Progress through G1 and S in relation to net protein accumulation in human NHIK 3025 cells

Tumour necrotisation in nude mice xenografts by the reversible protein synthesis inhibitor zilascorb(2H)

The deuterated benzaldehyde derivative zilascorb(2H), 5,6-O-benzylidene-d-L-ascorbic acid, was administered once daily by i.v. injection in nude mice with grafted tumours of a human malignant melanoma (E.E.) and ovarian carcinoma (OVCAR-3) origins. Like benzaldehyde, zilascorb(2H) has been shown to induce protein synthesis inhibition at otherwise non-toxic doses in cells grown in vitro, and acts reversibly in the sense that protein synthesis returns to normal shortly after removal of the drug. The present data indicate that daily injections with zilascorb(2H) induce a tumour volume growth inhibitory effect in both tumour xenografts studied. Furthermore, from histological examinations of each single tumour it was found that tumours of drug-treated animals, although smaller than those of placebo-treated (i.e. control) animals, had, on average, a higher necrotic fraction than control tumours. Thus, it is concluded that zilascorb(2H) induces tumour necrotisation and not just inhibition of the rate of tumour cell production. Continued measurement of tumour volume after ended treatment with zilascorb(2H) indicated that surviving tumour cells resumed their normal growth rate immediately. The reversibility of the effect induced by this compound, earlier observed in vitro only, is therefore here confirmed to be valid also in two different tumour xenografts in vivo. The present data accords well with the assumption that protein synthesis inhibition is the primary cellular effect of zilascorb(2H) in vivo. We therefore conclude that zilascorb(2H)-induced cancer cell lethality in tumour xenografts probably comes as a secondary consequence of prolonged protein synthesis inhibition.

Effect of two pyrimidine analogs on accumulation of tubulin in NHIK 3025 cells

Accumulation of tubulin as compared with the accumulation of total cellular protein in human NHIK 3025 cells treated with the sulfone 2-(2-thenyl)sulfonyl-5-bromopyrimidine (NY 4137) and the sulfoxide 2-(2-thenyl)sulfinyl-5-bromopyrimidine (NY 4138), two mitotic inhibitors, were investigated by two-parametric flow cytometry. Following a 4 h treatment with NY 4137 tubulin accumulation is inhibited while total protein continues to accumulate. After treatment for 4 h with NY 4138 the accumulation of total protein is approximately constant, while the accumulation of tubulin is reduced although not to the same degree as that found for NY 4137-treated cells. In addition, the percentage tubulin SH-groups (6.89 +/- 0.14) remaining after treatment of purified rat brain tubulin with NY 4137 or NY 4138 was determined. Treatment with 0.0125 mM NY 4137 reduced the number of tubulin SH-groups detectable with dithiobis benzoate or from 6.89 +/- 0.14 before treatment to about 4 after treatment. However, practically all SH-groups of tubulin remain detectable following treatment with the same concentration of NY 4138. From the results described in this report we infer that NY 4137 binds to tubulin SH-groups and that inhibition of tubulin accumulation follows as a secondary effect.

Effects of increased cell mass and altered gene dosage on the timing of initiation of macronuclear DNA synthesis in Paramecium tetraurelia. Implications for cell cycle regulation

In Paramecium, cell mass and macronuclear DNA content can vary substantially, and both variables affect the timing of initiation of macronuclear DNA synthesis. Cells normally begin macronuclear DNA synthesis at 0.25 in the cell cycle when the mean cell mass is about 119% of the initial value. Gene mutations were used to alter cell size by temporarily blocking cell cycle progression and to change DNA content by altering the segregation pattern of macronuclear material to daughter nuclei at fission. Changes in cell mass or macronuclear DNA content imposed at fission or in the subsequent G1 interval do not affect the timing of initiation of DNA synthesis in that cell cycle, but do affect the timing of initiation of DNA synthesis in the subsequent cell cycle. The progeny of cells with lower than average macronuclear DNA content tend to initiate DNA synthesis earlier than other cells. The G1 interval is proportionally shortened when initial cell mass is greater than normal, and no measurable G1 interval is present when initial cell mass equals or exceeds the normal cell mass present at initiation of DNA synthesis. These results suggest that the timing of initiation of DNA synthesis is established during the preceding cell cycle and that the 'timer' mechanism is not significantly affected by either drastic changes in gene dosage or gene concentration during the G1 interval.

