Search

Entry and progression through mitosis has traditionally been linked directly to the activity of cyclin-dependent kinase 1 (Cdk1). In this study we utilized low doses of the Cdk1-specific inhibitor, RO3306 from early G 2 phase onwards. Addition of low doses of RO3306 in G 2 phase induced minor chromosome congression and segregation defects. In contrast, mild doses of RO3306 during G 2 phase resulted in cells entering an aberrant mitosis, with cells fragmenting centrosomes and failing to fully disassemble the nuclear envelope. Cells often underwent cytokinesis and metaphase simultaneously, despite the presence of an active spindle assembly checkpoint, which prevented degradation of cyclin B1 and securin, resulting in the random partitioning of whole chromosomes. This highly aberrant mitosis produced a significant increase in the proportion of viable polyploid cells present up to 3 days post-treatment. Furthermore, cells treated with medium doses of RO3306 were only able to reach the threshold of Cdk1 substrate phosphorylation required to initiate nuclear envelope breakdown, but failed to reach the levels of phosphorylation required to correctly complete pro-metaphase. Treatment with low doses of Okadaic acid, which primarily inhibits PP2A, rescued the mitotic defects and increased the number of cells that completed a normal mitosis. This supports the current model that PP2A is the primary phosphatase that counterbalances the activity of Cdk1 during mitosis. Taken together these results show that continuous and subtle disruption of Cdk1 activity from G 2 phase onwards has deleterious consequences on mitotic progression by disrupting the balance between Cdk1 and PP2A.

Cyclin E1 is expressed at the G 1/S phase transition of the cell cycle to drive the initiation of DNA replication and is degraded during S/G2M. Deregulation of its periodic degradation is observed in cancer and is associated with increased proliferation and genomic instability. We identify that in cancer cells, unlike normal cells, the closely related protein cyclin E2 is expressed predominantly in S phase, concurrent with DNA replication. This occurs at least in part because the ubiquitin ligase component that is responsible for cyclin E1 downregulation in S phase, Fbw7, fails to effectively target cyclin E2 for proteosomal degradation. The distinct cell cycle expression of the two E-type cyclins in cancer cells has implications for their roles in genomic instability and proliferation and may explain their associations with different signatures of disease.

Proliferation and differentiation are tightly coupled processes, so that a final cell cycle is often linked to the initiation of cell differentiation. The flux in cell cycle proteins during this process is commonly assumed to simply control the final cell cycle exit. However it now appears that cell cycle proteins can also play a role in the decision to continue cycling or to terminally differentiate. A subset of the G1 to S phase transition proteins, D-type cyclins, Rb family proteins and the CDK inhibitors, are particularly involved in the commitment to differentiation. Cell cycle proteins can sequester or modify activators of differentiation pathways, while simultaneously performing their cell cycle functions as illustrated by their roles in terminal differentiation in mammary epithelium. G1 to S phase cell cycle proteins, particularly cyclin D1, are commonly altered in breast cancer and contribute to breast tumorigenesis, presumably by increasing proliferation. However the capacity for cell cycle proteins to also influence differentiation may influence tumour progression, and may alter the efficacy of differentiation-based therapeutics.