Abrogation of the Chk1-mediated G₂ checkpoint pathway potentiates temozolomide-induced toxicity in a p53-independent manner in human glioblastoma cells

Temozolomide (TMZ) produces O⁶-methylguanine in DNA, which in turn mispairs with thymine, triggering futile DNA mismatch repair (MMR) and ultimately cell death. We found previously that in p53-proficient human glioma cells, TMZ-induced futile DNA MMR resulted not in apoptosis but rather in prolonged, p53- and p21-associated G₂-M arrest and senescence. Additionally, p53-deficient cells were relatively more TMZ resistant than p53-deficient glioma cells, which underwent only transient G₂-M arrest before death by mitotic catastrophe. These results suggested that prolonged G₂-M arrest might protect cells from TMZ-induced cytotoxicity. In the present study, we therefore focused on the mechanism by which TMZ induces G₂-M arrest and on whether inhibition of such G₂-M arrest might sensitize glioma cells to TMZ-induced toxicity. U87MG glioma cells treated with TMZ underwent G₂-M arrest associated with Chk1 activation and phosphorylation of both cdc25C and cdc2. These TMZ-induced effects were inhibited by the Chk1 kinase inhibitor UCN-01. Although not in itself toxic, UCN-01 increased the cytotoxicity of TMZ 5-fold, primarily by inhibiting cellular senescence and increasing the percentage of cells bypassing G₂-M arrest and undergoing mitotic catastrophe. In addition to enhancing TMZ-induced cytotoxicity in p53-proficient cells, UCN-01 also blocked TMZ-induced Chk1 activation and transient G₂-M arrest in p53-deficient U87MG-E6 cells and similarly enhanced TMZ-induced mitotic catastrophe and cell death. Taken together, these results indicate that Chk1 links TMZ-induced MMR to G₂-M arrest. Furthermore, inhibition of the cytoprotective G₂ arrest pathway sensitizes cells to TMZ-induced cytotoxicity and may representa novel, mechanism-based means of increasing TMZ efficacy in both p53 wild-type and p53 mutant glioma cells.