Ionizing Radiation Effects on the Cell Cycle

It has been known for several decades that ionizing radiation leads to a prolongation of the cell cycle and can result in an arrest in the G1, G2, and S phases.7,19,20 Because ionizing radiation causes a variety of DNA damage, it was initially inferred that these cell-cycle arrests (now called checkpoints) were essential for the repair of these different types of DNA damage. However, over the past 10 to 15 years, the biology of the cell cycle has become better understood as a complex but finely regulated process involving many factors, particularly the cyclins and cyclin-dependent kinases (CDKs).19 Progression through the cell cycle is promoted by a number of CDKs that are complexed with specific regulatory proteins called cyclins, and these complexes drive the cell cycle. Additionally, there are a corresponding number of cell-cycle inhibitory proteins (CDKIs), which serve as negative regulators of the cell cycle. To date, at least nine structurally related CDKs have been identified along with more than 20 cyclins. The CDK-cyclin complexes themselves are activated by phosphorylation at specific sites, although not all these CDK-cyclin complexes have clearly defined cell-cycle regulatory roles. It is also now recognized that the G0 phase is not a "quiescent" phase as initially termed. Indeed, cellular growth functions occur during G0, and subsequent entry from G0 into the cell cycle (G1) is tightly regulated at the restriction point. This point is thought to divide the early and late G1 phase of the cell cycle. A current model of the human cell cycle and the major cyclins, CDKs, and CDKIs is depicted in Figure 3.7.

The arrest of cells at the G1 checkpoint following ionizing radiation damage is best understood at the present time. The retinoblastoma tumor suppressor gene product (Rb) governs the G1-S phase transition.21 In its active state, Rb is hypophosphorylated and forms an inhibitory complex with the E2F transcription factors. The activity of Rb is modulated by the sequential phosphorylation by CDK 4/6-cyclin D and CDK2-cyclin E. An ionizing radiation-induced G1 arrest results from a specific CDKI, p21waf1/cip1, which prevents key events such as the phosphorylation of RB and activation of E2F transcription factors. Importantly, p21waf1 is induced at the transcriptional level by wild-type p53, which accumulates in irradiated cells and causes a cell-cycle arrest in both G1 and G2.22,23 While the p53-mediated G1 arrest is primarily due to the induction of p21, a p53-mediated G2 arrest involves induction of both p21 and 14-3-3 s, a protein that normally sequesters cyclin B-Cdc2 complexes in the nucleus.

Following recovery of a G1 checkpoint, cyclin E binds to CDK2, and this active complex completely hyperphosphory-lates Rb (pRb), which releases the E2F complex and fully activates the E2F transcription factors.24 The irradiated cell then proceeds into S-phase transcription of a range of targets involved in chemotherapy-based radiosensitization. These drug-radiation targets include ribonucleotide reductase (RR), thymidylate synthase (TS), and thymidine kinase (TK). The interactions of radiosensitizing drugs such as RR inhibitors (gemcitabine, hydroxyurea) and TS inhibitors (fluoropyrim-idines), as well as drugs activated by TK (fluoropyrimidines, halogenated pyrimidine analogues), are used clinically to enhance radiation cytotoxicity. These drug-radiation combinations are discussed later in this chapter (see following section on Mechanisms of Interaction with Conventional Chemotherapy).

Early in S phase, cyclins D and E are targeted by ubiqui-tination for proteasome degradation. The production of cyclin A and the subsequent complex of cyclin A-CDK2 enables S-phase progression, with the production of other enzymes and proteins involved in DNA synthesis, including histones and proliferating cell nuclear antigen (PCNA). Ionizing radiation can also induce an S-phase (or replication) checkpoint, which involves activation of ataxia telangiectasia mutated (ATM) and ATM and Rad-3 related (ATR) kinases with subsequent activation of Chk1 and Chk2.25 The phosphorylation (activation) of Chk1 and Chk2 inhibits phorylation of Cdc2 and blocks progression into G2 and entry into mitosis (M phase).

figure 3.7. Current model of the cell cycle. The cell cycle is regulated by cyclins, cyclin-dependent kinases (CDKs), and cyclin-depen-dent kinase inhibitors (CDKIs). The cell cycle is divided into four distinct phases: Gj, S, G2, and M. G0 represents exit from the cell cycle in which the cell performs its routine functions, including the important function of cell growth. The progression of cell through the cell cycle is driven by CDKs, which are positively and negatively regulated by cyclins and CDKIs, respectively. The resection point governs the transition point beyond which a cell's progression through the cell cycle is independent of external stimuli.

figure 3.7. Current model of the cell cycle. The cell cycle is regulated by cyclins, cyclin-dependent kinases (CDKs), and cyclin-depen-dent kinase inhibitors (CDKIs). The cell cycle is divided into four distinct phases: Gj, S, G2, and M. G0 represents exit from the cell cycle in which the cell performs its routine functions, including the important function of cell growth. The progression of cell through the cell cycle is driven by CDKs, which are positively and negatively regulated by cyclins and CDKIs, respectively. The resection point governs the transition point beyond which a cell's progression through the cell cycle is independent of external stimuli.

Ionizing Radiation Cell Cycle

A clear understanding of the role of the G2 checkpoints to ionizing radiation damage and repair is lacking at the present time.26 From genetic studies in yeast using Saccharomyces cerevisiae, the RAD9, RAD17, RAD24, and MEC3 genes are required for a G2 arrest. In human cells, the DNA mismatch repair proteins MLH1 and MSH2 also appear to play a role in the G2 arrest following ionizing radiation damage.27 Based on the dramatic increase in our understanding of ionizing radiation effects on G1 and S-phase checkpoints over the past few years, it is anticipated that the signaling pathways for recognition of ionizing radiation damage during G2 and M will be better understood in the near future.19

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  • RUTH
    What part of the cell cycle does radiation affect?
    2 months ago

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