PTTG1IP Orchestrates Epithelial–Mesenchymal Transition and DNA Damage Response in Thyroid Cancer Progression

Background: Thyroid cancer progression involves cell-state plasticity in the form of epithelial–mesenchymal transition (EMT), and defects in DNA damage response (DDR), both of which are linked to metastasis and treatment failure. The role of pituitary tumor transforming 1 interacting protein (PTTG1IP/PBF) in these processes remains insufficiently defined. Methods: Transcriptomes from The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) (GSE138042) datasets were analyzed to prioritize EMT-associated genes and to assess correlations with EMT regulators, junction markers, and matrix metalloproteinases. PTTG1IP expression was measured by quantitative real‐time reverse transcriptase PCR (qRT-PCR) in thyroid cancer cell lines and normal thyroid HTori-3 cells. Cell viability (Cell Counting Kit-8 (CCK-8)) and apoptosis (terminal deoxynucleotidyl transferase dUTP nick end labeling, TUNEL) assays were performed following shRNA knockdown of PTTG1IP. The PTTG1IP–CTTN association was examined by co-immunoprecipitation and immunofluorescence. Chromatin immunoprecipitation (ChIP)-qPCR was used to investigate PTTG1IP occupancy at DDR gene promoters. Radiosensitivity was evaluated by loss of cell viability, flow-cytometric cell death, and phosphorylated H2AX (p-H2AX) immunofluorescence after irradiation. Results: PTTG1IP emerged as a top EMT-linked candidate across cohorts, with elevated mRNA and protein expression in thyroid cancers and cell lines. Silencing of PTTG1IP reduced viability and increased apoptosis in human thyroid cancer cell lines TPC-1 and KTC-1. PTTG1IP expression aligned with canonical EMT transcription factors and with DDR genes. Biochemically, PTTG1IP formed an endogenous complex with cortactin (CTTN) and co-localized in cancer cells. Mechanistically, PTTG1IP occupied the BRCA1, BRCA2, RAD51, RAD51-associated protein 1 (RAD51AP1), and ATM serine/threonine kinase (ATM) promoters. Functionally, depletion of PTTG1IP led to increased radiation-induced DNA damage and cell death, resulting in a greater post-irradiation loss of viability. Conclusions: PTTG1IP is a multifunctional node in thyroid cancer, coupling cytoskeletal programs with DDR control, and supporting cell growth and treatment tolerance. The targeting of PTTG1IP, particularly to enhance radiosensitivity, may provide a novel therapeutic strategy for thyroid cancer.