Cell cycle changes in transformed cells growing under serum-free conditions

Stimulating effects of insulin and low-density lipoprotein on cell growth and macromolecular syntheses of HeLa cells cultured in K(+)-depleted medium

The influence of the intracellular K+ concentration on the effects of growth factors (insulin, EGF, hydrocortisone, and transferrin) and LDL on growth of HeLa cells was investigated. Upon replacement of K+ in a chemically defined medium (K(+)-CDM) by Rb+ (Rb(+)-CDM), about 80% of the intracellular K+ was replaced by Rb+ within 24 h, but showed no further change in the next 24 h, irrespective of addition of dialyzed calf serum (5%) or growth factors to the medium. In Rb(+)-CDM, cell growth and DNA synthesis were greatly suppressed, although cell viability was not significantly altered for 72 h. The suppression of cell growth was partially restored by addition of serum, insulin (5 micrograms/ml), or LDL (2.5 mg/ml) to Rb(+)-CDM. A combination of serum and insulin or insulin and LDL stimulated cell growth to approximately the level in K(+)-CDM without any addition, but a combination of serum and LDL did not have more effect than that of serum alone. Unexpectedly, other factors were ineffective in stimulating growth in Rb(+)-CDM. In Rb(+)-CDM, the effect of insulin was lost in 24-48 h, whereas that of LDL persisted for at least 96 h. Insulin and LDL also enhanced growth in K(+)-CDM. After cessation of cell growth in Rb(+)-CDM for 24 h, addition of insulin and/or LDL markedly restored cell growth and DNA synthesis. Therefore, insulin and LDL may stimulate certain mechanisms required for cell growth that can operate in K(+)-deficient conditions.

Isolation and characterization of a cloned growth factor dependent macrophage cell line, BAC1.2F5

The SV40 transformed murine macrophage cell line, BAC1, proliferates in response to the colony stimulating factor, CSF-1 (Schwarzbaum et al., J. Immunol., 132:1158, 1984). In order to obtain a cell line suitable for biochemical and genetic studies of CSF-1 signal transduction, clones of BAC1 were established. Clones ranged from being completely autonomous to being completely dependent on CSF-1 for growth. Cells of one clone (2F5), which proliferated in response to either CSF-1 or granulocyte-macrophage CSF (GM-CSF) were characterized in detail. The kinetics of receptor-mediated internalization and intracellular destruction of CSF-1 were comparable to the kinetics observed with peritoneal exudate macrophages. CSF-1 was shown to regulate cell spreading, cell survival, protein degradation, and the duration of the G1 and S phases of the cell cycle. The 2F5 clone therefore exhibits a number of CSF-1 stimulated responses and is being used for genetic and biochemical studies of CSF-1 action.

The effects of glucose and amino acids on tumor and host DNA synthesis

Tumor-bearing animals provided with intravenous glucose and amino acids (TPN) exhibit enhanced response to S-phase-specific chemotherapeutic agents (H. M. Reynolds, J. M. Daly, B. Rowlands, S. J. Dudrick, and E. M. Copeland. Cancer 45: 3069, 1980; M. H. Torosian, J. L. Mullen, E. E. Miller, et al. J. Parenter. Enteral Nutr. 7: 337, 1983). To determine the mechanism of this response, DNA synthesis rate during starvation or a 48-hr infusion of glucose/amino acids (Glu/AA) was evaluated in tumor, liver, and terminal ileal cells of 68 rats. Tumor cells exhibited a rapid increase in DNA synthesis following the initiation of an infusion of Glu/AA. This increase was most marked after 2 hr of infusion and returned to control levels within 24 hr. Liver DNA synthesis rate increased in both starved and Glu/AA animals over 48 hr with a larger increase in animals receiving Glu/AA. Ileal DNA synthesis decreased equally in both groups. Short pulse Glu/AA produced transient increases in tumor DNA synthesis. Changes in host tissues occurred but followed a different temporal sequence. This may indicate the existence of a period of time following initiation of metabolic manipulation when tumor susceptibility to phase-specific chemotherapeutic agents will be enhanced while host tissues will be spared from increased toxicity.

