Figure 4 ZR-ATMi cells are more sensitive than ZR-ctr cells
to Rapamycin ic50 olaparib but not to iniparib. (A) ZR-75-1 cells were transfected with shATM-carrying vector (ZR-ATMi) and its siR5 negative control (ZR-ctr). ATM protein levels in ZR-ATMi and ZR-ctr cells were analyzed by Western blot. α-tubulin was used as an internal control. B-C ZR-ATMi and ZR-ctr cells were exposed to increased concentrations of olaparib (B) or iniparib for 72 hrs (C). Data are represented as mean ± SD. (D) Flow cytometry analysis of Ulixertinib cell-cycle distribution of ZR-ATMi and ZR-ctr cells treated with the indicated concentrations with olaparib or iniparib for 72 hrs. E-F Quantitative analyses of colony formation. The numbers of DMSO-resistant colonies in ZR-ATMi and ZR-ctr cells were set to 100, while olaparib (E) or iniparib (F) treated cel1s were presented as mean ± SD. Asterisks Palbociclib indicate statistical significant difference (*P < 0.1; **P < 0.05). In contrast with the sensitivity induced by ATM-depletion in MCF-7 cells, when treated with iniparib, both ZR-ATMi and ZR-ctr cells showed a substantial loss of viability that was independent of ATM, as indicated by the similarity of
their survival curves (Figure 4C) and cell cycle distribution (Figure 4D). These results were confirmed by the complete inhibition of colony formation induced by iniparib in ZR-75-1 cells, independent of their ATM status (Figure 4F). In addition, the different response between MCF-7 and ZR-75-1 cells to this drug suggests that ER expression and the wild-type status of BRCA1/2 and TP53 are not involved in the sensitivity to iniparib. These results might be explained by the recent observations indicating that the primary mechanism of action for iniparib is a nonselective modification of cysteine-containing proteins, rather then inhibition of PARP activity . Conclusions In a few hematological malignancies, ATM-deficiency was shown to confer sensitivity to PARP inhibitors, indicating that ATM might be included in the DDR factors whose mutation or loss of expression confer sensitivity to this class of drugs. Based on these observations, we asked whether
ATM deficiency plays a similar role in breast cancer, the solid tumor linked to ATM germline mutations. For this study, we employed two breast-cancer cell lines selected Anidulafungin (LY303366) among those showing the molecular feature we recently observed in the breast tumors arising in A-T heterozygotes. In addition, we selected two compounds, olaparib and iniparib, originally described as PARP inhibitors. We show that ATM-depletion confers sensitivity to olaparib in both cell lines and a mild sensitivity to iniparib in the MCF-7 cells indicating that ATM mutation/inactivation might be consider in the selection of breast cancers responsive to PARP inhibition. Acknowledgements We thank Dr. Tania Merlino for the proof reading of the manuscript and Dr. Lidia Strigari for statistical support.