D-type cyclins and the DNA damage response

Abstract

The cell cycle is strictly regulated to ensure the precise transmission of genetic information from one cell to its daughter cells. The DNA damage response is one of these regulatory pathways and is activated in response to both intrinsic and extrinsic genotoxic stress. Once activated, downstream signalling disrupts normal cell cycle progression, causing arrest or alternatively, cell death. One of the cell cycle proteins, cyclin D1, has been suggested to play a crucial role in integrating the cell cycle machinery and the DNA damage response. This investigation demonstrates that cyclin D1 expression responds in a dose-dependent manner to the UV-mimetic DNA damaging agent, 4-nitroquinolin 1-oxide (4NQO). Immunoblotting experiments revealed a biphasic response of cyclin D1 expression to 4NQO: cyclin D1 levels were largely unchanged following exposure of cells to low and high doses of 4NQO, whilst it was dramatically reduced after treatment with intermediate doses of 4NQO. The CDK inhibitor protein, p21, also responded in a dose-dependent manner, exhibiting elevated expression at low doses of 4NQO but was reduced or unchanged at intermediate and high doses, respectively. Exposure to IR or H2O2 triggered distinct responses by cyclin D1 and p21, indicating the likelihood that these DNA damaging agents activate distinct signalling pathways. Analyses of cellular responses by flow cytometry demonstrated that p21 expression primarily conferred resistance to damage-induced cell death whereas the level of cyclin D1 expression correlated with the mode of cell death. 4NQO-induced cyclin D1 downregulation was proteasome-dependent but was resistant to deregulation of the majority of the key damage-regulating proteins. This investigation also revealed that other members of D-type cyclin family, cyclins D2 and D3, also exhibit distinct responses to various DNA damaging agents. Preliminary observations indicated that responses of the three D-cyclins were not interdependent and they may play distinct roles in the DNA damage response