Mechanisms of the G2 cell cycle delay following radiation

Cell cycle delay is an almost universal response to DNA damage in eukaryotic cells. There is evidence that this delay may mediate survival after DNA damage. Cells with wild-type p53 function block in G1 due to the expression of WAF1/p21; however, many tumor cells lack wildtype p53 function and consequently block in G2. The mechanisms underlying the G2 delay are less well-characterized; the dominant theme of this thesis is to investigate these mechanisms. The first section deals with the construction and characterization of an inducible vector expressing cyclin B1 protein, with which the role of cyclin B1 in the radiation-induced G2 arrest was directly examined. The induced expression of cyclin B1 only partially abrogated the radiation-induced G2 arrest. While this provided direct evidence for the first time of the importance of the level of cyclin B1 protein during G2 delay, it also suggested that other mechanisms are at work. The second section builds on the results of the first and examines subcellular compartmentalization of p34$\rm\sp{cdc2}$ kinase activity in relationship to DNA damage and cyclin B1 protein expression. After irradiation, p34$\rm\sp{cdc2}$ kinase activity was substantially diminished. With time, p34$\rm\sp{cdc2}$ kinase activity appeared first in the cytoplasm before any nuclear activity is detectable. Cytoplasmic but not nuclear p34$\rm\sp{cdc2}$ kinase activity was increased with the induced expression of cyclin B1 and the expression of a mutant p34$\rm\sp{cdc2}$ protein lacking inhibitory phosphorylations; nuclear cdk1 activity appeared later after a delay. The third section examines the localization of and conditions under which phosphotyrosine-phosphorylated p34$\rm\sp{cdc2}$ accumulates after DNA damage. Both radiation and etoposide treatment resulted in the accumulation of phosphotyrosine-phosphorylated p34$\rm\sp{cdc2}$ in the nucleus and is abrogated by caffeine. These results demonstrate the complexity of mechanisms regulating the G2 delay after irradiation and provide evidence that multiple distinct but interrelated mechanisms are at work. These include cyclin B1 protein levels, p34$\rm\sp{cdc2}$ phosphorylation, and subcellular localization of p34$\rm\sp{cdc2}$ activity. These results also suggest that phosphorylation may help regulate the nuclear localization of p34$\rm\sp{cdc2},$ and possibly, the localization of active p34$\rm\sp{cdc2}$. A comprehensive model is presented. The fourth section provides evidence that modulation of the G2 delay can influence cell survival after DNA damage, highlighting the importance of fully understanding the mechanisms by which tumor cells regulate cell cycle delay after DNA damage