Isolation and characterization of revertant cell lines. II. Growth control of a polyploid revertant line derived from SV40-transformed 3T3 mouse cells

Persistence to anti-cancer treatments in the stationary to proliferating transition

The heterogeneous responses of clonal cancer cells to treatment is understood to be caused by several factors, including stochasticity, cell-cycle dynamics, and different micro-environments. In a tumor, cancer cells may encounter fluctuating conditions and transit from a stationary culture to a proliferating state, for example this may occur following treatment. Here, we undertake a quantitative evaluation of the response of single cancerous lymphoblasts (L1210 cells) to various treatments administered during this transition. Additionally, we developed an experimental system, a “Mammalian Mother Machine,” that tracks the fate of thousands of mammalian cells over several generations under transient exposure to chemotherapeutic drugs. Using our developed system, we were able to follow the same cell under repeated treatments and continuously track many generations. We found that the dynamics of the transition between stationary and proliferative states are highly variable and affect the response to drug treatment. Using cell-cycle markers, we were able to isolate a subpopulation of persister cells with distinctly higher than average survival probability. The higher survival rate encountered with cell-cycle phase specific drugs was associated with a significantly longer time-till-division, and was reduced by a non cell-cycle specific drug. Our results suggest that the variability of transition times from the stationary to the proliferating state may be an obstacle hampering the effectiveness of drugs and should be taken into account when designing treatment regimens.

Simvastatin but not pravastatin inhibits the proliferation of rat aorta myocytes

The in vitro effect of simvastatin and pravastatin, two competitive inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A reductase, on the proliferation of rat aortic smooth muscle cells was investigated. Simvastatin, but not pravastatin, inhibited the replication of arterial myocytes in concentrations ranging between 0.01 microM and 10 microM. The inhibition, evaluated as cell number and nuclear incorporation of [3H]thymidine, was dose-dependent and completely prevented by addition of mevalonate (100 microM), confirming the role of mevalonate or its products in regulating cell division and growth. The present results provide evidence that simvastatin, in addition to its effect on cholesterol biosynthesis, interferes in vitro with other processes involved in atherogenesis.

Effect of tumor necrosis factor/cachectin on the activity of the low density lipoprotein receptor on human skin fibroblasts

We have investigated the effects of recombinant human tumor necrosis factor (TNF)/cachectin on the cellular binding of human low density lipoprotein (LDL) to human skin fibroblasts. When recombinant TNF was added to cultured cells, LDL binding doubled after 24 h of incubation. The effect of TNF was dose-dependent and its maximal effect was observed at concentrations of 1-10 ng/ml. TNF also stimulated the growth of human skin fibroblasts 1.6-fold. These results indicate that TNF increases LDL-receptor activity, which might be related to its stimulatory effect on cell growth.

In vitro inhibition of arterial myocyte growth and stimulation of low density lipoprotein metabolism by SIM 6080, a new calcium antagonist

We have investigated the in vitro effect of the new calcium antagonist SIM 6080 on proliferation of rat aortic smooth muscle cells and on LDL receptor-mediated catabolism in human fibroblasts. Verapamil was used as the reference compound. SIM 6080 inhibited the proliferation of rat aortic myocytes in concentrations ranging between 1 and 20 microM. The inhibition, evaluated as cell number and nuclear incorporation of [3H]thymidine, was dose and time dependent; the cell doubling time increased with drug concentrations up to 69 h versus 20 h for controls. Similar results on both LDL pathway and smooth muscle cell proliferation were achieved with verapamil, but higher concentrations were needed. The specific uptake and degradation of 125I-LDL was evaluated in human fibroblasts after 48 h incubation with SIM 6080 (1-10 microM). The compound dose dependently enhanced the receptor-mediated 125I-LDL uptake, with a fourfold increase as a maximal effect (10 microM); LDL degradation was less sensitive to the drug. The present results provide evidence that the new calcium antagonist SIM 6080 interferes in vitro with processes involved in atherogenesis.

