ATM-depletion in breast cancer cells confers sensitivity to PARP inhibition

Abstract

Background: Mutations in the DNA damage response (DDR) factors, breast cancer 1 (BRCA1) and BRCA2, sensitize tumor cells to poly(ADP-ribose) polymerase (PARP) inhibitors. The ataxia telangiectasia mutated (ATM) kinase is a key DDR protein whose heterozygous germline mutation is a moderate-risk factor for developing breast cancer. In this study, we examined whether ATM inactivation in breast cancer cell lines confers sensitivity to PARP inhibitors.

Methods: Wild-type BRCA1/2 breast cancer cells (i.e., MCF-7 and ZR-75-1 lines) were genetically manipulated to downregulate ATM expression then assayed for cytostaticity/cytotoxicity upon treatment with PARP inhibitors, olaparib and iniparib.

Results: When ATM-depleted cells and their relative controls were treated with olaparib (a competitive PARP-1/2 inhibitor) and iniparib (a molecule originally described as a covalent PARP-1 inhibitor) a different response to the two compounds was observed. ATM-depletion sensitized both MCF-7 and ZR-75-1 cells to olaparib-treatment, as assessed by short and long survival assays and cell cycle profiles. In contrast, iniparib induced only a mild, ATM-dependent cytostatic effect in MCF-7 cells whereas ZR-75-1 cells were sensitive to this drug, independently of ATM inactivation. These latest results might be explained by recent observations indicating that iniparib acts with mechanisms other than PARP inhibition.

Conclusions: These data indicate that ATM-depletion can sensitize breast cancer cells to PARP inhibition, suggesting a potential in the treatment of breast cancers low in ATM protein expression/activity, such as those arising in mutant ATM heterozygous carriers.

Figure 1

MCF-7 transduction with shATM-carrying vectors elicits a phenotype compatible with ATM defective cells. (A) MCF-7 cells were transfected with shATM-carrying vector (MCF7-ATMi) and its siR5 negative control (MCF7-ctr). ATM protein levels in MCF-7-ATMi and MCF-7-ctr cells were analyzed by Western blot. α-tubulin was used as an internal control. B-D Cell viability of MCF7-ATMi and MCF7-ctr cells upon treatment with IR (B) and doxorubicin (C). (D) MCF7-ctr cells were pre-treated with ATM inhibitor KU 55933 or its solvent before addition of doxorubicin as in (C). Data are represented as mean ± standard deviation (SD). (E) Flow cytometry analysis of cell-cycle distribution of MCF7-ATMi and MCF7-ctr cells upon treatment with IR and doxorubicin at indicated times. Asterisks indicate statistical significant difference (*P < 0.1; **P < 0.05).

Figure 2

MCF7-ATMi cells are more sensitive than MCF7-ctr cells to olaparib. A-B MCF7-ATMi and MCF7-ctr cells were exposed to increased concentrations of olaparib for 72 hrs (A) or were treated with olaparib (5 μM) for up to 96 hrs (B). Data are represented as mean ± SD. (C) Flow cytometry analysis of cell-cycle distribution of MCF7-ATMi and MCF7-ctr cells treated with the indicated concentrations with olaparib for 48 hrs. (D) DNA synthesis was measured by BrdU incorporation assay 48 hrs after olaparib treatment. (E) Quantitative analyses of colony formation. The numbers of DMSO-resistant colonies in MCF7-ATMi and MCF7-ctr cells were set to 100, while olaparib treated cel1s were presented as mean ± SD. Asterisks indicate statistical significant difference (*P < 0.1).

Figure 3

MCF7-ATMi cells are more sensitive than MCF7-ctr cells to iniparib. (A) Quantitative analyses of colony formation. The numbers of DMSO-resistant colonies in MCF7-ATMi and MCF7-ctr cells were set to 100, while iniparib treated cel1s were presented as mean ± SD. (B) Flow cytometry analysis of cell-cycle distribution of MCF7-ATMi and MCF7-ctr cells treated with the indicated concentrations of iniparib for 48 hrs. (C) DNA synthesis was measured by BrdU incorporation assay 48 hrs after iniparib treatment. Data are represented as mean ± SD. Asterisks indicate statistical significant difference (*P < 0.1; **P < 0.05).

Figure 4

ZR-ATMi cells are more sensitive than ZR-ctr cells to 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 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 indicate statistical significant difference (*P < 0.1; **P < 0.05).