ASCITES TUMOURS FROM BHK.21 CELLS TRANSFORMED IN VITRO BY POLYOMA VIRUS

YAP mediates apoptosis through failed integrin adhesion reinforcement

Extracellular matrix (ECM) rigidity is a major effector of cell fate decisions. Whereas cell proliferation on stiff matrices, wherein Yes-associated protein (YAP) plays a pivotal role, is well documented, activation of apoptosis in response to soft matrices is poorly understood. Here, we show that YAP drives the apoptotic decision as well. We find that in cells on soft matrices, YAP is recruited to small adhesions, phosphorylated at the Y357 residue, and translocated into the nucleus, ultimately leading to apoptosis. In contrast, Y357 phosphorylation levels are dramatically low in large adhesions on stiff matrices. Furthermore, mild attenuation of actomyosin contractility allows adhesion growth on soft matrices, leading to reduced Y357 phosphorylation levels and resulting in cell growth. These findings indicate that failed adhesion reinforcement drives rigidity-dependent apoptosis through YAP and that this decision is not determined solely by ECM rigidity but rather by the balance between cellular forces and ECM rigidity.

p21 is a critical CDK2 regulator essential for proliferation control in Rb-deficient cells

Proliferation in mammalian cells is controlled primarily in the G1-phase of the cell cycle through the action of the G1 cyclin-dependent kinases, CDK4 and CDK2. To explore the mechanism of cellular response to extrinsic factors, specific loss of function mutations were generated in two negative regulators of G1 progression, p21 and pRB. Individually, these mutations were shown to have significant effects in G1 regulation, and when combined, Rb and p21 mutations caused more profound defects in G1. Moreover, cells deficient for pRB and p21 were uniquely capable of anchorage-independent growth. In contrast, combined absence of pRB and p21 function was not sufficient to overcome contact inhibition of growth nor for tumor formation in nude mice. Finally, animals with the genotype Rb+/-;p21(-/-) succumbed to tumors more rapidly than Rb+/- mice, suggesting that in certain contexts mutations in these two cell cycle regulators can cooperate in tumor development.

Transformation of human bronchial epithelial cells transfected by Harvey ras oncogene

Transfection of normal human bronchial epithelial (NHBE) cells with a plasmid carrying the ras oncogene of Harvey murine sarcoma virus (v-Ha ras) changed the growth requirements, terminal differentiation, and tumorigenicity of the recipient cells. One of the cell lines isolated after transfection (TBE-1) was studied extensively and shown to contain v-Ha ras DNA. Total cellular RNA from TBE-1 cells hybridized to v-Ha ras structural gene fragment probes five to eight times more than RNA from parental NHBE cells. The TBE-1 cells expressed phosphorylated v-Ha ras polypeptide p21, showed a reduced requirement for growth-factor supplements, and became aneuploid as an early cellular response to v-Ha ras expression. As the transfectants acquire an indefinite life-span and anchorage independence they became transplantable tumor cells and showed many phenotypic changes suggesting a pleiotropic mechanism for the role of Ha ras in human carcinogenesis.

Formation and growth of multicellular spheroids of human origin

Different types of human cells which normally grow as monolayers or suspension cultures were tested for their capacity to form and grow as spheroids. Sixteen out of the 27 tested tumour cell lines formed spheroids. Nearly all of these spheroids also grew. With only two exceptions the doubling times were longer when the tumour cells grew as spheroids than when they grew in conventional mass culture. Eleven out of 13 tested human non-tumour cells formed small spheroids but of these only the spheroids of lymphoid origin could grow. These lymphoid cells grew faster when aggregated to spheroids than when in single-cell suspension culture. None of the other non-tumour cells, which normally grew as monolayers, could grow as spheroids. The normally monolayer-cultured tumour cells formed symmetrical spheroids with smooth surfaces while the normally suspension-cultured cells formed irregular spheroids with rough surfaces. All large spheroids had a necrotic centre surrounded by a shell of viable cells. The thickness of the viable cell layer varied depending on cell type. The shape and organization of cells within the spheroids also varied largely. The results show that many types of human cells can be cultured as spheroids and that a wide spectrum of morphological appearances and growth rates can be obtained.

Spontaneous, mutagen-induced and adenovirus-induced anchorage independent tumorigenic variants of mouse cells

Normal C57 Black mouse embryo cells did not form colonies in agarose, but rare variant (ar+) cells able to grow in agarose were detected. Fluctuation analysis showed that ar+ variants arose by spontaneous mutation in the cultured cells. The frequency of ar+ variants was increased by treating cells with N-methyl-N'nitro-N-nitrosoguanidine or ethyl methane sulphonate, or by abortive infection by human adenovirus type 5. Induced ar+ cells were fibroblastic; most grew slowly and had slightly reduced saturation density and increased serum requirement, but formed colonies in agarose. Fourteen of twenty ar+ clones induced by Ad5 were T antigen negative and two of these were also negative when tested for viral DNA. Six clones contained a few cells that were T antigen positive when first tested, but were negative when retested later. The ar+ variants were tumorigenic in athymic and in normal syngeneic mice. The results suggest that the ar+ phenotype can arise by spontaneous or chemically-induced mutation, and can be induced by adenovirus by a process different from classical transformation.

