TIPRL1 and its ATM-dependent phosphorylation promote radiotherapy resistance in head and neck cancer

TIPRL1 (target of rapamycin signaling pathway regulator-like 1) is a known interactor and inhibitor of protein phosphatases PP2A, PP4 and PP6 - all pleiotropic modulators of the DNA Damage Response (DDR). Here, we describe a new role for TIPRL1 in the radiotherapy (RT) response of Head and Neck Squamous Cell Carcinoma (HNSCC). TIPRL1 expression was found increased in tumor versus non- tumor tissue, with high tumoral TIPRL1 expression associating with lower locoregional control and decreased survival of RT-treated patients. TIPRL1 deletion in HNSCC cells resulted in increased RT sensitivity, a faster but prolonged cell cycle arrest, increased micronuclei formation and an altered proteome-wide DDR. Upon irradiation, ATM phosphorylates TIPRL1 at Ser265, contributing to TIPRL1-mediated RT resistance. Mass spectrometry analysis identified DNA-PKcs, RAD51 and nucleosomal histones as novel TIPRL1 interactors. Histone binding, although stimulated by RT, was adversely affected by TIPRL1 Ser265 phosphorylation. Our findings underscore a clinically relevant role for TIPRL1 and its ATM-dependent phosphorylation in RT resistance through modulation of DNA damage checkpoint activation and repair.