Inhibitor SourceBook™ Second Edition

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 DNAdamage
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
of Ser 317 and Ser 345 in response to DNA damage. Once
activated, Chk1 phosphorylates Ser 123 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 Ser 216 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 DNAstrand-breaking
agents such as ionizing radiation
and topoisomerase inhibitors through the ATMdependent
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 Ser 20 at the amino-terminal activation
domain of p53 and regulates levels of p53 in response to
DNA double strand breaks. However, phosphorylation
of Ser 20 is not the only important event for p53 response
induced by UV light. Chk2 can also regulate p53 through
targeting several other phosphorylation sites.
Calbiochem • Inhibitor SourceBook
Phosphorylation/Dephosphorylation
Many current cancer treatments, including certain
classes of chemotherapeutics, induce cytotoxicity
by damaging DNA. However, many cancers become
resistant to these therapies. Thus, modulating DNAdamage
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.
References:
Sorenson, C.S., et al. 2005. Nat. Cell Biol. 7, 95.
Zhou, B.B.S. and Bartek, J. 2004. Nat. Rev. Cancer 4, 2 6.
Craig, A., et al. 2003. EMBO Rep. 4, 787.
O’Neill, T., et al. 2002. J. Biol. Chem. 277, 6 02.
Zhao, H., et al. 2002. Proc. Natl. Acad. Sci. USA 99, 4795.
Graves, P.R., et al. 2000. J. Biol. Chem. 275, 5600.
Hirao, A., et al. 2000. Science 287, 824.
Mailand, N., et al. 2000. Science 288, 425.
Zhou, B-B., et al. 2000. Nature 408, 433.
More online... www.calbiochem.com/inhibitors/checkpoint
Technical Support
Phone 800 628 8470
E-mail calbiochem@emdbiosciences.com