The serum growth and survival requirements of SV40-transformed 3T3 cells

Simian virus 40-transformed 3T3 cells are dependent on serum for survival and growth. This growth activity can be separated on a pH 2 Sephadex G100 column into two fractions: a high molecular weight activity and a low molecular weight substance that has recently been characterized as containing as its major agent, biotin. To replace the remainder of the serum requirement, hormones and other growth factors were tested. Both insulin at high, nonphysiological concentrations (200 to 500 ng/ml) and transferrin (5 X 10(-8) M) stimulate the growth rate in low serum medium (0.3% v/v bovine calf serum DME) individually and, when added together, are nearly as growth enhancing as 10% serum. The need for the residual serum in this medium can be eliminated by the use of crystalline trypsin during trypsinization. Under these serum-free conditions, biotin and transferrin supplementation provide for moderately good growth (20 to 30 hr population doubling time, 1 X 10(6) cells/3.2-cm dish final cell density). Insulin addition further stimulate the growth rate (16 to 20 hr) and the final density (1.5 X 10(6) cells). Although the protein growth factors, EGF (0.5 to 1.0 ng/ml) and FGF (4 to 10 ng/ml), also appear to enhance growth individually and additively, their effects are slight and very variable. Nevertheless, the complete serum-free medium (DME supplemented with biotin, transferrin, insulin, EGF, and FGF) yields growth comparable but still inferior to 10% serum supplementation (14-versus 12-hr population doubling time, 1 to 2 X 10(6) versus 2 to 3 X 10(6) cells final cell density).

The suppression of cellular proliferation in SV40-transformed 3T3 cells by glucocorticoids

Glucocorticosteroids, when added two hours after cell plating to SV40-transformed, 3T3 mouse fibroblasts in low serum (0.3% v/v), biotin-supplemented medium, suppress cellular proliferation by 24 hours. While some cell death probably occurs, the growth inhibition is not primarily due to cytotoxicity and cytolysis. This conclusion is supported by the following: 1) both dead and viable cell numbers are suppressed, 2) little cell debris is evident in the medium, and 3) very high concentrations of glucocorticoids do not cause an increase in the dead cell count. Furthermore, this growth suppression, which is specific for glucocorticoids since several non-glucocorticoid steroids have no inhibitory effect, is not permanent nor irreversible. Removal of the glucocorticoid and replacement with 10% serum restore rapid proliferation. Although higher concentrations (1% and 10%) of serum afford some protection against glucocorticoid inhibition, this protection is not simply a consequence of faster growth rates. SV3T3 cells can be grown in serum-free medium supplemented with biotin, transferrin, insulin, and epidermal growth factor (EGF). Under these conditions growth rates are comparable to high serum media, yet glucocorticoids are still powerful inhibitors. However, the omission of insulin from serum-free, glucocorticoid cultures does result in observable cell death and lysis. Flow microfluorometry and autoradiographic studies have determined that glucocorticoid-inhibited cells are partially blocked in G1. The proportions of S phase and G2 + M cells are greatly reduced with an accompanying accumulation of G1 cells. These results suggest that glucocorticoids regulate a biochemical step(s) in G1 which is critical for DNA initiation.

Growth and G1 arrest of sarcoma virus trnasformed cells in serum free media

Rous-sarcoma transformed BHK cells can be continuously cultured in a medium containing Eagle's Minimal Essential Medium, iron and biotin. The rate of cell multiplication increased when serine, or serine plus other non-essential amino-acids were added to the medium. With biotin deleted from the medium there is a reduction in DNA synthesis and most cells are blocked in G1.

