Calcium stimulation of plasminogen activator secretion/production by swiss 3T3 cells.

Actively growing Swiss 3T3 cells secret high levels of plasminogen activator which decreases after the cells become confluent. In contrast, simian virus 40-transformed 3T3 cells secrete large amounts of plasminogen activator independent of cell density (Chou, I.-N., O'Donnel, S.P., Black, P.H., and Roblin, R.O. (1977) J. Cell. Physiol. 91, 31-38). These results suggest a correlation between active cell multiplication and plasminogen activator secretion in both 3T3 and simian virus-transformed 3T3 cells. The data reported herein indicate that treatment of both subconfluent and confluent Swiss 3T3 cells with high concentrations of Ca2+ (final 3.0 to 4.9 mM) increases the amounts of both secreted and cell-associated plasminogen activator in a dose-dependent manner. In addition, the ionophore A23187 (19 to 95 nM) in the presence of a normal level of Ca2+ (1.8 mM) stimulates both production and secretion of plasminogen activator from growing 3T3 cells. The Ca2+ stimulation of plasminogen activator production/secretion may be related to the mitogenic effect of Ca2+.     

Polarized secretion of urokinase-type plasminogen activator by epithelial cells.

Numerous epithelial cell types produce and secrete plasminogen activators (PAs) and/or PA inhibitors (PAIs). When epithelial cells were grown on polycarbonate filters and their apical and basolateral secretion products analyzed, PA activity accumulated in a highly polarized fashion; depending upon the cell line, the compartment of PA accumulation was either apical (MDCK I cells and HBL-100 cells) or basolateral (LLC-PK1, CaCo-2, and HeLa cells). By contrast, PAI-1 was recovered in roughly equal amounts in both compartments. Basolateral accumulation of urokinase-type plasminogen activator (uPA), but not its apical targeting, required an acidic compartment and the integrity of the cytoskeleton. Polarity of uPA accumulation did not result from removal of the free enzyme from the opposite compartment through its binding to the cell surface. Transfection with wild-type or mutated murine uPA demonstrated that neither the "growth factor" domain nor the kringle domain is required for the appropriate sorting of the protein. We propose that polarized secretion of PAs is one mechanism whereby cells spatially control extracellular proteolysis.     

Synthesis and secretion of plasminogen activator inhibitor 1 by human endothelial cells in vitro. Effect of active site mutagenized tissue-type plasminogen activator

The effects of recombinant tissue-type plasminogen activator (rt-PA) and of an inactive mutant of rt-PA, obtained by mutagenesis of the active site Ser478 to Ala (rt-PA-Ala478), on the synthesis and secretion of plasminogen activator inhibitor-1 (PAI-1) by human umbilical vein endothelial cells (HUVEC) in culture were studied. Under base-line conditions, PAI-1 antigen secretion was 4.3 +/- 1.0 micrograms (mean +/- S.D., n = 8) per 10(6) cells in 24 h. This PAI-1 had a low specific activity (6,000 +/- 1,600 units/mg) and Mr of 50,000, which was not altered by addition of rt-PA. In HUVEC cultured with 2 micrograms/ml rt-PA-Ala478, PAI-1 antigen secretion was 2.1 +/- 0.8 micrograms (n = 5) per 10(6) cells in 24 h with a specific activity of 120,000 +/- 42,000 units/mg and Mr of 50,000. Addition of rt-PA to this conditioned medium resulted in generation of three main components: 16% migrated as an Mr 106,000 rt-PA.PAI-1 complex, 16% as an Mr 81,000 degraded rt-PA.PAI-1 complex and the remainder as an Mr 45,000 degradation product of PAI-1. HUVEC cultured with 2 micrograms/ml rt-PA secreted 3.9 +/- 0.6 micrograms (n = 8) PAI-1 antigen per 10(6) cells within 24 h, of which 20-50% occurred as intact or degraded complexes with t-PA (Mr 106,000 and 81,000) and the rest as an inactive Mr 45,000 degradation product of PAI-1. PAI-1 mRNA levels, determined by Northern blot analysis and expressed relative to beta-actin mRNA levels, were very similar for HUVEC cultured in the absence or the presence of rt-PA or rt-PA-Ala478. It is concluded that PAI-1 is secreted by HUVEC in culture in fully active form which spontaneously inactivates. PAI-1 can be stabilized by addition of rt-PA-Ala478 to the culture medium, resulting in a 20-fold increase in specific activity. Interaction of rt-PA with active PAI-1 produces both t-PA.PAI-1 complex and an inactive degradation product of PAI-1.     

