Targeting poly(ADP-ribose) polymerase activity for cancer therapy

Abstract

Poly(ADP-ribosyl)ation is a ubiquitous protein modification found in mammalian cells that modulates many cellular responses, including DNA repair. The poly(ADP-ribose) polymerase (PARP) family catalyze the formation and addition onto proteins of negatively charged ADP-ribose polymers synthesized from NAD(+). The absence of PARP-1 and PARP-2, both of which are activated by DNA damage, results in hypersensitivity to ionizing radiation and alkylating agents. PARP inhibitors that compete with NAD(+) at the enzyme's activity site are effective chemo- and radiopotentiation agents and, in BRCA-deficient tumors, can be used as single-agent therapies acting through the principle of synthetic lethality. Through extensive drug-development programs, third-generation inhibitors have now entered clinical trials and are showing great promise. However, both PARP-1 and PARP-2 are not only involved in DNA repair but also in transcription regulation, chromatin modification, and cellular homeostasis. The impact on these processes of PARP inhibition on long-term therapeutic responses needs to be investigated.

Fig. 1

The domain structure of the two DNA damage-dependent human PARPs, PARP-1 and PARP-2, showing the DNA-binding, the automodification and catalytic domains (domains A–F). The PARP signature sequence (in dark blue within the F catalytic domain) is the most conserved between the PARPs. Zn I and Zn II: Zinc-finger motifs. Zn III: Zinc ribbon domain. NLS nuclear localization signal; BRCT BRCA1 carboxy terminus; NoLS nucleolar localization signal

Fig. 2

Scheme summarizing the differential effect of PARP-1 inhibition versus silencing on SSB repair via the BER pathway. The experimental evidence would suggest that SSB rejoining proceeds via the LPR sub-pathway when the SPR is deficient because of a lack of PARP-1. In contrast, inhibition of poly(ADP-ribosyl)ation in PARP-1-proficient cells was found to result in the accumulation of PARP-1 and PCNA in the vicinity of DNA damaged sites, with a tenfold reduction of the bulk rate of SSBR. This does not impact on radiosensitivity in the G1 phase of the cell cycle, probably because cells have enough time to perform SSB repair. However, under such conditions in S phase collision of unrepaired SSBs with replication forks results in a increase of radiosensitivity due to the formation of a large number of DSBs (reproduced with permission from [87])

Fig. 3

Radiation survival in control, PARP-1-KD, and XRCC1-KD HeLa cells synchronized in S phase. Cells were synchronized by a double thymidine block, allowed to progress in S phase for 2 h and exposed to increasing doses of gamma rays. Following treatment, the cells were allowed to grow as colonies for 10–15 days, fixed, stained with Coomassie Blue, and then counted. The experimental data for control and XRCC1-KD lay within the same envelope of statistical deviation whereas the PARP-1-KD cells showed an enhanced radiation sensitivity (reproduced with permission from [87])