Low-density lipoprotein and scavenger receptor activities are modulated by secretory products derived from cells of the arterial wall

The atherosclerotic lesion consists of cholesterol-loaded macrophages, smooth muscle cells, and other cells of the arterial wall. Conditioned medium from human monocyte-derived macrophages (HMDM) stimulated both acetylated and native low-density lipoprotein (Ac-LDL and LDL, respectively) degradation in autologous cells by 25% and 90%, respectively, and this was due to an increase in the number of both LDL and Ac-LDL receptors. Macrophage conditioned medium also resulted in an approximate doubling of LDL degradation by human arterial smooth muscle cells (HASMC), endothelial cells (HEC), and skin fibroblasts (HSF). Macrophage degradation of both Ac-LDL and LDL was enhanced 15% to 45% by conditioned medium derived from HASMC and HSF, respectively, but not by HEC-conditioned medium. Conditioned medium from HASMC, like that from macrophages, could also enhance LDL degradation by smooth muscle cells, fibroblasts, endothelial cells, and macrophages. Thus, the current study demonstrated that arterial wall cells secretory products can affect cellular lipoprotein receptor activities. This phenomenon could lead to increased cellular cholesterol accumulation and foam cell formation.

Reversion to normal phenotype induced by SV40 in a spontaneously transformed malignant Chinese hamster cell line

By using a selection procedure that excluded the transforming effect of SV40, reversions to several properties of normal phenotype were for the first time obtained in a transformed Chinese hamster cell line after SV40 infection. The value of induction to recovery of contact inhibition was typical for SV40-induced reverse gene mutations. Thirteen of 15 isolated revertant clones were T-antigen positive, thus synthesizing the product of viral oncogene. Therefore, in the majority of clones reversion occurred in spite of the presence of viral transforming protein. Dot hybridization revealed the presence of SV40 DNA in all revertants including those expressing no T antigen. The virus rescued from one T-antigen positive and two negative clones proved to be infectious. Reversion to contact inhibition was followed by reversion as regards serum requirements and growth in soft agar. However, in all cases reversion was partial. Karyologic analysis of revertant clones showed that six clones maintained the hypodiploid karyotype of the parental clone, six revertants were near-tetraploid, and one was near triploid. The possible events underlying the SV40-induced reversions to normal phenotype and the role of virus-induced mutations in viral carcinogenesis are discussed.

Heparin-binding growth factor type one and platelet-derived growth factor are required for the optimal expression of cell surface low density lipoprotein receptor binding activity in human adult arterial smooth muscle cells

Purified heparin-binding growth factor-1 (HBGF-1) stimulated low density lipoprotein binding, internalization, and degradation in isolated human adult arterial smooth muscle cells. Exposure of quiescent cells to HBGF-1 in serum-free, defined medium increased both low density lipoprotein (LDL) receptor activity and de novo cholesterol biosynthesis. Both events preceded the onset of DNA synthesis by 6 to 9 h. HBGF-1 acted additively with platelet-derived growth factor (PDGF) to maximally stimulate cell surface LDL receptor binding activity and DNA synthesis in the smooth muscle cells. The presence of LDL was required for maximal mitogenic activity of HBGF-1 and PDGF. In the presence of LDL, growth factor-stimulated, proliferating human smooth muscle cells accumulated cholesterol ester and triglycerides. The results suggest that HBGF-1, PDGF, and LDL act together to promote the maximal proliferation of smooth muscle cells in culture. Chronic exposure to the three growth promoters may contribute to the smooth muscle cell hyperplasia and lipid accumulation observed in atherosclerotic lesions.

Transformation of precrisis human cells by the simian virus 40 cytoplasmic-localization mutant pSVCT3 is accompanied by nuclear T antigen