Cell cycle analysis of sodium butyrate and hydroxyurea, inducers of ectopic hormone production in HeLa cells

Sodium butyrate and hydroxyurea, effective inhibitors of DNA synthesis in HeLa cells, cause these cells to produce increased levels of the ectopic glycopeptide hormones human chorionic gonadotropin (hCG), follicle stimulating hormone (FSH), and free alpha chains for these hormones. The objective of this study was an assessment of the role of modulation of cell cycle events in the action of these two chemical agents. A variety of experimental approaches was employed to obtain a clear view of the drugs' effects on cells located initially in all phases of the cell cycle. Cells in early G1, G2, or M phase at time of addition of either inhibitor were not arrested at early time points, but by 48 hours became collected at a location characteristic for each drug, near the G1-S phase boundary. Flow microfluorometry (FMF) and thymidine labeling index revealed that butyrate-treated cells arrested late in G1 phase very close to S phase, while hydroxyurea-blocked cells continued to early S phase. Both inhibitors prevented cells originally in S phase from reaching mitosis. S cells exposed to hydroxyurea were killed by 48 hours, but those growing in 5 mM butyrate progressed to the end of S or G2 phase where they became irreversibly arrested although not removed from the monolayer. Analysis of the cell cycle location and viability of each subpopulation resulting from 48 hour exposure to butyrate or hydroxyurea is important for the study of the function of each cellular subset. Treatment of HeLa cells with lower concentrations of butyrate (1 mM) resulted in slowed yet exponential growth. Fraction labeled mitosis (FLM) analysis shows that this is a result of prolongation of the G1 phase.

Transformed cells have lost control of ribosome number through their growth cycle

Previous studies on the synthesis and function of the protein synthetic machinery through the growth cycle of normal cultured hamster embryo fibroblasts (HA) were extended here to a series of four different clonal lines of polyoma virus-transformed HA cells. Under our culture conditions, these transformed cells could enter a stationary phase characterized by no mitotic cells, very low rates of DNA synthesis, and arrest in a post-mitotic pre-DNA synthetic state. Cellular viability was initially high in stationary phase but, unlike normal cells, transformed cells slowly lost viability. The rate of protein synthesis in the stationary phase of the transformed cells fell to 25-30% of the exponential rate. Though this reduction was similar to that seen in normal cells, it was accomplished by different means. The specific reduction in the ribosome complement per cell to values below that of any cycling cell seen in normal cells, was not seen in any of the transformed lines. This observation, which implies a loss of normal control of ribisome synthesis through the growth cycle after transformation, was confirmed in normal Chinese hamster embryo fibroblasts and transformed CHO cell lines. Normal control of ribosome synthesis was restored in L-73 and LR-73, growth control revertants of one of the transformed CHO lines. The transformed lines reduced their protein synthetic rates in stationary phase either by a greater reduction in the proportion of functioning ribosomes than that seen in normal cells or by a decrease in the elongation rate of functioning ribosomes; the latter effect was not seen in the normal cells. A model for growth control of normal cells and its derangement in transformed cells is presented.

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.

T antigen and initiation of cell DNA synthesis in a temperature-sensitive mouse line transformed by an SV40tsA mutant and in heterokaryons of the transformed cells and chick erythrocytes

The role of SV40 gene A product in initiation of cellular DNA synthesis was investigated, using a mouse kidney line [mKSA207] transformed by SV40tsA207. mKSA207 cells were temperature sensitive for growth, lost SV40 T antigen (Tag) when incubated in low serum at 40degreeC, and accumulated Tag in the cytoplasm when fed 10% serum and incubated at the nonpermissive temperature (39.7degreeC). Following serum addition, the percentage of mKSA207 cells synthesizing DNA was essentially the same at nonpermissive (39.7 degrees C) and permissive temperatures (33.5degreeC). The cells entered S phase asynchronously at both temperatures, but most cells entered S within 16 h, and before Tag accumulated. mKSA207 synchronized by a double thymidine block also synthesized DNA at 39.7degreesC and entered a second S phase. Tag-depleted or Tag-synchronized mKSA207, when fused with chick erythrocytes (CE), activated CE DNA synthesis. At nonpermissive temperatures (39.7degreesC), 40% of CE nuclei in heterokaryons with Tag-depleted mKSA207 displayed 3H-thymidine--labeled nuclei 28--40 h after fusion, when only 12% of CE nuclei were Tag+. The experiments indicate that SV40 gene A product probably does not have a direct role as initiator of cellular DNA synthesis.