PNKP Targeting Engages the Autophagic Machinery to Enhance Ferroptosis and to Potentiate Anti-Tumor Efficacy of Doxorubicin in TNBC
The polynucleotide kinase/phosphatase (PNKP) is a DNA repair enzyme possessing bifunctional DNA 3'-phosphatase and DNA 5'-kinase activities. It plays an important role in rejoining of single- and double-strand DNA breaks and is considered as a potential therapeutic target for different cancer types. Here we show that PNKP is highly expressed in triple negative breast cancer (TNBC) and associated with poor prognosis and chemoresistance. Targeting of PNKP enhanced ferroptosis in TNBC, which was associated with increased labile iron pool and ROS and concomitantly decreased in intracellular glutathione, SCD1 and GPX4 levels. Transcriptomic profiling and mechanistic data indicate that PNKP targeting robustly enhances the lysosomal and the autophagic machinery by activating STING and concurrently inhibiting STAT3, thereby increasing ferrtinophagy, intracellular iron level and modulates the expression of key ferroptosis regulators. Importantly, PNKP and STAT3 are rapidly phosphorylated, colocalize, and interact upon ferroptosis induction or doxorubicin treatment, the first line treatment for TNBC patients. Targeting of PNKP together with doxorubicin synergistically inhibited TNBC growth in animal model and TNBC-patients derived organoids. These results offer a promising therapeutic combination for TNBC and highlight the clinical potential of PNKP targeting and ferroptotic death for TNBC therapy. Overall design: PNKP knock-down was induced in the breast cancer line MDA-MB-468 using lentiviral infection of pLKO vector harboring shRNA aimed for PNKP. Cells infected with empty pLKO served as control. Three independent repeats were carried out.