28. DNA Lesions V: Global Response

In addition to a local response, the phosphorylated CHK1s generated by a given lesion also contribute to a call for a global, cell-wide response--that response being cell cycle arrest at the intra-S-phase checkpoint. But phosphorylated CHK1s generated by one or a small number of lesions won't be enough to initiate such a response. Cell cycle arrest can only be initiated by a quantity of phosphorylated CHK1 in the nucleus that can only be reached if there are many lesions or other serious problems like, for example, a shortage of deoxyribonucleotide monomers in the cell. Cell cycle arrest gives the cell ample time to try to address whatever global replication problem might arise.

Here's how it works: Recall from chapter__ that the phases of the cell cycle are determined by proteins called cyclin-dependent kinases (CDKs) and their partner proteins: various cyclins roaming around the nucleus. The presence of specific CDK-cyclin pairs determines the present phase of the cell cycle.

S-phase, specifically, is driven (or "turned on") by CDK2 and cyclins E (earlier S-phase) and/or cyclin A (later S-phase). Thus, if the cell wants to arrest the cell cycle at the intra-S checkpoint because it senses serious replication stress, phosphorylated CHK1 would have to suppress CDK2 or these two cyclins (or all of the above).

Intra-S checkpoint. The cell gets a kind of "status report" at different points in the cell cycle called "checkpoints." The Intra-S checkpoint is focused on DNA replication. If there is significant replication stress within the cell as measured by the amount of activated CHK1 in the nucleus, the cell will arrest the cell cycle

In fact, CHK1 suppresses CDK2 activity indirectly rather than acting on cyclins themselves. When not in S phase, CDK2 is kept inactive by inhibitory phosphorylation at two conserved residues, Thr14 and Tyr15. When CDK2 activity is required, the phosphatase CDC25A removes these inhibitory phosphates, allowing CDK2 to become active.

During a global response to replication stress, activated CHK1 phosphorylates CDC25A at multiple sites, which both inhibits CDC25A activity and marks it for rapid degradation. As CDC25A levels fall, inhibitory phosphorylation of CDK2 accumulates, leading to CDK2 inactivation and enforcement of the intra-S-phase checkpoint. This, again, buys the cell time to either address the problem hindering replication or take steps to dismantle and then kill itself in a process called apoptosis.

Apoptosis is a fascinating process. I want to say a few words about it, even though it's a little bit of an aside. Apoptosis is often rerfered to as "programmed cell death." It is an internal process that can be initiated by any cell to, in effect, terminate its own existence. In fact, apoptosis is sometimes more evocatively called "programmed cell suicide."

Our focus is genome replication. So let's put it in that context. If a cell finds itself in serious trouble during genome replication--so serious that recovery isn't possible--then it can initiate a internal series of steps that methodically dismantles itself molecular piece by molecular piece, recycling biomolecules that can be used later by other cells. Apoptosis may seem extreme. But the risk with a highly damaged genome is that it can lead down a path towards malignancy, or cancer.

Again, though, the cell won't arrest S-phase unless it senses that there are replication problems at many, many sites on the genome. And, again it senses this based on the quantity of phosphorylated CHK1 in the nucleus. The amount of phosphorylated CHK1 determines the degree to which CDC25A is phosphorylated and subsequently inactivated or destroyed. And the extent to which this occurs determines the fate of CDK2 and thus the status of the cell cycle.