AI26 inhibits the ADP-ribosylhydrolase ARH3 and suppresses DNA damage repair.

The ADP-ribosylhydrolase ARH3 plays a key role in DNA damage repair, digesting poly(ADP-ribose) and removing ADP-ribose from serine residues of the substrates. Specific inhibitors that selectively target ARH3 would be a useful tool to examine DNA damage repair, as well as a possible strategy for tumor suppression. However, efforts to date have not identified any suitable compounds. Here, we used in silico and biochemistry screening to search for ARH3 inhibitors. We discovered a small molecule compound named ARH3 inhibitor 26 (AI26) as, to our knowledge, the first ARH3 inhibitor. AI26 binds to the catalytic pocket of ARH3 and inhibits the enzymatic activity of ARH3 with an estimated IC50 of ∼2.41 µm in vitro Moreover, hydrolysis of DNA damage-induced ADP-ribosylation was clearly inhibited when cells were pretreated with AI26, leading to defects in DNA damage repair. In addition, tumor cells with DNA damage repair defects were hypersensitive to AI26 treatment, as well as combinations of AI26 and other DNA-damaging agents such as camptothecin and doxorubicin. Collectively, these results reveal not only a chemical probe to study ARH3-mediated DNA damage repair but also a chemotherapeutic strategy for tumor suppression.

Targeting dePARylation for cancer therapy.

Poly(ADP-ribosyl)ation (PARylation) mediated by poly ADP-ribose polymerases (PARPs) plays a key role in DNA damage repair. Suppression of PARylation by PARP inhibitors impairs DNA damage repair and induces apoptosis of tumor cells with repair defects. Thus, PARP inhibitors have been approved by the US FDA for various types of cancer treatment. However, recent studies suggest that dePARylation also plays a key role in DNA damage repair. Instead of antagonizing PARylation, dePARylation acts as a downstream step of PARylation in DNA damage repair. Moreover, several types of dePARylation inhibitors have been developed and examined in the preclinical studies for cancer treatment. In this review, we will discuss the recent progress on the role of dePARylation in DNA damage repair and cancer suppression. We expect that targeting dePARylation could be a promising approach for cancer chemotherapy in the future.

Molecular basis for the MacroD1-mediated hydrolysis of ADP-ribosylation.

MacroD1 is an enzyme that hydrolyzes protein mono-ADP-ribosylation. However, the key catalytic residues of MacroD1 in these biochemical reactions remain elusive. Here, we present the crystal structure of MacroD1 in a complex with ADP-ribose (ADPR). The ß5-α10-loop functions as a switch loop to mediate substrate recognition and right orientation. The conserved Phe272 in the ß5-α10-loop plays a crucial role in the orientation of ADPR distal ribose, and a conserved hydrogen-bond network contributes significantly to hold and orient the catalytic water12, which mediates ADPR hydrolysis. Moreover, we found that MacroD1 was recruited to the sites of DNA damage via recognition of ADP-ribosylation at DNA lesions. The MacroD1-mediated ADPR hydrolysis is essential for DNA damage repair. Taken together, our study provides structural and functional insights into the molecular mechanism of MacroD1-mediated ADPR hydrolysis and its role in DNA damage repair.