News and Publications

Control of the Cell Cycle in Human Cells

C.H. McGowan, A. Blasina

Discovering the molecular mechanisms that regulate progression of the cell cycle is a major goal in cell biology and is essential for developing an understanding of how cell proliferation is governed in human disease. In mammals, the cell cycle is driven by the sequential activation of a number of cyclin-dependent kinases. Each of these kinases phosphorylates and alters proteins required for specific events in the cell cycle. The activity of cyclin-dependent kinases is regulated by changes in the abundance of the cyclin subunit, by inhibitory phosphorylation, and by association with inhibitory proteins. These regulatory mechanisms combine to delay progression of the cell cycle when cells are exposed to adverse conditions. For example, at least 2 delays in the cycle occur after damage to DNA.

The delays that allow coordination of independent functions of the cell cycle are called checkpoints. The checkpoint that controls the transition from G₁ to S depends on the induction of a cyclin-dependent kinase inhibitor by the protein p53 and prevents cells from replicating damaged DNA. A second checkpoint is involved in the transition from G₂ to M and prevents segregation of damaged or incompletely replicated DNA.

Work in our laboratory focuses on the regulation of the G₂-to-M transition in human cells. In human cells, the mitosis-inducing kinase Cdc2/cyclin B is inhibited by phosphorylation of threonine-14 and tyrosine-15. We have shown that phosphorylation of Cdc2 is essential for the operation of the checkpoint that prevents activation of Cdc2 in the presence of unreplicated or damaged DNA. Although many of the gene products that control the phosphorylation state of Cdc2 have been determined, the mechanism by which they respond to checkpoint controls is not well characterized. We recently identified 2 human checkpoint kinases that inhibit the activity of the mitotic inducer Cdc25. Cdc25 is a phosphatase that normally induces mitosis by dephosphorylating and activating Cdc2. We have shown that the activity of Cdc25 is decreased after DNA damage.

Ongoing studies are aimed at characterizing these and other elements of the signal transduction pathway that prevent mitotic progression in the presence of damaged or unreplicated DNA. The information gained from these studies may provide a rational basis for the improvement of radiation and chemotherapy.

PUBLICATIONS

Blasina, A., Paegle, E.S., McGowan, C.H. The role of inhibitory phosphorylation of CDC2 following DNA replication block and radiation-induced damage in human cells. Mol. Biol. Cell 8:1013, 1997.