Cancer metabolism in radiation sensitization: complementary roles of O-GlcNAc Transferase (OGT) and PARP1

For double-strand breaks (DSBs) formed by radiation, onset of 5' to 3' end resection is a deciding factor in repair pathway choice, favoring homologous recombination (HR) over non-homologous end-joining (NHEJ). Studying HR-proficient MCF7 breast cancer cells, we confirmed a role for PARP1 in promoting DSB repair and limiting resection stress and identify the hexosamine biosynthetic pathway (HBP)-dependent post-translational modification O-GlcNAcylation as an independent regulator. Using pharmacological and genetic perturbations of O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), we showed that O-GlcNAcylation can limit end resection as measured by BrdU and RPA staining, recruitment of HR proteins BRCA1 and RAD51, and accumulation of cytosolic DNA in S/G2-phase cells. These effects were independent of PARP1 but required the histone methyltransferase EZH2. Loss of OGT or EZH2 phenocopied PARP inhibition, leading to hyperresection after irradiation. The OGA inhibitor PUGNAc suppressed hyperresection due to PARP1 knockout while PARP inhibitor veliparib exacerbated defects in OGT- or EZH2-deficient cells. In each case, increased resection correlated with cytosolic DNA accumulation, suggesting a link to inflammatory signaling. These findings implicate the Warburg effect, via the HBP and O-GlcNAcylation, in favoring NHEJ over HR and suggest that disrupting EZH2 may sensitize HR-proficient tumor cells to radiation via resection-dependent mechanisms. Our results highlight the potential of targeting cancer-associated metabolic reprogramming to overwhelm HR repair and drive resection stress. Combining PARP inhibition with blockade of O-GlcNAcylation or EZH2 may offer a strategy to radiosensitize proliferating, HR-proficient cancers while sparing non-cycling normal tissues.