Gemcitabine sensitization by checkpoint kinase 1 inhibition correlates with inhibition of a Rad51 DNA damage response in pancreatic cancer cells

• Published in: Molecular cancer therapeutics Vol. 8; no. 1; pp. 45 - 54
• Main Authors: Parsels, Leslie A, Morgan, Meredith A, Tanska, Daria M, Parsels, Joshua D, Palmer, Brian D, Booth, R John, Denny, William A, Canman, Christine E, Kraker, Alan J, Lawrence, Theodore S, Maybaum, Jonathan
• Format: Journal Article
• Language: English
• Published: United States 01.01.2009
• Subjects: Biocatalysis; Carbazoles - pharmacology; Cell Line, Tumor; Cell Survival - drug effects; Checkpoint Kinase 1; Deoxycytidine - analogs & derivatives; Deoxycytidine - pharmacology; DNA Damage; Gemcitabine; Humans; Pancreatic Neoplasms - enzymology; Pancreatic Neoplasms - genetics; Pancreatic Neoplasms - pathology; Phosphorylation - drug effects; Protein Kinase Inhibitors - pharmacology; Protein Kinases - metabolism; Rad51 Recombinase - genetics; Rad51 Recombinase - metabolism
• ISSN: 1535-7163; 1538-8514
• DOI: 10.1158/1535-7163.MCT-08-0662

The protein kinase checkpoint kinase 1 (Chk1) has been implicated as a key regulator of cell cycle progression and DNA repair, and inhibitors of Chk1 (e.g., UCN-01 and EXEL-9844) potentiate the cytotoxic actions of chemotherapeutic drugs in tumor cells. We have examined the ability of PD-321852, a small-molecule Chk1 inhibitor, to potentiate gemcitabine-induced clonogenic death in a panel of pancreatic cancer cell lines and evaluated the relationship between endpoints associated with Chk1 inhibition and chemosensitization. Gemcitabine chemosensitization by minimally toxic concentrations of PD-321852 ranged from minimal (<3-fold change in survival) in Panc1 cells to >30-fold in MiaPaCa2 cells. PD-321852 inhibited Chk1 in all cell lines as evidenced by stabilization of Cdc25A; in combination with gemcitabine, a synergistic loss of Chk1 protein was observed in the more sensitized cell lines. Gemcitabine chemosensitization, however, did not correlate with abrogation of the S-M or G2-M checkpoint; PD-321852 did not induce premature mitotic entry in gemcitabine-treated BxPC3 or M-Panc96 cells, which were sensitized to gemcitabine 6.2- and 4.6-fold, respectively. In the more sensitized cells lines, PD-321852 not only inhibited gemcitabine-induced Rad51 focus formation and the recovery from gemcitabine-induced replication stress, as evidenced by persistence of gamma-H2AX, but also depleted these cells of Rad51 protein. Our data suggest the inhibition of this Chk1-mediated Rad51 response to gemcitabine-induced replication stress is an important factor in determining gemcitabine chemosensitization by Chk1 inhibition in pancreatic cancer cells.