Cell cycle controls in higher eukaryotic cells: resting state or a prolonged G1 period?

We express the viewpoint that control over cell growth in higher eukaryotes is achieved predominantly by regular transition of cells from proliferation to rest and vice versa as a result of coordinated interrelationship between intracellular growth inhibitors and extracellular growth factors. The resting state is considered as a special physiological state of a cell where the prereplicative reactions necessary for the onset of DNA synthesis are inhibited. Cells pass into a resting state at each successive cell cycle, with regard to the next cycle, once the threshold level of growth inhibitors has been attained. Cellular rest may thus initiate and proceed in parallel with conventional periods of the cell cycle but in a hidden way. Its termination strictly depends on the appropriate concentration of extracellular growth factors. In the absence of growth factors cells, after completing mitosis, pass into an overt state of rest metabolically different from any period of the cell cycle including G1.

Immediate effects of serum depletion on dissociation between growth in size and cell division in proliferating 3T3 cells

Proliferating nonconfluent 3T3 cells become committed to proceed through the cell cycle or to enter G0 during the first post-mitotic part of G1 (G1pm). The decision to proceed through G1pm is dependent on the presence of serum growth factors in the culture medium. Cells that have passed this particular growth-factor-dependent cell cycle stage are independent of serum growth factors and undergo mitosis on schedule. We report here that G1ps, S, and G2 cells cease to increase in size when serum is withdrawn. As a result the mitotic cell size after 8 hours serum starvation is reduced to approximately 60% of the normal mitotic cell. This reduced growth in cell size is due to a rapid decrease in protein synthesis and some increase in protein degradation. This dissociation between growth in size and cell-cycle progression within a single cell cycle provides a new approach to study the two processes separately.

Protein synthesis in 3T3 cells stimulated to proliferate at various rates

Reproducible conditions were defined for using rates of leucine incorporation as a valid measure of rates of de novo protein synthesis in mouse 3T3 cells. Upon stimulation of quiescent cultures, rates of de novo synthesis of proteins increased and pool levels of amino acids decreased in proportion to the concentration of serum in the stimulating medium. Rates of de novo protein synthesis (per cell) exhibited a biphasic pattern of increase. These rates approached a plateau value at the end of the lag phase and increased again as cells entered S phase. This pattern of behaviour helps to explain the observed relationships between cell growth (increase in mass) and cell proliferation (increase in cell number).

A gene function required for cell cycle progression during the G1 portion of the cell cycle and for maintenance of macronuclear DNA synthesis in Paramecium tetraurelia

The ccl mutation in Paramecium tetraurelia reversibly and rapidly blocks cell cycle progression and DNA synthesis at the restrictive temperature. Progression through the cell cycle is blocked during both the G1 and S portions of the cell cycle, while at the restrictive temperature there is neither residual cell cycle progression nor induction of excess delay of subsequent cell cycle events. DNA synthesis activity is reduced to 50% of the normal level in about 5 min and is completely blocked at 30 min after a shift to restrictive temperature. On return to permissive conditions, DNA synthesis is reactivated with similar kinetics.

Doubling of cell mass is not necessary in order to achieve cell division in cultured human cells

When exponentially growing NHIK 3025 cells were shifted from medium containing 30% serum to medium containing 0.03% serum the rate of net protein accumulation was reduced due to both a reduction in the rate of protein synthesis and an increase in the rate of protein degradation. This change in growth conditions increased the protein doubling time from 18 to 140 h. The cell cycle duration of cells synchronized by mitotic selection was, however, only increased from 17 to 26 h by this treatment. Therefore, when the cells divide by the end of the first cell cycle following synchronization, the cells shifted to 0.03% serum contained far less protein than those growing continuously in 30% serum. Hence, the attainment of a critical cell mass is probably not controlling cell division for cells growing in a balanced state.

