PARP9-mediated PARylation is essential for the cytotoxicity of talazoparib in colorectal cancer

Poly (ADP-ribose) polymerase (PARP) inhibitors have emerged as a cornerstone of targeted cancer therapy, yet their mechanisms beyond canonical DNA damage repair remain incompletely understood. Here, we investigated the role of ADP-ribosylation dynamics in mediating PARP inhibitor sensitivity in colorectal cancer. Transcriptomic profiling of tumor tissues from PARP inhibitor-treated xenografts revealed significant enrichment of NAD+-associated metabolic pathways alongside DNA damage responses, implicating NAD+-dependent ADP-ribosylation in the modulation of cytotoxicity. Among clinically approved PARP inhibitors, talazoparib exhibited the most potent cytotoxic effects across colorectal cancer cell lines, with HCT-8 cells displaying the highest sensitivity compared to HCT-116 and SW-620 cells, which showed limited responsiveness. Mechanistically, talazoparib treatment unexpectedly enhanced PARylation but not MARylation in sensitive cells, an effect associated with intracellular NAD+ accumulation. Expression profiling identified PARP9 and PARP14 as preferentially upregulated in HCT-8 cells relative to HCT-116 and SW-620 cells, and stable knockdown of either PARP9 or PARP14 attenuated talazoparib-induced cytotoxicity, accompanied by diminished NAD+ accumulation and PARylation alterations. Collectively, these findings uncover a previously underappreciated mechanism by which PARP inhibitors exert cytotoxic effects on cancer cells through NAD+-dependent PARylation dynamics mediated by non-canonical PARP family members, particularly PARP9 and PARP14, providing new insights into the therapeutic applicability of PARP inhibitors in colorectal cancer.