrix is inhibited by uPA and ATF, implying that cell motility decreases when uPA/uPAR acts as a VN receptor. In contrast, plasminogen activator inhibitor I (PAI-I) can stimulate melanoma cell migration on VN, presumably by inhibiting uPA/uPAR-mediated cell adhesion to VN and thereby releasing the inhibition of cell migration induced by uPA. Together, our data implicate components of the plasminogen activation system in the direct regulation of cell adhesion and migration, thereby modulating the behavior of malignant tumor cells.

P52PAI-1 gene expression in butyrate-induced flat revertants of v-ras-transformed rat kidney cells: mechanism of induction and involvement in the morphological response

Sodium n-butyrate-induced flat reversion in v-K-ras oncogene-transformed rat kidney (KNRK) cells is associated with transcriptional activation of the p52PAI-1 gene (which encodes the type-1 inhibitor of plasminogen activator). Butyrate-initiated expression of p52PAI-1 mRNA and protein correlated with induced cell spreading and preceded development of cell-to-substrate focal adhesions. Such undersurface matrix contact structures, which are absent from parental KNRK cells, require proximal PAI-1 deposition for their stabilization. Stimulated p52PAI-1 expression by flat revertants (approximating 25-fold that of control cells) and the accompanying cytoarchitectural reorganization appeared to be programmed responses to butyrate as both events required de novo RNA and protein synthesis, metabolic characteristics consistent with a secondary pathway of gene regulation. To assess the relevance of p52PAI-1 induction to the process of flat reversion more critically, a molecular genetic approach was designed to maintain high-level constitutive p52PAI-1 synthesis in the absence of butyrate. KNRK cells transfected with a Rc/CMVPAI plasmid construct, in which expression of a p52PAI-1 cDNA insert was driven by enhancer-promoter sequences from the immediate-early gene of human cytomegalovirus, formed colonies comprised of flat-revertant-like cells with a greater frequency than did cells transfected with the Rc/CMV vector alone (24.8% and 1.7% respectively). Comparative analysis of randomly selected Rc/ CMVPAI clones indicated that a 10-fold increase in immunoreactive p52PAI-1 protein, relative to Rc/CMV isolates, correlated with generation of the flat phenotype. These data suggest that induced p52PAI-1 expression probably functions in the development of morphological revertants in the KNRK cell system.

Plasminogen activator inhibitor type 2 in breast cancer

The serine protease urokinase plasminogen activator (uPA) is causally involved in cancer invasion and metastasis. Activity of this protease in vivo is controlled principally by two inhibitors, one of which is plasminogen activator inhibitor type 2 (PAI-2). In this study, we show that PAI-2 levels were significantly higher in primary breast carcinomas (n = 152) than benign breast tumours (n = 18). In the primary cancers, PAI-2 levels correlated weakly but significantly with those of uPA and PAI-1, but not with tissue type plasminogen activator (tPA) or uPA receptor (uPAR) levels. Using Northern blotting, mRNA for PAI-2 was found in 28.6% of 49 primary breast cancers. In contrast to findings at the protein level, PAI-2 mRNA levels failed to correlate with those for uPA or PAI-1. After immunocytochemistry with primary cancers, PAI-2 was detected predominantly in the malignant cells of primary carcinomas but was also present in stromal cells. Using the median value as a cut-off point, PAI-2 showed no significant relationship with either disease-free interval or overall survival. However, using an optimum cut-off value, patients with low levels of PAI-2 had a worse outcome than those with a high level. We conclude that, unlike PAI-1, high levels of PAI-2 may be a favourable prognostic marker in breast cancer.