title: p53-dependent Chk1 phosphorylation is required for maintenance of prolonged G2 Arrest.
abstract: Targeting checkpoint kinases has been shown to have a potential chemosensitizing effect in cancer treatment. However, inhibitors of such kinases preferentially abrogate the DNA damage-induced G2 checkpoint in p53-/- as opposed to p53+/+ cells. The mechanisms by which p53 (TP53) can prevent abrogation of the G2 checkpoint are unclear. Using normal human diploid p53+/+ and p53-/- fibroblasts as model systems, we have compared the effects of three checkpoint inhibitors, caffeine, staurosporine and UCN-01, on gamma-radiation-induced G2 arrest. The G2 arrest in p53+/+ cells was abrogated by caffeine, but not by staurosporine and UCN-01, whereas the G2 arrest in p53-/- cells was sensitive to all three inhibitors. Chk2 (CHEK1) phosphorylation was maintained in the presence of all three inhibitors in both p53+/+ and p53-/- cells. Chk1 phosphorylation was maintained only in the presence of staurosporine and UCN-01 in p53+/+ cells. In the presence of caffeine Chk1 phosphorylation was inhibited regardless of p53 status. The pathway of Chk1 phosphorylation --> Cdc25A degradation --> inhibition of cyclin B1/Cdk1 activity --> G2 arrest is accordingly resistant to staurosporine and UCN-01 in p53+/+ cells. Moreover, sustained phosphorylation of Chk1 in the presence of staurosporine and UCN-01 is strongly related to phosphorylation of p53. The present study suggests the unique role of Chk1 in preventing abrogation of the G2 checkpoint in p53+/+ cells.
authors: Department of Microbiology and Molecular Genetics, University of California Irvine, School of Medicine, Irvine, California 92697, USA.
authors: Xiao Qi
authors: XQ
authors: Wang
authors: Eric J
authors: EJ
authors: Stanbridge
authors: Xiaoyan
authors: X
authors: Lao
authors: Qi
authors: Q
authors: Cai
authors: Sheung Tat
authors: ST
authors: Fan
authors: J Leslie
authors: JL
authors: Redpath
mesh_terms: D002110:Caffeine / Q000494:pharmacology; D002460:Cell Line; D000071877:Checkpoint Kinase 1; D064447:Checkpoint Kinase 2; D015536:Down-Regulation / Q000528:radiation effects; D016195:G2 Phase / Q000187:drug effects / Q000528:radiation effects; D006801:Humans; D010766:Phosphorylation / Q000187:drug effects / Q000528:radiation effects; D011494:Protein Kinases / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D019311:Staurosporine / Q000031:analogs & derivatives / Q000494:pharmacology; D016159:Tumor Suppressor Protein p53 / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D020687:cdc25 Phosphatases / Q000378:metabolism
chemical_list: D016159:Tumor Suppressor Protein p53; D002110:Caffeine; C054852:7-hydroxystaurosporine; D011494:Protein Kinases; D064447:Checkpoint Kinase 2; C000605867:CHEK1 protein, human; C578634:CHEK2 protein, human; D000071877:Checkpoint Kinase 1; D017346:Protein Serine-Threonine Kinases; C517004:CDC25A protein, human; D020687:cdc25 Phosphatases; D019311:Staurosporine