PARP-1 and PARP-2: New players in tumour development

Abstract

Poly(ADP-ribose) polymerase-1 (PARP-1) and PARP-2 belong to a family of enzymes that, using NAD(+) as a substrate, catalyze poly(ADP-ribosyl)ation of proteins. PARP-1 and PARP-2 catalytic activity is stimulated by DNA-strand breaks targeting mainly proteins involved in chromatin structure and DNA metabolism, providing strong support for a dual role of both PARP-1 and PARP-2 in the DNA damage response as DNA damage sensors and signal transducers to downstream effectors. The DNA damage response has important consequences for genomic stability and tumour development. In order to manipulate DNA damage responses to selectively induce tumour cell death, a considerable effort is centred on defining the molecular mechanisms that allow cells to detect, respond to, and repair DNA damage. PARP inhibitors that compete with NAD+ at the highly conserved enzyme active site are arisen as new potential therapeutic strategies as chemo- and radiopotentiation and for the treatment of cancers with specific DNA repair defects as single-agent therapies. In the present review, we highlight emerging information about the redundant and specific functions of PARP-1 and PARP-2 in genome surveillance and DNA repair pathways. Understanding these roles might provide invaluable clues to design new cancer therapeutic approaches. In addition, we provide an overview of ongoing clinical trials with PARP inhibitors and the value of PARP-1 and PARP-2 expression as prognostic biomarkers in cancer.

Figure 1

Poly(ADP-ribosyl)ation reaction activated by DNA strand breaks. PARP-1 and PARP-2, rapidly recognizes DNA-strand breaks generated by genotoxic agents leading to their activation. Activated PARPs hydroiyse βNAD⁺, releasing nicotinamide (Nam) and one proton (H⁺) and catalyse the transfer of ADP-ribose moiety onto aminoacid residues of acceptor proteins. The proteins targeted are involved in numerous biological processes, including DNA repair, chromatin structure and transcription. Poly(ADP-ribosyl)ation of aceptor proteins has functional consequences such as DNA-break signalling, chromatin relaxation and recruitment of DNA repair proteins. The reaction is reversed by the activities of poly(ADP-ribose) giycohydroiase (PARG) and poly(ADP-ribose) hydrolase-3 (ARH3) that hydroiyse poly(ADP-ribose) into ADP-ribose units.

Figure 2

Structural characteristics of human PARP-1 and PARP-2. (A) Schematic representation of human PARP-1 and PARP-2 gene organisation and protein domains. The region that is homologous to the PARP signature (residues 859-908 of PARP-1 and 412-461 of PARP-2 in variant 2) as well as the crucial residue for polymerase activity ( glu-tamic acid 988 of PARP-1 and glutamid acid 545 of PARP-2 in variant 2) are indicated as darkened green box within the catalytic domain. Fl, Fll: zinc fingers motifs; FIN: zinc ribbon domain; BRCT: BRCA1 C-terminus motif; WGR: domain with unknown function; NLS: nuclear localization signal; NoLS: nucleolar localization signal. (B) Superposition of the catalytic domain structures of human PARP-1 (red) and human PARP-2 (green) in complex with PARP inhibitor ABT-888 [159] (http://www.pdb.org).

Figure 3

A model for the selective effects of PARP inhibitors on HR-deficient cells. PARP-1 and PARP-2 promote the repair of DNA single-strand breaks (SSBs) that result from various genotoxic insults such as oxidative damage by base excision repair (BER). When PARP-1 and PARP-2 are inhibited, these lesions are unresolved and large numbers of DNA SSBs persist and are encountered by DNA replication forks. These lead to replication fork arrest associated with DNA double-strand breaks (DSBs). Such DSBs are effectively repaired by homologous recombination (HR) in normal cells but not in HR-deficient cells (i.e. BRCA1- or BRCA2-deficient cells). Accordingly, PARP inhibition in these HR-deficient cells causes a high degree of genomic instability and cell death.