Stimulation of WI-38 cell cycle transit: effect of serum concentration and cell density

Synchronization of Mammalian Cell Cultures by Serum Deprivation

Mammalian cells are amenable to the study of regulatory mechanisms dictating cell cycle progression in vitro by shifting them into the same phase of the cycle. Procedures to arrest cultured cells in specific phases of the cell cycle may be termed in vitro synchronization. The procedure described here was developed for the study of primary astrocytes and a glioma cell line, but is broadly applicable to other mammalian cells. Its application allows astrocytes to re-enter the cell cycle from a state of quiescence (G₀) under carefully defined experimental conditions to move together into subsequent phases such as the G₁ and S phases. A number of methods have been established to synchronize mammalian cell cultures, which include counterflow centrifugal elutriation, mitotic shake off, chemically induced cell cycle arrest, and newer live cell methods, such as cell permeable dyes. Yet, there are intrinsic limitations associated with these methods. In the present protocol, we describe a simple, reliable, and reversible procedure to synchronize astrocyte and glioma cultures from newborn rat brain by serum deprivation. The procedure is similar, and generally applicable, to other mammalian cells. This protocol consists essentially of two parts: (1) proliferation of astrocytes under optimal conditions in vitro until reaching desired confluence; and (2) synchronization and G₀ phase arrest of cultures by serum down-shift. This procedure has been utilized to examine cell cycle control in astroglioma cells and astrocytes from injured adult brain. It has also been employed in precursor cloning studies in developmental biology, suggesting wide applicability.

Synchronization of mammalian cell cultures by serum deprivation

Mammalian cells are amenable to study the regulation of cell cycle progression in vitro by shifting them into the same phase of the cycle. Procedures to arrest cultured cells in specific phases of the cell cycle may be termed in vitro synchronization. The procedure described here was developed for the study of primary astrocytes and a glioma cell line, but is applicable to other mammalian cells. Its application allows astrocytes to reenter the cell cycle from a state of quiescence (G(0)), and then, under carefully defined experimental conditions, to move together into subsequent phases such as the G(1) and S phases. A number of methods have been established to synchronize mammalian cell cultures, which include physical separation by centrifugal elutriation and mitotic shake off or chemically induced cell cycle arrest. Yet, there are intrinsic limitations associated with these methods. In the present protocol, we describe a simple, reliable, and reversible procedure to synchronize astrocyte and glioma cultures from newborn rat brain by serum deprivation. The procedure is similar, and generally applicable, to other mammalian cells. This protocol consists essentially of two parts: (1) proliferation of astrocytes under optimal conditions in vitro until reaching desired confluence; and (2) synchronization of cultures by serum downshift and arrested in the G(0) phase of the cell cycle. This procedure has been extended to the examination of cell cycle control in astroglioma cells and astrocytes from injured adult brain. It has also been employed in precursor cloning studies in developmental biology, suggesting wide applicability.

Formation of MacroH2A-containing senescence-associated heterochromatin foci and senescence driven by ASF1a and HIRA

In senescent cells, specialized domains of transcriptionally silent senescence-associated heterochromatic foci (SAHF), containing heterochromatin proteins such as HP1, are thought to repress expression of proliferation-promoting genes. We have investigated the composition and mode of assembly of SAHF and its contribution to cell cycle exit. SAHF is enriched in a transcription-silencing histone H2A variant, macroH2A. As cells approach senescence, a known chromatin regulator, HIRA, enters PML nuclear bodies, where it transiently colocalizes with HP1 proteins, prior to incorporation of HP1 proteins into SAHF. A physical complex containing HIRA and another chromatin regulator, ASF1a, is rate limiting for formation of SAHF and onset of senescence, and ASF1a is required for formation of SAHF and efficient senescence-associated cell cycle exit. These data indicate that HIRA and ASF1a drive formation of macroH2A-containing SAHF and senescence-associated cell cycle exit, via a pathway that appears to depend on flux of heterochromatic proteins through PML bodies.

In vitro synchronization of mammalian astrocytic cultures by serum deprivation

The study of the regulation of cell division cycle in vitro requires cell cultures growing in the same phase of the cycle. The procedure by which cells are arrested in specific phases of the cell cycle is termed synchronization. Synchronization is particularly important in the study of astrocyte biology, as its application allows astrocytes to re-enter the cell cycle from a state of quiescence (G(0)), and, under carefully defined experimental conditions, move together into subsequent phases such as the G(1) and S phases. A number of methods have been established to synchronize mammalian cell cultures, including centrifugal elutriation, mitotic shake-off, and chemically induced cell cycle arrest. Yet there are intrinsic limitations associated with these methods. In the present protocol, we describe a simple, reliable, and reversible procedure to synchronize astrocytic cultures from newborn rat brains by serum deprivation. This protocol consists essentially of two parts: (1) proliferation of astrocytes under optimal conditions in vitro until reaching desired confluence; and (2) synchronization of cultures by serum down-shift and arrested in the G(0) phase of the cell cycle. This procedure has recently been extended toward the study of cell cycle control in astroglioma cells and astrocytes from injured adult brains. Since it was also employed in recent precursor cloning studies in developmental biology, this procedure will certainly find increasing use in future research.