Hormonal control of urokinase plasminogen activator secretion by sheep ovarian surface epithelial cells

Secretion of urokinase plasminogen activator (uPA) by ovarian surface epithelium (OSE) adjacent to the preovulatory ovine follicle has been implicated in apical tissue degradation and follicular rupture. In vitro experiments were designed to test the hypothesis that uPA release by OSE is under direct hormonal control. Epithelial cells were isolated from the ovarian surface of sheep using a polytetrafluorethylene scraper designed to dislodge adherent cells from culture flasks. Amidolytic cleavage of a uPA-specific chromogen (carbobenzoxy-L-gamma-glutamyl [alpha-ot-but]-glycyl-arginine-p-nitroanilide monoacetate) was used as a measure of enzymatic bioactivity in OSE-conditioned incubation media. Secretion of uPA by OSE suspensions from proestrous ewes was stimulated by exposure (2 h) to a preovulatory surge-like concentration of LH. OSE cells obtained during the luteal phase or anestrus were not responsive to LH. Baseline rates of uPA secretion and expression of estradiol receptors (in situ immunofluorescence detection) were not affected by reproductive status. Induction of uPA secretion by anestrous OSE was attained after priming (6 h) with estradiol-17beta; responsiveness was attributed to gonadotropin receptor (ligand binding) up-regulation. Monolayers of OSE established on polyethylene membranes secreted uPA predominately in a basal (i.e., toward the substratum) direction. We suggest that OSE in juxtaposition with the (hyperemic) wall of the preovulatory follicle is perfused by surge levels of LH, invoking uPA release into underlying ovarian tissues.     

Glycosylation and secretion of human tissue plasminogen activator in recombinant baculovirus-infected insect cells.

Cell lines established from the lepidopteran insect Spodoptera frugiperda (fall armyworm; Sf9) are used routinely as hosts for the expression of foreign proteins by recombinant baculovirus vectors. We have examined the pathway of protein glycosylation and secretion in these cells, using human tissue plasminogen activator (t-PA) as a model. t-PA expressed in Sf9 cells was both N glycosylated and secreted. At least a subset of the N-linked oligosaccharides in extracellular t-PA was resistant to endo-beta-N-acetyl-D-glucosaminidase H, which removes immature, high-mannose-type oligosaccharides. This refutes the general conclusion from previous studies that Sf9 cells cannot process immature N-linked oligosaccharides to an endo-beta-N-acetyl-D-glucosaminidase H-resistant form. A nonglycosylated t-PA precursor was not detected in Sf9 cells, even with very short pulse-labeling times. This suggests that the mammalian signal sequence of t-PA is efficiently recognized in Sf9 cells and that it can mediate rapid translocation across the membrane of the rough endoplasmic reticulum, where cotranslational N glycosylation takes place. However, t-PA was secreted rather slowly, with a half-time of about 1.6 h. Thus, a rate-limiting step(s) in secretion occurs subsequent to translocation and N glycosylation of the t-PA polypeptide. Treatment of Sf9 cells with tunicamycin, but not with inhibitors of oligosaccharide processing, prevented the appearance of t-PA in the extracellular medium. This suggests that N glycosylation per se, but not processing of the N-linked oligosaccharides, is required directly or indirectly in baculovirus-infected Sf9 cells for the secretion of t-PA. Finally, the relative efficiency of secretion decreased dramatically with time of infection, suggesting that the Sf9 host cell secretory pathway is compromised during the later stages of baculovirus infection.     

