Anticancer sensitivities and biological characteristics of <scp>HCT116</scp> cells resistant to the selective poly( <scp>ADP</scp> ‐ribose) glycohydrolase inhibitor

Poly(ADP-ribose) glycohydrolase (PARG) is a key enzyme involved in poly(ADP-ribose) (PAR) degradation and is considered a potential anticancer target. We previously investigated resistance mechanisms to the PARG inhibitor PDD00017273 in human colorectal cancer HCT116 cells and established an acquired PDD00017273-resistant HCT116R^PDD cell line. In this study, we analyzed the protein levels of enzymes associated with PAR metabolism in both parental HCT116 cells and resistant HCT116R^PDD cells using western blotting. PARG expression levels were similar between HCT116R^PDD and HCT116 cells. However, the levels of PARP1 and ARH3 were reduced in HCT116R^PDD cells compared to HCT116 cells. Nevertheless, intracellular PAR levels were elevated in HCT116R^PDD cells. Interestingly, HCT116R^PDD cells exhibited greater sensitivity to γ-ray irradiation and the nicotinamide phosphoribosyltransferase (NAMPT) inhibitor FK866 than the parental HCT116 cells, yet showed comparable sensitivity to 5-FU, cisplatin, and PARP inhibitors olaparib, talazoparib, and veliparib. Furthermore, we observed that HCT116R^PDD cells tended to maintain slightly higher levels of intracellular NAD⁺/NADH and ATP compared to parental HCT116 cells. These findings suggest that cancer cells employ a mechanism to regulate NAD⁺ and ATP levels, thereby avoiding cell death from intracellular PAR accumulation through coordinated PARP-PARG regulation.