The relation between protein accumulation and cell cycle traverse of human NHIK 3025 cells in unbalanced growth.

Human NHIK 3025 cells, synchronized by mitotic selection, were given 2 mM thymidine, which inhibited DNA synthesis without reducing the rate of protein accumulation. After removal of the thymidine the cells proceeded towards mitosis and cell division, with an S duration 2 hours shorter than, but a G2 and M duration nearly identical to that of the control cells. If cycloheximide (1.25 muM) was present together with thymidine, no net protein accumulation took place during the treatment, and the subsequent duration of S, G2, and M was similar to that of untreated cells. The shortening of S seen after treatment with thymidine alone would therefore indicate that the rate of DNA synthesis depended on the amount of some preaccumulated protein. The postreplicative period in thymidine-treated cells was lengthened by cycloheximide treatment although the protein content had already been doubled. This suggests that proteins required for the traverse of this part of the cell cycle might have to be synthesized after completion of DNA replication. Shortly after removal of thymidine, the rate of protein accumulation declined markedly, indicating the existence of some mechanism for negative control of cell mass. In addition, the daughters of thymidine-treated cells had their cell cycle shortened by 2 hours. As a result, the cells had returned to balanced growth already in the first cell cycle following the induction of unbalanced growth. In conclusion, our experiments suggest that NHIK 3025 cells might require a minimum time in order to traverse the cell cycle, which is independent of cell mass.