Studies on the mechanism of Ca2+ stimulation of plasminogen activator synthesis/release by Swiss 3T3 cells.

Stimulation of postconfluent Swiss 3T3 cells in serum-free medium with 4.3 mM Ca2+ results in marked increases in both released and cell-associated plasminogen activator (PA). Increased release of PA commenced approximately 10 to 12 hours post-stimulation and continued to increase steadily until 48 hours at which time the stimulates cells (4.3 mM Ca2+) released approximately 14 times more PA than control cells (1.8 mM Ca2+). Sr2+, like Ca2+, also stimulates PA synthesis/release either in the presence or in the absence of 1.8 mM Ca2+ whereas an excess of Mg2+ inhibits Ca2+ stimulation. Supranormal [Pi] in the medium stimulates PA synthesis/release in the presence of 1.8 mM mM Ca2+. Further, optimal stimulation by 4.3 mM Ca2+ requires a normal level of Pi (1.0 mM). Elevation of medium [Ca2+] or [Pi] results in an enhanced uptake of Ca2+. The facts that cycloheximide treatment completely abolishes the Ca2+ stimulatory effect and that an increase in cell associated PA precedes release indicate that PA release is coupled to synthesis of new PA. Ca2+ stimulation of PA synthesis/release also requires continuous energy production and RNA as well as protein synthesis. A hypothesis is proposed to explain the relationship between stimulation of PA production and its enhanced release from cells stimulated by elevated [Ca2+] or [Pi] in the media. The possibility that PA release may be an example of the phenomenon of membrane shedding as opposed to secretion is discussed.     

Decreased plasminogen activator inhibitor-1 secretion in hypoxic corneal epithelial cells is associated with increased urokinase plasminogen activator activity

The aim of this study was to determine the effects of hypoxia on mRNA levels, cell-associated and -secreted protein concentration, activity, and protein complex formation of urokinase-type plasminogen activator, its receptor, and plasminogen activator inhibitor type-1 in corneal epithelium. Non-transformed human corneal epithelial cells were cultured in 20% oxygen (normoxic conditions) or 2% oxygen (hypoxic conditions) for 1, 3, 5, or 7 days. Relative changes in mRNA levels of plasminogen activator, receptor, and plasminogen activator inhibitor-1 were determined using a cDNA expression array, chemiluminescence, and densitometry. Protein concentrations were determined using enzyme linked immunosorbent assays. Activity assays were also used. Protein complex formation was assayed using cell surface biotinylation, immunoprecipitation, and Western blot analysis. Hypoxic corneal epithelial cells demonstrated no significant differences in plasminogen activator or receptor mRNA. Cell-associated plasminogen activator and membrane-associated receptor protein levels were unchanged. In contrast decreases in mRNA and secreted plasminogen activator inhibitor-1 protein were observed in hypoxic cells. Concurrently, increased cell-associated plasminogen activator activity was observed in hypoxic cells. The formation of plasminogen activator/receptor/plasminogen activator inhibitor-1 complex at the cell surface was not inhibited by hypoxia. However, in hypoxic cells less plasminogen activator inhibitor-1 was associated with receptor. It is concluded that in corneal epithelium cultured in 2% oxygen plasminogen activator inhibitor-1 may be an important regulatory factor of the plasminogen activator system resulting in increased urokinase plasminogen activator activity.     

Analysis of cellular interactions in density-dependent inhibition of 3T3 Cell proliferation

Subpopulations of different proliferative status are determined during cell-density dependent proliferation of 3T3 cells. From these data the probability of conversion of proliferative to quiescent cells is derived and found to correlate well with published data on binding of growth-inhibiting factors secreted from growth-inhibited cells.Based on material presented at the Symposium Intercellular Communication Stuttgart, September 16–17, 1982     

Production of plasminogen activator by migrating cephalic neural crest cells.

