Studies on the division cycle of mammalian cells. II. Causal relationship between completion of DNA synthesis and onset of the G2 period

Kinetic differences between fed and starved Chinese hamster ovary cells

When Chinese hamster (CHO-K1) cells are grown as monolayer cultures, they eventually reach a population-density plateau after which no net increase in cell numbers occurs. The kinetics of aged cells in nutritionally deprived (starved) or density-inhibited (fed) late plateau-phase cultures were studied by four methods: (i) Reproductive integrity and cell viability were monitored daily by clonogenic-cell assay and erythrosin-b dye-exclusion techniques. (ii) Mitotic frequencies of cells from 18 day old cultures were determined during regrowth by analysing time-lapse video microscope records of dividing cells. (iii) Tritiated-thymidine ([3H]TdR) autoradiography was used to determine the fractions of DNA-synthesizing cells in cultures entering plateau phase and during regrowth after harvest. (iv) The rate of labelled nucleoside uptake and incorporation into DNA was measured using liquid scintillation or sodium iodide crystal counters after labelling with [3H]TdR or [125I]UdR. Non-cycling cells in starved cultures accumulate primarily as G1 phase cells. Most cells not in G1 phase had stopped in G2 phase. Very few cells (less than 2%) were found in S phase. In contrast, about half of the cells in periodically fed cultures were found to be in DNA-synthetic phase, and the percentage of these S phase cells fluctuated in a manner reflecting the frequency of medium replacement. Populations of both types of plateau-phase cultures demonstrate extremely coherent cyclic patterns of DNA synthesis upon harvest and reculturing. They retain this high degree of synchrony for more than three generations after the resumption of growth. From these data it is concluded that nutritionally deprived (starved) late plateau-phase cells generally stop in either G1 or G2 phase, whereas periodically fed late plateau-phase cultures contain a very large fraction of cycling cells. Populations of cells from these two types of non-expanding cultures are kinetically dissimilar, and should not be expected to respond to extracellular stimuli in the same manner.

The relation between protein accumulation and cell cycle traverse of human NHIK 3025 cells in unbalanced growth

Human NHIK 3025 cells, synchronized by mitotic selection, were given 2 mM thymidine, which inhibited DNA synthesis without reducing the rate of protein accumulation. After removal of the thymidine the cells proceeded towards mitosis and cell division, with an S duration 2 hours shorter than, but a G2 and M duration nearly identical to that of the control cells. If cycloheximide (1.25 muM) was present together with thymidine, no net protein accumulation took place during the treatment, and the subsequent duration of S, G2, and M was similar to that of untreated cells. The shortening of S seen after treatment with thymidine alone would therefore indicate that the rate of DNA synthesis depended on the amount of some preaccumulated protein. The postreplicative period in thymidine-treated cells was lengthened by cycloheximide treatment although the protein content had already been doubled. This suggests that proteins required for the traverse of this part of the cell cycle might have to be synthesized after completion of DNA replication. Shortly after removal of thymidine, the rate of protein accumulation declined markedly, indicating the existence of some mechanism for negative control of cell mass. In addition, the daughters of thymidine-treated cells had their cell cycle shortened by 2 hours. As a result, the cells had returned to balanced growth already in the first cell cycle following the induction of unbalanced growth. In conclusion, our experiments suggest that NHIK 3025 cells might require a minimum time in order to traverse the cell cycle, which is independent of cell mass.

Karyometric and cytophotometric studies on nucleic acids in the culture of human lymphocytes with phytohemagglutinin

Lymphocyte transformation with phytohemagglutinin (PHA) was analyzed by karyometry and cytophotometry of single cells. The former showed the distribution of various-sized transforming cells, and the latter the increase of DNA and RNA. Two different types of karyograms were obtained; one with three peaks corresponding to 2c, 4c, and 8c, and the other with only a 2c peak. Although small in number (8%), karyometry showed that DNA synthesis took place in the extremely early stage of transformation, 1 h after stimulation with PHA, which was proved by cytophotometry and ultrastructural findings. An increasing curve of DNA and RNA revealed that a lymphocyte continued to enlarge from 2c to 4c and divide again through the normal cell cycle in a suitable condition. The largest blast cell, a polyploid cell, may be induced by the lack of a sufficient amount of RNA and protein in need of cell division, and may be a degenerative cell in the strict sense. Among the large cells with blastlike appearance, there were a considerable number of so-called degenerative cells with a low amount of DNA. The ratio of RNA to DNA increased steeply in the actively DNA synthesizing S phase, which supports the morphologic findings of blast cell, the appearance of basophilic cytoplasm, and active-looking nucleoli.

Studies on the division cycle of mammalian cells. V. Modifications of time parameters by different steady-state culture conditions

In cultures of murine neoplastic mast cells, the duration of different phases of the division cycle (G(1), S, G(2), and mitosis [M]) was determined under optimal and several well-defined suboptimal growth conditions. Two methods of evaluation were applied to the same culture system: first, the relative number of G(1), S, G(2), and M cells was determined by pulse labeling of samples with thymidine-(3)H and subsequent radioautography in conjunction with a microfluorometric technique permitting rapid measurements of cellular DNA content; second, after pulse labeling with thymidine-(3)H, the variations with time of the mitotic labeling index were analyzed. Suboptimal culture conditions were obtained by reducing the concentration of single essential medium components (leucine, glucose, or serum) or by the addition of specific metabolic inhibitors (actinomycin D, amethopterin). Growth-limiting culture conditions resulted in increased generation times. Even under control conditions, the cell number doubling time exceeded the generation time, and this difference was more pronounced in suboptimal media. Under most of the suboptimal conditions tested, the increase in generation time was attributable primarily to an extended duration of the G(1) phase. Under certain growth-limiting conditions, however, other phases were also prolonged. In addition, the variabilities of the generation time and of certain cell cycle phases were increased under suboptimal culture conditions. Results obtained by the two methods of evaluation were, in general, in good agreement with each other. Some differences were, however, observed and interpreted in terms of cell death and/or asymmetric frequency distributions of cell cycle parameters.