pSVCT3 is a cytoplasmic-localization mutant of simian virus 40 (SV40) isolated from the SV40 adenovirus 7 hybrid virus (PARA) and cloned into plasmid PBR. The large T antigen of pSVCT3 accumulates in the cytoplasm of infected monkey cells instead of being transported to the nucleus. The sole change in CT3 large T antigen is amino acid residue 128 (Lys----Asn). Transformation of precrisis rodent cells by pSVCT3 is negligible, whereas the frequency of transformation of established rodent cell lines by pSVCT3 is comparable to that of wild-type SV40. According to the model, in which transformation of precrisis cells involves the combined oncogenic action of both nuclear and cytoplasmic gene products, we predicted that pSVCT3 would localize in the cytoplasm of human cells and would therefore at most only partially and rarely transform precrisis human cells. We have found that pSVCT3 is able to transform precrisis human cells at high frequency. Furthermore, pSVCT3-transformed human precrisis cells relocalized T antigen to their nuclei. The relocalization of large T antigen was not dependent on cell growth. Wild-type and pSVCT3-transformed human cell lines both have about five copies of integrated SV40 DNA. SV40 virus-specific proteins, including the 100,000-molecular-weight super large T antigen, were expressed in pSVCT3-transformed human cells. Our results suggest that molecules in precrisis human cells, but not cells of other species, are able to complement the cytoplasmic-localization defect of the CT3 mutant large T antigen.

Downregulation of high density lipoprotein receptor activity of cultured fibroblasts by platelet-derived growth factor

High density lipoprotein (HDL) receptor activity was decreased by the addition of platelet-derived growth factor (PDGF) to cholesterol-loaded cultured human skin fibroblasts. Effects were observed within 12 hours, coinciding with increases in tritiated thymidine incorporation into DNA. The PDGF-mediated decrease in HDL binding was associated with a decrease in cholesterol efflux promoted by HDL3. PDGF exerted reciprocal effects on HDL and low density lipoprotein (LDL) receptor activity. With fibroblasts not loaded with cholesterol, PDGF increased LDL binding without changing HDL receptor activity. With cholesterol-loaded cells, PDGF decreased HDL receptor activity without changing LDL receptor activity. These results show that a reduction in HDL receptor activity is an additional cellular response to PDGF. Decreased HDL-mediated cholesterol efflux may result in retention of cholesterol needed for membrane synthesis in cells stimulated to proliferate.

In vitro inhibition of rat arterial smooth muscle cell growth by extractive sulfated mucopolysaccharides

The effect of a sulfated mucopolysaccharide mixture of known composition, extracted from pig duodenum, was studied on the proliferation of rat arterial smooth muscle cells cultured from rat aorta. Cell growth, stimulated by fetal calf serum, was monitored by direct cell count and by determination of the mitotic index. The extractive mixture was studied in comparison with commercial heparin, with heparin with different electrophoretic mobilities in barium acetate and with dermatan and heparan sulfates. Heparins and the extractive mucopolysaccharide mixture inhibited cell growth measured at various time intervals, and in their presence the proliferation of smooth muscle cells plateaued at lower cell densities. Dermatan and heparan sulfates were either inactive or significantly less effective than the other mucopolysaccharides. A short preincubation (3 h) of smooth muscle cells with the extractive mixture, followed by incubation with the growing medium with no mucopolysaccharides added, slowed the cell growing rate, suggesting an interaction of the mixture components with the cell surface.

In vitro and in vivo effects of endothelial cell-derived growth factor

Endothelial cells in culture produce a growth factor with an apparent molecular weight of 10,000 to 30,000 daltons. This material, the endothelial cell-derived growth factor ( ECDGF ), can replace the requirement of 3T3 and smooth muscle cells for exogenous mitogens. ECDGF , added to plasma-derived serum, a platelet deficient preparation of plasma, supports growth of these cells to confluency. Lyophilized preparations of the ECDGF stimulate migration of smooth muscle cells into an in vitro wound. The same material induces an angiogenic response when implanted on the chick chorioallantoic membrane. Thus two requirements of an angiogenic response, migration and proliferation, are supplied by endothelial cell products.

Relationship of simian virus 40 tumor antigens to virus-induced mutagenesis

We analyzed the mutation frequency to 8-azaguanine (8AZ) resistance in rat FR3T3 cells acutely infected with simian virus 40 wild type and tsA and early deletion mutants and in a series of temperature-sensitive (N) and temperature-insensitive (A) transformants derived from Chinese hamster lung (CHL) cells. Upon acute infection, the frequency of mutation to 8AZ resistance was raised at most by two- to eightfold over the spontaneous frequency, and it was independent of the presence of a functional 90,000-molecular-weight T antigen or 20,000-molecular-weight t antigen or both. Similarly, in the stable transformants of CHL cells, no correlation was found between functional T antigens and mutation to 8AZ resistance. It therefore seems unlikely that simian virus 40-induced transformation results from any mutagenic activity of this virus.