Neural crest cells migrate extensively during embryonic development and differentiate into a wide variety of cell types. Our working hypothesis is that during migration, embryonic cells secrete proteases which modify local microenvironments, thereby facilitating directed cellular movements. In this communication, we report studies on the migration of cephalic neural crest cells in the avian embryo. We demonstrate that these cells produce high levels of the serine protease, plasminogen activator (PA), at the time of their initial migration from the neural tube and during their migration to and colonization of the developing head and neck.     

Cell surface changes correlated with density-dependent growth inhibition. Glycosaminoglycan metabolism in 3T3, SV3T3, and con A selected revertant cells.

A 35SO4-labeling/chromatography technique has been developed which facilitates quantitation of sulfated glycosaminoglycan (GAG) synthesis in mammalian cell cultures. The technique has been used to compare sulfated GAG biosynthesis, degradation, and turnover in three related cell lines with differing degrees of density-dependent inhibition of growth in vitro (Balb/c 3T3, SV3T3, and SV3T3 revertant cells). Viral transformation of Balb 3T3 cells is accompanied by a 2-5-fold decrease in cell associated sulfated GAG. SV3T3 revertant cells, which show partial reversion to low saturation density in vitro, show a 2.5-8-fold increase in cell-associated sulfated GAG compared to the parental SV3T3 cells from which they were selected. In addition, the distribution of 35SO4 and [3H]glucosamine among the different GAG species produced by SV3T3 revertant cells reverts so that it is similar to the distribution characteristic of untransformed 3T3 cells rather than SV3T3 cells. Mild trypsin treatment of 35SO4-labeled cells removed 68-84% of the cellular sulfated GAG, suggesting that at least this proportion of the total cellular sulfated GAG was located at the cell periphery. Removal of 35SO4-labeled cells from the Petri dish with a Ca2+ selective chelating agent revealed a fraction of the sulfated GAG that remained tightly bound to the Petri dish. A higher proportion of the total cell-associated sulfated GAG remained attached to the Petri dish in cultures of untransformed and revertant cells compared to that present in cultures of transformed cells. A role for sulfated GAG in density-dependent growth inhibition of fibroblast cultures is proposed and discussed in the light of the data obtained.     

Inhibition of plasminogen activator secretion by cyclic AMP in a macrophage-like cell line.

The continuous cell line, J774.2, exhibits many macrophage-like functions such as latex and Fc-mediated phagocytosis, antibody mediated phagocytosis, antibody mediated cytotoxicity, chemotaxis, and lysozyme secretion. Cyclic AMP stimulates Fc-mediated phagocytosis and inhibits the growth of J774.2. To further evaluate the relationship between cyclic AMP and the specialized functions exhibited by these cells. Variants deficient in phagocytosis, adenylate cyclase and cyclic AMP-dependent protein kinase were derived. We have now shown that J774.2 also secretes plasminogen activator and that this secretion is rapidly and specifically inhibited by 8-bromoadenosine 3':5'-cyclic monophosphoric acid (8 Br--cAMP) or cholera toxin under conditions where lysozyme secretion is unaltered. Utilizing protein kinase-deficient variants, the ability of cyclic AMP to inhibit plasminogen activator secretion was shown to be mediated by a cyclic AMP-dependent protein kinase. We conclude that cyclic AMP has diametrically opposing effects on two macrophage-like functions: Fc-mediated phagocytosis and plasminogen activator secretion.     

