Lack of correlation between fibrinolysis and the transformed state of cultured mammalian cells

Microenvironmental Regulation of the Initiated Cell

In the classical skin model of tumor initiation, keratinocytes treated once with carcinogen retain their normal appearance and growth behavior indefinitely unless promoted to growth into papillomas. Because many of the papillomas regress and may recur with further promotion, their cells can also be considered as initiated. The growth of initiated keratinocytes can be inhibited either in vitro or in vivo by close association with an excess of normal keratinocytes, but it is enhanced by dermal fibroblasts. Chick embryo fibroblasts (CEF) in culture produce transformed foci after infection with Rous sarcoma virus (RSV) on a background of normal CEF in a medium containing 10% or less calf serum (CS), but they retain normal appearance and growth regulation in 10% fetal bovine serum (FBS) or 20% CS. Transformation of a carcinogen-treated line of mouse embryo fibroblasts is prevented, and can be reversed, in high concentrations of FBS in the presence of an excess of normal cells. FBS has high, broad-spectrum antiprotease activity. Increased protease production occurs in a variety of transformed cells and is correlated with progression in tumors. Protease treatment stimulates DNA synthesis and mitosis in confluent, contact-inhibited normal cell cultures. Synthetic inhibitors of proteases suppress transformation in carcinogen-treated cultures and inhibit tumor formation in animals. Several different classes of protease may be overexpressed in the same transformed cells. It is proposed that excessive protease production accounts for major features of neoplastic transformation of initiated cells, but that transformation can be held in check by protease inhibitors present in serum and released from surrounding cells. It would be informative to determine whether high concentrations of FBS would inhibit the neoplastic development of initiated keratinocytes.

Plasminogen activators in normal and malignant oral epithelium in vivo and in vitro

Urokinase-type (uPA) and tissue-type (tPA) plasminogen activators were identified by fibrinolytic autography in the sulcus epithelium of human gingival mucosa but not in the orthokeratinized gingival epithelium. Fibrinolytic activity was present only over blood vessels in frozen sections of oral squamous cell carcinomas, the malignant epithelial cells showing no plasminogen activator activity. Plasminogen activators could not be demonstrated in either the sulcus or gingival epithelium by immunofluorescence, but both uPA and tPA were found in occasional squamous carcinoma cells. Fibrinolytic activity of culture fluids from epithelial explants grown in vitro from human gingival mucosa showed marked variation, but activity was much higher in the culture supernatants than in the cell lysates. Fibrinolytic activity of culture fluids from epithelial explants of squamous cell carcinomas was low both in supernatants and lysates. Zymogram overlays of sodium dodecyl sulphate-polyacrylamide electrophoretic gels from culture supernatants showed that the low fibrinolytic activity of culture supernatants of oral squamous cell carcinomas was due to the associated presence of plasminogen activator inhibitors. The fibrinolytic activity in the zymogram was due predominantly to uPA but some lysis was due also to tPA.

The role of urokinase-type plasminogen activator in aggressive tumor cell behavior

The correlation between urokinase-type plasminogen activator (uPA) expression and tumor cell invasion and metastasis has been well documented. Urokinase converts the zymogen plasminogen to plasmin, a trypsin-like enzyme with broad substrate specificities. Net uPA activity is determined not only by the amount of the enzyme itself, but also by its state of activation and the amount of specific plasminogen activator inhibitors (PAIs) present. Both uPA and its substrate, plasminogen, can bind to cells via specific membrane-associated receptors. Expression of uPA, uPA receptor (uPAR), and PAIs is regulated by growth factors, oncogenes, and other effector molecules. In the present review we discuss the interactions of uPA with its receptor, inhibitors, and substrate and how these interactions influence malignant behavior. We also review recent reports in which investigators have used anti-catalytic antibodies and/or gene transfection to demonstrate that uPA is directly involved in tumor cell invasion and metastasis.