Relationship of simian virus 40 tumor antigens to virus-induced mutagenesis

We analyzed the mutation frequency to 8-azaguanine (8AZ) resistance in rat FR3T3 cells acutely infected with simian virus 40 wild type and tsA and early deletion mutants and in a series of temperature-sensitive (N) and temperature-insensitive (A) transformants derived from Chinese hamster lung (CHL) cells. Upon acute infection, the frequency of mutation to 8AZ resistance was raised at most by two- to eightfold over the spontaneous frequency, and it was independent of the presence of a functional 90,000-molecular-weight T antigen or 20,000-molecular-weight t antigen or both. Similarly, in the stable transformants of CHL cells, no correlation was found between functional T antigens and mutation to 8AZ resistance. It therefore seems unlikely that simian virus 40-induced transformation results from any mutagenic activity of this virus.

Effect of positively charged sphingomyelin liposomes on cholesterol metabolism of cells in culture

When cultured human skin fibroblasts were incubated at 37 degrees C with sonically dispersed positively charged sphingomyelin liposomes, sphingomyelin accumulated within the cell. This resulted in stimulation of cholesterol synthesis by increasing 3-hydroxy-3-methylglutaryl Coenzyme A reductase activity. Activation was rapid and was not due to the efflux of cell cholesterol or to cell growth and proliferation. Neither low density lipoprotein cholesterol nor nonlipoprotein cholesterol could suppress the sphingomyelin-induced cholesterol synthesis or activate the acyl-CoA cholesterol acyltransferase, despite an increase in cell cholesterol content. In contrast, the response to 7-ketocholesterol or 26-hydroxycholesterol was not impaired. The effect of sphingomyelin on cholesterol synthesis was temporary and reversible. Twenty-four hours after removal of sphingomyelin, cholesterol synthesis returned to normal and could be suppressed by LDL. Accumulation of sphingomyelin in the cell decreased lysosomal cholesteryl ester hydrolase but had no effect on the microsomal cholesteryl ester hydrolase. These results suggest that accumulation of sphingomyelin in the cell markedly affects cellular cholesterol homeostasis. Resultant accumulation of cholesteryl esters in the presence of extracellular cholesterol could be relevant to atherogenesis.

Neovascular responses induced by cultured aortic endothelial cells

Neovascularization was studied in the chorioallantoic membrane of the chick embryo after implantation of bovine aortic endothelial and smooth muscle cells, Swiss and BALB/c 3T3 cells and human diploid fibroblasts cultured separately on microcarrier beads. Quantitative analysis of neovascularization indicated a 3 1/2-fold increase in the number of blood vessels responding to endothelial cells while smooth muscle cells induced a twofold increase when compared to the response of beads without cells. Skin fibroblasts and Swiss 3T3 cells did not elicit a comparable response. The marked angiogenic response induced by endothelial cells was characterized by a 137% increase in total vessel length and a 35% increase in average vessel area when compared to controls. Two of the properties required for an angiogenesis factor--stimulation of cellular migration and proliferation--can also be demonstrated using endothelial cell-conditioned medium in cell culture systems. Medium from cultured bovine aortic endothelium stimulates DNA synthesis, proliferation, and migration of smooth muscle cells. In addition, conditioned media from both endothelial cells and smooth muscle cells produced an angiogenic response in the chorioallantoic membrane assay, which was comparable to that produced by intact cells growing on microcarrier beads. Similar responses were not evident with medium conditioned by other cell types. These results indicate the potential importance of endothelial cells and endothelial cell products in regulating blood vessel growth.

