Cell cycle S phase

Cell cycle analysis by dedicate software (e g. Modfit or Flowjo ) [26] usually underestimates percentage of cells in S-phase, since Gi and G2/M peaks are fitted by a gaussian model with modelling of cell cycle phases, and early (ES) and late S-phase (LS) are included inside fitted peaks (Figure 4). 

The degree of sensitization achieved also appears to be profoundly affected by the cell-cycle phase. It is well established that cells in S phase are much more radioresistant than cells in other phases of the cell cycle (28). Interesting results from Latz et al. show quite clearly that cells that are pretreated with gemcitabine no longer show a progressive increase in radioresistance as they move toward DNA replication and therefore sensitization appears to be greatest in S phase (21). 

Cells in S-phase are most sensitive to the cytotoxic effects of methotrexate. RNA and protein synthesis also may be inhibited to some extent and may delay progression through the cell cycle, particularly from Gj to S. 

Doxorubicin binds tightly to DNA by its ability to intercalate between base pairs and therefore is preferentially concentrated in nuclear structures. Intercalation results in steric hindrance, hence production of single-strand breaks in DNA and inhibition of DNA synthesis and DNA-dependent RNA synthesis. The enzyme topoisomerase II is thought to be involved in the generation of DNA strand breaks by the anthracydines. Cells in S-phase are most sensitive to doxorubicin, although cytotoxicity also occurs in other phases of the cell cycle. 

It acts by inhibiting dihydrofolate reductase. It inhibits conversion of dihydrofolic acid to tetrahydrofolic which is essential for purine synthesis and amino acid interconversions. It primarily affects DNA synthesis but also RNA and protein synthesis. It has cell cycle specific action and kills cells in S phase. It is readily absorbed from gastrointestinal tract but larger doses are absorbed incompletely, little drug is metabolised and it is excreted largely unchanged in urine. 

BrdU 5-Bromodeoxyuridine (also abbreviated BUdR or BrdUrd) is a thymidine analog that will be incorporated into the DNA of cycling cells. Cells pulsed with BrdU can then be stained with anti-BrdU monoclonal antibodies to indicate which cells have been synthesizing DNA during the pulse period. BrdU staining is a more precise way to look at the proportion of cells in S phase than simple propidium iodide staining for DNA content. 

S phase S phase is the period of the cell cycle during which cells are in the process of synthesizing DNA in preparation for cell division. During S phase, cells have between the 2C amount of DNA normal to their species and the 4C amount of DNA, which is exactly double the 2C amount. It is the overlap of fluorescence intensity between cells in S phase and some of the cells with the 2C and 4C amounts of DNA that leads to uncertainty in the flow cytometric estimation of the S-phase fraction. 

Kaposi s sarcoma-associated herpesvirus Bcl-2 homolog low molar mass DNA cell cycle mitosis phase... 

To clarify the reason why different circadian schedules of 5-FU delivery have distinct cytotoxic effects, we used the cell cycle automaton model to determine the time evolution of the fraction of cells in S phase in response to different patterns of circadian drug administration, for a cell cycle variability of 15%. The results, shown in Fig. 10.5, correspond to the case considered in Fig. 10.4, namely, entrainment of a 22-h cell cycle by the circadian clock. The data for Fig. 10.5a clearly indicate why the circadian schedule with a peak at 4 a.m. is the least toxic. The reason is that the fraction of cells in S phase is then precisely in antiphase with the circadian profile of 5-FU. Since 5-FU only affects cells in the S phase, the circadian delivery of the anticancer drug in this case kills but a negligible amount of cells. 

Fig. 10.5 Explanation of the cytotoxic effect of various circadian schedules of5-FU delivery with peak at 4 a.m. (a), 10 a.m. (b), 4 p.m. (c), or 10 p.m. (d), and of continuous 5-FU delivery (e). Data are obtained for variability V = 15% and for a cell cycle duration of 22 h, in the presence of entrainment by the circadian clock. The hatched area shows the fraction of cells in S phase exposed to 5-FU and thus likely marked to exit the cell cycle at the next G2/M transition. The curves in Fig. 10.4 showing the cumulated number of cells killed indicate that the schedule with peak delivery at 4 a.m. is the one that causes minimal damage to the cells because the peak...

The cases of peak delivery at 10 a.m. (Fig. 10.5b) or 10 p.m. (Fig. 10.5d) are intermediate between the two preceding cases. Overlap between the peak of 5-FU and the peak of cells in S phase is only partial, but it is still greater in the case of the peak at 10 a.m., so that this pattern is the second most toxic, followed by the circadian delivery centered around 10 p.m. The comparison of the four panels Fig. 10.5a-d explains the results of Fig. 10.4a on the marked differences in cytotoxic effects of the four 5-FU circadian delivery schedules. The use of the cell cycle automaton helps clarify the dynamic bases that underlie the distinctive effects of the peak time in the circadian pattern of anticancer drug delivery. 

To some extent the idea of resonance is also present in the case of circadian 5-FU delivery. Indeed, the circadian patterns of 5-FU which peak at 4 a.m. or 4 p.m. correspond to oscillations that are, respectively, in antiphase or in corresponding phase with the circadian variation of the fraction of cells in S phase. This effect can be seen even for cell cycle durations that differ from 24 h, because of the entrainment of the cell cycle by the circadian clock. 

It is found (Fig. 11.4) that few cells enter S-phase before 12h, but by 18 h about 70% of the cells will be making DNA. All the cells divide shortly after 24 h. This is somewhat longer than the normal cycle time and this has been interpreted in two different ways, viz. ... 

DNA flow cytometric analysis of cells treated with resveratrol aglycone for 24 h. Each point represents the mean SE of three to seven independent experiments. Cell cycle was monitored by a DNA flow cytometric analysis indicating as follows , % of G1-phase cells , % of S-phase cells , % of G2/M-phase cells. p < 0.05 vs. vehicle controls. 

AAV transduction can occur in the absence of cell cycle however, transduction efficiency is markedly improved in cells in S-phase (22). Furthermore, activation of the cellular DNA repair machinery also supports second strand synthesis, thus improving AAV transduction (23,24). The latter suggests that transduction of terminally differentiated postmitotic cells may be hampered to some extent due to insufficient second strand synthesis. 

Fig. 3.1. (A) ElA-induced cell proliferation. ElA proteins release E2F from Rb, which subsequently induces CyclinE/cdk2 gene expression and pushes cells into S-phase. (B) ElA-induced cell cycle arrest and apoptosis counteracted by ElB. ElA pro-...

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