Tumorigenicity of herpesvirus-transformed cells correlates with production of plasminogen activator

Our studies first demonstrated that established hamster cell lines transformed in vitro by herpesviruses activate plasminogen more effectively than normal hamster fibroblasts. This ability is probably due to increased levels of the enzyme plasminogen activator (PA). In the studies described here, the 333-8-9 cell line, originally transformed by herpes simplex virus type 2 strain 333, was used to derive subclonal lines that maintained stable PA phenotypes over the course of long in vitro passage. We were interested in correlating tumor formation by the subclones with their fibrinolytic capacity. Cells were, therefore, single-cell subcloned twice, and resulting cultures were tested for ability to activate plasminogen in vitro. PA activity was then quantitated by [125I]fibrin lysis assay, and high- and low-activity subclones were isolated; these retained high- or low-activity phenotypes. Syngeneic newborn hamsters were inoculated with these subclones and observed for the appearance of palpable tumors. A strong correlation between enzyme activity and tumor formation was observed in four separate trials; animals receiving high-PA subclones developed tumors more rapidly than those inoculated with the parental cell line. Tumors were also excised from test animals, and the cell lines established from the tumors were tested in vitro at different passages for their ability to activate plasminogen. These tumor cells were then reinoculated into syngeneic animals to confirm the tumorigenicity of cell lines with high fibrinolytic activity. In these experiments, the positive correlation between PA production and tumorigenicity was confirmed.

Relationship between protease activity and a sialoglycopeptide inhibitor isolated from bovine brain

We have recently described the isolation and purification to homogeneity of a new sialoglycopeptide from bovine brain cell surfaces that reversibly inhibits protein synthesis and DNA synthesis of normal but not transformed cells. Active inhibitory preparations, however, were shown to contain a protease activity that was not lost upon purification. Several experiments were performed to establish the relationship between the proteolytic activity of the sialoglycopeptide and the biological inhibitory activity. Both the protease activity and inhibitory activity were stable at pH 6-8 but were reduced or completely destroyed below pH 4 and above pH 9. Acid inactivation was reversible and upon dialysis, both the biological inhibitory and protease activities were regained. Deglycosylation and CNBr cleavage indicated that the polypeptide backbone, rather than carbohydrate moiety, played an important role in the protease and biological inhibitory activities. Furthermore, chemical modification of amino and tyrosine groups indicated that both residues are essential for both activities. Thus, the biological inhibitory activity and protease activity are very closely related and most likely reside with the same polypeptide sequence.

Heterogeneity of plasminogen activator expression in various Moloney virus-induced tumor cell lines. Lack of correlation with tumor growth and cell phenotype

The aberrant expression of a plasminogen activator (PA) by Moloney virus (MuLV)-transformed mouse lymphocytes and its relation to cell phenotype and tumor growth have been studied. Nine cultured cell lines were established from neoplastic splenic and thymic tissues obtained from B10 congeneic mice inoculated with MuLV and killed when overtly leukemic. Cell surface markers were assayed by microcytotoxicity tests, the concentration of MuLV p30 and group-specific MuLV gp 70 was determined by radioimmunoassays and the expression of PA activity was assessed in a fibrin-agar plate method. PA activity of 24 h serum-free culture supernatant, intact cells or cell lysates (2 X 10(5) cells/ml) was expressed in International Units by reference to a urokinase standard curve. Tumor extension and cell morphology were investigated by histologic and cytomorphologic analysis. In all cases the cell lines were derived from T cells. PA activity is not expressed by normal lymphocytes, but variations in PA expression were observed in the transformed cells. Five out of nine transformed cell lines showed PA activity with a range of 1.3 to 9.9 IU/ml. No PA activity could be detected in the other cell lines. No correlation was found between PA expression and the cell-surface-expressed phenotype, neither was there any correlation between the PA content, the cytopathological features and the degree and type of organ infiltration. This lack of correlation indicates that there is no relation between PA activity and the expression of the transformed phenotype, and that the presence of PA activity seems to be irrelevant to the tumorigenic capacities of the transformed cell lines.

A polypeptide secreted by transformed cells that modulates human plasminogen activator production

A diffusible factor produced and secreted by malignant murine cells was capable of inducing plasminogen activator production by normal diploid human fibroblasts. The factor's ability to induce plasminogen activator was insensitive to treatment with nucleases, but its activity was destroyed by digestion with proteases. It is proposed that such a factor would play a role in malignancy if it would recruit normal cells that were adjacent to transformed cells to produce plasminogen activator which could result in tumor-promoted proteolysis.

