ATM inhibitor KU-55933 increases the TMZ responsiveness of only inherently TMZ sensitive GBM cells

Oct 12, 2012Journal of neuro-oncology

ATM inhibitor KU-55933 boosts temozolomide response only in glioblastoma cells already sensitive to temozolomide

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Abstract

KU-55933 significantly increased cell killing in TMZ-sensitive glioma lines compared to TMZ alone.

The survival rate of U251 cells decreased from 0.08 ± 0.01 to 0.004 ± 0.0015 with the addition of KU-55933 to TMZ.
U87 cells showed a similar trend, with survival dropping from 0.04 ± 0.002 to 0.02 ± 0.005 when treated with KU-55933 and TMZ.
Addition of KU-55933 elevated the fraction of U251 cells arrested in G2/M from 35 ± 0.8 % to 61.8 ± 1.1 %.
For U87 cells, G2/M fraction increased from 18.6 ± 0.4 % to 25 ± 0.2 % with KU-55933 combined with TMZ.
KU-55933 did not sensitize TMZ-resistant lines, and no G2/M arrest was observed in these resistant cells.
ATM inhibition may suppress the repair of TMZ-induced DNA damage in inherently sensitive glioma cells.

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Ataxia telangiectasia mutated (ATM) kinase is critical in sensing and repairing DNA double-stranded breaks (DSBs) such as those induced by temozolomide (TMZ). ATM deficiency increases TMZ sensitivity, which suggests that ATM inhibitors may be effective TMZ sensitizing agents. In this study, the TMZ sensitizing effects of 2 ATM specific inhibitors were studied in established and xenograft-derived glioblastoma (GBM) lines that are inherently sensitive to TMZ and derivative TMZ-resistant lines. In parental U251 and U87 glioma lines, the addition of KU-55933 to TMZ significantly increased cell killing compared to TMZ alone [U251 survival: 0.004 ± 0.0015 vs. 0.08 ± 0.01 (p < 0.001), respectively, and U87 survival: 0.02 ± 0.005 vs. 0.04 ± 0.002 (p < 0.001), respectively] and also elevated the fraction of cells arrested in G2/M [U251 G2/M fraction: 61.8 ± 1.1 % vs. 35 ± 0.8 % (p < 0.001), respectively, and U87 G2/M fraction 25 ± 0.2 % vs.18.6 ± 0.4 % (p < 0.001), respectively]. In contrast, KU-55933 did not sensitize the resistant lines to TMZ, and neither TMZ alone or combined with KU-55933 induced a G2/M arrest. While KU-55933 did not enhance TMZ induced Chk1/Chk2 activation, it increased TMZ-induced residual γ-H2AX foci in the parental cells but not in the TMZ resistant cells. Similar sensitization was observed with either KU-55933 or CP-466722 combined with TMZ in GBM12 xenograft line but not in GBM12TMZ, which is resistant to TMZ due to MGMT overexpression. These findings are consistent with a model where ATM inhibition suppresses the repair of TMZ-induced DSBs in inherently TMZ-sensitive tumor lines, which suggests an ATM inhibitor potentially could be deployed with an improvement in the therapeutic window when combined with TMZ.

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ATM inhibitor KU-55933 increases the TMZ responsiveness of only inherently TMZ sensitive GBM cells

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