Nonlytic simian virus 40-specific 100K phosphoprotein is associated with anchorage-independent growth in simian virus 40-transformed and revertant mouse cell lines

Normal fibroblasts display two distinct growth controls which can be assayed as requirements for serum or for anchorage. Interaction of mouse 3T3 fibroblasts with simian virus 40 (SV40) thus generates four classes of transformed cells. We have examined viral gene expression in these four classes of cell lines. Immunoprecipitation of [35S]methionine-labeled cell extracts with an antiserum obtained from tumor-bearing hamsters detected the SV40 large T and small t proteins (94,000 molecular weight [94K], 17K) and the nonviral host 54K protein in all cell lines tested. A tumor antigen with an apparent molecular weight of 100,000 was also found in some, but not all, lines. Similar "super T" molecules have been found by others in many rodent transformed lines. We carried out an analysis of the relation of phenotype to relative amounts of these proteins in cell lines of the four classes, using the Spearman rank correlation test. The amount of the 100K T antigen relative to the 94K T antigen or to total viral protein was well correlated with the ability to form colonies in semisolid medium. No significant correlation was found between quantities of labeled 94K T antigen, 54K host antigen, or 17K t antigen and either serum or anchorage independence. Mouse cells transformed with the small t SV40 deletion mutant 884 synthesized a 100K T antigen, suggesting that small t is not required for the production of this protein. The 100K T antigen migrated more slowly than lytic T. Since mixtures of extracts from cells expressing and lacking the 100K T antigen yielded the expected amount of this protein, it is unlikely that the 100K T derives from the 94K protein by a posttranslational modification.

Transport of 6-deoxy-D-glucose and D-xylose by untransformed and SV40-transformed 3T3 cells

Transport rates of the nonphosphorylated D-glucose analogs 6-deoxy-D-glucose and D-xylose were measured in quiescent and serum-stimulated cultures of mouse 3T3 cells, in SV40-transformed 3T3 cells (SV101), and in a density revertant cell line derived from SV101 (FI-SV101). Initial rates of both entry and exit of 6-deoxy-D-glucose and D-xylose were more than threefold higher in serum-stimulated 3T3 and in SV101 cells than they were in quiescent 3T3 cells, but transport rates were not higher in the transformed cells (SV101) than they were in serum-stimulated 3T3. Confluent cultures of FI-SV101 showed lower rates of transport than serum-stimulated FI-SV101, but not as low as quiescent 3T3 cells enter the quiescent G0 state, but emphasize that SV40-transformed 3T3 cells do not show higher activity of the D-glucose carrier than do actively growing 3T3 cells. Thus, enhanced glucose transport appears not to be a specific consequence of transformation, but a reflection of the active growth state of the cell.

Long-term effect of hyperlipidemic serum on the synthesis of glycosaminoglycans and on the rate of growth of rabbit aortic smooth muscle cells in culture

Rabbit aortic smooth muscle cells (SMCs) were successfully subcultured in 10% hyperlipidemic rabbit serum (HLS) for at least 9 passages. SMCs grown in HLS grew into higher cell densities than SMCs cultured in normolipidemic rabbit serum (NLS) for at least 4-5 passages in NLS and HLS, respectively. However, cells cultured in NLS and HLS for up to 7 passages had similar growth characteristics when they were trypsinised and seeded to grow in 10% fetal calf serum (FCS). Incorporation of [3H]glucosamine into GAGs was taken to represent their rate of synthesis. As compared with cultures incubated in 10% NLS, incubation of rabbit aortic SMCs in the presence of 10% HLS increased the synthesis of sulphated GAGs secreted into the pericellular space by 35% during the first 24 h of contact with HLS. After preincubation for one week in the presence of HLS the synthesis of sulphated GAGs secreted into the incubation medium and into the pericellular space was stimulated by 95% and 34%, respectively. The stimulation of the synthesis of sulphated GAGs by HLS continued for up to 4 weeks at least if the contact of the cells with HLS was maintained. When the cells were subcultured in the presence of NLS and HLS and seeded to grow in FCS after the 1st, 3rd and 7th trypsinisations, the synthesis of sulphated GAGs in cultures of cells from both sources was similar.