Plasminogen activator in differentiating mouse keratinocytes

The activity of the serine protease plasminogen activator (PA) was measured in cell lysates from primary mouse keratinocyte cultures as well as from a number of established mouse keratinocyte lines. Enzyme activity was generally higher in the transformed lines than in the primary cultures; however, among the lines tested, those that expressed the highest degree of morphologic differentiation had the highest levels of cell-associated PA. In both the normal (primary) and transformed (established) keratinocyte cultures, PA activity increased when cultures reached confluence and morphologic evidence of differentiation was noted. The highest specific activity of the enzyme was found in cells shed from differentiating cultures, which consisted predominantly of detergent-resistant cornified envelopes. As the cultures differentiated and these cells were shed from the culture surface, the total cell-associated PA activity of the culture decreased accordingly. In both the normal and transformed keratinocyte cultures, peak PA activity occurred at a time when DNA synthesis was declining. These findings indicate that as keratinocytes differentiate, their intracellular levels of PA increase. The modulation of this endogenous keratinocyte enzyme may play an important, although as yet undefined, role in the normal maturation and terminal differentiation of these cells.

Fibrinolytic activity in human tumor tissues

The fibrinolytic activity of 156 malignant and 36 benign solid tumors from autopsy and biopsy specimens was studied by the fibrin slide technique. The inhibitory activity against fibrinolysis was graded according to the lysis time of vascular tissues within the tumor. The results show that all malignant solid tumors, with the exception of prostate carcinoma, demonstrated varying degrees of inhibition of fibrinolysis. Persistently high inhibitory activity was found in squamous cell carcinoma of the esophagus, the respiratory tract, cervix uteri, and skin; carcinoma of uterus; colorectal carcinoma; small cell anaplastic carcinoma of lung; neuroblastoma, carcinoma of bile duct, while malignant tumors of the kidney show a lesser degree of inhibition. In contrast, with the exception of the hydatidiform mole, benign solid tumors show little or no inhibition. A similar absence of fibrinolytic activity is seen in metastatic disease. Further studies of the role of the fibrinolytic system in tumors seems warranted.

Tumor invasion and host extracellular matrix

In this review some of the major mechanistic pathways by which tumor cells are thought to invade host tissues are discussed. Tumor invasion has been conceived to be the result of pathological, close-range interactions between malignant cells and host stroma. The sequence of events that characterize invasion can be summarized as follows: (a) Tumor cell clusters break from the confinement of the primary tumor. Loss of intercellular junctions (desmosomes), alterations in the chemical composition and physical properties of the cell surface coat (loss of fibronectin and heparan sulfate; excessive amounts of hyaluronate), and loosening of cell-substrate interactions (loss of hemidesmosomes, fibronectin, and heparan sulfate), are among the most frequently listed causes of tumor cell shedding. (b) Increased proteolytic activities at the invasion front cause focal alterations in the surrounding extracellular matrix, thereby changing its physical properties. Collagenases and cathepsins, as well as elastase and other neutral proteinases are the enzymes most frequently associated with matrix destruction and invasion. In some tissues this process is effectively regulated by inhibitors of matrix-degrading, proteolytic enzymes. (c) Tumor cells migrate into the altered matrix, possibly moving as aggregates along guidance tracks provided by host structures (blood vessels, lymphatics, nerves) or matrix macromolecules (collagen and fibronectin tracks). Migration seems to be preceded by increased swelling of glycosaminoglycan (i.e., hyaluronate) in the matrix, ahead of the migrating cell population. Various host cell types (mast cells, fibroblasts, endothelial cells, macrophages, etc.) may participate in these events.

Characterization and partial purification of the plasminogen activator from human neuroblastoma cell line, SK-N-SH. A comparison with human urokinase

The plasminogen activator liberated by cells of human neuroblastoma strain SK-N-SH was purified up to 400-fold, with a 40% recovery of activity, by a relatively simple procedure. This involved (NH4)2SO4 precipitation, followed by chromatography on both Affi-Gel Blue and p-aminobenzaminidine-Sepharose. The SK-N-SH activator was shown to differ from human urokinase with respect to immunological specificity, the molecular weights and isoelectric points of their enzymatically active species, the ability to be activated by fibrin and their relative sensitivities to inactivation by diisopropyl fluorophosphate. The average molecular weights of the enzymatically active species derived from strain SK-N-SH were shown to be 66,500, 64,500, 60,500 and 37,500. In the presence of fibrin, the SK-N-SH plasminogen activator appeared to be stimulated approximately 16-fold, with no apparent stimulation of urokinase activity. Urokinase is inactivated by diisopropyl fluorophosphate at a rate 6.4-fold faster than that of the SK-N-SH activator.