The relative effects of different types of growth factors on DNA replication, mitosis, and cellular enlargement

It was recently demonstrated that growth in cell size can be dissociated from DNA synthesis and mitosis. 3T3 cells starved to quiescence in low serum concentration can be stimulated to undergo DNA synthesis and one cell division without growing in size (unbalanced growth) (42-44). We report here that in cells stimulated to undergo unbalanced growth, the cell nucleus undergoes balanced growth, i.e., nearly doubles in size prior to mitosis. The reduced ability to grow in cell size under unbalanced growth conditions is thus mainly ascribable to the cytoplasm. Furthermore, the extent to which cells grow in size prior to mitosis is dependent on the serum concentration in the tissue culture medium (44). This data suggests that some macromolecular factor or factors in serum are required for growth in cell size prior to mitosis. We report in this study that epidermal growth factor (EGF) alone exerts a small but significant stimulatory influence on DNA synthesis and mitosis but does not affect cellular enlargement. In contrast, insulin added at supraphysiological concentrations does not stimulate quiescent cells to enter S phase but instead stimulates growth in cell size in the small fraction of dividing cells. Furthermore, cells stimulated to proliferate by EGF could be induced to undergo balanced growth when insulin was added concomitantly. Finally, platelet-derived growth factor (PDGF) stimulates quiescent sparse 3T3 cells to undergo DNA synthesis and mitosis. PDGF also exerts a limited but significant effect on cellular enlargement. However, PDGF alone could not induce a complete balanced growth, i.e., a doubling in cell size prior to mitosis.

Cellular DNA replication is independent of the synthesis or accumulation of ribosomal RNA

We have used an antibody against RNA polymerase I to investigate the role of rRNA synthesis and/or accumulation in the control of cell proliferation. The antibody was microinjected directly into the nuclei of quiescent Swiss 3T3 cells that were subsequently stimulated with serum. Under the experimental conditions used, the microinjection of the antibody against RNA polymerase I (RNA pol I) caused a 50-70% decrease in nucleolar RNA synthesis that lasted for at least 17 h, a greater than 90% inhibition in the accumulation of nucleolar RNA, and a 70% inhibition in the accumulation of total cellular RNA. A control IgG, similarly microinjected into Swiss 3T3 cells had no inhibitory effect on either the synthesis or accumulation of nucleolar and cellular RNA. Despite the dramatic effect on the synthesis and accumulation of ribosomal RNA (rRNA) the antibody against RNA (rRNA) the antibody against RNA pol I was totally ineffective in inhibiting the entry into S phase of serum-stimulated Swiss 3T3 cells. Cells depleted of cellular RNA by metaphase arrest also entered S phase with subnormal amounts of cellular RNA. The results of these experiments clearly indicate that a normal rate of nucleolar RNA synthesis, and a normal rate of accumulation of total cellular RNA are not a prerequisite for the entry of cells into S phase.

Effects of benzaldehyde on survival and cell-cycle kinetics of human cells cultivated in vitro

Synchronized cells of the human line NHIK 3025 were used to study inactivating and cell-cycle inhibitory effects induced by benzaldehyde. Inactivation was measured as loss of colony-forming ability after treatment of exponentially growing or synchronized cells. Cell-cycle inhibition was measured by flow cytometric recordings of DNA-histograms and microscopic recordings of cell division in synchronized cells. Treatment with benzaldehyde for 4 or 24 hr showed that a marked decrease in survival took place for concentrations above 6.4 mM. Cell-cycle inhibition was observed at concentrations as low as 0.8 mM. Synchronized cells were treated with 3.2 and 6.4 mM benzaldehyde for 8 hr starting at various stages of the cell-cycle. Both the colony-forming ability and the rate of cell-cycle traverse was measured. No difference in sensitivity was found whether the treatment was given in G1, S or in G2. Thus the results show that there is no specific part of interphase where the cells are particularly sensitive with respect to either the inactivating or the cell-cycle inhibitory effects of benzaldehyde in concentrations up to 6.4 mM. When benzaldehyde was present during mitosis both the inactivating and the cell-cycle inhibitory effects were markedly enhanced as compared to the corresponding effects during interphase. It is concluded that benzaldehyde must affect some process within the cell which represents a general requirement for cell-cycle progression. In addition, there are effects on processes that take place only during the last few minutes before and/or during mitosis.