In the presence of DNA damage or incomplete DNA replication, eukaryotic cells activate cell cycle checkpoints that temporarily halt the cell cycle to permit DNA repair or completion of DNA replication to take place. In the presence of extensive damage or absence of timely repair, these checkpoint signaling pathways may also trigger a pathway that effects programmed cell death or apoptosis. DNA damage activated cell cycle checkpoints are regulated in part by the phosphoinositide kinase family of checkpoint components, including Rad3 in S. pombe, Mec1/Tel1 in S. cerevisiae, mammalian ATM and ATR, MEI-41 in drosophila, and ATM and ATR in xenopus. These checkpoint kinases regulate the activities of two downstream effector serine/threonine kinases, Chk2 and Chk1.
Both, Chk1 and Chk2 are structurally unrelated yet functionally overlapping serine/threonine kinases which relay the checkpoint signals from the proximal checkpoint kinases of the PI3K family, particularly ATM and ATR. Ionizing radiation, telomere erosion, and radiomimetic drugs induce double-strand DNA breaks and activate ATM kinase, which phosphorylates Chk2. In addition, other checkpoint proteins may coregulate Chk2 activation. These factors include a DSB-interacting protein 53BP1, DNA ends processing MRN nuclease complex (MRE11/Rad50/NBS1) and its newly identified binding partner MDC1. Chk2 has a key role in delaying cell cycle progression in response to DNA damage. Upon activation, Chk2 phosphorylates the mitosis inducing phosphatase CDC25C on an inhibitory site, blocks the progression from G2 to M-phase, and phosphorylates p53 on a regulatory site, which induces transcription of WAF1/p21/CIP1, resulting in the arrest of G1-phase of the cell cycle. After DNA damage, Chk2 phosphorylates p53 on ser20, attenuating the binding of p53 to MDM2 that targets p53 for degradation in the proteasome, and allowing accumulation and subsequent activation of p21/WAF1 and G1 arrest. The oncosuppressor protein BRCA1, the core of the BRCA1-associated super complex, physically interacts with Chk2 and is also a putative target of Chk2 activity. Phosphorylation of BRCA1 by Chk2 in response to DNA damage is required for survival after DNA damage.
Rad3 dependent activation of Chk1 leads to negative regulation of CDC25A. This negative regulation may occur by direct inhibition of CDC25 activity, prevention of the activation of CDC25 that occurs at the G2-M transition, or interference in the interaction between CDC25 and CDC2. Inhibitory phosphorylation of CDC2 is crucial for G2 DNA damage arrest in mammalian cells. Chk1 phosphorylation of CDC25C promotes the binding of a 14-3-3 protein, which sequesters CDC25C in the cytoplasm. CDS1 arrests cells in G2 by phosphorylating CDC25C on amino acid residues also targeted by Chk1.
Following their activation, Chk1 and Chk2 phosphorylate unique and overlapping downstream effectors that further propagate checkpoint signaling. Depending on the type of stress, velocity of DNA damage, and cellular context, this leads to switching of the stress-induced transcription program (E2F1, BRCA1, p53), direct or indirect initiation of DNA repair (BRCA1, p53), acute delay (degradation of CDC25A) and/or sustained block (CDC25C, p53, PLK3) of cell cycle progression, apoptosis (PML1, p53, E2F1), and modulation of the chromatin remodeling pathways (TLK1/2). Despite their overlapping roles in checkpoint signaling, the biological requirements for Chk1 and Chk2 function are strikingly different. Chk1 is essential for mammalian development and viability while Chk2 has several other important functions apart from targeting p53.