The use of agarose in the determination of anchorage-independent growth

At the present time, growth in agar suspension is one of the best in vitro correlates of tumorigenicity. Growth in agarose, however, has not been evaluated extensively as an in vitro criterion for tumorigenicity. In the present study we have tested 19 cell lines, including six mouse-human hybrids, for growth in agarose and agar in the presence and absence of exogenous hypoxanthine. None of the six nontumorigenic cell lines grew in agar or agarose. Ten of the 13 tumorigenic cell lines grew in both agar and agarose with about equal efficiency. The remaining three tumorigenic cell lines grew well in agarose but poorly or not at all in agar. Hypoxanthine did not stimulate the growth in agar or agarose of any of the cell lines except BHK. We conclude that growth in agarose may be a more sensitive marker for tumorigenicity than growth in agar and that BHK is exceptional in its sensitivity to supplemental purines.

Studies on a fractionated murine fibrosarcoma: proliferative potential of the separated cells

A transplantable methylcholanthrene-induced fibrosarcoma of female BALB/c mice (the MC-2 fibrosarcoma) was dissociated by combined mechanical and enzymatic means, then fractionated by isopycnic centrifugation in linear albumin gradients. In some experiments recovered cells were both cultured in soft nutrietn agar and inoculated subcutaneously into syngeneic recipients. In these experiments a highly significant correlation was observed between subsequtnt colony number and rapid growth phase tumor size suggesting identity of clonigenic and tumorigenic cells. It was consistently found that clonigenic cells were markedly depleted from the low density extremes of the cell density distribution profiles suggesting that the low density neoplastic cells had irreversibly left the growth fraction. With increasing tumor age, sequential studies showed that both total and clonigenic cell density distribution profiles were variable, showing no obvious trend, suggesting that in the age (13-35 days) and size (2-8 g) range studied growth fraction changes had little selective effect on cells of any specific density. These results imply that a marked selective depletion of low density clonigenic cells (or selective accumulation of low density non-proliferative cells) must mainly occur during an earlier phase of tumor growth. Studies on several other murine solid tumors also showed maximal depletion of clonigenic cells from the least dense fractions, suggesting that this situation may be common.

Physical and nutritional factors in gel culture of mammalian cells

The growth of human glioma cells, cultured as spherical colonies in agarose gel, stopped after about 10 days for both large and small colonies apparently due to an increased osmolality in the gel. When osmolality was kept under control by addition of distilled water, growth continued. However, a continuous increase in the population-doubling period, similar for both large and small colonies, then was observed. The increase persisted although excess amounts of nutrition were added. When the cells were cultured in liquid suspension above a thin layer of agarose gel and most of the medium was repeatedly changed, the colonies continued to grow beyond the limits in gel cultured. HeLa and hamster embryonic lung cell colonies showed a growth pattern in agarose gel similar to the glioma cells. The results imply that the osmolality must be kept under precise control to prevent growth inhibition. However, it seems difficult to ascertain optimal growth in gel culture for more than about 2 weeks probably because of the accumulation of toxic products.

Plasma membrane motility and proliferation of human glioma cells in agarose and monolayer cultures

Human glioma cells, growing as spherical colonies in agarose gel, or as monolayers on glass or plastic, were studied with time-lapse cinematography and electron microscopy. The cells in the agarose cultured colonies often had ruffling-like membrane structures which were similar in for, although smaller in size, than those observed on monolayer cultured cells. The ruffling-like structures were more frequent at the periphery than in the central regions of the colonies which was in parallel to the proliferative pattern. In time-lapse cinematography it was seen that pinocytotic vacuoles were formed from ruffling membranes in the monolayer cultures. In the transmission electron microscope, such vacuoles were also found near the ruffling-like structures in the agarose cultured cells. In dense monolayer cultures, ruffling and associated pinocytosis were to a large extent transferred from the margin to the upper surface of the cells. This capacity may be an important property for the ability of the malignant cells to attain extreme high cell densities in monolayer cultures and to grow as colonies in agarose cultures. As has been previously shown, the normal counterpart of the gliomas, the glia cells, cannot grow to high densities; they do not ruffle on their upper cell surface and are unable to grow in suspension or agarose culture.