Tumor production in the nude mouse, fibrinolytic activity and cross-reactivity with antimelanoma sera of various human tumor cell lines

It has been reported previously that antisera produced in non human primates against many cultured human melanoma lines recognize tumor-associated antigens on the melanoma cells. The reactivity correlated with other parameters of malignancy, such as rapidity of tumor growth in nude mice and release of plasminogen activator. In this study we have investigated whether these antisera also react with non melanoma cancer lines and searched for a correlation between this reaction and the other parameters of malignancy. There was cross-reactivity of the ten sera with 4-43% of the 46 non melanoma lines compared to 62-100% of the 16 melanoma lines, indicating a preferential reactivity with melanoma lines. For the non-melanoma lines no correlation was found between the release of plasminogen activator, the degree of tumorigenicity in the nude mouse or the reactivity of surface antigens as detected with antimelanoma sera.

Clonal variation in cultured neuroblastoma cells. I. Isolation and characterization of variants

Two spontaneously arising variant clones were selected from the N18 neuroblastoma cell line solely on the basis of their flattened morphology and tight adherence to the culture flask. Two other clones having the round loosely adherent morphology typical of the parent line were also selected, and flat variants were shown to arise in them upon prolonged cultivation. The flat variant clones have slower growth rates in culture, lower cloning efficiencies in suspension, and reduced acetylcholinesterase inducibility when compared with either the parent N18 line or the round cell clones. Cells of both morphologic types have high levels of plasminogen activator and are tumorigenic, although the variants have a slower growth rate in vivo, consistent with their slower growth rate in culture. SDS-polyacrylamide gel electrophoresis of total protein from the two cell types shows that the flat variants have increased amounts of a 200,000 molecular weight polypeptide that has tentatively been identified as the heavy chain of myosin. Round morphological revertants from one of the flat variant clones exhibited growth characteristics typical of the parent N18 line, but their content of myosin heavy chain, although reduced, was not so low as that in the round cell clones originally isolated. The possibility of a causal relationship between flat morphology, reduced suspension cloning efficiency, and increased content of myosin heavy chain is discussed.

Plasminogen activator synthesis accompanying chemical carcinogen-induced in vitro transformation of Syrian hamster and guinea-pig fetal cells

The production of the extracellular protease, plasminogen activator, in relationship to growth in semi-solid medium and tumorigenicity has been studied in Syrian hamster embryo and strain 2 guinea-pig embryo cell culture models of chemical carcinogenesis. Whereas normal hamster and guinea-pig embryo cells had negligible levels of plasminogen activator, neoplastically transformed cells derived from cultures exposed to chemical carcinogens had high levels of plasminogen activator and grew as progressively enlarging colonies in soft agar. The development of plasminogen activator secretion in relation to neoplastic transformation was further studied in the guinea-pig cells where the latent period between carcinogen exposure and neoplastic transformation ranged from 4 months to more than one year. The production of plasminogen activator in this system also exhibited a long latent period, and acquisition of extracellular plasminogen activator correlated temporally with growth in agar and tumorigenicity. Plasminogen activator and growth in semi-solid agar in concert are useful markers for identification of neoplastic cells transformed in culture following exposure to a chemical carcinogen.

Transformation phenotype of polyoma virus-transformed rat fibroblasts: plasminogen activator production is modulated by the growth state of the cells and regulated by the expression of an early viral gene function

The expression of two transformation parameters, namely, ability to grow in agar and plasminogen activator production, was studied in several rat fibroblasts transformed by either wild-type or thermo-sensitive (tsa and ts25) polyoma viruses. The production of plasminogen activator was found to be dependent upon the growth state of the infected cells during a period of several days after infection. The analysis of the transformed phenotype of 25 tsa transformants and of 19 ts25 transformants independently isolated under various growth conditions led to the conclusion that there is no correlation between the regulation processes involved in plasminogen activator production and ability to grow without anchorage. The results obtained also suggested that the production of plasminogen activator is under the control of a functional large T antigen.