Ability of endothelial cells to condition culture medium

Endothelial cell-conditioned medium contains two classes of factors distinguishable by behavior during dialysis and on specificity for cell type. One species, which diffuses through dialysis tubing with an exclusion limit of 6,000 to 8,000 daltons, supports growth of bovine aortic endothelial (BAE) cells in medium containing a growth limiting concentration of serum (0.2% serum). The production of this material appears to depend upon the presence of serum in the medium being conditioned. The activity increases with time of exposure of BAE cells to serum and with increasing concentration of serum present in the incubation medium. This activity cannot be replaced by exogenous epidermal growth factor, fibroblast growth factor, platelet-derived growth factor, insulin, or thymidine. The second species, the endothelial cell-derived growth factor (ECDGF), is retained by dialysis tubing with an exclusion limit of 6,000 to 8,000 daltons. ECDGF stimulates the growth of smooth muscle cells but does not support BAE cell growth in limiting serum concentrations. unlike the dialyzable species, the production of ECDGF is independent of previous incubation of BAE cell cultures in serum. These studies suggest that BAE cells are able to utilize serum components to produce conditioning factors for their own growth that are distinct from the higher molecular weight ECDGF.

Nonlytic simian virus 40-specific 100K phosphoprotein is associated with anchorage-independent growth in simian virus 40-transformed and revertant mouse cell lines

Normal fibroblasts display two distinct growth controls which can be assayed as requirements for serum or for anchorage. Interaction of mouse 3T3 fibroblasts with simian virus 40 (SV40) thus generates four classes of transformed cells. We have examined viral gene expression in these four classes of cell lines. Immunoprecipitation of [35S]methionine-labeled cell extracts with an antiserum obtained from tumor-bearing hamsters detected the SV40 large T and small t proteins (94,000 molecular weight [94K], 17K) and the nonviral host 54K protein in all cell lines tested. A tumor antigen with an apparent molecular weight of 100,000 was also found in some, but not all, lines. Similar "super T" molecules have been found by others in many rodent transformed lines. We carried out an analysis of the relation of phenotype to relative amounts of these proteins in cell lines of the four classes, using the Spearman rank correlation test. The amount of the 100K T antigen relative to the 94K T antigen or to total viral protein was well correlated with the ability to form colonies in semisolid medium. No significant correlation was found between quantities of labeled 94K T antigen, 54K host antigen, or 17K t antigen and either serum or anchorage independence. Mouse cells transformed with the small t SV40 deletion mutant 884 synthesized a 100K T antigen, suggesting that small t is not required for the production of this protein. The 100K T antigen migrated more slowly than lytic T. Since mixtures of extracts from cells expressing and lacking the 100K T antigen yielded the expected amount of this protein, it is unlikely that the 100K T derives from the 94K protein by a posttranslational modification.

Platelet-derived growth factor stimulates activity of low density lipoprotein receptors

Partially purified platelet-derived growth factor stimulates low density lipoprotein binding and degradation in cultured aortic smooth muscle cells of monkeys by increasing the number of available low density lipoprotein receptors. When platelet-derived growth factor was added to quiescent cells, low density lipoprotein binding increased within 4-8 hr. Stimulation of low density lipoprotein receptor activity preceded stimulation of DNA synthesis by platelet-derived growth factor by 8-12 hr. Enhancement of endogenous cholesterol synthesis by platelet-derived growth factor preceded stimulation of low density lipoprotein receptor activity. These findings suggest that the platelet-derived growth factor can increase both the exogenous and endogenous supplies of cholesterol to the cell for its use during cell proliferation.

An endothelial cell-derived growth factor

Cell-free plasma-derived serum (PDS) is deficient in the platelet-derived growth factor and will not support the growth of 3T3 cells, fibroblasts, or smooth muscle cells. However, when PDS-containing medium is preincubated with endothelial cells, the medium becomes modified so that it will support growth. The activity produced by the endothelial cells results from a polypeptide of 10,000 to 30,000 daltons which has several features that differ from those of the platelet-derived growth factor, including heat instability and lack of adsorption to CM Sephadex.