Neoplastic transformation induced by furylfuramide and nitromethylfuran of embryonic hamster cells in tissue culture

Secondary cultures of Syrian hamster embryonic fibroblasts were tested for transformation and neoplastic properties after exposure in vitro to furylfuramine (AF-2) and other nitrofurans. Typical morphological transformation was seen in five of six cultures between 30 and 186 days following treatment with 5-10X10(6) M AF-2 for 24 h. Transformation was seen in only one of four cultures 145 days after treatment for 6 h with AF-2. Treatment with 5-10X10(6) M NMF (5-nitro-2-methylfuran) for 24 h also induced transformation after 50 and 118 days in two cultures. In contrast, untreated cultures and cultures treated with 5-10X10(-6) M NFT [4-(5-nitro-2-furyl)thiazole] for 24 h were not transformed within 200 days. Three of the six lines transformed by AF-2 and both lines transformed by NMF also became tumorigenic 7-24 days after morphologic transformation. The other three transformed lines produced nodules which regressed within a few weeks of transplantation. Untreated and treated non-transformed lines did not produce tumors during an observation period of 6 months. The tumors were classified as fibrosarcomas. The ability to form colonies in soft agar was acquired by only one tumorigenic line.

Effects of bacterial agarase on agarose gel in cell culture

Bacterial agarase, concentrated and purified from culture filtrate of agar-degrading bacteria, has been used to clean cells cultured in soft agarose from gel residues. The enzyme also has been used to liquefy the gel directly in the dishes to facilitate the removal of cells. The sufaces of glioma cells from agarase-treated colonies could not be distinguished in the scanning electron microscope from surfaces of cells which had never been in contact with agarose or agarase. This implies that most agarose residues had been removed, and also that the treatment did not seriously alter the cell surfaces. The influence of the agarase treatment also was tested by comparison of the mitotic index and the incorporation of [3H]thymidine in agarase-treated and untreated cells. No effects of the treatment could be seen in these tests.

An Effective Immunodepressive Treatment for Demonstrating in Allogeneic Mice the Tumorigenic Properties of Cells Grown in Vitro

Short intensive treatment with N-diazoacetylglycine amide (DGA) before the inoculum, followed by prolonged daily administration of L-aspàraginase (Asnase), was tested for its ability to elicit tumorigenic properties of fibroblast-like cells cultured in vitro. With this treatment progressive tumor growth was obtained in allogeneic mice injected with cells of a transformed subline. Results show that combined use of DGA and Asnase affords a higher probability of proving in vivo the tumorigenic properties of injected cells than in newborn or X-irradiated recipients. Experimental data indicate that L-asparagine depletion does not inhibit the in vitro growth of fibroblast-like cells.

Neoplastic transformation in vitro of a clone of adult liver epithelial cells into differentiated hepatoma-like cells under conditions of nutritional stress

Differentiated epithelial cells in contact-inhibited monolayers derived from adult rat liver have been transformed in vitro into epithelioid neoplastic cells under conditions of nutritional stress. The transformed cells maintain their differentiated quality and manufacture serum proteins. They differ from control cultures in the following properties: They are aneuploid, can be agglutinated by wheat-germ agglutinin and concanavalin A, can grow in suspension, and are able to form colonies in semisoft agar. There is no intercellular communication at permeable junctions between the cells; this is demonstrable by electrical measurement or by injection of fluorescein. The cells show invasiveness in culture, and are not inhibited by contact with normal cells. The characteristics of the hepatocytes after transformation in vitro resemble those of epithelioid cells derived from a transplantable hepatoma.

Replication of herpes-type virus in a Burkitt lymphoma cell line

Replication of the herpes-type virus in the P3HR-1 Burkitt lymphoma cell line was studied. The cell cultures with 10(6) viable cells/ml were incubated at 33 C for 15 days. The amount of virus in both the cell and fluid portions of the cultures was determined by the loop-drop particle-counting procedure with electron microscopy. An apparent growth curve of the virus was constructed. The maximal cell-associated virus, 10(10) virus particles in an 80-ml culture, was observed after 9 days of incubation. The maximal extracellular virus, 2.5 x 10(9) particles per culture, was observed at the 12th day. About 10% of the released virus particles were enveloped. Under these conditions, there was little or no cell multiplication, but the percentage of immunofluorescent cells reactive to a selected human serum (probably indicating the presence of virus in the cells) increased to a maximum of 50% at the 9th day.

Immunofluorescence and herpes-type virus particles in the P3HR-1 Burkitt lymphoma cell line

A subline of the P3 (Jijoye) Burkitt lymphoma cell line, designated P3HR-1, initially contained 1 to 5% of cells which were positive by indirect immunofluorescence with selected human sera. After 4 months of propagation, this cell line regularly showed 15 to 40% reactive cells. Antigen(s) in the cell line which was reactive by immunofluorescence was similar or identical to that found in several other Burkitt tumor cell lines in previous studies. When the cells were incubated at 35 or 32 C for 9 to 15 days without refeeding, more than 50% of the cells became immunofluorescence-positive. Thirteen different cultures of P3HR-1 cells, which contained up to 75% immunofluorescence-positive cells, were thin-sectioned and examined by electron microscopy. The percentage of cells containing herpes-type virus particles in the cultures varied from <3 to 78%. There was generally a good correlation between the number of immunofluorescent cells and the number of cells containing virus particles. The number of virus particles per cell section ranged from 1 to more than 100. These results strongly support the hypothesis that the immunofluorescent antigen is related to the presence of the herpes-type virus particle in the cells.

Malignant conversion of cells in vitro by carcinogens and viruses

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