Lack of correlation between effects of tumor promoter TPA on plasminogen activator production, phosphatidyl choline synthesis, and hexose transport in mammalian cell culture systems

We have investigated the effects of the tumor promoter 12-0-tetradecanoyl-phorbol-13-acetate (TPA) on plasminogen activator production, hexose transport and metabolism, and the incorporation of choline into the acid soluble pool and into phosphatidylcholine in suspension cultures of mouse L, mouse P388 leukemia, human HeLa, and Chinese hamster ovary cells, and in monolayer cultures of baby hamster kidney (BHK), mouse 3T3, mouse 3T6, and mouse P388D1 macrophage-like cells. BHK, 3T3, P388D1, and P388 cells produced plasminogen activator constitutively, but no significant production was observed in the other cell lines. Plasminogen activator production was induced or stimulated by TPA only in P388 cells (10- to 20-fold by 100 ng TPA/ml). On the other hand, phosphatidylcholine synthesis was stimulated by TPA only in HeLa cells, and hexose transport, as measured with 3-0-methyl-D-glucose, only in 3T3 and P388D1 cells, as well as in human lymphocytes. The stimulation of hexose transport occurred more rapidly than the induction of plasminogen activator production and seemed to be part of the mitogenic response of cells to TPA treatment. A stimulation of deoxyglucose uptake was similarly limited to 3T3 and P388D1 cells. A significant decarboxylation of carbon 1 of deoxyglucose occurred in P388 and P388D1 cells, but not in Novikoff cells, and any decarboxylation that occurred was not stimulated by TPA. The results indicate that the various investigated responses of cells to TPA are unrelated and occur independent of each other. The time courses of the biochemical responses also differ significantly.

The plate-implant as a bioassay for the neoplastic potential of cultured cells

One objective of this study was to test the plate implant as an in vivo bioassay for the neoplastic state with pairs of cell lines that were non-tumorigenic and tumorigenic when inoculated in suspension at various sites. Each pair originated from a common cell pool or from one cell derived from normal rat or mouse embryonic tissue. The implantation of cells attached to polycarbonate plates was compared with the subcutaneous injection of comparable numbers of cells in suspension or the intraocular injection of 10(5) cells. In addition, some implants of glass helices with attached cells were made into syngeneic mice and compared with injection of comparable cell numbers intramuscularly into X-irradiated hosts. A total of 10 lines or clones were tested on plates. Of four lines that were non-tumorigenic when inoculated in suspension either subcutaneously or intraocularly, two were tumorigenic when implanted subcutaneously attached to plastic plates. The remaining two are the first permanent rodent lines that have not been tumorigc on plastic plates. Of the remaining six lines, all were tumorigenic on plates and two produced sarcomas at a higher frequency on plates than in suspension. Cytologic diagnoses of the cells implanted on plates, subcutaneously and intraocularly, correlated best with the results from plate implants. Of three mouse lines implanted on helices, two were equally tumorigenic in suspension and on helices, whereas the third was tumorigenic on helices only. The plate implant was more satisfactory technically than the helices and proved to be one of the most sensitive bioassays for the neoplastic state of cultured cells.

Phorbol ester-induced morphological changes in transformed chick fibroblasts: evidence for direct catalytic involvement of plasminogen activator

The tumor promoter phorbol myristate acetate (PMA) induces the production of the serine protease plasminogen activator (PA) in cultures of normal chick embryo fibroblasts (CEF) and synergistically enhances PA production in Rous sarcoma virus-transformed chick embryo fibroblasts (RSVCEF). Following PMA treatment of serum-free RSVCEF cultures, PA induction is accompanied by distinct morphological changes, including enhanced cell clustering and the formation of dense cellular aggregates. These alterations in the morphology of the PMA-treated transformed cells are inhibited by several protease inhibitors, including leupeptin, NPGB, SBTI, benzamidine and DFP, the specific inhibitor of serine enzymes. A number of protease inhibitors are ineffective in preventing the PMA-induced morphological changes; these include inhibitors of trypsin, chymotrypsin, elastase, thrombin and, most importantly, plasmin. The use of a fluorescent substrate to assay PA directly demonstrated that the pattern of inhibiton of PA activity correlates exactly with the inhibition of morphological changes. The of 3H-DFP to label and characterize serine zymes in the culture fluid from PMA-treated cells further indicated that PA is the serine protease responsible for the morphological changes. Thus PA itself can catalytically alter cellular behavior in culture independent of plasminogen, until not its only known natural substrate.

