Sunday 12 March 2006
CHKE1 gene encodes a protein kinase that is required for the DNA damage checkpoint. Checkpoint pathways control the order and timing of cell cycle transitions and ensure that critical events, such as DNA replication and chromosome segregation, are completed with high fidelity.
The regulation of G2-M and S-phase cell-cycle checkpoints
CHEK1 is a checkpoint kinase in mammals and regulates G2-M and S-phase cell-cycle checkpoints. CHEK1 is expressed in the S and G2 phases of proliferating cells and is absent or expressed at very low levels in quiescent and differentiated cells.
CHK1 is activated by phosphorylation at serine (Ser) 345 and Ser317 in response to various types of DNA damage in mammals, including damage that is induced by IR, ultraviolet (UV) light, hydroxyurea (HU) and topoisomerase inhibitors.
For optimal activation of CHK1, the presence of other checkpoint proteins such as claspin is required.
CHK1, in turn, phosphorylates Ser123 and several other serine residues of the phosphatase CDC25A which targets it for ubiquitin-mediated degradation. Consequently, CDC25A can not dephosphorylate and activate CDK2 and CDK1, so cells arrest in late G1, S and G2 phases.
Cells that fail to downregulate CDC25A after DNA damage are unable to arrest cell-cycle progression in a timely manner. CHK1 can also phosphorylate Ser216 of CDC25C, which prevents its activation at G2, at least partially, through 14-3-3-mediated translocation to the cytoplasm. Again, G2 arrest ensues because of the inability of CDC25C to dephosphorylate and activate CDK1.
CHK1 and DNA-damage-induced checkpoints
In addition to biochemical evidence, genetic results also indicate that CHK1 is also involved in DNA-damage-induced checkpoints.
Complete deficiency of Chk1 in avian somatic DT-40 lymphoma cells abolished DNA-damage-induced G2 arrest and undermined replication-checkpoint responses. Embryonic mouse stem cells that conditionally lack the Chk1 gene can not prevent mitotic entry in response to IR16, demonstrating that Chk1 is required for the G2-M checkpoint in mammals, as observed in lower organisms.
SiRNA-mediated knockdown of CHK1 in human cells revealed an essential role of this kinase in the control of CDC25A protein turnover and thereby in both normal S-phase and the intra-S-phase DNA-damage checkpoint.
SiRNA studies also confirmed the requirement for CHK1 in the G2-M checkpoint in response to IR20 and DNA-damaging drugs of different mechanisms. Furthermore, addition of the CHK1 inhibitor UCN-01 or CEP-3891 before DNA damage abrogated the G2-M and S-phase checkpoints.
UCN-01 was originally isolated from the culture broth of Streptomyces staurosporeus as a selective inhibitor of protein kinase C, but was later shown to be a potent CHK1 inhibitor.
CHEK1 can also inhibit AKT at slightly higher concentrations; therefore, interpretation of the UCN-01 data needs to take this relative selectivity into consideration.
candidate tumor suppressor gene in 11q21-q24 LOH
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