PubMed1994
To study the alteration of nuclear DNA content of cancer cells after peplomycin (PEP) treatment, DNA cytofluorometry was performed in combination with 3H-thymidine (3H-TdR) autoradiography using cultured A431 cells. The cells in the logarithmic growth were treated with PEP (1.25 micrograms/ml) for 24 hr, during the first 4 hr of which they were pulse-labeled with 3H-TdR (2.4 x 10(4) Bq/ml). After washing with PBS, the cells were then cultured without both PEP and 3H-TdR, fixed at different times and stained with propidium iodide (PI) for the auto-stage cytofluorometry, which enabled DNA content analysis for labeled and unlabeled cells by repeated scanning of the same cell population. The nuclear DNA content histograms demonstrated that A431 cells were mostly arrested in G2 phase of 4C stem line by treatment with PEP for 24 hr. This G2 block lasted up to 8 hr after removal of the drug, and thereafter, marked polyploidization associated with DNA synthesis occurred, showing almost no mitotic figures, while only a few cells returned to G1 phase via M phase. During the period of 72-120 hr, however, the fractions of advanced polyploid cells (DNA content > or = 8C) gradually decreased and the DNA content distribution pattern became eventually similar to the original one as seen before PEP treatment. From these results we hypothesized as follows: 1) At S-G2 boundary, there is some control mechanism that checks whether the cells, after S phase, can enter the M phase or not. 2) The cells, which are not permitted to enter mitosis by the control mechanism, show marked polyploidization. 3) Only the cells that enter into mitosis can live and proliferate, though the advanced polyploid cells die shortly. 4) This control mechanism might be related to the precision of DNA repair that is checked at the G2-M checkpoint.