Proteolytic enzymes in normal and transformed cells

Virally transformed cells show an increased production of proteolytic enzymes. These might be involved in transformation-dependent alterations of cell surface glycoproteins. The possibility arises that some of these proteases might be membrane-bound. To investigate this possibility, we have undertaken a comparative study of the reactivity of intact normal and transformed cells with the tritium labelled protease inhibitor diisopropylfluorophosphate, in parallel with fibrinolytic assays. Using these two approaches in concert, it was possible to identify and localize in the transformed cells several proteases which were present in the particulate cell fraction and were probably membrane bound. In particular, a diisopropylfluorophosphate-reactive polypeptide of 62,000 was increased 5--8-fold on transformation. It comigrated with a fibrinolytic activity. Other particle-bound activities were also detected. While diisopropylfluorophosphate-labelling can be useful for detecting proteases inside cells, it does not appear to be specific for surface proteases.

Behaviour of normal and neoplastic cultured mouse cells on the chorioallantoic membrane of the chicken

Cultivated normal and transformed fibroblasts of the mouse (short-term cultures of lung fibroblasts and L-cells) have been implanted onto the chorioallantoic membrane of the chick embryo (CAM). Both cell types formed macroscopically visible nodules on the CAM where they induced a weak angiogenic reaction. Labelling of the cells with activated charcoal or 3H-thymidine gave evidence of their invasion into the CAM mesoderm, where they induced the formation of new capillaries. The successive multiplication of the cells led to the formation of tumours, resp. tumour-like cellular accumulations in the hypertrophied mesoderm of the CAM. Treatment of L-cells with the protease-inhibitor Contrykal reduced the invasive properties of the cells. The results presented clearly demonstrate invasive and angiogenic properties of the normal and malignantly transformed cell cultures of the mouse used in our experiments.

Thromboplastic and fibrinolytic activities of cultured human cancer cell lines

Thromboplastic and fibrinolytic activities of 14 lines of cultured human cancer cells were estimated by modified Astrup's methods. High tissue thromboplastic activity was found in one line of urinary-bladder cancer, 2 lines of gastric cancer and one line of lung cancer, but no activity was found in 6 lines of lung cancer. High fibrinolytic activity was noted in one line of gastric cancer and 2 lines of lung cancer, but no activity was seen in 6 lines of lung cancer and one line of gastric cancer. No plasmin activity was found. The tumour cell lines could be classified into 3 groups on the basis of the 2 activities. Cancer cell lines could also be classified into 2 groups: with high or low release of thromboplastin into culture media. Fibrinolytic activity was found in the culture media of all cell lines with high fibrinolytic activity. Fibrinolytic activity, but not thromboplastic activity, seemed to be influenced by the constituents of culture media. No definite correlation was found between the 2 activities and the histological types of the parent tumours of the cultured cells.

Plasminogen activator synthesis by cultured human embryonic lung cells: characterization of the suppressive effect of corticosteroids

The synthesis of plasminogen activator (PA) by cultured human embryonic lung (HuEL) cells has been examined. The production of PA by these cells was found to be reversibly inhibited by physiological levels of glucocorticoids. The suppression of PA synthesis in HuEL cells was not accompanied by an inhibition of cell growth. Moreover, the glucocorticoid induced deinduction of plasminogen activator synthesis occurred in both growing and non-growing cells. The inhibition of PA production by corticosteroids appeared to have a requirement for DNA-dependent RNA synthesis since the inhibition of DNA-dependent RNA synthesis at the time of exposure of cells to corticosteroids prevented the deinduction of PA.

Rat hepatocyte primary cultures. IV. Maintenance in defined medium and the role of production of plasminogen activator and other proteases

Primary cultures of rat hepatocytes survived well for up to 4 days in defined medium in the presence of dexamethasone but not in its absence. The loss of viability was accompanied by a loss of ultrastructural features characteristic of hepatocytes. The cultures began producing plasminogen activator and a neutral protease after 24 hr in culture. Dexamethasone inhibited the production of both of these substances. The deterioration of the cultures appeared not to be related to plasminogen activator, but prolongation of survival by a variety of protease inhibitors suggested that the neutral protease might contribute to deterioration.

Effects of cocultivation with transformed cells on surface proteins of normal cells

Virally transformed fibroblasts do not have on their surface a major protein (large external transformation-sensitive, LETS) which is present in normal cells. Cocultivation of the transformed cells with normal cells whose surface proteins have been prelabelled induces an accelerated release of the LETS protein from the normal cells. We have investigated various conditions which affect this phenomenon. Our results show that alteration of cell surface proteins by cocultivation with the transformed cells is time and dose-dependent and requires cell contact. Serum was depleted at least 99% of plasminogen by affinity chromatography and used in the cocultivation experiments. It was found that activation of plasminogen was not required for the accelerated turnover of the LETS protein. Other diffusible proteases are also unlikely to be involved. The possibility that transformed cells have a membrane bound activity is discussed. The role of plasminogen activation was also tested for its relevance in transformation related proteolysis, growth and morphology of cells.

