Figure 4. Three clinical PARP inhibitors differ in their potency to poison PARP1 and PARP2 irrespective of their potency to inhibit PARP catalytic activity.

(A) Drug-induced PARylation inhibition. Western blotting of PAR levels in whole cell lysates from wild-type DT40 cells treated as indicated for 30 min. Blots were probed with anti-PAR antibody. Whole cell lysates from untreated PARP1−/− cells were used as a control. Asterisks indicate non-specific bands. (B) Representative PAR ELISA assays in DT40 cells treated with olaparib, veliparib or MK-4827 for 120 min. PAR level of untreated cells was set as 100%. IC₅₀ (inhibitory concentration 50%) of olaparib, veliparib and MK-4827 were 1.2 nM, 10.5 nM and 50.5 nM, respectively. (C–F) Differential cytotoxicity of the three PARP inhibitors. Viability was measured as in Figure 1D. Error bars represent SD (n=3). Survival curves of the wild-type cells (C), PARP1−/− cells (D), wild-type cells treated with a subtoxic MMS concentration (0.00025%; see Fig 2C) (E), and BRCA2-deficient DT40 cells (F). Sensitivity was determined as in Figure 1D. Error bars represent SD (n=3). The viability of PARP1−/− cell to 0.00025% MMS (22%) was shown with a horizontal dashed line with annotation (E). (G) Survival curves of DU145 treated with the indicated PARP inhibitors for 72 hours. (H) Survival curves of DU145 treated with MMS alone or MMS plus 1 μM olaparib or veliparib or MK-4827 for 72 hours.