Role of serum components in density-dependent inhibition of growth of cells in culture. Platelet-derived growth factor is the major serum determinant of saturation density

The effects of platelet-derived growth factor and plasma components on saturation density in cultures of 3T3 cells were investigated. Both of these components of whole blood serum affect saturation density; however, when 3T3 cells become quiescent at high density in medium containing whole blood serum, only platelet-derived growth factor and fresh whole blood serum are capable of stimulating proliferation. Addition of fresh plasma- derived serum has little effect on cell growth. These results suggest that the platelet factor is the major determinant of saturation density in cultures of 3T3 cells maintained in medium supplemented with whole blood serum. Experiments were performed to investigate the mechanism by which platelet-derived growth factor regulates saturation density. We investigated the possibilities of inactivation of growth factors by proliferating cells, and the effects of cell density on the response of 3T3 cells to platelet-derived growth factor. The amount of platelet- derived growth factor required to initiated DNA synthesis increases with increasing cell density. Some inactivation of growth factors by growing cells was detected, but this depletion was only evident at high cell density. We propose that density-dependent inhibition in cultured 3T3 cells is the result both of an increased requirement for the platelet- derived growth factor as the cultures become more crowded and of inactivation of growth factor activity by growing cells.

Characterization of cell lines showing growth control isolated from both the wild type and a leucyl-tRNA synthetase mutant of Chinese hamster ovary cells

The genetic approach to the problem of cellular growth control is limited by the availability of recessive mutations in cell lines which are capable of growth control in vitro. The CHO cell line has yielded many recessive mutations including, for example, tsH1, a temperature sensitive leucyl-tRNA synthetase mutant, which under non-permissive conditions rapidly shuts down protein synthesis and generates uncharged tRNA. Both CHO and tsH1 are transformed, however, and do not respond to environmental stimuli with the coordinated regulation of macromolecular processes observed in normal diploid fibroblasts. We describe here the isolation and characterization of growth control revertants obtained from both CHOwt and tsH1. The best of these GRC+L-73, isolated from tsH1, had 20 chromosomes, one less than tsH1, had normal fibroblastic morphology, would not grow in suspension, required high serum concentrations for growth, grew to relatively low cell densities at saturation in monolayer culture and showed a stationary phase characterized by arrest in a G1-like state with maintenance of high viability for several weeks. It is expected that this line as well as a ts revertant GRC+LR-73 will greatly facilitate the genetic investigation of growth control and, in particular, will help to elucidate the role of uncharged tRNA in the regulation of macromolecular synthesis in mammalian cells.

2-amino-isobutyric acid and 3-O-methyl-D-glucose transport in 3T3, SV 40-transformed 3T3 and revertant cell lines

In order to further investigate the connection between transport and growth control, 3T3 cells, SV40 transformed 3T3 cells (SV101), and three revertant cell lines derived from SV101 which have regained certain manifestations of growth control were used. Transport rates of 2-amino-isobutyric acid and 3-O-methyl-D-glucose were measured in sparse, confluent, serum-starved, and serum-stimulated cultures. As shown before, cessation of 3T3 cell growth in G0 under conditions of confluence or serum deprivation was associated with reduced rates of transport for both compounds, whereas the density and serum dependence of growth and transport was largely eliminated in SV101. The density revertant F1SV101, which has regained density regulation of growth similar to 3T3 cells, has also regained density regulation of transport. Neither growth nor transport were serum dependent. The serum revertants AgammaSV7 and LsSV6 have regained both density and serum regulation of growth, but not according to the original mechanism of 3T3 cells of entry into a Go state. Transport was high under conditions of confluence or serum deprivation. Thus for these cells rates of transport were not reduced simply as a consequences of slower cell growth nor were low transport rates responsible for growth arrest. The data are consistent with the possibility that growth arrest specifically in the G0 state could shut off a number of cellular activities, including transport.

Isolation and characterization of T antigen-negative revertants from a line of transformed rat cells containing one copy of the SV40 genome

Negative selection with FUdR produced revertants from the transformed rat line 14B, which contains one insertion of the SV40 viral genome (Botchan, Topp and Sambrook, 1976). 14B contains nuclear T antigen, grows to a high density, grows in low serum and is anchorage-independent. The revertants fall into three classes with regard to viral DNA sequences: the SV40 DNA is retained; the SV40 DNA is retained but has undergone a deletion; and the SV40 DNA is lost, generating a cured cell. This heterogeneity is not a result of long-term passage. The revertants arise with a frequency of one in 8.4 X 10(5) cells after as few as 12 passages. All three classes of revertants are T antigen-negative, density-sensitive, more serum sensitive than 14B and anchorage-dependent. These data argue for a direct role of the functioning viral genome in the maintenance of the transformed state, and that with 14B, the phenotypes of transformation are not virus gene dosage-dependent.

