SummaryFocal adhesion signaling and actin stress fibers are dispensable for progression through the ongoing cell cycle

Phosphatidylinositol 3-kinase (PI3-kinase) activity has been implicated in regulating cell cycle progression at distinct points in the cell cycle by preventing cell cycle arrest or apoptosis. In this study, the role of PI3-kinase activity during the entire G1 phase of the ongoing cell cycle was studied in Chinese hamster ovary (CHO) cells synchronized by mitotic shake-off. We show that inhibition of PI3-kinase activity during and 2 h after mitosis inhibited cell cycle progression into S phase. In the presence of the PI3-kinase inhibitor wortmannin or LY294002, cells were arrested during early G1 phase, leading to the expression of the cleaved caspase-3, a central mediator of apoptosis. These results demonstrate that PI3-kinase activity is required for progression through the M/G1 phase. In the absence of PI3-kinase activity, cells are induced for apoptosis in this particular phase of the cell cycle.

No abstract

Inhibition of PKB (protein kinase B) activity using a highly selective PKB inhibitor resulted in inhibition of cell cycle progression only if cells were in early G1 phase at the time of addition of the inhibitor, as demonstrated by time-lapse cinematography. Addition of the inhibitor during mitosis up to 2 h after mitosis resulted in arrest of the cells in early G1 phase, as deduced from the expression of cyclins D and A and incorporation of thymidine. After 24 h of cell cycle arrest, cells expressed the cleaved caspase-3, a central mediator of apoptosis. These results demonstrate that PKB activity in early G1 phase is required to prevent the induction of apoptosis. Using antibodies, it was demonstrated that active PKB translocates to the nucleus during early G1 phase, while an even distribution of PKB was observed through cytoplasm and nucleus during the end of G1 phase.