An automated system for cell cycle analysis

Mitotic disassembly of the Golgi apparatus in vivo

Populations enriched in prophase cells were obtained either by using a cell line with a temperature-sensitive mutation in the mitotic kinase, p34cdc2, or by treating cells with olomoucine, an inhibitor of this kinase. Both methods resulted in efficient and reversible block of the cells at the G2/M boundary. After cells were released from the cell cycle block, the morphological changes to the Golgi apparatus were characterised using both quantitative conventional electron microscopy and immuno-gold microscopy. The early mitotic phases were divided into six stages (G2 to pro-metaphase) based on the morphology of the nucleus. During prophase the cross-sectional length of Golgi stacks decreased prior to unstacking. At the same time, small vesicular profiles, typically 50–70 nm in diameter, accumulated in the vicinity of the stacks. The disappearance of Golgi stacks was accompanied by the transient appearance of tubular networks. By the time cells entered prometaphase, the stacks had completely disassembled and only clusters consisting of Golgi vesicles and short tubular elements were left. When cells were released from the G2/M boundary and pulsed briefly with [AlF4]- to prevent uncoating of transport vesicles, vesicular profiles with a morphology reminiscent of COP-coated vesicles appeared. These vesicular profiles were either associated with Golgi stacks or, at later stages, with clusters, but were formed at all stages of disassembly. Together these results provide further support for our model that continued budding of vesicles from the rims of Golgi cisternae is at least partly responsible for the disassembly of the Golgi apparatus.

Substructure in the radiation survival response at low dose: asynchronous and partially synchronized V79-WNRE cells

The radiation survival response of asynchronously-dividing populations of the cell line V79-WNRE indicates substructure in the low dose region that can be better characterized by separately fitting the data within the low and high dose regions to the linear-quadratic (LQ) equation. Flow cytometry and cell sorting techniques have been used to determine the response of both asynchronous populations and synchronous populations obtained by mitotic selection with or without an additional drug block. The statistically significant substructure which is present in the radiation response of asynchronous cells is absent in G1/S cells synchronized by mitotic selection followed by hydroxyurea (or aphidicolin) accumulation at the G1/S boundary. Such G1/S populations have a radiation response that is well characterized by a single LQ expression over 3 logs of cell inactivation. In contrast, the radiation response of cells synchronized by mitotic selection alone and irradiated in early G1 shows substantial substructure when fitted to the LQ equation, reflecting some radioresistance at high dose. While this response is well described by a single hit plus multitarget equation, it is possible that the small proportion (< 5%) of interphase cells in such mitotically-selected populations may be responsible for the resistance observed at higher doses.

Characterization of the cell cycle and synchrony in a CV-1 cell line

Synchronous populations of CV-1 cells have been prepared by the selective detachment of mitotic cells. An approach, using a combination of standard techniques, has allowed evaluation of the degree of synchrony throughout the cell cycle. CV-1 cells have a cycle of approximately 19-hr duration, throughout which synchronous populations progress with a dispersion of 3 hr, i.e., ie, greater than or equal to 90% of the cells enter S phase and subsequently divide within a 3-hr period. A major cause of this dispersion is the differential attachment times of individual cell within the population when a synchronous culture is initiated.

Difference between diploid and aneuploid Chinese hamster cells in replication at mid-S-phase

Following partial synchronization of the heteroploid Chinese hamster cell line V-79 and of normal diploid lung fibroblasts of the Chinese hamster in culture, their DNA replication during S-phase aws compared by means of a BrdU-incorporation/thymidine pulse technique and Hoechst-Giemsa differential staining of metaphase chromosomes. This comparison indirectly shows the S-phase of the heteroploid cells of V-79 to be 2 h shorter than the diploid cell S-phase. When the thymidine pulse is applied to diploid lung fibroblasts at mid-S-phase, differential staining colours metaphase chromosomes a pale blue. Performing the corresponding experiment with V-79 cells, neither a pale blue nor dark red staining is obtained, but rather an intermediate shade, showing prominently dark staining regions in parts. The pause in DNA synthesis observed at mid-S-phase of the diploid Chinese hamster lung fibroblasts seems to be omitted at mid-S-phase of the V-79 cells.

Tyrosyltubulin ligase and colchicine binding activity in synchronized Chinese hamster cells

Tyrosyltubulin ligase (TTL) was found to be present in CHO and V79 Chinese hamster cells grown in tissue culture. The enzyme is soluble and requires potassium, magnesium, and ATP for maximum activity and requires tubulin as a substrate. TTL was analyzed through the cell cycle of V79 and CHO Chinese hamster cells. The enzyme showed two peaks of activity in V79 cells at 4 h and 7 h after mitotic selection, corresponding to the early S and mid to late S phases of the cell cycle. In CHO cells the enzyme displayed a major peak of activity at mid S and a minor peak or plateau during early S. Tubulin, as measured by (3H)colchicine binding, was shown to increase through S phase and reach a maximum late in the cycle during G2 approx. 3 h after maximum TTL activity.

Carcinoplacental alkaline phosphatase. Base level and hormone-induced activity associated with events in the cell cycle

Using HeLa TCRC-1, a cell line which is monophenotypic with respect to the Regan isoenzyme of alkaline phosphatase, we have examined the factors which influence its expression in relation to events of the cell cycle. DNA synthesis is not required for hormone induction of the Regan isoenzyme as in the presence of hydroxyurea, a specific inhibitor of DNA synthesis, we found induction to occur. Additionally, when partially synchronised cells were allowed to leave the S period prior to hormone treatment, and hydroxyurea was added to prevent cells from entering the next S period, hormone induction of the Regan isoenzyme was still observed. This indicates that initiation of expression of hormone-induced carcinoplacental alkaline phosphatase occur prior to the DNA synthetic phase of the cell cycle. We propose a hypothetical two-step mechanism of hormone induction to interpret the present findings in relation to previous results.

Intermittent DNA synthesis and periodic expression of enzyme activity in the cell cycle of WI-38

Synchronous cultures of WI-38 were obtained using an automated system for detachment and partitioning of mitotic cells which operates without the use of inhibitors, altered medium, or lowered temperatures. The generation time in synchronous WI-38 is 19.5 h and the duration of S phase when determined from the percentage of labeled metaphase cells or nuclei is 12 h. DNA replication in WI-38 occurs in three temporally distinct and rapid bursts separated by intervals of greatly reduced synthesis within what is nominally described as the DNA synthetic (S) period. Lactate dehydrogenase (LDH) displayed maxima in G(1) between 2 and 4 h and again at 10 and 16 h. Peaks in LDH activity were coordinated with DNA replication in a fashion similar to that reported for diploid Chinese hamster cells. Oscillations in LDH activity are more pronounced in normal diploid fibroblasts than in established and neoplastic lines.