The secretion of urokinase-like plasminogen activator is inhibited by microtubule-interacting drugs

The secretion of proteinases into the extracellular matrix is one of the main features of tumour cells, as related to their invasive behaviour. Considering the role of the microtubule cytoskeleton, and particularly the action of microtubule-associated protein (MAPs) in mediating protein secretion, the effects of the anti-microtubule drugs estramustine and taxol, on the secretion of urokinase-type plasminogen activator (u-PA) and the 72 kDa gelatinase were investigated. Treatment of 5637 bladder carcinoma cells with estramustine and taxol inhibited u-PA secretion into the conditioned medium in a drug concentration-dependent fashion. This inhibition was confirmed by determinations of u-PA enzymatic activities and by measurements of the levels of immunoreactive activator. Studies using gelatin zymograms also showed an inhibition of another tumoural proteinase namely the 72 kDa gelatinase. Time-course uptake experiments showed that estramustine was incorporated into the cells, a process which depended on temperature. On the other hand, immunofluorescence studies indicated that the microtubule network was affected by taxol with the formation of bundles of microtubules at different cell domains. Minor effects were visualized after treatment of the cells with estramustine-phosphate, a drug that blocks primarily the action of microtubule-associated proteins. The studies provide a way to analyse the relationships between u-PA secretion and the integrity of the cytoskeletal network.     

Prostromelysin-1 (proMMP-3) stimulates plasminogen activation by tissue-type plasminogen activator.

Matrix metalloproteinase-3 (MMP-3 or stromelysin-1) specifically binds to tissue-type plasminogen activator (t-PA), without however, hydrolyzing the protein. Binding affinity to proMMP-3 is similar to single chain t-PA, two chain t-PA and active site mutagenized t-PA (Ka of 6.3 x 106 to 8.0 x 106 M-1), but is reduced for t-PA lacking the finger and growth factor domains (Ka of 2.0 x 106 M-1). Activation of native Glu-plasminogen by t-PA in the presence of proMMP-3 obeys Michaelis-Menten kinetics; at saturating concentrations of proMMP-3, the catalytic efficiency of two chain t-PA is enhanced 20-fold (kcat/Km of 7.9 x 10-3 vs. 4.1 x 10-4 microM-1.s-1). This is mainly the result of an enhanced affinity of t-PA for its substrate (Km of 1.6 microM vs. 89 microM in the absence of proMMP-3), whereas the kcat is less affected (kcat of 1.3 x 10-2 vs. 3.6 x 10-2 s-1). Activation of Lys-plasminogen by two chain t-PA is stimulated about 13-fold at a saturating concentration of proMMP-3, whereas that of miniplasminogen is virtually unaffected (1.4-fold). Plasminogen activation by single chain t-PA is stimulated about ninefold by proMMP-3, whereas that by the mutant lacking finger and growth factor domains is stimulated only threefold. Biospecific interaction analysis revealed binding of Lys-plasminogen to proMMP-3 with 18-fold higher affinity (Ka of 22 x 106 M-1) and of miniplasminogen with fivefold lower affinity (Ka of 0.26 x 106 M-1) as compared to Glu-plasminogen (Ka of 1.2 x 106 M-1). Plasminogen and t-PA appear to bind to different sites on proMMP-3. These data are compatible with a model in which both plasminogen and t-PA bind to proMMP-3, resulting in a cyclic ternary complex in which t-PA has an enhanced affinity for plasminogen, which may be in a Lys-plasminogen-like conformation. Maximal binding and stimulation require the N-terminal finger and growth factor domains of t-PA and the N-terminal kringle domains of plasminogen.     

Correlation between a specific molecular weight form of plasminogen activator and metabolic activity of 3T3 cells

In quiescent cultures of 3T3 cells, plasminogen activator (PA) is found predominantly as a 75,000 dalton species. When quiescent cells are exposed to mitogenic agents such as phorbol myristate acetate, Ca++, or 25% serum, the absolute levels of PA in cell lysates may either increase or decrease. However, a consistent observation is that in the stimulated cultures PA is found predominantly as a 49,000 dalton species. This also is the predominant form of PA in growing and transformed cells. Concomitant with the mitogen-induced stimulation of the 49,000 dalton PA in quiescent cultures is a change in morphology to one that is characteristic of growing and transformed cells. The data suggest that PA is not operative in causing the morphological change that occurs with activation; however, the 49,000 dalton PA in particular is closely related to the pleiotypic response accompanying growth stimulation and transformation.     

