Checkpoint Kinase Inhibitors
Eukaryotes have evolved elaborate sensory networks to detect and repair DNA damage and prevent alterations in their genetic material. In response to DNA damage, eukaryotic cells arrest either in G1 or S phase, to prevent replication of damaged genes, or in G2 phase to avoid segregation of defective chromosomes. Checkpoint kinases, Chk1 and Chk2, participate in various DNA-damage responses, including cell-cycle checkpoints, genome maintenance, DNA repair, and apoptosis. They phosphorylate several key proteins involved in cell cycle and block their activity.
Chk1, an evolutionarily conserved protein kinase, is expressed in the S and G2 phases of cell cycle of proliferating cells. It is activated by phosphorylation on Ser317 and Ser345 in response to DNA damage. Once activated, Chk1 phosphorylates Ser123 of Cdc25A, which targets it for ubiquitin-mediated degradation. The phosphorylated Cdc25A cannot dephosphorylate and activate Cdk1 and Cdk2, resulting in an arrest of cell cycle in the G1, S, and G2 phases. Chk1 also phosphorylates Ser216 (14-3-3 binding site) on Cdc25C and prevents its activation in the G2 phase. Phosphorylated Cdc25C cannot dephosphorylate and activate Cdk1. Recent research indicates that Chk1 is an ideal chemosensitization target and its inhibition can sensitize tumors, particularly those with p53- deficiency, to various chemotherapeutic agents.
Chk2 is structurally different from Chk1, but they share overlapping substrate specificities. Chk2 is activated following exposure to infrared light or topotecan, whereas Chk1 is activated by agents that interfere with DNA replication. This observation has lead to the belief that Chk1 blocks cell-cycle progression when replication is inhibited, whereas Chk2 acts when there are double-strand breaks. Chk2 is activated by DNA-strand-breaking agents such as ionizing radiation and topoisomerase inhibitors through the ATM-dependent pathway. The role of Chk2 in checkpoints is not clearly understood. However, it is reported to phosphorylate Cdc25A and inhibit its activity. Chk2 also phosphorylates Ser20 at the amino-terminal activation domain of p53 and regulates levels of p53 in response to DNA double strand breaks. Phosphorylation of Ser20 is not the only important event for p53 response induced by UV light. Chk2 can also regulate p53 through targeting several other phosphorylation sites.
Many current cancer treatments, including certain classes of chemotherapeutics, induce cytotoxicity by damaging DNA. However, many cancers become resistant to these therapies. Thus, modulating DNA-damage responses to selectively enhance the sensitivity of cancer cells to these therapies is highly desirable. Inhibitors of Chk1 and Chk2 have shown potential to enhance the efficacy of DNA-damaging cancer therapeutic agents by selectively increasing the sensitivity of tumor cells.
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