Surface glycoproteins and concanavalin-A-mediated agglutinability of clonal variants and tumour cells derived from SV40-virus-transformed mouse 3T3 cells

Cell strains isolated from an established line of SV40-transformed mouse 3T3 cells (SV3T3) showed large variations in the various parameters of transformation, viz. saturation density, serum requirement, contact inhibition of movement and of growth and Concanavalin-A-mediated agglutinability. These cell strains were studied for changes in elution profiles of fucose-labelled surface glycoproteins, using actively growing, untransformed 3T3 cells as controls. A cell strain established from a tumour arising after injection of wild-type SV3T3 cells and SV3T3 cells grown in vivo in diffusion chambers, was similarly studied. Changes in surface glycoproteins were observed only in the tumour-derived cell strain. The results suggest that alterations in surface glycoproteins are associated with the tumorigenic potential of the cells rather than with the transformed phenotype per se.

Isolation and characterization of revertant cell lines. VII. DNA synthesis and mitotic rate of serum-sensitive revertants in non-permissive growth conditions

The ability to synthesize DNA and enter mitosis was studied in Balb/c and Swiss 3T3 cells, SV40 and MSV-transformed 3T3 cells and revertants of these transformed cells in cultures of different serum concentrations and cell densities. Three ways were found by which cells were able to maintain a constant cell number in non-permissive growth conditions: cessation of DNA synthesis, synthesis of DNA coupled with failure to enter mitosis, and the slow traverse of the cell cycle coupled with cell shedding. Growth control of the revertant of an MSV-transformed Balb/3T3 cell most closely resembled that of Balb or Swiss 3T3. This line did not grow in 1% serum and did not synthesize DNA in either non-permissive condition. Serum-sensitive revertants of SV40-transformed 3T3 cells are also unable to grow in 1% serum and also do not grow beyond confluence in 10% serum, but these cells differ from 3T3 in the manner in which this growth arrest is accomplished. In 1% serum, revertants synthesize DNA but do not enter mitosis. At confluence in 10% serum, they slowly traverse the cell cycle, with dividing cells replacing cells that are shed into the medium.

Isolation and characterization of revertant cell lines. VI. Susceptibility of revertants to retransformation by simian virus 40 and murine sarcoma virus

The susceptibility of two classes of revertants of Simian virus 40 (SV40)-transformed 3T3 cells to retransformation by SV40 or murine sarcoma virus (MSV) was studied. Both serum-sensitive and density-sensitive revertants are not retransformable by SV40. MSV can transform both types of revertants. The MSV-transformed revertants grow to high cell densities and form colonies when suspended in semi-solid methylcellulose medium, but are unable to grow in 1% calf serum. The MSV-transformed revertants produce infectious MSV and murine leukemia virus and possess the same number of chromosomes as the untransformed revertants.

Selection of revertants of Kirsten sarcoma virus transformed nonproducer BALB-3T3 cells

Revertants of Kirsten sarcoma virus transformed nonproducer BALB/3T3 cells (KA31 cells) were isolated after exposing the transformed cells to 5-fluorodeoxyuridine at high cell density, or when suspended in methylcellulose. Revertants were also isolated by treating KA31 cells with the lectin, concanavalin A, which is manyfold more toxic to transformed cells than for normal cells. The revertants resemble BALB/3T3 cells in their morphology and growth characteristics in that they have a low saturation density, fail to grow in 1% calf serum or when suspended in methylcellulose, and cease to synthesize DNA after reaching their saturation density. Infection by murine leukemia virus rescues Kirsten sarcoma virus from only the concanavalin-A-selected variants, though all the revertants are susceptible to infection by leukemia virus. The concanavalin A revertants also become transformed after infection with murine leukemia virus. All the revertants can be transformed by Kirsten sarcoma virus but not by simian virus 40.