The long-term objective of this study is to improve the therapy of gliomas by better understanding the basis for sensitivity/resistance to temozolomide (TMZ), a chemotherapeutic agent important in the treatment of gliomas. The sensitivity/resistance of gliomas to TMZ is well known to be influenced by the extent of TMZ-induced DNA damage and by the ability of the tumor to repair this damage. The sensitivity/resistance of gliomas to TMZ is also, however, influenced by the G2 cell cycle checkpoint, activation of which can dissociate TMZ-induced DNA damage from TMZ-induced cytotoxicity. The TMZ-induced G2 checkpoint is controlled in part by the Chk1 pathway, activation of which leads to phosphorylation of the Cdc2/Cyclin B complex, G2 arrest, and protection from TMZ toxicity. We recently uncovered evidence that a second independent pathway involving the p38 stress kinases may also control TMZ-induced G2 arrest and, in turn, TMZ sensitivity/resistance. Despite its potential importance in controlling TMZ sensitivity/resistance, the activation and consequences of activation of the p38 pathway, as well as its interaction with the Chk1 pathway and potential for therapeutic manipulation remain undefined. We hypothesize that 1) the p38 pathway is activated following TMZ exposure by DNA mismatch repair-induced DNA single- strand breaks, 2) the DNA damage sensors c-abl, 53BP1, ATM, and/or ATR link MMR-induced DNA damage to p38 activation, 3) activated p38 contributes to the initiation of TMZ-induced G2 arrest by suppressing nuclear activity of Cdc25 B and/or Cdc25C, and Cdc2-cyclin B complexes, 4) p38 contributes to the maintenance of TMZ-induced G2 arrest by effects on p53- and p21-dependent Cdc2 activation, and 5) inhibition of the p38 and Chk1 pathways will additively or synergistically sensitize glioma cells to TMZ in vitro and in vivo. The results of these studies are expected to lead to a better understanding of the TMZ-induced G2 checkpoint and to identification of ways in which the TMZ-induced G2 checkpoint, and hence TMZ resistance, can be selectively reversed in gliomas.
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