Cyclic AMP potentiates phorbol ester stimulation of tissue plasminogen activator release and inhibits secretion of plasminogen activator inhibitor-1 from human endothelial cells

We have examined the effect of phorbol esters and cAMP elevating compounds on tissue plasminogen activator (tPA) and plasminogen activator inhibitor-1 (PAI-1) secretion. Phorbol esters induce a time- and dose-dependent increase in tPA release from endothelial cells, while forskolin, isobutylmethylxanthine, dibutyryl cAMP, and 8-bromo-cAMP had no significant stimulatory effect on tPA secretion. However, elevation of cAMP simultaneously with phorbol ester treatment potentiated the phorbol ester-induced release of tPA 6 times from 22.2 ng/ml with phorbol myristate acetate (PMA) alone to 122.1 ng/ml (PMA and forskolin). Potentiation was dose-dependent (half-maximal potentiation = 4 microM forskolin), and tPA release was enhanced at all stimulatory concentrations of PMA with no change in the PMA concentrations causing half-maximal or maximum tPA release. The kinetics of release was also similar in PMA versus PMA-forskolin-treated cells. A 4-h delay was observed, enhanced release was transient, and was followed by the onset of a refractory period. In contrast, elevation of cAMP reduced constitutive secretion of PAI-1 by 30-40% and prevented the increase in PAI-1 secretion stimulated by PMA. Elevated cAMP also decreased the rate of PAI-1 deposition into the endothelial substratum. These studies indicate that activation of a cAMP-dependent pathway(s) in coordination with phorbol ester-induced responses plays a central role in modifying the tPA and PAI-1 secretion from endothelial cells, leading to a profibrinolytic state in the endothelial environment.     

Hormonal regulation of tissue plasminogen activator secretion and mRNA levels in rat granulosa cells

Granulosa cells from immature rats produce tissue plasminogen activator (tPA) in response to follicle stimulating hormone (FSH) or luteinizing hormone (LH) both in vitro and in vivo. We have used the in vitro system to investigate the level at which the hormonal induction of tPA is regulated. Within 12 h following FSH addition, a dramatic but transient increase in tPA secretion occurs for by 24 h secretion returns to basal levels. This pattern of enzyme induction is similar with LH, but the onset of the increase is delayed. When steady-state tPA mRNA levels are examined after hormone treatment, the results mirror those obtained if one measures enzyme activity; a large increase in tPA mRNA followed by a decrease to basal levels is observed with both hormones, and the lag in induction by LH is also apparent. These results demonstrate that the regulation of tPA activity by gonadotropins occurs at the level of the steady-state concentration of the mRNA. In the presence of cycloheximide, the induction of tPA mRNA by FSH or LH is not greatly affected, indicating that this phase of the response to gonadotropins does not require the synthesis of new protein. However, the decrease in tPA mRNA levels observed 24 h after FSH treatment is affected by cycloheximide, in that the drug delays the reduction in mRNA levels seen with hormone alone.     

Modulation of synovial fibroblast plasminogen activator and plasminogen activator inhibitor production by protein kinase C.

Phorbol myristate acetate (PMA) added to human synovial fibroblast cultures caused a dose-dependent increase in the production of plasminogen activator inhibitor-type 1 (PAI-1). In addition, PMA inhibited endogenous and interleukin-1 (IL-1) induced plasminogen activator (PA) activity, while increasing mRNA PAI-1 levels. Other protein kinase C (PKC) activators, mezerein and teleocidin B4, caused similar effects. The simultaneous addition of the PKC antagonists, H-7 or staurosporine, prevented the inhibition of PA activity by PMA. This study shows that activation of PKC inhibits PA and stimulates PAI production in human synovial fibroblasts. These results suggest that activation of PKC may play an important role in regulating increased PA production associated with joint destruction in rheumatoid arthritis (RA).