RTRB-10TYROSINE KINASE RECEPTOR TIE2 REGULATES DNA REPAIR THROUGH THE PROTO-ONCOGENE ABL1 IN BRAIN TUMOR STEM CELLS

Abstract

Glioblastoma is the most common and aggressive primary brain tumor in adults with a median survival of 9-15 months. This tumor invariably recurs after therapy, which is largely caused by the striking radioresistance. In gliomas, we found the tyrosine kinase receptor TIE2 is overexpressed in brain tumor stem cells (BTSCs) and in human surgical glioma specimens in relation to malignancy. Here we find that ionizing irradiation (IR) treatment of mice bearing intracranial BTSCs-derived xenografts resulted in an unexpected TIE2 nuclear localization and increased level of its natural ligand, ANG1. These data was corroborated in BTSCs and glioma cultures upon IR or Ang1 stimulus, using immunofluorescence and confocal analysis, and subcellular fractionation followed by Western blot. Of clinical interest, nuclear TIE2 is associated with radioresistance of glioma cells, which was reverted by using Tie2 soluble receptor (which inhibits ANG1/TIE2 interaction) and after genetic Tie2 mutation of a newly discovered nuclear localization signal. At the molecular level, we also found that after IR treatment, TIE2 localized in the DNA-repair foci and complexed with gH2AX, suggesting a role of TIE2 in the DNA repair machinery. To validate our hypothesis, we used a fluorescent reporter construct in which GFP gene was reconstituted following a non-homologous end joining (NHEJ) event, and we observed that TIE2-expressing cells showed more efficient NHEJ repair than non-expressing TIE2 cells. Based on the recently reported role of ABL1 (cAbl) in the ATM and KAT5 mediated DNA damage repair, we explored the relationship between ABL1 and the TIE2-mediated radioresistance. Our data clearly showed that DNA repair efficiency significantly and specifically decreased by using ABL1 inhibitors or ABL1 siRNAs, but not after ABL2 inhibition. Collectively, our results should propel the development of preclinical studies on the combination of nuclear TIE2-targeting strategies and inhibition of ABL1 with radiotherapy for patients with glioblastomas.