Serum-independent regulation of initiation of DNA synthesis relating to temperature-sensitive defect in rat 3Y1tsD123 fibroblasts and its compensation by simian virus 40

Randomly proliferating 3Y1tsD123 cells are arrested in G1 phase within 24 h after a shift up to 39.8 degrees C (temperature arrest), yet the density-arrested cells (prepared at 33.8 degrees C) enter S phase at 39.8 degrees C with serum stimulation, with or without preexposure to 39.8 degrees C for 24 h (Zaitsu and Kimura 1984a). When the density-arrested 3Y1tsD123 cells were preexposed to 39.8 degrees C for 96 h, they lost the ability to enter S phase at 39.8 degrees C by serum stimulation and required a longer lag time to enter S phase at 33.8 degrees C by serum stimulation than did the cells not preexposed to 39.8 degrees C. Simian virus 40 induced cellular DNA synthesis at 39.8 degrees C in the density-arrested 3Y1tsD123 preexposed to 39.8 degrees C for 96 h. In the absence of serum after a shift down to 33.8 degrees C, the temperature-arrested 3Y1tsD123 cells entered S phase and then divided once. We postulate from these results that (1) the ts defect in 3Y1tsD123 is involved in a serum-independent process. Once this process is accomplished, its accomplishment is invalidated slowly with preexposure to 39.8 degrees C. This and the serum-dependent processes occur in parallel but not necessarily simultaneously. The accomplishment of both (all) processes is required for the initiation of S phase. The density-arrested 3Y1tsD123 cells have accomplished the serum-independent process related to the ts defect, but have not accomplished serum-dependent processes. In case of the temperature-arrested 3Y1tsD123 cells, the reverse holds true. The lag time for entry into S phase depends on the preparedness for the initiation of DNA synthesis (on the extent of accomplishment of each of all processes required for entry into S phase). (2) To induce cellular DNA synthesis, simian virus 40 stimulates directly the serum-independent process. However, we do not rule out the possibility that simian virus 40 stimulates serum-dependent processes simultaneously.

Advance toward S phase and retreat toward deeper "G0" states in resting 3Y1 cells with environmental changes

To elucidate conditions which affect the lag time for resting cells to enter S phase after serum stimulation, we used a wild-type 3Y1 rat fibroblast line and four temperature-sensitive mutants of 3Y1 (3Y1tsD123, 3Y1tsF121, 3Y1tsG125, and 3Y1tsH203). Among these five lines, in only tsG125 cells was there an obviously prolonged lag time with increase in time in resting state at 33.8 degrees C. The resting wild-type 3Y1 cells, preexposed to 39.8 degrees C, also showed a prolongation of lag time. The prolongation in tsG125 had a certain limit. Preexposure to 39.8 degrees C before serum stimulation accelerated such prolongation in tsG125 to its limit, but did not change the limit, per se. Resting tsG125 cells stimulated by serum at 39.8 degrees C, did not enter S phase, yet they did advance toward S phase. When they were kept at 39.8 degrees C, they retreated toward a deeper resting state ("G0") with time. These retreats correlated with the decrease in stimulating activity in the culture media. About 20% of the resting tsG125 cells stimulated by serum at 39.8 degrees C were committed to enter S phase, when the extent of commitment was examined at 33.8 degrees C. Most of the tsG125 cells committed at 33.8 degrees C did not enter S phase, when the extent of commitment was examined at 39.8 degrees C. More cells were committed after stimulation at 33.8 degrees C than at 39.8 degrees C, when the test was done at 33.8 degrees C. We suggest that resting cells may be reversibly changed within range of resting states, in either direction, that is, advance toward S phase or retreat toward deeper "G0." These changes may be determined by alterations in the balance between synthesis and decay of the preparedness for the initiation of DNA synthesis caused by cellular response to environmental changes (e.g., medium activity, temperature, etc.). The ts defect in tsG125 may affect the cell cycle progression, both before and after commitment by serum.

Arrest states in a set of mutants of rat 3Y1 cells temperature-sensitive for entering S phase

Four temperature-sensitive (ts) mutants of rat 3Y1 cells (3Y1tsD123, 3Y1tsF121, 3Y1tsG125, and 3Y1tsH203) are arrested at 39.8 degrees C mainly with a 2N DNA content (temperature-arrested cells). The states of these cells were compared with findings in case of cells arrested at 33.8 degrees C at saturation density (density-arrested cells), with regard to the ability to enter S phase after release from arrest or after serum stimulation at 39.8 degrees C. With the 3Y1tsD123, the ts defect is an event which seems essential for the initiation of S phase and occurs after mitosis but not after release from the density arrest. The defect in 3Y1tsF121 related to the efficiency of utilization of serum component(s). In case of 3Y1tsG125, the state of temperature arrest appeared to locate between the state of density arrest and the beginning of S phase. There was no significant difference between the density- and the temperature-arrested cells, in case of 3Y1tsH203.

Density-dependent limitation of growth and the regulation of cell replication by changes in the triggering level of the cell cycle switch

If the initiation/suppression of cell replication results from the operation of an intracellular switch, then proliferation control can be expected to occur through the modulation of its threshold setting or sensitivity. Assuming the existence of a threshold and the uptake of regulators by an equilibrium process, one can explain various types of dependency of the maximum cell density on the initial level of the regulator (e.g., serum) in the medium. Moreover it becomes theoretically possible to distinguish whether an altered threshold or equilibrium constant is responsible for modified proliferation characteristics. It is pointed out that some published data on transformed cells are more consistent with an altered threshold than the permeability change invoked. Other data suggests that transformation has produced more complex effects. The arguments presented indicate that while some cells may be more susceptible to control via changes in the relevant equilibrium constant, others may respond more markedly to threshold modulation. If control occurs through inhibition of replication, it is possible for the system to exhibit a critical "mass": if this is exceeded (in ways discussed) proliferation could continue indefinitely provided adequate nutrients are available. An oscillator concept of the cell cycle accounts for the existence of a threshold and also permits a general understanding of its modulation by multiple agents. It is concluded that at least some forms of cancer are the result of altered thresholds.