Characterization of plasminogen activator in human cervical cells

Plasminogen activator activity was detected in human gynecologic specimens using a synthetic fluorogenic peptide substrate assay and confirmed by an 125I-labeled fibrin plate assay. Epithelial cells in these samples contain enzymatic activity that biochemically resembles both the well-characterized plasminogen activator, urokinase, and the less-specific plasminogen activator, trypsin. Inhibition of the cervical cell activity by diisopropylfluorophosphate and p-nitrophenyl-p'-guanidinobenzoate demonstrates that, like urokinase and trypsin, this plasminogen activator is also a serine protease. Polyacrylamide gel electrophoresis of plasminogen that had been incubated with cervical cells indicated the same mechanism of plasminogen activation as exhibited by urokinase. We attempted to correlate plasminogen activator activity of each sample with cytomorphologic diagnosis. Three of the four dysplastic samples analyzed showed higher plasminogen activator activity than did the normal samples.

Characterization of normal human embryo cells grown to over 100 population doublings

Normal human embryonic cells were subcultured for over 100 population doublings without modification of the basic medium. The cells were evaluated for growth rate, confluent density, chromosome stability, growth in soft agar, ability to hydrolyze casein and tumorigenicity. The cells possessed the characteristics of normal cells. The batch of serum used to supplement the medium was found to be of primary importance in the long-term growth of this cell culture.

Co-cultivation of tumorigenic mouse melanoma cells with cells of a non-tumorigenic subclone inhibits plasminogen activator expression by the melanoma cells

Clone B559 mouse melanoma cells are highly tumorigenic and produce plasminogen activator. Cells of clone C3471, a line obtained by continued growth of B559 cells in medium containing 5-bromodeoxyuridine (1 microgram/ml), have no plasminogen activator and are non-tumorigenic. When B559 cells are co-cultivated with C3471 cells, the ability of B559 cells to activate plasminogen is suppressed. Under these conditions cell fusion occurs. Lack of expression of plasminogen activators is not a consequence of cell fusion, inhibition of cell division or release of soluble inhibitors of either plasminogen activators or plasmin. No inhibitors of plasminogen activators could be demonstrated in association with sub cellular fractions of C3471 cells or with the C-type viral particles released from C3471 cells. Close contact between cells of the two lines is shown to be essential for suppression of plasminogen activation.

Production of plasminogen activator by established cell lines of mouse origin

The correlation between malignant transformation and increased plasminogen activator synthesis has been studied in a variety of established cell lines. In contrast to the behavior of secondary mouse embryo cultures, which always show increased fibrinolytic activity when transformed, no such correlation was found within the BALB/c 3T3 line and its transformed derivatives. Cell lines were established from tumors initiated in BALB/c mice by several transformed cell lines. These lines were generally found to contain no more plasminogen activator than the cells used for inoculation. A correlation was found between transformation and plasminogen activator synthesis within Swiss 3T3 cell lines. However, the correlation was not maintained by serum revertants of transformed Swiss 3T3 cells.

Cell density-dependent secretion of plasminogen activator by 3T3 cells

The expression of extracellular fibrinolytic activity in untransformed 3T3 cell cultures depends on the growth state of the cells. Actively growing 3T3 cultures exhibit a relatively high level of fibrinolysis, which decreases progressively as the cells become confluent and density-inhibited. The low level of fibrinolytic activity in confluent 3T3 cultures is due to a diminution in secretion of plasminogen activator since the intracellular level of plasminogen activator remains high. The amount of plasminogen activator observed in growing 3T3 cultures varies depending upon whether the cells are passaged with trypsin/EDTA solution, or with Ca++ selective chelating agent, ethylene-bis (oxyethylenenitrilo) tetraacetic acid (EGTA). However, in cells passaged using either agent, the amount of plasminogen activator secreted is always greatest when the cells are actively growing and decreases thereafter. In contrast to confluent 3T3 cultures, dense cultures of SV40-virus transformed 3T3 cells continued to secrete relatively large amounts of plasminogen activator. The ability to decrease secretion of plasminogen activator as cells become dense may be an important characteristic of cells which demonstrate density-dependent inhibition of cell multiplication in vitro.