Abrogation of Chk1-mediated S/G2 checkpoint by UCN-01 enhances ara-C-induced cytotoxicity in human colon cancer cells.

AIM: To investigate whether 7-hydroxystaurosporine (UCN-01) affects cell cycle progression in arabinosylcytosine (ara-C) treated human colon carcinoma HT-29 cells.
METHODS: Cytotoxicity, DNA synthesis, cell cycle distribution, protein level, and kinase activity were determined by clonogenic assay, flow cytometry, DNA synthesis assay, immunoblotting, and kinase assays, respectively.
RESULTS: UCN-01 abrogated an S/G2-phase checkpoint in HT-29 cells treated with ara-C. When UCN-01 was added after treatment with ara-C, the rate of recovery of DNA synthesis was enhanced and colony-forming ability diminished. Thus, premature recovery of DNA synthesis was associated with increased cytotoxicity. Measurements of cyclin A and B protein levels, Cdk2 and Cdc2 kinase activities, Cdc25C phosphorylation, and Chk1 kinase activity were consistent with UCN-01-induced abrogation of the S/G2-phase checkpoint in ara-C treated cells.
CONCLUSION: The abrogation of the S/G2 checkpoint may be due to inhibition of Chk1 kinase by UCN-01. The enhanced cytotoxicity produced when UCN-01 was combined with ara-C suggested a rationale for the use of this drug combination for tumors that might be susceptible to cell cycle checkpoint abrogation.