p53-dependent change in replication timing of the human genome

The tumor suppressor gene p53 has roles in multiple cell-cycle checkpoints, including the G1/S transition, to prevent replication of cells with DNA damage. p53 is thought to be associated with regulation of replication timing during S-phase in the human genome. In the present study, we used p53-wild-type and p53-null HCT116 colon carcinoma cells to analyze p53-dependent changes in replication timing of the human genome. The percentage of HCT116 p53(−/−) cells in S-phase was higher than that of HCT116 p53(+/+) cells. We compared replication timing of human genes between the two cell lines using 25,000 human cDNA microarray. We identified genes that replicated earlier in HCT116 p53(−/−) cells than in HCT116 p53(+/+) cells. These genes included cell-cycle- and apoptosis-related genes. We propose that p53 plays a role in regulation of replication timing of the human genome through the control of cell-cycle checkpoints.     

Stimulatory effect of estrogen on the growth of endometrial cancer cells is regulated by cell-cycle regulators

Estrogen is known as a major risk factor in tumorigenesis of the endometrium. The aim of this study is to establish stable estrogen-responsive endometrial cancer cell lines and to investigate the mechanism of estrogen action, focusing on cell-cycle regulation. Human wild-type estrogen receptor cDNA was transfected into endometrial cancer cells (Ishikawa) and estrogen-responsive cell lines were cloned. Their estrogen responsiveness was evaluated by the effect of estrogen on cellular growth and progesterone receptor expression. It was quantitatively estimated by immunocytochemistry or immunoblotting how the expression of cell-cycle regulators such as cyclin D1, cyclin E, Cyclin A, p53, p21 and p27 was regulated by estrogen. A cell line stably responsive to estrogen was established, and cells proliferated and the glandular structure was formed by estrogen stimulation. Cyclin D1 expression increased at 6–24 h and cyclin A gradually increased until 48 h of estrogen treatment compared with untreated cells. On the other hand, p53 and p21 expressions decreased at 6–24 h, and p27 gradually decreased until 24 h by estrogen. Our results show that the stimulatory effect of estrogen on cell proliferation may be regulated by the up-regulation of cyclin D1 and cyclin A, and down-regulation of p53, p21 and p27. This cell line is useful to clarify the molecular mechanism of estrogen action on endometrial cancer.     

Changes in cell-cycle duration and growth fraction in the shoot meristem of Sinapis during floral transition

The cell-cycle duration and the growth fraction were estimated in the shoot meristem of Sinapis alba L. during the transition from the vegetative to the floral condition. Compared with the vegetative meristem, the cell-cycle length was reduced from 86 to 32 h and the growth fraction, i.e. the proportion of rapidly cycling cells, was increased from 30–40% to 50–60%. These changes were detectable as early as 30 h after the start of the single inductive long day. The faster cell cycle in the evoked meristem was achieved by a shortening of the G1 (pre-DNA synthesis), S (DNA synthesis) and G2 (post-DNA synthesis) phases of the cycle. In both vegetative and evoked meristems, both-the central and peripheral zones were mosaics of rapidly cycling and non-cycling cells, but the growth fraction was always higher in the peripheral zone.Abbreviations G1 pre-DNA synthesis phase - G2 post-DNA synthesis phase - GF growth fraction - M mitosis phase - PLM percentage-labelled-mitoses method - S DNA synthesis phase - TdR thymidine