Mitochondrial NADP(H)-dependent de novo dTMP biosynthesis counteracts the cytotoxicity of PARP inhibitors.

PARP inhibitors (PARPis), known to elicit mitochondrial protection in nononcological diseases by elevating the cellular NAD+ pool, exhibit potent cytotoxicity in selected human cancers. The role of mitochondrial metabolism in PARPi-mediated antitumor therapy remains unexplored. Here, we propose a causal link between mitochondrial NAD+ metabolism and PARPi responsiveness. In PARPi-non-responsive tumor cells, PARP inhibition specifically expands mitochondrial NADP(H) [mito-NADP(H)] pool, thereby facilitating de novo mitochondrial dTMP (mito-dTMP) biosynthesis and maintaining mitochondrial dTTP (mito-dTTP) pool to prevent uracil misincorporation into mitochondrial DNA (mtDNA), regardless of homologous recombination (HR) status. Mechanistically, loss of PTPN1 ADPRylation by PARPi abolishes its phosphatase activity toward STAT3, yielding enhanced STAT3 phosphorylation and the subsequent transactivation of FoxO1. FoxO1 modulates transcriptomic signature governing mitochondrial NADPH fluxes to de novo mito-dTMP generation. Our results uncover a fundamental vulnerability that can be leveraged by cotargeting STAT3 and PARP to trigger mitochondrial dysfunction.