PID1 IS A NOVEL SENSITIZER OF BRAIN TUMOR CELLS TO CHEMOTHERAPY

Abstract

BACKGROUND: PID1 is a phosphotyrosine binding domain-containing protein of unknown function in cancer. We recently provided the first report of PID1 in brain tumors (and in cancer). PID1 inhibits growth and proliferation and induces cell death, apoptosis and mitochondrial depolarization in glioblastoma, medulloblastoma and ATRT cell lines. PID1 siRNA had the opposite effect on mitochondrial depolarization. PID1 mRNA level was directly correlated with survival of patients with medulloblastoma or glioma: PID1 mRNA was lowest in tumor subgroups with the poorest prognosis and highest in the more favorable prognosis groups within each diagnosis. The mechanism by which PID1 affects these tumors is currently unknown, and is the focus of studies in our laboratory. METHODS: Experiments were conducted in tissue culture in primary and established cell lines using established methods to assess mRNA, proliferation and signal transduction. RESULTS: Since lower PID1 mRNA was associated with poorer outcome in medulloblastoma and gliomas and higher PID1 was associated with improved outcome, we hypothesized that PID1 level may affect responsiveness of these brain tumors to therapy: higher responsiveness in tumors with high PID1 and resistance when it is low. Indeed, while both cisplatin (10 µg/ml) or transient PID1 overexpression increased apoptosis of glioma and medulloblastoma cell lines (indicated by increased AnnexinV, caspase-3 cleavage and mitochondrial depolarization), combining cisplatin with PID1 caused a markedly higher effect than each alone. Moreover, knockdown of PID1 by siRNA inhibited the cisplatin-induced mitochondrial membrane depolarization and apoptosis (AnnexinV and caspase-3 cleavage), suggesting that PID1 may be required for cisplatin-induced apoptosis. This supports our hypothesis that PID1 may sensitize brain tumor cells to chemotherapy. Intriguingly, PID1 mRNA increased in response to cisplatin (5 µg/ml) as well as to etoposide (5 µg/ml) and vincristine (50ng/ml), in a time- and dose-dependent manner in the brain tumor cell lines. Further, etoposide and cisplatin also increased PID1 luciferase promoter activity. Interestingly, the increase in chemotherapy-induced PID1 promoter reporter activity as well as the chemotherapy-induced increase in PID1 mRNA were both blocked by inhibitors of NFκB, consistent with presence of an NBκB recognition site in the promoter of PID1, and indicating that regulation of PID1 may be an NFκB-dependent mechanism. CONCLUSIONS: Our data suggest that PID1 mediates glioma and medulloblastoma cell response to chemotherapy in an NFκB-dependent manner. Moreover, these data also suggest that PID1 sensitizes medulloblastoma and glioma to chemotherapy, providing the initial mechanistic step to understand the correlation between higher PID1 mRNA and survival in patients. SECONDARY CATEGORY: Pediatrics.