| Through remarkable advances in the molecular characterization of cell cycle regulation, we have begun to understand how the cell cycle is coupled with, and controlled by, various signal transduction pathways that mediate other cellular processes. The work presented here explored the roles of two major signaling cascades in progression into and through mammalian mitosis.; The first part of my work sought to determine if abnormal chromatin topology delays the G2/M transition by activating the p38 pathway. For this, chromatin condensation during the terminal stage of G2 was disturbed by a histone deacetylase (HDAC) inhibitor, which induces histone hyperacetylation. I found that inhibiting HDAC arrested or significantly delayed entry into mitosis. This G2/M delay was abolished when cells were pre-treated with a p38 inhibitor. These results revealed that the p38 pathway is responsive to changes in the higher-order chromatin structure, and it delays the G2/M transition in response.; The extracellular signal-regulated kinase (ERK)1/2 cascade is a major signal transduction pathway that promotes the G1/S transition in mammalian somatic cells. However, the role of this pathway in later stages of the cell cycle is ill defined and controvertial. Biochemical studies have suggested a requirement for ERK1/2 activity at the G2/M transition, mitotic Golgi fragmentation and the spindle assembly checkpoint, while other lines of evidence suggest that ERK1/2 activity is suppressed during mitosis. In the present study, I aimed to define the requirement for ERK signaling in the G2/M and M/A transitions by high-resolution time-lapse microscopy of living mammalian cells. I found that inhibition or abberrant activation of the ERK1/2 pathway during late G2 had no influence on progression into and through mitosis. Indeed, I found that ERK1/2 activity was depressed in unsynchronized mitotic cells. I also found that ERK1/2 was not involved in the stress-activated p38 checkpoint or the spindle assembly checkpoint pathways. Interestingly, however, inhibition of the ERK1/2 pathway caused a transient delay in early G2, and this delay was mediated by p53. These observations reveal that, contrary to previous reports, ERK1/2 is not required after mid G2 for the G2/M and M/A transitions in mammalian cells. |
