Vascular Smooth Muscle Cells Potentiate Plasmin Generation by Both Urokinase and Tissue Plasminogen Activator-Dependent Mechanisms: Evidence for a Specific Tissue-Type Plasminogen Activator Receptor on These Cells

Activation of the β-Catenin/T-Cell–Specific Transcription Factor/Lymphoid Enhancer Factor-1 Pathway by Plasminogen Activators in ECV304 Carcinoma Cells

Besides its involvement in clot lysis, the plasminogen activator (PA) system elicits various cellular responses involved in cell migration, adhesion, and proliferation and plays a key role in the progression of cancers. β-Catenin interacts with E-cadherins and functions as transcriptional coactivator of the Wnt-signaling pathway, which is implicated in tumor formation when aberrantly activated. We report that tissue-type plasminogen activator (tPA) elicited tyrosine phosphorylation and cytosolic accumulation of an active (non–serine-threonin phosphorylated, nonubiquitinated) form of β-catenin in ECV304 carcinoma cells. tPA-dependent β-catenin activation is mediated through epidermal growth factor receptor (EGFR) transactivation (via Src), suggested by the inhibitory effects of AG1478 and PP2 (specific inhibitors of EGFR and Src, respectively) and by the lack of β-catenin activation in EGFR-negative B82 fibroblasts. EGFR phosphorylation and β-catenin activation were inhibited by plasminogen activator inhibitor 1 and pertussis toxin, two inhibitors of the urokinase-type plasminogen activator (uPA)/uPA receptor system. β-Catenin activation was correlated with the phosphorylation of glycogen synthase kinase-3β through a phosphatidylinositol 3-kinase/Akt-dependent mechanism. Gel shift experiments revealed the activation of β-catenin/T-cell–specific transcription factor (Tcf)/lymphoid enhancer factor-1 (Lef) transcriptional complex, evidenced by an increased binding of nuclear extracts to oligonucleotides containing the cyclin D1 Lef/Tcf site. β-Catenin silencing through small interfering RNA and antisense oligonucleotides inhibited both the tPA-mediated cyclin D1 expression and cell proliferation. A similar activation of the β-catenin pathway was triggered by amino-terminal fragment, the NH2-terminal catalytically inactive fragment of tPA, thus suggesting that this effect was independent of the proteolytic activity of plasminogen activators. In conclusion, the β-catenin/Lef/Tcf pathway is activated by tPA and is involved in cell cycle progression and proliferation.

Characterization of Cell-Associated Plasminogen Activation Catalyzed by Urokinase-Type Plasminogen Activator, but Independent of Urokinase Receptor (uPAR, CD87)

The 55-kD urokinase (uPA) receptor (uPAR, CD87) is capable of binding uPA and may be involved in regulating cell-associated plasminogen activation and pericellular proteolysis. While investigating the relationship between uPAR levels and plasmin generation, we found that uPA-catalyzed plasminogen activation is stimulated by cells which do not express uPAR. This uPAR-independent mechanism appears to be at least as effective in vitro as uPAR-dependent stimulation, such that stimulation on the order of 30-fold was observed, resulting from improvements in both apparent kcat and apparent Km. The mechanism depends on simultaneous binding of both uPA and plasminogen to the cell and requires the presence of the amino-terminal fragment (ATF), available in single chain and two chain high-molecular-weight uPA, but not low-molecular-weight uPA. Stimulation was observed in all leukemic cell lines investigated at similar optimum concentrations of 106to 107 cells/mL and may be more general. A mechanism is proposed whereby uPA can associate with binding sites on the cell surface of lower affinity, but higher capacity than uPAR, but these are sufficient to stimulate plasmin generation even at subphysiologic uPA concentrations. This mechanism is likely to operate under conditions commonly used for in vitro studies and may have some significance in vivo.